**Synthesis of TiO2 Nanoparticles and Their Catalytic Activity**

Editor

**Wei Zhou**

Basel • Beijing • Wuhan • Barcelona • Belgrade • Novi Sad • Cluj • Manchester

*Editor* Wei Zhou Chemistry and Chemical Engineering Qilu University of Technology Jinan China

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Nanomaterials* (ISSN 2079-4991) (available at: www.mdpi.com/journal/nanomaterials/special issues/TiO2 nanoparticles).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

Lastname, A.A.; Lastname, B.B. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-8753-0 (Hbk) ISBN 978-3-0365-8752-3 (PDF) doi.org/10.3390/books978-3-0365-8752-3**

© 2023 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license. The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) license.

 He subsequently worked at Heilongjiang University, and he became a full professor in 2015. From 2020, he worked at Qilu University of Technology as an independent Principal Investigator. His research interests include mesoporous materials, semiconductor nanomaterials for solar energy conversion, photocatalysis, photothermal and photoelectrochemical performance. He has published over 150 peer-reviewed SCI papers as first author or corresponding author with over 15000 citations and has an H-index of 70.

## **Preface**

Solar light utilization technologies are largely promoted by the rapid development of material science, which have generated various applications in energy conversion and catalysis. In the past decades, we have experienced the rapid development of solar energy conversion research, resulting in a remarkable increase in energy conversion efficiency and photocatalytic performance. Due to their high stability, low cost, decent activity, and non-toxic properties, TiO2 nanomaterials have received considerable attention in photocatalytic water splitting, CO2 conversion, organic synthesis, etc., in terms of efficient utilization of abundant solar energy. Besides the photocatalytic energy conversions, TiO2 can also be deposited onto substances as bioactive coatings for bioimplant applications to effectively improve the generation of reactive oxygen species for antibacterial. Thus, the advances in catalytic applications by TiO2 nanomaterials could facilitate the utilization of photocatalysts in real conditions. We are delighted to introduce this Special Issue of "Synthesis of TiO2 Nanoparticles and Their Catalytic Activity", which includes 10 research and review articles that explore cutting-edge advances in the synthesis and applications of TiO2 nanomaterials in various fields. We would like to thank all the authors and reviewers for their contributions to this Special Issue. We also hope that the articles presented here are helpful and inspiring for their designs, concepts, and perspectives and can arouse novel understandings in the field of nanomaterials and catalysis.

> **Wei Zhou** *Editor*

## *Article* **Construction of Built-In Electric Field in TiO2@Ti2O3 Core-Shell Heterojunctions toward Optimized Photocatalytic Performance**

**Tingting Hu 1,2, Panpan Feng 3,\*, Liping Guo 2 , Hongqi Chu 2 and Fusheng Liu 1,\***

**Abstract:** A series of Ti2O3@TiO2 core-shell heterojunction composite photocatalysts with different internal electric fields were synthesized using simple heat treatment methods. The synthesized Ti2O3@TiO2 core-shell heterojunction composites were characterized by means of SEM, XRD, PL, UV–Vis, BET, SPV, TEM and other related analytical techniques. Tetracycline (TC) was used as the degradation target to evaluate the photocatalytic performance of the synthesized Ti2O3@TiO2 coreshell heterojunction composites. The relevant test results show that the photocatalytic performance of the optimized materials has been significantly enhanced compared to Ti2O3 , while the photocatalytic degradation rate has increased from 28% to 70.1%. After verification via several different testing and characterization techniques, the excellent catalytic performance is attributed to the efficient separation efficiency of the photogenerated charge carriers derived from the built-in electric field formed between Ti2O3 and TiO2 . When the recombination of electrons and holes is occupied, more charges are generated to reach the surface of the photocatalyst, thereby improving the photocatalytic degradation efficiency. Thus, this work provides a universal strategy to enhance the photocatalytic performance of Ti2O3 by coupling it with TiO2 to build an internal electric field.

**Keywords:** photocatalysis; Ti2O3@TiO2 core-shell heterojunction; built-in electric field

#### **1. Introduction**

Tetracycline (TC) has stable properties due to its four-complex benzene cycloalkyl groups [1,2]. It is highly susceptible to accumulation in the environment, and it is a difficultto-degrade organic pollutant. Tetracycline (TC) in the environment is persistent, easy to accumulate, and difficult to degrade, which causes serious harm to the food chain and human health [3,4]. In addition, due to its long application, drug resistance is a serious concern. It can be absorbed from the gastrointestinal tract, albeit not completely. About 60%~70% of the dose is residual in the environment. Therefore, the long-term residue of TC in the aquatic environment harms human health and the ecological environment. To alleviate this serious situation, applying photocatalysis technology to the degradation of water pollutants as soon as possible will greatly help with the purification of the water environment [5,6]. Compared with traditional chemical, physical, and biological technologies, semiconductor photocatalytic technology can efficiently degrade organic pollutants in wastewater, such as antibiotic wastewater, into small-molecule substances [7–9].

As the ninth most abundant element on Earth, titanium has been widely studied for its high thermal stability, low cost, and light-responsive ability in metal oxide composites such as titanium dioxide (TiO2) [10–12]. TiO2 can be stimulated by light to produce strong oxidizing h+ and strong reducing e−, which makes TiO2 highly efficient in terms

**Citation:** Hu, T.; Feng, P.; Guo, L.; Chu, H.; Liu, F. Construction of Built-In Electric Field in TiO2@Ti2O3 Core-Shell Heterojunctions toward Optimized Photocatalytic Performance. *Nanomaterials* **2023**, *13*, 2125. https://doi.org/10.3390/ nano13142125

Academic Editors: Barbara Bonelli and Andreu Cabot

Received: 1 June 2023 Revised: 12 July 2023 Accepted: 18 July 2023 Published: 21 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of its photocatalytic performance [13,14]. However, due to its inherent large bandgap (≈3 eV), typical titanium dioxide only responds to ultraviolet radiation with a wavelength of <378 nm [15–17]. In the past few decades, researchers have considerably reduced the bandgap to achieve titanium dioxide absorption of solar energy in the visible spectral range [18]. Research in the literature confirms that titanium dioxide's bandgap width can be reduced to 1.5 eV, while its optical response can be extended to 800 nm, achieving a utilization rate of 40% for total solar energy [19]. However, to design an efficient fullspectrum solar converter with high solar energy utilization, further modification of titanium oxide is needed to reduce the bandgap to less than 0.5 eV.

Magnéli phase titanium oxides were only identified as conductive compounds in the 1950s, with the general formula TinO2n−1 (3 < *n* < 10). This non-stoichiometric semiconductor titanium oxide, which contains structural vacancies in both the titanium and oxygen sublattices, exhibits excellent photocatalytic performance. Due to the limitations of the methods used to manufacture Magnéli phase materials, it is difficult to prepare individual pure phases [20,21]. Therefore, the Magnéli phase materials synthesized by heating TiO2 or Ti at different temperatures typically contain mixed phases. In addition, the reliability of the electrical and chemical properties of the phases obtained through separation methods is relatively low. Even the most common methods (such as lame synthesis methods, both electrochemical and chemical) for synthesizing titanium suboxides still have many problems, such as the low active surface of the obtained material and the difficulty in adjusting the size and structure. For example, processing must be carried out in a high-temperature and oxygen-free atmosphere. In addition, even at temperatures ranging from 425 to 525 ◦C, TinO2n−1 is unstable and decomposes into various upper structures. However, the enhanced part of the photocatalytic performance with regard to photocatalytic degradation comes from the nanostructures on the surface of bulk titanium suboxides. Ti3+ ions play a significant role in reducing the bandgap and effectively boosting the adsorption and activation of reactants, which markedly enhances the light-responsive ability [22]. The relatively small bandgap of Ti2O3 enables it to absorb solar energy within the full spectral range [23,24]. In addition, Ti2O3 is one of the most representative examples of the Mott insulation system, and its metal–insulator transition characteristics are derived from the electronic correlation effect [25,26]. Due to its narrow bandgap, Ti2O3 exhibits high theoretical photocurrent density and high carrier mobility. After modification, the light absorption ability of traditional Ti2O3 can be further enhanced. Among many methods, constructing an internal electric field at the interface of the photocatalysts as a driving force for charge separation is considered an efficient method for achieving maximum carrier separation and driving target surface reactions in photocatalytic systems [27]. Specifically, photogenerated electrons will undergo reverse transfer driven by an electric field, greatly accelerating the separation of electron–hole pairs [28,29]. For example, Cui et al. constructed an internal electric field by preparing p–n homojunction perovskite solar cells, which directionally promoted the transport of photogenerated carriers, greatly reducing the carrier recombination losses and improving the power conversion efficiency [30].

As titanium suboxides are usually prepared via heat treatment under an inert or reductive atmosphere, there is a demand for methods to prepare nanostructured titanium suboxides via hydrothermal or other methods. During these synthesis processes, the phase structure of the material is not easily controlled, making it even more difficult to construct controllable heterojunctions in an orderly manner. Research has shown that introducing TiO2 (anatase)-based symbionts into the low oxidation state structure can significantly improve the conductivity of the composite and the properties related to photoluminescence [31]. Therefore, in this work, we combined the TiO2 and Ti2O3 to construct an orderly core-shell heterojunction, creating an internal electric field. In addition, the band structures of Ti2O3@TiO2 heterojunctions were investigated by means of UV–Vis spectroscopy, the Mott–Schottky curve, and surface photovoltage (SPV). Constructing an internal electric field has been proven to promote the transfer efficiency of photogenerated charges and improve photocatalytic activity. The correlation between built-in electric fields

and photocatalytic performance was revealed based on multiple test characterizations. The possible enhanced photocatalytic degradation mechanism of tetracycline (TC) was proposed. This work provides a simple and universal method for regulating the built-in electric fields in photocatalysts that can be applied to environmental pollution purification.

#### **2. Materials and Methods**

#### *2.1. Chemicals*

The Ti2O3 chemicals and tetracycline (TC) were purchased from Aladdin. The ethanol and Nafion solution were purchased from Sigma–Aldrich. Deionized water was used in the experiments. The materials used were all of analytical grade (99%) and had not undergone purification treatment.

#### *2.2. Methods*

## Synthesis of the Ti2O3@TiO2 Heterojunction

The Ti2O3@TiO2 heterojunction was synthesized via a simple heat treatment method. First, using commercial Ti2O3 as the research object, a Ti2O3@TiO2 heterojunction system was constructed in an orderly fashion using heat treatment methods. Then, 1 g of Ti2O3 was dispersed in a porcelain boat. Program heating was used to raise the temperature at 2 ◦C min−1 from room temperature to different predetermined temperatures. The temperature was kept constant for 2 h and cooled to room temperature. The reaction conditions for the preparation of the TiO2@Ti2O3 heterojunction were explored. Reaction temperatures of 100 ◦C, 200 ◦C, 300 ◦C, 400 ◦C, 500 ◦C, 550 ◦C, 600 ◦C, and 700 ◦C were selected to explore the photocatalytic activity of TiO2@Ti2O3 toward TC decomposition. The prepared catalysts were named Ti2O3-100, Ti2O3-200, Ti2O3-300, Ti2O3-400, Ti2O3-500, Ti2O3-550, Ti2O3-600, and Ti2O3-700, respectively.

#### *2.3. Characterizations*

The crystalline phase structures of the Ti2O3@TiO2 samples were determined via X-ray diffraction (XRD) (Rigaku, Tokyo, Japan) with Cu Kα irradiation. Field emission scanning electron microscopy (SEM) (HITACHI, SU8010, Tokyo, Japan) and transmission electron microscopy (TEM) (JEM-ARM200F, Tokyo, Japan) were performed to reveal the surface topography of the Ti2O3@TiO2 samples. The adsorption isotherm of the Ti2O3@TiO2 samples was used to measure the specific surface area, and a pore size distribution analyzer (BET, Micromeritics, ASAP2460, Norcross, GA, USA) with N2 as the adsorption medium was used in combination with analysis via the Brunauer–Emmett–Teller (BET) method. The carrier recombination was evaluated based on the photoluminescence (PL) spectra (Edinburgh, Livingston, FLS 980, Scotland, UK) with using a Xenon lamp (excitation wavelength 375 nm). UV–Vis diffuse reflectance spectroscopy was used to analyze the spectral response range of the Ti2O3@TiO2 (DRS, Shimadzu, SolidSpec-3700, Tokyo, Japan). The energy gap (Eg) of the Ti2O3@TiO2 and Ti2O3 was obtained via the Tauc plot method. Thermogravimetric analysis (TGA) was used to monitor the thermal stability performance of the Ti2O3@TiO2 and Ti2O3 on an SDT Q600 (TA Instruments, New Castle, DE, USA). The SPV transient was recorded using a tunable Nd:YAG laser (EKSPLA, NT 342/1/UVE) excited by pulses with a duration of 5 ns and wavelengths between 420 and 720 nm, and a sampling oscilloscope (GAGE, CS14200, sampling rate of 100 Mm/s) was used by applying logarithmic readings without averaging to avoid potential accuracy loss in a short period. An electrochemical station (CHI660E, Chenhua, Shanghai, China) with a conventional three-electrode system was employed to carry out the electrochemical measurements. The three-electrode system contained working, counter (carbon rod) and reference (Ag/AgCl) electrodes in 1 M KOH alkaline medium. The steps for making the working electrodes were as follows: 4 mg of catalysts was dispersed in 1 mL of mixed solution (water:ethanol = 3:1) before 10 µL was taken and added dropwise to the surface of 1 × 1 cm−2 ITO and then left to dry naturally.

#### *2.4. Measurement of Photocatalytic Activity 2.4. Measurement of Photocatalytic Activity*

of 1 × 1 cm−2 ITO and then left to dry naturally.

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The photocatalytic performances of the Ti2O3@TiO2 and Ti2O3 were mainly confirmed by examining the photocatalytic degradation under simulated visible light. First, 100 mg of catalyst was added to 100 mL of TC solution (5 mg/L). A mechanical stirrer was used to stir the solution with a speed range of 0–1500 rpm. Before irradiation, a 20 min dark reaction was performed to achieve adsorption–desorption equilibrium. Then, the reaction solution was irradiated using a 300 W Xenon lamp with AM 1.5 (the current was 1.8 A; PLS-SXE300D, Perfect Light, Beijing, China). During the photocatalytic reaction, 4 mL of the solution was withdrawn every 20 min and the concentration of TC was measured. Correspondingly, the absorbance of TC in the supernatant solution was measured at a detection wavelength of 357 nm using a UV–Vis spectrophotometer corresponding to the maximum adsorption for the solution. The photocatalytic activities of the Ti2O3@TiO2 and Ti2O3 were studied by analyzing the degradation curves of the tetracycline. The photocatalytic performances of the Ti2O3@TiO2 and Ti2O3 were mainly confirmed by examining the photocatalytic degradation under simulated visible light. First, 100 mg of catalyst was added to 100 mL of TC solution (5 mg/L). A mechanical stirrer was used to stir the solution with a speed range of 0–1500 rpm. Before irradiation, a 20 min dark reaction was performed to achieve adsorption–desorption equilibrium. Then, the reaction solution was irradiated using a 300 W Xenon lamp with AM 1.5 (the current was 1.8 A; PLS-SXE300D, Perfect Light, Beijing, China). During the photocatalytic reaction, 4 mL of the solution was withdrawn every 20 min and the concentration of TC was measured. Correspondingly, the absorbance of TC in the supernatant solution was measured at a detection wavelength of 357 nm using a UV–Vis spectrophotometer corresponding to the maximum adsorption for the solution. The photocatalytic activities of the Ti2O3@TiO2 and Ti2O3 were studied by analyzing the degradation curves of the tetracycline.

solution (water:ethanol = 3:1) before 10 µL was taken and added dropwise to the surface

#### **3. Results and Discussion 3. Results and Discussion**

#### *3.1. The Results of the TGA 3.1. The Results of the TGA*

To investigate the changes in the quality and heat flux of the materials at different temperatures, TG tests were conducted on the samples. From the results (Figure 1), it can be seen that when the temperature reaches above 500 ◦C, the material mass gradually increases, which is due to the gradual oxidation of Ti2O3 to form TiO2. When the temperature reaches 750 ◦C, the mass of the sample tends to stabilize. The sample undergoes phase transformation at different temperatures, so it is highly feasible to construct a Ti2O3@TiO2 heterojunction system in an orderly manner using heat treatment methods. To investigate the changes in the quality and heat flux of the materials at different temperatures, TG tests were conducted on the samples. From the results (Figure 1), it can be seen that when the temperature reaches above 500 °C, the material mass gradually increases, which is due to the gradual oxidation of Ti2O3 to form TiO2. When the temperature reaches 750 °C, the mass of the sample tends to stabilize. The sample undergoes phase transformation at different temperatures, so it is highly feasible to construct a Ti2O3@TiO2 heterojunction system in an orderly manner using heat treatment methods.

**Figure 1.** Optical photos of the Ti2O3@TiO2 heterojunction at different heat treatment temperatures. **Figure 1.** Optical photos of the Ti2O3@TiO2 heterojunction at different heat treatment temperatures.

As shown in Figure 2, after treatment at different temperatures, the color of the Ti2O3 changed significantly, with the pure Ti2O3 turning black. When the temperature was below 600 °C, the color of the sample became lighter as the temperature increased. When the temperature was above 600 °C, the sample color turned yellow. Therefore, to further investigate the effect of temperature on the microstructures of the materials, scanning electron microscopy (SEM) was used to study the changes in the microstructure of the Ti2O3 under different heat treatment temperatures. Based on the results in Figures 1 and 2, the Ti2O3, Ti2O3-400, Ti2O3-500, Ti2O3-550, Ti2O3-600, and Ti2O3-700 were selected for the SEM characterization. As shown in Figure 2, after treatment at different temperatures, the color of the Ti2O3 changed significantly, with the pure Ti2O3 turning black. When the temperature was below 600 ◦C, the color of the sample became lighter as the temperature increased. When the temperature was above 600 ◦C, the sample color turned yellow. Therefore, to further investigate the effect of temperature on the microstructures of the materials, scanning electron microscopy (SEM) was used to study the changes in the microstructure of the Ti2O3 under different heat treatment temperatures. Based on the results in Figures 1 and 2, the Ti2O3, Ti2O3-400, Ti2O3-500, Ti2O3-550, Ti2O3-600, and Ti2O3-700 were selected for the SEM characterization.

**Figure 2.** Optical photos of Ti2O3 at different heat treatment temperatures. **Figure 2.** Optical photos of Ti2O3 at different heat treatment temperatures. *3.2. The Results of the SEM and XRD Analyses*

#### *3.2. The Results of the SEM and XRD Analyses 3.2. The Results of the SEM and XRD Analyses* From the results in Figure 3, it can be seen that the Ti2O3 before heat treatment has an

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From the results in Figure 3, it can be seen that the Ti2O3 before heat treatment has an irregular 3D particle structure with a smooth surface. When the temperature is below 500 °C, there is no significant change in the microstructure and size of the sample. When the temperature reaches 550 °C, the surface roughness of the sample significantly increases and small particles are generated on the outer surface. As the temperature further increases, the outer particle diameter continuously increases and agglomeration occurs. From the results in Figure 3, it can be seen that the Ti2O3 before heat treatment has an irregular 3D particle structure with a smooth surface. When the temperature is below 500 ◦C, there is no significant change in the microstructure and size of the sample. When the temperature reaches 550 ◦C, the surface roughness of the sample significantly increases and small particles are generated on the outer surface. As the temperature further increases, the outer particle diameter continuously increases and agglomeration occurs. irregular 3D particle structure with a smooth surface. When the temperature is below 500 °C, there is no significant change in the microstructure and size of the sample. When the temperature reaches 550 °C, the surface roughness of the sample significantly increases and small particles are generated on the outer surface. As the temperature further increases, the outer particle diameter continuously increases and agglomeration occurs.

action, the samples were characterized by measn of XRD. As shown in Figure 4, the diffraction peaks detected at 23.8°, 33.0°, 34.8°, 40.2°, 48.8°, and 53.7° are related to the Ti2O3 phase (JCPDS No. 43-1033) and can be attributed to the (012), (104), (110), (113), (024), and **Figure 3.** SEM images of Ti2O3 under different heat treatment temperatures: (**a**) Ti2O3; (**b**) Ti2O3-400; (**c**) Ti2O3-500; (**d**) Ti2O3-550; (**e**) Ti2O3-600; and (**f**) Ti2O3-700. **Figure 3.** SEM images of Ti2O3 under different heat treatment temperatures: (**a**) Ti2O3 ; (**b**) Ti2O3 -400; (**c**) Ti2O3 -500; (**d**) Ti2O3 -550; (**e**) Ti2O3 -600; and (**f**) Ti2O3 -700.

(116) crystal planes, respectively [32]. When the heat treatment temperature reaches 400 °C, new diffraction peaks begin to appear at 27.4° (110) and 36.0° (101), which belong to the rutile phase TiO2 (JCPDS No. 21-1276) [33]. With the further increase in temperature, the diffraction peaks attributed to the rutile phase TiO2 further appear at 27.4° (110), 36.0°

°C, new diffraction peaks begin to appear at 27.4° (110) and 36.0° (101), which belong to the rutile phase TiO2 (JCPDS No. 21-1276) [33]. With the further increase in temperature, the diffraction peaks attributed to the rutile phase TiO2 further appear at 27.4° (110), 36.0°

In order to investigate the crystal form changes in Ti2O3 after the heat treatment re-

action, the samples were characterized by measn of XRD. As shown in Figure 4, the diffraction peaks detected at 23.8°, 33.0°, 34.8°, 40.2°, 48.8°, and 53.7° are related to the Ti2O3 phase (JCPDS No. 43-1033) and can be attributed to the (012), (104), (110), (113), (024), and

In order to investigate the crystal form changes in Ti2O3 after the heat treatment reaction, the samples were characterized by measn of XRD. As shown in Figure 4, the diffraction peaks detected at 23.8◦ , 33.0◦ , 34.8◦ , 40.2◦ , 48.8◦ , and 53.7◦ are related to the Ti2O3 phase (JCPDS No. 43-1033) and can be attributed to the (012), (104), (110), (113), (024), and (116) crystal planes, respectively [32]. When the heat treatment temperature reaches 400 ◦C, new diffraction peaks begin to appear at 27.4◦ (110) and 36.0◦ (101), which belong to the rutile phase TiO2 (JCPDS No. 21-1276) [33]. With the further increase in temperature, the diffraction peaks attributed to the rutile phase TiO2 further appear at 27.4◦ (110), 36.0◦ (101), 39.2◦ (200), 41.2◦ (111), 44.0◦ (210), 54.3◦ (211), 56.6◦ (220), and 64.1◦ (310), and the peak intensity also increases with the thermal treatment temperature. When the temperature reaches 600 ◦C, all the Ti2O3 is converted into the TiO2 rutile phase. In addition, no diffraction peaks of impurities are observed. Therefore, Ti2O3 gradually undergoes phase transition into TiO2 during the heat treatment process. Combining XRD and SEM shows that the Ti2O3-450, Ti2O3-500 and Ti2O3-550 are composed of Ti2O3 and TiO2, forming a core-shell heterojunction. The experimental results show that the progress of TiO2 conversion can be controlled by changing the heat treatment temperature, thereby achieving the orderly construction of a Ti2O3@TiO2 heterojunction system. *Nanomaterials* **2023**, *13*, x FOR PEER REVIEW 6 of 16 (101), 39.2° (200), 41.2° (111), 44.0° (210), 54.3° (211), 56.6° (220), and 64.1° (310), and the peak intensity also increases with the thermal treatment temperature. When the temperature reaches 600 °C, all the Ti2O3 is converted into the TiO2 rutile phase. In addition, no diffraction peaks of impurities are observed. Therefore, Ti2O3 gradually undergoes phase transition into TiO2 during the heat treatment process. Combining XRD and SEM shows that the Ti2O3-450, Ti2O3-500 and Ti2O3-550 are composed of Ti2O3 and TiO2, forming a core-shell heterojunction. The experimental results show that the progress of TiO2 conversion can be controlled by changing the heat treatment temperature, thereby achieving the orderly construction of a Ti2O3@TiO2 heterojunction system.

**Figure 4.** XRD spectra of Ti2O3, Ti2O3-100, Ti2O3-200, Ti2O3-300, Ti2O3-400, Ti2O3-450, Ti2O3-500, Ti2O3- 550, Ti2O3-600, and Ti2O3-700. **Figure 4.** XRD spectra of Ti2O3 , Ti2O3 -100, Ti2O3 -200, Ti2O3 -300, Ti2O3 -400, Ti2O3 -450, Ti2O3 -500, Ti2O3 -550, Ti2O3 -600, and Ti2O3 -700.

#### *3.3. The Results of the TEM Analyses 3.3. The Results of the TEM Analyses*

In order to further investigate the microstructure and crystal plane changes of the Ti2O3-based samples during the high-temperature reaction, we conducted TEM testing on the Ti2O3 samples after high-temperature treatment at 550 °C. Figure 5a shows that after the high-temperature treatment, there are significant differences between the microstructure of the outer layer and the internal structure of the Ti2O3@TiO2 heterostructure, such as the particle accumulation and size. As shown in Figure 5b, electron diffraction can further indicate that the particulate matter comprises Ti2O3 and TiO2. The high-resolution transmission test results show that the lattice fringes are d = 0.27 nm and 0.373 nm, belonging to the (104) and (0.12) of Ti2O3, and d = 0.219 nm, belonging to the (111) of TiO2. The TEM results are consistent with the previous XRD and scanning test results, once again proving that the heat treatment method can construct Ti2O3@TiO2 heterojunctions. In order to further investigate the microstructure and crystal plane changes of the Ti2O3-based samples during the high-temperature reaction, we conducted TEM testing on the Ti2O3 samples after high-temperature treatment at 550 ◦C. Figure 5a shows that after the high-temperature treatment, there are significant differences between the microstructure of the outer layer and the internal structure of the Ti2O3@TiO2 heterostructure, such as the particle accumulation and size. As shown in Figure 5b, electron diffraction can further indicate that the particulate matter comprises Ti2O3 and TiO2. The high-resolution transmission test results show that the lattice fringes are d = 0.27 nm and 0.373 nm, belonging to the (104) and (0.12) of Ti2O3, and d = 0.219 nm, belonging to the (111) of TiO2. The TEM results are consistent with the previous XRD and scanning test results, once again proving that the heat treatment method can construct Ti2O3@TiO2 heterojunctions.

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**Figure 5.** (**a**,**b**) High-resolution TEM image of the Ti2O3-550 after high-temperature heat treatment. **Figure 5.** (**a**,**b**) High-resolution TEM image of the Ti2O3 -550 after high-temperature heat treatment. *3.4. The Results of the N2 Adsorption Analyses*

#### *3.4. The Results of the N2 Adsorption Analyses 3.4. The Results of the N2 Adsorption Analyses* N2 adsorption–desorption experiments were conducted to investigate the effects of different temperatures on the specific surface area and pore size of the composite materi-

N2 adsorption–desorption experiments were conducted to investigate the effects of different temperatures on the specific surface area and pore size of the composite materials. Figure 6a shows that the specific surface area of the Ti2O3-550 is the largest, approximately 0.796 m2/g, which is about 12 times that of the original Ti2O3 (approximately 0.067 m2/g). The pore volume of the Ti2O3 significantly increased from 0.000467 cm3/g to 0.0058 cm3/g (Ti2O3-550), demonstrating the reconstruction of the microstructure of the material during the transition from Ti2O3 to TiO2. In addition, in the Ti2O3@TiO2 heterojunction system, the Ti2O3-700 bulk phase has the highest content of TiO2, although its specific surface area is not the largest. This phenomenon proves that the expansion effect of the specific surface area in the system does not originate from the TiO2. Therefore, the research results indicate that the temperature has significant control over the microstructure of the system. In addition, from the pore size distribution map (Figure 6b), it can be seen that the pore size of the original Ti2O3 is mainly distributed around 37.5 nm and 50 nm. With the increase in the heat treatment temperature, the Ti2O3@TiO2 heterojunctions generate new pore structures in the bulk phase after retaining the original pore structure of the material, with the pore sizes mainly being concentrated in the range of 18–20 nm. The results suggest that the microstructure of the Ti2O3 did not undergo significant changes during the heat treatment process. N2 adsorption–desorption experiments were conducted to investigate the effects of different temperatures on the specific surface area and pore size of the composite materials. Figure 6a shows that the specific surface area of the Ti2O3-550 is the largest, approximately 0.796 m2/g, which is about 12 times that of the original Ti2O3 (approximately 0.067 m2/g). The pore volume of the Ti2O3 significantly increased from 0.000467 cm3/g to 0.0058 cm3/g (Ti2O3-550), demonstrating the reconstruction of the microstructure of the material during the transition from Ti2O3 to TiO2. In addition, in the Ti2O3@TiO2 heterojunction system, the Ti2O3-700 bulk phase has the highest content of TiO2, although its specific surface area is not the largest. This phenomenon proves that the expansion effect of the specific surface area in the system does not originate from the TiO2. Therefore, the research results indicate that the temperature has significant control over the microstructure of the system. In addition, from the pore size distribution map (Figure 6b), it can be seen that the pore size of the original Ti2O3 is mainly distributed around 37.5 nm and 50 nm. With the increase in the heat treatment temperature, the Ti2O3@TiO2 heterojunctions generate new pore structures in the bulk phase after retaining the original pore structure of the material, with the pore sizes mainly being concentrated in the range of 18–20 nm. The results suggest that the microstructure of the Ti2O3 did not undergo significant changes during the heat treatment process. als. Figure 6a shows that the specific surface area of the Ti2O3-550 is the largest, approximately 0.796 m2/g, which is about 12 times that of the original Ti2O3 (approximately 0.067 m2/g). The pore volume of the Ti2O3 significantly increased from 0.000467 cm3/g to 0.0058 cm3/g (Ti2O3-550), demonstrating the reconstruction of the microstructure of the material during the transition from Ti2O3 to TiO2. In addition, in the Ti2O3@TiO2 heterojunction system, the Ti2O3-700 bulk phase has the highest content of TiO2, although its specific surface area is not the largest. This phenomenon proves that the expansion effect of the specific surface area in the system does not originate from the TiO2. Therefore, the research results indicate that the temperature has significant control over the microstructure of the system. In addition, from the pore size distribution map (Figure 6b), it can be seen that the pore size of the original Ti2O3 is mainly distributed around 37.5 nm and 50 nm. With the increase in the heat treatment temperature, the Ti2O3@TiO2 heterojunctions generate new pore structures in the bulk phase after retaining the original pore structure of the material, with the pore sizes mainly being concentrated in the range of 18–20 nm. The results suggest that the microstructure of the Ti2O3 did not undergo significant changes during the heat treatment process.

**Figure 6.** (**a**) N2 adsorption–desorption isotherms of Ti2O3 at different heat treatment temperatures. (**b**) Pore size distribution curve of Ti2O3 at different heat treatment temperatures. **Figure 6.** (**a**) N2 adsorption–desorption isotherms of Ti2O3 at different heat treatment temperatures. (**b**) Pore size distribution curve of Ti2O3 at different heat treatment temperatures. **Figure 6.** (**a**) N2 adsorption–desorption isotherms of Ti2O3 at different heat treatment temperatures. (**b**) Pore size distribution curve of Ti2O3 at different heat treatment temperatures.

#### *3.5. The Results of the UV–Vis Analyses* ternal electric field. Compared with other reported materials, the synthesized materials

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UV visible absorption spectroscopy was used to study the regulatory effect of the built-in electric field on the optical properties of the Ti2O3 and Ti2O3@TiO2 heterojunctions. As shown in Figure 7, both the Ti2O3 and Ti2O3@TiO2 heterojunctions exhibit strong absorption from 200 to 500 nm. It is worth noting that the Ti2O3-550 sample exhibits excellent absorption capacity throughout the entire process and significant tail absorption under the irradiation of visible light (λ > 600 nm). Compared with the as-received Ti2O3, the absorption ability of the Ti2O3-550 sample in the visible light absorption region remains unchanged, or even slightly enhanced. The existence of an internal electric field can promote the absorption ability of the Ti2O3-550 with regard to visible light. In addition, the bandgaps of the Ti2O3 and Ti2O3@TiO2 heterojunctions can be estimated using the formula of αhv = A(hv-Eg)½ (Eg is the bandgap energy, α is the absorption coefficient, A is a constant, v is the optical frequency, and h is the Planck constant) [34]. In the graph of photon energy, the bandgap energy value is the intersection point between the extended dashed line and the horizontal axis of the coordinate system (the horizontal axis value is hv = 1240/wavelength, while the vertical axis value is (α Hv) ½) [35,36]. As shown in Figure 7b, the calculated bandgaps of the pure Ti2O3 and Ti2O3-550, Ti2O3-600, and Ti2O3-700 are 0.4 eV, 0.08 eV,1.82 eV, and 1.89 eV, respectively, indicating that the bandgaps of semiconductor materials can be effectively adjusted and optimized by constructing a reasonable in-

*3.5. The Results of the UV–Vis Analyses*

UV visible absorption spectroscopy was used to study the regulatory effect of the built-in electric field on the optical properties of the Ti2O3 and Ti2O3@TiO2 heterojunctions. As shown in Figure 7, both the Ti2O3 and Ti2O3@TiO2 heterojunctions exhibit strong absorption from 200 to 500 nm. It is worth noting that the Ti2O3-550 sample exhibits excellent absorption capacity throughout the entire process and significant tail absorption under the irradiation of visible light (λ > 600 nm). Compared with the as-received Ti2O3, the absorption ability of the Ti2O3-550 sample in the visible light absorption region remains unchanged, or even slightly enhanced. The existence of an internal electric field can promote the absorption ability of the Ti2O3-550 with regard to visible light. In addition, the bandgaps of the Ti2O3 and Ti2O3@TiO2 heterojunctions can be estimated using the formula of *α*hv = A(hv − Eg)1/2 (Eg is the bandgap energy, α is the absorption coefficient, A is a constant, v is the optical frequency, and h is the Planck constant) [34]. In the graph of photon energy, the bandgap energy value is the intersection point between the extended dashed line and the horizontal axis of the coordinate system (the horizontal axis value is hv = 1240/wavelength, while the vertical axis value is (α Hv) 1/2) [35,36]. As shown in Figure 7b, the calculated bandgaps of the pure Ti2O3 and Ti2O3-550, Ti2O3-600, and Ti2O3-700 are 0.4 eV, 0.08 eV, 1.82 eV, and 1.89 eV, respectively, indicating that the bandgaps of semiconductor materials can be effectively adjusted and optimized by constructing a reasonable internal electric field. Compared with other reported materials, the synthesized materials have relatively small bandgaps (Table 1). have relatively small bandgaps (Table 1). **Table 1.** Comparison of the bandgaps of Ti2O3@TiO2 and other reported studies. **Number Titanium-Based Oxides Bandgap References**  1 α-Ti2O3 1–2 eV [37] 2 csp-Ti2O3 1–2 eV [37] 3 Anatase TiO2 3.2 eV [38] 4 TiO2 3.3 eV [39] 5 α-Ti2O3 ≈0.1 eV [23] 6 TiO2 3.23 eV [40] 7 Ti2O3 3.18 eV [40] 8T-723 (TiOx@anatase) 3.04 eV [41] 9T-810 (TiOx@anatase) 3.1 eV [41] 10 Ti2O3@TiO2 0.08 eV This work

**Figure 7.** (**a**) Ultraviolet visible absorption spectra of the Ti2O3 and Ti2O3/TiO2 heterojunctions (Ti2O3- 550, Ti2O3-600, and Ti2O3-700) and (**b**) the corresponding Kubelka–Munk conversion reflection spectra. **Figure 7.** (**a**) Ultraviolet visible absorption spectra of the Ti2O3 and Ti2O3/TiO2 heterojunctions (Ti2O3 -550, Ti2O3 -600, and Ti2O3 -700) and (**b**) the corresponding Kubelka–Munk conversion reflection spectra.

**Table 1.** Comparison of the bandgaps of Ti2O3@TiO2 and other reported studies.

#### *3.6. The Results of the Optoelectronic Performance Analyses* tion systems, the Ti2O3-550 has the strongest photocurrent density, much higher than the

*Nanomaterials* **2023**, *13*, x FOR PEER REVIEW 9 of 16

*3.6. The Results of the Optoelectronic Performance Analyses*

Photoelectrochemical measurements in a typical three-electrode battery were used to study the electron generation and migration characteristics of the Ti2O3 and Ti2O3@TiO2 heterojunctions. As shown in Figure 8a, among the pure Ti2O3 and Ti2O3@TiO2 heterojunction systems, the Ti2O3-550 has the strongest photocurrent density, much higher than the original Ti2O3, confirming its excellent solar energy utilization ability. The above measurements intuitively prove that the material prepared at 550 ◦C has the best photoelectric performance. In addition, the relationship between the samples prepared at different heat treatment temperatures and the photocatalytic performance was systematically studied. The catalytic performance of the Ti2O3 and Ti2O3@TiO2 heterojunctions (Ti2O3-400, Ti2O3-500, Ti2O3-550, Ti2O3-600) under visible light was investigated using TC as the substrate. As shown in Figure 8b, after stirring under dark conditions for 20 min, the matrix adsorption–desorption equilibrium of all the samples was observed. The photocatalytic performance of the Ti2O3 and Ti2O3@TiO2 heterojunction composite materials was evaluated by measuring the degradation of the tetracycline in the sample under simulated visible light irradiation. From the results, it can be seen that Ti2O3 significantly enhances its ability to degrade tetracycline by constructing a Ti2O3@TiO2 heterojunction, with the degradation rate increasing from 28% to 70.1% (35.05 mg of tetracycline degraded/100 mg of photocatalyst). Compared with other reported materials, the synthesized materials have relatively good photocatalytic degradation efficiency (Table 2). As the temperature further increases, the catalytic efficiency decreases. The experimental results indicate that the excellent photocatalytic performance of the Ti2O3-550 can be attributed to the narrower bandgap, reasonable band structure, and larger specific surface area. More importantly, the proper construction of the heterojunction may result in the redistribution of the electron density to build an internal electric field in the Ti2O3@TiO2 and inhibit the recombination process of the electron–hole pairs. Therefore, by constructing heterojunctions reasonably and achieving precise regulation of the built-in electric fields, the photocatalytic activity of semiconductor materials can be optimized. original Ti2O3, confirming its excellent solar energy utilization ability. The above measurements intuitively prove that the material prepared at 550 °C has the best photoelectric performance. In addition, the relationship between the samples prepared at different heat treatment temperatures and the photocatalytic performance was systematically studied. The catalytic performance of the Ti2O3 and Ti2O3@TiO2 heterojunctions (Ti2O3-400, Ti2O3- 500, Ti2O3-550, Ti2O3-600) under visible light was investigated using TC as the substrate. As shown in Figure 8b, after stirring under dark conditions for 20 min, the matrix adsorption–desorption equilibrium of all the samples was observed. The photocatalytic performance of the Ti2O3 and Ti2O3@TiO2 heterojunction composite materials was evaluated by measuring the degradation of the tetracycline in the sample under simulated visible light irradiation. From the results, it can be seen that Ti2O3 significantly enhances its ability to degrade tetracycline by constructing a Ti2O3@TiO2 heterojunction, with the degradation rate increasing from 28% to 70.1% (35.05 mg of tetracycline degraded/100 mg of photocatalyst). Compared with other reported materials, the synthesized materials have relatively good photocatalytic degradation efficiency (Table 2). As the temperature further increases, the catalytic efficiency decreases. The experimental results indicate that the excellent photocatalytic performance of the Ti2O3-550 can be attributed to the narrower bandgap, reasonable band structure, and larger specific surface area. More importantly, the proper construction of the heterojunction may result in the redistribution of the electron density to build an internal electric field in the Ti2O3@TiO2 and inhibit the recombination process of the electron–hole pairs. Therefore, by constructing heterojunctions reasonably and achieving precise regulation of the built-in electric fields, the photocatalytic activity of semiconductor materials can be optimized.

Photoelectrochemical measurements in a typical three-electrode battery were used to

study the electron generation and migration characteristics of the Ti2O3 and Ti2O3@TiO2 heterojunctions. As shown in Figure 8a, among the pure Ti2O3 and Ti2O3@TiO2 heterojunc-

**Figure 8.** (**a**) Photogenerated current of the pure Ti2O3 and Ti2O3-400, Ti2O3-500, Ti2O3-550, and Ti2O3- 600, and (**b**) the photocatalytic degradation performance of tetracycline. **Figure 8.** (**a**) Photogenerated current of the pure Ti2O3 and Ti2O3 -400, Ti2O3 -500, Ti2O3 -550, and Ti2O3 -600, and (**b**) the photocatalytic degradation performance of tetracycline.

**Table 2.** Comparison of the degradation efficiency of Ti2O3@TiO2 and other reported studies. **Table 2.** Comparison of the degradation efficiency of Ti2O3@TiO2 and other reported studies.

5 Bi3O4Br 0.622/20 [44]

**Table 2.** *Cont.*

#### *3.7. Discussion on the Photocatalytic Mechanism*

In order to investigate the built-in electric field effect in the Ti2O3 and Ti2O3@TiO2 heterojunctions in depth, the surface photovoltage (SPV) was used to investigate the separation and migration process of the photogenerated charge carriers in the materials [48]. Figure 9a,b show that the SPV response signals of the original Ti2O3 and Ti2O3-400 were not detected. This may be due to the materials' low separation efficiency or the high recombination efficiency of the photogenerated carriers. When the heat treatment temperature was further increased to 500 and 550 ◦C (Figure 9c,d), significant photovoltage response signals were observed at 275–400 nm for the Ti2O3-500 and Ti2O3-550, which can be attributed to the electronic transitions between the valence band and conduction band [49,50]. Moreover, the SPV response is positive, indicating that the upward energy band bends toward the surface/interface [51]. It can be clearly seen that the SPV response values have increased to 4.1 µV and 2.33 mV, respectively. This increase is caused by the built-in electric field constructed by the Ti2O3 and TiO2, which continuously adjusts with the composition. With the increase in the heat treatment temperature, the amount of TiO2 in the outer layer from the conversion of the Ti2O3 gradually increases and the built-in electric field effect is continuously enhanced. Thus, the separation of the photoexcited charge carriers is promoted, resulting in changes in the surface photovoltage. When the direction of the applied electric field is changed, there is no significant change in the photogenerated voltage of the Ti2O3-500 (Figure 9c), indicating that the direction of the built-in electric field of the Ti2O3-500 is opposite to the direction of the positive applied electric field. As shown in Figure 9d, the photogenerated voltage of the Ti2O3-550 significantly increases with the applied electric field. When the direction of the applied electric field is changed, the photogenerated voltage of the Ti2O3-500 is suppressed and reduced, indicating that the direction of the built-in electric field of the Ti2O3-550 is the same as the direction of the positive applied electric field. However, when the heat treatment temperature is increased to 600 ◦C (Figure 9e), the outer TiO2 content further increases. The SPV response of the Ti2O3-600 decreases to 0.75 mV, indicating that in the presence of excessive TiO2, the built-in electric field effect decreases and the separation of the photoexcited charge carriers and the recombination of the photogenerated electron–hole pairs are suppressed. In addition, the photogenerated voltage of the Ti2O3-600 increases significantly with the increase in the applied electric field. When the direction of the applied electric field changes, the photogenerated voltage of the Ti2O3-600 is suppressed and reduced, indicating that the direction of the built-in electric field of the Ti2O3-600 is the same as that of the positive applied electric field. Photogenerated charge carriers can recombine through radiation or non-radiation decay. Therefore, by comparing the changes in the SPV response with the heat treatment temperature in the Ti2O3 and Ti2O3@TiO2 heterojunctions, it can be inferred that the Ti2O3-550 has the best built-in electric field effect, which can effectively suppress non-radiative decay, allowing for the more effective separation of the photogenerated charge carriers and transport of the photogenerated electrons to the catalyst surface to facilitate the degradation of the tetracycline [52,53].

**Figure 9.** Surface photovoltage (SPV) diagrams of the Ti2O3 and Ti2O3/TiO2 heterojunctions: (**a**) Ti2O3, (**b**) Ti2O3-400, (**c**) Ti2O3-500, (**d**) Ti2O3-550, and (**e**) Ti2O3-600. **Figure 9.** Surface photovoltage (SPV) diagrams of the Ti2O3 and Ti2O3/TiO2 heterojunctions: (**a**) Ti2O3 , (**b**) Ti2O3 -400, (**c**) Ti2O3 -500, (**d**) Ti2O3 -550, and (**e**) Ti2O3 -600.

As shown in Figure 10, the slope of the straight-line portion of the Mott–Schottky curve of the Ti2O3 and Ti2O3@TiO2 heterojunctions (Ti2O3-500, Ti2O3-550, and Ti2O3-600) within the passivation zone is positive, indicating that the semiconductor film formed by the composite materials in this solution is an n-type semiconductor [54,55]. When the Ti2O3@TiO2 heterojunction is not in contact with the solution, the material's Fermi energy level is higher than the solution's chemical potential. When the Ti2O3@TiO2 heterojunction enters the solution, the carriers, i.e., electrons, in the bulk phase will spontaneously transfer from the high energy level to the low energy level, that is, from the side of the Ti2O3@TiO2 to the solution, to balance the Fermi energy levels [56]. A space charge layer is formed on one side of the Ti2O3@TiO2 film, causing the bulk phase to carry opposite charges to the solution. Excess charges are distributed within the space charge layer. Due to the lower concentration of electrons in the Ti2O3@TiO2 bulk phase compared to the solution, electrons continue to enter the solution. Therefore, in the surface area on the semiconductor side, electrons are consumed, leaving only positive charges. This process causes the energy ratio near the surface to be corrected internally in the semiconductor, resulting in the energy bands of the Ti2O3 and Ti2O3@TiO2 heterojunctions bending upwards in this As shown in Figure 10, the slope of the straight-line portion of the Mott–Schottky curve of the Ti2O3 and Ti2O3@TiO2 heterojunctions (Ti2O3-500, Ti2O3-550, and Ti2O3-600) within the passivation zone is positive, indicating that the semiconductor film formed by the composite materials in this solution is an n-type semiconductor [54,55]. When the Ti2O3@TiO2 heterojunction is not in contact with the solution, the material's Fermi energy level is higher than the solution's chemical potential. When the Ti2O3@TiO2 heterojunction enters the solution, the carriers, i.e., electrons, in the bulk phase will spontaneously transfer from the high energy level to the low energy level, that is, from the side of the Ti2O3@TiO2 to the solution, to balance the Fermi energy levels [56]. A space charge layer is formed on one side of the Ti2O3@TiO2 film, causing the bulk phase to carry opposite charges to the solution. Excess charges are distributed within the space charge layer. Due to the lower concentration of electrons in the Ti2O3@TiO2 bulk phase compared to the solution, electrons continue to enter the solution. Therefore, in the surface area on the semiconductor side, electrons are consumed, leaving only positive charges. This process causes the energy ratio near the surface to be corrected internally in the semiconductor, resulting in the energy bands of the Ti2O3 and Ti2O3@TiO2 heterojunctions bending upwards in this region. As electrons continue to propagate, the energy band continues to bend upwards. If an external

region. As electrons continue to propagate, the energy band continues to bend upwards.

voltage is continuously applied to the Ti2O3 and Ti2O3@TiO2 heterojunctions, the charge distribution at the interface changes with the electron input and the band bending changes. When the electrons that flood into the space charge layer neutralize the excess positive charge there, the energy band of the semiconductor returns to the same level as when it was not in contact with the solution. At this point, the required voltage is the flat band voltage. Suppose there is a built-in electric field in the semiconductor. In that case, it will decrease the necessary external voltage required to determine the potential size of the built-in electric field. bending changes. When the electrons that flood into the space charge layer neutralize the excess positive charge there, the energy band of the semiconductor returns to the same level as when it was not in contact with the solution. At this point, the required voltage is the flat band voltage. Suppose there is a built-in electric field in the semiconductor. In that case, it will decrease the necessary external voltage required to determine the potential size of the built-in electric field.

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**Figure 10.** Mott–Schottky diagrams of the Ti2O3 and Ti2O3/TiO2 heterojunctions: (**a**) Ti2O3, (**b**) Ti2O3- 500, (**c**) Ti2O3-500, and (**d**) Ti2O3-600. **Figure 10.** Mott–Schottky diagrams of the Ti2O3 and Ti2O3/TiO2 heterojunctions: (**a**) Ti2O3 , (**b**) Ti2O3 - 500, (**c**) Ti2O3 -550, and (**d**) Ti2O3 -600.

As shown by the results, the flat band potentials of the Ti2O3, Ti2O3-500, Ti2O3-550, and Ti2O3-600Ti2O3-500 are −0.55 V, −0.49 V, −0.34 V, and −0.43 V, respectively. The results indirectly prove that in the Ti2O3@TiO2 heterojunction system, the built-in electric field of the Ti2O3-550 has the highest potential. In addition, the results of the Mott–Schottky curve indicate that the Ti2O3-550 has the highest carrier concentration, which is consistent with the conclusion obtained via the SPV. The Ti2O3-550 has a high Fermi energy level, which can endow the Ti2O3@TiO2 heterostructure with a more excellent electronic reservoir ca-As shown by the results, the flat band potentials of the Ti2O3, Ti2O3-500, Ti2O3- 550, and Ti2O3-600 are −0.55 V, −0.49 V, −0.34 V, and −0.43 V, respectively. The results indirectly prove that in the Ti2O3@TiO2 heterojunction system, the built-in electric field of the Ti2O3-550 has the highest potential. In addition, the results of the Mott–Schottky curve indicate that the Ti2O3-550 has the highest carrier concentration, which is consistent with the conclusion obtained via the SPV. The Ti2O3-550 has a high Fermi energy level, which can endow the Ti2O3@TiO2 heterostructure with a more excellent electronic reservoir capability and accelerate the carrier separation and conversion.

pability and accelerate the carrier separation and conversion. These experimental results indicate that under visible light irradiation, the photocatalytic reaction of the Ti2O3@TiO2 heterojunction is direct photocatalysis, where electrons are excited by visible light from the valence band of the TiO2 to the transition band of the Ti2O3, which is lower than the conduction band of the TiO2. Then, electrons can easily transfer from the Ti (III) site to the adjacent Ti (IV) sites through the valence excitation process [22]. Under visible light irradiation, photogenerated holes are injected into the conduction band of the photocatalyst by the built-in electric field. Subsequently, elec-These experimental results indicate that under visible light irradiation, the photocatalytic reaction of the Ti2O3@TiO2 heterojunction is direct photocatalysis, where electrons are excited by visible light from the valence band of the TiO2 to the transition band of the Ti2O3, which is lower than the conduction band of the TiO2. Then, electrons can easily transfer from the Ti (III) site to the adjacent Ti (IV) sites through the valence excitation process [22]. Under visible light irradiation, photogenerated holes are injected into the conduction band of the photocatalyst by the built-in electric field. Subsequently, electrons are captured to produce reactive substances and induce degradation reactions (Figure 11).

trons are captured to produce reactive substances and induce degradation reactions (Fig-

ure 11).

**References**

*382*, 122842.

**Figure 11.** Schematic diagram of photocatalytic TC degradation over the Ti2O3@TiO2 heterojunc-**Figure 11.** Schematic diagram of photocatalytic TC degradation over the Ti2O3@TiO2 heterojunctions.

#### tions. **4. Conclusions**

**4. Conclusions**  In summary, Ti2O3@TiO2 heterojunctions have been constructed via a sample heat treatment method as an efficient TC degradation system. The photocatalytic activity of the Ti2O3@TiO2 heterojunctions for TC degradation under visible light was investigated. Compared with the pure Ti2O3, the photocatalytic performance of the Ti2O3@TiO2 heterojunctions was significantly improved. After the heterojunction structure was constructed, the photodegradation rate increased from 28% to 70.1% (35.05 mg of tetracycline degraded/100 mg of photocatalyst). The relevant experimental results indicated that the excellent photocatalytic performance of the Ti2O3-550 can be attributed to the narrow bandgap and reasonable band structure derived from the built-in electric field. By using the SPV and Mott–Schottky methods, it was revealed that the construction of the heterojunctions in the Ti2O3@TiO2 resulted in the construction of an internal electric field, which suppressed the recombination process of the photogenerated electron–hole pairs. The most significant aspect here is that the built-in electric field can be controlled by changing In summary, Ti2O3@TiO2 heterojunctions have been constructed via a sample heat treatment method as an efficient TC degradation system. The photocatalytic activity of the Ti2O3@TiO2 heterojunctions for TC degradation under visible light was investigated. Compared with the pure Ti2O3, the photocatalytic performance of the Ti2O3@TiO2 heterojunctions was significantly improved. After the heterojunction structure was constructed, the photodegradation rate increased from 28% to 70.1% (35.05 mg of tetracycline degraded/100 mg of photocatalyst). The relevant experimental results indicated that the excellent photocatalytic performance of the Ti2O3-550 can be attributed to the narrow bandgap and reasonable band structure derived from the built-in electric field. By using the SPV and Mott–Schottky methods, it was revealed that the construction of the heterojunctions in the Ti2O3@TiO2 resulted in the construction of an internal electric field, which suppressed the recombination process of the photogenerated electron–hole pairs. The most significant aspect here is that the built-in electric field can be controlled by changing the content of Ti2O3 and TiO2. Overall, this work provides a simple method to precisely regulate the built-in electric field and optimize the photocatalytic activity of semiconductor materials by reasonably constructing heterojunctions.

the content of Ti2O3 and TiO2. Overall, this work provides a simple method to precisely regulate the built-in electric field and optimize the photocatalytic activity of semiconductor materials by reasonably constructing heterojunctions. **Author Contributions:** Conceptualization, F.L., P.F. and H.C.; methodology, T.H. and L.G.; formal analysis, T.H., F.L., P.F. and H.C.; investigation, T.H. and L.G.; writing—original draft preparation, T.H.; writing—review and editing, F.L., P.F. and H.C.; visualization, P.F.; supervision, F.L., P.F. and H.C. All authors have read and agreed to the published version of the manuscript.

**Author Contributions:** Conceptualization, F.L., P.F. and H.C.; methodology, T.H. and L.G.; formal analysis, T.H., F.L., P.F. and H.C.; investigation, T.H. and L.G.; writing—original draft preparation, T.H.; writing—review and editing, F.L., P.F. and H.C.; visualization, P.F.; supervision, F.L., P.F. and H.C. All authors have read and agreed to the published version of the manuscript. **Funding:** We gratefully acknowledge the support of the National Natural Science Foundation of China (22205124), the Shandong Province Natural Science Foundation (ZR2021QB070), Basic research projects for the pilot project of integrating science, education, and industry (2023PY024), the **Funding:** We gratefully acknowledge the support of the National Natural Science Foundation of China (22205124), the Shandong Province Natural Science Foundation (ZR2021QB070), Basic Research Projects for the Pilot Project of Integrating Science and Education and Industry of Qilu University of Technology (Shandong Academy of Sciences) (2023PY024), the Development Plan of the Youth Innovation Team in Colleges and Universities of Shandong Province, and Basic Research Projects for the Pilot Project of Integrating Science and Education and Industry of Qilu University of Technology (Shandong Academy of Sciences) (2023PX108).

Development Plan of the Youth Innovation Team in Colleges and Universities of Shandong Province, **Data Availability Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

and Basic research projects for the pilot project of integrating science, education, and industry (2023PX108). **Conflicts of Interest:** The authors declare no conflict of interest.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

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## *Article* **Fabrication of Z-Type TiN@(A,R)TiO2 Plasmonic Photocatalyst with Enhanced Photocatalytic Activity**

**Wanting Wang 1 , Yuanting Wu 1 , Long Chen 1 , Chenggang Xu 1 , Changqing Liu 1,2,\* and Chengxin Li 2**

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** Plasmonic effect-enhanced Z-type heterojunction photocatalysts comprise a promising solution to the two fundamental problems of current TiO2 -based photocatalysis concerning low-charge carrier separation efficiency and low utilization of solar illumination. A plasmonic effect-enhanced TiN@anatase-TiO2/rutile-TiO2 Z-type heterojunction photocatalyst with the strong interface of the N–O chemical bond was synthesized by hydrothermal oxidation of TiN. The prepared photocatalyst shows desirable visible light absorption and good visible-light-photocatalytic activity. The enhancement in photocatalytic activities contribute to the plasma resonance effect of TiN, the N–O bond-connected charge transfer channel at the TiO2/TiN heterointerface, and the synergistically Z-type charge transfer pathway between the anatase TiO2 (A-TiO2 ) and rutile TiO2 (R-TiO2 ). The optimization study shows that the catalyst with a weight ratio of A-TiO2/R-TiO2/TiN of approximately 15:1:1 achieved the best visible light photodegradation activity. This work demonstrates the effectiveness of fabricating plasmonic effect-enhanced Z-type heterostructure semiconductor photocatalysts with enhanced visible-light-photocatalytic activities.

**Keywords:** Z-type system; LSPR; photocatalyst; TiO2

### **1. Introduction**

Green nanotechnology driven by solar energy has attracted great interest in alleviating the environmental hazards of pesticides, organic dyes, toxic gases, and industrial wastewater [1–6]. Since Carey et al. [7,8] used semiconductors to degrade pollutants in 1976, TiO2 has proven to be a material that can be used for environmental purification. However, pure TiO2 possesses a wide band gap (about 3.2 eV) [9,10]. Due to this limitation, it only responds to UV light and has low solar energy utilization (about 4%). Furthermore, the recombination rate of photogenerated charge carriers generated after TiO2 excitation is much higher than that of interfacial charge transfer, resulting in low activity even under UV light [11]. Therefore, promoting solar utilization and charge carrier separation is the key to improving the photocatalytic performance of the catalysts.

Combining the localized surface plasmon resonance (LSPR) effect with semiconductor photocatalysts is a promising method to promote both the charge carrier separation efficiency and the responsive solar illumination range [12]. Till now, most plasmonic photocatalysts relied on noble metal nanostructures (such as Au or Ag) [13]. However, their potential for practical applications is limited due to their rarity, high cost, low thermal stability, and easy dissolution upon the exposure to air or humidity. Thus, novel plasmonic photocatalysts without noble metal components should be developed to overcome these problems.

Recently, TiN has emerged as an attractive competitor in photocatalytic applications due to its plasmonic resonance absorption properties [14]. In addition, the work function of TiN is about 4 eV versus vacuum [15], which is greater than or equal to the electron affinity

**Citation:** Wang, W.; Wu, Y.; Chen, L.; Xu, C.; Liu, C.; Li, C. Fabrication of Z-Type TiN@(A,R)TiO2 Plasmonic Photocatalyst with Enhanced Photocatalytic Activity. *Nanomaterials* **2023**, *13*, 1984. https://doi.org/ 10.3390/nano13131984

Academic Editor: Chiara Maccato

Received: 15 May 2023 Revised: 26 June 2023 Accepted: 27 June 2023 Published: 30 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of most semiconductor metal oxide photocatalysts, including TiO2. Therefore, TiN tends to form a favorable energetic alignment to promote hot carrier-enhanced solar energy conversion [16]. Naldoni et al. [17] explored the plasmonic-enhanced TiO2 photocatalysts by coupling with TiN, demonstrating that the LSPR effect of the TiN introduced an enhanced photocurrent generation and photocatalytic activity. Fakhouri et al. [18] demonstrated a significant photoactive improvement to bilayered RF magnetron-sputtering TiN/TiO2 thin films due to enhanced charge separation at the heterojunction. Clatworthy et al. [19] demonstrated enhanced photocatalytic activity of TiN-TiO2 nanoparticle composites and proposed that hot electrons migration can be promoted due to TiO2 photovoltage by combining visible light with UV light. However, there is usually a certain lattice mismatch between different semiconductors, therefore constructed heterostructures usually result in large lattice defects and interface resistance [20]. These lattice defects often form the capturing center of photogenerated carriers [21], and the interface resistance would restrict charge transfer and affect their stability [22], thus greatly affecting the efficiency of charge carrier separation. Therefore, a novel plasmonic photocatalyst without noble metal components could be developed if a nanostructured TiN/TiO2 composite with good contact could be created. Zhu et al. [23] found that the epitaxial growth of different semiconductors on conductive precursors and the regulation of growth conditions can significantly reduce the interface contact resistance, which can solve the challenge of building heterostructures to obtain high photogenerated charge separation characteristics. Li et al. [24] fabricated a TiN/TiO2 plasmonic photocatalyst by in situ growing TiO2 on TiN nanoparticles, demonstrating good visible light photocatalytic performance. Furthermore, Zhang et al. [25] significantly reduced the interface resistance and greatly improved their ability to photoelectrochemical decompose water by forming a strong interface contact of the S–O covalent bond at the interface of the Cu2S/Fe2O3 heterostructure. In our previous work [26], the Ti–O–Zr bonded TiO2/ZrTiO4 heterointerface was constructed by growing ZrTiO4 in situ on TiO2 to enhance the transport of photogenerated carriers. However, the possibility of forming a chemically bonded TiO2/TiN heterostructure and its synergistic enhancement of photocatalytic activities with the LSPR effect of TiN have yet to be explored.

In addition, fabrication of the direct Z-type heterojunction is an alternative strategy to obtain a semiconductor photocatalyst with high performance due to its advantage in charge carrier separation and utilizing the high-redox properties of each component [27]. For direct Z-type heterojunctions with staggered band structure, the photogenerated electrons on lower CB and the holes on the higher VB recombine. Meanwhile, the electrons and holes with stronger redox abilities are retained [28,29]. Thus, the charge carrier separation can be enhanced, and the highest redox potential of the heterojunction can be retained, thus contributing to the promoted photocatalytic activities. In our previous work, we successfully constructed a direct Z-type A-TiO2/R-TiO2 heterojunction by synergistically mediating oxygen vacancy contents and the band structure of the catalysts through Bdoping [30]. However, to our best knowledge, there is no report concerning the possibility of combining TiN plasmonic enhancement with direct Z-type TiO2-based heterojunction.

In this study, to solve the two fundamental problems of current TiO2-based photocatalysis on low charge carrier separation efficiency and low utilization of solar illumination, we optimized a unique plasmonic effect-enhanced Z-type TiN@A-TiO2/R-TiO2 photocatalyst with a strong interface of the N–O chemical bond through hydrothermal in situ oxidation of TiN to (A,R)-TiO2. In this photocatalyst system, the desirable visible light absorption could be attributed to the LSPR effect of the TiN component. The charge carrier separation efficiency could be enhanced by the Z-type charge transfer mode at the interface of the A-TiO2/R-TiO2 heterojunction. The obtained TiN@A-TiO2/R-TiO2 photocatalyst showed a distinct enhancement in visible light absorption, photocurrent generation, and photodegradation activities, demonstrating a simple way to promote the photoactive properties of semiconductor photocatalysts by fabricating plasmonic effect-enhanced Z-type heterostructures.

#### **2. Experimental Section**

#### *2.1. Chemicals*

Commercial titanium nitride (TiN, AR) were procured from Aladdin Reagent Co., Ltd., Shanghai, China. Ethanol (C2H5OH, CP); hydrogen peroxide (H2O2, 35 wt%, AR), rhodamine B (RhB, AR), and concentrated sulfuric acid (H2SO4, AR) were procured from the National Reagent company, Beijing, China. All reagents were used as received.

#### *2.2. Preparation of the Catalyst*

All samples were prepared by a simple hydrothermal process. Firstly, TiN powder was dispersed in 40 mL of deionized water and sonicated for 15 min to obtain TiN suspension. A total of 1 mL of H2SO4 (1 M) and a certain amount of H2O2 was added dropwise and stirred for 2 h. The suspension was then hydrothermally treated at 180 ◦C for 5 h, then washed and dried at 60 ◦C for 24 h to obtain the target samples. The hydrolysis degree of TiN was determined by the amount of H2O2 added. In this work, the mass fraction of added H2O2 is 0%, 0.5%, 1.0%, 2.5%, and 5.0%. The obtained catalysts were labeled as TiN, sample 1 (S1), sample 2 (S2), sample 3 (S3), and sample 4 (S4), respectively.

#### *2.3. Characterization Methods*

Compositions were recorded on a D/max-2200PC powder X-ray diffraction (XRD), with Cu Kα radiation over a 2θ ranging from 10◦ to 70◦ . Morphologies and microstructures were recorded by SEM (FEI Verios 460, Hillsboro, OR, USA), TEM, and HRTEM (FEI Tecnai G2 F20 S-TWIN, Hillsboro, OR, USA). XPS was studied on an X-ray photoelectron spectroscope (XPS, AXIS SUPRA, Manchester, UK) with a monochromatic Al Kα source. Comparing with the standard binding energy of adsorbed carbon (284.6 eV), charge correction was applied after the peak fitting using the CasaXPS analysis software. UV–Vis diffuse reflectance spectra (DRS) were tested on a UV–Vis-NIR spectrophotometer (Cary 5000, Santa Clara, CA, USA). Photoluminescence (PL) spectra were obtained by a fluorescence spectrophotometer (F-4600, Rigaku, Japan) with the excitation source at 345 nm. EPR spectra were conducted by a Bruker A300 spectrometer, during which DMPO was applied using a 300 W Xe lamp as the light source. The FT-IR spectrum (4000–500 cm−1 ) was obtained on Vertex70 Bruker FT-IR Spectroscopy.

Photoelectrochemical analysis was performed on a CHI760D electrochemical workstation equipped with a 300 W Xe lamp and a cut off filter (>420 nm), in which 20 µL of catalyst slurry on an FTO substrate of 2 cm × 2 cm was used as the working electrode. The slurry was prepared by dispersing 5 mg catalyst powder into polyvinylidene difluoride N-methyl pyrrolidone solution (0.5 g, 2 wt%) through ultrasonic vibration. A total of 0.5 M Na2SO4 solution, platinum, and Ag/AgCl were used as the electrolyte solution, counter, and reference electrode, respectively.

#### *2.4. Photocatalytic Performance*

The photocatalytic activity of various catalysts was evaluated by RhB photodegradation. Firstly, 30 mL of RhB solution (10 mg/L) was prepared, then 30 mg of catalyst was added under stirring. Then, the above suspension was illuminated under visible light for 120 min, collected, centrifuged, and measured at regular intervals of 30 min. The peak absorbency of the centrifuged RhB solution at 554 nm was applied to analyze its concentration using a UV–Vis spectrophotometer.

#### **3. Results and Discussion**

#### *3.1. Structural Characterization of the Photocatalysts*

Figure 1 shows the XRD results of samples obtained with various H2O2 content. Except for the single-phase TiN sample, all other samples show diffraction peaks ascribed to three phases—that is, A-TiO2, R-TiO2, and TiN [31]. Moreover, as the H2O2 content increases, the peak intensity of TiN decreases and that of TiO2 increases. The content of each phase was determined by the Rietveld method and shown in Table 1. With an increase

in the H2O2 content, the phase proportion of TiN gradually decreases accompanied by an increase in the TiO2 content, indicating the hydrolysis of TiN and its conversion into TiO2. Furthermore, the ratio of A-TiO2 to R-TiO2 increases accordingly. Specifically, for sample S2, the weight ratio of the three phases (A-TiO2:R-TiO2:TiN) is about 15:1:1. S1 0.5 80.9% 19.1% 4.25 21.1% S2 1.0 93.8% 6.2% 15.13 7.2% S3 2.5 97.3% 2.7% 36.40 1.7% S4 5.0 98.3% 1.7% 57.82 0.2%

TiN 0 - - - 100%

**TiO2/wt% TiN/wt% A-TiO2 R-TiO2 A-TiO2:R-TiO2**

S2, the weight ratio of the three phases (A-TiO2:R-TiO2:TiN) is about 15:1:1.

**Table 1.** Compositions and ratios of the A/R-TiO2 and TiN in the prepared samples.

Figure 1 shows the XRD results of samples obtained with various H2O2 content. Except for the single-phase TiN sample, all other samples show diffraction peaks ascribed to three phases—that is, A-TiO2, R-TiO2, and TiN [31]. Moreover, as the H2O2 content increases, the peak intensity of TiN decreases and that of TiO2 increases. The content of each phase was determined by the Rietveld method and shown in Table 1. With an increase in the H2O2 content, the phase proportion of TiN gradually decreases accompanied by an increase in the TiO2 content, indicating the hydrolysis of TiN and its conversion into TiO2. Furthermore, the ratio of A-TiO2 to R-TiO2 increases accordingly. Specifically, for sample

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 4 of 18

*3.1. Structural Characterization of the Photocatalysts*

**3. Results and Discussion**

**Samples Content of**

**H2O2 (wt%)**

**Figure 1.** XRD results of the prepared samples obtained with various H2O2 content. **Figure 1.** XRD results of the prepared samples obtained with various H2O2 content.

Figure 2 shows the FT-IR spectra of sample TiN and S2. In the spectra, the wide ab-**Table 1.** Compositions and ratios of the A/R-TiO2 and TiN in the prepared samples.

Figure 2 shows the FT-IR spectra of sample TiN and S2. In the spectra, the wide absorption band at 3440 cm−1 and the peaks around 1633 cm−1 are ascribed to the adsorbed water and hydroxyl groups [32], respectively. The NOx-determined peaks appeared at 1382 cm−1 and 1346 cm−1 [33]. Furthermore, peaks between 500–800 cm−1 are believed to be caused by the stretching vibration of Ti–O–Ti bonds [34]. Compared to the TiN sample, the maximum strength of the Ti–O–Ti bond increased significantly, and a new peak of NOx appeared, indicating the formation of TiO2 and the possible existence of a newly formed N–O bond in sample S2.

Figures S1–S3 show the SEM images, particle size distributions, and BET surface areas of all the samples. As can be seen, all the samples show uniform and fine particle distribution with an average particle size of about 50 nm, except for sample TiN and S2, which show a slightly smaller size (around 35 nm). Moreover, except for the comparison samples (P25 and TiN), all the other samples have close BET surface areas, suggesting that surface area is not the reason for the photocatalytic performance difference between various samples. Microstructure was studied through TEM analysis. In the TEM results (Figure 3a), uniformly distributed irregular nanoparticles including polygonal, spherical,

and rod-shaped particles can be observed. The length of rod particles is 50–200 nm, while the particle size of polygon and spherical particles is about 25 nm. In the HRTEM results (Figure 3b), spacings of 0.210, 0.324, and 0.352 nm of the lattice fringes, correspond to the (200) plane of TiN, the (110) plane of R-TiO2, and the (101) plane of A-TO2, respectively [35,36]. Moreover, the (A,R)-TiO2 are identified on the surface of TiN, and all three phases are in close contact. Furthermore, the energy spectra (Figure 3c–f) show the evenly distributed Ti, N, and O elements, indicating the potential formation of the TiN/(A,R)-TiO2 heterointerface. Therefore, it can be deduced that the in situ oxidation of TiN and growth of (A,R)-TiO2 could create heterojunctions with an intimate contact interface, improving charge transfer efficiency. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 5 of 18

**Figure 2.** FT-IR spectra of TiN and sample S2. **Figure 2.** FT-IR spectra of TiN and sample S2.

charge transfer efficiency.

Figures S1–S3 show the SEM images, particle size distributions, and BET surface areas of all the samples. As can be seen, all the samples show uniform and fine particle distribution with an average particle size of about 50 nm, except for sample TiN and S2, which show a slightly smaller size (around 35 nm). Moreover, except for the comparison samples (P25 and TiN), all the other samples have close BET surface areas, suggesting that surface area is not the reason for the photocatalytic performance difference between various samples. Microstructure was studied through TEM analysis. In the TEM results (Figure 3a), uniformly distributed irregular nanoparticles including polygonal, spherical, and rod-shaped particles can be observed. The length of rod particles is 50–200 nm, while the particle size of polygon and spherical particles is about 25 nm. In the HRTEM results (Figure 3b), spacings of 0.210, 0.324, and 0.352 nm of the lattice fringes, correspond to the (200) plane of TiN, the (110) plane of R-TiO2, and the (101) plane of A-TO2, respectively [35,36]. Moreover, the (A,R)-TiO2 are identified on the surface of TiN, and all three phases are in close contact. Furthermore, the energy spectra (Figure 3c–f) show the evenly dis-Figure 4a is the XPS survey spectrum of S2, demonstrating the presence of C, O, and Ti elements. The N element was not detected, suggesting that it may appear in the interior of the particles. The Ti 2p XPS spectrum was fitted into four peaks. The Ti 2p3/2 (458.1 eV) and the Ti 2p1/2 (464.1 eV) were for TiO2 [37–40]. The Ti 2p3/2 (457.3 eV) and Ti 2p1/2 (462.5 eV) were for partially oxidized TiN [41]. This observation confirms the creation of TiO2 from the oxidation of TiN and suggests the possibility of forming a chemical contact interface between TiN and TiO2. By further analysis of the O 1s spectrum of S2 (Figure 4c), four peaks can be fitted at 533.09, 531.54, 529.71, and 529.15 eV, which could be attributed to adsorbed H2O (AO), Ti–O in Ti2O3 suggests the existence of oxygen vacancies (VO), and oxygen in Ti–O–N and Ti–O lattices [42–44], respectively. Considering the relatively high area ratio of the Vo XPS peak, it can be deduced that there is a high content of VO in the sample. Based on the analysis of the O1s spectrum, it is clear that N–O bonds exist between TiN and its oxidation products TiO2, which contribute to the good contact interface of the formed TiN/TiO2 heterostructure.

tributed Ti, N, and O elements, indicating the potential formation of the TiN/(A,R)-TiO2 heterointerface. Therefore, it can be deduced that the in situ oxidation of TiN and growth of (A,R)-TiO2 could create heterojunctions with an intimate contact interface, improving

**Figure 3.** (**a**) TEM; (**b**) HRTEM; (**c**) HADDF images and EDS mappings of the elements N (**d**); O (**e**); Ti (**f**) for S2. **Figure 3.** (**a**) TEM; (**b**) HRTEM; (**c**) HADDF images and EDS mappings of the elements N (**d**); O (**e**); Ti (**f**) for S2. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 7 of 18

N–O bonds exist between TiN and its oxidation products TiO2, which contribute to the **Figure 4.** XPS spectra of S2: (**a**) full spectrum; (**b**) Ti 2p; and (**c**) O1s. **Figure 4.** XPS spectra of S2: (**a**) full spectrum; (**b**) Ti 2p; and (**c**) O1s.

#### good contact interface of the formed TiN/TiO2 heterostructure. *3.2. Photodegradation Performance 3.2. Photodegradation Performance*

without conversion into ·OH.

Figure 5 shows the photodegradation performance of the catalysts. As can be seen, all samples showed no obvious adsorption in the dark reaction. The sample TiN did not have a degrading effect on RhB, indicating that it is not the main catalytic carrier in the photocatalytic reaction process but a cocatalyst. Compared to TiN samples, the hydrolyzed samples showed obvious degrading behavior on RhB. With the deeper degree of hydrolysis, i.e., the decrease in TiN content and the increase in weight ratio of A-TiO2 to R-TiO2, the photodegradation rate increases first and then decreases. Among them, the S2 Figure 5 shows the photodegradation performance of the catalysts. As can be seen, all samples showed no obvious adsorption in the dark reaction. The sample TiN did not have a degrading effect on RhB, indicating that it is not the main catalytic carrier in the photocatalytic reaction process but a cocatalyst. Compared to TiN samples, the hydrolyzed samples showed obvious degrading behavior on RhB. With the deeper degree of hydrolysis, i.e., the decrease in TiN content and the increase in weight ratio of A-TiO2 to R-TiO2, the photodegradation rate increases first and then decreases. Among them, the S2

sample has the best degradation efficiency, reaching more than 97% in 90 min. As for the kinetics of RhB degradation, the degradation curves are well-fitted by a

first-order kinetics [35]. Figure 5b shows the relationship between ln (C0/C) and t for all experiments using different samples, where C0 is the initial RhB content and C is the RhB concentration at reaction time t. By regression analysis of the linear curve in the graph, the value of the apparent first-order rate constant can be directly obtained, in which the value of sample S2 is the highest 0.02272 min−1. In addition, the cycling experiment (Figure 5c) shows that the sample can maintain a degradation efficiency of more than 90% after five cycles, showing good stability. From the XRD analysis in Figure 5d, no detectable differences can be seen between the as-prepared and cycled S2, indicating a well-preserved crystalline structure of the catalyst after multiple photocatalytic cycles. Moreover, a few studies on the RhB photodegradation performance of TiO2-based photocatalysts are summarized in Table 2. The table shows that the visible-light degradation performance of RhB over the catalyst prepared in this work was enhanced, indicating

that the prepared TiN@(A,R)TiO2 is a promising visible-light photocatalyst.

Figure S4 shows the result of the free radical capture experiment. By adding IPA, TEOA, BQ, and AgNO3 as the capture agents of ·OH, h+, ·O2−, and e−, respectively, the effect of free radicals on photocatalysis was investigated [45]. The addition of TEOA and BQ has the greatest impact on the photodegradation rate, suggesting that the corresponding h+ and ·O2− may play the main role in the photodegradation process. EPR test was carried out on sample S2, and the result is shown in Figure 6. In the O2− free radical detection, the DMPO-·O2− signal peak of 1:1:1:1 was detected, and its intensity increased with prolonged irradiation time, confirming that the O2− radical is the main active species, whereas for ·OH radicals, no DMPO-·OH signal peak of 1:2:2:1 can be detected, suggesting that no ·OH radical can be produced during light irradiation. This result indirectly verified that h+ might participate in the following photodegradation reaction

sample has the best degradation efficiency, reaching more than 97% in 90 min. As for the kinetics of RhB degradation, the degradation curves are well-fitted by a mono-exponential curve, indicating that the photodegradation experiments follow the first-order kinetics [35]. Figure 5b shows the relationship between ln (C0/C) and t for all experiments using different samples, where C0 is the initial RhB content and C is the RhB concentration at reaction time t. By regression analysis of the linear curve in the graph, the value of the apparent first-order rate constant can be directly obtained, in which the value of sample S2 is the highest 0.02272 min−1 . In addition, the cycling experiment (Figure 5c) shows that the sample can maintain a degradation efficiency of more than 90% after five cycles, showing good stability. From the XRD analysis in Figure 5d, no detectable differences can be seen between the as-prepared and cycled S2, indicating a well-preserved crystalline structure of the catalyst after multiple photocatalytic cycles. Moreover, a few studies on the RhB photodegradation performance of TiO2-based photocatalysts are summarized in Table 2. The table shows that the visible-light degradation performance of RhB over the catalyst prepared in this work was enhanced, indicating that the prepared TiN@(A,R)TiO2 is a promising visible-light photocatalyst. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 8 of 18

**Figure 5.** (**a**) Photodegradation performance; (**b**) kinetics of all prepared catalysts; (**c**) cycling experiments of sample S2; and (**d**) XRD patterns of the as-prepared and cycled sample S2. **Figure 5.** (**a**) Photodegradation performance; (**b**) kinetics of all prepared catalysts; (**c**) cycling experiments of sample S2; and (**d**) XRD patterns of the as-prepared and cycled sample S2.

**Table 2.** Summary of recent relative works on the RhB photodegradation performance of TiO2-based heterojunction photocatalysts. **Photocatalyst C0 (mg/L) Dosage (mg) Light Source Degradation Rate Time (min) Kinetic Rate (min−1) Ref.**  Ag@TiO2 10 100 150 W Xe lamp 98.2% 120 0.0188 [46] TiO2 hollow boxes 100 50 Visible light 96.5% 240 0.0025 [47] Ag2O/TiO2 4.79 40 UV light 87.7% 80 0.0277 [48] Figure S4 shows the result of the free radical capture experiment. By adding IPA, TEOA, BQ, and AgNO3 as the capture agents of ·OH, h+ , ·O2 −, and e−, respectively, the effect of free radicals on photocatalysis was investigated [45]. The addition of TEOA and BQ has the greatest impact on the photodegradation rate, suggesting that the corresponding h + and ·O2 − may play the main role in the photodegradation process. EPR test was carried out on sample S2, and the result is shown in Figure 6. In the O2 − free radical detection, the DMPO-·O2 − signal peak of 1:1:1:1 was detected, and its intensity increased with prolonged irradiation time, confirming that the O2 − radical is the main active species, whereas for

Bi2WO6/TiO2/Pt 20 100 UV light 60.0% 40 0.0210 [51] g-C3N4/TiO2 50 5 Visible light 87.0% 300 0.0115 [52] A/R-TiO2 10 25 UV light About 100% 50 - [53] Au/A/R-TiO2 - - UV light 97% 60 0.0470 [54] TiN@(A,R)TiO2 10 30 Visible light 97.0% 90 0.0227 This work

Ag/ZnO/AgO/TiO2 10 30 350 W

·OH radicals, no DMPO-·OH signal peak of 1:2:2:1 can be detected, suggesting that no ·OH radical can be produced during light irradiation. This result indirectly verified that h+ might participate in the following photodegradation reaction without conversion into ·OH. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 9 of 18

**Figure 6.** EPR results of (**a**) DMPO-•O2−; (**b**) DMPO-•OH with sample S2. **Figure 6.** EPR results of (**a**) DMPO-•O2 −; (**b**) DMPO-•OH with sample S2.

#### *3.3. Photocatalytic Mechanism*

Figure 7a shows the UV–Vis DRS spectra of all prepared catalysts. TiN shows full spectrum absorption characteristics similar to those of metals. Compared to P25 and sample S4 with little TiN content showing no obvious visible light absorption, the other samples show obvious light absorption in the entire visible light region (390–780 nm). Moreover, with increasing hydrolysis degree of TiN, the light absorption intensity gradually decreases, confirming that component TiN plays a decisive role in the light absorption ability of the prepared photocatalyst. The result is consistent with the report that the presence of TiN contributed to improving the material's entire solar light absorption capability [31].

**Figure 7.** (**a**) UV–Vis DRS; (**b**) Plot of (αhv)1/2 versus hν; (**c**) PL spectra; (**d**) TP curves; (**e**) EIS plots; (**f**) band structures of the samples S1 (blue), S2 (red), S3 (purple), S4 (orange), P25 (yellowish-brown) and TiN (green).

(**f**) band structures of the samples S1 (blue), S2 (red), S3 (purple), S4 (orange), P25 (yellow-The band gap (Eg) is further obtained through the conversion of Formula (1) [55]:

**Figure 7.** (**a**) UV–Vis DRS; (**b**) Plot of (αhv)1/2 versus hν; (**c**) PL spectra; (**d**) TP curves; (**e**) EIS plots;

$$
\alpha \mathbf{h} \mathbf{v} = \mathbf{A} \left( \mathbf{h} \mathbf{v} - \mathbf{E} \mathbf{g} \right)^{\mathbf{n} \mathbf{2}},\tag{1}
$$

The band gap (Eg) is further obtained through the conversion of Formula (1) [55]: αhν = A (hν − Eg) n⁄2, (1) where A is a constant, n = 1 for indirect semiconductors [56], and α and h are the absorption coefficient and photon energy, respectively.

where A is a constant, n = 1 for indirect semiconductors [56], and α and h are the absorption coefficient and photon energy, respectively. With the decrease in the TiN content, the Eg of the sample gradually increases from 0.70 eV (S1) to 3.06 eV (P25). Therefore, the presence of TiN can effectively reduce the With the decrease in the TiN content, the Eg of the sample gradually increases from 0.70 eV (S1) to 3.06 eV (P25). Therefore, the presence of TiN can effectively reduce the band gap of the sample, thus significantly improving the capability of light absorbance and utilization.

ish-brown) and TiN (green).

Figure 7c shows the PL spectral analysis of the samples, in which a higher fluorescence intensity represents a higher carrier recombination rate [57]. TiN and P25 showed the lowest and strongest fluorescence intensity, respectively. The fluorescence intensity of the others gradually increased with increasing TiO2 phase content. In particular, sample S2 also maintains low fluorescence intensity, indicating its outstanding charge separation ability. From the instantaneous photocurrent results in Figure 7d, the highest transient photocurrent signal can be observed for sample S2. The photocurrent signal decreases remarkably when the TiO2 phase content is further increased. The above results demonstrate the photocurrent enhancement effect of TiN on TiO2.

Figure 7e shows the electrochemical impedance spectroscopy (EIS) analysis of the samples. In addition, the resultant Nyquist plots (insert in Figure 7e) were fitted with an equivalent circuit using Zman software. As is shown, the equivalent circuit consists of internal resistance (Rs), charge transfer resistance (Rct1, Rct2), Warburg impedance (W), and double-layer capacitance (CPE1, CPE2) [58]. Compared to P25, the charge transfer resistance of the other samples (Figure 7e) is reduced to a certain extent, suggesting that the presence of TiN could improve the conductivity of the samples [59]. In particular, the charge transfer resistance of sample S2 is the lowest, demonstrating the greatest charge transfer rate. The above results demonstrate that the best charge carries separation and transfer can be obtained in sample S2. The reason can be explained by its proper phase proportion, and the formation of the N–O bond at the interface of (A,R)-TiO2 and TiN, which can effectively reduce the interface contact resistance of the heterostructure.

The calculated flat band potentials (Efb) are shown in Figure S5. The Mott–Schottky curves and calculation process of Efb can be found in Figures S5 and S6. Considering that the ECB of the n-type semiconductor is about 0.1 eV higher than its Efb, the ECB of the samples can be further deduced [60]. Combined with the band gap values (Eg), their energy band structures can be obtained (Figure 7f) through the following formula (2) [61]:

$$\mathbf{E\_{VB}} = \mathbf{E\_{CB}} + \mathbf{E}\mathbf{g}.\tag{2}$$

It can be seen in Figure 7f that the presence of TiN can significantly decrease the Eg of the samples by improving the valence band (VB) potential. For sample S2, the Eg was reduced from 3.06 eV to 1.42 eV with the VB position changing from +2.04 to +0.82 eV, and CB position was slightly changed compared to P25. With a narrow band gap, sample S2 is more conducive to generating e− and h+ charge carriers, while it shows no ability to produce ·OH active species due to its high valence band position (VB), which is in good agreement with the result of the EPR test.

Considering the staggered band structures of the A-TiO2/R-TiO2, Type-II or Z-type charge transfer modes may occur in the heterojunction, as shown in Figure 8. The values of the CB and VB for (A,R)-TiO2 are obtained from the literature [62]. If Type-II mode is formed, electrons transfer to the CB of A-TiO2, the reduction potential of which is weak and cannot further reduce surface-adsorbed oxygen to generate ·O2 − for the following photodegradation process, and this situation is inconsistent with our experimental results. Therefore, the Z-type charge transfer pathway is preferred for the heterojunction constructed in our work. Specifically, according to the literature, for partially reduced samples, the VB and CB positions of R-TiO2 are higher than that of A-TiO2 and the work function of R-TiO2 (ϕ ≈ 4.3 eV) is smaller than that of A-TiO2 (ϕ ≈ 4.7 eV) [63,64]. When they are in contact, free electrons spontaneously flow from R-TiO2 to A-TiO2 to obtain their Fermi energy levels to reach equilibrium. At this time, there are a large number of negatively charged electrons near the A-TiO2 interface. In contrast, positive charges are gathered at the R-TiO2 interface, generating a built-in electric field. Due to the shift in the Fermi energy level, R-TiO2 will generate an upward band bending, while A-TiO2 will generate a downward band bending [65]. The Z-type electron transfer path is generated due to the formed electric field and the energy band bending. To further prove the formed heterojunction is a Z-type photocatalyst, the Ag nanoparticles were photo-deposited on the catalyst to track where the electrons flow to. Figure 9 shows the EDS, TEM, and HRTEM

images of the photodeposition of Ag nanoparticles on sample S2. It shows the uniform distribution of Ag, N, O, and Ti elements, and the Ag nanoparticles were isolated on R-TiO2 and apart from A-TiO2. The results suggest that the electrons were left on R-TiO2, confirming the Z-type charge transfer pathway in the formed heterojunction. TEM, and HRTEM images of the photodeposition of Ag nanoparticles on sample S2. It shows the uniform distribution of Ag, N, O, and Ti elements, and the Ag nanoparticles were isolated on R-TiO2 and apart from A-TiO2. The results suggest that the electrons were left on R-TiO2, confirming the Z-type charge transfer pathway in the formed heterojunction.

formed heterojunction is a Z-type photocatalyst, the Ag nanoparticles were photo-deposited on the catalyst to track where the electrons flow to. Figure 9 shows the EDS,

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 12 of 18

**Figure 8.** Schematic illustration of the possible charge carrier transfer mode in Type-Ⅱ and direct Z-type photocatalysts. **Figure 8.** Schematic illustration of the possible charge carrier transfer mode in Type-II and direct Z-type photocatalysts. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 13 of 18

**Figure 9.** (**a**) TEM; (**b**) HRTEM; (**c**) HADDF images and EDS mappings of the elements Ag (**d**); N (**e**); O (**f**); Ti (**g**) for S2 with photo-deposited Ag nanoparticles. **Figure 9.** (**a**) TEM; (**b**) HRTEM; (**c**) HADDF images and EDS mappings of the elements Ag (**d**); N (**e**); O (**f**); Ti (**g**) for S2 with photo-deposited Ag nanoparticles.

In this work, the plasmonic component TiN can broaden the absorbed light range and generate hot electrons due to the LSPR effect. Since nanostructured TiO2 was obtained in situ from TiN and charge transfer channel N–O bonds were formed between

between them. Furthermore, the work function of TiN is ~3.7 eV (φm), and the electron affinity of TiO2 is ~4.2 eV (φs). Considering the barrier energy (the lowest energy required for an electron in the metal to be injected into the semiconductor) can be calculated as φ = φm − φs [66], a negative value (−0.5 eV) can be obtained, suggesting the quick injecting of the hot electrons into TiO2. Therefore, the improved photocatalytic performance of TiN@A-TiO2/R-TiO2 heterojunction can be concluded and shown in Figure 10. First, the plasmonic properties of TiN greatly broaden the light absorption range, generating and injecting hot electrons into TiO2. Furthermore, the N–O bond contacted TiO2/TiN heterointerface can significantly reduce the contact resistance of the interface and improve the charge transfer efficiency. Moreover, the optimized three-phase ratio and the formed Z-type A-TiO2/R-TiO2 heterojunction with an intimate interface contribute to the charge carrier separation and retain its high redox capacity. Thus, more active species will par-

ticipate in the following photodegradation activities.

In this work, the plasmonic component TiN can broaden the absorbed light range and generate hot electrons due to the LSPR effect. Since nanostructured TiO2 was obtained in situ from TiN and charge transfer channel N–O bonds were formed between TiN and TiO2, the resulting intimate contacted interface benefits the electron transfer between them. Furthermore, the work function of TiN is ~3.7 eV (ϕm), and the electron affinity of TiO2 is ~4.2 eV (ϕs). Considering the barrier energy (the lowest energy required for an electron in the metal to be injected into the semiconductor) can be calculated as ϕ = ϕm − ϕs [66], a negative value (−0.5 eV) can be obtained, suggesting the quick injecting of the hot electrons into TiO2. Therefore, the improved photocatalytic performance of TiN@A-TiO2/R-TiO2 heterojunction can be concluded and shown in Figure 10. First, the plasmonic properties of TiN greatly broaden the light absorption range, generating and injecting hot electrons into TiO2. Furthermore, the N–O bond contacted TiO2/TiN heterointerface can significantly reduce the contact resistance of the interface and improve the charge transfer efficiency. Moreover, the optimized three-phase ratio and the formed Z-type A-TiO2/R-TiO2 heterojunction with an intimate interface contribute to the charge carrier separation and retain its high redox capacity. Thus, more active species will participate in the following photodegradation activities. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 14 of 18

**Figure 10.** (**a**,**b**) Schematic illustration of the formation and a possible photoinduced catalytic mechanism of the TiN@(A,R)TiO2 heterojunction. **Figure 10.** (**a**,**b**) Schematic illustration of the formation and a possible photoinduced catalytic mechanism of the TiN@(A,R)TiO2 heterojunction.

#### **4. Conclusions 4. Conclusions**

In this work, a plasmonic effect-enhanced TiN@A-TiO2/R-TiO2 direct Z-type heterojunction was fabricated through the simple hydrothermal reaction process. By regulating the amount of H2O2 oxidant, the proportion of TiN, anatase TiO2, and rutile TiO2 contents can be successfully adjusted and the interface charge transfer channel (N–O bond) has been constructed. Due to the Z-type charge transfer path between A-TiO2 and In this work, a plasmonic effect-enhanced TiN@A-TiO2/R-TiO2 direct Z-type heterojunction was fabricated through the simple hydrothermal reaction process. By regulating the amount of H2O2 oxidant, the proportion of TiN, anatase TiO2, and rutile TiO2 contents can be successfully adjusted and the interface charge transfer channel (N–O bond) has been constructed. Due to the Z-type charge transfer path between A-TiO2 and R-

R-TiO2, the N–O bond connected charge transfer channel at the TiN/TiO2 interface, and the synergistic plasma resonance effect of TiN, the optimized photocatalyst shows a dis-

**Supplementary Materials:** The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: SEM images of (a) TiN, (b) S1, (c) S2, (d) S3, (e) S4. Figure S2: Particle size distribution of all samples (a) TiN, (b) S1, (c) S2, (d) S3, (e) S4. Figure S3: N2 adsorption/desorption isotherm plots and pore size distribution of all samples. Figure S4: Free radical capture experiment of sample S2. Figure S5: Mott-Schottky curves of samples (a) P25, (b) TiN, (c) S1, (d) S2, (e) S3, (f) S4. Figure S6: Schematic diagram of flat band potential of the prepared samples. **Author Contributions:** Conceptualization, C.L. (Changqing Liu) and C.L. (Chengxin Li); Data curation, W.W. and L.C.; Formal analysis, C.X.; Funding acquisition, Y.W. and C.L. (Changqing Liu); Investigation, W.W.; Methodology, L.C.; Project administration, Y.W. and C.L. (Chengxin Li); Resources, Y.W. and C.L. (Chengxin Li); Supervision, Y.W. and C.L. (Changqing Liu); Validation, C.L. (Changqing Liu); Writing—original draft, W.W.; Writing—review & editing, C.L. (Changqing Liu).

All authors have read and agreed to the published version of the manuscript.

of semiconductor photocatalysts.

TiO2, the N–O bond connected charge transfer channel at the TiN/TiO2 interface, and the synergistic plasma resonance effect of TiN, the optimized photocatalyst shows a distinct increment in visible light absorption, photocurrent generation, and photocatalytic performance, demonstrating an effective approach to promote the photoactive properties of semiconductor photocatalysts.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13131984/s1, Figure S1: SEM images of (a) TiN, (b) S1, (c) S2, (d) S3, (e) S4. Figure S2: Particle size distribution of all samples (a) TiN, (b) S1, (c) S2, (d) S3, (e) S4. Figure S3: N2 adsorption/desorption isotherm plots and pore size distribution of all samples. Figure S4: Free radical capture experiment of sample S2. Figure S5: Mott-Schottky curves of samples (a) P25, (b) TiN, (c) S1, (d) S2, (e) S3, (f) S4. Figure S6: Schematic diagram of flat band potential of the prepared samples.

**Author Contributions:** Conceptualization, C.L. (Changqing Liu) and C.L. (Chengxin Li); Data curation, W.W. and L.C.; Formal analysis, C.X.; Funding acquisition, Y.W. and C.L. (Changqing Liu); Investigation, W.W.; Methodology, L.C.; Project administration, Y.W. and C.L. (Chengxin Li); Resources, Y.W. and C.L. (Chengxin Li); Supervision, Y.W. and C.L. (Changqing Liu); Validation, C.L. (Changqing Liu); Writing—original draft, W.W.; Writing—review & editing, C.L. (Changqing Liu). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been supported by the National Natural Science Foundation of China (Grant No. 51702194 and 52173214), the Natural Science Foundation of Shaanxi Province (Grant No. 2023-JC-YB-384), and the Youth Innovation Team of Shaanxi Universities (2022-70).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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## *Review* **A Review on Nano Ti-Based Oxides for Dark and Photocatalysis: From Photoinduced Processes to Bioimplant Applications**

**Christine Joy Querebillo**

Leibniz-Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstr. 20, 01069 Dresden, Germany<EMAIL_ADDRESS>
**Abstract:** Catalysis on TiO2 nanomaterials in the presence of H2O and oxygen plays a crucial role in the advancement of many different fields, such as clean energy technologies, catalysis, disinfection, and bioimplants. Photocatalysis on TiO2 nanomaterials is well-established and has advanced in the last decades in terms of the understanding of its underlying principles and improvement of its efficiency. Meanwhile, the increasing complexity of modern scientific challenges in disinfection and bioimplants requires a profound mechanistic understanding of both residual and dark catalysis. Here, an overview of the progress made in TiO2 catalysis is given both in the presence and absence of light. It begins with the mechanisms involving reactive oxygen species (ROS) in TiO2 photocatalysis. This is followed by improvements in their photocatalytic efficiency due to their nanomorphology and states by enhancing charge separation and increasing light harvesting. A subsection on black TiO2 nanomaterials and their interesting properties and physics is also included. Progress in residual catalysis and dark catalysis on TiO2 are then presented. Safety, microbicidal effect, and studies on Ti-oxides for bioimplants are also presented. Finally, conclusions and future perspectives in light of disinfection and bioimplant application are given.

**Keywords:** reactive oxygen species; photocatalytic efficiency; charge separation; light harvesting; black TiO2 ; titanium alloys; residual disinfection; dark catalysis; antibacterial; inflammation

#### **1. Introduction**

Titanium dioxide (TiO2), or titania, occurs naturally and forms spontaneously when bare Ti is exposed to air [1,2]. At room temperature, TiO2 is commonly considered an *n*-type semiconductor [3,4] with a bandgap (*Eg*) of around 3 eV (3.2 eV for anatase, 3 eV for rutile) [2,4]. Since the discovery of its photoelectrochemical (PEC) water-splitting ability by Fujishima and Honda [3,5], TiO2 remains today as the standard photocatalyst. The excitation of its electrons from the valence band (VB) to the conduction band (CB) upon exposure to light with energy *E* ≥ *Eg*, i.e., UV (to violet, 413 nm) light for the bulk form, results in photogenerated charge carriers (electrons and holes) which can be utilized in various processes, resulting in its photocatalytic activity. In addition, TiO2 exhibits desirable material properties, such as its wide availability, biocompatibility, chemical stability in an aqueous environment, and affordability [6–14], resulting in the application of TiO2 in different fields, such as catalysis, alternative/clean energy technologies, environmental cleaning, pollutant degradation, medicine, pharmaceuticals, disinfection, and biomedical implants [1–4,6–21].

The mechanism of photocatalysis on TiO2 depends on the type of catalytic reaction that is being investigated, though it primarily involves interfacial (and bulk) processes of the photoinduced electrons and holes, which occur at different time scales (Figure 1) [22–26]. The faster charge carrier generation due to photon absorption (1) compared to electron– hole recombination (2) enables the possibility of having CB electrons and VB holes that can be tapped for reductive (3) and oxidative (4) processes, respectively. However, the

**Citation:** Querebillo, C.J. A Review on Nano Ti-Based Oxides for Dark and Photocatalysis: From Photoinduced Processes to Bioimplant Applications. *Nanomaterials* **2023**, *13*, 982. https:// doi.org/10.3390/nano13060982

Academic Editors: Wei Zhou and Antonino Gulino

Received: 30 January 2023 Revised: 13 February 2023 Accepted: 24 February 2023 Published: 8 March 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

recombination process lies at the same time scale as these interfacial charge transfer (CT) processes, and therefore efforts to further delay the recombination process can improve the photocatalytic efficiency of TiO2. ever, the recombination process lies at the same time scale as these interfacial charge transfer (CT) processes, and therefore efforts to further delay the recombination process can improve the photocatalytic efficiency of TiO2.

electron–hole recombination (2) enables the possibility of having CB electrons and VB holes that can be tapped for reductive (3) and oxidative (4) processes, respectively. How-

*Nanomaterials* **2023**, *13*, x FOR PEER REVIEW 2 of 40

**Figure 1.** The core steps in photocatalytic mechanism include (1) absorption of light which forms charge carriers, (2) recombination of charge carriers, (3) reductive process with electrons in the conduction band (CB), (4) oxidative pathway undertaken by a valence-band (VB) hole, (5) further reactions (hydrolysis, reactions with reactive oxygen species (ROS), etc.), (6) trapping a VB hole at a Ti-OH group on the surface, and (7) trapping of CB electron at a Ti(IV) site to produce Ti(III). Redrawn from ref. [26] to include timescales obtained from [22–26]. The timescale in 3 and 4 are mainly from nontrapped electrons and holes (i.e., directly from the CB and VB). **Figure 1.** The core steps in photocatalytic mechanism include (1) absorption of light which forms charge carriers, (2) recombination of charge carriers, (3) reductive process with electrons in the conduction band (CB), (4) oxidative pathway undertaken by a valence-band (VB) hole, (5) further reactions (hydrolysis, reactions with reactive oxygen species (ROS), etc.), (6) trapping a VB hole at a Ti-OH group on the surface, and (7) trapping of CB electron at a Ti(IV) site to produce Ti(III). Redrawn from ref. [26] to include timescales obtained from [22–26]. The timescale in 3 and 4 are mainly from nontrapped electrons and holes (i.e., directly from the CB and VB).

Furthermore, the VB holes can transport quickly to hole trap sites at the surface, such as at surface Ti-OH groups (6), which is often the case for the photogenerated holes because of the fast femtosecond process. Consequently, free VB holes are scarcely present in TiO2 [24], and surface-trapped holes are usually responsible for the oxidation reaction which can occur faster in the ps–ns range [22–26]. The electrons, on the other hand, can get trapped at Ti(IV) sites (7) and form Ti(III), which can also participate in other redox processes [22–26]. Furthermore, the VB holes can transport quickly to hole trap sites at the surface, such as at surface Ti-OH groups (6), which is often the case for the photogenerated holes because of the fast femtosecond process. Consequently, free VB holes are scarcely present in TiO2 [24], and surface-trapped holes are usually responsible for the oxidation reaction which can occur faster in the ps–ns range [22–26]. The electrons, on the other hand, can get trapped at Ti(IV) sites (7) and form Ti(III), which can also participate in other redox processes [22–26].

Direct recombination of the photogenerated carriers usually does not occur and instead happens upon meeting trapped complementary charge carriers. For example, mobile electrons can recombine with trapped holes, decreasing the former's lifetime and reducing the photocatalytic performance of TiO2. As such, anatase usually performs better as a photocatalyst than rutile due to its longer electron lifetime (>few ms for anatase vs. ~24 ns for rutile) and stronger band bending [27,28]. Further photocatalytic reactions on TiO2 occur on the surface involving reactive oxygen species (ROS) (Figure 1, (5)) [22]. Direct recombination of the photogenerated carriers usually does not occur and instead happens upon meeting trapped complementary charge carriers. For example, mobile electrons can recombine with trapped holes, decreasing the former's lifetime and reducing the photocatalytic performance of TiO2. As such, anatase usually performs better as a photocatalyst than rutile due to its longer electron lifetime (>few ms for anatase vs. ~24 ns for rutile) and stronger band bending [27,28]. Further photocatalytic reactions on TiO2 occur on the surface involving reactive oxygen species (ROS) (Figure 1, (5)) [22].

In the past decades, most studies on TiO2 are focused on its photocatalytic activity (both in the bulk and nanomaterial form), and the literature is overflowing with strategies to improve its performance by addressing inherent deterrents, extending the spectral range for which TiO2-based photocatalysis can be used, or enhancing the light utilization through various geometries (nanotextures, structured arrays, etc.). These improvements are based mainly on the key steps in photocatalysis, focusing on light absorption, generation of charge carriers, their separation and transport, catalyst replenishment, and prevention of back and side processes. With the goal and steps clarified, TiO2 photocatalysis still faces a number of challenges considering that modern materials require multiple functionalities and therefore a balance of its properties. Hence, the influence of different TiO2 properties on its photocatalytic performance is also a major focus of research. In the past decades, most studies on TiO2 are focused on its photocatalytic activity (both in the bulk and nanomaterial form), and the literature is overflowing with strategies to improve its performance by addressing inherent deterrents, extending the spectral range for which TiO2-based photocatalysis can be used, or enhancing the light utilization through various geometries (nanotextures, structured arrays, etc.). These improvements are based mainly on the key steps in photocatalysis, focusing on light absorption, generation of charge carriers, their separation and transport, catalyst replenishment, and prevention of back and side processes. With the goal and steps clarified, TiO2 photocatalysis still faces a number of challenges considering that modern materials require multiple functionalities and therefore a balance of its properties. Hence, the influence of different TiO2 properties on its photocatalytic performance is also a major focus of research.

Almost overshadowed by the numerous works on photocatalysis but persisting mainly due to the excellent biocompatibility and oxidative bleaching ability of TiO2, research on TiO2 in the "dark" or in the absence of light has also been growing steadily over Almost overshadowed by the numerous works on photocatalysis but persisting mainly due to the excellent biocompatibility and oxidative bleaching ability of TiO2, research on TiO2 in the "dark" or in the absence of light has also been growing steadily over the years. Titanium, from which TiO2 can be grown, exhibits excellent mechanical properties which helped in establishing its place in the field of biomedical implants. For example, titanium and its alloys are standard materials used for bone implants, and strong research in medicine and engineering is focused on continuously improving their physicochemical properties, mechanical properties, and designability/processability [29–33], on top of added functionalities desired in modern biomaterials, such as its antimicrobial and regenerative properties [34–39]. The last two are also attributed to its catalytic ROS-forming ability, which should then be considered in bioimplant applications. After implantation surgery, i.e., in the postoperative phase, implant material surfaces are devoid of light. Understanding the mechanism of "dark" catalysis on TiO2 in the physiological condition is therefore important in addressing the current challenges and limitations not only for Ti and TiO2 but also for more advanced Ti alloys and Ti-based materials and their oxides for implant application.

Considering the vast use of TiO2, it is not surprising that the published works on TiO2 catalysis come from many different disciplines of different perspectives. Modern scientific and engineering challenges then often entail a multidisciplinary approach to answer the increasingly becoming more complex questions. Yet, if we look at the literature, it seems that there is still a need to consolidate this immense knowledge of TiO2 catalysis from a multidisciplinary perspective.

The different knowledge and experiences we gain by working on different topics involving titania can help us develop the skill of understanding catalysis on TiO2 nanomaterials from a more inclusive viewpoint. Different fields working on titania, for instance, electromagnetic field enhancement on semiconductors [40–50], dye photodegradation (see Section 2.2), and bioimplant applications [31,36,51–56] to name a few, may all involve catalytic properties of TiO2 nanomaterials, yet they also need a nuanced understanding of TiO2 in light of specific, targeted applications. Such background and experience can certainly help us easily understand the literature though regularly immersing oneself in various literature on TiO2 catalysis easily available to us nowadays, and constantly discussing with colleagues and peers can also help us be familiarized and updated with the progress on TiO2 catalysis. As such, despite the experience and background of the author, which certainly helped in the reading and analysis of the literature for this review, the method of regularly keeping up to date with the literature, engaging in scientific discussions, and a period of intensive gathering and reading of the literature on TiO2 catalysis was therefore adapted in preparing this review. Many excellent works and reviews helped in the preparation of a general survey, with emphasis on certain points of interest—namely, the advancements in photocatalytic enhancement, the photocatalytic and dark bactericidal activities, and the use of Ti (and Ti-based) oxide nanomaterials for bioimplants. In some topics, the readers are referred to excellent reviews available in the literature, and the scope is limited to oxides of Ti (mainly TiO2). On the other hand, the preparation of this review was conducted with the awareness that the answers to present-day complex scientific questions may still not be available in the literature, and due to the multidisciplinary nature of these questions and the explosion of the available literature on the internet, not all available review materials on the internet and in print can be included in this review. Nevertheless, inspired by the present challenges that the bioimplant community wishes to address, a review on photoand dark catalysis of TiO2 is presented here.

In this review, the catalysis of TiO2 nanomaterials, both in the presence of light (photocatalysis) and in the dark, is presented to give a general overview of the full spectrum of its catalytic activity. A section discussing the role of ROS in TiO2 photocatalysis, which is crucial in photo- and dark catalysis on TiO2, is included. As the understanding of the role of ROS in photocatalysis is more extensive, it is beneficial to look at it from a mechanistic perspective without going into details, as excellent reviews and articles also exist in the literature [22,57,58]. The influence of some properties of TiO2 on its performance in photocatalysis will then be presented to understand the surface engineering that has been carried out to advance the photocatalysis field. TiO2 nanoparticles (NPs), having been extensively developed for photocatalysis, also pose some risks, and the safety of using them will also be discussed. Together with this, the other side of the coin, the photocatalytic antibacterial property afforded by TiO2 NPs, is presented. Then, highlights

and advancements in the efforts to boost the photocatalytic efficiency of TiO2 mainly in terms of charge separation enhancement and improvements in light harvesting will be presented. Black TiO2 nanomaterials, a current hot topic in the field of TiO2 photocatalysis, will also be presented in light of their physics and photocatalytic activity.

Many studies on dark catalysis are investigations on the influence of Ti-based implants on the inflammatory response and vice versa. In addition, early studies on dark catalysis are observations carried out mainly as a reference to photocatalytic works and the residual effect after the removal of irradiation. These will be presented to serve as a bridge between photo- and "dark catalysis" and will be followed by studies addressing and contributing to the so-far understanding of the dark catalysis mechanism. Dark catalysis is important when looking at the inflammatory response which yields ROS, resulting in some microbicidal effect of TiO2 and improving the performance of biomedical implants. As there is a huge scientific community working on biomedical implants who are looking at improving the performance, regenerative ability, and other properties of Ti-based materials, discussions on Ti and Ti-based oxides for biomedical applications, the safety of these materials, and the inflammatory condition are also included. Finally, a conclusion/future perspective in terms of photo- and dark catalysis on Ti-based oxides for disinfection and bioimplant application is given.

#### **2. TiO2 Photocatalysis**

Since photocatalysis is mainly due to redox reactions of photogenerated charge carriers producing reactive surface species and that they mostly occur in the presence of water and/or oxygen, it is important to look at the role of reactive oxygen species (ROS).

### *2.1. Reactive Oxygen Species in TiO2 Photocatalysis*