To demonstrate the applicability and feasibility of the fabricated aptasensor for clinical samples, two different concentrations (47.60 and 238.10 µg mL−1 ) of GA solutions were spiked into the diluted serum. The recovery rates of low and high GA concentrations were 104% and 90%, respectively, as shown in Table 4, which were within the acceptable range with RSD values of 6.10% and 2.80%, respectively. The data showed that the aptasensor developed here can detect GA levels accurately. Therefore, the developed aptasensor is observed to be reliable and can be used as a potential tool for the detection of GA in real clinical samples. However, further validation experiment including the comparison of the developed biosensor and a reference method for real sample analysis is required prior to implementation in the clinical laboratory.

**Table 4.** Recovery assay of GA in human serum using the developed aptasensor.

a The relative standard deviations.

#### **3. Materials and Methods**

#### *3.1. Reagents and Materials*

The GA binding aptamer with a 33-nucleotide sequence of 50 -GG TGG CTG GAG GGG GCG CGA ACG TTT TTT TTT T–30 [49] was modified by an amino group (NH2) at 50 end of the aptamer sequence (Integrated DNA Technologies Pte. Ltd., Singapore). The aptamer was reconstituted using Tris-EDTA (TE) buffer (1X, pH 8.2). Dopamine hydrochloride, GA, and ethanolamine were purchased from Sigma-Aldrich (Singapore). Disposable screenprinted carbon electrodes (SPCEs, 30 × 12.5 mm) were purchased from Quasense, Thailand. As a redox indicator, potassium ferricyanide (K3[Fe(CN)6], 5 mM) together with 0.1 M potassium chloride (KCl) in phosphate-buffered saline (PBS) (1X, pH 7.4). All reagents were prepared in ultrapure deionized water (DI).

#### *3.2. Instruments*

The electrochemical detection system was completed by the transducer (the electrode sensor) and the detector. The transducer was a disposable SPCE with a threeelectrode system including a 3-mm working carbon electrode, a carbon counter electrode, and a silver/silver chloride reference electrode. The PalmSens4 potentiostat with PS Trace 5.8 software (PalmSens BV Co., Ltd., Houten, The Netherlands) was used for all

electrochemical experiments. The characterization of PDA-NPs was carried out by an Eppendorf BioSpectrometer® fluorescence (Hamburg, Germany), a Bruker TENSOR II ATR-FTIR spectrometer (Bruker, Germany), a transmission electron microscope (TEM, FEI, TECNAI G2 20, Nieuw-Vennep, The Netherlands), a scanning electron microscope (SEM, Jeol, JSM-IT200 InTouchScope™, Tokyo, Japan) and a dynamic light scattering (DLS, Malvern, UK).

#### *3.3. Synthesis of PDA-NPs and In Vitro Conjugation of GA Aptamer on PDA-NPs*

PDA-NPs were synthesized by simply dissolving 50 mg of dopamine hydrochloride into 100 mL of 10 mM Tris buffer (pH 8.5, 9.5, and 10.5). The suspension was stirred at 180 rpm for 20 h. The solution was then centrifuged at 16,100× *g* for 5 min at room temperature. PDA-NPs were washed twice with 1 mL of Tris buffer. The supernatant was discarded, and the remaining pellets were collected. The PDA-aptamer complex was prepared by mixing equal volumes of synthesized PDA-NPs and GA-specific aptamer. Aptamer conjugation was done 45 min after incubation.

#### *3.4. Fabrication of the Electrochemical Aptasensor*

The working carbon electrodes were coated with seven microliters of the PDA-NPs solution and dried in the oven at 50 ◦C for 30 min. To remove excess PDA-NPs, DI was used to thoroughly clean the electrodes. Then, 7 µL of 1 µM GA aptamer were immobilized and stabilized on the working electrode for 30 min to allow complete binding between the amino groups of the aptamers on the PDA surfaces. PBS (1X, pH 7.4) was used to completely wash the unbound aptamers. The surfaces were blocked with 0.1 M ethanolamine to reduce non-specific binding. After blocking for 15 min, the SPCEs were washed with PBS again. The layer-by-layer aptasensor was applied for the detection of GA in this study.

#### *3.5. Electrochemical Analysis*

The produced aptasensors were incubated with various GA concentrations for 30 min. Then, electrode surfaces were rinsed with PBS and 130 µL of 5 mM [Fe(CN)6] 3−/4− was added until the surfaces were immersed. For the characterization of the fabrication process, CV and EIS analyses were carried out after each immobilization procedure. Both electrode characterization and GA detection were measured by CV with a potential of −0.5 V to 0.9 V at a scan rate of 100 mVs−1 using a redox indicator. The impedance spectra were measured using the open circuit potential at the frequency range of 100 mHz to 100 kHz. The flow diagram of the proposed electrochemical aptasensor is illustrated in Scheme 1.

**Scheme 1.** Schematic diagram of the proposed PDA-NPs functionalized electrochemical aptasensor for GA protein detection (**a**) Synthesis and functionalization of PDA-NPs (**b**) Drop-coating PDA-NPs onto SPCEs (**c**) Immobilization of aptamer (**d**) Blocking with ethanolamine (**e**) Incubation with GA and (**f**) Voltametric measurements using 5 mM [Fe(CN)6 ] 3−/4− as a redox indicator.

#### *3.6. Real Sample Analysis*

After optimal conditions were obtained, the capability of the proposed electrochemical biosensor was investigated in real samples. Serum samples were spiked with GA at final concentrations of 47.60 and 238.10 µg mL−1 to test the clinical performance of the proposed biosensor. Known concentrations of GA spiked into diluted serum samples were determined by employing the electrochemical measurements that were described in the previous section.

#### **4. Conclusions**

PDA is quite a promising sensor material for a variety of analyses. In particular, it has been extensively applied for designing biochips for biomarker detection. In this study, we used functional PDA-NPs as a coating material on SPCE surfaces to create an electrochemical GA detector under a new concept, which has the potential to monitor glycemic control in DM patients. Here, we designed the sensing system to be convenient with a simple and facile fabrication process and label-free GA detection. The PDA aptasensor developed here showed excellent sensitivity, selectivity, and a low detection limit of 0.40 µg mL−1 for GA detection. This proposed technique shows its potential application in serum/plasma GA measurement to improve the screening and management of diabetic patients in the future. It can be also applied to new PDA-based nanoconstructs to modify the electrode surfaces for a variety of biomedical applications.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ijms232213699/s1, Table S1: Hydrodynamic diameter and polydispersity index (PDI) of PDA-NPs synthesized at different pH.

**Author Contributions:** Conceptual framework, P.M. and J.D.; Experimental design, N.N.S.A. and P.M.; Performing experiments, N.N.S.A. and P.M.; Data analysis and results interpretation, N.N.S.A. and P.M.; Drafting manuscript, P.M., O.B. and N.N.S.A.; Revision of the manuscript, P.M., S.D. and J.D.; Final approval version of the manuscript, P.B. and J.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is funded by the Young Researcher Development Project of Khon Kaen University Year 2022, Khon Kaen, Thailand.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Centre for Ethics in Human Research, Khon Kaen University (HE651209).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The author would like to acknowledge Yukifumi Nawa for editing the manuscript via the Publication Clinic, KKU, Thailand.

**Conflicts of Interest:** The authors declare no competing financial interest.

#### **References**

## *Article* **Electrochemically Activated CNT Sheet as a Cathode for Zn-CO2 Batteries**

**Daniel Rui Chen 1,† , Megha Chitranshi 2,† , Vesselin Shanov 1,3 and Mark Schulz 2,\***

**Abstract:** High demand for electrochemical storage devices is increasing the need for high-performance batteries. A Zn-CO2 battery offers a promising solution for CO2 reduction as well as energy storage applications. For this study, a Zn-CO2 battery was fabricated using a Carbon Nanotube (CNT) sheet as a cathode and a Zn plate as an anode. The electrochemical activation technique was used to increase the surface area of the CNT electrode by roughly 4.5 times. Copper (Cu) as a catalyst was then deposited onto the activated CNT electrode using electrodeposition method and different Cu loadings were investigated to optimize CO2 reduction. The final assembled Zn-CO2 battery has a 1.6 V output voltage at a current density of 0.063 mA/cm2 , which is higher than most devices reported in the literature. This study demonstrates the importance of activation process which enabled more catalyst loading on the cathode resulted in additional active sites for electroreduction process. This paper presents the activated CNT sheet as a promising cathode material for Zn-CO2 batteries.

**Keywords:** CNT/Cu composite; CO2 reduction; Zn-CO2 battery

## **1. Introduction**

The Metal-CO2 battery has attracted attention due to its unique approach to utilize CO2 economically and efficiently. The discharging process of the battery is based on the mechanism that naturally occurred the oxidation process from the metal anode provide electron to reduce CO2 to various hydrocarbon byproducts, electrons can pass through an external circuit to the cathode and generate electricity [1]. Although metal-CO2 battery research is still at a very early stage, it is considered to be a promising solution to capture and recycle CO2. This technology is intended to address two major global issues that are currently faced by humanity: the energy crisis and climate change caused by the rapid rise of CO2 levels.

Various successful metal-CO2 batteries have been demonstrated in recent years including Li-CO2 [2–5], Al-CO2 [6], Na-CO2 [7], and others. The most studied battery is Li-CO2 battery due to its high theoretical energy density of 1876 Wh Kg−1 [2], compared to other available batteries. However, these batteries have challenges including, use of high active metal anodes, solid discharge products, and cost limits the practical environmentally friendly applications [8–10]. Zn-CO2 provides a promising alternative due to its low toxicity, safety, cost, low active metal anodes, and is environmentally friendly [4,8,9,11,12].

Reutilizing CO2 to produce other high-value chemicals is deemed to be a more practical route. Electrochemical reduction of CO2 (ERC) has many advantages, which include producing high economic value products such as methanol, ethylene, and formic acid, operating at room temperature and atmospheric pressure, and the process can be powered using renewable sources of energy [13].

**Citation:** Chen, D.R.; Chitranshi, M.; Shanov, V.; Schulz, M. Electrochemically Activated CNT Sheet as a Cathode for Zn-CO2 Batteries. *Int. J. Mol. Sci.* **2022**, *23*, 12602. https://doi.org/10.3390/ ijms232012602

Academic Editor: Raghvendra Singh Yadav

Received: 8 August 2022 Accepted: 17 October 2022 Published: 20 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

However, ERC has also been associated with many constraints and disadvantages. These shortcomings are demonstrated in the following three points: (i). ERC has suffered from high electrode overpotential and low resultant current density [14,15]; (ii). The hydrogen evolution reaction competes with the ERC process at the high potential in the aqueous phase [16]; (iii). Product selectivity and low efficiency often plague the ERC process [15]. Using electrocatalysts has been suggested to lower the potential and increase the current density, and to improve the overall efficiency of the ERC process. Metal and semiconducting metals, such as Cu, Fe, Sn, Au, Pt, etc., are commonly used as cathode materials to reduce CO2 electrocatalytically [17,18].

Among various metal catalysts, Cu has been considered the most appropriate candidate for the ERC process. In an aqueous phase, Cu can electrocatalytically reduce CO2 to hydrocarbons with relatively low current density (around 5–10 mA cm−2 ) and high current efficiency (i.e., >69% at 0 ◦C) [19]. Furthermore, the selectivity of the products can be controlled by the atomic structure of Cu, for instance, a Cu (100) electrode with an optimal density of steps has shown higher selectivity for C-C coupling than other planar surfaces [20,21]; production of oxygenates is enhanced in the crystal with the plane of (100) and (110) [20,22]; recent DFT results also show that Cu (211) yields higher magnitude activity than other planes (100) and (111) for CO reduction to CH4 [23].

CNT materials have been known for their great electrical and mechanical properties [24,25], chemical inertness, and electrocatalytic stability [26], as well as high surface area [27]. All these advantages make CNT a good substrate material for Cu deposition. By utilizing the porous surface morphology of the CNT sheet, Cu materials can penetrate and distribute more evenly inside the nanotubes. The inherent mechanical property of CNT also provides CNT-Cu composite materials with a great degree of flexibility. To ensure the formation of micro-sized Cu particles, a slow-paced organic deposition method was used to deposit Cu on the CNT sheet. Two Zn-CO2 batteries were investigated using two tubular cathodes prepared from directly grown CNTs on copper and nickel mesh [28]. These batteries demonstrated excellent stability, high input voltage, and discharge performance. Another Zn-CO2 was fabricated using hollow CNT fiber exhibited high selectivity and efficiency [10]. The battery showed energy density of 288.3 Wh Kg−1 and was stable up to 8 days.

This paper discusses the use of CNT sheet as a cathode material for Zn-CO2 battery. The CNT sheet was first acid-treated to make it hydrophilic and then electrochemical activation was performed on the acid-treated CNT sheet to increase the surface area for Cu deposition. The organic deposition method was used to deposit the Cu on the activated CNT sheet, different Cu depositions were achieved by varying the deposition time of the process. Optimum deposition time in terms of CO2 reduction efficiency was studied. The aim of this research is to use activation process to increase the surface area of the electrode for more catalyst deposition. The additional area will provide more activation sites for electrochemical reduction activity and improve the performance of the device. The performance of Cu deposited CNT was then investigated and compared with non-activated CNT cathode electrode.

#### **2. Results & Discussion**

#### *2.1. CNT Activation and Cu Deposition*

During the final stage of the FCCVD process for CNT synthesis, the CNT sheet has lost a significant amount of surface area due to the densification process. Thus, electrochemical activation has been used as a practical approach to recover some of the losses. The activation is explained in detail in our previous paper [29]. Before the activation, CNTs sheets were acid-treated to increase their hydrophilicity The hydrophobicity of as-synthesized CNT sheet has prevented electrolytes from the wetting the electrode, which is mainly due to the nonpolar aromatic molecular structure. Acid treatment was used to functionalize the CNT sheet and promote the wetting [30]. Additionally, to not bring external chemical elements into the electrochemical cell, NaCl was used as the electrolyte to activate the CNT sheet. The duration of activation was limited to less than 10 min to prevent CNT structural damage

caused by excessive activation. During the activation process, electrolysis, and formation of H2 caused expansion and delamination of the CNT sheet. After 10 min activation, the activated CNT sheet exhibited sponge-like and multi-layer morphology, which was able to increase the inner surface area for the deposition of Cu. The illustration of activation and deposition process used for CNT sheet samples for CO2 reduction and battery part is shown in Figure 1. activate the CNT sheet. The duration of activation was limited to less than 10 min to prevent CNT structural damage caused by excessive activation. During the activation process, electrolysis, and formation of H2 caused expansion and delamination of the CNT sheet. After 10 min activation, the activated CNT sheet exhibited sponge-like and multilayer morphology, which was able to increase the inner surface area for the deposition of Cu. The illustration of activation and deposition process used for CNT sheet samples for CO2 reduction and battery part is shown in Figure 1.

The activation is explained in detail in our previous paper [29]. Before the activation, CNTs sheets were acid-treated to increase their hydrophilicity The hydrophobicity of assynthesized CNT sheet has prevented electrolytes from the wetting the electrode, which is mainly due to the nonpolar aromatic molecular structure. Acid treatment was used to functionalize the CNT sheet and promote the wetting [30]. Additionally, to not bring external chemical elements into the electrochemical cell, NaCl was used as the electrolyte to

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 3 of 15

**Figure 1.** The illustration of process steps used for preparing CNT samples for analysis. **Figure 1.** The illustration of process steps used for preparing CNT samples for analysis.

During the Cu deposition, organic electrolytes (*Cu (CH*3*COO)*2 *& CH*3*CN*) were used for deposition. Organic deposition has been used as a Cu seeding method in some publications, it has helped to achieve a uniform and less congregated Cu distribution on CNT substrate due to its low Faradaic efficiencies and slow deposition rate [31]. In comparison to an aqueous electrolyte deposition method (*CuSO*4), organic deposition can infiltrate into the activated area and provide more balanced internal and surface deposition [31]. In During the Cu deposition, organic electrolytes (*Cu (CH*3*COO)*2 *& CH*3*CN*) were used for deposition. Organic deposition has been used as a Cu seeding method in some publications, it has helped to achieve a uniform and less congregated Cu distribution on CNT substrate due to its low Faradaic efficiencies and slow deposition rate [31]. In comparison to an aqueous electrolyte deposition method (*CuSO*4), organic deposition can infiltrate into the activated area and provide more balanced internal and surface deposition [31]. In this research, organic deposition was used to deposit Cu onto activated CNT sheet to maximize its surface area.

Cu deposition was achieved by electrochemical deposition of Cu onto a CNT electrode with a constant voltage by using the organic deposition method. The organic deposition method (OD) has been used to promote Cu deposition within CNT wires and control surface deposition to obtain a uniform and continuous Cu matrix. OD is the initial seeding process that deposit nano-sized Cu particle, under the following heat treatment, these nanosized Cu particles can agglomerate into micro-sized Cu particles, which catalyze CO2 reduction [31,32]. During the CV measurement, the three electrode method was used to measure the cyclic voltammetry at different scan rates (10, 25, 50, 100, and 150 mV/s), the CV curves were presented in Figures S1A–D and S2 (Sections S1 and S2) to study the electrochemical activity of various samples, according to which there is no redox peak for the acid-treated sample, whereas redox peaks are visible for the 2, 5, 10, 20 h for the Cu

deposited sample. The onset of the reduction peak is an indication of the occurrence of the CO2 reduction reaction. Due to the absence of Cu catalyst, a redox reaction is unlikely to have occurred on the acid-treated CNT sample.

Additionally, the relationship between current density and potential is shown in Figure S2, as it indicated that current density increases with the increase in potential. However, the acid-treated CNT sample shows the lowest negative current density increase compared with other samples. The 10 h Cu deposited sample showed the highest negative current density increase especially below −1 V. Due to its great electrochemical activity at a negative potential, 10 h was considered the optimal OD time frame to provide most electrochemically active CNT sample for CO2 reduction.

#### *2.2. The Surface Area Comparison*

The electrochemical active surface area of acid-treated CNT as well as CNT after activation was estimated based on the double-layer capacitance (Cdl) by using a cyclic voltammetry (CV) graph. According to the classic capacitance equation for double-layer capacitance: *Cdl* = *ε*0 ∗ *A*/ *d*, where *A* is the surface area and d is the distance between layers [33], the surface area of the electrode is proportional to the capacitance value.

During the measurement, CV was taken between −1.0 and 2.0 V vs. RHE at scan rates of 10 mV/s and the electrolyte is 1 M NaCl saturated with CO2. According to the CV graph in Figure 2, the capacitance of the sample was estimated using *Cdl* = R *Idv*/*v* ∗ *m* ∗ ∆*V*, where R *Idv* is the integrated area under the CV curve, *v* is the scan rate, m is the weight of the electrode sample, and ∆*V* is the voltage window. Because *v*, m, and ∆*V* are the same for the acid-treated and activated samples, the surface area ratio between activated CNT and acid-treated CNT is equivalent to the ratio of the integrated area of their respective CV curve. Based on the calculation, R *Idv* of the activated CNT curve is around 4.5 times larger than the acid-treated CNT curve, which suggested a large increase in the electrochemical active area and higher surface roughness. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 5 of 15

**Figure 2.** CV curve of acid-treated and activated CNT. **Figure 2.** CV curve of acid-treated and activated CNT.

#### *2.3. Structural Analysis 2.3. Structural Analysis*

ceramic tube used in the synthesis reactor.

The SEM images of acid-treated and activated CNT with Cu deposition with different magnification as well as their EDX graph are shown in Figures 3 and 4, respectively. After organic deposition, Cu precipitant was first formed around CNT strands and deposited onto the surface of CNT sheet (Figure 3A). After the heat treatment, these precipitants started to coalescence and formed large size spherical particles, which were uniformly distributed on the CNT surface as shown in Figure 3B, while the cross-section is non-acti-The SEM images of acid-treated and activated CNT with Cu deposition with different magnification as well as their EDX graph are shown in Figures 3 and 4, respectively. After organic deposition, Cu precipitant was first formed around CNT strands and deposited onto the surface of CNT sheet (Figure 3A). After the heat treatment, these precipitants

vated (Figure 3C, D) and its compact structure prevents further infiltration of Cu. Additionally, EDX was used to verify the presence of Cu (Figure 3E). The presence of O indi-

used in the nanotube synthesis process. The elements of Mo and Cl are possibly from the

started to coalescence and formed large size spherical particles, which were uniformly distributed on the CNT surface as shown in Figure 3B, while the cross-section is non-activated (Figure 3C,D) and its compact structure prevents further infiltration of Cu. Additionally, EDX was used to verify the presence of Cu (Figure 3E). The presence of O indicates that Cu particles are partially oxidized, a small percentage of impurities such as Fe, S are from the catalyst source (Ferrocene) and growth promoter (Thiophene), respectively, used in the nanotube synthesis process. The elements of Mo and Cl are possibly from the ceramic tube used in the synthesis reactor. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 6 of 15

**Figure 3.** SEM images of acid-treated CNT with Cu organic deposition: surface morphology (**A**, **B**), as well as cross-sectional images (**C**, **D**), and EDX spectrum of the selected area (**E**). **Figure 3.** SEM images of acid-treated CNT with Cu organic deposition: surface morphology (**A**,**B**), as well as cross-sectional images (**C**,**D**), and EDX spectrum of the selected area (**E**).

winder that of non-activated ones (Figure 4A), the rough surface also gives rise to the rough Cu surface morphologies (Figure 4B). Past literature has shown that surface

In terms of the activated CNT sheet, due to the nature of the electrochemical activation process, which generates a copious amount of H2, H2 is not only able to delaminate

roughness has a direct relationship with activity and product selectivity of CO2 reduction [34,35]. There are more under-coordinated sites are present in the roughened surface that showed more adsorption energies compared to highly coordinated sites. Cross-section SEM images showed the multilayer and delaminated structure, which provide ample space for Cu deposition. An EDX spectrum of the cross-section indicates that Cu has a

large presence, and the presence of O indicates Cu is partially oxidized.

In terms of the activated CNT sheet, due to the nature of the electrochemical activation process, which generates a copious amount of H2, H2 is not only able to delaminate the CNT sheet, but surface roughness has also been increased due to the bulge formation in the CNT sheet from H2 generation. The distance between CNT strands appears to be winder that of non-activated ones (Figure 4A), the rough surface also gives rise to the rough Cu surface morphologies (Figure 4B). Past literature has shown that surface roughness has a direct relationship with activity and product selectivity of CO2 reduction [34,35]. There are more under-coordinated sites are present in the roughened surface that showed more adsorption energies compared to highly coordinated sites. Cross-section SEM images showed the multilayer and delaminated structure, which provide ample space for Cu deposition. An EDX spectrum of the cross-section indicates that Cu has a large presence, and the presence of O indicates Cu is partially oxidized. tential suggests high electrocatalytic activity of activated CNT/Cu for CO2 reduction. The comparison was made between Cu deposited acid-treated CNT and Cu deposited activated CNT, before the LSV measurement, NaCl electrolyte was saturated with CO2, and CO2 gas was continuously pumped into NaCl electrolyte during the experiment. The

A linear sweep voltammetry (LSV) graph was used to estimate the electrochemical reduction potential of the electrodes. Enhanced current density at increased negative po-

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 8 of 15

*2.4. Linear Sweep Voltammetry Graph and Potentiostatic Curve*

#### *2.4. Linear Sweep Voltammetry Graph and Potentiostatic Curve* range of sweeping potential was kept between 0.4 and −0.6 vs. RHE, current density was

A linear sweep voltammetry (LSV) graph was used to estimate the electrochemical reduction potential of the electrodes. Enhanced current density at increased negative potential suggests high electrocatalytic activity of activated CNT/Cu for CO2 reduction. The comparison was made between Cu deposited acid-treated CNT and Cu deposited activated CNT, before the LSV measurement, NaCl electrolyte was saturated with CO2, and CO2 gas was continuously pumped into NaCl electrolyte during the experiment. The range of sweeping potential was kept between 0.4 and −0.6 vs. RHE, current density was normalized by the geometric area of the CNT sample. It can be seen from Figure 5A that, for the Cu deposited activated CNT sample, a rapid shift in current and increased current density indicate an intensified reduction reaction. The reduction current density initially decreased and increased with further increase of reduction potential. However, in terms of Cu deposited only in the acid-treated sample, the current density appeared to be stabilized at around 0.5 mA/cm2 after the initial rapid decline, which indicates a minor occurrence of the electrochemical reduction reaction. normalized by the geometric area of the CNT sample. It can be seen from Figure 5A that, for the Cu deposited activated CNT sample, a rapid shift in current and increased current density indicate an intensified reduction reaction. The reduction current density initially decreased and increased with further increase of reduction potential. However, in terms of Cu deposited only in the acid-treated sample, the current density appeared to be stabilized at around 0.5 mA/cm2 after the initial rapid decline, which indicates a minor occurrence of the electrochemical reduction reaction. Potentiostatic testing was carried out and results are shown in Figure 5B. A constant voltage −0.4 vs. RHE was applied and the current density variation was measured during the 4 h running. Cu deposited activated CNT experienced initial decline and later became stabilized around 0.5 mA/cm2, while Cu deposited only acid-treated CNT was kept constant around 0.25 mA/cm2, which indicates that activated CNT provides extra space for Cu deposition to increase the activation sites, additionally rough surface morphology of Cu particles on the CNT sheet enhances the electrochemical activities.

**Figure 5.** LSV curve (**A**) and potentiostatic curve (**B**) of acid-treated CNT with 10 Hrs Cu organic deposition and activated CNT with 10 Hrs organic deposition. **Figure 5.** LSV curve (**A**) and potentiostatic curve (**B**) of acid-treated CNT with 10 Hrs Cu organic deposition and activated CNT with 10 Hrs organic deposition.

*2.5. Zn-CO2 Battery Performance*  The Zn-CO2 metal battery was constructed in the H-cell, which is schematically shown in Figure 6. Cu deposited activated CNT was used as the cathode material in the cathodic compartment, while the Zn plate was used as an anode in the anodic compartment. A Nafion membrane was used to stabilize the pH values of anolyte and catholyte. After NaCl was saturated with CO2, the gas was constantly bubbling into the electrolyte during the experiment. During the discharge process, the Zn plate can react with KOH to Potentiostatic testing was carried out and results are shown in Figure 5B. A constant voltage −0.4 vs. RHE was applied and the current density variation was measured during the 4 h running. Cu deposited activated CNT experienced initial decline and later became stabilized around 0.5 mA/cm2 , while Cu deposited only acid-treated CNT was kept constant around 0.25 mA/cm2 , which indicates that activated CNT provides extra space for Cu deposition to increase the activation sites, additionally rough surface morphology of Cu particles on the CNT sheet enhances the electrochemical activities.

#### form Zn (OH)2 and release electrons, at the same time, CO2 reduction occurred on the Cu *2.5. Zn-CO2 Battery Performance*

deposited CNT electrode, and Cu catalyzes to promote the CO2 conversion. The possible reaction occurred on both electrodes are shown below: *Cathode*: 2CO2 + 8H+ + 8e−1 ↔ CH3COOH +2H2O (1M NaCl, sat. CO2) (1) *Anode*: 4Zn + 8OH−– 8e−1 ↔ 4Zn (OH)2 (1M KOH) (2) The Zn-CO2 metal battery was constructed in the H-cell, which is schematically shown in Figure 6. Cu deposited activated CNT was used as the cathode material in the cathodic compartment, while the Zn plate was used as an anode in the anodic compartment. A Nafion membrane was used to stabilize the pH values of anolyte and catholyte. After NaCl was saturated with CO2, the gas was constantly bubbling into the electrolyte during the experiment. During the discharge process, the Zn plate can react with KOH to form Zn (OH)2 and release electrons, at the same time, CO2 reduction occurred on the Cu deposited CNT electrode, and Cu catalyzes to promote the CO2 conversion.

*Overall reaction*: 4Zn + 2CO2 + 8OH− +8H+ ↔ 4Zn (OH)2 + CH3COOH +2H2O (3)

**Figure 6.** Schematic diagram of the Zn-CO2 electrochemical cell. **Figure 6.** Schematic diagram of the Zn-CO2 electrochemical cell.

The potential stability of Zn-CO2 in terms of discharging time before and after the saturation of CO2 was measured. According to Figure 7A, the acid-treated CNT showed The possible reaction occurred on both electrodes are shown below:

stabilized around 1.3 V, after the saturation of CO2, the potential has seen a steady drop,

$$\text{Cathode:}\ 2\text{CO}\_2 + 8\text{H}^+ + 8\text{e}^{-1} \leftrightarrow \text{CH}\_3\text{COOH} + 2\text{H}\_2\text{O} \text{ (1M NaCl, sat. CO}\_2\text{)}\tag{1}$$

$$\text{Anode: } 4\text{Zn} + 8\text{OH}^- \cdot 8\text{e}^{-1} \leftrightarrow 4\text{Zn (OH)}\_2 \text{ (1M KOH)}\tag{2}$$

#### CO2, furthermore, Cu deposited activated-CNT shows the highest potential increase of 0.4 V after CO2 saturation, while Cu deposited acid treated-CNT has a 0.2 V increase after CO2 *Overall reaction*: 4Zn + 2CO2 + 8OH− +8H+ ↔ 4Zn (OH)2 + CH3COOH +2H2O (3)

saturation. During 20 min discharging process, the discharge current density was kept at 0.0625 mA/cm2 . The potential change at different current densities was also measured and presented in Figure 7E, the potential was reduced from 1.2 V at a current density of 0.122 mA/cm2 to 0.8 V at a current density of 0.37 mA/cm2 . In comparison, when NaHCO3 was used as electrolyte on both side of the cell, due to the low reactivity between Zn and Na-HCO3, output potential show negligible improvement when Cu-CNT was cathode (Supplementary Materials Figure S3 in Sections S3). The formation of acetic acid (CH3COOH) in the electrolyte has been verified by Nuclear Magnetic Resonance (NMR) (Figure 8A). NMR is used to determine the molecular structure at the atomic level, in which all nuclei have multiple spins and are electrically charged. Inside of NMR magnetic field, radio wave excites the nuclei from lower to higher energy lever in a single step and generates nuclear magnetic resonance, which can be detected by sensitive radio receiver. The resonance frequency gives access to the detailed information of the electronic structure and functional group of a molecule/chemical compounds [36,37]. The potential stability of Zn-CO2 in terms of discharging time before and after the saturation of CO2 was measured. According to Figure 7A, the acid-treated CNT showed the lowest potential output and was followed by a sharp potential drop, while inactivated CNT. In Figure 7B, before the saturation of CO2, the output potential of a cell has been stabilized around 1.3 V, after the saturation of CO2, the potential has seen a steady drop, which is likely due to the increased internal resistance caused by CO2 saturated electrolyte. Figure 7C,D, both demonstrate the potential to increase due to the introduction of CO2, furthermore, Cu deposited activated-CNT shows the highest potential increase of 0.4 V after CO2 saturation, while Cu deposited acid treated-CNT has a 0.2 V increase after CO2 saturation. During 20 min discharging process, the discharge current density was kept at 0.0625 mA/cm2 . The potential change at different current densities was also measured and presented in Figure 7E, the potential was reduced from 1.2 V at a current density of 0.122 mA/cm2 to 0.8 V at a current density of 0.37 mA/cm2 . In comparison, when NaHCO3 was used as electrolyte on both side of the cell, due to the low reactivity between Zn and NaHCO3, output potential show negligible improvement when Cu-CNT was cathode (Supplementary Materials Figure S3 in Sections S3). *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 10 of 15

(**C**) (**D**)

**Figure 7.** Discharging curve of acid-treated CNT for 20 min (**A**), activated CNT (**B**), Cu-deposited acid-treated CNT (**C**), Cu deposited activated CNT (**D**), discharging curve of Cu deposited activated

According to various literature, pathways towards C2+ products are considered to be very complex and generates debate among researchers performing theoretical studies. Compared with C1 products, C2+ hydrocarbon by-products contain two or more carbon atoms and have higher energy densities, although most theoretically proposed mecha-

nisms are based on density functional theory (DFT) simulations, however, the consensus **Figure 7.** *Cont*.

(**E**)

CNT at different current densities (**E**).

**Figure 7.** Discharging curve of acid-treated CNT for 20 min (**A**), activated CNT (**B**), Cu-deposited acid-treated CNT (**C**), Cu deposited activated CNT (**D**), discharging curve of Cu deposited activated CNT at different current densities (**E**). **Figure 7.** Discharging curve of acid-treated CNT for 20 min (**A**), activated CNT (**B**), Cu-deposited acid-treated CNT (**C**), Cu deposited activated CNT (**D**), discharging curve of Cu deposited activated CNT at different current densities (**E**).

According to various literature, pathways towards C2+ products are considered to be very complex and generates debate among researchers performing theoretical studies. Compared with C1 products, C2+ hydrocarbon by-products contain two or more carbon atoms and have higher energy densities, although most theoretically proposed mechanisms are based on density functional theory (DFT) simulations, however, the consensus The formation of acetic acid (CH3COOH) in the electrolyte has been verified by Nuclear Magnetic Resonance (NMR) (Figure 8A). NMR is used to determine the molecular structure at the atomic level, in which all nuclei have multiple spins and are electrically charged. Inside of NMR magnetic field, radio wave excites the nuclei from lower to higher energy lever in a single step and generates nuclear magnetic resonance, which can be detected by sensitive radio receiver. The resonance frequency gives access to the detailed information of the electronic structure and functional group of a molecule/chemical compounds [36,37].

According to various literature, pathways towards C2+ products are considered to be very complex and generates debate among researchers performing theoretical studies. Compared with C1 products, C2+ hydrocarbon by-products contain two or more carbon atoms and have higher energy densities, although most theoretically proposed mechanisms are based on density functional theory (DFT) simulations, however, the consensus is by transferring a concerted proton-electron (H+/e−) from the electrolyte to adsorbed species, which in this case is Cu surface, a CO2 molecule can reduce to carboxyl intermediate \*COOH and then to \*CO and water molecule, further reaction include protonation of \*CO to \*CH3O and following coupling with CO2· − to form to acetate [38], which is illustrated in Figure 8B due to the complicated nature of the reaction as well as this could well be one of the possible routes to form acetic acid, further studies are needed to fully confirm. The Zn-CO2 battery has a theoretical electromotive force of 4.557 V and a theoretical energy density of 915.75 Wh Kg−1 (Supplementary Materials Section S4). The comparison between this work and other works can be found in Table S1 in Section S5 which proves that this

the literature.

Zn-CO2 battery has a high open voltage of 1.6 V compared to other batteries reported in the literature. Zn-CO2 battery has a high open voltage of 1.6 V compared to other batteries reported in

) from the electrolyte to adsorbed spe-

to form to acetate [38], which is illustrated

/e−

cies, which in this case is Cu surface, a CO2 molecule can reduce to carboxyl intermediate \*COOH and then to \*CO and water molecule, further reaction include protonation of \*CO

in Figure 8B due to the complicated nature of the reaction as well as this could well be one of the possible routes to form acetic acid, further studies are needed to fully confirm. The

density of 915.75 Wh Kg−1 (Supplementary Materials Section S4). The comparison between this work and other works can be found in Table S1 in Section S5 which proves that this

**Figure 8.** 1H NMR graph of the by-product electrolyte (**A**) and acetic acid by possible product pathway from CO2 reduction (**B**). **Figure 8.** 1H NMR graph of the by-product electrolyte (**A**) and acetic acid by possible product pathway from CO2 reduction (**B**).

### **3. Materials and Methods**

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 11 of 15

is by transferring a concerted proton-electron (H+

to \*CH3O and following coupling with CO2·−

*3.1. The Synthesis and Activation of CNT Sheet*

The CNT sheet used in this experiment was synthesized from Floating Catalyst Chemical Vapor Deposition (FCCVD) method. The FC-CVD process is a one-step continuous process and can produce industrial-scale nanotube sheet. The process is substrate-free and provides flexibility to tune the process and customize the material according to a specific application. In this process, we introduced a feedstock at the inlet of the reactor. The feedstock is composed of methanol, n-Hexane, ferrocene, and thiophene. The CNT sock

(web of nanotubes) forms in the high-temperature synthesis zone of the reactor and is collected at the outlet of the reactor onto a rotating drum. A detailed description of the synthesis process tuning, process analysis, and characterization techniques can be found in our previously published research articles [39–43,43–46]. The as-synthesized CNT sheet materials are cleaned and expanded through an acid treatment and activation process. The acid treatment follows the steps of immersing the sample inside a mixture of 3:1 ratio of H2SO4 (*Fisher Chemical*) and HNO3 (*Fisher Chemical*) followed by heat reflux (90 ◦C) for 2–3 h.

For activation of the CNT sheet [29], a three-electrode method was employed using an electrochemical workstation (*Gamry, Interface 1000, Warminster, PA, USA*). A rectangular CNT sheet (*34 mm* × *48 mm*) served as the working electrode. Ag/AgCl and graphite rods were used as the reference and counter electrodes respectively. A 0.3 M NaCl solution was used as the electrolyte. A cyclic voltammetry (CV) sweep was conducted at a scan rate of 50 mV/s, with an applied voltage between −2 V and +2 V, to chemically activate which increases the surface area of the CNT sheet. Typically, 10–15 scan cycles were considered as the optimal number of cycles to fully activate the CNT without causing structural damage to the CNT sheet. The relationship between number of activation cycles and optimal electrochemical performance is given in [29].

#### *3.2. Organic Deposition of Copper on Activated CNT*

The Cu electrodeposition was carried out on a workstation equipped with a power supply (*Circuit Specialists, CSI5003X5, AZ, USA*), a voltage stabilizer (*XY-SK35 CNC buckboost, Econede, FL, USA*), and a time relay device (*XY-WJ01 Delay Relay Module, Hardware, TKXEC, China*) were used to ensure constant voltage supply and accurate control of the electrodeposition time. The electrodeposition setup consisted of an activated CNT sheet sample as the cathode material that was mounted on a glass slide with a Cu strip attached to the upper edge of the sample. The CNT cathode material and Cu anode material were immersed and separated each other at a distance of ~20 mm in the electrolyte. Anhydrous 0.0075 g copper acetate (*Cu (CH*3*COO)*2*, Sigma Aldrich, St. Louis, MO, USA*) in 150 mL acetonitrile (*CH*3*CN, Sigma Aldrich, St. Louis, MO, USA*) was used as the electrolyte for organic deposition. Deposition times were 2 h, 5 h, 10 h, and 20 h, respectively, and the excess electrolyte after Cu organic deposition was removed from the sample by rinsing in deionized water. The sample was then dried in the air for 1 h.

### *3.3. Metal-CO2 Battery Electrochemical Measurement*

Electrochemical activity for CO2 reduction experiments was conducted in a threeelectrode method using an electrochemical workstation (*Gamry, Interface 1000, Warminster, PA, USA*) and a CV/linear sweep voltammetry (*LSV*) graph was recorded. All electrochemical data was collected vs. the Ag/AgCl reference electrode. The electrolyte used in both compartments was 1 M NaCl solution. The Ag/AgCl electrode and the Platinum electrode were used as the reference and counter electrode. The following equation was used to convert from the Ag/AgCl scale to the reversible hydrogen electrode (RHE) standard:

$$E\ (\text{vs.}\ RHE) = E\ (\text{vs.}\ A\text{g/}A\text{gCl}) + 0.209V + 0.0591V \times pH\tag{4}$$

The pH value of the electrolyte in Equation (1) was measured using a digital pH meter (Digital pH tester pen, VIVUSUN, China).

A full Zn-CO2 battery was assembled in an H-type cell separated by a Nafion membrane. A Zn plate (*34 mm* × *48 mm*) and Cu deposited CNT were used as anode and cathode respectively. Before the experiment, ultra-pure CO2 (*99.99%, Wright Brother, Cincinnati, OH, USA*) was pumped into the cathode compartment for 45 min at a rate of 0.25 sL/min to saturate the electrolyte. The whole system is air-tight to prevent leakage and increase CO2 saturation concentration in the electrolyte (*1 M NaCl*).

#### *3.4. Material Characterization*

Scanning electron microscopy (SEM) (*FEI SCIOS, 5 kV, Themofisher Scientific, Waltham, MA, USA*) was used to characterize the surface morphology as well as the cross-section of the activated CNT/Cu sample. Nuclear magnetic resonance (*NMR*) (*Bruker NEO 400Billerica, MA, USA*), with Bruker Ascend 9.4 T, 54 mm (narrow) bore, BOSS-336 shim system, equipped with Bruker 5 mm SMART 15N- 31P and 19F observe with 1H decoupling and observe, Z-axis PFG, VT range from 150 to +150 ◦C, was used to analyze the by-product solution from the electrochemical reduction experiment.

#### **4. Conclusions**

A Zn-CO2 battery with copper deposited CNT as a cathode and a zinc plate as an anode is proposed. The area of the CNT electrode was increased by 4.5 times using the electrochemical activation technique. Different copper loadings were studied for the CO2 reduction process, and it was concluded that the ten hours of organic deposition is the optimum loading for our sample. A new approach for using activated CNT as the cathode is presented and the assembled battery showed an open circuit voltage of 1.6 V and a current density of 0.063 mA/cm2 , which is higher than reported for most device in the literature. Moreover, the oxidized Zn plate is easily replaceable which extends the lifetime of the battery. However, the reduction reaction is still ambiguous and needs a better understanding of the reaction mechanism.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ijms232012602/s1.

**Author Contributions:** Conceptualization, D.R.C. and M.C.; Formal analysis, D.R.C. and M.C.; Supervision, V.S. and M.S.; Validation, M.S.; Writing—original draft, D.R.C. and M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research study was supported by the National Institute for Occupational Safety and Health through the Pilot Research Project Training Program of the University of Cincinnati Education and Research Center Grant #T42OH008432.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

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

#### **References**

## *Article* **Crystal Design and Photoactivity of TiO2 Nanorod Template Decorated with Nanostructured Bi2S3 Visible Light Sensitizer**

**Yuan-Chang Liang \*, Shao-Yu You and Bo-Yue Chen**

Department of Optoelectronics and Materials Technology, National Taiwan Ocean University, Keelung 20224, Taiwan

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** In this study, TiO2 -Bi2S3 composites with various morphologies were synthesized through hydrothermal vulcanization with sputtering deposited Bi2O3 sacrificial layer method on the TiO2 nanorod templates. The morphologies of decorated Bi2S3 nanostructures on the TiO2 nanorod templates are controlled by the duration of hydrothermal vulcanization treatment. The Bi2S3 crystals in lumpy filament, nanowire, and nanorod feature were decorated on the TiO2 nanorod template after 1, 3, and 5 h hydrothermal vulcanization, respectively. Comparatively, TiO2 -Bi2S3 composites with Bi2S3 nanowires exhibit the best photocurrent density, the lowest interfacial resistance value and the highest photodegradation efficiency towards Rhodamine B solution. The possible Z-scheme photoinduced charge separation mechanism and suitable morphology of Bi2S3 nanowires might account for the high photoactivity of TiO2 nanorod-Bi2S3 nanowire composites.

**Keywords:** composites; vulcanization; photoactivity

#### **1. Introduction**

Nanorod arrays of TiO2 are beneficial to provide direct channels for electron transport, reducing the recombination probability of electrons in the transmission process. The rod morphology helps to improve the electron injection and collection efficiency of TiO2 semiconductor [1–5]. However, the wide band gap nature of TiO2 engenders poor response to visible light [6,7]. In order to improve the photoactivity of TiO2, the strategy of semiconductor coupling is often used to enhance the solar energy conversion and utilization of TiO2 [8]. Several cases of TiO2 coupled with a visible light sensitizer has been shown a promising approach to improve solar energy utilization efficiency. TiO2 microspheres coupled with CdS nanoparticles enhance the light harvesting ability and suppress the electron-hole recombination of TiO2 and CdS [9]. The electrospinning formed TiO2/CuO composite nanofibers enhance light absorbance and interparticle charge transfer and lower the band gap energy, thus promoting absorbance and utilization of photon energy from a broader light spectrum [10]. Precise control of Bi2O3 coverage layer phase composition realizes the high photoactivity of the one-dimensional TiO2-Bi2O3 composites [11]. The decoration of ZnFe2O4 crystallites onto TiO2 improves the photoactivity of TiO2 and enhance the photodegradation performance towards Methylene orange [12]. The TiO2/Bi2S3 core–shell nanowire arrays demonstrate improved photocurrent density than that of pristine TiO2 because of the broadened light absorption ability and the increased charge carrier separation efficiency [13]. Similarly, Bi2S3 nanowires/TiO2 nanorod arrays exhibit an excellent photoelectrochemical activity [14]. Rosette-rod TiO2/Bi2S3 shows substantial improvement in photoresponse in compared with pristine TiO2 photoanode [15]. Furthermore, hydrothermal method deposition of TiO2/Bi2S3 composite film is presented to be promising for applications of photoanode [16]. These examples clearly present the feasibility of construction of TiO2-Bi2S3 heterogeneous system to be used in photoactive devices with an improved efficiency.

**Citation:** Liang, Y.-C.; You, S.-Y.; Chen, B.-Y. Crystal Design and Photoactivity of TiO2 Nanorod Template Decorated with Nanostructured Bi2S3 Visible Light Sensitizer. *Int. J. Mol. Sci.* **2022**, *23*, 12024. https://doi.org/10.3390/ ijms231912024

Academic Editor: Raghvendra Singh Yadav

Received: 12 September 2022 Accepted: 6 October 2022 Published: 10 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

Among various visible-light sensitizers, Bi2S3 has attracted much attention because it is environmentally friendly and non-toxic. Bi2S3 has a high optical absorption coefficient and a suitable band gap, which can absorb most of the visible light spectrum [17]. Bi2S3 crystals with a large area growth and different morphologies have been realized through various chemical routes [18]. The photoactivity of Bi2S3 crystals is highly dependent on shape, size, and crystalline quality. How to precisely control the microstructures of Bi2S3 in order to fabricate TiO2-Bi2S3 composites with satisfactory photoactivity is an important issue. In this study, Bi2S3 crystals with various microstructures were decorated on TiO2 nanorod arrays via vulcanization of Bi2O3 sacrificial layer. The vulcanization of metal oxide to obtain metal sulfide has been shown to be a promising and easy approach to synthesize the metal sulfides with controllable microstructures [17,19]. Using metal oxide sacrificial layer to form the metal sulfide through vulcanization is an easy approach to control the microstructure of the as-synthesized metal sulfide. This approach is suitable for microstructural control of the sulfide crystals and thus tuning the physical properties of the samples. However, studies on synthesis of TiO2-Bi2S3 composites through vulcanization of Bi2O3 layer and their photoactivities are lacking. A detailed vulcanization process-dependent microstructure evolution and photoactive properties of TiO2-Bi2S3 composites are proposed in this study to realize the design of TiO2-Bi2S3 composites with high photoactive performance.

#### **2. Experiments**

The synthesis of TiO2 nanorod template on the F-doped SnO2 (FTO) substrate was realized through a hydrothermal method at 170 ◦C. Other detailed parameters and preparation procedures have been described elsewhere [4]. The Bi2O3 thin films were sputter deposited on the TiO2 nanorod template using a radio-frequency magnetron sputtering system. A metallic Bi disc was used as the target (99.9 wt%, 2 inches in diameter); the substrate temperature was fixed at 410 ◦C. The working atmosphere is mixed Ar/O2 with a ratio of 1/1, the working pressure is 20 mtorr and sputtering power is 30 W. The sputtering duration is 50 min. The Bi2O3 thin-film coated TiO2 nanorod templates are further immersed in a 20 mL reaction solution containing 0.1M thiourea and sealed in a Teflon-lined autoclave for a hydrothermal reaction at 160 ◦C for 1, 3, and 5 h to obtain TiO2-Bi2S3 composites (named as BT-1, BT-3, and BT-5, respectively.).

Crystallographic structures of as-synthesized samples were investigated by X-ray diffraction (XRD) analysis using Cu K*α* radiation with a two-theta scan range of 20–60◦ and scan rate of four degrees per min. The morphologies of the as-synthesized samples were investigated using a field emission scanning electron microscopy (FE-SEM). The morphology, high resolution images, crystallographic structure, and composition of BT-1 and BT-3 composite samples were investigated by high-resolution transmission electron microscopy (HR-TEM). X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental binding energies of the as-synthesized samples. The diffuse reflectance spectra of the as-synthesized samples were recorded by using UV-vis spectrophotometer (Jasco V750) at the wavelength range of 200–800 nm. Photoelectrochemical (PEC) and electrochemical impedance spectroscopy (EIS) properties of various photoanodes were investigated using the potentiostat (SP150, BioLogic, Orlando, FL, USA). During measurements, an Ag/AgCl electrode were used as the reference electrode herein. The 0.5 M Na2SO4 solution was used as electrolyte and the light irradiation source for measurements is excited from 100 W Xe arc lamp. The 10 mL RhB solution (5 × 10−5 M) was used as target dye solution for photodegradation experiments. The RhB solution containing various photocatalysts with different irradiation durations (0, 15, 30, 45, and 60 min) was carried out to understand the photocatalytic activity of various as-synthesized samples.

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

The morphology of initial pristine TiO2 rod array template was clearly observed from the SEM images in Figure 1a. As seen, the TiO2 rods are uniformly grown on the entire substrate; moreover, TiO2 rods have a regular cross-section and the side-wall surfaces are

smooth. Figure 1b displays the morphology of BT-1. A large number of lumpy filament-like crystals are covered on the top region of the TiO2 rod template. Figure 1c shows that abundant nanowires with around 0.8 µm length were grafted on the TiO2 rod template for the BT-3 sample. Such a morphology is similar to that of Bi2S3 nanowire-decorated TiO2 rod heterostructures synthesized via a facile two-step hydrothermal growth process reported by Liu et al. [14]. Compared to Figure 1c, after extending the hydrothermal duration to 5 h, the diameter of one dimensional Bi2S3 thickened, and the length increases from 0.8 µm to 1.2 µm as exhibited in Figure 1d. Some aggregations of one-dimensional Bi2O3 crystals appear on the TiO2 rod template. It has been shown that Bi2O3 oxide layer is easily etched and transfer to Bi2S3 phase during a vulcanization process with sulfur ions in the reaction solution [20]. like crystals are covered on the top region of the TiO2 rod template. Figure 1c shows that abundant nanowires with around 0.8 μm length were grafted on the TiO2 rod template for the BT-3 sample. Such a morphology is similar to that of Bi2S3 nanowire-decorated TiO2 rod heterostructures synthesized via a facile two-step hydrothermal growth process reported by Liu et al. [14]. Compared to Figure 1c, after extending the hydrothermal duration to 5 h, the diameter of one dimensional Bi2S3 thickened, and the length increases from 0.8 μm to 1.2 μm as exhibited in Figure 1d. Some aggregations of one-dimensional Bi2O3 crystals appear on the TiO2 rod template. It has been shown that Bi2O3 oxide layer is easily etched and transfer to Bi2S3 phase during a vulcanization process with sulfur ions in the reaction solution [20].

The morphology of initial pristine TiO2 rod array template was clearly observed from the SEM images in Figure 1a. As seen, the TiO2 rods are uniformly grown on the entire substrate; moreover, TiO2 rods have a regular cross-section and the side-wall surfaces are smooth. Figure 1b displays the morphology of BT-1. A large number of lumpy filament-

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 3 of 15

**3. Results and Discussion**

**Figure 1.** SEM images: (**a**) TiO2 template, (**b**) BT-1, (**c**) BT-3, and (**d**) BT-5. **Figure 1.** SEM images: (**a**) TiO2 template, (**b**) BT-1, (**c**) BT-3, and (**d**) BT-5.

Figure 2 shows the possible formation processes of Bi2S3 nanostructures on the TiO2- Bi2O3 composite rod template via different vulcanization processes in this study. Notably, in aqueous solution the solubility of Bi2S3 is much lower than that of Bi2O3. When the TiO2- Bi2O3 composite rods are placed in a reaction solution containing a large amount of S2− ions, there is a high possibility of S2− ions transfer toward to the Bi2O3 surface as exhibited in (a). Furthermore, due to solubility disparity, the formation of Bi2S3 nuclei on Bi2O3 will occur during the vulcanization process because of the ion exchange of S2− from the sulfur precursor solution and O2− from the Bi2O3 surface. The generation of Bi2S3 crystals will continually proceed with a result of Bi3+ reacting with S2− as exhibited in schematic (b). Further extending reaction duration, the Bi2S3 nuclei will slowly stack up to form loose filament structure at the given vulcanization condition of schematic (c). When an increase of the vulcanization reaction duration to 3 h, the loose and lumpy Bi2S3 clusters will transfer into abundant, distinguishable, and separable nanowire crystals covered on the TiO2 template as displayed in schematic (d). When the reaction duration reaches 5 h, the Figure 2 shows the possible formation processes of Bi2S3 nanostructures on the TiO2-Bi2O3 composite rod template via different vulcanization processes in this study. Notably, in aqueous solution the solubility of Bi2S3 is much lower than that of Bi2O3. When the TiO2-Bi2O3 composite rods are placed in a reaction solution containing a large amount of S2− ions, there is a high possibility of S2− ions transfer toward to the Bi2O3 surface as exhibited in (a). Furthermore, due to solubility disparity, the formation of Bi2S3 nuclei on Bi2O3 will occur during the vulcanization process because of the ion exchange of S2− from the sulfur precursor solution and O2− from the Bi2O3 surface. The generation of Bi2S3 crystals will continually proceed with a result of Bi3+ reacting with S2− as exhibited in schematic (b). Further extending reaction duration, the Bi2S3 nuclei will slowly stack up to form loose filament structure at the given vulcanization condition of schematic (c). When an increase of the vulcanization reaction duration to 3 h, the loose and lumpy Bi2S3 clusters will transfer into abundant, distinguishable, and separable nanowire crystals covered on the TiO2 template as displayed in schematic (d). When the reaction duration reaches 5 h, the one-dimensional Bi2S3 crystals continue to aggregate into a thicker rod structure, as shown in schematic (e).

one-dimensional Bi2S3 crystals continue to aggregate into a thicker rod structure, as shown

in schematic (e).

**Figure 2.** Schematic illustration of the formation processes of Bi2S3 nanostructures. **Figure 2.** Schematic illustration of the formation processes of Bi2S3 nanostructures.

Figure 3a–c demonstrate the XRD patterns of vulcanization-treated BT-1, BT-3, and BT-5 samples. In addition to the Bragg reflections originated from the FTO substrate, the characteristic diffraction peaks centered at approximately 26.61°, 36.18°, and 54.76°can be indexed to (110), (101), and (211) planes of rutile TiO2 (JCPDS No. 00-021-1276), revealing high crystallinity of the TiO2 template. Notably, after hydrothermal vulcanization, several visible Bragg reflections originated from orthorhombic Bi2S3 phase could be clearly observed (JCPDS No. 017-0320) in the BT-1, BT-3, and BT-5 samples. Figure 3a–c present that addition of thiourea in hydrothermal reaction solution can vulcanize the Bi2O3 sacrificial oxide layer to transfer into the corresponding sulfide phase in BT-1, BT-3, and BT-5. The prolonged vulcanization process (from 1 to 5 h) improves Bragg reflection intensity of Bi2S3 phase of the samples, revealing an improved crystallinity of the samples. This might be associated with crystal morphology transformation of the loose and lumpy filament-like Bi2S3 crystals of BT-1 to one-dimensional Bi2S3 rods of BT-5 with prolonged vulcanization process. No Bragg reflections associated with Bi2O3 and other impurity phases were detected in the XRD patterns, revealing the vulcanization processes herein successfully synthesized the TiO2-Bi2S3 composite structure, and the as-synthesized Figure 3a–c demonstrate the XRD patterns of vulcanization-treated BT-1, BT-3, and BT-5 samples. In addition to the Bragg reflections originated from the FTO substrate, the characteristic diffraction peaks centered at approximately 26.61◦ , 36.18◦ , and 54.76◦ can be indexed to (110), (101), and (211) planes of rutile TiO2 (JCPDS No. 00-021-1276), revealing high crystallinity of the TiO2 template. Notably, after hydrothermal vulcanization,several visible Bragg reflections originated from orthorhombic Bi2S3 phase could be clearly observed (JCPDS No. 017-0320) in the BT-1, BT-3, and BT-5 samples. Figure 3a–c present that addition of thiourea in hydrothermal reaction solution can vulcanize the Bi2O3 sacrificial oxide layer to transfer into the corresponding sulfide phase in BT-1, BT-3, and BT-5. The prolonged vulcanization process (from 1 to 5 h) improves Bragg reflection intensity of Bi2S3 phase of the samples, revealing an improved crystallinity of the samples. This might be associated with crystal morphology transformation of the loose and lumpy filament-like Bi2S3 crystals of BT-1 to one-dimensional Bi2S3 rods of BT-5 with prolonged vulcanization process. No Bragg reflections associated with Bi2O3 and other impurity phases were detected in the XRD patterns, revealing the vulcanization processes herein successfully synthesized the TiO2-Bi2S3 composite structure, and the as-synthesized composite structures are crystalline.

composite structures are crystalline. Figure 4a shows the low-magnification TEM image of the BT-1. It can be observed that Bi2S3 flakes decorated on the TiO2 rod. Figure 4b is the HRTEM image taken from the red square in Figure 3a. The distinct lattice fringes with a distance of approximately 0.28 nm are from the interplane spacing of orthorhombic Bi2S3 (2 2 1) and the lattice fringes with a spacing of 0.33 nm are from the interplane spacing of tetragonal TiO2 (1 1 0). Figure 4c showed the selected area electron diffraction (SAED) pattern of several BT-1. Several clear diffraction spots arranged in centric rings are associated with the orthorhombic Bi2S3 (2 1 1), (2 2 1), and (4 3 1) planes and the rutile TiO2 (1 1 0) plane, which agrees with the result observed by XRD analysis. This demonstrates the well-constructed TiO2-Bi2S3 heterogeneous structure of BT-1, and a good crystallinity of the composites. The EDS spectrum in Figure 4d shows Ti, O, Bi, and S are the main elements in this composite structure. Furthermore, the particular element line scan intensity distributions displayed in Figure 4e reveals the construction of Bi2S3 flakes on the TiO2 rod for the BT-1.

**Figure 3.** XRD patterns: (**a**) TiO2 template, (**b**) BT-1, (**c**) BT-3, and (**d**) BT-5. **Figure 3.** XRD patterns: (**a**) TiO2 template, (**b**) BT-1, (**c**) BT-3, and (**d**) BT-5.

**Figure 4.** (**a**) Low magnification TEM image of BT-1. (**b**) HRTEM image taken from the local region of the sample in (**a**). (**c**) The SAED pattern of BT-1 in (**a**). (**d**) EDS spectrum taken from the sample in (**a**). (**e**) EDS line-scanning profiles across the composite. purity of Bi2S3 composition of the nanowire. **Figure 4.** (**a**) Low magnification TEM image of BT-1. (**b**) HRTEM image taken from the local region of the sample in (**a**). (**c**) The SAED pattern of BT-1 in (**a**). (**d**) EDS spectrum taken from the sample in (**a**). (**e**) EDS line-scanning profiles across the composite.

Figure 5a shows the low-magnification TEM image of the BT-3 structure scratched from the sample. The sidewall of the composite showed an undulated feature because of the decoration of Bi2S3 crystals. However, a broken top region of the structure was

of the BT-3 composite is not observed herein, as revealed in the aforementioned SEM observation. Figure 5b displays the HRTEM image taken from the red square region in Figure 5a. The distinct lattice fringes with a distance of approximately 0.28 nm are associated with the interplanar spacing of Bi2S3 (221). Figure 5c demonstrates the EDS spectrum taken from the nanostructure in Figure 5a. In addition to Cu and C originated from the TEM grid, the Ti, O, Bi, and S are the main constituent elements in this composite structure, which proves that TiO2 and Bi2S3 phases coexist in the composite structure. Figure 5d presents a low-magnification image of the scratched nanowire structure from BT-3 sample. The nanowire has the diameter of 23 nm, and the surface is smooth. The nanowire shows a little bent state. Figure 5e shows the HRTEM image taken from the red square in Figure 5d. The distinct and ordered lattice fringes shows the single crystalline quality of the Bi2S3 nanowire. Similarity, a distinguishable Bi2S3 (221) lattice image in the one-dimensional Bi2S3 crystal in an orthorhombic structure has been shown in WO3/Bi2S3 composite synthesized via chemical bath deposition [21]. Figure 5f shows the EDS spectrum taken from the single Bi2S3 nanowire, where the EDS spectrum indicates high

Figure 5a shows the low-magnification TEM image of the BT-3 structure scratched from the sample. The sidewall of the composite showed an undulated feature because of the decoration of Bi2S3 crystals. However, a broken top region of the structure was observed in Figure 5a. Because the preparation of TEM sample with a scratched off method will destroy the integrity of the BT-3 composite, the distinct two-layered structure of the BT-3 composite is not observed herein, as revealed in the aforementioned SEM observation. Figure 5b displays the HRTEM image taken from the red square region in Figure 5a. The distinct lattice fringes with a distance of approximately 0.28 nm are associated with the interplanar spacing of Bi2S3 (2 2 1). Figure 5c demonstrates the EDS spectrum taken from the nanostructure in Figure 5a. In addition to Cu and C originated from the TEM grid, the Ti, O, Bi, and S are the main constituent elements in this composite structure, which proves that TiO2 and Bi2S3 phases coexist in the composite structure. Figure 5d presents a low-magnification image of the scratched nanowire structure from BT-3 sample. The nanowire has the diameter of 23 nm, and the surface is smooth. The nanowire shows a little bent state. Figure 5e shows the HRTEM image taken from the red square in Figure 5d. The distinct and ordered lattice fringes shows the single crystalline quality of the Bi2S3 nanowire. Similarity, a distinguishable Bi2S3 (2 2 1) lattice image in the one-dimensional Bi2S3 crystal in an orthorhombic structure has been shown in WO3/Bi2S3 composite synthesized via chemical bath deposition [21]. Figure 5f shows the EDS spectrum taken from the single Bi2S3 nanowire, where the EDS spectrum indicates high purity of Bi2S3 composition of the nanowire. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 7 of 15

**Figure 5.** (**a**) Low-magnification TEM image of scratched BT-3. (**b**) HRTEM image taken from the local region of the sample in (**a**). (**c**) The EDS spectrum of BT-3 in (**a**). (**d**) Low magnification TEM image of Bi2S3 nanowire scratched from BT-3. (**e**) HRTEM image taken from the local regions of the sample in (**d**). (**f**) The EDS spectrum of Bi2S3 nanowire in (**d**). **Figure 5.** (**a**) Low-magnification TEM image of scratched BT-3. (**b**) HRTEM image taken from the local region of the sample in (**a**). (**c**) The EDS spectrum of BT-3 in (**a**). (**d**) Low magnification TEM image of Bi2S3 nanowire scratched from BT-3. (**e**) HRTEM image taken from the local regions of the sample in (**d**). (**f**) The EDS spectrum of Bi2S3 nanowire in (**d**).

Figure 6 shows the characteristic XPS spectral lines of various samples. The main constituent elements of Bi, S, Ti, and O are detected in BT-1, BT-3, and BT-5, supporting the existence of Bi2S3 and TiO2 in the composite structure. Moreover, the relatively weak Ti signals from the spectra in comparison with that of the Bi signals, revealing the capping Figure 6 shows the characteristic XPS spectral lines of various samples. The main constituent elements of Bi, S, Ti, and O are detected in BT-1, BT-3, and BT-5, supporting the existence of Bi2S3 and TiO2 in the composite structure. Moreover, the relatively weak Ti signals from the spectra in comparison with that of the Bi signals, revealing the capping layer of Bi2S3 phase on the TiO2 template for the test samples, and this spectral feature has

layer of Bi2S3 phase on the TiO2 template for the test samples, and this spectral feature has widely been observed in the composite structure, having obvious layering characteristics

to ambient air, no impurity was detected in the as-synthesized samples. In order to further investigate the elemental binding states of the Bi2S3 capping layer, the XPS narrow scan spectra of Bi 4f for BT-1, BT-3, and BT-5 are displayed in Figure 7a–c, respectively. Two sharp and distinct peaks centered at approximately 157.3 eV and 162.6 eV, which are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi2S3, respectively [22]. The tiny peak located between the two distinct characteristic peaks of Bi 4f is originated from S 2p. The binding energies of Bi 4f core-level peaks corresponded to the characteristic binding state of Bi3+ in the Bi2S3, revealing the well formation of the Bi2S3 phase through vulcanizing the Bi2O3 layer. No

metallic Bi or Bi2O3 appeared after vulcanization.

**Figure 6.** XPS survey scan spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5.

widely been observed in the composite structure, having obvious layering characteristics [9]. In addition to the C signal that originated from the sample contamination on exposure to ambient air, no impurity was detected in the as-synthesized samples. In order to further investigate the elemental binding states of the Bi2S3 capping layer, the XPS narrow scan spectra of Bi 4f for BT-1, BT-3, and BT-5 are displayed in Figure 7a–c, respectively. Two sharp and distinct peaks centered at approximately 157.3 eV and 162.6 eV, which are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi2S3, respectively [22]. The tiny peak located between the two distinct characteristic peaks of Bi 4f is originated from S 2p. The binding energies of Bi 4f core-level peaks corresponded to the characteristic binding state of Bi3+ in the Bi2S3, revealing the well formation of the Bi2S3 phase through vulcanizing the Bi2O3 layer. No metallic Bi or Bi2O3 appeared after vulcanization. to ambient air, no impurity was detected in the as-synthesized samples. In order to further investigate the elemental binding states of the Bi2S3 capping layer, the XPS narrow scan spectra of Bi 4f for BT-1, BT-3, and BT-5 are displayed in Figure 7a–c, respectively. Two sharp and distinct peaks centered at approximately 157.3 eV and 162.6 eV, which are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi2S3, respectively [22]. The tiny peak located between the two distinct characteristic peaks of Bi 4f is originated from S 2p. The binding energies of Bi 4f core-level peaks corresponded to the characteristic binding state of Bi3+ in the Bi2S3, revealing the well formation of the Bi2S3 phase through vulcanizing the Bi2O3 layer. No metallic Bi or Bi2O3 appeared after vulcanization.

**Figure 5.** (**a**) Low-magnification TEM image of scratched BT-3. (**b**) HRTEM image taken from the local region of the sample in (**a**). (**c**) The EDS spectrum of BT-3 in (**a**). (**d**) Low magnification TEM image of Bi2S3 nanowire scratched from BT-3. (**e**) HRTEM image taken from the local regions of the

Figure 6 shows the characteristic XPS spectral lines of various samples. The main constituent elements of Bi, S, Ti, and O are detected in BT-1, BT-3, and BT-5, supporting the existence of Bi2S3 and TiO2 in the composite structure. Moreover, the relatively weak Ti signals from the spectra in comparison with that of the Bi signals, revealing the capping layer of Bi2S3 phase on the TiO2 template for the test samples, and this spectral feature has widely been observed in the composite structure, having obvious layering characteristics [9]. In addition to the C signal that originated from the sample contamination on exposure

sample in (**d**). (**f**) The EDS spectrum of Bi2S3 nanowire in (**d**).

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 7 of 15

**Figure 6. Figure 6.** XPS survey scan spectra: ( XPS survey scan spectra: (**a**) BT-1, ( **a**) BT-**b** 1, ( ) BT-3, and ( **b**) BT-3, and ( **c**) BT-5. **c**) BT-5.

**Figure 7.** XPS analysis of narrow scan Bi 4f spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5. **Figure 7.** XPS analysis of narrow scan Bi 4f spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5.

Notably, the narrow scan spectra of S 2p region of BT-1, BT-3, and BT-5 in Figure 8 reveal the characteristic peaks centered at approximately 162.6 eV and 160.1 eV, which are assigned to S 2p1/2 and S 2p3/2, respectively. The S 2p binding energies herein associated with the aforementioned Bi 4f binding states evidenced the formation of Bi-S bonds in the Notably, the narrow scan spectra of S 2p region of BT-1, BT-3, and BT-5 in Figure 8 reveal the characteristic peaks centered at approximately 162.6 eV and 160.1 eV, which are assigned to S 2p1/2 and S 2p3/2, respectively. The S 2p binding energies herein associated with the aforementioned Bi 4f binding states evidenced the formation of Bi-S bonds in the capping layer synthesized via the given vulcanization processes in this study [23].

capping layer synthesized via the given vulcanization processes in this study [23]. A comparison of the UV-vis absorption spectra of TiO2 template and various TiO2-Bi2S3 samples are displayed in Figure 9a. The absorption of the TiO2 template is mainly in the UV light range. The decoration of Bi2S3 extends the absorption range of the TiO2 template to the visible light range because of the narrow energy gap of Bi2S3. This is consistent with the feature of optical absorption spectra for Bi2S3 nanorod/TiO2 nanoplate composites [24]. Moreover, in the previous TiO2/Bi2S3 core–shell nanowire arrays synthesized via a successive ionic layer adsorption and reaction method, the absorption of the composite extends to cover the visible light range, and the decoration of Bi2S3 phase increases the light absorption ability of the pristine TiO2 nanowires [8]. Notably, the band gap energies of the pristine TiO2 and Bi2S3 are evaluated in Figure 9b,c, respectively, from Tauc plots based on

**Figure 8.** XPS analysis of narrow scan S 2p spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5.

TiO2/Bi2S3 heterostructure herein improves the light harvesting ability.

A comparison of the UV-vis absorption spectra of TiO2 template and various TiO2- Bi2S3 samples are displayed in Figure 9a. The absorption of the TiO2 template is mainly in the UV light range. The decoration of Bi2S3 extends the absorption range of the TiO2 template to the visible light range because of the narrow energy gap of Bi2S3. This is consistent with the feature of optical absorption spectra for Bi2S3 nanorod/TiO2 nanoplate composites [24]. Moreover, in the previous TiO2/Bi2S3 core–shell nanowire arrays synthesized via a successive ionic layer adsorption and reaction method, the absorption of the composite extends to cover the visible light range, and the decoration of Bi2S3 phase increases the light absorption ability of the pristine TiO2 nanowires [8]. Notably, the band gap energies of the pristine TiO2 and Bi2S3 are evaluated in Figure 9b,c, respectively, from Tauc plots based on Kubelka–Munk function [25]. The TiO2 has a band gap energy of approximately 3.02 eV, and the band gap energy of the reference Bi2S3 derived from the sputtering deposited Bi2O3 layer and then vulcanized using 0.1 M thiourea is approximately 1.3 eV. Based on the evaluated band gap energy, the construction of the

Kubelka–Munk function [25]. The TiO2 has a band gap energy of approximately 3.02 eV, and the band gap energy of the reference Bi2S3 derived from the sputtering deposited Bi2O3 layer and then vulcanized using 0.1 M thiourea is approximately 1.3 eV. Based on the evaluated band gap energy, the construction of the TiO2/Bi2S3 heterostructure herein improves the light harvesting ability. reveal the characteristic peaks centered at approximately 162.6 eV and 160.1 eV, which are assigned to S 2p1/2 and S 2p3/2, respectively. The S 2p binding energies herein associated with the aforementioned Bi 4f binding states evidenced the formation of Bi-S bonds in the capping layer synthesized via the given vulcanization processes in this study [23].

Notably, the narrow scan spectra of S 2p region of BT-1, BT-3, and BT-5 in Figure 8

**Figure 7.** XPS analysis of narrow scan Bi 4f spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5.

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 8 of 15

**Figure 8.** XPS analysis of narrow scan S 2p spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5. **Figure 8.** XPS analysis of narrow scan S 2p spectra: (**a**) BT-1, (**b**) BT-3, and (**c**) BT-5.

Tauc plots based on Kubelka–Munk function [25]. The TiO2 has a band gap energy of approximately 3.02 eV, and the band gap energy of the reference Bi2S3 derived from the **Figure 9.** (**a**) UV–vis absorbance spectra of various samples. Band gap evaluation of (**b**) pristine TiO2 and (**c**) pristine Bi2S3. **Figure 9.** (**a**) UV–vis absorbance spectra of various samples. Band gap evaluation of (**b**) pristine TiO2 and (**c**) pristine Bi2S3 .

sputtering deposited Bi2O3 layer and then vulcanized using 0.1 M thiourea is approximately 1.3 eV. Based on the evaluated band gap energy, the construction of the TiO2/Bi2S3 heterostructure herein improves the light harvesting ability. Figure 10a shows the transient photocurrent performance of various photoelectrodes under repeated on/off irradiation cycles at 0.5 V (vs. Ag/AgCl). It is known that the transient photocurrent performance is highly associated with the photoresponse, charge carrier transport speed, charge carrier separation efficiency, and charge carrier recombination rate of samples. When the light is on, the photocurrent densities of all samples swiftly rise to a stable value of 0.12, 0.81, 0.93, and 0.47 mA/cm2 for TiO2, BT-1, BT-3, and BT-5, respectively. When the light is off, the photocurrent densities of the samples drop to their initial dark current density value instantaneously. Such a fast rise and fall process of the photocurrents indicates that carrier transport and separation in the prepared photoelectrodes proceed quickly [11]. Furthermore, the photoresponse of the BT-3 increased approximately three times higher than that of the TiO2 template. The BT-1 and BT-5 also exhibited enhanced photoresponses in comparison with the TiO2 template. Among various composite structure, the BT-3 has the highest photocurrent response, indicating it has the lowest photogenerated electron/hole recombination rate. By contrast, the BT-5 exhibits a lower photocurrent response among various composites. This might be attributed to the following reason. Substantially excessive Bi2S3 deposition leads to an increase in the number of recombination centers, and as a result, a decrease in photocurrent density has been observed due to the loss of photogenerated electrons [15]. The results herein prove that decoration of Bi2S3 crystals onto the TiO2 template can Figure 10a shows the transient photocurrent performance of various photoelectrodes under repeated on/off irradiation cycles at 0.5 V (vs. Ag/AgCl). It is known that the transient photocurrent performance is highly associated with the photoresponse, charge carrier transport speed, charge carrier separation efficiency, and charge carrier recombination rate of samples. When the light is on, the photocurrent densities of all samples swiftly rise to a stable value of 0.12, 0.81, 0.93, and 0.47 mA/cm2 for TiO2, BT-1, BT-3, and BT-5, respectively. When the light is off, the photocurrent densities of the samples drop to their initial dark current density value instantaneously. Such a fast rise and fall process of the photocurrents indicates that carrier transport and separation in the prepared photoelectrodes proceed quickly [11]. Furthermore, the photoresponse of the BT-3 increased approximately three times higher than that of the TiO2 template. The BT-1 and BT-5 also exhibited enhanced photoresponses in comparison with the TiO2 template. Among various composite structure, the BT-3 has the highest photocurrent response, indicating it has the lowest photogenerated electron/hole recombination rate. By contrast, the BT-5 exhibits a lower photocurrent response among various composites. This might be attributed to the following reason. Substantially excessive Bi2S3 deposition leads to an increase in the number of recombination centers, and as a result, a decrease in photocurrent density has been observed due to the loss of photogenerated electrons [15]. The results herein prove that decoration of Bi2S3 crystals onto the TiO2 template can effectively improve the photoinduced charge separation efficiency of the TiO2 template, resulting in more efficient charge migration and

effectively improve the photoinduced charge separation efficiency of the TiO2 template, resulting in more efficient charge migration and higher photocurrent density of the TiO2-

with a suitable band alignment has been demonstrated in TiO2-Bi2O3 and ZnO-Sn2S3 heterostructures [11,25]. Figure 10b presents the Nyquist plots of various photoelectrodes. The semicircular radius of the Nyquist plots can reflect the charge transfer resistance of the photoelectrode materials. As show in Figure 10b, the arc radius of the TiO2-Bi2S3 composites herein are smaller than that of the TiO2 template, revealing that formation of TiO2/Bi2S3 heterojunction is beneficial for interfacial charge transfer. Comparatively, BT-3 has the smallest radius of the Nyquist curve among various samples. The possible equivalent circuits shown in Figure 10c are used to fit the EIS Nyquist results, where Rs, R1, R2, and CPE represent the solution resistance, semiconductor depletion layer resistance, charge transfer resistance, and chemical capacitance, respectively [26]. Herein, the R2 is evaluated from the fitting results of the Nyquist plots at the low frequency region. The R2 values of the pristine TiO2, BT-1, BT-3, and BT-5 are 3500, 583.3, 463.4, and 925 Ω, respectively. The BT-3 exhibited the smallest R2 value. As the vulcanization duration was further prolonged, a larger size of Bi2S3 crystallites deposition occurred, which might prolong the electron diffusion path in the BT-5 composite, resulting a larger R2 value among various composites. Notably, charge transfer resistances of the BT-1, BT-3, and BT-5 are much lower than that of TiO2. This indicates that decoration of Bi2S3 onto the TiO2

higher photocurrent density of the TiO2-Bi2S3 composite structure. A similar improved transient photocurrent performance because of decoration of visible light sensitizer onto the wide band gap semiconductor with a suitable band alignment has been demonstrated in TiO2-Bi2O3 and ZnO-Sn2S3 heterostructures [11,25]. Figure 10b presents the Nyquist plots of various photoelectrodes. The semicircular radius of the Nyquist plots can reflect the charge transfer resistance of the photoelectrode materials. As show in Figure 10b, the arc radius of the TiO2-Bi2S3 composites herein are smaller than that of the TiO2 template, revealing that formation of TiO2/Bi2S3 heterojunction is beneficial for interfacial charge transfer. Comparatively, BT-3 has the smallest radius of the Nyquist curve among various samples. The possible equivalent circuits shown in Figure 10c are used to fit the EIS Nyquist results, where Rs, R1, R2, and CPE represent the solution resistance, semiconductor depletion layer resistance, charge transfer resistance, and chemical capacitance, respectively [26]. Herein, the R2 is evaluated from the fitting results of the Nyquist plots at the low frequency region. The R2 values of the pristine TiO2, BT-1, BT-3, and BT-5 are 3500, 583.3, 463.4, and 925 Ω, respectively. The BT-3 exhibited the smallest R2 value. As the vulcanization duration was further prolonged, a larger size of Bi2S3 crystallites deposition occurred, which might prolong the electron diffusion path in the BT-5 composite, resulting a larger R2 value among various composites. Notably, charge transfer resistances of the BT-1, BT-3, and BT-5 are much lower than that of TiO2. This indicates that decoration of Bi2S3 onto the TiO2 could effectively reduce the photoinduced charge transfer resistance of the TiO2 template and enhance the separation efficiency of charge carriers. Both the transient photocurrent and Nyquist plot results herein support an effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer occurred on the BT-3 interface, which might lead to the enhanced photocatalytic performance. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 10 of 15 could effectively reduce the photoinduced charge transfer resistance of the TiO2 template and enhance the separation efficiency of charge carriers. Both the transient photocurrent and Nyquist plot results herein support an effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer occurred on the BT-3 interface, which might lead to the enhanced photocatalytic performance.

composite.

**Figure 10.** (**a**) Photocurrent density versus time curves of various samples at 0.5 V vs. Ag/AgCl under chopped illumination. (**b**) Nyquist plot of various samples at an open-circuit potential under illumination (**c**) Possible equivalent circuits used to fit R2 value of the pristine TiO2 and TiO2-Bi2S3 **Figure 10.** (**a**) Photocurrent density versus time curves of various samples at 0.5 V vs. Ag/AgCl under chopped illumination. (**b**) Nyquist plot of various samples at an open-circuit potential under illumination (**c**) Possible equivalent circuits used to fit R2 value of the pristine TiO2 and TiO2 -Bi2S3 composite.

Figure 11 shows the Mott–Schottky (M−S) plots of TiO2, Bi2S3, and TiO2-Bi2S3 composites performed at 1 kHz. All the samples were measured with a positive slope in the M−S plots, revealing an n-type nature of the composed semiconductors. The flat band potential (Efb) of various samples can be calculated from the x intercept of the linear region in the M−S plots according to the M−S equation [27]. Furthermore, the normal hydrogen electrode (NHE) potential can be converted from the Ag/AgCl reference electrode as NHE = V(Ag/AgCl)—0.197 V. The Efb of TiO2 and Bi2S3 referenced samples are −0.21 and −0.45 V vs. NHE, respectively. Comparatively, the flat band potential of the TiO2-Bi2S3 composites has a negative shift compared to the TiO2 template as exhibited in Figure 11c– e. Moreover, the tangent slope in the linear region of the M−S plots for the TiO2-Bi2S3 composites is smaller than the TiO2 template, demonstrating the decoration of Bi2S3 crystallites onto the TiO2 template increases carrier concentration and reduce the charge recombination rate of the composites. A more negative shift of the flat band potential with Figure 11 shows the Mott–Schottky (M–S) plots of TiO2, Bi2S3, and TiO2-Bi2S3 composites performed at 1 kHz. All the samples were measured with a positive slope in the M–S plots, revealing an n-type nature of the composed semiconductors. The flat band potential (Efb) of various samples can be calculated from the x intercept of the linear region in the M–S plots according to the M–S equation [27]. Furthermore, the normal hydrogen electrode (NHE) potential can be converted from the Ag/AgCl reference electrode as NHE = V(Ag/AgCl)—0.197 V. The Efb of TiO2 and Bi2S3 referenced samples are −0.21 and −0.45 V vs. NHE, respectively. Comparatively, the flat band potential of the TiO2-Bi2S3 composites has a negative shift compared to the TiO2 template as exhibited in Figure 11c–e. Moreover, the tangent slope in the linear region of the M–S plots for the TiO2-Bi2S3 composites is smaller than the TiO2 template, demonstrating the decoration of Bi2S3 crystallites onto the TiO2 template increases carrier concentration and reduce the charge recombination rate of the composites. A more negative shift of the flat band potential with respective to TiO2 template is observed for the BT-3 among various TiO2-Bi2S3 composites; moreover,

respective to TiO2 template is observed for the BT-3 among various TiO2-Bi2S3 composites; moreover, the smaller tangent slop in the linear region of the M−S plot was observed for

Bi2S3 composites. It has been shown that the crystallite size and number of visible-light sensitizers affect the photoinduced charge separation efficiency of the visible-light sensitizer-decorated TiO2 [28]. Moreover, the crystal quality of the semiconductor has been proved to affect its photoactivity [29]. The BT-3 because of suitable Bi2S3 crystallite quality and size shows the superior photogenerated carrier density and effective interface

transfer ability among various composites in this study.

the smaller tangent slop in the linear region of the M–S plot was observed for the BT-3. These reveal a superior electronic property of the BT-3 than that of other TiO2-Bi2S3 composites. It has been shown that the crystallite size and number of visible-light sensitizers affect the photoinduced charge separation efficiency of the visible-light sensitizer-decorated TiO2 [28]. Moreover, the crystal quality of the semiconductor has been proved to affect its photoactivity [29]. The BT-3 because of suitable Bi2S3 crystallite quality and size shows the superior photogenerated carrier density and effective interface transfer ability among various composites in this study. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 11 of 15

**Figure 11.** Mott–Schottky plots of various samples: (**a**) pristine TiO2, (**b**) pristine Bi2S3, (**c**) BT-1, (**d**) BT-3, and (**e**) BT-5. **Figure 11.** Mott–Schottky plots of various samples: (**a**) pristine TiO2 , (**b**) pristine Bi2S3 , (**c**) BT-1, (**d**) BT-3, and (**e**) BT-5.

Figure 12a shows the photodegradation level of various photocatalysts towards RhB solution. The percentage of photodegradation was calculated using the C/Co = It/Io. The Co and C are the initial and residual concentration of the RhB solution at t = 0 and at irradiation duration t, respectively, and can be evaluated from the intensity variation of absorbance spectra of the RhB solution with and without photocatalytic reaction [30]. Figure 12a demonstrates that the BT-1, BT-3, BT-5, and TiO2 degrades 60.3%, 65.6%, 50.8%, and 36.1% of RhB solution, respectively, after 30 min irradiation. Furthermore, the degradation level of RhB solution reached 77.8%, 87.2%, 71.2%, and 57.1% after 60 min irradiation for the BT-1, BT-3, BT-5, and TiO2, respectively. Notably, the dark balanced absorptions of various samples are also conducted to understand the initial catalysts' surface dye absorption efficiency. The C/Co of the BT-1, BT-3, BT-5, and TiO2 after 30 min dark balanced adsorption is 7.1%, 7.6%, 5.8%, and 4.2%, respectively. Comparatively, the BT-1 and BT-3 exhibit a slightly larger surface dye adsorption ability than that of BT-5 and TiO2. Figure 12b demonstrates the discoloration of RhB solution with BT-3 under different irradiation durations. Apparent discoloration appeared in the RhB solution containing BT-3 with an increased irradiation duration. To quantitatively compare the photocatalytic activity of the as-prepared photocatalyst samples, the photodegradation data were fitted to the pseudo-first-order kinetics equation: ln(Co/C) = kt, where k is the apparent firstorder rate constant [4]. Figure 12c presents the plot of ln(Co/C) versus t for various samples. The higher k value is observed for the TiO2-Bi2S3 composites than that of pristine TiO2. Moreover, the BT-3 demonstrates the highest k value of 0.03274 min−1 among various samples, confirming the higher photocatalytic activity of BT-3 composite. The suitable Figure 12a shows the photodegradation level of various photocatalysts towards RhB solution. The percentage of photodegradation was calculated using the C/Co = It/Io. The Co and C are the initial and residual concentration of the RhB solution at t = 0 and at irradiation duration t, respectively, and can be evaluated from the intensity variation of absorbance spectra of the RhB solution with and without photocatalytic reaction [30]. Figure 12a demonstrates that the BT-1, BT-3, BT-5, and TiO2 degrades 60.3%, 65.6%, 50.8%, and 36.1% of RhB solution, respectively, after 30 min irradiation. Furthermore, the degradation level of RhB solution reached 77.8%, 87.2%, 71.2%, and 57.1% after 60 min irradiation for the BT-1, BT-3, BT-5, and TiO2, respectively. Notably, the dark balanced absorptions of various samples are also conducted to understand the initial catalysts' surface dye absorption efficiency. The C/Co of the BT-1, BT-3, BT-5, and TiO2 after 30 min dark balanced adsorption is 7.1%, 7.6%, 5.8%, and 4.2%, respectively. Comparatively, the BT-1 and BT-3 exhibit a slightly larger surface dye adsorption ability than that of BT-5 and TiO2. Figure 12b demonstrates the discoloration of RhB solution with BT-3 under different irradiation durations. Apparent discoloration appeared in the RhB solution containing BT-3 with an increased irradiation duration. To quantitatively compare the photocatalytic activity of the as-prepared photocatalyst samples, the photodegradation data were fitted to the pseudo-first-order kinetics equation: ln(Co/C) = kt, where k is the apparent first-order rate constant [4]. Figure 12c presents the plot of ln(Co/C) versus t for various samples. The higher k value is observed for the TiO2-Bi2S3 composites than that of pristine TiO2. Moreover, the BT-3 demonstrates the highest k value of 0.03274 min−1 among various samples, confirming the higher photocatalytic activity of BT-3 composite. The suitable

morphology and decoration content of Bi2S3 crystallites on the TiO2 template might

of the CuO substantially affect the photocatalytic activity of CuO-ZnO nanocomposites [31]. Active groups of the photocatalytic degradation reaction were explored by the addition of free radical capture agents, as exhibited in Figure 12d. Herein, benzoquinone

− radical scavenger), tertiary butyl alcohol (t-BuOH, OH radical scavenger), and

(BQ, O2

morphology and decoration content of Bi2S3 crystallites on the TiO2 template might account for the observed result. This has also been supported by visible-light CuO sensitizer decorated ZnO composite photocatalysts that crystal morphology and content of the CuO substantially affect the photocatalytic activity of CuO-ZnO nanocomposites [31]. Active groups of the photocatalytic degradation reaction were explored by the addition of free radical capture agents, as exhibited in Figure 12d. Herein, benzoquinone (BQ, ·O2 − radical scavenger), tertiary butyl alcohol (t-BuOH, ·OH radical scavenger), and EDTA-2Na (EDTA-2Na, h+ radical scavenger) are used to explore the active groups of the photocatalytic degradation reaction. Notably, the photocatalytic efficiency of RhB solution with BT-3 was significantly inhibited after adding BQ, indicating that ·O2 − is the main active substance for RhB degradation. Furthermore, the results herein reveal that a single scavenger could not completely prevent the dye degradation, and the h+ and ·OH also contribute some degrees of photodegradation towards RhB dyes. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 12 of 15 EDTA-2Na (EDTA-2Na, h+ radical scavenger) are used to explore the active groups of the photocatalytic degradation reaction. Notably, the photocatalytic efficiency of RhB solution with BT-3 was significantly inhibited after adding BQ, indicating that O2 − is the main active substance for RhB degradation. Furthermore, the results herein reveal that a single scavenger could not completely prevent the dye degradation, and the h+ and OH also contribute some degrees of photodegradation towards RhB dyes.

**Figure 12.** (**a**) C/Co versus irradiation duration plot. (**b**) Discoloration of RhB solution with BT-3 under different irradiation durations. (**c**) ln (Co/C) versus irradiation duration plot. (**d**) The photocatalytic performance after adding various scavengers in BT-3/RhB solution. **Figure 12.** (**a**) C/Co versus irradiation duration plot. (**b**) Discoloration of RhB solution with BT-3 under different irradiation durations. (**c**) ln (Co/C) versus irradiation duration plot. (**d**) The photocatalytic performance after adding various scavengers in BT-3/RhB solution.

In the M-S analysis, the flat band potentials of the TiO2 and Bi2S3 are −0.21 and −0.45 eV, respectively. Furthermore, the ECB bottoms of the TiO2 and Bi2S3 can be evaluated to be −0.31 and −0.55 eV, respectively [32]. The valence band (VB) positions of the TiO2 and Bi2S3 are evaluated to be 2.71 and 0.75 eV, respectively. According to the band structures of TiO2 and Bi2S3, there are two possible migration mechanisms of photoinduced charge carriers in the TiO2/Bi2S3 heterojunction as shown in Figure 13. However, the potential of O2/·O2 − is −0.33 eV, which is more negative than CB of TiO2, and CB electrons are not easy to reduce O2. At the same time, the photoinduced holes on Bi2S3 VB cannot oxidize H2O to produce ·OH radicals, because the VB edge potential of Bi2S3 is more negative than the potential of H2O/OH (2.27 V), as shown in Figure 13a. Based on the aforementioned discussions, type II photodegradation mechanism is not suitable for the as-synthesized TiO2-Bi2S3 composites in this study. By contrast, a direct Z-scheme mechanism over the In the M-S analysis, the flat band potentials of the TiO2 and Bi2S3 are −0.21 and −0.45 eV, respectively. Furthermore, the ECB bottoms of the TiO2 and Bi2S3 can be evaluated to be −0.31 and −0.55 eV, respectively [32]. The valence band (VB) positions of the TiO2 and Bi2S3 are evaluated to be 2.71 and 0.75 eV, respectively. According to the band structures of TiO2 and Bi2S3, there are two possible migration mechanisms of photoinduced charge carriers in the TiO2/Bi2S3 heterojunction as shown in Figure 13. However, the potential of O2/·O2 − is −0.33 eV, which is more negative than CB of TiO2, and CB electrons are not easy to reduce O2. At the same time, the photoinduced holes on Bi2S3 VB cannot oxidize H2O to produce ·OH radicals, because the VB edge potential of Bi2S3 is more negative than the potential of H2O/OH (2.27 V), as shown in Figure 13a. Based on the aforementioned discussions, type II photodegradation mechanism is not suitable for the assynthesized TiO2-Bi2S3 composites in this study. By contrast, a direct Z-scheme mechanism

TiO2/Bi2S3 heterostructure can be proposed in Figure 13b. Upon irradiation, TiO2/Bi2S3 absorbs light greater than its band gap, electron-hole pairs are generated (Equations (1)

phenomenon of electron injection from RhB\* to the TiO2 CB has been proposed in the literature [33]. The electrons in the TiO2 CB position could migrate to the Bi2S3 VB,

over the TiO2/Bi2S3 heterostructure can be proposed in Figure 13b. Upon irradiation, TiO2/Bi2S3 absorbs light greater than its band gap, electron-hole pairs are generated (Equations (1) and (2)), and RhB will also be excited to RhB\* by light irradiation at the same time (Equation (3)). The RhB\* injects electrons into the CB of Bi2S3 (Equation (4)). A similar phenomenon of electron injection from RhB\* to the TiO2 CB has been proposed in the literature [33]. The electrons in the TiO2 CB position could migrate to the Bi2S3 VB, resulting in the effective separation of photoinduced charge carriers (Equation (5)). The RhB+ reacts with adsorbed dye to form intermediate products by the photosensitization process (Equation (6)). Parts of the holes in the TiO2 VB will react with the intermediate product to degrade RhB dyes (Equation (7)). Moreover, the residual photoinduced holes in the TiO2 VB will react with the H2O molecules to generate the ·OH radicals (Equation (8)), and the electrons are gathered on Bi2S3 CB to produce ·O2 − radicals (Equation (9)) [34]. Therefore, the RhB dyes are effectively photodegraded with BT-3 under irradiation (Equation (10)). A similar Z-scheme mechanism is shown in other TiO2-based composite systems decorated with visible-light sensitizers [35–37]. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 13 of 15 resulting in the effective separation of photoinduced charge carriers (Equation (5)). The RhB+ reacts with adsorbed dye to form intermediate products by the photosensitization process (Equation (6)). Parts of the holes in the TiO2 VB will react with the intermediate product to degrade RhB dyes (Equation (7)). Moreover, the residual photoinduced holes in the TiO2 VB will react with the H2O molecules to generate the ·OH radicals (Equation (8)), and the electrons are gathered on Bi2S3 CB to produce ·O2 − radicals (Equation (9)) [34]. Therefore, the RhB dyes are effectively photodegraded with BT-3 under irradiation (Equation (10)). A similar Z-scheme mechanism is shown in other TiO2-based composite systems decorated with visible-light sensitizers [35–37]. TiO2 + hυ ⟶ TiO2 e −(CB) + TiO2 h +(VB) (1)

$$\text{TiO}\_2 + \text{h}\nu \rightarrow \text{TiO}\_2\,\text{e}^-(\text{CB}) + \text{TiO}\_2\,\text{h}^+(\text{VB}) \tag{1}$$

$$\text{Bi}\_2\text{S}\_3 + \text{h}\nu \rightarrow \text{Bi}\_2\text{S}\_3 \text{ e}^- (\text{CB}) + \text{Bi}\_2\text{S}\_3 \text{ h}^+ (\text{VB}) \tag{2}$$

$$\text{RhB} + \text{h}\nu \rightarrow \text{RhB}^\*(\text{LUMO}) + \text{RhB}^+(\text{HOMO}) \tag{3}$$

$$\text{RhB}^\*(\text{LUMO}) + \text{Bi}\_2\text{S}\_3 \text{ e}^-(\text{CB}) \to \text{Bi}\_2\text{S}\_3 \text{ total e}^-(\text{CB}) \tag{4}$$

$$\text{TiO}\_2\text{ e}^-(\text{CB}) + \text{Bi}\_2\text{S}\_3\text{ h}^+(\text{VB}) \rightarrow \text{recombination} \tag{5}$$

$$\text{RhB}^+ (\text{HOMO}) + \text{RhB}\_{\text{(abs)}} \rightarrow \text{Intermediate} \tag{6}$$

Intermediate + TiO2 h +(VB) → degradation products (7) TiO2 h +(VB) + H2O ⟶ ∙ OH (8)

$$\text{TiO}\_2\text{ h}^+(\text{VB}) + \text{H}\_2\text{O} \to \cdot\text{OH}\tag{8}$$

$$\text{Bi}\_2\text{S}\_3\text{ e}^-(\text{CB}) + \text{O}\_2 \rightarrow \cdot \text{O}\_2{}^-\tag{9}$$

$$\text{RhB} + \text{O}\_2\text{ }^- \rightarrow \text{degradation products} \tag{10}$$

70

#### **4. Conclusions**

TiO2 nanorod array coated with Bi2O3 layer was used to vulcanize to form TiO2-Bi2S3 composites with various morphologies. The hydrothermal vulcanization duration profoundly affects the microstructure, optical properties, and photoactivity of TiO2-Bi2S3 composites. The decorated Bi2S3 crystals changed the morphology from filament, nanowire to nanorod with an increased vulcanization duration from 1, 3, and 5 h, respectively. The decoration of Bi2S3 crystals enhanced the light absorption capacity of the TiO2 nanorod template. The improved photodegradation performance of the composites can be reasonably attributed to the construction of the direct Z-scheme heterojunction between the TiO2 and Bi2S3. This study demonstrates that design of TiO2-Bi2S3 composites with suitable morphology of decorated Bi2S3 crystals can tune the photoactivity of the composites, and the findings in this study may be of great value for the development of oxide-sulfide composites for ideal photosensitive device applications.

**Author Contributions:** Methodology, Y.-C.L. and S.-Y.Y.; formal analysis, S.-Y.Y. and B.-Y.C.; investigation, S.-Y.Y. and B.-Y.C.; writing—original draft preparation, Y.-C.L. and S.-Y.Y.; supervision, Y.-C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science and Technology Council of Taiwan, grant number MOST 111-2221-E-019 -062-MY3.

**Data Availability Statement:** Not applicable.

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

#### **References**

## *Review* **Nanomaterials: A Review about Halloysite Nanotubes, Properties, and Application in the Biological Field**

**Giuseppa Biddeci, Gaetano Spinelli, Paolo Colomba and Francesco Di Blasi \***

Institute for Innovation and Biomedical Research (IRIB), 90146 Palermo, CNR, Italy

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +39-0916809514

**Abstract:** The use of synthetic materials and the attention towards environmental hazards and toxicity impose the development of green composites with natural origins. Clay is one of the candidates for this approach. Halloysite is a natural clay mineral, a member of the Kaolin group, with characteristic tubular morphology, usually named halloysite nanotubes (HNTs). The different surface chemistry of halloysite allows the selective modification of both the external surface and the inner lumen by supramolecular or covalent interactions. An interesting aspect of HNTs is related to the possibility of introducing different species that can be released more slowly compared to the pristine compound. Due to their unique hollow morphology and large cavity, HNTs can be employed as an optimal natural nanocarrier. This review discusses the structure, properties, and application of HNTs in the biological field, highlighting their high biocompatibility, and analyse the opportunity to use new HNT hybrids as drug carriers and delivery systems.

**Keywords:** halloysite nanotubes; biocompatibility; drug delivery system; biomedical application

#### **1. Introduction**

As emerging materials, nanomaterials have attracted much attention over the years due to their small size, but also due to the countless properties that distinguish them. In fact, the nanometric dimensions of material make it assume particular and different chemical-physical properties compared to conventional materials. These different properties, determined by the chemical composition, structure, surface, and increase in surface reactivity in relation to volume, solubility, and state of aggregation, have raised questions about potential human health and environmental risks [1]. In the past few years, there has been a growing interest in research aimed at the development of new organic or inorganic nanocomposites [2–5]. The attention of the scientific community has been drawn by nano clays, thanks to their natural origin, worldwide abundance, availability, biocompatibility, and sustainability [6–8]. Halloysite, largely known as halloysite nanotubes or HNTs, is a natural mineral clay composed of alternating layers of silica and alumina geologically laminated in mesoporous tubular particles with significant adsorption and loading capabilities [9]. Compared to other tubular nanomaterials, HNTs show some advantages in terms of processability and hydrodynamic properties [10]. In fact, for many years, much attention was focused on carbon nanotubes (CNTs) [11–14], showing that these nanotubes have a high cost, lower water dispersibility, and higher toxicity than HNTs [15]. The physicochemical properties of HNTs were fully described, disclosing their potential for various applications such as biomedicine and catalysis [16–19]. HNT-based composites are gaining interest in research aimed at the development of biomaterials for drug delivery vehicles in nanomedicine [20]. In this review, we aim to provide an overview of the properties and biomedical applications of halloysite nanotubes as drug delivery systems. We will discuss their possible application in biotechnology through a focus on HNT composites for biomedical applications. Herein, we aim to provide an overview of the structure, properties, and applicative aspects of halloysite nanotubes in the biomedical field. We start with a

**Citation:** Biddeci, G.; Spinelli, G.; Colomba, P.; Di Blasi, F. Nanomaterials: A Review about Halloysite Nanotubes, Properties, and Application in the Biological Field. *Int. J. Mol. Sci.* **2022**, *23*, 11518. https://doi.org/10.3390/ ijms231911518

Academic Editor: Christian M. Julien

Received: 2 August 2022 Accepted: 19 August 2022 Published: 29 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

brief background about HNTs then provide a brief discussion of their biocompatibility and application.

#### **2. Halloysite Nanotubes**

Halloysite is a two-layered aluminosilicate with a chemical composition similar to kaolinite (chemical formula: Al2Si2O5 (OH)4·nH2O) [21,22], and a hollow tubular structure in the sub-micrometre range [23–26]. Halloysite was first described in 1826 by the French chemist Pierre Berthier and was later given its name in honour of the Belgian geologist Omalius d'Halloy, who found it in the deposits of Angleur, Belgium [27]. Natural tubular halloysite clay has attracted great interest in materials development because it is one of the few inexpensive nanomaterials available in thousands of tons at a low price [28,29]. The nucleation of halloysite crystals occurs in different parts of the world, because of rock erosion due to weathering, pedogenesis, and hydrothermal alteration of ultramafic rocks [30,31]. New Zealand, Belgium, Brazil, France, China, Australia, and Turkey are rich in deposits of halloysite, and it has been shown that with the variation of the deposits it is possible to observe different characteristics that are maintained within the same deposit. Halloysite is defined as a 1:1 phyllosilicate in which a planar layer of tetrahedral silicates alternates with an octahedral geometry layer; these layers are bound together by oxygen bridges [32]. The fundamental unit for the octahedral sheet consists of three octahedrons. In particular, the siloxane groups are bonded via only one oxygen atom to octahedral rings at the outer part and the apical oxygen of tetrahedra becomes the vertices of octahedra [33]. However, under certain geological conditions, halloysite can also take on forms other than classical tubular. It is also possible to distinguish a spheroidal morphology, flat and almost rolled [34,35]. In the "Dragon Mine" deposits of Utah (USA), halloysite is characterized by a good degree of purity and looks like a white stone that can easily be transformed into soft, fine powder. In some deposits, the presence of metals as contaminants induces a colour change that becomes yellowish or brown [30]. Transmission electron microscopy (TEM) structural analysis has shown that halloysite with predominantly tubular morphology and heterogeneous dimensions is present in New Zealand, the United States, and Australian deposits [36]. A very interesting aspect is linked to the different chemical composition between the inner and outer surfaces, in which there are, respectively, aluminolic groups (Al-OH) that give a positive charge and siloxanes (Si-O-Si) that give a negative charge [37,38]. The charges that characterize the internal and external nanotube surfaces are due to the different dielectric and ionization properties of silicon oxides and aluminium. For pH values of 3–10, the positive charges are distributed in the inner lumen and the negative charges on the external surface; present on the edges is a negative/positive charge. In particular, the tubule lumen is positively charged with pH ≤ 8.5, and the outer surface is negatively charged with pH ≥ 1.5 [39]. Generally, because of the O and OH atoms that carry negative charges, the halloysite nanotube is negatively charged. As a result of the tubular shape, on the outer surface only a few hydroxyl groups are present; these are more concentrated in the internal lumen and therefore are more reactive. In fact, for halloysite, it is possible to classify three types of Al-OH, according to their positioning on the surface, at the ends, and between the octahedral and tetrahedral sheets, as shown in Figure 1. All can be reactive and dissociate according to the pH of the solutions, except those placed between the octahedral and tetrahedral sheets, due to steric hindrance [40].

The halloysite nanotubes' size may vary depending on the extraction site and the purification process they undergo, but they usually have an internal diameter of 10–30 nm, an external diameter of 40–70 nm [41], and a length between 200 and 2000 nm [42] (Figure 2a,b).

Nanotubes with a length between 3 and 5 µm have been found in some deposits, although those with smaller sizes are more interesting from a biological point of view, as they are more suitable for use as drug carriers [44]. Halloysite is chemically like kaolinite, but the halloysite double layers are separated by a monolayer of water molecules. In the hydrated form of halloysite *n* = 2 in the formula Al2(OH)4Si2O5·*n*H2O. One layer of water molecules is present between the multilayers and is named "halloysite-(10 Å)", where

"10 Å" indicates the d001-value of the layers. When the *n* value is 0 (*n* = 0), the halloysite is named dehydrated or "halloysite-(7 Å)" and may be obtained through the loss of the interlayer water molecules [30,45]. The water present in halloysite-(10 Å) can be classified as "hole water" and "associated water". In the first case, the water molecules are placed on the surface of the tetrahedral sheet with different orientations and make hydrogen bonds with the basal oxygens. The "associated" water has a greater degree of mobility at room temperature and is located at a different level in the interlayer space, with an ice-like configuration and forming hydrogen bonds with each other and/or with inner-surface hydroxyls [46–50]. Generally, HNT surfaces could be classified into three types. The inner lumen surface has a positive charge and is covered by Al-OH groups. It can undergo a variety of covalent modifications by which certain functional groups can be added. This method allows immobilizing several organic groups on the surface of the lumen stably. The external siloxane surface has a negative charge and can be used to establish covalent bonding with molecules such as organosilanes [51]. Moreover, it can be modified by coating with cationic substances, such as polymers, biopolymers, and surfactants. This type of modification can help to improve the dispersibility and biocompatibility of halloysite. Interstate surfaces, held together by hydrogen bridges, can be modified by direct or indirect intercalation of small organic molecules and some monovalent cationic salts. This, therefore, can lead to a weakening of the hydrogen bonds interstate and an increase in the surface between the various layers that can be understood as additional space for loading or adsorption [52]. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 3 of 19 *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 3 of 19

**Figure 1.** Detail of schematic illustration of the crystalline structure of halloysite nanotubes. **Figure 1.** Detail of schematic illustration of the crystalline structure of halloysite nanotubes. nm, an external diameter of 40–70 nm [41], and a length between 200 and 2000 nm [42] (Figure 2a,b).

**Figure 2.** (**a**) Atomic force microscopy; (**b**) TEM images of halloysite nanotubes precipitated from aqueous dispersion. Adapted from [43]. **Figure 2.** (**a**) Atomic force microscopy; (**b**) TEM images of halloysite nanotubes precipitated fromaqueous dispersion. Adapted from [43].

#### Nanotubes with a length between 3 and 5 μm have been found in some deposits, **3. Effects of Nanomaterial on Human Health**

**Figure 2.** (**a**) Atomic force microscopy; (**b**) TEM images of halloysite nanotubes precipitated from aqueous dispersion. Adapted from [43]. although those with smaller sizes are more interesting from a biological point of view, as they are more suitable for use as drug carriers [44]. Halloysite is chemically like kaolinite, but the halloysite double layers are separated by a monolayer of water molecules. In the Despite nanomaterials having potential benefits, their interaction with biologicalsystems may cause unpredicted risk to human life. One concern is the variation in dimensions of nanoparticles, which may affect chemical, thermodynamic, optical, biological,

Nanotubes with a length between 3 and 5 μm have been found in some deposits,

hydrated form of halloysite *n* = 2 in the formula Al2(OH)4Si2O5·*n*H2O. One layer of water molecules is present between the multilayers and is named "halloysite-(10 Å)", where "10

but the halloysite double layers are separated by a monolayer of water molecules. In the hydrated form of halloysite *n* = 2 in the formula Al2(OH)4Si2O5·*n*H2O. One layer of water molecules is present between the multilayers and is named "halloysite-(10 Å)", where "10 Å" indicates the d001-value of the layers. When the *n* value is 0 (*n* = 0), the halloysite is named dehydrated or "halloysite-(7 Å)" and may be obtained through the loss of the interlayer water molecules [30,45]. The water present in halloysite-(10 Å) can be classified as "hole water" and "associated water". In the first case, the water molecules are placed on the surface of the tetrahedral sheet with different orientations and make hydrogen bonds with the basal oxygens. The "associated" water has a greater degree of mobility at room temperature and is located at a different level in the interlayer space, with an icelike configuration and forming hydrogen bonds with each other and/or with inner-surface hydroxyls [46–50]. Generally, HNT surfaces could be classified into three types. The inner lumen surface has a positive charge and is covered by Al-OH groups. It can undergo a variety of covalent modifications by which certain functional groups can be added. This method allows immobilizing several organic groups on the surface of the lumen stably. The external siloxane surface has a negative charge and can be used to establish covalent bonding with molecules such as organosilanes [51]. Moreover, it can be modified by