# PHOTOCATALYSIS FOR ENVIRONMENTAL APPLICATIONS

EDITED BY : Fan Dong, Sen Zhang and Yuxin Zhang PUBLISHED IN : Frontiers in Chemistry

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# PHOTOCATALYSIS FOR ENVIRONMENTAL APPLICATIONS

Topic Editors:

Fan Dong, Chongqing Technology and Business University, China; University of Electronic Science and Technology of China, China Sen Zhang, University of Virginia, United States Yuxin Zhang, Chongqing University, China

Citation: Dong, F., Zhang, S., Zhang, Y., eds. (2019). Photocatalysis for Environmental Applications. Lausanne: Frontiers Media. doi: 10.3389/978-2-88945-902-5

# Table of Contents

Shiying He, Pengfu Hou, Evangelos Petropoulos, Yanfang Feng, Yingliang Yu, Lihong Xue and Linzhang Yang

*91 Photocatalytic Oxidation of Acetone Over High Thermally Stable TiO2 Nanosheets With Exposed (001) Facets*

Ting Shi, Youyu Duan, Kangle Lv, Zhao Hu, Qin Li, Mei Li and Xiaofang Li

*101 Compressible and Recyclable Monolithic g-C3N4 /Melamine Sponge: A Facile Ultrasonic-Coating Approach and Enhanced Visible-Light Photocatalytic Activity*

Ye Yang, Qian Zhang, Ruiyang Zhang, Tao Ran, Wenchao Wan and Ying Zhou

*111 Insights Into Highly Improved Solar-Driven Photocatalytic Oxygen Evolution Over Integrated Ag3PO4 /MoS2 Heterosttructures* Xingkai Cui, Xiaofei Yang, Xiaozhai Xian, Lin Tian, Hua Tang and Qinqin Liu

Wei Guan, Gaoge Sun, Lei Yin, Zhenghua Zhang and Shichao Tian

Jin Han, Kejian Li, Hanyun Cheng and Liwu Zhang

# Editorial: Photocatalysis for Environmental Applications

#### Fan Dong1,2 \*, Yuxin Zhang3 \* and Sen Zhang4 \*

1 Chongqing Key Laboratory of Catalysis and New Environmental Materials, Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, China, 2 Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China, 3 College of Materials Science and Engineering, Chongqing University, Chongqing, China, 4 Department of Chemistry, University of Virginia, Charlottesville, VA, United States

Keywords: photocatalysis, environmental catalysis, air pollution, reaction mechanism, nanomaterials

#### **Editorial on the Research Topic**

#### **Photocatalysis for Environmental Applications**

#### Edited by:

Simelys Hernández, Polytechnic University of Turin, Italy

#### Reviewed by:

Fatwa Abdi, Helmholtz-Zentrum Berlin für Materialien und Energie, Helmholtz-Gemeinschaft Deutscher Forschungszentren (HZ), Germany

#### \*Correspondence:

Fan Dong<EMAIL_ADDRESS>Yuxin Zhang<EMAIL_ADDRESS>Sen Zhang<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry

Received: 12 February 2019 Accepted: 16 April 2019 Published: 01 May 2019

#### Citation:

Dong F, Zhang Y and Zhang S (2019) Editorial: Photocatalysis for Environmental Applications. Front. Chem. 7:303. doi: 10.3389/fchem.2019.00303 Environmental pollution is one of the major challenges because of the rapid development of urbanization and industrialization. Considering this environmental challenge, providing a clean environment for human beings is very important for the sustainability. The nanostructured photocatalysts with intriguing physiochemical property have offered opportunities to solve the issue of environmental sustainability (Chen et al., 2019; Huo et al., 2019; Li J. et al., 2019). In recent years, significant advances have been witnessed on the synthesis and application of photocatalyst in environmental remediation (He et al., 2018a; Li et al., 2018c; Li X. et al., 2018, 2019; Wang et al., 2018c). These new photocatalysts have enabled wide applications in the air purification, wastewater treatment, and so on (Cui et al., 2018; He et al., 2018b; Li et al., 2018b; Xiong et al., 2018) The rapid development in catalysis science, nanoscience, and materials enable the significant advances in new strategies for the controlled preparation, photocatalysis reaction mechanism, and structure-activity relationship of photocatalysts (Dong et al., 2018a; Li et al., 2018a; Wang et al., 2018a,b). The structural features of photocatalysts can be further tuned to achieve enhanced photocatalytic performance in environmental applications (Dong et al., 2018b; Li X. et al., 2018; Wang et al., 2018d).

The rapid development in photocatalysis for environment has inspired this interesting Research Topic. We have invited scientists worldwide to contribute original research and review articles which could enhance our understanding of the key problems in environmental applications of nanostructured photocatalysts. The original articles describing the photocatalysts for environmental control, and for sustainable development have been accepted for publication after peered review. In this topic issue, the readers will find very interesting results covering the following aspects (1) design and synthesis of photocatalysts with new morphology and active catalytic sites; (2) photocatalysts for green synthesis; (3) photocatalysts for CO2 conversion to solar fules; (4) photocatalysts for wastewater treatment and air purification; and (5) revealing the photocatalysis reaction mechanism as applied in environmental problems.

For the g-C3N4 based photocatalysts, Guan et al. synthesized Ti4O7/g-C3N4 composites by a low temperature method. The enhanced photocatalytic activity for Ti4O7/g-C3N4 could be ascribed to the promoted charge separation and photoabsorption efficiency. Yang et al. fabricated a monolithic g-C3N4/melamine sponge by a cost-effective ultrasonic-coating method. The monolithic g-C3N4/melamine demonstrated high photocatalytic activity for NO removal and CO2 reduction. Guan et al. prepared the Ti4O7/g-C3N4 photocatalysts by a hydrolysis method. The Ti4O7/g-C3N4 exhibited remarkably improved photocatalytic activity for hypophosphite oxidation, which can be ascribed to the heterojunction structure of Ti4O7/g-C3N4 that enhanced charge carrier efficiency (Guan et al.).

Xu et al. prepared BiVO4 by a facile method and conducted a trapping experiment to study the free radical transformation mechanisms. They identified •OH and h+ as the main active radicals for oxidation. Han et al. developed a new photoelectrochemical (PEC) technology for simultaneous SO2 removal and H2 production. The enhanced H2 production and SO2 removal efficiency can be attributed to the improved charge carrier transfer after Mo doping (Han et al.). Regmi et al. reviewed recent advances on the microbial decontamination by photocatalysts and their possible mechanisms are highlighted.

Cui et al. fabricated the Ag3PO4/MoS2 nanocomposites and revealed that the improved performance of Ag3PO4/MoS2 can be ascribed to wide spectra response, efficient charge separation and enhanced oxidation capacity. He et al. developed a two-step ZnO-modified strategy to immobilize the catalyst on rGO sheets. The high ammonia degradation efficiency of ZnO/Cu/rGO can be attributed to the enhanced ROSs production efficiency and the activated interfacial catalytic sites. Shi et al. prepare high energy faceted TiO2 nanosheets by calcination of TiOF2 cubes. The 500◦ C-calcined sample exhibits the highest photocatalytic activity for removal of acetone owing to the high energy TiO2-NSs and the surface adsorbed fluorine.

Kim et al. synthesized the nitrogen doped TiO2 by a novel plasma electrolysis method. The 0.4 at.% N doped TiO2 catalyst showed the highest photocatalytic performance. Xu et al. developed a BiOCl/NaNbO3 p-n heterojunction by an in-situ method. The BiOCl/NaNbO3 composites exhibited much

#### REFERENCES

enhanced photocatalytic activity attributed to the formation of p-n junction between NaNbO3 and BiOCl that facilitated the charge separation (Xu et al.). Ren et al. synthesized the AgBr@Ag modified titanium phosphate composites. The AgBr@Ag/titanium phosphate exhibited higher photocatalytic activity and the photocatalytic degradation mechanisms were proposed.

She et al. reported selective activation of saturated C–H bond to generate the high-value-added chemicals on Ni-doped CdS nanoparticles. The high photocatalytic performance can be attributed to the cubic and hexagonal phases, Ni-doping and the charge carriers separation. Li et al. synthesized Au/BiFeO3 homojunctions via a simple method. The Au1.2-BFO showed efficient photocatalytic activity due to the hierarchical structure, SPR effect of Au particles, and the defects (Li et al.). Zhang and Liang fabricated the new 2D g-C3N4@BiOCl/Bi12O17Cl2 by a facile approach, which showed enhanced visible light absorption and electron-hole separation efficiency and thus highly enhanced photocatalytic activity for NO removal.

At last, as the Guest Editors of this topic issue, we would like to appreciate all the authors for the contributed articles and thank for all the referees for their comments on the manuscripts. We hope that the readers will find the results in articles of this topic issue interesting and useful for their research. Finally, we appreciate the editorial staff of Frontiers in Chemistry for their work in publishing of this topic issue.

#### AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Dong, Zhang and Zhang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Facile Synthesis of Ternary g-C3N4@BiOCl/Bi12O17Cl2 Composites With Excellent Visible Light Photocatalytic Activity for NO Removal

#### Wendong Zhang\* and Yi Liang

Chongqing Key Laboratory of Inorganic Functional Materials, Department of Scientific Research Management, Chongqing Normal University, Chongqing, China

#### Edited by:

Nicolas Keller, l'Environnement et la Santé (ICPEES), France

#### Reviewed by:

Dong Guohui, Shaanxi University of Science and Technology, China Bitao Liu, Chongqing University of Arts and Sciences, China Liwu Zhang, Fudan University, China

> \*Correspondence: Wendong Zhang<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry

Received: 19 August 2018 Accepted: 25 March 2019 Published: 11 April 2019

#### Citation:

Zhang W and Liang Y (2019) Facile Synthesis of Ternary g-C3N4@BiOCl/Bi12O17Cl2 Composites With Excellent Visible Light Photocatalytic Activity for NO Removal. Front. Chem. 7:231. doi: 10.3389/fchem.2019.00231 In this study, novel two-dimensional (2D) g-C3N4@BiOCl/Bi12O17Cl2 composites have been fabricated through a facile deposition-precipitation process. The as-prepared photocatalysts were characterized by XRD, SEM, TEM, XPS, UV-vis DRS, PL, Photocurrent, EIS, ESR, and N2 adsorption-desorption. The photocatalytic activities were investigated through NO removal test in gas under visible light irradiation (λ > 420 nm). The g-C3N4@BiOCl/Bi12O17Cl2 composites exhibit enhanced visible light absorption and photo-induced electron-hole separation efficiency, compared with pristine g-C3N4 and BiOCl/Bi12O17Cl2. The intimated contact interfaces between g-C3N4 and BiOCl/Bi12O17Cl2 nanosheets are responsible for the more efficient photochemical interactions. The present work provides a new direction to develop a class of ternary g-C3N4-based visible-light-driven photocatalysts for environmental purification.

Keywords: facile synthesis, g-C3N4@BiOCl/Bi12O17Cl2 , visible light, photocatalytic activity, nitrogen oxide (NO) removal

## INTRODUCTION

In the past decades, with the rapid development of modern industrial society, a large amount of highly harmful and toxic contaminants have been discharged into environmental system, which have been the focus of world attention (Han et al., 2017; Li et al., 2018). It remains a great challenge to completely achieve the degradation of environmental contaminants through the conventional treatment process, especially for the low-level concentrations of contaminants (Jiang et al., 2017, 2018; Zhong et al., 2017).

Photocatalysis, as a novel technique, have potential application in degradation low-level concentrations of environmental contaminants under visible-light irradiation (Xiong et al., 2015; Zheng and Zhang, 2016; Guan et al., 2017; Jin et al., 2018). Up to now, although a large number of photocatalysts have been explored for environmental purification, most of them still suffer from the limited utilization of solar light, resulting in quite low visible light photocatalytic activity (Chibac et al., 2017; Zhang et al., 2017). Hence, it is desirable to develop highly efficient visible-light-driven photocatalysts for satisfying the requirements of practical applications.

**8**

Currently, graphitic carbon nitride (g-C3N4), an organic semiconductor photocatalyst, has attracted intensive research interest in energy conversion and environmental remediation fields, largely due to its typical physicochemical properties, such as suitable band gap, good chemical and thermal stability, environmental friendly, etc (Wang et al., 2011; Dong and Zhang, 2013; Dong et al., 2015). However, the photocatalytic efficiency of g-C3N4 is far below the requirement of practical applications, mainly due to its fast recombination of photoexcited electron-hole pairs. So far, various strategies have been developed to improve the photocatalytic performance of g-C3N4, including electronic structure engineering, nanostructure optimization, and heterojunction construction (Cao et al., 2015; Zhao et al., 2015; Ong et al., 2016). Furthermore, it is well-known that the g-C3N4 nanosheets can provide a good two-dimensional surface and interface platform for growth of other nanostructured semiconductors. In particular, the heterojunction with other semiconductor photocatalysts has been regarded as an attractive and effective solution in enabling the efficient separation of photo-excited electronhole pairs, which not only helps to prolong the life-time of photo-excited charge carriers, but also endows g-C3N4-based heterojunctions with more abundant active sites, leading to the significantly enhancement of photocatalytic performance. Successful examples include g-C3N4/TiO2 (Wei et al., 2016), g-C3N4/MoS2 (Hou et al., 2013), g-C3N4/BiOBr (Sun et al., 2014), g-C3N4/WO3 (Huang et al., 2013), g-C3N4/CdS (Liu, 2015), g-C3N4/g-C3N4 (Dong et al., 2013), g-C3N4/(BiO)2CO3 (Zhang et al., 2014), g-C3N4/graphene (Kim et al., 2016), and g-C3N4/Ag (Olga et al., 2016), etc. However, little information about ternary g-C3N4@BiOCl/Bi12O17Cl2 heterojunction for visible light photocatalytic removal of NO has been reported.

Here, we report a highly cost-effective method based on the in situ self-assembly of BiOCl/Bi12O17Cl2 binary nanoplates onto the surface of g-C3N4 nanosheets at room temperature. It takes into consideration advantages of well-matched band structures

among g-C3N4, BiOCl and Bi12O17Cl2, the as-prepared ternary g-C3N4@BiOCl/Bi12O17Cl2 heterojunctions exhibit apparent characteristics including larger surface area, improved visible light absorption ability, and efficient separation of photo-induced charge carries, which are extremely favorable for improving the photocatalytic activity.

# Fabrication of g-C3N4@BiOCl/Bi12O17Cl2 Composites

The pure g-C3N4 nanosheets and TiO2 powders were fabricated according to the previous reports (Dong et al., 2011; Zhang et al., 2014), respectively. It's a typical synthesize that 1.33 g of BiCl3 and 0.25 g of as-obtained g-C3N4 were added to 50 mL absolute ethyl alcohol and then were ultrasonicated for 30 min. Afterwards, dripping 12.6 mL of NaOH solution (2.0 mol/L) dropwise into BiCl3 solution and then stirring vigorously for 4 h at room temperature. After that, filtering and washing the resulting precipitate for times with distilled water and ethanol. The final samples were obtained after drying under vacuum at 60◦C for 24 h. Five types of samples were prepared. The mass rations of BiOCl/Bi12O17Cl2 to g-C3N4 are 1:2, 1:4, 1:1, 2:1, and 4:1, respectively. Accordingly, the final samples were labeled as BOC-CN-1-2, BOC-CN-1-4, BOC-CN-1-1, BOC-CN-2-1, and BOC-CN-4-1, respectively. The pure BiOCl/Bi12O17Cl2 samples were synthesized under the same conditions without adding g-C3N4 nanosheets, and the g-C3N4@BiOCl/Bi12O17Cl2 sample was labeled as BOC.

## Materials Characterization

In a typical analysis of the crystal phase of the as-obtained samples, X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Japan) was applied. Meanwhile scanning electron microscope (SEM, JEOL model JSM-6490, Japan) and transmission electron microscopy (TEM: JEM-2010, Japan) were utilized to characterize the morphology and structure. And the surface properties were examined by X-ray photoelectron spectroscopy with Al Kα X-rays (hν = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA). By using a Scan UV-vis spectrophotometer (UVvis DRS: UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly and BaSO4 as reflectance sample, the UV-vis diffuse reflection spectra was gained. Nitrogen adsorption-desorption was conducted on a nitrogen adsorption apparatus (ASAP 2020, USA) to insure the specific surface areas and total pore volumes. Photoluminescence (PL: F-7000, HITACHI, Japan) was used to investigate the charge transfer properties. ESR spectrometer (FLsp920, England) was applied to detect the electron spin resonance (ESR) signals of •OH and •O − 2 , respectively. The photocurrent measurements (CHI 660B electrochemical system: Shanghai, China) and electrochemical impedance spectroscopy (EIS) were carried out to analyze the photo-generated charge separation properties. All the samples were degassed at 150◦C prior to measurements.

#### Appraisement of Photocatalytic Activity

The photocatalytic activity was investigated by removal of NO at ppb (1 × 10−9 ) levels in a continuous flow reactor at ambient

FIGURE 4 | SEM (a,b) and TEM (c,d) images of BOC-CN. EDS elemental mapping (e–k) of the same region, indicating the spatial distribution of Bi (g), C (h), Cl (i), N (j), and O (k), respectively.

temperature. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 × 15 × 10 cm). A 100-W commercial tungsten halogen lamp (THL100, Beijing, China) was vertically placed outside the reactor, and the light spectra range (see **Supplementary Figure 1**) from 350 to 2,400 nm. A UV cut-off filter (Aike UBG-420, Shenzhen, China) was adopted to remove UV light in the light beam. Photocatalyst (0.2 g) was coated onto a dish with a diameter of 12.0 cm, and the irradiance received by the photocatalyst powder is about 0.66 W/cm2 . The coated dish was then pretreated at 70◦C to remove water in the suspension. The catalyst adhesion on the dish was firm enough to avoid the erosion (or removal) of the catalyst during air flowing. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 × 10−6 NO (N2 balance, BOC gas). The initial concentration of NO was diluted to about 500 × 10−9 by the air stream. The desired relative humidity (RH) level of the NO flow was controlled at 50% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 2.4 L·min−1 by a mass flow controller. After the adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., 42i-TL), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 1.0 L·min−1 . The removal ratio (η) of NO was calculated by η (%) = (1-C/C0) ×100%, where C and C0 are concentrations of NO in the outlet stream and the feeding stream, respectively.

# RESULTS AND DISCUSSION

The phase structures of the obtained samples were investigated by XRD patterns. As shown in **Figure 1**, the BOC sample exhibits typical XRD peaks of BiOCl and Bi12O17Cl2 (He et al., 2016; Huang et al., 2016), thus suggesting the formation of BiOCl/Bi12O17Cl2 heterojunctions, the result is good agreement with the previous reports (Zhang et al., 2017). For the pure CN sample, the typical diffraction peaks appeared at 27.4 and 13.1◦ are indexed to the g-C3N4 (002) and (100) planes (Wang et al., 2011; Cao et al., 2015), respectively. Based on further observation, the peak intensity of the CN decreased with increasing BOC content, implying that interactions exist between the BOC and CN. Finally, two peaks of CN cannot be observed in the BOC-CN composites, which can be ascribed to a good dispersion of BOC onto the surface of CN.

The morphologies and microstructures of BOC, CN, and BOC-CN were characterized by SEM and TEM. For BOC samples (**Figure 2**), the layered and irregular microstructures consist of smooth nanosheets with different sizes. Moreover, there are two different fringes with the lattice spacing of 0.59 and 0.25 nm (**Figure 2d**), which can be indexed to the (006) crystal plane of Bi12O17Cl2 and (003) crystal plane of BiOCl, respectively. For CN samples (**Figure 3**), it presents the lamellar morphology is composed of numerous nanosheets with a much looser pore structure. As shown in **Figures 4a–d**, the BOC-CN composites also consist of a large number of layered nanosheets with different shapes. Obviously, the BOC nanosheets were in situ growth on the surface of g-C3N4, resulting in the formation of closely interface in g-C3N4@BiOCl/Bi12O17Cl2 composites, which is beneficial for the separation and transfer of the photo-induced electron-hole pairs. The surface element dispersion state of BOC-CN composites are studied by EDS mapping. As shown in **Figures 4e–k**, the Bi, C, Cl, N, and O elements are uniformly distributed in BOC-CN samples.

The XPS measurements were applied to verify the composition and chemical state of the elements. Two peaks at 159.1 and 164.3 eV are consistent with Bi4f7/2 and Bi4f5/2 (**Figure 5A**), respectively. The XPS spectrum for Cl shows two peaks at 197.8 and 199.4 eV attributed to Cl2p3/2 and Cl2p1/2 (**Figure 5B**), respectively. The peak centered at 530.3 eV that corresponds to the binding energy of O 1s (**Figure 5C**) (Bi et al., 2016; Zhang et al., 2017). The C peak at 284.8 eV can be ascribed to the adventitious carbon atom. **Figure 5D** shows one peak at 288.5 is identified as overlapped peaks of N-C=N and the O-C=O. Two different peaks are observed in **Figure 5E**, the N peak at 398.9 eV correspond to the C=N-C and the N

peak at 400.8 eV is attributed to the residual amino groups (Zhang et al., 2014).

The optical absorption property of the as-obtained samples was investigated by UV-vis DRS. As can be seen from **Figure 6A**, BOC and CN exhibit absorbance edge around 550 and 475 nm, respectively, displaying that BOC and CN possess good visible light absorption ability. Interestingly, BOC-CN-4-1 shows relatively stronger visible light absorption ability lies in the range of 550–800 nm, because of the synergetic effect between BOC and CN. **Figure 6B** shows the PL spectra for BOC, CN,

and BOC-CN-4-1 using the exciting light of 320 nm. Compared to CN, the PL intensity of BOC-CN-4-1 significantly decreases, demonstrating that the interface interaction between BOC and CN could inhibit the recombination rate of photo-generated electrons and holes.

The photocurrent and electrochemical impedance experiments were used to investigated the photo-generated charges separation and transfer property of BOC, CN, and BOC-CN-4-1 samples under visible light irradiation (Zhang et al., 2018a). Compared with the pure BOC and CN, it was interesting to find that BOC-CN-4-1 exhibits significantly enhanced photocurrent density (**Figure 7A**), suggesting that BOC-CN-4-1 possesses higher photo-generated charge separation property. As can be seen from the **Figure 7B**, the arc radius on the EIS Nyquist plot of BOC-CN-4-1 was smaller than that of the pure BOC and CN, demonstrating that BOC-CN-4-1 has much more efficient photo-generated charge separation and transfer property.

**Figure 8A** shows that the adsorption-desorption isotherms of all the samples are type IV according to the IUPAC classification, demonstrating that all the samples have mesopores (Dong et al., 2013; Zhang et al., 2014). However, the typical H3 hysteresis loop at low pressure can be ascribed to the aggregation of nanosheets with slit-like pores. **Figure 8B** further confirms the presence of mesopores in BOC, CN, and BOC-CN samples. In addition,

the specific surface area (SBET) and total pore volume (Vp) are 32.8 m2 /g and 0.176 cm3 /g for BOC-CN composites, which are higher than those of pure BOC (29.7 m2 /g and 0.168 cm3 /g) and CN (21.8 m2 /g and 0.132 cm3 /g), the enlarged SBET and Vp of BOC-CN composites can be attributed to the stack of nanosheets layered by layered. The enlarged SBET and Vp of BOC-CN composites can facilitate the reactants adsorb and transfer, and provide more active sites for the photocatalytic reaction.

The electron spin resonance (ESR) experiments were further used to confirm the active species during the photocatalytic reaction process under visible light irradiation. As shown in **Figures 9A,B**, the superoxide (•O − 2 ) radicals and hydroxyl (•OH) radicals have been successfully detected by the ESR technique, respectively. Moreover, the intensity of all peaks increase significantly with the irradiation time, demonstrating that •O − 2 and •OH are continuously generated during the reaction. The result shows that both •O − 2 and •OH are the main photocatalytic reaction active species. The formation of •O − 2 and •OH from the photochemical reaction shows in Equations 1–3 (Zhang et al., 2018a).

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textbullet O}\_2^- \tag{1}$$

$$\text{\textbullet O}\_2^- + 2\text{H}^+ + \text{e}^- \rightarrow \text{H}\_2\text{O}\_2 \tag{2}$$

$$\mathrm{H}\_{2}\mathrm{O}\_{2} + \mathrm{e}^{-} \rightarrow \bullet \mathrm{OH} + \mathrm{OH}^{-} \tag{3}$$

Furthermore, the possible mechanism for the photocatalytic reaction at the BOC-CN interface are presented in **Figure 10**. After the visible light irradiation, the Bi12O17Cl2 and g-C3N4 can be excited and then produce electron-hole pairs (Dong et al., 2013; Zhang et al., 2018b). On the one hand, the excited electrons in CB of g-C3N4 can directly transfer to CB of BiOCl (Zhang et al., 2013). On the other hand, the excited holes in VB of Bi12O17Cl2 can transfer to VB of g-C3N4, the excited electrons in CB of g-C3N4 can transfer to CB of Bi12O17Cl2, and the electrons can further transfer to the CB of BiOCl from the CB of Bi12O17Cl2, the band structures of the three components are well-matched resulting in efficient separation and transfer of the photo-induced carriers. Hence, the suitable band structures clearly show that the efficient electron-hole pair separation plays critical role in improving the photochemical reaction (Zhang et al., 2013, 2014).

The visible-light-induced photocatalytic activities of the TiO2, BOC, CN, and BOC-CN samples toward NO were revealed in **Figure 11A**. The pure BOC and CN only removed 36.2 and 14.6% of NO after 30 min visible light irradiation due to its fast recombination of photo-induced carriers, respectively. However, the visible-light-induced photocatalytic activities of the TiO2 sample can be neglected under the same conditions, indicating that the activity does not result from a UV-A light induced photocatalytic activity due to some trace of UV-A light after the cut-off filter. When the heterojunction was formed, the NO removal ratio over BOC-CN composites was increased to 46.8%. For ruling out and evidencing the NO degradation, the adsorption experiment of the optimized BOC-CN-4-1was carried out under dark condition, the result shows that adsorption property of NO over the BOC-CN-4-1-dark sample can also be ignored. The photocatalytic stability experiment of the BOC-CN-4-1 sample was evaluated by repeating the reaction for five runs under visible light irradiation. As shown in in **Figure 11B**, the photocatalytic performance shows slightly loss after five run, indicating that BOC-CN-4-1 photocatalyst possesses good photocatalytic stability. Interestingly, the BOC-CN composites exhibit even higher visible light photocatalytic activity than that of BiOBr/C3N4 (removal rate of 32.7%) and p-doped g-C3N4 (removal rate of 42.3%) (Sun et al., 2014; Zhang et al., 2016). The enhanced photocatalytic activity of BOC-CN can be attributed to the synergistic contribution of BOC and g-C3N4 with respect to the suitable band structure, enlarged SBET and Vp, improved visible light absorption, and efficient photo-induced carrier separation at the interface of BOC and g-C3N4 (Dong et al., 2013; Hou et al., 2013; Sun et al., 2014; Liu, 2015; Wei et al., 2016).

#### CONCLUSION

In summary, we have synthesized ternary BOC-CN heterojunctions with outstanding visible light photocatalytic

performance by self-assembly of BiOCl/Bi12O17Cl2 nanosheets on the surface of g-C3N4 nanosheets via a chemical depositionprecipitation method. The results reveal that g-C3N4, BiOCl, and Bi12O17Cl2 possess well-matched band structures, which is helpful to the separation and transport of photo-induced carriers. This work provides a new perspective for the design and fabrication of high performance and stable BiOCl/Bi12O17Cl2-based photocatalysts via a facile method at room temperature.

#### AUTHOR CONTRIBUTIONS

WZ: experiment, data analysis, and paper writing. YL: paper writing.

#### REFERENCES

#### FUNDING

This research is financially supported by the National Natural Science Foundation of China (No. 51708078), Natural Science Foundation of Chongqing (CSTC, 2018jcyjA1040), and the Chongqing Postdoctoral Science Foundation funded project (No. Xm2016027).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00231/full#supplementary-material

Supplementary Figure 1 | The light spectra range of tungsten halogen lamp.

macroscopic visible-light photocatalyst. Chin. J. Catal. 2, 313–320. doi: 10.1016/S1872-2067(16)62568-8

at room temperature. Ind. Eng. Chem. Res. 52, 6740–6746. doi: 10.1021/ie 400615f

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Zhang and Liang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# In situ Synthesis of Au-Induced Hierarchical Nanofibers/Nanoflakes Structured BiFeO3 Homojunction Photocatalyst With Enhanced Photocatalytic Activity

#### Yan'an Li 1 , Jiao Li 1 \*, Long Chen1 \*, Haibin Sun1 , Hua Zhang1 , Hong Guo2 and Liu Feng2

*1 School of Materials Science and Engineering, Shandong University of Technology, Zibo, China, 2 Analysis & Testing Center, Shandong University of Technology, Zibo, China*

In order to further improve the photocatalytic performance of BiFeO3 (BFO), novel Au-induced hierarchical nanofibers/nanoflakes structured BiFeO3 homojunctions (Au*x*-BFO, *x* = 0, 0. 6, 1.2, 1.8, 2.4 wt%) were *in situ* synthesized through a simple reduction method with assist of sodium citrate under the analogous hydrothermal environment. The effect of loading amount of Au nanoparticles (NPs) on the physicochemical properties and photocatalytic activity was investigated in detail. The Au1.2-BFO NFs sample show the best photocatalytic activity (85.76%), much higher than that for pure BFO samples (49.49%), mainly due to the hierarchical nanofibers/nanoflakes structured homojunction, the surface plasmon resonance (SPR) effect of Au NPs, as well as the presence of defects (Fe2+/Fe3+ pairs and oxygen vacancy). Furthermore, the possible formation mechanism of the unique homojunction and the enhanced photocatalytic mechanism for the degradation of methylene blue (MB) dye are proposed. It is proven that holes (h+) play the decisive role in the photocatalytic process. The present work provides a fascinating way to synthesize efficient homojunctions for the degradation of organic pollutes.

Keywords: bismuth ferrite, homojunction, SPR effect, defects, photodegradation

# INTRODUCTION

Nowadays, energy crisis and environmental deterioration issues are severely detrimental for economic development and human health (Wang et al., 2013). Semiconductor photocatalysis as an efficient and green technology can convert solar energy into chemical energy to dispose of these issues (Wang et al., 2013; Wang H. et al., 2017). In order to fully exploit the solar energy, it is necessary to explore visible light responsive photocatalysts with excellent photocatalytic activities (Zhang et al., 2017).

Among the variety of visible-light-driven semiconductor materials, BiFeO3 (BFO) has attracted a great deal of attention due to its narrow band gap (2.2∼2.5 eV), good chemical stability and low cost (Wang et al., 2013; Niu et al., 2015). In addition, the room temperature multiferroic property makes it easily recovered from the treated water to avoid the secondary pollution (Zhang et al., 2017). However, the rapid recombination rate of photogenerated electron-hole pairs and low quantum yield limit its practical application (Huo et al., 2011; Srivastav and Gajbhiye, 2012; Niu et al., 2015). Therefore, much efforts have been made to improve its photocatalytic performance, such as element doping (Wang et al., 2012; Ahmada et al., 2017; Irfan et al., 2017; Yang et al., 2018),

#### Edited by:

*Yuxin Zhang, Chongqing University, China*

#### Reviewed by:

*Dong Guohui, Shaanxi University of Science and Technology, China Xiaoying Liu, Chongqing Technology and Business University, China*

#### \*Correspondence:

*Jiao Li<EMAIL_ADDRESS>Long Chen<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *03 September 2018* Accepted: *12 December 2018* Published: *04 January 2019*

#### Citation:

*Li Y, Li J, Chen L, Sun H, Zhang H, Guo H and Feng L (2019) In situ Synthesis of Au-Induced Hierarchical Nanofibers/Nanoflakes Structured BiFeO3 Homojunction Photocatalyst With Enhanced Photocatalytic Activity. Front.

Compared to that of bulk materials, the reduced radial dimension and the extremely large surface-to-volume ratio of one-dimension (1D) nanofibrous structure can promote the rapid transfer of photogenerated charge carriers to the surface, and thus enhance the separation efficiency (Li S. et al., 2013). Furthermore, the perpendicular transport direction of photogenerated electrons and holes can further effectively inhibit the recombination of photogenerated electron-hole pairs (Mohan and Subramanian, 2013). Therefore, the synthesis of BFO sample with nanofibrous structure is in favor of enhancing the photocatalytic activity.

Noble metal (e.g., Ag, Au, Pt, etc.) deposition is also an efficient way to enhance the photocatalytic activity (Li et al., 2015; Niu et al., 2015; Zhang et al., 2015). Among them, Au nanoparticles (NPs) exhibit excellent a chemical stability and a characteristic absorption peak in the visible wavelength range due to the strong surface plasmon resonance (SPR) effect (Li S. et al., 2013; Li et al., 2015). The deposited Au NPs can not only act as electron-trapping centers to reduce the recombination rate of photogenerated electron-hole pairs, but also can act as light harvesters enhancing the light absorbing ability and as catalytic sites for the photocatalytic reaction (Lin et al., 2017; Chiu et al., 2018). Significantly, the formed Schottky junctions between Au NPs and BFO nanofibers (NFs) can adjust the interfacial band structure and thus promote the separation and transfer of photogenerated charge carriers.

In comparison with heterojunction built with different semiconductors. Besides the band alignment of two semiconductors, the creation of heterojunctions is also dependent on other properties of semiconductor, such as electron affinity and work function (Ye et al., 2017). The homojunction is constructed by the same semiconductor materials with different crystal phases, exposing facets or semiconductor types, etc., which can avoid the disadvantages of heterojunction fabrication (Huang et al., 2017). The enhanced photocatalytic activity of heterojunctions or homojunctions is mainly dependent on the space charge accumulation or depletion at the interfaces of two phases (Ye et al., 2017).The homojunction structure can also introduce an internal field to effectively facilitate surface charge separation, retard the recombination of photogenerated carriers, and thus remarkably improve the photocatalytic performance. However, there are few reports on the fabrication of BFO homojunction, especially for the aspect of in situ synthesis.

In this work, an distinctive Au NPs deposited BFO homojunction, in which BFO nanoflakes are assembled on the surface of BFO NFs, was in situ synthesized through a simple reduction method under the analogous hydrothermal environment. The effect of loading amount of Au NPs on the physicochemical properties and photocatalytic activity was investigated in detail. Furthermore, it proposed a possible formation mechanism of the unique homojunction and the enhanced photocatalytic mechanism for the degradation of methylene blue (MB) dye over Aux-BFO NFs under simulated solar light irradiation.

# EXPERIMENTAL

# Preparation of Aux-BFO NFs

BFO NFs (Aux-BFO NFs, x = 0 wt%) were prepared via a sol-gel method combined with an electrospinning technique. All the chemical reagents used for the synthesis were analytical grade. Bismuth nitrate (Bi(NO3)3·5H2O) and ferric nitrate (Fe(NO3)3·9H2O) with the Bi: Fe molar ratio of 1.11:1 (to make up for the loss of Bi ions during the calcination process) were dispersed in methylglycol by ultrasonic to form a homogeneous solution. Then the viscosity and pH value of the solution were adjusted by 2.5 mL of glacial acetic acid, called solution A. g of poly(vinylpyrrolidone) (PVP, Mw = 1,300,000) was added to the mixed solvent of N'N-dimethylformamide (DMF) and ethanol with the volume ratio of 2:1, called solution B. Solution B was added dropwise to solution A with constantly stirring for 24 h to form the pre-spinning solution which has a pH of 2.0. The pre-spinning solution was loaded into a plastic syringe connected with a stainless needle and then fixed onto the electrospinning system. An optimized high voltage of 15 kV was applied to the needle with the flow rate of 0.4 mL/h. The distance was 20 cm from the needle to the rotating drum collector. The as-spun NFs were dried at 60◦C for 4 h, calcined at 350◦C for 30 min with a heating rate of 2◦C/min, and then calcined at 600◦C for 120 min with a heating rate of 5◦C/min in air.

Au NPs deposited BFO NFs (Aux-BFO NFs, x = 0.6, 1.2, 1.8, 2.4 wt%) were synthesized by a simple reduction method with sodium citrate. In a typical procedure, 57.38 mg of BFO NFs were first dispersed in 50 mL of deionized water, followed by adding a certain amount of HAuCl4 (3 mmol/L) and sodium citrate (0.04 mol/L). Then the suspension was kept stirring at 150◦C for 30 min in oil bath. After cooled to room temperature naturally, the obtained products were centrifuged, washed with deionized water and absolute ethanol for several times, and then dried at 80◦C for 24 h. The loading amount of Au NPs on BFO NFs were 0.6, 1.2, 1.8, and 2.4 wt% (mass ratio of Au to BFO), which were defined as Au0.6-BFO NFs, Au1.2-BFO NFs, Au1.8-BFO NFs, and Au2.4-BFO NFs, respectively.

# Characterization

The phase structures were identified by X-ray diffraction (XRD; D8 Advance, Bruker AXS, German) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 50 mA in a 2θ ranging from 20 to 70◦ . The morphologies were observed by field emission scanning electron microscope (FE-SEM; Sirion 200, FEI, USA). The transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed at a field emission transmission electron microscope (TEM; Tecnai G2 F20 S-TWIN, FEI, USA) with an accelerating voltage of 200 kV. UV-vis diffuse reflectance spectra (DRS) were measured at room temperature by UV-Vis spectrometer (UV-Vis; UV-3600plus, Shimadzu, Japan) with the wavelength range of 350–800 nm using BaSO4 as a reflectance. The measurements of magnetic properties were carried out by a superconducting quantum interference device magnetometer (FM, MPMS XL-7, Qunatum Design, USA). The element chemical states were characterized by X-ray photoelectron spectrometer (XPS; PHI-5300, PHI, USA) with an Al Kα X-ray radiation. The photoluminescence (PL) spectra were measured on the fluorescence spectrophotometer (PL; Hitachi F-4600, Hitachi, Japan) with the excitation wavelength of 230 nm and a 150 W xenon lamp as the light source.

# Photocatalytic Activity Evaluation

Twenty five milligram of Aux-BFO NFs were added to 30 mL of 10 mg/L MB solution with ultrasound for 10 min. After stirring in the dark for 30 min to reach the adsorptiondesorption equilibrium, the suspension system was exposed to simulated solar light irradiation (CHF-XM-500W xenon lamp, Beijing Perfactlight Company, China). During the photocatalytic reaction process, the suspension was taken out at every 30 min interval and centrifuged to remove the photocatalyst. The MB concentration was measured by a UV-vis spectrophotometer at the wavelength of 666 nm.

# RESULTS AND DISCUSSION

# XRD Analysis

The XRD patterns of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) are shown in **Figure 1**. All the diffraction peaks of pure BFO NFs can be indexed to the rhombohedral perovskite structure with R3m space group of the BiFeO3 phase (JCPDS card no. 86–1518) (Chen et al., 2017). The strong diffraction peaks also imply the good crystallinity. In addition, the crystalline structures of BFO NFs are not changed after depositing Au NPs.

# Microstructures

**Figure 2** shows the SEM images of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%). It can be seen from **Figure 2A** that the pure BFO NFs exhibit an uniform nanofibrous structure with compactly packed and continuous nanoparticles. The diameter of BFO NFs is about 100–200 nm. As shown in **Figures 2B–E**, there is no

obvious change of the nanofibrous structure for BFO NFs with Au NPs deposition, but some thin nanoflakes are embedded in the surface of nanofibers, and the number of nanoflakes increases with the increase of the loading amount of Au NPs.

The EDS mapping is performed to clarify the chemical composition and element distribution in Au1.2-BFO NFs, as shown in **Figure 3**. It can be clearly seen that Bi, Fe, and O elements are uniformly distributed throughout the single nanofiber (**Figures 3A–D**). Au NPs are dispersed well on BFO NFs, instead of agglomerating together (**Figure 3E**). More importantly, The EDS result (**Figure 3F**) recorded at the "+" position in **Figure 3A** shows that the main elements of the nanoflake are Bi, Fe, Au, and O. It is worthy to note that the atomic ratio of Bi/Fe is almost 1:1, nearly corresponding to the composition of BFO. The trace amount of Au shows that Au NPs are also deposited on the nanoflake.

In order to further illustrate the morphology and microstructure of pure BFO NFs and Au1.2-BFO NFs, TEM technology is used. As shown in **Figure 4A**, the pure BFO sample is nanofibrous structure compactly packed with nanoparticles. The HRTEM image in **Figure 4D** is collected from the square area in **Figure 4A**. The interplanar spacing of 0.278 nm corresponds to the (110) lattice plane of the rhombohedral perovskite BFO structure. **Figure 4B** shows the TEM image

FIGURE 3 | (A) The SEM image of a single representative Au1.2-BFO NFs, the corresponding elemental mappings of (B) Bi, (C) Fe, (D) O, (E) Au elements, and (F) EDS pattern.

of Au1.2-BFO NFs. It can be seen that the morphology is still the nanofibous structure. However, the surface of the nanofiber becomes rougher and the composed nanoparticles appear to be more prominent after depositing the Au NPs on the surface of BFO NFs. Furthermore, new nanoflake-like structures are formed and embedded in the BFO NFs. In order to confirm the structure of nanoflakes, the HRTEM is used, and the result of which is shown in **Figure 4E**. The interplanar spacing of 0.397 nm is ascribed to the (012) lattice plane of the rhombohedral perovskite BFO structure, further implying that the new formed nanoflake-like structure is indeed BFO phase. Therefore, homojunctions will be formed between BFO NFs

and BFO nanoflakes. From **Figure 4B**, there is no obvious Au NPs deposited on the BFO surface. In order to clearly observe the deposited Au NPs, the magnified TEM image is shown in **Figure 4C**. It can be seen that Au NPs with the size of about 5 nm are indeed deposited uniformly on the BFO surface. The according HRTEM image is shown in **Figure 4F**. The interplanar spacing of 0.236 nm matches well with the (111) lattice plane of the cubic Au structure (Wang P. et al., 2017).

The possible formation mechanism of the Au NPs deposited hierarchical nanofibers/nanoflakes structured BFO homojunction is proposed in **Figure 5**. Firstly, the HAuCl4 is decomposed to AuCl3 and HCl in the aqueous solution. BiFeO3, being regarded as the product of a combination of Bi2O3 and Fe2O3, is soluble in the weak acidic environment (Sakar et al.,

FIGURE 7 | *M–H* hysteresis loops of Au*x*-BFO NFs (*x* = 0, 0.6, 1.2, 1.8, 2.4 wt%) measured at room-temperature; the inset shows the partially magnified curves.

2013). Therefore, the generated HCl can decompose the BiFeO3 to generate Bi3+ and Fe3+ ions in the solution (**Figure 5A**). In particular, the decomposition reaction prefers to occur at the defects on the surface of BFO NFs where the higher surface energy have. The defects may be generated during the oxidation decomposition of organic material (PVP) under the high calcination temperature. Secondly, the citrate present in the solution can act as a chelating agent to react with Bi3+, Fe3+, and Au3+ ions to form stable metal-chelate complexes (**Figure 5B**). In addition, due to the presence of defects, the citrate can anchor on the surface of BFO NFs (Mudunkotuwa and Grassian, 2010). Finally, the uniform Au NPs and nanoflake-like BFO structure are formed on the surface of BFO NFs under the effect of the citric acid in the analogous hydrothermal environment (**Figure 5C**). With the increase of the HAuCl4 content (the increase of the loading amount of Au NPs), more Bi3+ and Fe3+ ions pass from the defects on the surface of BFO NFs to the solution, and more nanoflake-like BFO structure are formed.

### XPS Analysis

The chemical state and elemental composition of Au1.2-BFO NFs are analyzed by XPS technology. According to the overall XPS survey (**Figure 6A**), only Bi, Fe, O, Au, and C elements can be detected in the sample. The peak of C 1s located at 284.5 eV is used for calibration (Dou et al., 2015; Chen et al., 2016). As shown in **Figure 6B**, the two peaks centered at around 158.11 eV and 163.42 eV can be assigned to the binding energy of Bi 4f7/2 and Bi 4f5/2, respectively. It is confirmed that the bismuth species in Au1.2-BFO NFs is the oxidation state of +3. From the high resolution XPS spectra of Fe 2p in **Figure 6C**, the three peaks located at 710.79, 717.12, and 723.20 eV can be ascribed to the Fe 2p3/2, satellite, and Fe 2p1/2 peaks, respectively. In addition, the Fe 2p3/2 peak can be fitted into two peaks located at 711.05 and 709.11 eV, which are attributed to Fe3+ and Fe2+ species, respectively (Wu et al., 2016). The presence of Fe2+ species may come from the reduction of Fe3+ species by citric acid under high temperature environment. In order to meet

the requirement of charge equilibrium, oxygen vacancy should be formed accompanied by the Fe2+ species. The O 1s curve in **Figure 6D** can be splitted into two peaks with Lorentzian-Gaussian function. The peaks at 531.51 and 528.96 eV can be ascribed to the surface adsorbed oxygen (Oads) and lattice oxygen (Olatt) species, respectively (Bharathkumar et al., 2016; Chen et al., 2016). The Oads species may originate from the –OH groups bonded to the metal cations in the oxygen defcient region (Guo et al., 2016; Sun et al., 2017), which can be converted to oxygenrelated free radicals to participate in the photocatalytic reaction. Moreover, the more Oads species usually mean the more oxygen vacancies it possesses, and also the better photocatalytic activity. From **Figure 6E**, the binding energies of 83.69 eV and 87.40 eV are ascribed to the Au 4f7/2 and Au 4f5/2 peaks, respectively, indicating that the gold is present in the form of metallic gold (Au0 ) in the Au1.2-BFO NFs. This can be ascribed to the reduction by citric acid under high temperature environment. Therefore, the metallic gold have been successfully deposited on the surface of BFO NFs. Whereas, the binding energies of Au 4f in the Au1.2-BFO NFs are 0.3 eV below the standard binding energy of a metallic gold foil (Jovic et al., 2013), indicating the existence of electron transfer from BFO NFs to Au NPs for the different Fermi levels (Li S. et al., 2013). In addition, the loading amount of Au NPs is measured to be 1.14%, almost equal to the theoretical value of 1.2%.

#### Ferromagnetism Analysis

In order to investigate the effect of Au on the magnetic property of Aux-BFO NFs, the room-temperature magnetic hysteresis (M-H) loops are shown in **Figure 7**. It can be seen that all BFO NFs exhibit the weak-ferromagnetic (FM) characters, which is due to the suppressed cycloidal spin structure and the coexistence of Fe3+ and Fe2+, thereby resulting in the FM spins in the system (Xu et al., 2014; Modak et al., 2016). The pure BFO NFs show the saturation magnetization (Ms) value of 0.32 emu/g. When Au NPs are deposited on the surface of BFO NFs, the Ms value is enhanced with the increase in the loading amount of Au NPs. The Ms values are 1.07, 1.19, 1.22, and 1.27 emu/g for BFO NFs with the loading amount of Au NPs of x = 0.6, 1.2, 1.8, and 2.4 wt%, respectively. The gradually enhanced magnetic properties of Aux-BFO NFs could be attributed to the new formed nanoflake-like structures, where the confined dimensions (15 nm) facilitate the enhancement of FM properties (Chauhan et al., 2012). In addition, the rather large magnetic moment of Au NPs explains the enhanced magnetic properties with increase in loading amount of Au NPs (Nealon et al., 2012). From the inset of **Figure 7**, an obvious off-center displacement in magnetic field axis is shown in the M–H curves. This can be ascribed to the exchange bias (EB) effect of antiferromagnet (AFM) - FM ordering in BFO NFs, where the AFM-core/FM-shelllike structure can be formed in the well-defined 1D structure (Sakar et al., 2016). Compared to that of the pure BFO NFs, the off-center displacement of Aux-BFO NFs is relatively less pronounced, which is ascribed to the enhanced FM spins caused by the emergence of nanoflake-like structures.

## Optical Properties

**Figure 8** shows the UV-vis DRS spectra of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) samples. It can be seen that the pure BFO NFs exhibit excellent absorption in both UV and visible light region. The absorption edge located at 575 nm is due to the bandgap transition. When the Au NPs are deposited on the surface of BFO NFs, the optical properties are obviously enhanced. In addition, the Aux-BFO NFs exhibit a weak absorption peak in the range of 600∼725 nm, which are attributed to the SPR effect of Au NPs (Villa et al., 2016; Wang P. et al., 2017). The enhanced absorption in visible light is in favor of boosting the photocatalytic activity. Moreover, the band gap energies can be calculated from the plots of the (αhν) 2 vs. the photon energy (hν) (Xu et al., 2015; Singh et al., 2018), the

results of which are shown in the inset of **Figure 8**. The band gap energies are calculated to be 2.11, 2.08, 2.02, 2.01, and 1.97 eV for the x = 0, 0.6, 1.2, 1.8, 2.4 wt%, respectively. It is obvious that the band gap energies decrease slightly with the increase in the loading amount of Au NPs. The possible reason is that the deposited Au NPs can produce some lower energy levels due to the formed Schottky junction at the interface between Au NPs and BFO NFs (Papadas et al., 2015).

# PL Analysis

Photoluminescence (PL) spectra are extensively used to study the transfer and separation of photogenerated electron-hole pairs in the semiconductor material. The peak intensity can reflect the capture, migration and transfer efficiency of photogenerated electron-hole pairs. The lower PL intensity usually means the lower recombination rate of photogenerated electron-hole pairs, and also the higher photocatalytic activity (Tang et al., 2018). It can be seen from **Figure 9** that the PL intensity decreases obviously once the Au NPs are loaded on the surface of BFO NFs. Significantly, the PL intensity of Au1.2-BFO NFs is the lowest. This indicates that the deposition of Au NPs on the surface of BFO NFs can efficiently inhibit the recombination of photogenerated charge carriers, which may be attributed to that the Schottky junction formed between Au NPs and BFO NFs can efficiently promote electron transfer from BFO NFs to Au NPs, and thus decreased the recombination rate of photogenerated charge carriers (Li S. et al., 2013). However, the PL intensity increases when the loading amount of Au NPs is too much. This can be ascribed to the fact that the excess amount of Au NPs can result in an increase of surface defects, which can act as recombination centers for photogenerated charge carriers, thus leading to the increase in the recombination rate. Therefore, it is necessary to deposit an appropriate amount of Au NPs on the surface of BFO NFs to promote the efficient separation of photogenerated electron-hole pairs, thus enhancing the photocatalytic activity.

#### Photocatalytic Activities

**Figure 10A** shows the photocatalytic degradation efficiencies of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) on the degradation of MB dye under simulated solar light irradiation. For contrast, the blank experiment without any photocatalyst is carried out under the same conditions. The result shows that the MB dye is hardly degraded under simulated solar light irradiation. The degradation efficiency is only 2.77% after irradiation of 3 h. When the pure BFO sample is added to the reaction system, the degradation efficiency reaches to be 49.49% after irradiation of 3 h. However, the photocatalytic activity is still relatively poor. Once Au NPs are deposited on the surface of BFO NFs, the photocatalytic activities of Aux-BFO NFs are obviously enhanced. The enhanced activities can be ascribed to the hierarchical nanofibers/nanoflakes structured homojunction, the SPR effect of Au NPs, as well as the presence of defects (Fe2+/Fe3+ pairs and oxygen vacancy). In addition, the photocatalytic activity is increased and then suppressed with the increase in the loading amount of Au NPs. When the loading amount of Au NPs is 1.2%, the photocatalytic activity is the best with the photocatalytic efficiency of 85.76% after irradiation of 3 h.

To evaluate the reaction kinetics of the degradation of MB, the plot of ln(C0/C) vs. time is plotted in **Figure 10B**, which shows the kinetics of the degradation process by the five photocatalysts which follow Langmuir-Hinshelwood pseudo first-order kineticsequation, ln(C0/C) = kt, where t is the irradiation time and k is the rate constant (Niu et al., 2015). The observed calculated rate constants k in the presence of blank sample, Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) are 8.28 × 10−3 , 200.23 × 10−3 , 411.46 × 10−3 , 623.27 × 10−3 , 552.85 × 10−3 , and 472.69 × 10−3 h −1 , respectively. which also indicates that the Au1.2-BFO NFs sample exhibits the strongest photocatalytic activity among the Aux-BFO NFs (x = 0, 0.6, 1.8, 2.4 wt%) samples.

In order to evaluate the stability of photocatalysts, the recycle experiment is carried out. After each recycle, the photocatalysts

are collected by simple filtration and washed with deionized water and absolute ethanol. As shown in **Figure 11**, there is no obvious loss in photocatalytic activity after five recycle test, indicating that Aux-BFO NFs photocatalysts have good stability during the photocatalytic degradation of MB dye under simulated solar light irradiation.

In order to explore the possible reaction pathways during the photocatalytic degradation of MB dye under simulated solar light irradiation, the trapping experiments are used to determine the main active species in this reaction system. Ethylenediaminetetraacetic acid (EDTA), tertbutyl alcohol (TBA) and benzoquinone (BQ) are used as scavengers for holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O − 2 ), respectively (Chen et al., 2016; Qiao et al., 2017; Ji et al., 2018). As can be seen from **Figure 12**, when the TBA or BQ is added, a slight reduction in MB degradation efficiency is occurred in comparison to the initial activity without addition of any scavengers, which means that •O − 2 and •OH radicals are not the main active species. However, the photocatalytic activity is severely suppressed when EDTA is added to the reaction system, implying that photogenerated holes are the main active species and play the decisive role in the photocatalytic activity.

## Photocatalytic Mechanism

On the basis of the aforementioned experimental and active species trapping results, the possible enhanced photocatalytic mechanism is illustrated in **Figure 13**. It can be ascribed to the following aspects: (1) Nanofibers/nanoflakes structured homojunction. It is well-known that nanoflakes with ultrathin structure possess higher valence band (VB) energy than that of nanofibers with larger particle size (Weng et al., 2014). After forming homojunctions between BFO NFs and BFO nanoflakes, the staggered band potentials would introduce a new internal electrical field to provide a driving force to reduce the charge transfer barrier. The formed homojunction can promote the transfer of photogenerated holes (h+) from the VB of BFO NFs to the VB of BFO nanoflakes, whereas photogenerated electrons (e−) from the conduction band (CB) of BFO nanoflakes to that of BFO NFs. (2) SPR effect of Au NPs. The enhanced local electric field in the near-surface region of BFO induced by the SPR effect of Au NPs results in the further separation of photogenerated electrons and holes. The photogenerated electrons are thermodynamically favorable to transfer from the CB of BFO to Au NPs due to the Fermi energy of Au NPs is more positive than the CB potential of BFO (Li S. et al., 2013). The formed Schottky junction at the interfaces can act as trapping centers for photogenerated electrons, contributing the accumulation of photogenerated electrons at the surface of Au NPs and preventing the recombination of photogenerated electron-hole pairs. (3) Defects. The presence of Fe2+/Fe3+ pairs located below the CB of BFO can act as electron/hole trapping centers and thus reduce the recombination rate (Zhang et al., 2014; Verma and Kotnala, 2016). In addition, the oxygen vacancy can introduce the defect state energy level and effectively regulate the band structure, thus significantly improve the separation of photogenerated charge carriers (Zhang C. et al., 2016). Under the effect of these aspects, the photocatalytic activities are remarkably enhanced.

During the process of photocatalytic reaction, a few photogenerated electrons in the CB of BFO nanoflakes can

Li et al. Au-Induced BiFeO3 Homojunction Photocatalyst

react with oxygen molecules adsorbed on the surface to form superoxide radicals (•O − 2 ). The photogenerated electrons transferred to the Au NPs can react with Fe2+/Fe3+ pairs and oxygen vacancy defects. Meanwhile, a little part of photogenerated holes in the VB of BFO NFs can oxidize the absorbed OH− and H2O to hydroxyl radicals (•OH) (Dong et al., 2016; Hu et al., 2017). Finally, the photogenerated holes transferred the VB of BFO nanoflakes as well as the formed •O − 2 and •OH species can oxidation decomposition of MB dye molecules.

# CONCLUSIONS

Novel Au-induced hierarchical nanofibers/nanoflakes structured BFO homojunction (Aux-BFO, x=0, 0.6, 1.2, 1.8, 2.4 wt%) were in situ synthesized by a simple reduction method under the analogous hydrothermal environment. The deposited Au NPs were distributed uniformly on the surface of BFO sample. The formed BFO nanoflakes were vertically assembled on BFO nanofibers to form a hierarchical architecture. The photocatalytic results showed that Au1.2-BFO NFs samples exhibited the best photocatalytic activity with the photocatalytic efficiency of 85.76% after irradiation of 3 h. The remarkable enhanced photocatalytic activity could be

# REFERENCES

mainly attributed to the hierarchical nanofibers/nanoflakes structured homojunction, the SPR effect of Au NPs, as well as the presence of defects (Fe2+/Fe3+ pairs and oxygen vacancy). This work provided a novel method to design efficient homojunctions with multiple functions in solar energy utilization.

## AUTHOR CONTRIBUTIONS

YL synthesized Aux-BFO NFs samples and analyzed part of characterizations. JL is the supervisor of this research work. LC and HS helped writing. HZ analyzed Ferromagnetism measurements. HG and LF performed SEM, EDS, and TEM imaging and analyses. All authors have contributed to preparing the review article.

#### FUNDING

This work was supported by the National Natural Science Foundation of China (Grant No. 51702189), the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2017BEM033), and Science & Technology Plan of the Education Department of Shandong Province (grant No. J18KA002).

BiFeO3 nanowires under visible-light. Chem. Commun. 49, 5856–5858. doi: 10.1039/c3cc40363g

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Li, Li, Chen, Sun, Zhang, Guo and Feng. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Titanium Phosphate Nanoplates Modified With AgBr@Ag Nanoparticles: A Novel Heterostructured Photocatalyst With Significantly Enhanced Visible Light Responsive Activity

#### Manli Ren, Jiaqiu Bao, Peifang Wang, Chao Wang and Yanhui Ao\*

*Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Guohong Wang, Hubei Normal University, China Qizhao Wang, Northwest Normal University, China*

#### \*Correspondence:

*Yanhui Ao<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *17 July 2018* Accepted: *25 September 2018* Published: *17 October 2018*

#### Citation:

*Ren M, Bao J, Wang P, Wang C and Ao Y (2018) Titanium Phosphate Nanoplates Modified With AgBr@Ag Nanoparticles: A Novel Heterostructured Photocatalyst With Significantly Enhanced Visible Light Responsive Activity. Front. Chem. 6:489. doi: 10.3389/fchem.2018.00489* AgBr@Ag modified titanium phosphate composites were fabricated through a two-step approach. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The optical properties of the composites were characterized by using UV-vis diffuse reflectance spectroscopy. The photocatalytic activities of the composites were investigated on the degradation of Rhodamine B and ciprofloxacin under visible light irradiation. AgBr@Ag/titanium phosphate was determined to exhibit considerably higher photocatalytic activity than the corresponding individual components. The mechanism on the enhancement of the photocatalytic activity was proposed based on the results of photoluminescence spectra and photocurrent measurements. Furthermore, the possible photocatalytic mechanisms of organic compounds degradation were also proposed.

 Therefore, it is urgent to solve this severe problem via green technology (Liu et al., 2012; Hao et al., 2016; Zou et al., 2016; Chen et al., 2018; Feng et al., 2018; Guo et al., 2018). Currently, photocatalysis technology has recieved intense attention as an efficient technology for solving the problem of environment (Fan et al., 2013; Ao et al., 2016; Wang et al., 2016; Dong et al., 2017; Hao et al., 2017). Furthermore, it is well known that composite photocatalysts always show higher activity than the corresponding single component (Ma et al., 2018; Wang et al., 2018; Zhang et al., 2018). The improved photocatalytic activity is due to the different electronic energy levels of different components, which create an internal electric field that can accelerate the separation rate of photogenerated electrons and holes, and reduce their recombination rate (Guo et al., 2017).

Recently, titanium phosphate has been widely studied for the treatment of organic compounds. Titanium phosphate [α-Ti(HPO4)2•H2O, hereafter TP] consists of layers of titanium atoms bonding on both plane sides of the monohydrogen phosphate groups. Water molecules located in the interlayer region to form a network of hydrogen-bonding with phosphate groups (Ekambaram et al., 2000; Guo et al., 2013; Guo and Han, 2014; Zhu et al., 2014). However, titanium phosphate can only absorb ultraviolet light (<5% of the sunlight). Furthermore, the high recombination rate of photoinduced electron-hole pairs is still exist, which would lead to lower

photocatalytic activity. Thus, it is urgent to find effective ways to improve the activity of the TP.

Recently, silver halide materials have received intense attention due to their excellent visible light responsive photocatalytic activity. Silver halide@silver nanoparticles (AgNPs) based heterojunctions have been found to be excellent photocatalysts (Chen et al., 2015; Bai et al., 2016; Li et al., 2018b; Xuan et al., 2018). It has been proved that plasmon AgNPs on the surface of AgX can enhance the light absorption of the composites and accelerate the separation rate of the photogenerated charges (Daupor and Wongnawa, 2014; Li et al., 2015a,b; Ding et al., 2018; Xiao et al., 2018). Moreover, it was also considered that AgNPs decrease the recombination rate of Ag+ with photogenerated electrons, and allow the formation of Br0 species (photogenerated holes react with Br− ions) that can degrade the organic compounds (Xia and Halas, 2005; Jiang et al., 2014; Zhang et al., 2015a). Therefore, it would be a good way to obtain higher visible light activated activity through the conbination of AgBr@Ag and TP.

In this work, we prepared visible light active AgBr@Ag/TP photocatalysts by a simple and feasible method. The morphology, crystal structure, optical properties, and composition of the photocatalysts were investigated. Moreover, the possible photocatalytic degradation mechanism of ciprofloxacin and the enhancement mechanism photocatalytic activity were proposed.

## EXPERIMENTAL

All experimental details are shown in **Supporting Information**.

Ag@AgBr/TP-3.

# RESULTS AND DISCUSSION

#### Characterization

The phase structure was characterized by XRD, and the obtained patterns are shown in **Figure 1**. The peaks at about 11.6, 21.0, 26.0, 35.4, and 35.8◦ are indexed to the (002), (200), (202), (006), and (020) directions, respectively. All peaks can be ascribed to TP (JCPDS Card No: 83-0109; Ortíz-Oliveros et al., 2014; Yada et al., 2014). Diffraction peaks are also detected at 26.73, 30.96, 44.35, 55.04, 64.49, and 73.26◦ , which can be indexed to the (111), (200), (220), (222), (400), and (420) directions of AgBr (JCPDS Card NO: 06-0438; Chen et al., 2015; Bai et al., 2016). Furthermore, with the increasing of AgBr mass, the intensity of the characteristic peaks of AgBr increased gradually. No diffraction peaks of Ag can be observed, possibly due to its low mass, high dispersion or small size (Wang et al., 2013, 2014).

The morphologies of the composites were investigated by TEM and SEM. The obtained SEM image of pure TP is displayed in **Figure 2a**. It can be seen that the TP sample is composed

of uniform plates with sub-micrometer width and 30∼40 nm thickness. The SEM image of AgBr@Ag/TP-3 taken from **Figure 2b** displays smaller AgBr@Ag nanoparticles decorating

FIGURE 5 | Photocatalytic degradation of CIP over the as-prepared samples under visible light irradiation.

the TP surface. The TEM image of pure TP is displayed in **Figure 2c**. It can also be seen that the titanium phosphate plate is hexagonal, which is in good agreement with the results of SEM. The TEM image of AgBr@Ag/TP is shown in **Figure 2d**. It can be seen that AgBr nanoparticles decorated the TP surface, indicating the formation of heterojunctions in the prepared samples (Cui et al., 2018). As shown in the inset of **Figure 2d**, the measured lattice spacings of 0.289 nm corresponds to the (200) lattice planes of AgBr.

The elemental composition and chemical states of the elements in the composites were analyzed by XPS. The peaks of Br 3d, P 2p, Ag 3d, C 1s, Ti 2p, and O1s can be observed in **Figures 3A,B** shows the electron binding energies of Ag 3d5/2 and Ag 3d3/2 orbitals, which can be further divided into 366.6/367.4 and 372.6/373.4 eV, respectively. The peaks at 366.6 and 372.6 eV indicate the presence of Ag+. Both the peak separation (6.0 eV) and peak positions (367.4 and 373.4 eV) indicate the presence of metallic form (Ag0 ; Yang et al., 2015; Tang et al., 2017a,b). As shown in **Figure 3C**, two peaks at 68.0 and 67.0 eV corresponding to the binding energies of Br 3d3/2 and Br 3d5/2 can be found (Wang et al., 2009). All above results prove the presence of AgBr and Ag in the composite. In **Figure 3D**, the Ti peaks observed at 459.0 and

464.8 eV can be attributed to Ti 2p3/2 and Ti 2p1/2. The peak separation of 5.8 eV for binding energy between the two peaks indicates that Ti was mainly Ti4+ in the composite (Yada et al., 2014).

The optical properties of photocatalysts significantly affect their photocatalytic performance (He et al., 2015). Therefore, optical properties of the composites were investigated by DRS and the obtained results are shown in **Figure 4**. Pure TP exhibited no absorption in the visible-light region, whereas the Ag@AgBr/TP composites had better absorption performance, possibly due to the deposition of AgBr and formation of AgNPs on the surface of the composites. Moreover, Ag@AgBr/TP-3 showed the highest absorption in visible light region, indicating that high absorption capability is ascribed to an optimal loading. In other words, when the loading of AgBr@Ag exceeds a critical value, the AgBr@Ag nanoparticles would agglomerate and decrease the absorption capability.

#### Photocatalytic Activity

To investigate the photocatalytic performance of the composites, we conducted experiments on the degradation of colorless organic pollutant (CIP) under visible light irradiation. As shown in **Figure 5**, AgBr@Ag/TP-3 displayed the highest photocatalytic activity for CIP degradation among all samples. Approximately 51.4, 44.2, 61.3, 71.5, and 65.4% of CIP was degraded in 180 min by AgBr@Ag, AgBr@Ag/TP-1, AgBr@Ag/TP-2, AgBr@Ag/TP-3, and AgBr@Ag/TP-4, respectively. Furthermore, the TP showed a low photocatalytic efficiency for CIP degradation. The degradation curve of CIP by TP exhibits a fluctuating trend. There may be some CIP molecules desorbed from the surface of TP. These results show that the Ag@AgBr/TP photocatalysts are generally effective for the degradation of colorless organic compounds.

#### Intermediates Analysis

Prior to the complete mineralization of CIP, there are many degradation intermediates in the photocatalytic process. The sample solution was detected by HPLC/MS during the photocatalytic degradation. Some intermediates in the photocatalytic process were identified: A1 (m/z = 362.2), A2 (m/z = 334.2), A3 (m/z = 306.2), A4 (m/z = 291.2), and A5 (m/z = 263.2). These results show that A1∼A5 are the main intermediates during the degradation process of CIP. As observed in many other oxidation system (Sturini et al., 2012), desethylene ciprofloxacin (A3) was dominant intermediate of CIP, and was formed by a net loss of C2H2 at the piperazinyl substituent of CIP. Prior to the formation of desethylene ciprofloxacin, two intermediates were detected. The hydroxyl keto-derivative of the piperazinyl substituent (A1) was detected to be the first oxidized intermediate on the way to the formation of A3. Apparently, the subsequent loss of CO from A1 results in the cleavage of the piperazinyl ring to form keto-derivatives A2 and A2', in which a carbonyl group remains on the N atom of aniline and alkylamine, respectively. A second loss of a CO molecule from A2 and A2' leads to the formation of desethylene ciprofloxacin (A3). The further oxidation of desethylene ciprofloxacin (A3) generated A4 with a keto group through the loss of a N atom. The decarbonylation of A4 forms aniline (A5), completely destroying the piperazinyl substituent of CIP. The probable pathways for CIP degradation are shown in **Figure 6**.

## Possible Photocatalytic Mechanism

Several experiments were performed to further understand the mechanism on the enhancement of photocatalytic performance for the AgBr@Ag/TP composites. The photoluminescence (PL) spectrum was measured to investigate the recombination rate of the photogenerated electron-hole pairs. The photoluminescence spectra of the prepared samples are shown in **Figure 7**. The intensity of PL is directly proportional to the electron-hole

irradiation.

recombination rate, and lower PL intensity means lower recombination rate (Zhang et al., 2015b; Shi et al., 2016). The main emission peak of pure TP centers at ∼470 nm, which was ascribed to the band gap recombination of electronhole pairs. The PL intensity decreases significantly after the deposition of AgBr@Ag. The decreasing emission intensity of the AgBr@Ag/TP suggests that the recombination of electronhole pairs can be suppressed after the deposition of AgBr@Ag (Xu et al., 2013). Furthermore, it can be seen that the PL intensity increase as the amount of Ag@AgBr increase to a certain value, and AgBr@Ag/TP-3 composite showed the lowest PL intensity, which means Ag@AgBr/TP-3 composite has the lowest recombination rate of photogenerated electrons-hole pairs. Therefore, it would lead to the highest photocatalytic performance. However, the PL intensity increase as the amount of Ag@AgBr increase further. It can be ascribed to the reason that superfluous Ag@AgBr nanoparticles would aggregate thus cannot form effective heterojunctions with TP. The above results demonstrate that the deposition of AgBr on TP and the formation of AgNPs on the surface of the composite can decrease the recombination efficiency and enhance the separation efficiency of the photogenerated electrons and holes.

It is well known that the efficiency of the electron-hole pairs separation significantly affects the photocatalytic performance (Jiang et al., 2011). The electrons in the valence band of photocatalysts can be activated and then transfer to the conduction band, leading to the formation of photocurrent. A higher photocurrent means higher electron-hole pairs separation rate, which induces to better photocatalytic performance (Xiang et al., 2011). **Figure 8** shows the photocurrents for different samples under 20 s intermittent visible-light irradiation with a bias of 0.3 V. The photocurrents of AgBr@Ag/TP composites are all much higher than that of TP. Moreover, AgBr@Ag/TP-3 exhibits the highest value. The results are in good agreement with the photocatalytic activity, indicating that the efficient separation of photo-induced electrons and holes results in the high photocatalytic activity.

It was demonstrated that the separation efficiency of photogenerated electrons and holes is important to the photocatalytic performance. Furthermore, the conduction band (CB) and valence band (VB) potential have a significantly effect on the separation of the photogenerated electron-hole pairs. The CB and VB potential of AgBr and TP can be obtained using Equations. (1) and (2) (Zhang et al., 2008):

$$\rm E\_{vb} = X - E\_0 + 0.5E\_{\rm g} \tag{1}$$

$$\mathbf{E\_{cb}} = \mathbf{E\_{vb}} - \mathbf{E\_{\mathcal{g}}} \tag{2}$$

**w**here Evb is the potential of VB, X is the absolute electronegativity of photocatalyst (X is 6.85 eV and 5.81 eV of TP and AgBr, respectively), Eg is the band gap energy and E0 is the energy of free electrons (∼4.5 eV). Based on these equations, the CB potential (Ecb) and VB potential (Evb) of TP are calculated as 0.43 and 4.28 eV, while Ecb and Evb of AgBr are calculated as 0.01 eV and 2.62 eV, respectively.

Therefore, a possible mechanism on the photocatalytic degradation of RhB by AgBr@Ag/TP under the irradiation of visible light is illustrated in **Figure 9**. The plasmon induced holes can react with Br− and form active Br0 atoms. The Br0 atoms exhibit an excellent oxidation capability to oxidize organic compounds. Meanwhile, plasmon induced electrons are formed on the surface of AgNPs, part of these electrons are trapped by O2 to produce •O − 2 , mainly contributing to the organic compounds removal (Ye et al., 2012; Zhang and Yates, 2012; Cheng et al., 2015; Ding et al., 2018). Besides, the other electrons would transferto the conduction band of AgBr, and then further transfer from AgBr to the conduction band of TP. Meanwhile, the h+

stored in valence band of AgBr and TP was also involved in the organic compounds photocatalytic degradation, though they are not the dominant active species. The relevant equations are shown as following:

$$\text{Ag} + \text{visible} - \text{light} \rightarrow \text{e}^- + \text{h}^+ \tag{3}$$

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textbullet O}\_2^- \tag{4}$$

$$\text{Br}^+ + \text{Br}^- \rightarrow \text{Br}^0 \tag{5}$$

$$\text{CIP} + \bullet \text{O}\_2^- \text{(orBr}^0) \rightarrow \text{degradation products} \tag{6}$$

Based on above results, the possible mechanism on the enhancement of photocatalytic activity for AgBr@Ag/TP composites can be proposed. AgBr@Ag nanoparticles deposited on TP improve the light absorption capability, as confirmed by UV-vis DRS studies. Furthermore, the heterojunctions between AgNPs, AgBr, and TP are conducive to improving the transfer rate and separation efficiency of the electron-hole pairs, and hindering the recombination of the pairs, thus inducing to the enhancement of the photocatalytic activity.

#### CONCLUSIONS

AgBr@Ag/TP composites exhibited higher photocatalytic performance than pure TP and AgBr@Ag, and can degrade **a**

#### REFERENCES

**typical antibiotic (CIP)** under the irradiation of visible light. The excellent photocatalytic activity can be ascribed to their enhanced visible-light absorption, low recombination and high transferring rate of the photo-generated charges, as shown by DRS, PL, and photocurrent experiments.

#### AUTHOR CONTRIBUTIONS

YA and CW designed the project and guided the work. MR and JB conducted the experiments and characterizations of the samples. PW revised and polished the manuscript.

#### ACKNOWLEDGMENTS

We are grateful for grants from National Science Funds for Creative Research Groups of China (No. 51421006), Natural Science Foundation of China (51679063), the Fundamental Research Funds for The Central Universities (2018B14514), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Key Plan for Research and Development of China (2016YFC0502203), and PAPD.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00489/full#supplementary-material

synergistic effect of improved charge separation efficiency and redox ability. Appl. Catal. B-Environ. 234, 90–99. doi: 10.1016/j.apcatb.2018.04.036

the remediation of organic pollutants. Chin. J. Catal. 38, 337–347. doi: 10.1016/S1872-2067(16)62570-6

visible light irradiation: a direct hole oxidation pathway. Chem. Eng. J. 274, 290–297. doi: 10.1016/j.cej.2015.03.077

into useful hydrocarbons through coupling photodegradation of MB with photoreduction of CO2. ACS Catal. 6, 6861–6867. doi: 10.1021/acscatal.6b 01729

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Ren, Bao, Wang, Wang and Ao. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Photocatalytic Activation of Saturated C–H Bond Over the CdS Mixed-Phase Under Visible Light Irradiation

#### Houde She1 , Liangshan Li 1 , Hua Zhou1 , Lei Wang1 , Jingwei Huang1 and Qizhao Wang1,2 \*

*1 College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, China, 2 Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Lanzhou, China*

Selective activation of saturated C–H bond in hydrocarbons to produce high-value-added chemicals is of great significance for chemical synthesis and transformation. Herein, we present a facile procedure to achieve Ni-doped CdS nanoparticles with mixed (cubic and hexagonal) phases, as well as its application to the photocatalytic activation of saturated primary C–H bond of toluene and its derivatives. The photocatalytic oxidation rate of toluene into benzaldehyde of formation reached up to 216.7 µmolh−1g −1 under visible light irradiation. The excellent photocatalytic performance of Ni(II)-doped CdS [Ni(II)/CdS] can be attributed to its unique structural assembly with cubic and hexagonal phases and also the addition of Ni ions, together taking effect in promoting the separation of photogenerated charge carriers. The possible reaction mechanism for the photocatalytic selective oxidation is illustrated in this work. The band width of the as-prepared mixed phase CdS is reduced, which can effectively expand the response range and improve photocatalytic performance.

Keywords: toluene, benzaldehyde, ion modify, mixed-phase, C–H bond, selective oxidation

# INTRODUCTION

The highly selective oxidation of inactive C–H bond in hydrocarbons to form high value-added oxygenated products under mild conditions is one of the major challenges in the industrial field (Pal et al., 2014; Yang et al., 2017). For example, toluene is a simple aromatic compound and can be selectively oxidized into benzaldehyde, benzyl alcohol, benzoic acid and benzyl benzoate, all of which are crucial intermediate for the manufacture of fine chemicals, such as pharmaceuticals, dyes, preservatives and perfume. At present, benzaldehyde is primarily synthesized via traditional route of chlorination hydrolysis of toluene. The discharges of toxic gases and organic waste bring on serious environmental concerns. Therefore, it is essential to develop an environment-friendly technique to address such problem. Photocatalysis is a promising alternative strategy that has been widely applied in the activation of C–H bond under mild conditions (Zhang et al., 2012; Hao et al., 2018) owning to its clean and low energy consumption, etc. For example, Yuan et al. reported that BiOBr/TiO2 can be used in oxidation of toluene into benzaldehyde under visible light irradiation (Yuan et al., 2013). He's group (He et al., 2018) presented a toluene selective oxidation that has been carried out by Cd3(C3N3S3)2/CdS porous composites under visible light irradiation. Cd3(C3N3S3)2/CdS exhibits excellent performance for the transformation of toluene into benzaldehyde.

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Zhu Gangqiang, Shaanxi Normal University, China Guidong Yang, Xi'an Jiaotong University, China Xiaole Weng, Zhejiang University, China*

#### \*Correspondence:

*Qizhao Wang<EMAIL_ADDRESS><EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *23 August 2018* Accepted: *14 September 2018* Published: *09 October 2018*

#### Citation:

*She H, Li L, Zhou H, Wang L, Huang J and Wang Q (2018) Photocatalytic Activation of Saturated C–H Bond Over the CdS Mixed-Phase Under Visible Light Irradiation. Front. Chem. 6:466. doi: 10.3389/fchem.2018.00466*

It is well-known that the CdS is economical and has been extensively employed in photocatalysis research because of its adequate band gap (2.45 eV), which is unique among photocatalytic materials (Wang et al., 2014; Cheng and Xiang, 2016; Xiang et al., 2016). Although CdS-based photocatalysts are widely studied in the field of photocatalysis (Yan et al., 2009; He et al., 2016b; Li et al., 2018) some inherent defects like high charge carrier recombination rate and photo-corrosion, debase their photocatalytic activity and practical application (He et al., 2016a). To hold back electrons-holes recombination and improve photocatalytic activity, various strategies, such as the loading of co-catalysts, forming heterojunctions have been scrupulously developed (Wang et al., 2013; Wei et al., 2017, 2018). In the past decade, Pt (Luo et al., 2015b), Au (Majeed et al., 2016; Wang et al., 2018b), Pd (Luo et al., 2015a), Ag (Gupta et al., 2017), and Ru (Nosheen et al., 2017) have been used as co-catalysts in various photocatalyst systems to enhance charge separation and surface reactions. Despite the high photocatalytic performance achieved by these noble metal-based co-catalysts contained catalyst, their use in mass production are limited due to the scarcity and high cost of them. Therefore, it is desirable to investigate new catalyst system with non-noble metal as co-catalyst, which can meet the general industrial requirements, such as low cost, high efficiency, and good durability. In this regard, many non-precious metals [such as Fe (Yan et al., 2017; Deka and Kalita, 2018), Cu (Kumar et al., 2018; Wang et al., 2018a), Co (Dong et al., 2017), Ni (Yang et al., 2018), Mo (Zhang et al., 2017)] materials have been studied as a co-catalyst to improve performance of main catalyst. In particular, nickel-based materials [such as metallic nickel, nickel ion (Song et al., 2017), nickel oxides and hydroxides (Zhang et al., 2015)] exhibit excellent activity and attract much attention as promoters for enhancing photocatalytic activity. Murugesan et al. (Murugesan et al., 2017) prepared Ba and Ni doped CdS by a spray pyrolysis method. Doping Ba and Ni can significantly improve optical performance of CdS. Besides, it has also been suggested that Ni doped CdS was prepared via an impregnation method (Yu et al., 2016), which doped samples can efficiently promotes separation of holes and electrons and consequently enhances the photocatalytic hydrogen production.

However, in the above works, only modified single phase CdS can enhance the photocatalytic activity. But, as reported by some researchers, the CdS base can also form a homojunction for enhancing its photocatalytic activity (Dai et al., 2014). The mixed-phase CdS has been reported by Zhao's group (Zhao et al., 2018), which demonstrated that the photoresponse range can be broadened to ameliorate photocatalytic activity.

In this work, we report the synthesis of Ni(II) doped CdS (Ni(II)/CdS) with mixed-phase (cubic and hexagonal) via a simple impregnation method. The Ni(II)/CdS was further used as photocatalyst in the activation of saturated primary C–H bond of toluene and its derivatives at room temperature and under two barometric pressure (0.2 MPa). The yield of benzaldehyde can reach up to 216.70 µmolh−1 g −1 , which is 7.7 times higher than that of mixed-phase CdS. Controlled experiments using different radical scavenger were also conducted to elucidate the possible reaction mechanism for the selective oxidation of C–H bond in toluene over the mix phase of Ni(II)/CdS.

## EXPERIMENTAL

#### Materials

All reagents were analytical grade and used without further purification. Benzotrifluoride (BTF) (>99%)was purchased from Aladdin Chemical Reagent Co., Ltd. Oleic acid (C18H34O2), Cadmium nitrate tetrahydrate [Cd(NO3)2·4H2O], 8% of ammonium sulfide [(NH4)2S], sodium borohydride (NaBH4), ethyl alcohol (C2H6O), toluene (>99.5%) and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

# Preparation

#### Synthesis of Mixed-Phase CdS

Typically, Cd(NO3)2·4H2O (1.0 mmol) and oleic acid (0.0375 ml) were first dissolved in deionized water (50 mL) in a beaker placed on a magnetic stirrer for 15 min at room temperature to form solution A. NaBH4 (4.0 mmol) was also dissolved in deionized water (250 mL) in a beaker placed on a magnetic stirrer at room temperature to form solution B. The two solutions were then mixed and quickly put into a microwave oven (M1-L213B, at a fixed frequency of 2,450 MHz, Guangdong midea kitchen appliances manufacturing Co., Ltd., China) followed by 20 s irradiation. The Cd nanoparticles (NPs) were collected and washed by ethyl alcohol three times, and dispersed in 20 mL ethyl alcohol to get sulfurized by adding certain amount of (NH4)2S (2.0 mmol) and followed by 1 h stirring at room temperature. Then the yellowish products were collected and washed by ethyl alcohol three times. Finally, the products were dried in drying oven at 60◦C for 4 h.

#### Preparation of Ni(II)-Mixed-Phase CdS Samples

The Ni(II) doped mixed phase CdS samples (Ni(II)/CdS) were synthesized by a simple impregnation method. Typically, CdS (200.0 mg) was dispersed into10.0 mL of NiCl2·6H2O solution and stirred at 80◦C for 2 h. The products was collected and washed by deionized water several times. The amount of Ni (the weight ratio of Ni(II) to CdS) was about 0.5, 1.0, 3.0, and 5.0 wt%, respectively. Finally, the samples dried in drying oven at 60◦C for 4 h.

#### Characterization

X-ray diffraction (XRD) patterns were acquired by a Rigaku D/MAX-2200/PCX-ray diffractometer in the angular range of 10–90◦ (2θ) with Cu Kα radiation (40 kV, 20 mA). UVvis diffuse reflectance spectra (DRS) were acquired from 230 to 800 nm by a UV-vis spectrophotometer (PuXin TU-1901). The morphologies and microstructures of the samples were obtained using field emission scanning electron microscope (FE-SEM, Ultra Plus, Carl Zeiss) and transmission electron microscope (TEM, F20, FEI). X-ray photoelectron spectroscopy (XPS) was used to study the chemical compositions and the valence states by a photoelectron spectrometer (PHI5702). The photoelectrochemical (PEC) performances of photoanodes were acquired by a three-electrode system (CHI-660D Co., Shanghai, China) under a LED lamp (λ > 420 nm, CEL-LED100) illumination. A Pt wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrodes were made on the fluride-tin oxide (FTO) conductor glasses. The samples (10 mg) were homogeneously dispersed in anhydrous ethanol and ultrasound for 60 min, which were slowly dripped on FTO glasses. The electrolyte was 0.5 M Na2SO4 (pH = 7.35) aqueous solution in a quartz ware 0.5 V of the bias voltage was used for photoelectrochemical testing. Illumination through the back-side of FTO was used with an illumination area of about 1.0 cm−2 . Photoluminescence (PL) spectra of the samples were recorded by a fluorescence spectrophotometer (PE, LS-55) at room temperature, using a 390 nm excitation wavelength.

#### Evaluation of Photocatalytic Activity

The photocatalytic reaction was performed in a 25 mL glass bottle under visible light irradiation in this work. Typically, 0.5 mmol of substrate and 80 mg of catalyst were dispersed in the 5.0 mL of solvent (BTF) in a 25 mL glass bottle. Then the glass bottle was transferred into a closed reactor (**Figure S1**) and stirred for 30 min in dark to make the catalyst fully contact with the solution and achieve adsorption equilibrium. Meanwhile, the reactor was bubbled into oxygen molecules from a gas cylinder for 30 min at absolute pressure of 0.2 MPa. A 300 W Xe arc lamp (CEL-HXF 300, Beijing Perfectlight Co. Ltd.) with a ultraviolet cutoff filter (λ < 420 nm) was used in the following photocatalytic reaction. After 2 h of irradiation, the mixture was centrifuged to remove catalyst powders at 8,000 rmp for 5 min by a centrifuge (TG16-WS). The liquid supernatant was analyzed by a gas chromatograph (GC-9600). A series of controlled experiments were carried out similar to the photocatalytic oxidation process of toluene, except that the radical scavengers (0.1 mmol) were added to the reaction system. The conversion (Con.) rate of substrates, yield of aldehyde, and selectivity (Sel.) were defined with the following equations:

$$\text{Con.(\%)} = [(\text{C}\_0 - \text{C}\_1)/\text{C}\_0] \times 100\% \tag{1}$$

$$\text{Yield(\%)} = \text{(C}\_{\text{aldehydde}}/\text{C}\_{\text{0}}) \times 100\text{\%} \tag{2}$$

$$\text{Sel.(\%)} = [\text{C}\_{\text{aldehydde}} / (\text{C}\_0 - \text{C}\_1)] \times 100\text{\%} \tag{3}$$

Where C0 is the initial concentration of substrates; C1 and Caldehyde are the concentration of the remaining substrates and the corresponding aldehyde at a certain time after the photocatalytic reaction, respectively.

#### RESULTS AND DISCUSSION

## DRS, XRD and Photoluminescence (PL) Spectra Analysis

**Figure 1A** shows the UV–vis diffuse reflectance spectra of mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS, 5.0 wt%Ni(II)/CdS samples. All samples of the spectra display absorption in the visible-light region. The mixedphase CdS NPs exhibit a absorption edges at 518 nm, while the

Ni(II)/CdS samples show red shift of absorption edges compared with prestine CdS sample. In particular, the absorption edges of 3.0 wt%Ni(II)/CdS NPs was observed at 555 nm. Hence, it is clear that the modification of Ni(II) co-catalyst can significantly affect the optical absorption feature of CdS NPs. The optical band gap value has been calculated via the equation: (αhv)2 = A(hv — Eg) (Su et al., 2013; Kuang et al., 2015), where α, h, A and v correspond to absorption coefficient, Planck constant, proportionality and light frequency, respectively. As shown in **Figure 1B**, the band gaps estimated are 2.39, 2.26, 2.24, 2.23 and 2.26 eV for mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt% Ni(II)/CdS, 3.0 wt%Ni(II)/CdS and 5.0 wt%Ni(II)/CdS, respectively. The above results indicate that the Ni(II)/CdS have changed the band gap of the mixed-phase CdS. Compared with the single-phase CdS of absorption edge, the absorption edge of the mixed phase CdS is red shift. Prior to this, the cubic CdS was synthesized by a solvothermal method. In addition, the UV–vis diffuse reflectance spectra and Tauc plot are shown in **Figure S2**.

The X-ray diffraction (XRD) pattern of as-prepared CdS sample is shown in **Figure 1C**. It is evident that the sample contain hexagonal phase CdS because the peaks at 24.8, 26.5, and 28.2◦ correspond to the (100), (002) and (101) crystal planes (hexagonal CdS, JCPDS No. 41–1049), respectively. Meanwhile, peaks at 26.5, 44.0 and 52.1◦ can be indexed to (111), (220), and (311) planes of cubic phase CdS (JCPDS No. 75-1546) or (002), (110), and (112) planes of hexagonal phase CdS. So it is clear that most of the strong diffraction peaks of JCPDS No. 75–1546 (cubic phase) overlap with the peaks of JCPDS No. 41–1049 (hexagonal phase) except for the peak at 30.7◦ , which corresponds to (200) plane of cubic phase CdS. But it is not safe to judge the existence of cubic phase CdS from the XRD patterns simply according to this distinctive peak because the intensity of (200) peak is only one fifth of that of (111) plane due to its weak and even indiscernible signal in the whole pattern. However, the nature of mixed phases of our samples can be alternatively judged from the intensity distribution of diffraction peaks, as suggested by the peak of 26.5◦ corresponds to the (111) crystal plane and overlaps with the (002) crystal plane of the hexagonal phase. Noting that peak of (002) plane is not the strongest line in JCPDS No. 41-1049, it is unreasonable to conclude that the strongest peak is given at 26.5◦ if our samples are hexagonal phase CdS, the only possible explanation is that our sample should be mixed-phase CdS so that the diffraction wave of (111) plane (cubic CdS) will stack up with that of (002) plane (hexagonal CdS) and presents the strongest peak at 26.5◦ . Another possibility for the presence of the strongest peak at 26.5◦ can be explained by anisotropic growth of hexagonal phase CdS. The dominant growth along (002) plane will give similar result even if our sample is not mixed-phase CdS. To rule out this possibility, SEM and TEM were performed on all the samples and the results (shown in **Figure 3**) do not support this supposition. So it is now safe to demonstrate that the CdS sample should be composed of cubic CdS and hexagonal CdS. According to the patterns shown in **Figure 1C**, the same diffraction peak intensity and width as mixed-phase CdS suggests that the appearance and crystallite size of CdS NPs are not affected by doping different amount of Ni(II). Photoluminescence (PL) spectroscopy is an effective method to study the electronic structure and optical properties of semiconductor materials. **Figure 1D** shows the PL spectra of all samples at the excitation wavelength of 390 nm in room temperature. It can be seen from **Figure 1D** that the catalyst samples have a strong signal peak at a wavelength of about 540 nm. In general, a high intensity of the photoluminescent signal indicates a high recombination probability of photogenerated electrons (e−) and holes (h+) as well as a low photocatalytic activity (Martha et al., 2013; Wu et al., 2014; Weng et al., 2016; Cui et al., 2017a; Nie et al., 2018; She et al., 2018). In this regard, it is seen from **Figure 1D** that the activity of the catalyst 3.0 wt%Ni(II)/CdS is significantly higher than that of the other samples, which is in a good agreement with the result of catalyst activity tests.

#### Morphology and Microstructures Analysis

**Figure 2** shows the SEM images of the mixed-phase CdS and 3.0 wt%Ni(II)/CdS. It is seen that the mixed-phase CdS sample consists of irregular particles with a size range of 20–100 nm in **Figures 2a,b**. As exhibited in the **Figures 2c,d**, 3.0 wt%Ni(II)/CdS sample is also constituted of irregular nanoparticle, and its microscopic structure is similar to that of mixed-phase CdS. But its surface is not so smooth as that of mixed-phase CdS due to the modification of Ni(II). The EDX analysis was also performed to confirm the existence of Ni element and the result is shown in **Figure 3f**. The SEM images of the 0.5 wt%Ni(II)/CdS, 1.0 wt% Ni(II)/CdS and 5.0 wt% Ni(II)/CdS are displayed in **Figure S3**, respectively. It can be clearly seen that all the samples are constituted of irregular nanoparticle with the same microscopic structure.

To study the microscopic structure and morphology of asprepared catalysts, TEM, HRTEM, SAED, and EDX analysis were performed and the results are shown in **Figure 3**. As shown in **Figure 3b**, the lattice spacing of 0.359 and 0.335 nm can be

FIGURE 2 | SEM images of the mixed-phase CdS (a,b), and 3.0 wt% Ni(II)/CdS (c,d).

ascribed to the (100) and (002) crystal face of hexagonal phase and the spacing of 0.290 nm can be ascribed to the (200) crystal face of cubic CdS. This image analysis further confirms that the as-prepared catalysts are composed of cubic and hexagonal phase CdS NPs and is consistent with the XRD results as discussed above. Compared with prestine CdS (**Figure 3a**), it is observed that the microscopic structure and morphology of 3.0 wt%Ni(II)/CdS (**Figure 3c**) is not significantly changed. The lattice spacing of Ni is not observed in **Figure 3d**, which could be attributed to the low dosage of Ni(II) ions. As shown in **Figure 3e**, the SAED pattern shows that the sample possesses a polycrystalline nature. In addition, the energy dispersive X-ray (EDX) analysis also suggests the coexistence of Cd, S and Ni elements in **Figure 3f**.

# XPS Analysis

The surface nature of the as-prepared CdS and 3.0 wt%Ni(II)/CdS samples were characterized by XPS measurements. As shown in **Figure 4A**, Cd and S elements are derived from the CdS phase, while the O element might come from H2O (Chen et al., 2015). We can see that the peaks at the binding energy of 404.9 and 411.7 eV correspond to Cd 3d5/2 and Cd 3d3/2 in **Figure 4B** (Jin et al., 2015). **Figure 4C** indicates the S 2p peak bifurcates as two peaks of S 2p3/2 and S 2p1/2 (Xue et al., 2018), corresponding to the binding energy of 161.4 and 162.7 eV (Li et al., 2015b), respectively. As shown in the **Figure 4D** (3.0 wt%Ni(II)/CdS), the peaks at about 855.5 and 873.1 eV are attributed to Ni 2p3/2 and Ni 2p1/2 of Ni(II) (Song et al., 2017), with a spin-energy separation of 17.6 eV. The peaks at 861.3 and 879.3 eV are ascribed to the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively (Oros-Ruiz et al., 2014, 2016). The above results also suggest that Ni(II) was successfully decorated on the surface of the CdS nanoparticles.

# Electrochemical Characterization

To study the efficiency of the photogenerated electron-hole, the transient photocurrent tests were carried out on all as-prepared samples (mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS and 5.0 wt%Ni(II)/CdS). As shown in **Figure 5A**, it is clear that all Ni(II)-doped samples have a higher photocurrent than that of prestine CdS, indicating that the separation efficiency of photoinduced charge carriers is improved by doping Ni(II) ions into CdS lattice. The results are consistent with the PL test results (**Figure 1D**; Zou et al., 2016). **Figure 5B** shows the electrochemical impedance spectroscopy (EIS) curves of the mixed-phase CdS and Ni(II)/CdS. Comparing the Nyquist plots circle radius of the mixed-phase CdS with that of 3.0 wt% Ni(II)/CdS, it can be observed that the radius of arc curve of 3.0 wt%Ni(II)/CdS is the shortest among all tested samples, suggesting that Ni(II) ions in CdS lattice can facilitate electron-hole separation and interfacial migration (Cui et al., 2017b). It can be also inferred that the Ni(II) ions act as a carrier for transferring electrons. **Figures 5C**, **Figure S4** show the Mott-Schottky curves of the as-prepared samples. The slopes of the two Mott-Schottky curves are all positive in the range of −0.5 and 1.2 V, indicating that the samples are n-type semiconductor, which is consistent with the reported results. The flat band potentials of CdS and 3.0 wt%Ni (II)/CdS are −0.56 and −0.51 V, respectively. The flat band potential is approximately equal to the conduction band (CB) edge for n-type semiconductors. Therefore, the CB positions of CdS and 3.0 wt%Ni (II)/CdS are approximately −0.56 and −0.51 V, respectively. The valence band (VB) edge of samples can be

determined by the following equation: Eg = EVB – ECB (Jin et al., 2015; Wang et al., 2018c), where ECB is the CB edge potential, EVB is the VB edge potential, and Eg is the band gap of the semiconductors. The calculated VB positions of CdS and 3.0 wt%Ni (II)/CdS are approximately 1.83 and 1.72 V, respectively.

## Photocatalytic Performance Experiment

To study the effect of amount of doping Ni(II) on the conversion level, we have tested the photocatalytic performance of the mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS, and 5.0 wt%Ni(II)/CdS for selective oxidation of toluene under visible light irradiation for 2 h. It can be seen from **Table 1** that the formation rate of benzaldehyde can be improved when the amount of Ni(II) increases from 0.5 to 3.0 wt%. The formation rate of benzaldehyde can reach to 216.70 µmolh−1 g −1 using 3.0 wt%Ni(II)/CdS as catalyst after 2 h irradiation. The result suggest that 3.0 wt%Ni(II)/CdS is highly active visiblelight-driven photocatalyst for the selective oxidation of primary C–H bond in toluene. As we known, metal ions may become the capture center of photogenerated electrons, and effectively suppress the recombination of photogenerated electrons and holes, thereby improving the photocatalytic activity. As the doping amount, the trapping sites of the carriers increase, which increases the carrier lifetime and improves the separation of photogenerated electron-holes. When the doping amount is increases, the carrier capture sites are increased and the carrier lifetime is extended, resulting in a great improvement in photoelectron-hole separation. Therefore, this process enhances the photocatalytic activity. However, when the doping amount is increased to a certain extent, the distance between the trapping sites of the trapping carriers becomes small, the doping ions evolve into a recombination center of electrons and holes, and the activity of the catalyst is lowered. However, when the doping amount is increased to a certain amount, the distance between the trapping sites of the trapping carriers becomes small, resulting in the doping ions evolve into a recombination center of electrons and holes. So, the photocatalyst exhibits lower photocatalytic activity. When the doping amount of 3 wt% is an optimum value, the photocatalyst exhibits the best photocatalytic activity.

To demonstrate the general applicability of the as-prepared Ni(II)/CdS photocatalysts for selective oxidation of such primary C–H bond in alkyl aromatics, the visible light photoactivity of 3.0 wt%Ni(II)/CdS toward the selective oxidation of other substituted toluenes were further tested, and the results are listed in **Table 2**. It is clear to see that the 3.0 wt%Ni(II)/CdS is also active for the oxidation of primary C–H bond of substituted toluenes to the corresponding aldehydes. The above photoactivity results suggest that the as-prepared 3.0 wt%Ni(II)/CdS, that is

TABLE 1 | Oxidation of toluene over the as-prepared samples under visible light irradiation for 2 h.

achieved by such a very simple method at room temperature, can be used as an efficient and visible-light-driven photocatalyst toward the selective activation of primary C–H bond in a variety of substituted toluenes.

# Cycle and Possible Reaction Mechanism Experiment

The 4 times cycling test was performed over the 3.0 wt%Ni(II)/CdS to verify its stability under the same conditions. After irradiation, the reaction mixture was centrifuged to separate the catalyst, which was then washed three times with ethanol and deionized water and dried before next cycle test. In **Figure 6A**, it can be seen that the conversion of tolueneand the selectivity of benzaldehyde are almost unchanged. To further

TABLE 2 | Oxidation of toluene and substituted toluenes over the 3.0

wt%Ni(II)/CdS under visible light irradiation for 2 h.

confirm the stability of our sample as catalyst, the XRD test were performed to measure the fresh 3.0 wt%Ni(II)/CdS as well as the used 3.0 wt%Ni(II)/CdS. As shown in **Figure 6B**, the

crystal structure of 3.0 wt%Ni(II)/CdS sample does not change significantly after the photocatalytic reaction. Furthermore, considering the XRD test on stability of our sample together with the SEM images (shown in **Figure S5**) of used the 3.0 wt%Ni(II)/CdS after photocatalytic reaction, it can be inferred that the morphology and structure of the sample as photocatalyst remain unchanged during reaction, indicating that the 3.0 wt% Ni(II)/CdS composite is a stable photocatalyst for the selective oxidation of substituted toluenes to the corresponding aldehydes under the experimental conditions. Furthermore, the valence of Ni and its effect on the photocatalytic process through XPS detection of the 3.0 wt%Ni(II)/CdS after photocatalytic reaction had been analyzed. According to XPS (**Figure S6**), Cd and S present undetectable change between fresh and used 3.0 wt%Ni(II)/CdS. As shown in **Figure S6D**, two weak peaks at 850.6 and 868.9 eV can be respectively attributed to the 2p3/2 and 2p1/2 peaks of Ni metal (Li et al., 2015a), indicating the formation of the nickel metallic state of the catalyst during the photocatalytic reaction. It is considered that photogenerated electrons in the CdS CB tended to transfer to Ni(II) clusters and then effectively reduce a portion of Ni(II) to Ni0 atoms (Weng et al., 2017). However, the combination of electrons and oxygen molecules quickly forms a process of the long-term transport of electrons. Therefore, more holes are combined with toluene to form an oxidation reaction process. This process facilitates efficient separation of carriers to ameliorate photocatalytic activity. As shown in **Figure 6C**, toluene oxidation over 3.0 wt%Ni(II)/CdS under visible light irradiation for 10 h can give a toluene conversion rate of 20.4%. It presents a great potential for the industrial synthesis for the fine chemicals. To study the role of photogenerated radical species involved in photocatalytic oxidation of toluene on 3.0 wt%Ni(II)/CdS sample under visible light irradiation and the reaction mechanism involved, a series of control experiments were carried out and the result is shown in **Figure 6D**. In the control experiments, all conditions were kept unchanged except that different scavengers (AO, BQ and AgNO3) were added to capture h+, •O − 2 and e−, respectively. As shown in **Figure 6D**, ammonium oxalate (AO) scavenger was added and the conversion of toluene was almost terminated under visible light irradiation. Adding benzoquinone (BQ) or AgNO3 to the reaction system also significantly suppressed the conversion of toluene. Although electrons cannot directly participate in the oxidation of toluene, it can affect the activation of oxygen molecules. A series of control experiments result suggest that the h+ play the important role in photocatalytic oxidation of toluene over 3.0 wt%Ni(II)/CdS. The O2 is activated by e− to form the superoxide radicals, which acts as the main oxidant for the photocatalytic toluene process. The above results suggest that the major active species for the photocatalytic selective oxidation of toluene is h+ rather than e−.

Thus, the possible mechanism is proposed, as given in **Figure 7**. Compared with the common CdS of band width (2.41 eV), the band width (2.39 eV) of the mixed phase CdS is reduced. Besides, doping Ni(II) to the photocatalyst is capable of greatly enhancing its electron transport and charge separation. The photogenerated electrons of CdS can be rapidly transferred to the Ni(II) promoter because of the positive potential of Ni2+/Ni0 (−0.23 V vs. SHE, pH = 0) than the CB potential of the mixed-phase CdS (about −0.56 V, **Figure 5C**; Ran et al., 2011; Meng et al., 2018), thereby forming a long-term electron transfer process. The toluene is adsorbed on the surface of the 3.0 wt%Ni(II)/CdS and oxidized by the holes to the corresponding cationic radicals. Meanwhile, the electrons react with adsorbed O2 to give activated oxygen species. The activated oxygen species then selectively oxidize the cationic radicals, finally leading to the formation of benzaldehyde.

According to the experimental and simulated results, the photocatalytic mechanism of oxidation of toluene coupling with was proposed as following scheme:

#### CONCLUSION

In summary, we have synthesized Ni(II)-doped cubic and hexagonal phases CdS semiconductor with nanosphere structure morphology by a simple method, which is able to be used as a well-photocatalyst to the activation of saturated primary C–H bond in toluene and substituted toluenes under mild conditions. The mixed phase CdS forms a homojunction, resulting in a reduction in its band width. It can effectively expand the response range and improve photocatalytic performance. Besides, doping Ni(II) to the photocatalyst is capable of greatly enhancing its electron transport and charge separation. The superior photocatalytic performance of Ni(II)-doped CdS is attributed to its unique structure assembly of specific morphology, which can be efficient transport and separation of photogenerated charge carriers under visible light irradiation. Mechanism research shows that Ni as a co-catalyst can improve the catalytic activity of the CdS. The as-prepared 3.0 wt%Ni(II)/CdS has highly active for the selective oxidation of inert primary C–H bond, which has great potential in photocatalytic selective activation of C–H bond to fine chemicals.

#### AUTHOR CONTRIBUTIONS

HS and LL designed and conducted the experiments. LL, HZ, and JH analyzed the data. HS, LL, LW, and QW wrote the paper.

#### FUNDING

This work was financially supported by the National Natural Science Foundation of China (21261021, 21663027, 51262028), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Program for the Young Innovative Talents of Longyuan (NWNU-LKQN-15-2) and the Undergraduate Academic Innovative Research Team of Northwest Normal University.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00466/full#supplementary-material

$$\mathbf{0\_2} + \mathbf{e}^\cdot \longrightarrow \longrightarrow \cdot \mathbf{0\_2}^\cdot \qquad (4)$$

# REFERENCES

over V2O5@TiO2 under simulated solar light irradiation. Green Chem. 19, 311–318. doi: 10.1039/C6GC02748B

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 She, Li, Zhou, Wang, Huang and Wang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# BiOCl Decorated NaNbO3 Nanocubes: A Novel p-n Heterojunction Photocatalyst With Improved Activity for Ofloxacin Degradation

#### Jingjing Xu1 \*, Bingbing Feng1 , Ying Wang1 , Yadi Qi 1 , Junfeng Niu2 and Mindong Chen1

*1 Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Engineering Technology Research Center of Environmental Cleaning Materials, Nanjing University of Information Science and Technology, Nanjing, China, 2 Research Center for Eco-Environmental Engineering, Dongguan University of Technology, Dongguan, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business*

#### *University, China* Reviewed by:

*Huogen Yu, Wuhan University of Technology, China Tianhua Zhou, Fujian Institute of Research on the Structure of Matter (CAS), China*

> \*Correspondence: *Jingjing Xu<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *12 July 2018* Accepted: *14 August 2018* Published: *02 October 2018*

#### Citation:

*Xu J, Feng B, Wang Y, Qi Y, Niu J and Chen M (2018) BiOCl Decorated NaNbO3 Nanocubes: A Novel p-n Heterojunction Photocatalyst With Improved Activity for Ofloxacin Degradation Front. Chem. 6:393. doi: 10.3389/fchem.2018.00393* BiOCl/NaNbO3 p-n heterojunction photocatalysts with significantly improved photocatalytic performance were fabricated by a facile *in-situ* growth method. The obtained BiOCl/NaNbO3 samples were characterized by UV-vis absorption spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), photocurrent (PC) and photoluminescence spectroscopy (PL). The photocatalytic activity of the BiOCl/NaNbO3 samples was investigated by the degradation of a typical antibiotic Ofloxacin (OFX). The experimental results showed that BiOCl/NaNbO3 composites exhibited much higher photocatalytic activity for OFX degradation compared to pure NaNbO3 and BiOCl. The degradation percent of OFX reached 90% within 60 min, and the apparent rate constant was about 8 times as that of pure NaNbO3 and BiOCl. The improved activity can be attributed to the formation of p-n junction between NaNbO3 and BiOCl. The formed p-n junction facilitated the separation of photogenerated holes and electrons, thereby enhancing photocatalytic activity. In addition, the composite photocatalyst showed satisfactory stability for the degradation of OFX. Due to the simple synthesis process, high photocatalytic activity, and the good recyclability of these composite photocatalysts, the results of this study would provide a good example for the rational design of other highly efficient heterojunction photocatalytic materials.

Keywords: photocatalysis, BiOCl, NaNbO3 , p–n junction, ofloxacin

## INTRODUCTION

The problem of water pollution has attracted increasing attention worldwide due to many toxic pollutants continue to enter into our water bodies (Ao et al., 2016b; Li et al., 2018; Yuan et al., 2018). As a green and sustainable technology, semiconductor photocatalysis can completely eliminate most pollutants in water and air (Zhang et al., 2016, 2017a,b; Chen et al., 2018; Guo et al., 2018; He et al., 2018; Yang et al., 2018). The contaminants are degraded by hydroxyl radical or other free radicals which are produced in the process after the absorption of light (Li et al., 2015; Cao et al., 2016; Xu et al., 2016; Yu et al., 2016; Zhang and Xiao, 2017; Han et al., 2018; Zhong et al., 2018). There are many semiconductors (such as oxides, sulfides, nitrates and silver based compounds etc.) can be served as photocatalysts to induce the degradation of pollutants.