Construction of All Solid State Bi2MoO6/Bi2WO6@Ti3C2 MXene Heterostructure System and the Mechanism Study of the Z-scheme Photocatalysis

Zhang, Dongfang; Liu, Junkun; Liu, Qiang; Wang, Jiaxun; Qu, Yang

doi:10.1590/1980-5373-MR-2024-0271

Abstract

In this work, all solid-state Z-scheme Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ MXene photocatalytic system was synthesized by a facile solvothermal method. For comparison, pure Bi₂MoO₆, Bi₂WO₆ and Bi₂MoO₆/Bi₂WO₆ samples were also synthesized. The structure and properties of the as-prepared catalytic materials were analyzed by X‑ray powder diffraction (XRD), transmission electron microscope (TEM) and high-resolution TEM (HRTEM), ultraviolet‑visible diffuse reflection spectroscopy (UV‑Vis DRS), fluorescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), N₂ adsorption–desorption measurements and photoelectrochemical tests, which were performed to analyze the phase composition, microstructure, morphology, optical properties, and recombination rate of photogenerated charge of the as-prepared samples. Crystal violet (CV)and p-nitrophenol (4-NP) were used as pollutants to simulate wastewater, and the photocatalytic degradation activity of the catalyst samples was evaluated under visible light. After irradiation for 80 min, the as-made Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ MXene has the best photocatalytic performance, and the degradation rate constant was 3~4 folds higher than that of pure Bi₂MoO₆ or Bi₂WO₆. Based on all the experimental characterizations, the corresponding Z-scheme photocatalytic mechanism of the Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ MXene system was further studied. The results showed that when introducing 2D Ti₃C₂ MXene, the light absorption range of the Bi₂MoO₆/ Bi₂WO₆ heterojunctions were significantly enhanced, and the separation of photogenerated charges were also improved.

Keywords:
Photocatalytic; Ti₃C₂ MXene; Z-scheme Heterostructure

1. Introduction

Along with the fast development of science and technology, environmental pollution has become an issue of increasing concern. Especially, industrial dyes and phenolic compounds constitute an important group of environmental pollutants found in surface and groundwater¹. Using semiconductor photocatalyst to degrade pollutants is one of the hot topics in dealing with environmental pollution. In 1976, Carey found that TiO₂ semiconductor particle aqueous solution system can completely mineralize the oxidative dechlorination of PCBs into small molecules such as H₂O and CO₂ under UV irradiation². Therefore, the research on photocatalytic degradation of pollutants become the main direction in the field of environmental remedy at present. With the progress of research, the problems of fast recombination of electrons and holes, narrow band gap and low solar energy utilization of single-phase catalyst seriously affect the photocatalytic activity of the photocatalytic system. In view of the above problems, constructing Z-type photocatalytic system is one of the effective ways to solve this problem, in which solid-state Z-type photocatalytic system is one of the research objects. Recently, Zhao and others constructed Ag₃VO₄/BiVO₄ solid-state Z-type photocatalytic system and degraded Bisphenol S by visible light, the solid-state Z-type system has developed rapidly in the field of photodegradation³-⁵.

Nowadays, as a new revolutionary material, graphene material is widely used in industry and life. The defect that there is no band gap between its conduction band and valence band has set off an upsurge of researchers' research on other two-dimensional materials with semiconductor properties. Combined with the advantages of Z-type photocatalytic system in inhibiting photogenerated electron recombination, the construction of Z-type photocatalytic system on two-dimensional structure or the construction of Z-type photocatalytic system with two-dimensional materials has attracted extensive attention. The synergistic effect of two-dimensional materials and Z-type photocatalytic system has stronger catalytic performance⁶,⁷.

Bismuth of photocatalyst is a common 2D materials, has a narrow band gap, and Bi 6s² lone pair electrons caused by intrinsic polarization favorable to the separation of electronic-hole pair and transfer of charge carriers. In addition, most bismuth based photocatalysts have unique layered structure. The synthesis of two-dimensional nanosheets by simple method (hydrothermal method) can greatly improve the photocatalytic performance of materials. Bismuth based photocatalysts often participate in the construction of Z-type photocatalytic system. For example, different kinds of 2D/2D Z-type nano photocatalysts (BiOBr/Bi₄O₅Br₂、Bi₂O₂CO₃/Bi₄O₅Br₂、AgBr/Bi₄O₅Br₂) based on 2D ultra-thin Bi₄O5Br₂ nanosheets has been constructed in the previous studies⁸-¹⁰. Because Bi₄O₅Br₂ has a unique graphene layered structure, a built-in electric field can be formed between the bismuth oxide layer and the bromine ion layer, and the photogenerated electrons and holes generated by the compound can be effectively separated. Thus,these Z-type photocatalytic systems have good photocatalytic activity for dye removal and N₂ fixation.

Taking Bi₂MoO₆ in two-dimensional bismuth photocatalytic materials as an example, its band gap width is about 2.6 eV and its utilization of visible light is relatively low. In order to improve this disadvantage, on the one hand, it can be layered and assembled into special nano materials to improve the absorption and utilization of visible light by using the reflection and refraction effects. On the other hand, starting from the material itself. At present, the catalytic performance of Bi₂MoO₆ can be improved from three aspects: element doping, defect structure and plasmon metal loading¹¹. While this work is aims to use the new two-dimensional laminar nano materials Ti₃C₂ MXene as carrier and cocatalyst, and solid-state Z-scheme Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ MXene composite materials could be constructed. On the one hand, the structure has uniform morphology and size. On the other hand, Ti₃C₂ MXene can provide a larger scale for active site reaction and good dispersion, which is favorable to the adsorption of degraded substrates in the photocatalytic process. Compared with the traditional heterojunction photocatalytic system, Z photocatalytic system can effectively accelerate the separation of electrons and holes and migration, inhibit the occurrence of adverse reaction, has higher catalytic activity. Based on two-dimensional photocatalytic materials unique space structure, Z-scheme photocatalytic system built on the 2D structure can enhance the spectral response ability of catalytic system and carrier separation efficiency, redox ability, anti-light corrosion stability.

In this paper, Z-scheme Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ MXene was prepared by a simple solvothermal method. The newly synthesized photocatalysts showed high photocatalytic behavior and the photocatalytic performance of the samples has been evaluated employing two pollutants. At first, CV was chosen as a model dye to evaluate the activity of catalysts. Besides, the p-nitrophenol as a target pollutant released by industrial wastewater discharge in urban areas was degraded by the as-prepared catalysts and the underlying mechanism that involved the photocatalytic oxidation process were also analyzed and investigated in detail, which is important for understanding the behavior of photo-generated charge on the interface as well as the possible mechanism of Z-type photocatalytic system. Moreover, the stability tests were also done to check the reuse performance of the composite material.

2. Materials and Methods

2.1. Photocatalysts synthesis

2.1.1. Preparation of Bi₂MoO₆ and Bi₂WO₆

The photocatalysts were prepared via solvothermal route. In a typical procedure for synthesis of Bi₂MoO₆, 1.4610 g bismuth nitrate pentahydrate and 0.3627 g sodium molybdate dihydrate, put them into a beaker (50 ml), add 0.5 ml polyethylene glycol was used as protective agent, then add 20 ml deionized water, add magnetons and stir them in DJ-1 titration mixer for 20 min. After mixing, it is transferred to a 50 mL Teflon-lined stainless steel autoclave and reacted in xmtd-8222 electric constant temperature blast drying oven at 160 °C for 3 hours. After the reaction, transfer the products to the beaker and washed the powders for several times using alcohol and deionized water, and then put the products into xmtd-8222 electric constant temperature blast drying oven at 80 °C for 24 h to obtain Bi₂MoO₆. The synthesis of Bi₂WO₆ was followed the similar procedure, and 1.4647 g bismuth nitrate pentahydrate and 0.4923 g sodium tungstate dehydrate were used as reaction agents.

2.1.2. Preparation of Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ mxene

Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ mxene composite material was synthesized through a one-pot solvothermal method. In a typical procedure, 2.929 g bismuth nitrate pentahydrate and 0.3627 g sodium molybdate dihydrate, 0.4923 g sodium tungstate dehydrate were used as starting reaction agents and dissolved in 0.5 ml polyethylene glycol and 20 ml deionized water to form solution A. Put 0.0303 g Ti₃C₂ mxene into the beaker, add 30 ml deionized water into the vessel to form solution B, then treat the solution in kq-500e ultrasonic cleaner for 10 min, make it disperse evenly, and then add solution A into it. Then stir the mixture in the DJ-1 titration stirrer for 30 min. After mixing, the above mixture is transferred to a 100 mL Teflon-lined stainless steel autoclave and reacted in xmtd-8222 electric constant temperature blast drying oven at 160 °C for 3 hours. After the reaction, transfer the final powders to the beaker, remove the supernatant with a rubber dropper, and then put the solids into xmtd-8222 electric constant temperature blast drying oven at 80 °C for 24 h to obtain Bi₂MoO₆ /Bi₂WO₆@Ti₃C₂ mxene composite materials.

2.2. Instrument and characterization

The structure and crystal phases of the as-prepared photocatalysts were determined by X-ray diffractometer (XRD) patterns with 2θ ranges from 10~80°. The equipment is Shimadzu XRD-7000 diffractometer with Cu Ka radiation (k = 1.5406 Å) and performed at 40 kV and 30 mA. Specific surface area of materials was determined using a Quadrasor SI instrument (Micromeritics Instruments, USA) after degassing at 300 °C for 3 h to remove physisorbed water. The measured data were processed according to the BET isotherm within the range of P/P_o = 0~1 at 77 K. The morphology of the catalyst was observed in the transmission electron microscope zone, and pictures were recorded on a Tecnai G2 F20 electron micro-scope running at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were measured on an ESCALAB 250 spectrometer. The binding energies were calibrated using C1s peak of contaminant carbon (BE = 284.8 eV) as an internal standard. Deconvolution of Bi 2p, V 3d, W 4f, Mo 3d, Bi 4f, Ti 2p and O 1s peak of catalysts was performed on origin 8.5 software. Diffuse reflectance (DR) UV-Vis spectra of the products were scanned on a Shimadzu UV-4100 spectrophotometer with BaSO₄ as reflectance standard. The photoluminescence (PL) spectra were conducted on an F-7000 spectrophotometer with excitation wavelength of 300 nm. The surface photovoltage (SPV) spectra were measured on the home-built apparatuses under a laser radiation pulse (450 nm). The photoelectrochemical measurements of the samples were performed on a CHI760D equipment, which used a standard three electrode system in 20 mL 0.2 M acetate buffer (pH 5.0). Then the resulting homogeneous ink and 0.5 vol% nafion solutions were dropwise-cast onto glassy carbon electrode or FTO glass in turn, dried under the infrared light. Photocurrent tests were conducted under a Xenon lamp (power: 300 W) with the same light intensity (100 mA cm⁻²).

2.3. Research for catalytic performance

The photodegradation experiment was carried out with crystal violet (CV) and p-nitrophenol (4-NP) as the model pollutants. A device for simulating visible light source (Chinese teaching Jinyuan, CEL-HXF300, 300 W xenon lamp with a filter of a cut-off wavelength of 400 nm) to conduct photocatalytic degradation experiments. The specific steps are as follows: 30 mg of prepared catalyst is added into CV or 4-NP solution (150 mL, 12 mg L^-1) in the photochemical reaction bottle and mixed evenly. Under the condition of no light, the magnetic stirrer is used to stir for 60 min to achieve the substrate adsorption-desorption equilibrium, and 5 ml of initial reaction solution is withdrawn from the reaction bottle for measurement. After that, turn on the xenon lamp light source to initiate photocatalytic reaction and start timing at the same time. Take 5 ml samples every 10 min and add them into the centrifugal tube respectively. Finally, the collected liquid samples were centrifuged at high speed (10000 rpm) for 3 minutes, and then take out the supernatant and measure the absorbance on a S22PC spectrophotometer.

2.4. Research for reaction mechanism

In order to detect the active substances that may produce in the photocatalytic process, different active inhibitors are added to the solution containing the products to be degraded. Ascorbic acid (AA), potassium iodide (KI) and p-benzoquinone (PBZQ) were used as hydroxyl radical (•OH), hole (h⁺) and superoxide radical (•O₂^–) inhibitors, respectively. The concentrations of all sacrificial agents are 8 mmol^-1.

3. Results and Discussion

3.1. Phase composition and chemical state

XRD technique was employed to study the phase composition and crystal structure of the synthesized catalysts. The results were displayed in Figure 1. As can be seen, for Bi₂MoO₆, all of the diffraction peaks were well matched with standard diffraction card (PDF No.21‑0102) of orthogonal crystal phase¹². The main peaks that appeared at 2θ values of 28.1^ο, 32.5^ο, 46.9^ο, 55.4^ο can be labeled as (131), (002), (062), (331) crystal facets, respectively. For Bi₂WO₆, its diffraction pattern was in accordance with diffraction card (PDF No.79‑2381) with the strongest diffraction peak (113) located at 28.36ο ¹³, which demonstrates an orthogonal crystal structure without impurities phase appearance. It can be found that the diffraction peaks of Bi₂MoO₆/Bi₂WO₆ sample contains both Bi₂WO₆ and Bi₂MoO₆ component without new phase formation. Besides, the characteristic peaks observed in XRD for Bi₂WO₆ and Bi₂MoO₆ indeed show similarities due to their membership in the same orthogonal crystal phase system characterized by a layered structure. However, there are subtle differences in the positions and intensities of these peaks within their respective XRD spectra. Specifically, the primary peak positions for Bi₂WO₆ and Bi₂MoO₆ in XRD are recorded as 28.36° and 28.1°, respectively. It is noteworthy that the diffraction peaks from the Bi₂MoO₆/Bi₂WO₆ sample encompass contributions from both Bi₂WO₆ and Bi₂MoO₆ components, with the principal peak centered at 28.25°. This suggests a successful composite formation of Bi₂WO₆ and Bi₂MoO₆. It can conclude that the Bi₂MoO₆/Bi₂WO₆ composite exhibit a coexistence of both Bi₂WO₆ and Bi₂MoO₆ phase. For comparison, the diffraction pattern of ternary Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ is similar to Bi₂MoO₆ or Bi₂WO₆ except the characteristic peaks (for example, the peaks appear at 18.8^ο and 60.5^ο can be assigned to (004) and (110) crystal planes, respectively) arising from Ti₃C₂-Mxene nanosheets, indicating that the introduction of Ti₃C₂-Mxene does not change the intrinsic structure Bi₂MoO₆/Bi₂WO₆ composite.

Figure 1
The XRD diffraction patterns of the as-prepared samples (2θ: 10-80^ο).

In addition, the FTIR spectra of Bi₂WO₆, Bi₂MoO₆, Bi₂WO₆/Bi₂MoO₆, Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ are depicted in Figure S1 (shown in supplements). As was displayed, absorption bands that appeared between 500~700 cm⁻¹ could be labeled to stretching modes of W-O-W, O-Bi and O-W. In detail, the stretching band of W-O mostly located at ~690 cm⁻¹ and the absorption bands arising from the deformation or stretching vibration of O-Bi modes between 520~600 cm⁻¹. The W-O-W bending vibration mode appeared at about 1315 cm⁻¹. The other peaks that were observed at 1633 and 3451 cm⁻¹ can be attributed to the O-H groups due to their deformation and stretching vibration modes. In addition, the vibration peaks are located at 942, 851 and 793 cm⁻¹ are related to Mo-O stretching mode, respectively¹⁴,¹⁵.

In order to analysis the surface chemical composition and valence states of the as-prepared ternary Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite, the XPS technique was used to investigate its structural properties, and the results are displayed in Figure 2. As can be seen, the appearance of two strong peaks at binding energies of 235.28 and 232.08 eV were attributed to Mo 3d_3/2 and Mo 3d_5/2 of Mo⁶⁺ (shown in Figure 2(a)). For the high resolution XPS spectrum of Bi³⁺ (shown in Figure 2(b)), two obvious peaks corresponding to the binding energy of 164.78 and 159.48 eV were appeared, which belongs to Bi 4f_5/2 and Bi 4f_7/2 of Bi³⁺ structure¹⁶. The characteristic peaks located at 37.88 and 35.78 eV can be ascribed to the W 4f_5/2 and W 4f_7/2 of W⁶⁺in the lattice (Figure 2(c))¹⁷. In the Figure 2(d), the binding energy related to oxygen can be deconvoluted into three peaks which suggests that three kinds of oxygen coexist in the Bi₂MoO₆/Bi₂WO₆ hybrid structure. The peaks corresponding to binding energy of 530.28 and 529.68 eV can be assigned to lattice oxygen in the Bi₂MoO₆ and Bi₂WO₆ crystal structure, whereas the peak located at about 531.1 eV can be attributed to hydroxyl group or adsorbed oxygen on the crystal surface. Figure 2 (e) and (f) present the characteristic peaks of binding energy of C and Ti comes from Ti₃C₂-Mxene, the peaks at 286.28 eV, 284.78 eV and 283.2 eV can be labeled as O-C, C-C and C-Ti chemical bonds, respectively. Moreover, the three main peaks located at 465.06, 462.52 and 459.1 eV can be attributed to Ti (IV)2p_1/2, Ti (II)2p_1/2 and Ti (IV)2p_3/2, individually. The above results suggests that the hybrid structure of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ are formed in the composite. In addition, the VB-XPS measurements were performed to investigate the band structure of the Bi₂MoO₆ and Bi₂WO₆ component, and the results were displayed in Figure 3. It can be drawn that the VB (valence band) position of Bi₂MoO₆ and Bi₂WO₆ samples were 1.97 and 3.26 eV (NHE as reference), respectively. The above results imply that the Bi₂MoO₆ and Bi₂WO₆ are photoreductive and photooxidative according to the energy level of their valence band, respectively.

Figure 2
The XPS spectra of the as-prepared samples: (a) Mo 3d, (b) Bi 4f, (c) W 4f, (d) O1s, (e) C1s and (f) Ti 2p.

Figure 3
The measured valence-band XPS spectra (VB-XPS) of the as-prepared samples.

3.2. Optical absorption properties

In order to study the optical properties of as-synthesized photocatalysts, the UV-vis diffused absorption spectra (UV-vis DRS) measurements were carried out and the results are displayed in Figure 4. As can be seen, the pure Bi₂MoO₆ and Bi₂WO₆ materials exhibit a fundamental absorption profile with absorption edge appear around 479 and 426 nm, respectively. These absorption profile mainly arising from the charge transfer response from VB (valence band) to CB (conduction band). Besides, the following formula was used to determine the band edge of the obtained materials more accurately. In the Kubelka-Munk equation: $α h v = ε {(h v - E_{0})}^{m}$ , the parameters of α stand for absorption coefficients, and m value is determined to be 0.5 according to the transition characteristic of Bi₂MoO₆ and Bi₂WO₆ semiconductors. By employing the K-M formula, the band gaps (E₀) of as-prepared samples were determined to be 2.92 eV and 2.61 eV for Bi₂WO₆ and Bi₂MoO₆ materials, respectively. In order to gain the energy level or position of band edge, the UV-vis DRS data was combined with the VB-XPS results. It thus can be concluded that the VB potential edges were 3.26 and 1.97 V for Bi₂WO₆ and Bi₂MoO₆, respectively. While the CB potential edges were calculated to be -0.64 and 0.34 V for Bi₂MoO₆ and Bi₂WO₆ herein.

Figure 4
The measured UV‑Vis diffuse reflectance spectra of the synthesized Bi₂MoO₆ and Bi₂WO₆ samples (a) and the corresponding band gap of the samples determined via K-M method (b).

3.3. Morphology and textural properties

The microstructures of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆, and Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ were investigated by conducting TEM and HRTEM analysis of the samples, and the results are displayed in. Figure 5. The results indicates that the as-prepared samples are irregular particles assembled by some nano-sheets and all formed a two-dimensional nanosheet structure. The nanosheets of pure Bi₂MoO₆ and Bi₂WO₆ particles are relatively large and have regular corners. In contrast, the nano-sheets of nano-sheet particles of composite materials are more loosely stacked, their edges are damaged and show irregular shapes, and the thickness is obviously thinner, thus the specific surface area of the samples is improved. From the HRTEM image of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂, it can be found that the lattice spacing of 0.32 nm corresponds to the (131) plane of Bi₂MoO₆, and the lattice fringes of 0.23 nm was assigned to Bi₂WO₆¹⁸,¹⁹. This again shows that Bi₂MoO₆ and Bi₂WO₆ are well deposited on Ti₃C₂. The BET measurements were conducted and N₂ adsorption–desorption isotherms of as-prepared materials are shown in Figure S2 (shown in supplements). As can be found, the as-prepared samples demonstrate type IV isotherms with obvious H3 hysteresis loops, which suggests the presence of mesopores and macropores. In addition, the BET surface areas were listed in Table S1 (shown in supplements), which demonstrates the test results on specific surface area of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆, and Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ samples. From the table, it can be found that the specific surface area of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite photocatalyst is larger than that of single Bi₂MoO₆ and Bi₂WO₆. This may be caused by the loading of Bi₂MoO₆ and Bi₂WO₆ nanoparticles on the two-dimensional nanosheet Ti₃C₂ with high specific surface area. The results indicate that the introduction of Ti₃C₂-Mxene nanosheets as substrate can increase the catalyst surface area in a certain degree. For comparison, the BET value for BMO is about 16.55 m²/g, whereas the BET value for ternary Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ is 58.59 m²/g. Generally, the greater the specific surface area of the material, the more active sites on the surface, and the better adsorption of activity and pollutant molecules, thus making the photocatalytic reaction more effective. Thus, the role of Ti₃C₂-Mxene nanosheets is enhance the catalyst surface area and provide more active sites for adsorption of target molecules and further to achieve photocatalytic degradation reaction effectively.

Figure 5
TEM images of (a) Bi₂MoO₆, (b) Bi₂WO₆, (c) Bi₂MoO₆/ Bi₂WO₆, (d) Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ and HRTEM image of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂.

3.4. Photoelectrochemical properties

Photoelectrochemical test can explore the process of separation and recombination of electron-hole pairs²⁰. In order to further explore the influence of heterostructure on photocatalytic activity, photoelectrochemical test can be used to investigate interface charge transfer process and separation efficiency. Figure S3 (shown in supplements) is a graph of the as-prepared samples in visible light transient photocurrent response in the multiple periods, photocurrent intensity of Bi₂MoO₆ is very low, which is attributed to the rapid recombination of photogenerated carriers and the band gap of Bi₂MoO₆ is narrow. On the contrary, the photocurrent of Bi₂MoO₆/Bi₂WO₆ composites is higher than that of pure materials. The results confirm the type II heterojunction energy band structures of are formed between Bi₂MoO₆ and Bi₂WO₆ components, which is benefit to the realization of separation for photo-generated electrons and holes. Moreover, the heterojunction structure can reduce the recombination efficiency and improve the photocatalytic performance. Besides, the introduction of Ti₃C₂-Mxene nanosheets can enhance the photocurrent intensity greatly, which imply that Ti₃C₂-Mxene nanosheets can effectively promote electrons and holes. Therefore, the separation and transfer of holes are helpful to the enhancement of photocatalytic activity for ternary Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite.

Surface photovoltage spectroscopy (SPS) technology as an effective tool can be used to explore the charge carrier separation or transfer process at the interface²¹. Herein, the surface photovoltage spectra of the as-prepared samples were characterized by SPS test and the results are displayed in Figure 6. It is known that the catalytic activity of photocatalyst is largely related to the surface of material or interface charge transport behavior. The basic properties of photogenerated charge and the separation of generated charges in space can be reflected in SPS under the light illumination. The signal of surface photovoltage (SPV) is generated by the change of surface potential after illumination. The intensity reflects the separation efficiency of photo-generated charges. The higher the signal intensity, the higher the photo-generated charge separation efficiency. Therefore, the surface photovoltage technique is used to study the charge transport behavior of semiconductor materials.

Figure 6
The measured SPV spectra from SPS response of the as-prepared samples.

From SPS diagram, it can be found that there are obvious photovoltaic response bands located between 300-500 nm. It is observed that the photovoltage intensity of composite material is relatively high and the response of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ is strongest, which means that the separation of photo-generated electron-cavity pair reaches to the maximum degree in the surface space charge region. The results indicate the incorporation of Mxene as well as the formation of heterostructure of Bi₂MoO₆/Bi₂WO₆ can lead to high separation efficiency of charge carriers²², therefore the photovoltaic response intensity of is strongest. SPS results show that the composite nanoparticles formed show higher photo-generated carrier separation efficiency than single Bi₂MoO₆ or Bi₂WO₆ component.

The separation probability of holes and electrons is a key factor for evaluate the photocatalytic activity of catalysts, so the samples were analyzed by fluorescence spectroscopy²³. The fluorescence spectrum is mainly generated by the photons released during the recombination process of electrons on the conduction band back to the valence band and holes on the semiconductor materials, that is, the fluorescence intensity of the sample can reflect the recombination efficiency of holes and electrons in the as-prepared material to a certain extent. Figure S4 (shown in supplements) gives the fluorescence spectra of the as-prepared samples. The results demonstrate that the fluorescence intensity of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ gradually decreases, that is, the hole-electron recombination efficiency of the four samples decreases sequentially, and the hole-electron separation efficiency sequentially rise. This indicates that the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ photocatalyst has superior photocatalytic activity. This is because the introduction of highly conductive electron transfer mediator Ti₃C₂ constitutes a Z-type photocatalytic system, which promotes the recombination of electrons and holes between the two photosystems, thereby improving the overall hole-electron separation efficiency.

3.5. The photodegradation experiments

The application of the photocatalytic oxidation process along with using different catalysts for the degradation of model pollutants in aqueous media was evaluated in the photodegradation experiments. By analyzing the photocatalytic degradation rate of crystal violet (CV) under visible light irradiation. The photocatalytic performance of the synthesized products was evaluated by the relationship between them, and the results were displayed in Figure 7. As can be seen, the activity of pure Bi₂MoO₆ was very low, and it was irradiated by light for 80 min. After that, the degradation rate was only 36%, while the degradation rate of Bi₂WO₆ was about 51%. It should be noted that after the introduction of Bi₂WO₆, the photocatalytic degradation performance of composite has been improved significantly and the degradation rate of Bi₂MoO₆/Bi₂WO₆ is about 70% under the same reaction condition. The introduction of Ti₃C₂-Mxene into the composite material can lead to the highest degradation rate, which can degrade about 87% of CV during 80 minutes. Based on the experimental results, the maximum light absorption characteristic peak of crystal violet is located at about 590 nm. With the extension of reaction time, the characteristic peak intensity of CV decreases gradually and shifts to the shorter wavelength direction. After 80 minutes, the solution has basically become a colorless and clear solution, indicating that most of crystal violet has been degraded.

Figure 7
The photocatalytic degradation efficiency (a), first order kinetic curves (b), UV-vis absorption of CV removal (c) and catalytic performance on CV degradation on the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ in the presence of various scavenger agents (d).

Generally speaking, the photocatalytic degradation of organic compounds conforms to Langmuir-Hinshlwood kinetic model, which is simplified for the convenience of calculation: ln(C₀/C_t) = kt, where k is the apparent rate constant of the first-order reaction, t is the reaction time, C₀ is the initial concentration after dark reaction, and C_t is the concentration of organic compounds at time t. In order to show the photocatalytic performance more clearly and intuitively, the rate constants of CV degradation of these samples was calculated. The degradation rate constants of Bi₂MoO₆ and Bi₂WO₆ are 0.0056 and 0.0089 min^-1, respectively. By comparison, the reaction rate constant of Bi₂MoO₆/Bi₂WO₆ is 0.01478 min^-1, whereas the degradation rate constant is the highest, 0.0232 min^-1 for Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite material, which is nearly four folds as much as that of pure Bi₂MoO₆. According to the first-order kinetic curve of the sample, the degradation rate constants are 0.0056, 0.0089, 0.01478 and 0.0232 min^-1 for Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆ and Bi₂MoO₆/Bi₂WO₆@Ti₃C₂, respectively, and their photocatalytic activities are Bi₂MoO₆/Bi₂WO₆@Ti₃C₂> Bi₂MoO₆/Bi₂WO₆> Bi₂WO₆> Bi₂MoO₆.

In addition, we choose p-nitrophenol as the research object of this experiment to investigate the photocatalytic degradation ability of the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite material to colorless organic pollutants, because p-nitrophenol (4-NP) is one of the toxic phenols that pollute water. Figure S5 (shown in supplements) gives the absorption spectra of 4-NP by photocatalytic degradation as a function of reaction time with the as-prepared Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ sample under illumination of visible light. It was observed that the concentration of 4-NP gradually decrease since the intensity of absorption spectra is weakened continuously. The results suggest that Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ sample has superior photocatalytic activity toward 4-NP removal under the identical conditions.

3.6. Exploration of photocatalytic mechanism

Photocatalytic process is generally considered as the photo-generation of electron-hole pairs and the active radicals caused by light illumination. Therefore, the main active substances in the photocatalytic system were identified, which is of reference value for studying photocatalytic mechanism. In order to explore the photocatalytic mechanism of the present Z-scheme system, the photocatalytic mechanism of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ catalyst was studied by trapping tests. In the experiment, the main substances produced during the decomposition of CV were investigated. Herein, ascorbic acid (AA), potassium iodide (KI) and p-benzoquinone (PBZQ) were adopted as trapping agents of free radicals such as hydroxyl radical (•OH), hole (h⁺) and superoxide radical (•O₂^–), and the concentrations of all sacrificial agents are 8 mmol^-1, so as to explore the effect of active substances on photocatalytic reaction. The photocatalysis reaction of the Z-scheme system was measured under the same conditions. The results were shown in Figure 7(d). It can be found that the photocatalytic activity was the best without any addition of sacrificial species, and the degradation rate of CV was 86%. After adding KI, the photocatalytic activity is somewhat weakened, and the degradation rate of CV was 66%. Nonetheless, in the presence of PBZQ and AA, there were obvious changes for the catalytic performance toward CV removal. The final degradation rate of CV was 30% and 23% with introduction of PBZQ and AA, while the degradation curve fluctuated slightly when PBZQ was added. It is probably because BQ has certain adsorption on catalyst surface. Compared to the no addition of scavenger agents, after adding AA and PBZQ, the degradation of CV was inhibited obviously. Therefore, it can be speculated that photoinduced •O₂^–and •OH were main active species. In other words, •O₂^–or •OH play a major role in photocatalytic degradation of CV, and h⁺ just plays a minor role in the catalytic process.

Based on all the above characterization and analysis, a possible photocatalytic mechanism of the Z-type Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ heterostructure can be proposed, as displayed in Figure 8. The Z-type photocatalytic system consists of two photosystems initially, which are a photooxidant (PS I Bi₂MoO₆) and a photoreductant (PS II Bi₂WO₆)²³,²⁴. The two photosystems are excited by visible light since Bi₂MoO₆ and Bi₂WO₆ belong to narrow band-gap semiconductors, and the photoinduced electrons can jump from the VB to CB. The holes leaved in the valence band have oxidizing ability, and the electron carriers in the conduction band have reducing ability, so they have photocatalytic activity. At the same time, the electrons in the conduction band of PS II Bi₂WO₆ recombine with the holes in the valence band of PS I Bi₂MoO₆ through the electron transport medium Ti₃C₂ MXene with strong conductivity, which greatly reduces the recombination probability of holes and electrons inside the two photosystems. The Z-scheme mechanism preserves the hole carriers in the conduction band of PS II Bi₂WO₆ and the electron carriers in the valence band of PS I Bi₂MoO₆, and the charge carriers from different PS system were separated effectively, which improves the separation efficiency of electrons and holes in the whole system. Compared with a single photocatalyst (I: Bi₂MoO₆ oxidation potential +1.97 V, reduction potential -0.64 V; II: Bi₂WO₆ oxidation potential +3.26 V, reduction potential +0.34 V), the constructed Z-type photocatalytic system has higher Redox ability (oxidation potential +3.26 V, reduction potential -0.64 V). The Z-type photocatalytic system can effectively inhibit the recombination of electrons and holes, and has stronger redox ability.

Figure 8
The schematic diagram for illustration of the possible Z-type mechanism of the photocatalytic reaction over the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ heterostructure system.

Subsequently, the photogenerated electrons in CB of Bi₂MoO₆ will react with O₂ molecules adsorbed on the surface of catalyst to generate·•O₂^– radicals since the chemical potential for O₂ transfer into •O₂^– is -0.33 V. In addition, h⁺ that accumulated on VB of Bi₂WO₆ is a strong oxidizing species, which can transfer OH^–or H₂O into •OH radicals, and CV can be directly decomposed by •OH radicals to achieve the effect of pollutant degradation. It also can be seen from the graph that the introduction of Mxene promotes separation of hole-electron pairs (e^–, h⁺) via Z-scheme route effectively. Thus, the recombination rate of electron and hole was inhibited greatly, and the migration of charge carrier is accelerated. At last, CV was decomposed into CO₂ and H₂O in the presence of these radicals. The 2D Mxene promotes separation of electron-hole pairs and interface electron transfer, and then enhances the photocatalytic performance.

3. 7. Stability test for catalytic reaction

The stability of catalyst is an important factor to determine whether it can be applied for actual situation or sewage treatment process. Moreover, the reuse rate is a major index to examine the practical performance of the catalyst. Therefore, cyclic experiment was used herein to analyze the stability of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ sample. The effect of the cyclic catalytic degradation of crystal violet of the composite sample is shown in Figure S6(a). After the degradation experiment, the residual catalyst was collected and rinse by deionized water for several times so that the sample can be reused for the next cycle catalytic reaction. As shown in the graph, after five successive cycles, the photocatalytic degradation ratio of CV can still be kept about 80%, and the activity was not obvious loss compared to the 1^st run. Figure S6 (b) shows the as-obtained XRD diffraction patterns of the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ sample before and after the cyclic experiments. The result shows that there is no obvious change in phase composition and crystal structure except the peak intensity of [004] crystal plane (2θ~19.2^ο) of the Mxene was decreased, indicating that the Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ composite has good photocatalytic stability and reuse performance.

4. Conclusions

In summary, a Z-scheme Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ mxene photocatalytic system have been constructed via a simple solvothermal route. This Z-scheme structure can avoid the weakening of electron reduction and hole oxidation ability and the catalytic performance of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ mxene photocatalytic system has a better and stronger catalytic ability for degradation of CV and 4-NP removal. Moreover, the reaction mechanisms involved in the photocatalytic process were studied in detail. The enhanced activity of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ mxene can be attributed to the widening of light absorption range and the formation of 2D composite heterojunction, close interface interaction and reduction of photo-generated charge recombination rate. Firstly, Ti₃C₂ MXene facilitate electron transfer in different photocatalytic systems and improve the catalytic efficiency. On the other hand, the microstructure of the target composite is lamellar, which reduces the recombination rate of electron-hole and creates more action sites. And so on, and the composite material also shows good stability and recyclability.

Supplementary material

The following online material is available for this article:

Table S1 –

The BET specific areas of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆ and Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ samples.

Figure S1 –

The FTIR spectra of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆ and Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ samples.

Figure S2 –

The N₂ adsorption–desorption isotherms of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/Bi₂WO₆ and Bi₂MoO₆/ Bi₂WO₆@Ti₃C₂ samples.

Figure S3 –

The obtained transient photocurrent responses (I-t curve) of Bi₂MoO₆, Bi₂WO₆, Bi₂MoO₆/ Bi₂WO_6, and Bi₂MoO₆/ Bi₂WO₆@Ti₃C₂ samples.

Figure S4 –

The obtained PL emission spectra of as-made different photocatalysts.

Figure S5 –

The evolution of time-dependent absorption spectra of 4-NP decomposed by Bi₂MoO₆/ Bi₂WO₆@Ti₃C₂ photocatalysts.

Figure S6 –

The recycle experiments for test the stability of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ material (a) and the corresponding XRD spectra of Bi₂MoO₆/Bi₂WO₆@Ti₃C₂ photocatalysts that are original without use and used after 5 st runs (b), respectively.

5. Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities (Project no: S202110504254).

^✤These authors contributed equally to this work.

7. References

¹ Eshaq G, Wang SB, Sun HQ, Sillanpää M. Core/shell FeVO4@BiOCl heterojunction as a durable heterogeneous Fenton catalyst for the efficient sonophotocatalytic degradation of p-nitrophenol. Separ Purif Tech. 2020;231:115915.
² Bahramian A, Raeissi K, Hakimizad A. An investigation of the characteristics of Al2O3/TiO2 PEO nanocomposite coating. Appl Surf Sci. 2015;351:13-26.
³ Zhao W, Feng Y, Huang HB, Zhou PC, Li J, Zhang LL, et al. A novel Z-scheme Ag3VO4/BiVO4 heterojunction photocatalyst: Study on the excellent photocatalytic performance and photocatalytic mechanism. Appl Catal B. 2019;245:448.
⁴ Liang Y, Chen YX, Lin L, Zhao MJ, Zhang L, Yan J, et al. An in situ ion exchange grown visible-light-driven Z-scheme AgVO ₃ /AgI graphene microtube for enhanced photocatalytic performance. New J Chem. 2020;44(4):1579-87.
⁵ Yang Q, Chen F, Li XM, Wang DB, Zhong Y, Zeng GM. Self-assembly Z-scheme heterostructured photocatalyst of Ag ₂ O@Ag-modified bismuth vanadate for efficient photocatalytic degradation of single and dual organic pollutants under visible light irradiation. RSC Advances. 2016;6(65):60291-307.
⁶ Zhang TT, Zhang DF, Tang YX, Wang KL, Pu XP, Shao X, et al. Facial synthesis of a novel Ag ₄ V ₂ O ₇ /g‐C ₃ N ₄ heterostructure with highly efficient photoactivity. J Am Ceram Soc. 2019;102(7):3897-907.
⁷ Yao XT, Zhang D, Liu YP, Chen YZ, Zhang DF, Liu JC, et al. One stone, three birds: up-conversion, photothermal and p-n heterojunction to boost BiOBr:Yb3+,Er3+/Cu3Mo2O9 full spectrum photodegradation. Front Chem Sci Eng. 2024;18(10):118.
⁸ Xu J, Mao YG, Liu T, Peng Y. Synthesis of a novel one-dimensional BiOBr–Bi ₄ O ₅ Br ₂ heterostructure with a high quality interface and its enhanced visible-light photocatalytic activity. CrystEngComm. 2018;20(16):2292-8.
⁹ Zhang LL, Wang ZQ, Li T, Hu C, Yang M. Ultrathin Bi ₄ O ₅ Br ₂ nanosheets with surface oxygen vacancies and strong interaction with Bi ₂ O ₂ CO ₃ for highly efficient removal of water contaminants. Environ Sci Nano. 2022;9(4):1341-52.
¹⁰ Chen YJ, Zhao CR, Ma SA, Xing PX, Hu X, Wu Y, et al. Fabrication of a Z-scheme AgBr/Bi ₄ O ₅ Br ₂ nanocomposite and its high efficiency in photocatalytic N ₂ fixation and dye degradation. Inorg Chem Front. 2019;6(11):3083-92.
¹¹ Yin GL, Jia YL, Lin YH, Zhang CY, Zhu ZH, Ma Y. A review on hierarchical Bi ₂ MoO ₆ nanostructures for photocatalysis applications. New J Chem. 2022;46(3):906-18.
¹² Dai WL, Yu JJ, Xu H, Hu X, Luo XB, Yang LX, et al. Synthesis of hierarchical flower-like Bi ₂ MoO ₆ microspheres as efficient photocatalyst for photoreduction of CO ₂ into solar fuels under visible light. CrystEngComm. 2016;18(19):3472-80.
¹³ Etogo A, Liu R, Ren JB, Qi LW, Zheng CC, Ning JQ, et al. Facile one-pot solvothermal preparation of Mo-doped Bi ₂ WO ₆ biscuit-like microstructures for visible-light-driven photocatalytic water oxidation. J Mater Chem A Mater Energy Sustain. 2016;4(34):13242-50.
¹⁴ Zhang JL, Zhang LS, Yu N, Xu KB, Li SJ, Wang HL, et al. Flower-like Bi ₂ S ₃ /Bi ₂ MoO ₆ heterojunction superstructures with enhanced visible-light-driven photocatalytic activity. RSC Advances. 2015;5(92):75081-8.
¹⁵ Hao YC, Dong XL, Zhai SR, Wang XY, Ma HC, Zhang XF. Towards understanding the photocatalytic activity enhancement of ordered mesoporous Bi ₂ MoO ₆ crystals prepared via a novel vacuum-assisted nanocasting method. RSC Advances. 2016;6(42):35709-18.
¹⁶ Wang DJ, Shen HD, Guo L, Wang C, Fu F, Liang YC. La and F co-doped Bi ₂ MoO ₆ architectures with enhanced photocatalytic performance via synergistic effect. RSC Advances. 2016;6(75):71052-60. http://doi.org/10.1039/C6RA12898J
» http://doi.org/10.1039/C6RA12898J
¹⁷ Xu YS, Zhang ZJ, Zhang WD. Inlay of Bi2O2CO3 nanoparticles onto Bi2WO6 nanosheets to build heterostructured photocatalysts. Dalton Trans. 2014;43:3660.
¹⁸ Zhang XB, Zhang L, Hua JS, Huang XH. Facile hydrothermal synthesis and improved photocatalytic activities of Zn ²⁺ doped Bi ₂ MoO ₆ nanosheets. RSC Advances. 2016;6(38):32349-57.
¹⁹ Ju P, Wang Y, Sun Y, Zhang D. Controllable one-pot synthesis of a nest-like Bi2WO6/BiVO4 composite with enhanced photocatalytic antifouling performance under visible light irradiation. Dalton Trans. 2016;45(11):4588-602.
²⁰ Liu X, Wang SK, Cao JH, Yu JH, Dong JX, Zhao YT, et al. Oxygen doping regulation of Co single atom catalysts for electro-Fenton degradation of tetracycline. J Colloid Interface Sci. 2024;673:434.
²¹ Kronik L, Shapira Y. Surface photovoltage phenomena: Theory, experiment, and applications. Surf Sci Rep. 1999;371:206.
²² Yang G, Liang YJ, Li K, Yang J, Xu R, Xie XJ. Construction of a Ce ³⁺ doped CeO ₂ /Bi ₂ MoO ₆ heterojunction with a mutual component activation system for highly enhancing the visible-light photocatalytic activity for removal of TC or Cr( vi ). Inorg Chem Front. 2019;6(6):1507-17. http://doi.org/10.1039/C9QI00302A
» http://doi.org/10.1039/C9QI00302A
²³ Wang SC, Wang LZ, Huang W. Bismuth-based photocatalysts for solar energy conversion. J Mater Chem A Mater Energy Sustain. 2020;8(46):24307-52. http://doi.org/10.1039/D0TA09729B
» http://doi.org/10.1039/D0TA09729B
²⁴ Guo LN, Huang HW, Mei LF, Li M, Zhang YH. Bismuth-based Z-scheme photocatalytic systems for solar energy conversion. Mater Chem Front. 2021;5(6):2484-505.

Publication Dates

Publication in this collection
27 Jan 2025
Date of issue
2024

History

Received
19 June 2024
Reviewed
03 Sept 2024
Accepted
20 Sept 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

[1] ¹ Eshaq G, Wang SB, Sun HQ, Sillanpää M. Core/shell FeVO4@BiOCl heterojunction as a durable heterogeneous Fenton catalyst for the efficient sonophotocatalytic degradation of p-nitrophenol. Separ Purif Tech. 2020;231:115915.

[2] ² Bahramian A, Raeissi K, Hakimizad A. An investigation of the characteristics of Al2O3/TiO2 PEO nanocomposite coating. Appl Surf Sci. 2015;351:13-26.

[3] ³ Zhao W, Feng Y, Huang HB, Zhou PC, Li J, Zhang LL, et al. A novel Z-scheme Ag3VO4/BiVO4 heterojunction photocatalyst: Study on the excellent photocatalytic performance and photocatalytic mechanism. Appl Catal B. 2019;245:448.

[4] ⁴ Liang Y, Chen YX, Lin L, Zhao MJ, Zhang L, Yan J, et al. An in situ ion exchange grown visible-light-driven Z-scheme AgVO ₃ /AgI graphene microtube for enhanced photocatalytic performance. New J Chem. 2020;44(4):1579-87.

[5] ⁵ Yang Q, Chen F, Li XM, Wang DB, Zhong Y, Zeng GM. Self-assembly Z-scheme heterostructured photocatalyst of Ag ₂ O@Ag-modified bismuth vanadate for efficient photocatalytic degradation of single and dual organic pollutants under visible light irradiation. RSC Advances. 2016;6(65):60291-307.

[6] ⁶ Zhang TT, Zhang DF, Tang YX, Wang KL, Pu XP, Shao X, et al. Facial synthesis of a novel Ag ₄ V ₂ O ₇ /g‐C ₃ N ₄ heterostructure with highly efficient photoactivity. J Am Ceram Soc. 2019;102(7):3897-907.

[7] ⁷ Yao XT, Zhang D, Liu YP, Chen YZ, Zhang DF, Liu JC, et al. One stone, three birds: up-conversion, photothermal and p-n heterojunction to boost BiOBr:Yb3+,Er3+/Cu3Mo2O9 full spectrum photodegradation. Front Chem Sci Eng. 2024;18(10):118.

[8] ⁸ Xu J, Mao YG, Liu T, Peng Y. Synthesis of a novel one-dimensional BiOBr–Bi ₄ O ₅ Br ₂ heterostructure with a high quality interface and its enhanced visible-light photocatalytic activity. CrystEngComm. 2018;20(16):2292-8.

[9] ⁹ Zhang LL, Wang ZQ, Li T, Hu C, Yang M. Ultrathin Bi ₄ O ₅ Br ₂ nanosheets with surface oxygen vacancies and strong interaction with Bi ₂ O ₂ CO ₃ for highly efficient removal of water contaminants. Environ Sci Nano. 2022;9(4):1341-52.

[10] ¹⁰ Chen YJ, Zhao CR, Ma SA, Xing PX, Hu X, Wu Y, et al. Fabrication of a Z-scheme AgBr/Bi ₄ O ₅ Br ₂ nanocomposite and its high efficiency in photocatalytic N ₂ fixation and dye degradation. Inorg Chem Front. 2019;6(11):3083-92.

[11] ¹¹ Yin GL, Jia YL, Lin YH, Zhang CY, Zhu ZH, Ma Y. A review on hierarchical Bi ₂ MoO ₆ nanostructures for photocatalysis applications. New J Chem. 2022;46(3):906-18.

[12] ¹² Dai WL, Yu JJ, Xu H, Hu X, Luo XB, Yang LX, et al. Synthesis of hierarchical flower-like Bi ₂ MoO ₆ microspheres as efficient photocatalyst for photoreduction of CO ₂ into solar fuels under visible light. CrystEngComm. 2016;18(19):3472-80.

[13] ¹³ Etogo A, Liu R, Ren JB, Qi LW, Zheng CC, Ning JQ, et al. Facile one-pot solvothermal preparation of Mo-doped Bi ₂ WO ₆ biscuit-like microstructures for visible-light-driven photocatalytic water oxidation. J Mater Chem A Mater Energy Sustain. 2016;4(34):13242-50.

[14] ¹⁴ Zhang JL, Zhang LS, Yu N, Xu KB, Li SJ, Wang HL, et al. Flower-like Bi ₂ S ₃ /Bi ₂ MoO ₆ heterojunction superstructures with enhanced visible-light-driven photocatalytic activity. RSC Advances. 2015;5(92):75081-8.

[15] ¹⁵ Hao YC, Dong XL, Zhai SR, Wang XY, Ma HC, Zhang XF. Towards understanding the photocatalytic activity enhancement of ordered mesoporous Bi ₂ MoO ₆ crystals prepared via a novel vacuum-assisted nanocasting method. RSC Advances. 2016;6(42):35709-18.

[16] ¹⁶ Wang DJ, Shen HD, Guo L, Wang C, Fu F, Liang YC. La and F co-doped Bi ₂ MoO ₆ architectures with enhanced photocatalytic performance via synergistic effect. RSC Advances. 2016;6(75):71052-60. http://doi.org/10.1039/C6RA12898J
» http://doi.org/10.1039/C6RA12898J

[17] ¹⁷ Xu YS, Zhang ZJ, Zhang WD. Inlay of Bi2O2CO3 nanoparticles onto Bi2WO6 nanosheets to build heterostructured photocatalysts. Dalton Trans. 2014;43:3660.

[18] ¹⁸ Zhang XB, Zhang L, Hua JS, Huang XH. Facile hydrothermal synthesis and improved photocatalytic activities of Zn ²⁺ doped Bi ₂ MoO ₆ nanosheets. RSC Advances. 2016;6(38):32349-57.

[19] ¹⁹ Ju P, Wang Y, Sun Y, Zhang D. Controllable one-pot synthesis of a nest-like Bi2WO6/BiVO4 composite with enhanced photocatalytic antifouling performance under visible light irradiation. Dalton Trans. 2016;45(11):4588-602.

[20] ²⁰ Liu X, Wang SK, Cao JH, Yu JH, Dong JX, Zhao YT, et al. Oxygen doping regulation of Co single atom catalysts for electro-Fenton degradation of tetracycline. J Colloid Interface Sci. 2024;673:434.

[21] ²¹ Kronik L, Shapira Y. Surface photovoltage phenomena: Theory, experiment, and applications. Surf Sci Rep. 1999;371:206.

[22] ²² Yang G, Liang YJ, Li K, Yang J, Xu R, Xie XJ. Construction of a Ce ³⁺ doped CeO ₂ /Bi ₂ MoO ₆ heterojunction with a mutual component activation system for highly enhancing the visible-light photocatalytic activity for removal of TC or Cr( vi ). Inorg Chem Front. 2019;6(6):1507-17. http://doi.org/10.1039/C9QI00302A
» http://doi.org/10.1039/C9QI00302A

[23] ²³ Wang SC, Wang LZ, Huang W. Bismuth-based photocatalysts for solar energy conversion. J Mater Chem A Mater Energy Sustain. 2020;8(46):24307-52. http://doi.org/10.1039/D0TA09729B
» http://doi.org/10.1039/D0TA09729B

[24] ²⁴ Guo LN, Huang HW, Mei LF, Li M, Zhang YH. Bismuth-based Z-scheme photocatalytic systems for solar energy conversion. Mater Chem Front. 2021;5(6):2484-505.