Photoelectrocatalytic (PEC) oxidation has been regarded as a potential approach of solving the problems of the organic pollution in aqueous solution using different semiconductor electrodes [1-3]. In the PEC process, the applied bias potential not only utilizes the electrochemical degradation of organic compounds but also improves the photocatalytic oxidation by accelerating the separation of electron-hole pairs. Early studies on photoelectrode materials for PEC reactions focused on TiO₂. However, TiO₂ can only absorb 3%-4% portion of UV light in sunlight due to the wide band gap (3.0-3.2 eV), which results in a very low PEC conversion efficiency. Consequently, a lot of visible-light-response photoelectrodes such as WO₃ (2.6 eV), Fe₂O₃ (2.1 eV), BiVO₄ (2.4 eV), have been widely developed to improve the utilization of solar energy [4-10]. Although many decisive breakthroughs have been made after the continuous development of PEC technology, the energy conversion efficiency still cannot match the practical applications. Thus, it is challenging to develop photoelectrodes with excellent performance, high stability and low cost.

In 2010, Zhu et al. [11] reported a new indirect bandgap semiconductor photocatalyst BiPO₄ with a band gap of 3.85 eV for the first time. And its photocatalytic degradation of methyl blue (MB) performance was much higher than that of P25. The high photocatalytic activity of BiPO₄ was ascribed to its high valence potential and the induction of PO₄^3–, which can promote the effective separation of electron-hole pairs. Due to the excellent photocatalytic oxidation property of BiPO₄, it might be a promising material for environmental treatment in the practical applications. However, most of studies, until now, were limited to the traditional powder system photocatalytic technology. Inherent limitations, such as the stability and reuse of the catalysts, were seldom explored [12, 13]. Recently, Zhang et al. [14] prepared BiPO₄ photoelectrodes through one-step cathodic electrodeposition approach, and they reported that BiPO₄ photoelectrode exhibited the excellent PEC degradation efficiency of organic pollutants. However, BiPO₄ can only be excited in the ultraviolet region, which leads to the low utilization efficiency of sunlight. Our early work first prepared BiOI/BiPO₄/FTO composite electrode by a simple two-step electrodeposition method to improve the light utilization and the photocatalytic performance. The PEC activity of BiOI/BiPO₄/FTO composite electrode for the removal of tetracycline under the visible light was 2.46 times as high as that of the BiPO₄/FTO electrode [15]. However, until now, a few papers have been reported about the preparation and properties of BiPO₄ or BiPO₄ based composite electrode. Searching for a new modification method to improve the PEC activity of BiPO₄ electrode is still a challenge and need further study.

Graphene is a two-dimensional material composed of sp² single-layer hexagonal carbon. Due to unique thermodynamic, electrical, optical and mechanical functions, it has drawn wide attention from many fields. Moreover, its application in the photocatalytic technology has been widely concerned [16-22]. The modification of photocatalytic materials with graphene can enhance the conductivity and improve the photocatalytic activity due to the quicker transfer of electrons. In particular, the combination of semiconductors and graphene is widely recognized as one of the most promising electrodes in photoelectrocatalysis. Furthermore, the deposition of the graphene film on the conductive substrate can further serve a substrate for the uniform growth of the crystal of the photocatalyst material to obtain a graphene-modified composite electrode material. Many graphene-modified photoelectrode semiconductor materials, such as ZnO/rGO, Cu₂O/rGO, BiVO₄/rGO and Fe₂O₃/rGO, showed the enhanced anode photocurrent and high PEC conversion efficiency [23-29].

In this work, a BiPO₄/rGO/FTO composite photoelectrode was prepared by a simple two-step electrodeposition method. The BiPO₄/rGO/FTO composite photoelectrode exhibits higher PEC activity than BiPO₄/FTO photoelectrode. The introduction of rGO substrate not only increased the light absorption of BiPO₄ but also promoted the separation and transfer of photogenerated electron-holes pairs. The effects of working voltage and the electrodeposition time on the degradation efficiency of methyl orange were investigated. The mechanism of enhancing PEC activity of BiPO₄/rGO/FTO composite photoelectrode was discussed. This two-step simple electrodeposition method for the preparation of photoelectrode material might have a potential for scale production, and this composite photoelectrode might be a promising material for the environmental treatment.

All the used chemicals were analytical reagents. All electrochemical deposition experiments were carried out with an electrochemical workstation (CHI760E, Shanghai Huachen Instrument Co., Ltd.). The resistance of the fluorine-doped tin oxide (FTO) glass resistor was 15 ohms and its area was 5 × 2.5 cm². FTO glass modification was performed according to the following steps. FTO glass was soaked in ethanol for ultrasonic cleaning for 30 min, soaked in 10% dilute nitric acid for 30 min, and then thoroughly washed with the mixture of ethanol and distilled water, and dried at 60 ℃ in an oven. The electrolyte of graphene oxide was prepared according to the following steps. The graphene oxide powder was ultrasonically dispersed in 100 mL of ultrapure water to configure a solution having a concentration of 2 mg/mL. Then, a certain amount of LiClO₄ was added to reach a concentration of 0.12 M. Electrodeposition of rGO/FTO electrode was performed according to the following steps. rGO/FTO electrode material was prepared by electrodeposition reduction in a three-electrode system with saturated calomel electrode (SCE) as the reference electrode, FTO as working electrode, and Pt wire as the counter electrode. The rGO/FTO composite electrode material was obtained by electrodeposition for 150 s at a bias voltage of -1.5 V.

BiPO₄ precursor electrolyte solution was prepared according to the following steps. The electrodeposition process of this composite electrode is shown in Fig. 1. Firstly, 10 mM ethylenediaminetetraacetic acid was dissolved into 100 mL of distilled water and then 10 mM lanthanum nitrate pentahydrate was added to adjust the pH to below 1. After that, 100 mM sodium phosphate decahydrate was added to adjust the pH to 1 and then 50 mL of 30% hydrogen peroxide was added. BiPO₄/rGO/FTO was prepared by the two-step electrodeposition method as following steps. In the three-electrode system, the saturated calomel electrode, the as prepared rGO/FTO electrode and the Pt wire were respectively used as the reference electrode, the working electrode, and the counter electrode. Before the electrodeposition of BiPO₄, the rGO/FTO electrode was rinsed with distilled water. The BiPO₄/rGO/FTO electrodes were prepared by electrodeposition of BiPO₄ precursor at a bias of -0.15 V for different deposition time. The obtained different BiPO₄/rGO/FTO electrodes were denoted as BiPO₄/rGO/FTO_{-x min} (x = 30, 45 and 60 min), respectively. For the comparison, the BiPO₄/FTO electrode was prepared by direct electrodeposition of BiPO₄ precursor on the surface of FTO under the same conditions.

The crystal structure of the electrode material was characterized by a German Bruker D8 Advance X-ray powder diffractometer (XRD). The Cu Kα target with λ = 1.5418 has an instrument power of 3 kV, a voltage of 40 kV, a current of 20 mA, and a scan range of 10° to 70°. With the SU8010 cold field emission scanning electron microscope (SEM), under the acceleration voltage of the electron beam of 100 kV, the microscopic morphology of the electrode material was characterized. The morphology and crystal structure of the electrode material was characterized by an FEI Tecnai G² F30 field emission transmission electron microscope (TEM) under an accelerating voltage of 200 kV. The Hitachi U-3900 UV-Vis spectrophotometer was used for the characterization of UV-vis diffuse reflectance spectra of the materials. The optical properties of the catalyst were characterized by the FLS980 fluorescence spectrometer (Edinburgh, UK) with an excitation wavelength of 225 nm. The Raman spectra of the materials were characterized by HORIBA HR 800 laser confocal micro-Raman spectroscopy. The elemental composition of the material was characterized by an X-ray photoelectron spectrometer (PHI Quantern, Japanese).

The photoelectric synergistic performance of materials was explored with an electrochemical workstation (CHI760E, Shanghai Huachen Instrument Co., Ltd.). In the three-electrode system, the saturated calomel electrode, the Pt wire and the BiPO₄/rGO/FTO electrode were used as the reference electrode, the counter electrode and the working electrode. With 10 ppm MB as an organic pollutant model, the PEC performance of the catalytic substrate was investigated. The light source was an ultraviolet lamp with an output intensity of 10 W. The change in the concentration of the MB solution was detected with a Hitachi U-3900 UV-vis spectrophotometer under an absorption wavelength of 464 nm. The volume of methyl orange solution in the quartz reactor was 100 mL and the effective area of ​​the BiPO₄/rGO/FTO electrode was 5 cm². The effect of deposition time of BiPO₄ on the PEC activity of composite electrode materials was investigated under a certain bias voltage. The effects of different working voltages on the PEC activity of the electrodes were investigated. The photocatalytic, electrocatalytic and photoelectrocatalytic removal efficiencies of the methyl orange solution by BiPO₄/rGO/FTO electrodes were studied. Electrochemical tests such as transient response photocurrent density, electrochemical impedance spectroscopy and linear sweep volt-ampere curve were explored in the same three-electrode system with an electrochemical workstation. The electrolyte solution was 100 mL of 0.1 M Na₂SO₄.

The SEM images of rGO/FTO, BiPO₄/FTO and BiPO₄/rGO/FTO photoelectrode are shown in Fig. 2. Layered ultrathin graphene films, formed by the reduction of graphene oxide under a certain negative bias, were uniformly deposited on the FTO glass sheet (Fig. 2(a)). Pure BiPO₄ nanorods were deposited on FTO glass (Fig. 2(b)). The morphology of pure BiPO₄ shows the regular nanorod with a length of 1-2 μm and a diameter of 400-600 nm. In Fig. 2(c) and 2(d), BiPO₄ nanorods with the length at the range of 2-2.5 μm and the diameter distribution range of 1.5-2 μm were uniformly distributed on the rGO nanosheets. Compared with the pure BiPO₄ nanorods, the BiPO₄ nanorods in the composite became larger and irregular, and the ratio of length to diameter was increased. This fact showed that introduction of rGO affected the growth of BiPO₄ crystals.

In order to further explore the crystal structure of the electrode material, the prepared BiPO₄/rGO/FTO materials were characterized by XRD. It can be seen that the characteristic peak of graphene oxide at 2θ = 11° is absent in pure rGO/FTO, indicating that the graphene oxide was reduced into the rGO during electrodeposition (Fig. 3). The major characteristic peaks of BiPO₄ such as (100), (101), (110), (200) and (102) were observed in the XRD patterns, which were consistent with the standard PDF card of hexagonal BiPO₄ (JCPDS No. 15-0766). For the BiPO₄/rGO/FTO composite photoelectrode, the characteristic peaks of monoclinic phases BiPO₄ at (011), (111), (200), (120), (202), (220) and (022) (JCPDS No. 15-0767) can also be observed, indicating that the formation of mixture crystal structure of hexagonal and monoclinic phases in this composite electrode. This fact shows that the introduction of rGO might affect the crystal growth of BiPO₄, resulting in the phase transfer from hexagonal crystal to monoclinic crystal.

The TEM and HRTEM images of BiPO₄/rGO/FTO composite photoelectrodes were shown in Fig. 4. BiPO₄ nanorods were uniformly deposited on the layer rGO film. The lattice fringes of BiPO₄ were observed in HRTEM images of the BiPO₄/rGO/FTO composite photoelectrode (Fig. 4(c)). The diffraction speckle pattern obtained by Fourier transform (Fig. 4(d)) showed that the lattice spacings correspond to (111), (220) and (022) of monoclinic phase of BiPO₄ crystal were respectively 0.3514, 0.2383, and 0.2328 nm, which were consistent with XRD results.

The Raman spectra of GO, rGO, BiPO₄ and BiPO₄/rGO/FTO composite photoelectrodes were shown in Fig. 5. The characteristic peaks of graphene (D peak and G peak) occurred at 1342 and 1587 cm^‒1 in the Raman spectra of GO and rGO. The D peak indicated the formation of defects in the associated graphite layer structure due to the introduction of oxygen-containing functional groups, whereas the G band indicated the symmetry and crystallinity of graphene. Compared with GO, rGO showed the higher I_D/I_G ratio, indicating that GO was reduced to rGO during electrodeposition. The O-Bi-O vibration peak of BiPO₄ appeared at 238 cm^‒1, and the vibration peaks of PO₄³⁺ were observed at 350, 562, 569, and 1093 cm^‒1. The composite electrode showed the vibration peak of PO₄³⁺ in BiPO₄ at 607 and 1098 cm^‒1 and the characteristic D peak and G peak of rGO at 1342 and 1587 cm^‒1, indicating that two materials of rGO and BiPO₄ were well combined in the BiPO₄/rGO/FTO composite photoelectrode. Moreover, the value of I_D/I_G was further increased indicating that the reduction degree of graphene was further improved during the electrodeposition process of BiPO₄. These changes increased the conductivity of rGO, accelerated the transfer of photogenerated charges of BiPO₄ and enhanced the PEC activity consequently. In addition, the G peak of graphene in the BiPO₄/rGO/FTO electrode shifted from 1587 to 1591 cm^‒1, indicating that a strong chemical interaction occurred between rGO and BiPO₄.

The XPS broad spectra of BiPO₄/FTO and BiPO₄/rGO/FTO photoelectrodes and the narrow spectra of Bi 4f, O 1s and C 1s are shown in Fig. 6. The peaks of Bi, P, O, C and other elements were observed in the wide spectra of two electrode materials (Fig. 6(a)). The two peaks of 159.4 and 164.7 eV in the BiPO₄/FTO electrode respectively represented the Bi 4f_7/2 and Bi 4f_5/2 peaks of the Bi-O bond (Fig. 6(b)). In the BiPO₄/rGO/FTO composite electrode, the peaks of Bi-O bond shifted to 160.1 and 165.4 eV, respectively. The increase in Bi³⁺ binding energy in BiPO₄/rGO/FTO composite photoelectrodes might be ascribed to the transfer of the outer electrons of BiPO₄ to rGO. It suggested that the electron cloud density of BiPO₄ was decreased. In the O 1s narrow spectra of BiPO₄/rGO/FTO and BiPO₄/FTO photoelectrodes, the peaks at 530 and 532 eV (Fig. 6(c)) corresponded to Bi-O and C-O peaks. In the narrow spectrum of C 1s of the BiPO₄/FTO and BiPO₄/rGO/FTO composite photoelectrodes, the peaks at 284.8 and 286.6 eV corresponded to the C-C and C-O bonds, respectively. However, it was odd that the peak of the C-O bond occurred in the BiPO₄/FTO photoelectrode. It was conjectured that some residual organic matters in EDTA did not removed clearly. Furthermore, compared with the BiPO₄/FTO electrode, the peak areas of the C-C and C-O bonds increased, which was due to the present of rGO in the composite. The O 1s narrow spectra of BiPO₄/rGO/FTO and BiPO₄/FTO photoelectrode at 530 and 532 eV were ascribed to Bi-O and C-O peak, respectively (Fig. 6(d)). Similarly, the C-O peak at 532.5 eV in the BiPO₄/FTO electrode was mainly ascribed to residual organic matters in EDTA during the synthesis of photoelectrode. The Bi-O peak area at 530.8 eV was decreased in the BiPO₄/rGO/FTO composite photoelectrode, whereas the C-O peak area at 532.5 eV was increased because the introduction of rGO could give occasion to the increase of the content of C-O bond.

The UV-vis diffuse reflectance spectra (DRS) of BiPO₄/FTO photoelectrode and BiPO₄/rGO/FTO composite photoelectrode under the different disposition time was shown in Fig. 7. BiPO₄ had light absorption only in the ultraviolet region and the absorption edge was about 375 nm. When the BiPO₄ nanorods were electrodeposited on the substrate of the rGO, the UV-vis diffuse absorption edge of the BiPO₄/rGO/FTO composite photoelectrode was red-shifted. In the composite electrode, the light absorption intensity and range varied with BiPO₄ electrodeposition time. When deposition time of BiPO₄ was 45 min, the obtained BiPO₄/rGO/FTO composite photoelectrode had the largest redshift range and photoresponse range, which improve the visible absorption of BiPO₄/FTO photoelectrode.

In order to explore the effects of rGO and the BiPO₄ deposition time on the PEC activity of the BiPO₄ nanorod photoelectrode, the PEC degradation experiments were carried out with 10 ppm methyl orange as the model pollutant and 10 W UV lamp as the light source. The BiPO₄/rGO/FTO composite electrode had a much better degradation effect on methyl orange than the BiPO₄/FTO photoelectrode under a constant applied bias voltage of 1.2 V (Fig. 8(a)), indicating that the introduction of film greatly enhanced the PEC degradation activity of BiPO₄ nanorods. As given in Fig. 8(b), the degradation reaction rate constant reactions of methyl orange by BiPO₄/rGO/FTO composite electrode under the different deposition time. All the composite photoelectrodes have the higher PEC activity than BiPO₄ nanorod photoelectrode. When the BiPO₄ deposition time was 45 min, the BiPO₄/rGO/FTO electrode had the optimal reaction rate constant (k = 0.281), which was 4.3 times higher than that of the BiPO₄/FTO electrode (k = 0.065). Further increasing the loading of BiPO₄, the reaction rate began to decrease. The reason is that the overloading of BiPO₄ might affect the light absorption of the underlying material and thus decreased the PEC activity.

It was given that the effect of working voltage on the PEC activity of BiPO₄/rGO/FTO_{–45 min} composite photoelectrode under different applied operating voltages. The operating voltage varied from 0.6 to 1.4 V. The removal rate of methyl orange increased at first and then decreased as the operating voltage increased. At the applied working voltage of 1.2 V, the BiPO₄/rGO/FTO composite photoelectrode had the optimum removal rate of methyl orange, indicating that 1.2 V was the optimum applied operating voltage for the PEC degradation. In the PEC degradation of pollutants, the applied bias voltage was one of the important factors affecting the degradation performance. The bias voltage could promote the transfer of photogenerated electrons to the platinum wire electrode through the external circuit. The electrons were effectively separated from holes in space, thus enhancing the quantum efficiency of photoelectroncatalysts. When the applied voltage reaches a certain value, the composite photoelectrode of the photogenerated electrons experienced directional movement and most of the photogenerated electrons were separated from holes to form a saturated photocurrent resulting in optimal photoelectric catalytic efficiency. When the voltage exceeded the optimum value, the space charge layer and the Hertz layer of the catalyst might be redistributed due to excessive working bias voltage, thus reducing photo-generated carriers and the PEC activity.

In order to prove the photoelectric synergy of BiPO₄/rGO/FTO composite photoelectrode, the photocatalytic, electrocatalytic and photoelectrocatalytic activities of this composite material were compared. After the 10 ppm methyl orange solution was irradiated for 5 h with a 10 W UV lamp without the catalyst, the pollutant concentration was not significantly changed, indicating that the self-degradation of methyl orange solution under the irradiation of UV lamp could be ruled out (Fig. 10(a)). The photocatalytic degradation rate of methyl orange by BiPO₄/rGO/FTO composite photoelectrode was only about 20% after 5 h under UV light irradiation. The electro-catalysis degradation rate of methyl orange was about 50% after 5 h under the bias voltage of 1.2 V. The PEC degradation efficiency of methyl orange in BiPO₄/rGO/FTO composite photoelectrode was increased to about 80%. However, the reaction rate constant of the BiPO₄/rGO/FTO composite photoelectrode was 0.281 k/h^–1, which was about 2 times higher than the electrocatalytic reaction rate constant and 5 times higher than photocatalytic reaction rate constant (Fig. 10(b)). The reaction kinetic constants indicated that the PEC degradation rate of methyl orange had the synergistic effect for the removal of organic compounds [30].

To further demonstrate the enhancement effect of photogenerated electrons on the synergistic catalytic performance, transient photocurrent response and electrochemical impedance spectroscopy (EIS) tests were performed. The photocurrent response intensity of the BiPO₄/FTO photoelectrode was about 0.1 mA/cm² (Fig. 11(a)). The BiPO₄/rGO/FTO_{–45 min} composite photoelectrode had the optimal photocurrent response intensity of about 0.4 mA/cm², which was 4 times that of the BiPO₄ electrode. The EIS (Fig. 11(b)) also showed that the BiPO₄/rGO/FTO_{–45 min} composite photoelectrode had the smallest internal resistance and efficient electron transfer. The results were consistent with the PEC activities of prepared electrodes. The composite electrodes had a stronger light absorption intensity and a wider light absorption range than the pure BiPO₄ electrode. The rGO film as a conductive medium could increase the conductivity of the electrode and accelerate the electron-hole separation, thus enhancing the catalytic activity of the photoelectric composite electrode material.

Based on the linear electrochemical voltammetric curve (LSV), the photoelectrochemical properties of various electro-optical materials were compared and shown in Fig. 12. The pure BiPO₄/FTO nanorod photoelectrode had the lowest photocurrent density under UV light in the bias voltage range of 0‒1.8 V. The introduction of the rGO film significantly increased the photocurrent response density of the photoelectrode, indicating that graphene accelerated the separation of photogenerated carriers. The results of LSV were agreed with the photocurrent density results.

As shown in Fig. 13, the BiPO₄/FTO thin film electrode had the strongest PL peak. After the combination of rGO (BiPO₄/rGO/FTO), the PL peak intensities were greatly reduced, indicating that the rGO film accelerated the separation and transfer of electron-hole pairs due to its excellent conductivity. The BiPO₄/rGO/FTO_{–45 min} composite photoelectrode had the lowest PL peak and the lowest electron-hole recombination efficiency, indicating the highest PEC activity.

The main active species during the degradation process were determined by free radical trapping experiments, and then the mechanism of photoelectric synergistic catalysis could be indirectly inferred. The degradation experiments were carried out under a 10W UV lamp and a bias voltage of 1.2 V. Methanol (MA), isopropanol (IPA) and p-benzoquinone (BQ) were used as the traps for the hole (h⁺), hydroxyl radical (^•OH) and superoxide radical (^•O₂^-) trapping agent, respectively. There was no significant decrease in the removal efficiency of methyl orange by BiPO₄/rGO/FTO composite photoelectrode after adding methanol (Fig. 14), indicating that holes (h⁺) did not directly participate in the degradation of methyl orange and was not the main active species in the PEC reaction. However, the degradation efficiency of BiPO₄/rGO/FTO composite photoelectrode decreased significantly after adding IPA, indicating that ^•OH was a main active species in the PEC reaction process. The removal efficiency of methyl orange by BiPO₄/rGO/FTO composite photoelectrode decreased significantly after the addition of p-benzoquinone, indicating that ^•O₂^- was another main active species in the PEC reaction process. From the above results, it could be inferred that holes did not directly play a role in the degradation of methyl orange. These active substances (^•OH and ^•O₂^-) dominated the photovoltaic synergistic catalytic reaction during the catalytic process.

Based on the experimental results, the mechanism of the BiPO₄/rGO/FTO electrode for the enhancement of the PEC activity was tentatively explained and shown in the Fig. 15. A large number of electron-hole pairs were generated in BiPO₄ under the UV light irradiation. As a good electron conductor, graphene rapidly transferred the photogenerated electrons to the FTO. The applied bias voltage formed an electric field to transfer electrons through the FTO glass to the Pt electrode, further accelerating electron-hole separation and transfer. A large number of photogenerated electrons were accumulated on the Pt electrode and reacted with adsorbed oxygen molecules to produce ^•O₂^- active species, which ultimately degraded organic pollutants. Correspondingly, the photogenerated holes generated by BiPO₄ could react with water molecules in the solution to produce another active species ^•OH. The ^•OH has strong oxidizing properties and can rapidly degrade pollutants. On the other hand, the introduction of rGO led to the BiPO₄ transition from the hexagonal phase to the monoclinic phase. The formation of a mixed crystal phase contributed to the separation of electrons and holes. In addition, graphene had a conjugated structure with a large π bond, which helped to increase the adsorption of pollutants on the catalyst surface and accelerated the photocatalytic oxidation reaction. The PEC activity of the BiPO₄/rGO/FTO composite photoelectrode finally enhanced [31].

A cyclic experiment was carried out in a static system to further investigate the stability of the BiPO₄/rGO/FTO electrode in the degradation process of methyl orange. From Fig. 16, the photoelectric catalytic oxidation effect of the BiPO₄/rGO/FTO electrode did not decrease significantly after four cycles of degradation reaction of 20 h, indicating that the BiPO₄/rGO/FTO electrode had the good stability.

In this work, BiPO₄/rGO/FTO composite photoelectrodes were prepared by a simple two-step electrodeposition method. The composite photoelectrodes had the excellent photoelectrocatalysis performance compared to the BiPO₄/FTO electrode, Graphene, as a good electron conductor, could accelerate the separation and transfer of electrons-hole pairs and improve light absorption of BiPO₄/FTO electrode. In addition, the introduction of rGO led to the formation of mixture phase of BiPO₄, further promoting the separation of photogenerated charges. Free radical inactivation experiments showed that ^•OH and ^•O₂^- might be the main active species in the photoelectrocatalytic degradation process. The composite electrode material exhibited extremely high stability and its photoelectric synergistic catalytic oxidation effect was not decreased significantly after four cycles of degradation reaction for 20 h. The prepared composite electrode has great application prospects in the field of photoelectrocatalytic oxidation for practical water purification.

Acknowledgment

This work was partly supported by the National Natural Science Foundation of China (21577132, 21978276), the Fundamental Research Funds for the Central Universities (2652018326, 2652018298, 2652018297), and the Beijing Municipal Education Commission Key Science and Technology Project Fund (KZ201910853043).