Decoration of Monodisperse Silver Vanadate on Biomass‐Derived Porous Carbon: A Dual‐Purpose Catalyst for Detection of p‐Nitrophenol and Photodegradation of Rhodamine B

Taking advantage of carbonaceous composite materials in electrocatalysis or photocatalysis is a promising pathway for transforming biomass resources into sustainable energy‐conversion systems. Herein, a carbonaceous composite catalyst based on the monodispersed silver vanadate nanoparticles (Ag3VO4 NPs) modification of biomass‐derived porous carbon (named as AVO/PC) is proposed for ultrasensitive and electrochemical rapid detection of p‐nitrophenol (p‐NP) under acidic condition. The radish‐derived three‐dimensional porous carbonaceous aerogels as supporter subtly introduced in the carbonization process of alkaline activation remarkably promote the physicochemical properties of the biomass‐derived porous carbon (PC), which can provide a favorable internal surface to load Ag3VO4 NPs. Subsequently, this electrochemical behavior of AVO/PC‐modified glassy carbon electrode (AVO/PC/GCE) exhibits high sensitivity (4.95 mA mM−1 cm−2) and a varied linear detection range (1 μmol L−1 ≈ 1 mmol L−1), as well as reproducibility and stability for p‐NP. Besides, the as‐prepared AVO/PC demonstrates the enhanced photocatalytic degradation performance of rhodamine B (RhB), and the photocatalytic rate reaches to 94% in visible light irradiation. The findings should be useful for designing and constructing dual‐purpose carbonaceous composite materials with potential applications in environmental monitoring and contaminant treatment.

Taking advantage of carbonaceous composite materials in electrocatalysis or photocatalysis is a promising pathway for transforming biomass resources into sustainable energy-conversion systems. Herein, a carbonaceous composite catalyst based on the monodispersed silver vanadate nanoparticles (Ag 3 VO 4 NPs) modification of biomass-derived porous carbon (named as AVO/PC) is proposed for ultrasensitive and electrochemical rapid detection of p-nitrophenol (p-NP) under acidic condition. The radish-derived three-dimensional porous carbonaceous aerogels as supporter subtly introduced in the carbonization process of alkaline activation remarkably promote the physicochemical properties of the biomass-derived porous carbon (PC), which can provide a favorable internal surface to load Ag 3 VO 4 NPs. Subsequently, this electrochemical behavior of AVO/PC-modified glassy carbon electrode (AVO/PC/GCE) exhibits high sensitivity (4.95 mA mM À1 cm À2 ) and a varied linear detection range (1 μmol L À1 % 1 mmol L À1 ), as well as reproducibility and stability for p-NP. Besides, the as-prepared AVO/PC demonstrates the enhanced photocatalytic degradation performance of rhodamine B (RhB), and the photocatalytic rate reaches to 94% in visible light irradiation. The findings should be useful for designing and constructing dual-purpose carbonaceous composite materials with potential applications in environmental monitoring and contaminant treatment.
organic dye [17] and electrochemical applications as electrode. [18] Due to the typical characteristics of silver-based catalyst, such as, excellent stability, narrow bandgap, catalytic activity, and good electrocatalysis characteristic. [17,18] However, the unavoidable agglomeration of nanoparticles and low quantum yield condition cause limitation in the use of Ag 3 VO 4 in the above field. To this end, much work has been done to improve its electroconductibility and photocatalytic activity. [19,20] Among them, finding suitable supporting materials is the simplest and most effective way to settle this matter.
The amount of carbonaceous resources available in biomass is enormous, but a large proportion of them are neglected or openly burned. In fact, their environmental treatment has been a great challenge for local residents and the government every year. [21] Therefore, some researchers have focused on converting biomass waste into carbon carbonaceous materials. [22][23][24] The research shows that many plants have high abundance of amino acids and proteins, which can lead to in situ doping of sulfur or nitrogen during carbonization process. The methods of carbonizing biomass are mainly concentrated in the temperature-, time-, and atmosphere control. Among these, hydrothermal is an effective and mild method, which provided the available biomass resources (e.g., fruit shells, watermelon rind, and lettuce) to dehydrate, polymerize, and precarbonization at low temperature. [25][26][27] After the hydrothermal carbonization, it can produce plentiful interconnected holes, which are beneficial for further chemical activation to generate large porosity. [28,29] In addition, carbonaceous materials with heteroatom-doped will improve the electrocatalytic performance, due to the redistribution of spin and charge densities, reduced resistivity, and increased number of active sites. [30,31] The porosity of hierarchical porous structures is a significant parameter that affects the catalytic performance of most electrochemical reactions. [30][31][32] The micropores (<2 nm) provide enough location of active sites, mesopores (2-50 nm) and macropores (>50 nm) facilitating the diffuson of both electrolytes and reactants. [32] Unfortunately, the single microporous structure of sustainable biomass materials results in a low surface area utilization although the specific surface area is high. [33,34] Therefore, the chemical activation with activation reagents (e.g., KOH and ZnCl 2 ) is a common approach to increase the specific surface area availability and thus increase the active sites of catalyst. [35][36][37][38] Wang et al. [39] prepared biomass-derived carbon with hierarchical porous structure by the KOH activation for supercapacitors and lithium-ion batteries. Furthermore, the enlarged surface area is fundamentally adaptable to be coupled with metal or semiconductor nanoparticles. It is worth noting that the porosity optimization reasonably combine with nanoparticles doping that can significantly improve the conductivity and capability of contaminants treatment. [40] Herein, we fabricate a novel porous carbon (abbreviated as PC) by using radish as precursor along with KOH as porogen and decorate monodisperse Ag 3 VO 4 nanoparticles on its surface (named as AVO/PC), as shown in Scheme 1. AVO/PC modified GCE was employed for electrochemical detection of p-NP by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is exhibited that a varied linear detection range (1 μmol L À1 % 1 mmol L À1 ), high sensitivity (4.95 mA mM À1 cm À2 ), as well as reproducibility and stability. The research using biomass as precursor is not limited to the research in the field of electrochemical detection. Furthermore, the AVO/PC catalysts showed photocatalytic activity toward RhB degradation under visible light irradiation, the photocatalytic degradation rate reached to 94% in 75 min.

Structural and Morphology Analysis
The crystal structure and phase composition of the pure and composite systems were investigated by XRD and Raman spectroscopy. As shown in Figure 1a [41] Compared with pure Ag 3 VO 4 , the XRD pattern of composite exhibits a broad diffraction peak around at 23°which corresponds to diffraction from carbon (002) plane. Moreover, the XRD patterns of AVO/PC suggested that the Ag 3 VO 4 NPs are completely dispersed on the surface of carbonaceous material, which also indicated that the incorporation into the porous carbonaceous material does not change the phase structure of Ag 3 VO 4 NPs.
Raman spectroscopy was performed to identify the structure of biomass-derived porous carbonaceous materials. In Figure 1b, two broad bands in Raman spectral were observed about 1340 and 1600 cm À1 for pure Ag 3 VO 4 and AVO/PC, demonstrating the D band (breathing mode at the sp 3 defective graphitic sites) and the G band (E 2g vibration mode of sp 2 C─C bonding), respectively. [42] The intensity ratios (I D /I G ) of AVO/PC material and carbonaceous aerogel were 1.06 and 0.99, respectively. These results revealed the presence of Ag 3 VO 4 NPs that leads to the Scheme 1. Schematic for preparation procedure of AVO/PC. www.advancedsciencenews.com www.advenergysustres.com increase of defect density. In addition, the D bands were removed slightly to the region of lower wave-number, representing a higher degree carbon curling because of strong interaction between Ag 3 VO 4 NPs and carbonaceous material. The 2D peaks were also witnessed at 2642 cm À1 , and the peak intensity was also enhanced appreciably. It established the formation of multilayer carbonaceous material which can protect the active of Ag 3 VO 4 NPs throughout the catalytic process. FTIR spectroscopy was utilized to analyze the information about the bonding state and the coordination of Ag 3 VO 4 , AVO/PC composites, as shown in Figure 1c. In the case of AVO/PC, a broad absorption and strong band compared to pure Ag 3 VO 4 at 2700-3600 cm À1 was contributed to O-H stretching vibrations, indicating the surface of porous carbonaceous aerogel with abundance of hydroxyl groups. The peaks in the region of 1200-1700 cm À1 were attributed to the C-O and C-H stretching vibration. The absorbance peak at about 920, 850, and 700 cm À1 was assigned to originate from the stretching vibration of V-O, Ag-O-V, Ag-V, respectively. [43] Combined with XRD and FTIR, it showed that AVO/PC was successfully prepared. Figure 1d and S1, Supporting Information, shows the N 2 adsorption/desorption isotherm before and after the activation of carbonaceous material, which was used to characterize the BET surface areas and BJH pore size distributions. The parameters of BET surface area, cumulative pore volume, and average pore diameter are displayed in Table S1, Supporting Information. It could be seen that the isotherms of carbonaceous material exhibited type-IV isotherm, indicating the existence of a mesoporous structure. However, the pore size before the activation of carbonaceous material is concentrated in the microporous regime, which has negative impact on diffusion and mass transfer. The pore size after the activation possessed well-developed porosity, and the average pore diameter reached to 14.8 nm. The plentiful micro/mesoporous structure can provide multidimensional activity sites and channels for surface reaction, which is an effective way to improve the accessibility of the active centers. What is more, when the carbonaceous material was activated by KOH, the BET surface area increased to 238.6 m 2 g À1 , which is almost 7 times the pristine carbonaceous aerogel material.
SEM was employed to examine the microstructure and morphologies of synthetic samples as shown in Figure 2. As shown in Figure 2a, the surface of biomass-based aerogel was of smooth microstructure. Compared with this microstructure, it can be clearly observed that the obtained porous carbonaceous material has abundant porous structure ( Figure 2b). The formation of the hierarchical porous skeleton could be attributed to the etching action between the KOH with strong corrosion and mesopore/macropore walls in the course of heat treatment with nitrogen gas. [44,45] The activation process between KOH and carbonaceous material was described in Equation (1) www.advancedsciencenews.com www.advenergysustres.com First, K 2 CO 3 formed was attributed to the etching between KOH and carbonaceous material at 400-600°C (Equation (1)). At a temperature of 700°C, K 2 CO 3 was decomposed into K 2 O and CO 2 (Equation (2)). With the increasing of temperature, the intermediate products K 2 CO 3 , CO 2 , and K 2 O could react with carbonaceous material over 700°C (Equation (3)-(5)). During the activation process, a large volume of gas is liberated, which would cause a homogeneous and continuous corrosion, this leads to an increase in BET surface area and the change in microscopic morphology. Figure 2c,d shows the microscopic morphology of pure Ag 3 VO 4 NPs and AVO/PC. The pure Ag 3 VO 4 NPs exhibited irregular structure of nanoparticle with an average size of 100 nm and showed hierarchical agglomeration phenomenon of nanoparticles. After hybridization, Ag 3 VO 4 NPs were distributed on the surfaces and interior of porous carbonaceous material. The TEM images are shown in Figure 2e,f. It can be seen that the edges of the biomass porous carbonaceous material were twisted and folded. Besides, the rough surface with irregular pore structure of carbon material can be observed in Figure 2f, which may be attributed to the formation of abundant micro/ mesoporous during KOH etching process. As shown in magnified image of Figure 2f, Ag 3 VO 4 NPs were about 10 nm in diameter and uniformly dispersed, which were in good accordance with the SEM images. The schematic diagram of dual-purpose application of AVO/PC composite material is shown in Figure 2g.

Electrochemical Characterizations
In order to select the optimization of experimental conditions, the electrochemical behavior of p-NP was investigated on different pH (pH = 0.3,1,2,3 and 4) with 1 mmol L À1 p-NP by CV. In Figure 3a, the electrochemical behavior of bare GCE, PC/GCE, AVO/GCE, and AVO/PC/GCE was carried out in the potential ranging between À0.8 and 0.8 V at a scan rate of 0.06 V s À1 . The bare GCE has no obvious response for 1 mmol L À1 p-NP, which indicated that bare GCE failed to detect p-NP. Despite the fact the distinguishable response toward p-NP was witnessed at AVO/GCE and PC/GCE, furthermore, AVO/PC/GCE exhibits remarkable enhancement in current of the redox peak. It can be seen that a well-defined reduction peak (R) at about À0.34 V and a pair of redox peaks (R 1 and O 1 ) at 0.44 and 0.56 V were, respectively, observed on this electrode. Two factors lead to the significant increase in the peak current of the AVO/PCmodified electrode for p-NP than AVO/GCE and PC/GCE: 1) the AVO/PC composite restricts the PC aggregation by the introduction of Ag 3 VO 4 NPs with more catalytic active sites and Ag 3 VO 4 NPs served as an electrocatalyst for accelerating electron transfer; 2) the Ag 3 VO 4 NPs in the surface of porous carbonaceous material are well dispersed, electrochemically accessible, and highly active. In order to further understand the redox peaks, AVO/PC/GCE was examined in the absence of p-NP. It can be seen that the redox peak disappeared, showing that the redox peak was originated by injecting p-NP into the www.advancedsciencenews.com www.advenergysustres.com solution. These data indicate that the AVO/PC-modified GCE has a good catalytic activity toward p-NP sensing compared to that of the bare GCE, Ag 3 VO 4 , and porous carbonaceous material. As shown in Figure 3b, the current and potential of oxidation and reduction peak shifted negatively as the changing of pH, demonstrating that the protons were involved in the redox reactions of p-NP. As can be clearly seen from the figure that the CV curve showed a significant redox peak when pH = 1, the peak current of the well-defined reduction peak at À0.45 V reaches 0.35 mA. Therefore, pH = 1 was selected as optimum pH in testing environment for further electroanalytical experiments. So as to further comprehend the electrochemical behavior of p-NP on the AVO/PC/GCE, the effect of the scan rate on reduction peak current (I R ), reduction peak potential (E R ), and oxidation peak current (I O1 ) was investigated by CV in optimum pH. Figure 3c displays the CV curves of AVO/PC/GCE with different scan rates from 0.02 to 0.2 V s À1 in 1 mmol L À1 of p-NP (pH = 1). It can be observed that with the increase in scan rate, the peak current also increased gradually. As shown in Figure 3d,e, I R /E R and I R1 /I O1 of p-NP were linearly regression equation to the square root of the scan rate (υ 1/2 ), which indicated that the irreversible reduction reactions of p-NP on the modified electrode surface were diffusion-controlled processes. The linear equations are described as follows (Equation (6)-(8)) As observed in figure, E R as a function of scan rate shifted negatively with increase in natural logarithm of the scan rate (lnυ) for detection of p-NP. The linear regression equations of E R and lnυ can be expressed as follows (Equation (9)) E RðpÀNPÞ =mA ¼ À 0:0369 ðν=V S À1 Þ 1 2 À 0.655ðR 2 ¼ 0.995Þ (9) When the acid is used as a supporting electrolyte, the hydrogen ions disperse the negative charge of oxygen in the nitro group, which is conducive to the electrochemical reduction reaction. The structure of p-NP is formula A, which may form formula B in a non-acid solution, and may form formula C in the acidic solution. The higher the hydrogen ions concentration formula A, the more is the production of formula C. Due to higher concentration of hydrogen ions in low pH, the peak current is larger and the peak potential is shifted. The possible reaction mechanism of p-NP redox reaction on the AVO/PC/GCE is described in Figure 3f. It was demonstrated that the electrons and protons participated in the electrochemical reaction process from the relationship of Ep and pH. Initially, p-NP was reduced to hydroxyl aminophenol through a four-electron and fourproton transfer step (R). Subsequently, a pair of reversible redox reactions (O 1 and R 1 ) occurred between p-NP and hydroxyl aminophenol. [46,47] Under optimal conditions, the effect of different concentrations and electrochemical impedance spectroscopy (EIS) were investigated for beyond electrochemical behavior in H 2 SO 4 (pH = 1) containing 1 mmol L À1 p-NP on the AVO/PC/GCE, respectively. Furthermore, it can be realized that a well-defined reduction peaks of different concentration p-NP were collected at À0.8 and 0.8 V, as shown in Figure 4a. The insert graph displayed partial enlarged detail of low concentration (1-10 μmol L À1 ). The calibration curve of p-NP was depicted in Figure 4b on the AVO/PC/GCE for p-NP detection revealing linear function in the wide range of 1 μM % 1 mM, which can be expressed as follows (Equation (10)): Clearly, sensitivity = slope/area, where slope refers to the slope of calibration curve of p-NP; area is the area of GCE with a diameter of 3 mm. Therefore, the sensitivity of AVO/PC/GCE was calculated to be 4.95 mA mM À1 cm À2 for p-NP, AVO/PC/ GCE has higher sensitivity compared to similar literature. [48] Moreover, the AVO/PC/GCE exhibited a wide linear detection range (1 μmol L À1 % 1 mmol L À1 ) for p-NP, and the peak current is about 0.53 mA at 1 μmol L À1 concentration of p-NP in the electrolyte. These results indicated that the AVO/PC/GCE has satisfactory advantage as an electrocatalyst for p-NP detection, and this was ascribed to excellent synergistic effect of AVO/PC. As shown in Figure 4c, a comparison of the Nyquist plots for AVO/GCE, PC/GCE, and AVO/PC/GCE and inset was partial magnification at high-frequency region. In impedance spectra, a semicircle at high frequency is related to the charge transfer resistance (R ct ), and a Warburg diffusion trend line at low frequency is related to the diffusion process. Especially, the R ct is an important parameter for exploring the electron transfer properties. As can be seen from the EIS, the AVO/PC/GCE with an extremely small semicircle at high frequency region, which indicates a significantly decreased charge-transfer resistance of electrode surfaces. That may be attributed to the synergistic effect of the Ag 3 VO 4 NPs' conductivity and porous carbonaceous material. This is in good consistent with the outcomes of CV cures www.advancedsciencenews.com www.advenergysustres.com ( Figure 3a). Furthermore, the linear portion at the low-frequency region of AVO/PC/GCE in comparison with that of AVO/GCE and PC/GCE displayed a steeper and shorter line gradient, which suggested that electron diffusion rate was accelerated. These results demonstrated that the AVO/PC/GCE can achieve efficient electron transfer and faster diffusion of electrolyte simultaneously. The reproducibility of AVO/PC/GCE was investigated at six independent electrodes in presence of 1 mmol L À1 p-NP. The calculation shows that the relative standard deviation (RSD) of the reduction peak (R) current was <3.8% ( Figure S2a, Supporting Information). It was found that the peak current of the AVO/PC/ GCE fluctuated only 1.8% after 200 CV sweeps (0.06 V s À1 ) in the potential range from À0.8 to 0.8 V. Furthermore, the stability of the AVO/PC/GCE electrode was investigated by measuring the current response of 1 mmol L À1 p-NP among 20 days. It was found that the current (R) responses retained more than 91% of its initial value ( Figure S2b, Supporting Information).

Photocatalytic Performance
The photocatalytic process was carried out in a cube reactor under a 500 W tungsten lamp. Figure 4d shows the UV-vis spectrum of RhB for AVO/PC with temporal evolution. The embedded picture represents the color changes of RhB over time during the photocatalytic degradation experiment. At 554 nm, it can be clearly seen from the figure that the maximum absorption peak of RhB spectrum gradually weakens. A gradual blue shift of maximum absorption peak was observed with increase in photocatalytic reaction time, demonstrating the step-by-step decomposition of chromophoric structures of RhB (ethyl groups). [49] The first broken down the ethyl groups of RhB followed by the cycloreversion happened in the process of degradation of RhB. Subsequently, according to the literature, dye contaminant was decomposed gradually into CO 2 , H 2 O, and other intermediate organic products. [50,51] An obvious change in color of RhB solution from red to colourlessness was observed, indicating the RhB degraded in almost 80 min. According to the previous report, [52] the dye RhB solution was difficult to degradation due to its stability under visible light irradiation alone in the absence of photocatalyst. As shown in Figure 4e, the photocatalytic degradation ratios of RhB with pure Ag 3 VO 4 and AVO/PC reached 48% and 94% within 75 min. In the meantime, the UVÀvis diffuse reflection spectra (DRS) of Ag 3 VO 4 and AVO/PC are shown in Figure S3a, Supporting Information. The absorption edge of Ag 3 VO 4 is at 574 nm. The visible light absorption of AVO/ PC went up compared to Ag 3 VO 4 , while the amplitude of visible light absorption also increased. Photoluminescence (PL) emission resulting from the recombination of photoexcited electrons/holes can be used to characterize the efficiency of photocatalysts for separating photogenerated carriers In Figure S3b, Supporting Information, exhibits lower PL intensity than Ag 3 VO 4 and AVO/PC, indicating that it can inhibit the recombination of photoexcited electron-hole pairs more efficiently. The enhanced photodegradation efficiency of AVO/PC is attributed to plentiful porous structure and narrow bandgap. On the one hand, the high BET surface area can supply more multidimensional activity sites and channels for photocatalytic degradation.
On the other hand, electronic transition of Ag 3 VO 4 NPs is easy due to its smaller bandgap. Additionally, the photogenerated electron-hole pairs as reactive species with robust redox capacities during the process of degrading the RhB. The superoxide radicals (•O 2 À ) and hydroxyl radicals (•OH) in the photocatalysis are mainly generated, subsequently degrading the RhB dye. The pathways are written as follows photocatalyst þ hv ! e À ðCBÞ þ h þ ðVBÞ For evaluating the durability of the AVO/PC photocatalyst, the recycling photodegradation of RhB was measured (Figure 4f ). It is observed that the degradation rate reduced by 8% after six cycles, indicating that AVO/PC has certain durability in the process of visible light photodegradation. The above results also indicated potential applications of AVO/PC composites in the field of photocatalytic degradation.

Conclusions
In summary, this work presents the rational syntheses of carbon materials as stable catalyst supporter, produced by carbonizing radish. After the precarbonization and KOH activation, the biomass-derived porous carbon was obtained with a hierarchical graphitic structure. Further, Ag 3 VO 4 NPs were monodispersed on biomass-derived porous carbon materials by in situ deposition, as a dual-purpose catalyst for detection of p-NP and photodegradation of RhB. These results indicated that AVO/PC/GCE had a fine linear in the wide range of 1 μmol L À1 % 1 mmol L À1 (R 2 = 0.999), and the sensitivity of AVO/PC/GCE was calculated to be 4.95 mA mM À1 cm À2 for p-NP. Additionally, the AVO/PC catalysts also showed photocatalytic activity toward RhB degradation under visible light irradiation; the photocatalytic degradation rate reached to 94%. The large hierarchical surface of the carbonaceous enabled served as a nanoengineered platform for immobilized Ag 3 VO 4 NPs, thus facilitated electronic transmission and promoted the separation of photogenerated electron from vacancies. This work provides an idea about the use of catalyst in different fields, which is an interesting and effective way for the practical applications.
Preparation of the AVO/PC Bifunctional Catalyst: The hydrothermal synthesis was employed to prepare the carbonaceous aerogels according to previous reports in our group. [25,53,54] The fresh radishes were cut and transferred to Teflon-sealed autoclave, which was placed in a 180°C for 15 h. After, the aerogel sample in deionized water were flash frozen by liquid nitrogen, and then kept in cryogenic refrigerator at least 24 h. The frozen samples were put into the chamber of a freeze dryer and dried for 12 h at <1 Pa. The temperature of the cold trap was set to À56°C.
www.advancedsciencenews.com www.advenergysustres.com Porous carbonaceous material was synthesized by KOH activation process. The mixture of aerogel and KOH (m aerogel : m KOH = 1:2) was soaked in 40 mL DI for 12 h, dried at 105°C to evaporate of DI, and calcined at 750°C (5°C min À1 ) for 2 h under N 2 atmosphere. Next, the as-obtained sample was washed until pH = 7. Finally, the porous carbon material was dried at 60°C for 24 h before proceeding to the next step. Subsequently, 50 mg porous carbon material was well dispersed in 20 mL DI and ultrasonic vibration for 30 min. Then, it was poured into 10 mL of DI containing AgNO 3 (58 mg) with stirring constantly, and then a required amount of Na 3 VO 4 ·12H 2 O (46 mg) solution was dropwise added (mass ratio of carbon material and Ag 3 VO 4 is 1/1). The mixed solution was stirred for 4 h, centrifuged, and dried overnight. The obtained sample was named as AVO/PC. Moreover, pure Ag 3 VO 4 was prepared under the same conditions without porous carbonous material. The diagrammatic drawing of synthetic process is shown in Scheme 1.
Electrochemical Detection Measurements: A three-electrode cell system was used to perform the electrochemical sensing measurements on CHI660E electrochemical workstation. A saturated calomel electrode and platinum electrode were employed as reference electrode and counter electrode, respectively. Glassy carbon electrode with a diameter of 3 mm was used as the working electrode. Meanwhile, 1 mmol L À1 of p-NP was dissolved in H 2 SO 4 solution (Ph = 1) as simulated wastewater and by aerating with high-purity N 2 for 15 min before use. The catalyst-modified glassy carbon electrodes (GCE) were prepared by casting 10 μL of the catalyst suspension (2 mg AVO/PC was ultrasonically dispersed in 1 mg ethanol and 10 μL of 5% Nafion) and dried for 12 h at room temperature before use. For comparison, the bare GCE, PC-modified GCE (PC/ GCE), and Ag 3 VO 4 NP-modified GCE (AVO/GCE) were prepared using the same procedure.
Photocatalytic Activity Measurement: Photodegradation activity was evaluated in a cylinder-type immersion photoreactor by degrading the simulated dye Rhodamine B (RhB) under the visible light irradiation. Cylindrical glass vessel contained 100 mL aqueous solution of RhB (20 mg L À1 ) in which 100 mg of catalyst was well dispersed. The vessel was further equipped with a water circulation system, in which air was continuous ventilation from the bottom of the reaction device. The light source of the reaction was a 500 W xenon lamp with 420 nm cut off filter. During the experiment, 5 mL reaction mixture was collected in a regular time interval and centrifuged to investigate the photocatalytic activity. The absorbance of RhB (about 554 nm) was measured by UV-vis spectrophotometer. The formula for photodegradation rate is described as follow Here C t and A t indicate the concentration and absorbance of the RhB solution at irradiation time of 't' min, respectively; C 0 and A 0 represent the initial concentration and absorbance at t = 0 min, respectively.
Structural Characterization: Various characterization techniques were used in order to investigate thoroughly the properties of as-prepared materials. X-ray diffraction (XRD) patterns of samples were obtained by D/Max-γA X-ray diffractometer (Bruker AXS Company, Germany). The functional groups detection was investigated using Fourier transform infrared (FTIR) spectra (Nicolet Nexus470, Thermo Electron, USA). Scanning electron microscopy (SEM) images were taken on a field emission electron microscope (JSM-7001F, JEOL, Japan) to observe the morphology of the prepared samples. Transmission electron microscopy (TEM) patterns were collected using a JEOL-JEM-2100 (JEOL, Japan) electron microscopy of 200 kV accelerating voltage. The vacuum freeze drier was used (FD-1C-50) Shanghai Bilon Instrument Manufacturing Co., Ltd during this experiment. Brunauer-Emmett-Teller (BET) surface area was measured with a Micromeritics NOVA2000 (Quantachrome Corporation, USA) nitrogen adsorption apparatus by N 2 physisorption at 77 K (the samples were outgassed at 423 K for %2 h under vacuum). Electrochemical measurements were characterized by an electrochemical workstation (CHI 660 E Chenhua Instruments, Inc., Shanghai, China). The equilibrium concentration of dyes was determined by the UV-Vis spectrometer (UV-2450; Shimadzu, Japan). UV-vis diffuse reflectance spectra (UV-2450, Shimadzu Corporation, Japan) were utilized by spectrophotometer equipped with a spherical diffuse reflectance accessory. The photoluminescence (PL) spectra were acquired by a QuantaMaster 40 (Photon Technology International, USA).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.