Photocatalytic Mineralization of Organic Acids over Visible-Light-Driven Au/BiVO4 Photocatalyst

1 Nanoscience Research Laboratory and Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 2Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4National Nanotechnology Center, Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Phathumthani 12120, Thailand 5 NANOTEC Center of Excellence, Chiang Mai University, Chiang Mai 50200, Thailand


Introduction
In the past few years, interest has been paid to research on water remediation with the application of an ideal "green" technology known as semiconductor photocatalysis.It has been widely accepted that this process successfully combines the principle of heterogeneous catalysis with a utilization of solar energy.By using this photocatalytic process, degradation of a wide range of organic pollutants into harmless carbon dioxide and water is made possible.Titanium dioxide, a well-known UV-light-active photocatalyst, has demonstrated an outstanding photocatalytic performance on degradation of various organic compounds [1][2][3].However, with its wide band gap energy of 3.2 eV, the application of TiO 2 is limited to UV light region which accounts for only 4% of the whole solar energy [4].Therefore, extensive research has currently been devoted to the development of visible-light-driven catalyst in order to effectively utilize the vast majority of the solar energy [4][5][6].Bismuth vanadate (BiVO 4 ) has long been recognized for its ferroelasticity [7] and its application as a nontoxic and bright yellow pigment [8].It has also been used as a gas sensing semiconductor, solid-state electrolyte, and cathode material in solid oxide fuel cells, and has recently been proved to be an active visible-light-responsive photocatalyst for water splitting [9] and organic pollutant decomposition [10,11].Since BiVO 4 is stable and neutral in water International Journal of Photoenergy without altering the solution pH, its application as photocatalyst for environmental treatment is extensively investigated.The photocatalytic property of BiVO 4 is strongly dependent on its morphology and crystalline form [12][13][14].Generally, synthetic BiVO 4 was found to exist in three crystalline phases including scheelite-monoclinic type, scheelitetetragonal type, and zircon-tetragonal type [15].Among the three polymorphs, the scheelite-monoclinic structure with band gap energy of 2.4 eV is reported to possess the highest photocatalytic activity [4,13,16].Therefore, many synthesis methods have been focused on the selective preparation of scheelite-monoclinic BiVO 4 photocatalyst.
Different synthetic routes have previously been employed to prepare the scheelite-monoclinic BiVO 4 such as traditional solid-state reaction [17] and hydrothermal method [18].However, these strategies have encountered similar problem in which the obtained BiVO 4 possessed very low surface area, normally in the range of less than 10 m 2 g −1 and, as a consequence, low photocatalytic performance has usually been attained.Apart from the low surface area of BiVO 4 , difficult separation of photogenerated electronhole pair was also reported to be one of the main reasons accounting for its poor photocatalytic efficiency [12,19].However, by loading BiVO 4 with only small amount of metals such as Pt [11], Au [12] Pd [19], Ag, Co, and Ni [20], enhanced photocatalytic activity was achieved possibly due to the metals acting as electron traps, thus promoting electron-hole separation and the interfacial charge-transfer process from catalyst to adsorbed substrate [12,19,20].However, there are few reports on the development of Au/BiVO 4 composite to affect photocatalysis under visiblelight irradiation.Recently, Cao et al. [21] reported that the Au/BiVO 4 composite showed superior visible-light activities in decomposing methyl orange dye.However, Long et al. [22] synthesized Au/BiVO 4 composite photocatalysts and found that the photocatalysts exhibited enhanced visiblelight photocatalytic activities on degradation of phenol.Since dicarboxylic acids are generally observed as intermediate products in the degradation pathways of various organic pollutants in real wastewaters [23][24][25], the influence of mutual interactions on the photocatalytic conversion process needs to be investigated.However, there has been no report that demonstrates the simultaneous detoxification of the dicarboxylic acids by Au/BiVO 4 composite.Herein, we report the preparation and photocatalytic performance of a visiblelight-driven Au/BiVO 4 catalyst.BiVO 4 with the scheelitemonoclinic structure was prepared by surfactant-assisted coprecipitation method and then subsequently impregnated with HAuCl 4 solution to finally obtain Au/BiVO 4 .Photocatalytic performances of the as-prepared scheelite-monoclinic Au/BiVO 4 samples were evaluated through the mineralizations of oxalic acid and malonic acid under visible light irradiation.SO 3 Na (SDBS), 98%, Aldrich), an anionic surfactant, was employed as a dispersant in this study.All chemicals were used as received without further purification.Firstly, 0.125 M each of Bi and V were separately prepared by dissolving Bi(NO 3 ) 3 ⋅5H 2 O in 4.0 M nitric acid solution and NH 3 VO 4 in 4.0 M ammonia solution.The as-prepared bismuth nitrate solution was then mixed with 0.1 M SDBS in ethanol.To this mixture, the vanadium precursor solution was slowly added and the solution was kept under stirring for 30 min.Then 4.0 M ammonia solution used as a precipitant was added drop-wise until pH 7 was attained.The resultant precipitate was washed with deionized water, centrifuged, and dried at 60 ∘ C for 12 h.The dried powder was then calcined at 400 ∘ C for 2 h to obtain BiVO 4 sample.The as-prepared BiVO 4 sample was then impregnated with aqueous solution of gold chloride (HAuCl 4 ⋅2H 2 O, ≤48% Aldrich) containing the nominal gold amounts of 0.10, 0.25, 0.50, 0.75, and 1.00 mol%.Then the impregnated powder was dried at 60 ∘ C for 3 h and subsequently calcined at 350 ∘ C for 2 h to finally obtain Au/BiVO 4 .

Characterization.
Powder X-ray diffraction (XRD) measurement was performed on an X-ray diffractometer (JEOL, JDX-3530) using Cu K radiation ( = 1.5418Å) and scanning from 10 ∘ to 75 ∘ .Specific surface area (SSA) of the particles was measured on Beckman Coulter SA 3100 according to the Brunauer-Emmett-Teller (BET) method.UV-Vis diffuse reflectance spectra were obtained on a UV-Vis spectrometer (PerkinElmer, Lambda 650S) using MgO as a reference and were converted to absorbance by Kubelka-Munk method [26].Particle morphology and chemical composition were also investigated on a scanning electron microscope (Hitachi, S3400N) equipped with an energy dispersive X-ray spectrophotometer (Oxford, ISIS300).

Photocatalytic Activity.
Visible light photocatalytic activities of the as-prepared Au/BiVO 4 powders were evaluated by the mineralization of oxalic acid (0.208 mM) and malonic acid (0.139 mM) in aqueous solution at ambient temperature and pressure.The photocatalytic studies were performed using a 100 mL spiral photoreactor equipped with a fluorescent lamp (Davis 33 cool white, 18 W, intensity of 4.39 mW/m 2 ), filtered with double-layer of Rosco E-colour UV filter to remove any UV component ( < 400 nm), in the middle of the reactor.Typically, 50 mL of 1.0 g L −1 catalyst suspension was prepared by dispersing the predetermined amount of catalyst in deionized water with ultrasonic probe for 15 min.The suspension pH was then adjusted to 3.0 ± 0.1 using 1 M perchloric acid solution before charging into the spiral reactor.Prior to catalytic testing, adsorbed carbon contaminants on the catalyst surface were firstly removed by illuminating the catalyst suspension with UV light for 1 h.Photocatalytic mineralizations of oxalic acid and malonic acid were then carried out by injecting 100 L of organic compound solution containing 500 g of carbon.Adsorption/desorption equilibrium of the organic substrates on the catalyst surface was attained by circulating the suspension for 30 min under the dark.Then the system was irradiated and the photocatalytic reaction was initiated.Dissolved carbon dioxide (CO 2 ) in water, generated from dicarboxylic acid, could be detected by online conductivity meter (Eutech 5000) as described by Abdullah et al. [27].XRD patterns of all samples presented similar profiles and the diffraction peaks matched well with scheelite-monoclinic BiVO 4 (JCPDS file no.14-0688).However, these obtained samples are not well-distorted scheelite-monoclinic BiVO 4 because there is no peak at 15 ∘ of 2 and the peaks at 18.5 ∘ , 35 ∘ and 46 ∘ of 2 are not well split [28].

Results and Discussion
Results from XRD suggested that the scheelite-monoclinic BiVO 4 , although with less distortion, was selectively prepared by using SDBS-assisted coprecipitation method.Upon loading pure BiVO 4 with Au, no significant change in diffraction patterns was observed which suggested that loading with Au up to 1.0 mol% did not affect the crystal structure of scheelite-monoclinic BiVO 4 .No other peak due to Au metal was found possib because the Au loading amount was small and high dispersion of Au was obtained as supported by EDX spectra in Figures 2(d) and 2(f) [29].BET specific surface area (SSA) of the resulted BiVO 4 was about 23 m 2 g.Upon loading the obtained BiVO 4 with Au from 0.1-1.0mol%, SSA of the sample was gradually increased from 23 m 2 g −1 to 31 m 2 g −1 .
Morphologies of pure BiVO 4 , 0.25 mol% Au/BiVO 4 , and 1.0 mol% Au/BiVO 4 as well as the presence of Au on the surface of Au/BiVO 4 samples were studied by SEM and EDX as illustrated in Figure 2. As seen from the SEM images in   2(f).The atomic percentage of each element is given in Table 1.The amount of Au in both samples was less than the loading amount since the Au nanoparticles were not evenly distributed on the BiVO 4 support; therefore, the selected areas had less amounts than the actual amounts.

Optical Absorption
Behavior.The inset of Figure 3(a) indicated that pure BiVO 4 had no absorption in the region of 550-800 nm; however, all Au/BiVO 4 samples showed enhanced absorption in this region upon increasing Au content.The absorption in this range can be ascribed to the surface plasmon resonance (SPR) of Au nanoparticles which is attributed to a collective of conduction electrons in response to optical excitation [30].In addition, the SPR peak was slightly shifted to longer wavelength as increasing Au content, possibly due to an increase of Au particle size [31].
Optical absorption near the band edge  and h] represent, a constant, absorption coefficient and the incident photon energy, respectively [4,32].The  value depends on the characteristics of the electron transition in a semiconductor.Since the electron transition in BiVO 4 is a direct transition, the value of  = 1 [4,19,32,33].The band gaps were estimated (h]) by using the intercept of the tangent to the -axis as illustrated in Figure 3(b).The estimated band gap energy of pure BiVO 4 was 2.53 eV.However, upon loading BiVO 4 with the nominal gold amount of 0.1−1.0mol%, a small shift towards lower band gap energy in the range of 2.48-2.50eV was observed, probably due to a charge-transfer transition between Au and BiVO 4 [12].Au/BiVO 4 .Further loading of Au more than 0.25 mol% resulted in a decreased photocatalytic activity.Therefore, an optimum Au loading amount in this study is 0.25 mol%.The existence of an optimum dopant concentration was previously explained by Zhang et al. [34].Therein, the author proposed that, at low dopant concentration, metal ion dopant can act as a trap for both electron and hole which then leads to a lengthening in the lifetime of the generated charge carriers and thus resulting in enhanced photocatalytic efficiency.However, at high dopant concentration, the charge trapping is high and as such, the charge carrier recombination through quantum tunneling is highly possible [34,35].Therefore, there exists an optimum dopant concentration.A comparison between Figures 4(a) and 4(b) indicated that longer irradiation time was required for malonic acid to attain complete mineralization.This is probably due to the longer hydrocarbon chain length of malonic acid compared with that of oxalic acid.
Photocatalytic mineralization rates of oxalic acid and malonic acid over 0.25 mol% Au/BiVO 4 were investigated and presented in Figure 4(b).The obtained results were found to fit well with Langmuir-Hinshelwood (LH) kinetics as evidenced by high correlation coefficient values ( 2 ) of 0.99 and 0.94 for degradations of oxalic acid and malonic acid, respectively.
The LH kinetic expression is given by [36,37] where  represents the initial mineralization rate of organic substrate, C is the concentration of the substrate at an illumination time t, and k and  are the mineralization rate constant and the adsorption coefficient of the reactant, respectively.Integration of (1) yields (2): where  0 is the initial concentration of the organic substrate and   in the concentration of the substrate at time .When  0 is very small, (2) can be reduced to (3), where  app is the initial apparent rate constant of a pseudofirst-order reaction.By plotting ln( 0 /  ) versus  as shown in Figure 4(b),  app values for photocatalytic mineralizations of oxalic acid over pure BiVO 4 , 0.25 mol% Au/BiVO 4 and malonic acid over 0.25 mol% Au/BiVO 4 can be obtained from slopes of the graphs.Good regression coefficients observed in this work indicated that the kinetics of dicarboxylic acid mineralizations followed a simplified Langmuir-Hinshelwood rate equation (3) with the pseudo-first order rate constants for oxalic acid degradation over pure BiVO 4 and 0.25 mol% Au/BiVO 4 of 0.0188 min −1 and 0.0487 min −1 , respectively, as shown in Figure 5.The  app of 0.25 mol% Au/BiVO 4 was more than twice higher compared to pure BiVO 4 .The  app of oxalic degradation was also found to be higher than that of malonic acid ( app = 0.0082 min −1 ).Generally, photocatalytic mineralization rate is governed by both structural and functional properties of the target molecule.In this work, the two acids used as the targeted substrates are dicarboxylic acid; therefore, functional characteristic could not be the main reason explaining the difference in rate constants.However, by considering the structures of these acids, oxalic acid (C 2 ) with shorter carbon chain length than malonic acid (C 3 ) could provide higher apparent rate constant.Our finding is in good agreement with that of Denny et al. [38].Therein, a decrease in 50% mineralization rate with increasing carbon chain length was observed and the authors ascribed this behavior to the increased complexity of the degradation mechanism of the longer chain hydrocarbons.Therefore, the decrease in  app observed in our study was likely attributed to an increase in carbon chain length of the targeted molecule.However, other factors including an increased steric hindrance effect, a decrease of the positive charge on the carbon of carboxyl group as increasing carbon chain length, thus making hydroxyl radical attraction more difficult may also have some contribution to the obtained apparent rate constant [38,39].

Conclusion
Significantly improved photocatalytic efficiency was observed upon loading pure monoclinic-scheelite BiVO 4 with low concentrations of gold dopant (0.1-1.0 mol%).Maximum apparent rate constant observed from 0.25 mol% Au/BiVO 4 was found to be more than twice higher compared to pure BiVO 4 .However, loading Au more than 0.25 mol% was detrimental to the photocatalytic activity since excess Au atoms may act as charge recombination centers, resulting in a decrease of charge carrier lifetime and low photocatalytic performance.The initial apparent pseudo-first-order rate constants of 0.25 mol% Au/BiVO 4 were found to be 0.0487 and 0.0082 min −1 for the degradations of oxalic acid and malonic acid, respectively.By considering structures of the two acids, lower pseudo-first order rate constant obtained in the case of malonic acid degradation was likely due to an increased complexity of the degradation mechanism of the longer chain acid.However, the fact that an enhanced steric hindrance effect and a decrease of the positive charge on the carbon of carboxyl group may affect the observed rate constantly could not be neglected.

3. 1 .
Physical Property of Au/BiVO 4 .Figure 1 illustrates XRD diffraction patterns of pure BiVO 4 and Au/BiVO 4 with different Au loading amounts.The XRD patterns revealed sharp peaks, indicating high crystallinity of the obtained particles.

3. 3 .
Photocatalytic Property.Photocatalytic activities of scheelite-monoclinic BiVO 4 and Au/BiVO 4 were evaluated by studying the mineralizations of two model organic compounds which were oxalic acid and malonic acid in aqueous solution under visible light illumination.Photocatalytic mineralizations of oxalic acid and malonic acid over the asprepared Au/BiVO 4 as a function of irradiation time are shown in Figures4(a) and 4(b), respectively.Results from Figure4(a) also indicated that a remarkable photocatalytic performance was obtained from 0.25 mol%

Table 1 :
The content of synthesized pure BiVO 4 and different Au/BiVO 4 powders determined by EDX analysis.