During the past decades, a large variety of advanced oxidation processes have been developed for degradation of various organic and inorganic materials [1-5]. These powerful oxidation methods are chemical, photochemical (PC), and electrochemical (EC) processes that are based on the in situ generation of highly reactive species [6-8]. Among these processes, photocatalysis is a promising advanced oxidation technology because of its ability to detoxify bacteria and photodegrade a large variety of contaminants cheaply and efficiently [9-11]. Thus, great attention has been paid to the photocatalytic reactions that occur on semiconductor materials [12, 13]. It is generally agreed that upon exposing a semiconductor surface to light of sufficient energy (hυ ≥ band gap, E_g), charge separation is induced, resulting in the formation of reactive electrons (e^-) and holes (h⁺). These charge carriers can initiate oxidation reactions either directly or by producing secondary oxidizing species such as hydroxyl radicals, hydroperoxyl radicals, hydrogen peroxide, and superoxide [14]. Considerable work in this area has been devoted to the oxidative degradation of pollutants on TiO₂, which suffers from the drawback of operating mainly under ultraviolet (UV) light irradiation [15, 16]. To overcome this drawback, an alternative suitable visible light-sensitive photocatalyst is required for practical applications. Bismuth vanadate (BiVO₄) is one of the most promising photoanode candidates that shows visible-light photoactivity [17-20]. The narrow E_g of BiVO₄ (∼2.4 eV) implies that it can absorb about 11% of the solar spectrum compared with 4% for a standard TiO₂ photocatalyst [17]. Moreover, the position of the valance band (VB) edge of BiVO₄ (located at ca. 2.4 V vs. RHE), provides sufficient overpotential for photogenerated holes to exhibit favorable photocatalytic activity for oxidation of materials [21].

Because of the quick recombination of excited electrons and holes, photoelectrocatalysis (PEC) has recently been demonstrated as an efficient way to further accelerate PC reactions. PEC involves applying an external potential to drive away electrons from the conduction band of a photocatalyst to a counter electrode, thereby increasing photocatalytic efficiency by preventing charge recombination.

Recently, Parkinson et al. [22] reported that TiO₂-based semiconductors could photo-oxidize chloride to perchlorate ions in aqueous solutions. They also mentioned that the potential of the oxygen p-band in most semiconducting oxides was very oxidizing, suggesting that other oxides such as hematite could also induce this reaction. In other work, You et al. [23] reported that photooxidation of ClO₃^-at the surface of a Cu-TiO₂/SiO₂ system resulted in the formation of ClO₄^-. As stated above, BiVO₄ has been identified as a promising photoanode material [24-28]. However, chlorate oxidation by BiVO₄ has not been reported. In the present work, a BiVO₄ film is prepared by electrodeposition followed by annealing. The film is optically and structurally characterized using different characterization methods and subsequently used for PEC oxidation of chlorate in aqueous solution.

PEC experiments were conducted in a single-compartment custom-made reactor. A micro-Autolab EC workstation (PG-STAT) was used to bias the BiVO₄ photoanode against a platinum-wire counter electrode and Ag/AgCl reference electrode. Chlorate oxidation experiments were carried out in a quartz cell containing test solution (30 mL) at room temperature. PC and PEC reactions were carried out under a light intensity of 200 mW/cm² and the photoactive area of the photoanode was 1 cm². Oxidation of chlorate was monitored by a spectrophotometric method according to a procedure previously described in the literature [29]. This method was based on the reaction of chlorate with a solution containing concentrated HCl and a small amount of bromide ions. The products of PEC oxidation of chlorate were monitored by a chromatograph (Knauer, Germany) equipped with a dual-piston pump, PRP-X110S analytical column (4.1 × 250 mm), and conductivity detector. An aqueous solution of Na₂CO₃ (9.0 mmol/L) at a flow rate of 1.3 mL/min was used as the eluent.

Bi metal was electrodeposited on the surface of a clean fluorine-doped tin oxide (FTO) substrate from a solution of Bi(NO₃)₃·5H₂O (20 mmol/L) in ethylene glycol. The deposition was carried out by passing a charge density of 0.04 C/cm² at a potential of -1.8 V vs. Ag/AgCl through the substrate, followed by a resting time of 2 s. This cycle was repeated eight times to pass a total charge of 0.32 C/cm² through the substrate. After electrodeposition, the film was carefully washed with ethanol and dried in air. To convert Bi to BiVO₄, VO(acac)₂ (56 μL, 0.15 mol/L) in dimethyl sulfoxide was placed on the surface of the Bi electrode (geometric area 1 cm²) to fully cover its surface. The film was dried at about 80 ℃ and then heated at 500 ℃ for 2 h in air (ramp rate 2 ℃/min). During the heating process, Bi metal and VO²⁺ oxidized and reacted with each other to form BiVO₄. Finally, after cooling to room temperature, the residual V₂O₅ was removed by soaking the electrode in NaOH (1.0 mol/L) solution for 30 min with stirring.

The structural and optical characterization of the as-grown BiVO₄ film was carried out using different techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman scattering, and UV-visible (UV-vis) spectroscopy. Fig. 1 presents SEM images of metallic Bi and BiVO₄ films. The morphologies of the Bi and BiVO₄ films were in good agreement with those reported elsewhere [30]. Fig. 1(a) reveals that the as-synthesized black Bi film was composed of uniform nanodendritic structures with dendrite trunks more than 1 μm long and branches with diameters of less than 100 nm. The highly ordered branches were distributed on both sides of each trunk to produce three-dimensional structures with high surface area. Moreover, a cross-sectional SEM image of the Bi film (Fig. 1(b)) confirmed the uniformity of this structure in the film with a thickness of about 1.4 μm. The structure and thickness of the BiVO₄ film differed from those of the original Bi film. As shown in Fig. 1(c), BiVO₄ formed a granular, more compact film with a thickness of about 400 nm (Fig. 1(d)). The BiVO₄ film contained some pores and cavities between grains, which may shorten the pathway for photogenerated holes to reach the surface of BiVO₄. The crystal structure of BiVO₄ was further evaluated by conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM), as illustrated in Fig. 1(e) and (f), respectively. The HR-TEM image reveals a lattice spacing of 0.31 nm, which can be assigned to the (121) crystalline plane of monoclinic scheelite BiVO₄ [18]. To investigate the crystal phase composition of the BiVO₄ film, XRD and Raman scattering measurements were also carried out.

It has been reported that BiVO₄ crystallizes in a monoclinic scheelite (clinobisvanite) phase upon calcining at temperatures higher than 400 ℃ [31]. XRD analysis of the film confirmed the high crystallinity of BiVO₄. As it can be seen in Fig. 2, the patterns of the BiVO₄ film were consistent with a monoclinic structure (JCPDS No.14-0688), which is considered the most active BiVO₄ phase for photocatalysis or PEC applications [32, 33]. The monoclinic nature of the film was confirmed from the two prominent peaks at 18.7° and a sharp peak at 30°, which correspond to the (011) and (112) reflections of BiVO₄, respectively. The presence of minor peaks that could be attributed to (101), (004), (121), (015), (204), and (116) reflections further confirmed the film was highly crystalline monoclinic BiVO₄ that was free from any other phases.

The Raman spectrum of the BiVO₄ film was consistent with XRD results and indicated that BiVO₄ had a monoclinic phase based on the characteristics of stretching and bending vibrations of VO₄^3- tetrahedra. Fig. 3(a) displays the Raman scattering spectrum of the crystalline BiVO₄ film, which main peaks at around 125, 206, 321, 362, and 820 cm^-1 are characteristics of the typical vibration bands of a pure monoclinic scheelite structure. In accordance with reported data [34-36], the most intense band at 820 cm^-1 was attributed to the symmetric stretching mode of V-O (A_g symmetry) and the weak bands at about 629 and 705 cm^-1 were assigned to the asymmetric V-O stretching mode (B_g symmetry). The symmetric and asymmetric bending modes corresponding to deformation of VO₄ tetrahedra were observed at 362 and 321 cm^-1, while the bands around 206 and 125 cm^-1 were attributed to external rotation and translation modes, respectively. Fig. 3(b) shows a Fourier transform infrared spectrum of the BiVO₄ film from 600 to 1500 cm^-1. The absorption bands in the 600-900 cm^-1 region were characteristic of the stretching vibrations of the V-O bond [37]. A broad, strong absorption band at 682 cm^-1 was assigned to the asymmetric stretching of VO₄^3-and the absorption band at 820 cm^-1 was assigned to the symmetric stretching of VO₄^3-.

To characterize the optical properties of the BiVO₄ film, we studied its UV-vis absorption properties (Fig. 4). The absorption spectrum of BiVO₄ agreed well with those previously reported for the monoclinic phase [38], exhibiting a prominent absorption edge in the visible region of the solar spectrum. The absorption edge of BiVO₄ was observed at around 520 nm, and its band gap was estimated to be around 2.35 eV according to the intercept on the wavelength axis (Fig. 4).

The EC, PC, and PEC processes for chlorate degradation in aqueous solutions were performed on the BiVO₄ electrode (Fig. 5). In the EC process, a bias of 1.5 V (DC mode) was applied to the BiVO₄ anode relative to the Ag/AgCl reference electrode. The EC and PC processes of chlorate oxidation over the BiVO₄ anode were negligible after 150 min, showing the high stability of chlorate ions under these conditions. Conversely, in the PEC process, about 76% of the chlorate was degraded after 150 min. These results indicate that the applied bias potential (1.5 vs. Ag|AgCl|KCl) effectively inhibited the recombination of the photogenerated e^--h⁺ pairs, prolonging the life of photogenerated charge carriers. In the PEC oxidation process, PEC oxygen evolution is the main side reaction, so the current efficiency will decrease as the concentration of chlorate decreases. This behavior can be seen clearly in Fig. 5(a). First-order kinetics were observed for the oxidation reaction (Fig. 5(b)), with first-order rate constants, k, for the PC, EC, and PEC processes of 0.0035, 0.085, and 0.61, respectively. It is clear that under the conditions used, the degradation rate of chlorate in the PEC process is much faster than those in the two other processes, which demonstrates the synergistic effect between the EC and PC processes. The synergistic factor between these two processes was quantified by the following equation [39]:

(1)

where, k_PEC, k_PC, and k_EC are the rate constants of PC, EC, and PEC, respectively. Using Eq. (1), F was calculated to be 6.9, which indicates there is a strong synergistic effect between the EC and PC processes.

The exact mechanism of EC, PC, and PEC of chlorate over the BiVO₄ photoelectrode and the chemical identity of the products are difficult to determine and beyond the scope of this study. However, as shown schematically in Fig. 6, under illumination, the photons absorbed by the BiVO₄ photocatalyst layer generated e^--h⁺ pairs (Reaction (2) ). In the absence of bias potential (PC process), the recombination of electrons and holes is the predominant reaction and oxidation of chlorate is negligible. However, applying an anodic bias to the electrode provides a potential gradient within the film to drive the photogenerated holes and electrons in different directions. Photoexcited electrons will transfer via the external circuit to the Pt counter electrode and reduce water to hydrogen. At the same time, highly oxidative photogenerated holes (oxidative energy) will transfer toward the surface of BiVO₄ and oxidize chlorate ions (Reaction (3) ).

The following simple reactions are involved in the oxidation process under light irradiation.

(2)

(3)

Another possible oxidation process involves production of hydroxyl radicals and their subsequent reaction with chlorate ions. It has been reported that oxidation of ClO₃^-can be accomplished through discharge of water molecules at relatively high potentials to give adsorbed hydroxyl radicals [40]. This method has been studied extensively using lead oxide substrates [41]. However, in the case of BiVO₄, hydroxyl radicals can be generated through photocatalytic reaction of water (or hydroxyl anions) with highly reactive holes in a one-electron transfer step (Reaction (4) ). The photogenerated hydroxyl radicals can subsequently oxidize chlorate according to Reactions (5) -(7).

(4)

(5)

(6)

(7)

It should be mentioned that oxidation of water to oxygen could compete with chlorate oxidation and perchlorate production. To provide evidence for the mechanism of chlorate oxidation, the solution after PEC oxidation of chlorate for 1 h was analyzed using ion chromatography. Fig. 7 shows that after the photocatalytic reactions on the BiVO₄ film, the chromatogram contains three observable peaks consistent with SO₄^2-, ClO₃^-, and ClO₄^-. The strongest peak is attributed to SO₄^2-, which was used as a supporting electrolyte for the PEC reaction. The broad peak at long retention time was related to ClO₄^-, implying that perchlorate was the main product obtained from the oxidation of chlorate. These results are in good agreement with our proposed mechanism for chlorate oxidation.

A BiVO₄ photoelectrode was synthesized by a simple EC method. The electrode was characterized using various techniques and subsequently used for oxidation of chlorate by EC, PC, and PEC processes. The results showed that EC and PC processes could not effectively oxidize chlorate over the BiVO₄ photoelectrode. However, their synergistic effect led to high PEC oxidation efficiency, with about 76% chlorate oxidation achieved after 150 min. Chlorate was oxidized by highly oxidizing holes in the VB produced from light irradiation on the BiVO₄ surface. The bias potential extended the lifetime of e^--h⁺ pairs. Using experimental data, a first-order rate constant and synergetic factor for chlorate oxidation over the BiVO₄ photoelectrode were obtained.