Revealing the primary role of the V⁴⁺/V⁵⁺ cycle in InVO₄ catalysts for promoting the photo-Fenton reaction

Abstract

Surface oxygen and metal defects can promote the performance of photocatalysts significantly, but their role in the activation of hydrogen peroxide (H₂O₂) for pollutant degradation is not clear. Herein, the surface oxygen vacancies (V_O) and vanadium vacancies (V_V: V⁴⁺/V⁵⁺) of InVO₄ catalysts were adjusted using different surfactants. Among them, a cationic surfactant with a hydrophilic head of quaternary ammonium ions facilitated the formation of surface vanadium vacancies (V⁴⁺/V⁵⁺) on the InVO₄ crystal. This V_V-dominated InVO₄ exhibited the optimal degradation rate of rhodamine B and microcystin-LR (MC-LR) with H₂O₂ (0.045 M) at neutral pH under visible-light irradiation, which was ∼8.2-times versus 3.2-times larger than that of the Vo-dominated InVO₄ system. The cycle of V⁴⁺/V⁵⁺ decomposed H₂O₂ to yield hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) continuously, which degraded organic pollutants efficiently. This study provides insights into the primary role of the V⁴⁺/V⁵⁺ cycle in InVO₄ catalysts for promoting the photo-Fenton reaction via adjuration of surfactants.

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Environmental significance

Surface oxygen and metal defects can promote the performance of photocatalysts significantly, but their role in the activation of H₂O₂ for pollutant degradation is not clear. Herein, the surface oxygen vacancies (V_O) and vanadium vacancies (V_V: V⁴⁺/V⁵⁺) of InVO₄ catalysts were adjusted using different surfactants. This study provides insights into the primary role of the V⁴⁺/V⁵⁺ cycle in InVO₄ catalysts for promoting the photo-Fenton reaction via adjuration of surfactants.

1. Introduction

Advanced oxidation processes have been used widely as efficient treatment for the removal of organic pollutants. Hydroxyl radical (·OH)-based Fenton reaction¹ with strong oxidation has attracted attention as a way to solve water pollution.² Among the many types of Fenton systems, heterogeneous catalysts (especially photo-Fenton catalysts) can be used for the degradation of organic contaminants, due to their recyclability,³ low metal dissolution,⁴ wide pH applicability⁵ and use of nature light.⁶ Vanadate catalysts⁷ are promising visible-light photocatalysts⁸ for CO₂ reduction,⁹ air purification,¹⁰ hydrogen production¹¹ and water treatment.¹² However, inadequate adsorption capacity,¹³ fast recombination of photogenerated electrons (e⁻) and holes (h⁺)¹⁴ and a narrow absorption¹⁵ range in visible light have hindered industrial application of vanadate photocatalysts. Hence, combining InVO₄ photocatalysis with the Fenton reaction for wastewater treatment is a rational approach.

Unfortunately, activating peroxides and producing reactive oxygen species (ROS) to degrade organic compounds using pure InVO₄ photocatalysis is difficult because of the high metal valence of vanadium (V⁵⁺). Ji and colleagues found that InVO₄–BiVO₄ having a high molar ratio of V⁴⁺/V⁵⁺ presented excellent photocatalytic activity in the presence of H₂O₂.¹⁶ In addition, Yao et al. found that V⁴⁺ rather than the oxygen vacancy (V_O) of InVO₄ could promote N₂ activation to NH₃, which was confirmed by control experiments and theoretical calculation.¹⁷ Hence, it was possible to form a high-efficiency photo-Fenton system for water treatment if surface (especially metal) defects¹⁸ (V_m) were introduced on the surface of photocatalysts.¹⁹ A surfactant was used commonly for the modification of photocatalytic materials and resulted in a vacancy on the material surface. Bi₂O₂CO₃ photocatalysis obtained surface Vo defects by the addition of hexadecyl trimethyl ammonium bromide (CTAB).²⁰ In addition, the photoelectric material BiVO₄ was found to have V_V defects on its surface during preparation using CTAB as a template. BiVO₄ with V_V defects showed better light absorption and higher efficiency of CO₂ reduction than that in an unmodified system.²¹

In the present study, we prepared a series of InVO₄ photocatalysts with different dominant surface defects (V_O or V_V) using different surfactants to investigate the effect of the surface defects of InVO₄ catalysts on photo-Fenton activity. In particular, the activation mechanism of H₂O₂ by these surface defects was analyzed. Notably, combining optical microscopy and HRTEM methods, we studied (for the first time) the relationship between the spatial structure of surfactants and types of surface defects. Our data helped to reveal the primary role of the V⁴⁺/V⁵⁺ cycle in an InVO₄ catalyst for promoting the photo-Fenton reaction.

2. Results and discussion

2.1 Characterizations

2.1.1 Comparison of surface V_V and V_O defects. As described in section 2.1 of the ESI,† C-InVO₄ and B-InVO₄ were prepared with N-hexadecyltrimethylammonium chloride (CTAC) and bromohexadecyl pyridine (CPB), respectively. The structure of CTAC and CPB are displayed in Fig. 1. The N-InVO₄ sample was synthesized without using a surfactant. Fig. 2a shows the XRD patterns of as-prepared InVO₄ samples. Similar to N-InVO₄ prepared without surfactant addition, C-InVO₄ and B-InVO₄ samples displayed sharp diffraction peaks at 20.85°, 31.07° and 33.05°, which could be assigned to the characteristic (020), (200) and (112) facets of the orthorhombic phase of InVO₄ (JCPDF: 48-0898), respectively. These data indicated that modification of a cationic surfactant did not affect the crystal structure of InVO₄ (which had good crystallinity). Notably, compared with N-InVO₄, samples of B-InVO₄ and C-InVO₄ showed a non-ignorable shift at 33.05° (Fig. S1a;† which has been demonstrated to be due to the surface defects of the InVO₄ lattice²²) and a shift to the right for C-InVO₄ and the left for B-InVO₄ (which might denote different surface defects).

Fig. 1 Structure of (a) CTAC and (b) CPB.

Fig. 2 (a) XRD patterns of InVO₄. XPS spectrum of as-prepared InVO₄ samples: (b) In3d, (c) V 2p_3/2 and (d) O1s.

XPS could be used to identify these defects. In the XPS spectra, a conspicuous doublet located at 444.5 eV (In 3d_3/2) and 452.1 eV (In 3d_5/2) were attributed to the surface In³⁺ species of the N-InVO₄ sample (Fig. 2b). The binding energy of In shifted to its lower value (443.9 eV, In 3d_3/2; 451.5 eV, In 3d_5/2) with C-InVO₄ and B-InVO₄. Meanwhile, the two bands at 515.7 and 517.1 eV were assigned to the surface V⁴⁺ and V⁵⁺ species with N-InVO₄, respectively (Fig. 2c). A similar lower value of V was observed on the surface of C-InVO₄ (516.6 eV, V⁵⁺; 515.4 eV, V⁴⁺) and B-InVO₄ (516.6 eV, V⁵⁺; 515.7 eV, V⁴⁺). The percentages of V⁴⁺ defects in C-InVO₄ and B-InVO₄ were calculated to be 18.3% and 12.57%, respectively, using an integral method (Table S1†),²³ which were much higher than that of N-InVO₄ (9.49%). These data indicated that the modification of cationic surfactants generated more surface Vv defects for the InVO₄ catalyst, but did not affect the crystal phase or crystallinity. To confirm this finding, the photoluminescence (PL) emission spectra of InVO₄ samples were also obtained. C-InVO₄ and B-InVO₄ samples showed a PL band at 469 nm, which was attributed to the intrinsic defects, including V_V and V_O (Fig. S1b†).²⁴

As shown in Fig. 2d, there were three types of oxygen species: lattice oxygen (O1), bridging hydroxyl (O2) and the adsorption of O₂ species on V_Os (O3). These were centered at 530.1, 531.4 and 532.8 eV, respectively.²⁵ The V_Os concentrations (Table S1†) of C-InVO₄ (25.47%) and B-InVO₄ (40.44%) were higher than that of N-InVO₄ (3.91%), confirming that modification with the cationic surfactant generated surface V_O defects, which was in agreement with the literature.²⁰ The formation of V_Os arose from the charge balance of V⁵⁺/V⁴⁺.²⁶

While V_O and V_V defects were present at the surfaces of C-InVO₄ and B-InVO₄, there were more V_V defects at the C-InVO₄ surface than those of B-InVO₄. In contrast, many fewer V_O defects of C-InVO₄ were present than those of B-InVO₄ (Table S1†). The differences between two samples in the concentration of surface defects aided distinguishing of the respective effects of V_O and V_V upon photocatalytic performance.

2.1.2 Structure of surfactant-mediated surface defects on InVO₄. We wished to gain insight into the formation mechanism of two types of surface defect. We divided the preparation process of InVO₄ into four steps, and monitored each step using an optical microscope (Fig. S2†). When the reacting ingredients were mixed adequately, particles were not observed in the presence of CTAC (Fig. S2b₁†) and CPB (Fig. S2c₁†), and sheet or cube crystals formed in the absence of a surfactant (Fig. S2a₁†). After adjustment of the pH to 9.28, an assembly structure was observed in the presence of the CTAC (Fig. S2b₂†) and CPB systems (Fig. S2c₂†), indicating that pH played a crucial part in the formation of precursors. In the non-surfactant system and cationic surfactant (CTAC and CPB) under hydrothermal treatment (Fig. S2a₃, a₄, b₃, b₄, c₃ and c₄†), eye-like and tree-like crystal nuclei formed gradually. They grew in an ordered way to form the tree-like crystals of InVO₄. In the absence of a surfactant, the tree-like crystals exhibited some short and fragmentary branches (Fig. S2a₄†). In the presence of CTAC, these crystals had “branches”, which then grew in an orderly manner along horizontal and vertical directions (Fig. S2b₃†). These phenomena could be explained by the formation of oil-in-water spherical micelles at this concentration (Fig. S3a†). In contrast, in the presence of CPB, tree-like branches had many slender and curved branches (Fig. S2c₄†) as rod micelles were formed (Fig. S3b†).

As described in Table S2,† the pore sizes of all samples ranged from 7 nm to 15 nm, and the pore volume increased from 0.008 cm³ g⁻¹ for N-InVO₄ catalyst to 0.019 cm³ g⁻¹ for B-InVO₄ and further to 0.084 cm³ g⁻¹ for C-InVO₄. In addition, the specific surface area of N-InVO₄, B-InVO₄ and C-InVO₄ was 3.91, 10.95 and 23.08 m² g⁻¹, respectively. These results indicated that CTAC was most effective in modifying the structure of C-InVO₄ to achieve a small crystal size (Fig. S4†), large porosity and good crystallinity. The largest specific surface area of C-InVO₄ was conducive to providing more active sites and improving the photocatalytic performance. HRTEM (Fig. 3) further confirmed that the presence of surfactants affected the morphology and surface structure of InVO₄ crystals. First, the images in Fig. 3a₁–c₁ showed that the particle size of C-InVO₄ was the smallest, consistent with the analysis stated above. The presence of a surfactant in the InVO₄ synthesis reduced the crystal size substantially, and the small crystals had a large surface to expose to reactants. Second, the HRTEM images of Fig. 3a₂–c₂ showed that the lattice spacing of N-InVO₄ and C-InVO₄ samples was 2.900 Å (assigned to the (200) plane of orthogonal InVO₄ (2.876 Å)), and the lattice spacing of B-InVO₄ sample was 4.246 Å (assigned to the (020) plane of orthogonal InVO₄ (4.259 Å)). The main exposed plane of N-InVO₄ and C-InVO₄ was the (200) facet, whereas that of B-InVO₄ was the (020) facet. These results indicated the one could tune the exposed facet of InVO₄ using different surfactants. Consequently, the V⁴⁺ defects of C-InVO₄ with a large surface area provided many active catalytic sites. V_O defects were predominantly the surface vacancies of B-InVO₄.

Fig. 3 TEM images of (a₁) N-InVO₄, (b₁) C-InVO₄ and (c₁) B-InVO₄. HRTEM images of (a₂) N-InVO₄, (b₂) C-InVO₄ and (c₂) B-InVO₄.

CPB and CTAC had the same hydrophobic tails of the hexadecyl group, and their hydrophilic head groups were different (Fig. 1). The hydrophilic head of the surfactant interacted with the crystal nuclei and crystal facets of InVO₄, which affected the growth of InVO₄ crystals but also regulated their predominate facets.²⁷ The hydrophilic head of CPB displayed a planar structure owing to a pyridine ring, whereas the hydrophilic quaternary ammonium ion of CTAC showed a tetrahedral spatial configuration. The planar structure head of CPB was favourable for forming InVO₄ crystals with (020) facets. In contrast, the tetrahedral spatial structure head of CTAC favored the formation of InVO₄ crystals with (200) facets. A possible formation mechanism is shown in Fig. 4.

Fig. 4 Tuning mechanism of a surfactant for the surface defects of (a) C-InVO₄ and (b) B-InVO₄.

2.1.3 Optical and electrochemical properties. The optical properties of C-InVO₄, B-InVO₄ and N-InVO₄ were illustrated in UV-vis DRS spectra (Fig. 5a). All samples had strong absorption in the visible-light region (520–580 nm). The bandgap energies (E_g) of InVO₄ photocatalysts were obtained by their corresponding Tauc plots from UV-vis DRS.²⁸ As shown in Fig. S5,† the bandgap E_g of C-InVO₄, B-InVO₄ and N-InVO₄ was 2.37, 2.17 and 2.07 eV, respectively. Photocurrent response spectra (Fig. 5b) showed that the C-InVO₄ photocatalyst exhibited the most significant photocurrent response, which resulted from the highest separation efficiency. The PL spectrum²⁹ also confirmed that the C-InVO₄ photocatalyst exhibited lower e⁻–h⁺ recombination³⁰ compared with that for N-InVO₄ and B-InVO₄ (Fig. S6a†). Furthermore, electrochemical impedance spectroscopy (EIS) was introduced to evaluate the charge-carrier separation. A smaller semi-circular radius of C-InVO₄ than that of N-InVO₄ and B-InVO₄ photocatalysts was documented (Fig. S6b†). This observation indicated that C-InVO₄ had low resistance and was more helpful for the transference of interfacial charge, which conformed to the results for the photocurrent response and PL. All these properties showed a better photocatalytic performance of the C-InVO₄ sample.

Fig. 5 (a) Spectra of UV-vis DRS and (b) curves for transient photocurrent intensity versus time.

2.2 Photocatalytic activity

Azo dyes are present in untreated dye water and pose a serious threat to life.³¹ Degradation experiments of rhodamine B (RhB) using as-prepared samples with H₂O₂ under λ > 420 nm visible-light irradiation were carried out. As shown in Fig. 6a, there was negligible degradation of RhB (C_t/C₀ = 5.6% and 18.7%, respectively) in blank experiments (only RhB or without catalyst) after 150 min illumination. Percent degradation of RhB using C-InVO₄, B-InVO₄ and N-InVO₄ was 98.8%, 29.6% and 28.5%, respectively (Fig. 6a). Notably, the reaction rate constant of C-InVO₄ (k = 0.0255 min⁻¹) was much higher than that of B-InVO₄ (k = 0.0031 min⁻¹), and unmodified N-InVO₄ (k = 0.0025 min⁻¹) (Fig. 6b). The photocatalytic activity of C-InVO₄ was nearly 8.2-times higher than that of B-InVO₄.

Fig. 6 (a) Degradation curve of RhB (1.5 × 10⁻⁵ M) using InVO₄/H₂O₂ (pH = 5.93) and (b) its pseudo-first-order kinetic curve. (c) Degradation curve of MC-LR (5 × 10⁻⁶ M) using InVO₄/H₂O₂ (pH = 6.73) and (d) the corresponding pseudo-first-order kinetic curve. Reaction conditions: [catalyst] = 0.4 g L⁻¹, [H₂O₂] = 0.045 M, T = 35.5 °C.

To show that the degradation stated above was photocatalysis rather than photosensitization, we conducted microcystin-LR (MC-LR) degradation (Fig. 6c). The photocatalytic activity of these samples was C-InVO₄ > N-InVO₄ > B-InVO₄, and their rate constant was 0.0132, 0.0078 and 0.0041 min⁻¹, respectively. In comparison, the constant for MC-LR/H₂O₂ system (without catalyst) was only 0.0004 min⁻¹ (Fig. 6d). The degradation ability of N-InVO₄ was higher than that of B-InVO₄ for MC-LR. It was contrary to the degradation of RhB because the overloaded bulk oxygen defects in B-InVO₄ acted as centers of charge recombination.¹⁷ This result demonstrated the improvement in the efficiency of degradation of organic pollutants caused by CTAC modification. Moreover, different photocatalysts for the removal of pollutants via a photo-Fenton system were compared (Table S3†). C-InVO₄ was quite comparable with other photocatalysts. In addition, dark adsorption had a negligible role in the overall reaction. The adsorption reaction of RhB and MC-LR could reach equilibrium within 120 min under dark conditions (Fig. S7a and b†). Fig. S7c† revealed the role of C-InVO₄ in H₂O₂ activation. Only 28.8% RhB was removed by C-InVO₄ in the absence of H₂O₂ after 150 min illumination, whereas nearly 100% degradation of RhB was achieved in C-InVO₄/H₂O₂ under visible-light irradiation. C-InVO₄ exhibited restrictive activity for RhB degradation but could activate H₂O₂ for degradation. In addition, the C-InVO₄/H₂O₂ system removed only 8.7% RhB in the dark (Fig. S7d†). Hence, H₂O₂ and visible light had crucial roles in RhB degradation.

2.3 Activation mechanism of H₂O₂ on the InVO₄ surface

2.3.1 C-InVO₄ exhibited the highest potential of H₂O₂ activation. Fig. 7a showed the decomposition curve of H₂O₂ under irradiation for 150 min, and the percent decomposition values of C-InVO₄, B-InVO₄ and N-InVO₄ samples were 4.98%, 2.49% and 3.18%, respectively, which were significantly higher than that of the control group (H₂O₂ only, 1.77%). Hence, InVO₄ samples modified by CTAC with higher V⁴⁺ defects could promote H₂O₂ decomposition. The total quantity of H₂O₂ decomposed under irradiation is illustrated in Fig. S8.† The content of H₂O₂ decomposed by C-InVO₄ (2.24 mM) was nearly two-times higher than that of B-InVO₄ (1.12 mM), which further confirmed that C-InVO₄ having more V⁴⁺ defects could activate H₂O₂. Interestingly, in the initial 40 min, the percent decomposition of H₂O₂ over C-InVO₄ or B-InVO₄ was negligible, much lower than that of N-InVO₄ (Fig. 7a). There appeared to be competing regeneration and decomposition of H₂O₂ in the two modified InVO₄ systems, and the regeneration rate of H₂O₂ over C-InVO₄ was higher than the decomposition rate between 30 min and 60 min. After 60 min, H₂O₂ decomposition in C-InVO₄/H₂O₂ and B-InVO₄/H₂O₂ systems proceeded rapidly. Their decomposition rate constants after the initial 60 min were 0.027 and 0.020 h⁻¹, respectively, which were much higher than that of N-InVO₄/H₂O₂ (0.012 h⁻¹) and the control experiment without a catalyst (H₂O₂ only, 0.00088 h⁻¹) (Fig. 7b), further confirming the higher capacity of H₂O₂ activation by C-InVO₄ owing to more surface V⁴⁺ defects.

Fig. 7 (a) Degradation curve of H₂O₂ with visible-light irradiation. (b) Pseudo-first-order kinetics with illumination from 60 to 150 min [catal] = 0.4 g L⁻¹, [RhB] = 1.5 × 10⁻⁵ M, T = 33.2 °C, [H₂O₂] = 0.045 M, pH = 5.94.

2.3.2 Activation of H₂O₂via the V⁴⁺/V⁵⁺ cycle on the C-InVO₄ surface. H₂O₂ decomposition generated ROS. Trapping agents were used to identify the ROS produced over C-InVO₄ (the most active catalyst). Ethylenediaminetetraacetic acid (EDTA),³² benzoquinone (BQ)³³ and isopropyl alcohol (IPA)³⁴ were used as trapping agents to capture metal ions, O₂·⁻ and ·OH, respectively. As depicted in Fig. S9,† only 37.6% of RhB was removed with EDTA addition, suggesting that metal ions (V⁴⁺) were important for RhB degradation. Meanwhile, the degradation efficiency was decreased to 85.4% in IPA, indicating that ·OH radicals were involved in the degradation of RhB. Interestingly, RhB degradation was accelerated with BQ. A similar phenomenon was found in the Fenton-like reaction.³⁵ During this reaction, the cycle of Fe²⁺/Fe³⁺ was constructed through p-xylenol/BQ and ROS, and promoted photocatalytic activity. Therefore, we proposed that the reaction in the C-InVO₄/H₂O₂ system followed a mechanism similar to the Fenton reaction, wherein the V⁴⁺/V⁵⁺ cycle was established. Furthermore, the successive production of ·OH and ·O₂⁻ in C-InVO₄/H₂O₂ with or without a light system using terephthalic acid (TA) and nitro blue tetrazolium (NBT) as probes, respectively, was observed.³ As illustrated in Fig. S10,† 21.9 μmol L⁻¹ of ·OH was produced, along with 2470 μmol L⁻¹ H₂O₂ (5.49%) consumption without illumination. However, the generation of ·O₂⁻ was only 7.2 μmol L⁻¹ in the dark. In the presence of visible light, much less H₂O₂ (4.98%, 2240 μmol L⁻¹) was decomposed to produce a higher amount of ·OH (53.49 μmol L⁻¹) and ·O₂⁻ (54.08 μmol L⁻¹). In general, the consumption of H₂O₂ could be promoted by light radiation and more active radicals generated. Our unusual discovery could be explained by the regeneration of H₂O₂ in the C-InVO₄ photo-Fenton, and this conclusion was demonstrated with the results shown in Fig. 7. In addition, the percentage of H₂O₂ converted to ·OH under visible light (2.39%) was 2.6-times higher than that in the dark (0.90%) (Fig. S10b†). The increasing concentration of ·OH and ·O₂⁻ in visible light also indicated the important role of two types of ROS in the degradation of organic pollutants.

The formation of active radicals was investigated further with ESR.³⁶ As illustrated in Fig. 8a, weak quadruple signals with an intensity ratio 1.0 : 2.0 : 2.0 : 1.0 for DMPO–·OH adducts were observed in the C-InVO₄/H₂O₂ system in the dark, and they increased significantly under visible-light irradiation due to greater generation of ·OH species. The generation of ·OH in the dark condition could indicate the V⁴⁺/V⁵⁺ cycle by H₂O₂ decomposition. In addition, DMPO/O₂·⁻ signals were not recorded in the dark, whereas DMPO/O₂·⁻ adducts appeared and increased (Fig. 8b) with increasing irradiation time, which indicated that visible light promoted the formation of O₂·⁻. O₂·⁻ reacted with V⁵⁺ and water to regenerate H₂O₂ and V⁴⁺. Therefore, H₂O₂ could be activated and multiplied via the V⁴⁺/V⁵⁺ cycle under visible-light irradiation.

Fig. 8 (a) ESR tests for DMPO–·OH in water and (b) DMPO–O₂·⁻ in methanol of the C-InVO₄/H₂O₂ system, [catal] = 0.4 g L⁻¹, [H₂O₂] = 0.045 M.

An acidic condition plays a crucial part in Fenton reactions.³⁷ We investigated the effect of pH upon RhB degradation. Fig. 9a showed that the removal of RhB using the C-InVO₄/H₂O₂ system could reach at least 82.11% in a wide range of pH (2.39 to 10.61). This finding indicated that the reaction mechanism did not merely involve the Fenton mechanism.

Fig. 9 (a) Photodegradation curves of RhB solution under different pH conditions. (b) Photodegradation curves of RhB solution with different amounts of dissolved O₂ (pH = 5.94). Reaction conditions: [catal] = 0.4 g L⁻¹, [RhB] = 1.5 × 10⁻⁵ M, T = 33.7 °C, [H₂O₂] = 0.045 M.

2.3.3 Surface V_O had a minimal role in H₂O₂ activation. It has been reported that V_O can activate the molecular O₂ adsorbed³⁸ on a surface to yield ROS,³⁹ thereby resulting the pollutant degradation. Based on previous analyses, two types of defects (V_V and V_O) were formed on the C-InVO₄ surface. In principle, the degradation ability of RhB could be improved with an increasing concentration of dissolved O₂. However, the dissolved oxygen concentration (DOC) in this suspension barely affected the degradation of RhB (8.79, 1.91 and 20.06 mg L⁻¹, respectively) (Fig. 9b). This finding indicated that activation of the O₂ adsorbed at V_O did not make an essential contribution to photocatalysis of the C-InVO₄/H₂O₂ system. This observation was also consistent with the poor performance of B-InVO₄, in which V_O predominated. We also validated that H₂O₂ instead of O₂ was the only source of ·O₂⁻.⁴⁰ Therefore, the V⁴⁺/V⁵⁺ cycle should be the primary driving force of photocatalysis. V_V defects were more important active sites for H₂O₂ activation than V_O defects.

XPS (V 2p_3/2) analysis of C-InVO₄, B-InVO₄ and N-InVO₄ samples after degradation (Fig. 10a) corroborated our conclusion. Degradation did not cause a substantial change in the content of V⁴⁺ defects in N-InVO₄ (9.49% → 7.71%) and B-InVO₄ (12.57% → 8.19%) systems. In contrast, surface V⁴⁺ defects of C-InVO₄ decreased considerably from 18.3% (Table S1†) to 5.7% after degradation (Table S4†). Meanwhile, V⁵⁺ increased by 12.6%. These synergistic changes in V⁴⁺ and V⁵⁺ defects suggested that V⁴⁺ reacted with H₂O₂ to yield V⁵⁺ on the C-InVO₄ surface.

Fig. 10 (a) XPS spectrum of C-InVO₄, B-InVO₄ and N-InVO₄ samples after degradation: V 2p_3/2. (b) Cycling performance of the C-InVO₄ sample upon addition of oxalic acid. (c) Rate constant and (d) XPS V 2p_3/2 spectrum of C-InVO₄ treated/not treated with oxalic acid. Reaction conditions: [catal] = 0.4 g L⁻¹, [RhB] = 1.5 × 10⁻⁵ M, T = 33.0 °C, [H₂O₂] = 0.045 M.

To provide further evidence for the proposed activation mechanism of H₂O₂, we undertook cyclic degradation experiments of RhB in the C-InVO₄/H₂O₂ system. For four cycles, the degradation efficiency become slower and slower (Fig. 10b) because V⁴⁺ was converted persistently to V⁵⁺, which could not activate H₂O₂. To verify this hypothesis, oxalic acid (reactant) was added in the fourth cycle to reduce V⁵⁺ to V⁴⁺. This additive significantly enhanced the degradation rate of RhB over C-InVO₄ in the presence of H₂O₂ (Fig. 10b). The reaction rate constant reached 1.88 h⁻¹, which was 3.13-times greater than that of the untreated C-InVO₄ system (k = 0.60 h⁻¹) (Fig. 10c). This finding demonstrated the function of surface V⁴⁺ defects for the degradation capacity of C-InVO₄, which reacted with H₂O₂ to yield V⁵⁺ and ROS. As shown in Fig. 10d, the higher concentration of V⁴⁺ defects (32.6%) of C-InVO₄ treated with oxalic acid after the first degradation experiments of RhB in the C-InVO₄/H₂O₂ system led to a higher degradation rate compared with that using untreated C-InVO₄ (5.7%) (Table S4†). These data verified the decisive influence of V⁴⁺ on activation of H₂O₂. We also examined other reducing agents (e.g. ascorbic acid, sodium sulfite, and sodium citrate) on the fourth-cycle experiment, and identical results were obtained (Fig. S11†).

Overall, the data suggested that the surface V⁴⁺ defects of C-InVO₄ reduced H₂O₂ to give ·OH, which was accelerated by irradiation. Simultaneously, visible-light irradiation enabled decomposition of H₂O₂ to yield ·O₂⁻. A tentative photocatalytic activation mechanism of H₂O₂ by surface V_V defects on C-InVO₄ was proposed using these equations:

V⁴⁺ + H₂O₂ → V⁵⁺ + ·OH + OH⁻ (1)

H₂O₂ + e⁻ → ·OH + OH⁻ (3)

·O₂⁻ + V⁵⁺ + 2H₂O → 2H₂O₂ + V⁴⁺ (5)

Surface V⁴⁺ defects reacted with absorbed H₂O₂ (ref. 41) on the C-InVO₄ surface to give ·OH (eqn (1)).⁴² A photogenerated e⁻–h⁺ pair was separated by visible-light irradiation⁴³ (eqn (2)), wherein H₂O₂ captured e⁻ (ref. 44) to produce ·OH (ref. 45–47) (eqn (3)). Meanwhile, visible-light irradiation promoted the reaction of H₂O₂ with the ·OH radical to yield ·O₂⁻ (eqn (4)).⁴⁰ The V⁵⁺ yielded reacted with ·O₂⁻ to regenerate H₂O₂ (eqn (5)), and the V⁴⁺/V⁵⁺ cycle was established (Fig. 11).

Fig. 11 Photocatalytic mechanism of C-InVO₄ in the presence of H₂O₂ under visible-light irradiation.

3. Conclusions

A series of InVO₄ photocatalysts with different dominant surface defects (V_O or V_V) was synthesized using different surfactants to investigate the effect of the surface defects of InVO₄ catalysts on photocatalytic activity. Among them, the CTAC surfactant with a hydrophilic head of quaternary ammonium cations facilitated small C-InVO₄ crystals with (200) facets enriched with surface V⁴⁺ defects. This V_V-dominated InVO₄ exhibited the optimal degradation rate of RhB and microcystin-LR with H₂O₂ (0.045 M) at neutral pH under visible-light irradiation, which was ∼8.2-times versus ∼3.2-times larger than that of the V_O-dominated InVO₄ system. These V_V defects could activate H₂O₂ to generate ·OH and O₂·⁻ under visible-light absorption, so these species could degrade organic pollutants efficiently. Our study provides insights into the primary role of the V⁴⁺/V⁵⁺ cycle in InVO₄ catalyst for promoting the photo-Fenton reaction via adjuration of surfactants.

Author contributions

X. Liu and D. Huang conceived this project. L. Jin and H. Liu performed the experiments. X. Liu, Y. Huang, L. Ye and D. Huang analyzed the data and wrote the manuscript. All authors provided scientific input as well as editing and approving the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported financially by NSFC (22136003, 21972073, 22076098, 21805166) and the 111 Project (D20015).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3en00776f

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