Constructing Z-Scheme 3D WO₃@Co₂SnO₄ Heterojunction as Dual-Photocathode for Production of H₂O₂ and In-Situ Degradation of Organic Pollutants

Abstract

As photoelectrochemical catalyst material, Z-scheme heterojunction 3D WO₃@Co₂SnO₄ composites were designed through a hydrothermal-calcination method. The morphology and structure were characterized by SEM, EDS, XRD, XPS, DRS, and Mott–Schottky analysis, and the photoelectrochemical properties were explored with the transient photocurrent and electrochemical impedance. The construction of Z-scheme heterojunction markedly heightened the separation efficiency of photogenerated electron-hole pairs of WO₃ and enhanced the light absorption intensity, retaining the strong redox ability of the photocatalyst. The 3D WO₃@Co₂SnO₄ was used as a photocathode for production of H₂O₂. Under the optimal reaction conditions, the yield of H₂O₂ can reach 1335 μmol·L⁻¹·h⁻¹. The results of free radial capture and rotating disc test revealed the existence of direct one-step two-electron and indirect two-step one-electron oxygen reduction to produce H₂O₂. Based on the excellent H₂O₂ production performance of the Z-scheme heterojunction photoelectrocatalytic material, 3D WO₃@Co₂SnO₄ and stainless-steel mesh were used to construct a dual-cathode photoelectric-Fenton system for in-situ degradation of a variety of pollutants in water, such as dye (Methyl orange, Rhodamine B), Tetracycline, sulfamethazine, and ciprofloxacin. The fluorescence spectrophotometry was used to detect hydroxyl radicals with terephthalic acid as a probe. Also, the photocatalytic degradation mechanism was revealed, indicating the dual-cathode photoelectron-Fenton system displayed satisfactory potential on degradation of different types of environmental pollutants. This work provided insights for designing high-activity photoelectrocatalytic materials to produce H₂O₂ and provided possibility for construction of a photoelectric-Fenton system without extra additions.

1. Introduction

Photoelectrocatalytic (PEC), as a new technology using solar energy, was developed not only for the catalytic production of energy, but also for the degradation of water pollutants, solving energy and environmental problems [1,2,3,4,5]. PEC can directly utilize the photogenerated electron-hole pairs in semiconductor catalysts to produce fuel and degrade pollutants, which was usually used for production of H₂ and reduction of CO₂ [6,7,8]. Furthermore, PEC production of hydrogen peroxide (H₂O₂) was also a new ideal solar energy conversion fuel technology [9,10,11]. As a kind of green and environmentally friendly chemical substance, H₂O₂ was widely used in the fields of pulp, chemical raw material synthesis and water purification because of its advantages, such as non-toxicity and high energy density, which was a promising clean fuel [12,13]. Nowadays, PEC production of H₂O₂ has been a widespread concern, though there are more problems, such as low stability of semiconductor catalysts, poor resistance to photo-corrosion, low photoelectric conversion efficiency, high recombination rate of photogenerated carriers, and so on [14]. Therefore, developing high efficiency, low-cost material for the production of H₂O₂ was an ideal way to promote clean energy utilization and reduce environmental pollution problems.

Tungsten trioxide (WO₃), an n-type metal oxide semiconductor, was considered a benign visible-light response material, with a band gap of approximately 2.8 eV [15], because of its simple preparation method, eco-friendly attributes and stability in acidic and oxidative conditions. WO₃ was considered one of the candidate semiconductor materials for PEC oxidation and degradation of organic pollutants due to its excellent absorption, extensive range of visible-light capability and efficient photogenerated electron-holes oxidative performance. Nevertheless, pure WO₃ was plagued by some defects, that is, fast recombination of photogenerated hole-electron pairs and poor migration rates of interfacial photogenerated species, which caused unsatisfied reaction kinetics. It was manifested from multiple sources that construction of heterojunction can help WO₃ to improve its visible-light adsorption property and accelerate the separation of photogenerated electron-holes, which was the effective action for enhancing the PEC performance [16,17,18,19,20].

Tin was a precious metal, and tin-based oxide was considered one of the most promising alternatives to precious metals, because of its high electrical conductance and optical properties [21,22,23]. The abundant active sites can be provided by the oxygen vacancies in Cobalt tin oxide (Co₂SnO₄), which was beneficial to increasing the conductance [24]. Meanwhile, Co₂SnO₄ can give good electrical conductivity and larger surface area when combined with other two-dimensional materials [25]. Herein, Co₂SnO₄ nanoparticles deposited and grew on the surface of WO₃ nanobricks to construct Z-scheme 3D WO₃@Co₂SnO₄ heterogeneous composites via a hydrothermal-calcination method. The PEC performances of WO₃@Co₂SnO₄ heterogeneous for multifunction application were investigated, including production of H₂O₂ and photocatalytic degradation of organic pollutants in water. The PEC system for producing H₂O₂ was focused using 3D WO₃@Co₂SnO₄ heterogeneous as a photocathode. Simultaneously, a dual-cathode photoelectron-Fenton system was investigated for PEC degradation of various environmental organic pollutants. The PEC reaction mechanisms were discussed in detail.

2. Materials and Methods

2.1. Synthesis of WO₃ Nanobricks

The reagents used in this experiment were of analytical purity and did not undergo any further purification. The hydrothermal-calcination technique was employed for the synthesis of WO₃ nanobricks. Initially, 0.1 g of NaWO₄·2H₂O (Sinopharm Chemical Reagent Co., Ltd., China) was added in deionized water (DI) with the addition of 4 mL HCl (Sinopharm Chemical Reagent Co., Ltd., China). Subsequently, a yellow flocculent precipitate was solubilized upon the introduction of 0.54 g of oxalic acid under continuous stirring. The homogeneous mixture underwent hydrothermal reaction at 180 °C for 8 h. Afterwards, the as-prepared products were centrifuged, washed three times with DI, and dried overnight at 60 °C. Finally, the precursors were calcined in a muffle furnace at 2 °C·min⁻¹ to reach 500 °C for 2 h. Consequently, WO₃ nanobricks in the form of light-yellow powder were obtained.

2.2. Synthesis of WO₃@Co₂SnO₄ Composites

The synthesis process of WO₃@Co₂SnO₄ was displayed in Scheme 1. The molar ratio of CoCl₂·6H₂O (Sinopharm Chemical Reagent Co., Ltd., China) to SnCl₄·5H₂O (Sinopharm Chemical Reagent Co., Ltd., China) in a 2:1 proportion was dissolved in 60 mL DI under continuous stirring. Subsequently, a blue precipitate was obtained upon the addition of 20 mL NaOH (Sinopharm Chemical Reagent Co., Ltd., China) solution (1 mol·L⁻¹). The previously prepared WO₃ nanobricks were introduced into the mixture with stirring for 30 min. The obtained dispersion was poured into a Teflon-lined steel autoclave to react at 180 °C for 10 h. Following centrifugation and triple DI washing, the resulting product, namely WO₃@Co₂SnO₄ composite, was dried overnight at 60 °C and then annealed at heating rate of 2 °C·min⁻¹ to reach 500 °C for a duration of 2 h. The WO₃@Co₂SnO₄ composites with varying content ratios (10%, 20%, 30%, and 50%, which is mass ratio of Co₂SnO₄ and WO₃) were prepared by adjusting the quantities of CoCl₂·6H₂O and SnCl₄·5H₂O, denoted as WCSO-1, WCSO-2, WCSO-3, and WCSO-5, respectively. Also, Co₂SnO₄ loaded WO₃ without heterojunctions (WO₃+Co₂SnO₄), that is Co₂SnO₄ mixed WO₃ simply, was used as another control group.

2.3. Preparation of Photocathode

The carbon paper (2 × 3 cm²) was activated following the methodology described in the literature [26]. Then, 3 mg of WO₃@Co₂SnO₄ composites were ultrasonically dissolved in 0.5 mL DI. The resulting WO₃@Co₂SnO₄ suspension was dropped onto the lower part of carbon paper, covering approximately two-thirds of the carbon paper’s area. After allowing the suspension to dry, the catalyst materials formed a uniform film on the drop-coated region of the carbon paper. The procedure was followed for the preparation of all the photocathode films.

2.4. Characterization

All the catalyst samples were characterized through Scanning electron microscopy (SEM) equipped with X-ray energy-dispersive spectroscopy (EDS) (HITACHI SU-8000, Japan) and X-ray diffraction (XRD) (Bruck D8 Siemens D5000, Germany); PHI 5700 (USA) was used to detect X-ray photoelectron spectroscopy (XPS) of the catalysts, the UV–Vis spectra (Hitachi U3010), and photoluminescence (PL).

2.5. PEC H₂O₂ Production

PEC H₂O₂ production was performed in a quartz reactor under 300 W Xenon light illumination (λ ≥ 420 nm). The performance of WO₃@Co₂SnO₄ photocathode was detected in a three-electrode configuration with a Pt counter electrode and Ag/AgCl reference electrode under −0.6 V (vs. Ag/AgCl) bias conditions. A 0.1 mol·L⁻¹ Na₂SO₄ solution (20 mL, pH = 3 with 0.5 mol·L⁻¹ HClO₄). Prior to PEC measurement, the electrolytes were purged with O₂ gas under continuous stirring for 30 min, which ensured the adsorption–desorption equilibrium. In the whole reaction process, the concentration of H₂O₂ was determined using a spectrophotometric method with potassium titanium oxalate.

2.6. PEC Degradation of Contaminations

The performance of the WO₃@Co₂SnO₄ photocathode in terms of PEC was assessed using a double cathode. A stainless-steel mesh (SSM) measuring 2 × 3 m² was utilized as the second cathode, while a platinum plate served as the anode, and an Ag/AgCl electrode functioned as reference electrode. The PEC degradation of organic pollutants was conducted under the following conditions: 0.1 M Na₂SO₄ as the supporting electrolyte containing an initial concentration of 10 mg·L⁻¹ organic pollutants, Methylene blue (MB), Rhodamine B (RhB), Tetracycline (TC), Sulfamerazine (SMR), and Ciprofloxacin (CIP); pH value at 3 adjusted by 0.5 mol·L⁻¹ HClO₄, the bias voltage as −0.6 V (vs. Ag/AgCl). Prior to PEC measurement, oxygen purging with continuous magnetic stirring for a duration of 30 min ensured adsorption–desorption balance. Subsequently, a 300 W Xenon lamp from Beijing Perfect Light Co., Ltd. (Beijing, China) served as the light source for illumination purposes. At specific time intervals, UV–Vis spectrophotometry monitored changes in concentrations of different organic pollutants.

3. Results

3.1. Characteristic of WO₃@Co₂SnO₄ Composite

The morphology of various materials is characterized in Figure 1a–c. Pure WO₃, obtained through the hydrothermal-calcination method, exhibited a smooth-surface stack of nanobricks measuring approximately 300–500 nm in length and width, and a height of about 50–300 nm (Figure 1a). Each nanobrick can serve as a support, providing an optimal contact interface for loading with other semiconductors to construct heterojunctions. Figure 1b displays the morphology of Co₂SnO₄, which was a was spherical nanoparticle in the diameter range of 20–30 nm. After the combination of WO₃ and Co₂SnO₄, numerous secondary Co₂SnO₄ nanoparticles can be observed to grow epitaxially on WO₃ nanobricks, making the surface of WO₃ nanobricks rough (Figure 1c). That indicated that the formation of a large number of heterojunction interfaces was conducive to shortening the ion diffusion and reaction distance. Subsequently, as illustrated in Figure 1d, the existence of W, Co, Sn, and O elements in the materials was ascertained via EDS analysis on the WO₃@Co₂SnO₄ composite, which suggested that WO₃ nanobricks were successfully combined with Co₂SnO₄ nanoparticles.

To examine the crystallographic composition of the samples, XRD analysis was conducted and presented in Figure 2a. The obtained results indicated a good match between pure WO₃ and monoclinic system WO₃ corresponding to JCPDS No. 43-1035 [27]. Additionally, besides the diffraction peak of WO₃, the XRD pattern of WO₃@Co₂SnO₄ composite also revealed distinct peaks associated with Co₂SnO₄ at (220), (222), (420), and (533) located at 29.56°, 38.25°, 52.21°, and 59.52°, respectively, according to JCPDS No.29-0514 [28]. These findings provided confirmation for the successful preparation of heterojunction composites consisting of WO₃@Co₂SnO₄.

XPS analysis was carried out to investigate the chemical constitution and surface states of WO₃, Co₂SnO₄, and WO₃@Co₂SnO₄. Figure 2b illustrates the survey XPS spectra obtained for WO₃, Co₂SnO₄, and WO₃@Co₂SnO₄. In the composite WO₃@Co₂SnO₄, distinct peaks corresponding to W, Sn, O, and Co peaks were observed consistently with the EDS results. Additionally, the reference peak at 284.8 eV in all survey spectra corresponded to C 1s. Figure 2c–f illustrates detailed scans of W 4f, Co 2p, Sn 3d, and O 1s peaks in pure materials and WO₃@Co₂SnO₄ composite in sequence. Two peaks at 34.88 eV and 36.98 eV in Figure 2c correspond well to the W⁶⁺ states of W 4f_5/2 and W 4f_7/2, respectively. The binding energy peaks at 780.28 eV and 796.98 eV in Figure 2d can be ascribed to Co 2p_3/2 and Co 2p_1/2 states, which was consistent with the results for Co²⁺ [29]. Furthermore, the satellite peaks located at 787.08 eV and 803.18 eV were associated with Co 2p_3/2 and Co 2p_1/2 states [26,30]. The peaks at 486.15 eV and 494.48 eV belonged to Sn 3d_5/2 and Sn 3d_3/2 states, respectively. The O 1s binding energy peaks were detected at three different values: 530.18 eV, 531.88 eV, and 533.08 eV, corresponding to Metal-O (M-O), O-H of H₂O, and absorbed O, respectively. Based on the XPS analysis results, it can be observed that Co₂SnO₄ was incorporated into the heterogeneous structure of WO₃. During the heterojunction formation process, there was an increase in electron cloud density around the semiconductor, resulting in a negative shift in XPS spectral peaks while the semiconductor exhibited positive shift in its XPS spectra peaks. In the case of WO₃@Co₂SnO₄ composite, there was a positive shift in binding energy for W 4f, and both the binding energy of Co 2p peaks and that of Sn 3d peaks were shifted negatively. All these XPS results collectively indicated successful construction of heterojunction.

The optical properties of semiconductor materials were crucial factors for the performance of PEC systems. Figure 3a illustrated the UV–Vis absorption spectra of WO₃, Co₂SnO₄, and WO₃@Co₂SnO₄, demonstrating enhanced absorption in the UV–Vis range after combing WO₃ with Co₂SnO₄. Additionally, there was a red shift in the absorption range along with a broad peak spanning from 550 to 650 nm. The incorporation of Co₂SnO₄ into WO₃ was inferred to significantly broaden the absorption region, indicating enhanced light-absorption capabilities. As displayed in Figure 3b, the band gap energy (E_g) for WO₃, Co₂SnO₄, and WO₃@Co₂SnO₄ calculated using the Kubelka–Munk function equation [31] were 2.75 eV, 1.62 eV, and 2.47 eV, respectively. This suggested introduction of Co₂SnO₄ reduced the band gap of WO₃ and noticeably improved its light-absorption ability.

The Mott-Schottky plots were conducted to estimate the semiconductor type and the flat-band potential (E_fb) values of pure WO₃ and Co₂SnO₄, as depicted in Figure 3c,d. Both WO₃ and Co₂SnO₄ exhibited n-type semiconducting behavior with a positive slope, confirming that WO₃ had an E_fb of 0.22 eV (vs. RHE) while Co₂SnO₄ had an E_fb of −0.42 eV (vs. RHE). Generally, for n-type semiconductors, the E_fb was typically 0.1–0.3 eV higher than the E_CB. Thus, the E_CB values for WO₃ and Co₂SnO₄ were determined to be 0.12 eV and −0.42 eV, respectively. The inset of Figure 3d illustrated the band gap relationship between WO₃ and Co₂SnO₄ (inset of Figure 3d), revealing a favorable alignment of band positions which facilitates efficient transfer of photogenerated electron-holes within the heterostructure.

In order to inquire about the separation and transfer of photogenerated carriers at the heterojunction interface, we measured the transient photocurrent response of samples under simulated sunlight (Xenon lamp). Generally, a higher photocurrent indicated better separation of photogenerated carriers and a faster migration rate. As depicted in Figure 3e, compared to WO₃ and Co₂SnO₄, the WO₃@Co₂SnO₄ composite displayed the highest photocurrent. This can be put down to the construction of a heterojunction and an internal electric field within the composite, which significantly improved the efficiency of separating photoexcited carrier. The Electrochemical Impedance Spectroscopy (EIS, Nyquist plots) analysis (Figure 3f) demonstrated that the WO₃@Co₂SnO₄ electrode displayed a significantly reduced arc radius in comparison to the WO₃ and Co₂SnO₄ electrodes, indicating its exceptional ability to efficiently separate and transport photogenerated carriers for enhanced photoelectrocatalytic reactions.

3.2. PEC Performances

3.2.1. In-Situ Generation of H₂O₂ on the WO₃@Co₂SnO₄ Photocathode

The PEC activities of various catalysts for in-situ generation of H₂O₂ were conducted under oxygen saturation condition with a pH of 3 and visible-light irradiation for 60 min. As depicted in Figure 4a, WCSO-2 exhibited the highest H₂O₂ production (1335.3 μmol·L⁻¹·h⁻¹) compared to pure WO₃ (33.13 μmol·L⁻¹·h⁻¹), pure Co₂SnO₄ (279.1 μmol·L⁻¹·h⁻¹), WCSO-1 (673.3 μmol·L⁻¹·h⁻¹), WCSO-3 (956.7 μmol·L⁻¹·h⁻¹), WCSO-3 (956.7 μmol·L⁻¹·h⁻¹), WCSO-5 (824.5 μmol·L⁻¹·h⁻¹), and WO₃+Co₂SnO₄(125.6 μmol·L⁻¹·h⁻¹), respectively. The low yield of H₂O₂ observed for the pure catalysts can be due to the high photogenerated carrier recombination efficiency and inappropriate conduction potential. However, the PEC production capability of H₂O₂ gradually increased with different loading amounts of Co₂SnO₄ in the WO₃@Co₂SnO₄ composites. This indicated that the introduction of Co₂SnO₄ can enhance the PEC properties, while insufficient Co₂SnO₄ was not conducive to constructing a heterojunction structure, and excessive Co₂SnO₄ may obstruct active sites on the catalyst. If there was no special illustration, WCSO-2 was selected as the catalyst material for the whole experiment. In Figure 4b, under the optimal PEC H₂O₂ production conditions on WO₃@Co₂SnO₄ photocathode, that is O₂-saturated solution with pH value at 3 with visible-light irradiation, the cumulative concentration of H₂O₂ was 3120 μmol·L⁻¹ for 240 min, which indicated that the WO₃@Co₂SnO₄ photocathode possessed excellent PEC H₂O₂ production performance.

The recycling stability for an electrode was one of the important criteria to evaluate the quality. The recyclability of the as-prepared WO₃@Co₂SnO₄ photocathode was investigated for four cycles in Figure 4c, which exhibited that the yield of H₂O₂ still can reach more than 1 mmol·L⁻¹ in the four cycles. Figure 4d demonstrated that the photocurrent remained stable for 36,000 s. The results above showed that the WO₃@Co₂SnO₄ photocathode had satisfied stability and can be reused many times.

3.2.2. PEC Degradation of Organic Pollutants on Double Photocathode

A double-cathode that is the WO₃@Co₂SnO₄ photocathode coupled with the SSM photoelectric-Fenton system was carried out to investigate PEC degradation of organic pollutants in water. Based on the excellent H₂O₂ production performance of the WO₃@Co₂SnO₄ photocathode, the generated H₂O₂ from WO₃@Co₂SnO₄ photocathode and the electrons (e⁻) by the SSM were in-situ converted into hydroxyl radial (·OH), that is H₂O₂ + e⁻ →·OH + OH⁻, to achieve degradation of common pollutants in water.

Firstly, the effect of different cathode systems, namely the single-cathode system and double-cathode system, was investigated for the degradation of tetracycline (TC). The PEC degradation of TC was conducted in a 20 mL solution containing 10 mg·L⁻¹ TC with saturated oxygen (0.1 mol·L⁻¹ Na₂SO₄, pH = 3). As shown in Figure 5a, the degradation efficiency of TC in a double-cathode system reached 96% for PEC 60 min and achieved complete degradation after PEC 75 min. In contrast, the single-cathode system only exhibited a degradation efficiency of 47% for PEC 90 min. These confirmed that the double-cathode configuration outperformed the single-cathode configuration in terms of PEC degradation of pollutants. Meanwhile, the constant of apparent reaction rates of the two different systems were calculated and illustrated in Figure 5b, which indicated that the PEC degradation reaction rate was obeyed by the linearly correlated slopes. The double-cathode system displayed the faster reaction rate, and the apparent kinetic rate constant was 0.07565 min⁻¹ which was obviously higher than the single-cathode system (0.01153 min⁻¹). It was illustrated that the introduction of SSM accelerated the PEC degradation reaction, and further verified that the double-cathode system can prompt the ·OH generation to improve the degradation effect of TC.

To further investigate the PEC capability of the double-cathode system, the degradation effect of organic pollutants (MO, RhB, SMR, and CIP) was examined in Figure 5c. After reaching adsorption-desorption equilibrium for 30 min, satisfactory degradation efficiencies were achieved by the double-cathode system. Figure 5d shows the degradation kinetics curves of four organic pollutants, which can be fitted into first-order kinetic model. Figure S1 displays UV-Vis spectra of organic dyes (MO and RhB), SMR, and CIP. It can be seen that the double-cathode system was fit for the degradation of different types of pollutants.

3.3. Deduced PEC Reaction Mechanisms

3.3.1. PEC H₂O₂ Production Mechanism

To investigate the mechanism of PEC H₂O₂ production, various sacrificial agents, Potassium Persulfate (K₂S₂O₈, scavenger of e⁻) [32], Ethyl Alcohol (CH₃CH₂OH, h⁺ scavenger) [33], 2,2,6,6-Tetramethylpiperidoxyl (TEMPO, scavenger of ∙O₂⁻) [34], and Isopropanol (IPA, ∙OH scavenger) [35] were added into the reaction system. It can be found in Figure 6a that yield of H₂O₂ decreased obviously upon the introduction of K₂S₂O₈ and TEMPO, indicating that e⁻ and ∙O₂⁻ played important roles during the process of PEC H₂O₂ production. When IPA was added, the yield of H₂O₂ only reduced 5.4%, suggesting ∙OH gave little inhibited effect on H₂O₂ production. The addition of CH₃CH₂OH slightly improved the yield of H₂O₂ because the recombination of photogenerated carriers was lowed after trapping h⁺. It can be found that the yield of H₂O₂ still remained at 294.5 mmol·L⁻¹ after trapping ∙O₂⁻ species, which showed that indirect ORR and direct ORR existed simultaneously, and indirect ORR was dominant in the PEC H₂O₂.

Furthermore, a rotating ring disk electrode (RRDE) was employed to record and calculate the number of e⁻ transfers (n) in the PEC H₂O₂ production process. The linear sweep voltammetry (LSVs) at different scan rates on RRED was measured in an O_2—saturated buffered of 0.1 mol·L⁻¹ Na₂SO₄ (pH of 3) with the scan rates range of 600–1600 rpm (Figure 6b). As displayed in Figure 6c, based on the Koutecky-Levich (KL) equation (1/j = 1/jk + kw − 1/2) [36,37], the number of e⁻ transfers (n) at the voltages of −0.585 V, −0.600 V, and −0.615 V was 1.4, 1.4, and 1.5, respectively, indicating both one-electron and two-electron transfer existed during the process of PEC H₂O₂ production. Namely, the direct one-step two-electron ORR coexisted with the indirect two-step one-electron ORR process. These results were consistent with those of free radical trapping experiments.

3.3.2. PEC Degradation of Organic Pollutants Mechanism

The fluorescent spectrometry with terephthalic acid (TA, 5 × 10⁻⁴ mol·L⁻¹) as a probe was used to detect the existence of ∙OH with λ_ex = 315 nm and λ_em = 340–600 nm. Figure 7a shows that the fluorescent intensity of TA increased continuously at 425 nm along with the time, indicating that the content of ∙OH was increasing. This confirmed the ∙OH in double-cathode system came from the transformation of H₂O₂ in the PEC process and as the main active substance. In order to further explore the primary active species in the double-cathode system, the trapping examinations were performed in Figure 7b. After capture of ∙OH by IPA, the degradation efficiency of TC decreased obviously, indicating that ∙OH was the dominating role during the PEC degradation of pollutants process. Meanwhile, the degradation efficiency of TC declined 22.6% after trapping h⁺ by CH₃CH₂OH, which demonstrated that h⁺ plays a subsidiary role.

Combined with the band gap of WO₃ and Co₂SnO₄ (inset of Figure 3b), it can be found that the E_CB of WO₃ and Co₂SnO₄ was +0.12 eV and −0.52 eV; the E_VB of WO₃ and Co₂SnO₄ was 2.87 eV and 1.10 eV. Under visible-light irradiation, both WO₃ and Co₂SnO₄ can be irradiated to generate electron-hole (e⁻-h⁺) pairs. WO₃+Co₂SnO₄ without heterojunctions cannot provide efficient way to transfer the photogenerated carriers and separate the photogenerated electron-hole pairs, which will hinder the PEC reaction. In addition, if WO₃ and Co₂SnO₄ formed a conventional WO₃-Co₂SnO₄ heterostructure, the generated e⁻ from the CB of Co₂SnO₄ will transfer to the CB of WO₃; meanwhile, the h⁺ on the VB of WO₃ can migrate to the VB of Co₂SnO₄. However, the potential of O₂/O₂⁻ was −0.33 eV, and the E_CB of WO₃ was insufficient to reduce O₂ to form ∙O₂⁻. Through the trapping experiment results, ∙O₂⁻ was one of the main active radicals. Based on the above results, the h⁺ on the VB of WO₃ and the photogenerated e⁻ on the CB of Co₂SnO₄ participated directly in the PEC degradation process, and also retained high redox properties. These testified WO₃@Co₂SnO₄ composites executed a new type of Z-scheme mode for PEC degradation of organic pollutants. As demonstrated in Scheme 2, the photogenerated e⁻ on the CB of WO₃ were combined directly with the h⁺ on the VB of Co₂SnO₄. Then the h⁺ on the VB of WO₃ and the generated e⁻ on the CB of Co₂SnO₄ can participate in the PEC process directly, that is, WO₃@Co₂SnO₄ composite followed the principal of Z-scheme carriers conduction mode. Based on the above analysis results, the deduced reaction equations are summarized below:

WO₃ + hv → h⁺_(VB) + e⁻_(CB)

Co₂SnO₄ + hv → e⁻_(CB) + h⁺_(VB)

O₂ + 2H⁺ + 2e⁻ → H₂O₂

O₂ + e⁻ → ∙O₂⁻;

O₂⁻ + 2H⁺ + e⁻ → H₂O₂

The degradation reaction equation was as follows:

H₂O₂ + e⁻ → ∙OH + OH

OH + Organic pollutants → CO₂ + H₂O

h⁺_{(VB of WO3)} + Organic pollutants → CO₂ + H₂O

Scheme 2. The deduced PEC mechanism for Z-scheme 3D WO₃@Co₂SnO₄ heterojunction composite under two-cathode photoelectron-Fenton system.

Scheme 2. The deduced PEC mechanism for Z-scheme 3D WO₃@Co₂SnO₄ heterojunction composite under two-cathode photoelectron-Fenton system.

4. Conclusions

(1)

Z-scheme 3D WO₃@Co₂SnO₄ heterojunction composites were prepared through a hydrothermal and then calcining technique. By employing the characterization methods of DRS, Mott-Schottky plots, Transient photocurrent, and electrochemical impedance spectroscopy, the bandgap structure of WO₃@Co₂SnO₄ and its separation capability of photogenerated carriers were detected.

(2)

As a photocathode, 3D WO₃@Co₂SnO₄ was one of the satisfied materials for production of H₂O₂. Under the optimal reaction conditions, the yield of H₂O₂ reached 1335 μmol·L⁻¹·h⁻¹. The reaction path of H₂O₂ was investigated by capturing experiments and the rotating disk electrode test, which was mainly an indirect two-step one-electron ORR process.

(3)

As the other cathode was designed to conduct a two-cathode photoelectron-Fenton system, 3D WO₃@Co₂SnO₄ as photocathode and stainless-steel mess was carried out in-situ PEC degradation of different organic environmental pollutants. The PEC degradation mechanism was explored, finding that it obeyed a new type of Z-scheme mode through fluorescence spectrophotometry and trapping agents experiments.

(4)

This work provided a new idea for preparation of multifunction materials, gave an economic and environmentally friendly way for photoelectrocatalysis production of H₂O₂, and established a photoelectron-Fenton system without additional additives.