English

• 0.   Introduction

  The increase in carbon dioxide (CO₂) concentration in the Earth′s atmosphere has significantly impacted global temperature, exacerbating the greenhouse effect. Therefore, slowing down the rise of carbon dioxide in the atmosphere and striving for carbon neutrality are two important global priorities in the forthcoming decades^[1]. Among diverse strategies to mitigate CO₂ concentration, solar-driven photocatalytic conversion of CO₂ to value-added chemical products/fuels is regarded as an auspicious method that can provide effective remedial measures for energy crisis and environmental predicaments^[2-3]. To this end, the development of efficient photocatalysts is a necessary prerequisite for achieving this goal. Over the past decades, various semiconducting photocatalysts, such as CdS^[4], ZnO^[5], BiOX^[6], NiCo₂O₄^[7], and g-C₃N₄^[8-9], have been exploited for CO₂ reduction since the pioneering work of Inoue et al^[10]. However, the efficiency of CO₂ conversion for existing photocatalysts is still far from meeting the requirements for industrial applications so far. Hence, designing and synthesizing novel photocatalysts has been a focus of research in the field of CO₂ reduction.

  As one of the most important spinel-type complex oxides, zinc ferrite (ZnFe₂O₄) has been widely used for photocatalytic degradation of pollutants, lithium-ion batteries, heterogeneous catalysis, photocatalysis, and biomedical applications owing to its unique physical, chemical, optical and electronic properties^{[7, 11-13]}. In terms of photocatalytic applications, ZnFe₂O₄ has been studied in the fields of photocatalytic hydrogen production, water splitting, and CO₂ conversion due to its narrow bandgap of ca. 1.9 eV, chemical stability, and unique electronic structure^[14-16]. For instance, Li et al.^[17] demonstrated that ultrafine ZnFe₂O₄ nanoparticles can convert CO₂ into ethanol and acetaldehyde under visible light illumination. To prevent ZnFe₂O₄ from agglomerating, Yan et al.^[18] in situ grew ZnFe₂O₄ nanoparticles on iron porphyrin covalent triazine-based frameworks via an ion-thermal method. With the assistance of Ru(bpy)₃, the as-prepared material exhibited higher photocatalytic CO₂ reduction activity under visible light compared to pure ZnFe₂O₄. However, the efficiency of photocatalytic CO₂ reduction of ZnFe₂O₄ is still far below the real demand due to the issues that arise from light harvesting, intensive recombination of electron-hole pairs, and insufficient conduction band potential^[19].

  Construction of heterojunction structures, especially S-scheme (or Z-scheme) photocatalytic systems^[20-21], can effectively overcome above mentioned problems by enhancing the separation of photoinduced charges, broadening the light absorption range, and widening the redox potentials^[22-23]. Recently, different ZnFe₂O₄-based heterojunction photocatalysts with an S-/Z-scheme charge transfer model have been synthesized and exhibit impressive CO₂ photoreduction activity^{[14, 24-25]}. WO₃, another photo-electronically active semiconductor, is a popular material in the field of photo(electro)catalysis due to its unique features, including narrow bandgap, non-toxicity, and chemical stability^[26]. In particular, WO₃ exhibits high oxidizing ability on account of its sufficient positive valence band (VB) maximum^[27]. However, to the best of our knowledge, there are currently no reports on the construction of ZnFe₂O₄/WO₃ heterojunctions for photocatalytic CO₂ reduction^[28].

  In the present study, we synthesized a series of heterojunction ZnFe₂O₄/WO₃ nanocomposites by loading different amounts of ZnFe₂O₄ nanoparticles on the surface of WO₃ nanoplates. The band structure of the typical ZnFe₂O₄/WO₃ composite was characterized and determined by UV-Vis diffuse reflection spectroscopy (UV-Vis DRS), Mott-Schottky plots, and X-ray photo-electron spectroscopy valence band (XPS-VB) spectrum. The photoreduction CO₂ performance of the as-prepared samples was measured in the presence of water vapor under the irradiation of sunlight simulated by xenon light. The results show that the optimized ZnFe₂O₄/WO₃ sample exhibits better photocatalytic CO₂ conversion activity than pure ZnFe₂O₄ and bare WO₃. The enhanced photocatalytic activity is ascribed to the effective separation of photoinduced charges and the widening of the redox potentials. An S-scheme charge transfer mode between ZnFe₂O₄ and WO₃ heterojunctions is further proposed.

  1.   Experimental

  All chemicals were used as received without any further purification.

  1.1   Synthesis of WO₃ nanosheets

  The WO₃ nanosheets were synthesized by a hydrothermal method^[29]. 1.979 g Na₂WO₄·2H₂O was dissolved in 90 mL of distilled water, and 90 mL of concentrated hydrochloric acid (36%-38%) was added dropwise with constant stirring. After continuing to stir for 1 h, the mixture was transferred to a hydrothermal reactor, and heated at 180 ℃ for 12 h. After centrifugation, the obtained products were washed three times with distilled water and dried at 100 ℃ for 12 h.

  1.2   Construction of ZnFe₂O₄/WO₃ heterostructure

  Before synthesizing ZnFe₂O₄/WO₃ nanocomposites, ZnFe₂O₄ was first synthesized via a facile solvent-thermal route^[30]. 1 mmol of ZnCl₂ and 2 mmol of FeCl₃ were dissolved in 80 mL of ethylene glycol while stirring. Subsequently, 8 mmol of NaOH was added to the solution at room temperature. The resulting mixture was then transferred to a Teflon-lined stainless autoclave and heated to 180 ℃ for a duration of 12 h. Afterward, the obtained precipitate was subjected to centrifugation, washing, and drying.

  To construct ZnFe₂O₄/WO₃ heterostructure, 0.232 g of the synthesized WO₃ powder was first dispersed in 30 mL deionized water, and then 0.5 g of maleic acid and different amounts of ZnFe₂O₄ were added to the suspension^[31]. It is worth noting that the introduction of maleic acid is to make the connection between ZnFe₂O₄ and WO₃ closer in subsequent treatment^[32]. After the mixture was stirred for 12 h at 60 ℃, the obtained products were collected, washed, and dried. Subsequently, the dried products were calcined at 300 ℃ for 3 h. The preparation process of ZnFe₂O₄/WO₃ composites is illustrated in Fig. 1.

  Figure 1

  

  Figure 1.  Schematic diagram of the preparation process of ZFO/WO composites

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  The added amounts of ZnFe₂O₄ relative to 1 mmol of WO₃ powder were set as 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 0.7, 1, and 2 mmol, respectively. For clarity, the correspondingly calcined products were labeled as xZFO/WO, where x=0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 0.7, 1, and 2, respectively.

  1.3   Characterization

  Powder X-ray diffraction (PXRD) measurements were performed with a PANalytical X′pert PRO diffractometer using Cu Kα₁ radiation (λ=0.154 06 nm) at 40 kV and 15 mA in a 2θ range of 10°-80° at a scanning rate of 5 (°)·min^-1. Microscopic morphology was examined via transmission electron microscopy (TEM, FEI Tecnai G² F20) at 200 kV and spot size 2. The light absorption properties of the catalysts were determined by a UV-Vis spectrophotometer (Agilent Cary 5000). The electrochemical workstation (Chen Hua CHI 660E) was used to perform the photoelectrochemical tests. Specific tests included transient photocurrent test (initial voltage E=1.0 V, total test time t=130 s, with or without light interval 10 s); impedance size test (initial voltage E=1.0 V, frequency 105 Hz); Mott-Schottky curve test (initial voltage E=-1 V, final voltage E=1.2 V, selected frequency of 1 000 Hz).

  1.4   Photocatalytic activity testing

  The photocatalytic CO₂ reduction activity of the catalysts was evaluated under full spectrum illumination, employing a 300 W Xenon lamp as the light source. Each time an experiment was performed, 20 mg of the catalyst was uniformly distributed onto a circular glass substrate, which was subsequently placed within a stainless-steel photocatalytic reactor. Prior to commencing the photocatalytic CO₂ reduction performance assessment, the reactor was evacuated using a vacuum pump, and a mixture of CO₂ and water vapor was introduced into the photocatalytic reactor. This purging process was iterated five times to remove air. After illumination for 5 h, gas chromatography (Agilent 7820A) equipped with a methanator, a flame ionization detector, and a thermal conductivity detector was utilized to quantify the yields of photocatalytic products.

  2.   Results and discussion

  2.1   Morphology and structure

  Fig. 2 shows the PXRD patterns for the representative sample 0.15ZFO/WO along with pure ZnFe₂O₄ and WO₃ for comparison. The specific diffraction peaks observed at 29.97°, 35.31°, 42.89°, 53.18°, 56.69°, and 62.25° can be attributed to the cubic ZnFe₂O₄ phase^[33] (PDF No. 01-089-4926), whereas the diffraction patterns of WO₃ closely match the characteristic peaks of the standard monoclinic phase WO₃^[34] (PDF No.01-072-1465). For 0.15ZFO/WO, all the XRD patterns can be well indexed to the cubic ZnFe₂O₄ phase and monoclinic phase of WO₃ with no appearance of other impurities.

  Figure 2

  

  Figure 2.  PXRD patterns of WO₃, ZnFe₂O₄, and 0.15ZFO/WO

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  The morphology and microstructure of the representative sample 0.15ZFO/WO were measured using TEM and HRTEM, and the images are displayed in Fig. 3. As discerned in Fig. 3a, WO₃ exhibited a well-defined nanoplate structure, with dimensions of approximately 200 nm in both length and width, presenting a notably smooth surface. In Fig. 3b, the fine ZnFe₂O₄ nanoparticles were conspicuously adhered to the surface and periphery of the nanoplates, with the interface delineated clearly in the high-resolution TEM (HRTEM) images (Fig. 3c and 3d).

  Figure 3

  

  Figure 3.  TEM (a, b) and HRTEM (c, d) images of 0.15ZFO/WO

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  The lattice fringes of ZnFe₂O₄ nanosheets and WO₃ nanoparticles were observed through HRTEM (Fig. 3c and 3d), in which the interplanar spacings of 0.342 and 0.370 nm can be indexed to the (012) and (020) planes of WO₃^[35], and the interplanar space of 0.254 nm can be indexed to the (311) plane of ZnFe₂O₄^[36]. On the other hand, the formation of heterojunctions could be observed at the interface between ZnFe₂O₄ and WO₃, as marked by the yellow frames in Fig. 3b and 3d.

  2.2   UV-Vis DRS spectra

  The light absorption characteristics of WO₃, ZnFe₂O₄, and xZFO/WO (x=0.05, 0.1, 0.15, 0.2, 0.25) samples were examined in a range of 200-800 nm via UV-Vis DRS, and the results are presented in Fig. 4. The absorption onset for pure WO₃ and ZnFe₂O₄ was observed at approximately 470 and 670 nm, respectively, in agreement with those reported^[37-38]. For the xZFO/WO samples, the absorption capacity increased proportionally with the increment in ZnFe₂O₄ loading content. Notably, both WO₃ and ZnFe₂O₄ exhibit characteristics of direct bandgap semiconductors^[39-40]. The direct bandgap of the samples was estimated from the Kubelka-Munk function^[41]. The function equation is (αhν)² = A(hν-E_g), where α, hν, E_g, and A represent absorption coefficient, photon energy, band gap (eV), and constant, respectively. By extrapolating this figure to (αhν)²=0, it was estimated that the bandgap of WO₃ and ZnFe₂O₄ was ca. 2.60 and ca. 1.83 eV, respectively (Fig. 4b). The results are consistent with those reported previously^{[39, 42]}.

  Figure 4

  

  Figure 4.  (a) UV-Vis DRS spectra of WO₃, ZnFe₂O₄, and xZFO/WO; (b) Tauc plots of WO₃ and ZnFe₂O₄

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  2.3   Photocatalytic CO₂ reduction performance

  To promote the formation of heterojunctions between ZnFe₂O₄ and WO₃, different amounts of maleic acid to central ions were introduced as a capping agent in the preparation process, followed by calcination treatment. The effects of adding a capping agent and heat treatment on the photocatalytic CO₂ reduction activity were evaluated. Fig. 5a shows the effects of adding different amounts of maleic acid as well as not adding maleic acid on the photocatalytic CO₂ reduction activities after illumination for 5 h. Considering that in CO₂ reduction products, producing 1 mol of methane requires 8 mol of electrons, while producing 1 mol of CO and H₂ requires only 2 mol of electrons, we used Eq. 1 to quantify the total number of electrons (N, μmol·g^-1) involved in the entire reaction to further compare photocatalytic activity. Fig. 5a shows the yields of CH₄, CO, and H₂ produced by 0.15ZFO/WO with different maleic acid loadings, 0.15ZFO/WO without maleic acid loading, and the physical mixture of ZnFe₂O₄ and WO₃, as well as the total number of electrons required.

  $ N=8 n_{\mathrm{CH}_4}+2 n_{\mathrm{CO}}+2 n_{\mathrm{H}_2} $

  (1)

  Figure 5

  

  Figure 5.  Effects of dosage of maleic acid (a) and calcination conditions (b) on the photocatalytic activity of the as-prepared samples

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  Without the addition of maleic acid, the CO and CH₄ yields of the obtained samples were 2.3 and 0.6 μmol·g^-1, respectively. By contrast, the CO and CH₄ yields of the samples added with maleic acid increased significantly. For instance, the yields of CO and CH₄ of 0.5ZFO/WO reached 7.87 and 4.88 μmol·g^-1, respectively. The increase in the yields may be attributed to the addition of maleic acid promoting the interphase interaction between ZnFe₂O₄ and WO₃^[32]. It is worth noting that adding more than 0.5 g of maleic acid will not further improve the photocatalytic CO₂ conversion efficiency, implying that under the conditions of this experiment, 0.5 g of maleic acid is sufficient to facilitate the grafting of ZnFe₂O₄ onto the surface of WO₃. However, the effect of maleic acid can only be manifested after calcination, as shown in Fig. 5b. Compared with the calcined sample, the CO and CH₄ yields of the uncalcined specimen and the physical mixed sample were significantly reduced, as shown in Fig. 5b. The consequences obtained from the total number of electrons are consistent with above results.

  The effects of loading amount of ZnFe₂O₄ on photocatalytic CO₂ activity were further studied (Fig. 6). For pure WO₃, almost no CO and CH₄ products were detected. Upon successful loading of ZnFe₂O₄ nanoparticles onto the surface of WO₃ nanosheets, the CO and CH₄ yields for all the samples increased. When the molar ratio of ZnFe₂O₄ to WO₃ was 0.15, the yields of CO and CH₄ reached their maximum values, attaining 7.87 and 4.88 μmol·g^-1, respectively. Further increasing the loading of ZnFe₂O₄, the yields of CO and CH₄ decreased instead. It is worthy of special attention that when the load amount of ZnFe₂O₄ exceeded 0.7, H₂ began to appear in the CO₂ reduction product. The reason may be the presence of single-phase ZnFe₂O₄. Accordingly, compared to the H₂O reduction reaction, the formation of xZFO/WO heterojunctions is beneficial for the CO₂ reduction reaction. From another perspective, the types and yields of CO₂ reduction products can be regulated by adjusting the composition of the materials, as shown in Fig. 6a and Fig. 6b.

  Figure 6

  

  Figure 6.  After illumination for 5 h, the yields of CH₄, CO, and H₂ as well as the total number of electrons for WO₃ (a), ZnFe₂O₄ (b), and xZFO/WO: (a) x=0.05, 0.1, 0.15, 0.2, 0.25, 0.3; (b) x=0.5, 0.7, 1, 2

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  To further evaluate photocatalytic activity, we also quantified the total number of electrons for ZnFe₂O₄ and xZFO/WO using Eq. 1. For all the composites, 0.15ZFO/WO achieved the highest electrons (54.8 μmol·g^-1), which was approximately 1.5 times that of pure ZnFe₂O₄ (36.2 μmol·g^-1). The enhancement of electron transfer number counts may be attributed to the formation of heterojunctions between the grain boundary of ZnFe₂O₄ and WO₃, which promotes the separation efficiency of photogenerated electrons and holes as shown below.

  To assess the stability, the 0.15ZFO/WO sample was selected, and conducted five consecutive photocatalytic cycles with each cycle lasting 5 h. Fig. 7a shows the cyclic test results for 0.15ZFO/WO. The yields of CO and CH₄ after three cycles remained at 7.23 and 3.17 μmol·g^-1, respectively, almost equivalent to the initial values. However, the CO and CH₄ yields after five cycles were 3.74 and 3.15 μmol·g^-1, indicating partial deactivation of the catalyst. Fig. 7b compares the XRD patterns of the 0.15ZFO/WO sample before and after five cycles of illumination. No obvious changes were found. Further in-depth research is needed on the underlying reasons for the partial deactivation of catalysts.

  Figure 7

  

  Figure 7.  (a) Photocatalytic cyclic activity of 0.15ZFO/WO; (b) PXRD patterns of 0.15ZFO/WO before and after five cycles of illumination

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  Blank and control tests were carried out to elucidate the source of C in CO and CH₄, and the test results of the 0.15ZFO/WO sample are shown in Fig. 8. When CO₂ in the reaction system was replaced by N₂, only H₂ was detected in the products, but no other products such as CO and CH₄ were detectable within the detection limit of our instrument. The results indicate that the products CO and CH₄ are produced from CO₂. In addition, under the conditions of no catalysts or in the dark, CO and CH₄ yields were extremely low, suggesting that photocatalysts and light are essential conditions for photocatalytic CO₂ reduction.

  Figure 8

  

  Figure 8.  Yields of CO, CH₄ and H₂ under different test conditions

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  2.4   Photocatalytic mechanism

  The band edge potentials of the composites are of critical importance because they have a direct impact on the redox reactions occurring at the catalyst surface^[43]. Firstly, the flat band potentials of WO₃ and ZnFe₂O₄ were determined using the Mott-Schottky analysis method. The obtained flat band potentials for WO₃ and ZnFe₂O₄ were 0.02 and -0.55 V (vs Ag/AgCl), respectively (Fig. 9a and 9c). It is known that the flat band potential of n-type semiconductors corresponds to the Fermi level (E_f). Considering the 0.197 V difference between the standard hydrogen electrode (NHE) and the Ag/AgCl electrode^[44], the E_f for WO₃ and ZnFe₂O₄ was calculated to be 0.22 and -0.35 V (vs NHE)^[38]. Secondly, the XPS-VB spectra of WO₃ and ZnFe₂O₄ were measured, and the results are displayed in Fig. 9b and 9d. According to the linear intersection method^[45], the differences between the E_VB and E_f for WO₃ and ZnFe₂O₄ were 2.53 and 1.60 eV, respectively. Consequently, the calculated VB potentials for WO₃ and ZnFe₂O₄ were determined to be 2.75 and 1.25 V (vs NHE). According to the equation: E_CB=E_VB-E_g^[46], the CB potentials for WO₃ and ZnFe₂O₄ can thus be calculated to be 0.15 and 0.58 V (vs NHE), respectively. Accordingly, the band energy of WO₃ and ZnFe₂O₄ before contact can be schemed (Fig. 10a).

  Figure 9

  

  Figure 9.  Mott-Schottky plots (a, c) and XPS-VB spectra (b, d) of ZnFe₂O₄ and WO₃

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  Figure 10

  

  Figure 10.  Schematic diagram of band structure of WO₃ and ZnFe₂O₄ before (a) and after (b) contact

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  The change in elemental binding energies reflects the alteration in electron density and thus can be used to examine the direction of carrier transfer in heterojunction photocatalysts^[47]. The results of XPS analysis of 0.15ZFO/WO showed that the binding energies of W4f_7/2 (35.6 eV) and W4f_5/2 (37.8 eV) shifted to lower binding energies compared to the pure WO₃ (Fig. 11)^[28]. This implies that the electrons in ZnFe₂O₄ flow to WO₃, and the built-in electric field is generated^[48].

  Figure 11

  

  Figure 11.  XPS spectra of W4f of WO₃ and 0.15ZFO/WO

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  When ZnFe₂O₄ is in contact with WO₃, the electrons in ZnFe₂O₄ flow towards WO₃ until the Fermi level flattens out due to the difference between Fermi levels^[49]. As a result, the electron band of ZnFe₂O₄ bends upward while that of WO₃ bends downward. Under light irradiation, the VB electrons of WO₃ and ZnFe₂O₄ are excited up and the separation process of the photoexcited charges can be drawn diagrammatically as Fig. 10b. Under the action of the internal electric field, the holes in the VB of ZnFe₂O₄ recombine with the electrons in the conduction band of WO₃. The electrons in the conduction band of ZnFe₂O₄ participate in the reduction reaction to reduce CO₂ to CO and CH₄, whereas the holes in the VB of WO₃ participate in the oxidation reaction to oxidize H₂O. Thus, an S-scheme charge transfer mode is formed between the heterojunctions of ZnFe₂O₄ and WO₃. The formation of S-scheme heterojunctions is not only beneficial for the spatial separation of photogenerated carriers but also promotes photocatalytic CO₂ reduction^{[34, 50]}.To verify the S-scheme mode, the tests of Pt photo-deposition were performed on ZFO/WO. TEM and HRTEM images are shown in Fig. 12. The Pt nanoparticles with lattice stripes of 0.226 nm, corresponding to the (111) crystallographic spacing of Pt, were present only on ZnFe₂O₄. It is reported that Pt nanoparticles are selectively deposited on the material zone with high electron density^[51]. The results show that the photogenerated electrons in the ZnFe₂O₄/WO₃ heterojunction are concentrated on ZnFe₂O₄, thus confirming the S-scheme charge transfer mode.

  Figure 12

  

  Figure 12.  TEM (a) and HRTEM (b) images of Pt/ZFO/WO

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  To assess the efficiency of photogenerated electron-hole separation and migration, transient photocurrent, and electrochemical impedance tests were conducted^[52]. Fig. 13a presents the transient photocurrent response of the samples. Compared to single-phase WO₃, xZFO/WO exhibited higher photocurrent intensities with the order: 0.15ZFO/WO > 0.2ZFO/WO > 0.25ZFO/WO > 0.1ZFO/WO > 0.05ZFO/WO > WO₃. The result suggests that the formation of a heterojunction is good for the separation of photogenerated carriers, especially when the ratio of two components is appropriate. The result is consistent with that reported previously^[28].

  Figure 13

  

  Figure 13.  Transient i-t curves (a) and electrochemical impedance spectra (b) of WO₃ and xZFO/WO (x=0.05, 0.1, 0.15, 0.2, 0.25)

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  Similarly, the results of the measured electrochemical impedance spectra for the composites also indicate the efficient separation of photogenerated electron-hole pairs^[53], as shown in Fig. 13b. The improvement in the efficiency of photogenerated charge separation ultimately leads to an increase in the CO₂ reduction activity of the materials.

  3.   Conclusions

  In summary, a series of ZnFe₂O₄/WO₃ heterojunctions were successfully constructed. Under irradiation, the optimal 0.15ZFO/WO sample exhibited the highest CO₂ reduction activity with the CO and CH₄ yields of 7.87 and 4.88 μmol·g^-1, respectively. The enhanced activity can be attributed to the formation of heterojunction with an S-scheme charge transfer mode. This charge transfer mode facilitates the efficient separation of photogenerated carriers while preserving the hole oxidation capability of WO₃. The results obtained herein offer a new avenue to synthesize S-scheme heterojunction ZnFe₂O₄-based materials for photocatalytic CO₂ reduction.

  
  Acknowledgments: We would like to thank the financial support of the Natural Science Foundation of Henan Province, China (Grant No.222300420526).
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  Figure 1  Schematic diagram of the preparation process of ZFO/WO composites

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  Figure 2  PXRD patterns of WO₃, ZnFe₂O₄, and 0.15ZFO/WO

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  Figure 3  TEM (a, b) and HRTEM (c, d) images of 0.15ZFO/WO

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  Figure 4  (a) UV-Vis DRS spectra of WO₃, ZnFe₂O₄, and xZFO/WO; (b) Tauc plots of WO₃ and ZnFe₂O₄

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  Figure 5  Effects of dosage of maleic acid (a) and calcination conditions (b) on the photocatalytic activity of the as-prepared samples

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  Figure 6  After illumination for 5 h, the yields of CH₄, CO, and H₂ as well as the total number of electrons for WO₃ (a), ZnFe₂O₄ (b), and xZFO/WO: (a) x=0.05, 0.1, 0.15, 0.2, 0.25, 0.3; (b) x=0.5, 0.7, 1, 2

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  Figure 7  (a) Photocatalytic cyclic activity of 0.15ZFO/WO; (b) PXRD patterns of 0.15ZFO/WO before and after five cycles of illumination

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  Figure 8  Yields of CO, CH₄ and H₂ under different test conditions

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  Figure 9  Mott-Schottky plots (a, c) and XPS-VB spectra (b, d) of ZnFe₂O₄ and WO₃

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  Figure 10  Schematic diagram of band structure of WO₃ and ZnFe₂O₄ before (a) and after (b) contact

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  Figure 11  XPS spectra of W4f of WO₃ and 0.15ZFO/WO

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  Figure 12  TEM (a) and HRTEM (b) images of Pt/ZFO/WO

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  Figure 13  Transient i-t curves (a) and electrochemical impedance spectra (b) of WO₃ and xZFO/WO (x=0.05, 0.1, 0.15, 0.2, 0.25)

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