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Article

Network Structured CuWO4/BiVO4/Co-Pi Nanocomposite for Solar Water Splitting

by
Ben Peng
1,
Mengyang Xia
2,
Chao Li
2,
Changshen Yue
1 and
Peng Diao
2,*
1
Central Research Institute of Building and Construction Co. LTD., Energy Convention and Environment Protection Co. LTD., State Key Laboratory of Iron & Steel industry Environmental Protection, Beijing 100088, China
2
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(12), 663; https://doi.org/10.3390/catal8120663
Submission received: 22 November 2018 / Revised: 12 December 2018 / Accepted: 13 December 2018 / Published: 17 December 2018
(This article belongs to the Special Issue Photocatalytic Nanocomposite Materials)
Browse Figures
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Abstract

:
A network structured CuWO4/BiVO4 nanocomposite with a high specific surface area was prepared from CuWO4 nanoflake (NF) arrays via a method that combined drop-casting and thermal annealing. The obtained CuWO4/BiVO4 exhibited high catalytic activity toward photoelectrochemical (PEC) water oxidation. When cobalt phosphate (Co-Pi) was coupled with CuWO4/BiVO4, the activity of the resulting CuWO4/BiVO4/Co-Pi composite for the oxygen evolution reaction (OER) was further improved. The photocurrent density (Jph) for OER on CuWO4/BiVO4/Co-Pi is among the highest reported on a CuWO4-based photoanode in a neutral solution. The high activity for the PEC OER was attributed to the high specific surface area of the composite, the formation of a CuWO4/BiVO4 heterojunction that accelerated electron–hole separation, and the coupling of the Co-Pi co-catalyst with CuWO4/BiVO4, which improved the charge transfer rate across composite/solution interface.

Graphical Abstract

1. Introduction

Photoelectrochemical (PEC) water splitting is a promising way to capture solar energy and to store it in the chemical bonds of two products, H2 and O2 [1]. There are two half-cell reactions for PEC water splitting: a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER). Between them, the OER is the more sluggish reaction because it involves the removal of four electrons from two water molecules. In previous studies, N-type semiconductors, such as TiO2 [2], ZnO [3,4], WO3 [5,6,7,8], Fe2O3 [6,9], Ag3PO4 [10], BiVO4 [11,12], and CuWO4 [13,14], whose valence band edges are higher than the redox potential of the O2/H2O couple, have been employed as the photoanode materials for such PEC OER. CuWO4 has been regarded as a promising photoanode material [13,14,15,16,17,18,19,20] due to its appropriate bandgap energy (2.3–2.4 eV), positive valence band edge potential (at ca. 2.8 V vs. reversible hydrogen electrode (RHE)) [13], low toxicity, and, most importantly, high stability in neutral and acid solutions. However, as compared with the highly active n-type semiconductors, the activity of CuWO4 for PEC OER is still low because of its indirect bandgap and poor charge transport property [18], which results in the accumulation of photoinduced holes, thereby increasing the recombination. Therefore, increasing both the electron–hole separation rate and the charge transfer rate across the CuWO4/solution interface are two possible ways of improving the activity of CuWO4 for PEC OER.
Constructing a heterojunction by using two n-type semiconductors whose band edges match well with each other is an effective strategy to accelerate the electron–hole separation [6,21]. For example, it has been demonstrated that the composites of WO3/Fe2O3 [6], CuWO4/CdS [19] WO3/CuWO4 [20], and WO3/BiVO4 [21,22] have exhibited enhanced activity toward PEC OER due to the formation of heterojunctions. BiVO4 is an n-type semiconductor with a band gap of ca. 2.3 eV, which is quite similar to that of CuWO4. The conduction and valence band edges of BiVO4 are located at ca. 0 V and 2.3 V versus RHE, respectively, making BiVO4 a good candidate to form a band-structure-matched heterojunction with other n-type semiconductors, such as WO3, Fe2O3, and CuWO4 [14,21,22,23,24]. The formation of WO3/BiVO4, Fe2O3/BiVO4, and CuWO4/BiVO4 heterojunctions significantly promotes the electron–hole separation rate and benefits the improvement of the PEC OER efficiency. Coupling a semiconductor with an electrochemical OER co-catalyst is another way to enhance the activity of photoanode materials for water oxidation, because the cocatalyst can improve the charge transfer rate at the electrode/solution interface. Cobalt phosphate (Co-Pi) is an efficient cocatalyst that has been widely used to boost the performance of photoanode materials for PEC OER [3,5,6,11,12].
In this work, we prepared CuWO4/BiVO4-composite nanoflakes (NFs) via a drop-casting and thermal annealing method, with CuWO4 NFs as templates. A layer of Co-Pi was deposited onto the surface of CuWO4/BiVO4 as a co-catalyst. We demonstrated that the obtained network-structured CuWO4/BiVO4/Co-Pi nanocomposite exhibited a significantly improved activity for PEC OER. The high activity of the nanocomposite was ascribed to the high specific surface area, the electron–hole separation rate enhanced by the CuWO4/BiVO4 heterojunction, and the accelerated charge transfer rate at the semiconductor/solution interface due to the presence of Co-Pi.

2. Results and Discussion

The network-structured CuWO4/BiVO4-composite NFs were prepared from CuWO4 NFs via a method that combined drop-casting and thermal annealing. The WO3 NFs grown on the fluorine-doped tin oxide (FTO) substrate were used as sacrificial templates to synthesis CuWO4 NFs via a thermal solid phase reaction [13]. The resulting CuWO4 NFs were used to fabricate a CuWO4/BiVO4 composite, and the procedure is shown schematically in Scheme 1a. Briefly, Bi precursor solution was drop-cast on the surface of the FTO-supported CuWO4 NFs, and the sample was annealed in air to obtain the CuWO4/BiVO4-composite NFs. Pure BiVO4 films were also prepared using the same method, in which the mixed precursor solution was drop-cast on bare FTO substrates (Scheme 1b). The Co-Pi layer was deposited on the surface of CuWO4/BiVO4 using a photo-assisted electrodeposition method described elsewhere [6].
Figure 1a–c show the typical scanning electron microscopic (SEM) images of the WO3 NFs, CuWO4 NFs, and the CuWO4/BiVO4-composite NFs, respectively. The WO3 sample, synthesized by hydrothermal growth, has a network structure, which was composed of the perpendicularly aligned WO3 NFs (Figure 1a). These WO3 NFs were used as sacrificial templates to synthesize CuWO4, and Cu(NO3)2 solution was drop casted onto the surface of WO3 NFs. During annealing, Cu(NO3)2 decomposed and produced CuO, which reacted with WO3 to form CuWO4. After annealing at 550 °C for 2 h, the WO3 NFs were completely converted into CuWO4 NFs [13]. As shown in Figure 1b, the obtained CuWO4 retained the microstructure of the WO3 NF templates. The size and thickness of the CuWO4 NFs was ca. 0.5~2 μm and 20~50 nm, respectively. The NF structure of CuWO4 provides a large specific surface area, which not only benefits the dispersion of the bismuth precursor (Bi(NO3)3) solution on the surface of CuWO4, but also contributes to the formation of a large area CuWO4/BiVO4 heterojunction. Figure 1c clearly demonstrates that, after the drop-casting of the Bi (NO3)3 precursor solution and the follow-up thermal annealing at 550 °C for 2 h, the resulting CuWO4/BiVO4 composite still kept the NF network structure. The high-magnification SEM image (the inset of Figure 1c) shows that, compared with pure CuWO4 NFs (see Figure 1b), the composite NFs became thicker and their surfaces became rougher, both suggesting the successful deposition of BiVO4 onto the surface of the CuWO4 NFs. In fact, the obtained CuWO4/BiVO4 nanocomposite can be regarded as a film that possesses a network structure composed of numerous intersected NFs.
High-resolution tunneling electron microscope (TEM) was employed to characterize the crystal structure of the composite NFs, and the result is presented in Figure 1d. The fringe with interplanar spacing of 0.290 nm and 0.386 nm can be clearly identified, which corresponded to the (040) plane of monoclinic BiVO4 and the (110) lattice plane of triclinic CuWO4, respectively. Moreover, the crystal boundaries between CuWO4 and BiVO4 can also be seen in Figure 1d, confirming the formation of a CuWO4/BiVO4 heterojunction. X-ray diffraction (XRD) was also used to characterize the crystal structure of the CuWO4/BiVO4-composite NFs, and the results are shown in Figure 2. The XRD spectrum of the composite NFs shows significantly enhanced diffraction intensity at 2θ of 19.0 and 28.9 degrees, where there happen to be the two most intense diffraction peaks (011) and (121) of monoclinic BiVO4. This result provides strong evidence that the enhanced XRD intensity of the composite was due to the diffraction contribution of BiVO4. Therefore, we can conclude that the CuWO4/BiVO4 composite was successfully prepared by the drop-casting and thermal annealing method.
Light absorption is one of the crucial factors influencing the photo-to-chemical conversion efficiency of a photoelectrode material for water splitting. We measured the light absorption property of the CuWO4/BiVO4-composite NFs by using UV–Vis diffuse reflectance spectroscopy, and the results are shown in Figure 3a. The pure CuWO4 and BiVO4 films exhibited an onset absorption wavelength at ca. 525 nm and 530 nm, corresponding to the band edge absorption of CuWO4 and BiVO4, respectively. The band gap energies for CuWO4 and BiVO4, which were obtained from Tauc plots (see Figure 3b), were 2.37 eV and 2.36 eV, respectively. These values were in good agreement with those reported in literature [11,12,13,14,15,16,17,18] and are much smaller than that of WO3 [6,25], ensuring that both CuWO4 and BiVO4 absorb more light than WO3. The quite similar bandgap energy between CuWO4 and BiVO4 implies that the formation of the CuWO4/BiVO4 heterojunction cannot extend the light absorption range. This is also confirmed in UV–Vis absorption spectra of the CuWO4/BiVO4 composite. As shown in Figure 3a, after the formation of the CuWO4/BiVO4 heterojunction, the composite still showed an onset absorption wavelength of ca. 530 nm, which is the same as that of BiVO4.
The PEC OER performances of the CuWO4, CuWO4/BiVO4, and CuWO4/BiVO4/Co-Pi composites were evaluated using linear sweep voltammetry (LSV) in the dark and under 100 mW·cm2 illumination. Figure 4a shows the LSV of the above photoanodes in a sodium phosphate buffer (pH = 7) solution in the dark and under irradiation. Without illumination, no oxidation current could be observed on all the photoanodes within the entire potential sweep region, suggesting that they were all electrochemically inert for water oxidation in the dark. However, when illuminated, each photoanode showed an oxidation photocurrent density (Jph), which had been proven to originate from the PEC OER [13,14,15,16,17,18,19]. The CuWO4/BiVO4 composite showed a much larger Jph than the CuWO4 NFs in the potential range from 0 V to 1.4 V versus RHE. As shown in Figure 4a, the OER onset potential of CuWO4/BiVO4 was 0.66 V versus RHE, which was negatively shifted ca. 0.1 V compared with CuWO4. The Jph value on CuWO4 NFs at 1.23 V versus RHE is ca. 0.33 mA·cm−2, which was in good agreement with the one reported previously [13]. However, after the formation of the CuWO4/BiVO4 heterojunction, Jph on the composite at the same potential was increased to 0.67 mA·cm−2, which was ca. 2 times higher than that on the CuWO4 NF photoanode.
We also investigated the influence of the deposition amount of BiVO4 on CuWO4, and the results are shown in Figure 4b. To elucidate the effect of the preparation procedure (drop-casting, drying and annealing) on the activity of CuWO4, Figure 4b presents the LSV curves of pure CuWO4 that experienced the same preparation steps as the CuWO4/BiVO4 composite (drop-casting 6 times) except that no Bi precursor (Bi (NO3)3) existed in the drop-casting solution. By comparing the photocurrent density of the pure CuWO4 in Figure 4a,b, we found that the aforementioned preparation procedure decreased the activity of CuWO4. However, the presence of BiVO4 not only compensated for the negative effect of the preparation procedure on the activity of CuWO4, but also led to a significantly improved photocurrent density on the resulting CuWO4/BiVO4 composite. This result provided further evidence that the formation of the CuWO4/BiVO4 heterojunction was crucial for the enhancement of the activity toward the PEC OER.
Figure 4b clearly demonstrates that, as the drop-casting times were increased, the photocurrent density of the CuWO4/BiVO4 composite first increased and then decreased, with the sample prepared by drop-casting 6 times exhibiting the highest activity. We believed that the thickness of the outer BiVO4 layer was the key factor controlling the activity of the CuWO4/BiVO4 composite. As is self-evident, more drop-casting times inevitably resulted in thicker BiVO4 layer, which improved the light absorption ability of the BiVO4 layer but impaired the light absorption ability of the CuWO4 layer. Therefore, there must be an optimal thickness of BiVO4 that balances the two opposite effects. We believe that the CuWO4/BiVO4 sample prepared by drop-casting 6 times had the optimal BiVO4 thickness, and as a result, it exhibited the highest activity toward the PEC OER.
It has been reported that Co-Pi and cobalt borate (Co-Bi) could be well coupled with BiVO4 as a co-catalyst, and Jph on the resulting BiVO4/Co-Pi (or Co-Bi) at 1.23 V versus RHE was increased to over 2.5–3 times the value on BiVO4 [11,12]. This work inspired us to combine the CuWO4/BiVO4 composite with Co-Pi, as in our study, the BiVO4 was the outermost layer that could be used to deposit Co-Pi. Figure 4a clearly shows that coupling CuWO4/BiVO4 with Co-Pi led to a significant improvement of Jph, and the value of Jph at 1.23 V versus RHE was 2.25 mA·cm−2, which is among the highest reported on CuWO4 [13,14,15,16,17,18,19,20] and CuWO4/BiVO4 photoanodes [14,24].
Figure 5 shows the schematic representation of the energy band structure of the CuWO4/BiVO4/Co-Pi composite and the corresponding charge transfer mechanism during illumination. We believe there were three main reasons for the high activity of the composite for the PER OER. First, the nano-network structure of the composite provided a large specific surface area that ensured a high probability of light–semiconductor interaction and a large composite/solution interface, both benefitting the improvement of the activity for the OER. Second, both the conduction and valence band edges of BiVO4 are higher than those of CuWO4, making a well-matched energy band structure for the CuWO4/BiVO4 heterojunction. The heterojunction led to a more efficient separation of the photoinduced electron–hole pairs, as the photoinduced holes were driven to the surface of BiVO4 and the electrons were injected into CuWO4 and then driven into FTO under applied potential. Third, the holes that moved to the surface of BiVO4 could be captured by Co-Pi and resulted in the generation of a high valence state of the Co species, such as Co (IV) [26,27], which were highly active toward the oxidized water.

3. Materials and Methods

3.1. Materials

Cupric nitrate trihydrate (Cu (NO)3·3H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Tungsten powder, tungstic acid (H2WO4), hydrogen peroxide (H2O2), absolute ethyl alcohol, isopropanol, potassium hydroxide (KOH), urea, acetonitrile, oxalic acid, hydrochloric acid (HCl), and acetic acid (C2H4O2) were purchased from Beijing Chemical Works (Beijing, China). All the chemicals were of analytical grade and used without further purification. All the aqueous solutions were prepared using deionized water (>18 MΩ). Fluorine-doped tin oxide (FTO) substrates (8 Ω⋅sq−1, transparency 80%) were purchased from Asahi Glass, Tokyo, Japan.

3.2. Preparation of Network-Structured CuWO4/BiVO4 and CuWO4/BiVO4/Co-Pi Nanocomposites

The WO3 NFs were first prepared on the FTO substrate via a nanoseed-assisted hydrothermal method [13], and then, they were used as sacrificial templates to synthesize CuWO4 NFs via a thermal solid phase reaction between WO3 NFs and CuO [13]. Then, the obtained CuWO4 NFs were used as supports to synthesize the network-structured CuWO4/BiVO4 nanocomposite. In detail, Bi (NO3)3·5H2O and vanadyl acetylacetonate were dissolved in a mixed solvent of glacial acetic acid and acetylacetone (20:1, volume ratio) to get a final concentration of 10 mM and 9.3 mM, respectively. Then, 20 μL of the mixed solution was drop-cast onto the FTO-supported CuWO4 NFs, and the sample was put into a 400 °C oven for 2 min to dry the drop-casting solution. This step was repeated for the required times to modulate the amount of bismuth precursor. Then, the dried samples were annealed in air at 550 °C for 2 h to obtain the CuWO4/BiVO4 composite. Pure BiVO4 films were also prepared using the same method in which the mixed precursor solution was drop-casted on bare FTO substrates. The Co-Pi layer was deposited onto the surface of CuWO4/BiVO4 using a photo-assisted electrodeposition method described in our previous work [6].

3.3. Characterization

The morphology of the WO3, CuWO4, and CuWO4/BiVO4-composite NFs were characterized by a Hitachi S-4800 field emission scanning electron microscope operating (Tokyo, Japan) at an accelerating voltage of 10 kV. High-resolution transmission electron microscopic images were obtained on a field emission JEM-2010F microscope (JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV. The X-ray diffraction (XRD) pattern of the CuWO4, BiVO4, and CuWO4/BiVO4-composite NFs were collected by an X-ray diffractometer (Rigaku, rint2000 advance theta-2theta powder diffractometer, Tokyo, Japan) with Cu Kα radiation. UV–Vis diffuse reflection spectra were recorded on a UV–Vis-NIR spectrophotometer (Shimadzu 3600, Kyoto, Japan).

3.4. Photoelectrochemical Measurements

Photoelectrochemical measurements were conducted on a CHI660D workstation (CH Instruments Co., Austin, TX, USA), while a 300 W xenon lamp served as the light source. The light intensity illuminated on the surface of all the photoelectrodes was calibrated to 100 mW·cm−2. All the electrochemical measurements were performed in a three-electrode photoelectrochemical cell in 1.0 M of Na2SO4 with 0.1 M of sodium phosphate buffer (pH = 7), and Pt and saturated calomel electrode (SCE) serving as counter and reference electrodes, respectively. All the potentials in this work were reported with reference to SCE and RHE.

4. Conclusions

In summary, we successfully prepared the CuWO4/BiVO4 film by using CuWO4 NFs as templates via a simple method that combined drop-casting and thermal annealing. The obtained CuWO4/BiVO4 film had a network structure that was composed of intersected CuWO4/BiVO4-composite NFs. The CuWO4/BiVO4 showed significantly improve activity for PER OER as compared with CuWO4. We further improved the activity by coupling co-catalyst Co-Pi with CuWO4/BiVO4. A photocurrent density of 2.25 mA·cm−2 was obtained on the CuWO4/BiVO4/Co-Pi composite, and this value was among the highest reported on CuWO4-based photoanodes in a neutral solution. The high activity for the PEC OER was attributed to the following three reasons: (1) The high specific surface area of the composite greatly increased the light absorption probability and the electrode/solution interface; (2) the CuWO4/BiVO4 heterojunction accelerated the separation of the photoinduced electron–hole pairs; and (3) the presence of the Co-Pi co-catalyst significantly improved the charge transfer across the composite/solution interface. Although CuWO4 is still no match for the highly efficient photoanode materials right now, this work demonstrates both the potential of CuWO4 as a photoanode for OER and the strategy to improve its performance.

Author Contributions

B.P. performed the tests, analyzed the data, and prepared the draft manuscript; M.X. worked on preparation and characterization; C.L. analyzed the data; C.Y. administered the project; and P.D. conceived of the paper and reviewed and edited the final manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC 51672017, 51872015, and 51604310), the Beijing Natural Science Foundation (2142020 and 2151001), and the Open Foundation of State Key Laboratory of Environmental Protection for Iron and Steel Industry (2016YZC03).

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 08 00663 sch001 550
Scheme 1. Schematic representation of the preparation of (a) the CuWO4/BiVO4-composite NFs and (b) the pure BiVO4 film on the fluorine-doped tin oxide (FTO) substrate via a method that involved drop-casting and thermal annealing.
Scheme 1. Schematic representation of the preparation of (a) the CuWO4/BiVO4-composite NFs and (b) the pure BiVO4 film on the fluorine-doped tin oxide (FTO) substrate via a method that involved drop-casting and thermal annealing.
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Figure 1. SEM images of the (a) WO3 NFs; (b) CuWO4 NFs, and (c) CuWO4/BiVO4-composite NFs; (d) high-resolution TEM image of the CuWO4/BiVO4-composite NFs.
Figure 1. SEM images of the (a) WO3 NFs; (b) CuWO4 NFs, and (c) CuWO4/BiVO4-composite NFs; (d) high-resolution TEM image of the CuWO4/BiVO4-composite NFs.
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Figure 2. XRD patterns of CuWO4, BiVO4, and the CuWO4/BiVO4-composite NFs.
Figure 2. XRD patterns of CuWO4, BiVO4, and the CuWO4/BiVO4-composite NFs.
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Figure 3. (a) UV–Vis absorption spectra of CuWO4, BiVO4, and the CuWO4/BiVO4 composite; (b) The corresponding Tauc plots of CuWO4 and BiVO4.
Figure 3. (a) UV–Vis absorption spectra of CuWO4, BiVO4, and the CuWO4/BiVO4 composite; (b) The corresponding Tauc plots of CuWO4 and BiVO4.
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Figure 4. (a) Linear sweep voltammograms of the CuWO4, CuWO4/BiVO4, and CuWO4/BiVO4/Co-Pi-composite photoanodes in 1.0 M of Na2SO4 with 0.1 M of sodium phosphate buffer (pH = 7) in the dark and under illumination (100 mW·cm2); (b) Variation of the photocurrent density as a function of the drop-casting times of the Bi precursor solution on CuWO4. The pure CuWO4 sample in (b) experienced the same preparation procedure as the CuWO4/BiVO4 composite (drop-casting 6 times) except that no Bi precursor existed in the drop-casting solution.
Figure 4. (a) Linear sweep voltammograms of the CuWO4, CuWO4/BiVO4, and CuWO4/BiVO4/Co-Pi-composite photoanodes in 1.0 M of Na2SO4 with 0.1 M of sodium phosphate buffer (pH = 7) in the dark and under illumination (100 mW·cm2); (b) Variation of the photocurrent density as a function of the drop-casting times of the Bi precursor solution on CuWO4. The pure CuWO4 sample in (b) experienced the same preparation procedure as the CuWO4/BiVO4 composite (drop-casting 6 times) except that no Bi precursor existed in the drop-casting solution.
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Figure 5. Schematic representation of the energy band structure of the CuWO4/BiVO4 heterojunction and charge transfer mechanism during illumination.
Figure 5. Schematic representation of the energy band structure of the CuWO4/BiVO4 heterojunction and charge transfer mechanism during illumination.
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MDPI and ACS Style

Peng, B.; Xia, M.; Li, C.; Yue, C.; Diao, P. Network Structured CuWO4/BiVO4/Co-Pi Nanocomposite for Solar Water Splitting. Catalysts 2018, 8, 663. https://doi.org/10.3390/catal8120663

AMA Style

Peng B, Xia M, Li C, Yue C, Diao P. Network Structured CuWO4/BiVO4/Co-Pi Nanocomposite for Solar Water Splitting. Catalysts. 2018; 8(12):663. https://doi.org/10.3390/catal8120663

Chicago/Turabian Style

Peng, Ben, Mengyang Xia, Chao Li, Changshen Yue, and Peng Diao. 2018. "Network Structured CuWO4/BiVO4/Co-Pi Nanocomposite for Solar Water Splitting" Catalysts 8, no. 12: 663. https://doi.org/10.3390/catal8120663

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