Photoelectrochemical oxygen evolution with cobalt phosphate and BiVO4 modified 1-D WO3 prepared by flame vapor deposition
Introduction
To alleviate energy crisis which the world is facing now, it is quite attractive to utilize two renewable resources of sunlight and water. The efficient photoelectrochemical (PEC) water splitting to convert solar energy into fuel is known as a “holy grail” [1]. Due to the slow oxygen evolution kinetics at the surface of photoanode in PEC, great efforts have been made to develop the applicable photoanode materials. Metal-oxide semiconductors are quite effective as photoanode for oxygen evolution reaction (OER), because of their high stability, excellent optoelectronic property [2] and more positive valence band [3]. Among them, tungsten oxide (WO3) has been intensively studied because of its superior electron transfer property, relatively moderate hole diffusion length (∼150 nm) and valence band edge with suitable overpotential enough for fast water oxidation [4].
To satisfy the demand of high solar to hydrogen efficiency, it is still required to enhance the photocatalytic activity of WO3 by increasing the absorption coefficient and OER kinetics and also by reducing the recombination rate between photo-generated electrons and holes. Especially, because the photocatalytic materials optimized for photon absorption or charge transport are not necessarily optimized for the stability and/or other required properties of good OER catalysts [5], rational design of heterojunction photocatalysts with dual band gap can provide a higher PEC performance by counterbalancing the drawbacks of those constituting photocatalysts. Bismuth vanadate (BiVO4) has the band structure with narrower bandgap (∼2.4 eV) matching with solar visible spectrum and the higher levels of both valence and conduction bands relative to those of WO3 and would be a good choice for heterojunction photocatalyst to compensate for the photocatalytic properties of WO3 [6]. The poor charge separation yield of BiVO4 can be overcome by the rapid separation of the holes and electrons through the heterojunction of WO3 and BiVO4. Still, the slow kinetic of water oxidation reaction is a common problem for many photoanodes and the photo-generated holes will be accumulated at the electrode-electrolyte interface, which would take place for WO3 and BiVO4 heterojunction photocatalysts. Development of non-precious metal-based OECs which are more stable and efficient is highly demanded [7], [8].
An effective strategy to develop more efficient water splitting catalysts is to duplicate photosynthesis [9]. In photosystem II (PS II) of natural photosynthesis, light is collected to generate the electron/hole pairs. The generated holes are fed to the oxygen evolution complex (Mn4CaO5 cluster), where the water is oxidized to generate oxygen and hydrogen ion by photosplitting reaction. The electrons generate the donor products of PS II and are moved to photosystem I to produce the nature’s form of hydrogen ion. Integrating the catalysts for artificial photosynthesis which is similar to Mn4CaO5 cluster of PS II can lower the kinetic barrier of OER and facilitate the charge transfer at the electrode-electrolyte interface and the development of artificial OER catalysts with low-cost and facile route has attracted great attention. Cobalt phosphate (Co-Pi) consisting of earth-abundant elements can be readily prepared [10], [11] and became well-known for high catalytic activities by assisting the OER catalysts after Nocera et al.’s investigation [10], [12], [13]. Co Pi can facilitate the photosplitting reaction of water to generate the oxygen and hydrogen ion, which is similar to PS II of natural photosynthesis.
The Co-Pi has been deposited on various photoanode materials by electrodeposition or photo-assisted electrodeposition. Several chemical processes were used to prepare the photoanode thin films: chemical vapor deposition (CVD) for Fe2O3 [5], electrodeposition for Fe2O3 [11], WO3 [14] and ZnO [15], metal-organic deposition for BiVO4 [16] and Mo:BiVO4 [17]. The Co-Pi modified photoanodes have shown higher PEC performances than the bare photoanodes and the PEC performances could have been improved more by using the controlled morphology of photoanodes. The easiest way for morphology control is to employ a template. Zhang et al. [18] prepared the controlled morphology of WO3 framework using polystyrene template and applied it to prepare inverse opal structured WO3/BiVO4/Co-Pi composite photoanode for the improved PEC performance.
If the framework of WO3/BiVO4 heterojunction photocatalyst is based on one-dimensional (1-D) nanostructured WO3 with high surface area, the light harvesting efficiency can be improved significantly [19], [20], [21], [22]. The PEC performance of 1-D WO3/BiVO4 heterojunction photoanode can also be improved remarkably by Co-Pi modification [23], [24]. Fig. 1 demonstrates the improved PEC performance by enhanced light harvesting and charge separation efficiencies in the WO3/BiVO4/Co-Pi composite photoanode with 1-D WO3 nanostructure on FTO. The 1-D nanostructure based on WO3/BiVO4/Co-Pi can combine various advantages of all constituting materials, which can improve the conversion efficiency for PEC applications remarkably. Therefore, the efficient preparation of 1-D nanostructured WO3 without the template is quite important for the proper application of WO3/BiVO4 heterojunction photocatalyst modified by Co-Pi. Hydrothermal [23], solvothermal [25], thermal evaporation [26] and CVD [27] processes could successfully provide 1-D nanostructured WO3, but the growth rates of thin film in these processes are relatively low. On the other hand, flame vapor deposition (FVD) process can provide the 1-D WO3 nanowires with high purity and crystallinity in a short time. The FVD process is easily scalable to prepare 1-D nanostructured WO3, because it is carried out at atmospheric pressure [19], [22].
In the present study, we have employed a rapid, facile and low cost FVD method to fabricate 1-D WO3 nanowires on fluorine-doped tin oxide (FTO) glass as photoanode framework. BiVO4 layer was spin coated on the WO3 nanowires to prepare 1-D WO3/BiVO4 heterojunction photoanodes. Co-Pi was deposited on the prepared 1-D WO3/BiVO4 photoanodes by photo-assisted electrodeposition process. PEC performances were evaluated and a detailed PEC study has revealed that applying Co-Pi to the 1-D WO3/BiVO4 OEC is a vital strategy to improve the overall water oxidation efficiency of composite photoanode.
Section snippets
Preparation of WO3 nanowires
WO3 nanowires were prepared on fluorine-doped tin oxide (FTO) substrate (1.5 cm 1.5 cm, 0.2 cm in thickness, 8 Ω/sq, Sigma-Aldrich) by FVD process. Firstly, FTO substrate was cleaned and crystalline substoichiometric WOx layer was grown for 15 min in FVD process incorporated with the constant tungsten wire feeder (wire feed rate: 5 μm/s). The premixed methane, nitrogen and oxygen gases with flow rate ratio of 1:3:1.7 were supplied in fuel rich condition (methane flow rate: 900 sccm). The tungsten wire
Results and discussion
Fig. 3 shows the SEM images of WO3 nanostructures prepared by this FVD process. The obtained WO3 thin film consists of high density 1-D nanowires with diameters of 130−170 nm throughout the film (Fig. 3a and b). The length of these nanowires is approximately observed to be 3 μm after the deposition time of 15 min (Fig. 3c) and the growth rate of nanowires is about 200 nm/min. Table 1 shows the comparison of WO3 nanowire growth rates for different processes and we can see that the FVD has the growth
Conclusions
We have proposed a heterojunction composite photoanode system of WO3/BiVO4/Co-Pi in which the 1-D WO3 nanowires serve as framework and the BiVO4 nanoparticle layer as cooperative photoabsorber and the Co-Pi as water oxidation catalyst. 1-D nanostructures of WO3 were prepared by FVD, followed by spin coating of BiVO4 and photo-assisted electro-deposition of Co-Pi OEC. It is shown that the OER performance of heterojunction composite photoanode can be improved successfully by synergetic effects of
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Mid-career Researcher Program through NRF funded by the MSIP (2019R1A2C1004716). Instrumental analysis was supported from the central laboratory of Kangwon National University.
References (39)
- et al.
Mater. Lett.
(2020) - et al.
Int. J. Hydrogen Energy
(2013) - et al.
Chem. Eng. J.
(2018) - et al.
Sol. Energy Mater. Sol. Cells
(2006) - et al.
Chem. Eng. J.
(2016) - et al.
Appl. Catal. B Environ.
(2014) - et al.
J. Catal.
(2013) - et al.
Cuihua Xuebao/Chin. J. Catal.
(2018) - et al.
Electrochim. Acta
(2015) - et al.
Acc. Chem. Res.
(1995)
APL Mater.
Photoelectrochemical Solar Fuel Production
Chem. Soc. Rev.
J. Am. Chem. Soc.
Science (80-)
ChemCatChem
Science (80-)
Science (80-)
Chem. Soc. Rev.
Cited by (15)
A TiO<inf>2</inf> nanotube photoanode (blue TiO<inf>2</inf> nanotube/RuO<inf>2</inf>/BiVO<inf>4</inf>) for efficient acetaminophen degradation and nitrogen removal
2024, Journal of Water Process EngineeringToward preparation of large scale and uniform mesoporous BiVO<inf>4</inf> thin films with enhanced photostability for solar water splitting
2023, Journal of Alloys and CompoundsOptical hydrogen sensing by MoO<inf>3</inf> films deposited by a facile flame synthesis method
2023, Applied Surface ScienceCurrent trending and beyond for solar-driven water splitting reaction on WO<inf>3</inf> photoanodes
2022, Journal of Energy ChemistryCitation Excerpt :Additionally, the CB and VB positions of this material are suitably positioned to construct a type-II heterostructure with WO3 [144]. Since BiVO4 has poor charge separation, the combination of its optoelectronic properties with those of WO3 can potentially overcome this issue by enabling rapid charge separation at the heterojunction interface [144–146]. This makes WO3/BiVO4 heterojunction one of the most explored systems in the literature [147].
Accelerating the controlled synthesis of WO<inf>3</inf> photoanode by modifying aerosol-assisted chemical vapour deposition for photoelectrochemical water splitting
2022, Chemical Engineering ScienceCitation Excerpt :The idea of using abundant natural resources such as sunlight and water for the generation of clean hydrogen energy via PEC cell has attracted the attention of many researchers (Shanmugasundaram et al., 2021). Hitherto, great efforts have been made to develop high-performance photoanode to overcome the sluggish oxygen evolution kinetics at the photoanode surface (Wang et al., 2020; Yoon et al., 2020). A wide range of semiconductors have been explored as photoanodic materials including metal oxides, (Bhat et al., 2020; Lo Vecchio et al., 2020; Mohamad Noh et al., 2020) nitrides (C3N4, Ta3N5, etc.) (Arzaee et al., 2020; Mohamed et al., 2020b) and sulphides (CdS, ZnS, etc.), (Wei et al., 2021).
- 1
These authors contributed equally to this work.