Packaging BiVO4 nanoparticles in ZnO microbelts for efficient photoelectrochemical hydrogen production
Graphical abstract
Introduction
Recently, photoelectrochemical (PEC) water splitting has attracted increasing attention, since it has been recognized as an efficient sustainable way to store solar energy in the form of hydrogen [[1], [2], [3], [4]]. Among the semiconductor family for PEC water splitting, BiVO4 has been known as one of the most attractive photoanode candidates, owing to its visible light absorption capability (band-gap energy: ∼2.4 eV), relatively high conduction band (CB) edge position (just below the water reduction potential) and its relative stability in near-neutral aqueous environments [[5], [6], [7], [8]]. However, BiVO4 often suffers from the rapid recombination rate of electron-hole pairs and poor charge transport properties, which greatly limits its photo conversion efficiency.
In particular, coupling BiVO4 with another suitable semiconductor to construct a heterojunction is widely used to prolong the photogenerated charge lifetime and then to promote its separation [[9], [10], [11], [12]]. It has been well established that ZnO is a favorable choice as the photoelectrode because of its facile synthesis process, low cost, and faster electron transfer compared with other semiconductors [[13], [14], [15]]. Nevertheless, ZnO has a wide bandgap (∼3.3 eV). This remarkably limits the light absorption in the visible region, which is considered as the fundamental obstacle to be applied in PEC water splitting. Accordingly, improving the visible light response of ZnO by coupling a narrow-band-gap semiconductor is highly sought-after for more efficient utilization of the solar energy [[16], [17], [18]]. Therefore, combining the BiVO4 and ZnO to build a ZnO-BiVO4 junction would be an excellent candidate for PEC water splitting, which can synchronously reduce the electron-hole recombination and enhance the light absorption. In this case, ZnO will serve as an electronic transmission channel, while BiVO4 will act as the robust light absorber. For instance, Balachandran et al. [19] reported the preparation of a nanobundle-shaped ZnO/BiVO4 photocatalyst by hydrothermal process followed by thermal decomposition towards the highly efficient degradation of AV 7 dye. Hu et al. [20] fabricated the ZnO/BiVO4 nanocomposite with different mole ratio through a wet-chemical process, which exhibited an unexpected performance for pollutant degradation and PEC water splitting; Yan et al. [21] deposited BiVO4 on ZnO nanorods to construct 1D ZnO/BiVO4 heterojunction photoanodes for efficient PEC water splitting.
In this paper, we present a facile strategy for the large-scale fabrication of ZnO/BiVO4 heterojunction with a novel nanostructure for photoelectrochemical hydrogen production. The isolated BiVO4 nanoparticles were packaged into ZnO microbelt matrix to form a ZnO/BiVO4 hybrid microbelt structure. It might provide a strategy for the rationally designed growth of heterojunctions with desired structures and compositions. Firstly, the precursor microbelts of BiVO4 were prepared by using a foaming-assisted electrospinning strategy. Secondly, different amounts of ZnO were deposited onto the BiVO4 precursor microbelts via atomic layer deposition (ALD) technique followed by heat-treatment to construct BiVO4@ZnO heterojunction microbelts in air. The obtained BiVO4@ZnO heterojunction microbelts exhibit a significantly enhanced PEC performance as well as favorable stability as compared to BiVO4 and ZnO alone no matter the sacrificial hole scavenger used or not。
Section snippets
Materials preparation
The BiVO4@ZnO hybrid microbelts are fabricated through a strategy including three sequential steps such as electrospinning, ALD and high-temperature annealing processes, as illustrated in Scheme 1.
- i)
Synthesis of Bi(NO3)3/VO(acac)2/DIPA/PVP precursor microbelts
The Bi(NO3)3/VO(acac)2/DIPA/PVP precursor microbelts were fabricated by a typical electrospinning method, which was performed through electrospinning the solution of high-molecular-weight poly (polyvinylpyrrolidone) (PVP-K90 Mw ≈ 1300000,
Results and discussion
Fig. 1 shows the X-ray diffraction (XRD) patterns of bare BiVO4 microbelts and the corresponding BiVO4@ZnO hybrid microbelts, disclosing that the bare BiVO4 microbelts match the standard peaks of monoclinic phase (JCPDS card No. 75-2480) with the lattice constants of a = 5.197 Å, b = 5.096 Å and c = 11.702 Å, respectively. As for the five BiVO4@ZnO hybrids, additional diffraction peaks are detected at 31.6°, 34.3° and 36.1°, which are indexed to the diffractions of (100), (002) and (101) planes
Conclusions
In summary, we have demonstrated that a novel structured BiVO4@ZnO heterojunction, that is packaging the isolated BiVO4 nanoparticles within the ZnO microbelt matrix, could be prepared by foaming-assisted electrospinning and subsequent ALD technique. The lifetime and the separation of photogenerated charge carriers of BiVO4 could be greatly enhanced after ZnO deposition, which is attributed to the transfers of photo-excited high-energy electrons from BiVO4 to ZnO at the interface of the type-II
Acknowledgements
This work was supported by National Natural Science Foundation of China (NSFC, Grant No. 51372122, 51372123, 51572133 and 51602163) and Natural Science Foundation of Ningbo Municipal Government (Grant No. 2016A610102, 2017A610002 and 2017A610005). W.-Y.W. thanks the Hong Kong Research Grants Council (HKBU 12302114), Areas of Excellence Scheme of HKSAR (AoE/P-03/08), National Natural Science Foundation of China (Grant No. 51573151), Ms Clarea Au (847S) and the Hong Kong Polytechnic University (
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