The Impact of Plasmonic Nanoparticles on Chemical Vapour Deposited Nano-Structured Heterojunction Water Splitting Photoanodes

Bismuth vanadate (BiVO₄)-coated tungsten trioxide (WO₃) is a visible light absorbing heterojunction system popular in the literature as an earth-abundant photoanode for water splitting. While the photoanode performance of BiVO₄ is limited by its short electron diffusion length and relatively large recombination rate, these factors are mitigated by forming a heterojunction with WO₃, which has complementary conduction and valence band energy levels for water oxidation. Additionally, nanostructuring is shown to improve charge carrier separation across the heterojunction interface, reducing recombination and improving the efficiency of the water oxidation reaction.¹ Employing plasmonic nanoparticles along with photocatalysts is an exciting approach that may enhance photocatalysis by improving light absorption and scattering, and enabling hot-electron injection, and plasmon-induced resonance energy transfer.²<br/><br/>Photocatalysts are often fabricated by spincoating, electrodeposition or photo-deposition, which offer useful control, but homogenous samples may be difficult to be achieved for larger-area depositions.³ We utilize a scalable atmospheric-pressure chemical vapour deposition (CVD) method for producing nanostructured photoanodes consisting of BiVO₄ coated onto WO₃ nanoneedles; optimal thickness and morphology is easily controlled by varying deposition parameters such as substrate temperature. WO₃ nanoneedles have previously been shown with transient absorption spectroscopy to have fast water oxidation kinetics.⁴<br/><br/>With a bandgap of 2.4 eV, BiVO₄ only has a maximum solar-to-hydrogen conversion efficiency of up to 9.2%, which may not be competitive with photovoltaic-coupled electrolysis for commercialisation.⁵ In this work, approaches in plasmonics-enhanced photocatalysis are employed to attempt to boost the conversion efficiency of nanostructured WO₃/ BiVO₄ heterojunction photoanodes beyond the 9.2% limit. The bare heterojunction photoanodes show up to 60% incident photon to current conversion efficiency when illuminated with light between 300-500 nm in wavelength under 1.23 V vs RHE of applied potential. Flat BiVO₄ photoanodes both with and without the heterojunction with WO₃, in contrast, show a much lower performance. One advantage of the CVD method is the possibility for sequential deposition over large substrate areas, and is a technique commonly used by industry for semiconductor fabrication. We demonstrate the deposition of Bi, Pd, and Au nanoparticles by CVD showing surface plasmon resonance enhancement that yields a potentially transformative route for fabricating efficient, large-area thin film photoelectrodes.<br/><br/>Acknowledgements: A.K. thanks the UK EPSRC for a Programme Grant (Grant No. EP/W017075/1) and B.T. thanks the UK Catalysis Meets Plasmonics - CPLAS research programme for funding his postdoctoral research.<br/><br/>References:<br/>1. S. Selim, J. R. Durrant, A. Kafizas et al., Chem. Sci. 2019, 10, 2643.<br/>2. E. Cortés, S. A. Maier et al., Chem. Rev. 2022, 122, 15082.<br/>3. B. Moss, O. Babacan, A. Kafizas, A. Hankin, Adv. Energy Mater. 2021, 11, 2008386.<br/>4. S. Corby, A. Kafizas, J. R. Durrant et al., J. Am. Chem. Soc. 2018, 140, 16168.<br/>5. B. Tam, O. Babacan, A. Kafizas, J. Nelson, Energy Environ. Sci., 2024, 17, 1677.