Photocatalytic and photoelectrochemical processes employing photoactive semiconductors are two key systems in harvesting sunlight for energy and environmental applications. Strategies have been formulated to improve the properties of the semiconductor for better performances. However, requirements to yield excellent performances are different in these two distinctive systems. Understanding of the underlying mechanism for the photoexcited charge transportation in relation to their photoactivities is of fundamental importance for rational design of high-performing photoactive materials, which may serve as a useful guideline for the fabrication of good photocatalysts or photoelectrodes toward sustainable solar fuels generation. Many photoactive semiconductors, including bismuth vanadate, are having inadequate charge transport properties such as low charge mobility, short lifetime, and slow surface kinetics. Standalone single photoactive semiconductors also faces the challenge of meeting the needs for large redox potentials with a sufficiently narrow bandgap for visible light absorption. Coupling two or more components into a photoactive composite may address some of these drawbacks by utilizing the strengths of the individual semiconductors. In this presentation, the charge transportation behavior of single component photoactive semiconductor based on bismuth vanadate, and the composite materials forming heterojunction with bismuth vanadate, will be examined and discussed (example of modulation of BiVO₄ photocatalyst in Figure 1).¹

Typically, crystal facet engineering, interfacing with non-photoactive secondary component, and narrowing the crystal size into quantum-confinement region, will be the three strategies to be shared with audience during the presentation. Preferential charge migration depending on the crystal facet has been reported in various photocatalysts. These promoted charge migration can be further boosted with spatially deposited cocatalysts on the selective redox sites. The manipulation of crystal facet allows the controlled electron transfer to induce 4-electron process (oxygen evolution) or 2-electron process (hydrogen peroxide evolution) in water oxidation. Nanosizing the bismuth vanadate into <10 nm region with distinctive yet interconnected crystal domains leads to the expansion of band structure. Conduction band of bismuth vanadate was expanded to energetic level capable of reducing proton into H₂. The traditional O2-evolving bismuth vanadate is now demonstrating H2-producing properties.

Figure 1: Time-resolved microwave conductivity (TRMC) spectroscopy signals measured for BiVO4 and its Power-law decay fittings.

Reference

1. H. Wu, R. Irani, K. Zhang, L. Jing, H. Dai, H. Y. Chung, F. F. Abdi, Y. H. Ng*, Unveiling Carrier Dynamics in Periodic Porous BiVO4 Photocatalyst for Enhanced Solar Water Splitting. ACS Energy Lett. 2021, 6, 3400-3407.