Trapping Behaviors of Photogenerated Electrons on the (110), (101), and (221) Facets of SnO₂: Experimental and DFT Investigations

Spatial separation of photogenerated charges between different crystal facets has been observed in some semiconductor photocatalysts; however, the charge separation mechanism is still ambiguous. As a characteristic parameter of crystal facet, surface energy may be a crucial factor to dictate the flow of photogenerated charges. In this work, the relationship between surface energy and the flow mode of photogenerated charges is investigated by using model photocatalysts, including lance-shaped SnO₂ particles and dodecahedral SnO₂ particles. The former are enclosed by two kinds of crystal facets with a big gap in surface energy, while the latter are composed of two types of crystal facets with nearly equal surface energy. However, the experimental results exhibit that the photogenerated electrons flow to all exposed crystal facets randomly in both two kinds of SnO₂ nanocrystals, which is opposite to what has been observed in extensively investigated semiconductor photocatalysts including TiO₂, SrTiO₃, BiVO₄, BiOCl, and Cu₂O. Our results disqualify surface energy as an appropriate descriptor of preferential charge flow. Furthermore, the experimental results are confirmed by trapping energies and work functions calculated with the first-principles methods, which are proved to be more relevant parameters for describing the charge flow direction. Additionally, the trapping sites on each crystal facet are determined by charge analysis.