Role of Surface States in Photocatalysis: Study of Chlorine-Passivated CdSe Nanocrystals for Photocatalytic Hydrogen Generation

Abstract

We examine the effects of chlorine-passivation of Cd surface atoms on photocatalytic H 2 O reduction by CdSe NCs. Transient absorption spectroscopy reveals that Cl passivation removes electron trap states in CdSe NCs, which is also reflected in an increase of photoluminescence quantum yield, e.g., from 9 to 22% after the Cl treatment. Size-tunable energy states in CdSe NCs enable the systematic investigation of surface defects and their effect on the photocatalytic hydrogen generation rate. It turns out that, depending on band-edge energy levels, the surface trap states may enhance or inhibit photocatalysis. Cl-treated CdSe NCs larger than 2.7 nm show a higher hydrogen evolution rate than untreated CdSe NCs of the same size as Cl treatment removes trap states with energy below the H 2 O reduction potential. In contrast, the same Cl treatment does not increase the photocatalytic rate of CdSe NCs smaller than 2.7 nm because both the conduction band edge and trap states are above the water reduction potential. The size-dependence of the effect of Cl treatment suggests that electron trap states in CdSe may promote photocatalytic activity by enhancing charge separation.

The Many “Facets” of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

“…(b) PLQY upon Cl-passivation, due to removal of electron traps upon saturation of Cd dangling bonds. Reproduced from ref ( 271 ). Copyright 2016 American Chemical Society.…”

“…Contrary to the picture assumed by Greaney et al, Kim et al proposed the removal of electron trap states upon Cl passivation of CdSe NC surfaces, as reflected by the improvement (from 9% to 22%; Figure 24 b) in the photoluminescence quantum yield (PLQY). 271 They resorted to the usual replacement of native organic ligands with Cl moieties through postsynthetic ligand exchange by a methanol solution of ammonium chloride. The improvement in the PLQY was ascribed to the effective passivation of the unsaturated surface Cd atoms, as the sterically hindered and thereby incomplete organic ligand passivation gives rise to nonradiative recombination sites.…”

Hydrogen Generation through Solar Photocatalytic Processes: A Review of the Configuration and the Properties of Effective Metal-Based Semiconductor Nanomaterials

“…For example, fluoride ions can adsorb on TiO 2 substituting the surface hydroxyl groups, and consequently, the number of surface hole traps is decreased [5]. It has been reported that a chlorine passivation decreases the electron trap surface states in CdSe nanocrystals for photocatalytic hydrogen generation, measuring an increase of photoluminescence quantum yield (from 9-22% after the Cl treatment) [143].…”

Faster Electron Injection and More Active Sites for Efficient Photocatalytic H2 Evolution in g‐C3N4/MoS2 Hybrid

“…Importantly, this measurement allows us to monitor the properties of individual nanostructure. fs‐TAS enables direct observation of the dynamics of charge carrier and nonradiative recombination process in picosecond time scale . In addition, it also helps us to explore the fundamental reasons for the photocatalytic activity .…”