Bottlenecks limiting efficiency of photocatalytic water reduction by mixed Cd-Zn sulfides/Pt-TiO2 composites

Abstract

Visible-light driven photocatalytic water reduction on composite materials consisting of platinized titania (Pt-TiO2) and transition metal sulfides (CdS or Cd0.5Zn0.5S) was investigated in detail. Sulfides were prepared by hydrothermal synthesis and room-temperature precipitation. The parameters limiting performance of these composite systems were elucidated. All composites with Pt-TiO2 demonstrated similar hydrogen evolution rates independent from their textural properties, bandgaps, electron transfer between components and intrinsic activities of the sulfides. Moreover, all platinized sulfides, except for the precipitated CdS, were more active than the corresponding composites with Pt-TiO2. This behavior – counterintuitive to the improved charge carrier separation found in the materials with heterojunctions – was rationalized by the low mobility of the conduction band electrons in TiO2. The slow electron transport severely limits efficiency of the investigated composite materials in the photocatalytic water reduction. This effect is especially apparent for highly active sulfides but less so for materials with inherently low activity. The low driving force of bare CdS toward water reduction results in an apparent synergy with Pt-TiO2 and makes the corresponding composites sensitive to Pt poisoning as the hydrogen evolution reaction predominantly takes place on Pt-TiO2. On the other hand, mixed sulfides, being more active water reduction photocatalysts, compete with Pt particles in this process making corresponding composites less sensitive to the state of Pt. The findings are discussed in terms of the intrinsic photocatalytic activity of sulfides, electron transfer from sulfides to titania, electrochemical potentials of conduction band electrons, poisoning of Pt nanoparticles, and charge carrier mobility.