The Effect of Anisotropic Physical Properties of Cu(InxGa1-x)(Se,S)2 (CIGS) Single Crystal Photocatalysts on Water Splitting

Abstract

The photoelectrochemical splitting of water to produce H2 and O2 is an attractive process for the large-scale production of clean, recyclable H2. The field of solar water splitting has gradually divided into either using conventional materials as thin films with stabilization layers or developing new materials that are themselves stable in electrolyte solutions under illumination. Divergent from these paths, we are investigating semiconductors in their single crystal form in order to take advantage of the physical properties unique to crystalline materials that are likely to cause a significant impact on photoactivity. Due to crystalline anisotropy, the crystallographic orientation of the exposed electrode surface is anticipated to affect the overall efficiency of the photocatalytic water-splitting process and the photostability of the material. Moderate band gap ternary chalcopyrites are a promising class of semiconductors for photoelectrochemical water splitting. Specifically, copper-indium-gallium dichalcogenides (diselenide or disulfide) [Cu(InxGa1-x)(Se,S)2]—abbreviated as CIGS—appear interesting due to their ideal, tunable light-absorption properties and band edge positions. To date, CIGS research has focused solely on thin film of nanoparticle materials. Electrodes based on these materials have demonstrated activity as photocathodes in photoelectrochemical cells (PEC) for water splitting. The role of crystallographic quality of CIGS in single crystal form has yet to be examined in PEC water splitting studies. The tetragonal crystal lattice structure of CIGS (Figure 1) gives a degree of asymmetry to the crystals, generating anisotropic properties in different lattice planes. Anisotropic variability has been shown to affect the optical, magnetic, and thermal expansion properties of CIGS. These differences in physical properties are expected to generate a crystal face dependence on the photoelectrochemical response of CIGS. In this presentation, we will report on the solid-state synthesis of oriented CIGS single crystal electrodes, along with the effects of crystallographic orientation on the efficiency of water-splitting. Figure 1: Tetragonal crystal structure of CIGS chalcopyrite semiconductors. View along the c-axis (left); view along the a-axis (right). Cu atoms are copper, In/Ga atoms are teal, and Se/S atoms are yellow. Figure 1