Improved H₂ Evolution in Quaternary SCIGS Chalcopyrite Semiconductors

In a search for improved photocathode materials for fuel-producing photoelectrochemical cells, quaternary Ag_xCu_1-xGa_yIn_1-yS₂ (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 1.0) p-type, chalcopyrite semiconductors (SCIGS) were prepared and tested for photochemical hydrogen evolution. The study reported here is based on the preparation of bulk phases of the quaternary system. In contrast to thin-film studies of this system, this approach enables enhanced control of the materials' chemical, structural, and electronic properties. Compared to ternary CIGS electrode materials, the quaternary SCIGS materials provide improved photoelectrochemical and electrocatalytic properties. The quaternary system allows for adjustment of the band structures and photocatalytic abilities to a finer degree than is possible in the Ag_xCu_1-xGaS₂ or Ag_xCu_1-xInS₂ ternaries, and several of the quaternary Ag_xCu_1-xGa_yIn_1-yS₂ compositions are found to show better water-splitting capability than the ternaries, even when a platinum co-catalyst is present in the ternary systems. Our work on quaternary compounds has led to finding unexpected optoelectric properties in ternary compounds of composition Ag_xCu_1-xGaS₂. Specifically, a "V-shaped" plot of semiconductor composition versus band gap is observed, which is not easily correlated with the observed variation in semiconductor structure. Even more interesting is the observation that the silver-rich branch of this "V" produces obviously higher H₂ evolution rates than observed with the copper-rich branch.