Synthesis and Characterization of N-Type CuGaS2 Nanoparticles and Films for Purpose of Photoelectrocatalytic Water Splitting

Abstract

In this work, n-type CuGaS2 (CGS) nanoparticles were synthesized using an original colloidal method in N-Methylimidazole solvent with GaCl3, Li2S and CuCl as Ga3+, S2− and Cu+ precursors, respectively. After annealing at 600 °C, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed an excess of gallium with a bulk Ga/Cu atomic ratio of 1.01 and a larger value at the surface. The CGS X-ray diffraction pattern is compatible with a chalcopyrite crystalline phase with crystallites size of about 17 nm. UV-visible spectroscopy measurements showed that CGS sample has a direct band gap energy of 2.41 eV. Capacitance measurements, carried out in aqueous 0.5 M KOH on CGS thin film, using electrochemical impedance spectroscopy, confirmed the material n-type conductivity. The Fermi level was found to be −3.93 eV and the majority charge carrier density is 1.93 × 1018 cm−3. The valence band higher energy level and the conduction band lower energy level are −6.31 eV and −3.90 eV, respectively. The energy level diagram of the n-CuGaS2/electrolyte junction suggests that photo-oxidation of OH− species to produce O2 gas would be favorable; however, a small external cathodic bias would be required to generate H2 gas at the platinum counter electrode.