Solar hydrogen evolution using metal-free photocatalytic polymeric carbon nitride/CuInS2 composites as photocathodes

Abstract

Polymeric carbon nitride (g-C₃N₄) films were synthesized on polycrystalline semiconductor CuInS₂ chalcopyrite thin film electrodes by thermal polycondensation and were investigated as photocathodes for the hydrogen evolution reaction (HER) under photoelectrochemical conditions. The composite photocathode materials were compared to g-C₃N₄ powders and were characterized with grazing incidence X-ray diffraction and X-ray photoemission spectroscopy as well as Fourier transform infrared and Raman spectroscopies. Surface modification of polycrystalline CuInS₂ semiconducting thin films with photocatalytically active g-C₃N₄ films revealed structural and chemical properties corresponding to the properties of g-C₃N₄ powders. The g-C₃N₄/CuInS₂ composite photocathode material generates a cathodic photocurrent at potentials up to +0.36 V vs. RHE in 0.1 M H₂SO₄ aqueous solution (pH 1), which corresponds to a +0.15 V higher onset potential of cathodic photocurrent than the unmodified CuInS₂ semiconducting thin film photocathodes. The cathodic photocurrent for the modified composite photocathode materials was reduced by almost 60% at the hydrogen redox potential. However, the photocurrent generated from the g-C₃N₄/CuInS₂ composite electrode was stable for 22 h. Therefore, the presence of the polymeric g-C₃N₄ films composed of a network of nanoporous crystallites strongly protects the CuInS₂ semiconducting substrate from degradation and photocorrosion under acidic conditions. Conversion of visible light to hydrogen by photoelectrochemical water splitting can thus be successfully achieved by g-C₃N₄ films synthesized on polycrystalline CuInS₂ chalcopyrite electrodes.