Photoelectrochemical Study of the Delafossite AgNiO2 Nanostructure: Application to Hydrogen Production

摘要

AgNiO_(2) is a semiconductor crystallizing in the delafossite structure; it is prepared by the hydrothermal route, and the photoelectrochemical properties are studied for the first time. The TG/DSC analyses show a low stability not exceeding 290 °C before its reduction into Ag and NiO. The direct bandgap energy of the bulk material is 0.87 eV, due to the d–d transition of Ag~(+) linearly coordinated. AgNiO_(2) is chemically stable in the pH region (4–14); a flat band potential of ?0.022 V_(RHE) with p-type behavior, inferred to oxygen insertion is reported in KOH solution (10~(?2) M). The holes density (1.92 × 10~(22) cm~(?3)) agrees with a semi-metallic behavior. Positive potentials give rise to surface oxidation of AgNiO_(2) in the diffusion plateau before oxygen evolution. The electrochemical oxygen insertion, investigated by chrono-amperometry, is found to be slow with a diffusion coefficient of ～8 × 10~(?16) cm~(2) s~(?1). The Nyquist plot exhibits a semicircle centered below the abscissa axis, whose diameter 4200 Ω cm~(2) decreases down to 760 Ω cm~(2) under visible illumination. Such results indicate dipolar and multi-relaxation processes and confirm the existence of the optical gap. The conduction band (?0.88 V_(RHE)) derived from Ag~(+): 4d orbital is more cathodic than the potential of H_(2)O/H_(2) (～?0.64 V_(RHE)) level and hydrogen is evolved under visible irradiation. An evolution rate of 1.43 mL g~(?1) min~(?1) at pH ～ 12.8 is obtained with a light-to-chemical energy efficiency of 2.40%.