Abstract

AgNiO2 is a semiconductor crystallizing in the delafossite structure; it is prepared by 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 band gap energy of the bulk material is 0.87 eV, due to d-d transition of Ag+ linearly coordinated. AgNiO2 is chemically stable in the pH region (4 – 14); a flat band potential of −0.022 VRHE with p-type behavior, inferred to oxygen insertion is reported in KOH solution (10−2 M). The holes density (1.92 × 1022 cm−3) agrees with a semi metallic behavior. Positive potentials give rise to surface oxidation of AgNiO2 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 cm2 s−1. The Nyquist plot exhibits a semicircle centred below the abscissa axis, whose diameter 4200 Ω cm2 decreases down to 760 Ω cm2 under visible illumination. Such results indicate dipolar and multi relaxation processes and confirm the existence of the optical gap. The conduction band (−0.88 VRHE) deriving from Ag+: 4d orbital, is more cathodic than the potential of H2O/H2 (∼ −0.64 VRHE) 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%.

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