Physical and photo-electrochemical characterizations of ZnO thin films deposited by ultrasonic spray method: Application to HCrO_4~-photoreduction

摘要

ZnO thin films, prepared by ultrasonic spray onto glass substrate, crystallize in the wurtzite structure. The XRD pattern shows preferential orientation along the [002] direction. The films deposited at 350 C consist of 60 nm crystallites with an average thickness of ～150nm and SEM images show rough surface areas. The gap increases with increasing the temperature of the substrate and a value of 3.25 eV is obtained for films deposited at 350 ℃. ZnO is nominally non-stochiometric and exhibits n-type conduction because of the native defects such as oxygen vacancies (V_o) and/or interstitial zinc atom (Zn_i) which act as donor shallows. The conductivity is thermally activated and obeys to an exponential type law with activation energy of 57 meV and an electron mobility of 7 cm~2 V~(-1) s~(-1). The capacitance-voltage (C~(-2) V) measurement in acid electrolyte (pH ～3) shows a linear behavior with a positive slope, characteristic of n-type conduction. A flat band potential of-0.70 V_(sce) and a donors density of 5.30 × 10~(16) cm~(-3) are determined. The Nyquist plot exhibits two semicircles attributed to a capacitive behavior with a low density of surface states within the gap region. The centre is localized below the real axis with a depletion angle of 16° ascribed to a constant phase element (CPE) due to the roughness of the film. The energy band diagram assesses the potentiality of ZnO films for the photo-electrochemical conversion. As application, 94% of chromate (3.8 × 10~(-4) M) is reduced after 6 h under sunlight (AM 1) with a quantum yield of 0.06% and the oxidation follows a first order kinetic.