Photocatalytic hydrogen evolution over the hetero-junction CoAl2O4/ZnO

Abstract

The photoevolution of hydrogen is demonstrated on the new hetero-junction CoAl2O4/ZnO. The nano sized spinel obtained from hydrotalcite calcination exhibits a high specific surface area (130 m2 g−1) and good photocatalytic activity. The optical transition of the spinel CoAl2O4 (1.56 eV), directly allowed, is due to Co2+: d–d transition. The capacitance measurements indicate a negative slope, characteristic of p type conduction with a flat band potential of −0.11 V SCE. The photoactivity is thermodynamically predicted from the energy band diagram. The potential of the conduction band (−1.51 V SCE) is more negative than the hydrogen level, leading to spontaneous hydrogen liberation. The spinel is activated by visible light and the electrons are transferred to water via ZnO. A reducing agent is required to prevent the photocorrosion, SO3 2− being particularly favorable and CoAl2O4 approaches 100 % stability. The hetero-junction consists of a mixture of powders (x%CoAl2O4/125 mg ZnO) exhibits self-sensitization for the hydrogen production in basic medium (pH~12.5) with an evolution rate of 2.04 × 10−2 cm3 h−1 (g catalyst)−1 under optimal conditions.