HYDROGEN EVOLUTION AT A PT-MODIFIED INP PHOTOELECTRODE - IMPROVEMENT OF CURRENT VOLTAGE CHARACTERISTICS BY HCL ETCHING

Hydrogen photoevolution at p-InP electrodes coated with platinum and palladium has been studied. Efficient and stable solar to chemical energy conversion has been achieved after etching the electrodes with concentrated HCl. The current-voltage (I-V) behavior of the as-prepared electrode covered with a continuous Pt layer is poor, due probably to the presence of defect states in InP. The photocurrent density of this electrode decreases with time under illumination, presumably due to an increase in the defect density. After etching of the electrode with concentrated HCl, the barrier height is increased to 1.0 V, and the I-V characteristics are improved remarkably, showing no degradation under illumination. SEM, XPS, and AES analyses show that the concentrated HCl solution dissolves the InP substrate in the InP/Pt interfacial region, and simultaneously part of the Pt is removed from the InP surface. A structural model of the Pt-modified and HCl-etched InP electrode is proposed, in which the InP surface is in direct contact with the solution due to the formation of minute Pt islands. By use of this model, the high barrier formation is well explained. The cell characteristics of the Pd-deposited electrodes are poor and become worse with hydrogen bubbling. The I-V characteristics of the Pt-deposited electrodes are unaffected by hydrogen or nitrogen bubbling and the reason is discussed.