Potential and pH dependence of photocurrent transients for boron-doped diamond electrodes in aqueous electrolyte

Abstract

Using illumination at energies below the intrinsic diamond energy gap, photocurrent transients have been recorded for boron-doped diamond (BDD) as an electrode in an aqueous electrolyte of 0.1M KH2PO4. The commercially-supplied BDD was in the form of a free-standing, polycrystalline film grown by chemical vapour deposition (CVD), with a boron acceptor concentration of ≥1020cm−3. The effects of mechanical polishing of the BDD, of electrochemical hydrogen evolution and of electrochemical oxygen evolution (in 0.1M KH2PO4), on the potential dependence of the photocurrent transients have been examined. Measurements of the cathodic photocurrent at light switch-on have been used to determine the photocurrent onset potential as a measure of the flatband potential. Comparison with and between related literature observations has shown broad agreement across considerably varying BDD/electrolyte systems. The flatband potential shifted positively following electrochemical oxygen evolution, indicating the formation of oxygen-containing groups on the diamond surface, these increasing the potential drop across the Helmholtz layer. For the electrochemically oxidised electrode, the cathodic photocurrent transient at a fixed potential changed reproducibly with changing solution pH, owing to the participation of the oxygen-containing surface groups in acid–base equilibrium with the solution. This clear demonstration of BDD as a photoelectrochemical pH sensor is in principle extendable to mapping the spatial variation in pH across a BDD surface by use of a focussed light spot.