The material class represented by GaN has been developed to high technological maturity for blue light emitting diodes and lasers, and is also a promising candidate for photoelectrochemical (PEC) water splitting. The bandgap of GaN can be tuned by alloying with Indium from the ultraviolet to the near infrared region, i.e. across the entire solar spectrum. In addition, the conduction and valence band edges have been predicted to straddle the H_⁺/H2 and O2/H2O redox potentials for Indium contents of up to 50%. So far, the PEC properties of GaN in an environment with neutral pH value have been investigated in chloride containing electrolytes, in which the oxidation of Cl- can compete with the oxidation of water. This competition may hinder a detailed analysis of the oxygen evolution reaction (OER) on GaN photoanodes. In this study, we investigate the charge carrier kinetics of the OER on GaN in phosphate buffered electrolyte (pH≈7). Furthermore, we employ as OER catalyst cobalt phosphate (Co-Pi), which has widely been used to enhance the performance of oxide photoanodes such as Fe2O3, WO3, BiVO4 [1]. We use nominally undoped n-type GaN thin films grown by hydride vapor phase epitaxy for PEC characterization by cyclic voltammetry under chopped light from a Xe lamp (intensity about 100 mW/cm²). Without Co-Pi, photocurrent spikes are observed especially at low bias potential (<0.7 V vs. RHE), which points to charge carrier accumulation and recombination processes via surface traps and slow kinetics of the OER. At more positive potentials these effects are suppressed, which allows a rough estimation of the energetic position of the responsible surface state. After modification of the GaN with Co-Pi, current spikes do not occur at all and the photocurrent increases. Optimization of the Co-Pi deposition leads to a photocurrent of 1.2 mA/cm² at 0.2 V vs. RHE, which is three times as high as without Co-Pi. Finally, we perform stability tests of GaN with and without Co-Pi. In both cases the photocurrent decreases with time, and the decrease rates are almost the same. These results indicate that Co-Pi does not work for GaN as a protection layer against photocorrosion but it increases the efficiency of the latter material as a photoanode. We will discuss the charge carrier processes at the GaN/electrolyte interface in terms of the energetics of the contributing reactions. These results are very useful for further improvements of solar water splitting with GaN and related materials.

Reference

[1] M.W.Kanan and D.G.Nocera, Science 321, 1072 (2008).