Investigations of the electrochemical stability of InGaN photoanodes in different electrolytes

InGaN is a promising material for direct solar water splitting due to its tuneable bandgap between 0.69 and 3.4 eV. Several investigations were carried out recently to test the suitability of InGaN in photoelectrochemical cells (PECs). These studies were performed in a multitude of different electrolytes and hence a comparison of the results is very difficult. Therefore, the electrochemical stability of InGaN was investigated and compared in different commonly used electrolytes in this work. The InGaN layers were deposited via metal-organic vapour phase epitaxy (MOVPE). Their structural properties were examined by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The long-term stability of the InGaN layers was studied in NaCl, HCl, HBr, H2SO4, NaOH and KOH. Etching was observed for the two bases after 48 h. Furthermore, the photoanodes were investigated via chronoamperometry. For these measurements, the samples were illuminated with a 55 W Xe lamp for 45 min and additionally biased with 0.5 V. A comparison of the different electrolytes reveals that only the samples in NaOH and HBr show a sufficient photocurrent. Additionally, stable InGaN surfaces were observed only in 1 mol/L NaCl and in 1 mol/L HBr. Since the use of HBr most likely leads to the formation of Br-2 instead of O-2, it is concluded that n-doped InGaN has to be used with co-catalysts in PECs to prevent the etching of the surface. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim