MOKSLINĖ KONFERENCIJA SCIENTIFIC CONFERENCE 2014 PHOTOELECTROCHEMICAL WATER SPLITTING: CASE STUDIES ON p-CuxO AND n-InxGa1-xN PHOTOELECTRODES Jurga Juodkazytė, Benjaminas Šebeka, Irena Savickaja Center for Physical Sciences and Technology, Department of Electrochemical Material Science el. p.: [email protected] Splitting of water into molecular hydrogen and oxygen in the photoelectrochemical cell (PEC) using solar light and semiconductor electrodes is very attractive from the viewpoint of sustainable hydrogenbased energy economy. Copper oxides Cu2O and CuO are among the most investigated p-type metal oxide solar absorbers due to their direct band gaps of ~ 2 eV and ~ 1.3 eV, respectively, which are close to the optimal energy for sunlight absorption, as well as due to the position of the conduction band edge, which is more negative + than the potential of H reduction, making these photocathodes the promising candidates for solar hydrogen generation [1]. n-GaN is wide band gap semiconductor, the Eg of which can be reduced by adding indium [2]. The InxGa1-xN is chemically stable material with the band gap ranging from 0.7 to 3.4 eV depending on indium content. Small Eg is beneficial for larger amount of light absorption, however, narrower potential range becomes available for ox/red reactions. As a result, water splitting and H2 evolution might be out of the potential range when photo-electrode is illuminated. Our studies were aimed at the investigation of the potential of these semiconductor materials for photoelectrochemical generation of hydrogen. Copper oxide electrode was prepared by means of chemical or electrochemical oxidation of copper foil. The procedure resulted in formation of nanostructures (Fig. 1a), the electrochemically active surface area of which exceeded geometric one by a factor of ~250 [3]. Samples of GaN were grown on 2-in (001) sapphire substrates in a close-coupled 3×2-in flip-top showerhead MOCVD reactor (Aixtron Ltd). 70 – 100 nm thick InxGa1-xN were grown on ~ 3.5 μm t hick GaN substrate. A thin 2–3 nm InGaN layer with constantly changing In concentration (1 to 10%) was used as a buffer to relax lattice mismatch induced stress. Performance of Cu/CuxO photocathode was studied in PEC with nanotubular titania photoanode, whereas in the case of InxGa1-xN photoanodes, Pt cathode was used. In both cases photoelectrochemical experiments were performed without applying external bias [4, 5]. High intensity discharge Xe-lamp with 6000 K spectrum and calibrated with a silicon diode to simulate AM 1.5 Sun -2 illumination (100 mW cm ) was used. The techniques employed in the investigations included X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, scanning electron microscopy and cyclic voltammetry. The processes involved in the performance of photoelectrochemical cells investigated along with the issues of photocorrosion of CuxO and GaN photoelectrodes (Fig. 1) as well as strategies for optimization of water photoelectrolysis will be discussed. a) b) c) d) Fig. 1. Evidence of photocorrosion: SEM images of Cu/CuxO photocathode before (a) and after (b) photoelectrolysis in PEC with Ti/TiO2 photoanode in 0.1 M KOH; SEM images of GaN photoanode before (c) and after (d) exploitation in PEC with Pt cathode in 0.1 M KOH; scale b ar: 1 μm References 1. 2. 3. 4. 5. Ch.-Y. Chiang, Y. Shin, K. Aroh, Sh. Ehrman, Int. J. Hydr. Energ. 37, 8232 (2012). J. Li, J. Y. Lin, H. X. Jiang, Appl. Phys. Lett. 93, 162107-1 (2008). J. Juodkazytė, B. Šebeka, I. Savickaja, A. Selskis, V. Jasulaitienė, P. Kalinauskas, Electrochim. Acta, 98, 109 (2013). J. Juodkazytė, B. Šebeka, I. Savickaja, A. Jagminas, V. Jasulaitienė, A. Selskis, J. Kovger, P. Mack, Electrochim. Acta, 137, 363 (2014). J. Juodkazytė, B. Šebeka, I. Savickaja, A. Kadys, E. Jelmakas, T. Grinys, S. Juodkazis, K. Juodkazis, T. Malinauskas, Sol. Energ. Mater. & Sol. Cells, 130, 36 (2014).