Formation of a p-n heterojunction on GaP photocathodes for H sub(2) production providing an open-circuit voltage of 710 mV

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 2; no. 19; pp. 6847 - 6853
• Main Authors: Malizia, Mauro, Seger, Brian, Chorkendorff, Ib, Vesborg, Peter CK
• Format: Journal Article
• Language: English
• Published: 01.04.2014
• Subjects: Devices; Electric potential; Heterojunctions; Photocathodes; Semiconductors; Titanium dioxide; Volatile organic compounds; Voltage
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c4ta00752b

Photocatalytic water splitting for the sustainable production of hydrogen using a two-photon tandem device requires careful optimization of the semiconductors used as photon absorbers. In this work we show how the open-circuit voltage of photocathodes for the hydrogen evolution reaction based on p-GaP was increased considerably by sputtering of different n-type metal oxides on the surface and thereby forming an effective p-n heterojunction. Both n-TiO sub(2) and n-Nb sub(2)O sub(5) increased the V sub(OC) of the photocathodes, with the latter giving an ultimate V sub(OC) of 710 mV using Pt as the cocatalyst. This value is unprecedented for a p-GaP-based HER photocathode operating in an acidic electrolyte under simulated 1 Sun illumination. An additional, but highly significant benefit of a TiO sub(2) layer is that it provides a remarkable operational stability of more than 24 h under constant operation. It was found that TiO sub(2) and Nb sub(2)O sub(5) overlayers, which were characterized by high donor density, caused a large built-in potential drop that is located almost exclusively in the p-type substrate. The large built-in potential drop in the GaP effectively separates charge carriers driving photogenerated electrons toward the surface of the electrode to perform the HER. According to this result, a further careful choice of materials having specific properties, such as optimal carrier concentration and band positions, could potentially increase V sub(OC) even more, paving the way for the realization of a non-assisted two-photon solar water splitting device.