Enhanced Photocatalytic Hydrogen Production By Surface Modification of p-Gap Photocathodes

Abstract

Photocatalytic water splitting is considered one of the most promising approaches for reducing both the reliance on fossil fuels and the emission of greenhouse gases such CO2 in the atmosphere. A working photocatalytic water splitting device must provide the voltage required for splitting water into hydrogen and oxygen (1.23 V) without external applied bias. It is therefore desirable that the photon absorbers utilized in such device provide the highest photovoltage possible together with a significant current density. GaP is a semiconductor material having 2.25 eV indirect bandgap and a theoretical maximum photocurrent density of about 12.5 mA/cm2. The best solar cells made of GaP show an open-circuit voltage of approximately 1.5 V and a maximum photocurrent density close to 2 mA/cm2. p-GaP utilized as a photocathode for hydrogen evolution shows significantly lower open-circuit voltage (+0.35 V RHE, with Pt cocatalyst), mainly because of inefficient charge separation at the semiconductor/electrolyte junction. Furthermore, this semiconductor suffers from corrosion in acidic conditions, thus requiring appropriate protection. One approach for improving charge separation and open-circuit voltage consists of forming a p-n heterojunction on GaP. We deposit different n-type metal oxides (TiO2, Nb2O5, ...) thus forming an heterojunction which significantly enhances charge separation upon light irradiation by forming a built-in potential at the junction interface. This built-in potential effectively drives electrons towards the surface of the photoelectrode with the hydrogen evolution reaction occurring at more positive potential compared to the bare p-GaP under the same operating conditions. The observed open-circuit voltage for the modified photocathodes is +0.70 V RHE, representing an increase of more than 300 mV compared to the pristine p-GaP semiconductor and marking an unprecedented value of open-circuit voltage for GaP-based photocathodes for hydrogen production. It is found that the high carrier density of the n-type oxides shifts the distribution of the built-in potential almost entirely towards the lightly doped p-type substrate and forms an asymmetric charge depletion region at the junction, as depicted in Figure 1. Moreover, TiO2shows excellent stability over long-time operation, unveiling its double role of brilliant material for both heterojunction formation and protection against corrosion of the substrate. Further improvement of the aforementioned system and a favorable coupling with an efficient photoanode could lead to a scenario where photocatalytic water splitting is carried out without any external applied bias under solar light irradiation. References: Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473 Grätzel, M. Photoelectrochemical cells. Nature 2001, 414, 338-344 Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution. J. Am. Chem. Soc. 2013, 135 (3), 1057-1064 Lu, X.; Huang, S.; Diaz, M.; Kotulak, N.; Hao, R.; Opila, R. & Barnett, A. Wide Band Gap Gallium Phosphide Solar Cells. IEEE J. Photovolt., 2012, 2, 214-220 Kaiser, B.; Fertig, D.; Ziegler, J.; Klett, J.; Hoch, S.; Jaegermann, W. Solar Hydrogen Generation with Wide-Band-Gap Semiconductors: GaP(100) Photoelectrodes and Surface Modification. ChemPhysChem 2012, 13, 3053-3060 Butler, M. A.; Ginley, D. S. P-Type GaP as a Semiconducting Photoelectrode. J. Electrochem. Soc. 1980, 127 (6), 1273-1278 [Formula]