Paper SS-WeM4
Formation of Heterogeneous Multiple-Oxide/Hydroxide Species on a GaP(111) Surface Tracked by In Situ Near-Ambient Pressure XPS

Wednesday, October 21, 2015, 9:00 am, Room 112

A photoelectrochemical (PEC) solar cell for water splitting converts solar energy into chemical energy and store it in the form of hydrogen, which is a promising candidate of sustainable and clean fuels. PEC solar cells consisted of phosphide-based III-V semiconductors are known to have a higher solar to hydrogen conversion efficiency than other materials. They are, however, usually limited by technological drawbacks such as photocorrosion or decreased electron extraction efficiency due to the formation of surface oxide species. The formation of surface oxides becomes critical when operating electrodes are exposed to aqueous electrolytes or to ambient conditions. Therefore, it is desirable to understand the interfacial processes of water interactions with semiconductors, and to elucidate possible oxidation and reduction mechanisms at the H₂O/semiconductor interface, especially under near realistic conditions.

In this study, water dissociative adsorption onto a GaP (111) surface was investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP XPS) at various pressures and temperatures [1]. This advanced technique allowed us to monitor the H₂O/semiconductor interfacial chemistry under operando conditions, which would not be feasible to be investigated by traditional surface sensitive techniques. The interfacial chemistry was tracked by recording high-resolution photoemission spectra of Ga 2p_3/2, O 1s, and P 2p. In the pressure-dependent study conducted at room temperature, ~300 K, the enhancement of surface Ga hydroxylation and oxidation was observed with an increase in H₂O pressures. This finding was also confirmed by changes observed in the photoemission spectra of O 1s. In the temperature dependent study, surface Ga hydroxylation and oxidation were further enhanced at temperatures below 673 K. While a large-scale conversion of surface O-Ga-OH species into Ga hydroxide, along with surface P oxidation, were observed at 773 K. Moreover, the formation of Ga and P oxide/hydroxide networks was suggested and a "phase diagram" that demonstrates the distribution of different chemical species under various experimental conditions has been generated (supplemental document). Our results led to a better understanding of the H₂O/semiconductor interfacial chemistry and the water splitting mechanism in the PEC solar cells.

Reference

1. Zhang, X., Ptasinska, S. Distinct and dramatic water dissociation on GaP(111) tracked by near-ambient pressure X-ray photoelectron spectroscopy. Phys. Chem. Chem. Phys., 2015, 17, 3909-3918.