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Photoelectrochemical activity on Ga-polar and N-polar GaN surfaces for energy conversion

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Abstract

Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical cells made of different facets of free-standing polar GaN system. To build the fundamental understanding at the differences of surface photochemistry afforded by the GaN {0001}and {0001}polar surfaces, we correlated the relationship between the surface structure and photoelectrochemical performance on the different polar facets. The photoelectrochemical measurements clearly revealed that the Ga-polar surface had a more negative onset potential relative to the N-polar surface due to the much negative flat-band potential. At more positive applied voltages, however, the N-polar surface yielded much higher photocurrent with conversion efficiency of 0.61% compared to that of 0.55% by using the Ga-polar surface. The reason could be attributed to the variation in the band structure of the different polar facets via Mott-Schottky analyses. Based on this work, understanding the facet effect on photoelectrochemical activity can provide a blueprint for the design of materials in solar hydrogen applications.

© 2013 Optical Society of America

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Figures (3)

Fig. 1
Fig. 1 (a) Scheme of the wurtzite crystallographic cell of GaN for Ga-polar and N-polar c-direction with the corresponding polarization field. (b) X-Ray Diffraction pattern of GaN for Ga-polar and N-polar.
Fig. 2
Fig. 2 (a) Photoelectrochemical response of the two polar surfaces in 1 M HCl solution in three-electrode system. (b) Photoconversion efficiency of the Photoelectrochemical cells for two polar surfaces as a function of applied potential in two-electrode system. Inset: Photoelectrochemical response of the two polar surfaces in 1 M HCl solution in two-electrode system.
Fig. 3
Fig. 3 Mott-Schottky plots of the (a) Ga-polar and N-polar surfaces, extrapolated to their corresponding flat band potentials. (b) The resulting band diagram for the Ga-polar and N-polar free standing thin film.

Equations (3)

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η e f f = [ j p ( E r e v o V C E ) I o ] * 100 %
1 C 2 = ( 2 e o ε ε o N d ) [ ( V a p p V f b ) k T e o ]
R exp ( ϕ k T )
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