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Solar & Alternative Energy

Hydrogen production from water on oxynitride photocatalysts

With the help of a metal oxide photocatalyst, overall water splitting can be driven by visible light to produce hydrogen and oxygen.
3 July 2006, SPIE Newsroom. DOI: 10.1117/2.1200606.0268

As part of the search for alternatives to coal, petroleum, natural gas, and other fossil fuels, recent studies have focused on hydrogen as a clean and renewable energy carrier. An ultimate goal in renewable energy research is to produce hydrogen using solar energy, and direct water splitting on a particulate photocatalyst using the sun would be a potential way to do this at a large scale.1

Research during the last two decades has shown that metal oxide photocatalysts are effective for overall water splitting, and some of these systems have achieved a quantum efficiency (QE) greater than 50%.2 Still, most of the metal oxide photocatalysts developed to date only function in the ultraviolet (UV) region due to their large band gaps (>3eV). Although a number of photocatalysts driven by visible light have been proposed as potential candidates for this purpose, a satisfactory material has yet to be devised. A successful material would need band edge positions suitable for overall water splitting, a band gap energy smaller than 3eV, and to be suitably stable during the photocatalytic reaction.

We have studied metal oxynitrides for their potential as visible-light-driven photocatalysts for this application. When N atoms are partially or fully substituted for O atoms in a metal oxide, the tops of the material's valence band must be shifted to a higher level—as compared to a corresponding metal oxide—without affecting the bottoms of the conduction band's level. As a result, the substitution of nitrogen for oxygen in a metal oxide causes the band gap of the corresponding oxynitride to be small enough to respond to visible light (<3eV). In our research on particulate photocatalysts for overall water splitting, we estimate the tentative QE value to be 30% at 600nm.

Oxynitrides, such as TaON, Ta3N5, and LaTiO2N, can be obtained by heating a corresponding metal oxide powder under a flow of NH3. In the presence of sacrificial reagents, the oxynitrides function as stable visible-light-driven photocatalysts for water reduction or oxidation.3,4 In 2005, Maeda et. al reported that the solid solution of GaN and ZnO, (Ga1-xZnx)(N1-xOx), with RuO2 modification, achieves overall water splitting under visible light.5,6 This was the first success using a non-oxide photocatalyst with a band gap in the visible light region (<3eV), but the QE of the conversion was very low (ca. 0.23%).6

The (Ga1-xZnx)(N1-xOx) solid solution alone shows little photocatalytic activity, but the loading of cocatalysts (typically metal or metal oxide nanoparticles) that promote H2 evolution helps the reaction proceed. In our attempts to develop a new, effective cocatalyst for overall water splitting on (Ga1-xZnx)(N1-xOx), we found that Rh2-yCryO3 nanoparticle loading increases the QE by a factor of 10 (ca. 2.5% at 420–440nm) compared to RuO2 loading, as reported previously.7Figure 1 shows the QEs of Rh2-yCryO3-loaded (Ga1-xZnx)(N1-xOx) for overall water splitting as a function of incident light wavelength. The QE decreases with increasing wavelength, and the longest wavelength available for the reaction coincides with the absorption edge of (Ga1-xZnx)(N1-xOx). This indicates that the reaction proceeds via light absorption by the material. Furthermore, this photocatalyst is stable in the overall water splitting reaction, in contrast to well-known non-oxide photocatalysts such as CdS and CdSe.


Figure 1. Quantum efficiency of Rh2-yCryO3-loaded (Ga1-xZnx)(N1-xOx) for overall water splitting as a function of the wavelength of the incident light.
 

Reproducible photocatalytic systems for this application have only been developed very recently, although several reports have claimed to demonstrate the decomposition of water under visible light. However, (Ga1-xZnx)(N1-xOx) actually functions as a suitable photocatalyst and has the highest recorded QE for this purpose. Although the material's performance is not high enough for practical applications, this is the first example of overall water splitting by a particulate photocatalyst with a band gap in the visible and so opens up the possibility for new non-oxide-type photocatalysts for energy conversion.