The decomposition of water by sunlight has long been recognized as a potentially important reaction to harvest and store solar energy.  This is because the process is a thermodynamically uphill reaction, one that can be powered by more than half of the photons in the solar spectrum, providing an effective method for energy storage.1  In addition, the process produces a clean energy carrier, hydrogen, which when used regenerates water without adversely altering the environment.  As water is largely transparent to most of the solar spectrum, the key challenge in carrying out the reaction efficiently and, more important, inexpensively is the lack of materials that can absorb sunlight broadly, transfer the energy to excited electrons, and catalyze water splitting with high specificity. 

Among various considerations about the “ideal” material for solar water splitting, stability takes the front seat because the conditions for the intended reaction are indeed harsh.  For this reason, metal oxides have received the most attention.  Although they can be good materials for water oxidation purposes, due to their bonding natures, oxides tend to exhibit electron affinities that are too high to reduce water.  This issue would prevent complete water splitting.  Such a challenge can, in principle, be addressed in at least two different ways.  First, one may try to change the electron affinity by altering the bonding natures through means such as introducing additional elements into the existing oxide crystal matrix.2  The difficulty of doing so lies in the lack of knowledge about the detailed correlations of various electronic properties.  Second, inspired by natural photosynthesis, one may complement the oxides with a more electron negative material.3  The stability of the added component in this approach remains an important concern. 

While it is not clear what a “winning” approach would be, the community at least agrees that significant efforts are necessary in order to meet our terra-watts needs using renewable solar energy.  And it is more than clear that materials research will play a very important role in meeting this daunting challenge.

By Dunwei Wang
Department of Chemistry
Boston College


1. Bolton, J. R.; Strickler, S. J.; Connolly, J. S., "Limiting and Realizable Efficiencies of Solar Photolysis of Water", Nature, 1985, 316, 495-500

2. Woodhouse, M.; Parkinson, B. A., "Combinatorial Approaches for the Identification and Optimization of Oxide Semiconductors for Efficient Solar Photoelectrolysis", Chem Soc Rev, 2009, 38, 197-210

3. Mayer, M. T.; Lin, Y.; Yuan, G.; Wang, D., "Forming Heterojunctions at the Nanoscale for Improved Photoelectrochemical Water Splitting by Semiconductor Materials: Case Studies on Hematite", Acc Chem Res, 2013, published online, doi: 10.1021/ar300302z