We would like to thank B. To and C. Xiao from NREL for the scanning electron micrographs. A. J. Nozik, J. Luther, D. Kroupa and Y. Yang are thanked for their helpful discussions. Y.Y. would like to acknowledge the support from the startup fund at New Jersey Institute of Technology. Y.Y., R.W.C., B.D.C., G.F.P., A.R.M. and M.C.B. acknowledge support from the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences within the Office of Science. J.G. and J.A.T. acknowledge the solar photochemistry programme within the division of Chemical Sciences,Geosciences, and Biosciences of the Office of Basic Energy Sciences within the Office of Science. All work is supported by the Department of Energy under contract No. DE-AC36-08GO28308 to NREL.