1. Contribution from the Institute for Materials Science, Darmstadt University of Technology Petersenstrasse 23, D-64287, Darmstadt, Germany, Zentrum für Sonnenenergie- und Wasserstoff-Forschung, Helmholtzstrasse 8, D-89081 Ulm, Germany, Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands, and Chemistry Department, George Washington University, Washington, DC 20052