Water Splitting DOI: 10.1002/anie.200701626 A Titanium Disilicide Derived Semiconducting Catalyst for Water Splitting under Solar Radiation—Reversible Storage of Oxygen and Hydrogen** PeterRitterskamp,AndriyKuklya,Marc-AndrØWüstkamp,KlausKerpen,ClaudiaWeidenthaler, and Martin Demuth* Dedicated to Professor Kurt Schaffner on the occasion of his 75th birthday Hydrogen and oxygen evolution from water using semi- conductors and light is an important issue in the exploitation of solar radiation as a sustainable energy. [1,2] However, a major drawback of most of the research in this field relates to the fact that appropriate semiconductors either are not readily accessible, absorb solar radiation inefficiently, [3–8] or producehydrogeninasacrificialmanneronly(i.e.thecatalyst is degraded). We present titanium disilicide (TiSi 2 ) as a prototype for the promising new class of silicide semiconduc- tors, [9] which have not, to date, been used for water splitting. [1] These semiconducting materials are inexpensive and abun- dant. One disadvantage might be their poor stability (in particular of TiSi 2 in water). However, we anticipated that sufficient passivation of TiSi 2 by limited oxide formation might render this project successful. [10] The light-absorption characteristics of TiSi 2 are ideal for solar applications: broad-band reflectance measurements show a band-gap range from 3.4 eV (ca. 360 nm) to 1.5 eV (ca. 800 nm) for TiSi 2 (Figure 1). This behavior is atypical of semiconductors since these materials usually exhibit small band-gap spreads. Determination of the quasi Fermi level of electrons at pH 7 of our catalyst showed values of 0.43 eV and 0.41 eV before and during reaction, respectively; [11] the latter energy level still fulfils the physical requirement for the reduction of protons to form hydrogen. [12] The broad-band water-splitting capacity has been ascer- tained by experiments run at individual wavelength ranges. For this purpose, Rayonet photoreactors were equipped with RUL 350 or 540 nm lamps (l max ;  60 nm emission range). Comparable water-splitting kinetics were obtained. Twophases(AandB)ofhydrogenevolutionareobserved when dark grey TiSi 2 powder (ca. 325 mesh, Alfa) is allowed to react at 55 8C under standard conditions (see the Exper- imental Section and Figure 2). Phase A starts at t = 0 with [H 2 ] = 0 and shows a nonlinear dependence. Phase B is characterized by the nearly linear part of the hydrogen evolution curve. We interpret the time dependence of hydro- gen evolution to be a consequence of simultaneously occur- ring reactions [Eqs. (1)–(3)]. The course of the reaction in TiSi 2 þ 6H 2 O ! TiSi 2 oxides þ 6H 2 ð1Þ H 2 O TiSi2 ðcat:Þ, hn  ! 1 = 2 O 2 þ 2H þ þ 2e  ð2Þ 2H þ þ 2e  ! H 2 ð3Þ Equation (1), which sacrifices the TiSi 2 by oxide formation, has been verified by runs in the dark at 50–85 8C. Initially, it shows a similar dependence as the reactions in light, but it levels off and does not show a linear dependnce as in phase B, which is attributed to water splitting, and hydrogen produc- tioncomestoastopafterapproximately150h(Figure6inthe Supporting Information). The course of hydrogen evolution in phase A depends strongly on the quality and composition of the catalyst, its particle size, the reaction temperature, and the pH value. [13] Figure 1. Band-gap range of the TiSi 2 -based semiconducting catalyst employed in this work. h + = hole with positive charge. [*] P. Ritterskamp, Dr. A. Kuklya, M.-A. Wüstkamp, Dr. K. Kerpen, Prof. M. Demuth Max-Planck-Institut für Bioanorganische Chemie 45413 Mülheim an der Ruhr (Germany) Fax: (+ 49)208-306-3951 E-mail: demuthm@mpi-muelheim.mpg.de Homepage: http://www.mpibac.mpg.de Dr. C. Weidenthaler [+] Max-Planck-Institut für Kohlenforschung 45470 Mülheim an der Ruhr (Germany) [ + ] XRPD/XP spectroscopy [**] We thank the Max-Planck-Gesellschaft for generous financial support of this work and Dr. Helmut Görner for helpful discussions. The mass spectrometry, performed by Dipl.-Biol. Katrin Beckmann and Dr. habil. Johannes Messinger, is gratefully acknowledged. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications 7770 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2007, 46, 7770 –7774