title: Hydrogen reduction of molybdenum oxide at room temperature.
abstract: The color changes in chemo- and photochromic MoO3used in sensors and in organic photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissociated at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a critical challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen reduction of MoO3thin films using atomic hydrogen in a water free environment. Hydrogen intercalates into MoO3forming HxMoO3, which slowly decomposes into MoO2+1/2 H2O as evidenced by the fast reduction of Mo6+into Mo5+states and slow but simultaneous formation of Mo4+states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temperature hydrogen reduction of bulk molybdenum oxide. The specific mechanism is a result of the balance between the reduction by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Advanced Analytical Technologies, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.|University of Zürich, Department of Chemistry, Winterthurer Strasse, 190, CH-8057 Zürich, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Borgschulte
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Advanced Analytical Technologies, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.|University of Zürich, Department of Chemistry, Winterthurer Strasse, 190, CH-8057 Zürich, Switzerland.
authors.forename: Olga
authors.initials: O
authors.lastname: Sambalova
authors.affiliation: Institute of Materials and Process Engineering (IMPE), ZHAW - Zürcher Hochschule für Angewandte Wissenschaften, Technikumstrasse 9, CH-8400 Winterthur, Switzerland.
authors.forename: Renaud
authors.initials: R
authors.lastname: Delmelle
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Sandra
authors.initials: S
authors.lastname: Jenatsch
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Roland
authors.initials: R
authors.lastname: Hany
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.|Institut des Matériaux, Ecole Polytechnique Fédéral de Lausanne, EPFL Station 12, CH-1015 Lausanne, Switzerland.
authors.forename: Frank
authors.initials: F
authors.lastname: Nüesch