Structural and Electronic Study of an Amorphous MoS3 Hydrogen-Generation Catalyst on a Quantum-Controlled Photosensitizer

The design and synthesis of catalysts, especially for the production of solar fuels, is a major challenge in developing sources of renewable energy. Catalyst development requires an understanding of the mechanism(s) involved and the nature of the active site. While platinum group metals have unrivalled activity for both hydrogen and oxygen evolution, they are scarce and expensive. Photocatalytic systems relying on earth-abundant materials are therefore desirable for large scale energy production. Herein, we examine the structure and electronic properties of an amorphous molybdenum sulfide species and its possible use for photocatalytic hydrogen evolution. The catalyst was grown on a seeded quantum-rod sensitizer, a model system for investigating the photophysics of solar fuel generation. This catalyst s activity is shown experimentally to be associated with under-coordinated molybdenum centers, and we document that a reduced form of MoS3 is an active species for hydrogen generation. Molybdenum sulfides are prevalent in both biological enzymes and industrial catalysts. Mo metalloenzymes are involved in carbon, nitrogen, and sulfur metabolism, while synthetic molybdenum sulfides serve as industrial hydrotreating catalysts and are proven electrocatalysts for the hydrogen evolution reaction (HER). MoS2, [13,14] incomplete cubane [Mo3S4] 4+ clusters, molecular molybdenum catalysts, and amorphous MoS2 made by a reduction of MoS3 [19] have been shown to be active HER catalysts. Highly active HER catalysts, including Pt, have a Gibbs free energy of H adsorption (DGH) close to zero. [14] Density functional theory calculations show that the equatorial sulfur atoms in Fe–Mo cofactors in nitrogenase enzymes as well as the bridging S atom on the edge sites of MoS2 bind H atoms with DGH 0. These calculations, coupled with scanning tunneling microscopy (STM) studies, have indicated that molybdenum sulfide based hydrodesulfurization and HER catalysts derive their activities from under-coordinated atoms. Recent investigations of MoS2 nanoparticles using STM combined with electrochemical measurements have revealed that HER activity scales with the number of edge sites, rather than nanoparticle area, adding substantial evidence that undercoordination is critical to activity. There is also substantial current interest in molecularly thick and structurally disordered metal oxide and sulfide layers supported on electrodes, surfaces, and nanoparticles as potential catalysts for the HER and oxygen evolution reaction (OER). Such ultrathin films can support a variety of unusual and possibly favorable bonding geometries and may retain flexibility in healing and recovering. Despite their potential, such systems remain very difficult to characterize, impeding reproducibility and the communication of results between groups. Mechanisms are difficult to pin down when structural and electronic characterization is lacking. In this work, we use X-ray absorption techniques to obtain structural information on a catalytically active disordered molybdenum chalcogenide species that was grown on a wellcontrolled seeded quantum rod photosensitizer system with very high surface area. The high surface area of the colloidal system enables us to employ a variety of X-ray characterization techniques. Yet the system is also well-defined: Amorphous layers of MoS3 are deposited on quantumcontrolled photosensitizers. We take advantage of recent work showing that cadmium chalcogenide nanocrystals can be engineered to systematically control the separation of photogenerated holes and electrons, thus allowing us to modulate the photochemical yield of hydrogen. Nanorods of CdS grown on CdSe seeds with varying diameters and pure CdS nanorods of differing length were synthesized by a seeded-growth method previously reported. These particles have been of interest as a model system for investigating photochemical HER because their [*] Dr. M. L. Tang, D. C. Grauer, Dr. L. Amirav, Prof. J. R. Long, Prof. A. P. Alivisatos Department of Chemistry, University of California, Berkeley Material Sciences, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu jyano@lbl.gov