Surface Chalcogen-based Metalate Clusters for Photocatalysis

One of the main challenges of todays society is the impending energy crisis, which is tightly linked to global warming and ever-increasing greenhouse gas emissions. A strong shift in our political and economic decisions is inevitable, as evidenced by the rapidly growing reliance on renewable energy sources and the corresponding research hype in the interdisciplinary field of materials chemistry. One solution for this issue is the use of sunlight energy and its direct conversion into so-called solar fuels. They can be generated by means of photo-catalysis: a process in which energy of light photons is used to convert stable abundant molecules like water and carbon dioxide into valuable chemicals such as hydrogen, methanol or methane. However, such photocatalytic reactions are highly complex and require a delicate design of the photo-catalyst the substance that is able to absorb light energy and direct it into these chemical transformations. To accomplish this multifaceted task in this project we propose a novel strategy that will complement the advantages of two main fields of photocatalysis. Homogeneous (molecular) photocatalysts are functionally and structurally tunable, allowing for in-depth understanding of reaction mechanisms and kinetics. Heterogeneous photocatalysts (solid-state materials) feature high oxidation-, thermal- and photostability. By combining the two material classes, new synergistic functions can be expected with the potential to take current photocatalytic systems to the next level. The strategy of the project is to wire all-inorganic molecular chalcogen-based metalate (MCM) catalysts, namely, polyoxometalates and polythiometalates, onto the surface (SURF) of functional visible-light active substrates, in order to create new hybrid (SURF-MCM) structures for bridging homogeneous and heterogeneous photocatalysis. We will realize this concept by (a) developing new attachment protocols for various clusters on functional semiconducting substrates, (b) conducting fundamental studies to evaluate how the attachment affects stability, structure and electronic properties of both components, and (c) exploring the potential of these new photo-catalysts towards water splitting and carbon dioxide-to-fuel conversion. This highly innovative project is located at the intersection of materials chemistry, photophysics and catalysis and well embedded within a close network of international collaborations. Our group at TU Vienna has extensive expertise in materials chemistry and 10 years hands-on experience in heterogeneous photocatalysis with the primary focus on fundamental aspects such as interface engineering, design of hetero-structures as well as development and understanding of co-catalysts. Our collaborators Prof. C. Streb from University of Ulm and Prof. Rompel from University of Vienna are world-renowned experts in the field of molecular (photo)catalysis and MCM cluster design and synthesis. We foresee that the scientific outcome of this project will advance the fields of photocatalysis and artificial photosynthesis as well as contribute to the development of other energy and environmental related applications such as electrocatalysis and sensing.

Photocatalysis is a process that utilizes the energy of sunlight to facilitate chemical reactions. In this way, photocatalysis can be used to split water (H2O) molecules into molecular hydrogen (H2) and oxygen (O2). Hydrogen generated in this way can be seen as a truly green and sustainable energy carrier. Typical photocatalysts - materials that allow this light-driven reaction - are solid-state nanoparticles and have poorly defined structures and surfaces. While their advantages include high oxidation-, thermal- and photostability, their undefined nature limits the degree of our catalytic understanding, so it becomes extremely challenging to design photocatalysts with high activity and selectivity. The original idea of this project was to introduce and develop a new approach toward designing photocatalysts by combining typical solid-state photocatalysts with structurally well-defined molecular clusters. Over the course of four years, we could demonstrate the feasibility of this approach by using two families of such clusters: thiometalates (metal-sulfur clusters) and polyoxometalates (metal-oxygen clusters). We have provided several comprehensive examples of (1) how these clusters can be anchored (supported) onto the surface of typical solid-state photocatalysts, (2) how this anchoring can be optimized, and (3) how the structure of these molecular clusters can change upon deposition. Besides our detailed research publications, we have provided the community with two comprehensive review papers able to draw the attention of other research fields to our strategy. Several of our hybrid photosystems have also been shown to be very active toward photocatalytic water splitting, which gives new directions for their implementation and development. We are convinced that these scientific outcomes will also contribute to the development of other energy- and environment-related applications, such as electrocatalysis and sensing.