Discovery of New Multi-phase Photocatalysts

Photocatalysts are solid materials capable of using light to initiate chemical reactions on their surfaces. They are currently used as self cleaning coatings in a wide range of commercial products (such as self cleaning windows, self cleaning fabrics) and to eliminate pollution in wastewater or the air. Globally the photocatalysis industry is predicted to grow to a value of US$1.7 billion by 2014, according to BCC Research. At present, effective photocatalysts can use only ultraviolet (UV) light. Far greater efficiency might be obtained, and new applications opened up, if a material could be found that works in visible light, as it is much more naturally abundant on Earth. One important future application is the use of sunlight by photocatalysts to split water, forming hydrogen; this has the possibility to contribute strongly to a renewable energy economy.

The proposed project will study new directions in photocatalytic material discovery, with the aim of finding new visible light active materials. The approach can be divided into two strands, each of which addresses a key problem in current research in this area:

Firstly, in this project epitaxial thin films will be used as vehicles for photocatalytic material discovery. The aim is to address the widespread use of poorly defined samples, such as nanopowders with inderterminable phase composition, dopant distribution, surface morphology and other properties that each contribute strongly to the catalytic properties of the material. In contrast epitaxial thin films act as model samples having well defined orientation, composition, surfaces and interfaces which make full characterisation and discovery of meaningful structure-function relationships possible. A variety of new techniques will be developed to study photocatalytic materials in epitaxial form.

Secondly, biomimetic Z scheme systems will be investigated. These use the same principle as biological photosynthesis, where two photosystems are coupled together to perform an overall reaction. In the artificial Z schemes studied here, two artificial photocatalyst materials will be coupled together in the solid state across a heterojunction, using a variety of different routes to synthesise the nanocomposite materials. The key advantage of a Z scheme is that it allows the energy of two photons to be combined. Therefore two low energy (visible light) photons can be used in place of one high energy (ultraviolet) photon to perform photocatalysis. Since visible light is much more abundant than UV light on Earth, this would mean a significant increase in catalyst efficacy.

Taken together, these two features represent a significantly novel approach to photocatlaysis, which aims to overcome long standing problems in the field, and generate reliable, well founded data as the basis for truly rational catalyst material design.

Who will benefit from the research and how will they benefit?

Photocatalysis has both current and potential future uses in a wide range of technologies. Improved photocatalyst materials, which is the objective of this project, will benefit producers and consumers of existing applications in the short to medium term. Longer term, several new applications may emerge, enabled by the improved materials discovered here. Each of these areas of impact are described in the following sections.

Short/medium term

Private sector companies using current photocatalysis technology, such as those producing self cleaning coatings, anitmicrobial coatings, wastewater treatments, and air pollution treatments, will be able to benefit from the new materials produced by the project. Globally this economic activity is predicted to reach US$1.7 billion by 2014. Current commercial photocatalysis technology is based on TiO2, which only utilises UV light. This is ideal for some applications where visible light transparency is needed (e.g. the window coatings developed by Pilkington Glass). However, in most cases visible transparency is not a requirement, and far greater photocatalytic reaction rates might be achieved if more abundant visible light was used. Development of such catalysts would enhance the functionality of existing semiconductor photocatalyst based technology, for example increasing degradation rate of organic molecules in wastewater, meaning less residence time is required in the reactor and increasing throughput, or allowing sunlight rather than UV lamps to be used to activate self cleaning properties of textiles or surface coatings. In addition, new applications of photocatalysts will become feasible if visible light activity is achieved. These may include antimicrobial coatings in healthcare environments which are activated by interior lighting. End users of these potential new technologies include hospitals and healthcare providers, state or municipal authorities who may employ air purification technology in cities, private companies who may use the technology to comply with increasing stringency in pollution/emission regulations. Internationally, the manufacturing industry in rapidly growing economies has a need for effective, in situ, low cost wastewater treatment to prevent significant environmental damage occurring during rapid growth of the sector. The materials produced in this project have the potential for significant impact in these countries.

Medium/long term

Photocatalysts may be used to split water forming H2, or reduce CO2 to form organic molecules. These are ways to convert sunlight into renewable chemical fuels (known as solar fuels), and may have a transformative impact on the energy economy, enabling the necessary shift away from a fossil fuel economy towards renewable sources. The UK Government has committed to achieving 15 % of our energy from renewable sources by 2020. To meet this aim a variety of methods will be required, with solar energy capture likely to be a major component. Private or state energy production companies may be major users, and successful development of this technology will have wide impacts on energy and environmental policy. Further ahead, solar fuels may act as the primary energy source in a future economy, or as a secondary source for specific applications, such as those where a battery is unsuitable. Potential beneficiaries include the wider public, who will benefit from clean, renewable energy, reduction in greenhouse emissions, and increased energy security.