Doped-Up: Bio-Inspired Assembly of Single Crystal Nanocomposites

The ability to tune the physical properties of materials is extremely attractive. All too often, the performance of a material is a compromise between two important properties such as high transparency and high conductivity or low thermal conductivity and high electrical conductivity. The obvious solution to this problem is to combine materials to generate composite structures. However, the creation of a new hybrid material by simply mixing materials with complementary properties rarely results in a net advantage. The key is to exert control over the assembly of the component materials over multiple length scales.

The goal of this project is to develop a robust and general methodology for the synthesis of a unique class of functional nanocomposites - single crystals containing a uniform distribution of inorganic nanoparticles. Our approach takes its inspiration from biominerals, such as bones, teeth and seashells, where these are invariably inorganic/ organic composites with hierarchical structures. Indeed, even single crystal biominerals are composites in which organic molecules are embedded within the crystal lattice. Nature therefore demonstrates that although crystallisation is a common means of purification, it is entirely possible to occlude additives within a crystal lattice given the appropriate pairing of the crystal and additive. Using the biologically-important mineral calcite (calcium carbonate) as a test system, we have made the exiting discovery that this biogenic strategy can be translated to synthetic systems to achieve efficient nanoparticle occlusion in single crystals.

We now wish to build on these preliminary results to develop our bio-inspired crystallisation strategy - in which copolymer-stabilised nanoparticles are used as simple crystal growth additives - for the synthesis of functional nanoparticle/ single crystal nanocomposites. This strategy delivers a number of key features. We are creating nanocomposites in which the nanoparticles are embedded within a single crystal, rather than the typical amorphous or polycrystalline matrix, and the nanoparticles are not aggregated. This provides a unique structure where the absence of grain boundaries is expected to enhance many physical properties. It is experimentally straightforward and amenable to scale-up, and we can easily produce sufficient material to determine structure/property relationships. We also benefit from the vast knowledge that is available concerning the crystallisation of traditional ionic compounds to control the size, shape and porosity of the nanocomposites.

Judicious design of the copolymer will provide control over the structures of the nanocomposites at the nano- and meso- length scales, and we will establish a tool-kit for controlling the nanoparticle loading, the inter-particle separations and the interfaces between the nanoparticles and the crystal host. As a suitable test-system we will focus on functional metal oxides containing noble metal nanoparticles/ quantum dots and study their transport and photocatalytic properties. Particular emphasis will be placed on evaluating the structure/property relationships, where the absence of grain boundaries and our ability to tune the structures of our materials is expected to provide us with unique information about their material properties. Our synthetic method is quite general however, and it is envisaged that it can be used as a platform for creating a broad spectrum of materials including capacitors, batteries, thermoelectrics and electrochromics.

Finally, while significant efforts have been made to identify the strategies by which organisms control crystallisation, these have seldom been applied to functional materials. This project will demonstrate the feasibility and potential of this approach, and will hopefully inspire other researchers to use bio-inspired crystallisation strategies to control the structure and properties of advanced materials.

This project will make an impact on UK industry, society and the economy in the principle areas of nanomaterials, composites, crystallisation and polymer science. Our overall goal is to develop a novel strategy for the formation of functional inorganic nanocomposites based on the polymer-directed occlusion of nanoparticles in single crystals. The work is undoubtedly fundamental science and highly adventurous. We therefore have no expectation of generating a product suitable for translation to industry within the 3 year time-frame of the project. However, the delivery of a new, flexible and scalable strategy for the creation of inorganic nanocomposites will undoubtedly have an impact on companies such as Toyota, Pilkington Glass and Johnson Matthey that generate advanced materials with eg. interesting optical, magnetic, conductivity, catalytic, thermoelectric and electrochromic properties.
As a suitable starting-point for our research the project will explore the formation of nanocomposites with photocatalytic properties, where these are relevant to companies such as BASF, CatalySystems, SA Evitech and KRONOS. Photocatalysis is a challenge of both national and global importance, driven by the need to reduce the consumption of fossil fuels and develop alternative energy sources with much smaller carbon footprints. Photocatalysts are also commonly employed for decontamination of water (e.g. harmful organic solutes), wastewate treatment, air treatment systems and in self-cleaning surfaces. However, the majority of photocatalysists on the market are based on UV light. The ability to extend further into the visible and improve quantum efficiency will be explored in our research.
Our research programme also relies upon cutting-edge synthetic polymer chemistry and the development of methods to control crystallisation processes. Both of these topics are hugely important to UK industry. There are many UK-based companies working in polymer manufacturing, including AkzoNobel, Lubrizol, Scott Bader, BP, and Ashland, and Armes has worked closely with 13 companies over the last 10 years. Crystallisation is fundamental to a wide range of technological processes, including the production of pharmaceuticals, foodstuffs and personal care products, as well as the synthesis of nanomaterials. It is also central to various natural phenomena, such as the formation of bones and teeth, the precipitation of ice in the atmosphere and the prevention of scale.
The relevance of our work to industry is also demonstrated through our industrial contacts. Armes is currently working with BASF, Lubrizol, P & G, AkzoNobel, Scott Bader, Ashland and GEO Specialty Chemicals, and sold a U. Sheffield patent application to DSM in 2007. Meldrum is/has worked with Unilever, Nexia Solutions, Imerys and P & G, while Critchley has worked with AstraZeneca, AF ChemPharm, Paraytech and Sekio-EPSON. He is also an inventor of a photochemically tunable surface system and also invented a method of monitoring UV degradation using photoelectron spectroscopy (see patents).

Immediate impact will be achieved through the training of early-stage researchers in synthetic polymer chemistry (Sheffield PDRA) and crystallisation/nanoscience (Leeds PDRA) for future careers in either academia or industry. The interdisciplinary nature of the project will be particularly effective in helping these two researchers to develop a flexible approach to solving problems and to working within a close-knit team. This project also lends itself to impact through public engagement and outreach. Leeds and Sheffield each have well-established outreach programmes and we will participate in a range of activities including the annual "Festivals of Science" where local schools take part in educational workshops related to science and engineering. Meldrum's group also has an "outreach" team, which is active in organising events, while Critchley's group contributes to the annual Leeds Light festival.