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Charge transport in nanoparticle-based materials underlies many emerging energy-conversion technologies, yet assessing the impact of nanometre-scale structure on charge transport across micrometre-scale distances remains a challenge. Here we develop an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe₂O₃ electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometre resolution across micrometre-scale distances, we have identified the existence of champion nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the present electrodes. Indeed, when electrodes are fabricated with a high proportion of these champion nanostructures, the electrodes achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water-splitting under 100 mW cm(-2) air mass 1.5 global sunlight.

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Even after a decade or so of research, the direct synthesis of highly crystalline mesoporous transition-metal oxides that are thermally stable and well ordered still constitutes a major challenge. Although various soft- and hard-templating approaches have been developed in the past, they usually suffer from multiple, tedious steps and often result in poor structure control. For many applications including power generation and energy conversion, however, high crystallinity and controlled mesoporosity are a prerequisite. To this end, here we report on an approach established for group-IV (titanium) and group-V (niobium) oxides, with potential applications to photovoltaic cells and fuel cells, respectively, which overcomes previous limitations. It gives direct access to the desired materials in a 'one-pot' synthesis using block copolymers with an sp2-hybridized carbon-containing hydrophobic block as structure-directing agents which converts to a sturdy, amorphous carbon material under appropriate heating conditions. This in situ carbon is sufficient to act as a rigid support keeping the pores of the oxides intact while crystallizing at temperatures as high as 1,000 degrees C.

Although silica nanoparticles are used as building blocks in nature and synthetic mesostructures, the influence of nanoparticle characteristics on the assembly and disassembly of mesostructured silica has not been investigated. We demonstrate that nanoparticle size and size distribution allow us to control the assembly of silica-type mesostructures and that, because of the discrete nature of nanoparticles, we can disassemble these mesostructures into a rich variety of structural building units. When assembling mesostructures, nanoparticles undergo size-dependent segregation once the nanoparticle diameter exceeds a critical size threshold, which is approximated by the root-mean-square end-to-end distance of the hydrophilic block of the block copolymer. Using this phenomenon, we direct gold-silica core-shell nanoparticles into the segregated regions of silica-type mesostructures, demonstrating the ability to precisely place nanoparticles and create compositionally heterogeneous, functional mesostructures. We further show that, because the mesostructures are composed of nanoparticles, they can be disassembled into nanotubes, hexapods and other complex, well-defined structural units, thereby introducing the concept of retrosynthesis to materials chemistry. Our results demonstrate how nanoparticle characteristics influence the structure and properties of nanoparticle-derived mesostructures. Size-dependent segregation and disassembly should improve structure control at the near-molecular level and should be applicable to a wide range of nanoparticle-derived mesostructures.