Simon R. Hall

The evolution of 'sol-gel' chemistry as a technique for materials synthesis

From its initial use to describe hydrolysis and condensation processes, the term ‘sol–gel’ is now used for a diverse range of chemistries. In fact, it is perhaps better defined more broadly as covering the synthesis of solid materials such as metal oxides from solution-state precursors. These can include metal alkoxides that crosslink to form metal–oxane gels, but also metal ion–chelate complexes or organic polymer gels containing metal species. What is important across all of these examples is how the choice of precursor can have a significant impact on the structure and composition of the solid product. In this review, we will attempt to classify different types of sol–gel precursor and how these can influence a sol–gel process, from self-assembly and ordering in the initial solution, to phase separation during the gelation process and finally to crystallographic transformations at high temperature.

Morphosynthesis of complex inorganic forms using pollen grain templates

Porous micron-sized particles of silica, calcium carbonate or calcium phosphate are prepared with complex morphologies by template-directed synthesis employing intact pollen grains; the materials adsorb and release drug molecules and can be functionalized with metallic or magnetic nanoparticles.

Transparent thin films and monoliths prepared from dye-functionalized ordered silica mesostructures

Micelle templating and co-condensation of tetraethoxysilane and 3-(2,4-dinitrophenylamino)propyl-triethoxysilane in the presence of hexadecyltrimethyl-ammonium bromide has been used to synthesize transparent yellow thin films and monoliths of organic dye-functionalized MCM-41 silica: these hierarchical materials consist of covalently linked inorganic–organic networks with mesoscale periodicity and macroscale organization.

Biopolymer-mediated synthesis of anisotropic piezoelectric nanorods

Alginate biopolymer was used to control the crystal growth of the lead-free piezoelectric material langasite. The piezo-response of these nanorods was demonstrated to be equivalent to that of ZnO. This is the first time that templated growth of langasite and its associate phases has been demonstrated.

Biotemplated syntheses of anisotropic nanoparticles

The technological advances predicted (or, perhaps, demanded) for the twenty-first century are intimately linked to the crystallochemically controlled synthesis of high-performance functional materials. To answer the new hendiatris of ‘smaller, faster, better’, the manufacture of these materials as nanoparticles has become a scientific noblesse oblige. Direct incorporation into the next generation of electronic devices will necessitate anisotropic forms of these materials, be they nanowires, nanotapes or nanotubes. Chemists have recently discovered that, in addition to the classical methods of anisotropic growth, new routes allow more complex materials to be synthesized in these morphologies. This review describes, using a series of examples, how the morphology of functional materials can be controlled using templated growth mediated by a biopolymer. By involving a biopolymer in the synthetic protocol, anisotropic nanoparticles and assemblages of even quite complex materials can be generated in syntheses that are simple, elegant and highly specific.

Fabrication of CaCO3–biopolymer thin films using supercritical carbon dioxide

The preparation of useful products based on the integration of carbon dioxide fixation and biomass utilization is important for the future development of environmentally harmonized materials technologies. Here we report the use of supercritical carbon dioxide and spin-coated cellulose/chitosan matrices to produce biopolymer films infiltrated with densely packed calcium carbonate (vaterite) particles. Composite films of high uniformity are prepared specifically in the presence of low concentrations of polyacrylic acid.

Template-directed synthesis of bi-functionalized organo-MCM-41 and phenyl-MCM-48 silica mesophases

Template-directed co-condensation was used to synthesize organo-functionalized MCM-41 silica hexagonal mesophases containing binary combinations of covalently linked phenyl and amino, thiol or allyl moieties; similar methods were used to prepare a phenyl-functionalized organo-MCM-48 silica with cubic structure.

A New General Synthetic Strategy for Phase‐Pure Complex Functional Materials

The ability of ionic liquids to solvate inorganic salts completely has to date never been employed in the synthesis of complex inorganic materials. Here, we demonstrate that complex functional oxides, even those traditionally considered extremely difficult to synthesize in bulk, such as quinternary superconductors, are produced with no impurity phases and on timescales that are much shorter than other synthetic techniques.

In Situ TEM Observation of a Microcrucible Mechanism of Nanowire Growth

Nanowire Growth Observed In the hypothetical microcrucible growth mechanism for nanowires, a molten catalytic particle located in a pore on a substrate continually feeds the outward growth of the wire. To observe such a mechanism requires the ability to examine nanowire growth in situ. Boston et al. (p. 623) studied various stages of Y2BaCuO5 nanowire growth using transmission electron microscopy and were able to observe a microcrucible growth mechanism directly. Nanowire growth from a microcrucible catalyst particle at the bottom of the wire is observed in situ. The growth of metal oxide nanowires can proceed via a number of mechanisms such as screw dislocation, vapor-liquid-solid process, or seeded growth. Transmission electron microscopy (TEM) can resolve nanowires but invariably lacks the facility for direct observation of how nanowires form. We used a transmission electron microscope equipped with an in situ heating stage to follow the growth of quaternary metal oxide nanowires. Video-rate imaging revealed barium carbonate nanoparticles diffusing through a porous matrix containing copper and yttrium oxides to subsequently act as catalytic sites for the outgrowth of Y2BaCuO5 nanowires on reaching the surface. The results suggest that sites on the rough surface of the porous matrix act as microcrucibles and thus provide insights into the mechanisms that drive metal oxide nanowire growth at high temperatures.

Alginate-mediated routes to the selective synthesis of complex metal oxide nanostructures

The exceptional electronic, magnetic, optical and catalytic properties demonstrated by many ceramic materials when confined to the nanoscale are well established. However, the synthesis of multicomponent metal oxide nanowires and nanoparticles is notoriously problematic due to the difficulty of controlling homogeneity and achieving the correct stoichiometry. In this paper, we demonstrate a selective route to nanowires or nanoparticles of a quaternary metal oxide product using sodium or ammonium alginate respectively. By pre-organizing metal cations within an alginate gel the nucleation and growth of precursor crystalline phases can be constrained to the nanoscale. On further calcination the alginate decomposition products prevent sintering of these precursor nanoparticles prior to conversion to the final product. The cooperative effect of polymer microstructure and decomposition products allows an exceptional level of control over nucleation, growth and transport of the intermediate phases and subsequently on the particle size and morphology of the final product.