Stacking disorder as the critical tuning parameter for achieving high-performance applications
Lead Research Organisation:
University College London
Department Name: Chemistry
Abstract
Very recent world-leading work on stacking disorder in ice and diamond1,2 places the CGS group in a unique position to now roll-out the know-how and knowledge into the wider materials arena. Guided by computational predictions, the student will prepare a wide range of stacking-disordered materials including AB materials, such as AgI, ZnS, GaN and SiC, small-molecule organic materials as well as metal-organic framework materials. Being able to control the stacking disorder in those materials will enable us to fine-tune their chemical and physical properties. Examples include the electronic band gaps, semiconductor properties, phosphorescence, solubility, hardness as well as (photo)catalytic and gas-storage properties.
Organisations
| Description | Silver iodide (AgI) is used for a wide range of applications from photocatalysis and antimicrobial coatings to photography and ice nucleation. By fitting powder X-ray diffraction patterns with MCDIFFaX, this project has shown that AgI displays a strong tendency to form stacking-disordered materials. The polytypism in AgI is determined by the silver cation to iodide molar ratio upon precipitation. Under iodide-rich conditions, fully hexagonal ß-AgI is obtained, whereas a maximal percentage of cubic stacking of 81% is obtained at a 2:1 molar ratio in the silver cation-rich system. These findings are explained based on a concentration-dependent competition between kinetically and thermodynamically favoured adsorption processes. The previously reported memory effects, observed upon transforming hexagonal and cubic AgI to the high-temperature superionic phase and back, are now followed quantitatively. An explanation for these findings is that the memory effects originate from excess ions at the surfaces of AgI crystals that stabilise the pyroelectricity of AgI associated with hexagonal stacking. The ability to "design" the polytypism of AgI by tuning the precipitation conditions provides the first example where the stacking disorder of a material can be controlled in a continuous fashion. (see paper: https://pubs.acs.org/doi/10.1021/acs.cgd.8b01715) Future studies are currently in process to clarify if this design principle can be applied to other materials and one such example to date has been copper iodide (CuI). The same design principles used for AgI (excesses of the cations and anions) did not result in different amounts of stacking disorder, instead fully cubic CuI was produced in all cases. Following this, heating experiments and measurements on the GEM beamline at ISIS have also been carried out. Studies have also been conducted on the AgI-CuI solid solutions, investigating the impact of different ratios of Ag and Cu as well as an excess of I. XPS and XRD has been conducted on all samples. Ball milling experiments on AgI, CuI and solid solutions of AgI-CuI have also taken place to investigate the effect that milling time has on the cubicities of the various samples. During the COVID-19 period, analysis has been conducted on any remaining data from the last period as well as new adaptations to MCDIFFaX to help fit a range of materials as well as developing the program further to include third order memory effects. International collaboration with multiple groups has also taken place investigating impact diamonds from the Popigai crater. These diamond samples have been characterised during this project with a range of techniques and the MCDIFFaX approach for analysing X-ray diffraction data has shown hexagonality indices up to 40%. (see paper: https://www.nature.com/articles/s41598-019-46556-3) A further collaboration on this diamond project has involved simulation of X-ray diffraction patterns of diamond/graphite structures using DIFFaX. The purpose of this collaboration was to investigate if so-called diaphite (diamond/graphite sp2-sp3 bonded) structures exist in these Popigai samples. Simulations were produced and compared to X-ray diffraction data. Results show that features in the X-ray diffraction patterns are consistent with extended graphitic regions and not diaphite effects. So, we conclude that the quantification using X-ray diffraction is challenging in the impact diamond samples that we have studied. (see paper: https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c00556) |
| Exploitation Route | The ability to "design" the polytypism of AgI by tuning the precipitation conditions provides the first example where the stacking disorder of a material can be controlled in a continuous fashion. This principle could be applied to other materials to enable the fine tuning of physical and chemical properties. Examples of these properties include hardness, electronic band gaps, semiconductor properties, solubility, (photo)catalytic and gas-storage properties and phosphorescence. |
| Sectors | Aerospace, Defence and Marine,Chemicals,Construction,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology |
| URL | https://www.nature.com/articles/s41598-019-46556-3 |