Templated electrodeposition of nanoscale materials

Abstract

Templated electrodeposition is an economic and efficient approach to fabricate nanostructures. Published work generally uses commercial membranes such as track-etched polycarbonate (PC) and porous anodic aluminium oxide (AAO) membranes with one-dimensional pore arrays as the hard templates for electrodeposition of nanoparticles and nanowires, which usually have diameter larger than 50 nm. In this thesis, mesoporous silica films with a three-dimensional accessible mesostructure are explored as the templates for electrodeposition of nanostructured gold, bismuth, tellurium and bismuth-tellurium compounds. The diameter of the mesopores is around 8 nm. Firstly, well-ordered mesoporous silica thin films with different pore structures were produced by using block copolymers F127 or Brij S10 as structure-directing agents through the evaporation induced self-assembly (EISA) approach. By adjusting surfactant concentration, the conditions of coating, aging and calcination process, mesoporous silica films with P6mm, Im 3 m and Fmmm, P6₃/mmc mesostructures were obtained. GISAXS was the main tool to characterise the mesostructure. Mesoporous silica films with Fmmm structure were chosen to be the hard templates for electrodeposition due to their well-ordered and three-dimensional accessible pore channels. FE-SEM showed the pore diameter of the templates was around 8 nm. The wall thickness was 5-8 nm and thickness of the film was 120-200 nm. The preparation of mesoporous silica films was optimised by fixing cracking and delamination problems. The diffusion of species in aqueous and DCM solvent in the mesochannels was investigated. Templated electrodeposition of gold was conducted from aqueous electrolyte solutions. Using the mesoporous silica films as the templates, two-step pulsed electrodeposition was conducted to obtain good gold nanostructure fillings of the mesopores. The effect of deposition conditions on the morphologies of deposited gold was investigated. FE-SEM showed that gold grew in the mesopores of the films, in any cracks in the films, and in any gap that may be present between films and substrates. To observe the deposited gold better, hydrofluoric acid vapour was used to etch away mesoporous silica films, and more gold particles were observed with diameters ranging from 20 to 80 nm. Using the same templates, electrodeposition of bismuth, tellurium and bismuth telluride were investigated with electrolytes containing [Nⁿ Bu₄][BiCl₄], [Nⁿ Bu₄][TiCl₆] and [Nⁿ Bu₄]Cl dissolved in non-aqueous dichloromethane solution. In situ GISANS was conducted to explore the process of templated bismuth electrodeposition and it indicated the electrodeposition of bismuth in the mesopores. FE-SEM showed there were a few Te nanowires even though most of the mesopores were not filled. This is a promising observation as it proves the possibility to prepare sub-10 nm nanowires by templated electrodeposition. A wide range of pulsed electrodeposition conditions were applied for bismuthtellurium electrodeposition. Most of the obtained particles were tellurium-rich and they showed trigonal tellurium structure. Apart from pulsed electrodeposition conditions, a variety of parameters including bismuth ratio (2.25 mM and 3 mM), deposition without templates and deposition duration were also investigated in pursuit of control over the composition of the deposit. Constant electrodeposition time is the essential parameter to influence the composition of the deposit. All samples from pulsed electrodeposition (0.3-5 s) show very low Bi:Te ratio (Bi/Te<0.1), whereas a sample electrodeposited for 5 min at -0.6 V achieved high Bi ratio (Bi/Te=0.8).

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ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

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