title: Challenges in the simulation of large scale, medium exposed, inorganic nanotubes
Creators: name: Poli, Emiliano
Publisher: name: University of Liverpool Repository
Year: 2016

description: Prompted by growing environmental and energy concerns, increasing research is dedicated to the development of technologies capable of sustainably converting sunlight into commercially viable forms of energy such as chemical fuels. Crucial to this energy conversion is the availability of materials, photo-catalysts (PCs), capable of promoting redox chemistry following absorption of light. Since the macroscopic efficiency of PCs is governed by the atomistic details of the PCs interfaces with media and reactants, increasingly large efforts have been dedicated to the characterization and understanding of these interfaces, also by mean of first principle simulations. In this context, we present a theoretical investigation of a rapidly growing class of onedimensional nanomaterial, Imogolite nanotubes (Imo NTs), whose potential for photocatalysis has so far been overlooked. The first Chapter of the Thesis provides an overview of the current state of the art for research in PCs, and the theoretical framework needed for atomic-scale understanding and informed development of PCs. The computational underpinning of the research carried out for this Thesis, namely Density Functional Theory (DFT) and its linear-scaling (LS) implementation in the ONETEP program is presented and discussed in Chapter 2. Ahead of presentation of the results of my original research, a small literature review on Imo NTs is provided in Chapter 3. Chapter 4 illustrates the applicability of LS-DFT to Imo NTs and, by mean of detailed benchmarking, sets best practice for simulations of these systems. The potential of Imo NTs as (co-)PCs is explored in Chapter 5, where an extensive study of the structure, wall-polarization, absolute band-alignment, band-separation, and optical properties of several Imo NTs is presented and discussed. The simulations suggest possible profitable use of Imo NTs for both photo-reduction and hole-scavenging purposes. The occurrence of (near-)UV charge-transfer excitations is also observed, which may be effective for electron-hole separation and enhanced photo-catalytic performances. Finally, the effects of the NTs’ wall-polarization on the absolute alignment of electron and hole acceptor states of interacting water (H2O) molecules are quantified and discussed. Chapter 6 reports an extensive study of defects in Imo NTs. Electronic and optical characterization of the defective Imo NTs suggests energetically favourable separation of photo-generated electrons and holes via relaxation to different defect-sites, with the ensuing possibility of defect-centred photo-redox activity in defective Imo NTs. The Thesis ends with the investigation of termination effects in Imo NTs. Chapter 7 presents results on the structural, electronic and optical characterization of representative finite Imo NTs models capable of simultaneous description of the NT-ends and bulk-like NT-core. The simulations reveal the presence of longitudinal band-bending and of NT core-end bandsseparations, which in turn suggests advantageous relaxation mechanisms for photo-generated e*-h pairs along the NT axis, to the potential benefit of Imo NTs photo-catalytic reactivity.