Novel inverse opal based nanostructures using atomic layer deposition for photoelectrochemical cell applications

Abstract

Three-dimensional (3D) inverse opal (IO) photonic crystals are potential candidate materials for nanostructured photoanode to realize highly efficient photoelectrochemical (PEC) solar energy harvesting. IO nanostructure possesses several beneficial characteristics for light harvesting such as direct electron transport paths for longer electron diffusion lengths, highly percolated pore structure for close interfacial contacts with the electrolyte, light trapping to improve photon absorptions and high surface area for greater photosensitizer loading. This thesis focuses on the design and fabrication of IO based nanostructures using atomic layer deposition (ALD) and investigation of PEC hydrogen generation using the IO nanostructures as photoanode of a PEC cell. Manly, detailed studies are performed to evaluate the optical and PEC performances of the IO based nanostructures and establish the key parameters which enable efficient PEC solar energy harvesting. TiO2 IO nanostructures are developed by controlled infiltrations of self-assembled polystyrene opal templates using ALD method. ALD infiltration kinetics is investigated using stop-flow process in comparison with conventional continuous flow (pulse-purge) process. The deposition in opal templates is found to be limited by the Knudsen flow of precursor gases into the nanostructure templates. Stop-flow process is shown to enable the controlled filling of high aspect ratio opal templates with filling fractions close to theoretical maximum and deposition cycle time one order magnitude lower than continuous-flow process. As fabricated TiO2 IO samples using stop-flow ALD are photosensitized with CdS quantum dots and applied as a photoanode for visible light driven PEC hydrogen generation. It is observed that TiO2 IO photoanodes possess fast photoresponse and efficient charge transfer properties. Photocurrent density and IPCE maxima are found to increase with decreasing pore size due to the increase in quantum dots loading. Bilayer photoanodes based on optically and electrically coupled ZnO nanowire array with TiO2 IO are developed using ALD to improve photon absorptions and light harvesting efficiency through photonic crystal light manipulations. Results have revealed that the bilayer nanoarchitectures contribute to the improved light harvesting through Bragg diffractions and slow light. Nevertheless, surface area shortage is observed to be the main limitation for PEC performance of these photoanodes. In order to address the surface area requirement, we have designed 3D ordered nanobushes by hierarchically integrating dense networks of ZnO nanowires with TiO2 IO. Tuning the diameter of the IO shells is shown to promote high diffuse scattering. A comparative PEC performance investigation has demonstrated the promise of the nanobushes photoanode for highly efficient PEC solar energy harvesting as supported by greatly improved specific surface area and strong light scattering.