Cobalt-doped zinc oxide thin films as model Fischer-Tropsch nano-catalysts grown by pulsed electron beam ablation

Abstract

The production of materials in thin film form with unique properties is of growing scientific and technological interest. Zinc oxide is a low cost, and environmentally benign wide band gap semiconductor, which makes it an excellent supporting material for nanoparticles with a plethora of potential applications. Upon doping with Co and other transition metals, ZnO exhibits room temperature ferromagnetic properties with enhanced performance and new functionalities when used in thin film devices. Zinc oxide-supported cobalt nano-composites are promising materials with desirable catalytic properties making it an interesting material for use as an efficient nano-catalyst in many important reactive processes such as Fischer-Tropsch synthesis (FTS), photocatalysis, hydrogen production and steam reforming. Pulsed electron beam ablation (PEBA) has recently emerged as a potential technique for the fabrication of superior quality thin films. The production of well controlled nano-sized particulates is a characteristic feature of PEBA, which has a strong bearing on the surface morphology of the deposited films. In the current work, the potential of PEBA in the deposition of Co-doped ZnO thin films has been assessed and the critical process conditions that affect the growth of the thin films on different substrates have been thoroughly investigated. The main objective of the current work is to deposit Co-doped ZnO thin films via PEBA, and assess the potential of the deposited films as model nano-structured catalysts for the synthesis of green liquid fuels from syngas. PEBA has several advantages including modest requirements for vacuum, control of film thickness, easy set-up, low capital cost, reduced operation and maintenance costs, small footprint, enhanced efficiency, and relative safety (no toxic gases as in pulsed laser ablation or potential noxious by-products as in solvo-thermal routes) over other film preparation techniques. In this project, Co:ZnO thin films have been synthesized from a single target on various substrates. Numerous process parameters have been assessed such as substrate material, deposition temperature, electron beam voltage, beam pulse frequency. Targets of varying cobalt loads viz., 5 w%, 10 w%, and 20 w% have been investigated as well. The effects of pre and post annealing on the physico-chemical properties of the thin films have also been studied. The deposited films have been characterized using complementary analytical techniques such as xray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), visible reflectance spectroscopy (VRS), and atomic force microscopy (AFM). Such comprehensive analyses have helped in assessing the quality of the films and in guiding the experimental strategy in the quest to find the optimal process conditions. Finally, the films have been evaluated for their potential as model nano-catalysts for FischerTropsch synthesis in a 3-phase continuously-stirred tank slurry reactor (3-φ-CSTSR) using a Robinson-Mahoney stationary basket (RMSB). The results have been described in terms of activity and selectivity of the thin film nano-catalysts.