Synthesis and optimisation of novel photocatalytic thin films for use in photo-assisted water splitting applications

• Main Author: Kundu, S.
• Published: University College London (University of London) 2012
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625981

The results of an investigation into the synthesis and optimisation of a novel photocatalytic thin film for use within a photochemical diode to produce hydrogen by water splitting are reported in this thesis. Initially, thin films of titanium dioxide were synthesised and applied to a range of substrates by a sol-gel dip-coating process. Metal nanoparticles were then introduced into the titanium dioxide films to alter the electronic properties of the photocatalytic thin film, allowing the film to harvest a greater proportion of the energy available in sunlight. The effect of thickness on the rate of photocatalytic dye degradation was compared to the effect of thickness on the photo-assisted evolution of oxygen and hydrogen within sacrificial systems1. It was found that the addition of metal nanoparticles enhanced the rate of both photocatalytic dye degradation and gas evolution using sacrificial solutions. Studies into the effect of varying photocatalytic thin film thickness showed that while the rate of photocatalytic dye degradation increased with increasing film thickness, the rate of evolution of both oxygen and hydrogen occurred within a specific thickness range. This observation was attributed to the fact that the two processes occur via very different mechanisms, the rate of one (dye degradation) relying simple on the amount of photocatalyst available for photoexcitation, and the rate of the other dependent on the correspondence of the thickness of the photochemical diode to the mobility of the photogenerated electron created. Following these initial investigations, various optimisation techniques were tested, such as the application of a noble metal undercoat on the substrate before a photocatalytic thin film is applied on top of this sputtered layer of noble metal. It was determined that the presence of a light layer of sputtered noble metal on the substrate prior to the application of the photocatalytic thin film enhanced the rate of hydrogen and oxygen evolution from sacrificial solutions. Dye degradation tests were not carried out beyond the initial investigative stages, as the aim of the project was ultimately to most efficiently produce hydrogen from water. It had already been established that dye degradation occurs via a very different mechanism to that of gas evolution from water splitting, therefore all further samples were tested for gas evolution alone. Following the investigation into the effect of a sputtered noble metal undercoat, the results of rates of photo-assisted gas evolution of photocatalytic thin films deposited onto substrates with different methods of surface roughening were studied. The rate of photo-assisted gas evolution for photocatalytic thin films deposited onto differently physically modified substrates are compared. Substrates with an increasing surface area are also tested in the same way. It was found that roughening the surface of the substrate prior to the deposition of a thin film enhanced the rates of hydrogen and oxygen evolution, although it was not conclusively proven whether this was due to an increase in the surface area of the substrate and consequent increase in the area of photocatalyst-substrate interface, or whether the enhancement was due to a better contact between substrate and photocatalyst as a result of the removal of any dirt or oxidised substrate material present on the surface of the substrate. Further studies into the effect of combining the effect of the application of a noble metal undercoat layer with the effect of surface roughening of a substrate before the application of a photocatalytic thin film were carried out, in order to determine whether combining two different methods of enhancement will have a multiplied enhancement of the rate of photocatalytic activity. It was found that by combining methods of chemical and physical modification of the substrate prior to the deposition of a photocatalytic thin film, the rate of hydrogen and oxygen evolution achieved was more enhanced than the rates achieved by either of the two previous methods of modification alone. Finally, the idea of extending the reach of the substrate further into the photocatalytic thin film was investigated, and the effect of the incorporation of a conductive network of metal fragments within the photocatalytic thin film on the rate of photo-assisted gas evolution was observed. Fine steel and bronze filings were added to the sols before deposition onto the substrates. While the rate of hydrogen and oxygen evolution was greater than for films with no metal filings present, the enhancement factor was not as great as that observed with previous physical and chemical modification methods.