Synthesis and study of porous and photocatalytic metal oxide foams for environmental remediation
: (Alternative Format Thesis)

Student thesis: Doctoral ThesisPhD


Current wastewater treatment methods are not able to effectively remove organic  micropollutants, present in low concentrations. This group of compounds includes a wide range of biologically active substances from a range of human  sources, including pharmaceuticals, pesticides, drugs, and caffeine that are found in concentrations of ng L-1 to µg L-1 in domestic wastewater. Photocatalysis is an advanced oxidation technique that can be applied as a tertiary treatment step after conventional treatment, utilising semiconductor materials activated by UV irradiation to degrade these compounds. Currently, most examples of photocatalysis focus on either nanoparticles slurries or immobilised catalyst  systems. Slurry systems require posttreatment downstream removal, while immobilised systems have lower surface areas and show lower activities than slurry systems. Recently, more interest has been paid to immobilising photocatalysts onto porous supports, however a remaining issue with this  approach is the potential leaching of catalyst and loss of nanoparticles from the support surface due to weak surface-catalyst interactions. Focusing on ZnO, the  aim of this thesis was to develop a novel method to produce porous structures made entirely of ZnO, thus removing the surface-catalyst interactions and removing the need for downstream removal, while combining the high surface  areas of a slurry system.

Novel foam-like structures were produced via direct air incorporation into a sol- gel reaction of zinc acetylacetonate and oxalic acid leading to the formation of macroscale zinc oxalate foams. These proto foams were then sintered to form ZnO foams, consisting of a continuous structure, free from discrete crystals or  particles. The resulting structures were termed molecular foams or MolFoams.

Control of the pore size was found to be possible by tuning the concentration of surfactant present in the gel, resulting in foams of smaller pore sizes and unique crystal morphologies, both of which proved to be beneficial to the photocatalytic activity of the foams. To analyse the photocatalytic activity of the foams, a bespoke recirculating batch reactor was developed to utilise the foams while making best use of the porous, hierarchical structure, consisting of a range of interconnected pores of different sizes. These foams were able to surpass the performance of slurry and supported photocatalyst systems for the removal of carbamazepine, with reaction rate constant of 5.43 x10-3 min-1 along with high electrical energy efficiencies and quantum efficiencies. 

ZnO is a widely used photocatalyst but is hindered by its low resistance to photocorrosion, and its large band gap means that it is unsuitable for solar applications. While research into doping of ZnO exists, it is focused on nanoparticles and films, with the few publications that report negative results failing to provide significant characterisation or interpretation to discern the cause of the reduced photocatalytic activity. The ZnO MolFoam synthetic method was modified for the incorporation of transition metal dopants into the structure to reduce the band gap of the material, such that it could be used under solar or visible light irradiation and to increase the resistance to photocorrosion that hinders wider usage. After initial testing revealed doping metal into the structure to have been successful at significantly reducing the photocorrosion of ZnO, with a 60-85 % reduction in Zn2+ leaching after irradiation but resulted in decreases in photocatalytic activity, focus was shifted towards analytical characterisation of the materials, including x-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy, to identify the cause of this decrease in activity as the shifting of band edge positions.

Also in this thesis is a meta-review of foams used in photocatalysis, which proposes an expansion of the terminology and literature surrounding porous materials, building upon the existing IUPAC terminology, and expanding it to include categorisation of foams and porous objects. This work also describes categorisation of foam materials with definitions based on synthesis method and includes a systematic analysis based on this categorisation, while also providing a semiquantitative comparison of the literature surrounding foams and  photocatalysis, considering the wide range of parameters involved in the publications in this area.

Finally, this work highlights the relevance of using multiple figures of merit in photocatalysis and proposes a short list of common best practices for the field. To conclude, this work has placed a primary focus on the development and adaptation of the MolFoam synthesis, leading to the production of wholly photocatalytic foams of ZnO, demonstrating good photocatalytic activity and  high efficiencies. The pore size of these foams was shown to be a key parameter  for the photocatalytic activity, with smaller pores leading to a higher reaction rate constant due to decreased distance required for pollutant to diffuse to the catalyst surface, and could be controlled using surfactant, CTAB. Doping of these  foams with transition metals was shown to significantly increase the resistance to photocorrosion, with higher dopant concentrations leading to significantly reduced Zn2+ leaching post irradiation, however the doping process was found to lead to shifting of the band edge positions of the material, such that the photocatalyst lacked the ability to produce hydroxyl radicals required to degrade carbamazepine.

The research presented in this thesis has led to three published papers and future work would be to refine the doping process for the ZnO foams, such that the resulting foams do not see the reduction in photocatalytic activity shown by the initial doping process. Additionally, it is of interest to examine the effect of doping of non-metal dopants such as nitrogen, assessing the impact on photocatalytic activity and stability, when compared to metal doped foams. The photocatalytic activity of these foams would also be investigated under visible light irradiation, in comparison to the UV irradiation described herein.
Date of Award29 Mar 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorDavide Mattia (Supervisor) & Jannis Wenk (Supervisor)

Cite this