2D Heterostructured Photocatalytic Materials for Water Treatment and Disinfection

Abstract

It is estimated that nearly 2 billion people commonly drink water that is not safe. There is a real and urgent need to develop technologies that can disinfect water at the point of use for household based water treatment in developing countries. Solar disinfection can be used to inactivate pathogenic microorganisms in water. However, some organisms are more resistant to solar photo-inactivation. The efficiency of solar disinfection may be improved using heterogeneous photocatalysis. The identification, design and development of semiconductors with band gap energies between 2-3 eV with band potentials suitable for the generation of reactive oxygen species (ROS) is critical to improving the solar efficiency of photocatalysis for environmental applications. The overall aim of this research was to join two narrow bandgap semiconductors in a z-scheme with complementary band edge potentials suitable for ROS generation and test for the solar driven photocatalytic disinfection of water.
Two-dimensional nanostructures of tungsten trioxide (WO3) and graphitic carbon nitride (g-C3N4) were fabricated as individual and heterostructures to study their performance for photocatalyst degradation of organic pollutants (formic acid and phenol) and inactivation of Escherichia. coli under simulated solar irradiation. Contrary to the published literature, the photocatalytic efficiency of the 2D heterostructures was poor compared to a standard benchmark photocatalyst P25-TiO2 (Evonik Aeroxide). The detailed physicochemical properties of the heterostructures were investigated to elucidate the reasons for the poor performance. Chemical probe studies indicated that the 2D heterostructures could work for photocatalytic oxidation reaction and generate hydroxyl radicals, but only with the presence of an electron scavenger. This suggested that the rate limiting step for the photocatalytic activity was the reduction of oxygen. Further investigations showed that WO3, g-C3N4 and the heterostructures were unable to photocatalytically reduce methyl viologen that was easily achieved with TiO2 under UV-Vis irradiation. XPS analysis indicated a structural defect in g-C3N4 with reducing nitrogen (N1s) atomic percentage after heterostructure formation. A surface defect arising from NH2 groups of g-C3N4 was associated with limited photocatalytic reduction. A photocurrent switching from anodic to cathodic was observed for g-C3N4 on sweeping potential from +1.0 to -1.0 V (SCE) through linear sweep voltammetry. The Mott-Schottky analysis in the dark and under UV-Vis irradiation suggested an electron trap between +0.5 to +0.8 V (SCE) for the g-C3N4. The M-S analysis also indicated the presence of the electron trapping state in the WO3/g-C3N4 heterostructure. This suggests that photoexcited conduction band electrons in the WO3 are transferred to the g-C3N4 but trapped in the defect state. The trapped electrons cannot be passed on to reduce molecular oxygen and thus this reaction is rate limiting.
From photoelectrochemical studies, an increased photocurrent density of 12.14 μA.cm-2 under a fixed potential of +1.0 V for WCN91 heterostructure electrode was observed compared to 1.82 μA.cm-2 for WO3 electrode, due to the improved electron-hole pair separation. The heterostructure electrode was tested as photoanode for the electrochemically assisted photocatalytic disinfection (EAP) of water and showed a 6-log reduction of E. coli (106 CFU.mL-1) in 150 min of UV-Vis irradiation as compared to a 5-log reduction with WO3 electrode in 240 min. The increased rate of EAP inactivation of E. coli with the 2D heterostructure electrode was encouraging as compared to the poor activity observed with the suspended photocatalytic system.

Date of Award	Jul 2020
Original language	English
Sponsors	Vice Chancellor's Research scholarship
Supervisor	Pilar Fernandez-Ibanez (Supervisor), Patrick Dunlop (Supervisor) & John Byrne (Supervisor)