_Applied_  
_sciences_  
Effect of the Surface Morphology of TiO2 Nanotubes on Photocatalytic Efficacy Using Electron-Transfer-Based Assays and Antimicrobial Tests  
**[PERSON] 1,*\({}^{\text{(\textregistered)}}\)**, **[PERSON] 2, [PERSON] 3, [PERSON] 3, [PERSON] 1, [PERSON] 1, [PERSON] 2, [PERSON] 2\({}^{\text{(\textregistered)}}\)** and [PERSON] 2  
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Received: 6 July 2020; Accepted: 28 July 2020; Published: 29 July 2020  
**Featured Application: Water availability is imperative to the Maltese islands. The lack of natural freshwater bodies and a climate characterized by low annual rainfall and high temperatures increase the pressure on the available freshwater resources to the ever-increasing population. The treatment of greywater to be used as second-class water for WC flushing and agricultural purposes could alleviate the demand put on the desalination plants and groundwater resources of the islands. In recent years, the application of titanium-dioxide-based photocatalytic systems has sparked a great deal of interest within the scientific community, due to its potential in removing pollutants and bacteria from air and water systems. The study at hand aims to provide an insight on the efficacy of TiO2 nanostructures having different morphologies to be used for this purpose and thus identify the ideal TiO2 surface morphology for implementation in a microscale photocatalytic greywater treatment system.**  
**Abstract:** The application of titanium oxide nanotubes for the removal of contaminants from freshwater is a rapidly growing scientific interest, especially when it comes to water conservation strategies. In this study we employed four different titanium oxide nanotube surfaces, prepared by a two-electrode anodic oxidation. Two of the surfaces were synthesised in aqueous media, while the other two surfaces were synthesised in ethylene glycol. One of the arrays synthesised in the organic medium was impregnated with silver nanoparticles, while the remaining surfaces were not. The chemical reactivity of the various surfaces was assessed using 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis (3-ethylbenzothazoline-6-sulfonic acid) (ABTS) as free electron sensitive probe molecules, in parallel with tann acid degradation and copper ion reducing capacity. The potential antimicrobial activity of the surfaces was assessed against a panel of microorganisms composed of yeast, fungi, Gram-positive and Gram-negative bacteria. Field emission scanning electron microscopy revealed that surfaces produced in the aqueous medium had a smaller tube length and a smaller tube diameter. It was noted that one of the materials using sodium sulfate as the supporting electrolyte had the most irregular nanostructure morphology with tubes growing to the side rather than vertically. The structural variation of the surfaces directly reflected both the chemical and biological activity, with the nanotubes formed in ethylene glycol showing the fastest rates in the stabilization of DPPH and ABTS radicals, the fastest tannic acid decomposition under various pH conditions and the fastest metal reducing activity. Furthermore, the surface containing silver and its bare counterpart showed the most effective antimicrobial activity, removing approximately 82% of Gram-negative bacteria, 50% of Gram-positive bacteria, 70% of yeast and 40% of fungi, with Gram-negative bacteria being the most susceptible to these surfaces.  
titanium oxide; nanotubes; DPPH; ABTS; CUPRAC; antimicrobial activity; water; photocatalytic