Stimuli-responsive heterojunctions based photo-electrocatalytic membrane reactors for reactive filtration of persistent organic pollutants

The design of semiconducting metal oxide heterojunctions is promising to overcome conventional limitations associated to photocatalysis or electrocatalysis, such as fast recombination of electron-hole pairs and poor longterm stability leading to low catalytic performance. A route to tackle this issue is to design catalysts at the atomic levels by arranging order and controlling nanoscale interfaces to yield catalytic materials with greater response rates and stability to dissolution or corrosion. The present study focuses on the formation of such nanoscale heterojunctions between TiO2 and ZnO via atomic layer deposition across conductive and porous stainless-steel substrates to develop enhanced photo-electro-responsive catalytic membrane reactors. The heterojunction nanosheet based structures produced higher density of electron and hole pairs and offered efficient separation of charges, longer lifetime of photo-generated electrons compared to single metal oxides, resulting in enhanced photocurrent efficiency. The tailoring of both the nanoscale dimensions of the metal oxide layers and the stacking of these inorganic nano-sheets led to the development of multi-heterojunctions, of a few tens of nanometres, deposited across conductive porous substrates. The high electron mobility across the heterojunction nano-sheets increased the oxygen evolution potential from 1.4 to 1.7 eV, leading to enhanced electrochemical reactions, as well as offered photocurrent densities 2-3 times higher than pristine single metal oxide membranes. The formation of type II heterojunction structures between TiO2 and ZnO leads to band alignment at the interface, yielding an efficient charge separation mechanism and high catalytic performance. A prototype of novel cross-flow filtration module was designed in this study to support the coupling of photo-electrocatalysis on the membrane surface and simultaneous pressure driven membrane processes. The designed 3D printed modules demonstrated highly enhanced degradation of several persistent organic pollutants, leading to reactivity up to 75 x 10 3 min 1 up to 500 % greater compared to single metal oxides generated with the same conditions. The intimate and synergistic interactions across the stacked metal oxide nano-sheets enabled high catalytic efficiency and stability, opening new avenues stimuli-responsive membranes scale up and implementation in wastewater remediation and compact reactors design, where sieving and reactivity are of prime importance.

Automated summary

The design of semiconducting metal oxide heterojunctions can overcome the limitations of conventional photocatalysis or electrocatalysis. This study focuses on the formation of nanoscale heterojunctions between TiO2 and ZnO, resulting in enhanced photocurrent efficiency and electrochemical reactions. By controlling the nanoscale dimensions and stacking of metal oxide nano-sheets, multi-heterojunctions with improved catalytic efficiency and stability are developed. The coupling of photo-electrocatalysis and pressure driven membrane processes in a novel cross-flow filtration module leads to highly enhanced degradation of persistent organic pollutants.