Photoelectrocatalytic degradation of tetracycline by highly effective TiO₂ nanopore arrays electrode

Abstract

The widely utilization of pharmaceutical and personal care products (PPCPs) in the pharmaceutical therapies and agricultural husbandry has led to the worldwide pollution in the environment. In this study, the photoelectrocatalytic (PEC) behaviors of typical PPCPs, tetracycline (TC), were performed via a highly effective TiO₂ nanopore arrays (TNPs) electrode, comparing with electrochemical (EC) and photocatalytic (PC) process. A significant photoelectrochemical synergetic effect in TC degradation was observed on the TNPs electrode and the rate constant for the PEC process of TNPs electrode was ∼6.7 times as high as its PC process. The TC removal rate achieved ∼80% within 3 h PEC reaction by TNPs electrode, which is ∼25% higher than that obtained for a conventional coated TiO₂ nanofilm electrode fabricated by sol–gel method. The possible mechanism involved in the PEC degradation of TC by TNPs electrode was discussed. Furthermore, the TNPs electrode also shows enhanced photocurrent response compared with that for the coated TiO₂ nanofilm electrode. Such kind of TiO₂ nanopores will have many potential applications in various areas as an outstanding photoelectrochemical material.

Introduction

TiO₂-based photocatalytic (PC) and photoelectrocatalytic (PEC) oxidation technique have been proven to be promising and highly efficient processes that can be used to degrade various recalcitrant organic pollutants, such as dyes, pesticides, herbicides, aromatics, etc., under UV light irradiation [1], [2], [3], [4], [5], [6], [7]. Upon UV illumination, the electrons are excited from the valence band to the conduction band, generating the electron/hole pairs. The positive holes are powerful oxidants for degrading the organic compounds adsorbed onto the electrode surface. The oxidation efficiency could be further improved by applying a positive bias potential across the electrode, which results in the improved separation of photogenerated electron/hole pairs and better use of photogenerated holes [2], [6]. In the past years, the colloidal and particulate titania were widely applied to photodecompose the organic compounds. However, the difficulty in separation and reuse of this catalyst powder from treated water limit its application in practice [8]. To avoid these problems, various pathways [9], [10], including sol–gel, sputtering, chemical vapor deposition, etc., have been developed to fabricate TiO₂ nanofilm on the solid supporting substrates. But these TiO₂ materials often suffer from structural disorder and vast grain boundaries, which may bring obstacles to electrons transport and hinder the charge separation efficiency within the electrode material [2]. In response to these deficiencies, most attempts [11], [12], [13], [14], [15] at synthesizing new TiO₂ structures (e.g. TiO₂ nanowires, nanorods, nanobelts, nanorings, nanotubes, etc.) or modifying TiO₂ (e.g. by depositing a noble metal on its surface, sensitizing it with dyes, doping it with transition metals or non-metal elements and complexes with matching semiconductors), have met with certain success.

We have previously reported on the fabrication and characterization of a novel kind of TiO₂ nanopore arrays (TNPs) electrode material with fast separation and transport of photogenerated electron/hole pairs and strong mechanical stability [16], [17]. In this paper, we report on an investigation on the PEC degradation of pharmaceutical and personal care products (PPCPs) using the highly effective TNPs electrode and compared with the PEC degradation results obtained using a conventional TiO₂ nanoparticulate film electrode prepared by sol–gel technique and tetracycline (TC) is used as a typical of PPCPs [18], [19] (Fig. S1, in Supplementary material). Since TC is widely prescribed in aquiculture and live stocking, can produce a series of pathological changes and disrupt ecosystem equilibrium [18].