Solvothermal fabrication and construction of highly photoelectrocatalytic TiO₂ NTs/Bi₂MoO₆ heterojunction based on titanium mesh

Abstract

The fabrication of TiO₂ NTs/Bi₂MoO₆ type-II heterojunction photocatalyst was carried out by a simple solvothermal method. Bi₂MoO₆ nanoparticles with nanosheet microstructures were successfully loaded on TiO₂ NTs surface through the adjustment of reaction intervals. The heterojunction photocatalyst showed excellent organic dye and heavy metal ion removal performances, and nearly 100%, 75%, 100% and 100% of MO, RhB, MB and Cr (VI) were removed by simulative sunlight irradiation for 3 or 2 h, respectively. The outstanding photocatalyic performance was mainly due to the formation of type-II heterojunction between TiO₂ and Bi₂MoO₆. The type-II heterojunction not only enhanced visible light response but also accelerated photogenerated charge carrier transfer and restrained the recombination of photogenerated electron-hole pairs with the assistance of internal electric field.

Introduction

Compared with TiO₂ nanopowder photocatalysts, one dimensional TiO₂ nanotube arrays (NTs) with the special tubular channel structure [1], [2], [3] have been extensively researched because of the excellent characteristics in UV response and photogengerated electron transfer [4], [5], [6], [7]. Therefore, one dimensional TiO₂ nanotubes are extensively applied in energy conversion and water treatment. For examples, Davide Spanu designed cascade-based H₂ evolution photocatalyst using TiO₂ NTs [8]. Yang et al. prepared mixed-ion perovskite solar cells based on TiO₂ nanorod array, and the champion power conversion efficiency achieved 19.33% [9]. Our team synthesized TiO₂ nanotube arrays modified with chrysanthemum-like BiOI nanoflowers via the successive ionic layer adsorption and reaction method for the removal of organic dyes and heavy metals [10]. How to prepare TiO₂ nanotubes with high photoelectrochemical ability has received extensive attention and excellent response. At present, the popular research methods include electrochemical deposition [11], sol-gel method [12] and anodic oxidation method [13], [14]. Among these preparation technologies, the anodic oxidation method is attractive because of its simple preparation and controllable tube diameter and thickness. Therefore, the TiO₂ NTs provided a good infrastructure to coupling other materials [14], [15]. For example, Low et al. prepared Ag-loaded TiO₂ NTs for photocatalytic CO₂ reduction, and TiO₂ NTs had huge activity improvement compared with pure TiO₂ because of the highly photocatalytic activity [16]. Ye et al. prepared photocatalytic fuel cell using TiO₂ NTs as the photoanode and Cu foil as cathode to remove aqueous micropollutant 4-chloro-2-methylphenoxyacetic acid [17]. Zheng and his group synthesized direct Z-scheme MoSe₂ decorating TiO₂ NTs photocatalyst for the water pollutant treatment [18].

To overcome the limited solar absorption disadvantage of TiO₂, various semiconductors such as BiOI, Bi₂O₃, CdS and PdO [19], [20], [21], [22] were widely used to sensitize the TiO₂ photocatalyst. Astoundingly, Bi-based semiconductors stood out from the visible light-driven materials because of the attractive visible light absorption, nontoxic, chemically stable and eco-friendly characteristics. Among many representative materials such as BiOX (X = Cl, Br, I), Bi₂WO₆, BiVO₄ and BiPO₄, Bi₂MoO₆ has attracted special attention because of the appropriate band gap and perovskite-like structure [23], [24]. The suitable band gap energy of Bi₂MoO₆ (2.7 eV) endows it outstanding visible light photocatalytic performance for energy production and pollutant degradation in waste water [25]. As well known that Bi₂MoO₆ is part of the Aurivillius oxide family, and shows layered structure consisting of the perovskite-like slab of MoO₆ inserted between (Bi₂O₂)²⁺ layers [26]. However, the practical business application of pure Bi₂MoO₆ was limited on account of low quantum yield and high recombination rate of photogenerated electron-hole [27]. To overcome the problems, various strategies such as F doping, MoS, Ag₃PO₄ and Ag₃VO₄ modification had been employed [28], [29]. Among the methods, the combination of Bi₂MoO₆ and TiO₂ can remarkably offer mutual benefit and achieve common photocatalytic target [30]. Up to now, Bi₂MoO₆ sensitized TiO₂ NTs based on titanium mesh has been little reported.

Herein, we first prepared TiO₂ NTs on Ti mesh substrate by a two-step anodic oxidation method, and then Bi₂MoO₆ nanosheets were deposited on the surface of NTs through a solvothermal method. The prepared TiO₂ NTs/Bi₂MoO₆ heterojunction photocatalyst not only showed excellent photocatalytic performances in organic dyes (MO, RhB and MB) but also heavy metal ions (Cr(VI)).