Photoelectrocatalytic oxidation of bisphenol A over mesh of TiO₂/graphene/Cu₂O

Highlights

• Graphene layers were grown in situ on soft TiO₂/Ti mesh through modified CVD.

• The soft TiO₂/graphene/Cu₂O mesh showed excellent photoelectrocatalytic properties.

• BPA was effectively degraded without generating toxic products.

• TiO₂/graphene/Cu₂O mesh can be conveniently incorporated in an apparatus.

Abstract

Mesh of TiO₂/graphene/Cu₂O was fabricated by chemical vapor deposition of graphene following electrochemical deposition of Cu₂O on anodized Ti soft wire bearing TiO₂ nanotubes. The mesh of TiO₂/graphene/Cu₂O was applied in photoelectrocatalytic oxidation of bisphenol A (BPA). The as-prepared TiO₂/graphene/Cu₂O mesh was used as both catalyst and electrode. Under visible light irradiation, BPA was effectively oxidized through photoelectrocatalysis over the TiO₂/graphene/Cu₂O mesh. Three main intermediates were evidenced during photoelectrocatalytic degradation of BPA, and no toxic products were determined. A detailed pathway of BPA degradation by TiO₂/G/Cu₂O is proposed based on the identified intermediates.

Introduction

Bisphenol A (2,2-bis(4-hydroxyphenyl) propane, BPA) is an organic chemical with various applications in the polymer industry. The global demand for BPA increased from 3.2 million tons in 2003 [1] to 5.5 million tons in 2011 [2]. With the extensive use of BPA, this compound has been detected at concentrations up to 21.5 μg L⁻¹ in urban wastewaters [3] and within the ranges of 1.92–11.1 μg L⁻¹ in industrial wastewater [4], 0.14–0.98 μg L⁻¹ in treated effluents [5], and 0.5–2 μg L⁻¹ in drinking water [6]. BPA has emerged as an environmental pollutant that causes diverse cellular responses even at low doses [7]. In particular, BPA acts as an endocrine disruptor [8], [9], [10], [11]. The predicted no-effect concentration of BPA for aquatic wild life has been revised and decreased from 100 μg L⁻¹ to 0.06 μg L⁻¹ [12]. Several studies have reported that exposure to very low BPA levels affects humans and may result in reduced fertility and increased incidence of breast, ovarian, and testicular cancers. Elimination of even low BPA concentrations from water becomes an urgent issue from the viewpoint of environmental remediation and human health [13], [14]. Moreover, BPA cannot be completely mineralized and its byproducts, which exhibit high endocrine-disrupting action, could be produced during the treatment process [15]. Thus, BPA must be completely transformed into CO₂ and removed from water.

Various methods, such as biological, chemical, electrochemical oxidation, and photocatalytic, have been developed to remove BPA from water [16], [17], [18], [19], [20], [21], [22]. As the most promising semiconductor catalyst, TiO₂ [23] and TiO₂-based [24] photocatalysts have been extensively investigated in the field of photocatalytic degradation. Given the difficulty of constructing solid-state, conductive semiconductor films, several reports documented that photoelectrocatalysis is an effective route to enhance photocatalytic degradation of BPA. In the last recent decade, anodic TiO₂ nanotube arrays grown on Ti substrate have been widely studied because of their superior advantages on constructing solid photocatalysts with high specific surface area and activity [25]. Taking TiO₂ nanotube arrays/Ti in photocatalysis can spare the operation of separating the photocatalyst in powder form from the treated water after degradation, which avoids the secondary pollution. In addition, one of the inherent outstanding advantages of anodic TiO₂ nanotube arrays/Ti is that it can be use both photocatalyst and electrode. As known, hole and electron are generated when TiO₂ is excited by the lights with their photoenergy being higher than the band gap of TiO₂. If the electrons and hoes immediately recombine, only heat is produced and the probability for desired redox reaction is lost. However, the recombination of hole and electron can be restrained through applying an external potential to the photocatalyst. Photoelectrocatalytic (PEC) degradation of organic pollutants has attracted great interest with TiO₂ films immobilized on conductive substrates [26], [27], [28], [29]. Assisted by a bias potential, the working electrode is positively biased to the counter electrode, the photogenerated electrons in TiO₂ anode can be conducted to the external circuit. This is more efficient in the use of electrons and holes and thereby improves the final photocatalytic efficiency over photocatalysis [26], [27], [28], [29].

In this work, highly ordered TiO₂ nanotubes were grown on soft Ti mesh which was weaved from Ti wires. The internal stress inside the anodic TiO₂ nanotubes grown on cylindrical Ti wires was lower than that on traditional Ti foils, thereby the physical stability of the long TiO₂ nanotubes grown on the Ti mesh are stronger than the one on traditional Ti foils. Then graphene was grown in situ on the TiO₂ nanotubes through chemical vapor deposition using Ni as a catalyst. As a highly conductive medium, graphene layers were formed on the external and inner walls of the nanotubes. Graphene was also formed in the space between two TiO₂ nanotubes, rendering the ordered TiO₂ nanotube arrays a conductive, integrated and immobilized electrode. To improve the response of the as-prepared TiO₂/graphene to visible light, we deposited Cu₂O particles to construct a ternary photocatalytic electrode designated as TiO₂/G/Cu₂O mesh which was expected to be a stable photoanode in PEC system because of its good conductivity and integrity.

The TiO₂/G/Cu₂O mesh was used as the photoelectrocatalyst to eliminate BPA from water. The influence of different factors, such as initial solution pH and bias potential were investigated. The photocatalytic and photoelectrocatalytic activities of the TiO₂/G/Cu₂O mesh were compared. Intermediates generated during the degradation were identified and the decomposition pathway of BPA is proposed.