Au–pd bimetallic alloy nanoparticle-decorated BiPO₄ nanorods for enhanced photocatalytic oxidation of trichloroethylene

Highlights

• BiPO₄/Au–Pd nanorods were prepared by a simple reduction method.

• Properties of BiPO₄/Au-Pd nanorods were confirmed by electrochemical measurements.

• Enhanced photocatalytic performance was measured under the same conditions.

• An optimum Au–Pd alloy weight percentage was found.

• The mechanism of the enhanced photocatalytic performance is discussed in detail.

Abstract

Photocatalytic oxidation reactions are regarded as promising green synthesis methods for the photodegradation of volatile organic compounds such as trichloroethylene. However, low photocatalytic efficiency and selectivity limit practical application of the technique. We report the synthesis of novel hexagonal BiPO₄/Au-Pd nanorods with diameters of 500 nm that display excellent performance in degrading trichloroethylene under visible-light irradiation. With co-catalytic Au–Pd bimetallic alloy nanoparticles deposited on the nanorod surfaces, the photocatalytic activity is enhanced approximately 25 times compared to that of the BiPO₄ nanorods. The dramatic enhancement of activity is attributed to the Au–Pd alloy nanoparticles effectively separating the oxidation and reduction sites, thereby promoting charge-separation efficiency and providing abundant catalytically active sites to enhance the reactivity of BiPO₄. The excellent photocatalytic performance is also attributed to the moderated conduction band position and an optimum Au–Pd weight percentage. This material provides a promising strategy for degradation of volatile organic compounds.

Introduction

Volatile organic compounds (VOCs) are gaseous indoor pollutants that damage human health [1], [2], [3]. Among them, trichloroethylene (TCE) is a well-known volatile organic solvent used in various industrial applications, such as the degreasing and cold cleaning of metal parts [4], [5], [6]; it is also an environmental pollutant. The photocatalytic oxidation of TCE was recently recognized as an efficient solution to environmental pollution [7], [8], [9]. In the past several decades, photocatalysis has attracted tremendous interest. Potential semiconductor photocatalysts have been the focus of research because they can be applied to the reduction of CO₂, environmental protection, and the production of hydrogen [10], [11], [12], [13], [14], [15]. Photocatalysis is also a promising technology for the treatment of low concentration of VOCs at ppm level, as it is highly effective and the operating conditions are much milder, with use of only air and light at room temperature.

TiO₂ is regarded as a promising photocatalyst because of its low cost, nontoxicity, and high photostability [16], [17], [18]. However, the high recombination rate of photogenerated electron–hole pairs and weak absorption of visible light cause low photoactivity, the worst problem for the practical application. Therefore, many studies have explored ways to find a new photocatalyst and thereby meet environmental and energy requirements [19], [20], [21], [22], [23].

As a wide band-gap semiconductor photocatalyst, BiPO₄ can act as a potential candidate for the photodegradation of trichloroethylene. BiPO₄ was first synthesized by Zhu’s group for the degradation of methylene blue (MB). The photocatalytic performance is about two to three times higher than that of P25. Pan et al. reported the photocatalytic performance of synthesized BiPO₄ oxy-acid salt in dye degradation and found that the inductive role of ${\text{PO}}_4^{-3}$ helped e⁻/h⁺ separation and enhanced its photocatalytic activity [24]. Afterwards, owing to its nontoxicity, low cost, and high efficiency, BiPO₄-based photocatalysts were widely researched for environmental remediation [25], [26], [27]. These studies are mainly focused in the following two directions. One is to control the performance of BiPO₄ through the preparation of BiPO₄ with a specific phase structure [28] or morphology [29]. Another is to extend the adsorption wavelength of visible light by combining it with narrow band-gap photocatalysts, such as g-C₃N₄ [30], Ag₃PO₄ [31], and CdS [32]. However, application of BiPO₄ is still limited in the degradation of dyes or phenol solution, which can be achieved by traditional P25 or ZnO with the lower efficiency than BiPO₄. Up till now, using BiPO₄ for the VOCs degradation is still rare.

The deposition of noble metal (Au and Ag) nanoparticles (NPs) on photocatalyst surfaces offers a new way to enhance the photocatalytic activity by the localized surface plasmon resonance (LSPR) effect. LSPR-induced improvement of semiconductor photocatalysts is mainly attributed to the extension of light absorption to longer wavelengths in the visible light spectrum and the resulting generation of more electron–hole pairs [33], [34], [35]. Bimetallic catalytic systems often exhibit tunable and synergistic effects compared to their monometallic counterparts [36], [37], [38]. It is well known that the electron band structures of single metals can be modified by guest metal particles. Up to now, many efforts have attempted the synthesis of various bimetallic plasmonic photocatalysts, such as Au–Ag, Au–Pd, Au–Pt, and Pd–Pt [39], [40], [41], [42]. When compared with monometallic photocatalysts, bimetallic catalysts showed enhanced performance.

Here, we present a novel Au–Pd/BiPO₄ composite photocatalyst for the degradation of TCE. First, we synthesized novel BiPO₄ nanorods. These were decorated with bimetallic Au–Pd alloy NPs to form Au–Pd/BiPO₄ plasmonic photocatalysts through a reduction method. The deposition of Au–Pd alloy NPs suppressed the recombination rate of photogenerated charges and facilitated photocatalytic reactions for TCE degradation. Simultaneously, the LSPR effect of Au broadens the range of light absorption and provides more electron–hole pairs to participate in photocatalysis. As expected, the Au–Pd/BiPO₄ composites displayed dramatically enhanced activity in the photodegradation of TCE. The aim of the work is to establish the possibility of removing VOCs via a green chemical method.