Graphene oxide based Pt–TiO₂ photocatalyst: Ultrasound assisted synthesis, characterization and catalytic efficiency

Abstract

An ultrasound-assisted method was used for synthesizing nanosized Pt–graphene oxide (GO)–TiO₂ photocatalyst. The Pt–GO–TiO₂ nanoparticles were characterized by diffused reflectance spectroscopy, X-ray diffraction, N₂ BET adsorption–desorption measurements, atomic force microscopy and transmission electron microscopy. The photocatalytic and sonophotocatalytic degradation of a commonly used anionic surfactant, dodecylbenzenesulfonate (DBS), in aqueous solution was carried out using Pt–GO–TiO₂ nanoparticles in order to evaluate the photocatalytic efficiency. For comparison purpose, sonolytic degradation of DBS was carried out. The Pt–GO–TiO₂ catalyst degraded DBS at a higher rate than P–25 (TiO₂), prepared TiO₂ or GO–TiO₂ photocatalysts. The mineralization of DBS was enhanced by a factor of 3 using Pt–GO–TiO₂ compared to the P–25 (TiO₂). In the presence of GO, an enhanced rate of DBS oxidation was observed and, when doped with platinum, mineralization of DBS was further enhanced. The Pt–GO–TiO₂ catalyst also showed a considerable amount of degradation of DBS under visible light irradiation. The initial solution pH had an effect on the rate of photocatalytic oxidation of DBS, whereas no such effect of initial pH was observed in the sonochemical or sonophotocatalytic oxidation of DBS. The intermediate products formed during the degradation of DBS were monitored using electrospray mass spectrometry. The ability of GO to serve as a solid support to anchor platinum particles on GO–TiO₂ is useful in developing new photocatalysts.

Introduction

As is well known, surfactants are widely used in various applications as diverse as in the removal of contaminants from the surface of human skin to the manufacture of textiles. They are also used in a range of industries including paper, polymers, pharmaceuticals and oil recovery [1]. Each year large quantities of surfactants are produced worldwide for the above purposes to supply the required demand. For example, the average annual production of detergent is ∼8 million tons from the western world alone [2]. Among the different synthetic surfactants manufactured, the alkyl-benzenesulfonates such as dodecylbenzenesulfonate (DBS) are one of the major surfactant groups produced. Usually these are released into the water system after use, and this introduces a serious pollution issue that has attracted considerable environmental concerns. DBS is non-biodegradable in nature and accumulates in natural water systems over a period of time [3] and so can affect aquatic life as well as humans. The conventional treatment methods for the total removal and degradation of DBS are still at a primitive stage.

Advanced oxidation technologies (AOTs) are alternative methods to conventional methods for effective and complete oxidation of various organic pollutants into carbon dioxide and water [4], [5], [6], [7], [8], [9], [10], [11]. Several studies deal with the oxidative degradation of surfactant using AOTs [1], [2], [3], [9], [12]. A number of different AOTs, photocatalysis, photochemical and Fenton-oxidation techniques have been employed for the oxidation of surfactants [1], [2], [13], [14], [15]. Han et al. [13] have reported maximum mineralization of DBS using photocatalytic degradation with TiO₂–Cu₂O binary photocatalysts under visible light irradiation. Similarly, Zhang et al. [14] have reported photocatalytic degradation and maximum removal of DBS with a silica–TiO₂ composite polymer membrane catalyst.

Recently, sonochemical degradation technique has emerged as a potential AOT [5], [7], [8], [9], [16], [17]. In sonolysis, there is no requirement to use any added chemicals or catalysts for the oxidation of organic pollutants; a simple ultrasonic transducer is sufficient for the complete oxidation of organics in water. The chemical reactions occurring from the ultrasonic irradiation of a solution are produced through the phenomenon of cavitation. The process of cavitation refers to the rapid growth and implosive collapse of bubbles in a liquid resulting in an unusual reaction environment within and in the vicinity of bubbles. The temperature produced during the cavitational collapse is as high as 4500 K within these collapsing bubbles [18], [19]. Generally, the sonochemical oxidation of organic compounds in aqueous solution occurs by two-reaction pathways: (i) volatile compounds evaporate into the cavity during the expansion cycle and degrade via pyrolytic reaction within the collapsing bubble; (ii) it proceeds by the reaction of OH radicals with the solute adsorbed at the bubble interface. The nature of the reaction pathway depends on the volatility, hydrophobicity, and surface activity of the compound [9]. Yang et al. [3] have reported the sonochemical oxidation of single and mixed surfactants including DBS and extensively studied using both pulsed and continuous mode of operation of ultrasonic transducers. Singla et al. [9] have reported the sonochemical oxidation of a non-ionic surfactant and proposed a few reaction mechanisms to explain the sonochemical oxidation process.

Following the synthesis of graphene oxide (GO) down to a few layers thick, GO has been used as a solid support for various application studies including photocatalysis, electrocatalysis, fuel cells, and solar cells, due to its exceptional physical and chemical properties [20], [21], [22], [23], [24], [25], [26], [27], [28]. Recently, TiO₂ synthesized along with GO was shown to be suitable as a photocatalyst for environmental remediation purposes [28]. The process of GO synthesis is relatively simple under laboratory conditions and inexpensive in comparison with other solid supports such as carbon nano-tubes (CNTs). In this study, we have prepared GO–TiO₂ by a hydrothermal method and then doped this with Pt. A schematic of Pt–TiO₂ on the surface of GO sheets is shown in Fig. 1. The prepared Pt–GO–TiO₂ catalyst was used for the degradation of DBS as a model pollutant under photocatalysis and sonophotochemical oxidation. In addition, the sonochemical degradation of DBS was also carried out in order to compare the efficiencies of the three processes.