Graphene oxide based Pt–TiO2 photocatalyst: Ultrasound assisted synthesis, characterization and catalytic efficiency
Highlights
► A new photocatalyst, graphene oxide (GO) based Pt–TiO2 has been synthesized. ► A remarkable enhancement on the photocatalytic mineralization of a surfactant is achieved. ► The mineralization efficiency of the new catalyst is three times higher than Degussa Pt–25 TiO2. ► A strong interaction between Pt and GO during the reduction of Pt is responsible for the higher efficiency of the catalysts.
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.
Section snippets
Materials
All chemicals used in this study were analytical grade and were used without further purification. Graphite powder (99.99%) was purchased from Alfa Aesar, USA. All solutions were prepared deionized water from a water purification system (Millipore, Synergy). For pH adjustment, 0.1 M HCl and 0.1 M NaOH solutions were used. A stock solution containing 1.0 mM of DBS was prepared and diluted to the required initial concentration.
Preparation of TiO2 and TiO2 based photocatalysts
The pH swing method (multi gelation method) was used to prepare TiO2 nano
Characterization of Pt–GO–TiO2 photocatalysts
DRS is a conventional technique used to find out the band gap as well as the light absorption edge of photocatalysts. The absorption edge of GO–TiO2 was very slightly shifted to the near visible region of light in comparison with bare TiO2 (DRS figure is not shown). Furthermore, GO–TiO2 doped with Pt also extended its absorption towards the longer visible region and the corresponding band gap values are reported in Table 1. The crystalline nature of the TiO2 photocatalysts was confirmed by the
Conclusions
The prepared Pt–GO–TiO2 photocatalyst showed a very high efficiency for the photocatalytic degradation of DBS in the presence of both UV and visible light irradiation. The adsorption of DBS onto the catalyst prior to light irradiation plays a key factor for the photocatalytic oxidation of DBS. Pt doped GO–TiO2 exhibited an enhanced rate of mineralization of DBS compared with GO–TiO2 and the prepared TiO2 photocatalysts. The solution pH did not influence the rate of the degradation of DBS under
Acknowledgment
We acknowledge financial support from the Australian Research Council (ARC) and the ARC Particulate Fluids Processing Special Research Centre for infrastructure support.
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