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Cu2O Nanoparticles Anchored on Amine Functionalized Graphite Nanosheet: A Potential Reusable Catalyst Amrita Chakravarty, Koushik Bhowmik, Arnab Mukherjee, and Goutam De Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00970 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 29, 2015
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Cu2O Nanoparticles Anchored on Amine Functionalized Graphite Nanosheet: A Potential Reusable Catalyst Amrita Chakravarty, Koushik Bhowmik, Arnab Mukherjee,* and Goutam De* Nano–Structured Materials Division, CSIR–Central Glass & Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata–700032, India
KEYWORDS: cuprous oxide, room temperature synthesis, amine functionalized, graphite nanosheet, methyl orange.
ABSTRACT: Synthesis of Cu2O–amine functionalized graphite nanosheet (AFGNS) composite has been accomplished at room temperature. In the first step, AFGNS is synthesized by wet chemical functionalization where the –NH2 groups formed on nanosheet surface helps to anchor the Cu2+ ions homogeneously through co‒ordinate bonds. Reduction of Cu2+ (3.4 x 102 mmol) in presence of NaBH4 (1.8 mmol) can be restricted to Cu+1 on AFGNS surface at room temperature. This leads to the formation of uniform Cu2O nanoparticles (NP) on AFGNS. The role played by the –NH2 groups in anchoring Cu2+ ions and followed by stabilizing the Cu2O NP on AFGNS was understood by controlled reactions in absence of –NH2 groups and without any graphitic support, respectively. The prepared Cu2O–AFGNS composite shows excellent catalytic activity towards degradation of an azo dye, methyl orange which is an environmental pollutant. The dye degradation proceeds with 1
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high rate constant value and the composite shows high stability and excellent reuse capability.
1. INTRODUCTION Graphite nanosheet (GNS) is a novel carbon material with unique physical and chemical properties. It possesses a honeycomb lattice of completely conjugated sp2 hybridized system with lateral thickness >5 nm.1 Covalent functionalization of GNS creates functional groups on its surface which helps in anchoring the metal, metal oxide and semiconductor nanoparticles (NP)2,3 as a result of which GNS can be effectively used as a support material. Earlier studies have reported the drawback of using the conventional carbon nanomaterials4,5 (such as carbon black and carbon nanotubes) as catalyst supports. In case of carbon black, reuse in repetitive catalyst cycles changes their inherent morphology leading to the aggregation of the metal and metal oxide NP thus decreasing their efficiency significantly.4 Carbon nanotubes (CNTs) on the other hand possess a columniform 1–D nanostructure. Therefore point contact exist between the metal oxide NP and CNT, as a result of which there is only a limited enhancement in the efficiency of their composites.5 Thus although both carbon black and CNTs possess graphitic framework, their efficiency as support materials are restricted. However, in case of GNS, the enhanced activity of the metal oxide NP–GNS composite is mainly attributed to the large surface area of GNS6 with greater interfacial contact between the NP and the nanosheet surface. Further, the high electrical conductivity of GNS enhances the charge transfer to the NP3,7,8 as a result of which, GNS can act as an efficient material as catalyst support for application in the field of catalysis.9,10 Along with this, this type of composites can also have application in the fields of nano–electronic devices, fuel cells and nanoenergetics.11-13
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Cu2O is an environment friendly and cheap p–type semiconductor having a band gap of 2.16 eV.14 Due to its unique electronic structure, high optical absorption coefficient and ease of availability, Cu2O has been extensively used in the fields of catalysis, solar energy conversion devices and H2 production.15-17 Owing to the small band gap of Cu2O, upon anchoring it on GNS surface, the charge transfer between Cu2O to GNS enhances the catalytic efficiency of Cu2O. The GNS support also creates a heterogeneous catalytic system for which the composite can be conveniently separated from the reaction medium and reused in subsequent reaction cycles. Thus Cu2O–GNS composite is expected to be a very promising functional material with strong potential application in the field of catalysis. Synthesis of Cu2O/graphite and graphene composites have been carried out mainly with graphite and graphene oxide (GO). The functional groups (–COOH, epoxide and –OH) present on their surface have been most extensively used for anchoring the Cu2+ ions. Accordingly Cu2O–graphite/graphene composites have been prepared using Cu2+ anchored GO dispersion by hydrothermal, solvothermal and ultrasound techniques at elevated temperatures (100 °C and above) in presence of reducing agents such as glucose, ethylene glycol or o‒anisidine.17–24 However, all the reported processes required the reduction of GO to reduced graphene oxide (RGO). In this process the damaged sp2 hybridized graphitic framework of GO can be restored for necessary charge transfer. But during the reduction, the functional groups on the surface of GO are also mostly eliminated. Due to elimination of functional groups the agglomeration of the NP on the nanosheet surface is expected to occur.22,24 It has been observed that the Cu2O NP formed on RGO surface in some of the earlier reports were large and the size ranged from 20–250 nm17,19,23 where as in cases where particles of 4–5 nm size range were formed, agglomeration due to self assembly occurred leading to formation of large agglomerates (25–250 nm) on the RGO surface.22,24 In a recent report a significant decrease of the catalytic efficiency of TiO2/RGO composite has been 3
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described upon reuse due to such agglomeration.25 Further, all the above mentioned processes require a high temperature condition (above 100 °C) which compromises with the energy efficiency of the synthesis methods. Therefore, synthesis of Cu2O–graphite/graphene composites at room temperature with the uniform distribution of Cu2O NP is of significant importance to the research community at present. But a major problem associated with Cu2O NP synthesis at room temperature in presence of water is the difficulty to restrict the disproportionation reaction of CuI state. A probable method of achieving this objective can be by using graphite or graphene samples functionalized by amine (–NH2) group as the matrix material. The –NH2 functional group plays a significant role as an active anchoring group for metal and metal oxide NP.26–29 Further, upon controlled functionalization of GNS, the conducting graphitic framework can be maintained to a great extent. This excludes the requirement of further reduction to regain back the conducting framework (as observed in case of RGO). Herein we report an energy efficient synthesis of single phase, crystalline Cu2O NP on amine (–NH2) functionalized graphite nanosheet (AFGNS) surface under room temperature condition. Stabilization of the CuI state by the –NH2 groups present on GNS surface in aqueous medium at room temperature is noteworthy. The –NH2 group has a very strong interaction with Cu2+ ion with the formation of Cu←:NH2 co–ordinate bond.28,29 This helps in anchoring the ions at the first instance. The –NH2 groups also play a very important role in anchoring and stabilizing the Cu2O NP after their formation30 which is difficult to stabilize otherwise with unfunctionalized GNS due to disproportionation reaction of Cu2O.31 Subsequently, the superior catalytic activity of the as prepared composite towards the degradation of toxic azo dyes (environmental pollutants present as contaminants in water)
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was studied to demonstrate the utility of the prepared composite. We have used methyl orange (MO) as a representative azo dye in the present article and have studied the efficiency 4
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of the Cu2O–AFGNS composite in the catalytic degradation of MO to non–toxic products. As the composite material is cost effective and prepared in an energy efficient method, it can have potential application in industrial scale removal of azo compound pollutants from nature. 2. EXPERIMENTAL SECTION 2.1 Materials. The following chemicals were used as received: graphite powder (
