Article pubs.acs.org/Langmuir
Stable Ni Nanoparticle−Reduced Graphene Oxide Composites for the Reduction of Highly Toxic Aqueous Cr(VI) at Room Temperature Koushik Bhowmik, Arnab Mukherjee, Manish Kr Mishra, and Goutam De* Nano-Structured Materials Division, CSIR−Central Glass and Ceramic Research Institute, 196 Raja S. C. Mullick Road, Kolkata 700032, India S Supporting Information *
ABSTRACT: Inherent properties of graphene can be experienced by integrating it with different nanomaterials to form unique composite materials. Decorating the surface of graphene sheets with nanoparticles (NPs) is one of the recent approaches taken up by scientists all over the world. This article describes a simple synthesis route to preparing stable Ni NP−reduced graphene oxide (Ni−RGO) composite material. The otherwise unstable bare Ni NPs are stabilized when embedded in the RGO sheets. This synthesized composite material has a potential application in the formic acid-induced reduction of highly toxic aqueous Cr(VI) at room temperature (25 °C). The reduction of dichromate using formic acid as a reducing agent is a well-known redox reaction. However, the rate of the reaction is very slow at room temperature, which can be enhanced very significantly in the presence of Ni−RGO by introducing an intermediate redox step with formic acid. The Ni−RGO composite material is an easy to prepare, cheap, stable, reusable material that has the potential to replace costly Pd NPs used in this context. Ni−RGO is also found to be very active in enhancing the rate of reduction of other metal ions in the presence of formic acid at room temperature.
1. INTRODUCTION In recent years, graphene has attracted comprehensive research interest because of its extraordinary thermal, mechanical, electronic, and optical properties.1−5 Metal−graphene nanocomposites are a newly explored subject of current research.6−18 The inherent properties of graphene can be enhanced by using it as a support for metal NPs.10−15 The large contact area provided by the planar structure of graphene sheets can act as a wonderful support for trapping guest NPs.16 Moreover, graphene sheets have the unique ability to promote fast electron-transfer kinetics for a wide range of electroactive species.17,18 Thus, metal−graphene composite materials have been widely used in different applications because of their high surface area, stability under ambient conditions,16−18 and faster electron-transport mechanism.11 Enhanced catalytic activities of several metal NPs supported on RGO have been reported recently.12−15 Among various water pollutants, hexavalent Cr is one of the most toxic, mutagenic, and carcinogenic components requiring removal from water at any cost.19 The primary sources of Cr(VI) contamination are the metal finishing industries dealing with the arc welding of stainless steel and chrome plating. It is well known that hexavalent chromium enhances the risk of lung cancer via chronic inhalation.19,20 Cr(III), however, is nontoxic, relatively inert, and a well-known human nutrient.19,21,22 Researchers have used bifunctionalized mesoporous silica23 and a combination of ferrous sulfate and sodium dithionite24 to reduce Cr(VI). Pd NP-supported mesoporous TiO2 was also used as a photocatalyst for the photoreduction of Cr(VI).25 An © 2014 American Chemical Society
easy method of Cr(VI) reduction by formic acid using Pd NPs as a catalyst has been reported by Sadik et al.26 and also by our group.19 It is well known that aqueous formic acid undergoes decomposition to produce H2 and CO2.27 In the presence of Pd NPs as a catalyst, this process becomes faster.28 The liberated H2 can be used in situ for different chemical reactions.29 Besides the very high cost of the Pd precursor, one of the main drawbacks of using Pd NPs as a catalyst is the requirement of high temperature.26 In this article, we report a very fast reduction of toxic Cr(VI) to nontoxic Cr(III) using formic acid as a reducing agent at room temperature in the presence of Ni NPs stabilized by reduced graphene oxide (Ni−RGO). In general, bare Ni NPs are very unstable and prone to oxidation in air, but in the Ni−RGO composite, the Ni NPs are stabilized by RGO. The RGO surface contains many free electrons.18,30 These free electrons help to stabilize the Ni in its zero-valent state. Because of this kind of electronic environment, the metallic Ni NPs present on the RGO surface are rarely oxidized to a higher oxidation state. Also, the high surface area of RGO prevents the Ni NPs from agglomerating into larger particles. Here, Ni NPs embedded on RGO behave as a heterogeneous reactant that promotes an intermediate redox step with HCOOH to decompose it to H• and CO2. The high electron-transport property of RGO facilitates the electron transfer from H• to Cr(VI) and also stabilizes the otherwise Received: September 9, 2013 Revised: February 24, 2014 Published: March 3, 2014 3209
dx.doi.org/10.1021/la500156e | Langmuir 2014, 30, 3209−3216
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was then heated in air to 380 °C for 1 h to obtain crystalline NiO NPembedded RGO sheets. In situ reduction of NiO NPs in the presence of RGO at 350 °C with a continuous flow of H2 gas (10% H2−balance N2) gives rise to Ni NP-embedded RGO sheets. The synthesis of RGO and bare Ni nanoparticle has been provided in Supporting Information (S1.1 and S1.2) 2.4. Reduction of Cr(VI). The formic acid-induced reduction of Cr(VI) in the presence of Ni−RGO was done by monitoring the reaction using a UV−visible spectrophotometer. Typically, a 27 mL aqueous reaction mixture was prepared by mixing 10 mL of K2Cr2O7 (1 mM), 16 mL of H2O, and 1 mL of formic acid (85%). The concentrations of K2Cr2O7 and formic acid are 0.37 mM and 0.8 M, respectively. The pH of the reaction mixture was 2. The absorption spectrum recorded just after the preparation of this solution is considered to be “0” min data. With respect to Ni, 0.34 mmol of Ni− RGO powder was then added to the mixture, and absorption spectra were recorded in 1 min intervals. After the first reaction cycle, the Ni− RGO composite material was recovered by centrifugation of the reaction mixture and filtration using a PVDF membrane of pore size 0.22 μm. The residual powder was heat treated in a H2 (10% H2, balance N2) atmosphere at 350 °C for 1 h to recover the original composite material. The detailed experimental conditions of the control experiments have been provided in the Supporting Information (S1.3).
unstable bare metallic Ni NPs in air. This material was found to be very cheap, recyclable, and highly effective for carrying out the reduction smoothly at room temperature.
2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as received. Synthetic graphite powder (particle size
