Decoration of silver nanoparticles on activated graphite substrate and their electrocatalytic activity for methanol oxidation

Silver nanoparticles (30-70 nm) have been impregnated on the activated graphite powder by an electroless plating method. The so prepared silver decorated graphite powders are characterized by field emission scanning electron microscopy, powder X-ray diffraction, energy dispersive X-ray and X-ray photoelectron spectroscopy.

Journal of Science: Advanced Materials and Devices (2019) 290e298 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Decoration of silver nanoparticles on activated graphite substrate and their electrocatalytic activity for methanol oxidation M.S Shivakumar a, G Krishnamurthy b, **, C.R Ravikumar c, *, Aarti S Bhatt d a Research Centre, Dept of Chemistry, ACS College of Engineering, Bangalore, 560074, India DOS in Chemistry, Bangalore University, Bangalore, 560001, India c Dept of Chemistry, East West Institute of Technology, Bangalore, 560091, India d Department of Chemistry, NMAM Institute of Technology, Nitte, 574110, India b a r t i c l e i n f o a b s t r a c t Article history: Received 21 December 2018 Received in revised form 31 May 2019 Accepted June 2019 Available online 10 June 2019 Silver nanoparticles (~30e70 nm) have been impregnated on the activated graphite powder by an electroless plating method The so prepared silver decorated graphite powders are characterized by field emission scanning electron microscopy, powder X-ray diffraction, energy dispersive X-ray and X-ray photoelectron spectroscopy The activated graphite powder displays a high surface coverage with tin which is essential, as this ensures a thorough and complete coating of the graphite powder with silver The electrochemical studies of Graphite, Sn/Graphite and successive decoration of AgeSn/Graphite powder have been carried out using cyclic voltammetry in the potential range between À1.2 and 0.0 V at a sweep rate of 50 mV sÀ1 and their electrocatalytic activity for methanol oxidation has been examined in alkaline medium The effective active surface area of Graphite and AgeSn/Graphite electrode are calculated to be 6.2479 Â 10À5 cm2 and 6.7886 Â 10À5 cm2, respectively The impedance spectrum of the AgeSn/Graphite electrode displays a depressed semicircle in the high-frequency region which corresponds to low charge resistance and high capacitance The results highlight the electrocatalytic behavior of the graphite supported silver nanoparticles, making them suitable for fuel cell applications © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Silver nanoparticles Electro catalyst Graphite powder Electroless deposition Methanol oxidation Introduction Electroless deposition of silver nanoparticles on various substrates is a well-known technique, which generally employed for this metal This method involves pretreatment of the substrate surface, sensitization and activation for increasing the rate of metal ions reduction on the surface of carbon substrate [1e3] The applications of silver nanoparticles (Ag NPs) thus obtained according to their properties like electric conductivity, reflectivity and other surface properties such as their ability to absorb as well as chemisorb, making them suitable candidates for catalysis applications [4e8] These properties also lend them unique biological, chemical and physical properties matching up to their macro-scaled counterparts No wonder, Ag NPs have also * Corresponding author ** Corresponding author E-mail addresses: drgkmurthy.bub@gmail.com (G Krishnamurthy), Ravicr128@ gmail.com (C.R Ravikumar) Peer review under responsibility of Vietnam National University, Hanoi been used in varied fields such as renewable energies, medicine and environment [9,10] An alternative for the reduction of silver nanoparticles is to introduce organic functional groups, for instance, carboxyl and hydroxyl groups onto the carbon surface [11,12] These organic functional groups act as binding sites to the metal ions which undergo reduction to ultimately form a metal layer on the carbon surface [13,14] Carbon materials such as graphite are widely used in industries and processing techniques due to their high conductivity and elevated specific surface area Also, the low cost of graphite adds to its preferences [15] Electrocatalysts are generally employed in fuel cells as it helps in improving the fuel oxidation However, the high cost of fuel cells and its low durability hinders its wide application In order to make these fuel cells affordable, research is being carried out to decrease the cost by decreasing the amount of expensive elements used or by substituting some of the expensive components with cheaper but more durable materials Simultaneously, an effort is being made to improvise its durability by developing high durable catalyst supports or it can also be done by using Pt alloys as electro catalysts [16] Carbon based materials make a better catalyst support; for https://doi.org/10.1016/j.jsamd.2019.06.001 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M.S Shivakumar et al / Journal of Science: Advanced Materials and Devices (2019) 290e298 instance XC-72 Vulcan carbon powder is a widely used support in the electrodes of several modern fuel cells The graphite submicron particles on the catalyst support aids in the oxidation of fuel cells application Our present work employs the electroless plating method to reduce silver nanoparticles by introducing organic functional groups on the activated graphite powder with electrocatalytic surfaces This serves as a better platform to obtain decorated silver nanoparticles on sensitized and activated graphite powders The oxidized graphite powders have been treated with reducing agent and immersed in Ag bath solution (pH 11) to obtain silver nanoparticles on graphite powders [17] The so-prepared decorated silver nanoparticles on sensitized and activated graphite have been tested for their electrocatalytic activity of methanol oxidation in alkaline solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) The surface morphology, structure, composition have been characterized by Field emission scanning electron microscopy (FESEM), Energy dispersive X-ray analysis (EDX), powder X-ray diffraction (PXRD) and X-Ray photoelectron spectroscopy (XPES) 291 coating and the extent of coverage were analyzed using a Carl Zeiss Ultra 55 FESEM The elemental analysis was done using Axis Ultra X-Ray Photoelectron spectroscopy The electrochemical studies were carried out using an Auto lab PGSTAT30 model with pilot integration controlled by GPES 4.9 software in a threecompartment cell The measurements were carried in the frequency range of Hz to MHz The experimentally obtained real and imaginary components were analyzed using the CH608E instrument 2.4 Preparation of working electrode For the preparation of carbon paste electrode, 500 mg of silver decorated graphite powder was thoroughly mixed with 20% of silicone oil The resulting paste was packed into a Teflon tube and a copper wire was inserted for external electric contact The surface was polished by butter paper When necessary, a fresh surface was obtained by pushing an excess and polishing the electrode surface mechanically using steel rod Results and discussion Materials and methods 3.1 The mechanism of Ag deposition on Sn/graphite All the chemicals have been procured from SigmaeAldrich and have been used without any further purification Graphite powder, silver nitrate, stannous chloride, polyethylene glycol, glucose, potassium hydroxide and methanol used have !99.99% purity The solvents sulphuric acid, nitric acid and aqueous ammonium hydroxide solution are of AR grade 2.1 Functionalization of graphite powder About g of graphite powder was treated in 200 cm3 of an acid mixture of conc HNO3 and conc H2SO4 (1:3 v/v) and refluxed at 110  C for h to produce eOH and eCOOH functionalized graphite powder The samples were then filtered, washed with distilled water and dried at 95 ±  C for about h Thus, the functionalized graphite powder was obtained The mechanism of deposition of silver on graphite substrate involves, firstly, the sensitization of graphite surface in the SnCl2 solution to enhance the absorption of silver ions in the subsequent activation process The stannous ions adsorbed on to the surface of functionalized graphite surface during sensitization act as seeds for the nuclei of the Ag nanoparticles growth during activation process The initial formation of the silver nanoparticles is based on the application of reducing and oxidizing agents on the graphite substrate surface, namely Sn2ỵand Agỵ This redox reaction proceeds as [8,18e20] Sn2ỵ ỵ 2Agỵ /Sn4ỵ ỵ 2Ag (1) In this method, which is a form of polyol method, ethylene glycol acts as a reducing agent Silver nanoparticles are synthesized during the reduction of silver ions while the hydroxyl groups of poly (ethylene glycol) are oxidized to aldehyde groups 2.2 Decoration of silver nanoparticles on graphite powder To obtain decorated silver nanoparticles on graphite by electroless plating, g of oxidized graphite powder (