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A scalable and facile synthesis of carbon nanospheres as a metal free electrocatalyst for oxidation of l -ascorbic acid: Alternate fuel for direct oxidation fuel cells
Accepted Manuscript A scalable and facile synthesis of carbon nanospheres as a metal free electrocatalyst for oxidation of l-ascorbic acid: Alternate fuel for direct oxidation fuel cells
Bhaskar R. Sathe PII: DOI: Reference:
S1572-6657(17)30207-2 doi: 10.1016/j.jelechem.2017.03.049 JEAC 3239
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
2 January 2017 12 March 2017 14 March 2017
Please cite this article as: Bhaskar R. Sathe , A scalable and facile synthesis of carbon nanospheres as a metal free electrocatalyst for oxidation of l-ascorbic acid: Alternate fuel for direct oxidation fuel cells. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi: 10.1016/ j.jelechem.2017.03.049
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ACCEPTED MANUSCRIPT A Scalable and Facile Synthesis of Carbon Nanospheres as a Metal free Electrocatalyst for Oxidation of L-Ascorbic Acid: Alternate Fuel for Direct Oxidation Fuel Cells
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Bhaskar R. Sathe* Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad- 431 004, (MH) India; Email: [email protected] Abstract:
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Large-scale, highly crystalline, high surface area carbon nanospheres (CNSs; ~250 nm) were synthesized on Cu substrate by optimized two step chemical vapour deposition (CVD) approach
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from a mixture of camphor and naphthalene as a source of carbon at 950 C using Ar as a carrier gas. On the basis of host of characterization techniques the mechanistic pathway proposed for the
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formation of CNSs having ordered graphite crystal structure. The present approach is capable of producing monodispersed, high yield and ultra high purity (no other carbon impurities)
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nanospheres. This work further report on the electrocatalytic performance of as-synthesized CNSs and acid treated CNSs for the ascorbic acid (AA) oxidation reaction as a model reaction
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for direct oxidation fuel cells. These CNSs based electrocatalytic systems exhibits enhanced current densities and lower oxidation overvoltages response, demonstrating excellent catalytic
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activities towards AA oxidation could be due to their advantageous structural features.
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Keywords: Metal-Free Electrocatalysis, Direct Oxidation Fuel Cells,
Carbon Nanospheres
acid oxidation. 1. Introduction:
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(CNSs), Acid Functionalized Carbon Nanospheres (F-CNSs), Cyclic voltammetry, L-Ascorbic
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Significant scientific efforts of material chemists have been undertaken in the recent years to develop cost effective preparative methods for carbon nanostructures, due to are having potential applications in various fields [1]. Among the most unpredicted and capable carbon materials, the interest in carbon nanospheres (CNSs) steams from their number of appealing properties, such as its low density, high corrosion resistance ability, tailorable surface properties, excellent thermal and mechanical stability, thermal insulation ability, which make them as a potential material in many applications [2]. In addition to this, there is an immense interest in the fabrication of CNSs because of having highly curved and reactive graphitic structures. Interestingly, among the various polymorphic forms of carbon (such as diamond, graphite, fullerenes and nanotubes), 1
ACCEPTED MANUSCRIPT CNSs had been previously used as an anode material in secondary Na/Li ion batteries, H2 storage, catalytic supports, templates for the synthesis of other useful hollow spheres, photonic crystals in photonics etc.[3-8]. The preparation methods mostly include carbonization of carbon precursors, replication synthesis with hard templates, synthesis of polymer spheres followed by a thermal treatment in an inert atmosphere, supra-molecular aggregate self-assembling approach and others [9-11]. The
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carbon materials have a various ways of aggregation during the reactive processes which leads to
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the formation of a range of morphologies/textures. These as-synthesized materials, which include
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the impurities of other nanosized carbon forms along with metal from catalytic materials. Furthermore, the products of such syntheses are in low yield, often non-homogeneous, difficult
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to purify and it makes system more expensive [temperature and high vacuum are usually uneasy to handle] [12]. Among all the methods, chemical vapor deposition (CVD) has been one of the
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efficient strategies for synthesizing high yield and with quality CNSs. This method consists of simply pyrolyzing the supplied specific hydrocarbon gases at their decomposing temperatures.
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Moreover, this has been stimulated with respect to other approaches, by its simplicity, straightforward scale-up, and its potential to be much less expensive. However, to date CVD
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routes to synthesize catalyst-free (to avoid the metal contamination) carbon nanospheres have not been explored, because of trace amount of catalyst will get incorporated in material [10, 13].
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Naphthalene is a natural hydrocarbon source which is regenerative, low cost, environmental friendly and its unique structure motivates the use as a carbon source [15, 16]. For example,
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mass spectra of naphthalene vapour have shown high abundance of hexagonal carbon rings. In order to suit for industrial production and their applications, to produce CNSs of having desired
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geometries and at controlled locations, a new method must be developed having capability to provide high-yield CNSs. Significantly, during pyrolysis, breaking of the 3D-carbon skeleton is less probable and it becomes a foundation for their spherical growth [14]. Novel methods for the fabrication of CNSs are especially desirable if they can produce samples of both; high purity and yield from eagerly available and economical starting materials. Accordingly, herewith, we report for the first time large-scale synthesis of CNSs by using pyrolysis of mixture of camphor and naphthalene on Cu substrate at 950-1000 C in a carrier gas flow of Ar using duel zone CVD approach. Further utility of this materials were tested for electrocatalytic oxidation of L-ascorbic
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ACCEPTED MANUSCRIPT Acid (L-AA) and it shows higher current density and lower overpotential, reflects it as an one of the natural fuel leading to direct oxidation fuel cells. More significantly, the choice given for L-AA is because of two basic reasons, having biological importance [17] in various metabolic systems and also natural fuel in various energy conversion engines including fuel cells [18]. Fascinatingly, L-AA is a well-known water soluble vitamin, effective antioxidant and it oftenly used as a chemical marker for the determination of
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quality of food [19]. It is also having clinical implications, for example, it is commonly used in the treatment and prevention of colds, mental illness etc. since it can assist the growth of cells,
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tissues, iron adsorption and many more developments in living systems. Unfortunately, the excessive level of L-AA in biological fluids may form oxalates and it assess the oxidative stress
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in human metabolism, and too much this stress has been linked to cancer, diabetics, hepatic disease, kidney stones as well as other diseases [20]. Another reason for choice of L-AA studies
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is includes, a number of fuel cells based on direct oxidative electrocatalytic systems have been designed and demonstrated to work using L-AA as a fuel in addition to existing fuels like
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methanol, ethanol, hydrazine, formic acid etc. [21].
However, in all of these cases, high efficiency in the fuel cells was obtained only when
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expensive noble metal (Pt, Au, Rh, Ir) and bimetallic (Pt-Ru, Pt-Rh, Pt-Cu, Pt-Sn) nanoparticles and it supported on carbon were used as an electrodes [22-27]. Furthermore, other than their
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unaccordable cost, most of these nanoparticle-based electrocatalysts have been known to their aggregation, surface passivation and quickly get poisoned during oxidation reaction towards
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positive potential and results into decrement in their electrocatalytic activities. On the other hand, it is known that among the various electrode processes, oxidation of L-AA is a notoriously
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sluggish reaction, and elucidation of the reaction mechanism in different medium is quite complicated. In view of this the use of the CNSs based metal free electrode materials features unique and innovative advantage due to their tunable molecular structures, and strong tolerance to acid/alkaline environments. On the other hand, so far, all experimental and theoretical studies on the electrocatalytic L-AA are exclusively focused on the surface properties of metallic catalysts due to the importance of metal-H bonds [28-30] in this process; whether a metal-free CNSs based material can exhibit similar catalytic behavior and be more active than metal-based electrocatalysts is still unknown. 2. Experimental: 3
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Chemicals: Camphor (99.9%), naphthalene (99.9%), ethylene glycol, sodium sulphate, ascorbic acid (LAA), isopropanol, acetone, sulphuric acid, nitric acid, hydrochloric acid were used as received from Merck (AR grade); deionized water (16 MΩ) from Milli-Q system was used for all experiments.
2.2.
Synthesis of Carbon Nanospheres (CNSs):
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Herein, the CNSs, were fabricated by using CVD set-up, (schematically in Figure 1d) capable of
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attaining temperature of 1200 C, comprising of a dual zone furnace by using similar approach
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reported earlier for fabrication of carbon nanostructures [31-32]. In brief, at the beginning, a stream of Ar was passed through the quartz tube (i. d. 34 mm) at the flow rate of 100 sccm
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(standard cubic centimeter) to drive out all impurities including adsorbed volatile impurities till both the zones achieved their respective temperatures (pre heater zone; 150 C, main zone; 950
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C). A mixture of naphthalene and camphor (90:10 wt %) was used as a precursor. This precursor was kept in the first (pre heater) zone of the dual furnace, maintaining the optimized
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temperature at ~180-200 C for ~1hr. to ensure the complete sublimation of carbon based precursors. The sublimed precursors were then passed into the main zone by means of Ar as a
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carrier gas with an optimized flow rate of 250 sccm. The temperature of the zone second of furnace was programmed such that it starts decreasing as soon as the vaporized gases entered the
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reaction zone without any external cooling aid. The carbon based materials deposited on Cu substrate which is kept at the center were obtained by subjecting the system of zone second to
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the temperature ramp from 950-1000 C at a natural cooling rate (~10 C/min). These assynthesized materials have been further characterized using various techniques like SEM, TEM,
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XPS, TGA, XRD and Raman spectroscopy in order to study their morphologies, chemical bonding, structure and other properties. 2.3.
Synthesis of Functionalized Carbon Nanospheres (F-CNSs): Initially, 200 mg of as-synthesised CNSs (diameter: ~250 nm,) were carboxylated using microwave (MW) treatment for 4 min at a power of 60 % of total 700 W (separated by 60 s offtime interval). An acid mixture of 98 % H2SO4 and 78 % HNO3 (1:1) was used for microwave treatment. The mixture was then filtered through polytetraethylene (PTFE) membrane (pore size of 200 nm) and the carbon residue was washed thoroughly with deionized water until the pH of
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ACCEPTED MANUSCRIPT the filtrate became neutral (pH 7). These F-CNSs were then dried at 100 ºC in an oven for 5 h and used further for electrocatalytic studies towards L-AA oxidation studies. 2.4.
Electrochemical Studies: All electrochemical studies were performed on an Autolab PGSTAT30 (Eco Chemie) instrument using a conventional three electrode test cell with a reversible hydrogen electrode (RHE) electrode and a platinum foil as the reference and counter electrodes, respectively. A 3 mm dia.
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glassy carbon (GC) electrode for working electrode was polished using 0.3 and 0.05 µm alumina
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powders, followed by washing with water and acetone. The working electrode was prepared as
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follows: 10 µL aliquot of the slurry made by sonication of 1 mg of the CNSs and F-CNSs catalyst in 1 mL isopropyl alcohol was drop-coated on glassy carbon electrode (GCE). After this,
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2 µL of 0.01 wt% Nafion diluted with ethanol was coated on the surface of the electrocatalyst layer to yield a uniform thin film. This electrode was then dried in air and was used as the
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working electrode for all electrochemical studies. An aqueous solution of 0.5 M Na2SO4 was used as the electrolyte throughout the measurements. Further, for L-AA oxidation studies were
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performed using different concentration of 0.1 to 50 mM L-AA in 0.5 M Na2SO4 as the supporting electrolyte on both CNSs and FCNSs based metal free electrocatalytic systems and
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prior to the electrochemical experiments the electrolyte was deaerated with N2 gas. The electrocatalytic performance of these both systems compared with GC (carbon form) instead of
Characterizations:
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The morphology of these carbon based materials were examined by a JEOL JSM-6700F SEM having a field emission source (FEG) and exhibiting a resolution of about ~2-3 nm, with no Cu impurities in/on CNSs; confirmed from EDS analysis. XRD studies were also performed in order
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2.5.
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other metal systems.
to understand the product crystallinity and presence of Cu using CuK (1.54 A°) radiation on a Philips PW1830 instrument operating at 40 kV and a current of 30 mA at a step of 0.02 (2h) at room temperature. In order to understand the surface defects along with other metal impurities present in CNSs sample, XPS measurements were carried out on a VG Micro Tech ESCA 3000 instrument at a pressure of >1x10-9 Torr (pass energy of 50 eV with an electron take off angle 600 and an overall resolution of 1 eV) using Al Kα radiation (1486 eV). The binding energy of C 1s peak was fixed to 284.6 eV and all peaks were calibrated with reference to this graphitic C 1s peak. The background was subtracted by Shirley method. Raman analysis was performed on a 5
ACCEPTED MANUSCRIPT Micro-Raman Spectrometer, LabRAM HR800 from JY Horiba, France, using 514.5 nm Ar ion LASER in order to obtain valuable information regarding defects and the extent of graphitization. 3. Results and Discussion: SEM and TEM images of the as-synthesized collected carbon based products shows welldispersed nanostructured carbon spheres. Accordingly, SEM and TEM images shown in Figures
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1(a) and (b) of typical CNSs are having diameters in the range of 250-300 nm. Among these
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more than 80 % of the CNSs were found to have the sizes of ~250 nm. Interestingly, these CNSs
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with uniform sizes can be collected from different parts of the Cu substrate which was kept at the center of the zone second of furnace. However, no carbon nanotubes or other carbon
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nanostructures were found in the collected carbon sample which affirms that only carbon nanospheres with very high uniformity were found on the Cu substrate. SAED pattern (Figure
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1c) of the solid CNSs are composed of crystalline graphene layers having interlayer distances about 0.33-0.35 nm, which is similar to the distance between crystal layers of graphite [33].
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Although the exact growth mechanism of CNS formation remains uncertain, it is believed that multiple factors must be involved. Significantly, when we replaced the carbon source of
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mixture of camphor and naphthalene with either any of them (camphor or naphthalene) other allotropic forms or their mixture with CNSs are observed (not shown here for brevity). The
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molecular structure of precursor has a detrimental effect on the morphology of the nanostructures grown [34-35]. For example, it reflects from literature, the linear hydrocarbons thermally
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decompose into atomic carbons or linear dimers/trimers of carbon, and generally produce straight hollow CNTs [36-37]. On the other hand, cyclic/byclic/polycyclic hydrocarbons such as
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benzene, xylene, cyclohexane and many more, produces relatively curved/hunched carbon nanostructures with the number of walls often bridged inside [38-39]. As compared to above precursors; naphthalene and camphor are carbon, hydrogen rich and controlled oxygen present in camphor which internally supports to remove other impurities (undissociated precursor molecules, amorphous carbon) at high temperature [40-43]. Significantly, presence of connected hexagonal and controlled pentagonal carbon, abundance of H2 and O2 in precursors may have a good coordination for the formation of high quality, uniform and highly stable amorphous carbon free CNSs.
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Figure. 1. A typical scanning electron microscopic (SEM) (a), TEM (b) images, and SAED pattern (c) for CNSs which confirms high yield, monodispersed (size ~250 nm) and crystalline nature, (d) schematic representation of probable mechanistic pathways for construction of CNSs using two steps modified chemical vapor deposition (CVD) method at 950 °C.
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ACCEPTED MANUSCRIPT Eloquently, naphthalene is a crystalline polyaromatic hydrocarbon consisting of two benzene rings and it could be vaporized at temperature of 218 C [44]. Meanwhile, camphor is another member of a natural hydrocarbon source having an unique structure consisting of hexagonal, pentagonal rings and methyl carbons and it could be vaporized at temperature of 180 C [41]. Under heating in controlled atmosphere (oxygen free carrier gas), methyl carbon of camphor can
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be easily detached, whereas breaking of the 3D-carbon skeleton is less probable. In support with
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this, it is reported that mass spectra of camphor vapour have shown high abundance of hexagonal and pentagonal carbon rings [45]. This could be the driving force to grow carbon into spherical
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form. With a certain molar ratio, a mixture of naphthalene and camphor could systematically and steadily be decomposed within a temperature range of their decomposition temperature.
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Significantly, because of lower vaporization temperature of camphor, initially the molecules reach at high temperature zone by carrier gas Ar. Therefore, after being attained to a designated
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temperature which is higher than their vaporizing temperature, the dissociation of the camphor would provide a certain amount of active hexagonal and pentagonal ring as nucleation sites.
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Similarly, molecules of the naphthalene vaporizes (218 C) and reaches at high temperature zone and then decomposed to smaller hydrocarbon clusters acts as a carbon source feed for the further
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growth of CNSs as shown schematically in the Figure 1(d). With an increase in the temperature of both furnaces, the generation rate of carbon (both for nucleation and its further growth) would
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be more enhanced due to its higher rate of decomposition and diffusion. Based on visualization and earlier studies which were reported elsewhere for synthesis of other carbon nanostructure, it
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could be confirmed that both naphthalene and camphor could be decomposed and turned into black gaseous products [46]. Meanwhile, these active gaseous molecules within a particular
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composition are responsible for autonomous formation of graphene sheets which is a fundamental building block of CNSs in second zone of furnace. Therefore, with the increasing time for deposition and also temperature, the self-assembling reaction of CNSs would be anticipated to become more promoted due to the increased amount of those precursors. Moreover, to confirm the as-synthesized CNSs are Cu free, the collected CNSs were further analyzed by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis and shown in Figure 2(a)-(c) respectively. Interestingly, a wide survey XPS spectrum of the materials shows that the samples mainly contain carbon (98 wt.%) and remaining oxygen (2 wt.%) from the surface of the carbon. The O 1s signal of the 8
ACCEPTED MANUSCRIPT peak at 533 eV is assigned to molecular oxygen (O2), which originates from air, adsorbed on the
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CNSs [47-49].
Figure 2. (a) Wide survey X-ray photoelectron (XP) spectrum, (b) Superimposed thermogravimetric analysis (TGA) curve and derivative plot and (c) Energy dispersive spectra (EDS) profile of the as-
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synthesized CNSs, to confirm Cu free CNSs having carbon, hydrogen and controlled oxygen.
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Moreover, TGA profile of CNSs indicating that CNSs are stable up to 600 °C in air showing it decomposes within short temperature range i.e. upto at ~650 °C. This also confirms from the
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decomposition profile, absence of any wt.% residue above ~650 °C temperature indicates the CNSs are completely free from Cu and other metallic impurities can be shown in Figure 2b.
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Further, EDS of as-synthesised CNSs is presented in Figure 2c. The spectrum shows the signal ~0.35 keV of carbon from CNSs indicates that the CNSs contain only carbon while there is an
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absence of Cu (from substrate) in the CNSs and is support to above results of XPS and TGA.
found (Figure 2).
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The results indicate that the constituent of the spheres is only carbon and no other elements are
To further improve its solubility and the activity of these CNSs for electrocatalytic studies,
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acid functionalization was carried to develop additional active sites on its surface by using our earlier reported procedure for other carbon nanostructures [31, 32, 47, 48]. It briefly, 200 mg of as-synthesized CNSs (diameter: ~250 nm) were dispersed in acid mixture of 98 % H2SO4 and 78 % HNO3 (1:1) and carboxylated using microwave (MW) treatment for 4 min at a power of 60 % of total 700 W (separated by 60s off-time interval). The mixture was then filtered through polytetrafluoroethylene (PTFE) membrane (pore size of 200 nm) and the CNSs residue was washed thoroughly with deionized water until the pH of the filtrate became neutral (pH 7). These acidified/functionalized CNSs (F-CNSs) were then dried at 100 °C in an oven for 5h and used further for electrocatalytic studies. Structural information on the local molecular environment 9
ACCEPTED MANUSCRIPT and extent of sp3 defect sites due to the formation of functional groups after acid functionalization of on CNSs can be obtained from XRD, FTIR, XPS and Raman analysis respectively and is shown in Figure 3. The superimposed X-ray diffraction (XRD) patterns of CNSs (I) and F-CNSs (II) are displayed in Figure 3(a). The XRD patterns show that both samples have Bragg reflections corresponding to the (002) and (10) planes of carbon. However, some subtle differences are also
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evident in the crystallographic features of the materials, specifically suggesting a slight increase
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in the intensity of the signals and in the broadening due to additional defects/functionalities.
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Accordingly, Figure S1 shows superimposed FTIR spectra for (a) CNSs and (b) acid treated CNSs, where the common bands C-H (2800-3000 cm-1) and C=C (1525 cm-1) are particularly
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informative about the nature of graphitic carbon from the CNSs assembly. Moreover, it also gives a clear evidence for the presence of sp3 defects after functionalization of CNSs surface.
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Another common and broad band in the high frequency region, at 3480 cm-1 corresponding to the free -O-H bonds is invariant with surface modification, although a slight shift (at 3660 cm-1) in
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the case of acid treated CNSs (b) is observed which could be attributed to intermolecular bonding (H-bonding) among various functional groups on sidewalls of the CNSs [50]. The
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fundamental bands corresponding to C=O (broad; in the range of 1680-1850 cm-1) might be attributed to -COO- groups while the -O-H (phenolic; 1150 cm-1) band along with a signal
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corresponding to –S-O and –C-SO3- observed for both acid treated CNSs. In conclusion, the appearance of an additional strong bands corresponding to surface oxidation of CNSs in case of
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acid treated CNSs is attributed to the presence of a -COO-, -SO3-,-OH groups [51].
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Figure 3 (a). XRD pattern of as-synthesized CNSs (I) and acid functionalized CNSs (II), showing (002)
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and (10) reflections corresponding to graphitic stacking of CNSs. (b) Raman spectrum of (I) as-
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synthesized CNSs and (II) acid functionalized CNSs taken using an Ar ion Laser with wavelength of 514.5 nm, which clearly shows distinct D and G bands corresponding to defect induced double resonant scattering for ordered graphitic carbon in as synthesized CNSs having progressive increment in intensity
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of D band. (C) X-ray photoelectron (XP) spectra of (I) C 1s of as-synthesized, and (II) acid functionalized CNSs, which show two different deconvoluted signals corresponding to SP2 and SP3 carbon having
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variation due to a range of functionalization steps (experimental data points are shown as circle, resultant fitting curves as continuous lines and individual fitted curves as dashed lines deconvulated by using
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Shirley fitting algorithm).
Accordingly, Figure 3(b), represents a comparison of the Raman spectra of the bare CNSs and acid functionalized CNSs, each of them consisting of two characteristic bands. The band at ~1325 cm-1 is the D band, which is caused by the induction of significant defects or disorder on the CNSs surface and the G band corresponding to the graphitic structure observed at ~1583 cm-1 , which is in good agreement with the signals reported for carbon nanostructures [50]. Further analysis from Raman spectra is supported by XP spectra shown in Figure 3c (I and II). In support with FTIR, Raman studies, the presence of additional and representative surface functionalities on acid treated CNSs along with as-synthesized CNSs is confirmed by high 11
ACCEPTED MANUSCRIPT resolved deconvoluted XPS analysis of C 1s of both the CNSs based samples have been carried out to analyse the systematic conversion of graphatic (sp2) to oxidised/defective (sp3) carbon with acid treatment on sp3 sites (Figure. 3(c) I-II). A sharp peak at 284.6 eV corresponds to a * feature associated with sp2 hybridized carbon. A Gaussian fit of the *-type peak of the C 1s spectrum indicates the presence of peaks at 284.6 and 285.9 eV, which can be assigned to the
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sp2-hybridized graphite-like carbons and molecular oxygen adsorbed sp3-hybridized carbons,
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respectively (Figure 2a). The generation and distribution of defect sites on the CNSs lattice is due to the formation of defects on some of the strained surface walls of the nanospheres during
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the oxidation process. These defects include the conversion of sp2 to sp3-hybridized carbon, with
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the formation of functional groups such as -COOH, -SO3H and -OH on the surface [52].
Figure 4: Oxidation of L-AA on different electrocatalytic materials in 0.5 M Na2SO4 solution; (a) superimposed cyclic voltammograms of (i) without L-AA on bare GC (red), (ii) with 0.1 mM L-AA on bare GC (blue), (iii) with 0.1 mM L-AA as-synthesized CNSs (black), (b) superimposed cyclic voltammograms of (i) CNSs (black) and (ii) F-CNSs (red) electrode of 0.1 mM AA in 0.5 M Na2SO4 at scan rate 50 mV/s and (c) schematic representation and probable electron transfer processes for
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ACCEPTED MANUSCRIPT illustrating the electrocatalytic L-AA oxidation on CNSs and (d) Cyclic Voltamogram on CNSs in 0.5 M Na2SO4 solution containing AA concentration (i) 0.1, (ii) 0.5, (iii) 1.0, (iv) 10.0 and (v) 50 mM using Pt foil and Hg/Hg2SO4 counter and reference electrodes respectively.
In order to explore the electrocatalytic properties of metal free CNSs, we have investigated their sensitivity to L-AA oxidation reactions, which is a surface sensitive reaction relevant to
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biological electron transfer [53] and oxidative energy conversion in fuel cells processes [54].
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Accordingly, typical superimposed cyclic voltamogram of bare GC (feature less), GC in 0.1 mM
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L-AA in 0.5 M Na2SO4 solution (week oxidation signal) and GC modified with CNSs (prominent oxidation signal) in the potential range of -1.3-0.7 V vs Hg/Hg2SO4 is depicted in
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Figure 4(a). Further the effect of acid treatment on CNSs (acid functionalized i.e. F-CNNSs) towards reactivity for L-AA oxidation reaction is shown in Figure 4(b). Accordingly, the anodic peak potential is noticed at onset potential at -0.36 V for F-CNSs modified GC electrode which
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is ~0.13 V less negative than the bare GC (-0.23 V), and the oxidation current density becomes ~7 times that of the latter is depicted in Figure 4(a). In case of CNSs the onset potential is –0.29
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V which gets slight shifted by ~0.06 V less negative value than the bare GC electrode and it can
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be due to availability of anchoring sites on F-CNSs for L-AA results into decrease in onset potential. The current density for the CNSs also increases by ~14 times compared to the bare GC
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in L-AA solution. In case of F-CNSs, the decreases in onset potential by ~ 0.13 V and broadened signal could be due to variable acidic functionalities (-COOH, -NO2,-SO3H, -OH) and different
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sites for interactions with L-AA molecules. Nevertheless, both CNSs and F-CNSs shows better electrocatalytic activity than blank GC. Probable electron transfer during electrocatalytic conversion is shown in Figure 4(c). In addition to this, to confirm the signals of L-AA and its
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support towards concentration dependent studies have been carried out using different concentrations of L-AA on F-CNSs. Accordingly, Figure 4(d) shows the effect of varying L-AA concentration on the oxidation peak current. The oxidative current increased with the increase in concentration of L-AA. Significantly, there is no any effect of concentrations on current densities of other low potential signals, which is in good agreement on earlier conclusion of the signals are corresponding to acid functionalities of CNSs. . Table 1: Cyclic voltammetric data for electrooxidation of L-AA (0.1mM) in Na2SO4 (0.5 M) at a sweep rate of 50 mV/s for CNSs, FCNSs along with GC electrodes. 13
ACCEPTED MANUSCRIPT Sr. No.
Electrodes 1.
F-CNSs
Onset Potential vs Current Density at Enhancement MMS peak potential factor 2 -0.36 V 81.6 μA/cm 710
2.
CNSs
-0.29 V
172.6 μA/cm2
1500
3.
GC
-0.23 V
12.1 μA/cm2
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As expected the features of the signal changes and having the trend of CNSs>>FCNSs>GC.
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Moreover, as reflected from literature it has been explored that the activity of the electrocatalyst
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based on enhancement factor [55-57] and which is defined as the ratio of the current densities of CNSs and F-CNSs versus that acquired on GC respectively and is summarized in Table 1. It has been observed after acid treatment CNSs surface becomes functional and represents
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defective/reactive platform for L-AA towards oxidation, this may be the reason for decrease in onset potential of FCNSs. Moreover, in case of FCNSs exhibits multiple signals towards
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negative potential with respect to signal of L-AA could be due to redox nature of different functionalities associated with its surface. In addition to this, functionalization of CNSs exhibits
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increase in capacitance but the L-AA oxidation signal is having very high current density and
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was not affected by their capacitance current. To find out the change in activity of CNSs after acid functionalization have been explained by combined results from XP and Raman spectra shown in Figure 3 (b and c).
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Moreover, the calculated respective intensity ratios of sp2/sp3 and/or D/G are informative in order to quantify the surface oxidation process along with their concomitant topographic effects
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[58]. Accordingly, the D/G values for the as-synthesized CNSs and F-CNSs are 1.07 and 1.25 respectively. The F-CNSs have a higher D/G ratio than the bare CNSs, which is indicative of
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oxidative functionalization. This presumably is related to the strong bond between the surface oxidized functionalities of the CNSs (surface carbon defects). Moreover, as shown in Figure 3(c), the sharp vibration band observed in the Raman spectrum after the acid functionalization of the CNSs illustrates the higher graphitized degree of order and is in good agreement with earlier reports [59].Considering the lower onset potentials and higher current densities from cyclic voltamograms, it can be concluded that CNSs and F-CNSs shows superior electro-oxidation of L-AA compared to all other existing materials. These CNSs based electrocatalysts are stable even after electrochemical measurements (see SEM image SI-II) indicating the morphology and
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ACCEPTED MANUSCRIPT off-course reactivity of CNSs and which is also unaffected by other internal reactive intermediate species generated during the L-AA oxidation. 4. Conclusions: In conclusion, we have developed method for synthesis of large scale CNSs on polycrystalline Cu substrates by using optimized CVD approach. The D-band, the peak position and intensity of the G-band in Raman spectroscopy was used to get both qualitative and
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quantative information of as synthesized CNSs and extent of acid functionality responsible for
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electrocatalytic oxidation of L-AA. To synthesize highly crystalline monodisperse CNSs on a Cu
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substrate, the ratio of carbon sources, partial pressure of carrier gas Ar, temperatures of both zones and growth time were optimized for synthesis of good quality CNSs. The lower anodic
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onset potential is noticed at -0.36V and -0.29 V for F-CNSs and CNSs which is ~0.13 V and ~0.06 V less negative than the bare GC (-0.23 V), similarly, the oxidation current density
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becomes ~7 (FCNSs) and ~14 (CNSs) times that of the bare GC. Further, it indicates acid functionalities available on the surface of CNSs responsible for the efficient electrocatalytic
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oxidation of L-AA. We believe that our approach for the synthesis of monodispersed CNSs with high crystallinity may be potentially useful for the development of many electrochemcial
5. Acknowledgements:
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devices.
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Author acknowledge the financial support provided by FAST TRACK DST-SERB (SERB/F/7963/2014-15) & EMR DST-SERB (Ref. F. No.: EMR/2016/003587) New Delhi
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(India) and DAE-BRNS, Mumbai (India) research project (Ref F. No. 34/20/06/2014BRNS/21gs). We are also thankful to the Department of Chemistry, Dr. Babasaheb Ambedkar
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Marathwada University for providing the laboratory facility.
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Graphical Abstract:
A Scalable and Facile Synthesis of Carbon Nanospheres as a Metal free Electrocatalyst for Oxidation of L-Ascorbic Acid: Alternate Fuel for Direct Oxidation Fuel Cells
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Bhaskar R. Sathe Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, 431 004, (MH) India; Email: [email protected]
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Electrochemical oxidation of L-ascorbic acid (L-AA) on carbon nanospheres (CNSs; ~250 nm) in neural media was investigated to use L-AA as an alternate fuel for direct oxidation fuel
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cells. Higher current density with lower onset potential was obtained for the oxidation of L-AA by modifying glassy carbon electrodes with CNSs based metal free electrocatalysts.
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Interestingly, these CNSs exhibits superior electrocatalytic performance and it could be due to
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due to their advantageous structural features.
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ACCEPTED MANUSCRIPT
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Highlights: 1. One-step strategy to synthesize carbon nanospheres (CNSs) is demonstrated using chemical vapor deposition. 2. Carbon nanospheres (CNSs) act as a highly efficient metal free electrocatalyst for oxidation of L-ascorbic acid. 3. Overall activity is either superior or comparable to recent catalytic systems reported in the literature with electrochemical, structural and morphological stability.
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Bhaskar R. Sathe PII: DOI: Reference:
S1572-6657(17)30207-2 doi: 10.1016/j.jelechem.2017.03.049 JEAC 3239
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
2 January 2017 12 March 2017 14 March 2017
Please cite this article as: Bhaskar R. Sathe , A scalable and facile synthesis of carbon nanospheres as a metal free electrocatalyst for oxidation of l-ascorbic acid: Alternate fuel for direct oxidation fuel cells. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi: 10.1016/ j.jelechem.2017.03.049
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ACCEPTED MANUSCRIPT A Scalable and Facile Synthesis of Carbon Nanospheres as a Metal free Electrocatalyst for Oxidation of L-Ascorbic Acid: Alternate Fuel for Direct Oxidation Fuel Cells
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Bhaskar R. Sathe* Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad- 431 004, (MH) India; Email: [email protected] Abstract:
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Large-scale, highly crystalline, high surface area carbon nanospheres (CNSs; ~250 nm) were synthesized on Cu substrate by optimized two step chemical vapour deposition (CVD) approach
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from a mixture of camphor and naphthalene as a source of carbon at 950 C using Ar as a carrier gas. On the basis of host of characterization techniques the mechanistic pathway proposed for the
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formation of CNSs having ordered graphite crystal structure. The present approach is capable of producing monodispersed, high yield and ultra high purity (no other carbon impurities)
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nanospheres. This work further report on the electrocatalytic performance of as-synthesized CNSs and acid treated CNSs for the ascorbic acid (AA) oxidation reaction as a model reaction
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for direct oxidation fuel cells. These CNSs based electrocatalytic systems exhibits enhanced current densities and lower oxidation overvoltages response, demonstrating excellent catalytic
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activities towards AA oxidation could be due to their advantageous structural features.
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Keywords: Metal-Free Electrocatalysis, Direct Oxidation Fuel Cells,
Carbon Nanospheres
acid oxidation. 1. Introduction:
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(CNSs), Acid Functionalized Carbon Nanospheres (F-CNSs), Cyclic voltammetry, L-Ascorbic
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Significant scientific efforts of material chemists have been undertaken in the recent years to develop cost effective preparative methods for carbon nanostructures, due to are having potential applications in various fields [1]. Among the most unpredicted and capable carbon materials, the interest in carbon nanospheres (CNSs) steams from their number of appealing properties, such as its low density, high corrosion resistance ability, tailorable surface properties, excellent thermal and mechanical stability, thermal insulation ability, which make them as a potential material in many applications [2]. In addition to this, there is an immense interest in the fabrication of CNSs because of having highly curved and reactive graphitic structures. Interestingly, among the various polymorphic forms of carbon (such as diamond, graphite, fullerenes and nanotubes), 1
ACCEPTED MANUSCRIPT CNSs had been previously used as an anode material in secondary Na/Li ion batteries, H2 storage, catalytic supports, templates for the synthesis of other useful hollow spheres, photonic crystals in photonics etc.[3-8]. The preparation methods mostly include carbonization of carbon precursors, replication synthesis with hard templates, synthesis of polymer spheres followed by a thermal treatment in an inert atmosphere, supra-molecular aggregate self-assembling approach and others [9-11]. The
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carbon materials have a various ways of aggregation during the reactive processes which leads to
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the formation of a range of morphologies/textures. These as-synthesized materials, which include
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the impurities of other nanosized carbon forms along with metal from catalytic materials. Furthermore, the products of such syntheses are in low yield, often non-homogeneous, difficult
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to purify and it makes system more expensive [temperature and high vacuum are usually uneasy to handle] [12]. Among all the methods, chemical vapor deposition (CVD) has been one of the
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efficient strategies for synthesizing high yield and with quality CNSs. This method consists of simply pyrolyzing the supplied specific hydrocarbon gases at their decomposing temperatures.
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Moreover, this has been stimulated with respect to other approaches, by its simplicity, straightforward scale-up, and its potential to be much less expensive. However, to date CVD
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routes to synthesize catalyst-free (to avoid the metal contamination) carbon nanospheres have not been explored, because of trace amount of catalyst will get incorporated in material [10, 13].
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Naphthalene is a natural hydrocarbon source which is regenerative, low cost, environmental friendly and its unique structure motivates the use as a carbon source [15, 16]. For example,
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mass spectra of naphthalene vapour have shown high abundance of hexagonal carbon rings. In order to suit for industrial production and their applications, to produce CNSs of having desired
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geometries and at controlled locations, a new method must be developed having capability to provide high-yield CNSs. Significantly, during pyrolysis, breaking of the 3D-carbon skeleton is less probable and it becomes a foundation for their spherical growth [14]. Novel methods for the fabrication of CNSs are especially desirable if they can produce samples of both; high purity and yield from eagerly available and economical starting materials. Accordingly, herewith, we report for the first time large-scale synthesis of CNSs by using pyrolysis of mixture of camphor and naphthalene on Cu substrate at 950-1000 C in a carrier gas flow of Ar using duel zone CVD approach. Further utility of this materials were tested for electrocatalytic oxidation of L-ascorbic
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ACCEPTED MANUSCRIPT Acid (L-AA) and it shows higher current density and lower overpotential, reflects it as an one of the natural fuel leading to direct oxidation fuel cells. More significantly, the choice given for L-AA is because of two basic reasons, having biological importance [17] in various metabolic systems and also natural fuel in various energy conversion engines including fuel cells [18]. Fascinatingly, L-AA is a well-known water soluble vitamin, effective antioxidant and it oftenly used as a chemical marker for the determination of
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quality of food [19]. It is also having clinical implications, for example, it is commonly used in the treatment and prevention of colds, mental illness etc. since it can assist the growth of cells,
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tissues, iron adsorption and many more developments in living systems. Unfortunately, the excessive level of L-AA in biological fluids may form oxalates and it assess the oxidative stress
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in human metabolism, and too much this stress has been linked to cancer, diabetics, hepatic disease, kidney stones as well as other diseases [20]. Another reason for choice of L-AA studies
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is includes, a number of fuel cells based on direct oxidative electrocatalytic systems have been designed and demonstrated to work using L-AA as a fuel in addition to existing fuels like
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methanol, ethanol, hydrazine, formic acid etc. [21].
However, in all of these cases, high efficiency in the fuel cells was obtained only when
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expensive noble metal (Pt, Au, Rh, Ir) and bimetallic (Pt-Ru, Pt-Rh, Pt-Cu, Pt-Sn) nanoparticles and it supported on carbon were used as an electrodes [22-27]. Furthermore, other than their
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unaccordable cost, most of these nanoparticle-based electrocatalysts have been known to their aggregation, surface passivation and quickly get poisoned during oxidation reaction towards
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positive potential and results into decrement in their electrocatalytic activities. On the other hand, it is known that among the various electrode processes, oxidation of L-AA is a notoriously
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sluggish reaction, and elucidation of the reaction mechanism in different medium is quite complicated. In view of this the use of the CNSs based metal free electrode materials features unique and innovative advantage due to their tunable molecular structures, and strong tolerance to acid/alkaline environments. On the other hand, so far, all experimental and theoretical studies on the electrocatalytic L-AA are exclusively focused on the surface properties of metallic catalysts due to the importance of metal-H bonds [28-30] in this process; whether a metal-free CNSs based material can exhibit similar catalytic behavior and be more active than metal-based electrocatalysts is still unknown. 2. Experimental: 3
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Chemicals: Camphor (99.9%), naphthalene (99.9%), ethylene glycol, sodium sulphate, ascorbic acid (LAA), isopropanol, acetone, sulphuric acid, nitric acid, hydrochloric acid were used as received from Merck (AR grade); deionized water (16 MΩ) from Milli-Q system was used for all experiments.
2.2.
Synthesis of Carbon Nanospheres (CNSs):
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Herein, the CNSs, were fabricated by using CVD set-up, (schematically in Figure 1d) capable of
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attaining temperature of 1200 C, comprising of a dual zone furnace by using similar approach
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reported earlier for fabrication of carbon nanostructures [31-32]. In brief, at the beginning, a stream of Ar was passed through the quartz tube (i. d. 34 mm) at the flow rate of 100 sccm
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(standard cubic centimeter) to drive out all impurities including adsorbed volatile impurities till both the zones achieved their respective temperatures (pre heater zone; 150 C, main zone; 950
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C). A mixture of naphthalene and camphor (90:10 wt %) was used as a precursor. This precursor was kept in the first (pre heater) zone of the dual furnace, maintaining the optimized
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temperature at ~180-200 C for ~1hr. to ensure the complete sublimation of carbon based precursors. The sublimed precursors were then passed into the main zone by means of Ar as a
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carrier gas with an optimized flow rate of 250 sccm. The temperature of the zone second of furnace was programmed such that it starts decreasing as soon as the vaporized gases entered the
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reaction zone without any external cooling aid. The carbon based materials deposited on Cu substrate which is kept at the center were obtained by subjecting the system of zone second to
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the temperature ramp from 950-1000 C at a natural cooling rate (~10 C/min). These assynthesized materials have been further characterized using various techniques like SEM, TEM,
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XPS, TGA, XRD and Raman spectroscopy in order to study their morphologies, chemical bonding, structure and other properties. 2.3.
Synthesis of Functionalized Carbon Nanospheres (F-CNSs): Initially, 200 mg of as-synthesised CNSs (diameter: ~250 nm,) were carboxylated using microwave (MW) treatment for 4 min at a power of 60 % of total 700 W (separated by 60 s offtime interval). An acid mixture of 98 % H2SO4 and 78 % HNO3 (1:1) was used for microwave treatment. The mixture was then filtered through polytetraethylene (PTFE) membrane (pore size of 200 nm) and the carbon residue was washed thoroughly with deionized water until the pH of
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ACCEPTED MANUSCRIPT the filtrate became neutral (pH 7). These F-CNSs were then dried at 100 ºC in an oven for 5 h and used further for electrocatalytic studies towards L-AA oxidation studies. 2.4.
Electrochemical Studies: All electrochemical studies were performed on an Autolab PGSTAT30 (Eco Chemie) instrument using a conventional three electrode test cell with a reversible hydrogen electrode (RHE) electrode and a platinum foil as the reference and counter electrodes, respectively. A 3 mm dia.
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glassy carbon (GC) electrode for working electrode was polished using 0.3 and 0.05 µm alumina
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powders, followed by washing with water and acetone. The working electrode was prepared as
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follows: 10 µL aliquot of the slurry made by sonication of 1 mg of the CNSs and F-CNSs catalyst in 1 mL isopropyl alcohol was drop-coated on glassy carbon electrode (GCE). After this,
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2 µL of 0.01 wt% Nafion diluted with ethanol was coated on the surface of the electrocatalyst layer to yield a uniform thin film. This electrode was then dried in air and was used as the
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working electrode for all electrochemical studies. An aqueous solution of 0.5 M Na2SO4 was used as the electrolyte throughout the measurements. Further, for L-AA oxidation studies were
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performed using different concentration of 0.1 to 50 mM L-AA in 0.5 M Na2SO4 as the supporting electrolyte on both CNSs and FCNSs based metal free electrocatalytic systems and
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prior to the electrochemical experiments the electrolyte was deaerated with N2 gas. The electrocatalytic performance of these both systems compared with GC (carbon form) instead of
Characterizations:
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The morphology of these carbon based materials were examined by a JEOL JSM-6700F SEM having a field emission source (FEG) and exhibiting a resolution of about ~2-3 nm, with no Cu impurities in/on CNSs; confirmed from EDS analysis. XRD studies were also performed in order
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2.5.
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other metal systems.
to understand the product crystallinity and presence of Cu using CuK (1.54 A°) radiation on a Philips PW1830 instrument operating at 40 kV and a current of 30 mA at a step of 0.02 (2h) at room temperature. In order to understand the surface defects along with other metal impurities present in CNSs sample, XPS measurements were carried out on a VG Micro Tech ESCA 3000 instrument at a pressure of >1x10-9 Torr (pass energy of 50 eV with an electron take off angle 600 and an overall resolution of 1 eV) using Al Kα radiation (1486 eV). The binding energy of C 1s peak was fixed to 284.6 eV and all peaks were calibrated with reference to this graphitic C 1s peak. The background was subtracted by Shirley method. Raman analysis was performed on a 5
ACCEPTED MANUSCRIPT Micro-Raman Spectrometer, LabRAM HR800 from JY Horiba, France, using 514.5 nm Ar ion LASER in order to obtain valuable information regarding defects and the extent of graphitization. 3. Results and Discussion: SEM and TEM images of the as-synthesized collected carbon based products shows welldispersed nanostructured carbon spheres. Accordingly, SEM and TEM images shown in Figures
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1(a) and (b) of typical CNSs are having diameters in the range of 250-300 nm. Among these
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more than 80 % of the CNSs were found to have the sizes of ~250 nm. Interestingly, these CNSs
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with uniform sizes can be collected from different parts of the Cu substrate which was kept at the center of the zone second of furnace. However, no carbon nanotubes or other carbon
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nanostructures were found in the collected carbon sample which affirms that only carbon nanospheres with very high uniformity were found on the Cu substrate. SAED pattern (Figure
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1c) of the solid CNSs are composed of crystalline graphene layers having interlayer distances about 0.33-0.35 nm, which is similar to the distance between crystal layers of graphite [33].
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Although the exact growth mechanism of CNS formation remains uncertain, it is believed that multiple factors must be involved. Significantly, when we replaced the carbon source of
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mixture of camphor and naphthalene with either any of them (camphor or naphthalene) other allotropic forms or their mixture with CNSs are observed (not shown here for brevity). The
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molecular structure of precursor has a detrimental effect on the morphology of the nanostructures grown [34-35]. For example, it reflects from literature, the linear hydrocarbons thermally
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decompose into atomic carbons or linear dimers/trimers of carbon, and generally produce straight hollow CNTs [36-37]. On the other hand, cyclic/byclic/polycyclic hydrocarbons such as
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benzene, xylene, cyclohexane and many more, produces relatively curved/hunched carbon nanostructures with the number of walls often bridged inside [38-39]. As compared to above precursors; naphthalene and camphor are carbon, hydrogen rich and controlled oxygen present in camphor which internally supports to remove other impurities (undissociated precursor molecules, amorphous carbon) at high temperature [40-43]. Significantly, presence of connected hexagonal and controlled pentagonal carbon, abundance of H2 and O2 in precursors may have a good coordination for the formation of high quality, uniform and highly stable amorphous carbon free CNSs.
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Figure. 1. A typical scanning electron microscopic (SEM) (a), TEM (b) images, and SAED pattern (c) for CNSs which confirms high yield, monodispersed (size ~250 nm) and crystalline nature, (d) schematic representation of probable mechanistic pathways for construction of CNSs using two steps modified chemical vapor deposition (CVD) method at 950 °C.
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ACCEPTED MANUSCRIPT Eloquently, naphthalene is a crystalline polyaromatic hydrocarbon consisting of two benzene rings and it could be vaporized at temperature of 218 C [44]. Meanwhile, camphor is another member of a natural hydrocarbon source having an unique structure consisting of hexagonal, pentagonal rings and methyl carbons and it could be vaporized at temperature of 180 C [41]. Under heating in controlled atmosphere (oxygen free carrier gas), methyl carbon of camphor can
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be easily detached, whereas breaking of the 3D-carbon skeleton is less probable. In support with
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this, it is reported that mass spectra of camphor vapour have shown high abundance of hexagonal and pentagonal carbon rings [45]. This could be the driving force to grow carbon into spherical
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form. With a certain molar ratio, a mixture of naphthalene and camphor could systematically and steadily be decomposed within a temperature range of their decomposition temperature.
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Significantly, because of lower vaporization temperature of camphor, initially the molecules reach at high temperature zone by carrier gas Ar. Therefore, after being attained to a designated
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temperature which is higher than their vaporizing temperature, the dissociation of the camphor would provide a certain amount of active hexagonal and pentagonal ring as nucleation sites.
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Similarly, molecules of the naphthalene vaporizes (218 C) and reaches at high temperature zone and then decomposed to smaller hydrocarbon clusters acts as a carbon source feed for the further
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growth of CNSs as shown schematically in the Figure 1(d). With an increase in the temperature of both furnaces, the generation rate of carbon (both for nucleation and its further growth) would
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be more enhanced due to its higher rate of decomposition and diffusion. Based on visualization and earlier studies which were reported elsewhere for synthesis of other carbon nanostructure, it
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could be confirmed that both naphthalene and camphor could be decomposed and turned into black gaseous products [46]. Meanwhile, these active gaseous molecules within a particular
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composition are responsible for autonomous formation of graphene sheets which is a fundamental building block of CNSs in second zone of furnace. Therefore, with the increasing time for deposition and also temperature, the self-assembling reaction of CNSs would be anticipated to become more promoted due to the increased amount of those precursors. Moreover, to confirm the as-synthesized CNSs are Cu free, the collected CNSs were further analyzed by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis and shown in Figure 2(a)-(c) respectively. Interestingly, a wide survey XPS spectrum of the materials shows that the samples mainly contain carbon (98 wt.%) and remaining oxygen (2 wt.%) from the surface of the carbon. The O 1s signal of the 8
ACCEPTED MANUSCRIPT peak at 533 eV is assigned to molecular oxygen (O2), which originates from air, adsorbed on the
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CNSs [47-49].
Figure 2. (a) Wide survey X-ray photoelectron (XP) spectrum, (b) Superimposed thermogravimetric analysis (TGA) curve and derivative plot and (c) Energy dispersive spectra (EDS) profile of the as-
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synthesized CNSs, to confirm Cu free CNSs having carbon, hydrogen and controlled oxygen.
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Moreover, TGA profile of CNSs indicating that CNSs are stable up to 600 °C in air showing it decomposes within short temperature range i.e. upto at ~650 °C. This also confirms from the
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decomposition profile, absence of any wt.% residue above ~650 °C temperature indicates the CNSs are completely free from Cu and other metallic impurities can be shown in Figure 2b.
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Further, EDS of as-synthesised CNSs is presented in Figure 2c. The spectrum shows the signal ~0.35 keV of carbon from CNSs indicates that the CNSs contain only carbon while there is an
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absence of Cu (from substrate) in the CNSs and is support to above results of XPS and TGA.
found (Figure 2).
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The results indicate that the constituent of the spheres is only carbon and no other elements are
To further improve its solubility and the activity of these CNSs for electrocatalytic studies,
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acid functionalization was carried to develop additional active sites on its surface by using our earlier reported procedure for other carbon nanostructures [31, 32, 47, 48]. It briefly, 200 mg of as-synthesized CNSs (diameter: ~250 nm) were dispersed in acid mixture of 98 % H2SO4 and 78 % HNO3 (1:1) and carboxylated using microwave (MW) treatment for 4 min at a power of 60 % of total 700 W (separated by 60s off-time interval). The mixture was then filtered through polytetrafluoroethylene (PTFE) membrane (pore size of 200 nm) and the CNSs residue was washed thoroughly with deionized water until the pH of the filtrate became neutral (pH 7). These acidified/functionalized CNSs (F-CNSs) were then dried at 100 °C in an oven for 5h and used further for electrocatalytic studies. Structural information on the local molecular environment 9
ACCEPTED MANUSCRIPT and extent of sp3 defect sites due to the formation of functional groups after acid functionalization of on CNSs can be obtained from XRD, FTIR, XPS and Raman analysis respectively and is shown in Figure 3. The superimposed X-ray diffraction (XRD) patterns of CNSs (I) and F-CNSs (II) are displayed in Figure 3(a). The XRD patterns show that both samples have Bragg reflections corresponding to the (002) and (10) planes of carbon. However, some subtle differences are also
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evident in the crystallographic features of the materials, specifically suggesting a slight increase
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in the intensity of the signals and in the broadening due to additional defects/functionalities.
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Accordingly, Figure S1 shows superimposed FTIR spectra for (a) CNSs and (b) acid treated CNSs, where the common bands C-H (2800-3000 cm-1) and C=C (1525 cm-1) are particularly
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informative about the nature of graphitic carbon from the CNSs assembly. Moreover, it also gives a clear evidence for the presence of sp3 defects after functionalization of CNSs surface.
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Another common and broad band in the high frequency region, at 3480 cm-1 corresponding to the free -O-H bonds is invariant with surface modification, although a slight shift (at 3660 cm-1) in
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the case of acid treated CNSs (b) is observed which could be attributed to intermolecular bonding (H-bonding) among various functional groups on sidewalls of the CNSs [50]. The
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fundamental bands corresponding to C=O (broad; in the range of 1680-1850 cm-1) might be attributed to -COO- groups while the -O-H (phenolic; 1150 cm-1) band along with a signal
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corresponding to –S-O and –C-SO3- observed for both acid treated CNSs. In conclusion, the appearance of an additional strong bands corresponding to surface oxidation of CNSs in case of
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acid treated CNSs is attributed to the presence of a -COO-, -SO3-,-OH groups [51].
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Figure 3 (a). XRD pattern of as-synthesized CNSs (I) and acid functionalized CNSs (II), showing (002)
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and (10) reflections corresponding to graphitic stacking of CNSs. (b) Raman spectrum of (I) as-
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synthesized CNSs and (II) acid functionalized CNSs taken using an Ar ion Laser with wavelength of 514.5 nm, which clearly shows distinct D and G bands corresponding to defect induced double resonant scattering for ordered graphitic carbon in as synthesized CNSs having progressive increment in intensity
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of D band. (C) X-ray photoelectron (XP) spectra of (I) C 1s of as-synthesized, and (II) acid functionalized CNSs, which show two different deconvoluted signals corresponding to SP2 and SP3 carbon having
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variation due to a range of functionalization steps (experimental data points are shown as circle, resultant fitting curves as continuous lines and individual fitted curves as dashed lines deconvulated by using
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Shirley fitting algorithm).
Accordingly, Figure 3(b), represents a comparison of the Raman spectra of the bare CNSs and acid functionalized CNSs, each of them consisting of two characteristic bands. The band at ~1325 cm-1 is the D band, which is caused by the induction of significant defects or disorder on the CNSs surface and the G band corresponding to the graphitic structure observed at ~1583 cm-1 , which is in good agreement with the signals reported for carbon nanostructures [50]. Further analysis from Raman spectra is supported by XP spectra shown in Figure 3c (I and II). In support with FTIR, Raman studies, the presence of additional and representative surface functionalities on acid treated CNSs along with as-synthesized CNSs is confirmed by high 11
ACCEPTED MANUSCRIPT resolved deconvoluted XPS analysis of C 1s of both the CNSs based samples have been carried out to analyse the systematic conversion of graphatic (sp2) to oxidised/defective (sp3) carbon with acid treatment on sp3 sites (Figure. 3(c) I-II). A sharp peak at 284.6 eV corresponds to a * feature associated with sp2 hybridized carbon. A Gaussian fit of the *-type peak of the C 1s spectrum indicates the presence of peaks at 284.6 and 285.9 eV, which can be assigned to the
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sp2-hybridized graphite-like carbons and molecular oxygen adsorbed sp3-hybridized carbons,
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respectively (Figure 2a). The generation and distribution of defect sites on the CNSs lattice is due to the formation of defects on some of the strained surface walls of the nanospheres during
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the oxidation process. These defects include the conversion of sp2 to sp3-hybridized carbon, with
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the formation of functional groups such as -COOH, -SO3H and -OH on the surface [52].
Figure 4: Oxidation of L-AA on different electrocatalytic materials in 0.5 M Na2SO4 solution; (a) superimposed cyclic voltammograms of (i) without L-AA on bare GC (red), (ii) with 0.1 mM L-AA on bare GC (blue), (iii) with 0.1 mM L-AA as-synthesized CNSs (black), (b) superimposed cyclic voltammograms of (i) CNSs (black) and (ii) F-CNSs (red) electrode of 0.1 mM AA in 0.5 M Na2SO4 at scan rate 50 mV/s and (c) schematic representation and probable electron transfer processes for
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ACCEPTED MANUSCRIPT illustrating the electrocatalytic L-AA oxidation on CNSs and (d) Cyclic Voltamogram on CNSs in 0.5 M Na2SO4 solution containing AA concentration (i) 0.1, (ii) 0.5, (iii) 1.0, (iv) 10.0 and (v) 50 mM using Pt foil and Hg/Hg2SO4 counter and reference electrodes respectively.
In order to explore the electrocatalytic properties of metal free CNSs, we have investigated their sensitivity to L-AA oxidation reactions, which is a surface sensitive reaction relevant to
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biological electron transfer [53] and oxidative energy conversion in fuel cells processes [54].
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Accordingly, typical superimposed cyclic voltamogram of bare GC (feature less), GC in 0.1 mM
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L-AA in 0.5 M Na2SO4 solution (week oxidation signal) and GC modified with CNSs (prominent oxidation signal) in the potential range of -1.3-0.7 V vs Hg/Hg2SO4 is depicted in
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Figure 4(a). Further the effect of acid treatment on CNSs (acid functionalized i.e. F-CNNSs) towards reactivity for L-AA oxidation reaction is shown in Figure 4(b). Accordingly, the anodic peak potential is noticed at onset potential at -0.36 V for F-CNSs modified GC electrode which
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is ~0.13 V less negative than the bare GC (-0.23 V), and the oxidation current density becomes ~7 times that of the latter is depicted in Figure 4(a). In case of CNSs the onset potential is –0.29
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V which gets slight shifted by ~0.06 V less negative value than the bare GC electrode and it can
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be due to availability of anchoring sites on F-CNSs for L-AA results into decrease in onset potential. The current density for the CNSs also increases by ~14 times compared to the bare GC
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in L-AA solution. In case of F-CNSs, the decreases in onset potential by ~ 0.13 V and broadened signal could be due to variable acidic functionalities (-COOH, -NO2,-SO3H, -OH) and different
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sites for interactions with L-AA molecules. Nevertheless, both CNSs and F-CNSs shows better electrocatalytic activity than blank GC. Probable electron transfer during electrocatalytic conversion is shown in Figure 4(c). In addition to this, to confirm the signals of L-AA and its
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support towards concentration dependent studies have been carried out using different concentrations of L-AA on F-CNSs. Accordingly, Figure 4(d) shows the effect of varying L-AA concentration on the oxidation peak current. The oxidative current increased with the increase in concentration of L-AA. Significantly, there is no any effect of concentrations on current densities of other low potential signals, which is in good agreement on earlier conclusion of the signals are corresponding to acid functionalities of CNSs. . Table 1: Cyclic voltammetric data for electrooxidation of L-AA (0.1mM) in Na2SO4 (0.5 M) at a sweep rate of 50 mV/s for CNSs, FCNSs along with GC electrodes. 13
ACCEPTED MANUSCRIPT Sr. No.
Electrodes 1.
F-CNSs
Onset Potential vs Current Density at Enhancement MMS peak potential factor 2 -0.36 V 81.6 μA/cm 710
2.
CNSs
-0.29 V
172.6 μA/cm2
1500
3.
GC
-0.23 V
12.1 μA/cm2
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As expected the features of the signal changes and having the trend of CNSs>>FCNSs>GC.
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Moreover, as reflected from literature it has been explored that the activity of the electrocatalyst
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based on enhancement factor [55-57] and which is defined as the ratio of the current densities of CNSs and F-CNSs versus that acquired on GC respectively and is summarized in Table 1. It has been observed after acid treatment CNSs surface becomes functional and represents
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defective/reactive platform for L-AA towards oxidation, this may be the reason for decrease in onset potential of FCNSs. Moreover, in case of FCNSs exhibits multiple signals towards
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negative potential with respect to signal of L-AA could be due to redox nature of different functionalities associated with its surface. In addition to this, functionalization of CNSs exhibits
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increase in capacitance but the L-AA oxidation signal is having very high current density and
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was not affected by their capacitance current. To find out the change in activity of CNSs after acid functionalization have been explained by combined results from XP and Raman spectra shown in Figure 3 (b and c).
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Moreover, the calculated respective intensity ratios of sp2/sp3 and/or D/G are informative in order to quantify the surface oxidation process along with their concomitant topographic effects
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[58]. Accordingly, the D/G values for the as-synthesized CNSs and F-CNSs are 1.07 and 1.25 respectively. The F-CNSs have a higher D/G ratio than the bare CNSs, which is indicative of
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oxidative functionalization. This presumably is related to the strong bond between the surface oxidized functionalities of the CNSs (surface carbon defects). Moreover, as shown in Figure 3(c), the sharp vibration band observed in the Raman spectrum after the acid functionalization of the CNSs illustrates the higher graphitized degree of order and is in good agreement with earlier reports [59].Considering the lower onset potentials and higher current densities from cyclic voltamograms, it can be concluded that CNSs and F-CNSs shows superior electro-oxidation of L-AA compared to all other existing materials. These CNSs based electrocatalysts are stable even after electrochemical measurements (see SEM image SI-II) indicating the morphology and
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ACCEPTED MANUSCRIPT off-course reactivity of CNSs and which is also unaffected by other internal reactive intermediate species generated during the L-AA oxidation. 4. Conclusions: In conclusion, we have developed method for synthesis of large scale CNSs on polycrystalline Cu substrates by using optimized CVD approach. The D-band, the peak position and intensity of the G-band in Raman spectroscopy was used to get both qualitative and
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quantative information of as synthesized CNSs and extent of acid functionality responsible for
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electrocatalytic oxidation of L-AA. To synthesize highly crystalline monodisperse CNSs on a Cu
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substrate, the ratio of carbon sources, partial pressure of carrier gas Ar, temperatures of both zones and growth time were optimized for synthesis of good quality CNSs. The lower anodic
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onset potential is noticed at -0.36V and -0.29 V for F-CNSs and CNSs which is ~0.13 V and ~0.06 V less negative than the bare GC (-0.23 V), similarly, the oxidation current density
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becomes ~7 (FCNSs) and ~14 (CNSs) times that of the bare GC. Further, it indicates acid functionalities available on the surface of CNSs responsible for the efficient electrocatalytic
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oxidation of L-AA. We believe that our approach for the synthesis of monodispersed CNSs with high crystallinity may be potentially useful for the development of many electrochemcial
5. Acknowledgements:
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devices.
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Author acknowledge the financial support provided by FAST TRACK DST-SERB (SERB/F/7963/2014-15) & EMR DST-SERB (Ref. F. No.: EMR/2016/003587) New Delhi
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(India) and DAE-BRNS, Mumbai (India) research project (Ref F. No. 34/20/06/2014BRNS/21gs). We are also thankful to the Department of Chemistry, Dr. Babasaheb Ambedkar
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Marathwada University for providing the laboratory facility.
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48. B. A. Kakade, S. Patil , B. R. Sathe, S. P. Gokhale,V. K. Pillai, Near-complete phase transfer of single-wall carbon nanotubes by covalent functionalization, J. Chem. Sci. 120 (2008) 599-
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49. X. Zou, X. Huang, A. Goswami, R. Silva, B. R. Sathe, E. Mikmeková, T. Asefa, Cobalt-
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ACCEPTED MANUSCRIPT 53. C. R. Raj, K. Tokuda, T. Ohsaka, Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate, Bioelectrochemistry 53 (2001)183-191. 54. M. Chouna, H. J. Leeb, J. Leea, Positively charged carbon electrocatalyst for enhanced power performance of L-ascorbic acid fuel cells, J. Energy Chem. 25 (2016) 793-797. 55. B. R. Sathe, B. K. Balan, V. K. Pillai, Enhanced electrocatalytic performance of
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56. B. R. Sathe, High aspect ratio rhodium nanostructures for tunable electrocatalytic performance, Phys. Chem. Chem. Phys. 25 (2013) 7866—7872.
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57. B. K. Balan, B. R. Sathe, Significant enhancement of formic acid oxidation using rhodium nanostructures, J.Nanosci. Nanotechno. 12 (2012)1-5.
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58. Y. Kim, K. Fujisawa, H. Muramatsu, T. Hayashi, M. Endo, T. Fujimori, K. Kaneko, M. Terrones, J. Behrends, A. Eckmann, C. Casiraghi, K. Novoselov, R. Saito, M. Dresselhaus,
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Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, A. K. Sood, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat.
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Graphical Abstract:
A Scalable and Facile Synthesis of Carbon Nanospheres as a Metal free Electrocatalyst for Oxidation of L-Ascorbic Acid: Alternate Fuel for Direct Oxidation Fuel Cells
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Bhaskar R. Sathe Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, 431 004, (MH) India; Email: [email protected]
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Electrochemical oxidation of L-ascorbic acid (L-AA) on carbon nanospheres (CNSs; ~250 nm) in neural media was investigated to use L-AA as an alternate fuel for direct oxidation fuel
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cells. Higher current density with lower onset potential was obtained for the oxidation of L-AA by modifying glassy carbon electrodes with CNSs based metal free electrocatalysts.
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Interestingly, these CNSs exhibits superior electrocatalytic performance and it could be due to
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due to their advantageous structural features.
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ACCEPTED MANUSCRIPT
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Highlights: 1. One-step strategy to synthesize carbon nanospheres (CNSs) is demonstrated using chemical vapor deposition. 2. Carbon nanospheres (CNSs) act as a highly efficient metal free electrocatalyst for oxidation of L-ascorbic acid. 3. Overall activity is either superior or comparable to recent catalytic systems reported in the literature with electrochemical, structural and morphological stability.
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