PdAg Bimetallic Nanoalloy-Decorated Graphene: A Nanohybrid with

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PdAg Bimetallic Nano Alloy Decorated Graphene: A Nanohybrid with Unprecedented Electrocatalytic, Catalytic and Sensing Activity Sajad Ahmad Bhat, Nusrat Rashid, Mudasir Ahmad Rather, Sarwar Ahmad Pandit, Ghulam Mohammad Rather, Pravin P. Ingole, and Mohsin Ahmad Bhat ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00510 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

PdAg Bimetallic Nano Alloy Decorated Graphene: A Nanohybrid with Unprecedented Electrocatalytic, Catalytic and Sensing Activity Sajad Ahmad Bhata, Nusrat Rashid b, Mudasir Ahmad Rather a, Sarwar Ahmad Pandit a, Ghulam Mohammad Rather a, Pravin P. Ingoleb and Mohsin Ahmad Bhata,* a,

*Department of Chemistry, University of Kashmir, Srinagar-190006, J & K, India. E-mail: [email protected]; Fax: +91 194-2414049;Tel: +91 194 2414049

b

Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India 110016.

Abstract The recent reports about the promising and tuneable electrocatalytic activity and stability of nanoalloys have stimulated an intense research activity towards the design and synthesis of homogenously alloyed novel bimetallic nano-electrocatalysts. We, herein present a simple one-pot facile wet-chemical approach for the deposition of high quality bimetallic palladiumsilver (PdAg) homogenous nanoalloy crystals on reduced graphene oxide (Gr) sheets. Morphological, structural and chemical characterization of the so crafted nanohybrids establishes a homogeneous distribution of 1:1 PdAg nanoalloy crystals supported over reduced graphene oxide (PdAg-Gr). The PdAg-Gr nanohybrids exhibit outstanding electrocatalytic, catalytic and electroanalytical performance. The PdAg-Gr samples were found to exhibit exceptional durability when subjected to repeated potential cycles or longterm electrolysis.The overpotential requirement for fuel cell reactions viz. methanol oxidation reaction and oxygen reduction reaction, and detoxification of environmental pollutants viz. electroreduction of methyl iodide and chloroacetonitrile, over PdAg-Gr was observed to be just -0.221 V, -0.297 V, -1.508 V, and-1.189 V (vs. Ag/AgCl, 3 M KCl) respectively. The potential of PdAg-Gr nanohybrid for simultaneous and sensitive electrochemical sensing and estimation of hydroxy-benzene isomers with very low detection limits (0.05 µM for hydroquinone, 0.06 µM for catechol, 6.7 nM for 4-aminophenol and 13.7 nM for 2aminophenol) is demonstrated. Additionally, the PdAg-Gr was observed to offer excellent solution phase catalytic performance in bringing about the reduction of notorious environmental pollutant 4-nitrophenol to pharmaceutically important 4-aminophenol with an apparent rate constant (kapp) of 3.106 x 10-2 s-1 and normalized rate constant (knor) of 6.21 x 102 s-1g-1. The presented synthetic scheme besides being high yielding, low cost and easy to carry out results in production of PdAg-Gr nanohybrids with stability and activity significantly better than most of the nanomaterials purposefully designed and testified so far by various groups.

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Key words: Graphene, Palladium-Silver nanoalloy (PdAg), methanol oxidation reaction (MOR), oxygen reduction reaction (ORR), halocarbons, hydroxy benzene isomers and 4nitrophenol. 1. Introduction Fabrication of nano structured metallic, bi-metallic, metal-alloy and metal-oxide electrocatalysts has received considerable attention of material chemists in last decade.1-6A large number of efforts have also been devoted towards the design of novel nanostructured carbon materials like nanotubes, carbon fibers, carbon xerogel, hollow carbon spheres, ordered mesoporous carbons1,2, 4-6 and graphene/reduced forms of graphene oxide4 as the host supports of such nano heterostructures. The carbon supports have been demonstrated to significantly boost the electrocatalytic/catalytic efficiency, durability, tolerance towards poisoning/fouling,

selectivity and sensitivity of metal nanodeposits.3-6The unique

physicochemical and electronic properties of graphene/reduced graphene oxide make it an excellent support material for the deposition of metal nanoparticles (NPs). Recent studies have established that both graphene/reduced graphene oxide not only act as conducting support of such nano-conjugates, these also offer additional tuneable features that can be exploited for enhancing the efficiency and selectivity of metal-carbon nanohybrids.1-2, 4-9 Among the diverse range of nanostructured electrocatalysts reported so far, the platinumbased ones always prove to be the most active in lab scale demonstrations. Hence, the nanosized platinum and its composites have been projected as most effective electro-catalysts for fuel cells,1-3 electrochemical sensors,10-11and electrochemical/chemical detoxifiers for environmental contaminants like organic halides,12-13 nitro-aromatics and dyes.14-15However, the high cost, poisoning/fouling tendency, durability and efficiency concerns associated with platinum based nanoelectrocatalysts pose some serious challenges in electrochemical setups. These concerns affect the cost, design, operation, stability and efficiency of electrochemical setups and hence hamper the large-scale use of platinum based electrocatalysts in commercialized electrochemical setups. Therefore, serious research efforts are going on towards the design and exploration of electrocatalytic performances of non-platinum based electro-catalysts/catalysts. In this context, the use of palladium in view of its availability, lower cost and strong tolerance towards carbon monoxide (CO) poisoning as a suitable alternative to Pt has been extensively investigated.16-17However, the lower

catalytic

performance and poor anti-poisoning property of Pd impose serious limitations on its role as a promising future electrocatalyst.18-19Alloying of Pd with other precious or non-precious 2|Page ACS Paragon Plus Environment

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metals like Au, Ag, Cu, Ru and Sn etc. has been found to enhance the electrocatalytic efficiency of Pd.17-24In this regard, many recent reports23-24have evidenced the use of Ag cocatalyst with Pd as an unsurpassed choice, both in terms of cost and catalytic activity. It is pertinent to mention that besides their use as electrocatalysts for fuel cell reactions, both Ag and Pd have been reported to offer excellent activity towards the electrochemical/chemical reduction of biologically toxic halo hydrocarbons and nitro-phenols and selective sensing of hydroxy-benzene isomers.10-15,25-27 An extra advantage in working for design of bimetallic electrocatalysts is that in addition to shape, size and geometry, these systems offer composition as an additional parameter that can be employed to tune their electrocatalytic activity. In fact, manoeuvring the desired shape and distribution of the constituents in these systems proves to be extremely difficult and often not reproducible. Previous reports regarding the synthesis of homogenous PdAg nano-alloy are either hard to reproduce or often result in end products with a Pd rich surface encapsulating Ag rich core (core-shell type).14For example, in graphene supported PdAg nanorings prepared through galvanic exchange approach, palladium ends up as PdO or Pd(OH)2 on ring surface with only a minimal of it present in its native form.28 Therefore, there is an urgent need for the development of a facile and reproducible synthetic strategy for preparation of PdAg nanoalloy with homogenous composition. In the present study, we report a highly reproducible, facile one-pot synthetic approach for the deposition of PdAg nanoalloy crystals with homogenous composition on graphene.The electrocatalytic performance of the resulting nanocomposite referred to as PdAg-Gr hereafter in the manuscript, was investigated for several important electrochemical reactions like methanol oxidation reaction (MOR), oxygen reduction reaction (ORR), electrochemical dehalogenation of methyl-iodide (CH3I) and chloroacetonitrile (ClACN). The PdAg-Gr was tested for its electrocatalytic performance for the selective sensing of hydroxybenzene isomers, and its catalytic activity for the reduction of 4-nitrophenol (4-NP). The PdAg-Gr nanohybrids were found to exhibit outstanding electrocatalytic, catalytic and electroanalytical performance.The presented synthetic approach besides being high yielding, low cost and easy to work out, results in production of PdAg-Gr nanohybrids with stability and activity significantly better than most of the similar nanomaterials so far reported by various groups. 2. Experimental Section 2.1

Reagents and Materials



Graphite powder with particle size < 50µm, CAS#7782-42-5 and hydrogen peroxide (30 %) were obtained from E-Merck. Sulphuric acid (98%), potassium permanganate, sodium hydroxide pellets, potassium nitrate, ammonia (30 %), methanol (CH3OH, 99%), methyliodide (CH3I), and chloroacetonitrile (ClACN) were purchased from Merck India. Silver nitrate (AgNO3), hydroquinone (H2Q), catechol (CC), 4-aminophenol (4-AP), 2-aminophenol (2-AP), 4-nitrophenol (4-NP) and sodium borohydride (NaBH4) of AR grade were obtained from SRL chemicals. Palladium acetate (Pd(OAc)2) and 25 % Nafion were Sigma Aldrich products. Tetrabutylammonium perchlorate (TBAP) and hydroxylamine hydrochloride (NH2OH.HCl) of AR grade were procured from High Media Laboratory Chemicals. Triply distilled water and dimethyl formamide (DMF) obtained from E-Merck were used as solvents for electrochemical investigations. 2.2 Synthesis of Nano-assemblies Graphene oxide was synthesized through improved Hummers method using acidified KMnO4 as oxidizing agent.29The details about synthesis of Ag-graphene nanocomposite are reported elsewhere.30Factors like standard reduction potentials of metals, strength of reducing agent and temperature are key to the formation of alloy nanoparticles. For the present work, we choose Pd(OAc)2 and AgNO3 as sources of required metal ions.The selection was made in view of the reported standard reduction potential of 0.590 V for PdCl42-/Pd2+, 0.951 V for Pd(OAc)2/Pd2+ and 0.799 V for AgNO3/Ag+, which suggest that the combination of Pd(OAc)2 and AgNO3 should favour the formation of Pd-Ag alloy nanoparticles.31The synthesis of PdAg-Gr nanoalloy was carried out through reduction of graphene oxide, Pd-salt and Ag-salt under suitable reducing conditions of hydroxyl amine (NH2OH) followed by a thermal treatment of the product at around 150 oC. The purpose of the thermal treatment was to promote the diffusion and fusion of Pd and Ag nano crystals to homogenize their distribution. The typical advantages about use of NH2OH, a strong reducing agent, in PdAgGr alloy synthesis have been discussed in our previous study.30In a typical experiment, 0.0625 grams of graphene oxide dissolved in 100 mL triple distilled water was subjected to one-hour ultra-sonication to obtain a homogeneous dispersion. The solution was kept on stirring at 60 oC in an oil bath followed by addition of 5 mM AgNO3 and Pd(OAc)2. After addition of 0.125 grams of NH2OH.HCl and 1 mL of 30 % aqueous ammonia, the reaction mixture changed its colour from yellow to dark brown. The temperature of the continually stirred reaction mixure was gradually raised to 150 oC with increments of 10 oC per 15 minutes and maintained at this value for about 30 minutes. The resultant product was 4|Page ACS Paragon Plus Environment

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centrifuged, washed several times and finally dried under vacuum. Pd-Gr was synthesized following the same procedure except the addition of AgNO3. 2.3. Instrumentation The PdAg-Gr nanocomposites were characterized through UV-Vis spectrophotometry, powder X-ray diffraction, TEM analysis and XPS. The details of these measurements and specification

of

instruments

employed

are

reported

elsewhere.30Electrochemical

measurements were performed with a Metrohm Autolab potentiostat/-galvanostat (PGSTAT100N). A three-electrode set-up with glassy carbon (GCE, 2 mm diameter) or GCE modified with Ag-Gr, Pd-Gr and PdAg-Gr as working electrode, platinum wire as counter electrode and Ag/AgCl 3M KCl as reference electrode in aqueous media, all from Metrohm Devices, Netherlands, were used for electrochemical investigations. A silver wire (99.9 %) and platinum (99.9 %) loop was used as quasi-reference (QRE) and counter electrode (CE) respectively in DMF as solvent. These were properly rinsed and cleaned with dilute HNO3, washed several times with Millipore water and wiped with soft tissue soaked in ethanol before connecting them in the electrochemical cell. 2.4. Electrode Fabrication For the preparation of Ag-Gr, Pd-Gr and PdAg-Gr modified GCE electrode, the GCE (2 mm diameter) was polished with alumina slurry (0.5 µm-0.05 µm), followed by washing with copious amount of triple distilled water and finally with ethanol. To ensure the complete removal of polishing powder from the electrode surface these were subjected to short period sonication cycles in water-ethanol mixture and thereafter a prolonged submersion in the said solvent mixture. Aqueous dispersion of the composite was prepared by dissolving 1 mg of Ag-Gr, Pd-Gr and PdAg-Gr per mL of water separately with addition of 10 µL of 25 % Nafion as binder and sonicating the mixtures for one hour at 25 oC. Appropriate volume of the aqueous dispersion was drop casted on GCE disk, and allowed to dry under ambient conditions for 12 hours. Prior to electrochemical measurements the electroanalyte solutions were purged with argon for 10 minutes and kept under Ar environment during the measurements. 2.5. Kinetic measurements For the kinetic study of catalytic reduction of 4-NP with NaBH4, fresh solutions of 4-NP and NaBH4 were prepared. For a typical kinetic run, 4-NP and NaBH4 at concentration of 0.125 5|Page ACS Paragon Plus Environment


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mM and 2 mM respectively in water were placed in a quartz cuvette, which led to formation of a yellow mixture. After the addition of 0.025 mg of PdAg-Gr as catalyst, the UV-Vis spectra were recorded instantaneously. The absorption peak intensity at 400 nm was recorded as a function of time. Following similar procedure, the kinetics of 4-NP reduction with NaBH4 in presence of different amounts of PdAg-Gr, Ag-Gr and Pd-Gr were investigated. 3. 3.1

Results and Discussions Characterization of Nano-assemblies

The details of structural elucidation of graphene oxide and Ag-Gr synthesized through the method employed for present work are reported in our earlier publication.30 The UV-Vis absorption characteristics for Pd-Gr and PdAg-Gr are presented in the form of figure 1 (A) and those for graphene oxide and Ag-Gr are depicted as figure S1 (A) in supporting information (SI). The UV-Vis spectrum of Pd-Gr (trace a, figure 1 (A)) displays a characteristic peak at around 270 nm, which can be ascribed to restoration of π-conjugation in graphene oxide upon reduction. In the UV-Vis spectrum of PdAgGr (trace b, figure 1 (A)) two bands, one in the range of 250-270 nm attributed to π-π* transition of C=C and the other in the range of 380-420 nm due to surface plasmon resonance (SPR) of AgNPs are noticed.30The SPR peak expected for nanodimensional Ag in PdAg-Gr was noticed to have broadened with a marked decrease in absorbance, this we attribute to substitution of Ag atoms by Pd atoms in the crystal lattice. These UV-Vis characteristics observed for PdAg-Gr, which precisely match with those reported for PdAg nanoalloy networks were the first indications to suggest the successful deposition of PdAg nano alloys on graphene.28,

32The

reduction of

graphene oxide to graphene and deposition of PdAg nanoalloys on its surface was also substantiated by the Raman spectra presented in figure S2 (SI). Increase in the intensities of D and G bands relative to that recorded for graphene established the deposition of PdAg NPs on graphene surface. Moreover, the intensity ratio of D and G bands (ID/IG) observed in the Raman spectra, a well-known signature of graphene oxide reduction was found to increase from 0.85 to 1.0 for the conversion of graphene oxide to PdAg-Gr.30 The crystallinity of the nano-assemblies was also confirmed by recording powder Xray diffraction (XRD) as shown in figure 1 (B). The XRD patterns of Pd-Gr (trace a, figure 1 (B))and PdAg-Gr (trace b, figure 1 (B)) both depict broad reflections at 2θ of around 25o from (002) planes of reduced graphene oxide.30 These broadened 6|Page ACS Paragon Plus Environment

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reflections, in accordance with observed UV-Vis attributes indicate the restoration of π-π conjugation in reduced graphene oxide upon its reduction. Besides, the XRD pattern of

Figure 1. (A) The UV-Vis spectra of Pd-Gr (red, a) depicts characteristic peak of C=C due to π-π transitions of graphene network. The UV-Vis spectra of PdAg-Gr (blue, b) depicts two broad bands, one in range of 250-270 nm due to π-π transitions of graphene network and the other in the range of 380-420 nm due to surface plasmon resonance (SPR) of AgNPs in PdAg-Gr. (B) The XRD spectra of Pd-Gr (black, a) and PdAg-Gr (black, b) both depict a broad peak due to (002) plane of graphene and characteristic peaks due to PdNPs (in Pd-Gr and PdAgGr) and AgNPs (in PdAg-Gr). In XRD spectra of PdAg-Gr the reflections exhibit a shift from that of Pd and Ag due to alloy formation.

Pd-Gr depict peaks at 2θ of 40.0o, 46.5o and 68.2o characteristic to reflections from (111), (200) and (220) planes of Pd nanoparticles (PdNPs) (JCSPDS No. 894897). In the XRD pattern of Ag-Gr, (figure S1 (B)) the reflections from (111), (200), (220) and (311) planes of Ag can be noticed at 2θ of 38.1o, 44.3o, 64.5o and 77.3o respectively (JCPDS No. 893722).The reflections from (111), (200), (220) and (311) planes for PdAg-Gr were observed at 2θ of 38.9o, 44.97o 65.0o and 77.9o. These 2θ values lie somewhere between the corresponding values observed for Ag (JCPDS No. 893722) and PdNPs (JCPDS No. 894897). These shifts observed in the diffraction peaks for the synthesized PdAg-Gr relative to those observed for pure Pd and Ag, establish its alloy character.28 Similar peak shifts in XRD pattern ascribed to alloy formation between Pd and Ag have been reported by other groups.24,33-34Moreover, the reflections due to oxide phase of silver, which although observed in XRD pattern of Ag-Gr30 (figure S1 (A)), are completely absent in PdAg-Gr. These recorded XRD characteristics for PdGr and PdAg-Gr are in good agreement with ones reported in literature.24, 33-34 In order to determine the composition of the PdAg-Gr nanocomposite, the elemental analysis was carried through EDAX and EDS measurements on the sample surface.



The corresponding maps observed for different elements are presented in figure S3 (AD)

Figure: 2. The TEM image of: (a) Pd nanoparticles on reduced graphene oxide with size range of 11-20 nm, (b)Ag nanoparticles on reduced graphene oxide with size range of 5-6 nm(c)and (d)PdAg alloy nanoparticles with size range 5-10 nm well dispersed on reduced graphene oxide. Inset of (d) magnified portions of PdAgGr.HR-TEM images of Pd-Gr (e) and PdAg-Gr (f).

(SI),which reveal the presence of carbon, oxygen, silver and palladium in PdAg-Gr in the relative percentages of ca.46 %, ca. 4 %, ca.26 % and ca.24 %, respectively by weight. The relative atomic composition observed for PdAg-Gr is presented as table S1 (SI). TEM and high-resolution TEM (HR-TEM) were employed to inspect the microscopic structures of Pd-Gr and PdAg-Gr and recorded sample images are shown in figure 2 (a) (Pd-Gr),(b) (Ag-Gr)and (c-d) (PdAg-Gr). From the recorded TEM images, the size of metallic nanoparticles was estimated to be in the range of 11-20 nm for PdNPs with an average size of 15 nm and in the range of 5-6 nm for AgNPs with an average size of 5.5 nm and a wide range of 5-10 nm for PdAg nanoalloys with an average size of 7 nm. In order to confirm the same, the size of PdAg nanoparticles was also calculated from XRD data using Scherrer equation24 which revealed the size of 8|Page ACS Paragon Plus Environment

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6.4 nm for the same. The estimated size for PdAg nanoalloys from XRD data does agree with TEM images very well. The size of PdAg NPs on Gr is smaller than Pd NPs on Gr but larger than Ag NPs on Gr which might be due to the differences in the reactivity of Ag and Pd precursors viz. Pd(OAc)2 and AgNO3 towards the reducing conditions of hydroxyl amine (NH2OH) used in the presented synthesis that would result into different nucleation and growth kinetics (through Ostwald ripening). From the recorded TEM images figure 2 (a-d) and inset image of (d), (magnified version of PdAg-Gr), it is apparent that the crystal structure, shape/morphology and size of metallic nanodeposits over graphene in PdAg-Gr (alloy deposit) are quite different from those in Pd-Gr and Ag-Gr. HR-TEM images presented in figure 2 (e-f) further establish that the metallic/alloy nanoparticles are crystalline with clear lattice structures. Figure 2 (e) reveals a lattice distance/fringe width of 0.220 nm which matches well with [111] fcc plane of Pd.33Figure 2 (f),the HR-TEM image of PdAg nanoalloys on graphene reveals lattice distances/fringe width of 0.298 nm. The reported lattice distance for [111] plane in PdNPs is 0.220 nm and for [111] plane in AgNPs it is around 0.230 nm.33-35The estimated fringe widths for PdAg nanoalloys (Figure 2 (f)) on graphene do not match with lattice distances either of Pd or Ag. This type of variation in lattice/interplaner distances is attributed to alloy formation between different metals.24,33,36To establish the chemical states of the metal deposits, characteristic XPS patterns were recorded for Ag-Gr, Pd-Gr and PdAg-Gr samples and the same are depicted as figure 3. For Ag-Gr sample (blue a, figure 3 (A)) two main peaks are observed in carbon region at 284.5 eV and 286.8 eV which can be attributed to C-C bond and C-bonded to oxygen respectively.16, 37

Figure 3.XPS spectra for the Ag-Gr, Pd-Gr and PdAg-Gr. (A) XPS spectra in carbon region: AgGr(blue, a),Pd-Gr (black, b) and PdAg-Gr (red, c). (B) XPS spectra in Ag region for Ag-Gr (red, a) and PdAg-Gr (black, b). The positive shift in binding energy of Ag in case of PdAg-Gr is indicative of alloy formation with Ag in native oxidation state. (C) XPS spectra in Pd region for Pd-Gr (red, a) and PdAgGr (black, b). The decrease in intensity and a slight negative binding energy shift is due to alloy formation between Pd and Ag.



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Though peak positioned at 284.5 eV is retained, the peak at 286.8 eV reduces to a hump for Pd-Gr sample (trace b, figure 3 (A)).Comparatively, while the intensity of CC peak in PdAg-Gr (trace c, figure 3 (A)) seems to have sharply increased, the peak due to C-bonded oxygen is almost invisible in the XPS spectrum of PdAg-Gr. The C 1s spectrum recorded for PdAg-Gr (trace c, figure 3 (A)) is very similar to that of highly ordered pyrolytic graphite (HOPG)38 suggesting that the nano-alloy is almost free from C-bonded oxygen. This is also supported by the ID/IG ratio observed in the Raman spectra recorded for the PdAg-Gr. These features suggest an increase in number of C-C bonds and a decrease in C-O functionalities in PdAg-Gr comparative to Ag-Gr and Pd-Gr nanocomposites. These observations noticed in the XPS spectra suggest the restoration of π-conjugation in graphene being facilitated by the presence of Pd and the restoration seems to be the highest in case of PdAg-Gr. The XPS patterns of Ag-Gr for the presence of Ag, shows two peaks at 366.91 and 372.91 eV (figure 3 (B), red a), which can be attributed to presence of Ag in its native and oxide forms.30,39The same peaks for Ag are shifted to 367.91 and 373.91 eV respectively (figure 3 (B), black b) in PdAg-Gr with an increased intensity. It has been reported that in metallic Ag, 3d peaks are centred at 367.6 and 373.90 eV28,

37

which exactly

match with the values observed for metallic Ag in PdAg-Gr. This leads us to infer that Ag is present in its native form with no Ag-oxides in PdAg-Gr, this is also supported by their XRD pattern. The absence of oxide phase of silver in PdAg-Gr can be attributed to its complete reduction by NH2OH that seems to have been facillitated by the presence of palladium.28,

30

The increase in peak intensity and positive shift in

binding energies (BE) noted for Ag characristic peaks in PdAg-Gr samples can be ascribed to alloy formation between Pd and Ag. Similar shifts in the metal characteristic XPS peaks on account of alloy formation have been reported in the past by several groups.24,36The XPS patterns of Pd-Gr show significant Pd 3d signals at BE of 335.1 eV and 340.35 eV in Pd-region (figure 3 C, trace a)indicating the presence of Pd in its native state.38-40The least intense peaks at 338.15 and 343.15 eV can be attributed to the presence of palladium as PdO or Pd(OH)2.16, 28 Compared to Pd-Gr, the native Pd 3d signals in XPS pattern of PdAg-Gr(figure 3 C, black b) are shifted negatively. Besides, figure 3 (C) also indicates that the intensity of Pd 3d signals corresponding to Pd(0) are more intense in Pd-Gr as compared to PdAg-Gr, a probable reason being replacement of some fraction of Pd by Ag in the crystal lattice of the 10 | P a g e ACS Paragon Plus Environment

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former. This BE shift and lower intensity of Pd 3d peaks in XPS pattern of PdAg-Gr can be attributed to alloy formation between Pd and Ag. The XPS pattern suggested a 1:1 atomic ratio of Pd:Ag in the PdAg-Gr. These observations regarding alloy formation between Pd and Ag are in good agreement with the earlier published reports.24,

36

Low intensity of Pd 3d peaks at 338.15 and 343.15 eV suggests a

negligible presence of PdO or Pd(OH)2 fraction in PdAg-Gr, implying that Pd must be predominantly in its metallic state in PdAg-Gr. This is in total contrast to the products of galvanic exchange method reported for synthesis of PdAg alloy nanorings on graphene wherein most of Pd has been found to be present as PdO or Pd(OH)2 with only traces of its metallic form in the surroudings of Ag.28

3.2. Electrochemical behaviour of Nano-assemblies The Ag-Gr, Pd-Gr and PdAg-Gr nanocomposites were tested for their electrochemical behaviour under both acidic and alkaline conditions. Figure 4 (A) depicts their representative CVs recorded with a potential sweep rate of 25 mV/s in 0.5 M H2SO4 at 298 K. Ag-Gr modified glassy carbon electrode in 0.5 M H2SO4 shows two redox peaks A1 and C1. The anodic peak A1 at 0.240 V is the outcome of oxidation of Ag (0) to Ag (I)-oxide and the cathodic peak C1 at 0.032 V for reduction of Ag (I)-oxide to Ag (0).30These electrochemical characteristics observed for Ag-Gr are in agreement with the previous reports.30,41For Pd-Gr, the representative CV (trace b, Figure 4 (A)) demonstrates two hydrogen desorption faradaic peaks in the anodic scan with their complimentary hydrogen adsorption peaks in the cathodic scan placed symmetrically on the potential axes. While the anodic peaks can be attributed to underpotential hydrogen adsorption (UPD-Hads) and overpotential hydrogen adsorption (OPD-Hads) the anodic peaks can be attributed to the complimentary hydrogen desorption processes.5 Moreover, in the anodic potential sweep, formation of Pd-oxides is observed in the range of 0.64 V to 1.02 V (labelled as peak A2) while in the reverse sweep, the reduction of these oxides to Pd is seen in the form of a cathodic peak (C2) at 0.456 V. For PdAg-Gr (trace c, Figure 4 (A)), characteristic faradaic peaks regarding the surface Ag as well as Pd are observed.These observations clearly establish the presence of both the Pd and Ag as electroactive components of the PdAgGr surface.The UPD-Hads and UPD-Hdes peaks with peak potentials of -0.042 V and -0.250 V as observed in the CV recorded over Pd-Gr are not clearly seen in the CV recorded over PdAg-Gr. This implies a strong dispersion of Pd in the PdAg-Gr leaving 11 | P a g e ACS Paragon Plus Environment


minimal binary and higher Pd-Pd domains required for exhibition of hydrogen adsorption/desorption.42 Besides, in anodic potential excursion in PdAg-Gr, an anodic peak labelled as A3 with peak potential of 0.260V which can be attributed to faradaic conversion of Ag (0) to Ag (I) oxide and the broad peak A4 in potential range of 0.61.1 V that can be attributed to Pd-oxidation are noticed.

Figure: 4 Cyclic voltammograms recorded for Ag-Gr (blue) (a), Pd-Gr (red) (b), and PdAg-Gr (green) (c) at 298 K with sweeping rate of 25 mV/s in 0.5 M H2SO4(A) and 0.1 M KOH (B) aqueous solutions showing the characteristic redox peaks for the pure metal nanoparticles of Ag, Pd and PdAg alloy catalysts supported on reduced graphene oxide. The exhibition of both Ag and Pd oxide reduction peaks indicates that the surface PdAg-Gr is composed of both metals.

In the negative potential scan, a broad peak C3 in the range 0.285-0.465 V attributed to reduction of Pd-oxides and a sharper peak C4 at 0.116 V, a faradaic response corresponding to reduction of Ag-oxides are observed in the CV recorded for PdAgGr. Compared to the voltammetric response of Ag-Gr, the oxidation peak potential of Ag (0) to Ag (I) and the reverse reduction peak on PdAg-Gr are both shifted to more positive potentials. This electrochemical observation is an indication of stabilization of native silver with less stable oxide phase in alloy form on graphene. To further support and validate this claim in favour of alloy formation between Pd and Ag, CVs for AgGr/GCE, Pd-Gr/GCE and PdAg-Gr/GCE were recorded in 0.1N KOH with scan rate of 25mV/s and the representative ones are presented as figure 4(B). The faradaic response recorded over Ag-Gr (figure 4(B), trace a) under alkaline conditions exhibits characteristic anodic peak for Ag-oxidation at 0.225 V and a cathodic peak for reduction of Ag-oxide at -0.03 V in the reverse scan. The trace b of figure 4 (B) displays the electrochemical oxidation of Pd to PdO/ Pd(OH)2 on Pd-Gr at 0.6-0.85 V and in the reverse scan the reduction of Pd-oxides is seen as a peak around -0.47 V. In voltammograms recorded over PdAg-Gr, (figure 4(B), trace c), characteristic peaks for the expected faradaic response of Ag as well as Pd were observed. In the positive 12 | P a g e ACS Paragon Plus Environment

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potential scan (trace c, figure 4(B)), formation of Ag and Pd-oxides in the form of transient current responses is noticed in the potential range of 0.1-0.66 V while in the reverse scan reduction of Ag and Pd-oxides as peaks centred around 0.029 V and 0.389 V respectively are observed. The faradaic responses over PdAg-Gr in alkaline medium are totally in contrast to that at PdAg-bimetallic alloy networks wherein only the faradaic response for Pd have been reported.24 These electrochemical observations which clearly imply surface presence of both Ag and Pd in PdAg-Gr, with support from other experimental and reported evidences,36 strongly authenticate that the surface of PdAg-Gr consists of both Pd and Ag with a homogenous alloy composition and rules out the possibility of formation of core-shell type nano alloy or heterogeneous deposition of Ag and Pd nanodeposits on graphene in the present study. The electrochemically active surface area (ECSA) of Pd-Gr and PdAg-Gr with respect to per gram of Pd-loading as estimated from the integration of reduction peak for Pdoxides (table 1)16 were estimated to be ca.6.7 m2/gPd and ca.21.6 m2/gPd, respectively.Thus, the ECSA for Pd on PdAg-Gr is more than three times that on PdGr. The increased ECSA of Pd in PdAg-Gr may be attributed to the smaller size of alloy clusters and the impact of the presence of Ag in the vicinity of Pd.36

3.3. Methanol Oxidation and Oxygen Reduction reactions Palladium and silver both in their bulk or nano-phase have been tagged as front runner substitutes to Pt- as electrode materials for MOR and ORR, two very crucial alkaline fuel cell reactions.1-3 The main advantage of alkaline medium comes from the fact that the electrode reaction kinetics in this medium is more facile than in the acid medium. Additionally for oxygen reduction at high pH unlike the case of low pH conditions, no specific chemical interaction between the catalyst and O2 or O2− is required; this permits the use of non-platinum based catalysts for electroreduction of O2 in alkaline conditions. Besides, Pd-based nanoarchitectures display superior electrocatalytic activity in alkaline medium as compared to acidic medium and diminishes the possibility of corrosion/poisoning of electrode surface.1-3 Therefore, electrocatalytic performance of PdAg-Gr was tested for MOR and ORR reactions under alkaline conditions and the observed activity was compared with those of Ag-Gr, Pd-Gr and bare-GCE. Figure 5(A) shows representative CV traces recorded with scan rate of 25 mV/s for MOR on different electrode systems in 0.1N NaOH at 298 K. A quantitative comparison of the observed electrocatalytic ability of different electrode systems towards MOR, the associated voltammetric parameters viz. peak potential (Ep), mass activity (A/gPd) and specific 13 | P a g e ACS Paragon Plus Environment


activity (µA/cm2) is presented in the form of table 1. No faradaic response was observed for electro-oxidation of methanol on bare-GCE (trace a, figure 5(A)) and AgGr/GCE (trace b, figure 5(A)). The voltammetric response recorded over PdGr/GCE(trace c, figure 5(A)), shows an anodic peak attributed to electrooxidation of methanol (positive potential) in forward scan and a less intense anodic peak in the reverse scan. The latter one is attributed to oxidation of carbonaceous intermediates adsorbed during the forward scan. Similar response was observed in the CVs recorded for methanol oxidation on PdAg-Gr/GCE (trace d, figure 5 (A)) albeit with voltammetric parameters suggestive of its electrocatalytic performance better than AgGr and Pd-Gr. The onset potential for methanol oxidation over Pd-Gr/GCE was observed to be -0.404 V with peak potential at -0.023 V. In comparison to Pd-Gr/GCE both the onset and peak potential for MOR were shifted negatively to -0.668 V and 0.221 V respectively over PdAg-Gr/GCE. Notably, the observed peak potential for MOR with PdAg-Gr in alkaline medium is more negative than those reported for graphene-silver (0.61 V),37 Pt/graphene (0.68 V),43Pt/CMK-5 (0.65 V),44 Pd-PANI nano-flower (-0.20 V),45Pd-rGO (-0.04 V),46Pd/Vulcan (-0.19 V), Pd/graphene (-0.17 V), Pd/PPy-graphene (-0.17 V),47 and Pd-Au (1:0.5)/rGO (-0.095 V).48 A comparison of catalytic efficiency of PdAg-Gr towards MOR with other catalysts is presented in table S2 (SI).

Figure 5:CVs recorded for MOR in 0.1 M NaOH and ORR in 0.1 M KOH aqueous solutions at 298 K with sweep rate of 25 mV/s. (A) Comparative CVs for MOR at changing electrode system: (a) Baseline for solvent (black), (b)Ag-Gr/GCE (red), (c)Pd-Gr/GCE (blue) and (d) PdAg-Gr/GCE (magenta). (B) MOR with changing scan numbers (1-30) at PdAg-Gr/GCE. The insets show variation of peak current (Ip) and peak potential (Ep) for forward peak with increasing scan numbers. (C) Comparative CVs recorded for ORR at various electrode systems: (a) Baseline for solvent (black), (b) Bare-GCE (red), (c) Ag-Gr/GCE (blue), (d) Pd-Gr/GCE (magenta) and (e) PdAg-Gr/GCE (green).

Besides the desired shifts in onset and peak potential, the mass activity (A/gPd) corresponding to faradic signal for methanol on PdAg-Gr/GCE was found to be about 5 times as compared to that observed for Pd-Gr/GCE. In the successive voltammetric 14 | P a g e ACS Paragon Plus Environment

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scans for MOR over PdAg-Gr/GCE, the peak current was observed to increase sharply with peak potential shifting negatively after every passing scan, and attaining a constant value after ca. 15 cycles(inset of figure 5 (B)). These observations suggest an increase in the efficiency of the electrocatalyst, which can be attributed to exposure of new catalytic sites on successive scans. Perversely, in most of the electrode systems designed for MOR, the peak height has been found to decrease after successive scans and it is attributed to poisoning of the electrocatalyst.2The ratio of forward peak current (If) to backward peak current (Ib) is an important parameter used to index the efficiency/tolerance of MOR electrocatalysts. A higher If/Ib indicates a more efficient removal of poisonous carbonaceous intermediates from the catalyst surface. The If/Ib value for MOR on PdAg-Gr/GCE was estimated to be ca. 28.7, more than four times that has been reported on Pd-Gr.47These results clearly suggest that PdAg-Gr/GCE exhibits better electrocatalytic performance and tolerance towards poisoning by electrogenerated carbonaceous intermediates during MOR.

Electrode ECSAPd -Ep m2/gPd (mV) Pd-Gr 6.7 23 PdAg-Gr 21.6 221

MOR Mass Specific Activity Activity A/gPd µA/cm2 7.8 113.71 39.78 261.14

-Ep (mV) 354 297

ORR Mass activity A/gPd 27.26 106.45

Specific Activity µA/cm2 885.98 1592.35

Table 1: Comparison of different parameters viz ECSAPd, peak potential (Ep), mass activity and specific activity for MOR and ORR performed on Pd-Gr and PdAg-Gr.

Comprehensive investigations reported for oxidation of CO on a Pt-surface have revealed a Langmuir-Hinshelwood type adsorption/reaction between CO and surface hydroxyl groups during MOR.49 CO adsorption being energetically more exothermic leads to its preferential adsorption on Pt-surface leaving behind lesser number of sites available for the-OH(ad). This in turn leads to a decrease in the probability of CO(ad)OH(ad) reaction and hence the blockade and reduced availability of electrode surface for the MOR. In alkaline media the oxidative removal of CO(ad) wherein the adsorption of hydroxide ion plays a key role has been proved to be the rate determining step of MOR.49 Therefore, to address the issue of electrocatalyst poisoning by CO(ad) during MOR, one of the strategies is mixing of the catalyst with a suitable co-catalyst

15 | P a g e ACS Paragon Plus Environment


possessing high affinity for hydroxyl groups. Appropriate inclusion of such a cocatalyst ensures a continuous availability of -OH- for its reaction with CO(ad) and thereby reducing the chances of its poisoning by the latter during MOR. Density functional studies reported by Ferrin and Mavrikakis50 in this regard suggest that the free energy of adsorbed CO on Ag is higher than on Pd while reverse is the case for adsorbed OH.Thus, the enhanced catalytic activity and stability of PdAg-Gr towards methanol oxidation as observed in present study can be explained on the basis of bifunctional mechanism theory.24,51The enhanced electrocatalytic performance of PdAgGr for MOR in light of bifunctional mechanism theory can be attributed to Ag specific adsorption of OH- over PdAg-Gr, that is expected to facilitate the removal of carbonaceous intermediates like CO(ad).24, 50, 51-52In the electrocatalytic pathway, the Ag in PdAg-Gr/GCE is therefore expected to act like a relay that provides hydroxyl ions to catalyst surface for oxidative removal of poisonous intermediates. This oxidative removal of CO(ad), besides ensuring a blockade free catalytic surface is expected to significantly electrocatalyse the MOR over PdAg-Gr/GCE surface. It is pertinent to mention, that although Pd and Ag possess similar crystal structure (fcc), the ionization energy as well as electronegativity of Pd is more than Ag. Hence, the substitution of Pd sites by Ag in the Pd lattice during alloy formation is expected to be associated with electronic charge transfer from Ag to Pd in PdAg-Gr nanoalloy, leaving the Pd sites negatively charged. This is also suggested by the relative position of metal peaks in the XPS spectra and CV characteristics recorded for the Ag-Gr, Pd-Gr and PdAgGr. It is a well-established fact, that negatively charged Pd facilitates the removal of intermediates like CO(ad) and hence presence of Ag in the PdAg-Gr nanoalloy shall make the Pd sites more tolerant towards CO poisoning during MOR.53-54 Similar results have been reported by Y-Lin et al24 for ethanol oxidation reaction over Pd-Ag bimetallic alloy networks. In recent past many research groups have also reported the enhanced catalytic activity of bimetallic PdXAgY/C over Pd/C for MOR,55 which has been attributed to ligand, geometry or ensemble effects. Figure 5 (C) shows comparative CVs recorded with sweep rate of 25 mV/s over Ag-Gr/GCE, Pd-Gr/GCE, PdAg-Gr/GCE and bare-GCE electrode systems for ORR in 0.1 M KOH. The associated voltammetric parameters viz peak potential (Ep), mass activity (A/gPd) and specific activity (µA/cm2) are enlisted in table 1.The over potential requirement for ORR on PdAg-Gr seems to be lower and catalytic activity much better than that reported for the said reaction at PdFe-Graphene,26 PdAg16 | P a g e ACS Paragon Plus Environment

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nanorings on graphene,28 ORR mediated by nanodroplet confined attomoles of Vitamin B-12,56Ag nanobelts,57 BN-Graphene,58 BCN-graphene,59sulphur doped graphene,60and [Co(acac)2] with N-Doped Graphene.61 The observed electrocatalytic activity of PdAg-Gr seems comparable to that reported for graphene nano-ribbon supported graphene quantum dots,62 graphene-porphyrin MOF composite,63and Co/CoO nanoparticles assembled on graphene.64A comparison of peak potentials for ORR at PdAg-Gr with other catalysts is presented in the form of table S3 (SI).Besides appreciable and desired shifts in peak potential for ORR, the mass activity (A/gPd) of PdAg-Gr/GCE was found to be about 4 times than that of Pd-Gr.The enhanced activity of Pd-Gr towards ORR has been attributed to the tensile strain effect for Pd supported over graphene.52 The tensile strain has been reported to result in electron exchange between Pd and graphene interfaces, thereby increasing the interaction states and transmission channels between these two while keeping enough pi-electrons available on graphene for conduction.52An increase in the tensile strain effect due to presence of Ag in PdAg-Gr is probably responsible for enhanced electrocatalytic performance of PdAg-Gr towards ORR in comparison to Ag-Gr and Pd-Gr. The enhanced electrochemical efficiency in such cases has been attributed to ensemble effects wherein more than one reactive sites, composed of different metal atoms or clusters catalyse various reaction steps in a multi-step reaction.36 To quantify the kinetic parameters associated with ORR, linear sweep voltammetry (LSV) measurements were carried out in an O2-saturated solution of 0.1 M KOH using a rotating disk electrode (RDE) setup. Figure 6 (A) shows ORR polarization curves recorded over (a) Ag-Gr/GCE, (b) Pd-Gr/GCE and (c) PdAg-Gr/GCE with disk rotating rate of 400 rpm and scan rate of 50 mV/s. The corresponding Tafel plots for ORR obtained from the LSVs are depicted as figure 6 (B).The estimated Tafel slopes with PdAg-Gr, Pd-Gr and Ag-Gr catalysts were ∼40 mV/dec, 141.5 mV/dec and 143.9 mV/dec, respectively. The Tafel slope of 40 mV/dec observed for PdAg-Gr as catalyst is much lower than that observed for Pd-Gr, Ag-Gr and Pt/C.3It is important to note that both the onset potential (Eonset = -0.170 V vs Ag/AgCl, 3 M KCl and Eonset = 0.806 V vs RHE) and half-wave potential (E1/2 = -0.320 V vs Ag/AgCl, 3 M KCl and E1/2 = 0.656 V vs RHE) for ORR with PdAg-Gr as catalyst are appreciably more positive than both Pd-Gr and Ag-Gr. The ORR exhibited a diffusion-limited current density of 6.7 mAcm-2(1600 rpm) at PdAg-Gr/GCE.The estimated parameters like Tafel slope,



onset potential (Eonset) half-wave potential (E1/2), and diffusion-limited current density for PdAg-Gr as catalyst are either far better or comparable to many catalytic materials like Pt (20 wt %)/C, Pt/C(Mo2C), Pt@Au/PyNG, Au(36 at.%)/Ag, Janus AgAu, Pd nanocube and Ag@Pt@Ag etc. reviewed recently for ORR in reference 3.A comparison of some of the parameters estimated for ORR at PdAg-Gr as catalyst with others is presented in the form of table S3 (SI).

Figure: 6 (A) LSV of O2-saturated solution of 0.1 N KOH at (a) Ag-Gr/GCE (b) Pd-Gr/GCE (c) PdAg-Gr/GCE working electrodes at disk rotating rate of 400 rpm and scan rate of 50 mV/s. (B) Tafel plot of corresponding LSVs. (C) LSVs of O2-saturated solution of 0.1 N KOH at PdAg-Gr/GCE working electrode at different disk rotating rates (100-2200 rpm) at scan rate of 50 mV/s. (D) Koutecky-Levich plots (J-1 vs ω-1/2) at different working electrodes viz Ag-Gr/GCE (at -0.85 V), Pd-Gr/GCE (at -0.8 V) and PdAg-Gr/GCE (at -0.5 V).

The LSVs for ORR were also recorded at changing disc rotating rates using all the three catalytic surfaces prepared for the present investigations. Figure 6 (C) shows ORR polarization curves at changing rotating rates (100-2200 rpm) with PdAg-Gr as catalyst. As expected, the current density increased with increase in disc rotating rate. The corresponding Koutecky-Levich (K-L) plots obtained for ORR over the Ag-Gr, Pd-Gr and PdAg-Gr catalytic surfaces (at potentials of -0.85 V, -0.80 V and -0.5 V vs Ag/AgCl 3 MKCl) are depicted in the form of figure 6 (D). It can be seen that the K-L plots display good linearity, indicating that the ORR over these catalytic surfaces follows a first order kinetics with


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respect to the concentration of dissolved O2. The kinetic parameters were estimated following Koutecky-Levich equations:3

= + = ω/ +

(1)

= 0.62 / ν/ =

(2) (3)

wherein is the measured current density, refers to diffusion limiting current density (Levich theory) and refers to kinetic limiting current density (Koutecky theory), ω is the angular speed of the rotating disk, is the overall number of electrons transferred in ORR, is the Faradays constant ( = 96485 C mol-1), is the bulk concentration and the diffusion coefficient of O2, ν is the kinematic viscosity of the electrolyte, and k is the electron transfer rate constant. From the slopes of these K-L plots the transferred number of electrons per O2 molecule at Ag-Gr, Pd-Gr and PdAg-Gr were estimated to be 3.5, 3.3 and 4.2 respectively. The number of electrons transferred per O2 molecule with PdAg-Gr at different potentials viz -0.45V, -0.5 V, -0.6 V and -0.7 V (vs Ag/AgCl 3 M KCl) were found to be 4.3, 4.2, 4.1 and 4.1 respectively. In view of these results, it can be inferred that the ORR over PdAg-Gr follows one-step 4e process, pathway desired for the fuel cell technology.3 Besides these parameters, the rate constants determined from the intercept of K-L plots for Ag-Gr, Pd-Gr and PdAg-Gr towards ORR in diffusion-controlled regions were found to be 0.78 x 102 cm/s, 0.81 x 102 cm/s and 2.61 x 102 cm/s respectively. These values indicate significantly better acceleration of ORR by PdAg-Gr in comparison to Ag-Gr and Pd-Gr.The increase in the ORR catalytic activity of nanometal, and nanoalloy composites by the presence of graphene has also been reported in the previous studies.65The enhanced activity of the PdAgGr towards MOR and ORR as observed in the present study can be attributed to the collective synergism among its metal and carbon components. Durability tests of PdAg-Gr were carried for both the MOR and ORR in alkaline conditions using chronoamperometry. The i-t curves recorded for MOR and ORR presented as figure S4 (A) and (B) (SI) imply that current corresponding to these reactions over PdAg-Gr remains unchanged even after 12 and 8 hrs of electrolysis. This suggests that PdAg-Gr suffers no degradation or loss of activity when employed for MOR and ORR and confirms its excellent stability for these reactions.

3.4 Electrocatalytic reduction of Halocarbons



Halocarbons form one of the main groups of environmental pollutants, with some of them being labelled as potent carcinogens.66Most of the halocarbons are resistant to physical, chemical or biological treatments. Among the most common methods advocated for their detoxification, electroreduction is considered as safe to operate and economically most viable approach for their remediation.66 Search for cheap, electrochemically stable and more efficient cathode materials is currently one of the focus areas of research related to electrodetoxification of halohydrocarbons. In this context the use of nano metal/metal oxide deposits like Ag, Cu and Pd have shown some encouraging results.12-13,27 In view of our observations vis.a vis. improved electrocatalytic activity of PdAg-Gr vs. Ag-Gr and Pd-Gr for MOR and ORR, we tested the electrocatalytic performance of these fabricated electrocatalysts towards electro-detoxification of methyl iodide (MeI) and chloroacetonitrile (ClACN). Figures 7 (A) and (B) display the representative CVs recorded for 2 mM MeI and ClACN in DMF over GCE (b) and Ag-Gr (c), Pd-Gr (d) and PdAg-Gr (e) modified GCE. The potential scale is with reference to E0 for ferrocene/ferrocenium (Fc/Fc+) couple in the employed electrolyte system. The CV recorded for MeI at bareGCE depicts a single reduction peak with peak potential of -2.906 V (vs. Fc/Fc+) in good agreement with reported value.67 Changing the electrode system to Ag-Gr/GCE under same experimental conditions shifts the reduction peak towards more positive potential of -2.416 V (vs. Fc/Fc+), indicating an accelerated electron transfer, on metal-graphene nanocomposite.27 However, the positive shift in the peak potential for electroreduction of MeI and ClACN over Pd-Gr/GCE and PdAg-Gr/GCE was much higher. The observed values of peak potential for MeI and ClACN over GCE, AgGr/GCE, Pd-Gr/GCE, and PdAg-Gr are enlisted in table 2. Assuming the peak potential to be a direct measure of the electrocatalytic activity, it seems that the electrocatalytic activity of the nanocomposites varies in the order of PdAg-Gr > Pd-Gr > Ag-Gr.


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Figure 7:Comparative CVs recorded for electroreduction of 2 mM methyl iodide (A) and 2 mM chloroacetonitrile (B) in 0.1 N TBAP at 298 K with scan rate of 25 mV/s.(a) Baseline for solvent (black), (b) Bare-GCE (red), (c) Ag-Gr/GCE (blue), (d) Pd-Gr/GCE (magenta) and (e) PdAg-Gr/GCE (green).

CH3I

ClACN

-Ep(mV)

-Ep(mV)

(vs. Fc/Fc+)

(vs. Fc/Fc+)

Bare-GCE

2.906

2.277

Ag-Gr/GCE

2.416

1.885

Pd-Gr/GCE

2.358

1.513

PdAg-Gr/GCE

1.508

1.189

Electrode

Table 2: Peak potentials observed for the electroreduction of methyl iodide and chloroacetonitrile performed on various electrode systems.

The peak potentials corresponding to electro-reduction of MeI at Pd-Gr/GCE and PdAg-Gr/GCE were -2.358 V and -1.508 V (vs. Fc/Fc+) respectively. A potential shift of ca. 1.4 V for MeI and ca. 1.088 V vs. Fc/Fc+ for ClACN was observed when PdAgGr/GCE was employed in place of bare-GCE. The observed peak potential for electroreduction of ClACN over PdAg-Gr is more positive in comparison to those reported for Cu/Ag nanoparticles (-1.354 V and -1.301 V vs. SCE),13 mediated electroreduction on silver electrode(-1.4 V vs. SCE),68and Au electrode (-2.25 V vs. Fc/Fc+).69 A comparison of peak potentials estimated for electroreduction of ClACN at PdAg-Gr as catalyst with other catalysts is presented in the form of table S4 (SI).These electrochemical signatures make us to infer that that deposition of Ag and Pd on graphene enhances their ability to electrocatalyse the dehalogenation of halocarbons. Further our results establish that alloying of Pd with Ag as nanoalloy deposits over 21 | P a g e ACS Paragon Plus Environment


graphene further boosts the electrocatalytic potential of these metals that can be attributed to their mutual synergistic coupling. Bian et.al. in their study related to electrocatalytic reduction-oxidation of chlorinated phenols have demonstrated that PdFe modified graphene catalyst is a better electrocatalyst in comparison to Pd/Gr.26 They have attributed the enhanced electrocatalytic activity of Pd-Fe/Gr to the presence of Fe which accelerates dechlorination and prevents the self-poisoning of Pd-surface. We attribute our observation vis.a.vis. comparative electrocatalytic potential of Ag-Gr, Pd-Gr and PdAg-Gr towards dehalogenation of halocarbons to synergistic activity of Ag, Pd and graphene similar to that of Pd-Fe modified graphene catalyst.26 Similar effects have been reported for Cu and Ag nanoparticles towards the electrochemical dehalogenation of many halocarbons by Durante et al.13

3.5. Simultaneous electrosensing of hydroxybenzene isomers Hydroxy benzene isomers like hydroquinone (H2Q) and catechol (CC), 2-aminphenol (2AP) and 4-aminophenol (4-AP) usually co-exist with each other. These isomers are frequently used as industrial reagents in the production of rubber, dyes, plastics, pharmaceuticals and cosmetics and are reported to have deleterious effects on environment and human health.10-11Similarities in their structure result in their very similar physicochemical attributes, which makes their simultaneous and sensitive estimation very challenging. Electrochemical methods in view of their very fast response, low operational cost, high selectivity and sensitivity especially with nanomaterial-based electrodes have proven to be very advantageous in the analysis of mixtures of such analytes (Table S5 and S6). In view of the literature we reviewed for entries of Tables S5 and S6, we tested PdAg-Gr nanocomposite for its electrosensing performance towards simultaneous and sensitive estimation of hydroxy-benzene isomers. Figure 8 (A) depicts CVs recorded with scan rate of 25 mV/s for mixture of 4 mM each of hydroquinone (H2Q) and catechol (CC) on bare-GCE (a) and Ag-Gr (b), Pd-Gr (c), PdAg-Gr (d) modified GCE in 0.1 N phosphate buffer of pH 7 at 298 K. The faradic response of the mixture of H2Q and CC to potential scan on bare-GCE (Figure 8A trace a) appears as a single oxidation peak at 0.552 V in forward direction and a reduction peak at -0.178 V. Comparatively the faradic responses over AgGr/GCE (Figure 8A, trace b) and Pd-Gr/GCE (Figure 8A, trace c) seem to be resolved (though not properly) with enhanced peak currents. Interestingly the faradaic responses of H2Q and CC in their mixture appear as well resolved peaks with significantly enhanced peak currents when sensed over PdAg-Gr/GCE (Figure 8A, 22 | P a g e ACS Paragon Plus Environment

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trace d).The voltammetric response to H2Q and CC over PdAg-Gr/GCE emerges as two anodic peaks at 0.163 V and 0.287 V in the forward scan and two cathodic peaks at 0.033 V and 0.177 V in the recorded CVs. The two anodic peaks are separated by ca. 124 mV and the cathodic peaks by ca. 144 mV, a separation sufficient enough for the simultaneous electrochemical sensing and quantification of these analytes from their mixtures.

Figure 8: (A)CVs recorded for mixture of 4 mM H2Q and CC with sweep rate of 25 mV/s. (B) DPVs recorded for mixture of 30 µM H2Q and CC (C) DPVs recorded for mixture of 10 µM 4-AP and 2-AP. DPVs are recorded at modulation amplitude of 0.05 mV and deposition time of 10 sec. Electrode/electrolyte system:Bare-GCE (a), Ag-Gr/GCE (b), Pd-Gr/GCE (c) and PdAg-Gr/GCE (d) in 0.1 N phosphate buffer of pH 7 at 298 K.

Differential pulse voltammetry (DPV) was employed to test the utility of PdAg-Gr for the simultaneous electrochemical sensing of H2Q and CC from their mixtures. Figure 8 (B) depicts DPVs recorded with optimised parameters for a mixture of 30 µM H2Q and CC each. A single oxidation peak is observed for the mixture on bare-GCE (Figure 8B, trace a). However, well separated oxidation peaks are observed at AgGr/GCE (Figure 8B trace b), Pd-Gr/GCE (Figure 8B trace c) and PdAg-Gr/GCE (Figure 8B, trace d).Besides, better separation of faradic responses (ca. 140 mV) at PdAg-Gr/GCE, the peak current for oxidation of H2Q and CC is enhanced by about 3.3 times and 4 times respectively compared to the peak currents at Ag-Gr/GCE. Both increase in peak intensities and peak separation clearly indicate improved sensitivity and selectivity of PdAg-Gr over Pd-Gr and Ag-Gr for electrochemical sensing of these analytes. We also tested the prospect of PdAg-Gr for electrochemical sensing of 4aminophenol and 2-aminophenol, two structurally related isomers, for which the faradaic responses overlap on most of the conventionally used electrode materials. Figure 8 (C) depicts the DPVs recorded for a mixture of 10 µM 4-AP and 2-AP at various electrode systems. As evident, the oxidation peaks at bare-GCE (Figure 8C, trace a), Ag-Gr/GCE (Figure 8C, trace b) and Pd-Gr/GCE (Figure 8C, trace c) are not 23 | P a g e ACS Paragon Plus Environment


well resolved. In comparison to these surfaces, the faradic signals are more prominent and well resolved at PdAg-Gr/GCE (Figure 8C, trace d) with peak separation of 93 mV. Thus, PdAg-Gr/GCE electrode system seems to be significantly better for simultaneous electrochemical sensing and estimation of 4-AP and 2-AP.

Figure 9: (A) DPVs recorded for changing concentration of H2Q ranging from 5.99-56.01 µM in presence of 30 µM CC. (B)DPVs recorded for changing concentration of CC ranging from 6.6-53.02 µM in presence of 30 µM H2Q. (C) DPVs recorded for changing concentration of 2-AP ranging from 1.33-22.41 µM in presence of 2 µM 4-AP. (D) DPVs recorded for changing concentration of 4-AP ranging from 1.33-22.41 µM in presence of 4 µM 2-AP. The calibration plots between Log Ip vs. Log of analyte concentrations are shown as inset of respective figures.

To estimate the sensitivity and detection limit of PdAg-Gr for the simultaneous electrochemical sensing of H2Q and CC or 4-AP and 2-AP in their mixtures, DPVs were recorded for samples variedly concentrated with these analytes. Figure 9 (A) depicts DPVs recorded at varying concentration of H2Q in presence of 30 µM CC. The traces reveal that peak current for oxidation of H2Q increases linearly with concentration in the range of 5.99-56.01 µM. The linear fit of Log Ip vs. log [H2Q] linearly fits the equation logIp (µA) = -1.446 + 0.5866 log [H2Q] µM with R2 =0.99 is shown as inset in figure 8 (A). A similar plot of log Ip vs. log [CC] of CC (6.6-53.02 µM) in presence of 30 µM H2Q fits the equation logIp (µA) = -1.0653 + 0.9494 log [CC] µM with R2=0.99 (inset to figure 9 (B)). The detection limits as calculated from the calibration curve in terms of 3sb/m (S/N=3), where sb is the standard deviation of the error of intercept and m is the slope of the calibration plot,30 are 0.05 µM for H2Q in presence of 30 µM CC and 0.06 µM for CC in presence of 30 µM H2Q. Similarly, 24 | P a g e ACS Paragon Plus Environment

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DPVs were recorded for changing concentrations of 2-AP (1.33-22.41 µM) in presence of 2 µM 4-AP and a representative of these results is shown in figure 9 (C) with log Ip vs. log [2-AP] (inset) found to follow the linear regression equation logIp (µA) = -1.29466 + 0.87228 log [2-AP] µM with R2 =0.99. Figure 9(D) displays DPVs of 4 µM 2-AP at varying concentration (1.33-22.41 µM) of 4-AP. The data for log Ip vs. log [4-AP] depicted in the form of inset of figure 9 (D) was found to follow two linear regression equations viz logIp (µA) = -1.79 + 1.84 log [4-AP] µM in low concentration range (1.33-3.99µM) and logIp (µA) = -1.25192 + 0.89209 log [4-AP] µM at high concentration range (3.99-22.41µM), with R2=0.99. Following the procedure as mentioned above, the detection limits of 4.89 nM (in low concentration range, 1.33-3.99µM) and 6.7nM (in high concentration range, 3.99-22.41µM) for 4-AP in presence of 4 µM 2-AP and 13.7 nM for 2-AP in presence of 2 µM 4-AP were estimated for the simultaneous determination of these isomers at PdAg-Gr/GCE. A comparison of estimated detection limits for the simultaneous detection of hydroxybenzene isomers at PdAg-Gr/GCE in the present study with those reported in already published works (table S5 and S6(SI)) reveals that the detection limit at PdAgGr/GCE is lower than at most of the materials reported so far.

3.6.

Homogenous catalytic reduction of 4-Nitrophenol

Reduction of 4-NP to 4-AP with NaBH4 is a typical reaction employed as a model surface catalyzed reaction to evaluate the catalytic performance of heterogeneous catalysts.14-15This reaction has been employed for assessing the catalytic performance of a range of metallic, bimetallic and metal-oxide nanoparticles of Ag, Au, Pt, Cu and Pd as such or supported on various carbon substrates like graphene as homogenous catalysts.14-15The reduction product, 4-AP is used for the synthesis of various analgesic and antipyretic drugs. Conventionally reduction of 4-NP is carried in the presence of hazardous iron-acid catalysts, raising the concerns of safety, cost and environmental impact. Therefore, designing of environmentally benign and efficient nano-catalysts for reductive conversion of 4-NP to 4-AP in presence of NaBH4 is desirable.70The catalytic performance of Ag-Gr, Pd-Gr and PdAg-Gr nanocomposites for NaBH4 reduction of 4-NP to 4-AP was tested following protocol mentioned in the experimental section. The absorption spectra of 4-NP (Figure 10 A-C, trace a, showing an absorption band at around 300nm) and time-dependence of its absorption in its mixture with NaBH4 (Figure 10A, strong absorption peak at 400 nm) plus Ag-Gr (Figure 10 B), Pd-Gr (Figure 10 C), PdAg-Gr (Figure 10 D) as catalysts were 25 | P a g e ACS Paragon Plus Environment


recorded. Shift of characteristic UV-Vis absorption band from 300 nm to 400 nm on addition of NaBH4 reflects the conversion of 4-NP to 4-nitrophenolate ions.70A decrease in the intensity of absorption band at around 400 nm characteristic to 4nitrophenolate ions marks its reduction to aminophenol. Addition of similar amounts (0.025 mg) of different catalysts to the reaction mixtures similarly concentrated with respect to 4-NP and NaBH4 was found to result in different responses.

Figure 10: Time-dependent UV-Vis spectral changes in recorded for NaBH4 induced reduction of 4NP: (A) Un-catalyzed,(B) Ag-Gr (0.025 mg) catalysed,(C) Pd-Gr(0.025 mg) catalysed, and(D) PdAgGr(0.025 mg) catalysed. (E) Pseudo-1st order kinetic fit between absorbance at wavelength of 400 nm and time in presence of same amount of different catalysts. (F) Comparison of apparent rate constants (kapp) in the form of bar graph which were obtained from the slope of Ist order fit.

Thus, while the absorption peak at 400 nm in presence of Ag-Gr (figure 10 (B)) remains unaltered even after 20 minutes implying a negligible or no production of amino phenol, decrease in the intensity of the said peak and emergence of a new peak at 300 nm upon addition of Pd-Gr (Figure 10 (C)) and PdAg-Gr (figure 10 (D)) to the reaction mixture marks the generation of 4-AP. Though in the presence of Pd-Gr, the reaction mixture shows presence of 4-nitrophenolate ions even after 20 minutes of reaction time (Figure 10 (C)), the reaction seems to be completed in less than 4 min in presence of PdAg-Gr(figure 10 (D)). For the quantification of kinetic parameters associated with 4-NP reduction by NaBH4 reaction mixtures wherein concentration of the latter was appreciably higher than that of former and were variedly concentrated with respect to synthesized catalysts, were followed for their absorption characteristics as a function of time. The recorded kinetic data were analysed for estimation of the apparent rate constant on the 26 | P a g e ACS Paragon Plus Environment

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assumption of pseudo-first-order kinetics.14-15,70Representative exponential fits of the recorded kinetic data as expected for the pseudo-first-order kinetic behaviour are depicted in figure 10 (E). The apparent rate constants (kapp) from the slope of kinetic fit were estimated to be 1.23±0.02 x 10-3s-1 for Pd-Gr and 9.05±0.032 x 10-3s-1 for PdAgGr (figure 10 (F)) indicating that kapp for PdAg-Gr catalysed reduction of 4-NP is more than 7 times higher than that of the Pd-Gr catalysed one. The effect of catalyst dosage on the reduction of 4-NP is depicted in the form of figure 11 (A-D). It is evident that increase in the catalyst amount to 0.05 mg decreases the reaction time to less than 1.5 min. The values of apparent rate constants (kapp) as a function of (PdAg-Gr) catalyst (table S7 (SI) and figure S5 (SI)) reveal that the rate constant increases exponentially with increase in the amount of the catalyst.

Figure 11: Time-dependent UV-Vis spectral changes in 4-NP catalyzed by different dosage of PdAgGr: (A) 0.0125 mg (B) 0.025 mg (C) 0.0375 mg (D) 0.05 mg.

Since, the carbon support in the form of graphene in PdAg-Gr is expected to play an important role in the reduction process and growth of metal nanoparticles, the catalytic role of graphene in the investigated reduction process cannot be completely ruled out. To reveal the role of host surface in such composite catalysts, estimation of normalized rate constant is recommended. The normalized rate constant sometimes referred to as catalytic factor is defined as the ratio of rate constant per unit mass of the catalyst i.e. knor = kapp/m. The evaluation of knor facilitates the comparison of catalytic efficiency of PdAg-Gr with other similar types of catalyst which have been used in past. The kapp and knor calculated for reduction of 4-NP after using 0.05 mg of PdAg-Gr as catalyst were found to be 3.11±0.02 x 10-2 s-1 and 6.21±0.01 x 102 s-1g-1 27 | P a g e ACS Paragon Plus Environment


respectively which are almost three times the values reported for PtAu-nanoflowers/rGO catalysed conversion of 4-NP to 4-AP by NaBH4.14A comparison of reported rate constants for different catalysts with present work is presented in the form of table S8(SI), showing that the rate constant observed with PdAg-Gr as catalyst is among the best. The very high catalytic performance of PdAg-Gr towards reduction of 4-NP as observed in the present work can be explained on the basis of the following facts: The catalytic reduction of nitrophenol with NaBH4 is a two-step process- (1) in the first and rate the determining step, nitrophenol diffuses and adsorbs on catalyst surface and (2) the next step involves catalyst mediated electron transfer from BH4- to nitrophenol. Compared to Pd, the Ag-O/Ag-N bond is easily formed, and Ag being more oxophilic, alloying of Pd with Ag is expected to facilitate the adsorption of nitrophenol to the catalyst surface. Besides this plausible mechanism the following synergistic factors may also contribute to higher catalytic activity of PdAg-Gr towards reduction of nitrophenol: (a) Modification of density of d-band electrons, due to charge redistribution which enhances both catalytic capacity and stability of these nanocatalysts (b) The structural irregularities of these bimetallic alloy nanoparticles that are expected to create bimetallic interfaces leading to enhanced catalytic sites (c) The graphene nano-surfaces which host the bimetallic alloy nanocatalysts possess very high surface area and ballistic electronic conductivity which facilitates both the adsorption and electron transfer from the catalyst to nitrophenol.

Conclusions In summary, we have succeeded in deposition of homogenous bimetallic palladiumsilver nanoalloy over graphene (PdAg-Gr) through an easy to carry out, facile, onepot, wet-chemical approach. Our studies reveal that alloying of Pd and Ag with deposition on graphene surface remarkably improves the electro/homogenous catalytic/analytical performance of the nanohybrid as compared to normal silvergraphene (Ag-Gr) or palladium-graphene (Pd-Gr) towards methanol oxidation reaction (MOR), oxygen reduction reaction (ORR), electrodetoxification of notorious environmental pollutants like halocarbons and nitro phenols and simultaneous detection of hydroxybenzene isomers. A comparison of the catalytic and analytical efficiency of PdAg-Gr with materials which have been purposefully designed and 28 | P a g e ACS Paragon Plus Environment

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tested for these applications, make us to conclude that alloying of Pd with less precious metal Ag in the form of homogenous alloy deposited over graphene makes it a novel material for such applications. Presented results clearly advocate that PdAg-Gr can be used as a successful anodic electrocatalyst for alkaline DMFCs, cathodic electrocatalyst for ORR, electrochemical dehalogenation of halocarbons, simultaneous and accurate determination of hydroxybenzene isomers and catalytic degradation of nitro aromatic compounds.

Supporting Information Figure S1: UV-Vis spectra of graphene oxide and Ag-Gr, Figure S2: Raman Spectra of Graphene oxide (a) and (b) PdAg-Gr, Figure S3: EDS maps observed for PdAg-Gr, Figure S4:i-Curves corresponding to stability tests for PdAg-Gr. Figure S5: Plot of Apparent rate,

kapp, constants as a function of catalyst (PdAg-Gr) dosage for reduction of 4-Nitrophenol to 4Aminophenol and Tables S1-S8.

Acknowledgements The authors thank and acknowledge Professor Allen J. Bard, University of Texas at Austin USA for his support regarding the XPS characterization of the samples. MAB thanks Department of Science and Technology, New Delhi, India, for the research grant no. SR/S1/PC-11/2009.MAR thanks CSIR for the financial assistance (09/25(0039)/2011-EMR-1). SAB thanks DST for financial assistance under DST INSPIRE Scheme (DST/6712/2013/711).

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PdAg Bimetallic Nanoalloy-Decorated Graphene: A Nanohybrid with

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