Journal of Colloid and Interface Science 426 (2014) 270–279

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

In situ fabrication of electrochemically grown mesoporous metallic thin films by anodic dissolution in deep eutectic solvents Anu Renjith, Arun Roy, V. Lakshminarayanan ⇑ Soft Condensed Matter Group, Raman Research Institute, Bangalore 560080, India

a r t i c l e

i n f o

Article history: Received 12 March 2014 Accepted 6 April 2014 Available online 18 April 2014 Keywords: Electrodeposition Calcination SERS HER Tafel slope EIS

a b s t r a c t We describe here a simple electrodeposition process of forming thin films of noble metallic nanoparticles such as Au, Ag and Pd in deep eutectic solvents (DES). The method consists of anodic dissolution of the corresponding metal in DES followed by the deposition on the cathodic surface. The anodic dissolution process in DES overcomes the problems associated with copious hydrogen and oxygen evolution on the electrode surface when carried out in aqueous medium. The proposed method utilizes the inherent abilities of DES to act as a reducing medium while simultaneously stabilizing the nanoparticles that are formed. The mesoporous metal films were characterized by SEM, XRD and electrochemical techniques. Potential applications of these substrates in surface enhanced Raman spectroscopy and electrocatalysis have been investigated. A large enhancement of Raman signal of analyte was achieved on the mesoporous silver substrate after removing all the stabilizer molecules from the surface by calcination. The highly porous texture of the electrodeposited film provides superior electro catalytic performance for hydrogen evolution reaction (HER). The mechanisms of HER on the fabricated substrates were studied by Tafel analysis and electrochemical impedance spectroscopy (EIS). Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Deep eutectic solvents (DES) are promising alternatives for conventional aqueous solutions or organic solvents which often require added supporting electrolytes to achieve a desired ionic conductivity [1]. The aqueous medium also suffers from a serious limitation due to copious oxygen and hydrogen evolution processes which restricts the useful electrochemical potential window. The DES medium is employed in electrochemical processes to counter these limitations of molecular solvents. Further, due to its significant ionic conductivity, DES can be used as a standalone media for electrochemical processes. While these features are also exhibited by ionic liquids, the DES has several advantages for practical applications owing to their insensitivity to water and lower cost. The DES was first introduced by Abbott et al. by complexing quaternary ammonium salt (choline chloride) with urea [2]. The hydrogen bonding interaction of chloride with urea decreases the electrostatic interaction within choline chloride thereby lowering the freezing point of the mixture often well below the room temperature. This concept of charge delocalization through hydrogen ⇑ Corresponding author. Fax: +91 80 361 0492. E-mail address: [email protected] (V. Lakshminarayanan). http://dx.doi.org/10.1016/j.jcis.2014.04.015 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

bonding was later extended to a wide range of quaternary ammonium salts and hydrogen bond donors such as carboxylic acid, diols and amines [3]. The properties of DES can be fine tuned by appropriate choice of the components, making them acceptable in applications such as organic and inorganic synthesis, solvent extraction, medium for enzymatic processes, and electrochemical metal finishing [4–9]. The wider acceptance of DES in metal/alloy electrodeposition processes has demonstrated its potential to be used for electrodeposition of metal nanoparticles too [1]. With the recent trend toward the development of environment friendly methods of nanoparticle synthesis, there is an increasing interest in DES as a medium of electrochemical nanoparticle synthesis. The DES and their ionic liquid counterparts often act as stabilizers and reducing agents in metal nanoparticle synthesis. This helps to avoid the use of additional stabilizers/reducing agents in the process [10]. Further, the exceptional physical properties of these liquids provide novel routes for the synthesis of metal nanoparticle. Since the DES has relatively low surface tension, it can enhance nucleation rates compared to that in aqueous medium leading to a decreased particle size [11,12]. The choline chloride-based deep eutectic solvents containing AgCl were used for the fabrication of nanoporous Ag films on copper alloy substrates by galvanic replacement reaction [13]. Wei et al. demonstrated a shape controlled

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

electrochemical method of synthesis of catalytically active platinum nanoparticles in DES without the addition of surfactants or seeds [14]. The stability of Pd nanoparticles electrodeposited in DES by cathodic reduction of K2PdCl4 on glassy carbon was studied recently by Hammons et al. [15]. Applications in the field of point of care devices, biosensors, electrocatalysts and SERS require stable and reproducible nanoparticle supported substrates. The method of electrodeposition of nanoparticles provides a simple straight forward route to fabricate ready to use substrates. The electrodeposited mesoporous films exhibit excellent adhesion on to the substrate and are often tolerant to organic solvents and electrolytes when used for various applications. The process involving anodic dissolution followed by electrodeposition has been well explored in molecular solvents and offers an advantage of obtaining highly pure, well-adherent nanoparticle films on conducting substrates. By manipulating current density and electrodeposition time, the size and the thickness of the nanoparticle film can be controlled. There have been several reports of electrochemical synthesis of nanoclusters of gold, copper, zinc oxide, cobalt, bimetallic nanoclusters of Pd–Pt, Pd–Ni and Ag in molecular solvents [16–19]. Recently, with the emergence of surface enhanced Raman scattering (SERS), as a versatile and a powerful analytical technique, enormous interest has been generated for the development of nanostructured materials of gold and silver with fine tuned morphology. Both gold and silver nanoparticles offer excellent choice as SERS substrates due to their strong localized surface plasmon resonance. The silver nanoparticles (AgNP) have added advantages over gold due to their lower plasmonic losses in visible spectrum [20] and also due to their proven biocompatibility. The silver nanoparticle supported substrates are obtained by forming silver film on polystyrene nanospheres, and drop casting the colloidal nanoparticles on glass. Besides, the method of nanoscale lithography, self-assembly of nanoparticle onto the substrates is also employed [21–25]. The colloid based substrates often cause irreproducibility in SERS due to aggregation of nanoparticles. Poor adhesion of nanoparticles onto substrate, instability of the active layer of SERS to organic solvents and also degradation due to continuous exposure to the laser beam are some of the limitations of conventional SERS substrates. The technique of nanoscale lithography, though requiring expensive fabrication procedures, produces highly reproducible SERS substrates. The nanostructured materials have easily accessible surface active sites which could be exploited for catalytic applications. The electrocatalytic properties of mesoporous substrates are extensively studied for reactions occurring in fuel cells. With hydrogen being pursued as a promising alternative clean energy source, the electrochemical reduction of water can be an ideal method for the sustainable production of hydrogen. There have been several efforts to identify robust electrocatalyst for hydrogen evolution reaction (HER) to produce large cathodic currents at low over potentials [26–28]. The palladium and gold based materials are being explored as effective alternatives to the existing Pt electrocatalyst [29,30]. In spite of all the advantages of the anodic dissolution method, there are several challenges involved in achieving nanoparticle deposition of Ag on solid substrates. The electrodeposition of AgNPs by anodic dissolution requires aprotic medium for electrodeposition, due to the passivation of anode in the presence of protons [31]. The mechanism of passivation of silver anode during anodic dissolution in protic solvents had been discussed by Murray et al. [32]. In protic medium, the protons are generated at the anode surface as a consequence of processes occurring at anode, i.e., dissolution of silver and oxidation of water. These protons cause direct oxidation of Ag to Ag+, AgO and finally Ag2O3, an insulating oxide. Thus, further progress of the reaction to the

271

synthesis of AgNPs can be achieved only by the complete removal of the passive silver oxide film. We feel that inherent ability of DES to solubilize the metal oxides makes it an excellent medium of choice for anodic dissolution of Ag and subsequent electrodeposition of mesoporous AgNPs. In what follows, we describe a simple method of preparing nanoparticle by anodic dissolution with the simultaneous formation of mesoporous films on the W and Pt/Rh substrate in the medium of DES. The application of the mesoporous silver substrate for SERS was also explored. The gold and palladium nanoparticle films were prepared by utilizing the intrinsic ability of DES to stabilize the nanoparticle. The mesoporous film was characterized by UV, SERS, XRD, SEM and cyclic voltammetry. We also studied the catalytic properties of electrodeposited nanoparticles of Ag, Pd and Au as potential candidates for HER. The electroactive properties of noble metallic mesoporous films in catalyzing HER were investigated by cyclic voltammetry, Tafel analysis and electrochemical impedance spectroscopy. 2. Experimental 2.1. Material used All the chemicals used in this study were of analytical grade (AR) reagents. Choline chloride, ((CH3)3N (Cl) CH2CH2OH) (Aldrich, 98%) was dried for 1 h at 100 °C prior to use. Ethylene glycol, urea, sodium hydroxide, (Merck), 4-mercapto phenyl boronic acid (Aldrich), sodium borohydride (Aldrich), and ethanol (Merck) were used as received. Methylene blue (MB) was used as analyte species in SERS studies. Silver, gold, palladium, platinum/rhodium thermocouple wire each of 0.5 mm diameter and tungsten wire (0.25 mm diameter) of 99.99% purity were purchased from Advent research materials. 2.2. Electrode pretreatment and DES preparation Metal wires which were used as electrodes were polished in aqueous slurries of grades 1 lm, 0.3 lm, 0.05 lm alumina successively, ultrasonicated in Millipore water and dried prior to use. The electrochemical synthesis and in situ deposition of metal nanoparticles were carried out in DES, which is a mixture of choline chloride and a hydrogen bond donor (HBD). Ethylene glycol and urea were used as HBD for the preparation of ethaline and reline respectively with a molar ratio of 1:2. The components were magnetically stirred at 80 °C until a homogeneous colorless liquid was formed. 2.3. Electrochemical studies A three compartment electrochemical cell was employed for cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and Tafel analysis. Pt was used as counter electrode and a Saturated Calomel Electrode (SCE) as a reference electrode. The hydrogen evolution reaction on silver was studied using Ag/AgCl/3 M NaCl as a reference electrode. 2.4. Instrumentation The electrochemical studies were carried out using a EG&G (Model 263A) potentiostat. The nanoparticle deposition was carried out in chronopotentiometric mode by controlling the applied current. The EIS studies were performed with a model 5210 lockin amplifier (Perkin–Elmer instruments) with Powersuite software (EG&G) interfaced with a PC. The UV–Vis absorption spectra of nanoparticles were measured using Perkin Elmer-Lambda 35 spectrophotometer. The X-ray diffraction studies (XRD) were carried

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

out using Cu Ka (l = 1.54 Å) radiation from a Rigaku Ultrax 18 rotating anode generator (5.4 kW) monochromated with a graphite crystal. The SEM images and EDAX were recorded with a field emission scanning electron microscope (Ultraplus, Carl Zeiss). Raman spectra were obtained with a high resolution triple Raman spectrometer (T64000, Horiba JobinYvon), using a He–Ne Laser (k = 632.8 nm) with a 50 objective. Laser power was maintained at 23 mW. Prior to acquiring spectra, the spectrometer was calibrated using a standard Si specimen. 2.5. Electrochemical synthesis and in situ deposition of metal nanoparticles The electrodeposition of nanoparticles was carried out in chronopotentiometric mode using two electrodes, kept parallel to each other at a distance of about 5 mm. The DES medium was kept under constant magnetic stirring in an electrochemical cell of 10 ml capacity. During the process, a mild gas evolution was seen at the cathode in DES medium. This is in contrast to the vigorous gas evolution occurring in aqueous solutions due to H2/O2 evolution at cathode and anode that adversely affects the adhesion of the nanoparticles on the substrate. Evolution of gases in ethaline medium had been previously observed at the cathode during electrolysis under extreme conditions of high current density of 12 A/dm2 for a long exposure time and was attributed to the formation of choline base [33,34]. The nanoparticle coated electrodes were scanned in 0.1 M H2SO4 prior to characterization to remove any organic species trapped within the porous substrates. 2.5.1. Silver For the formation of mesoporous film of silver, a Pt/Rh thermocouple wire and a W wire were used as substrates. A silver wire of diameter 0.5 mm was used as a sacrificial anode and Pt/Rh (or W) wire as a cathode. The tungsten wire was scanned in ethaline medium for the removal of tungsten oxide layer prior to electrodeposition. 4-Mercapto phenyl boronic acid (MPBA, 5 mM) was used as a stabilizer and sodium borohydride (0.15 M NaBH4) as a reducing agent for the electrochemical synthesis of silver nanoparticles in ethaline medium maintained at 50 °C. The electrolysis was carried out at a current density of 14 mA cm2. The reaction was accompanied by the formation of a black film of Ag2O on the Ag anode which subsequently dissolved in DES medium. The solubility of metal oxides in DES has been well known and is attributed to complexation of the solvent with metal oxides to form soluble species [35]. The color of the medium changes gradually from transparent to reddish brown, indicating the formation of Ag nanoparticles stabilized by MPBA. A simultaneous growth of mesoporous AgNPs was indicated by the formation of a black film at the cathode. The electrodeposition process was carried out for 30 min on Pt/ Rh wire while it was extended to 1 h for deposition onto W wire. The solution turned dark in color with time, which regained the original color after purification with ethanol through repeated sonication and decantation of the solvent. In view of its prospective application in SERS, the mesoporous silver films were calcined at 250 °C for 2.5 h to remove the organic stabilizers adhering to the nanoparticle. 2.5.2. Gold Electrodeposition of gold nanoparticles was carried out by using a gold wire of diameter 0.5 mm and gold disk electrode of 1 mm diameter as anode and cathode respectively in reline medium. Sodium borohydride (0.05 M NaBH4) was used as reducing agent for the electrochemical synthesis of gold nanoparticles in reline medium at 50 °C. The addition of an external stabilizer was avoided due to the intrinsic ability of the medium to stabilize the

nanoparticle formed [36]. The electrolysis was carried out at a current density of 0.3 A cm2 for a duration of 20 min. The color of the medium changed rapidly from transparent to grayish blue to black, indicating the formation of gold nanoparticles stabilized by the medium. In situ electrodeposition of AuNPs was apparent from the black film on the cathode. The nanoparticles obtained in the solution were diluted and purified in ethanol for the UV studies. 2.5.3. Palladium Electrochemical synthesis of Pd nanoparticle in solution and simultaneous electrodeposition on a Pd wire acting as a substrate was carried out in ethaline medium under the room temperature conditions. The medium employed in the method serves to reduce the metal ions to metal nanoparticle and also to stabilize the nanoparticles formed. A Palladium wire of diameter 0.5 mm was used as a sacrificial anode. The process of electrodeposition was carried out at a current density of 1 A/cm2 for 30 min. The solution turned black in a few minutes indicating the formation of Pd nanoparticles. The optical properties of the purified nanoparticles dispersed in ethanol were studied by UV–Vis spectroscopy. The surface enhanced Raman studies were carried out on the electrodeposited mesoporous AgNP and AuNP films for ligand characterization. The morphology and size of electrodeposited nanoparticles films were examined by SEM, EDAX and XRD. 3. Characterization The mechanism of electrodeposition of metal nanoparticle by anodic dissolution had been earlier reported by Reetz and Helbig [37]. By applying a current density, the metal was dissolved into the medium as ions (Mn+) which were subsequently reduced at the cathode. In addition to the electrochemical reduction at cathode surface, the presence of reducing agent in the medium also brings about chemical reduction of the metal ions leading to the formation of metal nanoparticles in the solution [38]. The broad surface plasmon bands of gold and silver nanoparticle solution shown in Fig. 1 are indicative of a polydispersed sample. The surface enhanced Raman spectra shown in Fig. 2a confirm the presence of MPBA monolayer formed on the mesoporous AgNP electrode. The Raman signal at 220 cm1 corresponding to AgAS bond, confirms the chemisorption of thiol monolayer on AgNPs [39]. A strong band at 707 cm1 corresponds to the ACAS stretching mode [23]. Raman signals at 999 cm1 (out of plane CACAC stretch), 1080 cm1, 1590 cm1 (aromatic ring vibrations) and

(a)

(b)

absorbance (a.u.)

272

300

400

500

600

700

500

550

600

650

wavelength ( λ nm ) Fig. 1. UV–Vis absorption spectra of purified (a) silver (b) gold nanoparticles in ethanol.

273

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

(a) intensity (a.u.)

Intensity (a.u.)

(b)

500

1000 1500 2000 -1 Raman Shift ( cm )

2500

500

1000

1500 2000 2500 -1 Raman shift (cm )

3000

Fig. 2. SERS of the mesoporous (a) AgNP (b) AuNP on Pt/Rh substrate.

the weak band at 1180 cm1 (ACH deformations) are characteristics of an aromatic ring. A peak at 1442 cm1 was observed due to the benzene ring vibrations in phenyl-boronic acid group linkage [40]. Raman spectrum of pure MPBA is provided in Fig. S1 (supporting information) for comparison. The adsorption of thiol monolayer onto surface was further confirmed by the absence of 2575 cm1 band corresponding to free ASH groups. The absence of the band in the SERS spectrum also indicated that free MPBA molecules did not contribute to the signal intensity in spectra. Assuming a monolayer film of MPBA on the surface of nanoparticles, the SERS signal enhancement due to mesoporous nature of AgNP film was calculated to be 106 compared to the normal Raman spectra of known concentration of MPBA dropcasted on a Si wafer. The presence of stabilizer ligands viz., urea and choline chloride (DES) on gold nanoparticles was confirmed from Raman spectrum of the mesoporous substrate (Fig. 2b). The nanoparticle modified surface exhibited significantly enhanced Raman signals of the ligands. The presence of urea was identified from amide II (1555 cm1), amide III (1238 cm1) and NAC@O in plane deformation (626 cm1) peaks in the Raman spectrum. Raman bands at 2945 cm1 corresponding to ACH2 stretching vibrations could be attributed to that arising from choline cation. A strong band at 800 cm1 due to the in phase ACACAO stretching mode further confirmed the presence of choline group [41,42]. The surface XRD patterns of the mesoporous substrates were obtained in the 2H range of 30–80° for an exposure time of 1 h. Fig. 3a displays the diffraction peaks of AgNP modified Pt/Rh substrate. The XRD peak at 38.3° and the humps at 44.5° and 64.5° were assigned to (1 1 1), (2 0 0) and (2 2 0) crystal planes, indicating that the mesoporous Ag film possess face centered cubic structure [43]. The additional sharp peaks observed in the XRD spectra correspond to Pt/Rh wire. The average particle size of the electrodeposited Ag nanoparticles was determined by XRD using Debye–Scherer’s equation and was found to be 34.5 nm. On calcination at 250 °C for 2.5 h, the XRD peaks (Fig. 3b) corresponding to fcc nanostructure of mesoporous Ag became more intense as compared to that prior to calcination. This is evidently due to the surface reorganization at high temperature during calcination. A similar observation had been reported when AgNPs were calcined to remove organic impurities [44]. The surface XRD pattern of mesoporous gold substrate shown in Fig. 3c can be indexed to fcc nanostructure from the peaks occurring at 38.3° (1 1 1), 44.5° (2 0 0), 64.2° (2 2 0), 77.7° (3 1 1), 81.8° (2 2 2) [16]. The average size of nanoparticle was calculated to be 28 nm. Fig. 3d displays the surface XRD pattern of Pd nanoparticles deposited on Pd. The mesoporous Pd exhibited face centered cubic

(fcc) pattern with peaks at 39.9° (1 1 1), 46.9° (2 0 0), 67.9° (2 2 0). The XRD pattern showed additional peaks at 39° and 45°, which cannot be indexed to Pd (fcc). These additional peaks were also observed earlier and were attributed to the lattice expansion of Pd on absorbing hydrogen [45,46]. The study also showed that complete desorption of trapped hydrogen from the lattice could not be achieved even by high temperature treatments. In our studies, the nanoparticle synthesis is not accompanied by hydrogen evolution. However, electrochemical scanning in acid medium prior to characterization as mentioned in Section 2.5 to remove trapped organic species could have resulted in the formation of surface hydrides [47]. The metal hydride formation observed in the case of Pd nanoparticle, under similar electrochemical pretreatment as that of gold and silver was due to the enhanced hydrogen absorption and adsorption ability of Pd over the latter. Fig. 4a shows the SEM images of gold substrate containing electrodeposited nanoparticles of 30 nm size. The elemental analysis of the same is presented in Fig. S2 (supporting information). The highly porous nature of the nanostructured Pd film on Pd substrate deposited for an electrodeposition time of 30 min is displayed in Fig. 4b. Pd is a metal well known for its ability to absorb hydrogen and the porous nature of the specimen obtained in our studies puts forth a potential application to be used as a hydrogen storage material. The EDAX spectrum and elemental composition of mesoporous Pd are given in Fig. S3 (supporting information). 4. Evaluation of mesoporous metallic films in applications The process of electrodeposition of metal nanoparticle facilitates the formation of well adherent mesoporous films on substrates thereby enabling its direct use in various applications. In what follows, we describe the potential application of the fabricated silver nanoparticle modified substrates in the field of SERS along with the evaluation of electrocatalytic properties of mesoporous substrates of Ag, Pd and Au with a specific example of hydrogen evolution reaction. 4.1. SERS activity of mesoporous substrate One of the limitations associated with SERS substrates is the signal interference from substrate composition or stabilizer molecules of nanoparticles in the SERS spectra. This may impose some constraints in the acquisition of spectra of analyte in a restricted wave number range i.e., excluding substrate signals. In our studies, it was evident from Fig. 2a, that the mesoporous AgNP film gives enhanced signals of MPBA, which may interfere with the Raman bands of analyte species. The possibility of any unspecific molecu-

274

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

Intensity (counts)

(a)

40

50

60

70

(b)

80

40

50

60

70

(c)

40

50

60

70

80

80

(d)

40

50

60

70

80

2θ (degree ) Fig. 3. XRD pattern of (a) AgNP (b) calcined AgNP on Pt/Rh wire (c) AuNP/Au (d) PdNP/Pd.

(a)

(b)

Fig. 4. SEM images of mesoporous (a) AuNP/Au (b) PdNP/Pd (1 lm scale-bar). Inset shows magnified images with 100 nm scale-bar.

lar interaction between analyte species with the existing MPBA layer on AgNP cannot also be neglected. To overcome this problem, calcination of the substrate was performed to remove organic moieties present on the mesoporous substrate.

(a)

SEM images given in Fig. 5a show the mesoporous deposition of silver on Pt/Rh wire. The dendritic growth of the Ag nanoparticles seen in the interior regions may correspond to initial stages of electrodeposition which grow as clusters in later stages. The elemental

(b)

Fig. 5. SEM images of the mesoporous AgNP substrate (a) before calcination (b) after calcination. Inset shows magnified images with 100 nm scale-bar.

275

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

composition of the substrate provided in Fig. S4a (supporting information) showed the presence of Pt/Rh. This indicates that underlying substrate is not covered entirely by the silver deposit. Fig. 5b shows the SEM images of AgNP film on Pt/Rh substrate calcined at 250 °C for 2.5 h. The morphology of the nanoparticle changes notably, as AgNPs aggregate to form a network like structure. A comparison of inset images, shows that the smaller nanoparticles coalesce together to form larger clusters of sizes ranging from 200 to 300 nm. The individual nanoparticles of 10–15 nm coalescing to form larger aggregates can be seen in the inset. The EDAX spectra of the calcined substrates shown in Fig. S4b (supporting information) confirm that Pt/Rh is completely covered by the mesoporous AgNP film. Fig. 6a (i) shows the SERS of calcined mesoporous silver substrate exhibiting almost featureless spectra except for the broad band corresponding to AgAO at 240 cm1. The AgO is formed on the surface of Ag nanoparticles due to high temperature used during calcination. The Raman spectra of the calcined sample confirm the complete removal of the stabilizers adsorbed during the deposition of AgNPs. The utility of the mesoporous substrate in SERS studies has been explored with methylene blue as the analyte. The substrates were dipped in aqueous solutions of methylene blue (37 x 108 M to 37  106 M) for 1 h. A reference sample for acquiring normal Raman spectrum was prepared by drop casting 2 ll of 0.1 mg/5 ml aqueous solution of methylene blue on Si wafer. The SERS substrates and reference samples were allowed to dry at room temperature and the spectra were collected over a range of 100–2000 cm1 with an exposure time of 1 s. The spectral intensity obtained was normalized by the corresponding AgAO peak intensity for all SERS substrates under study. A comparison of Fig. 6a and b, shows that the SERS spectra exhibit almost the same features as that present in the normal Raman spectra (NRS) of methylene blue. The SERS spectra are not associated with shifts in band positions, which are usually observed on chemisorption with the substrate. The intensity of the SERS spectra increases with the concentration of MB in the sample. The SERS associated with the least concentration (3.7  107 M) of the analyte displays only the prominent peaks of Raman signals of methylene blue. The strong peaks observed in the spectra are 1619 cm1 (stretching vibrations of CAC ring), 1393 cm1 (in-plane ring deformation). The peak observed at 447 cm1 with a shoulder at 476 cm1 in NRS is seen as split peaks at the same band position in SERS. Raman peak positions obtained in our studies were compared with literature values. Table 1 shows the assignment of the Raman bands in SERS and NRS of methylene blue. The enhancement factor (G) of the substrate was calculated by the comparison of SERS of methylene blue (37 lM) with that of

normal Raman spectrum (NRS) at the peak position 1619 cm1 by using the formula

G¼

where NSERS and Nref are the number of molecules contributing to the signal on the SERS substrate and reference sample respectively. ISERS and Iref are the integral area of the Raman signal at 1619 cm1 in SERS and NRS spectra respectively. The enhancement factor was found to be about 2  105. The SERS spectrum of methylene blue without normalization that was used for the calculation of enhancement factor is provided in Fig. S5 of supporting information along with the calculations. The porous AgNP film fabricated by electrodeposition was found to be well adherent on the surface and could endure electrochemical potential cycling, vigorous heat treatments, rinsing with aqueous and nonaqueous solvents. The silver nanoparticle film electrodeposited on metallic substrates provided stable and reproducible SERS signals even upon continuous irradiation with the laser beam, showing a good thermal stability. 4.2. Electrocatalytic activity of the mesoporous metallic substrates to hydrogen evolution reaction (HER) HER is one of the most investigated electrochemical reactions, which can be performed either in acidic or alkaline medium. The studies carried out till date, on the mechanism of HER in acidic medium involve primarily a proton discharge (Volmer step) at the metal surface, followed by either Tafel or Heyrovsky steps. k1

M þ H3 Oþ þ e ! MHads þ H2 O

ð1Þ

The Tafel step (2) consists of the chemical combination of adsorbed hydrogen atoms on metal whereas Heyrovsky step (3) involves electrochemical desorption. k2

2MHads ! M þ H2

ð2Þ

k3

MHads þ Hþ þ e ! 2M þ H2

ð3Þ

where M represents free metal sites at the electrode, MHads represents the adsorbed hydrogen atom on the metal surface, k1 (Eq. (1)), k2 (Eq. (2)), and k3 (Eq. (3)) are the rate constants of the Volmer, Tafel and Heyrovsky steps respectively. Thus, Volmer–Tafel and Volmer–Heyrovsky mechanism are commonly proposed pathways to explain the reduction of protons at the metal surface giving rise to hydrogen evolution. The rate

(b) Intensity (a.u.)

(a)

Intensity (a.u)

Nref ISERS NSERS Iref

iv

iii ii i 300

600

900 1200 -1 Raman shift (cm )

1500

300

600

900

1200

1500

-1 Raman shift (cm )

Fig. 6. (a) Normalized SERS spectra of calcined substrate adsorbed from aqueous methylene blue solutions of (i) 0 M (ii) 3.7  107 M (iii) 3.7  106 M (iv) 37  106 M (b) Raman spectra of methylene blue (2 ll of 0.1 mg/5 ml) dropcast on Silicon wafer (inset shows the structure of methylene blue).

276

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

Table 1 Comparison of Raman peak positions of methylene blue observed in our studies and reported results along with spectral assignments [23,48]. Observed peak position cm1

Reported peak positions cm1

Assignments

1619 1502

1618 1503

m (CAC ring) m (CAC) in

1427 1390 1152 948 894 768 596 501

1430 1396 1159 948 890 774 610 502

m (CAN) a (CAH)

plane

b (CAH)

cCAH CH3rock

cCAH d (CASAC) d (CANAC)

m stretching, a in plane ring vibrations, b in plane bending, d skeletal deformations.

determining step in either of the two pathways can be determined from the Tafel plot of HER (log I vs. E). The slope of the Tafel plot gives an idea on the rate determining step in either of the mechanisms while the intercept provides the exchange current density for the reaction. A Tafel slope of 30 mV/dec indicates Tafel mechanism while a slope of 120 mV/dec may be due to either Volmer or Heyrovsky being the rate determining step. In our work, we have carried out cyclic voltammetry, Tafel analysis and EIS to evaluate the potential electrocatalytic applications of mesoporous substrates and also to investigate the mechanism of HER followed in the process. The current densities reported in figures are calculated from the geometrical area. The electrolyte was purged with nitrogen to remove any dissolved oxygen present.

4.2.1. Voltammetric response of AgNP coated W wire The SEM images and EDAX spectra given in Figs. S6 and S7 (supporting information) of silver nanoparticle modified substrate indicate the underlying metal W is well covered with nanoparticles of silver. The XRD characteristics of AgNP on W shown in Fig. S8 (supporting information) were similar to that obtained on Pt/Rh substrate and confirmed fcc nanostructure of AgNP. The cyclic voltammetry in 0.25 M HClO4 at a scan rate of 100 mV was performed and the results were compared with that of bulk silver wire under identical conditions (Fig. 7). The onset potential for HER with bulk Ag electrode used in our studies occurs at 0.620 V. The variation in onset potential of HER with crystallographic orientation of Ag had been studied by Diesing et al. in neutral perchlorate medium. They extended these studies to different pHs and

2

Current ( A / cm )

0.0

a

-0.1

observed that onset potentials were not significantly affected by the crystallographic orientation in acidic medium [49]. The value of 0.62 V vs. Ag/AgCl obtained in our studies is comparable to the reported values on Ag (1 1 1) single crystal electrode in acidic medium of 0.4 V vs. NHE [50]. The voltammograms of porous AgNP film in 0.25 M HClO4 showed a lowering of onset potential for HER with respect to Ag wire electrode by 100 mV. The electrocatalytic applications of citrate capped AgNP modified basal plane pyrolytic graphite (BPPG) in HER had been studied by Campbell et al. [51]. The onset potential reported here is about +0.6 V more positive than the values of 1.25 V measured by them. The observed onset potential in their studies was found to be dependent on the surface coverage of AgNP on BPPG. To the best of our knowledge, there has not been any other report on the electrocatalytic applications of AgNPs in acidic medium for HER. The porous AgNP film exhibits a current density of 375 mA/cm2 for HER with respect to that of bare silver wire electrode (7 mA/cm2), an increase by a factor of about 50. A Tafel slope of 110 mV/decade was measured from the Tafel plot (Fig. 8a) obtained from the chronoamperometric data at different over potentials of hydrogen evolution for AgNP modified electrode A Tafel slope of 120 mV/decade indicates Volmer–Heyrovsky mechanism involving an initial proton discharge step followed by electrochemical desorption process. Recently, the mechanism of hydrogen evolution has been studied on (1 0 0) plane of silver by potentiostatic current transients at overpotentials g > 0.7 V in acidic solutions [52]. The reaction was shown to follow Volmer– Heyrovski mechanism with Volmer step being the rate determining step. The electrochemical impedance spectroscopy studies were carried out to get further insight onto the kinetics of the process. The EIS was performed at different potentials from 540 mV to 660 mV for a frequency range of 100 kHz–100 mHz at an ac amplitude of 10 mV. The Nyquist plot obtained for HER on porous AgNP electrode in 0.25 M HClO4 is shown in Fig. 8b. The diameter of the semicircle is indicative of the charge transfer resistance Rct, which decreased with applied cathodic over potential. This indicates faster charge transfer kinetics at higher overpotentials. The EIS data was fitted to an equivalent circuit consisting of charge transfer resistance in parallel with constant phase element Q represented by R(QR) equivalent circuit. The data is presented in Fig. S9 (supporting information). The constant phase element obtained by equivalent circuit fitting has an n value 0.75 in all cases with no significant change in CPE values (1  105 S secn). An inductive loop could be observed in Fig. 8b at high overpotentials and low frequencies. It was observed in HER of other metal surfaces too and was attributed to the formation of electrosorbed intermediate layers at the surface [53]. The low frequency inductive loop observed in HER was described by single adsorbed species mechanism by Harrington et al. [54]. The mechanism proposed had two distinct steps (Eqs. (1) and (3)) consisting of hydride formation and dehydrogenation involving electron transfer with the metal sites. Thus, EIS data, in addition to Tafel analysis further confirms that HER follows Volmer–Heyrovsky pathway on porous AgNP films on W.

-0.2

-0.3

b -0.4 -0.90

-0.75

-0.60

-0.45

-0.30

Potential ( V Vs Ag/AgCl) Fig. 7. Cyclic voltammogram of (a) Ag wire (b) AgNP deposited on W wire of at a scan rate of 100 mV/s in 0.25 M HClO4 vs. Ag/AgCl/3 M NaCl reference electrode.

4.2.2. Voltammetric response of AuNP coated on gold disk electrode The electroactive surface area (ECSA) of gold nanoparticle coated gold disk electrode was determined from the charge associated with stripping peak of gold oxide in 0.1 M H2SO4. Fig. 9a shows the cyclic voltammograms of mesoporous gold in 0.1 M H2SO4, with a well defined stripping peak corresponding to the reduction of gold oxide at 0.82 V. A highly porous nature of the surface was apparent from the measured charge of 550 lC corresponding to an ECSA of 1.45 cm2 for a geometric area of 0.00785 cm2. The electrocatalytic activity of mesoporous AuNP

277

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279 5

(b)

(a)

4

-0.60

3

3

Z" ( x 10 ohms )

Potential ( V Vs Ag/AgCl)

-0.62

-0.58

-0.56

2

1

-0.54 -3.75

-3.50

-3.25

-3.00

0

-3 log I (x10 A )

iii

ii

i

0

2000

4000

6000

8000

10000

3

Z' ( x 10 ohms)

Fig. 8. (a) Tafel plots for HER of porous AgNP coated electrode (b) Nyquist plots of porous AgNP coated W wire at (i) 660 mV (ii) 600 mV (iii) 540 mV in 0.25 M HClO4 under N2 atmosphere.

0.3

(a)

0.00

(b)

-0.05

i

0.1

2 Current (A / cm )

Current ( x10

-3 A)

0.2

0.0 -0.1 -0.2

-0.10

-0.15

-0.20

-0.3 -0.25

-0.4 -0.4

0.0

0.4

0.8

1.2

Potential ( V Vs SCE)

-1.0

ii -0.8

-0.6

-0.4

Potential

(V

-0.2

0.0

0.2

Vs SCE)

Fig. 9. (a) Cyclic voltammogram of AuNP/Au disk (b) HER at (i) Au disk (ii) AuNP/Au electrode at a scan rate of 100 mV/s in 0.1 M H2SO4 vs. SCE reference electrode.

film on gold disk electrode was evaluated for HER and compared with an unmodified gold disk electrode using cyclic voltammetry (Fig. 9b). The mesoporous substrate exhibited distinctly enhanced performance over its unmodified counterpart as is evident from the positive shift in the onset potential and also increased current densities. The onset potential for hydrogen evolution process on mesoporous gold substrate occurs at 360 mV when compared to 580 mV for an unmodified gold disk electrode. The current density for HER when normalized to the electroactive surface area corresponds to 1.5 mA/cm2. Plowman et al. prepared gold nanospikes by electrochemical route and evaluated their catalytic activity in HER in terms of onset potential (0.3 vs. Ag/AgCl) and current density (0.16 mA/cm2) [55].The value of measured current density in our case is about an order of magnitude higher than the value reported by them. Costa et al. also has studied the catalytic effect of AuNP modified on screen printed graphite electrodes by dropcasting gold nanoparticles in HCl and varying their concentration. The measured onset potential in the study was found to decrease with increasing concentrations of AuNP from pM to nM (0.8 V to 0.3 V vs. Ag/AgCl) [56]. The mechanism of HER on gold had been studied in detail by a number of groups with significant focus on their crystallographic orientation. The catalytic activities of low index planes of gold were compared by Perez et al. [57]. The mechanisms of HER for all crystal orientations were found to be the same, with (1 1 1) plane exhibiting a maximum catalytic activity. Further, the HER studies on gold till date yielded two Tafel slopes at low and high overpotentials respectively [58–60]. The Volmer–Tafel mechanism

has been the proposed pathway for HER, with the discharge process being the rate determining step. The process of hydrogen adsorption on Au is considered to be energetically unfavorable making the discharge process slow while the weak AuAH bond makes the desorption process faster [32]. In our studies, Tafel analysis (Fig. 10a) yielded two slopes of 90 mV and 112 mV at low and high overpotentials respectively. The electrochemical impedance studies showed lowering of Rct values with overpotential. A careful analysis of the Nyquist plot given in Fig. 10b at lower overpotential shows the absence of inductive loop, thus excluding Heyrovsky mechanism for HER. The constant phase element obtained by equivalent circuit fitting given in Fig. S10 (supporting information) has an n value of 0.8 in all cases with no significant change in CPE values (1  105 S secn). 4.2.3. Voltammetric response of PdNP coated on Pd wire Fig. 11a shows the cyclic voltammogram of Pd NP deposited on Pd wire in 0.1 M H2SO4 at a scan range of 0.4 V to 1 V exhibiting peaks during both the anodic and cathodic scans. The broad peak at a potential of about 0 V is attributed to the oxidation of adsorbed and absorbed hydrogen within the palladium lattice. The formation of PdO extending beyond 0.7 V is accompanied by potential oscillations that continue during the initial stages after the potential reversal. It is probable that the simultaneous oxidation of adsorbed hydrogen and palladium have contributed to the oscillations. A true surface area of 0.35 cm2 for a geometric area of 0.023 cm2 indicated the mesoporous nature of Pd substrate. The onset potential in the nanoparticle modified Pd wire is almost half of that of bulk Pd wire as can be seen in from Fig. 11b. The HER

278

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

(a)

(b) 0.9

ohms)

-0.39

3

-0.42

Z' ' ( x 10

Potential ( V Vs SCE)

-0.36

-0.45 -0.48

0.6

0.3

ii

i

-0.51

iii

0.0

-0.54 -2.7

-2.4

-2.1

-1.8

-1.5

-1.2

-0.9

-0.6

0

500

1000

-3 log I ( x10 A)

1500

2000

2500

Z' ohms

Fig. 10. (a) Tafel plots (b) Nyquist plots at (i) 500 mV (ii) 450 mV (iii) 400 mV for HER of porous AuNP coated Au disk electrode in 0.1 M H2SO4 under N2 atmosphere.

1

(a)

(b)

0.5

i ii

2

Current (A / cm )

Current ( x 10

-3

A)

0.0

0

-1

-0.5 -1.0 -1.5 -2.0

-2

-2.5 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.6

-0.3

Potential ( V Vs SCE )

0.0

0.3

0.6

Potential (V Vs SCE)

Fig. 11. (a) Cyclic voltammogram of PdNP/Pd (b) HER at (i) Pd wire (ii) PdNP/Pd electrode at a scan rate of 100 mV/s in 0.1 M H2SO4 vs. SCE reference electrode.

(a)

160

-0.27

(b)

120

-0.30

Z'' (ohm)

Potential ( V Vs SCE )

-0.24

-0.33

80

40 -0.36

0 -0.39

i -1.2

-1.0

-0.8

log I

-0.6

-0.4

-0.2

0.0

0.2

0

ii

iii 100

( x 10-3 A)

200

iv 300

400

500

Z' (ohm)

Fig. 12. (a) Tafel plots (b) Nyquist plots at (i) 400 mV (ii) 350 mV (iii) 300 mV (iv) 250 mV for HER of porous PdNP coated Pd wire electrode in 0.1 M H2SO4 under N2 atmosphere.

of the mesoporous Pd electrode commences at significantly lower potential (0.256 V) compared to that of unmodified Pd wire (0.52 V) vs. SCE. Another striking feature exhibited by these mesoporous substrate was the extremely low overpotential (50 mV) required for the onset of hydrogen evolution with respect to the open circuit potential (204 mV). This makes the Pd nanoparticle coated substrate more advantageous for the development of an efficient HER catalyst. The HER on bulk Pd electrodes had already been studied extensively, with many groups proposing Volmer–Tafel mechanism for the process. The Tafel analysis were performed within a potential range of 0.24 V to 0.4 V. A slope of 110 mV was obtained indicating proton discharge process to be the rate determining step

(Fig. 12a). The EIS analysis further points toward the same, excluding the possibility of Heyrovsky mechanism. A lowering of Rct with overpotential was evident from Fig. 12b. An equivalent circuit fitting given in Fig. S11 (supporting information) revealed that Rct decreased from a value of 800 ohms to 50 ohms at an overpotential of mere 400 mV.

5. Conclusions The electrochemical synthesis of noble metal nanoparticles and their in situ deposition on to metallic substrate have been successfully demonstrated by anodic dissolution technique. The

A. Renjith et al. / Journal of Colloid and Interface Science 426 (2014) 270–279

techniques reported here in the presence and also in the absence of external reducing agents and stabilizers provide a simple and efficient technique to generate mesoporous films of metal nanoparticles. The mesoporous films were quite adherent to electrodeposited substrates and could be effectively subjected to electrochemical treatments, solvent exposure and laser irradiation. The mesoporous silver film shows a SERS enhancement by factor of 105. We have shown that the electrodeposition of AgNP in an environmentally benign solvent such as DES and its subsequent sintering at high temperature establishes a simple and efficient technique for the fabrication of stable SERS substrates free from the interference from other associated organic moieties. The porous nanostructured films of silver, gold and palladium studied in this work exhibited a good electrocatalytic activity for HER, with substantial lowering of onset potential and increased current densities compared to the corresponding bulk metals. The overpotential for HER on PdNP modified substrate was quite small, of the order of a few milli volts with respect to the open circuit potential. Further applications of these porous metallic films in field of sensors are presently being explored. Acknowledgments We would like to thank Mr. A. Dhason for SEM characterization and Ms. K.N. Vasudha for her help in XRD and UV studies. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.04.015. References [1] A.P. Abbott, K.E. Ttaib, G. Frisch, K.J. McKenzie, K.S. Ryder, Phys. Chem. Chem. Phys. 11 (2009) 4269. [2] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun. (2003) 70. [3] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, J. Am. Chem. Soc. 126 (2004) 9142. [4] A.P. Abbott, T.J. Bell, S. Handa, B. Stoddart, Green Chem. 7 (2005) 705. [5] M. Krystof, M. Pérez-Sánchez, P. Domínguez, ChemSusChem 6 (2013) 630. [6] Y.H. Choi, J.V. Spronsen, Y. Dai, M. Verberne, F. Hollmann, I.W. Arends, G.J. Witkamp, R. Verpoorte, Plant Physiol. 156 (2011) 1701. [7] J.T. Gorke, F. Srienc, R.J. Kazlauskas, Chem Commun. (Camb.) 10 (2008) 1235. [8] D. Lindberg, M. de la Fuente Revenga, M. Widersten, J. Biotechnol. 147 (2010) 169. [9] Y.A. Sonawane, S.B. Phadtare, B.N. Borse, A.R. Jagtap, G.S. Shankarling, Org. Lett. 12 (2010) 1456. [10] H.G. Liao, Y.X. Jiang, Z.Y. Zhou, S.P. Chen, S.G. Sun, Angew. Chem. Int. Ed. Engl. 47 (2008) 9100. [11] A.V. Mudring, T. Alammar, T. Bäcker, K. Richter, ACS Symp. Ser.; ch. 12 (2010) 177. [12] K. Richter, P.S. Campbell, T. Baecker, A. Schimitzek, D. Yaprak, A.V. Mudring, Phys. Status Solidi B 250 (2013) 1152. [13] C.D. Gu, X.J. Xu, J.P. Tu, J. Phys. Chem. C 114 (2010) 13614. [14] Lu Wei, You-Jun Fan, Na Tian, Zhi-You Zhou, Xue-Qin Zhao, Bing-Wei Mao, ShiGang Sun, J. Phys. Chem. C 116 (2012) 2040.

279

[15] J.A. Hammons, T. Muselle, J. Ustarroz, Maria Tzedaki, M. Raes, A. Hubin, H. Terryn, J. Phys. Chem. C 117 (2013) 14381. [16] D.H. Nagaraju, V. Lakshminarayanan, J. Phys. Chem. C 113 (2009) 14922. [17] B. Stypuła, J. Banas´, M. Starowicz, H. Krawiec, A. Bernasik, A. Janas, J. Appl. Electrochem. 36 (2012) 1407. [18] B. Stypula, M. Starowicz, M. Hajos, E. Olejnik, Arch. Metall. Mater. 56 (2011) 287. [19] J.A. Becker, R. Schäfer, R. Festag, W. Ruland, J.H. Wendorff, J. Pebler, S.A. Quaiser, W. Helbig, M.T. Reetz, J. Chem. Phys. 103 (1995) 2520. [20] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824. [21] M. Fan, A.G. Brolo, Phys. Chem. Chem. Phys. 11 (2009) 7381. [22] G.H. Gu, J.S. Suh, J. Phys. Chem. C 114 (2010) 7258. [23] M. Rycenga, J.M. McLellan, Y. Xia, Chem. Phys. Lett. 463 (2008) 166. [24] G.D. Fleming, J.J. Finnerty, M.C. Vallette, F. Célis, A.E. Aliaga, C. Fredes, R. Koch, J. Raman Spectrosc. 40 (2009) 632. [25] Y. Bu, S. Lee, ACS Appl. Mater. Interfaces 4 (2012) 3923. [26] J. Yang, D. Voiry, S.J. Ahn, D. Kang, A.Y. Kim, M. Chhowalla, H.S. Shin, Angew. Chem. Int. Ed. Engl. 52 (2013) 13751. [27] K.R. Koboski, E.F. Nelsen, J.R. Hampton, Nanoscale Res Lett. 8 (2013) 528. [28] D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V.B. Shenoy, G. Eda, M. Chhowalla, Nano Lett. 13 (2013) 6222. [29] L. Wang, U. Stimming, M. Eikerling, Electrocatalysis 1 (2010) 60. [30] A. Safavia, S.H. Kazemic, H. Kazemia, Fuel 118 (2014) 156. [31] R. Sánchez, M.C. Blanco, M.A. Quintela, J. Phys. Chem. B 104 (2000) 9683. [32] B.J. Murray, Q. Li, J.T. Newberg, E.J. Menke, J.C. Hemminger, R.M. Penner, Nano Lett. 5 (2005) 2319. [33] K. Haerens, E. Mathinjs, K. Binnemans, B.V. der Bruggen, Green Chem. 11 (2009) 1357. [34] K. Haerens, E. Mathinjs, A. Chmielarz, B.V. der Bruggen, J. Environ. Manage. 90 (2009) 3245. [35] A.P. Abbott, G. Capper, D.L. Davies, K.J. McKenzie, S.U. Obi, J. Chem. Eng. Data 51 (2006) 1280. [36] M. Chirea, A. Freitas, B.S. Vasile, C. Ghitulica, C.M. Pereira, F. Silva, Langmuir 27 (2011) 3906. [37] M.T. Reetz, W. Helbig, J. Am. Chem. Soc. 116 (1994) 7401. [38] D.H. Nagaraju, V. Lakshminarayanan, Langmuir 24 (2008) 13855. [39] L. Skantarov, A. Orinak, R. Orinakova, F. Lofaj, Nano-Micro Lett. 4 (2012) 184. [40] Y. Erdogdu, M.T. Gulluoglu, M. Kurt, J. Raman Spectrosc. 40 (2009) 1615. [41] C.M. Nakul, M.A. Mihaela, G.Z. Michael, R.C. Paul, E.A. Vernon, J. Am. Chem. Soc. 126 (2004) 2399. [42] L. Daimay, B.C. Norman, G.F. William, G.G. Jeanette, The Handbook of Infrared and Raman Characteristic Frequencies of Organic molecules, Academic Press, San Diego, California, 1991. [43] S. Gurunathan, K. Kalishwaralal, R. Vaidyanathan, D. Venkataraman, S.R.K. Pandian, J. Muniyandi, N. Hariharan, S.H. Eom, Colloid Surface B 74 (2009) 328. [44] P. Mukherjee, M. Roy, B.P. Mandal, G.K. Dey, P.K. Mukherjee, J. Ghatak, A.K. Tyagi, S.P. Kale, Nanotechnology 19 (2008) 075103. [45] T.H. Phan, R.E. Schaak, Chem. Commun. (2009) 3026. [46] D. Jose, B.R. Jagirdar, Int. J. Hydrogen Energy 35 (2010) 6804. [47] Y. Gimeno, A. Hernandez Creus, S. Gonza´lez, R.C. Salvarezza, A.J. Arvia, Chem. Mater. 13 (2001) 1857. [48] G.N. Xiao, S.Q. Man, Chem. Phys. Lett. 447 (2007) 305. [49] D. Diesing, H. Winkes, A. Otto, Phys. Status Solidi 159 (1997) 243–254. [50] D. Eberhardt, E. Santos, W. Schmickler, J. Electroanal. Chem. 461 (1999) 76. [51] F.W. Campbell, S.R. Belding, B. Ronan, L. Xiao, R.G. Compton, J. Phys. Chem. C 113 (2009) 14852. [52] A. Ruderman, M.F. Juareza, G. Soldanoa, L.B. Avalleb, G. Beltramoc, M. Giesenc, E. Santosa, Electrochim. Acta 109 (2013) 403. [53] D.A. Harrington, B.E. Conway, Electrochim. Acta 32 (1987) 1703. [54] D.A. Harrington, P.V. Driessche P, J. Electroanal. Chem. 501 (2001) 222–234. [55] B. Plowman, S.J. Ippolito, V. Bansal, Y.M. Sabri, P. Anthony, O’Mullane, S.K. Bhargava, Chem. Commun. (2009) 5039. [56] M.M. Costa, A.E. Muñiz, A. Merkoçi, Electrochem. Commun. 12 (2010) 1501. [57] J. Perez, E.R. Gonzalez, H.M. Villullas, J. Phys. Chem. B 102 (1998) 10931. [58] A.T. Kuhn, M. Byrne, Electrochim. Acta 16 (1971) 391. [59] G.J. Brug, M. Sluyters-Rehbach, J.H. Sluyters, J. Elecroanal. Chem. 181 (1984) 245. [60] L.A. Khanova, L.I. Krishtalik, J. Electroanal. Chem. 660 (2011) 224.