Charge Density Modulated Shape-Dependent Electrocatalytic Activity

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Charge Density Modulated Shape-Dependent Electrocatalytic Activity of Gold Nanoparticles for the Oxidation of Ascorbic Acid Debranjan Mandal, Subrata Mondal, Dulal Senapati, Biswarup Satpati, and Marthi V Sangaranarayanan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07710 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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Charge Density Modulated Shape-Dependent Electrocatalytic Activity of Gold Nanoparticles for the Oxidation of Ascorbic Acid Debranjan Mandal1, Subrata Mondal1, Dulal Senapati2, Biswarup Satpati3, and M. V. Sangaranarayanan*1 1

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036,

India 2

Nanophotonics Group, Chemical Sciences Division, Saha Institute of Nuclear Physics,

Kolkata 700064, India 3

Surface Physics & Material Science Division, Saha Institute of Nuclear Physics,1/AF,

Bidhannagar, Kolkata 700064, India

1 ACS Paragon Plus Environment

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Abstract Electrocatalytic performance of noble metal nanoparticles depends upon their size, shape, composition and crystalline facets. Here we demonstrate the shape-dependent electrocatalytic activity of Au nanoparticles towards ascorbic acid oxidation in acidic medium, wherein the catalysis is strongly influenced by the shape of the nanoparticles. The synthesis of (popcorn, tetrapod and bipod shaped) Au nanoparticles was carried out using a systematic variation of the surfactant concentrations based on the seed mediated growth technique at room temperature. Due to the facile electrostatic interaction of the positively charged Au nanoparticles with glassy carbon electrode, the modification of the surface with variable-shaped Au nanoparticles is accomplished without involving any binding agents. Among variable-shaped face-centered cubic (fcc) crystalline AuNPs, bipod-shaped Au nanoparticles (GNBipd) exhibit a superior electrocatalytic performance over tetrapod-shape (GNTepd) and popcorn-shaped (GNPop) nanoparticles as inferred from the Differential Pulse Voltammetry and Electrochemical Impedance Spectroscopy. The results have been explained by invoking the relative surface free energy (γ) with preferentially exposed crystal planes, relative surface area (A), zeta potential (ξ), and the curvature induced charge density (σq) at the apex for individual variable-shaped gold nanoparticles.

Keywords: Gold nanoparticles, Charge density, Field lensing, Crystallinity, Ascorbic acid, Electrochemical Impedance Spectroscopy



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Introduction Ascorbic acid is a well known water soluble vitamin and is an effective antioxidant1. It is often utilized as a chemical marker for determining the quality of the food and has also significance in clinical studies.2,3 Further, it is used in treatment and prevention of cold, mental illness, etc since it can assist the development of healthy cells, growth of tissues, iron adsorption, etc.4 However the excessive levels of ascorbic acid may form oxalates and thereby developing kidney stones as well as causing other diseases.5,6 Among various electrode processes, oxidation of ascorbic acid is a notoriously sluggish reaction and the elucidation of the reaction mechanism in neutral as well as acidic media is quite complicated. Several attempts have been made to accelerate the kinetics of the electrode reaction by modifying the surface with different materials viz. polymer based,7 carbon paste,8 functional group modified,9 metal modified,10 etc. In the case of metals such as Pt, Ga, Hg and Au, the involvement of two electrons in the oxidation process has been established using electron paramagnetic resonance studies11 while a combination of microchannel band electrodes with hydrodynamic voltammetry12 has demonstrated the effect of pH on the mechanistic pathways. On account of their novel physicochemical and electronic properties in conjunction with enhanced surface areas, metal nanostructures exhibit impressive catalytic properties.13 In this context, gold nanoparticles (AuNPs) have attracted immense interest in multiple directions which include ultrasensitive DNA/RNA detection,14 antimicrobial activity against multi-drug resistant bacteria,15 oxidation of CO16 due to


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their unique shape-dependent properties and hence the modification of electrode surfaces with AuNPs is an active area of research.17 The influence of shapes on the electrocatalytic property of Au nanoparticles arises due their shape-dependent surface free energy, total surface charge on individual variable-shaped particles, average surface area which depends on their shape complexity and curvature induced charge density at the apex for individual variable-shaped gold nanoparticles. For a specific crystalline nature, the surface free energy of that crystal depends on the nature of their crystal facets. According to literature, the free energies associated with the low index crystallographic planes of an fcc metal increases in the order: γ(111) < γ(100) < γ(110).18 In the case of methanol oxidation, Au nanospheres with a preferential {111} orientation exhibited highly remarkable catalytic activity whereas Au nanorods with dominant {100} orientation were found superior towards oxygen reduction reaction19; cubic Au nanoparticles with {100} facet possess enhanced catalytic ability for the oxidation of glucose than {110}-bounded rhombic dodecahedral and {111}-bounded octahedral Au nanocrystals.20-21 These studies indicate that electrocatalytic performance of gold nanoparticles is dictated by the size, shape and crystalline nature. To understand the electrocatalytic activity of different crystalline AuNPs, the (electro) chemical modification of electrode surfaces is often carried out by immobilizing AuNPs using electrodeposition or covalent binding.22 It is customary to invoke zeta potential values (ξ) (or overall surface charges) for interpreting the stability of the nanoparticle suspensions. It is well known that nanoparticles with zeta potential greater than +30mV or less than 30mV have sufficient electrostatic repulsion so as to remain stable in solutions.21 Along with their colloidal stability, greater surface charges also facilitate oxidation and



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reduction process of analytes present in the system. In this study, since we are considering the oxidation of ascorbic acid, positively charged gold nanomaterials are the correct choices on account of their inherent electron withdrawal properties. Moreover, variable-shaped gold nanoparticles deposited on the GC electrode prevent any aggregation due to the reduction of surface charge on account of their electron withdrawal properties and thereby decreasing the effective zeta potentials during the catalytic oxidation. A schematic representation of GC electrode modification is depicted in Scheme 1. Hashimoto et al.23 have reported the photocatalytic decomposition of ethylamine ions by nitrogen doped TiO2 films and explained the dependence of decomposition rates on relative negative charges (controlled by nitrogen doping) on TiO2 surfaces. They observed a higher decomposition rate of ethylamine ions in aqueous solution for nitrogen doped TiO2 than that of pure TiO2. The exploitation of zeta potentials for efficient catalytic performance has been reported by several other groups24 and hence we should consider the surface charge of nanoparticles as a controlling factor for enhanced catalysis. Furthermore, the catalytic activity of a material increases with the available external surface area (A)25,26 of the material and for larger A values, larger is the diffusion of the reactants to the catalytically active sites. Recently, Chattopadhyay et al.27 have reported a surface area controlled (by varying the ratio between water and acetanilide as solvent) differential catalytic activities of citrate-stabilized gold nanoparticles for the reduction of 4-nitrophenol (4NP) into 4-aminophenol (4AP), using excess sodium borohydride. Along with their crystalline surface free energy, overall surface charge, and surface area, the surface charge density at the apex for different shaped nanostructures is also an important controlling factor for enhancing the catalytic


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activity. While there exist extensive studies correlating the surface area and relative surface free energy with their catalytic activity for different nanomaterials, very few studies are available where the surface charge has been investigated as an effective controlling factor for catalytic behavior of a nanomaterial. Contrary to the reports on surface area, crystalline surface free energy and nanoparticle surface charge, the curvature-induced charge density at the apex as a controlling factor for catalytic activity of nanomaterials is non-existent. Thus, the present study is, to our knowledge, the first report which explains the catalytic oxidation of ascorbic acid by variable-shaped gold nanomaterials from their respective curvature induced charge density at the apex for enhancing the catalytic activity of a material.

Scheme 1: Schematic representation for the modification of GC electrode (GCE) surface with gold nanoparticles for the oxidation of ascorbic acid. However, AuNPs-modified electrodes have been widely utilized in the past for oxidation of ascorbic acid,28-30 methanol31,32 and ethanol33; in the present study, we have investigated the oxidation of ascorbic acid using different shapes of AuNPs drop-casted 6 ACS Paragon Plus Environment


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onto the GC electrode and inferred that the electrocatalytic behavior is influenced by the shapes of AuNPs -analyzed using the Differential Pulse Voltammetry (DPV) as well as the Electrochemical Impedance Spectroscopy (EIS). Although the modification of GC surfaces with Au nanoparticles in the context of ascorbic acid oxidation is already known,32-33 we demonstrate the shape-dependent electrocatalytic behavior of Au nanoparticles for the first time. The different morphologies of AuNPs viz. popcorn (GNPop), tetrapod (GNTepd), and bipod (GNBipd) have been synthesized using different concentrations of cetyltrimethyl ammonium bromide (CTAB) according to the protocol developed by Senapati et al. [34] and the immobilization of the AuNPs on the surface is accomplished without any binding agents by exploiting the facile electrostatic interaction of the GC electrode with the cationic charge of the AuNPs surface. For the oxidation of ascorbic acid on electrode surfaces, the bipod shaped Au nanoparticles are shown to be more catalytically active than the tetrapod and popcorn shaped geometries. Experimental Section Chemicals The chemicals Trisodium citrate (Na3C6H5O7), Sodium borohydride (NaBH4), and Perchloric acid (HClO4) were procured from SRL Chemicals, India, while Cetyl trimethylammonium bromide or CTAB (C19H42BrN), Aurochloric acid (HAuCl4), Ascorbic acid (AA), (C6H8O6) and Silver nitrate (AgNO3) were obtained from Sigma Aldrich, India, and were used as received. The different shapes of AuNPs were synthesized using Milli Q water while all the electrochemical experiments were carried out using triple distilled water. Synthesis of different shaped AuNPs and modification of the GC electrode


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The different shapes of AuNPs were synthesized using an earlier procedure advocated by Senapati et al.,34 employing the seed mediated growth technique. In this method, Au seeds were first prepared by mixing 0.5mL of 0.01 M HAuCl4.3H2O with 0.2 mL of 25 mM of trisodium citrate (TSC) in 20 mL of Milli Q water where the final concentration of both HAuCl4.3H2O and trisodium citrate were at 2.5×10-4 M. An ice cooled freshly prepared 10-1M 60 µL NaBH4 solution was then added drop by drop. The solution turns from colorless to violet to deep red. This seed solution is kept for two hours in dark before we use it for synthesizing different shaped AuNPs. For the preparation of different shaped AuNPs, CTAB is ultra sonicated for 20 minutes at 30 °C. To this solution, 2mL of 10-2 M HAuCl4.3H2O is added with constant stirring. To this 300µL of 10-2 M AgNO3 was added followed by the addition of 320 µL of 10-1 M ascorbic acid drop wise. The solution turns colorless and immediately, 500 µL of the seed solution was added at a time. The different morphologies of AuNPs viz. star shape, tetrapod, and bipod were synthesized by varying the weight of CTAB in the following manner: (i) 0.049 gms, (ii) 0.49 gms, and (iii) 1.64 gms, in 50 mL respectively. The AuNPs synthesized by the aforementioned procedure were then washed three times by low speed centrifugation to remove unreacted CTAB, followed by drop casting onto the GC electrode. The electrode was initially cleaned by polishing with alumina powders of different grades and sonicated for five minutes in deionized water. The electrode was then dried in order to avoid hydration so that water cannot repel the AuNPs as well as cannot dilute the AuNPs concentration from the electrode surface. The electrode was kept vertically and the AuNPs of various shapes were drop casted on the electrode surface carefully in order to modify only the active part of the electrode surface.



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After drop casting the AuNPs on the GC electrode, the electrode was dried at room temperature for 12 hours. Electrochemical Measurements All the electrochemical measurements were performed in an one-compartment cell of three electrode configuration using AuNPs modified GC electrode as the working electrode (CH Instruments, USA) while the saturated calomel electrode (SCE) and Pt wire (Bioanalytical system, USA) served as the reference and counter electrode respectively. Differential pulse voltammetry (DPV) and Electrochemical Impedance Spectroscopy (EIS) experiments were performed in CH 660D Electrochemical work station (CH Instruments, USA). The oxidation of ascorbic acid was studied by using DPV on AuNPs-modified GC electrodes in 0.1 M HClO4 solution using a potential window of 0.2 to 0.8V. The EIS was carried out using AuNPs modified GC electrode employing a constant potential of ~ 0.4 V and the frequency range of 105 Hz to 10-2 Hz. The oxidation of ascorbic acid using different shaped modified GC electrode was carried out with a constant concentration of 4×10-4 M of ascorbic acid. All the electrochemical experiments were performed at a temperature of 30±1°C. Results and Discussion Structural Characterization and Surface Free Energy Measurement The structural characterization of different shaped AuNPs was carried out using transmission electron microscopy (TEM). Figure 1 depicts the TEM images of various shaped AuNPs along with the gold nanoseed. All the TEM images correspond to uniform distribution of different shaped AuNPs wherein the identical sizes of a particular morphology are observed.


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Figure 1: HRTEM images of different shaped Au Nanoparticles: (A) Gold nanopopcorn, GNPop, (B) Gold nanotetrapod, GNTepd, (C) Gold nanobipod, GNBipd, (D) Gold nanoseed, GNSeed (4.3 ± 1.4 nm) and (E1,E2,E3) Single particle TEM images of GNPop, GNTepd, and GNBipd respectively. Most noble metal nanocrystals (NCs) crystallize in the face-centered cubic (fcc) crystal structures and surface-energy considerations are crucial in predicting the morphology of noble-metal NCs. A close look into the formation mechanism of variable shaped gold nanoparticles was achieved by characterization using high resolution transmission electron microscopy (HRTEM). Three different nano-structures GNPop, GNTepd, and GNBipd at the CTAB concentration of 2.7×10-3 M, 2.7×10-2 M and 9×10-2 M along with the 4.3± 1.4 nm spherical GNSeed were analysed by FEI, Tecnai, F30-ST



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microscope operating at 300 kV to understand their crystalline nature, facet orientation of the tips, exposed crystal planes and are presented in Figure 1. The corresponding HRTEM and magnified image in the inset for 4.3± 1.4 nm spherical GNSeed (Figure 1D) clearly indicates the single crystal nature of the structure and (110) plane as the basal plane. In Figure 1A, an icosahedral GNPop synthesized from 4.3± 1.4 nm spherical gold nanoseeds (2.7×10-3 M CTAB-based product). Fast Fourier transform (FFT) pattern from all the tips can be indexed having [110] zone axis (two of them are shown in Figure 1A). As we increase the CTAB concentration from 2.7×10-3 M to 2.7×10-2 M and 9.0×10-2 M, resulting GNTepd (Figure 1B) and GNBipd (Figure 1C) stabilizes with different shape by keeping same zone-axis orientation [110] as evident from TEM images for all three structures. Zeta Potential (ξ) In order to obtain more insights into the overall surface charge and the stability of AuNPs, zeta potentials of different shaped gold nanoparticles were measured. The positive zeta potentials indicate the cationically charged surface of the AuNPs. Among these, popcorn shaped AuNPs have the highest zeta potential (+52 mV) than tetrapod s (+47mV) and bipod (+40 mV) shaped nanoparticles. At higher zeta potentials, the surface charge of the nanoparticles is also large so that the particles are precluded from proximity to each other and hence do not aggregate, thus providing increased stability. From the zeta potential values, it is also evident that the increasing order of dispersive stability follow the pattern: GNPop > > .

Charge Density at the Tip


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Along with their crystal stability and zeta potential (ξ), we have also performed a numerical charge density measurement at the sharp tips for the three different shaped gold nanoparticles. The methodology of estimating the charge density is depicted in Figure 2. As shown in Figure 2, we have first measured the average curvature at the tips for three different shaped nanomaterials in a statistical manner. If we draw the smallest circle with radius 'R' which perfectly fits the boundary of the tip, mathematically we can define the curvature (κ) of this sharp tip simply by the reciprocal of the radius, i.e., κ =

.35 This implies that the tips with smallest diameter have the highest curvature. Again, zeta potential measurements show that these nanostructures are positively charged and hence they form a charged layer on the surface which gets focused at their apex. In other words, we can expect a field lensing at the tips for each nanostructure. Depending on the extent of curvature at the tip, field lensing could be either deep or shallow. It is well known in the literature that the depth of field is infinite for a zero curvature lens or mirror and decreases as the curvature increases.36 Hence it is obvious that for a highly curved tip, all the field will be focused tightly in a point compared to a widely curved tip. As a result of this depth of field dependence, the charge density at the apex will be maximum for highest curvature tips. Mathematically we can define this lensing power or charge

density as: ∝ σ ∝ =   = 2κ, where 'P' is the lensing power, 'σq' is the charge density, 'f' is the focal length and '1/R' is the curvature of the tip.36



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Figure 2: Statistical analysis of curvature at the tip for three different shaped nanostructures. Small red circles in A1, B1, and C1 indicate their respective curvature for individual tips and large red circles in A and C indicate the region selected for detailed curvature study in a large area TEM image. A & A1: Bare GNPops & selected area curvature statistics of GNPops in image A; B & B1: Bare GNBipds & selected area curvature statistics of GNBipd in image B; and C & C1: Bare GNTepds & selected area curvature statistics of GNTepd in image C. From the above statistical analysis, it is clear that the average tip diameter (2R) for GNPops are 16 nm (tip radius, R = 8.0 nm) which corresponds to a curvature of 1.2×106 cm-1. Similarly, the average tip diameters for GNTepds and GNBipds are 9.75 nm (tip radius R = 4.87 nm) and 5.12 nm (tip radius R = 2.56 nm) and the calculated tip


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curvature for GNTepds and GNBipds are 2.1×106 cm-1 and 3.9×106 cm-1 respectively. Hence, the resulting average charge density at the tip for three different nanostructures are 2.4×106 cm-1, 4.2×106 cm-1, and 7.8×106 cm-1 for GNPops, GNTepds, and GNBipds correspondingly. Thus, it follows that the charge density at the tip increases approximately two times from GNPops to GNTepds and again approximately two times

from GNTepds to GNBipds and follow the order: σ

!"#

< σ

$%#

< σ

.

Active Surface Area for Different Shaped Nanoparticles Besides surface free energy, surface free charge, and average charge density at the tip for different shaped nanoparticles, we have also calculated the active surface area for each different shaped nanoparticle. In the present case, GNPop has the smallest tips with average length of 20nm and average base diameter (by considering the tips are as cone shaped) of 14nm (Figure 3A) but have at least seven (7) tips observable in a twodimensional image and more tips may be present in a true three-dimensional structure. Compared to the GNPop, GNTepd has only three (3) tips with a base (total 4 pods) in a two-dimensional image but the tips are much bigger with an average length of 32nm and relatively wider with average base diameter of 21nm (Figure 3B). Out of three different shaped nanoparticles, GNBipd has only two tips and each one is 50nm long and average base diameter of 22nm (Figure 3C). By ignoring any effective surface area contribution from the core and all the tips as cone shaped (Figure 3D),we can calculate the total surface area per particle for individual shaped nanoparticles from the equation, &'()*+ =

,-.- + √ℎ + - 2 , where 'r' is the average radius of the cone-shaped tip base and 'h' is the average length of the cone-shaped tip for a specific shaped nanoparticle as depicted in 14 ACS Paragon Plus Environment


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Figure 3. From the above mentioned dimensions of the tips, our measured total surface areas

are:A!456 = 77 9 619.6>nm = 4337 nm , A!456

$%#

4354 nm and A!456

!"#

= 73 9 1451.54>nm =

= 72 9 2148>nm = 4296 nm . This implies that although the

three different gold nanoparticles have different shapes, the effective surface areas remain almost constant.

Figure 3: Single particle TEM image from (A) GNPop, (B) GNTepd, and (C) GNBipd for the measurement of average tip length and tip diameters by considering each tip as a three dimensional cone (D), where '2r' is the average diameter of the cone-shaped tip base and 'h' is the average length of cone-shaped tip for a specific shaped nanoparticle. Electrocatalytic Oxidation of Ascorbic Acid Using DPV It is well-known that the electrochemical behavior of metal nanoparticles is significantly different from that of the bulk metal.37 However, it is not obvious whether the nano structures also possess different catalytic abilities if their morphologies are dissimilar. As mentioned earlier, the oxidation of ascorbic acid is a sluggish reaction [38] and extensive attempts have been made to catalyze this reaction in view of its importance in biological systems.4 In order to study the difference among bulk gold, gold nano particles and bare Glassy carbon (GC) electrodes, DPV was performed for the oxidation of ascorbic acid using these three electrodes (Figure1 of SI). As a preliminary 15 ACS Paragon Plus Environment

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investigation, GNTepd drop-casted onto the GC electrode was chosen. For the bare GC and Au electrodes, the peak potential for the oxidation of AA occurs at 0.72 V and 0.71 V respectively while for the AuNPs (GNTepd) coated GC electrode, the corresponding value is 0.53 V; furthermore, the oxidation current too increases four times in comparison with the bare gold and GC electrodes. These oxidation potentials indicate that the overpotential has decreased by ~0.2 V which suggests the enhanced catalytic ability of AuNPs modification of GC electrodes. In order to comprehend this behavior more quantitatively, further DPV analysis was performed utilizing the different shapes of AuNPs drop casted onto the GC electrodes and the differential pulse voltammograms are not identical (Figure 4A), thus implying that the kinetics of oxidation of AA is dependent upon the shapes of AuNPs. As the shape changes from popcorn-shaped to bipod-shaped, the potential shifts towards more negative values in conjunction with the enhancement in the oxidation current.

Influence of Variable-Shaped AuNPs on electrocatalytic investigations Figure 4A depicts the differential pulse voltammogram pertaining to the oxidation of ascorbic acid using different shaped Au nanoparticles modified GC electrodes. The anodic peak potential is noticed at 0.58V for popcorn-shaped AuNPs-modified GC electrode which is ~ 0.14 V less negative than the bare GC electrode and the oxidation current becomes twice that of the latter as depicted in Figure 4A. In the case of tetrapodshaped AuNP, the peak potential is observed at 0.53V which gets further shifted towards more negative values than the bare GC electrode. The current for the tetrapod-shaped AuNP coated GC electrode also increases by four times than the bare GC electrode



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(Figure 4A). For the bipod-shaped AuNPs modified GC electrode, the peak potential is 0.46 V which is 0.26 V lower than the bare GC electrode and the current increases by six times in comparison with the bare GC electrode.

Figure 4: Oxidation of ascorbic acid on variable-shaped AuNPs modified GC electrodes in 0.1 M HClO4 solution: (A) Differential Pulse Voltammograms in the potential window of 0.2 to 0.8 V with the potential increment = 4 mV, amplitude = 50 mV, pulse width = 0.05 sec and pulse period = 0.5 sec and (B) Nyquist plots in the frequency range 105 Hz to 10-2 Hz, at a potential of 0.4 V. This shape-dependent catalytic behavior of Au nanoparticles towards the oxidation of ascorbic acid is entirely new and hence warrants new theories for currentpotential behavior. Nevertheless, the observed catalytic behaviour can be interpreted in a qualitative manner by a plausible mechanism. From the DPV studies, the electrocatalytic activities of AuNPs towards ascorbic acid oxidation follow the trend as: GNbipd > GNTepd > GNPop. Prima facie, the differences in the catalytic activity can be 17 ACS Paragon Plus Environment

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comprehended with the help of four different controlling factors: (i) relative stability (or surface free energy) of the active crystalline planes for three different nanostructures, (ii) zeta potential (or surface charge) of individual variable-shaped gold nanomaterials, (iii) active surface area of different shaped nanoparticles, and (iv) curvature induced charge density at the apex for individual gold nanoparticles. According to the literature, the free energies associated with the crystallographic planes of an fcc metal increases in the order: γ(111) < γ(100) < γ(110).18 As we discussed in the previous section, though the increment of CTAB concentration stabilizes nanoparticles with different shapes, they maintain the zone-axis orientation {110} same, as evident from TEM images for all three structures; interestingly, GNBipd structures have all the tips oriented only in {110} direction (Figure 1C). Hence, the increasing amount of stabilizer which changes their shapes does effectively change their tip orientations and in the case of GNBipd, tips are in the highest free energy directions and hence exhibit the superior electrocatalytic ability over tetrapod-shape (GNTepd) and popcorn-shaped (GNPop) AuNPs. The high-energy facets of noble metal nanocrystals (NCs) possessing high density of atomic steps, edges, kinks etc and having more active sites for the breaking of chemical bonds (than common NCs) are excellent catalysts for organic reactions and surface chemical processes.39,40 In an analogous manner, although GNBipd exhibits superior catalytic property among the three different nanostructures, it is still not obvious which nanostructure will exhibit enhanced catalytic activity among GNTepd and GNPop as all the tips are not oriented in {110} direction. By considering the surface free energy as the controlling factor in dictating their catalytic activities (i.e., decrease in the oxidation potential and increase in the peak current) in comparison with the bare GC electrode, the catalytic activity (z)41 for



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{J}

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{J}

{J}

three different morphologies should follow the order: z$%# > z!"# and z . {J}

{J}

From figure 4A and Table 1, the catalytic activity follows the trend z$%# > z!"# > {J}

z . The above order demonstrates that the surface free energy of individual nanostructures is inadequate to explain the observed catalytic activity and urges a new explanation. In the present context, since we are considering the oxidation (donation of electrons) of AA by using variable-shaped gold nanomaterials coated electrodes, surface charge on individual gold nanomaterials may probably account for their relative catalytic activities. As discussed in the previous section (zeta potential analysis), it was shown that zeta potential (ζ) on individual variable-shaped gold nanoparticles varies as: ζ 7+52mV> > ζ!"# 7+47mV> > ζ$%# 7+40mV>,

electron

withdrawal

tendency should be maximum for GNPop and minimum for GNBipd. This should result in the maximum catalytic activity for GNPop and minimum for GNBipd whereby the expected order of catalytic oxidation activity is: z > z!"# > z$%# ; however, the observed order of the catalytic activity is exactly reverse. It is well known that the catalytic activity of particles increases with the specific surface area.38,41-43 If we compare the catalytic activity of different sized particles having same shapes, it is natural to expect more catalytic activity for smaller particles due to the availability of larger surface per unit mass. However, this is not applicable for different sized particles of varying shapes too. In order to eliminate any ambiguity related to the active surface area of the modified electrode, we have maintained the active surface area of all three different-shaped nanoparticles almost constant. In this investigation, the sizes of AuNPs are nearly same and the effective surface areas do not differ significantly 19 ACS Paragon Plus Environment

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among them (~ 4300nm2). Hence the surface to volume ratios remain constant, thereby leading to equal active surface areas for all the three shapes. Consequently, the roughness factor of the electrode may not play a significant role herein. Hence, the surface areas may not play a crucial role in determining the catalytic activity Since we are unable to explain unambiguously the observed catalytic activity of the variable-shaped gold nanomaterials with the help of their surface free energies, surface charges and effective surface areas, we analyze the charge density at their apex using the curvature statistics for individual variable-shaped gold nanoparticles. It is well known that most exciting opto-electronic activities of nanomaterials stem from their nano-scale sharp tips (e.g., lightning rod effect).44-46 This effect occurs especially at sharp features (e.g., corners, crease edges, cracks, branches, tips, etc.) of the nanoparticles because there are very few neighboring atoms surrounding corner atoms compared to edge atoms. In the case of our positively charged nanoparticles, the lack of surrounding atoms at their tips results in little restoring force acting on the charge cloud (+Ve charge) and a greater free-charge environment, which leads to the production of larger fields. More the curvature at the tip, lower is the atom density at the corner, thereby resulting in greater free-charge density. As discussed earlier, the estimated average charge density at the tip for three different nanostructures are 2.4×106 cm-1, 4.2×106 cm-1, and 7.8×106 cm-1 for

GNPops, GNTepds, and GNBipds respectively and follow the order σ !"#

σ

$%#

< σ

R O!

$%#

> R O!

.

This variation can be interpreted using the exchange current densities (i0) viz47 '

PQ' = RST

(1)

U

where ‘n’ denotes the number of electrons (here, n = 2) and hence i0 value for GNBipd is higher than all other shapes of AuNPs. Furthermore, the exchange current density is related to the standard heterogeneous electron transfer rate constant (k0) as47 7[α> α X\] ^J

J = VW&XYZ


(2)


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where ‘A’ denotes the active surface area and COx and CRed denote the bulk concentrations of ascorbic acid (AA) and dehydroascorbic acid (DAA) respectively, α 7[α>

being the symmetry factor. Assuming XYZ

α = X\] = X, the above equation leads to

J = VW&X^J .Hence it follows that the standard heterogeneous electron transfer rate

constant for oxidation of ascorbic acid at AuNPs modified GC electrode increases as: GNPop < < .

The impedance (Z) can be written in terms of the real 7_ ′ > and imaginary 7_ ″ >

parts47 as: a

_ ′ = PΩ + PQ' + σ` [b

(3)

a

_ ′′ = σ` [b + 2σ X]

(4)

where RΩ and RCT denote the solution resistance respectively and

and charge transfer resistance

ω denotes the angular frequency (= 2πf), σ being the Warburg

coefficient given by30 '

σ = Rb Sb c√ d

a b [cc] eff

+

a b efff [ccc]

i

(5)

DAA and DDAA denote respectively the diffusion coefficient of ascorbic acid (AA) and dehydroxy ascorbic acid (DAA) while [AA] and [DAA] represent the corresponding bulk concentrations. As the shapes of the AuNPs change, the active sites also vary, thus enhancing the oxidation of AA. This is consistent with the DPV data of Figure 4A. Since the extent of oxidation depends upon the shape of the AuNPs, the concentration of DAA ([DAA]) too increases from popcorn shaped AuNPs to bipod AuNPs. From equation (4) it is seen that the imaginary part (_ ′′ ) of the impedance is directly related to the Warburg


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coefficient, and hence the Warburg coefficient follows the trend as: σ > σ!"# > σ$%# . In order to obtain quantitative insights into the system behavior, the classical Randles equivalent circuit has been modified by incorporating the constant phase element48 (Figure 4B inset). The constant phase element represents the surface inhomogeneity of the electrode which arises from the porosity and nature of the electrode surface. The impedance of CPE is given as _Qjk = [l7m`>Ra ][, where Q denotes the parameter characterizing

the surface and electroactive species while n1 denotes the

porosity.48 The fitting of the impedance data to the equivalent circuit is shown in Figure 1 of SI while Table 2 provides the system parameters deduced from the fitting. As the morphology of the AuNPs varies, the nature of the modified electrode surface is also altered due to their different porosities , thereby changing n1 vis a vis CPE. Table 2: The System Parameters deduced from the Nyquist plot for the oxidation of AA Geometry

RCT (kΩ)

i0 (in 10-8Amp)

k0 (in 10-6 cm s-1) CPE (in 10-5 Ω-1s)

Bare GC

554.92

2.31

4.2

0.28

0.65

W (in 10-5 Ω s-1/2) 80.0

GNPop

114.20

11.2

20.1

0.98

0.90

8.0

GNTepd

65.28

19.6

32.9

0.80

0.88

20.0

GNBipd

43.25

29.6

54.2

0.78

0.85

7.5

.


n1


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Figure 5: The dependence of the electrode kinetic parameters on the shape of AuNPs towards the oxidation of AA. Yellow color bars denote the variation of the standard heterogeneous rate constants while the blue color bars indicate the changes in the charge transfer resistance. It is of interest to enquire whether other shapes of Au NPs such as nanospheres may provide additional insights regarding the catalytic activity. While this requires further systematic investigations, we note that CTAB has been employed here as the shape templating surfactant whose positive head group (NH4+) leads to an effective attachment of the AuNPs on the electrode. On the other hand, it is difficult to synthesize uniformly spherical naoparticles with the charge density proportional to the curvature (κ = 1/R), if CTAB is employed as the surfactant. Due to their shape-templating nature, CTAB-based


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AuNPs generate anisotropy and the extent of anisotropy depends upon the concentration of CTAB. Although one can generate spherical nanoparticles by other surfactants such as trisodium citrate (TSC), the latter cannot function as a shape-controlling agent due to the negative charges offered by the citrate group and hence precludes their attachment onto the electrode surface. For all the three structures, the core is considered to be spherical with the tips being aligned on it. We have considered the three shapes so as to study the influence of their anisotropic nature on the electrocatalytic effect. The immobilization of AuNPs on the GC electrode leads to the effective utilization of the binding agent on account of their facile electrostatic interaction with the surface. The present strategy offers a control in the catalytic activities of the nanoparticles due to the inequality in their shapes and the resultant field lensing at the tip. All the different shapes of AuNPs modified GC electrodes exhibit superior electrocatalytic activities than the bare electrodes. Among different morphologies of AuNPs, GNBipd with all the tips oriented only in {110} direction has the highest catalytic activity. The bipod geometry exhibits the maximum peak current for the oxidation of ascorbic acid and the peak potential also progressively shifts towards less positive potentials in comparison with the bare GC electrode and other morphologies. The standard heterogeneous rate constant (k0) for the oxidation of ascorbic acid is also significantly higher for the bipod geometry. The electrocatalytic ability of different shaped AuNPs towards ascorbic acid oxidation is interpreted by considering both the relative stability (or surface free energy) of the active crystalline planes and the charge density at the apex for individual AuNPs. The bipod geometry exhibited thirteen fold increase (Figure 5) in the rate constant than the bare GC electrode and this increase is significant, considering the sluggish nature of the electron



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transfer process. Although the electrocatalytic behaviors of AuNPs were carried out in acidic medium (pH = 1.4), we believe the similar trend to be valid in the buffer solution too. Among different morphologies, ascorbic acid oxidation is more favored on GNBipd geometry neither due to the extent of their free surface positive charge nor active surface area of different shaped nanoparticles, but due to their combined effect of relative stability (or surface free energy) of the active crystalline planes and curvature at the tips which induces field lensing. Furthermore, depending on the deep or shallow depth of charge focusing, they show a variable-shaped catalytic oxidation property. Conclusions Gold nanoparticles of popcorn-shaped, tetrapod and bipod geometries were synthesized using a surfactant-assisted seed mediated growth technique and these were drop casted onto the GC electrodes in order to analyze the electrocatalytic oxidation of ascorbic acid. The various morphologies of AuNPs alter the standard heterogeneous electron transfer rate constants in a significant manner. The bipod geometries have the marked influence on the electron transfer kinetics and this behavior is interpreted using their active crystalline planes and the charge density at the apex for different variableshaped gold nanoparticles. The electrocatalytic activity follows the trend : GNBipd >

GNTepd > GNPop. To the best of our knowledge, this is the first report demonstrating the electrocatalytic activity of variable-shaped gold nanoparticles by considering their curvature induced field lensing ability at the apex (σq) along with their relative surface free energy (γ) while the relative surface area (A) and zeta potential (ξ) on individual nanomaterials have minimal or no effect on their observed catalytic activities.


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Associated Content Supporting Information Differential pulse voltammograms for electrocatalytic oxidation of AA using AuNPs/GC, bare GC and bare Au electrodes. Nyquist plots for the oxidation of AA using bare GC and various shaped AuNPs modified GC electrodes and their equivalent circuit fitting. Acknowledgement The helpful comments of the reviewers are gratefully acknowledged. DS would like to express his sincere gratitude to BARD project (PIC No. 12-R&D-SIN-5.04-0103), DAE, Government of India, for their generous laboratory establishment funding. MVS thanks the Department of Science and Technology, Government of India for financial support. References [1] Yu, A. M.; Chen, H. Y. Electrocatalytic Oxidation and Determination of Ascorbic Acid at Poly(glutamic acid) Chemically Modified Electrode. Anal. Chim. Acta 1997, 344, 181-185. [2] Velisek, J.; Cejpek, K. Biosynthesis of Food Constituents: Vitamins. 2. Water-soluble Vitamins: Part 1 - a review Czech J. Food Sci. 2007, 25, 49-64. [3] Koshiishi, I.; Imanari, T. Measurement of Ascorbate and Dehydroascorbate Contents in Biological Fluids. Anal. Chem. 1997, 69, 216-220. [4] Levine, M.; Downing, D. New Concepts in the Biology and Biochemistry of Ascorbic Acid. J. Nutr. Environ. Med. 1992, 3, 361-362. [5] Qiu, S.; Gao, S.; Xie, L.; Chen, H.; Liu, Q.; Lin, Z.; Qiu, B.; Chen, G. An Ultrasensitive Electrochemical Sensor for Ascorbic Acid Based on Click Chemistry. Analyst 2011, 136, 3962-3966. 28 ACS Paragon Plus Environment


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Charge Density Modulated Shape-Dependent Electrocatalytic Activity

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