Green, Seed-Mediated Synthesis of Au Nanowires and Their Efficient

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Green, Seed-Mediated Synthesis of Au Nanowires and its Efficient Electrocatalytic Activity in Oxygen Reduction Reaction Sathiya BalaSubramanian, Anjaiah Sheelam, Kothandaraman Ramanujam, and Dhamodharan Raghavachari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07553 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces

Green, Seed-Mediated Synthesis of Au Nanowires and its Efficient Electrocatalytic Activity in Oxygen Reduction Reaction Sathiya Balasubramanian, Anjaiah Sheelam, Kothandaraman Ramanujam* and Raghavachari Dhamodharan* Department of Chemistry, I.I.T. Madras, Chennai 600 036, India

*Authors for correspondence *Raghavachari Dhamodharan; E-mail: [email protected] *Kothandaraman Ramanujam; E-mail: [email protected]

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Abstract A new, simple, green method for the synthesis of Au nanowires (average diameter 8 nm and several µm in length) using Au seeds prepared from bael gum (BG) is reported. The nanowires are characterized using UV-Visible absorption spectroscopy, PXRD, TEM and HRTEM. It is observed that the rate of the reduction process might be the decisive factor for the shape selectivity, as evident from the formation of nanowires at a particular concentration of seeds and NaOH. The polysaccharide present in BG is the active ingredient for the synthesis of Au nanowires while the small molecules present in BG, when used alone, did not result in nanowire formation. The TEM images of the precursor to the Au nanowires suggested that new, nucleated particles align in a linear manner and fuse with one another resulting in the nanowire. The linear fusion of the newly nucleated particles could be due to the lack of adequate protecting agent and the presence of Au complex adsorbed to the surface. The electrochemical activity of the nanowires for oxygen reduction reaction (ORR) is assessed and compared with that of nanotriangles and spherical nanoparticles of Au. The performance of Au nanowire is better than Au-nanomaterials (heat-treated as well as non heat-treated), Au seeds and clusters. The better efficiency of the nanowires when compared to the other reported catalysts is attributed to the presence of active (100) facets with numerous corners, edges and surface defects.

KEYWORDS: Gold Nanowires, Oxygen Reduction Reaction, Bael Gum, Electrocatalytic Activity, Green Synthesis, Seed-mediated Synthesis.


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Introduction Anisotropic nanoparticles have been intensely investigated due to their size and shape dependent properties.1 Among the nanoparticles of different metals, the place occupied by gold is irreplaceable as it plays an important role in different areas of research such as catalysis, sensors, diodes and diagnostics.2-5 The added advantages of using Au nanoparticles in different areas of research is its chemical inertness and bio-compatible nature. Especially, one dimensional Au nanowires exhibit excellent electrical, thermal conductivity and mechanical properties.6 They are mainly noted for their promising technological applications as an interconnect in nanoelectronics, as a light emitting diode in optoelectronics, field effect transistors, nonvolatile memory elements and as biosensors.7-13 Recently, it has been shown that gold nanowires when incorporated within a three-dimensional alginate scaffold, exhibit enhanced conductivity enabling the cardiac cells that grow on this scaffold to contract synchronously.14

In this

application, the gold nanowires connect the pore walls of the scaffold together and improve the electrical communication between the cardiac cells. This in turn enhanced the conductivity of the scaffold and allowed cells to bind in greater numbers and form thicker and better-aligned tissues.

Gold nanoparticles or nanowires are incorporated within biomaterials may have a

tremendous impact when applied in medicine.15 Such biocomposite materials may be used to engineer a multitude of other types of tissues, including neuronal and bone, or to improve the surface electrical properties and biocompatibility of many materials, such as implants. The most interesting observation is that these materials may also direct stem cell differentiation.16 It has also been shown that (100) faceted Au nanowires or cubes were the most active plane for Oxygen Reduction Reaction (ORR) under basic conditions.17 Nanowires can be synthesized by many methods such as template growth,18 seed mediated,19 photochemical,20 oxidation21 and top-down synthesis.22 However, predicting the mechanism by which nanowires form is still open to debate. Among the reported methods of synthesis, templated growth appears to be the best. However, a drawback of this method is the requirement of harsh conditions to remove the template and shape directing agent, postsynthesis, without changing the surface structure.23 A simple, template-less, green method would throw more light on the mechanism of nanowires formation and thereby enable its extensive use in different areas of application. Though, a simple green method for synthesis of nanowire using 3 ACS Paragon Plus Environment


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pomegranate, a natural product, was reported earlier, the product obtained seems to be a mixture of spheres and needle like structures.24 However this work does not throw light on the mechanism of formation of Au nanowire and especially in terms of the structure of active natural product that is involved.24 Here, we report a simple, green, seed-mediated method for the synthesis of Au nanowires. We have identified the active reagent, which directs the formation of the nanowire and propose a mechanism of formation in terms of nucleation and growth. Subsequently, the catalytic activity of the nanowires for the ORR vis-à-vis spherical and triangular gold nanoparticles is compared. The ORR is an important reaction in biology and in energy conversion processes. It has been demonstrated that the ORR activity of the materials can be greatly improved by designing nanoparticle with certain shapes and size.25, 26 Research on shape dependent catalytic activity may also enable to understand the fundamentals relative to the key questions of different surface facets and their reactivity. It is shown that Au nanowires exhibit superior catalytic activity in ORR when compared to Au nanotriangles and Au nanospheres prepared with BG. Experimental Methods UV-Visible spectra of Au seeds and wires were recorded using JASCO UV-530 spectrophotometer (Japan). FT-IR was recorded using JASCO FT-IR-4100 (Japan). PXRD patterns were obtained using Bruker D8 Advanced Powder X-ray Diffractometer with Cu Kα radiation of λ = 1.5406 Å being the X-ray source. SEM images were obtained using Hitachi S4800 HR-SEM.

TEM images were obtained using JEOL3010 transmission electron

microscope with an acceleration voltage of 200 kV. The current-potential (I-V) curves were obtained using the Autolab Potentiostat/Galvanostat 302 N connected to Pine RDE/RRDE instruments.


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Materials Bael fruits collected from IIT Madras campus were broken into two half. The gummy substance present as a part of the fruit was separated and filtered using cotton cloth to remove the seeds. It was placed in a hot air oven at 70 °C for 24 h. The preparation of dry powder (BG), which is brown in color, was reported earlier by our group.27 Synthesis of seeds using BG Aqueous HAuCl4 solution (0.2 weight percent) was prepared and used as stock solution for Au nanoparticles synthesis. In a vial, 20 mg of BG was taken and dissolved in 4.5 mL of water. Then, 0.5 mL of 0.2 w % HAuCl4 solution was added. The mixture was heated at 90 °C for 1 h. The solution was then cooled to room temperature. Synthesis of Au nanowire 0.1 mL of the above solution (seed) was added to 10 mL of HAuCl4 solution (prepared by diluting 1 mL of the stock solution to 10 mL). Then, 5 µL of 1 M aqueous NaOH was added. This was placed in an oil bath maintained at 90 °C for 1 h. During the reaction, the colorless solution turned deep violet, suggesting the formation of nanoparticles.28 Separation and purification of polysaccharide from BG BG was dissolved in 2 % v/v acetic acid and heated to 80 °C for 1 h. Then, it was precipitated using excess acetone. The precipitate was filtered and subjected to repeated dissolution and precipitation. This procedure was repeated atleast five times and pure polysaccharide was obtained.29 Synthesis of spherical Au nanoparticle using BG A mixture of 1:12.5, by weight of HAuCl4 : BG was taken and dissolved in 10 mL of distilled water. This solution was placed in an oil bath (maintained at 90 °C) for 24 h. It was then allowed to cool to room temperature and used as such.



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Synthesis of nanotriangles of Au using BG In a vial, 12.5 mg of BG was dissolved in 9 mL of water. Then, 1 mL of 0.2 w % HAuCl4 was added slowly, so that, the weight ratio of HAuCl4: BG is 1:6.25. This solution was left at RT for 2 days, upon which it turned purple and eventually a precipitate was observed at the bottom of the vial. The dispersion was observed to contain ≈1 µm sized gold nanotriangles as reported earlier.27 Electrocatalytic activity studies To study the electrochemical activity of Au nanoparticles of different shapes (nanotriangles, wires and spherical particles), 25 µL of each solution (which contains approximately 50 µg of Au catalyst) were deposited on glassy carbon electrode and allowed to dry under argon flow. 2 mL of 5 w % Nafion® solution was diluted with 5 mL of isopropyl alcohol. Then, 3 µL of this diluted Nafion solution was drop-cast above the catalyst layer. The electrochemical measurements were carried out in 0.1 M KOH solution saturated with high purity N2 and O2 in three electrode glass cell with reversible Hg/HgO, OH- as reference and Pt as the counter electrode. Results and Discussion Aegle marmelos, known as bael, is a tree that grows in tropical countries. Since ancient times, it is recognized well for its medicinal value. Especially, the fruit extract is used to treat scurvy, ulcer, diabetes and address high cholesterol related health issues. According to literature, the gum isolated from bael fruit (BG) consists of branched polysaccharide and a number of small organic molecules. The structure of the polysachcharide of BG comprises of monomers of galactose, arabinose, rhamnose and galactouronic acid.29 Some of the small molecules present in BG are imperatorin, imperatorin oxide, cymene, limonene, citronellal, octadiene, 2-octenol, acetoin, phellandrene, isoamyl acetate, linalool oxide, psoralen, marmesin, marmelosin, etc.30, 31 Towards the synthesis of Au nanowires, at first, the Au seed particles were synthesized using BG powder and characterized using UV-Visible spectroscopy, PXRD and TEM. The presence of a single surface plasmon resonance (SPR) with absorption maximum (λmax) at 531 nm (Supporting Information Figure S1a) suggested that the seed particles could be spherical in 6 ACS Paragon Plus Environment

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nature. The PXRD pattern of the seeds (Supporting Information Figure S1b) showed sharp reflections at 2θ values of 38, 44, 64, 77 and 82 degrees corresponding to (111), (200), (220), (311) and (420) planes of face centered cubic unit cell of gold. The ratio between the intensities of the reflections due to planes (111) and (200) was ≈ 2.3. This value is same as that observed for bulk gold suggesting that there is no specific orientation of the seeds with respect to the substrate.32-34 The broad peak observed at 2θ ~ 25 is from the microscope glass slide used as a substrate for recording the powder X-ray diffraction. The TEM image of the seeds are shown in Figures 1a and 1b and this also confirms its pseudo spherical nature. From the particle size distribution graph (Figure 1c), the mean size of the seeds (particles) is determined to be 10 ± 5 nm.

Figure 1. a) and b) TEM images of Au nanoseeds at different magnifications; c) Particle size distribution graph for the nanoseeds. The gold nanowires were synthesized from the Au seed particles described in the previous paragraph. To the seed solution (purple), 1 mL of 0.2 w % aqueous solution of HAuCl4 (light yellow) was added followed by the addition of aqueous NaOH, upon which the solution turned colorless. During the reduction process, the colorless solution gradually turned into deep violet and this indicated the formation of nanowires.28 The UV-Visible spectrum and PXRD pattern of Au nanowires thus formed are shown in Supporting Information Figure S2. The UVvisible spectrum displayed the presence of a broad SPR absorption that begins in the visible region and extends to the near infrared (NIR) region.

This could be either due to the

superposition of longitudinal resonance of Au nanowires of different aspect ratios or to the 7 ACS Paragon Plus Environment


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scattering of light that could arise from micrometer sized nanowires or both. The Au nanowires were not preferentially oriented towards any particular plane, with reference to the substrate, as there was no change in the intensity ratio of (111) and (200) plane from that of the bulk gold.32-34 The TEM images of Au nanowires are shown in Figure 2 from which the average diameter of the wire was determined to be 8 nm while the average length was several µm. The nanowires appear wavy with several kinks and straight line regions. The TEM image of a single nanowire (Figure 3a) and the selected area electron diffraction (SAED) pattern of the marked portion in Figure 3a is shown in Figure 3b. The HR-TEM image of the nanowire is shown in Figure 3c and indicates the presence of twinning, defects and planes such as (111) and (100). The defined and diffused spots of the FFT (Fast Fourier Transform) pattern correspond to single and poly crystalline region of the nanowire, which is also included as insets to Figure 3c. The SAED pattern confirmed the polycrystalline nature of the nanowires.

Effect of concentration of NaOH and seeds The formation of Au nanowires was noticed only at a particular concentration of the reducing agent and 5 µL of 1 M NaOH. By changing the concentration from this value either low yield of Au nanowire or formation of irregular wires or Au particles of other shapes such as dumb bell or spherical was observed. For example, under the same reaction conditions, when the concentration of NaOH was increased from 5 to 20 µL, the shape of the nanostructure changed into dumb bell and spherical particles as shown by the representative TEM images in Figure 4.


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Figure 2. a) and b) TEM images of Au nanowires at different magnifications.

a)

c)

b)

Figure 3. a) TEM image of Au nanowire; b) selected area electron diffraction pattern; c) HRTEM image along with the FFT of single and poly crystalline pattern of Au nanowire.

Figure 4. TEM images of a) Au nanowires (5µL NaOH); b) dumbbell and spherical particles (20 µL NaOH).



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The formation of irregular nanowires was observed when the weight ratio of HAuCl4: BG was altered from 1:20 (used in the synthesis of Au seed) to 1:5. The formation of spherical nanoparticles was observed when the weight ratio of HAuCl4: BG was increased from 1:20 to 1:40, even in the absence of NaOH. The TEM images of irregular nanowires and spherical nanoparticles are shown in Supporting Information Figure S3. These control experiments reveal that rate of reaction might be playing the decisive role in the formation of different shapes of the nanoparticles. Identification of active reducing and shape directing agent As mentioned earlier, BG is a mixture of polysaccharide and small organic molecules. To investigate the role of the polymer and the small molecules, they were separated as mentioned in the experimental section. The purified polysaccharide was employed as reducing agent for the synthesis of Au seeds, which in turn was utilized for the preparation of Au nanowires. The formation of Au nanowires as the end product when the purified polysaccharide was used, indirectly implied that the polysaccharide present in BG could be the active agent in the reaction mixture. This was further validated by employing gum acacia, another polysaccharide, with structure similar to that of BG. These experiments clearly showed that polymer is the active ingredient in the synthesis of nanowire. The TEM images of Au nanowires synthesized using purified BG and gum acacia is shown in Supporting Information Figure S4. To eliminate the role of small molecules, imperatorin oxide, a molecule known to be present in significant quantity in BG was isolated (details not given) and used for the synthesis of Au nanowires. The negative results, obtained under different experimental conditions, clearly demonstrated the importance of polymer in the formation of nanowires. The TEM images of Au seeds and the resultant particles obtained under different experimental conditions using imperatorin oxide as reducing agent are presented in Supporting Information Figure S5. Possible mechanism of gold nanowire formation The mechanism of nanowire growth is discussed in some detail here. Upon the addition of 5 µL of 1 M NaOH to chloroauric acid solution, the colour changed from yellow to colourless. This could be either due to the reduction of Au from +3 to +1 state or due to the formation of AuCl3-x(OH)x- or both. The formation of well dispersed fine Au colloids could be facilitated by 10 ACS Paragon Plus Environment

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the lower reduction potential of AuCl3-x(OH)x- as reported earlier.35 The growth of the nanowires was observed by recording the TEM of the reaction mixture at different time intervals (Figure 5). After 10 minutes of the reaction, new nucleation centres were observed (Figure 5a) and this might be due to the reduction of HAuCl4 or AuCl3-x(OH)x-. Since no new reducing agent was added and under the limited quantity of the reducing agent (BG) present already from the synthesis of seeds, the surface of Au seeds has to be partially covered by HAuCl4.36 In this case, it could be either HAuCl4 or AuCl3-x(OH)x- adsorbed on Au seed surface. The FT-IR spectrum of Au nanowires (Figure 6) showed the presence of the characteristic peaks of the polysaccharide of bael gum (at 3388, 2923, ~ 1630, ~ 1400 cm-1; the FTIR spectrum of the polysachcharide of bael gum not presented here) and confirmed its presence on the gold nanowires. In addition, the FTIR also showed the presence of characteristic peaks at 3455 cm-1 corresponding to O-H stretching and at 676 cm-1 specific to O-Au-O stretching37 suggesting that Au(OH)2 based complex could be present on the surface of Au nanowires. The –OH stretching frequency in bael gum is observed at 3388 cm-1 while it is observed at 3435 cm-1 in the case of the nanowire. The O-H stretching frequency is observed at 3405 cm-1 for liquid water at 25 °C and at 3387 cm-1 for water adsorbed to gold38, which takes place only at low temperarure (100 to 140 K). Hence this rules out the presence of free water in the liquid state on the surface of the nanowires. Thus the presence of Au-OH groups on the surface of the nanowires appears to be reasonably supported by data.

Figure 5. TEM images of Au nanowire intermediates taken at different time intervals; a) 10 mins; b) 20 mins; c) 30 mins and d) 60 mins.



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Figure 6. FT-IR spectrum of Au nanowires. Earlier, it was shown that HAuCl4 or AuCl3-x(OH)x- adsorbed on Au seeds were in uncharged state and this uncharged species might attract one another.35 Subsequently and during the coalescence process, the addition could proceed in a linear fashion or in a random manner. Earlier studies also suggested that steric effect might be preventing the nucleated particle to add in a random manner.39 A closer look at the TEM images, presented as Figure 5, suggested that the Au seeds could be acting as the starting point for the nanowires. Thus, in course of time, freshly added HAuCl4 is reduced and forms new nucleation centres around the seeds, which grow further to a size of around 6-8 nm. The rate and amount of capping agent present in the solution could be decisive factors in the size of the newly formed particles. This process is also assisted by the addition of NaOH and also by carrying out the reaction under boiling conditions. The faster reduction allows the formation of smaller size particles and the lack of adequate quantity of capping agent permits its fusion with the adjacent particle/cluster. It has been shown that when the distance between the two particles is less than 10 nm and if the particles are not protected by enough reducing agent, the van der Waals attractive forces allowed the particles to fuse with one another.35 The preferential adsorption of AuCl4- or AuCl4-x(OH)x- could also increase the van der Waals force of attraction between the particles. Hence, the newly formed nanoparticles arrange themselves in a linear manner and fuse with adjacent particle (Figures 5b and 5c). Then, with time, the fused particles undergo further collision with one another and this leads to the formation of nanowires (Figure 5d). 12 ACS Paragon Plus Environment

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Oxygen reduction reaction (ORR) The electrochemistry of oxygen is one of the important electrocatalysis reactions since it is one of the half reactions in the electrolysis of water and energy conversion devices such as fuel cells and metal-air batteries.40 Even though the Au in bulk is inactive towards ORR, it is known to oxidize CO to CO2 and reduce the oxygen to water when its size is at the nanometer scale. The kinetics of O2 reduction on (100) single crystal Au electrodes in alkaline medium was examined and also compared with the (111) and (110) faceted electrodes.41,42 Even though modest activity was observed initially, later it was found that the activity can be improved by tuning the size and shape of Au nanomaterials.43-45 It has been shown that smaller size and especially less than or equal to 3 nm sized spherical nanoparticles exhibit better catalytic activity towards ORR.46-48 In contrast, it was also reported that decreasing the thickness of the Au layer in alkaline medium resulted in a slight reduction in activity.49

In comparison to spherical

particles, certain shapes with particular facets seem to be performing well. For example, the activity of (111) faceted Au triangles and nanoperiwinkles were higher than spheres.44 Recently, the ORR activity of Au cubes, octahedron and spherical particles were compared. The specific activity was observed to be highest for (100) faceted cubes.50 It is well established that the ORR activity is structure sensitive with the order of activity by facets being (100) > (110) > (111).41, 42, 51, 52

With the existing literature knowledge, as described, the ORR catalytic activity of Au

nanowire was compared with two dimensional single crystalline nanotriangles and pseudo polycrystalline spherical particles. The TEM image of nanowires and spherical particles as well as the SEM image of nanotriangles are shown in Figure 7.

Figure 7. a) TEM image of Au nanowires; b) SEM image of Au nanotriangles and c) TEM image of spherical Au particles. 13 ACS Paragon Plus Environment


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The rotating disc electrode (RDE) polarization curves for the different Au particles (wire, triangle and spheres) at 1600 rpm, in oxygen saturated 0.1 N KOH is presented in Figure 8. The nanowires show on-set potential at 0.96 V vs. RHE, which is approximately 250 mV higher than that observed for the Au nanotriangles and Au nanospheres. The limiting current observed for all the catalysts are lower than that of the theoretical limiting current (Levich current density ~ 6 mA cm-2 expected from the O2 saturated 0.1 N KOH solution).53 The order of the limiting current density (iL) is 3.5 mA cm-2 (nanowires) > 2.4 mA cm-2 (spheres) > 1.3 mA cm-2 (triangles) indicating poor accessibility of the electrode area for ORR in the case of triangles and spheres. The reason for the lower ORR activity observed in the case of triangles could be due to the predominant presence of the least active (111) plane on the particles surface coupled with stacking of the plates. To probe this further, the ORR reaction was performed using lower concentration of triangles. A slight increase in activity was observed, which suggests that stacking fault is not the major cause that limits the activity. The capping of the BG on the spherical particles could create the barrier for the diffusion of the oxygen to the catalyst surface and thereby limiting the ORR current. In the case of the nanowires, the presence of (100) facets with numerous corners, edges and surface defects might be providing the active sites for ORR. In addition, the presence of Au-OH bonds on the surface might facilitate the stronger interaction with O2 resulting in best activity, as reported earlier.54

Figure 8. The current density versus applied potential for ORR at the RDE for the different Au particles (wire, triangle and spheres), at 1600 rpm, in oxygen saturated 0.1 N KOH. 14 ACS Paragon Plus Environment

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The ORR was also recorded at different rpms (Figures not shown) in which increase in the limiting current with increasing rotation rate of the electrode was observed indicating that it is a diffusion limited process. The number electrons estimated by K-L analysis for the gold nanowires, spheres and triangles are 3.6, 2.8 and 2.7, respectively (K-L analysis was performed at 0.47, 0.5, 0.52 and 0.55 V vs. RHE; data not shown). To understand the oxygen reduction pathways, rotating ring-disk electrode (RRDE) analysis was also carried out. The polarization curves obtained at the disk and ring for Au wires (a, a’), triangles (b, b’) and spheres (c, c’) are shown in Figure 9. The number of electrons (n) and % hydrogen peroxide ( ) generation during ORR was estimated using the equation 1 and 2. =

(1)

⁄. × ⁄.

=

(2)

⁄.

Where, Iring, Idisk and C.E stand for the ring current, disk current and the collection efficiency respectively. The C.E value (0.19) of the catalyst was assessed using the reported procedure.55



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Figure 9. (a), (b) and (c) Ring and disk polarization curves; number of electrons involved and percentage hydrogen peroxide production during the ORR on Au (a’) nanowires, nanotriangles (b’) and spherical nanoparticles (c’), respectively. The “n” values for oxygen reduction are observed to be 3.7, 2.7 and 2.5 for nanowire, triangle and sphere, respectively. If the O2 undergoes complete reduction into OH-, n would then be 4 and would be 0 %. If this number is under 4, the possibility of obtaining intermediates of the ORR (superoxide and peroxyl) is higher. The high “n” value obtained with nanowire


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indicates its good 4-electron selectivity in the alkaline aqueous media via either a direct fourelectron route or a “two plus two” route. The maximum peroxide production (around 85 %) was observed for the Au spheres, which is due to low “n” value observed. The exchange current density and Tafel slopes were taken as kinetic descriptors for the ORR (Figure 10). The exchange current density (io) observed for the nanowires, spheres and triangles (shown in Figure 10 a) are 1.0 × 10-4, 3.0 × 10-8 and 8.0 × 10-7 mA cm-2; the Tafel

slopes are 83, 86 and 78 mV dec-1 (Figure 10b), respectively. The Tafel slope (∝, contains the information about both “n” (number of electrons involved in the reduction) and the transfer coefficient (∝) while R, T and F are universal gas constant, absolute temperature and Faraday constant, respectively. Among the three catalysts studied, the highest exchange current density value of 1.0 × 10-4 mA cm-2 is observed for the nanowires. The nanowire also shows the highest ORR performance onset potential of 0.96 V vs. RHE and “n” value between 3.9-3.5 in the potential window of 0.96-0.6 V vs. RHE. For all the three materials, the observed Tafel slope was ~ 80 mV dec-1.

Assuming an ∝ value of 0.5, the number electrons involved in rate

determining step is ~ 1.5.

Figure 10. a) The exchange current density (where the respective lines intersect the Y-axis) of the Au nanowires, triangles and spherical nanoparticles; b) mass transfer corrected RDE plots and its Tafel slopes.



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Electrochemical active surface area calculation Cyclic voltammetry was conducted in the Cu-under potential deposition (Cu-UPD) range for Au nanowires, triangles and spherical particles in a solution containing 1 mM Cu2+, 50 mM H2SO4 and 1 mM of Na2SO4 (Figure 11). In the case of the Au nanowires, the Cu-UPD is observed at around 0.32 and 0.26 V vs. Ag/AgCl. Similarly, for Au triangles, it is observed at 0.22 V vs. Ag/AgCl with a shoulder around 0.27 V vs. Ag/AgCl. In contrast, Au spheres showed very broad and weak Cu-UPD signal at 0.48 V vs. Ag/AgCl. Cu-UPD charge of 1.31, 0.3 and 0.16 μC cm( were observed for the nanowires, nanotriangles and nanospheres, respectively.

The corresponding electrochemically active surface area are 52.33 cm g (, ,

11.2 cm g (, and 5.5 cm g (, assuming a Cu-UPD charge value of 445 μC cm( calculated for the UPD of a (1×1) Cu Monolayer (ML ) on Au (111) single crystal.56-58 Hence, exchange current density and the surface area of the catalyst act as deciding factors in the ORR activity. The average number of electrons involved in the ORR for the nanowires, from the RRDE, is 3.7 and is less than 2 % at 0.8 V and less than 23 % in the entire ORR region.In the case of nanotriangles and nanospheres was about 62 % and 84 %, indicating largely 2 electron pathway for ORR. The onset potential, n and of the nanocatalysts are compared against other Au-nanomaterials reported in the literature. The Au nanowire exhibits complete reduction of oxygen by 4e- process and high onset potential for ORR compared to several Au nanomaterials known in the literature (Table 1).44,46,56,59,60,61 The mass specific and area specific ORR activity of Au nanowires, nanotriangles and nanospheres are shown in Table 2. As can be seen, the mass activity of Au nanowires is an order of magnitude lower than that of Pt/C, which is due to low electrochemical surface area exhibited by Au nanowires (52 cm2/g) vs. Pt/C (30 – 100 m2 g-1).62-65


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Figure 11. Cu-UPD on different Au nanomaterials: a) wire; b) triangle and (c) sphere.



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Table 1. Comparison of electrochemical reduction of oxygen using different electrocatalysts.

χ

Onset

No. of

Shape/ Size

potential

electrons

(nm)

(V vs.

(@ 0.8 V vs.

(% @ 0.8

RHE)

RHE)

V vs.

H2O2

Material

Reference

RHE) Au-wire

Present wire/ 8

0.96

Au-triangle Triangle

3.92

2

work

2.75 (at 0.7 V)

62

Present

0.77

Au-sphere

work 2.79 (at 0.65

Present

Particle

0.71

V)

84

work

Cluster/ 3.5

0.95

3.92

5

(56)

nanoparticles

Cube/ 40

0.92

3.9

3

(44)

Gold clusters

Gold cluster/ 3

0.87

2

--

AuPC-1 Cubic Au

(46)

BNNS/Au

7

0.9

4

--

Nanosheets

0.6

2

98 (60)

Au-BNNS/Au

nanoparticles

0.65

2.5

80

Cluster/ 3-4

0.9

3.88

8

Gold nanoclusters coated on carbon nanosheets


(61)

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Table 2. Mass specific activity and area specific activity of Au nanowires, triangles and spherical nanoparticles. Gold nanomaterial

Mass activity (A/mg) @0.90

Au wire

Specific activity (mA/cm-2 )

@0.85 V vs. RHE

@0.90

@0.85 V vs. RHE

0.0086

0.0397

165

760

Au triangle

N/A

N/A

N/A

N/A

Au sphere

N/A

N/A

N/A

N/A

Stability study A preliminary assessment of the stability of the catalysts was carried out by a simple chronoamperometry response measured at 0.8 V vs. RHE for all the Au nanomaterials (Figure 12). It can be seen from this measurement that in the case of nanowires, nearly 75 % of the activity is retained even after 11 h of ORR reaction. Strictly speaking this should have been investigated by measuring the LSV polarization curves before and after ADT (10000 cycles or more), as reported in literature.

This experiment and electrocatalytic activity of the new

materials on carbon support shall be investigated at a future date.

Figure 12. Chronoamperometry response of the Au nanomaterials in 0.1 N KOH solution at 25 °C. 21 ACS Paragon Plus Environment


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Conclusions A new and green method for the synthesis of Au nanowires using bael gum (BG) polysaccharide is presented. The mechanistic study suggests that the rate of reaction and the absence of enough capping agent could be the prime factors for the formation of nanowires. Detailed experiments showed that polysaccharide present in BG is the active ingredient for the formation of Au nanowires. The presence of kinks, steps and the polycrystalline nature of the nanowires were exploited in electrocatalytic reduction of oxygen. As expected, polycrystalline Au nanowire displayed the highest onset potential and limiting current density when compared with nanotriangles and spherical nanoparticles of synthesized using BG. Au nanowires were observed to show better selectivity and high onset potential for ORR compared to the known materials such as heat-treated and non-heat-treated Au nanomaterials as well as clusters.

Supporting Information UV-Visible spectrum of Au seed solution and PXRD pattern of Au seeds on microscopic glass slide; SPR of Au nanowires and its PXRD pattern; TEM images of irregular Au nanowires (HAuCl4: BG 1:5 wt ratio) and pseudo spherical nanoparticles (1:40 wt ratio); TEM images of Au nanowire synthesized using purified polysaccharides obtained from BG and gum acacia; TEM images of Au seeds prepared using imperatorin oxide isolated from bael gum and Au particles prepared using different concentrations of the seed.

Author Information Corresponding Authors *Authors for correspondence *Raghavachari Dhamodharan; E-mail: [email protected] *Kothandaraman Ramanujam; E-mail: [email protected]


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Notes The authors declare no competing financial interest.

Acknowledgment SB thanks UGC for support in the form of a fellowship. The authors thank the Department of Materials and Metallurgical Engineering and especially Prof. S. Sankaran of , IIT Madras for the TEM facility. Th financial support of Indian IIT Madras is gratefully acknowledged.

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TOC:


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Green, Seed-Mediated Synthesis of Au Nanowires and Their Efficient

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