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Study of Shape Dependent Electrocatalytic Activity of Gold nanoparticles by Optically Targeted ElectroChemical Cell Microscopy (OTECCM): A comparison of different shapes towards hydrazine oxidation.
Structure-functionality relationship and stability (structural changes) over time of nanoparticles will be the determining routes for choosing any electrocatalyst. Optically targeted Electrochemical Cell Microscopy (OTECCM) has been applied to study the electrocatalytic performance of Au nanoparticles with different shapes towards hydrazine oxidation. Variations of electrocatalytic response had been observed on these shapes which force to think about structural changes over time. The result found from these experiments showed the capability of OTECCM to measure inseparable structure-function-stability relationship of any kind of nano materials as well as any reactions which makes it as emerging/prominent technique in the nearest future.
Metal nanoparticles have become the focus of extensive research because of their unique physiochemical properties such as optical, 1-4 magnetic, 5,6 and electronic 7,8 compared to those of the bulk material. These unique properties make them a suitable candidate to a variety of technical applications, including in sensing, 10,11,17-19 spectroscopy, 11-13 and in biomedical. 16, 17 In recent years metal nanoparticles have been also extensively applied to electrocatalytic process such as ORR and HER due to their high catalytic activity and uniquely high surface-to-volume ratio. 9
The performance of nanoparticles towards different kinds of electrocatalytic reaction largely depends on their size, shape, composition and/or crystalline structure1,2 and sometimes on state like aggregation. 3 For particles with similar shape, the size determined the mass current densities: smaller particles size led to greater catalytic current densities per unit mass because of the greater surface-to-volume ratio. Along with size, the shape of nanoparticles is an important parameter as most of the reactions in electrochemistry are surface sensitive. Shape helps us to identify which facets are exposed at the surface of nanoparticles. C.M. Sánchez-Sánchez et al. proved that Au NC’s are more active than spheres and rods towards ORR in basic media and Blake et al.18 proved that gold nanosphere showed higher catalytic activity to wards hydrazine oxidation in acidic media. However, there are other factors we need to consider before assessing the applicability of electrocatalyst.
To understand the effects of these two factors (size and shape), one has to visualize and probe electrochemical activity at the level of individual NPs. Studying the chemical behavior of individual nanoparticles would provide deep insights into how each of these factors individually affects performance and serve as a guide to synthesize novel materials with enhanced catalytic performance. Most of the classical methods can measure the total electrocatalytic reaction current of a large number of nanoparticles or ensemble current but these approaches are time consuming and can measure only the average catalytic activity. Moreover, these techniques are not able to explain the useful information between structures and function relationship.
To study the electrochemical behavior of individual nanoparticles several methodologies were developed. These includes optical techniques, such as surface plasmon resonance imaging to record single-particles electrochemistry, single molecule fluorescence imaging and electrochemical measurements at a metal NP attached to a small electrode,single-particle collision on an UMEand confined nanoelectrolyte or probe based techniques such as Scanning ElectroChemical Microscopy (SECM) and Scanning ElectroChemical Cell Microscopy (SECCM). Despite of being very effective techniques, sometimes these techniques have faced diverse challenges such as locating and isolating individual nanoparticles, probing them at sufficiently high temporal and spatial resolution, and most importantly unable to provide correlated structural information. Sometimes these limitations can be minimized using hybrid method. For example, SECM alone cannot give topographical information, but combined with other technique such as AFM or SICM can overcome this limitation. Fluorescence and Raman spectroscopy can give information of isolated and immobilized molecules but they do not provide information on size or shape. Also, above methods are suffering from high background current and make it difficult to differentiate this background current from individual nanoparticles.
In our previous work, we have demonstrated the capability of OTECCM to study the electrocatalytic activity of Au and Cu nanoparticles with higher lateral resolution and lower time scale which encourage us to study further with same technique. So, this present investigation aims to understand the Tri-relationship among the shapes/structure, functionality and stability of gold nanoparticles. Hydrazine oxidation was used as a model system.
2. Experimental Methods:
2.1 Synthesis of Nano sphere:
Au NS’s were synthesized using the method reported by Liz-Marzan et al. The initial seed solution was prepared by adding 50µL of 0.05M HAuCl4 with 5 mL of 0.1 M CTAC solution. After that 200 µL of freshly prepared 0.02M NaBH4 solution was added with continuous stirring for 3 minutes. The solution was then diluted 10 times with 100 mM CTAC. 40 µL of 0.1 M ascorbic acid and 900 µL of the seed solution were then mixed with 0.05 M HAuCl4 under vigorous stirring. The solution was kept at room temperature without any disturbance. This produced around 10 nm gold nanospheres. In a 50mL volumetric flask, 25 µL of previously prepared gold nanospheres and 40 µL of 0.1 M ascorbic acid were mixed with 25 mM CTAC solution. 10 µL of the seed (10nm sphere) solution and 50 µL of 0.05M HAuCl4 solution were added respectively. 10 µL of NaOCl and 10 µL of 0.05 M HAuCl4 were added to 10 mL of growth solution with vigorous stirring to etch the particles. The solution was left undisturbed for few minutes at 300C. Solution was monitored to see the etching effects using UV-Vis spectroscopy.
2.2 Synthesis of Nanotriangle:
Au nanoparticles with triangular prism geometry were prepared using the method of Liz-Marzan. For seed solution, 4.7 mL of 0.1M CTAC solution was mixed with 25µL of 0.05 M HAuCl4 solution followed by 300 µL of freshly prepared 0.01 M NaBH4 under vigorous stirring. The solution was kept for around 2h at room temperature to consume the excess borohydrade. Two growth solutions were then prepared. In growth solution (1), 8 mL DI water, 40µL of 0.05 M HAuCl4 and 0.01 M NaI solution was added with 1.6mL of 0.1 M CTAC. In growth solution (2), 500µL 0.05M HAuCl4 and 40 mL of 0.05 M CTAC was mixed with 300 µL of 0.01M NaI solution. Then 40 and 400 µL of 0.1 M ascorbic acid was added to both growth solutions respectively. 100 µL of diluted (10x in 0.1 M CTAC) seed solution was added to growth solution (1) and immediately 3.2 mL of growth solution (1) was added with growth solution (2). After stirring for few seconds manually, this growth solution (2) was kept undisturbed at room temperature for at least 1 hr. This AuNT was then purified by adding 0.175 M CTAC solution and kept overnight. The supernatant was removed and precipitant was then dispersed in 5mL of 0.1 M CTAC for future use.
2.3 Synthesis of Nanocube:
From which author?
First, to prepare the seed solution, in 50 mL round-bottom flask 9.75 mL 100mM CTAB and 250uL of 10mM HAuCl4 was mixed with freshly prepared 600 uL of 10mM NaH4. The solution was stirred for 3 minutes and kept for 3 hrs. in 270 C. 50uL of this seed solution was then added with 2 mL of 200mM CTAC and 1.5 mL of 100 mM AA. 2mL of 0.5 mM HAuCl4 was then injected in one shot. The solution was stirred for 15 minutes at 300rpm. After centrifugation twice, this prepared solution was dispersed first in 1 mL DI water and then in 1 mL of 20mM CTAC solution for future use. 6uL of these seed solution was then added with 6 mL of 100 mM CTAC,200 uL of NaBr, 390 uL of 10mM AA and 6 mL of 0.05mM HAuCl4. The solution was then stirred at 500rpm for 25 minutes. After centrifugation, this solution is dispersed in DIW twice in DI water.
2.4 Characterization of Au nanoparticles:
The synthesized nanoparticles were characterized via UV-absorption spectroscopy (name and model of UV-spectroscopy) and Transmission electron spectroscopy (FEI Tecnai G2 F20 TEM).
2.5 Sample preparation: with UV-ozone cleaning:
Samples of synthesized nanoparticles of each shape were prepared by drop-coating (5 µL) onto clear indium tin oxide (ITO) coated glass slide. The samples were then allowed to dry for 5 mins, rinsing with copious amount of water followed by 30 mins of UV-ozone cleaning to remove organic residue.
2.6 Electrochemical Measurement:
All electrochemical measurements (voltammetric SECCM) were performed/ carried out in a two-electrode configuration using a home built instrument. In all electrochemical measurements ITO dropcoated with nanoparticles serves as working electrode and Ag/AgCl as Quasi-reference counter electrode. 2 mM N2H4, 25 mM citric acid and 25 mM trisodium citrate was used as electrolyte. In this configuration, tip is mounted on a stage of a three piezoelectric positioners and it can be moved in three dimensions while the sample is held stationary. Using this piezo system this tip is lowered over the sample corresponding to individual nanoparticles which are detected optically. When the liquid meniscus (that forms at the end of the tip) makes a contact with the sample a sudden change in current will be observed. At this point, the probe will stop moving and localized electrochemical measurement will be carried out within the potential window (-0.5 V to +1.5 V) and with a sweep rate of 1000 mV/s. Upon completion of this measurement of individual measurement the probe will be retracted and move to a new particle.
3. Results and discussion:
Figure 1 (top panel) shows a representative TEM image as well as the particle size histogram of the gold nanospheres (Au NS’s) synthesized in this work. These Au NS’s shows a spherical shape with an average diameter of 61 nm. These Au NS’s can be considered as a poly-oriented catalyst material. The UV-Vis spectrum has only on SPR peak since no predominant facet is expected for a spherical NP’s. The gold nanotrinagles (Au NT’s) synthesized have an equilateral geometry and an average edge length of 81nm (middle panel). The absorption spectrum shows two bands: the longer wavelength SPR for the NT centered at around 653nm corresponding to triangular shape nanoparticles and another at shorter wavelength SPR centered at 540 nm corresponding to other non-triangular nanoparticles in the solution. The TEM and particle size histogram (bottom panel) of the synthesized gold Nanocube (Au NC’s) NP’s indicates that the particle size is 75nm and presence of preferential facets with (100) orientation.
Figure 2 represents the cyclic voltammogram of two individual nanoparticles of each shape. As our previous report on nanorods, two characteristics features is also present i.e. anodic peak between 0.7 V to +1.2 V and another peak in the cathodic sweep at around +0.7 V. Both arise due to hydrazine oxidation. Along with these two characteristics features we also observed the variation of current profile over time and it changes from particle to particle as well as shape to shape. In the case of nanosphere, in second cycle we saw an increase in current compared to first cycle which might attribute due to stripping of ligand layer attached to a sphere. And in following cycle it loses some of its activity due to losing some of its active sites. These features are also common in Nanotriangle and Nanocube but in both cases rounding/edge effects might play a dominant role. That’s why we see continuous decrease in current profile.( facet or indices effects???).
Figure 3: Variation of current profile and E1/2 over cycles.
Au NS’s (left panel), Au NT’s (middle panel) and Au NC’s (right panel).
From figure 3 and figure 4 which represent the response of current and the position of E1/2 confirms the explanation made above. In case of nanosphere, we see a constant current after each cycle (in second cycle little bit higher) but in case of Nanotriangle and Nanocube there is decrease in current profile. The E1/2 was at around 0.6 V after first cycle for every shape but shifted in negative direction by ~0.1 V in nanosphere but in other two shapes we saw more shifting in negative direction (that might represent high catalytic efficiency). Both Nanotriangle and Nanocube showed greater activity at first cycle but lose their activity in successive cycling. Like nanospheres, nano triangle shows the constant E1/2 and current in following two cycles. That means after three cycles Nanotriangle lose its geometry/shape and become sphere (rounding effect). Nanocube, the current keeps decreasing in following cycle and may attribute due to rounding effect. Further investigation should be carried on for the confirmation.
The current observed with these three shapes after each cycle of cyclic voltammetry was compared and Nanosphere showed stable current in three cycles. Though Nanocube showed higher current at its first cycle but lost around 68.31% and 80.72% after second and third cycle respectively. Whereas, Nanotriangle lost around 61.10% and 67.47% and Nanosphere lost around 10.07% after third cycle but increase about 7.26% after first cycle. This data will help us to choose electrocatalyst based on their performance and our requirements i.e. stability over time period.
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