Current Controlled Growth Mechanism of Surface Roughed Pt Nanowires and Their Electrochemical Property

Anodic aluminum oxide (AAO) membrane was purchased from Whatman (Anodisc 25, 0.2 μm). Pt electroplating solution (Platinum TP RTU, Techni Inc.) was directly used as purchased. Hydrogen peroxide was purchased from Acros Organics. Phosphate buffer solution (PBS, pH = 7.2) was prepared using sodium phosphate (Acros Organics). All other reagents were of analytical grade. DI water was prepared from a Barnstead Nanopure Water Purification System.

Electrodeposition and cyclic voltammetry (CV) were carried out on a VersaSTAT3 electrochemical system (Princeton Applied Research). The morphology and elements of the Pt nanowires were characterized by a JEOL JSM-7401F field emission scanning electron microscope (FE-SEM) equipped with an EDAX detector. X-ray diffraction (XRD, Scintag PAD X with Cu Kα radiation) with a scan rate of 2 degree per minute was used to characterize the crystal structure and grain size of the Pt nanowires. The nanowire surface roughness was quantified by atomic force microscope (AFM, Veeco Multimode) in a non-contact mode.

The Pt nanowires were fabricated by electrodeposition method using AAO membrane. The current density was controlled to prepare different surface roughness. For the nanowire growth, various current densities were utilized. The only variable in DC electrodeposition is current density (or applied voltage). For PC electrodeposition, there are three important parameters affecting the structure properties, i.e., the time period that the pulses are imposed (ON-time, T_ON) during which potential/current is applied, the relaxation time (OFF-time, T_OFF) during which zero current is applied, and peak current density (I_P).¹² PC will deposit metal at the same rate as DC provided that the average pulse current density equals the latter. The detailed nanowire preparation procedure can be found in our previous publications.^13–15

A glassy carbon electrode (GCE) (Bioanalytical Systems, Inc.) was polished with alumina particles (0.5 μm, Bioanalytical Systems, Inc.), then thoroughly rinsed with DI water, and ultrasonicated in DI water and ethanol for 2 min. After the electrode was dried in the air, 10 μL Pt nanowire suspension in ethanol was deposited onto the GCE surface using a pipette and dried in the air. Then, 5 μL 2.5% Nafion solution in ethanol (Fisher Science Education) was drop casted on the surface of the electrode, which was dried in the air for 30 min.

A three-electrode sensing setup was used, including nanowires modified GCE as the working electrode, Pt wire as the counter electrode (99.9%, Alfa Aesar) and Ag/AgCl as the reference electrode (Bioanalytical Systems, Inc.). The electrochemical measurement was carried out in a phosphate buffer solution (PBS) with the concentration of 0.1 M.

In this study, the nanowires are normally prepared in the size range of 200–300 nm in width and a few microns in length. For DC electrodeposition, the current density for the synthesis of Pt nanowires is in the range of 1 to 6 mA cm⁻². The surface morphology of the Pt nanowires is presented in Figure 1. Figure 1a and 1b show that the surfaces of the prepared Pt nanowires are very smooth at low current density of 1 and 2 mA cm⁻², which indicates that small nucleus formed and the grain size is small. The small nuclei are packed very well and thus the overall surface looks very smooth. The higher magnification SEM image with detailed surface morphology can be seen in Figure 2a. When the deposition current density increases to 3 mA cm⁻², some surface rough structure appears during the growth of the Pt nanowires along with some smooth surface as shown in Figure 1c. It is obvious in higher magnification SEM image (Figure 2b) that both rough and smooth surfaces can be seen from the prepared Pt nanowires. It is possible that the grain size increases while increasing the current density due to the growth of existing grains. The surfaces of the Pt nanowires become even rougher when the deposition current density is 4 mA cm⁻², as shown in Figure 1d. When the current density increases to 5 and 6 mA cm⁻², the surface of the Pt nanowires are completely rough (Figure 1e and 1f) due to the dominant growth of the existing grains, which was promoted by higher current density. Figure 2c gives a clear observation of the surface morphology with complete roughness across the entire nanowires. Based on the above results, it can be concluded that the higher current density resulted in rougher surfaces of the Pt nanowires. The element of the Pt nanowires was confirmed by the EDS result in Figure 2d. The spectra are exactly the same, indicating the same element of Pt for nanowires with different roughness, even though there is structural variation. The signal of Si is contributed by the Si substrate that supported the sample for imaging, while the C signal is probably due to the carbon tape that was used to prepare the sample or some molecules that were adsorbed on nanowire surfaces.

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The crystal structure of the above-mentioned Pt nanowire samples obtained from 6 deposition conditions were characterized by the XRD analysis, as shown in Figure 3a. The peaks at 39.8°, 46.2°, 67.4°, 81.2° and 85.7° can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystalline plane diffractions peaks, respectively, illustrating the face-centered cubic (fcc) phase of Pt.^16,17 Furthermore, it is observed that the spectra collected from the surface roughed nanowires have sharper peaks with better symmetry than the smooth Pt nanowires, which also indicates the larger grain size formed in the surface roughed Pt nanowires. The grain sizes of the Pt nanowires at different current densities are calculated from line broadening of the X-ray peaks according to Scherrer's formula, as shown in Figure 3b. The calculated grain size from the XRD results is ranged from 4 nm (1 mA cm⁻² deposition) to 9.8 nm (6 mA cm⁻² deposition), and shows a linear function of current density in this data range. The typical grain size measured from the TEM images shown in Figure 2 (2a and 2c), e.g., 4.26 nm for 1 mA cm⁻² deposition (Figure 2a) and 9.57 nm for 6 mA cm⁻² deposition (Figure 2c), are in good agreement with the calculated results from XRD measurements. In addition, the quantification of the surface roughness of the Pt nanowires has been measured by AFM, in which six points at different locations of the nanowires were used to get the average roughness of that sample. Figure 3b presents the change in the grain size (left side) and surface roughness (right side) as a function of current density. In general, the surface roughness is also a linear function of the current density. Based on these measurements, it can be inferred that there is an excellent correlation between the variations of surface texture and crystallite size.

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In the PC electrodeposition, the effect of three pulse electrodeposition parameters, I_P, T_ON and T_OFF is investigated, respectively.

The average current density (I_A) of pulse plating is defined as:

Prior to the PC electrodeposition, DC electrodeposition was conducted to determine the average current density which was then used in the PC deposition. As can be seen in Fig. 1 and Fig. 3, increase in the current density increased the roughness of the Pt nanowire surface and the grain size. The effect of peak current density on the morphology characteristics of Pt nanowires was examined by varying the current density at 8, 12 and 16 mA cm⁻² in PC electrodeposition. During these experiments, the values of current-on time (T_ON = 0.5s) and -off time (T_OFF = 1.5s) were kept constant. According to equation 1, the average current density was 2, 3 and 4 mA cm⁻², respectively. Figure 4 shows the effect of peak current density on the surface morphology of Pt nanowires. It is evident from the figure that smooth surface and correspondingly small grains of Pt nanowires were obtained at 8 mA cm⁻² (Figure 4a). Increasing the peak current density to 12 and 16 mA cm⁻² resulted in rougher surface and increase of the grain size (Figure 4b and 4c). Figure 4d displays the variation of grain size of three Pt nanowire samples at different current densities. The grain sizes are calculated from line broadening of the X-ray peaks according to Scherrer's formula. The grain size is roughly linear as a function of the pulsed current density, and the grain size values are comparable to these from the DC electrodeposition as shown in Figure 3b. This behavior is consistent with the results obtained from DC electrodepositon.

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The effect of pulse duration T_ON or current-off T_OFF on the surface morphology and grain size of Pt nanowires are illustrated in Figure 5 based on the condition of I_P = 12 mA cm⁻², T_ON = 0.5s and T_OFF = 1.5s. The micrographs on the left (Figure 5a, 5b, and 5c) show that an increase in T_ON, while keeping I_P = 12 mA cm⁻² and T_OFF = 1.5s, results in an increase in roughness and grain size of Pt nanowires. Smooth Pt nanowires are formed at T_ON = 0.05s (Figure 5a), while further increase of current on-time to 1s dramatically improves the roughness of the Pt nanowires, where all the nanowires are completely rough as shown in Figure 5c. The trend of rougher surface with increasing current on-time can best be explained by an increased average current density at longer current on-times. The SEM images on right side of the figure (Figure 5d, 5e, and 5f) illustrate the effect of current off-time on the surface morphology and grain size of Pt nanowires. The current off-time changed from 0.5 to 5s at the T_ON value (0.5s) and pulse current density of 12 mA cm⁻². The roughness of the Pt nanowires decreased gradually with increasing current off-time. The Pt nanowires become smooth when the current off-time increases to 5s. The formation of smooth nanowires at longer off-times can be explained by the decrease of the average current density.

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A series of FESEM images shown above represent the general trend in Pt nanowire morphology as a function of current density. The results from the analysis of the XRD data confirm that the grain size increases with increasing current density. It is believed that in electrodeposition the growth of Pt nanowires is the process of nucleation and crystallization. And the dominant function of nucleation or crystallization is determined by the free energy of the surface of a nucleus and the driving force (current density or potential). The crystalline nanowires will grow after the nucleus size exceeds the critical dimension, Nc.¹⁸ Nc is expressed as

where s, ɛ, z, and b are the area occupied by one metallic atom on the surface of the nucleus, the edge energy, the effective electron number, and a constant (b = π for circular), respectively; η is the overpotential. The overpotential is established by Tafel¹⁹ as follows:

where a and b are constants and i is the current density.

According to Eq. 3, the larger the current density applied, the higher the overpotential is. In the DC electrodeposition, the only parameter that can be changed is the overpotential η or current density i, and in the PC electrodeposition, the change of T_ON or T_OFF will result in the change of the average current density. So all the changes in both DC and PC electrodeposition are related to the change of the overpotential η or current density i. Under low deposition current density, nuclei form, since low deposition current density leads to large Nc, which prevents the formation of columnar grains. If the current density is high, then columnar grain growth is favored because Nc is small. In this study, the lower current density resulted in smooth surface nanowires, due to the nucleus formed by nucleation process; the smaller nuclei are packed well into nanowire structure due to the template constraint and thus formed nanowires with smooth surfaces. While at higher current density, nanowires with rough surfaces and larger grain sizes were formed due to the dominant growth of existing columnar grains, which was promoted by a higher current density. Our results shown in Figure 3 are in agreement with this scenario. At medium current density (such as 3 mA cm⁻²), a transient state existed, where both smooth nanowire surface and rough surface were observed, as shown in Figure 1c, 2b, 4b, 5b and 5e. From the systematic studies above, possible nanowire growth mechanism from the electrodeposition process is schematically illustrated in Figure 6.

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The accurate and rapid detection of hydrogen peroxide (H₂O₂) is very important because it is not only a by-product of various oxidases in countless biological processes, but also an essential compound in pharmaceutical research, clinical laboratory, medical diagnostics and environmental analysis.^20–22 Electrochemical sensors have been proven to be an accurate and sensitive way of H₂O₂ examination due to low-cost, high sensitivity, fast response and convenient operation. Platinum (Pt)-based electrodes have been widely employed for enzyme-free sensing of H₂O₂ due to its high electrocatalytic efficiency and selectivity for H₂O₂.^23–27

H₂O₂ detetion was studied on these well-prepared Pt nanowires. Typical CVs of H₂O₂ response with different Pt nanowires are presented in the Figure 7a, which were obtained in 0.1M PBS and 10mM H₂O₂ solution. It should be noted that similar amounts of Pt nanowires in weight were mounted for all modified GCE in this figure to compare the electro-catalytic performances. For the Nafion modified GCE (black curve), there is almost no current peaks of H₂O₂ reduction that can be observed, which indicates that Nafion has no electro-catalytic activity for H₂O₂ reduction. The shapes of all the other curves in Figure 7a shows a cathodic current peak in the negative sweep at around −0.45 V versus Ag/AgCl electrode, representing general electrochemical CV features for the electro-reduction reactions of H₂O₂. Figure 7b shows that the peak current increased with respect to higher current density deposited Pt nanowires and the trend is similar to the result shown in Figure 3, which indicates that higher current density deposited Pt nanowires were the preeminent catalyst for H₂O₂ reduction among all the prepared Pt nanowires. On all accounts, the increase of peak current density which indicates that the increased catalytic activity of the higher current deposited Pt nanowires electrode may be mainly attributed to the following two causes: (1) the rough Pt nanowires have more surface area than the smooth ones which will provide more space for the catalytic activity, and (2) it is possible that the large grain size of rough Pt nanowires results in higher conductance which leads to easier and faster electron transfer rate from the coated nanowire film to GCE.

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The CVs of 10mM H₂O₂ in 0.1M PBS with different scan rates and different concentrations with the same scan rate at a rough Pt nanowire modified GCE are presented in Figure 8a and 8c. It can be seen that the peak current for hydrogen peroxide reduction become larger with the increasing of either scan rate or H₂O₂ concentration.

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The relation between the peak current obtained from backward CV scan and square root of scan rates (υ) of CV is shown in Figure 8b, and a linear relationship is observed. It can be obtained that the process of electrocatalytic hydrogen peroxide reduction may be controlled by diffusion of hydrogen peroxide. The reduction peak current measured at −0.45 V versus Ag/AgCl for hydrogen peroxide with different concentrations was found to increase linearly. It can be inferred that the presence of the deposited rough Pt nanowires on the surface of the GCE facilitates the detection of hydrogen peroxide. As can be seen in Figure 8d, the anodic peak current density increased linearly as the H₂O₂ concentration increased. These features demonstrate that the Pt NW modified GCE is well suited for the detection of H₂O₂.