#### Abstract

Highly ordered and nanometer-scaled nickel wire arrays were successfully prepared by supercritical electrodeposition method using anodized aluminum oxide (AAO) template. The results show that the well-ordered and free-standing nickel nanowire arrays can be constructed uniformly on a titanium-coated silicon wafer after removing the AAO template. The diameter and length of the nickel nanowire in the arrays can be obtained, about 100 nm and 10 um, respectively. Based on Box-Behnken design and Response Surface Methodology (RSM), a regression model was built by fitting the experimental results with a polynomial equation. The current density, pressure, and temperature are critical important factors of the growth mechanism of deposited nanowires. The optimal length of nanowires, 10.03 *μ*m, can be achieved at the following conditions: current density 0.23 A/cm^{2}, pressure 107 bar, and temperature 53°C.

#### 1. Introduction

One of the most important challenges in materials science and technology today is the preparation of ordered nanostructure arrays with controlled properties and dimensions for industrial application [1]. It is of great interest in the trend toward miniaturization and densification of electronic parts and components which tend to become smaller in size with a larger number of individual devices [2]. Thus, porous alumina films and nanocomposites based on AAO membranes could be used as perfect model objects for a deeper understanding of processes on nanoscaled in spatially ordered systems [3].

Several reports have been published on nanowire arrays-filled porous AAO films, for materials including single-metal nanowires [4], ferromagnetic alloy nanowires [5], and multilayered nanowires [6]. Electrodeposition of metal or alloy in AAO pores needs to stabilize an electric current distribution during operation owing to the high aspect ratio of AAO pores [7]. A common problem exists during electroplating, which is the electric current also causing the dissociation of water in addition to the electrolysis of metal ions due to the handicap of solution transport in nanometer space. The dissociation of water may create several defects on the growth mechanism of deposited metal alloy [8]. Due to the low viscosity, high diffusivity, and zero surface tension nature of supercritical carbon dioxide (Sc-CO_{2}), plating technology with Sc-CO_{2} has attracted special attention because Sc-CO_{2}, in particular, can transport the solute into fine nanometer space of the materials without shrinking or causing other harm due to the interface tension that exists between liquids and gases [9].

The aim of this work was to elucidate a simple model of electroplating conditions to control the performance of deposited nanowire arrays using AAO template by supercritical electroplating process through statistical experimental method. Based on Box-Behnken design and Response Surface Methodology [10], a mathematics model was built by fitting the experimental results with the regression analysis to yield the most information by a minimum number of experiments.

#### 2. Materials and Methods

##### 2.1. Materials

A highly pure AAO/Ti/Si substrate (3.5 mm) was deposited on a p-type silicon substrate coated with a Ti film (about 300 nm) provided from Metal Industry Research & Development Center. The electroplating solution, so-called Watt bath, was composed of NiSO_{4} (280 g/L), NiCl_{2} (80 g/L), and boric acid (40 g/L). Carbon dioxide with a minimum purity of 99.9% was purchased from Toei Kagaku Co. Ltd. A nonionic block copolymer-poly(ethylene oxide)-poly(propylene oxide) [HO(CH_{2}CH_{2}O)_{100}CH_{3}(CHCH_{2}O)_{30}(CH_{2}CH_{2}O)_{100}H, 0.001 mol/L], PEOPPO, was employed as surfactants in our experiments. The chemicals used in this work were extra pure reagents without further purification.

##### 2.2. Experimental Apparatus

A high-pressure experimental apparatus used for electroplating was fabricated by ourselves and its outline was shown in Figure 1. The temperature variation of each run was observed to be less than 1.0°C. The maximum working temperature and the maximum pressure were 90°C and 250 bar, respectively. The integrated electroplating cell that had a volume of 200 mL was a stainless steel 316 vessel in a temperature-controlled air bath with an agitator. Both the anode and the cathode were attached using platinum wires to the reactor and were connected to a programmable power supply; model YPP15030, manufactured by Yamamoto-ms Co. Ltd. A typical electroplating reaction was performed in a constantly agitated ternary system of Sc-CO_{2}, the electroplating solution and a surfactant. The 100 mL nickel electroplating solution, and the surfactant both were put in a high-pressure cell. CO_{2} was introduced to the high-pressure cell using a pump and pressurized to a predetermined pressure. The ternary system was then constantly agitated at a speed of 400 rpm under a desirable constant temperature. Deposition was carried out under the controlled factors including pressure, temperature, and current density. The electrodeposition was kept running until deposited nanowires overflowed from nanoholes. The overflowed nanowires were mechanically polished with metallographic abrasive paper.

##### 2.3. Analysis

After the formation of nanowires, the specimens were cut to 2 mm width layers with diamond cutter, which is used commonly for TEM specimen preparation. The morphologies of the AAO film on Ti/Si substrate and nanowires arrays were examined by Hitachi Model HF-2000 Field Emission Transmission Electron Microscope operating at an accelerating voltage of 150 kV. The elemental analysis was performed with an energy dispersive X-ray analysis (EDX, Noran Pioneer TM X-ray detector) installed in connection with the TEM.

#### 3. Results and Discussion

##### 3.1. Surface Observation

The TEM images analysis of deposited nanowires arrays using AAO template obtained by supercritical electroplating (75 bar) and general electroplating (1 bar) operated at 45°C, 0.3 A/cm^{2} and 1.0 hour were presented in Figures 2 and 3, respectively. It is evident that highly ordered nickel nanowire arrays were successfully prepared by supercritical and general electrodeposition method using anodized aluminum oxide (AAO) template. The better performance of nickel nanowires arrays can be obtained by supercritical electroplating method. The electrodeposition by direct current only is not stable and uniform filling of the pores cannot be achieved smoothly. This is due to low ion concentrations and hydrogen evolution in the pores of AAO at cathode side, which leads to high overpotentials and can facilitate the dendritically growth. However, supercritical fluid is of lower mass transfer resistance to eliminate concentration polarization and hydrogen evolution in the pores of AAO. In our experiment, the hydrogen gas that was generated is miscible with Sc-CO_{2} and is eliminated with Sc-CO_{2} in the dynamic emulsion. It concerns the electrochemical reactions in emulsions formed by Sc-CO_{2} and aqueous electrolyte with surfactant PEOPPO. The emulsion particle size ranges typically from several nanometers to several millimeters and can be controlled with surfactants to within a relatively narrow size distribution. When the electroplating solution comes in contact with the cathode, the nucleation and the crystal growth can occur. However, when the Sc-CO_{2} comes in contact with the cathode, the nucleation and the crystal growth cannot occur. Therefore, the plating in the emulsion is similar to pulse plating and can facilitate the nickel nanowire arrays growth with high order [11].

##### 3.2. Optimization of the Significant Variables

The optimization of experimental conditions represents a critical step in the development of a supercritical nickel plating method for nanowire array application. In order to prepare excellent performance of nickel nanowires using anodized aluminum oxide (AAO) template, the electrodeposition step must be adjusted for layer-type growth mechanism not dendritically. During electroplating step, current density (), pressure (), and temperature () have essential effect on the growth mechanism of deposited nanowires [12]. Hence, these variables were used to find the optimized conditions for better performance of nickel nanowires from anodized aluminum oxide (AAO) template using Box-Behnken design and Response Surface Methodology (RSM). The length of nickel nanowires was selected as the response property. Each variable had three levels to be examined at high level (+1), medium level (0), and low level (−1). The high and low levels we selected for this study represented the extremes of normal operating ranges. The range and the levels of the experimental variables investigated in this study are given in Table 1. The Box-Behnken design and experimental results are tabulated in the Table 2. In developing regression model, the experimental variables were coded according to the following equation: where is the coded value of the variable , is the value of at the center point, and is the step change value.

Once the experiments were performed, the regression model could be constructed by fitting the experimental results with a second-order polynomial. The optimal conditions of selected variables were searched using the regression model and also by analyzing RSM. Analysis of variance (ANOVA) was used to obtain the interaction between the process variables and the response. The quality of the fit of polynomial model was expressed by the coefficient of determination and in (2), respectively. The statistical significance was checked with by the -test by (3) in the program [13]: where is the number of model parameters, is the number of experiments, DF refers to degrees of freedom, and SS is the total sum of squares. The terms SSB and SSE correspond to sum of squares between the groups and within the groups, respectively.

The experimental results were analyzed using statistical methods appropriate to the experimental design used. Statistical software package Design-Expert 7.1 was used to analyze the experimental results. According to the RSM methodology, a second-order polynomial model was used to fit the experimental variables for length of deposited nickel nanowires, , used as a response. Multiple regression analysis of the results was given by the following equation:

ANOVA results presented in Table 3 indicate that this polynomial model can be used to navigate the design space. It can be seen that the polynomial model -value of 37.16 implies that the model is significant for quality of deposited nickel nanowires by supercritical electroplating. The values of Prob. > less than 0.1000 indicate that the model terms are significant, whereas the values greater than 0.1000 are not significant. The results indicated that, the three linear, , , , a two-variable interaction and two quadric , terms had significant effect () on the length of nickel nanowires according to Prob. > values. The results identify that the current density, pressure, and temperature are primary factors on supercritical electrodeposition of nickel nanowires [14]. The characteristic of supercritical fluid, especially for density of Sc-CO_{2}, is determined by its pressure and temperature so as to exert an influence on the performance of supercritical electroplating [15].

Equation (4) should be used to visualize the optimization behavior of deposited nickel nanowires by supercritical electroplating. The checking of model adequacy is an important part of the data analysis procedure. The (determination coefficient) of the regression equation obtained from analysis of variance is 0.9531 (a value >0.75 indicates aptness of the model), which means that the model can explain 95.31% variation in the response. Figure 4 showed the predicted length of nickel nanowires versus studentized residuals which the residuals divided by the estimated standard deviation of that residual [16]. It measures the number of standard deviations separating the actual and predicted values. The plot in Figure 4 indicates whether the residuals follow a normal distribution, in which case the points will follow a straight line. Most of the standard residuals should lie in the interval of 2.60 and any observation with a standardized residual outside of this interval is potentially unusual with respect to its observed response. Therefore, the predicting response surface equation confirms that the equation gives an excellent fitting to the experimentally observed data. By comparisons of experimental value and predicted value of regression model, it was also observed that agreement was satisfactory in Table 2. Based on the regression model, the predicted maximum for length of deposited nickel nanowires was calculated to be 10.03 *μ*m from (4). The corresponding optimal variable conditions were of current density 0.23 A/cm^{2}, pressure 107 bar, and temperature 53°C.

#### 4. Conclusions

Highly ordered nickel nanowire arrays have been prepared using supercritical electroplating method. It is also highlighted that the developed method showed a wide range of variable parameters for obtaining nickel nanowire arrays with uniform characteristics using AAO template. Data from the present investigation has shown that nickel nanowires deposition is dependent mainly on current density, pressure, temperature. On the basis of a statistical optimization method with Box-Behnken design, a theoretical approach derived by numerical calculation resulted in agreement with the experimental data.

#### Acknowledgment

The authors sincerely appreciate the financial support of the Nation Science Council of the Republic of China (99-2221-E-242-007) for this work.