Variation of crystallinity of Cu and Cu2O nanowires arrays grown in various pores of porous alumina membrane

Various pore sizes of a porous alumina membrane were fabricated using H2SO4 and H2C2O4 electrolyte under different ionization voltages. Cu nanowire arrays with high aspect ratios, uniform pore size, and ordered pore arrangement were synthesized using the above porous alumina membrane (PAM). Moreover, Cu2O nanowire arrays were prepared through the oxidization of Cu metal nanowire arrays. From the microstructure and compositional analysis, it was observed that pores of different sizes, i.e. 20~30, 70~90 and 90~100 nm could be obtained by controlling various electrolytes and anodization voltage. The Cu nanowire synthesized with various pore sizes were found to be single crystal (20~90 nm) and polycrystalline (90~100nm) respectively. The single crystal Cu with (111) direction was occurred due to homogeneous current density distribution and relationship between current density (J) and nucleus radius (ro). After oxidation of Cu, the Cu2O nanowires with the pore sizes of 20~100 nm was found to be single crystal. The rearranged of Cu and O2 lattice sites promotes the enhancement of crystalline property. Copyright © 2017 VBRI Press.


Introduction
There are many intensive studies on nanostructure materials such as nanotubes, nanowires, nanosprings and nanorods for their unique electrical, chemical, optical, mechanical, sensing, and magnetic properties for possible applications in nanodevices [1][2][3][4]. The fabrication of different nanostructure using unique and innovative method is the interest of researcher worldwide to get specific nanostructure for specific application. Template synthesis is a one of the bottom-up technology widely applied to fabricate various kinds of nanostructure, especially nanoparticles, nanotubes, and nanowires. A well-known template called porous alumina membrane (PAM), featuring order pore arrays, narrow size distribution, and high aspect ratios [5], has attracted much attention for the synthesis of 1-D nanostructures [6,7]. PAM having pore size 30nm to few hundred are synthesized depending on the processing electrolyte and voltage [8][9] for different applications. In addition, much work has been devoted for synthesizing metal [10-12] and metal oxide nanowire arrays directly from PAM template [13][14][15][16][17]. PAM having better characteristics such as tunable pore size, uniform structure and high thermal stability become the most interesting material in nanofabrication.
The growth of metal and metal oxide nanowires directly from PAM plays important role for potential applications in devices [18]. A significant level of control over the growth of nanowires on PAM is necessary for the fabrication of devices. The crystallinity of metal and metal oxide nanowires plays unique role for this applications. Now Cu and Cu2O nanowires has emerged great attention due to its superior characteristics and applications [19][20][21]. However, the stability of Cu nanowires against oxygen, high temperatures, and chemical etching makes difficult for large scalable fabrication, before Cu nanowires can be fully integrated into commercial devices. The challenges remain also for the stabilization of Cu nanowires because they are prone to oxidation under ambient conditions. As a result, the growth of Cu and CuxO nanowires is more important for future applications. Cu are single crystal under low deposition potential and polycrystalline under high deposition potential. Polycrystalline Cu2O nanowire array growth via PAM was studied using three-electrode electrochemical deposition [22][23][24][25]. Oh et al. [22] fabricated Cu2O-based nanowires on PAM using aqueous CuSO4 and lactic acid solutions with a pH = 9. Choi et al. [23] observed that nanowire arrays of Cu reduced into Cu2O when the pH was changed from 6 to 10. Inguanta et al.
[24] synthesized Cu2O nanowire arrays at 55 o C with pH=6.5 and -0.2V vs. SCE (saturated calomel electrode) in copper acetate and sodium acetate solutions. They also explored the different potential form on pulse to avoid the co-deposition of Cu and Cu2O [25].
Many researchers have focused on the direct synthesis of Cu2O nanowire arrays by the electrochemical process; however, the prepared Cu2O nanowires were found to be polycrystalline in nature. The growth mechanism of Cu2O nanowires on PAMs is assumed to occur through particle agglomeration in the nano channels. It has been proposed that single crystal nanowires or nanorods provide direct paths for transporting electrons, with the electron transport rate in single crystals being several orders greater than that of their polycrystalline counterpart [26]. Hence, fabricating single crystals nanowire is paramount for improving electrical properties. In this regard, Xu et. al [27] reported the synthesis of CdS nanowire by controlling the pore size of PAMs under electrochemical deposition method. Ultimately, they observed single crystal CdS nanowires with a pore size diameter close to 8 nm. Single crystal CdS growth was achieved via atom-by-atom assembly of individual crystallites instead of the agglomeration of particles formed in the solution phase. Furthermore, in our previous work [28], we have synthesized Cu2O nanowire arrays by annealing Cu/PAMs composition under various partial oxygen atmospheres. Present work report the possibility of synthesis of Cu nanowire arrays by controlling the pore size of PAMs using electrochemical deposition and Cu2O single nanowires by heating the Cu/PAMs nanowires than those reported in the literature so far. The crystallinity of the nanowires is controlled depending on the size of the pore. The synthesized Cu nanowires may have potential application in devices such as electrostatic dissipative and current collector for batteries of Li and Cu2O can be a candidate for use in next generation memory devices.

Preparation of porous alumina membranes (PAMs)
PAMs was prepared using the well-known procedure using Al foil with anodization process as described elsewhere [16]. The Al foil sheet were annealed, cleaned and finally electro polished in a mixture of HClO4-C2H5OH (1:4 vol %) at 10  C, with a current density of 100mA/cm 2 for 1 min.
The standard anodization was carried out in 0.3 M sulfuric acid and 0.3 M oxalic acid at a constant voltage of 18, 40 and 60V using Pt foil as the counter electrode. The electrolyte was then vigorously stirred, with a temperature kept at 0  C during anodization. After 3~6 h of anodization, the alumina film was selectively etched away in a mixture of H3PO4-CrO3-H2O (2g-3.5 mL-100 mL) at 70  C for 40 min. Afterwards, the film underwent a second anodization process under the same conditions for 18~24 h. After the second anodization, the remaining Al was dissolved by saturated HgCl2 solution. Finally, the straight nano channel pores were handled by dissolving the barrier layer with 5% H3PO4 solution at 60 o C for 20 min.

Preparation of Cu/PAMs and Cu2O/PAMs nanowires arrays
A layer of Ti (10 nm) and Pt (100 nm) were coated onto one side of the membrane before deposition of nanowire arrays and adhered to the Cu substrate to serve as the working electrode (cathode). Pt foil (anode) and a saturated calomel electrode (SCE, 0.241V/NHE) served as the counter and reference electrodes, respectively, in a three-electrode electrochemical deposition system. A potentiostat (263A, Princeton Applied Research) was used to provide potential and also to record the current density-time (J-t) curve during fabrication of the Cu nanowire. The PAMs/Ti/Pt/Cu were degassed using a mixture of 0.2 M CuSO4 and 0.15 M H3BO3 solution with a pH = 2 (controlled by H2SO4) and deposited at -0.18 V/SCE for 2 h. After electrochemical deposition, the Cu/PAMs nanowire arrays were annealed in air at 400  C for 8 h to obtain the Cu2O/PAMs nanowire arrays.

Characterization of Cu/PAMs and Cu2O/PAMs nanowires arrays
The microstructure and chemical composition of the prepared Cu and Cu2O nanowire arrays were analyzed by employing field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and an energy dispersive spectrometer (EDS). For SEM observation, Cu/PAMs and Cu2O/PAMs were immersed in a 1M NaOH solution for 30 min, and then cleaned by deionized water several times. The HR-TEM samples were prepared by immersing the Cu/PAMs and Cu2O/PAMs in 3M NaOH for 5 min, and then washed by deionized water several times. The same step was repeated 3 times until the Cu/PAMs and Cu2O/PAMs became soft. Subsequently, the samples were dispersed in ethanol and collected onto the Ni grid.

Results and discussion
The various pore sizes of PAMs were prepared using sulfuric acid and oxalic acid under 18 V to 60 V anodization voltages. Many experimental parameters such as electrolyte concentration, the anodic voltage, anodic time, formation temperature, oxidation time, annealing temperature and etching process are used to get the required pore size of PAM [10]. The size of the pore can be easily controlled by changing the processing condition. Fig. 1 shows the SEM top-view morphology of PAM prepared under different conditions, which consists of uniform pore arrays. The pore size distribution of PAM using sulfuric acid at 18 V was around 20-40 nm (Fig. 1A). Using oxalic acid, the average pore size for anodization voltages of 40 V to 60 V was around 70~90 and 90~100 nm respectively (Figs. 1B-C). According to Lee et. al [29], the pore size distribution with respect to voltage exhibits a linear relationship in a mild anodization (MA) process, which is consistent with our results. Pore size of PAM is crucial for growth of nanostructure.  Since the various pore sizes of PAMs were obtained by controlling the electrolyte and anodization voltage, the Cu nanowires were embedded into the nano channel of PAMs using electrochemical deposition. Fig. 2 shows that the Cu nanowire arrays, which were initially embedded in various pore sizes of PAM, and became free standing after the PAMs were removed using a NaOH solution. As much, the size and arrangement of nanowires depend on the PAMs and pore sizes were found to be around 20-40, 70-90 and 90-100 nm respectively. The crystallinity of individual nanowires was investigated via TEM images, as shown in Fig. 3. Initially, to confirm the size of each nanowire, observed diameters measured 30, 70, and 90 nm (Figs. 3 A, C and E), which was similar to the SEM results. It can be seen that the nanowire shape and size were controlled directly by the template. The electron diffraction patterns show single crystal formations when the nanowire was fabricated with diameters of 20-40 and 70-90 nm. When the nanowire increased to 90 nm, a fibrillar structure was observed, and the nanowire crystallinity was polycrystalline (as shown in Fig 3F. Further, the d-spacing is found to be 2.43 Å, and the growth direction is along (111). The prefer orientation of Cu was (111) due to the lowest surface energy during growth process. To determine the Cu nanowire growth rate under various PAM pore sizes, the J-t curve at a constant potential of -0.18 V/SCE was plotted and shown in Fig. 4. Generally, synthesis of nanowire by template assistance consists of three steps, which are shown in Fig. 4A. In the first step, In the case of 20-30 nm and 70-90 nm assisted Cu nanowires; the variety of current density was smooth, which demonstrated the homogeneous behavior over the cathode surface. Additionally, by comparing the initial current density in various pore size assisted Cu nanowire growth, it is observed that the current density increases gradually with an increase in pore size. This result also implies that the nucleus radius decreases gradually with an increasing pore size, which influences the nanocrystallite size [33]. Thus, the crystallinity of 20-30 nm and 70-90 nm nanowires is single crystalline in nature. However, when increasing the pore size to 90-100 nm, the mechanism of nanowire growth is attributed to the agglomeration of Cu particles. The fibrillar structure observed in the 90-100 nm nanowire was due to the rapid growth as the nano crystallite did not have enough time to undergo nucleation. When the nanowire was fabricated over to be 90 nm in size, the nanowire was found to be polycrystalline. Xu et. al [27] also reported that single crystal nanowire growth was achieved via atom-by-atom assembly of individual crystallites instead of the agglomeration of particles formed in the solution phase, which is very similar to our work.
The above results indicated that ordered Cu/PAMs nanowire arrays were synthesized, the crystallinity of which was varied by controlling the PAM pore size. Fig. 5 shows the SEM images of the Cu2O nanowire arrays with various diameters obtained after annealing the Cu nanowire arrays at 400 o C for 8 h. Initially, the arrays were prepared inside PAMs and then became free standing after dissolving the PAMs by 1M NaOH for 30 min. Consequently, the size and shape of the nanowires correspond with the PAMs configuration. To confirm the composition of copper oxide, EDX measurement was carried out and, with Fig. 5 (D) offering the EDX analysis of the nanowire. Al and Ni elements were found due to the alumina template and Ni grid, respectively. EDX results indicate that the ratio between Cu and O is 2:1, which implies that the nanowires are indeed Cu2O. The crystallinity of individual nanowires was investigated from TEM and convergent beam electron diffraction (CBED) and select area electron diffraction (SAED) images as shown in Fig. 6. Compared to the SEM results, the TEM morphologies also implied the same results, with individual nanowire sizes were 20~30, 70~90 and 90~100nm. In addition, the observation of shell layer covered on the Cu2O nanowire shown in Fig. 6E was remaining Al2O3, which was removed incompletely. The crystallinity of different size nanowires can be observed from the ED images. It was found that single crystals were synthesized for nanowire size of 20~30, 70~90, and 90~100 nm. The electron diffraction patterns of (200), (220), and (002) were observed in each single crystal nanowire (Figs. 6B, D, F); also, the dspacing's d(200), d(220), and d(002) were found to be 2.06, 2.4, and 1.5 Å, respectively. Combined with the EDX analysis, the ED results also demonstrate the formation of FCC-Cu2O during the heat treatment of Cu. In addition, the d-spacing in different diameters of single crystal nanowires were 2.39, 2.38, and 2.41 Å. The results also imply that the Cu2O growth direction was (111) [34]. In comparison to the synthesis of Cu nanowire, here, the O2 atoms were diffused into the tetrahedral site of FCC-Cu during the heat treatment process to form Cu2O. We can observe that the crystallinity was improved during the oxidation process and we can easily get the single crystal Cu2O. The results further imply that not only did the O2 atoms diffuse into the Cu structure during heat treatment, but the Cu and O2 lattice sites may also rearranged during the formation of nanowires. Cu2O crystallizes in the cuprite structure where inside the unit cell the oxygen ions are located on a bcc sub-lattice, while copper ions on a fcc sub-lattice. The structure is favourable as it consists of two linkages of tetrahedral rotated 90 0 to each other and grows in preferential direction. During oxidation, atoms have sufficient energy to move to the correct lattice position, and crystalline grains are produced because of the low surface energy [35]. The major contribution to the oxidation reaction comes from lattice oxygen [36]. Therefore, the oxygen mobility in the crystal lattice determines the crystallinity of nanowires due to changes in the periodicity of the materials [37]. The possible control of the oxidation behavior of Cu leads to artificial structural modifications of the oxide nanostructures [38]. Based on above results, we can synthesize the appropriate Cu and/or Cu2O single crystal nanowires using PAM for future electronic device applications. A comparison of this work and reported literature work for growth of Cu/Cu2O nanowires is given in Table 1 for better understanding. Our grown nanowires have been shown to be advantageous than the other work for possible application.

Conclusion
We have successfully prepared various ordered pore sizes of PAMs by controlling the H2SO4 and H2C2O4 electrolytes. Pore sizes are found to be 20~30, 70~90 and 90~100 nm by increasing the anodization voltage from 18 to 60 V. The crystallinity of Cu nanowire is influenced by various pore sizes of PAMs, such as single crystal behavior at 20 to 90 nm, and polycrystalline at 90~100 nm. The slow diffusion of  2 aq Cu to the electrode surface caused the atom-by-atom agglomeration and thereby induced the single crystal behavior for nanowire sizes of less than 90 nm. The O2 is diffused into the Cu lattice during the heat treatment, and the template space limitation induced the formation of Cu2O phase. Moreover, the crystallinity is also improved in each condition due to the rearrangement of Cu and O2 lattice