Rare Metal Materials and Engineering Volume 40, Issue 7, July 2011
Online English edition of the Chinese language journal
Received date: July 07, 2010
Foundation item: Science and Technology Planning Project of Guangdong Province (2006B04001001); A Subsidized Project on Natural Science in Guangdong Province (04011311, 021286); A Funding Project of Shenzhen City (200450)
Corresponding author: Gong Xiaozhong, Master, College of Chemistry and Chemical Engineering, Shenzhen Key Laboratory of Functional Polymers, Shenzhen Univer-sity, Shenzhen 518060, P . R. China, Tel: 0086-755-26536141, E-mail: hxgxz@https://www.doczj.com/doc/3718370683.html,, cici_gxz@https://www.doczj.com/doc/3718370683.html,; Tang Jiaoning, Professor, Tel: 0086-755-26538536 Copyright ? 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV . All rights reserved.
The larger L/D (length diameter) ratio of magnetic metal nanowire array often leads to the magnetization direction along with the length axis of nanowire, and shows the strong aeolotropy of magnetic shape. Also, the coercivity and rema-nence ratio can be controlled in many ways. The medium can meet requirements for density perpendicular to magnetic re-cording [1,2], and it is an ideal one [3,4] for the recording of den-sity perpendicular magnetic. Due to its unique status in the RE field, the element Nd has been the hotspot in the market for several years [5]. Besides its higher magnetic moment and magnetic property, the nanowire of Nd possesses conductivity and mechanical properties, so it is of the great significance of investigation. Although the Nd is widely used in industrious fields and medical treatment, the reports on preparing Nd are relatively few [6]. So far, there is still no report on the methods for preparing nanowire of SS Nd in China. Among the several methods in preparing one-dimensional nanowire, the most successful one is the template method. In the restrictive me-dium environment, the method designs the template model whose size of apertures and channels is controllable. Further, the method can control the shape and size and prevent the ag-glomeration after various nano-particles are embedded in the holes. Therefore, the template method has become the most important one for synthesizing nano materials [7,8]. In this paper, the aluminum sheet of high purity was used. Through a two-step anodization process and without peeling off the film, an electrolysis manner was applied to corrode its barrier layer [9,10], and then the PAA template with the diameter of 60 nm was prepared. After that, in the low temperature melted salt system and with PAA as template, the method of constant electric potential was used to electrodeposit the nanowire ar-ray of Nd. We obtained the single substance Nd nanowire, of 81.18 wt%. Afterwards we discussed the influence of current density and electrodeposition duration on the components of nanowire array, and characterized the morphology, the com-ponents and the crystal form of the nanowire array.
1 Experiment
Main reagents used in the experiment were as follows: the neodymium chloride (from Baotou Xunbo New Mate-rial Co., Ltd., in purity ≥ 99.95%), the oxalic acid (from Tianjin Damao Chemical Reagent Factory, in purity ≥99.50%), the phosphoric acid (from Tianjin Fuchen Chemical Reagent Factory, in purity ≥ 99.50%), the car-bamide (from Tianjin Baishi Chemical Industry Co., Ltd.,
in purity ≥ 99.00%), the sodium bromide (from Tianjin Yaohua Chemical Reagent Co., Ltd.,in purity ≥ 99.00%), the potassium bromide (from Tianjin Fuchen Chemical Reagent Factory, in purity ≥ 99.00%), the formamide (from Tianjin Fuyu Fine Chemicals Co., Ltd.,in purity ≥ 99.50%), the perchloric acid (from Jinlu Chemical Industry Co., Ltd., with purity of 70%-72%), the liquid paraffin (from Shantou Guanghua Chemical Factory, with boiling point of 300 °C above), the anhydrous alcohol (from Tianjin Fuchen Chemical Reagent Factory, in purity ≥ 99.70%), the sodium hydroxide (from Tianjin Damao Chemical Reagent Factory, in purity ≥ 99.00%), the chromium trioxide (form Luoyang Chemical Reagent Factory, in purity ≥ 99.00%), the acetone (from Tianjin Fuyu Fine Chemicals Co., Ltd.,in purity ≥99.50%), and the polyvinyl alcohol #124 (from Shanghai Runjie Chemical Reagent Co., Ltd.).
The carbamide-NaBr-KBr-formamide system was the deposit liquid, and perchloric acid was used to adjust its pH to 2-3. Carbon rod was anode, while PAA template was cathode. The NdCl3·n H2O, after vacuum pumping for 5 h, was dissolved into the deposit liquid. The concentration of 0.2 mol·L-1 was set. In the ZKX-2b vacuum anaerobic and hydrophobic operation box, the nitrogen was filled for protection. With magnetic stirring, the MD-20 multi-functional impulse-electroplating power source was used to adjust, under the constant potential, the current density to 1000-5000 A·m-2 . The duration of electrode-position was 1000-2000 s. Electrodeposition should be observed many times. After the reaction, the template with deposited nanowires was taken out, and cleaned with ethanol in its surface in order to remove the residues that may be attached on the template. The template edges were cut away by scissors, and then the template was put into ethanol solution of HgCl2, so as to remove the residual aluminum. After the two PAA films were separated, the template was taken out and soaked in ethanol for conser-vation and test. Or, it was coated with polyvinyl alcohol as protective film to prevent oxidation.
The JSM—5910LV SEM from Japan JEOL was adopted to observe the morphology of nanowire array. The X-ray EDS, in the model of #7274 and made by British OXFORD INSTRUMENTS was adopted to determine the chemical composition of nanowire array surface. The X-ray diffracto-meter of x’pert PRO from Holland PANalypical was used to analyze phase of nanowire array. The applied X-ray was the Cu Kα (λ＝0.154 18 nm).
2 Results and Discussions
2.1 Electrodeposition of Nd nanowire
The typical electrode potential of RE Nd is extremely nega-tive. In the aqueous solution, it is hardly electrodeposited be-cause of the hydrogen evolution reaction. So we selected the low temperature melted salt system, and a single lanthanum metal film was successfully electrodeposited on silicon sub-strate. The result of the experiment can be explained from the electronegativity principle[11] of elements. The electronegativ-ity of Nd and Al are 1.14 and 1.61, respectively. Because the electronegativity indicates the element ability to pull electron around it in a compound, the larger the electronegativity, the stronger the ability is. If the Nd is deposited on Al base, then a certain amount of Nd-Al bonds shall be formed. According to the electronegativity principle, in the formation of Nd-Al bond, some electrons should be transferred from Nd to Al, and the formed bond should possess the nature of electrovalent bond. The RE elements should maintain some positive charges, and this will lead to deposition potential of Nd (Ⅲ) shifting posi-tively [12,13]; therefore the RE metal La can be deposited on Al base. On the other hand, the experiment result can be ex-plained from the under-potential deposition theory: the more reactive metal ions make the reduction on the substrate of less reactive metal ions. When the electrode potential is more posi-tive than the standard equilibrium potential of the deposit metal, the metal ions can be reduced under potential and de-posit. Generally speaking, because the electron work of the matrix material is greater than that of deposit metal ions, the under-potential deposition often occurs. For example, the elec-tron work of Al substrate is 5.986 eV, and that of deposited metal Nd is 5.49 eV; thus, the under-potential deposition may occur on Al substrate. As the electron works of two metal at-oms are different, the electrons shall be partially transferred from deposited metal atoms to matrix metal atoms, and the bond between the two shall possess a certain nature of elec-trovalent bond. Therefore, the deposited metal atoms remain some positive charges, and they will lead to deposit potential shifting positively.
2.2 Influence of current density on the morphology
and composition of Nd nanowire
The current density is adjusted to 1000-2000 A·m-2 under constant potential and the Nd nanowires are electrodeposited in carbamide-NaBr-KBr-formamide system for 1800 s. The X-ray EDS (#7274 type, Britain Oxford Instruments) is used to deter-mine the contents of each element in deposited film. A series of samples is prepared when the current density changes from 1000to 2000 A·m-2. The contents of pure metal Nd in nanowire array increase along with the rising of current density, but then reduce. The results are shown in Fig.1.
Fig.1 Influence of current density on contents of RE Nd nanowire
(with pH at 2 and deposition duration of 1800 s)
When the current density changes from 1000 to 2000 A·m -2, it can be found through SEM observation that the nanowires are very sparse at low current density (such as 1000 A·m -2), and flow over the template at high current density (such as 2000 A·m -2) with laid wires in a mass. The nanowires are or-derly only at the current density of 1800 A·m -2. It can be at-tributed to the fact that when the cathode current density is too low (such as 1000 A·m -2), the polarization of cathode is little, the deposited crystal grain is coarser, the deposition is sparse, and the contents of Nd is lower. With increasing of current density, the polarization of the cathode will be increased, and the deposited crystal will be refined and compact, so the con-tent of Nd will be higher. However, the cathode current den-sity can not be too high (such as 2000 A·m -2). If it exceeds the allowed maximum, as there are few metal ions around cathode, the nanowire will overflow and lodge, and a metal film like branch will be formed on its surface. Consequently, the con-tent of Nd will be reduced. As above mentioned, it is impor-tant to properly select the current density of 1500 A·m -2.
2.3 Influence of electrodeposition duration on the
morphology and composition of Nd nanowire
The current density is adjusted to 1500 A·m -2 under constant potential, and Nd nanowires are electrodeposited in car-bamide-NaBr-KBr-formamide system for 1000-2000 s. The X-ray EDS (7274 model and from Britain OXFORD INSTRUMENTS) was used to determine the content of each element in the deposited film. The results are shown in Fig.2. It can be seen from Fig.2 that the content of electrodeposi-tion metal Nd increases with the prolonging of electrodeposi-tion duration, and 1800 s is the limit value. If the electrode-position duration is continuously increased, the content of pure metal will not be increased since the concentration of ef-fective components in plating solution is reduced. And on contrary, the nanowire array will be oxidized more easily.
2.4 EDS analysis
Fig.2 Influence of electrodepositing time on contents of Nd
nanowire (with pH at 2, and current density of 1500 A·m -2)
The EDS was used to conduct qualitative and quantitative analysis of elements in the sample. The results are shown in Fig.3. At the current density of 1000 A·m -2 and the electrode-position for 1800 s, as nanowires are very sparse, the charac-teristic X-ray spectrum can not determine the content of Nd. When the current density is increased to 2000 A·m -2, and elec-trodeposition duration is 1800 s, since nanowires overflow from template and lodge, the deposited film will be rough, loose and more easily oxidized. But, only the contents of Nd and oxygen elements can be determined, while the content of Al can not be determined because of the coverage. At this time, the sediment is the mixture of the RE Nd and the neo-dymium oxide. The content of RE element Nd are 69.21 at%, or 49.07 wt%, as shown in Fig.3.
Only with the current density of 1500 A·m -2 and electrode-position duration of 1800 s, the nanowires electrodeposited in carbamide-NaBr-KBr-formamide system are orderly. The re-sults determined by characteristic X-ray spectrum are shown in Fig.4
It can be seen from Fig.4 that after the deduction of Al 2O 3 base, the percentage content of SS Nd is 96.19 at % and 91.54 at wt %.
2.5 Observation of PAA template and morphology of
nanowire array
Fig.5 shows the morphology of top surface of PAA tem-plate, which was prepared from an aluminum sheet of high purity through a two-step anodization process, without peeling the film and corrosion of its barrier layer by the method of electrolysis. It can be seen that the nano apertures on PAA template is approximately 60 nm, and the distance between pores are approximately 20 nm. These nanopores are orderly and evenly distributed, with similar diameters and hexago-nal-shaped mouth.
When the current density is 1500 A·m -2 and electrodeposi-tion duration is 1800 s, the nanowires prepared by electrode-position in carbamide-NaBr-KBr-formamide system are or-derly, as shown in Fig.6.
800 1200 1600 2000 2400
Current Density/A·m -2
8580757065
60
ωN d /%
800 1200 1600 2000 2400
Electrodeposition Time/s
8580757065
60
ωN d /%
Fig.3 EDS analysis result of nanowire (under current density of
2000 A·m -2 and deposition duration of 1800 s)
Fig.4 EDS analysis result of nanowire (under current density of
1500 A·m -2 and deposition duration of 1800 s)
Fig.5 SEM image of PAA template
As the nanowires fill the nanopores of PAA template, it is necessary to disclose some parts of the template, in order to make nanowire exposed to the surface for observation. Fig.6a shows that nanowires fully fill the nanopores of PAA template, with higher order degree. Fig.6b shows that the diameter of the nanowire is 60-70 nm, and its dimension is almost the same as the aperture of prepared PAA template.
2.6 TEM analysis of morphology of nanowire array
Fig.7 is a TEM image of Nd nanowire array, prepared by electrodeposition for 1800 s under the current density of 1500 A·m -2. It can be seen that the average length of Nd nanowires are more than 1000 nm, and the average diameter is about 60 nm.
2.7 Determination of nanowire structure
Fig.6 SEM images of nanowire (under current density of 1500
A·m -2 and electrodeposition duration of 1800 s)
Fig.7 TEM image of Nd nanowires with the diameter of 60 nm
We conducted the XRD analysis after nanowires are coated with protective film of polyvinyl alcohol (see Fig.8). In XRD pattern, the diffraction peaks of three diffraction angles 2θ = 29.16°, 30.27°, 32.03° correspond to crystal planes of hexago-
Fig,8
XRD pattern of Nanowire
a
b
I n t e n s i t y /c p s
10 20 30 40 50 60 70
2θ/(o)
10000
80006000
40002000
102 101
004101
002
100
110 112
201
0 2 4 6 8 10
Energy/keV
I n t e n s i t y /a .u .
O
Nd
Nd Nd
Nd Nd Elements wt% at%
O 7.27 41.39 Nd 92.73 58.61
I n t e n s i t y /a .u .
0 2 4 6 8 10 Energy/keV
O
Al Nd
Nd Nd Nd
Nd Elements wt% at%
O 37.14 58.00 Al 41.33 38.27 Nd 21.53 3.73
nal Nd in [101], [004], [102], location and strength of peaks are consistent with 65-5015 card of PDF card. It shows that RE metal Nd is of hexagonal crystal phase. Diffraction angles in XRD pattern are 2θ=26.85°，29.78°, 30.77°, 47.42°, 57.02°, 57.59°, and they are consistent with the location of character-istic diffraction peaks in hexagonal crystal planes of [100], [002], [101], [110], [112], [201] for Nd2O3(PDF 06-0408), but with weaker strength.
3 Conclusions
1) In oxalic acid solution of 0.5 mol/L and under constant voltage of 40 V, the nanopores of the prepared alumina tem-plate are orderly and evenly distributed, with similar diameters and hexagonal-shaped mouth, and the aperture is approxi-mately 60-70 nm.
2) Under electrolytic corrosion for 15 min with voltage of –1.8 V and at temperature of 10 °C, the removal of barrier layer is in good condition, the aperture of bottom surface after removal of barrier layer is almost the same as that of top sur-face before removal, and partially excessive corrosion does not appear.
3) The average diameter of nanowires is about 60 nanome-ters, which is almost the same as the aperture of PAA template prepared by secondary oxidation. The nanowires contain Nd and Nd2O3, in which SS Nd occupies 91.54% of total mass in deposit, and the atomic percentage is 96.19%. The deposit is composed of hexagonal SS Nd and hexagonal Nd2O3. Nd ox-ide do exist in deposit, and the method of coating polyvinyl alcohol on nanowire template for preventing oxidation needs to be improved.
References
1Whitney T M, Jiang J S, Searson P C. Science[J], 1993, 261:
1316
2Tie Chenyang(铁晨阳), Bao Jianchun(包建春), Xu Zheng(徐正). Materials Research Journal(材料研究学报)[J], 2002, 16(5): 463
3Zuo Juan(左鹃), Sun Lan(孙览), Lin Changjian(林昌健).
Electronic Components and Materials(电子元件与材料)[J], 2003, 22(12): 32
4Lei Shuhua(雷淑华), Lin Jian(林健), Huang Wenchan(黄文旵) et al. Materials Review(材料导报)[J], 2007, 21(1): 102
5Lin Jiansheng(林建生), Chen Yanan(陈亚男), Xia Zhu(夏铸).
Journal of Rare Earth(稀土)[J], 1998, 19(1): 25
6Liu Zhongxing(刘中兴), Zhang Hongguang(张宏光), Wu Yongfu(伍永福) et al. Nonferrous Metals(Extractive Metal-lurgy)(有色金属冶炼部分)[J], 2007(6): 28
7Xu Dongsheng, Chen Dapeng, Xu Yajie et al. Pure Appl Chem[J] 2000, 72(1-2): 127
8Schonenberger C, Vander Zande B M I, Fokkink L G J et al. J Phys Chem B[J], 1997, 101(28): 5497
9Xu Guorong(徐国荣), Ren Fenglian(任凤莲), Si Shihui(司士辉) et al.Acta Physico-Chimica Sinica(物理化学学报)[J], 2006, 22(3): 341
10Xu Guorong(徐国荣), Ren Fenglian(任凤莲), Si Shihui(司士辉) et al. Applied Chemistry(应用化学)[J], 2006, 23(8): 830
11Linus Pauling. Translated by Lu Jiaxi(卢嘉锡). The Nature of the Chemical Bond(3rd edition)(化学键的本质)[M]. Shanghai: Sci-ence Press of Shanghai, 1981
12Wang Yu(王宇), Liu Guankun(刘冠昆), Tong Yexiang(童叶翔).
Journal of the Chinese Rare Earth Society(中国稀土学报)[J], 2001, 19(1): 54
13Wang Yu(王宇), Liu Guankun(刘冠昆), He Fengrong(何凤荣) et al. Acta Physico-Chimica Sinica(物理化学学报)[J], 2001, 17(3): 206
无阻挡层模板法直流电沉积稀土单质Nd纳米线阵列
龚晓钟, 汤皎宁, 徐松志, 田鹏
(深圳大学，广东深圳 518060)
摘要：在草酸电解液中用二次阳极氧化法制备多孔阳极化氧化铝(PAA)模板。通过对各种工艺参数(如电流密度、氧化时间、电解液浓度等)的调整，最终获得孔洞分布均匀、孔径基本一致、孔口呈六边形的PAA模板。同时在不剥离膜的情况下去除阻挡层，使原残留的铝成为基底，起电极作用。在非水体系中，用模板直流电沉积成功地制备了稀土单质Nd纳米线。经过SEM观测，PAA模板孔径在60 nm 左右，去阻挡层前后变化不大，制备的Nd纳米线排列有序，尺寸一致；EDS测定表明纳米线为单质Nd及少量Nd2O3纳米线，单质Nd含量为92.73wt%；XRD分析图谱显示所得到的单质Nd及Nd2O3纳米线都为六方结构。
关键词：金属材料；纳米线；稀土单质钕；非水体系；电沉积；阻挡层；氧化铝模板
作者简介：龚晓钟，女，1956年生，硕士，深圳大学化学与化工学院，深圳市功能高分子重点实验室，广东深圳 518060，电话：0755-********，E-mail: hxgxz@https://www.doczj.com/doc/3718370683.html,; 通讯作者: 汤皎宁, 教授，电话：0755-********