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J. Phys. Chem. B 2005, 109, 23300-23303
Electrochemical Growth of Nickel Hollow Nanostructures on Copper Substrates Gao-Ren Li,*,† Lin-Gang Kay,‡ Guan-Kun Liu,† and Ye-Xiang Tong*,† School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China, and College of Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45219 ReceiVed: September 2, 2005; In Final Form: October 11, 2005
We have shown hollow Ni nanonodules with outer diameters of 80-200 nm and wall thicknesses of 5-25 nm could be prepared by electrochemical deposition in the NiCl2 + dimethyl sulfoxide + C2H4O3 + H2O system, and the products were high purity. In particular, Ni hollow nanonodule structures or highly assembling Ni hollow nanostructures can be selected by varying the compositions of the solvent mixture. Apart from the hollow grain size, the wall thickness can also easily be controlled by varying the electrochemical parameters, salt concentration, and deposition time. The typical coercivity of Ni hollow nanostructures with particle sizes of about 100-150 nm was much bigger than that of the bulk Ni.
1. Introduction Among many specific morphologies, hollow nanoscale structures have attracted more and more attention in recent years because of their promising properties for scale-dependent applications such as drug delivery, robust catalysts, photonic devices, surface functionalization, and contaminated waste removal materials.1-7 So far there have been two basic templating methods, with either soft or hard templates, for obtaining hollow nanoscale structures. Such efforts in the above methods include the use of colloid particles, fibers, organogelators, sacrificial cores, ionic organic surfactants, or nonionic polymeric surfactants.1-16 The difficulties in these methods lie in the control of the fabrication process, as well as of the homogeneity and thickness of the coating. Also all of these methods are laborintensive processes. For example, to create hollow centers, dissolution of the templated cores has to be done. To overcome the complexities associated with templating methods, a lot of template-free methods, such as self-assembly, for the preparation of hollow nanoscale structures have been developed recently, in which large quantities of inorganic additives such as NH4OH, (NH4)2SO4, KCl, or an ionic liquid, are involved.17-20 However, in these methods, it is hard to avoid inclusion of additive impurities and is difficult to control the diameter size and wall thickness of the hollow nanoscale spheres. Considering the disadvantages of these above-mentioned methods, much of our attention has been paid to the electrochemical methods for the preparation of hollow nanoscale structures. Electrochemistry is a powerful tool for making hollow materials because the grain size and wall thickness can be easily controlled by varying the electrochemical parameters such as current density, overvoltage, bath composition, temperature, and electrodepositon time. To the best of our knowledge, here for the first time we report an electrochemical method by which Ni hollow nanoscale structures can be prepared, and the challenge has been met with limited success. 2. Experimental Section In our experiments, a simple three-electrode cell was used. The working electrode used in cyclic voltammogram (CV) was † ‡
Sun Yat-Sen University. University of Cincinnati.
Figure 1. Cyclic voltammogram (CV) of Pt electrode in the 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O system at 290 K.
the Pt (99.99%, 1.0 cm2). The Pt foil with 2.6 cm2 was used as an auxiliary electrode. A saturated Calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system. The deposition solution consisted of dimethyl sulfoxide (DMSO), hydroxyctic acid (C2H4O3), NiCl2, and water. The electrochemical depositions were carried out on Cu substrates under galvanostatic conditions with current densities between 1 and 50 µA/cm2. The cyclic voltammetry and electrochemical deposition experiments were done with a Zahner Elektrik IM6e electrochemical workstation. All the experiments were carried out at room temperature. The deposits were characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID). 3. Results and Discussion The cyclic voltammogram (CV) of the 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O system on Pt electrode at 290 K is shown in Figure 1. It shows the cathodic reduction peak of Ni2+ at approximately 0.76 V, as well as Ni anodic stripping peak at +0.30 V. According to the standard electrode potential of Ni2+ in aqueous solution (0.25 V), we concluded that the Ni2+ was not dissociative in the NiCl2 +
10.1021/jp0549879 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/19/2005
Ni Hollow Nanostructures on Cu Substrates
J. Phys. Chem. B, Vol. 109, No. 49, 2005 23301
Figure 2. (a) SEM and (b) TEM images of the hollow nodulelike structures prepared on Cu electrode in 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O with current density of the 20 µA/cm2 system at 290 K.
C2H4O3 + DMSO + H2O system and formed complexes with C2H4O3 and DMSO, respectively. Hydroxyctic acid (C2H4O3) is a monodentate ligand, and it can ligate nickel atoms to form coordinate bonds through the oxygen atoms in OdC bonds. At the same time, the C2H4O3 molecules can aggregate to form large complex micelles by hydrogen bonds. This was proved by the cyclic voltammogram of 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL H2O shown in Figure 1, from which we did not see the cathodic reduction peak of H+ ionizing from hydroxyctic acid. Also dimethyl sulfoxide is a monodentate ligand, and it can ligate nickel atoms to form complexes by bonding reactions through the oxygen atoms in OdS bonds. Electrochemical deposition was carried out on Cu substrates in the 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O system under galvanostatic condition with current density of 20 µA/cm2. Surface morphology of the obtained thin film was characterized by field-emission scanning electron microscopy (FE-SEM). As seen from Figure 2a, the particles in the prepared sample showed long and interconnected nodulelike structures. At the same time we can observe that the nodulelike structures were composed of nanonodules with small holes on the surface. The sizes of the nanonodules were about 100-150 nm. The diameters of the small holes were about 1020 nm. Because the FE-SEM image could not reveal the inner structures, the transmission electron microscopy (TEM) image of the sample was recorded to reveal the actual inner structures of the nanonodules, as shown in Figure 2b. The contrast between the edge and the center shows the nanonodules were hollow. The wall thicknesses of the nanonodules were about 15 nm. However, the inner nanonodules did not communicate with each other; namely, there was a wall between every two inner nanonodules. The typical XRD patterns of the prepared product were shown in Figure 3. All peaks can be indexed as face-centered cubic (fcc) Ni(111), Ni(200), and Ni(220): in good accordance with JCPDS (Joint Committee of Powder Diffraction Standards). No peaks of any other impurities were detected. In addition, the relative intensities in XRD suggest texturing along a (111) direction. It has been known for a long time that the peak width of XRD can be used in the Scherrer equation to determine the average grain size.21 For this sample, we obtained an average grain size of about 150 nm. It was found from energy dispersive spectroscopy (EDS) elemental analysis that the hollow nanostructures were almost composed of nickel (99.9 at. %), indicating the high purity of the shell. Also, a little sulfur that comes from DMSO was found in the EDS analysis, meaning a little DMSO, qua solvent, exists inside the nickel hollow structure. Hydroxyctic acid, qua solvent, also existed inside the Ni hollow nanostructure, although the atoms of hydrogen and carbon cannot be found by normal EDS. Because the prepared
Figure 3. XRD patterns of hollow Ni nanostructures synthesized in 3 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O with 20 µA/cm2. The high intensity of the (111) peak shows texturing along the (111) direction.
Ni hollow nanostructure had a small hole on the surface, hydroxyctic acid can freely pass in and out via this hole in solution. To learn more about the formation of the hollow Ni structures, various synthetic conditions were tested. The wall thickness of Ni nanonodules could be controlled precisely by varying the electrochemical parameters and salt concentration. The wall thickness increased with salt concentration and current density. For example, a uniform wall thickness of about 10 nm can be prepared in the 1 µM NiCl2 + 10 mL of DMSO + 5 mL of C2H4O3 + 15 mL of H2O solution with a current density of 10 µA/cm2. However, the wall thickness can be increased to 15 nm when the NiCl2 concentration was 3 µM and the current density was 20 µA/cm2, as shown in Figure 2b. An inverse electrochemical parameters and salt concentration dependencies were observed for the outer diameters. For example, 100-150 nm of outer diameters in Figure 2a could be reduced to 50120 nm when the NiCl2 concentration was increased to 5 µM and the current density was increased to 30 µA/cm2. This may be because there were more hollow Ni nanoparticles assembling together in a shorter time as the Ni2+ deposition rate increased. However, the hollow nanoparticles would deteriorate when the current density was increased to 100 µA/cm2 or larger. The formation of surface morphologies of the prepared samples was sensitive to the compositions of the solvent mixture: when the experiment was carried out in the absence of C2H4O3 and deposition current density was 30 µA/cm2, solid Ni nanospheres were obtained as shown in Figure 4a. When C2H4O3 content and DMSO content were in a ratio of 1:2, a long Ni hollow nanonodule structures were obtained as show in Figure 2a. Half-assembling hollow nanosphere structures with holes on the surface could be fabricated when hydroxyctic acid content and DMSO content were in a ratio of 1:1, as shown in
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Figure 4. FE-SEM images of the surface structures electrodeposited in (a) 5 µM NiCl2 + 10 mL of DMSO + 15 mL of H2O, (b) 5 µM NiCl2 + 10 mL of DMSO + 10 mL of C2H4O3 + 15 mL of H2O, and (c) 5 µM NiCl2 + 5 mL of DMSO + 15 mL of C2H4O3 + 15 mL of H2O with a current density of 30 µA/cm2.
Figure 4b, and the nodulelike morphology will disappear. Highly assembling hollow nanosphere structures with holes on the surface could be fabricated with outer diameters of 80-100 nm and wall thicknesses of 5-10 nm when hydroxyctic acid content and DMSO content were in a ratio of 3:1, as shown in Figure 4c. Both the outer diameters and wall thicknesses of hollow nanostructures with holes on the surface could also be tuned by varying the aging time. For example, highly assembling hollow nanostructures with outer diameters of 100 nm could be synthesized when the deposition time was 6 h. A possible formation mechanism of the hollow nickel nanostructures with small holes on the surface was illustrated as follows. C2H4O3 can aggregate to form spherical micelles by hydrogen bonds, and DMSO cannot. After the initial nucleation, the newly formed Ni nuclei grow into nanoparticles, and then the newly formed nanoparticles have a tendency to aggregate at the interface of the spherical micelles and the substrate. Ni2+ was continuously electrodeposited to surround the complex micelle aggregates, which was considerably accelerated later and resulted in the formation of hollow Ni structures. With aging, Ni2+ complexes inside of the hollow structures were continuously electrodeposited on the inner surface to cause the concentrations of Ni2+ complexes inside to be lower than those outside as the Ni2+ complexes inside were limited. When the concentration differences between the inside and outside were very big, the spherical micelles would be broken and the complexes outside of the hollow structures gradually diffused inward, resulting in the formation of the small holes on the surface of hollow Ni nanostructures. The mixture of C2H4O3, DMSO, and water used as solvent could effectively affect the formation of the spherical micelles that were important for the formation of hollow spheres. When the experiment was carried out in higher concentration of C2H4O3, the number of the formed spherical micelles was more in solution, and we could get higher assembling hollow nanosphere structures. When the experiment was carried out in the absence of C2H4O3, the correspondingly spherical micelle aggregates could not be formed in solution as DMSO cannot form a hydrogen bond. In other words, the hollow Ni spherical nanostructures could not be obtained when electrodeposition was carried out in the absence of hydroxyctic acid. This conclusion was in accordance with the above experimental results that we only obtained solid Ni nanospheres when the electrodeposition was carried out in the 5 µM NiCl2 + 10 mL of DMSO + 15 mL of H2O solution. Thermal stability of the formed hollow nanostructures in our experiment was discussed. The inner and outer radii of hollow spherical structure were assumed r1 and r2, respectively. The chemical potentials of inner and outer surface atoms were
Figure 5. Hysteresis loop of the highly assembling Ni hollow nanostructures with particle sizes of about 100-150 nm.
assumed µ1 and µ2, respectively. The relationship between chemical potential (µ) and radii (r) is22
µ1{)µ0+2γΩ/(-r1)} < µ2{)µ0+2γΩ/r2}
(1)
The potential difference between the inner and outer surface atoms is
∆µ ) µ2 - µ1 ) 2γΩ(1/r2 + 1/r1)
(2)
where µ0 refers to the chemical potential of the atoms in the bulk material and γ refers to the interfacial energy per atom. The gradient atoms would gradually diffuse from the outer surface to the inner surface if the potential difference is very big, finally leading to an elimination of the void and to a solid nanosphere. So the hollow Ni nanostructures are more stable with inner and outer radii increasing. In our experiment the obtained hollow Ni nanostructures with outer diameters of 200 nm and wall thicknesses of 5 nm are more stable than those with outer diameters of 80 nm and wall thicknesses of 5 or 25 nm. Hysteresis loop measurements were performed on the various samples in an applied magnetic field ranging from 8000 to +8000 Oe at 200 K. The magnetic field applied was parallel to the (111) surface on the Ni hollow nanostructures. For each sample, the corresponding substrate contribution has been subtracted from the presented experimental magnetic data. The hysteresis loop of the obtained highly assembling Ni hollow nanostructures with particle sizes of about 100-150 nm in the magnetic field range of 340 to +340 Oe is shown in Figure 5. The typical coercivity was 43.73 Oe, which was enhanced compared to that of the bulk Ni (about 0.7 Oe (222.8 A m-1) for Ni).23 The saturation magnetization, Ms, of the highly
Ni Hollow Nanostructures on Cu Substrates
J. Phys. Chem. B, Vol. 109, No. 49, 2005 23303 electrochemical means and provided a novel procedure to prepare hollow nanoscale structures. Hollow Ni nanonodules with outer diameters of 80-200 nm and wall thicknesses of 5-25 nm have been prepared by electrochemical deposition in the NiCl2 + DMSO + C2H4O3 + H2O system, and the products were high purity. Apart from the hollow grain size, the wall thickness can also easily be controlled by varying the electrochemical parameters, salt concentration, and deposition time. It is expected that this electrochemical growth method can be extended to the synthesis of other hollow metal nanostructures. Acknowledgment. We appreciate that this work was supported by Natural Science Funds of Guangdong Province (Grant No. 031584) and the Science and Technology Development Project of Guangdong Province (Grant No. 2003C104030). References and Notes
Figure 6. (a) Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for the highly assembling Ni hollow nanostructures with particle sizes of about 100-150 nm. (b) Differential curve of ZFC.
assembling Ni hollow nanostructures was 16.56 emu g-1. The corresponding zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were shown in Figure 6a. A small field of 100 Oe was applied in our experiments. The differential curve of ZFC shown in Figure 6b indicates that the Curie temperature (Tc) was 579 K. The Tc value is much lower than that of bulk Ni (631 K). Figure 6b also shows the change in magnetization with temperature up to 750 K suggesting a long-range magnetic ordering transition at 579 K in these highly assembling Ni hollow nanostructures. 4. Conclusions In summary, we have shown for the first time that Ni hollow nanonodule structures or highly assembling Ni hollow nanostructures with holes on the surface could be prepared by
(1) (a) Mathlowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedl, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410. (b) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (2) Caruso, F.; Caruso, R.; Mo˜hwald, A. H. Science 1998, 282, 1111. (3) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622. (4) Sun, Y.; Xia, Y. Science 2002, 298, 2716. (5) Langer, R. Science 1990, 249, 1527. (6) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641. (7) Pathak, C. P.; Sawhney, A. S.; Hubbell, J. A. J. Am. Chem. Soc. 1992, 114, 8311. (8) Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (9) Caruso, R. A.; Schattka, J. H.; Greiner, A. AdV. Mater. 2001, 13, 1577. (10) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem. 2003, 115, 1987; Angew. Chem., Int. Ed. 2003, 42, 1943. (11) Blin, J. L.; LOonard, A.; Yuan, Z. Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem. 2003, 115, 2978; Angew. Chem., Int. Ed. 2003, 42, 2872. (12) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem. 2003, 115, 3135; Angew. Chem., Int. Ed. 2003, 42, 3027. (13) Collins, A.; Spickermann, M.; Mann, C. S. J. Mater. Chem. 2003, 13, 1112. (14) Zhu, J. J.; Xu, S.; Wang, H.; Zhu, J. M.; Chen, H. Y. AdV. Mater. 2003, 15, 156. (15) Yuan, Z. Y.; Ren, T. Z.; Su, B. L. AdV. Mater. 2003, 15, 1462. (16) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (17) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (18) Hah, H. J.; Kim, J. S.; Jeon, B. J.; Koo, S. M.; Lee, Y. E. Chem. Commun. 2003, 1712. (19) Guo, C. W.; Cao, Y.; Xie, S. H.; Dai, W. L.; Fan, K. N. Chem. Commun. 2003, 700. (20) Afanasiev, P.; Bezverkhy, I. J. Phys. Chem. B 2003, 107, 2678. (21) Scherrer, P. Goettinger Nachr. 1918, 2, 98-100. (22) Tu, K. N.; Go¨seleb, U. Appl. Phys. Lett. 2005, 86, 093111. (23) Chikazumi, S. Physics of Magnetism; John Wiley & Sons: New York, 1964.
