Formation of Nanoporous Nickel by Selective Anodic Etching of the

Jan 15, 2008 - By taking advantage of nickel passivation, selective anodic etching of copper from the Ni−Cu alloy produces nanohollow nickel tubes o...
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J. Phys. Chem. C 2008, 112, 1371-1376

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Formation of Nanoporous Nickel by Selective Anodic Etching of the Nobler Copper Component from Electrodeposited Nickel-Copper Alloys Jeng-Kuei Chang,*,† Shih-Hsun Hsu,‡ I-Wen Sun,*,‡ and Wen-Ta Tsai† Department of Materials Science and Engineering, National Cheng Kung UniVersity, Tainan, Taiwan, and Department of Chemistry, National Cheng Kung UniVersity, Tainan, Taiwan ReceiVed: September 10, 2007; In Final Form: NoVember 2, 2007

The preparation of nanoporous nickel films by electrochemical deposition of Ni-Cu alloy followed by the selective anodic etching of the less-active component (Cu) from the alloy was studied in an aqueous solution containing Cu(II) and Ni(II) at room temperature. Constant potential electrodeposition produced columnar Ni-Cu alloys, in which the Ni content increased as the deposition potential became more negative. X-ray diffraction and Auger mapping results indicate the presence of separated Cu-rich and Ni-rich phases in the alloys, with the Cu-rich phase being more concentrated in the middle of the column and surrounded by the Ni-rich phase. Cyclic voltammetric data indicates that anodic dissolution of nickel is retarded by passivation. By taking advantage of nickel passivation, selective anodic etching of copper from the Ni-Cu alloy produces nanohollow nickel tubes on indium-tin-oxide-coated glass substrates. The nanohollow tube structure obtained in this study is different from the interconnected bicontinuous nanopores that are usually obtained by dealloying the less noble component from a homogeneous solid solution alloy. The nanohollow tubes may have resulted from the fact that multiple phases columnar alloy deposits were produced by the electrodeposition step and from the limited mobility of nickel during the anodic etching step.

Introduction Nanoporous metal structures can be formed by a process known as dealloying, in which the less noble component A in a binary AxB1-x alloy is selectively removed either chemically or electrochemically. The preferential removal of A results in the formation of a porous structure of the less reactive component B. Numerous examples1-16 of dealloying have been published in the literature, and the mechanism which leads to the formation of pores through dealloying has also been proposed. The final porous structures formed by dealloying depend on the dissolution rate of A and the surface diffusivity of B. Generally, an interconnected bicontinuous network of pores is obtained. Porous nickel with a high surface area has been prepared from dealloying NiAl and NiZn alloys as a material for various applications.17-20 In a recent example, porous nickel was prepared by the electrochemical formation of NiZn surface alloys followed by subsequent dealloying in zinc chloride-alkali chloride molten salts at 350-450 °C.19 It was believed that the high temperature was required to enhance the surface diffusion of nickel in order to obtain the desired interconnected porous structure. While most of the studies have focused on the selective dissolution of the less noble element from a homogeneous binary solid solution alloy, Searson and co-workers21 recently demonstrated for the first time that, by taking advantage of the formation of a passive nickel oxide film in sulfamate aqueous solutions, nanoporous nickel could be prepared through selective electrochemical dissolution of the more noble copper rather than the less noble Ni from electrodeposited homogeneous Ni-Cu alloys. * To whom correspondence should be addressed. Phone: +886-62757575 ext. 62942 (J.-K.C.); +886-6-2757575 ext. 65355 (I.-W.S.). Fax: +886-6-2754395 (J.-K.C.). E-mail: [email protected] (J.K.C.); [email protected] (I.-W.S.). † Department of Materials Science and Engineering, National Cheng Kung University. ‡ Department of Chemistry, National Cheng Kung University.

In most studies, dealloying was performed with homogeneous single-phase binary metal solid solutions. Although electrodeposited phase-separated alloys were observed in the work of Searson,21 selective dissolution of such phase separated alloys was not investigated. For a multiple-phase alloy, selective dissolution may occur only at the A-rich phase, and porous structures different from those obtained by dealloying a singlephase alloy may be expected. This paper describes the formation of nanoporous nickel by electrodepositing Ni-Cu alloy films in a solution containing NiSO4 and CuSO4 followed by the selective etching of the less reactive copper from the alloys. Data from scanning electron microscopy (SEM), X-ray diffraction (XRD), and Auger mapping experiments indicate that the Ni-Cu alloys prepared in this work are columnar and may contain separated Cu- and Nirich multiple phases. Selective electrochemical etching of copper from the Ni-Cu alloys resulted in nanohollow nickel tube structures rather than the three-dimensional network of interconnected pores that were generally obtained in previous reports. Experimental Procedures The Ni-Cu alloy films were electrodeposited from a plating solution containing 1 M NiSO4, 0.01 M CuSO4, and 0.5 M H3BO3 (pH ) 4). The deposition process was performed at 25 °C in a three-electrode cell with a platinum counter electrode and a saturated calomel reference electrode (SCE, +0.241 V vs SHE). An indium-tin-oxide (ITO) coated glass with an exposed area of 1.4 cm2 was assembled as the working electrode. The alloy films were deposited under a constant potential condition, and the total cathodic passed charge was controlled to be 1 C. Selective dissolution of Cu from the alloy was then conducted in the same solution by applying an anodic potential of 0.5 V. The morphologies and chemical compositions of the samples were examined with a SEM (Philip XL-40 FEG) and auxiliary X-ray energy dispersive spectroscopy (EDS). XRD analyses were performed with a Shimadzu XRD-7000S dif-

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Figure 2. Ni content vs deposition potential for the Ni-Cu alloy films.

Figure 1. Cyclic voltammogram for an ITO electrode in solutions containing 0.5 M H3BO3 and (a) 1 M NiSO4, (b) and (c) 1 M NiSO4 + 0.01 M CuSO4 with a potential scan rate of 10 mV s-1.

fractometer to explore the crystal structure. Cu KR radiation with a wavelength of 1.5418 Å was used as the X-ray source. The detected diffraction angle (2θ) was scanned from 20 to 80° with a speed of 1° per minute. Auger mappings were recorded with a JEOL JAMP-9500F Auger electron spectrometer with a field emission electron gun. The probe diameter for Auger analyses was 8 nm. A base pressure of approximately 5 × 10-8 Pa in the AES main chamber was maintained during the operation. Results and Discussion Cyclic Voltammetry. Cyclic voltammetry was performed to understand the electrochemical behavior of Ni(II) and Cu(II) species on the ITO-coated glass electrode. The cyclic voltam-

Figure 3. SEM micrographs of the as-deposited Ni-Cu alloy films obtained by constant potential electrodeposition at: (a) -0.70 V, (b) -0.85 V, and (c) -1.0 V.

mogram recorded at an ITO electrode for a solution containing nickel is presented in Figure 1a. On the cathodic potential sweep,

Formation of Nanoporous Nickel

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Figure 5. XRD patterns of the Ni-Cu alloy films electrodeposited at different potentials.

Figure 4. Cross-section SEM micrographs of (a) an undeposited ITOcoated glass substrate and (b) a Ni-Cu deposited ITO-coated glass sample prepared by electrodeposition at -0.85 V for 1 C charge.

the reduction peak c1 of Ni(II) starts slowly at about -0.7 V and then increases rapidly at about -0.97 V. Upon scan reversal, a current loop that is usually associated with an overpotential driven nucleation process was observed prior to a small nickel anodic stripping peak a1 at about -0.3 V. The integrated charge under the anodic peak, Qa, is much smaller than that integrated under the cathodic peak, Qc, i.e., the ratio of Qa/Qc ) 0.34, indicating that passivation of Ni occurs during the anodic scan. Figure 1b shows the cyclic voltammogram of a solution containing 1 M NiSO4 and 0.01 M CuSO4 with a cathodic scan limit of -0.5 V. Only a single peak corresponding to the reduction peak c2 of Cu(II) to copper metal is seen at about -0.3 V and an anodic peak a2, due to the stripping of the deposited copper to Cu(II), is seen at 0.2 V during the reverse scan. This result is inconsistent with previous studies on the electrochemical reduction of Cu(II) in aqueous solutions.21,22 In the previous studies, copper was deposited through consecutive reductions of Cu(II) to Cu(I) and Cu(I) to copper. It is noticed that the charge integrated under the anodic peak is very close to that integrated under the cathodic peak (Qa/Qc ) 0.90), suggesting that the deposited Cu can be efficiently stripped off ITO. Figure 1c shows the cyclic voltammogram of the same solution as that in Figure 1b, but the cathodic scan limit is extended to -1.0 V; the deposition of Ni also occurs. In the

reverse scan of Figure 1c, significant changes can be observed. First, the Ni oxidation peak shown in Figure 1a almost disappears. Second, a new anodic wave a3 appears, which overlays the anodic stripping peak a2 of the copper deposit that was seen in Figure 1b. These results clearly indicate the formation of Ni-Cu alloy films on the ITO surface by electrodeposition; the anodic peak a2 at less positive potential can be attributed to the stripping of pure Cu. Because of the passivation of nickel, the stripping of copper from the Ni-Cu alloy is retarded and shifts to a more positive potential (peak a3). The total charge integrated under the anodic peaks is much smaller than that integrated under the cathodic peaks (Qa/Qc ) 0.46), suggesting that selective anodic stripping of Cu off the Ni-Cu alloys is possible. Electrodeposition and Characterization of Ni-Cu Alloy Films. Films of Ni-Cu alloy on ITO-coated glass substrates were grown by electrodeposition from solutions containing 1 M NiSO4, 0.01 M CuSO4, and 0.5 M H3BO3 (pH ) 4) at potentials under -0.7 V. The composition of Ni-Cu films was determined by EDS analysis and is plotted vs the deposition potential in Figure 2. The atomic percentage of Ni increases from about 20 at % at -0.7 V to a plateau value of about 90 at % at -1.1 V. This occurs because copper deposition is diffusion limited at these potentials so that the rate of copper deposition is fixed, whereas the nickel deposition rate increases as the deposition potential changes from the charge-transfer-limited region to the diffusion-limited region, and hence the nickel content in the deposited alloy film increases as the deposition potential becomes more negative. The copper content in the deposited film is higher than 50 at % when the deposition potential is less negative than -0.85 V. Previous studies have shown that to prepare a better porous structure by dealloying, the component being etched should generally be near or higher than 50 at %. This means that -0.85 V is an important deposition potential. The as-deposited surface morphologies of the electrodeposits were examined by SEM; some typical planeview micrographs are shown in Figure 3. As can be seen from the images, the morphology of the electrodeposits is dependent on the deposition potential. Figure 3a reveals that electrolysis at -0.70 V resulted in boulder deposits that clustered into protruding columnar structures. Figure 3b shows that when the deposition potential is lowered to -0.85 V, the number of columnar structures increases while their sizes decrease. This

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Figure 7. Current-time curves during the selective dissolution of a Ni-Cu alloy film electrodeposited at -0.85 V for 1 C charge.

Figure 6. Auger mappings of the Ni and Cu elements, respectively, in a Ni-Cu alloy film electrodeposited at -0.85 V.

is probably due to the increased nucleation density. Figure 3c indicates that the deposits become even denser when the deposition potential is further lowered to -1.0 V, where the deposition of nickel approaches the diffusion-limited region. The columnar structure of the electrodeposited Ni-Cu film is further illustrated in Figure 4 by the cross-section SEM micrograph of a sample prepared by electrolysis at -0.85 V for 1 C charge together with that of an undeposited ITO-coated glass substrate. This figure shows that, during the deposition, new particles overlaid on previous particles and grew into the columnar

structure. The fact that not all the columnar deposits have the same height and diameter implies that new nuclei were formed during the deposition. XRD patterns for Ni-Cu electrodeposits prepared at different potentials are shown in Figure 5. Both Ni and Cu have the facecentered cubic structure, and solid solutions were expected for Ni-Cu alloys. Figure 5 shows that the Ni-Cu alloy with a high Cu content (80 at %) prepared at -0.70 V exhibits a diffraction peak near the position corresponding to Cu(111), indicating the formation of a solid solution. However, it is interesting to note that the electrodeposit with a nickel content of about 40 at % obtained at -0.80 V exhibits significant peak splitting (or multiple peaks) in the diffraction pattern, indicating phase separation and hence the formation of inhomogeneous alloys that contain segregated copper-rich and nickel-rich phases. As shown in this figure, the reflection intensity due to the nickelrich phase in the deposited film increases as the deposition potential becomes more negative than -0.85 V. When the deposition potential is further changed to -1.0 V to produce Ni-Cu having high Ni content (80 at %), the multiple peaks merge into one, which further shifts to the position of Ni(111). As mention before, phase separation similar to that in Figure 5 was also observed by Searson21 but was not studied in detail. The segregated codeposition of Ni-Cu observed in Figure 5 was further studied using Auger electron spectroscopy. The Auger mappings of the Ni and Cu elements, respectively, in a Ni-Cu film electrodeposited at -0.85 V are shown in Figure 6. In this figure, the presence of Cu is illustrated with increasing concentration from the yellow region to the orange region, and the presence of Ni is illustrated by the light blue region. As illustrated in Figures 3 and 4, the film consists of columnar deposits. Figure 6 indicates that Cu is mostly concentrated within the central part of each columnar deposit, while Ni is distributed around the columnar structure. This figure clearly confirms the phase segregation that was observed by XRD patterns shown in Figure 5. The phase-segregated deposition of Ni-Cu is interesting and, as will be shown in the following section, determines the resulting porous structure after Cu is selectively etched from the alloy. Formation of Porous Nickel by Electrochemical Dissolution of Copper from the Ni-Cu Deposit. To study the formation of porous Ni, Ni-Cu films were electrodeposited by passing 1 C of charge at a constant potential of -0.85 V followed by anodic dissolution at 0.50 V to selectively remove Cu from the Ni-Cu film. Figure 7 shows a typical current vs time curve recorded for this dissolution experiment. This figure shows that the dissolution current stays high and relatively

Formation of Nanoporous Nickel

Figure 8. The (111) diffraction peak of Ni-Cu alloy films that were electrodeposited at -0.85 V with 1 C of charge (curve a) followed by selective dissolution at 0.50 V for 0.1 C (curve b), 0.3 C (curve c), and 0.5 C (curve d).

constant until 0.3 C of charge is passed, suggesting that during this period the rates of creation and removal of the active dissolution sites are approximately equal. When most of the copper atoms in the alloy film have been removed, the dissolution current decays quickly beyond 0.3 C and gradually flattens out when 0.5 C is passed. Because the total charge employed for the electrodeposition of Ni-Cu alloy film was 1 C and the Cu/Ni content ratio in this alloy was approximately 1/1, it was expected that approximately all the Cu would be etched when 0.5 C of anodic charge would be passed. Interestingly, the current did not reach zero at 0.5 C, indicating that not only Cu but also Ni is dissolved during the anodic

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1375 dissolution step. The dissolution rate of Ni is, however, fairly slow, as indicated by the low current. XRD patterns were recorded after the anodic dissolution of the Ni-Cu films under the same conditions as those in Figure 7. Figure 8 shows that the (111) diffraction peak of the Ni-Cu alloy film changed with different amounts of anodic passed charge. As shown in this figure, the peak intensity of the Curich phase in the Ni-Cu film decreases rapidly with increasing anodic charge, and the peak shifts to the position for pure nickel. Moreover, the peak intensity for Ni(111) also decreases somewhat, implying that Ni may be partially dissolved. The surface microstructure of the Ni-Cu film at different stages during the selective dissolution process was examined with SEM. The evolution of the porous nickel surface is illustrated by the SEM images shown in Figure 9 for samples prepared in the same manner as those for Figure 8. As illustrated in Figure 6, the Cu-rich alloy phase sits in the central part of the columnar Ni-Cu electrodeposit and is surrounded by the Ni-rich phase. Because of the passivation of nickel, the formation of a porous nickel surface starts with the selective removal of copper atoms from the central tip of the columnar Ni-Cu deposits where the Cu-rich alloy phase is located, leading to the formation of tiny pits on the tip (Figure 9a). As the pits were formed, more of the unetched Ni-Cu surface was exposed to the electrolyte, providing more surface area for further dissolution of copper. As the copper atoms in the columnar deposits were continuously removed, the pits grew in size to turn into pores (Figure 9b), and the diameter and depth of the pores increased with increasing etching charge, eventually resulting in a porous nickel film formed with hollow tubes 50200 nm in diameter (Figure 9c). A magnified SEM micrograph (Figure 9d) reveals that the wall thickness of the hollow tubes is about 20 nm.

Figure 9. SEM micrographs of Ni-Cu alloy films that were electrodeposited at -0.85 V with 1 C of charge followed by selective dissolution at 0.50 V for (a) 0.1 C, (b) 0.3 C, and (c) 0.5 C. (d) Magnified micrograph of (c).

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Chang et al. containing Cu(II) and Ni(II) followed by the selective anodic etching of the less-active component from the alloy in the same solution was studied at room temperature. Cyclic voltammetric data indicated that copper is thermodynamically more stable than nickel. However, anodic dissolution of nickel is retarded by passivation and selective anodic dissolution of the nobler component copper occurs instead. Constant potential electrodeposition produced columnar Ni-Cu alloys. The Ni content in the alloy increased as the deposition potential became more negative. XRD and Auger mapping results indicated the presence of separated Cu-rich and Ni-rich phases in the alloys. The Cu-rich phase was more concentrated in the middle of each columnar deposit and was surrounded by the Ni-rich phase. By taking advantage of nickel passivation, selective anodic etching of copper from the Ni-Cu alloy produces nanohollow tubes on the ITO-coated glass substrate. The nanohollow tube structure obtained in this study is different from the interconnected bicontinuous nanopores that are usually obtained by dealloying the less noble component from a homogeneous solid solution alloy. The resulting nanohollow tubes may be ascribed to the fact that multiple phases columnar alloy deposits were produced in the electrodeposition step and to the limited mobility of nickel during the anodic etching step. The nanohollow tube structure may be beneficial in applications such as electrocatalysis and supercapacitors, in which diffusion of reactants to the porous material is important. Acknowledgment. This work was supported by the National Science Council of the Republic of China, Taiwan (96-2113M-006-015-MY3). References and Notes

Figure 10. SEM micrographs of Ni-Cu alloy films that were electrodeposited at (a) -0.70 V, (b) -0.85 V, and (c) -1.0 V followed by selective dissolution at 0.50 V.

The effect of the deposition potential on the morphology is illustrated in Figure 10, which shows the plane-view SEM micrographs for Ni-Cu films after complete electrochemical etching at 0.5 V. Figure 10a shows that etching a Ni-Cu film deposited at -0.70 V (Cu content ) 80 at %) results in the formation of pores 200-300 nm in diameter, but the density of the pores is small. Figure 10b shows that the density of the columnar pores increased with the expense of the pore diameter when the Ni-Cu film was deposited at -0.85 V (Cu at % ) 50) and etched. Figure 10c shows that etching the Ni-Cu film deposited at -1.0 V (Cu at % ) 20) results in carrotlike deposits with tiny holes on their central parts. In comparison of Figure 10 and Figure 3, it is apparent that the morphology of the etched films is dependent on the original morphology of the alloys which was controlled by the deposition potential. Summary and Conclusions The preparation of nanoporous nickel films by electrochemical deposition of Ni-Cu alloy from an aqueous solution

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