Article pubs.acs.org/JPCC
Passivation of Nickel Nanoneedles in Aqueous Solutions Bowei Zhang,† Junsheng Wu,‡ Xiaogang Li,‡ Hai Liu,† Boluo Yadian,† R. V. Ramanujan,† Kun Zhou,§ Renbing Wu,§ Shiji Hao,† and Yizhong Huang*,† †
School of Materials Science and Engineering, Nanyang Technological University, Singapore Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, China § School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore ‡
ABSTRACT: Passivation, a typical electrochemical behavior, involves the formation of a thin layer on the surface of a bulky metal component, which has been assumed to be native, dense, and highly protective to the metal from deterioration. In the present paper, nickel was chosen and prepared into a nanoneedle. A duplex layered passive film grown around the surface of Ni nanoneedle in the alkaline solution (0.1 M NaOH) was observed in TEM, which has a thickness of ∼4 nm and consists of NiO inner layer and Ni(OH)2 outer layer. However, the surface of Ni nanoneedle only appears a single inner NiO layer with a thickness of 3 nm after passivation in a strong acid solution (0.1 M H2SO4). This could be due to the sufficient H+ that inhibits the formation of Ni(OH)2 outer layer. There may be still a mono- or bimolecular Ni(OH)2 layer beyond the resolution of TEM. The ultrathin NiO inner layer covers the entire nickel nanoneedle and was determined to be crystalline and coherently oriented with Ni substrate.
1. INTRODUCTION Nickel, a very active metal, is known to have strong corrosion resistance to aggressive environments due to its easy passivation behavior. The microstructure of passive film grown on the bulky nickel surface has been sufficiently investigated.1−5 It is commonly agreed that the passive film consists of double layers (NiO inner layer and Ni(OH)2 outer layer) in both alkaline solution and acid solution. However, the relative amounts of NiO and Ni(OH)2 in passive film are still in controversy. One view is that Ni(OH)2 is a monolayer by absorption6−8 while the other view is that a complete hydroxide layer exists which is developed on nickel oxide.9−11 In addition, the crystallographic order (i.e., crystalline structure), disorder (i.e., amorphous structure), or mixture of passive film has been still controversial. P. Marcus et al. have reported that nickel passive film of doublelayer structure formed in 0.05 M H2SO4 + 0.095 M NaOH (pH = 3) was observed in scanning tunneling microscopy (STM), i.e., crystalline NiO inner layer (which is epitaxial with the substrate lattice) and amorphous Ni(OH)2 outer layer,12,13 which is in agreement with the investigation by Takeshi Suzuki at al.14 In addition, the total thickness of passive film ranging from 0.7 to 2.5 nm, measured by various spectroscopic technologies,15,16 have been reported. All these uncertainties are the consequences of the lack of direct evidence of cross section. Transmission electron microscopy (TEM), which has been successfully applied to the observation of nanoscale oxidation layer formed on the surface of pure copper in alkaline solution,17 is capable of viewing the internal structure between the oxides and their adjacent substrates. In particular, this method allows the electrochemical behavior of materials when they are at nanosize to be studied. Therefore, a valuable © 2014 American Chemical Society
comparison can be made for the same materials with the size at both macro-, micro-, and nanoscales in terms of their response to aqueous environments. In the present work, we use a similar fabrication method to prepare a nickel nanostructure specimen using argon ion milling technique and then examine the passive film using ex situ TEM.
2. EXPERIMENTAL SECTION The preparation of nickel nanoneedles was performed by using a precise argon ion beam that sputters the grids of pure nickel wire (purity 99.99%). A nickel grid with a diameter of 3 mm was first mounted by a sample holder, which was then loaded into the chamber in a Gatan 691 precise ion milling system (PIPS) manufactured by Gatan, Inc., Warrendale, PA, United States. Two argon ion beams were set at an inclined angle of ±2° to bombard the center of the grid (Figure 1a) at a beam energy of 3 keV until it was perforated The subsequent ion polishing with a lower energy of 1 keV would taper the nickel wires producing multiple identical nickel nanoneedles. The detailed procedure for preparation of the nanoneedle is described in the literature.17 The electrochemical polarization to the nickel needle specimens was carried out in 0.1 M NaOH solution and 0.1 M H2SO4 solution at room temperature (25 ± 1°), respectively. A typical three electrode potentiostatic system of AUTOLAB PGSTAT 302N (Metrohm Pte Ltd., Switzerland) was used for the electrochemical measurements. The nickel grids were used as the working electrode, a platinum foil as the counter Received: February 21, 2014 Revised: April 9, 2014 Published: April 14, 2014 9073
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Figure 1. (a) A schematic diagram illustrating the process of nickel needle specimen preparation. (b) A bright-field TEM image of an example of nickel needles. (c) An HR-TEM image of the tip region of a nickel needle for passivation in 0.1 M NaOH solution and an FFT pattern (inset) taken from the white square A marked in the image. (d) An HR-TEM image of the tip region of a nickel needle for passivation in 0.1 M H2SO4 solution and an FFT pattern (inset) taken from the white square B marked in the image.
electrode, and an Ag/AgCl electrode (3 mol L−1 KCl solution) as the reference electrode in the electrochemical cell. All potential values are reported against the Ag/AgCl electrode in the present work. Prior to the measurement, the specimen was kept at −0.6 V for 10 min in order to electrochemically reduce the oxide film formed by exposure of the nickel surface to air. The polarization was then implemented by scanning in the reverse direction to a proper potential so that the entire passive region was presented. The scan rate was set to be 5 mV/s. On completion of the electrochemical passivation, the nickel grid was taken out of the cell rapidly followed by thorough rinsing with deionized water, and then dried 30 min in a vacuumed desiccator at room temperature. Finally, the sample was transferred into TEM (JEOL JEM-2100F, Japan) for observation at an accelerating voltage of 200 keV.
3. RESULTS AND DISCUSSION The nickel nanoneedle prior to electrochemical polarization was viewed in TEM immediately after it was prepared. As an example, Figure 1b is a bright-field TEM image of one nickel needle, and Figure 1c and Figure 1d are two tips of two nickel needles. The FFT patterns in Figure 1c and Figure 1d were performed by the selection of the square region marked as A and B. There is no oxide layer visible that covers around nanoneedle tips. The bright edge of the nanoneedle shown in Figure 1b and Figure 1d is due to the slight underfocus of the image in TEM. Potentiodynamic polarization was carried out to investigate the passive behavior of nickel nanoneedle in 0.1 M NaOH solution (as shown in Figure 2a). In the initial anodic potential sweep, a single and characteristic peak at −0.2 V is observed, which indicates that it is attributable to the nickel dissolution 9074
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Figure 2. (a) A potentiodynamic polarization curve of pure nickel grid electrode in 0.1 M NaOH solution. (b) A potentiodynamic polarization curve of pure nickel grid electrode in 0.1 M H2SO4 solution.
A thin film (Figure 3a) is seen to be covered around the nickel nanoneedle and consists of duplex layers (i.e., inner layer adjacent to Ni matrix and outer layer) in the large magnification as shown in Figure 3c, a high-resolution TEM image of the square region marked as “A” in Figure 3a. The inner layer shows clear lattice phase contrast. The inset D′ in Figure 3c is a reciprocal lattice image converted from the region D of the inner layer and is arranged by an ordered sharp spot array reflected from face centered cubic (fcc) NiO in a projection plane of (0 0 1). It is interesting to find that the thin NiO layer is coherently oriented with the Ni matrix, as indicated from the FFT pattern (inset C′) of the square region C. This is probably due to the growth of oxide being aligned with Ni matrix. As a result, the lattice structure is retained and originated well with the Ni matrix. The slight change of the orientation between the Ni matrix and NiO is likely due to the little diffusion of Ni outward, as proved by FFT patterns C′ and D′. They show a very small orientation rotation with respect to each other. The outer layer is formed by a number of nanocrystals that are decorated outside of the inner layer. They are identified to be hexagonal Ni(OH)2 and randomly oriented with respect to the inner layer/Ni matrix. As an example, the FFT pattern (inset
and the onset of passive film formation. The rapid decrease of anodic current with the increase of scanning voltage suggests the occurrence of passivation. The passive region starts at around −0.1 V until +0.5 V before the oxygen evolution takes place. Potentiostatic polarization for the nickel sample under TEM observation was carried out at +0.4 V (P1), a relatively high potential in the passive region, in 0.1 M NaOH solution for 30 min when the current density was stable. A similar polarization was carried out by scanning to +1.2 V with the same scan rate in 0.1 M H2SO4 solution. The measured curve indicates that passivation initiates at +0.2 V and comes to an end at around 1.2 V (as shown in Figure 2b). Potentiostatic polarization measurement for the nickel specimen was carried out at +0.8 V (P2) in 0.1 M H2SO4 solution for 10 min until the current density decreased to a plateau. The passivated nickel sample was immediately taken out from the solution and quickly transferred into the TEM chamber after being carefully treated with cleaning and drying. Bright-field TEM images of the nickel needles (Figure 1c and Figure 1d) after the electrochemical passivation in 0.1 M NaOH solution and 0.1 M H2SO4 solution are shown in Figure 3a and Figure 3b. 9075
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not discovered in the present observation. This is owing to the high concentration of H+. Ni(OH)2 compound is not stable in acid solution and merely forms a monomolecular or bimolecular layer, which is beyond the resolution of TEM. In contrast, the thickness of the Ni(OH)2 outer layer could reach 3 nm due to the existance of abundant OH−. Especially, the increase of the Ni(OH)2 outer layer thickness is at the expense of the thickness decrease of the NiO inner layer. Furthermore, the total thickness of the passive film formed on nickel in 0.1 M NaOH solution seems slightly larger than that formed in 0.1 M H2SO4 solution. Compared with the previous studies, TEM provides us direct structural information on cross section in this work, which presents the interface between substrate and the double-layer passive film. Meanwhile, crystalline NiO inner layer and Ni(OH)2 outer layer as well as their orientation relationship have been clarified.
4. CONCLUSIONS In the present work, TEM results show that an ultrathin inner layer consisting of crystalline NiO grows on the nickel nanoneedle surface by anodic polarization in both 0.1 M NaOH solution and 0.1 M H2SO4 solution. This dense NiO layer embraces the nickel surface compactly and is coherently oriented with nickel substrate. However, a randomly oriented crystalline Ni(OH)2 outer layer was discovered after Ni passivation in 0.1 M NaOH solution, which was hardly observed on the nanoneedle passivated in 0.1 M H2SO4 solution. In addition, the thickness of passive film formed on nickel is pH value dependent, which will enlarge slightly with the increase of pH value.
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AUTHOR INFORMATION
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[email protected]. Tel: (65) 6790 4345. Notes
The authors declare no competing financial interest.
■ Figure 3. (a) An HR-TEM image of the nickel needle specimen after passivation in 0.1 M NaOH solution at 0.4 V for 30 min. (b) An HRTEM image of the nickel needle specimen after passivation in 0.1 M H2SO4 solution at 0.8 V for 10 min. (c) An HR-TEM image of the white square A in panel a and the FFT patterns (C′, D′, and E′) taken from areas marked C, D, and E, respectively. (d) An HR-TEM image of the white square B in panel b and the FFT patterns (F′, G′, and H′) taken from areas marked F, G, and H, respectively.
ACKNOWLEDGMENTS This work was supported by SUG (Start-up funding in NTU: COE_SUG/RSS_20AUG10_11/14), Tier 1 (AcRF grant MOE Singapore: RG47/11), National Natural Science Foundation of China (Grants 50701006 and 51271031), Fundamental Research Funds for the Central Universities (FRF-SD-12-027A). The transmission electron microscopy work was performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore.
E′) generated from the nanoparticle in the square region E is indexed to be due to the reflection of Ni(OH)2 at a zone axis of [1 0 −1 1]. Nevertheless, only a single NiO inner layer was observed on the passivated surface of nanoneedle in 0.1 M H2SO4 solution (Figure 3d). This inner layer was also found to be coherently oriented with the Ni matrix, as confirmed by performing FFT from different regions of the thin film in comparison with the Ni matrix. For example, the inset F′ and H′ patterns taken from two regions F and H in the NiO inner layer appear indentical and are the same as the G′ pattern transformed from the matrix region H. Different from the previous work, Ni(OH)2 layer was
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