One-dimensional nickel borate nanowhiskers: characterization, properties, and a novel application in materials bonding

Abstract

The growth of nickel borate [Ni₃(BO₃)₂] nanowhiskers was successfully achieved by simply heating a mixture of nickel and boron oxide (B₂O₃) powders at 950 °C in air. The products were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The synthesized nanowhiskers, with diameters of 150–500 nm and lengths of 10–30 μm, possessed high aspect ratio, and were found to grow along the [012] crystallographic direction. Stacking faults, hollow interiors, and high-energy facets were found occasionally. The high-temperature stability of Ni₃(BO₃)₂ nanowhiskers was investigated. The nanowhiskers partially decomposed after being treated at 1200 °C. Based on the solid–liquid–solid mechanism, the possible growth process of Ni₃(BO₃)₂ nanowhiskers was discussed. Ni₃(BO₃)₂ nanowhiskers were successfully used for bonding nickel in air. The nickel/nanowhisker/nickel joint was characterized in detail by scanning electron microscopy and laser scanning confocal microscopy.

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1. Introduction

One-dimensional (1D) nanostructures, such as nanotubes,^1–3 nanowires,^4–7 nanorods,^8,9 and whiskers,^10–13 have attracted an extraordinary amount of attention due to their applications in optics, electronics, mechanics, etc. Metal borates, one member of the nanostructured materials family, are of interest because of their excellent mechanical properties, good chemical inertness, and low thermal expansion coefficient.^14–18 Among the known metal borates, nickel borate (Ni₃(BO₃)₂) has attracted significant interest because of its magnetic, catalytic, and phosphorescent properties.^19–22 In Menaka's research,²⁰ the obtained Ni₃(BO₃)₂ nanorods and nanospindles showed antiferromagnetic behaviour with a minor variation in the Néel temperature (44–47 K). Terry²² reported producing of fatty amines with a nickel borate catalyst within different temperature ranges. Efforts have been made to synthesize Ni₃(BO₃)₂ using different methods. Menaka et al. reported the synthesis of Ni₃(BO₃)₂ nanoparticles by the reverse micellar route¹⁹ and the precursor-mediated route.²⁰ Liu et al.²¹ prepared rod-like micro Ni₃(BO₃)₂ via a facile NaCl and NP-9 flux-assisted thermal conversion route. However, it is still difficult to control the synthesis of Ni₃(BO₃)₂ with 1D morphologies. Fabricating 1D Ni₃(BO₃)₂ with a high aspect ratio (nanowires or nanowhiskers) will be important for designing 1D Ni₃(BO₃)₂-based composites, nanodevices, and electronic ceramics. In particular, Ni₃(BO₃)₂ nanowhiskers possess significant potential for materials bonding to form joints with high-temperature applications at low cost. However, to date, few reports have been published concerning the synthesis of Ni₃(BO₃)₂ nanowhiskers.

In this study, Ni₃(BO₃)₂ nanowhiskers were synthesized by heating a mixture of Ni and B₂O₃ powders in air. The microstructure of the Ni₃(BO₃)₂ nanowhiskers was investigated in detail. Moreover, the high-temperature stability and the materials bonding application were tested. The possible formation mechanism of Ni₃(BO₃)₂ nanowhiskers and the effect of Ni₃(BO₃)₂ nanowhiskers on the bonding of nickel in air were discussed.

2. Experimental

Ni₃(BO₃)₂ nanowhiskers were prepared by heating the reactants in air. The source materials were a mixture of Ni (99.0%, Sinopharm Chemical Reagent Co., Ltd) and B₂O₃ (99.0%, Sinopharm Chemical Reagent Co., Ltd) powders with a mole ratio of 1 : 3. The powder mixture was first milled in a planetary ball mill for 120 min and then placed on a Ni holder in a muffle furnace. The furnace was heated at a rate of 10 °C min⁻¹ and kept at 950 °C for 240 min in air. After the furnace was cooled down to room temperature, a green layer of product was formed. High-temperature stability tests were carried out by heating the as-prepared product to 1100 °C and 1200 °C in air. To test the possibility of forming joints with Ni₃(BO₃)₂ nanowhiskers, bulk Ni was cut into 8 mm × 8 mm × 2 mm pieces with an electro-discharge machine. The surfaces to be joined were ground using 2000 grit SiC paper and then pre-oxidised in the furnace at 950 °C for 240 min. The bonding experiments were carried out in air.

The morphologies of the nanowhisker samples were examined using scanning electron microscopy (SEM, Hitachi S-4300). In addition, the samples were identified by X-ray diffraction (XRD, Bruker D8 ADVANCE) with Cu Kα radiation under an accelerating voltage of 40 kV. A few drops of ethanol solution containing nanowhiskers were deposited onto carbon-coated copper grids for transmission electron microscopy (TEM, FEI Tecnai G² F30) and high-resolution transmission electron microscopy (HRTEM) studies. The joined samples were sectioned perpendicular to the bond line, and then microstructural observations were conducted with a laser scanning confocal microscope (LSCM, Keyence VHX-1000E) and SEM.

3. Results and discussion

3.1 Characterization

Fig. 1(a) and (b) shows SEM images of the product synthesized at 950 °C. It was noted that the product consisted of abundant straight nanowhiskers with diameters ranging from 150 to 500 nm and lengths ranging from 10 to 30 μm, whereas a small fraction of the whiskers had thicker diameters of ∼1 μm. Fig. 1(c) shows the XRD pattern of the nanowhiskers formed on Ni substrate. The diffraction peaks can be indexed to the orthorhombic phase of Ni₃(BO₃)₂ (JCPDS no. 75-1809) with the lattice parameters of a = 0.54 nm, b = 0.45 nm, and c = 0.83 nm, Ni as well as NiO, the latter one was formed due to oxidation of Ni powder at high temperature in air.

Fig. 1 (a and b) SEM images of Ni₃(BO₃)₂ nanowhiskers synthesized at 950 °C for 240 min. (c) XRD pattern of Ni₃(BO₃)₂ nanowhiskers formed on Ni substrate.

The detailed structure of the Ni₃(BO₃)₂ nanowhiskers was further characterized by TEM examinations. The TEM image in Fig. 2(a) shows that the individual nanowhisker was straight, with an aspect ratio of more than 30 : 1, and that the surface of the nanowhisker was clean and smooth. The selected area electron diffraction (SAED) pattern in Fig. 2(b) indicates that the individual nanowhisker was single crystalline in nature. To analyze the growth direction of the nanowhisker, HRTEM image (Fig. 2(c)) was obtained. Fig. 2(c) indicates that the individual nanowhisker grew along the [012] direction (indicated with an arrow). Diffraction patterns taken from different parts of the nanowhisker show exactly the same pattern. More than 10 individual nanowhiskers were examined using this method, and the results indicate that the detected nanowhiskers were single crystalline and that they grew along the [012] direction. No dislocations or stacking faults were observed in the area examined.

Fig. 2 (a) TEM image of an individual Ni₃(BO₃)₂ nanowhisker. SAED pattern (b) and HRTEM image (c) of the selected nanowhisker shown in (a). The arrows in (a) and (c) indicated the growth direction of the nanowhisker.

However, it does not mean that there were no defects in the synthesised Ni₃(BO₃)₂ nanowhiskers. In some areas of the products, side steps could be observed, as shown in Fig. 3(a), which was possibly induced by the stacking faults.²³ The SAED pattern of a Ni₃(BO₃)₂ nanowhisker shown in Fig. 3(b) indicates that the stacking faults usually had a width of seven atomic layers. It is worth mentioning that some of the nanowhiskers displayed a clear hollow interior, namely tubular structure, as shown in Fig. 3(c) and (d). Although several kinds of nanowhiskers with tubular morphology have been successfully synthesized, such as titanate nanotubes² magnesium borate nanotubes,³ there have been no reports published about tubular structured Ni₃(BO₃)₂. Fig. 3(c) indicates that the cross-section of the hollow inner cavity was not a perfect circle but rather an ellipse. The clear contrast in Fig. 3(d) confirms the smooth inner fringe through the tubular structure, and the diameter of the inner cavity was approximately 100 nm. In addition to the tubular morphology, two other points of the nanowhisker in Fig. 3(c) are worth noting. The first point is the unique tip morphology, namely, a quadrilateral front facet that is different from the cross-section of the main part of the nanowhisker. Such variation in cross-section might be originated from the local temperature change during the nanowhisker growth. The second point is the small facet between adjacent large facets. It is well known that the surface energy is a function of the surface normal, r(n). For any element of the crystal surface, dA, the surface energy is r(n)dA, where n is normal to the element dA. Thus, the shape of a single crystal with a fixed volume in thermodynamic equilibrium is:¹⁵

E = ∫r(n)dA (1)

which minimises the total surface energy. Moreover, the energies of two orientations (r₁ and r₂) correspond to the perpendicular distances from the centre of the crystal to the facet (l₁ and l₂):¹⁵

l₁/l₂ = r₁/r₂ (2)

Fig. 3 (a) SEM image of nanowhiskers with stacking faults. (b) SAED pattern of an individual nanowhisker with stacking faults. (c) SEM and (d) TEM images of the tubular nanowhiskers, respectively. The yellow lines and red arrows in (c) illustrated the locations of high energy facets.

Therefore, it is easy to conclude that the small facets that were far away from the centre of the nanowhisker in Fig. 3(c) had a higher surface energy than the large facets. However, such high-energy facets (HEFs) did not have an adverse effect on the crystallization process of Ni₃(BO₃)₂ nanowhiskers.

3.2 High temperature stability

To study the high-temperature stability of Ni₃(BO₃)₂ nanowhiskers, the as-prepared Ni₃(BO₃)₂ nanowhiskers were heated to 1100 °C and 1200 °C for 120 min. After being treated at 1100 °C, the surfaces of the products began to be rough. When treated at 1200 °C for 120 min, most of the nanowhiskers decomposed. Fig. 4(a) and (b) show the SEM and TEM images of partially decomposed Ni₃(BO₃)₂ nanowhiskers after treated at 1200 °C. It can be seen that the nanowhiskers did not retain their original straight morphology, instead taking on a particle-chain structure. On the basis of the HRTEM image of a decomposed nanowhisker shown in Fig. 4(c), a bonded semi-coherent interface between (200)_NiO and (112)_Ni3(BO3)2 was observed. The lattice mismatch of the interface can be calculated according to the following equation:²⁴

F = 2(d₂ − d₁)/(d₂ + d₁) (3)

where the relationship between d₁ and d₂ is shown in Fig. 4(c). Thus, it can be determined that d₁/d₂ = 5/4, and the mismatch is determined to be −22.2%, which implies a high strain energy induced by the decomposition of nanowhiskers.

Fig. 4 (a) SEM images of decomposed nanowhiskers after treated at 1200 °C. (b) TEM images of a partial decomposed nanowhisker. (c) HRTEM image of NiO/Ni₃(BO₃)₂ interface in a decomposed nanowhisker.

3.3 Possible growth mechanism

The growth process of Ni₃(BO₃)₂ nanowhiskers has been divided into three stages. The scheme of the main growth steps that lead to the formation of Ni₃(BO₃)₂ nanowhiskers is shown in Fig. 5. In the first stage, two reactions occurred during the heating process: B₂O₃ (s) → B₂O₃ (l) and 2Ni (s) + O₂ (g) → 2NiO (s). The solid B₂O₃ melted when the temperature was higher than its melting point of 450 °C. Based on the analysis of metal borate nanowire fabrication processes,^5,7 the liquid B₂O₃ acted as both the transportation assistant and the reactive substance. After the oxidation of Ni to NiO during the heating process or at the nanowhisker growth temperature, liquid B₂O₃ spread on the surface of NiO, and the following reaction took place in the second stage: B₂O₃ (l) + 3NiO (s) → Ni₃(BO₃)₂ (s). The relatively low supersaturation is critical for whisker growth,²⁵ and the fact that low supersaturation facilitates the growth of the whiskers has been accepted in previous studies.^26,27 Thus, when the saturation solubility of Ni₃(BO₃)₂ in liquid B₂O₃ is reached, the nanowhiskers begin to nucleate and grow (the final stage). Although it is easy to form nanowires and whiskers with monoclinic structures, such as WO₃ nanowires,²⁸ K₂Ti₆O₁₃ nanowires,²⁹ and Cu₂S nanowires,³⁰ one-dimensional orthorhombic metal borates are most likely to form.^5,7,15 During the nanowhisker growth process, no catalyst was used, which is different from the process used to form carbon nanotubes. As is often the case, metal catalysts must be used to fabricate carbon nanotubes, due to their effects on the dissociation of hydrocarbons and the segregation of carbon atoms.^31,32 The size and shape of the catalysts control the morphology of the carbon nanotubes, which often have oblong forms. However, the metal oxides and B₂O₃ can directly react at high temperatures, which is called self-catalysis.⁷ Because no drops were found on the tips of the nanowhiskers, the growth of Ni₃(BO₃)₂ nanowhiskers might be dominated by the solid–liquid–solid (SLS) process.³³ The initial stacking process induced a quadrilateral tip to form on the nanowhiskers, as Fig. 3(c) shows, which indicated that the crystal growth would be anisotropic.^7,15 And the difference between the quadrilateral front facet and the cross-section of the main part of the nanowhisker was thought be originated from the local temperature change during the nanowhisker growth. During the subsequent growth process, Ni₃(BO₃)₂ molecules were conveyed to the tips of the Ni₃(BO₃)₂ nanowhiskers by liquid B₂O₃, causing the nanowhiskers to grow continuously. Because the Ni₃(BO₃)₂ particles could be synthesised at 750–800 °C,^19–21 this suggests that there is more energy available for the growth of Ni₃(BO₃)₂ nanowhiskers. This is why the Ni₃(BO₃)₂ nanowhiskers showed good crystallinity.

Fig. 5 Scheme of the main growth steps that lead to the formation of Ni₃(BO₃)₂ nanowhiskers.

3.4 Material bonding application

Because of the nonvacuum growth environment and the good crystallinity, Ni₃(BO₃)₂ nanowhiskers were used to bond bulk nickel in air. Fig. 6 shows the morphologies of the nickel/nanowhisker/nickel joints bonded at 950 °C for 240 min. From the cross-sectional images of the joint shown in Fig. 6(a) and (b) that were obtained by LSCM, it can be clearly seen that the pieces of bulk nickel were successfully bonded by Ni₃(BO₃)₂ nanowhiskers. Two types of bonding or connection relationships could be observed in different parts of the joint: relationship A (RA) and relationship B (RB). Based on the discussion above, when the gap between two matrixes was sufficiently small, the crossing of nanowhiskers that grew from the opposite NiO surface made an effective connection (RA) called a nanowhisker network structure, which is shown in Fig. 6(c). Fig. 6(d) shows that some nanowhiskers grew across the joint, forming a direct and stronger bonding of the matrixes (RB). Because the Ni₃(BO₃)₂ nanowhiskers can retain good 1D characteristics below 1100 °C, the nickel/nanowhisker/nickel joint has the potential to be used in high-temperature applications, as well as that the bonding process needs to be a low-cost process.

Fig. 6 (a) and (b) LSCM images of the nickel/nanowhiskers/nickel joint, revealing the successful application of nanowhiskers in materials joining. The white arrow illustrated the nanowhiskers area in the joint. (c) SEM image of bonding relationship A (see the blue dotted box) and (d) relationship B (see the blue arrows) of nanowhiskers in the joint, respectively.

4. Conclusions

In summary, single-crystal Ni₃(BO₃)₂ nanowhiskers with good crystallinity were synthesized by heating a Ni and B₂O₃ powder mixture in air. Ni₃(BO₃)₂ nanowhiskers with a high aspect ratio grew along the [012] direction, and the nanowhiskers had diameters ranging from 150 to 500 nm and lengths from 10 to 30 μm. In addition, some nanowhiskers were found to have stacking defects and a hollow inner cavity. Small facets with high energy were observed between the two adjacent large facets. The nanowhiskers began to decompose when treated at 1200 °C for 120 min. The growth process is thought to follow the SLS mechanism. The Ni₃(BO₃)₂ nanowhiskers were successfully used to bond nickel in air.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under grant no. 51275133. The authors would also like to acknowledge the China Postdoctoral Science Foundation under Grant no. 2013M531032. And the authors thank Prof. C. Y. Xu for useful discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01630k

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