Preparation and growth mechanism of one-dimensional NdB₆ nanostructures: nanobelts, nanoawls, and nanotubes

Abstract

Three kinds of one-dimensional (1D) neodymium hexaboride (NdB₆) nanostructures, including nanobelts, nanoawls, and nanotubes, have been synthesized through a chemical vapor deposition (CVD) process with a self-catalyzed mechanism. For the first time, we report the preparation of NdB₆ nanotubes. The morphology and crystalline structure are characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Transmission electron microscopy images show that they have different growth directions: [111], [001], and [110], respectively. In addition, detailed growth mechanisms of the nanobelts, nanoawls, and nanotubes are presented. A droplet induced self-catalyzed mechanism, self-catalyzed with a vapor–solid mechanism, and diffusion limited self-catalyzed mechanism are proposed to explain the growth of nanobelts, nanoawls, and nanotubes, respectively.

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1. Introduction

Since the discovery of carbon nanotubes in 1991, one-dimensional (1D) nanostructured materials such as nanowires, nanobelts, and nanotubes have attracted much attention because of their novel morphologies and excellent physical properties, as well as great potential technological applications.^1–4 They are expected to play important roles as both interconnects and building blocks of next-generation nanoscale electronic devices.⁵ As a kind of excellent material, 1D rare-earth hexaboride (RB₆) nanostructures have received sustained research interest because of their unique physical properties including low work function, high melting point, low volatility, high chemical stability, high mechanical strength and high aspect ratio.^6–9 Several RB₆ nanowires (e.g., LaB₆, CeB₆, and GdB₆) and nanotubes (PrB₆ and EuB₆) have recently been synthesized and proposed for advanced electron field emitters due to their low work function and the enhancement of the electric field at the nanosized tips.^10–16 Compared with widely used LaB₆ (work function 2.7 eV), NdB₆ has a lower work function (1.6 eV), which means higher field emission current density with a lower applied turn-on voltage when used as a electron emitter based on the Fowler–Nordheim (F–N) theory.^17–19

Recently, Wang et al. reported the synthesis of NdB₆ nanowires using palladium-nanoparticle catalyzed low pressure chemical vapor deposition (LPCVD) process, but the boron precursor (B₁₀H₁₄) was toxic and not safe to human.²⁰ Very recently, Xu et al. developed a catalyst-free chemical vapor deposition method to synthesize NdB₆ nanoneedles, which demonstrated a low turn-on (2.71 V μm⁻¹) electric field and a high current density (5.37 mA cm⁻²) at a field of 4.32 V μm⁻¹. However, the B₂H₆ they used was highly toxic and combustible. In addition, the reaction needed very low pressure.²¹ Therefore, a safer and simpler method to synthesize NdB₆ nanostructures should be developed. Moreover, the synthesis of NdB₆ nanotubes have not been reported yet.

In this paper, through controlling the reacting temperature and concentration gradient, we designed and synthesized 1D NdB₆ nanostructures: nanobelts, nanoawls, and nanotubes on Si substrates in a horizontal tube furnace. The synthesized method is an ordinary pressure chemical vapor deposition (OPCVD) process with a self-catalyzed mechanism by using Nd, BCl₃ (low toxicity) and H₂ as the reactants. For the first time, we report the preparation of NdB₆ nanotubes. The morphology and crystalline structure are characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM images show that the nanobelts, nanoawls, and nanotubes have different growth directions: [111], [001], and [110], respectively. Detailed growth mechanisms of the nanobelts, nanoawls, and nanotubes are presented.

2. Experimental section

The NdB₆ 1D nanostructures were prepared by a self-catalyzed mechanism with neodymium, boron trichloride and hydrogen as the reactants. The synthesis is based on the following chemical reaction: Nd(s) + 6BCl₃(g) + 9H₂(g) = NdB₆(s) + 18HCl(g).²² The synthesis was performed in a horizontal high-temperature tube furnace, which was equipped with a quartz tube of about 61 mm in inner diameter, 68 mm in outer diameter and 1170 mm in length (Shenjia Kiln, Luoyang, China). All the experiments were carried out at 1 atm. The real temperature in the flat-temperature zone (length of 20 cm) was measured by a thermocouple which was mounted at the center of the tube. The silicon ((100) plane, Empak, USA) substrates (8 × 8 mm²) were used for the experiments. The substrates were ultrasonically cleaned using distilled water and ethanol.

Precursor Nd metal powders (weight: 0.5 mg; purity: 99.9%; particle size less than 96 μm; melting point: 1024 °C) were well deposited on the Si substrates. The Si substrates were loaded inside the center of a quartz boat (length of 10 cm). The quartz boat was then quickly placed into the center of flat-temperature zone of the tube furnace. And the furnace tube was purged with high-purity gas mixture (50% H₂ + 50% Ar, 99.99%, Ruimin Gas, Guangzhou, China) for 2 h to eliminate oxygen in the furnace. Then the tube was heated to 990–1020 °C at 10 °C min⁻¹ with a 50 sccm (standard cubic centimeters per minute) continuous flow of gas mixture (50% H₂ + 50% Ar) and kept at this temperature for 30 or 50 min. When the desired temperature was reached, a steady BCl₃ (99.99%, Summit Specialty Gases, Tianjin, China) flow of 20 sccm was introduced to the tube. The corresponding duration times of BCl₃ for nanobelts, nanoawls and nanotubes are 30, 40 and 50 min, respectively. After the reaction, the tube was cooled down to room temperature in 5 h under mixed gas atmosphere. Then the products were washed with distilled water for several times to remove HCl and H₃BO₃. After drying at 60 °C for 1 h, the final gray products were obtained. Three representative samples with different morphologies (nanobelts, nanoawls, and nanotubes) are fabricated by varying some experimental parameters, and the detailed experiment conditions are presented in Table 1.

Table 1 Experiment conditions for 1D NdB₆ nanostructures

Morphology	Catalyst	Reaction temperature [°C]	Reaction time [min]	BCl3 duration time [min]
Nanobelts	No	990	30	30
Nanoawls	No	1000	50	40
Nanotubes	No	1020	50	50

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The products were characterized and analyzed by X-ray diffraction (XRD; TD-3500 with CuKα radiation), micro-Raman scattering spectroscopy (Renishaw RM-2000 confocal micro-Raman system; 633.0 nm excitation red laser), field emission scanning electron microscopy (FE-SEM; Nova NanoSEM430), high resolution transmission electron microscopy (HRTEM; JEM-2010HR at an accelerating voltage of 200 kV and Tecnai G2 F30 at an accelerating voltage of 300 kV) installed with energy dispersive X-ray spectroscopy (EDX; Oxford), and selected area electron diffraction (SAED; Tecnai G2 F30).

3. Results and discussion

3.1. Characterization

Fig. 1a shows the X-ray diffraction (XRD) patterns of obtained 1D NdB₆ nanostructures (nanobelts, nanoawls, and nanotubes). The XRD patterns agree well with the standard NdB₆ diffraction pattern from the Inorganic Crystal Structure Database (ICSD, no. 108069) (bottom curve), which reveals a primitive single-phase cubic NdB₆ with a space group of Pm3̄m. All the diffraction peaks can be indexed by using Dicvol program and assigned to the lattice planes of (100), (110), (111), (200), (210), (211), (220) for the corresponding d-spacing. Moreover, the following discussed TEM and SAED can also match the crystal structure of NdB₆, while EDX can confirm the existence of Nd and B elements. Fig. 1b presents the micro-Raman spectra of the NdB₆ 1D nanostructures acquired at room temperature with 633 nm excitation in ambient atmosphere. Due to the Pm3̄m symmetry of NdB₆, the lattice vibration modes can be obtained: Γ = A_1g + E_g + T_1g + T_2g + 2T_1u + T_2u, where the Raman-active phonons are A_1g, E_g, and T_2g. Two T_1u modes are infrared-active, and T_1g and T_2u are optically inactive.²³ Three observed peaks at around 688, 1162, and 1281 cm⁻¹ are the Raman-active phonons with the representations of T_2g, E_g, and A_g, respectively. Moreover, an extra peak at around 170 cm⁻¹ can be regarded as the vibration of rare-earth ions in the cage consisting of octahedral B₆ (see the crystal structure of NdB₆ in Fig. 1b inset), and can be assigned as the second-order Raman excitations of T_1u phonon at Brillouin zone boundary.²⁴ In addition, a broad peak at 1400 cm⁻¹ marked by the asterisk is assigned as the second-order excitation of the T_2g phonon.

Fig. 1 (a) XRD patterns and (b) Raman spectra of 1D NdB₆ nanostructures: nanobelts, nanoawls, and nanotubes.

The micro morphology of 1D NdB₆ nanostructures is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image (Fig. 2a) demonstrates that bulk of beltlike nanostructures synthesized at 990 °C are deposited onto the surface of the Si substrate. They have a rectangle like cross section with widths of 50 to 300 nm, width-to-thickness ratios of 3 to 5, and lengths of 2 to 10 μm (see Fig. 2a inset). Fig. 2b shows the SEM image of the NdB₆ nanoawls obtained on the Si substrates at 1000 °C with a reaction time of 50 min where the duration time of a flow of BCl₃ (20 sccm) is 40 min. The image reveals that the final product is composed of nanoawls in a uniform morphology. The nanoawls are straight and about 3 to 10 μm in length (see Fig. 2b inset). Most of the nanoawls are tapered, with a diameter ranging from approximately 50 to 300 nm at the roots and 20 to 70 nm at the tips (marked by yellow arrows). The diameter ratios of root-to-tip are 5 to 15. Interestingly, these nanoawls possessing extremely sharp tips are very important and beneficial for field-induced electron emission, such as NdB₆ nanoneedles,²¹ SiC nanotips,²⁵ and AlN nanocones.²⁶ By controlling the experimental conditions, the NdB₆ nanotubes were prepared on the Si substrates at 1020 °C. Fig. 2c depicts the typical NdB₆ tubular nanostructures with open ends (marked by yellow arrow). The nanotubes are well faceted with a circular cross section. But most of nanotubes grew aggregation, so the sample was dispersed onto a TEM Cu grid for further structural analysis. Fig. 2d is the TEM image of NdB₆ nanotubes with uniform wall thicknesses. The nanotubes have inner diameters ranging from 30 to 80 nm, and wall thicknesses between 5 and 20 nm. Moreover, the two ends of the nanotubes are open. The inset of Fig. 2d displays an individual nanotube with both ends open, which indicates the nanotubes are not terminated by any particles (marked by two yellow arrows).

Fig. 2 SEM images of (a) NdB₆ nanobelts synthesized at 990 °C (inset is a low-magnification image), (b) NdB₆ nanoawls obtained at 1000 °C (inset is a low-magnification image), and (c) NdB₆ nanotubes obtained at 1020 °C; (d) TEM image of NdB₆ nanotubes (inset is an individual nanotube).

To obtain detailed information regarding to the crystallinity, morphology, and growth direction of the 1D NdB₆ nanostructures, HRTEM and SAED analyses were performed, and representative results are discussed as follows. Fig. 3a shows a low-magnification TEM image of a typical NdB₆ nanobelt with width of 120 nm. Fig. 3d and inset respectively depicts the HRTEM lattice image of the nanobelt and the corresponding SAED pattern from the marked area in Fig. 3a, demonstrating the single crystalline nature of the nanobelt. The HRTEM image reveals the planar spacing along the growth direction is 0.238 nm, which corresponds to the (111) plane of NdB₆ cubic structure. According to the two d-spacing (0.412 nm and 0.291 nm), the corresponding (001) and (110) planes are confirmed. Fig. 3d inset is the corresponding diffraction pattern recorded in [1−10] axis. The growth direction of the nanobelt is determined to be the [111] direction. Therefore, single-crystalline NdB₆ nanobelts along [111] direction have been successfully prepared. Fig. 3b is the TEM image of an individual NdB₆ nanoawl which has a sharp tip with a diameter of about 40 nm. Moreover, no visible catalytic particles can be observed from the tip of the nanoawl. The corresponding HRTEM image and electron diffraction pattern of the tip of NdB₆ nanoawl are shown in Fig. 3e and inset, which reveals the [001] growth direction. In addition, NdB₆ nanotubes were characterized by TEM. Fig. 3c depicts a typical NdB₆ nanotube with a diameter about 77 nm. The wall thickness is estimated to be 11 nm and the inner diameter is approximately 55 nm.

Fig. 3 TEM images of single NdB₆ nanostructures: (a) nanobelt, (b) nanoawl, and (c) nanotube; (d), (e), and (f): the corresponding HRTEM images and SAED patterns of NdB₆ nanobelt, nanoawl, and nanotube.

The obvious contrast between the wall and the core region again confirms the hollow nature of the tube. The HRTEM lattice image (Fig. 3f) and the ring SAED pattern (inset) demonstrate that the nanotube possesses a polycrystalline structure. Moreover, HRTEM image shows the plane along the growth direction of the nanotube is (110), which reveals axial growth is along [110].

Different 1D morphologies have different preferred growth directions because they are obtained in different reaction conditions. From Table 1, the morphological structures of NdB₆ are found to be strongly influenced by the reaction temperatures, especially the temperature range of 990–1020 °C, which is a little lower than the melting point of bulk Nd (1024 °C). At 990 °C for nanobelt growth, the Nd droplets are in the solid–liquid equilibrium state with a high surface energy. During the nucleation period, nanobelts have a side nucleation with a (−211) plane (side of the nanobelt in Fig. 3d). It seems that surface energy minimization may play an important role in formation of nanobelt, and close-packed planes have relatively lower surface energies.²⁷ The growth plane (−211) of the nanobelt may have a relatively higher surface energy, which may lead to a faster growth along the [111] direction with relatively lower surface energy, forming a belt structure. At 1000 °C, growth of nanoawls along [001] direction is due to the closest-packed (001) plane within minimum surface energy. At a higher 1020 °C near the melting point of Nd, because the growth may undergo a special ring-shaped nucleation, hollow nanotube could be a polycrystalline structure. Hollow structures have more surfaces, so growing along [110] direction can reduce surface energy in these polycrystalline structures.

In order to confirm the growth directions of these 1D nanostructures, we have measured more than one sample. In our previous work,³³ the growth directions of two NdB₆ submicroawls are determined to be both the [001] directions. Thus, we have measured three different NdB₆ submicroawls or nanoawls with the same growth directions. So we conclude that NdB₆ nanoawls have the preferred [001] growth direction. Fig. 4 shows the TEM, HRTEM and SAED images of another NdB₆ nanobelt and nanotube. They demonstrate that the growth direction of nanobelt (Fig. 4a and b) is also [111], while nanotube (Fig. 4c and d) has the growth direction of [110]. Therefore, in different reaction conditions, different 1D morphologies have different preferred growth directions. In summary, three kinds of 1D NdB₆ nanostructures prepared with different reaction conditions have different growth directions: [111], [001], and [110]. The results indicate that the (111), (001) and (110) planes with low indices are the dominant terminating lattice planes with higher atomic density, and therefore, it would minimize the total energy for the crystal to grow in this lattice direction. This result agrees with the lowest energy principle in crystal growth. Moreover, lower work functions and thermal stability of these low index planes are beneficial for field-induced electron emission and thermionic emission.^28,29

Fig. 4 Another NdB₆ nanobelt with [111] direction: (a) TEM image; (b) HRTEM image and SAED pattern. Another NdB₆ nanotube with [110] direction: (c) TEM image; (d) HRTEM image and SAED pattern.

3.2. Analysis of growth mechanism

To explore the growth mechanism of the 1D NdB₆ nanostructures, the components on the tips of nanobelt, nanoawl, and nanotube are confirmed by EDX. The corresponding tips (marked areas by yellow circles) are shown in the insets of Fig. 5a–c. The representative EDX spectra shown in Fig. 5a–c reveal the existence of Nd and B elements in 1D NdB₆ nanostructures. The Cu signals come from the supporting Cu grids. No other signals (such as Si) can be identified from the tips of the nanostructures. Due to the high melting point of Si (1410 °C), Si could not serve as the catalyst in the growth of NdB₆ nanostructures. Thus, no substrates (Si) are contaminated with any catalytic material and they do not yield catalytic particles themselves. This result reveals that the growth of the nanostructures may not be governed by the conventional vapor–liquid–solid (VLS) mechanism.³⁰ In the present work, neither a catalyst nor a template is used, and no metallic particles are found at the ends of 1D NdB₆ nanostructures. Thus, the growths of 1D NdB₆ nanostructures are likely to follow a self-catalyzed mechanism, which means that metal Nd plays the roles of both raw material and catalyst simultaneously.³¹ Therefore, three different growth models related to self-catalyzed mechanism are proposed to explain their growth as shown in Fig. 6.

Fig. 5 EDX spectra collected from the tips (the marked areas in the insets) of the NdB₆ nanostructures: (a) nanobelt, (b) nanoawl, and (c) nanotube.

Fig. 6 Schematic presentation of the growth mechanisms of 1D NdB₆ nanostructures: (a) nanobelt, (b) nanoawl, and (c) nanotube.

3.2.1. Nanobelt. There are two mechanisms to explain the growth of nanobelts. First, Pan et al. proposed that the growth of semiconducting oxide nanobelts was governed by a vapor–solid (VS) process, in which the oxide vapor, evaporated from the starting oxide at a higher temperature zone, directly deposited on a substrate at a lower temperature region and grown into beltlike nanostructures.³ Second, Yan et al. presented that the growth of MgO nanobelt was induced by an ellipsoidal catalyst droplet, which was ascribed to the combination of two or more adjacent catalyst droplets.³² As to the first one, the VS mechanism is a catalyst-free physical vapor deposition process without chemical reaction. Unlike this mechanism, our growth of NdB₆ nanobelts is a chemical vapor deposition process with self-catalyzed vapor–liquid–solid (VLS) mechanism [Nd(l) + 6BCl₃(g) + 9H₂(g) = NdB₆(s) + 18HCl(g), 990 °C]. Here, like the second viewpoint, we propose that the growth of NdB₆ nanobelt is induced by an ellipsoidal Nd (instead of catalyst) droplet. For the understanding of this growth mechanism, the experiment conditions are very crucial. At 990 °C, the Nd droplets are in the solid–liquid equilibrium state, and the surfaces of Nd droplets are in the molten state but the interiors are partly solid. So the Nd droplets can be linked, but can't be merged completely. Fig. 6a gives a typical sketch map of the nanobelt growth induced by two linked Nd droplets. Step one is the formation of two separated Nd droplets on the substrate at 990 °C. At step two, the two droplets become large and link together with the interface of two linked circular planes. In order to prove this droplets-induced process, controlled experiments regarding reaction duration times of 2, 10, and 30 min were conducted. Fig. 7 shows time-dependent morphological evolution of the NdB₆ synthesized at 990 °C. Fig. 7a is the initial nucleation morphology, which shows single droplets and two or more linked droplets. At step three, the linked droplets further grow and form an ellipsoidal sphere. The elliptic plane would act as preferentially nucleated sites and induce the growth of nanobelts with rectangular cross section, which is in accord with the cubic crystal symmetry of NdB₆. Fig. 7b shows that the nanobelts grown at 10 min are sparse and short with rectangular cross section. Then the nanobelts grown at 30 min are dense and long, which reveals the growth process of NdB₆ nanobelts. At step four, the growth would continue until the reactants are exhausted in the end of reaction.

Fig. 7 The time-dependent morphological evolution of NdB₆ synthesized at 990 °C. SEM images of NdB₆ prepared with duration: (a) 2 minutes; (b) 10 minutes; (c) 30 minutes.

3.2.2. Nanoawl. Based on our previous work,^33,34 we propose that the combination of self-catalyzed and vapor–solid growth is responsible for the growth of NdB₆ nanoawls. Fig. 6b depicts the growth sketch map of a NdB₆ nanoawl. At 1000 °C, the vaporization of Nd droplets can generate Nd vapor (∼5.7 × 10⁻² Pa). Step one is the formation of Nd droplet and a little Nd vapor at 1000 °C. At step two, with the absorption of active B, nucleation and initial crystal grow by precipitation of NdB₆ from the droplet. Meanwhile, a little Nd vapor and active B may react for the lateral growth. At step three, with increasing the reaction time, the process of NdB₆ crystal growth from the nucleation site continues and NdB₆ crystal ascend atop the liquid droplet to form nanorod. Finally, when the BCl₃ precursor is switched off at 40 min, the local concentration at the growth zone decreases until the remaining BCl₃ flow is completely exhausted. This decrease in concentration gradient is expected to cause a decrease in the ratio of lateral vs. vertical growth, leading to the growth of tapered tops of the nanoawls.

3.2.3. Nanotube. As to the formation of tubular structure under self-catalyzed mechanism, the related reports are few. Xu et al. proposed that the growth of InN nanotubes was governed by a vapor–solid (VS) process.³⁵ But the growth here is self-catalyzed vapor–liquid–solid (VLS) process where Nd serves as catalyst for NdB₆ nanotube growth. Thus, a catalytic VLS growth model is similar to the growth process of NdB₆ nanotubes. It is interesting to note that the morphologies of the synthesized 1D NdB₆ nanostructures depend on the growth temperatures. For the growth of nanotubes, the reaction temperature may be the key factor because NdB₆ nanotubes can be only fabricated at 1020 °C, which is a critical temperature of solid–liquid equilibrium state. This result is similar with the synthesis of MgO nanotubes at the range of 580–670 °C, which is close to the melting point of Mg (650 °C).³² This can be understood by considering the kinetics-limited and diffusion-limited processes. A broad accepted growth model was proposed by Bakkers et al. in which they considered that the formation of hollow nanotube can be ascribed to the diffusion limit in the droplet at high temperature, which resulted in ring-shaped nucleation and induced the growth of nanotube.³⁶ A schematic illustration of the nanotube nucleation and growth process is shown in Fig. 6c. Step one is the formation of Nd droplet at 1020 °C. At step two, with the absorption of active B, a diffusion limited growth process leads to higher growth species concentration at the edges of the droplet/substrate interface. Consequently, those edges would reach supersaturation first, and the nucleation is ring like, leading to the growth of NdB₆ nanotube. Similar mechanism has been proposed in ref. 37 and 38 to understand the nanotube growth of different materials.

4. Conclusion

In summary, three kinds of 1D NdB₆ nanostructures, including nanobelts, nanoawls, and nanotubes, have been synthesized through a CVD process with a self-catalyzed mechanism. For the first time, we report the preparation of NdB₆ nanotubes. TEM shows that they have different growth directions: [111], [001], and [110], respectively. In addition, detailed growth mechanisms of the nanobelts, nanoawls, and nanotubes are presented. A droplets induced self-catalyzed mechanism, self-catalyzed with vapor–solid mechanism, and diffusion limited self-catalyzed mechanism are proposed to explain the growth of nanobelts, nanoawls, and nanotubes, respectively. The 1D NdB₆ nanostructures can provide potential applications including electron guns of TEM and SEM, flat panel displays, as well as other electronic devices that require high-performance electron sources.

Acknowledgements

The authors gratefully acknowledge the financial support of the projects from the National Natural Science Foundation of China (No. 51372089) and the Fundamental Research Funds for the Central Universities (No. 2014ZB0014).

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