Morphological Evolution of Neodymium Boride Nanostructure Growth by Chemical Vapor Deposition

Abstract

Nanoscale-driven design of electron emission materials can significantly increase their overall efficiency as cathodes for field-induced electron emission by taking advantage of the field enhancement effect. The refractory nature and low work function (1.6 eV) of neodymium hexaboride (NdB 6 ) suggest that high aspect ratio NdB 6 nanostructures are potential candidates as efficient field emission cathodes. Here we report the morphological evolution of one-dimensional neodymium boride nanostructures synthesized using palladium-nanoparticlecatalyzed chemical vapor deposition as a function of reaction temperature. Scanning electron microscopy data show that judicious choices of reaction temperatures (795-940 °C) can lead to the preferential growth of curly nanowires or high aspect ratio nanowires. Transmission electron microscopy and selected area electron diffraction reveal that the crystallinity of these nanostructures changes from amorphous, to polycrystalline, to single crystalline as the reaction temperature increases. At reaction temperatures above 900 °C, single-crystalline NdB 6 nanowires with preferential [100] growth direction were successfully synthesized. Energy dispersive X-ray spectroscopic data suggest that this morphological evolution was strongly influenced by the solubility profiles of Nd and B in the Pd catalyst nanoparticles at different reaction temperatures. The implication of these results on the criteria of catalyst choices for the growth of binary metallic boride nanomaterials is also discussed.

A versatile route for the convenient synthesis of rare-earth and alkaline-earth hexaborides at mild temperatures

“…For potential application in cathode materials, the chemical vapor deposition (CVD) method for the synthesis of one-dimensional hexaborides have been reported intensively in the recent years. Rare-earth halides or alkaline-earth oxides powders were commonly chosen to react with BCl 3 , 5,17 B 2 H 6 , 18-20 B 10 H 14 , 21,22 or nido-pentaborane (or nido-decaborane) 2 at temperatures ranging from 800 to 1150 C. Recently, a similar method was applied to prepare one-dimensional hexaborides by rare-earth or alkaline-earth metals and BCl 3 via a CVD process, however, the reactants are either sensitive to water or rather toxic. [23][24][25][26][27] Therefore, it is necessary to explore an alternative route for the preparation of crystalline RB 6 and AB 6 by using convenient chemicals via simple processes at mild temperatures.…”

Single-crystalline LaxNd1−xB6 nanowires: synthesis, characterization and field emission performance

“…8 Thus, a lot of relevant research has been carried out on this kind of material with nanostructure attributes, especially on the low work function of REB 6 . [13][14][15] Problems are also found using this method. 9-12 While on account of the severe hygroscopicity of RCl 3 in even low degrees of moisture circumstances, it's difficult to obtain large-scale, high quality RB 6 nanowires through this method.…”

“…Brewer et al used RECl 3 and B 10 H 14 as rare earth metal and boron sources, respectively, together with palladium (Pd) nanoparticles to serve as catalysts, using a similar CVD process. [13][14][15] Problems are also found using this method. In addition to the semblable issues that exist for RECl 3 , the solid precursor B 10 H 14 , which is not easy to control in the CVD flow, also hinders the synthesis of REB 6 .…”

Synthesis of single-crystalline NdB6 submicroawls via a simple flux-controlled self-catalyzed method

“…12 If single-crystalline NdB 6 nanostructures with high aspect ratio morphology, such as nanowires or nanotubes, can be fabricated, it may be an ideal candidate for the investigation and development of advanced eld emission theory, and efficient cathodes for vacuum electronic devices. Recently, Wang developed a palladium-nanoparticle-catalyzed chemical vapor deposition (CVD) method to synthesize NdB 6 nanoobelisks and nanowires, 13 but the boron precursor (B 10 H 14 ) was toxic and the Pd-Nd-B alloy drops on the tips would limit the electron emission and reduce the current density. 14 Very recently, Xu et al reported the synthesis of NdB 6 nanowires and nanoneedles via a catalyst-free CVD approach.…”