New route toward nanosized crystalline metal borides with tuneable stoichiometry and variable morphologies Guillaume Gouget, Patricia Beaunier, David Portehault, Clément Sanchez To cite this version: Guillaume Gouget, Patricia Beaunier, David Portehault, Clément Sanchez. New route toward nano- sized crystalline metal borides with tuneable stoichiometry and variable morphologies. Faraday Dis- cussions, Royal Society of Chemistry, 2016, 191, pp.511 - 525. 10.1039/C6FD00053C. hal-01440776 HAL Id: hal-01440776 https://hal.sorbonne-universite.fr/hal-01440776 Submitted on 19 Jan 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. New route toward nanosized crystalline metal borides with tuneable stoichiometry and variable morphologies† G. Gouget,a P. Beaunier,b David Portehaulta,* and Clément Sancheza,* Herein we highlight for the first time the ability to tune the stoichiometry of metal boride nanocrystals through nanoparticle synthesis in thermally stable inorganic molten salts. Two metal-boron systems are chosen as case studies: boron-poor nickel borides and boron-rich yttrium borides. We show that NiB, Ni4B3, Ni2B, Ni3B, and YB6 particles can be obtained as crystalline phases with good selectivity. Anisotropic crystallization is observed in two cases: the first boron-rich YB4 nanorods are reported, while boron-poor NiB nanoparticles show a peculiar crystal habit, as they are obtained as spheres with uniaxial defects related to the crystal structure. Crystallization mechanisms are proposed to account for the appearance of these two kinds of anisotropy at the nanoscale. Introduction The family of metal-boron alloys spans a wide range of compounds, including various stoichiometries. This diversity results from the high density of covalent bonds in metal borides structures. By increasing the boron content (Fig. 1), the structure evolves from linear B-B chains, to two- and three-dimensional boron frameworks including interconnected boron clusters1 in which B-B lengths are shorter than in one- dimensional chains. This structural observation is commonly related to the mechanical properties of 2 metal borides, such as super-hardness in WB4 or high temperature refractory behavior in metallic 3 ceramics as HfB2. Covalence also results in high melting temperatures of borides (2600 °C for YB6) and pure boron allotropes (2075 °C for β-B).4 Noteworthy, the metal-boron interaction also brings specific behaviors. For instance, in OsB, metal atoms bring high electron concentration, so that electrostatic repulsions prevent compression, thus yielding ultra-incompressibility.2 More generally, boron-poor Fig. 1 Crystal structures of NiB (space group Amam), YB4 (P4/mbm) and YB6 (Pm-3m). (elemental ratio M:B ≥ 1:1) borides of transition metals are drawing attention in a range of fields from magnetism5 to catalysis.6–9 In this latter case, B-poor phases such as NiB and CoB are potential low price catalysts for dehydrogenation and Fisher-Tropsch reactions.10 In these phases, boron may prevent coking and deactivation of the active sites by sequestering carbon atoms.11,12 On the other side, boron-rich borides are actively studied and used, such as hexaborides for field emission13 and thermoelectric energy 14 conversion. Richer borides comprising icosahedral B12 boron clusters, for instance YB66, are also strongly investigated for thermoelectricy.15 Thus, original properties of metal borides are deeply related to both 16 elements and the way they interact. This is particularly exemplified by MgB2 superconductivity : its remarkably high critical temperature of 39 K for classical conductors was not expected, given the apparent simplicity of the composition17 compared to usual cuprate superconductors. The 18 superconductivity of MgB2 is related to both elements being light, as suggested in the BCS theory, together with anisotropic boron organisation in metallic honeycomb layers19 stabilized by electron transfer from magnesium to boron. All in all, metal borides exhibit a large diversity of technologically relevant mechanical and transport behaviours (Fig. 2) that originate from the complex interplay between the covalent boron framework and the metal atoms. Interestingly, all the above mentioned properties are related to bulk phases. The example of other 20–22 23 materials families, including gold and chalcogenides, shows how properties, e.g. in catalysis and optics, can be impacted when nanoparticles are scrutinized. However, such studies are still extremely scant for crystalline metal borides.5,24 Such scarcity originates from the harduous synthesis of crystalline metal borides, which has been discussed recently.24 Only few phases are known at the nanoscale, especially rare earth and alkaline earth hexaborides, produced by chemical vapor deposition (CVD) 25,26, 27–29 30 5 solid state reactions, often under autogeneous pressure. Some transition metal borides (VB2 , FeB , 31 Co2B ) have been reported at the nanoscale from solid state chemistry, while MgB2 has been obtained via a range of methods involving the reactivity of gaseous magnesium.32–34 In most cases the synthetic processes cannot be generalized to a wide range of metal borides, does not enable size control (except CVD), and particle sizes are rarely below 30 nm, and often above one micrometer. Besides, the control of the final stoichiometry for nanoscaled crystalline metal borides is currently uncharted. Fig. 2 Attractive properties of metal borides, known for bulk phases. The periodic table of elements is restricted to boron and metals. 2 Fig 3. Synthesis of metal borides in inorganic molten salts. LiCl/KCl (45/55 wt.) melts at Tf = 353 °C. After cooling down the reactant medium and washing out the salts, various metal boride nanoparticles are obtained in term of compositions, stoichiometries and shapes. aResults from Portehault et al. 35. Recently, a route was proposed toward a wide range of metal boride nanocrystals,35 based on the reaction between metal salts and an alkali borohydride below 900 °C, at atmospheric pressure in an inorganic molten salt mixture, composed of LiCl/KCl at the eutectic composition (Tf = 353 °C). This one- pot process enables production of nanocrystalline metal borides at temperatures between 400 °C and 900 °C, with a readily available, thermally stable and no oxygen-containing solvent, easily eliminated after reaction by using water. In a typical synthesis schematized Fig. 3, the solid precursors and salts are heated together over the melting temperature of the reactant medium in argon atmosphere. After cooling down to room temperature, a black block is obtained. It contains salts that are dissolved afterwards with a polar solvent such as deionized water. Washing and drying the remaining black suspension lead to a black powder consisting of metal boride nanoparticles. The particle size could be controlled and decreased below 20 nm. However, each metal studied was associated to a precise M:B ratio, so that stoichiometry control for a given metal was not provided. Besides, all reported nanoparticles were round shaped. In this article we present the first evidence of a general synthesis of nanocrystalline metal borides with controlled stoichiometry using syntheses in inorganic molten salts, by focusing on the B-poor Ni-B and B- rich Y-B systems. Depending on the ratio between metal and boron precursors, the temperature and reaction time, NiB, Ni4B3, Ni2B, Ni3B, YB6 and YB4 can be selectively fabricated (Fig. 3). Furthermore, we highlight the occurrence of a peculiar anisotropic uniaxial defective crystal structure within spherical particles that originate from crystallization in confined space. Finally, we report for the first time the ability to reach anisotropic nanoparticles, as a first step toward morphological control of crystalline metal boride nanostructures. Results Briefly, all syntheses were performed under inert argon atmosphere. Prior to heating, metal chlorides and sodium borohydride were mixed under argon with the powder mixture of LiCl and KCl at the eutectic composition (45/55 wt.). After reaction and cooling, the frozen salt matrix was washed with deionized water, the remaining black powders were dried under vacuum and stored in air. 3 Nickel borides Starting from NiCl2 as metal precursor, the reaction medium was heated at 750 °C during 1.5 h. Different NiCl2:NaBH4 (Ni:B) molar ratios were investigated. The corresponding X-Ray diffraction (XRD) patterns (Fig. 4) show that for Ni:B = 1:2, NiB is the major product, with broad reflections suggesting nanosized crystallites. The apparent crystallite size calculated according to the Scherrer formula is 9±2 nm. Minor peaks of Ni4B3 are also observed. For Ni:B = 1:1.2; 1:1 and 0.65, Ni4B3, Ni2B and Ni3B are isolated as single crystalline phases, respectively. Minor parasite peaks are observed in the two last patterns. They could not be indexed along with a known phase. In contrast to NiB, Ni4B3, Ni2B and Ni3B patterns show narrow peaks that suggest larger particles. The calculated crystallite sizes for Ni4B3, Ni2B and Ni3B are 29±6, 33±3 and 40±2 nm, respectively. Fig. 4 Powder XRD patterns for NiCl2:NaBH4 molar ratios 1:2, 1:1.2, 1:1 and 1:0.65. Transmission electron microscopy (TEM) of the NiB (NiCl2:NaBH4 = 1:2) sample (Fig. S1) reveals spherical crystalline nanoparticles of 2 to 40 nm, in agreement with the crystallite size, dispersed within an amorphous matrix of 1 to 2 nm thickness. The smaller the particles, the more they tend to perfect spheres. Particles bigger than 20 nm are slightly faceted. High resolution TEM (HRTEM) (Fig. 5) highlights lattice fringes in a spherical NiB nanoparticle. Many defects are observed and demonstrate the polycrystalline nature of each particle.