Draft:Molybdenum boride

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Introduction

Molybdenum borides are compounds comprised of molybdenum (Mo) and boron (B) atoms, with different atomic compositions and material structures. They are a member of the family for transition metal borides (TMBs), as molybdenum is a transition metal located in group 6 and period 5 of the periodic table, and the electronegativity for molybdenum and boron, on the Pauling scale, are 2.16 and 2.04, respectively.[1]

Since the electronegativity of molybdenum and boron are relatively similar, indicating that both atoms are similarly capable of attracting electrons when bonded, the covalent character of the ionic Mo-B bonds is relatively strong. As a result, combined with B-B bonds that are covalent and Mo-Mo bonds that are metallic, the complex bonding interactions resulted in a wide range of atomic compositions and arrangements for molybdenum borides, as well as various unique material properties. Additionally, due to the fact that molybdenum is a d-block element with flexibility in its oxidation states, molybdenum borides demonstrate enhanced performances in optoelectronic and catalytic applications.[2]

Molybdenum borides that have been in the scope of research include, but not limited to, metal-rich molybdenum borides (such as Mo3B and Mo2B), molybdenum monoborides (α-MoB and β-MoB), molybdenum diborides (α-MoB2 and β-MoB2), and boron-rich molybdenum borides (such as MoB3, MoB4, and MoB5).[3][4][5][6]

Crystal structures

Metal-rich molybdenum borides

A diagram of the crystal structure of Mo2B (viewed along the [111] direction) using experimental values

In a research of Mo3B thin films with thicknesses of 6.48nm, the crystal structure of Mo3B was determined to be part of the hexagonal crystal system. Particularly, it had the space group of P63/mmc (No. 194) and lattice parameters a = 4.568 Å and c = 7.150 Å, with 14 atoms in the unit cell. In addition, the molybdenum atoms were found to occupy 2b and 4f Wyckoff sites while the boron atoms occupied 2c Wyckoff sites, when the crystal structure was projected from the top view perspective.[3]

For Mo2B, experimental results demonstrated that the compound had the I4/mcm (No. 140) space group of the tetragonal crystal system, with lattice parameters a = 5.547 Å and c = 4.739 Å, while the molybdenum atoms occupied 8h Wyckoff sites and the boron atoms occupied 4a Wyckoff sites.[4] Computationally, it was found that the crystal structure with the I4/m (No. 87) space group was more energetically favorable, which had lattice parameters a = 5.56 Å and c = 4.75 Å as the molybdenum atoms occupied 8h Wyckoff sites and the boron atoms occupied 4e Wyckoff sites.[6]

Molybdenum monoborides

A diagram of the crystal structure of α-MoB (viewed along the [111] direction)
A diagram of the crystal structure of β-MoB (viewed along the [111] direction)

For molybdenum monoborides, there are two phases that have been widely investigated, α-MoB and β-MoB. Particularly, it was found that the α-MoB phase was most stable at lower temperatures, while the β-MoB phase emerged at higher temperatures and became the only phase for molybdenum monoboride at temperatures higher than 2600K, which was verified by X-ray diffraction patterns for molybdenum monoborides synthesized at different temperatures.[5]

For the α-MoB phase, it had the I41/amd (No. 141) space group of the tetragonal crystal system, with lattice parameters a = 3.118 Å and c = 16.936 Å and both molybdenum and boron atoms at 8e Wyckoff positions with distinct fractional coordinates.[5]

On the other hand, for the β-MoB phase, it had the Cmcm (No. 63) space group of the orthorhombic crystal system, with lattice parameters a = 3.142 Å, b = 8.481 Å and c = 3.077 Å and both molybdenum and boron atoms at 4c Wyckoff positions with distinct fractional coordinates.[5]

Computational methods have also yielded similar results for both the α-MoB and β-MoB phases while the phase transition temperature was predicted to be around 1900K.[6]

Molybdenum diborides

A diagram of the crystal structure of α-MoB2 (viewed along the [111] direction)
A diagram of the crystal structure of β-MoB2 (viewed along the [111] direction)

For molybdenum diborides, the existence of two phases was found as well, which were α-MoB2 and β-MoB2 phases. Particularly, experimental results showed that the α-MoB2 phase had the P6/mmm (No. 191) space group in the hexagonal crystal system, with lattice parameters a = 3.046 Å and c = 3.071 Å. The molybdenum atoms were found to occupy 1a Wyckoff positions while the boron atoms were found at 2d Wyckoff positions.

For the β-MoB2 phase, it had the R3m (No. 166) space group of the trigonal crystal system, with lattice parameters a = 3.012 Å and c = 20.943 Å. In the β-MoB2 phase, both the molybdenum atoms and boron atoms occupy 6c Wyckoff positions with distinct fractional coordinates.[4]

Computational calculations verified the existence of both phases and showed that β-MoB2 was the phase with the lowest energy. An energy difference of 152 meV/atom was also calculated between the α-MoB2 and β-MoB2 phases.[6]

Boron-rich molybdenum borides

For boron-rich molybdenum borides, experimental characterization, of which X-ray diffraction (XRD) patterns have been exceptionally important, start to become complicated since the precise atomic arrangements of boron atoms that are much lighter than molybdenum atoms were difficult to be captured. As a result, computational methods have been shown to be helpful for determining the potential positions of the boron atoms in the lattices and predicting certain material properties for these compounds.

In a computational research, it was calculated that MoB3 had two crystal structures that were both metastable, one having the R3m (No. 166) space group and the other having the P63/mmc (No. 194) space group, with the latter becoming stabler at higher temperatures. For MoB4, computational results showed that the structure had the P63/mmc (No. 194) space group, while the calculated XRD patterns were distinct from experimental measurements of MoB4 samples. The research also predicted the existence of MoB5 through calculations, which was comprised of hexagonal prisms of Mo-B bonds and multiple boron clusters that result in a unique three dimensional structure. The structure for MoB5 was shown to have the Pmmn (No. 59) space group of the orthorhombic crystal system.[6]

Synthesis

Many synthesis processes were used to produce different stoichiometric compounds of molybdenum borides with various crystal structures.

Chemical vapor deposition

Thin films of molybdenum boride, particularly Mo3B, were synthesized by chemical vapor deposition, in which foils of molybdenum served as the source for forming the Mo3B crystals as a diboron dioxide vapor produced from high temperature heating and transported by hydrogen gas reacted with the foils. After deposition, the Mo3B films were extracted from the molybdenum foils using PMMA and acid treatment, and transferred to silicon dioxide substrates.[3]

Heat-pressure treatment

The heat-pressure treatment procedure involved the mixing of molybdenum metal with amorphous boron with different molar ratios, depending on the atomic compositions of the desired molybdenum boride products. After thorough grinding for the mixture to take the form of uniform powders, it underwent cold-pressing for optimized material shape and was treated under 5.2 GPa of pressure and temperatures ranging from around 1900K to 2100K for different periods of time, ranging from 15 minutes to 60 minutes. This method was utilized for the synthesis of Mo2B, α-MoB, α-MoB2, and β-MoB2 crystals.[4] For the synthesis of β-MoB crystals, which were stable at high temperatures, a temperature of 2600K was used for treating the samples for 15 minutes.[5]

Additionally, it was shown that pure heat treatment without high pressure could also result in the synthesis of molybdenum boride compounds, as heating around 1700K to 2100K with flowing Argon gases for prolonged periods of time, ranging from 7 hours to 60 hours, resulted in molybdenum diboride crystals and α-MoB and β-MoB phases.[7]

Arc-melting procedure

In this method, a high temperature furnace was no longer needed, as molybdenum and boron powders were cold-pressed and arc-melted using a range of currents applied between the sample and a tungsten electrode. The mixtures were repeatedly arc-melted at a high current (40 A to 60 A) with a lower current of 10 A in between. Using this method, the synthesis of Mo2B, α-MoB, β-MoB, and molybdenum diboride crystals were demonstrated.[8]

Molten-salt treatment

For treatment with molten salt, the temperature was increased to around 1200K to 1300K, lower than that required for the heat-pressure treatment, to maintain the liquid equilibrium phase for salts like potassium chloride (KCl) and sodium chloride (NaCl). Subsequently, molybdenum oxides and boron powders were added and reacted in the molten salts, producing molybdenum diborides and boron oxides. The desired molybdenum diborides were eventually extracted from the molten salts and boron oxides through dissolution and filtration processes. [9]

Layer-etching procedure

An important method for producing thin films and nanosheets of molybdenum boride compounds is the layer-etching procedure, in which MAB phases (with molybdenum as the M elements, boron as the B elements, and usually aluminum as the A elements) were etched in acids, such as hydrofluoric acid in an attempt to remove the layers with A elements. However, due to the structural difference between molybdenum aluminum borides (MoAlB) and molybdenum monoborides, the etching could only result in partial molybdenum boride nanosheets.[2]

Applications

Due to the wide range of atomic compositions and crystal structures for molybdenum boride compounds, the applications of such materials cover many important fields of modern research and industry.

Electrocatalysis

Molybdenum borides have been used extensively in the field of electrocatalysis, especially for the hydrogen evolution reaction (HER). Particularly, it was found that the catalytic activities increased in the order for MoB2, α-MoB, β-MoB2, and α-MoB2, as current densities reached -100 mA/cm2 at around -0.22 V vs. RHE for α-MoB2 in 0.5 M sulfuric acid. The reason for the enhanced electrocatalytic performance is due to the fact that α-MoB2 demonstrated high densities of active sites for catalysis on all interfaces, either molybdenum-terminated or boron-terminated. As a result, the active sites that are independent of the material's facets in α-MoB2 enhanced HER activity comparing to materials that only have active sites along certain facets.[9]

Additionally, thin films of Mo3B were also shown to have enhanced electrocatalytic performances for HER, as films synthesized at around 1100 K were optimal and were capable of achieving current densities of -20 mA/cm2 at around -0.25 V vs. RHE, comparing to a platinum foil that reached similar current densities at around -0.12 V vs. RHE in 0.5 M sulfuric acid. The Mo3B films also demonstrated strong stability over thousands of cycles of catalysis at HER potentials.[3]

Energy Storage

Using theoretical calculations for Mo2B, it was found that such compound was ideal for serving as the anodic electrode in lithium and sodium ion batteries, since the undesired formation of lithium and sodium metals during cycling of charging and discharging in the battery would be suppressed. Additionally, outstanding mechanical properties for molybdenum diborides made the compounds optimal for flexible electrodes. Molybdenum monoboride layers synthesized by layer-etching of MoAlB also demonstrated stable reversible capacity over many cycles.[2]

Spectroscopy Applications

Since molybdenum borides have complicated and unique atomic arrangements, they are capable for being used in surface-enhanced Raman spectroscopy (SERS) due to optimal charge transfers. It was found that molybdenum borides produced by the layer-etching procedure demonstrated enhanced performances during SERS, when compared to other SERS substrates. This is because the density of states near the Fermi level for the electrons of molybdenum boride nanosheets is high, making such material capable of sensing small signals during SERS.[2]

References

  1. Allred, A. L. (1961). "Electronegativity values from thermochemical data". J. Inorg. Nucl. Chem. 17: 215–221. doi:10.1016/0022-1902(61)80142-5.
  2. Rout, C. S.; et al. (2024). "Recent developments and future perspectives of molybdenum borides and MBenes". Adv. Sci. 11: 2308178. doi:10.1002/advs.202308178.{{cite journal}}: CS1 maint: article number as page number (link)
  3. Wang, X.; et al. (2017). "Ultrathin molybdenum boride films for highly efficient catalysis of the hydrogen evolution reaction". J. Mater. Chem. A. 5: 23471–23475. doi:10.1039/c7ta08597d.
  4. Chen, Y.; et al. (2017). "Highly active, nonprecious electrocatalyst comprising borophene subunits for the hydrogen evolution reaction". J. Am. Chem. Soc. 139: 12370–12373. doi:10.1021/jacs.7b06337.
  5. Tao, Q.; et al. (2019). "Modulating hardness in molybdenum monoborides by adjusting an array of boron zigzag chains". Chem. Mater. 31: 200–206. doi:10.1021/acs.chemmater.8b04154.
  6. Rybkovskiy, D.; et al. (2020). "Structure, stability, and mechanical properties of boron-rich Mo−B phases: a computational study". J. Phys. Chem. Lett. 11: 2393−2401. doi:10.1021/acs.jpclett.0c00242.
  7. Klesnar, H. (1996). "The diboride compounds of molybdenum: MoB2−x and Mo2B5−y". J. Alloys Compd. 241: 180–186. doi:10.1016/0925-8388(96)02294-3.
  8. Park, H. (2017). "Boron-dependency of molybdenum boride electrocatalysts for the hydrogen evolution reaction". Angew. Chem. Int. Ed. 56: 5575–5578. doi:10.1002/anie.201611756.
  9. Chen, H. (2020). "Intermetallic borides: structures, synthesis and applications in electrocatalysis". Inorg. Chem. Front. 7: 2248–2264. doi:10.1039/d0qi00146e.