Phase and Morphology Control of Hexagonal MoO₃ Crystals via Na⁺ Interactions: A Raman Spectroscopy Study

Abstract

We present the effect of sodium ions (Na + ) on the nucleation process and phase selectivity for the formation of hexagonal molybdenum trioxide crystals (h-MoO 3 ). The phase selectivity during the reaction is attributed to the interaction of Na + with the molecules in our precursor solution formed by metallic molybdenum dissolved in a mixture of hydrochloric and nitric acids. The vibrational characteristics of the precursor solutions were studied by Raman spectroscopy in combination with density functional theory modeling, showing the presence of [MoO 2 Cl 3 (H 2 O)] − ions within the solutions. The symmetric stretching vibration of the Mo−O bonds found at 962 cm −1 in [MoO 2 Cl 3 (H 2 O)] − proved that the addition of Na + (in the form of dissolved NaCl) to the precursor solutions resulted only in an electrostatic interaction with the aquo (H 2 O) and chloro (Cl − ) ligands in the complex. After heating the precursor solutions, Xray diffraction, Raman spectroscopy, and scanning electron microscopy of the obtained powders showed that adding NaCl contributed to the phase selectivity of the reaction, with the Na + ions playing a vital role in the formation of h-MoO 3 over other crystalline phases. Based on the nature of the molybdenum complexes found in the precursor solutions and the structural characteristics of the powders, a formation mechanism to obtain h-MoO 3 is proposed. Additionally, the phase stability of h-MoO 3 crystals was studied by calorimetry techniques, showing that h-MoO 3 transforms to α-MoO 3 at ∼649 K. These results provide important insights into phase control to selectively form hexagonal MoO 3 .

Predicting the Geometry of Core–Shell Structures: How a Shape Changes with Constant Added Thickness

“…Solid nanoparticles can be observed using techniques such as electron microscopy. 1,16,[20][21][22][23][24][25]28 Similarly, protein structures, and thus their shapes, may be experimentally determined using, for example, X-ray diffraction, NMR, and cryo-electron microscopy. 29−31 However, for both nanoparticles and proteins, we are limited to measurements of shape only after fully determining their structure with very little a priori understanding of the shape.…”

“…In many of these scenarios, the shape of the core/shell assembly is important to the function of the particle. ,,− Nanoparticles may form any number of shapes, and control of nanoparticle shapes is often a highly prized synthetic target. − Controlling shape may lead to consistent characteristics that enable properties to be tuned. , In solute physics, the shape of the cavity a solute occupies within a solvent is vital to understanding the solute–solvent interactions involved. ,,, This is especially true for proteins, where the shape is both extremely complex and vital to their function. , The first solvation shell around the particles or macromolecules itself may also be of great interest. For example, dynamic light scatteringa ubiquitous method for measuring particle sizetracks the hydrodynamic size of the particle and, therefore, includes not just the particle but also what diffuses with the particle …”

“…In all of these cases, little is known about how the interior and exterior shapes of core/shell-type objects are related. Solid nanoparticles can be observed using techniques such as electron microscopy. ,,− , Similarly, protein structures, and thus their shapes, may be experimentally determined using, for example, X-ray diffraction, NMR, and cryo-electron microscopy. − However, for both nanoparticles and proteins, we are limited to measurements of shape only after fully determining their structure with very little a priori understanding of the shape. This strictly post hoc view precludes a broader understanding of the shape and the impact of a shell.…”

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity

“…The Raman spectrum (Figure 2b) of compound 1 displays the characteristic peaks of h-MoO 3 , well in agreement with previous literature. 10,16 The Raman band at 976 cm −1 belongs to the symmetric stretching vibration of the Mo�O bond. The intense bands at 900 and 883 cm belonging to E g mode, and the relatively weak band at 315 cm −1 is assigned to the A 1g representation.…”

“…Molybdenum trioxide (MoO 3 ) is a low-cost, environmentally benign transition metal oxide known for the intriguing structural and functional properties of its various crystalline phases. , Various polymorphs of MoO 3 are known until date: the better known and explored, stable orthorhombic α-MoO 3 and three metastable phases, namely, monoclinic β-MoO 3, hexagonal MoO 3 , and the monoclinic high-pressure variant MoO 3 –II/ε-MoO 3 . , The basic {MoO 6 } octahedral unit in each of these phases is arranged differently and this arrangement is largely influenced by the synthetic conditions like temperature, pressure, pH, and counter cations. − α-MoO 3 is built by octahedral {MoO 6 } units sharing corners along the [100] direction and sharing edges along [001], resulting in a two-dimensional layered structure (ABA stacking) . β-MoO 3 , the monoclinic distorted phase, has a three-dimensional ReO 3 -related structure, based on corner-shared octahedral {MoO 6 } units .…”