Structural determination of MoO3 polymorphsDEST-synthesized MoO3 samples such as (1) am-MoO3-x solvolyzed at 60 °C in water (MOw60), (2) am-MoO3-x solvolyzed at 90 °C in water (MOw90) and (3) am-MoO3-x solvolyzed at 90 °C in iso-propanol (MOipa90) were subjected to detailed functional group analyses (FT-IR, Raman, UV–Vis, XRD, XPS), after their phase transition, to investigate the chemical bonding states between molybdenum and oxygen atoms.FT-IR spectra (Fig. 1) confirmed that MOw60 and MOipa90 were exhibiting almost similar characteristic responses like (i) a shallow peak at 3423 cm−1 corresponding to H–OH stretching vibration of firmly bounded water molecules in the sample19, (ii) less intense peaks at 2916 cm−1, 2845 cm−1 denoting C–H stretching, (iii) a prominent peak at 1612 cm−1 attributed to H–O–H bending vibration, (iv) then finger print region with some clear peaks at 970 cm−1, 906 cm−1 due to stretching of Mo=O double bond vibrations of hexagonal phase20, and a peak at 548 cm−1 due to single oxygen atom interaction with three molybdenum atoms21 (Fig. 1). MOw60 and MOipa90 samples having two peaks present at 2916 cm−1 and 2845 cm−1 were attributed to –CH2 vibration22 caused by any organic impurities in H2O or iso-propanol.Figure 1FTIR spectra of DEST-made MoO3 polymorphs materials.On the other hand, MOw90 sample did not show any peaks at higher wavenumbers (from 3500 cm−1 to 1700 cm−1) but at 1612 cm−1 and the finger print region, similar to MOw60 (Fig. 1). The finger print region in MOw90 was slightly blue shifted to (a) 987 cm−1, (b) 864 cm−1, (c) 601 cm−1, and (d) 466 cm−1, attributable to (a) Mo=O stretching vibration which is representing the stable, layered orthorhombic MoO3 phase, (b) orthorhombic MoO3 structure having Mo–O–Mo bonding with stretching vibrations of O3 atoms23, (c) Mo–O vibrations21, (d) Mo2–O4 bonding due to solvolyzing H2O molecules, respectively. From these results, it was confirmed that the samples MOw60 and MOipa90 were in hexagonal crystalline structure (h-MoO3) and MOw90 was in orthorhombic crystalline structure (α-MoO3). In order to confirm the phase transition further, the Raman Spectroscopy was applied next.The micro-Raman spectra of all three MoO3 samples (MOw60, MOw90, MOipa90) were collected under 523 nm excitation and recorded in the range from 1100 to 100 cm−1 as shown in Fig. 2. The bands appeared between 100 to 400 cm−1 and 600 to 1000 cm−1 were representing bending, stretching vibrations of Mo–O/Mo=O. These are the lattice modes giving rise to the bands in lower region of MoO6 octahedra in h-MoO3 and α-MoO3, respectively24. The peaks appeared from 100 to 300 cm−1 were due to the MoO6 skeleton base structure, the peaks between 300 and 700 cm−1 were assigned to O–Mo–O bonds and the absorption at high wave number range, 700 to 1000 cm−1, corresponded to Mo–O–Mo bonding. A well-defined sharp peak present at 820 cm−1, along with other main peaks 666 and 993 cm−1 were reflecting the fingerprint region of the stable α-MoO3 phase24,25,26. A tiny peak at 909 cm−1 present in MOw90, MOipa90 samples could be an indication of shared identity of a particular crystalline phase between these two samples. The detailed Raman vibrational analysis of bonds for h-MoO3 and α-MoO3 in comparison with literature evidences is shown in supplementary information file (SI); Tables S1 and S2. From this, the formation of h-MoO3 (MOw60, MOipa90) and α-MoO3 (MOw90) in our experimental conditions were confirmed. Further to determine the electronic states/band gaps in these polymorphs, the synthesized MoO3 samples (MOw60, MOw90, MOipa90) were subjected to UV–Vis absorption spectroscopy.Figure 2Raman spectra of DEST-made MoO3 polymorphs materials.The UV–Vis spectroscopic study, as illustrated in Fig. 3a, provides detailed insights into the optical properties of the samples, MOw60, MOw90, and MOipa90. A distinct excitonic absorption peak at 212 nm was observed in all three samples, indicating the presence of 2D MoO3 nanosheets27. This characteristic excitonic absorption is a hallmark feature of 2D materials. Furthermore, additional band edge absorptions were observed at different wavelengths for each sample: 300 nm for MOw60 (h-MoO3), 385 nm for MOw90 (α-MoO3), and 315 nm for MOipa90 (h-MoO3). These characteristic absorption wavelengths indicated variations in the electronic properties and band structures in these samples. Thus, confirming different phases of MoO3. Figure 3b showcases the determination of band gaps (Eg) through Tauc plot analysis using the UV–Vis spectrum. The calculated Eg values for MOw60 (h-MoO3), MOw90 (α-MoO3), and MOipa90 (h-MoO3) were 2.94 eV, 2.45 eV, and 1.51 eV respectively. These values did match with those values reported in literature by Lei Zheng et al.10, Hanmei Hu et al.28, and Yuehong Song et al.29, providing validation for the accuracy of the experimental measurements.Figure 3UV–Vis spectra of DEST-made MoO3 polymorphs materials. (a) Absorbance spectra, (b) Tauc plot for finding Eg.However, some more clarity was needed to distinguish MOw60 from MOipa90 as though they showed similar IR, Raman spectral lines, their UV–Vis spectral data revealed that there were some minute variations between them. Also, in Raman Spectroscopic data, there was an ambiguity in identifying crystalline phases, as all three samples showed the distinct peaks at 666 cm−1, 820 cm−1 and 993 cm−1. To sort this out, we went for an extensive crystal structural analysis of these MoO3 polymorphs using XRD technique.Crystal structural analysis of MoO3 phasesThe most important analysis to prove the phase transition in as-synthesized MoO3 samples was X-ray diffraction studies (XRD). Figure S1 shows the XRD patterns of commercially purchased stock MoO3 (bulk powder), physical vapor deposited MoO3-x (nanopowder), and then recrystallized MoO3 samples from water (MOw60, MOw90) and isopropanol (MOipa90), along with simulated diffraction patterns of h-MoO3 and α-MoO3. This direct comparison allowed us to assess the peak positions and intensities of the corresponding phases.The thin films of MoO3 (thickness of 150 nm) deposited on glass substrates by PVD, are usually amorphous and sub-stoichiometric (am-MoO3-x) in nature. The absence of sharp peaks and presence of broad hump (26.17°) in the XRD pattern confirmed the amorphous (am-) nature of as-deposited thin films (Fig. S1). This hump was due to weak diffraction, suggesting that the film possesses a combination of amorphous and crystalline characteristics (64% amorphous and 36% crystalline).The crystallinity percentage was calculated using the following formula;$${\text{Crystallinity }}\;(\%) = \frac{{\text{Area of crystalline peaks }}}{{{\text{Area of crystalline peak}} + {\text{Area of amorphous peak}}}} \times 100$$Contrary to am-MoO3-x, the recrystallized MoO3 samples; MOw60, MOw90, MOipa90, were exhibiting sharp distinct peaks in the XRD pattern, suggesting phase transition and enhanced crystallinity (Fig. S1, 4). The samples recrystallized using water (MOw60) exhibited a phase transition from amorphous (am-MoO3-x) to hexagonal structure (h-MoO3). However, with an increase in recrystallization temperature (MOw90), a complete phase transition from amorphous (am-MoO3-x) to thermodynamically stable orthorhombhic structure (α- MoO3) was observed. We also believe that this transition should have happened via the intermediate, which is metastable h-MoO3 form, as we already know that h- structure was the resultant phase at 60 ° C in H2O. Conversely, in the case of MoO3 sample recrystallized using iso-propanol (MOipa90), a mixed phases of h and α- MoO3 product was obtained, suggesting an initial phase transition from am-MoO3 -x to h-MoO3, followed by a partial phase transition from h-MoO3 to α-MoO3 occurred in this sample. The measured diffraction patterns of MoO3 phases were analyzed by using the Rietveld refinement technique (assisted by FULLPROF computer program).Figure 4X-ray diffraction patterns refined by Rietveld method for DEST-made MoO3 polymorphs materials: (a) MOw60 (h-MoO3), (b) MOw90 (α-MoO3), (c) MOipa90 (mixed phases of h and α-MoO3). Black and red lines (through the data points) are representing the experimental and the calculated patterns, respectively. The difference between the experimental and the calculated patterns is shown by the blue lines at the bottom of each panel. The vertical green bars represent the allowed nuclear Bragg positions.The Rietveld analysis of the XRD of MOw60 sample, synthesized at 60 °C in H2O, revealed a pure single-phase of h-MoO3, confirmed by the space group P 63/m and space group number 176 (Fig. 4a). This analysis counted the lattice parameters of h-MoO3 as follows: a = b = 10.6133(2), c = 3.7243(3), α = β = 90°, and γ = 120°. The fractional coordinates of the atomic positions are presented in Table S3.This analysis further revealed that the Mo ions reside in octahedral arrangement with the O atoms (Fig. 5a) and no monoclinic β-MoO3 structure (Fig. 5b) was obtained with any of the synthesized samples (MOw60, MOw90, MOipa90). The hexagonal crystalline structure (h-MoO3) of the MOw60 sample possess 1D tunnels along crystallographic c-axis, which is made up of the zigzag chains of MoO6 octahedra, the very building block of the crystal (Fig. 5c).Figure 5A model unit cell crystal structure of (a) MoO3, (b) β-MoO3, (c) h-MoO3 and (d) α-MoO3.The Rietveld analysis of the XRD of MOw90 sample, synthesized at 90 °C in H2O, showed again a single-phase, which is thermodynamically stable orthorhombic crystal symmetry (α-MoO3), confirmed by the space group P n m a (Fig. 4b). The lattice parameters of α-MoO3 phase were found to be a = 3.9591(2), b = 13.8611(6), c = 3.7243(3), α = β = γ = 90°. The fractional coordinates of the atomic positions are presented in Table S3. The orthorhombic structure of α-MoO3 consists of layered planes of the foundational octahedron (oh-MoO6) units, in double layer form (Fig. 5d). These stacked bilayers of distorted MoO6 octahedral units arranged in an ABAB… pattern with edge-sharing zigzag rows and corner-sharing rows, along the [001] and [100] directions.The Rietveld analysis of the XRD of MOipa90 sample, synthesized at 90 °C in iso-propanol ((CH3)2CHOH) confirmed the presence of (010) and #(020) of h-MoO3 and α-MoO3 phases respectively (Fig. 4c), indicating the composition of mixed phases, but predominately composed of h-MoO3 (space group P 63/m and space group number 176). This analysis (considering both phases) further suggested that the mixed phase contains 72.6% of h-MoO3 and 27.4% of α-MoO3. The refined crystal structural parameters of mixed phase are as follows (Table S4): a = b = 10.6134(2), c = 3.7207(3), α = β = 90°, and γ = 120° for h-MoO3, and a = 3. 9832(3), b = 13.8673(5), c = 3.7192(4), α = β = γ = 90° for α-MoO3.Thus, the XRD measurements have confirmed the phase transitions occurred in as-synthesized MoO3 samples, very lucidly and proved that they are polymorphs. XRD data also nullified the ambiguity that we had from UV–Vis and Raman spectral data, that (i) whether the phase change happened in MOw60 and MOipa90 are same or not, and (ii) the formation of α-MoO3 in MOw90. Henceforth, we mark the polymorph of MOipa90 as ‘h and α-MoO3’ instead of just ‘h-MoO3’. Therefore, the facile vacuum assisted solvothermal (VST) approach has yielded a very nice phase transition (am → h → α) even at very low temperature in the presence of a protic, green solvent like water.Surface morphology and topography analysisThe h-MoO3 and α-MoO3 represent two distinct polymorphs of MoO3 with unique structural and electronic properties. Understanding their surface morphology and topography is crucial for exploring their potential applications further. High Resolution Transmission Electron Microscopy (HRTEM) analysis was employed to understand the surface morphology and crystal structure of MoO3 polymorphs synthesized by DEST method. Images of low magnification, lattice fringes and SAED pattern of the materials (MOw60, MOw90, MOipa90) are shown in Fig. 6. All the samples were showing a two-dimensional structure in few hundreds of nanometer size. The interplanar spacing analysis of MoO3 polymorphs, conducted in lattice fringe images of HRTEM provides crucial insights into the material’s structural characteristics at the atomic level. By measuring the distances between adjacent crystal planes along various crystallographic directions, the arrangement of atoms within the MoO3 lattice can be understood.Figure 6HRTEM images of DEST-made MoO3 polymorph 2D nanosheet materials.The obtained interplanar spacing results for hexagonal molybdenum trioxide (h-MoO3) of MOw60, were d = 3.47 Å (011), d = 3.4 Å (120), and d = 3.05 Å (030). These (011), (120), and (030) planes confirmed the unique arrangement of atoms within the hexagonal lattice of MoO3. The orthorhombic molybdenum trioxide (α-MoO3) of MOw90 exhibited interplanar spacings of d = 3.2 Å (210) and d = 3.8 Å (101). The interplanar spacing of 3.2 Å along the (210) crystallographic plane indicated the short distance between adjacent planes oriented in this direction, revealing the compact nature of the atomic arrangement within this plane. Conversely, the interplanar spacing of 3.8 Å along the (101) plane signified a larger separation between adjacent planes. These findings shed light on the anisotropic nature of α-MoO3, where the interplanar spacings vary along different crystallographic directions. The mixed-phases of MOipa90 (h and α-MoO3) showed interplanar spacings of d = 2.2 Å (501) and d = 2.5 Å (121), which was smaller than what pure, single phase h-MoO3 and α-MoO3 showed.The bright pointed diffraction spots appeared in Selected Area Electron Diffraction (SAED) pattern confirmed high order of crystallinity in all three samples (MOw60, MOw90, MOipa90) (Fig. 6). From SAED pattern, the crystalline phases of MOw60, MOipa90, were confirmed as hexagonal (h-MoO3) structure. On the other hand, MOw90 was confirmed to be orthorhombic (α-MoO3) crystal structure. The samples showed different types of lattice fringes with respect to oxygen defects formed during heat treatments (w60, w90, ipa90) under vacuum. The crystal structures of MoO3 became irreversibly distorted as oxygen was removed initially from the lattice structure, followed by its refilling and the intercalation of H+ ions from solvents30,31. This proved our claim again that the DEST method could efficiently bring a phase transformation even at very low temperature providing optimized solvothermal conditions (VST) are used.Further, we have also investigated the microscopic visuals of 2D nanosheets of h-MoO3 (MOw60), α-MoO3 (MOw90) and mixed phases (h and α-MoO3) of MOipa90, using SEM and AFM techniques as shown in Figs. 7a–c and 8 respectively.Figure 7SEM images of DEST-made MoO3 polymorph 2D nanosheet materials: (a) MOw60 (h-MoO3), (b) MOw90 (α-MoO3), (c) MOipa90 (h and α-MoO3).Figure 8AFM images of DEST-made MoO3 polymorph 2D nanosheet materials: (a) MOw60 (h-MoO3), (b) MOw90 (α-MoO3), (c) MO ipa90 (h and α-MoO3).The scanning electron microscopy (SEM) images clearly revealed sheet-like structures with diameters ranging from 1 to 2 microns for all the samples, effectively suggesting two-dimensional (2D) morphology (Fig. 7). Also, the images suggested that the sheets were formed uniformly in dimension, thus our synthesis method offers the feasibility of controlling the dimensions of the MoO3 polymorphs at the micron scale level. This observation highlights the successful fabrication of MoO3 polymorphs in our desired 2D form which holds significant implications in potential applications. Figure 8 illustrates AFM images of MoO3 polymorphs. AFM imaging provided a high-resolution representation of the topography of the samples; MoO3 nanoparticles. From the images, we can clearly see the individual nanoparticles appearing as distinct features on the substrate. This allows us to determine the size, shape and arrangement at the nanoscale.Further, the oxidation states of Mo in the material and chemical bonding in the surface of samples (MOw60, MOw90, MOipa90) were investigated by the X-ray Photoelectron Spectroscopy (XPS) (Fig. 9a, b). The full energy survey spectrum (Fig. S2) of MoO3 polymorphs confirmed the presence of Mo4p, Mo3d, C1s, Mo3p and O1s peaks at their characteristic binding energies that validated the identity of MoO3 in the sample.Figure 9XPS spectra of DEST-made MoO3 polymorph 2D nanosheet materials (a) Mo-3d core level region, (b) O-1s region.In Fig. 9a, the deconvolution of the Mo spectrum scan revealed distinct peaks at (236.53 eV, 233.38 eV)32,33, and (236.98 eV, 233.78 eV)19,32,34, for MOw60 and MOw90 respectively. These peaks correspond to the spin–orbit doublets (3d5/2 and 3d3/2) of Mo6+, thereby confirming the presence of MoO3. However, these peaks slightly shifted to low binding energies (235.98 eV, 232.88 eV) for MOipa906,10,35,36. This shift is due to subtle differences in the local chemical environment which influences the binding energies of Mo-related peaks in the XPS spectra37,38. O-1s peaks (Fig. 9b) also showed similar shift (531.28 eV, 531.78 eV, 530.78 eV for MOw60, MOw90, MOipa90), indicating variations in the oxygen bonding environment among different polymorphs.To be specific, Table S5 shows that the Mo-3d3/2, Mo-3d5/2, O-1s peak positions, shifted 0.45 eV. 0.40 eV, 0.5 eV respectively, between the metastable h- and most stable α- phases of MoO3. Similarly shifts of 0.55 eV, 0.50 eV, 0.50 eV noticed for the same peaks correspondingly, between the metastable h- and mixed phases (h and α-MoO3) of the material. Further clear shifts of 1 eV, 0.9 eV, 1 eV were observed for the above-mentioned peaks respectively, between the most stable α-phase and mixed phases (h and α-MoO3) of the material. Overall, the changes in surface chemistry alter the electron density near the surface, influencing the binding energies of photoelectrons, which resulted in XPS peak shifts.We believe that iso-propanol mediated vacuum assisted solvothermal step (VST) could have caused different oxygen defect in MoO3 than the water mediated synthesis. Oxygen defect creates low valent Mo ions, if these low valent ions haven’t regenerated into Mo6+ completely, that could cause a slight shift in the binding energy. We also knew that the MOipa90 produced mixed phases of h-MoO3 and α-MoO3, thus iso-propanol acts very differently from water which produced solely the α-MoO3 at 90 °C.Hypothetical insights into the mechanism of phase transition in MoO3-x (am → h → α-MoO3) under VST conditionsOur aim was to achieve the phase transition at very low temperature using physico-chemical method rather than widely reported high temperature-physical methods. In physical methods, MoO3 is annealed at very high temperature (300–1500 °C) in flow furnaces or chambers, for its phase transition. Thus, consumes lot of energy during its production. We wanted to bring down this energy cost, by introducing a vacuum assisted solvothermal treatment (VST), where careful application of solvent can do a phase transition wonder. This very unique approach and the results, motivated us to discern the mechanistic details based on hypothetical premises.In our studies, defects implantation has been adopted as a nuanced technique for modifying the crystalline phase and its physico-chemical properties, in order to achieve desired energy storage application from it. Defects created in various locations of the lattices, could adjust band structure, electron density and bonding at different degrees. Oxygen vacancies, a kind of intrinsic defect in metal oxides, is one of the most preferred fine tuning at crystal level, in materials designing. Through controlled removal of oxygen atoms from the lattices of metal oxides, the lattices become disordered and low valence metal ions are created. Further, these low-valent metal ions will look for foreign ions or oxygen atoms present in the solution/environment to make new bonding to stabilize themselves. So that, the free energy of the crystal system can be brought down, for better stability. Thus, paving a way for phase transition in the crystalline materials. This will have a major impact on the crystal structure and its inherent characteristics39.Oxide materials are highly prone to induced oxygen vacancy defects, especially when the metal ions are at higher oxidation state. In our case (MoO3), the Mo is at +6 oxidation state, that can be reduced to lower oxidation states, e.g., +4, by creating oxygen vacancies in thermal or chemical environment. It is well-known fact that MoO3 could lose oxygen if it is heated under vacuum or other reducing environment conditions. Under these conditions, it should be noted that MoO3 can quickly break down into lesser oxides40,41,42, where the stoichiometry deviates from the ideal MoO3 composition into MoO3-x, due to the absence of oxygen atoms. These lesser oxides are known to have empirical compositions between MoO3 and MoO2, leading to the formation of low valent Mo4+ ions. Further, these low valent Mo4+ ions can be stabilized by supplying protons and the oxygen vacancies can be refilled by some fractions of oxygen from the protic solvents (Brønsted acids), to get back MoO3. This was the theoretical basis of our experiment and we supplied protons (H+) as foreign ions from water and iso-propanol, to quench the Mo4+ ions, for the regeneration MoO3 with modified phase.In this present work, we believe that the vacuum environment during PVD step, could have created some oxygen vacancies (Voi) already, with the formation of sub-stoichiometric molybdenum oxide, MoO3-x (150 nm thickness), but without crystallinity (Fig. S1). Hence, to complete the recrystallization, we have taken the VST approach, after the MoO3-x extraction step (USE). During VST also, there might be a creation of oxygen vacancies (Voii) due to vacuum, with the formation of MoO3-(x+y). If the thickness of the MoO3-x thin film deposition increased up to 350 nm, then positively charged structural defects would also increase due to the concentration of oxygen vacancies43,44,45.We denote these oxygen vacancies as,
(1)
Voi—Primary oxygen vacancies: Intermediate—amorphous (am-) MoO3-x, x—fraction of oxygen defected created during PVD.
(2)
Voii—Secondary oxygen vacancies: Intermediate—MoO3-(x+y); y—fraction of oxygen defected during VST.
Later, these oxygen defective intermediates were oxygen refilled to some extent and stabilized by protons through thermo-solvolysis at VST step. In VST, we have chosen the primary protic solvent, H2O, as our reagent, which is green (eco-friendly, low cost, renewable) in nature. In order to evaluate its comparative performance, we have also employed iso-propanol ((CH3)2CHOH), another weak protic solvent for solvolysis. In general, water is a highly polar solvent, which makes it effective for dissolving ionic compounds and facilitating reactions involving charged species. Many oxidation reactions are thermodynamically favorable in aqueous environments due to the high dielectric constant and hydrogen bonding capabilities of water. These properties stabilize charged intermediates and transition states, lowering the activation energy barrier for the reaction. After the oxidation reaction, water can be easily separated from the reaction mixture by distillation, evaporation, or other separation techniques. This simplifies product purification and reduces processing costs. iso-propanol is also a moderately polar solvent which can dissolve both polar and non-polar substances and again can stabilize the intermediates through hydrogen bonding or solvation effects, thereby influencing the reaction pathway and product distribution. Here, we mostly rely on the Brønsted–Lowry acidity and dissociation constant of the solvents to impact the hydrolysis of oxides.The vacuum assisted heating (VST) creates the secondary oxygen defects followed by proton-hungry molybdate anions (MoO42−) in the presence of these low-boiling solvents. What these solvents basically do is that, they push/shift the reaction equilibrium towards the right side of the equation, thereby inducing the formation of these anion intermediates according to Le Chatelier’s principle of chemical equilibrium. Thus, we end up with regeneration of MoO3 from the decomposition of molybdic acid, but with modified phase. Based on the product materials’ characterization results, it was confirmed that water mediated VST did yield a successful phase transition (am → h → α) in MoO3-x, whereas iso-propanol mediated VST delivered a partial phase transition (am → h → h and α), leaving a mixed phase product material. However, both the solvents yielded phase transition from am → h in MoO3-x thoroughly and effectively.We reasoned out that the physico-chemical properties of solvents play a major role in facilitating phase transition in MoO3-x during VST. For e.g., Water is a more protic solvent than iso-propanol, because it has ten thousand times higher dissociation constant (H2O: Ka = 1 × 10−14) than iso-propanol ((CH3)2CHOH): (Ka = 1 × 10−18). Hence, water would release more protons in a short span of time, which is needed for the phase transition in MoO3-x.The following equations show a possible hydrolysis reaction between water and MoO3-x containing oxygen vacancy, based on literature46.$${\text{MoO}}_{{3}} \;(\alpha) \to {\text{MoO}}_{{{3} – {\text{x}}}} \;(am)\quad ({\text{PVD}};\;{\text{bulk}} \to {\text{nano}}\;{\text{ size}},\;{\text{Implantation }}\;{\text{of}}\;{\text{Vo}}^{{\text{i}}} \;{\text{defect}})$$$${\text{2MoO}}_{{{3} – {\text{x}}}} \left( {am} \right) \to {\text{2MoO}}_{{{3} – ({\text{x}} + {\text{y}})}}^{{{\text{n}} + }} + { 1/2}\;{\text{ O}}_{{2}} \quad ({\text{VST}};\;{\text{ Implantation }}\;{\text{of}}\;{\text{Vo}}^{{{\text{ii}}}} \;{\text{defect}},\;{\text{at 6}}0{-}{9}0\;^{ \circ } {\text{C/vacuum}})$$$${\text{2MoO}}_{{{2}.{5}}}^{{{1} + }} + {\text{ H}}_{{2}} {\text{O}} \leftrightarrow {\text{MoO}}_{{4}}^{{{2} – }} + {\text{ MoO}}_{{2}} + {\text{ 2H}}^{{ + }{}} \quad ({\text{K}}_{{\text{a}}} = {1 } \times { 1}0^{{ – {14}}})$$$${\text{MoO}}_{{4}}^{{{2} – }} + {\text{ 2H}}^{ + } \leftrightarrow {\text{ H}}_{{2}} {\text{MoO}}_{{4}}$$$${\text{H}}_{{2}} {\text{MoO}}_{{4}} \to {\text{MoO}}_{{3}} \;\left( {h \, \;{\text{or}}\;\alpha } \right) + {\text{ H}}_{{2}} {\text{O}}$$$${\text{MoO}}_{{2}} + {\text{ 1/2 O}}_{{2}} \to {\text{MoO}}_{{3}} \;\left( {h \, \;{\text{or}}\;\alpha } \right)$$According to the above equations, the commercially acquired bulk α-MoO3 material was initially converted into an amorphous (am-) MoO3-x nanomaterial, with implanted primary oxygen vacancies (Voi), at the initial PVD step. Then, these am-MoO3-x nanomaterials were ultrasonically extracted (USE) and subjected to VST for implanting secondary oxygen vacancies (Voii), followed by the formation of a (MoO3-(x+y))n+ cation. This cation immediately got stabilized itself by reacting with solvent environment (H2O) to form intermediates like MoO2 and molybdate anion (MoO42−). Further this anion reacted with available protons to produce less stable molybdic acid followed by its decomposition into molybdenum trioxide, with a new crystal phase (hexagonal h-MoO3 or orthorhombic α-MoO3). The by-product MoO2 also reacted with left over oxygen atoms to produce MoO3 with new phase.In the case of iso-propanol, it released protons slowly or produced lesser number of protons in the medium as its dissociation was weak. Hence, the solvation of (MoO3-(x+y))n+ cation was also slow and weak. Because of the inadequate concentration of released protons (H+) in the medium, the hydrolysis of molybdate anions into molybdic acid, followed by its decomposition into α-MoO3 was only achieved partially according to the stoichiometric equations mentioned below. Therefore, the product was the mixed phases of h and α-MoO3, but dominated by h-phase as evidenced by XRD.$${\text{MoO}}_{{3}} \;\left( \alpha \right) \to {\text{MoO}}_{{{3} – {\text{x}}}} \;\left( {am} \right)\quad ({\text{PVD}}; \, \;{\text{bulk}} \to {\text{nano }}\;{\text{size}}, \, \;{\text{Implantation}}\;{\text{ of}}\;{\text{Vo}}^{{\text{i}}} \;{\text{defect}})$$$${\text{4MoO}}_{{{3} – {\text{x}}}} \;\left( {am} \right) \to {\text{4MoO}}_{{{3} – ({\text{x}} + {\text{y}})}}^{{{\text{n}} + }} + {\text{ O}}_{{2}} \quad ({\text{VST}};\;{\text{ Implantation }}\;{\text{of}}\;{\text{Vo}}^{{{\text{ii}}}} \;{\text{defect}},\;{\text{at 9}}0\;^{ \circ } {\text{C/vacuum}})$$$${\text{4MoO}}_{{{2}.{5}}}^{{{1} + }} + {2}\left( {{\text{CH}}_{{3}} } \right)_{{2}} {\text{CHOH }} + {\text{O}}_{{2}} \leftrightarrow {\text{ 2MoO}}_{{4}}^{{{2} – }} + {\text{2MoO}}_{{2}} + {\text{4H}}^{ + } + {2}\left( {{\text{CH}}_{{3}} } \right)_{{2}} {\text{C }}{ =} {\text{ O}} \ldots \left( {{\text{K}}_{{\text{a}}} = { 1 } \times {1}0^{{ – {18}}} } \right)$$$${\text{2MoO}}_{{4}}^{{{2} – }} + {\text{4H}}^{ + } \leftrightarrow {\text{2H}}_{{2}} {\text{MoO}}_{{4}}$$$${\text{2H}}_{{2}} {\text{MoO}}_{{4}} \to {\text{2MoO}}_{{3}} \left( {h \, \;{\text{and}}\;\alpha } \right) + {\text{2H}}_{{2}} {\text{O}}$$$${\text{2MoO}}_{{2}} + {\text{O}}_{{2}} \to {\text{2MoO}}_{{3}} \;\left( {h \, \;{\text{and}}\;\alpha } \right)$$Therefore, the release of protons from solvent dissociation was the rate determining step in both the case of water and iso-propanol, which largely affected the intermediate and final product formation with phase transition.$${\text{MoO}}_{{\text{3 – x}}} + {\text{ H}}_{{2}} {\text{O}}\mathop{\longrightarrow}\limits^{{ {6}0^\circ {\text{C}}}} h{\text{ – MoO}}_{{3}} \quad \quad \quad \quad \quad \quad \quad \quad\quad\left( {w60} \right)$$$${\text{MoO}}_{{\text{3 – x}}} + {\text{ H}}_{{2}} {\text{O}}\mathop{\longrightarrow}\limits^{{ {9}0^\circ {\text{C}}}}\alpha {\text{ – MoO}}_{{3}} \quad \quad \quad \quad \quad \quad \quad \quad\quad\left( {w90} \right)$$$${\text{MoO}}_{{\text{3 – x}}} + \, \left( {{\text{CH}}_{{3}} } \right)_{{2}} {\text{CHOH}}\mathop{\longrightarrow}\limits^{{ {9}0^\circ {\text{C}}}}h \, \;{\text{and}}\; \, \alpha {\text{ – MoO}}_{{3}} \quad \quad \quad \left( {ipa90} \right)$$We realize that our reasoning behind solvent-mediated mechanism is only at theoretical level and not experimentally proven yet. To negate this shortcoming, we will be doing in-situ spectroscopic measurements to understand the stoichiometric chemistry, reaction kinetics, and mechanism of this phase transition, in our future studies. These experiments will reveal the nature and distribution of oxygen vacancies, nature of intermediates, solvation level, proton concentration, nucleation etc.Discerning the oxygen defectsEffective control and design of oxygen vacancy sites within the material are crucial for intended applications. There are some indirect indications for the presence of oxygen vacancies (Voi or Voii) in our synthesized materials (MOw60, MOw90, MOipa90), as evidenced by spectroscopic characterizations. FT-IR spectra (Fig. 1) showed some peaks, due to single oxygen atom interaction with three molybdenum atoms (548 cm−1), Mo–O–Mo bonding with stretching vibrations of O3 atoms (864 cm−1), Mo–O vibrations (601 cm−1), and Mo2–O4 bonding by solvolyzing H2O molecules (466 cm−1). Similarly, Raman spectra of the samples also showed peaks for O–Mo–O bonds (300–700 cm−1) and Mo–O–Mo bonding (700–1000 cm−1). But again, these are all qualitative indications only.Oxygen vacancies (Voi, Voii), which alter the electronic environment at molecular level, is detectable, by observing changes in the binding energies of Mo-3d and O-1s peaks in XPS spectra too. Reduced oxidation states of Mo (from Mo6+ to Mo5+ or Mo4+) undergo shift in Mo-3d peaks, while the O-1s peaks undergo shift and change in shape as described in XPS data interpretation (Fig. 9, Table S5). However, quantitative analysis of these oxygen peaks only could reveal the concentration and distribution of oxygen vacancies, which is crucial for the interpretation of mechanism of phase transition.Hence, we have quantified the area under the O-1s peak for all 3 samples (Fig. 9b, Table S5), which conveys that the thermodynamically most stable α-MoO3 obtained via w90 treatment possess more oxygen concentration than the metastable h-MoO3 obtained from w60, and h and α-MoO3 from ipa90 treatments. Table S5 shows that more the oxygen content, better the stability and phase transition. Because, the oxygen deficient and defective (MoO3-(x+y))n+cation is hungry of oxygen atoms which has to be quenched by supplying fresh oxygen atoms and then be stabilized by protons, coming from either reagents or solvents. These results suggest that w90 treatment (solvolysis) could have efficiently refilled the oxygen vacancies in (MoO3-(x+y))n+, by the incoming oxygen atoms from H2O solvent and protonation of MoO42− intermediate by the same H2O, with the complete phase transition (am → h → α-MoO3). Whereas in w60, ipa90 treatments (solvolysis), the substitution of oxygen atoms from solvents: H2O at 60 °C, iso-propanol at 90 °C, was not that efficient, to fill the oxygen vacancies and for the protonated stabilization of MoO42− intermediate. Hence, they were left with incomplete and partial transitions like, only up to h-MoO3 and mixed h and α-MoO3 respectively. So, it was purely because of elevated temperature (90 °C) and more protic solvent like H2O, the phase transition propelled from h- to α-crystalline structure. Thus, from XPS data perspective, it was inferred that MOw60, MOipa90 samples possess more oxygen vacancies than MOw90, in which the vacancies were efficiently refilled.The real limitation here is that the identification of source of oxygen atoms in α-MoO3 and h-MoO3 product phases. For this, we need an in-depth spectroscopic (Mass/IR) study of O18 isotope labelled-H2O18 mediated VST of MoO3-x. Another way of characterizing the oxygen vacancies would be using in-situ EPR spectroscopy to find out the nature of oxygen defected and then refilled. This way the exact nature and distribution of oxygen vacancies can be characterized. We keep this task for our future work and would definitely want to report the scientific community later.Further, our strategy to control the oxygen defects was an indirect approach by measuring band gaps via UV–Vis spectroscopy. The principle is that introducing oxygen vacancies creates new energy levels and when their concentration exceeds 0.1%, significantly alters the energy bands in the material. In transition metal oxide semiconductors, oxygen vacancies modify the valence band, which consists mainly of the oxygen 2p orbital, that leads to changes in the material’s optical absorption properties47. Yuan et al. studied how different calcination rates for metal oxides influence the number of oxygen vacancy sites. Their findings suggest that increasing oxygen vacancies can reduce the band gap value of the material48.In our case, Fig. 3b, showcases the determination of band gaps (Eg) through Tauc plot analysis using the UV–vis spectrum. The calculated Eg values for MOw60 (h-MoO3), MOw90 (α-MoO3), and MOipa90 (h and α-MoO3) were 2.94 eV, 2.45 eV, and 1.51 eV respectively. Based on above mentioned Yuan et al’s postulate, the total oxygen vacancies (Vo = Voi + Voii) in our samples could be in the order for; MOipa90 > MOw90 > MOw60. This indicated that organic solvent like iso-propanol (ipa90) medium could have created more secondary oxygen vacancies (Voii) than an aqueous solvent (H2O) medium (w90), followed by w60 medium, during VST. However, the solvolysis efficiency of refilling oxygen in the vacancy sites followed by proton mediated stabilization of MoO42− intermediate was better at 90 °C, especially with H2O. Hence the degree of phase transition to α- phase, in our samples, was in the reverse order for; MOw90 > MOipa90 > MOw60. At 60 °C in H2O, both the Vo created as well as the solvolysis were less and inefficient. Thus, MOw60 ended up with phase transition (am → h) only up to metastable hexagonal phase (h), whereas MOw90 resulted in complete phase transition from am- to h-, then to α-MoO3, and finally MOipa90 yielding only partial transition (mixed phases of h and α-MoO3). Here comes a disagreement between, XPS and UV–Vis data for MOw60 sample, in which O-1s area suggested the presence of more Vo, whereas Eg value indicated less Vo in it. It may be because of the less expression of oxygen atoms from the meta stable h-MoO3 (MOw60) in XPS causing less sensitivity towards oxygen atoms. We will sort out this discrepancy in our future work.Thus, oxygen defects can be correlated to bang gaps (Eg) and vice-versa via UV–Vis spectroscopy. Therefore, the Eg values are of one kind of indirect indication for us, to change the synthesis parameters and have a control over the distribution and concentration of oxygen defects within the crystal lattice. This was our strategy to control the oxygen vacancies in quantitative terms.Others in reported methods, often employed high temperature annealing treatment in a reducing atmosphere to generate oxygen vacancies in metal oxide materials. The concentration of these vacancies can be controlled to some extent by adjusting parameters such as temperature, vacuum level, atmosphere composition, inert gas and other process variables. But we have controlled it through simple vacuum assisted solvolysis technique (VST) in a cost-effective way, in our present study. Thus, we believed that applying a controlled heat under vacuum and then neutralizing the positively charged defective MoO3 using protic solvents, could yield desired crystalline phase transition in our materials. This was the hypothesis behind our experimental planning and execution.Repeatability and reproducibility of DEST method for an eco-friendly, energy efficient, low temperature phase transition in MoO3
Wet-chemical syntheses would often result in issues like reproducibility and heterogeneity in the sample. We have carefully avoided these issues as much as possible, by paying more attention on exact repetition of operation parameters (precursor weight, temperature, time, vacuum level, solvent quantities) in multiple synthesis trials to confirm the repeatability and reproducibility (Fig. S3). SEM (Fig. 7a–c) showed the homogeneity within each sample in terms of particle size and morphological appearance.Currently we rely on the microscopic techniques and XRD to address sample heterogeneity if anything of that sort found. For e.g. The Rietveld analysis of the X-ray diffraction pattern of MOipa90 confirms the presence of (010) of h-MoO3 and #(020) of α-MoO3 phases, within the same sample (Fig. 4c). This indicates the heterogeneity and composition of mixed phases, but predominately composed of h-MoO3. The software reveals that the mixed phase contains 72.6% of h-MoO3 and 27.4% of α-MoO3. This ratio can be controlled by tuning the solvent amount, mixtures of solvents, ratios of solvents, material concentration etc. It is good to maintain the homogeneity and avoid heterogeneity in the samples prepared. But sometimes heterogeneity could also give us unexpected positive turn around in applications, due to synergy between multiple phases. So, it is not always undesirable to have heterogeneity in the synthesized samples.Figure 4 and Fig. S3 show the XRD spectra of samples from original batch (Batch 1) and reproduced materials (Batch 2) respectively. Table S6 shows the comparison of X-Ray diffraction angles (2θ) between two batches of samples. The results suggest that careful repetition of synthesis parameters (temperature, time, vacuum level, solvent volume, precursor amount) yielded same materials as the 2θ values were almost similar, except few fractions of degree deviation, here and there. Hence, this DEST synthesis protocol is reproducible. Thus, we believe that our experimental design is reliable and has clear control over all the variables involved in the products’ formation.Discrepancies between our findings and reported resultsTables S7, S8 show how our method of executing phase transition in MoO3 differs from other previously reported methods. Basically, our method doesn’t require very high temperature for phase transition and also employs facile procedure with eco-friendly low boiling solvents, as against the phase transitions which are often carried out at high temperatures (150–450 °C), that consumes lot of thermal energy and energy costs.It is widely reported that amorphous (non-crystalline/nano) materials are always good starting points for the formation of phase-pure materials of specific mono-morph, through controlled crystallization49. Our work involves in converting amorphous MoO3-x into h-MoO3, α-MoO3 and mixed phases (h and α) at the maximum 60–90 °C using either water or iso-propanol in a simple vacuum distillation unit. However, the reported methods start with crystalline precursor materials like NH4+, Na+ cation containing molybdate, heptamolybdate compounds. These cations play a catalytic/promotor role in nucleation and crystal growth steps of various forms of MoO3, which is not the case in our method. Other major difference is that they use non-eco-friendly, stochiometric reagents like highly corrosive, strong mineral acids e.g., HCl, HNO3 etc., in order to hydrolyze molybdate and heptamolybdate feed molecules, during hydrothermal syntheses. They also report that variety of crystalline phases can be formed at different temperatures and various ratios of precursor to acid solvent. But some of their lattice parameters21,35 and band gaps have notable changes from our values, for the given crystal structure. This comparative preparation and phase transition conditions for MoO3 declares that our method is better than the reported methods. Thus, we defend our work, as a reasonable contribution to nanomaterials synthesis field.The discrepancies between our method and reported methods can throw up new application possibilities in electrochemistry, energy storage, photocatalysis, solar cells, adsorbents, etc., as every method produce a material with unique properties such as lattice parameters, band gap, interlayer spacing etc. This also has lot of scope for extensive characterization of these materials which can impact overall trajectory of materials science research.Electrochemical investigations of DEST-made MoO3 polymorph 2D nanosheet materialsAny material in the universe should justify its existence for an application. Hence, we have investigated the synthesized materials for supercapacitor performance, which is our model application study. Though it is proclaimed that α-MoO3 is the most electroactive phase, still we wanted to have a comparison with h-MoO3 and mixed phase (h and α-MoO3), as they were prepared from new synthetic route. We never know what are all happening behind in an unknown synthesis method. So, it is good to make an attempt and see the results to have better confirmation.The electrochemical performance of DEST-made MoO3 polymorphs modified glassy carbon electrodes (GCE) were examined by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. There was no leaching of loaded material from GCE surface observed, in entirety of experiments. The area under the curve in CV (Fig. 10a) for α-MoO3 (MOw90) was significantly greater than that of other polymorphs; h-MoO3 and mixed h and α-MoO3 (MOw60, MOipa90) at the scan rate of 20 mV s−1, showing significantly increased charge/power density. Figure 10b, showing the CV curves exclusively for the best performing α-MoO3 (MOw90), recorded in the potential range, − 1.0 to 0 V and at different scan rates starting from 20 to 100 mV/s. The area under the curve increased when the scan rate increased and all CV curves exhibited quasi-rectangular shapes without any redox peaks, indicating the electrochemical stability of the material.Figure 10Electrochemical performances of DEST-made MoO3 polymorphs modified glassy carbon electrodes (GCE): (a) Cyclic Voltammetric (CV) curves for all polymorphs at the scan rate of 20 mV s−1, (b) CVs at different scan rates for α -MoO3 (MOw90), (c) Galvanostatic Charging-Discharging (GCD) characterization for all polymorphs at the current density of 0.125 A/g, (d) GCD profiles at different current densities for α -MoO3 (MOw90), (e) Electrochemical impedance spectroscopy (EIS) for all polymorphs, (f) Specific capacitance values of all polymorphs at different scan rate.Further, galvanostatic charge–discharge curves (GCD) were recorded at current density, 0.125 A/g, to assess the capacitance behavior of MoO3 polymorphs, h-MoO3 (MOw60), α-MoO3 (MOw90) and h and α-MoO3 (MOipa90), as shown Fig. 10c. All three MoO3 polymorphs were displaying a non-linear charge–discharge curve, demonstrating their pseudocapacitive nature. As expected, α-MoO3 (MOw90) showed slow and steady pace of charging and discharging, effectively suggesting the superior capacitance behavior than that of other polymorphs (based on the Eq. 1 in ‘Methods’ section). GCD measurements were also done at different current densities for the best performing α-MoO3 (MOw90) as shown in Fig. 10d. Current densities higher than 0.125 A/g was found to be detrimental in terms of capacitance measurement, thus suggesting 0.125 A/g was the optimum current density for the DEST-made MoO3 polymorphs.Electrochemical impedance spectroscopy (EIS) experiment was conducted in order to learn more about the electrocapacitive behavior of the aforementioned MoO3 polymorphs. Figure 10e displays the Nyquist plot for each sample. All three samples showed a no electrode resistance, but electrolyte resistance. The impedance response in the Nyquist plot can be split into two regions, (i) an initial semi-circular electrolyte resistance region, and further (ii) an upward straight line of resistance free region. The semicircle directly indicates the initial resistance that the material experiences, which is in the order for h-MoO3 (MOw60) > h and α-MoO3 (MOipa90) > α-MoO3 (MOw90). Thus, confirming the superior conducting and capacitive nature of α-MoO3 (MOw90) over other polymorphs.Figure 10f shows the calculated specific capacitance values at different scan rates for all three MoO3 polymorphs. Based on the calculations using Eq. (1) and Fig. 10f, the specific capacitance for MOw60 (h-MoO3), MOw90 (α-MoO3) and MOipa90 (h and α-MoO3) were found to be 14.3 Fg−1, 256 Fg−1, 41.8 Fg−1 respectively. Thus, MOw90 (α-MoO3) proved to be having 18 times better capacitance than its sister material MOw60 (h-MoO3) and 6 times better performing than its counterpart MOipa90 (h and α-MoO3). This result is at par excellence than other reported materials in the literature. The comparative performance analysis with existing α-MoO3 and non-MoO3 coated electrode materials in literature, is furnished in Tables S9, S10. Similar to earlier mentioned discrepancies in lattice parameters and band gaps, based on preparation method and phase transition temperature (Tables S7, S8), the reported capacitance values for α-MoO3 also differs from our value (256 Fg−1), but largely in decrement. It shows that the reported α-MoO3 materials, where the phase transition was achieved only above 400 °C, exhibit specific capacitance, only below 200 Fg−1. The electrode materials other than MoO3 (non-MoO3) also show similar trend i.e., < 200 Fg−1. Thus, our material is competitive and promising, comparatively with other reported MoO3 materials and conducting polymers.Structure–activity relationshipRole of oxygen defectsOxygen vacancies (Vo), which are the predominant point defects in metal oxides, have the tendency to increase the electron density in the local environment of the molecular structure. This increase is caused by electrons donated by the escaped oxygen atoms, leading to an improved electrical conductivity and catalytic performance of the product material. e.g., particularly in n-type MoO3 semiconductors50. Oxygen vacancies (Vo) can also significantly increase the interlayer spacing because of loss of oxygen atoms. This also enhances the electrochemical activity and thereby promoting faster charge storage kinetics51. For example, annealing in a reducing environment can increase oxygen vacancy concentration, improving lithium–ion intercalation kinetics in battery applications. Thus, the control over oxygen defect formation can result in desired electrochemical properties such as conductivity, capacitance etc. in the target material. In our case, MOipa90 (h and α-MoO3) sample has more oxygen vacancies (Vo) than MOw90 (α-MoO3) sample, based on the Eg values for MOw60 (h-MoO3), MOw90 (α-MoO3), and MOipa90 (h and α-MoO3) ; 2.94 eV, 2.45 eV, and 1.51 eV respectively (Vo: MOipa90 > MOw90 > MOw60). However, the electro-capacitance activity was higher (256 Fg−1) with MOw90 sample than MOipa90 sample (41.8 Fg−1). Hence, this suggests that more than oxygen vacancies, the lattice parameters in the crystalline phase and interlayer spacing were playing a major role in electrochemical responses of the MoO3 polymorphs.Role of crystal characteristicsCharacteristics like lattice parameters, density of states, surface area, and morphology vary for different crystal structures of the same compound. Lattice parameters influence ion intercalation and diffusion pathways, whereas the density of states near the Fermi level impacts electronic conductivity and charge storage. Surface and morphological features like area, roughness and defects affect the availability of catalytic active sites for charge storage. Hence, the overall crystal structure influences electrochemical properties consequently, in large.MoO3 exists in polymorphs, such as h-MoO3 (hexagonal), α-MoO3 (orthorhombic), β-MoO3 (monoclinic), and γ-MoO3 (cubic)4,5,6,7. The structure of different phases of MoO3 is established by how the foundational octahedral unit (MoO6) shares its corners and edges for ions’ interactions10. The configuration of MoO6 octahedrons (Fig. 11a) is influenced by external factors including temperature, pressure, and impurities52. The β-MoO3 structure (Fig. 5b) is made up of MoO6 octahedra that share corners in three dimensions and thus creating a monoclinic structure. It is similar to the structure of WO3 and related to distorted ReO3 type structure, thus classified as an empty A-site perovskite (ABO3). β-MoO3 holds only corner sharing oxygen atoms, whereas h-MoO3 and α-MoO3 having both corner and edge sharing oxygen atoms. The h-MoO3 is constructed by zigzag chains of same MoO6 octahedra (Fig. 11b,c), along [001] direction, which creates massive 1D tunnels. These chains are interlinked between them at cis-position. Whereas, α-MoO3 is composed with edge-sharing zigzag rows and corner-sharing rows of deformed MoO6 octahedra along the [001] and [100] directions respectively, to form a planar double layer (2D), giving rise to an anisotropic structure. Strong covalent and ionic bonds dominate the internal interactions in the deformed MoO6 octahedra, which are kept together in the vertical [010] direction by weak van der Waals’ forces, that causes the stratification in α-MoO3. Double layers of MoO6 octahedra are arranged in a (ABA) pattern and repeated in the ab plane along the b axis to produce a layered framework in the crystal structure of α-MoO39,26 (Fig. 11d,e). These structural differences between various crystalline phases can have influence over the electrochemical properties of MoO3 polymorphs. Figure 11A schematic presentation of MoO6 octahedron (a), composed of Mo and O atoms, forming MoO3 crystal structures; h -MoO3 (b,c), α -MoO3 (d,e). The unit cell dimensions are denoted by the grey lines.Among the MoO3 polymorphs, α-MoO3 generally provides the highest specific capacitance due to its thermodynamic stability and favorable layered structure that facilitates ion intercalation and de-intercalation11,15. However, the actual performance can also be influenced by other factors such as synthesis methods, morphology, particle size, thickness of the nanosheets, lattice parameters, interlayer spacing etc. In our case (Table 1), the α-MoO3 (MOw90) possess smaller lattice parameters (a = 3.95, b = 3.95, c = 3.72) than h-MoO3 (a = 10.61, b = 10.61, c = 3.72), and an enhanced interlayer spacing of 3.8 Å (101). This could have resulted more surface area in α- MoO3, than h-MoO3 (MOw60), as α-MoO3 was derived from the intermediate (MoO3-(x+y))n+ having more secondary oxygen vacancies ( Voii), caused by w90 treatment in VST. It is also believed that orthorhombic (α-) structure possess high fraction of surface exposed atoms53,54 than hexagonal structure (h-), due to smaller lattice and enhanced interlayer spacing. Hence it could have more active sites for charge storage. Thus, these structural differences between the metastable h-MoO3 and thermodynamically stable α-MoO3 have their direct influence on the capacitance values; former perfoms (14.3 Fg−1) poorer than latter (256 Fg−1), whereas the mixed phase, performs moderately (41.8 Fg−1).
Table 1 Comparison of crucial characterization data of MoO3 polymorphs to discern their electrochemical activity.We were expecting a synergistic effect from the mixed-phase MoO3 (h and α) viz-a-viz its electrochemical performance. But the result did not meet our expectation. The mixed phase usually causes variations in the local chemical environment, including differences in stoichiometry, surface defects, oxidation states, adsorbed species, and bonding configurations. These variations affect the electron density around Mo atoms. We also believe that there might be some unknown interfacial sites which could have distorted lattice parameters and modified density of states near the Fermi level. These interfaces at the phase boundaries can influence ion diffusion pathways and charge transfer, which will have an overall impact on electronic conductivity, charging-discharging ability of the material. To experimentally prove this interfacial phenomenon, pure h-MoO3 and pure α-MoO3 crystalline phases can be physically mixed in various known ratios and then be characterized by UV–Vis, XRD, XPS, CV, GCD. This might offer a preliminary understanding of how the bulk level-interface influence the electrochemical behaviour of the mixed phase. Based on this, MoO3 polymorphs materials possesing lattice level-interfaces can be synthesized and further scrutinized by Rietveld refinement technique in XRD using FULLPROF program.These new lattice level-interfaces can be created by tuning the synthesis parameters especially the solvents. We realize that the current ratio (72.6% of h-MoO3 and 27.4% of α-MoO3) in the mixed phase was not favorable for enhanced capacitance. Hence, for e.g. a synthesis of (50:50) of h-MoO3: α-MoO3 or a more of α-MoO3 and less of h-MoO3 combination (75:25, 60:40), could help in achieving the desired results, as we already know that α-MoO3 is relatively more electro-active and capacitive. We believe that this ratio can be adjusted by employing different organic solvents (ethanol, acetic acid etc.) for the creation of defective (MoO3-(x+y))n+ and its subsequent hydrolysis with different degrees of phase transition. This way a final product of lattice level-mixture of various ratios of MoO3 polymorphs can be obtained. This study could enable the most anticipated synergistic effect from the mixed-phase MoO3 (h and α) for its enhanced electrochemical performance.One interesting point to be noted here is that, even though the interlayer spacing in mixed-phase MoO3 (h and α) was lower (2.5 Å) than h-MoO3 (MOw60) (3.47 Å) (Table 1), it was more electro-active than h-MoO3. So, there is definitely a scope for enhancing its capacitance further through synergistic effect, given right preparation conditions are used. We keep this more nuanced work for our future activities.Factors affecting electrochemical behaviorBased on the results and literature evidences, the factors which might largely affect the electrochemical behavior of MoO3 are (a) synthesis methods, (b) oxygen vacancies, (c) crystal structure and interfaces, (d) morphology, (e) particle size, (f) thickness of the nanosheets, (g) interlayer spacing etc. However, the actual performance can also be influenced by factors other than these material properties; for e.g., specific conditions of electrochemical testing such as (a) amount of material loading on GCE, (b) choice of electrolyte, (c) electrodes’ configuration etc. Optimizing all these factors can enhance the specific capacitance of each mono-morph to the desired performance level. We will look into these details very elaborately in our future work.Future prospectsApart from the proposed future courses of actions mentioned in the discussion part, we would also like to scale up of the production of MoO3 and studying its long-term stability through longer hours experiments (charging-discharging), performing multiple cycles of GCD. We are excited to report our findings in our next communication.