Renewable synthesis of MoO3 nanosheets via low temperature phase transition for supercapacitor application

2D transition metal oxides have created revolution in the field of supercapacitors due to their fabulous electrochemical performance and stability. Molybdenum trioxides (MoO3) are one of the most prominent solid-state materials employed in energy storage applications. In this present work, we report a non-laborious physical vapor deposition (PVD) and ultrasonic extraction (USE) followed by vacuum assisted solvothermal treatment (VST) route (DEST), to produce 2D MoO3 nanosheets, without any complex equipment requirements. Phase transition in MoO3 is often achieved at very high temperatures by other reported works. But our well-thought-out, robust approach led to a phase transition from one phase to another phase, for e.g., hexagonal (h-MoO3) to orthorhombic (α-MoO3) structure at very low temperature (90 °C), using a green solvent (H2O) and renewable energy. This was achieved by implementing the concept of oxygen vacancy defects and solvolysis. The synthesized 2D nanomaterials were investigated for electrochemical performance as supercapacitor electrode materials. The α-MoO3 electrode material has shown supreme capacitance (256 Fg−1) than its counterpart h-MoO3 and mixed phases (h and α) of MoO3 (< 50 Fg−1). Thus, this work opens up a new possibility to synthesize electrocapacitive 2D MoO3 nanosheets in an eco-friendly and energy efficient way; hence can contribute in renewable circular economy.

Physical vapor deposition (PVD) is one of the important techniques to deposit MoO 3 on a chosen substrate because it produces highly crystalline and stratified structures with high yield 3 .In an oxygen-deficient environment, the reduction of MoO 3 gives rise to several phases of MoO 3-x (where 0 < x < 1) with a ReO 3 -type structure, such as Mo 9 O 26 , Mo 8 O 23 , and Mo 4 O 11 .These phases are categorized as part of the Magneli series, characterized by the composition Mo n O 3n-1 8 .The phase transition in MoO 3 can be achieved through thermal treatments at above 350 °C9 .h-MoO 3 can be obtained at 440 °C10 , whereas β-MoO 3 and stable α-MoO 3 phases can be obtained at 600 °C, 750 °C respectively 4 .Among these different phases, α-MoO 3 is getting more attention due to excellent catalytic activity and high electrochemical performance as reported in the literature 11 .
Regulating the physicochemical properties through engineering of oxygen defects is a powerful approach to significantly alter materials' characteristics in a controlled manner.Generally, oxygen defects play a crucial role in the optical and electrical properties of transition metal oxide materials.In the case of oxygen-deficient MoO 3-x , the presence of oxygen vacancies leads to the formation of energy states within the forbidden optical band gap of the growing thin film, facilitated by the presence of excess molybdenum (Mo) atoms [12][13][14] .The presence of oxygen vacancies in interlayer spacing followed by phase transition, which in turn will reflect in superior specific surface area, electroactive sites, electrical conductivity.The intercalation offers two times higher pseudocapacitive behavior for MoO 3-x than MoO 3 15 .Hence, oxygen defect mediated phase transition is essential to study in developing desired application.However, these phase transitions are often carried out at high temperatures, thus consuming lot of thermal energy and energy costs.Hence, in overall, it is important to study cost effective, eco-friendly phase transitions to support renewable, recyclable processes.
Based on the prior literature knowledge on MoO 3 syntheses and characterizations, we decided to develop a facile method (DEST) in which we aimed to convert the commercially acquired bulk MoO 3 material into an amorphous (am-) MoO 3-x nanomaterial first, by physical vapor deposition (PVD), then ultrasonically extract (USE) this nanomaterial into suitable green solvents, which will be the resource feed for the phase transition of MoO 3-x that we tried to achieve at very low temperature under vacuum assisted solvothermal treatment (VST) conditions.In this work, we report an oxygen defect mediated phase transition (am → h → α MoO 3 ) happening just at 60-90 °C, in water or iso-propanol, for the first time ever.VST can be done in rotary evaporation mode, which is a very basic technique to extract or remove solvents 16 .This simple, yet fruitive method is proposed for the potential use in the synthesis of metastable h-MoO 3 , stable α-MoO 3 and mixed h and α-MoO 3 from pre-synthesized am-MoO 3-x solutions.Moreover, these crystalline phases of MoO 3 can be finetuned using an appropriate selection of solvents, and operating modalities like temperature, vacuum level, rotation speed etc., during VST.We also believe that our DEST synthesis approach could bring down enormous energy and waste solvent management costs while scaling up the production of 2D MoO 3 nanosheets.Further, we have characterized the DEST-synthesized materials to understand the effect of oxygen vacancies and the mechanism of phase transition.The postulates have been detailed using hypothetical premises.Finally, the as-synthesized crystalline phases of MoO 3 were subjected to a model application; supercapacitor performance, which is an auxiliary study in this work.
Our primary focus in the present work is to convey the larger message to the scientific community that the phase transition of MoO 3 polymorphs can be done at lower temperatures using green solvents without any sophisticated equipment requirements.For e.g., Jadkar et al. reported a method which employed CVD technique with the operation temperature of 1500 ± 50 °C17 .In their method, 'Mo' filament was directly oxidized to α-MoO 3 .Also, there was no scope of intermediate phase transitions like, from hexagonal phase (h-) and orthorhombic phase (α-).Han et al.'s method required an electrospinning equipment and again high temperature, up to 300 °C was needed to produce α-MoO 3 from its precursor 18 .Whereas, our work involves in converting amorphous MoO 3-x into h-MoO 3 , α-MoO 3 and mixed phases (h and α) of MoO 3 at the maximum 60-90 °C using either water or iso-propanol in a simple vacuum distillation unit.The new insights we are aiming to provide here is the theoretical background of oxygen vacancies and its refilling by solvolysis, which can yield different crystalline phases of MoO 3 practically.
The other important reason why we study the phase transitions in MoO 3 crystal systems is that different phases of MoO 3 crystals could possess varying physico-chemical properties.Materials researchers would always want to exploit these properties, to suit their application that they are developing, for e.g., catalysis, adsorbents, composites, charge storage etc.Hence, we chose to carry out capacitance measurement studies in this present work.Making comparisons are part of research exercise.Thus, we compared the MoO 3 polymorphs for their electrochemical performance.As these polymorphs were prepared by a new route, we were inquisitive of knowing their electrochemical properties.We believe that our DEST synthesis approach could bring up new properties in the crystalline structure of 2D MoO 3 nanosheets.Therefore, it would offer a vibrant supercapacitor material in energy storage applications for the future.Also, our work can contribute to a renewable, circular economy in a longer run.
FT-IR spectra (Fig. 1) confirmed that MO w60 and MO ipa90 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 sample 19 , (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 phase 20 , and a peak at 548 cm −1 due to single oxygen atom interaction with three molybdenum atoms 21 (Fig. 1).MO w60 and MO ipa90 samples having two peaks present at 2916 cm −1 and 2845 cm −1 were attributed to -CH 2 vibration 22 caused by any organic impurities in H 2 O or iso-propanol.
On the other hand, MO w90 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 MO w60 (Fig. 1).The finger print region in MO w90 was slightly blue shifted to (a) 987 cm − From these results, it was confirmed that the samples MO w60 and MO ipa90 were in hexagonal crystalline structure (h-MoO 3 ) and MO w90 was in orthorhombic crystalline structure (α-MoO 3 ).In order to confirm the phase transition further, the Raman Spectroscopy was applied next.
The micro-Raman spectra of all three MoO 3 samples (MO w60 , MO w90 , MO ipa90 ) 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 MoO 6 octahedra in h-MoO 3 and α-MoO 3 , respectively 24 .The peaks appeared from 100 to 300 cm −1 were due to the MoO 6 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-MoO 3 phase [24][25][26] .A tiny peak at 909 cm −1 present in MO w90 , MO ipa90 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-MoO 3 and α-MoO 3 in comparison with literature evidences is shown in supplementary information file (SI); Tables S1 and S2.From this, the formation of h-MoO 3 (MO w60 , MO ipa90 ) and α-MoO 3 (MO w90 ) in our experimental conditions were confirmed.Further to determine the electronic states/band gaps in these polymorphs, the synthesized MoO 3 samples (MO w60 , MO w90 , MO ipa90 ) were subjected to UV-Vis absorption spectroscopy.
The UV-Vis spectroscopic study, as illustrated in Fig. 3a, provides detailed insights into the optical properties of the samples, MO w60 , MO w90 , and MO ipa90 .A distinct excitonic absorption peak at 212 nm was observed in all three samples, indicating the presence of 2D MoO 3 nanosheets 27 .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 MO w60 (h-MoO 3 ), 385 nm for MO w90 (α-MoO 3 ), and 315 nm for MO ipa90 (h-MoO 3 ).These characteristic absorption wavelengths indicated variations in the electronic properties and band structures in these samples.Thus, confirming different phases of MoO 3 .Figure 3b showcases the determination of band gaps (E g ) through Tauc plot analysis using the UV-Vis spectrum.The calculated E g values for MO w60 www.nature.com/scientificreports/(h-MoO 3 ), MO w90 (α-MoO 3 ), and MO ipa90 (h-MoO 3 ) 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.However, some more clarity was needed to distinguish MO w60 from MO ipa90 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 MoO 3 polymorphs using XRD technique.

Crystal structural analysis of MoO 3 phases
The most important analysis to prove the phase transition in as-synthesized MoO 3 samples was X-ray diffraction studies (XRD).Figure S1 shows the XRD patterns of commercially purchased stock MoO 3 (bulk powder), physical vapor deposited MoO 3-x (nanopowder), and then recrystallized MoO 3 samples from water (MO w60 , MO w90 ) and isopropanol (MO ipa90 ), along with simulated diffraction patterns of h-MoO 3 and α-MoO 3 .This direct comparison allowed us to assess the peak positions and intensities of the corresponding phases.
The thin films of MoO 3 (thickness of 150 nm) deposited on glass substrates by PVD, are usually amorphous and sub-stoichiometric (am-MoO 3-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  www.nature.com/scientificreports/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; Contrary to am-MoO 3-x , the recrystallized MoO 3 samples; MO w60, MO w90, MO ipa90 , were exhibiting sharp distinct peaks in the XRD pattern, suggesting phase transition and enhanced crystallinity (Fig. S1, 4).The samples recrystallized using water (MO w60 ) exhibited a phase transition from amorphous (am-MoO 3-x ) to hexagonal structure (h-MoO 3 ).However, with an increase in recrystallization temperature (MO w90 ), a complete phase transition from amorphous (am-MoO 3-x ) to thermodynamically stable orthorhombhic structure (α-MoO 3 ) was observed.We also believe that this transition should have happened via the intermediate, which is metastable h-MoO 3 form, as we already know that h-structure was the resultant phase at 60 ° C in H 2 O. Conversely, in the case of MoO 3 sample recrystallized using iso-propanol (MO ipa90 ), a mixed phases of h and α-MoO 3 product was obtained, suggesting an initial phase transition from am-MoO 3 -x to h-MoO 3 , followed by a partial phase transition from h-MoO 3 to α-MoO 3 occurred in this sample.The measured diffraction patterns of MoO 3 phases were analyzed by using the Rietveld refinement technique (assisted by FULLPROF computer program).
The Rietveld analysis of the XRD of MO w60 sample, synthesized at 60 °C in H 2 O, revealed a pure singlephase of h-MoO 3 , confirmed by the space group P 63/m and space group number 176 (Fig. 4a).This analysis counted the lattice parameters of h-MoO 3 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.
Area of crystalline peaks Area of crystalline peak + Area of amorphous peak × 100 This analysis further revealed that the Mo ions reside in octahedral arrangement with the O atoms (Fig. 5a) and no monoclinic β-MoO 3 structure (Fig. 5b) was obtained with any of the synthesized samples (MO w60 , MO w90 , MO ipa90 ).The hexagonal crystalline structure (h-MoO 3 ) of the MO w60 sample possess 1D tunnels along crystallographic c-axis, which is made up of the zigzag chains of MoO 6 octahedra, the very building block of the crystal (Fig. 5c).
The Rietveld analysis of the XRD of MO w90 sample, synthesized at 90 °C in H 2 O, showed again a singlephase, which is thermodynamically stable orthorhombic crystal symmetry (α-MoO 3 ), confirmed by the space group P n m a (Fig. 4b).The lattice parameters of α-MoO 3 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-MoO 3 consists of layered planes of the foundational octahedron (oh-MoO 6 ) units, in double layer form (Fig. 5d).These stacked bilayers of distorted MoO 6 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 MO ipa90 sample, synthesized at 90 °C in iso-propanol ((CH 3 ) 2 CHOH) confirmed the presence of (010) and #(020) of h-MoO 3 and α-MoO 3 phases respectively (Fig. 4c), indicating the composition of mixed phases, but predominately composed of h-MoO 3 (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-MoO 3 and 27.4% of α-MoO 3 .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-MoO 3 , and a = 3. 9832(3), b = 13.8673(5), Thus, the XRD measurements have confirmed the phase transitions occurred in as-synthesized MoO 3 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 MO w60 and MO ipa90 are same or not, and (ii) the formation of α-MoO 3 in MO w90 .Henceforth, we mark the polymorph of MO ipa90 as 'h and α-MoO 3 ' instead of just 'h-MoO 3 ' .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 analysis
The h-MoO 3 and α-MoO 3 represent two distinct polymorphs of MoO 3 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 The bright pointed diffraction spots appeared in Selected Area Electron Diffraction (SAED) pattern confirmed high order of crystallinity in all three samples (MO w60 , MO w90 , MO ipa90 ) (Fig. 6).From SAED pattern, the crystalline phases of MO w60 , MO ipa90 , were confirmed as hexagonal (h-MoO 3 ) structure.On the other hand, MO w90 was confirmed to be orthorhombic (α-MoO 3 ) crystal structure.The samples showed different types of www.nature.com/scientificreports/lattice fringes with respect to oxygen defects formed during heat treatments (w60, w90, ipa90) under vacuum.
The crystal structures of MoO 3 became irreversibly distorted as oxygen was removed initially from the lattice structure, followed by its refilling and the intercalation of H + ions from solvents 30,31 .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-MoO 3 (MO w60 ), α-MoO 3 (MO w90 ) and mixed phases (h and α-MoO 3 ) of MO ipa90 , using SEM and AFM techniques as shown in Figs.7a-c  and 8 respectively.
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 MoO 3 polymorphs at the micron scale level.This observation highlights the successful fabrication of MoO 3 polymorphs in our desired 2D form which holds significant implications in potential applications.Figure 8 illustrates AFM images of MoO 3 polymorphs.AFM imaging provided a high-resolution representation of the topography of the samples; MoO 3 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 (MO w60 , MO w90 , MO ipa90 ) were investigated by the X-ray Photoelectron Spectroscopy (XPS) (Fig. 9a, b).The full energy  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 MO w60 and MO w90 respectively.These peaks correspond to the spin-orbit doublets (3d 5/2 and 3d 3/2 ) of Mo 6+ , thereby confirming the presence of MoO 3 .However, these peaks slightly shifted to low binding energies (235.98 eV, 232.88 eV) for MO ipa90 6,10,35,36 .This shift is due to subtle differences in the local chemical environment which influences the binding energies of Mo-related peaks in the XPS spectra 37,38 .O-1s peaks (Fig. 9b) also showed similar shift (531.28 eV, 531.78 eV, 530.78 eV for MO w60 , MO w90 , MO ipa90 ), indicating variations in the oxygen bonding environment among different polymorphs.
To be specific, Table S5 shows that the Mo-3d 3/2 , Mo-3d 5/2 , 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 MoO 3 .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-MoO 3 ) 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-MoO 3 ) 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 MoO 3 than the water mediated synthesis.Oxygen defect creates low valent Mo ions, if these low valent ions haven't regenerated into Mo 6+ completely, that could cause a slight shift in the binding energy.We also knew that the MO ipa90 produced mixed phases of h-MoO 3 and α-MoO 3 , thus iso-propanol acts very differently from water which produced solely the α-MoO 3 at 90 °C.

Hypothetical insights into the mechanism of phase transition in MoO 3-x (am → h → α-MoO 3 ) under VST conditions
Our 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, MoO 3 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 characteristics 39 .
Oxide materials are highly prone to induced oxygen vacancy defects, especially when the metal ions are at higher oxidation state.In our case (MoO 3 ), 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 MoO 3 could lose oxygen if it is heated under vacuum or other reducing environment conditions.Under these conditions, it should be noted that MoO 3 can quickly break down into lesser oxides [40][41][42] , where the stoichiometry deviates from the ideal MoO 3 composition into MoO 3-x , due to the absence of oxygen atoms.These lesser oxides are known to have empirical compositions between MoO 3 and MoO 2 , leading to the formation of low valent Mo 4+ ions.Further, these low valent Mo 4+ 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 MoO 3 .This was the theoretical basis of our experiment and we supplied protons (H + ) as foreign ions from water and iso-propanol, to quench the Mo 4+ ions, for the regeneration MoO 3 with modified phase.
In this present work, we believe that the vacuum environment during PVD step, could have created some oxygen vacancies (Vo i ) already, with the formation of sub-stoichiometric molybdenum oxide, MoO 3-x (150 nm thickness), but without crystallinity (Fig. S1).Hence, to complete the recrystallization, we have taken the VST approach, after the MoO 3-x extraction step (USE).During VST also, there might be a creation of oxygen vacancies (Vo ii ) due to vacuum, with the formation of MoO 3-(x+y) .If the thickness of the MoO 3-x thin film deposition increased up to 350 nm, then positively charged structural defects would also increase due to the concentration of oxygen vacancies [43][44][45] .
We denote these oxygen vacancies as, 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, H 2 O, 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 ((CH 3 ) 2 CHOH), 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 (MoO 4 2− ) 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 MoO 3 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 MoO 3-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 MoO 3-x thoroughly and effectively.
We reasoned out that the physico-chemical properties of solvents play a major role in facilitating phase transition in MoO 3-x during VST.For e.g., Water is a more protic solvent than iso-propanol, because it has ten thousand times higher dissociation constant (H 2 O: K a = 1 × 10 −14 ) than iso-propanol ((CH 3 ) 2 CHOH): (K a = 1 × 10 −18 ).Hence, water would release more protons in a short span of time, which is needed for the phase transition in MoO 3-x .
The following equations show a possible hydrolysis reaction between water and MoO 3-x containing oxygen vacancy, based on literature 46 .
According to the above equations, the commercially acquired bulk α-MoO 3 material was initially converted into an amorphous (am-) MoO 3-x nanomaterial, with implanted primary oxygen vacancies (Vo i ), at the initial MoO 3 (α) → MoO 3−x (am) (PVD; bulk → nano size, Implantation of Vo i defect) ).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-MoO 3 or orthorhombic α-MoO 3 ).The by-product MoO 2 also reacted with left over oxygen atoms to produce MoO 3 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 (MoO 3-(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-MoO 3 was only achieved partially according to the stoichiometric equations mentioned below.Therefore, the product was the mixed phases of h and α-MoO 3 , but dominated by h-phase as evidenced by XRD.
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.
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 defects
Effective 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 (Vo i or Vo ii ) in our synthesized materials (MO w60 , MO w90 , MO ipa90 ), 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 O 3 atoms (864 cm −1 ), Mo-O vibrations (601 cm −1 ), and Mo 2 -O 4 bonding by solvolyzing H 2 O 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 (Vo i , Vo ii ), 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 Mo 6+ to Mo 5+ or Mo 4+ ) 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-MoO 3 obtained via w90 treatment possess more oxygen concentration than the metastable h-MoO 3 obtained from w60, and h and α-MoO 3 from ipa90 treatments.Table S5 shows that more the oxygen content, better the stability and phase transition.Because, the oxygen deficient and defective (MoO 3-(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 (MoO 3-(x+y) ) n+ , by the incoming oxygen atoms from H 2 O solvent and protonation of MoO 4 2− intermediate by the same H 2 O, with the complete phase transition (am → h → α-MoO 3 ).Whereas in w60, ipa90 treatments (solvolysis), the substitution of oxygen atoms from solvents: H 2 O at 60 °C, iso-propanol at 90 °C, was not that efficient, to fill the oxygen vacancies and for the MoO 3 (α) → MoO 3−x (am) (PVD; bulk → nano size, Implantation of Vo i defect) www.nature.com/scientificreports/protonated stabilization of MoO 4 2− intermediate.Hence, they were left with incomplete and partial transitions like, only up to h-MoO 3 and mixed h and α-MoO 3 respectively.So, it was purely because of elevated temperature (90 °C) and more protic solvent like H 2 O, the phase transition propelled from h-to α-crystalline structure.Thus, from XPS data perspective, it was inferred that MO w60 , MO ipa90 samples possess more oxygen vacancies than MO w90 , in which the vacancies were efficiently refilled.
The real limitation here is that the identification of source of oxygen atoms in α-MoO 3 and h-MoO 3 product phases.For this, we need an in-depth spectroscopic (Mass/IR) study of O 18 isotope labelled-H 2 O 18 mediated VST of MoO 3-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 properties 47 .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 material 48 .
In our case, Fig. 3b, showcases the determination of band gaps (E g ) through Tauc plot analysis using the UV-vis spectrum.The calculated E g values for MO w60 (h-MoO 3 ), MO w90 (α-MoO 3 ), and MO ipa90 (h and α-MoO 3 ) 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 = Vo i + Vo ii ) in our samples could be in the order for; MO ipa90 > MO w90 > MO w60 .This indicated that organic solvent like iso-propanol (ipa90) medium could have created more secondary oxygen vacancies (Vo ii ) than an aqueous solvent (H 2 O) 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 MoO 4 2− intermediate was better at 90 °C, especially with H 2 O. Hence the degree of phase transition to α-phase, in our samples, was in the reverse order for; MO w90 > MO ipa90 > MO w60 .At 60 °C in H 2 O, both the Vo created as well as the solvolysis were less and inefficient.Thus, MO w60 ended up with phase transition (am → h) only up to metastable hexagonal phase (h), whereas MO w90 resulted in complete phase transition from am-to h-, then to α-MoO 3 , and finally MO ipa90 yielding only partial transition (mixed phases of h and α-MoO 3 ).Here comes a disagreement between, XPS and UV-Vis data for MO w60 sample, in which O-1s area suggested the presence of more Vo, whereas E g value indicated less Vo in it.It may be because of the less expression of oxygen atoms from the meta stable h-MoO 3 (MO w60 ) 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 (E g ) and vice-versa via UV-Vis spectroscopy.Therefore, the E g 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 MoO 3 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 MoO 3
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 MO ipa90 confirms the presence of (010) of h-MoO 3 and #(020) of α-MoO 3 phases, within the same sample (Fig. 4c).This indicates the heterogeneity and composition of mixed phases, but predominately composed of h-MoO 3 .The software reveals that the mixed phase contains 72.6% of h-MoO 3 and 27.4% of α-MoO 3 .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 results
Tables S7, S8 show how our method of executing phase transition in MoO 3 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 crystallization 49 .Our work involves in converting amorphous MoO 3-x into h-MoO 3 , α-MoO 3 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 NH 4 + , Na + cation containing molybdate, heptamolybdate compounds.These cations play a catalytic/promotor role in nucleation and crystal growth steps of various forms of MoO 3 , 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, HNO 3 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 parameters 21,35 and band gaps have notable changes from our values, for the given crystal structure.This comparative preparation and phase transition conditions for MoO 3 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 MoO 3 polymorph 2D nanosheet materials
Any 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-MoO 3 is the most electroactive phase, still we wanted to have a comparison with h-MoO 3 and mixed phase (h and α-MoO 3 ), 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 MoO 3 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-MoO 3 (MO w90 ) was significantly greater than that of other polymorphs; h-MoO 3 and mixed h and α-MoO 3 (MO w60 , MO ipa90 ) 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-MoO 3 (MO w90 ), 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.
Further, galvanostatic charge-discharge curves (GCD) were recorded at current density, 0.125 A/g, to assess the capacitance behavior of MoO 3 polymorphs, h-MoO 3 (MO w60 ), α-MoO 3 (MO w90 ) and h and α-MoO 3 (MO ipa90 ), as shown Fig. 10c.All three MoO 3 polymorphs were displaying a non-linear charge-discharge curve, demonstrating their pseudocapacitive nature.As expected, α-MoO 3 (MO w90 ) 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-MoO 3 (MO w90 ) 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 MoO 3 polymorphs.
Electrochemical impedance spectroscopy (EIS) experiment was conducted in order to learn more about the electrocapacitive behavior of the aforementioned MoO 3 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-MoO 3 (MO w60 ) > h and α-MoO 3 (MO ipa90 ) > α-MoO 3 (MO w90 ).Thus, confirming the superior conducting and capacitive nature of α-MoO 3 (MO w90 ) over other polymorphs.
Figure 10f shows the calculated specific capacitance values at different scan rates for all three MoO 3 polymorphs.Based on the calculations using Eq. ( 1) and Fig. 10f, the specific capacitance for MO w60 (h-MoO 3 ), MO w90 (α-MoO 3 ) and MO ipa90 (h and α-MoO 3 ) were found to be 14.3 Fg −1 , 256 Fg −1 , 41.8 Fg −1 respectively.Thus, MO w90 (α-MoO 3 ) proved to be having 18 times better capacitance than its sister material MO w60 (h-MoO 3 ) and 6 times better performing than its counterpart MO ipa90 (h and α-MoO 3 ).This result is at par excellence than other reported materials in the literature.The comparative performance analysis with existing α-MoO 3 and non-MoO 3 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-MoO 3 also differs from our value (256 Fg −1 ), but largely in decrement.It shows that the reported α-MoO 3 materials, where the phase transition was achieved only above 400 °C, exhibit specific capacitance, only below 200 Fg −1 .The electrode materials other than MoO 3 (non-MoO 3 ) also show similar trend i.e., < 200 Fg −1 .Thus, our material is competitive and promising, comparatively with other reported MoO 3 materials and conducting polymers.

Structure-activity relationship
Role of oxygen defects Oxygen 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 MoO 3 semiconductors 50 .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 kinetics 51 .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, MO ipa90 (h and α-MoO 3 ) sample has more oxygen vacancies (Vo) than MO w90 (α-MoO 3 ) sample, based on the E g values for MO w60 (h-MoO 3 ), MO w90 (α-MoO 3 ), and MO ipa90 (h and α-MoO 3 ) ; 2.94 eV, 2.45 eV, and 1.51 eV respectively (Vo: MO ipa90 > MO w90 > MO w60 ).However, the electro-capacitance activity was higher (256 Fg −1 ) with MO w90 sample than MO ipa90 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 MoO 3 polymorphs.

Role of crystal characteristics
Characteristics 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.MoO 3 exists in polymorphs, such as h-MoO 3 (hexagonal), α-MoO 3 (orthorhombic), β-MoO 3 (monoclinic), and γ-MoO 3 (cubic) [4][5][6][7] .The structure of different phases of MoO 3 is established by how the foundational octahedral unit (MoO 6 ) shares its corners and edges for ions' interactions 10 .The configuration of MoO 6 octahedrons (Fig. 11a) is influenced by external factors including temperature, pressure, and impurities 52 .The β-MoO 3 structure (Fig. 5b) is made up of MoO 6 octahedra that share corners in three dimensions and thus creating a monoclinic structure.It is similar to the structure of WO 3 and related to distorted ReO 3 type structure, thus classified as an empty A-site perovskite (ABO 3 ).β-MoO 3 holds only corner sharing oxygen atoms, whereas h-MoO www.nature.com/scientificreports/crystal structure of α-MoO 3 9,26 (Fig. 11d,e).These structural differences between various crystalline phases can have influence over the electrochemical properties of MoO 3 polymorphs.
Among the MoO 3 polymorphs, α-MoO 3 generally provides the highest specific capacitance due to its thermodynamic stability and favorable layered structure that facilitates ion intercalation and de-intercalation 11,15 .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-MoO 3 (MO w90 ) possess smaller lattice parameters (a = 3.95, b = 3.95, c = 3.72) than h-MoO 3 (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-MoO 3 , than h-MoO 3 (MO w60 ), as α-MoO 3 was derived from the intermediate (MoO 3-(x+y) ) n+ having more secondary oxygen vacancies ( Vo ii ), caused by w90 treatment in VST.It is also believed that orthorhombic (α-) structure possess high fraction of surface exposed atoms 53,54 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-MoO 3 and thermodynamically stable α-MoO 3 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 ).
We were expecting a synergistic effect from the mixed-phase MoO 3 (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-MoO 3 and pure α-MoO 3 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, MoO 3 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-MoO 3 and 27.4% of α-MoO 3 ) in the mixed phase was not favorable for enhanced capacitance.Hence, for e.g. a synthesis of (50:50) of h-MoO 3 : α-MoO 3 or a more of α-MoO 3 and less of h-MoO 3 combination (75:25, 60:40), could help in achieving the desired results, as we already know that α-MoO 3 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 (MoO 3-(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 MoO 3 polymorphs can be obtained.This study could enable the most anticipated synergistic effect from the mixed-phase MoO 3 (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 MoO 3 (h and α) was lower (2.5 Å) than h-MoO 3 (MO w60 ) (3.47 Å) (Table 1), it was more electro-active than h-MoO 3 .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 behavior
Based on the results and literature evidences, the factors which might largely affect the electrochemical behavior of MoO 3 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 www.nature.com/scientificreports/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 prospects
Apart from the proposed future courses of actions mentioned in the discussion part, we would also like to scale up of the production of MoO 3 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.

Conclusion
In summary, physical vapor deposited (PVD) amorphous MoO 3-x nano powder was meticulously extracted by ultrasonication (USE) in strategically chosen green solvents.Further, the extracted MoO 3-x nanomaterials were recrystallized by an eco-friendly, energy efficient, vacuum assisted solvothermal (VST) approach at very low temperature in water or iso-propanol.The combination of operating parameters in this DEST synthesis has truly yielded h-MoO 3 even at 60 °C and a fascinating phase transition from h-MoO 3 to α-MoO 3 at very low temperature ca.90 °C, in just water.On the other hand, iso-propanol as a solvent, produced mixed phases (h and α) of MoO 3 at 90 °C.The different phases of synthesized MoO 3 were identified and confirmed by an extensive XRD spectroscopic scrutiny and other characterizations.We believe that the implantation of oxygen defects followed by its solvolysis with the protic nature of the solvent and optimally very low temperature selection, resulted in an efficient proton mediated phase transition (am → h → α-MoO 3 ) in first-of-its kind.Because, other reported methods have always used very high calcination temperature ranges (300-1500 °C) to achieve this phase transition in MoO 3 .
To utilize the synthesized 2D nanosheets of MoO 3 polymorph materials in real world application, a series of electrochemical investigations have been carried out under CV, GCD conditions.MO w90 (α-MoO 3 ) found to be exhibiting 18 times better specific capacitance performance than MO w60 (h-MoO 3 ) and six fold better activity than MO ipa90 (h and α-MoO 3 ).We reasoned out that, α-MoO 3 (MO w90 ) possess smaller lattice parameters and an enhanced interlayer spacing, which could have resulted more surface area and high fraction of surface exposed atoms in it, than h-MoO 3 (MO w60 ), leading to more active sites for charge storage.These results convey the message that, by finetuning the synthesis parameters, electrochemical conditions further, DEST produced α-MoO 3 2D nanosheets can be made as the most promising upcoming candidates for creating next generation high-performance supercapacitors.

Experimental Materials
Molybdenum Trioxide Extra Pure (MoO 3 ) and iso-propanol ((CH 3 ) 2 CHOH) were purchased from Loba Chemie.For the physical vapor deposition of MoO 3 precursor the thermal evaporation BC 300 box coater was used.Milli-Q water was used in entire experiment.

Preparation of molybdenum trioxide (MoO 3-x ) thin film by physical vapor deposition (PVD)
Physical vapor deposition (PVD) of commercially acquired MoO 3 powder (bulk material) was performed by using a less-expensive, homemade thermal box coater equipment.The glass substrates were cleaned before use, through Radio Corporation of America (RCA) procedure to remove organic and inorganic impurities settled on it 55 , followed by UV treatment to make the substrate's surface hydrophilic 56,57 .Then the target material, molybdenum trioxide powder was taken and placed on the molybdenum boat in the thermal-ultrahigh vacuum evaporation set-up.MoO 3 powder was evaporated at 700 °C and slowly deposited as thin films, layer by layer on the cleaned glass substrates, under the chamber pressure of 10 −7 Pa.The thickness of the MoO 3 films were controlled to be 150 nm by real time quartz crystal thickness monitor with constant rate of deposition ca.0.3 nm/ s 58,59 .Thus, the thin films of amorphous MoO 3-x materials were achieved (Fig. S4).

Collection of amorphous MoO 3-x by ultrasonic extraction (USE)
The extraction of PVD-made MoO 3-x thin films was carried out using lab ultrasonic bath (USE).The MoO 3-x thin film coated glass substrates were immersed in two different polar solvents; water (H 2 O) and iso-propanol ((CH 3 ) 2 CHOH), separately, to disperse the amorphous MoO 3-x flakes in respective solvents, by the application of sonication treatment for one hour.Thus, MoO 3-x thin films were peeled off carefully from the glass substrates in the form of nano powder and dissolved in above mentioned polar solvents.This led to get two transparent, homogeneous MoO 3-x solutions, separately (Fig. S4).

Recrystallization of MoO 3-x by vacuum assisted solvothermal treatment (VST)
Further, the above aqueous and alcoholic MoO 3-x solutions were subjected to vacuum assisted solvothermal treatments (VST) using lab rotavapor equipment.The rotary evaporator set up basically had MoO 3-x solution containing rotating round bottom (RB) flask heated by metallic water bath filled with Milli-Q water and a chiller unit maintained at 7 °C to condense the extracted solvent.The aqueous and alcoholic MoO 3-x solutions were heated at different temperatures (60, 90 °C and 90 °C respectively) under constant 120 mbar vacuum, with the rotation speed of 175 rpm.The process was run continuously until entire solvent got evaporated with the outcome of MoO 3-x recrystallization (Fig. S4).At the end, three different colors of MoO 3 (greyish green, blue and black) wet flakes were obtained for the respective treatments.These flakes were further vacuum dried in an oven at 60 °C to obtain the rigid solid flakes of (h-), (α-), (h and α-) MoO 3 materials, correspondingly.

Figure 4 .
Figure 4. X-ray diffraction patterns refined by Rietveld method for DEST-made MoO 3 polymorphs materials: (a) MO w60 (h-MoO 3 ), (b) MO w90 (α-MoO 3 ), (c) MO ipa90 (mixed phases of h and α-MoO 3 ).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.

Figure 10 .
Figure 10.Electrochemical performances of DEST-made MoO 3 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 -MoO 3 (MO w90 ), (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 -MoO 3 (MO w90 ), (e) Electrochemical impedance spectroscopy (EIS) for all polymorphs, (f) Specific capacitance values of all polymorphs at different scan rate.

Figure 11 .
Figure 11.A schematic presentation of MoO 6 octahedron (a), composed of Mo and O atoms, forming MoO 3 crystal structures; h -MoO 3 (b,c), α -MoO 3 (d,e).The unit cell dimensions are denoted by the grey lines.