Effect of Ablation Rate on the Microstructure and Electrochromic Properties of Pulsed-Laser-Deposited Molybdenum Oxide Thin Films

Exploring Vacuum-Assisted Thin Films toward Supercapacitor Applications: Present Status and Future Prospects

Demand for high-performance energy storage devices is growing tremendously. Supercapacitors possess an excellent candidature to fulfill the energy storage requisites such as high energy density when compared to conventional capacitors, high power density, and cycling stability as compared to batteries, though not only for large-scale devices for higher energy/power density applications but also for macro- to microdevices for miniaturized electrical components. With the aid of various routes, many materials have been explored with well-tuned properties with controlled surface architecture through various preparative parameters to find those best suited for supercapacitive electrodes. Growth of a thin film can be accomplished through chemical or physical (vacuum-assisted) routes. Vacuum-assisted (physical) growth yields high purity, precise dimensions with a line-of-sight deposition, along with high adhesion between the film and the substrates, and hence, these techniques are necessary to manufacture many macro- to microscale supercapacitor devices. Still, much effort has not been put forth to explore vacuum-assisted techniques to fabricate supercapacitive electrodes and energy storage applications. The present review explores the first comprehensive report on the growth of widespread materials through vacuum-assisted physical deposition techniques inclusive of thermal evaporation, e-beam evaporation, sputtering, and laser beam ablation toward supercapacitive energy storage applications on one platform. The theoretical background of nucleation and growth through physical deposition, optimization of process parameters, and characterization to supercapacitor applications from macro- to microscale devices has been well explored to a provide critical analysis with literature-reviewed materials. The review ends with future challenges to bring out upcoming prospects to further enhance supercapacitive performance, as much work and materials need to be explored through these routes.

First-Principles Calculation of MoO2 and MoO3 Electronic and Optical Properties Compared with Experimental Data

MoO₃ and MoO₂ systems have attracted particular attention for many widespread applications thanks to their electronic and optical peculiarities; from the crystallographic point of view, MoO₃ adopts a thermodynamically stable orthorhombic phase (α-MoO₃) belonging to the space group Pbmn, while MoO₂ assumes a monoclinic arrangement characterized by space group P2₁/c. In the present paper, we investigated the electronic and optical properties of both MoO₃ and MoO₂ by using Density Functional Theory calculations, in particular, the Meta Generalized Gradient Approximation (MGGA) SCAN functional together with the PseudoDojo pseudopotential, which were used for the first time to obtain a deeper insight into the nature of different Mo-O bonds in these materials. The calculated density of states, the band gap, and the band structure were confirmed and validated by comparison with already available experimental results, while the optical properties were validated by recording optical spectra. Furthermore, the calculated band-gap energy value for the orthorhombic MoO₃ showed the best match to the experimental value reported in the literature. All these findings suggest that the newly proposed theoretical techniques reproduce the experimental evidence of both MoO₂ and MoO₃ systems with high accuracy.

Electrochromic Performance of Sputtered NbTi-Based Mixed Metal Oxide Thin Films with a Metallic Seed Layer

An attempt has been made to promote the efficiency of the electrochromic (EC) windows to perform at a faster switching rate with good coloration and easy recyclability. In this work, ion-assisted pulsed DC unbalanced confocal magnetron sputtering is used to fabricate mixed metal oxide thin films of Nb and Ti (which are termed as NTO) for EC applications. Further, to increase the EC efficiency of this film, a very thin metallic seed layer is incorporated between the substrate and the film using the layer-by-layer (LBL) coating strategy. The film was prepared by two steps: (i) very thin metallic seed layer coating and (ii) mixed metal oxide NTO layer coating. The metal layer was made of a Nb metal, Ti metal, and NbTi mixed metal, which was coated with the substrate and NTO layer, resulting in three different films, namely, Nb-NTO, Ti-NTO, and NbTi-NTO. The EC properties of these three films were studied in 1 M LiClO₄ dissolved in propylene carbonate and compared with the as-prepared NTO film. The results showed that the Nb-NTO film showed better EC properties, fast switching, better stability, and good recyclability. To check the stability, the film was subjected to prolonged cycling of 500 cycles with a harsh anodic and cathodic sweep at the scan rate of 100 mV s^-1. The UV-Vis spectrum confirmed the Li⁺ trapping in the films after prolonged cycling. To detrap the ions from the host surface, galvanostatic detrapping is carried out called rejuvenation studies. The rejuvenation rate of films is studied at a constant current loading of ∼2 × 10^-5 A cm^-2. To check the commercialization of the EC window, we successfully fabricated the Nb-NTO device with a PEO-LiClO₄-based polymer gel electrolyte.

A Review on Mechanochemistry: Approaching Advanced Energy Materials with Greener Force

Mechanochemistry with solvent-free and environmentally friendly characteristics is one of the most promising alternatives to traditional liquid-phase-based reactions, demonstrating epoch-making significance in the realization of different types of chemistry. Mechanochemistry utilizes mechanical energy to promote physical and chemical transformations to design complex molecules and nanostructured materials, encourage dispersion and recombination of multiphase components, and accelerate reaction rates and efficiencies via highly reactive surfaces. In particular, mechanochemistry deserves special attention because it is capable of endowing energy materials with unique characteristics and properties. Herein, the latest advances and progress in mechanochemistry for the preparation and modification of energy materials are reviewed. An outline of the basic knowledge, methods, and characteristics of different mechanochemical strategies is presented, distinguishing this review from most mechanochemistry reviews that only focus on ball-milling. Next, this outline is followed by a detailed and insightful discussion of mechanochemistry-involved energy conversion and storage applications. The discussion comprehensively covers aspects of energy transformations from mechanical/optical/chemical energy to electrical energy. Finally, next-generation advanced energy materials are proposed. This review is intended to bring mechanochemistry to the frontline and guide this burgeoning field of interdisciplinary research for developing advanced energy materials with greener mechanical force.

Synthesis and characterization of MoxFe1−xO nanocomposites for the ultra-fast degradation of methylene blue via a Fenton-like process: a green approach

A detailed degradation study of methylene blue within 22 minutes by the green synthesis of MoxFe1−xO nanocomposites using Punica granatum peel extract.

Synthesis and defect engineering of molybdenum oxides and their SERS applications

Surface-enhanced Raman scattering (SERS) spectroscopy has been developed into a cross-disciplinary analytical technology through exploring various materials' Raman vibrational modes with ultra-high sensitivity and specificity. Although conventional noble-metal based SERS substrates have achieved great success, oxide-semiconductor-based SERS substrates are attracting researchers' intensive interest due to their merits of facile fabrication, high uniformity and tunable SERS characteristics. Among all the SERS active oxide semiconductors, molybdenum oxides (MoO_x) possess exceptional advantages of high Raman enhancement factor, environmental stability, recyclable detection, etc. More interestingly, the SERS effect of the MoO_x SERS substrates may involve both the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which is determined by the stoichiometry and morphology of the material. Therefore, the focus of this review will be on two critical points: (1) synthesis and material engineering methods of the functional MoO_x material and (2) MoO_x SERS mechanism and performance evaluation. First, we review recent works on the MoO_x preparation and material property tuning approaches. Second, the SERS mechanism and performance of various MoO_x substrates are surveyed. In particular, the performance uniformity, enhancement factor and recyclability are evaluated. In the end, we discuss several challenges and open questions related to further promoting the MoO_x as the SERS substrate for monitoring extremely low trace molecules and the theory for better understanding of the SERS enhancement mechanism.

Band structure tuning of α-MoO3 by tin intercalation for ultrafast photonic applications

van der Waals (vdW) transition metal oxides have attracted extensive attention due to their intriguing physical and chemical properties. However, primary drawbacks of these materials are the lack of band structure tunability and substandard optical properties, which severely hinder their implementation in nanophotonic applications. Atomic intercalation is an emerging structural engineering approach for two-dimensional vdW materials to engineer the atomic structure and modify the optical properties, thereby broadening their range of applications. Herein, we synthesized tin-intercalated ultrathin α-MoO3 (Sn-MoO3) nanoribbons via chemical intercalation method and then investigated the broadband nonlinear optics (NLO) of stable few-layer α-MoO3 by performing a Z-scan laser measurement and femtosecond-resolved transient absorption (TA) spectroscopy. Sn-MoO3 showed a stable structure of Mo-O-Sn-O-Mo and a shorter relaxation time than pristine MoO3, indicating the accelerated recombination process of electrons and holes. Furthermore, Sn-MoO3 nanoribbons were used as an optical saturable absorber for ultrafast photonics; a highly stable femtosecond laser with a pulse width of 467 fs was generated from a single-mode fiber in the telecommunication band (1550 nm). These results indicate that atomic intercalation is an effective way to modulate the band structure and nonlinear optical properties of α-MoO3, which hold a great potential in the generation of ultrafast mode-locked laser pulses for optical communication technologies.

Molybdenum-Suboxide Thin Films as Anode Layers in Planar Lithium Microbatteries

In this paper, we investigate the effects of operational conditions on structural, electronic and electrochemical properties on molybdenum suboxides (MoO3-δ) thin films. The films are prepared using pulsed-laser deposition by varying the deposition temperature (Ts), laser fluence (Φ), the partial oxygen pressure (PO2) and annealing temperature (Ta). We find that three classes of samples are obtained with different degrees of stoichiometric deviation without post-treatment: (i) amorphous MoO3-δ (δ < 0.05) (ii) nearly-stoichiometric samples (δ ≈ 0) and (iii) suboxides MoO3-δ (δ > 0.05). The suboxide films 0.05 ≤ δ ≤ 0.25 deposited on Au/Ti/SiO2/flexible-Si substrates with appropriate processing conditions show high electrochemical performance as an anode layer for lithium planar microbatteries. In the realm of simple synthesis, the MoO3-δ film deposited at 450 °C under oxygen pressure of 13 Pa is a mixture of α-MoO3 and Mo8O23 phases (15:85). The electrochemical test of the 0.15MoO3-0.85Mo8O23 film shows a specific capacity of 484 µAh cm−2 µm−1 after 100 cycles of charge-discharge at a constant current of 0.5 A cm−2 in the potential range 3.0-0.05 V.

Electrically Activated UV-A Filters Based on Electrochromic MoO3-x

Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO₃ and WO₃ are utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO_3-x-based electrically activated ultraviolet (UV) filters. The MoO_3-x grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H⁺ injection/extraction in MoO_3-x is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325-800 nm), with a peak of ∼60% in the UV-A range (350-392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm² C^-1 are achieved.

Interplay between Solubility Limit, Structure, and Optical Properties of Tungsten-Doped Ga2O3 Compounds Synthesized by a Two-Step Calcination Process

This work unfolds the fundamental mechanisms and demonstrates the tunable optical properties derived via chemical composition tailoring in tungsten (W)-doped gallium oxide (Ga₂O₃) compounds. On the basis of the detailed investigation, the solubility limits of tungsten (W⁶⁺) ion and associated effects on the crystal structure, morphology, and optical properties of W-doped Ga₂O₃ (Ga_{2-2 x}W _xO₃, 0.00 ≤ x ≤ 0.25, GWO) compounds are reported. GWO materials were synthesized via a conventional solid-state reaction route, where a two-step calcination is adopted to produce materials with a high structural and chemical quality. X-ray diffraction analyses of sintered GWO compounds reveal the formation of a solid solution of GWO compounds at lower concentrations W ( x ≤ 0.10), while unreacted WO₃ secondary phase formation occurs at higher concentrations ( x>0.10). Insolubility of W at higher concentrations ( x ≥ 0.15) is attributed to the difference in formation enthalpies of respective oxides, i.e., Ga₂O₃ and WO₃. GWO compounds exhibit an interesting trend in morphology evolution as a function of W content. While intrinsic Ga₂O₃ exhibits rod-shaped morphology, W-doped Ga₂O₃ compounds exhibit nearly spherical-shaped grain morphology. Increasing W content ( x ≥ 0.10) induces morphology transformation from spherical to faceted grains with different facets (square and hexagonal). Relatively larger grain sizes in GWO compounds might be attributed to vacancy assisted enhanced mass transport due to W incorporation and/or WO₃ induced liquid phase sintering. Our findings demonstrate a substantial red shift in band gap ( E_g), which is evident from the optical absorption spectra, enabling the wide spectral selectivity of GWO compounds. W 5d orbitals induced sp- d exchange interaction between valence band and conduction band electrons accounts for the substantial red shift in E_g of GWO compounds. Also, with increasing W, E_g decreases linearly, obeying Vegard law up to x = 0.15 and, at this point, an abrupt E_g drop prevails. The nonlinearity ( bowing effect) behavior in E_g beyond x = 0.15 is due to insolubility of W at higher concentrations. The fundamental scientific understanding of the interdependence of synthetic conditions, structure, chemistry, and band gap could be useful to optimize GWO materials for optical, optoelectronic, and photocatalytic device applications.

Mechanistic insights into the phenomena of increasing capacity with cycle number: using pulsed-laser deposited MoO2thin film electrodes

Thin film electrodes often feature fluctuations in capacity with cycle number. This work shows how electrode reactions and peeling off the current collector is a plausible mechanism for these fluctuations.

Molybdenum Oxides - From Fundamentals to Functionality

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.