Vanadium oxide nanostructures for chemiresistive gas and vapour sensing: a review on state of the art

Ir Nanoparticles Supported on Oxygen-Deficient Vanadium Oxides Prepared by a Polyoxovanadate Precursor for Enhanced Electrocatalytic Hydrogen Evolution

Developing highly active electrocatalysts is crucial for the application of electrocatalytic water splitting. In this study, we prepared vanadium oxide-graphene carbon nanocomposites (VxOy/C) with abundant defects using a carbon- and oxygen-rich hexavanadate derivative Na2[V6O7{(OCH2)3CCH3}4] as a precursor without the addition of an extra carbon source. Subsequently, the VxOy/C was used as a catalyst support to load a small amount of Ir, forming the Ir/VxOy/C nanoelectrocatalyst. This catalyst exhibited low hydrogen evolution overpotentials of only 18.90 and 13.46 mV at a working current density of 10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 electrolyte systems, outperforming the commercial Pt/C catalysts. Additionally, the catalyst showed excellent chemical stability and long-term durability. This work provides a new strategy for the design and synthesis of highly active electrocatalysts for water splitting.

Recent Advances of VO2 in Sensors and Actuators

Vanadium dioxide (VO2) stands out for its versatility in numerous applications, thanks to its unique reversible insulator-to-metal phase transition. This transition can be initiated by various stimuli, leading to significant alterations in the material's characteristics, including its resistivity and optical properties. As the interest in the material is growing year by year, the purpose of this review is to explore the trends and current state of progress on some of the applications proposed for VO2 in the field of sensors and actuators using literature review methods. Some key applications identified are resistive sensors such as strain, temperature, light, gas concentration, and thermal fluid flow sensors for microfluidics and mechanical microactuators. Several critical challenges have been recognized in the field, including the expanded investigation of VO2-based applications across multiple domains, exploring various methods to enhance device performance such as modifying the phase transition temperature, advancing the fabrication techniques for VO2 structures, and developing innovative modelling approaches. Current research in the field shows a variety of different sensors, actuators, and material combinations, leading to different sensor and actuator performance input ranges and output sensitivities.

Highly Tunable MOCVD Process of Vanadium Dioxide Thin Films: Relationship between Structural/Morphological Features and Electrodynamic Properties

The monoclinic structures of vanadium dioxide are widely studied as appealing systems due to a plethora of functional properties in several technological fields. In particular, the possibility to obtain the VO2 material in the form of thin film with a high control of structure and morphology represents a key issue for their use in THz devices and sensors. Herein, a fine control of the crystal habit has been addressed through an in-depth study of the metal organic chemical vapor deposition (MOCVD) synthetic approach. The focus is devoted to the key operative parameters such as deposition temperature inside the reactor in order to stabilize the P21/c or the C2/m monoclinic VO2 structures. Furthermore, the compositional purity, the morphology and the thickness of the VO2 films have been assessed through energy dispersive X-ray (EDX) analyses and field-emission scanning electron microscopy (FE-SEM), respectively. THz time domain spectroscopy is used to validate at very high frequency the functional properties of the as-prepared VO2 films.

Vanadium Oxide-Doped Laser-Induced Graphene Multi-Parameter Sensor to Decouple Soil Nitrogen Loss and Temperature

Monitoring nitrogen utilization efficiency and soil temperature in agricultural systems for timely intervention is essential for crop health with reduced environmental pollution. Herein, this work presents a high-performance multi-parameter sensor based on vanadium oxide (VOX )-doped laser-induced graphene (LIG) foam to completely decouple nitrogen oxides (NOX ) and temperature. The highly porous 3D VOX -doped LIG foam composite is readily obtained by laser scribing vanadium sulfide (V5 S8 )-doped block copolymer and phenolic resin self-assembled films. The heterojunction formed at the LIG/VOX interface provides the sensor with enhanced response to NOX and an ultralow limit of detection of 3 ppb (theoretical estimate of 451 ppt) at room temperature. The sensor also exhibits a wide detection range, fast response/recovery, good selectivity, and stability over 16 days. Meanwhile, the sensor can accurately detect temperature over a wide linear range of 10-110 °C. The encapsulation of the sensor with a soft membrane further allows for temperature sensing without being affected by NOX . The unencapsulated sensor operated at elevated temperature removes the influences of relative humidity and temperature variations for accurate NOX measurements. The capability to decouple nitrogen loss and soil temperature paves the way for the development of future multimodal decoupled electronics for precision agriculture and health monitoring.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Hydrothermal Synthesis of Vanadium Oxide Microstructures with Mixed Oxidation States

This review is based on hydrothermal synthetic procedures that generate different vanadium oxide microstructures with mixed oxidation states, where different vanadium (V5+) precursors (vanadate, vanadium oxide, vanadium alkoxide, etc.,) are used to obtain various types of morphologies and shapes, such as sea urchins, cogs, stars, squares, etc., depending on the amphiphilic molecules (usually surfactants) exhibiting a structural director role containing an organic functional group such as primary amines and thiols, respectively. The performance of sol–gel methodology, where intercalation processes sometimes take place, is crucial prior to the hydrothermal treatment stage to control the V4+/V5+. In every synthesis, many physical and chemical parameters, such as temperature, pH, reaction time., etc., are responsible for influencing the reactions in order to obtain different products; the final material usually corresponds to a mixed oxidation state structure with different content rates. This feature has been used in many technological applications, and some researchers have enhanced it by functionalizing the products to enhance their electrochemical and magnetic properties. Although some results have been auspicious, there are a number of projects underway to improve the synthesis in many ways, including yield, secondary products, size distribution, oxidation state ratio, etc., to achieve the best benefits from these microstructures in the large number of technological, catalytic, and magnetic devices, among other applications.

Kinetics of Catalyst-Free and Position-Controlled Low-Pressure Chemical Vapor Deposition Growth of VO2 Nanowire Arrays on Nanoimprinted Si Substrates

In the present article, the position-controlled and catalytic-free synthesis of vanadium dioxide (VO₂) nanowires (NWs) grown by the chemical vapor deposition (CVD) on nanoimprinted silicon substrates in the form of nanopillar arrays was analyzed. The NW growth on silicon nanopillars with different cross-sectional areas was studied, and it has been shown that the NWs' height decreases with an increase in their cross-sectional area. The X-ray diffraction technique, scanning electron microscopy, and X-ray photoelectron spectroscopy showed the high quality of the grown VO₂ NWs. A qualitative description of the growth rate of vertical NWs based on the material balance equation is given. The dependence of the growth rate of vertical and horizontal NWs on the precursor concentration in the gas phase and on the growth time was investigated. It was found that the height of vertical VO₂ NWs along the [100] direction exhibited a linear dependence on time and increased with an increase in the precursor concentration. For horizontal VO₂ NWs, the height along the direction [011] varied little with the growth time and precursor concentration. These results suggest that the high-aspect ratio vertical VO₂ NWs formed due to different growth modes of their crystal faces forming the top of the growing VO₂ crystals and their lateral crystal faces related to the difference between the free energies of these crystal faces and implemented experimental conditions. The results obtained permit a better insight into the growth of high-aspect ratio VO₂ NWs and into the formation of large VO₂ NW arrays with a controlled composition and properties.

Insights into first-principles characterization of the monoclinic VO2(B) polymorph via DFT + U calculation: electronic, magnetic and optical properties

We have studied the structural, electronic, magnetic, and optical properties of the VO2(B) polymorph using first-principles calculations based on density functional theory (DFT). This polymorph was found to display four optimized structures namely VO2(B)PP, VO2(B)LP, VO2(B)PPD, and VO2(B)LPD using the generalized gradient approximation (GGA) PBE exchange-correlation functional by including/excluding van der Waals interaction. Our derivation provides a theoretical justification for adding an on-site Coulomb U value in the conventional DFT calculations to allow a direct comparison of the two methods. We predicted a zero bandgap of the VO2(B) structure based on GGA/PBE. However, by GGA/PBE + U, we found accurate bandgap values of 0.76, 0.66, and 0.70 eV for VO2(B)PP, VO2(B)LP, and VO2(B)PPD, respectively. The results obtained from DFT + U were accompanied by a structural transition from the metallic to semiconductor property. Here, we verified the non-magnetic characteristic of the monoclinic VO2(B) phase with some available experimental and theoretical data. However, the debate on the magnetic property of this polymorph remains unresolved. Imaginary and real parts of the dielectric function, as computed with the GGA/PBE functional and the GGA/PBE + U functional, were also reported. The first absorption peaks of all considered geometries in the imaginary part of the dielectric constants indicated that the VO2(B) structure could perfectly absorb infrared light. The computed static dielectric constants with positive values, as derived from the optical properties, confirmed the conductivity of this material. Among the four proposed geometries of VO2(B) in this study, the outcomes obtained by VO2(B)PPD reveal good results owing to the excellent consistency of its bandgap, magnetic and optical properties with other experimental and theoretical observations. The theoretical framework in our study will provide useful insight for future practical applications of the VO2(B) polymorph in electronics and optoelectronics.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Highly effective and stable MWCNT/WO3 nanocatalyst for ammonia gas sensing, photodegradation of ciprofloxacin and peroxidase mimic activity

The present study discusses the ammonia (NH₃) sensing characteristics, photocatalytic degradation of emerging pollutants, and peroxidase mimic activity of multifunctional multi-walled carbon nanotube-tungsten oxide nanocomposite (MWCNT/WO₃) prepared by conventional solvothermal method. The prepared MWCNT/WO₃ nanocomposites were characterized by various analytical techniques like XRD, Raman, XPS, N₂ adsorption, FESEM with elemental analysis and diffuse reflection spectroscopy. The prepared 1% MWCNT/WO₃ nanocomposite showed better gas sensing performance for the NH₃ vapors at 10-100 ppm than the pristine WO₃ and the response and recover time of about 13 and 15s towards 20 ppm of ammonia (NH₃) was achieved. The photocatalytic activity of MWCNT/WO₃ towards organic dyes such as Rhodamine-B (Rh.B) methylene blue (MB) and pharmaceutical compound ciprofloxacin (CIP) were studied and achieved above 90% degradation at 160 min for CIP and 60 min for MB and Rho-B respectively. The radicle scavenging activity for MWCNT/WO₃ nanocomposite showed the predominant formation of hydroxyl (OH^•) and superoxide radicle (^•O^2-). Further, the MWCNT/WO₃ nanocomposite showed peroxidase mimic activity and exhibit the limit of detection (LOD) of about 321 nM. From the overall analysis, MWCNT/WO₃ hybrid seems to have potential characteristics that can be explored for multiple functional applications.

Pressure-Induced Structural Behavior of Orthorhombic Mn3(VO4)2: Raman Spectroscopic and X-ray Diffraction Investigations

The effect of high pressure on the structure of orthorhombic Mn₃(VO₄)₂ is investigated using in situ Raman spectroscopy and X-ray powder diffraction up to high pressures of 26.2 and 23.4 GPa, respectively. The study demonstrates a pressure-induced structural phase transition starting at 10 GPa, with the coexistence of phases in the range of 10-20 GPa. The sluggish first-order phase transition is complete by 20 GPa. Importantly, the new phase could be recovered at ambient conditions. Raman spectra of the recovered new phase indicate increased distortion and as a consequence the lowering of the local symmetry of the VO₄ tetrahedra. This behavior is different from that reported for isostructural compounds Zn₃(VO₄)₂ and Ni₃(VO₄)₂ where both show stable structures, although almost similar anisotropic compression of the unit cell is observed. The transition observed in orthorhombic Mn₃(VO₄)₂ could be due to the internal charge transfer between the cations, which favors the structural transition at lower pressures and the eventual recovery of the new phase even upon pressure release in comparison to other isostructural compounds. The experimental equation of state parameters obtained for orthorhombic Mn₃(VO₄)₂ match excellently with empirically calculated values reported earlier.

Hydrothermal synthesis of copper (׀׀) oxide-nanoparticles with highly enhanced BTEX gas sensing performance using chemiresistive sensor

In the present work, the cost effective and facile hydrothermal synthesis technique was adopted to synthesize the copper (׀׀) oxide (CuO)-Nanoparticles (NPs). Physico-chemical characterization of the synthesized CuO-NPs was done by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Ultraviolet-visible spectroscopy (UV-Vis), and scanning electron microscopy (SEM) analysis were carried out to study the structural, optical, and surface morphology of nanomaterial. XRD analysis revealed that the synthesized CuO-NPs had monoclinic structure and the average crystallite size is 20 nm. FTIR spectra indicate the vibrational bands of metal oxygen bonds (Cu-O). UV-visible absorption spectra were utilized to determine the energy band gap (E_g) of the CuO-NPs. In addition, we fabricated the chemiresistive sensor using synthesized CuO-NPs for detecting Volatile Organic Compounds (VOCs). These results demonstrate that CuO-NPs based chemiresistive sensor is ideal for qualitative detection of BTEX chemicals vapors (i.e. Benzene, Toluene, Ethylbenzene, and Xylene).

NO2 Gas Sensing Performance of a VO2(B) Ultrathin Vertical Nanosheet Array: Experimental and DFT Investigation

A VO₂(B) ultrathin vertical nanosheet array was prepared by the hydrothermal method. The influence of the concentration of oxalic acid on the crystal structure and room-temperature NO₂ sensing performance was studied. The morphology and crystal structure of the nanosheets were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Room-temperature gas sensing measurements of this structure to NO₂ with a concentration span from 0.5 to 5 ppm were carried out. The experimental results showed that the thickness of the vertical VO₂(B) nanosheet was lower than 20 nm and close to the 2 times Debye length of VO₂(B). The response of the sensor based on this structure to 5 ppm NO₂ was up to 2.03, and the detection limit was 20 ppb. Its high response performance was due to the fact that the target gas could completely control the entire conductive path by forming depletion layers on the surface of VO₂(B) nanosheets. Density functional theory was used to analyze the adsorption of NO₂ on the VO₂(B) surface. It is found that the band gap of VO₂(B) becomes narrower and the Fermi level moves to the valence band after NO₂ adsorption, and the density of states near the Fermi level increases significantly. This ultrathin vertical nanosheet array structure can make VO₂(B) detect NO₂ with high sensitivity at room temperature and therefore has potential applications in the field of low-power-consumption gas sensors.

Electronic and thermodynamic properties of native point defects in V2O5: a first-principles study

The formation of native point defects in semiconductors and their behaviors play a crucial role in material properties. Although the native defects of V2O5 include vacancies, self-interstitials, and antisites, only oxygen vacancies have been extensively explored. In this work, we carried out first-principles calculations to systematically study the properties of possible native defects in V2O5. The electronic structure and the formation energy of each defect were calculated using the DFT+U method. Defect concentrations were estimated using a statistical model with a constraint of charge neutrality. We found that the vanadyl vacancy is a shallow acceptor that could supply holes to the system. However, the intrinsic p-type doping in V2O5 hardly occurred because the vanadyl vacancy could be readily compensated by the more stable donor, i.e., the oxygen vacancy and oxygen interstitial, instead of holes. The oxygen vacancy is the most dominant defect under oxygen-deficient conditions. However, under extreme O-rich conditions, a deep donor of oxygen interstitial becomes the major defect species. The dominant oxygen vacancy under synthesized conditions plays an important role in determining the electronic conductivity of V2O5. It induces the formation of compensating electron polarons. The polarons are trapped at V centers close to the vacancy site with the effective escaping barriers of around 0.6 eV. Such barriers are higher than that of the isolated polaron hopping (0.2 eV). The estimated polaron mobilities obtained from kinetic Monte Carlo simulations confirmed that oxygen vacancies act as polaron-trapping sites, which diminishes the polaron mobility by 4 orders of magnitude. Nevertheless, when the sample is synthesized at elevated temperatures, a number of thermally activated polarons in samples are quite high due to the high concentrations of oxygen vacancies. These polarons can contribute as charge carriers of intrinsic n-type semiconducting V2O5.

Metal–Oxide Nanowire Molecular Sensors and Their Promises

During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-defined crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based sensor electronics.

Selective MOCVD synthesis of VO2 crystals on nanosharp Si structures

High-quality single VO₂ nanocrystals and ordered arrays of VO₂ nanorings were selectively synthesized by chemical vapor deposition (CVD) respectively on the tip apices and on the sidewall scallops.

A Review of Nanostructured Resistive-Based Vanadium Oxide Gas Sensors

Vanadium pentoxide (V2O5) is a transition metal oxide with features such as high availability, good catalytic activity, unique electrical properties and high conductivity which are appropriate for gas sensing applications. In this review, we discuss different gas sensing aspects of V2O5 in pristine, doped, decorated and composite forms. Depending on its synthesis procedure, morphology, sensing temperature and surface conditions, the V2O5-based gas sensors show different responses to target gases. Herein, we have discussed the behavior of V2O5-based gas sensors to different gases and associated sensing mechanisms. This review paper can be a useful reference for the researchers who works in the field of gas sensors.

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations. Part 1: 1D and 2D Nanostructures

This article discusses the main uses of 1D and 2D nanomaterials in the development of conductometric gas sensors based on metal oxides. It is shown that, along with the advantages of these materials, which can improve the parameters of gas sensors, there are a number of disadvantages that significantly limit their use in the development of devices designed for the sensor market.

Cadmium metavanadate mixed oxide nanorods for the chemiresistive detection of methane molecules

An energy band diagram of the V₂O₅–CdO thin film and illustration of the methane (CH₄) gas sensing mechanism with band bending.