A novel solution process for the synthesis of VO2 thin films with excellent thermochromic properties

Modulation of Structure and Optical Property of Nitrogen-Incorporated VO2 (M1) Thin Films by Polyvinyl Pyrrolidone

VO₂, as a promising material for smart windows, has attracted much attention, and researchers have been continuously striving to optimize the performance of VO₂-based materials. Herein, nitrogen-incorporated VO₂ (M1) thin films, using a polyvinylpyrrolidone (PVP)-assisted sol-gel method followed by heat treatment in NH₃ atmosphere, were synthesized, which exhibited a good solar modulation efficiency (ΔT_sol) of 4.99% and modulation efficiency of 37.6% at 2000 nm (ΔT_{2000 nm}), while their visible integrated transmittance (T_lum) ranged from 52.19% to 56.79% after the phase transition. The crystallization, microstructure, and thickness of the film could be regulated by varying PVP concentrations. XPS results showed that, in addition to the NH₃ atmosphere-N doped into VO₂ lattice, the pyrrolidone-N introduced N-containing groups with N-N, N-O, or N-H bonds into the vicinity of the surface or void of the film in the form of molecular adsorption or atom (N, O, and H) filling. According to the Tauc plot, the estimated bandgap of N-incorporated VO₂ thin films related to metal-to-insulator transition (E_g1) was 0.16-0.26 eV, while that associated with the visible transparency (E_g2) was 1.31-1.45 eV. The calculated E_g1 and E_g2 from the first-principles theory were 0.1-0.5 eV and 1.4-1.6 eV, respectively. The Tauc plot estimation and theoretical calculations suggested that the combined effect of N-doping and N-adsorption with the extra atom (H, N, and O) decreased the critical temperature (τ_c) due to the reduction in E_g1.

Infrared Transmission Characteristics of Phase Transitioning VO2 on Various Substrates

Infrared transmission characteristics of VO₂ thin films synthesized on multiple substrates, using a low-pressure direct oxidation technique, have been characterized. Material characterization of these films indicates high material quality, which resulted in large variation of electrical and optical properties at phase transition. A change in optical transmissivity greater than 80% was observed for these films utilizing infrared (IR) laser illumination at 1550 nm. Phase transition enabled by temperature change induced by a pulsed high-power laser beam resulted in modulated IR laser transmission with a low time constant in VO₂ on transparent quartz and muscovite substrates. Investigation of the effect of mechanical strain on phase transition in VO₂ grown on flexible muscovite substrate indicate shift in transition temperature to higher for tensile and lower for compressive strains.

Enhanced electrochemical performance of cobalt oxide layers coated LiNi0.8Co0.1Mn0.1O2 by polyvinylpyrrolidone-assisted method cathode for Li-ion batteries

LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) stands out among many cathode materials for lithium-ion batteries due to its high energy density. However, the Li-Ni mixing phenomenon and the side reactions between the active cathode material and the electrolyte during the charging and discharging process have hindered its commercial application. Co₃O₄ is a transition metal oxide with a spinel structure that can provide Li⁺ embedding sites for NCM811 and is considered a competitive coating material. Here, we use polyvinylpyrrolidone (PVP) as the assisting material and Co(NO₃)₂·6H₂O as the cobalt source to form a uniform and continuous Co₃O₄ coating on the surface the of NCM811 by a simple wet chemical method. The formed coating can avoid direct contact between NCM811 and the electrolyte, enhance the structural stability of the material surface, and reduce the polarization of the electrode during cycling. The experimental results demonstrate that the performance of the modified cathode materials is higher than that of NCM811, with a higher initial specific capacity (183.59 mAh·g^-1 at 0.1C), specific capacity (151.10 mAh·g^-1 at 2C) and cycle performance. The performances are better than those of many oxide coating materials at this research stage and provide a potential solution for the practical application of NCM811 lithium-ion batteries that provide high energy.

Influence of VO2 based structures and smart coatings on weather resistance for boosting the thermochromic properties of smart window applications

Vanadium dioxide (VO₂)-based energy-saving smart films or coatings aroused great interest in scientific research and industry due to the reversible crystalline structural transition of VO₂ from the monoclinic to tetragonal phase around room temperature, which can induce significant changes in transmittance and reflectance in the infrared (IR) range. However, there are still some obstacles for commercial application of VO₂-based films or coatings in our daily life, such as the high phase transition temperature (68 °C), low luminous transmittance, solar modulation ability, and poor environmental stability. Particularly, due to its active nature chemically, VO₂ is prone to gradual oxidation, causing deterioration of optical properties during very long life span of windows. In this review, the recent progress in enhancing the thermochromic properties of VO₂-hybrid materials especially based on environmental stability has been summarized for the first time in terms of structural modifications such as core–shell structures for nanoparticles and nanorods and thin-films with single layer, layer-by-layer, and sandwich-like structures due to their excellent results for improving environmental stability. Moreover, future development trends have also been presented to promote the goal of commercial production of VO₂ smart coatings.

VO₂ based energy saving smart coatings are of great interest in research and industry due to the reversible crystalline structural transition of VO₂ which can induce significant transmittance and reflectance changes in the infrared range.

Manipulating atomic defects in plasmonic vanadium dioxide for superior solar and thermal management

Vanadium dioxide (VO₂) is a unique active plasmonic material due to its intrinsic metal-insulator transition, remaining less explored. Herein, we pioneer a method to tailor the VO₂ surface plasmon by manipulating its atomic defects and establish a universal quantitative understanding based on seven representative defective VO₂ systems. Record high tunability is achieved for the localized surface plasmon resonance (LSPR) energy (0.66-1.16 eV) and transition temperature range (40-100 °C). The Drude model and density functional theory reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. We further demonstrate their superior performances in extensive unconventional plasmonic applications including energy-saving smart windows, wearable camouflage devices, and encryption inks.

A Study on the Coating Characteristics of VO₂ Nanoparticle Thin Film with Various Conditions of Ultrasonic Spray Coater

Research on smart windows is accelerating with a global trend that emphasizes efficient energy use. VO₂ is representativematerial for thermochromic smart windows that can reflect part of sunlight depending on the external environment. We attempted to produce thermochromic thin films by ultrasonic spray coating of VO₂ nano inks. Ultrasonic spray coating is a technique that is widely used to form thin and uniform thin films, but optimization has been required due to problems such as surface roughness and coffee-ring effect. In this study, we investigated the effects of ultrasonic spray coating process conditions on the quality of VO₂ thin films, and attempted to optimize condition.

A Universal Approach To Achieve High Luminous Transmittance and Solar Modulating Ability Simultaneously for Vanadium Dioxide Smart Coatings via Double-Sided Localized Surface Plasmon Resonances

Vanadium dioxide (VO₂)-based thermochromic coatings has attracted considerable attention in the application of smart windows as a result of their intriguing property of metal-insulator transition at moderate temperatures. However, the practical requirements of smart windows, i.e., the high luminous transmittance of T_lum > 60% and large solar modulating ability of ΔT_sol > 10%, are competing to a large extent and hardly satisfied simultaneously. Here, we proposed a facile and universal method to prepare VO₂ coatings for exceeding the criteria above using double-sided localized surface plasmon resonances (LSPRs), which are excited by the VO₂ nanoparticles dispersed evenly on both surfaces of the fused silica substrate. With subtle engineering of the sol-gel and heat treatment processes, the morphology of as-prepared VO₂ nanoparticles and corresponding LSPRs are controlled to achieve a high luminous transmittance (T_lum = 68.2%) and solar modulating ability (ΔT_sol = 11.7%) simultaneously. Further simulation suggests that the double-sided LSPRs can collectively enhance the performance of VO₂ smart coatings. Moreover, the double-sided VO₂ nanoparticle coatings demonstrate stable performance with no more than 1% degradation of T_lum and ΔT_sol after 1500 cycles. This study provides an alternative strategy to obtain high-quality VO₂ (M) solar modulating coatings.

Large Scale Synthesis of Nanopyramidal-Like VO₂ Films by an Oxygen-Assisted Etching Growth Method with Significantly Enhanced Field Emission Properties

The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO₂ thin films into pyramidal-like VO₂ nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO₂ crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO₂ structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO₂ films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO₂ films showed potential benefits for application such as temperature-regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO₂-based device applications.

Synthesis and thermochromic property studies on W doped VO2 films fabricated by sol-gel method

Tungsten-doped VO₂ thin films have been synthesized by a modified sol–gel process and followed by a post annealing. Vanadium pentoxide and tungstic acid as raw materials with the addition of hydrogen peroxide, concentrated hydrochloric acid (catalyst) and oxalic acid used as reducing agent were reacted in isobutanol. Finally, the uniform sol of vanadyl oxalate in isobutanol solvent was obtained as precursor. Detailed study suggested that W doped in VO₂ introduces additional electron carriers and induces the formation of V³⁺. Post annealing under vacuum promotes the releasing of chemical stress and generates oxygen vacancies in the samples. Temperature dependent transmittance study revealed that the releasing of chemical stress and deliberately introducing oxygen vacancies in W-doped VO₂ films have positive effects on enhancing its switching ability in the infrared transmittance as the metal-insulator transition (MIT) occurs. The largest switching of transmittance was obtained about 48% in the infrared range at 43 °C in 1.5%W doped VO₂ films, which is significantly larger than the reported ones. The findings in this work open a new way to synthesize the novel and thermochromic W doped VO₂ films with facility and low cost. Therefore, it has extensive application to construct smart windows and electronic devices.

Dual-Phase Transformation: Spontaneous Self-Template Surface-Patterning Strategy for Ultra-transparent VO2 Solar Modulating Coatings

Dual-phase transformation has been developed as a template-free surface patterning technique in this study. Ordered VO₂ honeycomb structures with a complex hierarchy have been fabricated via this method, and the microstructures of the obtained VO₂(M) coatings are tunable by tailoring the pertinent variables. The VO₂(M) honeycomb-structured coatings have excellent visible light transmittance at 700 nm (T_vis) up to 95.4% with decent solar modulating ability (ΔT_sol) of 5.5%, creating the potential as ultratransparent smart solar modulating coatings. Its excellent performance has been confirmed by a proof-of-principle demonstration. The dual-phase transformation technique has dramatically simplified the conventional colloidal lithography technique as a scalable surface patterning technique for achieving high-performance metal oxide coatings with diverse applications, such as catalysis, sensing, optics, electronics, and superwettable materials.

Preparation, Characterization and Thermo-Chromic Properties of EVA/VO₂ Laminate Films for Smart Window Applications and Energy Efficiency in Building

Thermochromic films based on vanadium dioxide (VO₂)/ethylene vinyl acetate copolymer (EVA) composite were developed. The monoclinic VO₂ particles was firstly prepared via hydrothermal and calcination processes. The effects of hydrothermal time and tungsten doping agent on crystal structure and morphology of the calcined metal oxides were reported. After that, 1 wt % of the prepared VO₂ powder was mixed with EVA compound, using two different mixing processes. It was found that mechanical properties of the EVA/VO₂ films prepared by the melt process were superior to those of which prepared by the solution process. On the other hand, percentage visible light transmittance of the solution casted EVA/VO₂ film was greater than that of the melt processed composite film. This was related to the different gel content of EVA rubber and state of dispersion and distribution of VO₂ within the polymer matrix phase. Thermochromic behaviors and heat reflectance of the EVA/VO₂ film were also verified. In overall, this study demonstrated that it was possible to develop a thermochromic film using the polymer composite approach. In this regard, the mixing condition was found to be one of the most important factors affecting morphology and thermo-mechanical properties of the films.

One-Step Hydrothermal Synthesis of W-Doped VO2 (M) Nanorods with a Tunable Phase-Transition Temperature for Infrared Smart Windows

Vanadium dioxide (VO₂), with reversible metal-semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO₂ nanoparticles are particularly superior to VO₂ films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO₂ (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO₂ underwent a two-stage phase evolution from the pure phase comprising VO₂ (A) nanobelts to VO₂ (M) nanorods, instead of the conventional three-stage B-A-M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO₂ (M). The phase-transition temperature of VO₂ (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO₂ (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO₂ (M) nanorods and provides significant insights into their applications in infrared smart windows.

Enhanced Visible Transmittance of Thermochromic VO₂ Thin Films by SiO₂ Passivation Layer and Their Optical Characterization

This paper presents the preparation of high-quality vanadium dioxide (VO₂) thermochromic thin films with enhanced visible transmittance (Tvis) via radio frequency (RF) sputtering and plasma enhanced chemical vapor deposition (PECVD). VO₂ thin films with high Tvis and excellent optical switching efficiency (Eos) were successfully prepared by employing SiO₂ as a passivation layer. After SiO₂ deposition, the roughness of the films was decreased 2-fold and a denser structure was formed. These morphological changes corresponded to the results of optical characterization including the haze, reflectance and absorption spectra. In spite of SiO₂ coating, the phase transition temperature (Tc) of the prepared films was not affected. Compared with pristine VO₂, the total layer thickness after SiO₂ coating was 160 nm, which is an increase of 80 nm. Despite the thickness change, the VO₂ thin films showed a higher Tvis value (λ 650 nm, 58%) compared with the pristine samples (λ 650 nm, 43%). This enhancement of Tvis while maintaining high Eos is meaningful for VO₂-based smart window applications.

Oxidative stress-mediated selective antimicrobial ability of nano-VO2 against Gram-positive bacteria for environmental and biomedical applications

Vanadium dioxide (VO2) is a unique thermochromic material as a result of its semiconductor-metal transition, holding great promise for energy-saving intelligent windows. Herein, pure nano-VO2 from discrete nanoparticles to continuous films were successfully deposited on quartz glass by controlling the sputtering parameters. It was demonstrated that, for Gram-positive S. aureus and S. epidermidis, the nano-VO2 could effectively disrupt bacteria morphology and membrane integrity, and eventually cause death. By contrast, the nano-VO2 did not exhibit significant toxicity towards Gram-negative E. coli and P. aeruginosa. To our knowledge, this is the first report on a selective antimicrobial effect of nano-VO2 materials on Gram-positive bacteria. Based on the experimental results, a plausible mechanism was proposed for the antimicrobial selectivity, which might originate from the different sensitivity of Gram-positive and Gram-negative bacteria to intracellular reactive oxygen species (ROS) level. Elevated intracellular ROS levels exceed the threshold that bacteria can self-regulate to maintain cellular redox homeostasis and thus cause oxidative stress, which can be alleviated by the intervention of glutathione (GSH) antioxidant. In addition, nano-VO2 did not produce significant cytotoxicity (hemolysis) against human erythrocytes within 12 h. Meanwhile, potential cytotoxicity against HIBEpiC revealed a time- and dose-dependent behavior that might be controlled and balanced by careful design. The findings in the present work may contribute to understanding the antimicrobial behavior of nano-VO2, and to expanding the new applications of VO2-based nanomaterials in environmental and biomedical fields.

Lowered phase transition temperature and excellent solar heat shielding properties of well-crystallized VO2 by W doping

Well-crystallized W-doped VO₂ with low phase transition temperature and excellent balance between T_c and latent heat.

Crystallized TiO2 (A)–VO2 (M/R) nanocomposite films with electrochromism–thermochromism dual-response properties

This work offers a method to combine the advantages of thermochromic (TC) and electrochromic (EC) smart windows by synthesizing TC–EC dual-response TiO₂–VO₂ nanocomposite films.

Solid-state-reaction synthesis of VO2 nanoparticles with low phase transition temperature, enhanced chemical stability and excellent thermochromic properties

Solid-state-reaction synthesis of VO₂ nanoparticles at 500 °C with different time.

Dual-functional sensor based on switchable plasmonic structure of VO2 nano-crystal films and Ag nanoparticles

Utilizing the insulator-metal phase transition of vanadium dioxide (VO2) crystal films, we develop a dual-functional sensor based on the coupling between VO2 nano-crystal films and Ag nanoparticles, which can probe fluorescence or Raman signals on the same substrate and it is switchable by changing temperature. At room temperature, the VO2 crystal films is insulator phase and the fluorescence signals of probe molecules (R6G) is detectable (Raman is in "off"). At high temperature (such as 85 °C), the VO2 crystal films become metallic phase. Ag nanoparticles interact with the metal phase of VO2 crystal films to produce stronger localized electric field. The stronger electric field can excite the Raman signals of probe molecules (R6G) and the coupled structure can also emit the Raman signals out efficiently (Raman is in "on"). The switchable probe of fluorescence and Raman signals would have potential applications in active photoelectric components, such as intelligent switch and multifunctional active sensor etc.

Anisotropic vanadium dioxide sculptured thin films with superior thermochromic properties

VO₂ (M) STF through reduction of V₂O₅ STF was prepared. The results illustrate that V₂O₅ STF can be successfully obtained by oblique angle thermal evaporation technique. After annealing at 550°C/3 min, the V₂O₅ STF deposited at 85° can be easily transformed into VO₂ STF with slanted columnar structure and superior thermochromic properties. After deposition SiO₂ antireflective layer, T_lum of VO₂ STF is enhanced 26% and ΔT_sol increases 60% compared with that of normal VO₂ thin films. Due to the anisotropic microstructure of VO₂ STF, angular selectivity transmission of VO₂ STF is observed and the solar modulation ability is further improved from 7.2% to 8.7% when light is along columnar direction. Moreover, the phase transition temperature of VO₂ STF can be depressed into 54.5°C without doping. Considering the oblique incidence of sunlight on windows, VO₂ STF is more beneficial for practical application as smart windows compared with normal homogenous VO₂ thin films.

Tunable assembly of vanadium dioxide nanoparticles to create porous film for energy-saving applications

Nanoparticle-assembled vanadium dioxide (VO2) films have been easily prepared with the assistance of cetyltrimethylammonium vanadate (CTAV) precursor which exhibits self-assembly properties. The obtained VO2 film has a micro/nano hierarchical porous structure, so its visible-light transmittance is significantly improved (∼25% increased compared to continuous film). The VO2 particle density as well as the film porosity can be facilely controlled by adjusting experimental parameters such as dip-coating speed. Accordingly, film optical properties can also be tuned to a large extent, in particular the visible transmittance (Tvis) and near-infrared switching efficiency (ΔTnir). These VO2 nanoparticle-assembled films prepared by this novel method provide a useful model to research the balance between Tvis and ΔTnir.

VO2 Films by Polymer-Assisted Deposition: Investigation of Thermal Decomposition of Precursor Gel and Control of Phase Transition Temperatures

Polymer-assisted deposition (PAD) can be employed to prepare metal oxide films by coordination between metal ions and polymers instead of hydrolysis and condensation reactions, providing a cheap and scalable alternative for sol-gel process. This work involves a TG/DTA-MS study on the thermal decomposition of the precursor gel to form VO2 films. The results show that polymers influence effectively the achievement of thermochromic VO2, supporting the film-forming mechanism we proposed in a previous work. W-doping shifted the MIT temperature from 58 to 33 °C while remained 73% of the modulating ability in infrared transmittance at 2000 nm.

Nanoporous thermochromic VO(2) films with low optical constants, enhanced luminous transmittance and thermochromic properties

Nanoporous thermochromic VO(2) films with low optical constants and tunable thicknesses have been prepared by polymer-assisted deposition. The film porosity and thickness change the interference relationship of light reflected from the film-substrate and the air-film interfaces, strongly influencing the optical properties of these VO(2) films. Our optimized single-layered VO(2) films exhibit high integrated luminous transmittance (T(lum,l) = 43.3%, T(lum,h) = 39.9%) and solar modulation (ΔT(sol) = 14.1%, from T(sol,l) = 42.9% to T(sol,h) = 28.8%), which are comparable to those of five-layered TiO(2)/VO(2)/TiO(2)/VO(2)/TiO(2) films (T(lum,l) = 45%, T(lum,h) = 42% and ΔT(sol) = 12%, from T(sol,l) = 52% to T(sol,h) = 40%, from Phys. Status Solidi A2009, 206, 2155-2160.). Optical calculations suggest that the performance could be further improved by increasing the porosity.

Thermochromic VO2 thin films: solution-based processing, improved optical properties, and lowered phase transformation temperature

This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO(2) thin film, the integral values of visible transmittance (T(int)) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (DeltaT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T(int) values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the DeltaT at 2000 nm remained high. These results represent the best data for VO(2) to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO(2) will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 microm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.