Tungsten-doped Vanadium Dioxide Research Articles

Optical, Electrical, Structural, and Thermo-Mechanical Properties of Undoped and Tungsten-Doped Vanadium Dioxide Thin Films.

The undoped and tungsten (W)-doped vanadium dioxide (VO2) thin films were prepared by electron beam evaporation associated with ion-beam-assisted deposition (IAD). The influence of different W-doped contents (3-5%) on the electrical, optical, structural, and thermo-mechanical properties of VO2 thin films was investigated experimentally. Spectral transmittance results showed that with the increase in W-doped contents, the transmittance in the visible light range (400-750 nm) decreases from 60.2% to 53.9%, and the transmittance in the infrared wavelength range (2.5 μm to 5.5 μm) drops from 55.8% to 15.4%. As the W-doped content increases, the residual stress in the VO2 thin film decreases from -0.276 GPa to -0.238 GPa, but the surface roughness increases. For temperature-dependent spectroscopic measurements, heating the VO2 thin films from 30 °C to 100 °C showed the most significant change in transmittance for the 5% W-doped VO2 thin film. When the heating temperature exceeds 55 °C, the optical transmittance drops significantly, and the visible light transmittance drops by about 11%. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to evaluate the microstructure characteristics of VO2 thin films.

Modeling and Analysis of a Radiative Thermal Memristor

This study presents a theoretical framework for a radiative thermal memristor (RTM), utilizing Tungsten-doped vanadium dioxide (WVO) as the phase-change material (PCM) and silicon carbide (SiC) in the far-field regime. The behavior of the RTM is depicted through a Lissajous curve, illustrating the relationship between net flux (Q) and a periodically modulated temperature difference ΔT(t). It is established that temperature variations in the memristance (M) of the RTM form a closed loop, governed by PCM hysteresis. The analysis explores the impact of thermal conductivity contrast (r) and periodic thermal input amplitude (θ) on the Q–ΔT curve and the M–ΔT curve and negative differential thermal resistance (NDTR), revealing notable effects on the curve shapes and the emergence of NDTR. An increasing r leads to changes in the Lissajous curve’s shape and enhances the NDTR influence, while variations in both r and (θ) significantly affect the Q values and Lissajous curve amplitudes. In the M–ΔT curve, the height is linked to thermal conductivity contrast (r), with increasing r resulting in higher curve heights.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries.

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50°C. Remarkably, even at an elevated temperature (80°C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

Quantitative 3D Temperature Rendering of Deep Tumors by a NIR-II Reversibly Responsive W-VO2@PEG Photoacoustic Nanothermometer to Promote Precise Cancer Photothermal Therapy.

Accurately monitoring the three-dimensional (3D) temperature distribution of the tumor area in situ is a critical task that remains challenging in precision cancer photothermal (PT) therapy. Here, by ingeniously constructing a polyethylene glycol-coated tungsten-doped vanadium dioxide (W-VO2@PEG) photoacoustic (PA) nanothermometer (NThem) that linearly and reversibly responds to the thermal field near the human-body-temperature range, the authors propose a method to realize quantitative 3D temperature rendering of deep tumors to promote precise cancer PT therapy. The prepared NThems exhibit a mild phase transition from the monoclinic phase to the rutile phase when their temperature grows from 35 to 45 °C, with the optical absorption sharply increased ∼2-fold at 1064 nm in an approximately linear manner in the near-infrared-II (NIR-II) region, enabling W-VO2@PEG to be used as NThems for quantitative temperature monitoring of deep tumors with basepoint calibration, as well as diagnostic agents for PT therapy. Experimental results showed that the temperature measurement accuracy of the proposed method can reach 0.3 °C, with imaging depths up to 2 and 0.65 cm in tissue-mimicking phantoms and mouse tumor tissue, respectively. In addition, it was verified through PT therapy experiments in mice that the proposed method can achieve extremely high PT therapy efficiency by monitoring the temperature of the target area during PT therapy. This work provides a potential demonstration promoting precise cancer PT therapy through quantitative 3D temperature rendering of deep tumors by PA NThems with higher security and higher efficacy.

Defect engineering of vanadium dioxide by W ion doping with enhanced electrochemical performance

Defects engineering, as an effective protocol to tune the microstructures and electrochemical properties of metal oxide, has been widely used in the fabrication of electrode materials. In this study, the tungsten-doped vanadium dioxide (Wx-VO2) nanorods have been fabricated via a one-step hydrothermal strategy combined with annealing to optimize the structure and performance as the cathode materials for aqueous batteries. The enlarged lattice distance, the formation of W–O bond and the controllable cation vacancies endow the Wx-VO2 nanorods with enhanced electrochemical performance and structural stability. As a result, the capacity retention of Wx-VO2 cathode can reach to 93.2% at 0.1 A g−1 after 1000 cycles, which is considerable compared with other electrode for aqueous batteries. This work offers an efficient strategy for the rational design and controllable fabrication of high-performance electrode materials towards aqueous batteries.

Preparation and durability evaluation of vanadium dioxide intelligent thermal insulation films

Monoclinic vanadium dioxide (VO2) is a new type of thermally induced phase-transition material that undergoes a significant change in infrared transmittance during its phase transition, which makes it an excellent material for potential applications in the field of passive thermochromic smart windows. Improving the optical modulation capabilities and the durability of VO2 are crucial objectives for smart window applications. In this paper, three kinds of ethanol dispersion solutions were prepared by a nanogrinding machine using the following nanoparticles: monoclinic VO2, tungsten-doped vanadium dioxide (WVO) and silicon dioxide-coated tungsten-doped vanadium dioxide (WVO@SiO2). Polyvinyl butyral was selected as the film-forming agent, and the VO2 composite functional films were prepared by a scraping method. The ability of the prepared composite functional films to modulate light during alternating heating and cooling cycles and to resist a reduction in activity were evaluated preliminarily by an ultraviolet light irradiation method. The results show that, compared with a pure-phase VO2 film, tungsten-doped films can significantly enhance light modulation and the durability of the composite films. In addition, surface encapsulation by SiO2 in combination with introducing antioxidants also contributes to significantly improved optical properties and stability of the composite films. For example, the prepared composite films retained an infrared modulation efficiency of 49.7% at 1500 nm with a visible light transmittance of more than 60% after 65 cycles of high- and low-temperature treatments, showing excellent smart thermochromic performance, cyclic stability and promising application potential. This study is expected to provide a theoretical basis and experimental data for the preparation of stable VO2 nanomaterials and their applications to smart window films.

All-weather thermal regulation coatings

Thermally responsive coatings can provide huge opportunities for energy savings in buildings when applied to walls, rooftops, and windows. Groundbreaking work on radiative cooling has emerged in the last decade. In a recent issue of <i>Science</i>, Junqiao Wu and coworkers as well as Long Yi and coworkers take this one step forward by using tungsten-doped vanadium dioxide to build a thermal regulatory structure. The temperature-responsive structures have been demonstrated to radiatively cool in the summer and trap heat in the winter. Their work sets the stage for all-weather thermal regulation coatings. In this preview, we discuss this scientific advancement in the context of multi-functional coatings for sustainability.

Human-Body-Temperature Triggerable Phase Transition of W-VO2@PEG Nanoprobes with Strong and Switchable NIR-II Absorption for Deep and Contrast-Enhanced Photoacoustic Imaging.

The immense potential of temperature-responsive nanomaterials for use as contrast agents has propelled much recent research and development in the field of photoacoustic (PA) imaging, while the exorbitant transition temperature exceeding the human-tolerable range and the low reversibility of the reported temperature-sensitive nanosystems are still two severe issues that hinder effective imaging and long-term monitoring in practical applications. Herein, we propose a high-performing thermoresponsive polyethylene glycol-coated tungsten-doped vanadium dioxide (W-VO2@PEG) nanoprobe (NP) with strong and switchable optical absorption in the near-infrared-II (NIR-II) biowindow (1000-1700 nm) near human-body temperature, to achieve deep and contrast-enhanced PA imaging. Our study shows that the PA signal amplitude of W-VO2@PEG NPs at 1064 nm increases up to 260% when the temperature increases from 35 °C to 45 °C, with a signal fluctuation of less than 10% after 10 temperature cycles, therefore enabling great potential of "off-to-on" dynamic contrast-enhanced imaging capability in deep-seated tissues. Experiments on tissue-mimicking phantoms and in vitro chicken breast showed that, by levering the prepared W-VO2@PEG NPs and dynamically modulating the temperature field with an external NIR optical stimulus, contrast-enhanced PA images of the target can be obtained with an imaging depth up to 1.5 cm. Furthermore, in vivo potential of the prepared thermoresponsive NPs for the detection and identification of deep-seated tumors by directly comparing to conventional "always on" NPs has been demonstrated. Our work will offer feasible guidance for the development of smart temperature-activatable PA NPs with improved imaging depth and imaging contrast.

Scalable thermochromic smart windows with passive radiative cooling regulation.

Radiative cooling materials spontaneously radiate long-wave infrared (LWIR) to the cold outer space, providing cooling power that is preferred in hot seasons. Radiative cooling has been widely explored for walls and roofs but rarely for windows, which are one of the least energy-efficient parts of buildings. We fabricated scalable smart windows using a solution process giving different emissivity (ε) at high (εLWIR-H of 0.61) and low (εLWIR-L of 0.21) temperatures to regulate radiative cooling automatically while maintaining luminous transparency and near-infrared (NIR) modulation. These passive and independent visible–NIR–LWIR regulated smart windows are capable of dynamic radiative cooling for self-adapting applications across different climate zones.

Flexible smart photovoltaic foil for energy generation and conservation in buildings

Building-integrated solar cells not only generate electricity but also impact the envelope thermal characteristics, thus changing the micro-climate of buildings. Therefore, the collective effects of energy conservation and generation on buildings should be considered in one design. In this work, a smart photovoltaic window foil with near-infrared (NIR) modulation and low long-wavelength IR emissivity has been fabricated by combining organic perovskite and inorganic tungsten doped vanadium dioxide nanoparticles (W-VO2 NPs). The W-VO2 offers solar modulation and promotes extraction and transport of photogenerated charges, giving peak power conversion efficiency of 15.4% at 25 °C and 16.1% at 45 °C and average visible transmission of 25.5%, comparable to the best reported semitransparent perovskite solar cell with an additional favorable NIR modulation of 10.7%. This new strategy to integrate energy-saving functionality into photovoltaic foil initiates a new design rule for smart window development.

Direct transfer of thermochromic tungsten-doped vanadium dioxide thin-films onto flexible polymeric substrates

In this study, we demonstrate the preparation of flexible thermochromic vanadium dioxide (VO2) thin films via the direct film transfer technique using adhesive polymer films. Phase-change VO2 films are prepared via solution-based deposition using sub-stoichiometric colloidal VOx nanoparticles (NPs) on mica substrates; subsequently, a high-temperature rapid thermal annealing process is employed to induce phase transformation to monoclinic VO2 (VO2(M)). Thereafter, the direct film transfer of VO2(M) on mica substrates is conducted using PET substrates coated with an acrylate adhesive to form flexible and transparent mica/VO2(M)/PET films. The reversible phase transition behaviors of the flexible VO2(M) films are observed, such as a high luminous transmittance (Tlum > 50%) and solar modulation ability (ΔTsol > 15%). In addition, doping with tungsten ions systematically decreases the phase transition temperature (Tc) such that it is approximately equal to the ambient temperature, depending on the doping concentration. We observed that VO2(M) doped with 1.3 at% of W exhibits a Tc of 29 °C with Tlum and ΔTsol values of 53% and 10%, respectively; these are the highest Tlum and ΔTsol values reported thus far for flexible VO2(M) thin films at a near-ambient Tc.

Thermophysical Properties of Metal-Insulator Transition Materials During Phase Transition for Thermal Control Devices

This study reports on the fabrication of metal-insulator transition materials and measurement of the temperature dependency of their thermophysical properties, namely, specific heat, thermal conductivity, and total hemispherical emittance to evaluate the potential of these materials as multifunctional thermal control devices. Perovskite-type manganese oxide, La0.8Sr0.2MnO3 (LSMO) and La0.8Pb0.2MnO3 (LPMO), and vanadium dioxide (VO2) are selected as candidate materials. LSMO and LPMO are prepared using the solution combustion and simple sintering methods, and VO2 is prepared using the spark plasma sintering method. The phase transition temperatures, specific heat capacities, and heat storage capabilities during the phase transition of these materials are measured via differential scanning calorimetry. The thermal conductivities are measured via AC calorimetric method. The total hemispherical emittances are measured using the steady-state calorimetric method. Results show that VO2 has the highest heat storage capability during phase transition. The large change in the total hemispherical emittances of LSMO and LPMO and the large change in the thermal conductivity of VO2 before and after phase transition are confirmed. Moreover, tungsten-doped vanadium dioxide (VWO2) is fabricated to reduce the phase transition temperature to much lower than that of VO2, and the measurement results of its thermophysical properties are also presented.

A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal-Insulator Transition.

Thermal radiation from a black body increases with the fourth power of absolute temperature (T4 ), an effect known as the Stefan-Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (εint ), is insensitive to temperature (|dεint /dT| ≈ 10-4 °C-1 ). The resultant radiance bound by the T4 law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which εint can be engineered to decrease in an arbitrary manner near room temperature (|dεint /dT| ≈ 8 × 10-3 °C-1 ), enabling unprecedented manipulation of infrared radiation. As an example, εint is programmed to vary with temperature as the inverse of T4 , precisely counteracting the T4 dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.

Transparent Wood Composites Fabricated by Impregnation of Epoxy Resin and W-Doped VO2 Nanoparticles for Application in Energy-Saving Windows.

Two types of transparent wood composites with anisotropic structure for energy-saving windows were successfully fabricated by infiltration of epoxy resin dispersion containing tungsten-doped vanadium dioxide nanoparticles (W-doped VO2 NPs) into the delignified wood template and subsequent polymerization. The well integration of the epoxy resin, W-doped VO2 NPs, and the pore-structured wood endowed the anisotropic composites with high visible transmittance (68.2% for the composite prepared from longitudinally cut trees (L-composite), 73.3% for the composite prepared from radically cut trees (R-composite)), obviously different mechanical performance (fracture stress of 74.57 MPa (L-composite) and 56.14 MPa (R-composite) and modulus of 1.47 GPa (L-composite) and 1.23 GPa (R-composite)), and low thermal conductivity (0.20 W·m-1 K-1 (L-composite) and 0.32 W·m-1 K-1 (R-composite)). Moreover, these two kinds of W/VO2 transparent wood composites both show an outstanding thermoregulation ability when they are used as windows. A significant amount of heat (from a simulated light source) was reflected by VO2 NPs, and as a result, the indoor temperature of a demo system had a significant slower temperature increase rate when compared with that for a similar system with a common glass panel applied. Novel transparent wood composites combining a low thermal conductivity wood template and thermochromic VO2 NPs provide a potential solution for replacement of heavy, high thermal conductivity, and infrared transparent glass but still meet indoor occupancy view perception.

Rapid thermal annealing assisted facile solution method for tungsten-doped vanadium dioxide thin films on glass substrate

Abstract Vanadium dioxide VO2 is an important thermochromic material with useful applications in smart energy devices. This paper describes the synthesis of high-quality vanadium dioxide thermochromic thin films directly on glass substrate without any barrier layer prepared using a simple solution method. We demonstrate that 50 nm thick VO2 films display excellent visible transmittance (76.9% on glass, 69.4% on fused silica) and a large NIR switching efficiency (33.8% on glass, 44.3% on fused silica) at 1600 nm. It is found that the metal to semiconductor phase transition temperature Tc of undoped films can be reduced from a theoretical value of 68 °C to 49.4 °C by rapid thermal annealing. With W doping a continuous drop in the Tc is observed and for VO2 with 2% W doping, a Tc of 25.8 °C is achieved. These good optical properties and the near room-temperature phase transition temperature suggest that solution processing with the rapid thermal annealing process is a feasible method to obtain this promising material on glass substrate for practical application in thermochromic smart windows.

Tungsten-doped vanadium dioxide thin film synthesis by alternate layer-by-layer growth and post-deposition annealing

Intrinsic vanadium dioxide (VO2) encounters a metal–insulator transition (MIT) sharply at ~67 °C. Incorporating W6+ into VO2 enables lowering the MIT temperature effectually. Here we have developed a novel two-step synthesis process of high-quality W-doped VO2 thin films along with a delicate doping control: deposition of nanoscale alternately-layered precursor films composed of V and V–W layers by successive sputtering and subsequent annealing. The produced films have been characterized comprehensively to understand the effects of W-doping and process parameters into MIT functionality and structural properties in detail. With W-doping, the change rate of the MIT temperature was determined as ~ −14.5 °C/at%W while the resistance change across MIT decreases considerably. This work would be of close relevance to rendering VO2-based devices operable in the wider temperature range.

Metal–Insulator Phase Transition in Tungsten-Doped Vanadium Dioxide Thin Films

The electrical conductivity of thin polycrystalline V(1 – x)WxO2 has been studied in a wide temperature range, which covers the regions of both the metallic and insulator phases. An increase in the tungsten concentration is shown to shift the metal–insulator phase transition toward lower temperatures, while the temperature range of the coexistence of the phases monotonically increases as the impurity concentration increases. The temperature dependence of the conductivity of the insulator phase of V(1 – x)WxO2 is explained using the hopping conduction model that takes into account the influence of thermal vibrations of atoms on the resonance integral. Parameter e in the dependence on the level of doping VO2 has been calculated.

Preparation and properties of tungsten-doped VO2 microcapsule intelligent temperature-control packaging paper

Because of their insufficient temperature-control and waterproof property, traditional thermal-insulation materials cannot accurately control the temperature range and cannot be used for the protection of chemicals, precision instruments, and other items. In this study, waterborne acrylic resin was used as the main film former, and tungsten-doped vanadium dioxide microcapsules (PCMs/W-VO2) were used as the filler. The PCMs/W-VO2 intelligent temperature-control waterborne coating was prepared by supplementing with a flatting agent, organosilicon waterproofing agent, and other agents. The PCMs/W-VO2 intelligent temperature-control packaging paper was made by applying a waterborne coating on an ivory board. The morphology, structure, and phase-change properties of the samples were analyzed using SEM, FTIR, DSC, XRD, EDS and other techniques. And the thermal-insulation temperature difference, contact angle (CA), and mechanical properties were also tested. The results show that the prepared packaging paper had a phase-change temperature at 45 °C, the infrared light reflectance increased by 32%, and the thermal-insulation temperature difference reached 10.7 °C, showing excellent thermal-insulation temperature-control property. The XRD and EDS show that the surface of intelligent temperature-control packaging paper contains PCMs/W-VO2 microcapsules and organosilicon waterproofing agent. The SEM images show that the microcapsules were evenly distributed on the paper surface. CA analysis shows that the contact angle increased significantly, indicating that the paper sheet had good hydrophobicity. The physical properties of the paper sheet were also improved, and considering the economic factors of intelligent temperature-control coatings, the optimum tensile index, tearing index, burst index and folding index were 6.84 kN/m, 22.69 mN·m2/g, 3.60 kPa·m2 g−1 and 188 times (Resolation: 14.7 N), respectively.

Theoretical framework of the thermal memristor via a solid-state phase change material

The thermal memristor is the equivalent of the electrical memristor but in the thermal domain. The defining characteristic of the electrical memristor is the pinched Lissajous-type i-v curve (current to voltage difference); therefore, analogous behavior in the q-T curve (heat flux to temperature difference) should be the distinguishing feature of a thermal memristor. Herein, we propose a theoretical framework to realize thermal memristor devices using a solid-solid phase change material, tungsten doped vanadium dioxide. We show that by adding a periodic thermal input to the device, it is possible to obtain the characteristic pinched Lissajous type q-T curve that is indicative of the existence of a thermal memristor.

Temperature-responsive tungsten doped vanadium dioxide thin film starves bacteria to death

Abstract The colonization of microorganisms on material surfaces, or namely biofouling, is a challenging matter in both the medical and marine industries. With the inspiration of the metabolism cascade of microorganisms, this study develops a new antifouling material surface that enables the drainage of extracellular electrons from the electron transfer chain in microbial metabolism, thereby interrupting the energy metabolism and subsequent microbial viability. This thought has been realized by tungsten-doped vanadium dioxide (VO2) thin film using customized magnetron-sputtering deposition. The aim of tungsten doping is to tune the semiconductor-to-metal phase change of the VO2 thin film and then endow the temperature-responsive electrical conductivity (band structure). While contacting with microorganisms, the electrically conductive VO2 can disrupt the membrane respiration function of bacteria. This antifouling phenomenon can be explained by a three-step mechanism. The initial step is the microbial adhesion onto the metallic VO2 film to form the direct microbe–VO2 physical contact, which leads to the destructive extraction of electrons from the transmembrane protein complex of the respiratory chain (a discharge process); this induces oxidative stress and energy starvation and, eventually, interrupts the microbial membrane function. Finally, the microbial membrane integrity is destroyed, which leads to intracellular matter leakage (electrocution). This study demonstrates that the temperature-dependent VO2 electrical conductivity or band structure serves as a key factor to modulate the antimicrobial capability of tungsten-doped VO2 thin film. It is believed that the current findings can provide a new insight for the development of new antifouling surfaces.