Direct synthesis of thermochromic VO2 through hydrothermal reaction

Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

Monoclinic-phase VO₂ (VO₂(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (T_c = 68 °C), the insulating VO₂(M) (space group P21/c) is transformed into metallic rutile VO₂ (VO₂(R) space group P42/mnm). VO₂(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO₂(R) at a higher T_c, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO₂(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO₂(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO₂(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO₂(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO₂(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.

Preparation of shape-controlling VO2(M/R) nanoparticles via one-step hydrothermal synthesis

In this study, we developed a facile one-step hydrothermal process that allows to synthesize high-purity VO2(M/R) nanoparticles with various morphologies such as nanorods, nanogranules, nanoblocks, and nanospheres. W dopants are successfully implanted in VO2(M/R) unit cells with high doping efficiency, which allows to regulate the size, morphology, and phase of obtained nanoparticles. The underlying regulation mechanism is presented in detail to reveal how hydrothermal products vary with W doping contents, which provides a synthetic strategy for the preparation of shape-controlling VO2(M/R) nanoparticles with high purity to satisfy different specific demands for corresponding applications in the field of thermochromic smart windows.

Synthesis and Morphological Control of VO2 Nanostructures via a One-Step Hydrothermal Method

The morphology of nanostructures is a vital parameter to consider in components comprised of materials exhibiting specific functionalities. The number of process steps and the need for high temperatures can often be a limiting factor when targeting a specific morphology. Here, we demonstrate a repeatable synthesis of different morphologies of a highly crystalline monoclinic phase of vanadium dioxide (VO₂(M)) using a one-step hydrothermal method. By adjusting the synthesis parameters, such as pH, temperature, and reducing agent concentration in the precursor, VO₂ nanostructures with high uniformity and crystallinity are achieved. Some of these morphologies were obtained via the choice of the reducing agent that allowed us to skip the annealing step. Our results indicate that the morphologies of the nanostructures are very sensitive to the hydrazine hydrate (N₂H₄.H₂O) concentration. Another reducing agent, dodecylamine, was used to achieve well-organized and high-quality VO₂(M) nanotubes. Differential scanning calorimetry (DSC) experiments revealed that all samples display the monoclinic-to-tetragonal structural transition (MTST) regardless of the morphology, albeit at different temperatures that can be interpreted as the variations in overheating and undercooling limits. VO₂(M) structures with a higher surface to volume ratio exhibit a higher overheating limit than those with low ratios.

Synthesis of Monoclinic Vanadium Dioxide via One-Pot Hydrothermal Route

Pure monoclinic vanadium dioxide nanoparticles (VO2 NPs) with a controlled uniform size are considered essential for the preparation of thermochromic smart window coatings on desired substrates. Herein, we report a facile one-step hydrothermal synthesis of VO2(M) NPs without post-treatment of annealing, which may induce unwanted aggregation of NPs. In contrast with the annealed sample, the one-step processed VO2(M) NPs exhibit superior thermochromic performance with the solar modulation efficiency of 11.8% and luminous transmittance of 37.3%.

VO2 Nanostructures for Batteries and Supercapacitors: A Review

Vanadium dioxide (VO₂ ) received tremendous interest lately due to its unique structural, electronic, and optoelectronic properties. VO₂ has been extensively used in electrochromic displays and memristors and its VO₂ (B) polymorph is extensively utilized as electrode material in energy storage applications. More studies are focused on VO₂ (B) nanostructures which displayed different energy storage behavior than the bulk VO₂ . The present review provides a systematic overview of the progress in VO₂ nanostructures syntheses and its application in energy storage devices. Herein, a general introduction, discussion about crystal structure, and syntheses of a variety of nanostructures such as nanowires, nanorods, nanobelts, nanotubes, carambola shaped, etc. are summarized. The energy storage application of VO₂ nanostructure and its composites are also described in detail and categorically, e.g. Li-ion battery, Na-ion battery, and supercapacitors. The current status and challenges associated with VO₂ nanostructures are reported. Finally, light has been shed for the overall performance improvement of VO₂ nanostructure as potential electrode material for future application.

In-Operando Diffraction and Spectroscopic Evaluation of Pure, Zr-, and Ce-Doped Vanadium Dioxide Thermochromic Films Derived via Glycolate Synthesis

Pure and doped vanadia (VO₂, V_0.98Zr_0.02O₂, V_0.98Ce_0.02O₂) samples were prepared by wet chemistry synthesis from vanadyl glycolate intermediate phase and tape casted into films. Combining in-operando grazing incidence synchrotron X-ray diffraction and Raman spectroscopy, we studied the structural evolution of the films under isothermal conditions. The setup allowed assessment of the thermochromic functionality with continuous monitoring of the monoclinic to tetragonal transition in pure and doped vanadia phases, responsible for the transmission and reflection of light in the infrared part of the solar spectrum. The materials characterisation by X-ray diffraction beamline (MCX) goniometer demonstrated ideal performance, combining flexible geometry, high resolution, and the potential to accommodate the multi-channel equipment for in-operando characterisation. This method proved viable for evaluating the relevant structural and physical, and thereof functional properties of these systems. We revealed that dopants reduce the transition temperature by 5 °C on average. The synthetic route of the films was held responsible for the observed phase separation. The more favourable behaviour of cerium-doped sample was attributed to cerium alkoxide behaviour. In addition, structural, microstructural, thermal, and spectroscopic characterisation on powder samples was performed to gain more insight into the development of the phases that are responsible for thermochromic features in a broader range of doping ratios. The influence of the dopants on the extent of the thermochromic transition (transmission to reflection hysteresis) was also evaluated using (micro) structural, thermal and spectroscopic methods of powder samples. Characterisations showed that zirconium doping in 2, 4, and 6 mol% significantly influenced the phase composition and morphology of the precursor. Vanadium oxides other than VO₂ can easily crystallise; however, a thermal treatment regime that allowed crystallisation of VO₂ as a single phase was established.

Two-Step Synthesis of VO2 (M) with Tuned Crystallinity

Highly crystallized monoclinic vanadium dioxide, VO₂ (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO₂ powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO₂ powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO₂ (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO₂ particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO₂, calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, V₂O₃ and V₂O₅ with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.

Star-shaped VO2 (M) nanoparticle films with high thermochromic performance

Star-shaped VO₂ (M) nanoparticles demonstrate adjustable phase transition temperature and excellent reversible infrared modulations in response to external voltage.