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.

On-off near-infrared absorbance based on thermal-responsive plasmonic coupling in vanadium dioxide arrays for thermochromic windows.

Vanadium dioxide (VO2) emerges as an attractive plasmonic material due to its unique reversible thermal-responsive phase transition and the promising application in energy-saving smart windows. Here, by optimizing the geometry of VO2 nano-cylinder arrays, we demonstrate a significant performance enhancement for energy-efficient thermochromic windows. Such a performance enhancement relies on the on-off behavior of plasmonic resonance in the extremely high packing density of VO2 nano-cylinder arrays. Different from the typical plasmonic material, silver, VO2 nano-cylinders are characterized to have strong absorbance in near-infrared spectrum with significantly weaker plasmonic coupling to their neighbors, making them suitable to be arranged with a high packing density. The VO2 nano-cylinder arrays exhibit a 160% luminous transmittance increment, comparing to a flat film with the same solar modulation of ∼10%. The work provides a better understanding of the plasmonic behavior on phase-change VO2 and an efficient method to enhance smart window performance.

In-plane aligned assemblies of 1D-nanoobjects: recent approaches and applications.

One-dimensional (1D) nanoobjects have strongly anisotropic physical properties which are averaged out and cannot be exploited in disordered systems. The goal of the present review is to describe the current methods for preparing macroscopic composite films in which the long axis of individual 1D-nanoobjects is more or less parallel to the x,y-plane of the substrate as well as to each other (alignment direction). Such structures are generally described as in-plane anisotropic and many of their physical properties show minima or maxima parallel to the alignment direction. Optical polarizers are a typical class of such materials, but anisotropic materials properties can enhance the performance of devices and materials over many length scales in various disciplines of materials science including electronic devices, environmental sensors, energy saving and energy generation applications, plasmonic devices, Surface-Enhanced Raman Scattering (SERS) and biological applications. The reviewed alignment methods fall into two categories: techniques in which all nanoobjects remain in the x,y-plane and the in-plane densities and alignment are controlled; and techniques allowing building complex architectures in which each stratum of multilayered or stacked films may differ in chemical nature or alignment direction or both. This review serves a purpose to provide a platform to inspire new alignment approaches with improved assembly quality and upscaling potential and new applications with enhanced performance by alignment.

Trisulfide-Bond Acenes for Organic Batteries.

The molecular design of organic battery electrodes is a big challenge. Here, we synthesize two metal-free organosulfur acenes and shed insight into battery properties using first-principles calculations. A new zone-melting chemical-vapor-transport (ZM-CVT) apparatus was fabricated to provide a simple, solvent-free, and continuous synthetic protocol, and produce single crystals of tetrathiotetracene (TTT) and hexathiapentacene (HTP) at a large scale. Single crystals of HTP showed better Li-ion battery performance and higher cycling stability than those of TTT. A two-step, three-electron lithiation mechanism instead of the commonly depicted two-electron mechanism is proposed for the HTP Li-ion battery. The superior performance of HTP is linked to unique trisulfide bonding scenarios, which are also responsible for the formation of empty channels along the stacking direction. In-depth theoretical analysis suggests that organosulfur acenes are potential prototypes for organic battery materials with tunable properties, and that the tuning of sulfur bonds is critical in designing these new materials.

Vanadium Dioxide: The Multistimuli Responsive Material and Its Applications.

The reversible, ultrafast, and multistimuli responsive phase transition of vanadium dioxide (VO2 ) makes it an intriguing "smart" material. Its crystallographic transition from the monoclinic to tetragonal phases can be triggered by diverse stimuli including optical, thermal, electrical, electrochemical, mechanical, or magnetic perturbations. Consequently, the development of high-performance smart devices based on VO2 grows rapidly. This review systematically summarizes VO2 -based emerging technologies by classifying different stimuli (inputs) with their corresponding responses (outputs) including consideration of the mechanisms at play. The potential applications of such devices are vast and include switches, memories, photodetectors, actuators, smart windows, camouflages, passive radiators, resonators, sensors, field effect transistors, magnetic refrigeration, and oscillators. Finally, the challenges of integrating VO2 into smart devices are discussed and future developments in this area are considered.

Materials in Radiative Cooling Technologies.

Radiative cooling (RC) is a carbon-neutral cooling technology that utilizes thermal radiation to dissipate heat from the Earth's surface to the cold outer space. Research in the field of RC has garnered increasing interest from both academia and industry due to its potential to drive sustainable economic and environmental benefits to human society by reducing energy consumption and greenhouse gas emissions from conventional cooling systems. Materials innovation is the key to fully exploit the potential of RC. This review aims to elucidate the materials development with a focus on the design strategy including their intrinsic properties, structural formations, and performance improvement. The main types of RC materials, i.e., static-homogeneous, static-composite, dynamic, and multifunctional materials, are systematically overviewed. Future trends, possible challenges, and potential solutions are presented with perspectives in the concluding part, aiming to provide a roadmap for the future development of advanced RC materials.

A solar/radiative cooling dual-regulation smart window based on shape-morphing kirigami structures.

The energy efficiency of buildings has become a critical issue due to their substantial contribution to global energy consumption. Windows, in particular, are often the least efficient component of the building envelope, and conventional smart windows focus solely on regulating solar transmittance while overlooking radiative cooling. Although several recent designs achieved dual-control of solar and radiative cooling, these windows still face limitations in terms of durability, limited modulation ability and energy-saving performance. To address these challenges, we propose a novel dual-control smart window design consisting of a reconfigurable kirigami structure and polydimethylsiloxane-laminated thermochromic hydrogel coated with silver nanowires. In summer, the thermochromic hydrogel turns translucent to suppress the solar heat gain, while the high emissivity kirigami structure covers the exterior surface of the window, promoting radiative cooling. In winter, the hydrogel becomes transparent to allow for solar transmission. Additionally, the kirigami structure undergoes an out-of-plane structural change, opening towards the outside environment to expose the underlying low-emissivity silver nanowires and suppress heat radiation. Our design achieves a promising solar transmittance modulation ability of ∼24% and a good long-wave infrared emissivity regulation ability of 0.5. Furthermore, it exhibits significantly improved durability, which is nine times longer than the lifespan of conventional smart hydrogels. Our novel approach offers a promising solution for constructing energy-efficient and durable smart windows and outperforms existing state-of-the-art solar/radiative cooling dual-regulation smart windows in the literature.

Physical crosslinked hydrogel-derived smart windows: anti-freezing and fast thermal responsive performance.

Thermochromic hydrogels are versatile smart materials that have many applications, including in smart windows, sensing, camouflage, etc. The previous reports of hydrogel smart windows have been based on covalent crosslinking, requiring multistep processing, and complicated preparation. Moreover, most research studies focused on enhancing the luminous transmittance (Tlum) and modulating ability (ΔTsol), while the structural integrity and antifreezing ability, which are essential in practical applications, have been compromised and rarely investigated. Herein, we develop a new physical (noncovalent crosslinked) hydrogel-derived smart window by introducing an in situ free radical polymerization (FRP) of N-isopropylacrylamide (NIPAM) in a glycerol-water (GW) binary solvent system. The noncovalent crosslinked PNIPAM GW solutions are facilely synthesized, giving outstanding freezing tolerance (∼-18 °C), a comparably high Tlum of 90%, and ΔTsol of 60.8%, together with added advantages of fast response time (∼10 s) and good structural integrity before and after phase transition. This work could provide a new strategy to design and fabricate heat stimulated smart hydrogels not limited to energy saving smart windows.

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 visible­NIR­LWIR regulated smart windows are capable of dynamic radiative cooling for self-adapting applications across different climate zones.

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.

Tunable Grain Orientation of Chalcogenide Film and Its Application for Second Harmonic Generation.

To date, the second harmonic generation (SHG) has a great effect on photonic devices. However, it is a formidable challenge to achieve reconfigurable SHG. Hereby, we experimentally demonstrate the SHG response from the oriented Ge2Sb2Te5 (GST) grains induced by polarized laser pulses for the first time. The orientation of GST grains is found to be perpendicular to the polarization direction of the pump laser. Such unique ordered structures result in a periodic change of SHG intensity with the input polarization angle of the pump laser rotating every 180°. These findings may pave avenues for generating nonlinear optical sources with a simple process, scalability, and switchable functionality.

Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations.

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (VyOx) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO2 is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO2 nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO2 nanograting structures could enhance the luminous transmittance (Tlum) compared with a pristine flat VO2 surface. It is worth mentioning that Tlum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced Tlum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO2. Femtosecond laser-patterned VO2 films exhibiting nanograting structures with an average periodicity of ≈500-700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO2 dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V2O3 and V2O5 increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔTNIR = 10.8%, ΔTsol = 10.9%) compared with a flat VO2 thin film (ΔTNIR = 8%, ΔTsol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO2 thin films and opens a new gateway for smart devices.

Self-Assembled VO2 Mesh Film-Based Resistance Switches with High Transparency and Abrupt ON/OFF Ratio.

Vanadium dioxide, a well-known phase transition material with abrupt resistance change during its transition temperature, is herein used to fabricate the transparent mesh film onto a glass slide through self-assembly mesh printing. A record high ON/OFF ratio up to 104 is achieved together with high visible transmittance of 86% compared to the normal glass slide with visible transmittance at 88%. The high transparent properties make the resistive switches applicable for next-generation electronics, such as see-through computing device and beyond. A simple and scalable mesh printing approach-integrated phase change material may provide a promising way to fabricate transparent resistance switches for next-generation electronics.