Ultrathin Nanosheets of Half-Metallic Monoclinic Vanadium Dioxide with a Thermally Induced Phase Transition

Air/liquid interfacial formation process of conductive metal-organic framework nanosheets

The air/liquid interface is a superior platform to create nanosheets of materials by promoting spontaneous two-dimensional growth of components. Metal-organic frameworks (MOFs)-intrinsically porous crystals-with π-conjugated triphenylene-based ligands show high electrical conductivities. Forming nanosheets of such conductive MOFs should enable their use in electronic devices. Although highly conductive MOF nanosheets have been created at the air/liquid interface, direct control of their continuity, morphology, thickness, crystallinity, and orientation directly influencing device performance remains as an issue to be addressed. Here, we present detailed insights into the formation process of electrically conductive MOF nanosheets composed of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and Ni2+ ions (HITP-Ni-NS) at the air/liquid interface. The morphological and structural features of HITP-Ni-NS strongly depend on the standing time-the time without any external actions involved, but leaving the interface undisturbed after setting the ligand solution onto the metal-ion solution. We find that the fundamental features of HITP-Ni-NS are determined by the standing time with conductivity sensitively influenced by such pre-determined HITP-Ni-NS characteristics. These findings will lead towards the establishment of a rational strategy for creating MOF nanosheets at the air/liquid interface with desired properties, thereby accelerating their use in diverse potential applications.

Thermochromic Energy Efficient Windows: Fundamentals, Recent Advances, and Perspectives

Thermochromic energy efficient windows represent an important protocol technology for advanced architectural windows with energy-saving capabilities through the intelligent regulation of indoor solar irradiation and the modulation of window optical properties in response to real-time temperature stimuli. In this review, recent progress in some promising thermochromic systems is summarized from the aspects of structures, the micro-/mesoscale regulation of thermochromic properties, and integration with other emerging energy techniques. Furthermore, the challenges and opportunities in thermochromic energy-efficient windows are outlined to promote future scientific investigations and practical applications in building energy conservation.

Progress on two-dimensional binary oxide materials

Two-dimensional van der Waals (2D vdW) materials have attracted much attention because of their unique electronic and optical properties. Since the successful isolation of graphene in 2004, many interesting 2D materials have emerged, including elemental olefins (silicene, germanene, etc.), transition metal chalcogenides, transition metal carbides (nitrides), hexagonal boron, etc. On the other hand, 2D binary oxide materials are an important group in the 2D family owing to their high structural diversity, low cost, high stability, and strong adjustability. This review systematically summarizes the research progress on 2D binary oxide materials. We discuss their composition and structure in terms of vdW and non-vdW categories in detail, followed by a discussion of their synthesis methods. In particular, we focus on strategies to tailor the properties of 2D oxides and their emerging applications in different fields. Finally, the challenges and future developments of 2D binary oxides are provided.

Synthesis of Phase-Pure Thermochromic VO2 (M1)

A highly reproducible, simple, and inexpensive synthesis method for obtaining phase-pure thermochromic monoclinic VO₂ (M1) is presented. Vanadium(III) oxide and ammonium metavanadate were used as starting materials and no additional reducing agents are required. Heating a mixture of these two components under an argon atmosphere at 750 °C for 2-4 h provides the direct formation of VO₂ (M1) without detectable impurity phases. The formation reaction of VO₂ (M1) was studied using in situ powder X-ray diffraction (PXRD), where a pressed pellet of the precursor material was heated during the continuous collection of PXRD data on a two-dimensional detector. The formation takes place via at least two crystalline intermediate phases where the first forms at 170-185 °C (likely an ammonium and oxygen deficient (NH₄)_1-δVO_3-δ phase), and the second at 230 °C (likely a more disordered phase due to the increased background intensity). We assume that the solid-state reaction between the unknown but likely disordered vanadate phase and vanadium(III) oxide starts at 395 °C in concert with the appearance of several other unknown crystalline phases. At 610-750 °C, phase-pure rutile VO₂ (P4₂/mnm) is obtained, which upon cooling converts to monoclinic VO₂ (M1). The product composition, microstructure, and homogeneity are characterized by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The synthesized VO₂ (M1) has a sharp reversible insulator-to-metal transition at 71.3 °C during heating and 59.5 °C during cooling, as characterized using differential scanning calorimetry, and resistivity and magnetic property measurements.

Surface Engineering of Defective and Porous Ir Metallene with Polyallylamine for Hydrogen Evolution Electrocatalysis

The design of defects and porous structures into metallene with functional surfaces is highly desired to improve its permeability, surface area, and active sites, but remains a great challenge. In this work, polyallylamine-encapsulated Ir metallene with defects and porous structure (Ir@PAH metallene) is easily fabricated by a one-step wet chemical reduction method. The Ir@PAH metallene exhibits excellent hydrogen evolution reaction (HER) performance with an overpotential of only 14 mV at 10 mA cm^-2 , a low Tafel slope of 31.2 mV dec^-1 , and almost no activity decay after stability test. The abundant defects and pores as well as several-atomic-layer nanosheet structures of Ir@PAH metallene provide a large specific surface area, high conductivity, and efficient mass transport/diffusion. In addition, surface-functionalized PAH molecules can modulate the electronic structure through strong Ir-N interaction and act as proton carriers to capture hydrogen ions, which is very beneficial for the HER in acidic media. This work provides a useful strategy for the synthesis of the defective and porous metallene with functionalized surfaces for various catalytic applications.

Generation of 2D nonlayered ferromagnetic VO2(M) nanosheets induced by strain engineering of CO2

Two-dimensional (2D) nonlayered ferromagnets displaying high Curie temperatures, sizable magnetic anisotropy levels, and large spin polarizations are emerging as promising 2D ferromagnetics. However, the difficulties in synthesizing 2D nonlayered intrinsic ferromagnets have largely limited their development. Herein, defect-rich 2D nonlayered VO₂(M) nanosheets have been fabricated by deploying straining engineering of CO₂ on the metal-insulator transition (MIT) of VO₂. Above T_MIT, the strong strain engineering of CO₂ in the R phase of VO₂ generated a very large number of atomic defects in its 3D crystal structure, and as a result facilitated conversion of the defective 3D network to 2D nanosheets along the c-axis. The as-prepared 2D defective VO₂(M) nanosheets displayed unique room-temperature ferromagnetism, attributed to the symmetry breaking triggered by the disordered atomic structure combined with the 3D-to-2D transformation.

Phase-Selective Synthesis of Ultrathin FeTe Nanoplates by Controllable Fe/Te Atom Ratio in the Growth Atmosphere

Phase controllable synthesis of 2D materials is of significance for tuning related electrical, optical, and magnetic properties. Herein, the phase-controllable synthesis of tetragonal and hexagonal FeTe nanoplates has been realized by a rational control of the Fe/Te ratio in a chemical vapor deposition system. Using density functional theory calculations, it has been revealed that with the change of the Fe/Te ratio, the formation energy of active clusters changes, causing the phase-controllable synthesis of FeTe nanoplates. The thickness of the obtained FeTe nanoplates can be tuned down to the 2D limit (2.8 nm for tetragonal and 1.4 nm for hexagonal FeTe). X-ray diffraction pattern, transmission electron microscopy, and high resolution scanning transmission electron microscope analyses exhibit the high crystallinity of the as-grown FeTe nanoplates. The two kinds of FeTe nanoflakes show metallic behavior and good electrical conductivity, featuring 8.44 × 10⁴ S m^-1 for 9.8 nm-thick tetragonal FeTe and 5.45 × 10⁴ S m^-1 for 7.6 nm-thick hexagonal FeTe. The study provides an efficient and convenient route for tailoring the phases of FeTe nanoplates, which benefits to study phase-sensitive properties, and may pave the way for the synthesis of other multiphase 2D nanosheets with controllable phases.

LiCl as Phase-Transfer Catalysts to Synthesize Thin Co2 P Nanosheets for Oxygen Evolution Reaction

Inorganic salts have been widely studied as templates for the synthesis of 2D layer structures. However, these salts normally can only serve as templates without any catalytic activity. Here, we report that LiCl used for the synthesis of ultrathin nanosheets not only serves as template for the synthesis of ultrathin Co₂ P nanosheets with a thickness of 0.7 nm but also acts as a catalyst that induces the phase-transfer from CoP to Co₂ P. The Co₂ P nanosheets have a high electrochemical performance for oxygen evolution reaction.

Self-Assembled Vanadium Oxide Nanoflakes for p-Type Ammonia Sensors at Room Temperature

VO₂(B), VO₂(M), and V₂O₅ are the most famous compounds in the vanadium oxide family. Here, their gas-sensing properties were investigated and compared. VO₂(B) nanoflakes were first self-assembled via a hydrothermal method, and then VO₂(M) and V₂O₅ nanoflakes were obtained after a heat-phase transformation in nitrogen and air, respectively. Their microstructures were evaluated using X-ray diffraction and scanning and transmission electron microscopies, respectively. Gas sensing measurements indicated that VO₂(M) nanoflakes were gas-insensitive, while both VO₂(B) and V₂O₅ nanoflakes were highly selective to ammonia at room temperature. As ammonia sensors, both VO₂(B) and V₂O₅ nanoflakes showed abnormal p-type sensing characteristics, although vanadium oxides are generally considered as n-type semiconductors. Moreover, V₂O₅ nanoflakes exhibited superior ammonia sensing performance compared to VO₂(B) nanoflakes, with one order of magnitude higher sensitivity, a shorter response time of 14–22 s, and a shorter recovery time of 14–20 s. These characteristics showed the excellent potential of V₂O₅ nanostructures as ammonia sensors.

Benign Fabrication of Fully Stereocomplex Polylactide with High Molecular Weights via a Thermally Induced Technique

A reproducible and environmentally friendly method for the preparation of high molecular-weight stereocomplex polylactide (HMW SCPLA) is achieved. Poly(l-lactide) and poly(d-lactide) were simply dissolved in an environmentally friendly solvent, dibasic ester (DBE), at 110 °C. Then, the two solutions were mixed and cooled to room temperature, and the HMW SCPLA spontaneously precipitated in the form of fine powder consequently. The presence of the DBE reduced the reaction temperature and improved the molecular mobility of the polymers; thus, the degradation problems and the molecular diffusion issue in the process of the formation of the stereocomplex could be overcome. The relationship among the concentration of the mixture, degree of stereo-complexation, and thermal properties of SCPLA powders was also established. Moreover, porous membrane and film SCPLA material with good thermal properties were also obtained using this thermally induced technique. This method could be a good approach to expand the SCPLA applications.

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.

Two-Dimensional SiO2/VO2 Photonic Crystals with Statically Visible and Dynamically Infrared Modulated for Smart Window Deployment

Two-dimensional (2D) photonic structures, widely used for generating photonic band gaps (PBG) in a variety of materials, are for the first time integrated with the temperature-dependent phase change of vanadium dioxide (VO₂). VO₂ possesses thermochromic properties, whose potential remains unrealized due to an undesirable yellow-brown color. Here, a SiO₂/VO₂ core/shell 2D photonic crystal is demonstrated to exhibit static visible light tunability and dynamic near-infrared (NIR) modulation. Three-dimensional (3D) finite difference time domain (FDTD) simulations predict that the transmittance can be tuned across the visible spectrum, while maintaining good solar regulation efficiency (ΔT_sol = 11.0%) and high solar transmittance (T_lum = 49.6%). Experiments show that the color changes of VO₂ films are accompanied by NIR modulation. This work presents a novel way to manipulate VO₂ photonic structures to modulate light transmission as a function of wavelength at different temperatures.

X-Ray Spectroscopy of Ultra-Thin Oxide/Oxide Heteroepitaxial Films: A Case Study of Single-Nanometer VO2/TiO2

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO₂/TiO₂ (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.

Regulating the electrical behaviors of 2D inorganic nanomaterials for energy applications

Recent years have witnessed great developments in inorganic 2D nanomaterials for their unique dimensional confinement and diverse electronic energy bands. Precisely regulating their intrinsic electrical behaviors would bring superior electrical conductivity, rendering 2D nanomaterials ideal candidates for active materials in electrochemical applications when combined with the excellent reaction activity from the inorganic lattice. This Concept focuses on highly conducting inorganic 2D nanomaterials, including intrinsic metallic 2D nanomaterials and artificial highly conductive 2D nanomaterials. The intrinsic metallicity of 2D nanomaterials is derived from their closely packed atomic structures that ensure maximum overlapping of electron orbitals, while artificial highly conductive 2D nanomaterials could be achieved by designed methodologies of surface modification, intralayer ion doping, and lattice strain, in which atomic-scale structural modulation plays a vital role in realizing conducting behaviors. Benefiting from fast electron transfer, high reaction activity, as well as large surface areas arising from the 2D inorganic lattice, highly conducting 2D nanomaterials open up prospects for enhancing performance in electrochemical catalysis and electrochemical capacitors. Conductive 2D inorganic nanomaterials promise higher efficiency for electrochemical applications of energy conversion and storage.

Ultrathin two-dimensional inorganic materials: new opportunities for solid state nanochemistry

CONSPECTUS: The ultimate goal of solid state chemistry is to gain a clear correlation between atomic, defect, and electronic structure and intrinsic properties of solid state materials. Solid materials can generally be classified as amorphous, quasicrystalline, and crystalline based on their atomic arrangement, in which crystalline materials can be further divided into single crystals, microcrystals, and nanocrystals. Conventional solid state chemistry mainly focuses on studying single crystals and microcrystals, while recently nanocrystals have become a hot research topic in the field of solid state chemistry. As more and more nanocrystalline materials have been artificially fabricated, the solid state chemistry for studying those nanosolids has become a new subdiscipline: solid state nanochemistry. However, solid state nanochemistry, usually called "nanochemistry" for short, primarily studies the microstructures and macroscopic properties of a nanomaterial's aggregation states. Due to abundant microstructures in the aggregation states, it is only possible to build a simple but imprecise correlation between the microscopic morphology and the macroscopic properties of the nanostructures. Notably, atomically thin two-dimensional inorganic materials provide an ideal platform to establish clear structure-property relationships in the field of solid state nanochemistry, thanks to their homogeneous dispersion without the assistance of a capping ligand. In addition, their atomic structures including coordination number, bond length, and disorder degree of the examined atoms can be clearly disclosed by X-ray absorption fine structure spectroscopy. Also, their more exposed interior atoms would inevitably induce the formation of various defects, which would have a non-negligible effect on their physicochemical properties. Based on the obtained atomic and defect structural characteristics, density-functional calculations are performed to study their electronic structures. Then, after the properties of the individual ultrathin two-dimensional materials or their assembled highly oriented thin film-based nanodevices are measured, the explicit relationship between atomic, defect, and electronic structure and intrinsic properties could be established. In this Account, we focus on our recent advances in the field of solid state nanochemistry, including atomic structure characterization of ultrathin two-dimensional inorganic materials by X-ray absorption fine structure spectroscopy, characterization of their different types of structural defects by positron annihilation spectra and electron spin resonance, and investigation of their electronic structure by density-functional calculations. In addition, we summarize the close correlation between atomic, defect, and electronic structure variations and the optoelectronic, electrical, magnetic, and thermal properties of ultrathin two-dimensional materials. Finally, we also propose the major challenges and opportunities that face solid state nanochemistry. We believe that all the past achievements in ultrathin two-dimensional materials could bring new opportunities for solid state nanochemistry.

Self-assembled metastable γ-Ga2O3 nanoflowers with hexagonal nanopetals for solar-blind photodetection

Metastable γ-Ga2O3 nanoflowers assembled from hexagonal nanopetals are successfully constructed by the oxidation of metallic Ga in acetone solution. The nanoflowers with a hollow interior structure exhibit a short response time and a large light-current-dark-current ratio under a relatively low bias voltage, suggesting an especially important potential application in solar-blind photodetection.

Atomically-thick two-dimensional crystals: electronic structure regulation and energy device construction

Atomically-thick two-dimensional crystals can provide promising opportunities to satisfy people's requirement of next-generation flexible and transparent nanodevices. However, the characterization of these low-dimensional structures and the understanding of their clear structure-property relationship encounter many great difficulties, owing to the lack of long-range order in the third dimensionality. In this review, we survey the recent progress in fine structure characterization by X-ray absorption fine structure spectroscopy and also overview electronic structure modulation by density-functional calculations in the ultrathin two-dimensional crystals. In addition, we highlight their structure-property relationship, transparent and flexible device construction as well as wide applications in photoelectrochemical water splitting, photodetectors, thermoelectric conversion, touchless moisture sensing, supercapacitors and lithium ion batteries. Finally, we outline the major challenges and opportunities that face the atomically-thick two-dimensional crystals. It is anticipated that the present review will deepen people's understanding of this field and hence contribute to guide the future design of high-efficiency energy-related devices.

Hydrothermal synthesis of Mo-doped VO2/TiO2 composite nanocrystals with enhanced thermochromic performance

This paper reports a one-step TiO2 seed-assistant hydrothermal synthesis of Mo-doped VO2(M)/TiO2 composite nanocrystals. It was found that excess Mo doping can promote formation of the VO2(M) phase, and rutile TiO2 seed is beneficial to morphology control, size reduction, and infrared modulation of Mo-doped VO2(M) nanocrystals. The Mo-doped VO2 nanocrystals epitaxially grow on TiO2 seeds and have a quasi-spherical shape with size down to 20 nm and a nearly 35% infrared modulation near room temperature. The findings of this work demonstrate important progress in the near-room-temperature thermochromic performance of VO2(M) nanomaterials, which will find potential application in constructing VO2(M) nanocrystal-based smart window coatings.