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Pure and Ti-doped vanadium dioxide (VO) thin films were deposited on muscovite substrates using an inorganic sol-gel method. The crystallinity, stoichiometry, surface morphology and phase transition properties of the films were investigated using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope and in situ FTIR measurement, respectively. The results indicated that all the VO films exhibited polycrystalline structure, with a (011) preferred orientation. But Ti doping affected the morphology of the films remarkably, which then led to varied phase transition properties. Particularly, the phase transition temperature elevated by increasing the doping concentration of Ti (from 61 °C for pure film to 71.5 °C for the film doped with 2.8 at.% Ti), and the hysteresis loop decreased simultaneously. This study proposed a new method to prepare Ti-doped VO film and provided considerable insights for the application of Ti-doped VO film due to its unique phase transition properties. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Vanadium oxide (VO2), as one of the classical strongly correlated oxides with a reversible and sharp insulator-metal transition (IMT), enables many applications in dynamic terahertz (THz) wave control. Recently, due to the inherent phase transition hysteresis feature, VO2 has shown favorable application prospects in memory-related devices once combined with metamaterials or metasurfaces. However, to date, VO2-based memory meta-devices are usually in a single-channel read/write mode, which limits their storage capacity and speed. In this paper, we propose a reconfigurable meta-memory based on VO2, which favors a two-channel read/write mode. Our design consists of a pair of large and small split-ring resonators, and the corresponding VO2 patterns are embedded in the gap locations. By controlling the external power supply, the two operation bands can be controlled independently to achieve at least four amplitude states, including "00", "01", "10", and "11", which results in a two-channel storage function. In addition, our research may provide prospective applications in fields such as THz switching, photon storage, and THz communication systems in the future. [ABSTRACT FROM AUTHOR]

Highlights: [•] Porous nano-structured VO2 films with different shapes particles are obtained via a simple process. [•] Systematic studied the effects of N2 flow rate on morphology and the thermochromic properties of VO2 films. [•] ΔT is increased to 39.8°C and T t is reduced to 58.3°C, the features of the S–M transition are different. [ABSTRACT FROM AUTHOR]

Abstract: Nanostructured VO2(B) was synthesized via a combined hydrothermal method using V2O5 as a source material and oxalic acid powder as a reductant. Especially, cetyltrimethylammonium bromide (CTAB) was used as template and then three different morphologies of the VO2(B): nanobelts, nanoflowers and nanoflakes were obtained through the change of the experimental conditions. The morphology and crystalline structure of the prepared products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Furthermore, the electrochemical charge–discharge cycling properties of the VO2(B) nanostructures in lithium-ion battery were investigated. The results indicated that the belt-like, flower-like and flake-like VO2(B) nanostructures have the initial specific discharge capacity of 205.2, 254.0 and 56.0mAhg−1, and that the morphology of VO2(B) nanostructures can deeply affect the service performance of batteries. According to the experiments, this CTAB-assisted hydrothermal method provides an insight into the preparation and application of nanostructured VO2(B) as cathode material in lithium-ion battery. [Copyright &y& Elsevier]

Abstract: In this work, we confirmed experimentally that the porous nano-structure of vanadium dioxide (VO2) films had significant effects on the features of the metal–insulator transition. Porous VO2 films had been synthesized on mica substrates via a simple solution process with inorganic precursor and cetyltrimethyl ammonium bromide (CTAB). SEM images showed a combination of homogenously distributed pores and VO2 nanoparticles. Metal to insulator transition (MIT) characteristics measurements revealed that, the porous nano-structure VO2 films had low transition temperature, wide hysteresis width and gentle slopes of hysteresis loop. Morphologies and MIT characteristics of the films could be controlled by simply varying CTAB concentration, which is of great importance in achieving practical applications. [Copyright &y& Elsevier]

Nanostructured Li 2 TiO 3 ceramics which may have effective thermal conductivity, excellent tritium release behaviour and good irradiation resistance are regarded as a promising solid tritium breeding material for the fusion reactor blanket of the International Thermonuclear Experimental Reactor (ITER). However, due to the limitations of the preparation technology, reports concerning Li 2 TiO 3 nanoceramics have been rare. In this paper, uniform nano-Li 2 TiO 3 powder particles which were essential to obtain nanostructured Li 2 TiO 3 ceramics pebbles were synthesised via a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method, and then rare, homogeneous nanostructured Li 2 TiO 3 ceramic pebbles were fabricated with the as-prepared powder particles. The mechanisms by which CTAB can reduce particle agglomeration and be of assistance in achieving a nanostructured Li 2 TiO 3 ceramic were also investigated. In addition, systematic experiments on the relationship between the added amount of CTAB and the mechanical properties of the Li 2 TiO 3 ceramic structure were also carried out. The results revealed that the desired Li 2 TiO 3 nanoceramic could be fabricated when 3% CTAB was introduced, as the Li 2 TiO 3 pebbles obtained had a small grain size (90 nm), high relative density (89.71%T.D.) and crush load (99.93 N), which were expected to show favourable potential as a promising tritium breeder material in the fusion reactor blanket. [ABSTRACT FROM AUTHOR]

Abstract: Mixture of cerium (III) nitrate hexahydrate and vanadium pentaoxide powder were used as precursor to prepare Ce-doped VO2 films on the muscovite substrate by inorganic sol–gel method. SEM, XRD and XPS were used to investigate the morphologies and structures of VO2 films. The results show that the VO2 films grow on the muscovite substrate with preferred orientated (011) plane and the Ce exists in the form of Ce4+ and Ce3+ replacing part of V atoms in the lattice. The infrared transmittance change was measured from room temperature to the temperature above the metal–insulator transition. The films have excellent thermochromic performance. The metal–insulator transition temperature of VO2 films changes appreciably with Ce doped, which decreases by 4.5°C per 1 at.% doping. Furthermore, Ce doping could remarkably reduce the particle size of VO2 films. [Copyright &y& Elsevier]

VO 2 , which exhibits semiconductor-metal phase transition characteristics occurring on a picosecond time scale, holds great promise for ultrafast terahertz modulation in next-generation communication. However, as of now, there is no reported prototype for an ultrafast device. The temperature effect has been proposed as one of the major obstacles. Consequently, reducing the excitation threshold for the phase transition would be highly significant. The traditional strategy typically involves chemical doping, but this approach often leads to a decrease in phase transition amplitude and a slower transition speed. In this work, we proposed a design featuring a highly conductive MXene interfacial layer between the VO 2 film and the substrate. We demonstrate a significant reduction in the phase transition threshold for both temperature and laser-induced phase transition by adjusting the conductivity of the MXene layers with varying thicknesses. Our observations show that the phase transition temperature can be decreased by 9 °C, while the pump fluence for laser excitation can be reduced by as high as 36%. The ultrafast phase transition process on a picosecond scale, as revealed by the optical-pump terahertz-probe method, suggests that the MXene layers have minimal impact on the phase transition speed. Moreover, the reduced phase transition threshold can remarkably alleviate the photothermal effect and inhibit temperature rise and diffusion in VO 2 triggered by laser. This study offers a blueprint for designing VO 2 /MXene hybrid films with reduced phase transition thresholds. It holds significant potential for the development of low-power, intelligent optical and electrical devices including, but not limited to, terahertz modulators based on phase transition phenomena.

Black TiO 2 formed by introducing lattice disorder into pristine TiO 2 has a narrowed band gap and suppresses the recombination of charge carriers. This provides a potential strategy for visible light photocatalysis. However, the microstructural design of black TiO 2 for a higher optimization of visible light is still in high demand. In this work, we proposed the preparation of black TiO 2 hollow shells with controllable cavity diameters using silica spheres as templates for the cavities and the NaBH 4 reduction method. The decreased cavity size resulted in a hollow shell with an enhanced visible-light absorption and improved photocatalytic performance. Moreover, we demonstrated that this cavity can be combined with gold nanoparticles (AuNPs) to form AuNPs@black TiO 2 yolk-shells. The AuNPs provided additional visible light absorption and promoted the separation of photogenerated carriers in the yolk-shell structures. This further improved the photocatalysis, the degradation rate of Cr(VI) can reach 0.066 min -1 . Our work evaluated the effect of the cavity size on the photocatalytic performance of hollow and yolk-shell structures and provided concepts for the further enhancement of visible-light photocatalysis.

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO 2 ) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO 2 film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

The dynamic control of terahertz (THz) wave transmission on flexible functional materials is a fundamental building block for wearable electronics and sensors in the THz range. However, achieving high-efficiency THz modulation and low insertion loss is a great challenge while maintaining the excellent flexibility and stretchability of the materials. Herein, we report a Ti 3 C 2 T x MXene/waterborne polyurethane (WPU) membrane prepared by a vacuum-assisted filtration method, which exhibits excellent THz modulation properties across stretching. The hydrophilic Ti 3 C 2 T x MXene and WPU enable the uniform 3D distribution of Ti 3 C 2 T x MXene in the WPU matrix. Particularly, the stretchability with the maximum strain of the membranes can reach 200%, accompanied by dynamic tuning of THz transmittance for more than 90% and an insertion loss as low as -4.87 dB. The giant THz modulation continuously decreases with MXene content per unit area, accompanied by a lower density of the MXene interface and diminished THz absorption during stretching. Such a design opens a pathway for achieving flexible THz modulators with a high modulation depth and low insertion loss, which would be used for THz flexible and wearable devices.

Optical physical unclonable function (PUF) is one of the most promising hardware security solutions, which has been proven to be resistant to machine learning attacks. However, the disordered structures of the traditional optical PUFs are usually deterministic once they are manufactured and therefore exhibit fixed challenge-response behaviors. Herein, a reconfigurable PUF (R-PUF) is proposed and demonstrated by using the reversible phase transition behavior of VO 2 nanocrystals combined with TiO 2 disordered nanoparticles. Both the simulation and experiment results show that the near-infrared laser speckle pattern of the R-PUF can be almost completely altered after the phase transition of VO 2 nanocrystals, resulting in a reconfigurable and reproducible optical response. The similarity of the response speckles shows an obvious hysteresis loop during the rise and drop of temperature, providing a simple way to regulate and control the response behaviors of the R-PUF. More importantly, the hysteretic characteristic provides a new dimension to describe the challenge-response behavior of the R-PUF besides the laser speckle, providing an effective way to improve the security and encoding capacity of the optical PUFs. The proposed R-PUF can be employed as a promising security primitive for high robustness and high-security authentication and encryption.

Phase change materials exhibit unique advantages in reconfigurable photonic devices due to drastic tunability of photoelectric properties. Here, we systematically investigate the thermal equilibrium process and the ultrafast dynamics of Ge 2 Sb 2 Te 5 (GST) driven by femtosecond (fs) pulses, using time-resolved terahertz spectroscopy. Both fs-pulse-driven crystallization and amorphization are demonstrated, and the threshold of photoinduced crystallization (amorphization) is determined to be 8.4 mJ/cm 2 (10.1 mJ/cm 2 ). The ultrafast carrier dynamics reveal that the cumulative photothermal effect plays a crucial role in the ultrafast crystallization, and modulation depth of volatile (nonvolatile) THz has switching limits up to 30% (15%). A distinctive phonon absorption at 1.1 THz is observed, providing fingerprint spectrum evidence of crystalline lattice formation driven by intense fs pulses. Finally, multistate volatile (nonvolatile) THz switching is implemented by tuning optical pump fluence. These results provide insight into the photoinduced phase change of GST and offer benefits for all optical THz functional devices.

Dynamic tuning of terahertz (THz) wave is vital for the development of next generation THz devices. Utilization of solar energy for tuning THz waves is a promising, eco-friendly, and sustainable way to expand THz application scenarios. Ti 2 O 3 with an ultranarrow bandgap of 0.1eV exhibits intriguing thermal-induced metal-insulator transition (MIT), and possesses excellent photothermal conversion efficiency. Herein, Ti 2 O 3 film was fabricated by a two-step magnetron sputtering method, and exhibited an excellent photothermal conversion efficiency of 90.45% and demonstrated temperature-dependent THz transmission characteristics with a wideband at 0.1-1 THz. We supposed to combine photothermal conversion characteristics with temperature-dependent THz transmission properties of Ti 2 O 3 film, which could introduce solar light as the energy source for tuning THz waves. Our work will provide new sight for investigating MIT characteristics of Ti 2 O 3 at THz regime and exhibit huge potential in the application of tuning terahertz waves in outdoor scenarios in the future., Competing Interests: The authors declare no competing interests., (© 2021 The Author(s).)

In this work, rapid chemical precipitation assisted annealing method is used to prepare flower-like NiCo 2 S 4 . And the flower-like structure after polyethylene glycol (PEG) modification yields an excellent specific capacitance (2198.9 F g -1 at 1 A g -1 ). And an asymmetric supercapacitor assembled with NiCo 2 S 4 (PEG-modified) and activated carbon (AC) shows an energy density of 38.2 Wh kg -1 at 400 W kg -1 , and outstanding stability (80% remained after 3000 cycles at 5 A g -1 ). Benefited by a larger specific surface area and suitable pore size of the aggregate structure, the specific capacitance of prepared NiCo 2 S 4 is increased by about 2 times. This uncomplicated preparation method is proved to be suitable for NiCo 2 S 4 with a high specific capacitance of supercapacitors.

Black TiO 2 has received tremendous attention because of its lattice disorder-induced reduction in the TiO 2 bandgap, which yields excellent light absorption and photocatalytic ability. In this report, a highly efficient visible-light-driven black TiO 2 photocatalyst was synthesized with a mesoporous hollow shell structure. It provided a higher specific surface area, more reaction sites and enhanced visible light absorption capability, which significantly promoted the photocatalytic reaction. Subsequently, the mesoporous hollow black TiO 2 with different lattice disorder-engineering degrees were designed. The structure disorder in the black TiO 2 obviously increased with reduction temperature, leading to improved visible light absorption. However, their visible-light-driven photocatalytic efficiency increased first and then decreased. The highest value can be observed for the sample reduced at 350 °C, which was 2-, 1.4- and 5-fold that of the samples reduced at 320 °C, 380 °C and 400 °C, respectively. This contradiction can be ascribed to the varied functions of the surface defects with different concentrations in the black TiO 2 during the catalytic process. In particular, the defects at low concentrations boost photocatalysis but reverse photocatalysis at high concentrations when they act as charge recombination centers. This study provides significant insight for the fabrication of high-efficiency visible-light-driven catalytic black TiO 2 and the understanding of its catalysis mechanism., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Ultrafast detection and switching of light are key processes in high-speed optoelectronic devices. However, the performances of VO 2 -based optoelectronics are strongly degraded by photothermal. The mechanism of the latter is still unclear. Here, by using femtosecond-laser (fs-laser) driven kinetic terahertz wave absorption, we quantitatively separate slow photothermal response and ultrafast photodoping response (e.g. light-induced insulator-to-metal transition) from second- to picosecond-timescales, and discover the competing interplay between them. With self-photothermal (mainly determined by fs-laser pulse repetition rate and pump fluence), the ultrafast transition time was degraded by 190% from 50 ps to 95 ps, the ultrafast transition threshold was decreased to 82% from 11mJ/cm 2 to 9mJ/cm 2 , while the amplitudes of the two photoresponse are competing. Percolation theory, along with the macroscopic conductivity response, is used to explain the competing interplay. Our findings are relevant for designing and optimizing VO 2 -based ultrafast optoelectronic devices.

Vanadium dioxide (VO 2 ), with reversible metal-semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO 2 nanoparticles are particularly superior to VO 2 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 VO 2 (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO 2 underwent a two-stage phase evolution from the pure phase comprising VO 2 (A) nanobelts to VO 2 (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 VO 2 (M). The phase-transition temperature of VO 2 (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO 2 (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 VO 2 (M) nanorods and provides significant insights into their applications in infrared smart windows., Competing Interests: The authors declare no competing financial interest.

We investigate the ionization and displacement effects of an electron-beam (e-beam) on amorphous Gd 2 Zr 2 O 7 synthesized by the co-precipitation and calcination methods. The as-received amorphous specimens were irradiated under electron beams at different energies (80 keV, 120 keV, and 2 MeV) and then characterized by X-ray diffraction and transmission electron microscopy. A metastable fluorite phase was observed in nanocrystalline Gd 2 Zr 2 O 7 and is proposed to arise from the relatively lower surface and interface energy compared with the pyrochlore phase. Fast crystallization could be induced by 120 keV e-beam irradiation (beam current = 0.47 mA/cm 2 ). The crystallization occurred on the nanoscale upon ionization irradiation at 400 °C after a dose of less than 10 17 electrons/cm 2 . Under e-beam irradiation, the activation energy for the grain growth process was approximately 10 kJ/mol, but the activation energy was 135 kJ/mol by calcination in a furnace. The thermally activated ionization process was considered the fast crystallization mechanism.

This study investigated ageing-resistant properties of carboxyl-terminated polyester (polyethylene glycol terephthalate) composites modified with nanoscale titanium dioxide particles (nano-TiO2). The nano-TiO2 was pretreated by a dry coating method, with aluminate coupling agent as a functional grafting additive. The agglomeration resistance was evaluated, which exhibited significant improvement for the modified nanoparticles. Then, the effects of the modified nano-TiO2 on the crosslinking and ageing-resistant properties of the composites were studied. With a real-time Fourier transform infrared (FT-IR) measurement, the nano-TiO2 displayed promoting effect on the crosslinking of polyester resin with triglycidyl isocyanurate (TGIC) as crosslinking agent. Moreover, the gloss retention, colour aberration and the surface morphologies of the composites during accelerated UV ageing (1500 hours) were investigated. The results demonstrated much less degree of ageing degradation for the nanocomposites, indicating an important role of the nano-TiO2 in improving the ageing-resistant properties of synthetic polymer composites.