The Hydric Effect in Inorganic Nanomaterials for Nanoelectronics and Energy Applications

Chemical Modulation of Metal-Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal-insulator transition (MIT) of vanadium dioxide (VO2 ) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO2 , endowing VO2 with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO2 nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO2 and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO2 MIT are also provided.

Insights into the Semiconductor SERS Activity: The Impact of the Defect-Induced Energy Band Offset and Electron Lifetime Change

The significant boost in surface-enhanced Raman scattering (SERS) by the chemical enhancement of semiconducting oxides is a pivotal finding. It offers a prospective path toward high uniformity and low-cost SERS substrates. However, a detailed understanding of factors that influence the charge transfer process is still insufficient. Herein, we reveal the important role of defect-induced band offset and electron lifetime change in SERS evolution observed in a MoO3 oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.

Designing of Nanomaterials-Based Enzymatic Biosensors: Synthesis, Properties, and Applications

Among the many biological entities employed in the development of biosensors, enzymes have attracted the most attention. Nanotechnology has been fostering excellent prospects in the development of enzymatic biosensors, since enzyme immobilization onto conductive nanostructures can improve characteristics that are crucial in biosensor transduction, such as surface-to-volume ratio, signal response, selectivity, sensitivity, conductivity, and biocatalytic activity, among others. These and other advantages of nanomaterial-based enzymatic biosensors are discussed in this work via the compilation of several reports on their applications in different industrial segments. To provide detailed insights into the state of the art of this technology, all the relevant concepts around the topic are discussed, including the properties of enzymes, the mechanisms involved in their immobilization, and the application of different enzyme-derived biosensors and nanomaterials. Finally, there is a discussion around the pressing challenges in this technology, which will be useful for guiding the development of future research in the area.

Hydrogen-Doping-Induced Metal-Like Ultrahigh Free-Carrier Concentration in Metal-Oxide Material for Giant and Tunable Plasmon Resonance

The practical utilization of plasmon-based technology relies on the ability to find high-performance plasmonic materials other than noble metals. A key scientific challenge is to significantly increase the intrinsically low concentration of free carriers in metal-oxide materials. Here, a novel electron-proton co-doping strategy is developed to achieve uniform hydrogen doping in metal-oxide MoO₃ at mild conditions, which creates a metal-like ultrahigh free-carrier concentration approaching that of noble metals (10²¹ cm^-3 in H_1.68 MoO₃ versus 10²² cm^-3 in Au/Ag). This bestows giant and tunable plasmonic resonances in the visible region to this originally semiconductive material. Using ultrafast spectroscopy characterizations and first-principle simulations, the formation of a quasi-metallic energy band structure that leads to long-lived and strong plasmonic field is revealed. As verified by the surface-enhanced Raman spectra (SERS) of rhodamine 6G molecules on H_x MoO₃ , the SERS enhancement factor reaches as high as 1.1 × 10⁷ with a detection limit at concentration as low as 1 × 10^-9  mol L^-1 , representing the best among the hitherto reported non-metal systems. The findings not only provide a set of metal-like semiconductor materials with merits of low cost, tunable electronic structure, and plasmonic resonance, but also a general strategy to induce tunable ultrahigh free-carrier concentration in non-metal systems.

Extraordinary Role of Bi for Improving Thermoelectrics in Low-Solubility SnTe-CdTe Alloys

As an environment-friendly alternative to traditional PbTe, many attempts have recently been made to improve thermoelectric SnTe. Effective strategies are mainly focused on valence band convergence, nanostructuring, interstitial defects, and alloying solubility. In particular, alloying SnTe with CdTe/GeTe triggers an inherent decline of valence band offset effectively owing to a high solubility of ∼20% of CdTe. However, to what level an additional element doping in low-solubility SnTe-CdTe alloys can play a role in enhancing the thermoelectric performance still remains a mystery. Here, a new strategy is shown that unexpected Bi doping, by alloying with only ∼3% CdTe, induces a significant enhancement of the thermoelectric figure of merit ZT to be ∼240% (ZT up to ∼1.3) at 838 K, which is mainly determined by the dramatically reduced lattice thermal conductivity above 800 K deriving from the exotic Bi doping and Cu-interstitial defects. More interestingly, combining experimental and theoretical evidences, the Bi-doping-driven band convergence is also beneficial to the improvement of thermoelectric performance below 800 K. The present findings demonstrate the extraordinary role of Bi for advancing the thermoelectric performance in SnTe alloys.

Electrochemical fabrication of FeSxfilms with high catalytic activity for oxygen evolution

Electrochemical decomposition of water to produce oxygen (O₂) and hydrogen (H₂) through an anodic oxygen evolution reaction (OER) and a cathodic hydrogen evolution reaction (HER) is a promising green method for sustainable energy supply. Here, we demonstrate that cauliflower-like S-doped iron microsphere films are materials that can efficiently decompose water as an electrocatalyst for the oxygen evolution reaction. FeS_x films are prepared by a simple one-step electrodeposition method and directly grow on copper foam from a deep eutectic solvent, ethaline (mixture of choline chloride and ethylene glycol), as a durable and highly efficient catalyst for the OER in 1.0 M KOH. The prepared FeS_x/CF, as an oxygen-evolving anode, shows remarkable catalytic performance toward the OER with a moderate Tafel slope of 105 mV dec⁻¹, and require an overpotential of only 340 mV to drive a geometrical catalytic current density of 10 mA cm⁻². In addition, this catalyst also demonstrates strong long-term electrochemical durability. This study provides a simple synthesis route for practical applications of limited transition metal nano catalysts.

Electrochemical decomposition of water to produce oxygen (O₂) and hydrogen (H₂) through an anodic oxygen evolution reaction (OER) and a cathodic hydrogen evolution reaction (HER) is a promising green method for sustainable energy supply.

Molybdenum-Incorporated Mesoporous Silica: Surface Engineering toward Enhanced Metal-Support Interactions and Efficient Hydrogenation

In heterogeneous catalysis, strong metal-support interactions are highly desired to improve catalytic turnover on metal catalysts. Herein, molybdenum is uniformly incorporated into mesoporous silica (KIT-6) to accomplish strong interactions with iridium catalysts, and consequently, active and selective hydrogenation of carbonyl compounds. Mo-incorporated KIT-6 (Mo-KIT-6) affords electronic interactions to improve the proportion of metallic Ir⁰ species, avoiding the easy surface oxidation of ultrafine metals in silica mesocavities. Owing to the effective H₂ activation and subsequent hydrogenation on metallic Ir⁰ sites, optimal Ir/Mo-KIT-6 with a high Ir⁰/Ir^δ+ ratio delivers prominent performance in the hydrogenation of amides to amines and α,β-unsaturated aldehydes to unsaturated alcohols. As for N-acetylmorpholine hydrogenation, the Ir/Mo-KIT-6 catalyst achieves efficient turnover toward N-ethylmorpholine with high selectivity (>99%) and exhibits activity that relies on the engineered chemical state of Ir sites. Such promotion is further proved to be universal in cinnamaldehyde hydrogenation. This work will provide new opportunities for catalyst design through surface/interface engineering.

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films

Vanadium dioxide (VO₂) with a metal-insulator transition (MIT) has been supposed as a candidate for optoelectronic devices. However, the MIT temperature ( T_MIT) above room temperature limits its application scope. Here, high-quality V_{1- x}W _xO₂ films have been prepared by pulsed laser deposition. On the basis of temperature-dependent transmittance and Raman spectra, it was found that T_MIT increases from 241 to 279 K, when increasing the doping concentration in the range of 0.16 ≤ x ≤ 0.20. The interband electronic transitions and orbital structures of V_{1- x}W _xO₂ films have been investigated via fitting transmittance spectra. Moreover, with the aid of first-principles calculations, an effective orbital theory has been proposed to explain the unique phenomenon. When the W doping concentration increases, the π* and d_II orbitals shift toward the π orbital. Meanwhile, the energy gap between the π* and d_II orbitals decreases at the insulator state. It indicates that the bandwidth is narrowed, which impedes MIT. In addition, the overlap of the π* and d_II orbitals increases at the metal state, and more doping electrons occupy the π* orbital induced by increasing W doping concentration. It manifests that the Mott insulating state becomes more stable, which further improves T_MIT. The present work provides a feasible approach to tune T_MIT via orbital variation and can be helpful in developing the potential VO₂-based optoelectronic devices.

Electronic-dimensionality reduction of bulk MoS2 by hydrogen treatment

A reduction in the electronic-dimensionality of materials is one method for achieving improvements in material properties. Here, a reduction in electronic-dimensionality is demonstrated using a simple hydrogen treatment technique. Quantum well states from hydrogen-treated bulk 2H-MoS2 are observed using angle resolved photoemission spectroscopy (ARPES). The electronic states are confined within a few MoS2 layers after the hydrogen treatment. A significant reduction in the band-gap can also be achieved after the hydrogen treatment, and both phenomena can be explained by the formation of sulfur vacancies generated by the chemical reaction between sulfur and hydrogen.

Ga-Doping-Induced Carrier Tuning and Multiphase Engineering in n-type PbTe with Enhanced Thermoelectric Performance

P-type lead telluride (PbTe) emerged as a promising thermoelectric material for intermediate-temperature waste-heat-energy harvesting. However, n-type PbTe still confronted with a considerable challenge owing to its relatively low figure of merit ZT and conversion efficiency η, limiting widespread thermoelectric applications. Here, we report that Ga-doping in n-type PbTe can optimize carrier concentration and thus improve the power factor. Moreover, further experimental and theoretical evidence reveals that Ga-doping-induced multiphase structures with nano- to micrometer size can simultaneously modulate phonon transport, leading to dramatic reduction of lattice thermal conductivity. As a consequence, a tremendous enhancement of ZT value at 823 K reaches ∼1.3 for n-type Pb_0.97Ga_0.03Te. In particular, in a wide temperature range from 323 to 823 K, the average ZT_ave value of ∼0.9 and the calculated conversion efficiency η of ∼13% are achieved by Ga doping. The present findings demonstrate the great potential in Ga-doped PbTe thermoelectric materials through a synergetic carrier tuning and multiphase engineering strategy.

In situ electrochemical development of copper oxide nanocatalysts within a TCNQ nanowire array: a highly conductive electrocatalyst for the oxygen evolution reaction

It is highly desired to develop efficient earth-abundant electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. In this communication, we report the in situ electrochemical conversion of a nanoarray of Cu(tetracyanoquinodimethane), Cu(TCNQ), an inorganic-organic hybrid, on Cu foam into CuO nanocrystals confined in a highly conductive nanoarray via anode oxidation. As a 3D catalyst electrode, the resulting CuO-TCNQ/CF shows high OER activity and demands an overpotential of only 317 mV to drive a geometrical catalytic current density of 25 mA cm^-2. Notably, this catalyst also demonstrates strong long-term electrochemical durability. This study provides us with a universal strategy toward topotactic room-temperature preparation of conductive nanoarrays with confined transition metal nanocatalysts for practical applications.

Growth of 2D Mesoporous Polyaniline with Controlled Pore Structures on Ultrathin MoS2 Nanosheets by Block Copolymer Self-Assembly in Solution

The development of versatile strategies toward two-dimensional (2D) porous nanocomposites with tunable pore structures draws immense scientific attention in view of their attractive physiochemical properties and a wide range of promising applications. This paper describes a self-assembly approach for the directed growth of mesoporous polyaniline (PANi) with tunable pore structures and sizes on ultrathin freestanding MoS₂ nanosheets in solution, which produces 2D mesoporous PANi/MoS₂ nanocomposites. The strategy employs spherical and cylindrical micelles, which are formed by the controlled solution self-assembly of block copolymers, as the soft templates for the construction of well-defined spherical and cylindrical mesopores in the 2D PANi/MoS₂ nanocomposites, respectively. With potential applications as supercapacitor electrode materials, the resultant 2D composites show excellent capacitive performance with a maximum capacitance of 500 F g^-1 at a current density of 0.5 A g^-1, good rate performance, as well as outstanding stability for charge-discharge cycling. Moreover, the 2D mesoporous nanocomposites offer an opportunity for the study on the influence of different pore structures on their capacitive performance, which helps to understand the pore structure-property relationship of 2D porous electrode materials and to achieve their electrochemical performance control.

Enhanced photogenerated carrier separation in CdS quantum dot sensitized ZnFe2O4/ZnIn2S4 nanosheet stereoscopic films for exceptional visible light photocatalytic H2 evolution performance

CdS quantum dot sensitized ZnFe₂O₄/ZnIn₂S₄ nanosheet stereoscopic films were synthesized through two sequential solvothermal processes and the ionic layer adsorption-reaction method. The hydrophilic ZnIn₂S₄ nanosheet stereoscopic film was pre-prepared to act as a suitable host material, and then ZnFe₂O₄ nanoparticles and CdS quantum dots were uniformly decorated on the surface of the ZnIn₂S₄ nanosheet stereoscopic film to form a ternary heterostructure stereoscopic film. The band structure difference in the ternary heterostructure can promote the spatial separation and transport efficiency of photogenerated charge carriers. Meanwhile, the composite stereoscopic film has significant structural advantages and can provide a large amount of reaction active sites and outstanding visible light utilization. The superhydrophilic surface contributes to interface contact of catalyst/solution and gas detachment. These positive factors led to a significantly enhanced photocatalytic H₂ evolution activity of the CdS/ZnFe₂O₄/ZnIn₂S₄ ternary heterostructure film in comparison with the pristine ZnIn₂S₄ and binary heterostructure film photocatalysts. The optimized CdS/ZnFe₂O₄/ZnIn₂S₄ ternary heterostructure film demonstrates the highest H₂ production rate of 79.0 μmol h^-1, which surpasses that of ZnIn₂S₄ by more than 3.2 times. This synthesis strategy can be applicable for the facile synthesis of other visible-light-driven composite film catalysts.

Simple but Effective Way To Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)2 Annihilation and Oxygen Vacancy Generation by Vacuum Annealing

This study presents an effective and the simplest method to substantially improve the photoelectrochemical water-splitting ability of hydrothermally grown ZnO nanorod arrays (NRAs). In the hydrothermal growth of ZnO NRAs, unwanted Zn(OH)₂ species are formed, which act as trapping sites of photoexcited charges. We found that those inherent charge-trapping sites could be annihilated by the desorption of the hydroxyl groups upon vacuum annealing above 200 °C, which resulted in an enhancement of the charge-separation efficiency and photocurrent density. Another drastic increase in the photocurrent density occurred when ZnO NRAs were treated with annealing at higher temperature (700 °C), which can be attributed to the introduced oxygen vacancies acting as shallow donors in the ZnO crystal lattice. The removal of the charge-trapping Zn(OH)₂ and the generation of oxygen vacancies were confirmed by photoluminescence (PL) and XPS analyses. The ZnO NRAs treated by this simple method yield a photocurrent density of 600 μA/cm² at 1.23 V_RHE under 1 sun illumination, which is 20 times higher than that obtained from as-grown ZnO NRAs. This study presents a highly efficient way of increasing the bulk electric conductivity and photoelectrochemical activity of metal oxide nanorods without requiring the introduction of any extrinsic dopants.

Intrinsic Defects and H Doping in WO3

WO₃ is widely used as industrial catalyst. Intrinsic and/or extrinsic defects can tune the electronic properties and extend applications to gas sensors and optoelectonics. However, H doping is a challenge to WO₃, the relevant mechanisms being hardly understood. In this context, we investigate intrinsic defects and H doping by density functional theory and experiments. Formation energies are calculated to determine the lowest energy defect states. O vacancies turn out to be stable in O-poor environment, in agreement with X-ray photoelectron spectroscopy, and O-H bond formation of H interstitial defects is predicted and confirmed by Fourier transform infrared spectroscopy.

A nitrogen-rich, azaindole-based microporous organic network: synergistic effect of local dipole–π and dipole–quadrupole interactions on carbon dioxide uptake

A new type of microporous organic polymer with azaindole units (N-PEINK) has been designed.