Nanosized metal (Pt or Pd)-decorated TiO2 nanofibers (NFs) were synthesized by a wet impregnation method. CdSe quantum dots (QDs) were then anchored onto the metal-decorated TiO2 NFs. The photocatalytic performance of these catalysts was tested for activation and reduction of CO2 under UV-B light. Gas chromatographic analysis indicated the formation of methanol, formic acid, and methyl formate as the primary products. In the absence of CdSe QDs, Pd-decorated TiO2 NFs were found to exhibit enhanced performance compared to Pt-decorated TiO2 NFs for methanol production. However, in the presence of CdSe, Pt-decorated TiO2 NFs exhibited higher selectivity for methanol, typically producing ～90 ppmg?1·h?1 methanol. The CO2 photoreduction mechanism is proposed to take place via a hydrogenation pathway from first principles calculations, which complement the experimental observations.
Although In2O3 nanofibers (NFs) are well-known candidates as active materials for next-generation, low-cost electronics, these NF based devices still suffer from high leakage current, insufficient on–off current ratios (Ion/Ioff), and large, negative threshold voltages (VTH), leading to poor device performance, parasitic energy consumption, and rather complicated circuit design. Here, instead of the conventional surface modification of In2O3 NFs, we present a one-step electrospinning process (i.e., without hot-press) to obtain controllable Mg-doped In2O3 NF networks to achieve high-performance enhancement-mode thin-film transistors (TFTs). By simply adjusting the Mg doping concentration, the device performance can be manipulated precisely. For the optimal doping concentration of 2 mol%, the devices exhibit a small VTH (3.2 V), high saturation current (1.1 × 10–4 A), large on/off current ratio (>108), and respectable peak carrier mobility (2.04 cm2/(V·s)), corresponding to one of the best device performances among all 1D metal-oxide NFs based devices reported so far. When high-κ HfOx thin films are employed as the gate dielectric, their electron mobility and VTH can be further improved to 5.30 cm2/(V·s) and 0.9 V, respectively, which demonstrates the promising prospect of these Mg-doped In2O3 NF networks for highperformance, large-scale, and low-power electronics.
Two-dimensional ZrS2 materials have potential for applications in nanoelectronics because of their theoretically predicted high mobility and sheet current density. Herein, we report the thickness and temperature dependent transport properties of ZrS2 multilayers that were directly deposited on hexagonal boron nitride (h-BN) by chemical vapor deposition. Hysteresis-free gate sweeping, metalinsulator transition, and T–γ (γ ~ 0.82–1.26) temperature dependent mobility were observed in the ZrS2 films.
To promote commercialization of perovskite solar cells (PSCs), low-temperature processed electron transport layer (ETL) with high carrier mobility still needs to be further developed. Here, we reported two-dimensional (2D) tin disulfide (SnS2) nanosheets as ETL in PSCs for the first time. The morphologies of the 2D SnS2 material can be easy controlled by the in situ synthesized method on the conductive fluorine-doped tin oxide (FTO) substrate. We achieved a champion power conversion efficiency (PCE) of 13.63%, with the short-circuit current density (JSC) of 23.70 mA/cm2, open-circuit voltage (VOC) of 0.95 V, and fill factor (FF) of 0.61. The high JSC of PSCs results from effective electron collection of the 2D SnS2 nanosheets from perovskite layer and fast electron transport to the FTO. The low VOC and FF are the results of the lower conduction band of 2D SnS2 (4.23 eV) than that of TiO2 (4.0 eV). These results demonstrate that 2D material is a promising candidate for ETL in PSCs.
Regulating the selectivity of CO2 photoreduction is particularly challenging. Herein, we propose ideal models of atomic layers with/without element doping to investigate the effect of doping engineering to tune the selectivity of CO2 photoreduction. Prototypical ZnCo2O4 atomic layers with/without Ni-doping were first synthesized. Density functional theory calculations reveal that introducing Ni atoms creates several new energy levels and increases the density-of-states at the conduction band minimum. Synchrotron radiation photoemission spectroscopy demonstrates that the band structures are suitable for CO2 photoreduction, while the surface photovoltage spectra demonstrate that Ni doping increases the carrier separation efficiency. In situ diffuse reflectance Fourier transform infrared spectra disclose that the CO2·? radical is the main intermediate, while temperature-programed desorption curves reveal that the ZnCo2O4 atomic layers with/without Ni doping favor the respective CO and CH4 desorption. The Ni-doped ZnCo2O4 atomic layers exhibit a 3.5-time higher CO selectivity than the ZnCo2O4 atomic layers. This work establishes a clear correlation between elemental doping and selectivity regulation for CO2 photoreduction, opening new possibilities for tailoring solar-driven photocatalytic behaviors.
As a newly discovered member of the tungstate family, InWO4 hollow nanospheres with a monoclinic wolframite structure were synthesized successfully. The crystal phase of InWO4 was investigated via a combination of CASTEP geometric optimization and experimental simulation. InWO4 has a space group of P2/c with two InWO4 formula units per unit cell. The optimized cell dimensions are a = 5.16 Å, b = 5.97 Å, and c = 5.23 Å, with α = 90°, β = 92.11°, γ = 90°, giving a unit cell volume of 161.10 Å3, which is consistent with the experimental measurements. More importantly, InWO4 was a promising host material for different Ln3+ (Ln = Eu and Yb/Er) ions. For InWO4:Yb3+/Er3+ excited at 980 nm, transitions from the 4G11/2 (384 nm), 2H9/2 (411 nm), and 4F7/2 (487 nm) levels to the ground state (4I15/2) of Er3+ were observed. In addition to the aforementioned properties, the InWO4 hollow nanospheres can be used to improve the performance of dye-sensitized solar cells, which is chiefly attributed to theirlight scattering.
An inverted planar heterojunction perovskite solar cell (PSC) is one of the most competitive photovoltaic devices exhibiting a high power conversion efficiency (PCE) and nearly free hysteresis in the voltage–current output. However, the band alignment between the transport materials and the perovskite absorber has not been optimized, resulting in a lower open-circuit voltage (Voc) than that of regular PSCs. To address this issue, we tune the band alignment in perovskite photovoltaic architecture by introducing bilayer structured transport materials, e.g., the hole transport material poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/V2O5. In this study, solution processed inorganic V2Ox interlayer is incorporated into PEDOT:PSS for achieving improved film surface properties as well as optical and electrical properties. For example, the work function (WF) was changed from 5.1 to 5.4 eV. A remarkably high PCE of 17.5% with nearly free hysteresis and a stabilized efficiency of 17.1% have been achieved. Electronic impedance spectra (EIS) demonstrate a significant increase in the recombination resistance after introducing the interlayer, associated with the high Voc output value of 1.05 V. Transient photocurrent and photovoltage measurements indicate that a comparable charge transport process and an inhibited recombination process occur in the PSC with the introduction of the V2Ox interlayer.
The size and density of Ag nanoparticles on n-layer MoS2 exhibit thicknessdependent behavior. The size and density of these particles increased and decreased, respectively, with increasing layer number (n) of n-layer MoS2. Furthermore, the surface-enhanced Raman scattering (SERS) of Ag on this substrate was observed. The enhancement factor of this scattering varied with the thickness of MoS2. The mechanisms governing the aforementioned thickness dependences are proposed and discussed.
Defect equilibrium diagrams have been constructed for intrinsic and extrinsic defects in GaN:Mg crystals using the quasi-chemical formalism, and the formation of (VN-MgGa)× and (VN-MgGa??) defect complexes has been analyzed. The results indicate that, under equilibrium conditions, VN vacancies in the charge state 3+ and Mg′Ga impurities form (VN-MgGa)?? defect complexes, which compensate acceptors. 相似文献
The oxygen reduction reaction (ORR) is essential in research pertaining to life science and energy. In applications, platinum-based catalysts give ideal reactivity, but, in practice, are often subject to high costs and poor stability. Some cost-efficient transition metal oxides have exhibited excellent ORR reactivity, but the stability and durability of such alternative catalyst materials pose serious challenges. Here, we present a facile method to fabricate uniform CoxOy nanoparticles and embed them into N-doped carbon, which results in a composite of extraordinary stability and durability, while maintaining its high reactivity. The half-wave potential shows a negative shift of only 21 mV after 10,000 cycles, only one third of that observed for Pt/C (63 mV). Furthermore, after 100,000 s testing at a constant potential, the current decreases by only 17%, significantly less than for Pt/C (35%). The exceptional stability and durability results from the system architecture, which comprises a thin carbon shell that prevents agglomeration of the CoxOy nanoparticles and their detaching from the substrate.
This is the first report on the thermoelectric properties of a catalyst-free polyol method used to prepare stoichiometric Bi2?xCuxS3 samples, x = 0.1, 0.2, 0.3, 0.4, via hot pressing. Various systematic approaches to arrive at in their stoichiometric compositions are explored precisely with introduction of excess precursor of S. X-ray diffraction data analysis using Rietveld refinement confirms a polyhedral orthorhombic crystal structure with a space group Pnma, in contrast to Pbnm reported earlier. Raman data further substantiates this. X-ray photoelectron spectroscopy confirms the valence states of the constituent elements (Bi3+, Cu2+, and S2?) and energy dispersive X-ray analysis corroborates their compositions. The particle sizes of the pure Bi2S3 nanoparticles were 20, 35, and 82 nm as determined from the Scherrer formula, atomic force microscopy, and dynamic light scattering, respectively. Their transmission electron microscopy image shows rod-like nanostructures elongated in the 〈010〉 direction with an average diameter of 23 nm and a length of several hundreds of nanometers. A 34% improvement in the thermoelectric figure of merit is observed for Bi1.6Cu0.4S3 as compared to pure Bi2S3 at 300 K.
We report the synthesis and electrochemical sodium storage of cobalt disulfide (CoS2) with various micro/nano-structures. CoS2 with microscale sizes are either assembled by nanoparticles (P-CoS2) via a facile solvothermal route or nanooctahedrons constructed solid (O-CoS2) and hollow microstructures (H-CoS2) fabricated by hydrothermal methods. Among three morphologies, H-CoS2 exhibits the largest discharge capacities and best rate performance as anode of sodium-ion batteries (SIBs). Furthermore, H-CoS2 delivers a capacity of 690 mA·h·g?1 at 1 A·g?1 after 100 cycles in a potential range of 0.1–3.0 V, and ～240 mA·h·g?1 over 800 cycles in the potential window of 1.0–3.0 V. This cycling difference mainly lies in the two discharge plateaus observed in 0.1–3.0 V and one discharge plateau in 1.0–3.0 V. To interpret the reactions, X-ray diffraction (XRD) and transmission electron microscopy (TEM) are applied. The results show that at the first plateau around 1.4 V, the insertion reaction (CoS2 + xNa+ + xe? → NaxCoS2) occurs; while at the second plateau around 0.6 V, the conversion reaction (NaxCoS2 + (4 ? x) Na+ + (4 ? x)e? → Co + 2Na2S) takes place. This provides insights for electrochemical sodium storage of CoS2 as the anode of SIBs.
The catalytic performance of metal nanoparticles is often affected by surface oxidation levels. Instead of post-synthesis oxidation/reduction, we propose an efficient method to modulate the oxidation levels by tuning the composition of bimetallic nanoparticles. Here we report a series of Pt–Re bimetallic nanoparticles synthesized via a facile thermal co-reduction process, with a uniform size of approximately 3 nm. The investigation of the growth of the Pt–Re nanoparticles suggests that the Re atoms were enriched on the surface, as confirmed by X-ray photoelectron spectroscopy. Furthermore, X-ray absorption spectroscopy showed that metallic Re was decreased and high-valency ReOx species were increased in particles with higher Re/Pt ratios. In the etherification of allylic alcohols catalyzed by Pt–Re nanoparticles of different compositions under ambient conditions, particles with higher Re/Pt ratios exhibited significantly better performances. The highest mass activity of Pt–Re bimetallic nanoparticles (127 μmol·g?1·s?1) was more than forty times that of the industrial catalyst CH3ReO3 (3 μmol·g?1·s?1). The catalytically active sites were associated with ReOx and could be tuned by adjusting the Pt ratio.
A facile hydrothermal synthetic method, followed by in situ reduction and galvanic replacement processes, is used to prepare PtCo-modified Co3O4 nanosheets (PtCo/Co3O4 NSs) supported on Ni foam. The prepared nanomaterial is used as an electrocatalyst for NaBH4 oxidation in alkaline solution. The morphology and phase composition of PtCo/Co3O4 NSs are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of PtCo/Co3O4 NSs is investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in a standard three-electrode system. Current densities of 70 and 850 mA·cm–2 were obtained at–0.4 V for Co/Co3O4 and PtCo/Co3O4 NSs, respectively, in a solution containing 2 mol·L–1 NaOH and 0.2 mol·L–1 NaBH4. The use of a noble metal (Pt) greatly enhances the catalytic activity of the transition metal (Co) and Co3O4. Besides, both Co and Co3O4 exhibit good B–H bond breaking ability (in NaBH4), which leads to better electrocatalytic activity and stability of PtCo/Co3O4 NSs in NaBH4 electrooxidation compared to pure Pt. The results demonstrate that the as-prepared PtCo/Co3O4 NSs can be a promising electrocatalyst for borohydride oxidation.
Hollow nanostructures have attracted considerable attention owing to their large surface area, tunable cavity, and low density. In this study, a unique flower-like C@SnOX@C hollow nanostructure (denoted as C@SnOX@C-1) was synthesized through a novel one-pot approach. The C@SnOX@C-1 had a hollow carbon core and interlaced petals on the shell. Each petal was a SnO2 nanosheet coated with an ultrathin carbon layer ~2 nm thick. The generation of the hollow carbon core, the growth of the SnO2 nanosheets, and the coating of the carbon layers were simultaneously completed via a hydrothermal process using resorcinol-formaldehyde resin-coated SiO2 nanospheres, tin chloride, urea, and glucose as precursors. The resultant architecture with a large surface area exhibited excellent lithium-storage performance, delivering a high reversible capacity of 756.9 mA·h·g–1 at a current density of 100 mA·g–1 after 100 cycles.
The fabrication of heterostructures of two-dimensional semiconductors with specific bandgaps is an important approach to realizing the full potential of these materials in electronic and optoelectronic devices. Several groups have recently reported the direct growth of lateral and vertical heterostructures based on monolayers of typical semiconducting transition metal dichalcogenides (TMDCs) such as WSe2, MoSe2, WS2, and MoS2. Here, we demonstrate the single-step direct growth of lateral and vertical heterostructures based on bandgap-tunable Mo1-xWxS2 alloy monolayers by the sulfurization of patterned thin films of WO3 and MoO3. These patterned films are capable of generating a wide variety of concentration gradients by the diffusion of transition metals during the crystal growth phase. Under high temperatures, this leads to the formation of monolayer crystals of Mo1-xWxS2 alloys with various compositions and bandgaps, depending on the positions of the crystals on the substrates. Heterostructures of these alloys are obtained through stepwise changes in the ratio of W/Mo within a single domain during low-temperature growth. The stabilization of the monolayer Mo1-xWxS2 alloys, which often degrade even under gentle conditions, was accomplished by coating the alloys with other monolayers. The present findings demonstrate an efficient means of both studying and optimizing the optical and electrical properties of TMDC-based heterostructures to allow use of the materials in future device applications.
Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS2@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS2 nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm2) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS2 and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.
T-Nb2O5/reduced graphene oxide nanohybrids were fabricated via the hydrothermal attachment of Nb2O5 nanowires to dispersed graphene oxide nanosheets followed by a high-temperature phase transformation. Electrochemical measurements showed that the nanohybrid anodes possessed enhanced reversible capacity and superior cycling stability compared to those of a pristine T-Nb2O5 nanowire electrode. Owing to the strong bonds between graphene nanosheets and T-Nb2O5 nanowires, the nanohybrids achieved an initial capacity of 227 mAh·g?1. Additionally, non-aqueous asymmetric supercapacitors (ASCs) were fabricated with the synthesized nanohybrids as the anode and activated carbon as the cathode. The 3 V Li-ion ASC with a LiPF6-based organic electrolyte achieved an energy density of 45.1 Wh·kg?1 at 715.2 W·kg?1. The working potential could be further enhanced to 4 V when a polymer ionogel separator (PVDF-HFP/LiTFSI/EMIMBF4) and formulated ionic liquid electrolyte were employed. Such a quasi-solid state ASC could operate at 60 °C and delivered a maximum energy density of 70 Wh·kg?1 at 1 kW·kg?1.
Superexchange effects play an important role in the determination of crystal structures; however, there has been much less reported on how they determine the stability of clusters. Using evolutionary search strategies and DFT+U (density functional theory with the Hubbard U correction) calculations, we investigate the global minimum-energy structures of Fe12On clusters. Among predicted Fe12On clusters, a cage-shaped Fe12O12 cluster with unexpected stability was observed. In addition, the bare Fe12O12 cluster is shown to possess an extremely large energy gap (2.00 eV), which is greater than that of C60, Au20 and Al13?clusters. Using a Heisenberg model, we traced the origin of the unexpected stability of the bare Fe12O12 cluster to magnetic competition between the nearest-neighbor exchange constant J1 and the next-nearest neighbor exchange constant J2 that was induced by the superexchange interactions. The bare Fe12O12 cluster is thus a unique molecule that is stable and chemically inert.
One-dimensional hollow nanostructures have potential applications in many fields and can be fabricated using various methods. Herein, a selective-oxidation route for the synthesis of unique TexSey nanotubes (STNTs) with a controlled morphology using TexSey@Se core–shell nanowires (TSSNWs) as a template is reported. Because of the lower redox potential of TeO2/Te compared to that of H2SeO3/Se, the Te in TSSNWs can be preferentially oxidized by an appropriate oxidant of HNO2 to form STNTs. The inner diameters and wall thicknesses of the STNTs can be tuned by modulating the core diameters and shell thicknesses of the TSSNWs, respectively. The STNTs can be assembled into a monolayer composed of well-arranged nanotubes using the Langmuir–Blodgett technique. A device based on films stacked with 10 STNT monolayers was fabricated to investigate the photocoductivity of the STNTs. The STNTs exhibited a good photoresponse over the whole ultraviolet–visible spectrum, revealing their potential for application in optoelectronic devices.
