ABSTRACT
Two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs7Pb6I19 nanosheets (NSs) were successfully developed for the first time by employing a structural recrystallization process with additional passivation of small organic sulfide molecules. The structure of Cs7Pb6I19 NSs is confirmed by powder X-ray diffraction measurements, atomically-resolved STEM measurements and atomic force microscopy (AFM) studies. Cs7Pb6I19 NSs with a specific n value of 6 exhibits unique absorption and emission spectra with intense excitons at 560 nm due to quantum confinement effects in 2D perovskite slabs. The formation mechanisms of 2D Cs7Pb6I19 NSs and 3D γ-CsPbI3 phases were investigated by in situ photoluminescence (PL) spectroscopy and the activation energies of their formation reactions were calculated to be 151 kJ mol-1 and 95.3 kJ mol-1, respectively. The phase stability of 2D Cs7Pb6I19 NSs can be maintained at temperatures below 14 °C for more than 4 weeks. The overall results indicate that 2D Cs7Pb6I19 NSs demonstrate unique optical properties and structural stability compared with other 3D perovskite materials. We have opened a new path to the future discovery of 2D perovskite structures with metastable phases by using this recrystallization method and the assistance of sulfur-derived organic molecules.
ABSTRACT
In contrast to the 2D organic-inorganic hybrid Ruddlesden-Popper halide perovskites (RPP), a new class of 2D all inorganic RPP (IRPP) has been recently proposed by substituting the organic spacers with an optimal inorganic alternative of cesium cations (Cs+ ). Nevertheless, the synthesis of high-membered 2D IRPPs (n > 1) has been a very challenging task because the Cs+ need to act as both spacers and A-site cations simultaneously. This work presents the successful synthesis of stable phase-pure high-membered 2D IRPPs of Csn+1 Pbn Br3n+1 nanosheets (NSs) with n = 3 and 4 by employing the strategy of using additional strong binding bidentate ligands. The structures of the 2D IRPPs (n = 3 and 4) NSs are confirmed by powder X-ray diffraction and high-resolution aberration-corrected scanning transmission electron microscope measurements. These 2D IRPPs NSs exhibit a strong quantum confinement effect with tunable absorption and emission in the visible light range by varying their n values, attributed to their inherent 2D quantum-well structure. The superior structural and optical stability of the phase-pure high-membered 2D IRPPs make them a promising candidate as photocatalysts in CO2 reduction reactions with outstanding photocatalytic performance and long-term stability.
ABSTRACT
Nitrate (NO3-) reduction reaction (NtRR) is considered as a green alternative method for the conventional method of NH3 synthesis (Haber-Bosch process), which is known as a high energy consuming and large CO2 emitting process. Herein, the copper nanodendrites (Cu NDs) grown along with the {200} facet as an efficient NtRR catalyst have been successfully fabricated and investigated. It exhibited high Faradaic efficiency of 97% at low potential (-0.3 V vs RHE). Furthermore, the 15NO3- isotope labeling method was utilized to confirm the formation of NH3. Both experimental and theoretical studies showed that NtRR on the Cu metal nanostructure is a facet dependent process. Dissociation of NO bonding is supposed to be the rate-determining step as NtRR is a spontaneously reductive and protonation process for all the different facets of Cu. Density functional theory (DFT) calculations revealed that Cu{200} and Cu{220} offer lower activation energy for dissociation of NO compared to that of Cu{111}.
ABSTRACT
Organometallic two-dimensional (2D) nanosheets with tailorable components have recently fascinated the optoelectronic communities due to their solution-processable nature. However, the poor stability of organic molecules may hinder their practical application in photovoltaic devices. Instead of conventional organometallic 2D nanosheets with low weatherability, an air-stable π-conjugated 2D bis(dithiolene)iron(II) (FeBHT) coordination nanosheet (CONASH) is synthesized via bottom-up liquid/liquid interfacial polymerization using benzenehexathiol (BHT) and iron(II) ammonium sulfate [Fe(NH4)2(SO4)2] as precursors. The uncoordinated thiol groups in FeBHT are easily oxidized, but the Fe(NH4)2(SO4)2 dissociation rate is slow, which facilitates the protection of sulfur groups by iron(II) ions. The density functional theory calculates that the resultant FeBHT network gains the oxygen-repelling function for oxidation suppression. In air, the FeBHT CONASH exhibits self-powered photoresponses with short response times (<40 ms) and a spectral responsivity of 6.57 mA W-1, a specific detectivity of 3.13 × 1011 Jones and an external quantum efficiency of 2.23% under 365 nm illumination. Interestingly, the FeBHT self-powered photodetector reveals extremely high long-term air stability, maintaining over 94% of its initial photocurrent after aging for 60 days without encapsulation. These results open the prospect of using organometallic 2D materials in commercialized optoelectronic fields.
ABSTRACT
Platinum diselenide (PtSe2) is a group-10 two-dimensional (2D) transition metal dichalcogenide that exhibits the most prominent atomic-layer-dependent electronic behavior of "semiconductor-to-semimetal" transition when going from monolayer to bulk form. This work demonstrates an efficient photoelectrochemical (PEC) conversion for direct solar-to-hydrogen (H2) production based on 2D layered PtSe2/Si heterojunction photocathodes. By systematically controlling the number of atomic layers of wafer-scale 2D PtSe2 films through chemical vapor deposition (CVD), the interfacial band alignments at the 2D layered PtSe2/Si heterojunctions can be appropriately engineered. The 2D PtSe2/p-Si heterojunction photocathode consisting of a PtSe2 thin film with a thickness of 2.2 nm (or 3 atomic layers) exhibits the optimized band alignment and delivers the best PEC performance for hydrogen production with a photocurrent density of -32.4 mA cm-2 at 0 V and an onset potential of 1 mA cm-2 at 0.29 V versus a reversible hydrogen electrode (RHE) after post-treatment. The wafer-scale atomic-layer controlled band engineering of 2D PtSe2 thin-film catalysts integrated with the Si light absorber provides an effective way in the renewable energy application for direct solar-to-hydrogen production.
ABSTRACT
Textured growth of ZnO nanorods with no restriction of the substrate material is beneficial to their applications. The approaches to grow ZnO nanorods with texture are based on preparing suitable surface structure on the growth substrate, e.g. using a crystalline substrate with a specific surface structures or pre-depositing seed layers by high-temperature annealing of precursors. In the aqueous nutrient solution of the chemical bath deposition (CBD) process for ZnO growth, the concentration of Zn2+ ions at the extended hydrophobic surface is sufficiently high for forming self-assembly nuclei with a preferred orientation, resulting in the subsequent textured growth of ZnO nanorods. In this research, the hydrophobic surface is prepared by modifying Si surface with a self-assembly octadecyltrimethoxysilane (OTMS) monolayer. The formation mechanism of the nuclei on this hydrophobic surface for the textured growth of ZnO nanorods is investigated. It is shown that the nuclei form at the beginning of the CBD process and later transform into the Wurtzite structure to seed ZnO growth. An alternative approach to prepare seed layers is therefore involved in the aqueous CBD process, which is applicable to a range of hydrophobic substrates for textured growth of ZnO nanorods.
ABSTRACT
Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS2 films. An intermediate liquid phase-Na2Mo2O7 is formed through a eutectic reaction of MoO3 and NaF, followed by being sulfurized into MoS2. The as-formed MoS2 seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 µm) flakes or full-coverage films (with a record-high average grain size of 450 µm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm2 V-1 s-1 for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 108) across a 1.5 cm × 1.5 cm region.
ABSTRACT
Silicon is expected to be a useful anode material in lithium ion batteries for future energy storage applications, because of its high theoretical charge storage density of Li+ ions. However, volume expansion due to lithiation fractures the Si anode material, leading to poor cycle stability of battery operation. The approaches to overcome the problem include using Si nanowires to relieve the stress induced by volume expansion and coating a protective layer on the Si anode to prevent delamination. In this study, we use in-situ scanning electron microscopy to monitor the morphological changes of 90 nm thick pristine Si nanowires and the Si nanowires coated with amorphous TiO2, respectively, during electrochemical lithiation. The results of in-situ observation show that both kinds of Si nanowires exhibit a larger thickness after 10 h lithiation and suffer fracture after 25 h. It is also found that the TiO2 layer is not strong enough to prevent Si nanowires from fracture. Since the TiO2 layer can not be elastically deformed, this surface shell fractures earlier in the lithiation process than pristine Si nanowires. Transformation of the crystalline Si nanowires to an amorphous phase and lithium composition detected in the nanowires support that the observed fracture indeed results from lithiation.
ABSTRACT
Perovskite α-CsPbI3 nanocrystals (NCs) with a high fluorescence quantum yield (QY) typically undergo a rapid phase transformation to a low-QY δ-CsPbI3 phase, thus limiting their optoelectronic applications. In this study, organic molecule hexamethyldisilathiane (HMS) is used as a unique surfactant to greatly enhance the stability of the cubic phase of CsPbI3 NCs (HMS-CsPbI3) under ambient conditions. The reaction kinetics of the phase transformation of CsPbI3 NCs are systemically investigated through in situ photoluminescence (PL), X-ray diffraction, and transmission electron microscope (TEM) measurements under moisture. The activation energy of HMS-CsPbI3 NCs is found to be 14 times larger than that of CsPbI3 NCs capped by olyelamine (OLA-CsPbI3 NCs). According to density functional theory calculations, the bonding between HMS and CsPbI3 NCs is stronger than that between OLA and CsPbI3 NCs, preventing the subsequent phase transformation. Our study presents a clear pathway for achieving highly stable CsPbI3 NCs for future applications.
ABSTRACT
Metal@semiconductor core-shell nanoparticles (NPs) are widely used in photocatalysts, sensors, and optical applications owing to their unique metal-semiconductor interface and the integration of the properties from both core and shell materials. Although many efforts have been made toward the precise synthesis of Au@Cu2O core-shell structures, the chemical stability of Au@Cu2O aqueous suspensions, which is of great significance in many related applications, is not mentioned in any published research. Herein we report the synthesis of Au@Cu2O core-shell NPs with small shell thickness from 2 to 40 nm through a wet-chemistry method. The UV-vis absorption properties are found to be tunable with Cu2O thickness in the range of 2-40 nm. Furthermore, the chemical stability of Au@Cu2O core-shell nanoparticle suspensions in water/ethanol mixed solvents is investigated. It is found that water/ethanol mixed solvents with a larger amount of water are more likely to deteriorate the stability of Au@Cu2O NPs by oxidizing Cu2O to CuO. The results from this work may provide useful information for the preparation of metal@Cu2O water-based suspensions that are expected to be used for SERS, photocatalyst, or photothermal applications.
ABSTRACT
Precise design of hollow nanostructures can be realized via various approaches developed in the last two decades, endowing nanomaterials with unique structures and outstanding performances, showing their usefulness in a broad range of fields. Herein, we demonstrate the formation of SnO2@SiO2 hollow nanostructures, for the first time, by interaction between liquid state Sn cores and SiO2 shell structures inside Sn@SiO2 core-shell nanoparticles with real-time observation via in situ transmission electron microscopy (TEM). Based on the in situ results, designed transformation of the nanoparticle structure from core-shell Sn@SiO2 to yolk-shell Sn@SiO2 and hollow SnO2@SiO2 is demonstrated, showing the controllable structure of core-shell Sn@SiO2 nanoparticles via fixing liquid-state Sn inside a SiO2 shell which has a certain Sn containing capacity. The present approach expands the toolbox for the design and preparation of yolk-shell and hollow nanostructures, thus providing us with a new strategy for fabrication of more complicated nanostructures.
ABSTRACT
We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN â¼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 â¼ 7 and Tsr2 â¼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near â¼2.1 T and a metamagnetic state near â¼5.9 T having â¼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ â¼ 7.6(4) mJ/mol K2 and θD â¼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 µB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.
ABSTRACT
Understanding the growth mechanism of heterojunctions in silicon-germanium alloy (Si-Ge) nanowires is helpful for designing adequate physical properties in the material for device applications. We examine the formation of the heterojunction in low Ge-content Si-Ge nanowires by an approach of thermal oxidation, which produces an atomically abrupt interface with an obvious concentration change. Forming heterojunctions in Si-Ge nanowires by this approach involves more complicated reaction routes than direct growth of heterojunction nanowires using the vapor-liquid-solid method. At the beginning of the oxidation process, the AuGeSi eutectic liquid at the nanowire tip significantly etches the Si-Ge alloy nanowires. Selective oxidation of Si results in a change of the relative amount of Ge to Si in the eutectic liquid, which further modulates the solubility of Ge and Si atoms. The compositional variation in the Au-Ge-Si ternary alloy system during the oxidation process accounts for the observed concentration profile in the heterojunction nanowire. The thermal oxidation approach is applied on a low Ge-content Si-Ge thin film that is coated with Au nanoparticles. Si-Ge nanodots, which exhibit a higher Ge concentration, are precipitated epitaxially in the film, as a result of compositional modulation in the AuGeSi eutectic liquid.
ABSTRACT
Using the focused ion beam (FIB) to prepare plan-view transmission electron microscopy (TEM) specimens is beneficial for obtaining structural information of two-dimensional atomic layer materials, such as graphene and molybdenum disulfide (MoS2) nanosheets supported on substrates. The scanning electron microscopy (SEM) image in a dual-beam FIB-SEM can accurately locate an area of interest for specimen preparation. Besides, FIB specimen preparation avoids damages and hydrocarbon contamination that are usually produced in other preparation methods, in which chemical etching and polymer adhesion layers are used. In order to reduce harmful ion-beam bombardment and re-deposition on the thin atomic layers during FIB specimen preparation, we develop a method to protect the atomic layers by making a "microcapsule" to insulate the sample surface. The method is applied respectively to prepare plan-view TEM specimens of a graphene sheet with multiple adlayers and MoS2 atomic layers. Useful electron diffraction results can be obtained from these specimens for understanding the interlayer orientation relationships in the two materials. Auger electron spectroscopy analysis further confirms that the sample surface is free from contamination under the sufficient protection given by the proposed method.
ABSTRACT
Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor-liquid-solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%) can be altered by thermal oxidation at 700 °C. During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (â¼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.
ABSTRACT
Recently, interest in aluminium ion batteries with aluminium anodes, graphite cathodes and ionic liquid electrolytes has increased; however, much remains to be done to increase the cathode capacity and to understand details of the anion-graphite intercalation mechanism. Here, an aluminium ion battery cell made using pristine natural graphite flakes achieves a specific capacity of â¼110 mAh g-1 with Coulombic efficiency â¼98%, at a current density of 99 mA g-1 (0.9 C) with clear discharge voltage plateaus (2.25-2.0 V and 1.9-1.5 V). The cell has a capacity of 60 mAh g-1 at 6 C, over 6,000 cycles with Coulombic efficiency â¼ 99%. Raman spectroscopy shows two different intercalation processes involving chloroaluminate anions at the two discharging plateaus, while C-Cl bonding on the surface, or edges of natural graphite, is found using X-ray absorption spectroscopy. Finally, theoretical calculations are employed to investigate the intercalation behaviour of choloraluminate anions in the graphite electrode.
ABSTRACT
Although the success of graphene research has opened up a new route for wearable electronic and optoelectronic devices, producing graphene with controllable quality and cost-effective growth on a large scale remains challenging due to the lack of understanding about its growth kinetics. Domain boundaries interrupt lattice continuity of graphene; therefore, lowering the nucleation density at the initial stage of graphene growth in the chemical vapor deposition (CVD) process is beneficial for improving the quality of graphene for applications. Herein, we show that by forming an oxide passivation layer on Cu substrates before CVD graphene growth, graphene nucleation density can be effectively decreased. The nucleation mechanism in the presence of an oxide passivation layer is of interest. The analysis of graphene growth kinetics suggests that the thickness of the boundary layer for mass transfer on the substrate surface plays an important role in controlling the reduction rate of the oxide passivation layer. A thick boundary layer created under slow gas flow causes slow reduction of the oxide passivation layer, making finite sites for graphene nucleation. The domain density in a graphene layer is therefore significantly reduced. Graphene sheets of various domain densities (ranging from 104 to 1 mm-2) can be fabricated by suitably choosing the growth parameters. The graphene sheet with a lower density of domain boundaries exhibits better electrical conductivities.
ABSTRACT
Hydrogen is considered as sustainable and environmentally friendly energy for global energy demands in the future. Here a Co-FeS2 catalyst with surface phosphide doping (P/Co-FeS2 ) for hydrogen evolution reaction (HER) in acidic solutions is developed. The P/Co-FeS2 exhibits superior HER electrochemical performance with overpotential of -90 mV at 100 mA cm-2 and Tafel slope of 41 mV/decade and excellent durability.
ABSTRACT
Dye-sensitized solar cells (DSSCs) present low-cost alternatives to conventional wafer-based inorganic solar cells and have remarkable power conversion efficiency. To further enhance performance, we propose a new DSSC architecture with a novel dual-functional polymer interlayer that prevents charge recombination and facilitates ionic conduction, as well as maintaining dye loading and regeneration. Poly(vinylidene fluoride-trifluoroethylene) (p(VDF-TrFE)) was coated on the outside of a dye-sensitized TiO2 photoanode by a simple solution process that did not sacrifice the amount of adsorbed dye molecules in the DSSC device. Light-intensity-modulated photocurrent and photovoltage spectroscopy revealed that the proposed p(VDF-TrFE)-coated anode yielded longer electron lifetime and improved the injection of photogenerated electrons into TiO2, thereby reducing the electron transport time. Comparative cyclic voltammetry and UV-visible absorption spectroscopy based on a ferrocene-ferrocenium external standard material demonstrated that p(VDF-TrFE) enhanced the power conversion efficiency from 7.67% to 9.11%. This dual functional p(VDF-TrFE) interlayer is a promising candidate for improving the performance of DSSCs and can also be employed in other electrochemical devices.
ABSTRACT
The combination of nonvolatile memory switching and volatile threshold switching functions of transition metal oxides in crossbar memory arrays is of great potential for replacing charge-based flash memory in very-large-scale integration. Here, we show that the resistive switching material structure, (amorphous TiOx)/(Ag nanoparticles)/(polycrystalline TiOx), fabricated on the textured-FTO substrate with ITO as the top electrode exhibits both the memory switching and threshold switching functions. When the device is used for resistive switching, it is forming-free for resistive memory applications with low operation voltage (<± 1 V) and self-compliance to current up to 50 µA. When it is used for threshold switching, the low threshold current is beneficial for improving the device selectivity. The variation of oxygen distribution measured by energy dispersive X-ray spectroscopy and scanning transmission electron microscopy indicates the formation or rupture of conducting filaments in the device at different resistance states. It is therefore suggested that the push and pull actions of oxygen ions in the amorphous TiOx and polycrystalline TiOx films during the voltage sweep account for the memory switching and threshold switching properties in the device.