Nanosheet Films Research Articles

Fabrication of multilayer films of N-doped titania nanosheets and hematite nanocubes via layer by layer assembly

Hybrid films of nitrogen-doped titania nanosheets and hematite nanocubes were fabricated successfully via layer-by-layer assembly method. The morphological and structural properties were studied by scanning electron microscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Scanning electron microscope (SEM) images confirmed that hematite nanocubes were deposited randomly between the N-doped titania nanosheets and disordered layer structure was formed. The consecutive buildup of the film (PEI/Ti 0.91O 2− x N x /PEI/Fe 2O 3) n was monitored by UV–vis absorption spectroscopy. The linear increase in absorbance as a function of the deposition number implies that the same amount of composites was absorbed in each deposition cycle.

Influence of pH condition on colloidal suspension of exfoliated graphene oxide by electrostatic repulsion

A facile chemical process is described to produce graphene oxide utilizing a zwitterions amino acid intermediate from graphite oxide sheets. 11-aminoundecanoic acid molecules were protonated to intercalate molecules into the graphite oxide sheets to achieve ion exchange, and the carboxyl groups were then ionized in a NaOH solution to exfoliate the graphite oxide sheets. In this way, the produced graphene oxide nanosheets were stably dispersed in water. The delaminated graphene nanosheets were confirmed by XRD, AFM, and TEM. XRD patterns indicated the d002-spacing of the graphite greatly increased from 0.380nm and 0.870nm. AFM and TEM images showed that the ordered graphite crystal structure of graphene nanosheets was effectively exfoliated by this method. The prepared graphene nanosheets films showed 87.1% transmittance and a sheet resistance of 2.1×103Ω/square.

Facile synthesis and photoelectrochemical performance of the bush-like ZnO nanosheets film

Bush-like ZnO nanosheets film is fabricated on conductive transparent oxide substrates using a facile hydrothermal method without any surfactant or further heat treatment. The morphology, structure and photoluminescence property of the products are characterized in detail. The ZnO nanosheets are composed of numerous grains with the 5 nm average diameter, and most of the sheets present the relative uniform arrangement. These nanosheets construct a porous network which is favorable for the photoelectrochemical properties and applications in dye-sensitized solar cells. The photoelectrochemical performances of the bush-like ZnO nanosheets film are investigated, and a high open-circuit voltage of 0.69 V is achieved.

Ultra-thin film electrodes of tetratitanate nanosheets

The lithium intercalation properties of ultra-thin films of tetratitanate nanosheets were examined to determine the intrinsic electrode performance and the influence of nanosheet layer lamination on the electrode properties. Ultra-thin films of tetratitanate nanosheets were prepared by layer-by-layer assembly directly onto a Pt current collector. The ultra-thin film of tetratitanate nanosheets prepared by single deposition operation exhibited a specific capacity of 0.13 μAh cm − 2 , which is a large value corresponding to the reduction and oxidation of approximately 87% of the Ti ions. A change in the rate controlling process from a charge transfer reaction to diffusion was observed by cyclic voltammetry at 50 mV s − 1 . The lithium ion diffusion coefficient was determined by analyzing the results computationally with a mathematical model, and it was found to be 3 × 10 − 10 cm 2 s − 1 . The nanosheets exhibited high reactivity in a thin film with an estimated thickness of 9.1 nm. Sufficient electronic conductivity of the electrode, whose thickness is below 10 nm, and sufficient lithium transportation through the electrode contribute to the high reactivity.

Hydrothermal Growth of Layered Titanate Nanosheet Arrays on Titanium Foil and Their Topotactic Transformation to Heterostructured TiO2 Photocatalysts

Layered titanate nanostructures have been recognized as an important intermediate in preparing TiO2 nanostructures due to their availability in various morphologies, among which ultrathin nanosheets have attracted particular attention due to their unique properties arising from their high surface area and confined thickness in nanoscale. Herein, we report a simple approach to fabricate heterostructured TiO2 nanosheet array film by (1) hydrothermal growth of sodium trititanate nanosheet array on Ti foil in alkaline solution, (2) ion exchange of Na+ in titanate naosheet with H+, and (3) topotactical transformation into TiO2 nanosheet via thermal annealing. XRD and Raman as well as HRTEM analyses proved the formation of a novel anatase/TiO2(B) heterostructured nanosheet array with coherent interface between the two phases, which exhibited superior photocatalytic efficiency under UV illumination as compared with the relevant commercial products P25 and anode oxidized TiO2 nanotube film of the same thickness but with greater mass of photocatalyst. The high photocatalytic activity of the TiO2 nanosheet array film can be ascribed to the synergistic effect of large surface area and high crystallinity as well as heterojunction between TiO2(B) and anatase phase.

Simultaneous increase in conductivity and Seebeck coefficient in a polyaniline/graphene nanosheets thermoelectric nanocomposite

Polyaniline/graphene nanosheets (PANI/GNs) thermoelectric (TE) bulk composite pellets and films have been prepared with PANI to GNs weight ratios ranging from 4:1 to 1:1. Their structure and morphology have been investigated by Raman spectroscopy, thermogravimetric analysis and field emission scanning electron microscopy. The TE properties of the pellets and films were measured at room temperature. As the weight ratio of PANI to GNs decreased from 4:1 to 1:1, both the electrical conductivity and the Seebeck coefficient of pellets and the films increased. This was attributed to a substantial increase in carrier mobility while carrier concentration was not significantly changed. As a result, the power factor of the pellets and films increased from 0.64 to 5.60 and 0.05 to 1.47μWm−1K−2, respectively. This is the first time the TE properties of PANI/GNs nanocomposites were reported.

A Precursor-Limited Nanoparticle Coalescence Pathway for Tuning the Thickness of Laterally-Uniform Colloidal Nanosheets: The Case of SnSe

The availability of high-quality colloidal nanosheets underpins a diverse range of applications and investigations into dimension-dependent physical properties. To facilitate this, synthetic methods that yield single-crystal colloidal nanosheets with regular shapes, uniform lateral dimensions, and tunable thicknesses are critically important. Most strategies that yield colloidal nanosheets achieve some, but not all, of these morphological characteristics. Here, we describe a synthetic pathway that generates colloidal nanosheets of SnSe with uniform lateral dimensions and tunable thicknesses. SnSe represents an excellent prototype system for studying the formation of colloidal nanosheets because of its layered crystal structure and the growing interest in its potential application as an absorption layer in low-cost photovoltaic devices. Freestanding colloidal SnSe nanosheets were synthesized by slowly heating a one-pot reaction mixture of SnCl(2), oleylamine, trioctylphosphine selenide (TOP-Se), and hexamethyldisilazane (HMDS) to 240 °C. The SnSe nanostructures adopt a uniform square-like morphology with lateral dimensions of approximately 500 nm × 500 nm, and the average nanosheet thicknesses can be tuned from approximately 10 to 40 nm by adjusting the concentrations of the SnCl(2) and TOP-Se reagents. Aliquot studies reveal fundamental insights into how the nanosheets form: they first "grow out" laterally via coalescence of individual nanoparticle building blocks to yield a single-crystal nanosheet template and then "grow up" vertically (through nanoparticle attachment to the nanosheet template) in a pseudo layer-by-layer fashion. Vertical growth is therefore limited, and can be controlled, by reagent concentration. Drop-cast films of the SnSe nanosheets are photoactive and have a bandgap of approximately 1 eV. These studies, demonstrated for SnSe but potentially applicable to other systems, establish a straightforward pathway for tuning the thicknesses of colloidal nanosheets while maintaining lateral uniformity.

MnO2 Nanosheets Thin-Film Electrodes for Electrochemical Capacitors

Electrochemical properties of MnO2 nanosheets thin-film electrode for electrochemical capacitors were examined. MnO2 nanosheets were prepared by exfoliation of birnessite-type MnO2. MnO2 nanosheets with small lateral sizes of 50-100 nm (Small-NS) and large lateral sizes of approximately 1 μm (Large-NS) were obtained by multistep centrifugation. Thin films of MnO2 nanosheets were prepared by electrophoretic deposition. Films of Large-NS had a homogeneous microstructure like a bulk. Small-NS had a heterogeneous microstructure containing micropore. The reversible capacities for the films with thickness of approximately 12 nm were approximately 100 mAh g-1(330 F g-1). Capacities of Large-NS films decreased with increasing amount of nanosheets. Capacities of Small-NS films were almost independent of the amount of nanosheets due to fast ion diffusion in the electrodes. Heterogeneous microstructure containing micropore contributes to the good electrode properties of Small-NS films.

Synthesis of ZnO films with a special texture and enhanced field emission properties

ZnO films with special textures are fabricated on Mo-coated Al2O3 ceramic substrates by the catalyst-free electron beam evaporation method, and the as-deposited films are treated by hydrogen plasma. It is found that the surface morphologies of the films are changed significantly after hydrogen plasma treatment and that the films consist of vertically standing and intersecting nanosheets. A lower turn-on field of 1.2 V/μm and an enhanced current density ∼0.11 mA/cm2 at 2.47 V/μm are achieved. The low threshold field and the high emission current density are attributed primarily to the unique shape and smaller resistivity of the ZnO nanosheet films.

SnO2 nanosheet as a photoanode interfacial layer for dye-sensitized solar cells

SnO2 nanosheet films about 200 nm in thickness are successfully fabricated on fluorine-doped tin oxide (FTO) glass by a facile solution-grown approach. The prepared SnO2 nanosheet film is applied as an interfacial layer between the nanocrystalline TiO2 film and the FTO substrate in dye-sensitized solar cells (DSCs). Experimental results show that the introduction of a SnO2 nanosheet film not only suppresses the electron back-transport reaction at the electrolyte/FTO interface but also provides an efficient electron transition channel along the SnO2 nanosheets, and as a result, increasing the open circuit voltage and short current density, and finally improving the conversion efficiency for the DSCs from 3.89% to 4.62%.

Performance of ZnO dye-sensitized solar cells with various nanostructures as anodes

Several types of ZnO nanostructured films, including nanoparticle films, ordered porous films, nanorod films, nanotube films and porous nanosheet films, were fabricated and used as photoanodes in dye-sensitized solar cells (DSSCs). Their performance was described and compared in detail. DSSCs based on ordered porous films presented relatively higher conversion efficiency (0.7%) than that of DSSCs based on nanoparticle films (0.2%). DSSCs based on nanorod films showed the maximal efficiency up to 1.3%, mainly attributed to the c-axis oriented structure which could provide the faster conduction pathway for charge transport. Although nanotube films with large surface area could absorb more dyes, the efficiency of DSSCs based on nanotube films was only 1.2% and wasn’t as high as that of nanorod DSSCs, which may be caused by the charge-carrier recombination losses at the defects formed in the etching. Moreover, the porous ZnO nanosheets photoanode showed an improved efficiency by 2.5 times as compared to ZnO nanosheets samples due to increase the dye loading and light harvesting.

Transfer of carbon nanosheet films to nongrowth, zero thermal budget substrates

Carbon-based nanostructures and materials have become a popular subject of research due to their unique thermal, mechanical, electrical, and optical properties. For example, the strong C–C bonds of graphene-based systems allow for excellent thermal conduction at room temperature and the conjugation of the sp2 lattice enables extremely high electron mobility. However, the use of carbon nanostructures as a component in polymer composites, sensors, mirco-electro-mechanical systems, and both rigid and flexible electronics has been limited by several factors, including the incompatibility with standard photolithography techniques, the high temperatures required for the nanostructure growth, and the presence of—or complication—of removing noncarbon species. Here, the authors report on a novel method for the transfer of carbon nanosheets to a low or zero thermal budget substrate while maintaining their original morphology and electrical properties. Four-point probe measurements’ post-transfer shows the retention of in-plane conductivity and scanning electron microscopy reveals the preservation of the original vertical morphology. Raman spectroscopy measurements confirm the retention of the graphitic structure of the post-transfer nanosheet film. This new transfer technique builds on the ability to conformally coat nanosheets while maintaining the original ultrahigh surface area morphology and the ability to fully incorporate nanosheets into several polymers while maintaining the original nanostructure separation. For a demonstration of the usefulness of polymer filling, carbon nanosheets were used as an ultrahigh surface area electrode for the photoactive polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] in proof of principle experiments of a nanosheet-based organic photovoltaic device.

Electro-conducting polymeric films prepared from the hybrids of carbon nanotubes and graphene nanosheets

Transparent and electro-conducting hybrid films were fabricated from unoxidized graphene nanosheets and single-walled carbon nanotubes (SWCNTs). A graphene dispersion was prepared in N-methylpyrrolidone (NMP) with ultrasonication. This ultrasonication method allows the preparation of unoxidized graphene nanosheets. Therefore, graphene nanosheets could be obtained with a low defect concentration and good electro-conductivity. In the hybrid films of graphene nanosheets and SWCNTs, the graphene nanosheets were interconnected by SWCNTs. A transparent hybrid film with relatively low sheet resistance could be prepared because this interconnected structure between the SWCNTs and graphene nanosheets can act as an electro-conducting pathway. As a result, the hybrid film showed improved electrical, transparent characteristics using unoxidized graphene, even though a small amount of SWCNTs were used. The hybrid films had 90% transmittance at 550 nm and a sheet resistance of 1.9 × 10 3 Ω/sq.

Photoinduced Electron Transfer between the Anionic Porphyrins and Viologens in Titania Nanosheets and Monodisperse Mesoporous Silica Hybrid Films

Photoinduced electron transfer between an anionic porphyrin derivative (tetrakis(p-carboxyphenyl)porphyrin; H(2)TCPP(4-)) and an electron accepting methyl viologen (MV(2+)) was investigated in two different nanoscale configurations, i.e., layered titania nanosheet (TNS) photocatalysts and ammonium-functionalized monodisperse mesoporous silica (AMMSS) particles. Cationic MV(2+) intercalated within the TNS interlayers while anionic H(2)TCPP(4-) was accommodated within AMMSS nanocavities to form (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films. Upon irradiation with UV light and excitation of the TNS in the (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films, the consumption of H(2)TCPP(4-) and the formation of a one-electron reduced MV(2+) (MV(+·)) were simultaneously observed. No consumption of H(2)TCPP(4-) was observed when an electrically insulating poly(styrene) (PS) was also introduced at the interface. These results suggest that photoinduced electron transfer occurred at the interface between the TNS and the AMMSS.

TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO2-coated SnO2 nanosheet (TiO2–SnO2 NS) films about 300nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl4 aqueous solution. The as-prepared TiO2–SnO2 NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO2–SnO2 NSs were analyzed by current–voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO2–SnO2 NSs can provide an efficient electron transition channel along the SnO2 nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.

A very facile, low temperature, one-step route to in situ fabricate copper sulfide nanosheet thin films

CuS nanosheet thin films with large areas were synthesized in situ on copper substrates through a facile, one-step treatment of copper foil and sulfur powder in the presence of N,N-dimethylformamide (DMF) at low temperature (60 °C or less). Compared with the previous reported solvothermal approach, the current low temperature route is simpler, low cost and low energy consumption. The effects of the reaction temperature on the synthesis and the optical property of CuS nanosheet films were investigated. The resulting CuS nanosheet films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and UV–vis spectrometer. In this study, we also demonstrated this method to prepare copper sulfide nanosheet films on ground-glass and indium tin oxide glass (ITO) substrates. We found that perfect nanosheet crystals could be obtained in all cases. The growth mechanism of CuS nanosheet-based films was proposed. The current method may open up a new way to prepare thin films of metal sulfide at low temperatures.

Titanoniobate and niobate nanosheet photocatalysts: superior photoinduced hydrophilicity and enhanced thermal stability of unilamellar Nb3O8nanosheet

Unique photocatalysts based on novel two-dimensional (2D) oxide nanosheets have been synthesized and their photochemical activity has been examined. Monolayer films of titanoniobate and niobate (TiNbO5, Ti2NbO7, Ti5NbO14, and Nb3O8) nanosheets, synthesized by exfoliating layered oxide precursors through a soft-chemical procedure, were fabricated on quartz glass substrate via a sequential adsorption method. All the nanosheet films exhibited good photoinduced hydrophilicity, while their oxidation activity was very low. This behavior can be regarded as inherent features of nanosheet-type photocatalysts having molecularly thin 2D anisotropy. Such ultrathin flexible structures are advantageous for facilitating photo-driven surface wettability change. Especially, the hydrophilic conversion property of Nb3O8 nanosheet was highly efficient, showing activity that was at least one order of magnitude superior to that of the widely used photocatalyst film of polycrystalline anatase TiO2. Moreover, the monolayer film of Nb3O8 nanosheet was found to have enhanced thermal stability and chemical resistance, particularly against diffusion of sodium ions at elevated temperature: Nb3O8 nanosheet film heated on sodium-rich glass (soda-lime glass) substrate maintained excellent hydrophilic conversion activity, whereas Ti0.87O2 nanosheet as well as anatase (TiO2) based photocatalysts was virtually deactivated. These features are a great advantage of Nb3O8 nanosheet photocatalysts for developing the practical super-hydrophilic applications where post-annealing is indispensable.

Facile solution deposition of ZnIn2S4 nanosheet films on FTO substrates for photoelectric application

In this paper, ZnIn(2)S(4) perpendicular nanosheet films have been directly deposited on FTO substrates by a facile hydrothermal method and investigated as the electrode materials for solar cells. The crystal structure, morphology, and optical properties of the obtained ZnIn(2)S(4) films were characterized by measurements such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrum (EDS), X-ray photoelectron spectrum (XPS) and UV-vis spectra. The results revealed a uniform perpendicular ZnIn(2)S(4) film with thickness of 4 μm and with an average nanosheet thickness of about 30 nm on FTO substrate, along with the band gap of 2.35 eV. The reaction conditions influencing the formation of ZnIn(2)S(4) films, such as the substrate treatment and reaction time were investigated. A possible mechanism for the formation of ZnIn(2)S(4) films on FTO substrates under hydrothermal conditions has been proposed. Furthermore, after heat treatment, the ZnIn(2)S(4) film electrode exhibited a photoelectrical conversion efficiency of 0.23% in FTO/ZnIn(2)S(4)/polysulfide/Au liquid-junction solar cell under AM 1.5 (100 mW cm(-2)).

Studies on mechanism of carbon nanotube and manganese oxide nanosheet self-sustained thin film for electrochemical capacitor

Multilayer films of multi-walled carbon nanotubes (MWCNTs) and manganese oxide nanosheets (MONSs) are assembled on ITO glass substrates by an electrostatic layer-by-layer assembly technique. Using scanning microscopy and atomic force microscopy, we demonstrate that the framework is composed of MWCNTs and MONSs self-sustained thin films with randomly interconnected network structure. In addition, the electrochemical performance of these films as electrodes has been investigated using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The roles of each monolayer including the dual character of PDDA, especially for the crucial effects of MWCNTs in the multilayer films are investigated. The present study indicates the potential of a new type of composite material for electrochemical capacitors with a high specific capacitance and energy density.

Field emission characteristics of thin-metal-coated nano-sheet carbon films

Nano-sheet carbon films (NSCFs) coated with very thin (≈5-nm-thick) metal layers were fabricated on Si wafer chips by means of quartz-tube-type microwave-plasma chemical-vapour-deposition method with hydrogen–methane gas mixture and an electron beam evaporation method. Field emission (FE) current densities obtained at a macroscopic average electric field, E, of ≈10V/μm changed from 13mA/cm2 for NSCF with no coated metal to 1.7, 0.7 and 30mA/cm2 for Ti-, Al- and Au-coated NSCFs, respectively, while the threshold E varied from 4.4V/μm for the former one to 5.3, 5.4 and 2.0V/μm for the corresponding latter ones, respectively. As the FE currents of Au-coated NSCFs tended to be saturated in a higher E region, compared to those of NSCFs with no coated metal, no simple Fowler–Nordheim (F–N) model is applicable. A modified F–N model considering statistic effects of the FE tip structures and a space-charge-limited-current effect is successfully applied to an explanation for the FE data observed in the low and high E regions.