Bipolar resistive switching in p-type Co3O4 nanosheets prepared by electrochemical deposition

Metal oxide nanosheets have potential applications in novel nanoelectronics as nanocrystal building blocks. In this work, the devices with a structure of Au/p-type Co₃O₄ nanosheets/indium tin oxide/glass having bipolar resistive switching characteristics were successfully fabricated. The experimental results demonstrate that the device have stable high/low resistance ratio that is greater than 25, endurance performance more than 200 cycles, and data retention more than 10,000 s. Such a superior performance of the as-fabricated device could be explained by the bulk film and Co₃O₄/indium tin oxide glass substrate interface effect.     

One-step synthesis of ZnO nanosheets: a blue-white fluorophore

ABSTRACT: Zinc oxide is synthesised at low temperature (80[DEGREE SIGN]C) in nanosheet geometry using a substrate-free, single-step, wet-chemical method and is found to act as a blue-white fluorophore. Investigation by atomic force microscopy, electron microscopy, and X-ray diffraction confirms zinc oxide material of nanosheet morphology where the individual nanosheets are polycrystalline in nature with the crystalline structure being of wurtzite character. Raman spectroscopy indicates the presence of various defects, while photoluminescence measurements show intense green (centre wavelength approximately 515 nm) blue (approximately 450 nm), and less dominant red (approximately 640 nm) emissions due to a variety of vacancy and interstitial defects, mostly associated with surfaces or grain boundaries. The resulting colour coordinate on the CIE-1931 standard is (0.23, 0.33), demonstrating potential for use as a blue-white fluorescent coating in conjunction with ultraviolet emitting LEDs. Although the defects are often treated as draw-backs of ZnO, here we demonstrate useful broadband visible fluorescence properties in as-prepared ZnO.     

Three-dimensional hierarchical ZnCo2O4 flower-like microspheres assembled from porous nanosheets: Hydrothermal synthesis and electrochemical properties

In this work, three-dimensional hierarchical ZnCo₂O₄ flower-like microspheres have been synthesized on a large scale via a facile and economical citrate-mediated hydrothermal method followed by an annealing process in air. The as-synthesized ZnCo₂O₄ flower-like microspheres are constructed by numerous interweaving porous nanosheets. According to the experimental results, a formation mechanism involving the assembly of the nanosheets from nanoparticles into flower-like microsphere is proposed. As a virtue of their beneficial structural features, the ZnCo₂O₄ flower-like microspheres exhibit a high lithium storage capacity and excellent cycling stability (1136 mA h g⁻¹ at 100 mA g⁻¹ after 50 cycles). This remarkable electrochemical performance can be ascribed to the hierarchical structure and porous structures in the nanosheets, which effectively increases the contact area between the active materials and the electrolyte, shortening the Li⁺ diffusion pathway and buffering the volume variation during cycling.     

Fabrication of dendritic silver nanostructure using an integration of holographic lithography and electrochemical deposition

Holographic lithography in combination with electrochemical deposition provides a novel and readily controlled method for preparing dendritic three-dimensional silver nanostructure with a large surface area. The scanning electron microscope and the synchrotron radiation X-ray three-dimensional imaging technique are used to characterize the as-prepared silver nanomaterial and demonstrate that the obtained silver has a three-dimensional dendritic morphology. The observed typical widths between the neighbor branches of the dendritic silver nanostructure are 30–50 nm. The configuration of the silver nanostructure is greatly related to the width of gold wires which are fabricated using holographic lithography and serve as substrate of the silver nanostructure. The three-dimensional silver architecture is formed only when silver is electrochemically deposited on 400-nm-width gold.     

Aptamer-based electrochemical detection of protein using enzymatic silver deposition

In this work an electrochemical protein detection based on enzymatic silver deposition has been proposed and applied to the detection of thrombin. The target protein, thrombin, was first captured by thrombin binding thiolated aptamer self-assembled monolayers (SAMs) on the gold electrode surface, and then sandwiched with another biotinylated thrombin binding aptamer for the association of alkaline phosphatase (Av-ALP). The attached Av-ALP enzymatically converts the nonelectroactive substrate p-aminophenyl phosphate (p-APP) to p-aminophenol (p-AP) which can reduce silver ions in solution leading to deposition of the metal onto the electrode surface. Finally, linear sweep voltammetry (LSV) is used to detect the amount of deposited silver. The peak current during the anodic scan was found to reflect the amount of the target protein captured into the sandwich configuration. The proposed approach has been successfully implemented for the detection of thrombin in the range of 0.1 nM to 1 μM. Therefore, the current work has demonstrated enzymatic silver deposition for detection at aptamer-modified electrode.     

One-step synthesis of silver nanoparticles embedded with polyethylene glycol as thin films

Silver nanoparticles (Ag NPs) embedded and stabilized with polyethylene glycol (PEG) were synthesized as colloids by heating and exposure to sunlight (direct and indirect) irradiation as green method. The deposition of Ag NP-PEGs onto Si-wafers was also made using the electrospray ionization deposition technique. The generating of Ag NP-PEGs as colloids was examined by UV–visible spectroscopy (UV–Vis) and transmission electron microscopy (TEM). The chemical composition of the resulted nanocomposites was evaluated by Fourier transform infrared (FTIR) and that of thin-film surfaces by X-ray photoelectron spectroscopy. Structure–property relationships of Ag-PEG nanocomposites prepared by heating were discussed in dependence on the time of heating. The UV–visible results confirmed the successful synthesis of spherical Ag NPs with absorption peaks at a wavelength of λ = 413 nm for the heating method and at λ = 418 as well as 449 nm for direct and indirect exposure to the sunlight. Ag-PEG nanocomposite thin films showed excellent antimicrobial activity. These results revealed that the Ag-PEG nanocomposites thin films can be used as potential materials in biomedical applications.     

Well-integrated ZnO nanorod arrays on conductive textiles by electrochemical synthesis and their physical properties

We reported well-integrated zinc oxide (ZnO) nanorod arrays (NRAs) on conductive textiles (CTs) and their structural and optical properties. The integrated ZnO NRAs were synthesized by cathodic electrochemical deposition on the ZnO seed layer-coated CT substrate in ultrasonic bath. The ZnO NRAs were regularly and densely grown as well as vertically aligned on the overall surface of CT substrate, in comparison with the grown ZnO NRAs without ZnO seed layer or ultrasonication. Additionally, their morphologies and sizes can be efficiently controlled by changing the external cathodic voltage between the ZnO seed-coated CT substrate and the counter electrode. At an external cathodic voltage of −2 V, the photoluminescence property of ZnO NRAs was optimized with good crystallinity and high density.     

Photoelectrochemical properties of hematite thin films grown via a two-step electrochemical deposition method

In this study, Fe₂O₃ thin films were deposited via one-step and two-step electrochemical deposition methods. The one-step method used a constant growth potential, and the two-step method used two continuous growth potentials during the deposition. The morphological, electrical, and structural properties of the thin films and the relationships of each property with the photoelectrochemical properties of the thin films from the two methods were analyzed and compared. We determined that the samples grown by the two-step method have better photoelectrochemical properties than those grown by the one-step method. In this study, we attempted to determine the optimum growth potentials for the one-step and two-step methods and the growth durations of the first and second stages in the two-step method in terms of the photoelectrochemical properties. The sample formed at ?0.05 V for 30 s in the first step and ?0.25 V for 2 min 30 s in the second step in the two-step method has the highest photocurrent density value of 0.28 mA/cm² (at 0.5 V vs. SCE), which is higher than that of the samples grown by the one-step method.     

Fabricating boron nitride nanosheets from hexagonal BN in water solution by a combined sonication and thermal-assisted hydrolysis method

Boron nitride nanosheets (BNNSs) have unique and excellent thermal, electrical, and mechanical properties. But, their low efficiency in the production methods restricts their applications in the study of researchers. Hence, we reported a study in which BNNSs were successfully produced through a simple, affordable, and high-efficiency method. In this approach, hexagonal boron nitride block (h-BN) was firstly examined in aqueous solution under sonication (120 min) to produce hydroxyl functionalization and; furthermore, to penetrate water molecules between the layers. Then, with the explosion of the water molecules into the h-BN layers in the presence of heat, the space between the layers was increased. Finally, with the exfoliation of bulk h-BN layers through the ultrasonic irradiation, the ultrathin BNNSs were produced with an efficiency of 37%. The TEM and AFM images confirmed that the obtained BNNSs have mainly consisted of nanosheets with a thickness in the range of 3–8 nm. The results of UV–Vis analysis of BNNSs compared to h-BN showed a strong absorption peak at 204 nm^?1, which confirmed the presence of nanosheets. Also, other analyses, including XRD, BET, and FT-IR, confirmed the structure of BNNSs.     

Desulfurization of gasoline by a new method of electrochemical catalytic oxidation

A new deep desulfurization process for gasoline was obtained by means of electrochemical catalytic oxidation with an electrochemical fluidized-bed reactor on particle group anode. The particle group anode was activated carbon-supported cerium dioxide (CeO₂/C), and the electrolyte was aqueous cerium nitrate solution, and copper pillar was cathode in the electrochemical reactions. The CeO₂/C particle group anode could remarkably accelerate the electrochemical reaction rate and promote the electrochemical catalysis performance for the electrochemical desulfurization reaction. Thermodynamics feasibility analysis clarified that the theoretical decomposition voltage ranged from 0.1–0.5 V in pure acid electrolyte system and desulfurization reactions could not spontaneously carry out, but the reactions were spontaneous when aqueous cerium nitrate solution serves as electrolyte. And the rule of the gasoline desulfurization by the means of electrochemical catalytic oxidation was investigated. The experimental results indicated that the optimal desulfurization conditions were as follows: the cell voltage, concentration of the Ce³⁺ ions, feed volume flow rate and the CeO₂ percentage by weight were 3.2 V, 0.08 mol l⁻¹, 300 ml min⁻¹ and 5.0 wt%, respectively. Under these conditions the concentration of sulfur in gasoline was reduced from 310 to 50 parts per million by weight (ppmw). Based on these experimental results, a mechanism of indirect electrochemical oxidation was also proposed.     

In-situ study of electrophoretic deposition of zinc oxide nanosheets and nanorods

In the present work, the effects of two different morphologies of zinc oxide nanoparticles (nanosheets and nanorods) were investigated by in-situ measurement of deposition weight, and current density. ZnO nanosheets and nanorods were synthesized by microwave-assisted method using co-surfactant route. The average thickness of obtained nanosheets, and the average diameter of nanorods were measured to be about 26 nm and 139 nm, respectively. ZnO films were obtained by electrophoretic deposition from suspension of nanoparticles in ethanol under different voltages. Results indicated that ZnO nanosheets tend to have greater deposition rate than ZnO nanorods under similar conditions. The compactness of the film obtained from nanosheet suspension was higher than the one obtained from nanorod suspension. However, the film obtained from ZnO nanorods displayed more uniformity at different voltages in comparison to the film obtained from ZnO nanosheets, which can be due to different active surface area, and also different way of motion under hydrodynamic forces in the suspension.     

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag⁺, ClO₄⁻ and Ag(111)/Ag⁺, SO₄²⁻. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag⁺ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu²⁺. The enhanced local Ag dissolution is explained by a reduced Ag⁺ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag⁺ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.     

Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene

Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO₂/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO₂/Si.     

Synthesis, crystal structure and electrochemical properties of a novel tetranuclear silver(I) cluster based on 1,1′-bis[(2-hydroxybenzylidene)thiocarbonohydrazono-1-ethyl] ferrocene (FcL)

A novel tetranuclear silver(I) heterocyclic cluster, which is of nano-hole structure, based on 1,1′-bis[(2-hydroxybenzylidene)thiocarbonohydrazono-1-ethyl] ferrocene (FcL) has been prepared and structurally characterized by single-crystal X-ray diffractions. In addition, its electrochemical properties are also discussed. The results confirm that the receptor FcL is responsive to Ag⁺.     

Facile one-step synthesis of pellet-press-assisted saddle-curl-edge-like g-C3N4 nanosheets for improved visible-light photocatalytic activity

Graphitic carbon nitride (g-C₃N₄) has attracted increasing interest as a visible-light-active photocatalyst. In this study, saddle-curl-edge-like g-C₃N₄ nanosheets were prepared using a pellet presser (referred to as g-CN P nanosheets). Urea was used as the precursor for the preparation of g-C₃N₄. Thermal polymerization of urea in a pellet form significantly affected the properties of g-C₃N₄. Systematic investigations were performed, and the results for the modified g-C₃N₄ nanosheets are presented herein. These results were compared with those for pristine g-C₃N₄ to identify the factors that affected the fundamental properties. X-ray diffraction analysis and high-resolution transmission electron microscopy revealed a crystallinity improvement in the g-CN P nanosheets. Fourier-transform infrared spectroscopy provided clear information regarding the fundamental modes of g-C₃N₄, and X-ray photoelectron spectroscopy (XPS) peak-fitting investigations revealed the variations of C and N in detail. The light-harvesting property and separation efficiency of the photogenerated charge carriers were examined via optical absorption and photoluminescence studies. The valence band edge and conduction band edge potentials were calculated using XPS, and the results indicated a significant reduction in the bandgap for the g-CN P nanosheets. The Brunauer–Emmett–Teller surface area increased for the g-CN P nanosheets. The photocatalytic degradation performance of the g-CN P nanosheets was tested by applying a potential and using the classical dye Rhodamine B (RhB). The RhB dye solution was almost completely degraded within 28 min. The rate constant of the g-CN P nanosheets was increased by a factor of 3.8 compared with the pristine g-C₃N₄ nanosheets. The high crystallinity, enhanced light absorption, reduced bandgap, and increased surface area of the saddle-curl-edge-like morphology boosted the photocatalytic performance of the g-CN P nanosheets.     

Template assisted solid state electrochemical growth of silver micro- and nanowires

We report on a template based solid state electrochemical method for fabricating silver nanowires with predefined diameter, depending only on the pore diameter of the template. As templates we used porous silicon with pore diameters in the μm range and porous alumina with pore diameters in the nm range. The template pores were filled with silver sulfide (a mixed silver cation and electronic conductor) by direct chemical reaction of silver and sulfur. The filled template was then placed between a silver foil as anode (bottom side) and a microelectrode (top side) as cathode. An array of small cylindrical transference cells with diameters in the range of either micro- or nanometers was thus obtained. By applying a cathodic voltage to the microelectrode silver in the form of either micro- or nanowires was deposited at about 150 °C. The growth rate is controllable by the electric current.     

Electrodeposition of nanocrystalline silver films and nanowires from the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate

We report in this paper on the electrodeposition of nanocrystalline silver films and nanowires in the air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIm]TfO containing Ag(TfO) as a source of silver. The study was performed by means of cyclic voltammetry and chronoamperometry, and the electrodeposits were characterized by SEM-EDX and XRD. The cyclic voltammetry behaviour showed typical reduction and oxidation peaks corresponding to the deposition and stripping of silver in the employed electrolyte. XRD patterns of the electrodeposited silver layers revealed the characteristic peaks of crystalline silver with crystallites in the nanosize regime. Silver nanowires with average diameters and lengths of about 200 nm and 3 μm, respectively, were prepared by potentiostatic deposition within a commercial nuclear track-etched polycarbonate template.     

Fabrication and electrochemical characterization of zinc selenide thin film by pulsed laser deposition

High-quality crystalline ZnSe thin film has been successfully fabricated by using pulsed laser ablation of mixed target of Zn and Se. The physical, electrochemical and spectroelectrochemical properties of the as-deposited thin film at different substrate temperatures have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the charge/discharge, cyclic voltammetry (CV) and in situ absorbance spectra measurements. The thin film deposited at room temperature consisted of a mixture of Zn and Se. When the substrate temperature was elevated to 300 °C, the as-deposited thin film was composed of crystallized ZnSe. The cycle performance of ZnSe/Li cell is well than that of Zn-Se/Li cell. A couple of reduction and oxidation peaks at 0.51 and 1.4 V from CV curves of ZnSe/Li cell was found, indicating the reversible formation and decomposition of Li₂Se. Both classical alloying/dealloying processes and the partially selenidation/reduction of nano-sized metal zinc were proposed in lithium electrochemical reaction of ZnSe. The high degree of crystallinity in the zinc alloys and the slow kinetics should be responsible for a large irreversible capacity loss and poor cyclability.     

Hierarchically nanostructured vanadium nitride microspheres assembled with porous nanosheets fabricated by a template-free route

In this research, hierarchically nanostructured vanadium nitride (VN) microspheres were successfully prepared by a template-free method. ZnO, behaving as a sacrificial phase, was selectively leached out during nitridation of Zn₃(OH)₂(V₂O₇)·(H₂O)₂ microspheres with NH₃, inducing a network structure. The VN microsphere was assembled with porous nanosheets. Extraordinary, the VN microsphere mimicked the morphology of the initial precursor and exhibited a rose-like morphology. The diameter of the microsphere was in the range of 1–3 µm, and the thickness of the porous nanosheets was typically approximately 30 nm. In addition, this facile and novel synthetic strategy can be further extended to the preparation of other hierarchically nanostructured transition metal nitrides.     

液相化学还原法制备单分散的纳米银粒子

以月桂酸为修饰剂,水合肼为还原剂,银氨溶液为银源,在水相中利用液相化学还原法制备了单分散的粒径分布均匀的纳米银粒子。利用透射电子显微镜(TEM)、X射线衍射(XRD)对样品的形貌和结构进行了分析,研究表明,修饰剂与硝酸银的质量比、反应温度对纳米银形貌及粒径有很大影响。当修饰剂与硝酸银的质量比为1.2∶1、反应温度为室温时,能够制备平均粒径为8 nm、粒径均匀、单分散的纳米银粒子。另外,UV光谱也证实,所制的溶胶为粒径均匀的纳米银溶胶。