Synthesis of FeP nanotube arrays as negative electrode for solid-state asymmetric supercapacitor.

Recently, metal phosphides have attracted considerable attention as promising electrode materials for supercapacitors. In this work, FeP nanotube arrays have been successfully synthesized on carbon cloth using ZnO nanorod arrays as the sacrificial templets, via a phosphidation process. The dimensions of the FeP nanotubes are characterized using SEM and TEM showing the diameter to be approximately 200 nm and with a wall thickness of 50-100 nm. The tubular structure of FeP nanotubes provides a facile ion pathway and reduced inner inactive material, thus they are favorable for supercapacitor applications. As a result, the as-synthesized FeP nanotube arrays deliver an improved specific capacitance of 149.11 F g-1 and a high areal capacitance of 300.1 mF cm-2 at a current density of 1 mA cm-2. Furthermore, an MnO2//FeP solid-state asymmetric supercapacitor was fabricated with a high areal capacitance of 142 mF cm-2, which indicates the great potential of FeP nanotube arrays to be a high-performing negative electrode material for supercapacitors.

One-step synthesis of TiO2 nanorod arrays on Ti foil for supercapacitor application.

Titanium dioxide (TiO2) nanorod arrays grown directly on Ti metal foil were prepared by a facile one-step hydrothermal method, in which the Ti foil serves as both substrate and precursor. The nanorods are tetragonal rutile single crystal with growth orientation along the [001] direction. The electrochemical properties of the TiO2 nanorod arrays were systematically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy using a three-electrode system. As a result, the TiO2 nanorod arrays exhibit good areal specific capacitance and excellent cyclic stability by retaining more than 98% of the initial specific capacitance after 1000 cycles. In addition, a good flexibility of the Ti foil with TiO2 nanorod arrays was demonstrated by the stable electrochemical performance under different bending angles, which indicates that TiO2 nanorod arrays grown on Ti foil could be a promising electrode material for flexible supercapacitor application.

Characterizing the role of iodine doping in improving photovoltaic performance of dye-sensitized hierarchically structured ZnO solar cells.

We report two novel types of hierarchically structured iodine-doped ZnO (I-ZnO)-based dye-sensitized solar cells (DSCs) using indoline D205 and the ruthenium complex N719 as sensitizers. It was found that iodine doping boosts the efficiencies of D205 I-ZnO and N719 I-ZnO DSCs with an enhancement of 20.3 and 17.9 %, respectively, compared to the undoped versions. Transient absorption spectra demonstrated that iodine doping impels an increase in the decay time of I-ZnO, favoring enhanced exciton life. Mott-Schottky analysis results indicated a negative shift of the flat-band potential (V(fb)) of ZnO, caused by iodine doping, and this shift correlated with the enhancement of the open circuit voltage (V(oc)). To reveal the effect of iodine doping on the effective separation of e(-)-h(+) pairs which is responsible for cell efficiency, direct visualization of light-induced changes in the surface potential between I-ZnO particles and dye molecules were traced by Kelvin probe force microscopy. We found that potential changes of iodine-doped ZnO films by irradiation were above one hundred millivolts and thus significantly greater. In order to correlate enhanced cell performance with iodine doping, electrochemical impedance spectroscopy, incident-photon-current efficiency, and cyclic voltammetry investigations on I-ZnO cells were carried out. The results revealed several favorable features of I-ZnO cells, that is, longer electron lifetime, lower charge-transfer resistance, stronger peak current, and extended visible light harvest, all of which serve to promote cell performance.

Controllable in situ synthesis of magnetite coated silica-core water-dispersible hybrid nanomaterials.

Magnetite nanoparticle coated silica (Fe3O4@SiO2) hybrid nanomaterials hold an important position in the fields of cell imaging and drug delivery. Here we report a large scale synthetic procedure that allows attachment of magnetite nanoparticles onto a silica surface in situ. Many different silica nanomaterials such as Stöber silica nanospheres, mesoporous silica nanoparticles, and hollow silica nanotubes have been coated with a high density layer of water-dispersible magnetite nanoparticles. The size and attachment efficiency of the magnetite nanoparticle can be well tuned by adjusting the precursor concentration and reflux time. The functionalization of Fe3O4@SiO2 nanoparticles with dye molecules and biocompatible polymers impart optical imaging modality and good colloidal stability in either buffer solution or serum. The functionalized materials also exhibited strong potential as negative contrast agents in T2 weighted magnetic resonance imaging.

CuO nanorod arrays by gas-phase cation exchange for efficient photoelectrochemical water splitting.

CuO has been considered a promising candidate for photoelectrochemical water splitting electrodes owing to its suitable bandgap, favorable band alignments, and earth-abundant nature. In this paper, a novel gas-phase cation exchange method was developed to synthesize CuO nanorod arrays by using ZnO nanorod arrays as the template. ZnO nanorods were fully converted to CuO nanorods with aspect ratios of 10-20 at the temperature range from 350 to 600 °C. The as-synthesized CuO nanorods exhibit a photocurrent as high as 2.42 mA cm-2 at 0 V vs. RHE (reversible hydrogen electrode) under 1.5 AM solar irradiation, demonstrating the potential as the photoelectrode for efficient photoelectrochemical water splitting. Our method provides a new approach for the rational fabrication of high-performance CuO-based nanodevices.

A large quantity synthesis of ultra-thin ZnO nanobelts induced from stacking faults.

Thin ZnO nanobelts with average width of 7.5 nm have been synthesized using vapor phase transport method. It was found that stacking faults directed the growth of the thin nanobelts along the (0110) direction with {2110} top/bottom surfaces and {0001} side surfaces. The {0002} stacking fault with translation of 1/3(0110) extends throughout entire length of the ZnO nanobelts. The growth steps at the {0110} growth fronts resulted from the {0002} stacking fault are believed to direct fast axial growth of the thin ZnO nanobelts. The thin ZnO nanobelts are expected to be promising candidates for highly sensitive chemical and biological sensor applications.

The detection of H2S at room temperature by using individual indium oxide nanowire transistors.

In(2)O(3) nanotransistors for gas sensor applications were fabricated using individual In(2)O(3) nanowires prepared by chemical vapor deposition. The nanosensors demonstrate characteristics of high sensitivity to H(2)S, and fast response and recovery, with the detection limit at 1 ppm at room temperature. The high sensitivity might be attributed to the strong electron accepting capability of H(2)S to the nanowires and the high surface-to-volume ratio of the nanowires. In addition, the nanosensors show a good selective detection of H(2)S under exposure to NH(3) and CO even at 1000 ppm; they are highly promising for practical applications in detection of low concentration H(2)S at room temperature.

Precise voltage contrast image assisted positioning for in situ electron beam nanolithography for nanodevice fabrication with suspended nanowire structures.

In this paper, we demonstrate precise voltage contrast image positioning for in situ electron beam (e-beam) nanolithography to integrate nanowires into suspended structures for nanoswitch fabrication. The positioning of the deflection electrodes on the nanowires can be well controlled using a precise voltage contrast image positioning technique, where the error can be minimized to about 10 nm. Using such a method, dispersed nanowires can be sandwiched between two layers of resist and suspended by one e-beam nanolithography process without any etching. The in situ e-beam nanolithography eliminates the stage movement error by preventing any movements of the stage during the nanolithography process; hence, a high precision laser stage and alignment marks on the substrate are not needed, which simplifies the traditional e-beam nanolithography process. The nanoswitches fabricated using this method show ON and OFF states with the changes of applied voltages. This simplified process provides an easy, low cost and less time-consuming route to integrating suspended nanowire based structures using a converted field emission scanning electron microscope e-beam system, which can also be customized to fabricate multi-layer structures and a site-specific nanodevice fabrication.

PEDOT coated iron phosphide nanorod arrays as high-performance supercapacitor negative electrodes.

PEDOT coated iron phosphide nanorod arrays are synthesized and demonstrated as high-performance negative electrodes for supercapacitors with high areal specific capacitance and significantly improved cycling stability. A MnO2//FeP/PEDOT aqueous asymmetric supercapacitor is fabricated with a high volumetric capacitance of 4.53 F cm-3 and an energy density of 1.61 mW h cm-3.

Three-Dimensional Cobalt Phosphide Nanowire Arrays as Negative Electrode Material for Flexible Solid-State Asymmetric Supercapacitors.

Despite the great progress that has been accomplished in supercapacitors, the imbalance of the development of positive and negative electrode materials still remains a critical issue to achieve high energy density; therefore, exploring high-performance negative electrode materials is highly desirable. In this article, three-dimensional cobalt phosphide (CoP) nanowire arrays were synthesized on a carbon cloth and were utilized as a binder-free supercapacitor negative electrode. The as-synthesized CoP nanowire arrays presented a high capacitance of 571.3 mF/cm2 at a current density of 1 mA/cm2. By using CoP nanowire arrays as the negative electrode and MnO2 nanowire arrays as the positive electrode, a flexible solid-state asymmetric supercapacitor has been fabricated and has exhibited excellent electrochemical performance, such as a high energy density of 0.69 mWh/cm3 and a high power density of 114.2 mW/cm3. In addition, the solid-state asymmetric supercapacitor shows high cycle stability with 82% capacitance retention after 5000 charge/discharge cycles. This work demonstrates that CoP is a promising negative electrode material for high-performance supercapacitor applications.

Influence of different aluminum salts on the photocatalytic properties of Al doped TiO2 nanoparticles towards the degradation of AO7 dye.

Three kinds of Al-TiO2 samples and pure TiO2 samples were synthesized via a modified polyacrylamide gel route using different aluminum salts, including Al2(SO4)3∙18H2O, AlCl3, and Al(NO3)3∙9H2O under identical conditions. The influence of different aluminum salts on the phase purity, morphologies, thermal stability of anatase and photocatalytic properties of the as-prepared Al-TiO2 nanoparticles were studied. The energy gap (Eg) of Al-TiO2 nanoparticles decreases due to Al ion doping into TiO2. The photocatalytic activities of the Al-TiO2 samples were investigated by the degradation of acid orange 7 dye in aqueous solution under simulated solar irradiation. The Al-TiO2 nanoparticles prepared from Al(NO3)3∙9H2O exhibit the best photocatalytic activity among the four kinds of samples, followed in turn by the Al-TiO2 nanoparticles prepared with AlCl3, Al2(SO4)3∙18H2O and pure TiO2. The different performances are attributed to complex effects of Eg, particle size, surface morphology, phase purity and the defect sites of the Al-TiO2 nanoparticles.

Drug-loaded, magnetic, hollow silica nanocomposites for nanomedicine.

Magnetic, hollow silica nanocomposites (MHSNC), including nanospheres and nanotubes, have been successfully synthesized using a coating of Fe(3)O(4) magnetic nanoparticles (NPs) ( approximately 10 nm) and silica on nanosized spherical and nanoneedle-like calcium carbonate (CaCO(3)) surfaces under alkaline conditions. The nanosized CaCO(3) surfaces were used as nanotemplates, and tetraethoxysilane and magnetic NPs were used as precursors. The as-synthesized MHSNC were immersed in an acidic solution to remove the CaCO(3), forming magnetic, hollow silica nanospheres and nanotubes. The MHSNC were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray powder diffraction, and superconducting quantum interference device (SQUID) magnetometer. SEM and TEM results showed that a smooth surface of MHSNC and a thin layer of silica ( approximately 10 nm) embedded with the magnetic NPs were successfully formed, and that the CaCO(3) nanotemplates appeared to be dissolved. SQUID measurement demonstrated that magnetization of MHSNC was dependent on temperature, exhibiting superparamagnetism. The MHSNC were immersed in ibuprofen solution. The amount of the loaded drug was determined to be 12 wt% for nanospheres, and 8 wt% for nanotubes by UV measurement, respectively. Drug-loaded MHSNC have potential applications in nanomedicine.

Synthesis and characterization of the rare-earth Dion-Jacobson layered perovskites, APrNb2O7 (A = Rb, Cs and CuCl).

The new double-layered perovskites, APrNb(2)O(7) (A = Rb, Cs) have been prepared by a high temperature ceramic method. Rietveld refinement of X-ray powder diffraction data confirmed the orthorhombic and tetragonal structures, respectively; RbPrNb(2)O(7) was refined in space group Imma (a = 5.4534(7) Å, b = 22.012(1) Å, c = 5.4549(7) Å) and CsPrNb(2)O(7) in P4/mmm (a = 3.8668(2) Å, c = 11.163(1) Å). (CuCl)PrNb(2)O(7), topochemically prepared by replacement of Rb(+)/Cs(+) with CuCl(+), contains a 2D Cu-Cl network between the PrNb(2)O(7) slabs (orthorhombic space group Pbam, a = 7.7328(6) Å, b = 7.7113(4) Å and c = 11.6706(3) Å). The parent compounds both show paramagnetic behavior µ(eff) (Rb) = 3.34(1)µ(B) and µ(eff) (Cs) = 3.60(2)µ(B) while the (CuCl)PrNb(2)O(7), paramagnetic (µ(eff) = 4.020(8)µ(B)) down to 20 K, exhibits antiferromagnet-like behavior below 20 K.

Piezo-phototronic Effect Enhanced UV/Visible Photodetector Based on Fully Wide Band Gap Type-II ZnO/ZnS Core/Shell Nanowire Array.

A high-performance broad band UV/visible photodetector has been successfully fabricated on a fully wide bandgap ZnO/ZnS type-II heterojunction core/shell nanowire array. The device can detect photons with energies significantly smaller (2.2 eV) than the band gap of ZnO (3.2 eV) and ZnS (3.7 eV), which is mainly attributed to spatially indirect type-II transition facilitated by the abrupt interface between the ZnO core and ZnS shell. The performance of the device was further enhanced through the piezo-phototronic effect induced lowering of the barrier height to allow charge carrier transport across the ZnO/ZnS interface, resulting in three orders of relative responsivity change measured at three different excitation wavelengths (385, 465, and 520 nm). This work demonstrates a prototype UV/visible photodetector based on the truly wide band gap semiconducting 3D core/shell nanowire array with enhanced performance through the piezo-phototronic effect.

Carrier separation and transport in perovskite solar cells studied by nanometre-scale profiling of electrical potential.

Organometal-halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells-which are the fundamental mechanisms of device operation and critical factors for power output-by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p-n junction structure at the TiO2/perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.

Surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film.

A surface acoustic wave (SAW) resonator with ZnO/SiO2 (ZS) composite film was used as an ammonia sensor in this study. ZS composite films were deposited on the surface of SAW devices using the sol-gel method, and were characterized using SEM, AFM, and XRD. The performance of the sensors under ammonia gas was optimized by adjusting the molar ratio of ZnO:SiO2 to 1:1, 1:2 and 1:3, and the sensor with the ratio of ZnO to SiO2 equaling to 1:2 was found to have the best performance. The response of sensor was 1.132 kHz under 10 ppm NH3, which was much higher than that of the sensor based on a pristine ZnO film. Moreover, the sensor has good selectivity, reversibility and stability at room temperature. These can be attributed to the enhanced absorption of ammonia and unique surface reaction on composite films due to the existence of silica.

Evidencing the existence of intrinsic half-metallicity and ferromagnetism in zigzag gallium sulfide nanoribbons.

The achievement of half-metallicity with ferromagnetic (FM) coupling has become a key technology for the development of one-dimensional (1D) nanoribbons for spintronic applications. Unfortunately, in previous studies, such a half-metallicity always occurs upon certain external constraints. Here we, for the first time, demonstrate, via density functional theory (DFT), that the recent experimentally realized gallium sulfide nanoribbons (GaSNRs) can display an intrinsic half-metallic character with FM coupling, raised from Ga-4s, Ga-4p and S-3p states at the Ga-dominated edge. Furthermore, the novel half-metallic behavior with FM coupling here is rather robust, especially for GaSNRs with large width and thickness, and can be sustained to the room temperature. Thus, our results accidentally disclose a new 1D spin nanomaterial, which allows us to go beyond the current scope limited to the graphene, boron nitride (BN), zinc oxide (ZnO) and molybdenum sulfide (MoS2) nanoribbons, toward more realistic spintronic applications.

Nearly lattice matched all wurtzite CdSe/ZnTe type II core-shell nanowires with epitaxial interfaces for photovoltaics.

Achieving a high-quality interface is of great importance in core-shell nanowire solar cells, as it significantly inhibits interfacial recombination and thus improves the photovoltaic performance. Combining thermal evaporation of CdSe and pulsed laser deposition of ZnTe, we successfully synthesized nearly lattice matched all wurtzite CdSe/ZnTe core-shell nanowires on silicon substrates. Comprehensive morphological and structural characterizations revealed that a wurtzite ZnTe shell layer epitaxially grows over a wurtzite CdSe core nanowire with an abrupt interface. Further optical studies confirmed a high-quality interface and demonstrated efficient charge separation induced by the type-II band alignment. A representative photovoltaic device has been demonstrated and yielded an energy-conversion efficiency of 1.7% which can be further improved by surface passivation. The all-wurtzite core-shell nanowire with an epitaxial interface offers an attractive platform to explore the piezo-phototronic effect and promises an efficient hybrid nano-sized, energy harvesting system.

Highly sensitive room-temperature surface acoustic wave (SAW) ammonia sensors based on Co3O4/SiO2 composite films.

Surface acoustic wave (SAW) sensors based on Co3O4/SiO2 composite sensing films for ammonia detection were investigated at room temperature. The Co3O4/SiO2 composite films were deposited onto ST-cut quartz SAW resonators by a sol-gel method. SEM and AFM characterizations showed that the films had porous structures. The existence of SiO2 was found to enhance the ammonia sensing property of the sensor significantly. The sensor based on a Co3O4/SiO2 composite film, with 50% Co3O4 loading, which had the highest RMS value (3.72), showed the best sensing property. It exhibited a positive frequency shift of 3500 Hz to 1 ppm ammonia as well as excellent selectivity, stability and reproducibility at room temperature. Moreover, a 37% decrease in the conductance of the composite film as well as a positive frequency shift of 12,500 Hz were observed when the sensor was exposed to 20 ppm ammonia, indicating the positive frequency shift was derived from the decrease in film conductance.

Vertically aligned CdSe nanowire arrays for energy harvesting and piezotronic devices.

We demonstrated the energy harvesting potential and piezotronic effect in vertically aligned CdSe nanowire (NW) arrays for the first time. The CdSe NW arrays were grown on a mica substrate by the vapor-liquid-solid process using a CdSe thin film as seed layer and platinum as catalyst. High-resolution transmission electron microscopy image and selected area electron diffraction pattern indicate that the CdSe NWs have a wurtzite structure and growth direction along (0001). Using conductive atomic force microscopy (AFM), an average output voltage of 30.7 mV and maximum of 137 mV were obtained. To investigate the effect of strain on electron transport, the current-voltage characteristics of the NWs were studied by positioning an AFM tip on top of an individual NW. By applying normal force/stress on the NW, the Schottky barrier between the Pt and CdSe was found to be elevated due to the piezotronic effect. With the change of strain of 0.12%, a current decreased from 84 to 17 pA at 2 V bias. This paper shows that the vertical CdSe NW array is a potential candidate for future piezo-phototronic devices.