This Article focuses on the fabrication of highly ordered nanotubes and some novel nanostructures of titania (TiO2) with a two-step anodization method. The first-step anodization was actually a pretreatment of the Ti foil surface and provided well-ordered imprints that served as a template for the further growth of nanotubes. As a result, the TiO2 nanotubes growing in the second-step anodization appreciably outperformed those fabricated with the conventional one-step Ti anodization in terms of size uniformity and arrangement orderliness. The parameters of the anodization were then modulated to obtain more complex structures. When the voltage in the second-step anodization was lower than that in the first-step anodization, a lotus root-shaped TiO2 nanostructure, in which each imprint contained several smaller nanopores, was achieved. When the second anodization was further divided into two stages, double-layered nanotube arrays were synthesized. They contained two distinctly separated parts, i.e., the bamboo-shaped upper one and the smooth-walled lower one. These results have demonstrated the effectiveness and controllability of the two-step anodization method in producing high-quality TiO2 nanotubes, which are believed to have potential applications in such fields as solar cells, photonic crystals, and hydrogen storage.

In a microwave tube the energy of electrons in an electron beam, which are usually generated by a thermionic cathode, is modulated such that it is transformed into electromagnetic wave energy. In this paper, we modulated the field emission using AC signals. A field emission current with AC components was attained by adding a small AC voltage to a high extracting DC voltage. We observed two phenomena related to the AC components of the field emission current. First, when the DC component of the extracting voltage was low, the current measured in the circuit was dominated by the displacement current. Second, besides the fundamental frequency component, higher order harmonics were also observed in the field emission current.

In this paper we described our study of the behaviors of field emitters driven by square-wave voltages. We observed phenomena under pulsed voltages that generally do not manifest themselves under direct-current voltages. We interpreted these phenomena with the cathode and anode combined treated as equivalent to a resistor and a condenser in series connection. First, because of the delay caused by the charging process of the condenser, the waveform of the voltage across the cathode-anode gap was remarkably distorted. Second, the resistor led to considerable attenuation in field emission, which was clearly observable within each pulse and became more dramatic with increasing repetition frequency of the pulses. Furthermore, the field emission currents under direct-current voltages were lower than those under pulsed voltages. This disparity is attributed to rising resistance in the circuit with rising temperature. We also discussed the restrictions that the waveform distortion and current attenuation could impose on potential field emitter applications.

•For field emission, 1D nanomaterials often have high aspect ratio but blunt tips.•2D nanomaterials have sharp layer edges but low aspect ratio.•A compound structure of TiO2 nanorods (1D) and MoS2 nanofilms (2D) is fabricated.•The advantages of 1D and 2D nanomaterials in field emission are utilized.Heterostructures of 2-dimensional (2D) layered MoS2 nanofilms on ordered 1-dimensional (1D) TiO2 nanorods (MoS2@TiO2 heterostructures) are synthesized on Si substrates using a simple and low-cost hydrothermal method and then used as field emitters. Results of comparative experiments show that the MoS2@TiO2 heterostructures outperform the as-synthesized 1D TiO2 nanorods in field emission. This advantage is mainly attributed to three factors. First, in MoS2@TiO2 heterostructures, besides the high aspect ratio of the TiO2 nanorods, the sharpness of the MoS2 layer edges also contributes to the enhancement of the local electric field. Second, the large number of the MoS2 layer edges gives rise to more emission sites. Third, more electrons are available in the conduction band of MoS2.

•A novel flexible fiber-type dye-sensitized solar cell with multi-working electrodes is designed.•This novel structure can harvest incident light from all directions.•The conversion efficiency is quite stable under an extreme mechanical deformation.•The conversion efficiency of this novel structure reaches 9.1%.Novel flexible fiber-type dye-sensitized solar cells (FF-DSSCs) with multi-working electrodes (MWFF-DSSCs) have been developed. In each MWFF-DSSC, all the components are assembled into a flexible plastic capillary tube. A Pt microwire along the axis of the tube is used as the sole counter electrode and a number of Ti microwires surrounding it, which are all covered with highly ordered titanium dioxide (TiO2) nanotube arrays, are jointly used as the working electrodes. This new configuration brings about good flexibility, capability of harvesting light from all directions and a conversion efficiency competitive with those of the conventional DSSCs. The photovoltaic performances of the MWFF-DSSC with six working electrodes (MWFF-DSSC(6)) are better than those of the MWFF-DSSCs with two to five working electrodes (MWFF-DSSC(x), x=2, 3, 4, or 5) and the FF-DSSC with a single working electrode (SWFF-DSSC). When the as-prepared TiO2 nanotube arrays are used as the working electrodes, a 6.6% conversion efficiency is obtained from an MWFF-DSSC(6). When the TiO2 nanotube arrays are treated in the niobium isopropoxide solution, the conversion efficiency is further raised to 9.1% and only suffers a 6.6% relative decrease even under a 180° bending.

In contrast to the main-stream strategy of growing convex nanostructures upward from the substrates and using them as cold electron sources, it is illustrated in this article that growing concave nanostructures downward into substrates also results in configurations suitable for field emission. Well-ordered TiO2 nanotube arrays were developed on the titanium foils in two-step anodizations. Simultaneously, arrays of sharp nanotips, which resembled the Spindt emitter arrays in appearance, also manifested themselves on the outmost surface of the foils. These nanotips were actually the remainder of the titanium foil surfaces that survived dissolution during anodization. Annealing transformed the amorphous TiO2 nanotips into anatase crystals and further to rutile. Despite the lack of an overall large aspect ratio, the sharpness of these nanotips guaranteed sufficiently strong electric fields for electron extraction. As a result, field emission was readily obtained from the TiO2 nanotip arrays, either before or after annealing. Photoelectron spectroscopy of the samples demonstrated that the majority of the emitted electrons came from local states in the band gap. Annealing at an appropriate temperature increased these local states and improved the field-emission capability of the samples.Keywords: annealing; anodization; field emission; TiO2 nanotip; X-ray photoelectron spectrum;

Tungsten oxide (W18O49) nanorods were grown by directly heating tungsten foils covered with potassium bromide (KBr) in low-pressure wet oxygen. The approach featured such advantages as convenient manipulation, low cost and rapid accessibility to high temperatures. A solid-liquid-solid (SLS) mechanism is believed to have dominated the growth process, in which the W18O49 nanorods segregated from eutectic droplets of potassium tungstate and tungsten oxide. The ultraviolet photoelectron spectroscopy (UPS) analysis disclosed that the valence band maximum (VBM) of these nanorods was approximately 9 eV below the vacuum level. The feasibility of using the such-fabricated nanorods as field emitters was tested and the related mechanism was also discussed.

Interaction between metallic zinc and Si wafer at moderately high temperatures was found to generate either concave wells or hollow pyramid-shaped protrusions on the (1 0 0) wafer surface. The formation of the wells is attributed to the dissolution of silicon at temperatures much lower than its melting point in the form of Zn–Si eutectic. When the temperature was raised, Zn atoms in the eutectic droplets evaporated and Si atoms in them precipitated accordingly. If a droplet contained plenty of Si atoms, these Si atoms could encounter each other during precipitation at the surface of the droplet and condense into a hollow pyramid-shaped protrusion.

Arrays of TiO2 nanotubes were fabricated by the anodization of Ti foils and then used in assembling dye-sensitized solar cells (DSSCs). The role of the morphologies of the TiO2 nanotubes in the photovoltaic performances of the DSSCs was studied in terms of the surface topography and the tube length. The necessity of removing the nanoporous films from the surface of the nanotube arrays for good DSSC performance has been demonstrated. Also, it has been shown that appropriately increasing the tube length was an effective measure for enhancing both the short-circuit current density and the conversion efficiency of the DSSCs.

A hierarchical nanostructure of wurtzite zinc oxide (ZnO) was synthesized by the thermal evaporation of Zn/ZnO powders. It had a hexagonal prism as the center and six clusters of nanorods as the branches. The formation of this ZnO nanostructure is attributed to three sequential catalyst-free reactions. First, a Zn prism was deposited on the substrate as the result of the condensation of Zn vapor; then, oxygen was introduced in an appropriate temperature range and the Zn prism was oxidized; finally, secondary ZnO nanorods grew on the ZnO surfaces of the central prism and the snowflake-shaped nanostructure was attained. This work has revealed the perspective of achieving hierarchical ZnO nanostructures through simple and controllable gas-phase reactions.

The multiwalled carbon nanotubes (MWCNTs) fabricated in a fast-heating chemical vapor deposition process were stretched using a microprobe system in a scanning electron microscope. The clean and straight MWCNTs survived multiple telescoping extension and eventually evolved into a gradually sharpened configuration. Some of the so-made telescopic MWCNTs were fabricated into atomic force microscopy probes. Their slender shape enabled them to image the deep holes on an anodic aluminum oxide template surface well. Also, their telescopic structure helped avoid mechanical instability and yield a good amplitude response curve.

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environment was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre-deposited transitional ZnO film on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10 μA/cm2 was achieved at the fields of 5.7 and 6.2 V/μm from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post-test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined.

Arrays of randomly oriented zinc oxide (ZnO) nanowires were fabricated on silicon wafers via a simple thermal evaporation method. During the fabrication, the temperature around the substrate was below 500 °C. The products were analyzed by conventional means and determined to be single crystals of wurtzite-type ZnO that grew along the c-axis. These nanowires were 10–100 nm in diameter and 10–100 μm in length, suggesting a possible high field enhancement factor. The dependence of the field emission current on the anode–cathode voltage (I–V behavior) of the ZnO nanowire arrays was measured in a lab-built ultrahigh vacuum system with a base pressure of 10−7 Pa. After surface cleaning by heat treatment, two characteristic electric fields, under which 10 μA/cm2 and 1 mA/cm2 current densities were extracted, were measured to be 4.0 and 4.7 V/μm, respectively. As observed with a transparent anode, emission occurred uniformly over the whole sample surface. A 72 h-long test on emission stability was performed under a constant voltage of 2.75 kV. The current dropped occasionally to approximately 80% of the initial value during the test owing to the poor adherence of the nanowires to the substrate. These preliminary results have shown the perspective of, as well as a major drawback to, a ZnO nanowire array being developed into a cold electron source to be used in future electronic devices.

Zinc oxide (ZnO) nanowires with an average diameter of 15 nm were grown using a vapor phase transport process. Field emission was achieved from these nanowires in spite of their random orientation. The electric field for the extraction of a 10 μA/cm2 current density was measured to range from 4.4 to 5.0 V/μm, and that for a 1 mA/cm2 current density from 7.6 to 8.7 V/μm, depending on whether the sample was submitted to a heat treatment. The results exhibit the potential application of ZnO nanowires as field emitters in future flat panel displays.