Electronic characteristics of fluorene/TiO2 molecular heterojunctions

The electronic properties of molecular junctions of the general type carbon/molecule/TiO2Au were examined as examples of "molecular heterojunctions" consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing Al2O3 instead of fluorene, and those with only the TiO2 layer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiO2 layer, and occurred in all three junction types studied. Conduction in PPFTiO2Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This "memory" effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to "dynamic doping" of the TiO2 layer by the imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.     

Construction of 2D-2D TiO2 nanosheet/layered WS2 heterojunctions with enhanced visible-light-responsive photocatalytic activity

Constructing nanocomposites that combine the advantages of composite materials, nanomaterials, and interfaces has been regarded as an important strategy to improve the photocatalytic activity of TiO₂. In this study, 2D-2D TiO₂ nanosheet/layered WS₂ (TNS/WS₂) heterojunctions were prepared via a hydrothermal method. The structure and morphology of the photocatalysts were systematically characterized. Layered WS₂ (～4 layers) was wrapped on the surface of TiO₂ nanosheets with a plate-to-plate stacked structure and connected with each other by W=O bonds. The as-prepared TNS/WS₂ heterojunctions showed higher photocatalytic activity for the degradation of RhB under visible-light irradiation, than pristine TiO₂ nanosheets and layered WS₂. The improvement of photocatalytic activity was primarily attributed to enhanced charge separation efficiency, which originated from the perfect 2D-2D nanointerfaces and intimate interfacial contacts between TiO₂ nanosheets and layered WS₂. Based on experimental results, a double-transfer photocatalytic mechanism for the TNS/WS₂ heterojunctions was proposed and discussed. This work provides new insights for synthesizing highly efficient and environmentally stable photocatalysts by engineering the surface heterojunctions.     

Depositional characteristics of metal coating on single-crystal TiO2 nanowires

Ultralong single-crystalline TiO(2) nanowires were prepared by a simple, low-cost solvothermal process. Silver nitrate, neodymium chloride, ceric nitrate, stannic chloride hydrate, and cadmium chloride were used as metal sources and deposited by reduction on the surface of TiO(2) nanowires. The composites were subsequently characterized by transmission electron microscopy, and their coverage was compared. The nature of the coatings on the TiO(2) nanowires varies from metal to metal. A novel approach on modified one-dimensional nanostructures with metal coating was developed, which has great potential applications in catalysts, sensors, and nanoscale devices.     

双层分等级TiO_2纳米线制备及其光伏性能研究

采用两步溶剂热反应制备了底层为分等级锐钛矿的TiO_2纳米线阵列,上层为分等级锐钛矿的TiO_2纳米线薄膜的双层结构电极.通过XRD和SEM对其组成和形貌进行了表征,并考察了纳米线薄膜对染料敏化太阳电池(DSSC)光伏性能的影响.实验结果表明,分等级锐钛矿的TiO_2纳米线作为DSSC的光阳极,光电转换效率为4.39%,其效率高于光滑的TiO_2纳米线光阳极电池效率(2.07%).     

TiO2纳米管与纳米线的制备及光电化学研究

利用钛箔表面沉积一层TiO2纳米粒子作为晶种,与NaOH反应,通过改变反应温度制备了TiO2纳米管与纳米线.在170℃,48 h的条件下合成了TiO2纳米管.在180℃时得到另一种一维的TiO2纳米线.并用XRD,SEM,SAED,EDS及HRTEM等分析手段对两种产物的成分、形貌、结构进行表征.对TiO2纳米管电极的光电化学性能进行了研究.结果表明,TiO2纳米管与纳米线为锐钛矿型和金红石型的混晶结构.TiO2纳米管单色光的光电转化效率达到10.38%.与钛酸盐纳米管相比,混晶结构TiO2纳米管显示出优良的光电转化性能.     

Rutile TiO2 nanowires on anatase TiO2 nanofibers: a branched heterostructured photocatalysts via interface-assisted fabrication approach

A water-dichloromethane interface-assisted hydrothermal method was employed to grow rutile TiO(2) nanowires (NWs) on electrospun anatase TiO(2) nanofibers (NFs), using highly reactive TiCl(4) as precursor. The water-dichloromethane interface inhibited the formation of rutile NWs in water phase, but promoted the selective radial growth of densely packed rutile NWs on anatase NFs to form a branched heterojunction. The density and length of rutile NWs could be readily controlled by varying reaction parameters. A formation mechanism for the branched heterojunction was proposed which involved (1) the entrapment of rutile precursor nanoparticles at water-dichloromethane interface, (2) the growth of rutile NWs on anatase NFs via Ostwald ripening through the scavengering of interface-entrapped rutile nanoparticles. The heterojunction formed at anatase NF and rutile NW enhanced the charge separation of both under ultraviolet excitation, as evidenced by photoluminescence and surface photovoltage spectra. The branched TiO(2) heterostructures showed higher photocatalytic activity in degradation of rodamine B dye solution than anatase NFs, and the mixture of anatase NFs, and P25 powders, which was discussed in terms of the synergistic effect of enhanced charge separation by anatase-rutile heterojunction, high activity of rutile NWs, and increased specific area of branched heterostructures.     

退火气氛对掺银TiO2薄膜结构和光催化性能的影响

本文采用sol-gel法制备了掺银的TiO2/glass纳米光催化薄膜,并分别在空气和真空条件下对薄膜进行退火处理, 结构和光催化性能的测试结果表明, 退火气氛对薄膜的结构和薄膜对染料溶液的光催化降解效率都有影响。经真空退火处理的TiO2及Ag-TiO2薄膜较空气中退火处理的同样薄膜的光催化性能都低, 但适量掺银的Ag-TiO2薄膜的光催化活性较TiO2有不同程度的增强。     

Direct synthesis of nanowires with anatase and TiO2-B structures at near ambient conditions

In this study, we present a new approach toward titanium oxide nanowires. In this approach, the growth formation of the wires sets in at a temperature as low as 40 degrees C under ambient pressure. Moreover, we provide evidence that nanowires with distinctive TiO2-anatase and TiO2-B structures can be directly produced without further thermal treatment using controlled reaction conditions.     

Fabrication of p/n heterojunctions by electrochemical deposition of Cu2O onto TiO2 nanotubes

This work describes the TiO₂ nanotubes formation by anodic oxidation under different experimental conditions and the following attempts to optimize the electrochemical deposition of Cu₂O within the nanotubular structure. The as-formed Cu₂O/TiO₂ interface is a p/n junction that could be used in various light-assisted devices. The effect of the F^– concentration, the polarization duration and the post-thermal treatment on the TiO₂ nanotubes is recalled. Then the electrochemical deposition of cuprous oxide is investigated by cyclic voltammetry and the influence of the pH on the morphology and crystal structure is studied. Finally, potential pulses were applied to optimize the Cu₂O film morphology in order to improve the nanotubes covering.     

Addition of TiO2 nanowires in different polymorphs for dye-sensitized solar cells

TiO₂ (B) and TiO₂ anatase nanowires were prepared at 150 °C for 120 h by a hydrothermal method followed by calcination in air at 400 °C for 2 h and at 700 °C for 2 h for TiO₂ (B) and TiO₂ anatase, respectively. Although dye-sensitized solar cells (DSC) with fully nanowire electrodes showed a rather low light-to-electricity conversion efficiency of 1.33 % for TiO₂ (B) and 2.42% for TiO₂ anatase, 10 wt % nanowire-dispersed electrodes in a P-25 TiO₂-nanoparticle matrix demonstrated improved efficiency of 6.17 % for TiO₂ (B) and 6.53% for TiO₂ anatase, these exceeding that of pure P-25 electrodes in this work (η=5.59%). The dominant mechanisms of the improvement at 10 wt% for the two different polymorphs are thought to be different, i.e., a light-scattering and film-thickness increment for the TiO₂ (B) system, whereas there is an improved conduction path through the matrix for the TiO₂ anatase system.      

SiO2 nanowires growing on hexagonally arranged circular patterns surrounded by TiO2 films

An effective approach has been demonstrated for the synthesis of novel composite architectures, SiO2 nanowires (NWs) growing on hexagonally arranged circular patterns surrounded by TiO2 films on Si substrate. First, a solution-dipping template strategy is used to create TiO2 films with hexagonally arranged pores on Au-coated Si substrate, resulting in hexagonally arranged circular patterns of catalysts surrounded by TiO2 films. Then the patterned catalysts guide the growth of SiO2 NWs with the original TiO2 films preserved, realizing the composite structures. Such composite architectures combine the photoluminescence (PL) properties of the two components, and also present more favorable PL property, laying a foundation for future advanced nano-optoelectronic devices.     

Facile synthesis and kinetic mechanism of Ag-doped TiO2/SiO2 nanoparticles for phenol degradation under visible light irradiation

Research on Chemical Intermediates - Ternary Ag–TiO2–SiO2 nanocomposite photocatalysts were synthesized via a facile sol–gel method and characterized by X-ray diffraction,...     

Synthesis and study of plasmon-induced carrier behavior at Ag/TiO2 nanowires

Nanocomposites of Ag/TiO(2) nanowires with enhanced photoelectrochemical performance have been prepared by a facile solvothermal synthesis of TiO(2) nanowires and subsequent photoreduction of Ag(+) ions to Ag nanoparticles (AgNPs) on the TiO(2) nanowires. The as-prepared nanocomposites exhibited significantly improved cathodic photocurrent responses under visible-light illumination, which is attributed to the local electric field enhancement of plasmon resonance effect near the TiO(2) surface rather than by the direct transfer of charge between the two materials. The visible-light-driven photocatalytic performance of these nanocomposites in the degradation of methylene blue dye was also studied, and the observed improvement in photocatalytic activity is associated with the extended light absorption range and efficient charge separation due to surface plasmon resonance effect of AgNPs.     

Photoelectrochemical study on charge transfer properties of TiO2-B nanowires with an application as humidity sensors

One-dimensional (1-D) TiO2-B nanowires have been synthesized via a facile solvothermal route. The morphology and crystalline structures of the nanowires were characterized by using powder X-ray diffraction, low/high-resolution transmission electron microscopy, and Brunauer-Emmett-Teller methods. It is important with the calcination treatment at 350 degrees C to maintain 1-D morphologies of the material in the form of single-crystalline TiO2-B nanowires. In addition, a simple method was used to study the photogenerated charge transfer and photoelectrochemical properties of the TiO2-B nanowires in comparison with commercial TiO2 P25 nanoparticles based on the experimental data from the electric field-effected photocurrent action spectrum and Mott-Schottky measurements. It was revealed that TiO2-B nanostructures played an important role in the photoelectrochemical processes. The synthetic TiO2-B nanowire electrode exhibited unique electronic properties, e.g., favorable charge-transfer ability, negative-shifted appearing flat-band potential, existence of abundant surface states or oxygen vacancies, and high-level dopant density. Moreover, the obtained TiO2-B nanowires were found to display excellent humidity sensing abilities as functional materials in the humidity sensor application. With relative humidity increased from 5% to 95%, about one and half orders of magnitude change in resistance was observed in the TiO2-B nanowire-based surface-type humidity sensors.     

Controlled synthesis of Ag/TiO2 core-shell nanowires with smooth and bristled surfaces via a one-step solution route

Ag/TiO2 core-shell nanowires were synthesized via a one-step solution method without using a template. Interestingly, the shell morphologies can be controlled to be smooth or bristled by altering the reaction temperature. Moreover, the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), energy-dispersive X-ray analysis (EDS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the resultant Ag/TiO2 core-shell nanowires. Moreover, the absorption peaks of our samples are significantly red-shifted compared with those of the uncoated pure silver nanowires, indicating that interaction between the core and shell occurred. On the basis of the experimental results, we proposed a template-induced Oswald ripening mechanism to explain the formation of the Ag/TiO2 core-shell nanowires.     

Maximizing the synergistic effect on accelerated photoelectron transfer of TiO2 by double Z-scheme heterojunctions and specific oxygen vacancies

Photocatalysis is an ideal technology for environmental applications, but its efficiency is severely limited by slow kinetics and low efficiency of carrier separation. Herein, a photocatalyst (TiO₂–Bi₂Ti₂O₇-600 °C) with homotypic growth possessing double Z-scheme heterojunctions was successfully synthesized by the growth of mixed crystalline phase TiO₂ on the surface of Bi₂Ti₂O₇. The carrier separation efficiency of the heterojunctions was enhanced by the photo-ferroelectricity of Bi₂Ti₂O_7. Simultaneously, the interface-fused TiO₂/Bi₂Ti₂O₇ provided stability for carrier transport between heterojunctions. Modern instrumental characterization confirmed that oxygen vacancies mainly exist in Bi–O structural units provided by Bi₂Ti₂O₇ and that the carrier separation efficiency of the double Z-scheme heterojunctions was significantly higher than that of the single Z-scheme heterojunctions. Density-of-states calculations based on the first principle confirmed that the carrier separation efficiency was higher when the oxygen vacancies presented in the Bi–O structural units than they presented in the Ti–O structural units. TiO₂–Bi₂Ti₂O₇-600 °C could accomplish the complete degradation of Rh–B (10 mg/L) in aqueous environment within 80 min, instead of only contributing to the destruction of conjugated chromogenic groups in Rh–B. This photocatalyst with stable structure, multiple carrier transport channels, and sufficient oxygen vacancies enables the photoelectrons to concentrate on the confinement effect, opening a novel avenue for the design strategy of new-generation photocatalysts in environmental wastewater applications.     

Characteristics of Ga and Ag-doped ZnO-based nanowires for an ethanol gas sensor prepared by hot-walled pulsed laser deposition

Pure ZnO and Ga (3 % w/w) and Ag (3 % w/w)-doped ZnO nanowires (NWs) have been grown by use of the hot-walled pulse laser deposition technique. The doping characteristics of Ga and Ag in ZnO NWs were analyzed by use of photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) and the results were compared with those for pure ZnO NWs. We also fabricated gas sensors by use of pure ZnO and Ga and Ag-doped ZnO NWs. Among the NW sensors, the Ag-doped NW sensor was most sensitive. We synthesized the NWs on sapphire substrates under different conditions, for example temperature, time, gas flow, and distance between target and substrate. The diameter and length of NWs were <100 nm and several microns, respectively. To analyze the effect of Ag doping on ZnO NWs, we investigated the near band edge emission by use of low-temperature PL and XPS. Significant changes in resistance and sensitivity were observed. When the sensors were used at 300 °C for detection of 1 ppm ethanol vapor, the sensitivity of the pure ZnO and the Ga and Ag-doped ZnO NW gas sensors was 97, 48, and 203 %, respectively.     

Substitution mechanism and structural study of Ag-doped LiCu2O2

Plate-like stoichiometric crystals of Ag-doped LiCu₂O₂ have been grown by slowly cooling Li₂CO₃·4(1 – x)CuO·4xAgNO₃ (0 ≤ x ≤ 0.5) melts. X-ray single crystal diffraction has shown that the crystals are isostructural with LiCu₂O₂ and contain around 5 at % Ag (relative to the Cu atoms). The addition of silver to lithium cuprate crystals significantly increases their electrical conductivity but has little effect on the temperature behavior of their magnetic moment. The possible substitution mechanism is determined which supports Ag⁺ ↔ Cu⁺, rather than Ag⁺ ↔ Li⁺ in the Ag-doped LiCu₂O₂ crystals.