Fabrication of transparent p-n junction composed of heteroepitaxially grown p-Li<Subscript>0.15</Subscript>Ni<Subscript>0.85</Subscript>O and n-ZnO films for UV-detector applications

Thin films of high-quality p-type Li_0.15Ni_0.85O (LNO) and n-type ZnO were heteroepitaxially grown on MgO(111) substrate by pulsed laser deposition technique to form transparent wide bandgap heterojunctions. The epitaxial nature of this p-LNO/n-ZnO/MgO heterojunction was confirmed to be (111)LNO||(0001)ZnO||(111)MgO (out-of-plane) and (002)LNO||(1002)ZnO||(002)MgO (in-plane) by X-ray diffraction. Optical transmittance spectrum and I–V characteristics were obtained at room temperature. The heterojunction exhibits reasonable optical transmittance of 50–60% on average in the whole infrared and visible region, and highly asymmetric electrical rectification with a turn-on voltage of about 1.0 V and a small leakage current. The highest photoresponsivity for a wavelength of 350 nm is 3.4×10³ V/W when the junction is irradiated under 5 μW UV illumination. The spectral response peak is obtained in the UV region and a reasonable large responsivity is shown for this p-LNO/n-ZnO/MgO heterojunction, which suggests the possibility of an inexpensive transparent oxide UV detector in a wide variety of electronics applications. PACS 68.55.Jk; 81.05.Dz; 81.15.Fg     

The structure,composition, and dimensions of TiO<Subscript>2</Subscript> and ZnO nanomaterials in commercial sunscreens

TiO₂ and ZnO nanomaterials are widely used to block ultraviolet radiation in many skin care products, yet product labels do not specify their dimensions, shape, or composition. The absence of this basic information creates a data gap for both researchers and consumers alike. Here, we investigate the structural similarity of pigments derived from actual sunscreen products to nanocrystals which have been the subject of intense scrutiny in the nanotoxicity literature. TiO₂ and ZnO particles were isolated from eight out of nine commercial suncare products using three extraction methods. Their dimension, shape, crystal phase, surface area, and elemental composition were examined using transmission and scanning electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller (BET) specific surface area analysis, energy dispersive X-ray and inductively coupled plasma optical emission spectroscopy. TiO₂ pigments were generally rutile nanocrystals (dimensions ~25 nm) with needle-like or near-spherical shapes. ZnO pigments were wurtzite rods with a short axes less than 40 nm and longer dimensions often in excess of 100 nm. We identify two commercial sources of TiO₂ and ZnO nanocrystals whose physical and chemical features are similar to the pigments found in sunscreens. These particular materials would be effective surrogates for the commercial product and could be used in studies of the health and environmental impacts of engineered nanomaterials contained in sunscreens.     

TiO<Subscript>2</Subscript> Nanorod Arrays Sensitized with CdS Quantum Dots for Solar Cell Applications: Effects of Rod Geometry on Photoelectrochemical Performance

CdS quantum dot (QD) sensitized TiO₂ nanorod array (NRA) film electrodes with different rod geometries were fabricated via a solvothermal route followed by a sequentialchemical bath deposition (S-CBD) process. By controlling the solution growth conditions, the rod geometries, especially the tip structures, of the TiO₂ NRAs were tuned. The results indicated that the vertically aligned hierarchical NRAs possessed conically shaped tip geometry, which was favorable for film electrodes due to the reduced reflectance, enhanced light harvesting, fast charge-carrier separation and transfer, suppression of carrier recombination, sufficient electrolyte penetration and subsequent efficient QD assembly. CdS QD sensitized TiO₂ NRA film electrodes with tapered tips exhibited an enhanced photoelectrochemical (PEC) performance, a photocurrent intensity of 5.13 mA/cm² at a potential of 0 V vs. saturated calomel electrode, an open-circuit potential of −0.68 V vs. saturated calomel electrode and an incident photon to current conversion efficiency (IPCE) of 22% in the visible-light region from 400 to 500 nm. The effects of rod geometry on the optical absorption, reflectance, hydrophilic properties and PEC performance of bare TiO₂ and CdS QD sensitized TiO₂ NRA film electrodes were investigated. The mechanism of charge-carrier generation and transfer in these CdS QD sensitized solar cells based on vertically aligned TiO₂ nanorods is discussed.     

Photoactivity passivation of TiO<Subscript>2</Subscript> nanoparticles using molecular layer deposited (MLD) polymer films

Pigment-grade anatase TiO₂ particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO₂ particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al₂O₃ film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO₂ particles.     

Pure and Sn-, Ga- and Mn-doped ZnO gas sensors working at different temperatures for formaldehyde,humidity, NH<Subscript>3</Subscript>, toluene and CO

ZnO and Sn-, Ga- and Mn-doped ZnO nanoparticles were prepared by a coprecipitation method, and characterized by scanning electron microscopy (SEM), energy dispersive spectra (EDS), X-ray diffraction (XRD) and Raman spectra. The gas sensing properties were studied using formaldehyde, relative humidity, NH₃, toluene and CO as the probes. The results show that all particles have wurtzite ZnO phase, though Sn–ZnO has a relatively smaller particle (and crystallite) size than the other three samples. Gas sensing property tests reveal that the temperature where the gas sensing maximum is gained (T _M) is changed by different dopants: Sn–ZnO and Mn–ZnO have relatively lower T _M (∼100°C lower) compared with that of pure ZnO, while Ga–ZnO has the same T _M as pure ZnO except in CO sensing. Thermoluminescence (TL) spectra were used to investigate the mechanism of T _M change. The peak positions of Ga–ZnO and ZnO are the same at 300–350°C, while that of Sn–ZnO shifts to 250–300°C, which might contribute to the same T _M of Ga–ZnO and pure ZnO and relatively lower T _M of Sn–ZnO. In the case of Mn–ZnO, the luminescence emission is evidently limited by its black color.     

Preparation of TiO<Subscript>2</Subscript>-coated LiCo<Subscript>1/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by electrostatic self-assembly method and its properties

A precursor of TiO₂–LiCo_1/3Ni_1/3Mn_1/3O₂ was prepared by electrostatic self-assembly method. The final product was obtained by heating the precursor at 400–450 °C for 4–6 h in air. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical tests were used to examine the structural, morphology, elementary valence, and electrochemical characteristics. XRD indicated that the TiO₂-coated material can be indexed by α-NaFeO₂ layered structure, which belongs to hexagonal-type space group R3m. XPS results confirmed the existence of TiO₂ compound on the surface of the coated sample. The SEM image showed that the material had spherically porous morphology with the uniform size about 6 μm. The initial charge–discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ material was 168.8/160.0 mAh/g. After 60 cycles, the discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ sample was 147.0 mAh/g, and the coulombic efficiency was 94.0%. Compared with the uncoated sample, the electrochemical performance of TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ was improved.     

Light intensity and temperature dependence on performance of a photoelectrochemical cells of ITO/TiO<Subscript>2</Subscript>/PVC-LiClO<Subscript>4</Subscript>/graphite

The photovoltaic characteristics of a photoelectrochemical cell of ITO/TiO₂/PVC-LiClO₄/graphite are reported. This paper is concerned with the influence of light intensity and temperature on short-circuit current density, J_sc and open-circuit voltage, V_oc of the device. The photoelectrochemical cell material was a screen-printed layer of titanium dioxide onto an ITO-covered glass substrate, which was used as a working electrode of the cell. The solid electrolyte was polivinylchloride-lithium perchlorate. The graphite film serves as a counter electrode of the cell. The current density–voltage characteristics of the device under an illumination of 20, 40, 60, 80 and 100 mW cm⁻² light from a tungsten halogen lamp were recorded at 40 °C as well as under an illumination of 100 mW cm⁻² at 30, 35, 40, 45 and 50 °C, respectively. It was found that the short-circuit current density, J_sc of the device increases with both light intensity and temperature. The J_sc obtained at 100 mW cm⁻² was 1.0 μAcm⁻² and that at 50 °C was 0.7 μAcm⁻².     

Improved efficiency of dye-sensitized solar cells applied with nanostructured N–F doped TiO2 electrode

Dye-sensitized solar cells (DSSCs) were fabricated with N–F-doped TiO₂ electrodes. The XRD pattern of the N–F-doped TiO₂ is almost the same as that of pure TiO₂, showing that N and F doping has little influence on the formation of anatase titania. The influence of dopant N and F on band energetics and photoelectrochemical properties of nanostructured TiO₂ electrodes were investigated. Compared with pure TiO₂ electrodes, the E_fb of N–F-doped TiO₂ electrodes shifted a little in electrolytes containing LiClO₄. However the total trap densities were remarkably decreased as TiO₂ electrodes were doped with N and F. Finally the N–F-doped TiO₂ electrodes were sensitized with N3 and their photoelectrochemical properties were studied. Experimental results showed that the photoelectric conversion efficiency of N3 sensitized N–F-doped TiO₂ electrodes was 8.61% under irradiation of 100 mW cm^?2 white light, about 17.1% higher than that of a pure TiO₂ electrode.     

Self-assembly of rutile (α-TiO<Subscript>2</Subscript>) nanoclusters into nanorods in microemulsions at low temperature and their photocatalytic performance

A novel method – inverse microemulsion has been developed not only for synthesizing low cost TiO₂ nanocrystals but also for the first time making these nanocrystals self-assemble into various nanoparticles at 85°C. By variation of the volume ratios of oil to water in reverse microemulsions, the morphologies of obtained samples turned from nanoclusters to nanospherules, then grew into nanodumbbells, and became nanorods at last. It could be observed by transmission electron microscope (TEM) directly. The resulting materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and high-resolution transmission electron microscope (HRTEM). The photocatalytic activity of TiO₂ was tested with photodegradation of Methyl Orange (MO) in water. The catalyst consisting of nanorods showed the highest photocatalytic activity, which is due to its large surface area. Furthermore, the mechanism of self-assembly of TiO₂ nanocrystals was discussed in detail.     

Temperature dependence of the lowest frequency <Emphasis Type="BoldItalic">E</Emphasis><Subscript><Emphasis Type="Bold">g</Emphasis></Subscript> Raman mode in laser-synthesized anatase TiO<Subscript>2</Subscript> nanopowder

Nanosized titanium dioxide (TiO₂) powder was prepared by a laser-induced pyrolysis. Specific surface area of the as-grown powder measured by BET method was 109 m²/g. The grain size (14.5 nm) estimated from these data coincides well with the crystallite size (12.3 nm) determined by XRD measurements. The average grain size (∼35 nm) obtained from the subsequent SEM measurements refers to considerable agglomeration of nanoparticles. Raman spectroscopy has been used to investigate the structural properties of TiO₂ nanopowder and its anatase structure is confirmed. The blueshift and broadening of the lowest frequency E_g Raman mode at temperature range ∼25–550 K have been analyzed using a phonon-confinement model. Dominant influence of the strong anharmonic effect at higher temperatures was demonstrated. PACS 81.07.Wx; 78.30.-j; 63.22.+m     

Synthesis and nanostructural investigation of TiO<Subscript>2</Subscript> nanorods doped by SiO<Subscript>2</Subscript>

TiO₂ Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200°C using TiO₂ powder as the source material. In the present work, the synthesis of TiO₂ nanorods in anatase, rutile and Ti₇O₁₃ phases and synthesis of TiO₂ nanorods by incorporating SiO₂ dopant, using the sol–gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO₂ nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti–O–Si bonds having four-fold coordination with oxygen in SiO^4 −.     

Synthesis,characterization of Cr-doped TiO<Subscript>2</Subscript> nanotubes with high photocatalytic activity

Cr-doped TiO₂ nanotubes (Cr/TiO₂ NTs) with high photocatalytic activity were prepared by the combination of sol–gel process with hydrothermal treatment. XRD, TEM and UV–vis DRS techniques were employed for microstructural characterization. TEM images show that Cr/TiO₂ NTs are in good tubular structure and have diameter of about 10 nm. The Cr doping induces the shift of the absorption edge to the visible light range and the narrowing of the band gap. The photocatalytic experiment reveals that the photocatalytic performance of TiO₂ NTs can be improved by the doping of chromium ions.     

Rectifying and photovoltage characteristics of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript>/Nb-doped SrTiO<Subscript>3</Subscript> heterojunction

A heterojunction was fabricated by growing a layer of Bi₂Sr₂Co₂O _y thin film on the 0.7 wt% Nb-doped SrTiO₃ substrate. Such heterojunction showed good rectifying characteristics over a wide temperature range, and its transport mechanism under the forward bias can be attributed to a space charge limited conduction mechanism via defects near the interface of the heterojunction. Photovoltaic properties of the heterojunction were studied by using both continuous-wave and pulsed irradiations and the results can be well explained by the photovoltaic effect of a p–n junction.     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Preparation of nanosized Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> using different content of starting materials and their photoactivities

A series of nanosized Bi₂WO₆ catalysts was synthesized using various starting materials, and they were characterized by X-ray diffractometry, transmission electron microscopy, and diffuse reflectance spectroscopy. Rhodamine-B (RhB) photodegradation in aqueous medium was employed as a probe reaction to test the photoactivity of the as-prepared samples. Dependence of the photocatalytic activities on different contents of the starting materials was examined under visible irradiation (λ > 400 nm). The sample prepared in the following conditions: reaction time 24 h, the pH of the solution 7, the Bi³⁺ amount in the start precipitates 5 mmol — exhibited the highest photochemical activity when the hydrothermal temperature was settled at 180°C. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 243–249, Martch–April, 2009.     

Photoactive materials prepared by homogeneous hydrolysis with thioacetamide: Part 2—TiO2/ZnO nanocomposites

Photocatalytic active titanium dioxide (TiO₂)/zinc oxide (ZnO) composite was prepared by homogeneous hydrolysis of a mixture of titanium oxo-sulphate and zinc sulphate in aqueous solutions with thioacetamide and subsequent annealing at the temperature of 600 °C. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission microscopy (HRTEM). Nitrogen adsorption-desorption was used for surface area (Brunauer-Emmett-Teller—BET) and porosity determination. The photoactivity of the prepared TiO₂/ZnO samples was assessed by the photocatalytic decomposition of Orange II dye in an aqueous slurry under irradiation of 254 and 365 nm wavelengths. Under the same conditions, the photocatalytic activity of a commercially available photocatalyst (Degussa P25), the pure anatase TiO₂ nanoparticles and cubic ZnO were examined.     

Efficient visible light-induced photoelectrocatalytic degradation of rhodamine B by polyaniline-sensitized TiO<Subscript>2</Subscript> nanotube arrays

Self-organized anodic TiO₂ nanotube arrays were sensitized with polyaniline by a simple electrodeposite method. The morphological and structural properties studied by scanning electron microscopy and fourier transform infrared spectroscopy reveal the successful deposition of polyaniline on the nanotube arrays. The polyaniline-sensitized TiO₂ nanotube arrays exhibit a distinguishable red shift on the absorption spectrum. Electrochemical impedance investigation attested to a significant improvement of the interfacial electron-transfer kinetics for promoted electron–hole effective separation. The as-prepared samples showed a high efficiency for the photoelectrocatalytic degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The enhanced photoelectrocatalytic activity could be attributed to the extended absorption in the visible-light region by the polyaniline and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the polyaniline/TiO₂ nanotube arrays interface.     

Photoluminescence in anatase titanium dioxide nanocrystals

Titanium dioxide (TiO₂) nanocrystals were prepared by a hydrolysis process of tetrabutyl titanate. X-ray diffraction and Raman scattering showed that the as-prepared TiO₂ nanocrystals have anatase structure of TiO₂, and that the monophase anatase nanocrystals can be achieved through a series of annealing treatments below 650 °C. We measured photoluminescence (PL) spectra of the TiO₂ nanocrystals. Under 2.41–2.71 eV laser irradiation, the TiO₂ nanocrystals displayed strong visible light emission with maxima of 2.15–2.29 eV even at excitation power as low as 0.06 W/cm². To identify the PL mechanism in the TiO₂ nanocrystals, the dependences of the PL intensity on excitation power and irradiation time were investigated. The experimental results indicated that the radiative recombination is mediated by localized levels related to surface defects residing in TiO₂ nanocrystallites. Received: 7 April 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

Effect of ethylene carbonate plasticizer and TiO<Subscript>2</Subscript> nanoparticles on 49% poly(methyl methacrylate) grafted natural rubber-based polymer electrolyte

The effect of plasticizer and TiO₂ nanoparticles on the conductivity, chemical interaction and surface morphology of polymer electrolyte of MG49–EC–LiClO₄–TiO₂ has been investigated. The electrolyte films were successfully prepared by solution casting technique. The ceramic filler, TiO₂, was synthesized in situ by sol-gel process and was added into the MG49–EC–LiClO₄ electrolyte system. Alternating current electrochemical impedance spectroscopy was employed to investigate the ionic conductivity of the electrolyte films at 25 °C, and the analysis showed that the addition of TiO₂ filler and ethylene carbonate (EC) plasticizer has increased the ionic conductivity of the electrolyte up to its optimum level. The highest conductivity of 1.1 × 10⁻³ Scm⁻¹ was obtained at 30 wt.% of EC. Fourier transform infrared spectroscopy measurement was employed to study the interactions between lithium ions and oxygen atoms that occurred at carbonyl (C=O) and ether (C-O-C) groups. The scanning electron microscopy micrograph shows that the electrolyte with 30 wt.% EC posses the smoothest surface for which the highest conductivity was obtained.