Mesoporous TiO2 nano-spheres: Electrospray combined sol-gel fabrication and application to organic phosphorus degradation

In this work, electrospray technique combined sol-gel method was used to prepare porous TiO2 film. X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD), and Scanning Electron Microscope (SEM) analyses were conducted to examine the chemical composition, phase structure, and surface morphology of the sprayed TiO2 film. After calcined at 450℃ in air atmosphere for 2 h, mesoporous TiO2 nano-spheres clusters were formed on the surface of silicon wafer and the average size of nano-spheres was 250 nm. Ti presented as Ti 4+ oxidation state in TiO2 film, and the TiO2 film exhibited the anatase phase. The sprayed porous TiO2 films were employed as photocatalyst to degrade organic phosphorus in water samples. Compared with the TiO2 film prepared by Sol-Gel spin-coating method, the porous TiO2 film deposited by electrospray combined sol-gel method showed higher photocatalytic activity.     

Synthesis of Sn doped ZnO/TiO2 nanocomposite film and their application to H2 gas sensing properties

Tin doped Zinc oxide/Titanium oxide nanocomposite (TZO/TiO₂) was prepared by two methods: TiO₂ nanotube (Nt) arrays are grown by anodic oxidation of titanium foil and TZO films was deposited on the TiO₂ Nt obtained by hydrothermal process. The morphological characteristics and structures of ZnO/TiO₂ and TZO/TiO₂ were examined by (scanning elecron miscroscopy) SEM, (X rays diffraction) XRD and (energy dispersive spectroscopy) EDS analysis. The diameter of TiO₂ Nts was ranged from 40 nm to 90 nm with wall thicknesses of approximately 10 nm. The anatase structure of Titania, the hexagonal Zincite crystal of zinc oxide and tetragonal structure of tin oxide were identified by XRD. EDS analysis revealed the presence of O, Zn, Ti and Sn elements in the obtained deposits.These nanocomposites have been used as active layer in hydrogen gas sensing application. The hydrogen sensing characteristics of the sensor was analyzed by measuring the sensor responses in the temperature of 100 °C and 160 °C. The highest gas response is approximately 1.48 at 160 °C.The sensing mechanism of the nanocomposite sensor was explained in terms of H₂ chimisorption on the highly active nanotube surface.     

Large-area,high-quality self-assembly electron transport layer for organic optoelectronic devices

We propose a self-assembly method for forming large-area high-quality solution-processed titanium oxide (TiO₂) films as efficient electron transport layer for organic solar cells. The self-assembled solution-processed TiO₂ layers are highly ordered and significantly improved in surface morphology over commonly-used spin-coating process resulting in better charge collection and significant material saving. When incorporated into polymer solar cells, the TiO₂ device shows enhanced performance. Furthermore, we demonstrate the TiO₂ can form large-area films, and achieve very uniform and improved device performances. Consequently, the self-assembled TiO₂ films can be efficient and low-cost electron transport layer potentially for large-area organic optoelectronics.     

Sono‐ and Photochemical Routes for the Formation of Highly Dispersed Gold Nanoclusters in Mesoporous Titania Films

A sono‐ and photochemical approach has been developed to incorporate highly dispersed gold nanoclusters into mesoporous TiO₂ films. The first step involves the sonication of a TiO₂ film immersed in a gold chloride solution. This effectively removes the air trapped in the porous film matrix and drives the gold chloride into the pore channels, leading to a homogeneous adsorption of ionic Au in the TiO₂ mesoporous matrix. The second step takes advantage of the photocatalytic property of TiO₂ to reduce the adsorbed Au ions to Au⁰. As the gold nanoclusters thus produced are stabilized by the TiO₂ mesonetwork, no organic capping molecules are required. Highly dispersed Au/TiO₂ nanoheterojunction arrays can be obtained using this interesting approach.     

Response time and mechanism of Pd modified TiO2 gas sensor

In this work, gas response properties of Pd modified TiO₂ sensing films are discussed when exposed to H₂ and O₂. TiO₂ films are surface modified in PdCl₂-containing solution by the dipping method and treated for different treatment times to get different surface states. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Kröger–Vink defect theory are used to characterize the sensing films. The gas response properties indicate that the sensor response time which related to the rate of change of sensor resistance is affected by the activation energy (E). In particular, the sensor treated at 900 °C for 2 h exhibits a response time of about 20–240 ms when exposed to H₂ and 40–130 ms when exposed to O₂ at 500–800 °C.     

ZnO-Coated TiO2 Nanotube Arrays for a Photoelectrode in Dye-Sensitized Solar Cells

In dye-sensitized solar cells, highly ordered TiO₂ nanotube arrays as a photoelectrode have higher charge collection efficiencies than a nanoparticle-based structure due to their faster charge percolation and slower recombination of electrons. Highly ordered TiO₂ nanotube arrays were grown by anodic oxidation of 0.5-mm-thick titanium foil. To increase the conversion efficiency of dye-sensitized solar cells with TiO₂ nanotube arrays, the surface of the TiO₂ nanotube arrays was modified by zinc oxide thin films. The ZnO thin film was formed by atomic layer deposition. The thin film was conformal on the inner and outer walls of TiO₂ nanotube arrays. ZnO thin film improved the short circuit current (J _sc) and open circuit voltage (V _oc) due to increasing specific surface area from particulates of ZnO thin film and increasing the surface charge induced from the isoelectric point. The power conversion efficiency of dye-sensitized solar cells with ZnO thin film on 4.5-μm-thick TiO₂ nanotube arrays was 1.43%. Microstructure and phase were observed by scanning electron microscopy, x-ray diffractometry, and transmission electron microscopy.     

Inorganic Macroporous Films from Preformed Nanoparticles and Membrane Templates: Synthesis and Investigation of Photocatalytic and Photoelectrochemical Properties

Colloidal dispersions of titania, zirconia, tin oxide, indium oxide, and ceria have been successfully used to impregnate membrane templates and form the respective metal oxide (MO) porous films. The use of alumina and iron oxide sols in the same procedure, however, resulted in compact structures. By mixing different nanoparticle solutions before impregnation, final inorganic films containing two metal oxides, of variable metal oxide ratios, were obtained. The porous inorganic materials were analyzed in terms of surface area, pore size, film thickness, and crystallinity. The mechanism of nanoparticle infiltration and particle adsorption to the template walls is proposed based on the stability of the inorganic film and a study of the influence of either the sol concentration or washing times on the amount of inorganic substance incorporated in the hybrid material. The photocatalytic decomposition of an organic pollutant, 2‐chlorophenol, was demonstrated for the porous titania material along with the structures containing mixtures of titania with zirconia, indium oxide, and tin oxide. A ratio of 9:1 TiO₂/MO gave the highest photocatalytic activity, which was higher than the activity of Degussa P25 for the TiO₂/In₂O₃ and TiO₂/SnO₂ systems under the same conditions. The titania films have also been attached to substrates—glass or indium tin oxide (ITO) surfaces—and the photoelectrochemical properties of the porous film attained. A comparison with a spin‐coated titania film (prepared from the same colloidal dispersion) showed that the structured porous inorganic film has two times the photoelectrochemical efficiency as the spin‐coated film.     

Surface Control of Planarization Layer on Embossed Glass for Light Extraction in OLEDs

We developed a highly refractive index planarization layer showing a very smooth surface for organic light‐emitting diode (OLED) light extraction, and we successfully prepared a highly efficient white OLED device with an embossed nano‐structure and highly refractive index planarization layers. White OLEDs act as an internal out‐coupling layer. We used a spin‐coating method and two types of TiO₂ solutions for a planarization of the embossed nano‐structure on a glass substrate. The first TiO₂ solution was TiO₂ sol, which consists of TiO₂ colloidal particles in an acidic aqueous solution and several organic additives. The second solution was an organic and inorganic hybrid solution of TiO₂. The surface roughness (R_a) and refractive index of the TiO₂ planarization films on a flat glass were 0.4 nm and 2.0 at 550 nm, respectively. The J–V characteristics of the OLED including the embossed nano‐structure and the TiO₂ planarization film were almost the same as those of an OLED with a flat glass, and the luminous efficacy of the aforementioned OLED was enhanced by 34% compared to that of an OLED with a flat glass.     

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

Rational Design of Highly Porous SnO2 Nanotubes Functionalized with Biomimetic Nanocatalysts for Direct Observation of Simulated Diabetes

1D metal‐oxide nanotube (NT) structures have attracted considerable attention for applications in chemical sensors due to their high surface area and unique chemical and physical properties. Moreover, bimodal pores, i.e., meso‐ and macro‐sized pores, which are formed on the shell of NTs, can further facilitate gas penetration into the sensing layers, leading to much improved sensing properties. However, thin‐walled NTs with bimodal pore distribution have been rarely fabricated due to the limitations of synthetic methods. Here, Ostwald ripening‐driven electrospinning combined with sacrificial templating route using polystyrene (PS) colloid and bioinspired protein is firstly proposed for producing both bi‐modal pores and catalyst‐loaded thin‐walled SnO₂ NTs. Homogeneous catalyst loading on porous SnO₂ NTs is achieved by the protein cage that contains catalysts and PS colloids and protein shells are thermally decomposed during calcination of electrospun fibers, resulting in the creation of dual‐sized pores on NTs. Pt catalyst decorated porous SnO₂ NTs (Pt‐PS_SnO₂ NTs) show exceptionally high acetone gas response, superior selectivity against other interfering gases, and very low limit of detection (10 ppb) to simulated diabetic acetone molecules. More importantly, sensor arrays assembled with developed porous SnO₂ NTs enable the direct distinction between the simulated diabetic breath and normal breath from healthy people.     

Preparation of nanocomposite thin films by sol-gel and photochemical metal–organic deposition of Ba0.5Sr0.5TiO3 and PbZr0.48Ti0.52O3

The process for generating nanocomposite films constructed from alternate thin film layers of Ba_0.5Sr_0.5TiO₃ and PbZr_0.48Ti_0.52O₃ by photochemical metal–organic deposition and sol-gel has been investigated. By spin coating the appropriate metal organic precursors to Ba_0.5 Sr_0.5TiO₃ followed by photolysis a single layer of amorphous Ba_0.5Sr_0.5TiO₃, is produced. Subsequent spin coating of the appropriate metal organic precursors of PbZr_0.48Ti_0.52O₃ and photolysis led to the formation of a layer of amorphous PbZr_0.48Ti_0.52O₃. By repeating this procedure a material composed of alternating layers of BST and PZT was constructed. In an analogous process alternate coating and heating of the appropriate sol-gel precursors was used to make similar structures. These nanocomposite materials were formed as amorphous layered materials but could be made to crystallize by heat treatment. Heat treatment resulted in crystallization of the films although the resultant diffraction pattern was dependent upon the thickness of the layers. X-ray diffraction patterns of both BST and PZT were apparent in crystalline films formed from layers of more than 25 nm in thickness. The crystallization of films formed from layers less than 25 nm in thickness showed only a lattice constant intermediate between that expected for BST and PST consistent with the interdiffusion of these components. Atomic force microscopy indicated that the amorphous nanocomposite films were near featureless while the crystalline films had a much higher surface roughness.     

Effect of sputtering power on optical properties of prepared TiO2 thin films by thermal oxidation of sputtered Ti layers

In this research, TiO₂ thin films prepared via thermal oxidation of Ti layers were deposited by RF-magnetron sputtering method at three different sputtering powers. The effects of sputtering power on structure, surface and optical properties of TiO₂ thin films grown on glass substrate were studied by X-ray diffraction (XRD), atomic force microscopic (AFM) and UV–visible spectrophotometer. The results reveal that, the structure of layers is changed from amorphous to crystalline at anatase phase by thermal oxidation of deposited Ti layers and rutile phase is formed when sputtering power is increased. The optical parameters: absorption coefficient, dielectric constants, extinction coefficient, refractive index, optical conductivity and dissipation factor are decreased with increase in sputtering power, but increase in optical band gap is observed. The roughness of thin films surface is affected by changes in sputtering power which is obtained by AFM images.     

Improvement of the morphological and electrical characteristics of Al3+, Fe3+ and Bi3+-doped TiO2 compact thin films and their incorporation into hybrid solar cells

Compact titanium dioxide (TiO₂) hole-blocking layers are commonly employed in organic-inorganic solar cells, however, their importance in terms of morphology and electrical conductivity is frequently overlooked in this novel type of solar energy converters. In this work, single TiO₂ thin films were prepared by a sol-gel method, observing large pinhole densities and low electrical conductivities. As a means to solve the morphological issue, the deposition of a second TiO₂ film was explored, which effectively reduced the surface irregularities obtained in single oxide films. The limited electrical conductivity of single and double layers was successfully increased by doping with the trivalent cations of aluminum, iron (III) and bismuth (III), observing an increase from 2.48 × 10⁻⁸ S/cm for an undoped TiO₂ double layer to 51.41 × 10⁻⁸ S/cm for a Fe³⁺-doped TiO₂ double layer. The incorporation of these hole blocking layers in hybrid solar cells led to further insights in the important role of trivalent doping cations in the transference and transport of electrons on the surface and in the bulk of the prepared TiO₂ compact films.     

High‐speed atmospheric atomic layer deposition of ultra thin amorphous TiO2 blocking layers at 100 °C for inverted bulk heterojunction solar cells

Ultrafast, spatial atmospheric atomic layer deposition, which does not involve vacuum steps and is compatible with roll‐to‐roll processing, is used to grow high quality TiO₂ blocking layers for organic solar cells. Dense, uniform thin TiO₂ films are grown at temperatures as low as 100 °C in only 37 s (~20 nm/min growth rate). Incorporation of these films in P3HT‐PCBM‐based solar cells shows performances comparable with cells made using TiO₂ films deposited with much longer processing times and/or higher temperatures. Copyright © 2013 John Wiley & Sons, Ltd.     

Porous Silicon‐Based Optical Microsensors for Volatile Organic Analytes: Effect of Surface Chemistry on Stability and Specificity

Sensing of the volatile organic compounds (VOCs) isopropyl alcohol (IPA) and heptane in air using sub‐millimeter porous silicon‐based sensor elements is demonstrated in the concentration range 50–800 ppm. The sensor elements are prepared as one‐dimensional photonic crystals (rugate filters) by programmed electrochemical etch of p⁺⁺ silicon, and analyte sensing is achieved by measurement of the wavelength shift of the photonic resonance. The sensors are studied as a function of surface chemistry: ozone oxidation, thermal oxidation, hydrosilylation (1‐dodecene), electrochemical methylation, reaction with dicholorodimethylsilane and thermal carbonization with acetylene. The thermally oxidized and the dichlorodimethylsilane‐modified materials show the greatest stability under atmospheric conditions. Optical microsensors are prepared by attachment of the porous Si layer to the distal end of optical fibers. The acetylated porous Si microsensor displays a greater response to heptane than to IPA, whereas the other chemical modifications display a greater response to IPA than to heptane. The thermal oxide sensor displays a strong response to water vapor, while the acetylated material shows a relatively weak response. The results suggest that a combination of optical fiber sensors with different surface chemistries can be used to classify VOC analytes. Application of the miniature sensors to the detection of VOC breakthrough in a full‐scale activated carbon respirator cartridge simulator is demonstrated.     

Efficient organic photovoltaics using solution-processed,annealing-free TiO2 nanocrystalline particles as an interface modification layer

A solution-processed, annealing-free TiO₂ nanocrystalline particles (TiO₂ NPs) as an interface modification layer was inserted in organic photovoltaics (OPVs), in which the widely used polymer poly (3-hexyl thiophene) (P3HT), a low band gap alkoxylphenyl substituted [1,2-b:4,5-b′] dithiophene-based polymer (PBDTPO-DTBO), and a soluble small molecule benzodithiophene derivative (TIBDT) were used as the donor material, respectively. The annealing-free TiO₂ NPs could be easily spin-coated upon the surface of organic active layers, and showed comparable properties to thermal-annealed ones. The power conversion efficiencies (PCEs) of OPV devices could be enhanced dramatically with inserting an annealing-free TiO₂ NPs layer. The PCEs of OPV devices based on P3HT:PC₆₁BM, PBDTPO-DTBO:PC₇₁BM and TIBDT:PC₆₁BM bulk heterojunctions were improved by 28%, 15% and 27%, respectively, with an annealing-free TiO₂ NPs layer, in which the highest PCE of 5.76% was achieved in PBDTPO-DTBO:PC₇₁BM OPVs. The solution-processed, annealing-free TiO₂ NPs thin films show great potential applications in the fabrication of large-area OPVs by printing or coating techniques on flexible polymer substrates. In particularly, it would promote to fabricate solution-processed, annealing-free OPV devices with suitable hole transport layer and organic/polymer active materials.     

Room temperature NO2 gas sensing properties of DBSA doped PPy–WO3 hybrid nanocomposite sensor

We demonstrate the chemiresistive NO₂ gas sensor based on DBSA doped PPy–WO₃ hybrid nanocomposites operating at room temperature. The sensor was fabricated on glass substrate using simple and cost effective drop casting method. The gas sensing performance of sensor was studied for various toxic/flammable analytes like NO₂, C₂H₅OH, CH₃OH, H₂S and NH₃. The sensor shows higher selectivity towards NO₂ gas with 72% response at 100 ppm. Also the sensor can successfully detect low concentration of NO₂ gas upto 5 ppm with reasonable response of 12%. Structural, morphological and compositional analyses evidenced the successful formation of DBSA doped PPy–WO₃ hybrid nanocomposite with uniform dispersion of DBSA into PPy–WO₃ hybrid nanocomposite and enhance the gas sensing behavior. We demonstrated that DBSA doped PPy–WO₃ hybrid nanocomposite sensor films shows excellent reproducibility, high stability, moderate response and recovery time for NO₂ gas in the concentration range of 5–100 ppm. A gas sensing mechanism based on the formation of random nano p–n junctions distributed over the surface of the sensor film has been proposed. In addition modulation of depletion width takes place in sensor on interaction with the target NO₂ gas has been depicted on the basis of schematic energy band diagram. Impedance spectroscopy was employed to study bulk, grain boundary resistance and capacitance before and after exposure of NO₂ gas. The structural and intermolecular interaction within the hybrid nanocomposites were explored by Raman and X-ray photoelectron spectroscopy (XPS), while field emission scanning electron microscopy (FESEM) was used to characterize surface morphology. The present method can be extended to fabricate other organic dopent-conducting polymer–metal oxide hybrid nanocomposite materials and could find better application in the gas sensing.     

In Situ Synthesis of Molybdenum Carbide Nanoparticles Incorporated into Laser-Patterned Nitrogen-Doped Carbon for Room Temperature VOC Sensing

Carbon laser-patterning (CLaP) is emerging as a new tool for the precise and selective synthesis of functional carbon-based materials for on-chip applications. The aim of this work is to demonstrate the applicability of laser-patterned nitrogen-doped carbon (LP-NC) for resistive gas-sensing applications. Films of pre-carbonized organic nanoparticles on polyethylenetherephthalate are carbonized with a CO₂-laser. Upon laser-irradiation a compositional and morphological gradient in the films is generated with a carbon content of 92% near the top surface. The specific surface areas of the LP-NC are increased by introducing sodium iodide (NaI) as a porogen. Electronic conductivity and surface area measurements corroborate the deeper penetration of the laser-energy into the film in the presence of NaI. Furthermore, impregnation of LP-NC with MoC_1−x (<10 nm) nanoparticles is achieved by addition of ammonium heptamolybdate into the precursor film. The resulting doping-sensitive nano-grain boundaries between p-type carbon and metallic MoC_1−x lead to an improvement of the volatile organic compounds sensing response of ΔR/R₀ = −3.7% or −0.8% for 1250 ppm acetone or 900 ppm toluene at room temperature, respectively, which is competitive with carbon-based sensor materials. Further advances in sensitivity and in situ functionalization are expected to make CLaP a useful method for printing selective sensor arrays.     

An Experimental Insight into the Structural and Electronic Characteristics of Strontium‐Doped Titanium Dioxide Nanotube Arrays

The possibility of in situ doping during electrochemical anodization of titania nanotube arrays is demonstrated and the mechanism and variations in structural and electronic characteristics of the nanotube arrays as after doping is systematically explored. In the presence of strontium as the dopant, bulk analysis shows strontium mainly incorporated into the lattice of TiO₂. Surface analysis, however, reveals phase segregation of SrO in the TiO₂ matrix at high Sr doping levels. The near edge X‐ray absorption fine structure (NEXAFS) spectroscopy analysis reveals that Sr²⁺ doping only alters the Ti and O ions interaction in the TiO₂ lattice on the surface with no effect on their individual charge states. An in‐depth understanding of the dopant incorporation mechanism and distribution into TiO₂ nanotube arrays is achieved using high resolution transmission electron microscopy (HRTEM) and the high angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) coupled with the electron energy loss spectroscopy (EELS) measurements on the surface and bulk of the nanotubes. Upon their use to photoelectrochemically split water, the Sr‐doped TiO₂ nanotube film shows incident photon conversion efficiencies (IPCE) as high as 65%. The enhanced light activity in conjunction with the ordered one‐dimensional morphology makes the fabricated films promising candidates for water photoelectrolysis.     

Sulfate‐Assisted Interfacial Engineering for High Yield and Efficiency of Triple Cation Perovskite Solar Cells with Alkali‐Doped TiO2 Electron‐Transporting Layers

Facile electron injection and extraction are two key attributes desired in electron transporting layers to enhance the efficiency of planar perovskite solar cells. Herein it is demonstrated that the incorporation of alkali metal dopants in mesoporous TiO₂ can effectively modulate electronic conductivity and improve the charge extraction process by counterbalancing oxygen vacancies acting as nonradiative recombination centers. Moreover, sulfate bridges (SO₄^2?) grafted on the surface of K‐doped mesoporous titania provide a seamless integration of absorber and electron‐transporting layers that accelerate overall transport kinetics. Potassium doping markedly influences the nucleation of the perovskite layer to produce highly dense films with facetted crystallites. Solar cells made from K:TiO₂ electrodes exhibit power conversion efficiencies up to 21.1% with small hysteresis despite all solution coating processes conducted under ambient air conditions (controlled humidity: 25–35%). The higher device efficiencies are attributed to intrinsically tuned electronic conductivity and chemical modification of grain boundaries enabling uniform coverage of perovskite films with large grain size.