Nanostructured Anatase TiO2 Research Articles

Controlled growth of monocrystalline rutile nanoshuttles in anatase TiO2 particles under mild conditions

A controlled growth of heterogeneous nanostructure of well-dispersed 1D monocrystalline rutile and 0D anatase TiO2 is achieved under mild conditions by acid peptization of lamellar titanate which formed after NaOH treatment of the hydrolysate of titanium tetra-n-butoxide.

Role of Molecular Anchor Groups in Molecule-to-Semiconductor Electron Transfer

The dynamics of heterogeneous electron transfer (ET) from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum. Data from ultraviolet photoelectron spectroscopy and from linear absorption spectroscopy showed that the donor state of the chromophore was located around 900 meV above the lower edge of the conduction band. With the wide band limit fulfilled the rate of the heterogeneous ET reaction was only controlled by the strength of the electronic coupling and not reduced by Franck-Condon factors. Two different time constants for the electron transfer, i.e., 13 and 28 fs, were measured with carboxylic acid and phosphonic acid as the respective anchor groups. The difference in the ET time constants was explained with the different extension of the donor orbital onto the respective anchor group to reach the empty electronic states of the semiconductor. The time constants were extracted by means of a simple rate equation model. The validity of applying this model on this ultrafast time scale was verified by comparing the rate equation model with an optical Bloch equation model.

Preparation and photoactivity of nanostructured anatase, rutile and brookite TiO2 thin films

Photoactive films consisting of pure anatase, brookite or rutile TiO2 were prepared by dip coating from water dispersions obtained by using TiCl4 as the precursor under similar mild experimental conditions.

Adsorption and Solar Light Decomposition of Acetone on Anatase TiO2 and Niobium Doped TiO2 Thin Films

Adsorption and solar light decomposition of acetone was studied on nanostructured anatase TiO2 and Nb-doped TiO2 films made by sol-gel methods (10 and 20 mol % NbO2.5). A detailed characterization of the film materials show that films contain only nanoparticles with the anatase modification with pentavalent Nb oxide dissolved into the anatase structure, which is interpreted as formation of substituted Nb=O clusters in the anatase lattice. The Nb-doped films displayed a slight yellow color and an enhanced the visible light absorption with a red-shift of the optical absorption edge from 394 nm for the pure TiO2 film to 411 nm for 20 mol % NbO2.5. In-situ Fourier transform infrared (FTIR) transmission spectroscopy shows that acetone adsorbs associatively with eta1-coordination to the surface cations on all films. On Nb-doped TiO2 films, the carbonyl bonding to the surface is stabilized, which is evidenced by a lowering of the nu(C=O) frequency by about 20 cm(-1) to 1672 cm(-1). Upon solar light illumination acetone is readily decomposed on TiO2, and stable surface coordinated intermediates are formed. The decomposition rate is an order of magnitude smaller on the Nb-doped films despite an enhanced visible light absorption in these materials. The quantum yield is determined to be 0.053, 0.004 and 0.002 for the pure, 10% Nb:TiO2, and 20%Nb:TiO2, respectively. Using an interplay between FTIR and DFT calculations we show that the key surface intermediates are bidentate bridged formate and carbonate, and H-bonded bicarbonate, respectively, whose concentration on the surface can be correlated with their heats of formation and bond strength to coordinatively unsaturated surface Ti and Nb atoms at the surface. The oxidation rate of these intermediates is substantially slower than the initial acetone decomposition rate, and limits the total oxidation rate at t>7 min on TiO2, while no decrease of the rate is observed on the Nb-doped films. The rate of degradation of key surface intermediates is different on pure TiO2 and Nb-doped TiO2, but cannot explain the overall lower total oxidation rate for the Nb-doped films. Instead the inferior photocatalytic activity in Nb-doped TiO2 is attributed to an enhanced electron-hole pair recombination rate due to Nb=O cluster and cation vacancy formation.

Nanostructured ZrO2-Coated TiO2 Electrodes for Dye-Sensitised Solar Cells

Thin films of ZrO2 were deposited on nanostructured anatase TiO2 electrodes via sol-gel route and utilised in the assembly of a dye-sensitised solar cells (DSSCs) forming nanostructured core-shell networks. The ZrO2-coated TiO2 electrodes were characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and constructing a DSSC. The characterisation concluded that core-shell morphologies were produced with varying ZrO2 shell thickness without altering the anatase TiO2 core. When a DSSC was constructed from the ZrO2:TiO2 core-shell electrode, the efficiency increased to 2.27% from 0.42% for the uncoated TiO2 electrode. As the ZrO2 shell thickness increased, the cell efficiency was reduced.

Excited State Electron Transfer from Ru(II) Polypyridyl Complexes Anchored to Nanocrystalline TiO2 through Rigid-Rod Linkers

Rigid-rod linkers varying in length were used to bind Ru(II) polypyridyl complexes to the surface of TiO2 (anatase) and of ZrO2 nanoparticle thin films. The linkers were made of p-phenyleneethynylene (Ph-E)n bridges carrying two COOR anchoring groups at the end and were capped with Ru(II) polypyridyl complexes as the sensitizing chromophores. Two series of rigid-rod sensitizers were prepared: Ru complexes having bpy or 4,4‘-(Cl)2-bpy as the ancillary ligands. In the first series, the excited state was localized on the rigid-rod linker, in the second series, the excited states were localized on the 4,4‘-(Cl)2-bpy ligands. The rigid-rod sensitizers with Ru(bpy)2 complexes did bind strongly (Kad ∼ 105 M-1) with high surface coverages (∼10-8 mol/cm2) on the nanostructured metal oxide films, ZrO2 and anatase TiO2. The length of the fully conjugated rigid-rod linker influences the photophysical properties of the sensitizer, and nanosecond transient absorption measurements indicated long-lived metal-to-ligand charge-transfer (MLCT) excited states (∼2 μs) with evidence for delocalization onto the rigid-rod linker. The interfacial electron transfer behavior on TiO2 was found to be dependent on the Brønsted acidity or basicity of the surface. On base pretreated TiO2, the excited state electron injection yields were low and could be increased by addition of LiClO4 to an external CH3CN solution. Under these conditions, a fraction of the injection process could be time resolved on a 10 ns time scale. On acidic TiO2, ultrafast excited state electron injection was observed for both series. Recombination was found to be second order with average rate constants independent of which rigid-rod sensitizer was excited. For rigid-rod sensitizers with Ru(4,4‘-(Cl)2-bpy)2, there was evidence for a direct interaction between the 4,4‘-(Cl)2-bpy ligands and the TiO2 surface. Photophysical and interfacial electron transfer properties of these Cl-substituted complexes were nearly independent of the rigid-rod length.

From nanostructures to porous silicon: sensors and photocatalytic reactors

A rapid, reversible, and sensitive porous silicon (PS) gas sensor, based upon a uniquely formed highly efficient electrical contact to a nanopore covered microporous array, is modified to introduce selectivity and for the purpose of a novel heterogeneous photocatalytic reactor. Utilizing photoluminescence induced electroless metallization as a means of obtaining a highly efficient electrical contact we demonstrate the detection of HC1, NH3, CO and NO at the ppm level. The response of this device, which operates with limited sensitivity at a bias voltage of 1–10 mV, is rapid and reversible. The form of metallization used to establish a low resistance, 20 Ω, contact also provides the basis for more efficient electroluminescent devices. After an electroless gold treatment, the impedance response of the device to ammonia increases by 2.5 times, while the CO and NO responses are unchanged. With an electroless tin coating, the room temperature responses to NH3, NO, and CO are all amplified. With an FFT analysis, a gas response can now be acquired and filtered on a drifting baseline, further increasing sensitivity. These sensor suites are now being extended to develop microreactors in which nanoscale quantum dot (QD) photocatalysts will be placed within the pores of PS and excited using PS electroluminescence or photoluminescence to excite visible light absorbing QDs. Visible light absorbing titania-based QDs have been developed. Using a nanoscale exclusive synthesis route, we directly treat TiO2 nanocolloids and, in seconds, at room temperature, we produce nitrogen doped, stable, and environmentally benign TiO2–xNx photocatalysts whose optical response, now not limited to the ultraviolet, can be tuned across the entire visible region. This synthesis, which can be simultaneously accompanied by metal atom seeding, can be accomplished through the direct nitration of anatase TiO2 nanostructures with alkyl ammonium salts. Tunability throughout the visible depends on the degree of TiO2 nanoparticle agglomeration and the influence of metal seeding. No organics are incorporated into the final TiO2–xNx products. These visible light absorbing photocatalysts readily photodegrade methylene blue and gaseous ethylene. They can be transformed from liquids to gels and placed on the surfaces of sensor and microreactor based configurations 1) to produce an improved photocatalytically induced solar based sensor response, and 2) with a goal to facilitate catalytically induced disinfection of airborne pathogens. In contrast to a nitridation process which is facile at the nanoscale, we find little or no direct nitridation of micrometer sized anatase or rutile TiO2 powders at room temperature. Thus, we demonstrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron particles. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

XAS investigation of tantalum and niobium in nanostructured TiO2 anatase

XAS investigation of tantalum and niobium in nanostructured TiO2 anatase

Highly Efficient Formation of Visible Light Tunable TiO2‐xNx Photocatalysts and Their Transformation at the Nanoscale.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale

Using a simple nanoscale exclusive synthesis route, TiO2-xNx photocatalysts that can be tuned to absorb across the visible region are produced in seconds at room temperature. The photocatalysts are formed by employing the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. Depending on the degree of TiO2 nanoparticle agglomeration, catalytically active TiO2-xNx anatase structured particles are obtained whose absorption onset extends well into the visible region at λ ∼ 550 nm. The introduction of a small quantity of palladium in the form of the chloride or nitrate facilitates further nitrogen uptake, appears to lead to a partial phase transformation, displays a counterion effect when compared also to the acetate, and produces a material absorbing well into the near-infrared. The introduction of palladium via the chloride also facilitates the formation of small tetrahedral and octahedral palladium-based crystallites throughout the TiO2-xNx lattice. Surprisingly, no organics appear to be incorporated into the final TiO2-xNx products. The resulting photocatalysts readily photodegrade methylene blue and lead to the catalytic oxidation of ethylene as they are placed as gels on surfaces. In contrast to the current nitridation process, which is quite facile at the nanoscale, we observe a much slower nitration of Degussa P25 nanopowders and little or no direct nitridation of micrometer-sized anatase or rutile TiO2 powders at room temperature. We thus demonstrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron materials.

Preparation of Composite Anatase TiO2 Nanostructure by Precipitation from Hydrolyzed TiCl4 Solution Using Anodic Alumina Membrane

Composite anatase TiO2 nanostructure, consisting of both tubules and rods, has been synthesized by precipitation from hydrolyzed TiCl4 solution using an anodic alumina membrane (AAM). SEM analyses show that the structure consists of rods of 200−250-nm diameter and tubules of about 500-nm diameter. Each tubule may be visualized as a single sheet of 60−80 nm thickness rolled into a cylinder of diameter about 2.5 times the pore size of the AAM. The thus-prepared TiO2 nanomaterial is characterized and a possible mechanism of its formation is discussed.

Highly Efficient Formation of TiO2-XNX–Based Photocatalysts - Potential Applications For Active Sites in Microreactors, Sensors, and Photovoltaics

ABSTRACTWe have produced nitrogen doped, stable, and environmentally benign TiO2-xNx photocatalysts whose optical response can be tuned across the entire visible region using a nanoscale exclusive synthesis route, performed in seconds at room temperature. This synthesis, which can be simultaneously accompanied by metal atom seeding, that can be accomplished through the direct nitration of anatase TiO2 nanostructures with alkyl ammonium salts. Tunability throughout the visible spectrum depends on the degree of TiO2 nano- particle agglomeration and the influence of metal seeding. The introduction of a small quantity of palladium, in the form of the chloride or nitrate, promotes further nitrogen uptake, appears to lead to a partial phase transformation, displays a counter ion effect with the acetate, and produces a material absorbing well into the near infrared. The nitridation process also induces the formation of oxygen hole centers. Silver introduced as the nitrate into a TiO2 or TiO2-xNx nanostructure framework, forms seeded AgxO - TiO2 or TiO2-xNx nanostructure mixtures which can be induced to self-assemble and to agglomerate into nano-needle and planar arrays using select metal probes. Surprisingly, no organics are incorporated into the final TiO2-xNx products. These visible light absorbing photocatalysts readily photodegrade methylene blue and when used to create photocatalytic sites on a surface based microreactor induce ethylene oxidation with visible light. They can be transformed from liquids to gels and placed on the surfaces of sensor and microreactor based configurations to 1) facilitate a photocatalytically induced solar pumped sensor response, and 2) provide a possible means for the catalytically induced disinfection of airborne pathogens. In contrast to a process which is facile at the nanoscale, we find little or no direct nitridation of micrometer sized anatase or rutile TiO2 powders at room temperature. Thus, we demonstrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron materials.

Formation of nanostructured TiO2 by flame spraying with liquid feedstock

A liquid feedstock flame spraying system was developed to deposit the nanostructured TiO2 coating using butyl titanate (Ti(OC4H9)4) and ethanol solution as precursor. The resultant deposit was characterized by infrared spectrum (IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that during spraying, butyl titanate had been completely decomposed and converted into nanostructured anatase TiO2, having a narrow size distribution from 10 to 20 nm.

The Nature of Electron Migration in Dye-Sensitized Nanostructured TiO2

The relationship between the morphology of nanostructured anatase TiO2 films and electron migration in these electrodes has been investigated. Toward this end, three different TiO2 powders have been studied, all of which have a unique microstructure. Photocurrent dynamics is used to elucidate electron transport in nanostructured TiO2 networks. In particular, intensity-modulated photocurrent spectroscopy proves to be a powerful characterization technique. In all cases the transport of electrons through a TiO2 network can be described with a macroscopic diffusion model. By fitting the data to this model it is possible to determine the absorption coefficient, the diffusion coefficient, and the lifetime of the electrons. Upon illumination, electrons accumulate in the nanostructured film. A fraction of these electrons are stored in deep surface traps. Another fraction of the electrons reside in the conduction band and are free to move. The average concentration of these excess mobile conduction band electrons amounts to one electron per nanoparticle, irrespective of the type of electrode, the film thickness, or the irradiation intensity. Equilibration of the quasi-Fermi level combined with infinite fast extraction of electrons at the substrate explains this saturation effect.

Electrochromic Properties of bis(phthalocyaninato)lutetium(III) Sensitized Nanostructured Anatase TiO[sub 2] Thin Films

Electrochromic Properties of bis(phthalocyaninato)lutetium(III) Sensitized Nanostructured Anatase TiO2 Thin Films

In-Situ X-ray Diffraction Study of Lithium Intercalation in Nanostructured Anatase Titanium dioxide

AbstractElectrochemical lithium intercalation in nanostructured anatase TiO2 is investigated with in-situ X-ray diffraction. A complete and reversible phase transformation from tetragonal anatase TiO2 to orthorhombic anatase Li0.5TiO2 is observed. The difference of the XRD spectra before and after insertion can be fitted with the lattice parameters of the two phases as fit parameters. The maximum amount of lithium that can be dissolved in anatase TiO2 before the phase transformation occurs is found to be very small.