WS<Subscript>2</Subscript> layer formation on multi-walled carbon nanotubes

Time-dependent powder X-ray-diffraction analyses reveal that the conversion of WO₃ into WS₂ on carbon nanotube surfaces in the presence of H₂S is a one-step process. The WS₂ layers grow simultaneously along the tube in the radial and axial directions. Received: 17 June 2002 / Accepted: 19 June 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-1273/677-196, E-mail: d.walton@sussex.ac.uk     

Photogreying of TiO<Subscript>2</Subscript> nanoparticles

Photogreying, the change in brightness on UV irradiation in the absence of oxygen, of TiO₂ nanoparticulate dispersions is shown to depend on the nature of the liquid, consistent with a surface reaction. Measurements on a series of TiO₂ particles (mainly 75×10 nm) dispersed in, e.g., alkyl benzoate correlate well with those on the same TiO₂’s dispersed in a second liquid (e.g. propan-2-ol). Photogreying in propan-2-ol is paralleled by photocatalytic-oxidation activity, indicating a common origin – UV-generation of charge carriers. Further, photogreying parallels Ti³⁺ formation. Hence, although appearance and the visible spectra of photogreyed particles both differ from those of Ti³⁺ in ≤10 nm colloidal TiO₂, we suggest that photogreying is caused by capture of UV excited electrons to form Ti³⁺. Surface treatment reduces photogreying, and we speculate that differences between uncoated samples reflect differences in the number of potentially reducible Ti’s.     

Anatase TiO<Subscript>2</Subscript> nanoparticles for lithium-ion batteries

Anatase TiO₂ nanoparticles were prepared by a simple sol-gel method at moderate temperature. X-ray powder diffraction (XRD) and Raman spectroscopy revealed the exclusive presence of anatase TiO₂ without impurities such as rutile or brookite TiO₂. Thermogravimetric analysis confirmed the formation of TiO₂ at about 400 °C. Particle size of about 20 nm observed by transmission electron microscopy matches well with the dimension of crystallites calculated from XRD. The electrochemical tests of the sol-gel-prepared anatase TiO₂ show promising results as electrode for lithium-ion batteries with a stable specific capacity of 174 mAh g^?1 after 30 cycles at C/10 rate. The results show that improvement of the electrochemical properties of TiO₂ to reach the performance required for use as an electrode for lithium-ion batteries requires not only nanosized porous particles but also a morphology that prevents the self-aggregation of the particles during cycling.     

Structural properties of amorphous TiO<Subscript>2</Subscript> nanoparticles

Structural properties of amorphous TiO₂ spherical nanoparticles have been studied in models with different sizes of 2 nm, 3 nm, 4 nm and 5 nm under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. Structural properties of an amorphous nanoparticle obtained at 350 K have been analyzed in detail through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Moreover, we show the radial density profile in a nanoparticle. Calculations show that size effects on structure of a model are significant and that if the size is larger than 3 nm, amorphous TiO₂ nanoparticles have a distorted octahedral network structure with the mean coordination number Z_Ti-O ≈6.0 and Z_O-Ti ≈3.0 like those observed in the bulk. Surface structure and surface energy of nanoparticles have been obtained and presented.     

Carboxylic species adsorption on TiO<Subscript>2</Subscript> nanoparticles

We present the results of a quantum-chemical study of the interface formed by titania nanoparticles and a set of carboxylic moieties, namely, benzoic and bi-isonicotinic acids and a tris-(2, 2′-dcbipyridine) Fe (II) complex placed on the surface of either rutile or anatase polymorphs. The calculations were performed in the spd basis using semiempirical quantum-chemical codes, both sequential and parallel. The results are mainly addressed to the geometry optimization of the adsorbed molecules on the surface, as well as to the adsorption mechanism and the energy of adsorption. The text was submitted by the authors in English.     

Thermostable photocatalytically active TiO<Subscript>2</Subscript> anatase nanoparticles

Anatase is the low-temperature (300–550 °C) crystalline polymorph of TiO₂ and it transforms to rutile upon heating. For applications utilizing the photocatalytic properties of nanoscale anatase at elevated temperatures (over 600 °C) the issue of phase stabilisation is of major interest. In this study, binary TiO₂/SiO₂ particles were synthesized by a flame aerosol process with TiCl₄ and SiCl₄ as precursors. The theoretical Si/Ti ratio was varied in the range of 0.7–1.3 mol/mol. The synthesized TiO₂/SiO₂ samples were heat treated at 900 and 1,000 °C for 3 h to determine the thermostability of anatase. Pyrogenic TiO₂ P25 (from Evonik/Degussa, Germany) widely applied as photocatalyst was used as non-thermostabilized reference material for comparison of photocatalytic activity of powders. Both the non-calcinated and calcinated powders were characterized by means of XRD, TEM and BET. Photocatalytic activity was examined with dichloroacetic acid (DCA) chosen as a model compound. It was found that SiO₂ stabilized the material retarding the collapse of catalyst surface area during calcination. The weighted anatase content of 85% remains completely unchanged even after calcination at 1,000 °C. The presence of SiO₂ layer/bridge as spacer between TiO₂ particles freezes the grain growth: the average crystallite size increased negligibly from 17 to 18 nm even during the calcination at 1,000 °C. Due to the stabilizing effect of SiO₂ the titania nanoparticles calcinated at 900 and 1,000 °C show significant photocatalytic activity. Furthermore, the increase in photocatalytic activity with calcination temperature indicates that the titania surface becomes more accessible either due to intensified cracking of the SiO₂ layer or due to enhanced transport of SiO₂ into the necks thus releasing additional titania surface.     

Electrical conduction mechanism in Fe<Subscript>70</Subscript>Pd<Subscript>30</Subscript> catalyzed multi-walled carbon nanotubes

Carbon nanotubes (CNTs) are synthesized by the catalytic decomposition of acetylene using low pressure chemical vapour deposition method (LPCVD) at 800 °C and at a chamber pressure of 10 Torr over a supported catalyst film of Fe₇₀Pd₃₀. Morphology of these CNTs is studied using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). From HRTEM image of these multi-walled carbon nanotubes (MWNTs), it is clear that these MWNTs do not possess a co-axial cylindrical structure, but are composed of imperfect and broken graphite cylinders of different sizes. The average diameter and length of the nanotubes varies between 20–70 nm and 5–60 μm respectively. Electrical transport measurements of these MWNTs are studied over a temperature range of 298–4.2 K. The results have been interpreted in terms of variable-range hopping (VRH) over the entire temperature range of 298–4.2 K. Three-dimensional variable-range hopping (VRH) is suggested for the temperature range (298–125 K), while two-dimensional VRH is observed for the temperature range (125–4.2 K).     

Radiolabelling of TiO<Subscript>2</Subscript> nanoparticles for radiotracer studies

Industrially manufactured titanium dioxide nanoparticles have been successfully radiolabelled with ⁴⁸V by irradiation with a cyclotron-generated proton beam. Centrifugation tests showed that the ⁴⁸V radiolabels were stably bound within the nanoparticle structure in an aqueous medium, while X-ray diffraction indicated that no major structural modifications to the nanoparticles resulted from the proton irradiation. In vitro tests of the uptake of cold and radiolabelled nanoparticles using the human cell line Calu-3 showed no significant difference in the uptake between both batches of nanoparticles. The uptake was quantified by Inductively Coupled Plasma Mass Spectrometry and high resolution γ-ray spectrometry for cold and radiolabelled nanoparticles, respectively. These preliminary results indicate that alterations to the nanoparticles’ properties introduced by proton bombardment can be controlled to a sufficient extent that their further use as radiotracers for biological investigations can be envisaged and elaborated.     

Formation of well-packed TiO<Subscript>2</Subscript> nanoparticles on multiwall carbon nanotubes using CVD method to fabricate high sensitive gas sensors

Titanium oxide nanoparticles were coated on multiwall carbon nanotubes (MWCNTs) using an atmospheric pressure chemical vapor deposition (CVD) to achieve highly compact nanoparticles of about 5 nm on CNT structure. The CNTs with a diameter of about 50 nm were grown by plasma enhanced CVD. Gas sensitivity of the fabricated structure was investigated and compared with TiO₂/CNT composite-based gas sensors. The effect of the structural interaction between the nanoparticles and the CNT wall on sensing mechanism of the as-prepared gas sensors was investigated. Ultrasensitive gas sensors were obtained by TiO₂/CNT nanostructures with strong interaction between the MWCNT and the TiO₂ nanoparticles. The measurements show high chemical activity and exceptional electrical response of the as-prepared structure being exposed to gases. Scanning and transmission electron microscopy and X-ray diffraction analysis were used to obtain structural information.     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).     

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.     

Symmetry and models of single-walled TiO<Subscript>2</Subscript> nanotubes with rectangular morphology

The formalism of line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for symmetry analysis of single-walled titania nanotubes (SW TiO₂ NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of anatase structure. Either six- or twelve-layer (101) slabs have been cut from TiO₂ crystal in a stable anatase phase. After structural optimization, the latter keeps the centered rectangular symmetry of initial slab slightly compressed along a direction coincided with large sides of elemental rectangles. We have considered two sets of SW TiO₂ NTs with optimized six- and twelve-layer structures, which possess chiralities (−n, n) and (n, n) of anatase nanotubes. To analyze the structural and electronic properties of titania slabs and nanotubes, we have performed their ab initio LCAO calculations, using the hybrid Hartree-Fock/Kohn-Sham exchange-correlation functional PBE0. The band gaps (Δɛ _gap ) and strain energies (E _strain ) of six-layer nanotubes have been computed and analyzed as functions of NT diameter (D _NT). As to models of 12-layer SW TiO₂ NTs of both chiralities, their optimization results in structural exfoliation, i.e., the multi-walled structure should be rather formed in nanotubes with such a number of atomic layers.     

Anodic TiO<Subscript>2</Subscript> nanotubes powder and its application in dye-sensitized solar cells

An increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. TiO₂ semiconductor material is expected to play an important role in helping solve the energy crisis through effective utilization of solar energy based on photovoltaic devices. Dye-sensitized solar cells (DSSCs) are potentially lower cost alternative to inorganic silicon-based photovoltaic cells. In this study, we report on the fabrication of DSSCs from anodic TiO₂ nanotubes (NT) powder, produced by rapid breakdown potentiostatic anodization of Ti foil in 0.1 M HClO₄ electrolyte, as photoanode. TiO₂ NT powders with a typical NT outer diameter of approximately 40 nm, wall thickness of approximately 8–15 nm, and length of about 20–25 μm, have been synthesized. The counter electrode was made by electrodeposition of Pt from an aqueous solution of 5 mM H₂PtCl₆ onto fluorine-doped tin oxide (FTO) glass substrate. The above front-side illuminated DSSCs were compared with back-side illuminated DSSCs fabricated from anodic TiO₂ NTs that were grown on the top of Ti foil as photoanode. The highest cell efficiency was 3.54% under 100 mW/cm² light intensity (1 sun AM 1.5G light, Jsc = 14.3 mA/cm², Voc = 0.544 V, FF = 0.455). To the best of our knowledge, this is the first report on the fabrication of DSSC from anodic TiO₂ NTs powder. The TiO₂/FTO photoanodes were characterized by FE-SEM, XRD, and UV–Visible spectroscopy. The catalytic properties of Pt/FTO counter electrodes have been examined by cyclic voltammetry.     

The influence of Fe,Cu, SiO<Subscript>2</Subscript>, TiO<Subscript>2</Subscript>, and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in aqueous solution on proton relaxation times

Measurements of proton nuclear spin-spin and spin-lattice relaxation times are applied for determining the concentration of solid-phase nanoparticles in nanofluids. This approach is tested for metal oxides SiO₂, TiO₂, Al₂O₃ and metal-carbon nanoparticles of 3d-metals Fe and Cu. It is shown that the sensitivity of the method for determining concentrations of 3d-metals is much higher than for oxides (by 2–4 orders of magnitude). It is revealed that measurement of the proton spin-spin relaxation time allows one to determine the concentration of Cu nanoparticles to 0.0001 mg/ml and that of Fe nanoparticles to 0.00001 mg/ml.     

Minimizing of the boundary friction coefficient in automotive engines using Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and TiO<Subscript>2</Subscript> nanoparticles

Minimizing of the boundary friction coefficient is critical for engine efficiency improvement. It is known that the tribological behavior has a major role in controlling the performance of automotive engines in terms of the fuel consumption. The purpose of this research is an experimental study to minimize the boundary friction coefficient via nano-lubricant additives. The tribological characteristics of Al₂O₃ and TiO₂ nano-lubricants were evaluated under reciprocating test conditions to simulate a piston ring/cylinder liner interface in automotive engines. The nanoparticles were suspended in a commercially available lubricant in a concentration of 0.25 wt.% to formulate the nano-lubricants. The Al₂O₃ and TiO₂ nanoparticles had sizes of 8–12 and 10 nm, respectively. The experimental results have shown that the boundary friction coefficient reduced by 35–51% near the top and bottom dead center of the stroke (TDC and BDC) for the Al₂O₃ and TiO₂ nano-lubricants, respectively. The anti-wear mechanism was generated via the formation of protective films on the worn surfaces of the ring and liner. These results will be a promising approach for improving fuel economy in automotive.

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Graphical Abstract Minimizing of the boundary friction coefficient in automotive engines using Al₂O₃ and TiO₂ nanoparticles

     

Computational study on the structural and surface properties of amorphous hydroxylated TiO<Subscript>2</Subscript> spherical nanoparticles

Monte Carlo simulations were carried out on amorphous titanium dioxide (TiO₂) for both bulk and hydroxylated nanoparticles with particle sizes ranging from 1 to 10 nm. The potential developed by the Matsui and Akaogi (MA) was used to model the interatomic interactions of TiO₂ in both cases (bulk and nanoparticles). Besides, Angular and Morse potentials proposed by the Tether, Cormack, Du et. al. (TCD) were introduced to model the interactions of hydroxyl groups on the TiO₂ surfaces, i.e., the Ti-O-H groups with an experimental and theoretical angles of 125 ^o . The bulk system was developed using periodic boundary conditions. The TiO₂ nanoparticles were extracted by applying a spherical cut section in the bulk TiO₂ melt structure to obtain the required size. Free valences on the nanoparticle surfaces were saturated via additional hydroxyl groups and then quenched to 300 K under free boundary conditions. The bulk and surface properties of the nanoparticles were calculated at 300 K and zero pressure and characterized via radial distribution functions, bond angle distributions, bond distances, coordination numbers, OH group concentrations and radial density profiles. In addition, to understand the difference in properties of amorphous hydroxylated TiO₂ nanoparticles and bulk amorphous TiO₂, a comparative study was done at the same thermodynamic conditions. The study shows that the bulk properties of amorphous hydroxylated TiO₂ nanoparticles are strongly size-dependent and different from those of the bulk TiO₂. As expected, increasing the particle size leads to an approach of the particle’s bulk properties to the bulk properties of the (quasi) infinite system. The size effects show that decreasing the particle size results in increasing the surface effects and surface OH group concentrations. Accordingly, small-sized TiO₂ nanoparticles have higher surface OH group concentrations and larger surface effects than large-sized TiO₂ nanoparticles. Larger surface effects result significant changes in their bond angles, bond distances, and coordination numbers. The simulation results of the surface properties reveal that the surface titanium atoms in the TiO2 nanoparticles have the capability of accommodating up to 5 hydroxyl groups. The mean surface hydroxyl group density of the amorphous TiO₂ spherical nanoparticles is estimated to be around 8.1/nm ², which lies in the range of 8–16/nm ², found by experimental and other simulation studies. Details of the modelling, simulations results and the study are presented in this paper.     

Assembly,characterization, and photocatalytic activities of TiO<Subscript>2</Subscript> nanotubes/CdS quantum dots nanocomposites

The semiconductor quantum dots (QDs) can be very efficient to tune the response of photocatalyst of TiO₂ to visible light. In this study, CdS QDs formed in situ with about 8 nm have been successfully deposited onto the surfaces of TiO₂ nanotubes (TNTs) to form TNTs/CdS QDs nanocomposites by use of a simple bifunctional organic linker, thiolactic acid. The diffuse reflectance spectroscopy (DRS) spectra of as prepared samples showed that the absorption edge of the TNTs/CdS composite is extended to visible range, with absorption edge at 530 nm. The photocatalytic activity and stability of TNTs/CdS were also evaluated for the photodegradation of rhodamine B. The results showed that when TNTs/CdS QDs was used, photocatalytic degradation of RhB under visible light irradiation reached 91.6%, higher than 45.4 and 30.5% for P25 and TNTs, respectively. This study indicated that the TNTs/CdS QDs nanocomposites were superior catalysts for photodegradation under visible light irradiation compared with TNTs and P25 samples, which may find wide application as a powerful photocatalyst in environmental field.     

Magnetic properties of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

The low-temperature dependences of magnetic characteristics (namely, the coercive force H _c , the remanent magnetization M _r , local magnetic anisotropy fields H _a, and the saturation magnetization M _s ) determined from the irreversible and reversible parts of the magnetization curves for Fe₃C ferromagnetic nanoparticles encapsulated in carbon nanotubes are investigated experimentally. The behavior of the temperature dependences of the coercive force H _c (T) and the remanent magnetization M _r (T) indicates a single-domain structure of the particles under study and makes it possible to estimate their blocking temperature T _B = 420–450 K. It is found that the saturation magnetization M _s and the local magnetic anisotropy field H _a vary with temperature as ～T ^5/2.     

Synthesis,characterization, and thermal stability of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript>/CR-Ag multilayered nanostructures

The controllable synthesis and characterization of novel thermally stable silver-based particles are described. The experimental approach involves the design of thermally stable nanostructures by the deposition of an interfacial thick, active titania layer between the primary substrate (SiO₂ particles) and the metal nanoparticles (Ag NPs), as well as the doping of Ag nanoparticles with an organic molecule (Congo Red, CR). The nanostructured particles were composed of a 330-nm silica core capped by a granular titania layer (10 to 13 nm in thickness), along with monodisperse 5 to 30 nm CR-Ag NPs deposited on top. The titania-coated support (SiO₂/TiO₂ particles) was shown to be chemically and thermally stable and promoted the nucleation and anchoring of CR-Ag NPs, which prevented the sintering of CR-Ag NPs when the structure was exposed to high temperatures. The thermal stability of the silver composites was examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Larger than 10 nm CR-Ag NPs were thermally stable up to 300 °C. Such temperature was high enough to destabilize the CR-Ag NPs due to the melting point of the CR. On the other hand, smaller than 10 nm Ag NPs were stable at temperatures up to 500 °C because of the strong metal-metal oxide binding energy. Energy dispersion X-ray spectroscopy (EDS) was carried out to qualitatively analyze the chemical stability of the structure at different temperatures which confirmed the stability of the structure and the existence of silver NPs at temperatures up to 500 °C.     

Impedance spectroscopy study of Na<Subscript>1/2</Subscript>Sm<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> ceramic

Complex impedance analysis of a valence-compensated perovskite ceramic oxide Na_1/2Sm_1/2TiO₃, prepared by a mixed oxide (solid-state reaction) method, has been carried out. The formation of single-phase material was confirmed by X-ray diffraction studies, and it was found to be an orthorhombic phase at room temperature. In a scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with that of impedance analysis. Alternating current impedance measurements were made over a wide temperature range (31–400 °C) in an air atmosphere. Complex impedance and modulus plots helped to separate out the contributions of grain and grain boundaries to the overall polarization or electrical behavior. The physical structure of the samples was visualized most prominently at higher temperatures (275 °C) from the Nyquist plots showing inter- and intragranular impedance present in the material. The frequency dependence of electrical data is also analyzed in the framework of the conductivity and modulus formalisms. The bulk resistance, evaluated from the impedance spectrum, was observed to decrease with rise in temperature, showing a typical negative temperature coefficient of resistance-type behavior like that of semiconductors. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by the impedance data. PACS 77.22.Ch; 77.22.Ej; 77.22.Gm; 77.22.Jp; 77.84.Bw