Colloidal TiO2 Research Articles

Combining the UV-Switchability of Keggin Ions with a Galvanic Replacement Process to Fabricate TiO2–Polyoxometalate–Bimetal Nanocomposites for Improved Surface Enhanced Raman Scattering and Solar Light Photocatalysis

While the decoration of TiO2 surfaces with metal nanoparticles has been well-established, the modification of the composition of metal nanoparticles postdeposition onto TiO2 surfaces and applicability of such bimetallic systems for surface enhanced Raman scattering (SERS) and photocatalysis has not hitherto been investigated. In this work, we, for the first time, combine the unique UV-switchability of the Keggin ions of 12-phosphotungstic acid (PTA) to directly form metal nanoparticles (Ag and Cu) onto the colloidal TiO2 surface, with a galvanic replacement process to convert these predeposited metal nanoparticles into bimetallic systems (Ag/Au, Ag/Pt, Cu/Au, Cu/Pt, and Cu/Ag). We further demonstrate the applicability of these novel TiO2-polyoxometalate-bimetal nanocomposites toward improved SERS and solar light photocatalysis.

Porous TiO2/C Nanocomposite Shells As a High-Performance Anode Material for Lithium-Ion Batteries

Porous TiO2/C nanocomposite shells with high capacity, excellent cycle stability, and rate performance have been prepared. The synthesis involves coating colloidal TiO2 nanoshells with a resorcinol-formaldehyde (RF) layer with controllable thickness through a sol-gel-like process, and calcining the composites at 700 °C in an inert atmosphere to induce crystallization from amorphous TiO2 to anatase and simultaneous carbonization from RF to carbon. The cross-linked RF polymer contributes to the high stability of the shell morphology and the porous nature of the shells. A strong dependence of the capacity on the amount of incorporated carbon has been revealed, allowing the optimization of the electrode structure for high-rate cell performance.

Plastic dye-sensitized solar cells with enhanced performance prepared from a printable TiO2 paste

The presented work demonstrates a novel technique to prepare printable TiO2 paste, employing a (Sb, In) doped SnO2 sol (SITO-sol) as a crosslinking agent and binder, to construct plastic DSCs. Porous and robust hybrid TiO2 films with enhanced conductivity and electrochemical properties were obtained using the newly developed TiO2 paste. Compared with DSCs prepared from a pristine TiO2 colloid, plastic DSCs fabricated by the presented process show a significant improvement (49%) in power conversion efficiency (6.75% vs. 4.54%). The mechanism for the enhanced performance was investigated.

Surfactant-induced thermomechanical and morphological changes in TiO2-polystyrene nanocomposites

Nanocomposites of polystyrene and TiO2 colloidal nanorods with different loadings have been prepared by mixing pre-synthesized oleic acid capped colloidal TiO2 nanorods into commercial polystyrene via solvent blending using chloroform. The microstructure and morphology of the nanocomposites was evaluated by wide angle X-ray diffraction and transmission electron microscopy. The observations revealed that the surfactant plays an important role for interactions between the polymer and the filler. Differential scanning calorimetry showed that the glass transition temperature of the nanocomposites decreased which is consistent with the surfactant acting as a plasticizer in the polystyrene matrix. Thermogravimetric analysis revealed that the nanocomposites show no significant improvement in thermal stability as compared to the bare PS up to a temperature of 400°C. However, after 400°C, the TGA curve shifts a little to higher temperature as compared to the bare PS. The dynamic mechanical properties of the nanocomposites indicate that the storage modulus, loss modulus, and glass transition temperature do not change with increasing nanorods content of 2 and 4wt% but decrease afterward for 8wt%. Transmission electron microscopy images clearly show debonding characteristics in polystyrene matrix.

Interaction between Nano TiO2 and Simulacra of Dissolved and Colloidal Substances in the Process Water of a Paper Machine

In the papermaking process, the removal and control of dissolved and colloidal substances (DCS) is a key issue for reducing the usage of fresh water. The use of nano TiO2 for removal of dissolved substances (DS) and colloidal substances (CS) was investigated through monitoring the titration process of nano TiO2 colloids to sodium laurate (C11H23COONa, DS simulacra) and stearic acid (C17H35COOH, CS simulacra) solution with COD (chemical oxygen demand), DLS (dynamic light scattering), SEM (scanning electron microscope), and zeta potential, respectively. The results indicated that most of the simulacra molecules could be removed from the aqueous solution by the flocculation with nano TiO2 colloids. The removal of CS by nano TiO2 colloid arose from heterocoagulation rather than from charge neutralization, in which nano TiO2 was adsorbed onto the surface of CS particles and bridged CS to form flocs. While in the removal process of DS by nano TiO2, the negative-charged portion of the DS molecule was adsorbed onto the surface of nano TiO2 particles with a mono- or multilayer, eliminating the repulsive force between nano TiO2 particles and resulting in their flocculation.

Enhanced Photocatalytic Performances of CeO2/TiO2 Nanobelt Heterostructures

CeO2 /TiO2 nanobelt heterostructures are synthesized via a cost-effective hydrothermal method. The as-prepared nanocomposites consist of CeO2 nanoparticles assembled on the rough surface of TiO2 nanobelts. In comparison with P25 TiO2 colloids, surface-coarsened TiO2 nanobelts, and CeO2 nanoparticles, the CeO2 /TiO2 nanobelt heterostructures exhibit a markedly enhanced photocatalytic activity in the degradation of organic pollutants such as methyl orange (MO) under either UV or visible light irradiation. The enhanced photocatalytic performance is attributed to a novel capture-photodegradation-release mechanism. During the photocatalytic process, MO molecules are captured by CeO2 nanoparticles, degraded by photogenerated free radicals, and then released to the solution. With its high degradation efficiency, broad active light wavelength, and good stability, the CeO2 /TiO2 nanobelt heterostructures represent a new effective photocatalyst that is low-cost, recyclable, and will have wide application in photodegradation of various organic pollutants. The new capture-photodegradation-release mechanism for improved photocatalysis properties is of importance in the rational design and synthesis of new photocatalysts.

Synthesis of composite Au/TiO2 nanoparticles through pulsed laser ablation and study of their optical properties

This paper is a report on the synthesis of composite Au/TiO2 nanoparticles (NPs) using a novel and chemically clean method by ablating a gold target immersed in colloidal TiO2 NPs using 1064 nm Nd:YAG laser pulses. Further, the NPs were characterized through ultraviolet–visible spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. In addition, the nonlinear optical response of the synthesized samples was studied under continuous wave He–Ne laser irradiation.

Preparation of titanium dioxide porous thin films via photopolymerization of the colloidal TiO2 dispersion

Titanium dioxide (TiO2) thin films with porous structures have been successfully achieved by photopolymerization stimulating the particle dispersion combined TiO2 colloid with photoinitiator and monomer. The photopolymerization monomer Pentaerythritol Triacrylate (PETA) and photoinitiator 2-Hydroxy-2-methyl-propiophenone (HMPP) are added to the TiO2 colloidal solution to form the new colloidal TiO2 dispersion. After coating the dispersion on the substrate, the coating is cured with UV light. With the evaporation of solvent and the formation of microgels induced by polymerization, a phase separation occurs in the system. A porous TiO2 film with various pore sizes from mesoporous to macroporous could be produced after heat treatment. The morphology and size of pores could be tuned by changing the process of phase separation, which is controlled by the composition of colloidal TiO2 dispersion. The content of photoinitiator and the temperature of heat treatment were decided by UV–vis absorption spectra and thermal analysis (DTA–TGA), respectively; Real-Time Fourier-transform infrared (RT-FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffractometer (XRD) and high-resolution transmission electron microscopy (HR-TEM) were used to characterize the TiO2 films. The as-prepared TiO2 porous films exhibited higher photocatalytic activity for degradation methylene blue in comparison with conventional TiO2 dense film due to its porous structure.

Study of the fragmentation phenomena of TiO2 nanoparticles produced by femtosecond laser ablation in aqueous media

Since last decade, Pulsed Laser Ablation in Liquid (PLAL) has become an increasingly important technique for the production of the nanoparticles (NPs) since it usually provides high purity nanoparticle systems. This paper reports on the production and fragmentation of titanium oxide TiO2 nanoparticles by pulsed laser ablation of a titanium target immersed in Sodium Dodecyl Sulfate (SDS) solution using an ultrafast Ti:Sapphire laser. After the production of TiO2 nanoparticles for 30min of laser irradiation, second harmonics of the laser wavelength are re-applied for different energies (180,120, 60µJ) to SDS solution containing TiO2 colloids in order to fragment relatively large pieces to obtain smaller ones. It was found that size of nanoparticles after the treatment is independent of the initial characteristics of colloids, but depends strongly on laser parameters especially pulse energy and on the presence of chemically active species in the solution. It was reported that particle size and size distribution range can be decreased using second harmonics of Ti:Sapphire laser wavelengths by using different values of energy. Re-irradiation process at average energy value of 180μJ decreased average particle size from 185nm to 110nm. Characterization of the NPs was studied by applying various techniques such as UV–visible (UV–vis.), Transmission Electron Microscope (TEM), Dynamic Light Scattering (DLS) and Fourier Transform Infra-Red (FTIR).

Antibacterial property and characterization of cotton fabric treated with chitosan/AgCl–TiO2 colloid

The antibacterial activity of cotton fabric was studied by using chitosan/AgCl–TiO2 colloid. Different blend ratios of chitosan to AgCl–TiO2 colloid were used to investigate the efficacy of antibacterial activity against Staphylococcus aureus (gram positive) and Escherichia coli (gram negative) and its effect on physical properties of cotton fabric. Our study shows that the combination of chitosan with AgCl–TiO2 colloid produced better antibacterial activity than the fabric treated without chitosan; 100% bacterial reduction against S. aureus and E. coli obtained with chitosan/AgCl–TiO2 colloid at concentrations of 4g/L and 10g/L respectively. Moreover, the treated cotton indicates improved tensile strength and wrinkle recovery angle (WRA). Increasing chitosan concentration slightly affected the fabric stiffness and whiteness. The treated cotton fabrics were further characterized by Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), energy dispersive X-ray (EDX) and wide angle X-ray (WAXD). We can expect a direct industrial application of our proposed work because it is simple one go pad-dry-cure method and the low cost commercial grades of chitosan and AgCl–TiO2 are conveniently available in the market.

The role of surface defect sites of titania nanoparticles in the photocatalysis: Aging and modification

A study of photocatalytic activity of bare as prepared, bare aged and dopamine surface modified colloidal TiO2 nanoparticles was obtained following degradation reaction of herbicide RS-2-(4-chloro-o-tolyloxy)propionic acid under UV light irradiation. Results showed that the most active photocatalyst is bare aged TiO2 and the least active are dopamine modified nanoparticles. Results are discussed in the light of surface structure of TiO2 nanoparticles. The study of surface modification of TiO2 nanoparticles (4.5nm, TiO2 NPs) with dopamine was also performed. The formation of inner-sphere charge-transfer complexes results in red shift of semiconductor absorption threshold (600nm), compared to bare TiO2 NPs (380nm). Effective band gap energy of 3.2eV for bare TiO2 NPs is reduced to 2.1eV for TiO2/dopamine charge transfer complexes. The binding structure was investigated by UV-vis and FTIR spectroscopy. The obtained optimal geometry for binding of dopamine to surface Ti atoms was binuclear bidentate-bridging. From the Benesi–Hildebrand plot, stability constant of the order 103M−1 has been determined.

Targeted Disinfection of E. coli via Bioconjugation to Photoreactive TiO2

The selective control of pathogenic bacteria is an ongoing challenge. A strategy is proposed that combines targeted binding of the bacterium, using antibodies, with their photoactivated oxidative destruction. Photoactive colloidal TiO2 was first derivatized with E. coli antibodies (EA-TiO2). When mixtures of the organisms E. coli and Pseudomonas putida ( P. putida ) were exposed to modified EA-TiO2, the particles preferentially selected E. coli for surface binding. Two consequences arose from surface bioconjugation: bacteria were found to flocculate upon mixing at appropriate ratios of EA-TiO2/ E. coli , and EA-TiO2-bound E. coli underwent cell death after exposure to UV light. In the former case, flocculation of the bacteria was optimal at ~50 EA-TiO2 particles per E. coli . Selective flocculation provides an alternative strategy for pathogen removal. With respect to UV disinfection, as few as 26 EA-TiO2 particles per E. coli gave a 10 000-fold decrease in viable bacteria. Thus, it is possible to selectively target and kill one type of bacteria in a mixture of pathogens. The results give support to the proposal that photocatalytic TiO2 most effectively delivers an oxidizing agent when the titania is bound to the bacterial surface.

Predictions of TiO2-driven migration of Se(IV) based on an integrated study of TiO2 colloid stability and Se(IV) surface adsorption

Colloids (ex. nanoparticles) may be relevant in contaminant migration in groundwater if the contaminant is strongly adsorbed onto colloids and they are stable in the environment.An integrated study of Se(IV) sorption and TiO2 colloid stability was performed in this work. This integrated study includes the experimental analysis and modelling of Se(IV) sorption onto TiO2 particle surface and the evaluation of the influence of adsorbed Se(IV) on TiO2 stability.Results indicated that low ionic strength and acid pH favour both high Se(IV) sorption and TiO2 colloid stability. However, at medium–high surface occupancy, sorbed Se(IV) highly affected TiO2 colloid stability. Anion adsorption decreases the positive surface charge shifting the TiO2 isoelectric point, thus promoting particle aggregation under acid pH.Nevertheless, the application of the single particle counting technique allowed determination of the presence of a significant fraction of TiO2 nanoparticles which are highly susceptible to migrate even under favourable conditions for aggregation. Therefore, although part of the TiO2 is disabled for migration by sorbed Se at acid conditions, still a considerable percentage that remains disaggregated could move together with Se. The developed Se(IV) sorption model can inform about the extent and type of sorption under specific geochemical conditions.

Improvements to the photocatalytic efficiency of polyaniline modified TiO2 nanoparticles

Polyaniline/TiO2 nanocomposite powders were successfully synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in water without the addition of acid, in the presence of colloidal TiO2 nanoparticles (d∼4.5nm). The morphological, structural, and optical properties of the synthesized nanocomposites prepared at initial TiO2/aniline mole ratios of 50, 100, and 150 were studied using transmission electron microscopy, X-ray powder diffraction, Raman and UV–vis spectroscopies. The presence of emeraldine salt and the base form of linear polyaniline (PANI) chains as well as the presence of phenazine units, branched PANI chains, oligomeric structures and short PANI chains in PANI/TiO2 nanocomposites was confirmed by Raman spectroscopy. The anatase crystal structure of TiO2 nanoparticles in all the nanocomposites was confirmed by X-ray powder diffraction measurements and by Raman spectroscopy. The photocatalytic activities of PANI/TiO2 nanocomposites were evaluated using the photocatalytic degradation of Rhodamine B and Methylene blue as model compounds and compared with the activity of bare colloidal TiO2 nanoparticles. Enhanced degradation efficiencies in the dyes used were observed. A possible explanation for the PANI/TiO2 photocatalytic activities was suggested, taking into account the influence of the molecular structures of the used dyes and PANI in PANI/TiO2 nanocomposites.

Fabrication of flexible all-inorganic nanocrystal solar cells by room-temperature processing

We demonstrate the fabrication of all-inorganic heterostructured n–p junction devices made of colloidal PbS quantum dots (QDs) and TiO2 nanorods (NRs). The entire device fabrication procedure relies on room-temperature processing, which is compatible with flexible plastic substrates and low-cost production. Through Kelvin Probe Force Microscopy and femtosecond pump and probe spectroscopy we decipher the electron transfer process occurring at the interface between the colloidal PbS QDs and TiO2 anatase NRs. Overall we demonstrate a high power conversion efficiency of ∼3.6% on glass and ∼1.8% on flexible substrates, which is among the highest reported for entirely inorganic-nanocrystal based solar cells on plastic supports.

Surface modification of anatase nanoparticles with fused ring salicylate-type ligands (3-hydroxy-2-naphthoic acids): a combined DFT and experimental study of optical properties

The surface modification of nanocrystalline TiO2 particles (45 Å) with salicylate-type ligands consisting of an extended aromatic ring system, specifically 3-hydroxy-2-naphthoic acid, 3,5-dihydroxy-2-naphthoic acid and 3,7-dihydroxy-2-naphthoic acid, was found to alter the optical properties of nanoparticles in a similar way to salicylic acid. The formation of the inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and a reduction in the band gap upon the increase in the electron delocalization when including an additional ring. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of a salicylate-type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From both absorption measurements in methanol/water = 90/10 solutions and steady-state quenching measurements of modifier fluorescence upon binding to TiO2 in aqueous solutions, stability constants in the order of 10(3) M(-1) have been determined at pH 2 and pH 3. Fluorescence lifetime measurements, in the presence and absence of colloidal TiO2 nanoparticles, indicated that the fluorescence quenching process is primarily static quenching, thus proving the formation of a nonfluorescent CT complex. The binding structures were investigated by using FTIR spectroscopy. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain the vibrational frequencies of charge transfer complexes, and the calculated values were then compared with the experimental data.

New insight into singlet oxygen generation at surface modified nanocrystalline TiO2 – the effect of near-infrared irradiation

The generation of singlet oxygen in aqueous colloids of nanocrystalline TiO2 (anatase) modified by organic chelating ligands forming surface Ti(IV) complexes was studied. Detailed studies revealed a plausible and to date unappreciated influence of near-infrared irradiation on singlet oxygen generation at the surface of TiO2. To detect (1)O2, direct and indirect methods have been applied: a photon counting technique enabling time-resolved measurements of (1)O2 phosphorescence, and fluorescence measurements of a product of singlet oxygen interaction with Singlet Oxygen Sensor Green (SOSG). Both methods proved the generation of (1)O2. Nanocrystalline TiO2 modified with salicylic acid appeared to be the most efficient photosensitizer among the tested materials. The measured quantum yield reached the value of 0.012 upon irradiation at 355 nm, while unmodified TiO2 colloids appeared to be substantially less efficient generators of singlet oxygen with the corresponding quantum yield of ca. 0.003. A photocatalytic degradation of 4-chlorophenol, proceeding through oxidation by OH˙, was also monitored. The influence of irradiation conditions (UV, vis, NIR or any combination of these spectral ranges) on the generation of both singlet oxygen and hydroxyl radicals has been tested and discussed. Simultaneous irradiation with visible and NIR light did not accelerate OH˙ formation; however, for TiO2 modified with catechol it influenced (1)O2 generation. Singlet oxygen is presumably formed according to Nosaka's mechanism comprising O2˙(-) oxidation with a strong oxidant (hole, an oxidized ligand); however, the energy transfer from NIR-excited titanium(iii) centers (trapped electrons) plays also a plausible role.

Photolithographic processing of silver loaded dielectric coatings based on preformed colloidal TiO2 nanoparticles dispersed in a mesoporous silica binder

Titanium dioxide is a well known photocatalyst for reactions involving surface trapped photogenerated carriers. Noble metal photo-reduction may be used for the processing of silver/TiO2 nanocomposite coatings that may exhibit interesting optical and electrical properties. We present here results of our investigations performed on an original system consisting of preformed colloidal TiO2 nanoparticles homogeneously dispersed within a mesoporous silica host matrix. Light irradiation of samples immerged in an aqueous silver salt solution leads to the homogeneous deposition of silver islands in the vicinity of the TiO2 particles and throughout the film thickness. The silver volume fraction is directly controlled by the irradiation dose up to a value of about 16 vol.%. Films exhibit tunable plasmonic properties that correspond to silver nanoparticles in interaction, and a percolation threshold is observed at 8–10 vol.%, leading to films with a conductivity of about 40 S cm−1. The major interest of this method lies in the high silver reduction quantum efficiency (about 50%) and the possibility to modulate optical and electronic properties by light irradiation while the low temperature of processing permits the photolithographic deposition of metallic patterns on organic flexible substrates.

Swelling of Elastomers in Solutions of TiO2 Nanoparticles

Elastomers used in protective gloves can be sensitive to the action of solvents used to disperse commercial solutions of nanoparticles. These effects may include the swelling of the polymer, leading to a modification of its mechanical and chemical properties. Modifications to the properties of the polymer will impact the protection provided by the protective gloves. The goal of this work was therefore to study the swelling of several elastomers when exposed to commercial solutions of nanoparticles. The study involved four elastomers and three commercial solutions of colloidal titanium dioxide (TiO2). Swelling was assessed by measurements of mass gain and length change. Tests were also performed with technical and ultrapure solvents corresponding to the liquid carriers. The solutions had a significant effect on the swelling of nitrile rubber, latex, and neoprene. A large mass gain was recorded for short immersion times, indicating a possible penetration of the nanoparticle liquid carrier into these elastomers. Length change measurements revealed a swelling anisotropy effect with nitrile rubber and latex in the solutions of colloidal TiO2. No effect was measured with butyl rubber. The results show that great care must be taken when selecting protective gloves for the handling of nanoparticle dispersions.

Simple inkjet-printed, UV-activated oxygen indicator

The preparation and characterisation of a novel, simple, UV-activated, water-based, colorimetric indicator for oxygen is described, comprising a redox dye (methylene blue, MB), a colloidal semiconductor photocatalyst (SC = TiO2), and a sacrificial electron donor (SED = tartaric acid). The colloidal anatase TiO2 particles, with diameter ∼5 nm, are made via the hydrothermal treatment of titanium isopropoxide, with tartaric acid as the peptizing agent, and SED. This ink is inkjet-printed on polyester film, resulting in a physically robust, colorimetric oxygen-indicating film with a thickness of ∼63 nm. Upon exposure to 3 min UVA light (4 mW cm−2), the indicator is readily photobleached when MB is photoreduced to leuco-methylene blue, LMB. The photobleached film recovers its colour in ∼12 h under ambient conditions (∼21 °C, ∼60%RH, 21% O2), but did not recover its original colour in the absence of O2. The rate of recovery in air (from cylinders) exhibits some dependence upon relative humidity at 21 °C, but little at 4 °C and rate of recovery is found to be linearly dependent upon the %O2 present. The rate of colour recovery is reduced upon reducing the temperature from 21 to 4 °C, and has an estimated activation energy for the reaction of the LMB photoreduced dye with O2, Ea = 41 ± 6 kJ mol−1. Potential uses of this simple UV light activated indicator are discussed briefly.