Colloidal TiO2 Research Articles

Highly stable colloidal TiO2 nanocrystals with strong violet-blue emission

Improved sol–gel method has been applied to prepare highly stable colloidal TiO2 nanocrystals. The synthesized titania nanocrystals exhibit strong emission in the violet-blue wavelength region. Very long evolution time was obtained by preventing the sol to gel conversion with reflux process. FTIR, XRD, UV–vis absorption, photoluminescence and high resolution transmission electron microscope (HRTEM) were used to study the optical properties, crystalline phase, morphology, shape and size of prepared TiO2 colloidal nanocrystals. HRTEM showed that the diameter of TiO2 colloidal nanocrystals is about 5nm. Although the PL spectra show similar spectral features upon excitation wavelengths at 280, 300 and 350nm, but their emission intensities are significantly different from each other. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280nm excitation wavelength which is in agreement and better than reported before. Obtained results confirm that the prepared colloidal TiO2 sample has enough potential for optoelectronics applications.

Electrocatalytic activity of composite Fe/TiO2 electrodeposits for hydrogen evolution reaction in alkaline solutions

Fe/TiO2 composite coatings were obtained by electrodeposition using a methanesulfonate plating bath containing colloidal TiO2 particles. Iron/titania electrodeposited composites exhibited enhanced electrocatalytic activity towards hydrogen and oxygen evolution reactions (HER and OER) in an alkaline solution. The electrocatalytic properties of Fe/TiO2 composites towards the HER were studied by means of voltammetry, electrochemical impedance spectroscopy and chronoamperometry methods. An increase in TiO2 content in iron matrix resulted in an increase in the electrocatalytic activity. The kinetic parameters were calculated on the basis of Tafel plots and EIS measurements and the possible mechanisms of the HER and OER were considered. The hydrogen evolution reaction on Fe/TiO2 electrodes obeyed Volmer–Heyrovsky mechanism. It was concluded that the increase in the rate of the HER and OER on Fe/TiO2 composite electrodes may be associated with the emergence of the redox couples on the electrode surface and the growth of the concentration of the surface active sites. The Fe/TiO2 electrocatalysts showed a good catalytic stability in the HER and OER in 1 M NaOH solution.

Nitrite Reduction to Nitrous Oxide and Ammonia by TiO2 Electrons in a Colloid Solution via Consecutive One-Electron Transfer Reactions.

The mechanism of nitrite reduction by excess electrons on TiO2 nanoparticles (eTiO2(-)) was studied under anaerobic conditions. TiO2 was loaded with up to 75 electrons per particle, induced by γ-irradiation of acidic TiO2 colloid solutions containing 2-propanol. Time-resolved kinetics and material analysis were performed, mostly at 1.66 g L(-1) TiO2. At relatively low nitrite concentrations (R = [eTiO2(-)]o/[nitrite]o > 1.5), eTiO2(-) decays via two consecutive processes; at higher concentrations, only one decay step is observed. The stoichiometric ratio Δ[eTiO2(-)]/[nitrite]o of the faster process is about 2. This process involves the one-electron reduction of nitrite, forming the nitrite radical (k1 = (2.0 ± 0.2) × 10(6) M(-1) s(-1)), which further reacts with eTiO2(-) (k2) in competition with its dehydration to nitric oxide (NO) (k3). The ratios k2/k3 = (3.0 ± 0.5) × 10(3) M(-1) and k2 > 1 × 10(6) M(-1) s(-1) were derived from kinetic simulations and product analysis. The major product of this process is NO. The slower stage of the kinetics involves the reduction of NO by eTiO2(-), and the detailed mechanism of this process has been discussed in our earlier publication. The results reported in this study suggest that several intermediates, including NO and NH2OH, are adsorbed on the titanium nanoparticles and give rise to inverse dependency of the respective reaction rates on the TiO2 concentration. It is demonstrated that the reduction of nitrite by eTiO2(-) yields mainly N2O and NH3 via consecutive one-electron transfer reactions.

Excited-state dynamics of size-dependent colloidal TiO2-Au nanocomposites.

The ultrafast excited-state dynamics of size-dependent TiO2-Au nanocomposites synthesized by reducing gold nanoclusters to the surface of colloidal TiO2 nanoparticles are studied using pump-probe transient absorption spectroscopy with 400 nm excitation pulses. The results show that the relaxation processes of the plasmon depletion band, which are described by electron-phonon and phonon-phonon scattering lifetimes, are independent of the gold nanocluster shell size surrounding the TiO2 nanoparticle core. The dynamics corresponding to interfacial electron transfer between the gold nanoclusters and the TiO2 bandgap are observed to spectrally overlap with the gold interband transition signal, and the electron transfer lifetimes are shown to significantly decrease as the nanocluster shell size increases. Additionally, size-dependent periodic oscillations are observed and are attributed to acoustic phonons of a porous shell composed of aggregated gold nanoclusters around the TiO2 core, with frequencies that decrease and damping times that remain constant as the nanocluster shell size increases. These results are important for the development of improved catalytic nanomaterial applications.

Electrophoretic deposition of colloidal TiO2 nanorods towards nano-porous thin-films

Nanostructured and nano-porous thin films, offering elevated active surface, are of significant importance for a wide variety of technological applications. Here we report on the electrophoretic deposition – on a conductive substrate – of colloidal TiO2 rod-like shaped nanocrystals dispersed in an organic solvent. Colloidal synthesis allowed obtaining a careful control over the morphological properties and the crystallographic phase of nanoparticles, which resulted in rod-shaped anatase nanocrystals. Compared to spherical nanocrystal, the rod-shape offers enhanced surface/volume ratio, which limits the charge recombination. The electrophoretic deposition drives nanorods toward the immobilization on the conductive surface of the substrate, forming homogeneous nanoporous thin-films composed by densely packed nanorods. Indeed, SEM analysis endorsed that the morphology of NRs is preserved in the thin-film. The approach reported here – colloidal synthesis followed by electrophoretic deposition – could be extended to the functionalization of different surfaces with the peculiar properties inferred by the nanoparticles.

Porous TiO2 Assembled from Monodispersed Nanoparticles.

Porous TiO2 were assembled by evaporating or refluxing TiO2 colloid, which was obtained by dispersing the TiO2 nanoparticles with a crystallite size (dXRD) of 3.2 nm into water or ethanol without any additives. Porous transparent bulk TiO2 was obtained by evaporating the TiO2-C2H5OH colloid at room temperature for 2 weeks, while porous TiO2 nanospheres were assembled by refluxing the TiO2-H2O colloid at 80 °C for 36 h. Both of the porous TiO2 architectures were pore-size-adjustable depending on the further treating temperature. Porous TiO2 nanospheres exhibited enhanced photocatalysis activity compared to the nanoparticles.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1372-2) contains supplementary material, which is available to authorized users.

The characteristics of novel bimodal Ag–TiO2 nanoparticles generated by hybrid laser-ultrasonic technique

Silver–titania (Ag–TiO2) nanoparticles with smaller Ag nanoparticles attached to larger TiO2 nanoparticles were generated by hybrid ultrasonic vibration and picosecond laser ablation of Ag and Ti bulk targets in deionised water, for the first time. The laser has a wavelength of 1064 nm and a pulse duration of 10 ps. It was observed that without the ultrasonic vibration, Ag and TiO2 nanoparticles did not combine, thus the role of ultrasonic vibration is essential. In addition, colloidal TiO2 and Ag nanoparticles were generated separately for comparison under the same laser beam characteristics and process conditions. The absorption spectra of colloidal Ag–TiO2 cluster nanoparticles were examined by UV–Vis spectroscopy, and size distribution was characterised using transmission electron microscopy. The morphology and composition of Ag–TiO2 nanoparticles were examined using scanning transmission electron microscopy in high-angle annular dark field, and energy-dispersive X-ray spectroscopy. The crystalline structures were investigated by X-ray diffraction. The size of larger TiO2 particles was in the range 30–150 nm, and the smaller-sized Ag nanoparticles attached to the TiO2 was mainly in the range of 10–15 nm. The yield is more than 50 % with the remaining nanoparticles in the form of uncombined Ag and TiO2. The nanoparticles generated had strong antibacterial effects as tested against E. coli. A discussion is given on the role of ultrasonic vibration in the formation of Ag–TiO2 hybrid nanoparticles by picosecond laser ablation.

Photophysics and photochemistry of uranyl ions in aqueous solutions: Refining of quantitative characteristics

The photochemistry and photophysics of aqueous solutions of uranyl nitrate have been investigated by nanosecond laser photolysis with excitation at 266 and 355 nm and by time-resolved fluorescence spectroscopy. The quantum yield has been determined for (UO 2 2+ )* formation under excitation with λ = 266 and 355 nm light (φ = 0.35). The quantum yield of uranyl luminescence under the same conditions is 1 × 10–2 and 1.2 × 10–3, respectively, while the quantum yield of luminescence in the solid state is unity, irrespective of the excitation wavelength. The decay of (UO 2 2+ )* in the presence of ethanol is biexponential. The rate constants of this process at pH 3.4 are k 1 = (2.7 ± 0.2) × 107 L mol–1 s–1 and k 2 = (5.4 ± 0.2) × 106 L mol–1 s–1. This biexponential behavior is explained by the existence of different complex uranyl ion species in the solution. The addition of colloidal TiO2 to the solution exerts no effect on the quantum yield of (UO 2 2+ )* formation or on the rate of the reaction between (UO 2 2+ )* and ethanol. The results of this study have been compared with data available from the literature.

TiO2 Nanocrystals Decorated CVD Graphene Based Hybrid for UV-Light Active Photoanodes

In this work, the manufacturing and characterization of an optically transparent and UV-light photoactive anode, formed of graphene grown by chemical vapor deposition (CVD) decorated with a close packed multilayer nanostructured layout of colloidal TiO2 nanocrystals (NCs), has been reported. The hybrid material was prepared by a facile solution-based procedure, which relays on incubating the CVD graphene in a solution of 1-pyrene butyric acid (PBA)-surface coated TiO2 NCs. Pyrene undergoes π-π stacking interactions, anchoring the NCs to the graphene platform with retention of the NC geometry and composition. Concomitantly, the NCs immobilize onto graphene preserving the structure of the aromatic platform. Photoelectrochemical investigation shows that the composite material exhibits a photoelectric response 50% higher than that of bare graphene based electrodes.

Hydrophilic carbon/TiO2 colloid composite: a potential counter electrode for dye-sensitized solar cells

Abstract: A new type of counter electrode (CE), composed of hydrophilic carbon (HC) particle and TiO2 colloid (HC/TiO2), was successfully prepared using doctor blade technique on fluorine-doped tin oxide substrate for dye-sensitized solar cell (DSSC). Properties of HC/TiO2 CE, including crystal structure, surface morphology, roughness, conductivity, and catalytic activity, were analyzed. Results showed that a HC/TiO2 CE with an average thickness of 1 µm contributed to high surface roughness. Cyclic voltammetry further revealed that HC/TiO2 CE displayed good catalytic activity similar to that of Pt electrode, which is mainly attributed to an addition of TiO2 in the electrode. DSSC was fabricated with HC/TiO2 CE. Under one sun illumination (AM 1.5, Pin of 100 mWcm−2), the device exhibited an energy conversion efficiency of 1.9 %, which is comparable to 3.6 % of the cell with Pt electrode under the same experimental conditions. These findings suggest that HC/TiO2 CE is a promising alternative CE for low-cost DSSCs. Graphical Abstract: [Figure not available: see fulltext.

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups.

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO2 has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO2 colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S2-S1 internal conversion. On the film heterogeneous electron transfer occurred in 80 fs, much faster compared to all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron transfer times were measured for different linkers. The measurements are compared to a quantum-mechanical model that accounts for electronic-vibronic coupling and finite band width for the acceptor states. We conclude that HET occurs into a distribution of transition states that differs from regular surface states or bridge mediated states.

TiO2–SiO2 films from organic-free colloidal TiO2 anatase nanoparticles as photocatalyst for removal of volatile organic compounds from indoor air

Different types of TiO2–SiO2 composites in the form of powder or films have been reported as efficient photocatalysts for decomposition of organic compounds in liquid and gas phase. Herein, we report for the first time on the formation of efficient TiO2–SiO2 films made from acidic organic-free colloidal solution of TiO2 anatase nanoparticles (AS) and mesoporous SiO2 (SBA-15) with 100% loading (TiO2: SiO2 molar ratio 1:1) under simple and low cost procedure. AS was prepared from metatitanic acid precursor using a novel, environmentally friendly approach of TiO2 nanoparticles precipitation with NaOH and peptization with HCl. These AS/SBA-15 films, immobilized by brush deposition on glass carriers, showed total decomposition of toluene and 91% decomposition of formaldehyde as model VOCs in gas phase, at room temperature under UVA irradiation in lab-made batch photoreactor. The trend of photocatalytic efficiency for decomposition of formaldehyde was AS/SBA-15>P25>PC500>AS. The adsorption capability of the AS/SBA-15 was higher in comparison to its pure TiO2 analogues (AS, P25, PC500) in case of toluene. Turnover frequency (TOF) of the AS/SBA-15 was approximately six times higher in comparison to its pure TiO2 analogue AS for toluene and formaldehyde. The potential of AS/SBA-15 for the use in air cleaning devices is finally discussed.

Photoelectrochemical activity of colloidal TiO2 nanostructures assembled at polarisable liquid/liquid interfaces

Photoelectrochemical responses arising from the heterogeneous hole-transfer from colloidal TiO2 nanoparticles to ferrocene species across the polarizable water/1,2-dichloroethane (DCE) interface are investigated as a function of the formal redox potential of the electron donor. The interfacial assembly of electrostatically stabilized 5nm TiO2 colloids was monitored by impedance measurements at various Galvani potential difference across the liquid/liquid interface. The onset potential of the photocurrent responses is close to the potential at which the excess interfacial charge increases due to the assembly of the TiO2 nanoparticles. However, a closer examination of the potential dependence of these two parameters show that the interfacial excess charge is not solely dependent on the adsorption of charged nanoparticles at the interface. We also provide strong evidence that the photoelectrochemical responses are determined by the relationship between rate of electron capture at the nanoparticle surface and surface recombination processes, rather than the interfacial oxidation of the ferrocene derivatives. Second order surface recombination constants of the order of 10−3cm2s−1 were estimated, which are consistent with a ~0.6 quantum yield for the heterogenous hole-transfer.

One step preparation of highly dispersed TiO2 nanoparticles

A facile approach was developed to prepare highly dispersed TiO2 nanoparticles with selected phase. The crystallization phase of the nanoparticles can be easily tuned from anatase to rutile by the dosage of hydrochloric acid in the reaction system. The crystallite size of the as-prepared anatase TiO2 nanoparticles was ca. 3.2 nm with high dispersion. A transparent TiO2 colloid was obtained by dispersing the as-prepared anatase TiO2 nanoparticles in deionized water without any organic additives added. The concentration of TiO2-H2O colloid can be as high as 1600 g/L. The optical transmittance of TiO2-H2O colloid with a low concentration was nearly 100% in the visible region. Furthermore, anatase TiO2 nanoparticles(TiO2-NPs) showed superior photocatalytic performance compared to rutile TiO2-NPs.

Influence of TiO2 nanoparticles on formation mechanism of PANI/TiO2 nanocomposite coating on PET fabric and its structural and electrical properties

Polyester fibers gained significant share from all other man-made fibers, but despite numerous advantages a characteristic static electricity generation is still seeking innovative solutions. In order to improve electrical properties of poly(ethyleneterephthalate) (PET) fabric, coating consisting of polyaniline/TiO2 (PANI/TiO2) nanocomposite was deposited on its surface. Coating was synthesized in acidic medium by the chemical oxidative polymerization of aniline with ammonium peroxydisulfate (APS) in the presence of colloidal TiO2 nanoparticles (d~4.5nm) and PET fabric. The morphology, molecular structure and dielectric properties of PANI/TiO2 nanocomposites were studied by scanning electron microscopy (SEM), Raman and dielectric spectroscopies. SEM analysis revealed the formation of non-uniform PANI coating on the reference PET sample. In contrast, uniform PANI/TiO2 coating on the surface of PET fabric was generated when TiO2 nanoparticles were engaged. The existence of conductive emeraldine salt form of PANI and phenazine units was confirmed by Raman spectroscopy. Formation mechanisms of PANI and PANI/TiO2 coatings on PET fabrics were proposed. The presence of TiO2 nanoparticles significantly affected the dielectric properties (permittivity and loss tangent) and AC conductivity of PANI/TiO2 coated PET fabrics. PET fabrics coated by PANI/TiO2 nanocomposite showed up to two orders of magnitude higher conductivity in the measured frequency region compared to PANI coated PET fabric.

Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals

Solution-processed films of colloidal aliovalent niobium-doped anatase TiO2 nanocrystals exhibit modulation of optical transmittance in two spectral regions-near-infrared (NIR) and visible light-as they undergo progressive and reversible charging in an electrochemical cell. The Nb-TiO2 nanocrystal film supports a localized surface plasmon resonance in the NIR, which can be dynamically modulated via capacitive charging. When the nanocrystals are charged by insertion of lithium ions, inducing a well-known structural phase transition of the anatase lattice, strong modulation of visible transmittance is observed. Based on X-ray absorption near-edge spectroscopy, the conduction electrons localize only upon lithium ion insertion, thus rationalizing the two modes of optical switching observed in a single material. These multimodal electrochromic properties show promise for application in dynamic optical filters or smart windows.

Formation and properties of polyelectrolytes/TiO2 composite coating on wood surfaces through layer-by-layer assembly method

Abstract A transparent and protective multilayer coating composed of poly(allylamine hydrochloride) (PAH), poly(styrene sulfonic acid) sodium salt (PSS), and nano-TiO2 films was fabricated on wood surfaces by layer-by-layer assembly method. The coating was formed through pre-immobilization of cationic PAH layer on wood substrate, followed by alternate soaking cycles in pH-controlled anionic PSS solution and TiO2 colloidal solution. The structure and properties of the assembled coating were characterized through scanning electron microscopy (SEM), energy disperse X-ray analysis (EDXA), UV reflection and absorption, colorimetry, and contact angle (CA) measurement. Results revealed that the coverage uniformity and thickness were improved with increasing number of PSS/TiO2 bilayers. The coating masked the cell wall ultrastructure while leaving the microscale features intact. The anatase TiO2 in the assembled coating enhanced the UV stability of wood and resulted in a lowered photochromism. Furthermore, the photocatalytic capability of the nano-TiO2 films in degrading dyes of rhodamine B and methylene blue was verified. The nano-TiO2 film on the top surface of the coating embodied a superhydrophilicity, showing self-cleaning and anti-fog capabilities. Stearic acid modification altered the superhydrophilicity to hydrophobicity with CA of 140°.

A novel fabrication of silver-modified TiO2 colloidal-assembled microstructures and enhanced visible photocatalytic activities

Silver-modified TiO2 colloidal-assembled microstructures (STCMs) have been synthesized through a novel and direct hydrothermal hydrochloric acid (HCl) etching combined with soaked Ag-loading process, in which the unique silver-modified 3D spherical hierarchy microstructures were obtained and also amorphous colloidal TiO2 became further crystalline to form unique angular nanoparticles. The physical and chemical properties of STCMs were studied by FESEM, XRD and UV–vis diffuse reflectance spectra (DRS). The photocatalytic activity of the as-prepared STCMs was evaluated by photocatalytic reduction of aqueous solution under visible light irradiation. The experimental results showed that the STCMs exhibited much better photocatalytic activity than that of TCMs and PT (pure TiO2 microspheres) we previously prepared to make a comparison.

Experimental investigation on the use of highly charged nanoparticles to improve the stability of weakly charged colloidal system

The present work highlighted on the implementation of a unique concept for stabilizing colloids at their incipiently low charge potential. A highly charged nanoparticle was introduced within a coagulated prone colloidal system, serving as stabilizer to resist otherwise rapid flocculation and sedimentation process. A low size asymmetry of nanoparticle/colloid serves as the new topic of investigation in addition to the well-established large size ratio nanoparticle/microparticle study. Highly charged Al2O3 nanoparticles were used within the present research context to stabilize TiO2 and Fe3O4 based colloids via the formation of composite structures. It was believed, based on the experimental evidence, that Al2O3 nanoparticle interact with the weakly charged TiO2 and Fe3O4 colloids within the binary system via absorption and/or haloing modes to increase the overall charge potential of the respective colloids, thus preventing further surface contact via van der Waal’s attraction. Series of experimental results strongly suggest the presence of weakly charged colloids in the studied bimodal system where, in the absence of highly charged nanoparticle, experience rapid instability. Absorbance measurement indicated that the colloidal stability drops in accordance to the highly charged nanoparticle sedimentation rate, suggesting the dominant influence of nanoparticles to attain a well-dispersed binary system. Further, it was found that the level of colloidal stability was enhanced with increasing nanoparticle fraction within the mixture. Rheological observation revealed that each hybrid complexes demonstrated behavior reminiscence to water with negligible increase in viscosity which serves as highly favorable condition particularly in thermal transport applications.

Immobilization of self-assembled pre-dispersed nano-TiO2 onto montmorillonite and its photocatalytic activity

The immobilization of pre-dispersed TiO2 colloids onto the external surface of the clay mineral montmorillonite (Mt) was accomplished and regulated via a self-assembly method employing the cationic surfactant cetyltrimethylammonium bromide (CTAB). The role of CTAB in the synthesis process was investigated by preparing a series of TiO2-CTAB-Mt composites (TCM) with various CTAB doses. The results indicated that a uniform and continuous TiO2 film was deposited on the external surface of montmorillonite in the composite synthesized with 0.1wt.% of CTAB, and the TCM nano-composites showed much higher values for specific surface area, average pore size and pore volume than the raw montmorillonite clay. Then, the formed TCM materials were applied in photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) in aqueous solution. The degradation efficiency reached as high as 94.7%. Based on the degradation intermediates benezoquinone, fumaric acid and oxalic acid identified by LC–MS analysis, a mechanism for the photocatalytic oxidation of 2,4-DCP on TiO2/Mt nano-composites is proposed.