Minimizing electron‐hole recombination in modified TiO2 photocatalysis: electron transfer to solution as rate‐limiting step in organic compounds degradation

4‐Stilbenecarboxaldehyde (4SCA) at pH 3 was added to TiO₂ anatase to form a new catalyst where the aldehyde carbonyl group reacts with the TiO₂‐OH to form the corresponding acetal (4SCA‐TiO₂). 4SCA‐TiO₂ significantly retards the electron recombination when it is illuminated with ultraviolet B light because of the formation of a stable radical anion·^?4SCA‐TiO₂ that we have detected spectroelectrochemically. The light excited electron on the catalysis is transferred relatively slow to solution. Therefore, the electron transfer to solution is the rate‐limiting step for water‐dissolved organic compound degradation when 4SCA‐TiO₂ is used as photocatalyst. For instance, degradation rate constants using naphthalene (Naph) and p‐nitrophenol (PNP) in an ample pH range support the proposal. Accordingly, rate constants are faster when the standard redox potential of the involved electron acceptor in the solution increases. In fact, this condition can be tuned to promote reactivity. The affinity between the organics being degraded and 4SCA‐TiO₂ also influences on the degradation rate constants.     

Investigation on SERS of different phase structure TiO2 nanoparticles

In this paper, anatase and rutile TiO₂ nanoparticles as well as their mixed crystal phase structure TiO₂ nanoparticles were synthesized by a sol‐hydrothermal method, and were served as active substrates for surface‐enhanced Raman scattering (SERS) study. The results show that the 4‐mercaptobenzoic acid probe molecules exhibit different degree SERS enhancements on the surface of different phase structure TiO₂ nanoparticles. The mixed crystal structure TiO₂ with an appropriate proportion of anatase and rutile phase is favourable to SERS enhancement of adsorbed molecules. These are mainly attributed to the contributions of the TiO₂‐to‐molecule charge transfer mechanism and the mixed crystal effect. Copyright © 2015 John Wiley & Sons, Ltd.     

Evolution of TiO2 coating layers on lamellar sericite in the presence of La and the pigmentary properties

TiO₂-coated sericite powders were prepared by the chemical deposition method starting from lamellar sericite and TiCl₄ in the presence or absence of La³⁺ cations. After calcination at 900 °C for 1 h, the resultant TiO₂ nanoparticles on the sericite surfaces existed in anatase phase. The light scattering indexes of the TiO₂-coated lamellar sericite powders were dozens of times higher than that of the naked lamellar sericite powders. The presence of La³⁺ in the deposition solution was beneficial to the formation of the small-sized anatase TiO₂ nanoparticles, resulting in the formation of the dense and uniform island-like TiO₂ coating layers in a large range of the weight ratios of TiO₂ to sericite from 5% to 20%. The TiO₂-coated lamellar sericite powders prepared in the presence of La³⁺ had higher light scattering index than that prepared in the absence of La³⁺. XPS analysis shows that when La³⁺ cations were absent in the reaction solution, TiO₂ coating layers anchored at the sericite surface via the Ti-O-Si and Ti-O-Al bonds. The presence of La³⁺ cations caused the formation of Si-O-La and Al-O-La bonds at the sericite surface and Ti-O-La bond at the surface of TiO₂ coating layers. After coating TiO₂ on the sericite surface, the yellowness of the TiO₂-coated sericite powders obviously increased and the brightness slightly decreased.     

Surface enhanced Raman scattering of nano diamond using visible‐light‐activated TiO2 as a catalyst to photo‐reduce nano‐structured silver from AgNO3 as SERS‐active substrate

In this work, we demonstrate nano‐structured silver particles photo‐reduced from silver nitride (AgNO₃) solution using visible‐light‐activated titanium dioxide (TiO₂), which can be a convenient and effective substrate for surface enhanced Raman spectroscopy (SERS) observation. Visible‐light‐activated carbon‐containing TiO₂ nanoparticles are applied to photo‐reduce and form nano‐structured silver from AgNO₃ upon visible‐light illumination. Photo‐reduced nano‐structured silver is used as an active substrate for SERS studies of TiO₂ as well as nano diamond and TiO₂. The photo reduction of AgNO₃ and SERS observation can be obtained by simultaneously using the same visible laser excitation. The coexistence of the anatase phase with small admixture of the rutile phase in the TiO₂ can be observed using SERS. The carbon structure in the carbon‐containing TiO₂ was determined to be sp² type carbon bonding by SERS. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural role of polyethylene glycol in the formation of anatase nanocrystalline titania at low temperature

Anatase nanocrystalline TiO₂ thin films were obtained by a sol–gel dip‐coating method, in which the nanocrystallization is effected by a simple hot water treatment of the deposited films at temperatures below 90 °C under atmospheric pressure for 1 h. The dip‐coating sol was prepared by reacting titanium tetra‐n‐butoxide [Ti(OⁿBu)₄] with polyethylene glycol (PEG) in ethanol. Films obtained from a sol that do not contain PEG show no sign of crystallization, demonstrating the importance of PEG in the crystallization process. Raman studies of reaction dynamics show that PEG undergoes a nucleophilic substitution reaction replacing butoxy groups in Ti(OⁿBu)₄. Stoichiometric reactions of Ti(OⁿBu)₄ with PEG in polar and nonpolar solvents were performed, and they yielded different titanium–PEG hybrid polymers, which were isolated and characterized by various spectroscopic techniques such as IR, Raman, solid‐state NMR and MALDI‐TOF‐MS. NMR studies evidenced the location and the way in which PEG is bonded with titanium atoms in the titanium–PEG hybrid polymers. On the basis of these studies, we have proposed structures for these polymers. It is demonstrated that the structure of the obtained polymers plays an important role in the formation of anatase TiO₂ nanoparticles in hot water at temperatures below 90 °C under atmospheric pressure. Copyright © 2009 John Wiley & Sons, Ltd.     

Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation,sonochemical and hydrothermal approach

In this work we prepared TiO₂ nano-powders and TiO₂/Pt nano-composites via three synthesis methods (sol-precipitation, sonochemical method, hydrothermal method) starting with the same precursors and media. To evaluate and compare the physical properties of the prepared materials, X-ray diffraction analysis, BET measurements, FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron microscopy (TEM, HRTEM, SAED) were applied. The results showed changes to the TiO₂ phase composition and crystallinity, the specific surface area as well as the platinum’s particle shape and size, depending on the method of synthesis. To determine the photocatalytic efficiency of the prepared materials, the photocatalytic discoloration of the methylene blue solution was evaluated using UV–Vis spectroscopy. The important properties required for a high photocatalytic activity, related to the surface characteristics and the phase composition, were determined in terms of the synthesis method. It was concluded that the optimum characteristics were obtained when using the hydrothermal approach, where the TiO₂ had two phases, i.e., – anatase and rutile, a Pt-phase in the form of nanoparticles and adsorbed Pt-molecular species, as well as the presence of available free surface hydroxyl groups. Such characteristics had a critical influence on the photocatalytic activity of the final material.     

Adsorption study of 4‐MBA on TiO2 nanoparticles by surface‐enhanced Raman spectroscopy

In this paper, the adsorption of 4‐mercaptobenzoic acid (4‐MBA) on TiO₂ nanoparticles was studied mostly by surface‐enhanced Raman spectroscopy (SERS) and UV‐vis spectroscopy, at different pH values as well as under different temperatures and concentrations. The results show that the 4‐MBA molecules are bonded to the TiO₂ surface both through the sulfur atoms and COO⁻ groups at neutral or alkaline pH, but only through the sulfur atom at acidic pH. Furthermore, the 4‐MBA molecules possess high adsorptive stability on TiO₂ at a comparatively high temperature (150 °C). Concentration‐dependent SERS experiments show that the saturation concentration for 4‐MBA adsorbed on TiO₂ is about 10⁻³ M in natural case (pH = 6). Copyright © 2009 John Wiley & Sons, Ltd.     

Study on transformation mechanism of lithium titanate modified with hydrochloric acid

The effects of the concentration of hydrochloric acid and treatment time on the transformation of Li₂TiO₃ were studied in detail. The results demonstrate that lithium ions are easily removed from the (?133) and (?206) planes. In contrast, Li⁺ extraction requires a longer time for the (002) and (?131) planes. A mixture of the anatase and rutile phases, pure rutile, and pure anatase can be generated by treating Li₂TiO₃ with a suitable concentration of hydrochloric acid for an appropriate amount of time. The phase(s) that are present significantly affect the cyclic adsorption performance of a titanium lithium ion sieve and the dissolution of Ti. The transformation from H₂TiO₃ particles to TiO₂ primarily occurs via the dissolution-recrystallization process. The electrophilic H⁺ and highly electronegative Cl^? affect the Ti–O bond, resulting in the destruction of the Ti–O bond in TiO₆ octahedrons, promoting the structural rearrangement of anatase to rutile TiO₂.     

Formation of single-crystalline TiO2 nanomaterials with controlled phase composition and morphology and the application in dye-sensitized solar cell

Single-crystalline TiO₂ nanomaterials with controlled phase composition and morphology were synthesized by hydrothermal transformation of H-titanate nanotubes under different pH. Rutile rectangle nanorods with two four-side tapered tips were produced at pH of 0, whereas anatase nanoparticles with mainly of rhombic shape were obtained at pH from 2 to 7 and their average particle size increased with pH. The transformation mechanisms at different pH were discussed. The single-crystalline anatase nanoparticles obtained at pH of 2 had ca. 12 nm in average particle size, and the powder possessed as large as 112 m²/g specific surface areas; the conversion efficiency of the dye-sensitized solar cell based on the nanoparticles was increased by over 40% as compared with that of the cell based on P25.     

Influence of pH on the preparation of dispersed Ag–TiO<Subscript>2</Subscript> nanocomposite

In this paper, data concerning the effect of pH on the morphology of Ag–TiO₂ nanocomposite during photodeposition of Ag on TiO₂ nanoparticles is reported. TiO₂ nanoparticles prepared by sol–gel method were coated with Ag by photodeposition from an aqueous solution of AgNO₃ at various pH levels ranging from 1 to 10 in a titania sol, under UV light. The as-prepared nanocomposite particles were characterized by UV–vis absorption spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and N₂ adsorption/desorption method at liquid nitrogen temperature (−196 °C) from Brunauer–Emmett–Teller (BET) measurements. It is shown that at a Ag loading of 1.25 wt.% on TiO₂, a high-surface area nanocomposite morphology corresponding to an average of one Ag nanoparticle per titania nanoparticle was achieved. The diameter of the titania crystallites/particles were in the range of 10–20 nm while the size of Ag particles attached to the larger titania particles were 3 ± 1 nm as deduced from crystallite size by XRD and particle size by TEM. Ag recovery by photo harvesting from the solution was nearly 100%. TEM micrographs revealed that Ag-coated TiO₂ nanoparticles showed a sharp increase in the degree of agglomeration for nanocomposites prepared at basic pH values, with a corresponding sharp decrease in BET surface area especially at pH > 9. The BET surface area of the Ag–TiO₂ nanoparticles was nearly constant at around a value of 140 m² g⁻¹ at all pH from 1–8 with an anomalous maximum of 164 m² g⁻¹ when prepared from a sol at pH of 4, and a sharp decrease to 78 m² g⁻¹ at pH of 10.     

Comparing the Contribution of Visible‐Light Irradiation,Gold Nanoparticles,and Titania Supports in Photocatalytic Nitroaromatic Coupling and Aromatic Alcohol Oxidation

Under visible‐light irradiation, gold nanoparticles (Au NPs) supported by titania (TiO₂) nanofibers show excellent activity and high selectivity for both reductive coupling of nitroaromatics to corresponding azobenzene or azoxylbenzene and selective oxidation of aromatic alcohols to corresponding aldehydes. The Au NPs act as active centers mainly due to their localized surface plasmon resonance (LSPR) effect. They can effectively couple the photonic energy and thermal energy to enhance reaction efficiency. Visible‐light irradiation has more influence on the reduction than on the oxidation, lowering the activation energy by 24.7 kJ mol^?1 and increasing the conversion rate by 88% for the reductive coupling, compared to merely 8.7 kJ mol^?1 and 46% for the oxidation. Furthermore, it is found that the conversion of nitroaromatics significantly depends on the particle size and specific surface area of supported Au NPs; and the catalyst on TiO₂(B) support outperforms that on anatase phase with preferable ability to activate oxygen. In contrast, for the selective oxidation, the effect of surface area is less prominent and Au NPs on anatase exhibit higher photo‐catalytic activity than other TiO₂ phases. The catalysts can be recovered efficiently because the Au NPs stably attach to TiO₂ supports by forming a well‐matched coherent interface observed via high‐resolution TEM.     

Influence of phonon confinement,surface stress,and zirconium doping on the Raman vibrational properties of anatase TiO2 nanoparticles

We present a comprehensive analysis of the Raman spectra of pure and zirconium‐doped anatase TiO₂ nanoparticles. To account for the wavenumber shifts of the E_g(ν₆) mode as a function of particle size (L) and dopant concentration (x), a modification of the standard phonon confinement model (PCM) is introduced, which takes into account the contribution of surface stress by means of the Laplace–Young equation. Together with X‐ray diffraction (XRD) and transmission electron microscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Raman wavenumber is of the same magnitude as the spatial phonon confinement effect. Annealing experiments show that Zr‐doped nanoparticles exhibit retarded grain growth and delayed anatase‐to‐rutile phase transition by up to 200 K compared to pure anatase TiO₂. XRD shows that Zr doping leads to a unit cell expansion of the anatase structure. Applying the modified PCM to the x‐dependent variations of the E_g(ν₆) Raman mode, the mode‐Grüneisen parameter is found to increase abruptly at x > 0.07 with a concomitant mode softening. This coincides with the x range over which the Zr cations are reported to be displaced from their position in the tetrahedral lattice, and where Zr precipitation occurs upon annealing. The results have implications for the interpretation of Raman spectra of ionic metal oxide nanoparticles and how these are modified upon cation doping. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrochemical properties of thin TiO2 electrode on Li1 + xAlxGe2 − x(PO4)3 solid electrolyte

A fabrication of all-solid-state thin-film rechargeable lithium ion batteries by sol-gel method is expected to achieve both the simplification and cost reduction for fabrication process. TiO₂ thin film electrode was prepared by PVP (polyvinylpyrrolidone) sol-gel method combined with spin-coating on Li_1 + xAl_xGe_2 − x(PO₄)₃ (LAGP) solid electrolyte which has wide electrochemical window. The thin film was composed of anatase TiO₂ that is the most active phase for Li insertion and extraction and contacted well with LAGP substrate. In the cyclic voltammogram, a redox couple was observed at 1.8 V vs. Li/Li⁺ assigned to Li insertion/extraction into/from anatase TiO₂, indicating that the thin film worked as electrode for lithium battery. The charge and discharge test in various charge and discharge rates revealed that the discharge process (delithiation) is thought to be faster than charge process (lithiation). It is attested that the sol-gel process, which derives both simplification and cost reduction for fabrication process, can be applied to thin film battery using LAGP solid electrolyte.     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.     

TiO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> nanoparticles synthesized using He/Ar thermal plasma and their effectiveness on low-concentration mercury vapor removal

Oxygen-vacant titanium dioxide (TiO_2−x ) nanoparticles were synthesized using thermal plasma as a heating source at various applied plasma currents and He/Ar ratios. Samples with diverse characteristics were developed and the mercury removal effectiveness was subsequently evaluated. TiO₂ nanoparticles possessing high purity and uniform particle sizes were successfully synthesized using metal titanium and O₂ as precursors and Ar as plasma gas. TiO_2−x in anatase phase with a particle size at 5–10 nm was formed at the He/Ar volume ratio of 25/75. Further increasing the He/Ar ratio elevated the plasma temperature, causing the tungsten to melt, vaporize from the cathode, and then dope into the formed TiO₂ nanoparticles. The doped W appeared to inhibit the growth of nanoparticles and decrease the crystallinity of formed anatase. The effectiveness of oxygen-vacant sites on Hg⁰ removal under the visible light circumstance was confirmed. Hg⁰ removal by the TiO_2−x nanoparticles was enhanced by increasing the O₂ concentration. However, moisture reduced Hg⁰ capture, especially when light irradiation was applied. The reduction in Hg⁰ capture may be resulted from the competitive adsorption of H₂O on the active sites of TiO_2−x with Hg⁰ and transformed Hg²⁺.     

Preparation and photocatalytic activity of bicrystal phase TiO2 nanotubes containing TiO2-B and anatase

Bicrystal phase TiO₂ nanotubes (NTS) containing monoclinic TiO₂-B and anatase were prepared by the hydrothermal reaction of anatase nanoparticles with NaOH aqueous solution and a heat treatment. Their structure was characterized by XRD, TEM and Raman spectra. The results showed that the bicrystal phase TiO₂ NTS were formed after calcining H₂Ti₄O₉·H₂O NTS at 573 K. The bicrystal phase TiO₂ NTS exhibit significantly higher photocatalytic activity than the single phase anatase NTS and Dessuga P-25 nanoparticles in the degradation of Methyl Orange aqueous solution under ultraviolet light irradiation, which is attributed to the large surface and interface areas of the bicrystal phase TiO₂ NTS.     

Estimation of contact area of nanoparticles in chains using continuum elastic contact mechanics

In this research, we have studied the doping behaviors of eight transition metal ion dopants on the crystal phase, particle sizes, XRD patterns, adsorption spectra, anatase fraction, and photoreactivity of TiO₂ nanoparticles. The pristine and ion-doped TiO₂ nanoparticles of 15.91-25.47 nm were prepared using sol–gel method. Test metal ion concentrations ranged from 0.00002 to 0.2 at.%. The absorption spectra of the TiO₂ nanoparticles were characterized using UV-Visible spectrometer. The wavelength of the absorption edge of TiO₂ was estimated using the spectra derivative-tangent method. The photoreactivities of pristine and ion-doped TiO₂ nanoparticles under UV irradiation were quantified by the decoloring rate of methyl orange. XRD patterns were recorded using a Rigaku D/MAX-2500 V diffractometer with Cu Kα radiation (50 kV and 250 mA), and particle size and anatase fraction were calculated. Results reveal that different ion doping exhibited complex effects on the studied characteristics of TiO₂ nanoparticles. In general, red shift occurred to ion-doped TiO₂ nanoparticles, but still with higher TiO₂ photoreactivities when doped with Fe³⁺ and Ni²⁺ ions. Among the ions investigated, Ni-doped TiO₂ nanoparticles have shown highest photoreactivity at the concentration of 0.002 at.%, about 1.9 times that of the pristine TiO₂. Ion doping was shown to reduce the diameter and influence the fraction of anatase. Data also indicated that the combination of anatase diameter and ion radius might play an important role in the photoreactivity of TiO₂ nanoparticles. This investigation contributes to the understanding of complex ion doping effects on TiO₂ nanoparticles, and provides references for enhancing their environmental application.     

Temperature dependence of pre‐edge features in Ti K‐edge XANES spectra for ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase

XANES (X‐ray absorption near‐edge structure) spectra of the Ti K‐edges of ATiO₃ (A = Ca and Sr), A₂TiO₄ (A = Mg and Fe), TiO₂ rutile and TiO₂ anatase were measured in the temperature range 20–900 K. Ti atoms for all samples were located in TiO₆ octahedral sites. The absorption intensity invariant point (AIIP) was found to be between the pre‐edge and post‐edge. After the AIIP, amplitudes damped due to Debye–Waller factor effects with temperature. Amplitudes in the pre‐edge region increased with temperature normally by thermal vibration. Use of the AIIP peak intensity as a standard point enables a quantitative comparison of the intensity of the pre‐edge peaks in various titanium compounds over a wide temperature range.     

Catalyst-free low temperature synthesis of discrete anatase titanium dioxide nanocrystals with highly thermal stability and UVC-cut capability

A novel, facile, catalyst-free, and low temperature process for the synthesis of discrete anatase TiO₂ nanocrystals has been developed in the absence of stabilizing agent. The product was shown to be discrete anatase TiO₂ nanocrystals with a mean diameter of 4.97 ± 0.9 nm and a specific surface area of 393 m²/g. By varying the water content and precursor concentration, the particle size could be tuned. Also, the resultant colloid solution was quite stable even in the absence of stabilizing agent because of the coverage of EG molecules on the particle surface. In addition, the anatase TiO₂ nanocrystals obtained in this work had highly thermal stability even at temperatures up to 800 °C. Also, as compared to Degussa P25 TiO₂ powders, they exhibited stronger absorption at 200–350 nm and higher transmittance in the visible light region. Thus, the new approach proposed in this work was practicable for the synthesis of anatase TiO₂ nanocrystals, particularly for those requested to have highly thermal stability and UVC-cut capability.

     

Characterization of TiO2 Nanoparticles in Langmuir-Blodgett Films

In this work we have synthesized TiO₂ nanoparticles, using either a sol–gel base catalysed process in the interior of CTAB reversed micelles (TiO₂ CTAB sol), or the neutralization of a TiO₂/H₂SO₄ solution in the interior of AOT reversed micelles. From the absorption and emission data of the TiO₂ nanoparticles it is possible to conclude that in the sol–gel route there remains alkoxide groups in the structure, originating transitions lower than the energy gap of TiO₂ semiconductor. These transitions disappear in the neutralization procedure, where the alkoxide groups are absent in the structure. We have assigned the observed indirect and direct optical transitions according to the anatase band structure. TiO₂ Langmuir-Blodgett (LB) films were prepared either by direct deposition of titanium isopropoxide or by deposition of the TiO₂ CTAB sol. These films showed photoluminescence, which was attributed to band-gap emission and to surface recombination of defect states.