Photocatalytic Activity of Amorphous−Anatase Mixture of Titanium(IV) Oxide Particles Suspended in Aqueous Solutions

Charge transport-driven selective oxidation of graphene

Due to the tunability of the physical, electrical, and optical characteristics of graphene, precisely controlling graphene oxidation is of great importance for potential applications of graphene-based electronics. Here, we demonstrate a facile and precise way for graphene oxidation controlled by photoexcited charge transfer depending on the substrate and bias voltage. It is observed that graphene on TiO2 is easily oxidized under UV-ozone treatment, while graphene on SiO2 remains unchanged. The mechanism for the selective oxidation of graphene on TiO2 is associated with charge transfer from the TiO2 to the graphene. Raman spectra were used to investigate the graphene following applied bias voltages on the graphene/TiO2 diode under UV-ozone exposure. We found that under a reverse bias of 0.6 V on the graphene/TiO2 diode, graphene oxidation was accelerated under UV-ozone exposure, thus confirming the role of charge transfer between the graphene and the TiO2 that results in the selective oxidation of the graphene. The selective oxidation of graphene can be utilized for the precise, nanoscale patterning of the graphene oxide and locally patterned chemical doping, finally leading to the feasibility and expansion of a variety of graphene-based applications.

Engineering the Absorption and Field Enhancement Properties of Au-TiO2 Nanohybrids via Whispering Gallery Mode Resonances for Photocatalytic Water Splitting

Recently, surface plasmon resonance (SPR) effects have been widely used to construct photocatalysts which are active in the visible spectral region. Such plasmonic photocatalysts usually comprise a semiconductor material transparent in the visible range (such as TiO2) and plasmonic nano-objects (e.g., Au nanoparticles (Au NPs)). Specific SPRs, though, only partially cover the visible spectrum and feature weak light absorption. Here, we explore the unique role played by whispering gallery mode (WGM) resonances in the expression of the photocatalytic activity of plasmonic photocatalysts. Using numerical simulations, we demonstrate that, by solely exploiting a proper geometrical arrangement and WGM resonances in a TiO2 sphere, the plasmonic absorption can be extended over the entire visible range and can be increased by more than 40 times. Furthermore, the local electric field at the Au-TiO2 interface is also considerably enhanced. These results are experimentally corroborated, by means of absorption spectroscopy and Raman measurements. Accordingly, such WGM-assisted plasmonic photocatalysts, when employed in water splitting experiments, exhibit enhanced activity in the visible range. Our findings show a promising and straightforward way to design full solar spectrum photocatalysts.

Remarkable Charge Separation and Photocatalytic Efficiency Enhancement through Interconnection of TiO2 Nanoparticles by Hydrothermal Treatment

Although tremendous effort has been directed to synthesizing advanced TiO2 , it remains difficult to obtain TiO2 exhibiting a photocatalytic efficiency higher than that of P25, a benchmark photocatalyst. P25 is composed of anatase, rutile, and amorphous TiO2 particles, and photoexcited electron transfer and subsequent charge separation at the anatase-rutile particle interfaces explain its high photocatalytic efficiency. Herein, we report on a facile and rational hydrothermal treatment of P25 to selectively convert the amorphous component into crystalline TiO2 , which is deposited between the original anatase and rutile particles to increase the particle interfaces and thus enhance charge separation. This process produces a new TiO2 exhibiting a considerably enhanced photocatalytic efficiency. This method of synthesizing this TiO2 , inspired by a recently burgeoning zeolite design, promises to make TiO2 applications more feasible and effective.

Aqueous arsenite removal by simultaneous ultraviolet photocatalytic oxidation-coagulation of titanium sulfate

This study explored the efficacy and efficiency of a simultaneous UV-catalyzed oxidation-coagulation process of titanium sulfate (UV/Ti(SO4)2) for efficient removal of As(III) from water. It revealed that, As(III) could be oxidized to As(V) during the UV catalyzed coagulation of Ti(SO4)2 with highly efficient As(III) removal in the pH range 4-6. The UV catalyzed oxidation-coagulation showed surprisingly effective oxidation of As(III) to As(V) within a short time. XPS indicated that 84.7% of arsenic on the coagulated precipitate was in the oxidized form of As(V) after the UV/Ti(SO4)2 treatment of As(III) aqueous solutions at pH 5. Arsenic remaining in solution at high pH was in the oxidized form As(V). Removal efficiencies of As(III) were investigated as a function of pH, Ti(SO4)2 dosage, initial As(III) concentration and irradiation energy. As(III) could almost completely be removed (>99%) by the photocatalytic oxidation-coagulation process with a moderate dose of Ti(SO4)2 in the pH range 4-6 at an initial arsenic concentration of 200 μg/L. The mechanisms of the photocatalytic coagulation oxidation of Ti(SO4)2 are similar to those of UV/crystalline TiO2 particles, involving the formation and reactions of the hydroxyl radical OH and superoxide HO2/O2(-).

Size matters--The phototoxicity of TiO2 nanomaterials

Under solar radiation several titanium dioxide nanoparticles (nano-TiO2) are known to be phototoxic for daphnids. We investigated the influence of primary particle size (10, 25, and 220 nm) and ionic strength (IS) of the test medium on the acute phototoxicity of anatase TiO2 particles to Daphnia magna. The intermediate sized particles (25 nm) showed the highest phototoxicity followed by the 10 nm and 220 nm sized particles (median effective concentrations (EC50): 0.53, 1.28, 3.88 mg/L). Photoactivity was specified by differentiating free OH radicals (therephthalic acid method) and on the other hand surface adsorbed, as well as free OH, electron holes, and O2(-) (electron paramagnetic resonance spectroscopy, EPR). We show that the formation of free OH radicals increased with a decrease in primary particle size (terephthalic acid method), whereas the total measured ROS content was highest at an intermediate particle size of 25 nm, which consequently revealed the highest photoxicity. The photoactivities of the 10 and 220 nm particles as measured by EPR were comparable. We suggest that phototoxicity depends additionally on the particle-daphnia interaction area, which explains the higher photoxicity of the 10 nm particles compared to the 220 nm particles. Thus, phototoxicity is a function of the generation of different ROS and the particle-daphnia interaction area, both depending on particle size. Phototoxicity of the 10 nm and 25 nm sized nanoparticles decreased as IS of the test medium increased (EC50: 2.9 and 1.1 mg/L). In conformity with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory we suggest that the precipitation of nano-TiO2 was more pronounced in high than in low IS medium, causing a lower phototoxicity. In summary, primary particle size and IS of the medium were identified as factors influencing phototoxicity of anatase nano-TiO2 to D. magna.

Influence of the porous texture of SBA-15 mesoporous silica on the anatase formation in TiO2–SiO2 nanocomposites

TiO₂–SiO₂ composites with 44 wt% of anatase exhibit a better MO degradation rate than those with fully crystallized anatase.

Factors affecting oxygen evolution through water oxidation on polycrystalline titanium dioxide

A linear correlation between crystallite size and O₂ evolution rate was obtained, due to the efficient spatial separation of photogenerated carriers.

Ag Nanorods Coated with Ultrathin TiO2 Shells as Stable and Recyclable SERS Substrates

TiO₂-coated Ag nanorods (Ag@TiO₂ NRs) have been fabricated as multifunctional surface-enhanced Raman scattering (SERS) substrates. Uniform TiO₂ shells could sufficiently protect the internal Ag NRs against oxidation and sulfuration, thus the temporal stability of SERS substrates was markedly improved. Meanwhile, due to the synergetic effect between crystalline TiO₂ and Ag, the nanocomposites could clean themselves via photocatalytic degradation of the adsorbed molecules under ultraviolet irradiation and water dilution, making the SERS substrates renewable. Such Ag@TiO₂ NRs were shown to serve as outstanding SERS sensors featuring high sensitivity, superior stability and recyclability.

Development of photocatalysts for selective and efficient organic transformations

One of the main goals of organic chemists is to find easy, environmentally friendly, and cost effective methods for the synthesis of industrially important compounds. Photocatalysts have brought revolution in this regard as they make use of unlimited source of energy (the solar light) to carry out the synthesis of organic compounds having otherwise complex synthetic procedures. However, selectivity of the products has been a major issue since the beginning of photocatalysis. The present article encompasses state of the art accomplishments in harvesting light energy for selective organic transformations using photocatalysts. Several approaches for the development of photocatalysts for selective organic conversions have been critically discussed with the objective of developing efficient, selective, environmental friendly and high yield photocatalytic methodologies.

Electronic structures and current conductivities of B, C, N and F defects in amorphous titanium dioxide

Energetics and charge conductivities of defect states in amorphous titanium dioxide induced by second-row elements (B, C, N and F doping) were investigated by DFT calculations and Marcus theory.

Review of one-dimensional and two-dimensional nanostructured materials for hydrogen generation

Hydrogen is an attractive alternative to fossil fuels in terms of environmental and other advantages.

Synthesis and characterization of nanocrystalline TiO2 with application as photoactive coating on stones

Self-cleaning photocatalytic coatings for biocalcarenite stones, based on TiO2 nanoparticles obtained by sol-gel processes at different pH values and also adding gold particles, have been investigated. The selected test material is a biocalcarenite named "pietra di Lecce" (Lecce stone), outcropping in Southern Italy. Scanning electron microscopy with energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, and Raman investigations were carried out to characterize the TiO2 nanoparticles and coatings. Nanocrystalline anatase and, to a lesser extent, brookite phases are obtained. Photocatalytic activity of the TiO2 sols and of the coatings on "pietra di Lecce" was assessed under ultraviolet irradiation, monitoring methyl orange (MeO) dye degradation as a function of time. To evaluate the harmlessness of the treatment, colorimetric tests and water absorption by capillarity were performed. The results show good photodegradation rates for titania nanosols, particularly when putting in Au particles, whereas a satisfactory chromatic compatibility between the sol and the surface of the calcarenite is found only without Au addition.

HIGHLIGHTS

Sols of nanocrystalline titania at different pH values and with Au particles were prepared and characterized. Satisfactory photodegradation of MeO by the sols in solution and on calcarenite-coated surfaces is obtained. The addition of Au particles improves the photodegradation activity but gives poor chromatic results on "pietra di Lecce."

Influence of a silica interlayer on the structural and magnetic properties of sol-gel TiO₂-coated magnetic nanoparticles

Superparamagnetic iron oxide nanoparticles coated with titanium dioxide have been synthesized, growing the titanium dioxide directly either on the magnetic nuclei or on magnetic nanoparticles previously coated with a semihydrophobic silica layer. Both coatings have been obtained by sol-gel. Since it is well-known that the existence of the intermediate silica layer influences the physicochemical properties of the material, a detailed characterization of both types of coatings has been carried out. The morphology, structure, and composition of the synthesized nanomatrices have been locally analyzed with subangstrom spatial resolution, by means of aberration corrected transmission electron microscopy (HRTEM and STEM-HAADF). Besides magnetization measurements, proton relaxivity experiments have been also performed on water suspensions of the as-synthesized nanoparticles to investigate the role of the silica interlayer in the relaxometric properties. The silica interlayer leads to nanoparticles with much higher water stability and to higher relaxivity of the suspensions.

A facile low-temperature approach to designing controlled amorphous-based titania composite photocatalysts with excellent noble-metal-free photocatalytic hydrogen production

A microporous amorphous-based titania composite photocatalysts has been fabricated using a facile low-temperature (120 °C) synthetic method. Notably, we have successfully prepared the various stages of the amorphous/crystalline heterostructure by simply adjusting the pH value. The high-pH sample favors the formation of amorphous based titania composite structure. Additionally, the BET surface area of the sample increases with the increasing of the pH value, reaching a maximum of 358 m(2) g(-1) when the pH value is 12. Unexpectedly, the H2 productivity of amorphous-based composite photocatalyst without noble metal co-catalyst increases significantly with the increasing pH value, which is attributed to the quickly increasing amorphous, and the highly active catalytic centers created by the synergistic effect between crystalline TiO2 and amorphous ZnO. This study demonstrates that it is possible to improve the properties of the amorphous-based composite photocatalyst by properly modifying the synthesis conditions. The approach presented herein can be applied to the research of controlled amorphous-based composite photocatalytic systems.

Physical characterization of titanium dioxide nanoparticles

OBJECTIVE

The objective of this study was to review six measurement methods (X-ray line broadening, electron microscopy, static light scattering, dynamic light scattering, X-ray sedimentation and surface area determination), which are widely used for the characterization of ultrafine inorganic oxides used in cosmetic formulations. Depending on the processing that they have received and the system in which they are examined, these oxides can exist as primary particles, strongly bound aggregates or weakly bound agglomerates.

METHODS

The example of titanium dioxide, TiO2 , is used to consider which type of particle is being measured in a particular case, and the factors which influence the 'size' that is generated by a particular method. Where appropriate, a correlation is made between results of different measurements.

RESULTS

Results for a particular set of four cosmetic grade TiO2 's are presented and examined, in the context of a much broader set of measurements taken from the scientific literature.

CONCLUSION

In general, X-ray line broadening, electron microscopy and surface area measurements led to estimates of the size of primary particles. By contrast, both sedimentation and light scattering measurements measured the size of the secondary particles, and the figures which were generated depended on the dispersion conditions used for preparation of the measurement samples. For poorly dispersed or lightly milled samples, the size may be dominated by the presence of weakly bound agglomerates, but even when the sample is well dispersed or heavily milled, the reported size cannot be less than that of the aggregates.

Photocatalysts with internal electric fields

The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

Morphological and structural behavior of TiO2 nanoparticles in the presence of WO3: crystallization of the oxide composite system

Composite TiO₂–WO₃ oxide materials were prepared by a single pot microemulsion method and studied during calcination treatments under dry air in order to analyze the influence of tungsten on the behavior of the dominant titania component.

Effect of doping on electronic structure and photocatalytic behavior of amorphous TiO2

Visible light photocatalysts based on doped crystalline forms of titanium dioxide (TiO2) have attracted significant scientific attention in recent decades. Amorphous TiO2, despite many merits over crystalline phases, has not been studied as thoroughly. In this paper, an in-depth analysis of the electronic properties of doped amorphous TiO2 is performed using density functional theory with Hubbard's energy correction (DFT + U). Monodoping with p-type (N) and n-type (Nb) dopants shows appreciable bandgap reduction, but leads to recombination centers due to the presence of uncompensated charges. To resolve this issue, charge compensation via codoping is attempted. The charge compensated codoping not only reduces the bandgap by 0.4 eV but also eliminates the bandgap states present in monodoped systems responsible for charge carrier recombination. Furthermore, the localized tail states present in the aTiO2 system are eliminated to a large extent which leads to a decrease in the charge recombination and an increase in the charge migration. Thus, appropriate doping of amorphous TiO2 may lead to an alternative route for the development of visible light photocatalysts.

Template-free synthesis of a porous organic-inorganic hybrid tin(IV) phosphonate and its high catalytic activity for esterification of free fatty acids

Here we have synthesized an organic-inorganic hybrid mesoporous tin phosphonate monolith (MLSnP-1) with crystalline pore walls by a template-free sol-gel route. N2 sorption analysis shows Brunauer-Emmett-Teller (BET) surface area of 347 m2 g(-1). Wide-angle powder X-ray diffraction (PXRD) pattern shows few broad diffraction peaks indicating crystalline pore wall of the material. High-resolution transmission electron microscopic (HR TEM) image further reveals the crystal fringes on the pore wall. Framework bonding and local environment around phosphorus and carbon were examined by Fourier transform infrared (FT IR) spectroscopy and solid-state MAS NMR spectroscopy. The material exhibits remarkable catalytic activity for esterification of long chain fatty acids under mild reaction conditions at room temperature.