Formation of TiO2 nanoparticles by reactive-layer-assisted deposition and characterization by XPS and STM

Black N/H-TiO2 Nanoplates with a Flower-Like Hierarchical Architecture for Photocatalytic Hydrogen Evolution

A facile two-step strategy was used to prepare black of hydrogenated/nitrogen-doped TiO₂ nanoplates (NHTA) with a flower-like hierarchical architecture. In situ nitriding and self-assembly was realized by hydrothermal synthesis using tripolycyanamide as a N source and as a structure-directing agent. After thorough characterization, it was found that the hydrogenation treatment did not damage the flower-like architecture but distorted the anatase crystal structure and significantly changed the band structure of NHTA owing to the increased concentration of oxygen vacancies, hydroxyl groups, and Ti³⁺ cations. Under AM 1.5 illumination, the photocatalytic H₂ evolution rate on the black NHTA was approximately 1500 μmol g^-1  h^-1 , which was much better than the N-doped TiO₂ nanoplates (≈690 μmol g^-1  h^-1 ). This improvement in the hydrogen evolution rate was attributed to a reduced bandgap, enhanced separation of the photogenerated charge carriers, and an increase in the surface-active sites.

A study of the transition between the non-polar and bipolar resistance switching mechanisms in the TiN/TiO2/Al memory

Thermochemical and electronic trapping/detrapping mechanism-based resistance switching in TiO2 is one of the most extensively researched topics in the field of resistance-switching random access memory (ReRAM). In this study, the subtle correlation between the formation and rupture of the Magnéli-based conducting filament (CF), which is the mechanism of non-polar thermochemical-reaction-based switching, and the electron trapping/detrapping at the defect centers, which is the mechanism of bipolar electronic switching, is examined in detail. The chemical interaction between the TiN top electrode and the TiO2 layer generates a stable and immobile electron trapping layer, which is called a "switching layer", whereas the thin region between the just-mentioned switching layer and the remaining Magnéli CF after the thermochemical reset comprises a non-switching layer. The seemingly very complicated switching behavior with respect to the bias polarity, compliance current, and detailed biasing sequence could be reasonably explained by the phenomenological model based on the combined motions of the CF, switching layer, and non-switching layer. Light-induced detrapping experiments further supplement the suggested switching model.

Highly Efficient Photocatalytic Hydrogen Evolution in Ternary Hybrid TiO2/CuO/Cu Thoroughly Mesoporous Nanofibers

Development of novel hybrid photocatalysts with high efficiency and durability for photocatalytic hydrogen generation is highly desired but still remains a grand challenge currently. In the present work, we reported the exploration of ternary hybrid TiO2/CuO/Cu thoroughly mesoporous nanofibers via a foaming-assisted electrospinning technique. It is found that by adjusting the Cu contents in the solutions, the unitary (TiO2), binary (TiO2/CuO, TiO2/Cu), and ternary (TiO2/CuO/Cu) mesoporous products can be obtained, enabling the growth of TiO2/CuO/Cu ternary hybrids in a tailored manner. The photocatalytic behavior of the as-synthesized products as well as P25 was evaluated in terms of their hydrogen evolution efficiency for the photodecomposition water under Xe lamp irradiation. The results showed that the ternary TiO2/CuO/Cu thoroughly mesoporous nanofibers exhibit a robust stability and the most efficient photocatalytic H2 evolution with the highest release rate of ∼851.3 μmol g(-1) h(-1), which was profoundly enhanced for more than 3.5 times with respect to those of the pristine TiO2 counterparts and commercial P25, suggesting their promising applications in clean energy production.

Inverse Oxide/Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments

Inverse oxide/metal catalysts have shown to be excellent systems for studying the role of the oxide and oxide-metal interface in catalytic reactions. These systems can have special structural and catalytic properties due to strong oxide-metal interactions difficult to attain when depositing a metal on a regular oxide support. Oxide phases that are not seen or are metastable in a bulk oxide can become stable in an oxide/metal system opening the possibility for new chemical properties. Using these systems, it has been possible to explore fundamental properties of the metal-oxide interface (composition, structure, electronic state), which determine catalytic performance in the oxidation of CO, the water-gas shift and the hydrogenation of CO2 to methanol. Recently, there has been a significant advance in the preparation of oxide/metal catalysts for technical or industrial applications. One goal is to identify methods able to control in a precise way the size of the deposited oxide particles and their structure on the metal substrate.

Synthesis of Carbon Nanotube-Nanotubular Titania Composites by Catalyst-Free CVD Process: Insights into the Formation Mechanism and Photocatalytic Properties

This work presents the synthesis of carbon nanotubes (CNTs) inside titania nanotube (TNTs) templates by a catalyst-free chemical vapor deposition (CVD) approach as composite platforms for photocatalytic applications. The nanotubular structure of TNTs prepared by electrochemical anodization provides a unique platform to grow CNTs with precisely controlled geometric features. The formation mechanism of carbon nanotubes inside nanotubular titania without using metal catalysts is explored and explained. The structural features, crystalline structures, and chemical composition of the resulting CNTs-TNTs composites were systematically characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The deposition time during CVD process was used to determine the formation mechanism of CNTs inside TNTs template. The photocatalytic properties of CNTs-TNTs composites were evaluated via the degradation of rhodamine B, an organic model molecule, in aqueous solution under mercury-xenon Hg (Xe) lamp irradiation monitored by UV-visible spectroscopy. The obtained results reveal that CNTs induces a synergestic effect on the photocatalytic activity of TNTs for rhodamine B degradation, opening new opportunities to develop advanced photocatalysts for environmental and energy applications.

On/off-switchable anti-neoplastic nanoarchitecture

Throughout the world, there are increasing demands for alternate approaches to advanced cancer therapeutics. Numerous potentially chemotherapeutic compounds are developed every year for clinical trial and some of them are considered as potential drug candidates. Nanotechnology-based approaches have accelerated the discovery process, but the key challenge still remains to develop therapeutically viable and physiologically safe materials suitable for cancer therapy. Here, we report a high turnover, on/off-switchable functionally popping reactive oxygen species (ROS) generator using a smart mesoporous titanium dioxide popcorn (TiO₂ Pops) nanoarchitecture. The resulting TiO₂ Pops, unlike TiO₂ nanoparticles (TiO₂ NPs), are exceptionally biocompatible with normal cells. Under identical conditions, TiO₂ Pops show very high photocatalytic activity compared to TiO₂ NPs. Upon on/off-switchable photo activation, the TiO₂ Pops can trigger the generation of high-turnover flash ROS and can deliver their potential anticancer effect by enhancing the intracellular ROS level until it crosses the threshold to open the ‘death gate’, thus reducing the survival of cancer cells by at least six times in comparison with TiO₂ NPs without affecting the normal cells.

Characterization of engineered TiO₂ nanomaterials in a life cycle and risk assessments perspective

For the last 10 years, engineered nanomaterials (ENMs) have raised interest to industrials due to their properties. They are present in a large variety of products from cosmetics to building materials through food additives, and their value on the market was estimated to reach $3 trillion in 2014 (Technology Strategy Board 2009). TiO2 NMs represent the second most important part of ENMs production worldwide (550-5500 t/year). However, a gap of knowledge remains regarding the fate and the effects of these, and consequently, impact and risk assessments are challenging. This is due to difficulties in not only characterizing NMs but also in selecting the NM properties which could contribute most to ecotoxicity and human toxicity. Characterizing NMs should thus rely on various analytical techniques in order to evaluate several properties and to crosscheck the results. The aims of this review are to understand the fate and effects of TiO2 NMs in water, sediment, and soil and to determine which of their properties need to be characterized, to assess the analytical techniques available for their characterization, and to discuss the integration of specific properties in the Life Cycle Assessment and Risk Assessment calculations. This study underlines the need to take into account nano-specific properties in the modeling of their fate and effects. Among them, crystallinity, size, aggregation state, surface area, and particle number are most significant. This highlights the need for adapting ecotoxicological studies to NP-specific properties via new methods of measurement and new metrics for ecotoxicity thresholds.

Direct synthesis of pure single-crystalline Magnéli phase Ti8O15 nanowires as conductive carbon-free materials for electrocatalysis

The Magnéli phase Ti8O15 nanowires (NWs) have been grown directly on a Ti substrate by a facile one-step evaporation-deposition synthesis method under a hydrogen atmosphere. The Ti8O15 NWs exhibit an outstanding electrical conductivity at room temperature. The electrical conductivity of a single Ti8O15 nanowire is 20.6 S cm(-1) at 300 K. Theoretical calculations manifest that the existence of a large number of oxygen vacancies changes the band structure, resulting in the reduction of the electronic resistance. The Magnéli phase Ti8O15 nanowires have been used as conductive carbon-free supports to load Pt nanoparticles for direct methanol oxidation reaction (MOR). The Pt/Ti8O15 NWs show an enhanced activity and extremely high durability compared with commercial Pt/C catalysts.

High photocatalytic activity of carbon doped TiO2 prepared by fast combustion of organic capping ligands

Carbon doped TiO₂ prepared by fast combustion of oleylamine ligands exhibit much higher photocatalytic activity for hydrogen production than those prepared by conventional methods.

Exclusion of metal oxide by an RF sputtered Ti layer in flexible perovskite solar cells: energetic interface between a Ti layer and an organic charge transporting layer

The effects of a titanium (Ti) layer on the charge transport and recombination rates of flexible perovskite solar cells were studied.

Sol–gel synthesis of nanocrystal-constructed hierarchically porous TiO2 based composites for lithium ion batteries

Hierarchically porous TiO₂ based composites have been synthesized by a facile sol–gel method. As anode materials for lithium-ion batteries (LIBs), which exhibit excellent cycling stability and superior rate capability.

Synthesis and electrochemical performance of Li4Ti5O12/TiO2/C nanocrystallines for high-rate lithium ion batteries

A Li₄Ti₅O₁₂/TiO₂/carbon (Li₄Ti₅O₁₂/TiO₂/C) nanocrystalline composite has been successfully synthesized by a facile sol–gel method and subsequent calcination treatment.

A green method to prepare TiO2/MWCNT nanocomposites with high photocatalytic activity and insights into the effect of heat treatment on photocatalytic activity

We demonstrate a green method to prepare TiO₂/MWCNT with uniform loading and high photocatalytic activity, and study the effect of heat treatment on the photocatalytic activity.

Effective photocatalysis of functional nanocomposites based on carbon and TiO2 nanoparticles

A unique nanocomposite C-TiO2 was prepared by the growth of TiO2 on carbon nanoparticles using a simple hydrothermal procedure. Transmission electron microscopic (TEM) measurements showed that the nanocomposites exhibited an average core diameter of approximately 5 nm with a rather well-defined lattice space (0.4 nm) that was somewhat larger than that (0.38 nm) of the (100) crystalline planes of anatase TiO2. This lattice expansion was accounted for by the formation of surface defect dipoles of the nanosized TiO2 particles. X-ray photoelectron spectroscopic (XPS) measurements suggested that partial charge transfer occurred from carbon nanoparticles to TiO2 by the interfacial Ti-O-C linkages, which led to effective diminishment of the C-TiO2 photoluminescence as compared to that of pure TiO2 or carbon nanoparticles, suggesting intimate electronic interactions between the carbon and TiO2 components in the nanocomposites. Such unique characteristics were then exploited for the effective photocatalytic degradation of organic pollutants, as exemplified by methylene blue, by C-TiO2 under UV photoirradiation. Experimental measurements showed that the photocatalytic activity of C-TiO2 nanocomposites was about twice that of TiO2 alone, whereas little activity was observed with carbon nanoparticles. This was attributed to the electron-accepting sites on the carbon nanoparticles that facilitated interfacial charge separation.

Preparation and characterization of TiO2/acid leached serpentinite tailings composites and their photocatalytic reduction of chromium(VI)

Composite TiO2/acid leached serpentine tailings (AST) were synthesized through the hydrolysis-deposition method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and surface area measurement (BET). The XRD analysis showed that TiO2 coated on the surface of acid leached serpentine tailings was mixed crystal phases of rutile and anatase, the grain size of which is 10-30 nm. SEM, TEM, and EDS analysis exhibited that nano-TiO2 particles were deposited on the surface and internal cavities of acid leaching serpentine tailings. The XPS and FT-IR analysis demonstrated that the coating process of TiO2 on AST was a physical adsorption process. The large specific surface area, porous structure, and plentiful surface hydroxyl group of TiO2/AST composite resulted in the high adsorption capacity of Cr(VI). The experimental results demonstrated that initial concentration of Cr(VI), the amount of the catalyst, and pH greatly influenced the removal efficiency of Cr(VI). The removal kinetics of Cr(VI) at a relative low initial concentration was fitted well with Langmuir-Hinshelwood kinetics model with R(2) value of about unity. The as-prepared composites exhibited strong adsorption and photocatalytic capacity for the removal of Cr(VI), and the possible photocatalytic reduction mechanism was studied. The photodecomposition of Cr(VI) was as high as 95% within 2h, and the reusability of the photocatalysis was proven.

Reactive-layer-assisted deposition mechanism and characterization of titanium oxide films

The growth mechanism of TiO(2) films and their morphology are reported using the reactive-layer-assisted deposition (RLAD) method under ultrahigh vacuum conditions. The oxide film formation involves Ti atom deposition on top of amorphous solid water (ASW) condensed on a SiO(2)/Si(100) support at 90 K. Subsequent annealing leads to the desorption of all nonreacted buffer molecules, resulting in the deposition of the titanium oxide film. Employing mass spectrometry and using D(2)O as a buffer, we detected the evolution of deuterium molecules during titanium atom deposition. A solid state sol-gel-like formation mechanism of titanium oxide is proposed on the basis these observations. The morphology of the oxide films is characterized by AFM as a rather uniform amorphous thin film at room temperature. Upon further annealing above 750 K, crystallization of the titanium oxide film has set in, coinciding with a dewetting process of the oxide layer, and information obtained from similar growth procedure on an amorphous carbon-covered TEM grid. It was shown that these films are rather insensitive to the underlying substrate at temperatures below 500 K.

An economic method to prepare vacuum activated photocatalysts with high photo-activities and photosensitivities

This study reports a simple and economic method to modify Degussa P25 with a vacuum activated procedure, resulting in its high photo-activity and photosensitivity, which suggests this method to be a starting point for the extension of its application to photocatalysis.

Photodegradation of methyl orange by photocatalyst of CNTs/P-TiO(2) under UV and visible-light irradiation

A novel nanoscale photocatalyst CNTs/P-TiO(2) was successfully prepared by hydrothermal method. The morphology and the physicochemical properties of the prepared samples were investigated using TEM, XPS, XRD, BET, FTIR, TG-DSC and UV-vis DRS spectroscopy. The photocatalytic activity was evaluated by degradation of methyl orange (MO) dye. The results demonstrated that CNTs/P-TiO(2) nanoparticles could effectively photodegrade MO not only under UV irradiation but also under visible-light (VL) irradiation. The MO degradation performance on CNTs/P-TiO(2) was superior to that of the commercial P25. The optimal mass ratio of CNTs to P-TiO(2) in the nanocomposite catalyst was 5:100. The synergetic effect was discussed in terms of different roles played by phosphorus doping and introducing CNTs into the composite catalysts.

Degradation of methyl orange by composite photocatalysts nano-TiO2 immobilized on activated carbons of different porosities

Composite photocatalysts TiO(2) immobilized on granular activated carbons with different porosities (TiO(2)/AC) were prepared by a novel approach, dip-hydrothermal method using peroxotitanate as precursor. The TiO(2)/AC composites were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and the nitrogen absorption. Their photocatalytic activity was evaluated by degradation of methyl orange (MO). The results showed that nano-TiO(2) particles of anatase type were well deposited on the activated carbon surface. The porosity of activated carbon had significant influence on the adsorption, the amount of TiO(2) deposited on the external surface of AC and the activity of composite photocatalysts. The composite TiO(2)/AC made from proper mesoporosity AC exhibited higher catalytic activity than the mixture of powdered TiO(2) with AC. Furthermore, the mechanism of synergistic effect of AC adsorption and TiO(2) photocatalysis was discussed.

Preparation of TiO2 nanocrystallites by oxidation of Ti-Au111 surface alloy

Ti-Au surface alloy oxidation is used to form nanocrystals of TiO(2) on Au(111). In situ scanning tunneling microscopy (STM) studies show that the approach yields arrays of 8-11 nm wide crystals with relatively narrow size dispersion and uniform crystallography. STM imaging shows that their crystallographic form is rutile with a triangular or hexagonal geometry. Scanning tunneling spectroscopy indicates that the crystals have a well-developed band gap, comparable to that in bulk TiO(2).

Scanning tunneling microscopy study of titanium oxide nanocrystals prepared on Au(111) by reactive-layer-assisted deposition

We report on an scanning tunneling microscopy study of the nanocrystallite phases of TiO(2) formed via reactive-layer-assisted deposition in ultrahigh vacuum. The synthesis used reaction of a thin layer of water, on a Au(111) substrate at 130 K, with low-coverage vapor-deposited Ti. The effects of annealing temperature and reactant coverage were investigated. Large-scale (>20 nm) patterns in the surface distribution of nanoparticles were observed with the characteristic length-scale of the pattern correlating with the thickness of the initial layer of H(2)O. The phenomenon is explained as being due to the formation of droplets of liquid water at temperatures between 130 and 300 K. After the surface was annealed to 400 K, the individual titania nanoparticles formed by this process had diameters of 0.5-1 nm. When the surface was annealed to higher temperatures, nanoparticles coalesced and for annealing temperatures of 900 K compact nanocrystals formed with typical dimensions of 5-20 nm. Three distinct classes of nanocrystallites were observed and their atomic structure and composition investigated and discussed.

Comparative study on the acute pulmonary toxicity induced by 3 and 20nm TiO(2) primary particles in mice

The acute pulmonary toxicity induced by 3-nm TiO(2) primary particles was preliminary investigated after they were intratracheally instilled at doses of 0.4, 4 and 40mg/kg into lungs of mice. The biochemical parameters in bronchoalveolar lavage fluid (BALF) and pathological examination were used as endpoints to assess their pulmonary toxicity at 3-day postexposure. As such, the pulmonary toxicity assessment of 20-nm TiO(2) primary particles was performed using the same method. It was found that the 3-nm TiO(2) primary particles induced no pulmonary toxicity at dose of 0.4mg/kg, moderate toxicity at 4mg/kg and lung overload at 40mg/kg, and this kind of particles did not produce more pulmonary toxicity than the 20-nm ones at any instilled doses. As regards physicochemical characteristics of the two TiO(2) particles, their pH values in medium, other than particle size, surface area and aggregation, may play important role in affecting their pulmonary toxicity.