Anatase TiO2 Nanocrystals Research Articles

Crystal-plane effects of anatase TiO2 on the selective hydrogenation of crotonaldehyde over Ir/TiO2 catalysts

Supported Ir/TiO2 catalysts with the anatase TiO2 nanocrystals exposing {101}, {100} and {001} planes were tested for selective hydrogenation of crotonaldehyde. The Ir/TiO2-{101} catalyst dominantly exposing {101} plane gave a initial reaction rate of 166.1 μmol gIr-1 s−1 at 80 °C and a turnover frequency of crotyl alcohol formation of 0.022 s−1, which is 5 and 7 times respectively higher than those obtained on the Ir/TiO2-{001} catalyst dominantly exposing {001} plane even though they have similar Ir particle sizes (ca. 1.3 nm). The reaction behaviors were related to the surface oxygen vacancy, which served as surface acid sites and provide adsorption sites for CO bond in the CRAL molecule. Therefore, the higher concentration of oxygen vacancy in the Ir/TiO2-{101} accounted for the improved performance compared to the Ir/TiO2-{001}. However, strong adsorption of crotonaldehyde and/or products on the catalyst surface, as well as the CO poisoning via decarbonylation of crotonaldehyde, were the main reasons for the catalyst deactivation.

Effect of metal ions doping on structural, optical properties and photocatalytic activity of anatase TiO2 thin films

The metal ion‐doped (Fe3+, Co2+, Ni2+ and Ag+) TiO2 nanocrystalline thin films were spin‐coated on glass substrates. The grazing incidence X‐ray diffractometer (GIXRD), SEM, energy‐dispersive X‐ray Spectroscopy (EDX), Raman, UV–VIS and a fluorescence spectrometer were employed to study the structural, compositional and optical properties of metal‐doped TiO2 films. As revealed by GIXRD, all the metal‐doped TiO2 thin films were found to anatase tetragonal crystal structure. The deposited films were optimized using annealing temperature (550°C) to get high‐quality crystalline films with the anatase phase. The thin films with the specific surface area of Fe‐, Co‐, Ni‐ and Ag‐doped anatase TiO2 nanocrystals are 152.18, 88.25 96.68 and 178.38 m2/g, respectively, which has been calculated by GIXRD results. Optical transmittance spectra were used to determine the thickness, band gap energy and refractive index of metal‐doped TiO2 thin films. The characteristic vibrational Raman modes also revealed that the films were found to be anatase phase. The intensity of luminescence emission decreases with the doping of metal ions. Metal‐doped TiO2 films exhibit strong photocatalytic performance than undoped TiO2 films. The degradation efficiencies typically increase with the different dopants for the visible‐light degradation of methylene blue (MB) and methyl orange (MO). Moreover, the photocatalytic activity (PCA) of silver ion‐doped titania thin film is found to be better than that of Fe3+‐, Co2+‐, and Ni2+‐doped TiO2 films.

Selective CO2 Photoconversion on Low-Coordinate TiO2

The selective conversion of CO2 into desired hydrocarbon products remains a challenge in photocatalysis. In this issue of Chem, Feng and colleagues report a strong-magnetic-field-assisted strategy for forming active low-coordinate (100) facets on TiO2 nanotubes, enabling selective photoconversion of CO2 into the C2 product C2H5OH under visible light.

Defect-induced anisotropic surface reactivity and ion transfer processes of anatase nanoparticles

Surface reactivity and ion transfer processes of anatase TiO2 nanocrystals were studied using lithium bis(trifluoromethylsulfone)imide (LiTFSI) as a probing molecule. Analysis of synthesized anatase TiO2 by electron microscopy reveals aggregated nanoparticles (average size ~8 nm) with significant defects (holes and cracks). With the introduction of LiTFSI salt, the Li+-adsorption propensity towards the surface along the anatase (100) step edge plane is evident in both x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis. Ab initio molecular dynamics (AIMD) analysis corroborates the site-preferential interaction of Li+ cations with oxygen vacancies and the thermodynamically favorable transport through the (100) step edge plane. Using 7Li nuclear magnetic resonance (NMR) chemical shift and relaxometry measurements, the presence of Li+ cations near the interface between TiO2 and the bulk LiTFSI phase was identified, and subsequent diffusion properties were analyzed. The lower activation energy derived from NMR analysis reveals enhanced mobility of Li+ cations along the surface, in good agreement with AIMD calculations. On the other hand, the TFSI– anion interaction with defect sites leads to CF3 bond dissociation and subsequent generation of carbonyl fluoride-type species. The multimodal spectroscopic analysis including NMR, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS) confirms the decomposition of TFSI– anions near the anatase surface. The reaction mechanism and electronic structure of interfacial constituents were simulated using AIMD calculations. Overall, this work demonstrates the role of defects at the anatase nanoparticle surface on charge transfer and interfacial reaction processes.

Rhodium as efficient additive for boosting acetone sensing by TiO2 nanocrystals. Beyond the classical view of noble metal additives

Anatase TiO2 nanocrystals were prepared by solvothermal synthesis and modified by in- situ generated Rh nanoparticles, with a starting nominal Rh:Ti atomic concentration of 0.01 and 0.05. After heat-treatment at 400 °C the TiO2 host was still in the anatase crystallographic phase, embedding Rh nanoparticles homogeneously distributed and whose surface had been oxidized to Rh2O3, as established by X-ray diffraction, Transmission Electron Microscopy and X-ray Photoelectron spectroscopy. Moreover, Rh seemed also homogeneously distributed in elemental form or as Rh2O3 nanoclusters. The acetone sensing properties of the resulting materials were enhanced by Rh addition, featuring a response increase of one order of magnitude at the best operating temperature of 300 °C. Moreover, Rh addition enlarged the detection range down to 10 ppm whereas pure TiO2 was not able of giving an appreciable response already at a concentration as high as 50 ppm. From the sensing data, the enhancement of the sensor response was attributed to the finely dispersed Rh species and not to the oxidized Rh nanocrystals.

Maximizing the Catalytically Active {001} Facets on Anatase Nanoparticles

In applications such as photocatalysis and NOx removal, it is advantageous to have a large exposure of the high-energy {001} facets of anatase TiO2 nanocrystals. However, a large-scale synthesis of...

Manipulation of Electron Transfer between Pd and TiO2 for Improved Electrocatalytic Hydrogen Evolution Reaction Performance.

The urgent need of catalysts with improved performances toward the hydrogen evolution reaction (HER) is still one of the crucial issues for the water splitting electrocatalysis. Herein, we exhibit that the HER activity of the Pd nanocubes could be improved by selecting the appropriately shaped titania nanocrystals as support. In particular, we used Pd nanoparticles with (100)-facet exposed to show that the HER performance of Pd cubes can be improved in both acidic and alkaline electrolyte media when combined on the anatase TiO2 nanocrystals. Furthermore, we have also investigated the facet effect of TiO2 on the performance in detail, which indicated stronger catalytic activity when (001)-TiO2 was used rather than (mix 101/001)-TiO2 and (101)-TiO2. The electron-transfer-induced improvement of HER activity of Pd/TiO2 was assessed by electron energy loss spectroscopy (EELS). Thereafter, the combined support materials with suitable facet exposed can give an additional adjusting path to regulate the HER activities of Pd nanocatalysts, which henceforth can further contribute to a novel way for tuning other catalysts with good electrocatalytic properties.

Vapor pressure-assisted synthesis of chemically bonded TiO2/C nanocomposites with highly mesoporous structure for lithium-ion battery anode with high capacity, ultralong cycling lifetime, and superior rate capability

To date, development of a lithium-ion battery anode with high capacity, long cycling life over ten thousands of cycles, and fast lithium ion storage within a few seconds for each cycle is still a challenge. Here, a highly mesoporous TiO2/C nanocomposite consisting of ultrathin carbon layers tightly coating on the surface of anatase TiO2 nanocrystals with an average size of 20 nm via interfacial chemical bonds of Ti-O-C is obtained by a simple and facile vapor pressure-assisted synthesis route that pyrolysis of the solution of tetrabutyl orthotitanate tetramer dissolved in ethanol in a sealed vessel. As anode for lithium ion battery, the TiO2/C nanocomposite shows a reversible capacity of 630.3 mAh g−1 at 0.5C after 400 cycles, which is unprecedent for the ever reported TiO2/C electrode at similar cycle number. Besides, the TiO2/C nanocomposite also exhibits ultralong cycling lifetime (over 10000 cycles without capacity loss at 120C) and ultrahigh rate capability (20.5 mAh g−1 at 144C). The outstanding lithium ion storage performances are ascribed to its unique structures, i.e., ultrathin carbon coating, ultrasmall TiO2 particles, the formation of interfacial chemical bonds of Ti-O-C, and highly mesoporous structure.

Low temperature synthesis of anatase TiO2 nanocrystals using an organic-inorganic gel precursor

The purpose of this research was to establish a novel approach to synthesize nano-crystalline anatase TiO2 powder through a low temperature solution process and extracting TiO2 from the solution via freeze-drying procedure. The amorphous TiO2 precursors were first prepared using an aqueous TiCl4 solution and N, N-dimethylformamide as a TiO2 structure directing agent at 40 °C. The freeze-drying treatment of the DMF-derived amorphous Ti(IV) gel precursors produced organic–inorganic hybrids, including crystallized tiny TiO2 particles. The size, shape, and crystal phase of the TiO2 particles were determined by using transmission electron microscopy and X-ray diffraction. Most notably, anatase TiO2 nanocrystals were successfully obtained at less than near-room temperature. Furthermore, a sintering process of the gel precursors at 500 °C produced anatase TiO2 nanoparticles with three-dimensional network morphology. The resulting nanoparticles were found to have approximate grain sizes of 30–50 nm, with the organic-inorganic gel supporting the crystallization as a template.

Surface Sensitivity of Hydrogen Evolution and Formaldehyde Reduction on Differently Oriented TiO2 Anatase Nanocrystals

Selectivity of nanocrystalline anatase electrodes with different preferential surface orientation in formaldehyde reduction was assessed as a model of oxide-based catalyst for electrochemical CO2 valuation. Cathodic behavior of TiO2 (anatase)-based electrodes observed in formaldehyde reaction integrates, in fact, several processes including hydrogen evolution, formaldehyde reduction, and proton insertion into anatase structure. The electrochemical activity of the anatase-based cathodes is, regardless of the surface orientation, dominated by proton insertion. The proton insertion is more pronounced on {001}-oriented anatase than on {101}-oriented nanocrystals due to anisotropy of the proton transport in the anatase which is more facile in the (001) direction. Aside from the proton insertion, both anatase orientations also differ in selectivity in the formaldehyde reduction. While {101} surface orientation produces primarily hydrogen and methanol, the same process on {001}-oriented surfaces shows the ability to produce aside of methanol also hydrocarbons most likely methane. The overall activity towards the reduction of organics is, however, lower than that of metals.

Green approach and ease synthesis of C/N-codoped TiO2 nanocrystals for photodegradation of endocrine

Carbon species and nitrogen–codoped anatase TiO2 (C/N-codoped TiO2) nanocrystals with uniform particle size and high crystallinity were synthesized, employing hexamethylenetetramine (HMTA) as a nitrogen and carbon source throughout solvothermal reaction of TiCl4 and HMTA at different contents (0.5 g, 1 g, 1.5 g, and 2 g HMTA). The synthesized photocatalysts were assessed by photocatalytic degradation of resorcinol as a probe of potent endocrine-disrupting chemicals (EDCs) compared with commercial P-25 photocatalyst. Raman spectra and XRD results confirmed that the obtained C/N-codoped TiO2 is anatase TiO2 phase with 10–20 nm nanocrystallite sizes. For resorcinol photo-oxidation in aerated aqueous solution, C/N-codoped TiO2 nanocrystals synthesized at 0.5 g HMTA (H0.5) showed the superior photocatalytic efficiency among all synthesized photocatalysts. H0.5 photocatalyst exhibited the outstanding photocatalytic performance, which is 1.2 and 2 times greater than that of commercial P-25 and pure TiO2. In addition, the rate constant of the H0.5 sample was enhanced 7.5 and 3 times than that of commercial P-25 and pure TiO2, respectively. The C/N-codoped TiO2 nanocrystals achieved high photocatalytic efficiency of up to 96% within 90 min. In contrast, the photocatalytic efficiency based on the commercial P-25 was 54%. The photocatalytic efficiency of C/N-codoped TiO2 was considerably enhanced, owing to its increase of photogenerated charge separation rate, smaller crystal size, narrower bandgap energy, and stronger light harvesting in wide visible region. Detailed mechanistic and kinetic studies through photodegradation of resorcinol in water were addressed.

Diffusion doping of cobalt in rod-shape anatase TiO2 nanocrystals leads to antiferromagnetism.

Cobalt(ii) ions were adsorbed to the surface of rod-shape anatase TiO2 nanocrystals and subsequently heated to promote ion diffusion into the nanocrystal. After removal of any remaining surface bound cobalt, a sample consisting of strictly cobalt-doped TiO2 was obtained and characterized with powder X-ray diffraction, transmission electron microscopy, UV-visible spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy, SQUID magnetometry, and inductively-coupled plasma atomic emission spectroscopy. The nanocrystal morphology was unchanged in the process and no new crystal phases were detected. The concentration of cobalt in the doped samples linearly correlates with the initial loading of cobalt(ii) ions on the nanocrystal surface. Thin films of the cobalt doped TiO2 nanocrystals were prepared on indium-tin oxide coated glass substrate, and the electrical conductivity increased with the concentration of doped cobalt. Magnetic measurements of the cobalt-doped TiO2 nanocrystals reveal paramagnetic behavior at room temperature, and antiferromagnetic interactions between Co ions at low temperatures. Antiferromagnetism is atypical for cobalt-doped TiO2 nanocrystals, and is proposed to arise from interstitial doping that may be favored by the diffusional doping mechanism.

Three-dimensional morphology of anatase nanocrystals obtained from supercritical flow synthesis with industrial grade TiOSO4 precursor

Anatase TiO2 (a-TiO2) nanocrystals are vital in catalytic applications both as catalysts (e.g. photodegradation) and as a carrier material (e.g. NOx removal from exhaust). The synthesis of a-TiO2 nanocrystals and their properties have been heavily scrutinized, but there exists a clear gap between the scientific literature, and the scale and price expectation of industrial application. Here it is demonstrated that the industrially most attractive Ti precursor, titanyl sulfate (TiOSO4), can be combined with the green, scalable and fast supercritical flow method to produce phase pure and highly crystalline a-TiO2 nanoparticles with high specific surface area. Control of the nanocrystal morphology is important since it is known that certain facets substantially promote catalytic activity. It is, however, in itself challenging to determine nanocrystal morphology to provide a rational basis for the synthesis control. Here we advocate the use of advanced Rietveld refinement of powder X-ray diffraction data including anisotropic size broadening models in aiding to establish the sample three-dimensional morphology. This relatively quick and robust method assists in overcoming the often encountered ambiguity inherent in two-dimensional to three-dimensional reconstruction of selected particle morphologies with transmission electron microscopy and tomography techniques.

Microwave-Assisted Synthesis of High-Energy Faceted TiO2 Nanocrystals Derived from Exfoliated Porous Metatitanic Acid Nanosheets with Improved Photocatalytic and Photovoltaic Performance.

A facile one-pot microwave-assisted hydrothermal synthesis of rutile TiO2 quadrangular prisms with dominant {110} facets, anatase TiO2 nanorods and square nanoprisms with co-exposed {101}/[111] facets, anatase TiO2 nanorhombuses with co-exposed {101}/{010} facets, and anatase TiO2 nanospindles with dominant {010} facets were reported through the use of exfoliated porous metatitanic acid nanosheets as a precursor. The nanostructures and the formation reaction mechanism of the obtained rutile and anatase TiO2 nanocrystals from the delaminated nanosheets were investigated. The transformation from the exfoliated metatitanic nanosheets with distorted hexagonal cavities to TiO2 nanocrystals involved a dissolution reaction of the nanosheets, nucleation of the primary [TiO6]8− monomers, and the growth of rutile-type and anatase-type TiO2 nuclei during the microwave-assisted hydrothermal reaction. In addition, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated through the photocatalytic degradation of typical carcinogenic and mutagenic methyl orange (MO) under UV-light irradiation at a normal temperature and pressure. Furthermore, the dye-sensitized solar cell (DSSC) performance of the synthesized anatase TiO2 nanocrystals with various morphologies and crystal facets was also characterized. The {101}/[111]-faceted pH2.5-T175 nanocrystal showed the highest photocatalytic and photovoltaic performance compared to the other TiO2 samples, which could be attributed mainly to its minimum particle size and maximum specific surface area.

Efficient Z-scheme photocatalysts of ultrathin g-C3N4-wrapped Au/TiO2-nanocrystals for enhanced visible-light-driven conversion of CO2 with H2O

Z-scheme-type photocatalysts with two photochemical systems of graphic-C3N4 (g-C3N4) and anatase TiO2 (A-TiO2) nanocrystals combined with Au nanoparticles (NPs) [(Au/A-TiO2)@g-C3N4] were successfully fabricated. The surface heterojunction between coexposed {001} and {101} facets in A-TiO2 nanocrystals improves separation efficiency of photoelectrons and holes. Supported Au NPs gather and transfer the stimulated electrons originated from A-TiO2 to g-C3N4. Wrapped g-C3N4 nanosheets can not only trap the photoelectrons, but also its surface π bond can improve adsorption capabilities for CO2. (Au/A-TiO2)@g-C3N4 catalysts with enriched surface photoelectrons and CO2 exhibit high photocatalytic activity for visible-light-driven conversion of CO2, i.e., the formation rates of CH4 and CO over (Au/A-TiO2)@C3N4-5 catalyst are 37.4 and 21.7 μmol g−1 h−1, respectively. The selectivity for CO2 reduction is higher than 99%, and the selectivity of CH4 product increases from 1.4% to 63.3%. The innovation of Z-scheme-type photocatalysts is expected to provide new inspiration for artificial simulation photosynthesis.

Particle Consolidation and Electron Transport in Anatase TiO2 Nanocrystal Films

A sequence of chemical vapor synthesis and thermal annealing in defined gas atmospheres was used to prepare phase-pure anatase TiO2 nanocrystal powders featuring clean surfaces and a narrow particle size distribution with a median particle diameter of 14.5 ± 0.5 nm. Random networks of these nanocrystals were immobilized from aqueous dispersions onto conducting substrates and are introduced as model systems for electronic conductivity studies. Thermal annealing of the immobilized films at 100 °C < T < 450 °C in air was performed to generate particle-particle contacts upon virtual preservation of the structural properties of the nanoparticle films. The distribution of electrochemically active electronic states as well as the dependence of the electronic conductivity on the Fermi level position in the semiconductor films was studied in aqueous electrolytes in situ using electrochemical methods. An exponential distribution of surface states is observed to remain unchanged upon sintering. However, capacitive peaks corresponding to deep electron traps in the nanoparticle films shift positive on the potential scale evidencing an increase of the trapping energy upon progressive thermal annealing. These peaks are attributed to trap states at particle-particle interfaces in the random nanocrystal network (i.e., at grain boundaries). In the potential region, where the capacitive peaks are detected, we observe an exponential conductivity variation by up to 5 orders of magnitude. The potential range featuring the exponential conductivity variation shifts positive by up to 0.15 V when increasing the sintering temperature from 100 to 450 °C. Importantly, all films approach a potential- and sintering-temperature-independent maximum conductivity of ∼10-4 Ω-1·cm-1 at more negative potentials. On the basis of these results we introduce a qualitative model, which highlights the detrimental impact of electron traps located on particle-particle interfaces on the electronic conductivity in random semiconductor nanoparticle networks.

An Environmental Transmission Electron Microscopy Study of the Stability of the TiO2 (1 × 4) Reconstructed (001) Surface

The anatase TiO2 nanocrystals with dominant (001) facets attracted tremendous attention in the past decade. However, the reported intrinsic property of the (001) surface is still a cause of controversy. A crucial reason is that the (001) surface usually undergoes a (1 × 4) reconstruction, which may result in a remarkable difference in property. Herein, we performed an in situ environmental transmission electron microscopy (ETEM) study regarding the formation and stability of the (1 × 4)-(001) surface of TiO2 nanocrystals. The systematic ETEM studies confirmed that the (1 × 4)-(001) surface could be generated at elevated temperature (above 300 °C) when the surface contaminants were removed and the formed reconstruction could survive under different conditions, which indicate the surface reconstruction should be taken into account in related property research. In addition, an oriented layer-by-layer beam damage process on the (001) surface is confirmed, and optimal imaging conditions were also investigated,...

Morphology-Engineered Highly Active and Stable Ru/TiO2 Catalysts for Selective CO Methanation.

Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2 - and H2 -rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non-deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2 -{100} and Ru/TiO2 -{101} are very stable, while Ru/TiO2 -{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal-support interactions (MSIs). The stronger MSIs on the defect-rich TiO2 -{100} and TiO2 -{101} supports stabilize flat Ru nanoparticles, while on TiO2 -{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2 -{001}.

Hydrothermal synthesis of Mn2+ doped titanate nanotubes: Investigation of their structure and room temperature ferromagnetic behavior

Hydrothermal synthesis of Mn2+ doped titanate nanotubes (TNTs), which exhibited room temperature ferromagnetism (RTFM), is reported. Dispersions of 1 and 5 at.% Mn2+ doped anatase TiO2 nanocrystals were used as precursors. Size and shape of Mn2+ doped TNTs and precursor nanocrystals were studied by transmission electron microscopy (TEM). The relatively uniform size distribution of transverse dimension of nanotubes of about 10 nm was observed while their lengths varied up to few hundred nanometers. The X-Ray Diffraction (XRD) analysis and Raman spectroscopy of resultant powder confirmed the hydrogen dititanate (H2Ti2O5 x H2O) crystal phase of Mn2+ doped TNTs with the presence of small amount of sodium titanates. Electron paramagnetic resonance (EPR) experiments were performed to probe the local atomic and electronic structure of Mn in the nanotubes. Room temperature ferromagnetic ordering with saturation magnetic moment (Ms) in the range of 0.6–1.5 μB per Mn atom was observed.

Lithium storage performance of {010}-faceted and [111]-faceted anatase TiO2 nanocrystals

As a popular anode material for lithium-ion batteries, anatase TiO2 nanoparticles with exposed {001} facets usually exhibit exceptional lithium storage performance owing to more accessible sites and fast migration of lithium ions along the good crystalline channels. However, there are few researches on the lithium storage capability of TiO2 nanocrystals with other high-energy facets owing to lack of effective synthesis method for controlling crystal facets. Herein, anatase TiO2 nanocrystals with exposed {010}- and [111]-facets are successfully prepared by using the delaminated tetratitanate nanoribbons as precursors. The electrochemical properties of these TiO2 nanocrystals with high-energy surfaces and the comparison with commercial TiO2 nanoparticles (P25) are studied. It is found that the cycle and rate performance of TiO2 nanocrystals is highly improved by reducing the particle size of nanocrystals. Moreover, TiO2 nanocrystals with exposed {010}- and [111]-facets exhibit better lithium storage capacities in comparison with P25 without a specific facet though P25 has smaller particle size than these TiO2 nanocrystals, indicating that the exposed facets of TiO2 nanocrystals have an important impact on their lithium storage capacity. Therefore, the synthesis design of high-performance TiO2 materials applied in the next-generation secondary batteries should both consider the particle size and the exposed facets of nanocrystals.