Surface Ti Atoms Research Articles

Visible light absorption of TiO2 nanoparticles surface-modified with vitamin B6: A comparative experimental and DFT study

Surface modification of titanium dioxide nanoparticles (TiO2 NPs) with the biologically active molecule pyridoxine hydrochloride (vitamin B6) was found to alter the optical properties. Microstructural characterization involving transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis revealed that the anatase TiO2 NPs had a narrow size distribution with an average diameter of 45 ?. The absorption onset of the surface-modified TiO2 samples was red-shifted by about 0.4 eV compared to the unmodified particles. The mode of binding between vitamin B6 and the surface Ti atoms was investigated by Fourier transform infrared spectroscopy (FTIR). From the Benesi? Hildebrand plot, the stability constant of the surface complex was found to be of the order 102 M-1. The experimental findings were supported by detailed quantum chemical calculations based on the density functional theory (DFT). Agreement was found between the experimentally measured absorption spectra of the TiO2 NPs surface-modified with vitamin B6 and the theoretically calculated electronic excitation spectra of the corresponding model system.

Nanočestice titan-dioksida površinski modifikovane iminom kao punioci za epoksidnu smolu

The surface of anatase TiO2 nanoparticles was modified with in situ synthesized imine, based on 3,4-dihydroxybenzaldehyde and octadecylamine. The modified TiO2 nanoparticles were investigated using FTIR and UV-Vis spectroscopy, which confirmed the formation of the charge transfer complex between imine and surface of TiO2 nanoparticles by the reaction of surface Ti atoms with hydroxyl groups of a modifying agent. TiO2 nanoparticles modified with imine were further applied as a filler for the preparation of epoxy based nanocomposites. Thermal and mechani­cal properties of the prepared nanocomposites were examined using the dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis and hardness measurements. The obtained results have shown that modified TiO2 nanoparticles have no significant influence on the glass transition temperature and that they slow down thermo-oxidative degradation of epoxy resin in the first stage and increase hardness of the crosslinked epoxy resin.

Low-temperature activation of methane on doped single atoms: descriptor and prediction.

Catalytic transformation of methane under mild conditions remains a grand challenge. Fundamental understanding of C-H activation of methane is crucial for designing a catalyst for the utilization of methane at low temperature. Recent experiments show that strong methane chemisorption on oxides of precious metals leads to facile C-H activation. However, only a very few such oxides are capable (for example, IrO2 and PdO). Here we show for the first time that strong methane chemisorption and facile C-H activation can be accomplished by single transition-metal atoms on TiO2, some of which are even better than IrO2. Using methane adsorption energy as a descriptor, we screened over 30 transition-metal single atoms doped on TiO2 for the chemisorption of methane by replacing a surface Ti atom with a single atom of another transition metal. It is found that the adsorption energies of methane on a single atom of Pd, Rh, Os, Ir, and Pt doped on rutile TiO2(110) are greater than or similar to those on rutile IrO2(110), a benchmark for the chemisorption of methane on transition oxides. Electronic structure analysis uncovered orbital overlap and mixing between methane and the single atom, as well as significant localization of the charge between the molecule and the surface, demonstrating chemical bonding of CH4 to doped single atoms. Facile C-H dissociation has been found on the single-atom sites with the transition state energies lower than desorption energies. Our computational studies predict that Pd, Rh, Os, Ir, and Pt single atoms on rutile TiO2(110) can activate C-H of methane at a temperature lower than 25 °C.

Hydrogen adsorption, dissociation and diffusion on two-dimensional Ti2C monolayer

The adsorption, dissociation and diffusion of hydrogen on two-dimensional (2D) Ti2C monolayer with and without carbon vacancies have been investigated by first-principles calculations based on density functional theory. Weak molecular physisorption was observed for the adsorption of H2 on 2D Ti2C monolayer with end-on configuration. However, for the side-on configuration, the H2 molecule decomposes spontaneously and then the two H atoms prefer to bond to the surface Ti atoms at the monolayer. Based on the chemical bonding analysis, the mechanism of the dissociation of H2 has been elucidated. Moreover, the hydrogen diffusion on the Ti2C surface with and without carbon vacancies was investigated. A better hydrogen diffusion property was found for the Ti2C monolayer with carbon vacancies. The energetics for H penetration from surface to the solute site in the bulk was also studied.

Chemical Origin of Termination-Functionalized MXenes: Ti3C2T2 as a Case Study

MXenes represent a burgeoning family of two-dimensional (2D) functional materials with a variety of applications that highly rely on termination-mediated surface functionalization, but the understanding of termination is limited. Here, we take Ti3C2T2 (T = O, F, OH, and H) as an example of MXenes, to demonstrate how termination stabilizes the Ti3C2 monolayer matrix by saturating the nonbonding valence electrons of the surface Ti atom by the low-energy orbitals of the termination. This is achieved by orbitally resolved density of states analysis by simply yet efficiently manipulating the internal coordination of the octahedral crystal field to match exactly the Cartesian coordination. Highly degenerate 3d orbitals of surface Ti split in such a way that it exhibits pseudogaps whose widths predict a stability order Ti3C2O2 > Ti3C2F2 > Ti3C2(OH)2 > Ti3C2H2 > Ti3C2, consistent with Bader charge analysis, thermodynamic calculations, and experimental results. This new criterion could have implications in the gen...

Charge-transfer complex formation between TiO2 nanoparticles and thiosalicylic acid: A comprehensive experimental and DFT study

Under normal conditions, titanium dioxide does not absorb visible light photons due to large band gap. Nevertheless, when titanium dioxide nanoparticles (TiO2 NPs) are surface-modified with thiosalicylic acid (TSA), their optical properties are altered owing to the formation of charge transfer complex that initiates absorption in the visible spectral range. Colloidal and sol-gel techniques were used to synthesize uniform TiO2 NPs of different sizes (average diameters in the range 4–15 nm), and effects of their subsequent modification by TSA molecules were compared with effect of modification of commercial Degussa TiO2 powder. Thorough microstructural characterization of TiO2 nanoparticulates was performed including transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis, as well as nitrogen adsorption-desorption isotherms. Optical measurements revealed that all surface-modified TiO2 samples with TSA have similar spectral features independent of their morphological differences, and, more importantly, absorption onset of modified TiO2 samples was found to be red-shifted by 1.0 eV compared to the unmodified ones. The mode of binding between TSA and surface Ti atoms was analyzed by infrared spectroscopy. Finally, the quantum chemical calculations, based on density functional theory, were performed to support optical characterization of surface-modified TiO2 with TSA.

First-principles prediction of the morphology of L10 FePt nanoparticles supported on Mg(Ti)O for heat-assisted magnetic recording applications

We perform first-principles calculations to predict the morphology of $L{1}_{0}$ ordered FePt nanoparticles grown on Mg(Ti)O substrates with relevance to application in heat-assisted magnetic recording (HAMR) media. We show how incorporation of Ti into MgO substrates reduces the FePt adhesion energy from $\ensuremath{-}1.29$ (pure MgO) to $\ensuremath{-}2.35\phantom{\rule{4pt}{0ex}}\mathrm{J}/{\mathrm{m}}^{2}$ (pure TiO). This effect is due to the formation of strong Fe-Ti bonds at the interface. Consistent with experimental observations, the predicted equilibrium morphology of supported FePt nanoparticles is significantly changed, corresponding to increased wetting. This behavior is undesirable for HAMR media since it promotes grain growth which limits the storage density. We show how passivation of surface Ti atoms (e.g., with MgO) is sufficient to restore the wetting observed for pure MgO substrates offering a viable strategy for optimization of next generation recording media.

Effects of single metal atom (Pt, Pd, Rh and Ru) adsorption on the photocatalytic properties of anatase TiO2

The effects of single metal atom (Pt, Pd, Rh and Ru) adsorption on the photocatalytic properties of anatase TiO2 are investigated by means of the first-principles calculations based on density functional theory (DFT). Our results show that the most stable adsorption site for single metal atom on anatase TiO2 (101) surface is the bridge site formed by two twofold coordinated oxygen (O2c) atoms at the step edge. Due to the charge transfer from metal atoms to anatase TiO2 (101) surface, the work function of adsorbed surface is significantly smaller than the clean one, indicating enhanced surface activity. Fukui functions are highly localized around the isolated metal atoms, indicating that single metal atoms on anatase TiO2 (101) surface serve as the active reduction and oxidation sites in the photocatalytic process. Photo-induced electrons in the electronically excited TiO2 photocatalyst can be transferred to target species through the deposited single atoms. The band structures of host TiO2 are almost unchanged upon the adsorption, and the metal induced states are located in the band gap of the host. Remarkably, due to the metal atoms adsorption, the upward shift of conduction band edge will improve the reducing capacity of anatase TiO2. Moreover, when single metal atoms are adsorbed, potential energy of topmost surface Ti atoms turns to get close to the vacuum level, which significantly facilitates the electron transfer for hydrogen evolution. Results in this work provide new insights into improving the photocatalytic performance by single metal atoms adsorption.

Surface-modified TiO2 nanoparticles with ascorbic acid: Antioxidant properties and efficiency against DNA damage in vitro

The antigenotoxic and antioxidative properties of surface-modified TiO2 nanoparticles (NPs) with ascorbic acid (AA) were compared with those of constituents (free AA and bare TiO2 NPs). Colloids consisting of the TiO2 NPs with anatase crystal structure were prepared by acidic hydrolysis of TiCl4. The synthesized TiO2 NPs were characterized using transmission electron microscopy and X-ray diffraction analysis. The charge transfer (CT) complex formation between surface Ti atoms and AA is indicated by immediate appearance of red color. Composition and stability constants of CT complex were determined using Job’s method and Banesi-Hildebrand analysis, respectively. The surface structure of CT complex was determined from infra-red spectra of free and bound AA to the surface Ti atoms. The experimental data were supported with quantum chemical calculations based on density functional theory (DFT). The antigenotoxic potential of CT complex was evaluated in leukocytes of whole blood cells in vitro by comet assay method. For evaluation of antioxidant properties, total antioxidant status (TAS) and total oxidant status (TOS) were determined in human serum pool in vitro.The presented results indicate that bare TiO2 NPs have more pronounced antigenotoxic effects in comparison with either surface-modified TiO2 NPs with AA or free AA. No significant differences between the antigenotoxic and antioxidative properties of free and bound AA on the TiO2 NPs were noticed in the investigated concentration range. It seems that surface-modified TiO2 NPs with AA and/or similar compounds can be used to maintain its beneficial activities.

Exploring Rutile (110) and Anatase (101) TiO2 Water Interfaces by Reactive Force-Field Simulations

We have investigated static/structural as well as dynamical properties of anatase (101) and rutile (110) TiO2 interfaces with liquid bulk water by reactive force fields (ReaxFF). Layered, well-organized structure of water in the interface region was clearly observed within 6.5 A of the surfaces. The first-hydration layer molecules adsorbed to unsaturated surface Ti atoms undergo spontaneous dissociation leading, rather controversially, to full coverage of O2c/Ob by H+ and partial coverage of Ti5c by OH–. Expected large variations of intrinsic electric field on the interfaces, and drop of electrostatic potential, were detected. Interfacial water was found to be heavily confined with a self-diffusion constant of 2 orders of magnitude lower than 2.28 × 10–9 m2/s measured in the bulk water region. Moreover, the rotational movement of adsorbed water molecules was found to be considerably hindered as well. On the other hand, the calculated hydrogen-bond lifetime on the interface was shorter than in bulk water f...

High photocatalytic hydrogen production on Cu(II) pre-grafted Pt/TiO2

A series of Pt/Cu/TiO2 photocatalysts, showing very high performance in photocatalytic hydrogen production from methanol/water vapour mixtures, were prepared under mild conditions by Cu(II) grafting on commercial P25 TiO2, with nominal Cu/TiO2 ratios ranging from 0.05 to 0.5wt.%, followed by 0.5wt.% Pt nanoparticles deposition by the deposition-precipitation method in the presence of urea. The structural features of the so obtained materials were fully characterized by X-ray absorption spectroscopy, which provided information on the oxidation state of the two metals and on the metal–metal and metal-TiO2 interactions, and by EPR analysis, which evidenced electron transfer phenomena involving copper under irradiation. The photocatalysts showed a volcano-shaped photoactivity trend in hydrogen production with increasing nominal Cu content, the maximum rate of H2 evolution (27.2mmolh−1gcat−1) being attained with the photocatalyst containing 0.1wt.% of copper. In this sample CuO nanoclusters appear to be intimately coordinated with surface Ti atoms in a surface structure that partially stabilizes pre-grafted copper in metallic form, possibly acting as an electron-transfer bridge at the interface between CuO nanoclusters and TiO2. Synergistic effects in H2 photocatalytic production are clearly induced by the co-presence of grafted Cu nanoclusters and Pt nanoparticles on the TiO2 surface, with the copper oxidation state switching under UV–vis irradiation, facilitating electron transfer to adsorbed protons.

Interfacial Charge-Transfer Transitions in BaTiO3 Nanoparticles Adsorbed with Catechol

Interfacial charge-transfer (ICT) transitions between wide band gap inorganic semiconductors such as titanium dioxide (TiO2) and organic materials are characteristic electronic transitions that enable the absorption of lower energy photons than the band gap of inorganic semiconductors and the HOMO–LUMO gap of organic compounds and also direct photocarrier injection to inorganic semiconductors. Despite these features, inorganic semiconductors that show ICT transitions are quite limited. Here we report ICT transitions in barium titanate (BaTiO3) nanoparticles with a pseudocubic structure using 1,2-benzenediol (catechol) that is a representative aromatic bidentate ligand to induce ICT transitions. BaTiO3 nanoparticles are instantaneously colored orange by the addition of the colorless solution of catechol, showing a broad absorption band with an onset at ca. 620 nm. The observed FT-IR spectrum indicates that catechol molecules chemisorb on BaTiO3 surfaces by chelate coordination with a surface Ti atom more p...

An ab initio study on atomic and electronic structures of two-dimensional Al3Ti at Al/TiB2 interface

The atomic and electronic structures of a two-dimentional (2D) Al3Ti layer at Al/TiB2 interface has been investigated using first-principle calculations. The result reveals the 2D-Al3Ti adopts the structure of bulk Al3Ti. There exists a strong Ti(3d)–Al(3p) hybridization between Ti and Al atoms of the 2D-Al3Ti, as well as between surface Ti atoms of TiB2 and Al atoms of 2D-Al3Ti. It leads to a stronger covalent Ti–Al bonding at the Al/2D–Al3Ti/TiB2 interface than at the Al/TiB2 interface, which is responsible for the stability of 2D-Al3Ti.

Structural and magnetic properties of Ti12M clusters (M=Sc to Zn)

The geometries, electronic, and magnetic properties of the 3d atom doped icosahedron (ICO) Ti12M (M=Sc to Zn), where a dopant atom replaces either the centra l(Ti12Mc) or surface (Ti12Ms) Ti atom in ICO Ti13 cluster, have been systematically investigated by using the density functional theory. The structures of all the optimized Ti12Mc and Ti12Ms clusters are distorted ICO. Sc, Ni, Cu, and Zn atoms prefer to displace surface Ti atom, V, Cr, Mn, and Fe atoms prefer to displace central Ti atom. The position of impurity atom depends on the strength of the interaction between the central atom and the surface atoms. As compared to the pure Ti13 cluster, Ti12Mc and Ti12Ms (M=V, Fe, Co, and Ni) clusters are more stable, Ti12Mc and Ti12Ms (M=Sc, Cr, Mn, Cu, and Zn) are less stable. Both Ti12Nis and Ti12Nic are magic clusters, which originate from their electronic as well as geometric closed shells. Because the exchange interaction prevails over the crystal field in Ti12M clusters, the valence electrons fill molecular orbitals in terms of Hund’s rule of maximum spin.

Oxygen Vacancies of Anatase(101): Extreme Sensitivity to the Density Functional Theory Method.

We present a systematic investigation of the influence of theoretical parameters on the characterization of surface and subsurface oxygen vacancies in anatase with the 101 facet exposed. This metastable phase of titania continues to resist a facile description of its defects, particularly, in the reduced state. Nine nonequivalent sites were examined under varying levels of theory with characterization of formation energies, geometry, and electronic states extracted from Bader charges, charge density, and density of states. At DFT+U levels of theory, these sites remain nonequivalent. We note a new surface oxygen vacancy minimum related to localization of electrons at surface and a subsurface Ti atoms, rather than the more favorable localization at neighboring surface Ti atoms.

Metal-Organic Frameworks as Promising Photosensitizers for Photoelectrochemical Water Splitting.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Anatase TiO2 nanosheets with surface acid sites for Friedel–Crafts alkylation

The solvothermal synthesis route to mesoporous TiO2 using a vinylphenol polymer structure-directing agent (SDA) was optimized to provide a high yield of bilayer nanosheets. The resultant TiO2 material exhibited a high BET area of 420 m2 g−1 after the removal of SDA. Structural analysis of the nanosheets was carried out using powder X-ray diffraction, high resolution-transmission electron microscopy, Raman spectroscopy and solid-state NMR. The results indicated that the TiO2 material consisted mainly of the bilayer nanosheets with (010) facet-dominant anatase crystal structure. The thickness of the nanosheet was 0.75 nm, corresponding to two unit-cell parameters along the b-axis in the anatase crystal system. More than 10% of the surface Ti atoms were terminated with bridging O–H groups and coordinatively unsaturated Ti atoms, which acted as Brönsted and Lewis acid sites. Accordingly, the anatase nanosheets exhibited catalytic performance in the liquid-phase Friedel–Crafts alkylation of aromatic compounds.

Visible light absorption of surface modified TiO2 powders with bidentate benzene derivatives

TiO2 powders with different specific surface area were prepared using reproducible, sol–gel synthetic route and their ability to form hybrids with catechol and 5-amino salicylic acid was compared with the commercially available Degussa P25 TiO2 powder. Microstructural characterization involving transmission electron microscopy, X-ray diffraction analysis and nitrogen adsorption–desorption isotherms indicated that TiO2 samples cover reasonably wide size and/or specific surface area range (50–115 m2/g). The surface modification of TiO2 powders with catechol and 5-amino salicylic acid induced significant shift of absorption to the visible spectral region due to charge transfer complex formation. It should be emphasized that tunable optical properties of TiO2 in powder form have never been reported in the literature. The largest red shift of the absorption onset was observed for sample with the largest specific surface area upon surface modification with both ligands. The binding of the modifier molecules to the surface Ti atoms was studied using Fourier transform infrared spectroscopy.

TiO2 colloidal nanocrystals surface modification by V2O5 species: Investigation by 47,49Ti MAS-NMR and H2, CO and NO2 sensing properties

TiO2 and TiO2–V2O5 nanocrystals were prepared by coupling sol–gel and solvothermal methods, followed by heat-treatment at 400°C, after which the mean nanocrystal size was about 5nm. The materials were characterized by X-ray diffraction, transmission electron microscopy and solid state nuclear magnetic resonance spectroscopy. It was shown that while the TiO2 phase was always anatase even after heat-treatment at 500°C, the presence of the vanadium oxide species enhanced the surface re-configuration of the Ti ions. Hence the coordination environment of surface Ti atoms was drastically changed, by formation of further bonds and imposition of a given local geometry. The final hypothesis was that in pure titania surface rearrangement occurs, leading to the new NMR signal, but this modification was favored in the TiO2–V2O5 sample, where the Ti surface atoms were forced into the final configurations by the bonding with V atoms through oxygen. The materials heat-treated at 400°C were used to process chemoresistive sensors, which were tested to hydrogen, CO and NO2, as examples of gases with peculiar sensing mechanisms. The results evidenced that the surface deposition of V2O5 onto the anatase TiO2 nanocrystals was effective in modifying the adsorption properties of the anatase nanocrystals.

Theoretical studies of fluorine substituent effect on organic photo-sensitizers in dye sensitized solar cells

Based on titanium dioxide model and LJBs sensitizers (triphenylamine as the donor, 3,4-ethylene dioxy thiophene plus various functionalized phenylenes as the π-spacer, and cyanoacrylic acid as the anchoring group) with different connection types on TiO2 substrate, the reasonable dye-TiO2 connection has been located. The results show that the dissociative adsorption of LJBs is more thermodynamically favorable than the neutral molecule bound configurations. LJBs adsorb onto TiO2 via interaction between 3d orbital of surface Ti atom and 2p orbital of N/O atoms in the acceptors. This result is confirmed in both neutral and dissociation forms of LJBs molecules. According to our calculations, adding a meta-fluorine substituent to the phenyl group of cyanoacrylic acid (LJB-Fm) may damage the planarity and conjugation. Consequently, the light harvesting efficiency decreased, and that is particularly unfavorable for the DSSCs application. The Ortho F-substituted dye (LJB-Fo), however, exhibited enhanced light absorption and more efficient intra-molecular charge transport. The bigger Jsc, Voc values of LJB-Fo system predicts its superior DSSC performance. Additionally, LJBs anchored on TiO2 surface via group –COO (LJB-H and LJB-Fo) lead to an indirect mechanism for electron injection. While LJB-Fm preferred to direct electron injection mechanism due to the strong orbital-coupling between sensitizer and the TiO2 substrate.