Deficient TiO2 Research Articles

Oxygen Deficient TiO<sub>2</sub> Photoanode for Photoelectrochemical Water Oxidation

Titanium dioxide (TiO2) has been widely used as photoanodes in photoelectrochemical (PEC) water splitting. However, the typically high density of bandgap trap states results in fast charge carrier recombination and poor electrical conductivity, and thereby weak PEC performance. Rational creation of oxygen vacancy (Vo) in TiO2 has been demonstrated as an effective method to modify the electronic and optical properties, as well as improved PEC performance. Different strategies have been developed to fabricate oxygen deficient TiO2 photoanodes, such as hydrogen treatment, thermal annealing, electrochemical reduction, flame reduction, and chemical reduction. In conjunction with oxygen vacancy creation, doping of TiO2 with elements further enhances the PEC activity by introducing other bandgap states. Various techniques, including ultrafast laser spectroscopy, have been employed to probe the chemical nature and associated charge carrier dynamics of the bandgap states.

Optimal levels of oxygen deficiency in the visible light photocatalyst TiO2−x and long-term stability of catalytic performance

The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO2 (TiO2−x) prepared by Ar/H2 plasma surface treatment on the degree of oxygen deficiency (x) was assessed to determine the deficiency region associated with highest performance. The highest activity was obtained at x=0.06 (TiO1.94). The maximum visible light activity for this material, estimated from the formaldehyde (HCHO) removal rate, was three times higher than that exhibited by nitrogen-doped TiO2 (TiO2−xNx). The catalytic ability was found to decrease over the first week after fabrication of the material, after which it became stable, and the performance of TiO2−x at this point was found to be nearly equal to that of TiO2−xNx. The results of ab initio calculations of density of states for TiO2−x suggest that new oxygen deficiency states emerge at almost the exact center between the valence and conduction bands when x>0.06, which increases the recombination rate between electrons and holes. Therefore the declining performance of TiO2−x at larger x values is attributed to the emergence of new oxygen deficient states.

Group 4 Metal Oxynitride Nanocatalysts for Oxygen Reduction Reaction in Acidic Media

Titanium dioxide (TiO2) has attracted significant attention for the use in various applications; photocatalysts, dye-sensitized solar cells, lithium/sodium ion batteries, sensors, etc. Since the discovery of high visible-light photocatalytic activity by Asahi et al. in 2001,1 substitution of oxygen atoms in TiO2 by heteroatoms has been the hot topic in these fields. Recent work of Ota et al. opened a new application of TiO2: oxygen deficient TiO x showed the oxygen reduction reaction (ORR) activity in acidic media that simulate polymer electrolyte fuel cell (PEFC) cathodes.2 Other oxide catalysts containing group 4 or 5 metals also showed ORR activity in acidic media, whereas TiO2 is the most promising from the viewpoint of natural abundance. The oxygen defects in oxide catalysts have been acknowledged as active sites for ORR and they have been incorporated using various routes such as the pyrolysis under reductive atmosphere containing hydrogen,2 so-called carbothermal-reduction3 and nitrogen-doping.4-8 Rutile is more active for ORR than anatase,3,6,7 and this trend is opposite to that in photocatalytic activity. In this work, oxygen defects were incorporated into nanoparticles of rutile TiO2 and other group 4 metal oxides by substitutional nitrogen-doping using a recently developed solution phase combustion route.7,8 X-ray photoelectron spectroscopy analyses on TiO2 catalysts revealed that (1) charge imbalance caused by doped nitrogen atoms were compensated by the incorporation of oxygen defects, not the decrease in the valence of titanium on the surface and (2) the surface rutile was formed on TiN, yielding a core-shell structure. Many transition metal nitrides are well known to be unstable in water or even in air moisture, except for Ta3N5;9,10 however, the surface rutile successfully protected the TiN-core in 0.1 mol dm–3 H2SO4 solution not to change the ORR mechanism represented by Tafel slope after 20,000 potential cycles between 0.6 and 1.0 V versus the reversible hydrogen electrode. The active sites will be discussed at the meeting by comparing the activity of TiO2 and other group 4 metal oxides. Acknowledgments The authors gratefully acknowledge Mr. Yusei Tsushima for his help with acquisition of transmission electron microscopy images. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (26420132) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; an Adaptable and Seamless Technology Transfer Program through Target-driven R&D grant (AS262Z02037) from the Japan Science and Technology Agency; a research grant from the Takahashi Industrial and Economic Research Foundation of Japan; a research grant from the Naoji Iwatani Foundation of Japan; and a grant for chemical research from the Foundation for Japanese Chemical Research. The X-ray photoelectron spectra were acquired with the support by Nanotechnology Platform, 12024046 of the MEXT of Japan. References (1) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science, 293, 269 (2001). (2) J. H. Kim, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Electrochim. Acta, 52, 2492 (2007). (3) M. Chisaka and N. Itagaki, Electrochim. Acta, in press. Doi:10.1016/j.electacta.2015.10.184. (4) M. Chisaka, H. Sasaki and H. Muramoto, Phys. Chem. Chem. Phys., 16, 20419 (2014). (5) M. Chisaka, A. Ishihara, K. Suito, K. Ota and H. Muramoto, Electrochim. Acta, 88, 697 (2013). (6) M. Chisaka and H. Muramoto, ChemElectroChem, 1, 544 (2014). (7) M. Chisaka, Y. Ando and H. Muramoto, Electrochim. Acta, 183, 100 (2015) (8) M. Chisaka, Y. Ando and N. Itagaki, J. Mater Chem. A, in press. Doi:10.1039/C5TA08235H (9) M. Hara, E. Chiba, A. Ishikawa, T. Takata, J. N. Kondo and K. Domen, J. Phys. Chem. B, 107, 13441 (2003). (10) M. Chisaka, A. Ishihara, N. Uehara, M. Matsumoto and K. Ota, J. Mater Chem. A, 3, 16414 (2015).

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m2/g in pure to 405.1m2/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol–TiO2 interface and finally interact with phenol molecule and degrade it.

Photocatalytic Reduction of CO2 with H2O By Engineered TiO2 Nanocomposites

Photocatalytic conversion of CO2 with H2O to solar fuels is a promising technology to recycle CO2, a major greenhouse gas. However, the CO2-to-fuel conversion efficiency is typically low, mainly due to the fast recombination of photo-excited electron-hole pairs and limited absorption of sunlight by wide band-gap photocatalysts like TiO2. In addition, some scientific challenges have been overlooked including the photocatalyst’s ability to adsorb CO2 and the adsorption/desorption kinetics on the catalyst surface. To promote charge transfer, visible light response, and CO2 adsorption and activation, we have engineered a series of TiO2 nanocomposites including Cu/TiO2, oxygen deficient TiO2, TiO2 nanocrystals with mixed exposed facets, porous microsphere MgO/TiO2, and nano-TiO2 dispersed on MgAl-layered oxide materials. Photocatalytic CO2 reduction experiments have been conducted under UV and visible light irradiation, respectively, with CO and CH4 being the major products. Furthermore, we have compared the CO2 adsorption and reduction intermediates and reaction kinetics at near room temperature and near flue gas temperature (~150 oC), using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The higher reaction temperature enhanced intermediate/product desorption, and the addition of basic materials (MgO and MgAl-LDO) improves CO2 adsorption particularly in the presence of water vapor, thus enhancing the catalytic activity and stability of the photocatalyst.

Interface of Pt with SrTiO3(001); A combined theoretical and experimental study

Density Functional Theory and X-ray standing wave atomic imaging are combined to develop models of platinum overlayers, patches and small particles on (001) surfaces of SrTiO3. The higher coverage (0.3 ML nominal Pt) experimental result matches best to a simple 1 × 1 surface model. For lower coverage (0.1 ML) a Ti deficient TiO2 double layer termination is identified as most favorable. A simple model is developed by which XSW amplitudes can be used to infer the aspect ratio of surface structures.

Oxygen defect dependent variation of band gap, Urbach energy and luminescence property of anatase, anatase–rutile mixed phase and of rutile phases of TiO2 nanoparticles

TiO2 nanoparticles are prepared by a sol–gel method and annealed both in air and vacuum at different temperatures to obtain anatase, anatase–rutile mixed phase and rutile TiO2 nanoparticles. The phase conversion from anatase to anatase–rutile mixed phase and to rutile phase takes place via interface nucleation between adjoint anatase nanocrystallites and annealing temperature and defects take the initiate in this phase transformation. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis and photoluminescence spectroscopy (PL). Anatase TiO2 exhibits a defect related absorption hump in the visible region, which is otherwise absent in the air annealed samples. The Urbach energy is very high in the vacuum annealed and in the anatase–rutile mixed phase TiO2. Vacuum annealed anatase TiO2 has the lowest emission intensity, whereas an intense emission is seen in its air annealed counterpart. The oxygen vacancies in the vacuum annealed samples act as non-radiative recombination centers and quench the emission intensity. Oxygen deficient anatase TiO2 has the longest carrier lifetime. Time resolved spectroscopy measurement shows that the oxygen vacancies act as efficient trap centers of electrons and reduce the recombination time of the charge carriers.

The formation and stability of Rh nanostructures on TiO2(1 1 0) surface and TiOx encapsulation layers

Rh overlayers formed on slightly oxygen deficient TiO2(110) single crystals, as well as on ultrathin encapsulation titania layers prepared on Rh multilayers were characterized by AES, LEIS, XPS, TPD and work function (WF) measurements. Rh deposition on TiO2(110) below 0.1 monolayer (ML) Rh coverage led to electron transfer from the metal toward the TiO2(110) surface. Annealing of Rh multilayers up to 950K in UHV resulted in the surface diffusion of titanium and oxygen ions into a TiOx encapsulation layer of definite stoichiometry and a thickness of a few atomic layers. The accompanying 0.3–0.6eV WF enhancement at ΘRh=2–6ML can be attributed to the smoothing of the Rh overlayer and the formation of a continuous TiOx dipole layer consisting of positively charged titanium ions at the metal–oxide interface and negative oxygen ions in the topmost layer. De-encapsulation of Rh particles was observed on a TiO2 sample less reduced in its bulk, revealing the roles of bulk and surface substrate stoichiometry on the decoration process. Increasing the thickness of Rh multilayers supported on the TiO2(110) single crystal hampered ion diffusion and consequently, it led to an increase in the temperature characteristic of the completion of the encapsulation. Deposition of additional Rh on the TiOx encapsulation layers covering Rh multilayers resulted in the growth of Rh particles having a similar height up to 1ML. LEIS data indicated that the decoration of the second metal layer by titania was hindered. It occurred at a temperature more than 100K higher than that characteristic of the TiO2(110) surface at the same Rh coverage. Aspects of the produced structures in relation to the formation of protective oxide layers, modification of surface work function, catalysis and metal–insulator–metal (MIM) devices are considered.

Effect of the oxygen vacancy gradient in titanium dioxide on the switching direction of bipolar resistive memory

Resistive memory switching behavior depending on voltage sweep direction is studied by intentionally creating oxygen vacancies within titanium dioxide (TiO 2). By inserting a reactive Ti layer on the TiO 2, oxygen deficient TiO 2− x layer is created, which then causes TiO 2− x /TiO 2 which has an oxygen vacancy gradient. This gradient of oxygen vacancy makes it possible to create an insulating TiO 2 layer on the bottom electrode during the first reset with a negative bias at the top electrode. This insulating layer makes counterclockwise directional bipolar switching more stable. On the other hand, under the clockwise directional voltage sweeping, the first set switching is prevented by the insulating TiO 2 layers created during the first and second reset, which leads to a short circuit due to local heating eventually.

Ab initio study of magnetism at the TiO2/LaAlO3 interface

In this paper we study the possible relation between the electronic and magnetic structure of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO$_3$. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and -1, respectively. As a consequence of that, an oxygen deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab-initio calculations for superlattices with TiO2/AlO2 interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbour Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the LSDA and LSDA+U approximations.

Theoretical calculation and experimental study of influence of oxygen vacancy on the electronic structure and hemocompatibility of rutile TiO2

In this work, the relationship between electronic structure and hemocompatibility of oxygen deficient rutile TiO 2− x was studied by both theoretical calculation and experimental study. Based on the local density functional theory, first-principals method was performed to calculate the electronic structure of rutile TiO 2 with different oxygen vacancy concentration. In the range of less than 10% of (or equal) physically realistic O vacancy concentration, the band gap of rutile TiO 2 increases with increasing O vacancy concentration, leading the TiO 2 changes from a p-type to an n-type semiconductor. The valance band of TiO 2 is predominated by O 2 p orbital, while the conduction band is occupied by Ti 3 d orbital for different O vacancy concentration. The O vacancy results in the occupation of electrons at the bottom of conduction band of TiO 2 , and the donor density increases with increasing O vacancy concentration. When materials come in contact with blood, the n-type semiconductor feature of oxygen deficient TiO 2− x with the bottom of conduction band occupied by electrons would prevent charge transfer from fibrinogen into the surface of materials, thus inhibiting the aggregation and activation of platelets, therefore improving the hemocompatibility of rutile TiO 2- x .

Degradation of C 2H 2 with modified-TiO 2 photocatalysts under visible light irradiation

Photocatalytic degradation of C 2H 2 by nitrogen-doped TiO 2 (N-TiO 2), nitrogen and carbon-doped TiO 2 (NC-TiO 2), BA-PW25 (commercialized as oxygen deficient TiO 2 (TiO 2− δ ) photocatalyst), and P25 (conventional TiO 2) were analyzed. Only BA-PW25 photocatalyst decomposed C 2H 2 into CO 2 under visible light ( λ > 400 nm), while the other photocatalysts did not change C 2H 2. From the analyses of the byproduct formation and the reaction rate of C 2H 2 degradation, it is inferred that the active species effective for C 2H 2 degradation were formed on the mid-gap level of BA-PW25 at the potentials between the valence-band top of conventional TiO 2 and the N-induced mid-gap level of N-TiO 2.

Effects of Oxygen Content on Bioactivity of Titanium Oxide Films Fabricated on Titanium by Electron Beam Evaporation

The titanium oxide films were fabricated on titanium metal by e-beam deposition technique in various oxygen partial pressures in order to investigate the effects of oxygen content in titanium oxide film on the bioactivity of titanium implant. The nano-sized titanium oxide particles were observed on the surface of specimens. Raman spectra showed that titanium oxide films deposited by e-beam evaporator had oxygen deficient TiO2 structure. The oxygen content in oxide films was calculated from the high resolution XPS spectra of Ti 2p. The densities of HA particles formed on the sample surfaces after immersion test in SBF became higher as the contents of oxygen in titanium oxide films increased. We concluded that the degree of hydroxyl group formation in SBF depended on the stoichiometry of TiO2, which enhanced the bioactivity of titanium.

A DFT + U description of oxygen vacancies at the TiO 2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO 2 rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti 3+ sites. We consider the electronic structure and geometry of the oxygen deficient TiO 2 rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the + U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (⩽4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (⩾4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.

Influence of the target composition on the discharge voltage during magnetron sputtering

The discharge voltage measured during magnetron sputtering is inversely proportional with the Ion Induced Electron Emission (ISEE) coefficient of the target material. In this way, it is possible to investigate the influence of the target composition on the ISEE coefficient by measuring the discharge voltage at constant current for targets composed of a mixture of metals and/or oxides. Three different targets were investigated. A special designed Al / Cu target built from Cu and Al parts was used to investigate the generally accepted idea that the ISEE coefficient of a target is a linear composition of the ISEE coefficient of its components. This proposition seems to hold as the inverse of the discharge voltage is indeed linearly dependent on the target composition. Also for the second series of targets, oxygen deficient TiO 2 targets, a linear relation between the target composition and the inverse of the discharge voltage was found. However, for a third series of targets, mixtures of Zr and Y, a deviation from this linear behaviour was noticed.

Solid state reduction of a natural ilmenite

An ilmenite concentrate was reduced with graphite under argon gas between 1250 °C and 1350 °C by using a thermogravimetric analysis (TGA) system. Specimens were reacted for various periods of time during which the mass loss values were recorded. Samples representing different reduction stages were examined by optical microscopy, image analyzer, X-ray diffraction and SEM (EDAX) techniques. Reduction of Fe 3+ to Fe 2+, Fe 2+ to Fe and Ti 4+ to Ti 3+ took place within 50% reduction during which a secondary oxide phase was formed and replaced the natural ilmenite. Reduction of Ti 3+ to Ti 2+ took place after 50% reduction which altered the composition of the secondary oxide phase by decreasing its oxygen content. Reduction of Ti 2+ took place while the oxygen deficient TiO 1− x phase formed. Formation of any titanium carbide or oxycarbide phases depended on the completion of the reduction which was not possible at the specified temperatures.

Photocatalytic remote oxidation with various photocatalysts and enhancement of its activity

Photocatalytic remote oxidation with various photocatalysts, anatase TiO2, oxygen deficient TiO2, brookite TiO2, ZnO, WO3, metal (Au, Pt, Ag and Pd)-loaded anatase TiO2 and metal (Au, Pt and Ag)-loaded ZnO, were investigated. All these photocatalysts exhibited activities for remote oxidation. Among them, Pt-loaded TiO2 exhibited the highest activity; it took 2 min to make the octadecyltriethoxysilane-modified glass surfaces super-hydrophilic, while bare anatase TiO2 took 24 min. Since the remote oxidation activity of a TiO2 film that was prepared without organic compounds was almost the same as that of a normal TiO2 film, which contains organic impurities, the active species essential to the remote oxidation is not generated photocatalytically from the organic impurities but from ambient species such as O2 and H2O. The photocatalytic remote oxidation by TiO2 and Pt–TiO2 could be applied to poly(vinylidenefluoride) and many other materials but not to Teflon. The same trend was observed for Fenton and H2O2-UV reactions, in which hydroxyl radical is generated, suggesting that an oxidizing species as strong as a hydroxyl radical is involved in the remote oxidation.

Structure and growth of ultrathin titanium oxide films on Ru(0 0 0 1)

Structure, chemical composition and thermal stability of TiO x films with a thickness of up to 3 ML grown on a Ru(0 0 0 1) substrate were investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The films were prepared by evaporation of Ti in 10 −7 mbar O 2 onto Ru(0 0 0 1) at 640 K, followed by annealing in 10 −7 mbar O 2 or in UHV at temperatures between 700 K and 1000 K. Depending on the deposition and post-annealing conditions, we find several different structures of the Ti oxide, with the layers post-annealed in an O 2 environment being generally better defined than those post-annealed in vacuum. The layers consist of triangular units which coalesce upon annealing. O 2 annealing of monolayer films leads to an oxygen deficient TiO 2 layer. Annealing to 900–1000 K changes the structure and composition of the film, a coincidence structure with a Moiré pattern is observed upon O 2 annealing.

Electronic excitations at oxygen deficient TiO 2(110) surfaces: A study by EELS

TiO 2(110) surfaces with controlled oxygen deficiency introduced by 160 eV electron bombardment have been studied by XPS and EELS. Stoichiometry was monitored by the growth of core peaks due to Ti 3+ states in XPS. Oxygen desorption is characterised by an initial cross section of 3 × 10 −21 cm 2 that decreases with increasing oxygen loss, tending toward a limiting composition Ti 4O 7. The oxygen deficient surfaces display sub-bandgap excitations in electronic EELS, whilst in the vibrational region there is a selective downward shift and attenuation of the highest energy phonon loss. This is attributed to modification of the effective background dielectric constant by the defect excitations. Quantitative consideration of the changes in HREELS leads to an estimate of 0.1 for the oscillator strength of the defect electronic excitations. The high value supports the idea that electrons at oxygen deficient TiO 2 surfaces occupy states that are pulled down below the conduction band by polaronic self trapping.

Electronic excitations at oxygen deficient TiO 2(110) surfaces: A study by EELS

Electronic excitations at oxygen deficient TiO 2(110) surfaces: A study by EELS