Interaction and ordering of lattice defects in oxygen-deficient rutile TiO2-x

The influence of bulk stoichiometry on near-ambient pressure reactivity of bare and Pt-loaded rutile TiO2(110)

The interaction of catalyst particles with reducible support materials can drastically change their reactivity. On rutile TiO2, processes like particle encapsulation (caused by the "strong metal-support interaction", SMSI) have long been known to depend on the initial reduction state of the oxide. Despite this knowledge, sample stoichiometry has rarely been controlled in a reproducible manner in the surface science literature. Here, we use scanning tunnelling microscopy (STM) to explore systematically how near-ambient pressures (0.1-1.0 mbar) of O2, H2, CO and CO2 affect blank and Pt-loaded rutile TiO2(110) surfaces of different bulk stoichiometry at 600 K. To this end, we present preparation recipes that result in a sample stoichiometry always converging back to the same value, which allows us to use the same samples with constant reduction state over hundreds of preparation cycles. Comparing a highly reduced and a near-stoichiometric TiO2 sample, we find that surface reactivity to all four gasses differs even without Pt loading. Most surprisingly, we find that the highly reduced TiO2(110) is oxidized by CO2, but this reaction is completely inhibited on the near-stoichiometric sample. Pt nanoparticles, in turn, become encapsulated after vacuum annealing on the reduced, but not on the near-stoichiometric sample. Encapsulation on the near-stoichiometric sample is achieved only after exposing it to 0.1 mbar H2 at 600 K. Interestingly, we also see a further modification of the already encapsulated particles on the reduced sample under the same conditions, such that they become embedded deeper in the TiO2(110) surface.

In situ studies on defect formation dynamics in flash-sintered TiO2

Flash-sintered (FS) ceramics have shown promising mechanical deformability at room temperature compared to conventional sintered ceramics. One major contributing factor to plasticity is high-density defects, such as dislocations, stacking faults and point defects, resulted presumably from the high electrical field during flash sintering. However, such direct experiemtnal evidence for defect formation and evolution under the electric field remains lacking. Here we performed in situ biasing experiments in FS and conventionally sintered (CS) polycrystalline TiO2 in a transmission electron microscope (TEM) to compare the defect evolution dynamics. In situ TEM studies revealed the coalescence of point defects under the electrical field in both FS and CS TiO2 and the subsequent formation of stacking faults, which are often referred to as Wadsley defects. Surprisingly, under the electrical field, the average fault growth rate in the FS samples is 10 times as much as that in the CS TiO2. Furthermore, the Magnéli phase, a 3D oxygen-deficient phase formed by the aggregation of Wadsley defects, is observed in the FS samples, but not in the CS samples. The present study provides new insights into defect dynamics in FS ceramics.

Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering

Both oxygen vacancies and surface hydroxyls play a crucial role in catalysis. Yet, their relationship is not often explored. Herein, we prepare two series of TiO₂ (rutile and P25) with increasing oxygen deficiency and Ti³⁺ concentration by pulsed laser defect engineering in liquid (PUDEL), and selectively quantify the acidic and basic surface OH by fluoride substitution. As indicated by EPR spectroscopy, the laser-generated Ti³⁺ exist near the surface of rutile, but appear to be deeper in the bulk for P25. Fluoride substitution shows that extra acidic bridging OH are selectively created on rutile, while the surface OH density remains constant for P25. These observations suggest near-surface Ti³⁺ are highly related to surface bridging OH, presumably the former increasing the electron density of the bridging oxygen to form more of the latter. We anticipate that fluoride substitution will enable better characterization of surface OH and its correlation with defects in metal oxides.

Light-Induced Migration of Spin Defects in TiO2 Nanosystems and their Contribution to the H2 Evolution Catalysis from Water

The photocatalytic activity for H₂ production from water, without presence of hole scavengers, of thermally reduced TiO₂ nanoparticles (H-500, H-700) and neat anatase were followed by in-situ continuous-wave light-induced electron paramagnetic resonance technique (CW-LEPR), in order to correlate the H₂ evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e^- +h⁺ ) recombination and were the crucial electronic factor affecting catalysis. In the best-performant system (H-500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti³⁺ sites, including interstitial Ti³⁺ , and singly ionized electrons trapped in oxygen vacancies (V_O ^. ). The interplay of these species fed successfully surface exposed Ti⁴⁺ sites, which became a catalytically active, fast reacting Ti⁴⁺ ⇄Ti³⁺ state that was key for the H₂ evolution process.

Methanol photo-reforming with water on pure titania for hydrogen production

The behaviour of titania for the photo-reforming of methanol with water at ambient temperature has been examined. It is shown that the reactivity is very poor, compared with metal-loaded catalysts at low methanol levels in solution, but the rate becomes much higher at high methanol levels, such that the difference from metal-loaded samples is much less. The optimum yield is with approximately a 1 : 1 methanol/water solution. The reaction also proceeds well in the gas phase. During all such catalysis, the titania becomes blue, due to light absorption increasing across the range 400-800 nm. However, this does not result in visible range activity for the photo-reforming and is due to the reduction of the material in the presence of light and the formation of anion vacancies and Ti³⁺ centres. These anion vacancies are only very slowly re-oxidized in air on P25 titania, taking days to recover the original whiteness of the oxide. The performance of anatase, rutile and the mixed phase is compared. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Local crystallographic shear structures in a[201] extended mixed dislocations of SrTiO3 unraveled by atomic-scale imaging using transmission electron microscopy and spectroscopy

Recently, extended mixed dislocations were observed at a [001]/(100) low-angle tilt grain boundary of a SrTiO3 bicrystal because of a slight twist between the two crystal parts. The b = a[201]/(100) mixed dislocations at the grain boundary dissociate into three dislocations with Burgers vector b of a/2[101], a[100], and a/2[101], respectively. A structure model has been proposed in particular for the dislocation cores of the two partials with b = a/2[101] based on the high-angle annular dark-field (HAADF) images acquired by scanning transmission electron microscopy (STEM). However, the details of the atomic structure and the chemical composition of the dislocation cores remain unexplored, especially for the b = a[100] dislocation that is evidently disassociated into two b = a/2[101] partial dislocations. In this work, we study the further atomic details of the extended mixed dislocations, in particular the local chemistry, in a SrTiO3 bicrystal using STEM, electron energy loss spectroscopy (EELS), and energy dispersive X-ray (EDX) spectroscopy techniques. By these atomic-scale imaging techniques, we reveal a unique feature for the atomic structure of the b = a[201]/(100) extended mixed dislocation, which we named as local crystallographic shear (LCS) structures. In addition, we identify a rock salt FCC-type TiOx (x = 0.66-1.24) phase at the locations of the extended mixed dislocations. In contrast to the insulating TiO2 phases, the TiOx phase is known to exhibit very low electrical resistivity of only several μΩ cm. In this regard, the extended mixed dislocations of SrTiO3 comprising the FCC TiOx phase may function as the conducting filament in resistive switching processes by completion and disruption of the TiOx phase along the dislocation cores through electrically stimulated redox reactions.

Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production

Black TiO₂ nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO₂, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure-activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO₂ photocatalysts for hydrogen production such as (i) defects formation in reduced TiO₂, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO₂ photocatalysts are outlined.

KPFM/AFM imaging on TiO2(110) surface in O2 gas

We have carried out high-speed imaging of the topography and local contact potential difference (LCPD) on rutile TiO₂(110) in O₂ gas by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We succeeded in KPFM/AFM imaging with atomic resolution at 1 frame min^-1 and observed the adsorbate on a hydroxylated TiO₂(110) surface. The observed adsorbate is considered to be oxygen adatoms (O_a), hydroperoxyls (HO₂), or terminal hydroxyls (OH_t). After adsorption, changes in the topography and the LCPD of the adsorbate were observed. This phenomenon is thought to be caused by the charge transfer of the adsorbate. This technique has the potential to observe catalytic behavior with atomic resolution.

EPR approaches to heterogeneous catalysis. The chemistry of titanium in heterogeneous catalysts and photocatalysts

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

Ferromagnetic Interactions in Highly Stable, Partially Reduced TiO2 : The S=2 State in Anatase

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO₂ anatase obtained by the mild oxidation of TiB₂ under hydrothermal conditions. Continuous-wave and pulse EPR spectroscopy at X and Q band frequencies provide compelling evidence for the presence of S=2 states, stable in a wide range of temperatures up to room temperature. A tentative model, corroborated by spin-polarized DFT calculations, is proposed, which consists of four ferromagnetically interacting Ti³⁺ ions with distances ranging from 0.5 nm to 0.8 nm and tetrahedral arrangement.

Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy

Metal oxides are abundant in nature and they are some of the most versatile materials for applications ranging from catalysis to novel electronics.

EPR investigation of TiCl3 dissolved in polar solvents--implications for the understanding of active Ti(III) species in heterogeneous Ziegler-Natta catalysts

Multi-frequency continuous-wave and pulsed EPR techniques are employed to investigate Ti(III)-chloro complexes obtained by dissolving TiCl3 in anhydrous and hydrated methanol. Two distinctly different species, characterized by different g matrices are observed in the two cases. Hyperfine sublevel correlation (HYSCORE) spectroscopy is found to be a powerful method to identify the type of nuclei surrounding the Ti(3+) ion. For the first time, the hyperfine and nuclear quadrupole data of Ti(III)-bound (35/37)Cl nuclei are reported together with (1)H and (13)C hyperfine data of the coordinated methanol molecules. DFT modelling allows interpreting the measured spin Hamiltonian parameters in terms of microscopic models of the solvated species. The theoretical observable properties (g matrix, (35/37)Cl, (1)H and (13)C hyperfine tensors) are in quantitative agreement with the experiments for two families of complexes: [TiCln(CH3OH)6-n]((3-n)+) (with n ranging from 1 to 3) and [Ti(CH3OH)5(OH)](2+) or [Ti(CH3OH)5(OCH3)](2+). The first complex is observed in anhydrous methanol, while the second type of complex is observed when water is added to the solution, the presence of OH(-) and/or CH3O(-) species being promoted by water hydrolysis. The results obtained for the frozen solutions are critically compared to EPR spectra recorded for a MgCl2-supported Ti-based Ziegler-Natta model catalyst.

Vanadium Doped Nanostructured TiO2 Dielectrics

Dielectric properties of titanium oxide ceramics are strongly influenced by the microstructural features and concentration of dopants and impurity ions. Electrical conductivity (via insulation resistance) of vanadium doped nanostructured titanium dioxide (TiO2) ceramics was measured as a function of donor concentration and temperature. In order to further clarify the effect of the dopants on the microstructural development and resultant dielectric properties of TiO2, electron paramagnetic resonance (EPR) spectroscopy was employed. Vanadium-doped TiO2 exhibited well-defined hyperfine splitting characteristics of the 51V nuclei indicating that the dopant ions are dispersed within the grains and not preferentially segregated at the grain boundaries.

Magnetic resonance study of annealed and rinsed N-doped TiO2 nanoparticles

Nanoparticles of nitrogen-modified TiO2 (N-doped TiO2) calcined at 300°C and 350°C, have been prepared with and without water rinsing. Samples were characterized by x-ray diffractrometry (XRD) and optical spectroscopy. The electron paramagnetic resonance (EPR) spectra from centers involving oxygen vacancies were recorded for all samples. These could be attributed to paramagnetic surface centers of the hole type, for example to paramagnetic oxygen radicals O−, O2−etc. The concentration of these centers increased after water rising and it further increased for samples annealed at higher temperature. Additionally, for samples calcined at 300°C, and calcined at 350°C and rinsed, the EPR spectra evidenced the presence of magnetic clusters of Ti3+ ions. The photocatalytic activity of samples was studied towards phenol decomposition under unltraviolet-visible (UV-Vis) irradiation. It was found that, in comparison to the starting materials, the rinsed materials showed increased photocatalytic activity towards phenol oxidation. The light absorption (UV-Vis/DRS) as well as surface Fourier transform infrared/diffuse reflectance spectroscopy (FTIR/DR) studies confirmed a significantly enhanced light absorption and the presence of nitrogen groups on the photocatalysts surfaces, respectively. A significant increase of concentration of paramagnetic centers connected with oxygen vacancies after water rising has had an essential influence on increasing their photocatalytic activity.

Molecular imaging of reductive coupling reactions: interstitial-mediated coupling of benzaldehyde on reduced TiO2(110)

We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface-a diolate formed from benzaldehyde coupling on TiO(2)(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to ∼400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.

The role of Ti(3+) interstitials in TiO(2)(110) reduction and oxidation

Here we describe results which teach us much about the mechanism of the reduction and oxidation of TiO(2)(110) by the application of scanning tunnelling microscopy imaging at high temperatures. Titania reduces at high temperature by thermal oxygen loss to leave localized (i.e.  Ti(3+)) and delocalized electrons on the lattice Ti, and a reduced titania interstitial that diffuses into the bulk of the crystal. The interstitial titania can be recalled to the surface by treatment in very low pressures of oxygen, occurring at a significant rate even at 573 K. This re-oxidation occurs by re-growth of titania layers in a Volmer-Weber manner, by a repeating sequence in which in-growth of extra titania within the cross-linked (1 × 2) structure completes the (1 × 1) bulk termination. The next layer then initiates with the nucleation of points and strings which extend to form islands of cross-linked (1 × 2), which once again grow and fill in to reform the (1 × 1). This process continues in a cyclical manner to form many new layers of well-ordered titania. The details of the mechanism and kinetics of the process are considered.

Density functional theory based first-principle calculation of Nb-doped anatase TiO2 and its interactions with oxygen vacancies and interstitial oxygen

The structure and electronic properties of Nb-doped anatase (TNO) were studied from first principles using the density functional theory based band structure method. Four independent types of unit cells were studied; i.e., pure anatase, anatase with Nb dopant at Ti sites (Nb(Ti)), and cells with either interstitial oxygen (O(i)) or oxygen vacancies (V(O)). In addition, a unit cell with a Nb(Ti) and O(i), and a cell with Nb(Ti) and V(O) were investigated to clarify the role of nonstoichiometry in TNO. From the calculated results, the importance of the adjacent Nb(Ti)-V(O) and Nb(Ti)-O(i) structures was pointed out, and the experimental observation of the relationship between nonstoichiometry and electronic conductivity was rationalized. The shape of the impurity states found in these structures was used to comprehend the experimental observation of carrier concentration and the charge state of Nb dopant. The changes in lattice constants supported the existence of these structures as well. On the contrary, the cell with a simple Nb(Ti) did not show significant changes in structure and electronic properties, other than the emission of an electron in the conduction band. A stabilization of the impurity state was observed in the adjacent Nb(Ti)-V(O) structure compared to the V(O). The possibility of an essential role of this state in electric conduction was discussed. The formation of the adjacent Nb(Ti)-O(i) structure by O(2) gas annealing was discussed using statistical mechanics. The Gibbs free energies were calculated for O(i) atoms in TNO and compared to that of O(2) molecules in the gas phase. The analysis was qualitatively consistent with experimental behavior under the assumption of the Nb(Ti)-V(O) structures.

Quantum conductance oscillations in metal/molecule/metal switches at room temperature

We apply pressure-modulated conductance microscopy to metal/molecule/metal switches. Apart from pressure-induced conductance peaks that indicate nanoscale conducting pathways, we also observe dips and oscillations for devices with conductance between 1 and 2 conductance quantum. The conductance oscillations arise from interfering electron waves along one or two quantum conductance channels between two partially transmitting electrode surfaces at room temperature, underscoring these devices' potential as coherent, atomic-scale switches.

Anisotropy in the Electrical Conductivity of Rutile TiO2 in the (110) Plane

The bulk electrical conductivity of a TiO(2)(110) single crystal has been measured in all azimuths parallel to the (110) plane using a four-point probe. A distinct anisotropy in the conductivity has been found, with the highest bulk conductivity direction being parallel to the 001 direction.

Monitoring Hole Trapping in Photoexcited TiO2(110) Using a Surface Photoreaction

The hole-induced photodesorption of chemisorbed O2 from a TiO2(110) single crystal has been employed to monitor the kinetics of electron-hole pair (e-h) formation and hole trapping. Excitation is produced by 3.4 +/- 0.05 eV photons at 110 K. Two separate O2 desorption processes have been found which are characteristic of low photon fluxes and high photon fluxes. At a critical photon flux, Fhnu(crit), the slow O2 photodesorption process suddenly converts to a fast process, signaling the saturation of hole traps in the TiO2 crystal. Consequently, this allows photogenerated holes to more efficiently reach the surface, causing more rapid O2 photodesorption. The estimated bulk concentration of hole traps is approximately 2.5 x 10(18) cm(-3), involving a fraction of about 3 x 10(-5) of the atomic sites in the bulk. Both the slow and fast O2 photodesorption processes are described by a rate law that is proportional to Fhnu(1/2), indicating that the steady-state concentration of holes, [h], is governed by second-order e-h pair recombination kinetics. Effective use is made of a hole scavenger molecule, adsorbed methanol (CH3OH), to probe the role of added hole traps on the rate of the photodesorption of adsorbed O2 molecules and on the magnitude of Fhnu(crit).