Rutile TiO2 Surface Research Articles

First-principles study on the mechanism of photocatalytic reduction of nitrobenzene on the rutile TiO2(110) surface.

Photocatalytic synthesis of organic compounds has attracted more and more attention recently. In this work, we present a theoretical study on the molecular mechanism of the photocatalytic reduction of nitrobenzene to aniline on the rutile TiO2(110) surface. We have studied the adsorption and conversion of nitrobenzene at both the surface Ti site and the oxygen vacancy (Ov) site. The full reaction pathways at these two sites were calculated. The rate-limiting step and possible intermediates were identified. The results suggest that Ov is more active in the adsorption and conversion of nitrobenzene. Interestingly, we found that the chemistry of nitrobenzene on the rutile TiO2(110) surface, especially the breaking of the N-O bond, is closely related to the number of excess electrons available. Based on the calculation, we have proposed a full molecular mechanism which is compatible with the existing experiments. The results should be helpful for the design of more efficient photocatalysts for the conversion of nitrobenzene.

Adsorption of acetylene on a rutile TiO2(110) surface: a low temperature STM study.

Low temperature scanning tunneling microscopy (LT-STM) has been applied to investigate the adsorption of acetylene (C2H2) on the rutile TiO2(110) surface at 77 K. Through in situ exposing and ex situ annealing experiments we unambiguously demonstrate that C2H2 molecules adsorb weakly on top of the Ti5c ions. Moreover, our data clearly reveal that the oxygen vacancy (Ov) cannot directly adsorb C2H2 molecules but can activate its next-nearest Ti5c ions which register as a binding site for C2H2 molecules with enhanced stability. Further increasing the C2H2 coverage leads to the stepwise formation of c-(4 × 2) and c-(2 × 2) ordered structures before final saturation at around 0.68 monolayer. The saturated film is characterized by dominating C2H2 dimers and other minority linear clusters whose intermolecular distances are slightly larger than the intervals between Ti5c ions yet significantly shorter than the C2H2 dimers in the gas phase. We propose that such clustered C2H2 may serve as potential precursors for the polymerization reactions on metal oxide surfaces.

Elucidating the charge state of an Au nanocluster on the oxidized/reduced rutile TiO2 (110) surface using non-contact atomic force microscopy and Kelvin probe force microscopy.

The charge state of Au nanoclusters on oxidized/reduced rutile TiO2 (110) surfaces were investigated by a combination of non-contact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultra-high vacuum. We found that the Au nanoclusters supported on oxidized/reduced surfaces had a relatively positive/negative charge state, respectively, compared with the substrate. In addition, the distance dependence of LCPD verified the contrast observed in the KPFM images. The physical background of charge transfer observation can be explained by the model of charge attachment/detachment from multiple oxygen vacancies/adatoms surrounding Au nanoclusters. These results suggest that the electronic properties of the Au nanoclusters are dramatically influenced by the condition of the support used.

The effect of DMPO on the formation of hydroxyl radicals on the rutile TiO2(110) surface.

Unraveling the formation mechanism of hydroxyl radicals (OH˙) is one of the outstanding issues in photocatalytic reactions, where 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is widely utilized as a trapping agent to detect OH˙ radicals in experiments. In this study, we carry out density functional theory calculations to reveal the origin and formation process of OH˙ radicals by investigating the interaction of water with DMPO on a rutile TiO2(110) surface. Our results clearly show that the OH˙ radicals trapped by DMPO stem from water upon illumination. The charge compensation mechanism dominates the formation of DMPO-OH from the reaction between DMPO and water on the rutile TiO2(110) surface. These findings provide new insights into the photocatalytic mechanism and may achieve new frontiers in photocatalytic research.

Atomic structures of V–Ti–O intermixed oxide monolayer on rutile TiO2(011) substrate predicted by extensive structural search

The formation of various structural V–Ti–O intermixed oxide monolayers on the rutile TiO2(011) substrates have been observed in experiments. The atomic structures and formation mechanism of them are still unclear. To this end, a global search of V–Ti–O ordered structures on rutile TiO2(011) surface is performed based on the evolutionary method. Nine energetically stable or metastable structures of different compositions are predicted, which can be considered as V-doping in different type of 2 × 1 reconstruction of rutile TiO2 (011) surface. The calculated electronic structures indicate that the V atoms in most structures are not fully oxidized and introduce localized impurity states in the band gap. The oxidation states of the V atoms and surface Ti atoms are determined. The stabilization regions of them are determined based on the calculated surface energies. Our results are instructive for the experimental synthesis of different structural V–Ti–O monolayers, and significant for deep understanding of the photo- or electro-catalytic properties of them.

FePc and FePcF16 on Rutile TiO2(110) and (100): Influence of the Substrate Preparation on the Interaction Strength.

Interface properties of iron phthalocyanine (FePc) and perfluorinated iron phthalocyanine (FePcF16) on rutile TiO2(100) and TiO2(110) surfaces were studied using X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and low-energy electron diffraction (LEED). It is demonstrated that the interaction strength at the interfaces is considerably affected by the detailed preparation procedure. Weak interactions were observed for all studied interfaces between FePc or FePcF16 and rutile, as long as the substrate was exposed to oxygen during the annealing steps of the preparation procedure. The absence of oxygen in the last annealing step only had almost no influence on interface properties. In contrast, repeated substrate cleaning cycles performed in the absence of oxygen resulted in a more reactive, defect-rich substrate surface. On such reactive surfaces, stronger interactions were observed, including the cleavage of some C–F bonds of FePcF16.

Rutile TiO2 supported single atom Au catalyst: A facile approach to enhance methanol dehydrogenation

Rutile TiO2 supported single atom Au catalyst for promoting methanol dehydrogenation activity in direct methanol fuel cell (DMFC) has been investigated by using density functional theory calculations. Rutile Au1/TiO2 (110) and non-doped rutile TiO2 (110) surfaces are conducted due to their lowest index and energy. Fruitfully, rutile Au1/TiO2 (110) surface exhibits the excellent catalytic performance on methanol dehydrogenation, being prone to reform under alkali conditions; its lattice gold, titanium and oxygen atoms are involved in the whole methanol dehydrogenation process, along with the intermediate adsorbates. Methanol, via the activation of OH and C–H bonds on rutile Au1/TiO2 (110) surface, proceeds the subsequent stripping of the possible intermediate products: H2, HCHO, CO and CO2 to gas phase separately. Despite the most difficult step among them, CO2 stripping benefits considerably from the unique rutile TiO2 (110) surface and alkali participation.

Towards a transferable design of solid-state embedding models on the example of a rutile TiO2 (110) surface.

In this work, we present general and robust transferable principles for the construction of quantum-mechanically treated clusters in a solid-state embedding (SSE) approach, beyond the still prevalent trial and error approach. Thereby, we probe the quality of different cluster shapes on the accuracy of chemisorption energies of small molecules and small polaron formation energies at the rutile TiO2 (110) surface as test cases. Our analyses show that at least the binding energies and electronic structures in the form of the density of states tend to be quite robust already for small, nonoptimal cluster shapes. In contrast to that, the description of polaron formation can be dramatically influenced by the employed cluster geometry possibly leading to an erroneous energetic ordering or even to a wrong prediction of the polaronic states themselves. Our findings show that this is mainly caused by an inaccurate description of the Hartree potential at boundary and surrounding atoms, which are insufficiently compensated by the embedding environment. This stresses the importance of the cluster size and shape for the accuracy of general-purpose SSE models that do not have to be refitted for each new chemical question. Based on these observations, we derive some general design criteria for solid state embedded clusters.

Nitrate-group-grafting-induced assembly of rutile TiO2 nanobundles for enhanced photocatalytic hydrogen evolution

In this study, an acid-induced assembly strategy for a rutile TiO2 photocatalyst was proposed on the basis of the treatment of lamellar protonated titanate with a concentrated HNO3 solution. Nitrate groups were successfully grafted onto a TiO2 surface and induced the assembly of rutile TiO2 nanorods into uniform spindle-like nanobundles. The resulting TiO2 product achieved a photocatalytic hydrogen evolution rate of 402.4 μmol h−1, which is 3.1 times higher than that of Degussa P25-TiO2. It was demonstrated that nitrate group grafting caused the rutile TiO2 surface to become negatively charged, which is favorable for trapping positive protons and improving charge carrier separation, thereby enhancing photocatalytic hydrogen production. Additionally, surface charges were crucial to structural stability based on electrostatic repulsion. This study not only developed a facile surface modification strategy for fabricating efficient H2 production photocatalysts but also identified an influence mechanism of inorganic acids different from that reported in the literature.

Unusual adsorption behavior of hydrogen molecules on Zr–doped perfect and oxygen–vacancy defective rutile TiO2(110) surfaces: Periodic DFT study

Adsorptions of Zr atom onto the perfect rutile TiO2(110) and the oxygen vacancy rutile TiO2 (110) ([TiO2+Vo]) to form Zr–TiO2 and Zr‒[TiO2+Vo] were studied using periodic density functional theory (DFT) method. Three configurations of both Zr–TiO2 and Zr‒[TiO2+Vo] surfaces were found and binding energies of Zr atom of the most stable Configurations of Zr–TiO2 and Zr‒[TiO2+Vo] surfaces are respectively −3.36 and −3.26 eV. The most stable Configurations of the Zr–TiO2 and Zr‒[TiO2+Vo] surfaces were selected in hydrogen adsorption study. Adsorption energies of single H2 molecule on the most stable Zr–TiO2 and Zr‒[TiO2+Vo] are −1.43 and −1.45 eV, respectively. Based on the second H2 molecular adsorption on the hydrogen pre‒adsorbed Zr–TiO2 and Zr‒[TiO2+Vo] surfaces, adsorption energies of −1.90 and −2.55 eV were found, respectively. The second H2 molecule adsorption was found to be much stronger than the first H2 molecule adsorbed onto the Zr–TiO2 and Zr‒[TiO2+Vo] surfaces by 32.9% and 75.9%, respectively. Either the Zr–TiO2 or Zr‒[TiO2+Vo] surface is suggested as hydrogen–storage material and the Zr–TiO2 can be proposed as an electrical resistance‒based hydrogen sensor.

Identification of Atomic Defects and Adsorbate on Rutile TiO2(110)-(1 × 1) Surface by Atomic Force Microscopy

Atomic force microscopy (AFM) has been developed as a powerful tool to study surface topography and surface chemistry with atomic resolution, whereas straightforward identification of surface point defect/adsorbate is far from trivial in AFM imaging due to its complex mechanism. In this work, we successfully demonstrate a reliable way to discriminate surface point defect/adsorbate on a rutile TiO2 surface by AFM. Each surface point defect/adsorbate can positively respond to the applied bias voltage, but the specific response value is different. The present methodology is reliable to distinguish the surface atomic defect and adsorbates on metal oxides, no matter what kinds of tip apex termination, and useful for continuous investigation of catalytic reactions on the surface.

Preparation of phthalocyanine blue/rutile TiO2 composite pigment with a ball milling method and study on its NIR reflectivity

The phthalocyanine blue/rutile TiO2 composite pigments were fabricated with a dry ball milling method, and they were characterized by transmission electron microscope (TEM) and fourier transform infrared spectroscopy (FT-IR). The results showed that the hydrophilic/hydrophobic property of rutile TiO2 played a key role on the structure of composite pigment and the phthalocyanine blue (PB) pigment preferred to encapsulate on the surface of hydrophobic rutile TiO2 to form a core-shell structure. And by comparison with the PB pigment, the composite pigment showed 10%–17% higher reflectivity in the wavelength range of 800–1100 and 1200–1600 nm, and the NIR reflectivity in the wavelength range of 1200–1600 nm reached up to 90%.

Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation.

Titanium oxide (TiO2) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H2 with TiO2 surface plays an important role. However, the activation of hydrogen over rutile TiO2 surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H2 as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO2 orientations.

Chemical nature and thermal decomposition behavior of tartaric acid multilayers on rutile TiO2(110)

R,R-tartaric acid (RR-TA) thermal stability and decomposition on the rutile TiO2(110) surface was investigated by temperature programmed desorption. The authors show that a majority of RR-TA molecules are desorbed intact from multilayers at around 340 K, while they decompose from the first chemisorbed layer between 460 and 480 K. Complementary information on the chemical nature of RR-TA in the multilayer regime was gained by x-ray photoelectron spectroscopy, which shows that biacid molecules form the multilayer while they are monotartrate at the interface.

Computational study of surface orientation effect of rutile TiO2 on H2S and CO sensing mechanism

The adsorption of hazardous CO and H2S on four low-index surfaces of rutile TiO2 surfaces was investigated using first-principle calculations, in an effort to distinguish the difference of surface reactivity on these surfaces in sensing process. We predicted that rutile TiO2 possessed excellent capacity for H2S detection than CO based on adsorptive structure, density of states (DOS) analysis and charge density difference (CDD) plots. Particularly, in H2S adsorption, the dissociative adsorption process occurred on the (001) surface of TiO2 with largest adsorption energy as 1.17 eV and noticeable modification in the electronic structure, rendering high sensitivity on this surface. The decomposition of H2S yielded HS and H species, whose bonding features were also analyzed. It was suggested that 3p states of HS were able to mix with surface titanium and the 1 s states of H were considerably overlapped with 2p states of surface bridging oxygen, forming OH radicals on (001) surface. The reason for different sensing properties on these surfaces was pinpointed, and the significance of rutile (100) facet in H2S monitoring was underlined. These preliminary investigations would provide a basic understanding for CO and H2S adsorption for pursuing high-performance sensor toward targeted gas.

Activation of Small Organic Molecules on Ti2+-Rich TiO2 Surfaces: Deoxygenation vs. C–C Coupling

Abstract Rutile TiO2 is an important model system for understanding the adsorption and conversion of molecules on transition metal oxide catalysts. In the last decades, point defects, such as oxygen vacancies and Ti3+ interstitials, exhibited an important influence on the reaction of oxygen and oxygen-containing molecules on titania surfaces. In brief, partially reduced TiO2 containing a significant amount of Ti3+ is often more active for the conversion of such molecules. In this study, we investigate an even higher reduced surface prepared by argon ion bombardment of a rutile TiO2 (110) single crystal. By X-ray photoelectron spectroscopy we show that, besides Ti4+, this surface is almost equally dominated by Ti3+ and Ti2+. To probe the reactivity of these highly reduced surfaces, we have adsorbed two different classes of oxygen-containing molecules and utilized temperature programmed reaction spectroscopy to investigate the conversion. While alcohols (in this case methanol) already show a defect-dependent partial conversion in a deoxygenation reaction on the (stochiometric or slightly reduced) rutile TiO2 (110) surface, ketones (e.g. acetone) are usually not converted on the rutile TiO2 (110) surface independent on the bulk defect density. Here, we present a nearly full conversion for both molecules via deoxygenation reactions and reductive C–C coupling, forming different hydrocarbons at different temperatures between 375 K and 640 K on the sputtered Ti2+ rich surface.

Effects of surface hydration on the electron injection rate from graphene to anatase and rutile TiO2 surfaces

The effect of hydration of anatase (0 0 1), and rutile (0 0 1), (1 0 1) and (1 1 0) surfaces of TiO2 on the rate of electron capture from an adsorbed graphene sheet is elucidated by means of computation at a level of tight-binding density functional theory. Initially, the clean surfaces are allowed to interact with bulk water. Following an equilibration period, the first solvation shell is exposed to vacuum at 300 K for a sufficiently long period of time to allow any weakly bound water molecules to evaporate or be displaced from the surface. The resultant hydroxylated and originally clean surfaces are used for studying the interaction with a graphene sheet. Comparison of the calculated surface-graphene interaction energies and electron transfer probabilities of a cold electron to clean and hydroxylated surfaces shows that the water-sourced OH layer of these surfaces acts as an insulator in interfacial charge transport of the cold electron.

Adsorption of a chiral modifier on an oxide surface: Chemical nature of tartaric acid on rutile TiO2 (110)

Tartaric acid (TA) is one of the most known chiral modifier used in heterogeneous enantioselective catalysis and understanding how it interacts with oxide supports is of significant importance for the design of efficient supported metal catalysts. This work presents a detailed surface science characterisation of (R,R)-TA adsorption on a rutile TiO2(110) surface from a chemical and structural point of view. X-ray Photoelectron Spectroscopy (XPS), High Resolution Electron Energy Loss Spectroscopy (HREELS), Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM) were used to decipher the adsorption mode and geometrical arrangement of TA on TiO2(110). For XPS analysis, spectra were compared to references recorded on model metal surfaces, namely Au(111) and Cu(110), on which a better understanding of the TA chemical nature is available. On TiO2, both XPS and HREELS converged to show the monotartrate chemical nature (only one acidic group is deprotonated) of the adsorbed TA molecules, while STM and LEED evidenced a (2 × 1) 2D network arrangement. TA molecules are anchored via both oxygen atoms of one carboxylate group binding directly to two adjacent Ti5c of the underlying oxide surface. No chiral ordered domains were observed whatever the coverage, suggesting that surface chirality is induced at the molecular level.

Influence of oxygen adsorption for vacancy-induced d0 magnetism in rutile TiO2 (001) surface

Abstract Effect of O2 adsorption on the magnetism of the rutile TiO2 (001) surface under different O2 coverages were investigated. The surfaces that we considered for O2 adsorption included the defect-free surface and the O-deficient surfaces with the charge states of 0, +1 and +2. Nonmagnetic defect-free surface is difficult to absorb the O2 molecule. Stabilities of oxygen vacancies with various valences at the top layer depend on their charge states, and the magnetic of the defective surface is related to VO+ instead of VO0 and VO2+. At low O2 coverage, the endothermic adsorptions of O2 molecule may introduce a nonzero local magnetic moment in the neutral reduced surface while its exothermic adsorption cannot. For the +1 or +2 O-deficient surface, the O2 molecule is exothermically adsorbed on the surface and obtains some electrons to form a superoxide anion, which leads to the existence of magnetism. At high O2 coverage, the chemisorptions of new O2 molecule on the neutral O2-preadsorbed substrate are related to the adsorption sites and O O bond oriented. Endothermic adsorption of new O2 molecule induces the magnetic moment on nonmagnetic O2-preadsorbed substrate, while the substrate with the charge state of +1 or +2 inhibits more O2 molecule adsorptions.

Imaging the surface potential at the steps on the rutile TiO2(110) surface by Kelvin probe force microscopy.

Although step structures have generally been considered to be active sites, their role on a TiO2 surface in catalytic reactions is poorly understood. In this study, we measured the contact potential difference around the steps on a rutile TiO2(110)-(1 × 1) surface with O2 exposure using Kelvin probe force microscopy. A drop in contact potential difference was observed at the steps, indicating that the work function locally decreased. Moreover, for the first time, we found that the drop in contact potential difference at a <1−11> step was larger than that at a <001> step. We propose a model for interpreting the surface potential at the steps by combining the upward dipole moment, in analogy to the Smoluchowski effect, and the local dipole moment of surface atoms. This local change in surface potential provides insight into the important role of the steps in the catalytic reaction.