Rutile Surface Research Articles

The Effect of Surface Electronic Structure on the Bioactivity of Neutral Dopant Si, Ge, and Sn on TiO2 (110): A DFT Study

In this paper, first principles calculations based on density functional theory were used to study the electronic properties and adsorption behavior of modified rutile surfaces with neutral element dopants (Si, Ge, Sn). The results showed that Sn doped surface has the smallest work function (7.07 eV) and Ge doped surface has the lowest conduction band edge (1.28 eV). Furthermore, the adsorption behavior of H2O and Arg‐Gly‐Asp (RGD) molecules on modified rutile surfaces was calculated to estimate the biocompatibility of modified dental implant surfaces. For the purpose of simulating the real solution environment, the free energy of solvation for adsorbates, calculated by Gaussian 09 program code, was taken into account. The results of the adsorption calculations of H2O indicate that Sn doped surface has a more negative adsorption energy (−3.15 eV) and a higher charge transfer (−0.1) than any other modified surface, which is consistent with its low work function. The results of the adsorption calculations of Arg‐Gly‐Asp demonstrate that Ge doped surface also has a shorter distance (1.95 Å) between Ti atom and O atom in RGD, which is similar with the partial density of states results. All of the results show that the electronic structure of an implant surface has a significant influence on its bioactivity and is also useful for the design of modification methods on dental implant surfaces.

Photocatalytic C[sbnd]C bond cleavage in ethylene glycol on TiO2: A molecular level picture and the effect of metal nanoparticles

Polyol conversion to value-added products is of great interest for the bio-diesel industry. Photocatalytic oxidation processes may offer a green approach for polyol conversion; however the lack of comprehensive mechanistic understanding from an interdisciplinary perspective limits or even misleads the design of highly selective and efficient photocatalysts for such process. Here we have studied the photocatalytic polyol conversion on pristine TiO2 and metal (Au, Pd, and Pt) nanoparticles (NPs) decorated TiO2 using ethylene glycol (EG) as the model compound. We have developed a mechanistic picture at molecular level by coupling in-situ surface science study on rutile (110) surface with in-situ vibrational-mass spectrometry study on TiO2 nanopowders. The CC bond cleavage was found to be the only pathway in EG photo-conversion under deaerated conditions, leading to the formation of formaldehyde and hydrogen. We rationalized that the desorption of the surface adsorbed H (Hads) to be the rate determining step (RDS), making pristine TiO2 a poor photocatalyst that only catalyze the EG conversion at very low surface coverages. The addition of metal NPs on TiO2 surface promotes the desorption of Hads significantly, thus leading to an enhanced CC bond cleavage performance at higher surface coverages that is more applicable.

Application of the Lany–Zunger polaron correction for calculating surface charge trapping

Defect calculations, using density functional theory in a local or semi-local approximation, in transition metal oxides are severely handicapped by the electron self-interaction error. The underestimation of the band gap may cause incorrect occupation of defect states and wrong formation energies, and the underestimated localization of the states disfavors the formation of small polarons. These problems can be avoided by using higher level approximations (GW or a correctly chosen hybrid functional), but those methods are computationally too expensive to be used for calculating surface defects in a periodic slab model. Lany and Zunger have suggested a convenient (low-cost) solution for solving the band-gap and charge delocalization problem, by applying a correction scheme to the standard local or semi-local approximations. Most importantly, the linearity of the total energy as a function of the fractional occupation numbers, is restored, leading to the fulfillment of the generalized Koopmans’ theorem. The method works well in the bulk but, as we show here, it is not accurate on the surface due to the different screening environment. We also show that by making the atom- and angular-momentum dependent parameters of the Lany–Zunger polaron-correction also coordination dependent, it is possible to correctly describe charge trapping in small polaron states on the anatase (1 0 1) and rutile (1 1 0) surfaces at a low computational cost.

Van der Waals Interaction Really Matters: Energetics of Benzoic Acid on TiO2 Rutile Surfaces

Density functional theory (DFT) has been applied to elucidate the adsorption structures and energetics of benzoic acid on TiO2 (110), (100), and (011) rutile surfaces. We demonstrate that ab initio calculations of interacting carboxylic acids require an exchange-correlation functional with van der Waals (vdW) correction to yield reliable results, even for very small aliphatic species like acetic acid. On the (110) surface, benzoates dimerize due to intermolecular vdW interaction and form a 2 × 2 superstructure, which explains experimental findings already reported in the literature. The lateral vdW attraction between benzoates is even enhanced on (100) and (011) surfaces because of advantageous substrate periodicities, resulting essentially in only one geometrical adsorbate species.

Band positions of Rutile surfaces and the possibility of water splitting

It is well known that both the band gap and the band edge positions of oxide semiconductors are important for the photocatalytic water splitting. In this study, we show that different surface terminations of the same crystalline solid lead to considerable variations of the band gaps and band edges. As an example, we investigate the low-index surfaces of rutile TiO2. A series of hybrid methods based on the PBE exchange-correlation functional, PBE0, HSE06 and HISS, are employed to study the effect of long-range exchange on the electronic properties. In aqueous solution, the oxide particles employed in photocatalysis are fully covered with water molecules.We therefore study the influence of molecularly and dissociatively adsorbed water on the band positions. It is found that water adsorption leads to significant shifts of the band edge positions due to changes of the electrostatic potential at the surface atom positions.

The Activation Mechanism of Bi3+ Ions to Rutile Flotation in a Strong Acidic Environment

Lead hydroxyl compounds are known as rutile flotation of the traditional activated component, but the optimum pH range for flotation is 2–3 using styryl phosphoric acid (SPA) as collector, without lead hydroxyl compounds in slurry solution. In this study, Bi3+ ions as a novel activator was investigated. The results revealed that the presence of Bi3+ ions increased the surface potential, due to the specific adsorption of hydroxyl compounds, which greatly increases the adsorption capacity of SPA on the rutile surface. Bi3+ ions increased the activation sites through the form of hydroxyl species adsorbing on the rutile surface and occupying the steric position of the original Ca2+ ions. The proton substitution reaction occurred between the hydroxyl species of Bi3+ ions (Bi(OH)n+(3−n)) and the hydroxylated rutile surface, producing the compounds of Ti-O-Bi2+. The micro-flotation tests results suggested that Bi3+ ions could improve the flotation recovery of rutile from 61% to 90%, and from 61% to 64% for Pb2+ ions.

Canonical, Deprotonated, or Zwitterionic? A Computational Study on Amino Acid Interaction with the TiO2 (101) Anatase Surface

The interaction of 11 amino acids with the TiO2 (101) anatase surface was investigated by means of PBE-D2* periodic simulations, both from a static and dynamic points of view. Several adsorption states, with the amino acid in its canonical, zwitterionic, or deprotonated forms, were considered. The strongest interactions correspond to dative interactions between the electron pairs of N or O atoms of the amino acids and the Ti atoms of the surface. For glycine, the most stable configuration corresponds to the deprotonated (N,O) binding mode, at variance to that observed for the adsorption of glycine on rutile (110) surface, for which the dissociative (O,O) binding mode was determined to be the preferred one. For the remaining amino acids, the following general trend was identified: those amino acids with acidic or a basic lateral chain groups, except Arg, exhibit a deprotonated (N,O) binding mode as glycine, because the additional dative interactions with the side chain overcome the destabilizing effect ind...

Phase quantification by X-ray photoemission valence band analysis applied to mixed phase TiO2 powders

A method of quantitative phase analysis using valence band X-ray photoelectron spectra is presented and applied to the analysis of TiO2 anatase-rutile mixtures. The valence band spectra of pure TiO2 polymorphs were measured, and these spectral shapes used to fit valence band spectra from mixed phase samples. Given the surface sensitive nature of the technique, this yields a surface phase fraction. Mixed phase samples were prepared from high and low surface area anatase and rutile powders. In the samples studied here, the surface phase fraction of anatase was found to be linearly correlated with photocatalytic activity of the mixed phase samples, even for samples with very different anatase and rutile surface areas. We apply this method to determine the surface phase fraction of P25 powder. This method may be applied to other systems where a surface phase fraction is an important characteristic.

The distribution of excess carriers and their effects on water dissociation on rutile (110) surface

The excess carrier widely exists in the metal oxides, while its effect on the water splitting is still unclear. Here, the first-principle calculations were carried out to investigate how the effects of excess carriers affect the water dissociation on rutile (110). The result shows that the excess electron and hole play different roles in the reactions because of the distinct behaviors. The excess electron is easily to be trapped in the lattice Ti atom, and the trapped electron slightly changes the reaction barrier of water dissociation. However, the excess hole prefers to stick with the hydroxyl radical, which can obviously lower the energy barrier of water dissociation from 0.39eV to 0.24eV. The results further show that the dissimilar behaviors of excess electron and hole mainly come from their abilities of trapping and distinct interaction with proton in rutile TiO2. Such work helps to understand carrier’s distribution and its effect on water splitting, which should help to reveal the role of excess electron and hole in TiO2.

Rutile TiO2 Flocculent Ripples with High Antireflectivity and Superhydrophobicity on the Surface of Titanium under 10 ns Laser Irradiation without Focusing.

We report on the formation of rutile TiO2 flocculent laser-induced periodic surface structures (LIPSSs) with high antireflectivity and superhydrophobicity on the surface of titanium under 10 ns 1064 nm laser irradiation without focusing. The center part of the Gaussian laser beam is used to deposit flocculent structure and the edge part used to produce LIPSSs. The melt and modification thresholds of titanium were determined first, and then, the melt and modification spot-overlap numbers, several responsible for the formation of flocculent structure and LIPSSs, were introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in laser fluence and spot-overlap number, contributing to the production of flocculent LIPSSs. LIPSSs are obtained with the modification spot-overlap number above 300, and the amount of flocculent structures increases with an increase in the peak laser fluence and spot-overlap number. Then, considering that the fine adjustment of the melt and modification spot-overlop numbers in one-time line scanning is quite difficult, the composite structure, of which both LIPSSs and flocculent structures are distinct, was optimized using laser line scanning twice. On this basis, a characterization test shows the sample full of the flocculent LIPSSs represents best antireflectivity with the value around 10% in the waveband between 260 and 2600 nm (advance 5 times in infrared wavelengths compared to the initial titanium surface), and shows the no-stick hydrophobicity with the contact angle of 160° and roll-off angle of 25° because of the pure rutile phase of TiO2.

Ion Exchange Thermodynamics at the Rutile-Water Interface: Flow Microcalorimetric Measurements and Surface Complexation Modeling of Na-K-Rb-Cl-NO3 Adsorption.

Flow microcalorimetry was used to investigate the energetics associated with Rb+, K+, Na+, Cl-, and NO3- exchange at the rutile-water interface. Heats of exchange reflected differences in bulk hydration/dehydration enthalpies (Na+ > K+ > Rb+, and Cl- > NO3-) such that exchanging Na+ or Cl- from the surface was exothermic, reflecting their greater bulk hydration enthalpies. Exchange heats were measured at pH 2, 3.25, 5.8, and 11 and exhibited considerable differences as well as pH dependence. These trends were rationalized with the aid of a molecularly constrained surface complexation model (SCM) that incorporated the inner-sphere binding observed for the cations on the rutile (110) surface. Explicitly accounting for the inner-sphere binding configuration differences between Rb+, K+, and Na+, as well as accompanying differences in negative surface charge development, resulted in much better agreement with measured exchange ratios than by considering bulk hydration enthalpies alone. The observation that calculated exchange ratios agreed with those measured experimentally lends additional credence to the SCM. Consequently, flow microcalorimetry and surface complexation modeling are a useful complement of techniques for probing the energetics associated with ion exchange and adsorption processes and should also serve to help validate molecular simulations of interfacial energetics.

Structural Dependence of Competitive Adsorption of Water and Methanol on TiO2 Surfaces

AbstractEmploying TiO2 anatase (001)‐(1 × 4), rutile (110) and rutile (011)‐(2 × 1) single crystal surfaces, we comprehensively studied the effects of TiO2 surface structures on the competitive adsorption of water and methanol by means of low energy electron diffraction, thermal desorption spectra and X‐ray photoelectron spectroscopy. The relative adsorption strengths of chemisorbed methanol and water vary with the TiO2 surface structures and the adsorption sites. This leads to TiO2 surface structure‐dependent competitive adsorption of water and methanol. The chemisorption of CH3OH on TiO2 anatase (001)‐(1 × 4) surface is seldom affected by pre‐covered water at low coverages but is affected by pre‐covered water at high coverages; the chemisorption of CH3OH on TiO2 rutile (110) surface is seldom affected by pre‐covered water; and the chemisorption of CH3OH on TiO2 rutile (011)‐(2 × 1) surface is affected by pre‐covered water even at low coverages. These results deepen the fundamental understandings of surface chemistry on TiO2 surfaces.

Comparative study of CO adsorption on planar and tetrahedral Pt nanoclusters supported on TiO2(110) stoichiometric and reduced surfaces

TiO2 is a widely used material due to its electronic and catalytic properties, which are of interest for technological applications. In catalysis it is generally used as support for different catalyzers, such as for example Pt subnanoclusters given that they improve the efficiency of the material.In this work we use an ab initio DFT+U modeling method to study the structure and energetic of Pt4 clusters deposited on rutile TiO2(110) stoichiometric and reduced surfaces. For the Pt-titania system we examine the relative stability between the flat versus 3D tetrahedral Pt4 structures, and characterize the cluster/substrate interaction. We determine their equilibrium geometries, adsorption energies, charge transfer effects and electronic density of states to characterize different aspects of the metal-oxide interaction. For both, the stoichiometric and reduced rutile TiO2(110), we find that the flat square configuration is preferred, as experiments indicate. In particular, we are interested in the potential activity of these cluster-supported systems for the oxidation of CO adsorbed on Pt. To examine this behavior we evaluate the structure, electronic DOS properties and charge transfer effects for the adsorption of CO on both the flat and tetrahedral Pt4 isomers over the stoichiometric and reduced TiO2 rutile surfaces. The results point to the planar cluster on the stoichiometric surface as the most stable configuration for CO adsorption, while for the CO conversion to CO2 the tetrahedral Pt4 cluster on the stoichiometric TiO2 surface would be the most favorable catalytic substrate.

Electronic structures of rutile (011)(2 × 1) surfaces: A many-body perturbation theory study.

Using the GW method within many-body perturbation theory, we investigate the electronic properties of the rutile (011) surfaces with different reconstruction patterns. We find that keeping the Ti:O ratio on the reconstructedsurface to 1:2 enlarges the bandgap of the rutile (011) surface to ca. 4.0 eV. Increasing the content of O atoms in the surface can turn rutile into a semi-metal. For some surfaces, it is important to apply self-consistent GW calculation to get the correct charge distributions for the frontier orbitals, which are relevant to the photocatalytic behavior of TiO2.

TiO2 crystalline structure and electrochemical performance in two-ply yarn CNT/TiO2 asymmetric supercapacitors

Solid-state flexible energy storage devices play a crucial role in the development of wearable electronic textiles. In this study, we fabricated flexible asymmetric two-ply yarn supercapacitors from carbon nanotube yarns and surface-oxidized titanium filament. The crystalline structure of the TiO2 surface layer can be adjusted to amorphous, anatase and rutile states by altering the annealing temperature. The titanium filament with a rutile TiO2 surface layer produced at high annealing temperature showed far superior electrochemical performance over the filaments with amorphous and anatase TiO2 surface layers. The as-prepared asymmetric two-ply yarn supercapacitors in aqueous gel electrolyte can achieve a durable operating voltage up to 1.4 V, with a maximum energy density of 11.7 Wh kg−1 and a maximum power density of 2060 W kg−1. The asymmetric two-ply yarn supercapacitors exhibited excellent flexibility and cycling stability over 1200 cycles at straight, twisted and bent states.

100-fold improvement in carrier drift mobilities in alkanephosphonate-passivated monocrystalline TiO2 nanowire arrays

Single crystal rutile titania nanowires grown by solvothermal synthesis are actively being researched for use as electron transporting scaffolds in perovskite solar cells, in low detection limit ultraviolet photodetectors, in photoelectrochemical water-splitting, and in chemiresistive and electrochemical sensing. The electron drift mobility (μn) in solution-grown TiO2 nanowires is very low due to a high density of deep traps, and reduces performance in optoelectronic devices. In this study, the effects of molecular passivation of the nanowire surface by octadecylphosphonic acid (ODPA), on carrier transport in TiO2 nanowire ensembles, were investigated using transient space charge limited current measurements. Infrared spectroscopy indicated the formation of a highly ordered phosphonate monolayer with a high likelihood of bidentate binding of ODPA to the rutile surface. We report the hole drift mobility (μp) for the first time in unpassivated solvothermal rutile nanowires to be 8.2 × 10−5 cm2 V−1 s−1 and the use of ODPA passivation resulted in μp improving by nearly two orders of magnitude to 7.1 × 10−3 cm2 V−1 s−1. Likewise, ODPA passivation produced between a 2 and 3 order of magnitude improvement in μn from ∼10−5–10−6 cm2 V−1 s−1 to ∼10−3 cm2 V−1 s−1. The bias dependence of the post-transit photocurrent decays in ODPA-passivated nanowires indicated that minority carriers were lost to trapping and/or monomolecular recombination for small values of bias (<5 V). Bimolecular recombination was indicated to be the dominant recombination mechanism at higher bias values.

The effect of silica thickness on nano TiO2 particles for functional polyurethane nanocomposites

In order to help reduce the agglomeration of TiO2 nanoparticles in polyurethane coatings while enhancing their photoactivity and mechanical/physical properties, this work examined encapsulating TiO2 nanoparticles in a thin layer of SiO2, prior to their nanocomposite polymerization. By applying a Stöber process, varying thicknesses of SiO2 were successfully coated onto the surface of anatase and rutile TiO2 nanoparticles. The methylene blue results showed that different loadings of SiO2 onto the TiO2 surface significantly influenced their photocatalytic activity. When the loading weight of SiO2 was lower than 3.25 wt%, the photocatalytic activity was enhanced, while with higher loadings, it gave lower photocatalytic activity. When the rutile phase TiO2 surface was fully covered with SiO2, an enhanced photocatalytic activity was observed. When these silica coated nanoparticles were applied in polyurethane coatings, increasing the amount of SiO2 on the titania surface increased the coatings contact angle from 75° to 87° for anatase phase and 70°–78° for rutile phase. The Young’s modulus was also increased from 1.06 GPa to 2.77 GMPa for anatase phase and 1.06–2.17 GPa for rutile phase, attributed to the silica layer giving better integration. The thermal conductivity of the polyurethane coatings was also successfully decreased by encapsulating SiO2 on the titania surface for next generation high performance coatings.

Electronic Structure and Charge Transfer in the TiO2 Rutile (110)/Graphene Composite Using Hybrid DFT Calculations

Composite systems of TiO2 with nanocarbon materials, such as graphene, graphene oxide, and carbon nanotubes, have proven to be efficient photocatalyst materials. However, a detailed understanding of their electronic structure and the mechanisms of the charge transfer processes is still lacking. Here, we use hybrid density functional theory calculations to analyze the electronic properties of the ideal rutile (110)–graphene interface, in order to understand experimentally observed trends in photoinduced charge transfer. We show that the potential energy surface of pristine graphene physisorbed above rutile (110) is relatively flat, enabling many possible positions of graphene above the rutile (110) surface. We verify that tensile and compressive strain has a negligible effect on the electronic properties of graphene at low levels of strain. By analyzing the band structure of this composite material and the composition of the valence and conduction band edges, we show that both the highest occupied states a...

Interfacial Interaction of Titania Nanoparticles and Ligated Uranyl Species: A Relativistic DFT Investigation.

To understand interfacial behavior of actinides adsorbed onto mineral surfaces and unravel their structure-property relationship, the structures, electronic properties, and energetics of various ligated uranyl species adsorbed onto TiO2 surface nanoparticle clusters (SNCs) were examined using relativistic density functional theory. Rutile (110) and anatase (101) titania surfaces, experimentally known to be stable, were fully optimized. For the former, models studied include clean and water-free Ti27O64H20 (dry), partially hydrated (Ti27O64H20)(H2O)8 (sol) and proton-saturated [(Ti27O64H20)(H2O)8(H)2]2+ (sat), while defect-free and defected anatase SNCs involving more than 38 TiO2 units were considered. The aquouranyl sorption onto rutile SNCs is energetically preferred, with interaction energies of -8.54, -10.36, and -2.39 eV, respectively. Energy decomposition demonstrates that the sorption is dominated by orbital attractive interactions and modified by steric effects. Greater hydrogen-bonding involvement leads to increased orbital interactions (i.e., more negative energy) from dry to sol/sat complexes, while much larger steric interaction in the sat complex significantly reduces the sorption interaction (i.e., more positive energy). For dry SNC, adsorbates were varied from aquo to aquo-carbonato, to carbonato, to hydroxo uranyl species. Longer U-Osurf/U-Ti distances and more positive sorption energies were calculated upon introducing carbonato and hydroxo ligands, indicative of weaker uranyl sorption onto the substrate. This is consistent with experimental observations that the uranyl sorption rate decreases upon raising solution pH value or adding carbon dioxide. Anatase SNCs adsorbing aquouranyl are even more exothermic, because more bonds are formed than in the case of rutile. Moreover, the anatase sorption can be tuned by surface defects as well as its Ti and O stoichiometry. All the aquouranyl-SNC complexes show similar character of molecular orbitals and energetic order although differing in highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps and orbital energy levels, but changes can be accomplished by adding carbonato and hydroxo ligands.

Unpredictable adsorption and visible light induced decolorization of nano rutile for the treatment of crystal violet

Photocatalysts containing different ratios of anatase and rutile are prepared via heat treatment of Degussa P-25 titania. X-ray diffraction (XRD), Bruuauer-Emmett-Teller (BET), ultraviolet–visible light diffuse reflectance spectra (DRS), Raman spectra (Raman), positron annihilation lifetime spectra (PAL) and temperature-programmed desorption (TPD) are applied to investigate the phase composition of the synthesized catalysts. Using crystal violet (CV) as the target pollutant, the unexpected visible light decolorization of rutile is observed. Despite the decreased specific surface area, the as-synthesized rutile samples exhibit much higher adsorption capability of CV than P-25 does, which in turn leads to improved photoreaction efficiency. Since the rutile samples can't absorb the visible light, the degradation under visible light irradiation is attributed to self-sensitization of CV on the surface of rutile.