Computational study of the adsorption of dimethyl methylphosphonate (DMMP) on the (0 1 0) surface of anatase TiO2 with and without faceting

The adsorption of dimethyl methylphosphonate (DMMP) on the (0 1 0) surface of anatase TiO₂, which is isostructural with the (1 0 0), has been studied using density functional theory and two-dimensionally-periodic slab models. The experimentally-observed faceting of this surface has, for the first time, been included in the modeling. The relaxations of bare surfaces both with and without faceting are similar, leading to an atomic-scale roughening due to inward (outward) displacement of fivefold-coordinated Ti_5c (sixfold-coordinated T_6c) atoms together with outward displacement of threefold-coordinated O_3c atoms. Molecular adsorption occurs by formation of a Ti_5c?OP dative bond with one or more CH?O_2c bonds between CH₃ groups and unsaturated, twofold-coordinated (O_2c) sites. The energies for molecular adsorption, obtained using the B3LYP functional, are virtually identical (about ?21.0 kcal/mol) for the two surfaces and are also close to those found elsewhere for the rutile (1 1 0) and anatase (1 0 1) surfaces. A possible first step in the dissociative adsorption of DMMP has also been modeled and is found to be thermodynamically favored over molecular adsorption to a degree which depends on faceting.     

Unique geometric and electronic structure of CO adsorbed on Ge(100): A DFT study

CO adsorption on the Ge(100) surface has been investigated using a slab model with density functional theory implemented in SIESTA. CO was found to be exclusively adsorbed on the asymmetric dimer with C attaching on the lower Ge dimer atom. The adsorption process is barrierless. The calculated adsorption energy and vibration frequencies are comparable to previous experimental results. The crystal orbital Hamilton analysis showed that the bonding between Ge and CO is mainly attributable to the Ge 4p_z orbital overlapping with C 2 s, or with CO molecular orbitals 3σ and 4σ. The repulsive energy between adsorbed CO molecules is less than 1 kcal/mol. The diffusion barrier of CO on the Ge(100) surface is about 14 kcal/mol.     

A model for the dissociative adsorption of N2O on Cu(1 0 0) using a continuous potential energy surface

For the first time, a continuous ab initio potential energy surface (PES) taking into account all molecular degrees of freedom is built and used to model the reaction of a polyatomic molecule with a surface. DFT slab calculations are used to sample the configuration space of the system N₂O/Cu(1 0 0), and the PES function is built with a method of Manzhos and Carrington [J. Chem. Phys. 127 (2007) 014103] using dimensionality reduction from only 4300 single-point energies. Molecular dynamics simulations are performed on the PES to calculate the probability of dissociative adsorption.     

Periodic density functional theory studies of the VOx/TiO2 (anatase) catalysts: Structure and stability of monomeric species

Periodic density functional theory has been utilized to investigate the structure and stability of monomeric HVO_x species on anatase support. The three most stable surfaces of anatase were investigated, namely the (001), (100) and (101) surfaces. Unlike previous theoretical studies it was found that on the (001) surface vanadia species with five-coordinated vanadium atom are more stable than those with tetrahedrally coordinated vanadium atom. On the other hand, on the (100) and (101) surfaces, the vanadium atom in the vanadia species is still tetrahedrally coordinated. The stability of different VO_x/TiO₂ structures which are not fully dehydrated has been systematically studied and the results show that the vanadia species on the three surfaces follow an order of TiO₂ (001) > TiO₂ (100) > TiO₂ (101). This can be understood from the acidity and basicity of the three anatase surfaces. The results suggest that monomeric VO_x species may be better stabilized if the support exposes more (001) surfaces. Our analyses on electronic structure of the most stable VO_x/TiO₂ structure (D₀₀₁) suggest that its bridging V–O–Ti oxygen atoms may have higher reactivity than the terminal vanadyl oxygen atoms.     

Adsorption of L-cysteine on rutile TiO2(110)

We have used X-ray photoelectron spectroscopy to study the adsorption of L-cysteine on a rutile TiO₂(110) surface at room temperature and ? 65 °C. For the molecules in direct contact with the surface our results suggest that the molecules bind dissociatively to the fivefold-coordinated Ti atoms of the surface through their deprotonated carboxylic groups. A second, dissociative interaction occurs between the molecular thiol groups and the surface. It is attributed to a dissociative bond to the bridging oxygen vacancies. Most likely, the thiol groups are deprotonated and a bond is formed between the thiolates and defects. In an alternative scenario, the C–S bond is cleaved and atomic sulfur binds to the defects. With regard to the molecular amino groups, they remain neutral at the lowest investigated coverages (0.3–0.5 ML), but already starting from around 0.7 ML nominal coverage protons are being transferred to them. The fraction of protonated amino groups increases with coverage and becomes dominating in multilayers prepared at room temperature and ? 65 °C. In these multilayers the carboxylic groups are deprotonated.     

Adsorption of Xe atoms on the TiO2(1 1 0) surface: A density functional study

The interaction of xenon atoms with the TiO₂(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO₂(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO₂(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.     

Microcalorimetry of O2 and NO on flat and stepped platinum surfaces

Using the single-crystal adsorption calorimeter (SCAC), coverage-dependent heats of adsorption and sticking probabilities are reported for O₂ and NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} at 300 K. At low coverage, oxygen adsorption is dissociative for all Pt surfaces. The highest initial heat of adsorption is found on Pt{2 1 1}, with a value of 370 kJ/mol, followed by those on Pt{4 1 1} (310 kJ/mol) and Pt{1 1 1} (300 kJ/mol). We attribute this relatively large difference in the dissociative heat of adsorption at low coverage to the step character of the {2 1 1} surface. Initial sticking probabilities, s_o, are similar for the three surfaces, 0.22 on Pt{1 1 1}, 0.17 on Pt{2 1 1} and 0.18 on Pt{4 1 1}, rapidly decreasing as the oxygen coverage increases. For nitric oxide, the initial heats of adsorption are very similar and consistent with either dissociative or molecular adsorption, with values of 182 kJ/mol on Pt{1 1 1}, 192 kJ/mol on Pt{2 1 1} and 217 kJ/mol on Pt{4 1 1}. The s_o value is virtually identical for all three systems, with values ranging from 0.82 to 0.85, suggesting that the initial sticking probability is insensitive to the surface structure and adsorption is intrinsically precursor mediated. SCAC data are also used to evaluate pre-exponential factors, ν, for first-order desorption at high coverage where adsorption is non-dissociative. Values of 3 × 10¹⁸, 6 × 10¹⁸ and 2 × 10¹⁸ s^?1 for O₂, and 4 × 10¹⁹, 6 × 10¹⁷ and 2 × 10²⁰ s^?1 for NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1}, respectively, are found. These unexpectedly high values are rationalised in terms of conventional transition state theory entropy changes.     

DFT-GGA study of NO adsorption on the LaO (001) surface of LaFeO3

In order to demonstrate the adsorption of the nitrogen monoxide molecule (NO) on the LaO (001) surface of LaFeO₃, we perform simulations based on density functional theory. The generalized gradient approximation (GGA) for the exchange-correlation energy functional indicates that the electronic state of the LaFeO₃ bulk is an anti-ferromagnetic insulator with a local magnetic moment of 4.1 μ_B at each Fe atom. Using the ultrasoft pseudo-potential method with spin-polarized GGA, fully optimized internal parameters as well as charge and spin density are determined for the NO-adsorbed structure prepared in a slab model. The calculated adsorption energy of NO is around ? 1.4 eV on the LaO (001) surface of LaFeO₃. This value decreases down to ? 4.46 eV at an oxygen vacancy site, where the nitrogen atom of NO is embedded in the 1st LaO layer forming a bond with Fe in the 2nd FeO layer.     

Evidence for non-thermalized vibrationally excited molecule production during azomethane pyrolysis in methane desorption from Cu(001)

A Cu(001) surface was exposed to products of an azomethane pyrolysis doser at varying temperatures. In addition to methyl radical adsorption, for certain doser conditions one or more doser emergent species can undergo an activated adsorption on the copper face. Directly after exposures, temperature programmed desorption between 170 K and 500 K was used to indicate the relative concentrations of adsorbed atomic hydrogen and methyl species, and thermally induced surface reactions. Two methane desorption features were invariably observed, indicating the presence of adsorbed methyl groups (CH₃) and transient adsorbed atomic hydrogen. The deduced relative surface concentrations levels of both H and CH₃ depend on the total exposures and the operating temperatures of the azomethane pyrolysis doser. The initial H concentrations apparent at surface temperatures between 275 K and 375 K are shown to arise from defect-related methyl decomposition and, at high operating doser temperatures, from the initial adsorption of one or more activated Cu incident species. It is proposed that the distributions of vibrational energies of emergent molecular hydrogen or methane species from higher temperature dosers are non-thermal. Hence, with doser temperatures of 800 °C or above, the effects of subsequent dissociative molecular adsorption on the copper surface can dominate over Cu defect chemistries.     

Adsorption CO2 on the perfect and oxygen vacancy defect surfaces of anatase TiO2 and its photocatalytic mechanism of conversion to CO

The adsorption energies for physisorption and the most stable chemisorption of CO₂ on the neutral charge of perfect anatase [TiO₂] (0 0 1) are −9.03 and −24.66 kcal/mol on the spin-unpolarized and −12.98 and −26.19 kcal/mol on the spin-polarized surface. The small activation barriers of 1.67 kcal/mol on the spin-unpolarized surface and of 6.66 kcal/mol on the spin-unpolarized surface were obtained. The adsorption mechanism of CO₂ on the oxygen vacancy defect [TiO₂ + V_O] surface of anatase TiO₂ using density functional theory calculations was investigated. The energetically preferred conversion of CO₂ to CO was found either on the spin-unpolarized or spin-polarized surfaces of oxygen vacancy defect surface [TiO₂ + V_O] as photocatalyst.     

Investigation of hydrogen bonds and temperature effects on the water monolayer adsorption on rutile TiO2 (110) by first-principles molecular dynamics simulations

Density Functional Theory (DFT), based on both static and Born–Oppenheimer Molecular Dynamics approaches, has been used to investigate the effect of hydrogen bonds and temperature on the water monolayer adsorption on the rutile TiO₂ (110) face. It was demonstrated that the difference between some previous theoretical results and experimental data is due to too slim slab thickness model and/or too small surface area. According to the present static calculations, water monolayer adsorbs molecularly on the five-fold titanium atoms of an optimised five-layer slab thickness, due to the stabilising lateral hydrogen bonds between molecules. From the molecular dynamics simulations, two adsorption mechanisms were described as a function of temperature. Finally, it was pointed out that the dynamics of water adsorption is strongly influenced by the structural model used. When temperature increases, the monolayer dissociates gradually. However, because of the periodic boundary conditions, the 1 × 1 surface unit needs to be extended to at least 2 × 5 to get an accurate representation of the monolayer dissociation ratio. In these conditions, this ratio is around 20%, 25% and 33% at 270, 350 and 425 K, respectively.     

Ab-initio study of Sr-doping effects on nitric oxide adsorption on the LaO (001) surface of LaFeO3

Density-functional calculations of molecular nitric oxide (NO) on defective (La,Sr)O (001) surfaces of (La,Sr)FeO_3 ? δ using slab models are performed to elucidate the oxygen vacancy formation problem on the LaO (001) surface of LaFeO_3 ? δ.From the estimation of the NO adsorption energy, NO adsorption is found on (La,Sr)O surfaces of (La_0.83,Sr_0.17)FeO_3 ? δ with δ = 0 or 0.25.The absolute value of the NO adsorption energy shows a remarkable increase at oxygen vacancies in the top surface layer, where the nitrogen atoms of the adsorbed molecules are embedded in the first (La,Sr)O layer, because a bond with Fe in the second FeO₂ layer is formed.Our data shows that Sr doping promotes formation of oxygen vacancies, which keep the NO adsorption ability high.Thus, we conclude that if Sr doping increases the number of oxygen vacancy sites by a charge compensation effect, NO adsorption on LaFeO₃ is enhanced, which provides an explanation for several experimental observations.     

The preparation and characterization of nanoparticle TiO2/Ti films and their photocatalytic activity

Nanoparticle TiO₂/Ti films were prepared by a sol–gel process using Ti(OBu)₄ as raw material, the as-prepared film samples were also characterized by TG-DTA, XRD, TEM, SEM, XPS, DRS, PL, SPS and EFISPS testing techniques. TiO₂ nanoparticles experienced two processes of phase transition, i.e. amorphous to anatase and anatase to rutile at the calcining temperature range from 450 to 700 °C. TiO₂ nanoparticles calcined at 600 °C had similar composition, structure, morphology and particle size with the internationally commercial P-25 TiO₂ particles. Thus, the conclusion that 600 °C might be the most appropriate calcining temperature during the preparation process of nanoparticle TiO₂/Ti film photocatalysts could be made by considering the main factors such as the properties of TiO₂ nanoparticles, the adhesion of nanoparticle TiO₂ film to Ti substrate, the effects of calcining temperature on Ti substrate and the surface characteristics and morphology of nanoparticle TiO₂/Ti film for the practice view. The Ti element mainly existed on the nanoparticle TiO₂/Ti(3) film calcined at 600 °C as the chemical state of Ti⁴⁺, while O element mainly existed as three kinds of chemical states, i.e. crystal lattice oxygen, hydroxyl oxygen and adsorbed oxygen with increasing band energy. Its photoluminescence (PL) spectra with a peak at about 380 nm could be observed using 260 nm excitation, possibly resulting from the electron transition from the bottom of conduction band to the top of valence band. The PL peak position was nearly the same as the onset of its diffuse reflection spectra (DRS) and surface photovoltage spectroscopy (SPS), demonstrating that the effects of the quantum size on optical property were greater than that of the Coulomb and surface polarization. The PL spectra with two peaks related to the anatase and rutile, respectively, could be observed using the excited wavelength of 310 nm. Weak PL spectra could be observed using the excited wavelength of 450 nm, resulting from surface states. In addition, during the experimental process of the photocatalytic degradation phenol, the photocatalytic activity of nanoparticle TiO₂/Ti film with three layers calcined at 600 °C was the highest.     

The interaction between hexahydroxytriphenylene and the rutile TiO2(110)-(1 × 1) surface at UHV conditions

The interaction between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and the rutile TiO₂(110)–(1 × 1) surface under ultrahigh vacuum (UHV) conditions was investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The NEXAFS results showed that HHTP molecules formed a submonolayer and a monolayer that aligned along the [001]-direction with, respectively, a more or less flat downward orientation and a more upright orientation to the TiO₂ surface. The HHTP molecules that aligned along the [001]-direction were most likely grafted onto the TiO₂(110) surface by a bidentate bridge between each of the oxygen atoms of one of the catechol units within the HHTP molecule and two adjacent Ti(5f)⁴⁺ ions on the TiO₂(110) surface. The coordination is non-dissociative in the case of the submonolayer, but dissociative in the monolayer, according to the analysis of the C1s XPS, UPS, C1s NEXAFS data and complementary DFT calculations.     

Understanding 4-bromostyrene Adsorption on the Si(001)-(1 × 2) surface: A density functional theory study

Adsorption properties of 4-bromostyrene (Br–Sty) on the Si(001)-(1 × 2) surface are investigated by ab initio calculation based on density functional theory (DFT). For the adsorption of Br–Sty molecule on the Si(001)-(1 × 2) surface, we have assumed two possible cases within: (i) binding on the partially H-terminated surface and (ii) binding on the clean surface. For the first case, we have estimated two different binding sides: (i) Bromine-terminated bindings and (ii) Carbon-terminated binding. The adsorption energies of Br-terminated and C-terminated binding were found as 0.36 eV and 3.76 eV, respectively. In the same manner, we have also assumed two possible binding sides for the clean surface: (i) Br-terminated binding and (ii) ring-shaped binding. We have found adsorption energies for Br-terminated and ring-shaped binding as 0.14 eV and 1.10 eV on the clean surface, respectively. Moreover, the nudged elastic band method (NEB) was used to reveal the adsorption pathway of these binding models. These results serve to understand the possibility of the adsorption of Br–Sty molecules onto different kind of silicon surfaces into different reaction conditions.     

In-situ imaging of the nucleation and growth of epitaxial anatase TiO2(001) films on SrTiO3(001)

The growth of TiO₂ anatase films on Nb‐doped SrTiO₃(001) molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski–Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO₂. The epitaxial islands subsequently nucleate and coalesce into large commonly oriented crystallites. The reconstruction observed by reflection high-energy electron diffraction (RHEED) is shown to result from the coexistence of individual (4 × 1) and (1 × 4) reconstructions present on different crystallite surfaces. The anatase grows in units of bilayers, resulting in a step height of 2 ML (~ 0.5 nm). This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4 × 1) and (1 × 4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies.     

The super-hydrophobic IR-reflectivity TiO2 coated hollow glass microspheres synthesized by soft-chemistry method

The super-hydrophobic and IR-reflectivity hollow glass microspheres (HGM) was synthesized by being coated with anatase TiO₂ and a super-hydrophobic material. The super-hydrophobic self-cleaning property prolong the life time of the IR reflectivity. TBT and PFOTES were firstly applied and hydrolyzed on HGM and then underwent hydrothermal reaction to synthesis anatase TiO₂ film. For comparison, the PFOTES/TiO₂ mutual-coated HGM (MCHGM), PFOTES single-coated HGM (F-SCHGM) and TiO₂ single-coated HGM (Ti-SCHGM) were synthesized as well. The MCHGM had bigger contact angle (153°) but smaller sliding angle (16°) than F-SCHGM (contact angle: 141.2°; sliding angle: 67°). Ti-SCHGM and MCHGM both showed similar IR reflectivity with ca. 5.8% increase compared with original HGM and F-SCHGM. For the thermal conductivity, coefficients of F-SCHGM (0.0479 W/(m K)) was basically equal to that of the original HGM (0.0475 W/(m K)). Negligible difference was found between the thermal conductivity coefficients of MCHGM-coated HGM (0.0543 W/(m K)) and Ti-SCHGM (0.0546 W/(m K)).     

Study of CO diffusion on stepped Pt(1 1 1) surface by scanning tunneling microscopy

We have used a time-dependent tunneling current mode based on scanning tunneling microscopy/spectroscopy (STM/STS) to study the tracer diffusion of CO molecules along steps and on terraces of Pt(1 1 1). The results show that the hopping rate of CO molecules along steps is about 10 times faster than that on terraces in the measured temperature range. The diffusion activation energies are 5.1 kcal/mol and 3.8 kcal/mol on terraces and along steps, respectively. The lower activation energy and faster hopping rate for CO molecules diffusing along steps provide evidence that steps provide fast diffusion channels for CO molecules on stepped Pt(1 1 1) surfaces.     

Ultrasonic synthesis and photocatalytic characterization of H3PW12O40/TiO2 (anatase)

A novel H₃PW₁₂O₄₀/TiO₂ (anatase) composite photocatalyst was prepared by a high-intensity ultrasonic method using a lower temperature (80 °C) and was characterized by XRD and FT-IR. Its photocatalytic activity, using solar light, was evaluated through the degradation of organic dye methylene blue (MB) in aqueous. When MB solution (50 mg/l, 200 ml) containing H₃PW₁₂O₄₀/TiO₂ (anatase) (0.4 g) was degraded by solar irradiation after 90 min, the removal of concentration and TOC of MB reached 95% and 73%, respectively. The photocatalyst activity of H₃PW₁₂O₄₀/TiO₂ (anatase) was much higher than TiO₂ which was prepared in the same way. H₃PW₁₂O₄₀/TiO₂ remained efficient after five repeated experiments.     

A comparative DFT study of adsorption and catalytic performance of Au nanoparticles at anatase and brookite TiO2 surfaces

We have compared the adsorption properties of small Au_n (n = 1–8) nanoparticles on the defect-free (stoichiometric) and defective (partially reduced) brookite TiO₂(210) and anatase TiO₂(101) surfaces using density functional theory calculations. The interaction between Au atoms and anatase TiO₂(101) was determined to be quite weak and small Au_n particles grown at defects (O vacancies) prefer extended 2D structures. By contrast, dispersion and 3D configurations appear to be favored at brookite TiO₂(210) for Au_n nanoparticles due to their strong interaction. Calculations of CO oxidation at Au_n (n = 6–8) particles supported at defective brookite TiO₂(210) show that occurrence of protruding low-coordinated Au atoms is essential for favorable CO adsorption and subsequent reaction with O₂. In particular, the configuration of the Au_n nanoparticles can determine the energetics in the formation of active Au atoms, and their mobility also affects the reaction between CO and O₂ (or O).