Mechanisms behind the generalized synchronization conditions

A universal mechanism underlying generalized synchronization conditions in unidirectionally coupled stochastic oscillators is considered. The consideration is carried out in the framework of a modified system with additional dissipation. The approach developed is illustrated with model examples. The conclusion is reached that two types of the behavior of nonlinear dynamic systems known as generalized synchronization and noise-induced synchronization, which are viewed as different phenomena, actually represent a unique type of the synchronous behavior of stochastic oscillators and are caused by the same mechanism.     

A surface science perspective on TiO2 photocatalysis

The field of surface science provides a unique approach to understanding bulk, surface and interfacial phenomena occurring during TiO₂ photocatalysis. This review highlights, from a surface science perspective, recent literature that provides molecular-level insights into photon-initiated events occurring at TiO₂ surfaces. Seven key scientific issues are identified in the organization of this review. These are: (1) photon absorption, (2) charge transport and trapping, (3) electron transfer dynamics, (4) the adsorbed state, (5) mechanisms, (6) poisons and promoters, and (7) phase and form. This review ends with a brief examination of several chemical processes (such as water splitting) in which TiO₂ photocatalysis has made significant contributions in the literature.     

Methylene bromide chemistry and photochemistry on rutile TiO2(110)

The chemistry and photochemistry of methylene bromide (CD₂Br₂) on the rutile TiO₂(110) surface was probed using temperature programmed desorption (TPD). CD₂Br₂ desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η¹-CD₂Br₂ species and a bridging η²-CD₂Br₂ species, respectively. The latter two TPD states presumably involve binding of CD₂Br₂ molecules to the surface through Br coordination at five-coordinate Ti⁴⁺ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti⁴⁺ site density (5.2 × 10¹⁴ cm^? 2). No thermal decomposition of CD₂Br₂ was observed on either the clean surface or with preadsorbed O₂. UV irradiation of CD₂Br₂ on TiO₂(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD₂Br₂ photodesorption from TiO₂(110) occurred with a low cross section (~ 2 × 10^? 21 cm²) similar to that expected from direct optical excitation of CD₂Br₂. This observation suggests that charge carriers generated in TiO₂(110) were no more effective in activating adsorbed CD₂Br₂ molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD₂Br₂ on TiO₂ may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO₂ by bandgap excitation.     

Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

TiO2表面氧空位对NO分子吸附的作用

采用程序升温热脱附(TPD)实验方法测定了NO在TiO2表面吸附后的脱附谱，利用分子轨道理论研究了TiO2吸附NO的原子簇模型及吸附前后的原子簇能级变化．结果表明，NO在TiO2表面吸附后可在两个峰值温度450和980K脱附出N2．TiO2表面经预覆氧处理后，N2的脱附量降低．吸附时NO中的O能够占据TiO2表面氧空位并与N脱离，而N原子则相互结合成为N2脱附．分子轨道理论计算证明在TiO2(110)表面能够存在氧空位并具备吸附NO的结构条件．     

Electrical properties and defect chemistry of anatase (TiO2)

The electrical properties of pure Anatase are investigated by impedance spectroscopy as function of temperature and oxygen partial pressure. The experimental results are fully interpreted by point defect chemistry. A transition from predominant Schottky to Frenkel cation disorder is observed when the temperature increases and/or the oxygen partial pressure decreases. The reduction enthalpies are near those obtained for Rutile in previous studies.     

三氯苯酚在TiO_2(101)面吸附特征的理论研究

本文利用密度泛函理论,研究了三氯苯酚(TCP)在锐钛矿型TiO_2(101)晶面的吸附特征,优化得到了四种稳定的吸附构型,计算了最佳吸附位点、吸附能、电荷密度等.研究发现:四种稳定吸附构型都具有化学吸附特征,并且当三氯苯酚羟基上的氢原子被吸附在TiO_2(101)晶面的三配位的氧原子上时,吸附能最大,吸附结构最稳定,吸附位相邻的C-C键长和C-Cl键长均有增加,键得到了活化,从而增加了该位置断键的可能性.本研究工作将为后续三氯苯酚的开环降解研究提供理论依据.     

Coordination chemistry and kinetics of preferential etching on surfaces of TiO2 (rutile)

Fused KHSO₄ can be used as both a polish and a preferential etchant for the {001} surfaces of rutile, depending on the temperature of the system. Polishing occurs at temperatures above 550°C. At temperatures below 550°C, three types of etch pit are observed, two well defined with regular shapes (sides parallel to the <110> and <100> and <130> directions, respectively) and one irregularly shaped. Photochemical deposition of silver [1] onto the crystal surface and subsequent removal (HNO₃) prior to etching causes the irregularly shaped pits to be etched more rapidly than in the case of a surface region that was not predeposited. A chemical reaction mechanism is presented to interpret the observed etching behavior. The crystal structure (i.e. the different ionic composition of different crystallographic planes) and surface chemistry of rutile are considered in the formulation of this mechanism. It is suggested that etchant molecules form surface complexes on the TiO₂ (after dehydration of surface OH groups) through coordination of Ti⁴⁺ via terminal and bridged out-of-plane O^2? positions. Evidence is presented to show that the etching rate-determining reaction is the dissolution of these soluble surface complexes. The observed photochemical effects are explained on the basis of the trapping of photogenerated holes or electrons at certain crystal defects.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

Atomic scale Kelvin probe force microscopy studies of the surface potential variations on the TiO2(110) surface

From an interplay of simultaneous Kelvin probe force microscopy and noncontact atomic force microscopy we study atomic-scale variations in the electronic surface potential on TiO(2)(110). Both imaging channels reveal an atomic contrast reflected by the geometry and charged state of the alternating rows of Ti and O surface atoms. From a thorough cross-section analysis we add significant trust to the concept of a local contact potential difference, and determine from this the chemical identity of individual surface species and their role in setting up the local surface potential.     

First principles study of oxygen adsorption on the anatase TiO2 (101) surface

The main objective of this study is to understand the sensing origin of anatase TiO₂ and provide an insight into why gas sensitivity of anatase TiO₂ is not so excellent.

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奥硝唑在TiO_2(101)晶面上吸附特征的理论研究

奥硝唑残留是一种新兴污染物,对环境和人类健康具有巨大的威胁.采用密度泛函理论,研究了奥硝唑在锐钛矿TiO_2(101)晶面的吸附特性.优化了奥硝唑在锐钛矿TiO_2(101)晶面的吸附结构,计算了最佳吸附位点,吸附能,态密度,电子结构图.结果表明,当咪唑环上N(3)原子吸附在TiO_2的Ti(5)原子上时,吸附能最大,为最稳定的吸附构型.通过对吸附构型的分析,我们发现C(2)-N(3)键呈现变弱趋势,我们推测奥硝唑在TiO_2表面降解的可能性以及反应活性位点就是咪唑环上C-N键.     

Pyrocatechol as a surface capping molecule on rutile TiO2 (110)

A ‘cap and dip’ method of adsorbing ruthenium di-2,2′-bipyridyl-4,4′-dicarboxylic acid diisocyanate (N3 dye) on a rutile TiO₂ (110) surface was investigated using pyrocatechol as a capping molecule. This method involves cleaning the rutile surface in ultra-high vacuum (UHV), depositing pyrocatechol onto the surface to ‘cap’ the adsorption sites, removing from vacuum, ‘dipping’ in an N3 dye solution and returning to vacuum. Photoemission measurements following the return of the crystal to vacuum suggest that the pyrocatechol keeps the surface free from contamination on exposure to atmosphere. Photoemission spectra also indicate that the pyrocatechol capping molecules are replaced by the N3 dye in solution and that the N3 dye is adsorbed intact on the rutile TiO₂ (110) surface. This technique may allow other large molecules, which are thermally unstable to evaporation in UHV, to be easily deposited onto TiO₂ surfaces.     

The location of excess electrons on H2O/TiO2(110) surface and its role in the surface reactions

Excess electrons play a key role in many of the properties of Titanium dioxide (TiO₂). Understanding their behaviour is important for improving the performance of TiO₂ in energy-related applications. Here, we describe a DFT + U study of the locations of the unpaired electron (UPE) on rutile TiO₂(110) (R-TiO₂(110)) surface and H₂O/R-TiO₂(110) surface. Our results show that the subsurface are preferred with R-TiO₂(110) surface. In contrast to previous studies, we find that the UPE tends to migrate to the surface H₂O-Ti_5c (the five-coordinated titanium (Ti_5c) at surface with H₂O adsorption) with the increasing of H₂O coverage and UPE concentration. In addition, we have shown that the UPE plays an important role in the O-H bond dissociation and other important elementary reactions in photo-catalytic H₂O dissociation on R-TiO₂(110) such as H, OH and H₂ desorption. Specifically, it enhances the O-H bond dissociation, as well as H and H₂ desorption from bridging hydroxyl and Ti_5c-OH (the Ti_5c with OH adsorption), but hinders the OH and H desorption from Ti_5c. We believe our results afford a further understanding of the adsorbent dependent UPE migration, and the role of UPE in the surface reactions.     

Dispersion of V2O5 supported on a TiO2 by x-ray photoelectron spectroscopy surface

The dispersion of V₂O₅ in V₂O₅/TiO₂ catalysis has been studied by x-ray photoelectron spectroscopy, transmission electron microscopy and a thermal gravimetric analysis for samples with various V₂O₅ content. The coverage, thickness and dispersion of vanadium oxide were calculated by a quantitative x-ray photoelectron spectroscopy (XPS) analysis. The experimental results show that V₂O₅ is well dispersed on a TiO₂ surface at low VEOs content. The coverage increases rapidly with the V₂O₅ content up to 10 wt%. Beyond that, the coverage increases very slowly, and considerable parts of the TiO₂ surface remain bare even at a V₂O₅ content as high as 40 wt%. The vanadium oxide is probably present on the TiO₂ surface as isolated patches or islands. This investigation shows that x-ray photoelectron spectroscopy is also a powerful tool for obtaining information about the dispersion of supported species in catalytic research.     

ESDIAD and LEED studies of the clean TiO2(100) surface

Electron-stimulated desorption ion angular distribution (ESDIAD) and LEED were used to investigate the structure changes at the TiO₂(100) surface. The angular distribution of O⁺ ions from the (1 × 3) reconstructed surface is consistent with the microfacet model proposed from X-ray diffraction and STM studies. The (1 × 3) reconstructed surface can be transferred back to the (1 × 1) surface after annealing at 950 K in oxygen, through a stage where the surface consists of (1 × 1) and (1 × 3) domains which are smaller than the coherent width of the LEED electron beam. Evidence for surface reconstruction on the (1 × 1) surface is also found.