欠电势沉积Bi-Te基体系热电材料的平衡热力学分析

基于普通的能斯特方程, 建立了单原子层平衡电势的热力学模型. 据此, 分析了单原子层覆盖度以及电吸附价与欠电势之间的相互关系, 获得了沉积物与衬底之间干涉特性. 并且分析了Bi-Te基体系欠电势沉积热力学特性. 通过对Bi欠电势沉积在几个不同的金属衬底体系的分析阐明了功函数随覆盖度的变化机制. 研究了铋离子的浓度变化对铋的欠电势及覆盖度的影响关系, 结果表明, 铋在铂上欠电势沉积的体系在整个欠电势范围内具有恒定的电吸附价, 而铋在覆盖了一层碲的铂衬底上欠电势沉积的体系其电吸附价随覆盖度的增加而降低, 从热力学理论角度对铋在碲覆盖的衬底上导致欠电势负移的特性给予了解释.     

Preparation of bismuth telluride thin film by electrochemical atomic layer epitaxy (ECALE)

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Bi and Te on cold rolled silver substrate have been performed. The voltammetric analysis of underpotential shift demonstrates that the initial Te UPD on Bi-covered Ag and Bi UPD on Te-covered Ag fitted UPD dynamics mechanism. A thin film of bismuth telluride was formed by alternately depositing Te and Bi via an automated flow deposition system. X-ray diffraction indicated the deposits of Bi₂Te₃. Energy Dispersive X-ray Detector quantitative analysis gave a 2: 3 stoichiometric ratio of Bi to Te, which was consistent with X-ray Diffraction results. Electron probe microanalysis of the deposits showed a network structure that results from the surface defects of the cold rolled Ag substrate and the lattice mismatch between substrate and deposit. Translated from Chinese Journal of Applied Chemistry, 2005, 22 (11) (in Chinese)     

The influence of organic adsorbates on the UPD process. Oxidation of formic acid at UPD lead-modified platinum electrodes

The oxidation of formic acid at Pt electrodes in the presence of underpotentially deposited (UPD) Pb has been studied using an electrochemical quartz crystal microbalance (EQCM). Although the current associated with the UPD process is largely obscured by current from the oxidation of formic acid, the mass response is dominated by the changes in UPD coverage. Thus examination of mass responses accompanying cyclic voltammetric and constant-potential experiments reveals both variations in UPD coverage and the manner in which the underpotential deposits are affected by adsorbates derived from formic acid. At low concentrations of formic acid there is some suppression of the underpotential deposit and data suggest that strongly adsorbing intermediates form most rapidly in the hydrogen adsorption region of potential. Mass responses also indicate slight increases in UPD coverage upon removal of strongly adsorbed species by oxidation. Oxidation of high concentrations (0.1 M) of formic acid induces a significant positive shift in the potential for removal of the UPD deposit on the positive scan, and on the subsequent negative scan the rapid reaction between the oxidized Pt surface and formic acid removes the oxide at a higher potential than normal and consequently allows the UPD process to begin at a more positive potential. Adsorption of Pb²⁺ at oxidized Pt surfaces is also inhibited by the presence of formic acid.     

Theoretical study of phase transitions and underpotential deposition on a liquid|solid interface

The underpotential deposition of metals on a single crystal electrode in the presence of anions has been studied in the framework of a triangular lattice gas model with pair- and three particle lateral interactions. Interactions between the adsorbed species cause the formation of the ordered (√3 × √3) structures in accordance with the symmetry of the lattice of adsorption sites. Using the mathematical equivalence between the lattice gas model and Ising and Blume-Emery-Griffiths spin models, we apply the method of the real space renormalization group for investigation the phase diagram of the system and the processes of deposition and desorption of the ions. It is shown that the phase transitions, occurring in the system, influence the underpotential deposition of metal ions strongly and are observed as sharp peaks in the voltammograms. We calculate coulometric and voltammetric curves and estimate the order of the lateral interaction magnitude between the adsorbed ions.     

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

Understanding the pH dependent shift of the oxidation peak of the underpotential deposited hydrogen (H_upd) in cyclic voltammograms on the Pt surface is of significance in terms of both the fundamentals of electrochemistry and the rational design of catalysts for the hydrogen oxidation/evolution reactions (HOR/HER). In this work, we provide compelling evidence that the pH dependent shift in the H_upd peak on Pt surfaces is driven by the structure of interfacial water rather than the specific adsorption of cations on the electrode surface. Combined cyclic voltammetric and surface enhanced spectroscopic investigations using an organic cation and crown‐ether chelated alkali metal cations show that specific adsorption of metal and organic cations on the Pt surface at the conditions relevant to the HOR/HER is unlikely. The vibrational band corresponding to strongly bound water is monitored when the electrode potential is varied in the H_upd range in both acid and base.     

Electrodeposition of mercury on gold from very dilute mercury(II) solution

Flameless atomic absorption spectrometry (flameless a.a.s.) was applied to study the state of mercury deposited on a gold plate electrode from very dilute mercury(II) solution by controlled-potential electrolysis. A stable monolayer is formed on the gold electrode by the electrolysis at a potential about 200 mV more positive than the reversible Nernst potential for the reduction of mercury(II) to mercury(0). After the monolayer formation, bulk mercury is deposited on the monolayer at the reversible potential and an adatom layer is also found. The difference of activation free energies between the evaporation of mercury from the monolayer and that from bulk mercury corresponds to the underpotential shift for the electrodeposition of mercury on the gold electrode.     

Probing electronic coupling in excitonically coupled heterodimer complexes by two-color three-pulse photon echoes

Following the earlier work of Yang et al. [J. Chem. Phys. 110 (1999) 2983] analytical expressions for the downhill and uphill resonant two-color three-pulse photon echo peak shift (2C-3PEPS) of a heterodimer system are derived in the impulsive limit. It is shown how to obtain information about coupling between the components of the dimer from the combined one- and two-color peak shift measurements. Further analytical relations are derived which enable site specific information about the environment of the components, including the relative difference of the inhomogeneity and the difference between the energy-gap correlation functions on the heterodimer sites to be obtained. The simulations show only a very small influence of the laser pulse length on the measured values of coupling coefficient and other relevant quantities suggesting that current 2C-3PEPS measurements can find practical application in directly measuring couplings in excitonically coupled heterodimer complexes.     

Lead underpotential deposition on gold single-crystal surfaces: The (100) face and its vicinal faces

Results concerning underpotential deposition of lead on gold single-crystal faces are presented. The (100) face and its vicinal faces are studied by cyclic voltammograms, potential step measurements and phase shift measurements. The explanation of the complex CV of the (100) face is attempted; adsorption, reconstruction and nucleation processes are discussed. Some details concerning the single-crystal techniques are given in the Appendix.     

Strategic synergy: advances in the shape control of bimetallic nanoparticles with dilute alloyed surfaces

Bimetallic nanoparticles with dilute alloyed surfaces are promising materials for many applications, especially catalysis. However, the preparation of nanoparticles composed of catalytically advantageous metal pairs is complicated by challenges, such as lattice mismatch and dissimilar reduction potentials. This review presents recent advances in nanoparticle shape and composition control, and these syntheses can be classified as ‘two-step’ or ‘one-step.’ Two-step techniques use a premade particle that is modified by the deposition of the secondary metal via galvanic exchange, direct reduction, or underpotential deposition. In contrast, one-step methods such as seed-mediated coreduction, sequential coreduction, and underpotential deposition involve the simultaneous deposition of both metals. A perspective on ongoing synthetic challenges is also provided, as well as promising directions for future work.     

Cadmium atomic layers on tellurium electrodes

The process of electrodeposition of atomic layers of cadmium Cd_ad at order of magnitude polycrystalline tellurium electrodes at potentials in excess of equilibrium potential E _Cd ²⁺/_Cd (underpotential deposition, UPD) is studied. In acid sulfate solutions (0.1 M H₂SO₄ + 0.05 M CdSO₄), the magnitude of underpotential (underpotential shift) ΔE ^UPD = 0.41 V. In cyclic voltammograms, the anodic oxidation of Cd_ad is recorded in the form of two peaks of anodic current that are displaced relative to one another in the potential scale (weakly and strongly bound adatoms). The cadmium UPD process on tellurium is accompanied by the diffusion of cadmium into the bulk of tellurium and a transition from a 2D structure Cd_ad/Te to 3D CdTe nanostructures, which are distributed in the tellurium matrix. In contrast with tellurium, the Te/CdTe_nano heterostructure exhibits photoelectrochemical activity, i.e. it generates a cathodic photocurrent.     

The influence of concentration of ethylenediamine and acetate ions on the occurrence of underpotential effects in the system Cu(111)/Pb+2

The occurrence of lead on copper underpotential effects is dependent on concentration of ethylenediamine and acetate ions in the solution and also on solution pH. The cause of variations in intensity of occurrence of underpotential effects is the change in the state of chemical stability of the system Cu(111)/Pb⁺² + CH₃ COO⁻ + ethylenediamine + H⁺.     

X-ray photoelectron spectroscopic evidence of distinctive underpotential deposition states of Ag and Cu on Pt substrates

The underpotential deposition (u.p.d.) of copper and silver ions on platinum electrodes has been studied using X-ray photoelectron spectroscopy (x.p.s.). A significant chemical shift of the x.p.s. binding energies of the u.p.d. copper and silver atoms relative to bulk copper and silver metal is obtained. Analogous spectral results are obtained for in situ vapor deposition of copper and silver atoms on clean platinum substrates which reveal the metallic nature of the u.p.d. species.     

A Modulation QCM Applied to Copper Electrodeposition and Stripping

A fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM−D) was applied to copper electrodeposition and subsequent stripping. Accumulation brings the frequency noise down to the mHz range, corresponding to 0.1 % of a monolayer. With this precision, the apparent mass transfer rate as determined from the time-derivative of the frequency shift can be directly compared to the current. Small but systematic deviations between the two can be attributed to nanoscale roughness. In the voltage range of underpotential deposition (UPD), the apparent mass transfer rate shows peaks and shoulders. The plating additive benzotriazole (BTA) leaves the magnitude of electrogravimetric signals unchanged, but shifts the UPD onset potential. The additive thiourea (TU) promotes UPD and strongly increases the bandwidth.     

Application of work function measurements to the characterization of thin films and solid surfaces

Two different methods of electron work function measurements, the diode and the onset method, which can easily be incorporated in existing analytical equipment are described. For the diode method the sample is used as the anode of a diode arrangement, and the work function changes of the specimen are obtained from the shift of the break points of characteristic retarding field lines. The onset method uses the shift of the onset of the secondary electron energy distribution due to work function changes.Both methods were applied to Ta₂O₅ layers of 500 Å thickness which were produced by electrochemical oxidation of polycrystalline tantalum substrates. In particular, bombardment induced work function changes during sputter removal of these layers have been investigated.The onset method was also applied to differently oriented grains of a polycrystalline Ta specimen. Work function differences of 0.1 eV between two different grain surfaces were safely detectable.In addition, a work function difference of 0.5 eV between Au and Mo was determined which agrees well with literature values.     

Investigation of the role of the C-PCM solvent effect in reactivity indices

It has been shown recently, how the coupling between electronic degrees of freedom and vibrational modes is reflected in the properties of molecules. The necessary derivatives have been analyzed and their thermodynamic relations were demonstrated. This present work is focused on the analysis of a molecular system, under the influence of C-PCM induced solvent effect. The analysis is based on reactivity indices derived from DFT. The shift of frequency for diatomic molecules has been obtained. It has been identified as chemical force effect. The role of nuclear reactivity indices has been emphasized. This concept has been extended to obtain regional chemical potential values within C-PCM model for polyatomic molecules.     

Cs的覆盖度对Cs/石墨烯的结构稳定性和场发射性能影响

采用密度泛函理论方法,对Cs以不同覆盖度吸附在石墨烯表面时系统的结构稳定性和场发射性能进行了研究。计算结果表明单原子Cs吸附在石墨烯表面的碳六元环的中心位置时系统的能量最低。随着Cs的覆盖度增加,碱金属Cs与石墨烯之间的吸附作用逐渐增强,(4×4)R 0°和(2×2)R 0°结构是稳定的。由于碱金属Cs的修饰作用,Cs/石墨烯体系的功函明显下降,且随着碱金属Cs的覆盖度增大,系统的功函逐渐减小。态密度的计算结果表明功函的下降主要与Cs和石墨烯之间的电子转移有关。随着覆盖度的增大,石墨烯的电子态逐渐向低能方向移动,系统的费米能级升高并导致材料的功函下降。     

Cs的覆盖度对Cs/石墨烯的结构稳定性和场发射性能影响

采用密度泛函理论方法,对Cs以不同覆盖度吸附在石墨烯表面时系统的结构稳定性和场发射性能进行了研究.计算结果表明单原子Cs吸附在石墨烯表面的碳六元环的中心位置时系统的能量最低.随着Cs的覆盖度增加,碱金属Cs与石墨烯之间的吸附作用逐渐增强,(4×4)R 0°和(2×2)R 0°结构是稳定的.由于碱金属Cs的修饰作用,Cs/石墨烯体系的功函明显下降,且随着碱金属Cs的覆盖度增大,系统的功函逐渐减小.态密度的计算结果表明功函的下降主要与Cs和石墨烯之间的电子转移有关.随着覆盖度的增大,石墨烯的电子态逐渐向低能方向移动,系统的费米能级升高并导致材料的功函下降.     

Inner filter effect and the onset of concentration dependent red shift of synchronous fluorescence spectra

The phenomenon of concentration dependent red shift, often observed in synchronous fluorescence spectra (SFS) of monofluorophoric as well as multifluorophoric systems at high chromophore concentrations, is known to have good analytical advantages. This was previously understood in terms of large inner filter effect (IFE) through the introduction of a derived absorption spectral profile that closely corresponds to the SFS profile. Using representative monofluorophoric and multifluorophoric systems, it is now explained how the SF spectral maximum changes with concentration of the fluorophore. For dilute solutions of monofluorophores the maximum is unchanged as expected. It is shown here that the onset of red shift of SFS maximum of both the mono as well as the multifluorophoric systems must occur at the derived absorption spectral parameter value of 0.32 that corresponds to the absorbance value of 0.87. This value is unique irrespective of the nature of the fluorophore under study. For monofluorophoric systems, the wavelength of derived absorption spectral maximum and the wavelength of synchronous fluorescence spectral maximum closely correspond with each other in the entire concentration range. In contrast, for multifluorophoric systems like diesel and aqueous humic acid, large deviations were noted that could be explained as to be due to the presence of non-fluorescing chromophores in the system. This work bridges the entire fluorophore concentration range over which the red shift of SFS maximum sets in; and in the process it establishes the importance of the derived absorption spectral parameter in understanding the phenomenon of concentration dependent red shift of SFS maximum.     

Spectrum Correction in Study of Solvation Dynamics by Fluorescence Non-collinear Optical Parametric Ampli cation Spectroscopy

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy can extract the curve of spectral gain from its parametric superfluorescence. This unique spectrum correction method enables fluorescence non-collinear optical parametric amplification spectroscopy acquiring the genuine transient fluorescence spectrum of the studied system. In this work we employ fluorescence non-collinear optical parametric amplification spectroscopy technique to study the solvation dynamics of DCM dye in ethanol solution, and confirm that genuine solvation correlation function and shift of peak frequency can be derived from transient fluorescence spectra after the spectral gain correction. It demonstrates that fluorescence non-collinear optical parametric amplification spectroscopy can benefit the research fields, which focuses on both fluorescence intensity dynamics and fluorescence spectral shape evolution.     

碲在金衬底上的不可逆吸附行为及其欠电位沉积机制的研究

研究了碲在金衬底上的不可逆吸附行为特征及其对碲原子欠电位沉积行为的影响. 同时也探讨了碲原子于金衬底上的欠电位沉积机制. 结果显示在开路条件下碲原子在金衬底表面具有不可逆的吸附行为, 证实了在金的双电层范围内很难将这种碲的吸附物移走. 为了完全移走碲的吸附物, 需要采用特定的电化学清洗程序. 发现碲的吸附物移走发生在电位循环至金的氧化区域, 且在该区域这种碲的吸附物移走与金的表面氧化同时发生. 扫描速率分析结果证实碲欠电位沉积在金表面符合Sanchez-Maestre模型的三个标准, 说明碲原子于金衬底上欠电位沉积符合二维形核和生长机制.