Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

和频振动光谱(SFG-VS)研究中由基团在界面上取向所引起的光谱增强或相消的干涉现象为研究分子在界面上的绝对取向提供了一种直接的测量方法. 这一方法比SFG实验中复杂的相位测量方法更为直接和简单可行. 以在空气/水界面取向已知的对羟基苯腈(PCP)分子的氰基(-CN)基团为相位参考来获得3,5-二甲基对羟基苯腈(35DMHBN)和2,6-二甲基对羟基苯腈(26DMHBN)分子在空气/水界面的取向信息. 通过对这三种分子的水溶液和它们两两混合溶液界面上-CN基团和频振动光谱强度的比较,发现在空气/水界面的3     

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

ABSTRACT

We have examined the impact of intermolecular vibrational coupling effects of the O-H stretch modes, as obtained by the surface-specific velocity-velocity correlation function approach, on the simulated sum-frequency generation spectra of the water/air interface. Our study shows that the inclusion of intermolecular coupling effects within the first three water layers, i.e. from the water/air interface up to a distance of 6?Å towards the bulk, is essential to reproduce the experimental SFG spectra. In particular, we find that these intermolecular vibrational contributions to the SFG spectra of the water/air interface are dominated by the coupling between the SFG active interfacial and SFG inactive bulk water molecules. Moreover, we find that most of the intermolecular vibrational contributions to the spectra originate from the coupling between double-donor water molecules only, whereas the remaining contributions originate mainly from the coupling between single-donor and double-donor water molecules.     

金电极上Cu,Zn-SOD的表面增强拉曼光谱

应用原位共焦拉曼光谱技术在经表面增强处理后的金电极上,观察Cu,Zn-SOD分子的吸附方式及其随电位变化的过程。实验结果表明,在400mV-700mV的电位区间内,Cu,Zn-SOD分子始终以氨基酸残基上COO-和NH2在电极表面吸附,使活性中心所处的平面远离电极表面,且随电位负移发生脱附。上述结果为解释SOD分子在金电极表面无法实现快速电子传递提供了直接依据。     

Surface enhanced Raman scattering of water on an Ag electrode

Surface enhanced Raman scattering of adsorbates on an Ag electrode in various electrolytes (e.g., 0.1–1.0 M KF, KCl, KBr, KI, K₃PO₄, and NaN₃) has been investigated in an effort to elucidate the mechanism of the enhancement of water compared to that for other adsorbates. (It is well known, for example, that pyridine exhibits large enhancement in 0.1 M KCl while SERS from water is not detectable unless the salt concentration is raised to almost 1 M.) Use of an optical multichannel analyzer allowed rapid recording of Raman spectra, and SERS intensities of adsorbates could therefore be monitored simultaneously during a continuous oxidation-reduction cycle. Potential dependencies of SERS intensities when the electrode potential is cycled in a non-faradiac potential range immediately following oxidation and reduction indicate that adatoms are partially responsible for the Raman enhancement. Furthermore, the anions in the electrolyte play an important role in stabilizing these “active sites.” For this reason, the degree of enhancement is influenced by the solubility of the Ag compound formed during oxidation and the specific adsorption of the anions to the Ag surface. Preferential alignment of H₂O molecules with their oxygen ends facing the surface at positive potentials, the tendency of anions in the electrolyte to disrupt hydrogen bonding with the water network, and the weak hydrogen bonding of H₂O with the anions give rise to a SERS lineshape from adsorbed H₂O molecules which is narrower than, and thus distinguishable from, the Raman line of bulk water. Thus, the degree to which the particular anions in the electrolyte disrupt hydrogen bonding among water molecules and reform hydrogen bonding between the anions and H₂O molecules influences the SERS lineshape and the apparent enhancement of the H₂ Raman emission.     

Potential-induced structure transitions in self-assembled monolayers: ethanethiol on Au(1 0 0)

We have characterized the structural behaviour of ethanethiol self-assembled monolayers (SAMs) on Au(1 0 0) in 0.1 M H₂SO₄ as a function of electrode potential, using in-situ scanning tunneling microscopy (STM). After modification of the Au(1 0 0) electrode in an ethanolic solution of ethanethiol, STM images in air reveal a disordered thiol adlayer and a surface that is covered by 25% of monoatomic high gold islands, which originate from lifting of the (hex) reconstruction during thiol adsorption. In contrast to alkanethiol SAMs on Au(1 1 1), no vacancy islands are seen on the Au(1 0 0) surface. After contact of the SAM-covered Au(1 0 0) electrode with 0.1 M H₂SO₄ under potential control, two different structures are observed, depending on the potential range positive or negative of +0.3 V vs. SCE. In both cases the emerging ordered structures are quadratic, their unit cells being rotated by 45° with respect to the main crystallographic axes of the substrate. However, the ordered structure at negative potentials is more densely packed than the one at positive potentials, and in addition the surface reveals an almost 50% coverage of monoatomic high gold islands. The structure of the SAM changes reversibly with the electrode potential, the long range order gradually decreasing with each transition. Concomittant with this structure transition monoatomic deep holes are created when the potential is stepped from the cathodic to the anodic region. The experimental observations are rationalized by a high mobility of the gold thiolate moiety, causing the surface density of the SAM-covered gold to change drastically with potential.     

SFG experiment and ab initio study of the chemisorption of CN on low-index platinum surfaces

A dual analysis is proposed in order to have a better understanding of the adsorption of the cyanide ions on a platinum electrode. The SFG (Sum Frequency Generation) spectroscopy allows the in situ vibrational study and the SFG spectra of the CN⁻ species adsorbed on single crystal Pt electrode allow a systematic study of the low-index platinum surfaces. This experimental work is supported by ab initio calculations using density functional theory and cluster models. For each surface orientation and each geometry, a cluster model of 20-30 Pt atoms has been built in order to interpret the chemisorption of the CN⁻ ions through four kinds of adsorption geometry: on-top or bridge site, bonding via C or N atoms. Geometries have been optimized and adsorption energies, electronic properties and vibrational frequencies have been computed. From the electronic properties, we can propose an analysis of the bonding mechanism for each studied kind of adsorption.The SFG spectra of the CN⁻/Pt(1 1 1) system present an unique resonance owing to the top C adsorption. It is mainly the same for the CN⁻/Pt(1 0 0) system. It is also the case for the SFG spectra of the CN⁻/Pt(1 1 0) system recorded at negative electrochemical voltage; at more positive voltage, a second resonance appears at a lower frequency, owing to the top N adsorption.Experimental and theoretical values of the C-N stretching frequencies are in excellent agreement.     

Potential dependent thiocyanate adsorption on gold electrodes: a comparison study between SERS and SHINERS

Potential dependent adsorption of target molecules on electrode surface has long been analyzed by several analytical techniques at the electrochemical interfaces. Here, the adsorption of thiocyanate (SCN⁻) on gold electrodes [Au (111) and Au (poly)] is investigated by electrochemical shell isolated nanoparticle‐enhanced Raman spectroscopy (EC‐SHINERS) and surface‐enhanced Raman spectroscopy. Based on the experimental observation, C − N stretching mode of N‐bound SCN⁻ can be observed around 2080 cm⁻¹ throughout the whole potential range. The band corresponding to ν_C−N of S‐bound SCN⁻ appears as a shoulder at more negative potentials, and as a well‐defined band are more positive potentials. However, the overlapped bands provoke a negative shift in the frequency of S‐bound thiocyanate. Therefore, a change in the calculated Stark slope is observed. Interestingly, SHINERS has been employed to demonstrate the thiocyanate orientation and its effect on Raman spectra. Our results widen the opportunities of SHINERS to unravel the potential‐dependent adsorption behavior of target molecules on single‐crystal electrode surfaces. Copyright © 2016 John Wiley & Sons, Ltd.     

Photoelectron spectroscopy on silver electrodes on yttria stabilized zirconia under applied potential

Silver electrode layers of different porosity on yttrium stabilized zirconia (YSZ) were investigated by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS resp.). By applying different potentials in a three electrode arrangement oxygen could be transported from a gaseous reservoir to the working electrode. The change in the work function of the working electrode as a function of applied potentials was monitored by UPS. It showed significant different behavior for dense and porous silver electrodes. On a dense silver electrode a change in work function of up to 0.6 eV was observed at potentials at which a considerable current passed through the electrolyte. In the UP spectra of the valence band an additional structure appeared at a binding energy of 3.3 eV. These changes were also reflected in the appearance of oxygen O 1s signals in the XP spectra at 529 eV assigned to subsurface oxygen. On a porous silver electrode the work function at equilibrium (open circuit) was higher than that of metallic silver and increased linearly with increasing potential, although yet no current through the electrolyte could be observed. With the onset of considerable currents at larger anodic potentials the work function increase became stronger. As reason for the linear increase of the measured work function contributions of the solid electrolyte YSZ are considered. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Probing the interaction of ionic liquids with graphene using surface‐enhanced Raman spectroscopy

We report an in situ measurement of the interaction of an imidazolium‐based room temperature ionic liquid with both pure silver and a graphene‐over‐silver electrode under an applied electrochemical potential. At a negative applied potential, overall signal intensity increased indicating enhanced ionic liquid concentration at both silver and graphene electrodes. Vibrational modes associated with the imidazolium ring exhibited greater intensity enhancements and larger peak shifts compared with the anion indicating that the cation adsorbs with the ring and alkyl chain parallel to the electrode surface for both silver and graphene. In contrast to the silver, the surface enhanced Raman spectra of the ionic liquid near graphene showed shifts in the cation peaks even at no applied potential because of the strong π–π interaction between the ionic liquid and the graphene. Furthermore, the intensity of the graphene peak decreased in the presence of ionic liquid possibly due to the interaction between the ionic liquid and graphene. These results illustrate the effectiveness of surface‐enhanced Raman spectroscopy to investigate electrolyte interactions with graphene at the liquid/electrode interface. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical sum-frequency emission from langmuir-blodgett films of variable thickness: effects of the substrate and polar orientation of fatty acids in the films

The nonlinear optical response arising from a model multilayer structure, i.e., Langmuir-Blodgett (LB) films comprised of different numbers of per-protonated (H) and per-deuterated (D) fatty acid layers on solid substrates, has been evaluated by sum-frequency generation (SFG) spectroscopy. The SFG signals depend significantly on the absolute polar orientation of the fatty acids in the individual layers and on the nonlinear optical response of the substrate. The SFG spectra on gold and fused quartz substrates demonstrate a totally different dependence on the number of the contributing H layers, which it is possible to analyze quantitatively. The results provide important information for understanding the origin of the nonlinear optical responses from ordered systems composed of multiple interfaces and therefore for extracting exact structural information about each interface from the observed SFG signals.     

Molecular structure of polystyrene at Air/Polymer and Solid/Polymer interfaces

IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.     

半胱胺单分子膜结构的表面增强拉曼散射研究(英文)

在金属电极表面所形成的有机分子的单分子膜或薄膜对于基础研究和实际应用都有着极其重要的意义。以化学吸附形式在金电极表面所形成的半胱胺单分子膜,常常用于蛋白质等生物大分子在金属表面进行吸附的连接层,以避免这些生物大分子在金属表面直接吸附而造成的变性、失活现象的发生。本文报道了我们采用表面增强拉曼散射光谱方法研究在金电极表面吸附的半胱胺单分子膜的结构特征。研究结果表明,在金电极表面所形成的半胱胺单分子膜中,半胱胺分子主要的构型为扭转构型。在与金表面的相互作用中,由于除巯基的结合作用以外,还存在半胱胺分子中端基氨基和金表面较强的亲和性,使得以扭转构型吸附在金表面的半胱胺单分子膜相当稳定。这是金电极表面的半胱胺单分子膜结构的主要行征。当考察外加电势对此单分子膜结构的影响时,可以发现有关扭转构型的特征谱峰其相对强度随着电位负移而减小。这一结构随电位的变化关系可以通过表面电势的变化对氨基氮原子上孤对电子与金属表面间相互作用的影响来加以阐释。     

氯化亚锡吸附与配位作用下L-肌肽的表面增强拉曼光谱

利用在电极表面强化学吸附的氯化亚锡与被测有机/生物分子之间的相互作用,不仅可以有效消除被测分子的碳化和氧化现象,还可以获得特别的增强信号,可应用于反应活性强的有机/生物分子的表面增强拉曼光谱分析。还报道了肌肽的电化学表面增强拉曼散射(SERS)光谱的研究结果,重点考察了加入氯化亚锡前后和改变电极电势对肌肽吸附SERS光谱的影响。无氯化亚锡时,肌肽的SERS光谱存在严重的碳化信号的干扰;氯化亚锡存在下,调控电极电势可引起吸附的氯化亚锡与肌肽在电极表面发生配位作用,借助这种表面相互作用,可将肌肽固载于基底表面,从而获得L-肌肽的高质量表面增强拉曼光谱,完全消除碳化现象的干扰。     

Irreversible loss of adatoms on Ag electrodes during potential cycling determined from surface enhanced raman intensities

Surface enhanced Raman scattering of adsorbates (Ag⁰-Cl^?, H₂O and pyridine) on Ag electrodes in 1M KC1 and 1M KCl+0.05M pyridine electrolytes was monitored continuously with an optical multichannel analyzer system as the electrode potential was cycled over various ranges within nonfaradaic regions of the oxidation-reduction cycle. A systematic investigation was performed of the potential dependence of SERS of pyridine in 1M KX + 0.05M pyridine electrolytes, where X = F, Cl, Br and I. Since the surface coverage of the adsorbates is reversible with potential cycling within a potential range, it was possible to determine potential dependences of the irreversible loss in the SERS enhancement factor which occurs as the electrode potential is ramped toward the potential of zero charge (PZC). The results provide strong support for the role of adatoms on the electrode surface in the overall enhancement mechanism. There is evidence that the strongly bound adsorbates immobilize the adatoms at positive potentials but allow the adatoms to migrate and become lost at surface defects as the potential approaches the PZC where the adsorbates are less tightly bound.     

电化学汞齐法处理金表面的增强拉曼散射研究

采用汞盐的电化学还原法可通过控制电解的时间及速度来控制在金表面还原生成的金属汞的量,从而使金表面获得可控制的刻蚀,形成适当的粗糙度。以２,２ 联吡啶作为探针化合物吸附于在不同条件下汞齐化处理的金表面,收集其拉曼信号以确定最佳的刻蚀条件。经电化学汞齐化刻蚀活化的金表面比用通常的氧化还原循环处理的金表面具有更好的增强效果。将聚噻吩沉积在以上述刻蚀法粗糙化的金电极表面可获得信噪比高的亚单分子层的聚噻吩的ＳＥＲＳ谱,并表明在三氟化硼乙醚中沉积的聚噻吩较之在通常的有机溶剂中沉积的聚噻吩具有更有序的排列结构。     

Anisotropy of the surface properties of germanium in contact with an electrolyte containing gold ions

Recombination curves and polarization curves as well as the differential capacitance of an interface layer and the deflection of energy-band surfaces as functions of the electrode potential have been measured at the interface between n-germanium and 0.1 M Na₂SO₄ + (10^?8 to 10^?6) M Au³⁺ by a complex method. The lattice planes (111), (110), and (100) of germanium were examined. It has been shown that the parameters of states implanted during gold deposition depend on the surface orientation. A correlation has been discovered between the ratio of the hole-entrapment section to the electron-entrapment section and the electrode potential of plane-energy bands. This relation indicates that the parameters of recombination centers depend on the potential distribution across the electric double layer. The study was also concerned with the kinetics of recombination centers generated during gold deposition. It is suggested that the surface states at germanium surfaces in contact with a neutral electrolyte are of the same nature, whether or not such surfaces have been doped with gold.     

电化学体系中吸附在金电极表面吡啶的紫外拉曼光谱研究

观察到波长为325nm的紫外光激发下吸附在金电极表面的吡啶的表面增强拉曼光谱(SERS)。结合其SERS谱随电位的变化关系,分析了其中的增强机制并表明随电位的负移吡啶的吸附方式发生改变。     

现场拉曼光谱研究乙腈在金电极上的解离吸附行为

利用共焦显微拉曼系统、结合合适的电极表面粗糙方法研究了非水体系 0 1mol/LLiClO4 /CH3CN溶液中 ,乙腈分子在金表面的吸附和解离行为。结果表明非水体系中乙腈可在金表面发生还原反应 ,产物CN- 离子与电极表面作用形成的表面配合物可在较宽的电位区间吸附于电极表面。溶液中的微量水、激光照射以及电极电位均对该反应有较大的影响。通过拉曼谱图的比较得出乙腈分子解离出的CN- 在金电极表面比在银电极表面有更强的吸附作用。     

Mercury induced reorientation of alkanethiolates adsorbed on gold

3 -(CH₂)₂₁-SH) were investigated by IR-vis sum frequency generation (SFG). Well-ordered monolayers and films with submonolayer coverage with a high number of gauche conformations were prepared on polycrystalline gold substrates. Exposure to a saturated mercury atmosphere induces a major reorganization of the adsorbate phase indicated by a decrease of the average cant angle. Additionally, well-ordered films show an increase of gauche defects upon amalgamation. In contrast, films with submonolayer coverage exhibit an improvement of molecular order. The experiments suggest that amalgamation flattens the corrugation potential of the substrate and yields a separation of the thiol layer into areas of low thiol density with highly disordered alkyl chains and of higher density than on gold with an orientation of the molecules similar to what is observed on silver substrates. Received: 13 November 1998     

Cu,Zn-SOD在银电极上的原位表面增强Raman光谱

本文应用原位共焦拉曼光谱技术观察了Cu ,Zn -SOD分子在经表面增强处理后的银电极上 ,吸附状态随电位跃迁变化的过程。结果表明在 -0 2V～ 0 1V的电位区间内 ,Cu,Zn -SOD分子经由羧酸根离子吸附于银电极表面 ,而在 0 2～ 0 4V电位区间内发生脱附 ,表面吸附的物种改变为缓冲溶液中的磷酸根离子。上述结果为SOD分子在固体电极表面电子传递机制的探索提供了依据。