Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Pt-skeleton surfaces

The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and after immersion to an electrolyte. In contrast all sputtered surfaces formed Pt-skeleton outermost layers due to dissolution of transition metal atoms. Therefore, these three different near-surface compositions (Pt-skin, Pt-skeleton, and pure polycrystalline Pt) all having pure-Pt outermost layers are found to have different electronic structures, which originates from different arrangements of subsurface atoms of the alloying component. Modification in Pt electronic properties alters adsorption/catalytic properties of the corresponding bimetallic alloy. The most active systems for the electrochemical oxygen reduction reaction are established to be the Pt-skin near-surface composition, which also have the most shifted metallic d-band center position versus Fermi level.     

Pt-skin的Pt基双金属电催化的氧还原设计

过去几十年,能源储存转化领域取得重大的进展. 而Pt-skin的Pt基双金属电催化剂在调控电催化剂的电子结构具有巨大的前景,特别是对于氧还原反应而言. 本工作主要综述了最近几年关于Pt-skin的Pt 基双金属电催化剂的设计制备,以及其性能. 本文的主要重点在于系统的综述了Pt-skin的Pt 基双金属电催化剂的合成方法,以及其对于氧还原反应的机理研究.     

Pt-表层限域的表面FeOx结构用于低温CO氧化反应

通过简单的浸渍-还原并随后在酸性溶液中处理制备了Pt-Cu双金属催化剂. 利用电感耦合等离子体发射光谱、X射线衍射和X射线光电子能谱对不同处理条件下的Pt-Cu纳米粒子的结构和组成进行了表征. 研究发现, Pt-Cu催化剂在高温H₂中还原形成了PtCu₃合金结构, 酸洗处理后形成了包含Pt-骨架(Pt-skeleton)的表面结构和PtCu₃合金核的纳米粒子. 而Pt-骨架结构又可通过在H₂中退火转变为规整的Pt-表层(Pt-skin)结构. Pt-表层表面修饰Fe氧化物后在CO选择氧化反应中表现出较好的催化性能. 通过此方法制备的三金属Pt-Cu-Fe催化剂可达到与Pt-Fe相近的高活性, 并且催化剂中Pt的用量大大降低.     

Pt(111) 表面上表层 Fe (FeO) 结构和次表层 Fe 结构的可控转变

 利用扫描隧道显微镜 (STM) 和 X 射线光电子能谱 (XPS) 对 Pt(111) 表面制备的 Fe 单层薄膜及其在不同环境气氛条件下的多种结构进行了研究. 在温度为 487 K 的 Pt(111) 表面制备出了完整的 Fe 单层薄膜Fe/Pt(111). 对 Fe/Pt(111) 依次升高温度进行超高真空退火, STM 和 XPS 结果表明退火温度高于 800 K 时, 表面 Fe 原子扩散到次表层区域, 形成次表层 Fe 结构Pt/Fe/Pt(111). Pt/Fe/Pt(111) 在 O2 氧化气氛中经 850 K 退火可转变成表面 FeO 薄膜FeO/Pt(111). FeO/Pt(111) 结构在温和的 H2 还原气氛中 (600 K) 转变成表面 Fe 结构, 进一步的还原处理 (800 K) 则可以重新生成 Pt/Fe/Pt(111). 控制样品的环境气氛在 O2 和 H2 之间切换, 使得表面 Fe (FeO) 和次表面 Fe 可以重复地转变. 本研究实现了多种 Fe-Pt 表面结构的可控制备, 可为合理地设计高效、价廉的催化剂提供借鉴.     

氧气气氛下Pt3Ni(111)表面结构变化的从头算原子热力学研究

采用从头算原子热力学方法系统研究了Ni-rich和Pt-rich条件下Pt₃Ni(111)在不同偏析、表面化学吸附氧覆盖度下560个可能结构的相对稳定性,构建了氧气气氛下Pt₃Ni(111)表面结构演化、直至满覆盖化学吸附氧的热力学相图.结果表明,随着氧的化学势的升高,在热力学上仅出现两类稳定的结构,主要包括没有化学吸附氧的干净Pt-skin表面,以及在很低氧的化学势下就形成的含有化学吸附氧的Ni-skin表面,而有化学吸附氧的PtNi表面合金化的中间结构则处于亚稳态.仔细分析发现,这些结构的形成主要由金属的偏析能、氧与两种金属成键强弱的差别、氧的化学势的高低三个因素共同决定.     

Precisely Controlled Synthesis of Pt-Pd Octahedral Nanoframes as a Superior Catalyst towards Oxygen Reduction Reaction

Pt-based nanoframes represent a class of promising catalysts towards oxygen reduction reaction. Herein, we, for the first time, successfully prepared Pt-Pd octahedral nanoframes with ultrathin ridges less than 2 nm in thickness. The Pt-Pd octahedral nanoframes were obtained through site-selected deposition of Pt atoms onto the edge sites of Pd octahedral seeds, followed by selective removal of the Pd octahedral cores via chemical etching. Due to that a combination of three-dimensional opens geometrical structure and Pt-skin surface compositional structure, the Pt-Pd octahedral nanoframes/C catalyst shows a mass activity of 1.15 A/mg_Pt towards oxygen reduction reaction, 5.8 times enhancement in mass activity relative to commercial Pt/C catalyst (0.20 A/mg_Pt). Moreover, even after 8000 cycles of accelerated durability test, the Pt-Pd octahedral nanoframes/C catalyst still exhibits a mass activity which is more than three times higher than that of pristine Pt/C catalyst.     

Oxygen reduction reaction activities of Ni/Pt(111) model catalysts fabricated by molecular beam epitaxy

Oxygen reduction reaction (ORR) activities were evaluated for clean Pt(111) and Ni/Pt(111) model catalysts fabricated by molecular beam epitaxy. Exposure of clean Pt(111) to 1.0 L CO at 303 K produced linear-bonded and bridge-bonded CO-Pt IR bands at 2093 and 1858 cm^? 1. In contrast, 0.3-nm-thick Ni deposited on Pt(111) at 573 K (573 K-Ni_0.3 nm/Pt(111)) produced broad IR bands for adsorbed CO at around 2070 cm^? 1; the separation of reflection high-energy electron diffraction (RHEED) streaks is slightly wider for 573 K-Ni_0.3 nm/Pt(111) than for the clean Pt(111). For 823 K-Ni_0.3 nm/Pt(111), the separation of the RHEED streaks is the same as that for the Pt(111), and a single sharp IR band due to adsorbed CO is located at 2082 cm^? 1. The results suggest that for the 823 K-Ni_0.3 nm/Pt(111), a Pt-enriched outermost surface (Pt-skin) was formed through surface segregation of the substrate Pt atoms. ORR activities for the 573 K- and 823 K-Ni_0.3 nm/Pt(111) as determined from linear sweep voltammetry curves were five times and eight times higher than that for clean Pt(111), respectively, demonstrating that Pt-skin generation is crucial for developing highly active electrode catalysts for fuel cells.     

Adsorption and Exchange Dynamics in Aging Hydroxyethylcellulose Layers on Silica

The adsorption kinetics of hydroxyethylcellulose (HEC) on silica and relaxations in adsorbed HEC layers were probed using total internal reflectance fluorescence and near-Brewster reflectivity. Like many random-coil polymers, HEC was found to adsorb at the transport-limited rate. Relaxations occurred at nearly constant interfacial mass when HEC layers were exposed to aqueous solvent, causing the subsequent exchange of chains between the layer and the free solution to become increasingly hindered. Eventually, on the time scale of a day, layers became immobilized and unable to accommodate chains from free solution. A continued fluorescence decay, beyond time scales that could be probed with self exchange, suggested further relaxations of the adsorbed HEC. The polydisperse HEC system (with an average molecular weight near 450,000) behaved qualitatively similar to molecular weight standard polyethylene oxide (PEO) layers on silica. For instance, relaxations in PEO layers occurred on a time scale of 10-20 h, like the HEC layers. Young layers of the latter, however, exhibited self-exchange kinetics that were an order of magnitude slower than PEO layers of similar age. This difference in adsorbed layer dynamics was attributed to HEC's stiffer backbone, compared with flexible PEO. Copyright 2000 Academic Press.     

Planar array of 1D gold nanoparticles on ridge-and-valley structured carbon

For fabrication of a planar array of 1D chains of gold nanoparticles prepared by a chemical process, the faceted (110) planes of sodium chloride crystals were used as templates to produce nanoscale ridge-and-valley structured carbon layers by a vacuum process. When these carbon layers loaded on copper grids were dipped in toluene solution of 3.4 nm gold nanoparticles followed by natural dry, a planar array of 1D chains of gold nanoparticles were formed on carbon layers, where the nanoparticles were immobilized predominantly in valleys and partly on ridges of carbon layers.     

贝壳的显微激光拉曼光谱研究

本文采用显微激光拉曼光谱对马氏珍珠贝和三角帆蚌贝壳进行了研究，结果表明，三角帆蚌贝壳珍珠层与棱柱层中的无机相皆与文石相，其中的有机相为类胡萝卜素类，马氏珍珠棱柱层中的无机相为方解石相，两种贝壳棱柱层晶体间的有机质中皆可能存在超微细的碳酸钙无机相。     

Alternate deposition of oriented calcite and amino acid layer on calcite substrates

Material synthesis inspired by novel nacre architecture and mechanism is popular and has attracted more and more attention. In this paper, iso-oriented calcite tablets/layers and amino acid layers were formed alternately on calcite wafers. It is interesting that the neonatal calcite tablets/layers have the same crystal orientation with their inorganic substrates through amino acid layers. It is quite possible that the amino acid layers in this study could transfer crystal orientation from formed inorganic layers to neighboring neonatal layers due to their fixed and appropriate structures, which may imply the process of nacre formation, and the role of aligned organic matrix sheets in nacre. Moreover, it could provide a new way to produce oriented calcite tablets/layers.     

金属纳米颗粒-聚电解质多层膜的X射线反射率研究

用静电自组装技术制备了不同层数的Au纳米颗粒-聚电解质多层膜,用X射线反射及原子力显微镜对膜的微结构进行了表征.研究发现,当Au纳米颗粒下面的聚电解质层较薄时,膜中无清晰的界面结构;随着Au纳米颗粒下面的聚电解质层的增厚,金属-聚电解质多层膜的界面变得越来越清晰.     

STS investigations to describe corrosion procedures on thin CrN layers

To represent the corrosion characteristics of thin hard material layers, CrN layers, which were magnetron sputtered, have been exposed to solutions with chloride or sulfate ions. The alterations of the layer surfaces were examined. Static immersion tests were carried out ex-situ in 1 mol L(-1) NaCl or 1 mol L(-1) Na(2)SO(4) solution over a period of 14 months. For in-situ measurements, the layers were inserted into the electrochemical cell of an EC STM as the working electrode. The sample potential was altered cyclically. At the same time, voltammograms were taken and the topography alteration was observed.Alterations in the surface structure were documented as hybrid information resulting from changes in topography and the chemical consistency or the thickness of the oxide layers. These modifications were demonstrated with scanning tunnelling spectroscopy before and after the treatment with solutions. Several chemical phases (initial state and oxide layers) were distinguished with raster tunnelling spectroscopy (measurements I(V) and I(z)). The evaluation of the I(z) spectroscopy showed that the tip was in mechanical contact with the sample if an oxide layer of greater thickness has existed.Exemplary XPS investigations demonstrated corrosive alterations of the CrN layers due to oxidation. Asymmetry and peak locations suggest that in addition to Cr(2)O(3) and CrN a complex mixture of different chromium oxides such as Cr(2)O(3) and Cr(OH)( x) exists.     

Lipophilicity determination of some 3,5-dinitro-benzoic-acid esters on unimepregnated cellulose layer

Unimpregnated cellulose layers proved to be suitable for the determination of the lipophilic character of 14 3,5-dinitro benzoic acid esters under reversed-phase, thin-layer chromatographic conditions. Lipophilic characters measured on unimpregnated cellulose layers correlated well with similar values determined on impregnated silica and on impregnated alumina. Minor differences in retention of the three types of layers were observed.     

单晶硅表面贵金属晶粒层的制备

将预处理过的单晶硅p-Si(100)浸入含贵金属盐的HF溶液, 制备了Ag, Au, Pd和Pt的晶粒层. 用原子力显微镜(AFM)、开路电位(OCP)、循环伏安(CV)和交流阻抗(A. C. Impedance)方法对晶粒层性能进行了考察. 形貌显示, 在浸镀20 s后, Ag和Pd晶粒层基本上覆盖了硅基底, Ag颗粒致密, Pd颗粒之间仍有空隙且晶粒较Ag大. Au晶粒层部分覆盖了基底, 而Pt只有极少数的晶粒. 60 s后, Ag, Pd和Au晶粒层都完全覆盖了基底, 而Pt晶粒仍然较少, 但晶粒有所长大. 循环伏安显示, Pd的溶出峰电流比Ag, Au, Pt高1个数量级. 交流阻抗测量表明, Pd晶粒层阻抗最小. 结果表明, Ag, Pd和Au都能用浸入沉积的方法在单晶硅上短时间内制备出晶粒层, 而Pt不能, 选用哪种晶粒层, 需要根据后续工序和实际需要而定.     

Reverse Phase Thin-Layer Chromato-Graphy of some Metal Ions on Butyl-Amine Impregnated Silica Gel G in Formic Acid Medium

Abstract

Reversed phase thin layer chromatography of metal ions have been performed on s-butylamine and t-butylamine impregnated silica gel thin layers in formic acid and sodium formate systems. Separation potential of impregnated silica gel thin layers was explored. Impregnated silica gel layers were found to be more selective (strongly sorbing) than unimpregnated silica gel chromo-toplates. Several important binary and ternary separations were achieved on impregnated thin layers.     

Infrared dielectric mirrors based on thin film multilayers of polystyrene and polyvinylpyrrolidone

Thin film polymer multilayers were prepared by spin coating alternating layers of polystyrene and polyvinylpyrrolidone. Samples with 10, 20, 30, 40, and 50 layers were prepared with individual layer thickness values in the range 223–508 nm. These samples were measured using a Fourier transform infrared spectrometer and were found to display narrow photonic band gaps (～ 0.04 to 0.06 μm wide) in their spectral response over the wavelength range 1.6 ? 2.6 μm. The position of the photonic band gaps was controlled by varying the thickness of the individual layers within the multilayer structures. This was achieved by varying the spin speed used during the deposition of the polymer layers. The peak reflectance of the multilayers was controlled by varying the number of layers within the multilayer samples giving values in the range 20–80% (corresponding to transmittances of 80–20%). Calculated transmittance spectra were also obtained using an optical transfer matrix method. These calculated spectra were shown to be in good agreement with the experimental data obtained. These experiments demonstrate a facile approach to the production of low cost dielectric mirrors that have tailored photonic properties over a range of wavelengths that are currently important for applications in fibre optic based telecommunications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

PS/PMMA mixed polymer brushes on the surface of clay layers: Preparation and application in polymer blends

Polystyrene (PS) and poly(methyl methacrylate) (PMMA) mixed polymer brushes on the surface of clay layers were prepared by using in situ free radical polymerization. Free radical initiator molecules with two quaternary ammonium groups at both ends were intercalated into the interlayer spacing of clay layers. The amount of polymer brushes grafted on the surface of clay layers can be controlled by controlling the polymerization time. Thermogravimetric analysis, X‐ray diffraction, and high‐resolution transmission electron microscope results indicated successful preparation of the mixed polymer brushes on the surface of clay layers. The kinetics of the grafting of the monomers was also studied. The mixed polymer brushes on the surface of clay layers were used as compatibilizers in blends of PS and PMMA. In the blends, the intercalated clay particles tend to locate at the interface of two phases reducing the interfacial tension. In the meanwhile, PMMA homopolymer chains tend to intercalate into clay layers. The driving force for the intercalation is the compatibility between homo‐PMMA chains and PMMA brushes on the surface of clay layers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5329–5338, 2007     

盐湖地区暴露25个月的碳钢表面锈层分析

通过扫描电镜(SEM)、电子探针(EPMA)、X射线衍射(XRD)、红外光谱(IRS)和电化学测试技术分析了碳钢在盐湖大气环境下暴露25个月后上下表面的锈层特征. 结果表明, 碳钢上下表面腐蚀量相似, 锈层中均富有Cl、Mg和Si等外来元素. 两个表面的腐蚀产物都主要由β-FeOOH、Fe8(O,OH)16Cl1.3和少量的γ-FeOOH组成, 最外部分的锈层中还检测到Fe3O4和δ-FeOOH. 电化学测试结果表明, 锈层对基体腐蚀具有抑制作用, 锈层保护性随暴露时间增加而增加.     

缓蚀剂对船用钢在潮湿大气中的防蚀研究

模拟 90 7A钢在海水的潮湿大气环境和表面覆有薄层缓蚀剂液膜状态下其腐蚀受抑制的情况 ,利用恒电量仪连接ACM控头 ,监测其腐蚀状态 ,结合电化学阻抗谱 (EIS)的测量 ,探讨 90 7A钢在薄层缓蚀剂液膜下的腐蚀受到抑制机理 .结果表明 :此时大气腐蚀反应已转变为阳极过程控制占优势 ,在浸润后期的EIS呈现出由腐蚀介质扩散和电化学放电混合控制特征 ,如果利用恒电量激励下的等效电路模型解析 ,则可以更清楚地获知薄层缓蚀剂液膜下金属表面膜层的信息