Control of the stability, electron-transfer kinetics, and pH-dependent energetics of Si/H2O interfaces through methyl termination of Si(111) surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.     

Covalent attachment of acetylene and methylacetylene functionality to Si(111) surfaces: scaffolds for organic surface functionalization while retaining Si-C passivation of Si(111) surface sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.     

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.     

Chemical and electrical passivation of silicon (111) surfaces through functionalization with sterically hindered alkyl groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH3)2CH- (iso-propyl), (CH3)3C- (tert-butyl), and C6H5- (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH(3)-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, -2.5 V vs Ag+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.     

Facet-dependent lithium intercalation into Si crystals: Si(100) vs. Si(111)

Fast Li transport in battery electrodes is essential to meeting the demanding requirements for a high-rate capability anode. We studied the intercalation of a Li atom into the surface and subsurface layers of Si(100) and Si(111) using density functional calculations with a slab representation of the surfaces. We suggest that the Li atom migrates on the Si surfaces and is subsequently inserted into the inside for both Si(100) and Si(111). The rate-determining steps are the surface incorporation and subsurface diffusion in Si(100) and Si(111), respectively. Our diffusion rate calculations reveal that, once the Li atom is incorporated into the Si surface, Li diffuses faster by at least two orders of magnitude along the <100> direction than along the <111> direction. The importance of careful treatment of the slab thickness for the study of impurity insertion into subsurface layers is also stressed.     

Electrochemical behaviour of oxocarbons on single crystal platinum electrodes: Part V. Tetrahydroxy-p-benzoquinone in 0.5 M sulphuric acid medium

The electrochemical behaviour of tetrahydroxy-p-benzoquinone (THQ) on Pt(111), Pt(100) and Pt(110) surfaces has been studied in sulphuric acid solutions by cyclic voltammetry. The experimental results show that THQ is the oxocarbon having a lesser tendency to form CO-like species after adsorption on platinum surfaces. THQ is directly adsorbed on Pt(111) and Pt(100), although it is not stable and undergoes a slow reaction to CO adspecies on Pt(100) surfaces. Clear evidence of CO adspecies has been found only on Pt(100) electrodes.     

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.     

Synthesis and Temperature-dependent Electrochemical Properties of Boron-doped Diamond Electrodes on Titanium

On the sand-blasting-treated titanium(Ti) substrate, the boron-doped diamond(BDD) electrodes with a wide potential window were prepared by microwave plasma chemical vapor deposition(MPCVD). The electrochemical oxidation ratios of phenol at BDD/Ti electrodes at elevated temperatures(from 20 ℃ to 80 ℃) were examined by the chemical oxygen demand(COD) of phenol electrolyte during electrolysis. The results show that the COD removal was increased at high temperatures and the optimized temperature for enhancing the electrochemical oxidation ratio of phenol is 60 ℃. The mechanism for the temperature-dependent electrochemical oxidation ratios of phenol at the electrodes was investigated. The study would be favorable for further improving the performance of BDD/Ti electrodes, especially working at high temperatures.     

Shape-dependent electrocatalysis: methanol and formic acid electrooxidation on preferentially oriented Pt nanoparticles

Reactivity towards methanol and formic acid electrooxidation on Pt nanoparticles with well characterised surfaces were studied and compared with the behaviour of single crystal electrodes with basal orientations. Polyoriented and preferential (100), (111) and (100)-(111) Pt nanoparticles were synthesised, cleaned preserving its surface structure, characterised and employed to evaluate the influence of the surface structure/shape of the Pt nanoparticles on these two relevant electrochemical reactions. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity of both oxidation processes, and thus the electrocatalytic properties strongly depend on the surface structure/shape of the nanoparticles, in particular on the presence of sites with (111) symmetry. These findings open the possibility of designing new and better electrocatalytic materials using decorated shape-controlled Pt nanoparticles as previously described with Pt single crystal electrodes.     

Spectroscopic investigation of the adsorption and oxidation of thiourea on polycrystalline Au and Au(111) in acidic media

In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO4(2-) were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)2(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.     

一种新的测定微粒的电化学技术

设计了一种由2个石墨电极短路相连组成工作电极的新的电化学池装置.操作时首先通过外力按压使极少量固体微粒粘附在其中一个石墨电极表面上,然后在溶液存在下将微粒夹紧并固定在2个石墨电极表面之间进行电化学测定.电化学转化过程中生成的可溶性物质被封闭在2个石墨电极表面之间而得到测定.用该技术对钯沉积在氧化铝上而组成的催化剂的电化学行为以及黄铁矿的电化学行为进行了研究.结果表明,其兼具可电解粘合剂碳糊电极和固体微粒伏安法(voltammetry of microparticles)技术的优点而避免了各自的缺点:即不使用粘合剂,从而消除了粘合剂中杂质产生的氧化或还原电流的影响;可测定电化学转化过程中生成的可溶性物质;分辨率好、易于操作.     

Si(111) Electrode/Electrolyte Interracial Studied by in-situ Second Harmonic Generation

Si(111) electrode has been widely used in electrochemical and photoelectrochemical studies. The potential dependent measurements of the second harmonic generation (SHG) were performed to study Si(111) electrode interface. At different azimuthal angles of the Si(111) and under different polarization combinations, the curve of the intensity of SHG with extern potential has a different form of line or parabola. Quantitative analysis showed that these differences in the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si(111) electrode. The change in the isotropic and anisotropic contribution of the Si(111) electrode may be attributed to the increase in the doping concentration of Si(111) electrodes.     

ATR-FTIR Study of Si(110) Surface Hydrogenated in NH4F-Based Mixtures

RCA (Radio Corporation of America) cleaning has been the important and critical step in semiconductor manufacturing for more than 30 years^[1]. As the electronics devices are shrinking and gate oxide is getting thinner, stringent requirements on metallic impurities,organic contamination and surface roughness on silicon wafer after wet chemical cleaning have attracted more attention in the mechanism of wet etching processes on Si(111) and Si(100) surfaces^[2=11]. In the past few years wet chemical and electrochemical etching of Si(110) in NH₄F solutions has been studied by using scanning tunneling microscopy (STM)^[12] and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR)^[13-14]. In the present work, we extend our study to Si(110) surface in NH₄F/HCl mixtures by using polarized attenuated total reflection FTIR (ATR-FTIR). We have compared the ex-situ ATR-FTIR results on Si(110) in various NH₄F/HCl solutions using Ge prism. Effects of potential on hydrogen-terminated structures on Si(110) surfaces have been investigated by employing in-situ electrochemical ATR-FTIR with double side polished single crystal silicon as a prism. Our ATR-FTIR spectra are correlated with the results obtained with in-situ STM.     

Unsupported palladium nanoparticles for ethanol cyclic voltammetric sensing in alkaline media

Carbon-supported palladium nanostructures have had a recent rise in their use for ethanol oxidation applications. In this work, we present the use of unsupported palladium nanoparticles (PdNPs), synthesized by sodium borohydride chemical reduction method, for ethanol electrochemical sensing. The unsupported PdNPs were studied for ethanol oxidation in alkaline media by cyclic voltammetry, and additionally were characterized using transmission electron microscopy, and x-ray photoelectron spectroscopy. The performance of unsupported PdNP-modified glassy carbon electrodes for the electrochemical ethanol oxidation in 1.0 M potassium hydroxide (KOH) solution was studied by cyclic voltammetry. These electrochemical results demonstrated that the unsupported PdNPs have very promising catalytic activity towards the oxidation of ethanol in alkaline media with good detection performance in the concentration range of 2304 to 288 ppm (i.e., 50.00 to 6.25 mM). The detection limit and linear correlation coefficient were 49.3 ppm (1.10 mM) and 0.9998, respectively. The unsupported PdNP-modified glassy carbon electrodes presented good cyclic voltammetric stability for ethanol sensing application in alkaline media.     

Adsorbed intermediates of formaldehyde oxidation and their role in the reaction mechanism

Formaldehyde oxidation was studied on the basal planes of platinum single crystals. Electrochemical and IR spectroscopy data give new information on the mechanism of oxidation. Formaldehyde oxidation at platinum electrodes is a surface-sensitive reaction. From the three basal planes of Pt(hkl), Pt(111) is the most active one. The less active surfaces Pt(100) and Pt(110) are blocked by adsorbed carbon monoxide at the initial stages of the reaction as the formaldehyde is admitted in the solution with the electrode polarized at 0.05 V. Besides CO(ad), other adsorbed species are formed. From these, methylene glycolate, H2COO(ad), is the intermediate of the fast oxidation pathways forming CO2 and HCOOH as soluble products. According to IR data the yields of soluble products at Pt(111) were calculated at 0.6 V, giving 63% for HCOOH and 37% for CO2. At 0.05 V the Pt(111) surface becomes slowly blocked by CO(ad), as observed when the electrode was left in contact with the formaldehyde solution over a period of several minutes. The same blockage occurs during a cyclic voltammogram, which causes a lowering of activity during the second potential scan. A general scheme of the reaction is proposed.     

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.     

Effects of the surface mobility on the oxidation of adsorbed CO on platinum electrodes in alkaline media. The role of the adlayer and surface defects

The oxidation of adsorbed CO on Pt single crystal electrodes has been studied in alkaline media. The surfaces used in this study were the Pt(111) electrode and vicinal stepped and kinked surfaces with (111) terraces. The kinked surfaces have either (110) steps broken by (100) kinks or (100) steps broken by (110) kinks and different kink densities. The voltammetric profiles for the CO stripping on those electrodes show peaks corresponding to the oxidation of CO on the (111) terraces, on the (100) steps/kinks and on the (110) steps/kinks at very distinctive potentials. Additionally, the stripping voltammograms always present a prewave. The analysis of the results with the different stepped and kinked surfaces indicates that the presence of the prewave is not associated with defects or kinks in the electrode surface. Also, the clear separation of the CO stripping process in different peak contributions indicates that the mobility of CO on the surface is very low. Using partial CO stripping experiments and studies at different pH, it has been proposed that the low mobility is a consequence of the negative absolute potential at which the adlayers are formed in alkaline media. Also, the surface diffusion coefficient for CO in these media has been estimated from the dependence of the stripping charge of the peaks with the scan rate of the voltammetry.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

金纳米粒子的二步电解合成及表面增强拉曼散射活性研究

在超声条件下采用二步电解方法在十六烷基三甲基溴化铵/丙酮/水三组分体系中合成金纳米粒子. 首先采用恒电流或电位的阶跃方法, 使体系中生成较小的金纳米粒子并作为晶种; 接着采用电位双阶跃方法, 使金纳米粒子在原来基础上继续生长, 控制电解电量可获得不同大小的金纳米粒子. 通过静电作用在洁净的单晶硅片表面组装金纳米粒子, 获得具有不同形貌的硅片, 并以此作为表面增强拉曼散射基底, 以吡啶为探针分子, 研究了不同基底的表面增强拉曼散射活性, 结果表明吡啶谱峰强弱与纳米粒子在硅片表面的排列形貌有关.     

Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.