单晶电极表面研究中的超高真空技术

阐述了超高真空和电化学联合技术的性能,这种技术是获取电极-溶液界面微观信息的一种手段.给出了有关样品制备和表面分析技术在实验中的方方面面,并选取了一些单晶电极表面研究的最近成果--电化学池中重构的Au(100)-hex和Pt(100)-hex单晶电极的相转变以及Pt在Ru(0001)和Ru(1010)表面,RuO2(100)在Ru(0001)上外延生长的结构研究.     

甲酸在Pt(100)单晶电极表面解离吸附过程的动力学

有机小分子在电催化剂表面的解离吸附,是燃料电池阳极氧化过程中发生自毒化现象的主要原因.事实上这类解离吸附是一种表面分子过程,包括有机分子在电极表面吸附,分子内断键,生成新的吸附分子或基因等步骤.Sun等研究了甲醇等在一系列铂单晶电极上的解离吸附,发现这类过程极强地依赖于电极表面原子排列结构.虽然已有大量文献报导了运用原位红外光谱检测各类有机小分子解离吸附物种,但迄今仍未见到动力学方面的研究结果.显然,对这种在电化学条件下表面分子反应过程的动力学研究,必将进一步揭     

单晶电极界面反应过程的电化学原位拉曼光谱研究

自20世纪70年代起,人们利用原位光谱技术（拉曼、红外等）对电化学界面进行了系统的研究,进而发展出原位谱学电化学的研究方向,由于其具有良好的表面灵敏度和能量分辨率,可以提供更多的表面反应信息,进而从微观上揭示反应机理. 伴随着纳米技术的兴起,表面增强拉曼光谱技术取得了快速的发展. 近来,壳层隔绝纳米粒子增强拉曼光谱（shell-isolated nanoparticle-enhanced Raman spectroscopy,SHINERS）技术更是得到人们的广泛关注. SHINERS的出现为研究单晶模型电极上的催化反应提供了一种非常好的原位光谱技术. 本文主要对原位电化学SHINERS技术在单晶电极界面研究的具体应用及其发展前景进行相关论述.     

Au单晶表面氟表面活性剂FSN自组装膜的电化学行为

研究Au(111)和Au(100)表面非离子型氟表面活性剂FSN自组装膜的电化学行为.电化学扫描隧道显微术和循环伏安法测试表明,在0～0.8 V电位区间,FSN自组装膜未发生氧化还原,均一性好,可稳定地存在于电极表面,并显著抑制硫酸根离子在电极表面的吸附和Au单晶表面的重构.在FSN自组装膜Au单晶电极的初始氧化阶段,Au(111)表面有少量突起,而Au(100)表面呈现台阶剧烈变化,但FSN自组装膜的吸附结构没有改变.与Au(100)表面相比,Au(111)表面形成的FSN自组装膜可更有效地抑制Au表面的氧化.     

Pt单晶(210)、(310)和(510)阶梯晶面上CO2电催化还原的表面过程

研究Pt单晶 (210)、(310)和(510)三个阶梯晶面上CO₂电催化还原的表面过程. 通过改变处理条件获得单晶电极不同的表面结构. 研究结果指出, 当铂单晶电极表面保持其确定原子排列结构时, 对CO₂还原的电催化活性随晶面上(110)台阶密度的降低而减小, 即Pt(210) > Pt(310) > Pt(510); 当三个电极表面发生氧的吸附导致原子排列结构重建时, 其电催化活性均有不同程度的提高. 虽然其活性顺序未发生变化, 但(110)台阶位密度越大的表面其电催化活性增加的程度越高. 研究指出Pt单晶电极的表面结构越开放, 其电催化活性也越高, 并且在外界条件诱导下更易于转变为具有更高反应活性的表面结构. 而相对有序的表面结构则比较稳定. 研究结果从微观层次获得CO₂与Pt单晶电极表面相互作用的规律, 深化了对CO₂电催化还原表面过程的认识.     

交流电方法用于单晶n-CdTe光电极性能的研究

本文应用循环伏安法研究了n型碲化镉单晶电极的光致腐蚀行为; 并运用PAR M368电化学阻抗系统测得了此电极在不同电位下的交流复阻抗图, 估算了此电极表面态密度及其它参数。实验结果表明, n-CdTe/液体结具有Sehottky结的特征, 此电极的Fermi能级可能有"钉扎"现象。     

基于超微电极的原位电化学拉曼光谱

原位电化学拉曼光谱是一种重要的光谱电化学技术.基于超微电极的原位电化学拉曼光谱将拉曼光谱反映的结构信息与电极表面的电化学过程从实验上严格对应和关联,为深刻理解电化学反应机理提供依据.本文综述了采用超微电极作为工作电极的原位电化学拉曼光谱的研究方法和应用进展,总结了应用超微电极作为工作电极开展电化学拉曼光谱实验的方法和具有表面增强拉曼活性的超微电极制备方法,展示了如何利用在超微电极表面获得的拉曼光谱与界面电化学过程的严格关联研究单个锌颗粒电化学氧化过程、吡啶分子在Au电极表面的电化学吸附过程,以及如何利用该技术能以高的信噪比和灵敏度同时测量光电流与分子反应这一特性研究对巯基苯胺选择性光氧化反应.采用超微电极作为工作电极的原位电化学拉曼光谱技术极大拓展了拉曼光谱技术的研究范围,有望成为探索(光)电化学反应的有力工具.     

铂单晶电极表面不可逆反应动力学I.Pt(100)单晶电极上甲酸氧化的现场红外反射光谱研究

运用电化学暂态方法和现场时间分辨ＦＴＩＲ反射光谱研究甲酸在Ｐｔ（１００）单晶电极上的解离吸附和氧化过程,深入认识了甲酸解离吸附的反应速率在－０．２５至０．２５Ｖ电位区间呈火山形变化的规律。根据电化学现场时间分辨红外光谱的研究结果,提出在研究反动力学时避免甲酸解离吸附干扰的方法,为进一步研究甲酸在Ｐｔ（１００）电极表面经活性中间体直接氧化至ＣＯ２的反应动力学奠定了基础。     

铂单晶电极表面不可逆反应动力学──Ⅰ.Pt(100)单晶电极上甲酸氧化的现场红外反射光谱研究

运用电化学暂态方法和现场时间分辨FTIR反射光谱研究甲酸在Pt（100）单晶电极上的解离吸附和氧化过程．深入认识了甲酸解离吸附的反应速率在-0．25至0．25V电位区间呈火山形变化的规律．根据电化学现场时间分辨红外光谱的研究结果，提出在研究反应动力学时避免甲酸解离吸附干扰的方法，为进一步研究甲酸在Pt（100）电极表面经活性中间体直接氧化至CO2的反应动力学奠定了基础．     

光电化学水氧化用多孔单晶金红石TiO2纳米棒阵列光阳极的设计和构建

光电化学电池(如染料敏化太阳能电池、量子点敏化太阳能电池以及光电化学水分解电池)是实现太阳能转化及存储的有效手段之一.其中,光电极是光电化学电池的核心组成部分,它集光吸收、光生电荷输运及转移等决定光转化效率的关键过程于一身,因此构筑高活性半导体光电极以实现高效太阳能转化利用引起研究者广泛关注.多孔TiO2纳米颗粒堆垛薄膜光阳极因具有大的比表面积,可提供更多的染料(量子点)担载和反应活性位点,在光电化学电池中表现出优异活性而被广泛研究.然而, TiO2纳米颗粒间大量存在的晶界对光生电荷有较强的散射作用,降低了光生电荷的收集效率.英国牛津大学Snaith研究小组利用模板辅助水热过程首次获得了(001)晶面占优的多孔单晶锐钛矿TiO2微米颗粒,这种多孔单晶TiO2微米颗粒在具有大比表面积的同时,其单晶结构还能有效去除晶界对电荷的散射作用,因而具有优异的电荷输运特性.利用这种多孔单晶TiO2微米颗粒组建的光阳极用于染料敏化太阳能电池中,展现出优异的太阳能光电转化性能.受该工作启发,各种形貌的多孔单晶TiO2微米颗粒作为光催化剂和光电化学分解水用光阳极材料被广泛研究,并表现出优异活性.在单晶微米颗粒堆垛成的薄膜光电极中,虽然单个单晶微米颗粒中晶界对电荷的散射作用被有效抑制,但是单晶颗粒间的晶界仍然存在并影响光生电荷的收集效率.为了彻底抑制晶界对光生电荷的散射作用,每个单晶颗粒都应该贯穿整个薄膜,例如一维TiO2纳米棒单晶阵列薄膜.虽然一维单晶阵列薄膜能够有效提高光生电荷的收集效率,但相对于多孔薄膜具有较小的比表面积,限制了担载染料(量子点)和反应位点的数量.为了增大TiO2单晶纳米棒阵列薄膜的比表面积,目前主要的手段包括调控纳米棒长径比、表面修饰TiO2纳米颗粒以及二次生长构建TiO2枝晶阵列.本文首次提出通过制备多孔单晶TiO2纳米棒单晶阵列薄膜来获得高比表面积和高光生电荷收集效率的光阳极,提高光电化学电池的效率.在透明导电薄膜(FTO)表面利用水热生长TiO2纳米棒阵列薄膜之前,预先在FTO基体上沉积一层SiO2球密堆模板, TiO2纳米棒单晶阵列在从FTO表面向上生长过程中,会将SiO2球模板包裹进TiO2纳米棒中,再通过碱溶液将SiO2球模板溶解,首次在FTO基体上原位生长出多孔单晶TiO2纳米棒阵列薄膜.将所得多孔单晶金红石TiO2纳米棒阵列薄膜作为光电化学分解水电池光阳极,其光电化学分解水活性相对于实心单晶金红石TiO2纳米棒阵列提高了2.6倍.多孔单晶金红石TiO2纳米棒阵列光阳极性能的提升可归因于：(1)多孔结构赋予多孔单晶金红石TiO2纳米棒阵列薄膜更大的比表面积,可提供更多的反应活性位点;(2)多孔结构能够有效缩短单晶金红石TiO2纳米棒中光生电荷体相输运距离,提高光生电荷的收集效率;(3)多孔结构通过对光多次反射吸收可有效增强光吸收,产生更多光生电荷参与水分解反应;(4)在制备过程中引入Si掺杂,导致多孔单晶金红石TiO2纳米棒带隙扩大了0.1 eV,带隙增大归因于导带位置负移0.1 eV,光生电子具有更强的还原能力,光电流起始电位相应负移约0.1 V.     

Sb在Pt(100),Pt(110),Pt(111)及Pt(320)上不可逆吸附的电化学特性

研究了Sb在Pt(1 0 0 ) ,Pt(1 1 0 ) ,Pt(1 1 1 )和Pt(32 0 )单晶面上不可逆吸附的电化学特性 .发现当扫描电位的上限Eu≤ 0 .45V时 ,Sbad可以稳定地吸附在Pt(1 0 0 ) ,Pt(1 1 0 )和Pt(1 1 1 )表面 ,而Sbad在Pt(32 0 )表面稳定的电位较低 ,为Eu≤ 0 .40V .从饱和吸附Sb的铂单晶电极出发 ,通过改变电位扫描上限Eu 和电位扫描圈数可以获得不同Sb覆盖度 (θSb)的电极 .根据Sb和H在铂单晶电极表面共吸附的定量数据 ,对Sb在不同铂单晶面上饱和吸附的模型进行了初步探讨 .     

Studies of surface processes of electrocatalytic reduction of CO_2 on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)>Pt(310)>Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a cer- tain extent. Although the activity order remains unchanged, the electrocatalytic activity has been en- hanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhib- its higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a micro- scopic level, and thrown new insight into understanding the surface processes of electrocatalytic re- duction of CO2.     

Studies of surface processes of electrocatalytic reduction of CO<Subscript>2</Subscript> on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)＞Pt(310)＞Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a certain extent. Although the activity order remains unchanged, the electrocatalytic activity has been enhanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhibits higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a microscopic level, and thrown new insight into understanding the surface processes of electrocatalytic reduction of CO2.     

Kinetics of dissociative adsorption of formic acid on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes in perchloric acid solutions

The kinetics of dissociative adsorption of HCOOH on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes has been investigated. The oxidation of dissociative adsorbate (DA) derived from the dissociative adsorption of HCOOH was used as a probe reaction together with the potential-step technique of short time windows. The quantity Q_ad of DA produced at a given potential E_ad and in a defined time window t_ad of adsorption has been determined systematically. At fixed t_ad peaked curves of Q_ad versus E_ad in the potential range between −0.25 and 0.25 V/SCE have been obtained on all four electrodes. The maximum rate of dissociative adsorption of HCOOH decreases in the order Pt(110) Pt(210) Pt(610) Pt(100).     

Electrocatalytic properties of Pt(111), Pt(332), Pt(331) and Pt(110) single crystal electrodes towards ethylene glycol oxidation in sulphuric acid solutions

In the present paper four platinum single crystal electrodes, two basal planes of Pt(111) and Pt(110) and two stepped surfaces of Pt(332) and Pt(331), were prepared and used in the study of electro-oxidation of ethylene glycol (EG). All of these Pt single crystal electrodes belong to the [1 0] zone of crystallography, and exhibit on their surface (111) symmetry sites or certain combinations of terraces of (111) symmetry with steps of (111) symmetry type. It has been found that as a result of a favourable steric matching of surface sites the Pt(110) electrode manifested a higher activity both for EG dissociative adsorption and oxidation than that of the Pt(111) electrode. The stepped surfaces of Pt(332) and Pt(331) operated with certain combinations of characteristics of Pt(111) and Pt(110). The best electrocatalytic properties have been obtained with a Pt(331) electrode, and this is attributed both to the configuration of the atomic arrangement and to the stability of this surface.In summary, the above results show that the performance of a given Pt single crystal electrode in EG oxidation at a potential below 1.0 V may be evaluated by three factors.

1. (1) The ability to resist self-poisoning (AB) which describes the difficulty of EG dissociative adsorption on the electrode surface.

2. (2) The activity for EG oxidation (AC). In considering that the threshold potential for EG oxidation on all electrodes is at 0.3 V and that the self-poisoning is encountered in PGPS, the activity for EG oxidation may be reasonably characterized by the intensity of the peak current acquired in NGPS near 0.6 V, which corresponds to the maximum current of EG oxidation on an activated (non-poisoned) surface of the electrode.

3. (3) The stability of activity during potential cycling (SA) between 0.05 and 1.0 V, which describes the resistance to the decrease of intensity of the EG oxidation current during voltammetric cycling.

For the two basal planes studied, the AB and SA of Pt(111) are higher than those of Pt(110), but its AC is much lower than that of Pt(110). These differences are clearly related to the surface atomic arrangement of the two electrodes. As has been discussed above, the surface of Pt(111) is atomically smooth and stable during voltammetric cycling. The surface of Pt(110) presents, however, atomic steps and is reconstructed under experimental conditions, i.e. certain steric configurations are encountered on the Pt(110) surface. The high AC and the low AB may be assigned to a favourite stereographic matching during EG adsorption and oxidation on Pt(110).The two electrodes with stepped surfaces, Pt(332) and Pt(331), contain different densities of (110) sites, which are formed on the border between terrace and step, as shown in Fig. 8. The AB of these two electrodes has been observed at a moderate range between that of Pt(111) and the AB of Pt(110). With a majority of (111) sites on its surface, the electrode of Pt(332) operates at a relatively higher AC than Pt(111) does, and its SA is not as good as that of Pt(111) but is much better than the SA of a Pt(110) electrode. In all cases the highest AC and SA are obtained with a Pt(331) electrode. It may be seen from the profile of a (331) plane (shown by the cross-section of A-A in Fig. 8) that all atoms on the top of the surface participated in forming (110) sites, and the atom on the step has two functions — one is to form a (110) site with an atom located in the terrace of second layer and the other is to form a (111) site in the terrace of the same layer. It has been mentioned in the above discussions that the Pt(110) electrode keeps a higher AC due to favourite stereographic matching in EG adsorption and oxidation, but its SA is the worst, due to the instability of the surface. The highest AC and SA obtained with Pt(331) may be ascribed not only to the high density of (110) sites existing on the surface, but also to the stabilization of these (110) sites, and moreover, the synergy generated by the atomic arrangement of the Pt(331) surface may also contribute to the performance of the Pt(331) electrode.     

Adsorption of formaldehyde on Pt(111) and Pt(100) electrodes: cyclic voltammetry and scanning tunneling microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.     

Electrochemical behaviour of Pt(100), Pt(111) and Pt polycrystalline surfaces in hydrogencarbonate solution

It was demonstrated that adsorbed CO is obtained from the reduction of NaHCO₃ solution when Pt(100), Pt(110), disordered Pt(111) and polycrystalline electrodes are employed. Reduction of CO₂ coming from the dissociation of the hydrogencarbonate anion is proposed as the reaction that produces CO. By using Fourier transform infrared spectroscopy, linear and multi-bonded CO were detected on polycrystalline platinum electrodes. The shape of the band associated with linearly adsorbed CO is monopolar as a consequence of the partial overlapping, at lower wavenumbers, of the absolute bands at both potentials (0.05 and 0.35 V).     

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.     

An influence of pretreatment conditions on surface structure and reactivity of Pt(100) towards CO oxidation reaction

We present a combined electrochemical and in situ STM study of the surface structure of Pt(100) single crystal electrodes in dependence on the cooling atmosphere after flame annealing. The following cooling conditions were applied: Ar/H₂ and Ar/CO mixtures (reductive atmosphere), argon (inert gas) and air (oxidative atmosphere). Surface characterization by in-situ STM allows deriving direct correlations between surface structure and macroscopic electrochemical behavior of the respective platinum electrodes. We investigated the influence of defect type and density as well as long range surface order on the kinetics of the CO electro-oxidation reaction. The defect-rich Pt(100) electrodes as cooled in air or Ar, and followed by immersion in the hydrogen adsorption region display higher activities as compared to the rather smooth Pt(100)-(1 × 1) electrode cooled in an Ar/H₂-atmosphere.     

Electrochemical and FTIRS characterisation of NO adlayers on cyanide-modified Pt(111) electrodes: the mechanism of nitric oxide electroreduction on Pt

We report here a study, using cyclic voltammetry and FTIRS, of NO irreversibly adsorbed on a cyanide-modified Pt(111) electrode. NO adlayers were formed by immersion of the cyanide-modified Pt(111) electrode in an acidic solution of KNO(2). The behaviour of NO adsorbed on the cyanide-modified electrode is very similar to that of NO on the clean Pt(111) surface, suggesting that adsorbed cyanide (saturation coverage theta(CN) = 0.5) behaves simply as a third body, blocking some of the surface sites but leaving the free Pt sites unaffected. Comparison of the voltammetric profile for NO electroreduction on Pt(111) and on cyanide-modified Pt(111) electrodes has allowed us: (i) to confirm that the reduction of three-fold hollow NO and atop NO on Pt(111) electrodes occurs in two distinct reduction peaks, as previously proposed by Rosca et al. (Langmuir, 2005, 21, 1448); (ii) to suggest that the reduction of irreversibly adsorbed NO layers on Pt electrodes can proceed through two possible paths, one involving an EE mechanism in which the rate-determining step (rds) is an Eley-Rideal reaction, with a direct proton transfer from the solution to adsorbed NO, and the other involving an EC mechanism in which the rds is a Langmuir-Hinshelwood reaction of adsorbed NO with adsorbed H. The availability of adsorbed hydrogen determines which path is followed by the reaction; (iii) to identify the smallest atomic ensemble for the reduction of NO on Pt as being composed of two adjacent Pt atoms.