单层 FeO 薄膜表面周期性氧缺陷结构的形成

 采用扫描隧道显微镜和 X 射线光电子能谱对含有次表层 Fe 的 Pt 表皮结构 (Pt skin), 即 0.4 ML Fe 的 Pt/Fe/Pt(111) 表面, 在 1.1 × 10?7 kPa 氧气气氛退火过程中的变化进行了研究. 结果表明, 当退火温度为 600 K 时, 氧气在 Pt/Fe/Pt(111) 表面上解离吸附并诱导表面局域结构的重构; 750 K 时次表层 Fe 可以扩散到表面并被氧化; 当升至 850 K 时, 在样品表面形成单层 FeO 结构, 并且 FeO 表面具有周期性的缺陷. 这种缺陷是由于单层 FeO 薄膜的摩尔条纹单胞中 fcc 位上一个或多个氧原子缺失形成的, 其中多原子空位被确定为缺失 6 个氧原子所致. FeO 表面缺陷结构的研究为理解 Fe-Pt 催化剂在氧化气氛中的结构稳定性以及构造表面活性位提供一定的基础.     

CeO2(111)和CeO2(100)的界面调控合成及在Pt(111)上的结构转变

氧化铈基催化材料在催化反应中存在显著的晶面效应,为了在分子尺度上理解其催化化学,需要可控合成具有明确表面结构的氧化铈.因此,我们研究了Pt(111)上氧化铈纳米结构和薄膜的生长.人们通常使用金属-氧化物之间的强相互作用来解释Pt/CeO_x催化剂上的催化过程,然而对于Pt与CeO_x之间的强相互作用仍旧缺乏原子尺度上的了解.我们的结果表明, Pt与氧化铈之间的相互作用可以影响氧化铈的表界面结构,这可能会进而影响Pt/CeO_x催化剂的性质.在Pt(111)上生长的氧化铈薄膜通常暴露CeO_2(111)表面.我们发现Pt(111)表面厚度在三层以内的氧化铈薄膜,其结构是高度动态且随着退火温度升高而变化的,这种动态结构变化可归因于Pt和氧化铈间的界面电子作用.当氧化铈薄膜的厚度增大到三层以上,其负载的氧化铈团簇开始表现出迥异于三层以下氧化铈纳米岛的优异的热稳定性,表明Pt与CeO_x之间的界面电子作用主要影响厚度在三层以内的氧化铈纳米结构.采用常规的反应沉积方法难以获得完全覆盖Pt(111)衬底的规整氧化铈薄膜,而我们通过采取一种两步的动力学限制生长方法,制备出了完全覆盖Pt(111)衬底的氧化铈薄膜.对于Pt(111)上厚度约为3-4层的氧化铈薄膜,在超高真空中于1000 K退火会导致氧化铈薄膜表面形成CeO_2(100)结构.这是因为高温还原促进了c-Ce_2O_3(100)缓冲层的形成,该缓冲层被Pt的界面电子转移以及相匹配的超晶格所稳定,并进一步成为顶层CeO_2(100)结构生长的模板.进一步在900 K的氧气中处理则可将薄膜CeO_2(100)表面完全转变为CeO_2(111)表面.因此, Pt(111)上氧化铈纳米岛和薄膜所展现的结构动态变化是由Pt-CeO_x界面作用与氧化铈层间作用相互竞争所决定.本研究提供了对氧化铈负载Pt催化剂的原子级理解,虽然Pt/CeO_2催化剂活性增强的原因常被简单归结于界面强相互作用,我们的研究在原子尺度上进一步表明Pt/CeO_2在还原条件下易形成界面Ce_2O_3层.此外,本研究提供了不同晶面二氧化铈模型催化剂的构筑方法,可将对氧化铈晶面效应和Pt/CeO_x催化剂的研究推进到分子尺度.     

铈基模型催化剂的纳米结构:有序CeO2(111)薄膜上的Pt-Co纳米颗粒

制备并表征了原子分散的模型体系:氧化铈负载的Pt-Co核壳催化剂.采用超高真空物理气相沉积法制备了有序CeO2(111)膜上的Pt@Co和Co@Pt核壳纳米结构,并用同步辐射光电子能谱和共振光发射光谱对其进行了研究.在低Co覆盖率(0.5 ML)下Co在CeO2(111)上沉积生成Co-CeO2(111)固溶体,然后在更高Co覆盖率下生长为金属Co纳米粒子.Pt@Co和Co@Pt两种模型结构在300-500 K温度范围内都能稳定地抗烧结.在500 K退火后, Pt@Co纳米结构含有接近纯的钴壳,而Co@Pt中的铂壳部分被金属钴覆盖.在550 K以上,在Pt@Co和Co@Pt纳米结构中近表面区域的重新排序中产生了次表层的Pt Co合金和富铂外壳.对于Co@Pt纳米粒子,近表面区域的化学有序性取决于沉积铂壳的初始厚度.无论初始铂壳的厚度如何,在有氧存在下对Co@Pt纳米结构进行退火,都会导致Pt-Co合金的分解以及Co的氧化.Co的逐步氧化与吸附质诱导的Co偏析共同导致在负载的Co@Pt纳米结构表面形成厚的Co O层.这一过程伴随着CeO2(111)薄膜的裂解,以及在550K以上氧气中退火后CeO2包裹氧化的Co@Pt纳米结构.很明显,于不同温度下在氧气和氢气的氧化-还原循环过程中,无论铂的初始厚度是多少,负载的Co@Pt纳米颗粒的结构和化学成分的变化主要是由氧化所致,而还原处理的影响则很小.     

铈基模型催化剂的纳米结构:有序CeO_2(111)薄膜上的Pt-Co纳米颗粒（英文）

制备并表征了原子分散的模型体系:氧化铈负载的Pt-Co核壳催化剂.采用超高真空物理气相沉积法制备了有序CeO_2(111)膜上的Pt@Co和Co@Pt核壳纳米结构,并用同步辐射光电子能谱和共振光发射光谱对其进行了研究.在低Co覆盖率(0.5 ML)下Co在CeO_2(111)上沉积生成Co-CeO_2(111)固溶体,然后在更高Co覆盖率下生长为金属Co纳米粒子.Pt@Co和Co@Pt两种模型结构在300-500 K温度范围内都能稳定地抗烧结.在500 K退火后, Pt@Co纳米结构含有接近纯的钴壳,而Co@Pt中的铂壳部分被金属钴覆盖.在550 K以上,在Pt@Co和Co@Pt纳米结构中近表面区域的重新排序中产生了次表层的Pt Co合金和富铂外壳.对于Co@Pt纳米粒子,近表面区域的化学有序性取决于沉积铂壳的初始厚度.无论初始铂壳的厚度如何,在有氧存在下对Co@Pt纳米结构进行退火,都会导致Pt-Co合金的分解以及Co的氧化.Co的逐步氧化与吸附质诱导的Co偏析共同导致在负载的Co@Pt纳米结构表面形成厚的Co O层.这一过程伴随着CeO_2(111)薄膜的裂解,以及在550K以上氧气中退火后CeO_2包裹氧化的Co@Pt纳米结构.很明显,于不同温度下在氧气和氢气的氧化-还原循环过程中,无论铂的初始厚度是多少,负载的Co@Pt纳米颗粒的结构和化学成分的变化主要是由氧化所致,而还原处理的影响则很小.     

预处理条件对Pt/Al2O3催化还原NO的活性影响

用溶胶-凝胶法制备Al2O3载体,浸渍法制备质量分数为0.5%的Pt/Al2O3催化剂.研究焙烧气氛和温度对选择性催化还原NO反应活性和Pt价态的影响.结果表明,H2焙烧的活性温度区间最宽,随着焙烧温度的升高,温度区间变化很小,523K下O2焙烧的催化剂活性最好,且活性区间向高温方向移动.活性现象用程序升温脱附实验(NO-TPD,NO-O2-TPD)进行了解释.XPS研究表明,523K下O2用焙烧Pt的主要价态是Pt2+,而523K下H2和N2焙烧Pt的主要价态为Pt0.     

CO2加氢活性与担载铁催化剂表面Fe—O键强度的关系

采用浸渍－还原法,制备了AI2O3、TiO2、ZrO2担载铁催化剂,并在温度623K、压力1．5MPa、空速800－1000h^-1、原料气组成CO2／H2为1：2等实验条件下,考察了不同还原温度的催化剂在CO2加氢制低碳（C2－C5）烃中的反应活性。结果表明,以CO2转化率计,各催化剂均存在最佳还原温度（Fe/AI2O3:873K;Fe/TiO2:77K;Fe/ZrO2:723K)。将此温度与催化剂表面Fe-O键强度相关联,发现在相同条件下,与载体M－O键长有关的Fe-O键越强,催化剂越容易被还原,最佳还原温度越低,反应活性越好。     

FeO/Pt(111)与FeO2/Pt(111)的几何、电子结构及表面氧活性的第一性原理研究

采用密度泛函理论系统研究了超薄氧化物膜/金属体系FeO/Pt和FeO₂/Pt及其表面不同区域(FCC,HCP和TOP)的几何结构、电子性质及氧的活性.研究发现,表面O-Fe高度差δ_z作为一个重要的特征结构参数直接影响局域表面静电势和表面氧的结合能: δ_z越大,静电势越大,氧的结合能越弱.计算发现,在FeO/Pt体系中,δ_z顺序为FCC > HCP > TOP,而FeO₂/Pt中是FCC > TOP > HCP.此外,在FeO/Pt中,电荷转移方向是从氧化物膜到衬底,Fe的表观价态为+2.36,表面功函较纯Pt(111)的变化可忽略; 而FeO₂/Pt中,电荷转移的方向是从衬底到氧化物,Fe的表观价态为+2.95,表面功函较纯Pt增加1.24 eV.进一步分析了电荷转移和表面偶极对电子性质的作用机制.这些研究结果对于认识超薄氧化物薄膜对表面几何结构、电子性质、表面氧活性的调制具有重要的启示意义.     

单原子分散的Au/Cu(111)表面合金的表面结构与吸附性质

Au-Cu双金属合金纳米颗粒对包括CO氧化和CO2还原等在内的多个反应有较好的催化活性,然而关于其表面性质的研究却相当匮乏。在此工作中,我们通过对低覆盖度的Au/Cu(111)和Cu/Au(111)双金属薄膜退火,制备出了单原子级分散的Au/Cu(111)和Cu/Au(111)合金化表面,并利用高分辨扫描隧道显微镜(STM)和扫描隧道谱(STS)进一步研究了掺杂原子的电子性质及其对CO吸附行为的影响。研究发现,分散在Cu(111)表面的表层和次表层Au单原子在STM上表现出不同衬度。在-0.5 e V附近,前者表现出相较于Cu(111)明显增强的电子态密度,而后者则明显减弱。吸附实验表明表层Au单原子对CO的吸附能力并没有得到增强,甚至会减弱其周围Cu原子的吸附能力。与Au在Cu(111)表面较好的分散相反,Cu原子倾向于钻入Au(111)的次表层,并且形成多原子聚集体。且Cu原子受Au(111)衬底吸电子作用的影响,其对CO的吸附能力明显减弱。这个研究结果揭示了合金表面的微观结构与性质的关联,为进一步阐明Au-Cu双金属催化剂的表面反应机理提供参考。     

PtRu合金薄膜结构及其催化性能

采用离子束溅射技术(IBS)在碳纤维布基底上制备PtRu/C合金薄膜作为燃料电池电极催化材料. 应用XPS、XRD、GIXD、AFM等分析手段研究了PtRu薄膜表面的成分、化学状态、表面形貌以及PtRu薄膜的表层、次表层和体相的结构. 结果表明, 在双束离子沉积过程中, 由于溅射产生的Pt+和Ru+之间的相互作用, 使薄膜表面的化学状态和薄膜表层(15-40 nm范围内)结构发生了变化, 并影响PtRu薄膜的催化性能. 当xPt/xRu=0.64时, PtRu薄膜出现Ru固溶体在表层富集, 并在表层诱发形成Pt39Ru61非晶相.     

酸洗Pt-Fe和Pt-Co催化剂在CO氧化反应中的对比研究

负载型纳米催化剂表面结构与其催化性能之间关系的研究一直受到广泛关注.由于其结构复杂使得人们在研究催化剂构效关系时遇到了很多困难.近年来,大量研究发现反转催化剂在众多反应中表现出优越的催化性能.反转催化剂是将过渡金属氧化物负载于其它金属表面.和传统金属/氧化物催化剂相比,反转催化剂更能突出氧化物在催化反应中的重要作用.众多研究表明,在氧化物-金属界面处存在特殊的作用,这种作用可以改变氧化物的电子特性和化学性质,进而产生较高的催化性能.傅强等人创建了金属氧化物负载于Pt表面的反转催化体系,其表现出了高的低温CO氧化反应性能.在氧化物和Pt之间的界面限域效应可以稳定氧化物中配位不饱和的金属阳离子.这种配位不饱和的氧化物提供了活化O2的活性位.目前,反转催化剂的研究主要集中在单晶模型体系中,在负载型催化剂中的研究还较少.我们以炭黑(CB)为载体,将还原后的Pt-Fe和Pt-Co催化剂经过酸洗制备了一种表面富Pt核为合金的结构.考察了酸洗后的Pt-Fe和Pt-Co催化剂经过不同温度氧化后的结构变化,并讨论了其结构与CO完全氧化反应(COOX)和CO选择氧化反应(CO-PROX)性能的关系.X射线粉末衍射(XRD),电感耦合等离子体发射光谱(ICP),透射电镜(TEM)和X射线光电子能谱(XPS)表征结果表明,还原后的Pt基催化剂经过酸洗可以选择性去除纳米粒子表面的3d过渡金属,形成表面富Pt体相为合金的结构.将酸洗后的Pt-Fe和Pt-Co催化剂在不同温度下空气中氧化,发现近表层的Fe(Co)会扩散到粒子表面上,形成过度氧化的Fe2O3(Co3O4)表面结构.氧化后的催化剂在COOX和CO-PROX反应中表现出截然不同的催化性能.酸洗后的Pt-Fe(Pt-Co)催化剂经过不同温度氧化后在COOX反应中活性都较差,室温下的CO转化率只有不到30%,CO完全转化的温度超过100oC,相当于纯Pt催化剂的活性.这说明Pt表面过度氧化的Fe2O3(Co3O4)对CO氧化反应的促进作用不明显.而氧化后的催化剂在CO-PROX反应中表现出较高的活性,250oC(或350oC)氧化后的酸洗Pt-Fe催化剂室温下的CO转化率接近100%,250oC(或350oC)氧化后的酸洗Pt-Co催化剂室温下的CO转化率也达到了70%.结合表征和反应结果,我们认为氧化处理形成的表面过度氧化的金属氧化物(Fe2O3,Co3O4)在COOX的催化性能较差.通入CO-PROX反应气后,气氛中大量H2的存在和Pt表面的氢溢流效应可以使得表面Fe2O3,Co3O4在室温下被还原成配位不饱和的FeO,CoO.这种配位不饱和的氧化物在表面Pt的限域作用和大量H2气氛下比较稳定,并且具有较强的活化解离O2的能力,进而提高了CO-PROX反应的活性.为了进一步证实催化剂表面氧化物与其催化性能的关系,我们在室温下进行了两种反应气的循环实验测试.测试结果表明,对于氧化后的酸洗Pt-Fe催化剂,COOX反应中的表面Fe2O3和CO-PROX反应中的表面FeO可以通过变换反应气氛实现两种氧化物的相互转变,并表现出完全不同的催化性能.对于氧化后的酸洗Pt-Co催化剂,CO-PROX反应中形成的CoO表面结构在COOX反应中也比较稳定,在两种反应气中表现出相似的催化性能.     

Ultrathin TiO(x) films on Pt(111): a LEED, XPS, and STM investigation

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.     

窄带隙IV-VI族半导体PbTe(111)的表面氧化及氧的热脱附机理

利用X射线光电子能谱(XPS)、扫描隧道显微镜(STM)以及低能电子衍射(LEED),对PbTe(111)薄膜的表面氧化及氧的热脱附机理进行了研究.结果表明:PbTe(111)薄膜经500VAr+轰击加上250℃高温退火循环处理,可得到呈(1×1)周期性排列的清洁表面.将此清洁表面暴露于大气两天后,表面被氧化形成了PbO2、PbO和TeO2,氧化层的厚度大于2个单原子层(ML),与清洁PbTe(111)表面相比,被氧化的PbTe(111)表面的Te3d5/2与Pb4f7/2芯态谱峰的面积比明显减小,表明被氧化的PbTe(111)表面是富Pb的.在热脱附处理过程中,PbO2和TeO2的芯态峰消失,且O1s芯态峰的强度迅速减弱,表明加热处理不仅使PbO2和TeO2发生了分解,同时也使氧发生了脱附,但PbO即使在350℃退火仍吸附于PbTe(111)表面.     

Epitaxial BaTiO3(100) films on Pt(100): a low-energy electron diffraction, scanning tunneling microscopy, and x-ray photoelectron spectroscopy study

The growth of epitaxial ultrathin BaTiO(3) films on a Pt(100) substrate has been studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The films have been prepared by radio-frequency-assisted magnetron sputter deposition at room temperature and develop a long-range order upon annealing at 900 K in O(2). By adjusting the Ar and O(2) partial pressures of the sputter gas, the stoichiometry was tuned to match that of a BaTiO(3)(100) single crystal as determined by XPS. STM reveals the growth of continuous BaTiO(3) films with unit cell high islands on top. With LEED already for monolayer thicknesses, the formation of a BaTiO(3)(100)-(1 × 1) structure has been observed. Films of 2-3 unit cell thickness show a brilliant (1 × 1) LEED pattern for which an extended set of LEED I-V data has been acquired. At temperatures above 1050 K the BaTiO(3) thin film starts to decay by formation of vacancy islands. In addition (4 × 4) and (3 × 3) surface reconstructions develop upon prolonged heating.     

Adsorption of oxygen on potassium-covered Pt(111) and Fe(110)

Photoemission measurements (UPS, XPS) show that potassium oxide is not formed when adsorbing oxygen on potassium-covered Pt (111) and Fe(110). Two oxygen states are observed. One is chemisorbed oxygen on the surface. The other can be interpreted as being due to either incorporated atoms or oxide.     

Scanning tunneling microscopy studies of pulse deposition of dinuclear organometallic molecules on Au(111)

Ultrahigh-vacuum scanning tunneling microscopy (STM) was used to study trans-[Cl(dppe)2Ru(C Triple Bond C)6Ru(dppe)2Cl] [abbreviated as Ru2, diphenylphosphinoethane (dppe)] on Au(111). This large organometallic molecule was pulse deposited onto the Au(111) surface under ultrahigh-vacuum (UHV) conditions. UHV STM studies on the prepared sample were carried out at room temperature and 77 K in order to probe molecular adsorption and to characterize the surface produced by the pulse deposition process. Isolated Ru2 molecules were successfully imaged by STM at room temperature; however, STM images were degraded by mobile toluene solvent molecules that remain on the surface after the deposition. Cooling the sample to 77 K allows the solvent molecules to be observed directly using STM, and under these conditions, toluene forms organized striped domains with regular domain boundaries and a lattice characterized by 5.3 and 2.7 A intermolecular distances. When methylene chloride is used as the solvent, it forms analogous domains on the surface at 77 K. Mild annealing under vacuum causes most toluene molecules to desorb from the surface; however, this annealing process may lead to thermal degradation of Ru2 molecules. Although pulse deposition is an effective way to deposit molecules on surfaces, the presence of solvent on the surface after pulse deposition is unavoidable without thermal annealing, and this annealing may cause undesired chemical changes in the adsorbates under study. Preparation of samples using pulse deposition must take into account the characteristics of sample molecules, solvent, and surfaces.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Comparing Ullmann Coupling on Noble Metal Surfaces: On‐Surface Polymerization of 1,3,6,8‐Tetrabromopyrene on Cu(111) and Au(111)

The on‐surface polymerization of 1,3,6,8‐tetrabromopyrene (Br₄Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br₄Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C?Cu?C bonds. After annealing at 473 K, the C?Cu?C bonds were converted to covalent C?C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self‐assembled two‐dimensional (2D) patterns stabilized by both Br?Br halogen and Br?H hydrogen bonds were observed upon deposition of Br₄Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C?Br bonds and the formation of disordered metal‐coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br₄Py on the different substrates.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Interface-controlled synthesis of CeO2(111) and CeO2(100) and their structural transition on Pt(111)

Ceria-based catalytic materials are known for their crystal-face-dependent catalytic properties. To obtain a molecular-level understanding of their surface chemistry, controlled synthesis of ceria with well-defined surface structures is required. We have thus studied the growth of CeO_x nanostructures (NSs) and thin films on Pt(111). The strong metal-oxide interaction has often been invoked to explain catalytic processes over the Pt/CeO_x catalysts. However, the Pt-CeO_x interaction has not been understood at the atomic level. We show here that the interfacial interaction between Pt and ceria could indeed affect the surface structures of ceria, which could subsequently determine their catalytic chemistry. While ceria on Pt(111) typically exposes the CeO₂(111) surface, we found that the structures of ceria layers with a thickness of three layers or less are highly dynamic and dependent on the annealing temperatures, owing to the electronic interaction between Pt and CeO_x. A two-step kinetically limited growth procedure was used to prepare the ceria film that fully covers the Pt(111) substrate. For a ceria film of ~3–4 monolayer (ML) thickness on Pt(111), annealing in ultrahigh vacuum (UHV) at 1000 K results in a surface of CeO₂ (100), stabilized by a c-Ce₂O₃(100) buffer layer. Further oxidation at 900 K transforms the surface of the CeO₂(100) thin film into a hexagonal CeO₂(111) surface.     

Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.