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。

Atomic Structure and Adsorption Property of the Singly Dispersed Au/Cu(111) Surface Alloy

Atomic-scale characterization of the atomic structure as well as molecular adsorption on an alloy surface plays a vital role in elucidating the catalytic mechanism of effective catalysts. Au-Cu alloy nanoparticles have important applications in catalyzing CO oxidation and CO₂ reduction. However, the atomic-scale properties of Au-Cu alloy surfaces are rarely investigated. In particular, the physical and chemical properties of singly-dispersed doping atoms, either Au in Cu or vice versa, as well as their influence on the overall surface properties, have not been studied in detail. In response, we first prepared low-coverage bimetallic Au/Cu(111) and Cu/Au(111) films, which were then annealed at high temperature to realize single atomically-dispersed Au/Cu(111) and Cu/Au(111) surface alloys (SA). We characterized the surface structures and adsorption properties by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). For the SA-Au/Cu(111) system, we found that Au atoms can be incorporated in both the skin and subsurface layer of the Cu(111) substrate. These species can be readily distinguished from the topography contrast in STM. Moreover, STS measurements showed clear differences between the electronic states of doped Au atoms and the Cu host. In particular, we found that Au in the skin layer was strengthened while the subsurface Au showed weakened filled states at approximately −0.5 eV compared with the Cu(111) surface, which corresponds to the characteristic Shockley state of an Au surface. These altered electronic properties at the sites of doped atoms are also reflected by changes in the interactions with probe molecules. Adsorption experiments showed that Au atoms in the top surface prevented the binding of CO molecules, causing various adsorption vacancies in the CO adlayer. In contrast, the subsurface Au atoms had little influence on surface binding with CO molecules. For the SA-Cu/Au(111) system, we found that Cu atoms tend to aggregate into small clusters in the subsurface region of the Au(111) substrate. Only few Cu atoms can be stabilized at the elbow positions of the reconstructed top surface of Au(111). Adsorption experiments showed that only Cu atoms in the skin layer can adsorb CO molecules at liquid nitrogen temperature, while the subsurface Cu atoms cannot. On the other hand, the Au atoms around the doped Cu atoms do not seem to be influenced at all, possibly because of the weak effect of Cu. These experimental results provide details on the atomistic aspects of Au-Cu alloy surfaces, which can improve our understanding of the catalytic mechanism of Au-Cu alloy catalysts.