| 摘要: | 本研究結合了掃描穿遂顯微鏡(Scanning Tunneling Microscopy, STM)及循環伏安法(Cyclic voltammetry, CV)觀察在含硫酸及鹽酸溶液中，銅在鉑(111)及銠(111)電極的低電位沉積(Underpotential Deposition, UPD)。在純的硫酸中以1 mV/s掃描速率所得的CV結果顯示，銅在鉑(111)電極的UPD分成兩個階段，導致在0.65和0.61 V有明顯的電流變化；在銠(111)電極上卻只有一清楚的特徵峰於0.44 V，這些巨觀的電化學特色可用原子解像STM直接觀察。就鉑(111)而言，這些銅的UPD特色對應於電鍍銅所產生的兩個原子結構，他們是(√3 × √3)R30°以及一不規則的結構，但在形成多層銅之前，STM顯示一排列良好的(√3 × √7)硫酸根結構，意味著銅在鉑上形成一(1 × 1)的結構。對銠(111)電極而言，無論銅沉積量的多寡，最上層的硫酸根皆以(√3 × √7)的結構吸附在電極表面；我們取得了一系列隨時間變化的STM圖像用以說明銅的UPD過程，初期銅是在台階上方開始沈積並快速的長成島狀物，這些特色逐漸融合形成一均勻的銅吸附層。在所有的電鍍過程中，硫酸根總是在最上層覆蓋整個表面，而沉積的銅原子被夾在鉑(111)或銠(111)電極和硫酸根之間。在大量沈積銅的電位下，有4-5層銅沉積在銠(111)電極後STM顯示表面仍有(√3 × √7)的結構，而其外觀和硫酸根吸附於銅(111)類似。 在硫酸溶液中加入少量的氯離子造成了鉑(111)電極上銅UPD的劇大變化，兩個UPD特徵峰的間距由50增加到200 mV，較正的UPD峰向正移100 mV，而較負的UPD峰向負移100 mV，這些CV結果顯示氯離子使得銅更易被還原。於0.7 V左右(第一根UPD特徵峰時)，STM顯示一(4 × 4)的結構，它應是吸附在銅層上面的氯離子所造成，雖然STM沒有顯示銅原子的結構，但它很有可能也是形成一間距為0.37 nm的(4 × 4)結構，此一原子的排列方式和 CuCl的(111)面結構相同；於0.65 V（第一UPD特徵峰結束），氯的結構轉變為(√7 × √7)R19.1°，由於高解像的STM結果顯示在此結構下有一六方排列的銅原子層，其間距為0.28 nm，因此這樣的變化是由更多銅沉積所造成；當電位降至0.45 V（第二UPD特徵峰結束），氯保持(√7 × √7)R19.1°的結構，但STM結果顯示銅原子的間距縮短為0.26 nm，因此第二UPD特徵峰應是於銅的繼續沉積，而非氯的脫附，這些STM結果對爭論多時的銅UPD現象提供了重要的訊息。 This work reports in situ scanning tunneling microscopy (STM) results of underpotential deposition (UPD) of copper at well-ordered Pt(111) and Rh(111) electrodes in sulfuric acid solution. With a 1 mV/s scan rate cyclic voltammograms of Pt(111) reveal two well-defined UPD peaks at 0.65 and 0.61 V,while only one UPD peak at 0.44 V is observed for Rh(111). Real-time STM imaging along the UPD processes of Cu on Pt(111) reveal that these features correspond to the formation of a (√3 ×√3)R30°structure and a disorder phase. The entire surface was covered with an (√3 × √7)oblique structure at the onset of bulk deposition. These STM results are tentatively assigned to the (bi)sulfate anions coadsorbed with Cu adatoms sandwiched between the Pt electrode and sulfate anions. In contrast, a monolayer of Cu is plated in a single step on Rh(111), which was always accompanied by (bi)sulfate anions arranging in a well-ordered ( √3 × √7)oblique structure. A series time-dependent in situ STM images were acquired to unveil the deposition processes of Cu, which clearly show the initiation of deposition at defect sites, particularly upper step ledges, and the subsequently rapid lateral growth of Cu islands to cover nearly the whole surface. Decreasing potential to the bulk deposition region led to the formation of local Cu islands with a thickness of 4-5 layers of Cu, on which a well ordered (√3 × √7)oblique structure was observed. All the STM results indicate that sulfate anions actively participated in the UPD processes of Cu at these two electrodes. The contrast between these two electrodes illustrates the important roles of anions and interfacial forces existing among the electrodes, adatoms and anions. The cyclic voltammograms for well-ordered Pt(111) electrodes in 0.05 M HCl + 1 mM Cu(ClO4)2 show a marked effect of chloride on UPD of Cu, as two CV feature are total resolved .In situ STM imaging at 0.7 V(1st UPD peak),indeed indicate the formation of an incommensurate (4 × 4) structure, presumably due to the upper chloride layer, whereas the lower Cu layer remained hidden. At 0.65V,in situ STM revealed (√7 × √7)R19.1°. Because this structure appeared at the end of the 1st peak, the structural transformation from (4 × 4) to (√7 ×√7)R19.1°was induced by deposition of more Cu atoms. Changing imaging condition led to revelation of a hexagonal array with a lattice constant of 0.28 nm, attributable to the lower Cu layer.In situ STM imaging at 0.45 V still revealed a well-ordered (√7 × √7)R19.1°-Cl adlayer, but a hexagonal array with a lattice constant of 0.256 nm.The 2nd peak could be continuous deposition of Cu, resulting in from a pseudomorphoric to a close-pack(1 × 1) structure. |
