| 摘要: | 摘要 本研究利用掃描式電子穿隧顯微鏡(STM)和循環伏安法(Cyclic Voltammetry, CV)，研究鹵素吸附在汞薄膜電極上的結構。汞薄膜電極是以電解的方式，將汞離子還原後電鍍在高規則度的銥(111)電極上，以此過程所通過的電量計算，估計約有30層汞原子組成一薄膜，此類薄膜的電化學特性並不會隨膜厚而有所改變。 汞薄膜電極在0.1 M過氯酸和硫酸中顯現相同的CV特色，例如在0 V (vs. Ag/AgCl)附近有一對高度對稱的氧化還原波，可能來自於陰離子(過氯酸根及硫酸根)的吸附與脫附。在10 mM的鹵化鉀中，所有的CV均顯現兩組特徵，分別為在負電位時的鹵素吸附與脫附，和較正電位時鹵化亞汞的形成與還原。從各個CV圖形中，較正電位的特色所含之電荷量計算，除了在氟化鉀中不會形成氟化亞汞薄膜外，在其他鹵化鉀中會形成約8 ? 9層的鹵化亞汞薄膜。 STM (Scanning Tunneling Microscope)結果顯示，汞薄膜具有典型固態電極的台階與平台的表面特色，但也具有液態金屬的高移動性，因此我們無法得到一穩定的高解析STM圖像，但在含特異性吸附的鹵素離子(氯、溴、碘)溶液中，這三種離子的吸附，可導致原子的圖像。同時發現汞薄膜，雖然其厚度可能有30個原子層，但其表面型態與銥(111)載體是相關的，因為STM結果顯示汞薄膜表面的台階方向和平台大小與銥(111)載體相同。 一系列連續變化的STM影像顯示碘離子的吸附過程，碘離子先從台階下緣吸附，再擴展到平台。除了弱吸附的氟離子外，其它鹵素離子都能得到原子級的STM影像。碘離子的吸附有兩種矩形的結構，其覆蓋度為0.52和0.61，電位分別在-150和0 mV。溴離子與氯離子的吸附結構接近六方形，而晶格大小也類似它們的凡得瓦直徑(分別為3.6和3.4 Å)。STM結果顯示氯離子的吸附具明顯的水合現象，在較負電位時，氯的陰離子特性增加，導致鉀離子的共吸附。從STM結果得到碘化亞汞生成的電位在200 mV，在局部區域堆疊並產生具晶格結構的薄膜，最後碘化亞汞的堆疊使得電極表面粗糙化。 在10 mM氟化鉀中，汞的氧化並無氟化亞汞產生。STM影像顯示在0.15 V時汞會有溶解的現象。推測汞會溶解形成亞汞離子，由於亞汞離子在酸性中很穩定，因此不會自身氧化還原而形成汞原子與汞離子。在汞大量溶解後，從STM影像仍然可觀察到良好的台階與平台。當電位在-650 mV(比PZC更負)時，我們觀察到一二維結構。雖然影像無法很清楚顯示在真實空間的細部結構，但由於在此電位氟及鉀離子應不會和汞電極作直接的接觸吸附，他們應以水合離子的型態吸附，這可能是第一次以STM直接觀察到金屬表面上的水分子層。 Abstract The electrified interfaces of mercury electrodes potentiostatically deposited onto a well-ordered Ir(111) electrode has been examined with cyclic voltammetry and in situ scanning tunneling microscopy (STM). A thickness of 30 monolayer of the mercury film is estimated from the amount of charges passed during the reduction of Hg2+. This filmy Hg electrode exhibits similar electrochemical behavior as that of bulk Hg. The cyclic voltammograms (CV) obtained in 0.1 M perchloric and sulfuric acid solutions are alike. The CV profile is essentially featureless between –0.5 and 0 V (vs. Ag/AgCl), which is ascribed to be the double-layer charging region. Adsorption of anions, such as perchlorate and sulfate, give rise to a slight increase of current at potential positive of 0 V. In contrast, the cyclic voltammograms, obtained in solutions containing 10 mM potassium halide, exhibit two pairs of features, ascribable to the adsorption/desorption of halides and the formation/stripping of mercury(I) halide (Hg2X2) compounds. The potential regions over which these processes occur vary with the strength of the surface bonding between halide and Hg and the standard potential of Hg2X2. They appear at most negative potential for iodide, followed by bromide and chloride, and finally fluoride. Coulometric results integrated from the reduction wave of the Hg2X2 films (X = I-, Br-, Cl-) indicate the films are 8-9 layers thick, while no such surface film forms in potassium fluoride. Although it is rather difficult, if not impossible, to image bulk Hg with STM in solutions, we are able to achieve STM atomic resolution of the as-prepared Hg films covered with specifically adsorbed halide. It is remarkable to note that the Hg films exhibit typical terrace-and-step characteristics of a solid, although bulk Hg is known to be a liquid metal at room temperature. It is also surprising to see that the surface morphology of the Hg films bear a strong resemblance to that of the Ir(111) substrate. Because the orientations of steps and breath of the terraces are essentially identical to those of the Ir(111) substrate, it seems that the Hg films, despite being as thick as 30 atomic layers, grow under the guidance of the substrate. On the other hand, the morphological features changes rapidly with time, suggesting that the Hg atoms are rather mobile at the potential of zero charge (PZC) in perchloric acid solution. The high-resolution STM scans are always noisy, we were not able to achieve atomic resolution of the bare Hg surfaces under these conditions. Time-sequenced STM images are acquired to show the adsorption events of iodide anions, preferentially starting at the lower edges of steps and then creeping onto the terraces. This process proceeds in a nucleation-and-growth, rather than random fashion, suggesting the attractive interaction of the adsorbed halides. STM atomic resolutions are acquired for all the halide, except fluoride because it binds weakly with Hg. Despite adsorbing in ordered superlattices, they are mostly incommensurate. Iodide anions are adsorbed in two unexpected square-like arrays with coverage of 0.52 and 0.61 at potential of –150 and 0 mV, respectively. In contrast, the adlattices of bromide and chloride anions are mostly hexagonal with lattice constants resembling their van der Waals diameters (3.6 and 3.4 Å for Br- and Cl-, respectively). The results of chloride adlayers suggest the possibility of surface hydration, along with the coadsorption of potassium cations, as the anionic character of the chloride increases at more negative potential. STM atomic resolutions were also obtained for the Hg2I2 species formed at potential positive of 200 mV. It grew locally in a layered and crystalline fashion, leading to substantial roughening of the surface morphology. Oxidation of Hg in 10 mM potassium fluoride does not result in crystalline Hg2F2. Rather, real-time STM imaging of mercury films reveals pronounced dissolution of metallic Hg at potential positive of 0.15 V. Presumably, Hg dissolves as Hg22+ cations, which do not seems to undergo disproportionation to Hg and Hg2+. The Hg22+ cations are likely to be stable in acidic solutions. STM imaging of the Hg electrode after extensive dissolution of Hg reveals still well-defined steps and terraces without noticeable precipitation. Furthermore, we were able to obtained near atomic resolution of the water layers formed at -650 mV more negative than the PZC. Although the images are not clear to decipher the real-space structures of water molecules, tentatively, this is the first direct view of a water layer on metal surfaces. |
