| 摘要: | 本研究利用掃描式電子穿隧顯微鏡( In-situ Scanning Tunneling Microscopy, STM )及循環伏安儀( Cyclic Voltammetry, CV )探討多層汞( bulk )在乾淨的及碘修飾的銥(111)、鉑(111)與銠(111)電極上之沈積過程。 銥(111)表面的成份能操控汞膜的型態，碘吸附層的存在使得汞在三維島狀物產生前以層狀方式( layer-by-layer )沈積約 7 層的薄膜；同時，高品質的 STM 原子圖像證明排列規則的碘原子層吸附於汞膜之上，因此汞膜應具一定之晶格結構。在乾淨的銥(111)上，汞同樣以層狀方式沈積但是只有兩層原子的厚度，且此汞薄膜會有高密度的坑洞，在此情況下，我們無法得到原子圖像。 在銥(111)電極上，吸附於多層汞薄膜上的碘原子形成高度規則的二維結構，其排列的方式視電位而定，因為碘的覆蓋度會隨電位逐漸增加。在220 mV時，碘的吸附層為 (Ö3 x Ö3)R30°，? = 0.33；在300 mV 時轉變為 (11 x Ö3-R30°)，? = 0.36；至 500 mV 時形成一最密的 (4 x 4) 結構，? = 0.44；在 550 mV 銥載體上的汞薄膜開始氧化並形成多層的碘化亞汞島狀特徵。另外，根據 STM 觀測的碘原子最密結構，其間距為 4.5 Å，故可反推汞原子大小為 3.1 Å。 鉑(111)電極上碘原子層導致前三層汞原子是以層狀方式沈積，之後汞沈積轉變成三維的方式長成奈米大小的汞滴。高解析之 STM 圖像顯示在 100 ~ 250 mV碘原子為 (Ö13 x Ö13)R13.9° 的結構；然而在250 mV汞膜已開始氧化，但 STM 並未發現有類似銥(111)上之碘化亞汞結構。在銠(111)上僅有一層的汞結構，隨後即以三維島狀物成長，這種粗糙的表面提高了原子圖像的困難度。綜合以上的結果，載體之化性主宰了汞沈積機制；汞沈積皆遵守S-K (Stranski- Krastanov)的成長模式，但平整汞膜的厚度應是控制於載體效應和汞原子與載體之間的化學鍵強度，故在銥(111)、鉑(111)及銠(111)上汞-金屬的鍵強可能依次遞減，因此汞薄膜之厚度為分別為 7 層、 3 層及 1 層原子大小。 我們將鉛鍍在以銥為載體的單層汞膜上，發現鉛可形成平整的鉛膜，其厚度約為 4 層原子，一平整的鉛膜可解釋在以汞薄膜分析重金屬剝除時，其靈敏度和滴汞電極相比有明顯的改善。至於我們利用陽極剝除法( Anodic Stripping Voltammetry, ASV )與較厚的汞膜電極來分析微量鉛金屬，結果顯示，鉛離子氧化剝除所產生的電流量和溶液中鉛離子的濃度成正比，鉛離子的濃度越高則產生的電流量越大。未來若能針對實驗過程稍加改良，例如：鍍汞的濃度與時間、鍍汞時的轉速、還原鉛離子的電位等，應該可以同時做到定性與定量分析。 In-situ Scanning Tunneling Microscopy (In-situ STM) and Cyclic Voltammetry (CV) are used to study the bulk deposition process of mercury on iodine-modified and bare Ir(111) electrodes. We also perform similar deposition experiments on Pt(111) and Rh(111) to investigate the effect of substrate on the electrodeposition of Hg films. The compositions of Ir(111) surface can dominate the morphology of Hg film. An iodine adlayer render layer-by-layer deposition of Hg till the 7th layer when 3D islands growth prevail. At the mean time, high-quality STM atomic resolution identifies a well-ordered iodine overlayer residing on top of the Hg film, strongly suggesting that the Hg film is crystalline. At a bare Ir(111), Hg deposition was also layer-by-layer but only up to two atomic layers. Without the iodine adlayer, the Hg film was however seriously pitted and it was not possible to achieve atomic resolution. The coverage and real-space structure of iodine adatoms on Ir(111)-supported Hg thin films vary with potential, similar to the “electrocompression” previously observed at Au(111). Making potential positively render a series of structures, starting with (Ö3 x Ö3)R30° , ? = 0.33, transforming to (11 x Ö3-R30°), ? = 0.36 at 0.3 V, and finally ends up with (4 x 4), ? = 0.44 at 0.5 V. This Ir-supported Hg film began to oxide to produce islands consisting of multilayers of Hg2I2. On the other hand, since iodine adatoms on Hg film is highly ordered and close-packed, the spacing between two nearest iodine atoms is 4.5 Å, from which the Hg interatomic spacing of 3.1 Å is deduced. The iodine overlayer on Pt(111) results in layer-by-layer deposition of Hg till the 3th layer, where 3D islands growth takes control to produce nanometer-scaled Hg drops. The iodine adlayer floating atop the Hg films is highly ordered, identified as (Ö13 x Ö13)R13.9° . However, Hg film begins to oxide at 0.25 V, which occurs anomalously 250 mV more negative than that of Ir(111). This oxidation event does not produce well packed Hg2I2 on Pt(111). Hg electrodeposited on Rh(111) is uniform for only one layer before 3D islands growth predominates. It’s not possible to achieve atomic resolution on this system. In summary, the identity of substrates dominates the mechanisms of Hg electrodeposition, as Hg form uniform films amount to 7, 3 and 1 layer thick on Ir(111), Pt(111) and Rh(111), respectively. Electrodeposition of Pb on Ir-supported monolayer Hg films has been examined with STM. Pb thin films are atomically smooth up to a thickness of about 4 layers. This result suggests that the resolution of Hg thin film electrodes used for heavy metal stripping analysis can produce higher resolution than traditional mercury drop electrode, because Pb adatoms tend to stay at the surface of Hg films. As the use of anodic stripping voltammetry (ASV) and thicker Hg film in analyzing Pb in aqueous samples, the results showed the peak current of Pb oxidative stripping was directly proportional to the Pb2+ concentration in the solution. Further improvement of this method include optimization of deposition, the concentration and time of deposited Hg, the rotating speed in depositing solution, and potential of reduced Pb2+. |
