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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/6372

    Title: 利用即時掃描式電子穿隧顯微鏡和高能電子繞射研究汞膜電鍍在鉑(111)和銥(111)單晶電極上的結構;In-Situ Electrochemical STM and RHEED Study of Mercury Films Electrodeposited on Pt(111) and Ir(111) Single Crystal Electrodes
    Authors: 吳恆良;Heng-Liang Wu
    Contributors: 化學研究所
    Keywords: 電鍍;鉑(111);銥(111);即時掃描式電子穿隧顯微鏡;高能電子繞射;汞膜;RHEED;Hg;Pt(111);Ir(111);in-situ stm
    Date: 2008-06-20
    Issue Date: 2009-09-22 10:18:25 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 掃描式電子穿隧顯微鏡 (in situ scanning tunneling microscopy, STM) 和循環伏安法 (cyclic voltammetry, CV) 以及高能量電子繞射 (reflection high energy electron diffraction, RHEED) 被用以探討電鍍在單結晶銥(111)和鉑(111)電極上的汞膜形貌,以及汞膜上的分子吸附和金屬沈積的介面過程。在0.1 M 過氯酸中, STM 圖像顯示汞膜在銥(111)和鉑(111)電極上是以層狀的成長機制成長,在鉑(111)電極上的汞膜(大約1~2層的汞膜),隨著電位移動我們可以發現三種不同的結構,電位0.4 V時,汞吸附在鉑(111)上的結構呈現條紋狀結構,電位0.25 V時,汞吸附在鉑(111)上的結構呈現條波浪狀結構,電位0.1 V時,汞吸附在鉑(111)上的結構呈現(√31 × √31)R8.9°結構,此三種結構會隨著不同電位而改變並且為可逆反應,除此之外我們發現有些區域是合金結構,在X光繞射實驗中,在鉑(111)電極上的汞膜(大約100層的汞膜),我們發現PtHg4的訊號非常的尖銳,這代表PtHg4是一個大範圍規則且完整的結構,在高能量電子繞射實驗中發現,鉑(111)上低覆蓋度的汞膜是和載體鉑(111)夾30°的方向排列上去,在高覆蓋度的汞膜中,我們可以發現PtHg4以及PtHg2 在鉑(111)上是沿著(201)和(1-10)方向成長,我們認為當汞電鍍在鉑載體時表面能量過高,形成鉑-汞合金後將會降低表面能量。 在X光繞射實驗中,在銥(111)電極上的汞膜(大約100層的汞膜),我們得到液態汞的訊號且沒有任何銥-汞合金的訊號,根據 STM 和 CV 的結果,在0.1 M 過氯酸中,電位定在0.2 V,我們可以得到平整且多層的汞膜,借著升高碳針的電位直到1.5 V去蝕刻汞膜,用以證明我們確實得到多層的汞膜,利用升高碳針電位去蝕刻薄膜,我們預期我們可以用不同的電位去蝕刻不同的金屬薄膜,或許可以利用探針來分辨不同的金屬。碘吸附汞膜上在-0.35 V 到 0.4 V的電位區間有許多結構,在負電位-0.35 V時,碘的吸附結構為六方結構,但是每個碘原子並不是吸附在同一個位置上,所以造成表面仍有高低起伏的特徵,當電位在正電位0.1 V時,碘的吸附結構為(8 × 2√31)結構,當我們將電位移到0.4 V時,碘會和汞氧化成碘化亞汞(Hg2I2),我們認為正是因為汞有完整的晶格結構,所以在汞膜上吸附的碘也有完整的晶格結構,根據碘的吸附結構,我們反推下層汞的原子間距為0.31nm,這和文獻中汞原子的原子間距相當吻合。汞膜對於氫氣的產生有較大的過電壓,將一些重金屬電鍍在汞膜上,可以做成薄膜和是一些合金材料,所以利用汞膜去分析重金屬銅和鎘,我們發現銅和鎘都是以層狀的方式電鍍在汞膜上,將鎘電鍍上在汞膜上可以形成大範圍且完整的結構,這個結果對於製造 MCT 這種材料是非常重要的發現,根據 STM 的結果,鎘會和汞形成合金,造成剝除後的汞膜相當粗糙,但是銅卻沒有這些現象,CV的結果顯示鎘在汞膜上的剝除峰比銅還要寬大,我們比較兩者的半高寬,鎘的半高寬為90 mV而銅的半高寬為30 mV,這是因為鎘和汞形成合金後,鎘陷入汞膜裡面,當電位到達鎘的剝除電位之後,鎘必須從汞膜中剝除,銅只是從汞膜表面剝除,所以剝除鎘需要較寬的剝除峰。 In situ scanning tunneling microscopy (STM), cyclic voltammetry (CV) and reflection high energy electron diffraction (RHEED) have been used to examine mercury films electrochemically deposited onto well-ordered single crystalline electrodes of Pt(111) and Ir(111). In situ STM imaging reveals atomically flat surface morphology of mercury films deposited on ordered Pt(111) and Ir(111) electrodes, implying layer-by-layer growth mode of Hg on these electrodes in 0.1 M HClO4 + 1 mM Hg(ClO4)2. Depending on the amount of Hg deposit on Pt(111) electrode, in situ STM reveals a number of ordered arrays, which could derive from the production of intermetallic compounds with unidentified chemical compositions. These structures could be associated with PtHg2 and PtHg4 characterized by X-ray diffraction methods in previous studies. These ordered arrays of PtHg could be transformed reversibly by the modulation of potential, which could reflect surface alloy and dealloy as a function of potential. Reflection high energy electron diffraction (RHEED) was used to characterize mercury film electrodeposited Pt(111) electrode at room temperature. Depending on the amount of Hg deposit, two different growth modes were observed. At low Hg coverage, crystalline (0001)Hg adlayer accompanied by 30◦-rotated (111)-Pt patches was found on Pt(111). Deposition of multilayer Hg resulted in layered PtHg2 and PtHg4 amalgams, which grew epitaxially by aligning their (201) and (1-10) planes, respectively, parallel to the Pt(111) substrate. The preference of these epitaxial relationships for the electrochemically formed Pt–Hg intermetallic compounds on Pt(111) could result from minimization of the surface energy. In situ STM was also used to examine multilayered Hg film prepared by electrochemical deposition onto well-ordered Ir(111) single crystal electrode in 0.1 M HClO4 + 1 mM Hg(ClO4)2. Topography STM scans discerned the surface morphology of the as-prepared Hg film, but it was difficult to achieve atomic resolution. In potassium iodide solution atomic resolution STM images were readily obtained on the Hg film, revealing the structures of an iodine monolayer adsorbed on this Hg film. Iodine adatoms formed an incommensurate hexagonal array with a nearest neighbor spacing of 0.43 nm at -0.35 V. Shifting potential positively transformed the iodine adlattice into a commensurate (8 ? 2?3)rect structure, which is further converted into another incommensurate structure before the Hg film was oxidized to Hg2I2-like species. The formation of highly ordered iodine adlayers on Hg film indicates that the Hg substrate had to be crystalline at room temperature, which highlights the most intriguing finding of this study. The lattice constant of the Hg deposit is estimated to be 0.31 nm from the structure of iodine adlattice. This is consistent with separate electron diffraction measurements. To gain insights into the electrodeposition of heavy metal on Hg film, we used in situ STM to examine electrodeposition of Cd and Cu on Ir-supported Hg films. The as-produced Cd and Cu films were atomically smooth up to a thickness of 3 - 4 layers. The surface morphology of the Hg film appeared to become roughened after the Cd deposit, but remained mostly unchanged after the deposition and removal of Cu. We found that the surface roughness of Hg film hinges on how much Cd was deposit and how long the Cd deposit was allowed to stay on the Hg film. To further substantiate this issue, we obtained STM results with Hg film electrode coated with sub-monolayer of Cd and Cu. Cu is deposited on Hg film in a layered fashion, producing crystalline Cu film, as revealed by STM atomic resolution. It did not amalgamate with the Hg substrate. Cd is deposited in a layered fashion, producing Cd film on Hg film. It amalgamates with Hg film easily, which is responsible for the broader stripping peak, as compared to Cu. These STM results are useful in improving the understanding of the use Hg as the electrode for stripping analysis.
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