利用掃描式電子穿隧顯微鏡觀察汞薄膜在銥(111)電極上鹵素的吸附結構

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廖健成(Chien-Cheng Liao) 查詢紙本館藏

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利用掃描式電子穿隧顯微鏡觀察汞薄膜在銥(111)電極上鹵素的吸附結構
(An in-situ STM study of halogens adsorbed on a Ir(111) based mercury electrode.)

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摘要(中)

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。

摘要(英)

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.

關鍵字(中)

★ 銥(111)
★ 鹵素
★ 掃描式電子穿隧顯微鏡

關鍵字(英)

★ STM
★ Ir(111)
★ halogen

論文目次

目錄
第一章緒論……………………………………………………………..1
1–1 掃描式電子穿隧顯微鏡的原理…………………………………..1
1–2 石英晶片微量天秤的原理………………………………………..1
1–3 過去汞電極的研究方法…………………………………………..3
1–3–1 電雙層的簡介………………………………………………3
1–3–2 PZC的簡介………………………………………………….4
1–3–3 特異性吸附的介紹…………………………………………5
1–4汞電極的應用………………………………………………………6
1–4–1剝除分析法的簡介…………………………………………..6
1–4–2 滴汞電極與汞薄膜電極的比較……………………………7
1–5 相關的研究報導…………………………………………………..8
第二章實驗部分………………………………………………………11
2–1藥品部分…………………………………………………………..11
2–2氣體部分…………………………………………………………..12
2–3金屬部分………………………………………………….……….12
2–4儀器部分……………………………………….…………………12
2–5 實驗步驟…………………………………………………………13
2–5–1 銥(111)電極的實驗部分………………….………….……13
2–5–2 陽極剝除伏安法的實驗部分……………….……….……16
2–5–3 電化學石英晶片微量天秤的實驗部分…….……….……16
第三章結果與討論……………………….…………………….……..18
3–1 乾淨汞薄膜電極的電化學行為……………………………..…..18
3–1–1 銥(111)電極的循環伏安圖…………………………..……18
3–1–2 汞薄膜電極的循環伏安圖…………………………….….19
3–1–3 汞薄膜電極的STM影像…………………………………20
3–2 鹵素在汞薄膜電極上的吸附……………………………………22
3–2–1 循環伏安圖………………………………………………..22
3–2–2 汞薄膜電極在10 mM碘化鉀溶液中的STM影像………27
3–2–3 汞薄膜電極在10 mM溴化鉀溶液中的STM影像………36
3–2–4 汞薄膜電極在10 mM氯化鉀溶液中的STM影像………37
3–2–5 汞薄膜電極在10 mM氟化鉀溶液中的STM影像………41
3–3 在汞薄膜電極上鍍鉛的電化學行為……………………………45
3–3–1 汞薄膜電極在1 mM過氯酸鉛中的循環伏安圖………...45
3–3–2 汞薄膜電極在1 mM過氯酸鉛中的STM影像………….46
3–4 利用汞薄膜電極檢測水溶液中微量的鉛………………………48
3–5 電化學石英晶片微量天秤的研究………………………………49
3–5–1 黃金石英晶片在乾淨過氯酸中的EQCM圖…………….49
3–5–2 黃金石英晶片在碘化鉀溶液中的EQCM圖…………….50
3–5–3 黃金石英晶片鍍銅的EQCM圖………………………….51
第四章結論…………………………………………………………53
第五章參考文獻……………………………………………………56
圖目錄
圖1、石英晶片的製作原理……………………………………………58
圖2、石英晶片的震盪模式及頻率範圍………………………………59
圖3、電雙層的結構圖…………………………………………………60
圖4、特異性吸附的介紹……………………………………………….61
圖5、剝除分析法的簡介……………………………………………….62
圖6、滴汞電極與汞薄膜電極的比較…………………………………..63
圖7、液態金屬的相關研究……………………………………………64
圖8、汞薄膜電極的SHG結果…………………………………………65
圖9、EQCM的裝置圖…………………………………………………66
圖10、銥(111)電極鍛燒的照片……………………………………….67
圖11、鍍汞的實驗槽…………………………………………………..68
圖12、乾淨銥(111)電極在0.1 M過氯酸溶液中之CV圖…………..69
圖13、汞薄膜電極在不同濃度過氯酸溶液中之CV圖….………….70
圖14、汞薄膜電極在0.1 M硫酸溶液中之CV圖…….……………..71
圖15、乾淨銥(111)電極與汞薄膜電極的比較………….…………….72
圖16、汞薄膜電極在0.1 M HClO4中的STM連續變化圖………….73
圖17、汞薄膜電極在過氯酸中之STM影像………………………….74
圖18、汞薄膜電極在不同鹵素溶液中的CV圖…………….………75
圖19、鹵化亞汞還原電位實驗值與理論值的比較………………..…76
圖20、汞薄膜電極在10 mM KI中不同掃描速率的CV圖…………77
圖21、汞薄膜電極在10 mM KBr中不同掃描速率的CV圖………..78
圖22、汞薄膜電極在10 mM KCl中不同掃描速率的CV圖…….…79
圖23、汞薄膜電極在10 mM KF中不同掃描速率的CV圖……..….80
圖24、碘吸附的STM連續變化影像及示意圖……..………………..81
圖25、計算碘的覆蓋速度及碘吸附方式示意圖………..……………82
圖26、觀察電位改變對STM影像的影響……………………………83
圖27、碘吸附的STM連續變化影像…………………………………84
圖28、多層碘化亞汞的STM影像……………………………………85
圖29、電位為-150 mV時碘吸附的STM影像……………………….86
圖30、碘吸附在汞薄膜上的STM影像………………………………87
圖31、碘吸附在汞薄膜上大範圍的STM影像………………..……..88
圖32、碘吸附在汞薄膜上小範圍的STM影像…………….…………89
圖33、碘吸附在汞薄膜上的結構圖……………………..……………90
圖34、碘吸附在(111)上的STM影像…………………………………91
圖35、碘吸附在(111)上的結構圖………………….…………………92
圖36、溴吸附在汞薄膜上大範圍的STM影像………..…………….93
圖37、溴吸附在汞薄膜上小範圍的STM影像………..…………….94
圖38、汞薄膜在氯化鉀中不同電位的STM影像…………………….95
圖39、氯在不同電位下結構變化之STM影像………………………96
圖40、在190 mV下氯吸附結構之STM影像………………………..97
圖41、在180 mV下氯吸附結構之STM影像…………….…………98
圖42、在160 mV下氯吸附結構之STM影像……….………………99
圖43、在150 mV下氯吸附結構之STM影像……………………….100
圖44、在130和120 mV下氯吸附結構之STM影像………………101
圖45、氯吸附結構在不同電位下距離與角度的變化………….……102
圖46、鉀離子或水分子吸附在汞薄膜上大範圍的STM影像……..103
圖47、鉀離子或水分子吸附在汞薄膜上小範圍的STM影像…….104
圖48、汞薄膜在10 mM KF中不同電位的STM連續影像…….….105
圖49、氟吸附在汞薄膜上之STM連續變化影像…………………..106
圖50、氟吸附在汞薄膜上之STM影像………….…………………..107
圖51、汞薄膜在1 mM過氯酸鉛溶液中之CV圖………………….108
圖52、鉛離子吸附在汞薄膜電極上之STM影像…….………….…109
圖53、鉛沉積在汞薄膜上之STM影像…………………………..…110
圖54、汞薄膜檢測不同濃度鉛溶液的ASV圖……..……………….111
圖55、黃金石英晶片在0.1 M HClO4中的EQCM圖………………112
圖56、黃金石英晶片在0.1 M KI中的EQCM圖…………………..113
圖57、黃金石英晶片在1 mM CuSO4中的EQCM圖………………114

參考文獻

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指導教授

姚學麟(Shueh-Lin Yau)

審核日期

2002-7-16

推文