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| 題名: | 即時電化學掃描式電子穿隧顯微鏡對Au(111), Au(100) 和 Pt(100) 電極反應的研究;In Situ Electrochemical Scanning Tunneling Microscopy Study of Electrode Processes on Au(111), Au(100), and Pt(100) Electrodes |
| 作者: | 許家豪;Chia-Haw Shue |
| 貢獻者: | 化學研究所 |
| 關鍵詞: | 掃描式電子穿隧顯微鏡;STM |
| 日期: | 2005-07-01 |
| 上傳時間: | 2009-09-22 10:11:24 (UTC+8) |
| 出版者: | 國立中央大學圖書館 |
| 摘要: | 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) 被用以探討在單結晶 Au(111)、 Au(100) 和 Pt(100) 電極上的表面重構、分子吸附及金屬沈積的介面過程。 在0.1 M 過氯酸中，高解像之 STM 圖像顯示，調整 Au(111) 和 Au(100) 電極之電位可反覆控制其表面重排和未重排之間的相轉變，清晰之原子圖像可詳細地檢測其表面原子結構及原子間距的變化。電位在 0 V (對可逆氫電極)時， STM 結果顯示 Au(111) 的表面呈現魚骨狀的重排結構，兩鄰近亮紋的間距為6.3 奈米。隨電位增加至 0.4 V 時，亮紋的間距由6.3增加到6.83 奈米，此一距離的增加集中於面心立方堆積的區域，而體心立方和過度區間之原子間距並無明顯的變化。在達到 0.7 V 時，Au(111) 的重排條紋完全消失且出現奈米島狀物。相同的，隨電位增加，Au(100) 的表面結構從 “hex” 轉變為 (1 × 1) 時，亦出現奈米島狀物，其總量大約和兩者的原子密度相合，這些變化反應出電極上的電荷(或電位)主控電極之表面結構。 在含 50 µM 氰化鉀之過氯酸溶液中 (pH11)，於 -0.6 V 時，以即時顯像的 STM 可發現 Au(111) 和 Au(100) 表面的溶解和沈積過程具有方向性，此一特性來自於兩者的表面原子結構；換言之電極表面的能量具有方向性，和原子密排方向平行的台階最為穩定，因此金的溶解和沈積均沿特定方向進行。在酸性氰化鉀溶液中，雖然氰化氫為主要的物種，但當其吸附在金電極表面時會進行去氫反應而形成陰離子的氰酸根，在 Au(111) 上其吸附結構包含 (6 ? 6)、(?7 ? ?7)R19.1° 和 oblique(?7 ? ?21) 三種結構，在 Au(100) 上為 (3?2 ? 3?2)R45° 結構。在 Au(100) 上氰化物可能吸附在一個金原子的上端，而在 Au(111) 電極上它可能鍵結於三個金原子中間。介於 0.7~0.8 V 之間，吸附之氰化物造成金表面的結構變化，例如可能形成 Au+CN- 的吸附狀態，此一吸附結構為金溶解的先驅體，因此當電位高於 0.8 V 時，Au(111) 和 Au(100) 的表面開始全面溶解而造成表面的粗糙，此一高速率的過程和低速率的層狀剝解形成強烈對比。 金電極對一氧化碳的催化反應為另一研究的重點。STM顯示在含飽和一氧化碳的過氯酸中，於 0.1 到 0.7 V 之間，一氧化碳在 Au(111) 和 Au(100) 電極上形成有序的結構；Au(111) - (9 ? ?3) - 5CO 的結構，在 Au(100) 上為一未定義結構，但清晰之 STM 分子圖像顯示一氧化碳可能形成數種鍵結的方式。在 0.1 V 時，無 Au(111) - (?3 ? 22) 的重排結構；當電位由 0.1 改變到 -0.1 V 時，在一氧化碳分子之吸附狀態下的金原子層出現 Au(111) 的重排特徵，其特徵寬度相較典型的 Au(111) 重排大約增加 19% (7.5和6.3奈米之差)；相似的情形也出現在 Au(100) 電極的波浪特徵(1.7和1.49奈米之差)。當電位提高至 0.8 V 時，一氧化碳開始氧化，只有 Au(111) 表面發生緩慢的重構現象，由 (9 ? ?3) - 5CO 變成 (7 ? ?7) - 3CO 的結構，一氧化碳覆蓋度由 0.28變成 0.14，在後者的結構中，一氧化碳分子形成間隔0.85奈米寬的分子帶狀結構，此一結構的形成有可能為接納另一反應分子 - 水，因此一氧化碳和水分子可能共吸附，然後在金電極上反應而形成的二氧化碳；換言之，一氧化碳的氧化機制尋一 Langmuir-Hinshelwood 模式進行。 最後，利用高解析的 STM 和 CV 研究在電化學環境中的 Pt(100) 電極反應，包含自組裝之單層苯硫酚分子膜、雙層銀原子的沈積以及利用探針控制銅離子的沈積。STM 顯示苯硫酚在 Pt(100) 表面上形成一高規則之 (?2 ? ?2)R45° 的結構，兩鄰近分子間距為0.39奈米，苯硫酚分子以硫頭和四個鉑原子鍵結而垂直站立於 Pt(100) 表面上。 就銀的低電位電鍍 (UPD) 而言，CV 結果顯示在 1.15 V 出現第一個銀的UPD 的特徵峰，其電荷量為 189 ?C/cm2，並由 STM 證實此時在 Pt(100) 表面有單層的銀原子沈積。電位降低至 0.7 V 時，出現兩種沈積過程，其電荷量分別為 116 和40 ?C/cm2 ，約有半層的銀先沈積形成一 (?2 ? ?2)R45° 的結構，之後銀再沈積重構成近似六方的原子結構。 利用銅的沈積過程，說明在電雙層中 STM 的探針和鉑電極雙重電層的重疊現象。在探針電位的影響下，位於探針下的局部鉑電極，其電位異於其他受電位儀所操控之常態電位，只有探針末端影響鉑電極的電位，這一探針-載體之間的互相作用是一靜電現象，由於局部之銅離子的濃度受探針電位的影響而變化，因此連帶的影響鉑電極的電位。由於 Pt(100) 電極上被單層碘所覆蓋，銅的沈積會選擇性地發生在碘鍵結能量最弱的位置。 In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) have been used to examine the reconstruction, molecular adsorption, and metal deposition at the interfaces of well-ordered Au(111), Au(100), and Pt(100) electrodes. The atomic structures of well-ordered Au(111) and Au(100) surfaces were examined meticulously in 0.l M HClO4, aiming to unraveling the potential-controlled phase transition from the reconstruction to the unreconstruction. In the case of Au(111) in situ STM results reveal typical herringbone structures with pairwise protruded stripes repeating in every 6.3 nm at the negative potential (0 V vs. RHE). This characteristic spacing of 6.3 nm increased with more positive potential (0.4 V) to 6.83 nm before each pair of stripe was diminished to produce nanometer size gold islands on the surface of Au(111). The spacing between the paired lines, however, did not change with potential, which implied that the lifting of reconstruction occurred via the expansion of the face-centered-cubic (fcc) domain with no change in the hexagonal-close-packing (hcp) domain. However, this expansion of atomic lattice of two corrugated “ropes” did not appear on the Au(100) surface. On both Au(111) and Au(100), the number of protruded islands increased with more positive potential. On the other hand, the herringbone structures on Au(111) and “waves” on Au(100) decreased from the reconstruction to the unreconstruction. In situ STM imaging of Au(111) and Au(100) revealed anisotropy in surface dissolution and deposition at -0.6 V in pH11, 0.1 M KClO4 containing 50 ?M KCN. This anisotropic phenomenon largely originated from the intrinsic anisotropy in the surface energy of the reconstructed surface structures of both electrodes. Cyanide species were predominant anions on the surfaces of Au(111) and Au(100), even in acidic KCN solutions. The predominant species in acidic cyanide solution, HCN, is proposed to undergo oxidative adsorption resulting in dehydrogenation in this process. Cyanide ad-species were found to adsorb in (6 ? 6), (?7 ? ?7)R19.1°, and oblique(?7 ? ?21) on Au(111), and (3?2 ? 3?2)R45° on Au(100) in 0.1 M HClO4 containing 50 ?M KCN. All molecular features in the STM images of Au(100) produced the identical corrugation height, implying that CN- ad-species resided at atop sites, which was consistent with the results of spectroscopic studies. The 3-fold fcc and hcp sites were the most likely coordination sites of CN- ad-species on Au(111). At 0.7~0.8 V, a compound phase of Au+CN- adlayer was formed on both electrodes. Presumably, the production of this phase is driven by the positive potential which pulls cyanide into the gold lattice. Gold dissolves universally once the potential is made more positive than 0.8 V. Surfaces of Au(111) and Au(100) appeared to be rough under this condition, which contrasted markedly with the atomically flat pseudo layer-by-layer dissolution observed at negative potentials. Immersion of Au(111) or Au(100) electrodes under potential control at 0.1~0.7 V in a CO-saturated perchloric acid generated several ordered structures, but they gradually transformed into a more stable (9 ? ?3) - 5CO structure on Au(111) and a hitherto undetermined structure on Au(100). Molecular-resolution STM revealed CO molecules of different corrugation heights, implying CO molecules were adsorbed on different symmetric sites. Dosing Au(111) with a CO-saturated HClO4 at 0.1 V caused lifting of the Au(111) - (?3 ? 22) reconstruction, but adjusting the potential from 0.1 to -0.1 V rendered the reconstructed Au(111) to reform, with increasing the periodicity of the herringbone feature from 6.36 to 7.5 nm. This phenomenon is similar to the wavy features seen on Au(100) from 1.49 to 1.7 nm. Molecular-resolution STM revealed that not all CO molecules have the same corrugation heights, unlike implies CO molecules are adsorbed on different sites. Raising the potential from 0.7 to 0.8 V resulted in a slow phase transition from (9 ? ?3) - 5CO to (7 ? ?7) - 3CO on Au(111), as CO molecules were partially desorbed or electrooxidized. The coverage decreased from 0.28 to 0.14 as a consequence of these restructuring events. These results suggest that electrooxidation of CO proceeded via the Langmuir-Hinshelwood mechanism on Au electrodes. Finally, high-resolution STM and CV techniques were employed to examine the self-assembled monolayers (SAMs) of organosulfur (benzenethiol, BT) admolecules, the deposited bilayer of silver, and tip-induced of copper deposition on Pt(100) electrodes in electrochemical environment. The BT admolecules are adsorbed in a highly ordered adlattice on Pt(100), identified as (?2 ? ?2)R45° by molecular-resolution STM imaging. Two neighboring admolecules are separated by only 0.39 nm within this ordered array, necessitating vertically molecular configuration with its sulfur-headgroup directly bonded to 4-fold hollow sites on Pt(100). Prior to the commencement of bulk deposition, underpotential deposition (UPD) of Ag proceeded stepwise depositing bilayer of Ag metal on Pt(100). The first step resulted in a sharp peak at 1.15 V in the CV with a charge of 189 ?C/cm2 which is close to that needed for the deposition of a monolayer of Ag. STM imaging results also supported this reasoning, as a pseudo-morphic Ag adlayer was observed. The second and third Ag UPD steps are close in potential, both commencing near 0.4 V more negative than the first one. They contain 116 and 40 ?C/cm2 charges, which added up to account for the deposition of another monolayer of Ag. STM discerns a (?2 ? ?2)R45? structure, which transformed to a pseudo-hexagonal lattice before bulk deposition. The overlap of electric double layers between the STM tip and Pt electrodes is illustrated by using Cu deposition as a probe. It is shown that the local potential at an Pt(100) electrode under the tip differed from the remaining area whose potential was pinned mainly by the potentiostate. The dimension of the tip-dominated area varied with the scan size, indicating that only the very end of the tip was influential to the electrode potential. This tip-and-substrate interaction is likely to be electrostatic. In addition, since the Pt(100) surface was coated with the iodine adatoms, the deposited Cu adatoms would selectively go to those iodine sites of the weakest bonding. |
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