利用掃描式電子穿隧顯微鏡探討陰離子及pH值對電沉積鈷、鎳於鉑(111)電極上之影響

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題名:	利用掃描式電子穿隧顯微鏡探討陰離子及pH值對電沉積鈷、鎳於鉑(111)電極上之影響
作者:	郭妍均;Kuo,Yen-chun
貢獻者:	化學學系
關鍵詞:	掃描穿隧式電子顯微鏡;鈷;鎳;STM;Cobalt;Nickel;electrodeposition
日期:	2014-07-25
上傳時間:	2014-10-15 14:37:29 (UTC+8)
出版者:	國立中央大學
摘要:	本研究利用掃描式電子穿隧顯微鏡( in situ scanning tunneling microscopy，in situ-STM )和循環伏安法( cyclic voltammetry，CV )探討兩個主題：不同陰離子及pH值對電沉積鈷、鎳於鉑(111)電極上之影響。第一部分為單純硫酸鉀溶液下( 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4, pH3 )，在-0.35V氫氧根離子伴隨著鈷的沉積而形成蜂窩狀結構，將電位往負至-0.6V時，發現此結構消失，推測OH-被還原而鈷原子轉為不規則排列。在有氯離子時，鈷和氯離子共同吸附而形成無序的吸附層，但含溴離子下，則與鈷形成不同的蜂窩狀結構。陰離子和鈷共吸附強度為：Br- > Cl- > OH-。於含溴離子硫酸鉀溶液中，鈷的沉積效率最佳，氯離子溶液次之，純硫酸鉀溶液中效率最差。在單純硫酸鉀溶液中，在5層原子厚度內鈷以二維層狀方式沉積於鉑(111)電極上，形成有規則高低起伏波浪狀結構，由這些規則結構可推算出鈷原子的間距由0.257 nm減少至0.254 nm。在第六層之後，螺旋狀的島狀結構開始出現。在純硫酸鉀溶液中，島狀物有清楚的三角形形狀，而在氯離子的存在下，則造成不規則的島狀特徵。若溶液中含溴離子，鈷從第二層開始就以三維（Volmer-Weber mode）的方式成長。第二部分著重於pH值對鈷鎳沉積效率的影響，探討( pH2、3及6 )之硫酸鉀溶液中，電沉積鈷、鎳於鉑(111)電極上。鈷的沉積效率於pH6下最佳，可達72.5 %，而效率最差為pH2溶液下；而鎳的沉積效率：pH3 > pH2，因於酸性溶液中，氫離子濃度較高，所消耗的電荷較多用於氫離子還原成氫氣，而不利於鈷及鎳的沉積。在pH2中，第一層鈷及鎳是以fractal-like growth中的extended fractal growth的方式沉積33；而於pH3下，第一層鈷及鎳皆會與氫氧根離子共吸附在鉑(111)電極上，形成一蜂窩狀結構，當此結構於較負電位時，由於氫氧根離子會脫附，因而造成此結構轉為不規則排列；在pH6硫酸鉀溶液中，第一層鈷膜也會形成蜂窩狀結構，但會有少部分不規則區塊。在沉積多層的部分，於pH2及3之硫酸鉀溶液中，5層以內鈷膜及鎳膜皆是以二維層狀方式沉積於鉑(111)電極上，且皆有moiré pattern結構，當第六層則轉為三維島狀的形式成長，此成長模式皆為SK mode；而於pH6下，鈷的成長模式為VW mode。由STM結果顯示，於pH3溶液下，鈷原子的沉積方式是以FCC堆疊；而於pH6下，鈷原子則是傾向HCP的方式堆疊。此結果與文獻結果相符合36，鈷的沉積在pH值較低下會傾向於FCC；而於較高pH值下，則會以HCP的方式堆疊。 ;In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine two topics: Effects of anions and pH on the electrodeposition of cobalt on Pt(111) electrode. Part I was the electrodeposition of cobalt onto a Pt(111) electrode in 0.1 M K2SO4 + 1 mM H2SO4 (pH3). Cobalt deposited tended to nucleate uniformly on the terrace and accompanied by hydroxide adsorption, then assembled into ordered honeycomb-like structured film at -0.34 V( vs. Ag/AgCl ). While the potential was shifted to -0.6 V, hydroxide was reduced and local ordered moiré patterns were displaced by rough patches immediately. Chloride ions and cobalt were coadsorbed to form a disordered bilayer. Cobalt monolayer accompanied with a layer of bromide to arrange in honeycomb structure. Adsorption strength of anion and cobalt: Br- > Cl- > OH-. In potassium sulfate solution containing chloride and bromide, the deposition efficiency of cobalt was highest in presence of bromide, followed by chloride, and potassium sulfate yielded the worst efficiency. For the first five layers of cobalt, deposition proceeded mainly in a two-dimensional mode, producing a regular moiré pattern in potassium sulfate solution. The in-plane interatomic spacing between cobalt atoms decreased gradually from 0.257 to 0.254 nm. Spiral islands were observed starting from the sixth layer, yielding well-defined, stacked triangules defined by <110> aligned steps. By contrast, islands with irregular shapes were found in the presence of chloride. In the presence of bromide, cobalt was deposited in three-dimensional (Volmer-Weber mode). Electrodeposition of cobalt onto a Pt(111) electrode in potassium sulfate solutions made of unlike pH. Deposition efficiency reached as high as (72.5 %) in pH6 solution, by opposed to 8.0 % found in pH2 solution. Similarly, nickel was deposited more efficiently in pH3 than pH2 solution. Due to the higher proton concentration in acidic solution, charge wasw mostly consumed by protons to yield hydrogen, leading to lower reduce deposition efficiency of nickel and cobalt. It was observed that the first layer of cobalt or nickel grew in fractals and in extended fractal at pH2. In addition, the deposition of cobalt or nickel accompanied by hydroxide adsorption into ordered honeycomb-like structure at pH3 solution. At more negative potential, hydroxide was desorbed, producing partially ordered honeycomb-like structure. The dgree of ordering of honeycomb-like structure was poorer in pH6 electrolyte. Multilayer deposition of cobalt or nickel in pH2 and pH3 potassium sulfate solution occurred in two-dimensional, producing regular moiré patterns until the sixth layer, where three-dimensional islands were imaged by the STM. However, the deposition at pH6 followed the VW mode. Cobolt deposit could pack in fcc at pH2, whereas it adopted hcp stacking at higher pH(~6) sulfate solution.
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