Adsorption Kinetics of 1-Alkanethiols on Hydrogenated Ge(111)

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• 作者：Madhava R. Kosuri ; Roya Cone ; Qiming Li ; Sang M. Han ; Bruce C. Bunker ; and Thomas M. Mayer
• 刊名：Langmuir
• 出版年：2004
• 出版时间：February 3, 2004
• 年：2004
• 卷：20
• 期：3
• 页码：835 - 840
• 全文大小：106K
• 年卷期：v.20,no.3(February 3, 2004)
• ISSN：1520-5827

文摘

We have investigated the liquid-phase self-assembly of 1-alkanethiols (HS(CH₂)_n-1CH₃, n = 8, 16, and18) on hydrogenated Ge(111), using attenuated total reflection Fourier transform infrared spectroscopyas well as water contact angle measurements. The infrared absorbance of C-H stretching modes ofalkanethiolates on Ge, in conjunction with water contact angle measurements, demonstrates that the finalpacking density is a function of alkanethiol concentration in 2-propanol and its chain length. Highconcentration and long alkyl chain increase the steady-state surface coverage of alkanethiolates. A criticalchain length exists between n = 8 and 16, above which the adsorption kinetics is comparable for all longalkyl chain 1-alkanethiols. The steady-state coverage of hexadecanethiolates, representing long-chainalkanethiolates, reaches a maximum at approximately 5.9 × 10¹⁴ hexadecanethiolates/cm² in 1 M solution.The characteristic time constant to reach a steady state also decreases with increasing chain length. Thischain length dependence is attributed to the attractive chain-to-chain interaction in long-alkyl-chain self-assembled monolayers, which reduces the desorption-to-adsorption rate ratio (k_d/k_a). We also report theadsorption and desorption rate constants (k_a and k_d) of 1-hexadecanethiol on hydrogenated Ge(111) atroom temperature. The alkanethiol adsorption is a two-step process following a first-order Langmuirisotherm: (1) fast adsorption with k_a = 2.4 ± 0.2 cm³/(mol s) and k_d = (8.2 ± 0.5) × 10^-6 s^-1; (2) slowadsorption with k_a = 0.8 ± 0.5 cm³/(mol s) and k_d = (3 ± 2) × 10^-6 s^-1.