以超臨界二氧化碳輔助分散銀粒子及以異硬脂酸金屬鹽為前趨物應用濕式化學法製備奈米銀及奈米硫化金屬之研究

Deaggregation of Silver Powders Assisted by Supercritical CO2, Synthesis of Silver Nanoparticles and Metal Sulfide Nanocrystals by Wet-Chemical Method Using Metal Isostearate as Precursor

謝賢德(Hsien-Te Hsieh)

指導教授 ： 金惟國

國立清華大學/工學院/化學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00011

摘要

本研究之目的為發展製備奈米材料之技術，包括(一)超臨界二氧化碳輔助分散銀粒子、(二)二氧化碳膨脹液體法製備奈米銀粒子及(三)以異硬脂酸金屬鹽為前趨物製備奈米硫化金屬等三部分，其研究內容之摘要分述如下：

一、超臨界二氧化碳輔助分散銀粒子
將盛有銀粒子/有機溶劑/分散劑溶液之高壓反應器以二氧化碳建壓至800 ~ 2000 psi，經所需時間後，再進行快速洩壓程序，探討銀粒子聚集體之解團聚現象。其中，有機溶劑之選擇為toluene、hexane及ethyl acetate，分散劑之選擇為isostearic acid及dodecanethiol，操作溫度為25~50°C，並藉由動態雷射光散射儀鑑定產物之平均粒徑。
經超臨界二氧化碳輔助分散程序後，銀粒子之平均粒徑約為1000 nm左右或更小。若採用ethyl acetate溶劑及isostearic acid分散劑時，對銀粒子聚集體之解團聚效果最佳，其平均粒徑約為500 nm。不過，收集的銀粒子溶液會在收集瓶內壁及溶液表面形成薄膜狀之聚集體，且有部分銀粒子聚集沈澱。推究其原因，應為二氧化碳之反溶劑效應，或為分散劑濃度不足提供銀粒子表面吸附之所需。此外，在快速洩壓過程時，二氧化碳之體積大幅膨脹導致溶劑產生霧化，僅能收集少許的產物。因此，必須採用超大體積容器方可收集大部分產物，故為此方法之缺點。
二、二氧化碳膨脹液體法製備奈米銀粒子
以異硬脂酸鈉鹽與硝酸銀進行陽離子交換反應，合成異硬脂酸銀鹽(AgISt)之新型銀前趨物。經ATR-FTIR、1H-NMR、XRD、DSC及TGA分析，結果顯示分支行烷鏈具有立體障礙效應，進而阻礙AgISt生成規則排列之層狀結構，故於有機溶劑中具有較高的溶解度。本研究提出具創新性的方法，以AgISt作為前趨物，氫氣作為還原劑，二氧化碳膨脹液體作為反應媒介，製備奈米銀粒子。其中，氫氣及二氧化碳之操作壓力分別為14 ~ 800 psi及200 ~ 800 psi。
反應溫度為40°C時，可製備粒徑為2 ~ 7 nm之奈米銀粒子。提高氫氣及二氧化碳之壓力時，奈米銀粒子之粒徑分佈變小且生成速率會增加。經HRTEM、SAED及ATR-FTIR分析，結果顯示奈米銀粒子具有面心立方之晶體結構，且表面吸附異硬脂酸，使其能穩定分散於溶劑中。提升反應溫度至60及80°C，奈米銀粒子之粒徑分佈變大且生成速率趨緩，推究其原因為溶液的體積膨脹度減少，以致氫氣的質傳阻力增加所導致。
三、以異硬脂酸金屬鹽為前趨物製備奈米硫化金屬材料
以異硬脂酸鈉鹽分別與硫酸鋅、醋酸鎘及氯化銅進行質子交換反應，合成異硬脂酸鋅鹽(ZnISt2)、鎘鹽(CdISt2)及銅鹽(CuISt2)。經XRD及DSC分析，結果顯示ZnISt2、CdISt2及CuISt2不具規則排列之層狀結構，故於有機溶劑中具有較高的溶解度，可作金屬前趨物，與硫化氫進行硫化反應，以濕式化學法製備奈米硫化鋅、硫化鎘及硫化銅材料。以ZnISt2作為前趨物，在40~120°C反應溫度下，可生成硫化鋅奈米線；在160°C下，可生成硫化鋅奈米棒。以CdISt2作為前趨物，在40~120°C下，可生成不同尺寸之一維奈米棒、bipod、tripod及tetrapod等型態之硫化鎘奈米棒。經HRTEM分析，顯示硫化鎘奈米棒及支架型奈米棒中心轉折點，分別屬於六角晶系之纖鋅結構及立方晶系之閃鋅結構。提高溫度至160°C，可生成類球狀粒子、長徑比較小的一維奈米棒及少部分的蟲型奈米棒。以CuISt2作為前趨物，在40°C下，會生成不規則型態之硫化銅聚集體；在80~160°C下會生成圓形碟狀、三角形及六角形型態之奈米硫化銅。經XRD圖譜分析，結果顯示奈米硫化銅具有六方晶系之結構(CuS, covellite)。

關鍵字

超臨界二氧化碳 ； 二氧化碳膨脹液體 ； 奈米銀 ； 奈米硫化金屬

並列摘要

The main purpose of this study was to develop the techniques of the preparation of nanocrystals. In this dissertation, three techniques including (I) the deaggregation of silver powders assisted by supercritical CO2, (II) the synthesis of silver nanoparticles in CO2-expanded liquids, and (III) the synthesis of metal sulfide nanocrystals using wet-chemical method had been studied.

System I. The deaggregation of silver powders assisted by supercritical CO2
The mixture of silver particles/organic solvent/dispersing agent in the reactor was pressurized with CO2 ranging from 800 to 2000 psi for a period of time, followed by the depressurization through a nozzle rapidly. The organic solvents of toluene, hexane and ethyl acetate and the dispersing agents of isostearic acid and dodecanethiol were used. The process temperature was ranged from 25 to 50°C. After the process of depressurization, the silver particles solution was investigated by dynamic laser scattering (DLS).
It was found that the operation with the pressurized CO2, especially in the supercritical condition, could help the deaggregation of silver powders and the size of deaggregated silver particles was less than 1000 nm. However, part of the deaggregated silver particles tended to assemble into thin films on the surface of solution and the wall of receiver. The anti-solvent effect induced by adding CO2 or insufficient amount of dispersing agent to cap the surface of silver particles might be the reasons. In addition, during depressurization through nozzle, the volume of gas expanded greatly leading to the nebulization of organic solvent. Thus, the huge receiver to collect the nebulizing solvent droplets was required.

System II. The synthesis of silver nanoparticles in CO2-expanded liquids
A soluble form of silver carboxylate, silver isostearate (AgISt), was synthesized and characterized. The results of ATR-FTIR, 1H-NMR, XRD, DSC and TGA indicated that the methylated branched alky chains in AgISt exhibited a steric hindrance to impede the growth of layered structure of AgISt molecules, which led to the high solubility of AgISt in non-polar solvents. A novel technique to synthesize silver nanoparticles (AgNPs) using CO2-expanded liquids as the processing medium was proposed. AgISt and hydrogen (H2) were utilized as silver precursor and reducing agent, respectively. The operative pressure of H2 and CO2 were ranged from 14 to 800 psi and from 200 to 800 psi, respectively.
At 40°C, the averaged size of synthesized AgNPs was ranged from 2 to 7 nm. While the applied pressures of H2 and CO2 were increased, the size distribution of AgNPs was narrower and the formation rate of AgNPs was increased. The investigations of HRTEM, SAED, ATR-FTIR showed that AgNPs were grown in face-centered cubic phase and capped with isostearic acid, which was derived from the reduction of AgISt with H2. Further increase the reaction temperature to 60 or 80°C, the formation rate of AgNPs was reduced and the size distribution of AgNPs became broader. The reason might be that the resistance of mass transfer of H2 in CO2-expanded liquids limited the reduction reaction of AgISt and H2 as temperature was increased.

System III. The synthesis of metal sulfide nanocrystals using wet-chemical method
Metal isostearates including zinc isostearate (ZnISt2), cadmium isostearate (CdISt2), and copper isostearate (CuISt2) were synthesized by the cation exchange reaction of sodium isostearate with the corresponding metal ions. The results of XRD and DSC indicated that no layered structure was form in metal isostearate, which led to their high solubility in non-polar solvents. Metal isostearates were employed as precursors to react with H2S to synthesis metal sulfide nanocrystals in wet-chemical method. By using ZnISt2 as precursor, ZnS nanowires were formed at 40~120°C, whereas nanorods were formed at 160°C. By using CdISt2 as precursor, rod, bipod, tripod, and tetrapod shapes of CdS nanocrystals were formed at 40~120°C. The investigation of HRTEM indicated that the arms and cores of multipod-shaped CdS were grown in wurtzite phase and zinc blende phase, respectively. Further increased the temperature to 160°C, spherical, rod-like and warm-like CdS nanocrystals were formed. By using CuISt2 as precursor, irregular aggregated CuS were form at 40°C, whereas circular, triangular, and hexagonal CuS nanocrystals were form at 80~160°C. The XRD pattern indicated that CuS nanocrystals were grown in covellite phase.

並列關鍵字

supercritical CO2 ； CO2-expanded liquids ； silver nanoparticle ； metal sulfide nanocrystal

參考文獻

1. Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A., Nanocrystal and nanowire synthesis and dispersibility in supercritical fluids. J Phys Chem B 2004, 108 (28), 9574-9587.

2. Jessop, P. G.; Ikariya, T.; Noyori, R., Homogeneous catalysis in supercritical fluids. Chemical Reviews 1999, 99 (2), 475-493.

3. Eastoe, J.; Gold, S.; Steytler, D. C., Surfactants for CO2. Langmuir 2006, 22 (24), 9832-9842.

4. Liu, Z. T.; Wu, J.; Liu, L.; Sun, C. A.; Song, L. P.; Gao, Z. W.; Dong, W. S.; Lu, J., Solubilities of AOT analogues surfactants in supercritical CO2 and HFC-134a fluids. J Chem Eng Data 2006, 51 (5), 1761-1768.

5. Tan, B.; Lee, J. Y.; Cooper, A. I., Ionic hydrocarbon surfactants for emulsification and dispersion polymerization in supercritical CO2. Macromolecules 2006, 39 (22), 7471-7473.