利用微波加熱法純化奈米碳管與在碳管上合成白金觸媒之探討

Abstract

奈米碳管具有獨特且優異的化學及物理特性，深具潛力應用於各個領域，例如：場發射顯示器、儲氫材料、化學感測器、奈米電子元件與複合材料等。合成奈米碳管主要的方法有：雷射蒸發法、電弧法與化學氣相沉積法，在這些方法中，過渡金屬(Fe, Co, Ni)常用來作為奈米碳管之成長之觸媒，觸媒存在奈米碳管中會降低奈米碳管的應用特性，本研究論文利用微波輔助酸處理製程去除多層管壁奈米碳管中之金屬觸媒。多層管壁奈米碳管試樣先經過超音波震盪分散後，再以微波輔助消化系統酸處理去除金屬觸媒，研究結果顯示，以熱重分析儀量測純化前後奈米碳管中殘存之金屬觸媒量。在5M硝酸中，微波消化15分鐘，金屬觸媒含量可由 10.39 wt%快速減低至1.52 wt%。利用穿透式電子顯微鏡來觀察純化前後碳管的結構與金屬觸媒，而拉曼光譜儀則探討奈米碳管之鍵結特性。結果顯示，硝酸可以快速吸收微波之熱與能，可以避免因碳管長時間浸泡於酸溶液中，而對管壁所造成的破壞，進而達到快速純化奈米碳管。另外，本論文研究並探討使用微波聚醇法有效且快速被覆白金粒子於奈米碳管上。結果顯示，添加分散劑與保護劑使白金粒子均勻有效被覆於奈米碳管表面，先對奈米碳管進行純化步驟，再利用微波聚醇法以使具有氯鉑酸之乙二醇溶液內之白金觸媒粒子被覆在奈米碳管上。為了快速形成具有高密度且均勻被覆白金粒子的奈米碳管，添加適當聚乙烯吡咯烷酮(Polyvinylpyrrolidone, PVP)與十二烷基硫酸鈉(Sodium Dodecyl Sulfate, SDS)當界面活性劑與分散劑，結果顯示微波聚醇法所合成之白金粒子約4.1nm，其粒徑分佈集中且均勻分散於奈米碳管表面。添加SDS可達到有效分散奈米碳管表面白金粒子，而不會產生聚集現象。微波聚醇法所需反應時間約1.5min，白金被覆量約54 wt%，所以微波聚醇法添加SDS、PVP可有效且快速均勻被覆白金粒子於奈米碳管上。

Carbon nanotubes (CNTs) have excellent and unique performances and versatile applications, such as field emission display, hydrogen storage, chemical sensor, nano-electronic devices and composite materials. High-quality and well-aligned carbon nanotubes are essential to the potential applications. The synthesis methods of CNTs are arc-discharge, laser vaporization, and chemical vapor deposition. Transition metal (Fe, Co, Ni) are known to be catalysts for vapor grown CNTs synthesis. CNTs may found their limited applications as they contain metal catalysts in tubes. In this thesis, microwave-assisted acidic treatment process is used to purify multi-walled carbon nanotubes. Ultrasonic shaking was first applied to disperse CNTs samples. A acidic treatment with microwave-assisted digestion system was then used to dissolve metal catalysts. The result indicated that the amounts of catalyst metals reduced from 10.39 wt% to 1.52 wt% within 15 min of microwave digestion acid treatment with 5 M HNO3. A thermo-gravimetric analysis (TGA) was used to estimate the amount of catalyst metal in CNTs. The structure of carbon nanotubes and catalyst before/after purification is characterized and TEM. The bonding characteristics of CNTs are studied by using a Raman spectrometer. That HNO3 can rapidly absorb microwave heat and energy to shorten the acid treatment time prevents the structures of CNTs from destruction. A high-yield of multi-walled carbon nanotubes with high purity is then obtained. In the second part of the thesis, CNTs are first purified by microwave digestion and Pt nanoparticles in ethylene glycol solution with Pt precursor (H2PtCl6) are coated on CNTs by microwave polyol process. In order to briskly synthesize uniform Pt nanoparticles of high density dispersed on CNTs, poly(vinylpyrrolidone) (PVP) and sodium dodecyl sulfate (SDS) are adequately added in mixture reactants to act as the protective reagents and dispersants. The results show that Pt nanoparticles with narrow distribution and suitable diameter of about 4.1 nm were highly dispersed and loaded on the surface of CNTs by microwave digestion method. SDS was added to increase the dispersion of Pt on CNTs and to reduce the aggregation phenomenon. The microwave polyol processing duration was below 1.5min and the loading amount of Pt on CNTs was about 52 wt%. Pt nanoparticles can be uniformly and rapidly dispersed on CNTs by microwave polyol process with the addition of SDS and PVP.