介孔洞模板輔助熱化學汽相沉積法成長均一性碳奈米管

Abstract

本研究目的在用觸媒輔助熱化學氣相沉積法去成長高均一性尺寸的碳管，並且去探討其結構和性質。反應氣體包含不同流量比例的乙炔、氫氣和氮氣。以silica gel和MCM-41介孔洞粉末分別作為碳管成長的基材和模板。經使用二氧化矽前驅物 (矽酸鈉) 和介面活性劑 (十六烷基三甲基溴化銨) 當原料的標準製程來準備MCM-41，而silica gel是經由商業管道購買。MCM-41先藉由加入孔洞導入劑 (二氯甲烷) 把碳管觸媒前驅物 (硝酸鐵) 滲入其孔洞內，而silica gel是當作基材來沉積碳管觸媒前驅物 (硝酸鐵) 。接著將這些滲入孔洞或覆蓋表面觸媒前驅物的粉末在熱化學氣相沉積系統上成長碳管。這些粉末和碳管的結構和性質由BET、SEM、TEM、XRD、FTIR和拉曼光譜技術來分析。從這些實驗結果可以得到以下的結論。 silica gel和MCM-41的平均孔洞大小分別為 ~ 6 nm和 ~ 2.8 nm。對於使用silica gel當基材的試片主要成長出多壁碳管，其管徑大約分為兩個部份 (8 ~ 60 nm) 接近管徑尺寸和遠大於管徑尺寸。接近管徑尺寸可能是觸媒落在管口附近而受到限制。結果顯示在中間成長溫度 ( ~ 850℃) 時，有最大的碳管數量密度。XRD的結果指出大於成長溫度800℃時碳管有很好的石墨化。 關於加入氮氣及無氫氣條件下對碳管成長的影響，結果顯示加上氮氣並未產生竹節狀碳管，雖然文獻上報導氮氣通常有助於竹節狀碳管的形成。還發現到沒有加入氫氣和氮氣於反應氣體中所成長出的碳管平均直徑較小。 使用MCM-41當模板去控制管徑尺寸的可能性,其結果顯示所成長出碳管的平均直徑大約為2.5 nm，它接近MCM-41的孔洞尺寸 ( ~ 2.8 nm) 。換句話說，使用MCM-41當作模版可成功長出雙壁碳管。

The purposes of this work were to synthesize carbon nanotubes (CNTs) with high uniformity in sizes by the catalyst-assisted thermal chemical vapor deposition (CVD) method and to examine their structures and properties. The reaction gases include C2H2, H2 and N2 under different flow ratios. The MCM-41 and silica gel are mesoporous powders, which were used as template and substrate for CNTs growth, respectively. The MCM-41 powders were prepared by a standard process, using SiO2 precursor (Na2SiO3) and surfactant (C16TMAB) as raw materials, and the silica gel powders were purchased from commercial source. The pore channels of the MCM-41 powders were first infiltrated the CNTs catalyst precursor (Fe(NO3)3) by adding pore penetration agent (CH2Cl2), and the surfaces of silica gel powders were used as substrate to deposit the CNTs catalyst precursor (Fe(NO3)3) for some of the specimens. The catalyst precursor-infiltrated or coated powders were followed by CNTs deposition in thermal CVD system. The structures and properties of powders and CNTs were characterized by BET, SEM, TEM, XRD, FTIR and Raman spectroscopy. From the experimental results, the following conclusions can be drawn. The average pore sizes of the silica gel substrates and MCM-41 templates are ~ 6 nm and ~ 2.8 nm, respectively. For specimens with silica gel as substrates, the deposited CNTs are mainly multi-walled carbon nanotubes (MWNTs) networks, where the tube diameters (8 ~ 60 nm) can be roughly divided into two groups – tube diameters close to or much greater than the pore size of the silica gel. The group of the smaller tubes may be grown under the limitation of pore doors. The results also show that there is the maximum tube number density at medium deposition temperature ( ~ 850℃). The XRD results indicate that a good graphitization of CNTs can be obtained with deposition temperatures greater than 800℃. Regarding to effects of adding nitrogen and adding no hydrogen on CNTs growth, it is appealed that nitrogen addition does not activate formation of bamboo-like CNTs, though nitrogen is generally believed to have greater tendency to form bamboo-like CNTs. It is found that the average diameter of the deposited CNTs is much smaller by adding no hydrogen and nitrogen in the reaction gases. To examine the possibility of using MCM-41 as templates to control the tube sizes, the results show that the average diameter of the deposited double-walled carbon nanotubes (DWNTs) is about 2.5 nm, which is close to the average pore size of MCM-41 (~ 2.8 nm). In other words, the MCM-41 can be successfully used as template to grow DWNTs.