以回流加熱法製備二氧化鈰奈米顆粒

Abstract

二氧化鈰奈米材料(ceria nanomaterial)因為其尺寸小、比表面積大、還有化學性質活潑等因素，再加上取得容易，因此為稀土元素中應用最為廣泛的金屬氧化物，其應用於觸媒、半導體、電化學、與生醫等領域皆有出色的發展。多種形狀的二氧化鈰奈米顆粒之製備方式以及其適合的應用領域之研究已經被大量的發表，然而多數的二氧化鈰奈米材料皆使用水熱合成或是溶劑熱合成法來進行製備，因此本研究從製備方式改變，使用回流加熱法來進行合成，希望能以較低的技術門檻與較短的反應時間，同時保留其穩定性的方式來製備二氧化鈰奈米材料。 首先，本研究使用回流加熱法，在PVP和乙二醇的輔助下以140℃反應2小時製備出平均粒徑45奈米的二氧化鈰奈米球(ceria nanosphere)，接著我們觀察反應前驅物濃度對反應造成的影響，透過調整前驅物和乙二醇的濃度，成功製備出單分散(monodisperse)，平均粒徑20.6奈米與91.7奈米的奈米球，最後我們分別探討PVP、溶劑與前驅物對反應的影響，透過調整上述三種反應參數，我們分別得到三種聚集且不規則狀的奈米顆粒，證明PVP做為界面活性劑以及高分子分散劑對產物形態所帶來的益處。 除此之外，我們同樣使用回流加熱法，在製備奈米球的系統中加入氫氧化鈉，以同樣的反應溫度以及時間，成功製備均勻(uniform)且分散的二氧化鈰奈米棒(ceria nanorods)，達到了形狀的控制，也改善了在鹼性環境下合成二氧化鈰奈米棒容易聚集的問題，進一步發現在PVP和乙二醇的輔助下，只需要0.5小時就能製備出二氧化鈰奈米棒，且若沒有在系統中添加PVP與乙二醇，所得之二氧化鈰奈米棒之聚集程度高出許多，甚至難以辨識其輪廓，在奈米棒的周邊也會有許多雜質生成。上述的現象可證明PVP能夠加速奈米晶粒的自組裝(self-assembly)，且有助於二氧化鈰奈米棒的分散。最後為了控制二氧化鈰奈米棒的長度與長寬比，我們嘗試了不同的反應參數。當我們改變了反應時間及溫度，觀察反應時間2-24小時，反應溫度100-180℃，發現隨著反應時間由2小時上升到6小時，奈米棒有變長的情形，但同時聚集的程度也變得更加明顯，且可以明顯看出有許多不同形態的產物，當反應時間從6小時拉長到24小時，奈米棒似乎反而有變短的趨勢，但是一樣有許多不同型態的產物生成。當我們改變了前驅物的濃度，發現當前驅物濃度降低時，二氧化鈰奈米棒的長度有些微的上升，同時寬度並沒有改變。最後當我們改變氫氧化鈉的濃度，發現奈米棒的長度有顯著的上升，因此我們可以認定若要改變二氧化鈰奈米棒的長度，增加氫氧化鈉濃度是比較有效率的做法。

Cerium dioxide (ceria) nanomaterials is the most widely applied metal oxide among rare earth family due to their small size, large specific surface area, high chemical activity, and they are the most abundant element in lanthanide metal. Ceria has excellent development on catalyst, semiconductor, electrochemistry, and biomedical science. Researches of synthesis processes, different shapes of ceria nanoparticles and their shape dependent applications have been published several times. However, most of the ceria nanomaterials are synthesized by hydrothermal method and solvothermal method, therefore, this study changed from the synthesis method, using reflux heating method to synthesize, we hope that we can prepare ceria nanomaterials employed lower technical equipment and shorter reaction time while retaining the stability. First, we use the reflux heating method, and with the aid of PVP and ethylene glycol , we successfully synthesize ceria nanospheres at temperature 140℃ and reaction time 2 hours, which with the average radius of 45nm. Next, we investigate the effect of the precursor concentration on the reaction. By adjusting the concentration of the precursor and ethylene glycol, we successfully prepared monodisperse nanospheres with average particle sizes of 20.6 nm and 91.7 nm. Last, we separately discuss the effects of PVP, solvent and precursor on the reaction. By adjusting the above three reaction parameters, we obtained three aggregated and irregular nanoparticles, proving the effect of PVP as a surfactant and soft template on the product morphology. In addition, again, we used reflux heating method, and add sodium hydroxide to the system for preparing nanospheres. We successfully prepared uniform and dispersed ceria nanorods at the same reaction temperature and time, which means we achieve the shape selective synthesis, and also improved the common problem of aggregation when preparing ceria nanorods in alkaline environment. It was further found that with the aid of PVP and ethylene glycol, it took only 0.5 hours to prepare ceria nanorods, and if without PVP and ethylene glycol, the degree of aggregation of the obtained ceria nanorods is much higher, and it is even difficult to identify the rod shape, what’s more, many impurities are generated around the nanorods. The above phenomenon can explain that PVP and ethylene glycol can push the speed of self-assembly of nanocrystal grains and contribute to the dispersion of ceria nanorods. Finally, with the purpose of control the length and aspect ratio of the ceria nanorods, we tried different reaction parameters. As we change the reaction time at the range of 2-24 hours and reaction temperature at the range of 100-180℃, it was found that as the reaction time increased from 2 hours to 6 hours, the nanorods became a little bit longer, but at the same time the degree of aggregation became more obvious, and it can be clearly seen that there are some different shapes of products. When the reaction time extended from 6 hours to 24 hours, the nanorods seem to have a tendency to become shorter, but there are still many different shapes of products. When we changed the concentration of the precursor, we found that as the concentration of the precursor decreased, the length of the ceria nanorods increased slightly, while the width did not change. Finally, when we changed the concentration of sodium hydroxide, we found that the length of the nanorods increased significantly. Therefore, we can assume that it is more efficient to increase the concentration of sodium hydroxide if we want to change the length of the ceria nanorods.