Multiferroic material is a subject of increasing interest recently. Not only because BiFeO3, with high Curie temperature (TC = 810-830℃) and Neel temperature (TN = 370℃), is one of the multiferroic material, but also because there is another phase, Bi2Fe4O9, exists that has variety of applications. In the present work, the syntheses, characterization, and study of optical properties of single-crystalline BiFeO3 and Bi2Fe4O9 have been done. The two ternary bismuth ferrite compounds can be obtained by conventional hydrothermal method with simple experimental procedure. Besides, the experimental results illustrate that Bi2Fe4O9 with different morphologies can be exactly controlled by adding ethylenediamine as surfactant or by controlling the molar ratio of potassium ions and sodium ions (K+/Na+). Optical absorption properties were confirmed by UV-Vis. The energy band gap of Bi2Fe4O9 is in the visible light-region. Therefore, it has the potential application to be one of the good candidates for visible-light photocatalyst. Moreover, Bi2Fe4O9 has also attracted much attention because of the possible application in photodegredation. In our experiment, we optimize the experimental procedure and provide a simple way to synthesize and growth pure single-crystalline BFO and Bi2Fe4O9 submicron particles with different morphologies.

本研究主要針對鐵酸鉍的兩種相－BiFeO3及Bi2Fe4O9進行合成、表面形貌與結構分析以及對於光學性質的研究。近幾年來，許多研究團隊投入研究具備多鐵性質的BiFeO3薄膜。由於BiFeO3的尼爾溫度與居禮溫度均高於室溫，因此在室溫下，可同時具備鐵電性質與反鐵磁性。故BiFeO3具備特殊的性質，在壓電、磁紀錄、鐵電、terahertz等領域都有很大的潛力可望取代現在的the state-of-the-art。 此外，根據許多文獻指出，當BiFeO3與Bi2Fe4O9小至微奈米尺度，其能階約落在可見光範圍，未來可應用於可見光光觸媒的領域，以取代貴金屬如鉑、鈀等。
　　在本研究之實驗結果中的合成部分，透過改變攪拌速率、礦化劑(mineralizer)之滴定速率、溶液酸鹼值、加熱時間、反應體積以及前驅物的溶解度等多方變數之控制，進行實驗步驟最優化(optimization)，成功透過傳統水熱法合成BiFeO3及Bi2Fe4O9　微奈米結構，且均為單晶結構。此外，實驗結果證實添加乙二胺當作界面活性劑可以成長出不同表面形貌之單晶Bi2Fe4O9。當該系統的酸鹼值逐漸減少時，Bi2Fe4O9　的表面形貌從平板狀逐漸轉變為不規則片狀薄片，甚至產生了如花瓣片狀的特殊結構。這類型的結構具備大量的表面積，有利於未來應用在光觸媒的領域。而實驗結果亦發現，透過調變礦化劑中鈉離子與鉀離子的比例，可得出不同厚度的Bi2Fe4O9片狀結構。以上不同表面形貌的鐵酸鉍系列結構均透過UV-Visible量測，測得此材料的能隙均落於可見光黃光至綠光的波段，且隨著Bi2Fe4O9片狀結構厚度的減少，能隙增加。
藉由改變上述提到之參數，吾人利用水熱法成功合成BiFeO3及Bi2Fe4O9　微奈米結構。而透過研究鈉離子與鉀離子的比例對於Bi2Fe4O9片狀結構厚度影響的機制，可更進一步了解鐵酸鉍系列材料的合成及有關成長機制。相信本實驗結果可提供有關資訊給後續欲用化學方法合成金屬氧化物的研究人員。

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