一維奈米結構釩氧化物VO2， V2O3和b-NaxV2O5之成長及其鑑定

Abstract

在這份研究報告當中，我們提出一套新穎的合成方法來沉積接近垂直成長之奈米線薄膜。能經由此套合成方法製備出的材料有R相之釩氧化二釩(VO2(R))、B相之二氧化釩(VO2(B))、三氧化二釩(V2O3))以及□相的鈉釩氧化物(□-NaxV2O5)奈米線，這套合成方法能細分成兩個部分。在第一部分當中，V2O5奈米線是作為一前驅產物，先一步的由熱蒸鍍合成法製備而得。接著再進一步的將V2O5奈米線在還原氣體的氛圍下加熱，進行還原反應。在此還原反應中，有影響的可變參數為還原溫度、還原反應時間以及還原氣體的組成。經由準確的調控反應參數能夠成功的製備出三個具有不同結晶性釩氧化物的奈米線，其中包括VO2(R)，VO2(B)和 V2O3。在還原反應之後，獲得的釩氧化物能夠順利得保有原先的奈米線構型，且奈米線也仍然是近乎垂直的排列在基板上。在研究奈米線的結晶性的過程當中，我們發現被還原的釩氧化物其奈米線的成長方向會受到最原始的V2O5奈米線的成長方向所影響。因此，在此論點的基礎下，我們考慮各個奈米線的成長方向，成功的建構出在還原反應裡可能進行的轉變機制。 再另一個部分中，在原有的熱蒸鍍合成法中加入「基板前處理」後，□-NaxV2O5奈米線就能成功的被製備獲得。在此系統中，矽酸鈉鹽是作為提供鈉離子來源的反應前驅物，因此我們在不含鈉的基板上先一步的沉積一層矽酸鈉鹽薄膜並精準的控制其鈉的含量。接著在熱蒸鍍合成法中，釩氧錯合物會被蒸氣攜帶至基板上再與基板上的鈉離子反應，進一步的生成出□-NaxV2O5奈米線。經由X光粉末繞射與穿透式電子顯微鏡的量測之後，能夠推測出□-NaxV2O5奈米線是沿著[100]的方向成長。在實驗的過程當中，我們發現到會影響此反應的可變實驗參數有反應溫度，熱蒸鍍時所使用的釩氧離子溶液的濃度以及所使用的矽酸鈉鹽溶液的濃度。經由調控這些參數，能夠控制成長在基板上的奈米線長度。最後在研究報告當中，我們將提出其可能的成長機制。 經由探討各個奈米線材的電子場發射性質，顯示出各個材料本身的性質與其場發射特性具有相當的關連性。此報告中所製備出的奈米線皆具有低觸發電壓(turn-on field)、高最大發射電流密度(emission current density)。製備出的奈米線材中，V2O3奈米線是有較好的電子場發射性質，其觸發電場值為5.2V/□m，而在電場值8.3V/□m的作用底下所測得的最大發射電流密度為8.3mA/cm2。這些研究結果皆顯示出，釩氧化物奈米現在未來的場發射元件上具有相當大的發展潛力。

We report a deposition process to prepare nearly aligned binary and ternary vanadium oxides nanowires, including VO2(R), VO2(B), V2O3, and □-NaxV2O5. In the first section, VO2(R), VO2(B), and V2O3 nanowires were prepared by reducing the precursor of V2O5 nanowires with controlled concentration of reducing gas flow and reduced period. The large portions of NWs microstructure are successfully preserved during the reduction, and are still nearly vertical-aligned on the surface. The growth directions of these reducing products were influenced by the growth direction of the original phase, V2O5 NWs. On the basis of the growth direction of each product, possible mechanisms of conversion during reduction reaction are proposed. In the second section, □-NaxV2O5 nanowires were prepared via a route combined the original thermal evaporation procedure with additive pre-treatment of the surfaces of substrates. The sodium silicate, source of the sodium, was first coated on the substrates and the amount of precursor was carefully controlled on a Na+-free substrate. The length of the □-Na0.24V2O5 wires could be controlled in the range of 5-25□m by changing the reaction condition, which grew along the [100] direction. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) were used to confirm the crystallinity of oxide nanowires. The effect of the controlled amount of deposited material and morphology were studied. A possible formation mechanism was proposed. The field emission properties of these binary and ternary vanadium oxides nanowires have been investigated. The results show that the as-prepared NWs possess excellent field emission performances with low turn-on field, large emission current density and linear Fowler-Nordheim behaviors. The studies suggest that these 1D nanomaterials could serve as promising candidates for future field emission devices.