奈米二氧化鈦基系統之氣體感測行為研究

The Gas Sensing Properties of Nanocrystalline TiO2 based Systems

楊宗燁(Tsung-Yeh Yang)

指導教授 ： 林鴻明

大同大學/工程學院/材料工程學系所/博士(2005年)

摘要

本研究是藉由紫外光照射，強化奈米二氧化鈦對氣體感測感測行為的研究，樣品的製備是在 5-mbar 的氦氣和氧氣氣氛下利用氣相沉積技術，並且淬火至液態氮溫度形成奈米二氧化鈦。初合成的奈米二氧化鈦具有均勻尺寸與多孔性，利用X光繞射 ( XRD ) 光譜對初合成及經過熱處理的樣品做相鑑定；利用電子穿透顯微鏡 ( TEM ) 量測微粒的結晶結構和陶瓷的顯微結構；利用原子力顯微鏡 ( AFM ) 分析微粒的表面形態與微粒的尺寸。並且研究二氧化鈦的結構、表面形態以及紫外光照射對於二氧化鈦感測器感測特性的影響。
初步X光繞射光譜的結果顯示二氧化鈦的結構，在 600℃ 的燒結溫度下，二氧化鈦仍是銳鈦礦相 (anatase)；經過 800℃ 的燒結後二氧化鈦相變化產生銳鈦礦與金紅石相 (rutile) 兩相共存；經過 1000℃ 的高溫下二氧化鈦由低溫的銳鈦礦完全相變化轉換成金紅石相。為了要對奈米二氧化鈦在退火之後，在結構特性上有更佳的瞭解，利用X光吸收光譜技術，能夠詳細的定義氧或鈦區域原子排列的狀況。根據 XAS 結果，鈦 K-edge 的近緣圖譜並沒有發生明顯的變化，由這可推斷奈米二氧化鈦微粒的結構轉換，可能是氧在微粒裡遷移並在燒結後所引起。另一個原因是當鍛燒溫度增加時，只使得氧化現象更加明顯，經過鍛燒之後，氧的峰值位置仍不改變。這些結果顯示從 600℃ 加熱到 1000℃ 的熱處理過程使得奈米二氧化鈦的結構更加穩定，這對感測材料是一個很重要的資訊。
多孔性奈米二氧化鈦的表面很容易吸附水氣，當二氧化鈦表面溼度增加時會使得電阻減小。在 CO 不同濃度範圍，阻值的變化可由電阻與時間變化的關係得到。奈米二氧化鈦氣體感測，經紫外光照射其特性可提升已被證實。實驗證實在 CO 氣體、紫外光照射提升下靈敏度被提升30倍，並且可利用紫外光激發產生光催化反應的模型來解釋其物理意義。
另一個結果顯示基於基本理論的假設下，我們發現二氧化鈦氣體感測器在高溫與在紫外光照射下，可明顯的提升其導電性與靈敏度，靈敏度隨著紫外光照射流量密度的增加而增加。除此之外，我們提出一個新的模型解釋二氧化鈦表面與氣體分子之間反應的反應機制。
此外另一個研究顯示，利用紫外光提升二氧化鈦感測器特性，除了可以增加導電性外亦可促進反應速率。 對一氧化碳氣體而言在 200-800 ppm 紫外光照射下，感測器有好的靈敏度。在光照的情況下反應時間會增長且回復時間會隨濃度的增加而延長。
表面 FTIR 光譜學證明了對於及時偵測氣體/固體表面相互作用的影響與發生在感測器上的及時反應是一個強而有力的技術。 這個技術證明了氣體感測器的表面反應，取決於紫外光的活化條件。 除此之外，為了確保氣體感測器有更佳的反應效率、好的再線性、高的靈敏度和多的選擇性，研究上必須小心地製備氣體感測器的材料並且控制好操作的參數。

關鍵字

感測器 ； 原子力顯微鏡 ； 電子穿透顯微鏡 ； X光繞射光譜 ； 奈米二氧化鈦

並列摘要

The gas sensing properties of nanocrystalline TiO2 are enhanced by submitting the sensor system to UV light irradiation. Titanium dioxide nanoparticles are produced by gas condensation technique under 5-mbar of helium and oxygen atmosphere, and then quenched to the liquid nitrogen temperature. The as prepared titanium dioxides of nanoparticles are porous free with uniform size. The powder X-ray diffraction (XRD) spectrum confirms that the as-prepared and after sinter treatments samples. The crystal structure and ceramic microstructure of the powders are determined by transmission electron microscope (TEM). The size of granules and surface morphology are analyzed by atomic force microscopy (AFM). Structural and morphological studies as well as UV irradiation are carried out in order to investigate their influence on the sensing properties of TiO2 sensor.
Preliminary X-ray diffraction results indicate that the structure of TiO2, that is transformed to the rutile phase up 1000℃. There are two phases exist at 800℃, one is anatase phase and another one is rutile phase. An anatase phase exist at 600℃. In order to understand the better distinguish the structural properties of the titanium dioxide Nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or titanium can be determined. According to the XAS result, the shape of the Ti K-edge undergoes no distinct changes. This infers that structure transformation of titanium dioxide nanoparticle may be caused by the migration of oxygen after sintering. The other reason is when increases of calcine temperature caused the oxidation more obvious. After calcine process the position of the oxygen doesn't shift. These results indicate that heat treatment to approximately from 600℃ to 1000℃ produced stable structures of nanocrystalline of TiO2. It is important information to the sensing material.
It is easily to adsorb the water vapor on the surface of porous nano-TiO2. The resistance decreases while the surface of TiO2 capacitance increases with highly humidity. The gas sensing properties can be obtained from the changes of the resistance as a function of time during different CO concentrations range. The gas-sensing properties of nano-TiO2 have been demonstrated to be enhanced by UV irradiation. The sensitivity to gaseous CO increases by a factor of 30 with UV light enhancement. The model of photocatalysis can be used to interpret the enhancement of the sensing properties by UV stimulation. The UV enhancement of the gas sensing property can be explained by the decrease of the activity energy of surface reactions.
The results of vacuum environment experiment indicate the effect of UV radiation on the sensing mechanism of TiO2 gas sensor can be carried out based on theoretical assumptions. The UV radiation can significantly enhance the conductivity and sensitivity of a TiO2 gas sensor at high temperature. The sensitivity increases with the increasing UV radiation flux density. Also, the current of the nano TiO2 thin films due to the UV irradiation at lower temperature is comparable with that of the nano TiO2 at high temperature without UV irradiation. A new model has been created in this study to explain the mechanism of gas/soild interaction between surface of TiO2 and CO molecular.
The light activates the TiO2 gas sensor increasing both the changes in conductance also increasing the rate of reaction. The sensor exhibited good sensitivity to CO (200-800 ppm) with UV irradiated. It shows that the response times increased slightly and the recovery times increased with increasing CO concentration. Surface FTIR spectrometry proved to be a powerful technique for studying in situ detection the gas/soild surface interactions and electric response occurring in nano TiO2 gas sensor. This technique has proved that surface reactivity of gas sensor depends on the activation conditions of UV source. In the other words, to ensure better response, reproducibility, higher sensitivity and selectivity of the gas sensor, the material of gas sensor must be carefully prepared and the working conditions shall well control.

並列關鍵字

AFM ； Gas sensor ； Nanocrystalline TiO2 ； TEM ； XRD

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