Novel Cathodic Deposition of TiO2 Thin Film: Deposition Mechanism, Material Characterization, and Morphology Control

黃靜君(Ching-Chun Huang)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/碩士(2010年)

摘要

In this study, a novel and fast cathodic deposition process is developed to fabricate a titanium dioxide thin film from an aqueous solution containing titanium trichloride and excess sodium nitrate. The reaction mechanism is studied by linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical quartz crystal microbalance (EQCM). It is found that the redox reaction NO3 and Ti3+ in the precursor solution accelerates the cathodic deposition of TiO2 thin film drastically. The Ti to O ratio of as-prepared thin film is confirmed to be 1 to 2 by high-resolution X-ray photoelectron spectroscope (HRXPS). Besides, the surface morphologies and crystalline phase are characterized by field-emission transmission electron microscopic (FETEM), field-emission scanning electron microscopic (FESEM), X-ray diffraction (XRD) and atomic force microscope (AFM). The surface morphology of the as-prepared deposit plated at room temperature has been confirmed to be rough and porous, which is mainly full of micropores from the BET results of nitrogen adsorption/desorption isotherms.
This study also demonstrates that the microstructure of TiO2 film can be designed and controlled by adjusting the temperature and cycle number of cathodic deposition in the previous, specially designed solution. The poor electron conductivity of TiO2 depresses the generation rate of OH from the reduction of NO3 and N2, reducing the TiO2 deposition rate. The morphology and size of TiO2 aggregates are strongly influenced by varying the deposition temperature from 5 to 50 oC. A maximal rate of TiO2 deposition is obtained at 25-35 oC because a higher OH concentration favorable for TiO2 formation and more N2 bubbles crowding out TiO2+ and TiO2 primary particulates occur simultaneously at the cathode surface vicinity with increasing the bath temperature. TiO2 deposited at 25 oC is roughest with roughness factor (Ra) = 66.847 nm. This study provides a useful method to control the
morphology and deposition rate of TiO2 film for practical applications.
Besides, in this work, two different annealing strategies: annealing in air and microwave-assisted hydrothermal annealing (MAHA) on the crystallinity of TiO2 deposits were compared. A novel and promising strategy is proposed for obtaining well-crystalline TiO2 in the anatase phase (A-TiO2) at a low annealing temperature of 150 oC in a very short annealing time. The porous titanium dioxide films were treated by the MAHA process to increase the crystal size of A-TiO2 at 150 oC. Through FESEM and FETEM analyses, the morphology of TiO2 particles remains grainy without obvious size change. From selected-area electron diffraction (SAED) and Raman characterization, the amorphous as-deposited TiO2 films were effectively transformed into crystalline TiO2 of the anatase phase in the MAHA environment at 150 oC. Effects of the applied power density and MAHA time on the A-TiO2 crystal size at 150 oC were preliminarily investigated. TiO2 with the best crystalline quality was obtained under an applied power density of 120 W for 2 min, very similar to A-TiO2 annealed in air at 300 oC for 1 hr.

關鍵字

二氧化鈦 ； 陰極沉積 ； 反應機制 ； 型態控制 ； 銳鈦礦

並列摘要

本研究利用三氯化鈦與過量硝酸鈉混合水溶液為新穎鍍液，透過陰極沉積法將二氧化鈦薄膜快速地製備於電極表面。藉由線性伏安掃描(LSV)、循環伏安掃描(CV)及電化學石英天平(EQCM)的分析結果，我們推導出此陰極沉積系統的反應機構；研究發現於電化學掃描時，前驅物溶液中硝酸根離子與三價鈦離子的氧化還原反應會顯著地加速二氧化鈦的沉積速率。透過高解析度X光光電化學光譜(HRXPS)，所得薄膜之鈦原子與氧原子的成分比例是2比1，進而確定二氧化鈦的生成。此外，經由場發射式穿透式電子顯微鏡(FETEM)、場發射式掃描式電子顯微鏡(FESEM)、X光繞射光譜(XRD)及原子力顯微鏡(AFM)的鑑定，室溫下沉積之二氧化鈦薄膜的表面型態是粗糙且多孔性的，且經氮氣吸脫附實驗測量證實氧化鈦鍍層之孔洞大多為微孔級(micropores)孔洞。
本研究亦藉由調控沉積溫度與循環掃描圈數來達到二氧化鈦薄膜微結構的設計與控制。我們發現當沉積圈數越多時，由於前期沉積上去的二氧化鈦薄膜導電性不佳，會減緩後期電極表面的氫氧根離子生成反應，進而降低二氧化鈦的沉積速度。此外，二氧化鈦薄膜的微結構受沉積溫度的影響很大，而最大沉積速率發生於25-35 oC 而非50 oC。經過線性伏安掃描曲線的觀察，高溫(例如50 oC)下，雖然氫氧根離子的產生量會增加，但同時間產生的大量氮氣氣泡亦會將TiO2+離子推擠出電極表面區域，導致二氧化鈦的沉積出現困難。室溫(25 oC)下製備之二氧化鈦薄膜，經由AFM測定，其粗糙因子(roughness factor, Ra)為66.847奈米。簡言之，本研究提供了一個可以因應不同應用而控制二氧化鈦薄膜微觀結構的有效方法。
此外，本研究透過兩種不同的途徑來增加陰極沉積之二氧化鈦薄膜的結晶性:空氣環境中直接以高溫鍛燒(DHT)與水相環境中以微波輔助水熱方式鍛燒(MAHA)。其中，微波輔助水熱的方式由於可以在相對低溫和相對短的時間內達到好的結晶效果，因此不僅具新穎性且具前瞻性。藉由微波輔助水熱的途徑，銳
鈦礦(anatase)晶相的多孔性二氧化鈦薄膜僅需150 oC的溫度便可得到。經FESEM與FETEM的分析後，二氧化鈦薄膜中的組成粒子維持球狀且大小不變。從非結晶相到銳鈦礦結晶相的轉變是透過FETEM的選區繞射(SAED)及拉曼光譜(Raman)來確認；同時藉由拉曼光譜的訊號強度比較，我們研究了微波能量密度及反應時間對結晶程度的影響。初步的研究結果顯示，微波輔助法僅需150 oC及2分鐘，便可達到傳統高溫鍛燒法在300 oC及1小時下所得的結晶度。

並列關鍵字

titanium dioxide ； cathodic deposition ； reaction mechanism ； morphology control ； anatase

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