奈米級含氮及含氮鐵二氧化鈦製備及可見光催化活性研究

Preparation, Characterization and Visible-light Responsive Photocatalytic Activity of Iron/Nitrogen Doped Titanium Dioxide Nanocrystalline

徐寶崇(Bao-Chrung Hsu)

指導教授 ： 陳孝行 ； 蘇昭瑾

國立臺北科技大學/工程學院/工程科技研究所/博士(2009年)

https://doi.org/10.6841/NTUT.2009.00533

摘要

本研究以一種簡單、創新的程序，密閉系統中，煅燒 Deguessa P-25 二氧化鈦和氯化銨之混和物，以製備含氮二氧化鈦 (TiOxNy)，由化學分析能譜儀(ESCA)可知，氮成功的摻雜在二氧化鈦上，其中在煅燒溫度400 °C且二氧化鈦和氯化銨比為 1：6 時，改質後之 TiOxNy (TiOxNy -1:6-400) 有最高之含氮量 19.22 at.%，氮摻雜的二氧化鈦利用紫外線/可見光（UV-Visible）吸收光譜儀進行分析，會產生紅位移。由X-ray繞射儀（XRD）分析結果可知，在較高溫度及較高氯化銨/二氧化鈦比值時，二氧化鈦之結晶相易從銳鈦礦相（anatase phase）逐漸轉為金紅石相（rutile phase），推測係因高溫下形成的HCl(g)導致。另經由亞甲基藍的光催化降解，製備之TiOxNy在煅燒溫度400 °C且二氧化鈦/氯化銨為 1：6 時具有最佳之光催化活性。依據ESCA、UV-Visible、XRD和比表面積分析（BET）可知氮摻雜量、二氧化鈦結晶相成分及能隙是影響改質後二氧化鈦光催化活性之主要影響因子。
同樣地，亦可經由類似程序，煅燒 DP-25、氯化銨和零價鐵（ZVI）之混和物，以製備含氮鐵二氧化鈦 (Fe/N-TiO2)，由ESCA分析可知，氮和鐵成功的摻雜在二氧化鈦上，形成 N-Ti-O鍵結及Fe2O3，其中氮摻雜會降低鈦的束縛能，鐵的摻雜則會提高鈦的束縛能，由XRD分析顯示，ZVI的添加會形成二個結晶相-pseudo-brookite (Fe2TiO5)和Fe2O3，其中Fe2O3其作用類似形成複合半導體，且易在二氧化鈦表面上鍵結形成Fe2TiO5，而增加結晶的大小，此外，鐵摻雜會抑制二氧化鈦之結晶相從anatase phase轉為rutile phase。由UV-Visible吸收光譜分析可知，氮和鐵摻雜在二氧化鈦上會產生顯著的紅位移，經由亞甲基藍的光催化降解，製備之Fe/N-TiO2-1:6-0.01 (即TiO2 和 NH4Cl 重量比為1:6，且ZVI添加量為0.01g，煅燒溫度400 °C) 時具有最佳之光催化活性 (kapp = 0.068)，且其速率常數約為DP-25的4倍。
利用循環伏安法（cyclic voltammetry, CV）量測改質二氧化鈦之電化學特性，顯示ZVI的添加會降低改質後之Fe/N-TiO2的價電子帶（valence band）和導電帶（conduction band），另從光催化反應結果可推知，直接經由光生電洞產生之氫氧自由基的總氧化效應大於間接經由高氧自由基所產生之氫氧自由基，且要達到最佳之光催化活性之鐵的摻雜量在1.9at.%（即ZVI為0.01g）。
亞甲基藍光降解機制符合Langmuir-Hinswood 模式且遵循一次反應動力學，經由Langmuir-Hinswood 模式線性關係可求得反應動力常數k及吸附常數K值如下：
For Fe/N-TiO2 （ZVI為0.01g）：k = 1.73545 mg/l hr and K = 0.64578 l/mg
For DP-25：k = 0.42769 mg/l hr and K = 1.43660 l/mg
顯示Fe/N-TiO2反應動力常數約為DP-25的4倍，且DP-25吸附常數K大於Fe/N-TiO2吸附常數，此結果與比表面積（BET）實驗所得結果吻合，即DP-25的比表面積大於Fe/N-TiO2。

關鍵字

零價鐵 ； 氯化銨 ； 含氮鐵二氧化鈦 ； 可見光 ； 光催化活性

並列摘要

A simple and novel procedure was developed for preparing nitrogen doped titanium dioxide （TiOxNy）by calcinating a mixture of Deguessa P-25 TiO2 and NH4Cl under an airtight system. Analysis of electron spectroscopy for chemical analysis (ESCA) indicated the molecular state nitrogen was incorporated into TiO2 lattice leading to the observable shift of the absorption edge to longer wavelength region with higher absorption intensity. the highest nitrogen content in TiOxNy is 19.22 at. %, occurred in the TiOxNy -1:6-400 sample with TiO2 : NH4Cl weight ratio of 1:6 and calcination temperature of 400 oC. X-ray Diffraction (XRD) results show that the anatase-to-rutile phase transformation occurs at high temperature calcination and/or high NH4Cl/TiO2 ratio. These TiOxNy samples exhibit photocatalytic activity for methylene blue (MB) decomposition in aqueous solution under visible light irradiation. The optimum photocatalytic activity for the degradation of MB was achieved for the sample of TiOxNy-1:6-400. The crystal phase composition and band gap energy are the major factors responsible for TiOxNy photocatalytic activity. The correlation between the photocatalytic activity of TiOxNy and nitrogen content, rutile phase composition, and surface area was investigated and discussed in this thesis.
Doping nitrogen in TiO2 enhances photoresponse in visible light region while doping iron reduces the recombination of photo-induced electrons and holes. In the second part of this thesis, a similar procedure was applied to prepare nitrogen and iron co-doped titian nanocrystalline (Fe/N-TiO2) by calcinating the mixture of Degussa P-25 TiO2, zero-valent iron (ZVI), and NH4Cl at the temperature of 400 oC under airtight condition. Characterization of Fe/N-TiO2 was investigated by ESCA, XRD, and UV/Vis spectroscopy. The ESCA measurement showed that both N and Fe were successfully doped into TiO2 lattice forming the N-Ti-O linkage and Fe2O3 in the TiO2 crystal. The N doping lowered the Ti binding energy, while the Fe doping increased the Ti binding energy. The XRD results showed the formation of two new phases: pseudo-brookite (Fe2TiO5) and Fe2O3. The Fe2O3 acts as composite semiconductor which is easy to bond and produce Fe2TiO5 adsorbing onto the TiO2 surface, increasing the crystallite size. The Fe doping can inhibit the anatase to rutile transformation of TiO2 during the calcinations. Finally, the absorption wavelength of Fe/N-TiO2 was shifted to the visible light region for maximum of 561 nm. The samples of Fe/N-TiO2 also exhibit photocatalytic activity for MB decomposition in aqueous solution under visible light irradiation.
The electro-chemical properties measured with cyclic voltammetry (CV) showed that the incorporation of ZVI led to lower valence band and lower conduction band in ZVI-adding from 0.005g to 0.05g. The results from the photocatalytic reaction can be extrapolated that total oxidation effect of OH radicals generated directly by photo-generated holes is higher than that of OH radicals generated indirectly by high-oxygen free radicals. There is an optimal dopant concentration of Fe3+ at 1.9 at.% corresponding to 0.01g ZVI-adding in this study.
The MB photo-degradation mechanism is consistent with Langmuir-Hinshelwood (L-H) model and obeys first-order kinetics. The linear transformation of L-H model allows to calculate the kinetic rate constant k and adsorption constant K. These values are given below:
For Fe/N-doped TiO2: k = 1.73545 mg/l hr and K = 0.64578 l/mg.
For DP-25 TiO2: k = 0.42769 mg/l hr and K = 1.43660 l/mg.
This clearly indicates that the kinetic rate constant k of Fe/N-TiO2 is about fourfold than that of DP-25 and the adsorption constant K of DP-25 is greater than that of Fe/N-TiO2, consistent with the previous BET results that the specific DP-25 surface area is greater than that of Fe/N-TiO2.

並列關鍵字

Zero valent iron ； ammonia chloride ； Fe/N-TiO2 ； Visible light

參考文獻

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