核殼結構奈米金或銀/二氧化鈦之合成及其在光催化反應之應用; Photocatalytic Destruction of Methylene Blue on Au@TiO2 or Ag@TiO2: Effect of Core Size and Shell Thickness

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Title:	核殼結構奈米金或銀/二氧化鈦之合成及其在光催化反應之應用;Photocatalytic Destruction of Methylene Blue on Au@TiO2 or Ag@TiO2: Effect of Core Size and Shell Thickness
Authors:	陳昱碩;Yu-Shou Chen
Contributors:	化學工程與材料工程研究所
Keywords:	光觸媒;核殼結構;亞甲基藍降解;奈米金;奈米銀;二氧化鈦;膠體溶膠法;水熱法;gold;silver;hydrothermal method;methylene degradation.;sol-gel method;photocatalyst;titanium dioxide;core/shell structure
Date:	2011-06-16
Issue Date:	2012-01-05 11:15:39 (UTC+8)
Abstract:	本研究之目的在於發展高催化活性之核殼結構光觸媒，並將其應用於有機汙染物之分解。摻雜貴金屬於二氧化鈦表面可以提高光催化效率，由於金屬在表面形成電子活性點以促進界面電荷轉移。此種觸媒結構雖然活性好，但容易造成暴露在外的金屬與其他表面介質產生作用，使金屬容易溶解或腐蝕，導致觸媒活性衰退。核殼結構可以克服此缺點，將貴金屬置於內核，而二氧化鈦當作殼層。本研究利用膠體凝膠法合成金/二氧化鈦及銀/二氧化鈦核殼結構光觸媒，並利用不同製備條件 (CTAB濃度、聯胺與硝酸銀比例、貴金屬前驅物濃度與水熱溫度) 來達到控制貴金屬核的粒徑大小、貴金屬擔載量以及二氧化鈦的結晶性。觸媒鑑定方面，主要是以紫外可見光光譜儀(UV-vis)、動態散射粒徑分析儀(DLS)、感應偶合電漿質譜分析儀(ICP)、X光繞射儀(XRD)、穿透式電子顯微鏡(TEM)、高解析穿透式電子顯微鏡(HRTEM)與X光電子能譜儀(XPS)進行鑑定與分析。並以兩支8w波長為254 nm的紫外燈管為光源進行亞甲基藍的光催化反應測試。在TEM圖、紫外可見光光譜以及動態散射粒徑分析儀結果顯示，改變CTAB的濃度能有效控制Au@TiO2中金核的顆粒大小在6.6到32.37 nm之間，然而在Ag@TiO2中則需要改變聯胺和硝酸銀的比例才能夠控制銀核的顆粒大小，其大小則在6.82到15.35 nm之間。在X光繞射分析顯示隨著水熱的溫度增加，二氧化鈦的結晶性與結晶大小也會增加。光反應活性之鑑定以10 ppm亞甲基藍水溶液為光反應標準物，以兩支8 w 波長為254 nm 的紫外光燈管當作光源，光降解樣品取樣利用紫外可見光光譜儀(UV-vis)分析濃度。在不同貴金屬核顆粒大小中，小的粒徑由於有較好的電子捕捉性因此有較好的活性，但活性變化不大。在Au@TiO2 與Ag@TiO2中最適化的金屬擔載量分別為1wt.% 及0.5wt.%。此外光催化活性也與水熱溫度成正比，其主要由於增加水熱溫度，會增加二氧化鈦的結晶性，而晶型結構明顯的二氧化鈦，可以增加光子進入內核的量子效率。總結上述結果，在Metal@TiO2核殼型光觸媒中，貴金屬核的大小並不是影響活性的主要因素，而是與二氧化鈦殼層的結晶性、貴金屬的種類與擔載量有關。 The purpose of this study was to develop a catalyst with high photocatalytic activity and had core/shell structure. It could be applied to the decomposition of organic pollutants under UV light illumination. The literature shows that doping precious metal on the surface of titanium dioxide can enhance the photocatalytic activity because noble metal can form active sites to promote the electronic charge transfer in the interface of metal and titanium dioxide. Although the activity of this kind of structure is high, the exposed metal is easy to dissolve or corrosive, leading to catalyst decay. Core/shell structure can be used to overcome this shortcoming, noble metals located in the core, while titanium dioxide is in the shell. In this study, Au@TiO2 and Ag@TiO2 catalysts with core/shell structure were synthesized by sol-gel method with hydrothermal treatment. Different preparation parameters lead to different sizes of metal core, different crystal size of TiO2 and different crystallinity of TiO2. These catalysts were characterized by UV-vis spectroscopy (UV-vis), Dynamic light scattering analyzer (DLS), inductively-coupled plasma-mass spectrometry (ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). TEM micrographs, UV-vis spectra and DLS results showed the Au core size of Au@TiO2 could effectively be controlled between 6.6 and 32.37 nm by different CTAB concentration. However, the Ag core size of Ag@TiO2 could just be controlled between 6.82 and 15.35 nm by different ratio of hydrazine to silver nitrate. XRD patterns exhibited the crystallinity of TiO2 increased with increasing the temperature of hydrothermal process. The photoreaction was carried out in a 10 ppm methylene blue solution with two 8w 254 nm UV light as the light source. The concentration of MB in the degradation samples were measured by UV-vis spectrometer (UV-vis). The effect of various metal core sizes, various noble metals loading amount and the hydrothermal temperature were investigated. The results showed the small metal core size had slightly higher activity than the larger ones, and the optimal amounts of Au and Ag loading were 1wt. % and 0.5 wt. %, respectively. Furthermore, the sample with the highest hydrothermal temperature had the highest activity due to the highest crystallinity. From these results, the photocatalytic activity of Metal@TiO2 catalyst mainly depended on the crystallinity of TiO2, the amount of noble metal loading and the kinds of cocatalyst rather than the size of noble metal core.
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