奈米金之侷域性表面電漿共振效應對光催化反應之研究

Abstract

本研究探討奈米金顆粒的侷域性表面電漿共振(Localized Surface Plasmon Resonance, LSPR)對二氧化鈦光觸媒催化活性的影響。於實驗中建立兩套光催化反應系統，分別為亞甲基藍溶液降解及光催化水分解產氫，用以比較各種觸媒的光催化活性。 在亞甲基藍降解實驗中，能引發奈米金LSPR效應的可見光光源波長約在530 nm左右。實驗結果顯示，在能促使奈米金產生LSPR效應的可見光光源(波長為400-700 nm，光強度為185k lux)照射6小時後，使得亞甲基藍達31.5%降解。而在另外附加光過濾片(只能讓250-450 nm的波長通過，光強度為185k lux)的光源照射下，無法產生LSPR效應的奈米金僅促使19%的亞甲基藍降解。奈米金的LSPR效應加強了亞甲基藍的降解。 此外，本研究成功地以熱水解法製備了厚度約300 nm的二氧化鈦薄膜及以檸檬酸鈉還原法製備奈米金懸浮液。並將奈米金顆粒負載於二氧化鈦薄膜(Au/TiO2)。另外，也成功地製備了殼層厚度為3 nm二氧化矽的奈米金-二氧化矽核殼結構，並將其負載於二氧化鈦薄膜(Au@SiO2/TiO2)。添加上述各種不同的觸媒於亞甲基藍溶液中，在紫外光(365 nm, 光強度為5.5 mW/cm2)及可見光(400-700 nm, 光強度為185k lux)同時照射5小時後，觀察亞甲基藍降解程度，比較各種觸媒對光催化活性。結果顯示，亞甲基藍降解程度：Au@SiO2/TiO2 > Au/TiO2 > TiO2 ，降解效果最好的Au@SiO2/TiO2觸媒可使亞甲基藍溶液在光源照射5小時後降解達95%。3 nm二氧化矽殼層增強了奈米金的LSPR效應。 而在水分解產氫實驗中，以光沈積法成功地製備了3wt.% Au/TiO2。3wt.%Au/TiO2在單獨紫外光(254 nm, 光強度為30 mW/cm2)或紫外光(254 nm, 光強度為30 mW/cm2)與可見光(400-700 nm, 光強度為185k lux)同時照射7小時下的產氫量分別為35.04 μmol/g-cat及53.75 μmol/g-cat。在紫外光及可見光同時照射下，能引發LSPR效應的觸媒可得到最大產氫量。由於奈米金能延緩光激發所產生的電子電洞對再結合，此外，由於其LSPR效應，因而提昇了氫氣的產量。而產生的氫氣與氧氣符合水分解之化學劑量比例2:1。 最後，利用有限元素分析法來模擬電場強度的變化，用以輔助說明實驗結果。當奈米金存在時，奈米金特殊的LSPR效應確實增強了其周圍的電場強度，我們認為此電場增強現象可視為一外加電場，因而促致光催化效果增加。另外，由模擬結果也可證實當奈米金被一層適當厚度的二氧化矽包覆後並負載於光觸媒時，確實有助於增強奈米金的LSPR效應，因而比起單獨只有奈米金負載於光觸媒更能增加光催化活性。

This research studies the LSPR (Localized Surface Plasmon Resonance) effect of gold nanoparticles on the photoactivity of TiO2. Two photocatalytic reaction systems, methylene blue (MB) photodegradation and hydrogen production by photocatalytic water splitting, the photocatalytic activity of various photocatalysts were compared. In the MB photodegradation, the wavelength of visible light that can induce the LSPR effect of gold nanoparticles is around 530 nm. The experimental results showed that the gold nanoparticles with LSPR effect can achieve 31.5% of MB photodegradation after 6 h of visible light irradiation (the wavelength of light is 400-700 nm, the intensity of light is 185k lux). Whereas under filtered light (only wavelength of 250-450 nm can pass through, the intensity of light is 185k lux), the MB degradation only reached 19%. The gold nanoparticle’s LSPR effect enhanced the photodegradation of methylene blue. Furthermore, a 300 nm-thick TiO2 film was successfully prepared by thermal hydrolysis method. Gold nanoparticle dispersion was prepared by sodium citrate reduced method and was later loaded unto the TiO2 film (Au/TiO2). Also, the gold nanoparticles coated by 3 nm-thick SiO2 shell was similarly loaded onto the TiO2 film (Au@SiO2/TiO2). The photocatalytic activities were compared by MB photodegradation again. Under 5 h of UV light (365 nm, the intensity of light is 5.5 mW/cm2) and visible light (400-700 nm, the intensity of light is 185k lux) irradiation simultaneously, the MB degradation results: Au@SiO2/TiO2 > Au/TiO2 > TiO2. The maximum MB photodegradation which was achieved by Au@SiO2/TiO2 was 95%. In short, the 3 nm of SiO2 shell enhanced the LSPR effect of gold nanoparticles. 3wt.%Au/TiO2 was successfully fabricated by photodeposition method. In water splitting, 3wt.%Au/TiO2 enhanced the production of H2 to 35.04 μmol/g-cat under 7 h of UV light irradiation (254 nm, the intensity of light is 30 mW/cm2). Under 7 h of UV light (254 nm, the intensity of light is 30 mW/cm2) and visible light (400-700 nm, the intensity of light is 185k lux) irradiation at the same time, 53.75 μmol/g-cat of H2 was produced. Because visible light can induce the LSPR effect of gold nanoparticles, it yielded a better production of the two. Gold nanoparticles can retard the recombination of electron and hole, additionally because of the LSPR effect, the production of H2 was increased. The ratio of H2 and O2 production corresponded to the stoichiometric ratio of water splitting. Finally, we simulated the change of electric field intensity with finite element method (FEM). When gold nanoparticles were present, the LSPR effect of gold nanoparticles enhanced the surrounding electric field intensity. We consider this enhancement to be an extra electric field, which improved the photocatalytic activity. Also, the simulation result proved that when gold nanoparticles were coated with a 3-nm SiO2 shell, the LSPR effect was further enhanced compared to bare gold nanoparticle.