半導體奈米異質結構之製備及其催化特性

Abstract

本論文主要關注於半導體奈米異質結構材料如半導體－類金屬的ZnO-rGO、半導體－金屬的ZnSe0.5(N2H4)-Au奈米帶、半導體－半導體－金屬的In2O3-TiO2-Pt奈米帶及蛋黃－蛋殼結構之半導體－金屬的Au@Cu7S4等的催化特性研究。因複合材料間能帶結構的差異，在光源照射下，半導體上的激發電子會自發性的傳遞至另一具有較低導帶或功函數的半導體/金屬導體上，產生高效率的載子分離特性，不論是在光催化降解汙染物或是光電化學上光能轉換為電能的應用都有明顯的提升效果。激發電子於異質結構界面間的傳遞現象可藉由時間解析螢光光譜(time-resolved photoluminescence spectroscopy，TRPL)技術來量測，透過實際量化界面間電子傳遞所得的載子動力學結果，以建立樣品之相對能帶結構、界面間載子動力學與載子被導出利用效率之間的關連性。另一方面，利用合成出蛋黃－蛋殼結構之Au@Cu7S4做為類似過氧化酶催化劑，在過氧化氫的環境下，探討其產生氫氧自由基進行後續3,3',5,5'-四甲基聯苯胺氧化反應的特性。 首先是利用Hummers化學法合成出氧化石墨烯(graphene oxide，GO)，再藉由水熱法的方式成長ZnO於石墨烯上，同時在此高溫高壓的環境下還原氧化石墨烯(reduced graphene oxide，rGO)，得到ZnO奈米晶體接枝於還原氧化石墨烯的複合材料。過程中可藉由調控加入之GO的含量，有效得控制rGO上的ZnO奈米晶體的分佈密度。由於能帶結構的差異，ZnO上的激發電子會自發性的轉移到rGO，而在時間解析螢光光譜上可觀察到電子傳遞速率常數隨著rGO含量的增加而提升，顯示出ZnO 半導體上的激發載子能有效的分離且在後續反映中被有效利用，接著透過揮發性有機物─乙醛氣體分子的降解現象來更進ㄧ步的探討ZnO-rGO複合材料的光催化特性。此外，ZnO-rGO較純ZnO的樣品在白光照射產生水氧化電流的表現上，更提升了約50%，也顯示出ZnO-rGO複合材料不論在光催化亦或是光電化學電池上的都具有應用上的潛力。 半導體－金屬的合成是在聯胺溶液的輔助下，藉由水熱法反應合成出ㄧ維的無機－有機混合半導體ZnSe0.5(N2H4)。藉由調控反應中水和聯胺的相對比例，能夠有效的控制ZnSe0.5(N2H4)的形貌，包括奈米線(nanowires)、奈米帶(nanobelts)和奈米片(nanoflakes)。由TRPL量測分析中發現ZnSe0.5(N2H4)奈米線具有相對較長的載子生命週期且因具有高度非等向性的結構，在可見光降解羅丹明B 染料(Rhodamine B，RhB)的反應中，較奈米帶和奈米片有更佳的光催化特性。而相較於ZnSe奈米顆粒和ZnSe單晶奈米線，此無機－有機混合半導體在可見光照射下仍表現出絕佳的光催化效果。在將金奈米顆粒接枝於此奈米線表面後，ZnSe0.5(N2H4) 奈米線之光催化活性可被進一步提升；而在TRPL 量測中也顯示出約有40%的激發電子能由ZnSe0.5(N2H4)端轉移至表面的金顆粒上，提升了載子分離的效果，讓更多的激發載子有機會被利用於後續的催化反應上，同時從RhB光降解反應中也應證了修飾金顆粒於ZnSe0.5(N2H4)表面能有效提升其光催化活性。 接著是合成出三成份的半導體－半導體－金屬的系統，並且研究其內部的載子動力學。首先以水熱法合成出的TiO2奈米帶為主體，並在其表面上利用沉積－鍛燒的方法接枝In2O3奈米顆粒，下ㄧ步再藉由光沉積的方法選擇性地將Pt顆粒還原於In2O3−TiO2奈米帶的TiO2表面上，得到In2O3-TiO2-Pt三成份奈米帶。對於In2O3-TiO2而言，因為其兩材料間的能帶結構差異，In2O3端所產生的光激發電子會自發性得轉移至TiO2，產生載子分離的效果。而在引入Pt顆粒於TiO2表面後，In2O3端產生之光激發電子便會透過TiO2再傳遞到Pt端，使In2O3-TiO2-Pt達到更顯著的載子分離特性，而此顯著的載子分離特性也表現於相對應的光電流量測上。藉由TRPL量化分析In2O3與TiO2之間的電子傳遞現象以及當Pt沉積於TiO2上時對於此電子傳遞現象的影響。隨著Pt的修飾，In2O3−TiO2−Pt奈米帶的電子犧牲速率常數也顯著的增加，而此結果與在光催化降解RhB的反應相對呼應，即ㄧ正比的關係。 第二部分是先合成Au@Cu2O 核殼結構來作為模板，再進一步加入硫化鈉，透過Kirkendall 效應形成蛋黃－蛋殼之Au@Cu7S4結構，並且研究其類似過氧化酶的催化特性。由於其結構的關係，Au在此能相較於核殼結構裸露出更多的比表面積，能更有效的與過氧化氫接觸產生氫氧自由基;另一方面，氫氧自由基亦可藉由過氧化氫與Cu元素透過Fenton反應產生，因此中空的Cu7S4的結構相較於實心的Cu2O能裸露更多的比表面積，有較佳的氫氧自由基產生效率。而在將此複合結構應用於RhB染料降解的實驗中可以發現，在過氧化氫的環境下此材料能夠有效的分解染料分子，顯示出其產生之氫氧自由基具有進一步分解有機汙染物的應用特性。

In this dissertation, we focused on the catalytic properties of semiconductor heterostructures, which included semiconductor-graphene: ZnO-rGO composites, semiconductor-metal: ZnSe0.5(N2H4)-Au nanowires, semiconductor- semiconductor- metal: In2O3-TiO2-Pt nanobelts and semiconductor-metal: Au@Cu7S4 yolk-shell nanoparticles. Due to the difference in band structure between the constituents, the excited electrons in one of the semiconductor domain would preferentially transfer to the other semiconductor or metal with lower conduction band or work function under light illumination, leading to remarkable charge separation property. The enchantment in photocatalytic activity of pollutant degradation or photoconversion efficiency in photoelectrochemical application therefore can be achieved. The charge transfer behavior at the interface of heterostructures was explored by using time-resolved photoluminescence (TRPL). Through quantitatively analyzing the electron transfer event with TRPL, the correlation between the charge transfer dynamic and the difference in band diagram for semiconductor heterostrucutres could be further established. In addition, the Au@Cu7S4 yolk-shell nanoparticles (NPs) were synthesized using Au@Cu2O core-shell NPs as a sacrificial template and served as peroxidase mimics to investigate their peroxidase-like catalytic properties by using 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide. First, the ZnO-rGO composites were synthesized by hydrothermal method. The graphene oxide (GO) sheets were first prepared by Hummers method, followed by the growth of ZnO on the sheets during the hydrothermal process; in the meantime, the graphene oxide could be reduced to reduced graphene oxide (rGO). By modulating the amount of GO nanosheets added, the content of rGO in the resultant ZnO–rGO composites can be readily controlled. Due to the difference in band structures for ZnO–rGO composites, the photoexcited electrons of ZnO would preferentially transfer to rGO, leading to charge carrier separation. TRPL spectra revealed that an increased electron-transfer rate constant was observed for ZnO–rGO with increasing rGO contents, suggesting that an increased number of photoexcited charge carriers were separated and available for photocatalysis utilization. The photocatalytic properties of the ZnO–rGO composites were investigated by using gaseous acetaldehyde (CH3CHO), a typical volatile organic compound (VOC), as the test pollutant. Furthermore, the photoactivity of ZnO–rGO toward electrolytic water oxidation was also evaluated, which revealed 50% increase in water oxidation current over pure ZnO under white light illumination. The demonstration from this work may facilitate the use of ZnO–rGO composites in photodegradation of VOCs as well as for photoelectrochemical applications. Second, ZnSe0.5(N2H4) inorganic-organic hybrids were synthesized using a facile hydrazine-assisted hydrothermal method. By modulating the volume ratio of hydrazine hydrate to deionized water employed in the synthesis, the morphology of the grown ZnSe0.5(N2H4) can be varied, which included nanowires, NBs and nanoflakes. With the relatively long exciton lifetime and highly anisotropic structure, ZnSe0.5(N2H4) nanowires performed much better in the photodegradation of rhodamine B (RhB) than the other two counterpart products. As compared to pure ZnSe NPs and single-phase ZnSe nanowires obtained from further processing ZnSe0.5(N2H4), the ZnSe0.5(N2H4) hybrid nanowires exhibited superior photocatalytic performance under visible light illumination. With further decorated Au particles, TRPL spectra showed that the photoexcited electrons in ZnSe0.5(N2H4) nanowires can be transported to the decorated Au, which enabled a fuller extent of participation of charge carriers in the photocatalytic process and thus conduced to a significant enhancement in the photocatalytic activity. Third, we investigated the interfacial charge carrier dynamics of the three-component semiconductor−semiconductor−metal heterojunction system. The samples were prepared by selectively depositing Pt NPs on the TiO2 surface of In2O3-decorated TiO2 nanobelts (In2O3−TiO2 nanobelts (NBs)). For In2O3−TiO2 NBs, because of the difference in band structures between In2O3 and TiO2, the photoexcited electrons of In2O3 nanocrystals would preferentially transfer to TiO2 NBs to cause charge carrier separation. With the introduction of Pt on TiO2 surface, a fluent electron transfer from In2O3, through TiO2, and eventually to Pt was achieved, giving rise to the increasingly pronounced charge separation property. TRPL spectra were measured to quantitatively analyze the electron transfer event between In2O3 and TiO2 for In2O3−TiO2 NBs and its dependence on Pt deposition. Upon the deposition of Pt, In2O3−TiO2 NBs showed an increased apparent electron-scavenging rate constant, fundamentally consistent with the result of their performance evaluation in photocatalysis. In the final part, we presented the peroxidase-like catalytic properties of Au@Cu7S4 yolk-shell NPs synthesized by using Au@Cu2O core-shell nanostructures as templates via Kirkendall effect. The TMB oxidation results revealed that the Au@Cu7S4 yolk-shell nanopartilces had remarkable activity in the generation of ∙OH radicals in the existence of H2O2. Due to their unique structure, the Au core can provide sufficient active sites for H2O2 adsorption as compared to Au@Cu2O core-shell samples. On the other hand, the Cu7S4 hollow structures for Au@Cu7S4 yolk-shell samples exposed higher surface area than that of Cu2O for Au@Cu2O core-shell samples. The RhB degradation test demonstrated that the present yolk-shell samples could practically be applied in degradation of organic pollutant with hydrogen peroxide as well.