标题: 半导体奈米异质结构之制备及其催化特性
Semiconductor Nano-heterostructures: Synthesis and Their Catalytic Applications
作者: 陈宇志
Chen, Yu-Chih
徐雍蓥
Hsu, Yung-Jung
材料科学与工程学系所
关键字: 光催化;载子动力学;太阳光能量转换;载子分离现象;photocatalysis;charge carrier dynamics;solar energy conversion;charge separation
公开日期: 2015
摘要: 本论文主要关注于半导体奈米异质结构材料如半导体-类金属的ZnO-rGO、半导体-金属的ZnSe0.5(N2H4)-Au奈米带、半导体-半导体-金属的In2O3-TiO2-Pt奈米带及蛋黄-蛋壳结构之半导体-金属的Au@Cu7S4等的催化特性研究。因复合材料间能带结构的差异,在光源照射下,半导体上的激发电子会自发性的传递至另一具有较低导带或功函数的半导体/金属导体上,产生高效率的载子分离特性,不论是在光催化降解污染物或是光电化学上光能转换为电能的应用都有明显的提升效果。激发电子于异质结构界面间的传递现象可藉由时间解析萤光光谱(time-resolved photoluminescence spectroscopy,TRPL)技术来量测,透过实际量化界面间电子传递所得的载子动力学结果,以建立样品之相对能带结构、界面间载子动力学与载子被导出利用效率之间的关连性。另一方面,利用合成出蛋黄-蛋壳结构之Au@Cu7S4做为类似过氧化酶催化剂,在过氧化氢的环境下,探讨其产生氢氧自由基进行后续3,3',5,5'-四甲基联苯胺氧化反应的特性。
首先是利用Hummers化学法合成出氧化石墨烯(graphene oxide,GO),再藉由水热法的方式成长ZnO于石墨烯上,同时在此高温高压的环境下还原氧化石墨烯(reduced graphene oxide,rGO),得到ZnO奈米晶体接枝于还原氧化石墨烯的复合材料。过程中可藉由调控加入之GO的含量,有效得控制rGO上的ZnO奈米晶体的分布密度。由于能带结构的差异,ZnO上的激发电子会自发性的转移到rGO,而在时间解析萤光光谱上可观察到电子传递速率常数随着rGO含量的增加而提升,显示出ZnO 半导体上的激发载子能有效的分离且在后续反映中被有效利用,接着透过挥发性有机物─乙醛气体分子的降解现象来更进ㄧ步的探讨ZnO-rGO复合材料的光催化特性。此外,ZnO-rGO较纯ZnO的样品在白光照射产生水氧化电流的表现上,更提升了约50%,也显示出ZnO-rGO复合材料不论在光催化亦或是光电化学电池上的都具有应用上的潜力。
半导体-金属的合成是在联胺溶液的辅助下,藉由水热法反应合成出ㄧ维的无机-有机混合半导体ZnSe0.5(N2H4)。藉由调控反应中水和联胺的相对比例,能够有效的控制ZnSe0.5(N2H4)的形貌,包括奈米线(nanowires)、奈米带(nanobelts)和奈米片(nanoflakes)。由TRPL量测分析中发现ZnSe0.5(N2H4)奈米线具有相对较长的载子生命周期且因具有高度非等向性的结构,在可见光降解罗丹明B 染料(Rhodamine B,RhB)的反应中,较奈米带和奈米片有更佳的光催化特性。而相较于ZnSe奈米颗粒和ZnSe单晶奈米线,此无机-有机混合半导体在可见光照射下仍表现出绝佳的光催化效果。在将金奈米颗粒接枝于此奈米线表面后,ZnSe0.5(N2H4) 奈米线之光催化活性可被进一步提升;而在TRPL 量测中也显示出约有40%的激发电子能由ZnSe0.5(N2H4)端转移至表面的金颗粒上,提升了载子分离的效果,让更多的激发载子有机会被利用于后续的催化反应上,同时从RhB光降解反应中也应证了修饰金颗粒于ZnSe0.5(N2H4)表面能有效提升其光催化活性。
接着是合成出三成份的半导体-半导体-金属的系统,并且研究其内部的载子动力学。首先以水热法合成出的TiO2奈米带为主体,并在其表面上利用沉积-锻烧的方法接枝In2O3奈米颗粒,下ㄧ步再藉由光沉积的方法选择性地将Pt颗粒还原于In2O3−TiO2奈米带的TiO2表面上,得到In2O3-TiO2-Pt三成份奈米带。对于In2O3-TiO2而言,因为其两材料间的能带结构差异,In2O3端所产生的光激发电子会自发性得转移至TiO2,产生载子分离的效果。而在引入Pt颗粒于TiO2表面后,In2O3端产生之光激发电子便会透过TiO2再传递到Pt端,使In2O3-TiO2-Pt达到更显着的载子分离特性,而此显着的载子分离特性也表现于相对应的光电流量测上。藉由TRPL量化分析In2O3与TiO2之间的电子传递现象以及当Pt沉积于TiO2上时对于此电子传递现象的影响。随着Pt的修饰,In2O3−TiO2−Pt奈米带的电子牺牲速率常数也显着的增加,而此结果与在光催化降解RhB的反应相对呼应,即ㄧ正比的关系。
第二部分是先合成Au@Cu2O 核壳结构来作为模板,再进一步加入硫化钠,透过Kirkendall 效应形成蛋黄-蛋壳之Au@Cu7S4结构,并且研究其类似过氧化酶的催化特性。由于其结构的关系,Au在此能相较于核壳结构裸露出更多的比表面积,能更有效的与过氧化氢接触产生氢氧自由基;另一方面,氢氧自由基亦可藉由过氧化氢与Cu元素透过Fenton反应产生,因此中空的Cu7S4的结构相较于实心的Cu2O能裸露更多的比表面积,有较佳的氢氧自由基产生效率。而在将此复合结构应用于RhB染料降解的实验中可以发现,在过氧化氢的环境下此材料能够有效的分解染料分子,显示出其产生之氢氧自由基具有进一步分解有机污染物的应用特性。
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.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT079918816
http://hdl.handle.net/11536/125891
显示于类别:Thesis