【作者】 李颖；

【导师】 严辉；

【作者基本信息】 北京工业大学 ， 材料物理与化学， 2007， 硕士

【摘要】 氧化锌(ZnO)是一种具有六方纤锌矿晶体结构的多功能金属氧化物材料,在光电、压电、热电、铁电、铁磁等方面都具有优异的性能。60meV高激子结合能的半导体特性使ZnO在紫外光探测器、蓝紫波段发光二极管(LEDs)和短波长激光二极管(LDs)以及光信息存储领域有着广阔的应用前景。3.37eV宽禁带的特点也使ZnO成为半导体光催化剂的高效催化材料。本文主要通过一种低成本、操作简单、化学组分易控制的合成方法—热汽相沉积法,在非真空条件下制备纳微结构ZnO,并对其生长机理、结构形貌及光催化性能进行较为系统的研究。以金属锌粉为蒸发源物质,在氩气气氛中加热,在无外加催化剂的条件下成功的在Si衬底上制备了大量形貌均一的四足状ZnO(T-ZnO)。测试结果说明四足状产物有较高的结晶质量,这和从中心核伸展生长的“无衬底”生长形式有关,因此产物很容易在基体材料中实现均匀的三维分布,有可能成为较为理想的复合材料的补强增韧剂。系统比较载气类型及流量、沉积时间和衬底温度对产物形貌的影响,发现在生长过程中热汽相沉积过程对于O2的浓度十分敏感,O2不仅影响ZnO颗粒大小,而且使ZnO的极性生长面发生变化,对产物形貌有决定性作用。此外,通过反应过程的热力学分析,得出在高温时调节氧分压是控制反应趋势的有效手段。沉积时间的增加和衬底温度的升高使衬底上逐渐出现二维纳米片,通过理论分析认为是二维形核率提高所导致。利用VS机理和晶体的形核生长理论,探讨了存在于T-ZnO及棒状ZnO (R-ZnO)中“塔状”形貌的生长机理,认为其形成过程主要决定于ZnO各晶面表面自由能的差异和极性晶体ZnO沿C轴方向生长的固有生长习性,并相应提出了“阶段性外延形核”的生长模式。采用自制光催化性能实验装置对制备的T-ZnO光催化性能进行测试,产物在紫外灯下具有降解甲基橙有机溶液的光催化性能。在对原装置引入水浴冷却槽、并减少测试溶液量之后,测试条件进一步得到优化,解决了先前测试结果中降解率突降及效果不明显的问题,产物催化效率显著。实验结果表明水浴温度是测试ZnO催化降解率过程中的重要因素之一,其原因主要是有效降低了由紫外灯照射产生的热辐射对催化性能的影响。首次采用热汽相沉积和射频磁控溅射相结合的“两步法”成功制备了银/棒状ZnO(Ag/R-ZnO)复合催化剂,针对其光催化性能与单相R-ZnO进行了比较。溅射时间较短的Ag/R-ZnO复合产物其催化效率高于R-ZnO,但溅射时间较长的却低于R-ZnO,分析认为,这是由于溅射时间影响R-ZnO表面Ag负载量所导致的结果。Ag作为光生电子(e-)的接收器,可促进复合系统界面的载流子输运,有效的减少电子空穴对的复合率,从而提高光催化剂反应活性。但在负载量较高时,负载在R-ZnO表面的Ag可能形成一层连续膜,阻碍R-ZnO光照程度,因此无法产生电子-空穴对,造成复合产物光催化性能降低的结果。进一步利用上述方法成功制备了分散性较好的银/四足状ZnO(Ag/T-ZnO)复合催化剂,探讨了溅射功率对产物光催化性能的影响,再一次验证了Ag在T-ZnO表面的沉积量是影响产物光催化性能的关键因素。更多还原

【Abstract】 ZnO, which has hexagonal wurtzite structure, is recognized as one of the promising multifunctional metal oxide materials because of its excellent physical properties. ZnO nanostructure is an ideal material for the fabricating of blue and ultraviolet light-emitting diodes, low wavelength laser diodes and optical information storage due to its wide band gap of 3.37 eV and large exciton binding energy of 60 meV. ZnO has also great potential in applications in photocatalysis. The ZnO prepared in this paper is synthesized by a low cost and easy handled method—thermal vapor phase transport and condensation, (or thermal evaporation). The work presented in this dissertation is mostly focused on the fundamental study about the fabrication, morphology, growth mechanism and photocatalysis of ZnO. Furthermore, a two step technique based on thermal evaporation and RF magnetron sputtering for the preparation of Ag/ZnO compound structure was applied for the first time.Massive nanostructure tetrapod-like ZnO (T-ZnO) has been synthesized on Si (100) by thermal evaporation from the source of 99.9% pure metal zinc powder without the employments of metallic catalysts and vacuum conditions. According to the results of XRD, SEM and TEM, it was indicated that T-ZnO has good crystallinity and three-dimensional distribution, which may be a promising elasticizer of compound materials. In the research of different parameters in preparation of ZnO, thermal evaporation process is fairly sensitive to the concentration of O2. The oxygen flow rate plays an important role either in grain size or in the change of polar plane of T-ZnO.The main mechanism of Pyramids-like ZnO (P-ZnO) with micro dimension discovered in T-ZnO and Rod-like ZnO (R-ZnO) was discussed in detail. The analysis indicated that the products are T-ZnO with hexagonal structure, and the growth process is consistent with vapor-solid (VS) mechanism and crystal nucleation theory. The morphology of P-ZnO results from the c-axis orientation growth habit and the difference of surface free energy of ZnO. We conclude the phasic epitaxial re-nucleation in higher temperature is the reason of the formation of P-ZnO.The photocatalytic property of the products was tested in our home-made instruments. To improve the photocatalytic efficiency of ZnO in decoloration of methyl orange, water bath was applied in measurement for cooling the reaction solution. The testing results illustrated that the photocatalysis of ZnO is more remarkable than before. The heat radiation generated in ultraviolet exposure is the key factor that influences the photocatalysis of the products.Ag/R-ZnO and Ag/T-ZnO composite catalyst was fabricated individually by a two-step coupled technique based on thermal evaporation and RF magnetron sputtering for the first time. The results show that the Ag/R-ZnO composite catalyst exhibits better photocatalytic properties in contrast with R-ZnO. The effects of noble metal modification on surface composition, photoinduced charge transfer behavior and photocatalytic activity of Ag/ZnO samples were mainly investigated. And with Ag loading amount decreasing, the photocatalysis increases in general , indicating that the photoinduced electrons are easily captured by adsorbed oxygen via noble metal clusters so that the recombination of photoinduced electron and hole pairs is effectively inhibited. The relationship between Ag loading amount and sputtering power or deposition time were also discussed. The results above demonstrated the feasibility of non-solution loading technique. This two-step technique can better prevent aggregation of clusters and is of higher percentage of metal particle coverage than traditional solution loading methods, making it well suited for various applications in photocatalytic field.更多还原