氧化钨基异质结和氧缺陷结构的构筑及其应用研究

发布时间：2018-03-31 02:19

本文选题：超临界CO_2　切入点：TiO_2/WO_3·H_2O　出处：《郑州大学》2017年硕士论文

【摘要】：氧化钨材料作为一种典型的n型半导体,可广泛应用于光催化、电催化及电致变色智能窗等方面,已成为环境与能源领域的研究热点之一。氧化钨材料价格低廉,来源丰富,具有良好的化学稳定性和耐光腐蚀性,是一种可用于大规模生产的能源材料。氧化钨材料种类繁多,既有化学计量的WO_3,又有含有氧缺陷的亚化学计量WO_(3-x),另外还有含结晶水的水合氧化钨(WO_3·nH_2O)。在实际应用中,形貌、维度和结构等因素都会极大地影响到氧化钨材料的性能。与块体材料相比,低维度纳米材料具有更大的比表面积和载流子传输性能;而结构的不同,同样能反映出性能的差异,与WO_3相比,亚化学计量的WO_(3-x)结构中氧空位充当浅层供体,可以提高导电性和供体密度,从而增强表面物种(例如CO_2,H_2,NO_2等)的吸附。此外,单纯的氧化钨材料在应用于光催化剂时,受光催化机理的限制,会发生光生电子和空穴的复合,影响光催化的效率。而通过与其他半导体(如TiO_2)构筑半导体/半导体复合异质结构则能很好的促进光生电子和空穴的有效分离,避免光生电子-空穴对的复合。基于以上这些因素,本文主要进行了以下研究:(1)我们利用超临界CO_2辅助剥离得到具有少层结构的WO_3·H_2O纳米片。与块体材料相比,维度调控得到的该超薄纳米片具有大的比表面积,可以暴露出更多的活性位点。之后我们运用一种简单且新颖的方法,即在超临界CO_2环境中将该超薄纳米片与本组之前工作制得的TiO_2纳米片复合构筑得到类范德华异质结-TiO_2/WO_3·H_2O异质结。两者的纳米片层结构使得其发生充分堆叠。在用作光催化剂时,相较于单纯Ti O_2和WO_3·H_2O纳米片,该异质结表现良好的光电流响应和对甲基橙高效的降解效率。经分析发现,在光照下,TiO_2的光生电子会转移到WO_3·H_2O中,而WO_3·H_2O的空穴则会转移到TiO_2中,这样便避免了光生电子-空穴对的复合,促进了两者的分离,进而提高了光催化活性。(2)出于安全考虑,我们用NaBH4代替常用的H_2对商用WO_3在高温下进行氢化反应,通过这一简单的方法制备得到含有氧缺陷的亚化学计量WO_(3-x)。结果表明,在对WO_3进行高温氢化后,边缘形成无序区域,说明了氧空位的生成,并且随着氢化温度的提高(400℃),边缘无序区域扩大,表明氧空位含量增大。根据紫外-可见光-近红外测试发现,氢化之后,WO_(3-x)在近红外区域1050 nm处出现一个明显的特征峰,表现出明显的表面等离子共振效应。之后对样品进行电催化析氢性能测试,结果表明400℃下氢化的WO_(3-x)样品析氢效率明显提高,Tafel斜率为66 mV dec-1,远低于本体WO_3(113 mV dec-1)。并且在与文献报道的氧化钨基材料的Tafel斜率比较发现该样品仍具有很大的优势。
[Abstract]:Tungsten oxide, as a typical n-type semiconductor, can be widely used in photocatalysis, electrocatalysis and electrochromic smart windows, and has become one of the research hotspots in the field of environment and energy. With good chemical stability and photo-corrosion resistance, tungsten oxide is a kind of energy material which can be used in mass production. There are not only stoichiometric WO3s, but also sub-stoichiometric WOs with oxygen defects, as well as tungsten oxide hydrated with crystalline water, WO _ 3nH _ 2O. In practical applications, the morphology, Factors such as dimension and structure can greatly affect the properties of tungsten oxide. Compared with bulk materials, low-dimensional nanomaterials have greater specific surface area and carrier transport properties, while different structures can also reflect the differences in performance. Compared with WO_3, the oxygen vacancy in the substoichiometric WOSP 3-x structure acts as a shallow donor, which increases the conductivity and donor density, thus enhancing the adsorption of surface species (e.g., CO2T / H2S / NO2, etc.). In addition, simple tungsten oxide materials are used in photocatalysts. Due to the limitation of photocatalytic mechanism, the combination of photogenerated electrons and holes will occur. By constructing semiconductor / semiconductor composite heterostructures with other semiconductors (such as TiO-2), the effective separation of photogenerated electrons and holes can be promoted. To avoid photogenerated electron-hole pair compounding. Based on the above factors, the following studies have been carried out in this paper: 1) We use supercritical CO_2 assisted stripping to obtain WO_3 H2O nanocrystals with low layer structure. Compared with bulk materials, The dimensionally regulated nanocrystals have a large specific surface area to expose more active sites. Then we use a simple and novel approach. In the supercritical CO_2 environment, the ultrathin nanocrystals were combined with the previously worked TiO_2 nanostructures to obtain the van der Waals heterojunction-TiO-2 / WO / 3H _ 2O heterostructures. The nanostructures of the two nanostructures made them stack sufficiently. When used as photocatalysts, Compared with simple TiO-2 and WO_3 H2O nanochips, the heterojunction exhibits good photocurrent response and high degradation efficiency to methyl orange. It is found that the photogenerated electrons of tio _ 2 are transferred to WO_3 H _ 2O under light, while the holes of WO_3 H _ 2O are transferred to TiO_2. In this way, the combination of photogenerated electron-hole pairs is avoided, the separation of the two is promoted, and the photocatalytic activity is improved. (2) for safety reasons, we use NaBH4 to hydrogenate commercial WO_3 at high temperature instead of the commonly used H-2. By this simple method, sub-stoichiometric WOCs with oxygen defects were prepared. The results showed that after hydrogenation of WO_3 at high temperature, disordered regions formed at the edges, indicating the formation of oxygen vacancies. With the increase of hydrogenation temperature, the edge disordered region expands, which indicates that the oxygen vacancy content increases. According to the UV-Vis and NIR measurements, there is a characteristic peak at 1050 nm in the near infrared region after hydrogenation. It showed obvious surface plasmon resonance effect. Then the electrocatalytic hydrogen evolution performance of the sample was tested. The results showed that the hydrogen evolution efficiency of the hydrogenated WO _ 2 _ (3-x) sample was significantly increased by 66mV dec-1, which was far lower than that of the bulk WO_3(113 _ (MV) _ (dec-1). Compared with the reported Tafel slope of the tungsten oxide based material in the literature, it was found that the sample still had a great advantage.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ136.13

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