【作者】 王华；

【导师】 全燮；

【作者基本信息】 大连理工大学 ， 环境工程， 2008， 博士

【摘要】 零维C₆₀和一维碳纳米管被发现以后,纳米结构碳材料的研究及应用引起了人们的广泛关注,但二维纳米结构的碳纳米墙的研究却较少。碳纳米墙可用于电子器件、超导、场发射等方面,特别是由于具有自支撑的开放式结构和高的表面积,非常适合作为催化剂的载体。TiO₂和WO₃均是半导体材料,因具有较强的光催化氧化能力和高的光化学稳定性,所以广泛地作为催化剂用于多相光催化中。但TiO₂和WO₃的光催化效率有待进一步提高以达到实用化目的,同时光催化剂在使用中遇到粉体光催化剂难于分离回收,而负载于致密载体的催化剂的比表面积较小。利用碳纳米墙的表面结构特点,将TiO₂和WO₃光催化剂与碳纳米墙复合,可有效提高光催化剂比表面积,而且光催化剂与碳纳米墙间形成的异质结构会有利于光致电子-空穴的分离。为此,采用偏压等离子体增强热丝化学气相沉积方法制备的碳纳米墙为基体,用化学气相沉积方法制备了TiO₂/碳纳米墙和WO₃/碳纳米墙复合材料,并对材料进行了系统表征和光催化性能的研究。本论文主要开展了以下几个方面的工作:（1）独立设计并建造了偏压等离子体增强热丝化学气相沉积装置,该装置的加热功率为3kW,热丝和基体间的外加偏压可以达到300V,最大成膜面积20cm^-2。利用该装置,以金属钛为基体,选用H₂和CH₄作为反应气体,可以大量制备质地均匀的碳纳米墙。反应过程中甲烷和氢气的流量分别为18sccm和6sccm,体系压强为3000Pa,基体温度为600℃,基体偏压100V,沉积时间为0.5h。采用扫描电子显微镜对样品进行分析结果表明,碳纳米墙具有自支撑的碳纳米片网状结构,完全独立的垂直于基体方向向外扩展。单个碳纳米墙的厚度约为10nm,长度在500 nm到1μm之间,高度约为2μm。透射电镜选区电子衍射分析和拉曼光谱分析表明碳纳米墙是以sp²杂化形式为主的石墨类物质。碳纳米墙的形成主要决定于直流偏压所形成的电场,而等离子体氛围中碳粒子和原子氢的浓度也是形成碳纳米墙的关键因素。（2）以碳纳米墙为基体,使用加热钨丝的化学气相沉积方法,保持真空反应室内空气的压强为3000 Pa,控制加热电流为20 A,加热时间为5 min,可将WO₃非常均匀的包覆在碳纳米墙外壁,制备出WO₃/碳纳米墙复合材料。这种复合材料的形成得益于碳纳米墙所具有的开放的形貌结构。拉曼光谱和X射线衍射分析表明沉积在碳纳米墙上的WO₃的晶体结构为单斜晶系的WO₃。紫外-可见漫反射测试表明WO₃/碳纳米墙复合材料具有一定的光响应。在相同实验条件下,WO₃/碳纳米墙电极的光电流密度及光催化降解对硝基苯酚的降解效率高于所对照的三氧化钨纳米带阵列电极。（3）以碳纳米墙为基体,选择钛酸四丁酯为钛源,用金属有机化学气相沉积方法,制备出TiO₂/碳纳米墙复合材料。在TiO₂沉积过程中,通入500 sccm的氩气作为载气,沉积温度控制在320℃,钛酸四丁酯的用量为0.05 mL min^-1。当沉积过程结束后,将反应器的温度以2℃min^-1的速度从320℃升至430℃,保温1h。随着TiO₂沉积时间的延长,TiO₂/碳纳米墙的厚度可以从几十纳米增至近200nm。拉曼光谱和X射线衍射分析表明沉积在碳纳米墙上TiO₂为锐钛矿相晶体。手工半导体参数仪所测定的电流-电压（I-V）曲线表明TiO₂和碳纳米墙之间可以形成异质结构。表面光电压作用谱和光电流密度分析确定TiO₂和碳纳米墙间形成的异质结构有利于减少光生电子和空穴的复合。选择苯酚作为目标污染物,光催化降解苯酚的实验表明TiO₂/碳纳米墙电极的光催化活性高于二氧化钛纳米管阵列电极。TiO₂/碳纳米墙光电极所具有的开放式的高比表面积及异质结构可能是其具有高光催化性能的主要原因。更多还原

【Abstract】 Carbon nanostructures, such as zero-dimensional fullerenes and one-dimensional carbon nanotubes, have attracted great interests because they are often superior to the conventional bulk carbon materials. However, as compared with the intensive research of fullerenes and carbon nanotubes, only a few studies have been carried out on two-dimensional carbon nanowalls （CNWs）. Actually, the CNWs have unique field-emission and electron transport properties. In particular, the CNWs are the good candicates for catalyst supporting because they can form self-supported graphitic carbon network structure with an open boundary. TiO₂ and WO₃ have been regarded as the most attractive semiconductor materials with high Photocatalytic ability and long life stability. However, enhancing the Photocatalytic efficiency of photocatalyst to meet the practical application requirement is still a challenge, mostly because of low quantum yield caused by the rapid combination of photogenerated electrons and holes. Meanwhile, the Photocatalytic oxidation technology always suffers from the difficulties of separating suspended photocatalyst particles from aqueous solution as well as the surface area of supported photocatalyst exposed to the solution is lower than that of suspended photocatalyst in solution. In order to increase the Photocatalytic efficiency, attempt has been made to cover the TiO₂ and WO₃ on CNWs for increasing the surface area and the forming of a heterojunction which could provide a potential driving force for the separation of photogenerated charge carriers. Therefore, in present work, the TiO₂/CNW and the WO₃/CNW composite materials were prepared by chemical vapor deposition, and the Photocatalytic activity of these photocatalysis was evaluated. In this dissertation, the following several parts of work have been done:（1） A plasma enhanced hot filament chemical vapor deposition （PE-HFCVD） system was self-designed and self-made. The heat power, substrate bias voltage, and deposition area of this system is 3 kW, 300 V and 20 cm^-2, respectively. This system was employed to prepare the CNWs. The substrate is a Ti sheet, and the hydrogen and methane is used as source gases. The gas flow rates of the H₂ and CH₄ are controlled at 6 and 18 sccm, respectively. The reactor is evacuated using a rotary vacuum pump, and the pressure of the system is kept at approximately 3000 Pa during the whole experimental process. When the substrate temperature is estimated by a thermocouple to be about 600℃, a negative substrate bias of 100 V is initiated between the hot filament （anode） and the substrate holder （cathode）. The typical deposition time lasted 30 min. The CNWs appeare to distribute uniformly over the whole Ti sheet surface and each nanowall stood perpendicularly on the substrate, they can form self-supported graphitic carbon network structure with an open boundary. These self-aligned CNWs grow up to nearly 2μm and the length is in the range of 500 nm to 1μm. The thickness of the CNW is approximately 10 nm. Trassion electron micrography and Raman spectrum indicate that the CNWs are graphite with sp² hybird. The growth of CNWs depends on the electric field occurred by a DC source, and the concentration of carbon particals and hydrogen are the key factors.（2） The deposition of WO₃ on CNWs is carried out in a HFCVD system using tungsten filament as tungsten source. The WO₃ could be covered on CNWs uniformly by controlling the deposition duration. The formation of WO₃/CNWS is benefit from the form the self-supported graphitic CNW structure with an open boundary. Raman spectroscopy and X-ray diffraction indicate that the crystal phase of the WO₃ coating is monoclinic. The UV-vis diffusion reflection spectrum reveals that the WO₃/CNWS have Photocatalytic ability under visable light. The photocurrent density and the photocatalystic degradation rate of p-nitrophenol are higher for WO₃/CNW electrode than WO₃ nanobelt array electrode.（3） The deposition of TiO₂ on CNWs is carried out in a metal-organic chemical vapor deposition system （MOCVD） using titanium isopropoxide as titanium source. The CNW substrate is placed in a tubular-furnace quartz reactor. The argon is used as the carrier gas with a constant flow rate of 500 sccm. When the substrate temperature reach 320℃, the solution of the titanium isopropoxide is fed continuously into the tubular quartz reactor through a capillary at a rate of 0.05 mL min^-1. After the deposition process, the argon flow is stopped and the film is annealed in the air at 430°C for 1 h with a heating rate of 2°C min^-1. The excellent uniformity of TiO₂ has been obtained on the entire CNWs to form a TiO₂/CNW composite material by controlling the deposition duration, and the thinkness of TiO₂/CNWS increase from several ten nanometers to nearly 200 nm with deposition duration increasing. Raman spectroscopy and X-ray diffraction indicate that the crystal phase of the TiO₂ coating is anatase. The asymmetry of the current-voltage plot for the material reveals that a heterojunction is formed between the TiO₂ and the carbon nanowall. As a result of this heterojunction, enhanced separation of photogenerated electrons and holes is confirmed by surface photovoltage and photocurrent measurements. The investigation of Photocatalytic ability shows that the TiO₂/CNW electrode has a higher Photocatalytic activity than TiO₂ nanotube electrode for the degradation of phenol.更多还原