【作者】 张健；

【导师】 刘孝恒；

【作者基本信息】 南京理工大学 ， 材料与工程， 2014， 博士

【摘要】 能源危机和环境恶化已经成为当今人类社会面临的两个重大问题。利用可再生资源制备出对环境无污染并且可持续利用的新型能源是解决这两个问题的根本途径之一。在众多的新能源中,氢能具有来源广泛、利用形式多样、对环境无污染、较高的化学能以及可以循环利用等优点。因此氢能也被广大学者公认为21世纪最有潜力的新能源。光催化技术通过利用光催化剂吸收太阳能激发光生电子-空穴进而诱发水的氧化还原反应来制备氢气。与众多的人工合成的光催化剂相比,使用天然产物作为光催化分解水制备氢气的研究很少。我们研究了具有天然纳米结构并且天然掺铁的粘土矿物凹凸棒土(ATP)以及蛭石(VMT)作为光催化剂,可见光下高效分解水制取氢气。同时作为对比,还研究了Ag/ZnO纳米复合物的制备以及有机染料敏化下的光催化产氢性能。具体研究内容及重要结论摘示如下：1)将天然的硅酸盐矿物ATP直接作为催化剂,以曙红(EY)等有机染料为光敏剂,可见光照射下还原水制取氢气。通过x射线衍射、透射电镜、紫外可见吸收、X射线光电子能谱等测试仪器对样品进行表征。对不同ATP样品以及蒙脱土(一种具有类似晶胞结构但是不存在铁掺杂的硅酸盐矿物)的对比考察证实了ATP的催化主体作用。四种有机染料均具有一定的敏化ATP产氢的能力,但是EY对ATP敏化能力最好,少量Ag纳米晶负载后量子效率达到了10.8%。进一步的化学分析和理论计算表明Fe掺杂(两个ATP晶胞中掺杂一个Fe原子)可以提高光催化性能。在本章的研究中,我们提出了一种极为简便的产氢方法,几乎不需要任何人工合成的步骤,也不用考察催化剂合成、表征、掺杂以及形貌控制等常规研究内容。结合实测禁带宽度及价带光电子能谱数据提出ATP在有机染料EY敏化下可见光降解水产氢的机理。2)采用简单的溶液浸渍法制备出不同CdS纳米晶负载量的ATP样品,进行CdS纳米晶敏化ATP可见光分解水制氢测试。3wt%CdS负载量的ATP样品可以超出单独CdS催化活性的200%,同时还将ATP样品用刮涂的方法制备成电极,进行光电化学分解水的测试,在可见光照射下得到的最高光电流为0.35mA cm-2。基于ATP与CdS的能带位置,提出了CdS/ATP纳米复合物可见光下高效产氢的机理。3)在ATP的基础上将一维天然矿物的研究拓展至二维的蛭石(VMT),将VMT作为催化剂,有机染料敏化下可见光直接降解水制取氢气,采用简单的水热法制备出Pt纳米粒子负载的VMT样品,作为对比进行了产氢性能测试,发现单纯的VMT的产氢能力可以达到220μmol h-1,负载3%的Pt纳米晶后催化能力提升至330μmol h-1,量子产率达到了22.8%。将纯的VMT制备成电极,通过对电极正面及背面的光辐射进行光电化学分解水的测试,得到的光电流大小分别为0.2mA cm-2和0.4mA cm-2。通过理论计算对VMT的合理结构进行了优化,提出最接近实际的天然掺铁的蛭石晶胞结构。4)同样以蛭石为原料,采用化学浸渍法将CdS量子点负载到具有纳米片状结构的VMT表面,考察了CdS的不同负载量对光解水产氢速率和光电化学分解水能力的影响。由于CdS量子点的敏化作用,可见光的响应范围增大,电荷有效传递得到提升。发现5wt%CdS的样品具有最高的产氢速率(92μmol h-1)和量子效率(17.7%)。5)采用晶种法制备出具有特殊形貌及特殊活性晶面暴露的ZnO纳米晶体,对含有银晶种和不含有银晶种的体系分别进行了考察,发现当没有Ag晶种时,在长链脂肪胺(十八胺)的作用下生成纳米颗粒,而在Ag晶种作用下就会诱导ZnO特定晶面的生长,既而表现出特殊的纳米六棱锥形貌。复合后样品具有较好的有机染料敏化可见光降解水制氢的能力。我们将合成出的半导体材料ZnO及Ag/ZnO纳米复合材料进行有机染料敏化产氢测试,将结果与敏化矿物对比,发现矿物具有相对较高的催化活性。更多还原

【Abstract】 Nowadays, the world is facing serious energy and environmental problems, so it is very urgent to produce a clean and sustainable energy using renewable energy source to solve such problems. Among the new energys, Hydrogen energy has many advantages, such as:it could be produced from many hydrogen-containing resources and utilized in various forms; water is the only combustion product and it has excellent chemical energy; moreover, the water produced from the burning of it can be reused to generation hydrogen. Considering above merits, hydrogen has been regarded as the most promising alternative energy in21st century by the majority of scholars around the world. Photocatalysis technique is to ues the photogenerated electrons and holes after absorbing the sunlight of the photocatalast to split water into hydrogen and oxygen. Indeed, so far there have been many reports related to different approaches for the water splitting, but to our knowledge, there have hardly investigations on natural products. We used attapulgite (ATP) and vermiculite (VMT), common and cheap clay mineral but as a high quality nanosized material with large surface area, layered and natural Fe-doping, to effectively split water in visible light irradiation and sensitized by organic dyes or CdS nanocrystals. By contrast, we have also investigated the preparation and performance of Ag/ZnO nanocomposites under the same condition.The contents of this study and important conclusions were summarized as follows:1. Organic dyes sensitized natural silicate (ATP) has been used as photocatalyst for hydrogen production from water under visible light. The samples including purified ATP, unpurified ATP as well as Ag loaded ATP were characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible absorption spectrum, and X-ray photoelectron spectroscopy. By contrast, we could not get any hydrogen when changed ATP into montmorillonoid which is another silicate with similar composition (absence of Fe) under the same conditions, which demonstrate ATP was the host photocatalyst. We have tried four organic dyes under the same experimental conditions, and found that all of them have reduction ability, while EY is best. Loading Ag Nanocrystals (NCs), ATP showed the highest rate of hydrogen production with an apparent quantum efficiency (QE) of10.8%. Further chemical analysis as well as computational simulation proposed the natural Fe-doping (two ATP cells sharing one Fe atom.) can promote the photocatalytic precess. We have found a convenient and cheap route to produce hydrogen. In this way, no (or very small) artificial treatments including chemical synthesis, doping and nanoparticle morphology control are needed at all. Combined with the results of theoretical calculation and VB X-ray photoelectron spectroscopy, the mechanism of hydrogen evolution over EY-ATP photocatalyst was proposed.2. A series of CdS/ATP nanocomposites were synthesized via a green method. The CdS/ATP photocatalysts show high efficiency for water splitting under visible light irradiation. A loading amount3wt%CdS NCs over ATP leads to a200%increase in the photocatalytic activity. At the same time, an efficient photoelectrode was prepared by blade coating. The saturated photocurrent achieved by the CdS/ATP (3wt%) electrode under the illumination of visible light is0.35mA cm-2. Based on the Eg of ATP and CdS, the mechanism of hydrogen evolution over CdS/ATP photocatalyst was proposed.3. Inspired by the work above, we extended the host photocatalyst from1D natural nanomaterials (ATP) to2D natural nanomaterials (VMT). In this section, we use hydrothermal to combine Pt NCs to VMT and investigate the effect of the Pt NCs content on the rate of photocatalytic hydrogen evolution sensitized by EY under visible light irradiation. The rate was further increased to330μmol h-1with a high QE of22.8%when the pure VMT was coupled with3wt%of Pt NCs. VMT photoelectrode was prepared by blade coating. The photocurrent densities rise steeply up to approximately0.4mA cm-2and0.2mA cm-2respectively when irradiated from both front and back side of the electrode. Compared to the total energy (by using VASP code), we found that the most stable cell structure of VMT.4. In this study, we use chemical impregnation to combine CdS quantum dots (QDs) to VMT and investigate the effect of the CdS QDs content on the rate of photocatalytic hydrogen evolution under visible light irradiation. The as-prepared CdS-modified VMT exhibited enhanced hydrogen production activity and photoelectrochemical properties, which benefitted from the extended light absorption and the improved interfacial charge-transfer properties of VMT. The optimized hydrogen evolution rate over CdS/ATP nanocomposites (5wt%CdS) is92μmol h-1with a high QE of17.7%.5. As comparison, Ag and Ag/ZnO hybrid NCs were prepared and used as photocatalysts at the same condition. The controlled synthesis of Ag/ZnO hybrid NCs was based on a seed-mediated growth process. When Ag NCs was absent, octadecylamine (ODA) were employed as reducing agent and capping agents, the ZnO nanoparticles formed in situ in this system were protected by ODA. However, With Ag NCs as seeds, the pyramid shaped Ag/ZnO hybrid NCs were obtained with Ag nanoparticles decorated on the surface of nanopyramid. We found that dyes sensitized Ag/ZnO hybrid NCs exhibited excellent photocatalytic activities under visible light. However, by contrast, organic dyes sensitized natural silicate has a relatively high photocatalytic activity than Ag/ZnO nanocomposites under the same condition.更多还原