氧化铁的深层还原处理及其在光解水中的应用

发布时间：2018-05-05 00:41

本文选题：氧化铁 + 光解水　；参考：《苏州大学》2015年硕士论文

【摘要】：光催化分解水制备氢气是将太阳能转化为氢能的一项具有挑战且意义重大的课题。在众多半导体材料催化剂中,我们选择研究氧化铁体系,主要是氧化铁具有很多独特的优势,如容易获得、溶液中性能稳定、能带宽度合适等。尽管氧化铁理论计算得到的光解水效率很高,但目前的实际效率较低,主要原因包括氧化铁本身的光生电子-空穴复合速度快,不利于形成光电流的电荷迁移,空穴传输距离短,不足2 nm,差的导电性等。此外,一个限制氧化铁效率的重要因素是其能带位置不合适,需要施加一个偏压让氢气还原,从而使整个反应进行。氧化铁光解水催化过程中的各种其它因素也会导致偏压的产生,降低效率。降低氧化铁光解水过程中的偏压是目前相关领域一个重要的研究方向。在本论文中我们发展了多种方法来提高氧化铁光解水的性能,主要通过控制氧化铁表面还原处理的深度来降低偏压,取得了很好的效果。以下是本论文的具体内容:我们首先对氧化铁的水热合成方法进行了改进,进一步控制其形貌和重复性(第二章)。性能良好的未进行掺杂或其他修饰处理的空白氧化铁样品是进一步处理手段取得良好效果的前提,是一项基础但非常重要的工作。水热法目前在制备空白氧化铁样品上具有独特的优势,通常用简单的方法可以得到目前报道中性能较高的产物,因此得到了广泛应用。我们在实验过程中发现,目前的方法需要比较严格的条件,温度、压力以及浓度等变化的时候往往不能得到重复性好的高性能空白氧化铁样品。我们通过研究水热法前驱溶液中反应物NaNO3和FeCl3.6H2O浓度的影响,去除了以往方法中必须添加的HCl,并减小两种反应物浓度,在不改变其它条件的情况,合成出厚度合适、分布均匀的颗粒状氧化铁,值得一提的是,这种方法得到的空白样品具有良好的性能而且重复性非常高。在改进空白样品的制备条件之后,我们基于课题组前期氧空穴掺杂的实验研究,发展了氨硼烷水溶液热分解产氢来对氧化铁进行还原处理。这种处理方法同样可以形成表面氧空穴掺杂来提高效率。对比以前的方法,这种在氧化铁表面滴加氨硼烷溶液直接加热还原的方法操作简单,最佳条件下处理的氧化铁光电流比空白样提高近两倍,改善效果明显。更有意义的是,在加热过程中我们发现样品覆盖坩埚后进行还原处理得到的样品光解水效果更好,且起始电压(偏压)有明显改善。深入的研究证明这不是浓度增加的效果,而是加热过程中坩埚覆盖引起压力增大导致的深层处理的效果。通过增加压力,氧化铁表面还原深度也会增加,达到近表面的层次,实验证实近表面层次的还原处理能够有效增加光解水效果,同时也是偏压降低的重要原因。氧化铁还原程度太大的时候,会产生光催化活性低的Fe3O4,从而影响氧化铁的催化性,因此在表面处理中增加过大的浓度会降低效率,但压力增加的情况下继续向近表面层处理则可以提高效率。我们的实验还比较了利用缺氧气氛来产生氧空穴的方法,这种方法实现了体相掺氧空穴(近似体相还原),但效果比表面处理更差。因此合适的还原处理的深度(或氧空穴的分布深度)既不是在表面层,也不是在体相,而是近表面层比较合适。只有产生合适深度的氧空穴,才能有最佳催化活性。该结果可能对其他偏压降低的过程有一定的借鉴意义,同时提出了氧化铁修饰过程中深度处理的重要性。在本论文的第四章我们还进行了用不同方法将镍元素掺杂进氧化铁的研究。我们使用sputtering技术在样品表面电镀镍元素,之后退火处理。然而结果表明,这种处理方法并不是有效的,甚至阻碍了其本身的光催化性。可能是电镀过程破坏了氧化铁自身的纳米结构。我们将在未来的工作中深入研究其具体原因。
[Abstract]:The preparation of hydrogen by photocatalytic decomposition of water is a challenging and significant subject to convert solar energy into hydrogen energy. In a large number of semiconductor materials, we choose to study iron oxide system, mainly the iron oxide has a lot of unique advantages, such as easy to obtain, solution neutrality stability, and the suitable band width. The efficiency of the photolysis water is very high, but the current efficiency is low, the main reason is that the photoelectron electron hole compound speed is fast, it is not good for the charge migration of the photocurrent, short hole transmission distance, less than 2 nm, and the conductivity of difference. In addition, an important factor limiting the efficiency of iron oxide is its energy band. The position is not suitable, a partial pressure is required to reduce the hydrogen to make the whole reaction proceed. Other factors in the catalytic process of ferric oxide photolysis will also lead to the generation of bias, reducing the efficiency. Reducing the bias in the photolysis of iron oxide is an important research direction in the related field. In this paper, we develop A variety of methods to improve the performance of ferric oxide photolysis water, mainly by controlling the depth of the iron oxide surface reduction to reduce the bias voltage, has achieved good results. The following is the specific content of this paper: we first improved the hydrothermal synthesis method of iron oxide, control its morphology and repeatability (second chapters). Good performance. The blank iron oxide samples without doping or other modification are the prerequisite for further treatment. It is a basic but very important work. The hydrothermal method has a unique advantage in the preparation of blank iron oxide samples. In the course of the experiment, we found that the current method needs more stringent conditions, and the temperature, pressure and concentration often do not get good reproducible high performance blank iron oxide samples. We have studied the effects of the concentration of NaNO3 and FeCl3.6H2O on the reactant in the hydrothermal solution. The HCl, which must be added in the previous method, is removed and the concentration of two reactants is reduced. The granular iron oxide with suitable thickness and uniform distribution is synthesized without changing other conditions. It is worth mentioning that the blank samples obtained by this method have good performance and high repeatability. The preparation conditions of blank samples are improved. Then, based on the experimental study of the initial oxygen vacancy doping in the project group, we developed the reduction of hydrogen from the aqueous solution of the amborane water solution to the iron oxide reduction treatment. This method can also form the surface oxygen vacancy doping to improve the efficiency. Compared with the previous method, the solution is directly heated and reduced by the solution of the borane on the surface of the iron oxide. The method of operation is simple, the photoelectric current of iron oxide treated under the best condition is nearly two times higher than that of blank sample, and the improvement effect is obvious. It is more meaningful that in the heating process we found that the sample covered crucible after the reduction treatment was better, and the initial voltage (bias voltage) was obviously improved. It is not the effect of increasing concentration, but the effect of deep treatment caused by increasing the pressure of crucible during the heating process. By increasing the pressure, the reduction depth of the iron oxide surface will also increase to reach the near surface level. It is proved that the reduction treatment in the near surface layer can increase the effect of photodissociation water, and also the bias is reduced. Important reasons. When the iron oxide reduction is too large, the Fe3O4 of low photocatalytic activity will be produced, which will affect the catalytic activity of iron oxide, so increasing the concentration in the surface treatment will reduce the efficiency, but the high efficiency can be carried out to the near surface treatment under the condition of increasing pressure. Our experiment also compares the use of oxygen deficiency. The method of producing oxygen vacancies, this method realizes the oxygen vacancy of the body phase (the reduction of the approximate body phase), but the effect is worse than the surface treatment. Therefore, the appropriate depth of the reduction treatment (or the depth of the distribution of oxygen vacancies) is neither in the surface layer nor in the body phase, but in the near surface layer. Only the oxygen vacancies at the right depth are produced, only It may have the best catalytic activity. This result may be of reference to the process of other partial pressure reduction and the importance of depth treatment in the process of modification of iron oxide. In the fourth chapter of this paper, we also carried out the research on the doping of nickel in the iron oxide by different methods. We use the sputtering technology on the sample surface. Electroplating nickel and then annealing treatment. However, the results show that this treatment is not effective or even hinders its photocatalytic activity. It may be that the electroplating process destroys the nanostructure of iron oxide itself. We will study the specific reasons in future work.

【学位授予单位】：苏州大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ116.2;O643.36

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