Influence of a pressure induced piezoelectric field on the recombination processes in photocatalytically active nanoparticles

半导体材料的光催化需要由于光的吸收而激发自由电子和空穴。在半导体表面，它们可以被用来产生羟基自由基，这些自由基参与所谓的高级氧化过程(AOP)中的化学反应，例如用于空气和水的净化。因此，光催化可以消除常规方法无法过滤或生物降解的污染物。这些光催化剂的效率可以使用纳米级的半导体颗粒来提高，这是以其大的比表面积为特征的。然而，由于粒子内部不存在分离电子和空穴的自然驱动力，复合过程导致光催化效率显著降低。因此，抑制这些复合过程是生产效率显著提高的光催化剂的一种有前途的方法。因此，可以实现低成本和环境友好型建筑，而不需要在水处理过程中添加对环境有害的化学品。在本研究项目中，在纳米尺度的压电光催化系统(例如，氧化锌、硫化镉)中施加一个明确定义的外部驱动力(压力)，在粒子内部诱导电场。这种电场不仅在空间上将电子和空穴分开，而且还将它们从内部传输到相对的表面区域，在那里它们可以用于光催化过程。除了降低复合几率和提高催化活性外，空间分离还显著抑制了反应物的化学反应。这些效应应通过压力依赖的光致发光(复合)和光催化活性来检测。

Photocatalysis with semiconductor materials requires the excitation of free electrons and holes due to the absorption of light. At the surface of the semiconductor they can be utilized to generate hydroxyl radicals, which take part at chemical reactions in so called advanced oxidation processes (AOP) e.g. for air and water purification. Photocatalysis can thus eliminate pollutants, which can neither be filtered nor degraded biologically by conventional methods. The efficiency of these photocatalysts can be enhanced using nanoscale semiconductor particles, which are characterized by their large specific surface area. However, since no natural driving force for the separation of electrons and holes exists inside the particles, recombination processes cause a significant reduction of the photocatalytic efficiency. The suppression of these recombination processes thus represents a promising approach to produce photocatalysts with significantly improved efficiency. As a result low cost and environmentally friendly constructions can be realized without adding environmentally harmful chemicals during water treatment. In the present research project a well-defined external driving force (pressure) is impressed to nanoscale piezoelectric photocatalytic systems (e.g. ZnO, CdS) inducing an electrical field inside the particles. This electrical field not only separates the electrons and holes spatially, but also transports them from inside to opposite surface areas where they are available for photocatalytic processes. Besides decreasing the recombination probability and increasing the catalytic activity, chemical back reactions of the reactants is also significantly inhibited by the spatial separation. These effects shall be detected by pressure dependent photoluminescence (recombination) as well as photocatalytic activity.