Fabrication And Photoelectrochemical Properties Of Si Nanostructure Arrays

Hydrogen energy is regarded as an ideal green energy. Since Fujishima and Honda first demonstrated using TiO2as a photoanode for hydrogen evolution, photoelectrochemical water splitting is one of the most promising ways to produce hydrogen by using the renewable solar energy. Si nanostructure array has excellent anti-reflection property, which improves the efficiency of the absorption of solar energy light on the electrode. Additionally, Si nanostructure array possesses larger specific area improving the transfer of electrical charges across the length of nanostructures, which improves the efficiency of photoelectrochemical water splitting. Thus, Si nanostructure array have become the research focus of photoelectrochemical field for their excellent characteristics in recent years.In this thesis, several Si nanostructure array have been fabricated, respectively. We also have successfully synthesized hybrid Pt/Si nanowire arrays, Pt/Si nanohole arrays and Si/ZnO core/shell nanowire arrays by decorating the surfaces of silicon nanostructure obtained by using the metal assisted chemical etching. The growth conditions, morphologies, microstructures and water splitting performance of as-prepared samples were systematically studied. The summary of this thesis is listed as follows:(1) High density aligned silicon nanowire (SiNW) arrays decorated with discrete platinum nanoparticles (PtNPs) have been fabricated by metal assisted electroless etching followed by an electroless platinum deposition process, and systematic investigations of photoelectrochemical behavior of Pt/SiNW were also reported in this study. From the experiment, it can be seen clearly that large-area aligned SiNWs are perpendicular to the Si surface. The diameter distribution of the nanowires is in the range of30-200nm, and the length can be well controlled by etching time. Coating of PtNPs on SiNW sidewalls yielded a more positive onset potential (Vos), which enhances the photoelectrochemical hydrogen generation performance of the photoelectrodes. It also shows that excessive PtNPs deposition leads to a decreased photocurrent. Additionally, we have demonstrated that the photoelectrode consisting of longer SiNWs yielded a higher limiting current. However, when the length of SiNWs was increased further to>4μm, the limiting current dramatically reduced. The reason of the behavior was preliminarily explored.(2) Si/ZnO core/shell nanowire (NW) arrays were fabricated using atomic layer deposition (ALD) of ZnO shell on SiNW arrays prepared by metal assisted electroless etching method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy were utilized to characterize the core/shell structures. Water splitting performance of the core/shell structures was preliminarily studied. The Si/ZnO core/shell NW arrays yielded significantly higher photocuirent density than the planar Si/ZnO structure due to their low reflectance and high surface area. We studied the effect of the length of the core/shell array and the thickness of ZnO shell on photoelectrochemical H2generation. It is found that the longer NW arrays could produce higher photocurrent density, and the photoelectrochemical performance could be further improved by optimizing the thickness of ZnO shell.(3) Large-area highly ordered silicon nanohole (SiNH) arrays on Si substrate have been fabricated by the combination of nanosphere lithography and metal assisted electroless etching. The diameter, length of nanoholes, and the center-to-center distance of adjacent nanoholes, can be accurately controlled by nanosphere lithography and metal assisted electroless etching conditions. The sub-wavelength structure of SiNH arrays had excellent antireflection property. Here is the first report to investigate hydrogen generation on Pt/SiNH photocathode. Compared to the planar Si, the SiNH samples exhibited a higher photoelectrochemical hydrogen generation performance. Furthermore, decorating the SiNH arrays with platinum nanoparticles (PtNPs) by electroless platinum deposition method yielded a significantly high photovoltage of0.12V, which enhances the photoelectrochemical hydrogen generation performance of the photoelectrodes.