Pursuing solar energy conversion devices with low cost and high efficiency has become the major task of scientists and engineers in our age. Si nanowire (SiNW) and porous Si (PSi) provide the exciting future for achieving this aim. In this thesis, we obtain SiNW and PSi through cost-effective nano-technique ‘metal assisted chemical etching’ (MaCE). In case of SiNW, we find that during MaCE of ‘dirty’ Si, superior purification effect is observed, and metal impurities inside ‘dirty’ Si can be effectively removed. We further apply SiNWs from ‘dirty’ Si in photoelectrochemical water splitting. SiNWs show significant efficiency enhancement for H2 evolution. In case of PSi, we successfully obtain meso-PSi with high uniformity. The porosity can be flexibly tuned. Compared with traditional etching methods, this simple new technique has a fast etching rate. Thus, it can be directly applied in layer transfer photovoltaics while significantly reducing process complexities.