Hydrogen is regarded to be one of the most promising green energy source in the Industry 4.0. One promising approach to produce hydrogen is photocatalytic water splitting. To split water into hydrogen and oxygen, a minimum energy of 1.23 eV is required. Silicon is one of the best studied semiconducting material and the second most abundant element in the earth crust. However, the band gap of bulk silicon is 1.12 eV which is too small to provide electrons and holes with enough energy to split water. Herein, we report the top-down formation of porous silicon nanostructures with a sufficient large optical band gap to drive the photocatalytic hydrogen production. We successfully synthesized silicon nanostructures with a very high porosity while maintaining the crystallinity of the silicon. The high porosity leads to an increased active surface area and also to an enlarged optical band gap. XANES investigations near the L2,3 core level of silicon and K core level of oxygen as well as XPS studies were performed at the Helmholtz Zentrum Berlin (BESSY II) with the use synchrotron radiation at the Russian-German beamline. The highly porous silicon nanostructures show an optical band gap of up to 1.8-2.3 eV. XANES and XPS studies exhibits that the nanostructures are covered by a very thin SiO2 layer. Furthermore, we observed an increased hydrogen production under additional light irradiation with generation rate of 20,000µl/h at least for the first 4 hours.