Preparation And Photoelectrochemical Performance For The Electrodes Of The TiO₂ Nanotube And Si Nanowire Arrays

One-dimensional ?ID? nanostructured semiconductors have widely applied in the field of the photoelectrochemical ?PEC? water-splitting due to its photogenerated electrons directed transmission, large specific surface areas, and high length-to-diameter ratios. Two kinds of traditional ID semiconductor materials, TiO₂ nanotube and Si nanowire arrays have been chosen as the substrate of the photoelectrodes, respectively. There are less reactive sites on the surface of the TiO₂ nanotube arrays, and the valence band position of silicon is higher than the position of the potential for the oxygen evolution reaction, which lead to low photocatalytic efficiency of them. In this work, the heterogeneous structure were built in order to improve their PEC performance.Highly-ordered TiO₂ nanotube arrays ?TNTAs? on Ti foil were fabricated by electrochemical anodization. Cobalt-phosphate ?Co-Pi? fabricated by electrodeposition and Pt nanaoparticles ?Pt? fabricated by photochemical reduction were decorated onto TNTAs to improve the photoelectrochemical ?PEC? performance. The photocurrent density of the TNTAs/Co-Pi/Pt photoelectrode could reach 0.185 mA/cm2 at 1.23 V vs RHE under UV lamp illumination ?1 mW/cm2?, which is 2.65 times that of the pure TNTAs photoelectrode. The working mechanism of the TNTAs/Co-Pi/Pt photoelectrode was discussed in detail. Pt could promote the reduction of Co?IV?-oxo intermediate to Co?II? which facilitated the charge transfer in the PEC performance.Well aligned and perpendicular Si nanowire arrays ?SiNWs? on the Si substrate were prepared by the metal-catalyzed chemical etching method. Hematite and Pt nanoparticles modified onto the SiNWs by thermal decomposition. The SiNWs/20mMFe₂O₃-Pt photoelectrode exhibited a low photocurrent onset potential of 0.5 V vs RHE and a high photocurrent density of 0.87 mA/cm2 at 1.23 V vs RHE, while the photocurrent density of the SiNWs/20mMFe₂O₃ photoelectrode was only 0.37 mA/cm2. Mott-Schottky plots of the photoelectrodes suggested that Pt modified could reduce the flat band of the SiNWs/Fe₂O₃-Pt photoelectrode, which would result in lower photocurrent onset potentials.TiO₂ thin passivating layer was deposited on SiNWs/Fe₂O₃ photoelectrode by atomic layer deposition ?ALD?. Significant PEC performance improvement is observed when the TiO₂ prepared with 40-ALD cycles and annealing at 500?, and the photocurrent density of the SiNWs/Fe₂O₃/TiO₂ photoelectrode could reach 1.75 mA/cm2 at 1.23 V vs RHE. The ?-? curves of the SiNWs/Fe₂O₃/TiO₂ photoelectrode with different immersed time in the 1 M NaOH solution were essentially consistent, which indicated that TiO₂ passivating layer significantly improve the chemical stability of the photoelectrode in alkaline solution. Compared with the SiNWs/Fe₂O₃ photoelectrode, the photocurrent density of the SiNWs/Fe₂O₃/TiO₂ photoelectrode increased 0.23 times at 1.23 V vs RHE, and the photocurrent onset potential reduced 0.4 V vs RHE in an aqueous 0.5 M Na2SO4 solution.