The other side of Ag particle facing the Si would works as the catalyst to oxidize Si and generate electron, which generate H+ and electrons (reaction 6). The reactions at cathode (Ag facing the electrolyte) and the anode (Si contacting with Ag) sites are outlined as follow . (4) (5) (6) (7) The potential of the cathode site (EH2O2 = 1.77 V vs. SHE) is higher than that of the anode site (ESi =1.2 V vs. SHE), thus a local corrosion current would flow from the cathode site to the anode site. In this case, the catalytic Ag particle would work as a redox center and act as a short-circuited VX-680 supplier galvanic cell with an
electron flow inside the Ag particle, while H+ would migrate outside the Ag particle from the anode site to the cathode site. The H+ gradient across the Ag particle from the anode site to cathode site would build-up of an electric field which would propel Ag particles (with negative charge) toward the anode site, thus, the Ag particles deposited on the surface and side of SiNWs would migrate in a vertical or horizontal direction, respectively, as shown by the yellow arrows in Figure 6. It can satisfactorily explain the perpendicular longitudinal and lateral etching pore channel in Figure 5C. Figure 6 Ag particle migration in bulk Si Crenolanib supplier driven by self-electrophoresis mode. An electric field is
built with the presence of H+ gradient across the Ag particle from the anode site to cathode site, which can propel Ag particles toward the anode site. The formation process of mesoporous structures
within the SiNWs may be consistent with that of macroporous structures, both are caused by the lateral etching of silicon, i.e., lateral motility of Ag particles. The four steps are proposed to describe the PSiNWs formation in the HF/AgNO3/H2O2 etching system. When silicon wafers were Liothyronine Sodium immersed into the etchant, Ag nanoparticles were deposited on silicon surface, as depicted in Figure 7A. According to the self-electrophoresis mode, the nucleated Ag particles would migrate down and form the SiNWs, the duration of the redox reaction couple of reactions 4 and 6 lead to the growth of SiNWs. In addition, the reaction of silver ion deposition (Ag+ + e− → Ag) is still present during the growth of SiNWs. Thus, some of the silver particles would grow into dendrite and cover the surface of SiNWs, just as Ag dendrite form in the one-step MACE . As the standard reduction potential of H2O2 (1.77 eV) is larger than that of Ag (0.78 eV), the growing Ag dendritic layer can simultaneously be oxidized into Ag+ ions by H2O2 (reaction 2). The generated Ag+ ions could renucleate throughout the nanowires, as shown in Figure 7B. The horizontal and vertical migrations of Ag particles driven by self-electrophoresis finally induce perpendicular pore channels (Figure 7C).