Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders

Abstract

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065–88%), pore size (3–100 nm), specific surface area (20–310 m2·g–1), and specific pore volume (0.05–1.05 cm3·g–1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10–50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.

Formation and Dissolution of Mesoporous Layer during Metal-Particle-Assisted Etching of n-Type Silicon

“…Band structure at the metal/Si interface is a key factor in the metal-assisted etching. 31,36,37,40,41,45,46 For example, Kolasinski's group 36,45 showed that, for both the n-and p-Si, the band structures at the Au/Si and Pt/Si interfaces are favorable for the hole injection to the valence band of Si, but not at the Ag/Si interface. In the case of the Ag-assisted etching, positive holes produced by the H 2 O 2 reduction can be consumed at the Ag/Si interface without the hole injection into bulk Si enough to induce the anodic polarization.…”

“…They also proposed a possibility that the Ag particles are positively charged by the H 2 O 2 reduction and the electric field induces the etching of Si in the vicinity of them. 36,45 As another possibility, we suppose that the Si oxidation is catalyzed by the Ag particles, i.e. the Si oxidation with the electron injection can be a driving force of the Ag-assisted etching.…”

“…Here, we focus on the accumulation of holes at the metal/Si interface, which plays an important role for the preferential etching under the metal catalyst. 46 According to the calculation by Kolasinski's group, 36,45 the accumulation layer of holes is formed at the valence band of Si by the contact with Au and Pt. Then, the accumulation layer under the Au catalyst is smaller than that under the Pt catalyst, indicating that the spread of excess holes into the bulk Si during the Au-assisted etching is more significant than that during the Pt-assisted etching.…”

“…Metal catalyst plays an essential role for determining the etching behavior. 18,[28][29][30][31][32][33][34][35][36] Recently, Tamarov et al 36 comprehensively studied the etching behavior using various kinds…”

Metal Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

“…SiNWs produced by MACE can be either solid-core or porous [60] depending largely on the resistivity of the Si substrate. Low-doped Si regardless of whether it is n type or p type leads to solid core SiNW and highly doped Si succumbs to remote etching and the formation of porous SiNW [36,[61][62][63][64]. Not only can SiNWs be modified with fluoroalkylsilanes to exhibit superhydrophobicity [65], they can also have their rate of biodegradability regulated by controlling their termination [61].…”