Metal-assisted catalytic etching (MACE) involving Ag deposited on Si particles has been reported as a facile method for the production of Si nanowires (Si NWs). We show that the structure of Si particles subjected to MACE changes dramatically in response to changing the loading of the Ag catalyst. The

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.

The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless etching (ReEtching) was used to produce photoluminescent porous Si directly from Si powder. As etched, the material was H-terminated. The intensity and peak wavelength were greatly affected by the rinsing protocol employed. The highest intensity and bluest PL were obtained when dilute HCl(aq) rinsing was followed by pentane wetting and vacuum oven drying. Roughly half of the hydrogen coverage was replaced with –RCOOH groups by thermal hydrosilylation. Hydrosilylated porous Si exhibited greater stability in aqueous solutions than H-terminated porous Si. Pickling of hydrosilylated porous Si in phosphate buffer was used to increase the PL intensity without significantly shifting the PL wavelength. PL intensity, wavelength and peak shape responded linearly with temperature change in a manner that was specific to the surface termination, which could facilitate the use of these parameters in a differential sensor scheme that exploits the inherent inhomogeneities of porous Si PL response.

Injection of H2O2 to control both the rate and extent of etching allowed us to discover a new regime of MACE with extremely small quantities of deposited metal as a catalyst: low-load MACE (LL-MACE).1 The structure of particles subjected to LL-MACE is completely different compared to conventional MACE. Si nanowires and mesoporous micro- or nano-particles are produced with high yield, low cost and controlled properties suitable for applications in e.g. lithium-ion batteries, drug delivery, as well as biomedical imaging and contrast enhancement. Different metal catalysts behave in distinct manners than can be traced to differences in nucleation and reaction kinetics.2 Not only Ag, Au, Pd and Pt but also inexpensive Cu can be used to perform LL-MACE. Previously, controlled reactant injection and acetic acid were used to transform stain etching into the improved process known as regenerative electroless etching (ReEtching).3 Here injection and acetic acid are exploited to enhance MACE and extend it to the low-load regime. Ag, Au, Pd, Pt and Cu can be galvanically deposited onto Si powder out of solutions of HF(aq). Nucleation of deposited metal nanoparticles is made more uniform by dispersing the Si powder in acetic acid and injecting the dissolved metal ions at a controlled rate. The use of a syringe pump to deliver not only metal ions during deposition but also the oxidant (H2O2) during MACE is essential for increased product uniformity and yield. Temperatures of ~0–50 °C and grades of Si (i.e. electronics grade Si with various doping levels or metallurgical grade Si) produced significantly different pore size distributions. Doping levels and the associated changes in band bending and space charge layer width4 are linked to changes in the pore size distribution. As shown in Fig. 1(A), high loading of Ag resulted in deposition of dendrites and Ag particles. These broke into ~50–100 nm Ag nanoparticles that etched in a correlated fashion to generate parallel etch track pores. The uniform size distribution of the Ag nanoparticles was generated dynamically during etching. This conventional high-load MACE (HL-MACE) did not produce Si nanowires (Si NWs) directly. However, etch track pore walls were cleaved readily by ultrasonic agitation to form Si NWs. Etching far removed from the metal nanoparticles (remote etching) also occurred for highly doped Si. As shown in Fig. 1(B), reducing the loading of Ag by a factor of 100 to 1000 compared to HL-MACE created 10–30 nm nanoparticles that etched in an uncorrelated fashion and generated randomly directed etch track pores. Significant etching far removed from the Ag catalyst also occurred. The combination of localized and remote etching resulted in a bimodal distribution of mesoporosity peaking at ~4 nm from remote etching and 10–30 nm from localized etching. The bimodal distribution was confirmed by the BJH method of nitrogen adsorption porosimetry. The pore size distribution could be controlled by changing the process temperature, the grade of Si and the metal composition. Whereas Ag, Au, Pd and Pt performed well in both the HL and LL limits, Cu generated porosification only in the LL limit. Funded by grants #314552 and #314412 from Academy of Finland and the National Science Foundation award #1825331. The microscopy studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). 1 K. Tamarov, J. D. Swanson, B. A. Unger, K. W. Kolasinski, A. T. Ernst, M. Aindow, V.-P. Lehto, and J. Riikonen, ACS Appl. Mater. Interfaces 12, 4787 (2020). 2 K. W. Kolasinski, W. B. Barclay, Y. Sun, and M. Aindow, Electrochim. Acta 158, 219 (2015). 3 K. W. Kolasinski, N. J. Gimbar, H. Yu, M. Aindow, E. Mäkilä, and J. Salonen, Angew. Chem., Int. Ed. Engl. 56, 624 (2017). 4 E. Torralba, S. Le Gall, R. Lachaume, V. Magnin, J. Harari, M. Halbwax, J.-P. Vilcot, C. Cachet-Vivier, and S. Bastide, ACS Appl. Mater. Interfaces 8, 31375 (2016). Figure 1