Mesopore Formation and Silicon Surface Nanostructuration by Metal-Assisted Chemical Etching With Silver Nanoparticles.

This article presents a study on Metal-Assisted Chemical Etching (MACE) of silicon in HF-H2O2 using silver nanoparticles as catalysts. Our aim is a better understanding of the process to elaborate new 3D submicrometric surface structures useful for light management. We investigated MACE over the whole range of silicon doping, i.e., p++, p+, p, p-, n, n+, and n++. We discovered that, instead of the well-defined and straight mesopores obtained in p and n-type silicon, in p++ and n++ silicon MACE leads to the formation of cone-shaped macropores filled with porous silicon. We account for the transition between these two pore-formation regimes (straight and cone-shaped pores) by modeling (at equilibrium and under polarization) the Ag/Si/electrolyte (HF) system. The model simulates the system as two nanodiodes in series. We show that delocalized MACE is explained by a large tunnel current contribution for the p-Si/Ag and n-Si/HF diodes under reverse polarization, which increases with the doping level and when the size of the nanocontacts (Ag, HF) decreases. By analogy with the results obtained on heavily doped silicon, we finally present a method to form size-controlled cone-shaped macropores in p silicon with silver nanoparticles. This shape, instead of the usual straight mesopores, is obtained by applying an external anodic polarization during MACE. Two methods are shown to be effective for the control of the macropore cone angle: one by adjusting the potential applied during MACE, the other by changing the H2O2 concentration. Under appropriate etching conditions, the obtained macropores exhibit optical properties (reflectivity ~3 %) similar to that of black silicon.

Bimetallic Cu-Rh Nanoparticles on Diazonium-Modified Carbon Powders for the Electrocatalytic Reduction of Nitrates.

4-Benzenethiol-functionalized high-surface-area graphite powder was prepared and decorated with bimetallic Cu100-xRhx nanoparticles (NPs) to serve as electrocatalysts for the reduction of nitrates. In the first step, the HSAG powder was grafted with in-situ-generated diazonium compounds from 4-aminothiophenol (ATP) in an acidic medium using NaNO2 for the diazotization process. The surface composition was tuned using different initial quantities of ATP. The surface XPS-determined S/C atomic ratio was found to increase stepwise with the initial quantity of amine. In a second step, the grafted and untreated HSAG powders were decorated with Cu100-xRhx NPs by a wet chemical method and the elemental composition of the end composites was assessed by EDS-SEM and ICP, whereas TEM and EDS-TEM served to characterize the NP morphology and their composition on the nanometer scale. In all cases, the NP size was invariably found to be ∼1.7 nm but with a size distribution becoming narrower under an increasing grafting rate and the global composition enriched in copper. Voltammetry was performed with a cavity microelectrode to evaluate the electrocatalytic performance of the composites for nitrate reduction. Increasing diazonium grafting led to a progressive reduction of the peak current intensity and a shift of the peak potentials toward cathodic values. The maximum intensity was obtained for 0.005 µmol of diazonium salt per mg of HSAG, with a gain of 40% in comparison to the best untreated sample. This improvement and a change in the voltammogram characteristics after grafting seem to result from modifications of the local composition at the level of NPs that differ from the global composition. This work conclusively shows that diazonium surface modification is important not only in attaching electrocatalytic NPs to carbon supports but also in providing a narrower size distribution of the electrocatalysts together with finely tuned catalytic properties.

3D Patterning of Si by Contact Etching With Nanoporous Metals.

Nanoporous gold and platinum electrodes are used to pattern n-type silicon by contact etching at the macroscopic scale. This type of electrode has the advantage of forming nanocontacts between silicon, the metal and the electrolyte as in classical metal assisted chemical etching while ensuring electrolyte transport to and from the interface through the electrode. Nanoporous gold electrodes with two types of nanostructures, fine and coarse (average ligament widths of ~30 and 100 nm, respectively) have been elaborated and tested. Patterns consisting in networks of square-based pyramids (10 × 10 µm2 base × 7 µm height) and U-shaped lines (2, 5, and 10 µm width × 10 µm height × 4 µm interspacing) are imprinted by both electrochemical and chemical (HF-H2O2) contact etching. A complete pattern transfer of pyramids is achieved with coarse nanoporous gold in both contact etching modes, at a rate of ~0.35 µm min-1. Under the same etching conditions, U-shaped line were only partially imprinted. The surface state after imprinting presents various defects such as craters, pores or porous silicon. Small walls are sometimes obtained due to imprinting of the details of the coarse gold nanostructure. We establish that np-Au electrodes can be turned into "np-Pt" electrodes by simply sputtering a thin platinum layer (5 nm) on the etching (catalytic) side of the electrode. Imprinting with np Au/Pt slightly improves the pattern transfer resolution. 2D numerical simulations of the valence band modulation at the Au/Si/electrolyte interfaces are carried out to explain the localized aspect of contact etching of n-type silicon with gold and platinum and the different surface state obtained after patterning. They show that n-type silicon in contact with gold or platinum is in inversion regime, with holes under the metal (within 3 nm). Etching under moderate anodic polarization corresponds to a quasi 2D hole transfer over a few nanometers in the inversion layer between adjacent metal and electrolyte contacts and is therefore very localized around metal contacts.

Tunable Surface Structuration of Silicon by Metal Assisted Chemical Etching with Pt Nanoparticles under Electrochemical Bias.

An in-depth study of metal assisted chemical etching (MACE) of p-type c-Si in HF/H2O2 aqueous solutions using Pt nanoparticles as catalysts is presented. Combination of cyclic voltammetry, open circuit measurements, chronoamperometry, impedance spectroscopy, and 2D band bending modeling of the metal/semiconductor/electrolyte interfaces at the nanoscale and under different etching conditions allows gaining physical insights into this system. Additionally, in an attempt to mimic the etching conditions, the modeling has been performed with a positively biased nanoparticle buried in the Si substrate. Following these findings, the application of an external polarization during etching is introduced as a novel efficient approach for achieving straightforward control of the pore morphology by acting upon the band bending at the Si/electrolyte junction. In this way, nanostructures ranging from straight mesopores to cone-shaped macropores are obtained as the Si sample is biased from negative to positive potentials. Remarkably, macroscopic cone-shaped pores in the 1-5 µm size range with a high aspect ratio (L/W ∼ 1.6) are obtained by this method. This morphology leads to a reduction of the surface reflectance below 5% over the entire VIS-NIR domain, which outperforms macrostructures made by state of the art texturization techniques for Si solar cells.

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Bottom-up preparation of MgH2 nanoparticles with enhanced cycle life stability during electrochemical conversion in Li-ion batteries.

A promising anode material for Li-ion batteries based on MgH2 with around 5 nm average particles size was synthesized by a bottom-up method. A series of several composites containing MgH2 nanoparticles well dispersed into a porous carbon host has been prepared with different metal content up to 70 wt%. A narrow particle size distribution (1-10 nm) of the MgH2 nanospecies with around 5.5 nm average size can be controlled up to 50 wt% Mg. After a ball milling treatment under Ar, the composite containing 50 wt% Mg shows an impressive cycle life stability with a good electrochemical capacity of around 500 mA h g(-1). Moreover, the nanoparticles' size distribution is stable during cycling.

Cavity microelectrode for studying manganese dioxide powder as pH sensor.

The potential-pH response of an electrolytic manganese dioxide is investigated by means of a cavity microelectrode (CME). The potential-pH curves show a complex evolution that could be explained by the disporportionation of MnOOH species, leading to the formation of Mn(2+) ions on the MnO(2) surface. Such a behaviour is not suited for pH sensor application. However when the tip of the electrode is coated by a Nafion membrane, the potential-pH evolution shows a unique slope close to -60 mV pH(-1). In addition, the sensor exhibits short time responses to pH variations, a good selectivity, and it can be easily renewed compared to classical sensors.