Activity of Ag/CeZrO2, Ag+K/CeZrO2, and Ag-Au+K/CeZrO2 Systems for Lean Burn Exhaust Clean-Up

Herein, the activity of Ag and bimetallic Au-Ag catalysts, supported over Ce0.85Zr0.15O2 (CZ), was investigated in a complex stream, whose components included CO, C3H8, NO, O2, and, optionally, an injection of water vapor. In such a stream, three of the possible reactions that can occur are CO oxidation, propane combustion, and NO oxidation. The aim of these studies was to explore whether silver, due to its strong affinity to oxygen, will counteract the stabilization of oxygen by potassium. The effect of the presence of potassium ions on the activity of the monometallic silver catalysts is beneficial in the complex stream without water vapor in all three studied reactions, although it is negligible in the model CO stream. It has been shown that water vapor strongly suppresses the activity of the Ag+K/CZ catalyst, much more so than that of the Ag/CZ catalyst. The second purpose of the work was to determine whether the negative effect of potassium ions on the activity of nanogold catalyst can be countered by the addition of silver. Studies in a model stream for CO oxidation have shown that, for a catalyst preloaded with gold, the effect of potassium is nulled by silver, and the activity of AuAg + 0.15 at%K/CZ and AuAg + 0.30 at%K/CZ is the same as that of the monometallic Au catalyst. Conversely, when the reaction is carried out in a complex stream, containing CO, C3H8, NO, O2, and water vapor, the presence of water vapor leads to higher CO conversion as well as increased NO2 formation and slightly suppresses the C3H8 combustion.

High-Yield Growth and Tunable Morphology of Bi2Se3 Nanoribbons Synthesized on Thermally Dewetted Au

The yield and morphology (length, width, thickness) of stoichiometric Bi2Se3 nanoribbons grown by physical vapor deposition is studied as a function of the diameters and areal number density of the Au catalyst nanoparticles of mean diameters 8–150 nm formed by dewetting Au layers of thicknesses 1.5–16 nm. The highest yield of the Bi2Se3 nanoribbons is reached when synthesized on dewetted 3 nm thick Au layer (mean diameter of Au nanoparticles ~10 nm) and exceeds the nanoribbon yield obtained in catalyst-free synthesis by almost 50 times. The mean lengths and thicknesses of the Bi2Se3 nanoribbons are directly proportional to the mean diameters of Au catalyst nanoparticles. In contrast, the mean widths of the Bi2Se3 nanoribbons do not show a direct correlation with the Au nanoparticle size as they depend on the contribution ratio of two main growth mechanisms—catalyst-free and vapor–liquid–solid deposition. The Bi2Se3 nanoribbon growth mechanisms in relation to the Au catalyst nanoparticle size and areal number density are discussed. Determined charge transport characteristics confirm the high quality of the synthesized Bi2Se3 nanoribbons, which, together with the high yield and tunable morphology, makes these suitable for application in a variety of nanoscale devices.

Study of metal-assisted chemical etching of silicon as an alternative to dry etching for the development of vertical comb-drives

Metal-assisted chemical etching (MaCEtch) has recently emerged as a promising technique to etch anisotropic nano- and microstructures in silicon by metal catalysts. It is an economical wet chemical etching method, which can be a good alternative to deep-reactive ion etching (DRIE) process in terms of verticality and etch depth. In the present study, gold is used as a metal catalyst and deposited using physical vapour deposition. It has already been demonstrated that (100) p-type Si wafer can be etched with vertical and smooth side walls. Effects of varying concentrations of etchant constituents and various other parameters, that is, porosity of deposited Au, surface contaminants, oxide formation, metal catalyst, etching time, role of surface tension of additives on the etch depth and surface defects are studied and discussed in detail. By increasing the hydrofluoric acid (HF) concentration from 7.5 M to 10 M, lateral etching is reduced and the microstructure’s width is increased from 17 µm to 18 µm. Porous defects are suppressed by decreasing the hydrogen peroxide (H2O2) concentration from 1.5 M to 1 M. On increasing the etching time from 30 min to 60 min, the microstructures are over-etched laterally. Smoother side walls are fabricated by using the low-surface-tension additive ethanol. The maximum etch depth of 2.6 µm is achieved for Au catalyst in 30 min. The results are encouraging and useful for the development of vertical comb-drives and Micro-Electro-Mechanical Systems (MEMS).