Ultrasmall Metal Research Articles

Ultrasmall Metal Oxide Particles: Preparation, Photophysical Characterization, and Photocatalytic Properties

AbstractThis paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2‐propanol is described. Quantum (Q)‐size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower‐lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue‐shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pHzpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface‐bound molecules and free radical intermediates is presented to explain these differences in reactivity.

Control of cluster size in nozzle jet expansion

Recent reports showed that ultrasmall metal and semiconductor clusters (up to 1000 atoms per cluster) can be produced during the jet expansion of hot vapor through a small nozzle without the use of carrier gas. Cluster beams are potentially useful in very large-scale integrated (VLSI) thin film processing. In this report, the kinetics of the nonsteady state, stepwise cluster growth in a jet beam with metal vapor is solved quantitatively based on the classical liquid–drop theory. In our calculation for the jet expansion of silver vapor, we found that cluster size up to more than 100 atoms can be formed. This result is consistent with the notion that the nucleation barrier for metal and semiconductor cluster formation can still be small despite their high surface tension. We found that the cluster size distribution depends very much on the stagnation pressure (P0) before the expansion and the nozzle geometry, particularly the expansion angle (θ). For sufficiently large P0 (several Torr) and small θ (<30°), the cluster size distribution is peaked at a value which increases exponentially with the terminal Mach number Mt —the parameter characterizing the expansion conditions.

Ultra-small metal particle arrays produced by high resolution electron-beam lithography

Arrays of silver and gold-palladium particles smaller than 10 nm in diameter and center-to-center spacings as low as 25 nm have been fabricated by electron-beam contamination lithography in a scanning transmission electron microscope and ion milling.