A Rapid Prototyping Approach to Ag Nanoparticle Fabrication in the 10–100 nm Range

The fabrication of metal nanoparticles has drawn considerable attention because of both fundamental interest and the potential for applications. Depending on the exact particle size to be synthesized, elaborate and often time-consuming processes can be required to achieve highly uniform particles. Herein, we demonstrate a simple, inexpensive, and rapid method for making metal nanoparticles ranging between 10 and 100 nm in size. The process can be competed in approximately 11 min without the use of a clean-room environment or vacuum techniques. The method works by using a thin supported TiO2 film as a photocatalyst and an alumina membrane as a template. The nanoparticle array formed in this manner is nearly monodisperse and the particle surface density and geometry are uniform over large areas. The templates are reusable and the technique may be amenable for biosensing as well as catalytic applications. Metal nanoparticles possess unique physical, magnetic, optical, electrical, and catalytic properties. Gaining control over their size and geometry during the fabrication process is of central interest. This has led to remarkable achievements in the preparation of nearly monodisperse particles up to approximately 10 nm in diameter by employing synthetic methods. Moreover, photolithographic methods now make it possible to precisely control geometries down to the 100 nm scale. There are, however, fewer methods for easily fabricating highly uniform metal nanoparticle arrays on an intermediate length scale. Previous fabrication efforts on the 10 to 100 nm scale have included reduction synthesis and nanolithography. Also, template approaches have been used to synthesize nanotubes, nanorods, nanoelectrodes, and nanoparticles. Some of these techniques depend on wet electrochemical methods to deposit metal onto a uniform nanoporous template, while others apply thermal or electron-beam evaporation followed by etching procedures. Anodic aluminum oxide (AAO) templating has been widely exploited in these applications because of its uniformity, rigidity, and size-controllable properties. It should be noted that the approaches described above require a vacuum chamber for evaporating metal, a clean-room environment, sophisticated preparation, relatively specialized equipment, and/or etching steps. It was therefore deemed desirable to design a bench-top technique that could be performed rapidly with minimal equipment and without specialized facilities, while still achieving large arrays of highly uniform particles. Herein, a simple templating method is described for photoreducing metal from aqueous solutions onto TiO2 thin films through a nanoporous filtration membrane. TiO2 has well-studied photocatalytic properties, which makes it an excellent choice for reducing metal ions from solution by UV illumination. Most often it has been used in the form of nanoparticles for reducing metal ions in solution. More recently the process has been demonstrated with a thin TiO2 film on a silica substrate. [40] In both cases the metal particles that are synthesized are typically 50 to 200 nm, where the average size can be tuned to some extent by the solution conditions. Although the particles are not monodispersed, it should be emphasized that the process can be performed under mild conditions (i.e., at room temperature in aqueous solution and neutral pH). Our strategy for forming uniform size arrays of metal nanoparticles is outlined in Figure 1. In the first step, a TiO2 precursor was deposited onto the surface of a Pyrex slide in a dropwise fashion from an alcohol solution. The precursor solution consisted of 1 g of titanium(IV) isopropoxide, 0.15 g of HCl, and 8.0 g of isopropyl alcohol. Approximately 150 lL of this mixture quickly and completely wetted the surface of a 2.5 cm × 2.5 cm planar Pyrex substrate. Next, a circular nanoporous alumina filtration membrane with 18 nm diameter pores was placed on top of the wet glass slide and the system was allowed to dry under ambient conditions for 6 min. As the isopropyl alcohol evaporated, the TiO2 film formed spontaneously on the Pyrex and its presence was directly verified using X-ray photoelectron spectroscopy (XPS). Profilometry measurements showed that the film was approximately 200 nm thick. Atomic force microscopy (AFM) measurements revealed that the coating was very flat with a rootmean-square roughness of 0.72 nm over a 1 lm × 1 lm area (Fig. 2a). This surface roughness was identical to a control experiment where the precursor solution was allowed to evaporate from the substrate in the absence of the alumina membrane. Indeed, no evidence of TiO2 pillar formation caused by the nanoporous template could be detected. Moreover, only trace amounts of TiO2 could be found on the alumina surface using XPS. It should be noted that the alumina template was C O M M U N IC A TI O N S