Template Deposition of Metals

Department of Physical Chemistry, University of Venice, Santa Marta 2137, 30123 Venice, Italy 

Template synthesis is a relatively simple and easy procedure which has made the fabrication of rather sophisticated nanomaterials accessible to almost any laboratory. Template synthesis reqnires access to instmmentation capable of metal sputtering and electrochemical deposition. The characterization of the fabricated nanostructures can be done using instmmental techniques including spectrophotometry, voltanunetry, optical microscopy, atomic force microscopy, and electronic microscopies (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). 

The method is based on the simple but effective idea that the pores of a host material can be used as a template to direct the growth of new materials. Historically, template synthesis was introduced by Possin (1) and refined by WilUams and Giordano (2) who prepared different metallic nanowires with widths as small as 10 nm within the pores of etched nuclear damaged tracks in mica. It was further developed by Martin s group (3-5) and followed by others (6) with the number of examples and applications (7) continually increasing. The nanoporous membranes usually employed as templates are alumina or track-etched polymeric membranes which are widely used as ultrafiltration membranes. Recently, metal nanostmctures have also been obtained using the pores created by self-assembly in block copolymer structures under the influence of electric fields and high temperatures (8,9). 

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Figure 16.2.5 Electrochemical template deposition of metals (A) scheme of the electrochemical cell and (B) sequence of the growth of the template for the preparation of single metal continuous nanowires (sequences 1-4) or segmented nanoparticles (sequences 1, 2, 5, and 6). Detailed steps (1) metal sputtering to provide a conductive layer for the subsequent electrodeposition (2) electrodeposition of the same metal to form the fibers (3) growth of the fibers (4) etching of the template (5) electrodeposition of another metal (6) composite structure after the etching of the foundation metal. Part (B) redrawn with permission from reference (25).

Conditions and materials used for the electrochemical template deposition of metals... 

Douglas and coworkers were the first one that described a bottom-up approach based on S-layers as templates for the formation of perfectly ordered arrays of nanoparticles [128]. The S-layer lattice was used primarily to generate a nanometric lithographic mask for the subsequent deposition of metals. In this approach a thin Ta-W film was deposited... 

Synthetic lipids and peptides have been found to self-assemble into tubules [51,52]. Several groups have used these tubules as templates [17,51,53-56]. Much of this work has been the electroless deposition of metals [51,54]. Electrolessly plated Ni tubules were found to be effective field emission cathode sources [55]. Other materials templated in or on self-assembled lipid tubules include conducting polymer [56] and inorganic oxides [53]. Nanotubules from cellular cytoskeletons have also been used for electroless deposition of metals [57]. 

Deposition of metals may lead to well dispersed metal nanoparticles, as discussed in the previous section, but also to special metal structures. Using a Xi02 nanotube array prepared by anodic oxidation as a template and electrodepositing An onto the template. An nanonets could be prepared. 

Recently, we have successfully used these thermosensitive core-shell microgel particles as templates for the deposition of metal nanoparticles (Ag, Au, Pd, Pt, and Rh) [29, 59, 60], The reduction to metallic nanoparticles in the presence of microgel particles was done at room temperature via the addition of NaBPL and could be followed optically by the color change of the suspensions, as shown in Fig. 3. The immobilization of metal nanoparticles might be due to the strong localization of... 

Nanostructured microporous catalysts or catalyst supports offer intensified catalysis as they provide enhanced surface area accessible to the reactants and products. In nonstructured catalysts, although the surface area may be large, they are often inaccessible as a result of surface fouling and diffusion resistance can slow down the rate of reaction. In a recent development, microporous materials were used as templates for the solution deposition of metals, which were subsequently heat treated to obtain porous metallic structures, where the size of the pores ranged from 10 pm to lOnm. " The relative phase volume of these two regions can be controlled and the overall porosity can be in excess of 50%. Fig. 7 illustrates the size scale of structures ranging from 10 pm to 10 nm. 

In this study we summarize the recent developments in catalyst development in which nano-porous catalytic sites are accessible through a network of arterial micro-pores. These catalysts are obtained through a solution deposition of metals on a micro-porous polymeric template which is subsequently heat-treated to obtain porous metallic structures where the size of the pores ranged from tens of micrometers to tens of nanometers thus eliminating the problems of accessibility and rapid pore fouling and closure. The technique differs fundamentally from the compression-based systems where the porosity is reduced as a result of compaction. It also differs from the well-known wash-coating or chemical vapor deposition techniques. Furthermore, the mechanisms of metal deposition within micro-pores and nano-structure formation are novel. The importance and current fabrication techniques of porous metallic systems can be found in Refs. l... 

Fig. 5.1 Templated electrodeposition of metal oxides. (1) The electrochemical refiUing process starts at the FTO surface and progresses trough the voided DG channels. The deposition process is restricted to areas that are not covered by SU-8, thereby creating a visible design pattern in the electroplated V2O5. (2) Removal of the styrenic template yields the free-standing, mesoporous DG-structured metal oxide network...

Recently, we have successfully used thermosensitive core-shell microgel particles as a template for the deposition of metal nanoparticles (Ag, Au, Pd, Pt, and Rh).25, These microgel particles consist of a PS core onto which a shell... 

Porous silicon is a promising template for the preparation of metal nanostructures by eleetroehem-ical deposition. Because porous siheon is a semiconduetive porous electrode, eleetroehemieal deposition of metals oeeurs not only at the bottom of pores but also on the pore wall and pore openings. Thus, the control of electrochemical deposition within porous siheon has been a challenging issue. Eleetroehemieal deposition on porous siheon is influenced by illumination condihons. Metal deposition on porous siheon is possible by displacement deposihon. Many studies have reported on electrochemical deposition of metal for prachcal appheations. In this chapter, electrodeposition under polarization is firstly reviewed. Secondly, displacement deposition on porous siheon is explained. Finally, the microscopic structure formation by electrodeposition on porous siheon is summarized. 

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