Assembling the Nanocatalysts on Substrates

The main challenge that limits the usefulness of nanoparticles in catalysis is their recycling, which is difficult due to their small size. Different solutions 23/06/2014 08:15:00. Published on 12 June 2014 on http://pubs.rsc.org | doi:10.1039/9781782621034-00172

were proposed to overcome the handling problems. One of the suggested solutions is to fabricate the nanoparticles on the surface of the substrate by lithography such as electron beam lithography (EBL)⁸⁴or nanosphere lith-ography (NSL).⁸⁵The lithography technique involves fabrication of a poly-mer template of the nanoparticle on the surface of the substrate. The nanomaterials are deposited inside these polymer templates, which are then dissolved, leaving the nanoparticles on the surface of the substrate. In EBL, the electron beam is used to make these templates on a polymer thin film that coats the surface of the substrate. Although use of this technique led to the fabrication of nanoparticles on the surface of a substrate, the high cost, technical difficulty, limited morphological scope, and the poly-crystallinity of the product limits the scale-up of the technique. NSL involves the monolayer self-assembly of nano and microsphere polymer beads on the surface of a substrate. This produces a prismatic template in between the polymer beads. This technique can only produce prisms but does allow for size control by varying the size of the polymer beads.

Traditional colloidal chemical methods are the most efficient approach for synthesizing nanoparticles. Many shapes, sizes, compositions, and crystal structures have been prepared by these techniques. However, this method presents problems when dealing with the stability and handling.

The following two methods are used to make colloidal nanoparticles more applicable.

(1) Loading the nanoparticles on the surface of micro-size particle sup-ports such as solid state particles or polymer beads. This method can be carried out by two different techniques: in situ reduction of metal ions onto the surface of the support or loading of already synthesized nanoparticles onto the support. Reduction of the metal ions on the support produces different shapes and sizes of nanoparticles. Recently, the solvent-controlled swelling and heterocoagulation technique was introduced^86–88 to load dif-ferent kinds of nanoparticles onto the surface of a polystyrene polymer bead (PS) support. This method involves swelling the PS by an organic solvent such as tetrahydrofuran or chloroform. Pores and channels are formed on the surface of the PS beads due to the increase in size (size expansion is up to 200%). The capping materials of the nanoparticles anchor inside the pores of the PS. After the PS is washed in a non-swelling solvent, it shrinks and the nanoparticles are fixed on the surface of the PS through the capping ma-terials. This technique was used to coat the PS with different nanoparticles such as 30 nm and 80 nm polyvinylpyrrolidone (PVP) capped gold nano-spheres. Figures 9.2(A) and (B) show the SEM images of 10 mm PS coated with 30 and 80 nm gold nanoparticles, respectively. Figures 9.2(E) and (F) are the dark field scattering images for the PS coated with 30 and 80 nm gold nanospheres, respectively. This technique is applicable to other shapes, such as silver nanocubes capped with PVP as seen in Figure 9.2(C) (SEM) and Figure 9.2(G) (dark field). This technique is not only valid for nanoparticles coated with polymers, but also for other cationic capping materials such as trimethyltetradecylammonium bromide (TTAB). Figure 9.2(D) shows the Catalysis with Colloidal Metallic Hollow Nanostructures: Cage Effect 177

23/06/2014 08:15:00. Published on 12 June 2014 on http://pubs.rsc.org | doi:10.1039/9781782621034-00172

TEM image of 20 nm platinum nanocubes and the SEM image of the PS coated with platinum nanocubes capped with TTAB.

(2) The Langmuir–Blodgett (LB) technique is another method for as-sembling colloidal nanoparticles onto substrates. This technique produces a particle monolayer but requires that the nanoparticles be dispersed in a volatile solvent immiscible with the sub-layer liquid filling the LB trough (such as chloroform solvent and a water sub-layer). The nanoparticles in a volatile solvent are sprayed over the liquid sub-layer by micro-syringe. Due to the surface tension of the sub-layer liquid, the nanoparticles will arrange into a monolayer on the surface of the sub-layer as the solvent evaporates (as postulated by Langmuir). The inter-particle separation distance between the nanoparticles can be controlled by varying the available area that the nanoparticles are dispersed on. The LB film can be transferred to a substrate by the dipping method. Figure 9.3(A) shows AuNCs (dispersed in chloro-form) sprayed over the surface of the water sub-layer of the LB trough. The mechanical barrier separates the nanoparticles, which are blue in color (left), from the cleaned water surface (right). The AuNCs are transferred to the surface of a glass slide substrate by dipping the slide into the end of the LB trough and slowly pulling it out. Figure 9.3(B) shows the glass substrate after coating with the AuNCs from the SEM image shown in Figure 9.3(C).

The concentration of nanoparticles on the surface affects the surface tension of the water sub-layer. Thus the coverage density can be measured by a Wilhelmy plate attached to a pressure sensor. However, the relationship between the surface pressure and the value of the sub-layer area that the nanoparticles are distributed (isotherm) over determines how the nano-particles interact with the sub-layer surface and with one another.⁸⁹

Although recycling of the nanocatalyst at the end of the reaction is eco-nomically useful, keeping the activity of the catalyst after assembling it on the surface of the substrate is also an important issue. There is no change in the Figure 9.2 SEM images of nanoparticles coating 10 mm polystyrene beads: (A) 30 nm gold nanospheres, (B) 80 nm gold nanospheres, (C) 60 nm silver nanocubes, (H) platinum nanocubes. (D) TEM image of platinum nano-cubes. Dark field images of 10 mm PS beads covered by metal NPs:

(E) 30 nm AuNPs, (F) 80 nm AuNPs, and (G) 60 nm AgNCs.

23/06/2014 08:15:00. Published on 12 June 2014 on http://pubs.rsc.org | doi:10.1039/9781782621034-00172

activation energy of the catalysis reaction catalyzed by platinum nanocubes in colloids and supported on the surface of a PS bead substrate.⁸⁸However, the rate of the reaction is decreased since one-sixth of the surface of the cube is covered by the PS polymer. Unlike the silica and alumina support, this could affect the activity of the catalyst supported on their surfaces.⁵⁴