Controlling Interconnected Silver Network Structure in Sol–Gel Nanocomposite Via Shrinkage‐Induced Stress

Abstract

Materials synthesized via solution route sol-gel method are suitable for applications that rely on selectivity in diffusion, [1] size exclusion and filtration, [2] and encapsulation [3,4] because of their intrinsic nanoporous structures. These pore structures that occur spontaneously during the gelation reaction are highly interconnected. Many literatures have reported on how pore diameters, pore structures, and pore volumes can be controlled. [5,6] However, using these nanoscale pores as the template to produce secondary interconnected structures has been difficult due to that limited diffusion and the existing defects usually result in uneven distribution. We have reported that by incorporating low molecular weight polyethylene glycol (PEG), silver nanoparticles and silver chloride nanoparticles inside the pores of sol-gel materials remain mobile and they are able to transport to the surface when the sol-gel material is under shrinkage-induced stress. [7] It is known that silica materials synthesized by sol-gel method tend to shrink during drying. Solvent evaporation causes the pore structure of sol-gel materials to collapse and shrink, and such shrinkage often induce stress, resulting in cracks. This is particularly common on films made by sol-gel processes as cracks are often thickness-dependent. [8] Here we demonstrate that the microcracks also occur during the drying of a monolithic sol-gel sample and such microcrack surfaces can be utilized as the template to form interconnected silver structures (Scheme 1a). We also show that the density of silver structure is related to the volume shrinkage rate in the drying process while the density is not related to the final annealing stage where the induced stress and reductive polymer environment facilitate the formation of interconnected silver network without causing the sample to disintegrate (Scheme 1b).The general procedure for creating interconnected silver network in sol-gel nanocomposites includes preparation of wet gels, followed by aging and drying to form xerogels, and then annealing heat-treatment at 160 8C. The drying step was carried out by heat treatment at various temperatures (40-80 8C) to achieve various degrees of drying states prior to the final annealing step at 160 8C. During the annealing treatment, black samples became sliver reflective color, indicating metallic silver layers have been produced. We have previously reported that interconnected silver layers appear on both surface and internal space of sol-gel materials, and their interconnectivity is related to the amount of polymer in the samples. [7] Here, the sample dimension at each stage was shown in order to evaluate the degree of drying for the drying process (Figure 1). All aged gels retained 85% of the original size of wet gel volume and, respectively, shrank to 78, 72, and 61% of the wet gel size when drying for 4 h at 40, 60, and 80 8C. The final size after annealing is %55% of the wet gel and this value is independent of the drying process. Therefore COMMUNICATION [*] Dr.An inter...

Thermal Conductivity of TiO2 Nanoparticles Based Aqueous Nanofluids with an Addition of a Modified Silver Particle

“…Until now, considerable attention has been paid to the synthesis of nanocomposites by incorporating different nanoparticles owing to their exceptional properties. − Recently, “Ag” nanoparticles have been mostly used as a supporting material to synthesize the composite materials , benefiting from its very high thermal and electrical conductivity . “Ag” nanoparticles are also utilized in a wide range of practical applications such as in biosensors, biomedical imaging, Raman scattering, catalysts, as an antibacterial proxy, and in lithography. − However, no particular reports are available on the thermal characteristics of Ag/TiO 2 nanocomposite based nanofluids.…”