Scope, Mechanism, and Applications of High Temperature Entrenchment of Metal Nanoparticles on Amorphous Silicon Oxide Supports

Gosav, Abha Anand

doi:https://doi.org/10.21985/n2-2k5p-6x79

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Scope, Mechanism, and Applications of High Temperature Entrenchment of Metal Nanoparticles on Amorphous Silicon Oxide Supports

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Metal nanoparticles supported on oxides are versatile systems. Ordered arrays of multimetallic nanoparticles of different sizes and surface densities can be synthesized using block copolymer-mediated nanolithography techniques. Metal nanoparticles on planar supports like silica can be utilized for catalyst discovery. Under reaction conditions and at high temperatures, the changing surface energy equilibria at the metal-oxide interface causes a rearrangement of the metal nanoparticles and/or the supporting oxide. This often leads to catalyst deactivation by sintering where metal nanoparticle may migrate along the surface and coalesce or atoms from the nanoparticles migrate to other nanoparticles to form larger and fewer nanoparticles. Partially embedding uniform arrays of nanoparticles within the support can prevent their sintering. When some metal nanoparticles, supported on amorphous silica, are heated to temperatures above 1000 ºC under inert conditions, nanopores are formed in the silica surface. This behavior is observed for gold, silver, copper, palladium, and platinum nanoparticles but not cobalt or nickel nanoparticles. Furthermore, nanopore formation is unique to planar, amorphous silica and does not occur on planar quartz, hafnia, titania, and alumina as well as on nanoscale spherical silica. The mechanism that governs this behavior relies heavily on the diffusion of metal ions into the supporting silica at elevated temperatures. The dissolution of metal ions in the amorphous silica matrix lowers its glass transition temperature (Tg). At temperatures above the Tg (~1000 ºC), the supporting silica exhibits viscoelastic behavior and allows the nanoparticles. Crystalline oxide supports that do not undergo glass transition (like quartz and alumina) prevent nanopore formation. Moreover, metals with very low diffusivities in amorphous silica (like nickel and cobalt) do not form nanopores at 1040 ºC while silver, that has a very high diffusivity in silica forms nanopores readily even at 800 ºC. The shape of the supporting silica also plays a crucial role in nanopore formation. Spherical geometries have high internal stresses that hamper diffusive processes at the surface. On such supports, since metal ion diffusion into the support is significantly lowered, no nanopore formation is observed at high temperatures. The selective nature of nanopore formation is applied to design and fabricate modified tri-layer supports consisting of silica on alumina on silicon wafers. The thin layers of silica allow nanopore formation while the alumina layer acts as a barrier to this behavior. On these tri-layer supports, the depth to which nanoparticles ( of sizes 10-15 nm) will entrench can be controlled between 3-5 nm. The partially embedded nanoparticles are location stabilized and could have potential applications as sinter-resistant catalysts. Nanopore formation by thermal entrenchment of metal nanoparticles occurs above a specific minimum temperature which depends on the composition of metal nanoparticles. While Au and Pd nanoparticles form nanopores above 1000 ºC, Ag nanoparticles entrench above 800 ºC. Additionally, the ramp rates employed to attain these high temperatures also control nanopore formation. Lower ramp rates (<1 ºC/s) cause partial embedding and nanoparticle encapsulation whereas higher ramp rates (>1 ºC/s) lead to formation of clear nanopores. Thermal entrenchment competes with other surface phenomena, including nanoparticle agglomeration, diffusion, and encapsulation, which also occur at high temperatures. The dominant behavior, amongst these many competing phenomena arising at the metal-oxide interface, depends on the initial surface distribution of the nanoparticles. While well-spaced, large nanoparticles will each form a nanopores of comparable diameters, densely packed, small nanoparticles will agglomerate to form larger nanoparticles before entrenchment. There are also some distributions, where nanoparticle mobility is not favored, for which encapsulation of nanoparticles within the silica support is observed. Nanopore formation in silica supports is accompanied by the formation of oxide ridges around the nanopores. These are caused by oxide transport along the metal-oxide interface as the system attains to equilibrate at the high temperatures. The height of these oxide ridges depends on the nanoparticle composition as well as on the size. Silver nanoparticles form the tallest oxide ridges when compared to those created by gold and palladium nanoparticles of similar sizes. Moreover, highly mobile nanoparticles that aggregate before nanopore formation create very short oxide ridges as the metal-oxide interface is not stationary. Thus, nanopore with tunable diameters and oxide ridges can be generated by controlling the size and surface density of metal nanoparticles. These nanopores may be potentially used in separations and sensing applications.

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