Enzymatic Synthesis and Nanostructural Control of Gallium Oxide at Low Temperature

Abstract

The molecular mechanisms underlying the biological synthesis of nanostructured mineral/organic composites have long been recognized to offer exciting prospects for materials science.[1±8] In addition to their benign conditions for synthesis (including neutral pH, low temperature, low pressure, and the absence of caustic chemicals), these mechanisms often reveal a precision of nanostructural control not yet achievable in anthropogenic syntheses. Investigations into such mechanisms have shown that protein filaments occluded within the silica skeletal elements of a marine sponge consist of structure-directing enzymes capable of catalyzing, in vitro, the hydrolysis and polycondensation of molecular precursors of silica, silsesquioxanes, [5,9±11] and titania. [12] We show here that these protein filaments are not only capable of the hydrolysis and polycondensation of a gallium oxide molecular precursor to yield (depending on the reaction conditions) either gallium oxo-hydroxide (GaOOH) or spinel gallium oxide (c-Ga 2 O 3 , a gassensing semiconductor) at room temperature, but also to direct their resulting structures. This control is seen in the defined orientation of nanocrystals with respect to the surface of the protein, suggesting that structural determinants on the surface of the protein catalyze the formation of the c-Ga 2 O 3 polymorph at low temperatures and may direct its crystallographic orientation. These results demonstrate the feasibility of a low-temperature catalytic route to the synthesis and COMMUNICATIONS 314

Biomimetic and Aggregation-Driven Crystallization Route for Room-Temperature Material Synthesis: Growth of β-Ga2O3 Nanoparticles on Peptide Assemblies as Nanoreactors

“…This specific interaction that stabilizes and controls the kinetics of the intermediate phases during the synthesis leads to yield in unique crystal structures. 15 Although enzymes in nature can mineralize metals and semiconductors whose growths are hardly achieved at room temperature, those enzymes are not exactly designed to grow them in uniform size and shape. For example, silicatein moieties, the silica skeletal elements of a marine sponge, were applied to catalyze the hydrolysis and the polycondensation of γ-Ga 2 O 3 .15 In this biomimetic approach, the hydrolytic enzyme motif from the silicateins, consisting of nucleophilic hydroxyl of serine and amine of histidine, catalyzed the hydrolysis of the gallium precursors and resulted in both GaOOH and γ-Ga 2 O 3 nanocrystal formation with a wide size range.…”

Enzyme-mediated synthesis of ceramic materials

“…In addition, silicatein was found to be active in the synthesis of TiO 2 , 50)52) ZrO 2 , 51) Ga 2 O 3 , 53) or BaTiOF 4 , 54) which is of significance because site-selective precipitation of SiO 2 or TiO 2 has been difficult by conventional techniques. An example of silicateinmediated TiO 2 is shown in Fig.…”

The Biomimetic Mineralization Closer to a Real Biomineralization

“…However, Morse et al believe that protein filaments that catalyzed and templated synthesis of nanocrystalline TiO 2 (Sumerel, et al 2003) and Ga 2 O 3 (Kisailus, et al 2005) will incorporate the carbon impurities and degrade the performance of the materials for device applications that require high purity materials. In order to prevent the carbon impurities originating from the use of organic template, at the same time to capture the advantage of the slow catalysis and anisotropic, vectorial control of biocatalytic crystal growth, they developed a low-temperature, solution-based method employing the slow diffusion of ammonia vapor as a catalyst or hydrolysis of metal-containing molecular precursors.…”

“…The interface between chemically distinct selfassembled monolayer domains provided the necessary juxtaposition of nucleophilic (hydroxyl) and hydrogen-bonding (imidazole) agents to catalyze the hydrolysis of a gallium oxide precursor and template the condensed product to form GaOOH and the defect spinel, -Ga 2 O 3 . These methods use the similar principles borrowed from biomimetic synthesis www.intechopen.com routes (Sumerel, et al, 2003;Kisailus, et al, 2005;Schwenzer, et al, 2006): 1) slow catalysis of synthesis from molecular precursors provides the opportunity for kinetic control; and 2) crystal growth is vectorially regulated by a template, operating in concert with kinetic control to provide spatial and temporal control of crystal polymorph, orientation and morphology. And the results indicate that the kinetic control can provide an opportunity to get the materials with special structural features.…”

“…So it is meaningful for both understanding biomineralization phenomena and instructing biomimetic synthesis of inorganic functional materials. Nanocrystalline TiO 2 (Sumerel, et al, 2003) and Ga 2 O 3 (Kisailus, et al, 2005) were fabricated using a biomimetic synthesis method where silicatein, a catalytically active, structuredirecting enzyme (Morse, 1999), was used as a catalyst and template for the hydrolysis and subsequent polycondensation of water stable molecular complexes of titanium and gallium to form nanocrystalline TiO 2 (Sumerel, et al, 2003) and Ga 2 O 3 (Kisailus, et al, 2005), respectively. Ferroelectric BaTiO 3 nanocrystals were also synthesized by using an assembly of bola-amphiphile peptide, a linear type of peptide monomer with two amide head groups connected by a hydrocarbon tail group (Nuraje, et al, 2006).…”