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45283-AC10
Porous and Biphasic Materials through Solid State Reactions

Ram Seshadri, University of California (Santa Barbara)

We are investigating routes to porous and functional inorganic materials and bifunctional composites via solid-solid and solid-vapor reactions. By utilizing spontaneous, template-free processes, we are developing versatile, rapid routes which are broadly applicable, as evidenced by the wide variety of materials systems already investigated. These materials may be utilized in many fields, including filtration, catalysis, magnetic data storage, charge trapping, etc.

Reactive dip coating

Utilizing local interdiffusion of transition metal species on the surface of porous oxides has permitted the formation of nanoscale ternary oxide precipitates on a polycrystalline matrix. After dipping porous NiO in a solution of lanthanum acetate, firing at moderate temperatures results in a coating of La₂O₃ particles on the pore walls. Subsequent firing at higher temperatures fully converts the sub-micron coating particles into La₄Ni₃O₁₀. This diffusion-driven process leads to precipitates which are much stabler and well-adhering than conventional nanoparticles deposited on inert supports for catalytic applications.

Vapor-phase leaching

In the quest to produce porous materials via volume loss, pore-opening must occur at temperatures where nano-sized pores will remain open. Heating oxides in a flowing stream of 5% H₂/N₂ provides a reducing environment that allows conversion between transition metal oxides and metals. Reduction of Mn₃O₄ to MnO occurs at about 450°C, and is accompanied by significant volume loss. This volume loss is manifested in an open network of 20-100 nm pores. Because the oxygen sublattice is conserved during the process, extensive reconstruction of the grains does not occur. The crystalline orientation is therefore maintained, producing pores with aligned, square edges over the extent of each grain.

After reduction of macroporous Mn₃O₄ to MnO, we have closed the induced micropore network by reoxidation to Mn₂O₃. The macropore network is maintained, and subsequent reduction of Mn₂O₃ back to MnO reopens the micropores. This is a truly novel display of morphological regeneration in an oxide.

By applying hydrogen reduction to mixed-metal ternary oxides, we have selectively reduced individual components. Solid-state reduction of the inverse spinel Zn₂TiO₄ at 800°C leads to evaporation of Zn from the structure while the TiO₂ is retained:

The intermediate microstructure (Fig. 1a) consists of highly crystalline rutile TiO₂ dendrites on the surface of the partially reduced spinel grains, and reduction proceeds until a network of nanosized TiO₂ nanorods remains (Fig. 1b).

When both components remain in the structure, as in the case of NiMn₂O₄, we demonstrate the facile conversion of a single-phase material into an intimately mixed, highly-interfacial composite. At 725°C, the the reduction reaction proceeds in two distinct steps as:

The resulting composite has 25 nm Ni nanoparticles both on the surfaces and completely encased inside the porous MnO matrix. We observe magnetic exchange bias due to intimate contact between ferromagnetic Ni and the antiferromagnetic MnO. Because the redox process allows egress and ingress of metal nanoparticles, we can easily produce catalytic metals supported on oxides.

Thin film applications

The precision and ordering of the porosity in vapor-phase leached materials is especially apparent in our single-crystalline MnO thin films. ZnMn₂O₄ was deposited hydrothermally on (100) and (111) MgAl₂O₄ substrates. Reduction of these epitaxial films in 5% H₂/N₂ at 450-650°C removes Zn and O, resulting in MnO films shown in Fig. 2a with an open network of micropores. On (100) substrates, all pore faces are aligned exactly parallel or perpendicular to the substrate. On (111), the pore edges are rotated by 54.75° with respect to the substrate (Fig. 2b). Pore sizes are 50-100 nm on (100), and 30-50 nm on (111). The epitaxial agreement between MgAl₂O₄ and ZnMn₂O₄ allows for tailoring of the pore orientation and size simply by choosing the plane of growth. Because the reduction occurs at temperatures much less than the melting point of MnO (1840°C), pore channels remain fully open during firing, and the microstructure does not depend on film thickness.

Personal Impact

Since receiving support from the PRF, Ram Seshadri has received tenure in the UCSB Materials Department. Eric S. Toberer has earned his PhD in Materials. He is currently in a postdoctoral position at Caltech and was awarded a Beckman Fellowship. Graduate student Daniel P. Shoemaker has been researching functional biphasic materials since Fall 2006.

Fig. 1. (a) Intermediate reduction in 5% H₂/N₂ of Zn₂TiO₄ results in TiO₂ dendrites supported on an unreduced core. (b) Complete reduction leads to arrays of TiO₂ nanorods.

Fig. 2. Epitaxial ZnMn₂O₄ films on MgAl₂O₄ are reduced to MnO, resulting in open, aligned porosity while preserving crystallinity. Films grown on (a) the (100) plane and (b) (111) plane of the substrate display different pore sizes and orientations with respect to the growth plane.

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