Research

 

We design smart materials that organize themselves

 

The next generation of nanoscale devices need new nanomaterials

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Our limited fossil fuel supplies and environmental concerns motivate the international quest for improved sustainable energy sources. Key energy technologies, including photovoltaics, batteries, pseudocapacitors, fuel cells, and solar fuels all rely upon electrochemistry with reactions occurring at controlled interfaces and multiple species migrating along continuous pathways.  Here devices with sufficient power for practical devices require fast reaction rates and thus rely upon nanostructured materials having ample active interfacial area.  The controlled formation of nanoscale functional materials is thus critical to advance numerous platforms for alternative energy.

 

pmt_animationSmart materials ‘know’ how to self-assemble

Block copolymers are a fascinating class of molecules that can self-assemble into a diverse range of structures.  They can be thought of as chain-like molecules called homopolymers, for example A and B, that have been connected together.  Generally, the enthalpic cost for A-B contacts drives phase separation; however, since the blocks are tethered together, they can only phase separate on the length scale of the chain, usually 1-100 nm. Researchers have brought this structure control to functional materials by including inorganic components. Numerous materials and morphologies have been made starting since the 1990s, so what challenges remain?

 

Making the right stuff

The nanomaterials revolution in the battery research community taught us a lot about architecture-performance relationships. Namely, length scale matters, a lot – a material that seemed useless yesterday can become tomorrow’s high performer, but only if the right nanoscale architecture is used. But what is the “right architecture?” All electrochemical devices convolve multiple concurrent processes so answering this question requires consideration of multiple parameter dimensions. A guess-and-check approach is valid, but it turns out to be very difficult to fabricate tunable series of nanomaterials. A more efficient approach is to make the right series of materials with specific constants that explore nanoscale phenomena along isolated dimensions. This approach allows one to hone in on new architecture-performance relationships for isolated processes such as charge insertion or charge transport. Thus our ability to learn more and eventually produce the right architectures comes down to a materials chemistry challenge: how can we make it?

 

Persistent Micelle Templating (Patent Pending)

tia-schemeThe self-assembly of block copolymers is a powerful tool to realize functional materials with nanoscale features. The resulting porous materials all have a morphology with 2 characteristic dimensions : the inorganic dimension and the pore dimension. However, since the seminal demonstrations these two characteristic parameters have been coupled together where there is limited ability to vary one without the other one also changing. With either of the two formation pathways the use of equilibrating conditions is fundamentally incompatible with decoupled control – i.e. any change to the inorganic also results in changes to the porosity. Breaking free of this constraint requires the use of non-equilibrating conditions that are kinetically trapped.
We invented Persistent Micelle Templating to enable the next generation of energy devices. Here we use block copolymer micelles to template inorganic nanoparticles. You can add as much or as little inorganic as desired without the micelles responding to the changing thermodynamic conditions. This control allows us to realize systematic series of materials where e.g. the pore size and morphology are held constant while the inorganic wall thickness may be continuously varied with remarkable granularity. Such tunable isomorphic architectures uniquely enable the search for the next generations of nanomaterials.

 

Adding another nanoscale dimension with Atomic Layer Deposition (ALD)

ALD is a unique way to build nanoscale devices based upon conformal depositions. With ALD, a series of chemical pulses are used to deposit materials one atomic layer at a time.  For example the vapor of a metal precursor can selectively react with surface hydroxyl groups.  The following pulse is generally a hydrolysis agent that cleaves the remaining ligands and regenerates a surface hydroxyl termination for repeated deposition cycles.  A powerful feature of ALD is that it can deposit into high surface area materials to build functional, layered devices.  Different series of pulses can be used to deposit different metals, for example, doped materials or heterojunctions.  We recently reported the first ALD of a multinary oxide for solar water splitting.  There are endless possibilities to build new nanomaterials with ALD.