Nanotechnology for Sustainable Water and Energy

Objective

We synthesize and characterize catalytic nanoparticles and nanoparticle-enhanced membranes for more efficient water treatment and alternative energy technologies. We systematically alter the nano- and microscale structure of our materials (at least one dimension equal to or less than 100 nm) to change particular properties thought to affect material performance. We then use a suite of characterization tools to measure specific material properties. These measurements enable correlation between nanoscale structure, material properties and material performance, reducing costly and slow trial-and-error processes. We also help to develop new measurement tools. Our larger goal is to enable industry to design and develop sustainable technology based on structure-property-performance relationships that allow improved material performance, lifetime and cost.  

• Nanostructured materials can make water processing and energy production more sustainable, but are often inadequately characterized and poorly understood. As a result, the process of improving material design and performance is limited to trial-and-error.
• The Environmental Protection Agency estimates up to $450 billion needs to be invested in wastewater infrastructure and up to $475 billion in drinking water infrastructure, including new and upgraded treatment plants with advanced technology (including membranes) that can treat alternative water sources, including salt, reused and polluted water. 
• The Department of Energy estimates gross U.S. revenues from the automotive fuel cell industry alone are predicted to reach upwards of $80 billion a year by 2030, with the addition of more than 900,000 new jobs. Methane-based fuel cells could eliminate U.S. dependence on oil and coal. 
• Nanoparticles and membranes are already used in commercial water treatment and energy technologies and undergo continual development; design based on structure-property-performance relationships will accelerate technology from the research and development stage to commercial products.
• Customers and stakeholders include 3M Fuel Cell Components Program, Proton OnSite, Hydration Technology Innovations LLC, Trussell Technologies, Inc., multiple water utilities, Bureau of Reclamation, National Center for Atmospheric Research, Colorado School of Mines, Arizona State University and University of Connecticut.

Approach

Our nanoparticle synthesis focuses on wet chemistry methods, with a particular emphasis on aqueous-based synthesis techniques and metallic nanoparticles. Iron and other metal core-shell nanoparticles are synthesized with a shell of either a native oxide or an additional metal, such as nickel or palladium. Nanoparticle-enhanced polymer composite membranes are synthesized through phase inversion, where the particles are synthesized either ex situ and dispersed in the membrane casting solution or in situ within the casting solution or the cast membrane. Metallic nanoparticles are known to react with and degrade a wide variety of water contaminants and are also used as catalysts for energy applications such as fuel oxidation, energy storage, energy conversion and electrolysis. Iron is an increasing interest as industry seeks low-cost alternatives to catalysts such as platinum or palladium. In most commercial technologies, nanoparticles are immobilized on or within a support structure to control nanoparticle location, reactivity, life time and bulk material properties. As a result, we characterize nanoparticles alone and within a composite structure to understand (1) how synthesis parameters affect nanoparticle reactivity, and (2) how nanoparticles embedded in a composite structure affect nanoparticle and bulk material properties and performance. Properties including size, shape, surface functionalization and chemical composition are thought to affect nanoparticle reactivity. When embedded in a membrane or other support structure, these properties will also affect the resulting composite material. To connect material properties to performance, characterization focuses on organic-inorganic interfaces, chemical composition, material reliability, material lifetime and performance, and phenomena occurring at the micro- and nanoscale.

Publications

LF Greenlee, JD Torrey, RL Amaro, JM Shaw. Oxidation of stabilized zero valent iron nanoparticles, Environmental Science & Technology 46(23), 12913-12920 (2012).  

N Goldstein, LF Greenlee, Influence of synthesis parameters on iron nanoparticle size and zeta potential, J. Nanoparticle Research 14, 760-774 (2012).  

LF Greenlee, SA Hooker, Development of stabilized zero valent iron nanoparticles, Desalination and Water Treatment 37, 114-121 (2012).

Nanostructured materials offer unprecedented opportunity for sustainable water and energy production but are often inadequately characterized and, therefore, poorly understood. If, however, materials are well-characterized, material properties can be correlated to material performance, and future design can be based on these property-performance relationships. To determine these relationships, we change particular properties, including size, shape, crystallinity, chemical composition, surface structure, and interface structure, that affect material performance (e.g., nanoparticle reactivity or membrane filtration performance). Thus far, the field of nanoparticle synthesis has made few attempts to correlate stabilizer properties to the resulting properties of the nanoparticles produced by wet chemistry. Therefore, our current focus within nanoparticle synthesis is to correlate specific stabilizer properties, such as molecular size, chelation strength and the presence of specific functional groups (i.e., carboxylate versus phosphate or sulfate groups), with nanoparticle size and chemical and colloidal stability in solution. We are able to synthesize iron metal nanoparticles varying in size from 2 nm to 150 nm (Figure 1) with the use of specific organophosphate molecules as stabilizers in an aqueous synthesis technique. Preliminary results indicate that both the molecular size and the chelation strength of the stabilizer play important roles in the material properties and reactivity of the metallic nanoparticle.

Figure 1. Organic stabilizers, such as ATMP, are used to synthesize nanoparticles of a particular size, modify nanoparticle surface charge, and enable pH-controlled reversible agglomeration.

We have also synthesized iron-nickel core-shell nanoparticles using our wet chemistry technique and are further developing the method to apply to other metal shells that have importance for both environmental and alternative energy applications (e.g., palladium, platinum, gold and cobalt). We are particularly interested in characterizing the resulting core-shell structure and understanding the roles of the metallic shell and surface stabilization on nanoparticle reactivity and stability. While iron metal nanoparticles are attractive for many catalyst applications due to their high reactivity, Quartz crystal microbalance results have shown that the presence of a nickel shell on an iron core nanoparticle significantly reduces the extent of oxidation of the core nanoparticle (Figure 2), which suggests that core-shell nanoparticles can be designed to have extended lifetimes within an aqueous environment. This work includes the use of both dry, high-vacuum electron microscopy and novel nicroscopy techniques (e.g., wet cell microscopy and transmission scanning electron microscopy) to characterize nanoparticles.

Figure 2. Quartz crystal microbalance is used to quantify nanoparticle oxidation reactivity over time in oxygenated water, as a function of the molar ratio of nickel to iron, where iron is the nanoparticle core, and nickel is the nanoparticle shell.

Nanoparticle reactivity and particle-organic molecule interfaces are also critical in the material design and performance of composite membranes. The location of the nanoparticles in the membrane is a function of multiple parameters, including membrane polymer, the use of membrane pore formers (i.e., polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG)), and the nanoparticle properties (e.g., size, concentration, stabilizer molecule). While membrane pore formers are often used to change the porosity of a polymer membrane, our results show these compounds affect nanoparticle dispersions and the location of the nanoparticle at the particle-polymer interface (Figure 3). Preliminary results also indicate nanoparticles may be used to tune membrane structure and filtration performance.

Figure 3. Scanning electron micrographs reveal the internal structure of cast nanoparticle-polymer composite membranes (left image) and indicate the role of casting solution composition in the resultant nanoparticle location and polymer-particle interface (middle image, without PVP; right image, with PVP).