ORNL Researchers Study Impact of Water Monolayers on Nanomaterial Properties

Researchers from the Oak Ridge National Laboratory (ORNL) of the US Department of Energy are studying how the characteristics of water molecules on metal oxide surfaces can be used for controlling these minerals and using them for manufacturing products such as sound vehicle glass in frost and fog, highly efficient semiconductors for solar cells and organic light emitting diodes, and eco-friendly chemical sensors for industrial applications.

Water behavior at a mineral’s surface is based on the ordered atomic array in that region known as the interfacial region. However, when mineral particles or particles of crystalline solids are nanometer-sized, interfacial water can change the particles’ crystalline structure, manipulate interactions between particles resulting in their aggregation or encapsulate the particles strongly allowing them to remain for long periods in the environment. Since water is present in abundance in the atmosphere, it is normally observed on nanoparticle surfaces that are exposed to air.

Developing techniques to observe the interfacial region closely and understanding how that governs nanoparticle properties, is a huge scientific challenge. Using the key strengths of ORNL, computational and neutron sciences, the researchers have attempted to understand the impact of very few monolayers of water on material behavior.

In several papers published in the Journal of Physical Chemistry C and the Journal of the American Chemical Society, the research team analyzed cassiterite (SnO₂), which is a tin oxide), symbolizing a large group of isostructural oxides, including rutile (TiO₂).

These minerals are normally found in nature and their surfaces are made wet with water. Water behavior confined on metal oxide surface relates readily to applications in areas such as protein folding, heterogenous catalysis, environmental remediation, light-energy conversion in solar cells and mineral growth and dissolution.

According to a research scientist presently at the ORNL–University of Tennessee Joint Institute for Neutron Sciences, Hsiu-Wen Wang, metal oxide nanoparticles, when they are produced, adsorb water spontaneously from the atmosphere linking it to their surface. He conducted this research while doing a postdoctoral fellowship in the Chemical Sciences Division (CSD) at ORNL.

This water can hinder the role of SnO₂-containing products in strange ways, which are difficult to foresee. In order to comprehend the role that bound water plays in SnO₂ nanoparticle stability and to understand about the structure and dynamics of bound water, neutron scattering was used by Wang’s team at ORNL’s Spallation Neutron Source (SNS).

Wang stated that neutrons are ideal for studying light elements such as oxygen and hydrogen, the elements in water, and molecular dynamics simulations are a perfect tool to support the observations. Hydrogen is invisible to electron beams and X-ray but strongly scatters neutrons making ineleastic scattering and neutron diffraction suitable tools for studying water properties and properties of other hydrogen-bearing species.

When we drive all the water off the surface of the nanoparticles, this destabilizes the structure of the nanoparticles, and they grow larger. The lifetime of engineered nanoparticles in the environment is an important environmental safety and health issue. We show that water sorbed on the nanoparticles, which naturally happens when they are exposed to normal humid air, prolongs their lifetimes as nanomaterials, thus prolonging their potential environmental impacts. In addition, the high surface area of nanoparticles is desirable. If the particles grow, which happens as they are heated and dehumidified, their surface area drops rapidly.

David J. Wesolowski, a co-author

The nanoparticles are subjected to heat under vacuum to remove sorbed water. Water dissipation starts at around 250°C (which is 500°F). In order to totally remove water from the nanoparticles, considerable energy is needed. The nanoparticles remain stable even at these high temperatures due to the presence of bound water. Destabilization starts once water begins dissipating. Before study completion, researchers were not aware as to what degree water removal would cause destabilization.

It may be that the surfaces without water have different and useful chemical properties, but because water is everywhere in the environment, it is very important to know that the surfaces of oxide nanoparticles are likely to be already covered with a few molecular layers of water.

David J. Wesolowski, a co-author

Water structure on the surface of cassiterite nanoparticles and the structure of the particles themselves was determined using SNS’s Nanoscale-Ordered Materials Diffractometer (NOMAD) instrument. NOMAD is committed to local structure studies of a number of materials from liquids to nanoparticles using the neutron scattering pattern produced at the time of experiments, said Mikhail Feygenson, NOMAD instrument scientist.

“The combination of the high neutron flux of SNS and the wide detector coverage of NOMAD enables rapid data collection on very small samples, like our nanoparticles,” Feygenson said. “NOMAD is much faster than similar instruments around the world. In fact, the measurements of our samples that took about 24 hours of NOMAD time could have required as much as a full week on a similar instrument at another lab.”

The next stage of the study was done at SNS on the Fine-Resolution Fermi Chopper Spectrometer (SEQUOIA), which enables forefront research on dynamic processes in materials. “This part of the study focuses on the role of surface hydrogen bonds and the surface water vibrational properties,” said Alexander Kolesnikov, SEQUOIA instrument scientist.

The NOMAD and SEQUOIA studies caused the research team to confirm created computational models to completely capture the structural ordering of the surface-bound water on the SnO2 nanocrystals. Combining neutron scattering experiments with classical and first principles, molecular dynamics simulations offered evidence that strong hydrogen bonds, as strong as in water under ultrahigh pressure of more than 500,000atm, drive water molecules to disintegrate at the interfaces and cause a feeble interaction of hydrated SnO2 surface with further water layers.

“The results are significant in demonstrating many new features of surface-confined water that can provide general guidance into tuning of surface hydrophilic interactions at the molecular level,” said Jorge Sofo, professor of physics at Pennsylvania State University.

The US Department of Energy’s Office of Science supported the research. The research team for this project also included Rick Paul of the National Institute of Standards and Technology; Mark J. DelloStritto, Nitin Kumar, and James D. Kubicki of Pennsylvania State University; and Thomas Proffen, Paul R.C. Kent, Lukas Vlcek, Wei Wang, Lawrence Allard, Alexander Kolesnikov, and Lawrence Anovitz from ORNL. The Spallation Neutron Source is a DOE Office of Science User Facility.

References

• Water, Water, Everywhere — Controlling the Properties of Nanomaterials