Water vapor pressure affects nanocatalyst shape

Nanoparticles of zinc oxide (ZnO) are important catalysts, with applications such as in gas sensing and solar cells. The shape of a nanocatalyst affects its reactivity. To improve the functionality of ZnO nanocatalysts, scientists must understand the surface chemistry of its exposed crystal facets in different environmental conditions.

Previous work relied on surface energy diagrams for evaluating the thermodynamic stability of water adsorbate phases on ZnO. However, this neglected certain effects which are important, but much more difficult to evaluate: the zero-point vibrational energy, thermal excitation of phonons, and entropies of adsorbate phases. In The Journal of Chemical Physics, scientists showed that these effects are very important for making correct predictions concerning the stability of ZnO facets in the presence of water, and the phase of the surface water itself.

Co-author Stephane Kenmoe said they discovered that water vapor pressure can be used to manipulate the shape of ZnO nanoparticles to optimize them for particular applications.

The authors calculated adsorbate phase diagrams of ZnO surfaces in equilibrium with water vapor, finding that phonon contributions and entropies have important effects on the stability range of water adsorbate phases. In the case of phonon contributions, zero-point vibrational energy and increasing thermal excitations enlarge the Gibbs free energy of the surface. Meanwhile, the configurational entropy of adsorbate phases reduces the surface Gibbs free energy as temperature increases.

The configurational entropy contribution is small relative to the contributions from phonons and translations, but it becomes significant at higher temperatures and may determine the ultimate thermodynamic stability of a certain adsorbate phase. The researchers plan to use this same technique to study additional oxide systems.

Source: “Water adsorbate phases on ZnO and impact of vapor pressure on the equilibrium shape of nanoparticles,” by Stephane Kenmoe and P. Ulrich Biedermann, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5016122 .