Interaction of Zinc Oxide Nanoparticles with Water: Implications for Catalytic Activity

Abstract

Novel technological applications in catalysis and bactericidal formulation have emerged for zinc oxide (ZnO) nanoparticles owing to their ability to generate reactive oxygen species by fostering H2O dissociation. Rational improvement of those properties requires a mechanistic understanding of ZnO nanoparticle reactivity, which is currently lacking. Here, we determine the structural and electronic properties of nanometer-sized ZnO, determine the binding energetics of H2O adsorption, and compare to an extended macroscopic surface. We show that the electronic density of states of ZnO nanoparticles is size-dependent, exhibiting a decreasing bandgap with the increase of nanoparticle diameter. The electronic states near the Fermi energy dominantly arise from O 2p states, which are spatially localized on “reactive” surface O atoms on the nanoparticle edges that are doubly coordinated. The frontier electronic states localized at the low coordinated atoms induce a spontaneous dissociation of H2O at the nanoparticle edges. The surface Zn and O atoms have inhomogeneous electronic and geometrical/topological properties, thus providing nonequivalent sites for dissociative and molecular H2O adsorption. The free energy of H2O binding is dominated by the electronic DFT interaction energy, which is site-dependent and correlated with the Bader charge of surface Zn atom. Entropy is found to stabilize the bound form, because the increase in the vibrational contribution is greater than the decrease in the translational and rotational contribution, whereas solvation stabilizes the unbound state. The absence of rough edges on an extended, macroscopic ZnO surface prevents spontaneous dissociation of a single H2O. This study underlies the importance of coupling geometrical and electronic degrees of freedom in determining the reactivity of nanoparticles and provides a simple elucidation of the superior catalytic activity of ZnO nanoparticles compared to ZnO in macroscopic forms.

SDS-PAGE for Monitoring the Dissolution of Zinc Oxide Bactericidal Nanoparticles (Zinkicide) in Aqueous Solutions

“…Agriculture-grade Zinkicide (4.5% metallic Zn) was obtained from our collaborator TradeMark Nitrogen Inc. (TMN, Tampa, FL). 27 The material was studied as received without further purification. Figure S1 shows the representative HRTEM image of Zinkicide.…”

“…Lastly, small-angle neuron scattering (SANS) was found to be suitable to measure the particle size in aqueous suspension but requires extensive preparation and suffers from limited access to the sophisticated characterization machinery. 27 However, this technique is time-consuming and determining the size of the NPs is arduous in materials prepared with highly saturated solutions. Because of the high concentration needed for SANS characterization, mimicking the application rate for in-field concentration conditions and analyzing dissolution of the particles will be challenging.…”

“…Because of increased dissolution rates of ultrasmall ZnO NPs in solution, the use of synchrotron X-ray absorption/fluorescence measurements is not suitable for NPs dispersed in solution. Lastly, small-angle neuron scattering (SANS) was found to be suitable to measure the particle size in aqueous suspension but requires extensive preparation and suffers from limited access to the sophisticated characterization machinery . However, this technique is time-consuming and determining the size of the NPs is arduous in materials prepared with highly saturated solutions.…”

Pushing Nanomaterials past the Kilogram Scale—a Targeted Approach Integrating Scalable Microreactors, Machine Learning and High-Throughput Analysis

“…The antibacterial effects of ZnO stem from a collection of physical and chemical interactions with E. coli. ZnO surface defects catalyze the production of radical oxygenated species (ROS) and H2O2 that damage the cellular envelope and components, solubilized Zn 2+ enters cells and disrupts internal processes, and ZnO nanostructures electrostatically interact with cell membranes, causing them to rupture 40,58 . In the antibacterial test used, diffusivity across the agar surface is also an important factor.…”

An Atomistic Molecular Dynamics Study of Titanium Dioxide Adhesion to Lipid Bilayers

“…Intermetallic and metal/metal oxide nanoparticles (NPs) have unique physicochemical and electronic properties that find widespread applications for designing new classes of materials with unique catalytic, energetic, and optical properties. − These properties arise from the dramatic increase in surface area per volume as the size of the particles decreases, leading to surface atoms dominating the NP properties and electron confinement. − The promising properties of NPs have increasingly propelled their use in cosmetics, pigments/paints, pharmaceuticals, and drug delivery applications; however, the biological risks associated with many synthetic NPs have not been comprehensively evaluated…”