Combined Theoretical and Experimental Study on the Adsorption of Methanol on the ZnO(101̅0) Surface

Understanding the Role of Solvent on the Growth of Zinc Oxide: Insight from Experiment and Molecular Dynamics Simulations

The controlled synthesis of nanoparticles with tailored shapes and morphologies has garnered significant attention, driven by the ever-growing demand for advanced materials with defined properties. In nanoparticle formation, various parameters influence the final product, and among these, the solvent plays a pivotal role, as it constitutes the major component of the reaction medium. In this work, the critical role of solvents in controlling the growth of zinc oxide (ZnO) nanoparticles was investigated, with a focus on simple primary alcoholic solvents as the reaction medium. A model reaction based on the direct solvolysis of anhydrous zinc acetylacetonate was employed to probe the influence of different primary alcohols, specifically methanol, ethanol, and their mixture. A substantial difference in the preferential growth direction of the ZnO nanocrystals in methanol and ethanol was observed through XRD and was further proven through TEM. Thereby, in ethanol, a preferential growth in the [001] direction was observed, resulting in short nanorods as primary particles, while this growth was inhibited in methanol, leading to platelet- or sheet-like primary particles. To unravel the underlying mechanisms responsible for the observed solvent-dependent variations, molecular dynamics (MD) simulations were employed using an optimized interface force field to model the ZnO-alcohol interaction. These simulations provide valuable insights into the preferential adsorption of the solvent molecules onto the polar (0001) and (0001̅) and nonpolar (101̅0) ZnO surfaces, shedding light on the fundamental interactions driving the shape control phenomenon. Essentially, the experimental observations on primary particle morphology could be explained well by the adsorption behavior determined by the MD simulations. Furthermore, this report provides an extensive comparison with various similar reaction systems for ZnO synthesis, deriving correlations with the findings from the model system. These insights contribute to a deeper understanding of the intricate interplay between solvent properties and nanoparticle growth, offering a valuable toolkit for designing and optimizing the synthesis of ZnO nanoparticles with specific shapes and functionalities.

Lessons from a Challenging System: Accurate Adsorption Free Energies at the Amino Acid/ZnO Interface

We undertake steps to overcome four challenges that have hindered the understanding of ZnO/biomolecule interfaces at the atomic scale: parametrization of a classical force field, ZnO surface termination and amino acid protonation state in methanol, and convergence of enhanced sampling molecular dynamics simulations. We predict adsorption free energies for histidine, serine, cysteine, and tryptophan in remarkable agreement with experimental measurements obtained via a novel indicator-displacement assay. Adsorption is driven by direct surface/amino-acid interactions mediated by terminal hydroxyl groups and stabilized by strongly structured methanol solvation shells.

Adsorption of sulfur mustard on clean and water-saturated ZnO(101¯0): Structural diversity from first-principles calculations

We investigate the adsorption of a chemical warfare agent, namely sulfur mustard (SM), on clean and water-saturated ZnO(101¯0) surfaces using density functional theory calculations to understand the first step of its efficient neutralization to less toxic chemical compounds. We determine the relative stability of various SM conformers adsorbed at different sites on both ZnO surfaces. The unique hydrogen bonding patterns obtained for the idealized clean and the more realistic water-saturated ZnO surface are analyzed and their influence on the stability of the SM@ZnO structures is demonstrated. We find that absolute values of the calculated binding and interaction energies are significantly higher for the clean than for the water-saturated ZnO surface due to the formation of Cl⋯Zn and S⋯Zn contacts. The high adsorptive reactivity of the clean ZnO surface is also evident from the strong structural changes of the initial local energy minimum gas-phase conformations of the SM molecules upon adsorption. This phenomenon is not observed for the water-saturated ZnO surface, which has almost no impact on the SM conformation after adsorption, leaving it as it exists in the gas phase. The insights from the results obtained provide a missing piece toward the understanding of the complex mechanism of SM neutralization on ZnO surfaces.

Exploring the Interactions of Oxygen with Defective ZnO

Exploring the interactions of oxygen with defective oxide is of importance to understand the microscopic process and performance of ZnO-based oxygen sensors. The interactions of environmental oxygen with vacuum-annealed defective ZnO have been studied by electrical methods, vacuum Fourier transform infrared spectroscopy, and in situ adsorption experiments. It was found that the vacuum-annealed defective ZnO exhibits varied electrical response at different temperatures, which, by vacuum IR investigation, was ascribed to the subtle balance between formation of oxygen vacancies and their interactions with environmental oxygen. Further studies showed that two microscopic steps including surface adsorption and bulk diffusion were dominating the interactions between defective ZnO and environmental oxygen, and the corresponding apparent activation energies were estimated to be 0.093 and 0.67 eV through in situ adsorption experiments. The quite low activation barrier of oxygen adsorption on the defective ZnO was proposed to be responsible for the extreme high sensitivity of ZnO-based oxygen sensors.

Surface chemistry of methanol on different ZnO surfaces studied by vibrational spectroscopy

The adsorption and reactions of CH₃OH on nonpolar mixed-terminated ZnO(101[combining macron]0), polar O-ZnO(0001[combining macron]) and Zn-ZnO(0001) surfaces have been studied systematically using high-resolution electron energy loss spectroscopy (HREELS) in conjunction with temperature programmed desorption (TPD). For all three ZnO surfaces, exposure to methanol at room temperature leads to (partially) dissociative adsorption resulting in the formation of hydroxyl and methoxy species. Upon heating to higher temperatures, the dissociated and intact methanol species on ZnO(101[combining macron]0) predominantly undergo molecular desorption releasing CH₃OH at 370 and 440 K. The Zn-O dimer vacancies are responsible for the decomposition of a small fraction of methanol yielding H₂, CH₂O and CO at 540 and 565 K. The interaction of methanol with polar O-ZnO and Zn-ZnO surfaces is dominated by thermal decomposition of CH₃OH to produce CH₂O, H₂, CO, CO₂ and H₂O at elevated temperatures. The high chemical reactivity of both polar surfaces is related to the high abundance of different types of surface defects formed via massive restructuring. Importantly, the reconstructed Zn-ZnO surface exhibits high selectivity for hydrogen production at 520 K, which was not observed for the polar O-ZnO surface. The HREELS data revealed that this low-temperature hydrogen evolution on Zn-ZnO results from methoxy oxidation to a formate species occurring at O-terminated step-edge sites.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Cu/ZnO nanocatalysts in response to environmental conditions: surface morphology, electronic structure, redox state and CO2 activation

Methanol synthesis is one of the landmarks of heterogeneous catalysis due to the great industrial significance of methanol as a clean liquid fuel and as a raw material for industry.

On the stabilization mechanisms of organic functional groups on ZnO surfaces

Density functional theory (DFT) calculations have been employed to investigate the interaction between ZnO-(101[combining macron]0) and (12[combining macron]10) surfaces and organic functional groups. We analyze the influence of the surface coverage on the geometries and binding energies under a dry environment. Our calculations show that coverages θ = 1 ML are favored under ligand-rich conditions and a dry environment. However, based on thermodynamic considerations we suggest that these ligands may not be stable against adsorption of liquid water and water vapor.