From Anhydrous Zinc Oxide Nanoparticle Powders to Aqueous Colloids: Impact of Water Condensation and Organic Salt Adsorption on Free Exciton Emission

Abstract

Variations in the composition and structure of ZnO nanoparticle interfaces have a key influence on the materials’ optoelectronic properties and are responsible for high number of discrepant results reported for ZnO-based nanomaterials. Here, we conduct a systematic study of the room-temperature photoluminescence of anhydrous ZnO nanocrystals, as synthesized in the gas phase and processed in water-free atmosphere, and of their colloidal derivatives in aqueous dispersions with varying amounts of organic salt admixtures. A free exciton band at hv = 3.3 eV is essentially absent in the anhydrous ZnO nanocrystal powders measured in vacuum or in oxygen atmosphere. Surface hydration of the nanoparticles during colloid formation leads to the emergence of the free exciton band at hv = 3.3 eV and induces a small but significant release in lattice strain as detected by X-ray diffraction. Most importantly, admixture of acetate or citrate ions to the aqueous colloidal dispersions not only allows for the control of the ζ-potential but also affects the intensity of the free exciton emission in a correlated manner. The buildup of negative charge at the solid—liquid interface, as produced by citrate adsorption, increases the free exciton emission. This effect is attributed to the suppression of electron trapping in the near-surface region, which counteracts nonradiative exciton recombination. Using well-defined ZnO nanoparticles as model systems for interface chemistry studies, our findings highlight water-induced key effects that depend on the composition of the aqueous solution shell around the semiconducting metal oxide nanoparticles.

Green Synthesis of Zinc Oxide Nanoparticles: Fortification for Rice Grain Yield and Nutrients Uptake Enhancement

“…It is proved that the obtained zeta potential value of ZnO NPs stabilized via the steric and electrostatic repulsive force effects of brown algae is −38.7 mV, with a single peak signifying the presence of repulsion among the synthesized nanoparticles, as depicted in Figure 3 A. As the literature documented, the particles in suspension have a large negative zeta potential tend to repel with each other, and there will be no tendency of the particles to assemble [ 35 ]. As shown in Figure 3 B, ZnO NPs have high zeta potential values, thus, excellent force originates from brown algae, to act as a stabilizing agent to prevent the particles from coming together and flocculating and increases the stability of the prepared ZnO NPs.…”

“…The data for the average hydrodynamic size of ZnO NPs is shown in Figure 3 A as 41 nm, and their PDI is 0.264. The increase of particle sizes of ZnO NPs determined in an aqueous solution (41 nm), compared to that determined from XRD data in dry solution (31 nm), elucidates that the ZnO NPs adsorb water as hydrated water on their surfaces, which increases their sizes [ 35 ]. Moreover, the single peak obtained ( Figure 3 A) suggests that the consistency of the ZnO NPs synthesized via brown algae was reasonable.…”

“…As shown in Figure 3 B, ZnO NPs have high zeta potential values, thus, excellent force originates from brown algae, to act as a stabilizing agent to prevent the particles from coming together and flocculating and increases the stability of the prepared ZnO NPs. Thanks to the helpful role of brown algae for long-term stability, ZnO NPs exceeded the recommended zeta potentials for the stable suspensions at above +25 or −25 mV [ 35 ].…”

Hydrothermal Synthesis of (001) Facet Highly Exposed ZnO Plates: A New Insight into the Effect of Citrate

“…Zeta potential of different samples were measured, as displayed in Table 2. The adsorption of citrates can decrease the zeta potential of particles [33,34]. Compared with TS precursor dispersed in water and TS precursor dispersed in citrate solution, the OS precursor shows the most negative zeta potential (−25.8 mV), which was caused by the adsorption between citrates and the precursor.…”

Solvent assisted evolution and growth mechanism of zero to three dimensional ZnO nanostructures for dye sensitized solar cell applications

“…Solvent properties like dipole strength, coordination capacity, dielectric constant, etc. play important roles in controlling the aggregation rate 13 , 30 . Here we present a simple synthetic strategy for the preparation of ZnO nanostructures that can be used as the photoanode material in the fabrication of DSSC.…”