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

Krisztina Kocsis,Oliver Diwald,Thomas Berger,Johannes Bernardi,Matthias Niedermaier,Günther Redhammer,Michel Bockstedte,Vít Kasparek

doi:10.1021/acs.langmuir.9b00656

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.

Highlights

Synthesis and colloidal processing of semiconducting oxide nanostructures matter once their optical and electronic properties depend on their interfaces
Prior to the water adsorption experiments, which will be described below, the ZnO nanoparticle surfaces are free from adsorbed solvent molecules, inorganic ions, surfactants, and other synthesisrelated remnants. (An earlier Auger electron spectroscopy study revealed residual carbon species with a surface concentration of up to 5% as the only impurity present.42,43)
Once aqueous ZnO nanoparticle dispersions with particle concentrations of 0.1 mg/mL were prepared and showed stability during investigation and for a minimum of 2 h, the particle size distribution functions were determined by dynamic light scattering (DLS)

Summary

■ INTRODUCTION

Synthesis and colloidal processing of semiconducting oxide nanostructures matter once their optical and electronic properties depend on their interfaces. Langmuir other inorganic or organic adsorbates.[34−37] Apart from a number of studies that focus on the impact of specific molecules,[38−41] a general understanding of how adsorbed surfactants and the solvents’ dielectric properties can affect energy and intensity of photoluminescence bands is far from complete With this very first systematic comparison of the free exciton emission of ZnO nanocrystals, which were grown in the gas phase, processed, and measured under water-free conditions, with identical nanoparticles in contact with a condensed water phase, we discuss important optical property changes that can originate from the conversion of the anhydrous powder into an aqueous colloidal dispersion. The crystallite size broadening was handled by allowing a Lorentzian type, while microstrain was handled by a Gaussian-type component convolution

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES