Band gap engineered zinc oxide nanostructures via a sol-gel synthesis of solvent driven shape-controlled crystal growth.

Klinton Davis,Jamel White,Michael Froeschle,Hemali Rathnayake,Ryan Yarbrough

doi:10.1039/c9ra02091h

Klinton Davis, Jamel White + Show 3 more

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https://doi.org/10.1039/c9ra02091h

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Abstract

A reliable sol–gel approach, which combines the formation of ZnO nanocrystals and a solvent driven, shape-controlled, crystal-growth process to form well-organized ZnO nanostructures at low temperature is presented. The sol of ZnO nanocrystals showed shape-controlled crystal growth with respect to the solvent type, resulting in either nanorods, nanoparticles, or nanoslates. The solvothermal process, along with the solvent polarity facilitate the shape-controlled crystal growth process, augmenting the concept of a selective adhesion of solvents onto crystal facets and controlling the final shape of the nanostructures. The XRD traces and XPS spectra support the concept of selective adhesion of solvents onto crystal facets that leads to yield different ZnO morphologies. The shift in optical absorption maxima from 332 nm in initial precursor solution, to 347 nm for ZnO nanocrystals sol, and finally to 375 nm for ZnO nanorods, evidenced the gradual growth and ripening of nanocrystals to dimensional nanostructures. The engineered optical band gaps of ZnO nanostructures are found to be ranged from 3.10 eV to 3.37 eV with respect to the ZnO nanostructures formed in different solvent systems. The theoretical band gaps computed from the experimental XRD spectral traces lie within the range of the optical band gaps obtained from UV-visible spectra of ZnO nanostructures. The spin-casted thin film of ZnO nanorods prepared in DMF exhibits the electrical conductivity of 1.14 × 10−3 S cm−1, which is nearly one order of magnitude higher than the electrical conductivity of ZnO nanoparticles formed in hydroquinone and ZnO sols. The possibility of engineering the band gap and electrical properties of ZnO at nanoscale utilizing an aqueous-based wet chemical synthesis process presented here is simple, versatile, and environmentally friendly, and thus may applicable for making other types of band-gap engineered metal oxide nanostructures with shape-controlled morphologies and optoelectrical properties.

Highlights

Shape-controlled nanocrystal growth is governed by: (1) the classical crystal growth kinetics of Ostwald ripening theory and the “oriented attachment” mechanism where a sol of nanocrystals with shared crystallographic orientations directly combine together to form larger ones;33–35 (2) the relative surface energy of crystal facets and selective adhesion of solvents/surfactants onto crystal facets; and (3) the crystal growth regime, which depends on the monomer concentration and temperature
The concept of selective adhesion of surfactants onto crystal facets has been widely tested for the shaped-controlled synthesis of a variety of metal oxide nanostructures
As evidenced by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, the growth of crystal facets is governed by the relative surface energy of crystal facets and selective adhesion of solvents onto crystal facets

Summary

Introduction

Zinc oxide nanostructures are functional materials that can tailor their morphology through a variety of synthesis methods to yield a wide range of morphologies such as nanowires,1,2 nanorods,3–5 nanobelts,6 nanocombs,7 nanorings,8 and nanocages.9 Owing to the lack of a centre-of-symmetry in the wurtzite crystal structure, and high exciton binding energy (60 meV), nanostructured ZnO possess strong piezoelectric10 and pyroelectric11 properties and acts as a wide band gap (3.37 eV) semiconductor for short wavelength optoelectronic devices.12 In recent research advancements, ZnO has received considerableAmong dimensional ZnO nanomaterials, one-dimensional (1D) ZnO nanostructures with defect free high crystallinity are a particular interest due to their unique and inherent intrinsic chemical, electrical, physical, and mechanical properties compared to that of bulk and thin lm counterpart.10 synthesis of defect free 1D ZnO nanostructures, with desired morphology and composition, has been challenging as most growth techniques involve either high-cost fabrication processes or high temperature wet-chemical syntheses performed in highly toxic solvents. We demonstrate a reliable sol–gel approach, which combines the formation of a ZnO nanocrystals sol and a solvent driven shape-controlled in situ crystal growth process to form well-organized ZnO nanostructures at low temperature (

Results

Conclusion