Optical Properties of Hydrothermally Grown ZnO Nanoflowers
Authors: Samanta Pijus Kanti 
Published: May 2022

Abstract:
A simple hydrothermal method has been successfully employed to synthesize flower-like ZnO
nanostructure. X-ray diffraction data confirm the formation of ZnO with a Wurtzite structure. FESEM
images show the flower-like morphology of the synthesized nanostructures. The energy dispersive X-ray
spectroscopic analysis confirms the stoichiometric composition.. X-ray fluorescence spectrum shows no
impurity element in the synthesized ZnO. The synthesized ZnO exhibits low absorption in the visible
region of wavelength. Band gap enhancement was also observed owing to the quantum confinement effect.
The synthesized ZnO nanoflowers exhibit strong room-temperature photoluminescence with a broad
emission peak at 429 nm arising due to the recombination of electrons at zinc interstitials and holes in the
valence band. This defect-related photoluminescence is very important in the context of understanding the
defect dynamics in ZnO.

Background:
Zinc oxide (ZnO) is a well-known multifunctional material possessing unique structural,
electrical, and optical properties that are very useful in various device applications. Being a high and direct
band gap semiconductor, it is potentially being used in various UV light sources and detectors fabrication.
However, the emission and absorption properties strongly depend on the size of the ZnO nanoparticles
which in turn depends on the morphology of the nanostructure. Therefore, it is very much important
to understand the structure-property relationship for a predictable device performance.

Objectives:
Our objective of this work is to synthesize flower-like ZnO nanostructures using a simple
hydrothermal method. The flower-like ZnO morphology offers a large surface area that will be very suitable
for designing gas and chemical sensor devices. Another objective of this work is to study the crystallography
of ZnO. Next, the optical properties (emission and absorption) have been investigated to understand
the defect-related photoluminescence mechanism.

Method:
A simple hydrothermal method has been deployed to synthesize flower-like ZnO nanostructures.
A chloride decomposition scheme has been used to produce zinc hydroxide ions that will produce
ZnO nuclide. At the onset of saturation, ZnO nanocrystals start to grow. The entire reaction was performed
inside a Teflon cell stainless steel autoclave. The autoclave was placed in a horizontal tube furnace
and maintained at 150 °C for 2 hr. resulting in the formation of white powder-like material.

Results:
The X-ray diffraction data confirm the formation of polycrystalline ZnO having a Wurtzite structure.
Flower-like morphology was clearly observed in FESEM images. The EDS data confirm the composition
of ZnO with proper stoichiometry. Gibb’s free energy calculation favors the reaction under the
experimental condition. The absorption spectrum was used to calculate the band gap of the synthesized
ZnO nanoflowers. The Tauc plot revealed the band gap of the synthesized ZnO to be~ 3.69 eV. This enhancement
of band gap compared to bulk ZnO occurs due to the quantum confinement effect. The synthesized
ZnO nanoflowers exhibit broad photoluminescence peaked at 429 nm owing to the presence of interstitial
zinc.

Conclusion:
A hydrothermal method has been successfully used to synthesize well-crystalline ZnO
nanoflowers of proper stoichiometry. The flower-like nanostructure exhibits band gap enhancement due
to the quantum confinement effect. Room temperature visible photoluminescence was observed from the
ZnO nanoflowers with a board emission peak at 429 nm. This emission arises due to the presence of
deep-level zinc interstitial states. This finding will be very useful in understanding the role of defects in
the visible emission from ZnO nanostructures.