RSC Advances
PAPER
Facile solvothermal synthesis of high refractive
index ZrO2 spheres: estimation of the enhanced
light extraction efficiency
Cite this: RSC Adv., 2015, 5, 81915
a
b
a
Jin Young Park,† G. Seeta Rama Raju,† Byung Kee Moon and Jung Hyun Jeonga
*
Sub-micron sized ZrO2 spheres were synthesized by a facile solvothermal reaction method. The influence
of Zr(OPr)4 concentration on the size and size distribution of ZrO2 spheres were examined by taking the
scanning electron microscope and transmission electron microscope images. When the concentration
of Zr(OPr)4 was increased from 0.031 to 0.045 mol, the average size of the spheres also increased from
305 to 920 nm. The diffused reflectance spectra of the calcined ZrO2 spheres exhibited the higher
reflectivity than that of as-prepared ZrO2 spheres. As compared to the bare LEDs, light extraction
efficiency was improved to 23% when the ZrO2 spheres were coated on the ITO surface, however,
70.12% was improved when the ZrO2 spheres were coated on the GaN surface due to the spherical
morphology induced photon escape cone enhancement.
Received 16th July 2015
Accepted 21st September 2015
DOI: 10.1039/c5ra13284c
reported the synthesis of sub-micron size TiO2 spheres (200–500
nm) via co-precipitation method and used the calcination
process over 500 ꢀC. However, the structure of as-prepared TiO2
Introduction
Self-assembly is a promising method to fabricate mono-
dispersed spheres based photonic crystals in the visible and
near-infrared wavelength ranges.1–3 Particularly, sub-micron
sized monodispersed spheres are necessary for their applica-
tions in visible range photonics. Silicon dioxide (SiO2), poly-
styrene (PS) and poly(methyl methacrylate) (PMMA) spheres are
widely used to assemble the opal-like photonic crystals because
of their ease of synthesis with monodispersion properties and a
size distribution of less than 5%.4–7 Monodispersed silica
sphere (n ¼ 2.488 (anatase), and n ¼ 2.6 (rutile)) is amorphous.
Aer annealed at 500 ꢀC, TiO2 spheres were crystallized into
anatase phase but the refractive index reduced to 2.06. The
lower refractive index is due to the mesoporous nature of TiO2
spheres, which yields the less scattering of light. Wang et al.15
reported the synthesis of sub-micron size ZnO spheres (200–500
nm) by using Nd:YAG laser, however, ZnO spheres could not be
synthesized in large scale.
Among the various sizes of metallic spheres, high refractive
index material such as monodispersed submicron ZrO2 spheres
(n ¼ 2.0) are of particular interest and comparative particle size
to an optical wavelength, which make them ideal candidates for
photon-related application such as solar cells and photonic
crystals. Unfortunately, only few researchers have been able to
prepared the monodispersed ZrO2 spheres with submicron size.
Widoniak et al. reported the synthesis of ZrO2 spheres with 0.2–
2 mm by a sol–gel process.16,17 In this method, ZrO2 particles
were synthesized by adding various salts or polymers. The
structure of as-prepared ZrO2 particles is amorphous. Aer
annealed at 450 C, they are crystallized into tetragonal phase.
In this context, we have focused on the development of high
refractive index ZrO2 spheres by a facile route.
Herein, we reported a more convenient and cost effective
method to fabricate the monodispersed ZrO2 spheres with sub-
micro meter sized diameter by solvothermal method. This
process facilitate the direct formation of cubic phase ZrO2 submi-
cron spheres. Subsequently, the sizes of spheres were modied
with a functional factor of the Zr(OPr)4 precursor concentration.
Furthermore, light extraction efficiency and far-eld emission
colloidal spheres with tetraethyl orthosilicate (TEOS) hydro-
8
¨
lyzing methods were rst prepared by Stober et al. in 1968.
Polymer colloidal spheres such as PS and PMMA have been
produced by an emulsion polymerization method.9 However,
the properties of the prepared photonic crystals by using these
colloidal spheres are not shown to be impressive due to the low
refractive indices of SiO2 (refractive index (n) ¼ 1.45), PS (n ¼
1.58) and PMMA (n ¼ 1.49).10–12
To overcome the low refractive index problem, one of the
ways is to synthesize the spheres (without visible light absorp-
tion) with high refractive index because the high refractive
index spheres scatters more light at the large scattering angles
than that of low refractive index spheres.13 By the literature
survey, many research methods were developed to synthesize
the submicron spheres with high refractive index. Jiang et al.14
ꢀ
aDepartment of Physics, Pukyong National University, Daeyon 3-dong, Nam-gu, Busan
608-737, Republic of Korea. E-mail: bkmoon@pknu.ac.kr
bDepartment of Electronics and Radio Engineering, Kyung Hee University, 1 Seocheon-
dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea
† equally contributed to this work.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 81915–81919 | 81915