SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai
Bright green emission from nanostructured Gd2Zr2O7
Santosh K. Guptaa, Chithrambary Reghukumar, N. Pathaka, R.M. Kadam
a. Radiochemistry Division, Bhabha Atomic Research Centre Trombay, Mumbai-400085, India
b. Institute of science education and research Centre,
Email
Addresses-
santufrnd@gmail.com,
Santiniketan, West Bengal
chithrambary12@gmail.com,
-731235
nmpathak4@gmail.com,
rmkadam2003@yahoo.co.in
1. Introduction:
Rare earth doped luminescence materials in most cases are very expensive and non-environmentally benign.
Other class of luminescence materials is semiconductors and defect based inorganic compounds. However many
semiconductors possess intrinsic hazard and also some of them are potential threat to environment and that’s
why their usage is very limited Defect related emission without any activator ion has been the emerging area of
research and lots of attention has been given on how to replace traditional phosphor by rare earth free one.
As far as scientific and technological applications is concerned; rare earth zirconates (Re 2Zr2O7) have been
identified as the attractive candidates for TBC (thermal barrier coatings) applications, high temperature heating
devices and host materials for luminescence applications. There are two kinds of different crystal structures for
Re2Zr2O7, as the pyrochlore structure, and the fluorite structure. Among them, gadolinium zirconate (Gd2Zr2O7)
stand out as a material with a distinctively low thermal conductivity and high phase stability. In that context we
have synthesized gadolinium zirconate pyrochlore nanostructure using gel-combustion synthesis and it’s
characterized systematically using XRD, FTIR, SEM, Raman and photoluminescence spectroscopy.
2. Materials and Methods:
Gd2Zr2O7 was prepared by the method of sol-gel combustion. Gadolinium oxide [Gd2O3], Zirconyl nitrate [ZrO
(NO3)2.H2O] and Citric acid [C6H8O7.H2O] were used as the precursor materials. Stoichiometric compositions of
metal oxide, metal nitrate and fuel were calculated based upon propellant chemistry. Then they were mixed
together in a glass beaker of 100ml capacity. This resultant solution was kept over a magnetic stirrer (at 80oC for
6-7 hr) for obtaining a homogenous solution. With continuous stirring, the mixed solution resulted as an opal
gel. This gel was dried under infrared (IR) lamp which leads to dehydration and resulted into formation of
highly condensed porous network. Then the beaker was transferred into a furnace and heated to 400 0C for 2-3
hr. This results in formation of highly fluffy black masses. These were crushed into a fine powder with a mortar
and pestle and placed in alumina crucibles to be heat treated to 8000C for 6 hrs.
3. Significant result and discussion:
3.1. X-ray diffraction and SEM studies
Figure 1a shows the XRD patterns of undoped Gd2Zr2O7. There are five main diffraction peaks of (111), (200),
(220), (311) and (222). They are in agreement with the reflections of defect fluorite structure. The broad XRD
peak reflects the nanodimension of Gd2Zr2O7 crystallite. The average crystallite size was determined using
Scherrer’s formula from half width of the most intense peak (2θ=29. 8o) which comes out be 21 nm. SEM
studies showed that, the particles are uniformly distributed and mostly of spherical morphology as can be seen
from Figure 1b. Furthermore, the particle size is in a narrow range, the average grain size of them is about 50
nm in diameter. These images exhibited homogeneous aggregates with a sphere-like aspect, which are
composed of large number of small grains. Since particles consist of large number of crystallites, size obtained
using SEM is always going to be greater than one obtained using Scherer's formula.
3.2. Photoluminescence
The emission spectrum (Figure 2a) shows a broad band at around 530 nm (visible region) in green region of
electromagnetic spectrum. CIE coordinates value obtained in this case was found to be 0.200 and 0.196 (Figure
2b) showing Gd2Zr2O7 to be a strong green emitter. It is an important development because it is vary difficult to
find a suitable green phosphor because wide band gap is required, and the naked eye sensitivity is quite low in
the green spectral region. Such visible light emission on UV excitation in sample without any activator ion can
be ascribed to presence of defects in the lattice viz. oxygen related defect. Normally in nanomaterials based on
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preparation methodology and conditions; various kinds of defects are present viz. cation vacancy, cation antisite,
oxygen vacancy, oxygen antisite, cation interstitial, and oxygen interstitial. Even in oxygen vacancy there are
three different types; neutral, singly ionized and doubly ionized depending upon the number of electron it has
trapped zero, one or two.
Such intense green emission can arise in undoped Gd2Zr2O7 from transition of electron which is formed
during photon irradiation near to conduction band (CB) to single ionized oxygen vacancy centres (V o+).
Emission in such case arise from recombination process wherein an excited electron of the conduction band
(CB) loses its energy and re-occupied the energy levels of an electron hole in the valence band (VB) through the
localized defect levels. The schematic of defect induced emission is shown in Figure 2c.
Figure 1: (a) XRD pattern and (b) SEM micrograph of Gd2Zr2O7 nanostructure
Figure 2: (a) Emission spectrum (b) CIE chromaticity diagram and (c) Figurative model of probable mechanisms
responsible for observed emission of Gd 2Zr2O7
4. Conclusions:
This work reports on the Gel-combustion synthesis, characterization and photoluminescence spectroscopy of
Gd2Zr2O7 pyrochlore. Despite the absence of any activator, undoped Gd 2Zr2O7 showed distinct excitation and
emission spectra attributed to presence of local defect in the gadolinium zirconate phase. On irradiating the
Gd2Zr2O7 nanosphere with UV light, it is showing an intense green emission without the presence of any
activator ion which is attributed to presence of oxygen vacancy in gadolinium zirconate nanostructure. Presence
of oxygen vacancy was further confirmed by comparing the intensity of emission of sample annealed in
completely reducing and completely oxidizing atmosphere with that of as prepared sample. This can be a
promising green phosphor under near UV for white light emitting diodes.
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