The effect of Co2+ and Co3+ on photoluminescence characteristics of ZnS nanocrystallines

Article abstract of DOI:10.1016/S0022-3697(00)00287-0

Free-standing powder of doped ZnS nanoparticles has been synthesized by using a chemical co-precipitation of Zn²⁺, Co²⁺, Co³⁺ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ～2.6 ± 0.2 nm. Unique luminescent properties have been observed from ZnS nanoparticles doped with Co²⁺ and Co³⁺. The effect of Co³⁺ on emission spectra of doped samples is remarkably different from that of Co²⁺. Very strong and stable visible-light emission have been observed from ZnS nanoparticles doped with Co²⁺. However, the fluorescence intensity of Co³⁺-doped ZnS nanoparticles is much weaker than that of ZnS nanoparticles. Nanoparticles can be doped with cobalt during the synthesis without altering the X-ray diffraction pattern and emission wavelength. However, doping makes the fluorescence intensity vary. The fluorescence intensity of doped sample is about five times of that of pure ZnS nanocrystallines when the doped mole ratio of Co²⁺ is 0.5%. The fluorescence efficiencies of samples decreases as doped mole ratio of Co²⁺ and Co³⁺ increases. When Co³⁺ is doped in ZnS nanoparticles, the fluorescence efficiencies of doped ZnS nanoparticles almost died away. Therefore, Co³⁺ leads to fluorescence decay of ZnS nanocrystallites.

Full text of DOI:10.1016/S0022-3697(00)00287-0

Journal of Physics and Chemistry of Solids 62 (2001) 1181±1184
www.elsevier.nl/locate/jpcs
Technical note
The effect of Co21 and Co31 on photoluminescence characteristics
of ZnS nanocrystallines
a
a,
a
a
P. Yang , M. LuÈ , D. XuÈ , D. Yuan , C. Song , G. Zhou
a
b
*
aState Key Laboratory of Crystal Material, Shandong University, Jinan 250100, People's Republic of China
bShandong Supervision and Inspection Institute for Product Quality, Jinan 250100, People's Republic of China
Received 15 August 2000; accepted 9 November 2000
Abstract
Free-standing powder of doped ZnS nanoparticles has been synthesized by using a chemical co-precipitation of Zn21, Co21
,
Co31 with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ,2.6 ^ 0.2 nm.
Unique luminescent properties have been observed from ZnS nanoparticles doped with Co21 and Co31. The effect of Co31 on
emission spectra of doped samples is remarkably different from that of Co21. Very strong and stable visible-light emission have
been observed from ZnS nanoparticles doped with Co21. However, the ¯uorescence intensity of Co31-doped ZnS nanoparticles
is much weaker than that of ZnS nanoparticles. Nanoparticles can be doped with cobalt during the synthesis without altering the
X-ray diffraction pattern and emission wavelength. However, doping makes the ¯uorescence intensity vary. The ¯uorescence
intensity of doped sample is about ®ve times of that of pure ZnS nanocrystallines when the doped mole ratio of Co21 is 0.5%.
The ¯uorescence ef®ciencies of samples decreases as doped mole ratio of Co21 and Co31 increases. When Co31 is doped in ZnS
nanoparticles, the ¯uorescence ef®ciencies of doped ZnS nanoparticles almost died away. Therefore, Co31 leads to ¯uorescence
decay of ZnS nanocrystallites. q 2001 Elsevier Science Ltd. All rights reserved.
Because the prospect of applications of luminescence
materials is decided directly by their ¯uorescence ef®cien-
cies, enhancing of ¯uorescence ef®ciencies becomes very
important in the research of luminescence materials.
Doped nanocrystals of semiconductors are a new class of
luminescent materials [1]. They create new opportunities for
luminescent study and the applications of nanometer scale
materials.
[2±5]. Therefore, the acting mechanism of Co21 and Co31
cannot be explained by conventional theories
The precipitation of doped ZnS nanoparticles was
performed starting from homogeneous solutions of zinc
acetate (Zn(CH3COO)22H2O), Co(CH3COO)24H2O (the
32
color of aqueous solution of Co21 is pink), CoꢀNO2†
6
32
(the color of aqueous solution of CoꢀNO2† is yellow) at
6
0.5 M and TAA at 0.5 M for each precipitation reaction. The
32
Previously, there has not been much work engaged in
luminescent study of semiconductor nanoparticles doped
with Co21 and Co31. Here, we report the photoluminescence
(PL) characteristics of the ®rst realization of ZnS nanopar-
ticles doped with Co21 and Co31 for the ®rst time. We have
found that Co21 enhanced the ¯uorescence ef®ciencies of
ZnS nanocrystallines remarkably. Also, we have found that
Co31 is a ¯uorescence decay agent, although it only reduces
an electron as combining with Co21. Nanosized particles
have higher surface/volume ratio and more surface states.
They have many novel physical and chemical properties
aqueous solutions of CoꢀNO2† is obtained by reacting of
6
Co(CH3COO)24H2O and NaNO2. The reaction temperature
was ®xed at 808C, and the solutions were acidi®ed to control
the decomposition of TAA that generated the sul®de anions.
In each experiment, the salt solutions and the TAA solution
were heated separately to 808C before mixing in a batch
reactor with no agitation. A bluish tint appeared after
5 min, indicating the beginning of the nucleation step. The
reaction time was ®xed at 30 min, after which the hydrolytic
reaction of TAA was terminated by rapidly cooling the solu-
tion to ,108C in an ice bath. As-precipitated ZnS nanopar-
ticles doped with Co21 and Co31 were centrifuged at
4000 rpm, washed three times with water to eliminated
TAA, and washed three times with isopropyl alcohol. The
* Corresponding author.
E-mail address: mklu@icm.sdu.edu.cn (M. LuÈ).
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0022-3697(00)00287-0

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P. Yang et al. / Journal of Physics and Chemistry of Solids 62 (2001) 1181±1184
cleaned powders were dried for about 10 h at 808C until
complete evaporation of the solvent was achieved.
A Japan Hitachi M-850 ¯uorescence spectrophotometer
was used to measure the photoluminescence of the particles.
Good spectral data was recorded with the slits set at 10 nm.
The samples were pressed into thin slices and then were
placed in a special support for all measurements (at ®rst,
the wavelength of excitation was measured, then the emis-
sion spectra of samples were recorded). X-ray powder
diffraction (XRD) patterns were recorded using a Japan
Rigaku D/max-rA X-ray diffractometer by use of Cu Ka
Fig. 1. X-ray diffraction patterns of doped and undoped samples
(mean doped mole percent ratio).
Ê
irradiation(l ˆ 1.5418 A) at 40 kV/100 mA with a second-
ary graphite crystal monochromator. XPS analysis of ZnS
nanocrystallites doped with Co21 and Co31 were obtained
by using PHI5300 of Perkin-Elmer. The absorption spectra
of samples were recored by using UV-vis spectrophot-
ometer.
is the high dispersity of nanocrystalline systems, i.e. the
number of atoms at the surfaces or grain boundaries of
the crystalline regions is comparable to that of those
located in the crystalline lattice itself. Physical and
chemical properties, which are usually determined by
the molecular structure of the bulk lattice, become
increasingly dominated by the defect structure of the
surface [5±8]. The second phenomenon occurs only in
nanometer-sized semiconductors and metals. The de
Broglie wavelength of the electrons (and holes) becomes
comparable to the crystallite size [5]. Under these condi-
tions, the charge carriers have to be treated quantum
mechanically as `particles in a box', where the size of
the box is given the dimensions of the crystallites. In this
novel state of matter, the valence band and conduction
band split into discrete electronic levels, with the spacing
between these levels and the band gap increasing with
decreasing particles size. Therefore, luminescent
mechanism cannot be explained by conventional
theories.
The doped mole percent ratios of Co21 are 0.5, 1.0, 1.2
and 1.5, respectively. The doped mole percent ratios of Co31
are 0.2, 0.5, 1.0, respectively. XRD patterns of samples are
shown in Fig. 1. This reveals that the particles exhibit a zinc-
blende crystal structure. The three diffraction peaks corre-
spond to (111), (220), and (311) planes of the cubic crystal-
line ZnS, respectively. Due to the size effect, the XRD peaks
tend to broaden and their widths become larger as the parti-
cles become smaller. From XRD analysis, no characteristic
peaks of impurity phases have been observed. The average
size of particles doped, estimated from the Debye±Scheerer
formula, is 2.6 ^ 0.2 nm. The average size of ZnS nanopar-
ticles is about 1.35 nm. Fig. 2 shows the absorption spectra
of doped and undoped ZnS nanocrystallites. It reveals that
the absorption spectra of ZnS nanoparticles do not varied
when Co21 and Co31 are doped in ZnS nanoparticles.
Fig. 3 shows the emission spectra of pure ZnS nanopar-
ticles and ZnS nanocrystallites doped with different mole
ratio of Co21 and Co31. Their excitation wavelength is
337 nm and doped concentration is also given in Fig. 3.
The effect of Co21 on the luminescence properties of ZnS
nanoparticles is different from that of Co31, although the
emission wavelength is almost the same as that of pure
ZnS nanoparticals. When Co21 is doped in ZnS nanoparti-
cles, the ¯uorescence intensity of samples has been drama-
tically enhanced. The ¯uorescence intensity of ZnS
nanocrystallites doped with 0.5% mole Co21 is about ®ve
times of that of ZnS nanoparticles. However, the ¯uores-
cence intensity of ZnS nanocrystallites doped with Co31 is
almost non-existent.
In the Co-doped ZnS nanocrystallines, luminescent
mechanism and processes are very complex. According to
XPS analysis shows Zn (2P), S (2P) have no signi®cant
changes at different doped mole ratios. However, the XPS
pro®le of Co31 remarkably differs from the XPS pro®le of
Co21. Figs. 4 and 5 show the variation on the XPS pro®le of
ZnS nanocrystallines doped with Co21 and Co31
.
The most striking feature of nanosized materials is that
their chemical and physical properties differ remarkably
from those of the bulk solids. The reasons for this beha-
vior can be attributed to two basic phenomena. The ®rst
Fig. 2. The absorption spectra of samples (in colloidal solution,
mean doped mole percent).

P. Yang et al. / Journal of Physics and Chemistry of Solids 62 (2001) 1181±1184
1183
Fig. 4. XPS pro®le of Co31 in ZnS nanocrystals.
Fig. 5. XPS pro®le of Co21 in ZnS nanocrystals.
edly. We have found the same conclusion in each experi-
ment and examination.
In conclusion, as Co21 is a sensitizing agent in ZnS nano-
crystallines the ¯uorescence ef®ciencies of the samples, are
obviously increased. Particularly, the ¯uorescence ef®cien-
cies of doped sample are about ®ve times of those of pure
ZnS nanocrystallines when the doped mole ratio of Co21 is
0.5. Also, the ¯uorescence ef®ciencies of samples decreases
as the amount of Co21 and Co31 increases. The effect of
Co31 on emission spectra of doped samples is remarkably
different from that of Co21. Because Co31 deep center inhi-
bits more electrons (holes) and makes them unexcited, the
¯uorescence ef®ciencies of Co31-doped samples are
obviously lower than those of pure ZnS samples. Also, the
¯uorescence ef®ciencies of Co31-doped samples are very
weak (almost died away). So Co31 leads to ¯uorescence
decay of ZnS nanocrystallites.
Fig. 3. Emission spectra of samples (lexcitation ˆ 337 nm), mean
doped mole percent ratio of Co21 and Co31
.
the experimental results reported in this paper, Co21 is a
sensitizing agent. And the radiative recombination
processes are enhanced. So, the ¯uorescence ef®ciencies
of doped ZnS nanocrystallites are higher than those of
pure ZnS nanocrystallines. As semiconductor materials,
impurity effect would obviously reduce emission intensity.
Therefore, the ¯uorescence ef®ciencies decreases as Co21
-
doped mole ratio increases. When Co31 ions are doped in
ZnS nanocrystallines, Co31 deep centers are formed. Co31
deep centers inhibit more electrons (holes) which could not
be excited. So the nonraditive recombination processes are
enhanced by Co31 deep center. The ¯uorescence ef®ciencies
decreases as Co31-doped mole ratio increases. We have
done this experiment and examination of properties repeat-
Acknowledgements
The authors are grateful for the support of the Natural
Science Foundation of ChinaÐnumber 69890230.

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P. Yang et al. / Journal of Physics and Chemistry of Solids 62 (2001) 1181±1184
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