Size-dependent optical and electrochemical band gaps of ZnS nanorods fabricated through single molecule precursor route

Article abstract of DOI:10.1063/1.2734385

The colloidal, highly crystalline, and size-controlled ZnS nanorods have been prepared through a thermal decomposition of single-source-precursor approach in oleylamine solution. The length and width of nanorods can be controlled simply by varying the molar ratio of zinc 2-mercaptobenzothiazole complex and oleylamine or prolonging the anneal duration. Optical and electrochemical properties of different sizes of ZnS nanorods are discussed and size-dependent optical and electrochemical band gaps are evidently observed, which show a direct correlation between the electronic spectra and electrochemical band gap.

Full text of DOI:10.1063/1.2734385

Size-dependent optical and electrochemical band gaps of ZnS nanorods fabricated
through single molecule precursor route
B. Y. Geng, X. W. Liu, J. Z. Ma, Q. B. Du, and L. D. Zhang
Citation: Applied Physics Letters 90, 183106 (2007); doi: 10.1063/1.2734385
View online: http://dx.doi.org/10.1063/1.2734385
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APPLIED PHYSICS LETTERS 90, 183106 ͑2007͒
Size-dependent optical and electrochemical band gaps of ZnS nanorods
fabricated through single molecule precursor route
B. Y. Geng,a͒ X. W. Liu, J. Z. Ma, and Q. B. Du
Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science,
Anhui Normal University, Wuhu 241000, People’s Republic of China
L. D. Zhang
Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
͑Received 6 February 2007; accepted 3 April 2007; published online 1 May 2007͒
The colloidal, highly crystalline, and size-controlled ZnS nanorods have been prepared through a
thermal decomposition of single-source-precursor approach in oleylamine solution. The length and
width of nanorods can be controlled simply by varying the molar ratio of zinc
2-mercaptobenzothiazole complex and oleylamine or prolonging the anneal duration. Optical and
electrochemical properties of different sizes of ZnS nanorods are discussed and size-dependent
optical and electrochemical band gaps are evidently observed, which show a direct correlation
between the electronic spectra and electrochemical band gap. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2734385͔
Recently, one-dimensional ͑1D͒ nanocrystalline materi-
als, including nanorods and nanowires, have been intensively
investigated for their unique properties, which are derived
from their low dimensionality.1,2 As one of the most impor-
tant group II-VI semiconductors, ZnS with a band gap of
3.66 eV at room temperature is a traditional phosphor, which
was widely used in flat-panel display, electroluminescent de-
vices, infrared windows, sensors, and lasers.3,4 Several dif-
ferent synthetic methods have been used to synthesize 1D
ZnS nanostructures.5–11 However, the synthesis of high qual-
ity ZnS nanostructures with uniform size, shape, and good
optical properties is still a challenge. Nanocrystals, synthe-
sized using a wet chemistry technique, can grow continually
in the solution by coarsening or epitaxial attachment due to
the scarcity of a strong repelling organic layer on the particle
surface and lead to aggregation or precipitation over a few
days.12 In this letter, we report the preparation of colloidal,
highly crystalline, and size-controlled ZnS nanorods through
a thermal decomposition of single-source-precursor approach
in oleylamine ͑OA͒ solution with a similar process reported
by Zhang et al.,13 as well as their size-dependent optical and
was also dissolved in 10 ml ethanol. The above two solutions
were mixed together under stirring; the mixture was put into
an ice-water bath for 30 min and filtered. The precipitate was
collected, washed with de-ionized water, and dried in vacuo
at 60 °C overnight.
The ZnS nanorods were fabricated from the precursor
Zn͑Mer͒2 obtained from the above experiment. For sample 1,
8 ml OA was added in a three-necked flask at room tempera-
ture, and then the solvent was removed under vacuum at
about 100 °C for 30 min. After the solvent was cooled to
room temperature, 0.58 mmol Zn͑Mer͒ was added to the
2
flask, and then Ar was bubbled through the solution for
20 min. The flask was then flash heated to 270 °C and aged
for 7 h in inert atmosphere. After centrifugation, pale white
powder was obtained. The pale white powder was washed
with ethanol several times in order to remove excrescent OA
in the products and dried in vacuo for further characteriza-
tion. Sample 2 was synthesized under similar synthetic con-
ditions with an anneal duration of 3.5 h, while sample 3 was
synthesized using 15 ml OA with an anneal duration of 7 h.
The synthesized products were characterized by x-ray
diffraction ͑XRD͒ ͑MAC Science MXP 18AHF͒, selected-
area electron diffraction ͑SAED͒ and transmission electron
microscopy ͑TEM͒ ͑JEM 200CX͒, high-resolution TEM
͑HRTEM͒ ͑JEOL 2010͒, room temperature UV/visible ab-
sorption ͓U-3010 UV/visible spectrophotometer ͑220 V,
50 Hz͔͒, and photoluminescence ͑PL͒ ͑F-4500͒ using
250 nm as the excitation wavelength with a Xe arc lamp at
room temperature. The electrochemical properties of the na-
norods were investigated by cyclic voltammetry ͑CV͒ at a
Au electrode with tetra-n-butylammonium perchlorate
͑TBAP͒ as supporting electrolyte in freshly distilled CH2Cl2.
A three-electrode configuration was used where Pt coil and
Ag/AgCl electrode served as the counter and reference elec-
trodes, respectively.
electrochemical
properties.
In
this
case,
zinc
2-mercaptobenzothiazole complex ͓Zn͑Mer͒ ͔ was used as
2
single-source precursor to fabricate ZnS nanorods. Zn͑Mer͒
2
is air stable and synthesizes easily using a similar process
reported by Rockenberger et al.14 The length and width of
nanorods can be controlled simply by varying the molar ratio
of Zn͑Mer͒ and OA or prolonging the anneal duration.
2
Moreover, we study the electrochemical properties of ZnS
nanorods and demonstrate a direct correlation between their
electrochemical band gap and the electronic spectra in
CH2Cl2, as the electrochemical properties of semiconductors
are investigated poorly because the limited solvent window
of many solvent/electrolyte systems, the instability and the
insolubility of nanocrystals.
The synthetic procedure of Zn͑Mer͒ can be simply de-
The TEM images ͑Fig. 1͒ reveal that the sizes of the
nanorods can be controlled simply by varying the molar ratio
2
scribe as follows: 0.02 mol 2-mercaptobenzothiazole was
dissolved in 20 ml ethanol, and 0.01 mol Zn ͑CH3COO͒
of Zn͑Mer͒ and OA or prolonging the anneal duration. The
2
2
HRTEM images of the nanorods indicate the highly crystal-
line nature of these nanorods. They also reveal that the inter-
planar distances along the growth axis are 0.311 nm, which
a͒
Author to whom correspondence should be addressed: FAX: ϩ86-553-
3869303; Electronic mail: bygeng@mail.ahnu.edu.cn
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0003-6951/2007/90͑18͒/183106/3/$23.00 90, 183106-1 © 2007 American Institute of Physics
142.244.5.147 On: Thu, 27 Nov 2014 03:50:23

183106-2
Geng et al.
Appl. Phys. Lett. 90, 183106 ͑2007͒
FIG. 2. XRD patterns taken from as-synthesized products. ͑a͒ 3.5 h, 8 ml
OA; ͑b͒ 7 h, 15 ml OA; and ͑c͒ 7 h, 8 ml OA.
crystals, which is a general size-dependent phenomenon in
nanoparticles. Compared to samples 1 and 3, peaks of
sample 2 are not very obvious for a shorter anneal duration.
Figures 3͑a͒ and 3͑b͒ show the UV/visible absorption
and room temperature PL ͑excitation at 250 nm͒ spectra of
the as-prepared ZnS nanorods. The absorption peaks are lo-
cated around 310 nm for sample 1, 300 nm for sample 2, and
305 nm for sample 3, which clearly exhibit blueshifted about
30, 40, and 35 nm, respectively, compared to the bulk band-
gap wavelength ͑340 nm͒, which may be caused by excitonic
transition. The long absorption tail of all samples was due to
scattering by the crystal. Figure 3͑b͒ illustrates the room
temperature PL of as-prepared ZnS nanorods with different
sizes. The emission maximum appears at 364 nm ͑3.41 eV͒
for sample 1, 354 nm ͑3.51 eV͒ for sample 2, and 360 nm
FIG. 1. TEM and HRTEM images of ZnS nanorods. Reaction conditions:
7 h, 8 ml OA for ͑a͒ and ͑b͒; 3.5 h, 8 ml OA for ͑c͒ and ͑d͒; and 7 h, 15 ml
OA for ͑e͒ and ͑f͒. The inset in ͑b͒ is the SAED pattern of sample 1.
is consistent with the interplanar distance of the ͑110͒ plane
of the hexagonal wurtzite structure of ZnS. The SAED pat-
tern ͓inset in Fig. 1͑b͔͒ can be indexed to the hexagonal
structure, which is consistent with the corresponding XRD
data. When 8 ml OA was used and aged for 7 h at 270 °C,
the obtained nanorods had average size of about 5 nm
͑diameter͒ϫ24 nm ͑length͒. If the reaction was kept at
270 °C for 3.5 h, the shorter nanorods with length lower
than 12 nm were obtained ͓Figs. 1͑c͒ and 1͑d͔͒. It is impor-
tant to note that some preformed nanorods, which contained
a few primary particles, have been observed, which may
demonstrate that the nanorod formation may base on the ori-
ented attachment mechanism.15 If 15 ml OA was used, the
products are shown in Figs. 1͑e͒ and 1͑f͒. The nanorods have
a smaller size with a diameter of about 3 nm and 17 nm in
length, indicating that the concentration of OA plays an im-
portant role in rod-growth process. Furthermore, by changing
the synthetic conditions, the sizes of ZnS nanorods can be
controlled accurately, which are shown in Table I.
The typical XRD patterns of the as-synthesized products
are shown in Fig. 2, and all of the detectable diffraction
peaks are indexed to those from hexagonal ZnS,16 revealing
the pure sample phase. All XRD patterns show obvious size-
broadening effects, indicating the finite size of these nano-
TABLE I. Sizes of the as-prepared ZnS nanorods under different conditions.
Zn
OA
Size
No. ͑Mer͒ ͑mmol͒ ͑mL͒ T ͑°C͒ t ͑h͒ ͑diameterϫlength͒ ͑nm2͒
2
1
2
3
4
5
6
0.58
0.58
0.58
0.58
0.58
0.58
8
8
270
270
270
270
270
270
7
3.5
12
7
5ϫ24
5ϫ11
5.5ϫ32
3ϫ17
2ϫ14
3ϫ25
8
15
22
15
7
FIG. 3. ͑a͒ UV-visible spectra of the as-prepared ZnS nanorods dispersed in
cyclohexane. ͑b͒ Photoluminescence spectra of as-prepared ZnS nanorods
12
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dispersed in cyclohexane.
142.244.5.147 On: Thu, 27 Nov 2014 03:50:23

183106-3
Geng et al.
Appl. Phys. Lett. 90, 183106 ͑2007͒
nanoparticles, electron transfer to the particle cannot cause a
reduction reaction for the stability of thiol-capped gold nano-
particles and charging the particle double-layer discretely re-
sults in electrochemical analogs of Coulomb staircase charg-
ing. However, transition metal sulfide nanoparticles are not
as stable as Au nanoparticles, and electron transfer process
can cause a series of chemical reactions.19
Figure 4͑b͒ illustrates the CVs of different samples, from
which the different peak-to-peak separations are obviously
observed. The shapes of all CVs are similar, and the current
potentials are shifted slightly as the sizes are varied. The
peak-to-peak separation of sample 1 is about 3.18 V, 3.38 V
for sample 2, and 3.26 V for sample 3, indicating that the
gap between HOMO and LUMO is size dependent, which is
perfectly consistent with the optical band gap.
In summary, ZnS nanorods with different sizes have
been synthesized through a single-source-precursor approach
by controlling the molar ratio of Zn͑Mer͒ and OA or the
2
reaction duration. Optical and electrochemical properties of
different sizes of ZnS nanorods are discussed and size-
dependent optical and electrochemical band gaps are evi-
dently observed, which show a direct correlation between the
electronic spectra and electrochemical band gap.
This work was supported by the National Natural Sci-
ence Foundation of China ͑20671003͒, the Foundation of
Key Project of Natural Science of Anhui Education Commit-
tee ͑2006kj041a͒, the Young Teacher Sustentation Project of
Anhui ͑Grant No. 2005jq1041zd͒, and the Education Depart-
ment of Anhui Province ͑2006KJ006TD͒.
FIG. 4. ͑Color online͒ ͑a͒ CV response in presence of OA-capped ZnS
nanorod ͑1 mg/ml of sample 1͒ at a Au electrode. Sweep rate=100 mV/s
and ͓TBAP͔=0.05 M. ͑b͒ CVs illustrating the electrochemical band gap ͑
A1 −B1 peak separation͒ for all samples. ͓ZnS nanorod͔=1 mg/mL,
͓TBAP͔=0.05 M, and sweep rate=100 mV/s.
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