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Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods from zinc(II) acetylacetonate monohydrate
Author's Accepted Manuscript
Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods from zinc(II) acetylacetonate monohydrate Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee
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S0022-2313(14)00246-4 http://dx.doi.org/10.1016/j.jlumin.2014.04.007 LUMIN12648
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Journal of Luminescence
Received date: 24 February 2013 Revised date: 26 March 2014 Accepted date: 5 April 2014 Cite this article as: Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee, Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods from zinc(II) acetylacetonate monohydrate, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2014.04.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 1
Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods
2
from zinc(II) acetylacetonate monohydrate
3
Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee*
4
Department of Chemistry, Assam University, Silchar 788011, Assam, India
5
*Corresponding author. Tel: +91-03842-270848; fax: +91-03842-270342
6
Email: [email protected]
7
Abstract
8
Zinc oxide (ZnO) nanorods were synthesized via a low-temperature thermal decomposition of
9
zinc(II) acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O. A relatively inexpensive surfactant,
10
octadecylamine (C18H37NH2) served both as reaction solvent and capping agent during the
11
synthesis of ZnO nanorods. The synthesized nanorods were characterized by powder X-ray
12
diffraction (XRD), transmission electron microscopy (TEM), FT-IR, UV-visible, and
13
photoluminescence (PL) studies. The XRD spectrum furnished evidence for the hexagonal
14
wurtzite structure of ZnO. The TEM images revealed the material to be rod shaped having
15
diameter 30 nm and length 200 nm. The HRTEM image showed the lattice fringes between the
16
two adjacent planes are 0.244 nm apart, which corresponds to the interplanar separation of the
17
(101) plane of hexagonal ZnO. The electron diffraction (ED) pattern confirmed the single
18
crystalline nature of the nanorods. The PL spectrum showed two UV emissions at 356 nm
19
(~3.48eV) and 382 nm (~3.25eV), respectively. ZnO nanorods also showed very weak blue
20
bands at 445, 453 and 470 nm, respectively.
21
Keywords: Zinc oxide; Nanorods; Thermal decomposition; Photoluminescence.
22 23
2 24
1. Introduction
25
One-dimensional (1D) ZnO nanostructures such as nanowires, nanofibers, nanotubes, and
26
nanorods has attracted enormous current interest owing to their wide applications in nanodevices
27
such as light-emitting diodes [1, 2], field-effect transistors [3, 4], ultraviolet lasers [5, 6],
28
chemical sensors [7, 8] and solar cells [9, 10]. Various chemical synthetic routes like sol-gel
29
[11], hydrothermal [12, 13], thermal decomposition [14], reverse microemulsion [15] etc. have
30
been devised for the synthesis of ZnO nanorods. Amongst all, thermal decomposition emerged as
31
a quite popular synthetic option, as it is simple, low-cost, and yields high purity materials. ZnO
32
nanostructures exhibit luminescent properties in the near ultraviolet and visible regions.
33
However, due to surface defects the emission properties of ZnO in the visible region usually
34
depend on their synthesis method. Deep hole traps associated with the presence of oxygen
35
vacancies can cause green emission above 500 nm [16]. Shorter wavelength emissions in the
36
blue region are usually related to various defects such as interstitial zinc [17] or OH- groups at
37
the surface of the particles [18]. Zinc(II) acetylacetonate has been rather efficiently utilized by
38
several researchers previously as a precursor for the synthesis of ZnO nanomaterials of various
39
sizes and shapes. Thermal decomposition of zinc(II) acetylacetonate in oleylamine has been
40
earlier shown to yield monodispersed ZnO nanoparticles [19]. ZnO nanoparticles of size 12-20
41
nm were previously accessed via thermal decomposition of zinc(II) acetylacetonate in the
42
presence of surfactants oleylamine and triphenylphosphine [20]. Gross et al. developed a facile
43
and reproducible route to nanostructured ZnO by controlling the hydrolysis and condensation of
44
zinc(II)
45
zinc(II) acetylacetonate monohydrate at and above 200oC afforded ZnO nanoparticles of size 20-
46
40 nm [22]. Another strategy employed one-pot synthesis by refluxing an oversaturated solution
acetylacetonate
in
alkaline
conditions
[21].
Thermal
decomposition
of
3 47
of zinc(II) acetylacetonate monohydrate in 1-butanol, isobutanol or tert-butanol that yielded ZnO
48
nanomaterials of varying sizes and morphologies like nanorods, coral-like structures and
49
nanospheres [23, 24]. Though, ZnO nanomaterials of various morphologies were obtained earlier
50
using zinc(II) acetylacetonate precursor under different reaction conditions there appears to be no
51
report yet describing the synthesis of ZnO nanorods via thermal decomposition of
52
zinc(II)acetylacetonate. As a part of our continued interest in accessing metal oxide
53
nanomaterials from simpler metal precursors [25-27], we report herein the synthesis of ZnO
54
nanorods via a low-temperature thermal decomposition of zinc(II) acetylacetonate monohydrate
55
in the presence of a relatively inexpensive surfactant, octadecylamine (C18H37NH2).
56
2. Experimental
57
2.1. Materials and Physical measurements
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All chemicals used were of analytical grade and used as received. The precursor zinc(II)
59
acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O was prepared according to the literature
60
procedure [28]. FT-IR spectra were recorded on a Shimadzu Varian 4300 spectrometer on KBr
61
pellets. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris Diamond
62
thermal analyzer maintaining flow rate of 20mL/min and heating rate of 100C/min in air. X-ray
63
diffraction (XRD) measurements were carried out on a Bruker AXS D8-Advance powder X-ray
64
diffractometer with Cu-Kα radiation (λ=1.5418Å) with a scan speed of 20/min. Transmission
65
electron microscopy (TEM) images were obtained on a JEOL, JEM2100 equipment. The sample
66
powders were dispersed in n-hexane under sonication and TEM grids were prepared using a few
67
drops of the dispersion followed by drying in air. Absorption spectrum was taken with a
68
Shimadzu 1601 PC UV-visible spectrophotometer. Photoluminescence (PL) spectrum was
69
recorded on a Shimadzu RF-5301 PC spectrofluorophotometer.
4 70
2.2. Synthesis of ZnO nanorods
71
An amount of 0.6g of [Zn(C5H7O2)2].H2O and 10 ml of octadecylamine was heated in a round
72
bottomed flask for 1h at 1400C on an oil bath to get [Zn(C5H7O2)2(octadecylamine)2] complex.
73
Thereafter the reaction temperature was raised to 2000C. The light yellow solution gradually
74
became hazy indicating the formation of colloidal suspension. The solution was aged at 2000C
75
for 1h and cooled to room temperature. The white product was precipitated by adding excess
76
ethanol to the solution and washed with ethanol several times. The solid could be easily re-
77
dispersed in nonpolar organic solvents like n-hexane or toluene (Scheme 1).
78
Scheme 1.
79
3. Results and discussion
80
The current synthesis is a modified version of the procedure developed by Hyeon and others for
81
metal and oxide nanocrystals that utilizes thermal decomposition of transition metal complexes
82
[20, 29]. In the present synthesis (Scheme 1), zinc oxide nanorods were accessed by the thermal
83
decomposition of zinc(II) acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O precursor in the
84
presence of surfactant octadecylamine. The decomposition temperature (2000C) has been chosen
85
based on the TGA behavior of [Zn(C5H7O2)2].H2O at which the precursor compound degraded to
86
ZnO. Octadecylamine served both as reaction media and capping agent during the synthesis of
87
nanorods. The synthesized nanorods were characterized by XRD, TEM, FT-IR, UV-visible, and
88
PL studies. The TGA curve (Fig.1) revealed that the precursor zinc(II) acetylacetonate
89
monohydrate undergo decomposition in two steps. The first step is the dehydration, which
90
commences at 700C and completes at 1000C accompanied by a weight loss of ~7.3%, due to the
91
removal of one water molecule. A further weight loss of ~74.1% in the second step in the
5 92
temperature range 115-1950C furnished clear evidence for the loss of two acetylacetonate (acac-)
93
ligands affording ZnO as end residue.
94
Fig. 1.
95
The FT-IR spectrum of zinc(II) acetylacetonate (Fig.2(a)) showed a broad band centered
96
at about 3180 cm-1 due to ν(O-H) of lattice water. The peaks at 2995 and 2920 cm-1 arose due to
97
the C-H stretching vibration ν(C-H) of the methyl group. Coupling of the stretching vibrational
98
modes of C=O and C=C groups was observed around 1600 and 1515 cm-1. The band at 420 cm-1
99
can be assigned to the stretching vibrations of Zn-O. The FT-IR spectrum of the synthesized ZnO
100
(Fig.2(b)) showed a strong band at 488 cm-1, characteristics of Zn-O stretching vibrational mode.
101
Besides, C-N stretching mode of octadecylamine was observed at 1120 cm-1. A weak broad band
102
(3100-3700 cm-1) was observed due to the N-H stretching, those at 2920 and 2845 cm-1
103
attributable to the C-H stretching mode of octadecylamine, clearly indicated some
104
octadecylamine molecules have been adsorbed on the surface of the synthesized ZnO nanorods.
105
Fig. 2.
106
The XRD spectrum (Fig.3) of the synthesized material showed diffraction peaks, which can be
107
well indexed to the hexagonal phase of ZnO. The obtained ZnO possess the wurtzite structure.
108
The relative intensities of the diffraction peaks matched well with the standard diffraction pattern
109
of crystalline ZnO (space group P63mc, JCPDS File No. 89-1397). The sharp diffraction peaks
110
also indicated high crystallinity of the material. The XRD spectrum also showed two extra peaks
111
(asterisk marked), which are not characteristics of any other impurity phases. In fact these peaks
112
correspond well with the peaks of octadecylamine reported in the literature [30]. It is pertinent to
113
mention here that the calcined material was thoroughly washed with ethanol several times to
114
remove the excess surfactant molecules. Thus, the extra peaks in the XRD spectrum may be
6 115
attributed to the surfactant octadecylamine remaining at the surfaces of ZnO nanorods. The
116
average diameter of the nanorods estimated by the Debye-Scherrer formula using a Gaussian fit
117
was around 24 nm.
118
Fig. 3.
119
The TEM image (Fig.4) of the synthesized nanomaterial revealed them to be rod shaped
120
200 nm long possessing 30 nm diameter. The surfactant octadecylamine effectively controlled
121
the size of the nanorods by limiting the growth at the nucleation stage. Since the surfaces of the
122
nanorods were capped with the surfactant octadecylamine, these could be well dispersed in most
123
hydrophobic solvents. The HRTEM image showed the lattice fringes between the two adjacent
124
planes to be 0.244 nm apart, which corresponds to the interplanar separation of the (101) plane of
125
hexagonal ZnO. This indicated one of the growth planes of the nanorods is along the (101) plane.
126
The ED pattern indicated single crystalline nature of the synthesized nanorods.
127
Fig. 4.
128
Earlier Klabunde and coworker synthesized nanocrystalline ZnO utilizing organometallic
129
compound diethylzinc (Zn(C2H5)2) as precursor [31]. The synthesis involved transformation of
130
Zn(C2H5)2 into an alkoxide prior to hydrolysis and thermal treatment. Moreover, ZnO
131
nanocrystallites were found to agglomerate into bigger spherical particles having a diameter of
132
260 nm. High-temperature thermal decomposition of Zn(C2H5)2 in the presence of
133
trioctylphosphine oxide (TOPO) and alkylamines also known to afford ZnO nanocrystals [32].
134
Wachnicki et al. compared effectiveness of dimethylzinc (Zn(CH3)2) and diethylzinc (Zn(C2H5)2)
135
as precursors for monocrystalline zinc oxide films grown by atomic layer deposition (ALD) [33].
136
Due to the pyrophoric nature of Zn(CH3)2 and Zn(C2H5)2 precursors the reactions were often
137
required to be performed under inert atmosphere. In contrast air stable zinc(II) acetylacetonate
7 138
monohydrate precursor utilized here can be easily accessed under mild reaction conditions.
139
Moreover, it is a single source precursor, which provides both zinc and oxygen species via
140
decomposition. Using metal organic chemical vapor deposition (MOCVD) technique with
141
diethylzinc (Zn(C2H5)2) precursor vertically aligned (c-axis oriented) ZnO nanorods were grown
142
on sapphire (001) substrate [34]. A very high deposition temperature (>6000C) and inert
143
condition were necessary to grow the nanorods. The nanorods were found to grow almost
144
perpendicularly on the substrate and their lengths were about 3 µm. However, ZnO nanorods
145
synthesized in the present study lacks vertical alignment and possess comparatively much
146
smaller length than those obtained by MOCVD technique [34]. The present synthesis is based on
147
low decomposition temperature (~2000C) and non-inert condition.
148
Fig. 5.
149
Fig. 6.
150
The UV-visible spectrum (Fig.5) showed a strong absorption at around 375 nm (band gap
151
= 3.31 eV), which corresponds to the bulk value of the band gap of ZnO. No blue shift was
152
observed in the spectrum of ZnO nanorods, which indicated the nanorods to be too large to show
153
any quantum confinement related effects. The strong excitonic absorption observed is possibly
154
due to the effective surface passivation of ZnO by the surfactant octadecylamine and the more
155
efficient dispersion of the nanorods. As a result light scattering is reduced causing the long
156
wavelength tail to extend up to 800 nm. A similar absorption feature was observed earlier for the
157
oleic acid passivated ZnO nanotetrapods [35]. The PL spectrum (Fig.6) of ZnO nanorods excited
158
at 320 nm showed two UV emissions at 356 nm (~3.48eV) and 382 nm (~3.25eV), respectively.
159
The emission peak at 356 nm may be attributed to the band gap luminescence as it is blue shifted
160
compared to the optical absorption. The near-band edge (NBE) emission peak at 382 nm is
8 161
assigned to the recombination of free excitons [36]. In addition to the UV emissions, a broad
162
shoulder in the range 400-425 nm and very weak defect-related blue emissions at 445, 453 and
163
470 nm were also observed. The observance of blue bands were also reported earlier in ZnO
164
nanostructures [37-40] and believed to be associated with various intrinsic or extrinsic lattice
165
defects. Due to the effective surface passivation by the surfactant octadecylamine, ZnO nanorods
166
showed very weak defect-related visible blue emissions and were less intense compared to the
167
UV emissions [41-43]. ZnO nanorods grown on Al203 (0001) substrate by the MOCVD
168
technique with Zn (C2H5)2 showed NBE emission as well as quite high defect-related visible
169
emissions [44]. In contrast, the PL spectrum of ZnO nanorods synthesized in the present study
170
showed considerably quenched defect-related visible emissions, which indicated high optical
171
quality of the nanorods. This inference was further corroborated by the XRD and TEM
172
observations. Very recently, improvement of the optical and structural properties of ZnO
173
nanorods grown by ALD on the seed ZnO nanorods has been observed [45]. A high-temperature
174
annealing is also utilized to improve the optical and structural properties of ZnO nanorods [46,
175
47]. ZnO nanorods accessed in the present study without any additional surface treatment or
176
high-temperature processing exhibited good optical and structural quality.
177
4. Conclusion
178
A low-temperature thermal decomposition of zinc(II) acetylacetonate monohydrate in the
179
presence of an inexpensive surfactant octadecylamine successfully resulted ZnO nanorods.
180
Though, zinc(II) acetylacetonate has been exploited earlier as precursor for accessing ZnO
181
nanomaterials, the formation of nanorods as reported herein appear to be the first of its kind. The
182
synthetic strategy adopted is simple and should serve as a paradigm to access other metal oxide
183
nanomaterials of specific size and morphology.
9 184
Acknowledgement
185
Authors are thankful to SAIF, NEHU, Shillong for providing TEM facility. DDP thanks
186
University Grants Commission, Government of India for Research Fellowship Scheme for
187
Meritorious Students (RFSMS). We are thankful to DBT e-Library Consortium (DeLCON) of
188
Bioinformatics Centre, Assam University, Silchar.
189
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Figure captions
256
Fig. 1. TGA curve of zinc(II) acetylacetonate monohydrate.
257
Fig. 2. FT-IR spectrum of (a) zinc(II) acetylacetonate monohydrate and (b) ZnO nanorods.
258
Fig. 3. XRD spectrum of ZnO nanorods.
259
Fig. 4. (a) TEM image (b, c) HRTEM image and (d) ED pattern of ZnO nanorods.
260
Fig. 5. UV-visible spectrum of ZnO nanorods.
261
Fig. 6. Photoluminescence spectrum of ZnO nanorods.
262
Scheme 1. Illustration of the formation of ZnO nanorods.
263 264
13
Weight loss (mg)
5
4
3
2
1
0
100
200
300
400 0
Temperature ( C) 265 266 267 268 269 270
Fig. 1.
500
600
14
Transmittance (%)
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
-1
271 272 273
Wavenumber (cm ) Fig. 2.
275
276
2000
30 40 50
274
Fig. 3.
**
60
2θ (degree) 70
(202)
(004)
(201)
(200)
(112)
(103)
(101)
6000
(110)
(100)
4000
(102)
(002)
Intensity (a.u.)
15
0 80
16
277 278 279 280
Fig. 4.
17
Absorbance (a.u.)
3
2
1
0 400
500
600
Wavelength (nm) 281 282 283
Fig. 5.
700
800
18
20
Intensity (a.u.)
15
10
5
0
350
400
450
500
Wavelength (nm) 284 285 286 287 288 289 290 291 292 293 294
Fig. 6.
550
600
19
295 296
Scheme 1.
297 298 299 300 301 302 303 304
Highlights
305 306
Low temperature thermal decomposition of zinc(II) acetylacetonate monohydrate gave zinc oxide nanorods.
307 308
Powder XRD showed hexagonal wurtzite structure of ZnO having average diameter about 24 nm.
309 310
The TEM images revealed the material to be of rod shape having diameter 30 nm and length 200 nm.
311 312
ZnO showed band gap luminescence at 356 nm, excitonic emission at 382 nm and defect related blue bands.
313 314
The synthesis is simple and can act as paradigm for obtaining various metal oxide nanomaterials.
315
Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods from zinc(II) acetylacetonate monohydrate Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee
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S0022-2313(14)00246-4 http://dx.doi.org/10.1016/j.jlumin.2014.04.007 LUMIN12648
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Journal of Luminescence
Received date: 24 February 2013 Revised date: 26 March 2014 Accepted date: 5 April 2014 Cite this article as: Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee, Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods from zinc(II) acetylacetonate monohydrate, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2014.04.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 1
Surfactant controlled low-temperature thermal decomposition route to zinc oxide nanorods
2
from zinc(II) acetylacetonate monohydrate
3
Debraj Dhar Purkayastha, Bedabrat Sarma, Chira R. Bhattacharjee*
4
Department of Chemistry, Assam University, Silchar 788011, Assam, India
5
*Corresponding author. Tel: +91-03842-270848; fax: +91-03842-270342
6
Email: [email protected]
7
Abstract
8
Zinc oxide (ZnO) nanorods were synthesized via a low-temperature thermal decomposition of
9
zinc(II) acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O. A relatively inexpensive surfactant,
10
octadecylamine (C18H37NH2) served both as reaction solvent and capping agent during the
11
synthesis of ZnO nanorods. The synthesized nanorods were characterized by powder X-ray
12
diffraction (XRD), transmission electron microscopy (TEM), FT-IR, UV-visible, and
13
photoluminescence (PL) studies. The XRD spectrum furnished evidence for the hexagonal
14
wurtzite structure of ZnO. The TEM images revealed the material to be rod shaped having
15
diameter 30 nm and length 200 nm. The HRTEM image showed the lattice fringes between the
16
two adjacent planes are 0.244 nm apart, which corresponds to the interplanar separation of the
17
(101) plane of hexagonal ZnO. The electron diffraction (ED) pattern confirmed the single
18
crystalline nature of the nanorods. The PL spectrum showed two UV emissions at 356 nm
19
(~3.48eV) and 382 nm (~3.25eV), respectively. ZnO nanorods also showed very weak blue
20
bands at 445, 453 and 470 nm, respectively.
21
Keywords: Zinc oxide; Nanorods; Thermal decomposition; Photoluminescence.
22 23
2 24
1. Introduction
25
One-dimensional (1D) ZnO nanostructures such as nanowires, nanofibers, nanotubes, and
26
nanorods has attracted enormous current interest owing to their wide applications in nanodevices
27
such as light-emitting diodes [1, 2], field-effect transistors [3, 4], ultraviolet lasers [5, 6],
28
chemical sensors [7, 8] and solar cells [9, 10]. Various chemical synthetic routes like sol-gel
29
[11], hydrothermal [12, 13], thermal decomposition [14], reverse microemulsion [15] etc. have
30
been devised for the synthesis of ZnO nanorods. Amongst all, thermal decomposition emerged as
31
a quite popular synthetic option, as it is simple, low-cost, and yields high purity materials. ZnO
32
nanostructures exhibit luminescent properties in the near ultraviolet and visible regions.
33
However, due to surface defects the emission properties of ZnO in the visible region usually
34
depend on their synthesis method. Deep hole traps associated with the presence of oxygen
35
vacancies can cause green emission above 500 nm [16]. Shorter wavelength emissions in the
36
blue region are usually related to various defects such as interstitial zinc [17] or OH- groups at
37
the surface of the particles [18]. Zinc(II) acetylacetonate has been rather efficiently utilized by
38
several researchers previously as a precursor for the synthesis of ZnO nanomaterials of various
39
sizes and shapes. Thermal decomposition of zinc(II) acetylacetonate in oleylamine has been
40
earlier shown to yield monodispersed ZnO nanoparticles [19]. ZnO nanoparticles of size 12-20
41
nm were previously accessed via thermal decomposition of zinc(II) acetylacetonate in the
42
presence of surfactants oleylamine and triphenylphosphine [20]. Gross et al. developed a facile
43
and reproducible route to nanostructured ZnO by controlling the hydrolysis and condensation of
44
zinc(II)
45
zinc(II) acetylacetonate monohydrate at and above 200oC afforded ZnO nanoparticles of size 20-
46
40 nm [22]. Another strategy employed one-pot synthesis by refluxing an oversaturated solution
acetylacetonate
in
alkaline
conditions
[21].
Thermal
decomposition
of
3 47
of zinc(II) acetylacetonate monohydrate in 1-butanol, isobutanol or tert-butanol that yielded ZnO
48
nanomaterials of varying sizes and morphologies like nanorods, coral-like structures and
49
nanospheres [23, 24]. Though, ZnO nanomaterials of various morphologies were obtained earlier
50
using zinc(II) acetylacetonate precursor under different reaction conditions there appears to be no
51
report yet describing the synthesis of ZnO nanorods via thermal decomposition of
52
zinc(II)acetylacetonate. As a part of our continued interest in accessing metal oxide
53
nanomaterials from simpler metal precursors [25-27], we report herein the synthesis of ZnO
54
nanorods via a low-temperature thermal decomposition of zinc(II) acetylacetonate monohydrate
55
in the presence of a relatively inexpensive surfactant, octadecylamine (C18H37NH2).
56
2. Experimental
57
2.1. Materials and Physical measurements
58
All chemicals used were of analytical grade and used as received. The precursor zinc(II)
59
acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O was prepared according to the literature
60
procedure [28]. FT-IR spectra were recorded on a Shimadzu Varian 4300 spectrometer on KBr
61
pellets. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris Diamond
62
thermal analyzer maintaining flow rate of 20mL/min and heating rate of 100C/min in air. X-ray
63
diffraction (XRD) measurements were carried out on a Bruker AXS D8-Advance powder X-ray
64
diffractometer with Cu-Kα radiation (λ=1.5418Å) with a scan speed of 20/min. Transmission
65
electron microscopy (TEM) images were obtained on a JEOL, JEM2100 equipment. The sample
66
powders were dispersed in n-hexane under sonication and TEM grids were prepared using a few
67
drops of the dispersion followed by drying in air. Absorption spectrum was taken with a
68
Shimadzu 1601 PC UV-visible spectrophotometer. Photoluminescence (PL) spectrum was
69
recorded on a Shimadzu RF-5301 PC spectrofluorophotometer.
4 70
2.2. Synthesis of ZnO nanorods
71
An amount of 0.6g of [Zn(C5H7O2)2].H2O and 10 ml of octadecylamine was heated in a round
72
bottomed flask for 1h at 1400C on an oil bath to get [Zn(C5H7O2)2(octadecylamine)2] complex.
73
Thereafter the reaction temperature was raised to 2000C. The light yellow solution gradually
74
became hazy indicating the formation of colloidal suspension. The solution was aged at 2000C
75
for 1h and cooled to room temperature. The white product was precipitated by adding excess
76
ethanol to the solution and washed with ethanol several times. The solid could be easily re-
77
dispersed in nonpolar organic solvents like n-hexane or toluene (Scheme 1).
78
Scheme 1.
79
3. Results and discussion
80
The current synthesis is a modified version of the procedure developed by Hyeon and others for
81
metal and oxide nanocrystals that utilizes thermal decomposition of transition metal complexes
82
[20, 29]. In the present synthesis (Scheme 1), zinc oxide nanorods were accessed by the thermal
83
decomposition of zinc(II) acetylacetonate monohydrate, [Zn(C5H7O2)2].H2O precursor in the
84
presence of surfactant octadecylamine. The decomposition temperature (2000C) has been chosen
85
based on the TGA behavior of [Zn(C5H7O2)2].H2O at which the precursor compound degraded to
86
ZnO. Octadecylamine served both as reaction media and capping agent during the synthesis of
87
nanorods. The synthesized nanorods were characterized by XRD, TEM, FT-IR, UV-visible, and
88
PL studies. The TGA curve (Fig.1) revealed that the precursor zinc(II) acetylacetonate
89
monohydrate undergo decomposition in two steps. The first step is the dehydration, which
90
commences at 700C and completes at 1000C accompanied by a weight loss of ~7.3%, due to the
91
removal of one water molecule. A further weight loss of ~74.1% in the second step in the
5 92
temperature range 115-1950C furnished clear evidence for the loss of two acetylacetonate (acac-)
93
ligands affording ZnO as end residue.
94
Fig. 1.
95
The FT-IR spectrum of zinc(II) acetylacetonate (Fig.2(a)) showed a broad band centered
96
at about 3180 cm-1 due to ν(O-H) of lattice water. The peaks at 2995 and 2920 cm-1 arose due to
97
the C-H stretching vibration ν(C-H) of the methyl group. Coupling of the stretching vibrational
98
modes of C=O and C=C groups was observed around 1600 and 1515 cm-1. The band at 420 cm-1
99
can be assigned to the stretching vibrations of Zn-O. The FT-IR spectrum of the synthesized ZnO
100
(Fig.2(b)) showed a strong band at 488 cm-1, characteristics of Zn-O stretching vibrational mode.
101
Besides, C-N stretching mode of octadecylamine was observed at 1120 cm-1. A weak broad band
102
(3100-3700 cm-1) was observed due to the N-H stretching, those at 2920 and 2845 cm-1
103
attributable to the C-H stretching mode of octadecylamine, clearly indicated some
104
octadecylamine molecules have been adsorbed on the surface of the synthesized ZnO nanorods.
105
Fig. 2.
106
The XRD spectrum (Fig.3) of the synthesized material showed diffraction peaks, which can be
107
well indexed to the hexagonal phase of ZnO. The obtained ZnO possess the wurtzite structure.
108
The relative intensities of the diffraction peaks matched well with the standard diffraction pattern
109
of crystalline ZnO (space group P63mc, JCPDS File No. 89-1397). The sharp diffraction peaks
110
also indicated high crystallinity of the material. The XRD spectrum also showed two extra peaks
111
(asterisk marked), which are not characteristics of any other impurity phases. In fact these peaks
112
correspond well with the peaks of octadecylamine reported in the literature [30]. It is pertinent to
113
mention here that the calcined material was thoroughly washed with ethanol several times to
114
remove the excess surfactant molecules. Thus, the extra peaks in the XRD spectrum may be
6 115
attributed to the surfactant octadecylamine remaining at the surfaces of ZnO nanorods. The
116
average diameter of the nanorods estimated by the Debye-Scherrer formula using a Gaussian fit
117
was around 24 nm.
118
Fig. 3.
119
The TEM image (Fig.4) of the synthesized nanomaterial revealed them to be rod shaped
120
200 nm long possessing 30 nm diameter. The surfactant octadecylamine effectively controlled
121
the size of the nanorods by limiting the growth at the nucleation stage. Since the surfaces of the
122
nanorods were capped with the surfactant octadecylamine, these could be well dispersed in most
123
hydrophobic solvents. The HRTEM image showed the lattice fringes between the two adjacent
124
planes to be 0.244 nm apart, which corresponds to the interplanar separation of the (101) plane of
125
hexagonal ZnO. This indicated one of the growth planes of the nanorods is along the (101) plane.
126
The ED pattern indicated single crystalline nature of the synthesized nanorods.
127
Fig. 4.
128
Earlier Klabunde and coworker synthesized nanocrystalline ZnO utilizing organometallic
129
compound diethylzinc (Zn(C2H5)2) as precursor [31]. The synthesis involved transformation of
130
Zn(C2H5)2 into an alkoxide prior to hydrolysis and thermal treatment. Moreover, ZnO
131
nanocrystallites were found to agglomerate into bigger spherical particles having a diameter of
132
260 nm. High-temperature thermal decomposition of Zn(C2H5)2 in the presence of
133
trioctylphosphine oxide (TOPO) and alkylamines also known to afford ZnO nanocrystals [32].
134
Wachnicki et al. compared effectiveness of dimethylzinc (Zn(CH3)2) and diethylzinc (Zn(C2H5)2)
135
as precursors for monocrystalline zinc oxide films grown by atomic layer deposition (ALD) [33].
136
Due to the pyrophoric nature of Zn(CH3)2 and Zn(C2H5)2 precursors the reactions were often
137
required to be performed under inert atmosphere. In contrast air stable zinc(II) acetylacetonate
7 138
monohydrate precursor utilized here can be easily accessed under mild reaction conditions.
139
Moreover, it is a single source precursor, which provides both zinc and oxygen species via
140
decomposition. Using metal organic chemical vapor deposition (MOCVD) technique with
141
diethylzinc (Zn(C2H5)2) precursor vertically aligned (c-axis oriented) ZnO nanorods were grown
142
on sapphire (001) substrate [34]. A very high deposition temperature (>6000C) and inert
143
condition were necessary to grow the nanorods. The nanorods were found to grow almost
144
perpendicularly on the substrate and their lengths were about 3 µm. However, ZnO nanorods
145
synthesized in the present study lacks vertical alignment and possess comparatively much
146
smaller length than those obtained by MOCVD technique [34]. The present synthesis is based on
147
low decomposition temperature (~2000C) and non-inert condition.
148
Fig. 5.
149
Fig. 6.
150
The UV-visible spectrum (Fig.5) showed a strong absorption at around 375 nm (band gap
151
= 3.31 eV), which corresponds to the bulk value of the band gap of ZnO. No blue shift was
152
observed in the spectrum of ZnO nanorods, which indicated the nanorods to be too large to show
153
any quantum confinement related effects. The strong excitonic absorption observed is possibly
154
due to the effective surface passivation of ZnO by the surfactant octadecylamine and the more
155
efficient dispersion of the nanorods. As a result light scattering is reduced causing the long
156
wavelength tail to extend up to 800 nm. A similar absorption feature was observed earlier for the
157
oleic acid passivated ZnO nanotetrapods [35]. The PL spectrum (Fig.6) of ZnO nanorods excited
158
at 320 nm showed two UV emissions at 356 nm (~3.48eV) and 382 nm (~3.25eV), respectively.
159
The emission peak at 356 nm may be attributed to the band gap luminescence as it is blue shifted
160
compared to the optical absorption. The near-band edge (NBE) emission peak at 382 nm is
8 161
assigned to the recombination of free excitons [36]. In addition to the UV emissions, a broad
162
shoulder in the range 400-425 nm and very weak defect-related blue emissions at 445, 453 and
163
470 nm were also observed. The observance of blue bands were also reported earlier in ZnO
164
nanostructures [37-40] and believed to be associated with various intrinsic or extrinsic lattice
165
defects. Due to the effective surface passivation by the surfactant octadecylamine, ZnO nanorods
166
showed very weak defect-related visible blue emissions and were less intense compared to the
167
UV emissions [41-43]. ZnO nanorods grown on Al203 (0001) substrate by the MOCVD
168
technique with Zn (C2H5)2 showed NBE emission as well as quite high defect-related visible
169
emissions [44]. In contrast, the PL spectrum of ZnO nanorods synthesized in the present study
170
showed considerably quenched defect-related visible emissions, which indicated high optical
171
quality of the nanorods. This inference was further corroborated by the XRD and TEM
172
observations. Very recently, improvement of the optical and structural properties of ZnO
173
nanorods grown by ALD on the seed ZnO nanorods has been observed [45]. A high-temperature
174
annealing is also utilized to improve the optical and structural properties of ZnO nanorods [46,
175
47]. ZnO nanorods accessed in the present study without any additional surface treatment or
176
high-temperature processing exhibited good optical and structural quality.
177
4. Conclusion
178
A low-temperature thermal decomposition of zinc(II) acetylacetonate monohydrate in the
179
presence of an inexpensive surfactant octadecylamine successfully resulted ZnO nanorods.
180
Though, zinc(II) acetylacetonate has been exploited earlier as precursor for accessing ZnO
181
nanomaterials, the formation of nanorods as reported herein appear to be the first of its kind. The
182
synthetic strategy adopted is simple and should serve as a paradigm to access other metal oxide
183
nanomaterials of specific size and morphology.
9 184
Acknowledgement
185
Authors are thankful to SAIF, NEHU, Shillong for providing TEM facility. DDP thanks
186
University Grants Commission, Government of India for Research Fellowship Scheme for
187
Meritorious Students (RFSMS). We are thankful to DBT e-Library Consortium (DeLCON) of
188
Bioinformatics Centre, Assam University, Silchar.
189
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Figure captions
256
Fig. 1. TGA curve of zinc(II) acetylacetonate monohydrate.
257
Fig. 2. FT-IR spectrum of (a) zinc(II) acetylacetonate monohydrate and (b) ZnO nanorods.
258
Fig. 3. XRD spectrum of ZnO nanorods.
259
Fig. 4. (a) TEM image (b, c) HRTEM image and (d) ED pattern of ZnO nanorods.
260
Fig. 5. UV-visible spectrum of ZnO nanorods.
261
Fig. 6. Photoluminescence spectrum of ZnO nanorods.
262
Scheme 1. Illustration of the formation of ZnO nanorods.
263 264
13
Weight loss (mg)
5
4
3
2
1
0
100
200
300
400 0
Temperature ( C) 265 266 267 268 269 270
Fig. 1.
500
600
14
Transmittance (%)
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
-1
271 272 273
Wavenumber (cm ) Fig. 2.
275
276
2000
30 40 50
274
Fig. 3.
**
60
2θ (degree) 70
(202)
(004)
(201)
(200)
(112)
(103)
(101)
6000
(110)
(100)
4000
(102)
(002)
Intensity (a.u.)
15
0 80
16
277 278 279 280
Fig. 4.
17
Absorbance (a.u.)
3
2
1
0 400
500
600
Wavelength (nm) 281 282 283
Fig. 5.
700
800
18
20
Intensity (a.u.)
15
10
5
0
350
400
450
500
Wavelength (nm) 284 285 286 287 288 289 290 291 292 293 294
Fig. 6.
550
600
19
295 296
Scheme 1.
297 298 299 300 301 302 303 304
Highlights
305 306
Low temperature thermal decomposition of zinc(II) acetylacetonate monohydrate gave zinc oxide nanorods.
307 308
Powder XRD showed hexagonal wurtzite structure of ZnO having average diameter about 24 nm.
309 310
The TEM images revealed the material to be of rod shape having diameter 30 nm and length 200 nm.
311 312
ZnO showed band gap luminescence at 356 nm, excitonic emission at 382 nm and defect related blue bands.
313 314
The synthesis is simple and can act as paradigm for obtaining various metal oxide nanomaterials.
315