Materials Research Bulletin Manuscript Draft Manuscript Number: MRB-10-1004R1 Title: Fabrication of PbS nanoparticle coated amorphous carbon nanotubes: Structural, thermal and field emission properties Article Type: Research Paper Keywords: A. Chalcogenides, Composite; B. Chemical Synthesis; C. Electron microscopy; D. Microstructure Abstract: A simple chemical route for the synthesis of PbS nanoparticle coated amorphous carbon nanotubes (aCNTs) was described. The nanocomposite was prepared from an aqueous suspension of acid functionalized aCNTs, lead acetate (PbAc), and thiourea (TU) at room temperature. The phase formation and composition of the samples were characterized by X-ray diffraction and energy dispersive analysis of X-ray studies. The Fourier transformed infrared spectra analysis revealed the attachment of PbS nanoparticles on the acid functionalized aCNT surfaces. Morphology of the samples was analyzed with field emission scanning electron microscopy. UV-Vis study also confirmed the attachment of PbS nanoparticles on the walls of aCNTs. Thermal gravimetric analysis showed that the PbS coated aCNTs are more thermally stable than functionalized aCNTs. The PbS coated aCNTs showed enhanced field emission properties with a turn-on field 3.34 V µm-1 and the result is comparable to that of pure crystalline CNTs.
1
Fabrication of PbS nanoparticle coated amorphous carbon
nanotubes: Structural, thermal and field emission properties
S. Jana
a), D. Banerjee
b), A. Jha
b) and K. K. Chattopadhyay
a,b)*
a)Thin Films and Nanoscience Laboratory, Department of Physics
b)School of Material Science and Nanotechnology
Jadavpur University, Kolkata 700 032, India
Abstract
A simple chemical route for the synthesis of PbS nanoparticle coated amorphous carbon
nanotubes (aCNTs) was described. The nanocomposite was prepared from an aqueous
suspension of acid functionalized aCNTs, lead acetate (PbAc), and thiourea (TU) at room
temperature. The phase formation and composition of the samples were characterized by X-ray
diffraction and energy dispersive analysis of X-ray studies. The Fourier transformed infrared
spectra analysis revealed the attachment of PbS nanoparticles on the acid functionalized aCNT
surfaces. Morphology of the samples was analyzed with field emission scanning electron
microscopy. UV-Vis study also confirmed the attachment of PbS nanoparticles on the walls of
aCNTs. Thermal gravimetric analysis showed that the PbS coated aCNTs are more thermally
stable than functionalized aCNTs. The PbS coated aCNTs showed enhanced field emission
properties with a turn-on field 3.34 V µm-1
and the result is comparable to that of pure crystalline
CNTs.
Keywords: A. Chalcogenides, Composite; B. Chemical Synthesis; C. Electron microscopy; D.
Microstructure
…………………………………………………………………………………
*Corresponding author. [email protected]
1. REVISED MANUSCRIPT: ALL CHANGES IN BOLDClick here to view linked References
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1. Introduction
Carbon nanotubes (CNTs) have attracted wide interest over the past decade due to their
remarkable structural, mechanical, thermal and electrical properties [1, 2]. Also it can be used as
a very good material for forming different composites [3]. So far research works were mainly
focused on the crystalline CNTs such as with single and multiwall CNTs with perfect concentric
graphene layers. But the introduction of defects in the carbon networks are expected to lead more
interesting properties and hence developments of new potential nanodevices. For example, this
type of nanotubes displayed a semiconductor band gap that is inversely proportional to the
diameter compared to the corresponding crystalline tubes [4]. They could be used in
nanoelectronics and sensor devices due to the absence of chirality problems [5]. Also all the
conventional methods for producing crystalline CNTs need suitable catalysts and a high
temperature. Recently, amorphous carbon nanotubes (aCNTs) become another focus of
researchers because of their low temperature synthesis process and large yield of production [6,
7]. In our previous work we have reported a low temperature and large scale synthesis of
amorphous carbon nanoneedle like structure where many nanotubes were agglomerated to form
those needles and studied their field emission properties [8]. Moreover, there is a great research
interest in attaching organic and inorganic compounds on the surfaces of CNTs to optimize their
performance in various potential applications [9, 10]. Especially nanocrystalline semiconductors
have attracted much attention in decorating CNTs due to their size dependent tunable optical,
structural and electronic properties, which may provide novel nanocomposites with the combined
properties of two functional nanoscale materials to achieve wide range of applications. So far
various semiconductor nanocrystals such as CdS [11], TiO2 [12], CdSe/ZnS [13], ZnO [14, 15],
SnO2 [16], ZnS [17] and PbSe [18] have been attached on the surfaces of CNTs. PbS is one of
3
the most important IV-VI semiconductors because of its large exciton Bohr radius and relatively
narrow band gap which can be blue shifted from the near infrared (IR) to the visible region by
forming nanocrystallites [19]. Consequently, PbS nanoparticles have exhibited novel and
excellent optical and electrical properties and applications in nonlinear optical devices such as IR
detectors [20], display devices [21], Pb2+
ion selective sensors [22] and solar control coatings
[23]. PbS nanoparticles can be used in electroluminescent devices such as light emitting diodes
and in optical devices such as optical switches due to their exceptional third order nonlinear
optical properties [24]. Therefore it is well expected that the composites of CNTs and PbS may
have excellent optical and electrical properties. There are very few reports concerning the
fabrication of PbS nanoparticle modified CNTs [25]. Recently, Fernandes et al. [26, 27] have
reported PbS filled multiwalled CNTs for multiband infrared detection. In all the cases,
crystalline CNTs were taken for the composite formation and more importantly, the thermal and
field emission properties of PbS nanoparticle coated aCNTs have not been investigated yet.
Herein, we have reported a very simple low temperature chemical method for large scale
synthesis of aCNTs and for the first time PbS nanoparticle coated on the surfaces of these
aCNTs. The structural, thermal and field emission properties of the PbS/aCNTs heterostructure
have been studied in details. It is seen that when PbS nanoparticle is attached with the aCNTs
both the thermal and field emission properties improved significantly and field emission become
comparable to that of pure crystalline CNTs.
2. Experimental
2.1 Synthesis
The large scale preparation of aCNTs has been reported elsewhere [8]. Briefly speaking,
2 gm of ferrocene and 4 g of ammonium chloride were mixed and taken in an alumina boat.
4
After being heated at 250 oC for 30 min. in an air furnace, the boat was allowed to cool naturally
and a black powder was obtained. Then the powder was washed with dilute HCl and di-ionized
water consecutively for several times and finally dried at 80 oC for 24 h. The obtained purified
aCNTs were functionalized by immersing in sulfuric-nitric acid solution (H2SO4:HNO3,
volumetric ratio=3:1) at room temperature and treated in an ultrasonic bath for 2 h followed by
14 h of constant stirring. After that HCl was added to the solution and subsequently, this
solution was neutralized with ammonium hydroxide and filtered. The aCNTs were washed
several times with deionized water and then dried at 80 oC for 24 h. This acid treatment method
inserts carboxyl groups (-COOH) on the surface of aCNTs and helped aCNTs to get easily
dispersed in water.
The coating of PbS nanoparticle on the surfaces of functionalized aCNTs (f-aCNTS) was
achieved by reaction between Pb(CH3COOH)2 and thiourea in f-aCNTs dispersed aqueous
solution. 25 mg of f-aCNTs were dispersed in 25 ml deionized water by ultrasonication. Then
0.62 mmol of lead acetate was added with the previous f-aCNTs dispersed deionized water
solution and stirred by a magnetic stirrer at 60 °C. After 10 min, 1.47 mmol of thiourea
(NH2CSNH2) was added to it while the solution was continuously stirred for 2 h. The obtained
black precipitate was filtered, washed with distilled water and dried at 70 oC.
2.2 Characterizations
X-ray diffraction pattern of the as prepared samples were recorded on a BRUKER D8
ADVANCE X-ray diffractometer using CuKα radiation of wavelength λ= 1.54 Å in the 2θ range
20o – 70
o. The detail morphological features were investigated using Hitachi S-4800 field
emission scanning electron microscope (FESEM). The composition of aCNTs-PbS sample was
analyzed by energy dispersive X-ray analysis (EDX) system equipped with the FESEM. The
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Fourier transformed infrared spectra were recorded on a Shimadzu-8400S FT-IR spectrometer
using a KBr-disc method. Transmittance spectra were measured in a Perkin-Elmer lambda 35
UV-Vis spectrophotometer. Thermogravimetric analyses (TGA) were carried out using a Pyris
Diamond TG/DTA (Perkin-Elmer) thermal analyzer. The field emission measurements were
performed by using home made high vacuum field emission set up.
3. Results and discussion
3.1. Morphological analysis/FESEM study
The morphologies of the as-prepared aCNTs (pristine aCNTs), f-aCNTs and PbS coated
aCNTs samples were investigated from FESEM images. Fig. 1a and b show the FESEM images
of pristine aCNTs having average diameter of ~100-120 nm and a length several microns. The
tubular structure of each aCNTs is also clearly visible from fig. 1b with inner diameter ~ 60-80
nm and outer diameter of 90-120 nm. The FESEM images of f-aCNTs are shown in Fig. 1c and
d. It is observed from Fig. 1d that the diameters of the tubes increase after acid treatment and
become 120-150 nm which suggest the attachment of acid functional groups on the surfaces of
aCNTs. At present the exact reason for the diameter increment after functional group
attachment to CNTs is not known. Swelling of the CNTs walls due to interactions of the
functional groups with carbon is one of the responsible factors. There are some reports
indicating the increase of diameter of carbon nanotubes after acid oxidation. Recently Jha
et al. [28] reported the increment of the diameter of a-CNTs after stearic acid treatment.
Chen et al. [29] also functionalized multiwalled carbon nanotubes with poly (L- lactic acid)
(PLLA) and the PLLA coated MWNT became thicker and more uniform.
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Fig. 1e and f show the FESEM images of the aCNTs-PbS samples. It is clearly observed
from the Fig. 1e that the dense PbS nanocrystals are attached on the surface of aCNTs and the
external diameter of the PbS coated aCNTs is nearly 200-300 nm. This observation also
demonstrates that a relatively thick layer of PbS nanoparticles are attached on the aCNTs
surfaces. The highly magnified FESEM image (Fig. 1f) displays the morphology of the attached
PbS nanocrystals in detail. Most of the PbS nanocrystals are cubic in shape with average side
length 50 nm.
3.2. XRD and EDAX studies
Fig. 2 shows the X-ray diffraction patterns of the as-prepared samples. Fig. 2a shows the
XRD pattern of acid treated aCNTs which confirms the amorphous nature of the sample. The
XRD patterns of the PbS coated aCNTs (aCNT-PbS) (Fig. 2b) and pure PbS powder (Fig. 2c)
show several strong diffraction peaks at 2θ values of 25.9°, 30o, 43
o, 51
o, 53.3
o, 62.4
o and 68.8
o.
All these are assigned to the diffraction lines originated from (111), (200), (220), (311), (222),
(400) and (331) planes of the face-centre-cubic rock salt structured PbS with a lattice constant a
= 0.593 nm (JCPDS card File No 78-1901). These results confirm the formation of PbS crystals
on the surfaces of aCNTs. It is also observed that the intensity of the peaks corresponding to pure
PbS is much stronger than that of aCNTs-PbS sample which further indicates the formation of
PbS coated aCNTs sample.
The elemental composition of the aCNTs-PbS sample was obtained using energy dispersive
analysis of X-ray (EDAX) spectrum as shown in Fig. 3. The strong peaks, attributed to C, Pb and
S are clearly present in the EDAX spectrum and no other impurities were detected in the
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spectrum, confirming high purity of the aCNTs-PbS product. Moreover, according to
quantitative analysis of EDAX, the molar ratio of Pb to S is 1:1.09, which is almost consistent
with stoichiometric PbS within experimental error. It should be mentioned that presence of
carbon peak in the EDX spectra may also originate from carbon-based contaminations on
the sample or from SEM chamber. Hence presence of C was further confirmed by FTIR
study. Also the peaks for Pb and S are found to be very close in the EDX spectrum and
hence the atomic percentages as given by the EDX software may have ±10 % error.
3.3. FTIR study
Fig. 4a represents the FTIR spectrum of the f-aCNTs which exhibits a number of
characteristic peaks such as: sp3 hybridized (CH3)3 group [30] at 1395 cm
-1, sp
2 hybridized
C=C bond at around 1620 cm-1
, symmetrical and asymmetrical stretching of –CH2 vibrational
bonds at 2858 and 2922 cm-1
. Another peak appeared at 1730 cm-1
indicates the introduction
of carboxyl C=O groups [31] on the surfaces of aCNTs due to surface oxidation by
concentrated acids. This peak disappears after coating aCNTs with PbS (Fig. 4b) suggesting
PbS is attached to the aCNTs surfaces on carboxyl groups. Moreover, two extra weak peaks at
990 cm-1
and 1160 cm-1
for Pb-S bond [32] are detected. Generally, the Pb-S bond is mainly an
electrovalent bond so the FTIR spectra containing PbS do not show strong bands associated with
Pb-S stretching and bending vibrations. Peaks observed at 2360 cm-1
in all the spectra are due to
the presence of CO2 due to contamination from atmosphere and another intense peak, at 3450
cm-1
corresponds to the deformation vibrations of H2O due to the absorbed water molecule by
KBr matrix.
3.4. UV-Vis spectroscopic study
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The UV-Vis transmittance measurements of the as prepared samples were carried out
with the samples dispersed in alcohol at room temperature. Fig. 5 (a-d) shows the alcohol
dispersion of pristine aCNTs, f-aCNTs, only PbS nanoparticles and aCNTs-PbS respectively
after 2 h ultrasonication followed by keeping them overnight undisturbed. It is clearly seen that
both the pristine aCNTs (Fig. 5a) and PbS nanoparticles (Fig. 5c) get precipitated because of the
large Van der Waal force. But f-aCNTs (Fig. 5b) and aCNTs-PbS (Fig. 5d) remain well
dispersed due to functionalization of CNTs. Also better dispersion in water or in alcohol is an
indirect proof of successful functionalization of CNTs. Fig. 6 illustrates the UV-vis transmittance
spectra of f-aCNT, PbS nanoparticles and aCNT-PbS samples. The UV-vis spectrum of f-aCNTs
shows a typically featureless profile which is consistent with the previous report [33]. In contrast,
the transmittance spectrum of PbS nanoparticles shows absorption edges at around 240 nm and
320 nm. These results showed a large blue shift from the direct band gap (0.41 eV) of bulk
PbS crystals. Because the size of the sample (~ 50 nm) is bigger than the Bohr exciton
radius of the PbS (18 nm), hence strong quantum confinement is not possible, although
weak confinement may takes place. But that cannot explain so large shift in the band gap.
Small effective mass of the carriers in nanocrystalline PbS is one of the reasons for such
blue shift. Similar large shifts were reported by many other groups. Cao et al. [34]
observed a large blue shift in its absorption edge to the UV region ( 5.04-4.57 eV) for PbS
nanocubes with average size of about 80-40 nm due to the small effective mass of PbS. The
spectrum of aCNTs-PbS sample gives a similar absorption edges attributed to that of the PbS
nanoparticles. Similar kind of results was also obtained by Feng et al. [11] in their CdS
nanoparticle coated MWCNTs samples. They have attributed these results to the fact that there
was no charge diffusion or electronic interaction between the MWCNTs and CdS NPs in their
9
ground state. Here also we can conclude that the PbS nanoparticles attached to the f-aCNTs
causing no significant modification of the energy states of the f-aCNTs.
3.5. Thermal study
The thermal stability studies were performed from weight loss measurements by using
thermogravimetric analysis (TGA) in the temperature range 30-900 oC at a heating rate of 15
oC/
min. in N2 atmosphere. Fig. 7 shows a comparison of mass losses of pure PbS, f-aCNTs and
aCNT-PbS composites. For reference the pure PbS sample was prepared via the reaction
between lead acetate and thiourea in aqueous solution without using f-aCNTs keeping all
other conditions the same as used in coating the aCNTs. Pure PbS samples are very stable and
weight loss of 15 wt. % is observed in the range of 30-900 oC. f-aCNTs display a sudden
decrease in mass of 10 % at a temperature of 100 o
C due to water vapor. After that a steady
decease of mass occurs in the range of 100-900 oC and weight percentage decreases to 27.2 wt.
%, but, in case of aCNTs-PbS the weight percentage dropped from 99.9 % to 64 wt. %. This
result indicates that aCNTs become more thermally stable after coated with PbS nanoparticles.
3.6. Field emission study
The macroscopic electric field (E) is obtained from the applied voltage divided by the
inter-electrode distance. The simplified F-N equation for field electron emission is given by [35,
36]
]/[ln/ln 12/3212 EsbraEJ (1)
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Here J is the macroscopic current density, ϕ is the local work function, β the field
enhancement factor, a and b are respectively the 1st and 2
nd Fowler-Nordheim constants
having values a=1.54 × 10-6
A eV V-2
and b = 6.83 × 109 eV
-3/2 V m
-1. The plot of lnJ/E
2 vs.
1/E should be a straight line and r and s are appropriate values of the intercept and slope
correction factors, respectively. Typically, s is of the order of unity, but r may be of order
100 or greater. 8a shows the experimental J-E curves for pristine aCNTs, f-aCNTs and aCNTs-
PbS. In inset of Fig. 8a, shows the same for pristine aCNTs separately. The corresponding F-N
plots are shown in Fig. 8b. The straight line nature of the F-N plots suggests that electrons are
emitted due to cold field emission. It is seen that the field emission characteristics get much
enhanced in case of acid treated CNTs compared to the pristine CNTs and becomes the best for
aCNTs-PbS. There are some reports related with the improvement of field emission properties
for crystalline CNTs when attached with nanocrystals. For example, Uh et al. found similar
results for their titanium coated CNTs [37]. Again Chen et al. have shown that field emission
characteristics of CNTs can get better if a thin film of RuO2 is deposited over it [38]. The turn-on
field (ET), which we defined as the macroscopic field needed to get an emission current density 2
µA/cm2 for three samples is shown in Table 1. The enhancement factor β and effective work
function φeff for all these samples are obtained from the slopes (m) of F-N plots using the
relation:
β = -bφ3/2
/m (2.a)
φeff = φ/ β2/3
(2.b)
taking φ = 5 and 3.9 eV for carbon materials and PbS respectively. The values of these two
parameters are also shown in Table 1. It is noteworthy that f-aCNTs shows better field emission
than pristine aCNTs, due to the fact that, as-prepared aCNTs remained agglomerated (Fig. 4a–d)
11
thus the field screening effect due to close proximity is much greater which reduces the field
emission activity. When pristine aCNTs are treated with concentric acid the agglomeration is
much lesser thus screening effect also reduces resulting better field emission. The further
enhancement of field emission for the aCNTs-PbS samples may be attributed to the fact that
when aCNTs are coated with PbS nanoparticles, the roughness of the individual tubes enhanced
and thus the geometrical enhancement of the field is much higher as shown by Chen et al [35]. A
lesser value of work function and electron affinity for PbS compared to that of CNT are the other
key factors for the observed improved field emission from aCNTs-PbS. Very recently Chen et al.
[39] has found a much improved field emission from their IrO2 coated CNT field emitter arrays
and attributed this fact to both the lower work function of IrO2 and an improved field
enhancement factor after coating with IrO2. The coating or attachment of nanoparticles of a
suitable material on crystalline CNTs is a conventional process to obtain an improved field
emission from CNTs. The same technique may be employed to aCNTs and here CNTs-PbS
composite nanostructure showed comparable field emission properties with that of crystalline
CNTs [40]. Table 2 summarizes some of the recent field emission data from crystalline CNTs
field emitters for comparing the obtained result.
4. Conclusions
PbS nanoparticles coated aCNTs were synthesized by a simple chemical method. The
structure, composition, bonding information and morphology of the as prepared samples were
characterized by XRD, EDAX, FTIR and FESEM analysis. UV-Vis spectroscopic analysis
showed that the PbS nanoparticles are attached to the aCNTs and this does not cause a significant
modification of the energy states of the f-aCNTs. The PbS coated aCNTs are found to be more
12
thermally stable compared to f-aCNTs after attachment of PbS nanoparticles and thus suitable
for high temperature device fabrication. The field emission characteristics have been studied
extensively and it is seen that the PbS coated aCNTs give improved field emission and the result
is almost comparable to crystalline CNTs.
Acknowledgements
The authors wish to thank the University Grants Commission (UGC), the Government of India
for financial support.
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Table caption
Table 1. Different parameters of field emission for aCNTs, f-aCNTs and aCNTs-PbS samples
Table 2. Comparison of field emission characteristics of CNT based composites reported by
other groups to that of present work.
15
Table 1
Sample Turn-on field Enhancement Effective work
[J= 2µ A/cm2] (V/µm) factor β function φeff (eV)
Pristine aCNTs 18.7 3054 0.023
Acid treated f-aCNTs 4.86 7636 0.013
Pure PbS 4.28 5950 0.012
aCNT-PbS 3.34 10522 0.008
Table 2
Sample Turn on field (V/µm) Defined at Reference
CNT Composites
CNT-ZnO 5.58 3.72
I= 1 µA [14]
Ti-CNT 2.8 2 J= 0.1µA/cm2 [31]
RuO2-CNT 5 3.3 J= 1mA/cm2 [32]
IrO2-CNT 1.4 0.7 J= 0.1µA/cm2 [33]
aCNTs-PbS 18.7 3.34 J= 2 µA/ cm2 Present Work
16
Figure captions
Fig. 1. FESEM images of: (a) and (b) pristine aCNTs, (c) and (d) f-aCNTs, (e) and (f) aCNT-PbS
Fig. 2. XRD patterns of: (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS nanoparticles
Fig. 3. EDAX spectrum of PbS coated aCNTs sample
Fig. 4. FTIR spectra of: (a) f-aCNTs and (b) aCNT -PbS
Fig. 5. Dispersion of (a) Pristine aCNTs, (b) f-aCNTs, (c) PbS nanoparticles and (d) aCNT-PbS
into alcohol
Fig. 6. UV-Vis transmittance spectra of (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS
nanoparticles (Inset shows transmittance spectrum of pure PbS separately)
Fig. 7. TGA curves of (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS nanoparticles
Fig. 8. (a) Plot of emission current density (J) versus macroscopic field (E) for pristine
aCNTs, f-aCNTs, PbS and aCNT-PbS (Inset shows the J-E curve for pristine aCNTs
separately for determining the turn-on field) and (b) corresponding F-N plots
17
Fig. 1. S. Jana et al.
18
Fig. 2 S. Jana et al
19
Fig. 3 S. Jana et al.
.
20
Fig. 4 S. Jana et al.
(b)
21
Fig. 5 S. Jana et al.
a b
c d
Precipitation
Precipitation
22
Fig. 6 S. Jana et al.
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Fig. 7 S. Jana et al.
24
Fig. 8 S. Jana et al.
1
Fabrication of PbS nanoparticle coated amorphous carbon
nanotubes: Structural, thermal and field emission properties
S. Jana
a), D. Banerjee
b), A. Jha
b) and K. K. Chattopadhyay
a,b)*
a)Thin Films and Nanoscience Laboratory, Department of Physics
b)School of Material Science and Nanotechnology
Jadavpur University, Kolkata 700 032, India
Abstract
A simple chemical route for the synthesis of PbS nanoparticle coated amorphous carbon
nanotubes (aCNTs) was described. The nanocomposite was prepared from an aqueous
suspension of acid functionalized aCNTs, lead acetate (PbAc), and thiourea (TU) at room
temperature. The phase formation and composition of the samples were characterized by X-ray
diffraction and energy dispersive analysis of X-ray studies. The Fourier transformed infrared
spectra analysis revealed the attachment of PbS nanoparticles on the acid functionalized aCNT
surfaces. Morphology of the samples was analyzed with field emission scanning electron
microscopy. UV-Vis study also confirmed the attachment of PbS nanoparticles on the walls of
aCNTs. Thermal gravimetric analysis showed that the PbS coated aCNTs are more thermally
stable than functionalized aCNTs. The PbS coated aCNTs showed enhanced field emission
properties with a turn-on field 3.34 V µm-1
and the result is comparable to that of pure crystalline
CNTs.
Keywords: A. Chalcogenides, Composite; B. Chemical Synthesis; C. Electron microscopy; D.
Microstructure
…………………………………………………………………………………
*Corresponding author. [email protected]
*2. REVISED MANUSCRIPT: CHANGES NOT IN BOLDClick here to view linked References
2
1. Introduction
Carbon nanotubes (CNTs) have attracted wide interest over the past decade due to their
remarkable structural, mechanical, thermal and electrical properties [1, 2]. Also it can be used as
a very good material for forming different composites [3]. So far research works were mainly
focused on the crystalline CNTs such as with single and multiwall CNTs with perfect concentric
graphene layers. But the introduction of defects in the carbon networks are expected to lead more
interesting properties and hence developments of new potential nanodevices. For example, this
type of nanotubes displayed a semiconductor band gap that is inversely proportional to the
diameter compared to the corresponding crystalline tubes [4]. They could be used in
nanoelectronics and sensor devices due to the absence of chirality problems [5]. Also all the
conventional methods for producing crystalline CNTs need suitable catalysts and a high
temperature. Recently, amorphous carbon nanotubes (aCNTs) become another focus of
researchers because of their low temperature synthesis process and large yield of production [6,
7]. In our previous work we have reported a low temperature and large scale synthesis of
amorphous carbon nanoneedle like structure where many nanotubes were agglomerated to form
those needles and studied their field emission properties [8]. Moreover, there is a great research
interest in attaching organic and inorganic compounds on the surfaces of CNTs to optimize their
performance in various potential applications [9, 10]. Especially nanocrystalline semiconductors
have attracted much attention in decorating CNTs due to their size dependent tunable optical,
structural and electronic properties, which may provide novel nanocomposites with the combined
properties of two functional nanoscale materials to achieve wide range of applications. So far
various semiconductor nanocrystals such as CdS [11], TiO2 [12], CdSe/ZnS [13], ZnO [14, 15],
SnO2 [16], ZnS [17] and PbSe [18] have been attached on the surfaces of CNTs. PbS is one of
3
the most important IV-VI semiconductors because of its large exciton Bohr radius and relatively
narrow band gap which can be blue shifted from the near infrared (IR) to the visible region by
forming nanocrystallites [19]. Consequently, PbS nanoparticles have exhibited novel and
excellent optical and electrical properties and applications in nonlinear optical devices such as IR
detectors [20], display devices [21], Pb2+
ion selective sensors [22] and solar control coatings
[23]. PbS nanoparticles can be used in electroluminescent devices such as light emitting diodes
and in optical devices such as optical switches due to their exceptional third order nonlinear
optical properties [24]. Therefore it is well expected that the composites of CNTs and PbS may
have excellent optical and electrical properties. There are very few reports concerning the
fabrication of PbS nanoparticle modified CNTs [25]. Recently, Fernandes et al. [26, 27] have
reported PbS filled multiwalled CNTs for multiband infrared detection. In all the cases,
crystalline CNTs were taken for the composite formation and more importantly, the thermal and
field emission properties of PbS nanoparticle coated aCNTs have not been investigated yet.
Herein, we have reported a very simple low temperature chemical method for large scale
synthesis of aCNTs and for the first time PbS nanoparticle coated on the surfaces of these
aCNTs. The structural, thermal and field emission properties of the PbS/aCNTs heterostructure
have been studied in details. It is seen that when PbS nanoparticle is attached with the aCNTs
both the thermal and field emission properties improved significantly and field emission become
comparable to that of pure crystalline CNTs.
2. Experimental
2.1 Synthesis
The large scale preparation of aCNTs has been reported elsewhere [8]. Briefly speaking,
2 gm of ferrocene and 4 g of ammonium chloride were mixed and taken in an alumina boat.
4
After being heated at 250 oC for 30 min. in an air furnace, the boat was allowed to cool naturally
and a black powder was obtained. Then the powder was washed with dilute HCl and di-ionized
water consecutively for several times and finally dried at 80 oC for 24 h. The obtained purified
aCNTs were functionalized by immersing in sulfuric-nitric acid solution (H2SO4:HNO3,
volumetric ratio=3:1) at room temperature and treated in an ultrasonic bath for 2 h followed by
14 h of constant stirring. After that HCl was added to the solution and subsequently, this
solution was neutralized with ammonium hydroxide and filtered. The aCNTs were washed
several times with deionized water and then dried at 80 oC for 24 h. This acid treatment method
inserts carboxyl groups (-COOH) on the surface of aCNTs and helped aCNTs to get easily
dispersed in water.
The coating of PbS nanoparticle on the surfaces of functionalized aCNTs (f-aCNTS) was
achieved by reaction between Pb(CH3COOH)2 and thiourea in f-aCNTs dispersed aqueous
solution. 25 mg of f-aCNTs were dispersed in 25 ml deionized water by ultrasonication. Then
0.62 mmol of lead acetate was added with the previous f-aCNTs dispersed deionized water
solution and stirred by a magnetic stirrer at 60 °C. After 10 min, 1.47 mmol of thiourea
(NH2CSNH2) was added to it while the solution was continuously stirred for 2 h. The obtained
black precipitate was filtered, washed with distilled water and dried at 70 oC.
2.2 Characterizations
X-ray diffraction pattern of the as prepared samples were recorded on a BRUKER D8
ADVANCE X-ray diffractometer using CuKα radiation of wavelength λ= 1.54 Å in the 2θ range
20o – 70
o. The detail morphological features were investigated using Hitachi S-4800 field
emission scanning electron microscope (FESEM). The composition of aCNTs-PbS sample was
analyzed by energy dispersive X-ray analysis (EDX) system equipped with the FESEM. The
5
Fourier transformed infrared spectra were recorded on a Shimadzu-8400S FT-IR spectrometer
using a KBr-disc method. Transmittance spectra were measured in a Perkin-Elmer lambda 35
UV-Vis spectrophotometer. Thermogravimetric analyses (TGA) were carried out using a Pyris
Diamond TG/DTA (Perkin-Elmer) thermal analyzer. The field emission measurements were
performed by using home made high vacuum field emission set up.
3. Results and discussion
3.1. Morphological analysis/FESEM study
The morphologies of the as-prepared aCNTs (pristine aCNTs), f-aCNTs and PbS coated
aCNTs samples were investigated from FESEM images. Fig. 1a and b show the FESEM images
of pristine aCNTs having average diameter of ~100-120 nm and a length several microns. The
tubular structure of each aCNTs is also clearly visible from fig. 1b with inner diameter ~ 60-80
nm and outer diameter of 90-120 nm. The FESEM images of f-aCNTs are shown in Fig. 1c and
d. It is observed from Fig. 1d that the diameters of the tubes increase after acid treatment and
become 120-150 nm which suggest the attachment of acid functional groups on the surfaces of
aCNTs. At present the exact reason for the diameter increment after functional group attachment
to CNTs is not known. Swelling of the CNTs walls due to interactions of the functional groups
with carbon is one of the responsible factors. There are some reports indicating the increase of
diameter of carbon nanotubes after acid oxidation. Recently Jha et al. [28] reported the
increment of the diameter of a-CNTs after stearic acid treatment. Chen et al. [29] also
functionalized multiwalled carbon nanotubes with poly (L- lactic acid) (PLLA) and the PLLA
coated MWNT became thicker and more uniform.
6
Fig. 1e and f show the FESEM images of the aCNTs-PbS samples. It is clearly observed
from the Fig. 1e that the dense PbS nanocrystals are attached on the surface of aCNTs and the
external diameter of the PbS coated aCNTs is nearly 200-300 nm. This observation also
demonstrates that a relatively thick layer of PbS nanoparticles are attached on the aCNTs
surfaces. The highly magnified FESEM image (Fig. 1f) displays the morphology of the attached
PbS nanocrystals in detail. Most of the PbS nanocrystals are cubic in shape with average side
length 50 nm.
3.2. XRD and EDAX studies
Fig. 2 shows the X-ray diffraction patterns of the as-prepared samples. Fig. 2a shows the
XRD pattern of acid treated aCNTs which confirms the amorphous nature of the sample. The
XRD patterns of the PbS coated aCNTs (aCNT-PbS) (Fig. 2b) and pure PbS powder (Fig. 2c)
show several strong diffraction peaks at 2θ values of 25.9°, 30o, 43
o, 51
o, 53.3
o, 62.4
o and 68.8
o.
All these are assigned to the diffraction lines originated from (111), (200), (220), (311), (222),
(400) and (331) planes of the face-centre-cubic rock salt structured PbS with a lattice constant a
= 0.593 nm (JCPDS card File No 78-1901). These results confirm the formation of PbS crystals
on the surfaces of aCNTs. It is also observed that the intensity of the peaks corresponding to pure
PbS is much stronger than that of aCNTs-PbS sample which further indicates the formation of
PbS coated aCNTs sample.
The elemental composition of the aCNTs-PbS sample was obtained using energy dispersive
analysis of X-ray (EDAX) spectrum as shown in Fig. 3. The strong peaks, attributed to C, Pb and
S are clearly present in the EDAX spectrum and no other impurities were detected in the
7
spectrum, confirming high purity of the aCNTs-PbS product. Moreover, according to
quantitative analysis of EDAX, the molar ratio of Pb to S is 1:1.09, which is almost consistent
with stoichiometric PbS within experimental error. It should be mentioned that presence of
carbon peak in the EDX spectra may also originate from carbon-based contaminations on the
sample or from SEM chamber. Hence presence of C was further confirmed by FTIR study. Also
the peaks for Pb and S are found to be very close in the EDX spectrum and hence the atomic
percentages as given by the EDX software may have ±10 % error.
3.3. FTIR study
Fig. 4a represents the FTIR spectrum of the f-aCNTs which exhibits a number of
characteristic peaks such as: sp3 hybridized (CH3)3 group [30] at 1395 cm
-1, sp
2 hybridized C=C
bond at around 1620 cm-1
, symmetrical and asymmetrical stretching of –CH2 vibrational bonds at
2858 and 2922 cm-1
. Another peak appeared at 1730 cm-1
indicates the introduction of carboxyl
C=O groups [31] on the surfaces of aCNTs due to surface oxidation by concentrated acids. This
peak disappears after coating aCNTs with PbS (Fig. 4b) suggesting PbS is attached to the aCNTs
surfaces on carboxyl groups. Moreover, two extra weak peaks at 990 cm-1
and 1160 cm-1
for Pb-
S bond [32] are detected. Generally, the Pb-S bond is mainly an electrovalent bond so the FTIR
spectra containing PbS do not show strong bands associated with Pb-S stretching and bending
vibrations. Peaks observed at 2360 cm-1
in all the spectra are due to the presence of CO2 due to
contamination from atmosphere and another intense peak, at 3450 cm-1
corresponds to the
deformation vibrations of H2O due to the absorbed water molecule by KBr matrix.
3.4. UV-Vis spectroscopic study
8
The UV-Vis transmittance measurements of the as prepared samples were carried out
with the samples dispersed in alcohol at room temperature. Fig. 5 (a-d) shows the alcohol
dispersion of pristine aCNTs, f-aCNTs, only PbS nanoparticles and aCNTs-PbS respectively
after 2 h ultrasonication followed by keeping them overnight undisturbed. It is clearly seen that
both the pristine aCNTs (Fig. 5a) and PbS nanoparticles (Fig. 5c) get precipitated because of the
large Van der Waal force. But f-aCNTs (Fig. 5b) and aCNTs-PbS (Fig. 5d) remain well
dispersed due to functionalization of CNTs. Also better dispersion in water or in alcohol is an
indirect proof of successful functionalization of CNTs. Fig. 6 illustrates the UV-vis transmittance
spectra of f-aCNT, PbS nanoparticles and aCNT-PbS samples. The UV-vis spectrum of f-aCNTs
shows a typically featureless profile which is consistent with the previous report [33]. In contrast,
the transmittance spectrum of PbS nanoparticles shows absorption edges at around 240 nm and
320 nm. These results showed a large blue shift from the direct band gap (0.41 eV) of bulk PbS
crystals. Because the size of the sample (~ 50 nm) is bigger than the Bohr exciton radius of the
PbS (18 nm), hence strong quantum confinement is not possible, although weak confinement
may takes place. But that cannot explain so large shift in the band gap. Small effective mass of
the carriers in nanocrystalline PbS is one of the reasons for such blue shift. Similar large shifts
were reported by many other groups. Cao et al. [34] observed a large blue shift in its absorption
edge to the UV region ( 5.04-4.57 eV) for PbS nanocubes with average size of about 80-40 nm
due to the small effective mass of PbS. The spectrum of aCNTs-PbS sample gives a similar
absorption edges attributed to that of the PbS nanoparticles. Similar kind of results was also
obtained by Feng et al. [11] in their CdS nanoparticle coated MWCNTs samples. They have
attributed these results to the fact that there was no charge diffusion or electronic interaction
between the MWCNTs and CdS NPs in their ground state. Here also we can conclude that the
9
PbS nanoparticles attached to the f-aCNTs causing no significant modification of the energy
states of the f-aCNTs.
3.5. Thermal study
The thermal stability studies were performed from weight loss measurements by using
thermogravimetric analysis (TGA) in the temperature range 30-900 oC at a heating rate of 15
oC/
min. in N2 atmosphere. Fig. 7 shows a comparison of mass losses of pure PbS, f-aCNTs and
aCNT-PbS composites. For reference the pure PbS sample was prepared via the reaction between
lead acetate and thiourea in aqueous solution without using f-aCNTs keeping all other conditions
the same as used in coating the aCNTs. Pure PbS samples are very stable and weight loss of 15
wt. % is observed in the range of 30-900 oC. f-aCNTs display a sudden decrease in mass of 10 %
at a temperature of 100 o
C due to water vapor. After that a steady decease of mass occurs in the
range of 100-900 oC and weight percentage decreases to 27.2 wt. %, but, in case of aCNTs-PbS
the weight percentage dropped from 99.9 % to 64 wt. %. This result indicates that aCNTs
become more thermally stable after coated with PbS nanoparticles.
3.6. Field emission study
The macroscopic electric field (E) is obtained from the applied voltage divided by the
inter-electrode distance. The simplified F-N equation for field electron emission is given by [35,
36]
]/[ln/ln 12/3212 EsbraEJ (1)
10
Here J is the macroscopic current density, ϕ is the local work function, β the field enhancement
factor, a and b are respectively the 1st and 2
nd Fowler-Nordheim constants having values a=1.54
× 10-6
A eV V-2
and b = 6.83 × 109 eV
-3/2 V m
-1. The plot of lnJ/E
2 vs. 1/E should be a straight
line and r and s are appropriate values of the intercept and slope correction factors, respectively.
Typically, s is of the order of unity, but r may be of order 100 or greater. 8a shows the
experimental J-E curves for pristine aCNTs, f-aCNTs and aCNTs-PbS. In inset of Fig. 8a, shows
the same for pristine aCNTs separately. The corresponding F-N plots are shown in Fig. 8b. The
straight line nature of the F-N plots suggests that electrons are emitted due to cold field emission.
It is seen that the field emission characteristics get much enhanced in case of acid treated CNTs
compared to the pristine CNTs and becomes the best for aCNTs-PbS. There are some reports
related with the improvement of field emission properties for crystalline CNTs when attached
with nanocrystals. For example, Uh et al. found similar results for their titanium coated CNTs
[37]. Again Chen et al. have shown that field emission characteristics of CNTs can get better if a
thin film of RuO2 is deposited over it [38]. The turn-on field (ET), which we defined as the
macroscopic field needed to get an emission current density 2 µA/cm2 for three samples is shown
in Table 1. The enhancement factor β and effective work function φeff for all these samples are
obtained from the slopes (m) of F-N plots using the relation:
β = -bφ3/2
/m (2.a)
φeff = φ/ β2/3
(2.b)
taking φ = 5 and 3.9 eV for carbon materials and PbS respectively. The values of these two
parameters are also shown in Table 1. It is noteworthy that f-aCNTs shows better field emission
than pristine aCNTs, due to the fact that, as-prepared aCNTs remained agglomerated (Fig. 4a–d)
thus the field screening effect due to close proximity is much greater which reduces the field
11
emission activity. When pristine aCNTs are treated with concentric acid the agglomeration is
much lesser thus screening effect also reduces resulting better field emission. The further
enhancement of field emission for the aCNTs-PbS samples may be attributed to the fact that
when aCNTs are coated with PbS nanoparticles, the roughness of the individual tubes enhanced
and thus the geometrical enhancement of the field is much higher as shown by Chen et al [35]. A
lesser value of work function and electron affinity for PbS compared to that of CNT are the other
key factors for the observed improved field emission from aCNTs-PbS. Very recently Chen et al.
[39] has found a much improved field emission from their IrO2 coated CNT field emitter arrays
and attributed this fact to both the lower work function of IrO2 and an improved field
enhancement factor after coating with IrO2. The coating or attachment of nanoparticles of a
suitable material on crystalline CNTs is a conventional process to obtain an improved field
emission from CNTs. The same technique may be employed to aCNTs and here CNTs-PbS
composite nanostructure showed comparable field emission properties with that of crystalline
CNTs [40]. Table 2 summarizes some of the recent field emission data from crystalline CNTs
field emitters for comparing the obtained result.
4. Conclusions
PbS nanoparticles coated aCNTs were synthesized by a simple chemical method. The
structure, composition, bonding information and morphology of the as prepared samples were
characterized by XRD, EDAX, FTIR and FESEM analysis. UV-Vis spectroscopic analysis
showed that the PbS nanoparticles are attached to the aCNTs and this does not cause a significant
modification of the energy states of the f-aCNTs. The PbS coated aCNTs are found to be more
thermally stable compared to f-aCNTs after attachment of PbS nanoparticles and thus suitable
12
for high temperature device fabrication. The field emission characteristics have been studied
extensively and it is seen that the PbS coated aCNTs give improved field emission and the result
is almost comparable to crystalline CNTs.
Acknowledgements
The authors wish to thank the University Grants Commission (UGC), the Government of India
for financial support.
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Table caption
Table 1. Different parameters of field emission for aCNTs, f-aCNTs and aCNTs-PbS samples
Table 2. Comparison of field emission characteristics of CNT based composites reported by
other groups to that of present work.
15
Table 1
Sample Turn-on field Enhancement Effective work
[J= 2µ A/cm2] (V/µm) factor β function φeff (eV)
Pristine aCNTs 18.7 3054 0.023
Acid treated f-aCNTs 4.86 7636 0.013
Pure PbS 4.28 5950 0.012
aCNT-PbS 3.34 10522 0.008
Table 2
Sample Turn on field (V/µm) Defined at Reference
CNT Composites
CNT-ZnO 5.58 3.72
I= 1 µA [14]
Ti-CNT 2.8 2 J= 0.1µA/cm2 [31]
RuO2-CNT 5 3.3 J= 1mA/cm2 [32]
IrO2-CNT 1.4 0.7 J= 0.1µA/cm2 [33]
aCNTs-PbS 18.7 3.34 J= 2 µA/ cm2 Present Work
16
Figure captions
Fig. 1. FESEM images of: (a) and (b) pristine aCNTs, (c) and (d) f-aCNTs, (e) and (f) aCNT-PbS
Fig. 2. XRD patterns of: (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS nanoparticles
Fig. 3. EDAX spectrum of PbS coated aCNTs sample
Fig. 4. FTIR spectra of: (a) f-aCNTs and (b) aCNT -PbS
Fig. 5. Dispersion of (a) Pristine aCNTs, (b) f-aCNTs, (c) PbS nanoparticles and (d) aCNT-PbS
into alcohol
Fig. 6. UV-Vis transmittance spectra of (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS
nanoparticles (Inset shows transmittance spectrum of pure PbS separately)
Fig. 7. TGA curves of (a) f-aCNTs, (b) aCNT-PbS and (c) pure PbS nanoparticles
Fig. 8. (a) Plot of emission current density (J) versus macroscopic field (E) for pristine aCNTs, f-
aCNTs, PbS and aCNT-PbS (Inset shows the J-E curve for pristine aCNTs separately for
determining the turn-on field) and (b) corresponding F-N plots
17
Fig. 1. S. Jana et al.
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Fig. 2 S. Jana et al
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Fig. 3 S. Jana et al.
.
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Fig. 4 S. Jana et al.
(b)
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Fig. 5 S. Jana et al.
a b
c d
Precipitation
Precipitation
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Fig. 6 S. Jana et al.
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Fig. 7 S. Jana et al.
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Fig. 8 S. Jana et al.
To
The Editor
Materials Research Bulletin
Date: 25.4.2011
Sub: Submission of a revised manuscript to Materials Research Bulletin (Ref: MRB-
10-1004)
Dear Professor,
Please find the manuscript of our paper entitled “Fabrication of PbS nanoparticle
coated amorphous carbon nanotubes: Structural, thermal and field emission properties”
by. S. Jana et al. which is now modified according to reviewers‟ comments and
suggestions. Please also find „reply to reviewers’ comments‟ along with this. Please
notice that the changed portions are in BOLD the revised manuscript.
I shall be happy if you kindly acknowledge the same.
With regards,
Yours sincerely,
K.K. Chattopadhyay
Department of Physics
Jadavpur University
Kolkata – 700 032
India
Cover Letter
Reply to the Reviewers’ comments (MRB-10-1004):
Reviewer #1
1) The manuscript still contains a number of grammatical mistakes and poorly
constructed sentences. These should be revised and corrected before publication.
Reply: We have tried to improve the English and avoided grammatical mistakes in the
revised manuscript.
2) G Fernandes et al have recently reported on a similar system of PbS-CNTs (G E
Fernandes et al 2010 Nanotechnology 21 465204; and J. Phys. Chem. C, 2010, 114 (51),
pp 22703-22709). These should be cited in the current manuscript.
Reply: The authors thank the reviewer for his valuable suggestion and the suggested
references are now cited in the revised manuscript. [Ref. 26-27]
3) In the Results and discussion section, 3.1 - XRD and EDAX studies, the authors point
to the C peak in the EDAX spectrum. Many EDAX spectra (even of samples that do not
intentionally contain C) will show a C peak due to C-based adsorbents on the sample
/SEM chamber, which are practically always present. Therefore EDAX results are not
normally taken as proof of the presence of C in the samples, and Raman/FTIR are more
commonly used for this. Such a disclaimer should be made by the authors.
Reply: We agree with the reviewer that presence of carbon peak in the EDX spectra is
not a confirmatory proof of the presence of carbon in the sample as carbon may come
from carbon-based adsorbent on the sample or SEM chamber. Presence of C was
confirmed by FTIR study.
These are now mentioned in the revised manuscript. This disclaimer is made as per
reviewer’s suggestion in the revised manuscript. [Sec. 3.2; p.7, line 3-5]
4) In the same section, (3.1), the authors refer to the quantitative analysis of the EDAX
measurements that yield 1:1.09 for the molar ratio of Pb and S. However, the peaks for
Pb and S seem to practically overlap in the spectra shown in Fig. 2, and the capability to
distinguish between Pb and S seems therefore to be beyond the resolution of the EDAX
equipment used. The authors should explain this and possibly provide error estimates for
deduced molar ratio.
Reply: The reviewer is right in pointing out this. We have now mentioned this limitation
of EDX clearly in the revised manuscript. [Sec. 3.2; p.7, line 5-7]
5) In section 3.2 - FTIR study, labels such as C=O, (CH3)3, etc. should also be placed
near the corresponding peaks in figure 3 for easier viewing/comparison.
*Detailed Response to Reviewers
Reply: As per reviewer’s suggestion the labels have been placed near the corresponding
peaks in figure 4 (FTIR) spectra for easier viewing/ comparison in the revised
manuscript.
6) Also in section 3.2, the labeling of the peaks seem accurate, however spectra 3(b) and
3(c) do not show any clear differences between the PbS-only and PbS-aCNT samples
other than transmission intensity. Therefore these spectra do not seem to seem to add to
the author's argument that the sample in spectrum 3(b) of Fig. indeed contains both C and
PbS. The authors should highlight this distinction, if any, or else they should consider
dropping this section from the manuscript.
Reply: We performed FTIR study to confirm incorporation of carboxyl C=O groups on
the surface of a- CNTs after acid oxidation. These are responsible for the attachment of
PbS nanoparticles on a-CNTs surfaces and that was also confirmed from the FTIR study.
So as per reviewer’s suggestion we dropped the FTIR spectrum of only PbS and Fig. 4 is
replaced by the new one. The corresponding changes are also made in section 3.3.
7) A final comment for section 3.2 is that nowhere in the FTIR spectra is the absorption
edge of (bulk) PbS observed. Such a feature would be expected (at ~0.41 eV or ~3548
cm-1
) since the coating on the CNTs is apparently sufficiently thick (judging from the
authors discussion of the SEM images of the samples) that, at least for part of the PbS
contained in the sample, quantum confinement effects would not be present.
Reply: From Fig. 1f it is clearly seen that PbS layer contained nanosized crystallites (~
50 nm). In section 3.4 we have seen that the absorption edge of the PbS appeared at
around 320 nm (which not fall in the IR region and is beyond the FTIR measurement
range). Small effective mass of the carriers in nanocrystalline PbS are responsible for
such blue shift. [p.8, line 12-19]
8) Section 3.3 should probably be presented before the other results, as it contains SEM
images of the samples and give the reader a general feel for the composition/topology of
the samples, which is important in this work, particularly in the interpretation of the field
emission results.
Reply: As per reviewer’s suggestion section 3.3 is now presented before the other results
and becomes section 3.1 in the revised manuscript. The other sections and figure numbers
are also changed accordingly.
9) Also in section 3.3, the sentence "the diameters of the tubes increase after acid
treatment." is confusing and should be further explained. How does the attachment of
functional groups on the CNT lead to an increase in the CNT diameter?
Reply: Chemical functionalization is a common procedure to oxidize the surface of
CNTs to improve CNTs interaction and dispersion. Due to the acid oxidation / treatment
several functional groups are attached on CNTs. At present the exact reason for the
diameter increment after functional group attachment to CNTs is not known. There may
be swelling of the CNTs walls due to interactions of the functional groups with carbon.
There are some reports indicating the increase of diameter of carbon nanotubes after acid
oxidation. A. Jha et al. [28] shows the increament of the diameter of a-CNTs after stearic
acid treatment. G. X. Chen et al. [ 29] also functionalized multiwalled carbon nanotubes
with poly (L- lactic acid) (PLLA) and the PLLA coated MWNT became thicker and more
uniform.
This discussion is now included in the revised manuscript. [ sec. 3.1, p.5, line 15-21]
10) In section 3.4 the identified PbS band edges at 240 nm (5.1 eV) and 320 nm (3.9 eV)
seem fairly large compared with most reported values for PbS quantum dots. The authors
should possibly discuss this providing some estimate of the mean diameters of the PbS
nanocrystals in their samples that would lead to such band edge values. The authors
should also double check that their optical components/cuvettes used in these
measurements do not absorb UV.
Reply: UV-vis absorption spectra of the as-synthesized PbS nanocrystals exhibited the
absorption edges at around 240 nm (5.1 eV) and 320 nm (3.9 eV). These results showed a
large blue shift from the direct band gap (0.41 eV) of bulk PbS crystals. Because the size
of the sample (~ 50 nm) is bigger than the Bohr exciton radius of the PbS (18 nm), hence
strong quantum confinement is not possible, although weak confinement may takes place.
But that cannot explain so large shift in the band gap. However similar large shifts were
reported by many other groups. Cao et al. [34] observed a large blue shift in its
absorption edge to the UV region (5.04-4.57 eV) from their synthesized PbS nanocubes
with average size of about 80-40 nm due to the small effective mass of PbS. This
discussion is now included in the revised manuscript. [sec. 3.4, p.8, line 12-19]
We repeated the transmittance measurement by dispersing the samples in equal amounts
in alcohol. It shows the highest transmittance of PbS only sample with absorption edges
at around 240 nm and 320 nm which is similar as that of CNT-PbS samples while CNT
sample gave featureless spectrum. This proves that the absorption peaks comes from the
samples not from the optical components/cuvettes used in these measurements.
11) In section 3.5 (and before in the manuscript) the authors should clarify whether the
PbS-only reference sample was prepared via the same reaction as the one used in coating
the CNTs.
Reply: The PbS-only reference sample preparation is now included in section 3.5. [ p. 9,
line 7-9]
12) The authors should clearly identify all the parameters in equation 1, section 3.6,
instead of only pointing the reader to a reference.
Reply: In section 3.6, all the parameters in equation 1 are now clearly identified. [ p.10,
line 1-6]
Reviewer #2
1) Why the authors name carbon nanotube as "amorphous carbon nanotubes" while the
carbon nanotube structure has good crystalline structure? Please define the phrase
"amorphous carbon nanotubes".
Reply: The walls of the most carbon nanotubes composed of perfect crystallized
concentric graphene layers such as single wall CNTs and multiwall CNTs. Whereas
amorphous carbon nanotubes (aCNTs) are those whose walls have amorphous structure
with short distance order or long distance disorder in the graphene/ carbon network. It is
one of the novelties of this work that these CNTs produced by us were amorphous in
nature.
2) The author state "PbS is one of the most important IV-VI semiconductors because of
its large exciton Bohr radius and relatively narrow band gap which can be blue shifted
from the near infrared (IR) to the visible region by forming nano crystallites ", so how
about the amorphous PbS particles instead of nano crystallites? Can the author tell the
reason why the curve b has more sensitivity than both curve a and curve c as in the Figure
6.
Reply: Amorphous semiconductors in general have tailing of bands whereas crystalline
semiconductors have sharper absorption tails. Also amorphous semiconductors do not
have true band gap, but have mobility gap. Hence although amorphous PbS may be
synthesized but that will not have the useful features for device applications like
nanocrystalline PbS.
The UV-Vis transmittance measurement were carried out by dispersing the samples into
ethanol but the samples were not taken in exactly equal amounts, so the curve b (CNT-
PbS) has more sensitivity than both curve a (CNT) and curve c (PbS). Now for
comparison, we repeat the experiment by dispersing exactly same amounts of samples
and get the plots are shown in fig. 6 which is now replaced by the previous one. The
result shows that the transmittance of PbS> aCNT-PbS> f-aCNT.
3) Since the cathode of field emission device always works at room temperature or lower,
please tell the reasons and background of thermally stable study of the cathode.
Reply: During field emission as the current density at the nano emitting sites is
significantly high and hence by Joule heating rise of temperature takes place. It is
reported that local evaporation of CNTs takes place. Hence it is important to study the
thermal stability of the synthesized structure. [ ref. Dean, K. A.; Burgin, T. P.; Chamala,
B. R. Appl. Phys. Lett. 2001, 79, 1873-1875 ]
4) It is very hard to understand why both PbS and PbS/CNT have obvious advantage field
emission properties over CNT? Please explain it according to the work function and
morphology.
Reply: The work function of PbS is 3.9 eV, which is much smaller than the work
function of carbon (5 eV). Hence the field emission performance of a-CNT-PbS should
be expected to be better than that of a-CNT, which indeed we found in our data.
Morphology of a particular sample will control the enhancement factor and that will vary
from sample to sample.
Reviewer #3
1) References are not cited properly. For example, on Page 2, the second line from the
bottom: a paper published in 2009 is cited as reference for ZnO attached CNTs. But ZnO-
CNT composites have been made early in 2007 by [M. H. Yang, T. Liang, Y. C. Peng, Q.
Chen, Acta Phys. -Chim. Sin., 2007, 23 (2), 145-151.]
Reply: In the introduction part we have tried to include new references and now another
relevant paper for ZnO-CNT composites is added in the revised manuscript. [ref. 14-15]
2) The evidences supporting the sample is aCNT-PbS are not strong.
No direct evidence is given showing the nanotube is carbon nanotube. SEM shows the
tube shape and XRD confirm amorphous character. But there is no evidence showing the
amorphous tube is carbon tube.
Reply: We have explained this point in the reply of question 3 of reviewer 1. This may
kindly be seen.
3) Some expressions are not accurate.
Page 4, the forth line from the bottom: the sentence "It was operated at 40 kV and 40
mA" should be deleted.
Page 7, the first paragraph: "All PbS nanocrystals are cubic in shape." is not correct, as
shown in Fig. 4f, although most of the particles have round shape, there are also some
particles (such as the one near the bottom right corner) show rod shape.
Reply: The sentence "It was operated at 40 kV and 40 mA" is now deleted from the
manuscript.
It is clearly seen from the fig. 1f that not all the PbS particles are cubic but most of them
have cubic shape and the manuscript has been corrected.
4) Page 9. As the work function of PbS is 3.9 eV, which is much smaller than the work
function of carbon (5 eV), it is not surprised that the field emission of a-CNT-PbS is
easier than that of a-CNT. The field emission of aCNT-PbS should be compared with that
of PbS.
Reply: As per reviewer’s suggestions now in Fig. 8 we have incorporated the data for
field emission from PbS sample also. It is clearly seen that field emission performance of
the a-CNT-PbS composite is much better than only PbS. Also it can be mentioned that
there is no published report of field emission from PbS so far. [p. 24, fig. 8]
Finally, the authors are grateful to the reviewers for their fruitful comments.
Graphical abstracts:
Simple chemical synthesis of PbS nanoparticle coated amorphous carbon nanotubes have shown
better thermal stability and enhanced electron field emission properties
*Graphical Abstract
Research Highlights
PbS nanocrystals coated amorphous carbon nanotubes have been synthesized
through a simple chemical route at low temperature
The composite is thermally more stable than amorphous CNTs
Composite have shown excellent cold cathode field emission property
*Research Highlights
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