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.

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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. kalyan_chattopadhyay@yahoo.com

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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

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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.

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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

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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)

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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

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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.

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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. kalyan_chattopadhyay@yahoo.com

*2. REVISED MANUSCRIPT: CHANGES NOT 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.

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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 ground state. Here also we can conclude that the

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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)

thus the field screening effect due to close proximity is much greater which reduces the field

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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

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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.

23

Fig. 7 S. Jana et al.

24

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