Effect of sulfur on enhancing nitrogen-doping and magnetic properties of carbon nanotubes.

0.71 MB·English

by  Cui, Tongxiang

Cuietal.NanoscaleResearchLetters2011,6:77
http://www.nanoscalereslett.com/content/6/1/77
NANO EXPRESS Open Access
Effect of sulfur on enhancing nitrogen-doping
and magnetic properties of carbon nanotubes
Tongxiang Cui1, Ruitao Lv1, Zheng-hong Huang1, Feiyu Kang1*, Kunlin Wang2, Dehai Wu2
Abstract
Sulfur (S) is introduced as an additive in the growth atmosphere of carbon nanotubes (CNTs) in the range of 940-
1020°C. CNT products with distorted sidewalls can be obtained by S-assisted growth. Moreover, many fascinating
CNT structures can also be found in samples grown with S addition, such as bamboo-like CNTs, twisted CNTs,
arborization-like CNTs, and bead-like CNTs. Compared with CNTs grown without S, more nitrogen-doping content
is achieved in CNTs with S addition, which is beneficial for the properties and applications of nitrogen-doped CNTs.
In addition, S can also enhance the encapsulation of ferromagnetic materials and thus improve the soft magnetic
properties of CNTs, which is favorable to the applications of CNTs in the electromagnetic wave-absorbing and
magnetic data storage areas.
Introduction controlling chirality and crystallinity of CNTs [14], lead-
Sulfur (S) is an important additive for controlling the ing to n-type CNTs [15], and improving the field emis-
structures and properties of carbon nanotubes (CNTs). sion performance of CNTs [16-18]. The effective doping
Many interesting carbon nanostructures, such as Y-junc- of nitrogen into CNTs is mainly achieved in situ during
tion CNTs [1], sea-urchin-like CNTs [2], long single- CNT growth [12-16,19], and N-doping content is
walled CNT (SWCNT) strands [3], double-walled CNT usually controlled by adjusting the ratio of carbon
films [4], amorphous CNTs [5], large-diameter SWCNTs source to nitrogen source [16,19], which is disadvanta-
[6], ultra-short CNTs [7], and small-diameter multi- geous for increasing the doping content of CNTs pro-
walled CNTs [8] can be obtained by S-assisted growth. duced with liquid carbon source. For example, N-doped
Meanwhile, enhanced mechanical properties [3], water- CNTs can be synthesized by mixture of xylene and pyri-
solubility [6], catalyst support performance [6], electro- dine, and CNTs with different N-doping contents are
chemical properties [7], and photovoltaic performance obtained from different xylene/pyridine ratios, but their
[9] can all be achieved by CNT products with S-assisted doping contents cannot be larger than that with pure
growth. Therefore, the introduction of S into growth pyridine [19]. Therefore, it is crucial to develop an effi-
atmosphere of CNTs has attracted much research inter- cient way to improve the N-doping content in CNTs
est because of their promotional effect on synthesis of from the view of both theoretical and applied research.
carbon nanostructures with novel morphologies and In this study, S was introduced as an additive in the
properties. growth of CNTs. CNTs with distorted walls were
Nitrogen-doping (N-doping) is an effective way to obtained, which might be good supports for catalyst
modify the properties of CNTs. Nitrogen-doped (N- nanoparticles. Many other fascinating CNT structures
doped) CNTs demonstrate negative differential resis- were also obtained in this study’s samples, such as bam-
tance behavior [10], high electrocatalytic activity for oxy- boo-like CNTs, twisted CNTs, arborization-like CNTs,
gen reduction [11], and can also be used as suitable and bead-like CNTs. When S was used as growth pro-
support for uniform distribution of Pt catalyst [12,13]. moter, the improved CNTs with high N-doping content
In addition, N-doping has a significant effect on could be achieved. In addition, the effect of S addition
on enhanced soft magnetic properties of CNTs was also
*Correspondence:[email protected] demonstrated.
1LaboratoryofAdvancedMaterials,DepartmentofMaterialsScienceand
Engineering,TsinghuaUniversity,Beijing100084,PRChina
Fulllistofauthorinformationisavailableattheendofthearticle
©2011Cuietal;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttribution
License(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,
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Experimental The TEM images of the as-grown products are shown
Experimental setup and procedure are similar to that in Figure 2. Typical morphology of CNTs grown with-
described in the previous report by the authors about out S is shown in Figure 2a, and their sidewalls are
N-doped CNT arrays [18]. Ferrocene and pure S pow- straight as those shown in many previous reports
ders were dissolved in acetonitrile to form a solution [11,18,20]. However, in the case of products obtained by
(20 mg/ml), and fed into chemical vapor deposition the S-assisted method, it can be clearly seen from Figure
(CVD) reactor by a syringe pump at a constant rate of 2b, c, d that CNTs with distorted sidewalls can be
0.4 ml/min for 0.5-1 h. A mixture of Ar (2000 sccm) obtained at different growth temperatures. Owing to the
and H (300 sccm) acts as the carrier gas. In order to existence of abundant defect in surface, this kind of
2
investigate the effect of S on CNT growth at a relatively CNTs might supply more active positions for efficient
low temperature, the temperature is set in the range of immobilization of catalyst nanoparticles, and thus be
940-1020°C. The S concentration in the catalyst (atomic useful for catalyst support applications [21]. Previous
ratio of S:Fe) is 1:10. CNTs without S additive are also studies have demonstrated that S is favorable for the
prepared for comparison. formation of pentagon and heptagon carbon rings, indu-
The resulting CNTs were characterized using scanning cing curvature and further influencing the CNT mor-
electron microscope (SEM, JOEL JSM-6460 LV SEM), phology [1,2]. Many pentagonal and heptagonal rings
transmission electron microscope (TEM, JEM-200 CX), exist in the sp2 carbon lattice, which are supposed to
microscopic confocal Raman spectrometer (Renishow cause the distort-walled CNTs formation. Furthermore,
RM 2000, using 632.8-nm laser excitation), Auger elec- many fascinating CNT structures can also be found in
tron spectroscopy (AES, PHI-700), and X-ray diffract- the CNT samples of this study with S, such as bamboo-
ometer (XRD, Bruker D advence). The magnetization like CNTs (Figure 2e), twisted CNTs (Figure 2f),
8
measurements were performed on a vibrating sample arborization-like CNTs (Figure 2g), and bead-like CNTs
magnetometer (LakeShore VSM-7307) at room tempera- (Figure 2h).
ture. Thermogravimetric analysis (TGA) results were Raman spectroscopy has been proved to be a perfect
obtained using 6-mg samples in air flow at a heating tool to evaluate the crystallinity and defects in carbon
rate of 20°C/min. structures [18,20]. The Raman spectra of the CNTs
grown at different temperatures with and without S are
Results and discussion shown in Figure 3a, b. The strong bands around 1330,
Figure1showstheSEMimagesofas-grownCNTsatdif- 1580, and 2650 cm-1 can be assigned to D-band,
ferent temperatures with and without S. It can be seen G-band, and 2D-band, respectively. Taking D- and
fromFigure1a,b,c,d,e,fthatalignedCNTsareobtained G-bands into consideration, little difference is found
at all the three temperatures(940°C, 980°C, and 1020°C) between the CNTs with and without S. The obvious dif-
withandwithoutS.AlignedCNTsproducedbyacetonitrile ference between their 2D-bands is shown in Figure 3. If
without S have been reported inour previous study [18], S is introduced into the CVD reactor, then 2D-band
andtheresultsofthisstudydemonstratethattheintroduc- downshifts approximately 30 cm-1. This might be
tionofSisnondestructivetothealignmentofCNTs. attributed to the improved N-doping content [22].
Figure1SEMimagesofas-grownCNTsatthreedifferenttemperatureswithandwithoutS.(a-c).DifferenttemperatureswithS:(a)940°C,
(b)980°C,(c)1020°C;(d-f)differenttemperatureswithoutS:(d)940°C,(e)980°C,(f)1020°C.
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Figure 2 TEM images of as-grown CNTs at three different temperatures with and without S. (a) 980°C without S; (b-d) different
temperatureswithS:(b)940°C,(c)980°C,(d)1020°C;(e-h)novelCNTstructureswithS:(e)bamboo-likeCNTs,(f)twistedCNTs,(g)arborization-
likeCNTs,(h)bead-likeCNTs.
The 2D-band of CNTs without S is almost symmetrical, of the two CNTs samples produced at 980°C with S and
but 2D-band is irregular with S. This may be due to the without S, respectively. Almost no signals of Fe were
difference between their CNT walls, because 2D-band is detected in both of the samples, indicating that Fe cata-
sensitive to the stacking of graphene sheets [23]. lyst particles are fully covered by carbon layers. The
The improvement of N-doping content is also con- CNTs produced with S consist of C (95.2 at.%), N (2.4
firmed by AES. Figure 3c shows the surface AES results at.%), and O (2.4 at.%), while CNTs without S consist of
Intensity (a.u.) a 1329.1 1583.0 1908200OOCCwith su2628.8lfur Intensity a.u.(cid:11)(cid:12) b 1330.5 1582.6 1908200 O COCwithout su2657.5lfur
940 OC 940OC
1000 1500 2000 2500 3000 1000 1500 2000 2500 3000
Raman Shift (cid:11)cm-1(cid:12) Raman Shift (cid:11)cm-1(cid:12)
c
8000
with sulfur
4000
without sulfur
C/s 0 N1O1
-4000
-8000
-12000 C1
0 200 400 600 800 1000 1200 1400 1600 1800
Kinetic Energy (eV)
Figure 3 Raman spectra and surface AES of CNTs. (a, b) Raman spectra of CNTs grown at different temperatures with and without S:
(a)withS,(b)withoutS;(c)SurfaceAESoftheCNTsproducedat980°CwithandwithoutS.
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C (96.9 at.%), N (1.2 at.%), and O (1.9 at.%). The pre- Table 1The saturation magnetization (M, emu/g)of
s
sence of O can be attributed to the exposure of the CNTsproduced atdifferent conditions
CNTs in the air atmosphere [18,24]. It can be seen that Temperature(°C) 940 980 1020
S has an effect on increasing N-doping content in WithS 21.2 17.9 13.1
CNTs. This might be because S induces pentagon and WithoutS 5.9 6.2 7.9
heptagon in sp2 carbon lattice [1,2], and these hetero-
cyclic rings are supposed to be favorable to the
part of TGA curves at 600-900°C range can be used to
enhancement of N-doping.
determine the content of ferromagnetic material inside
Hysteresis loops and the saturation magnetization
(M ) of CNTs produced at different temperatures with the CNTs [26]. It can be seen that the introduction of S
s
can remarkably increase the encapsulation of ferromag-
and without S are shown in Figure 4 and Table 1,
respectively. As shown in Table 1, the M of CNTs with netic material compared with that without S.
s
The XRD analysis proved the existence of pure Fe and
S is enhanced by 1.5-3.5 times compared with that with-
out S. As proved in our previous study, the M improve- FeC3 in the CNTs grown with and without S, as shown
s
in Figure 6a, b. Auger depth-resolved chemical analysis
ment of CNTs can be attributed to the increased
was carried out by sputtering the catalyst particle seek-
encapsulation of ferromagnetic materials into their inner
cavities [25]. Conversely, the enhanced M also indicates ing with SEM using an in situ 1 keV Ar-ion gun, (sput-
s
tering speed: 13.5 nm/min, probe diameter 1.8 nm), and
that S can increase the encapsulation of ferromagnetic
the resulting Auger depth profiles of CNTs grown at
materials into CNTs.
980°C with and without S are shown in Figure 6c, d.
In order to prove the enhanced encapsulation of ferro-
The major difference between Figure 6c, d is that S
magnetic material, TGA was carried out, as shown in
peak exists in Figure 6c. This indicates the existence of
Figure 5. The residual weight percentage in the platform
25 25
a b
20 20
15 15
with sulfur without sulfur
10 10
g) 5 5
emu/ 0 0 980oC
M ( -5 1020 oC -5
-10 -10
-15 980oC -15 940oC 1020oC
-20 -20
940oC
-25 -25
-10000 -5000 0 5000 10000 -10000 -5000 0 5000 10000
H (Oe) H (Oe)
Figure4HysteresisloopsofCNTsproducedatdifferenttemperatureswithandwithoutS:(a)withS,(b)withoutS.
100 100
a b
1020 oC 1020 oC
980 oC 980 oC
%) 80 with sulfur 940 oC 80 without sulfur 940 oC
ght ( 60 60
ei
W
ple 40 21.9 wt% 40
m
a 25.0 wt% 12.0 wt%
S
8.0 wt%
20 20
20.8 wt%
7.3 wt%
0 0
0 200 400 600 800 0 200 400 600 800
Temperature (oC) Temperature (oC)
Figure5TGAcurvesofCNTsproducedatdifferenttemperatureswithandwithoutS:(a)withS,(b)withoutS.
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ntensity a C (002)(cid:13)111(cid:11)(cid:12) (200)(cid:13)(201)(cid:84) (cid:13)(102)(220)(cid:13) (212)(cid:84)Fe (200)(cid:13)(040) withFe (220)(cid:13) (cid:84)s(cid:29)(cid:29)(cid:3)(cid:3)uFFlefe1u30SCr220 oC b C (002) (102)(cid:13)(cid:13)(220) Fe (200)(004)(cid:13) withoFe (220)u(cid:13)t (cid:29)sFu10lef23uC0r oC
I
980oC
980 oC
940 oC
940 oC
20 40 60 80 100 20 40 60 80 100
2 theta (degree) 2 theta (degree)
98 100
c with sulfur d without sulfur
C
%(cid:12) C
n (cid:11) 97 99
o
ati
ntr
ce 96 98
n
c co 3 Fe
omi 2 1 Fe
At
1
S
0 0
0 5 10 15 0 5 10 15
Sputter Time (cid:11)min(cid:12) Sputter Time (min)
Figure6XRDandAugerdepthscanningspectraofCNTs.(a,b)XRDspectraofCNTsproducedatdifferenttemperatureswithandwithout
S:(a)withS,(b)withoutS;(c,d)AugerdepthscanningspectraofCNTsproducedat980°CwithandwithoutS:(c)withS,(d)withoutS.
iron S compound in CNTs produced with S, which is in Abbreviations
accordance with the XRD result (Figure 6a). CNTs are AES:Augerelectronspectroscopy;CNTs:carbonnanotubes;CVD:chemical
vapordeposition;N:nitrogen;S:sulfur;SEM:scanningelectronmicroscope;
good templates for housing metals or compounds, and
SWCNT:single-walledCNT;TGA:thermogravimetricanalysis;TEM:
these nanostructures combine the properties of metals/ transmissionelectronmicroscope;XRD:X-raydiffraction.
compounds and CNTs together [25]. S can enrich the
Acknowledgements
categories and enhance the amount of encapsulation in
TheauthorsaregratefulforthefinancialsupportreceivedfromChina
CNTs, which is favorable to the applications of CNTs in PostdoctoralScienceFoundation(GrantNo.20090450021)andtheNational
the electromagnetic wave-absorbing [25], and magnetic NaturalScienceFoundationofChina(GrantNo.50902080,50632040).
data storage [27] areas.
Authordetails
1LaboratoryofAdvancedMaterials,DepartmentofMaterialsScienceand
Engineering,TsinghuaUniversity,Beijing100084,PRChina2KeyLaboratory
Conclusions
forAdvancedManufacturingbyMaterialsProcessingTechnologyand
CNTs with distorted sidewalls are synthesized by the S- DepartmentofMechanicalEngineering,TsinghuaUniversity,Beijing100084,
PRChina
assisted CVD method. S is favorable to the formation of
pentagonandheptagoncarbonrings, andtheseringsare Authors’contributions
supposed to result in the formation of CNTs with dis- TCcarriedoutthemostofexperimentsanddraftedthemanuscript.RL
participatedintheCNTsynthesisandmanuscriptpreparation.ZHparticipated
torted sidewalls. This kind of CNTs might supply more
intheanalysisofRamanandXRDspectra.FKdesignedtheexperimentsand
activepositionsforcatalystnanoparticlesandhavepoten- revisedthemanuscript.KWandDWdiscussedandanalyzedtheexperimental
tial usage in catalyst support area. It is also found that S results.Allauthorsreadandapprovedthefinalmanuscript.
addition can enhance the N-doping content of CNTs,
Competinginterests
resulting fromheterocyclicrings, which are favorablefor Theauthorsdeclarethattheyhavenocompetinginterests.
theenhancementofN-doping.TheeffectofSonenhan-
Received:4September2010 Accepted:12January2011
cingthesoftmagneticpropertiesofCNTsisalsodemon-
Published:12January2011
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