Stably dispersed carbon nanotubes covalently bonded to phthalocyanine cobalt(II) for ppb-level H2S sensing at room temperature

Article abstract of DOI:10.1039/c5ta09213b

Cost-efficient, highly sensitive and stable sensing materials play a key role in developing H₂S sensors. Herein, tetra-β-carboxyphenyloxyphthalocyanine cobalt(ii) (cPcCo) has been successfully bonded on the surface of acidified multiwalled carbon nanotubes (aCNTs) by a facile two-step condensation reaction in the presence of N,N′-dicyclohexylcarbodiimide (DCC), using hydroquinone (HQ), p-aminophenol (PAP), and p-phenylenediamine (PPD) as linking molecules, respectively. The obtained cPcCo-B-aCNT (B= HQ, PAP, and PPD) hybrids display good dispersibility in ethanol, which is beneficial to construct uniform sensing devices. The cPcCo-B-aCNT sensors, with a loose network-like structure, present abundant exposed sensing sites, oriented transmission of charges, and unimpeded pathways for H₂S diffusion, which endow the cPcCo-B-aCNT hybrids with excellent sensing performance, in terms of sensitivity, reliability, reproducibility, and detection limit. The detection limit of the sensors composed of cPcCo-B-aCNT hybrids towards H₂S reaches the ppb-level at room temperature, which is about the same as the odor threshold level for humans. For the cPcCo-HQ-aCNT sensor, the response to H₂S varies linearly with respect to its concentration from 20 to 160 ppb and from 320 to 2560 ppb, with the highest gas response of 2.5% to 80 ppb H₂S and a low detection limit of 5 ppb. Furthermore, the linking molecules play a critical role in the sensitivity of H₂S, as evidenced from the current-voltage characteristics. The systematic study developed here provides a valid way to fabricate other high-efficient H₂S sensors.

Full text of DOI:10.1039/c5ta09213b

Journal of
Materials Chemistry A
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PAPER
Stably dispersed carbon nanotubes covalently
bonded to phthalocyanine cobalt(II) for ppb-level
Cite this: DOI: 10.1039/c5ta09213b
H S sensing at room temperature†
2
a
a
a
a
a
a
Hao Wu, Zhimin Chen,* Jialin Zhang, Feng Wu, Chunying He, Bin Wang,
Yiqun Wu and Zhiyu Ren*a
ab
2
Cost-efficient, highly sensitive and stable sensing materials play a key role in developing H S sensors.
Herein, tetra-b-carboxyphenyloxyphthalocyanine cobalt(II) (cPcCo) has been successfully bonded on the
surface of acidified multiwalled carbon nanotubes (aCNTs) by a facile two-step condensation reaction in
0
the presence of N,N -dicyclohexylcarbodiimide (DCC), using hydroquinone (HQ), p-aminophenol (PAP),
and p-phenylenediamine (PPD) as linking molecules, respectively. The obtained cPcCo–B–aCNT (B¼
HQ, PAP, and PPD) hybrids display good dispersibility in ethanol, which is beneficial to construct uniform
sensing devices. The cPcCo–B–aCNT sensors, with a loose network-like structure, present abundant
2
exposed sensing sites, oriented transmission of charges, and unimpeded pathways for H S diffusion,
which endow the cPcCo–B–aCNT hybrids with excellent sensing performance, in terms of sensitivity,
reliability, reproducibility, and detection limit. The detection limit of the sensors composed of cPcCo–B–
aCNT hybrids towards H
odor threshold level for humans. For the cPcCo–HQ–aCNT sensor, the response to H
with respect to its concentration from 20 to 160 ppb and from 320 to 2560 ppb, with the highest gas
response of 2.5% to 80 ppb H S and a low detection limit of 5 ppb. Furthermore, the linking molecules
play a critical role in the sensitivity of H S, as evidenced from the current–voltage characteristics. The
systematic study developed here provides a valid way to fabricate other high-efficient H S sensors.
2
S reaches the ppb-level at room temperature, which is about the same as the
2
S varies linearly
Received 13th November 2015
Accepted 26th November 2015
2
DOI: 10.1039/c5ta09213b
2
www.rsc.org/MaterialsA
2
sensors, they still suffer from some drawbacks, such as poor
gas-sensing selectivity, low response, high operating tempera-
1
. Introduction
Hydrogen sulphide (H S) is a common and extremely dangerous ture and high power consumption.
2
gas, which oen results from bacterial activity in anaerobic
Metal phthalocyanines (MPcs), owing to their unique
wastewater, volcanic gas, coal mines, oil and natural gas conjugated 18p-electron structure, diversity of central atoms,
industries, and is released as
a
by-product in some and easily tailored functional groups at the outer phthalocy-
1–6
24–26
manufacturing industries. In daily life, exposure to H
2
S is anine ring, are well-known organic thin lm gas sensors.
because it may stimulate the eyes and Many studies indicate that MPc-based sensors could work at
7
–9
potentially fatal,
respiratory tract, cause dizziness and nausea, and even lead to room temperature with high sensitivity, short response time
suffocation, lung injury and sudden death. Therefore, it is and good selectivity. For example, F. I. Bohrer et al. revealed
important to develop sensors based on novel materials with that MPcs with different central atoms can be used as sensing
2
7
high sensitivity and selectivity, low detection limit, and energy materials to detection of volatile organic compounds (VOCs).
10–15
saving for monitoring H
2
S. Although various metal oxides,
S. Altun et al. disclosed that MPcs can serve as sensing mate-
and composite materials of metal rials for inorganic gases (such as SO , CO and CO ) by regu-
16–18
metal-doped oxides,
2
2
19–23
28
oxides
have been successfully developed as electrical H
2
S
lating their substituted groups. Signicantly, for MPc-based
sensors, critical shielding and an operational environment are
necessary owing to the intrinsically high impedance of MPcs
or else the response and reproducibility of pristine MPc
a
Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education of
China), School of Chemistry and Materials Science, Heilongjiang University, Harbin sensors would be relatively poor. In addition, pristine MPc-
1
50080, People's Republic of China. E-mail: zmchen@siom.ac.cn; zyren@hlju.edu.cn
based sensors with compact lm possess small accessible
surface areas and less exposed active sites for sensing, which
also limit the efficiency and sensitivity seriously. Hence, the
current detection results of pristine MPc-based sensors are not
as good as expected.
b
Shanghai Institutes of Optics and Fine Mechanics, Chinese Academy of Sciences,
Shanghai 201800, People's Republic of China
†
Electronic supplementary information (ESI) available: Experimental details,
additional structural and compositional characterizations, and comparison of
S-sensing performance. See DOI: 10.1039/c5ta09213b
H
2
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To overcome the above-mentioned drawbacks, an effective
29,30
solution is to introduce conductive agents.
Carbon nano-
tubes (CNTs) are one of the ideal conductive agents for
enhanced gas sensing. Owing to the CNTs' unique one-dimen-
sional structure, high electrical conductivity, excellent thermal
31–34
stability and mechanical properties,
electrons of MPc can be
readily accepted by CNTs and then be transported under nearly
35
ideal conditions along the tubular CNT axis. Therefore, the
resistance change arising from the charge transfer between the
adsorbed gas molecules and MPc/CNT hybrids becomes more
obvious. Moreover, in the MPc/CNT sensors, CNTs are inter-
laced with one another forming a web-like structure, producing
a large surface area on which more MPc molecules are exposed
as active sites for high sensitivity. Actually, some MPc/CNT
hybrids synthesized via non-covalent modication have been
designed to detect dangerous gases, because the at, planar Scheme 1 A schematic illustration of the synthesis procedure for
cPcCo–B–aCNT hybrids.
aromatic structure of MPcs is ideal for p–p stacking interac-
36–38
tions with the sidewalls of CNTs.
Due to the synergistic
effect of MPcs and CNTs, MPc/CNT hybrids really yield high
efficiency at room temperature, oen outperforming pristine
MPc and CNTs in terms of sensitivity, selectivity, and response
2
. Experimental
2.1 Reagents
39,40
time.
However, due to the indenable p–p interactions and
Multi-walled carbon nanotubes (CNTs, purity >95%, 5–15 mm in
length, 10–20 nm in diameter) purchased from Shenzhen Nano-
tech Port Co., Ltd. were puried to remove impurities by the
van der Waals force between MPcs and CNTs, it is difficult to
control the architecture and properties of MPc/CNT hybrids,
and the dispersion in the matrix of gas sensors accurately,
further resulting in poor gas-sensing reproducibility and
stability. Evidently, how to fabricate the MPc/CNT hybrids for
efficient gas sensors is still one of the main themes of current
research in this eld.
45,46
established methods.
4-Nitrophthalonitrile (99% purity), p-
hydroxybenzoic acid (99% purity) and DBU (98% purity) were
0
purchased from Sigma-Aldrich Co. LLC. N,N -Dicyclohexyl-
carbodiimide (DCC) (99% purity) and 4-dimethylaminopyridine
(
DMAP) (99% purity) were purchased from Aladdin Co. LLC. Ultra-
Compared with non-covalent modication, covalent bonding
directly to the surface of CNTs with MPcs, through a specic
chemical reaction, offers accurate tailoring of the composition
pure water (resistivity 18.2 MU cm) was obtained from a Milli-Q
Water System (Millipore Corp., Bedford, MA, USA) and was used in
the whole experimental process. Acidied multi-walled carbon
nanotubes (aCNTs) were prepared by a well-established method
41,42
and architecture.
It has been proposed that appropriate
linking molecules not only facilitate the combination of MPcs
and CNTs, but also accelerate the charge transfer from MPcs to
(see the ESI† for experimental details). Tetra-b-carboxyphenyloxy-
35,43,44
phthalocyanine cobalt(II) (cPcCo) was synthesized by the common
template reaction of 4-(4-carboxyphenyloxy)phthalonitrile with
anhydrous cobalt(II) chloride in the presence of 1,8-diazabicyclo
CNTs, and thus improve the gas sensing performance.
However, there are apparently very few studies focused on CNTs
covalently bonded with MPcs for gas sensing, and fewer studies
[5,4,0]undec-7-ene (DBU) (see the ESI† for experimental details).
2
on their application in H S gas sensors. Moreover, no systematic
All other reagents in this work were of analytical grade and used
without further purication.
study has worked on the effect of linking molecules on the gas-
sensing performance of MPc/CNT hybrids.
On the basis of the above consideration, in this work, acid-
ied multi-walled carbon nanotubes (aCNTs) covalently bonded
2.2 Preparation of cPcCo–B–aCNT hybrids
with tetra-b-carboxyphenyloxyphthalocyanine cobalt(II) (cPcCo)
0
via a condensation reaction in the presence of N,N -dicyclo- The cPcCo–HQ–aCNT hybrid was prepared by a two-step
hexylcarbodiimide (DCC), using hydroquinone (HQ), p-amino- condensation reaction at room temperature using DCC as the
phenol (PAP), and p-phenylenediamine (PPD) as linking condensation agent. Briey, HQ (3.0 mmol) was dissolved in
molecules (as show in Scheme 1), respectively have been used a solution of aCNTs (0.10 g) in anhydrous N,N-dimethylmetha-
2 2
for H S-sensing. As expected, compared with other H S-sensing namide (DMF) (20 mL). It was then sonicated at room temper-
materials, the obtained hybrids (cPcCo–B–aCNT, B¼ HQ, PAP, ature for 30 min. Aer that, DCC (0.30 g) and DMAP (0.050 g)
and PPD) show enhanced sensitivity, response time, reproduc- were added and the mixture was sonicated at room temperature
ibility and selectivity. In particular, the sensor composed of for another 30 min. The resulting black mixture was ltered
the cPcCo–HQ–aCNT hybrid shows the highest gas response through a 0.45 mm polytetrauoroethylene (PTFE) membrane
2
of 2.5% to 80 ppb H S, and a low detection limit of 5 ppb. lter, and washed continuously with methanol, acetic acid,
Furthermore, the inuence of linking molecules on the DMF and acetone until the ltrate was colorless. Aer that, the
ꢀ
dispersion and H
2
S-sensing performance of the cPcCo–B–aCNT paste-like lter-cake was dried in a vacuum oven at 50 C for 1 h,
hybrids has also been systematically discussed.
affording the HQ modied CNTs. Subsequently, cPcCo (0.20 g)
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was condensed with the HQ modied CNTs in a similar fashion Company) excited by a laser with 457.9 nm wavelength. UV/Vis
and using the same conditions, and nally the cPcCo–HQ– absorption spectra were recorded with a Lambda 35 UV/Vis
aCNT hybrid was obtained as a black powder. cPcCo–PAP–aCNT spectrometer (Perkin-Elmer). Thermogravimetric (TG) analysis
and cPcCo–PPD–aCNT hybrids were fabricated using similar was performed on a TA Q 600 under a stream of nitrogen at
ꢀ
ꢁ1
procedures except for the replacement of HQ with PAP and PPD, a heating rate of 10 C min . X-ray photoelectron spectroscopy
respectively. As a control, aCNTs modied with cPcCo (cPcCo/ (XPS) measurements were performed with an AXIS ULTRA DLD.
aCNTs) were synthesized by using a solution self-assembly Scanning electron microscopy (SEM) images were recorded with
method based on non-covalent interactions (see the ESI† for a Hitachi S-4800 eld emission scanning electron microscope
experimental details).
operating at 15 kV. Transmission electron microscopy (TEM)
images were obtained using a JEM-3010 electron microscope
(JEOL) with an acceleration voltage of 300 kV. Nitrogen
2.3 Sensor assembling and sensing measurements
adsorption–desorption isotherms at 77 K were collected on
a Micromeritics ASAP 2010 nitrogen adsorption apparatus. The
Brunauer–Emmett–Teller (BET) equation was used to calculate
the specic surface area. The gaseous products collected aer
A complete description of the gold interdigitated electrodes
(
IDEs) and the gas sensor testing device was already reported in
38
our previous study.
To prepare gas sensors composed of cPcCo–B–aCNT hybrids,
the as-prepared cPcCo–B–aCNT hybrids were dispersed in
2
the cPcCo–HQ–aCNT sensor was exposed to H S were moni-
tored by gas chromatography-mass spectrometry (GC-MS, AGI-
ꢁ1
ethanol to obtain a uniform suspension of 1.0 mg mL by
ultrasonication for 30 min, and then approximately 50 mL of the
homogeneous dispersion was sprayed on the IDEs using an
airbrush tool. To remove the solvent of the sensor and improve
the electrical contact between the hybrids and the gold elec-
trodes, the sensor devices were annealed in a vacuum oven for 2
ꢀ
LENT, 6890-5973N) using a DB-5 column from 50 to 250 C at
ꢀ
ꢁ1
ꢁ1
a heating rate of 10 C min and a N ow rate of 1 mL min
.
2
The current–voltage (I–V) characteristics of the fabricated
sensors were measured using a Keithley 4200 semiconductor
parameter analyzer with a two-point probe setup via sweeping
ꢁ1
the potential between ꢁ1 and +1 V under a 0.01 V s scan rate.
ꢀ
h at 50 C before sensing tests. For comparison, gas sensors,
Electrochemical impedance spectroscopy (EIS) was carried out
on a CHI760D electrochemical working station (CH Instru-
ments) at room temperature, the range of frequency was 0.01 Hz
to 100 kHz and the excitation amplitude was 0.2 V.
composed of the cPcCo/aCNT hybrid and aCNTs were also
fabricated by similar procedures. For the sensor based on
cPcCo, the as-prepared cPcCo was dissolved in acetone to obtain
ꢁ1
a 5.0 mg mL solution rst, and then approximately 50 mL of
the solution was dispensed on the IDEs using a trace sampler
(
100 mL). The resulting sensor was dried in a vacuum oven at 50
C for 2 h.
ꢀ
3. Results and discussion
A typical sensing test cycle consisted of three sequential 3.1 Characterization of cPcCo–B–aCNT hybrids
steps and in order to speed up the recovery of the sensor, a non-
ozone type mini ultraviolet (UV) lamp (10 V/3 W, 254 nm) was
used in the course of removal of the gas. First, an air ow was
introduced into the sensing test chamber to record a baseline.
Then, a target gas with certied concentrations was injected to
register sensor signals. Finally, the sensor was recovered in an
air ow under UV light. A constant DC voltage of 0.5 V was
applied to the electrode gap bridged by the hybrids and all
According to the design strategy, three cPcCo–B–aCNT hybrids
were synthesized by a condensation reaction in the presence of
DCC as the condensation agent. The typical FT-IR spectra of
cPcCo, aCNTs, and cPcCo–HQ–aCNT hybrids are shown in
Fig. 1A. The typical bands of the carboxyl group on aCNTs
ꢁ1
ꢁ1
47,48
appear at 1720 cm (y C]O) and 3435 cm (y O–H).
cPcCo
ꢁ1
exhibits characteristic vibrations at 1599 cm (y C]C) and
ꢁ1
1
237 cm (y Ar–O–Ar). Compared with the FT-IR spectrum of
ꢀ
ꢀ
measurements were performed at 28 C ꢂ 0.5 C with a relative
humidity of 60% ꢂ 2%. In this study, sensitivity (S) is dened by
the relative resistance change, as follows:
ꢁ1
ꢁ1
cPcCo, two distinct peak-shis (1513 cm and 1163 cm
)
appear in the spectrum of the cPcCo–HQ–aCNT hybrid, which
may be ascribed to the interaction between the phthalocyanine
41,49
DR
R
g
ꢁ R
0
macrocycle and CNT backbone.
The covalent bonding of
S ¼ R ꢃ100% ¼
ꢃ100%
0
R
0
cPcCo with aCNTs was also evidenced by Raman spectra. As
0
shown in Fig. 1B, the D-, G-, and D -bands of the aCNTs appear
where R
0
is the sensor resistance in the initial air ow which was
is the sensor resistance aer
ꢁ1
at 1360, 1571, and 1610 cm , respectively, which can be
attributed to the defects and disorder-induced modes, in-plane
used as the background and R
g
being exposed to a certain concentration of target gas. Response
and recovery times are dened as the time needed for 90% of
total resistance change on exposure to the target gas and air,
respectively.
50
E
2g zone-center mode, and the disorder in carbon nanotubes.
In contrast to the case of aCNTs, the D- and G-bands of the
ꢁ1
cPcCo–HQ–aCNT hybrid, which appear at 1368 and 1580 cm
are found to be slightly shied to the high wavenumbers. The
/I of the cPcCo–HQ–aCNT hybrid is almost the same as that
of aCNTs (2.08 vs. 2.00). The results indicate that cPcCo has
,
I
G D
2.4 Characterization
FT-IR spectra were recorded on a Nicolet FT-IR NEXUS spec- succeeded in bonding onto the surface of aCNTs via various
trometer (Thermo Scientic). The Raman spectra were acquired linking molecules, and the attachment does not affect the
51,52
using a Raman spectrophotometer (HR800, HORIBA Jobin Yvon structure of aCNTs.
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Fig. 2 High-resolution N1s XPS spectra (A) and C1s XPS spectra (B) of
cPcCo–HQ–aCNT, cPcCo–PAP–aCNT and cPcCo–PPD–aCNT
hybrids, respectively.
and cPcCo–PPD–aCNT hybrids, which issues from the nitrogen
of the amide group (–NH–CO–) being used for bonding between
57
2
the carboxyl and substituted amino (–NH ) groups. Accord-
ingly, the peaks at 286.2 eV assigned to the carbon of the amide
Fig. 1 (A) FT-IR spectra, (C) UV-Vis spectra, and (D) TGA profiles of
cPcCo, aCNTs, and cPcCo–HQ–aCNTs hybrid; (B) Raman spectra of
aCNTs, and cPcCo–HQ–aCNTs hybrid obtained at lexc ¼ 457.9 nm.
group (–NH–CO–) are also observed in the C1s XPS spectra of
58
cPcCo–PAP–aCNT and cPcCo–PPD–aCNT hybrids (Fig. 2B). All
these ndings are indicative of the formation of a covalent bond
Fig. 1C shows the UV-Vis spectra of cPcCo, aCNTs and between cPcCo and aCNTs using HQ, PAP, and PPD as the
cPcCo–HQ–aCNTs hybrid in DMF, respectively. Apparently, the linking molecules, respectively.
characteristic Q-band of cPcCo (664 nm) is observed in the UV-
The microstructures of the aCNTs and the cPcCo–B–aCNT
Vis spectrum of the cPcCo–HQ–aCNTs hybrid, accompanied by hybrids were also characterized by SEM and TEM. The typical
a red shi of about 10 nm, suggesting the charge transfer from SEM images of aCNTs, cPcCo–HQ–aCNT, cPcCo–PAP–aCNT and
53,54
the cPcCo ring to aCNTs.
2
TG analysis under a N atmosphere cPcCo–PPD–aCNT hybrids are shown in Fig. 3A–D. Aer spray-
was used to determine the content of cPcCo in the hybrids ing on the IDEs, both aCNTs and cPcCo–B–aCNT hybrids appear
ꢀ
(
Fig. 1D). In the range of 320–600 C, the weight loss of cPcCo as three-dimensional network-like structures. Compared with
and aCNTs is about 31.53% and 6.98%, respectively, which aCNTs, the lm of the cPcCo–HQ–aCNT hybrid is more uniform
originates from the destruction of the phthalocyanine periph- and looser, due to better dispersion. In contrast, there are some
eral substituent group and the residual amorphous carbon, and “knots” in the cPcCo–PAP–aCNT and cPcCo–PPD–aCNT hybrids,
the decarboxylation of the oxidized species. For the cPcCo–HQ– which are mainly due to the presence of amino (–NH) groups in
aCNT hybrid, the weight loss is only 8.31% at the same the cPcCo–PAP–aCNT and cPcCo–PPD–aCNT hybrids.
40,55
temperature range.
Thus, a corrected weight loss of 1.33%
The amino group with a strong electron-donating nature not
can be obtained, which comes from cPcCo in the cPcCo–HQ– only effectively connects cPcCo and aCNTs, but also improves the
aCNT hybrid. Considering the actual amount of cPcCo chemi- polarity of the hybrids and enhances the intermolecular force,
cally bonded to the surface of aCNTs, a real ratio of 4.22% resulting in the re-aggregation of hybrids and generating the
(1.33%/31.53%) can be calculated. Meanwhile, the FT-IR, “knots”. The inuence of linking molecules on the dispersion of
Raman, UV-Vis spectra and TGA analysis of cPcCo–PAP–aCNT the cPcCo–B–aCNT hybrids in ethanol was also studied. As
and cPcCo–PPD–aCNT hybrids were also obtained (Fig. S1–S4†), shown in Fig. 3G–J, aCNT and cPcCo–B–aCNT hybrids can
illustrating that cPcCo–PAP–aCNT and cPcCo–PPD–aCNT disperse in ethanol well and form black mixtures aer sonicating
hybrids were prepared successfully. The ratio of cPcCo in at room temperature for 10 minutes, which is very benecial to
cPcCo–PAP–aCNTs and cPcCo–PPD–aCNTs is 4.10% and 4.38%, fabricating uniform lms by the spraying technology for use as
respectively, which are basically similar to that of the cPcCo– gas sensors. However, aer standing for 3 days at room temper-
HQ–aCNT hybrid.
ature, aCNT and cPcCo–PPD–aCNT hybrid slowly settled down
As a further proof, XPS measurements were performed, and the ethanol became clear. On extending the standing time to
which could provide the chemical composition and states of 15 days, the cPcCo–PAP–aCNT hybrid precipitated at the bottom
surface elements. In the survey spectra of cPcCo–B–aCNT of the container; while the cPcCo–HQ–aCNT hybrid still
hybrids, N and Co species could be found, further revealing the remained as a good dispersion even aer standing for 60 days.
reaction between cPcCo and aCNTs (Fig. S5†). The high-reso- Evidently, the long-term dispersion and stability of cPcCo–HQ–
lution XPS spectrum of N in the cPcCo–HQ–aCNT hybrid can be aCNTs can be attributed to the covalent graing of cPcCo onto
mainly deconvoluted into two sub-peaks located at about 398.9 the surface of aCNTs via the linking molecule HQ. A discontin-
and 400.1 eV, ascribed to the aza-bridging nitrogen and pyrrole uous and irregular layer appears outside the distinct core and
56
nitrogen, respectively (Fig. 2A). Signicantly, a new peak at walls of carbon nanotubes (Fig. 3E and F), which probably arises
01.60 eV can be obviously observed in both cPcCo–PAP–aCNT from cPcCos coated onto aCNTs through covalent bonds.
4
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Fig. 3 (A–D) The typical SEM image of aCNTs (A), cPcCo–HQ–aCNT (B), cPcCo–PAP–aCNT (C) and cPcCo–PPD–aCNT hybrids (D),
respectively; (E and F) the TEM images of cPcCo–HQ–aCNT hybrid; (G–J) optical photographs of the equivalent aCNT and cPcCo–B–aCNT
hybrids dispersed in ethanol after sonicating at room temperature for 10 minutes (G), and after standing at room temperature for 3 days (H), 15
days (I), and 60 days (J). ((a)–(d): aCNT, cPcCo–HQ–aCNT, cPcCo–PAP–aCNT, and cPcCo–PPD–aCNT hybrids, respectively).
Meanwhile, cPcCo–B–aCNT hybrids show a large surface area of of H
2
S was also investigated (as shown in Fig. 4B), and all
2
ꢁ1
1
32.7–135.3 m g , being larger than that of the aCNT hybrid cPcCo–B–aCNT sensors exhibit two good linear responses
2
ꢁ1
(114.0 m
g
, Fig. S6†). Such an increase in surface area is (Fig. S7†). In particular, the cPcCo–HQ–aCNT sensor shows the
S for concentrations
mainly due to covalent graing of cPcCo onto the surface of best sensor response, 0.0325% per ppb H
2
aCNTs. Due to the easily accessible active sites for gas adsorption, ranging from 20 to 160 ppb, and 0.0061% per ppb H S for
2
and the permeable channels for diffusion of gas molecules,
cPcCo–B–aCNT hybrids are conducive to accelerating the inter-
action between the hybrids and adsorbed gas molecules, giving
10
rise to the high sensing activity.
3
.2 H S sensing properties of cPcCo–B–aCNT hybrids
2
2
To assess the H S-sensing activity of cPcCo–B–aCNT hybrids,
gas-sensor devices were fabricated using IDEs, according to the
experimental part. A constant DC voltage of 0.5 V was applied to
the electrode gap (50 mm) covered by the hybrids. The signal
resistor was continuously recorded, as the sensor sample was
subjected to the target gas exposure in an air-tight chamber,
and then recovered under UV light at room temperature. As
a control, the sensing response of aCNTs and cPcCo towards
2
H S were also studied. The detailed dynamic responses of the
sensors upon repeated H S exposure/removal cycles are shown
2
in Fig. 4A. Nine cycles were successively recorded at room
temperature, corresponding to H
120 ppb. It can be seen that the response intensities of all the
sensors increase as the concentration of H S gas is increased.
Although the cPcCo sensor displays fast response and recovery,
its response intensity is so weak that no response can be aCNT, cPcCo–HQ–aCNT, cPcCo–PAP–aCNT and cPcCo–PPD–
aCNT sensors to varying concentrations of H S; (C) response of
cPcCo–HQ–aCNT sensor exposure to 320 ppb H S and recovery
under UV light or without UV light; (D) ten sensing cycles of the
cPcCo–HQ–aCNT sensor to 80 ppb H S (inset: reproducibility char-
acteristic of the cPcCo–HQ–aCNT sensor to 80 ppb H S at different
2
S concentrations from 20 to
5
Fig. 4 (A) Response of cPcCo, aCNT, cPcCo–HQ–aCNT, cPcCo–
PAP–aCNT and cPcCo–PPD–aCNT sensors upon exposure to varying
2
2
concentrations of H S; (B) relationship of the response of cPcCo,
2
observed at less than 320 ppb H S, while for the aCNT sensor,
2
2
both the response intensity and the recovery time are poor. By
contrast, the cPcCo–B–aCNT sensors generate remarkable
2
2
responses towards H S. The relationship of the response of
2
cPcCo, aCNT, and cPcCo–B–aCNT sensors to the concentration durations).
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concentrations ranging from 320 to 2560 ppb, and the lowest
theoretical detection limit (S/N ¼ 3) for H S is approximately 5
2
ppb (Table S1†). The ppb-level sensing of cPcCo–HQ–aCNTs,
reaching the odor threshold level for humans, is superior to
most of the reported H S sensors (Table S2†).
2
It is clear that the synergistic effect between cPcCo and
aCNTs enhances the sensing performance. cPcCo covalently
bonded on the surface of aCNTs offers more sensing sites for
2
H S reaction, and aCNTs as a conductive agent can accelerate
the charge transfer. It can further be evidenced by the H2S
response of the cPcCo/aCNT sensor prepared by non-covalent
interactions. Compared to the cPcCo–B–aCNT hybrids, the
cPcCo/aCNT sensor shows a lower intensity of response and
higher limit of detection (Fig. S8 and Table S1†). Consequently,
the covalent linkage is conducive to the efficient combination of
MPcs and CNTs, and thus facilitates the charge transfer from
MPcs to CNTs and improves the gas sensing performance. In
addition, for all the cPcCo–B–aCNT hybrids, the surface area
and cPcCo loading are similar, but the cPcCo–HQ–aCNT sensor
displays the greatest response among the hybrids, suggesting
Fig. 5 Cross-sensitivities to various gases for the aCNT sensor (A) and
cPcCo–HQ–aCNT sensor (B) at room temperature; MeOH ¼ meth-
anol, EtOH ¼ ethanol, DMK ¼ acetone, DCM ¼ dichloromethane, TCM
2
that the linking molecule plays an important role in H S
sensing. The inuence of linking molecules on the sensing
property will be discussed below.
¼
trichloromethane, CTC ¼ carbon tetrachloride, PhH ¼ benzene, Tol
The effect of UV irradiation on the recovery property of the ¼ toluene, THF ¼ tetrahydrofuran, MA ¼ methyl aldehyde, AA ¼ acetic
cPcCo–HQ–aCNT sensor was also studied. In Fig. 4C, the cPcCo– acid, DEE ¼ diethyl ether, and EA ¼ ethyl acetate.
HQ–aCNT sensor was exposed to 320 ppb of H S, and then
2
recovered with or without UV radiation at room temperature. The
response time of the cPcCo–HQ–aCNT sensor was about 58
seconds, and the recovery time was about 614 seconds under UV
radiation; nevertheless, the recovery time lengthened signicantly
without UV radiation. The results illustrate that the recovery of the
cPcCo–HQ–aCNT sensor strongly depends on UV radiation. In
should be pointed out that the cPcCo–B–aCNT sensors exhibit
high responses to H
the same concentration (2.56 ppm), which is usually not ach-
ieved easily. Moreover, in contrast to H S and NH , the resis-
2 3 2
S and low responses to NH , NO and NO at
2
3
tances of cPcCo–B–aCNT sensors reduce when they are exposed
to the oxidizing atmospheres, NO and NO . As is well known,
2
this case, the photogenerated electrons can react with O
2
in air
both cPcCo and aCNTs are typical p-type semiconductors.
According to the charge-transfer mechanism, NO (or NO ) can
2
attract the electrons from cPcCo–B–aCNT hybrids and generate
more holes in the hybrids, leading to the reduced resistance.
Further, the cPcCo–HQ–aCNT sensor shows a negligible
response to VOCs even at high concentrations (1000 ppm).
producing adsorbed oxygen, and then the residual holes form an
59
accumulation layer on the surface of cPcCo–HQ–aCNTs. There-
fore, the resistance of cPcCo–HQ–aCNTs reduces and the sensor is
recovered. To further verify the reproducibility and reliability, ten
response cycles of the cPcCo–HQ–aCNT sensor to 80 ppb H
room temperature were recorded (Fig. 4D). It can be seen that,
with the on–off H S signal, the sensor shows a fast response time
less than 30 s) and recovery time (less than 520 s under UV
2
S at
Therefore, it is promising as a H S sensor to achieve quantita-
2
2
tive detection of H S in an indoor atmosphere, industrial
2
(
petroleum/gas, automobile exhaust gas, and so on.
radiation), and there is no change in the ten continuous cycles.
The long-term stability of the cPcCo–HQ–aCNT sensor was also
evaluated over 60 days by determining the response to 80 ppb H2S 3.3 H S sensing mechanism of the cPcCo–B–aCNT sensors
2
and storing in air at room temperature when not in use, and the
results are shown in the inset of Fig. 4D. A slight response drop
2
The sensitivity of the cPcCo–B–aCNT sensors towards H S
accords with the classical charge-transfer theory. In general, the
sensing effect is most likely to occur at the cPcCo layer, which
(only about 10%) could be observed aer storing for 60 days.
Clearly, the cPcCo–HQ–aCNT sensor has good stability and
reproducibility, which is crucial for further applications.
For gas sensors, reduced cross-sensitivity from intervening
gases is one of the important indexes for practical application.
To probe the selectivity of the cPcCo–B–aCNT sensors, the
representative gases in air and volatile organic compounds
60
covalently bonds onto the surface of aCNTs. Herein, H S
2
ꢁ
interacts with the main adsorbed oxygen species (O ) at room
temperature, and releases a large number of electrons as
2
61
indicated in the following equation:
2ꢁ
ꢁ
2 2 2
2H S (g) + 3O (ads) / 2H O (g) + 2SO (g) + 6e
(VOCs) were separately injected into the test chamber in turns at
room temperature (as shown in Fig. 5 and S9†). Compared to
the aCNT sensor, all the prepared cPcCo–B–aCNT sensors show
The electrons generated are trapped by the cPcCo–B–aCNT
good selectivity, especially the cPcCo–HQ–aCNT sensor. It hybrids through the phthalocyanine layer (layer 2) and linking-
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molecule layer (layer 1) in sequence. During the electron-
transfer process, electrons recombine with holes and decrease
the density of the hole-accumulation layer, which results in
increased resistance, as illustrated in Scheme 2.
To validate the H
composition and states of surface elements in the cPcCo–HQ–
aCNT sensor aer exposure to H S were probed by XPS
measurement (as shown in Fig. S10†). Apparently, aer expo-
sure to H S at room temperature, two weak peaks ascribed to
2
S sensing mechanism, the chemical
2
2
S2s (ꢄ225 eV) and S2p (156–175 eV) can be observed in the XPS
Fig. 6 (A) I–V curves and (B) Nyquist plots of aCNT and various
survey spectrum of the cPcCo–HQ–aCNT sensor. In the corre- cPcCo–B–aCNT sensors.
sponding S2p XPS spectrum, the two binding energies of 168.1
eV and 164.2 eV belong to the characteristic peaks of SO
2
and
S, respectively. As further evidence, the distinct signal of Table 1 Fitted impedance parameters of cPcCo–HQ–aCNT, cPcCo–
SO
appears in the GC-MS spectra (Fig. S11†). These results PAP–aCNT and cPcCo–PPD–aCNT electrodes
conrm that H
above sensing mechanism. In addition, compared with NH
NO, NO , and VOCs, the excellent selectivity of cPcCo–B–aCNT
9,15
H
2
2
2
2
S is oxidized to SO , in accordance with the
ꢁ
2
Samples
R
b
(U)
C (F cm
)
Rct (U)
3
,
ꢁ7
ꢁ
2
cPcCo–HQ–aCNT
4.21
4.46
4.56
4.70
5.82 ꢃ 10
5.49 ꢃ 10
4.59 ꢃ 10
4.35 ꢃ 10
12.50
15.36
17.45
20.99
7
hybrids for H S can be mainly ascribed to the strong interaction cPcCo–PAP–aCNT
2
2
ꢁ
ꢁ7
cPcCo–PPD–aCNT
between H S molecules and the adsorbed oxygen ions (e.g. O
,
2
ꢁ7
ꢁ
ꢁ
62
aCNT
O , and O ) on the surfaces of the cPcCo–B–aCNT hybrids.
2
As mentioned above, the linking molecules dramatically
inuence the dispersion and sensing performance of the
cPcCo–B–aCNT hybrids. In order to explore the effect of the
linking molecules, the I–V characteristics of the cPcCo–B–aCNT
sensors with the same mass loading on IDEs were investigated
aCNT hybrids is ascribed to the different linking molecules.
Amino (–NH) groups have stronger electron-donating capability
64
than –OH groups. Relative to HQ, the linking molecules PAP
and PPD containing –NH groups can provide more electrons to
combine with the holes in p-type semiconductors of cPcCo and
aCNTs. The combination of electrons and holes can reduce the
carrier concentration of the system, leading to further increase
in the resistances of cPcCo–PAP–aCNT and cPcCo–PPD–aCNT
(Fig. 6A). The linearity and symmetry of I–V curves suggest an
ohmic contact between the samples and gold electrodes.
Evidently, the device current of various hybrids at the same
voltage decreases in the order aCNTs > cPcCo–HQ–aCNTs >
cPcCo–PAP–aCNTs > cPcCo–PPD–aCNTs. Low current means
the sensor has a relatively high resistance. For cPcCo–B–aCNT
hybrids, the decrease in device current is possibly caused by the
intrinsically high resistance of cPcCo, and the donation of
electrons from the p-type cPcCo to the p-type aCNT backbone by
2
hybrids. Hence, their responses towards H S are not as good as
that of the cPcCo–HQ–aCNT hybrid (Fig. 4A).
The EIS of the cPcCo–HQ–aCNT, cPcCo–PAP–aCNT and
cPcCo–PPD–aCNT electrodes were also measured to investigate
their kinetic processes (Fig. 6B). The Nyquist plots are tted by
63
means of linking molecules. Due to the similar cPcCo content,
65
an appropriate electric equivalent circuit (insets of Fig. 6B),
and the obtained impedance parameters are given in Table 1.
Here, R is the uncompensated bulk resistance of the electro-
the distinct difference in device current of the three cPcCo–B–
b
lyte, separator and electrode, Rct is the charge-transfer resis-
tance at the active material interface, and C is the constant
phase angle element, involving double layer capacitance. It is
clear that the values of R and R are 4.21 U and 12.50 U for
b
ct
cPcCo–HQ–aCNTs, which are lower than those of cPcCo–PAP–
aCNTs (R
4
2
b
¼ 4.56 U and Rct ¼ 15.36 U), cPcCo–PPD–aCNTs (R
.60 U and Rct ¼ 20.99 U) and aCNTs (R ¼ 4.70 U and Rct
0.99 U). Both R
b
¼
b
¼
b
and Rct are related to the electrode's
conductivity and the electron transportation for the gas sensor.
The lower the values of R and Rct, the stronger the electron
b
transport. Therefore, the cPcCo–HQ–aCNT sensor displays
a higher sensing response and lower detection limit than the
other sensors.
4. Conclusions
A series of cPcCo–B–aCNT hybrids were successfully synthe-
Scheme 2 A schematic illustration of the gas sensing mechanism of
cPcCo–B–aCNT sensors upon interaction with H
2
S.
sized by covalent bonding using HQ, PAP, and PPD as linking
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molecules. Compared to many reported H
2
S sensors, the ob- 10 J. M. Ma, L. Mei, Y. J. Chen, Q. H. Li, T. H. Wang, Z. Xu,
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X. C. Duan and W. J. Zheng, Nanoscale, 2013, 5, 895–898.
2
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high sensitivity (2.5% to 80 ppb H S with a detection limit of 5
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2
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Acknowledgements
2
This work was nancially supported by the National Natural
Science Foundation of China (51002046, 51102082, 51202061
and 21573062), the Natural Science Foundation for the
Returned Overseas Scholars of Heilongjiang Province
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