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Journal of Hazardous Materials 265 (2014) 124–132
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Effective decoration of Pd nanoparticles on the surface of SnO2
nanowires for enhancement of CO gas-sensing performance
Do Dang Trung a , Nguyen Duc Hoa a,∗ , Pham Van Tong a , Nguyen Van Duy a ,
T.D. Dao b , H.V. Chung b , T. Nagao b , Nguyen Van Hieu a,∗
a
b
International Training Institute for Materials Science, Hanoi University of Science and Technology, No. 1 Dai Co Viet Road, Hanoi, Viet Nam
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Pd
nanoparticles decoration on
nanowire’s surface was done via a
simple route.
• The size and distribution of Pd
nanoparticles was controlled by
in situ reduction of the Pd complex.
• The coating Pd enhanced capable
detection of hazardous CO gas of onchip growth SnO2 nanowires.
• Pd-decorated SnO2 nanowires sensors have good stability to hazardous
CO gas.
a r t i c l e
i n f o
Article history:
Received 20 August 2013
Received in revised form 18 October 2013
Accepted 24 November 2013
Available online 2 December 2013
Keywords:
Pd NPs decoration
SnO2 nanowire
CO gas sensor
a b s t r a c t
Decoration of noble metal nanoparticles (NPs) on the surface of semiconducting metal oxide nanowires
(NWs) to enhance material characteristics, functionalization, and sensing abilities has attracted increasing interests from researchers worldwide. In this study, we introduce an effective method for the
decoration of Pd NPs on the surface of SnO2 NWs to enhance CO gas-sensing performance. Singlecrystal SnO2 NWs were fabricated by chemical vapor deposition, whereas Pd NPs were decorated on
the surface of SnO2 NWs by in situ reduction of the Pd complex at room temperature without using any
linker or reduction agent excepting the copolymer P123. The materials were characterized by advanced
techniques, such as high-resolution transmission electron microscopy, scanning transmission electron
microscopy, and energy-dispersive X-ray spectroscopy. The Pd NPs were effectively decorated on the
surface of SnO2 NWs. As an example, the CO sensing characteristics of SnO2 NWs decorated with Pd
NPs were investigated at different temperatures. Results revealed that the gas sensor exhibited excellent
sensing performance to CO at low concentration (1–25 ppm) with ultrafast response-recovery time (in
seconds), high responsivity, good stability, and reproducibility.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Carbon monoxide (CO) is a colorless, odorless, and tasteless but
highly toxic gas. The toxicity of CO gas is mainly caused by the
inhalation. Breathing CO gas can result in health problem because
∗ Corresponding authors. Tel.: +84 4 38680787; fax: +84 4 38692963.
E-mail addresses: ndhoa@itims.edu.vn (N.D. Hoa), hieu@itims.edu.vn (N.V. Hieu).
0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jhazmat.2013.11.054
it displaces oxygen in the blood and deprives the heart, brain, and
other vital organs of oxygen. In blood, CO is believed to combine
with hemoglobin to produce carboxyhemoglobin, which usurps
the space in hemoglobin leading to ineffective delivering oxygen to
bodily tissues [1]. Carbon monoxide is a byproduct of an incomplete
combustion process of organic substances, such as petroleum and
fuel, in internal combustion engines caused by the insufficient supply of oxygen. Exposure to a low concentration of approximately
35 ppm can cause headache and dizziness, whereas exposure to
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D.D. Trung et al. / Journal of Hazardous Materials 265 (2014) 124–132
a higher concentration (∼0.64%) can result in convulsion, respiratory arrest, and even death. Thus, Occupational Safety and Health
Administration limits CO concentration for long-term exposure to
less than 50 ppm. Early monitoring or alarm of CO at low concentration is very important not only in the environment protection
but also for human health safety. The current trend for CO sensor
development is to increase sensitivity and reduce response time,
as well as lowering the detection limit.
Nanostructured metal oxide semiconductors (MOS) are fundamental in studies and practical applications of photocatalysts
[2], batteries [3], biosensors [4], and gas sensors because of their
unique optical, electrical, and functional properties [5,6]. Many
attempts have been devoted for the design, synthesis, processing,
characterization, and device applications of MOS at the nanoscale.
Extensive studies have been concentrated on the development of
new materials with novel nanostructures and excellent properties to improve the performance of gas sensors [7–11]. Different
types of gas sensors have been developed including of optical sensors, electrochemical sensors, calorimetric sensors, and resistive
gas sensors. The resistive gas sensors which operated based on
the variation in their electrical resistance upon chemical reaction
and/or adsorption between the analytical gas molecules and the
surface of sensing layers, appear to be advantages not only because
of their low cost, simple completion, and good reliability for realtime control systems, but also the diverse practical applications in
environmental monitoring, transportation, security, defense, space
missions, energy, agriculture, medicine, etc. [7].
Among the materials used in resistive gas sensors, MOS such as
SnO2 , ZnO, WO3 , CuO, Co3 O4 , and NiO have attracted great attention because of their reasonable sensitivities to various gases, such
as NO2 , NH3 , CO, H2 , and C2 H5 OH [8–10]. Comparing with the thin
film or the bulk forms of metal oxide, the nanowire structure was
reported to exhibit significant advantages such as the small diameter compatible with Debye length and the anisotropic quantum
confinement effect, those promise excellent sensitivity as well as
cost effective for devices fabrication [7,8]. Generally, resistive type
of MOS-based gas sensors cannot sufficiently identify an unknown
gas or mixture of gases because of their poor selectivity, but they are
designed for monitoring some reducing gases and usually respond
to many compounds [10,11]. High detection limit also reduces the
number of potential applications of conventional gas sensors. One
of the key techniques to improve the functionalities, sensitivity, and
selectivity of MOS is decoration or doping of noble metal nanoparticles (NPs) on their surface [12–14]. Different metal NPs such as Pt,
Au, Ag, and Pd have been used in the decoration or functionalization
on the surface of MOS to enhance sensor performance [15]. Each
metal exhibits significantly enhanced response to a specific gas or to
some gases [16,17]. For instance, decoration of Pt NPs on the surface
of bead-like SnO2 nanowires (NWs) significantly enhances the H2
sensing performance of the NWs but reduces the response to NO2
[18]. Pt and Pd NPs were decorated on the surface of In2 O3 flowerlike nanobundles to enhance their sensitivities and responses to CO
gas at room temperature [19]. The decoration of Pd nanodots on the
surface of networked ZnO nanowires was also reported to exhibit
significant high response to CO gas at room temperature [20]. The
room temperature CO sensing was also demonstrated on the Ptloaded SnO2 porous nanosolid [21]. Pd NPs and NiO were decorated
on the surface of SnO2 NWs to significantly enhance their NO2 and
H2 S gas-sensing responses [22,23]. Highly responsive H2 gas sensor based on the metal–insulator transition mechanism was also
observed in V2 O5 NWs decorated with Pd NPs [24].
Decoration or functionalization of noble metal NPs on the surface of nanomaterials can be performed by a vacuum deposition
[25], drop-casting [26], supercritical fluid deposition [27], thermal
pyrolysis [28], or self-assembly [29]. Each decoration method has
some advantages and disadvantages. The self-assembly method
125
is complicated because noble metal NPs are first prepared and
stabilized in polymer before they can be decorated on the surface of nanostructured metal oxides [30]. Kolmakov et al. [31]
demonstrated an in situ vacuum deposition of Pd on the surface
of an individual SnO2 NW and investigated its gas-sensing characteristics. Moon et al. [32] reported a new route to enhance the
NO2 sensitivity of nanocrystalline TiO2 fiber-based gas sensors by
doping with Pd, in which the Pd-doped TiO2 fibers were fabricated
by electrospinning and subsequent annealing. Yuasa et al. [33]
prepared Pd-loaded SnO2 nanocrystals by photochemical deposition of Pd2+ onto SnO2 in an aqueous solution to achieve a highly
sensitive hydrogen gas sensor. Kim et al. [34] prepared In2 O3 -Pd
core–shell NWs by direct sputter deposition of Pd layer on bare
In2 O3 NWs to enhance their NO2 sensing characteristics. They
demonstrated that the improvement of sensing properties is caused
by the enhanced adsorption or dissociation of NO2 and the associated spillover effects. One-step hydrothermal synthesis was also
applied to fabricate Pd-ZnO nanoflowers, wherein Pd NPs and ZnO
nanoflowers were simultaneously grown in a sealed Teflon-lined
stainless steel autoclave [35]. However, this method has difficulties in decoration of noble metal NPs on the surface of different
MOS. Zhang et al. reported the self-assemblies of Pd NPs on the
surfaces of single crystal ZnO nanowires for enhanced H2 S gas performances [36], whereas Li et al. demonstrated an enhancement
of H2 S gas-sensing properties by assembly of Pd NPs on the surface of SnO2 NWs [37]. Chang et al. [38] decorated ZnO nanorod
arrays with Pd NPs by solution routes of aqueous chemical growth
and successive photochemical reduction to improve CO gas-sensing
characteristics. In their study, ZnO nanorods were prepared first
by a hexamethylenetetramine-assisted hydrothermal method, and
the decoration of Pd NPs was performed by ultraviolet irradiation
reduction of PdCl2 in ethanol solution. Development of a general,
low-temperature facile and effective method that does not require
a vacuum, linker, or reduction agent for the decoration of Pd NPs on
a wide range of MOS in gas-sensing applications is still a challenge
and an interesting task.
In this study, we introduce a vacuum-less method for the decoration of Pd NPs on the surface of SnO2 NWs at room temperature to
enhance their CO gas-sensing performance. Decoration of Pd NPs on
the surface of MOS was archived by direct reduction of Pd complex
in the presence of pluronic surfactant. This method is a facile and
effective route that enables the decoration of Pd NPs on the surface
of different MOSs, such as SnO2 , ZnO, Co3 O4 , and NiO, for gassensing applications. The CO-sensing properties of the fabricated
SnO2 NWs decorated with Pd NPs were investigated and found to
exhibit significantly enhanced performance. The fabricated materials also have potential applications in catalytic and/or bio-sensing
applications.
2. Experimental
2.1. On-chip growth of SnO2 NWs
Single-crystal SnO2 NWs were prepared directly on the sensor chips by thermal evaporation. The sensor chips have two Au
electrodes and heater that were screen-printed on the front and
back sides of an Al2 O3 substrate (Fig. 1A). The sensor chips used for
the in situ growth of SnO2 NWs were pre-deposited with a ∼5 nm
thick gold layer on the front side. The SnO2 NW process was conducted as previously described [23,39]. In a typical synthesis, 0.3 g
Sn powder was loaded on an alumina boat, which was then loaded
in a quartz tube of a thermal evaporation system [23]. The sensor chips were placed on the same quartz tube ∼2 cm away from
the boat. The temperature in the furnace was rapidly ramped up
to 750 ◦ C for 15 min. During the process, the vacuum of the quartz
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Fig. 1. (A) Al2 O3 substrate with screen-printing Au electrodes, (B) electrodes after growth of SnO2 NWs; (C) low and (D) high magnifications of FE-SEM images of the SnO2
NW grown on the substrate.
tubes was maintained at ∼2 × 10−2 torr by a rotary vacuum pump.
When the furnace reached the set-point temperature, 0.5 sccm
oxygen gas was introduced through the quartz tube for 15 min.
The furnace was cooled down to room temperature to collect the
samples.
spectroscopy (EDS) using an X-ray analyzer integrated with a TEM
instrument. Crystal structures of materials were studied through a
wide-angle powder X-ray diffraction (XRD) using Cu K␣ X-radiation
with a wavelength of 0.154178 nm.
2.3. Gas-sensing characterizations
2.2. Decoration of Pd NPs on the surface of SnO2 NWs
Decoration of the Pd NPs on the surface of nanostructured MOS
was performed by in situ reduction of the Pd complex in the presence of pluronic (P123) surfactant at room temperature without
using any further linker and reducing agent [40] and/or assistance
of photochemical reduction [41]. Copolymer P123 acts as a surfactant and a reducing agent to reduce Pd complex and stabilize
the NPs, thereby preventing aggregation and enhancing the active
surface of materials. The Pd complex was first prepared by dissolving PdCl2 in aqueous NaCl solution to obtain a complex solution of
20 mg/mL (Na2 [PdCl4 ]). At least three SnO2 NW sensors were added
to the complex solution with further stirring. After a few minutes,
a solution of 2 g pluronic dissolved in 40 mL H2 O was added to the
solution to reduce Pd2+ into Pd NPs. The reduction process was
performed for 4 h in ambient condition. After the products were collected and washed with deionized water and ethanol, the samples
were dried in an oven at 45 ◦ C overnight.
Characterization of the synthesized materials was performed
using several techniques, such as high-resolution transmission
electron microscopy (HRTEM), scanning transmission electron
microscopy (STEM), and selective area electron diffraction (SAED).
Elemental analyses were performed by energy-dispersive X-ray
The sensors were dried and then heat-treated at 500 ◦ C for
at least 1 h to stabilize sensor resistance. The gas-sensing properties were then measured by a flow-through technique with a
standard flow rate of 400 sccm for both dry air balance and analytic gases using a home-made system. This sensing system enabled
rapid switching (in a second) from dried air to analytic gas to
investigate the response-recovery time of the sensors. Prior to
measurements, the dry air was flown through the sensing chamber until the stability of sensor resistance was reached. During
sensing measurement, resistance of the sensors was continuously
measured using a Keithley (2700) instrument interfaced with a
computer while the dried air and analytic gases were switched
on/off in each cycle. In our experiment, we used the standard gas
concentration of 1000 ppm CO balanced in nitrogen and mixed
with dry air as carrier using a series of mass flow controllers to
obtain a lower concentration. Gas concentration was calculated as
follows: C(ppm) = Cstd (ppm) × f/(f + F), where f and F are the flow
rates of analytic gas and dry air, respectively, and Cstd (ppm) is
the concentration of the standard gas used in the experiment.
Sensor response was defined as S = Rair /Rgas , where Rair and Rgas
are sensor resistances in dry air and analytic CO gas, respectively
[10].
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3. Results and discussion
3.1. Material characterizations
Optical images of sensor chips before and after the growth of
SnO2 NWs are shown in Fig. 1A and B, respectively. The gap between
two electrodes was about 400 ␮m, and they were stable after the
growth of SnO2 NWs. White wool-like products were found on
the sensor chip substrates as a result of the catalytic growth of
the nanowires. The SnO2 nanowires grew on the electrodes and
on the substrate (gap between two electrodes) to form a mat-like
nanowires. These entangled mat-like nanowires will act as conducting paths for electrons to flow in the sensing characterization
127
(Fig. S1, ESI). The morphology of SnO2 NWs grown on the chips
was characterized by FE-SEM (Fig. 1C and D). As shown in the figures, a thick layer of SnO2 NWs was successfully deposited on the
chips. The as-grown SnO2 NWs have an average diameter of about
50–150 nm, and a length between 5 and 50 ␮m. The NW surface
was very smooth because of the VLS growth mechanism for the
crystalline SnO2 NWs with gold as the catalyst in NW growth [39].
Morphological characterization of Pd NPs decorated on the surface of SnO2 NWs was investigated using HRTEM, STEM, and EDS.
Fig. 2 shows the HRTEM images of the undecorated (A and B) and Pd
NP-decorated (C and D) SnO2 NWs. The undecorated SnO2 NWs had
a smooth surface with a uniform diameter along their axis despite
the variation in size of each NW from ∼40 nm to 80 nm (Fig. 2A).
Fig. 2. HRTEM images of pristine SnO2 NWs (A and B) and Pd NCs decorated on the surface of SnO2 NWs (C and D); Pd NC (E and F).
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The HRTEM image and respective SAED pattern demonstrated that
the SnO2 NW was a single crystal with tetragonal rutile structure
(Fig. 2B) [42]. After decoration with Pd NPs, the shape of the SnO2
NW was not damaged, indicating that the SnO2 material is stable
in the synthesis environment. Small Pd NPs were homogenously
decorated on the surface of the SnO2 NW (Fig. 2C). In the photoreduction method for the decoration of Pd NPs on the surface of
MOS nanorods, the Pd NPs agglomerated on the surface of the NWs
but were not homogenously decorated on the surface of materials. This method is ineffective for gas sensor application because
the agglomeration of Pd NPs might decrease the height of Schottky
barrier formed at the interface between Pd NPs and NWs, thereby
reducing sensor performance [33,38]. In our study, neither the surface functionalization nor the linker reagent was applied for the
decoration of MOS [43] with Pd NPs excepting the P123 surfactant, but the Pd NPs were homogenously and strongly attracted on
the surface of SnO2 NWs. Strong binding between Pd NPs and the
surface of SnO2 NWs generated the barrier height at the interface,
thus forming the conduction depletion region that contributed to
the sensitivity of materials. In the vacuum deposition of Pd on the
surface of SnO2 NWs, controlling the deposition rate and time is
important to prevent the formation of a continuous layer of noble
metal, just the core–shell structure, which lowers the sensing performance of the device [18,31]. As shown in the HRTEM image
(Fig. 2C), the density of Pd NPs dispersed on the surface of SnO2
NW was approximately 0.22 NPs/␮m2 . The size, morphology and
dispersion of Pd NPs on the surface of SnO2 nanowires could be
controlled by varying the concentration of palladium complex and
the synthesis time, and such characteristics may influent on the
gas-sensing performances, but they are out of the context of this
study. Pd NPs have a nearly spherical shape with an average diameter of approximately 6 nm. A high-magnification HRTEM image
(Fig. 2E) demonstrated that Pd NP is highly crystalline and that the
gap between the lattice fringes was approximately 0.22 nm, which
belongs to the space of the (1 1 1) planes (d(1 1 1) = 0.2246 nm). SAED
analysis (Fig. 2F) indicated that Pd NP is a single crystal, in which the
diffraction pattern was indexed to the (1 1 1) reflection of a cubic
structure of Pd (JCPDS card No. 05-0681) [44]. Some stacking faults
were also observed in the HRTEM images of SnO2 NW and Pd NP.
Further characterization of Pd NPs decorated on the surface of
SnO2 NWs by STEM image and EDS mapping is shown in Fig. 3. STEM
image also confirmed that Pd NPs were homogenously decorated
on the surface of the SnO2 NWs (Fig. 3A). Elemental analysis by EDS
mapping revealed the existence of Sn, O, and Pd, in which Sn and O
were originally from the SnO2 NWs, whereas Pd was originally from
the Pd NPs. O is expected to be higher than Sn in the EDS mapping,
but the results indicated otherwise (Fig. 3B and C). This instance
has not been clarified. Thus, the use of EDS mapping to calculate the
composition of material was omitted in this study. The distribution
of Pd NPs on the surface of SnO2 NWs was homogenously, and no
aggregation of Pd NPs was observed.
Crystal structures of the SnO2 NWs decorated with Pd NPs and
the bare Pd NPs investigated by XRD are shown in Fig. 4. The XRD
pattern of the Pd-SnO2 NWs exhibited strong diffraction peaks that
were perfectly indexed to the tetragonal rutile structure of SnO2 ,
which is consistent with the reported data from the JCPDS card
(77-0450) [45]. The standard profiles of cubic Pd (JCPDS, 05-0681)
and PdO (JCPDS, 41-1107) are also potted in Fig. 4. However, none
diffraction peak of the standard cubic Pd is observed marching with
the XRD pattern of Pd-SnO2 NWs, whereas the strongest peak (1 0 1)
of PdO is overlapped with the (2 0 0) of SnO2 thus it is difficult to
index or confirm the existence of Pd related phases in this XRD pattern. This result can be attributed to the small size and low content
of Pd NPs in the sample, resulting in a much lower intensity at the
diffraction peaks compared with those of single crystal SnO2 NWs.
To confirm the existence and successful synthesis of Pd NPs, an XRD
pattern of the pure Pd NPs synthesized under the same condition as
the Pd decoration process but without the addition of SnO2 NWs is
shown in Fig. S2 (ESI). The XRD pattern indicated the face-centered
cubic crystal structure of Pd NPs, with a space group of Fm3m (space
Fig. 3. STEM image (A) and EDS mapping (C and D) of Pd-decorated SnO2 NWs.
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Pd-SnO 2 NWs
JCPDS, No. 77-0450
40
30
(310)
(112)
(301)
60
70
2θ (deg.)
40
50
60
(220)
(103)
(200)
(112)
(200)
Pd (JCPDS 05-0681)
PdO (JCPDS 41-1107)
(110)
(002)
Intensity
20
50
(111)
30
(101)
20
(002)
(220)
(110)
(111)
(211)
(200)
Intensity (a.u.)
(101)
D.D. Trung et al. / Journal of Hazardous Materials 265 (2014) 124–132
70
Fig. 4. XRD patterns of (A) SnO2 NWs decorated with Pd NPs, and standard profiles
of Pd and PdO.
group number: 225) and cell parameters a = b = c ≈ 0.388 nm. These
data are consistent with the standard JCPDS card No. 05-0681.
Moreover, the intensity of the (1 1 1) diffraction peak was higher
than that of the (2 0 0) diffraction peak. This result suggests that
the Pd NPs are highly crystalline and mainly bound by {1 1 1} facets
[46]. These features are advantageous for the CO-sensing application because the Pd NPs with preferred {1 1 1} crystal facets exhibit
significantly better catalytic activity for CO catalytic oxidation than
the {1 0 0} facets [47].
3.2. CO gas-sensing performance
In our study, the CO gas-sensing characteristics of the synthesized Pd NP-decorated SnO2 NWs were investigated and compared
with those of pristine SnO2 NWs (Fig. 5(A)). The transient resistance
response to CO gas of the pristine SnO2 NWs and the Pd-decorated
SnO2 NWs demonstrated that both sensors can detect CO gas. Both
sensors exhibited decreased resistance upon exposure to CO gas
indicating that the Pd-SnO2 NWs possess typical n-type gas-sensing
properties similar to pristine SnO2 NWs [48]. However, decoration
of Pd NPs on the surface of SnO2 NWs strongly enhanced the CO gassensing performance of SnO2 NW sensors. The response S (Rair /Rgas )
to 400 ppm CO at 400 ◦ C was 7 for the Pd NP-decorated SnO2 NWs
and 3.9 for the pristine SnO2 NWs. Pd NP decoration also accelerates the adsorption and desorption rates of analytic gas molecules,
thereby resulting in ultrafast response-recovery time of the sensors. The 90% response and recovery time of the pristine SnO2 NW
sensors at 400 ◦ C were 10 and 60 s, respectively, whereas those of
the Pd NP-decorated SnO2 NW sensor were approximately 5 and
40 s, respectively. The improvement of response-recovery speed
of the Pd-SnO2 NW sensor was possibly due to the acceleration
of interaction rate between CO molecules and pre-adsorbed oxygen (O2 − , O− , or O2− ) under the catalytic activity of Pd NPs. The
enhanced responsivity of the Pd-SnO2 NWs compared with that
of the pristine SnO2 NWs can be explained based on the spillover
and the formation of Schottky barrier between Pd NPs and SnO2
Fig. 5. Comparison of the CO-sensing characteristics of the pristine SnO2 NW and
Pd-decorated SnO2 NW sensors.
NWs, which is similar to the mechanism of the Pd-ZnO arrays sensor
[40]. A cartoon illustrates the depletion region and the interaction
between analytic gas molecules and pre-adsorbed oxygen on the
surface of Pd-SnO2 NW is shown in Fig. 5(B). The catalytic activity of Pd NPs accelerates the dissociation of oxygen molecules and
causes a spillover of the ion absorbed oxygen on the surface of SnO2 .
More ion adsorbed oxygen on the surface of SnO2 provides more
sensing sites and enhances the sensitivity. In addition, SnO2 is an ntype semiconductor with a work function of 4.5 eV, which is higher
than that of Pd (∼5.5 eV). Therefore, when Pd NPs make contact
with SnO2 , electrons flow from the SnO2 NWs to the Pd NPs and
an electron-depletion region or the Shottky barrier forms between
SnO2 and Pd. The interaction of CO molecules with pre-adsorbed
oxygen releases electrons back to Pd-SnO2 NWs and significantly
reduces the height of Schottky barrier, resulting in a decrease in
sensor resistance [31].
For practical applications, we considered the response-recovery
time, responsivity, stability, and reproducibility of the sensors.
Thus, three sensors were fabricated under optimum synthesis
conditions, and their CO gas-sensing characteristics were investigated and compared. The transient resistance response to CO
(25–400 ppm) measured at different temperatures (350–450 ◦ C)
are shown in Fig. 6A–C and in Fig. S3 (ESI). The three sensors showed
similar trends of sensing behavior at all temperatures, in which the
resistance of the sensors decreased rapidly upon exposure to CO.
After refreshing the sensing chamber with dry air, the sensor resistance recovered to the initial values. This result indicates that the
adsorption of CO molecules on the surface of Pd-decorated SnO2
NWs is reversible. The response and recovery times of the sensors
calculated for the 90% of the variation measured at different temperatures ranged from 4 s to 6 s and from 20 s to 40 s, respectively.
The response time of about 210 s was reported in Pd-ZnO nanowires
for the CO gas at room temperature [20]. Higher working temperature required shorter recovery time in the measured ranges. The
temperature dependence of sensor response to 100 ppm CO shown
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400ppm
400
(A)
140
120
Sensor1 @ CO
200ppm
200
100ppm
50ppm
100
25ppm
75
Resistance(kΩ)
50
o
25
@350 C
150
100
50
o
@400 C
(A)
100
80
60
10 ppm
25 ppm
40
200
150
o
20
o
@450 C
0
100
200
300
400
500
600
700
800
0
5
400
425
450
(B) @ 100 ppm
4
Response, S (Rair/Rgas)
375
Sensor1
Sensor2
Sensor3
2
10
(C) @400oC
6
Sensor1
Sensor2
Sensor3
4
2
0
50
100
150
5
10
15
20
CO conc. (ppm)
25
o
120
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8
1.6
SnO2-Pd, 25 ppm CO @ 400 C
(B)
140
Resistance (kΩ)
350
2.0
Time (s)
160
o
2.4
100 200 300 400 500 600 700 800
Time (sec.)
Operating Temp. ( C)
1 ppm
2.8
SnO2-Pd, CO @ 400 C
100
50
2.5 ppm
5 ppm
Response, S(Rair /Rgas)
600
Resistance (k Ω)
Conc. (ppm)
130
200
250
300
350
400
CO conc. (ppm)
Fig. 6. The CO response of the Pd-SnO2 NWs sensors: (A) the change in resistance of
sensors upon exposure to different concentrations of CO measured at 350, 400, and
450 ◦ C; (B) temperature dependence and (C) concentration dependence of sensors
response, respectively.
in Fig. 6B demonstrated that the optimal working temperature is
400 ◦ C. Higher or lower than this working temperature, the sensor showed a decrease in responsivity. Based on the gas-sensing
characteristics in Fig. 6A and in Fig. S3, the responsivities of three
sensors were plotted as functions of CO concentration (Fig. 6C). All
the sensors showed a nearly linear dependence of responsivity to
CO concentration, and no significant difference in responsivity of
the three sensors was found, indicating a good reproducibility of
sensors.
Development of gas sensors that can detect CO gas at low concentration is also important. In this study, the low-concentration
CO-sensing characteristics of the Pd NP-decorated SnO2 NW sensor from 1 ppm to 25 ppm were also investigated. The transient
resistance responses to CO gas measured at an optimal temperature of 400 ◦ C are shown in Fig. 7A. The sensor exhibited very
good sensing characteristics even at a low concentration of CO
(e.g., 1 ppm). The sensor also showed very fast response-recovery
time from 10 s to 40 s. Summary of CO sensing performances of
different sensors is presented in Table 1 (see ESI). Our developed PdSnO2 nanowire sensor exhibited a better CO sensing performance
100
80
60
40
7 cycles
20
0
200
400
600
800
1000
Time (s)
Fig. 7. Low concentration CO sensing characteristics of the Pd-SnO2 NWs at 400 ◦ C:
(A) the change in sensor resistance upon exposure to different concentrations of CO
and (B) stability of the sensor after seven cycles of switching on and off, from dry air
to CO gas and back to dry air.
than ZnO nanorods, SnO2 single nanowire, Pd/SnO2 nanoparticles
and NiO/SnO2 single nanowire, but comparable with In/Pd-doped
SnO2 nanoparticles and Pd/ZnO nanowire junctions. For instance,
according to literature, CO sensing based on SnO2 NW sensor has
a detection limit of approximately 5–10 ppm. A single SnO2 NW
sensor has a detection limit of 5 ppm at approximately 300 ◦ C
with a response time of 100 s (AC measurement) [49]. Kuang et al.
[50] reported that the detection limit of the pristine and NiOfunctionalized SnO2 NW-based sensors (FET type) to CO is 100 ppm
at 250 ◦ C. Qian et al. [51] demonstrated that the CO detection limit
of a sensor with a single SnO2 nanobelt decorated with Au NPs can
reach approximately 5 ppm. Our results demonstrated that Pd NPs
decorated on the surface of SnO2 NWs are not only promising in
screening CO at high concentration but also effective in detecting
at low ppm level (1–25 ppm), thereby satisfying the environmental
monitoring requirement. In addition to high responsivity, stability
and response-recovery time are two of the most important parameters of gas sensors for real-life applications. The stability of the
Pd-SnO2 NW sensor was investigated for seven switching on/off
cycles, from dry air to CO gas and back to dry air, as shown in Fig. 7B.
Author's personal copy
D.D. Trung et al. / Journal of Hazardous Materials 265 (2014) 124–132
131
Table 1
Summary of CO sensing performances of different sensors.
Materials
Methods
SnO2 single nanowire
SnO2 quantum dots
ZnO nanorods
NiO/SnO2 FET single nanowire
Pt/SnO2 porous solid
Pd/ZnO nanowire junctions
Pd/SnO2 NPs
In/Pd-doped SnO2 NPs
Pd/SnO2 nanowires
Gas (ppm)
Spray pyrolysis
Sono-chemical
Hydrothermal
Sputtering deposition of NiO
Solvothermal hot press
␥-Ray radiolysis of Pd
Infiltration process
Sol–gel
In situ reduction of Pd NPs
CO, 4 ppm
CO, 1000 ppm
CO, 30 ppm
CO, 500 ppm
CO, 100 ppm
CO, 0.1 ppm
CO, 18 ppm
CO, 1 ppm
CO, 1 ppm
The sensor exhibited stable signals after seven cycles, and no significant distortion in responsivity was observed. This result might be
attributed to the high crystalline feature and high thermal stability
of Pd-SnO2 NWs [31].
Selectivity is important in gas sensors. Thus, we present
the response of Pd-decorated SnO2 NW sensors to various
types of gases, including CO (1–25 ppm), H2 (10–50 ppm), NH3
(10–50 ppm), and CO2 (1000–15,000 ppm) at 400 ◦ C. The response
as a function of gas concentration is presented in Fig. 8. Apparently, the Pd-decorated SnO2 NW sensor exhibited a remarkably
improved selectivity to the investigated interference gases. The
responsivities to CO gas (1–25 ppm) ranged from 1.8 to 2.8, whereas
these values to H2 , NH3 , and CO2 with higher concentrations only
ranged from 1.1 to 1.6. Our recent work has reported that the pristine SnO2 NW sensor has very poor selectivity to the measured
gases [52,53]. If we define the selectivity factor = S/C where S is
sensor response, and C is gas concentration, then the selectivity
factor to CO (0.24) is higher than that of H2 (0.17), NH3 (0.106)
and CO2 (0.001). This result confirms that Pd decoration enhances
the response and selectivity to CO gas of SnO2 NW sensors. The
better response toward CO than that of H2 and other gases of the
synthesized Pd-SnO2 NWs was not clear yet and this needs further
investigation for clarifying. However, Liewhiran et al. reported the
higher response to H2 than to CO of the Pd-loaded SnO2 nanoparticles prepared by flame-spray method [54], whereas Zhang et al.
reported a much higher response to CO of SnO2 doped with Pd and
In than other gases such as C2 H5 OH, CH4 , H2 , CH3 COCH3 , NO2 , C2 H2 ,
NH3 [55].
Temp.
◦
250 C
225 ◦ C
400 ◦ C
250 ◦ C
Room temp.
Room temp.
60 ◦ C
140 ◦ C
400 ◦ C
Response (S = Rair /Rgas )
Refs.
1.019 (Rgas /Rair )
147
1.1
15.9
64.5
1.02
∼1.9
∼3
1.8
Sens. Actuators B 138 (2009) 160–167
Sens. Actuators B 145 (2010) 7–12
Sens. Actuators B 181 (2013) 529–536
J. Phys. Chem. C 112 (2008) 11,539–11,544
Sens. Actuators B, 184 (2013) 33–39
Sen. Actuators B, 168 (2012) 8
Sens. Actuators B 177 (2013) 770–775
Sens. Actuators B 139 (2009) 287–291
This work
4. Conclusions
We have introduced an effective method of decorating Pd NPs
on the surface of SnO2 NWs to enhance CO-sensing performance.
SnO2 NWs were directly grown on the sensor chips, whereas the
decoration of Pd NPs was performed by in situ reduction of the Pd
complex in the presence of pluronic P123 surfactant. This method
is very facile and effective, and can be applied to decorate Pd NPs
on the surface of various MOSs, such as ZnO and NiO. Gas-sensing
characterization demonstrated enhanced CO-sensing performance
of SnO2 NWs decorated with Pd NPs. Pd NP-decorated SnO2 NW
sensors can detect a very low concentration of CO, down to below
ppm level, with good response-recovery time and high stability.
Decoration of Pd NPs on the surface of MOSs is effective for detection of CO and potential for the next generation of gas sensors. It
can also be used in catalytic applications.
Acknowledgments
This work was finished through the collaboration between
the International Training Institute for Materials Science (iSensors
group, ITIMS) and the National Institute for Materials Science
(MANA) under the Memorandum of Understanding signed in 2012.
This research was funded by the Vietnam National Foundation for
Science and Technology Development (Code: 103.02-2011.40) and
VLIR-UOS under the Research Initiative Project (ZEIN2012RIP20).
Appendix A. Supplementary data
3.2
CO
H2
Response, S(Rair/Rgas)
2.8
NH3
CO2
2.4
2.0
1.6
1.2
0
20
40
5000
10000
15000
CO conc. (ppm)
Fig. 8. The CO, H2 , NH3 , and CO2 gases response of the Pd-SnO2 NWs sensor as a
function of gases concentration at temperature of 400 ◦ C.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jhazmat.
2013.11.054.
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