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Nanoparticles-assembled Co3O4 nanorods p-type nanomaterials: One-pot synthesis and toluene-sensing properties
Accepted Manuscript Title: Nanoparticles-assembled Co3 O4 Nanorods p-type Nanomaterials: One-pot Synthesis and Toluene-sensing Properties Author: Lili Wang Jianan Deng Zheng Lou Tong Zhang PII: DOI: Reference:
S0925-4005(14)00485-7 http://dx.doi.org/doi:10.1016/j.snb.2014.04.074 SNB 16840
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-11-2013 21-4-2014 23-4-2014
Please cite this article as: L. Wang, J. Deng, Z. Lou, T. Zhang, Nanoparticlesassembled Co3 O4 Nanorods p-type Nanomaterials: One-pot Synthesis and Toluene-sensing Properties, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.04.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanoparticles-assembled Co3O4 Nanorods p-type Nanomaterials: One-pot Synthesis and Toluene-sensing Properties Lili Wang, Jianan Deng, Zheng Lou, Tong Zhang*
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State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
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an
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and Engineering, Jilin University, Changchun 130012, PR China
*Corresponding author: E-mail address: [email protected] (T. Zhang) Tel.: +86 431 85168385; Fax: +86 431 85168270.
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Abstract One dimensional (1D) Co3O4 nanorods have been synthesized by a simple one-pot solvothermal method. Field emission scanning electron microscopic and transmission
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electron microscopic results revealed that the Co3O4 samples were rod-like structure
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with a diameter of 50 nm, where the subunits were irregular-shaped nanoparticles.
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The response to 10 ppm toluene of the rod-like Co3O4-based sensor was 6.0 at 200ºC, which was about 6 times higher than that of the commercial Co3O4 powder.
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Interestingly, the rod-like Co3O4-based sensor exhibited not only a high response but also faster response and recovery speeds to toluene gases. The good sensing
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performance suggests that this unique rod-like Co3O4 nanostructure could be a
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promising candidate as a sensing material for gas sensor.
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Keywords P-type, Co3O4, 1D nanostructure, Toluene sensing
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1. Introduction One-dimensional (1D) semiconducting metal oxide nanostructures have attracted great attention due to their small grain size and large surface area-to-volume ratio for
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optics, electronics, gas sensors and so on [1-5]. Various metal oxide 1D
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nanostructures that used in gas sensors, such as SnO2 [6], ZnO [7], TiO2 [8], In2O3 [9],
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WO3 [10], Fe2O3 [11], and Zn2SnO4 [12], have been demonstrated to have high sensitivity and fast response speed due to their large surface area-to-volume ratio and
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rapid electron transport. However, to date, most efforts in the field of metal oxide gas sensors have been devoted to n-type semiconductors, while the sensing properties of
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the p-type metal oxide semiconductors have scarcely been investigated. Cobalt oxide (Co3O4) is an important p-type semiconductor with normal spinel
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structure, and has received considerable attention in the past few years to its application potential in many technological areas such heterogeneous catalysts [13],
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anode materials in Li-ion rechargeable batteries [14], solid-state sensors [15], electrochromic devices [16], solar energy absorbers [17], and optical and magnetic materials [18-19]. Among these potential applications, Co3O4 could also be a good candidate in gas sensing field because it has been used in industry as an oxidation agent for incompletely combusted gases and toxic gases, due to its high catalytic reactivity [20]. Recently, gas sensors based on 1 D Co3O4 nanostructures have been reported by several groups. For example, Co3O4 nanorods have been reported to have high CO sensing response, which enhanced three times in response to CO compared with the commercial Co3O4 powder [21]. Lee et al. have reported the Co3O4
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nanofibers-based sensor with high sensitivity and good selectivity to ethanol [22]. Similarly, Safty et al. have prepared macroporous Co3O4 nanorods which also showed good sensing performance to ethanol [23]. However, to the best of our knowledge, the
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solvothermal synthesis of Co3O4 nanorods for toluene gas sensor application is rarely
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reported.
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In this work, we report a simple strategy for the synthesis of Co3O4 nanorods through a solvothermal process with subsequent calcination of the obtained precursor.
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The toluene sensing properties of as-synthesized nanorods based sensor are investigated. In comparison to the commercial Co3O4 powder, improved gas response
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and selectivity of the Co3O4 nanorods towards toluene have been achieved. In this study, the effects of carrier and reference gases on the sensing properties of Co3O4 are
Experiment
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2.
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also investigated to understand the sensing mechanisms of sensors.
2.1. Materials.
All the reagents were of analytical grade and used without further purification.
Cobalt nitrate (Co(NO3)2•2H2O), sodium citrate, ethanol and ammonia were
purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). 2.2. Synthesis Process. In a typical procedure, 0.1 g sodium citrate was dissolved in 20 mL ethanol to form a transparent solution. After that, 5 mL 1 M Co(NO3)2•2H2O aqueous solution was dropped into the mixture solution under magnetic stirring. The pH value of the
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solution was adjusted to 13 by concentrated ammonia hydroxide (28%). After vigorous stirring for 30 min, the final solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 170ºC for 12 h in an
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electric oven. After the reaction was finished, the resulting solid products were
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centrifuged, washed with distilled water and ethanol to remove the ions possibly
were calcined at 450ºC for 3 h in a tube furnace.
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2.3. Characterizations
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remaining in the final products, and then dried at 60ºC in air. Finally, the products
X-ray diffraction patterns (XRD) were conducted on a Rigaku D/Max-2550
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diffractometer with Cu Kα radiation (λ= 0.15418 nm, 40 kV, 200 mA) in the range of 20-80° (2θ) at a scanning rate 6° min-1. Field emission scanning electron microscope
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(FESEM) equipped with an energy-dispersive X-ray spectrometer (EDX) was performed on a XL 30 ESEM FEG. The transmission electron microscopic (TEM)
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imaging was performed on a JEOL JEM-3010 TEM microscope (200 kV). 2.4. Fabrication and measurement of gas sensor Gas sensors were fabricated as follows: Fig. 1a-b shows a structure of the
as-fabricated sensor and a photograph of the sensor on the socket, respectively. The samples were mixed with deionized water at a weight ratio of 4:1 to form a paste. The paste with the thickness of about 500 μm was coated on a ceramic tube to form a gas sensor, and the sensor was dried in air at 60 ºC for 5 h. A Ni-Cr heating wire was used as a resistor. The different operating temperatures of the sensor were obtained by adjusting heating current of CGS-8 series Intelligent Test Meter. After the solvent was
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evaporated, the morphology of a Co3O4 sensing film on the ceramic tube was rod-like structures as shown in the FESEM images in the inset of Fig. 1b. The detailed fabrication processes of gas sensor were detailedly described in these reports [12, 24].
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The electrical properties of the sensor were measured by a CGS-8 series Intelligent
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Test Meter (China). The working voltage (VS) is 5 V and the reference resistance (RL)
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is 1 kΩ～1 MΩ. The response (S=Rg/Ra) of the sensor was defined as the ratio of sensor resistance in a target gas (Rg) to that in dry air (Ra) at the operating temperature
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between 120ºC and 260ºC. The time taken by the resistor to range from Ra to Ra+90% (Rg-Ra) is defined as the response time, when the sensor is exposed to the target gas.
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The time taken by the resistor to change from Rg to Rg-90% (Rg-Ra) is defined as the
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recovery time, when the sensor is retrieved from the target gas.
Results and discussion
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3.1. Structural and morphological characteristics The crystallinity and phase information of the products after heat treatment at
450ºC for 3 h in air were characterized by XRD. Fig. 2 is the XRD patterns of the rod-like and commercial Co3O4 powder. It can be clearly seen that the positions and
relative intensity of the main diffraction peaks of as-prepared Co3O4 nanorods (Fig. 2a) and commercial Co3O4 powder (Fig. 2b) were rather similar, and all of the diffraction peaks could be perfectly indexed to the cubic Co3O4 phase, which were in good consistence with those from the standard card (JCPDS 42-1467). Moreover, the high intensity of the diffraction peaks indicates that the Co3O4 material is highly
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crystalline. The morphologies and structures of the Co3O4 nanorods and commercial Co3O4 powder were examined with FESEM and TEM, respectively. From the
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low-magnification FESEM image in Fig. 3a, it is obvious that the as-prepared
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precursors before heat treatment at 450ºC have uniform nanorod shape without
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nanoparticles appeared. As shown in Fig. 3b, a large quantity of rod-like Co3O4 nanostructures with a length of about 1-3 µm are formed after calcining at 450ºC. The
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magnified FESEM image in Fig. 3c also clearly shows that these rod-like nanostructures are oriented assembly of many fine nanoparticles with a width of
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40-50 nm. In contrast, Fig. 3d-e illustrates the FESEM images of commercial Co3O4 powder. From these FESEM images, we find that the commercial Co3O4 powder
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exhibit an aggregation phenomenon, which indicated that they are composed of nanoparticles with the sizes in the range of several tens nanometer. The nanostructures
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of the Co3O4 nanorods and commercial Co3O4 powder were further investigated by
TEM. TEM images in Fig. 3f show that the Co3O4 samples are rod-like nanostructures with numerous nanoparticles. The high-magnification TEM image of an individual Co3O4 nanorod (Fig. 3g) clearly demonstrates that the individual Co3O4 nanorod has loose interiors structures and well-aligned porous structures, which agrees with FESEM observations. Fig. 3h is a HRTEM image taken from a single nanoparticle within the individual Co3O4 nanorod. The typical lattice fringe spacing is
determined to be 0.286 nm, corresponding to the (220) d spacing of the cubic spinel Co3O4 phase, which clearly demonstrate that the nanorod is consist of the single
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crystalline nanoparticles. By comparison, the commercial Co3O4 powder exhibited aggregation structures (Fig. 3i). Composition analysis was measured by energy dispersive X-ray analysis (EDX), as shown in Fig. 4. The result indicates that the
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prepared Co3O4 nanorods are composed of only two elements: Co and O.
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3.2. Gas-sensing properties
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Toluene is one of the most toxic, dangerous and neurotoxic compound among the volatile organic compounds (VOCs) widely present in paints, adhesives, rubber and in
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some printing. Toluene is also one of indoor air pollutants resulting in sick building syndrome (SBS) and harmful to human beings even at very low concentrations [25].
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Therefore, measurement and control of toluene emissions are becoming more and more important. The World Health Organization (WHO) has set a standard that the
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lowest level of chronic occupational toluene exposure unequivocally associated with
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neurobehavioral functional decrements is 332 mg/m3 (88 ppm) [26]. Fig. 5a shows the
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responses of the two sensors to 200 ppm of toluene at operating temperatures from 120 to 260ºC. The response of the sensor increased rapidly with an increase in the operating temperature. At 200ºC, the gas response is peaked to its maximum value of about 35. Above 200ºC, the gas response decreased as the operating temperature increased further. In contrast, when the commercial Co3O4 powder was used, the gas response to 200 ppm of toluene gas was relative low, with the maximum value of about 3.8 at the operating temperature of 200ºC. Thus, we choose 200ºC as our
operating temperature to proceed with the subsequent detections. Moreover, it can also be seen that the gas response of the Co3O4 nanorods to 200 ppm toluene is about
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10 times greater than that of commercial Co3O4 powder. These results show that the sensor based on the rod-like Co3O4 structures has an excellent toluene sensing properties. Fig. 5b shows the resistance-temperature behavior in air of Co3O4 sensors.
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In the whole operating temperature range, the resistance values of all the sensors
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decrease with increasing operating temperature due to the intrinsic characteristics of
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the semiconductor [21]. The resistance values of as-prepared Co3O4 nanorods and commercial Co3O4 powder sensors were 3.0 kΩ and 1.4 kΩ at the temperature of
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200ºC, respectively.
Fig. 6a is the dynamic response-recovery curves of the two sensors to different
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toluene concentrations (10-200 ppm) at the operating temperature of 200ºC. Evidently, the Co3O4 nanorods-based sensor (solid line) has the highest response amplitudes
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towards each concentration. The response magnitude of the sensor based on the as-synthesized Co3O4 nanorods was improved dramatically with increasing
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concentration of the toluene gas and was much higher than that of the commercial powder (dotted line). This means that the Co3O4 nanorods are more sensitive to toluene than that of the commercial powder. This could be attributed to the smaller size and higher surface area of the Co3O4 nanorods. The responses of the Co3O4 nanorods-based sensors are about 6, 14, 26, 35 to 10, 50, 100 and 200 ppm toluene, respectively. Even in low concentration level of toluene gases (e.g. 10 ppm), the Co3O4 nanorods-based sensor shows an acceptable response (6.0) from the view of the practical application. Response and recovery times are two important factors of gas sensors. As shown in Fig. 6b, the Co3O4 nanorods gas sensor exhibits faster
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response and recovery than the commercial powder. When exposed to 200 ppm toluene, the response and recovery times are about 90 and 55 s for the Co3O4 nanorods-based sensor (Fig. 6b), and 157 and 88 s for the commercial powder-based
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sensor (Fig. 6c), respectively. A comparison between the toluene performances of the
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Co3O4 nanorods-based sensors and some previously reported is summarized in Table
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1 [27-31]. The sensor exhibits higher response, lower operating temperature in the presence of toluene gas than most others, likely due to the 1D nanostructure and the
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unique properties of Co3O4 for the toluene gas.
We then investigated the response of these sensors at different toluene
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concentrations. Fig. 7 shows the response of Co3O4 nanorods (●) sensor increased rapidly with further increases in toluene concentration from 10 to 200 ppm, but
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increased more slowly from 200 to 1000 ppm as the sensor began to saturate. The gas responses of Co3O4 nanorods-based sensor to 10-1000 ppm toluene are ranging from
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6 to 47, which were much higher than that of commercial Co3O4 powder (■). The inset of Fig. 7 shows that the increase in the responses depends nearly linear on the
gas concentrations in the range from 10 to 200 ppm, which further confirms that the present Co3O4 nanorods are more favourable to detect toluene with low concentration. Fig. 8 depicts the histogram of the responses of the sensor based on as-prepared the
Co3O4 nanorods and commercial Co3O4 powder to various gas vapours, including reducing gases: C7H8, C6H6, C8H10, C3H6O, CO, C2H2, NH3, H2 and oxidizing gases (inset of Fig. 8): NO2, Cl2, CH4. All of the gases were tested at an operating temperature of 200ºC with a concentration of 200 ppm. Clearly, the responses of
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Co3O4 nanorods-based sensor to all gases are all improved compared with the other, and the largest increase is only observed for C7H8, implying the good selectivity of
3.3 Gas sensing mechanism of rod-like Co3O4 nanostructures
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the sensor for C7H8.
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Here, we suggest a possible mechanism to explain the toluene sensing properties of
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rod-like Co3O4 nanostructures. For most semiconducting oxide gas sensors, they are based on the conductivity changes caused by the adsorption and desorption of the gas
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molecules on the surface of the sensing structure [32-33]. Generally speaking, the majority of charge carriers near the surfaces of oxide semiconductors are significantly
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different in p- and n-type oxide semiconductors [21, 34]. As is known to all, Co3O4 is an important p-type semiconductor with hole carriers. The toluene sensing process is
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based on the changes in the conductivity of the rod-like Co3O4 nanostructures which is controlled by the toluene species and the amount of the chemisorbed oxygen on the
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surface.
By considering Co3O4 nanorods, it can be observed (in the Fig. 3b) that many
nanoparticles self-assembled into rod-like Co3O4 nanostructures with loose interiors structures. In air, oxygen molecules can be adsorbed on the surface of the Co3O4 nanorods to form chemisorbed oxygen (O-, O2-) by capturing free electrons from the
conduction band. The reaction kinematics may be explained by the following reactions in Eq (1-4) [35]:
O2 (gas) → O2 (ads)
(1)
O2 (ads) + e- → O2- (ads)
(2)
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O2- (ads) + e- → 2O-(ads)
(3)
O-(ads) + e- → O2-(ads)
(4)
For p-type Co3O4, the adsorption of the oxygen molecules results in an increase in
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the carrier density on the surface of the nanorods (Fig. 9a). As a consequence, an
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increase in the conductivity of the Co3O4 is observed. When the Co3O4 nanorods are
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exposed to toluene gas, the toluene molecules are easy to spread inward through well-aligned porous structures. Therefore both the effective contacts surface area and
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the amount of toluene molecules absorbed onto the surface of the rod-like Co3O4 will increase in comparison with the agglomeration structures (Fig. 9b). Then, the toluene
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molecules interact with pre-adsorbed oxygen as depicted in Eq (5). The interaction between toluene and pre-adsorbed oxygen releases free electrons. Those released
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electrons neutralize the holes in the Co3O4, which results in a decrease in the number of hole carrier in Co3O4, and consequently, an increase in sensor resistance. Thus, the
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response of the rod-like Co3O4 nanostructures is distinctly higher than that of the
commercial Co3O4 powder.
C7H8 + 2O− → C7H6O− + H2O + e−
4.
(5)
Conclusion
In summary, large yields of nanoparticles-assembled Co3O4 nanorods have been successfully synthesized using the solvothermal method and without using any structure-directing agents. The synthesized nanorods were found to be effective in the detection of toluene. The maximum gas response to 200 ppm of toluene is 35 at an
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operating temperature of 200ºC. The synthesized Co3O4 nanorods-based sensor shows low detection limit of 10 ppm to toluene. These results indicate that the Co3O4
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nanostructure, which show a potential application as toluene sensors.
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nanorods can significantly improve the toluene sensing properties due to the 1D
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Acknowledgement
This research work was financially supported by the Program for Chang Jiang
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Scholars and Innovative Research Team in University (no. IRT1017). Project
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Supported by Graduate Innovation Fund of Jinlin University (Grant No. 20121108).
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References [1] J. Gao, L.L. Wang, K. Kan, S. Xu, L.Q. Jing, S.Q. Liu, P.K. Shen, L. Li, K.Y. Shi, One-step synthesis of mesoporous Al2O3-In2O3 nanofibres with remarkable
ip t
gas-sensing performance to NOx at room temperature, J. Mater. Chem. A 2 (2014)
cr
949-956.
us
[2] N. Du, H. Zhang, D.R. Yang, One-dimensional hybrid nanostructures: synthesis via layer-by-layer assembly and applications, Nanoscale 4 (2012) 5517-5526.
an
[3] L.L. Wang, X.L. Luo, X.J. Zheng, R. Wang, T. Zhang, Direct annealing of electrospun synthesized high-performance porous SnO2 hollow nanofibers for gas
M
sensor, RSC Adv. 3 (2013) 9723-9728.
[4] J. Moona, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber
te
d
networks for gas sensor applications, Sens. Actuators B 149 (2010) 301-305. [5] Y.H. Li, J. Gong, G.H. He, Y.H. Deng, Synthesis of polyaniline nanotubes using
Ac ce p
Mn2O3 nanofibers as oxidant and their ammonia sensing properties, Synthetic Met.
161 (2011) 56-61.
[6] Y. Zhang, X. He, J. Li, Z. Miao, F. Huang, Fabrication and ethanol-sensing properties of micro gas sensor based on electrospun SnO2 nanofibers, Sens. Actuators B 132 (2008) 67-73.
[7] L. Peng, P.F. Qin, Q.R. Zeng, H.J. Song, M. Lei, J.J.N. Mwangi, D.J. Wang, T.F. Xie, Improvement of formaldehyde sensitivity of ZnO nanorods by modifying with Ru(dcbpy)2(NCS)2, Sens. Actuators B 160 (2011) 39-45. [8] I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive
14
Page 14 of 32
chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009-2013. [9] W. Zheng, X. Liu, W. Wang, Z. Li, H. Zhang, W. Wang, Z. Wang, C. Wang, A highly sensitive and fast-responding sensor based on electrospun In2O3 nanofibers,
ip t
Sens. Actuators B 142 (2009) 61-65.
cr
[10] G. Wang, Y. Ji, X. Huang, X. Yang, P.I. Gouma, M. Dudley, Fabrication and
sensing, J. Phys. Chem. B 110 (2006) 23777-23782.
us
characterization of polycrystalline WO3 nanofibers and their application for ammonia
an
[11] L.L. Wang, T. Fei, Z. Lou, T. Zhang, Three-Dimensional Hierarchical Flowerlike
Interfaces 3 (2011) 4689-4694.
M
α-Fe2O3 Nanostructures: Synthesis and Ethanol-Sensing Properties, ACS Appl. Mater.
[12] Y. Zeng, K. Zhang, X.L. Wang, Y.M. Sui, B. Zou, W.T. Zheng, G.T. Zou, Rapid
te
d
and selective H2S detection of hierarchical ZnSnO3 nanocages, Sens. Actuators B 159 (2011) 245-250.
Ac ce p
[13] L.H. Hu, Q. Peng, Y.D. Li, Selective synthesis of Co3O4 nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion, J. Am. Chem. Soc. 130 (2008) 16136-16137. [14] W.Y. Li, L.N. Xu, J. Chen, Co3O4 nanomaterials in lithium-ion batteries and gas
sensors, Adv. Funct. Mater. 15 (2005) 851-857. [15] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Highly selective CO sensor using indium oxide doubly promoted by cobalt oxide and gold, J. Elec-trochem. Soc. 144 (1997) 158-160. [16]
F.
Svegl,
B.
Orel,
M.G.
Hutchins,
K.
Kalcher,
Structural
and
15
Page 15 of 32
spectroelectrochemical investigations of sol-gel derived electrochromic spinel Co3O4 films, J. Electrochem. Soc. 143 (1996) 1532-1539. [17] M.G. Hutchins, P.J. Wright, P.D. Grebenik, Comparison of different forms of
ip t
black cobalt selective solar-absorber surfaces, Solar Energy Mater. 16 (1987) 113-131.
cr
[18] N.A.M. Barakat, M.S. Khil, F.A. Sheikh, H.Y. Kim, Synthesis and optical
us
properties of two cobalt oxides (CoO and Co3O4) nanofibers produced by electrospinning process, J. Phys. Chem. C 112 (2008) 12225-12233.
an
[19] G.X. Wang, X.P. Shen, J. Horvat, B. Wang, H. Liu, D. Wexler, J. Yao, Hydrothermal synthesis and optical, magnetic, and supercapacitance properties of
M
nanoporous cobalt oxide nanorods, J. Phys. Chem. C 113 (2009) 4357-4361. [20] A.M. Cao, J.S. Hu, H.P. Liang, W.G. Song, L.J. Wan, X.L. He, X.G. Gao, S.H.
te
d
Xia, Hierarchically structured cobalt oxide (Co3O4): the morphology control and its potential in sensors, J. Phys. Chem. B 110 (2006) 15858-15863.
Ac ce p
[21] D. Patil, P. Patil, V. Subramanian, P.A. Joy, H.S. Potdar, Highly sensitive and fast responding CO sensor based on Co3O4 nanorods, Talanta 81 (2010) 37-43. [22] J.W. Yoon, J.K. Choi, J.H. Lee, Design of a highly sensitive and selective C2H5OH sensor using p-type Co3O4 nanofibers, Sens. Actuators B 161 (2012)
570-577.
[23] H. Nguyen, S.A. Safty, Meso- and macroporous Co3O4 nanorods for effective VOC gas sensors, J. Phys. Chem. C 115 (2011) 8466-8474. [24] L.L. Wang, H.M. Dou, Z. Lou, T. Zhang, Encapsuled nanoreactors (Au@SnO2): a new sensing material for chemical sensor, Nanoscale 5 (2013) 2686-2691.
16
Page 16 of 32
[25] H. Ma, Y.M. Xu, Z.M. Rong, X.L. Cheng, S. Gao, X.F. Zhang, H. Zhao, L.H. Huo, Highly toluene sensing performance based on monodispersed Cr2O3 porous microspheres, Sens. Actuators B 174 (2012) 325-331.
ip t
[26] WHO Regional Office for Europe, Air quality guidelines for Europe, 2000,
cr
Copenhagen, Denmark.
us
[27] Q. Qi, T. Zhang, L. Liu, X.J. Zheng, Synthesis and toluene sensing properties of SnO2 nanofibers, Sens. Actuators B 137 (2009) 471-475.
an
[28] J.R. Huang, X.J. Xu, C.P. Gu, W.Z. Wang, B.Y. Geng, Y.F. Sun, J.H. Liu Size-controlled synthesis of porous ZnSnO3 cubes and their gas-sensing and
M
photocatalysis properties, Sens. Actuators B 171 (2012) 572-579. [29] J.R. Huang, X.J. Xu, C.P. Gu, W.Z. Wang, B.Y. Geng, Y.F. Sun, J.H. Liu,
te
d
Effective VOCs gas sensor based on porous SnO2 microcubes prepared via spontaneous phase segregation, Sens. Actuators B 173 (2012) 599-606.
Ac ce p
[30] C.P. Gu, X.J. Xu, J.R. Huang, W.Z. Wang, Y.F. Sun, J.H. Liu, Porous flower-like SnO2 nanostructures as sensitive gas sensors for volatile organic compounds detection,
Sens. Actuators B 174 (2012) 31-38. [31] Y. Zeng, T. Zhang, L.J. Wang, M.H. Kang, H.T. Fan, R. Wang, Y. He, Enhanced toluene sensing characteristics of TiO2-doped flowerlike ZnO nanostructures, Sens. Actuators B 140 (2009) 73-78. [32] J.C. Belmonte, J. Manzano, J. Arbiol, A. Cirera, J. Puigcorbe, A. Vila, N. Sabate, I. Gracia, C. Cane, J.R. Morante, Micromachined twin gas sensor for CO and O2 quantification based on catalytically modified nano-SnO2, Sens. Actuators B 114
17
Page 17 of 32
(2006) 881-892. [33] Z. Lou, L.L. Wang, T. Fei, T. Zhang, Enhanced ethanol sensing properties of NiO-doped SnO2 polyhedra, New J. Chem. 36 (2012) 1003-1007.
cr
semiconductors: Overview, Sens. Actuators B 192 (2014) 607-627.
ip t
[34] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide
Ac ce p
te
d
M
an
thick films, Sens. Actuators B 111 (2005) 58-62.
us
[35] K. Arshak, I. Gaidan, Gas sensing properties of ZnFe2O4/ZnO screen-printed
18
Page 18 of 32
Biographies
Lili Wang received her MS degree in Chemistry from Jilin University, China in 2010.
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She entered the PhD course in 2010, majoring in microelectronics and solid-state
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electronics. She is studying the synthesis and characterization of semiconducting,
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functional materials and gas sensors.
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Jianan Deng received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2011. As an MS student, his research interest is
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gas sensors based on semiconducting functional materials.
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Zheng Lou received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2009. He entered the PhD course in 2011 As a
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PhD student, his research interest is gas sensors based on semiconducting functional materials.
Tong Zhang completed her MS degree in semiconductor materials in 1992 and her PhD in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors.
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TABLE CAPTIONS
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Table 1. Toluene sensors in the present study and those reported in the literature.
FIGURE CAPTIONS
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Figure 1. (a) Schematic diagram showing the structure of a typical gas sensor, (b)
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photograph and FESEM image (inset, b) of the Co3O4-based sensor, and (c) theoretic diagram of the test circuit (VH: heating voltage, VS: circuit voltage and RL: load
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resistance).
Figure 2. XRD patterns of (a) Co3O4 nanorods calcined at 450ºC and (b) commercial
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Co3O4 powder.
Figure 3. FESEM and TEM images of the Co3O4 products: (a) un-thermal treatment
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precursors (b-c) Co3O4 nanorods calcined at 450 ºC; (d-e) commercial Co3O4 powder; (f-g) TEM images of the Co3O4 nanorods; (h) the corresponding HRTEM image of the
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Co3O4 nanorods; (i) TEM image of commercial Co3O4 powder. Figure 4. The corresponding EDX patterns of the Co3O4 nanorods and commercial Co3O4 powder.
Figure 5. (a) Responses of two sensors to 200 ppm toluene at different operating temperatures, (b) the relationship between the resistance in air and the operating temperatures of the sensor based on Co3O4 nanorods and commercial Co3O4 powder, respectively. Figure 6. (a) Responses of Co3O4 nanorods (solid line) and commercial Co3O4 powder (dotted line) versus different concentrations of toluene at 200ºC. Dynamic
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toluene sensing curves of the (b) Co3O4 nanorods and (c) commercial Co3O4 powder. Figure 7. Responses of Co3O4 nanorods (●) and commercial Co3O4 powder (■) at 200 ºC to versus toluene concentrations. The inset shows the sensitivity on the toluene
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concentration in the range of 10-200 ppm.
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Figure 8. Responses of Co3O4 nanorods and commercial Co3O4 powder versus 200
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ppm of various gas vapours (reducing gases) at 200ºC. Inset: responses of two sensors to oxidizing gases.
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Figure 9. Schematic diagrams on the gas sensing mechanism of the products: (a)
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Co3O4 nanorods and (b) commercial Co3O4 powder.
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Concentration/ppm
Temprature/ºC
200
200
SnO2 nanofibers [27]
200
350
ZnSnO3 nanocube [28]
200
SnO2 nanocube [29]
200
SnO2 flower-like [30]
200
ZnO flower-like [31]
200
Response 35 6
300
15
240
16
260
12
390
15
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This work
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Sensing material
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Table 1.
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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Figure 8.
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Figure 9.
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Graphical Abstract Nanoparticles‐assembled Co3O4 1D nanorods have been synthesized by a solvothermal method and exhibited higher toluene (C7H8) response than that of the commercial Co3O4 powders.
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S0925-4005(14)00485-7 http://dx.doi.org/doi:10.1016/j.snb.2014.04.074 SNB 16840
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-11-2013 21-4-2014 23-4-2014
Please cite this article as: L. Wang, J. Deng, Z. Lou, T. Zhang, Nanoparticlesassembled Co3 O4 Nanorods p-type Nanomaterials: One-pot Synthesis and Toluene-sensing Properties, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.04.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanoparticles-assembled Co3O4 Nanorods p-type Nanomaterials: One-pot Synthesis and Toluene-sensing Properties Lili Wang, Jianan Deng, Zheng Lou, Tong Zhang*
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State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
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and Engineering, Jilin University, Changchun 130012, PR China
*Corresponding author: E-mail address: [email protected] (T. Zhang) Tel.: +86 431 85168385; Fax: +86 431 85168270.
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Abstract One dimensional (1D) Co3O4 nanorods have been synthesized by a simple one-pot solvothermal method. Field emission scanning electron microscopic and transmission
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electron microscopic results revealed that the Co3O4 samples were rod-like structure
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with a diameter of 50 nm, where the subunits were irregular-shaped nanoparticles.
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The response to 10 ppm toluene of the rod-like Co3O4-based sensor was 6.0 at 200ºC, which was about 6 times higher than that of the commercial Co3O4 powder.
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Interestingly, the rod-like Co3O4-based sensor exhibited not only a high response but also faster response and recovery speeds to toluene gases. The good sensing
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performance suggests that this unique rod-like Co3O4 nanostructure could be a
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promising candidate as a sensing material for gas sensor.
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Keywords P-type, Co3O4, 1D nanostructure, Toluene sensing
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1. Introduction One-dimensional (1D) semiconducting metal oxide nanostructures have attracted great attention due to their small grain size and large surface area-to-volume ratio for
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optics, electronics, gas sensors and so on [1-5]. Various metal oxide 1D
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nanostructures that used in gas sensors, such as SnO2 [6], ZnO [7], TiO2 [8], In2O3 [9],
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WO3 [10], Fe2O3 [11], and Zn2SnO4 [12], have been demonstrated to have high sensitivity and fast response speed due to their large surface area-to-volume ratio and
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rapid electron transport. However, to date, most efforts in the field of metal oxide gas sensors have been devoted to n-type semiconductors, while the sensing properties of
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the p-type metal oxide semiconductors have scarcely been investigated. Cobalt oxide (Co3O4) is an important p-type semiconductor with normal spinel
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structure, and has received considerable attention in the past few years to its application potential in many technological areas such heterogeneous catalysts [13],
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anode materials in Li-ion rechargeable batteries [14], solid-state sensors [15], electrochromic devices [16], solar energy absorbers [17], and optical and magnetic materials [18-19]. Among these potential applications, Co3O4 could also be a good candidate in gas sensing field because it has been used in industry as an oxidation agent for incompletely combusted gases and toxic gases, due to its high catalytic reactivity [20]. Recently, gas sensors based on 1 D Co3O4 nanostructures have been reported by several groups. For example, Co3O4 nanorods have been reported to have high CO sensing response, which enhanced three times in response to CO compared with the commercial Co3O4 powder [21]. Lee et al. have reported the Co3O4
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nanofibers-based sensor with high sensitivity and good selectivity to ethanol [22]. Similarly, Safty et al. have prepared macroporous Co3O4 nanorods which also showed good sensing performance to ethanol [23]. However, to the best of our knowledge, the
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solvothermal synthesis of Co3O4 nanorods for toluene gas sensor application is rarely
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reported.
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In this work, we report a simple strategy for the synthesis of Co3O4 nanorods through a solvothermal process with subsequent calcination of the obtained precursor.
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The toluene sensing properties of as-synthesized nanorods based sensor are investigated. In comparison to the commercial Co3O4 powder, improved gas response
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and selectivity of the Co3O4 nanorods towards toluene have been achieved. In this study, the effects of carrier and reference gases on the sensing properties of Co3O4 are
Experiment
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2.
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also investigated to understand the sensing mechanisms of sensors.
2.1. Materials.
All the reagents were of analytical grade and used without further purification.
Cobalt nitrate (Co(NO3)2•2H2O), sodium citrate, ethanol and ammonia were
purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). 2.2. Synthesis Process. In a typical procedure, 0.1 g sodium citrate was dissolved in 20 mL ethanol to form a transparent solution. After that, 5 mL 1 M Co(NO3)2•2H2O aqueous solution was dropped into the mixture solution under magnetic stirring. The pH value of the
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solution was adjusted to 13 by concentrated ammonia hydroxide (28%). After vigorous stirring for 30 min, the final solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 170ºC for 12 h in an
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electric oven. After the reaction was finished, the resulting solid products were
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centrifuged, washed with distilled water and ethanol to remove the ions possibly
were calcined at 450ºC for 3 h in a tube furnace.
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2.3. Characterizations
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remaining in the final products, and then dried at 60ºC in air. Finally, the products
X-ray diffraction patterns (XRD) were conducted on a Rigaku D/Max-2550
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diffractometer with Cu Kα radiation (λ= 0.15418 nm, 40 kV, 200 mA) in the range of 20-80° (2θ) at a scanning rate 6° min-1. Field emission scanning electron microscope
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(FESEM) equipped with an energy-dispersive X-ray spectrometer (EDX) was performed on a XL 30 ESEM FEG. The transmission electron microscopic (TEM)
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imaging was performed on a JEOL JEM-3010 TEM microscope (200 kV). 2.4. Fabrication and measurement of gas sensor Gas sensors were fabricated as follows: Fig. 1a-b shows a structure of the
as-fabricated sensor and a photograph of the sensor on the socket, respectively. The samples were mixed with deionized water at a weight ratio of 4:1 to form a paste. The paste with the thickness of about 500 μm was coated on a ceramic tube to form a gas sensor, and the sensor was dried in air at 60 ºC for 5 h. A Ni-Cr heating wire was used as a resistor. The different operating temperatures of the sensor were obtained by adjusting heating current of CGS-8 series Intelligent Test Meter. After the solvent was
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evaporated, the morphology of a Co3O4 sensing film on the ceramic tube was rod-like structures as shown in the FESEM images in the inset of Fig. 1b. The detailed fabrication processes of gas sensor were detailedly described in these reports [12, 24].
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The electrical properties of the sensor were measured by a CGS-8 series Intelligent
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Test Meter (China). The working voltage (VS) is 5 V and the reference resistance (RL)
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is 1 kΩ～1 MΩ. The response (S=Rg/Ra) of the sensor was defined as the ratio of sensor resistance in a target gas (Rg) to that in dry air (Ra) at the operating temperature
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between 120ºC and 260ºC. The time taken by the resistor to range from Ra to Ra+90% (Rg-Ra) is defined as the response time, when the sensor is exposed to the target gas.
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The time taken by the resistor to change from Rg to Rg-90% (Rg-Ra) is defined as the
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recovery time, when the sensor is retrieved from the target gas.
Results and discussion
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3.1. Structural and morphological characteristics The crystallinity and phase information of the products after heat treatment at
450ºC for 3 h in air were characterized by XRD. Fig. 2 is the XRD patterns of the rod-like and commercial Co3O4 powder. It can be clearly seen that the positions and
relative intensity of the main diffraction peaks of as-prepared Co3O4 nanorods (Fig. 2a) and commercial Co3O4 powder (Fig. 2b) were rather similar, and all of the diffraction peaks could be perfectly indexed to the cubic Co3O4 phase, which were in good consistence with those from the standard card (JCPDS 42-1467). Moreover, the high intensity of the diffraction peaks indicates that the Co3O4 material is highly
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crystalline. The morphologies and structures of the Co3O4 nanorods and commercial Co3O4 powder were examined with FESEM and TEM, respectively. From the
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low-magnification FESEM image in Fig. 3a, it is obvious that the as-prepared
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precursors before heat treatment at 450ºC have uniform nanorod shape without
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nanoparticles appeared. As shown in Fig. 3b, a large quantity of rod-like Co3O4 nanostructures with a length of about 1-3 µm are formed after calcining at 450ºC. The
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magnified FESEM image in Fig. 3c also clearly shows that these rod-like nanostructures are oriented assembly of many fine nanoparticles with a width of
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40-50 nm. In contrast, Fig. 3d-e illustrates the FESEM images of commercial Co3O4 powder. From these FESEM images, we find that the commercial Co3O4 powder
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exhibit an aggregation phenomenon, which indicated that they are composed of nanoparticles with the sizes in the range of several tens nanometer. The nanostructures
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of the Co3O4 nanorods and commercial Co3O4 powder were further investigated by
TEM. TEM images in Fig. 3f show that the Co3O4 samples are rod-like nanostructures with numerous nanoparticles. The high-magnification TEM image of an individual Co3O4 nanorod (Fig. 3g) clearly demonstrates that the individual Co3O4 nanorod has loose interiors structures and well-aligned porous structures, which agrees with FESEM observations. Fig. 3h is a HRTEM image taken from a single nanoparticle within the individual Co3O4 nanorod. The typical lattice fringe spacing is
determined to be 0.286 nm, corresponding to the (220) d spacing of the cubic spinel Co3O4 phase, which clearly demonstrate that the nanorod is consist of the single
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crystalline nanoparticles. By comparison, the commercial Co3O4 powder exhibited aggregation structures (Fig. 3i). Composition analysis was measured by energy dispersive X-ray analysis (EDX), as shown in Fig. 4. The result indicates that the
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prepared Co3O4 nanorods are composed of only two elements: Co and O.
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3.2. Gas-sensing properties
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Toluene is one of the most toxic, dangerous and neurotoxic compound among the volatile organic compounds (VOCs) widely present in paints, adhesives, rubber and in
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some printing. Toluene is also one of indoor air pollutants resulting in sick building syndrome (SBS) and harmful to human beings even at very low concentrations [25].
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Therefore, measurement and control of toluene emissions are becoming more and more important. The World Health Organization (WHO) has set a standard that the
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lowest level of chronic occupational toluene exposure unequivocally associated with
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neurobehavioral functional decrements is 332 mg/m3 (88 ppm) [26]. Fig. 5a shows the
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responses of the two sensors to 200 ppm of toluene at operating temperatures from 120 to 260ºC. The response of the sensor increased rapidly with an increase in the operating temperature. At 200ºC, the gas response is peaked to its maximum value of about 35. Above 200ºC, the gas response decreased as the operating temperature increased further. In contrast, when the commercial Co3O4 powder was used, the gas response to 200 ppm of toluene gas was relative low, with the maximum value of about 3.8 at the operating temperature of 200ºC. Thus, we choose 200ºC as our
operating temperature to proceed with the subsequent detections. Moreover, it can also be seen that the gas response of the Co3O4 nanorods to 200 ppm toluene is about
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10 times greater than that of commercial Co3O4 powder. These results show that the sensor based on the rod-like Co3O4 structures has an excellent toluene sensing properties. Fig. 5b shows the resistance-temperature behavior in air of Co3O4 sensors.
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In the whole operating temperature range, the resistance values of all the sensors
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decrease with increasing operating temperature due to the intrinsic characteristics of
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the semiconductor [21]. The resistance values of as-prepared Co3O4 nanorods and commercial Co3O4 powder sensors were 3.0 kΩ and 1.4 kΩ at the temperature of
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200ºC, respectively.
Fig. 6a is the dynamic response-recovery curves of the two sensors to different
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toluene concentrations (10-200 ppm) at the operating temperature of 200ºC. Evidently, the Co3O4 nanorods-based sensor (solid line) has the highest response amplitudes
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towards each concentration. The response magnitude of the sensor based on the as-synthesized Co3O4 nanorods was improved dramatically with increasing
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concentration of the toluene gas and was much higher than that of the commercial powder (dotted line). This means that the Co3O4 nanorods are more sensitive to toluene than that of the commercial powder. This could be attributed to the smaller size and higher surface area of the Co3O4 nanorods. The responses of the Co3O4 nanorods-based sensors are about 6, 14, 26, 35 to 10, 50, 100 and 200 ppm toluene, respectively. Even in low concentration level of toluene gases (e.g. 10 ppm), the Co3O4 nanorods-based sensor shows an acceptable response (6.0) from the view of the practical application. Response and recovery times are two important factors of gas sensors. As shown in Fig. 6b, the Co3O4 nanorods gas sensor exhibits faster
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response and recovery than the commercial powder. When exposed to 200 ppm toluene, the response and recovery times are about 90 and 55 s for the Co3O4 nanorods-based sensor (Fig. 6b), and 157 and 88 s for the commercial powder-based
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sensor (Fig. 6c), respectively. A comparison between the toluene performances of the
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Co3O4 nanorods-based sensors and some previously reported is summarized in Table
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1 [27-31]. The sensor exhibits higher response, lower operating temperature in the presence of toluene gas than most others, likely due to the 1D nanostructure and the
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unique properties of Co3O4 for the toluene gas.
We then investigated the response of these sensors at different toluene
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concentrations. Fig. 7 shows the response of Co3O4 nanorods (●) sensor increased rapidly with further increases in toluene concentration from 10 to 200 ppm, but
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increased more slowly from 200 to 1000 ppm as the sensor began to saturate. The gas responses of Co3O4 nanorods-based sensor to 10-1000 ppm toluene are ranging from
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6 to 47, which were much higher than that of commercial Co3O4 powder (■). The inset of Fig. 7 shows that the increase in the responses depends nearly linear on the
gas concentrations in the range from 10 to 200 ppm, which further confirms that the present Co3O4 nanorods are more favourable to detect toluene with low concentration. Fig. 8 depicts the histogram of the responses of the sensor based on as-prepared the
Co3O4 nanorods and commercial Co3O4 powder to various gas vapours, including reducing gases: C7H8, C6H6, C8H10, C3H6O, CO, C2H2, NH3, H2 and oxidizing gases (inset of Fig. 8): NO2, Cl2, CH4. All of the gases were tested at an operating temperature of 200ºC with a concentration of 200 ppm. Clearly, the responses of
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Co3O4 nanorods-based sensor to all gases are all improved compared with the other, and the largest increase is only observed for C7H8, implying the good selectivity of
3.3 Gas sensing mechanism of rod-like Co3O4 nanostructures
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the sensor for C7H8.
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Here, we suggest a possible mechanism to explain the toluene sensing properties of
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rod-like Co3O4 nanostructures. For most semiconducting oxide gas sensors, they are based on the conductivity changes caused by the adsorption and desorption of the gas
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molecules on the surface of the sensing structure [32-33]. Generally speaking, the majority of charge carriers near the surfaces of oxide semiconductors are significantly
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different in p- and n-type oxide semiconductors [21, 34]. As is known to all, Co3O4 is an important p-type semiconductor with hole carriers. The toluene sensing process is
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based on the changes in the conductivity of the rod-like Co3O4 nanostructures which is controlled by the toluene species and the amount of the chemisorbed oxygen on the
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surface.
By considering Co3O4 nanorods, it can be observed (in the Fig. 3b) that many
nanoparticles self-assembled into rod-like Co3O4 nanostructures with loose interiors structures. In air, oxygen molecules can be adsorbed on the surface of the Co3O4 nanorods to form chemisorbed oxygen (O-, O2-) by capturing free electrons from the
conduction band. The reaction kinematics may be explained by the following reactions in Eq (1-4) [35]:
O2 (gas) → O2 (ads)
(1)
O2 (ads) + e- → O2- (ads)
(2)
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O2- (ads) + e- → 2O-(ads)
(3)
O-(ads) + e- → O2-(ads)
(4)
For p-type Co3O4, the adsorption of the oxygen molecules results in an increase in
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the carrier density on the surface of the nanorods (Fig. 9a). As a consequence, an
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increase in the conductivity of the Co3O4 is observed. When the Co3O4 nanorods are
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exposed to toluene gas, the toluene molecules are easy to spread inward through well-aligned porous structures. Therefore both the effective contacts surface area and
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the amount of toluene molecules absorbed onto the surface of the rod-like Co3O4 will increase in comparison with the agglomeration structures (Fig. 9b). Then, the toluene
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molecules interact with pre-adsorbed oxygen as depicted in Eq (5). The interaction between toluene and pre-adsorbed oxygen releases free electrons. Those released
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electrons neutralize the holes in the Co3O4, which results in a decrease in the number of hole carrier in Co3O4, and consequently, an increase in sensor resistance. Thus, the
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response of the rod-like Co3O4 nanostructures is distinctly higher than that of the
commercial Co3O4 powder.
C7H8 + 2O− → C7H6O− + H2O + e−
4.
(5)
Conclusion
In summary, large yields of nanoparticles-assembled Co3O4 nanorods have been successfully synthesized using the solvothermal method and without using any structure-directing agents. The synthesized nanorods were found to be effective in the detection of toluene. The maximum gas response to 200 ppm of toluene is 35 at an
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operating temperature of 200ºC. The synthesized Co3O4 nanorods-based sensor shows low detection limit of 10 ppm to toluene. These results indicate that the Co3O4
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nanostructure, which show a potential application as toluene sensors.
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nanorods can significantly improve the toluene sensing properties due to the 1D
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Acknowledgement
This research work was financially supported by the Program for Chang Jiang
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Scholars and Innovative Research Team in University (no. IRT1017). Project
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Supported by Graduate Innovation Fund of Jinlin University (Grant No. 20121108).
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References [1] J. Gao, L.L. Wang, K. Kan, S. Xu, L.Q. Jing, S.Q. Liu, P.K. Shen, L. Li, K.Y. Shi, One-step synthesis of mesoporous Al2O3-In2O3 nanofibres with remarkable
ip t
gas-sensing performance to NOx at room temperature, J. Mater. Chem. A 2 (2014)
cr
949-956.
us
[2] N. Du, H. Zhang, D.R. Yang, One-dimensional hybrid nanostructures: synthesis via layer-by-layer assembly and applications, Nanoscale 4 (2012) 5517-5526.
an
[3] L.L. Wang, X.L. Luo, X.J. Zheng, R. Wang, T. Zhang, Direct annealing of electrospun synthesized high-performance porous SnO2 hollow nanofibers for gas
M
sensor, RSC Adv. 3 (2013) 9723-9728.
[4] J. Moona, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber
te
d
networks for gas sensor applications, Sens. Actuators B 149 (2010) 301-305. [5] Y.H. Li, J. Gong, G.H. He, Y.H. Deng, Synthesis of polyaniline nanotubes using
Ac ce p
Mn2O3 nanofibers as oxidant and their ammonia sensing properties, Synthetic Met.
161 (2011) 56-61.
[6] Y. Zhang, X. He, J. Li, Z. Miao, F. Huang, Fabrication and ethanol-sensing properties of micro gas sensor based on electrospun SnO2 nanofibers, Sens. Actuators B 132 (2008) 67-73.
[7] L. Peng, P.F. Qin, Q.R. Zeng, H.J. Song, M. Lei, J.J.N. Mwangi, D.J. Wang, T.F. Xie, Improvement of formaldehyde sensitivity of ZnO nanorods by modifying with Ru(dcbpy)2(NCS)2, Sens. Actuators B 160 (2011) 39-45. [8] I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive
14
Page 14 of 32
chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009-2013. [9] W. Zheng, X. Liu, W. Wang, Z. Li, H. Zhang, W. Wang, Z. Wang, C. Wang, A highly sensitive and fast-responding sensor based on electrospun In2O3 nanofibers,
ip t
Sens. Actuators B 142 (2009) 61-65.
cr
[10] G. Wang, Y. Ji, X. Huang, X. Yang, P.I. Gouma, M. Dudley, Fabrication and
sensing, J. Phys. Chem. B 110 (2006) 23777-23782.
us
characterization of polycrystalline WO3 nanofibers and their application for ammonia
an
[11] L.L. Wang, T. Fei, Z. Lou, T. Zhang, Three-Dimensional Hierarchical Flowerlike
Interfaces 3 (2011) 4689-4694.
M
α-Fe2O3 Nanostructures: Synthesis and Ethanol-Sensing Properties, ACS Appl. Mater.
[12] Y. Zeng, K. Zhang, X.L. Wang, Y.M. Sui, B. Zou, W.T. Zheng, G.T. Zou, Rapid
te
d
and selective H2S detection of hierarchical ZnSnO3 nanocages, Sens. Actuators B 159 (2011) 245-250.
Ac ce p
[13] L.H. Hu, Q. Peng, Y.D. Li, Selective synthesis of Co3O4 nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion, J. Am. Chem. Soc. 130 (2008) 16136-16137. [14] W.Y. Li, L.N. Xu, J. Chen, Co3O4 nanomaterials in lithium-ion batteries and gas
sensors, Adv. Funct. Mater. 15 (2005) 851-857. [15] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Highly selective CO sensor using indium oxide doubly promoted by cobalt oxide and gold, J. Elec-trochem. Soc. 144 (1997) 158-160. [16]
F.
Svegl,
B.
Orel,
M.G.
Hutchins,
K.
Kalcher,
Structural
and
15
Page 15 of 32
spectroelectrochemical investigations of sol-gel derived electrochromic spinel Co3O4 films, J. Electrochem. Soc. 143 (1996) 1532-1539. [17] M.G. Hutchins, P.J. Wright, P.D. Grebenik, Comparison of different forms of
ip t
black cobalt selective solar-absorber surfaces, Solar Energy Mater. 16 (1987) 113-131.
cr
[18] N.A.M. Barakat, M.S. Khil, F.A. Sheikh, H.Y. Kim, Synthesis and optical
us
properties of two cobalt oxides (CoO and Co3O4) nanofibers produced by electrospinning process, J. Phys. Chem. C 112 (2008) 12225-12233.
an
[19] G.X. Wang, X.P. Shen, J. Horvat, B. Wang, H. Liu, D. Wexler, J. Yao, Hydrothermal synthesis and optical, magnetic, and supercapacitance properties of
M
nanoporous cobalt oxide nanorods, J. Phys. Chem. C 113 (2009) 4357-4361. [20] A.M. Cao, J.S. Hu, H.P. Liang, W.G. Song, L.J. Wan, X.L. He, X.G. Gao, S.H.
te
d
Xia, Hierarchically structured cobalt oxide (Co3O4): the morphology control and its potential in sensors, J. Phys. Chem. B 110 (2006) 15858-15863.
Ac ce p
[21] D. Patil, P. Patil, V. Subramanian, P.A. Joy, H.S. Potdar, Highly sensitive and fast responding CO sensor based on Co3O4 nanorods, Talanta 81 (2010) 37-43. [22] J.W. Yoon, J.K. Choi, J.H. Lee, Design of a highly sensitive and selective C2H5OH sensor using p-type Co3O4 nanofibers, Sens. Actuators B 161 (2012)
570-577.
[23] H. Nguyen, S.A. Safty, Meso- and macroporous Co3O4 nanorods for effective VOC gas sensors, J. Phys. Chem. C 115 (2011) 8466-8474. [24] L.L. Wang, H.M. Dou, Z. Lou, T. Zhang, Encapsuled nanoreactors (Au@SnO2): a new sensing material for chemical sensor, Nanoscale 5 (2013) 2686-2691.
16
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[25] H. Ma, Y.M. Xu, Z.M. Rong, X.L. Cheng, S. Gao, X.F. Zhang, H. Zhao, L.H. Huo, Highly toluene sensing performance based on monodispersed Cr2O3 porous microspheres, Sens. Actuators B 174 (2012) 325-331.
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[26] WHO Regional Office for Europe, Air quality guidelines for Europe, 2000,
cr
Copenhagen, Denmark.
us
[27] Q. Qi, T. Zhang, L. Liu, X.J. Zheng, Synthesis and toluene sensing properties of SnO2 nanofibers, Sens. Actuators B 137 (2009) 471-475.
an
[28] J.R. Huang, X.J. Xu, C.P. Gu, W.Z. Wang, B.Y. Geng, Y.F. Sun, J.H. Liu Size-controlled synthesis of porous ZnSnO3 cubes and their gas-sensing and
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photocatalysis properties, Sens. Actuators B 171 (2012) 572-579. [29] J.R. Huang, X.J. Xu, C.P. Gu, W.Z. Wang, B.Y. Geng, Y.F. Sun, J.H. Liu,
te
d
Effective VOCs gas sensor based on porous SnO2 microcubes prepared via spontaneous phase segregation, Sens. Actuators B 173 (2012) 599-606.
Ac ce p
[30] C.P. Gu, X.J. Xu, J.R. Huang, W.Z. Wang, Y.F. Sun, J.H. Liu, Porous flower-like SnO2 nanostructures as sensitive gas sensors for volatile organic compounds detection,
Sens. Actuators B 174 (2012) 31-38. [31] Y. Zeng, T. Zhang, L.J. Wang, M.H. Kang, H.T. Fan, R. Wang, Y. He, Enhanced toluene sensing characteristics of TiO2-doped flowerlike ZnO nanostructures, Sens. Actuators B 140 (2009) 73-78. [32] J.C. Belmonte, J. Manzano, J. Arbiol, A. Cirera, J. Puigcorbe, A. Vila, N. Sabate, I. Gracia, C. Cane, J.R. Morante, Micromachined twin gas sensor for CO and O2 quantification based on catalytically modified nano-SnO2, Sens. Actuators B 114
17
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(2006) 881-892. [33] Z. Lou, L.L. Wang, T. Fei, T. Zhang, Enhanced ethanol sensing properties of NiO-doped SnO2 polyhedra, New J. Chem. 36 (2012) 1003-1007.
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semiconductors: Overview, Sens. Actuators B 192 (2014) 607-627.
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[34] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide
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te
d
M
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thick films, Sens. Actuators B 111 (2005) 58-62.
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[35] K. Arshak, I. Gaidan, Gas sensing properties of ZnFe2O4/ZnO screen-printed
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Biographies
Lili Wang received her MS degree in Chemistry from Jilin University, China in 2010.
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She entered the PhD course in 2010, majoring in microelectronics and solid-state
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electronics. She is studying the synthesis and characterization of semiconducting,
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functional materials and gas sensors.
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Jianan Deng received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2011. As an MS student, his research interest is
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gas sensors based on semiconducting functional materials.
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Zheng Lou received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2009. He entered the PhD course in 2011 As a
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PhD student, his research interest is gas sensors based on semiconducting functional materials.
Tong Zhang completed her MS degree in semiconductor materials in 1992 and her PhD in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors.
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TABLE CAPTIONS
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Table 1. Toluene sensors in the present study and those reported in the literature.
FIGURE CAPTIONS
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Figure 1. (a) Schematic diagram showing the structure of a typical gas sensor, (b)
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photograph and FESEM image (inset, b) of the Co3O4-based sensor, and (c) theoretic diagram of the test circuit (VH: heating voltage, VS: circuit voltage and RL: load
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resistance).
Figure 2. XRD patterns of (a) Co3O4 nanorods calcined at 450ºC and (b) commercial
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Co3O4 powder.
Figure 3. FESEM and TEM images of the Co3O4 products: (a) un-thermal treatment
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precursors (b-c) Co3O4 nanorods calcined at 450 ºC; (d-e) commercial Co3O4 powder; (f-g) TEM images of the Co3O4 nanorods; (h) the corresponding HRTEM image of the
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Co3O4 nanorods; (i) TEM image of commercial Co3O4 powder. Figure 4. The corresponding EDX patterns of the Co3O4 nanorods and commercial Co3O4 powder.
Figure 5. (a) Responses of two sensors to 200 ppm toluene at different operating temperatures, (b) the relationship between the resistance in air and the operating temperatures of the sensor based on Co3O4 nanorods and commercial Co3O4 powder, respectively. Figure 6. (a) Responses of Co3O4 nanorods (solid line) and commercial Co3O4 powder (dotted line) versus different concentrations of toluene at 200ºC. Dynamic
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toluene sensing curves of the (b) Co3O4 nanorods and (c) commercial Co3O4 powder. Figure 7. Responses of Co3O4 nanorods (●) and commercial Co3O4 powder (■) at 200 ºC to versus toluene concentrations. The inset shows the sensitivity on the toluene
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concentration in the range of 10-200 ppm.
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Figure 8. Responses of Co3O4 nanorods and commercial Co3O4 powder versus 200
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ppm of various gas vapours (reducing gases) at 200ºC. Inset: responses of two sensors to oxidizing gases.
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Figure 9. Schematic diagrams on the gas sensing mechanism of the products: (a)
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Co3O4 nanorods and (b) commercial Co3O4 powder.
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Concentration/ppm
Temprature/ºC
200
200
SnO2 nanofibers [27]
200
350
ZnSnO3 nanocube [28]
200
SnO2 nanocube [29]
200
SnO2 flower-like [30]
200
ZnO flower-like [31]
200
Response 35 6
300
15
240
16
260
12
390
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Table 1.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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Figure 8.
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Figure 9.
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Graphical Abstract Nanoparticles‐assembled Co3O4 1D nanorods have been synthesized by a solvothermal method and exhibited higher toluene (C7H8) response than that of the commercial Co3O4 powders.
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