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molybdenum disulfide nanocomposite toward methane gas sensing at low temperature
Sensors and Actuators B 252 (2017) 624–632
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of platinum-loaded cobalt oxide/molybdenum disulfide nanocomposite toward methane gas sensing at low temperature Dongzhi Zhang a,∗ , Hongyan Chang a , Yan’e Sun a , Chuanxing Jiang a , Yao Yao b , Yong Zhang a a b
College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, China College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China
a r t i c l e
i n f o
Article history: Received 22 February 2017 Received in revised form 7 June 2017 Accepted 10 June 2017 Available online 10 June 2017 Keywords: Pt-Co3 O4 /MoS2 nanocomposite Layer-by-layer self-assembly Methane gas detection Operating temperature
a b s t r a c t A novel methane sensor based on platinum (Pt)-loaded cobalt oxide (Co3 O4 )/molybdenum disulfide (MoS2 ) nanocomposite was reported in this paper. The sensor was fabricated via layer-by-layer (LbL) self-assembly method for the first time, and was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), elemental mapping, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The gas sensing properties of the as-prepared Pt-Co3 O4 /MoS2 composite toward methane gas was investigated under various operating temperature, and the optimal working temperature of 170 ◦ C was determined. The Pt-Co3 O4 /MoS2 sensor exhibits superior gas sensing performance toward methane as compared to the Co3 O4 , Co3 O4 /MoS2 counterparts. The underlying gas sensing mechanism of the Pt-Co3 O4 /MoS2 sensor was systematically discussed, which demonstrates that the enhanced sensing performance of the sensor is attributed to the good synergistic effect of the ternary materials, including high availability of oxygen species, active catalytic effect, and special interactions at MoS2 /Co3 O4 heterojunction. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The monitoring and detecting of toxic and flammable gases in various fields, such as coal mine, has attracted considerable attention in the past decade. Methane (CH4 ), a kind of colorless but combustible gas, is the most important component of mine gas. As we all known, high concentration ( > 5%) of CH4 in coal mine is dangerous and may cause the explosion [1,2]. Therefore, reliable and large range concentration detection of CH4 is extremely necessary. Currently, metal oxide semiconductors (MOS) such as zinc oxide (ZnO), tin oxide (SnO2 ), titanium dioxide (TiO2 ), indium oxide (In2 O3 ) cobalt oxide (Co3 O4 ) and tungsten trioxide (WO3 ), have been known as promising gas sensing materials owing to their distinctive performance, including nano-size, easy fabrication, low cost, as well as stable physical-chemical properties [3,4]. Among them, Co3 O4 , as a p-type semiconductor, has generated lots of interest for detecting various kinds of gases, such as NH3 [5], NO2 [6], CO [7,8], ethanol [9], toluene [10], acetone [11] and water vapor [12]. However, few literatures have reported that pure Co3 O4 possess
∗ Corresponding author. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.snb.2017.06.063 0925-4005/© 2017 Elsevier B.V. All rights reserved.
satisfactory performance for detecting CH4 below 200 ◦ C [13]. With the appeal of reducing low power consumption in sensor development, it is highly worthy of enhancing the sensing performance of MOS-based sensors toward methane gas at lower temperature. In recent years, single-layer graphene has emerged as a typically two-dimensional (2D) nanomaterial and attracted tremendous interest owing to its unique atomically thin-layered structure and excellent electrical properties [14–16]. However, its application has been limited by its inability to function as a semiconductor, which is critical for the on-off switching operations performed by electronic devices. To overcome this shortcoming, the researchers turned to another emerging 2D nanomaterial, molybdenum disulfide (MoS2 ). MoS2 possesses a layered structure similar to graphene, has been extensively investigated as a promising candidate in various applications due to its exceptional properties [17–20]. Compared with graphene whose band gap is 0, MoS2 layered structure with bandgap varies from 1.2 eV (bulk MoS2 ) for indirect-gap to 1.8 eV (monolayer) for direct-bandgap, leading to stronger effect on electrical properties produced by molecule adsorption, fundamentally improving its sensitivity [21,22]. It is possible to form p-n heterojunction with other semiconducting materials as a new type of electronic devices, rendering MoS2 a capability in detecting various gases at low temperature. For instance, Zhao et al. showed
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MoS2 -decorated TiO2 nanotube gas sensor exhibited excellent sensing performance toward ethanol gas at working temperature of 150 ◦ C [23]. Liu et al. reported MoS2 thin films grown on p-type Si substrates by DC magnetron sputtering technique and exhibited obvious sensing properties to NH3 at room temperature [24]. In addition, the decorating or doping with noble metals also was recently considered to be alternative effective method to improve the sensing performance of MOS-based gas sensors at low temperature [25,26]. Noble metal nanoparticles (NPs) such as Ag, Au, Pt and Pd, have unique chemical and catalytic properties, which could enhance the adsorption of gas molecules and accelerate the electron exchange between the sensor and the target gas [26,27]. For instance, C. Kuru et al. reported that MoS2 -Pd composite by fabricating field effect transistor (FET) devices exhibited much higher sensor response with shorter response and recovery times than graphene-Pd composite at room temperature [28]. Wang et al. reported Pt-loaded SnO2 composite-based sensor had a high response to CO at room temperature as well as a good selectivity [29]. Xiang et al. presented a hydrogen sensor based on Pd NPs doped TiO2 nanotubes, which exhibited short response/recovery time, outstanding selectivity as well as good reproducibility at room temperature [30]. This work reports a methane gas sensor based on Pt-loaded Co3 O4 /MoS2 nanocomposite fabricated by layer-by-layer (LbL) selfassembly method. The morphologies, microstructures and compositional characteristics of the Pt- Co3 O4 /MoS2 nanocomposite were sufficiently examined by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), elemental mapping, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The methane gas-sensing performance of the as-prepared sensor was investigated and compared with pure Co3 O4 and Co3 O4 /MoS2 counterparts. Experimental results reveal that the sensing behavior of the Pt-Co3 O4 /MoS2 sensor is superior to the other two sensors, in terms of high response, fast response/recovery time as well as outstanding repeatability. Furthermore, the potential sensing mechanism of the sensor toward methane gas was explored in detail.
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for heating at 200 ◦ C for 24 h. The Co3 O4 and MoS2 solutions were collected for use after washing with DI water several times to remove excess ions. In order to enhance the electrostatic interaction of Co3 O4 and MoS2 , an equal volume of PDDA and PSS was added into the Co3 O4 and MoS2 solutions as cationic and anionic polyelectrolyte, respectively. Fig. 1(c) shows the LbL self-assembly fabrication of the PtCo3 O4 /MoS2 nanocomposite film. Firstly, two precursor layers of PDDA/PSS were self-assembled on the interdigital electrode (IDE) device with FR4 as substrate for surface modification. Afterwards, the Co3 O4 and MoS2 layers were alternately deposited through solution immersion for five cycles via LbL self-assembly technique, in which MoS2 layer was arranged as top layer. Here, the immersing time of PDDA and PSS were 10 min, Co3 O4 and MoS2 were 20 min. Rinsing with DI water and drying with nitrogen gas after each monolayer assembly are required for strengthening the interconnection between layers. Next, the device was immersed into 3 mmol/L H2 PtCl6 ·6H2 O solution over 1 h to achieve Pt-loading on Co3 O4 /MoS2 nanocomposite. Finally, the Pt-Co3 O4 /MoS2 device was heated at 80 ◦ C for 12 h. In order to highlight this work, the comparative devices of pure Co3 O4 and Co3 O4 /MoS2 were fabricated. The Co3 O4 film sensor was fabricated by drop-casting Co3 O4 solution on the IDE device, and the Co3 O4 /MoS2 film sensor was fabrication by LbL self-assembly route as the above-mentioned, but without the immersion of H2 PtCl6 ·6H2 O solution. 2.3. Instrument and analysis
Sodium molybdate (Na2 MoO4 ·2H2 O), thioacetamide (CH3 CSNH2 ), oxalic acid (C2 H2 O4 ), cobalt nitrate hex(Co(NO3 )2 ·6H2 O, >98.5%), trisodium phosphate ahydrate (Na3 PO4 ·12H2 O, >98%), hydrazine hydrate (N2 H4 ·H2 O) and chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, >99%) were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Additionally, poly (dimethyl diallyl ammonium chloride) (PDDA) and poly (sodium 4-sty-renesulfonate) (PSS) were obtained from Sigma-Aldrich Inc. All the chemicals were used as received without further purification.
The surface structure and morphology of the as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku D/Max 2500PC), field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100). The chemical composition analysis was performed by Hitachi S-4800 equipped with an energy dispersive spectrometer (EDS). The elemental mapping was examined by Merlin FE-SEM from Carl Zeiss (Germany) at 2–5 kV range accelerating voltage. Xray photoelectron spectroscopy (XPS) was performed to examine the elemental composition and chemical state of the as-prepared sample by a Thermo Scientific K-Alpha XPS spectrometer. The gas-sensing measurement was performed by exposing the prepared sensors to CH4 gas, and the resistance response was recorded via Agilent 34970A data-logger. The sensing device was placed in a sealed chamber equipped with appropriate inlet and outlet for gas. Air used as background gas and a known volume of CH4 gas was injected inside the testing chamber to achieve the desired concentration for test. The relative humidity of the atmosphere where the gas sensing tests performed is 45%RH. The working temperature for the sensor is controlled through applying a voltage to a heating resistor by a power source (GPD-4303S). The response value (R) was determined by R = |Ra -Rg |/Ra × 100%, where Ra is the resistance of the sensor in air and Rg is the resistance of the sensor in the target gas.
2.2. Fabrication
3. Results and discussion
The fabrication of Pt-Co3 O4 /MoS2 nanocomposite is performed by combining hydrothermal synthesis with LbL self-assembly method. Fig. 1(a) and (b) demonstrate the process for synthesizing Co3 O4 and MoS2 nanomaterials via hydrothermal method [5,31]. For Co3 O4 fabrication, an mount of Co (NO3 )2 ·6H2 O, Na3 PO4 ·12H2 O and N2 H4 ·H2 O were dissolved into deionized (DI) water with stirring for 1 h and then transferred to a stainless-steel autoclave for heating at 180 ◦ C for 12 h. For MoS2 fabrication, Na2 MoO4 ·2H2 O, thioacetamide and oxalic acid were dissolved into DI water and stirred over 1 h, and then transferred to a stainless-steel autoclave
3.1. Materials characterizations
2. Experiment 2.1. Materials
Fig. 2 shows the XRD patterns of Co3 O4 , MoS2 and PtCo3 O4 /MoS2 samples. The diffraction peaks in the XRD pattern of Co3 O4 are in good agreement with those on the standard card of cubic spinel Co3 O4 (JCPDS No. 76-1802). The reflection peaks of MoS2 are corresponding to the (002), (100), (103) and (110) reflections of MoS2 (JCPDS 37-1492). The XRD pattern of the Pt-Co3 O4 /MoS2 sample illustrates all the distinct lattice planes corresponding to the MoS2 , Co3 O4 and Pt nanoparticles, confirming
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Fig. 1. (a), (b) Hydrothermal synthesis of Co3 O4 and MoS2 ; (c) LbL self-assembly fabrication of Pt-Co3 O4 /MoS2 nanocomposite film.
Fig. 2. X-ray diffractograms of Co3 O4 , MoS2 and Pt-Co3 O4 /MoS2 samples.
the presence of individual components of MoS2 Co3 O4 and Pt. Such an observation suggests the successful fabrication of the PtCo3 O4 /MoS2 nanocomposite. The surface morphologies of as-prepared samples were characterized using field emission scanning electron microscopy (FE-SEM), and the results were shown in Fig. 3(a)–(c). Fig. 3(a) and (b) shows that the Co3 O4 exhibits nanorod-shape and MoS2 has flake-shape, respectively. Fig. 3 (c) shows the SEM image of the PtCo3 O4 /MoS2 nanocomposite, indicating that it composes of MoS2 nanosheets, Co3 O4 nanorods and Pt nanoparticles [32,33]. Additionally, EDS measurement was accomplished further to confirm the element composition of the Pt-Co3 O4 /MoS2 nanocomposite. Fig. 3(d) shows the observed elements exist in the Pt-Co3 O4 /MoS2 nanocomposite. Fig. 4 shows the observed elemental mapping results, indicating that all of the elements are uniformly distributed throughout the composite. However, the EDS spectrum and the EDS maps alone can not guarantee the presence of Mo and S, due to the fact that the EDS peaks overlap. Stoichiometric calculations from XPS spectra were further performed.
The elemental composition and chemical state of the PtCo3 O4 /MoS2 sample were further examined by XPS measurement, and the results are shown in Fig. 5. In the survey spectrum as shown in Fig. 5(a), we can find the Mo, S, Co, O and Pt elements coexist in the as-prepared sample. In Fig. 5(b), the peaks appear at 232.1 eV and 228.9 eV are assigned to Mo 3d3/2 and Mo 3d5/2 , respectively, which is attributed to the Mo4+ of MoS2 . The peak at 225.8 eV is indexed to S2− 2s. Meanwhile, the peak at 235.4 eV is indexed to Mo 3d3/2 of Mo6+ , which may be originated from the slight oxidation of MoS2 . In Fig. 5(c), two peaks at 162.9 eV and 161.8 eV are observed, corresponding to S 2p1/2 and S 2p3/2 of S2− , respectively. In Fig. 5(d), the atom of Co in the as-prepared sample has two valence states, tetrahedral Co2+ and octahedral Co3+ from the Co3 O4 . The major peaks at 796.6 eV and 780.6 eV are indexed to Co3+ 2p, and the major peaks at 802.4 eV and 786.5 eV are indexed to Co2+ 2p, respectively [34]. The O 1 s peak shown in Fig. 5(e) located at 531.1 eV, indicating that the oxygen atoms existed as O2− species in the compounds. Fig. 5(f) shows that the binding energies of Pt 4f5/2 and Pt 4f7/2 are 74.7 eV and 71.5 eV, respectively, which indicates the existence of metallic Pt in the as-prepared sample [35]. We also performed stoichiometric calculation of the Mo-to-S ratio (nA : nB ) from XPS spectra, which is calculated by dividing the peak areas (IA , IB ) after adjusting for their sensitivity factors (SA , SB ), nA : nB = (IA /SA ): (IB /SB ). The peak areas of 115498 and 43861 CPS·eV are obtained from the XPS spectra, corresponding to Mo 3d and S 2p, respectively. The sensitivity factors of Mo 3d and S 2p are 2.75 and 0.54, respectively [36]. Consequently, the Mo-to-S ratio of 1:1.93 was obtained, which is slightly substoichiometric, indicating sulfur vacancies in the MoS2 nanosheets. In order to further visualize the morphologies and nanostructures of as-prepared samples, the transmission electron microscope (TEM) images were shown in Fig. 6. Fig. 6(a)–(c) exhibits the TEM micrographs of the Pt-Co3 O4 /MoS2 nanocomposite, indicating the MoS2 nanosheet are decorated with Co3 O4 nanorods and Pt nanoparticles. Furthermore, high-magnification TEM images were shown in Fig. 6(d) and (e), we can find the Co3 O4
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Fig. 3. SEM images of (a) Co3 O4 , (b) MoS2 , (c) Pt-Co3 O4 /MoS2 samples; (d) EDS spectrum of Pt-Co3 O4 /MoS2 sample.
Fig. 4. Elemental mappings of Mo, S, Co, O and Pt.
nanorods and Pt nanoparticles distributed on the MoS2 nanosheets with the lattice fringe of 0.24 nm, 0.47 nm and 0.22 nm [3,37,38], which are ascribed to the (311), (111) plane of Co3 O4 and (111) plane of Pt, respectively. As shown in Fig. 6(f), a few layers (1–7) of MoS2 sheets with an interlayer spacing of 0.64 nm [39,40] were observed at the folded edges of MoS2 nanosheets.
3.2. Methane sensing properties Fig. 7 shows the response of the Pt-Co3 O4 /MoS2 and Co3 O4 film sensor toward 1000 ppm CH4 as a function of operating temperature. The response of Pt-Co3 O4 /MoS2 sensor is 7.43% at 170 ◦ C, and Co3 O4 sensor is 5.41% at 300 ◦ C. This comparison result confirms the role of MoS2 /Pt in lowering the working temperatures
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Fig. 5. XPS spectra of Pt-Co3 O4 /MoS2 sample: (a) survey spectrum, (b) Mo 3d core level XPS spectrum, (c) S 2p core level XPS spectrum, (d) Co 2p core level XPS spectrum, (e) O 1 s core level XPS spectrum and (f) Pt 4f core level XPS spectrum.
and enhancing the response to methane. It is obviously observed that the response of Pt-Co3 O4 /MoS2 sensor exhibits an “increasemaximum-decrease” tendency with the operating temperature, and the highest response toward methane was observed at 170 ◦ C. The working temperature of the sensor plays a crucial role in adjusting the dynamic adsorption/desorption behaviors and competitive chemisorption between methane molecules and atmospheric oxygen species [41]. The increasing of the working temperature is beneficial to the interaction of oxygen species on the sensing materials surface with methane molecules and enhance the response of the sensor. However, as the temperature increases further, the increase of desorption kinetics and surface reactivity on the sensing material makes the gas adsorption difficult, reducing the response
of the sensor. Thereby, the optimal operating temperature of 170 ◦ C was chosen to carry out the subsequent tests. To compare the sensing properties of pure Co3 O4 , Co3 O4 /MoS2 and Pt-Co3 O4 /MoS2 sensors, Fig. 8 shows the dynamic responserecovery curves of the three sensors exposed to 1000 ppm and 3000 ppm CH4 at 170 ◦ C. The Pt-Co3 O4 /MoS2 sensor exhibits the highest response among the three sensors, and the Co3 O4 /MoS2 sensor shows much higher response than the pure Co3 O4 sensor. This comparative results indicate the introduction of MoS2 and Pt is significantly to improve the sensing properties toward methane for the Co3 O4 counterpart. The Pt-Co3 O4 /MoS2 hybrid sensor achieved the best response due to the synergistic effect of the ternary materials.
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Fig. 6. (a)-(c) TEM micrographs and (d)-(f) HRTEM micrographs of Pt-Co3 O4 /MoS2 sample.
Fig. 8. The responses of Pt-Co3 O4 /MoS2 , Co3 O4 /MoS2 and Co3 O4 film sensors toward 1000 ppm and 3000 ppm methane at operating temperature of 170 ◦ C. Fig. 7. Response of Pt-Co3 O4 /MoS2 nanocomposite and Co3 O4 film sensors toward 1000 ppm methane under various operating temperatures.
Fig. 9(a) shows the real-time resistance measurement of the PtCo3 O4 /MoS2 nanocomposite sensor toward step-wisely increased methane concentration, ranging from 100 to 30000 ppm. The resistance of the sensor decreases monotonically as the increase of CH4 concentration. Fig. 9(b) shows the response-recovery curves of the Pt-Co3 O4 /MoS2 nanocomposite sensor upon exposure to various concentration of methane. The response values of the sensor are 1.49%, 3.61%, 7.49%, 12.31%, 18.05%, 27.81%, 31% and 34.22% toward 100, 500, 1000, 3000, 5000, 10000, 20000 and 30000 ppm methane gas, respectively. The response and recovery time were calculated to be 20–30 s and 15–25 s towards 100–30000 ppm methane. Fig. 9(c) shows the response as a function of methane concentration. The fitting function between the sensor response Y and methane concentration X can be depicted as Y = −32.4186e−X/6722 + 33.9226, and the correlation coefficient, R2 ,
is 0.99162. Fig. 9(d) shows the repeatability of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward various methane concentrations. The measurement results suggest the sensor possesses good repeatability. The selectivity of the Pt-Co3 O4 /MoS2 nanocomposite sensor was investigated by exposing the sensor to H2 , CO, C2 H6 and CH4 gas. The sensor response to the gas species under different concentrations of 500 ppm, 1000 ppm and 3000 ppm were tested at the operating temperature of 170 ◦ C. As shown in Fig. 10, the PtCo3 O4 /MoS2 nanocomposite sensor exhibits a good selectivity for CH4 detection. 3.3. Methane gas-sensing mechanism The above experimental results confirmed the Pt-Co3 O4 /MoS2 sensor has good response toward methane at 170 ◦ C, is a good candidate for detecting methane. Co3 O4 as a p-type semiconductor
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Fig. 9. (a) The real-time resistance measurement of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward step-wisely increased methane concentration. (b) The responserecovery curves of the Pt-Co3 O4 /MoS2 nanocomposite sensor upon exposure to various concentration of methane. (c) The response as a function of methane concentration. (d) The repeatability of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward various methane concentrations.
n-type MoS2 surface were ionized into negatively charged oxygen species via capturing electrons from the MoS2 nanosheets according to the reaction of Eq. (3) and (4) [21,42–44]. When methane gas is adsorbed on the surface, it reacts with the oxygen species and releases electrons back to MoS2 nanosheets (Eq. (5)), leading to a decrease of its surface resistance [45]. MoS2 has direct band gap and excellent electrical properties, playing an important role in low-temperature gas sensing of MoS2 -based gas sensors. Fig. 11 (a) shows the synergistic effect of the ternary materials in the Pt-Co3 O4 /MoS2 nanocomposite. The loading of Pt can not only promote the electrons transportation and carrier concentration change due to its larger work function, also serves as active catalyst with active catalytic effect for enhancing the interaction between adsorbed gas molecules and sensing materials [46–49].
Fig. 10. Selectivity of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward 500 ppm, 1000 ppm, 3000 ppm of H2 , CO, C2 H6 and CH4 gas at 170 ◦ C.
with high availability of oxygen species, the resistance of Co3 O4 film sensor increased when exposed to methane at higher working temperature. Compared with pure Co3 O4 , the Pt-Co3 O4 /MoS2 sensor shows n-type behavior to methane, mainly due to the composition and nanostructure for the nanocomposite. The dominant material is MoS2 and MoS2 is assembled on the top layer of the composite material. During the MoS2 preparation, thioacetamide (CH3 CSNH2 ) was hydrolyzed to produce the intermediate product of H2 S, which reacted with sodium molybdate (Na2 MoO4 ) to form the MoS2 . The chemical reaction for the formation of MoS2 can be depicted by Eqs. (1) and (2). The oxygen molecules adsorbed on the
CH3 CSNH2 + 2H2 O → H2 S + CH3 COOH + NH3
(1)
4Na2 MoO4 +9H2 S → 4MoS2 + Na2 SO4 + 6NaOH + 6H2 O
(2)
−
−
O2 (ads) + e ↔ O2 (ads)
(3)
O2 − (ads) + e− ↔ 2O− (ads)
(4)
O− (ads) + CH4 ↔ CO2 + H2 O + e−
(5)
On the other hand, the gas sensing characteristics was ascribed to the formation of p-n heterojunction formed at the interfaces of Co3 O4 and MoS2 . Fig. 11 (b) shows the schematic energy diagram of the heterojunction between Co3 O4 and MoS2 . Co3 O4 behaves p-type semiconductor with band-gap of 2.07 eV [50,51]. MoS2 is a kind of n-type semiconductors and its band-gap of 1.2 eV [52]. When p-type Co3 O4 and n-type MoS2 contact each other, the interdiffusion of both dominant carriers at the interface leads to the
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Fig. 11. (a) The effects of MoS2 and Pt in the Pt-Co3 O4 /MoS2 nanocomposite. (b) p–n heterojunction formed at the interface of Co3 O4 and MoS2 .
formation of p-n heterojunction and self-built depletion layer. The width modulation of the depletion layer for the p-n heterojunction occurs owing to the adsorption and desorption of methane [53,54]. When the sensor was exposed to methane gas, the holes on the surface of Co3 O4 decrease, whereas the electrons on the surface of MoS2 increase because the interaction between adsorbed oxygen species and methane molecules. The electrons transfer from MoS2 to Co3 O4 leads to the contraction of the depletion layer, resulting in a decrease of sensor resistance subsequently. 5. Conclusions In conclusion, a methane sensor based on Pt-Co3 O4 /MoS2 nanocomposite has been successfully fabricated using LbL selfassembly technology. XRD, SEM, EDS, TEM, XPS and element mapping measurements were employed to characterize the asprepared sample. A series of experiments of the presented sensor were performed upon exposure to various methane concentrations. The experimental results showed that the Pt-Co3 O4 /MoS2 sensor possesses not only high response, but also short response/recovery time, excellent repeatability and outstanding selectivity toward methane at an optimal working temperature of 170 ◦ C. The probable sensing mechanism of the sensor was mainly interpreted using special interactions at p-n heterojunction, high surface area of MoS2 and outstanding catalytic performance of Pt. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51407200), the Science and Technology Plan Project of Shandong Province (Grant No. 2014GSF117035), the Fundamental Research Funds for the Central Universities of China (No. 15CX05041A), and the Science and Technology Development Plan Project of Qingdao (Grant No.16-6-2-53-nsh). References [1] W. Lu, G. Jing, X. Bian, H. Yu, T. Cui, Micro catalytic methane sensors based on 3D quartz structures with cone-shaped cavities etched by high-resolution abrasive sand blasting, Sens. Actuators A 242 (2016) 9–17. [2] D. Zhang, N. Yin, B. Xia, Facile fabrication of ZnO nanocrystalline-modified graphene hybrid nanocomposite toward methane gas sensing application, J. Mater. Sci. 26 (2015) 5937–5945. [3] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B 192 (2014) 607–627. [4] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: a review, Sens. Actuators B 204 (2014) 250–272. [5] J.N. Deng, R. Zhang, L.L. Wang, Z. Lou, T. Zhang, Enhanced sensing performance of the Co3 O4 hierarchical nanorods to NH3 gas, Sens. Actuators B 209 (2015) 449–455.
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Biographies Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D. degree from South China University of Technology in 2011. He is currently an associate professor at China University of Petroleum (East China), Qingdao, China. His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics. Hongyan Chang received her B.S. degree from Shandong University of Technology in 2012. She is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, microelectro-mechanical systems (MEMS) and nanotechnology. Yan’e Sun received her B.S. degree in measurement & control technology and instrumentation from Ludong University in 2014. Currently, she is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments. Chuanxing Jiang received her B.S. degree in measurement & control technology and instrumentation from Yantai University in 2015. Currently, she is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments. Yao Yao received his B.S. degree from Southwest Jiaotong University, Sichuan, China in 2006, and obtained Ph.D. degree from the Southwest Jiaotong University, Sichuan, China in 2013. He is currently an associate professor at Chengdu University of Information Technology. His current research interests include carbon based electronics and acoustic sensors. Yong Zhang received his Ph. D degree in ocean information detection and treatment from Ocean University of China in 2008. Currently, he is an associate professor at China University of Petroleum (East China), Qingdao, China. His main research interests are precision measurement technology and instruments.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of platinum-loaded cobalt oxide/molybdenum disulfide nanocomposite toward methane gas sensing at low temperature Dongzhi Zhang a,∗ , Hongyan Chang a , Yan’e Sun a , Chuanxing Jiang a , Yao Yao b , Yong Zhang a a b
College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, China College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China
a r t i c l e
i n f o
Article history: Received 22 February 2017 Received in revised form 7 June 2017 Accepted 10 June 2017 Available online 10 June 2017 Keywords: Pt-Co3 O4 /MoS2 nanocomposite Layer-by-layer self-assembly Methane gas detection Operating temperature
a b s t r a c t A novel methane sensor based on platinum (Pt)-loaded cobalt oxide (Co3 O4 )/molybdenum disulfide (MoS2 ) nanocomposite was reported in this paper. The sensor was fabricated via layer-by-layer (LbL) self-assembly method for the first time, and was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), elemental mapping, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The gas sensing properties of the as-prepared Pt-Co3 O4 /MoS2 composite toward methane gas was investigated under various operating temperature, and the optimal working temperature of 170 ◦ C was determined. The Pt-Co3 O4 /MoS2 sensor exhibits superior gas sensing performance toward methane as compared to the Co3 O4 , Co3 O4 /MoS2 counterparts. The underlying gas sensing mechanism of the Pt-Co3 O4 /MoS2 sensor was systematically discussed, which demonstrates that the enhanced sensing performance of the sensor is attributed to the good synergistic effect of the ternary materials, including high availability of oxygen species, active catalytic effect, and special interactions at MoS2 /Co3 O4 heterojunction. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The monitoring and detecting of toxic and flammable gases in various fields, such as coal mine, has attracted considerable attention in the past decade. Methane (CH4 ), a kind of colorless but combustible gas, is the most important component of mine gas. As we all known, high concentration ( > 5%) of CH4 in coal mine is dangerous and may cause the explosion [1,2]. Therefore, reliable and large range concentration detection of CH4 is extremely necessary. Currently, metal oxide semiconductors (MOS) such as zinc oxide (ZnO), tin oxide (SnO2 ), titanium dioxide (TiO2 ), indium oxide (In2 O3 ) cobalt oxide (Co3 O4 ) and tungsten trioxide (WO3 ), have been known as promising gas sensing materials owing to their distinctive performance, including nano-size, easy fabrication, low cost, as well as stable physical-chemical properties [3,4]. Among them, Co3 O4 , as a p-type semiconductor, has generated lots of interest for detecting various kinds of gases, such as NH3 [5], NO2 [6], CO [7,8], ethanol [9], toluene [10], acetone [11] and water vapor [12]. However, few literatures have reported that pure Co3 O4 possess
∗ Corresponding author. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.snb.2017.06.063 0925-4005/© 2017 Elsevier B.V. All rights reserved.
satisfactory performance for detecting CH4 below 200 ◦ C [13]. With the appeal of reducing low power consumption in sensor development, it is highly worthy of enhancing the sensing performance of MOS-based sensors toward methane gas at lower temperature. In recent years, single-layer graphene has emerged as a typically two-dimensional (2D) nanomaterial and attracted tremendous interest owing to its unique atomically thin-layered structure and excellent electrical properties [14–16]. However, its application has been limited by its inability to function as a semiconductor, which is critical for the on-off switching operations performed by electronic devices. To overcome this shortcoming, the researchers turned to another emerging 2D nanomaterial, molybdenum disulfide (MoS2 ). MoS2 possesses a layered structure similar to graphene, has been extensively investigated as a promising candidate in various applications due to its exceptional properties [17–20]. Compared with graphene whose band gap is 0, MoS2 layered structure with bandgap varies from 1.2 eV (bulk MoS2 ) for indirect-gap to 1.8 eV (monolayer) for direct-bandgap, leading to stronger effect on electrical properties produced by molecule adsorption, fundamentally improving its sensitivity [21,22]. It is possible to form p-n heterojunction with other semiconducting materials as a new type of electronic devices, rendering MoS2 a capability in detecting various gases at low temperature. For instance, Zhao et al. showed
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MoS2 -decorated TiO2 nanotube gas sensor exhibited excellent sensing performance toward ethanol gas at working temperature of 150 ◦ C [23]. Liu et al. reported MoS2 thin films grown on p-type Si substrates by DC magnetron sputtering technique and exhibited obvious sensing properties to NH3 at room temperature [24]. In addition, the decorating or doping with noble metals also was recently considered to be alternative effective method to improve the sensing performance of MOS-based gas sensors at low temperature [25,26]. Noble metal nanoparticles (NPs) such as Ag, Au, Pt and Pd, have unique chemical and catalytic properties, which could enhance the adsorption of gas molecules and accelerate the electron exchange between the sensor and the target gas [26,27]. For instance, C. Kuru et al. reported that MoS2 -Pd composite by fabricating field effect transistor (FET) devices exhibited much higher sensor response with shorter response and recovery times than graphene-Pd composite at room temperature [28]. Wang et al. reported Pt-loaded SnO2 composite-based sensor had a high response to CO at room temperature as well as a good selectivity [29]. Xiang et al. presented a hydrogen sensor based on Pd NPs doped TiO2 nanotubes, which exhibited short response/recovery time, outstanding selectivity as well as good reproducibility at room temperature [30]. This work reports a methane gas sensor based on Pt-loaded Co3 O4 /MoS2 nanocomposite fabricated by layer-by-layer (LbL) selfassembly method. The morphologies, microstructures and compositional characteristics of the Pt- Co3 O4 /MoS2 nanocomposite were sufficiently examined by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), elemental mapping, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The methane gas-sensing performance of the as-prepared sensor was investigated and compared with pure Co3 O4 and Co3 O4 /MoS2 counterparts. Experimental results reveal that the sensing behavior of the Pt-Co3 O4 /MoS2 sensor is superior to the other two sensors, in terms of high response, fast response/recovery time as well as outstanding repeatability. Furthermore, the potential sensing mechanism of the sensor toward methane gas was explored in detail.
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for heating at 200 ◦ C for 24 h. The Co3 O4 and MoS2 solutions were collected for use after washing with DI water several times to remove excess ions. In order to enhance the electrostatic interaction of Co3 O4 and MoS2 , an equal volume of PDDA and PSS was added into the Co3 O4 and MoS2 solutions as cationic and anionic polyelectrolyte, respectively. Fig. 1(c) shows the LbL self-assembly fabrication of the PtCo3 O4 /MoS2 nanocomposite film. Firstly, two precursor layers of PDDA/PSS were self-assembled on the interdigital electrode (IDE) device with FR4 as substrate for surface modification. Afterwards, the Co3 O4 and MoS2 layers were alternately deposited through solution immersion for five cycles via LbL self-assembly technique, in which MoS2 layer was arranged as top layer. Here, the immersing time of PDDA and PSS were 10 min, Co3 O4 and MoS2 were 20 min. Rinsing with DI water and drying with nitrogen gas after each monolayer assembly are required for strengthening the interconnection between layers. Next, the device was immersed into 3 mmol/L H2 PtCl6 ·6H2 O solution over 1 h to achieve Pt-loading on Co3 O4 /MoS2 nanocomposite. Finally, the Pt-Co3 O4 /MoS2 device was heated at 80 ◦ C for 12 h. In order to highlight this work, the comparative devices of pure Co3 O4 and Co3 O4 /MoS2 were fabricated. The Co3 O4 film sensor was fabricated by drop-casting Co3 O4 solution on the IDE device, and the Co3 O4 /MoS2 film sensor was fabrication by LbL self-assembly route as the above-mentioned, but without the immersion of H2 PtCl6 ·6H2 O solution. 2.3. Instrument and analysis
Sodium molybdate (Na2 MoO4 ·2H2 O), thioacetamide (CH3 CSNH2 ), oxalic acid (C2 H2 O4 ), cobalt nitrate hex(Co(NO3 )2 ·6H2 O, >98.5%), trisodium phosphate ahydrate (Na3 PO4 ·12H2 O, >98%), hydrazine hydrate (N2 H4 ·H2 O) and chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, >99%) were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Additionally, poly (dimethyl diallyl ammonium chloride) (PDDA) and poly (sodium 4-sty-renesulfonate) (PSS) were obtained from Sigma-Aldrich Inc. All the chemicals were used as received without further purification.
The surface structure and morphology of the as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku D/Max 2500PC), field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100). The chemical composition analysis was performed by Hitachi S-4800 equipped with an energy dispersive spectrometer (EDS). The elemental mapping was examined by Merlin FE-SEM from Carl Zeiss (Germany) at 2–5 kV range accelerating voltage. Xray photoelectron spectroscopy (XPS) was performed to examine the elemental composition and chemical state of the as-prepared sample by a Thermo Scientific K-Alpha XPS spectrometer. The gas-sensing measurement was performed by exposing the prepared sensors to CH4 gas, and the resistance response was recorded via Agilent 34970A data-logger. The sensing device was placed in a sealed chamber equipped with appropriate inlet and outlet for gas. Air used as background gas and a known volume of CH4 gas was injected inside the testing chamber to achieve the desired concentration for test. The relative humidity of the atmosphere where the gas sensing tests performed is 45%RH. The working temperature for the sensor is controlled through applying a voltage to a heating resistor by a power source (GPD-4303S). The response value (R) was determined by R = |Ra -Rg |/Ra × 100%, where Ra is the resistance of the sensor in air and Rg is the resistance of the sensor in the target gas.
2.2. Fabrication
3. Results and discussion
The fabrication of Pt-Co3 O4 /MoS2 nanocomposite is performed by combining hydrothermal synthesis with LbL self-assembly method. Fig. 1(a) and (b) demonstrate the process for synthesizing Co3 O4 and MoS2 nanomaterials via hydrothermal method [5,31]. For Co3 O4 fabrication, an mount of Co (NO3 )2 ·6H2 O, Na3 PO4 ·12H2 O and N2 H4 ·H2 O were dissolved into deionized (DI) water with stirring for 1 h and then transferred to a stainless-steel autoclave for heating at 180 ◦ C for 12 h. For MoS2 fabrication, Na2 MoO4 ·2H2 O, thioacetamide and oxalic acid were dissolved into DI water and stirred over 1 h, and then transferred to a stainless-steel autoclave
3.1. Materials characterizations
2. Experiment 2.1. Materials
Fig. 2 shows the XRD patterns of Co3 O4 , MoS2 and PtCo3 O4 /MoS2 samples. The diffraction peaks in the XRD pattern of Co3 O4 are in good agreement with those on the standard card of cubic spinel Co3 O4 (JCPDS No. 76-1802). The reflection peaks of MoS2 are corresponding to the (002), (100), (103) and (110) reflections of MoS2 (JCPDS 37-1492). The XRD pattern of the Pt-Co3 O4 /MoS2 sample illustrates all the distinct lattice planes corresponding to the MoS2 , Co3 O4 and Pt nanoparticles, confirming
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Fig. 1. (a), (b) Hydrothermal synthesis of Co3 O4 and MoS2 ; (c) LbL self-assembly fabrication of Pt-Co3 O4 /MoS2 nanocomposite film.
Fig. 2. X-ray diffractograms of Co3 O4 , MoS2 and Pt-Co3 O4 /MoS2 samples.
the presence of individual components of MoS2 Co3 O4 and Pt. Such an observation suggests the successful fabrication of the PtCo3 O4 /MoS2 nanocomposite. The surface morphologies of as-prepared samples were characterized using field emission scanning electron microscopy (FE-SEM), and the results were shown in Fig. 3(a)–(c). Fig. 3(a) and (b) shows that the Co3 O4 exhibits nanorod-shape and MoS2 has flake-shape, respectively. Fig. 3 (c) shows the SEM image of the PtCo3 O4 /MoS2 nanocomposite, indicating that it composes of MoS2 nanosheets, Co3 O4 nanorods and Pt nanoparticles [32,33]. Additionally, EDS measurement was accomplished further to confirm the element composition of the Pt-Co3 O4 /MoS2 nanocomposite. Fig. 3(d) shows the observed elements exist in the Pt-Co3 O4 /MoS2 nanocomposite. Fig. 4 shows the observed elemental mapping results, indicating that all of the elements are uniformly distributed throughout the composite. However, the EDS spectrum and the EDS maps alone can not guarantee the presence of Mo and S, due to the fact that the EDS peaks overlap. Stoichiometric calculations from XPS spectra were further performed.
The elemental composition and chemical state of the PtCo3 O4 /MoS2 sample were further examined by XPS measurement, and the results are shown in Fig. 5. In the survey spectrum as shown in Fig. 5(a), we can find the Mo, S, Co, O and Pt elements coexist in the as-prepared sample. In Fig. 5(b), the peaks appear at 232.1 eV and 228.9 eV are assigned to Mo 3d3/2 and Mo 3d5/2 , respectively, which is attributed to the Mo4+ of MoS2 . The peak at 225.8 eV is indexed to S2− 2s. Meanwhile, the peak at 235.4 eV is indexed to Mo 3d3/2 of Mo6+ , which may be originated from the slight oxidation of MoS2 . In Fig. 5(c), two peaks at 162.9 eV and 161.8 eV are observed, corresponding to S 2p1/2 and S 2p3/2 of S2− , respectively. In Fig. 5(d), the atom of Co in the as-prepared sample has two valence states, tetrahedral Co2+ and octahedral Co3+ from the Co3 O4 . The major peaks at 796.6 eV and 780.6 eV are indexed to Co3+ 2p, and the major peaks at 802.4 eV and 786.5 eV are indexed to Co2+ 2p, respectively [34]. The O 1 s peak shown in Fig. 5(e) located at 531.1 eV, indicating that the oxygen atoms existed as O2− species in the compounds. Fig. 5(f) shows that the binding energies of Pt 4f5/2 and Pt 4f7/2 are 74.7 eV and 71.5 eV, respectively, which indicates the existence of metallic Pt in the as-prepared sample [35]. We also performed stoichiometric calculation of the Mo-to-S ratio (nA : nB ) from XPS spectra, which is calculated by dividing the peak areas (IA , IB ) after adjusting for their sensitivity factors (SA , SB ), nA : nB = (IA /SA ): (IB /SB ). The peak areas of 115498 and 43861 CPS·eV are obtained from the XPS spectra, corresponding to Mo 3d and S 2p, respectively. The sensitivity factors of Mo 3d and S 2p are 2.75 and 0.54, respectively [36]. Consequently, the Mo-to-S ratio of 1:1.93 was obtained, which is slightly substoichiometric, indicating sulfur vacancies in the MoS2 nanosheets. In order to further visualize the morphologies and nanostructures of as-prepared samples, the transmission electron microscope (TEM) images were shown in Fig. 6. Fig. 6(a)–(c) exhibits the TEM micrographs of the Pt-Co3 O4 /MoS2 nanocomposite, indicating the MoS2 nanosheet are decorated with Co3 O4 nanorods and Pt nanoparticles. Furthermore, high-magnification TEM images were shown in Fig. 6(d) and (e), we can find the Co3 O4
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Fig. 3. SEM images of (a) Co3 O4 , (b) MoS2 , (c) Pt-Co3 O4 /MoS2 samples; (d) EDS spectrum of Pt-Co3 O4 /MoS2 sample.
Fig. 4. Elemental mappings of Mo, S, Co, O and Pt.
nanorods and Pt nanoparticles distributed on the MoS2 nanosheets with the lattice fringe of 0.24 nm, 0.47 nm and 0.22 nm [3,37,38], which are ascribed to the (311), (111) plane of Co3 O4 and (111) plane of Pt, respectively. As shown in Fig. 6(f), a few layers (1–7) of MoS2 sheets with an interlayer spacing of 0.64 nm [39,40] were observed at the folded edges of MoS2 nanosheets.
3.2. Methane sensing properties Fig. 7 shows the response of the Pt-Co3 O4 /MoS2 and Co3 O4 film sensor toward 1000 ppm CH4 as a function of operating temperature. The response of Pt-Co3 O4 /MoS2 sensor is 7.43% at 170 ◦ C, and Co3 O4 sensor is 5.41% at 300 ◦ C. This comparison result confirms the role of MoS2 /Pt in lowering the working temperatures
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Fig. 5. XPS spectra of Pt-Co3 O4 /MoS2 sample: (a) survey spectrum, (b) Mo 3d core level XPS spectrum, (c) S 2p core level XPS spectrum, (d) Co 2p core level XPS spectrum, (e) O 1 s core level XPS spectrum and (f) Pt 4f core level XPS spectrum.
and enhancing the response to methane. It is obviously observed that the response of Pt-Co3 O4 /MoS2 sensor exhibits an “increasemaximum-decrease” tendency with the operating temperature, and the highest response toward methane was observed at 170 ◦ C. The working temperature of the sensor plays a crucial role in adjusting the dynamic adsorption/desorption behaviors and competitive chemisorption between methane molecules and atmospheric oxygen species [41]. The increasing of the working temperature is beneficial to the interaction of oxygen species on the sensing materials surface with methane molecules and enhance the response of the sensor. However, as the temperature increases further, the increase of desorption kinetics and surface reactivity on the sensing material makes the gas adsorption difficult, reducing the response
of the sensor. Thereby, the optimal operating temperature of 170 ◦ C was chosen to carry out the subsequent tests. To compare the sensing properties of pure Co3 O4 , Co3 O4 /MoS2 and Pt-Co3 O4 /MoS2 sensors, Fig. 8 shows the dynamic responserecovery curves of the three sensors exposed to 1000 ppm and 3000 ppm CH4 at 170 ◦ C. The Pt-Co3 O4 /MoS2 sensor exhibits the highest response among the three sensors, and the Co3 O4 /MoS2 sensor shows much higher response than the pure Co3 O4 sensor. This comparative results indicate the introduction of MoS2 and Pt is significantly to improve the sensing properties toward methane for the Co3 O4 counterpart. The Pt-Co3 O4 /MoS2 hybrid sensor achieved the best response due to the synergistic effect of the ternary materials.
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Fig. 6. (a)-(c) TEM micrographs and (d)-(f) HRTEM micrographs of Pt-Co3 O4 /MoS2 sample.
Fig. 8. The responses of Pt-Co3 O4 /MoS2 , Co3 O4 /MoS2 and Co3 O4 film sensors toward 1000 ppm and 3000 ppm methane at operating temperature of 170 ◦ C. Fig. 7. Response of Pt-Co3 O4 /MoS2 nanocomposite and Co3 O4 film sensors toward 1000 ppm methane under various operating temperatures.
Fig. 9(a) shows the real-time resistance measurement of the PtCo3 O4 /MoS2 nanocomposite sensor toward step-wisely increased methane concentration, ranging from 100 to 30000 ppm. The resistance of the sensor decreases monotonically as the increase of CH4 concentration. Fig. 9(b) shows the response-recovery curves of the Pt-Co3 O4 /MoS2 nanocomposite sensor upon exposure to various concentration of methane. The response values of the sensor are 1.49%, 3.61%, 7.49%, 12.31%, 18.05%, 27.81%, 31% and 34.22% toward 100, 500, 1000, 3000, 5000, 10000, 20000 and 30000 ppm methane gas, respectively. The response and recovery time were calculated to be 20–30 s and 15–25 s towards 100–30000 ppm methane. Fig. 9(c) shows the response as a function of methane concentration. The fitting function between the sensor response Y and methane concentration X can be depicted as Y = −32.4186e−X/6722 + 33.9226, and the correlation coefficient, R2 ,
is 0.99162. Fig. 9(d) shows the repeatability of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward various methane concentrations. The measurement results suggest the sensor possesses good repeatability. The selectivity of the Pt-Co3 O4 /MoS2 nanocomposite sensor was investigated by exposing the sensor to H2 , CO, C2 H6 and CH4 gas. The sensor response to the gas species under different concentrations of 500 ppm, 1000 ppm and 3000 ppm were tested at the operating temperature of 170 ◦ C. As shown in Fig. 10, the PtCo3 O4 /MoS2 nanocomposite sensor exhibits a good selectivity for CH4 detection. 3.3. Methane gas-sensing mechanism The above experimental results confirmed the Pt-Co3 O4 /MoS2 sensor has good response toward methane at 170 ◦ C, is a good candidate for detecting methane. Co3 O4 as a p-type semiconductor
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Fig. 9. (a) The real-time resistance measurement of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward step-wisely increased methane concentration. (b) The responserecovery curves of the Pt-Co3 O4 /MoS2 nanocomposite sensor upon exposure to various concentration of methane. (c) The response as a function of methane concentration. (d) The repeatability of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward various methane concentrations.
n-type MoS2 surface were ionized into negatively charged oxygen species via capturing electrons from the MoS2 nanosheets according to the reaction of Eq. (3) and (4) [21,42–44]. When methane gas is adsorbed on the surface, it reacts with the oxygen species and releases electrons back to MoS2 nanosheets (Eq. (5)), leading to a decrease of its surface resistance [45]. MoS2 has direct band gap and excellent electrical properties, playing an important role in low-temperature gas sensing of MoS2 -based gas sensors. Fig. 11 (a) shows the synergistic effect of the ternary materials in the Pt-Co3 O4 /MoS2 nanocomposite. The loading of Pt can not only promote the electrons transportation and carrier concentration change due to its larger work function, also serves as active catalyst with active catalytic effect for enhancing the interaction between adsorbed gas molecules and sensing materials [46–49].
Fig. 10. Selectivity of the Pt-Co3 O4 /MoS2 nanocomposite sensor toward 500 ppm, 1000 ppm, 3000 ppm of H2 , CO, C2 H6 and CH4 gas at 170 ◦ C.
with high availability of oxygen species, the resistance of Co3 O4 film sensor increased when exposed to methane at higher working temperature. Compared with pure Co3 O4 , the Pt-Co3 O4 /MoS2 sensor shows n-type behavior to methane, mainly due to the composition and nanostructure for the nanocomposite. The dominant material is MoS2 and MoS2 is assembled on the top layer of the composite material. During the MoS2 preparation, thioacetamide (CH3 CSNH2 ) was hydrolyzed to produce the intermediate product of H2 S, which reacted with sodium molybdate (Na2 MoO4 ) to form the MoS2 . The chemical reaction for the formation of MoS2 can be depicted by Eqs. (1) and (2). The oxygen molecules adsorbed on the
CH3 CSNH2 + 2H2 O → H2 S + CH3 COOH + NH3
(1)
4Na2 MoO4 +9H2 S → 4MoS2 + Na2 SO4 + 6NaOH + 6H2 O
(2)
−
−
O2 (ads) + e ↔ O2 (ads)
(3)
O2 − (ads) + e− ↔ 2O− (ads)
(4)
O− (ads) + CH4 ↔ CO2 + H2 O + e−
(5)
On the other hand, the gas sensing characteristics was ascribed to the formation of p-n heterojunction formed at the interfaces of Co3 O4 and MoS2 . Fig. 11 (b) shows the schematic energy diagram of the heterojunction between Co3 O4 and MoS2 . Co3 O4 behaves p-type semiconductor with band-gap of 2.07 eV [50,51]. MoS2 is a kind of n-type semiconductors and its band-gap of 1.2 eV [52]. When p-type Co3 O4 and n-type MoS2 contact each other, the interdiffusion of both dominant carriers at the interface leads to the
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Fig. 11. (a) The effects of MoS2 and Pt in the Pt-Co3 O4 /MoS2 nanocomposite. (b) p–n heterojunction formed at the interface of Co3 O4 and MoS2 .
formation of p-n heterojunction and self-built depletion layer. The width modulation of the depletion layer for the p-n heterojunction occurs owing to the adsorption and desorption of methane [53,54]. When the sensor was exposed to methane gas, the holes on the surface of Co3 O4 decrease, whereas the electrons on the surface of MoS2 increase because the interaction between adsorbed oxygen species and methane molecules. The electrons transfer from MoS2 to Co3 O4 leads to the contraction of the depletion layer, resulting in a decrease of sensor resistance subsequently. 5. Conclusions In conclusion, a methane sensor based on Pt-Co3 O4 /MoS2 nanocomposite has been successfully fabricated using LbL selfassembly technology. XRD, SEM, EDS, TEM, XPS and element mapping measurements were employed to characterize the asprepared sample. A series of experiments of the presented sensor were performed upon exposure to various methane concentrations. The experimental results showed that the Pt-Co3 O4 /MoS2 sensor possesses not only high response, but also short response/recovery time, excellent repeatability and outstanding selectivity toward methane at an optimal working temperature of 170 ◦ C. The probable sensing mechanism of the sensor was mainly interpreted using special interactions at p-n heterojunction, high surface area of MoS2 and outstanding catalytic performance of Pt. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51407200), the Science and Technology Plan Project of Shandong Province (Grant No. 2014GSF117035), the Fundamental Research Funds for the Central Universities of China (No. 15CX05041A), and the Science and Technology Development Plan Project of Qingdao (Grant No.16-6-2-53-nsh). References [1] W. Lu, G. Jing, X. Bian, H. Yu, T. Cui, Micro catalytic methane sensors based on 3D quartz structures with cone-shaped cavities etched by high-resolution abrasive sand blasting, Sens. Actuators A 242 (2016) 9–17. [2] D. Zhang, N. Yin, B. Xia, Facile fabrication of ZnO nanocrystalline-modified graphene hybrid nanocomposite toward methane gas sensing application, J. Mater. Sci. 26 (2015) 5937–5945. [3] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B 192 (2014) 607–627. [4] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: a review, Sens. Actuators B 204 (2014) 250–272. [5] J.N. Deng, R. Zhang, L.L. Wang, Z. Lou, T. Zhang, Enhanced sensing performance of the Co3 O4 hierarchical nanorods to NH3 gas, Sens. Actuators B 209 (2015) 449–455.
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Biographies Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D. degree from South China University of Technology in 2011. He is currently an associate professor at China University of Petroleum (East China), Qingdao, China. His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics. Hongyan Chang received her B.S. degree from Shandong University of Technology in 2012. She is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, microelectro-mechanical systems (MEMS) and nanotechnology. Yan’e Sun received her B.S. degree in measurement & control technology and instrumentation from Ludong University in 2014. Currently, she is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments. Chuanxing Jiang received her B.S. degree in measurement & control technology and instrumentation from Yantai University in 2015. Currently, she is graduate student at China University of Petroleum (East China), Qingdao, China. Her fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments. Yao Yao received his B.S. degree from Southwest Jiaotong University, Sichuan, China in 2006, and obtained Ph.D. degree from the Southwest Jiaotong University, Sichuan, China in 2013. He is currently an associate professor at Chengdu University of Information Technology. His current research interests include carbon based electronics and acoustic sensors. Yong Zhang received his Ph. D degree in ocean information detection and treatment from Ocean University of China in 2008. Currently, he is an associate professor at China University of Petroleum (East China), Qingdao, China. His main research interests are precision measurement technology and instruments.