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zinc oxide nanocomposite film
Journal of Alloys and Compounds 698 (2017) 476e483
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Room-temperature high-performance ammonia gas sensor based on layer-by-layer self-assembled molybdenum disulfide/zinc oxide nanocomposite film Dongzhi Zhang*, Chuanxing Jiang, Yan'e Sun College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2016 Received in revised form 16 December 2016 Accepted 17 December 2016 Available online 19 December 2016
A high-sensitivity ammonia gas sensor based on molybdenum disulfide/zinc oxide (MoS2/ZnO) nanocomposite film was reported in this paper. The multilayered film of MoS2/ZnO was fabricated on a PCB substrate with interdigital electrodes (IDE) via layer-by-layer self-assembly technique. The surface morphology, nanostructure and elemental composition of the MoS2/ZnO nanocomposite were described by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that ZnO and MoS2 had good contact in terms of electric and structure. Ammonia-sensing properties of the sensor were investigated by exposing it to various concentrations of ammonia at room temperature. The experimental results revealed that the MoS2/ZnO nanocomposite film sensor exhibited ultra-high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/recovery characteristics. Moreover, the sensing device exhibited significantly enhancement in ammonia gas sensing compared to the drop-casted pure ZnO and layer-by-layer selfassembled MoS2/polymer sensors. These results render the MoS2/ZnO nanocomposite a promising candidate for high-performance ammonia sensing at room temperature. The underlying sensing mechanisms of the MoS2/ZnO sensing device towards ammonia gas were ascribed to the excellent electrochemical properties and special nanostructure of the nanocomposite. © 2016 Elsevier B.V. All rights reserved.
Keywords: Layer-by-layer self-assembly MoS2/ZnO nanocomposite film Ammonia gas sensor Sensing mechanism
1. Introduction As a kind of ubiquitous gas in the atmosphere, ammonia gas plays an important role in many fields. For example, in industry production, ammonia gas is used as precursor for synthesizing various nitrogen compounds. And in the food industry, ammonia gas is an important indicator for protein decomposition processes [1]. Besides, ammonia gas is a highly toxic and explosive gas [2]. For humans, many organs would be injured in the high concentration of ammonia gas (ca. >300 ppm). At concentrations of ca. 15e28% by volume in air, ammonia is combustible [3]. Therefore, the detection of ammonia gas is vital in terms of industrial, agricultural and health monitoring. In recent years, many kinds of gas sensors have been fabricated for detecting ammonia gas, such as surface acoustic wave sensors [4e6], electrochemical sensors [7e9], metal semiconductor sensors [10,11], and emerging 2D nanomaterials-based
* Corresponding author. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.12.222 0925-8388/© 2016 Elsevier B.V. All rights reserved.
sensors [2,12e16]. Among the above-mentioned sensors, metal oxides semiconductors (such as ZnO, SnO2, TiO2, MoO3, V2O5 and In2O3) have attracted considerable attention due to its low cost, nano-size and easy to integration [17e22]. ZnO is one of the most versatile materials due to its excellent inherent properties of wide bandgap (3.37 eV), large exciton binding energy (60 MeV), and high chemical stability [23]. However, pure ZnO has suffered from some drawbacks, such as poor selectivity, lower response, and high operating temperature, which are common problems for the metal oxide semiconductor sensors [24,25]. To the best of our knowledge, graphene has achieved a great development in energy storage, biomedicine, electronics biomedicine, electronics and other fields since its discovery [26]. Moreover, due to its high specific surface area and unique electrical properties such as high mobility and low electrical noise, graphene is used as sensing materials for fabricating gas sensors [26,27]. However, recent advances have found, without proper surface functionalization, graphene is more like a semi-metal due to its zero band-gap, thus significantly restricting its application in gas sensor [28,29]. Compared to graphene, MoS2 as a kind of layered transition metal
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dichalcogenides (TMDs), it has attracted much attention due to its similar structure with graphene and band gap [1.2 eV (indirect band-gap) for bulk and 1.8 eV (direct band-gap) for monolayer] [30]. These unique properties could complement graphene and make it become a promising candidate for constructing gas sensors [31e35]. Furthermore, some researchers have found that the modification of MoS2 with metal oxide semiconductors can effectively enhance the performance of the gas sensor. For instance, Yan et al. investigated the sensing behavior of the sensor based on ZnO nanoparticles-coated MoS2 nanosheets towards ethanol vapors, and the results showed that the sensor had a superior gas-sensing performance [36]. Zhao et al. prepared the sensor based on MoS2TiO2 nanocomposite, which exhibited excellent sensing performances towards ethanol vapors at 150 C compared with the pure TiO2 sensor at the same environment [37]. Cui et al. reported a novel nanohybrid of SnO2 nanocrystal-decorated crumpled MoS2 nanosheets (MoS2/SnO2), which showed a greatly enhancement towards nitrogen dioxide sensing in comparison with the pure MoS2 and SnO2 counterparts at room temperature [29]. Currently, ammonia gas sensor based on MoS2 modified ZnO nanocomposite has not been investigated. In this paper, we demonstrated a facile and cost-effective method for fabricating MoS2/ZnO nanocomposite film sensor with layer-by-layer (LbL) self-assembly technique. The surface morphology, nanostructure and elemental composition of the MoS2/ZnO sample were examined by XRD, SEM and EDS. The sensing performances of the MoS2/ ZnO nanocomposite film sensor were investigated at room temperature. Experimental results showed that the self-assembled MoS2/ZnO film sensor exhibited greatly enhancement in ammonia-sensing properties, and presented distinct advantages of ultra-high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/recovery characteristics. Finally, the underlying sensing mechanism of the MoS2/ZnO film sensor towards ammonia gas was discussed. 2. Experiment 2.1. Materials The reagents of sodium molybdate dehydrate, zinc nitrate hexahydrate, thioacetamide and oxalic acid were obtained from Sinopharm Chemical Reagent Co. Ltd (shanghai, China). Sodium hydroxide was offered by West Long Chemical Limited by Co. Ltd (Guangdong, China). Polycation and polyanion used for layer-bylayer self-assembly were 1.5 wt% poly (diallyldimethylammonium chloride) [PDDA, (Sigma-Aldrich)] and 0.3 wt% poly (sodium 4-styrenesulfonate) [PSS, (Sigma-Aldrich)] with 0.5 M NaCl in both for ionic strength. All the chemicals were received without further purification. 2.2. Sensing film fabrication A facile hydrothermal method was used for synthesizing MoS2 and ZnO [38,39]. for the preparation of MoS2, sodium molybdate dehydrate (1.0 g) and thioacetamide (1.2 g) were firstly dissolved into 80 mL deionized (DI) water and stirred for 0.5 h. And next, 0.4 g oxalic acid was added into the above mixed solution with stirring for another 0.5 h. And then, the resulting mixture was transferred to a 100 mL Teflon stainless-steel autoclave, and heated for 24 h at 200 C. The high-purity MoS2 was obtained after washing several times with DI water. The MoS2 suspension was mixed with an equal volume of PSS solution (0.3 wt%). The mixed MoS2-PSS solution was slightly stirred and then ultrasonicated for 10 min. The synthesis of ZnO nanorods was similar to that of MoS2. Zinc nitrate hexahydrate (2.08 g) was dissolved into 140 mL DI water with stirring for 1 h,
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and 20 mL NaOH (4 mol/L) was added into the resulting solution with stirring for 0.5 h. After that, the resulting mixture was hydrothermally treated at 120 C for 12 h. When the autoclave cooled down to room temperature, the final product of ZnO suspension was washed with DI water for several times and then collected for further use. The sensor device was fabricated on a PCB substrate with coillike interdigitated electrodes, as reported in our previous work [39]. The schematic diagram of the sensor device and its structure are shown in Fig. 1. The sensing film was fabricated on the sensor device using LbL self-assembly method, as shown in Fig. 2. Two bilayers of PDDA/PSS as precursor layers were firstly self-assembled on the substrate surface for charge enhancement, followed by alternative turns of the device immersion into ZnO and MoS2-PSS solutions for five cycles. The deposition times used were 10 min for polyelectrolytes and 15 min for ZnO and MoS2-PSS. Moreover, an intermediate rinsing with DI water and drying under a stream of nitrogen were required after each monolayer assembly to reinforce the interconnection between layers. The incorporation of MoS2/ ZnO was achieved by the alternative immersion and deposition in ZnO and MoS2-PSS solutions under electrostatic force. Finally, the sensor device was dried in the oven at 50 C for 2 h. For making comparisons, the MoS2/PDDA sensing film was prepared by the same manner with PDDA used instead of ZnO, and another sensor based on pure ZnO film was drop-casted. 2.3. Instrument and analysis The ammonia-sensing experiment was performed at room temperature of 25 C. The measurement was carried out by exposing the MoS2/ZnO film sensor to various ammonia gas concentrations varying from 0.25 ppm to 100 ppm, which were achieved by injecting a certain volume of ammonia gas into a sealed container with a syringe [39]. The ammonia gas sample was supplied by Nanjing Special Gas Factory Co. Ltd (Nanjing, China). Ambient air was used as carrier gas, in which the relative humidity is around 35% RH. The resistance response of the presented sensors was recorded by using an Agilent 34970A data logger coupled with a computer. The normalized response of the sensor is defined as S¼ (R0-Rg)/R0 100%, where R0 and Rg are the electrical resistance of the sensor under the air and the given concentration of ammonia gas, respectively. 3. Results and discussion 3.1. Characterization results The XRD measurements of MoS2, ZnO and MoS2/ZnO samples were performed with an X-ray diffractometer (Rigaku D/Max 2500PC) using Cu Ka radiation (l ¼ 1.5418 Å) at room temperature. Fig. 3 shows the XRD spectrums of the samples. The black curve in Fig. 3 shows the pure hexagonal phase of MoS2 (JCPDS 37-1492) with lattice constants of a ¼ b ¼ 3.16 Å, c ¼ 12.29 Å [40]. The diffraction peaks of MoS2 are observed at 2q of 13.51, 34.06 , 39.31 and 59.2 , which are corresponding to the (002), (100), (103), and (110) planes of the MoS2 nanocrystal, and there are no peaks for other impurities being found in the spectra, illustrating that there is only MoS2 with a high purity and was well crystallized. The blue curve in Fig. 3 shows the major diffraction peaks for the ZnO nanocrystals (JCPDS89-1397) are observed, attributing to the (100), (002), (101), (102) and (110) planes of the ZnO [41]. The red curve in Fig. 3 shows the XRD pattern of the MoS2/ZnO nanocomposite, which contains the characteristic peaks of the MoS2 and ZnO, confirming the presence of MoS2 and ZnO in the MoS2/ZnO nanocomposite. The results of XRD characterization indicate the
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Fig. 1. Schematic diagram of the ammonia sensor and its structure.
successful preparation of the MoS2, ZnO and MoS2/ZnO samples. The surface morphology of as-prepared MoS2, ZnO and MoS2/ ZnO samples was analyzed by field emission scanning electron microscopy (SEM, Hitachi S-4800). Fig. 4 (a) shows the hydrothermally synthesized ZnO has nanorod-like shape, and Fig. 4 (b) displays that MoS2 has small pieces shape. Fig. 4 (c) illustrates that ZnO and MoS2 have good contact in the MoS2/ZnO nanocomposite. Furthermore, the element composition of the MoS2/ZnO nanocomposite was analyzed by energy dispersive spectroscopy (EDS, Ametek). The EDS spectrum is shown in Fig. 4 (d). Only the elements Mo, S, Zn, and O are detected in the MoS2/ZnO nanocomposite, no other impurity elements are observed. Brunauer-Emmett-Teller (BET) measurement was performed on the MoS2/ZnO sample by a surface area analyser (Micromeritics, ASAP 2020M). The MoS2/ZnO sample is composed of ZnO nanorods and MoS2 nanosheets which are interconnected with each other,
and form mesoporous structure with much more specific surface area. Fig. 5 shows the N2 adsorptiondesorption isotherm measurement. A distinct hysteresis loop at high relative pressure (P/P0) of 0.65e0.98 is observed, suggesting its mesoporous structure. The BET specific surface area of the MoS2/ZnO sample is calculated to be 110.27 m2/g, which is much higher than the reported ZnO and MoS2 with specific surface areas of 46.95 m2/g and 27.7 m2/g, respectively [42,43]. 3.2. Ammonia-sensing properties Fig. 6 demonstrates the real-time resistance measurement of the MoS2/ZnO film sensor against ammonia gas with concentration from 0.25 to 100 ppm at room temperature. The corresponding normalized response values are about 9.28%, 13.3%, 17.45%, 24.38%, 37.43%, 45.80% and 61.92% when the sensor exposed to 0.25, 0.5, 1,
Fig. 2. Layer-by-layer fabrication process of MoS2/ZnO nanocomposite film.
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Fig. 5. Nitrogen adsorption-desorption isotherm of MoS2/ZnO nanocomposite. Fig. 3. XRD patterns for MoS2, ZnO, and MoS2/ZnO nanocomposite.
5, 10, 50 and 100 ppm, respectively. Moreover, it is obviously found that the resistance monotonically decreases with the increases of gas concentration. The phenomenon could be interpreted as that the MoS2/ZnO film exhibited n-type semiconductor characteristics to ammonia gas [44]. Furthermore, the limit of detection (LOD) is defined as the sensor with three times the standard deviation of its noise according to IUPAC [45]. A low LOD of 12 ppb can be determined for the MoS2/ZnO film sensor towards ammonia sensing. It indicates that the MoS2/ZnO nanocomposite film sensor is very sensitive to ammonia gas. The repeatability of the MoS2/ZnO film sensor was measured through four cycles repeatedly switching exposure to ammonia gas concentration of 5 ppm, 10 ppm, 50 ppm and air at room
temperature, and the results are shown in Fig. 7. A satisfactory stability and repeatability were observed in ammonia sensing. Gas sensing selectivity is a vital parameter for gas sensors. Fig. 8 presents the response of MoS2/ZnO film sensor to various relative humidity (RH). The measured RH range is from 11% to 97%, and the resistance changed synchronously with different RH, indicating that humidity has some influence on the sensor resistance. Therefore, humidity compensation should be made when the sensor is used at different RH. Fig. 9 shows the gas responses of the MoS2/ZnO nanocomposite film sensor towards 5 ppm ammonia, carbon monoxide, carbon dioxide, hydrogen and dimethylmethane at room temperature, respectively. It can be clearly found that the response to ammonia is significantly higher than that of other tested gases, indicating that
Fig. 4. SEM characterization of (a) ZnO nanorods, (b) MoS2 nanosheets and (c) MoS2/ZnO nanocomposite. (d) EDS for MoS2/ZnO nanocomposite.
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Fig. 6. Resistance measurement of the MoS2/ZnO film sensor towards ammonia gas ranging from 0.25 to 100 ppm at room temperature. Fig. 9. Selectivity of the MoS2/ZnO film sensor to 5 ppm of ammonia, carbon monoxide, carbon dioxide, hydrogen and dimethylmethane at room temperature.
Fig. 7. Repeatability curve of the MoS2/ZnO nanocomposite film sensor towards ammonia gas of 5, 10, 50 ppm at room temperature.
the MoS2/ZnO nanocomposite sensor has high selectivity for ammonia detection. One possible reason may be due to the ammonia has one unique lone electron pair, making it more polarized than other gases [46]. Fig. 10 shows the long-term stability of the MoS2/ZnO film sensor upon exposure to 5, 10 and
Fig. 8. Response of the MoS2/ZnO film sensor as a function of relative humidity (RH).
50 ppm ammonia gas. The response of the sensor was recorded every five days over a month. The observed results indicated that the sensor response did not change significantly in the passing of time, verifying a good stability. To further investigate the advantages of the MoS2/ZnO film sensor, its sensing properties towards ammonia gas were compared with that of LbL self-assembled MoS2/PDDA and drop-casted pure ZnO film sensors. These sensors were tested against ammonia gas concentration from 0.25 to 100 ppm. Fig. 11 shows the normalized response of the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors. The MoS2/ZnO film sensor exhibits the highest response compared to the other two sensors. For instance, the normalized response values are 23.96%, 19.63% and 5.97% for the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors upon exposure of 5 ppm ammonia gas, respectively. The time taken by a sensor to achieve 90% of the total resistance change is defined as the response/recovery time. As shown in the inset of Fig. 11, the response/recovery time of the MoS2/ZnO nanocomposite film sensor is about 10/11 s for 50 ppm ammonia sensing, indicating the sensor possesses fast response and recovery characteristics. Fig. 12 plots the normalize responses of the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors as
Fig. 10. Long term stability of the MoS2/ZnO film sensor upon exposure to 5, 10 and 50 ppm ammonia gas.
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Furthermore, the ammonia-gas sensing properties of the MoS2/ ZnO film sensor compared with the published works are listed in Table 1 [15,47e50]. We compared our work with the previous works in which the sensors were measured against the same concentration of NH3 (50 ppm), and the gas sensitive response was calculated by the terms of (R0-Ra)/Ra 100%. The existing sensors made from reduced graphene oxide, metal oxide and MoS2 based sensing materials by various methods, such as drop-coating, spraying and sol-gel. The comparative results demonstrated that the LbL self-assembled MoS2/ZnO nanocomposite film showed superior gas sensing performances over the reported sensors towards ammonia sensing at room temperature. 3.3. Ammonia-sensing mechanism
Fig. 11. Normalized responses of MoS2/ZnO, MoS2/PDDA and ZnO film sensors exposed to various ammonia gas concentrations at room temperature, and the inset indicates the response and recovery characteristics of these sensors exposed to 50 ppm ammonia gas.
Fig. 12. Normalize responses of the MoS2/ZnO, MoS2/PDDA and ZnO film sensors as functions of ammonia gas concentration.
functions of ammonia concentration. The fitting equations for the sensor response Y and gas concentration X are represented as Y ¼ 19.507lgXþ17.723, Y ¼ 13.860lgXþ13.787 and Y ¼ 3.906lgX þ4.163 for the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensor, respectively. The sensor response exhibits a good linearity with the common logarithm of ammonia gas concentration, and the slopes of fitting equations for the three sensors are 19.507, 13.860 and 3.906 for the MoS2/ZnO, MoS2/PDDA and ZnO film sensor, respectively, suggesting the MoS2/ZnO film sensor possessed the highest sensitivity among the three samples.
The MoS2/ZnO nanocomposite film sensor exhibited an excellent response, fast response/recovery characteristics and good selectivity towards ammonia gas at room temperature, which could be attributed to the excellent electrochemical characteristics of both MoS2 and ZnO, and their synergistic effects. The MoS2/ZnO nanostructure offers quantities of efficient electron pathways and active centers for gas sensor. The BET measurement indicates that the MoS2/ZnO nanocomposite has much higher specific surface area than that of MoS2 and ZnO, which is significantly beneficial to the absorption and diffusion of ammonia molecules, and thus enhances the response of the sensor. MoS2 possesses natural band gap as well as high charge carrier mobility, which can provide direct conduction paths for the carriers transport. When the n-type ZnO semiconductor is exposed to air atmosphere, oxygen molecules are absorbed onto the surface of ZnO, acting as electron acceptors and capturing free electrons from the conduction band of the sensing material [51]. It is widely known that oxygen molecules are 2 adsorbed to form O (>300 C) 2 (<100 C), O (100e300 C) and O by capturing conductive electrons from the surface of metal oxide [52]. Thus, at room temperature in our experiment, the adsorbed oxygen species is mainly O 2(ads). When the sensor is exposed to reducing gas such as ammonia, gas molecules will react with O 2(ads), and releasing the trapped electrons back into the conduction band, thereby decreasing the measured resistance of the sensor. The sensing mechanism illustration of MoS2/ZnO nanocomposite is shown in Fig. 13 (a), and the reaction is expressed as follows [49,53]:
O2ðgasÞ /O2ðadsÞ
(1)
O2ðgasÞ þ e /O2ðadsÞ
(2)
4NH3 þ 5O2ðadsÞ /4NO þ 6H2 O þ 5e
(3)
The self-assembled MoS2/ZnO nanocomposite film exhibits a good bonding between MoS2 and ZnO. The work functions for the ZnO and MoS2 are W1 ¼ 4.43eV and W2 ¼ 4.33eV, respectively. The ZnO film has a larger work function than the MoS2 film. Thus, an energy barrier (DEB ¼ W1W2) is created at the interface between
Table 1 Performance of the presented sensor in this work compared with previous work. Sensing material
Fabrication method
Operating temperature
concentration
Response
Ref.
MoS2/Au rGO/ZnO rGO/TiO2 rGO/AgNWs ZnO MoS2/ZnO
Drop-coating Spraying Drop-coating Solution process Solegel LbL self-assemble
60 C RT RT RT 150 C RT
50 50 50 50 50 50
2.2% 3.05% 15.9% 7.5% 18% 46.2%
[15] [47] [48] [49] [50] This work
ppm ppm ppm ppm ppm ppm
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Universities of China (No. 15CX05041A), the Science and Technology Development Plan Project of Qingdao (Grant No.16-6-2-53nsh), and the Science and Technology Project of Huangdao Zone, Qingdao, China (Grant No. 2014-1-51). References
Fig. 13. (a) The sensing mechanism illustration of MoS2/ZnO nanocomposite. (b) Schematic illustration of the energy-band structure of the sensor (E0 is vacuum-energy level, W is work function, Eg is energy band gap, EF is Fermi-energy level, EC is the conduction band, and EV is the valence band).
MoS2 and ZnO when the heterojunction is formed [54]. Fig. 13 (b) shows the schematic illustration of the sensing mechanism of the MoS2/ZnO film sensor. When the sensor is exposed to NH3, gas molecules react with the O 2(ads) on the surface of ZnO and release considerable electrons to the ZnO film. These electrons can increase the concentration of the electron carriers in the ZnO film, and the Fermi level of the ZnO film shifts toward the conduction band and the barrier height at the MoS2/ZnO interface decreases. Consequently, the MoS2/ZnO film sensor exhibits a decreased resistance when it is exposed to NH3. The above reason may be attributed to the prominent sensing performances of the MoS2/ZnO nanocomposite sensor towards ammonia. 4. Conclusions In summary, the MoS2/ZnO nanocomposite film sensor was fabricated using LbL self-assembly technique. The surface morphology, nanostructure and elemental composition of the asprepared MoS2/ZnO sample were analyzed by XRD, SEM and EDS. The sensor was measured by exposing to different concentration of ammonia gas at room temperature, and experimental results showed that the sensor exhibited a high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/ recovery characteristics towards low concentration ammonia gas, which was superior to the pure ZnO and MoS2/PDDA counterparts. Finally, the possible sensing mechanism was discussed in detail. These observed results demonstrate that MoS2 modified ZnO nanocomposite is a promising block for constructing highperformance ammonia gas sensor. 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
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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. From 2009 to 2011, he worked as a visiting scholar of Mechanical Engineering at the University of Minnesota, U.S.A. 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.
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.
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.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Room-temperature high-performance ammonia gas sensor based on layer-by-layer self-assembled molybdenum disulfide/zinc oxide nanocomposite film Dongzhi Zhang*, Chuanxing Jiang, Yan'e Sun College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2016 Received in revised form 16 December 2016 Accepted 17 December 2016 Available online 19 December 2016
A high-sensitivity ammonia gas sensor based on molybdenum disulfide/zinc oxide (MoS2/ZnO) nanocomposite film was reported in this paper. The multilayered film of MoS2/ZnO was fabricated on a PCB substrate with interdigital electrodes (IDE) via layer-by-layer self-assembly technique. The surface morphology, nanostructure and elemental composition of the MoS2/ZnO nanocomposite were described by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that ZnO and MoS2 had good contact in terms of electric and structure. Ammonia-sensing properties of the sensor were investigated by exposing it to various concentrations of ammonia at room temperature. The experimental results revealed that the MoS2/ZnO nanocomposite film sensor exhibited ultra-high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/recovery characteristics. Moreover, the sensing device exhibited significantly enhancement in ammonia gas sensing compared to the drop-casted pure ZnO and layer-by-layer selfassembled MoS2/polymer sensors. These results render the MoS2/ZnO nanocomposite a promising candidate for high-performance ammonia sensing at room temperature. The underlying sensing mechanisms of the MoS2/ZnO sensing device towards ammonia gas were ascribed to the excellent electrochemical properties and special nanostructure of the nanocomposite. © 2016 Elsevier B.V. All rights reserved.
Keywords: Layer-by-layer self-assembly MoS2/ZnO nanocomposite film Ammonia gas sensor Sensing mechanism
1. Introduction As a kind of ubiquitous gas in the atmosphere, ammonia gas plays an important role in many fields. For example, in industry production, ammonia gas is used as precursor for synthesizing various nitrogen compounds. And in the food industry, ammonia gas is an important indicator for protein decomposition processes [1]. Besides, ammonia gas is a highly toxic and explosive gas [2]. For humans, many organs would be injured in the high concentration of ammonia gas (ca. >300 ppm). At concentrations of ca. 15e28% by volume in air, ammonia is combustible [3]. Therefore, the detection of ammonia gas is vital in terms of industrial, agricultural and health monitoring. In recent years, many kinds of gas sensors have been fabricated for detecting ammonia gas, such as surface acoustic wave sensors [4e6], electrochemical sensors [7e9], metal semiconductor sensors [10,11], and emerging 2D nanomaterials-based
* Corresponding author. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.12.222 0925-8388/© 2016 Elsevier B.V. All rights reserved.
sensors [2,12e16]. Among the above-mentioned sensors, metal oxides semiconductors (such as ZnO, SnO2, TiO2, MoO3, V2O5 and In2O3) have attracted considerable attention due to its low cost, nano-size and easy to integration [17e22]. ZnO is one of the most versatile materials due to its excellent inherent properties of wide bandgap (3.37 eV), large exciton binding energy (60 MeV), and high chemical stability [23]. However, pure ZnO has suffered from some drawbacks, such as poor selectivity, lower response, and high operating temperature, which are common problems for the metal oxide semiconductor sensors [24,25]. To the best of our knowledge, graphene has achieved a great development in energy storage, biomedicine, electronics biomedicine, electronics and other fields since its discovery [26]. Moreover, due to its high specific surface area and unique electrical properties such as high mobility and low electrical noise, graphene is used as sensing materials for fabricating gas sensors [26,27]. However, recent advances have found, without proper surface functionalization, graphene is more like a semi-metal due to its zero band-gap, thus significantly restricting its application in gas sensor [28,29]. Compared to graphene, MoS2 as a kind of layered transition metal
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dichalcogenides (TMDs), it has attracted much attention due to its similar structure with graphene and band gap [1.2 eV (indirect band-gap) for bulk and 1.8 eV (direct band-gap) for monolayer] [30]. These unique properties could complement graphene and make it become a promising candidate for constructing gas sensors [31e35]. Furthermore, some researchers have found that the modification of MoS2 with metal oxide semiconductors can effectively enhance the performance of the gas sensor. For instance, Yan et al. investigated the sensing behavior of the sensor based on ZnO nanoparticles-coated MoS2 nanosheets towards ethanol vapors, and the results showed that the sensor had a superior gas-sensing performance [36]. Zhao et al. prepared the sensor based on MoS2TiO2 nanocomposite, which exhibited excellent sensing performances towards ethanol vapors at 150 C compared with the pure TiO2 sensor at the same environment [37]. Cui et al. reported a novel nanohybrid of SnO2 nanocrystal-decorated crumpled MoS2 nanosheets (MoS2/SnO2), which showed a greatly enhancement towards nitrogen dioxide sensing in comparison with the pure MoS2 and SnO2 counterparts at room temperature [29]. Currently, ammonia gas sensor based on MoS2 modified ZnO nanocomposite has not been investigated. In this paper, we demonstrated a facile and cost-effective method for fabricating MoS2/ZnO nanocomposite film sensor with layer-by-layer (LbL) self-assembly technique. The surface morphology, nanostructure and elemental composition of the MoS2/ZnO sample were examined by XRD, SEM and EDS. The sensing performances of the MoS2/ ZnO nanocomposite film sensor were investigated at room temperature. Experimental results showed that the self-assembled MoS2/ZnO film sensor exhibited greatly enhancement in ammonia-sensing properties, and presented distinct advantages of ultra-high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/recovery characteristics. Finally, the underlying sensing mechanism of the MoS2/ZnO film sensor towards ammonia gas was discussed. 2. Experiment 2.1. Materials The reagents of sodium molybdate dehydrate, zinc nitrate hexahydrate, thioacetamide and oxalic acid were obtained from Sinopharm Chemical Reagent Co. Ltd (shanghai, China). Sodium hydroxide was offered by West Long Chemical Limited by Co. Ltd (Guangdong, China). Polycation and polyanion used for layer-bylayer self-assembly were 1.5 wt% poly (diallyldimethylammonium chloride) [PDDA, (Sigma-Aldrich)] and 0.3 wt% poly (sodium 4-styrenesulfonate) [PSS, (Sigma-Aldrich)] with 0.5 M NaCl in both for ionic strength. All the chemicals were received without further purification. 2.2. Sensing film fabrication A facile hydrothermal method was used for synthesizing MoS2 and ZnO [38,39]. for the preparation of MoS2, sodium molybdate dehydrate (1.0 g) and thioacetamide (1.2 g) were firstly dissolved into 80 mL deionized (DI) water and stirred for 0.5 h. And next, 0.4 g oxalic acid was added into the above mixed solution with stirring for another 0.5 h. And then, the resulting mixture was transferred to a 100 mL Teflon stainless-steel autoclave, and heated for 24 h at 200 C. The high-purity MoS2 was obtained after washing several times with DI water. The MoS2 suspension was mixed with an equal volume of PSS solution (0.3 wt%). The mixed MoS2-PSS solution was slightly stirred and then ultrasonicated for 10 min. The synthesis of ZnO nanorods was similar to that of MoS2. Zinc nitrate hexahydrate (2.08 g) was dissolved into 140 mL DI water with stirring for 1 h,
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and 20 mL NaOH (4 mol/L) was added into the resulting solution with stirring for 0.5 h. After that, the resulting mixture was hydrothermally treated at 120 C for 12 h. When the autoclave cooled down to room temperature, the final product of ZnO suspension was washed with DI water for several times and then collected for further use. The sensor device was fabricated on a PCB substrate with coillike interdigitated electrodes, as reported in our previous work [39]. The schematic diagram of the sensor device and its structure are shown in Fig. 1. The sensing film was fabricated on the sensor device using LbL self-assembly method, as shown in Fig. 2. Two bilayers of PDDA/PSS as precursor layers were firstly self-assembled on the substrate surface for charge enhancement, followed by alternative turns of the device immersion into ZnO and MoS2-PSS solutions for five cycles. The deposition times used were 10 min for polyelectrolytes and 15 min for ZnO and MoS2-PSS. Moreover, an intermediate rinsing with DI water and drying under a stream of nitrogen were required after each monolayer assembly to reinforce the interconnection between layers. The incorporation of MoS2/ ZnO was achieved by the alternative immersion and deposition in ZnO and MoS2-PSS solutions under electrostatic force. Finally, the sensor device was dried in the oven at 50 C for 2 h. For making comparisons, the MoS2/PDDA sensing film was prepared by the same manner with PDDA used instead of ZnO, and another sensor based on pure ZnO film was drop-casted. 2.3. Instrument and analysis The ammonia-sensing experiment was performed at room temperature of 25 C. The measurement was carried out by exposing the MoS2/ZnO film sensor to various ammonia gas concentrations varying from 0.25 ppm to 100 ppm, which were achieved by injecting a certain volume of ammonia gas into a sealed container with a syringe [39]. The ammonia gas sample was supplied by Nanjing Special Gas Factory Co. Ltd (Nanjing, China). Ambient air was used as carrier gas, in which the relative humidity is around 35% RH. The resistance response of the presented sensors was recorded by using an Agilent 34970A data logger coupled with a computer. The normalized response of the sensor is defined as S¼ (R0-Rg)/R0 100%, where R0 and Rg are the electrical resistance of the sensor under the air and the given concentration of ammonia gas, respectively. 3. Results and discussion 3.1. Characterization results The XRD measurements of MoS2, ZnO and MoS2/ZnO samples were performed with an X-ray diffractometer (Rigaku D/Max 2500PC) using Cu Ka radiation (l ¼ 1.5418 Å) at room temperature. Fig. 3 shows the XRD spectrums of the samples. The black curve in Fig. 3 shows the pure hexagonal phase of MoS2 (JCPDS 37-1492) with lattice constants of a ¼ b ¼ 3.16 Å, c ¼ 12.29 Å [40]. The diffraction peaks of MoS2 are observed at 2q of 13.51, 34.06 , 39.31 and 59.2 , which are corresponding to the (002), (100), (103), and (110) planes of the MoS2 nanocrystal, and there are no peaks for other impurities being found in the spectra, illustrating that there is only MoS2 with a high purity and was well crystallized. The blue curve in Fig. 3 shows the major diffraction peaks for the ZnO nanocrystals (JCPDS89-1397) are observed, attributing to the (100), (002), (101), (102) and (110) planes of the ZnO [41]. The red curve in Fig. 3 shows the XRD pattern of the MoS2/ZnO nanocomposite, which contains the characteristic peaks of the MoS2 and ZnO, confirming the presence of MoS2 and ZnO in the MoS2/ZnO nanocomposite. The results of XRD characterization indicate the
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Fig. 1. Schematic diagram of the ammonia sensor and its structure.
successful preparation of the MoS2, ZnO and MoS2/ZnO samples. The surface morphology of as-prepared MoS2, ZnO and MoS2/ ZnO samples was analyzed by field emission scanning electron microscopy (SEM, Hitachi S-4800). Fig. 4 (a) shows the hydrothermally synthesized ZnO has nanorod-like shape, and Fig. 4 (b) displays that MoS2 has small pieces shape. Fig. 4 (c) illustrates that ZnO and MoS2 have good contact in the MoS2/ZnO nanocomposite. Furthermore, the element composition of the MoS2/ZnO nanocomposite was analyzed by energy dispersive spectroscopy (EDS, Ametek). The EDS spectrum is shown in Fig. 4 (d). Only the elements Mo, S, Zn, and O are detected in the MoS2/ZnO nanocomposite, no other impurity elements are observed. Brunauer-Emmett-Teller (BET) measurement was performed on the MoS2/ZnO sample by a surface area analyser (Micromeritics, ASAP 2020M). The MoS2/ZnO sample is composed of ZnO nanorods and MoS2 nanosheets which are interconnected with each other,
and form mesoporous structure with much more specific surface area. Fig. 5 shows the N2 adsorptiondesorption isotherm measurement. A distinct hysteresis loop at high relative pressure (P/P0) of 0.65e0.98 is observed, suggesting its mesoporous structure. The BET specific surface area of the MoS2/ZnO sample is calculated to be 110.27 m2/g, which is much higher than the reported ZnO and MoS2 with specific surface areas of 46.95 m2/g and 27.7 m2/g, respectively [42,43]. 3.2. Ammonia-sensing properties Fig. 6 demonstrates the real-time resistance measurement of the MoS2/ZnO film sensor against ammonia gas with concentration from 0.25 to 100 ppm at room temperature. The corresponding normalized response values are about 9.28%, 13.3%, 17.45%, 24.38%, 37.43%, 45.80% and 61.92% when the sensor exposed to 0.25, 0.5, 1,
Fig. 2. Layer-by-layer fabrication process of MoS2/ZnO nanocomposite film.
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Fig. 5. Nitrogen adsorption-desorption isotherm of MoS2/ZnO nanocomposite. Fig. 3. XRD patterns for MoS2, ZnO, and MoS2/ZnO nanocomposite.
5, 10, 50 and 100 ppm, respectively. Moreover, it is obviously found that the resistance monotonically decreases with the increases of gas concentration. The phenomenon could be interpreted as that the MoS2/ZnO film exhibited n-type semiconductor characteristics to ammonia gas [44]. Furthermore, the limit of detection (LOD) is defined as the sensor with three times the standard deviation of its noise according to IUPAC [45]. A low LOD of 12 ppb can be determined for the MoS2/ZnO film sensor towards ammonia sensing. It indicates that the MoS2/ZnO nanocomposite film sensor is very sensitive to ammonia gas. The repeatability of the MoS2/ZnO film sensor was measured through four cycles repeatedly switching exposure to ammonia gas concentration of 5 ppm, 10 ppm, 50 ppm and air at room
temperature, and the results are shown in Fig. 7. A satisfactory stability and repeatability were observed in ammonia sensing. Gas sensing selectivity is a vital parameter for gas sensors. Fig. 8 presents the response of MoS2/ZnO film sensor to various relative humidity (RH). The measured RH range is from 11% to 97%, and the resistance changed synchronously with different RH, indicating that humidity has some influence on the sensor resistance. Therefore, humidity compensation should be made when the sensor is used at different RH. Fig. 9 shows the gas responses of the MoS2/ZnO nanocomposite film sensor towards 5 ppm ammonia, carbon monoxide, carbon dioxide, hydrogen and dimethylmethane at room temperature, respectively. It can be clearly found that the response to ammonia is significantly higher than that of other tested gases, indicating that
Fig. 4. SEM characterization of (a) ZnO nanorods, (b) MoS2 nanosheets and (c) MoS2/ZnO nanocomposite. (d) EDS for MoS2/ZnO nanocomposite.
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Fig. 6. Resistance measurement of the MoS2/ZnO film sensor towards ammonia gas ranging from 0.25 to 100 ppm at room temperature. Fig. 9. Selectivity of the MoS2/ZnO film sensor to 5 ppm of ammonia, carbon monoxide, carbon dioxide, hydrogen and dimethylmethane at room temperature.
Fig. 7. Repeatability curve of the MoS2/ZnO nanocomposite film sensor towards ammonia gas of 5, 10, 50 ppm at room temperature.
the MoS2/ZnO nanocomposite sensor has high selectivity for ammonia detection. One possible reason may be due to the ammonia has one unique lone electron pair, making it more polarized than other gases [46]. Fig. 10 shows the long-term stability of the MoS2/ZnO film sensor upon exposure to 5, 10 and
Fig. 8. Response of the MoS2/ZnO film sensor as a function of relative humidity (RH).
50 ppm ammonia gas. The response of the sensor was recorded every five days over a month. The observed results indicated that the sensor response did not change significantly in the passing of time, verifying a good stability. To further investigate the advantages of the MoS2/ZnO film sensor, its sensing properties towards ammonia gas were compared with that of LbL self-assembled MoS2/PDDA and drop-casted pure ZnO film sensors. These sensors were tested against ammonia gas concentration from 0.25 to 100 ppm. Fig. 11 shows the normalized response of the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors. The MoS2/ZnO film sensor exhibits the highest response compared to the other two sensors. For instance, the normalized response values are 23.96%, 19.63% and 5.97% for the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors upon exposure of 5 ppm ammonia gas, respectively. The time taken by a sensor to achieve 90% of the total resistance change is defined as the response/recovery time. As shown in the inset of Fig. 11, the response/recovery time of the MoS2/ZnO nanocomposite film sensor is about 10/11 s for 50 ppm ammonia sensing, indicating the sensor possesses fast response and recovery characteristics. Fig. 12 plots the normalize responses of the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensors as
Fig. 10. Long term stability of the MoS2/ZnO film sensor upon exposure to 5, 10 and 50 ppm ammonia gas.
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Furthermore, the ammonia-gas sensing properties of the MoS2/ ZnO film sensor compared with the published works are listed in Table 1 [15,47e50]. We compared our work with the previous works in which the sensors were measured against the same concentration of NH3 (50 ppm), and the gas sensitive response was calculated by the terms of (R0-Ra)/Ra 100%. The existing sensors made from reduced graphene oxide, metal oxide and MoS2 based sensing materials by various methods, such as drop-coating, spraying and sol-gel. The comparative results demonstrated that the LbL self-assembled MoS2/ZnO nanocomposite film showed superior gas sensing performances over the reported sensors towards ammonia sensing at room temperature. 3.3. Ammonia-sensing mechanism
Fig. 11. Normalized responses of MoS2/ZnO, MoS2/PDDA and ZnO film sensors exposed to various ammonia gas concentrations at room temperature, and the inset indicates the response and recovery characteristics of these sensors exposed to 50 ppm ammonia gas.
Fig. 12. Normalize responses of the MoS2/ZnO, MoS2/PDDA and ZnO film sensors as functions of ammonia gas concentration.
functions of ammonia concentration. The fitting equations for the sensor response Y and gas concentration X are represented as Y ¼ 19.507lgXþ17.723, Y ¼ 13.860lgXþ13.787 and Y ¼ 3.906lgX þ4.163 for the MoS2/ZnO, MoS2/PDDA and pure ZnO film sensor, respectively. The sensor response exhibits a good linearity with the common logarithm of ammonia gas concentration, and the slopes of fitting equations for the three sensors are 19.507, 13.860 and 3.906 for the MoS2/ZnO, MoS2/PDDA and ZnO film sensor, respectively, suggesting the MoS2/ZnO film sensor possessed the highest sensitivity among the three samples.
The MoS2/ZnO nanocomposite film sensor exhibited an excellent response, fast response/recovery characteristics and good selectivity towards ammonia gas at room temperature, which could be attributed to the excellent electrochemical characteristics of both MoS2 and ZnO, and their synergistic effects. The MoS2/ZnO nanostructure offers quantities of efficient electron pathways and active centers for gas sensor. The BET measurement indicates that the MoS2/ZnO nanocomposite has much higher specific surface area than that of MoS2 and ZnO, which is significantly beneficial to the absorption and diffusion of ammonia molecules, and thus enhances the response of the sensor. MoS2 possesses natural band gap as well as high charge carrier mobility, which can provide direct conduction paths for the carriers transport. When the n-type ZnO semiconductor is exposed to air atmosphere, oxygen molecules are absorbed onto the surface of ZnO, acting as electron acceptors and capturing free electrons from the conduction band of the sensing material [51]. It is widely known that oxygen molecules are 2 adsorbed to form O (>300 C) 2 (<100 C), O (100e300 C) and O by capturing conductive electrons from the surface of metal oxide [52]. Thus, at room temperature in our experiment, the adsorbed oxygen species is mainly O 2(ads). When the sensor is exposed to reducing gas such as ammonia, gas molecules will react with O 2(ads), and releasing the trapped electrons back into the conduction band, thereby decreasing the measured resistance of the sensor. The sensing mechanism illustration of MoS2/ZnO nanocomposite is shown in Fig. 13 (a), and the reaction is expressed as follows [49,53]:
O2ðgasÞ /O2ðadsÞ
(1)
O2ðgasÞ þ e /O2ðadsÞ
(2)
4NH3 þ 5O2ðadsÞ /4NO þ 6H2 O þ 5e
(3)
The self-assembled MoS2/ZnO nanocomposite film exhibits a good bonding between MoS2 and ZnO. The work functions for the ZnO and MoS2 are W1 ¼ 4.43eV and W2 ¼ 4.33eV, respectively. The ZnO film has a larger work function than the MoS2 film. Thus, an energy barrier (DEB ¼ W1W2) is created at the interface between
Table 1 Performance of the presented sensor in this work compared with previous work. Sensing material
Fabrication method
Operating temperature
concentration
Response
Ref.
MoS2/Au rGO/ZnO rGO/TiO2 rGO/AgNWs ZnO MoS2/ZnO
Drop-coating Spraying Drop-coating Solution process Solegel LbL self-assemble
60 C RT RT RT 150 C RT
50 50 50 50 50 50
2.2% 3.05% 15.9% 7.5% 18% 46.2%
[15] [47] [48] [49] [50] This work
ppm ppm ppm ppm ppm ppm
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Universities of China (No. 15CX05041A), the Science and Technology Development Plan Project of Qingdao (Grant No.16-6-2-53nsh), and the Science and Technology Project of Huangdao Zone, Qingdao, China (Grant No. 2014-1-51). References
Fig. 13. (a) The sensing mechanism illustration of MoS2/ZnO nanocomposite. (b) Schematic illustration of the energy-band structure of the sensor (E0 is vacuum-energy level, W is work function, Eg is energy band gap, EF is Fermi-energy level, EC is the conduction band, and EV is the valence band).
MoS2 and ZnO when the heterojunction is formed [54]. Fig. 13 (b) shows the schematic illustration of the sensing mechanism of the MoS2/ZnO film sensor. When the sensor is exposed to NH3, gas molecules react with the O 2(ads) on the surface of ZnO and release considerable electrons to the ZnO film. These electrons can increase the concentration of the electron carriers in the ZnO film, and the Fermi level of the ZnO film shifts toward the conduction band and the barrier height at the MoS2/ZnO interface decreases. Consequently, the MoS2/ZnO film sensor exhibits a decreased resistance when it is exposed to NH3. The above reason may be attributed to the prominent sensing performances of the MoS2/ZnO nanocomposite sensor towards ammonia. 4. Conclusions In summary, the MoS2/ZnO nanocomposite film sensor was fabricated using LbL self-assembly technique. The surface morphology, nanostructure and elemental composition of the asprepared MoS2/ZnO sample were analyzed by XRD, SEM and EDS. The sensor was measured by exposing to different concentration of ammonia gas at room temperature, and experimental results showed that the sensor exhibited a high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/ recovery characteristics towards low concentration ammonia gas, which was superior to the pure ZnO and MoS2/PDDA counterparts. Finally, the possible sensing mechanism was discussed in detail. These observed results demonstrate that MoS2 modified ZnO nanocomposite is a promising block for constructing highperformance ammonia gas sensor. 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
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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. From 2009 to 2011, he worked as a visiting scholar of Mechanical Engineering at the University of Minnesota, U.S.A. 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.
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.
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.