Solid-state chemical synthesis and xylene-sensing properties of α-MoO3 arrays assembled by nanoplates

Accepted Manuscript Title: Solid-state chemical synthesis and xylene-sensing properties of ␣-MoO3 arrays assembled by nanoplates Author: Haiyu Qin Yali Cao Jing Xie Hui Xu Dianzeng Jia PII: DOI: Reference:

S0925-4005(16)31879-2 http://dx.doi.org/doi:10.1016/j.snb.2016.11.081 SNB 21291

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

23-6-2016 13-11-2016 16-11-2016

Please cite this article as: Haiyu Qin, Yali Cao, Jing Xie, Hui Xu, Dianzeng Jia, Solid-state chemical synthesis and xylene-sensing properties of ␣-MoO3 arrays assembled by nanoplates, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.11.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solid-state chemical synthesis and xylene-sensing properties of α-MoO3 arrays assembled by nanoplates Haiyu Qin, Yali Cao*, Jing Xie, Hui Xu, Dianzeng Jia Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China.

Corresponding author. Tel.: +86-991-8583083. Fax: +86-991-8588883. E-mail addresses: [email protected].

Hightlights 1. The special α-MoO3 arrays constructed by nanoplates were simply synthesized by solid-state chemical reaction. 2. The green method was excuted no solvents. 3. The α-MoO3 arrays exhibited excellent gas-sensing properties for xylene.

Abstract The special α-MoO3 arrays constructed from nanoplates were synthesized using a simple solid-state chemical reaction. The obtained products were characterized by x-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy and Brunauer-Emmett-Teller analysis. Gas-sensing characteristics of the products toward noxious gases were investigated. The α-MoO3 arrays with high consistency were self-assembled in-situ by two-dimensional nanoplates due to the crystal face preferred orientation and the appropriate protection for specific crystal surface. The arrays exhibited good response to xylene in a wide range of concentrations, and the response value to 100 ppm xylene reached 19.2 at the optimum operation temperature of 370oC, which is three times higher than that of unassembled α-MoO3 nanoplates. The good xylene response was attributed to the preferential exposure of the active crystal faces with high reactivity and the relatively large specific surface area of arrays, which provided more active sites and adsorbed more oxygen molecules for reaction with xylene molecules. The excellent response properties of the α-MoO3 nanoplates arrays make it a good candidate as a gas-sensing material for xylene detection. Key words: α-MoO3; Nanoarrays; Solid-state synthesis; Gas sensors

1. Introduction Semiconducting gas sensors are widely developed and used in the detection of toxic and inflammable gases [1-2]. Among volatile organic compounds, xylene is one of the most serious pollutants. It is hazardous to human health and environmental quality owing to its toxicity and carcinogenicity. As a results, it is important to develop xylene-sensing material to detect it effectively. Oxide semiconductors, including ZnO, SnO2, Fe2O3, WO3 and In2O3 [3-7] can respond to various gases at elevated temperature and have the characteristics of low cost, simple production, good physical and chemical stability and long service life. Molybdenum oxide (MoO3) as an important oxide semiconductors, show a promising application as a gas-sensing materials. MoO3 has three fundamental crystalline phases: the thermodynamically stable orthorhombic phase (α-MoO3), the mestastable monoclinic phase (β-MoO3) and the hexagonal phase (γ-MoO3) [8]. Among them, the α-MoO3 has attracted considerable attention due to its peculiar two-dimensional layered structure, in which the layers are hold together by weak van der Waals forces, which result in peculiarities for sensing applications: more excellent characteristics than two other crystal phases [9-10]. At present, α-MoO3 is gradually becoming one of popular n-type semiconductor gas-sensing materials for the detection of organic toxic and inflammable organic gases. There are reports about the synthesis of α-MoO3 nanomaterials, using the thermal evaporation method [11], the sol-gel method [12], the chemical vapor deposition method [13], the sputtering method [14], the electrodeposition method [15,

16] and liquid phase exfoliation [17, 18]. These methods produce low-dimensional nanomaterials and assembled nanostructures. However, usually such synthesis processes require harsh experimental condition or complex synthetic processes. Oxides nanoparticles, nanorods and nanotubes as gas-sensing materials have been synthesized by solid-state chemical reaction method in recent years by our group [19-21]. In this work, the solid-state chemical reaction method is employed to fabricate α-MoO3 nanoplates and assembled arrays. The gas-sensing properties of the as-prepared α-MoO3 nanomaterials are investigated to explore their potential applications in the field of gas sensing.

2. Experimental 2.1 Synthesis All

syntheses

were

conducted

in

air.

Ammonium

molybdate

((NH4)6Mo7O24·4H2O), oxalic acid (H2C2O4·2H2O), polyethylene glycol (PEG-400) and other reagents used in this work were all analytically pure grade. The typical synthetic procedure of α-MoO3 was as follows. First, 10 mmol of solid ammonium molybdate was weighed and grounded for approximately 10 min in an agate mortar, and then 6 mL of PEG-400 was added. After complete mixing, 50 mmol of solid oxalic acid was weighted and ground for 30 min in an agate mortar at room temperature, and then the mixture was transferred to a 50 mL conical flask and reacted in a 60oC water bath for 24 hours. After that, the mixture was washed with distilled water and alcohol for several times to remove the by-products and PEG-400, and then dried it in air. Finally, the prepared precursor was calcined at 450oC for 1 h

in air atmosphere. Then, the α-MoO3 nanoplate arrays were obtained and are denoted as S2. Another product was synthesized following the same process without using PEG-400, and it was denoted as S1. 2.2 Characterization The crystalline phases of the obtained target products were characterized by X-ray diffraction (XRD) employing a Bruker D8 X-ray diffractometer at a scanning speed of 2º min-1 with Cu-Kα radiation (λ=1.5418Å) in the range of 10º-80º. Fourier transform infrared (FT-IR) spectra were obtained on a Bluker Vertex 70 spectrometer using KBr pellets in the range of 4000-400 cm-1. Transmission electron microscope (TEM) images were collected on a Hitachi H-600 transmission electron microscope with an accelerating voltage of 100 kV. Field emission scanning electron microscopy (FESEM) images were performed on a Hitachi S-4800 scanning electron microscope with an accelerating voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were obtained by a JEOL JEM-2010F transmission electron microscope with an accelerating voltage of 200 kV. BET surface areas and pore diameters were measured with nitrogen adsorption isotherms, which were taken on an ASAP2020 instrument. 2.3 Measurements of gas-sensing properties Gas-sensing properties of the products were measured on a WS-30A gas sensing measurement system (Zhengzhou Winsen Electronics Co. Ltd., PR China). The gas sensors were prepared in a similar method which was referred in literatures [22, 23].

The details were as follows. First, the as-prepared products were mixed with ethanol to form a paste. Then, the paste was coated on a ceramic tube which was attached with platinum wire electrodes. A Ni-Cr alloy wire was passed through the tube and used as a resistance heater to provide the operating temperature. Finally, the as-prepared gas sensor was soldered onto a Bakelite substrate and inserted into the test chamber (1800 cm3). To ensure the stability and repeatability of the gas sensors, they were aged at 300oC for 5 days in air before testing. The sensor response is defined as follows [24]: Re sponse 

Rair Rgas

(1)

Where Rair is the resistance of the gas sensor in the air and Rgas is the resistance of the gas sensor in the mixture containing the target gas. The response time is defined as the time required by the sensor to reach 90% of the its maximum response after the target gas is injected, and the recovery time is the time for the sensor needs to fall to 10% of its minimum response value in air [25, 26].

3. Results and discussion 3.1 Structural characterization of the products Fig. 1(a) shows the XRD pattern of the precursor prepared by the solid-state chemical reaction technique. As shown in the picture, the peaks of the starting material disappear completely, and new characteristic peaks appear in the diffraction pattern of the precursor, which indicates that the solid-state chemical reaction occurred between the two reactants, and that the precursor MoO3·xH2O was obtained. Fig. 1(b) shows the XRD patterns of the final solid products prepared by the thermal

decomposition of the precursor. All of the diffraction peaks in Fig. 1(b) can be exactly indexed to the orthorhombic structure of α-MoO3 [27] (JCPDS card No.35-0609) with cell constants: a=3.962 Å, b=13.858 Å and c=3.697 Å. The fine and sharp diffraction peaks imply the high crystallinity of the products. Moreover, it can be distinctly seen that the intensities of the diffraction peaks (040) and (020) of S2 are lower than those of S1, which indicates that the growth along (040) and (020) of S2 are inhibited by PEG-400. No characteristic peaks of impurities were detected in either of the products, which confirms the high purity of the products. Fig. 1(c) gives the IR spectra of S1 and S2. In the IR spectra, the peaks at approximately 1643 and 3450 cm-1 are attributed to the O-H stretching vibration pattern of adsorbed water on the surface. The bond of H-O-H stretching vibration at approximately 1400 cm-1 and the -OH stretching vibration at approximately 2900 cm-1 suggest the existence of crystalline water [28, 29]. The strong adsorption peaks at approximately 999 cm-1 are attributed to the stretching vibration of Mo=O, the band at approximately 858 cm-1 is attributed to the doubly coordinated oxygen (Mo2-O) stretching vibration, and the absorption band at around 615 cm-1 corresponds to the stretching vibration of O in the MoO3 molecular [30]. No peaks for by-products were detected in the FT-IR spectra, which suggests that the as-prepared α-MoO3 is of high purity. EDS was used to characterize the composition of α-MoO3. Fig. 1(d) displays the EDS of the samples. The results show that the final product is composed of Mo and O with an atomic ratio approximately 1:2.7, which is close to the theoretical atomic ratio of molybdenum oxide (1:3), providing further evidence that MoO3 was

obtained. 3.2 The morphology and microstructure of the products SEM observations were used to gain detailed insight into the morphology of the products S1 and S2, as seen in Fig. 2(a)-(d). It can be seen that the two obtained samples exhibit different morphologies. Product S1 displays irregular nanoplates with a certain degree of agglomeration. Product S2 shows orderly arrays assembled by the nanoplates with thicknesses about 50 nm. It can be seen that the α-MoO3 arrays with high consistency were self-assembled in-situ by the two-dimensional nanoplates. In this work, we also obtained products by calcining S1 and S2 at 500oC and 600oC, respectively (the SEM images of S1 and S2 are given in Fig. S2). It was found that the products increase in size and become more seriously agglomerated with increasing calcining temperature, which results in the specific surface area decreasing and the gas sensing properties worsening. The obtained products displayed a different morphology upon adding PEG-400 to the reaction system. It is well known that PEG is suitable for the synthesis of specific structures in solution [23,31]. Similarly, on the basis of our XRD and SEM results, we speculate that the surfactant PEG-400 plays an important role in the formation of α-MoO3 nanoplates arrays. In the solid-state chemical reaction, it is considered to be a crystal face inhibitor, which inhibits the growth of some crystal faces of S2 [32]. Moreover, it is used as a structure-directing agent in this solid-state chemical reaction. It acts as a diamond-structured soft template due to its chain molecular structure, and it can promote the product particles to grow into

one-dimensional nanoplates. At the same time, the plates attract each other owing to intermolecular hydrogen bonding interaction. As a result, the highly consistent α-MoO3 arrays were self-assembled in-situ by the two-dimensional nanoplates [33-36]. To further investigate the micromorphology of the α-MoO3 nanostructures, TEM and HRTEM combined with selected area electron diffraction (SAED) analysis were performed. As shown in Fig. 3(a) and (b), S1 is composed of nanoplates in a non-ordered state. Unlike S1, S2 shown in Fig. 3(d) and (e), is built up of a series of irregular nanoplates and have a neat and orderly structure. These results are consistent with the results of SEM. Fig. 3(c) and (f) shows the HRTEM images of the samples, and the results demonstrate that the obtained samples are composed of highly ordered lattice belts. As demonstrated in the Fig. 3(c), the obtained α-MoO3 nanoplates have distinct lattice stripes, where the distances of lattice stripes are approximately 0.326 nm and 0.355 nm, which corresponds to the {021} and {040} crystal faces of the S1, respectively. In Fig. 3(f), the distances of the lattice stripes are about 0.681 nm, 0.326 nm and 0.388 nm, which belong to the {020}, {021} and {110} crystal facets of the α-MoO3 arrays, respectively. The SAED images are shown in the inset of Fig. 3(c) and (f), and they demonstrate that the obtained samples present in orderly square diffraction spots, which suggests that the samples are single-crystal structures in the orthorhombic phase of MoO3. Fig. 4 reveals the nitrogen adsorption-desorption isotherms of the S1 and S2. We

can see that the BET surfaces are 8.82 m2/g and 13.15 m2/g for S1 and S2, respectively. The relatively large specific surface area for S2 may provide more active reaction sites for the adsorption and transportation of gas molecules to enhance the gas response [37]. To understand the formation of the unique array structure, the precursor of S2 was calcined at different temperatures. We observed the morphologies of the as-obtained samples prepared at different temperatures. As shown in Fig. 5, it can be seen that the morphology of the precursor changed gradually with the rise in the calcination temperature. At first, the precursor disintegrated into numerous tiny primary crystallites. These tiny granules have a very low boiling point, and will sublimate into vapor. When the density of the vapor is too high, the primary crystallites precipitate and form an increasing number of small particles. Then, from these additional tiny nanoparticles, the complete nanoplates are gradually formed. Therefore, it is thought that these precursors undergo an inside-out Ostwald ripening upon gradually increasing the calcination temperature [38, 39]. Finally, an increasing number of nanoplates join closely together and grow along the structure-directing agent PEG-400. High temperature can provide a greater driving force, which is beneficial in forming the regular array structure. As a result, the highly consistent α-MoO3 arrays self-assembled in situ from the two-dimensional nanoplate arrays at 450oC. 3.3 Gas-sensing properties of the products The gas-sensing properties of the as-prepared α-MoO3 nanoplates and arrays were investigated toward six different noxious gases including of ethanol, acetone,

xylene, toluene, ethybenzene and benzene at different operation temperature in this study. As is shown in Fig. 6(a), it can be clearly seen that S2 has a maximum response value of 19.2 at 370oC to 100 ppm xylene. This response value of S2 to xylene is higher than the value reported in the literature [40-42]. The result also indicates that the sensor based on S2 has different optimum operation temperatures for different target gases. It can be used to detect different gases at their respective optimum working temperatures. The selective response is an important parameter to evaluate the properties of a gas sensor. In this study, we discuss the response properties of the samples to 100 ppm of different gases at the optimum working temperature of 370oC. As depicted in Fig. 6(b), it can be distinctly seen that the samples have the high response to xylene: moderate response to ethanol, methanol and ethylbenzene; low response to benzene, acetone and toluene. This result demonstrates that the gas sensor based on α-MoO3 arrays has good selectivity toward xylene, and the α-MoO3 arrays can be used as a promising sensitive material for detecting xylene. To compare the gas response of the sensors based on S1 (nanoplates) and S2 (arrays) to 100 ppm xylene, the relation between the operation temperature and the response is shown in Fig. 7(a). It can be seen that the response value of the sensors based on S1 and S2 increases with working temperature at low temperature. It reaches a maximum response value at the optimum operation temperature of 370 oC for S2, and then the response decreases with increasing temperature after reaching the maximum value. From this, it can be seen that the response value of the sensor is closely related to the operation temperature. At the lower operation temperature,

physical adsorption occurs on the surface of the samples, the energy is lower, the absorbed target gas molecules cannot be sufficiently activated with enough energy to surpass the energy barrier to react with the oxygen absorbed on the surface of the sensors and therefore the sensors show low response values. With increasing operation temperature, chemical adsorption occurs, the target gas molecules have enough energy to react with the adsorbed oxygen and the sensors exhibit high response values. When the temperature is higher than the optimum value, the absorbed target gas molecules quickly desorb, which causes a decrease in the resistance of the gas sensors, and thus the response value decreases [43, 44]. In addition, we can see from the figure that S2 has a higher response value than S1 at the optimum operation temperature. However, the sensor based on S2 shows good response but a higher working temperature, which may be due to the close and tight array structure of S2. This structure leads to electron conduction that is slower and more difficult at relatively low operating temperatures, and thus it may require higher energy to promote the response. The particular nanoplate array structure of S2 can provide larger specific surface areas, which results in a stronger interaction between the xylene vapor and the surface active sites. Fig. 7(b) provides the response curve of the samples to different concentrations of xylene at 370oC. It can be seen that the gas response increases with the concentration of xylene ranging from 10 to 1000 ppm, and the response value of two samples with 1000 ppm xylene reaches 86.9 and 21.5, respectively. Compared with S1, S2 displays a greater response to xylene at every tested concentrations, the

response of S2 to 1000 ppm xylene is four times that of S1. Furthermore, the response of S2 increases rapidly as the xylene concentration increases and remains unsaturated at 1000 ppm, which suggests a very wide xylene detection range. More importantly, the response value of S2 to 10 ppm is still able to reach 2.6, which can perfectly meet the low-concentration requirements of practical applications. It is well known that the response and recovery time and the long-term stability are important characteristics of gas sensor. In this work, the response/recovery time and the long-term stability of the samples were studied, and the result is displayed in the Fig.8. From Fig. 8(a), the response and recovery time are approximately 1 s and 20 s for the S1, respectively, and 1 s and 15 s for the S2, respectively. This result indicates that both S1 and S2 display fast response and recovery properties, which may be attributed to the unique layered structure of α-MoO3. The self-assembled layered structure of the α-MoO3 arrays can provide a direct transport path for gas molecule adsorption and disadsorption [45]; therefore, α-MoO3 arrays (sample S2) display a relatively faster response and recovery speed. The dynamic responses of the sensors with different concentrations of xylene are given in Fig. S3, which illustrate that the sensors have rapid response and recovery times even at very high concentrations. The long-term stability of the samples was tested over 30 days, and the highest response of the two samples occurred on the fifth day, the response displayed a sharp drop on the tenth day, and the downward trend ended on the twenty day. Further study into methods for improving the performance of the samples, such as doping with noble metals or rare earth elements, is needed.

3.4 The xylene-sensing mechanism of the products For a typical n-type semiconductor, the gas-sensing behavior can be interpreted by the classical electron depletion theory [46, 47]. That is, the gas-sensing performance of α-MoO3 is related to the adsorption and desorption process of gas molecules on the surface. Due to the layered structure of MoO3, H+ from the test gases will intercalate into the layers of MoO3, which are held together by van der Waals forces, and then bonded edge-shared oxygen and terminal oxygen atoms forms OH2 groups. The OH2 groups are not stable under high temperature and are eventually released from their original positions in the crystal lattice, leaving oxygen vacancies and subsequently forming substoichiometric 2D MoO3-x [33, 34]. As we know that there are oxygen vacancies on the surface of the α-MoO3 samples, when the α-MoO3 samples are exposed to air, the oxygen molecules will adsorb on the surface of the α-MoO3 samples, and these adsorbed oxygen molecules can capture free electrons from the bulk to form O-, as shown in Eq.(2). Hence, the thickness of the depletion layer on the surface of the α-MoO3 samples increases, which leads to a decrease in the carrier concentration and an increase in the resistance of the gas sensor. Then, when the α-MoO3 samples are exposed to xylene, the xylene molecules will react with the adsorbed O- (420 K-670 K) or O2- (above 670 K), as displayed in Eq.(3) and (4), and the trapped electrons are released back into the bulk in this process, causing a decrease in the depletion layer and an increase in the carrier concentration in the α-MoO3 samples. As a result, the resistance of the gas sensor decreases [48-50]. O2 + 2 e- → 2 O-(adsorbed) / O2-(adsorbed) + h0

(2)

C6H4 (CH3)2 + 21 O-(adsorbed) → 8 CO2 + 5 H2O(gas) + 21 e-

(420 K-670 K)

(3)

C6H4 (CH3)2 + 21 O2-(adsorbed) → 8 CO2 + 5 H2O(gas) + 42 e- (above 670 K)

(4)

Based on our experimental results, xylene molecules can be considered to undergo the above process, and the response behavior of the sensor is directly related to the change in electric resistance during this process. The greater the change in the resistance value, the larger the gas response value.

4. Conclusion In summary, the α-MoO3 nanoplates arrays have been successfully synthesized by a simple and efficient solid-state chemical reaction method. The α-MoO3 nanoplates arrays have not only higher response values and better selectivity, but also wider concentration ranges in the detection of xylene compared with unassembled nanoplates. The response value to 100 ppm xylene of α-MoO3 nanoplate arrays is three times higher than that of unassembled nanoplates. The outstanding gas-sensing properties of the nanoplates arrays are attributed to the preferentially exposure of the active crystal face and the relatively larger specific surface area. The as-prepared α-MoO3 nanoplate arrays can be regarded as a novel and promising gas-sensing material for the detection of xylene.

Acknowledgements This work was financially supported by the Natural Science Foundation of Xinjiang Province (No. 2014211A013).

Biographies Haiyu Qin is a postgraduate at the Xinjiang University of China. Her research work is concentrated on the synthesis for gas sensing materials by solid-state chemical reaction technique. Yali Cao is a professor at Institute of Applied Chemistry, Xinjiang University of China. Her research work is focused on the synthesis for nanometer functional materials and their application. Jin Xie is a doctoral student at the Xinjiang University of China. Her research work is concentrated on the synthesis for nanoscale oxides and their application on photocatalyst. Hui Xu is a postgraduate at the Xinjiang University of China. Her research work is concentrated on the synthesis for nanoscale oxides and their application on gas sensor. Dianzeng Jia is a professor at Institute of Applied Chemistry, Xinjiang University of China. His research field is the studies on nanoscale functional materials and photochromic materials.

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Legend to illustrations Fig. 1. XRD patterns of (a) the precursor and (b) the samples; (c) the FT-IR of the samples; (d) the EDS of the sample. Fig. 2. The SEM images of samples: (a)(b)(c) S1; (d)(e)(f) S2. Fig. 3. The typical TEM, HRTEM and SAED images: (a)(b)(c) S1; (d)(e)(f) S2. Fig. 4. Nitrogen adsorption isotherms and corresponding pore size distributions (insert) of (a) S1 without PEG 400 and (b) S2 using PEG 400. Fig. 5. The SEM images of the precursor calcined at different temperature: (a)(e) 150oC; (b)(f) 250oC; (c)(g) 350oC; (d)(h) 450oC. Fig. 6. (a) The response of sample S2 to 100 ppm gas at the different working temperature; (b) the response of the samples to different concentrations of gas. Fig. 7. (a) The response of the samples to 100 ppm xylene at different working temperature; (b) The response of samples to different concentrations of xylene. Fig. 8. (a) The dynamic response curve and (b) the long-term stability of the samples to 100 ppm xylene at a working temperature of 370oC.

Fig. 1

Fig. 1. XRD patterns of (a) the precursor and (b) the samples; (c) the FT-IR of the samples; (d) the EDS of the sample.

Fig. 2

Fig. 2. The SEM images of samples: (a)(b)(c) S1; (d)(e)(f) S2.

Fig. 3

Fig. 3. The typical TEM, HRTEM and SAED images: (a)(b)(c) S1; (d)(e)(f) S2.

Fig. 4

Fig. 4. Nitrogen adsorption isotherms and corresponding pore size distributions (insert) of (a) S1 without PEG 400 and (b) S2 using PEG 400.

Fig. 5

Fig. 5. The SEM images of the precursor calcined at different temperature: (a)(e) 150oC; (b)(f) 250oC; (c)(g) 350oC; (d)(h) 450oC.

Fig. 6

Fig. 6. (a) The response of sample S2 to 100 ppm gas at the different working temperature; (b) the response of the samples to different concentrations of gas.

Fig. 7

Fig. 7. (a) The response of the samples to 100 ppm xylene at different working temperature; (b) The response of samples to different concentrations of xylene.

Fig. 8

Fig. 8. (a) The dynamic response curve and (b) the long-term stability of the samples to 100 ppm xylene at a working temperature of 370oC.