Enhanced Gas-sensing Properties for Trimethylamine at Low

Subscriber access provided by STEPHEN F AUSTIN STATE UNIV

Functional Nanostructured Materials (including low-D carbon)

Enhanced Gas-sensing Properties for Trimethylamine at Low Temperature Based on MoO3/Bi2Mo3O12 Hollow Microspheres Fang-Dou Zhang, Xin Dong, Xiaoli Cheng, Ying-Ming Xu, Xianfa Zhang, and Li-Hua Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22132 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhanced Gas-sensing Properties for Trimethylamine at Low Temperature Based on MoO3/Bi2Mo3O12 Hollow Microspheres Fangdou Zhang, Xin Dong, Xiaoli Cheng*, Yingming Xu, Xianfa Zhang and Lihua Huo*

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Most reported TMA sensors have to operate at high temperature, which will consume energy highly. In order to detect TMA at low temperature, it is necessary to modify the existing materials or develop new materials. In this paper, the sensor based on MoO3/Bi2Mo3O12 hollow microspheres can work at low operating temperature of 170oC, which were prepared via a simple solvothermal route. The phase and morphology of the product were characterized by X-ray diffraction meter, scanning electron microscope and transmission electron microscope. The surface chemistry of the MoO3/Bi2Mo3O12 sensor was studied with X-ray photoelectron spectroscope to investigate the TMA sensing mechanism. The MoO3/Bi2Mo3O12 sensor (S=25.8) had higher response to 50 ppm TMA than those of MoO3 hollow spheres (S=10.8) and Bi2Mo3O12 sensors (S=4.8) at 170oC. In contrast to the pure MoO3 and Bi2Mo3O12 sensors, the MoO3/Bi2Mo3O12 sensor exhibited an obviously enhanced gas-sensing property for TMA, which might be due to the heterostructure formed between MoO3 and Bi2Mo3O12 and the hollow morphology. It is the first time for MoO3/Bi2Mo3O12 to apply in gas sensors, which might take an important step in the application of MoO3/Bi2Mo3O12 or Bi2Mo3O12 in the field of gas sensing. Keyword: MoO3/Bi2Mo3O12 hollow microspheres, heterostructure, low temperature, TMA sensing property, gas sensing mechanism

2


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Trimethylamine (TMA), an organic amine, is a toxic gas. Once inhaled a certain amount of TMA, people would have a series of health problems such as breathing difficulty, headaches, lung edema nausea and irritation of the upper respiratory system.1 The allowable exposure time to TMA as low as 15 ppm is less than 15 min.2 In addition, monitoring the concentration of TMA is an effective means to determine the freshness of seafood.3 Hence, it is necessary to study for real-time monitoring the concentration of TMA in the environment to ensure people’s safety. The traditional techniques applied in detecting TMA include ion mobility spectrometry, mass spectrometry, liquid chromatography, pH test, and so on.4 But most of them need long time in preparing samples, require complex equipment and have low precision. In contrast, the metal oxide semiconductor (MOS) gas sensors can detect rapidly and accurately, with low energy consumption and low cost.5 So MOS gas sensors are often selected as practical means for analysis of the target gases on-site and real-time. Over the last couple of years, various metal oxide based TMA sensors have been investigated, such as ZnO,5 In2O3,6 MoO3,2,7 WO3,8 CeO2,9 Fe2O3,3,10 and SnO2.11 Among of them, molybdenum trioxide (MoO3) is a remarkable candidate in gas-sensing, especially in TMA detection, which can react with basic TMA strongly due to its abundant Lewis-acid sites.12,13 For instance, the sensor based on MoO3 nanoplates synthesized via ultrasonic spray pyrolysis exhibited an excellent sensitivity (373.74) towards 5 ppm TMA at 300oC.2 The MoO3 microrods were prepared by means of ultrasonic probe, which could detect 0.01 ppm TMA at 300oC.7 The MoO3 nanopapers fabricated via a hydrothermal route displayed an admirable sensitivity to 5 ppm TMA of 121 at the optimal working temperature of 325 oC.13 In order to further improve the application of molybdenum oxide in TMA gas-sensing, some significant efforts have been made, such as doping element,14 loading noble metal12 and adjusting morphology.15 3



Moreover, heterostructure formation is an effective method to promote the enhancement of gas-sensing performances as well. For example, MoO3/SnO2,16 MoO3/TiO2,17 MoO3/CuO18 and MoO3/Fe2O319 have been prepared successfully and exhibited enhanced gas sensing performances compared with the pure components. In recent years, heterostructure formation with multicomponent oxides has become the hotspot owing to their more freedom, in which the multicomponent oxides, changing the compositions, can regulate properties of physics and chemistry of materials.20 Li et al.21 reported that MoO3/Fe2(MoO4)3 heterostructure nanomaterial, fabricated via a hydrothermal method, displayed better sensitivity to toluene than bare MoO3 nanobelts. In addition, Gao et al.22 reported that the MoO3/Fe2(MoO4)3 yolk/shell nanostructures exhibited higher response to 1 ppm H2S compared with pure MoO3 nanorods. More recently, Li et al.23 fabricated CoMoO4/MoO3 heterojunction which showed four times response to 100 ppm TMA than pure MoO3 at 220oC, mainly due to the formation of junction between CoMoO4 and MoO3. However, most of MoO3-based TMA gas sensors have to operate at high temperature more than 200oC. It needs to exploit new TMA sensing materials to detect TMA more conveniently with lower operating temperatures. Bismuth molybdate, one of the multicomponent oxides, involving three phases of α-Bi2Mo3O12, βBi2Mo2O9 and γ-Bi2MoO6, was paid more attention on account of the wide availability and low cost of their component materials.24 Many investigations have proved that bismuth molybdate is suitable as a gas-sensing material.20,25-28 Although there is no report on the application of β-Bi2Mo2O9 and α-Bi2Mo3O12 in the field of gas sensing, the α-Bi2Mo3O12 possesses a special structure in the three phases, in which the ordered oxygen vacancies disperse in every three Bi sites. The special structure is good for the active lattice oxide ions to migrate rapidly, as well as it enhances the ability of reduction-reoxidation.29 This might make α-Bi2Mo3O12 become a good candidate for gas sensing. Liu et al.30 once reported that the Bi2Mo3O12 coated MoO3 heterostructural nanowires exhibited excellent photocatalytic 4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance than pure Bi2Mo3O12. The composited MoO3/Bi2Mo3O12 heterostructure might exhibit better gas sensing properties. In addition, the hollow morphology not only can provide larger space and more effective active sites, but also facilitate the diffusion of gas molecules. If designing the MoO3/Bi2Mo3O12 heterostructure with a hollow morphology, it might enhance the TMA sensing property of the sensing material. With this background, the MoO3/Bi2Mo3O12 hollow microspheres were synthesized via a simple solvothermal method. It is the first report on application of MoO3/Bi2Mo3O12 in gassensing performance. By the way, the optimal operating temperature of the MoO3/Bi2Mo3O12 sensor decreased to 170oC. The sensor showed good selectivity, repeatability, long-term stability and linear relationship to TMA. And environmental humidity had little effect on the sensor. Experimental section Materials and Reagents Molybdenum oxide (MoO3), bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O) and ethylene glycol ((CH2OH)2 ) were purchased from Tianjin Chemical Reagent Co. Ltd. (China), and are analytical grade and used without further purification. Synthesis of MoO3/Bi2Mo3O12 hollow microspheres Firstly, 0.35 mmol Bi(NO3)3·5H2O and 1.05 mmol commercial MoO3 were dispersed in 5 mL deionized water. After stirring 30 min, 30 mL ethylene glycol was added under stirring. The mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave after stirring for another 30 min and heated at 160oC for 48 h. Finally, the precipitation was washed several times with deionized water and ethanol and dried overnight at 80oC. After calcination at 500oC for 120 min in air, MoO3/Bi2Mo3O12 hollow microspheres can be obtained. For comparison, pure Bi2Mo3O12 was also prepared by the same process with addition of 0.70 mmol Bi(NO3)3·5H2O and 1.05 mmol commercial MoO3. 5



Characterization The phase and crystallization of the product were measured by X-ray powder diffraction (XRD, Bruker D8 Advance, Germany) in a range of 5 ~ 80o, using Cu-Kα radiation (λ=0.154056 nm). The morphology and fine structure of the samples were observed with scanning electron microscope (SEM, Hitachi S-4800, Japan) and transmission electron microscope (TEM, FEI Tecnai G2 F20 S-Twin, Japan). The surface chemistry and valence bands of the samples were tested using the X-ray photoelectron spectroscope (XPS, ULTRA AXIS DLD). And the electron binding energy values were corrected with reference to C 1s (284.6 eV). The specific surface areas were measured via N2 adsorption-desorption analysis system (TriStar II 3020) at 77 K. The UV-Vis absorption spectra (Lambda 900, USA) were used to obtain the band gaps of the products. Sensor fabrication and measurement The product mixed with terpineol was coated on the surface of Al2O3 ceramic tube, on which two Au electrodes were pre-deposited (the picture of the ceramic tube is given in Figure 1a). After the coated film drying at 80oC for 30 min and calcining at 300oC for 60 min, the tube with a Ni-Cr wire passing through was welded to the base (see Figure 1b). Before gas-sensing test, the sensor was aged at 170oC for 3 days. The gas-sensing properties were evaluated by JF02E test system (Kunming, China). The schematic diagram of gas-sensing measurement device is given in Figure 2. Firstly, the target gas was injected into a vacuum glass vessel (10 L) via a microsyringe, afterwards, fresh air was poured into the vessel to balance inside and outside pressure of the vessel. The sensor was inserted into the vessel when its resistance was stable in air. After its resistance stabilized in target gas, the sensor was carried out.31 The gas response (S) is S = Ra / Rg, in which Ra and Rg are the sensor resistance in air and gases, respectively. The selectivity coefficient (KTMA/X) is STMA / SX, in which the responses towards TMA and other gases are STMA and SX. 6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The response and recovery times are defined as the time that the sensors achieved 90% of the total resistance change when the sensors adsorb and desorb target gas. During gas-sensing measurement, the indoor temperature and relative humidity were about 25 ± 1oC and 30 ± 5 RH%, respectively. The effect of the relative humidity on gas response was evaluated with water vapor of various saturated salt solutions including KNO3 (94%), KCl (85%), NaCl (75%), CuCl2 (67%), Mg(NO3)2 (54%), K2CO3 (43%), MgCl2 (33%), CH3COOK (23%) and LiCl (11%). The response toward different humidity is the ratio between sensor resistance in air (Ra) under test conditions (the relative humidity was 30 ± 5 RH%) and humid air (Rh). Results and discussion Morphology and microstructure The peaks at 12.7o, 23.3o, 25.7o, 27.3o, 33.8o and 38.9o match with (020), (110), (040), (021), (211) and (060) planes of MoO3 (Figure 3a, JCPDS card No. 89-5108). And the peaks of product at 11.2o, 12.7o, 14.1o, 18.1o, 27.9o, 29.2o, 31.0o, 32.1o, 36.1o, 45.2o and 48.3o correspond to (011), (100), (-111), (012), (-221), (023), (211), (041), (123), (-244) and (242) planes of Bi2Mo3O12 (Figure 3b, JCPDS card No. 78-2420), respectively. The Figure 3c displays mixed diffraction peaks of MoO3 and Bi2Mo3O12 which indicates that the product is MoO3/Bi2Mo3O12. In order to further investigate the morphology of the product, SEM images are provided in Figure 4a and Figure 4b. The morphology of the precursor is hollow microsphere with the diameter of about 1.2 μm. The hollow microspheres are composed of nanoparticles with the size of 30 ~ 50 nm. The calcined product can maintain the hollow spherical structure even at a high calcination temperature of 500oC. But the spherical surface is a little smoother than the precursor. It indicates that the primary building blocks grow larger during the crystallization at 500oC. In addition, the TEM image (Figure 4c) also shows that the calcined MoO3/Bi2Mo3O12 microspheres are hollow and the shell thickness is about 85 nm. The 7



building blocks are 80 ~ 200 nm in size and some pores can be observed on the surface. The morphology of the hollow microspheres can provide a large specific surface area for the target gas. This will produce more active sites, making the gas sensing properties better. The HRTEM image (Figure 4d) displays clear lattice fringes with the characteristic spacings of 0.231 nm and 0.318 nm, matching with the (060) lattice plane of MoO3 and the (-221) lattice plane of Bi2Mo3O12, respectively. It further proves that the phase of the calcined product is MoO3/Bi2Mo3O12. The formation process of MoO3/Bi2Mo3O12 hollow spheres was investigated by SEM and TEM. Bi(NO3)3·5H2O and commercial MoO3 sheets (Figure 5a) were used as the raw materials in the experiment. It is obvious from Figure 5b that the surface of the solid raw material becomes rough and the sheets integrate together when the reaction proceeds to 1 h. As the reaction time reaches to 4 h, the smooth spheres appear, but there are still some unbroken-up spherical accumulations (Figure 5c). As the reaction is up to 12 h, welldispersed smooth solid spheres form completely (see Figure 5d and Figure 5e). When the reaction time further prolongs to 24 h, the product is still spheres with solid core but many nanoparticles grow and accumulate on the spheres surface (Figure 5f). When the reaction time is further extended, solid cores begin to consume from the inside to outside (Oswald ripening process), meanwhile, the solid spheres turn into hollow double-shelled ones at reaction time of 36 h (Figure 5g). With double-shelled spheres continued growth, the small hollow spheres inside begin to grow to large hollow spheres outside. When reaction time is 48 h, completely hollow spheres which are composed of nanoparticles form finally (Figure 5h). Gas sensing properties To investigate the gas sensing performance of the sample, the operating temperature needs to be determined first. Once the working temperature is lower than 170oC, the resistance of 8


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the sensor will exceed the test range. So there is no more information about the gas response below the optimal temperature of 170oC. The responses of the three sensors towards 50 ppm TMA were investigated at the operating temperatures higher than 170oC (170oC, 217oC, 252oC, 290oC and 330oC). The responses of the sensors decrease as the operating temperature increases (Figure 6a). At 170oC, the MoO3, Bi2Mo3O12 and MoO3/Bi2Mo3O12 sensors reach the maximum responses to 50 ppm TMA, which are 10.8, 4.8 and 25.8, respectively (the synthesis and the morphology of MoO3 (Figure S1a) and Bi2Mo3O12 (Figure S1b) are shown in supporting information). So the operating temperature is chosen as 170oC. And the response of the MoO3/Bi2Mo3O12 sensor towards 50 ppm TMA is about 2.5 times and 5.5 times that of the MoO3 and Bi2Mo3O12 sensors at 170oC. It indicates that the heterostructure formed between MoO3 and Bi2Mo3O12 helps to improve the TMA detection of the sensor greatly. When compared to the TMA sensing property of the reported oxide-based sensors and oxide/oxide composite sensors (Table 1), the working temperature of the MoO3/Bi2Mo3O12 sensor is lower than most of the reported sensors. To investigate the selectivity of the MoO3/Bi2Mo3O12 sensor, the responses towards eight 100 ppm gases were tested at 170oC. The results (Figure 6b) show that the MoO3/Bi2Mo3O12 sensor is more sensitive to TMA (45.39) than the other seven gases (NH3 (1.45), CH3OH (2.34), CH3CH2OH (4.61), TEA (8.42), H2S (24.96), C6H7N (8.34) and C6H6 (1.57). The selectivity coefficients (KTMA/X) of TMA towards the other seven gases are 31.28, 19.38, 9.83, 5.51, 1.82, 5.44, and 28.90, respectively. It’s obvious that the MoO3/Bi2Mo3O12 sensor has a good selectivity to TMA. The real-time gas response to 100 ppb ~ 100 ppm TMA at 170oC was investigated (Figure 6c). The response of MoO3/Bi2Mo3O12 sensor increases with the increasing of TMA concentration. The MoO3/Bi2Mo3O12 sensor has an excellent linear relationship (R2 = 0.9942) for 0.1 ~ 100 ppm TMA. In addition, the response time of the MoO3/Bi2Mo3O12-based sensor to 100 ppm TMA is as short as 7.1 s. 9



The five consecutive responses of the MoO3/Bi2Mo3O12 sensor to 10 ppm TMA at 170oC are 7.08, 7.12, 7.22, 7.02 and 7.19, respectively (Figure 6d). The average relative deviation is 0.9%. It indicates that the MoO3/Bi2Mo3O12 sensor has good reproducibility for TMA detection. For purpose of evaluating the long-term stability of the MoO3/Bi2Mo3O12 sensor, the responses towards 50 ppm TMA were tested once every 5 days in the duration of 3 months (Figure 7a). The relative standard deviation of response values is within 0.52%, which means the sensor can remain the original response. The anti-humidity ability of the MoO3/Bi2Mo3O12 sensor was also tested (Figure 7b). The responses of the MoO3/Bi2Mo3O12 sensor towards different relative humidity are all less than 1.3. It implies that influence of environment humidity on the MoO3/Bi2Mo3O12 sensor can be ignored at 170oC. Gas sensing mechanism To evaluate MoO3/Bi2Mo3O12 sensor’s TMA sensing mechanism, the surface of the MoO3/Bi2Mo3O12 sensor before and after exposure to TMA at 170oC was characterized by XPS. The detailed test procedure is as follows: Firstly, the MoO3/Bi2Mo3O12 sensor was inserted into 100 ppm TMA test vessel at the optimal temperature of 170oC and kept for 2 min when the resistance was stable. Next, the working temperature was adjusted to room temperature, meanwhile, the sensor was taken out of the vessel. Finally, the XPS test was completed immediately. The Bi 4f5/2 and Bi 4f7/2 peaks appear at 164.4 eV (164.7 eV) and 159.4 eV (159.4 eV) (Figure 8a, 8d) and the peaks of Mo 3d3/2 and Mo 3d5/2 appear at 235.7 eV (235.7eV) and 232.6 eV (232.6 eV) (Figure 8b, 8e) before (after) the sensor exposure to TMA. These data mean that the binding energies of Bi 4f and Mo 5d do not change when the sensor contacts with the TMA molecules. It implies that there are no valence state changes in Bi and Mo elements, which suggests that Bi and Mo elements do not participate in the TMA sensing reaction. Similar phenomenon has been already reported in the previous work.28 The O 1s spectra (Figures 8c and 8f) was deconvoluted into three peaks 10


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at 534.3/533.8 eV, 532.2/532.3 eV and 530.6/530.7 eV, which can be assigned to hydroxyl oxygen, surface adsorbed oxygen and lattice oxygen, respectively. The proportion of surface adsorbed oxygen decreases from 26.82% to 13.33% after contacting with TMA gas. It implies that the surface adsorbed oxygen ions involve in TMA sensing reaction on the sensing material surface rather than MoO3/Bi2Mo3O12 itself. The possible reactions occurred on MoO3/Bi2Mo3O12 surface are deduced as below 3: O 2 ( a d s ) + e − → O −2 O −2

(a d s )

(ads)

(< 100 o C)

(1)

+ e − → 2O − ( a d s ) (100 ~ 300 o C)

4N(CH3 )3 + 21O− (ads) → 2N2 + 9H2O + 6CO2 + 42e−

(2) (3)

As described in sensing mechanism schematic (Figure 9a), the surface adsorbed oxygen (O2(ads)) combines with electrons (e−) to form oxygen anion (O−(ads)) on the surface of the sensing material at 170oC. And then TMA (N(CH3)3) reacts with oxygen anion (O−(ads)), generating nitrogen (N2), water (H2O) and carbon dioxide (CO2) and releasing electrons (e−) back to the material. The enhancement of gas-sensing of MoO3/Bi2Mo3O12 might be attributed to the two reasons. Firstly, the hollow morphology provides larger surface specific area of 32.86 m 2g-1 than those of MoO3 (9.56 m2g-1) and Bi2Mo3O12 (13.31 m2g-1) (the N2 sorption isotherms and pore size distribution are shown in Figure S2). A large surface specific area, generating more effective active sites for the TMA molecules to diffuse and further react with the surface adsorbed oxygen, might enhance the gas sensing response. Secondly, the heterostructure formed between MoO3 and Bi2Mo3O12 particles (both of them are n-type semiconductors) provides a synergistic catalytic effect: (1) There is an influence of heterostructure formation on the conductivity of the sensor. The results of conductivity test show that the Ra values of MoO3, Bi2Mo3O12 and MoO3/Bi2Mo3O12 are about 700 MΩ, 48000 MΩ and 11000 MΩ at 170oC, respectively. In terms of Bi2Mo3O12, the resistance decreases after the heterostructure 11



formation, which means that the conductivity of the material is improved. On the other hand, the Ra value of MoO3/Bi2Mo3O12 is larger than that of MoO3, implying the conductivity of the material is poorer than MoO3. For MoO3, the higher resistance might facilitate the enhancement of gas response, because the resistance will change greatly once the TMA molecules adsorb on the surface of the material.41 (2) We investigated the energy band diagram at the interface between MoO3 and Bi2Mo3O12 particles, and gave the band gaps and valence bands by UV-vis diffuse reflectance spectra and XPS VB spectra.42,43 The band gap is estimated to be 2.67 eV for MoO3 (Figure S3a) and 2.73 eV for Bi2Mo3O12 (Figure S3b). From XPS VB spectra (Figure S3c and Figure S3d), the VBs of MoO3 and Bi2Mo3O12 are about 3.09 eV and 1.69 eV. So the CBs of MoO3 and Bi2Mo3O12 are calculated to be ~0.42eV and ~-1.04 eV. The electrons might transfer from Bi2Mo3O12 to MoO3, because the CB value of MoO3 is greater than that of Bi2Mo3O12, resulting in the appearance of depletion layer at the interface between MoO3 and Bi2Mo3O12 particles. The energy band diagram at the interface between MoO3 and Bi2Mo3O12 particles is given in Figure 9b. As a result, the electrons are easily trapped by the surface-adsorbed oxygen on the hetero-interface to form chemisorbed oxygen ions. Once the MoO3/Bi2Mo3O12 sensor is in TMA atmosphere, the chemisorbed oxygen ion will oxidize TMA molecules rapidly. Conclusions In summary, the MoO3/Bi2Mo3O12 hollow microspheres were fabricated by a solvothermal route followed by calcining at 500oC. Both the untreated and calcined products are spherical with a diameter of about 1.2 μm, which implies that the morphology can maintain even at a high calcanition temperature of 500oC. In contrast with the sensors based on MoO3 or Bi2Mo3O12, the response of MoO3/Bi2Mo3O12 sensor towards 50 ppm TMA is about 2.5 times and 5.5 times higher, respectively. One reason of the enhanced TMA sensing property might be that the heterostructure formed between MoO3 and Bi2Mo3O12 can provide a synergistic 12


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effect. In addition, the hollow morphology is good for the TMA molecules diffusion and reaction, which may be another reason for improving TMA sensing property. Moreover, the MoO3/Bi2Mo3O12 sensor can measure 100 ppb ~ 100 ppm TMA at low temperature of 170oC with a good linear relationship (R2 = 0.9942). The sensor also has a good selectivity, longterm stability, repeatability and anti-humidity ability. This work might reveal that the MoO3/Bi2Mo3O12 or Bi2Mo3O12 is a great candidate in TMA detection. AUTHOR INFORMATION Corresponding Authors *(X. L. Cheng) E-mail: [email protected] *(L. H. Huo) E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (21771060, 21547012 and 21305033), International Science & Technology Cooperation Program of China (2016YFE0115100), Program for Science and Technology Project of Heilongjiang province (B2015008), Youth Foundation of Harbin (2015RQQXJ047). ASSOCIATED CONTENT Supporting information Preparation of MoO3 hollow spheres; The SEM images of MoO3 and Bi2Mo3O12 hollow spheres; Nitrogen adsorption-desorption isotherms and pore size distribution of MoO3, Bi2Mo3O12 and MoO3/Bi2Mo3O12 hollow spheres; UV-vis absorption spectra and corresponding band gap energies of MoO3 and Bi2Mo3O12; The XPS VB spectra of MoO3 and Bi2Mo3O12. 13



References [1] Lee S.H.; Lim J.H.; Park J.; Hong S.; Park T.H.. Bioelectronic Nose Combined with A Microfluidic System for the Tetection of Gaseous Trimethylamine. Biosens.Bioelectron 2015, 71, 179–185. [2] Cho Y.H.; Ko Y.N.; Kang Y.C.; Kim I.D.; Lee J.H.. Ultraselective and Ultrasensitive Detection of Trimethylamine Using MoO3 Nanoplates Prepared by Ultrasonic Spray Pyrolysis. Sens. Actuators B 2014, 195, 189–196. [3] Wang P.; Zheng Z.K.; Cheng X.L.; Sui L.L.; Gao S.; Zhang X.F.; Xu Y.M.; Zhao H.; Huo L.H.. Ionic Liquid-assisted Synthesis of α-Fe2O3 Mesoporousnanorod Arrays and Their Excellent Trimethylamine Gas-sensing Properties for Monitoring Fish Freshness. J. Mater. Chem. A 2017, 5, 19846–19856. [4] Qi Q.; Zou Y.C.; Fan M.H.; Liu Y.P.; Gao S.; Wang P.P.; He Y.; Wang D.J.; Li G.D.. Trimethylamine Sensors with Enhanced Anti-humidity Ability Fabricated from La0.7Sr0.3FeO3 Coated In2O3-SnO2 Composite Nanofibers. Sens. Actuators B 2014, 203, 111–117. [5] Li C.; Lin Y.; Li F.; Zhu L.H.; Sun D.M.; Shen L.; Chen Y.; Ruan S.P.. Hexagonal ZnO Nanorings: Synthesis, Formation Mechanism and Trimethylamine Sensing Properties. RSC Adv. 2015, 5, 80561–80567. [6] Lu X.F.; Yu Q.Q.; Wang K.; Shi L.C.; Liu X.. Synthesis, Characterization and Gas Sensing Properties of Flowerlike In2O3 Composed of Microrods. Cryst. Res. Technol. 2010, 45, 557–561. [7]. Chu X.F; Liang S.M.; Sun W.Q.; Zhang W.B.; Chen T.Y.; Zhang Q.F.. Trimethylamine Sensing Properties of Sensors Based on MoO3 Microrods. Sens. Actuators B 2010, 148, 399–403. [8] Cho Y.H.; Kang Y.C.; Lee J.H.. Highly Selective and Sensitive Detection of Trimethylamine Using WO3 Hollow Spheres Prepared by Ultrasonic Spray Pyrolysis. Sens. Actuators B 2013, 176 , 971–977. [9] Kumar K.L.A.; Durgajanani S.; Jeyaprakash B.G.; Rayappan J.B.B.. Nanostructured Ceria Thin Film for Ethanol and Trimethylamine Sensing. Sens. Actuators B 2013, 177, 19–26. [10] Yang T.Y.; Du L.Y.; Zhai C.B.; Li Z.F.; Zhao Q.; Luo Y.; Xing D.J.; Zhang M.Z.. Ultrafast Response and Recovery Trimethylamine Sensor Based on α-Fe2O3 Snowflakelike Hierarchical Architectures. J. Alloys. Compd. 2017, 718, 396–404. 14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[11] Chu X.F.; Han Y.L.; Zhou S.M.; Shui H.F.. Trimethylamine Sensing Properties of Nano-SnO2 Prepared Using Microwave Heating Method. Ceram. Int. 2010, 36, 2175– 2180. [12] Zhang J.; Song P.; Li Z.Q.; Zhang S.; Yang Z.X.; Wang Q.. Enhanced Trimethylamine Sensing Performance of Single-crystal MoO3 Nanobelts Decorated with Au Nanoparticles. J. Alloys. Compd. 2016, 685, 1024–1033. [13] Li H.Y.; Huang L.; Wang X.X.; Lee C.S.; Yoon J.W.; Zhou J.; Guo X.; Lee J.H.. Molybdenum Trioxide Nanopaper as A Dual gas Sensor for Detecting Trimethylamine and Hydrogen Sulfide. RSC Adv., 2017, 7, 3680–3685. [14] Li Z.Q.; Wang W.J.; Zhao Z.C.; Liu X.R.; Song P.. Molybdenum Trioxide Nanopaper as A Dual Gas Sensor for Detecting Trimethylamine and Hydrogen Sulfide. Mat. Sci. Semicon. Proc. 2017, 66, 33–38. [15] Song P.; Wang Q.; Li J.; Yang Z.X.. Morphology-controllable Synthesis, Characterization and Sensing Properties of Single-crystal Molybdenum Trioxide. Sens. Actuators B 2013, 181, 620–628. [16] Xin L.L.; Yuan S.; Chen Z.H.; Chen Y.J.; Xue X.Y.. Enhanced Gas Sensing Performance of SnO2/α-MoO3 Heterostructure Nanobelts. Nanotechnology 2011, 22(6), 225502–225507. [17] Chen Y.J.; Xiao G.; Wang T.S.; Zhang F.; Ma Y.; Gao P.; Zhu C.L.; Zhang E.; Xu Z.; Li Q.H.. α-MoO3/TiO2 Core/shell Nanorods: Controlled-synthesis and Low Temperature Gas Sensing Properties. Sens. Actuators B 2011, 155, 270–277. [18] Wang T.S.; Wang Q.S.; Zhu C.L.; Ouyang Q.Y.; Qi L.H.; Li C.Y.; Xiao G.; Gao P.; Chen Y.J.. Synthesis and Enhanced H2S Gas Sensing Properties of α-MoO3/CuO p-n Junction Nanocomposite. Sens. Actuators B 2012, 171–172, 256–262. [19] Jiang D.; Wei W.; Li F.; Li Y.; Liu C.; Sun D.; Feng C.; Ruan S.. Xylene Gas Sensor Based on α-MoO3/α-Fe2O3 Heterostructure with High Response and Low Operating Temperature. RSC Adv. 2015, 5, 39442–39448. [20] Cui X.L.; Xu Y.M.; Zhang X.F.; Cheng X.L.; Gao S.; Zhao H.; Huo L.H.. In-situ Deposited Flower-like Bi2MoO6 Microspheres Thin Film Based Sensors for Highly Selective Detection of ppb-level H2S at Low Temperature. Sens. Actuators B 2017, 247, 681–690.

15



[21] Li J.T.; Wang L.W.; Liu H.J.; Zhao J.; Li X.; Wei H.; Han Y.F.. Synthesis and Enhanced Toluene Gas Sensing Properties of 1-D α-MoO3/Fe2(MoO4)3 Heterostructure. J. Alloys. Compd. 2017, 694, 939–945. [22] Gao X.M.; Li C.Y.; Yin Z.X.; Chen Y.J.. Synthesis and H2S Sensing Performance of MoO3/Fe2(MoO4)3 Yolk/shell Nanostructures. RSC Adv., 2015, 5, 37703–37709. [23] Li Z.Q.; Song P.; Yang Z.X.; Wang Q.. In Situ Formation of One-dimensional CoMoO4/MoO3 Heterojunction as An Effective Trimethylamine Gas Sensor. Ceram. Int. 2018, 44, 3364–3370. [24] Chen T.; Wang M.H.; Ma X.Y.. Preparation in Acidic and Alkaline Conditions and Characterization of α-Bi2Mo3O12 and γ-Bi2MoO6 Powders. J. Electron Mater. 2016, 45(8), 4375–4379. [25] Sun M.H.; Yin Y.Y.; Song C.W.; Wang Y.G.; Xiao J.K.; Qu S.C.; Zheng W.B.; Li C.; Dong W.; Zhang L.. Preparation of Bi2MoO6 Nanomaterials and Theirs Gas-Sensing Properties. J. Inorg. Organomet. Polym. Mater. 2016, 26, 294–301. [26] Tao P.; Xu Y.L.; Zhou Y.C.; Song C.W.; Qiu Y.H.; Dong W.; Zhang M.H.; Shao M.H.. Nitrogen Oxide (NO) Gas-sensing Properties of Bi2MoO6 Nanosheets Synthesized by A Hydrothermal Method. Materials Research 2017, 20(3), 786–790. [27] Pei Y.; Li X.G.; Chu H.; Ge Y.C.; Dong P.; Baines R.T.; Pei L.Y.; Ye M.X.; Shen J.F.. Anion-exchange Engineering of Cookie-like Bi2S3/Bi2MoO6 Heterostructure for Enhanced Photocatalytic Activities and Gas-sensing Properties. Talanta 2017, 165, 44–51. [28] Zhang F.D.; Xu Y.M.; Zhang X.F.; Sui L.L.; Hu P.; Zheng Z.K.; Cheng X.L.; Gao S.; Zhao H.; Huo L.H.. Highly Selective ppb-level H2S Sensor Based on the Walnut-like Bi2MoO6 at Low Temperature. Sens. Actuators B 2018, 277, 312–319. [29] Cao S.Y.; Chen C.S.; Liu T.G.; Tsang Y.H.; Zhang X.M.; Yu W.W.; Chen W.W.. Synthesis of Reduced Graphene Oxide/α-Bi2Mo3O12 @ β-Bi2O3 Heterojunctions by Organic Electrolytes Assisted UV-excited Method. Chem. Eng. J. 2014, 257, 309–316. [30] Liu T.X.; Li B.X.; Hao Y.G.; Yao Z.Y.. MoO3-nanowire Membrane and Bi2Mo3O12/MoO3 Nano-heterostructural Photocatalyst for Wastewater Treatment. Chem. Eng. J. 2014, 244, 382–390. [31] Yang M., Zhang X.F., Cheng X.L., Xu Y.M., Gao S., Zhao H., Huo L.H.. Hierarchical NiO Cube/Nitrogen-doped Reduced Graphene Oxide Composite with Enhanced H2S Sensing Properties at Low Temperature. ACS Appl. Mater. Interfaces 2017, 9, 26293– 26303. 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[32] Zhang S.; Song P.; Zhang J.; Li Z.Q.; Yang Z.X.; Wang Q.. In2O3-functionalized MoO3 Heterostructure Nanobelts with Improved Gas-sensing Performance. RSC Adv. 2016, 6, 50423–50430. [33] Chu X.F.; Liang S.M.; Chen T.Y.; Zhang Q.F.. Trimethylamine Sensing Properties of CdO-Fe2O3 Nano-materials Prepared Using Co-precipitation Method in the Presence of PEG400. Mater. Chem. Phys. 2010, 123, 396–400. [34] Zhang R.; Wang L.L.; Deng J.N.; Zhou T.T.; Lou Z.; Zhang T.. Hierarchical Structure with Heterogeneous Phase as High Performance Sensing Materials for Trimethylamine Gas Detecting. Sens. Actuators B 2015, 220, 1224–1231. [35] Lou Z.; Li F.; Deng J.N.; Wang L.L.; Zhang T.. Branch-like Hierarchical Heterostructure (α-Fe2O3/TiO2): A Novel Sensing Material for Trimethylamine Gas Sensor. ACS Appl. Mater. Interfaces 2013, 5, 12310–12316. [36] Li F.; Gao X.; Wang R.; Zhang T.; Lu G.Y.; Barsan N.. Design of Core-Shell Heterostructure Nanofibers with Different Work Function and Their Sensing Properties to Trimethylamine. ACS Appl. Mater. Interfaces 2016, 8, 19799–19806. [37] Li C.; Feng C.H.; Qu F.D.; Liu J.; Zhu L.H.; Lin Y.; Wang Y.; Li F.; Zhou J.G.; Ruan S.P.. Electrospun Nanofibers of p-type NiO/n-type ZnO Heterojunction with Different NiO Content and Its Influence on Trimethylamine Sensing Properties. Sens. Actuators B 2015, 207, 90–96. [38] Lee C.S.; Kim I.D.; Lee J.H.. Selective and Sensitive Detection of Trimethylamine Using ZnO-In2O3 Composite Nanofibers. Sens. Actuators B 2013, 181, 463–470. [39] Kwak C.H.; Woo H.S.; Lee J.H.. Selective Trimethylamine Sensors Using Cr2O3Decorated SnO2 Nanowires. Sens. Actuators B 2014, 204, 231–238. [40] Zhang W.H.; Zhang W.D.. Fabrication of SnO2-ZnO Nanocomposite Sensor for Selective Sensing of Trimethylamine and the Freshness of Fishes. Sens. Actuators B 2008, 134, 403–408. [41] Hu X.; Zhu Z.; Li Z.; Xie L.; Wu Y.; Zheng L.. Heterostructure of CuO Microspheres Modified with CuFe2O4 Nanoparticles for Highly Sensitive H2S Gas sensor. Sens. Actuators B 2018, 264, 139–149. [42] Zhang Y. ; Park, S.J.. Formation of Hollow MoO3/SnS2 Heterostructured Nanotubes for Efficient Light-driven Hydrogen Peroxide Production. J. Mater. Chem. A 2018, 6, 20304–20312. 17



[43] Pang Y.; Xu G.; Feng Q.; Liu J.; Lv J.; Zhang Y.; Wu Y.. Synthesis of αBi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application. Langmuir 2017, 33(36), 8933–8942.

18


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Captions Fig. 1. The pictures of the ceramic tube (a) and the coated ceramic tube welded on the base (b). Fig. 2. The schematic diagram of gas-sensing measurement device. Fig. 3. XRD patterns of the products (a) MoO3, (b) Bi2Mo3O12 and (c) MoO3/Bi2Mo3O12. Fig. 4. SEM images of the precursor (a) and the calcined product (b). TEM (c) and HRTEM (d) images of the calcined product. Fig. 5. SEM images of commercial MoO3 (a) and precursors obtained at 1 h (b), 4 h (c) and 12 h (d). TEM images of precursors obtained at 12 h (e), 24 h (f), 36 h (g) and 48 h (h). Fig. 6. (a) The responses of the sensors based on MoO3, Bi2Mo3O12 and MoO3/Bi2Mo3O12 towards 50 ppm of TMA at different operating temperatures. (b) The responses of MoO3/Bi2Mo3O12 sensor towards 100 ppm of various gases. (c) The gas response-recovery characteristics of the MoO3/Bi2Mo3O12 sensor to 100 ppb ~ 100 ppm of TMA measured at 170oC and the linear relationship between the responses of the MoO3/Bi2Mo3O12 sensor and TMA concentration at 170oC. (d) The reproducibility and responses of the MoO3/Bi2Mo3O12 sensor to 10 ppm TMA. Fig. 7. The long-term stability of the MoO3/Bi2Mo3O12 sensor to 50 ppm TMA (a) and the responses of the sensor towards various relative humidity (b) at 170oC. Fig. 8. Bi 4f, Mo 3d, and O 1s XPS spectra of the MoO3/Bi2Mo3O12 sensor before (a, b, c) and after (d, e, f) exposure to TMA at 170oC. Fig. 9. Schematic diagram of TMA sensing mechanism of MoO3/Bi2Mo3O12 sensor (a) and energy band structure for MoO3/Bi2Mo3O12 heterostructure (b). 19



Fig. 1

Fig. 2

20


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 3

Fig. 4

21



Fig. 5

Fig. 6

22


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 7

Fig. 8

23



Fig. 9

24


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of the MoO3/Bi2Mo3O12 sensor to TMA with other reported oxide/oxide composite sensors. TMA Sensing material

Response concentration (ppm)

Working temperature (oC)

Reference

In2O3/SnO2

10

22.0

25

[4]

CoMoO4/MoO3

10

25.0

220

[23]

In2O3/MoO3

10

31.69

260

[32]

CdO/Fe2O3

1

2.9

230

[33]

α-Fe2O3/ZnO

10

2.0

260

[34]

α-Fe2O3/TiO2

10

6.8

250

[35]

In2O3/SnO2

10

7.1

280

[36]

NiO/ZnO

10

54

260

[37]

ZnO/In2O3

5

133.9

300

[38]

Cr2O3/SnO2

5

9.9

450

[39]

SnO2/ZnO

1

7.0

330

[40]

MoO3/Bi2Mo3O12

10

7.2

170

This work

25



Table of Contents Graphic

MoO3/Bi2Mo3O12 hollow microspheres were prepared via a simple solvothermal route. The MoO3/Bi2Mo3O12 sensor exhibited an obviously enhanced gas-sensing property for TMA at low operating temperature of 170oC. It is the first time for MoO3/Bi2Mo3O12 to apply in gas sensors, which might take an important step in the application of MoO3/Bi2Mo3O12 or Bi2Mo3O12 in the field of gas sensing.

26


Page 26 of 26

Enhanced Gas-sensing Properties for Trimethylamine at Low

Recommend Documents