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Au-loaded hierarchical MoO3 hollow spheres with enhanced gas sensing performance for the detection of BTX (benzene, toluene and xylene) and the sensing mechanism Li-Li Sui, Xian-Fa Zhang, Xiaoli Cheng, Ping Wang, Ying-Ming Xu, Shan Gao, Hui Zhao, and Li-Hua Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

Au-loaded hierarchical MoO3 hollow spheres with enhanced gas sensing performance for the detection of BTX (benzene, toluene and xylene) and the sensing mechanism Lili Sui,†,‡ Xianfa Zhang,† Xiaoli Cheng,† Ping Wang,†,‡ Yingming Xu,*,† Shan Gao,† Hui Zhao,† and Lihua Huo*,† † Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China

‡ School of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, 161006, China

Corresponding Author *E-mail address: [email protected] (Y. M. Xu), [email protected] (L. H. Huo)

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ABSTRACT: Monodisperse, hierarchical α-MoO3 hollow spheres were fabricated using a facile template-free solvothermal method combined with subsequent calcination. Various quantities of Au nanoparticles (NPs) were deposited on the α-MoO3 hollow spheres to construct hybrid nanomaterials for chemical gas sensors and their BTX sensing properties were investigated. The 2.04 wt% Au-loaded α-MoO3 sensor can detect BTX effectively at 250 °C, especially, its responses to 100 ppm toluene and xylene are 17.5 and 22.1, respectively, which are 4.6 and 3.9 times higher than those of pure α-MoO3 hollow spheres at 290 °C. Besides, Au loading decreased the response times to toluene and xylene from 19 and 6 s to 1.6 and 2 s, respectively, lowered the working temperature from 290 to 250 °C as compared with those of pure α-MoO3. The surface status of Au/α-MoO3 hollow spheres before and after contacting with toluene at 250 °C was analyzed through XPS technique. Possible oxidization product of toluene was confirmed by GC for the first time. The gas-sensing mechanism of the Au-αMoO3 was speculated as the oxidation of toluene to water and carbon dioxide by chemisorbed oxygen and lattice oxygen. The possible reason related with improved gas sensing properties of the Au-functionalized α-MoO3 was discussed.

KEYWORD: Au/α-MoO3 hollow spheres, hierarchical structure, load, BTX sensor, sensing mechanism

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1. INTRODUCTION Inorganic functional materials with hierarchical structures have attracted widespread attention because of their improved electrical and optical characteristics in the field of photocatalysts,1 gas sensors,2,

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Li-ion batteries4 and solar cells5 when compared to solid

counterparts. Among these, three-dimensional (3D), hierarchical metal oxides with hollow nanostructures are promising sensors. Such peculiar architectures possess thin shell layers, abundant surface active sites and low agglomerated configurations, which provide fast access for gas molecules diffusion and short pathways for charge transport when target gases diffuse onto the entire sensing layer via nanoscale shell. These unique characteristics are beneficial to enhance the gas sensing properties of metal oxides. It is known that the adsorption and reaction of analyte gases on the sensing layer play a decisive role in gas sensing process. So, the surface reaction should be also promoted except for enhancing the gas diffusion. Metal oxide semiconductors functionalized with noble metals have been realized an effective method to achieve fast and high gas responses, in which the metal additives act as catalyst of the surface reaction and adsorbed locations of oxygen to improve the electrochemical reactions of tested gases on the sensor surface. Au NPs have been frequently introduced because of their lower cost than Pd and Pt as well as better thermal stability than Ag. It has been proved that Au-decorated metal oxides with hollow nanostructures, such as Au-loaded ZnO6, 7 and Au/SnO28, 9 microspheres, could promote the selectivity, sensitivity or response rate because the gas sensing reactions carried out effectively on both their inner and outer layers.

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MoO3 is a n-type semiconductor metal oxide (Eg=3.1 eV) and has been widely used in the fields of catalysts,10 gas sensors,3 electrochemistry11 and field emission12 for its stable physical-chemical characteristics. Especially, the α-MoO3 possesses particular layered crystal texture, making it advantageous for gas diffusion. Thus, α-MoO3 is suitable for the application in gas sensor area. Till now, most of the investigations about its sensing property are concentrated on low-dimensional nanomaterials. Although various 3D hierarchical MoO3 structures have been successfully prepared, the reports about the fabrication of hierarchical MoO3 hollow spheres are relatively less. Li et al.13 prepared α-MoO3 hollow spheres via a template method, whose sizes were about 80 µm. Wang et al.14 prepared α-MoO3 hollow microspheres by a hydrothermal route and subsequent calcination where ethanol and PEG were employed, the diameters of the hollow spheres were 1-2 µm. MoO3 hollow microspheres with diameter of 0.6–1 µm were also prepared by a solvothermal treatment followed by a calcination treatment.15 However, the above processes are generally complex and need templates or surfactants, and the as-obtained hollow spheres are large, usually with poor dispersity and uniformity. In addition, although MoO3 loaded by noble metals including Ag, Au, Pt and Pd have been studied, these hybrid materials are normally used as catalysts.16, 17

As far as we know, there has been little detailed exploration on the sensing behaviors for

the pure α-MoO3 and Au/α-MoO3 hollow spheres sensors, especially for their detection of hazardous BTX (benzene, toluene, and xylene) with high sensitivity and selectivity, as well as the corresponding investigation of the gas sensing mechanism. BTX are the most dangerous pollutants among volatile organic compounds (VOCs) and are commonly employed as organic solvent in the industrial production of paints, lacquers,

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adhesives, detergents, dyes and preservatives.18 But they are volatile, toxic and combustible and can generate risks to human and environment when they are released indoors and outdoors. Among BTX, benzene is carcinogenic, while toluene and xylene can cause similar symptoms of sick building syndromes even at sub-ppm level.19, 20 The threshold limit value (TLV) of benzene for 8 h exposure is 0.5 ppm established by American Conference of Governmental Industrial Hygienists (ACGIH),20 and The Occupational Safety and Health Administration recommends permissible exposure limits (PEL) for both toluene and xylene are 100 ppm as the 8 hour time-weighted average (TWA) concentration.21 Usually, it is difficult for oxide semiconductor sensor to test BXT selectively due to their similar physical and chemical properties. Thus, the distinction between benzene and methyl benzene is still challenging for the detection of BTX-related environmental pollutions. Herein, the α-MoO3 hollow spheres with hierarchical structures were successfully prepared by a simple solvothermal method and subsequent calcination. Then Au NPs were decorated on their surface by a facile solution method. Au-loaded α-MoO3 hollow spheres are multifunctional combinations of the gas accessible hollow nanostructures and catalytic promotion of Au, demonstrating increased selectivity, enhanced sensitivity, decreased working temperature and shortened response time toward methyl benzene compared with bare α-MoO3. Moreover, the methyl benzene sensing mechanism of Au/α-MoO3 hollow spheres was further explored, and the promotion of Au NPs assisted surface reaction in gas sensing was also discussed. 2. EXPERIMENTAL SECTION 2.1 Fabrication of hierarchical α-MoO3 hollow spheres. All chemicals were analytical

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grade and used as received. Hierarchical α-MoO3 hollow spheres were synthesized through a solvothermal process. Typically, 0.14 g of molybdenyl acetylacetonate was dissolved in 30 mL n-butyl alcohol under vigorous stirring. 5 mL, 1 mol L-1 of HNO3 was then added dropwise into above mixture to form homogenous yellow solution. After stirring for 1 h, the resulting solution was poured into 50 mL Teflon-lined stainless autoclave and sealed. The autoclave was heated at 220 °C for 12 h and then cooled to ambient temperature. The achieved black solid precipitate was collected by several rinse-centrifugation cycles with ultrapure water and ethanol, respectively. After being dried at 60 °C and annealed at 400 °C in air for 2 h, white sample of α-MoO3 hollow spheres were obtained. 2.2 Fabrication of Au-loaded α-MoO3 hollow spheres. Before preparation of Au/α-MoO3 hollow spheres, the Au colloid was prepared in advance. HAuCl4 solution (0.01 g mL-1, 4.12 mL) was dissolved in 93 mL ultrapure water and heated at 120 °C under stirring for 30 min. Then, 2 mL trisodium citrate solution (1.0 mmol L-1) was rapidly added and kept boiling for 30 min, and wine-colored Au colloid was obtained. Au/α-MoO3 hollow spheres were synthesized by surfactant-modified approach. Typically, 40 mg of as-obtained α-MoO3 powders were added into a mixed solution containing 15 mL ethanol and 1mL 3-aminopropyltriethoxysilane (APTES) to be functionalized. The mixture was stirred slowly for 12 h, and subsequently was centrifuged, rinsed several times with ethanol to remove redundant surfactant. After that, the modified α-MoO3 microspheres were dispersed in 10 mL ethanol. A suitable amount of Au colloid was dropwise introduced into the above solution and kept stirring for 1 h. The precipitates, Au/APTES/α-MoO3 microspheres, were centrifuged and rinsed five times with ethanol. After dried at 60 °C, the

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samples were heat-treated at 400 °C for 2 h. The as-obtained Au-loaded α-MoO3 hollow spheres with the Au/MoO3 addition ratio of 1.0, 2.0, and 4.0 wt% were determined to be 0.85, 2.04 and 4.16 wt%, respectively, by inductively coupled plasma (ICP) mass spectroscopy. The pure, the 0.85, 2.04 and 4.16 wt% Au-loaded α-MoO3 hollow spheres are labeled as α-MoO3, 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 in the following discussion, respectively. 2.3 Characterization. X-ray powder diffraction (XRD) patterns were obtained on D/MAX-III-B-40KV diffractometer with Cu Kα (λ=1.5406 Å) radiation. The scanning electron microscopy (SEM) images were recorded on HITACHI S-4800 instrument with an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) analysis was conducted using JEOL JEM-2100 apparatus operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was measured on ULTRA AXIS DLD system with monochromatic Al Kα source and C 1s peak at 284.6 eV as an internal standard. The intermediate gaseous products after the Au/α-MoO3 sensor exposed to toluene and xylene were characterized by gas chromatography spectrometer (GC, 112A, China) using a molecular sieve column (2 m × 3 mm, TDX01, China) at 100 °C in H2 flow with the velocity of 40 mL min-1. 2.4 Fabrication and measurement of gas sensors. The as-obtained pure α-MoO3 and Au-loaded α-MoO3 hollow spheres were uniformly dispersed into a drop of terpilenol to form slurry, respectively, and then were coated on the surface of Al2O3 microtubes (length = 4 mm, internal diameter = 0.8 mm, external diameter = 1.2 mm) with a pair of Au electrodes attached with Pt wires. A Ni–Cr wire was inserted into the Al2O3 microtube to form a side-heated gas sensor. The gas sensors were fabricated followed our reported literature.3, 22

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The gas-sensing properties of the sensors were measured in a static test system of JF02E (Kunming, China), and the temperature was controlled via a DC method. Detailed operating steps were followed as Refs. 23, 24. The response of the sensor (S=Ra/Rg) is calculated as the ratio of the sensor resistance in air (Ra) to that in test gas (Rg). The response/recovery time is defined as the time to achieve 90% of the total resistance change. 3. RESULTS AND DISCUSSION 3.1 Composition and morphology. The structure and crystal phase of MoO2 precursor, α-MoO3 and Au/α-MoO3 microspheres were investigated by XRD. The XRD pattern of the precursor is shown in Figure S1, the diffraction peaks can be indexed to a MoO2 phase (JCPDS card No. 50-0739). For the samples of pure and Au-loaded α-MoO3 hollow spheres after heat-treatment (Figure 1), all the diffraction peaks show standard diffraction pattern of orthorhombic α-MoO3 (JCPDS card No. 05-0508). No other phases could be observed in the 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 hierarchical nanostructures, possibly due to the lower Au content in the Au/α-MoO3 microspheres compared to MoO3. This is similar with the previously reported studies of SnO2 microspheres or In2O3 nanofibers sensitized by Au NPs.9, 25 The electronic state and the composition of Au/α-MoO3 hollow spheres were discussed by XPS detection (Figure 2). The spectra of all the Au/α-MoO3 microspheres (Figure 2a) confirm the presence of Mo, O, C and Au elements. In Mo 3d XPS spectra (Figure 2b), all the samples show two symmetric peaks, the binding energies of Mo 3d5/2 and Mo 3d3/2 are around 232.4 and 235.5 eV, respectively, consisting with those of Mo6+ in MoO3. In Au 4f XPS spectrum (Figure 2c), the intensity of the Au 4f5/2 and Au 4f7/2 peaks at about 87.3 and

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83.6 eV all increase with the rising loaded amount of Au, indicating that the Au NPs with Au0 valence state are loaded onto the surface of α-MoO3 hollow spheres.26 The morphology and refinement structure of the as-obtained MoO2 precursor, pure α-MoO3 and 2.04Au/α-MoO3 products are analyzed through SEM and TEM techniques. Figures 3a-1, b-1 and c-1 show the low-magnification FESEM images of the MoO2 precursor, unloaded α-MoO3 and 2.04Au/α-MoO3. It can be seen that the overall morphologies of the three samples are monodisperse, homogeneous and well-defined microspheres with the average diameters of about 400-600 nm. And from the high-magnification SEM images (Figures 3a-2, b-2 and c-2), the surface conversion of the hierarchical nanostructures can be clearly observed. The hollow architectures can be confirmed by a cracked MoO2 precursor depicted in Figure 3a-2, from which we find that the microsphere possesses rough external surface, the sphere is assembled by numerous of particles with dimensions of tens of nanometers. From the SEM image of an individual α-MoO3 microsphere (Figure 3b-2), it can be seen that the sphere-like morphology of α-MoO3 still maintains after calcination, while the building-blocks transform from primary nanoparticles to nanorods, which stack up with a diameter of about 40 nm. From its broken shell, the inner hollow space of the sphere could be clearly discerned, and the shell becomes looser, the pores become larger than the precursor. After Au NPs loading, the morphology of primary α-MoO3 microsphere is still maintained, and the white dots of Au are distributed on the surface of the microspheres (see Figure 3c-2), implying the successful deposition of Au NPs. The fine nanostructures of the above three samples were performed by TEM (Figure 4). The MoO2 precursor displays the dark solid edge and the pale hollow space in Figure 4a-1,

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confirming its hollow structures. The hollow sphere is 450 nm in diameter and 46 nm in thickness of shell. The constructed hierarchical nanostructure is assembled by dense nanoparticles with the diameters of 10-25 nm (Figure 4a-2). The spacing of adjacent lattice fringes shown in Figure 4a-3 was determined to be 0.24 nm, which can be indexed to the interplanar spacing of the (200) planes of MoO2. The corresponding selected area electron diffraction (SAED) pattern (Figure 4a-4) demonstrates that these hollow spheres are polycrystalline of MoO2. After calcination, the TEM image of α-MoO3 sphere (Figure 4b-1) shows that the hollow structure can be perfectly maintained. The shell of α-MoO3 hollow sphere is 62 nm in thickness and the building blocks of nanorods with diameters of about 40 nm can be clearly seen in Figure 4b-2. The HRTEM image (Figure 4b-3) reveals clear lattice fringes with lattice spacing of 0.37 and 0.40 nm, corresponding to the interplanar distance of (001) and (100) lattice planes in the α-MoO3 orthorhombic phase, respectively. The SAED pattern (Figure 4b-4) of the sample shows the typical polycrystalline structure. TEM analysis was also performed to confirm that the Au NPs have been successfully deposited on the surface of α-MoO3 spheres. As shown in Figure 4c-1, the 2.04Au/α-MoO3 sample exhibits a hollow spherical structure, the size of the sphere does not change and no agglomerations occur during the Au assembly process. Also, a few dark spots with a size of about 15 nm can be observed on shell of the sphere (Figure 4c-2). The dark spots and the shell exhibit the lattice fringes with interplanar spacing of approximate 0.23 and 0.36 nm (Figure 4c-3) which agree well with the crystallographic planes of Au (111) and α-MoO3 (001), respectively, further demonstrating the presence of the Au NPs on the surface of α-MoO3 spheres. The SAED pattern (Figure 4c-4) indicates the polycrystalline characteristics of the Au/α-MoO3

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spheres. 3.2 Formation mechanism of α-MoO3 hollow spheres. To explore the growth process of α-MoO3 hollow spheres, time-dependent experiments were performed and investigated through SEM and TEM techniques (Figure 5). At the reaction early stage (2 h), smooth sphere with diameter of about 300 nm forms (Figure 5a); the sphere is complete a solid one as shown in its TEM image (Figure 5f). When the reaction time is extended to 3 h, the sample is still microsphere with solid core and abundant small nanoparticles appear on its surface, which is attributed to the recrystallization preferentially carried out at the solid–liquid interface (Figures 5b and g). Upon increasing the reaction time to 4 h, crystallites pack more loosely in the interior than those packed along the external surface, then the yolk–shell spherical structure with a gap distance of about 70 nm forms (Figures 5c and h). At the same time, the outer crystalline shell of the solid particles grows further, because of the consumption of the inner crystallites and redeposition on the outer layer to reduce the relatively high interfacial energy. As the reaction time is prolonged to 12 h, the cavity of the hollow sphere forms, the hollow sphere is quite coarse and many nanoparticles (diameter of ca. 20 nm) closely pack together and form the outer shell (Figures 5d and i). When the as-prepared MoO2 precursor converts into α-MoO3 after calcination at 400 °C for 2 h, the hollow nanostructure of the microsphere is perfectly preserved, except for the building blocks of nanoparticles change into nanorods, which makes the surface coarser than ever and pores between the nanorods become bigger (Figures 5e and j). To sum up, the formation mechanism of α-MoO3 hollow sphere is believed to result from an inside-out Ostwald-ripening process, the formation of which might experience the following evaluation

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process: solid sphere → yolk–shell spherical structure → hollow sphere. 3.3 Gas-sensing performance. As we know that the optimum working temperature can dominate the gas sensing property of the sensor. Accordingly, the responses of α-MoO3, 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 sensors to 100 ppm toluene vapor were operated at the temperatures ranging from 217 to 330 °C, as shown in Figure 6a. Obviously, all the responses show the same “increase-maximum-decay” tendency with the operating temperature increasing, every sensor has its optimum working temperature (OWT), at which the sensors show the maximum response to toluene gas. This might be ascribed to the adsorption/desorption quantities of toluene molecules on the sensing layer at that temperature. As these sensors are exposed to 100 ppm toluene gas at their OWT, more toluene molecules prefer to adsorb on the surface of the sensor and are oxidized, demonstrating high chemical reactivity at the OWT. However, the temperature further increasing would accelerate toluene molecules desorption from the sensors’ surface, thus the effective adsorption quantities of toluene decrease. It also can be observed that the OWT for 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 sensors is 250 °C, which decreases 40 °C compared to 290 °C for pure α-MoO3 one. It may be ascribed that the Au NPs as catalyst could decrease the activity energy of the redox reaction. Besides, the highest responses of Au NPs functionalized α-MoO3 sensors to toluene gas are all higher than that of pure α-MoO3 sensor. The responses of 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 sensors to 100 ppm toluene are 6.3, 17.5 and 8.4 at 250 °C, about 1.6, 4.6 and 2.2 times bigger than that of pure α-MoO3 sensor (3.8) at 290 °C, respectively. The highest response increases with Au content increasing at first, then gradually decreases, and reaching the

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maximum when the Au amounts up to 2.04 wt%, which probably because too much loading proportion of Au would blot out the active center for sensing rather than improving its catalytic capability in the response of toluene. The above results verify that the highest response and OWT are improved by the deposition of Au NPs. Selectivity is an important parameter for chemical sensor in practical application. Therefore, the responses of the sensors based on the pure and Au/α-MoO3 to 100 ppm of formaldehyde, ethanol, chlorobenzene, benzene, toluene and xylene were detected at their relative OWTs identified by toluene detection. Figure 6b shows a bar graph of the responses of the sensors to those gases, there is no remarkable selectivity for the pure α-MoO3 sensor to the six gases. However, the responses of the Au-loaded α-MoO3 sensors towards these vapors are all improved compared with the pure one. Especially, the responses of the 2.04Au/α-MoO3 sensor to BTX, including benzene (5.3), toluene (17.5) and xylene (22.1) are significantly improved, which are about 3.8, 4.6 and 3.9 times higher than those of the pure α-MoO3 sensor (1.4, 3.8 and 5.6), respectively. For the selectivity detection, the responses of 2.04Au/α-MoO3 sensor to BTX and other six gases are compared in Figure 6c. The sensor shows very weak sensitivity to formaldehyde, ethanol, hydrogen, acetone, ammonia and chlorobenzene gases, the responses to them are 1.8, 5.0, 2.1, 1.4, 1.6 and 3.9, respectively, which are all less than 5.0. Usually, ethanol is considered as a major interference gas, and most oxide semiconductors are sensitive to it. Here, the selectivity of toluene and oxylene to ethanol of 2.04Au/α-MoO3 sensor is improved greatly. The selectivity coefficients of toluene and oxylene to ethanol are increased from 1.2 and 1.7 in the pure α-MoO3 sensor to 3.5 and 4.4 in the 2.04Au/α-MoO3 sensor. Both of them are remarkably higher than those of the

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reported materials including Co3O4,27 Co-doped ZnO,28 In-loaded ZnO29 and Sb-doped ZnO.30 The selectivity and sensitivity of Au-loaded α-MoO3 sensor to methyl benzene enhanced more significantly than ethanol, It might be attributed to the loaded Au nanoparticles more likely coordinating with aromatic rings to make them dissociate, especially with the methyl groups replaced ones, e.g. toluene, xylene, in comparison with the other groups with σ bond, e.g. ethanol. The correlation between BTX concentrations and responses of the 2.04Au/α-MoO3 sensor operated at 250 °C is shown in Figure 6d. The responses all increase with the three gases (benzene, toluene and xylene) concentration increasing in the range of 0.1-100 ppm, and which are up to 5.3, 17.5 and 22.1 toward 100 ppm benzene, toluene and xylene, respectively. Benzene is known to be inertial among BTX, and in case of toluene and xylene, the -CH3 groups acted as electron donating group will increase the electron density on benzene ring. The redox reaction, that is the oxidation of the -CH3 group on aromatic ring, is more likely to occur in toluene and xylene as compared to direct dissociation of the benzene ring. Thus, the responses of the 2.04Au/α-MoO3 sensor to 100 ppm benzene, toluene and xylene should increase along with the methyl quantity increasing at 250 °C. But we find the device offers higher response to toluene than that in xylene at lower concentration range (0.1-30 ppm), which might be resulted from the stereo-hindrance effect of two methyl groups in xylene. It decreases the coordination between the aromatic rings and Au nanoparticles in certain extent. After the concentration increases to higher than 30 ppm, the concentration effect will play a more important role than the stereo-hindrance one, making more aromatic rings to coordinate with Au nanoparticles, then results in the higher response at higher concentration range of

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xylene. Moreover, there are good linear relationship (R2) between the response of 2.04Au/α-MoO3 sensor and BTX, R2 = 0.9973 for 5-100 ppm benzene, 0.9922 for 1-100 ppm toluene and 0.9987 for 0.5-100 ppm xylene, respectively. Accordingly, 2.04Au/α-MoO3 sensors can real-time monitor sub-ppm levels of methyl benzene with excellent wider and linear detection concentration range in monitoring the pollutions of chemical industry and indoor air. Figure 7 depicts the dynamic resistivity transient of the typical 2.04Au/α-MoO3 sensor toward benzene, toluene and xylene in the concentration range of 5-100 ppm, 0.1–100 ppm and 0.5-100 ppm at 250 °C, respectively. The resistances decrease to a minimum immediately when exposed to these three gases, conforming to the gas-sensing nature of n-type semiconductor oxides. Obviously, these transients display rapid response property and excellent reversibility. The detection limits of the present sensor for the three gases are 5, 0.1 and 0.5 ppm, and corresponding responses are 1.3, 1.6 and 1.3, respectively, indicating its capability to distinguish toluene (or xylene) from benzene when the concentration of BTX is less than 5 ppm. As the sensor is exposed to 5 ppm toluene and xylene, the responses are 2.5 and 5.1, and response/recovery time are calculated to be 11/57 s and 118/289 s, respectively. Furthermore, the corresponding response times of the sensor to 100 ppm toluene and xylene are 1.6 and 2 s which are shorter than 19 and 6 s of pure α-MoO3 sensor (Figure S2). In consideration of practical application, the long-term stability of 2.04Au/α-MoO3 sensor was measured to 100 ppm toluene at 250 °C (Figure S3). After 6 months storage, the sensor remains almost the original response value, whose response change is within 5%, confirming satisfactory long-term stability of the sensor. It implies that the 2.04Au/α-MoO3 hollow

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spheres are an excellent candidate for rapid detection of trace methyl benzene at 250 °C. 3.4 Gas sensing mechanism. The XPS and GC techniques are introduced to speculate the sensing mechanism of 2.04Au/α-MoO3 sensor to toluene. The chemical state change of the 2.04Au/α-MoO3 sensor before and after it was exposed to 100 ppm toluene was measured by XPS (see Figure 8). Before the sensor contacts to toluene, two peaks at 232.4 and 235.6 eV are assigned to Mo6+ of MoO3 in the Mo 3d spectrum (Figure 8a),31 and the deconvoluted three peaks of O 1s (530.0, 531.7 and 533.5 eV, see Figure 8c) can be attributed to lattice oxygen, surface adsorbed oxygen and hydroxyl oxygen, respectively.32 A higher percentage of surface adsorbed oxygen (36%) on the 2.04Au/α-MoO3 sensor surface can be found in comparation with that (21%) of the pure α-MoO3 sensor (Figure S4), indicating that the Au functionalized α-MoO3 effectively improved the quantity of adsorbed oxygen. After the sensor is exposed to 100 ppm toluene at 250 °C, the peaks of Mo 3d5/2 and 3d3/2 appeared at 231.5 and 234.3 eV31 (Figure 8b) manifest the generation of Mo5+. It indicates that Mo6+ is partially reduced to Mo5+ after the sensor exposure to 100 ppm toluene at 250 °C. Meanwhile, the surface adsorbed oxygen obviously decreases from the original 36% to 18% (Figure 8d). So, the main sensitive reaction between Au/α-MoO3 and toluene might be the catalytic oxidation of the toluene molecules by chemisorbed and lattice oxygen. Besides, it is essential to confirm the oxidation product after toluene contacts to Au/α-MoO3 so as to explore the sensing mechanism of Au/α-MoO3 to toluene. To date, there have been no effective methods to verify the reaction product of toluene in gas sensing detection due to its gaseous state and trace amount. In this study, we investigated the gas components and seized

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the direct evidence of the gaseous intermediates through GC technique after the 2.04Au/α-MoO3 sensor was exposed to toluene (Figure S5). As can be seen, the chromatographic peaks at 1.115 and 1.482 min correspond to the retention time (Rt) of H2O and CO2 in toluene oxydation product, respectively. The intensity of these two peaks are much stronger than those of H2O (Rt = 3.543 min) and CO2 (Rt=3.845 min) in outdoor air, indicating the clearly increased amount of H2O and CO2 after toluene contacts to Au/α-MoO3, which provides the direct evidence for the gas sensing mechanism of toluene sensor reported till now.33, 34 In summary, in view of our research and reported results, the gas sensing behaviors of Au/α-MoO3 to toluene are guided by two courses: the surface (reaction with adsorbed oxygen) and bulk (reaction with lattice oxygen) reaction process. The operating principle of Au/α-MoO3 sensor is directed by the conductivity changes of the sensors when the semiconducting sensing materials interact with target gas molecules. When the sensor is kept in air at 250 °C, oxygen molecules adsorb on the sensor surface (Eq. 1) and capture electrons from the conduction band to form chemisorbed oxygen species of O2−, O− or O2− on grain boundaries (Eqs. 2-4).35 As a consequence, electron depletion layer forms on the surface region of the sensor increases the potential barrier. Subsequently, the resistance of the sensor increase. As the toluene is introduced at 250 °C, the reductive gas molecules adsorb onto the sensor surface (Eq. 5) and react with adsorbed oxygen anions (Eq. 6), leading to the diminishment of electron depletion layer, thus, the bulk reaction process is promoted and proceeds along with reaction (7). Therefore, toluene is oxidized by O− and Oo, respectively, and transforms into CO2 and H2O, the electrons produced by the above two courses are

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transferred backwards to the conduction band of α-MoO3, leading to the increase in the concentration of charge carrier and the decrease of sensor resistance. O2 (gas) ↔ O2 (ads) O2 (ads) + e− ↔ O2− (ads) O2− (ads) + e− ↔ 2O− (ads)

(1)

(T < 100 °C)

(2)

(100 °C < T < 300 °C)

(3)

O− (ads) + e− ↔ O2− (ads)

(T > 300 °C)

(4)

C6H5CH3 (gas) ↔ C6H5CH3 (ads)

(5)

C6H5CH3 (ads) + 18O− ↔ 7CO2 + 4H2O + 18e−

(6)

C6H5CH3 + 18Oo(s) ↔ 7CO2 + 4H2O + 36e− + 18Vo2+

(7)

where Oo is the lattice oxygen, Vo2+ is the oxygen vacancy in the lattice and (ads) indicates surface sites. The gas sensing performance of α-MoO3 hollow spheres to BTX significantly improved by Au NPs functionalization may be ascribed to three main aspects. Firstly, the "spillover effect" provided by metal NPs should be taken into account. For the functionalized α-MoO3 hollow spheres, oxygen molecules are more favorable to adsorb on the Au NPs and dissociated into adsorbed oxygen species by the assistance of Au NPs.6, 8, 36 The activated oxygen species will then transport and spill onto the surface of α-MoO3, resulting in more reactive oxygen species which are favorable to accelerate the reaction of analyte gas molecules and chemisorbed oxygen species. The response speed is therefore fastened. Secondly, the catalytic activity of Au NPs facilities more adsorbed oxygen diffuse faster to both the outer and inner surfaces of α-MoO3 hollow spheres at the optimal temperature of 250 °C, and then more electrons should be captured from the conduction band, generating broader electron depletion layer. Besides,

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the difference of work functions between α-MoO3 (3.1 eV) and Au (5.3 eV) enables the electrons transfer from α-MoO3 to Au, forming a Schottky barrier and an extra depletion layer at the Au/α-MoO3 interface, thus the resistance of Au-loaded α-MoO3 is 877.1 MΩ (Figure 7b) which increases for about 1.5 times larger as compared with the 575.8 MΩ of pristine one (Figure S2). So, when the sensor contacts with BTX, more electrons are delivered back to conduction band of α-MoO3, which generates greater change in the resistance and eventually a higher response. Thirdly, the uniform 3D hollow structure with nanoscale shell affords high gas accessibility for gas molecules to diffuse onto the entire sensing layer quickly, and its rough surface and porous structure can also supply more efficient diffusion channels, making it more convenient for charge transport from the gas to the surface of the metal oxide, thus, the sensor becomes more active for BTX. 4. CONCLUSIONS Pure and Au-loaded α-MoO3 with hierarchical hollow nanostructures were prepared, characterized, and evaluated as chemiresistive toluene and xylene gas sensors. The sensor based on Au decorated α-MoO3 hollow spheres exhibits improved gas sensing performance to BTX, especially the enhanced selectivity to methyl benzene (toluene and xylene) with negligible cross-responses to interference gases of ethanol, formaldehyde, benzene and chlorobenzene. The maximum response is increased, the optimum working temperature and response time to 100 ppm toluene and xylene are lowered by 2.04 wt% Au loading. The enhanced gas sensing properties might be ascribed to the “spillover effect” and catalytic effect of Au NPs, as well as the hierarchical hollow nanostructure of the nanomaterials. The gas sensing mechanism of Au/α-MoO3 towards toluene is addressed as the toluene oxidation

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into H2O and CO2 via the two simultaneous processes: the toluene reacted with the chemisorbed oxygen and the lattice oxygen on the surface of the sensing materials. Moreover, the 2.04Au/α-MoO3 sensor can detect toluene and xylene ultraselectively with discerning of benzene when its concentration is below 5 ppm. The work is significant for exploring other Au doped MOS with such hollow nanostructures and setting up a proper method to detect other environmental pollutions. ASSOCIATED CONTENT Supporting information. XRD pattern of MoO2 precursor. Sensing transients of α-MoO3 hollow spheres sensor to toluene and xylene. The stability image of the 2.04Au/α-MoO3 sensor after storage in air for different time periods. O 1s XPS spectrum of α-MoO3 sensor in air. Gas chromatogram of the intermediate gaseous product after the 2.04Au/α-MoO3 sensor exposure to toluene. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *(Y. M. Xu) 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 (61271126, 21547012 and 21305033), Program for Science and Technology Project of

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FIGURE CAPTIONS Figure 1. XRD patterns of α-MoO3 and Au/α-MoO3 microspheres with different Au mass ratios. Figure 2. Survey (a), Mo 3d (b) and Au 4f (c) high resolution XPS spectra of Au/α-MoO3 microspheres with different Au mass ratios (a: 0.85, b: 2.04, c: 4.16 wt%) Figure 3. Typical SEM images of precursor (a), α-MoO3 (b) and 2.04Au/α-MoO3 (c) hollow spheres. Figure 4. Typical TEM (x-1 and x-2, x=a, b, c) and HRTEM (x-3, x=a, b, c) images and SAED patterns (x-4, x=a, b, c) of precursor (a); α-MoO3 (b); and 2.04Au/α-MoO3 (c) hollow spheres. Figure 5. The SEM (a-d) and TEM (f-i) images of a single sphere obtained at different solvothermal reaction times; the SEM (e) and TEM (j) images of a single sphere after reaction for 12 h and followed by calcination. Figure 6. The responses of α-MoO3, 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 sensors versus the working temperature to 100 ppm toluene gas (a), versus 100 ppm various gases at their relative OWTs (b); 2.04Au/α-MoO3 sensor versus 100 ppm various gases (c), versus the concentration of BTX (d) at 250 °C. Figure 7. The sensing transients of 2.04Au/α-MoO3 sensor to different concentrations of benzene (a), toluene (b) and xylene (c) at 250 °C. Figure 8. Mo 3d/O 1s XPS spectra of 2.04Au/α-MoO3 sensor before (a)/(c) and after (b)/(d) the sensor exposure to100 ppm toluene at 250 °C.

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FIGURES

Figure 1.

Figure 2.

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Figure 3.

Figure 4.

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Figure 5.

Figure 6.

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Figure 7.

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Figure 8.

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Au-loaded hierarchical MoO3 hollow spheres with enhanced gas sensing performance for the detection of BTX (benzene, toluene and xylene) and the sensing mechanism Lili Sui,†,‡ Xianfa Zhang,† Xiaoli Cheng,† Ping Wang,†,‡ Yingming Xu,*,† Shan Gao,† Hui Zhao,† and Lihua Huo*,† † Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China

‡ School of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, 161006, China

Corresponding Author *E-mail address: [email protected] (Y. M. Xu), [email protected] (L. H. Huo)

Monodisperse, hierarchical α-MoO3 hollow spheres were fabricated via a facile template-free solvothermal route combined with subsequent calcination, and Au nanoparticles (NPs) were loaded to improve their gas sensing performances toward BTX (benzene, toluene and xylene) gases. Remarkably, the 2.04 wt% Au-loaded α-MoO3 sensor manifest enhanced sensing properties to methyl benzene. 33



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Figure 1. XRD patterns of α-MoO3 and Au/α-MoO3 microspheres with different Au mass ratios. 80x63mm (300 x 300 DPI)


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Figure 2. Survey (a), Mo 3d (b) and Au 4f (c) high resolution XPS spectra of Au/α-MoO3 microspheres with different Au mass ratios (a: 0.85, b: 2.04, c: 4.16 wt%) 160x140mm (300 x 300 DPI)



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Figure 3. Typical SEM images of precursor (a), α-MoO3 (b) and 2.04Au/α-MoO3 (c) hollow spheres. 160x74mm (300 x 300 DPI)


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Figure 4. Typical TEM (x-1 and x-2, x=a, b, c) and HRTEM (x-3, x=a, b, c) images and SAED patterns (x-4, x=a, b, c) of precursor (a); α-MoO3 (b); and 2.04Au/α-MoO3 (c) hollow spheres. 160x103mm (300 x 300 DPI)



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Figure 5. The SEM (a-d) and TEM (f-i) images of a single sphere obtained at different solvothermal reaction times; the SEM (e) and TEM (j) images of a single sphere after reaction for 12 h and followed by calcination. 160x48mm (300 x 300 DPI)


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Figure 6. The responses of α-MoO3, 0.85Au/α-MoO3, 2.04Au/α-MoO3 and 4.16Au/α-MoO3 sensors versus the working temperature to 100 ppm toluene gas (a), versus 100 ppm various gases at their relative OWTs (b); 2.04Au/α-MoO3 sensor versus 100 ppm various gases (c), versus the concentration of BTX (d) at 250 °C. 160x134mm (300 x 300 DPI)



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Figure 7. The sensing transients of 2.04Au/α-MoO3 sensor to different concentrations of benzene (a), toluene (b) and xylene (c) at 250 °C. 80x181mm (300 x 300 DPI)


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Figure 8. Mo 3d/O 1s XPS spectra of 2.04Au/α-MoO3 sensor before (a)/(c) and after (b)/(d) the sensor exposure to100 ppm toluene at 250 °C. 160x138mm (300 x 300 DPI)



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160x42mm (300 x 300 DPI)


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Au-Loaded Hierarchical MoO3 Hollow Spheres ... - ACS Publications

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