SnO2 Nanoflakes with n-n Junctions for Sensing H2S

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Porous MoO3/SnO2 Nanoflakes with n-n Junctions for Sensing H2S Xinming Gao, Qiuyun Ouyang, Chunling Zhu, Xitian Zhang, and Yujin Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00308 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Nano Materials

Porous MoO3/SnO2 Nanoflakes with n-n Junctions for Sensing H2S Xinming Gao,† Qiuyun Ouyang,† Chunling Zhu,*,§ Xitian Zhang, ‡ and Yujin Chen*,† † Key Laboratory of In-Fiber Integrated Optics, Ministry of Education and College of Science, Harbin Engineering University, Harbin 150001, China.

§

College of Material Science and Chemical Engineering, Harbin Engineering

University, Harbin 150001, China ‡ Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China. KEYWORDS: Gas sensor, low working temperature, H2S, n-n junctions, large surface area

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ABSTRACT: Currently, the detection of hydrogen sulfide is important due to its negative effect on environment and human health. In this paper, the porous MoO3/SnO2 nanoflakes (NFs) with n-n junctions, composed of the interconnected nanoparticles with diameter of less than 10 nm, were successfully fabricated via a facile method. In comparison to SnO2 NFs, the gas sensing properties of the porous MoO3/SnO2 NFs showed enhanced H2S sensing performance, including higher response values, lower working temperature and relatively fast response and recovery rates. The improved H2S sensing property is attributed to existence of n-n junctions at the interface between MoO3 and SnO2, larger surface area and bigger pore volume. The strategy presented in this work opens a new way for preparation of other types of porous metal oxide semiconductors with hetero-structures for high-performance gas sensor.


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INTRODUCTION With the rapid development of industry techniques, the environmental pollution issues are increasingly serious.1,2 To overcome the issues, various strategies were developed, including designing high-performance sensors to monitor poisonous gases, exploring highly active catalysts to convert poisonous gases, and improving recycling efficiency of the nature resources and so on.3-8 Among these strategies, developing gas sensor with excellent response value and good selectivity as well as long-term stability is one of the most effective methods. Hydrogen sulfide (H2S) is a colorless, toxic, flammable and rotten egg smells gas, which is very harmful to human beings body even in a low concentration.9-15 Usually, H2S produced in coal mines, petroleum refining, natural gas exploration, sewage treatments and automobile vehicle exhaust.16-25 According to security criterion proclaimed by American Conference of Government Industrial Hygienists, threshold limit value of H2S was stipulated as 10 ppm.18 Thus, developing efficient H2S gas sensor with high sensitivity, excellent selectivity and rapid response is essentially urgent. In past decades, metal oxide semiconductors (MOSs), such as SnO2, MoO3, ZnO, Fe2O3, NiO and Co3O4 .etc, have been widely researched as sensing materials due to their rich resource, low cost, and good integrated circuit compatibility.26-40 In spite of obvious advances in the applications of these oxides as sensing materials, some inherent shortcomings such as unsatisfied sensitivity and high operating temperature are still remained. For example, Chen et al. reported that CuO nanowire array sensor could detect 0.5-1000 ppm H2S at 160°C. However, when gas 3 ACS Paragon Plus Environment


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concentration exceeded 5 ppm, the response was unrecoverable.11 Li et al. manufactured MoO3 nanobelts with a response value of approximately 15 toward 20 ppm H2S at 170 °C; however the response rate was sluggish.28 Yang and co-workers reported the synthesis of self-assembly gridding α-MoO3 nanobelts which exhibited enhanced gas sensing performance at 327 °C.41 However, achievement of the detection of H2S at ppb level still remain a challenge. Recently, in order to maximize the sensitivity of the sensors to H2S, the following methods have been adopted. A common route to improve the gas sensor property is the surface modification by adding some noble metals as the catalyst into the metal oxide semiconductor, such as Pd, Pt, Ag and Au. For example, Lee et al. found that loading Pd on the Co3O4 surface could provide reaction areas for oxygen adsorption and combustion.42 The reason is in that the energy potential barriers of the gas adsorption and gas desorption could be decreased by the Pd at the surface. Meanwhile, as an efficient catalyst, Pd could also promote and accelerate the combustion reaction process. However, the large-applications of the noble metal are limited by their expensive costs and scarce reserves. Another route is constructing hetero-structures consisting of two or more kinds of metal oxides. On the one hand, the synergetic effect from different metal oxide in the hetero-structures would improve the sensing properties. On the other hand, the change of hetero-junction barrier generated between contacted interfaces under different atmospheres plays a crucial part in sensitive behavior of hetero-structures. To date, metal oxide nanocomposites such as SnO2/ZnO, Fe2O3/SnO2, SnO2/CuO and ZnO/Fe2O3 have been reported as sensing materials.43-48 4 ACS Paragon Plus Environment

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These nanocomposites have displayed enhanced sensing properties compared to their individual counterparts. For instance, Jiang and co-worker demonstrated the ZnO/Fe2O3 exhibited higher sensitivity compared to Fe2O3 and ZnO, respectively.47 Thus, the hetero-structures based on MOS materials are very promising application in gas sensors. In addition, reducing the size or making pore in the sensing materials could enhance their sensing performances.1,5 In this paper, a simple method is adopted to prepare porous MoO3/SnO2 NFs using graphene sheets (G) as templates. The porous MoO3/SnO2 NFs sensors exhibit an excellent H2S sensing performance at relatively low temperature. Meanwhile, the relative sensing mechanism of MoO3/SnO2 NFs is also discussed. EXPERIMENTAL SECTION Synthesis of porous SnO2 NFs. The G/SnO2 NFs were synthesized by a wet-chemical method.49 Typically, graphene sheets (15 mg) were dispersed in water (100 ml) under stirring for 10 minutes. After sonication for 20 minutes, SnCl2·2H2O (1.4 g) and HCl (1.4 mL) were added to the above suspension, respectively. Next, the mixture was maintained at room temperature for 36 h under continuous stirring. Black precipitates were washed and dried through a freeze-drying process. The obtained powder was thermally treated at 500oC for 2 h under air atmosphere. Synthesis of porous MoO3/SnO2 NFs. Porous MoO3/SnO2 NFs were synthesized using the graphene sheets as sacrificed templates, as illustrated in Scheme 1. G/SnO2 (40 mg) was dispersed in ethanol (40 mL). Next, 50 mg of molybdenyl 5 ACS Paragon Plus Environment


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acetylacetonate (C10H14MoO6) was added. At last, the mixture was stirred till the ethanol was completely evaporated. The obtained powder was calcined at 500 oC for 2 h in air. In addition, two additional samples were also prepared under the same conditions except the added amount of C10H14MoO6 is different. The samples obtained with added weights of C10H14MoO6 of 20, 50, and 80 mg were named as MoSn-S1, MoSn-S2, and MoSn-S3, respectively. Scheme 1. Illustration for the preparation process of MoO3/SnO2 NFs.

Characterizations. The crystal structure information of samples were collected by powder X-ray diffraction (XRD) (D/max 2550 V, Cu Kα radiation) and its scanning rate was 5° min-1. The morphology and size of samples were characterized by scanning electron microscope (HITACHI SU8000) and transmission electron microscope (FEI Tecnai-F20 equipped with a Gatan imaging filter). Nitrogen adsorption/desorption analysis was performed to 6 ACS Paragon Plus Environment

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analyze the surface areas of the samples (TRISTAR II3020). Surface chemical analysis of samples was carried out by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA System). Assembly of sensors. The sensors were assembled according to our previous method.18,49 Schematic diagram of device is described in Figure S1. Figure S1(a) illustrates the configuration of sensor. The corresponding equivalent circuit is displayed in Figure S1(b). Rl and Rs are denoted as a constant load resistor and resistance of material, while Vh represents a heating voltage.

Figure 1. XRD patterns of SnO2 (a), MoSn-S1 (b), MoSn-S2 (c), and MoSn-S3 (d). RESULTS AND DISCUSSION Structure characterization of samples. Figure 1(a) displays XRD pattern of SnO2 NFs. The diffraction peaks at 2θ of 26.61°, 33.89°, 37.94°, 51.78° and 65.93° correspond to (110), (101), (200), (211) and (301) lattice planes of SnO2 (JCPDs No. 41-1445), respectively. No other impurity peaks 7 ACS Paragon Plus Environment


are detected by XRD, indicating the high purity phase of the SnO2 NFs. Figure 1(b-d) show the XRD patterns of MoO3/SnO2 NFs with different content of MoO3. With increasing MoO3 concentration, the intensities of diffraction peaks from SnO2 obviously decrease and the full widths at half-maximum (FWHM) become slightly broad in comparison with the pure SnO2 NFs. According to the Scherrer equation, the crystal size in the samples can be calculated. The calculated average crystal sizes of the SnO2 particles in the SnO2, MoSn-S1, MoSn-S2 and MoSn-S3 NFs were approximately 8.5, 8.1, 7.7 and 7.3 nm using data of the (110) lattice plane, respectively (summarized in Table S1). This result implies that the introduction of MoO3 leads to the slightly decreased size of the SnO2 particle. The energy dispersive spectroscopy (EDS) patterns (Figure S2) indicates that Mo, Sn and O elements are existed in MoSn NFs, and the atomic ratios of Mo to Sn for MoSn-S1, MoSn-S2 and MoSn-S3 NFs are 1: 3, 1: 1.9 and 1: 1, respectively.

Figure 2. SEM images of SnO2 (a,b), MoSn-S1 (c,d), MoSn-S2 (e,f), and MoSn-S3 (g,h). The structures of the samples were investigated via SEM and TEM. Figure 2(a,b) displays that the SnO2 NFs are 1.5-3 μm in lateral size. MoSn-S1, MoSn-S2 and 8 ACS Paragon Plus Environment

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MoSn-S3 NFs have similar morphologies to that of the SnO2 NFs, as shown in Figure 2(c-h). However, the nanoflakes will suffer from agglomeration as the Sn/Mo ratio exceeds 1: 1 (Figure 2g,h). Figure 3(a,b) show TEM images of SnO2 NFs. It can be found that SnO2 NFs are composed of interconnected particles with porous feature. The interconnected particles have a diameter in range of 8-12 nm, matching well with XRD data. Figure 3(c) displays high-resolution (HR) TEM images of SnO2 NFs. The labeled lattice spacings are 0.334, 0.268 and 0.230 nm, corresponding to (110), (101) and (111) lattice planes of tetragonal SnO2, respectively. The lattice defects marked by the blue circles in Figure 3(c) appear at the interfaces between adjacent particles, suggesting the formation of the homo-junctions. Selected area electron diffraction (SAED) pattern clearly indicates crystalline nature of SnO2 NFs (Figure 3d). The labeled diffraction rings can be assigned to (110), (101) and (211) planes of tetragonal SnO2, respectively. The SEM, TEM, and SAED analyses demonstrate the porous SnO2 NFs are successfully fabricated.



Figure 3. TEM images of porous SnO2 NFs. (a) and (b) low-magnification TEM images, (c) HRTEM image. Besides, blue circles show lattice defects in the SnO2 NFs (d) corresponding SAED pattern.

Figure 4. (a) and (b) low-magnification TEM images (the inset image corresponding particle-size distribution diagram of MoSn-S2), (c) corresponding SAED pattern, (d) HRTEM images, and (e-h) EDS elemental mapping analysis of MoSn-S2. TEM images (Figure 4a and 4b) indicate that MoSn-S2 NFs have similar shape and porous feature to those of the SnO2 NFs. The average diameter of interconnected nanoparticles in MoSn-S2 NFs is approximately 7.8 nm, as revealed by the particle-size distribution diagram (the inset Figure 4b). The labeled diffraction rings correspond to (021) and (010) planes of MoO3, and (111) and (220) planes of SnO2, respectively, confirming coexistence of MoO3 and SnO2 in the porous MoSn-S2 NFs 10 ACS Paragon Plus Environment

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(Figure 4c). The lattice fringes are clearly identifiable in Figure 4(d), revealing the crystalline nature of porous MoSn-S2 NFs, consistent with the XRD and SAED results. The d-spacings with 0.334 and 0.264 nm marked in the HRTEM image match well with (110) and (101) planes of SnO2. Besides, the lattice fringes with interplanar spaces of 0.348 and 0.343 nm correspond to the (040) and (120) plane of orthorhombic MoO3, respectively. The lattice distortions (indicated by red circles) at the interfaces between the adjacent MoO3 and SnO2 nanoparticles can be observed, suggesting that the MoO3-SnO2 n-n junctions exist in the MoSn-S2 NFs. The EDX elemental mappings of individual SnMo-S2 NF indicate that Mo, Sn and O elements distribute uniformly throughout the NFs, which also confirms the possibility of formation of n-n junctions (Figure 4 e-h).

Figure 5. XPS spectra of the porous MoO3/SnO2 NFs. (a) Survey spectrum, (b) Sn 3d (c) Mo 3d, and (d) O 1 spectra. 11 ACS Paragon Plus Environment


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The XPS spectra were performed to study of the valence state and surface composition of the MoSn-S2 NFs. C 1s at 284.6 eV as intrinsic criterion was adopted to adjust the shift of binding energy. The XPS survey spectra indicate that the MoSn-S2 NFs contain Sn, Mo and O elements (Figure 5(a)). In Sn 3d XPS spectrum (Figure 5b), the peaks at 494.9 eV and 486.5 eV can be assigned to Sn 3d3/2 and Sn 3d5/2 binding energies of Sn4+ species, respectively.16,22,50 Mo 3d coupled peaks at 232.5 and 235.6 eV with an binding energy separation of 3.1 eV (Figure 5c) reveal that the valence state of Mo is 6+. 18,51 In the O 1s spectrum (Figure 5(d)), the binding energies at 530.4 and 531.5 eV are assigned to lattice oxygen and oxygen vacancies or defects (Ovac), respectively.52,53 The SEM and TEM images indicate that the SnO2 and MoSn NFs possess porous features, suggesting that they have large specific surface area. To investigate their porosity and specific surface area, the N2 adsorption-desorption isotherms and pore size distribution analysis were carried out. As shown in Figure S3(a-d), the Brunauer-Emmett-Teller (BET) surface areas are calculated to be 74.11, 98.14, 129.07 and 32.28 m2 g-1 for the SnO2, MoSn-S1, MoSn-S2 and MoSn-S3 NFs, respectively. The great reduced surface area of the MoSn-S3 NFs is due to the serious agglomeration among the nanoflakes (Figure 2g,h). The corresponding pore size distribution curves (the insets in Figure S3) indicate that main pore sizes are 5.2, 7.2, 8.6, and 10.0 nm for the SnO2, MoSn-S1, MoSn-S2, and MoSn-S3 NFs, respectively. In terms of desorption isotherms, cumulative pore volume for the MoSn-S2 NFs is 0.41 cm3 g-1, greatly larger than that of SnO2 NFs (0.15 cm3 g-1), MoSn-S1NFs (0.21 12 ACS Paragon Plus Environment

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cm3 g-1) and MoSn-S3 NFs (0.09 cm3 g-1). Large specific surface area and pore volume facilitate the enhancement of MoSn-S2 NFs in sensing performance.

Figure 6. (a) Sensor responses of as-prepared samples as a function of different operating temperatures to 10 ppm H2S concentration, (b) dynamic response and recovery curves of SnO2, MoSn-S1, MoSn-S2 and MoSn-S3 toward changed concentrations of H2S at 115oC, respectively, (c) typical sensor responses of SnO2, MoSn-S1, MoSn-S2 and MoSn-S3 toward 100 ppb and 500 ppb H2S gas at optimal working temperature, and (d) response and recovery times curve of MoSn-S2 NFs to 10 ppm H2S at 115oC. H2S properties of samples. The sensor responses of all samples to 10 ppm H2S were measured at different temperatures (Figure 6(a)). The sensor response value for the SnO2 NFs sluggishly 13 ACS Paragon Plus Environment


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increases at temperature below 145oC, and then decreases with further increase of temperature. For the MoSn NFs, the response value increases at temperature below 115oC, and then gradually decreases with further increase of temperature. Thus, the optimal working temperatures for the SnO2 NFs and MoSn NFs are determined to be 145oC and 115oC, respectively. Thus, the MoSn NFs have a lower optimal working temperature than the SnO2 NFs. Furthermore, the sensor responses of the MoSn-S1, MoSn-S2 and MoSn-S3 at 115oC are 16.2, 43.5 and 31.8, respectively, while only 9.1 for the SnO2 NFs at 145oC. Based on the results above, the MoSn NFs exhibit enhanced H2S sensing property in comparison to the SnO2 NFs to 10 ppm, including reduced optimal work temperature and increased sensor response value. To further assess the sensing property, the responses of the samples to H2S with different concentration were examined at their optimal temperature. In Figure 6(b), it can be found that the larger concentration of H2S is applied, the higher response of the sensor has for all the samples. Remarkably, MoSn-S2 NFs show superior H2S sensing property to other three samples in the H2S concentration from 1 to 50 ppm. Typically, the sensor response for the MoSn-S2 NFs to 10 ppm is 43.5, approximately 4.8, 2.7 and 1.4 times higher than that of SnO2 NFs, MoSn-S1 NFs and MoSn-S3 NFs, respectively. Therefore, the optimal Sn/Mo ratio for the MoO3/SnO2 nanoflakes as H2S sensing materials is 1.9: 1. Furthermore, the MoSn-S2 also displays linear relationships with the H2S concentration in the region of 1-50 ppm (Figure S4), which facilitates its practical applications. Meanwhile, the detection limit is also significant indicator. As shown in Figure 6(c), the sensor response of MoSn-S2 to 100 ppb H2S 14 ACS Paragon Plus Environment

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can also reach 2.2; however, the SnO2 NFs have almost not responses. Compared to the other reported metal oxides, the MoSn-S2 NFs exhibit enhanced H2S sensing property (Table S2). In addition, MoSn-S2 NFs have fast response and recovery times toward H2S gas. For instance, response and recovery times of MoSn-S2 NFs toward 10 ppm H2S are about 22 s and 10 s, respectively (Figure 6d).

Figure 7. (a) Reproducibility of MoSn-S2 sensor upon exposure to 1 ppm H2S at 115oC, (b) the corresponding change of resistance upon 1 ppm H2S, (c) the response of four samples to 10 ppm of various test gases, and (d) long-term stability of MoSn-S2 to10 ppm H2S. As for practical applications, the sensors are not only needed to have high sensitivity and rapid response rate toward target gases, but also satisfying 15 ACS Paragon Plus Environment


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reproducibility, excellent selectivity and good stability. In order to explore the reliability, the sensing performance of the MoSn-S2 toward 1 ppm H2S at 115oC was also investigated. As depicted in Figure 7(a), after 6 cycles testing the MoSn-S2 still holds the almost same response. The corresponding resistances change of the MoSn-S2 sensor is displayed in Figure 7(b). Apparently, the resistances change displays a decreased trend upon exposure to H2S, demonstrating a typical n-type semiconductors characteristics of the MoSn-S2. Notably, SEM image of the MoSn-S2 after 6 cycles testing indicates that the morphology of flakes were still maintained (Figure S5), revealing the structural stability of the MoSn-S2 as H2S sensing materials. To study the selectivity, the sensing properties of porous MoSn-S2 NFs to ethanol, hydrogen, ammonia, methanol, acetone and nitrogen dioxide (concentration: 10 ppm) were tested. The corresponding response values are displayed in Figure 7(c). It is clearly found that the MoSn-S2 NFs have larger response values to H2S than the other tested gases. Thus, the MoSn-S2 NFs possess excellent selectivity for H2S. The reasons for the selectivity are relatively complicated and may be related to the follow factors. i) Because the kinetics of the reaction occurred at the surface of the sensing materials with the different targeted gases are quite different, leading to the different response values of the sensing materials to different gases.6, 11, 49 ii) The metal oxides may react with H2S at 115oC, resulting in the formation of metal sulfide on the surface. The conductivity of metal sulfide is often higher than that of the corresponding metal oxide.49 Therefore, the MoO3/SnO2 nanoflakes with n-n junctions exhibited good selectivity for H2S. In order to verify this assumption, XPS spectra of MoSn-S2 16 ACS Paragon Plus Environment

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before and after the H2S exposure (the sensors were exposed to 10 ppm H2S at 115 oC for 15 min, and the XPS spectra were carried out). The peaks related to S species were not found in the XPS spectrum of the pristine MoSn-S2 (Figure S6a); however, a weak peak located at 162.1 eV appeared after the MoSn-S2 was exposed to H2S, which corresponds to the S(II) species (Figure S6b). The content of the S species was estimated to be about 1.63 at%. In addition, a small amount of S species (MoSn-S2) was detected by the EDS measurements after the MoSn-S2 was exposed to H2S, as shown in Figure S7. Thus, the transformation of metal oxide to metal sulfide is attributed to the good selectivity of MoO3/SnO2 nanoflakes. Furthermore, the peak positions of the Sn and Mo species changed little before and after the H2S-sensing process, indicative of the high stability of this nanomaterial (Figure S6c-f). As previously reported, the formation of metal sulfide usually leads to the saturation of the sensor response.54 To clarify if the saturation behavior occurs in the MoO3/SnO2 nanoflakes, we measured the sensor response of the MoSn-S2 to 100 – 200 ppm H2S. As shown in Figure S8, the sensor response values are 351, 400, and 405 for 100, 150, and 200 ppm H2S, implying the saturation behavior occurs as the H2S concentration exceeds 150 ppm. The long-term stability of the gas sensor was investigated at 115oC. As shown in Figure 7(d), the porous MoSn-S2 NFs-based sensors exhibit a nearly constant response toward 10 ppm H2S for 30 days detection. In addition, the effect of humidity on the sensing property of the MoSn-S2 NFs was investigated. As shown in Figure S9, when the relative humidity reaches to 50%, the response values of the MoSn-S2 toward H2S are reduced to some extent. Nevertheless, the MoSn-S2 can still 17 ACS Paragon Plus Environment


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detect 1 ppm H2S. Based on the above results, porous MoSn-S2 NFs have very promising application in H2S sensor.

Figure 8. Diagram of energy band structure of MoSn-nanocomposites in (a) air and (b) H2S. The sensing mechanism of the MoO3/SnO2 nanoflakes. As shown in Figure 7(b), the MoSn-S2 NFs show a typical n-type semiconductor characteristic43,47. In general, when a n-type metal oxide semiconductor is exposed to air, oxygen molecules will be adsorbed onto the surfaces of sensing material and ionized into species such as O2–, O– and O2–

by capturing electrons from its

conductive band (Eqs. 1-4).55 O2 (gas) ↔ O2 (ads)

(1)

O2 (ads) + e– ↔ O2– (ads)

(2)

O2– (ads) + e–↔ 2O– (ads)

(3) 18

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O– (ads) + e– ↔ O2– (ads)

(4)

Upon H2S exposure at an appropriate temperature, the surface oxygen species will react with H2S, and then release the electrons back to conductive band. Consequently, the resistance will reduce, dependent on H2S concentration (Eqs.5, 6), H2S (gas) ↔ H2S (ads) H2S + 3O– (ads) ↔ H2O + SO2 + 3e–

(5) (6)

Based on the above sensing mechanism, the reasons for enhanced sensing property of the MoO3/SnO2 NFs can be explained by the following factors. Firstly, the formation of nanoscale n-n junctions at the interfaces between MoO3 and SnO2 has positive effect on the sensing performance. MoO3 and SnO2 are n-type semiconductors with band gap of 3.3 eV and 3.6 eV, respectively.16,22,33 Because the MoO3 with work function of 5.3 eV is larger than SnO2 (4.7 eV), the part electrons transfer from SnO2 to MoO3.22 Consequently, energy band of MoO3/SnO2 hetero-junctions can be schematically depicted in Figure 8(a). φeff represents the effective barrier height. The conductivity (G) of heterostructures under changing atmosphere can be given by, G=G0 exp(-qφeff./kBT)

(7)

where G0 can be considered as a constant parameter, q represents charge of an electron, kB is Boltzmann’s constant, and T is absolute temperature. Under air surrounding, owing to the loss of electrons, the surface band of both MoO3 and SnO2 bends upward, which will lead to the increase of φeff. Under H2S surrounding, in view of the reactions between the ionized oxygen species and targeted gas, the captured electrons can be free back to conduction band of MoO3 and SnO2, resulting in the 19 ACS Paragon Plus Environment


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decrease of the height of potential barrier at their interfaces.56,57 Therefore, the change of the effective barrier additionally contributes to the enhanced H2S sensing performance of the MoO3/SnO2 NFs. To verify the contribution of the n-n junction barrier to the sensing performance, we mixed the pure SnO2 NFs physically with the pure MoO3 nanoparticles with the same ratio Mo to Sn (1: 1.9) as that in the porous MoSn-S2 NFs. As shown in Figure S10, the MoO3 only distributes on the surface of SnO2 NFs via physically mixed method. In the case, it is difficult to form n-n junctions in the physical mixture due to the absence of the tightly contacted interfaces between MoO3 and SnO2. As seen in Figure S11, the physical mixture shows greatly inferior H2S sensing property to that of the porous MoSn-S2 NFs. For example, the sensor response of the physical mixture toward 5 ppm H2S is 9.9, 2 times lower than that of the porous MoSn-S2 NFs. Thus, the formation of n-n junctions plays a crucial role in enhanced sensing property of MoSn-S2 NFs. Second, the BET surface area of the MoSn-S2 NFs is approximately 1.8 times larger than that of the SnO2 NFs. This means that more gas molecules can take part in surface reactions according to space-charge model, leading to larger change in the resistance when the MoSn-S2 NFs are exposed to different gas atmosphere. Third, the MoSn-S2 NFs have bigger pore volume than that of the SnO2 NFs. The bigger pore volume favors in the faster gas diffusion, facilitating the improvement of H2S sensing performance. As for the inferior H2S sensing properties of the MoSn-S1 and MoSn-S3 NFs to the MoSn-S2 NFs, it may be due to their smaller BET surface areas and pore volume.


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CONCLUSIONS In summary, the porous MoO3/SnO2 NFs composed of interconnected nanoparticles are fabricated through a mild route. The MoO3/SnO2 NFs display strengthened H2S response in comparison to SnO2 NFs. Furthermore, the detection limit of MoO3/SnO2 NFs for H2S is 100 ppb at 115oC. The enhancement in the H2S sensing property of the MoO3/SnO2 NFs can be explained by the existence of n-n junctions at the interfaces, large surface area and big pore volume. Our experimental results indicate that porous MoO3/SnO2 NFs are potential candidates for high-performance H2S sensing materials. ASSOCIATED CONTENT Supporting Information Available: Figure S1-Figure S11 and Table S1-Table S2. The schematic illustrations of sensor configuration; the EDS pattern of the as-prepared porous MoSn-S1, MoSn-S2 and MoSn-S3; Nitrogen adsorption-desorption isotherms of (a) SnO2 (b) MoSn-S1, (c) MoSn-S2, and (d) MoSn-S3; sensor response of MoSn-S2 as a function of H2S gas concentration at 115oC; the SEM images of MoSn-S2 NFs after several sensing cycles; XPS spectra of the MoSn-S2 before and after H2S sensing test; Dynamic response and recovery curves of MoSn-S2 sensor toward changed concentrations of H2S at 115oC under 50% relative humidity, respectively; the SEM images of the physically mixed MoO3/SnO2; dynamic response-recovery behaviors of the physically mixed MoO3/SnO2 sensor toward various concentrations of H2S at 115 oC; detailed comparison of the sensing property of MoSn-S1, MoSn-S2 and MoSn-S3 to others.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No. 51572051), the Natural Science Foundation of Heilongjiang Province (E2016023), the Fundamental Research Funds for the Central Universities (HEUCF201708), and also the Open Project Program (PEBM 201703 and PEBM201704) of the Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China. REFERENCES (1)Mirzaei, A.; Kim, S. S.; Kim, H. H. Resistance-Based H2S Gas Sensors Using Metal Oxide Nanostructures: A Review of Recent Advances. J. Hazard. Mater. 2018, 357, 314–331. (2) Bao, M.; Chen, Y. J.; Li, F.; Ma, J. M.; Lv, T.; Tang, Y. J.; Chen, L. B.; Xu, Z.; Wang, T. H. Plate-like P–N Heterogeneous NiO/WO3 Nanocomposites for High Performance Room Temperature NO2 Sensors. Nanoscale 2014, 6, 4063–4066. (3) Keshtkar, S.; Rashidi, A.; Kooti, M.; Askarieh, M.; Pourhashem, S.; Ghasemy, E.;


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Table of Contents Graphic and Synopsis

Porous MoO3/SnO2 nanoflakes with n-n junctions exhibited good selectivity and robust long-term stability and relatively rapid response time toward H2S.


SnO2 Nanoflakes with n-n Junctions for Sensing H2S

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