Large-scale synthesis of hierarchically porous ZnO hollow tubule for

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Functional Inorganic Materials and Devices

Large-scale synthesis of hierarchically porous ZnO hollow tubule for fast response to ppb-level H2S gas Hui-Bing Na, Xian-Fa Zhang, Zhaopeng Deng, Ying-Ming Xu, Li-Hua Huo, and Shan Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00173 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Applied Materials & Interfaces

Large-scale synthesis of hierarchically porous ZnO hollow tubule for fast response to ppb-level H2S gas Hui-Bing Na, Xian-Fa Zhang, Zhao-Peng Deng,* Ying-Ming Xu, Li-Hua Huo, and Shan Gao* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China

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ABSTRACT: Response and recovery time to toxic and inflammable hydrogen sulfide (H2S) gas is an important index for metal oxide sensors in real-time environmental monitoring. However, large-scale production of ZnO-based sensing materials towards fast response to ppb-level H2S has been rarely reported. In this work, hierarchically porous hexagonal ZnO hollow tubule was simply fabricated by zinc salt impregnation and subsequently calcination using absorbent cotton as template. The influence of the calcination temperature on the corresponding morphology and sensing properties is also explored. The hollow tubules calcined at 600 ℃ are constructed from abundant cross-linked nanoparticles (~20 nm). Its BET surface area is 31 m2·g-1 and the meso- and macro-porous sizes are centered at 35 and 115 nm. The sensor with a lower detection limit of 10 ppb exhibits fast response speed of 29 s towards 50 ppb H2S rather than those of the reported intrinsic and doped ZnO-based sensing materials. Furthermore, the sensor shows a wide linear range (10-1000 ppb), good reproducibility and stability. Such excellent trace ppb-level H2S performances are mainly related to the inherent characteristics of hierarchically porous hollow tubular structure and the surface adsorbed oxygen controlled type mechanism. KEYWORDS: ZnO, natural template, H2S, hierarchical pore, hollow tubule


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1. INTRODUCTION Semiconductor metal oxide (SMO) sensors with portable convenience, low cost and fast response have played an important role in the protection of environment and human health due to their detection of toxic and inflammable gases.1 Since the commercially available gas sensors of SnO2 in 1968,2 the important n-type semiconductor ZnO (Eg = 3.37 eV) has been one of the most potential applications of gas sensing materials owing to its rich resource, low price, non-toxicity, high conductivity and good stability. Up to now, nanostructural intrinsic and doped ZnO-based sensors with various shapes and morphologies have shown excellent performances in the detection for reduction and oxidation gases, such as alcohol,3,4 H2S,5,6 NO2,7,8 NH3,9,10 CO,11 H2,12,13 etc. Among the detection of these gases, hydrogen sulfide (H2S), as a well-known corrosive, flammable, and poisonous colorless gas, can paralyze the nerves and cause tissue hypoxia and even death.14,15 The acceptable ambient levels of H2S are 20-100 ppb.16 Meanwhile, in recent years, trace ppb-level H2S can also be utilized as biomarker in early diagnosis of lung diseases like asthma17 and has the therapeutic capabilities of potential anti-cancer agents18 and anti-inflammatory drugs.19 Accordingly, the study on the fabrication of fast and highly sensitive ZnO-based sensing materials to detect ppb-level H2S is very urgent. Recently, pure ZnO-based sensors with 1D nanowire5,22 and hierarchically porous micro/nanostructures (nanosphere,21 cross-linked nanosheet20) have been realized to detect 5-50 ppb H2S at the working temperature of 150-300 ℃ (Table 1). For example, the sensor based on 20 nm ZnO nanowires through hydrothermal method reported by Chen et al in 2017, could detect the H2S as low as 5 ppb, but the working temperature was high up to 300 ℃ with long response and recovery times of 241 and 144 s to 50 ppb.22 To the best of our knowledge, up to now, only both of sensors based on porous CuO-ZnO nanofilms at 225 ℃ in 201423 and cross-linked porous ZnO nanosheets at 200 ℃ in 201520 have proved to be fast response time of 35 s to 500 ppb H2S.



However, the cross-linked porous ZnO-based sensor by in-situ wet chemical reaction exhibited very long recovery time of 196 s, and the authors also admitted that the contact between ZnO nanosheets and Au electrodes played a certain role on the fast response towards trace H2S.20 These reported results also illustrate that the hierarchically porous structure is beneficial to improve electron transfer, absorb and diffuse of H2S gas molecules and oxygen species, thus generating the aforementioned better gas sensing properties. Unfortunately, it can be seen from Table 1 that the response time and recovery time in most reported ZnO-based and composites sensors towards trace ppb-level H2S is larger than 100 s. And also, fine controlling production for the sizes of these aforementioned nanowires/nanosheets is difficult and complicated in practical production. Therefore, simple, economical and large-scale fabrication of hierarchically porous ZnO micro/nanostructure with fast response and high sensitivity to trace ppb-level H2S is challenging work. Natural biomaterials provide unique hierarchical structure with multiple sizes ranging from nanoscale to macroscale due to the development and optimization through a long-term evolutionary process.29 Nowadays, hierarchically porous SMOs prepared by biomass template method have been widely used in gas sensors.30,31 For example, in 2014, Zhang et al. fabricated porous SnO2 microtubules by utilizing butterfly wings as biotemplate, which showed fast response time of 2 s to ammonia (R. T.).30 In 2016, Zhang et al. synthesized folded hierarchical SnO2 micro/nanostructure by using pomelo peel as template, which exhibited the detection limit of 100 ppb at 200 ℃ towards formaldehyde.31 However, there is only one report on H2S detection based on the ZnO based gas sensor that synthesized from biomass template. In 2009, Liu et al. prepared hierarchically porous ZnO materials by using wood as template, which presented the sensitivity of 200 to 50 ppm (only testing this concentration) H2S at high working temperature of 332 ℃.32 Furthermore, considering the selective detection of ppb-level H2S in practical applications of environmental monitoring and disease diagnosis, multifunctional ZnO has been recognized as the next-generation sensing materials. Therefore, simple and large-scale production of hierarchically porous ZnO micro/nanostructure by utilizing economical biomass as template and


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enhancing its fast response and high sensitivity to trace ppb-level H2S has important practical meanings. In this paper, hierarchically porous ZnO hollow tubule was successfully synthesized by using cheap and friendly absorbent cotton as template. The sensor based on ZnO calcined at 600 ℃ has a low detection limit of 10 ppb, and exhibits fast response time of 29 s towards 50 ppb H2S. Meanwhile, this sensor also shows good linear relationship, reproducibility and stability. The hierarchically porous ZnO hollow tubule could act as an excellent candidate of sensing material for fast detecting trace ppb-level H2S in practical application. 2. EXPERIMENTAL SECTION 2.1 Synthesis and Characterization. Absorbent cotton (AC) was obtained from Xuzhou Lierkang sanitary materials Co. Ltd, China. Analytical-grade zinc nitrate hexahydrate (CAS number: 10196-18-6) was purchased from Tianjin Kermel Chemical Group Co. Ltd. All the other reagents were directly used. Deionized water (18.2KΩ) was obtained from laboratory manufacture (PL5122). Absorbent cotton was washed with deionized water for some times before use. Bio-template synthesis process of hierarchically porous ZnO hollow tubule was shown in Scheme S1. In a typical synthesis, the absorbent cotton (2 g) was immersed in 100 mL of 0.10 mol·L-1 Zn(NO3)2 solution at room temperature for 6 h. After drying at 60 ℃ for 12 h (named Zn2+-AC), it was then placed in a crucible, and calcined in air atmosphere at 500, 600 and 700 ℃ for 2 h, respectively. Finally, the white products of ZnO were obtained, and named as ZnO-500, ZnO-600 and ZnO-700 for clarity. The crystalline phase, microstructure, nitrogen adsorption-desorption analysis, as well as the surface chemistry of the products were carried out according to our previous report (Supporting Information).33 2.2. Gas sensor fabrication and measurement. The thick film gas sensor was fabricated by a similar method as reported.33 The gas sensing properties of the ZnO-based sensor were tested with the JF02E measurement system (Kunming Guizhou Metallurgical Technology Co., Ltd, China) using a static test method. The testing gases used in this work contain ethanol, acetone, formaldehyde, aniline, triethylamine, chlorobenzene, ammonia, hydrogen, nitrogen dioxide and carbon



monoxide (Supporting Information). 3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The absorbent cotton is composed of polysaccharide cellulose after degreasing. As shown in Figure S1, the absence of characteristic Zn-O vibration reveals that Zn2+ ions interact with hydroxyl group and carboxyl group in cellulose and adhere to the surface of absorbent cotton fiber in the process of soaking absorbent cotton with Zn(NO3)2 solution. After the pyrolysis of absorbent cotton template in air atmosphere, the appearance of the strong absorption peak at 438 cm-1 indicates the formation of hexagonal ZnO. The existence of adsorbed O-H could modulate the defect of ZnO surface and is beneficial to performance of the resistive SMO sensor.34 In addition, in order to investigate the effect of calcination temperature on the microstructure, crystallinity and gas-sensing properties of pure ZnO, the Zn2+-AC samples are calcined at 500, 600 and 700 ℃, and the obtained samples are recorded as ZnO-500, ZnO-600 and ZnO-700, respectively. The XRD patterns of the products calcined at different temperatures are shown in Figure 1a. The diffraction peaks for three products are completely consistent with those of hexagonal ZnO (JCPDS Card No.36-1451) and become sharp and strong with the increase of calcination temperature, which indicates more complete crystallinity and increasing grain size of ZnO-700. These results may decrease the surface energy and the defects on the surface of ZnO-700, and also may not conducive to the improvement of the gas response to H2S.35 The full survey XPS spectrum of ZnO-600 in Figure 1b indicates that the product contains Zn and O elements with the peaks located at 530.1 eV (O 1s), 1044.3 eV (Zn 2p1/2) and 1021.1 eV (Zn 2p3/2), further proving that the calcined product is pure hexagonal ZnO. As shown in the low-magnification SEM images (Figure S2), the morphologies of ZnO products retain the original fiber microstructure of absorbent cotton, and with the increase of calcination temperature, the diameter of the fiber becomes gradually thinner. From the SEM images of the single ZnO fiber (Figure 2a,c,e), the hollow tubular structure is composed of a large amount of cross-linked nanoparticles with small pores on the surface. The inner diameter of the hollow tubule is 2-3 μm and the


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wall thickness is 0.6-1.7 μm. Furthermore, with the increase of calcination temperature, both the inner diameter and the wall thickness of the tubule gradually decrease. These hollow tubular features are beneficial to the transportation of target gas molecules. High-magnification image of the ZnO hollow tubule (Figure 2b) shows that the surface microstructure of ZnO-500 is ruffled with unclear small nanoparticles, indicating its poor crystallinity. This result is also demonstrated by the weak and border diffraction peaks in the corresponding XRD pattern (Figure 1a). When the calcination temperature is raised up to 700 ℃, the surface microstructure of ZnO-700 becomes smooth with large accumulated nanoparticles and less pore distribution (Figure 2f). In comparison, ZnO-600 shows clearly cross-linked nanoparticles and abundant pore distributions on the surface (Figure 2d). The structural features of ZnO-600 were further confirmed by TEM observations. As shown in Figure 2g, the low-magnification TEM image exhibits hollow tubular structure, which is composed of irregular cross-linked nanoparticles with average size of ~20 nm (Figure 2h). Meanwhile, multistage pores formed by the cross-linkage of nanoparticles are observed on the surface of the tubule. Such porous characteristics are beneficial to the contacts between the surface of nanoparticles and H2S gas, and allow the diffusion of more target gas molecules into or out of the tubules, which improve the sensitivity and shorten the response time of the sensors.20 High-magnification TEM image in Figure 2i shows the lattice fringes of 0.248 and 0.261 nm, corresponding to the (101) and (002) crystalline planes of hexagonal ZnO. The marked diffraction rings in the selected-area electron diffraction (SAED) pattern (inset in Figure 2i) correspond to the (100), (002), (101) and (102) crystalline planes of polycrystalline ZnO with a hexagonal structure. The specific surface area of hierarchically porous ZnO-600 hollow tubule is 31 m2·g-1 (Figure 3), which is obviously larger than those of the reported sphere-like, cauliflower-like and sisal-like ZnO.21 The relative larger specific surface area provides more active sites to adsorb H2S and oxygen species. The type IV isotherm with a H3 type hysteresis loop indicates the existence of pores in the material, which are mainly from the cross-linkage of nanoparticles. The meso- and macro-pore size distribution curve also



shows that the pore size of ZnO-600 is centered at 35 and 115 nm, most of which fall in the range of 100-150 nm. Therefore, according to the aforementioned structural features, it can be predicted that the inherent characteristics of proper crystallinity, relative small nanoparticles, unique hollow porous tubule and relative large specific surface area of ZnO-600 will be helpful to the adsorption and diffusion of the target gases and oxygen species, can further improve the fast response to trace H2S gas. 3.2. Gas sensing performance. The gas sensing performances of ZnO-based sensors to 10 ppm H2S at different heating temperature are shown in Figure 4a, in which all the three devices show high sensitivity to H2S at 217 ℃, and the sensitivities of ZnO-500, ZnO-600 and ZnO-700 are 50.91, 85.04 and 23.15, respectively. The relative low sensitivities for ZnO-500 and ZnO-700 originate from the poor crystallinity of the former and large aggregate nanoparticles and fewer defects caused by the perfect crystallinity of the latter. In contrast, the high response of ZnO-600 may be related to the proper crystallinity with suitable size of nanoparticles (~20 nm), large specific surface area and abundant hierarchically porous structure. It should be pointed that the resistance values of the three sensors are out of the measured range of the instrument below 217 oC. With the increase of working temperature, the sensitivities of the three devices gradually decrease. These are mainly attributed to the easily desorption of the H2S gas molecules and oxygen species from the surface of sensing materials before the redox reaction at high working temperature. Moreover, after 250 ℃, the sensitivity of ZnO-600 is slightly smaller than that of ZnO-500, which may cause by the accumulation of nanoparticles as increasing temperature. Since ZnO-600 presents high sensitivity to H2S gas at optimum temperature of 217 ℃, therefore, the following experiments are focused on the gas sensing performances of ZnO-600. The sensitive histogram of ZnO-600 sensor towards 10 ppm eleven different target gases at 217 ℃ is shown in Figure 4b. The responses to the above gases are 4.49, 1.42, 1.69, 2.10, 1.54, 1.56, 2.08, 1.04, 1.16, 1.07 and 85.04, respectively. The selective coefficients (KAB) of the H2S gas to the other gases are 18.9, 59.73, 50.3, 40.5, 55.2, 54.5, 40.9, 81.7, 73.3 and 79.5, respectively, the largest relative standard deviation (RSD) is less than 3.95%, showing good selectivity


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towards H2S and nearly no response to residual gases. This is mainly ascribed to the fact that the bond energy of H-SH in H2S molecule (381 kJ/mol) is smaller than those of other testing gases,32 resulting in the easily broken of the H-SH bond to react with surface adsorbed oxygen species. Therefore, the ZnO-600 sensor has excellent sensitivity and selectivity towards H2S at 217 ℃. As shown in Figure 5a, with the increasing concentration of H2S, the gas responses of the sensor increase in a good linear relationship (R2 = 0.994) for a concentration ranging from 10 to 1000 ppb and the largest RSD is less than 3.52%, indicating that the ZnO-600 sensor possesses a good linear relationship with the detection limit of 10 ppb (1.30). As shown in Table 1, the present detection limit is the same as those (10 ppb) of cross-linked ZnO nanosheets,20 CuO/ZnO nanospheres25 and net-like SnO2/ZnO.6 In contrast to the assistance of contact between ZnO nanosheet and Au electrodes for cross-linked ZnO nanosheets and heterojunctions in the two composites, the low detection limit of ZnO-600 originates from its inherent characteristics of hierarchically hollow porous tubule. Although the detection limit of ZnO-600 is slightly higher than that (5 ppb) of 20 nm ZnO nanowires at 300 ℃ reported by Chen et al,22 the present sensor exhibits fast response to ppb-level H2S in very short time at 217 ℃. It is well known that the response time and recovery time are two important parameters in evaluating the gas sensing performance of sensor. Therefore, the dynamic response-recovery curves are measured. As illustrated in Figure 5b and c, the response times of ZnO-600 are 61 and 29 s to 10 and 50 ppb H2S at 217 ℃, respectively. Although the response time for 50 ppb is faster than that of 10 ppb H2S, the relatively low gas concentration results in short recovery time (59 s), while high gas concentration needs longer time (98 s) to recover. It should be pointed that the fast response time and recovery time of ZnO-600 in ppb-level H2S concentrations are obviously shorter than most of reported pure ZnO and ZnO-based composite sensors listed in Table 1. In addition, as shown in Figure S3, the sensitivity of ZnO-600 sensor towards 10 ppm H2S is 85.04 with significantly shorter response time of 8 s. Furthermore, the gas responses of ZnO-600 sensor also exhibit a good linear range from 10 ppb to 50 ppm (Figure S3).



Figure 5d is the reproducibility test to 10 ppm H2S at 217 ℃ by ZnO-600 sensor. The gas sensor still retains the initial response amplitude after 5 cycles, indicating satisfactory reproducibility of the sensor towards H2S. After the first test, the response of the sensor to 10 ppm H2S gas was carried out every 10 days to investigate the long-term stability of the sensor (Figure S4a). The standard error of the sensor after 60 days is 2.36%, which indicates the sensor has good long-term stability. In order to further prove the stability of the sensor, we carried out the TEM and XPS characterizations of the sample on the Al2O3 tube after the stability tests with 1 ppm H2S. As shown in Figure 6, the above sample maintains the tubular morphology and polycrystalline ZnO with a hexagonal structure. The morphology after the stability tests is accordance with that of the pristine ZnO-600. XPS spectrum of the sample after the stability tests was illustrated in Figure S5, in which only Zn and O elements are observed and their binding energies are consistent with the pristine ZnO-600. It should be indicated that the percentage of surface adsorbed oxygen (25.83%) is nearly same as that of pristine ZnO-600 (25.66%, Figure 7a). These results ensure the reproducibility and long-term stability of the ZnO-600 sensor. In addition, the sensor response values of different humidity change from 1.4 to 1.6 in (Figure S4b). That is, the baseline resistance of 651 MΩ has minor changes from 407 to 465 MΩ, which indicates that the influence of relative humidity on the sensor is almost neglected at 217 ℃. 3.3. Gas sensing mechanism. As is known to all, SMO-based gas sensors have been usually operated by the surface controlled type sensing mechanism, that is, the reaction between the target reductive gases and ionized oxygen species on the surface of SMOs causes the decrease of resistance.36 For ZnO-based H2S sensor, in 2011 and 2015, Zheng et al.5 and Yong et al.37 proposed a new sensing mechanism of sulfuration-desulfuration reaction: ZnO(s) ↔ ZnS(s), in which the formation of metastable ZnS intermediate and its desulfuration back to ZnO can enhance the response to H2S, but obviously increase the recovery time.22 In order to better understand the fast response and recovery of ZnO-600 to trace ppb-level H2S gas, PXRD pattern and of XPS spectrum the samples after contacting


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with certain concentration of H2S at 217 ℃ were carried out (Figure S6 and S7). The diffraction peaks of ZnS are not observed in the PXRD pattern (Figure S6) even at the concentration of 100 ppm H2S, however, it can still not exclude the existence of trace amount of ZnS on the surface of sensing materials. The full survey of XPS spectrum (1 ppm, Figure S7) shows that only Zn and O elements are observed. In contrast, additional S element is detected in the XPS spectrum (50 ppm), and the mass percentage of S element is 4.01 %. The high S mass % may indicate the formation of small amount of metastable ZnS intermediate through sulfuration reaction despite of the Zn 2p being no change (Figure S8). This is also demonstrated by the short response time and long recovery time of ZnO-600 sensor towards ppm-level H2S (Figure S3b).5,37 As shown in Figure 7, the O 1s of ZnO-600 could be deconvoluted into three peaks at 532.6/532.3 (1 ppm)/532.4 (50 ppm) eV, 531.4/531.2 (1 ppm)/531.3 (50 ppm) and 530.0/530.0 (1 ppm)/530.1 (50 ppm) eV. These peaks can be designated as O(HO-), O2(ad) and O2-, respectively. The percentage of surface adsorbed oxygen decreases from 25.66%, 21.23% to 19.04% after contacting with the 1 and 50 ppm H2S, respectively. These indicate the reactions between (ppb~ppm)-level H2S molecules and the surface adsorbed oxygen species. According to the references,38 the oxygen species on the surface of ZnO-600 are atomic ion Oduring the temperature range of 100-300 °C. Compared with the molecular ion O2-, the high activity of atomic ion O- can promote the fast response of ZnO-600 to H2S gas. Furthermore, the only one peak at 163.3 eV corresponding to SO2 is observed in the XPS spectrum of ZnO-600 contacting with 1 ppm H2S (Figure 7d). These results reveal that the surface adsorbed oxygen controlled type sensing mechanism play a dominant role in determining the fast response of ZnO-600 towards ppb-level H2S. However, after being exposed to 50 ppm H2S at 217 °C, the high-resolution XPS spectrum of S 2p could be deconvoluted into two peaks (Figure 7e), involving the peaks at 164.4 and 162.2 eV. The former peak corresponds to SO2 and the latter peak is assigned to the sulfides (generated trace ZnS),5 respectively. Therefore, the appearance of the two S 2p peaks not only indicates that H2S is oxidized to SO2, but also implies the occurrence of the sulfuration-desulfuration reaction: ZnO(s) ↔



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ZnS(s). These results demonstrate that the surface adsorbed oxygen control and the metastable ZnS intermediate are responsible for the fast response to ppm-level H2S. According to the aforementioned analytical results, the schematic representation for material interface interaction with the interfacial molecule is shown in Scheme 1 to illustrate the fast response of ZnO-600 to ppb and ppm-level H2S. When ZnO-600 sensor is exposed to air, the adsorbed oxygen molecules can easily capture the electrons on the surface of ZnO sensing material and form atomic O- ions at 217 °C (Eq. 1).38 Then, the adsorption of O- ions enlarge the depletion layer boundary with reduced carrier concentration, which results in an increase in the potential barrier across the contacts between ZnO grains (Ec), thus enhancing the resistance of material. O2 (ad) + 2e- → 2O-(ad) (490 K)

(1)

After contacting with 1 ppm H2S, the surface adsorbed oxygen species (O- ions) react with the H2S molecule on the surface of sensing material (Eq. 2), which results in the release of the trapped electrons into the conduction band of ZnO, thus reducing the potential barrier and resistance of the sensor. Meanwhile, the XPS spectrum of ZnO-600 exposed in 1 ppm H2S exhibits the decrease of the surface adsorbed oxygen instead of the existence of sulfides. Therefore, the fast response of ZnO-600 sensor towards ppb-level H2S can be ascribed to the hollow porous tubular structure and the surface adsorbed oxygen control. H2S (g) + 3O- (ad) ↔ SO2 (g) + H2O (g) + 3e-

(2)

In addition, when the sensor is exposed to 50 ppm H2S at 217 °C, H2S molecules not only react with surface adsorbed oxygen species (O- ions, Eq. 2), but also react with ZnO and convert into ZnS (Eq. 3). The speculation is proved by the XPS spectrum of ZnO-600 exposed in 50 ppm H2S, including the decrease of the surface adsorbed oxygen and the formation of ZnS. The metallic ZnS can increase the conductivity and the surface active sites of the sensing material, further reducing the potential barrier and resistance of the sensor. Thus, the fast response of ZnO-600 sensor towards ppm-level H2S originates from the combination of the hollow porous tubular structure, the surface adsorbed oxygen control, as well as the metastable ZnS


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intermediate. After subsequent exposure to air, the metastable ZnS transfers into ZnO according to the reaction in Eq. 4. In this sense, the formation of ZnS on the surface leads to a resistance change owing to its high conductivity than that of ZnO, further shortening the response time of ZnO-600 towards ppm-level H2S through sacrificing the recovery time. ZnO(s) + H2S(g) ↔ ZnS(s) + H2O(g)

(3)

2ZnS + 3O2 →2ZnO + 2SO2

(4)

Furthermore, the very short response time and recovery time towards ppb-level H2S for the porous ZnO hollow tubule calcined at 600 ℃ can also be understood from the following structural characteristics: first, the relative small nanoparticles (~20 nm) and proper crystallinity could result in the defects on the surface of the tubular structure caused by surface adsorbed oxygen molecules, which improve electrons transporting efficiency and provide numerous absorption sites to absorb H2S and oxygen species, implying that the sensor may present fast response and recovery in short time to ppb-level H2S;1 second, the hierarchically porous structure is beneficial to the quick diffusion and desorption of more target gases and takes effect to adsorb and fix the target gases inside, thus increasing the contacts between the oxygen species and H2S gas, and further shortening the response time and recovery time of the sensors;39 third, the relative large specific surface area of 31 m2·g-1 provides more active sites to adsorb H2S and oxygen species, and hollow tubule and macropores formed by the cross-linked nanoparticles perform the role of transport channels for target gases, and then improve the sensing performances, especially the fast response and

recovery

towards

trace

ppb-level

H2S

gas.

In

addition,

the

sulfuration-desulfuration reaction can also promote the fast response of the sensor to ppm-level H2S. Therefore, the excellent ppb-level H2S gas-sensing properties of ZnO-600 sensor are mainly ascribed to the synergetic effects of the inherent characteristics of hierarchically porous hollow tubule, large specific area and surface adsorbed oxygen species. 4. CONCLUSIONS In summary, hierarchically porous hexagonal ZnO hollow tubule with well



crystallinity was simply and large-scale synthesized using absorbent cotton as template. The tubular structure is composed of abundant cross-linked nanoparticles with successive meso- and macro-pores and large specific surface area (31 m2·g-1, calcinated at 600 ℃). At 217 ℃, the sensor based on ZnO-600 exhibits excellent sensing performance to H2S, especially the fast response time of 29 s to 50 ppb H2S and a low detection limit of 10 ppb. Moreover, it shows a wide linear range from 10 to 1000 ppb, satisfactory reproducibility and long-term stability. These excellent gas sensing properties are related to the inherent characteristics of hierarchically porous hollow tubule, large specific area and surface adsorbed oxygen species. This study also provides a possible strategy for large-scale production of other SMOs with hierarchically porous hollow tubule for fast response to ppb-level target gases. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Pulications websit at DOI: 10.1021/acsami.8b07795. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] ORCID Shan Gao: 0000-0001-6370-4994 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is financial supported by the International Science & Technology Cooperation Program of China (2016YFE0115100), the Young Innovation Talents of college in Heilongjiang Province (UNPYSCT-2016074), and the Scientific and Technological Innovation Talents of Harbin (2016RAQXJ005). We thank the Key


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Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University for supporting this study.

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L. Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine. Chem. Soc. Rev. 2016, 45, 3479-3563.

FIGURE CAPTIONS Figure 1. The XRD patterns of the products calcined at different temperatures (a) and the full survey XPS spectrum of ZnO-600 (b). Figure 2. SEM images of ZnO calcined at different temperature: 500 oC (a, b); 600 oC (c, d); 700 oC (e, f) and TEM images of ZnO calcined at 600 oC (g-i) (Inset in Figure 2i: selected-area electron diffraction pattern). Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution (inset) of ZnO-600. Figure 4. The response of ZnO calcined at different temperature towards 10 ppm H2S (a) and the response of ZnO-600 towards 10 ppm different gases at 217 oC (b). Figure 5. (a) The relationship between the responses and ppb-level concentrations of H2S and the response-recovery characteristics of ZnO-600 sensor to different concentrations of H2S at 217 °C. The dynamic response-recovery curves to 10 (b) and 50 ppb (c) H2S. (d) The reproducibility test to 10 ppm H2S at 217 °C. Figure 6. TEM images of sample on the Al2O3 tube after the stability tests. Figure 7. O 1s and S 2p XPS spectra of ZnO-600 before (a) and after exposure to 1 ppm (b, d) and 50 ppm (c, e) H2S at 217 °C. Scheme 1. Schematic representation of the gas sensing mechanism for ZnO-600 towards different concentrations of H2S.


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

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Table 1 Comparison of sensing performances for reported ZnO and composites to ppb-level H2S. Materials and Morphologies

Working temperature (℃)

Response and recovery times towards H2S a C (ppb)

t Res (s)

t Rec (s)

LOD b(ppb)

Refs.

hierarchical porous ZnO hollow tubule

217

10 50

61 29

59 98

10

this work

ZnO cross-linked nanosheet c

200

500

35

196

10

20

ZnO nanosphere

220

500

219

227

50

21

ZnO nanowire (50 nm) c

150

50

189

32

50

5

ZnO nanowire (20 nm) c

300

50

241

144

5

22

porous CuO/ZnO nanofilm

225

500

35

80

500

23

CuO/ZnO nanoparticle

225

100

50

130

100

24

CuO/ZnO nanosphere

125

10

78

70

10

25

Net-like SnO2/ZnO c

100

10

130

40

10

6

CuO/ZnO hollow sphere

336

1000

300

900

1000

26

CuO/ZnO nanowire

200

500

260

150

500

27

NiO/ZnO nanotube c

215

1000

160

96

1000

28

Note: a Response and recovery times were defined as the times needed for 90% of total change in resistance on exposure to test gas and ambient air, respectively; b Limit of detection; c Estimated according to the corresponding references.


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Graphical Abstract:

Large-scale synthesis of hierarchically porous ZnO hollow tubule for fast response to ppb-level H2S gas Huibing Na, Xian-Fa Zhang, Zhaopeng Deng,* Yingming Xu, Lihua Huo, and Shan Gao* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China



Figure 1. The XRD patterns of the products calcined at different temperatures (a) and the full survey XPS spectrum of ZnO-600 (b). 174x73mm (300 x 300 DPI)


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Figure 2. SEM images of ZnO calcined at different temperature: 500 oC (a, b); 600 oC (c, d); 700 oC (e, f) and TEM images of ZnO calcined at 600 oC (g-i) (Inset in Figure 2i: selected-area electron diffraction pattern). 303x240mm (300 x 300 DPI)



Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution (inset) of ZnO-600.


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Figure 4. The response of ZnO calcined at different temperature towards 10 ppm H2S (a) and the response of ZnO-600 towards 10 ppm different gases at 217 oC (b).



Figure 5. (a) The relationship between the responses and ppb-level concentrations of H2S and the responserecovery characteristics of ZnO-600 sensor to different concentrations of H2S at 217 °C. The dynamic response-recovery curves to 10 (b) and 50 ppb (c) H2S. (d) The reproducibility test to 10 ppm H2S at 217 °C.


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Figure 6. TEM images of sample on the Al2O3 tube after the stability tests.



Figure 7. O 1s and S 2p XPS spectra of ZnO-600 before (a) and after exposure to 1 ppm (b, d) and 50 ppm (c, e) H2S at 217 °C.


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Scheme 1. Schematic representation of the gas sensing mechanism for ZnO-600 towards different concentrations of H2S. 338x190mm (96 x 96 DPI)


Large-scale synthesis of hierarchically porous ZnO hollow tubule for

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