Ordered Mesoporous Tin Oxide Semiconductors with Large Pores and

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Ordered Mesoporous Tin Oxide Semiconductors with Large Pores and Crystallized Wall for High-Performance Gas Sensing Xingyu Xiao, Liangliang Liu, Junhao Ma, Yuan Ren, Xiaowei Cheng, Yongheng Zhu, Dongyuan Zhao, Ahmed A. Elzatahry, Abdulaziz Alghamdi, and Yonghui Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18830 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

Ordered Mesoporous Tin Oxide Semiconductors with Large Pores and Crystallized Wall for High-Performance Gas Sensing †

†

†

†

†

‡

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Xingyu Xiao , Liangliang Liu , Junhao Ma , Yuan Ren , Xiaowei Cheng , Yongheng Zhu , Dongyuan Zhao , Ahmed # § †⊥ A. Elzatahry , Abdulaziz Alghamdi , Yonghui Deng *

†

Department of Chemistry, State Key Laboratory of Molecular Engineering of Polymers, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and iChEM, Fudan University, Shanghai 200433, China

‡

College of Food Science and Technology, Shanghai Ocean University, Shanghai 201306, China

⊥

State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China #

Materials Science and Technology Program, College of Arts and Sciences, Qatar University, PO Box 2713, Doha, Qatar §

Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

Email: [email protected]

Keywords:

mesoporous materials, tin oxides, gas sensing, block copolymer, hydrogen disulfides

ABSTRACT: Owing to their distinct chemical and physical properties, mesoporous metal oxide semiconductors have shown great application potential in catalysis, electrochemistry, energy conversion and energy storage. In this study, mesoporous crystalline SnO2 materials have been synthesized through an evaporation-induced co-assembly (EICA) method using poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers as the template, tin chlorides as the tin sources and THF as the solvent. By controlling conditions of the co-assembly process and employing a carbon-supported thermal treatment strategy, highly ordered mesoporous SnO2 materials with hexagonal mesostructure (space group P63/mmc) and crystalline pore walls can be obtained. The mesoporous SnO2 is employed in fabricating gas sensor nanodevices which exhibit an excellent sensing performance towards H2S with high sensitivity (170, 50 ppm) and superior stability, owing to high surface area (98 m2/g), well-connected mesopores of ca. 18.0 nm, and high density of active sites in the crystalline pore walls. The chemical mechanism study reveals that both SO2 and SnS2 are generated during the gas sensing process on the SnO2-based sensors. INTRODUCTION Semiconducting metal oxide nanostructures with various compositions and structures have stimulated considerable interests owing to their distinct physical and chemical properties, as well as their potential applications for electrochemistry 1-4, catalysis 5-7 and sensors 8-15. Among them, SnO2 (band gap: 3.6 eV) is of particular interest due to its unique properties in various fields such as gas sensing 16-20 and electrochemical devices 21-25. In an attempt to improve their application performance, it is highly desired to fabricate SnO2 nanomaterials with ordered mesoporous structures because the uniform and well-connected pores can facilitate molecular diffusion, while the high surface area from high porosity can provide abundant accessible active sites 26-29. Various methods have been explored to synthesize high-surface-area SnO2 nanomaterials, such as hydrothermal reaction 30-32, sol-gel procedures 33-35, spray drying 36-37, chemical vapor deposition (CVD) 38-39 and

soft-templating method. Soft-templating method based on the co-assembly of block copolymers (or amphiphilic surfactants) and inorganic (or organic) precursors has been widely used to synthesize ordered mesoporous silica and metal oxides 40-45. Different from other methods, soft-templating method usually leads to a continuous porous nanostructure with adjustable pore sizes. However, the synthesis of ordered mesoporous SnO2 materials has rarely been reported because the assembly of tin precursors and their crystallization during the treatment at high temperature is difficult to control. The main reason is that the common inorganic tin sources (e.g. SnCl4) are highly water sensitive and prone to rapid conversion into macroscopic precipitate in synthesis solutions. Most reports employ the commercial Pluronic triblock copolymers like poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (for example, PEO20-PPO70-PEO20 (P123) and PEO106-PPO70-PEO106 (F127)) as the template. For example, Yang et al. 46 reported the synthesis of mesoporous tin oxides with partially crystallized framework by using the

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solvent evaporation-induced self-assembly (EISA) method. The synthetic system consisted of SnCl4 (as the precursor), P123 (as the template), and ethanol (as the solvent). Lee group 47 fabricated hexagonal and cubic mesoporous tin oxide thin films using Pluronic F127 as the template by the spin-coating method. However, the Pluronic templating methods still have some drawbacks. For example, owing to the limited chain length of the hydrophobic PPO segments, the pore sizes of the obtained mesoporous tin oxide materials are usually smaller than 10 nm using the available commercial PEO-PPO-PEO triblock copolymers as the template. Moreover, shrinkage and collapse of the porous nanostructures usually take place during thermal treatment to remove the template or crystallization. To avoid these situations, Smarsly et al. 48 reported the synthesis of crystallized 3D cubic mesoporous SnO2 films through a dip-coating method using poly(ethylene-co-butylene)-block-poly(ethylene oxide) (commercially called KLE) as the template. The obtained material also possesses larger pore size (ca. 14.2 nm) than those obtained by Pluronic templates. However, this method strongly relies on the dip-coating of very dilute precursor solutions, which leads to film-like mesoporous materials and may restrict the mass production and potential applications of the mesoporous materials. In this study, we demonstrated a solvent evaporation induced co-assembly (EICA) method, in combination with robust carbon-supporting synthesis strategy, to synthesize crystalline mesoporous tin oxide materials in an acidic THF/H2O solution with poly(ethylene oxide)-b-polystyrene diblock copolymers (PEO-b-PS) as the structure-directing agent and tin (IV) chlorides as the tin precursors. Different from our previous work on mesoporous tungsten oxides 49, in this study, the tin precursors were first hydrolyzed into tin oxide nanoclusters. Here we employed acetylacetone to retard the further aggregation of tin oxide nanoclusters with its chelating effect. Pyrolysis at 350 °C in nitrogen was performed on the organic-inorganic hybrid composites obtained after EICA, which can induce the transformation of PS segments into amorphous carbon in the mesopores and the simultaneous crystallization of pore walls with well-retained ordered mesoporous structures supported by carbon residuals. The final step of calcination in air can remove the carbon residuals, giving rise to ordered mesoporous SnO2 with high surface areas (ca. 98 m2/g), large uniform pore size (ca. 18.0 nm) and highly crystalline pore wall with well-ordered large-pore mesopores. The sensors fabricated using ordered mesoporous SnO2 as the sensing materials show a rapid response (ca. 11 s), superior sensitivity (Ra/Rg > 170) in H2S of 50 ppm and excellent selectivity to H2S, due to the high surface areas, large pore size, tinny crystal size and intrinsic property of SnO2. In air, oxygen species are absorbed on the surface of the nanograins with a small size, which leads to the formation of potential barriers between the adjacent nanagrains and the increase of the resistance of the materials. Different from conventional

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sensing mechanism, upon exposure to H2S, both absorbed oxygen species and the mesoporous SnO2 materials react with H2S, which leads to the decrease of potential barriers as well as the decrease of the resistance. Therefore, the small crystal size plays an important role in improving the sensitivity of the mesoporous SnO2-based sensors. EXPERIMENT Chemicals Monomethyl poly (ethylene oxide) (Mw: 5000 g/mol) (designed as PEO5000) was purchased from Aldrich. Copper (I) bromide, styrene, pyridine, Al2O3, HCl solution (37 wt %) and acetylacetone were purchased from Shanghai Chemical Reagent Co. Ltd. N, N, N′, N″, N″ -Pentamethyl diethylenetriamine (PMDETA) was purchased from Acros. SnCl4·xH2O was purchased from Alfa Aesar Chemical Reagent Co. Ltd. Tetrahydrofuran (THF) was purchased from Sino-Pharm Chemical Reagent Co. Ltd. Commercial SnO2 were purchased from Aladdin Chemical Reagent Co. Ltd. All chemicals were used as received without any further purification. Synthesis of Ordered Mesoporous SnO2 Mesoporous SnO2 materials were synthesized through EICA method using PEO114-b-PS248 as the structure-directing agent. In a typical synthesis, high-molecular weight PEO-b-PS block polymers were synthesized through an ATRP method according to our previous work 49. It has an approximate molecular composition of PEO114-b-PS248 with a narrow molecular weight distribution (polydispersity index, PDI = 1.09) according to 1H NMR and gel permeation chromatography (GPC) measurements (Mn=30792 g mol-1). In a typical synthesis, 0.1 g of PEO114-b-PS248 was dissolved in 5.0 mL of THF. SnCl4·xH2O (0.5 g) was dissolved in acetylacetone (0.5 ml). The two solutions mentioned above were mixed under continuous magnetic stirring. Concentrated HCl (0.2 ml, 37 wt %) was then added into the resulting mixed solution. After stirring for another 2 h, the obtained homogeneous transparent solution was cast onto Petri dishes to evaporate the solvent at 20 °C overnight in a hood, followed by further heating at 100 °C in an oven for 24 h to remove solvent completely and solidify the organic-inorganic composites. Then, the as-made organic-inorganic composites were scraped from Petri dishes and submitted to thermal treatment at 350°C for 3 h in nitrogen with a ramp of 1 °C min-1. A black powder-like carbon-supported mesoporous tin oxides was obtained by the carbonization of PEO-b-PS with rich sp2 hybrid carbon 49-50. After that, the sample was further thermally treated at 450 °C (or higher temperature of 550 and 650 °C) in air for another 0.5 h to remove the carbon species, resulting in white mesoporous tin oxides. To optimize the synthesis, various weight ratios of SnCl4·xH2O/PEO-b-PS were used to synthesize mesoporous tin oxides through the procedure mentioned above, and the obtained samples were denoted as


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SnO2-x-y-z, where x, y and z refer to the weight ratio of SnCl4·xH2O/PEO-b-PS, thermal treatment temperature and the atmosphere, respectively. Non-porous SnO2 was synthesized through the same procedure as mesoporous SnO2 materials without the addition of PEO-b-PS. Characterization and Measurements Small-angle X-ray scattering (SAXS) patterns were collected on a Nanostar U small angle X-ray scattering system (Bruker, Germany) using Cu Kα radiation (40 kV, 35 mA). Field-emission scanning electron microscopy (FESEM) measurements were performed on the Hitachi Model S-4800 field-emission scanning electron microscope. Transition electron microscopy (TEM) characterization was performed on a JEOL 2011 microscope (Japan) operated at 200 kV. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D4 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Dynamic light scattering measurements were conducted on a Malvern Zetasizer ZS-90 analyzer. Raman spectra were collected at room temperature on an XploRA Raman system, and the excitation wavelength is 532 nm. Thermogravimetric analysis (TGA) was carried out on a SDTQ600 analyzer from 30 to 700 °C under air with a heating rate of 10 °C min-1. Fourier-transform infrared (FTIR) spectra were collected on a Nicolet Fourier spectrophotometer using KBr pellets. Nitrogen adsorption-desorption isotherms were measured at 77 K with a Micromeritics Tristar 3020 analyzer. Gas chromatograph-mass spectrometry (GC-MS) was recorded on the Thermoelectron focus DSQ gas chromatograph-mass spectrometry. X-ray photoelectron spectroscopy (XPS) was recorded on an AXIS ULTRA DLD XPS System with MONO Al source (Shimadzu Corp.). All of the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. Gas Sensing Properties Referring to previous work 51-52, measurements of gas sensing properties were conducted on the HW-30A gas sensing system (Hanwei Electronics Co. Ltd., P. R. China). Before measurement, 10 mg of mesoporous SnO2 powder was mixed with 0.1 ml of deionized water. The mixture was grounded in an agate mortar, and the resulting paste was painted on an alumina tube printed with Au electrodes and Pt wires in advance, followed by annealing at 300 °C for 2 h to solidify the coatings. Then the heater, a Ni-Cr alloy coil, was put into the tube to provide the operating temperature of the sensing test. After that, the sensor components were welded to a holder, which could be connected to the electric circuit. (Figure S1 (a) (c)) To enhance the stability, the as-fabricated gas sensors were annealed at 350 oC for 1 week. A static test system was employed to test the response of the gas sensors. In the electric circuit of the gas sensing tests (Figure S1 (b)), the gas sensor was connected in series with a load resistor (RL). Herein, RL was a constant. The circuit voltage (Vc) was set at 5 V, while the voltage of the load resistor (Vout)

was recorded as the output voltage. The heating voltage (VH) was variable to control the operating temperature. In a typical measurement, the test gases were injected into a chamber and diluted with air. The gas response of the tested sensor was defined as S = Ra/Rg (for reducing gases) or S=Rg/Ra (for oxidizing gases). Here Ra referred to the resistance of the tested sensor in air, while Rg referred to that in test gas. The response/recovery time was defined as the time taken by the sensor output to achieve 90 % of total voltage change after injecting/releasing the gas. The measurements were conducted with the temperature of about 20 °C and the humidity of about 40 %. RESULTS AND DISCUSSION Scheme 1 presents the solvent evaporation induced co-assembly (EICA) process for ordered mesoporous tin oxide materials. Since tin chloride is extremely sensitive to water and can be hydrolyzed quickly 40, a certain amount of concentrated hydrochloride solution and chelating agent acetylacetone are added to retard the hydrolysis of SnCl4 and generate acetylacetone-stabilized tin oxide nanoclusters. Meanwhile, THF is a good solvent for both PS and PEO segments, while water is poor solvent for PS but it can well dissolve PEO. After added to the system, SnCl4 was first quickly hydrolyzed into tin oxide nanoclusters. In the presence of acetylacetone, further aggregation can be effectively suppressed, and the nanoclusters can be well controlled within a small size. PEO-b-PS copolymers can interact with tin oxide nanoclusters via hydrogen bond and further co-assemble into spherical micelles as THF evaporates (Step 1) at the liquid-solid interface on Petri dishes. In such a composite micelle, the PS core solvated by THF was surrounded by a corona of PEO-tin oxide species. With further evaporation of THF, the core-corona micelles gradually pack into three dimensional ordered mesostructure (Step 2, Figure S2). The ordered mesostructure can be further fixed by annealing at 100 ºC. The obtained organic-inorganic composites (as-made mesostructured composites) were treated at 350 °C in nitrogen atmosphere. In this procedure, the PS blocks can be carbonized into amorphous carbon residuals in mesopores which act as rigid support to retain the mesoporous structure (Step 3, Figure S3). Finally, calcination at 450 °C in air is conducted to remove the carbon species in the mesopores, leading to crystalline mesoporous SnO2 materials with uniform and well connected spherical mesopores (Step 4).



100

LnI (a.u.)

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Scheme 1. The synthesis of ordered mesoporous crystalline SnO2 via solvent evaporation induced co-assembly (EICA) combined with a carbon-supported crystallization strategy. Step 1, PEO-b-PS copolymers co-assemble with tin oxide nanoclusters into spherical micelles via hydrogen bonds as THF evaporates. Step 2, the organic-inorganic micelles further assemble into ordered mesostructure after complete evaporation of THF. Step 3, the PS blocks are carbonized into amorphous carbon residuals in mesopores during thermal treatment at 350 oC in nitrogen. Step 4, crystalline mesoporous SnO2 materials are obtained after removal of the supporting carbon species in the mesopores via calcination at 450 oC in air. According to previous reports 53, the volume ratio of hydrophilic and hydrophobic part of the effective amphiphilic block copolymers (herein they refer to tin oxides-associated PEO and the PS, respectively) affects the mesostructure of the as-assembled nanocomposites. In this study, a series of nanocomposites was synthesized by adjusting the weight ratio of tin precursor and PEO-b-PS to achieve a controlled co-assembly process. Small angle X-ray scattering (SAXS) technique was employed to monitor the structure evolution of the as-prepared PEO-b-PS/SnO2 composites, and it was found that the nanocomposites (i.e. SnO2-x) obtained with a weight ratio of tin chlorides/PEO-b-PS ranging from 4 to 6 (x = 4-6) show multiple scattering peaks (Figure 1). Particularly, the composites obtained with a weight ratio of 5:1 (x = 5) exhibits the most resolved peaks at q-values of 0.213, 0.370 and 0.427 nm-1 (Figure 1b), which can be indexed to the 100, 110 and 200 reflections of the hexagonal mesostructure with the space group P63/mmc, respectively. The d-spacing and unit cell parameter (a0) calculated based on the strongest 100 peak were 29.4 nm (d100 = 2π/q) and 34.0 nm (a0 = 2d100/√3), respectively. After thermal treatment in nitrogen at 350 °C and subsequent calcination in air at 450 °C, the obtained SnO2-5-450-air sample shows a highly ordered mesostructure with a uniform pore size (~ 15.0 nm). The thickness of the pore wall was estimated to be about 10.0 nm from FESEM observation (Figure 2a, inset). The ordered mesoporous structure is in agreement with hexagonal arrangement of spherical pores (Figure 2a).

110

c

210

b a 0.0

0.5

1.0

1.5

2.0

-1

q (nm ) Figure 1. SAXS patterns of as-made mesoporous SnO2 samples with different weight ratio (tin chlorides versus PEO-b-PS) of 4 (a), 5 (b) and 6 (c).

Figure 2. (a) FESEM images and (b) (c) TEM images of ordered mesoporous crystalline SnO2-4-450-air sample. (d) HRTEM image of ordered mesoporous crystalline SnO2-4-450-air sample. The inset in panel (a) is the FESEM images with a high magnification. The inset in (c) is the corresponding SAED pattern, and the red hexagon marks the hexagonal array of mesopores. TEM characterizations further indicate the existence of hexagonally aligned mesoporous structure with highly crystalline walls and large uniform pores (Figure 2b-d). By contrast, the SnO2-4-450-air and SnO2-6-450-air samples synthesized with SnCl4/PEO-b-PS ratio of 4 and 6 respectively exhibit less ordered mesostructure (Figure S4). According to the TEM and HRTEM images (Figure 2b-d), the pore walls consist of numerous tinny nanocrystals. The crystal size is estimated to be about 3.0 nm from TEM image (Figure 2d), and the interplanar spacing is about 0.34 nm, corresponding to the (110) plane of the bulk tetragonal cassiterite (JCPDS 41-1445). The


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selected-area electron diffraction (SAED) patterns (inset of Figure 2c) show diffraction rings assigned to the typical bulk tetragonal cassiterite phase of tin oxides, and no spotty patterns are observed. It indicates a polycrystalline structure of the pore walls.

Figure 3. XRD patterns of (a) the as-made organic-inorganic PEO-b-PS/SnO2 composites after thermal treatment at 100 °C for 1 day (the as-made SnO2-5), (b) the sample SnO2-5-350-N2 obtained after carbonization in N2 at 350 °C, and the samples SnO2-5-450 (550, 650)-air obtained after further calcination at (c) 450 °C, (d) 550 °C and (e) 650 °C in air, respectively. Wide-angle XRD patterns were recorded for the samples obtained at different thermal treatment conditions (Figure 3). The as-made organic-inorganic PEO-b-PS/SnO2 composites (SnO2-5) obtained after 100°C annealing shows weak diffraction peaks at 26.7, 33.9 and 51.8 degrees, implying presence of small amount crystallized microdomains in the composites (Figure 3a). After thermal treatment at 350 °C in nitrogen, typical (110) (101) (211) reflection peaks of tetragonal cassiterite (JCPDS 41-1445) appear in the obtained SnO2-5-350-N2 sample (Figure 3b). Well-resolved diffraction peaks can be clearly observed in the patterns of SnO2-5-450-air samples after further calcination at 450 °C in air (Figure 3c), implying a highly crystalline phase. As the calcination temperature further increases to 550 °C and 650 °C, the intensity of diffraction peaks only slightly increases, implying a good thermal stability of the pore wall (Figure 3d, e). Using the Scherrer equation based on the strongest diffraction peak of (100), the crystallite size of the samples obtained after calcination at 450, 550 and 650 °C is calculated to be 2.16, 3.40 and 3.86 nm, respectively. The small crystal size could be attributed to the presence of acetylacetone in the synthesis system which helps to stabilize the SnO2 nanoclusters formed in the initial stage of the precursor solution and prevent the aggregation of these nanoclusters. To demonstrate the effect of AcAc, dynamic light scattering (DLS) technique was employed to monitor the SnCl4/THF/AcAc/HCl solution (i.e. the simulated precursor solution without block copolymer

template). In the presence of acetylacetone, the particle size in the solution is 1.2 nm in the beginning (t = 0 h) and 2.5 nm after stirring for 2 h (t = 2 h) (Figure S5). In comparison, with the absence of acetylacetone, the particle size is measured to be 2.3 nm (t = 0 h) and 845.5 nm (t = 2 h), respectively. It can be concluded that acetylacetone can effectively retard the growth of SnO2 nanoparticles via the chelation effect. In this study, in order to prevent the collapse of the mesostructure, the as-made organic-inorganic composites were intentionally treated at 350 °C for 3 h in nitrogen, because the carbon species in situ derived from PEO-b-PS can support the pore walls constructed by tin oxide nanoclusters. To verify the supporting effect of the carbon species, the black SnO2-5-350-N2 sample was etched by 2 M NaOH solution for 24 h, and the obtained black solution was analyzed by TEM observation, which reveals the presence of carbon particles of about 10 nm (Figure S6). The residues mainly consist of partially graphitized carbon as revealed by the Raman spectrum of the collected sample after etching (Figure S7). The existence of the carbon particles can effectively support the crystalline wall and prevent the ordered mesostructure from collapsing during further thermal treatment at higher temperatures. Thermogravimetric (TG) analysis conducted in air shows a distinct weight loss of about 15 wt% in the range of 400-500 °C for sample SnO2-5-350-N2 due to the combustion of carbon residuals; while almost no further weight loss is detected at temperature higher than 450 °C (Figure S8). It indicates the carbon residues can be completely removed after thermal treatment at 450 °C in air. As shown by Fourier-transform infrared spectroscopy (FTIR, Figure S9), compared to the as-prepared PEO-b-PS/SnO2 composites, almost no absorption peaks at 2922 cm-1 (-C-H) and 3024 cm-1 (=C-H) can be observed in the FTIR spectrum of the sample SnO2-5-450-air, which further confirms a successful removal of carbon species.

Figure 4. (a) Nitrogen adsorption-desorption isotherms recorded at 77 K and (b) the corresponding pore-size distribution curve of the crystalline ordered mesoporous SnO2-5-450-air. The inset in (a) shows the structure model of the materials with large pores and large window size. Nitrogen adsorption-desorption isotherms of the calcined SnO2-5-450-air sample show a type IV curve with a



H2-type hysteresis loop, thus confirming that the sample possesses mesoporous structures (Figure 4a). The pronounced hysteresis suggests that the spherical mesopores are connected through smaller pores. In the adsorption band, the sharp increase at P/P0 ≈ 0.85 indicates a uniform mesoporous structure with a large pore size. As confirmed by pore size distribution using BdB method, the pore size is uniform and centered at about 18.0 nm, (Figure 4b). Meanwhile, the sharp decrease in the desorption curve beyond P/P0 ≈ 0.85 indicates the existence of plenty of windows between adjacent pores, which can facilitate molecule diffusion within the mesoporous frameworks. The surface area of the ordered mesoporous SnO2 is calculated to be 98 m2 g-1. Considering the high density of tin oxides (6.95 g cm-3), the specific surface area is noticeably high. The large specific surface area and connective pore structure of mesoporous SnO2 materials can supply abundant active sites, making them ideal candidates for various applications including catalysis, gas sensing and so on.

Figure 5. (a) Response and recovery curve of the mesoporous crystalline SnO2-5-450-air based sensor to H2S at different concentrations (5-100 ppm) at the operating temperature of 350 °C. (b) The relationship between response and the concentration of H2S at 350 °C. (c) Cycle curve of the crystalline mesoporous SnO2 based sensor to H2S at the concentration of 50 ppm for four times. (d) Responses of the crystalline mesoporous SnO2 based sensor to different gases at 50 ppm. Herein, in this study, inspired by the highly connected porous structure and unique semiconducting properties of the mesoporous tin oxides, we explored the potential applications of the mesoporous crystalline tin oxide semiconductors in gas sensing. Hydrogen sulfide, as a dangerous and toxic gas, can cause personal distress at low concentration and even result in death in the case its concentration is over 250 ppm. To determine the appropriate working temperature, the mesoporous SnO2-coated sensor was tested towards 50 ppm H2S at different temperatures, and it was found that the recovery is too slow (at least 500 s) when the heating temperature

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is below 350 oC (e. g. 290, 250 and 400 oC). Therefore, the operating temperature is chosen to be 350 oC in order to obtain higher response (Figure S10, Figure S11). The electrical response of the crystalline mesoporous SnO2 sensor to different concentrations of H2S gas (5-100 ppm) is shown in Figure 5a. As the H2S gas with different volume was injected into the tested chamber, the voltage of the load resistance rapidly increased, indicating the increase of the electrical response of the gas sensors. Upon opening the chamber, the output voltage can quickly recover to its initial value, which indicates a good reversibility of the mesoporous SnO2-based gas sensors. For 10 and 20 ppm H2S, the sensor shows relatively long response time. At relatively low concentrations (e.g. 10-20 ppm), it is difficult for H2S gas to diffuse to the inner structure of the sensors quickly. As a result, the H2S would interact with the surface of the sensors which leads to the initial response platform. In a short time, H2S diffuse through the mesoporous structure into the interior of the sensors, resulting in the sharp increase of the response. The response of the SnO2 sensors increases with the rise of the concentration of H2S gas (Figure 5b). The value of the response is calculated based on the resistance-time curve (Figure S12). When exposed to 50 ppm H2S, the response of the sensors can reach about 170, which is much better than the SnO2–based materials reported previously (Table 1). The response and recovery time of the sensor based on crystalline mesoporous SnO2 towards 50 ppm H2S is 11 s and 165 s, respectively (Figure S13). Although the response is quite fast, the recovery time tends to be relatively long probably due to the high surface area with numerous active sites. In comparison, the response of commercial SnO2 based sensors to 50 ppm H2S is 4.7, and the response and recovery time is 24 and 25 s, respectively. While the lab-made non-porous SnO2 based sensor shows a response of 2.7 to 50 ppm H2S with a response and recovery time of 43 and 30 s, respectively. Therefore, the mesoporous SnO2 based sensors perform much better than the commercial SnO2 and lab-made nonporous SnO2 based sensors. The relatively long time during recovery process is due to the strong interaction between H2S molecules and mesopore walls. The performance of the SnO2-based sensors towards 50 ppm H2S for four times was tested to examine the stability and reproducibility, and the corresponding results were shown in Figure 5c. The response and recovery curves were very similar, which showed the sensing properties of the sensor remained constant in the tests. For practical application, the selectivity of the gas sensor to a certain gas is also important. The responses to multiple interfering gases, such as NH3, acetone, formaldehyde, toluene, methanol, ethanol, H2, CH4 and CO, were measured and compared at constant concentration of 50 ppm at the same temperature 350 °C (Figure 5d). The response of our mesoporous SnO2 based sensor to H2S gas is much higher than any other gas, indicating an impressive selectivity.


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ACS Applied Materials & Interfaces Scheme 2. The H2S sensing mechanism of the sensors based on mesoporous SnO2 materials exposure in air and H2S-air mixture (Ec, conduct band edge; Ef, Fermi energy).

Table 1. Comparison of H2S sensing performance of the SnO2-based sensors with various nanostructures Working

Response/recovery

Temperature/°C

H2S concentration/ppm

Sensitivity

Cu-doped SnO2 film

180

100

25.3

~10.1/~42.4 s

38

CuO/SnO2 hollow spheres

300

1

22.4

15 s/not mentioned

54

SnO2 nanofibers/rGO

200

5

34

< 198/< 114 s

55

SnO2 wires/rGO

22

50

33

2/298 s

56

SnO2-CuO nanowires

300

10

~15

9/8 s

57

Au/hollow SnO2 spheres

400

5

17.4

18 s/not mentioned

58

Pt doped SnO2 nanofibers

500

20

40.6

1/160 s

59

SnO2-CNT

RT

50

4

~60/~60 s

60

SnO2 multitube arrays

RT

5

1.5

14/30 s

61

quasi-2D Cu2O/SnO2

RT

50

1.8

180/500 s

62

Pr6O11-doped SnO2

300

50

74

4/30 s

63

CuO/SnO2

250

5

~4

150/~200 s

64

SnO2/CeO2

210

150

704

Not mentioned

65

Cd-doped SnO2

275

10

31

~20/~200 s

66

SnO2 quantum dots

70

50

29

37/127 s

67

SnO2 quantum dot/MWCNT

70

50

108

23/44 s

68

Structures

As can be seen from the voltage-time curves (Figure 5a, 5c), the voltage of the loading resistor turns to be relatively low without the injection of test gases (for example, H2S). That is, before the injection of test gases, the SnO2 sensors showed pretty high resistance. It may result from the tinny crystal size of the tin oxides. As illustrated by Scheme 2, many SnO2 homojunctions form between two abutting nanograins in the crystalline pore walls of the mesoporous materials. In air, oxygen molecules can diffuse through the mesopores and interspace of the nanograins, resulting in a complete cover of the surface of nanograins. The adsorbed oxygen species can extract electrons from the nanograins, therefore, the electron depletion layers are formed on the surface of SnO2 nanograins, leading to the formation of potential barriers on the boundaries. The existence of the potential barriers contributes to the restriction of the flow of electrons through the boundaries. Upon exposure to H2S, the voltage of the loading resistor increases rapidly

time

Ref.

due to the return of electron from H2S to SnO2 via surface reaction, corresponding to the decrease of the resistance of the SnO2 sensors. In order to monitor the reaction mechanism between H2S and the gas sensors, gas chromatograph-mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) were employed to investigate the products. The product, SO2, was detected in the chamber. 2 min after the injection of 50 ppm H2S, the gas in the chamber was extracted by a syringe and it was then injected into the GC-MS. Without mesoporous SnO2 sensors inside the chamber, little SO2 was detected (Figure S14a). While in the presence of mesoporous SnO2-based sensors, a strong peak of SO2 was monitored (Figure S14b). Meanwhile, the chemical composition of SnO2 coatings of the sensors was analyzed by XPS spectrum after injection of 50 ppm H2S for 2 min reaction (Figure S15). The S 2p peak located at 165 eV reveals the



presence of tin disulfide (SnS2). The formation of metal sulfides during gas sensing has been reported previously 69, 70 . These results imply that the chemisorbed oxygen species and SnO2 nanograins both react with H2S during sensing measurement to form SO2 and SnS2. The reaction formula can be concluded as:

2H2S + 3O2 → 2H 2O + 2SO2 2H2S + SnO 2 → SnS2 + 2H2O During the procedure, plenty of electrons could be released with the reduction of the oxygen species, which reduces the height of the potential barriers formed between the adjacent nanograins and results in decrease of the resistance at the SnO2-SnO2 homojunctions. Therefore, it is reasonable to conclude that the tinny size of the tin oxides in the mesoporous materials significantly amplifies the contrast of the resistance before and after the injection of H2S. On the other hand, due to the much narrower band gap of SnS2 (2.77 eV) 71 than that of SnO2 (3.6 eV), the existence of SnS2 can reduce the body resistance of the nanograins. Both the effects lead to sensitivity enhancement of the gas sensor. However, it can be difficult for SnS2 to recover to SnO2, which may have a negative impact on the recovery properties considering the relatively long recovery time (~165 s).

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ASSOCIATED CONTENT Supporting Information. The photograph of the equipment and the electric circuit for gas sensing analysis, TEM image of as-made PEO-b-PS/SnO2 nanocluster composite, TEM and SEM images of SnO2-4/6-350-N2 nanocomposites. TG curve of the SnO2-5-350-N2 sample, the recovery time-temperature and response-temperature curves of the mesoporous SnO2-based sensors, FTIR spectrum of the SnO2-5-450-air sample and as-made composites. Dynamic light scattering measurements of SnO2 precursors. TEM image and Raman spectrum of hollow carbon spheres obtained from SnO2-5-350-N2. Response and recovery curves of the mesoporous SnO2-based sensor. The response and recovery curves of the mesoporous crystalline SnO2-5-450-air based sensor to 50 ppm H2S gas at the working temperature of 250-400 o C. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions

CONCLUSIONS In summary, mesoporous crystalline SnO2 materials were synthesized via a solvent evaporation induced co-assembly (EICA) method, in combination with robust carbon-supporting synthesis strategy by using amphiphilic PEO-b-PS copolymers as the structure-directing agent and THF as the solvent. Acetylacetone was utilized as the chelating agent to avoid fast hydrolysis of SnCl4 and the aggregation of tin oxide nanoclusters, so as to achieve a controllable organic-inorganic co-assembly. PEO-b-PS copolymers not only direct the formation of ordered mesostructure, but also serve as the carbon source for generation of amorphous carbon in the mesopores to prevent the mesostructure from collapsing during thermal treatment in inert atmospheres. The obtained mesoporous tin oxides possess a large pore size (ca. 18.0 nm) and high surface areas (ca. 98 m2 g-1) with crystalline frameworks. The sensor based on mesoporous tin oxides were found to have excellent sensing performance with high sensitivity, superior stability and low detection limit towards H2S owing to the uniform pore size, high surface areas, sufficient active sites and small crystal size. The chemical mechanism study reveals that both the oxygen species absorbed on the surface of the mesoporous SnO2 materials and the materials react with H2S during the gas sensing process, which is different from conventional sensing mechanism. The crystalline mesoporous tin oxides with high porosity are promising sensing materials for fabricating portable gas sensors in environment monitoring, industrial detection and so on.

The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NSF of China (51372041, 51422202, 51402049 and 51432004), the “Shu Guang” Project (13SG02) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation, Youth Top-notch Talent Support Program of China, and the state key laboratory of Transducer Technology of China (Grant No. SKT1503). The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP# 0094. The authors thank Prof. Guangrong Zhou for assistance in TEM characterization. REFERENCES (1) Luo X.; Morrin A.; Killard A. J.; Smyth M. R. Application of nanoparticles in electrochemical sensors and biosensors. Electroanalysis, 2006, 18, 4, 319-326. (2) Ghicov A.; Schmuki P. Self-ordering electrochemistry: a review on growth and functionality of TiO2 nanotubes and other self-aligned MOx structures. Chem. Commun., 2009, 20, 2791-2808.


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Ordered Mesoporous Tin Oxide Semiconductors with Large Pores and

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