Mesoporous SnO2 Nanotubes via ElectrospinningâEtching Route

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Mesoporous SnO2 Nanotubes via Electrospinning-Etching Route: Highly Sensitive and Selective Detection of H2S Molecule Peresi Majura Bulemo, Hee-Jin Cho, Nam-Hoon Kim, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05241 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017

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ACS Applied Materials & Interfaces

Mesoporous SnO2 Nanotubes via ElectrospinningEtching Route: Highly Sensitive and Selective Detection of H2S Molecule Peresi Majura Bulemo, Hee-Jin Cho, Nam-Hoon Kim and Il-Doo Kim* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea.

ACS Paragon Plus Environment

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ABSTRACT In this work, we report the facile synthesis of thin-walled SnO2 nanotubes (NTs) with numerous clustered pores (pore radius 6.56 nm) and high surface area (125.63 m2/g) via selective etching of core (SiO2) region in SiO2-SnO2 composite nanofibers (NFs), in which SnO2 phase preferentially occupies the shell while SiO2 is concentrated in the center of the composite NFs. The SiO2-etched SnO2 NTs are composed of ultrasmall crystallites (~6 nm in size) originating from crystal growth inhibition by small SiO2 domains, which are partially segregated in the polycrystalline SnO2 shell during calcination. These features account for efficacious diffusion and innumerable active sites, which maximize interaction between background gas (air) and analyte gas (H2S). Evaluation of gas-sensing performance of the porous SnO2 NTs before and after decorating the exterior and interior walls with Pt nanoparticles (NPs) reveals exceptional selectivity and superior response (Ra/Rg) of 154.8 and 89.3 to 5 ppm and 1 ppm of H2S, respectively. Excellent gas-sensing characteristics are attributed to the porous topography, nanosized crystallites, high surface area and the catalytic activity of Pt/PtOx NPs.

KEYWORDS electrospinning, etching, semiconductor metal oxide, exhaled breath, gas sensor


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Introduction Among semiconducting metal oxides (SMOs), SnO2 has been extensively investigated for gas-sensing applications1-8 owing to its large band gap, nonstoichiometric nature,9 excellent electronic mobility,10 and stability.11 Numerous approaches regarding its synthesis into useful nanostructured morphologies such as powders,12-13 nanorods,14-15 nanowires (NWs),15-16 nanofibers (NFs),17 nanotubes (NTs),18-19 core-shell composites,20 and nanospheres21-22 have been explored to tailor its gas-sensing functionalities. Despite the diverse options of structures, SnO2-based chemiresistive sensors still lack appreciable response to analyte gas and selectivity for accurate discrimination of interfering gas molecules.23 Continued efforts to search for and implement novel and viable synthetic methods for desirable nanostructural morphologies are necessary to overcome these inherent shortfalls. In practice, enhancement of surface reactions demands robust porous nanostructures composed of fine nanocrystalline particles, which offer high surface area required for superior interaction of air adsorbates with analyte gas and easy gas diffusion into the porous sensing layers. The topology of fine nanocrystallites encompassing catalytically active surfaces via functionalization with nanoparticles (NPs) like silver,16 palladium,24-26 gold,27-28 and platinum28-29 dictates efficacious utilization of free electrons in the nonstoichiometric SnO2 and thereby tune Schottky barrier modulation at grain boundaries and selectivity to analyte gas. Besides, surface accessibility of these structures by diffusing air and analyte gas into the bulk of SnO2 is a significant factor to afford adsorption and desorption characteristics. Fortunately, electrospun one-dimensional (1-D) metal oxide nanotubular structures offer most peculiar ultrafine features characterized with high surface area-to-volume ratio and interconnectivity of nanoparticles. These features are imperative in attempt to fully exhaust


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manifold benefits of gas-sensing materials because they are associated with ultrahigh modulation of Schottky barrier during sequential exposure of sensing surface to air and analyte gas. For synthesis of electrospun SnO2 NTs, heat driven Ostwald ripening,28 Kirkendall effect,30 and coaxial electrospinning employing sacrificial polymeric (core) templates31 are the most exploited techniques. However, it is rare to attain a remarkable porosity by using these techniques because pore formation relies almost solely on interparticle spaces after removal of the sacrificial template and evaporation of solvent.32 In addition, NTs synthesized via these techniques exhibit rather low surface area28, 32 since growth of crystals is dependent on calcination conditions and/or incorporation of noble metals as grain growth inhibitors.26 Exceptionally simple strategy employing polystyrene (PS) colloids embedded in electrospinning solution as sacrificial templates33 is deemed a versatile approach to introduce extra pores in a wide range of meso- as well as macro-pores. However, size and concentration of PS colloids to be embedded in solution to attain interspersed porosity without interfering the formation mechanism and integrity of the tubular morphology are the limiting factors.34 To realize a hollow porous morphology, and yet attain controlled crystal growth for high surface area nanotubular structures, a robust dual strategy is hereby suggested. Herein, we introduce the electrospinning-etching route to successfully synthesize mesoporous SnO2 NTs. First, a solution containing a uniform dispersion of tin (Sn) and silicon (Si) precursors as well as a sacrificial polymer (PVP) is electrospun using a single-nozzle spinneret. Then asspun NFs are transformed into SiO2-cored SnO2 NFs containing fortuitous SiO2 in the shell upon calcination. Finally, the SiO2 phase is etched in alkali solution to form mesoporous SnO2 NTs. To the best of our knowledge, this approach has never been demonstrated in the synthesis of mesoporous SnO2 NTs. In principle, phase incompatibility and disparity in crystallization


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temperatures of SnO2 and SiO235-37 enlighten the possibility to synthesize heteronanostructures composed of SiO2 and SnO2 with core-shell topology. Apparently, preferential oxidation of Sn ionic species relative to their Si counterparts serves as prerequisite for formation of core-shell structures during calcination.37 In the course of calcination, the exterior of a NF experiences exposure to higher temperature and more abundant air than the interior. As a consequence, SiO2-cored SnO2 nanostructures are formed in virtue of the Kirkendall effect.38 On the basis of gradual crystallization of SnO2 with temperature,35-36 it is reasonable to ascertain that at calcination step, trapped SiO2 between crystals of SnO2 limits their growth39-41 prior to complete separation and crystallization of SnO2. Further, etching of SiO2 forming the core and segregated SiO2 in the shell by using strong alkali42 results in porous SnO2 NTs with large surface area and high density of surface pores. This structural architecture is significant to tune gas-sensing characteristics of SnO2 and entails merits connected with small amount of catalyst loading. The synthesized materials are tested for selective detection of hydrogen sulfide (H2S) against ethanol (CH3CH2OH), hydrogen (H2), acetone (CH3COCH3), toluene (C7H8), pentane (C5H12), carbon monoxide (CO) and ammonia (NH3). These reducing gases are well known disease specific metabolites43-45 in human exhaled breath when their concentrations exceed certain limits.46-47 Particularly H2S, which is a well-known biomarker for halitosis, occurs in exhaled breath of a patient human at concentration levels in the range of 80 ppb-2 ppm. Halitosis affects an estimated 10-30% of a given population,48-49 where approximately 90% of cases are linked to activity of micro-bacterial metabolism of amino acids and proteins in the digestive track50 and periodontal disease from bacterial infections.51-52 Commendable accuracy and selectivity in detecting the trace concentrations of H2S in breath are indispensable for diagnosis of halitosis and suitability for early intervention measures.44 Pt catalysts are well-known chemical sensitizers


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when deposited on metal oxide surface and has been reported to provide high response toward H2S sensing.53-54 The catalytic activity of Pt is very effective through activation and dissociation of air and analyte gas molecules on its surface and the subsequent spillover effect.55 To realize enhancement in response toward H2S sensing, the synthesized NTs are decorated with Pt nanoparticles on the interior and exterior surfaces of the shell. We then show that enhancement of selectivity to H2S can be achieved by residual SiO2 in the NTs.

Experimental Section Materials Tin

(ii)

chloride

dihydrate

(SnCl2·2H2O),

tetraethyl

orthosilicate

(TEOS),

N,N-

dimethylformamide (DMF), ethanol, polyvinylpyrrolidone (PVP, MW = 1300000 g/mol and MW = 10000 g/mol), sodium hydroxide (NaOH) solution (50% w/w), chloroplatinic acid (H2PtCl6·6H2O) and ethylene glycol (EG) were purchased from Sigma Aldrich. Hydrochloric acid (HCl) (37%) was purchased from Junsei Chemical Co. Ltd, whereas acetone was purchased from Sumchun Chemicals. All chemicals were used as received without further purification.

Synthesis of SnO2 NTs The schematic of synthesis setup is illustrated in Figure 1a. In a typical experimental procedure, 1-D NFs containing Sn and Si precursors and PVP (MW = 1300000 g/mol) were fabricated via a simple and versatile single-nozzle electrospinning technique. To prepare the electrospinning solution, 0.3 g of SnCl2·2H2O was dissolved in a mixed solution of DMF/ethanol (weight ratio = 1:1) under magnetic stirring at room temperature for 1 h. Then, 100 µL of TEOS and 30 µL of HCl were added to the dissolved solution of Sn precursor and stirred for 90 min.


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Finally, 0.3 g of PVP was added and the mixture was stirred further for 6 h to obtain a clear and transparent electrospinning solution. The resulting solution was loaded into a 12 mL syringe for electrospinning at a flow rate of 0.1 mL/min under electrostatic voltage of 12 kV. The collector-needle tip distance was maintained at 17 cm. The as-spun NFs were calcined in air to form SiO2-cored SnO2 NFs at a temperature increment of 5 °C/min from room temperature to 600 °C, and sustained at this temperature for 2 h. Chemical etching of SiO2 from the calcined NFs was performed via impregnation of the NFs into NaOH solution (pH 12) maintained at 55 °C in a bath of silicon oil for 7 h. After centrifugation and removal of the supernatant, collected SnO2 NTs were washed in deionized water and finally dried in an oven at 50 °C for 12 h.

Synthesis of Pt nanoparticles Polyol reduction method was used to synthesize well-dispersed quasi-spherical Pt NPs with size distribution of 3.5 - 10 nm.56 In a typical synthesis, 45 mL of EG (used as solvent and reducing agent) was heated to reaction temperature of 150 °C. At this temperature, 0.6 g of H2PtCl6·6H2O dissolved in 5 mL of EG solution was injected dropwise into the reaction medium at a constant rate using a syringe. Then 20 mL of EG solution containing 0.5 g of PVP (MW = 10000 g/mol) was added to the above heated solution at a constant rate and the reaction was allowed to proceed for 1 h. Acetone was added to the mixture at a volume ratio of 5:1 (acetone/Pt solution) followed by collection of NPs under centrifugation at 3000 rpm for 5 min. Finally, the NPs in a centrifuge were isolated and thoroughly washed four times in deionized water. After complete isolation, the as-obtained NPs were dispersed in ethanol to make 1823 mg/L of Pt NPs.


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Incorporation of Pt nanoparticles into SnO2 NTs Pt NPs were incorporated into post-etched SnO2 NTs as follows: 4.5 mg of SnO2 NTs were dispersed in 300 µL of ethanol under sonication. The dispersed SnO2 NTs were decorated with Pt NPs via physical mixing of SnO2 NTs with NPs at varying concentrations corresponding to 0.01, 0.02 and 0.04 Pt wt% to make three sensing pastes. Then, 1 µL of as-prepared pastes was drop-coated on alumina substrates (heated at 60 °C) to prepare uniform sensing layers followed by calcination in air at a ramping rate of 10 °C/min from room temperature to 500 °C, where the temperature was maintained for 1 h. Three sensors designated as 0.01% Pt-decorated SnO2 NTs, 0.02% Pt-decorated SnO2 NTs and 0.04% Pt-decorated SnO2 NTs to represent sensors derived from post-etched SnO2 NTs functionalized with Pt NPs at 0.01, 0.02 and 0.04 wt% respectively were prepared. For comparison purposes, sensors based on SiO2-cored SnO2 NFs prepared without etching process and post-etched SnO2 NTs were also prepared.

Materials characterization Synthesized materials were characterized using scanning electron microscopy (SEM, XL-30 SFEG, Philips), high resolution transmission electron microscopy (FE-TEM, Tecnai G2 F30 S-Twin, FEI) working at 300 kV and equipped with an energy-dispersive X-ray spectrometer (EDS), X-ray powder diffraction (XRD, D/Max-2500, Rigaku) employing CuKα radiation corresponding to λ = 1.5406 Å, X-ray fluorescence technique (XRF, ZSX Primus II, Rigaku), X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific), Fourier transform


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infrared radiation spectroscopy (FT-IR, Nicolet iS50, Thermo Fisher Scientific), and nitrogen adsorption-desorption Brunauer-Emmett-Teller method (BET, TriStar II 3020).

Measurement of gas-sensing properties Gas-sensing properties of the five prepared sensors (0.01% Pt-decorated SnO2 NTs, 0.02% Pt-decorated SnO2 NTs, 0.04% Pt-decorated SnO2 NTs, SiO2-cored SnO2 NFs, and postetched SnO2 NTs) were measured in a highly humid ambient condition (95% RH) using our homemade sensor measurement system equipped with data acquisition system (34972A, Agilent) to measure resistance change and a DC power supply system (U8031A, Agilent) for temperature control. Humidity was measured using a commercially available humidity sensor (605-H1, Testo Inc.) The alumina substrates (soldered) on sensor-chip holder were inserted in an 8-channel multiplexer and measurements were executed at 250 °C, 300 °C, 350 °C, 400 °C and 450 °C. In order to determine the performance of the sensors, gas sensing tests were carried out three times at a constant temperature and relative humidity. At each working temperature, pure air and varying trace concentrations (1 ppm, 2 ppm, 3 ppm, 4 ppm and 5 ppm) of analyte gas (H2S, CH3CH2OH, H2, CH3COCH3, C7H8, C5H12, CO and NH3) were sequentially introduced in the gas-sensing chamber on the basis of 10-min pure air and 10-min analyte gas to make a complete cycle. Air and varying concentrations of analyte gas for each cycle were introduced and controlled by mass flow controllers (MFCs) operated by LabVIEW software as reported elsewhere.44 To determine the response to gas concentration < 1 ppm, measurements were conducted by introducing 0.1 ppm, 0.2 ppm, 0.4 ppm and 0.6 ppm of analyte gas at an optimal working temperature. At optimum sensing conditions, the effect of humidity was studied by varying the ambient humidity at 55%, 75% and 95% RH. Finally, we treated humidity (95% RH)


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as interfering gas and the effect to sensing performance was investigated. The response of the sensor was evaluated as Ra/Rg, where Ra and Rg are sensor resistances in presence of atmospheric air and in presence of analyte gas, respectively.

Results and Discussion Morphology and microstructural analyses Figure 1a shows a schematic illustration of the procedure for the synthesis of SiO2-cored SnO2 NFs and Pt-decorated SnO2 NTs. The underlying formation mechanism shown in the schematic can be described. Precursors (SnCl2.2H2O and TEOS) and PVP dissolved in the solvent to form a homogenous electrospinning solution. Using a single-nozzle spinneret, homogenous as-spun NFs could be achieved as shown. Upon calcination, PVP decomposed while the precursors oxidized into incompatible SiO2 and SnO2 phases. As the oxides formed, SnO2 preferentially occupied the shell owing to its lower enthalpy energy of formation and standard free energy change than SiO2.37, 57-58 In principle, abundance of air and higher temperature on the exterior than the interior of the NFs effectuated initial formation of SnO2 on the surface while creating a concentration gradient of SnO2 increasing toward the surface. At this point, instantaneous diffusion of solubilized Sn2+ ions of the Sn precursor toward the surface became inevitable owing to the Kirkendall effect,38 with concomitant formation of SiO2 in the interior. In a concentrated solution of NaOH (pH 12), the core was selectively etched to form a water soluble sodium silicate,42 which was removed via washing in excess water leading to mesoporous SnO2 NTs. Lastly, the resultant SnO2 NTs were functionalized via decoration of Pt NPs on the exterior and interior walls.


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Figures 1b-e depict the morphology of the synthesized structures. Smooth morphology of as-spun NFs (Figure 1b) turned somewhat rough after calcination (Figure 1c). The morphology of post-etched SnO2 NTs was elucidated by SEM observation (Figure 1d) and corroborated by TEM observation (Figure 1e) after decoration with Pt NPs. In both cases, clear hollow morphologies were observed. In Figure 1f, energy-dispersive X-ray spectroscopy (EDS) maps of the Pt-decorated SnO2 NTs sample demonstrates a homogeneous distribution of constituent elements. We observed existence of residual Si in the entire length and cross-section of the NTs. To ascertain assumed positions of the incompatible components in the core-shell structure prior to etching, we employed TEM observation. Only a dense morphology similar to SEM image shown in Figure 1c was observed, with indistinguishable contrast between SiO2 and SnO2 components. Such a dense morphology signifies inconsiderable resultant porosity following the decomposition of PVP and organic matter from precursors. Alternatively, cross-sectional compositional line profile of elemental distribution (Figure 2a) was taken in arbitrarily chosen SiO2-cored SnO2 NF. Notably, disparities in the nature of intensity profiles of the constituent elements (Sn, O and Si) can be clearly observed. A parabolic-like intensity profile of Si distribution shows that during calcination, SiO2 was predominantly formed in the core,59 thus generating a hard sacrificial template for the hollow morphology obtained after chemical etching. Determination of chemical composition in post-etched SnO2 NTs using XRF technique revealed existence of a relatively small amount of residual SiO2 (0.854 wt%) compared to SnO2 (99.1 wt%). Microstructural XRD characterization of SiO2-cored SnO2 NFs and post-etched SnO2 NTs over 2θ = 20-80° (Figure 2b) indicates Bragg diffraction peaks attributable to crystalline cassiterite SnO2 (JCPDS No. 41-4145). Absence of characteristic diffraction peaks corresponding to SiO2 in


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SiO2-cored SnO2 NFs is symptomatic of its existence in amorphous form under our experimental conditions.37 Nonetheless, the amorphous nature of SiO2 upon calcination (at 600 °C) was anticipated in concurrence with previous literature35-36, 60 and is deemed to emanate from delayed crystallization.37,

57-58

For the case of post-etched SnO2 NTs, absence of diffraction peaks

attributable to SiO2 can be explained by the amorphous state of residual SiO2 and/or its presence in quantity beyond detection limit of XRD. These findings correlate well with absence of FTIR spectra at 803 cm-1 and at around 1100 cm-1 for the post-etched SnO2 NTs unlike the SiO2-cored SnO2 NFs (Figure 2c). These spectra are respectively assignable to vibrations of symmetric and asymmetric stretch of the Si-O-Si bond in SiO2.61-63 In our findings, we noticed that a continuous hollow core of several micrometers through the length of the sample (Figure S1, Supporting Information) was successfully achieved after etching under our defined experimental conditions. Accordingly, we presume that the residual SiO2 originates from fortuitous SiO2 in the shell, which existed as a localized and separately trapped phase between grains of SnO2, and possibly, was partially etched.64 In fact, the trapped phase played a significant role in our structure. Calculated average grain size using the Scherrer equation revealed small crystallites (~6 nm) of SnO2 in both SiO2-cored SnO2 NFs and post-etched SnO2 NTs. This size is comparable to twice the space charge length in typical SnO2.55 It is certain that fortuitous SiO2 in the shell effectively inhibited the growth of SnO2 crystallites during calcination.39-40, 64 Moreover, EDS maps of Ptdecorated SnO2 NTs confirmed complete removal of SiO2 from the core during etching, while fortuitous SiO2 in the shell was partially etched leading to formation of mesoporous NTs. In order to investigate the composition and microstructure of the synthesized Pt NPs and Pt-decorated SnO2 NTs, we carried out TEM analysis. Figure 3a shows a typical TEM image of the synthesized metallic Pt NPs. The magnified image in Figure 3a shows crystal structure with


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interlayer spacing of 0.227 nm, in consistency with (111) lattice fringe of Pt (JCPDS No. 65-2868). TEM image in Figure 3b shows a clear morphology of Pt-decorated SnO2 NT with average shell thickness of 25 nm. Surface observation of the HRTEM image reveals intermittent clusters of pores opening through the shell of the NTs (Figure 3c). These pores resulted from etching of fortuitous SiO2 diffused to the surface and/or near-surface region upon calcination of as-spun NFs. Close observation of the HRTEM micrograph in Figure 3d shows lattice fringes with interplanar spacing of 0.339 nm and 0.265 nm assignable to (110) and (101) planes of cassiterite SnO2 respectively. No lattice fringes corresponding to SiO2 are observed, probably due to its amorphous state under our synthesis conditions. The magnified image in Figure 3d shows lattice fringes of anchored NPs on SnO2 NTs, with characteristic interplanar spacing of 0.228 nm and 0.258 nm indexed to (020) and (011) planes of PtO2 (JCPDS No. 23-1306) respectively. Since Pt readily oxidizes upon exposure to air,28, 65 it is not surprising that Pt NPs decorated on SnO2 NTs were, in part, oxidized to higher oxidation states upon heat treatment of the sensing layer. Hence, it is reasoned that observed selected area electron diffraction (SAED) patterns (Figure S2, Supporting Information) stem from crystalline SnO2, Pt NPs and/or PtOx. XPS is a powerful tool for analyzing the surface chemistry of materials. Thus, to complement TEM analysis, the nature of chemical composition and bonding states of constituent elements in Pt-decorated SnO2 NTs were investigated via XPS analysis (Figure 4). Observation of high resolution XPS spectra in Figure 4a, reveals asymmetrical O 1s spectrum, which can be deconvoluted into three Gaussian peaks, with the main peak corresponding to O2- of deficient oxygen in crystal lattice of SnO2 occurring at 530.4 eV while the other two peaks attributed to chemically adsorbed O- and O2- are observed at 531 eV and 532.2 eV, respectively.56 As depicted in Figure 4b, splitting of Sn 3d XPS spectrum into symmetric Sn 3d3/2 and Sn d5/2 spectral peaks


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is observed at 495.25 eV and 486.82 eV, respectively. The two peaks exhibit the doublet signature assignable to Sn4+ in the lattice of SnO2 and are characterized with a spin-orbit energy gap which approximates the standard value reported in literature.66 Absence of other chemical states of Sn indicates that no impurities were incorporated into the lattice of SnO2. To study the chemical states of Pt, we examined the XPS spectral peaks of Pt 4f5/2 and Pt 4f7/2 spin-orbit components in the Pt 4f core level (Figure 4c). The peaks corresponding to Pt 4f5/2 and Pt 4f7/2 levels are separated by a spin-orbit coupling energy of 3.24 eV, which approximates the reported value.28 Spectra corresponding to Pt, PtO, and PtO2 are located at 70.93 eV, 72.16 eV67 and 75 eV68 respectively for Pt 4f7/2. As observed, presence of dominant pure Pt is an indication of partial oxidation of Pt NPs upon heat treatment, probably due to high ramp rate.28 The Si 2p XPS core level (Figure 4d) consists of Si 2p3/2 and Si 2p1/2 spectra centered at 102.3 eV and 103.2 eV respectively, with characteristic area and intensity ratios of 2.69-70 The binding energy of the Si 2p3/2 peak is lower than reported values for SiO2.70-72 The low binding energy has previously been attributed to formation of ionic networks like in soda glass.71

Gas-sensing analysis The gas-sensing characteristics of sensors based on SiO2-cored SnO2 NFs, pure SnO2 NTs, 0.01% Pt-decorated SnO2 NTs, 0.02% Pt-decorated SnO2 NTs and 0.04% Pt-decorated SnO2 NTs were evaluated using 1-5 ppm of analyte gas (H2S, CH3CH2OH, H2, CH3COCH3, C7H8, C5H12, CO and NH3) at constant temperature in a highly ambient humidity (95% RH). High humidity was introduced to mimic similar conditions of moisture in human exhaled breath. In order to understand temperature dependence of response and variation in performance, gas sensing tests were conducted three times under identical conditions by setting the temperature of


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the sensing layer at 250 °C, 300 °C, 350 °C, 400 °C and 450 °C. Figure 5a shows the response vs. temperature characteristics of the fabricated sensors on exposure to 5 ppm of H2S. Exceptionally, 0.02% Pt-decorated SnO2 NTs sensor exhibits the highest response compared to other sensors at optimal working temperature of 300 °C. Upon exposure of the sensor to 5 ppm of H2S, its response (Ra/Rg) increases sharply from 6.9 to 154.8 with a change of temperature from 250 °C to 300 °C, after which it drops off sharply at higher working temperatures (> 300 °C). Observations from other sensors confirm similar trends. Interestingly, a notable response (Ra/Rg = 89.3) to 1 ppm of H2S is observed for 0.02% Pt-decorated SnO2 NTs sensor at 300 °C compared to its Pt-decorated counterparts (Figure S3, Supporting Information). Exemplary high response of 0.02% Pt-decorated SnO2 NTs sensor stems from a substantial increase in concentration of adsorbed oxygen ions following catalytic dissociation of oxygen molecules by anchored Pt NPs73 and contribution of the probable p-n junction at PtOx-SnO2 interfaces. The adsorbed oxygen ions are reportedly to be dominant in form of O- in a temperature range from 200 °C to 400 °C.12 In the case of SnO2 NTs sensor, it displays a lower response (Ra/Rg = 52.8) to 5 ppm of H2S at 300 °C than Pt-decorated SnO2 NTs sensors but with a highest response (Ra/Rg = 45.3) at 250 °C than all other sensors. It is observed that SiO2-cored SnO2 NFs sensor displays a very low response (Ra/Rg < 5) at all temperatures possibly due to high content of poorly conducting SiO2.41 Time-dependent response-recovery transients at 300 °C (Figure 5b) show that a remarkable modulation of resistance is achieved when high concentration of H2S interact with SnO2 surface. As the concentration lowers, only a fraction of O- species is consumed, not sufficient to induce a significant resistance change as accentuated by a progressive decay in response. All fabricated sensors follow this trend while maintain n-type


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characteristic of metal oxide semiconductors. The sensing mechanism is presented in the Supporting Information (Figure S4). Intriguing ultrahigh selectivity to H2S against other interfering gases is another feature that demonstrates distinguishable sensing qualities of our material. Figures 5c-d compare selectivity of 0.02% Pt-decorated SnO2 NTs sensors toward different reducing gases at 5 ppm and 1 ppm, respectively. We have noticed that 0.02% Pt-decorated SnO2 NTs sensor exhibits the most enhanced response pattern to H2S incomparable to other gases. In fact, its threshold of detection is 0.1 ppm (Figure 5e), thus, demonstrating outstanding resolution to low concentration of H2S molecules. This peculiarity in selectivity to H2S can be explained by the hydrophilic nature of SiO2 residues. Ideally, localized SiO2 on SnO2 crystallites not only inhibits growth of SnO2 nanocrystallites, but also acts as active centers for adsorption of humidity.40 Presence of hydrophilic SiO2 favors the direct chemisorption of H2S on SnO2 surface through donation of lone pair electrons from sulfur (S) into SnO2. This preferential chemisorption of H2S is due to a lower electronegativity difference between hydrogen and sulfur constituting its H-S bond than between hydrogen and oxygen atoms in humidity.74-76 Since SiO2 is dielectric and does not participate in sensing,41 it plays humidity adsorption role similar to that of NiO reported by Kim et al.,77 and thus, interference of humidity with H2S sensing remains insignificant as long as SiO2 loading is optimal and its segregated phases on SnO2 crystallites remain isolated.40-41 It is thought that hydrophilic nature of amorphous SiO2 in a high ambient humidity (95% RH) is responsible for low sensor response to CH3CH2OH, H2, CH3COCH3, C7H8, C5H12, CO and NH3 molecules. Although H2S has the lowest H-S bond energy (381 kJ/mol) compared to these interfering gases,78 this energy is comparable to that of the C-H (393 kJ/mol) bond in acetone and pentane and therefore similar response would be expected with acetone and pentane.


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However, it is thought that the aforementioned factors render high response and selectivity toward H2S in the presence of these chemical molecules even though their bond energies are comparable. To further determine the effect of humidity on H2S sensing, we conducted H2S sensing tests at 55%, 75% and 95% RH using 0.02% Pt-decorated SnO2 NTs sensor. In general, the sensing results (Figure S5a, Supporting Information) shows a small increase in response from 154.8 at 95% RH to 157.1 and 163.3 at 75% RH and 55% RH respectively, when the sensors are exposed to 5 ppm of H2S. However, inconsistent trend was observed at 1 ppm where the response was 89.3, 88.5 and 87.5 at 95%, 75% and 55% RH, respectively. Small differences in response at the three conditions of humidity signify that the effect of humidity is not very significant. By introducing humidity (95% RH) separately as analyte gas, only a small response (Ra/Rg = 2) was observed (Figure S5b, Supporting Information), which is much lower than obtained for H2S at the same condition. It is noteworthy to mention that response times (58.1 s and 99.5 s at 5 ppm and 1 ppm respectively) of 0.02t% Pt-decorated SnO2 NTs sensor displayed in Figure 5f are still high for real time sensing although they are shorter than the recovery times (122.3 s and 111.6 s at 5 ppm and 1 ppm respectively). The response times of undecorated SnO2 NTs sensor are 297 s and 127.6 s at 5 ppm and 1 ppm respectively, whereas the recovery time is 71.2 s at 5 ppm, with no recovery at 1 ppm (Figure S6, Supporting Information). The performance of 0.02% Pt-decorated SnO2 NTs sensor compared to reported SnO2-based H2S sensors is shown in Table 1. Although the response of our fabricated sensor is high compared to reported sensors, it can be observed that it exhibits high selectivity and response to sub-ppm concentration of H2S, with low limit of detection.


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In addition to sensing mechanism, structural architecture of our fabricated NTs offers unique features for enhanced gas sensing performance in two aspects. First, small crystallites of SnO2 facilitate enhanced functionalities in gas sensing. Presence of fortuitous SiO2 in the shell (prior to etching) leads to inhibition of crystal growth of SnO2 during calcination, whereas etched SiO2 phase is a reason for the tubular morphology and mesopores necessary for diffusion of analyte gas (see the schematic, Figure 6a). For pristine SnO2, the crystal size typically ranges in the order of 12-16 nm for samples calcined at 600 °C.40, 55 In contrast, introduction of SiO2 in our case, we achieved average crystal size of ~6 nm, which is essential for a dramatic change of resistance in virtue of possible extension of electron depletion layer into the entire bulk of SnO2 crystals when exposed to air. Second, post-etched SnO2 NTs are characterized with considerably large surface area (125.63 m2/g), small average pore diameter (r = 6.56 nm) and large pore volume (0.206 cm3/g) (Figure 6b). The exhibited large surface area provides innumerable reaction sites required to increase air adsorption and interaction with the analyte gas. It is reasoned that the small pore size exhibited here is inferior to effect a meaningful diffusion of analyte gas (since r ∝ Dk, Dk = Knudsen diffusion coefficient). However, their cumulative effect has a tremendous impact. In this regard, the large pore volume of our post-etched SnO2 NTs is therefore thought to originate from the cumulative diffusion of analyte gas through the tube as well as countless clusters of pores formed on the surface of NTs as confirmed by TEM observation (Figure 3c). Moreover, ultrasmall thickness (~25 nm) of the shell effectively facilitates diffusion of air and analyte gas, favoring a substantial modulation of inter-grain flow of electrons. Such features merit comprehensive sensing performance using small amount of catalyst as observed.

Conclusions


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In summary, for the first time, we have synthesized mesoporous SnO2 NTs with shell thickness of 25 nm, containing residual SiO2 by selective chemical etching of amorphous SiO2 from calcined single-nozzle electrospun SnO2-cored SnO2 NFs. The 0.02% Pt-decorated SnO2 NTs sensor exemplifies exceptional sensing property even to trace concentrations as low as 0.1 ppm of H2S. It is reasoned that the high response is attributed to the hollow morphology of synthesized NTs, small grain size, existence of clusters of pores on the surface as well as functionalization with Pt/PtOx NPs. These features facilitate fast diffusion of H2S and efficient reaction of H2S with adsorbed oxygen ions. The formation of 1-D core-shell structures demonstrated in this work is solely dependent on incompatibility of SiO2 and SnO2 phases and existence of a concentration gradient of SnO2 during calcination. For that reason, we ascertain that part of SiO2 phase was trapped and remained segregated between grains of SnO2, and possibly, it was partially etched resulting in mesoporous NTs. We have further noticed that presence of SiO2 residues in mesoporous SnO2 NTs provides a robust gas-sensing material with enhanced selectivity toward H2S. Therefore, considering the peculiar features of our material and the superior sensing qualities demonstrated in this work, the material proves potential suitability for application as a sensing layer in chemiresistive sensors for breath analysis, specifically, for diagnosis of halitosis.

ASSOCIATED CONTENT Supporting Information Additional TEM image of SnO2 NTs (Figure S1), SAED patterns of Pt-decorated SnO2 NTs (Figure S2), sensor response vs temperature at 1 ppm (Figure S3), gas-sensing mechanism (Figure S4), graphical representation of humidity effects (Figure S5), response/recovery times of


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SnO2 NTs sensor (Figure S6), variation of baseline resistances with temperature (Figure S7). This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Wearable Platform Materials Technology Center (WMC) funded by the National Research Foundation (NRF) of Korea Grant of the Korean Government (Ministry of Science, ICT and Future Planning) (No. 2016R1A5A1009926). This work was also supported by Biomedical Treatment Technology Development Project (No. 2015M3A9D7067418), National Research Foundation (NRF) of Korea Grant (No. NRF-2015R1A2A1A1A6074901) and Global Frontier Project (CISS-2011-0031870), all funded by the Ministry of Science, ICT and Future Planning.


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Figure 1

Figure 1. (a) Schematic representation of the procedure for synthesis of SiO2-cored SnO2 NFs and Pt-decorated SnO2 NTs. (b) As-spun NFs. (c) Dense morphology of obtained SiO2-cored SnO2 NFs after calcination of as-spun NFs at 600 °C for 2 h. (d) Obtained SnO2 NTs following chemical etching of SiO2 from SiO2-cored SnO2 NFs using NaOH solution. (e) TEM image of Pt-decorated SnO2 NTs. (f) EDS mapping of elemental distribution in Ptdecorated SnO2 NTs.


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Figure 2

Figure 2. (a) Compositional line profile of constituent elements in calcined electrospun SiO2-cored SnO2 NFs. (b) XRD patterns of SiO2-cored SnO2 NFs and SnO2 NTs. (c) Collected FTIR spectra of SiO2-cored SnO2 NFs and SnO2 NTs corresponding to bonding states before and after chemical etching respectively.


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Figure 3

Figure 3. (a) TEM image of well-dispersed Pt NPs synthesized by Polyol reduction method. (b) Morphology of Pt-decorated SnO2 NTs. (c) HRTEM image showing clusters of pores on the surface of Pt-decorated SnO2 NTs. (d) HTREM image of Pt-decorated SnO2 NTs.


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Figure 4

Figure 4. High resolution XPS scan profile spectra of constituent elements in Pt decorated SnO2 NTs sample showing (a) O 1s peak. (b) Sn 3d region. (c) Pt 4f region. (d) Si 2p region of residual SiO2.


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Figure 5

Figure 5. (a) Variation of sensor response with operating temperature at 5 ppm of H2S. (b) Time-dependent response-recovery profiles of sensors at varying concentrations of H2S at 300 °C. (c) and (d) Selectivity to H2S in the presence of interfering gases at 5 ppm and 1 ppm of H2S respectively at a working temperature of 300 °C. (e) Variation of sensor response with low concentration of H2S at 300 °C. (f) Variation of response and recovery times of 0.02% Pt-decorated SnO2 NTs sensor at varying concentrations of H2S.


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Figure 6

Figure 6. (a) Schematic illustration of mesopores and diffusion of H2S in Pt-decorated SnO2 NTs. (b) Pore size distribution of SnO2 NTs measured by N2 adsorption-desorption method.


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Table 1

Table 1. Reported H2S sensing performance of SnO2-based chemiresistive sensors Temperature (°C)

Response (Ra/Rg)

Response/ Recovery time (s)

Relative humidity (%)

Response (Ra/Rg) at detection limit

References

SnO2 quantum wire/rGO

22

33 to 50 ppm

2/292

56-60

1.02 to 43 ppb

7

CuO-SnO2 hollow spheres

300

22.4 to 1 ppm

15/--

80

1.2 to 100 ppb

8

Au-SnO2 hollow spheres

400

17.4 to 5 ppm

18/--

90

5.7 to 100 ppb

27

Au-SnO2 NTs

300

34 to 5 ppm

~35/--

90

1.2 to 30 ppb

28

rGO-SnO2 NFs

200

33.7 to 5 ppm

~115/~100

85-95

5.1 to 1 ppm

45

Pt-decorated SnO2 NTs

300

89.3 to 1 ppm

99.5/111.6

95

3.0 to100 ppb

This work

Material


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Mesoporous SnO2 Nanotubes via ElectrospinningâEtching Route

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