Macroporous SnO2

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Functional Nanostructured Materials (including low-D carbon) 2

Facile Synthesis of Pt-Functionalized Meso/Macroporous SnO Hollow Spheres through In-Situ Templating with SiO for HS Sensors 2

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Peresi Majura Bulemo, Hee-Jin Cho, Dong-Ha Kim, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00901 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

Facile Synthesis of Pt-Functionalized Meso/Macroporous SnO2 Hollow Spheres through In-Situ Templating with SiO2 for H2S Sensors Peresi Majura Bulemo, Hee-Jin Cho, Dong-Ha 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.

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ABSTRACT Whereas single-nozzle electrospraying seems a versatile technique in synthesis of spherical semiconducting metal oxide (SMO) structures, the synthesized structures find limited use in gas-sensing applications due to their thick and dense morphology, which minimizes accessibility of their inner surfaces. Herein, unprecedented spherical SiO2@SnO2 core-shell structures are synthesized upon calcination of single-nozzle as-electrosprayed spheres (SPs) containing tin (Sn) and silicon (Si) precursors. Subsequent etching of SiO2 in NaOH (pH 12) affords meso/macroporous SnO2 hollow spheres (HSPs) with short diffusion length (31.4 ± 3.1 nm), small crystallites (15.5 nm), and large BET surface area (124.8 m2 g-1). Apart from surface meso/macropores, diffusion of gases into porous SnO2 sensing layers is realized through inner interconnection of voids of the SnO2 HSPs into a three-dimensional (3D) network. Functionalization of the postetched SnO2 HSPs with platinum nanoparticles (Pt NPs) at 0.08 wt% yields gas-sensing materials with outstanding response (Ra/Rg = 1.6, 10.8, and 105.1 to 0.1, 1, and 5 ppm of H2S, respectively), and selectivity toward H2S against interfering gas molecules at 250 °C. The SiO2 phase in the postcalcined SiO2@SnO2 SPs acts as a sacrificial template for pore creation, and crystal growth inhibition, whereas small amount of SiO2 residues in HSPs enhances selectivity.

KEYWORDS electrospraying, core-shell, etching, SnO2 hollow spheres, gas sensor


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1. Introduction Spherical hollow SMOs surpass their solid counterparts in gas-sensing applications due to high surface reaction sites that are readily accessible by target gas molecules. With regard to thick and solid spheres, the associated surface and/or decaying ‘Knudsen type’ diffusion of gases render inferior surface reactions and relatively low sensor sensitivity.1-3 Among SMOs, SnO2 has attracted attention for gas-sensing applications because of its nonstoichiometry, wide band gap, good stability, and high electronic mobility properties.4-6 Most strenuous efforts have been devoted to improving its sensitivity and poor intrinsic selectivity. Moreover, unlike SMOs such as WO3,7-8 and Co3O4,9 gas sensors based on SnO2 have been reported to exhibit cross-sensitivity to humidity, which affects its sensing performance.10-11 With regard to spherical hollow structures therefore, meso- and macroporous SnO2 structures, particularly when functionalized with catalysts, are most desirable to overcome these limitations. Mesopores and macropores are responsible for maximized Knudsen and molecular diffusion of gases into the sensing structure, respectively.12 Several synthesis approaches for spherical hollow SnO2 gas-sensing structures have been attempted, particularly through hard and soft templating,13-19 precipitation,20 templatefree hydrothermal synthesis,21-23 and spray pyrolysis.24-26 Although coaxial electrospraying is a straightforward technique, it is almost entirely dedicated to synthesis of core-shell drug carriers, usually in the order of micrometer-range sizes.27-30 On the other hand, reports utilizing singlenozzle electrospray method to synthesize spherical hollow SMOs for gas-sensing applications are scarce. In general, electrospraying (i.e., atomization of solution into spherical droplets of like charges) is an intriguing technique capable of producing non-agglomerated SPs with high packing density and intersphere contacts. During electrospraying, the applied electrostatic force


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induces the droplets at the tip of the ejecting nozzle with positive charge, and consequently, enabling self-dispersion and size control as they traverse to the grounded collecting substrate.30-32 Typically, a suspended Taylor cone undergo Coulombic fission as the applied electrostatic forces reaches the Rayleigh limit.33 As the droplets traverse to the substrate, the solvent evaporates in line with increasing charge density.30 Further splitting into offspring droplets takes place concomitant with increasing surface area and fast drying as more solvent evaporates. Eventually, dried droplets get deposited on the substrate in form of spherical particles. Although one would consider SMO-based electrosprayed SPs as ideal for intersphere electrons flow during gas sensing, SPs electrosprayed from metal precursor solutions are usually thick and dense since electrospray solutions must contain small amount of sacrificial polymer to avoid formation of nanofibers, and thus no significant mesopores and macropores are created upon decomposition of the polymer at calcination stage. Although mesospheres can be electrosprayed from solutions containing presynthesized nanoparticles,34 the particles are large in size and unfavorable for gassensing applications. Therefore, there remains a need to enhance porosity in spherical SnO2 structures synthesized via electrospraying to maximize accessibility of their inner surfaces for improved gas-sensing performance. The present contribution introduces, for the first time, a strategy to synthesize meso/macroporous SnO2 HSPs via a combination of single-nozzle electrospray and selective wet etching techniques. Here, we take advantage of differences in heat of formation of SiO2 and SnO2, and their incompatibility to synthesize SiO2@SnO2 core-shell structures (hereafter referred to as SiO2@SnO2 SPs) during calcination of as-electrosprayed SPs,35-36 after which the core (SiO2) is etched away using NaOH solution to form meso/macroporous SnO2 HSPs. The electrospray solution is a homogeneous dispersion of Si and Sn precursors. This facile in-situ


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templating approach not only forms the core-shell configuration but also eliminates the conventional templating step.37 In our erstwhile treatise,38 an embodiment of a similar approach was thoroughly demonstrated by employing a combination of electrospinning and wet etching techniques, and provided a compelling proof that the method works perfectly well in the case of synthesizing mesoporous SnO2 NTs, which exhibited large surface area and superior gas-sensing qualities toward H2S. Functionalization (through decoration) of pristine SnO2 NTs with Pt NPs enhanced the response/recovery characteristics in H2S sensing, similar to past report by Dong and coworkers.39 Motivated by the uniqueness of the erstwhile approach, we intuitively evaluate the newly synthesized SnO2 HSPs and their Pt-functionalized derivatives as materials for chemiresistive sensing of disease biomarkers in human exhaled breath, in particular hydrogen sulfide (H2S) against interfering gases namely ethanol (CH3CH2OH), hydrogen (H2), toluene (C7H8), acetone (CH3COCH3), ammonia (NH3), carbon monoxide (CO), and pentane (C5H12). Finally, we show that sensors fabricated from these materials demonstrate superior response and selectivity toward H2S.

2. Experimental Section 2.1

Materials Tin(II) chloride dihydrate (SnCl2·2H2O), tetraethyl orthosilicate (TEOS, ≥ 99.0% GC), N,N-

Dimethylformamide (DMF, anhydrous, 99.8%), ethanol, polyvinylpyrrolidone (PVP, MW = 1,300,000 g mol-1), sodium hydroxide solution (NaOH, 50% w/w) were purchased from Sigma Aldrich. All chemicals were reagent grade; no further purification was needed.

2.2

Synthesis of SiO2@SnO2 SPs


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A homogeneous solution containing SnCl2.2H2O, TEOS, PVP, and DMF was electrosprayed into SPs using an electrospinning machine employing a 25-gauge single-nozzle syringe needle. Briefly, the electrospray solution was prepared at room temperature as follows: 0.15 g of SnCl2.2H2O was first dissolved in 1.35 g of DMF under stirring at 500 rpm for 1 h. Afterwards, 50 µL of TEOS was added to the above solution and further stirred for 1 h. Finally, 0.15 g of PVP was added, and the solution was maintained under identical stirring condition for 4 h. The obtained solution was transferred into a 12-mL syringe, and was fed through the needle by a syringe pump during electrospraying. Electrospraying conditions were adjusted as follows: the tip of the solution-ejecting needle was set at a potential of 9.5 kV relative to the grounded stainless steel substrate separated by 10 cm, while the flow rate of the solution from a suspended Taylor cone (at the tip of the needle) was maintained at 0.05 mL min-1 under the applied potential. As-electrosprayed SPs were transformed into SiO2@SnO2 core-shell SPs (hereafter referred to as SiO2@SnO2 SPs) following calcination at 600 °C for 2 h.

2.3

Synthesis of SnO2 HSPs SnO2 HSPs were synthesized through wet chemical etching of SiO2 in postcalcined core-shell

SPs (i.e., SiO2@SnO2 SPs). Here, the encapsulated SiO2 was used as a template for realization of the porous hollow morphology. To etch away the template, SiO2@SnO2 SPs were introduced into a disposable plastic vial containing deionized (DI) water and NaOH solution adjusted to pH 12.40 The vial and its contents were left in a bath of silicon oil at a constant temperature (50 °C). After 4 h, the as-obtained SnO2 HSPs were collected by using a centrifuge (set at a speed of 3,000 rpm). Then, the HSPs were thoroughly cleaned in DI water through two cycles of centrifugation and redispersion to remove excess etchant and as-formed soluble silicates. The


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cleaned postetched SnO2 HSPs were dried overnight under air atmosphere in a laboratory oven at 50 °C.

2.4

Functionalization of SnO2 HSPs with Pt NPs Experimental polyol synthesis procedure of Pt NPs (1,823 mg L-1) is available in the

Supporting Information. To prepare Pt functionalized SnO2 HSPs, 2.25 mg of postetched SnO2 HSPs was first dispersed by sonication in 120 µL of ethanol for 5 min. Appropriate amount of as-prepared Pt NPs solution was added to the above suspension, and the two solutions were intermixed through sonication (for 1 min) to make a Pt-SnO2 paste comprising 0.04 wt% of Pt. To facilitate binding of Pt NPs to the HSPs, 1 drop of PVP (5 wt% in ethanol) from 1 µL pipette volume was also added to the prepared paste. The procedure was repeated for preparation of other Pt-SnO2 pastes at 0.08, 0.12, and 0.16 wt% Pt loading. The pastes were used for preparation of gas-sensing layers.

2.5

Sensor Fabrication Gas-sensing layers were prepared on alumina substrates. The substrates (2.5 mm long,

2.5 mm wide, and 0.2 mm thick) consisted of two gold electrodes (25 µm thick and spaced at 70 µm) on the top surface and Pt microheater on the bottom surface. Each terminal of gold electrodes and Pt microheater was joined to individual Pt microwires (≈4 mm long) to make four terminals, and thereafter, the prepared pastes were drop-coated, dried, and annealed. Typically, individual pastes (1 µL) were drop-coated on the top surface of the alumina substrates to make uniform sensing layers followed by 10 min drying on a hot plate at 65 °C, and 30 min annealing


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in air in a furnace at 500 °C (ramp rate = 10 °C min-1 from room temperature). Four sensors based on Pt-functionalized SnO2 HSPs were fabricated, and for simplicity denoted as Pt-SnO2 HSPs (0.04 wt%), Pt-SnO2 HSPs (0.08 wt%), Pt-SnO2 HSPs (0.12 wt%), and Pt-SnO2 HSPs (0.16 wt%), where the respective wt% of Pt NPs are shown in parentheses. Two more sensors based on non-functionalized forms of postcalcined SPs and postetched HSPs were also fabricated and denoted as SiO2@SnO2 SPs, and SnO2 HSPs, respectively. Finally, each alumina substrate (coated with postannealed sensing layer) was mounted on a sensor holder through the four Pt microwires before insertion into a test chamber for gas-sensing measurements.

2.6

Gas-Sensing Measurements Gas-sensing tests were implemented using target gases namely H2S, CH3CH2OH, H2, C7H8,

CH3COCH3, NH3, CO, and C5H12. Highly humid environment was maintained (relative humidity, RH = 90%) during the measurements to mimic the condition of human exhaled breath. The sensing equipment consisted of data acquisition (34972A, Agilent) system, temperature control (E3647A, Agilent) unit, and a 16-channel sensor test chamber (34902A, Agilent). The equipment was homemade, similar to that reported elsewhere.41-42 Resistance variation during air-target gas exposure cycles was acquired via the acquisition system that triggered the sensor response (Ra/Rg; where Ra and Rg are resistances upon exposure of the sensor to ambient air and target gas, respectively). Sensor measurements were investigated by setting the DC power supply to 3.0, 3.6, 4.3, 5.0, and 5.7 V corresponding to sensing temperatures of 200, 250, 300, 350, and 400 °C, respectively. At each individual temperature, the sensors were tested using a constant continuous flow of target gas at concentrations of 1, 2, 3, 4, and 5 ppm introduced in turn (that is, at a 10 min time lapse for each turn), with intermediate introduction of air (for other 10 min)


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after each turn. All measurements were taken at constant temperature and relative humidity. To further investigate the gas-sensing capability of the fabricated sensors, sensing tests were carried out using 0.1, 0.2, 0.4, and 0.6 ppm of target gas at optimal operating temperature. It is noteworthy to mention that sensing layers showed stable adhesion to alumina substrates even after gas sensing tests at 400 °C (Figure S1, Supporting Information). To determine accuracy and repeatability of measurements of prepared sensors, characterization was carried out three times at each test condition. The flow and concentrations of target gas were automated with the aid of solenoid valves and mass flow controllers run by LabVIEW program as reported elsewhere.41, 43 The target (inlet) gas was diluted with air (20 µmol of balance air per mole of target gas), and water vapour was introduced to control humidity. To deduce the effect of relative humidity at optimum sensing conditions, supporting measurements were taken at a lower relative humidity (RH = 50 %), and the results were compared with those obtained at 90% RH. The relative humidity was estimated using commercially available humidity sensor (605-H1, Testo Inc.). The scheme of the sensing equipment is available in the Supporting Information (Figure S2).

2.7

Materials Characterization Morphologies and lattice parameters of synthesized structures were analyzed by employing

scanning electron microscopy (SEM, XL-30 SFEG, Philips), and field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30 S-Twin, FEI). The FE-TEM was equipped with energy-dispersive spectrometry (EDS) facility for elemental characterization. The crystal structure of the samples was probed using X-ray diffraction (XRD, D/Max-2500, Rigaku) employing CuKα radiation, λ = 1.5406 Å, whereas the pore size distribution and characteristic surface areas of the samples (SiO2@SnO2 SPs and postetched SnO2 HSPs) were measured using


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Barrett-Joyner-Halender (BJH) and Brunauer-Emmett-Teller methods (BET, TriStar II 3020), respectively. To gain a deeper understanding on composition and bonding states in the Pt-SnO2 HSPs samples, elemental X-ray photoelectron spectra were collected by X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific) while X-ray fluorescent spectroscopy (XRF, ZSX Primus II, Rigaku) enabled the determination of chemical composition in the samples.

3. Results and Discussion 3.1 Morphology and Structural Characterization Figure 1a illustrates processing steps employed in the formation of postetched SnO2 hollow spheres functionalized with Pt NPs (hereafter referred to as Pt-SnO2 HSPs). The proposed formation mechanism of Pt-SnO2 HSPs follows a similar fashion as in the synthesis of SnO2 NTs described elsewhere.38 In brief, SiO2@SnO2 SPs were achieved upon calcination of aselectrosprayed SPs, where Si and Sn precursors oxidized into incompatible SiO2 and SnO2 phases, respectively. During the process, SnO2 preferentially occupied the shell owing to its lower enthalpy energy of formation than that of SiO2,36 which formed the core. Of critical significance here is the higher abundance of air and temperature on the surface of the aselectrosprayed SPs during calcination, which favored the formation of SnO2 on the surface before it formed in the interior, leading to a concentration gradient. Under this condition, a subsequent diffusion of Sn ionic species toward the exterior occurred due to the Kirkendall effect. Finally, the diffused Sn ionic species oxidized, whilst SiO2 concomitantly formed in the interior resulting in SiO2@SnO2 SPs as shown in the schematic. Structurally robust SnO2 HSPs


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were formed upon wet etching of core SiO2 in postcalcined core-shell SPs using NaOH solution.14, 40 Thermal gravimetric analysis and differential thermal gravimetric analysis (TGA/DTG) of as-electrosprayed SPs (Figure S3) revealed that thermal decomposition of PVP and organic residues occurred below 550 °C.44 At temperatures above 550 °C, a constant weight of the sample was observed, indicating that all organic matters had decomposed to form SiO2@SnO2 SPs. From this observation, it was confirmed that a temperature of 600 °C was sufficient to calcine the as-electrosprayed SPs, similar to previous report.45 Figure 1b elucidates the morphology of postcalcined SiO2@SnO2 SPs. Nearly monodisperse postcalcined SPs with welldefined sphericity were synthesized. Postetched structures appeared spherical with hollow interiors (Figure 1c,d). As suggested in the formation mechanism, SnO2 HSPs were formed after etching away the in-situ generated SiO2 core template. Core-shell configuration of the SiO2@SnO2 SPs was investigated by STEM linescan across the cross-section of a randomly selected postcalcined SP. The resulting linescan profiles (Figure S4) revealed a parabola-like distribution of Si, which confirmed the formation of a core-shell structure. The spatial distribution of elemental components in a Pt-SnO2 HSP indicated uniform distribution of Sn, O, Si, and Pt elements (see EDS maps in Figure 1e). The observed Si maps are attributed to SiO2 residues that remained after removal of core SiO2 template. It is suspected that during calcination of as-electrosprayed SPs, SiO2 was largely formed in the core while a small fraction of SiO2 was also partially formed and internally segregated in the shell. During the etching step, some of the SiO2 layers in the shell were partly etched or completely remained unetched. In addition to the achieved hollow morphology, another intriguing feature of postetched structures was the interconnection of inner voids into a three-dimensional (3D) network. The


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formation mechanism is schematically illustrated in Figure 1f. Typically, droplets ejected from the Taylor cone (at the tip of the needle) experience simultaneous actions of gravitational and electrostatic forces as they move toward the substrate. Upon deposition of the spherical droplets of precursors on the substrate (or to be precise, on top of their predecessors), their contact points somewhat overlap. It is obvious that during calcination, cores of adjacent SiO2@SnO2 SPs overlapped at contact points. Etching away of the encapsulated SiO2 in the core and overlapping regions (at sphere-sphere contacts) led to the 3D hollow morphology as displayed in Figure 1g. The origin of exterior macro-openings on postetched SnO2 HSPs has not been explicitly established at this point, but we intuitively suppose they originated, apparently, at the thinnest portions of the shell that severely weakened and collapsed during etching. It is also possible that detachment and complete separation of HSPs could occur at the originally overlapped regions resulting in the observed macro-openings. Evaluation of phases in SPs and HSPs was, in part, accomplished using XRD (Figure S5). The X-ray diffraction peaks of postcalcined SiO2@SnO2 SPs, postetched SnO2 HSPs, and Ptfunctionalized derivative of postetched sample (i.e., Pt-SnO2 HSPs loaded with Pt at 0.08 wt%, and annealed at 500 °C for 30 min) exhibited tetragonal SnO2 structure (JCPDS No. 41-4145). No phases belonging to SiO2, and pure Pt/PtOx were observed due to the amorphous nature of SiO2 and very low loading amount of Pt NPs. Quantitative XRF measurements of elemental composition provided content of SiO2 residues at 0.99, 1.03, 1.08, and 1.01 wt% in 0.04, 0.08, 0.12, and 0.16 wt% Pt-functionalized SnO2 HSPs, respectively (hereafter referred to as Pt-SnO2 HSPs (0.04 wt%), Pt-SnO2 HSPs (0.08 wt%), Pt-SnO2 HSPs (0.12 wt%), and Pt-SnO2 HSPs (0.16 wt%), respectively). From calculations using the Scherrer’s equation, the average size of SnO2 crystallites (based on (110), (101) and (211) peaks) was 15.5 nm in both SiO2@SnO2 SPs


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and SnO2 HSPs samples, and 15.9 nm in Pt-SnO2 HSPs (0.08 wt%) sample. As observed, the change in size was relatively small, and we attribute it to additional annealing of the Ptfunctionalized sample in air (500 °C for 30 min). It is inferred that coexistence of SiO2 with SnO2 entails structural merits in terms of size control of SnO2 crystallites similar to past literature.38, 46 TEM analyses were performed to investigate structural and crystalline characteristics of synthesized samples. As-synthesized quasi-spherical Pt NPs (6.4 ± 1.1 nm in size) consisted of interplanar spacing of 0.225 and 0.195 nm belonging to the (111) and (200) planes, respectively (Figure S6). The dense morphology of postcalcined SiO2@SnO2 SPs was evident, even at high magnification (Figure S7). As for Pt-SnO2 HSPs, an etched structure (62.8 ± 6.2 nm thick) was clearly observed at a high TEM magnification (Figure 2a). HRTEM image of the Pt-SnO2 HSPs (0.08 wt%) (Figure 2b) comprised lattice fringes spaced at 0.335 and 0.264 nm, which were characteristic of (110) and (101) planes of tetragonal SnO2, respectively. No phases of anchored Pt NPs were observed at this magnification, possibly due to small amount of Pt catalyst. We carried out XPS analysis to investigate chemical bonding characteristics of Pt-SnO2 HSPs. Figure 2c shows the Sn 3d signal resolved into Sn 3d5/2 and Sn 3d3/2 doublet components with peak maxima well-centered at 486.5 and 495.0 eV, respectively, which were representative signature of Sn4+ in tetragonal SnO2.47-48 However, for non-functionalized SnO2 HSPs, the Sn 3d5/2 and Sn 3d3/2 doublet maxima were centered at 486.49 and 494.89 eV, respectively, which were a little lower than observed for Pt-SnO2 HSPs (Figure S8). The O 1s peak in Figure 2d was coherently fitted with three peaks located at 530.3, 531.0, and 532.0 eV corresponding to deficient oxygen in the stoichiometry of tetragonal SnO2 and chemisorbed O-, and O2- species, respectively.47 As displayed in Figure 2e, Pt 4f core level showed a pair of Pt 4f7/2 and Pt 4f5/2


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peaks separated at a spin-orbit coupling energy gap of 3.25 eV.48 The extent of oxidation was evaluated by observing the Pf 4f7/2 peak, which confirmed presence of two distinct oxidized forms of Pt at 72.0 eV,49 and 74.5 eV,50 corresponding to PtO (12.48%) and PtO2 (1.17%), respectively, while a large percent of Pt NPs (86.35%) remained in zerovalent state (Pt0) upon annealing. A large number of NPs remained unoxidized due to rapid annealing (rate =10.0 °C min-1), in agreement with previous observations.38, 50 Another interesting observation was the significant shift of the Pt0 signal toward lower binding energy of 70.6 eV,51 inconsistent with that of bulk Pt.52 This behavior might be attributed to the electrons transfer between SnO2 and anchored Pt NPs, triggered by the higher work function of Pt compared to SnO2.51, 53-55 For the case of residual Si (Figure 2f), the Si 2p3/2 and Si 2p1/2 peaks were centered at 102.7 and 103.6 eV, respectively, with area and intensity ratios corresponding to characteristic peaks of SiO2.56-57 Nitrogen adsorption/desorption measurements showed that SnO2 HSPs exhibited type 4 isotherm feature typical for mesoporous materials, with a BET surface area of 124.8 m2 g-1 (Figure 3a). The observed H2-type hysteresis of SnO2 HSPs relates to disorderly arrangement and inhomogeneity of interconnected mesopores.58 Unlike postetched SnO2 HSPs, postcalcined SiO2@SnO2 SPs exhibited type 1 isotherm feature with negligibly small hysteresis typical for microporous materials. It is surprising that SiO2@SnO2 SPs exhibited a larger BET surface area (197.9 m2 g-1) compared to their postetched SnO2 HSPs counterparts, radically different from reported findings.59-60 The smaller BET surface area of SnO2 HSPs is arguably attributed to two reasons. First, during electrospraying, apparent instability of an electrospray jet could result in sprinkling of fine droplets of the solution onto the already formed SPs, which remained as particles after calcination. The so-formed particles were tiny with high surface energy, and easily


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aggregated (Figure S9a). The possibility exists that shells of the aggregated particles disintegrated during etching resulting in much finer particles. Second, the shells of postcalcined SPs partially collapsed into fragments upon etching, and during formation of exterior macroopenings (Figure S9b). The fragments formed in both cases aggregated and filled the intersphere spaces, resulting in smaller surface area of SnO2 HSPs. Figure 3b shows a plot of pore volume against pore size. The Barrett-Joyner-Halender (BJH) desorption average diameter of pores in SnO2 HSPs was 6.3 nm while it was 2.8 nm for the case of SiO2@SnO2 SPs. The decomposition of sacrificial PVP (the sphere-forming template) during calcination of as-electrosprayed SPs effectuated the formation of micropores in individual SPs, and effective macropores (interstitial spaces between neighboring SPs) due to packing while the etching process accounted for extra porosity in form of mesopores within shells, the hollow morphology, exterior macro-openings, and the interior intersphere macro-openings as already shown in Figure 1g. Here, the porosity resulting from the two processes was categorized as primary porosity and secondary porosity, respectively. The postcalcined SiO2@SnO2 SPs exhibited the former, while postetched HSPs exhibited both categories. Although postcalcined SiO2@SnO2 SPs exhibited larger BET surface area, their BJH desorption cumulative volume of pores in the range of 1.7–300 nm diameter was only 0.13 cm3 g-1, which was much less than 0.2 cm3 g-1 of SnO2 HSPs in the same range. Higher pore density and larger pore size observed for postetched SnO2 HSPs evince the role of etching in pores formation.

3.2 Gas-Sensing Results Figure 4 exhibits typical gas-sensing characteristics of the six fabricated sensors (SiO2@SnO2 SPs, SnO2 HSPs, Pt-SnO2 HSPs (0.04 wt%), Pt-SnO2 HSPs (0.08 wt%), Pt-SnO2


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HSPs (0.12 wt%), and Pt-SnO2 HSPs (0.16 wt%) sensors) measured in a humid environment (relative humidity, RH = 90%). All sensors reached maximal responses to 5 ppm of H2S in a range of working temperature between 200 and 400 °C (Figure S10). A difference in response to target gas was clearly observed at 0.04 and 0.08 wt% of Pt loading, which could be attributed to the increase in catalytic activity of Pt NPs.61 In general, low Pt content is insufficient to notably catalyze adsorption and desorption reactions of gas molecules on Pt-SnO2 HSPs when compared with higher contents, which could be the reason for lower sensor response observed at 0.04 wt% of Pt loading.48 Optimal amount of Pt loading (with respect to SnO2) was achieved at 0.08 wt% (sensor response, Ra/Rg = 105.1 and Ra/Rg = 10.8 at 5 ppm and 1 ppm, respectively), above which the response decreased (Figure 4a). The same trend was observed over the entire range of gas concentration (1-5 ppm). A lower response to 1-5 ppm of H2S upon Pt loading > 0.08 wt% was rather counter-intuitive. It is speculated that amount of Pt in the order of ≈0.08 wt% set a limit of catalysis, and that the effect of additional Pt NPs beyond this limit became trivial following saturation of the surface with NPs. Saturated surfaces are prone to aggregation of NPs, which in turn leads to deterioration in catalytic activity of the NPs.62 Moreover, as already shown, the average size of pores in postetched SnO2 HSPs was nearly of the same order of magnitude as that of the Pt NPs (6.3 nm versus 6.4 nm, respectively). It is possible that as the Pt content increased, some of the NPs fortuitously blocked these pores, hence acting as barriers to restrain accessibility of the diffusing gas to internal crystallites constituting the shell. A comparison of BET results revealed that both SnO2 HSPs and Pt-SnO2 HSPs (0.12 wt%) exhibited type 4 isotherm feature (see Figure 3a and Figure S11a), and their average pore sizes were comparable. However, the Pt-SnO2 HSPs (0.12 wt%) sample exhibited a BET surface area of 77.3 m2 g-1, which was much lower than that of SnO2 HSPs (i.e., 124.8 m2 g-1). In addition,


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the BJH desorption cumulative volume of pores of Pt-SnO2 HSPs (0.12 wt%) sample in the range of 1.7–300 nm diameter was only 0.12 cm3 g-1 compared to 0.2 cm3 g-1 of SnO2 HSPs in the same range. Likewise, as shown in Figure S11b, the volume of mesopores for Pt-SnO2 HSPs (0.12 wt%) sample decreased compared to that of SnO2 HSPs shown in Figure 3b. The occurrence of pore blockage, apparently, may be the reason for lower response of postetched SnO2 HSPs at Pt loading > 0.08 wt% as observed. Test results of the Pt-SnO2 HSPs (0.08 wt%) sensor to trace concentrations of target gas (Figure 4b) demonstrated that the sensor response scaled linearly with the gas concentration. By linear extrapolation, we expect a response of 1.1 to as low as 10 ppb. This concentration is much less than typical concentrations of H2S in healthy and patient human, which are typically in the range of 50-80 ppb and > 1.0 ppm, respectively.43, 63

The Pt-SnO2 HSPs (0.08 wt%) sensor exhibited rather slow response of (300.0 s) to 5 ppm of

H2S while the recovery was a little quicker (48.7 s) (Figure S12). A slightly lower response time (192.4 s) was observed at 1 ppm compared to response times of 199.8 s, and 328.2 s for its postcalcined SiO2@SnO2 SPs and postetched SnO2 HSPs counterparts, respectively, while its recovery times were consistently lower in the entire range of H2S concentration (1-5 ppm). For the nonfunctionalized SiO2@SnO2 SPs and SnO2 HSPs sensors, distinct sensing attributes were very obvious for postcalcined SPs contained poorly conducting SiO2 enriched cores. Wet etching of the SiO2 cores, and fortuitously dispersed SiO2 throughout the SnO2 shells led to mesoporous thin-walled HSPs characterized with short diffusion length (31.4 ± 3.1 nm), ubiquitous gas diffusion, and enhanced conductivity.2 As already shown in Figure 1g, the hollow structures entailed additional macroscale exterior openings and interconnected inner voids of adjacent HSPs that maximized accessibility of internal surfaces by gas molecules, giving rise to high modulation of intergranular resistance. The sensing mechanism is described in detail in the


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Supporting Information (Figure S13). The response-recovery plots (as already seen in Figure 4a) show that modulation of intergranular resistance became strong when high concentrations of H2S interacted with SnO2 surface. At low concentration however, the consumption of O- adsorbates lowered, and insufficient electrons were released incapable of triggering a comprehensive resistance change. This concentration-dependent desorption behavior is symptomatic of the observed response patterns. Figure 4c shows the response of Pt-SnO2 HSPs (0.08 wt%) sensor to 1 ppm of different gases at 250 °C, respectively. The origin of a better selectivity toward H2S against interfering gases is related to coexistence of SiO2 residues with SnO2 in the prepared materials. H2S interacts with SnO2 through donation of localized lone pair electrons on the sulfur atom, whereas humidity (H2O) and the interfering gases do not albeit identical conditions of exposure.64-66 More precisely, humidity effects were minimized since H2S easily adsorbs on SnO2 in preference to humidity due to low energy of the H-S bond.64-65 Moreover, the dielectric nature and nonfunctional property of SiO2 toward gas sensing imply that the number of charge carriers escaping onto SnO2 crystallites from humidity1,

67-68

considerably reduced since humidity

adsorption sites were localized and constrained only on SiO2 aggregates, and within the SiO2-SnO2 interface region as for the case of NiO reported elsewhere.10 To investigate the selectivity property of the Pt-SnO2 HSPs (0.08 wt%) sensor against oxidizing gases, NO and NO2 were tested in the concentration range of 1-5 ppm at the same conditions as reducing gases. The sensor showed a response of 1.4 and 1.2 toward 1 ppm of NO2 and NO, respectively, similar to responses observed for toluene (C7H8), acetone (CH3COCH3), ammonia (NH3), carbon monoxide (CO), and pentane (C5H12). (Figure S14a). Indeed, negligibly weak cross-sensitivity to humidity is corroborated by excellent recovery of the SiO2@SnO2 SPs, SnO2 HSPs, and Pt-SnO2


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HSPs (0.08 wt%) sensors to baseline resistances of 150 kΩ, 200 kΩ, and 6 MΩ, respectively, during the H2S in-out cycle at 5 ppm (Figure 4d and Figure S14b). To further verify this, we tested for stability of the Pt-SnO2 HSPs (0.08 wt%) sensor in highly humid condition (RH = 90%) for 15 cycles of exposure to 5 ppm of H2S. The sensor indicated a fairly stable repeatability of response with minimal variation (i.e., Ra/Rg = 105.4 ± 3.9) (Figure 4e). In addition, the performance of the sensor toward H2S was investigated, this time, at a lower relative humidity (50% RH) in the concentration of range of 1-5 ppm while other conditions were the same as for the 90% RH case. Tests conducted at the two conditions of relative humidity (i.e., 50% and 90% RH, at 250 °C) evidenced little variation in response of the sensor to 1-5 ppm of H2S although the response values at 50% RH were a little higher (Figure 4f). Nevertheless, Pt NPs are renowned for their catalytic suppression of moisture poisoning,11 and we suppose they also played a major role here. Deduction from these observations showed that small content of SiO2 residues as determined in this report, had little deteriorating effect to gas-sensing performance similar to past literature.46 What’s more, these structures showed superior sensing performance toward H2S, unlike many recent works that have reported similar morphology but showing selectivity to target gases other than H2S (see Table 1). Finally we emphasize that observed mesopores and macropores played a major role to facilitate relatively easy Knudsen and molecular diffusion, respectively,2 thus ensuring massive diffusion of H2S past SnO2 crystallites coupled with a remarkable consumption of O- adsorbates. In the case of postcalcined SiO2@SnO2 SPs, inconsiderable primary mesoporosity suggested that microsphere interiors were less accessible by gas molecules, posing a poor contribution to sensor response. On the contrary, meso/macroporosity, thin shell thickness, and short diffusion length of


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postetched SnO2 HSPs were vital in gas sensing owing to enormous easily accessible reaction sites by diffusing target gas.

4. Conclusions In summary, we have demonstrated in-situ templating of SnO2 HSPs with SiO2 upon calcination of single-nozzle as-electrosprayed SPs containing Sn and Si precursors. The incompatibility between the two different oxidized phases of precursors, and the factors enabling the Kirkendall effect during calcination provided a firmer basis to form SiO2@SnO2 SPs. Reproducible thin-walled SnO2 HSPs were obtained by selective wet etching of the core SiO2 template from postcalcined SiO2@SnO2 SPs, offering high potential for scalable synthesis of core-shell SPs, and HSPs. Characterization of synthesized structures via analytical tools revealed that postetched SnO2 HSPs exhibited 62.8 ± 6.2 nm shell thickness, small crystallites (15.5 nm), large BET surface area (124.8 m2 g-1), meso/macroporous structure, and 3D intersphere macroopenings, which are essential features for enhancement of oxygen adsorption-desorption kinetics especially when coupled with catalysts (Pt NPs). Pt-SnO2 HSPs (0.08 wt%) sensor exhibited remarkably high response of up to 1.6, 10.8, and 105.1 at 0.1, 1, and 5 ppm, respectively, and outstanding selectivity toward H2S. Threefold role of SiO2 was thoroughly established as (i) templating to afford the hollow morphology and extra mesoporosity (ii) grain growth inhibitor, and (iii) selectivity enhancer. The synthesized structures are envisioned as ideal gas sensing layers for applications in chemiresistive-type halitosis detectors. As far as it is known, this is the first report utilizing a single-nozzle electrospray assisted etching strategy to synthesize SnO2 HSPs. Moreover, the facile synthesis approach demonstrated in this work paves the way to a


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significant platform for synthesis of SnO2 HSPs, and one can expect that it can be extended to synthesis of other metal oxides-silica core-shell systems.

ASSOCIATED CONTENT Supporting Information SEM images showing adhesion of sensing layers on alumina substrates (Figure S1), schematic of homemade gas-sensing equipment (Figure S2), TGA and DTG curves of as-electrosprayed sample (Figure S3), STEM image of SiO2@SnO2 SP and the corresponding linescan profiles of elemental components (Figure S4), XRD patterns (Figure S5), synthesis procedure of Pt NPs and the corresponding TEM image of Pt NPs (Figure S6), high magnification TEM image of SiO2@SnO2 SP (Figure S7), Sn 3d XPS spectra for non-functionalized SnO2 HSPs and Pt-SnO2 HSPs (Figure S8), Additional SEM images of SiO2@SnO2 SPs and SnO2 HSPs (Figure S9), responses of sensors to 5 ppm of H2S in the temperature range of 200-400 °C (Figure S10), BET characterization of Pt-SnO2 HSPs (0.12 wt%) sample (Figure S11), response and recovery times of sensors at 250 °C (Figure S12), gas-sensing mechanism and the corresponding schematic (Figure S13), response of Pt-SnO2 HSPs (0.08 wt%) sensor to oxidizing gases, and the stability plot for SiO2@SnO2 SPs, SnO2 HSPs, and Pt-SnO2 HSPs (0.08 wt%) sensors in air (Figure S14). This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the Ministry of Science, ICT and Future Planning of the Korea government for financial support provided through the Wearable Platform Materials Technology Center (funded by the National Research Foundation of Korea, under grant No 2016R1A5A1009926), Biomedical Treatment Technology Development Project (grant No. 2015M3A9D7067418),

National

Research

Foundation

of

Korea

(grant

No.

NRF-

2015R1A2A1A1A6074901, and BRL Program through grant No. 2014R1A4A1003712), and Global Frontier Project (CISS-2011-0031870).


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64. Marrocchelli, D.; Yildiz, B. First-Principles Assessment of H2S and H2O Reaction Mechanisms and the Subsequent Hydrogen Absorption on the CeO2 (111) Surface. J. Phys. Chem. C 2012, 116, 2411-2424. 65. Mickelson, W.; Sussman, A.; Zettl, A. Low-Power, Fast, Selective Nanoparticle-Based Hydrogen Sulfide Gas Sensor. Appl. Phys. Lett. 2012, 100, 173110. 66. Liu, H.; Zhang, W.; Yu, H.; Gao, L.; Song, Z.; Xu, S.; Li, M.; Wang, Y.; Song, H.; Tang, J. Solution-Processed Gas Sensors Employing SnO2 Quantum Dot/MWCNT Nanocomposites. ACS Appl. Mater. Interfaces 2015, 8, 840-846. 67. McAleer, J. F.; Moseley, P. T.; Norris, J. O. W.; Williams, D. E.; Tofield, B. C. Tin Dioxide Gas Sensors. Part 2. - The Role of Surface Additives. J. Chem. Soc., Faraday Trans. 1 1988, 84, 441-457. 68. Ma, N.; Suematsu, K.; Yuasa, M.; Kida, T.; Shimanoe, K. Effect of Water Vapor on PdLoaded SnO2 Nanoparticles Gas Sensor. ACS Appl. Mater. Interfaces 2015, 7, 5863-5869.


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Figure 1

Figure 1. (a) Schematic illustration of the synthesis of Pt-SnO2 HSPs. (b) SEM image of postcalcined SiO2@SnO2 core-shell SPs. The core-shell configuration is achieved upon calcination of as-electrosprayed SPs containing Sn and Si precursors. (c) SEM image of postetched SnO2 HSPs. (d) Low magnification TEM image of Pt-SnO2 HSPs. (e) EDS elemental maps of Pt-SnO2 HSPs. (f) Schematic illustration of formation of interconnected inner voids following etching of SiO2 from SiO2@SnO2 core-shell SPs using NaOH solution (pH 12). (g) SEM image depicting interconnected inner voids, interior and exterior macro-openings of postetched SnO2 HSPs.


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Figure 2

Figure 2. (a) High magnification TEM image and (b) HRTEM image of Pt-SnO2 HSPs. (c) High resolution XPS spectra of tin (Sn 3d), (d) oxygen (O 1s), (e) platinum (Pt 4f), and (f) silicon (Si 2p) for the prepared Pt-SnO2 HSPs.


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Figure 3

Figure 3. BET surface area characterization. (a) Nitrogen (N2) adsorption-desorption isotherms, and (b) corresponding BJH desorption pore size distribution plots of postcalcined SiO2@SnO2 SPs and postetched SnO2 HSPs.


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Figure 4

Figure 4. Gas-sensing characteristics of the prepared sensors. (a) Response-recovery characteristics of the sensors upon cyclic exposure to air and 1-5 ppm of H2S at 250 °C. (b) Response of Pt-SnO2 HSPs (0.08 wt%) sensor to low concentration of H2S (0.1-0.6 ppm). (c) Response of Pt-SnO2 HSPs (0.08 wt%) sensor to 1 ppm of different target gases at 250 °C. (d) Recovery of SiO2@SnO2 SPs, SnO2 HSPs, and Pt-SnO2 HSPs (0.08 wt%) sensors to their baseline resistances upon exposure to 5 ppm of H2S at 250 °C.(e) Response of Pt-SnO2 HSPs (0.08 wt%) sensor to 5 ppm of H2S for 15 repeated exposure cycles at 250 °C (f) Response of PtSnO2 HSPs (0.08 wt%) sensor to 1-5 ppm of H2S at 250 °C in 50% and 90% RH ambient.


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Table 1. Sensing performance of recently reported SMO-based HSPs materials to different target gases. *The response is defined as Rg/Ra. synthesis route

material

temperature

concentration

response

response/recovery

[°C]

[ppm]

[Ra/Rg]

time [s]

target gas

C-doped WO3 hollow spheres

carbon templating

300

0.9

5.1

-

Fe-doped WO3 hollow spheres

carbon templating

120

0.01

1.3

Co3O4 microspheres

hydrothermal

200

100

SnO2 HSPs

SiO2 templating

200

SnO2 HSPs

PS sphere templating

SnO2 HSPs

ref.

Acetone

[7]

52/73

NO2

[8]

*38.2

0.1/0.7

ethanol

[9]

50

16

5/7

acetone

[16]

300

0.5

≈3.0

1.8/5.4

CH2O

[17]

carbon templating

260

100

135.8

10/8

ethanol

[18]

Pd/SnO2 HSPs

hydrothermal

200

100

14.7

5/92

CO

[23]

CuO-SnO2 HSPs

ultrasonic spray pyrolysis

300

1

22.4

15/-

H2S

[25]

Co3O4-SnO2 HSPs

spray pyrolysis and galvanic replacement

275

5

18.6

243/-

xylene

[26]

Pt-SnO2 HSPs

electrosprayetching

250

1

10.8

192.4/76.5

H2S

this work

porous


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Table of Contents


33

Macroporous SnO2

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