Bi-modally Porous WO3 Microbelts Functionalized with Pt Catalysts for

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Functional Inorganic Materials and Devices

Bi-modally Porous WO3 Microbelts Functionalized with Pt Catalysts for Selective H2S Sensors Min-Hyeok Kim, Ji-Soo Jang, Won-Tae Koo, Seon-Jin Choi, Sang-Joon Kim, Dong-Ha Kim, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Bi-modally Porous WO3 Microbelts Functionalized with Pt Catalysts for Selective H2S Sensors Min-Hyeok Kim, † Ji-Soo Jang, † Won-Tae Koo, † Seon-Jin Choi, †,§ Sang-Joon Kim, † DongHa 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 305-701, Republic of Korea §

Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts

Avenue,Cambridge, Massachusetts 02139, United States

*Corresponding author e-mail: [email protected]

KEYWORDS catalyst, porous nanobelt, WO3, exhaled breath sensor, gas sensor

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ABSTRACT

Bi-modally meso- (2–50 nm) and macro-porous (>50 nm) WO3 microbelts (MBs) functionalized with sub-3 nm Pt catalysts were fabricated via electrospinning technique followed by subsequent calcination. Importantly, apoferritin (Apo), tea saponin (TS) and polystyrene (PS)–colloid spheres (750 nm) dispersed in electrospinning solution acted as forming agents for producing meso- and macro-pores on WO3 MBs during calcination. Particularly, mesopores provide not only numerous reaction sites for effective chemical reactions, but also facilitate gas diffusion into interior of WO3 MBs, dominated by Knudsen diffusion. Macropores further accelerate gas permeability in the interior and on the exterior of WO3 MBs. In addition, Pt nanoparticles (NPs) with mean diameters of 2.27 nm were synthesized by using biological protein cages, i.e., Apo, to further enhance gas sensing performance. Bi-modally porous WO3 MBs functionalized by Pt catalysts showed remarkably high hydrogen sulfide (H2S) response (Rair/Rgas = 61 @ 1 ppm) and superior selectivity to H2S against other interfering gases, i.e., acetone (CH3COCH3), ethanol (C2H5OH), ammonia (NH3), and carbon monoxide (CO). These results demonstrate high potential for feasibility of catalyst-loaded meso- and macro-porous WO3 MBs as new sensing platforms for possibility of real-time diagnosis of halitosis.


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1. INTRODUCTION Among various semiconducting metal oxides (SMOs), tungsten oxide (WO3) has attracted tremendous attention due to its fascinating features such as wide bandgap,1 excellent electronic mobility,2 and chemical stability.3,4 For this reason, many efforts have been attempted to apply WO3 in diverse research fields such as catalysts,5 solar cells,6 and gas sensors.7 In particular, WO3, as one of the most fascinating materials for chemi-resistive gas sensors, has received the spotlight in the field of early diagnosis and environmental toxic gas monitoring due to high reactivity with target gases.7-9 However, pristine WO3 based chemiresistive gas sensors, i.e., WO3 film10 and WO3 microfiber,11 have suffered from low sensitivity and poor selectivity. In order to further enhance sensing performance, activation of surface reactions by facilitating porous nanostructures and functionalization of catalysts for electronic or chemical sensitizers is essential. In general, a number of methods have been suggested for the synthesis of nanostructured materials,12 which include chemical vapor deposition growth of nanostructure,13 atomic layer deposition with templating route,14 hydrothermal,15 template method,16-17 and solvothermal method.18 However, there have been critical challenges in productivity of nanostructure and homogeneous functionalization of catalytic nanoparticles (NPs) on SMO nanostructures, which result in severe degradation of catalytic sensitization. Recently, the electrospinning technique has been widely applied owing to its simplicity and versatility along with outstanding mass productivity and reproducibility to obtain SMO based 1-dimensionl (1D) nanostructures with controlled shape and size such as nanorod, nanofiber (NF), and microbelt (MB). In particular, MB is an adequate structure for gas sensors due to its high specific surface area, efficient pathways for the electron transportation, and effective depletion layer modulation.19-21 For example, Lv et al.22 reported ethanol gas sensor based on the Fe2O3 and LaFeO3 belt structure via electrospinning. In addition, Yan et al.23 synthesized hollow SnO2 MB structure with increased surface area via solvent 3 ACS Paragon Plus Environment


evaporation and phase separation during the electrospinning, which resulted in the excellent acetone sensing performance. Furthermore, Shen et al.24 synthesized mesoporous In2O3 MBs with enhanced ethanol gas sensing properties. However, these structures include mainly mesopores in the MB, thus full activation, i.e. entire modulation of surface depletion layer, of belt structure with thickness of sub-hundreds micrometer is not achieved due to limited gas diffusion in the interior sensing layers. Therefore, rational design of ideal sensing layers with many gas reaction sites as well as optimized gas accessibility including both mesopore (2–50) and macropore (>50 nm) is crucial. In addition, the electrospinning technique is an easy synthetic method for functionalization of catalyst NPs on 1D SMO nanostructures.25 Noble metal catalysts such as Pt,26 Pd,27 Ag,28 and Au NPs29 have been widely used to enhance gas sensing characteristics.30-31 However, agglomeration of the catalytic NPs during high temperature calcination adversely affects the gas sensing performance. To solve this issue, Kim et al. proposed new catalyst-immobilization route onto electrospun metal-oxide NFs by using bioinspired hollow protein template, which is composed of 24 different protein subunits with hollow nanocage structures. Protein templating route is highly robust for synthesis of approximately 2 nm-sized particles due to the limited core space (< 7–8 nm) of hollow nanocage and its high dispersibility. Moreover, after calcination process, mesopores are easily formed in 1D layers since the protein shell is decomposed at high temperature.32 In this work, we report highly sensitive and selective H2S gas sensors by using bi-modally meso- and macro-porous Pt-loaded WO3 MBs (B_Pt_WO3 MBs) that were synthesized by electrospinning process combined with sacrificial templating route. Herein, triple sacrificial templates, i.e., apoferritin (Apo), tea samponin (TS), and polystyrene (PS)-colloid spheres, were utilized to generate meso- and macro-sized pores through the entire surface of WO3 MBs after calcination process. In particular, the MB structure showed broad width and thin thickness, which are highly suitable for improved reaction between sensing material and 4 ACS Paragon Plus Environment

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target gas. On the basis of the bimodal pores distributed in MB structures, gas diffusion can be facilitated from the surface to the inner sensing layers. Along with high gas diffusion, the functionalization of nano-sized Pt catalyst induced by Apo dramatically increased the sensing properties. In other words, the combination of bimodal pores on MBs and Pt catalysts induces maximized resistance modulation, thereby showing remarkable gas sensing performance. To the best of our knowledge, this is the early report on bi-modally porous SMO MB structures which possess tremendous structural advantages in high surface area and gas accessibility.

2. MATERIALS AND METHODS 2.1. Materials. Ammonium metatungstate hydrate [AMH, (NH4)6H2W12O40·xH2O], polyvinylpyrrolidone (PVP, Mw=1,300,000 g/mol), tea saponin (TS, C57H90O2, Sapogenin 20–35%), chloroplatinic acid hexahydrate (H2PtCl6·xH2O), sodium borohydride (NaBH4), sodium hydroxide (NaOH), 2.5 wt% PS latex spheres with diameters of 750 nm dispersed in de-ionized (DI) water, and 0.2 µm filtered Apo from equine spleen were purchased from Sigma-Aldrich (St. Louis, USA). All chemicals were used without further purification. 2.2. Synthesis of Pt NPs encapsulated in apoferritin. The Pt NPs were prepared by using Apo which was obatined from equin spleen (0.2 µm filtered sample). The 1 g of Apo solution was mixed with 1 M NaOH solution to adjust the pH of solution at around 8.5. Then, aqueous solution containing 12 mg of H2PtCl6·xH2O was added to Apo solution and slowly stirred at 100 rpm for 1 h to allow the peneration of Pt4+ ions into the inner cage of Apo (Figure S1a). Subsequently, to reduce Pt4+ ions embedded in the hollow cavity of Apo, 1.5 mg/ml NaBH4 solution was added to the final solution to form Pt NPs encapsulated in Apo. Lastly, Pt NPs encapsulated in Apo (hereafter, Apo-Pt NPs) were centrifuged at 12,000 rpm for 10 min and re-dispersed in deionized water (DI water). 2.3. Synthesis of bi-modally meso- and macro-porous Pt-loaded WO3 MBs. 5 ACS Paragon Plus Environment


The B_Pt_WO3 MBs were prepared via the electrospinning as illustrated in Figure 1a. To prepare the electospinning solution, 0.35 g of AMH and 0.45 g of TS were dissolved in 1.5 g of 2.5 wt% PS-colloid spheres dispersed in DI water. And then, the prepared Apo-Pt NPs were added to above solution with concentration of 0.01, 0.02, and 0.05 wt% and stirred at room temperature (RT) for 1 h. Then, 0.25 g of PVP and ethanol (EtOH) were added to the above solution and additionally stirred at RT for 7 h. The electrospinning was carried out at a feeding rate of 0.5 mL/h. A constant voltage of 8 kV was applied between the needle tip (27 gauge) and stainless steel current collector substrate. The electrospun belts were collected on the collector, which was placed 15 cm away from needle tip. The as-spun MBs were calcined at 600ºC for 1 h in air ambient. The bi-modally meso- and macro-porous WO3 MBs (B_WO3 MBs) were prepared by the same experimental procedure without using Apo-Pt NPs. In addition, the mesoporous WO3 MBs (M_WO3 MBs) were synthesized by the same experimental process without using PS–colloid sphere dispersion in DI water and Apo-Pt NPs. 2.4. Gas sensing characterization. To evaluate the gas sensing characteristics of the M_WO3 MBs, B_WO3 MBs, and B_Pt_WO3 MBs, we firstly drop-coated the sensing materials on alumina substrate (area = 2.5 mm × 2.5 mm), which has parallel gold sensing electrode on the top surface (width = 25 µm, gap size = 70 µm) and a Pt micro-heater on the back side (Figure S2). The sensing layers were prepared using the same weight (6 mg) of sensing materials (B_Pt_WO3 MBs, B_WO3 MBs, and M_WO3 MBs) dispersed in 100 µL of ethanol. The dispersions were drop-coated on the substrate in equal amounts to achieve the same thickness of materials on sensing electrodes. The thickness of coated sensing layer on the alumina substrate varied a little, but generally ranged from 188–200 µm (Figure S3). Lastly, the sensor devices were loaded in the sensor chamber for sensing tests. Gas sensing characteristics were investigated toward hydrogen sulfide (H2S), acetone (CH3COCH3), ethanol (C2H5OH), ammonia (NH3), and carbon monoxide (CO) in the temperature range of 300–450°C using three independent sensors. For 6 ACS Paragon Plus Environment

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stabilization of the sensor, the air was repeatedly injected for 10 min, followed by exposure of analytic gas with concentrations ranging from 0.4 ppm to 5 ppm for 10 min in highly humid atmosphere (95% RH). In addition, stability test was carried out by evaluating the sensor response to 10 repeated cycles of exposure to 5 ppm of H2S. Responses of each cycle were identical, exhibiting high stability of the fabricated sensors. The resistance changes of the sensors were measured using acquisition system (34972, Agilent) and they were transformed into the response (Rair/Rgas), where Rair is the sensor resistance in the baseline air and Rgas is the resistance measured during the exposure to analytes. The operating temperature was modulated by adjusting the voltage of the Pt micro-heater patterned on the backside of the sensor substrates by using DC power supply (E3647A, Agilent). 2.5. Material characterization. The morphologies of samples were evaluated by field emission scanning electron microscopy (Nova230, FEI). The microstructure was investigated by field emission transmission electron microscopy (Tecnai G2 F30 S-Twin, FEI). The crystal structure was examined by powder X-ray diffraction (D/MAX-2500, Rigaku) analysis using Cu Kα radiation (λ = 1.5418 Å). The X-ray photoelectron spectroscopy (Sigma Probe, Thermo VG Scientific)

analysis

was

conducted

to

investigate

the

chemical

bonding

states.

Thermogravimetric analysis (Labsys Evo, Setaram) was carried out to analyze the thermal behavior of samples.

3. RESULTS AND DISCUSSION Figure 1a shows schematic illustration of formation mechanism for the MB structrure. Generally, solvents contained in as-spun polymeric nanofibers (PNFs) evaporate during electrospinning. During evaporation, the difference in evaporation rates between DI water and ethanol leads to the formation of shell-dry skin region and relatively core- wet region. As the 7 ACS Paragon Plus Environment


difference in evaporation rate between shell and core layers increases, the cylindrical fibers collapse to form belt structure due to continuous evaporation of the solvent from the coreregion of the polymeric fiber. According to a previous study,33-35 the morphological evolution of the MB structure is attributed to the formation of dry skins induced by the evaporation of the solvents. The ethanol initially evaporates, resulting in the formation of dry skins surrounding the as-spun polymeric nanofibers (PNFs). Subsequently, the evaporation of the DI water with relatively slow evaporation rate occurs inside the PNFs. Given that TS is highly hydrophilic, evaporation of DI water is greatly hindered, leading to the relatively thin dry skin region. Thus, the consequent collapse of the shape from NF to MB structure occurs resulting from continuous evaporation of the DI water within PNFs. In other words, the presence of TS results in the formation of dry skins caused by different evaporation rates of the mixed solvents, which lead to the belt shaped structure. Scanning electron microscopy (SEM) was employed to investigate the morphology of the PNFs (Figure 1b–e) with different TS concentration and the corresponding calcined samples (Figure 1f–i). The as-spun PNFs without TS exhibited cylindrical structure with average width of 1.31 ± 0.11 µm (Figure 1b). On the contrary, the as-spun PNFs with 0.45 g of TS showed a distinctive belt structure with increased average width of 3.01 ± 0.35 µm and decreased thickness of 883 ± 145 nm, respectively (Figure 1e). During high temperature calcination at 600ºC for 1 h, these PNFs were decomposed and W precursor was oxidized, resulting in the formation of WO3 MBs. As a result, the average width and thickness of as-spun PNFs with 0.45 g of TS decreased to 1.23 ± 0.19 µm and 383 ± 45.3 nm, respectively (Figure 1i). Based on these observations, we concluded that the TS played an important role on the morphological evolution of WO3 MBs. A schematic illustration of the Pt NP encapsulated in Apo is shown in Figure S1a. The Apo has a hollow structure with an empty space with 7–8 nm size in the core, which can encapsulate Pt ions; thus, the Pt catalyst with a few nm sizes can be embedded in hollow protein shell. The average size of Apo-Pt NPs was examined by transmission electron 8 ACS Paragon Plus Environment

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microscopy (TEM) analysis. The synthesized Apo-Pt NPs have an average size of 2.27 nm and showed crystallinity with the crystal plane of Pt (111), corresponding to the interplanar spacing of 0.226 nm (Figure S1b).36 Figure S1c shows a size distribution histogram of counting 45 different Apo-Pt NPs, which exhibits a narrow size distribution with the Apo-Pt NPs diameter ranging between 1.5–3.5 nm. In addition, the Pt catalytic NPs have a good dispersion property owing to the repulsion force of the positively charged protein surface.37 Figure 2a illustrates the synthetic process of bi-modally meso- and macro-porous Pt-loaded WO3 MBs (B_Pt_WO3 MBs). The B_Pt_WO3 MBs were synthesized by single nozzle electrospinning of W precursor and PVP solution including the PS–colloid spheres (Figure S4), Here, Apo-Pt NPs and TS served as forming agents for producing porous MBs structure after calcination process. As shown in Figure 2b, the as-spun MBs consisting of PS-colloids, TS, Apo-Pt NPs, W precursor, and PVP showed belt structure with bumpy surface caused by the decomposition of PS-colloids embedded within MBs. The embedded PS-colloids in the PNFs were clearly observed in magnified SEM image (Figure 2c). Then, the mesopores and macropores-loaded B_Pt_WO3 MBs were obtained after the calcination at 600ºC for 1 h (Figure 2d and 2e). On the other hand, the mesoporous WO3 MBs (M_WO3 MBs) were obtained without using PS-colloids, which are forming agents of macropores (Figure S5). To further investigate the thermal decomposition behavior of the sacrificial templates during the calcination, the thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) analysis were carried out with the samples of PVP/W precursor/PScolloid/TS/Apo-Pt composite polymeric MBs (PS_Apo-Pt_PMBs) (Figure 3), PVP/W precursor NFs (W_PNFs) (without all sacrificial templates, Figure S6a), and PVP/W precursor/TS/Apo-Pt MBs (Apo-Pt_PMBs) (without PS-colloid, Figure S6b) in the temperature range of 30–600ºC. The slight weight loss was observed in all samples, which is caused by evaporation of residual solvent in W_PNFs at section Ι (below 120ºC). To verify the decomposition temperature of TS, DTG of W_PNFs (without TS) and Apo-Pt_PMBs 9 ACS Paragon Plus Environment


(containing TS) samples was compared at section ΙΙ (120–245ºC). As shown in Figure S6, the larger weight loss and an additional DTG peak were observed in the Apo-Pt_PMBs. This result indicates that the decomposition of TS occurred at section ΙΙ. The DTG peak related to thermal decomposition of Apo was not clearly observed due to very small loading amount of Apo. In general, the Apo is completely decomposed at temperature below 250ºC.38 At section ΙΙΙ, weight loss curve was observed for all three samples which is attributed to the thermal decomposition of PVP side chains. At section ΙV, the DTG peak was observed only for the PS_Apo-Pt_PMBs, which is a clear evidence for thermal decomposition of PS-colloids, as similar to the previous study.38 At section V, the decomposition of residual carbon backbone from PVP and crystallization of WO3 by thermal oxidation of W precursor were observed. At temperature above 550ºC, no more weight loss curve was observed. According to the TG/DTG analysis, all the sacrificial templates were completely decomposed during calcination at 600ºC for 1 h, allowing the effective generation of meso- and macro-pores on WO3 MB structures. In order to analyze the crystal structure of M_WO3 MBs, B_WO3 MBs, and B_Pt_WO3 MBs, XRD analysis was performed (Figure S7a). The XRD peaks of the three samples revealed the (002), (020), (200), (120), (112), (022), and (202) planes for monoclinic WO3 crystal structure (JCPDS no. 43–1035). The average grain sizes of WO3 in B_Pt_WO3 MBs, B_WO3 MBs, and M_WO3 MBs were 26.49 ± 3.60, 31.38 ± 7.03, and 46.53 ± 12.57 nm, respectively, which were calculated using the Scherrer formula based on the major planes of (200), (020), and (002) (Figure S7b). Figure 4 presents TEM micrographs, energy dispersive spectroscopy (EDS) mapping images, HRTEM lattice images, and selected area electron diffraction (SAED) patterns of B_Pt_WO3 MBs. EDS mapping images showed that the Pt NPs were homogeneously distributed on the entire WO3 MBs (Figure 4a). The meso- (yellow squares in Figure 4a) and macro-pores (red circles in Figure 4a), which were generated by thermal decompositions of Pt-Apo and PS10 ACS Paragon Plus Environment

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colloids, were observed on B_Pt_WO3 MBs. HRTEM lattice images showed polycrystalline WO3 MBs composed of crystal planes of (020), (200), and (-112) with lattice spacing of 0.376, 0.364, and 0.311 nm, respectively. In addition, Pt (111) lattice fringes with interplanar spacing of 0.226 nm were clearly observed. Pt NPs were well-dispersed in the WO3 MBs without noticeable aggregation (Figure 4b). SAED patterns also revealed polycrystalline WO3 structure with lattice planes of (020), (112), and (202) as well as the crystal planes of (111) for Pt. The low concentration of Pt NPs in the composite MBs resulted in the weak intensity of Pt peaks in SAED pattern (Figure 4c). To further investigate the chemical binding state of B_Pt_WO3 MBs, we performed X-ray photoelectron spectroscopy (XPS) analysis. Figure 4d–f showed the XPS peaks of W 4f, O 1s, and Pt 4f, respectively. The high resolution spectrum of W 4f peaks exhibited two major peaks with binding energies of 37.7 eV and 35.5 eV that correspond to the W6+ 4f7/2 and 4f5/2 states, respectively (Figure 4d).39 As shown in Figure 4e, O 1s spectrum exhibited two peaks consisted of major peaks of O2– at 530.4 eV and O– at 531.4 eV,40 which are related to the chemisorbed oxygen species on the surface of WO3. Furthermore, the Pt 4f spectrum exhibited the peaks with binding energy of 71.0 eV and 74.2 eV corresponding to the Pt 4f7/2 and Pt 4f5/2, respectively. These peaks were confirmed as the Pt0 state. Additional smaller peaks were observed confirming oxidation state of Pt, i.e., PtO, at 72.8 eV for 4f7/2.41 (Figure 4f). According to the previous studies, some of Pt NPs partially oxidized to PtO at around 500°C.42-43 The XPS result identified that W precursor was completely oxidized to WO3 and Pt partially oxidized to PtO while majority of NPs maintained in Pt0 metallic state. In order to investigate the morphological and catalytic effects of B_Pt_WO3 MBs compared with M_WO3 MBs and B_WO3 MBs, the gas sensing characteristics were evaluated in highly humid atmosphere (95% RH), similar condition to human breath (Figure 5). The gas sensing characteristics are hugely affected by operating temperature because gas sensing responses are based on the surface reactions between target gas and chemisorbed oxygen species. In 11 ACS Paragon Plus Environment


addition, the loading amount of catalysts significantly influences the gas response. When the amount of catalyst is relatively low, catalytic property is not fully activated on the sensing layer. On the contrary, in case of the excessive amount of catalysts, the depletion layer, which was formed around the catalyst, can be overlapped, leading to the deterioration of catalytic effect.44 Therefore, it is important to distribute the proper amount of catalyst uniformly on the sensing layer. In this work, the operating temperature of sensors (300–450°C) and the loading amount of catalyst (0.01, 0.02, and 0.05 wt%) were carefully evaluated to optimize sensing properties (Figure S8). As shown in Figure 5a, highest gas response of B_Pt_WO3 MBs with 0.05 wt% loading amount of catalyst (B_0.05 wt% Pt_WO3 MBs) was observed at 365°C. With increasing the amount of Pt catalyst, the chemisorbed oxygen species can be increased by catalytic effects, i.e., spill-over effect and activation, leading to the increased baseline resistance. The sensing performance was evaluated in the concentration range of 0.4–5 ppm toward H2S at an optimized operating temperature of 365°C. The B_0.05 wt% Pt_WO3 MBs exhibited the remarkably improved response (Rair/Rgas = 378.12 ± 5.90) toward H2S at 5 ppm compared with M_WO3 MBs (Rair/Rgas = 5.31 ± 1.10) and B_WO3 MBs (Rair/Rgas = 11.73 ± 2.25) (Figure 5b). The response times of B_Pt_WO3 MBs, B_WO3 MBs, and M_WO3 MBs were 6.08, 368.4, and 7.96 sec respectively, whereas their recovery times were 288.2, 32.2, and 260.4 sec, respectively. This response shows that B_Pt_WO3 MBs is one of the superior H2S sensors when compared with sensors reported in recent studies (Table S1). In addition, B_0.05 wt% Pt_WO3 MBs exhibited stable response following 10 repeated cycles of exposure to 5 ppm of H2S under humid atmosphere (95% RH) (Figure S9). To verify the repeatability of the sensors, we re-synthesized B_0.05 wt% Pt_WO3 MBs (Figure S10), and the newly synthesized sensors also exhibited outstanding H2S response (Rair/Rgas > 348 @ 5 ppm H2S) similar with the previously fabricated samples (Rair/Rgas = 378.12 ± 5.90) (Figure S11). The long-term stability tests of B_Pt_WO3 MBs was investigated by comparing the sensing results of 1-month old, 6-months old, and 8-months old samples. Although the response of the 12 ACS Paragon Plus Environment

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sensors was decreased to 288.04 ± 4.17 at 5 ppm level H2S, the sensors exhibited stable response toward 1 ppm of H2S (Figure S12). It means that B_Pt_WO3 MBs can stably detect the sub-ppm level H2S gas molecules with great long-term stability. The sensors exhibited stable response toward 5 ppm and 1 ppm of H2S. No significant changes in grain sizes of WO3 sensing layers were observed before and after gas sensing tests. (Figure S13). In general, the sensing performance of metal oxide based sensors is affected by humidity. To study the effect of humidity, we carried out sensing tests in different humid atmospheres (95%, 40% RH, and dry condition). As shown in Figure S14, B_Pt_WO3 MBs exhibited the highest H2S response (Rair/Rgas = 847 @ 5 ppm) in dry atmosphere compared with its responses in 95% RH atmosphere (Rair/Rgas = 384 @ 5 ppm) and 40% RH atmosphere (Rair/Rgas = 495 @ 5 ppm). Since adsorbed water species react with chemisorbed oxygen species on WO3 surface, the sensor response decreases in highly humid environment.45 The B_Pt_WO3 MBs showed superior selectivity to H2S at 1 ppm against other interfering gases, i.e., acetone (CH3COCH3), ethanol (C2H5OH), ammonia (NH3), and carbon monoxide (CO) (Figure 5c). In order to investigate the feasibility of sensor arrays consisted of M_WO3 MBs, B_WO3 MBs, and B_0.05 wt% Pt_WO3 MBs as exhaled-breath analyzers, principal component analysis (PCA) was carried out. Based on the result, the hydrogen sulfide was clearly classified from other simulated biomarker gases, i.e., acetone, ethanol, ammonia, and carbon monoxide, without overlapping (Figure 5d). The bi-modally porous WO3 MBs sensitized by Pt NPs exhibited improved gas sensing performance by enhanced gas permeability, increased surface area, and Pt-PtO catalytic effects, i.e., chemical and electronic sensitization. The mesopores, which were obtained by thermal decomposition of TS and Apo, can provide numerous reaction sites for effective chemical reactions as well as easy gas penetration into interior of WO3 MBs by Knudsen diffusion.46 In addition, the macropores induced by PS-colloid template also provide gas flow pathway, resulting in facile penetration of H2S into inner sensing layer. In other words, both 13 ACS Paragon Plus Environment


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meso-and macro-pores effectively increased the active gas reaction sites inside the WO3 MBs. Thus, gas molecules readily diffuse into the entire sensing layer through bi-modal pore. When H2S gas is exposed to the sensing layers, the depletion layers can be narrowed due to the chemical reaction between oxygen adsorbates (i.e., O– and O2–) and H2S molecules, leading to the release of electrons back into the conduction band, which can be explained by the following Equations.47

H2S (gas) + 3O– (ads) → H2O + SO2 + 3e–

(1)

H2S (gas) + 3O2– (ads) → H2O + SO2 + 6e–

(2)

These reactions release electrons, leading to the resistance changes of M_WO3 MBs, B_WO3 MBs, and B_Pt_WO3 MBs, shown in Figure 6a. Since macropores formed on the MBs can block the electron path, the base resistance of B_WO3 MBs (18 MΩ) is higher than that of M_WO3 MBs (13 MΩ). To confirm the effect of the bi-modal pores in the MBs, we conducted the pore distribution and Brunauer-Emmett-Teller (BET) analyses (Figure 6b). Although the B_WO3 MBs showed slightly lower pore volume than that of M_WO3 MBs at the pore size ranging from 2 nm to 100 nm due to the decreased surface area by influence of fully open macropore formed on the MBs (Figure 6b), macrosized pores (over 100 nm) on B_WO3 MBs can reduce the dead sites of active sensing layers through the high gas permeability. In this sense, the response of B_WO3 MBs can be improved compared with M_WO3 MBs. On the other hand, B_Pt_WO3 MBs showed higher BET (6.79 m2/g) and pore volume than B_WO3 MBs (5.5 m2/g) because of increased mesopores induced by thermal decomposition of Apo. As a result, the bi-modal pores can provide effective reactions between more chemisorbed oxygen species and target gases, resulting in improved gas response caused by thickening and thinning of depletion layer. In addition to morphological effect, functionalization of catalytic Pt NPs can further enhance the gas sensing performance (Figure 14 ACS Paragon Plus Environment

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6c). Firstly, the Pt NPs are well known as effective chemical sensitizers. When the WO3 MBs are stabilized in air, the oxygen molecules can be dissociated into chemisorbed oxygen species, i.e, O2– and O–, by spill-over effect, which is the mechanism of dissociating gas molecules by Pt NPs, leading to enhanced surface reaction between oxygen molecules and WO3 MBs.48 Consequently, the reaction with H2S gas can be increased. To demonstrate the spill-over effect of Pt NPs, we conducted XPS analysis in the vicinity of oxygen peaks with two samples that is M_WO3 MBs and B_0.05 wt% Pt_WO3 MBs (Figure S15a–b). The characteristic peaks of chemisorbed oxygen species (O-), lattice state (O2-), and the ratio between O- and O2-, i.e., O-/O2-, were compared as shown in Table S2. The B_0.05 wt% Pt_WO3 MBs exhibited 0.349 of O-/O2- value, which is 1.22-fold enhanced value than that (0.287) of the M_WO3 MBs. The result clearly exhibit the role of catalytic Pt NPs that is facilitating the dissociation of oxygen molecules to enhance the H2S sensing characteristics. Secondly, the Pt NPs can be partially oxidized to PtO during calcination process as verified by XPS (Figure 4f). PtO and WO3 are known as p-type and n-type,49-50 respectively; thus, the depletion layers can be formed on WO3 MBs by p-n junction. Lastly, the difference in work function (1.09 eV) between Pt (5.65 eV) and WO3 (4.56 eV) can induce energy band bending,7 which forms a Schottky barrier at the interface, resulting in thickening of depletion layer on WO3 MBs caused by electron transferring from WO3 to Pt.7, 51 From these reasons, the resistance of B_Pt_WO3 (117 MΩ) is dramatically increased compared with that of B_WO3 MBs, resulting in the effective resistance modulation upon the exposure to H2S.

4. CONCLUSIONS In this work, we synthesized bi-modally meso- and macro-porous Pt-loaded WO3 microbelts (B_Pt_WO3 MBs) via electrospinning technique combined with diverse sacrificial templates, i.e., tea saponin, PS-colloid, and apoferritin (Apo)-templated Pt catalysts. The bi-modally 15 ACS Paragon Plus Environment


porous structure was obtained by thermal decomposition of sacrificial templates during high temperature calcination process. The mesopores can improve surface sites for gas reaction between oxygen species and H2S. Furthermore, the open macropores can enhance gas permeability into the sensing layers. The nanoscale catalytic Pt particles (c.a. 2.27 nm) were synthesized and well distributed on WO3 MBs via Apo templating route. Taking advantages of the bi-modally porous structure combined with highly effective catalyst, the B_0.05 wt% Pt_WO3 MBs exhibited outstanding response (Rair/Rgas = 372) toward H2S at 5 ppm in a highly humid condition (95% RH). In addition, superior H2S selectivity characteristics were obtained against other interfering gases such as acetone, ethanol, ammonia, and carbon monoxide. Furthermore, the H2S gas was clearly classified by using the principal component analysis with sensor arrays of M_WO3 MBs, B_WO3 MBs, and B_0.05 wt% Pt_WO3 MBs. The gas sensing results confirmed selective and sensitive detection of H2S, demonstrating a potential suitability of using the sensor for non-invasive and simple diagnosis of halitosis.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available online from the http://pubs.acs.org or from the author. Figures showing additional SEM images; TEM images; PXRD analysis results; TG/DSC data; supplementary H2S sensing characteristics; and approximation curve of limit of detection

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the center for Integrated Smart Sensors funded by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926), and the NRF of Korea grant funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A2A1A16074901). This work was also supported by the Ministry of Science, ICT & Future

Planning

as

Biomedical

Treatment

Technology

Development

Project

(2015M3A9D7067418).

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Figure Captions Figure 1. (a) Schematic illustration of mechanism for the formation of the microbelt structure, typical SEM images of the as-spun precursor fibers of samples (b) without TS, (c) with 0.15 g of TS, (d) with 0.30 g of TS, (e) with 0.45 g of TS, and (f)-(i) samples after calcination at 600°C for 1 h, respectively. Figure 2. (a) Synthetic illustration for synthetic method of porous WO3 MBs. (b) SEM image of as-spun TS/PVP/W precursor composite microbelts (MBs) decorated with PS-colloid template and Apo-Pt and (c) magnified SEM image of (b). (d) SEM image of meso- and macroporous Pt-loaded WO3 MBs (B_Pt_WO3 MBs) after calcination at 600°C for 1 h and (e) magnified SEM image of (d). Figure 3. Thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) analysis of the PVP/W precursor/PS-colloid/TS/Apo-Pt composite MBs in the temperature range of 30–600oC. Figure 4. (a) EDS elemental mapping of B_Pt_WO3 MBs, (b) HRTEM image of B_Pt_WO3 MBs, and (c) SAED pattern of B_Pt_WO3 MBs. XPS analysis using high resolution spectra of B_Pt_WO3 MBs in the vicinity of (d) W 4f, (e) O 1s, and (f) Pt 4f. Figure 5. H2S sensing characteristics of M_WO3 MBs, B_WO3 MBs, and B_0.05 wt% Pt_WO3 MBs: (a) Temperature-dependent H2S response property of B_0.05 wt% Pt_WO3 MBs in a temperature range of 300–450°C, (b) H2S response property of M_WO3 MBs, B_WO3 MBs, and B_0.05 wt% Pt_WO3 MBs at 0.4–5 ppm, (c) selective gas detection property of B_0.05 wt% Pt_WO3 MBs, (d) principal component analysis (PCA) of simulated gases (1–5 ppm) using M_ WO3_MBs sensor, B_WO3_MBs sensor, and B_0.05 wt% Pt_WO3 MBs sensor. Figure 6. (a) Dynamic resistance changes of M_WO3 MBs, B_WO3 MBs, and B_Pt_WO3 MBs at 5 ppm of H2S and operating temperature of 365°C, (b) pore volume distribution of M_WO3 MBs, B_WO3 MBs, and B_Pt_WO3 MBs by using N2 vapor, (c) schematic illustration of H2S sensing mechanism of B_Pt_WO3 MBs.


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Bi-modally Porous WO3 Microbelts Functionalized with Pt Catalysts for

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