33 mins ago - PtO2 nanocatalysts-loaded SnO2 multichannel nanofibers (PtO2-SnO2 MCNFs) were synthesized by single-spinneret electrospinning combined with apoferritin and two immiscible polymers, i.e., polyvinylpyrrolidone and polyacrylonitrile. The a
Jul 9, 2009 - (1-6) However, enhancing the photocatalytic efficiency of TiO2 to meet the ... /TiO2, MxOy (ZnO,(16) SnO2,(17) etc.)/TiO2 .... The photoreactor was designed with an internal light source (50 W high pressure ...... 2003, 83, 1566.
Jan 23, 2014 - ABSTRACT: In this paper, we present H2 gas sensors based on hollow and filled, well- aligned electrospun SnO2 nanofibers, operating at a ...
Nov 24, 2015 - From all the above results, we can conclude that the proposed nanoscale multichannel closed BPE array sensing platform is suitable for ...
Jan 23, 2014 - Strategies for designing metal oxide nanostructures. Ziqi Sun , Ting Liao , Liangzhi Kou. Science China Materials 2017 60 (1), 1-24 ...
May 11, 2016 - Highly selective detection, rapid response ( 50) against specific target gases, particularly at the 1 ppm level, still remain considerable challenges in gas sensor applications. We propose a rational design and facile synthesis concept
Nov 24, 2015 - Two driving electrodes were inserted into the cell between the two ends of the BPE to supply the power to induce the wireless faradaic reactions (Scheme 1). The PET ... NaOH, KCl, Na2HPO4, and NaH2PO4 were purchased from Beijing Chemic
Nov 24, 2015 - desire by NaOH, and then Au nanofibers were decorated in the inner region of the .... 2+ were purchased from Biocell Company (Zhengz-.
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Article
Nanoscale PtO2 Catalysts Loaded SnO2 Multichannel Nanofibers Toward Highly Sensitive Acetone Sensor Yong Jin Jeong, Won-Tae Koo, Ji-Soo Jang, Dong-Ha Kim, Min-Hyeok Kim, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16258 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017
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Multichannel Nanofibers Toward Highly Sensitive Acetone Sensor Yong Jin Jeong, Won-Tae Koo, Ji-Soo Jang, Dong-Ha Kim, Min-Hyeok 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
PtO2 nanocatalysts-loaded SnO2 multichannel nanofibers (PtO2-SnO2 MCNFs) were synthesized by single-spinneret electrospinning combined with apoferritin and two immiscible polymers, i.e., polyvinylpyrrolidone and polyacrylonitrile. The apoferritin which can encapsulate nanoparticles within small inner cavity (8 nm) was used as a catalyst loading template for effective functionalization of PtO2 catalysts. Taking advantage of the multichannel structure with high porosity, effective activation of catalysts on both the interior and exterior site of MCNFs was realized. As a result, under high humidity condition (95% RH), PtO2-SnO2 MCNFs exhibited remarkably high acetone response (Rair/Rgas = 194.15) toward 5 ppm acetone gases, superior selectivity to acetone molecules among various interfering gas species, and excellent stability during 30 cycles of response and recovery toward 1 ppm acetone gases. In this work, we firstly demonstrate the high suitability of multichannel semiconducting metal oxides structure functionalized
by
apoferritin
encapsulated catalytic nanoparticles as highly sensitive and selective gas sensing layer.
1. INTRODUCTION Semiconducting metal oxides (SMOs) based chemiresistive gas sensors have attracted much attention given their great advantages such as simple fabrication, portability, and sensitivity.1,
2
Considering these features, SMOs based gas sensors have been applied to
various areas including monitoring of toxic gases,3 air pollution,4 and diagnosis of diseases.5 In particular, as health care becomes more important, gas sensors for early and daily diagnosis of diseases are emerging as next-generation medical technologies. To realize this system, research on exhaled breath analysis has obtained great attention due to the potential possibility of facile diagnosis of diseases. Since the biomarkers in human exhaled breath indicate specific diseases, we can diagnose them by detecting target breath maker gases. However, human exhaled breath contains H2O vapors and various interfering gases, which react with sensing materials, resulting in the difficulty in sensitive and selective detection of target biomarker gases.6-8 To overcome this limitation, catalytic functionalization of sensing materials with large surface area should be achieved for effectively promoting surface reaction of specific target gases.9 In this sense, the nanostructured SMOs functionalized by catalysts are essential to improve sensing performance. Among various synthetic routes for nanomaterials, electrospinning is a versatile tool to synthesize one dimensional (1D) nanostructures.10,
11
Since 1D nanostructures offer large
surface area and high porosity, many researchers have studied electrospun nanostructures, i.e., nanofibers (NFs),12 nanotubes (NTs),13 and multichannel NFs (MCNFs).14 Very recently, MCNFs have been widely used for various areas such as batteries,15 biosensors,16 and gas sensors17 due to their unique structure. For example, Wu et al. synthesized multichannel porous TiO2 hollow nanofibers as anodes for sodium-ion batteries.15 The TiO2 MCNFs exhibited improved sodium storage performance due to their porous structure which enables effective access of the liquid electrolyte and Na+. In addition, Kim and co-workers reported multichannel carbon nanofiber transducer for Bisphenol A sensing.16 Due to the enhanced 3 ACS Paragon Plus Environment
specific surface area, the interactions between Bisphenol A and the transducer were improved. These properties of MCNFs such as large surface area and high accessibility are also suitable for gas sensor due to the enhanced surface reaction of target gases. Therefore, there have been many reported gas sensors based on MCNFs such as 2D WS2-loaded multichannel carbon NFs,18 ZnO hollow NFs,19 and hollow SnO2 NFs.17 However, they exhibited insufficient performances to detect under 5 ppm level gases, thus leading to difficulty in actual use. To improve sensing characteristics of MCNFs, functionallization of nanoscale catalysts should be achieved. Nevertheless, facile synthesis of nanocatalysts-loaded SMOs MCNFs by using single-spinneret electrospinning is still challenging. Although nanoscale catalysts on SMOs-based gas sensors improve sensing properties, they are readily aggregated on the surface of SMOs, leading to the decrease of sensing performance.20 To address this issue, various catalyst loading methods such as metal-organicframework (MOF),21 polyol,22 and apoferritin (AF)23 templating route have been suggested. In particular, AF has attracted great attention due to its unique cavity structure and high dispersibility. For instance, Kim et al. synthesized mesoporous WO3 NFs with nanoscale catalysts by single-spinneret electrospinning using AF encapsulated catalysts.24 Although this work reported that AF templated nanoscale catalyts can improve sensing properties, the rational design of highly efficient nanobuilding block is needed for further effective sensitization of nanocatalysts. To the best of our knowledge, AF templated nanocatalysts have not yet been employed in the multichannel fibrous structure. In this work, we demonstrate the facile combination of SnO2 multichannel nanofibers with catalytic PtO2 NPs (PtO2-SnO2 MCNFs). The PtO2-SnO2 MCNFs were synthesized by single-spinneret electrospinning combined with AF encapsulated Pt NPs (Pt-AF) and two immiscible polymers. During the electrospinning, as phase separation of two immiscible polymers occurs, numerous discontinuous phases were elongated inside continuous phase of NFs.15 Through subsequent calcination step, polymers of discontinuous phase were 4 ACS Paragon Plus Environment
decomposed, leaving hollow channels, and nanoscale catalysts were uniformly immobilized on both the interior and exterior sites of MCNFs.25 The synthesized multitubular structure has numerous hollow sites and multi-channels which can facilitate gas diffusion into the NFs, leading to the effective sensitization of AF templated nanoscale PtO2 catalysts. In this work, the synergistic sensing effect of PtO2-loaded SnO2 MCNFs, which is driven by the creation of hollow multichannels in 1D fiber structures and sensitization of protein-templated PtO2 catalysts, is discussed in terms of highly sensitive and selective acetone sensing characteristics.
2. MATERIALS AND METHODS 2.1. Materials. Polyvinylpyrrolidone (PVP, Mw ~ 1,300,000 g mol–1), polyacrylonitrile (PAN, Mw ~ 150,000 g mol-1), and sodium borohydride (NaBH4, ≥ 96%) were purchased from Aldrich. Tin (II) chloride dihydrate (SnCl2·2H2O), N,N-dimethylformamide (DMF, 99.8%), chloroplatinic acid hydrate (H2PtCl6·H2O), and apoferritin (0.2 µm filtered state from equine spleen) were purchased from Sigma-Aldrich. All materials were used without further purification.
2.2. Synthesis of SnO2 MCNFs and SnO2 NFs. SnO2 MCNFs and SnO2 NFs were synthesized as control samples for comparison with PtO2-functionalized SnO2 MCNFs and PtO2-functionalized SnO2 NFs. In case of the MCNFs, 0.12 g of PVP and 0.15 g of SnCl2·2H2O were dissolved in 2 ml of DMF. The solution was stirred at room temperature for 2 h followed by adding 0.18 g of PAN. After the solution was stirred at 70 °C for 10 h to dissolve the PAN, the electrospinning was carried out at a voltage of 15 kV and a feeding rate of 0.15 ml min-1. The syringe needle of 21 gauge was used and distance between needle and collecter was 15 cm. To form the SnO2 MCNFs, two step heat treatments were performed. The first step was carried out at 280 °C for 1 h to stabilize NFs and second step was conducted at 600 °C for 1 h to decompose organic components and oxidize inorganic 5 ACS Paragon Plus Environment
precursor. All heat treatments were carried out with a regular ramping rate (5 °C min-1) in air atmosphere. On the other hand, in case of the NFs, 0.30 g of PVP and 0.15 g of SnCl2·2H2O were dissolved in 2 ml of DMF. After the solution was stirred at room temperature for 8 h, the electrospinning was conducted with the same conditions for MCNFs. To form SnO2 NFs, heat treatment for 1 h at 600 °C with a ramping rate of 5 °C min-1 was carried out in air condition.
2.3. Synthesis of PtO2-SnO2 MCNFs and PtO2-SnO2 NFs. Pt encapsulated apoferritin (Pt-AF) was synthesized for functionalization of PtO2 nanocatalysts onto the SnO2 NFs. First of all, 0.5 g of AF was treated with NaOH (0.1 M) solution to adjust pH to 8.5. Subsequently, separately prepared H2PtCl6·H2O aqueous solution (1.6 wt%) was added into the AF solution and stirred at 100 rpm for 1 h at room temperature. During the stirring, Pt4+ ions were diffused into the inner cavity of AF. Then, the solution was treated with NaBH4 aqueous solution (0.1 M) to reduce Pt4+ ions to Pt NPs and further stirred at 200 rpm for 10 min at room temperature. Subsequently, the stirred solution was filtered by centrifugation to separate Pt-AF from the solution. The separated Pt-AF was dispersed in 2.5 g of DMF. Then, the DMF solution containing Pt-AF was added into the electrospinning solution that has same composition with solutions prepared for the synthesis of SnO2 MCNFs and SnO2 NFs. To synthesize PtO2-SnO2 MCNFs and PtO2-loaded SnO2 nanofibers (PtO2-SnO2 NFs), the electrospinning and heat treatment were conducted with the same conditions for SnO2 MCNFs and SnO2 NFs, respectively.
2.4. Sensor fabrication and gas sensing test. To investigate gas sensing properties, we prepared a sensing substrate which is described elsewhere.26 The substrate which has an area of 2.5 mm х 2.5mm and thickness of 0.2 mm was patterned with parallel two Au electrodes on the front side and a Pt heater on the back side. To coat the sensing materials onto the substrates, 10 mg of SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2-SnO2 MCNFs was 6 ACS Paragon Plus Environment
dispersed in 0.5 mL of EtOH. Then, the dispersed solutions were drop-coated on the sensing substrates. Prior to sensing test, the fabricated sensors were stabilized in humid air atmosphere for 3 h at sensor operating temperature (400 °C). The operating temperature was controlled by applying a specific voltage to Pt microheater of sensing substrate. Then, gas sensing test was conducted toward various gas species with concentrations ranging from 0.4 ppm to 5 ppm in a way injecting air for 10 min and analyte gas for 10 min. Response of sensors was evaluated by the resistance change, i.e., response (Rair/Rgas). Response time was caculated as the time to decrease resistance by 90% of resistance difference (Rair – Rgas) after injecting target gases. Selectivity toward acetone gases was measured by comparing acetone sensitiviy with the response toward other interfering gases at same condition. In addition, stability was evaluated by conducting sensing test for long time to identify reliability of the sensor without degradation. The pattern recognition analysis was conducted from the data of a sensor array of SnO2 MCNFs and PtO2-SnO2 MCNFs using IBM Statistical Package for the Social Sciences Statistics.
2.5. Characterization. To confirm morphologies of synthesized materials, the field emission scanning electron microscopy (Nova230, FEI) was used. The analysis of microstructures and properties of PtO2-SnO2 MCNFs was conducted by using field emission transmission electron microscopy (Tecnai G2 F30 S-TWIN, FEI). The high resolution powder X-ray diffractometer (Rigaku) with Cu Kα 1 radiation (λ = 1.5406 Å) was used to analyze the crystal structure and phase of samples. The chemical binding states of each component were identified by using X-ray photoelectron spectroscopy (K-alpha, Thermo VG Scientific). The pore size distribution and Brunauer-Emmett-Teller (BET) surface area of samples were confirmed by N2 adsorption/desorption isotherms (Tristar 3020, Micromeritics) at 77 K.
3. RESULTS AND DISCUSSION 7 ACS Paragon Plus Environment
Pt-AF was synthesized for effective functionalization of PtO2 nanocatalysts onto the SnO2 MCNFs. After injecting Pt ions in the AF cavity, Pt ions were reduced to Pt NPs, as previously reported (More detail in the Materials and Methods section).27 The synthesized PtAF with Pt (111) crystal plane was confirmed by high resolution transmission electron microscopy (HRTEM) images (Figure S1). AF with very small inner cavity (8 nm) limits the size of encapsualted NPs. Moreover, positively charged outer shell of AF effectively prevents bulk aggregation of NPs.28 Thus one can expect the homogeneous sensitization of welldispersed nanoscale catalysts onto the MCNFs. We prepared the electrospinning solution by adding synthesized Pt-AF into the solution composed of PVP, PAN, tin (II) chloride dihydrate (SnCl2·2H2O), and DMF. Due to the immiscibility between PVP and PAN, PVP and Sn precursor coexist as continuous phase and PAN occupies discontinuous phase in the solution (Scheme 1a).29 Then, PVP/PAN/Pt-AF/Sn precursor composite NFs were produced by using electrospinning. During the electrospinning, materials are divided into the matrix and droplet phase in the NFs. According to the previous reports,30, 31 high viscosity polymer (PAN) and low viscosity polymer (PVP) occupy droplet and matrix phase, respectively (Scheme 1b). Subsequently, PtO2-SnO2 MCNFs were obtained after two-step heat-treatments. First step was carried out at 280 °C for 1 h in air to stabilize NFs for the uniform multichannel structure. Second step was conducted at 600 °C for 1 h in air to decompose organic components (PVP, PAN, and AF) and form SnO2 multichannel structure, resulting in the formation of SnO2 MCNFs functionalized by PtO2 NPs (Scheme 1c). To identify the formation of multichannel structure and well-dispersed nanocatalysts, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis were conducted. As-synthesized PVP/PAN/Pt-AF/Sn precursor composite NFs showed fiber diameters from 700 to 900 nm (Figure 1a). In addition, magnified SEM image exhibited bumpy surface morphologies, proving the occurrence of phase separation (Figure 1b). However, Pt-AF was not observed due to its nanoscale size. After heat treatment, PtO2-SnO2 8 ACS Paragon Plus Environment
MCNFs with the reduced diameter of 300–400 nm were uniformly synthesized by the decomposition of organic components and densification during the formation of oxide NFs (Figure 1c). Basically, small amount of metal precursors in comparison with polymer leads to partially broken NFs because they cannot fill all the vacancies caused by decomposed polymer during calcination. However, by using the mixture of PVP and PAN (total weight of 0.3 g) and a small amount of Sn precursor (0.15 g), uniform MCNFs structures were obtained because the Sn precursor can be filled in a smaller space upon oxidation in contrast to dense NFs. Moreover, multichannel structure was clearly identified in magnified SEM images (Figure 1d). To verify the multichannel hollow structure, we analyzed the TEM image of PtO2-SnO2 MCNFs. The dark field scanning TEM image showed distinct multichannel structure (Figure 1e). In addition, we identified the SnO2 crystal lattices of (110) and (101) planes and the existence of PtO2 NP pointed by red arrow in HRTEM image (Figure 1f). During calcination, AF was decomposed and exposed Pt NPs were oxidized to PtO2 onto the SnO2 MCNFs. Selected area electron diffraction (SAED) patterns showed the SnO2 crystal planes of (110), (101), (200), and (112). On the other hand, the crystal planes of PtO2 NPs were not identified due to the small amount of PtO2 catalysts (Figure 1g). To clearly verify the existence of PtO2 NPs, we conducted EDS elemental mapping analysis. The mapping image evidently exhibited well dispersed PtO2 NPs without aggregation on the SnO2 MCNFs (Figure 1h). In addition, sequential up and down shape of line profile, whch means the repetition of filled part and hollow part, indicates the formation of multichannel structure (Figure 1i). For comparison, pristine SnO2 NFs were synthesized by electrospinning of the solution composed of PVP, Sn precursor, and DMF followed by subsequent calcination. As-spun NFs with the average diameter of 250 nm showed smooth surface morphologies unlike the case of MCNFs (Figure S2a). Then, the calcination at 600 °C for 1 h in air was conducted to decompose PVP and form the SnO2 NFs. Calcined SnO2 NFs exhibited partially broken structure because equal weights of Sn precursor and polymer were used in order to attain similar synthesis 9 ACS Paragon Plus Environment
conditions as in the case of MCNFs. In addition, calcined SnO2 NFs were composed of densely packed nanograins without channel and showed the diameter from 150 to 250 nm (Figure S2b). These results clearly demonstrate the critical role of immiscible copolymers for forming multichannel structure. To investigate the crystal structure of pristine SnO2 NFs, SnO2 MCNFs, and PtO2-SnO2 MCNFs, we carried out X-ray diffraction (XRD) analysis (Figure 2a). Based on the result, three samples equally showed the rutile (hexagonal) SnO2 (JCPDS no. 41-1445) and polycrystalline characteristics. However, the crystal planes of PtO2 particles were not observed in XRD analysis due to the low amounts of PtO2 catalysts. In addition, we calculated average grain size of three samples from (110), (101), and (211) XRD peaks by using Scherrer equation. The grain size of PtO2-SnO2 MCNFs (10.95 nm) and SnO2 MCNFs (11.14 nm) was noticeably smaller than that of pristine SnO2 NFs (18.06 nm), corresponding well to the grain size shown in SEM images (Figure 1d and S2b). Since droplet phase (PAN) in the middle of as-spun NFs can inhibit grain growth of SnO2, MCNFs have smaller grains. In addition, well dispersed AF has also pinning effect to prevent grain growth, as previously reported.24 To clearly identify the reasons for reduced grain sizes, we conducted thermogravimetric and differential scanning calorimetry analysis for as-spun PVP/Sn precursor composite NFs and PVP/PAN/Pt-AF/Sn precursor composite NFs. As shown in Figure S3a, for PVP/Sn precursor composite NFs, two distinct peaks exist at around 300 °C and above 400 °C, respectively. According to a previous report,32 PVP was decomposed with three steps. First decomposition of PVP occurs at 300 °C and second decomposition occurs at 400 °C. Last step appears at around 450 °C decomposing all PVP component. In addition, crystallization of SnO2 starts at about 400 °C.33 That is, a peak at 300 °C means decomposition of PVP and a peak above 400 °C exhibits decomposition of PVP and crystallization of Sn precursor. On the other hand, in the case of PVP/PAN/Pt-AF/Sn precursor composite NFs, an additional peak exists at 260 °C which exhibits the cyclization of 10 ACS Paragon Plus Environment
PAN and decomposition of AF (Figure S3b).34, 35 Besides, the weight loss of PVP/PAN/PtAF/Sn precursor composite NFs above 400 °C is much bigger than that of PVP/Sn precursor composite NFs because decomposition of PAN occurs at temperature above 450 °C.36 In this sense, reduced grain size of MCNFs can be explained by two main reasons, i.e., (i) PAN in the middle of NFs can endure up to 450 °C or higher temperature, hindering grain growth of SnO2, (ii) heat transfer to SnO2 is reduced because heat is used to decompose PAN. However, in the case of pristine NFs, PVP and Sn precursor coexist and grain growth of SnO2 occurs freely without inhibition of PAN and AF. To confirm the chemical binding states of each component, X-ray photoelectron spectroscopy (XPS) analysis was carried out for PtO2-SnO2 MCNFs. The high-resolution spectrum of Sn 3d showed two distinct peaks located at 486.5 and 494.9 eV corresponding to the Sn4+ 3d5/2 and 3d3/2 state, respectively (Figure 2b). In the case of the O 1s XPS spectrum, two peaks corresponding to O2- and O- state were observed at 530.3 and 531.5 eV (Figure 2c). These two states are chemically adsorbed forms of oxygen on the surface of SnO2 MCNFs. As shown in Figure 2d, high-resolution spectrum of Pt 4f revealed two characteristic peaks at 74.8 and 78.1 eV that correspond to PtO2 4f7/2 and 4f5/2.37 This result confirms that Pt catalysts were oxidized to PtO2 during the calcination.38 To identify the effect of multichannel structure and nanocatalysts, we measured sensing characteristics of pristine SnO2 NFs, SnO2 MCNFs, and PtO2-SnO2 MCNFs toward acetone gases under highly humid atmosphere (95% RH). Basically, gas sensing performance is dependent on the operating temperature and the loading amount of catalysts. The operating temperature affects the adsorption and desorption properties of target analytes as well as charge transport characteristics on the surface of SMOs.39, 40 The ionosorbed forms of oxygen as well as diffusion rate of the target gas and oxygen into the sensing layer are also dependent on the working temperature.40 Therefore, it is imperative to determine the temperature for optimum sensor performance. In addition, the excessive loading of catalysts on SMOs causes 11 ACS Paragon Plus Environment
aggregation of catalysts, leading to the degradation of sensing performance.9 Therefore, to find the optimized operating temperature and amount of catalysts, we carefully varied loading amount of catalysts (0.04–0.16 wt%) and carried out acetone sensing tests at various temperatures (300–450 °C). After drop-coating of pristine SnO2 NFs, SnO2 MCNFs, and 0.04–0.16 wt% of Pt loaded SnO2 MCNFs on the sensing substrates, we conducted sensing tests using a homemade sensing equipment. Based on the result, 0.12 wt% of Pt decorated SnO2 MCNFs showed the highest acetone response (Rair/Rgas = 194.15) toward 5 ppm at 400 °C (Figure S4). Then, to certainly prove the effect of multichannel structure, we also prepared the same amount of Pt catalysts (0.12 wt%) loaded SnO2 NFs as control group (Figure S5). Accordingly, we evaluated dynamic acetone sensing properties of pristine SnO2 NFs, SnO2 MCNFs, 0.12 wt% Pt-SnO2 NFs, and 0.12 wt% Pt-SnO2 MCNFs in the concentration range of 0.4–5 ppm at 400 °C. The 0.12 wt% Pt-SnO2 MCNFs, 0.12 wt% PtSnO2 NFs, SnO2 MCNFs, and pristine SnO2 NFs showed response of 194.15, 23.37, 14.48, and 10.32 toward 5 ppm of acetone gases (Figure 3a). These results clearly verify the remarkable effect of the multichannel structure and nanoscale PtO2 catalysts to increase response to the acetone molecules. In addition, we calculated response time of pristine SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2-SnO2 MCNFs toward each concentration of acetone at 400 °C (More details in the Materials and Methods section). As shown in Figure 3b, multichannel structure exhibited much faster response time (< 12 s) than dense nanofiber structure (< 28 s). This result means that multichannel structure facilitates gas penetration into the sensing layers, leading to immediate decrease in resistance. Moreover, to evaluate the selectivity of PtO2-SnO2 MCNFs, we conducted gas sensing tests toward various gas species, i.e., acetone (CH3COCH3), ethanol (C2H5OH), methane (CH4), carbon monoxide (CO), formaldehyde (HCHO), ammonia (NH3), toluene (C6H5CH3), and hydrogen sulfides (H2S) at 400 °C under highly humid condition (95% RH). The PtO2-SnO2 MCNFs showed superior selectivity toward acetone gases (Rgas/Rair = 21.94 at 1 ppm) against other interfering gases 12 ACS Paragon Plus Environment
(Rgas/Rair < 9.1 at 1 ppm) (Figure 3c). Considering this result, the PtO2-SnO2 MCNFs are suitable sensing materials for the selective detection of the acetone molecules in complex atmosphere. Furthermore, the PtO2-SnO2 MCNFs sensor showed excellent stability without huge degradation of performance during the 30 cycles of response and recovery toward 1 ppm of acetone at 400 °C for 10 h (Figure 3d). Compared with first cycle response (22.07), 30th cycled PtO2-SnO2 MCNFs sensor showed just 5.6 % decreased response (20.82). These sensing performances are outstanding among recently reported SMOs-based sensors for detecting acetone gases (Table 1),25, 26, 28, 33, 41-43 demonstrating that the nanoscale catalysts loaded multichannel structure has great potential as high performance acetone sensor. The operating principle of SMOs based gas sensor is based on the resistance change when the surface reaction with target gases occurs on the sensing layers.41 When n-type SMOs are exposed to the air, oxygens are chemically adsorbed on the surface of SMOs (O2 + 2e- → 2O-(ads)) under high operating temperature,44 leading to the formation of electron depletion region on the surface of SMOs. Then, when reducing gas such as acetone is exposed to the SMOs, the trapped electrons in chemisorbed oxygens are donated back to the SMOs by following reaction : CH3COCH3(gas) + 2O-(ads) → CH3O- + C+H3 + CO2(gas) + 2e-,45 decreasing the resistance of SMOs-based gas sensors. Therefore, the large surface area of SMOs is an essential factor to increase sensing performance because more oxygen adsorption-desorption reactions can occur on the surface of SMOs.20 In addition, the functionalization of catalysts improves the sensing performance by electronically and chemically sensitizing SMOs. In the present work, Pt nanocatalysts exist as oxidized forms (PtO2) which are p-type material, leading to the p-n junction at the interface of SnO2 and PtO2.46 Therefore, PtO2 increases the resistance change of SnO2 as an electronic sensitization. Moreover, according to previous studies,47, 48 reducing gases partially reduce PtO2 (especially for surface PtO2) to Pt, which function as chemical sensitizers when acetone gas is injected. The Pt NPs are capable of dissociating O2, C-H, and O-H bonds, leading to selective detection of acetone.49 However, 13 ACS Paragon Plus Environment
selectivity toward acetone was reduced at dry condition since H2O vapors predominantly react with surface lattice oxygen of SnO2 in Pt doped case (Figure S6).50 This reaction leads to more oxygen vacancies on the surface of SnO2 which induce much more surface adsorbed oxygen ions (O–), thus enhancing reactions with acetone molecules at highly humid condition.50 To clearly prove the mechanism, we conducted sensing test toward 5 ppm of acetone at 95% RH, 45% RH, and dry condition. The results indicate that PtO2-SnO2 MCNFs exhibited higher response to acetone as humidity increases (Figure S7a). In addition, comparison of baseline resistance verified enhanced formation of O– at higher humidity condition (Figure S7b). In fact, metal oxides functionalized with Pt catalyst are well known for selective detection of acetone in highly humid condition.51,
52
Thus, well dispersed
apoferritin templated PtO2 nanocatalysts loaded SnO2 multichannel structure provided high gas response toward acetone (Figure 4a). Since target gas can easily penetrate to the multichannel structure,18 the surface reaction of chemisorbed oxygen and target gas is effectively activated by PtO2 catalysts on both the interior and exterior sites of NFs. To clearly verify the effect, we investigated the variation of base resistance for pristine SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2-SnO2 MCNFs. The PtO2-SnO2 MCNFs had significantly higher base resistance (3146.43 kΩ) compared with SnO2 NFs (58.18 kΩ), SnO2 MCNFs (1128.34 kΩ), and PtO2-SnO2 NFs (2630.03 kΩ) (Figure 4b). The porous MCNFs exhibited higher baseline resistance than dense NFs due to the larger contact resistance caused by high porosity. The huge difference of baseline resistance between pristine SnO2 NFs (58.18 kΩ) and SnO2 MCNFs (1128.34 kΩ) revealed the evidence for the effect of multichannel structure. In addition, smaller grain size (11.14 nm) of SnO2 MCNFs than that (18.06 nm) of SnO2 NFs induces larger baseline resistance because smaller grains cause more contacts with oxygen leading to larger electron depletion region on the surface of SnO2. That is, enhanced modulation in electron depleted region can be obtained in case of the multichannel structure, thereby leading to increased resistance variation under target gas 14 ACS Paragon Plus Environment
exposure. On the other hand, the core part of densely packed pristine SnO2 NFs is not active; thus, it is not able to participate in the surface gas reactions. In addition, despite of the same amount of catalysts (0.12 wt%), higher resistance of PtO2-SnO2 MCNFs (3146.43 kΩ) compared with PtO2-SnO2 NFs (2630.03 kΩ) indicates the effective activation of catalysts on the multichannel structure. To further verify the increased surface area and pore distribution of multichannel nanofiber structure, we investigated Brunauer-Emmett-Teller (BET) surface area and pore size distribution for pristine SnO2 NFs and SnO2 MCNFs. Based on the result, BET surface area of SnO2 MCNFs (28.11 m2 g-1 STP) is almost three fold larger than that of pristine SnO2 NFs (9.90 m2 g-1 STP) (Figure 4c). Additionally, the cumulative pore volume of SnO2 MCNFs (0.143 cm3 g-1) is much larger than that of SnO2 NFs (0.057 cm3 g-1) (Figure 4d). In particular, the volume of mesopores about 20 nm was extremely increased due to the generation of small multichannels in the SnO2 MCNFs. These elongated pores with a narrow diameter (20 nm) are beneficial for facilitating Knudsen diffusion of gas molecules. Namely, in the small channels, target gases collide with the channel walls more frequently than with each other, leading to effective reaction with sensing material in the SnO2 MCNFs system. To investigate the selective detection property toward acetone molecules, we conducted the principal component analysis (PCA) which is a well-known pattern recognition algorithm system, using a sensor array consisted of SnO2 MCNFs and PtO2-SnO2 MCNFs (Figure 5). Since the human breath is highly humid and contains thousands of interfering gases, PCA was conducted with regard to eight different gases with the concentration of 1 ppm, i.e., acetone (CH3COCH3), ethanol (C2H5OH), methane (CH4), carbon monoxide (CO), formaldehyde (HCHO), ammonia (NH3), toluene (C6H5CH3), and hydrogen sulfides (H2S) at 400 °C under highly humid condition (95% RH). For each species, we performed sensing tests three times. The acetone test results indicated by three red squares were assembled in a small area and obviously classified from the other interfering gases which are expressed by black circles. This result demonstrates the potential feasibility of apoferritin templated nanoscale PtO2 15 ACS Paragon Plus Environment
catalysts loaded SnO2 MCNFs based gas sensor for the innovative future sensing system in terms of selective detection of acetone molecules. In addition, by varying metal precursors and catalysts, the strategy of this work can be extended to various metal oxide-catalyst multichannel nanofibers, thus enabling detection of diverse gas molecules.
4. CONCLUSIONS In this work, we demonstrated the facile synthesis of the nanoscale PtO2 catalysts loaded multichannel SnO2 NFs by using two immiscible polymers and sacrificial apoferritin template. During the electrospinning, the phase separation of two immiscible polymers occurred and calcined NFs exhibited the multichannel 1D oxide structure with enhanced surface area and meso-porosity. In addition, well-dispersed PtO2 nanocatalysts were achieved by utilizing the advantages of apoferritin with small inner cavity and positive charge of shell. Thus, multichannel SnO2 structure functionalized by apoferritin encapsulated Pt catalysts showed the superior activation effect of catalysts on both the interior and exterior sites of the NFs. As a result, the PtO2-SnO2 MCNFs exhibited superior response toward acetone gases (Rair/Rgas = 194.15 at 5 ppm), high cross selectivity toward acetone molecules among various interfering gases, and high reliability during 30 cycles of response and recovery. In particular, a sensor array of SnO2 MCNFs and PtO2-SnO2 MCNFs clearly distinguished acetone molecules from other interfering gases by PCA analysis. These results verified the potential feasibility of the PtO2-SnO2 MCNFs as new sensing materials platforms for exhaled breath analyzer.
ASSOCIATED CONTENT Supporting Information. Additional SEM images, thermal properties and sensing characteristics. These materials are available free of charge via the Internet at “http://pubs.acs.org.” 16 ACS Paragon Plus Environment
AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Biomedical Treatment Technology Development Project (No. 2015M3A9D7067418). This work was also 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), National Research Foundation (NRF) of Korea Grant (No. NRF2015R1A2A1A1A6074901), and Global Frontier Project (CISS-2011-0031870), all funded by the Ministry of Science, ICT and Future Planning. REFERENCES (1) Sun, Y. F.; Liu, S. B.; Meng, F. L.; Liu, J. Y.; Jin, Z.; Kong, L. T.; Liu, J. H. Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review. Sensors 2012, 12, 2610– 2631. (2) Park, S.; Kim, S.; Kheel, H.; Lee, C. Oxidizing Gas Sensing Properties of the n-ZnO/pCo3O4 Composite Nanoparticle Network Sensor. Sens. Actuators, B 2016, 222, 1193– 1200. (3) Utriainen, M.; Kärpänoja, E.; Paakkanen, H. Combining Miniaturized Ion Mobility Spectrometer and Metal Oxide Gas Sensor for the Fast Detection of Toxic Chemical Vapors. Sens. Actuators, B 2003, 93, 17–24.
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Figure Captions Scheme 1. Schematic illustration of (a) the copolymer-electrospinning, (b) as-spun PVP/PAN/Pt-AF/Sn precursor composite NFs, and (c) PtO2-SnO2 MCNFs. Figure 1. SEM images of (a), (b) as-spun PVP/PAN/Pt-AF/Sn precursor composite NFs and (c), (d) PtO2-SnO2 MCNFs. TEM analysis of PtO2-SnO2 MCNFs: (e) TEM image, (f) HRTEM image, (g) SAED patterns, (h) EDS elemental mapping images, and (i) line profile. Figure 2. (a) XRD patterns of pristine SnO2 NFs, SnO2 MCNFs, and PtO2-SnO2 MCNFs. XPS spectra of PtO2-SnO2 MCNFs: (b) Sn 3d, (c) O 1s, and (d) Pt 4f. Figure 3. (a) Dynamic acetone sensing performance of pristine SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2-SnO2 MCNFs in the concentration range of 0.4–5.0 ppm at 400 °C, (b) response time of pristine SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2-SnO2 MCNFs toward each concentration of acetone at 400 °C, (c) selectivity of PtO2-SnO2 MCNFs toward acetone gases among various 1 ppm gas species at 400 °C and 95% RH, and (d) stability of PtO2-SnO2 MCNFs toward 1 ppm acetone gases during 30 cycles of response and recovery for 10 h at 400 °C. Figure 4. (a) Schematic illustration of sensing mechanism of PtO2-SnO2 MCNFs, (b) variation of base resistance for pristine SnO2 NFs, SnO2 MCNFs, PtO2-SnO2 NFs, and PtO2SnO2 MCNFs toward 1 ppm acetone gases at 400 °C, (c) BET surface area of pristine SnO2 NFs and SnO2 MCNFs, and (d) pore size distribution of pristine SnO2 NFs and SnO2 MCNFs. Figure 5. Pattern recognition based on PCA analysis using a sensor array of SnO2 MCNFs and PtO2-SnO2 MCNFs. Table 1. Recent publications about SMOs-based gas sensors for detecting acetone molecules under highly humid condition.
