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Bio-inspired Cocatalysts Decorated WO3 Nanotube Toward Unparalleled Hydrogen Sulfide Chemiresistor Dong-Ha Kim, Ji-Soo Jang, Won-Tae Koo, Seon-Jin Choi, HeeJin Cho, Min-Hyeok Kim, Sang-Joon Kim, and Il-Doo Kim ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00210 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018
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Bio-inspired Cocatalysts Decorated WO3 Nanotube Toward
Unparalleled
Hydrogen
Sulfide
Chemiresistor
Dong-Ha Kim, † Ji-Soo Jang, † Won-Tae Koo, † Seon-Jin Choi, ¶ Hee-Jin Cho, † Min-Hyeok Kim, † Sang-Joon 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 ¶
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139, United States
*Corresponding author e-mail:
[email protected] KEYWORDS Bio-templates, cellulose nanocrystals, apoferritin, WO3 nanotube, chemical sensor 1 ACS Paragon Plus Environment
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ABSTRACT Herein, we incorporated dual bio-templates, i.e., cellulose nanocrystals (CNC) and apoferritin into electrospinning solution to achieve three distinct benefits, i.e., (i) facile synthesis of WO3 nanotube by utilizing self-agglomerating nature of CNC in the core of asspun nanofibers, (ii) effective sensitization by partial phase transition from WO3 to Na2W4O13 induced by interaction between sodium doped CNC and WO3 during calcination, and (iii) uniform functionalization with monodispersive apoferritin-derived Pt catalytic nanoparticles (2.22 ± 0.42 nm). Interestingly, sensitization effect of Na2W4O13 on WO3 resulted in highly selective H2S sensing characteristics against 7 different interfering molecules. Furthermore, synergistic effects with bio-inspired Pt catalyst induced remarkably enhanced H2S response (Rair/Rgas = 203.5), unparalleled selectivity (Rair/Rgas < 1.3 for the interfering molecules), and rapid response (< 10 s)/recovery (< 30 s) time at 1 ppm of H2S under 95% relative humidity level. This work paves the way for a new class of co-sensitization route to overcome critical shortcomings of SMOs-based chemical sensors, thus providing a potential platform for diagnosis of halitosis.
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Semiconducting metal oxides (SMOs) based gas sensors have been widely utilized for applications in environmental toxic gas monitoring,1 military defense,2 and food safety.3 More recently, miniaturized SMOs sensors have attracted much attention for potential use in noninvasive diagnosis of specific diseases such as diabetes (acetone),4 lung cancer (toluene),5 and halitosis (hydrogen sulfide),6 through detection and analysis of the respective markers in exhaled breath. Furthermore, their facile and rational design, low cost, portability, and ability to detect a large number of gas molecules is particularly intriguing for exhaled breath sensor applications, but still critical challenges remain. Their inherent limitations associated with low sensitivity and sluggish response/recovery speed, particularly at high humidity and/or low concentration of analyte gases have been partially overcome by the use of nanostructured sensing layers, including one-dimensional nanostructures synthesized by an electrospinning technique. Nevertheless, sensing layers based on electrospun nanofibers can offer high gas accessibility and large surface area, resulting in active interaction of air adsorbates with gas analytes.7-11 Moreover, catalystic sensitization of gas sensing layers provides much enhanced gas reaction speeds.12 Thus, homogeneous functionalization of non-aggregated catalysts on nanostructured gas sensing layers is essential for improving both gas sensitivity and gas response/recovery speed.13-16 Even though highly sensitive and fast responding gas sensors (< 10 s) have been successfully demonstrated in many literatures,17 limited selectivity characteristics of SMOs sensors have hindered their use in high accuracy and high precision sensing devices. Since gas sensing mechanism is based on the resistance change in air vs. target gas, many gas species that reach the surface of SMOs participate in reaction, leading to poor selectivity. To somehow overcome this limitation, pattern recognition technique using cross-reactive multi-sensing layers, so called sensor arrays, have been widely adopted to offer higher accuracy in gas detection.18 However, high manufacturing cost caused by the use of multiple sensor arrays
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and the use of pattern recognition tool such as principle component analysis limit the common and widespread use of such devices. Even though SMOs gas sensors have been successfully commercialized for applications in environmental toxic gas monitoring, the use of SMOs as exhaled breath sensor platforms have been greatly hindered by their poor selective detection of sub-ppm concentrations of breath marker molecules. To address this issue, many researches have explored the utilization of dual catalyst systems, i.e., chemical and/or electronic sensitizers that are much more effective than their single catalyst counterparts. For example, Koo et al. reported metal-organic framework driven PdO@ZnO cocatalysts decorated onto WO3 nanofibers as highly sensitive and selective toluene sensing layers.19 In addition, Jang et al. synthesized Co3O4-PdO cocatalyst loaded SnO2 hollow nanocubes using galvanic replacement reaction to prepare sensing layers for high performance acetone sensors.20 Also, Kim et al. reported CO detectors by utilizing reduced graphene oxide and Au co-functionalized SnO2 nanofibers as a sensing matrix.21 Nonetheless, sensing characteristics cannot offer exceptional selectivity factor (i.e., Star./Sinter., where Star. and Sinter. denote response toward target gas and the most reactive interfering gas, respectively), particularly under low ppm gas concentration. In this work, we newly fabricated superior H2S sensing layers using dual sacrificial biotemplates, i.e., cellulose nanocrystals and apoferritin, in a one-pot synthesis of cocatalysts (PtNa2W4O13) functionalized WO3 nanotubes using electrospinning technique. This work highlights the highly efficient electrospinning route using dual soft templates, i.e., (i) sodium doped cellulose nanocrystals template for simultaneous realization of morphology control (nanotube scaffold) as well as compositional modification (Na2W4O13), and (ii) homogeneous sensitization of nanotubes with apoferritin encapsulated catalytic Pt nanoparticles (NPs). As a result, unprecedented tailored design of dual bio-templates for unparalleled hydrogen sulfide (H2S) selectivity, i.e., selectivity factor larger than 150 to 1 ppm of H2S was realized via chemical reaction between Na-H2S and Pt-H2S in gas sensing structures. Furthermore, the gas 4 ACS Paragon Plus Environment
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sensing results exhibited one of the highest ranking H2S sensing characteristic under high humidity in comparison with reported state-of-the-art works. The microstructural and compositional evolution of WO3 nanotubes co-sensitized with triclinic Na2W4O13 and Pt NPs are discussed in terms of functionalization with Na-doped cellulose nanocrystals and Ptencapsulated apoferritin templates during electrospinning, and their decomposition during subsequent calcination.
EXPERIMENTAL SECTION Preparation
of
precursor
[(NH4)6H2W12O40·xH2O],
materials.
polyvinylpyrrolidone
Ammonium (PVP,
Mw =
metatungstate 1,300,000
hydrate g
mol-1),
chloroplatinic acid hydrate (H2PtCl6·6H2O), sodium borohydride (NaBH4), were purchased from Sigma-Aldrich (St. Louis, USA). Cellulose nanocrystals (CNC) were purchased from CelluForce (Montreal, Canada). All the chemicals were used without further purification. Synthesis of catalytic Pt nanoparticles. The Apo-Pt NPs were prepared by using protein nanocages, i.e., apoferritin template. First, horse spleen apoferritin solution (1 g) was adjusted to a pH of about 8.5 by using 0.01 M NaOH (aq.). Then, 15 mg of catalyst precursor (H2PtCl6·6H2O) was dissolved in 1 g of DI water. Subsequently, as-prepared aqueous solution of catalyst was mixed with apoferritin solution and stirred with a magnetic bar at 100 rpm for 1 h at room temperature to penetrate the metal ions into the inner cavity of the apoferritin shells. Then, subsequent reduction was conducted by adding 0.1 M NaBH4 solution (96 %, Sigma Aldrich) dropwise, and stirred for 30 min to form Apo-Pt NPs encapsulated in apoferritin nanocages. The Pt NPs solution was then filtered by centrifugation at 12,000 rpm for 5 min, and consequently dispersed in 2 g of DI water. Synthesis of Pt-Na2W4O13 loaded WO3 nanotubes. Firstly, 0.2 g of AMH, 0.25 g of PVP were dissolved in 2 g of 5 wt % CNC dispersed in DI water. The solution was vigorously stirred at 500 rpm for 6 h at room temperature. Afterward, as synthesized Apo-Pt NPs were 5 ACS Paragon Plus Environment
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added to the above solution, and the mixture was vigorously stirred at 500 rpm for 2 h at room temperature. Electrospinning was carried out with the prepared solution at a constant DC voltage of 17 kV between the stainless steel collector and the syringe needle (25 gauge) at a feeding rate of 0.10 mL min-1. The as-spun Apo-Pt NPs decorated CNC/W precursor/PVP composite NFs were calcined at 600 °C for 1 h at a ramping rate of 5 °C min-1 in air to obtain Pt-Na2W4O13 loaded WO3 NTs. Na2W4O13 loaded WO3 NTs were prepared by the same experimental procedure except that Apo-Pt NPs were not added to the electrospinning solution. Likewise, dense WO3 NFs were synthesized without addition of CNC solution and Apo-Pt NPs to the electrospinning solution while for Pt loaded WO3 NFs CNC solution was not added. Materials Characterization. The morphologies of the synthesized materials were analyzed by field-emission scanning electron microscopy (FE-SEM, Nova 230). The microstructural characteristics, Energy-dispersive spectrometry (EDS) mapping analysis, and selected area diffraction (SAED) patterns were analyzed by transmission electron microscopy (TEM) (Tecnai F30 S-Twin, FEI). The crystal structures were investigated by XRD (D/MAXRX 12 kW, Rigaku) using Cu Kα (λ=1.54 Å) radiation. The chemical bonding states were investigated by X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific) with Al Kα radiation (1486.6 eV). Thermal stability was examined by thermal gravimetric analysis (TGA, Labsys Evo, Setaram). Ultraviolet photoelectron spectroscopy (UPS) was conducted to analyze the work function of the samples. Evaluation of gas sensing performance. By using a homemade gas sensor testing system described elsewhere, the gas sensing characteristics were measured (Figure S1). The resistance of the sensor was measured at intervals of 4 s, using data acquisition system (34972A, Agilent) with a 16 channel multiplexer (34902A, Agilent). The measured resistance was converted into the response i.e., resistance ratio of Rair/Rgas, where Rair is the sensor resistance in background air and Rgas is the sensor resistance in analyte gas ambient. The 6 ACS Paragon Plus Environment
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response data was investigated for dense WO3 NFs, Na2W4O13 loaded WO3 NTs, and PtNa2W4O13 loaded WO3 NTs based sensors. Before the gas sensing measurement, all the sensors were stabilized for 3 h in background air, and highly humid atmosphere (95% RH), which is similar atmosphere to human exhaled breath. The gas analytes, i.e., hydrogen sulfide (H2S), acetone (CH3COCH3) and toluene (C6H5CH3), were used as simulated breath biomarkers. Cyclic exposure of the sensors to analyte gas for 10 min and background air for 10 min was conducted with gas concentration ranging from 0.15 ppm to 5 ppm. The operating temperature of the sensors was controlled by applying a DC voltage to the microheater by using DC power supply (E3647A, Agilent). The microheater is positioned on the back side of the alumina substrate onto which the sensing layers are drop-coated. The photograph and optical micrographic images exhibit that the sensing layers are uniformly coated on the alumina substrate (Figure S2a–b).
RESULTS AND DISCUSSION In this work, dual sacrificial bio-templates, i.e., sodium impurity-loaded cellulose nanocrystals (CNC) and Pt NP-encapsulated apoferritin (hereafter, Apo-Pt) were used. Figure S3a shows schematic illustration for preparation of CNC template, which is extracted from wood and chemically modified by sodium hydroxide and sulfuric acid. Thus, the molecular formula of CNC includes sulfate functional groups as well as sodium impurity. Morphological feature of CNC was analyzed by scanning electron microscopy (SEM) as shown in Figure S3b–c. High resolution SEM image in Figure S3c exhibits that CNC templates were partially aligned and agglomerated due to the existence of abundant hydroxyl groups on their surfaces. The size of CNC was about 20–30 nm in diameter and 100 nm in length. Figure S4a shows schematic illustration of apoferritin encapsulated Pt NPs. By utilizing the unique characteristics of apoferritin template, i.e., (i) partially positively charged nature of protein 7 ACS Paragon Plus Environment
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shell and (ii) hollow spherical scaffold with cavity size and outer diameter of 8 and 12 nm, respectively, ultrasmall Pt catalytic NPs encapsulated in a protein shell can be synthesized.22 As indicated in high resolution transmission electron microscopy (HRTEM) image in Figure S4b–c, Apo-Pt NPs exhibited finely dispersed and highly crystalline characteristics, showing lattice planes of Pt (111) and (200). By counting 70 different Apo-Pt NPs, the average size distribution was analyzed showing the mean diameter of 2.22 ± 0.42 nm (Figure S4d). In this work, tailored combination of dual sacrificial bio-templates, i.e., CNC and apoferritin, were introduced simultaneously during electrospinning to facilely synthesize cocatalysts (Pt NPs and Na2W4O13) decorated WO3 NTs (hereafter, Pt-Na2W4O13 loaded WO3 NTs). To investigate phase transformation of chemically modified CNC during high temperature calcination, we carried out powder X-ray diffraction (PXRD) analysis. After annealing in air at 600 °C for 1 h, amorphous CNC turned into sodium sulfate crystal structures characterized as Na2SO4 phase (Figure S5a). While the CNC itself burned out, a residual phase due to oxidation of Na-S elements was created, which exhibited crystalline Na2SO4 structure as shown in SEM image (Figure S5b). In the case of CNC-loaded electrospun NFs schematically illustrated in Figure 1a, CNC self-aligned and clustered in a bundle at the core of as-spun composite NFs, leading to core-shell nanofiber structure composed
of
CNC
in
the
core
and
W
precursor
[(NH4)6H2W12O40·xH2O]/polyvinylpyrrolidone (PVP) in the shell upon electrospinning. This intriguing morphological feature is attributed to unique characteristics of CNC template, i.e., (i) self-alignment due to formation of nematic liquid crystal phases under high applied voltage,23, 24 (ii) self-agglomeration owing to strong hydrogen bonding driven by abundant hydroxyl groups on CNC surface,25 and (iii) evaporation of the solvent (DI water) with concomitant shift of dissolved W precursor and PVP toward the outer shell of the as-spun composite NFs, whereas non-soluble CNC mainly remained in the core. After subsequent calcination at 600 °C for 1 h, CNC bundle in the core and PVP matrix completely 8 ACS Paragon Plus Environment
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decomposed while the W precursor oxidized to form tubular WO3 scaffold. Figure 1b shows morphological SEM image of as-spun CNC/W precursor/PVP composite NFs. Since thickness of as-spun NFs is directly proportional to viscosity of electrospinning solution,26 addition of highly viscous CNC solution led to as-spun CNC/W precursor/PVP composite NFs exhibiting thick morphology (620 ± 6 nm in diameter). After calcination at 600 °C for 1 h, CNC decomposed to form WO3 NTs scaffold containing crystalline phases of Na2W4O13, i.e., Na2W4O13 loaded WO3 NTs (Figure 1c–d). The average grain size of WO3 in Na2W4O13 loaded WO3 NTs was calculated to be 55.4 nm by using Scherrer equation. For the synthesis of Pt-Na2W4O13 loaded WO3 NTs, Apo-Pt NPs were added to the electrospinning solution (i.e., CNC, W precursor, and PVP, all dissolved in DI water). Highly viscous CNC solution led to thick as-spun Apo-Pt/CNC/W precursor/PVP composite NFs (605 ± 29 nm in diameter) similar to CNC/W precursor/PVP composite NFs (Figure 1e). After calcination at 600 °C for 1 h, decomposition of CNC in the core generated tubular WO3 nanostructures as shown in SEM image (Figure 1f). In addition, high resolution SEM image in Figure 1g clearly exhibited Pt-Na2W4O13 loaded WO3 NTs with diameter of 299 ± 9 nm, and the average grain size of 52.9 nm, which is slightly smaller than that of Na2W4O13 loaded WO3 NTs because of the pinning effect of Apo-Pt. As a reference sample, as-spun NFs and postcalcined dense WO3 NFs without using dual bio-templates (CNC and apoferritin) were synthesized under the same synthetic method, and analyzed by SEM. Figure 1h shows, morphological SEM image of as-spun W precursor/PVP composite NFs with an average diameter of 423 ± 5 nm. After high temperature calcination at 600 °C for 1 h, PVP polymeric matrix decomposed while W precursor oxidized to form dense WO3 NFs with a diameter of 291 ± 11 nm as shown in SEM images (Figure 1i–j). The WO3 NFs with bumpy surface morphology were composed of densely packed WO3 nanograins with an average size of 30.1 nm, distinctively different from the characteristics of CNC templated WO3 NTs. From SEM images, it is observed that ApoPt/CNC/W precursor/PVP composite NFs severely shrunk in diameter (~50 %) compared to 9 ACS Paragon Plus Environment
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W precursor/PVP composite NFs (~31 %) due to thermal decomposition of core CNC template as well as polymeric templates. In order to emphasize the importance of the calcination temperature, thermal decomposition behavior of CNC template was discussed by thermal gravimetric and differential scanning calorimetry analyses. The results exhibited that the 600 °C of calcination temperature is appropriate to thoroughly decompose CNC template toward formation of tubular WO3 nanostructures (see details in supporting information of Figure S6 and S7). In addition, the BET surface area and pore distribution analyses were conducted for the Pt-Na2W4O13 loaded WO3 NTs (Figure S8). The results exhibited that the BET surface area and pore volume were measured to be 3.3254 m2 g-1 and 0.0335 cm3 g-1, respectively. In order to clearly investigate the detailed microstructures of WO3 NTs, we carried out TEM analysis. Figure 2a demonstrates hollow morphology of Pt-Na2W4O13 loaded WO3 NTs. High resolution TEM (HRTEM) images were taken to further confirm lattice fringes of WO3, Na2W4O13, and Pt NPs (Figure 2b–c). HRTEM image in Figure 2b indicates polycrystalline characteristics of WO3 with lattice planes of (202), (020), and (200) corresponding to the interplanar distances of 2.62 Å, 3.77 Å, and 3.65 Å, respectively. Furthermore, lattice fringes with a interplanar spacing of 3.79 Å assigned to (001) plane of triclinic Na2W4O13 was also identified. In addition, HRTEM image in Figure 2c indicates additional (110) and (002) lattice planes of WO3, which correspond to interplanar distances of 5.24 Å and 3.84 Å, respectively. Also, uniform distribution of catalytic Pt NPs was observed while their lattice fringes were not clear because of overlapping with grains of background WO3. The SAED patterns confirmed the existence of Pt (111) lattice planes as well as polycrystalline WO3 (002), (112), and (202) (Figure 2d). EDS elemental mapping analysis was carried out to investigate high dispersibility of the cocatalysts (Pt NPs and Na2W4O13). In the dark field scanning TEM (STEM) image, hollow tubular structure was noticeably observed (Figure 2e). In addition, EDS elemental mapping analysis confirmed finely dispersed W, O, Na, and Pt, demonstrating 10 ACS Paragon Plus Environment
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homogeneous dispersion of elements in the entire WO3 NT (Figure 2f). The exact composition ratio and concentration of Na in CNC solution was characterized through inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Consequently, the concentration of Na was evaluated to be 491 mg kg-1, which indicated that the composition ratio of Na was 0.52 wt% with respect to W in Na2W4O13 loaded WO3 NTs. Relative deficiency of W, O, and Pt NPs in the core of Pt-Na2W4O13 loaded WO3 NT scaffold was proved by the EDS line profile, demonstrating the effectiveness of CNC as a nanotubeforming template (Figure S9a–b). The EDS line profile was also conducted for Na2W4O13 loaded WO3 NTs, indicating the tubular scaffold through deficiency of W and O in the core of the NTs (Figure S9c–d). To further study the crystal structures of WO3 NFs, Na2W4O13 loaded WO3 NTs, and PtNa2W4O13 loaded WO3 NTs, PXRD analysis was carried out. The PXRD data revealed that all three samples were composed of monoclinic WO3 (JCPDS# 43-1035) crystal structures (see black lines in Figure S10). Importantly, both Na2W4O13 loaded WO3 NTs and PtNa2W4O13 loaded WO3 NTs represented new crystal peaks belonging to triclinic Na2W4O13 (JCPDS# 27-1425) crystal structure (see purple lines in Figure S10), which originated from the thermal reaction between W precursors and Na species immobilized on the surface of chemically modified CNC. However, characteristics peaks related to Pt were not observed because the loading amount was below the detection limit of PXRD. To verify the effect of cocatalysts toward superior gas sensing characteristics stemming from synergistic effects, gas-sensing performances of pristine WO3 NFs, Na2W4O13 loaded WO3 NTs, and Pt-Na2W4O13 loaded WO3 NTs were compared. The sensor operating temperatures (350–500 °C) and loading concentrations of the bio-inspired Pt catalytic NPs (0.01, 0.02, 0.05, and 0.10 wt%) were carefully optimized in a highly humid atmosphere (95% RH). In the temperature range of 350–500 °C, the temperature dependent response variation was investigated at 5 ppm of H2S gas (Figure 3a). The results showed that the optimized 11 ACS Paragon Plus Environment
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operating temperature for Na2W4O13 loaded WO3 NTs was 500 °C with a response (Rair/Rgas, where Rair and Rgas are the resistances of the sensors in air and analyte gas, respectively) of 28.73 ± 4.42, whereas Pt-Na2W4O13 loaded WO3 NTs exhibited lower optimal operating temperature (450 °C) with a response of 570.38 ± 44.55. The lowering of the optimum temperature is thought to be induced by the Pt catalytic effect which lowers the activation energy of the surface reaction between H2S molecules and chemisorbed oxygen species.27 To figure out the optimized concentration of Pt catalytic NPs, Na2W4O13 loaded WO3 NTs loaded with four different amounts of Pt NPs (i.e., 0.01, 0.02, 0.05, and 0.10 wt%) were tested in the H2S concentration of 5–1 ppm at 450 °C (Figure S11). The results revealed that the 0.05 wt% of catalytic Pt NPs was an optimal loading amount. It is reported that H2S is the biomarker gas for halitosis, where the concentration of H2S in exhaled breath of halitosis patient is 0.8–2 ppm and it is less than 150 ppb for a healthy person.28, 29 In this regard, the H2S sensing characteristics were evaluated in the range of 0.15–5 ppm, which is far beyond the halitosis diagnosis threshold. The sensing tests were conducted at 450 °C with 3 sensors based on 0.05 wt% Pt-Na2W4O13 loaded WO3 NTs, Na2W4O13 loaded WO3 NTs, and dense WO3 NFs in highly humid atmosphere (95% RH) (Figure 3b). By measuring dynamic resistance variation, 0.05 wt% Pt-Na2W4O13 loaded WO3 NTs showed the highest response (Rair/Rgas) of 570.43 ± 44.61 to 5 ppm of H2S compared to that of Na2W4O13 loaded WO3 NTs (Rair/Rgas = 24.11 ± 4.01 at 5 ppm) and dense WO3 NFs (Rair/Rgas = 11.45 ± 0.16 at 5 ppm), which were 23.65-fold and 49.82-fold improvement in response, respectively. Significantly, the sensor with 0.05 wt% Pt-Na2W4O13 loaded WO3 NTs showed remarkably high response (Rair/Rgas = 10.47) even to H2S concentrations as low as 150 ppb, proving its great potential for detection of subppm H2S molecules exhaled by halitosis patients. Furthermore, the detection limit was calculated to be 3 ppb with a noticeable response (Rair/Rgas = 1.2) (Figure S12). To prove the catalytic effect of Na2W4O13 phase and morphological effect (tubular structure) induced by incorporation of CNC, we prepared 0.05 wt% Pt loaded WO3 NFs without utilizing sodium 12 ACS Paragon Plus Environment
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doped CNC and tested their response toward H2S. Pt loaded WO3 NFs exhibited the response of 10.42 ± 1.35 to 5 ppm of H2S at 450 °C (Figure S13). The Pt loaded WO3 NFs were also tested for H2S sensing at 350 °C, which was the optimal operating temperature. Here, Pt loaded WO3 NFs exhibited a response of 196.78 ± 5.13 to 5 ppm of H2S, which is still 2.9 fold lower than that of Pt-Na2W4O13 loaded WO3 NTs. In addition to the response, the response and recovery times of Pt-Na2W4O13 loaded WO3 NTs were significantly reduced to 5–10 s and 30–60 s, respectively, upon exposure to 1–5 ppm of H2S at 450 °C (Figure 3c–d). These figures represented 2–6 fold enhanced response/recovery speed compared to the response/recovery speed of pristine WO3 NFs (response time of 10–20 s and recovery time of 120–150 s) and Na2W4O13 loaded WO3 NTs (response time of 10–25 sec and recovery time of 120–160 s). The sensing characteristics of SMOs based chemiresistive gas sensors depend on the concentrations of chemisorbed oxygen species (O2–, O–, and O2–) and gas analytes to which sensing layers are exposed. Chemisorbed oxygen species trap electrons from WO3 NTs, enlarging electron depletion regions at grain boundaries. When sensing layers are exposed to reducing gas analyte such as H2S, the target gas reacts with the chemisorbed oxygen species, releasing trapped electrons from the depletion region back to the conduction band of WO3 under the following chemical reactions (H2S + 3O– → SO2 + H2O + 3e– ).30 Thus, the depletion regions diminish and the resistance of the sensing layer decreases. The graph in Figure 4a indicates that the baseline resistance for Na2W4O13 loaded WO3 NTs is about 2.12 MΩ, which is 2.33-fold higher than that of dense WO3 NFs (0.91 MΩ). This was possibly caused by formation of numerous heterojunctions between dominant WO3 (band gap = 2.8 eV) and partially dispersed Na2W4O13 (band gap = 3.1 eV).31 In order to verify the effect of the Na2W4O13 phases on energy-state shift, ultraviolet photoelectron spectroscopy (UPS) spectrum of pristine WO3 NFs and Na2W4O13 loaded WO3 NTs were conducted using He I radiation (21.22 eV) (Figure S14a). The energy-state shift (0.2 eV) toward the conduction 13 ACS Paragon Plus Environment
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band are observed in UPS spectrum (Figure S14b–c) due to electron transfer from Na2W4O13 to WO3, like an n-type doping effect. Thus, abundant electrons can create higher amount of chemisorbed oxygen species on the surface of the sensing layers. As indicated in Figure S15a–c, a comparison of Na2W4O13 loaded WO3 NTs with pristine WO3 NFs indicated that characteristic peaks of chemisorbed oxygen species (O– and O2–) as well as the ratio between O– and O2–, i.e., O–/O2–, increased for the case of Na2W4O13 loaded WO3 NTs, hence inducing enhanced H2S response property. Interestingly, when H2S gas is introduced onto Na2W4O13 loaded WO3 NTs, the Na2W4O13 phase is largely converted to metallic Na2S phase32 according to the following suggested reaction (Na2W4O13 + H2S → Na2S + H2O + 4WO3). Since Na2S phase is highly reactive toward oxygen molecules, Na2SO4 phase can be easily formed at high temperature (> 400 °C) by the following reaction mechanism (Na2S + 2O2 → Na2SO4).33, 34 Moreover, SO2 abundant ambient can cause Na2SO4 predominant state in Na2SNa2SO4 phase system.35 Since, Na2SO4 phase is highly soluble in water, sensing test under highly humid ambient (95% RH) results in partial dissolution of the Na2SO4 phase that gives conductive sodium ions and sulfate states, leading effective dissolution of the n-n heterojunctions at the Na2W4O13/WO3 interfaces and dramatic resistance variation during injection of H2S. In addition, the n-n heterojunctions can be restored during injection of baseline air, i.e., recovery process, thereby the baseline resistance of Na2W4O13 loaded WO3 NTs returns to its original value. To confirm the stability of 0.05 wt% Pt-Na2W4O13 loaded WO3 NTs toward reversible detection of H2S, 25 cycles of response (1 ppm of H2S exposure for 10 min) and recovery (baseline air exposure for 10 min) were conducted at 450 °C (Figure S16). The result exhibited remarkable stability without degradation of the response during the cyclic sensing measurement. For Pt-Na2W4O13 loaded WO3 NTs, high baseline resistance of about 14.45 MΩ was observed, which is mainly attributed to the functionalization with catalytic Pt NPs, which act as effective chemical sensitizers. Typically, Pt catalytic NPs play a critical role of dissociating larger amount of oxygen and H2S molecules, which in turn 14 ACS Paragon Plus Environment
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chemisorb/desorb on the surface of sensing layers following the spillover effect,36 thus leading to significant sensitization of two chemical reactions, i.e., Na with H2S and chemisorbed oxygen species with H2S. In addition, the difference in work function between Pt NPs (5.65 eV)37 and WO3 (4.53 eV) can partially enlarge electron depletion regions by transferring electrons from WO3 to Pt NPs. The transfer of electrons induces higher baseline resistance of the sensing layers, thus increasing the response to H2S. As can be seen in Figure 4b, outstanding H2S detection characteristic of Pt-Na2W4O13 loaded WO3 NTs is based on (i) facilitated H2S gas penetration, taking advantage of tubular scaffold, (ii) heterojunction effect of Na2W4O13 in heterogeneous Na2W4O13-WO3 system and its chemical reaction with H2S molecules, and (iii) chemical sensitization effect of Pt NPs with excellent size control (2.22 ± 0.42 nm). To investigate the chemical states of Pt-Na2W4O13 loaded WO3 NTs, X-ray photoelectron spectroscopy (XPS) analysis was conducted. As shown in Figure S17a, two distinct peaks attributed to W 4f indicate binding energies of 35.5 eV and 37.7 eV for 4f7/2 and 4f5/2 of W6+ oxidation states, respectively.38 Oxygen related peaks in Figure S17b indicate oxygen peaks at binding energies of 530.2 eV (O2- 1s peak), 531.0 eV (O- 1s peak), and 532.2 eV (O2- 1s peak), which are related to lattice state, chemisorbed state, and physisorbed state, respectively.39 Furthermore, Pt NPs dominantly remained as metallic Pt0 with binding energy of the Pt 4f7/2 peak observed at 71.2 eV (Figure S17c). Note that the spin-orbit coupling energy distance between Pt 4f7/2 and 4f5/2 is approximately 3.3 eV. To further investigate the origin of the dramatically enhanced sensing characteristics of PtNa2W4O13 loaded WO3 NTs toward H2S molecules, ex-situ XPS analysis was also carried out. In the vicinity of Na 1s peak, a single distinct peak was shown before exposure to H2S gas, indicating binding energy of 1071.4 eV for Na+ oxidation state attributed to the Na2W4O13 phase (Figure 4c). After exposure to H2S, an additional peak was observed with binding energy of 1071.2 eV, which was induced by the formation of Na2SO4 phase (Figure 4d).40 To 15 ACS Paragon Plus Environment
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reinforce the claim for the formation of Na2SO4 phase, the ex-situ XPS analysis in the vicinity of S 2p peak was also analyzed before and after exposure to H2S gas. No distinct peak related to S 2p was observed before exposure to H2S gas (Figure 4e). However, following exposure to H2S gas, sulfur related distinct peaks were observed at binding energies of 168.8 and 161.5 eV for S 2p3/2 peaks of sodium sulfate (Na2SO4) and sodium sulfide (Na2S) phases, respectively (Figure 4f).41 Note that the spin orbit coupling energy between S 2p3/2 and 2p5/2 is approximately 1.3 eV. For comparison, the ex-situ XPS analysis related to S 2p peak was also conducted for pristine WO3 NFs before and after exposure to H2S gas. However, no sulfur peaks attributed to sodium sulfide and sodium sulfate were observed (Figure S18a–b). These ex-situ XPS analyses strongly support the suggested chemical reaction between Na2W4O13 and H2S, which is essential for highly sensitive and selective H2S detection characteristics. One of the striking features of the Pt-Na2W4O13 loaded WO3 NTs is an extraordinary selectivity. In fact, one of the most inherent limitations of SMOs based gas sensors is poor selectivity. As a proof of concept, dense WO3 NFs revealed gas sensing results with a poor selectivity, showing high response (Rair/Rgas > 10) toward three gas molecules namely H2S, CH3COCH3, and C7H8 (Figure 5a). Furthermore, non-negligible response was obtained toward interfering molecules, i.e., HCHO, C2H5OH, CO, NH3, and CH4. On the other hand, Na2W4O13 loaded WO3 NTs exhibited superior selectivity behavior, i.e., enhanced response of up to 24.1 toward H2S while the response (Rair/Rgas < 1.5) toward interfering molecules (CH3COCH3, C7H8, HCHO, C2H5OH, CO, NH3, and CH4) dramatically deteriorated (Figure 5b). The tuning of unparalleled selectivity toward H2S molecules originated from heterojunction effect of Na2W4O13 in heterogeneous phase system (Na2W4O13-WO3), and mainly attributed to chemical reactions between sodium impurity and H2S molecules as confirmed with ex-situ XPS analysis. To verify the concept in more details, we also synthesized pristine Na2W4O13 NFs and sensing properties were observed which strongly support the suggested chemical reactions between H2S and Na (see details in supporting 16 ACS Paragon Plus Environment
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information of Figure S19 and S20). In addition, Pt-Na2W4O13 loaded WO3 NTs showed consistent superiority in terms of selectivity as well as exceptionally high response toward H2S gas (Figure 5c), which proves ultrahigh performance of cocatalysts (Pt NPs and Na2W4O13) toward selective detection of H2S molecules. Besides, the comprehensive evaluation of the characterized pattern of exhaled breath, i.e., breath fingerprint, is important for high accuracy analysis of the specific target molecules among many breath biomarkers. In this sense, principal component analysis (PCA), a powerful data processing tool based on statistics, was carried out to investigate the feasibility of pattern recognition of multiple biomarker species using sensing layers of Na2W4O13 loaded WO3 NTs and Pt-Na2W4O13 loaded WO3 NTs. The PCA results showed that 8 biomarker species with various concentrations (1–4 ppm) were classified into 8 clusters without overlapping, thus proving their excellent selective gas sensing characteristics (Figure 5d). The Pt-Na2W4O13 loaded WO3 NTs synthesized in this work demonstrate a breakthrough sensing layers as superior H2S detectors which exhibited one of the highest H2S response value (Figure 5e) as well as selectivity factor (Figure 5f) compared to reported state-of-the-art sensors operating in highly humid atmosphere (75–95% RH). In addition, fast response/recovery speed (10.5 s/26.7 s at 1 ppm) demonstrate strong potential feasibility of the sensor for real-time and on-site exhaled breath analysis (Table S1).13, 14, 27, 30, 42-50
CONCLUSIONS In summary, we newly suggested one-pot synthetic technique of cocatalysts (Pt NPs and Na2W4O13) functionalized WO3 NTs by utilizing dual sacrificial bio-templates, i.e., CNC and apoferritin, combined with an electrospinning technique. The outstanding gas sensing results of Pt-Na2W4O13 loaded WO3 NTs stemmed from synergistic effects of (i) controlled synthesis of NTs scaffold via thermal decomposition of self-agglomerated CNC in the core of as-spun 17 ACS Paragon Plus Environment
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NFs, (ii) building-up of heterogeneous n(Na2W4O13)-n(WO3) junctions due to the creation of n-type crystal (Na2W4O13) originated from interaction between WO3 and sodium doped CNC during calcination, and (iii) uniformly decorated bio-inspired Pt catalysts. Upon exposure to 1 ppm of H2S, the Pt-Na2W4O13 loaded WO3 NTs exhibited superior response (Rair/Rgas = 203.5), remarkably enhanced selectivity (Rair/Rgas < 1.3) against interfering gases (CH3COCH3, C7H8, HCHO, C2H5OH, CO, NH3, and CH4), ultrafast response/recovery time (10 s /30 s), and remarkable stability (25 cycles) in highly humid atmosphere (95% RH). In addition, pattern recognition by PCA resulted in clear discrimination of the 8 different biomarker gases. Importantly, the tailored combination of dual bio-templates (CNC and apoferritin) can be applied for other tubular catalyst-loaded 1D metal oxide composite nanostructures suitable for various potential applications such as energy storage, catalyst, and chemical sensor.
ASSOCIATED CONTENT Supporting Information. SEM, TEM and EDS mapping images, XRD data, sensing data, XPS data, UPS data, schematic illustration of gas sensor test system, and the table of the stateof-the-art hydrogen sulfide sensors. These materials are available free of charge via the internet at “http://pubs.acs.org.” Notes The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] ORCID Il-Doo Kim: 18 ACS Paragon Plus Environment
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ACKNOWLEDGMENTS This work was supported by the Wearable Platform Materials Technology Center (WMC) (NRF-2016R1A5A1009926) funded by the NRF of Korea government (Ministry of Science, ICT and Future Planning). 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 the dual bio-templates assisted electrospinning fabrication process for Pt-Na2W4O13 loaded WO3 NTs, SEM images of (b) as-spun CNC/W precursor/PVP composite NFs, (c) Na2W4O13 loaded WO3 NTs after calcination, and (d) high resolution SEM image of Na2W4O13 loaded WO3 NTs, (e) as-spun Apo-Pt/CNC/W precursor/PVP composite NFs, (f) Pt-Na2W4O13 loaded WO3 NTs, and (g) magnified image of (f), (h) as-spun W precursor/PVP composite NFs, (i) dense WO3 NFs after calcination, and (j) magnified image of (i). Figure 2. (a) TEM image of Pt-Na2W4O13 loaded WO3 NTs, (b,c) high-resolution (HR) TEM images of Pt-Na2W4O13 loaded WO3 NTs, (d) selected area electron diffraction (SAED) pattern of Pt-Na2W4O13 loaded WO3 NTs, (e) STEM image of Pt-Na2W4O13 loaded WO3 NTs, and (f) energy-dispersive spectroscopy (EDS) elemental mapping images of Pt-Na2W4O13 loaded WO3 NTs. Figure 3. Hydrogen sulfide sensing performance of dense WO3 NFs, Na2W4O13 loaded WO3 NTs, and Pt-Na2W4O13 loaded WO3 NTs. (a) temperature-dependent hydrogen sulfide 22 ACS Paragon Plus Environment
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response property in a temperature range of 350-500 °C, (b) dynamic hydrogen sulfide response characteristics in the concentration range of 5-0.15 ppm, (c) response time and (d) recovery time toward hydrogen sulfide at 450 °C. Figure 4. (a) Dynamic resistance transition of dense WO3 NFs, Na2W4O13 loaded WO3 NTs, and Pt-Na2W4O13 loaded WO3 NTs, (b) schematic illustration of hydrogen sulfide sensing mechanism for Pt-Na2W4O13 loaded WO3 NTs. Ex-situ XPS analysis using high resolution spectra of Pt-Na2W4O13 loaded WO3 NTs in the vicinity of Na 1s (c) before exposure to hydrogen sulfide and (d) after exposure to hydrogen sulfide as well as in the vicinity of S 1s (e) before exposure to hydrogen sulfide and (f) after exposure to hydrogen sulfide. Figure 5. Selective detection characteristics toward 5 ppm of biomarker gas molecules with sensing layers of (a) dense WO3 NFs, (b) Na2W4O13 loaded WO3 NTs, and (c) Pt-Na2W4O13 loaded WO3 NTs. (d) Pattern recognition based on principal component analysis (PCA) using sensor arrays of Na2W4O13 loaded WO3 NTs and Pt-Na2W4O13 loaded WO3 NTs for various concentrations (1–4 ppm) of 8 biomarker gas molecules, (e) gas response (Rair/Rgas) and (f) gas selectivity factor(Star./Sinter.) of various SMOs based sensors toward hydrogen sulfide under highly humid atmosphere (75–95% RH), as reported in recent literatures.
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