Self-Assembly Template Driven 3D Inverse Opal Microspheres

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Self-Assembly Template Driven 3D Inverse Opal Microspheres Functionalized with Catalysts Nanoparticles Enabling Highly Efficient Chemical Sensing Platform Tianshuang Wang, Inci Can, Sufang Zhang, Junming He, Peng Sun, Fangmeng Liu, and Geyu Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19641 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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ACS Applied Materials & Interfaces

Self-Assembly Template Driven 3D Inverse Opal Microspheres Functionalized with Catalysts Nanoparticles Enabling Highly Efficient Chemical Sensing Platform Tianshuang Wang1, Inci Can2, Sufang Zhang1, Junming He1, Peng Sun*,1, Fangmeng Liu1, and Geyu Lu*,1 1. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China. 2. Institute of Physical Chemistry, University of Tuebingen, Auf der Morgenstelle 15, 72076 Tuebingen, Germany. E-mail: [email protected]

KEYWORDS: three-dimensional inverse opal microspheres; one-step ultrasonic spray pyrolysis; self-assembly sulfonated PS spheres; PdO@In2O3 composites; high performance acetone gas sensor

ABSTRACT: The design of semiconductor metal oxides (SMOs) with well-ordered porous structure has attracted tremendous attention owing to their larger specific surface area. Herein, three-dimensional inverse opal In2O3 microspheres (3D-IO In2O3 MSs) were fabricated through one-step ultrasonic spray pyrolysis (USP) employed self-assembly sulfonated

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polystyrene (S-PS) spheres as a sacrificial template. The spherical pores observed in the 3D-IO In2O3 MSs were with diameters of about 4 nm and 80 nm. Subsequently, the catalytic palladium oxide nanoparticles (PdO NPs) were loaded on 3D-IO In2O3 MSs via a simple impregnation method and their gas sensing properties were investigated. By comparing with pristine 3D-IO In2O3 MSs, the 3D-IO PdO@In2O3 MSs exhibited a 3.9 times higher response (Rair/Rgas = 50.9) to 100 ppm acetone at 250 °C and a good acetone selectivity. The detection limit for acetone could extend down to ppb level. Furthermore, the 3D-IO PdO@In2O3 MSs-based sensor also possess well long-term stability. The extraordinary sensing performance can be attribute to the novel 3D periodic porous structure, highly three-dimensional interconnected, larger surface-to-volume ratio, size-tunable (meso- and macro-scale) bimodal pores and PdO NPs catalysts. 1. INTRODUCTION In the past few years, the environmental pollution and health care have become hot topics, which lead to the increasing demand of developing high performance gas sensors for real-time monitoring of the toxic and harmful gases and biomarker gases. Among different kinds of gas sensors, SMOs-based chemiresistive gas sensors have attracted a great attention, due to their key advantages in terms of miniaturization, low cost, simple operation, and real-time measurement.1-2 Therefore, various strategies have been used to improve gas sensing properties of SMOs-based gas sensors, including morphological changes,3-4 catalyst doping/loading,5-6 and construction of heterostrucutre.7-9 To date, diverse efforts have been reported to be deployed on designing gas sensing materials with precisely controlled micro- and nano- structure to improve their gas sensing


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properties.3-4,

9-13

Among them, porous sensing materials have received great attention

because of their special properties such as higher surface area, well-ordered porous architecture, high gas accessibility and rapid gas diffusion.6, 10, 13 Therefore, several strategies have recently been demonstrated to fabricate different porous sensing materials.4, 10, 13-16 On the one hand, solvothermal/hydrothermal self-assembly reactions were widely used to prepare porous sensing material.4, 14-15, 17 However, these methods are quite difficult to tune the size and distribution of pores, because the level of control is relatively low during the reactions. On the other hand, some research groups have focused on utilizing gravitational settling method combining PS spheres template to fabricate sensing materials with inverse opal structure (a kind of porous structure) on specific substrates.13,

18-20

However, such

method is tedious and time-consuming, as well as the self-assembly large-scale ordered structures are easily disrupted as they are transferred to the target substrates. Furthermore, some groups have also focused on preparing porous sensing material by combining USP method with colloidal spheres templates.10-12,

21

For instance, Shimizu and co-workers

synthesized porous SnO2 spheres for H2 gas sensor through USP method, and PMMA microspheres is used as sacrificial template.12 However, compared to a three-dimensional inverse opal structure, the transfer of gas molecules to the inner part in such ordinary porous spheres will be hampered, because of the poorer interconnectivity between the spherical pores.14 Therefore, we have developed a facile approach to fabricate three-dimensional inverse opal In2O3 microspheres by using USP method employed self-assembly S-PS spheres as a template. During fabrication procedure, PS spheres will undergo a sulfonation process, which will cause the S-PS spheres to self-assemble within the microspheres to form an opal



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structure.22 Finally, after high temperature annealing, it is successfully construct three-dimensional inverse opal structure on every microspheres. This fabrication strategy is simple, efficient and does not require any substrates. The main breakthrough of the present study is combining USP method with sulfonated polymer spheres template to construct three-dimensional inverse opal structure on sensing materials, in order to achieve the significantly enhanced interconnectivity between spherical pores. Therefore, the sensing material with 3D-IO MSs structure could achieve enhanced gas sensing properties.13 In addition, the enlargement of surface area accompanies with the sensitization of nanocatalyst can further promote the reaction between target gas and sensing materials, which will result in the effective improvement of gas sensing properties.1,

5-6, 23-24

Xing and

co-workers first reported the Au nanoparticles loaded on the 3D In2O3 inverse opals films via traditional vertical deposition method employing PMMA spheres as a template.23 They first grew sensing material with 3D inverse opal structure on a glass substrate, and then scraped down 3DIO films from the glass substrate to fabricate corresponding 3DIO gas sensors. Such method is very complex and less repeatable. In addition, the three-dimensional inverse opal structure will be destroyed during the scraping process.25 Wang et al. have synthesized Au-loaded mesoporous WO3, demonstrating much higher sensitivity and selectivity to n-butanol.24 However, the mesoporous materials-based gas sensors generally have relatively large resistance and slow recovery speed.26 Jang et al. have successfully fabricated Pt or Au@SnO2 nanotubes trough the protein templating route, and they found that the materials exhibited a dramatic enhancement in response to acetone.27 However, it is difficult to use for large-scale industrial production due to the complicated and expensive synthetic process.


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Therefore, it is still a challenge to design and synthesize the nanocatalytic@3D-IO SMOs based sensing materials by a simple and facile strategy. Herein, we first reported that the synthesize of honeycomb-like 3D-IO microspheres by using the combining of one-step USP method with self-assembly S-PS spheres template. Since Indium oxide (In2O3, Eg = 2.5-2.8 eV) has been widely investigated in gas sensor application, we use it as sensing material in this work.11, 23 It is possible to construct three-dimensional inverse opal structure on each microsphere by utilizing self-assembly S-PS spheres as a template, such novel structure provides both inside and outside reaction sites with high gas accessibility.7 On the basis of 3D-IO In2O3 MSs, the loading catalytic PdO NPs can further enhance the gas sensing properties by donating or depriving electrons to sensing layers.28 From this perspective, these two advantages, (i) three-dimensional inverse opal microspheres with plenty reaction sites and (ii) nanocatalyst functionalization, are key points for developing outstanding gas sensor. Thus, the sensor based on 3D-IO PdO@In2O3 MSs exhibited ultra-high gas sensing performance toward acetone gas, it meant that this sensor could be used for early diagnosis of diabetes.23 Besides, this simple synthetic route also presents a feasible solution for the design of 3D inverse opal MSs materials used in other material science. 2. EXPERIMENTAL SECTION 2.1 Chemical Reagents Poly (styrene sulfonic acid) sodium salt (M. W. 70000, Alfa Aesar, Shanghai, China). Styrene (C8H8, 99%), Sodium hydrogen carbonate (NaHCO3, 99.5%), Potassium peroxydisulfate (K2S2O8, 99.5%) and Indium nitrate hydrate (In(NO3)3·4.5H2O, 99.5%) were provided by Sinopharm Chemical Reagent Co. Ltd of China. Palladium nitrate dihydtrate



(Pd(NO3)2·2H2O, Aladdin). Sulfuric acid (H2SO4, 95%), hydrochloric acid (HCl, 38%) and hydrogen peroxide solution (H2O2, 30%). All chemicals were used without further purification. 2.2 Fabrication of Sulfonated PS Spheres (S-PS Spheres) First of all, PS spheres used in this article were synthesized by an emulsion-free polymerization method. Then, the prepared PS spheres powders were immersed in concentrated sulfuric acid (95%) and stirred at 40 °C for 6 h to form S-PS spheres, which were separated by centrifugation (20000 rpm, 40 min) from the system. Finally, the resulting product was sequentially washed with distilled water and ethanol five times, and then placed in 60 °C oven for 1 day. In order to confirm that the PS spheres translated into S-PS spheres after sulfonation, the powders of PS spheres before and after sulfonation have been measured thoroughly using Fourier transforms infrared (FT-IR) spectroscopy (Figure S1). The SEM image of S-PS spheres was shown in Figure S2, and the mean diameter of S-PS spheres was about 100 nm. 2.3 Preparation of Gas Sensing Materials 3D-IO In2O3 MSs were obtained via the USP method. First of all, the ultrasonic spray solution for the 3D-IO In2O3 MSs were prepared by dissolving In(NO)3·4.5H2O (0.24 g) in 15 ml of deionized water solution containing the well-dispersed S-PS spheres powders (0.175 g), hydrogen peroxide solution (0.3 ml, 30%) and 0.2 M hydrochloric acid (38%, HCl) (0.1 ml). And, the mixtures were stirred at room temperature for 4 h. Then, the aqueous droplets of spray solutions were produced by an ultrasonic atomizer (resonant frequency=1.7 MHz), and the mists were transported to a tubular furnace (700 °C) by N2 flowing (flow rate=500 sccm).


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As illustrated in Figure 1(a), the aqueous droplets would go through two processes: (i) solvent evaporated, metal particle formation and S-PS spheres consolidation, and (ii) decomposition of metal salt and most S-PS spheres (700 °C). The precursor powders were gathered in water-filled conical flask and dried in air at 80 °C for 10 h. Finally, the dried precursor powders were annealed in air at 600 °C for 3 h with temperature ramp of 10 °C min−1 to completely remove the template.



Figure 1. Schematic of the process to fabricate: (a) 3D-IO In2O3 MSs with well-ordered porous structure; and (b) PdO-loaded 3D-IO In2O3 MSs. The preparation process of the 3D-IO PdO@In2O3 MSs is shown in Figure 1(b). PdO NPs were loaded on 3D-IO In2O3 MSs through the impregnation method. In a typical process, 10 mg of 3D-IO In2O3 MSs powders were dissolved into 2 ml of ethanol by ultrasonic dispersion. Then, definite amounts of Pd(NO3)2·2H2O were added to as-prepared solution with the Pd/In atom ratios of 0 %, 5.2 %, 10.4 % and 20.8 %. After that, the solution was continuously stirred at room temperature until it was dried. Finally, the precursor powders with different amounts of Pd(NO3)2 were annealed in air at 350 °C for 1 h with temperature ramp of 1 °C min−1 to obtain 3D-IO PdO@In2O3 MSs. The final Pd/In atom ratios in S2-S4 samples were determined to be 4.04 %, 8.09 % and 18.07 % via X-ray photoelectron spectroscopy (XPS) analysis, which were marked as S1–S4 respectively for convenience. 2.4 Materials Characterization Fourier transforms infrared (FT-IR) spectra were recorded on Thermal Scientific Nicolet IS 10 FT-IR spectrometer with an average of 16 scans. Crystal phase was analyzed by the X-ray diffraction analysis (XRD; Rigaku D/Max 2550) using Cu Kα radiation (λ = 1.5418 Å). XPS measurement was carried out by ESCALAB 250 X-ray photoelectron spectrometer using X-ray source (Al Kα hυ = 1486.6 eV). The detail microstructure was observed by field-emission scanning electron microscopy (FESEM; JEOL JSM-7500F), transmission electron microscopy (TEM; JEOL JSM-2100F) and high-resolution transmission electron microscopy (HRTEM; JEOL JSM-2100F). The surface area and pore-size distribution were measured by Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods


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using N2 adsorption-desorption isotherms (Micrometrics Gemini VII). 2.5 The Fabrication of Gas Sensor and Gas Sensing Measurement Gas sensor device is illustrated in supplementary Figure S3 and gas sensor was fabricated as below: Firstly, the homogeneous slurry was obtained by mixing a certain amount of the powders (1.5 mg) with an appropriate amount of isopropanol (25 µl) in an agate mortar. Secondly, the paste was coated on the alumina tube using a small brush to form a thick sensing film. Thirdly, a Ni−Cr alloy coil as a heater was inserted into the alumina tube, and the operation temperature can be controlled by tuning current. Finally, the gas sensor was aged at 200 °C for 5 days. A static testing system (30 ± 5 RH%) was used to investigate the gas sensing properties.8 The response was defined as Rair/Rgas (Reducing gas, such as C2H5OH, C6H6, etc.). Sensing speed (response time and recovery time) was measured according to the change in resistance value. 3. RESULTS AND DISCUSSION 3.1 Morphology and Structure Property of 3D-IO In2O3 MSs Samples The as-prepared pristine 3D-IO In2O3 MSs possessed sufficiently interconnected and well-ordered 3D inverse opal (IO) skeletons, as well as a relatively smooth surface (Figure 2(a) and (b)). In addition, the spherical pores were originated from the morphology of the S-PS spheres, but the diameter of S-PS spheres was slightly larger than the pore size (60~90 nm), probably due to the shrinkage of spheres diameters during calcination.11 When the S3 sample was characterized through TEM (Figure 2(c)), the 3D IO structure among the whole microsphere could be easily observed. It was also noted worthy that, after the decomposition of S-PS spheres and the precursors, the shape of sphere still remained and the oxide walls



were stably formed among these spherical pores, indicating that the S-PS spheres were effectively utilized as a template to introduce the 3D inverse opal structure into In2O3 microspheres. A HRTEM image (Figure 2(d)) revealed the In2O3 (422) and In2O3 (332) crystal plane corresponding to the interplanar spacing of 2.07 Å and 2.13 Å, respectively. The corresponding selected-area electron diffraction (SAED) pattern was shown in Figure 2(e), a set of single-crystal electron diffraction spots corresponding to the cubic phase In2O3. In addition, the SEM image of the precursor powders for the 3D-IO In2O3 MSs (before annealed at 600 °C for 3 h under an air atmosphere) was shown in Figure S4. Only a small number of spherical pores were existed on the surface of most precursor microspheres, and there was no three-dimensional inverse opal structure. It may be due to the time of spray pyrolysis is insufficient to complete the decomposition of S-PS spheres.10

Figure 2. (a) and (b) High magnification SEM images of 3D-IO In2O3 MSs; (c) TEM image of 3D-IO In2O3 MSs; (d) HRTEM image of 3D-IO In2O3 MSs; (e) SAED image of 3D-IO In2O3 MSs.


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3.2 Morphology and Structure Properties of 3D-IO PdO@In2O3 MSs Samples Figure 3(a) and (b) exhibited the morphological and structural observations of the as-prepared 3D-IO PdO@In2O3 MSs (S3 sample). As shown in Figure 3(a), we could find that the 3D inverse opal microspheres structure maintained after the process of loading PdO NPs. However, a small portion of Pd NPs would agglomerate during the high-temperature annealing process, it mainly due to the strong attractive interactions between nanoparticles.29 In addition, compared with the surface of 3D-IO In2O3 MSs (Figure 2(b)), it was found that the 3D inverse opal skeleton surface of 3D-IO PdO@In2O3 MSs were substantially coarsened (Figure 3(b)), which revealed a large number of PdO NPs were distributed within the interior and exterior of the 3D-IO structure. As shown in Figure 3(c), it was also found that the S3 sample consisted of many uniform 3D IO microspheres structures with nearly same diameter. Besides, Figure 3(d) exhibited the size distribution of S3 sample, median particle diameter (d50) of S3 sample was 636.6 nm with a narrow peak, indicating that the 3D-IO PdO@In2O3 MSs particles had better dispersibility without noticeable aggregation.



Figure 3. (a) and (b) High magnification SEM images of the S3 sample; (c) Low magnification SEM image and (d) corresponding size distribution of the S3 sample. The XRD patterns of the obtained S1-S4 samples were exhibited in Figure S5. The XRD peaks of S1 sample (3D-IO In2O3 MSs) could be well matched with body-centered cubic phase In2O3 (JCPDS card 06-0416). The as-prepared 3D-IO PdO@In2O3 MSs (S2-S4 samples) were mixtures of cubic In2O3 and palladinite PdO crystallites, and the peak intensity of palladinite PdO (JCPDS card 41-1107) gradually appeared with increasing the amount of PdO NPs loading. Besides, the diffraction peaks became narrower with increasing the amounts of Pd, implying the mean size of the catalytic PdO particles increase.30 We further investigated the average sizes of PdO NPs in S2-S4 samples according to the XRD peaks. The results revealed that the average sizes of PdO NPs in S2, S3 and S4 samples calculated from the Debye–Scherrer formula (D = 0.89 λ/β cos(θ), D: average grain size; λ: X-ray wavelength 0.154056 nm; θ: Bragg diffraction angle; β: the full width at half-maximum) based on the (002) peaks were found to be 7.87 nm, 8.05 nm and 8.21 nm, respectively.


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Detailed information about the morphology of the as-prepared 3D-IO PdO@In2O3 MSs were further obtained by TEM analysis (Figure 4(a)). All spherical pores were well developed inside the microspheres and continuously connected each other, further indicating the S3 sample still possessed highly ordered porous system. In addition, HRTEM image exhibited the PdO catalyst functionalized on In2O3 with the crystal plane (101) of palladinite PdO, which corresponded to interplanar spacing of 2.64 Å (Figure 4(b)). The spherical dark spots with the size of 8-10 nm in the In2O3 were PdO NPs (red circle in the Figure 4(b)). The lattice fringes of the (222) plane of the cubic phase In2O3 was also observed in the 3D-IO PdO@In2O3. The existence of In2O3 polycrystalline of the body-centered cubic structure and PdO NPs was clearly observed in the SAED patterns in Figure 4(c). In addition, the SAED patterns of PdO (101) was relatively week due to the low concentration of PdO in 3D-IO In2O3 MSs. Figure 4(d) exhibited the energy-dispersive X-ray spectroscopy (EDS) elemental spectrum to confirm the chemical composition of S3 sample. Because most of the catalytic PdO NPs in the 3D-IO PdO@In2O3 MSs were not clearly observed in SEM images, we carried out EDS analysis to precisely confirm the distribution of catalytic PdO NPs. As shown in EDS elemental mapping images (Figure 4(f-h)), indicating that the catalytic PdO NPs were well-distributed functionalization without noticeable aggregation on the whole 3D-IO MSs structure.



Figure 4. (a) TEM image of the S3 sample; (b) HRTEM images of the S3 sample; (c) SAED patterns of the S3 sample; (d) EDS elemental spectrum of the S3 sample; TEM image of an individual (e) S3 sample and its EDS elemental mapping images of (f-h) In, O and Pd, respectively. 3.3 Pore Size Distribution and Surface Area Analysis Nitrogen physisorption experiments were performed on the 3D-IO In2O3 MSs (S1 sample) and 3D-IO PdO@In2O3 MSs (S3 sample). The S1 and S3 samples all exhibited a shape that is between type II and type IV isotherm and H1 hysteresis loops, which is the characteristic of mesoporous structure (Figure 5(a) and (d)). The BET surface areas of the S1 and S3 samples were determined to be about 30.3 and 26.7 m2g-1, respectively (as shown in Figure 5(b) and (e)). The pore size distributions of the S1 and S3 samples were shown in


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Figure 5(c) and (f), indicating that all samples had two different sizes (approximately 4 and 80 nm) of pores. Among them, pore with a mean diameter of about 4 nm was attributed to the outgassing produced by the decomposition of the S-PS spheres templates.10 Thus, we could conclude that the loading process had barely influence on the microstructure of 3D-IO In2O3 MSs.24 This special microstructure would be helpful for improve utilization factor due to larger surface area, porous structure, as well as sufficient gas diffusion and mass transport.23

Figure 5. Nitrogen adsorption-desorption isotherms of the (a) 3D-IO In2O3 MSs (S1 sample) and (d) 3D-IO PdO@In2O3 MSs (S3 sample); The BET surface areas of the (b) S1 and (e) S3 samples; The pore-size distributions of the (c) S1 and (f) S3 samples. 3.4 Surface States and Interface Interaction The chemical composition and bonding state of S3 sample was measured by XPS analysis (Figure 6). In the full range spectra Figure 6(a), C, O, In and Pd peaks were clearly observed and no impurities could be found.



Figure 6. (a) The complete XPS spectra of S3 sample; XPS spectra in the vicinity of the (b) Pd 3d, (c) In 3d and (d) O 1s peaks. The detailed XPS data of In 3d and Pd 3d core level region of S3 sample were exhibited in Figure 6(b) and (c), respectively. The spectrum of Pd 3d revealed two characteristic peaks of 3d3/2 at 342.2 eV and 3d5/2 at 336.9 eV, which were identified as the Pd2+ state.7 The spectrum of In 3d revealed two characteristic peaks of 3d3/2 at 452 eV and 3d5/2 at 444.4 eV.31 The O 1s XPS spectrum of S3 sample was shown in Figure 6(d), and it could be fitted to three kinds of oxygen species (lattice oxygen OL, oxygen vacancy regions OV and chemisorbed or dissociated oxygen or -OH species OC) by Gaussian simulation peaks.32 3.5 Gas Sensing Characteristics In general, the specific interaction between gases and the sensing material is highly influenced by the operating temperature. The variation of sensor temperatures not only changes the gas response/recovery kinetics but also the selectivity in the gas sensing reaction.6,

31

Figure 7(a) exhibited the response vs temperature characteristics of S1-S4


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sensors on exposure to 100 ppm of acetone. The Fuji-mountain-shaped correlation between gas response and operating temperature was observed from every sensor,6,

8

and the gas

sensor reached its maximum response at an intermediate temperature. The acetone response for S2-S4 sensors were 25.8, 50.9 and 30.5 at the optimal operating temperature of 250 °C, compared to S1 sensor (13.2 at 275 °C), respectively. Among them, the S3 sensor exhibited the highest response toward acetone, which was almost four times higher than that of the S1. While the final Pd/In atom ratios was less than 8.09 %, the response of 3D-IO PdO@In2O3 MSs toward acetone gradually improved with increasing PdO loaded, because of the sensitization effect of the PdO NPs.28, 33 On the other hand, as the final Pd/In atom ratios exceeded 18.07 %, the ready agglomeration of the tiny PdO NPs would hamper the effective contacts between PdO and In2O3, thus leading to a seriously decrease in acetone sensing properties.34 A shift of operating temperature can be attributed to the process of availability of the free electrons due to the reduction of Pd2+ to Pd at lower operating temperature.33 Selectivity is another important feature in distinguishing gas sensing qualities for practical applications. Therefore, the gas responses of S1-S4 sensors toward 100 ppm of various target gases were investigated at their respective optimum operating temperatures (250 or 275 °C). As shown in Figure 7(b), all of the 3D-IO PdO@In2O3 MSs samples (S2-S4 sensors) exhibited high selectivity toward acetone, and the acetone selectivity of pure 3D-IO In2O3 MSs sample (S1 sensor) was poorer. The result indicated that the catalytic PdO NPs could obviously improve the selectivity of the pure In2O3 to acetone. At lower operating temperature, the S3 sensor could exhibite the highest response to acetone and quiet lowest responses to other interference gases. In addition, the gas responses of S1 and S3 sensors



toward different 100 ppm test gases (acetone and ethanol) at 175-300 °C were depicted in Figure 7(c) and (d). The radar plots demonstrated that the optimal operating temperature for ethanol and acetone were different, possibly due to the different values of the lowest unoccupied molecule orbit energy for gas molecules.35-36 The enhanced selectivity to acetone could be attributed to the fact that PdO NPs promotes the reforming of acetone molecule into smaller and more active species.28, 37 Thus, the reaction between the activated fragments of acetone gas and chemisorbed oxygen molecules was facilitated. In addition, the operating temperature also had a considerable influence on the selectivity.36 In addition, the dynamic gas sensing transients of S1-S4 sensors to different concentration range of acetone at their respective optimum operating temperatures are displayed in Figure 7(e) and (f). All gas sensors exhibited clear response curves with low background noise in the studied range. Dynamic sensor response measurements showed that the highest acetone response of 50.9 was obtained with S3 sensor at 100 ppm, which was 3.9-fold better than that observed with 3D-IO In2O3 MSs (S1 sensor). Furthermore, the S3 sensor possessed the lowest detection limit (500 ppb) toward acetone. Simultaneously, Figure 7(g) exhibited the gas responses of S1-S4 sensors to acetone as a function of concentration. It could be found that all 3D-IO PdO@In2O3 MSs samples (S2-S4 sensors) exhibited a slower growth trend under low acetone concentrations (0.5-10 ppm). However, when the acetone concentration was above 20 ppm, the S2-S4 sensors showed a rapid increase in response with a linear trend under high concentrations (20-100 ppm). This phenomenon can be ascribed to the catalytic effect of PdO NPs and the dissociation of acetone molecules during gas sensing reaction.38-39 Meanwhile, based on the criterion for gas


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detection was set to Rair/Rgas ˃ 1.2,37 the acetone detection limit of S3 sensor was calculated to be 83.3 ppb (Figure S6).



Figure 7. (a) Responses of S1-S4 sensors to 100 ppm of acetone at 175-300 °C; (b) Selectivity of S1-S4 sensors to 100 ppm of various gases at their corresponding optimal operating temperature; Radar graphs and gas responses (Rair/Rgas) of (c) S1 and (d) S3 sensors to acetone and ethanol gases at 175-300 °C, respectively; Dynamic response-recovery curves of S1-S4 sensors as-fabricated sensors to acetone in the range of (e) 0.5-5 ppm and (f) 10-100 ppm under their optimal operating temperatures; (g) Dependence of the gas sensors response under different concentrations of acetone. The transients of S1-S4 sensors on exposure to 100 ppm acetone at their optimal operating temperature were shown in Figure 8(a). All sensors exhibited typical n-type sensing behaviors in which the sensor resistance decreased when exposed to a reducing gas (acetone) and returned to the original resistance in an air atmosphere. The baseline resistances of 3D-IO PdO@In2O3 MSs (S3 sensor) was significantly higher (932.5 KΩ) than that of 3D-IO In2O3 MSs (S1 sensor) (68.4 KΩ). As observed by HRTEM and XPS, the oxide phases such as PdO generated during the calcination could contribute to increase depletion widths and result in the increase in resistance, because the PdO acts as a strong acceptor of electrons from the In2O3.34 Therefore, the change in resistance is larger as compared to the pristine oxide case, leading to an increase in response. The corresponding response time and recovery time of S1-S4 sensors were calculated based on Figure 8(a). And the related acetone gas sensing performances were summarized in Table 1. The S3 sensor possessed a high response and fast recovery process compared with the other sensors. The recovery time of 3D-IO PdO@In2O3 MSs (26 s) was much shorter than the 3D-IO In2O3 MSs (49 s), which was attributed to the introduction of PdO NPs catalytic. Besides, the recovery properties of 3D-IO


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PdO@In2O3 MSs are also improved due to the addition of PdO NPs. The recovery time of S3 sensor (26 s) was faster than that of S1 sensor (142 s) to 100 ppm acetone at 250 °C. Table 1. Comparison of acetone sensing performances of S1-S4 sensors. Materials

Ra [Kohm]

Res. [Ra/Rg]

Treco. [s]

Temp. [°C]

S1

68.3

13.2

51

275

S2

343.1

25.8

75

250

S3

932.5

50.9

26

250

S4

2258.3

30.5

29

250

Ra: resistance in air; Res.: response; Treco.: recovery time; Temp.: temperature; The 3D-IO PdO@In2O3 MSs (S3 sensor) possessed excellent repeatability (Figure S7(a)). And the results in Figure S7(b) and (c) showed that that the S3 sensor tended to remain relatively stable during a long-term stability measurement of 30 days. Furthermore, a comparison of the acetone sensing ability between the S3 sensor made in this work and SMOs-based sensors reported in previous literatures was summarized in Table 2. From the table, it was obvious that the 3D-IO PdO@In2O3 MSs (S3 sensor) showed relatively faster response speed and lower optimal operating temperature.23, 40-47



Figure 8. (a) Dynamic resistance change transients of S1-S4 sensors toward 100 ppm acetone at their corresponding optimal operating temperature, respectively; (b) Schematic illustration showing the acetone sensing mechanism.


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Table 2. Comparison of acetone sensing ability of different gas sensors. Materials

Tres. [s]

Ref.

12.5

17

40

200

13

9

41

100

340

64.9

7

42

100

250

16.8

3

43

ZnO-In2O3 nanotubes

60

280

43.2

5

44

rGO/α-Fe2O3 particles

100

225

13.9

2

45

Au-In2O3 nanospheres

10

320

53.1

4

46

3DIO Au/In2O3 IO films

5

340

42.4

11

23

Pt-loaded α-Fe2O3 porous

100

220

27.2

1

47

100

250

50.9

1

This

ZnSnO3 hollow

Conc.

Temp.

Res.

[ppm]

[°C]

[Ra/Rg]

50

240

30

polyhedrons ZnFe2O4 Porous nanospheres Ni-SnO2 hollow nanofiber α-Fe2O3/SnO2 hierarchical flowers

nanospheres 3D-IO PdO@In2O3 microspheres

work

Conc.: concentration; Temp.: temperature; Res.: response (Rair/Rgas); Tres.: response time; Ref.: reference. 3.6 Acetone sensing mechanism As shown in the schematic illustration in Figure 8(b), the improvement in acetone gas



Page 24 of 38

sensing properties of 3D-IO PdO@In2O3 MSs can be explained as follows: First, the microstructure is an important factor affecting gas sensing performance. From the N2 adsorption-desorption isotherms results, it can be clearly observed that the 3D inverse opal microspheres structure possessed a large surface area, that provides more active sites to adsorb more chemisorbed oxygen species. Moreover, the highly interconnected, periodic and bimodal pores could provide excellent channels and surface accessibility for the transfer of gas molecule.10, 13, 26 Hence, the gas sensors based on the 3D-IO MSs could display enhanced acetone response, as well as fast response and recovery speed. Generally, the gas sensing properties of SMOs depends on the surface reaction between chemisorbed oxygen species (O2–, O–, and O2–) and analytes. It has been proved that below 150 °C the molecular species (O2-) dominates and above this temperature the ionic species (O– and O2–) dominate.48 When the n-type sensing materials are exposed to fresh air, chemisorbed oxygen species form on their surface by capturing free electrons from the conduction band.2,

31

Thus, electron

depletion layers are formed leading to a higher resistance (Rair). Thereafter, when the n-type sensing materials are exposed to reducing gases such as acetone, the acetone can react with the chemisorbed oxygen species to generate H2O and CO2, and the electrons are released back into the conduction band of n-type sensing materials, leading to a dramatic decrease in resistance.6 The reaction is

CH3COCH3+8O-(ads) → 3CO2+3H2O+8e-

(1)

Second, this superior acetone sensing performance also could be attributed to the electronic sensitization effect of PdO NPs.34 A p-n heterojunction is formed when the PdO


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NPs (p-type) are incorporated into 3D-IO In2O3 MSs, the PdO acts as a strong acceptor of electrons from the In2O3, therefore an electron (surface) depletion layer is formed near the interface of 3D-IO In2O3 MSs.2, 7 Moreover, the addition of PdO NPs could increase the concentration of chemisorbed oxygen species on the surface of [email protected] As a result, the baseline resistances of 3D-IO PdO@In2O3 MSs (S2-S4 samples) are much larger than that of 3D-IO In2O3 MSs (S1 sample) (Figure S8), leading to the large resistance changes during acetone sensing measurement and the indirect improvement of acetone sensitivity.23,

33

Furthermore, we also noted that a part of PdO NPs are reduced to the Pd0 state during the acetone sensing measurement (Figure S9). The reduced Pd NPs acts as a catalyst for lowing the activation energy of the reaction between acetone and chemisorbed oxygen species.34, 49 Therefore, more oxygen will be chemisorbed and eventually react with more numbers of acetone molecules to increase the sensor response. 4. CONCLUSION In this work, 3D-IO PdO@In2O3 MSs with well-ordered porous structure were first synthesized using facile spray pyrolysis method and self-assembly S-PS spheres. The 3D-IO PdO@In2O3 MSs possessed a regular honeycomb-like morphology, larger surface area and highly periodic ordered macroporous structure. Such novel structure could provide a high gas permeability, increases the adsorption sites of gas molecules, and accelerate the diffusion of gas molecules. The sensing ability of 3D-IO In2O3 MSs was investigated before and after functionalization with PdO catalyst nanoparticles. The result demonstrated that the 3D-IO PdO@In2O3 MSs exhibited excellent selectivity to acetone, giving a high response of 49.7 to 100 ppm after a long-term stability test at 250 °C. The improvement in sensing performances



could be ascribed to the combination of the novel 3D-IO microstructure with the catalysts nanoparticles.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional FT-IR spectra of PS spheres and S-PS spheres (Figure S1), SEM image of S-PS spheres (Figure S2), schematic diagram of gas sensor device (Figure S3), SEM image of the precursor sample (Figure S4), XRD patterns of S1-S4 samples (Figure S5), detection limit of S3 sensor (Figure S6), repeatability and long-term stability of S3 sensor (Figure S7), dynamic resistance transitions of S1-S4 sensors (Figure S8), ex situ XPS analysis spectra of 3D-IO PdO@In2O3 MSs (Figure S9) (PDF).

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. E-mail: [email protected] ORCID Peng Sun: 0000-0002-9509-9431 Notes The authors declare no competing financial interest.


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ACKNOWLEGMENTS This research was supported by the National Key Research and Development Program of China (No. 2016YFC0207300), National Nature Science Foundation of China (No. 61722305, No. 61503148, No. 61520106003 and No. 61327804), National High-Tech Research and Development Program of China (863 Program, No. 2014AA06A505), Science and Technology Development Program of Jilin Province (No. 20170520162JH), China Postdoctoral Science Foundation funded project (No. 2017T100208 and No. 2015M580247).



FIGURE CAPTIONS Figure 1. Schematic of the process to fabricate: (a) 3D-IO In2O3 MSs with well-ordered porous structure; and (b) PdO-loaded 3D-IO In2O3 MSs. Figure 2. (a) and (b) High magnification SEM images of 3D-IO In2O3 MSs; (c) TEM image of 3D-IO In2O3 MSs; (d) HRTEM image of 3D-IO In2O3 MSs; (e) SAED image of 3D-IO In2O3 MSs. Figure 3. (a) and (b) High magnification SEM images of the S3 sample; (c) Low magnification SEM image and (d) corresponding size distribution of the S3 sample. Figure 4. (a) TEM image of the S3 sample; (b) HRTEM images of the S3 sample; (c) SAED patterns of the S3 sample; (d) EDS elemental spectrum of the S3 sample; TEM image of an individual (e) S3 sample and its EDS elemental mapping images of (f-h) In, O and Pd, respectively. Figure 5. Nitrogen adsorption-desorption isotherms of the (a) 3D-IO In2O3 MSs (S1 sample) and (d) 3D-IO PdO@In2O3 MSs (S3 sample); The BET surface areas of the (b) S1 and (e) S3 samples; The pore-size distributions of the (c) S1 and (f) S3 samples. Figure 6. (a) The complete XPS spectra of S3 sample; XPS spectra in the vicinity of the (b) Pd 3d, (c) In 3d and (d) O 1s peaks. Figure 7. (a) Responses of S1-S4 sensors to 100 ppm of acetone at 175-300 °C; (b) Selectivity of S1-S4 sensors to 100 ppm of various gases at their corresponding optimal operating temperature; Radar graphs and gas responses (Rair/Rgas) of (c) S1 and (d) S3 sensors to acetone and ethanol gases at 175-300 °C, respectively; Dynamic response-recovery curves of S1-S4 sensors as-fabricated sensors to acetone in the range of (e) 0.5-5 ppm and (f) 10-100


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ppm under their optimal operating temperatures; (g) Dependence of the gas sensors response under different concentrations of acetone. Figure 8. (a) Dynamic resistance change transients of S1-S4 sensors toward 100 ppm acetone at their corresponding optimal operating temperature, respectively; (b) Schematic illustration showing the acetone sensing mechanism. Table 1. Comparison of acetone sensing performances of S1-S4 sensors Table 2. Comparison of acetone sensing ability of different gas sensors.



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Graphical TOC Entry



Graphical TOC Entry


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Self-Assembly Template Driven 3D Inverse Opal Microspheres

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