Direct Organization of Morphology-Controllable Mesoporous SnO2

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Direct Organization of Morphology-Controllable Mesoporous SnO2 using Amphiphilic Graft Copolymer for Gas Sensing Application Won Seok Chi, Chang Soo Lee, Hu Long, Myoung Hwan Oh, Alex Zettl, Carlo Carraro, Jong Hak Kim, and Roya Maboudian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07823 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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ACS Applied Materials & Interfaces

Direct Organization of Morphology-Controllable Mesoporous SnO2 using Amphiphilic Graft Copolymer for Gas Sensing Application

Won Seok Chi1,2, Chang Soo Lee3, Hu Long4, Myoung Hwan Oh5, Alex Zettl4,6,7, Carlo Carraro1, Jong Hak Kim3, Roya Maboudian1,2 1 Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. 2 Berkeley Sensor & Actuator Center, University of California at Berkeley, Berkeley, CA 94720, USA. 3 Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, South Korea. 4 Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA. 5 Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720, USA. 6 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

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7 Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. KEYWORDS. Amphiphilic graft copolymer, Sol-gel, Mesoporous materials, SnO2, Microheater, Gas sensor.

Abstract A simple and flexible strategy for controlled synthesis of mesoporous metal oxide films using an amphiphilic graft copolymer as sacrificial template is presented and the effectiveness of this approach for gas sensing applications is reported. The amphiphilic graft copolymer, poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) is used as a sacrificial template for the direct synthesis of mesoporous SnO2. The graft copolymer self-assembly is shown to enable a good control over the morphology of the resulting SnO2 layer.

Using this approach,

mesoporous SnO2 based sensors with varied porosity are fabricated in-situ on a microheater platform. This method reduces the interfacial contact resistance between the chemically sensitive materials and the microheater, while providing a simple fabrication process. The sensors show significantly different gas sensing performances depending on the SnO2 porosity, with the highly mesoporous SnO2 sensor exhibiting high sensitivity, low detection limit and fast response and recovery towards hydrogen gas. This printable solution-based method can be used reproducibly to fabricate a variety of mesoporous metal oxide layers with tunable morphologies on various substrates for high performance applications.


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Introduction Porous materials are attractive for many applications such as batteries1-3, supercapacitors4-7 and gas separators8-11, owing to their remarkable structure and morphology, such as tunable pore size and volume, and high surface area. The ability to tailor the pore size is critical for many of these applications. The smaller the pore size, the higher is the surface area for gas/ion penetration or for absorption onto active sites. However, very small pore size may inhibit molecular diffusion or surface reaction, resulting from bottleneck phenomena of large flux of molecules or deficiencies in active sites. Among the porous materials, mesoporous materials with 2 - 50 nm pore size offer excellent properties including high surface area and proper pore size, leading to high performance in many applications.12 Mesoporous metal oxides have been widely pursued as promising candidates for gas sensing applications due to their ease of fabrication, good chemical/thermal stability, and high sensitivity. Based on gas absorption and diffusion mechanisms, the mesoporous metal oxides architectures significantly promote the mass transfer of gaseous reactants toward active sites through rapid gas diffusion enabled by their suitable pore size with extremely high surface area, thus resulting in high sensitivity. Various mesoporous metal oxide nanostructures have been developed such as hierarchical nanostructures13-14, nanotubes15-16, hollow spheres17-18, or nanowires19-20 to achieve rapid response and fast electron transport through interconnected, porous, 2- or 3-dimentional structures, allowing excellent responses for gas sensing applications. However, metal oxides need to be heated to elevated temperatures (250-500 oC) to activate or enhance their sensing performance. Giving that many sensing applications require low-power consumptions and small device footprints21-22, it remains a challenge to effectively integrate presynthesized mesoporous nanomaterials with tailored porosity onto low-power microheaters. The typical integration method is drop casting the nanomaterials dispersed in solution onto the


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microheater chip, a process that is hard to control, resulting in variable structure of the deposited materials, in a reduction of active surface area due to aggregation, and in poor interfacial contact with the sensing electrodes. There have been recent reports on new strategies for direct mesoporous metal oxide deposition such as hydrothermal growths of chemically sensitive materials23-25, chemical vapor deposition methods26-28, and in-situ polymerization techniques of sensing materials29-30. Although these novel approaches show outstanding gas sensing performances, they suffer from a number of limitations such as complicated control processes during fabrication, difficulty to scale up for commercialization, and the need of special instrumentation or environmental conditions. Therefore, it is crucial to develop a simple and scalable strategy for the synthesis of well-organized mesoporous materials with controllable morphology on suitable substrates for different applications. In contrast to previous methods, the use of a sacrificial template to fabricate mesoporous materials offers an easier approach towards control over morphology and pore size of mesoporous materials. Here, we report a novel technique based on the direct formation of morphology-controllable mesoporous metal oxide layers using amphiphilic graft copolymers as a sacrificial soft template for gas sensing applications. This new method uses metal oxide salt as a precursor together with graft copolymer as a sacrificial soft template to generate mesoporous metal-oxide layers in situ on a low-power microheater platform, which significantly reduces the interfacial contact resistance between chemically sensitive materials and microheater with a simple fabrication process. The self-assembly of microphase-separated amphiphilic graft copolymer leads to controllable nanostructure, pore distribution, and surface area of the active materials. Poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) and SnO2 are chosen as representative sacrificial template and metal oxide, respectively, to demonstrate the validity of


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this strategy. The resulting SnO2 gas sensors with varying mesopores present significantly different gas sensing performances, with the highly mesoporous SnO2 sensor exhibiting high sensitivity, low detection limit and fast response and recovery towards hydrogen gas.

Experimental section Synthesis of PVC-g-POEM graft copolymer. PVC-g-POEM graft copolymers were synthesized via an atomic transfer radical polymerization (ATRP) process, schematically shown in Figure S1(a).31-32 This graft copolymer was chosen due to its relatively simple and low-cost synthesis compared to other microphase-separated copolymers (i.e., block copolymers). Briefly, 6.0g of poly(vinyl chloride) (PVC) was dissolved in 50 ml N-methyl-2-pyrrolidone (NMP) in a 250 ml round flask and 15 g of poly(oxyethylene methacrylate) (POEM) was added to the solution. After stirring for several hours, 0.10 g of CuCl and 0.23 ml of 1,1,4,7,10,10hexamethyltriethylenetetramine (HMTETA) were added and the flask was sealed with a rubber septum. After N2 purging for 30 min, the reaction flask was immersed in an oil bath at 90 oC. The reaction was allowed to proceed for 24 h, and then, the solution was cooled down. After passing the solution through a column with activated Al2O3 to remove the catalyst, it was precipitated into methanol (MeOH). The polymer was purified by repeated washing in MeOH to eliminate all residues. Finally, the PVC-g-POEM graft copolymers were dried in a vacuum oven at 50 oC for 24h. Preparation of mesoporous SnO2 precursor solution. There are several crucial parameters that can control the pore size and structure of mesoporous SnO2, such as polymer concentration, amount of metal oxide precursor and solvent composition. Two kinds of mesoporous SnO2 precursor solutions are presented here. 1) Less mesoporous SnO2 (LM-SnO2) precursor solution: 0.0125 g of PVC-g-POEM was completely dissolved in 2.5 ml THF solvent. 0.2 g of SnCl2 was


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added to as-prepared polymer solution, stirring for 3-4 hours to homogenize the solution. 2) Highly mesoporous SnO2 (HM-SnO2) precursor solution: 0.025g of PVC-g-POEM was fully dissolved in 5 ml THF solvent. Then, 0.2 g of SnCl2 was added to as-prepared polymer solution and after 10 min, 0.1 ml of deionized (DI) water was added to precursor solution. This solution was stirred for 3-4 h to make a homogeneous solution. Materials characterization. To confirm successful polymerization of PVC-g-POEM graft copolymer, the synthesized graft copolymer was characterized via FT-IR spectra (Excalibur Series FT-IR, DIGLAB Co.), 1H-NMR measurement with a 600-MHz high-resolution NMR spectrometer (AVANCE 600 MHz FT-NMR, Germany, Bruker), and differential scanning calorimetry (DSC) measurement (DSC 2920, TA Instruments, Inc.) by second sequence from 80 to 200 oC at a heating rate of 20 oC/min under air atmosphere. Thermogravimetric analysis (TGA, Mettler Toledo TGA/SDTA 851e, Columbus, OH) was conducted under an air atmosphere to determine thermal stability of PVC-g-POEM graft copolymer. The morphology and crystal structure of the mesoporous SnO2 layers were analyzed by scanning electron microscopy (SEM, SUPRA 55VP, Carl Zeiss), transmission electron microscopy (TEM, FEI G2 20 Technai microscope), Raman spectroscopy (Horiba Jobin Yvon LabRam), and X-ray diffraction (XRD, D8 Advance instrument, Bruker). For imaging the mesoporous SnO2 layer, the precursor solutions were casted onto a substrate and dried onto hotplate by increasing the temperature with 3 oC/min heating rate to 500 oC and kept it for 2 hours. For the polymer and precursor TEM imaging (LIBRA 120 microscope), the solutions were drop casted onto copper grid and dried in oven at 50 oC for complete solvent evaporation. Fabrication of Gas Sensor. The as-prepared mesoporous SnO2 precursor solutions were directly deposited onto a commercial microheater (Cambridge CMOS, CCS301) by drop-casting.


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After THF solvent evaporated at room temperature, microheater temperature was increased with 3 oC/min heating rate to 500 oC and kept at 500 oC for 2 hours, to completely remove the polymer template. This process mimics the procedure used for the samples prepared for imaging and structural analyses. Gas sensor testing. The gas sensor was placed inside a home-made gas flow chamber. A LabView-controlled gas delivery system was used to individually control the concentration of gases flown in the chamber. Hydrogen (Praxair, 5% in N2), propane (Praxair, 5% in N2) and methane (Praxair, 5% in N2) were used for gas sensing tests at a constant flow rate of 300 sccm. The house air was purged and stream balanced to reduce humidity and other contaminants. The performance of the gas sensor was measured using a Keithley 2602 source-meter, by applying voltages to the sensor and heater channels and measuring their currents to monitor the sensor response and heater temperature (using the calibration data provided by the manufacturer), respectively. Zephyr, an open-source Java-based measurement software, was used to collect date from the source-meter and gas delivery system including gas concentrations and flow rates.

Results and discussion Characterization of PVC-g-POEM graft copolymer and mesoporous SnO2. Amphiphilic PVC-g-POEM graft copolymer is composed of the hydrophobic PVC as main chains and the hydrophilic POEM as sides chains (schematically shown in Figure S1(b)). The FT-IR analyses are presented in Figure 1(a) for pure PVC, POEM monomers and PVC-g-POEM graft copolymer. The PVC-g-POEM graft copolymer exhibits two different functional bands from PVC and POEM, respectively. The carbonyl (C=O) and ether (C-O-C) stretching vibration bands due to the POEM are detected at 1727 and 1100 cm−1, respectively, and the chloride (CCl) vibration band derived from PVC is found at 610 cm−1. Furthermore, the alkene (C=C)


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vibration band in POEM at 1637 cm−1 completely disappears after graft polymerization of PVCg-POEM. The spectra indicate that the PVC-g-POEM graft copolymer is successfully synthesized using methacrylate group for propagation. The 1H-NMR analysis is conducted to determine the actual weight ratio of PVC and POEM in PVC-g-POEM graft copolymer. Shown in Figure 1(b), the strong signal at 4.6 ppm can be assigned to the protons in the CHCl groups of PVC, whereas the signals at 3.6 and 3.4 ppm are attributed to the protons in the CH2CH2O groups and CH3 groups of POEM, respectively. The actual grafting ratio is calculated by integrating the peak area assigned to each group. The weight ratio of PVC to POEM in PVC-g-POEM graft copolymer is found to be approximately 2:1. The amphiphilic PVC-g-POEM graft copolymer possesses glassy PVC main chains and rubbery POEM side chains simultaneously, which can produce well-developed microphase-separation. Due to these characteristics, the PVC-g-POEM graft copolymer shows two different glass transition temperatures (Tg) in Figure 1(c). The Tg at 83 oC originates from PVC main chains mobility, whereas Tg at -58 oC results from POEM side chains mobility. Moreover, the absence of endothermic melting temperature (Tm) indicates the amorphous state of PVC-g-POEM graft copolymer. This amphiphilic PVC-g-POEM graft copolymer with well-developed microphase-separation is a good sacrificial template to fabricate mesoporous structure metal oxides.33-35 To be used as a sacrificial template, thermal stability of PVC-g-POEM graft copolymer is investigated by TGA measurement in Figure 1(d). The PVC-g-POEM graft copolymer is thermally stable up to 250 o

C without any noticeable weight loss change. Two-step decomposition pattern is observed at

around 300 and 400 oC. Less thermally stable groups (i.e., chlorine) in PVC-g-POEM graft copolymer start to decompose at 300

o

C, whereas more thermally stable groups (i.e.,


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hydrocarbons) in PVC-g-POEM graft copolymer start to decompose 400 oC. At 500 oC, PVC-gPOEM graft copolymer is decomposed completely. These results reveal that PVC-g-POEM graft copolymer can be used as a sacrificial template which requires heating temperature of ≥ 500 oC to remove all the polymer.

Figure 1. (a) FT-IR spectra of PVC, POEM monomers, and PVC-g-POEM graft copolymer, (b) 1

H-NMR spectrum of PVC-g-POEM graft copolymer, (c) DSC analysis of PVC-g-POEM graft

copolymer, (d) TGA curve of PVC-g-POEM graft copolymer.


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Transmission electron microscopy is employed to understand the effect of various components in the sol-gel process on the resulting morphology. Figure 2(a) shows the TEM image of pure PVC-g-POEM graft copolymer in THF solvent. The THF solvent has neutral nature and thus, it is able to dissolve both hydrophobic PVC and hydrophilic POEM. In this way, the PVC-gPOEM graft copolymer develops evident microphase-separation in THF solvent. The dark regions represent the PVC domain and the bright regions represent the POEM domain, due to higher electron density of chlorine in PVC than in POEM. The morphology of PVC-g-POEM graft copolymer can be controlled by addition of H2O based on polymer-solvent interactions. Since H2O is a strongly hydrophilic polar solvent that has a good interaction with the POEM domain, it leads to well-developed POEM domains in PVC-g-POEM graft copolymer as shown in Figure 2(b). The PVC-g-POEM/H2O/THF mixture not only produces continuous interconnectivity between POEM domains, but also enhances the solubility of hydrophilic SnCl2 precursor. These synergistic effects help to fabricate highly well-developed mesoporous SnO2 structure. Based on the above characterizations, the amphiphilic PVC-g-POEM graft copolymer is used as a soft template through sol-gel process for the synthesis of mesoporous SnO2 layer. Figure 2(c) shows the morphology of PVC-g-POEM/SnCl2/THF mixture without water which possesses less-ordered microphase-separation, attributable to low solubility of SnCl2 in THF solvent. The dissolution of hydrophilic SnCl2 in PVC-g-POEM graft copolymer with THF solution needs several hours to adhere to POEM domain and develop self-assembly. However, when SnCl2 is added in PVC-g-POEM/H2O/THF solution, SnCl2 dissolves immediately due to the presence of H2O. Thus, the PVC-g-POEM/H2O/SnCl2/THF exhibits well-organized microphase-separation, as shown in Figure 2(d), compared to PVC-g-POEM/SnCl2/THF. These two recipes are used for


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the fabrication of mesoporous SnO2 layers with different morphology, referred to as, less mesoporous SnO2 (LM-SnO2) from Figure 2(c) and highly mesoporous SnO2 (HM-SnO2) from Figure 2(d).

Figure 2. TEM images of (a) PVC-g-POEM/THF, (b) PVC-g-POEM/H2O/THF (c) PVC-gPOEM/SnCl2/THF, and (d) PVC-g-POEM/H2O/SnCl2/THF. The two prepared solutions are drop-casted on silicon substrates and then the substrate temperature is raised at a rate of 3 oC/min and held at 500 oC for 2 h to obtain mesoporous layers


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with different morphologies. The LM-SnO2 and HM-SnO2 layers show comparatively controlled mesoporous structures with crack-free conditions, displayed in Figure S2 in low magnified images. Figures 3 (a, b) present SEM images of LM-SnO2 layer under different magnifications, where closely-aggregated SnO2 layer with less-developed mesoporous structure is observed. The pore size of LM-SnO2 is around 5-10 nm and pores are observed sparsely. In contrast, HM-SnO2 layer exhibits well-developed mesoporous structure with 30-40 nm pore size as shown in Figures 3(c, d). Moreover, the SnO2 skeleton is observed to compose of interconnected small SnO2 nanoparticles in HM-SnO2, exhibiting large density of nanopores with small diameters with high surface area, while LM-SnO2 layer shows much denser skeleton.

Figure 3. SEM images of (a, b) LM-SnO2 and (c, d) HM-SnO2 with different magnifications.


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To characterize the mesoporous SnO2 layers using TEM, the SnO2 layers are synthesized using the same deposition method directly onto TEM grids consisting of high temperature-resistant silicon nitride (Si3N4) window. The properties of the LM-SnO2 and HM-SnO2 layers, including porosity, domain size, and the lattice d-spacing value, are clearly detectable in TEM images (Figure 4). The TEM image of LM-SnO2 layer (Figure 4(a)) shows aggregated and large SnO2 domains with low surface area. This is consistent with the TEM image presented in Fig. 2(c) showing aggregation in microphase separation of PVC-g-POEM/SnCl2, and with the SEM image presented in Fig. 3(b), showing closely-aggregated SnO2 domains in LM-SnO2. The aggregated domains much larger than 20 nm are observed in magnified TEM image in Figure 4(b), with the inset showing the Fourier transform (FT) pattern of selected area. The (110) plane of LM-SnO2 is clearly observed corresponding to lattice spacing of 0.34 nm which confirms the formation of crystalline SnO2. In comparison with LM-SnO2, the TEM image of HM-SnO2 in Figure 4(c) exhibits a large number of small domains with extremely large surface area, matched with welldeveloped microphase-separation in PVC-g-POEM/H2O/SnCl2 (Fig. 2(d)) and well-organized, interconnected structure in HM-SnO2 (Fig. 3(d)). In Figure 4(d), the domains less than 10 nm show multiple active sites and contacts between adjacent domains, which are advantageous for gas absorption and conductive sensing path. The HM-SnO2 presents 0.34 nm lattice spacing in accordance with (110) plane of SnO2 through FT pattern in the selected area of TEM image, demonstrating that LM-SnO2 and HM-SnO2 possess identical crystal structures.


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Figure 4. TEM images of (a, b) LM-SnO2 and (c, d) HM-SnO2 with different magnifications. The insets of FT patterns reveal the plane of LM-SnO2 and HM-SnO2 The crystal structure of mesoporous SnO2 layer is also characterized by Raman and X-ray diffraction measurements. Figure 5(a) shows the Raman spectra of LM-SnO2 and MH-SnO2 layers, both exhibiting the SnO2 characteristic peaks, located at 475, 632, and 777 cm−1 attributed to the Eg, A1g, and B2g vibration mode of mesoporous SnO2, respectively. Furthermore, the peak found at 591 cm−1 indicates Sn-O surface vibration mode. Figure 5(b) presents the XRD spectra,


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showing that both LM-SnO2 and HM-SnO2 exhibit similar characteristic peaks corresponding to polycrystalline rutile SnO2 phase (JCPDS No. 41-1445). Applying Scherer’s equation to the strongest peak (110) which is also the exposed plane from the TEM analysis, the average crystalline size of mesoporous SnO2 is approximately 17 nm, consistent with the aggregated particles size observed from SEM and TEM images. The Raman spectra and XRD pattern results reveal that two types of mesoporous SnO2 have identical crystalline phase but different morphologies.

Figure 5. (a) Raman and (b) XRD spectra of mesoporous SnO2 layers (LM-SnO2 and HMSnO2). Gas sensor performance. For gas sensor applications, heating the sensing materials to high temperature is often used to activate sensing mechanism and enhance the response and recovery rates. To demonstrate the application of the synthesis method, a commercial low power microheater is used, as shown in Figure S3. (The power consumption characteristics of the microheater is presented in Figure S4.) The mesoporous SnO2 sol-gel solution is drop-casted onto the microheater at room temperature. The THF solvent evaporates because of its low boiling point and it can help with


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the evaporation of water resulting from the favorable interaction between THF and water. The mesoporous SnO2 layer is then directly generated with gradual increase of microheater temperature up to 500 oC and maintained for several hours to remove polymer template and develop SnO2 materials. This mesoporous SnO2 fabrication process provides good interfacial contact with sensor and chemical sensitive materials, which can reduce the sensor resistance. The as-prepared mesoporous SnO2 based sensors are tested towards hydrogen gas. Hydrogen is an important industrial gas used in a number of applications, such as a replacement for fossil fuel and an alternative fuel for transportation. Since hydrogen is odorless, colorless, but flammable, it is critical to install hydrogen gas monitoring systems that can detect potentially explosive situations. Speed and accuracy are needed to detect it in short time, because even at low concentration (lower explosive limit = 4%) it can lead to explosion. Operation temperature is a one of the key factors for gas sensor, due to its major influence on sensing performance. The response to hydrogen gas is measured at various heater temperatures to determine optimum temperature. Figure 6(a) shows the effect of temperature on the sensor response, calculated as (Rair/Rgas), where Rair is the average resistance in air and Rgas is the average resistance during exposure to 3% H2 in air. Both LM-SnO2 and HM-SnO2 record highest sensitivity with the microheater at 300 oC. Consequently, this temperature is chosen as the optimum temperature and further sensing tests are conducted at this temperature.


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Figure 6. (a) Sensitivity of the LM-SnO2 and HM-SnO2 sensor to 3% hydrogen gas at different temperature, real time resistance changes of (b) the LM-SnO2 and (c) the HM-SnO2 sensor to different hydrogen gas concentration, (d) selectivity of the LM-SnO2 and HM-SnO2 sensor toward hydrogen and hydrocarbon gases at 3% concentration. Figures 6(b, c) represent real-time resistance changes of the mesoporous SnO2 sensors to various hydrogen concentrations at microheater operating temperature of 300 oC. The HM-SnO2 based sensor shows higher sensitivity, and faster response and recovery times during exposure to various hydrogen concentrations. Regardless of hydrogen gas concentration, the resistance recovers to the baseline level after stopping H2 exposure. The average time to reach 90% of the stable sensor signal (t90) for the response is 5 s and for recovery is 97 s. In comparison, the LM-


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SnO2 based sensor shows slower response and recovery times, with t90 for the response of about 36 s and the t90 for recovery of about 165 s. The sensitivity of HM-SnO2 based sensor shows approximately linear dependence on hydrogen concentration, shown in Figure S5. The detection limits, calculated using the conventional signal-to-noise of 3, are 70 ppm for HM-SnO2 and 140 ppm for LM-SnO2 sensors. Thus, the HM-SnO2 shows superior sensing performance, including higher sensitivity, lower detection limit, and shorter response and recovery times, compared to LM-SnO2. We attribute these characteristics to a number of effects.36-41 One factor is the greater porosity of HM-SnO2 sensing layer consisting of well-connected SnO2 nanoparticles. Although the high surface area might not necessarily lead to more active sites due to Weisz limit42 in gas sensor, these pores provide more channels for gas diffusion toward the mesoporous SnO2 surface and greater likelihood for sorption. Furthermore, the HM-SnO2 sensing layer retains good interconnectivity between SnO2 nanoparticles, which can provide faster electron transport, leading to better sensor performance. Another factor leading to the improved sensing characteristics of HM-SnO2 based device is the better electrical contact between the HM-SnO2 sensing material and the electrodes. The selectivity of mesoporous SnO2 based sensors is also investigated with two hydrocarbon gases, namely propane and methane. Figure 6(d) shows the response to these gases and H2 at 3% concentrations, for both HM-SnO2 and LM-SnO2 sensors. The responses to propane and methane gases are significantly lower than to hydrogen. Clearly, the sensor shows excellent selectivity to H2 over these other gases. Although the exact mechanism behind this selectivity is not clear, it is well known that a sensing material’s surface structure and electronic characteristics strongly affect its interaction with various gases. There are thermodynamic considerations in that hydrocarbons commonly need more active sites and energy to react than hydrogen. There are


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also kinetic considerations in that hydrogen has much faster rate of diffusion and smaller kinetic size than hydrocarbon gases, allowing more sensitive gas response. As shown in Figure 6, the resistance of the mesoporous SnO2 material decreases when the sensor is exposed to H2, consistent with SnO2 exhibiting n-type sensing behavior. When mesoporous SnO2 is exposed to air at high temperature (~ 300 oC), oxygen molecules are chemisorbed on the mesoporous SnO2 surface. The adsorbed oxygen molecules trap electrons, creating an electron-depleted region, which leads to a barrier at the interface between contacting nanomaterials.43-45 The introducing hydrogen gas reacts with adsorbed oxygen molecules on mesoporous SnO2 surface, resulting in the formation of water molecules and the release of the trapped electrons. This results in a decrease in resistance in n-type materials in response to H2. Higher porosity of HM-SnO2 provides far more gas absorption sites, leading to higher reactivity with hydrogen gases compared LM-SnO2, allowing better performance. Consequently, the mesoporous SnO2 allows selective gas sensing performance in regarding to multiple gases, depending on their structure which can be easily tuned by self-assembly formation of PVC-gPOEM graft copolymer template.

Conclusion A new approach for direct synthesis and integration of mesoporous SnO2 for gas sensing applications is demonstrated using amphiphilic PVC-g-POEM graft copolymer as a sacrificial template. The pore-controllable mesoporous SnO2 exhibits high surface area and multiple pores with great connectivity by materials self-assembly from well-developed microphase-separated PVC-g-POEM graft copolymer. The sensing performance SnO2 depends on the pore structures as dictated by the sacrificial template. Highly mesoporous SnO2 based gas sensors exhibit high


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sensitivity toward hydrogen, low detection limit (60 ppm), fast response time (~ 10 s) and recovery time (~ 54 s), and excellent selectivity against other hydrocarbon gases at 300 oC. This mesoporous material synthesis and integration method can be used to fabricate other types of metal oxide or metal/metal oxide complexes for a variety of gas sensing applications. Furthermore, this strategy suggests the possibility to design commercial ink for inkjet printing based gas sensor. Commonly, ink is prepared by as-prepared nanomaterials and polymer in solvent. It requires many steps such as synthesis of nanomaterials, preparation of ink, and fabrication of gas sensor. However, the novel method presented here, based on graft polymer self-assmebly and nanomaterials precursor, can produce gas sensors efficiently with easy control, resulting in high surface area, excellent porosity, and inter-connectivity. In addition, the graft copolymer and nanomaterials precursor ink can be used for long periods without special treatment, due to its stability with homogeneous condition. The condition of precursor solution is maintained for several weeks without any change and the mesoporous SnO2 derived from the precursor solution after long times shows identical structure with that from fresh precursor solution on the microheater for gas sensors.

ASSOCIATED CONTENT Supporting Information. Synthesis procedure of PVC-g-POEM graft copolymer and schematic illustration of PVC-g-POEM graft copolymer; low-magnification SEM images of mesoporous SnO2; Microheater platform with cross-sectional schematic image and top-view optical images showing heater and sensing electrodes; characterization of microheater power consumption; sensitivity of mesoporous SnO2 toward hydrogen gas with various concentration.


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AUTHOR INFORMATION Corresponding Author *R. Maboudian. Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the Center for Advanced Meta-Materials (CAMM) (NRF2014M3A6B3063716) funded by the National Research Foundation (NRF) of Korean government. It was also supported by the National Science Foundation (NSF grant # IIP 1444950) and Berkeley Sensor & Actuator Center (BSAC) Industrial Members.

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Direct Organization of Morphology-Controllable Mesoporous SnO2

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