Ordered Large-Pore Mesoporous Cr2O3 with Ultrathin Framework for

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Ordered Large-Pore Mesoporous CrO with Ultrathin Frameworks for Formaldehyde Sensing Chang Ding, Yulei Ma, Xiaoyong Lai, Qingfeng Yang, Ping Xue, Fang Hu, and Wangchang Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Ordered Large-Pore Mesoporous Cr2O3 with Ultrathin Frameworks for Formaldehyde Sensing Chang Ding,†, a, c Yulei Ma,†, a, c Xiaoyong Lai,*, a, c Qingfeng Yang,a, c Ping Xue,a, c Fang Hu,d and Wangchang Geng*, b a

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, People’s Republic of China. b

School of Science, Key Laboratory of Space Applied Physics and Chemistry of Ministry of

Education, Northwestern Polytechnical University, Xi’an 710072, People's Republic of China. c

College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, People’s Republic of China.

d

School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, People's Republic of China.

* Corresponding author. E-mail address: [email protected] (X. Lai); [email protected] (W. Geng) † These two authors contributed equally to this work.

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Abstract A series of ordered mesoporous chromium oxides (Cr2O3) were synthesized by replicating bicontinuous cubic Ia3d mesoporous silica (KIT-6), where a controlled mesostructural transformation from Ia3d to I4132 symmetry during the replication from KIT-6 to Cr2O3 was done by reducing the pore size and interconnectivities of KIT-6, accompanied with the increase of pore size from 3 to 12 nm and the decrease of framework thickness from 8.6 to 5 nm of the resultant Cr2O3 replicas. The gas sensing behavior of Cr2O3 replicas toward formaldehyde (HCHO) were sequentially investigated. Ordered mesoporous Cr2O3 with both large accessible pores (12 nm) and ultrathin frameworks (5 nm) exhibits best sensing performances, whose response (Rgas/Rair=119) toward 9 ppm of HCHO is 4.4 times higher than that (Rgas/Rair=27) of its counterpart with small pores and thick frameworks. Moreover, it possesses excellent selectivity for detecting HCHO against other interference gases such as CO, benzene, toluene, p-xylene, NH3, H2S, moisture. The significantly enhanced sensing performance of ordered large-pore mesoporous Cr2O3 with ultrathin frameworks suggests its great potential for the selective detection of HCHO.

Keywords. Gas sensor, formaldehyde, Cr2O3, large mesopore, ultrathin framework, nanocasting.


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Introduction. Chemoreceptive gas sensors are of great importance in wide fields such as monitoring air quality, detecting volatile organic compounds (VOCs) or other toxic gases, and controlling fossil fuel byproducts from industrial sources and vehicle exhausts.1-11 Ordered mesoporous metal oxides are viewed as a class of promising sensing materials for gas sensors because of their high specific surface areas and excellent accessibility.12, 13 Hyodo et al. reported that the response of ordered mesoporous SnO2 materials toward H2 significantly increased with their specific surface areas and larger mesopores can ensure a higher response with the same specific surface area.14, 15 It is notable that these ordered mesoporous SnO2 only possess very small mesopores below ~3 nm, which may affect the diffusion of gases in the resultant sensors and limit the further improvement of response. Waitz et al. synthesized ordered mesoporous In2O3 by replicating from SBA-15 and KIT-6 and demonstrated that their response toward CH4 tended to increase with the specific surface area (38~90 m2·g−1) and pore size (5.0~6.0 nm), but decrease with the framework thickness (3.9~5.9 nm).16 Similarly, we also synthesized In2O3 replicas from SBA-15 with different pore sizes, whose response toward formaldehyde (HCHO) sharply increased with reducing the framework thickness (4.0~9.0 nm).17 Compared with those n-type mesoporous semiconductors, p-type semiconductive counterparts such as NiO, Cr2O3, CuO, Co3O4, usually exhibit relatively low gas response under identical conditions, although these p-type semiconductors are still viewed as promising material platforms for gas sensors because of their remarkable activities for catalytically oxidizing VOCs. In our previous work,18 we first reported that ordered mesoporous NiO with thin framework of 5.1 nm and bimodal pore system of both 3.7 and 12 nm mesopores possess significantly higher response toward HCHO than that for those counterparts with thicker framework of 8.5 nm and 3 ACS Paragon Plus Environment


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small mesopores of 3.7 nm, but still less than that for those n-type mesoporous semiconductors.19-24 Chromium oxide (Cr2O3) is also a well-known wide bandgap (3.4 eV) ptype semiconductor, which can be used in catalysis,25, electrochemical devices.32,

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gas sensing,27-29 magnetism,30,

31

Ordered mesoporous Cr2O3 materials have been synthesized in

previous literatures,34-39 and their potential applications in magnetism,30, 31 catalysis,25, 26, 40-42 and Li-ion battery,43,

44

have been also investigated. For example, Liu et al. synthesized ordered

mesoporous Cr2O3 with mesopores of 2.8 and 3.3 nm by replicating KIT-6 and SBA-15 respectively,44 which exhibited the enhanced response superior over bulk Cr2O3. Nevertheless, it is still necessary for practical applications to more finely tailor the mesostructural parameters of ordered mesoporous Cr2O3 (such as pore size, framework thickness) for further improving its gas-sensing properties. Larger accessible pores (more than 10 nm) could facilitate the diffusion of gases and speed the response and recovery, whereas ultrathin frameworks would allow for more carriers being influenced by gas exposure and lead to a higher response. However, ordered mesoporous Cr2O3 materials simultaneously satisfying both two requirements have been rarely reported. Herein, ordered mesoporous Cr2O3 materials with controlled mesostructures have been synthesized by replicating mesoporous silica KIT-6, which possesses an Ia3d bicontinuous cubic mesoporous network with tunable pore size and interconnectivity. With decreasing the pore sizes and interconnectivities of KIT-6, the mesostructural symmetry of the resultant mesoporous Cr2O3 replicas transformed from Ia3d to I4132, whose pore sizes increased from 3 nm to 12 nm, accompanied by the reduction of framework thickness from 8.6 to 5 nm. The gas sensing properties of all the Cr2O3 specimens for HCHO were sequentially investigated and ordered


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mesoporous Cr2O3 with both large accessible pores of 12 nm and ultrathin frameworks of 5 nm exhibited a faster and higher response to HCHO than other Cr2O3 replicas. Experimental Section Synthesis. A series of mesoporous silica (KIT-6) were synthesized at different hydrothermal aging temperatures (T=40, 80, 100 or 130 °C) according to the previous literature.45 The resultant products were denoted to as KIT-6-T. Mesoporous Cr2O3 replicas were synthesized by using KIT-6-T as hard templates and denoted as Cr2O3-T. Typically, 1 g of mesoporous silica aged at 40 °C (KIT-6-40) and 0.82 g of Cr(NO3)3·9H2O were added in 10 mL of ethanol under stirring at 40 °C. After ethanol was completely evaporated, the resulting powder was heated to 500 °C and kept at this temperature for 3 h, in order to decompose Cr(NO3)3·9H2O into Cr2O3. The resultant silica/Cr2O3 composite was treated for several times at room temperature with concentrated alkaline solutions (2 M NaOH). Finally, the mesoporous Cr2O3 material was collected from the solutions by centrifugation, washed by water and ethanol and dried at 70 °C for 12 h. For comparison, we also synthesized bulk Cr2O3 particles by directly decomposing Cr(NO3)3·9H2O without silica templates at 800 °C. Characterization. Morphological investigation was performed with a KYKY-2800B scanning electron microscope (SEM). Transmission electron microscopy (TEM) was carried out on a Hitachi H-7650 microscope. Powder X-ray diffraction (XRD) was conducted on a Bruker AXS D8 equipment (Cu-Kα radiation, 40 kV and 40 mA). N2 physisorption was performed on an Micromeritics ASAP 2020 HD system at −196 °C, where all the samples were degassed for 4 h at 200 °C. Total pore volumes were evaluated from the adsorbed volume at a relative pressure of 0.99. The specific surface areas were calculated through the Brunauer-Emmet-Teller (BET) 5 ACS Paragon Plus Environment


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algorithm by using the adsorption branches. The pore size distributions of Cr2O3 samples were obtained through the Barrett-Joyner-Halenda (BJH) method by using the desorption branches, whereas those of KIT-6 samples were calculated by nonlocal density functional theory (NLDFT) algorithm. Sensing Test. All the Cr2O3 samples were fabricated into gas sensors and tested their gas-sensing performance according to the procedure reported in our recent work.18 Typically, 5 µL of Cr2O3 suspension (30 mg· mL−1) was dropped on the interdigital electrode to form the sensing layer. This procedure was repeated once and the thickness of the resultant sensing layer was about 3.5 µm (Figure S1). A quartz tube furnace was utilized to provide the operating temperature for sensors. Dry air and analyte gas were alternately introduced into the quartz tube. The electrical response of the sensor in different environmental atmospheres was recorded with an automatic test system.

Results and Discussion Ordered mesoporous silica (KIT-6) with varied pore size, pore volume and interconnectivities (Figure S2 and S3) were used for producing ordered mesoporous Cr2O3 materials. Representative TEM images of mesoporous Cr2O3 replicas templated from different KIT-6 are showed in Figure 1. All the mesoporous Cr2O3 materials exhibit periodically ordered coupled or uncoupled subframeworks, whose thickness gradually increased from 5.0 to 8.6 nm with the mesopore sizes of the silica template from 5.7 to 9.4 nm. Predominant uncoupled subframeworks, analogously to those for CMK-1,46 could be observed in the products of Cr2O340 (Figure 1a), which reflects that the relatively poor interconnectivity of KIT-6-40, the preferential growth of Cr2O3 in one of their two sets of enantiomeric mesochannels and the 6 ACS Paragon Plus Environment

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mesostructure degradation from Ia3d to I4132 during replication. On the contrary, Cr2O3-130 almost exhibits the same mesostructure symmetry (Ia3d) with its parent template (Figure 1d). For Cr2O3-80 and Cr2O3-100, both uncoupled and coupled subframeworks could be observed because of the medium interconnectivities of their parent templates. Low-angle XRD data could also confirm the mesostructural change (Figure 2). For example, (211), (220) and (332) diffraction peaks are clearly observed in the low-angle XRD pattern of Cr2O3-130, which are similar with those of its parent template, suggesting the successful mesostructural transfer from KIT-6 into Cr2O3 replica. With the decrease of aging temperature for KIT-6, the diffraction peaks for the corresponding Cr2O3 replica gradually become weak, suggesting the degradation of their ordered mesostructures. For Cr2O3-40, an additional diffraction peak appeared in the lower angle, which could be indexed as (110) plane of the space group I4132 characterizing the symmetry of a single Cr2O3 subframework, similar with CMK-1 described by Solovyov et al.46 Wide-angle XRD data for all the Cr2O3 samples are showed in Figure 3, where all the diffraction peaks could be indexed to the hexagonal eskolaite phase (JPCDS No. 38-1479). Nitrogen physisorption for all the ordered mesoporous Cr2O3 replicas are showed in Figure 4. Three different kinds of pores centered at 3, 12 and 60 nm could be observed from their pore size distributions (the inset of Figure 4). Generally, KIT-6 synthesized at higher hydrothermal aging temperature (e.g. KIT-6-130) has large pore size and good interconnectivity, which allows for the easy penetration of chromium nitrate precursors within their two sets of enantiomeric mesochannels. As a result, Cr2O3 growth could cover both two sets of enantiomeric mesochannels and result in a replica with predominant coupled subframeworks (e.g. Cr2O3-130), whose pore size is close to the pore wall thickness of KIT-6-130 (about 3 nm). With the decrease of hydrothermal aging temperature for KIT-6, their pore size and interconnectivity usually 7 ACS Paragon Plus Environment


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reduced (e.g. KIT-6-40) and thus Cr2O3 trends to preferentially grow in one of two sets of enantiomeric mesochannels of KIT-6 and generates a replica (e.g. Cr2O3-40) with predominant uncoupled subframeworks and decreased mesostructure symmetry of I4132, whose pore contains these spaces originally occupied by the two walls of KIT-6-40 and the mesopore between them (about 12 nm). For those Cr2O3 replicas (Cr2O3-80 and Cr2O3-100) templated from KIT-6 with moderate pore sizes and interconnectivities, typical bimodal pore size distributions including both 3 and 12 nm mesopores should be expectable, since they possess both uncoupled and coupled subframeworks. However, the mesochannels of KIT-6 could not be completely filled up with Cr2O3 through one impregnation and calcination circle and the resultant Cr2O3 replicas exhibited relatively smaller particle size (100−300 nm, see Figure 1) than that for KIT-6 (Figure S4), which resulted in the formation of textural pores (about 60 nm) between small Cr2O3 particles. With the reducing pore size and interconnectivity of silica templates resulted from the decreased aging temperature, the subframework thickness and the proportion of large mesopores of the resultant Cr2O3 replicas gradually decreased and increased respectively. It is notable that the pore size of KIT-6 in the Liu’s report was about 8.8 nm, relatively close to that for KIT-6130, which resulted in mesoporous Cr2O3 replica with small pore size of 2.8 nm and relatively thick framework.44 On the contrary, Cr2O3-40 possesses both large accessible mesopore (12 nm) and ultrathin frameworks (5 nm) and may be of great interest in many fields such as gas sensor, catalysis. Formaldehyde (HCHO) is a well-known carcinogenic for humans. The occupational safety and health act (OSHA) has set a permissible exposure limit (PEL) of HCHO at 0.75 ppm and immediately dangerous to life or health limit (IDLH) at 20 ppm respectively.4 The potential of all the mesoporous Cr2O3 replicas as sensing materials for the detection of HCHO was investigated. 8 ACS Paragon Plus Environment

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Figure 5a~e shows dynamic response of all the sensors based on mesoporous Cr2O3 samples exposed to different HCHO concentration from 0.2 to 90 ppm where bulk Cr2O3 sample (Figure S5-6) served as a reference. The resistance immediately increased to a maximum value after exposure to HCHO and completely recovered to the initial value after replacing HCHO with dry air, which showed good reversibility during the cycles between the introduction and exhaust of HCHO. Cr2O3 is a typical p-type semiconductor where holes are major carriers because of the intrinsic cation vacancy, and its sensing principle is mainly based on the change of hole concentration and conductivity by the interaction between the semiconductor surface and absorbed gases (see Figure 6). When Cr2O3-based sensors are put in oxidative atmosphere (e.g. air), some electrons in its conduction band could be captured by oxidative gases (e.g. O2 in air) absorbing on the surface, thus leading to the increase of hole concentration and the formation of hole accumulation layer with high conductivity as well as the resistance decrease. When air containing HCHO gas is introduced, reductive HCHO could react with chemisorbed oxygen and release electrons back into conduction band, resulting in the hole concentration decrease and the thickness reduction of hole accumulation layer as well as the resistance increase. The response can be defined as Rgas/Rair (Rgas, the resistance of sensor in air containing HCHO gas; Rair, the resistance of sensor in air). The response of Cr2O3-130, Cr2O3-100, Cr2O3-80 and Cr2O3-40 toward HCHO gradually increased in the whole testing concentration range from 0.2 to 90 ppm (see Figure 5f), which all were superior over bulk Cr2O3 particle. According to the core–shell model for large particles proposed by Barsan et al.,47 bulk Cr2O3 particle is insensitive to the small change of hole concentration resulting from gas exposure (see Figure 6). With decreasing the size of Cr2O3 to twice its Debye length LD (which corresponds to its hole accumulation layer, ∼130 nm),48 more holes in Cr2O3 could be influenced during gas exposure, thus resulting in an 9 ACS Paragon Plus Environment


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enhanced response. Mesoporous Cr2O3 replicas are composed of some subframeworks with the thickness of 5∼8.6 nm, which is much less than its LD. Therefore, hole accumulation layer possibly extends to the whole framework and the high surface-to-volume ratio derived from ultrathin framework can enhance the interaction of surface and gases and increase the proportion of extrinsic holes, which can indeed lead to the response enhancement. As a result, Cr2O3-40 with ultrathin frameworks of 5 nm exhibits a highest response of 771 toward 90 ppm of HCHO among of all the mesoporous Cr2O3, which is also significantly superior over bulk Cr2O3 and even comparable with those n-type semiconductive sensors (Table S1).17, 24, 49-60 Response and recovery times are usually defined as the time that the sensor resistance reach 90% of the final equilibrium value, which are 136 and 31 s for Cr2O3-40, 203 and 44 s for Cr2O3-80, 214 and 66 s for Cr2O3-100, 345 and 83 s for Cr2O3-130, respectively (Figure S7). The response and recovery for gas sensors are largely determined by the diffusion of gases and it is well-known for mesoporous materials that the type of gases diffusion within their confined mesochannels is mainly Knudsen diffusion, whose diffusion coefficient linearly depends on pore size.12 Therefore, Cr2O3-40 with large mesopores exhibits a shortest response and recovery time, whereas Cr2O3130 with small mesopores shows a longest response and recovery time. The diffusion of gases within the former is about 2.5 times faster than that within the latter. For bimodal mesoporous replicas Cr2O3-80 and Cr2O3-100, the response and recovery with medium speed, related to the proportion of large mesopores, should be expectable. Furthermore, we primarily tested the stability of the sensor based on Cr2O3-40 and did not observe the significant response degradation during five consecutive cycles between 9 ppm HCHO and air (Figure 7). For comparison, we also tested the response of the sensor based on Cr2O3-40 toward other interference gases such as CO, benzene, toluene, p-xylene, NH3, H2S, which all were lower than 10 ACS Paragon Plus Environment

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that toward HCHO (Figure 8). That may be due to the superior catalytic activity of ordered mesoporous Cr2O3 for oxidizing HCHO. For example, Xia et al. demonstrated that complete HCHO oxidation was easily achieved at 130 °C by using ordered mesoporous Cr2O3 as catalysts,61 whereas Sinha et al. reported that complete toluene oxidation requires higher reaction temperature of 280–300 °C under the presence of ordered mesoporous Cr2O3.25 Moreover, the effect of moisture on the gas-sensing properties of Cr2O3-40 was also evaluated, whose response toward moisture (Relative humidity=99%) was about 12 and less than that for 0.2 ppm of HCHO (14). All the above-mentioned results suggest the great potential of mesoporous Cr2O3 with both large accessible pores and ultrathin frameworks in the selective detection of HCHO. Conclusions In summary, ordered mesoporous Cr2O3 replicas with controlled mesostructures were synthesized via the nanocasting route by using KIT-6 as templates, where an ordered mesoporous Cr2O3 with both large accessible pores (12 nm) and ultrathin frameworks (5 nm) was achieved for the first time by a controlled mesostructural transformation from Ia3d to I4132 symmetry during the replication from mesoporous silica to Cr2O3. The resultant ordered mesoporous Cr2O3 with both large accessible pores and ultrathin frameworks possessed a higher and faster response toward formaldehyde (HCHO) than other common mesoporous Cr2O3 replicas, suggesting its great potential for gas sensors in environmental monitoring. Further investigation on ordered mesoporous Cr2O3–based replicas with tailored framework composition will be worthwhile for the detection of other dangerous gases. Acknowledgements.



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Financial support by the National Natural Science Foundation of China (No. 51362024, 21006116 and 51672138) and the West Light Foundation of the Chinese Academy of Sciences is gratefully acknowledged. Supporting Information Available. Cross-sectional image of gas sensor, nitrogen physisorption and SEM images of KIT-6 and bulk Cr2O3, response transients of gas sensors towards 1 ppm of HCHO and comparison on various HCHO sensors.


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Figure captions Figure 1. TEM images of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130. The “S” and “L” indicate small and large mesopore respectively. Figure 2. Low-angle XRD patterns of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130. Figure 3. Wide-angle XRD patterns of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100, (d) Cr2O3-130 and (e) bulk Cr2O3. Figure 4. Nitrogen physisorption isotherms and the corresponding pore size distributions (inset) of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130. For clarification, the data of isotherms and pore size distributions are given with an offset of 150 and 0.6 cm3·g−1 for Cr2O3-40, 60 and 0.4 cm3·g−1 for Cr2O3-80, 40 and 0.2 cm3·g−1 for Cr2O3-100, respectively. Figure 5. Typical dynamic response curves of gas sensors fabricated from ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100, (d) Cr2O3130 and (e) bulk Cr2O3, during cycling between increasing concentration of HCHO and ambient air at 230 °C, and (f) their corresponding response versus HCHO concentration. Figure 6. Illustration of the dependence of the response on the mesostructural parameters of Cr2O3 sensing materials. Figure 7. Dynamic response reproducibility of gas sensor fabricated from Cr2O3-40 toward 9 ppm of HCHO. Figure 8. Response of gas sensor fabricated from Cr2O3-40 toward 1 ppm of HCHO, CO, benzene, toluene, p-xylene, NH3, H2S as well as moisture (Relative humidity=99%) at 230 °C.



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Figure 1. TEM images of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130. The “S” and “L” indicate small and large mesopore respectively.


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Figure 2. Low-angle XRD patterns of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130.

Figure 3. Wide-angle XRD patterns of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100, (d) Cr2O3-130 and (e) bulk Cr2O3.



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Figure 4. Nitrogen physisorption isotherms and the corresponding pore size distributions (inset) of ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100 and (d) Cr2O3-130. For clarification, the data of isotherms and pore size distributions are given with an offset of 150 and 0.6 cm3·g−1 for Cr2O3-40, 60 and 0.4 cm3·g−1 for Cr2O3-80, 40 and 0.2 cm3·g−1 for Cr2O3-100, respectively.


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Figure 5. Typical dynamic response curves of gas sensors fabricated from ordered mesoporous chromium oxides from different KIT-6: (a) Cr2O3-40, (b) Cr2O3-80, (c) Cr2O3-100, (d) Cr2O3130 and (e) bulk Cr2O3, during cycling between increasing concentration of HCHO and ambient air at 230 °C, and (f) their corresponding response versus HCHO concentration. 23 ACS Paragon Plus Environment


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Figure 6. Illustration of the dependence of the response on the mesostructural parameters of Cr2O3 sensing materials.


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Figure 7. Dynamic response reproducibility of gas sensor fabricated from Cr2O3-40 toward 9 ppm of HCHO.

Figure 8. Response of gas sensor fabricated from Cr2O3-40 toward 1 ppm of HCHO, CO, benzene, toluene, p-xylene, NH3, H2S as well as moisture (Relative humidity=99%) at 230 °C.



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Ordered Large-Pore Mesoporous Cr2O3 with Ultrathin Framework for

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