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Metal-free cataluminescence gas sensor for hydrogen sulfide based on its catalytic oxidation on silicon carbide nanocages Liqian Wu, Lichun Zhang, Mingxia Sun, Rui Liu, Lingzhu Yu, and Yi Lv Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04566 • Publication Date (Web): 26 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Analytical Chemistry

Metal-free cataluminescence gas sensor for hydrogen sulfide based on its catalytic oxidation on silicon carbide nanocages Liqian Wu, † Lichun Zhang, † Mingxia Sun, † Rui Liu, † * Lingzhu Yu, ‡ Yi Lv† * †

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, China ‡

Engineering Research Center in Biomaterials, Sichuan university, Chengdu, Sichuan, 610064,

China

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Cataluminescence (CTL)-based sensor is one of the most charming and effective tools for gas sensing, owing to the its efficient selectivity, high sensitivity, and rapidity. As sensing materials of CTL-based sensors, metal-based catalysts easily bring high cost and environmental pollution of heavy metals. More importantly, the long-term stability of metal-based catalysts is usually rather poor. Metal-free catalysts have unique advantages such as environmental friendliness, low cost and long-term stability, which are promising materials for CTL-based sensors. Herein, we fabricated a CTL sensor based on metal-free catalyst. F-doped cage-like SiC was synthesized via wet chemical etching. The as-prepared product showed a rapid, stable, highly selective and sensitive cataluminescent response to H2S. The stability of the sensor was demonstrated to be fairly good for at least 15 days. After CTL tests, F-doped cage-like SiC remained original morphology, structure and chemical composition. And for all we know, this is the first report of metal-free CTL sensor. Metal-free catalysts are environmental friendly, low cost and long-term stable, which may open a new avenue of CTL sensing.

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1. INTRODUCTION Cataluminescence (CTL) is chemiluminescence emitted on the surface of solid catalysts during a catalytic oxidation reaction in oxygen atmosphere.1-2 CTL sensor is one of the most charming and effective tools for chemical analysis, owing to its efficient selectivity, high sensitivity, and rapidity.3-8 Metal-based catalysts, such as ZnO,9 Mn3O4,10 Co3O411 are employed to accelerate the oxidation reaction of analytes in CTL-based sensors and sensor arrays, which gain great success in the detection of volatile organic compounds12-14, inorganic gas.15-18 Meanwhile, real-time in vivo clinical analysis is a new strategy based on CTL. For instance, Hu and coworkers have developed a real-time sevoflurane (SVF) sensor for exhaled breath based on nano-SrO, which is potential useful in liver disease diagnosis.19 However, metal-based catalysts in CTL sensors also bring high cost and environmental pollution of heavy metals. More importantly, the long-term stability of metal-based catalysts is usually rather poor. The sensitivities of CTL sensor are often decreased after long-term usage. For instance, copper oxides are easily poisoned to copper sulfides after long-term usage, which brings a great challenge for hydrogen sulfide (H2S) sensing.20 Metal-free catalysts, which emerge as green catalytic materials, have drawn extensive attention in recent years. As a type of metal-free catalysts, nanocarbon catalysts have the advantages of environmental friendliness, low cost, high selectivity and long-term stability, compared with metal-based catalysts in many catalytic processes.21 By far, many carbon materials have been used as metal-free catalysts, such as activated carbon,22 graphite,23-25 carbon nanotubes26 and carbon nanofibers.27 Silicon carbide (SiC), as one of the most promising carbon material, has distinct catalytic potential, thanks to its remarkable physical-chemical characteristics such as medium surface area, high thermal conductivity and high mechanical strength.28-30 To improve the catalytic property, 3



controllable morphology and chemical doping are two important ways for the synthesis of SiC. On one hand, hierarchical structures have an ultra-high specific surface area which can enhance their catalytic activities. SiC materials with one-, two-, and three-dimensional structures such as wires,31 tubes,32 films33 and hollow spheres34 have been fabricated by many synthetic methods including carbothermal reduction,35 template-directed process,36-37 chemical vapor deposition,33 laser irradiation38 and arc discharge.39 On the other hand, chemical doping in SiC using heteroatoms (e. g. B,40 N,41 O42 and Al43) is a common approach to effectively modulate its surface properties. Target gas molecules are prone to adsorb on the surface to improve its catalytic activity. Fluorination a effective chemical way to modify materials’ surface properties, owing to the high electronegativity of fluorine atoms. And strong bonds are formed easily between fluorine and materials.44-45 Therefore, advanced SiC materials combining ion doping and morphology modification are promising metal-free catalysts for CTL-based sensors. Herein, we fabricated a CTL sensor based on metal-free catalyst. F-doped cage-like SiC was synthesized via wet chemical etching. Pores were introduced into the prepared SiC products and the specific surface areas reached 170 m2/g. More importantly, the incorporation of F dopant modified the overall electronic properties and its catalytic activity. Owing to unique characteristics mentioned above, the as-prepared material was successfully applied in CTL sensor. H2S, a kind of toxic, obnoxious smell gas, was analyzed as a model analyte in the proposed metal-free CTL sensor. F-doped SiC nanocages showed a rapid, stable, highly selective and sensitive cataluminescent response to H2S. The sensor remained stable for at least 15 days. To our best knowledge, this is the first report of metal-free CTL sensor.

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2. EXPERIMENTAL SECTION 2.1 Materials and instrumentations. All reagents were analytical grade and used without further purification. Raw silicon carbide (SiC) was purchased from Nanjing Aipurei Nano-Material Company (Nanjing, China). HNO3 (65%), HF (40%) were purchased from Tianjing Kermel Chemical Reagent Company (Tianjin, China). Na2S.9H2O, H2SO4 (98%) were purchased from Chengdu Kelong Chemical Reagent Company (Chengdu, China). Water was obtained and further purified by a Milli-Q water purification system (ULUPURE, Chengdu, China). Kipp's apparatus was selected as the preparation device of H2S. Na2SO4, Na2S9H2O and dilute H2SO4 were chosen as desiccant and reactants in sequence. Firstly, Na2SO4 and Na2S9H2O were added in Kipp's apparatus under vacuum state. Then, a sufficient amount of dilute H2SO4 was added to the above solid by micrometer syringe. After reaction, the entire preparation device was full of pure H2S. 2.2 Synthesis of F-doped SiC nanocages. Schematic illustration of F-doped SiC nanocages was shown in Fig. S1. 0.5 g 3C-SiC powders with average sizes of 40 nm used as precursor was added in a Teflon-lined stainless steel autoclave. 2.5 mL HNO3 (65%), 7.5 mL HF (40%) were added into the solid above and ket at 100 °C for 2 h. After cooling to room temperature, black products were washed by distilled water and treated with ultrasonic vibration to avoid gather. Then, the treated SiC was immersed in HF (40%) for 24 h to remove remaining SiO2, and the resulted products were obtained by centrifugation and washed by distilled water until neutral. Afterwards, the products were dried in a vacuum oven at 80 oC for 12 h. Owing to simple construction, low power consumption and ambient working conditions, dielectric barrier discharge (DBD) plasma was applied to remove amorphous carbon. And its reactor system was set up based on our previous study.17 The black products were placed in the DBD reactor and atmospheric pressure plasma was emitted under an 5



input voltage of 70 V, using air as working gas. The DBD plasma device schematic was shown in Fig. S2 After DBD plasma treatment, the color of products changed to yellow. 2.3 Characterization and apparatus. X-ray diffraction (XRD) data was obtained on an X’Pert Pro X-ray diffractometer (Philips) using Cu Kα (λ = 1.5406 Å) radiation to characterize the structure of the as-prepared products. The size and surface morphology of as-prepared SiC were investigated by scanning electron microscopy (SEM, Hitachi, S3400). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were acquired on a transmission electron microscope (Tecnai G2F20 S-Twin, FEI Co., America) at an accelerating voltage of 200kV. X-ray photoelectron spectroscopy (XPS) were carried out on an X-ray photoelectron spectrometer using monochromatic Al Kα (1361 eV) to analyze the surface composition of the products. Specific surface area was measured by the Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption method on a micromeritics surface area analyzer (Quantachrome instrument). The pore size distribution was measured by the Barrett-Joyner-Halenda (BJH) method. Raman spectra were tested on a Raman Micro-Scope (Lab RAM HR) with a 632.8 nm laser. Fourier transform infrared (FT-IR) spectra was recorded on a Nicolet IS10 FTIR spectrometer (Thermo Inc., America). 2.4 Cataluminescence sensing measurements. The CTL responses of the as-prepared materials towards H2S were investigated by a CTL system based on our previous study.10 Briefly, 0.03 g SiC sample and ethanol were mixed to get a suspension. Then cylindrical ceramic heater was covered with SiC suspension and dried in vacuum oven at 80 °C. Subsequently, the cylindrical ceramic heater was placed into a quartz tube (100 mm × 10 mm i. d. × 12 mm o. d.) in our previous study. While air was used as carrier gas, gaseous H2S was driven into the quartz tube and rapidly oxidized catalytically on the surface of as-prepared material. And air flow rate was controlled by the precise 6


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flowmeter. The temperature of cylindrical ceramic heater was adjusted by a voltage regulator. At the same time, CTL emission was monitored by a photomultiplier tube (Hamamatsu, Japan). And the CTL intensity was obtained by a BPCL ultra-weak luminescence analyzer (Institute of Biophysics, Academia Sinica, Beijing, China). Data integration time was set at 0.1 s per spectrum, and the work voltage was set as -850 V.

3. RESULTS AND DISCUSSION 3.1 Characterization of F-doped SiC nanocages. The compositions and structures of prepared products were characterized via X-ray diffraction (XRD). The XRD patterns of raw SiC, etched SiC and DBD treated SiC were exhibited in Fig. 1a. Three strong diffraction peaks of samples could be well corresponded to the (111), (220) and (311) facets of cubic SiC with lattice constants a = b= c = 4.359 Å (JCPDS card no. 29-1129). This phenomenon illustrated that the incorporation of F dopant didn't change the phase structure of 3C-SiC. After removing amorphous carbon from etched products by DBD plasma, XPS was obtained to investigate surface compositions and chemical state of SiC samples. As-prepared products had Si, C, O and F four elements. The Si 2p XPS spectra were shown in Fig. 1b. The peaked located at 100.8 eV was Si-C bonds. And the bind energy peak at 103.8 eV was assigned to SiO2, owing to oxidizing SiOxCy during the plasma treatment, and similar experimental phenomenon has been stated by Victor et al.46 It could be seen that a small peak emerged at 102.5 eV , which belonged to Si-F bonds.47 The C 1s spectrum presented in Fig. 1c revealed that the peaks at 292.1 eV, 283.7 eV and 283.2 eV, assigned to C-F, SiOxCy and C-Si bonds, respectively.48 Moreover, we investigated the F 1s spectrum (Fig. 1d), and two peaks at 686.9 eV and 689.3 eV belonged to F-Si bonds and F-C bonds, 7



respectively. It was obvious that F atoms were incorporated into the SiC lattice to form Si-F bonds and C-F bonds. At the same time, we acquired the XPS data of etched sample. As displayed in Fig. S3, we found that the etched products possess Si-F, C-F bonds. It was caused by the reason that F atoms were incorporated into the SiC lattice to form Si-F bonds, C-F bonds, during the wet etching process. After removing amorphous carbon from etched product by DBD plasma treatment, F atoms also remained in the SiC nanostructures in the form of Si-F bonds, C-F bonds. According to XPS data, the atomic percent of F in the etched products was 5.31%. After removing amorphous carbon from etched product, the atomicpercent of F rose to 11.45%. It was possible that the mass of sample reduced after cleaning amorphous carbon, and F content increased. Meanwhile, FT-IR spectra were obtained to check the XPS results, which was exhibited in Fig. S4. The morphology of the as-prepared products was exhibited by SEM and TEM techniques. The SEM images of etched SiC, DBD treated SiC and raw SiC were shown in Fig.2, and we can find that raw SiC nanostructures exhibited grain shape with inhomogeneous size, and the average size was about 40 nm (Fig. 2a). After wet etching process, the morphology changed enormously to cage-like, with loose and porous surface (Fig. 2b, 2c). Moreover, removing amorphous carbon by plasma treatment didn't change the morphology of SiC nanostructures (Fig. 2d). And the single nanocage morphology of etched SiC and DBD treated SiC were shown in the TEM images (Fig. 2e, 2f). The HRTEM images of these two samples were exhibited in Fig. 2g, 2h. Both of them were highly crystallized. And the crystalline interplanar spacing was 0.252 nm, which was belonged to (111) plane of cubic SiC. And the growth mechanism was explored by fabricating samples at different acid volume ratios, reaction temperatures and reaction times (Fig. S5-S9).

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To further explore the structure properties of as-prepared materials, the specific surface area, pore volume and pore size were exhibited as Fig. S10. According to the IUPAC classification, all isotherms belonged to type IV with H3 hysteresis loops at high P/P0. The BET specific surface area (SBET) of raw SiC was 42 m2/g, with a pore volume of 0.16 cm3/g. After etching process, the SBET increased to 193 m2/g, with a pore volume of 0.70 cm3/g, which was more than four times enlarged compared with that of raw SiC. Hence, the etching process brings SiC larger specific surface area and pore volume. After removing amorphous carbon by DBD plasma, the SBET reduced to 170 m2/g. Amorphous carbon maybe make contributions to the specific surface area, owing to strong adsorbability. After cleaning it, the specific surface area reduced a little. Besides, the pore size increased to 9.3 nm after plasma treatment, which was larger than that of etched SiC. It may be caused by the reason that removing amorphous carbon by DBD plasma opens the plugged pores. 3.2 Construction of CTL-based H2S sensor. Hydrogen sulfide (H2S), a kind of colorless, toxic, obnoxious smell gas, is released from crude oil, natural gas industries, coal gasification, metal smelting process as well as food processing industries.49 H2S affects several different systems in the human body even at low concentration. When the concentration of H2S is higher than 250 ppm, human body will face rapid death.50 Since Zhang’s groups first reported CTL-based H2S gas sensors with Fe2O3 nanoparticles in 2004,51 various researches have emerged to fabricate specific sensing materials as H2S sensors with high sensitivity and good selectivity. On one hand, previous studies have indicated that morphological and structural features of sensing materials was closely related to their CTL performance. For instance, Zhang et al. had demonstrated that the controllable α-Fe2O3 nanotubes showed more superior CTL activities for H2S detection than α-Fe2O3 nanoparticles.52 Similarly, our group have controllably synthesized In2O3 hierarchical hollow microsphere and ZnO 9



enclosed hollow tubular, respectively.18, 53 On the other hand, the synthesis of composites based on metal oxides has demonstrated to be an efficient way to enhance the CTL performance. Na et al. adopted a simple one-step way to synthesize SnO2-carbon nanotube (CNT) nanocomposites which could be used as catalysts for H2S recognition.54 Additionally, our previous study has certified g-C3N4 as catalyst support to prepare α-Fe2O3/g-C3N4 composites and g-C3N4-Mn3O4 composites, which showed highly selective and sensitive cataluminescent response towards gaseous H2S.16, 17 To date, most researches have focused on the improvement of sensitivity and selectivity. However, the long-term stability of metal oxides or composites based on metal oxides is still a great challenge, because metal oxides are easily poisoned to metal sulfides after long time using. Therefore, it is imperative to prepare novel long- term stable sensing materials for the detection of H2S. Based on the unique characteristics of F-doped cage-like SiC, we used it as sensing material to detect H2S. CTL response curves based on SiC samples were exhibited in Fig. 3a. It was obvious that F-doped SiC nanocages showed a good response to H2S. And the CTL relative intensity was about eight times higher than that of raw SiC. Meanwhile, the CTL relative intensity of F-doped SiC nanocages was higher than etched SiC, after removing amorphous carbon from etched products by DBD plasma. We found treating raw SiC by DBD plasma only caused a slight enhancement of CTL response, owing to SiO2 layer. In order to find out a suitable SiC material for H2S sensor, the CTL responses to H2S gas based on the products etched using different acid volume ratios, different reaction temperatures and different reaction times were optimized at the same test conditions. In Fig. 3b, the CTL response to H2S was best when the acid volume ratio of HF and HNO3 was 3:1. Besides, the CTL responses varied with the reaction temperatures, and when reaction temperature was 100 °C, the CTL response was best (Fig. 3c). While optimizing the reaction time (Fig. 3d), the CTL response 10


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was optimum when the reaction time was 2 h. Therefore, we chose the product synthesized in the reaction condition of VHF : VHNO3 =3:1, 100 °C, 2 h and removing amorphous carbon by DBD plasma, as the suitable sensing material for H2S detection. To acquire high performance of F-doped SiC nanocages as sensing material, CTL test conditions were optimized. The operating wavelength of CTL emission was firstly investigated by optical filters in the range of 400-575 nm (400, 425, 440, 460, 490, 535 and 575 nm). In Fig. 4a, with the increase of wavelength, both CTL signal and signal to noise (S/N) decreased, and the maximum CTL wavelength was around 400 nm. To investigate the influence of flow rate, we tested the responses to 24.3 ppm H2S and the results were presented in Fig. 4b. While air flow rate increased from 0.05 to 0.1 L min-1, the CTL intensity increased distinctly. However, the CTL intensity declined at higher flow rates. This could be explained by the fact that a lower flow rate makes the oxidation process occur under diffusion-controlled conditions. Based on the results above, 0.1 L min-1 was selected as optimal flow rate. As a major parameter, temperature would play a significant role on the capability of CTL sensing material. As shown in Fig. 4c, the signal and S/N ratio sharply rose with the increase of temperature and we didn’t find the optimal temperature. It was possible that the relative CTL intensity increased, and the background signal produced by the heat radiation changed a little under 400 nm filter. For safety reasons, 298 °C was chosen for subsequent studies. 3.3 Analytical figure of merits. Analytical characteristics of cataluminescent H2S sensor based on F-doped SiC were observed under optimal experimental conditions. Fig. 5a showed CTL spectra and the responses to a series of different H2S concentration. And the calibration curves of CTL intensities versus H2S concentrations was exhibited in Fig. 5b. A linear relationship was acquired between the response and the concentration of H2S in the range of 6.1-30.4 ppm, and the limit of detection (S/N = 11



3) was 3.0 ppm. The linear regression was I = 23.77C - 67.516 (R = 0.9992), where I was average relative CTL intensity and C was the concentration of H2S. Besides, fast response and recovery time are key parameters to a gas sensor. The CTL response profile of F-doped SiC towards H2S at three different concentrations can be seen in Fig. 5c. The response and recovery time were less than 0.6 s and 1.0 s, respectively. Meanwhile, the high selectivity was also necessary for sensing materials in practical application. It was obvious that the as-prepared product demonstrated a high selectivity towards H2S by investigating a series of possible interfering substances including ethanol, isobutanol, tert butyl alcohol, n-pentanol, methanol, acetone, cyclohexanone, diacetone, n-heptane, epichlorohydrin, formic acid, acrylic acid, 1-thioglycerol, 1-dodecanethiol, carbon tetrachloride, methylbenzene, and formaldehyde, with the same concentrations of 64 ppm (Fig. 5d). And there was no evident interference for H2S sensing. The high selectivity towards H2S may ascribe the following reasons. (1) Possible interfering volatile organic compounds (VOCs) can’t occur catalytic oxidation reaction on the surface of F-doped SiC nanocages. (2) VOCs may be oxidized, but light emitting from excited intermediates possibly can’t be detected by BPCL ultra-weak luminescence analyzer. (3) F-doped SiC nanocages have showed a certain response towards H2S, 1-thioglycerol and 1-dodecanethiol. And we infer that -SH groups may have some influence on the CTL selectivity. Gas molecules with -SH groups may absorb easily on the surface of materials. Due to the steric effect, VOCs molecules with -SH groups may cause weaker adsorption than H2S molecules. Therefore, F-doped cage-like SiC was a promising sensing material to selectively detect H2S. Meanwhile, F-doped SiC nanocages also showed good reproducibility towards H2S. The relative standard deviation (RSD) of CTL response is 4% (Fig. S11).

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The long-term stability of the sensor based on F-doped SiC nanocages has been examined by continuously working at 298 °C for 15 days with 24.3 ppm H2S passing through the sensing material (Fig. 6). We found that the CTL intensity remained stable within 15 days. After many CTL tests, the used SiC was collected and characterized by XRD, FT-IR and SEM. The XRD patterns (Fig. 7a) showed that the material kept the original structures after CTL experiment. FT-IR was measured to investigate surface compositions and chemical state of materials (Fig. 7b). After CTL measurement, the F-doped SiC nanocages had similar spectra to that before CTL measurement. The SEM images were shown in Fig. 7c, Fig. 7d, and the morphology of material kept same to that before CTL measurement. These results above demonstrated SiC was a recyclable sensing material for H2S detection. 3.4 Possible mechanism. The possible cataluminescent reaction is a following process. O2 molecules from carrier gas were adsorbed on the surface of F-doped SiC nanocages. H2S molecules were subsequently carried into the reaction cell by air and catalytically oxidized on the surface of materials. Excited SO2* were the possible intermediate catalytic products which were generated during the catalytic oxidation reaction of H2S. Subsequently, strong luminescence emitted when SO2* returned to the ground state. It is well accepted that SO2* has the main emission peak near 400 nm, which is consistent with experimental optimal wavelength of CTL emission.51 And the luminescence intensity of SO2* is linear with the concentration of H2S. Compared with raw SiC, the obvious CTL response enhancement of F-doped SiC may ascribe the following reasons. Firstly, the specific surface area increases sharply via etching process, and more active sites can adhere to SiC surface. Secondly, based on pore size data (Fig. S10), the pore size increases from 3.8 nm to 9.3 nm after DBD plasma treatment, and porous SiC possibly absorb more H2S and O2 molecules to enhance 13



the CTL response. Thirdly, surface moieties have great influence on H2S sensing. Based on the FT-IR and XPS data, Si-O bonds exist on the surface of F-doped SiC nanocages, owing to oxidizing SiOxCy during the plasma treatment. Covalent bonds or van der Waals force may exist between Si-O bonds and O2 molecules, which will bring greater adsorption of O2 molecules on the surface of material. Finally, doping SiC with fluorine atoms produces more defects, which are characterized by Raman spectra. According to Sennik et al., these defects may increase the gas sensing properties.55 The high electronegativity of the surface fluorine also brings greater adsorption of O2 molecules on the favorable sites at fluorine atoms of the materials.56-57 Therefore, high specific surface area, porous nanostructures, surface moieties and F dopant are possibly crucial factors for the enhancement of CTL response.

4. CONCLUSIONS In summary, a CTL sensor was designed based on metal-free catalyst. F-doped cage-like SiC was successfully synthesized by wet chemical etching. During wet chemical etching process, fluorine atoms were incorporated into the SiC nanostructures in the form of Si-F bonds and C-F bonds. As-prepared product can be used as a superior CTL catalyst for H2S gas sensing, with a highly selective and sensitive response. More importantly, we successfully improved the long-term stability of the CTL-based sensor by using metal-free catalyst. The F-doped cage-like SiC has superior lifetime for H2S detection. After CTL tests, it remained original morphology, structure and chemical composition. Metal-free materials may intrigue great interests in the field of gas sensing, due to their environmental friendliness, low cost and long-term stability.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following figures are displayed: Schematic illustration (Fig. S1). Dielectric barrier discharge (DBD) plasma device schematic (Fig. S2); XPS spectra of etched SiC (Fig. S3); FT-IR data (Fig. S4); The SEM images of SiC etched by pure acid, HNO3 or HF (Fig. S5); The SEM images of SiC etched by different acid volume ratios (Fig. S6); The SEM images of SiC etched by different reaction temperatures (Fig. S7); The SEM images of SiC etched by different reaction times (Fig. S8); Raman spectra (Fig. S9). BET, BJH and pore volume. (Fig. S10); Reproducibility towards H2S (Fig. S11).

AUTHOR INFORMATION Corresponding authors Email: [email protected]; [email protected]; Tel. & Fax +86-28-8541-2798 Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOELEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21675113 and 21375089) and Science & Technology Department of Sichuan Province of China (2015JY0272). We thank Prof. Yingying Su and Dr. Shanlin Wang in Analytical & Testing Center of Sichuan University for technical assistance. 15



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Figure 1. (a) XRD patterns, black: raw SiC, blue: SiC etched by the mixture of HF and HNO3 (HF:HNO3 = 3:1), red: remove amorphous carbon from etched products by DBD plasma; (b)-(d) XPS spectra of as-prepared SiC after removing amorphous carbon, (b) Si 2p (c) C 1s (d) F 1s.

Figure 2. SEM images, (a) raw SiC, (b) (c) SiC etched by the mixture of HF and HNO3 (HF:HNO3 = 3:1), (d) removing amorphous carbon from etched products by DBD plasma. TEM images, (e) etched SiC, (f) removing amorphous carbon from etched product. HRTEM pattern of samples, (g) etched SiC, (h) removing amorphous carbon from etched product.

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Figure 3. (a) Response curves of the CTL sensor based on raw SiC, raw SiC treated by DBD plasma, etched SiC and F-doped SiC nanocages; (b)-(d): CTL behavior of SiC synthesized at different conditions, (b) acid volume ratio, (c) reaction temperature, (d) reaction time. Conditions: wavelength, 400 nm; temperature, 298 °C; air flow rate, 0.1 L min-1 and the concentration of H2S, 24.3 ppm.

Figure 4. The CTL response of F-doped SiC nanocages towards 24.3 ppm H2S at different (a) wavelengths, (b) air flow rates and (c) temperatures.

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Figure 5. (a) Typical CTL response to a series of different H2S concentration. (b) Calibration curves of CTL intensities versus H2S concentrations. I: the average relative CTL intensity, C: the concentration of H2S. (c) The CTL response and recovery time at different H2S concentrations (400 nm, 298 °C, 0.1 L min-1). (d) Selectivity towards H2S based on F-doped SiC nanocages. Conditions: wavelength, 400 nm; temperature, 298 °C; air flow rate, 0.1 L min-1 and the concentration of H2S, 24.3 ppm.

Figure 6. The stability of the sensor based on F-doped SiC nanocages. Conditions: wavelength, 400 nm; temperature, 298 °C; air flow rate, 0.1 L min-1 and the concentration of H2S, 24.3 ppm.

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Figure 7. (a) XRD patterns of F-doped SiC nanocages, black: before CTL test, red: after CTL test. (b) FT-IR spectra of F-doped SiC nanocages, red: before CTL test, black: after CTL test. (c)-(d) SEM image of F-doped SiC nanocages, (c) before CTL test, (d) after CTL test.

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for TOC only

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