UV-Assisted Cataluminescent Sensor for Carbon Monoxide Based on

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UV-assisted Cataluminescence Sensor for Carbon Monoxide based on Oxygen Functionalized g-C3N4 Nanomaterials Li Li, Dongyan Deng, Shixu Huang, Hongjie Song, Kailai Xu, Lichun Zhang, and Yi Lv Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02532 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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Analytical Chemistry

UV-assisted Cataluminescence Sensor for Carbon Monoxide based on Oxygen Functionalized g-C3N4 Nanomaterials

Li Li, † Dongyan Deng, † Shixu Huang, † Hongjie Song, † Kailai Xu, † Lichun Zhang, † Yi Lv †‡*

†

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

Chemistry, Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

*

Corresponding Author. Email: [email protected]; Tel. & Fax: +86-28-8541-2798

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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) is one of the most important sensing transduction principles for real-time monitoring of atmospheric pollutants. Highly sensitive CTL-based CO detection still remains a challenge because of relatively poor reactivity of CO and low catalytic efficiency of the catalysts. Herein, combining the ultraviolet (UV) light activation and chemical modification of the sensing element, we have successfully established a UV-assisted CTL sensor based on g-C3N4 for gaseous CO with high sensitivity, selectivity and stability. The UV irradiation can efficiently activate the CO molecules and induce the generation of reactive oxygen species (ROS) for CO oxidation. Besides, the carboxyl groups greatly facilitate the chemisorption of CO on functionalized g-C3N4 nanomaterials, thus enhancing the CTL sensitivity. The influences of experimental conditions and the possible catalytic mechanism of CO on functionalized g-C3N4 have been investigated in detail. Under the optimal experimental conditions, the proposed CTL sensor presents a detection limit (3σ) towards CO of 0.008 µg mL-1, which is much lower than the maximum allowable emission concentration of CO in atmospheric condition (0.030 µg mL-1). The UV-CTL system is green, sensitive, stable, and low cost, which possesses great potential application in gas sensing.

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1. INTRODUCTION Nowadays, air pollution is one of the major global problems threatening the life of human beings and other living things. Carbon monoxide (CO) is a poisonous gas which is harmful to public healthy and is also the leading cause of air pollution. As such, highly sensitive detection and effective prevention of CO poisoning in homes or factories is an urgent demand. Despite a variety of sensing methods and devices based on electrics,1-5 optics,6 etc. had been extensively established for gaseous CO, such approaches suffer from several drawbacks including the time-consuming or complicated procedures, matrix effects, and poor sensitivity.7 Thereafter, finding a simple, sensitive and green method for CO sensing is still ongoing. Cataluminescence (CTL) is one of the most important sensing transduction principles for real-time monitoring of atmospheric pollutants,8-11 due to the advantages of simplicity,12-16 high stability,17-22 reversible response and durability.23-25 The gaseous molecules are oxidized on the surface of catalysts and thus emit intense luminescence.26-29 CTL phenomenon has hitherto been reported when CO was catalytically oxidized on precious metal catalysts.30 This offers a new optical approach by using CTL-based sensing system for potential portable CO detection application. Unfortunately, the CTL responses of the reported sensors were rather weak, resulting from the poor reactivity of CO and the inefficiency and deactivation of catalysts. In this case, there is a real need to seek a pathway to expand the application field of this technique. Recently, Zhang’s group described a pioneering work by coupling plasma assisted catalysis (PAC), a very useful technique for decomposition of volatile organic compounds (VOCs), to CTL so as to enhance the sensitivity of CTL-based sensors for the CO detection,31 except for the BTEX (benzene, toluene, ethylbenzene, and xylene)32 and gaseous hydrocarbons.33 Moreover, a CTL sensor based on the catalytic oxidation of CO on Mn/SiO2 nanomaterials can be utilized at ambient condition.34 Inspired by these studies, we hypothesized that ultraviolet (UV) irradiation could also be employed as an effective assistant for CTL sensing of VOCs, since it can provide highly intense light energy with a broad wavelength range of 200-400 nm, which is beneficial for VOCs activation or decomposition. UV irradiation has been widely used for years in the field of water 3



disinfection via reactive oxygen species (ROS) induced inactivation,35 and air cleaning technologies on the basis of photocatalytic oxidation of VOCs.36 Specifically, hydrogen sulfide can be converted to sulfate dioxide directly upon UV irradiation.37 Accordingly, with UV light assistance, it could give sufficient activation energy or a certain ROS needed for the catalytic oxidation of CO molecules on catalysts, thereby enhancing the CTL response of CO even at low temperature, which is of great significance for CO sensing. On the other hand, the sensing materials used in CTL sensors or electrical sensors for CO detection usually involved metal-based catalysts, which usually suffer from the shortcomings of high-cost, environmental pollution and poor stability. In recent years, metal-free catalysts, as known as green catalytic materials, have gained extensive attentions. Graphitic carbon nitride (g-C3N4) materials is considered as one of the most promising metal-free catalysts because of its unique electronic properties, face termination for electron localization, and high specific surface area, it has been widely used in a variety of material science applications,38-39 e.g. as a catalyst for selective oxidation of alkanes,40 olefins,41 and alcohols,42 or as a membrane material for gas storage.43 Under an air atmosphere, the unpaired electrons at the N sites of g-C3N4 might serve as active catalytic sites to capture O2 molecules producing ROS.44 Importantly, it was proven that mesoporous g-C3N4 was successfully employed to interact with various double and triple bond reactants, such as activating C=O,45 rendering g-C3N4 a promising candidate catalyst for CO oxidation. Furthermore, SiC, a similar type of metal-free catalyst, has demonstrated excellent CTL sensing performance for H2S in our recent work,46 which greatly supports the above assumption. Herein, with the assistance of UV irradiation, we report for the first time a CTL sensor for CO using g-C3N4, a green metal-free catalyst, as sensing material. Under the illumination of UV light, CO molecules can be activated by additional energy. The as-prepared g-C3N4 was further chemically modified to be anchored with oxygen functional groups to facilitate the chemisorption of CO molecules, thus promoting the catalytic CO oxidation. According to the possible catalytic mechanism exploration,47-48 oxygen radicals are shown to play a pivotal role in the UV-assisted CTL system. The proposed method displayed superior characteristics of high sensitivity, fine 4


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durability, simple construction, as well as low cost for CO detection. It will possess potential in environment monitoring, or exhaust catalysis researches.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals obtained were of analytical grade, without further purification. The chemicals including melamine, guanidine hydrochloride, trimesic acid, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), as well as HNO3 were commercially available. Deionized water (18.2 MΩ cm−1) was always used during this work. 2.2. Synthesis of materials. Preparation of g-C3N4-tri-s and g-C3N4-s. The preparation of g-C3N4-tri-s was through the traditional method of calcinating melamine in air,49 while the synthesis of g-C3N4-s was via the mean of our laboratory (the details can be seen in supporting information). Preparation of g-C3N4-tri-s-acid and g-C3N4-s-acid. g-C3N4-tri-s-acid and g-C3N4-s-acid were synthesized by the method of HNO3 treatment,50 which can be seen in supporting information. Characterization and apparatus. The data of powder x-ray diffraction (XRD) were gained on a Philips X’pert Pro MPD diffractometer (Philips, Netherlands) making use of Cu Kα radiation source to investigate the structures of g-C3N4-tri-s, g-C3N4-s as well as g-C3N4-tri-s-acid. X-ray photoelectron spectroscopy (XPS) were characterized by an X-ray photoelectron spectrometer which used monochromatic Al Kα (1361 eV) to explore the surface compositions and the information of bonds of the above-mentioned products. Scanning electron microscope (Hitachi, S3400) was applied to study the scanning electron microscopy (SEM) images of g-C3N4-tri-s, g-C3N4-s and g-C3N4-tri-s-acid. Transmission electron microscopy (TEM) images were received on a transmission electron microscope (Tecnai G2F20 S-Twin, FEI Co., Hillsboro, OR) at an accelerating voltage of 200 kV to characterize the particle size and morphology of g-C3N4-tri-s-acid. In-situ DRIFTS experiments were carried out by a Nicolet IZ10 FTIR spectrometer, equipped with a liquid-nitrogen-cooled MCT detector. 32 scans were averaged for each spectrum, which were recorded at a resolution of 4 cm-1. Fourier transform infrared spectra 5



(FTIR) were obtained on a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA). The spectrum of electron paramagnetic resonance (EPR) was gained on a Bruker EMX-plus X-band continue wave EPR spectrometer with subsequent instrument settings: microwave frequency and power respectively as 9.83 GHz and 2.0 mW, in order to explore the probable mechanism in CO catalytic reaction. 2.4. Analytical methods. The CTL characteristics of the proposed CO sensor were investigated on a CTL system once reported51 with the modification of being under UV irradiation (the emission wavelength mainly focused on 365 nm and the power was 400 W). The gaseous CO molecules were firstly inserted by the microinjector and then flowed with air to the cylinder which could be activated by UV-light. Subsequently, the activated CO was oxidized on the surface of the functional g-C3N4 and the CTL signal was recorded using an analyzer (Scheme S1 which could be seen in details in supporting information). 3. RESULTS AND DISCUSSION 3.1 UV-assisted cataluminescence of CO. The effect of UV irradiation on the CTL systems was firstly investigated by employing two structural units of g-C3N4, respectively labeled as g-C3N4-s and g-C3N4-tri-s (Figure S1 and Figure S2), as sensing materials. From Figure S1 the two peaks at 16.3o and 27.5o were the characteristic peaks of g-C3N4-s, while the peaks at 13.2o and 27.3o were the characteristic peaks of g-C3N4-tri-s. Notably, compared to g-C3N4-tri-s, the two characteristic peaks of g-C3N4-s was a little blue shift, and the peak at 27.5o was lower in crystallinity. As we know, the peak at 27.5o is resulted from the lattice plane of 002, representing the accumulation peak of the conjugated aromatic ring, that is, connection orders of the structural unit of g-C3N4. Thereafter, the different connection orders of the structural unit of g-C3N4 leads to the weaker peak at 27.5 o of g-C3N4-s than that of g-C3N4-tri-s. As illuminated in Figure 1, a weak CTL signal can be observed when CO passed through g-C3N4-s or g-C3N4-tri-s catalysts in the absence of UV light. Once upon UV light irradiation, the CTL signals of CO were remarkably enhanced. In addition, the enhanced effect of UV irradiation on CTL response of g-C3N4-tri-s towards CO was superior to that of g-C3N4-s. It may be the reason that the structural unit of g-C3N4-tri-s has more active N sites than g-C3N4-s, which favors the electron transfer and O2 6


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adsorption (more details were discussed in theoretical calculation). These results indicated that UV light played a vital role in the catalytic CO oxidation. It should be noted that there were two peaks for each CO injection in Figure 1c-f. Accordingly, the two peaks might be ascribed to the two Eley-Rideal reaction pathways:52 (1) CO + ·O2-→ CO2 + O, CO can directly react with adsorbed O2 at the N site to form a CO2 molecule and an oxygen atom remains on the surface of g-C3N4; (2) CO + O → CO2, CO continually reacts with the adsorbed O atom to form a CO2 molecule. Therefore, it went through twice luminescence which arose from CO2*, leading to the double peaks. To account for the reasons for the obvious differences of the sensing materials in catalytic CO oxidation, the theoretical calculation was carried out. According to Lin’s studies about nitrogen-doped carbon nanotube as a potential metal-free catalyst for CO oxidation, O2 molecule could be directly chemisorbed and partially reduced on the C-N sites,53 forming the ·O2-, which consequently facilitate the oxidation of CO molecules. That is, the surface electron density on catalysts of these carbon and nitrogen materials has a significant influence on CO catalytic behaviors. Therefore, we employed DFT methods in the Dmol3 software of the Materials Studio package to calculate the electron density on the two sensing materials in our work.54-55 The vital parameters of the electron density including electrophilic Fukui function and nucleophilic Fukui function were calculated. As displayed in Figure 2a, the whole color of nucleophilic Fukui function of g-C3N4-tri-s showed yellow and green with the value ranging from -0.10 to +0.17, whereas g-C3N4-s displayed more yellow within the value between +0.009 and +0.32, which indicated the value of g-C3N4-tri-s was relatively low and the activity of nucleophilic reaction was insufficient; that is, the surface of g-C3N4-tri-s had higher abundant electrons. Further, comparing to g-C3N4-tri-s whose whole color of electrophilic Fukui function demonstrated yellow and green within the value between -0.058 and -0.310, the whole color of electrophilic Fukui function in g-C3N4-s displayed more red and yellow with the value ranging from +0.12 to +0.40 in Figure 2b, showing the value of g-C3N4-tri-s was relatively high and the activity of electrophilic reaction was sufficient, which further demonstrated the higher electron density on the surface of g-C3N4-tri-s. 7



In-situ DRIFTS experiment was performed to validate the theoretical computation results by studying chemical states of the bonding species during the catalytic CO oxidation. As can be seen in Figure S3, with CO flowing through g-C3N4-tri-s or g-C3N4-s, two peaks at 2110 cm-1 and 2180 cm-1 were both detected, corresponding to the stretching vibration of linear and bonded CO on the catalysts.56 The intensities of CO adsorption peaks on g-C3N4-tri-s were much higher than that on g-C3N4-s, revealing CO molecules could be more easily adsorbed on the surface of g-C3N4-tri-s with higher electron density. Moreover, the high electron density could also benefit the electron transfer from g-C3N4 to chemisorbed O2 to produce ·O2- (proved by later discussion in Section of 3.4), which was the key to CO oxidation.57 The better CTL performance of g-C3N4-tri-s may be attributed to the reason that the structural unit of g-C3N4-tri-s has more active sites of N than those of g-C3N4-s, which favors the electron transfer and O2 adsorption. In this case, the UV irradiation have a synergistic effect on promoting the activation and adsorption of CO molecules, and thus generate better CTL performance. In view of the superior properties of g-C3N4-tri-s and its CTL response to CO, it is recommendable to be chosen as the sensing element for CO detection. Interestingly, as shown in Figure 3, the CTL intensities were markedly increased after the as-prepared g-C3N4-tri-s was further treated with HNO3 refluxing, and g-C3N4-tri-s-acid displayed the best performance towards CO among the four materials no matter the temperature or flow rate was altered (Figure S4). Compared with those modification methods such as doping with metal58 or nonmetal elements59 and hybridizing with some metal oxides or nonmetal oxides60-61 in previous reports, there are two overwhelming advantages about using acid to modify g-C3N4-tri-s. Firstly, it neither brings in impurities, nor changes the structure of g-C3N4-tri-s during the acid treatment process. Secondly, after the treatment of refluxing with HNO3, the g-C3N4-tri-s contains a large number of functional carboxyl groups, which significantly facilitate CO molecules to adsorb on the surface of g-C3N4-tri-s-acid via hydrogen-bonding, thus resulting in better CTL performance of CO (which can be seen in later discussion). Therefore, in order to develop highly sensitive CTL sensor for CO, g-C3N4-tri-s-acid was employed as sensing material in subsequent studies. 3.2. Characterization of the sensing material. The compositions, structures and 8


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morphologies of g-C3N4-tri-s-acid were characterized by XRD, FT-IR, XPS, SEM, as well as TEM. The XRD pattern of g-C3N4-tri-s-acid in Figure 4a displayed that either relative intensity or the position of the two main characteristic diffraction peaks were well consistent with the standard structural unit of g-C3N4-tri-s (JCPDS no.87-1256), demonstrating that the treatment of refluxing with HNO3 did not change the structures of g-C3N4-tri-s. However, the peak of g-C3N4-tri-s-acid at 27.5o was in higher crystallinity (Fig S1), inferring that the -NH2 and some non-polymer might disappear after refluxing with HNO3. In addition, after the treatment with HNO3, the protonation and the interaction of hydrogen bonding became stronger, leading to the more obvious layer spacing. In order to investigate the chemical groups on surface of g-C3N4-tri-s-acid, FT-IR was carried out. According to the FT-IR spectra in Figure 4b, the characteristic peak at 807 cm-1 was ascribed to the tri-s-triazine unit of g-C3N4-tri-s and the peaks ranged from 1200 to 1600 cm-1 were the stretching vibration of C-N. Meanwhile, the broad peaks at 3000-3500 cm-1 were assigned to the amino and hydroxyl groups and the peak at 1684 cm-1 was the stretching vibration of carbonyl group, which proved that g-C3N4-tri-s-acid was modified with various oxygen functional groups after the treatment of refluxing with HNO3 (Figure S5). XPS characterization was conducted to study the surface compositions and chemical states of the g-C3N4-tri-s-acid. From the whole spectra of g-C3N4-tri-s-acid in Figure S6a, it could be seen that only carbon, nitrogen and oxygen were present, while no other elements especially metal elements were found in as-prepared g-C3N4-tri-s-acid, showing no impurity substances were brought in during the acid treatment process. Nevertheless, the oxygen content in g-C3N4-tri-s-acid was much higher than that in g-C3N4-tri-s, which is in agreement with the results in FT-IR that many oxygen functional groups were introduced. More specifically, Figure S6b showed that the C 1s spectrum of g-C3N4-tri-s-acid included five peaks: the bonds of sp2 C-C (284.6 eV), C–OH bonds (286.7 eV), C=O bonds (287.9 eV), sp2 carbon in N–C=N bonds (288 eV) as well as O=C-OH (289.1 eV). The peaks at 398.4 eV, 400.0 eV and 400.1 eV in Figure S6c were respectively attributed to the bonds of C=N–C, C–N–H and N-(C)3. While the peaks at 533.2 eV, 532.3 eV and 531.7 eV in Figure S6d were confirmed to be the bonds of oxygen functional groups of C-OH, O=C-OH and C=O. 9



Meanwhile, SEM and TEM were used to further explore the morphology as well as film-structures of g-C3N4-tri-s-acid. From the images of SEM (Figure 4c), it could be seen that there was a large number of fragments of film-structures. Comparing to g-C3N4-tri-s (Figure S7a), g-C3N4-tri-s-acid showed the relatively thinner and smaller layers. Moreover, the images of TEM in Figure 4d also demonstrated that the film-structures of g-C3N4-tri-s-acid were very small and distributed uniformly. Then it could be concluded that the treatment of g-C3N4-tri-s with HNO3 refluxing not only introduced the oxygen functional groups but also altered the morphology to be smaller and thinner. 3.3. Analytical performance of the CTL-based CO sensor. The effects of experimental conditions including wavelength, temperature and flow rate on the CTL performance of as-prepared g-C3N4-tri-s-acid toward CO were optimized in detail. As seen in Figure 5a, both the CTL signal and the signal-to-noise (S/N) ratio gradually increased and then decreased with the increasing wavelength, and the optimal wavelength was 460 nm. In terms of temperature, an important parameter for CTL sensor, it displayed that both the CTL signal and S/N reached maximum at 187 oC (Figure 5b). It might be because a lower temperature resulted in insufficient energy for CO and oxygen to chemisorb on g-C3N4-tri-s-acid, while a higher temperature might lead to desorption. The moderate temperature of 187 oC was not only energetic for the chemisorption of CO and O2 on g-C3N4-tri-s-acid but also beneficial for the catalytic CO oxidation. Thus, 187 oC was chosen as the optimal temperature. In addition, we tested the influence of flow rate on CTL response. As shown in Figure 5c, the CTL signal increased drastically with flow rate in the range of 0.05-0.1 L min-1. This can be explained that a lower flow rate allows the oxidation process to occur effectively under diffusion-controlled conditions. However, the CTL signal gradually decreased at higher flow rates, which may arise from insufficient reaction time between O2 and CO as well as the dilution of CO. Therefore, 0.1 L min-1 was selected for further study. Under the optimal experimental conditions, analytical characteristics of the proposed CTL CO sensor based on g-C3N4-tri-s-acid were studied. Representative and reproducible CTL signals towards CO with different concentrations were present in Figure 6a. The calibration curve in 10


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Figure 6b showed a linear range of 0.0625-6.25 µg mL-1, and a limit of detection (LOD) of 0.008 µg mL-1, which was much lower than the maximum allowable emission of CO concentration (0.030 µg mL-1). Additionally, fast response and recovery time, high selectivity, as well as high stability are several key features for a gas sensor. The CTL response profiles of g-C3N4-tri-s-acid towards CO at three different concentrations were shown in Figure 5d. The three CTL profiles were highly identical, and the response and recovery times were less than 0.7 and 30 s, respectively. As illustrated in Figure 6c, a high selectivity for CO detection was obtained based on the different CTL signals for CO, methane, ethane, propane, butane, formaldehyde, acetaldehyde, etc. Last but not the least, according to the interval test of 2 days within 14 days (RSD < 3%, n=21), it could be seen that the detection of CO was of well stability (Figure 6d). Moreover, the XPS spectra of g-C3N4-tri-s-acid in Figure S8 showed that the material remained its original chemical states after the CTL reaction, indicating that g-C3N4-tri-s-acid could be recyclable and used as a sensing material for CO detection. In order to explore the potential application of the proposed CTL-based CO sensor in environmental analysis, an artificial CO sample containing methane, ethane, propane, butane, formaldehyde, acetaldehyde, etc, was tested. As shown in Figure S9, the signals of CO between the pure CO sample and the artificial sample showed no remarkable differences. In other words, the CTL-based CO sensor has a great promise to be applied in real environment for routine analysis. 3.4. Possible mechanism. Catalytic oxidation of CO, hydrocarbons, and other pollutants has gained much attention for their potentials in industrial and environmental controls. To date, various carbon-based metal-free catalysts have yet been explored to be a green alternative to precious metal catalysts as well as to promote the reaction efficiency and catalysts stability. For instance, porous graphene oxide was reported to catalytically oxidize amines to imines with enhanced catalytic activity, which was attributed to the synergistic effect of the carboxyl groups of graphene oxide and the site of margin with the generation of radical of ·O2-.62 In addition, Li et al. reported the micro-porous g-C3N4 as a catalyst for the selective catalytic oxidation of the α-H of toluene wherein the radical of ·O2- played an important role.63 Then apart from the results of 11



the previous studies, we therefore propose that the reactive oxygen species might play a pivotal role in our UV-assisted CTL system, considering some ROS can be generated under UV irradiation64 and the unpaired electrons at the N sites of g-C3N4 might represent active catalytic sites to capture and activate O2 molecules. To better understand the catalytic active parts of the g-C3N4 and to provide evidence for the existence of ·O2- when exposed to UV irradiation, we used EPR technology to characterize the process of catalytic CO oxidation using DMPO as a particular target molecule to detect the radical of ·O2-. Experimental results were illustrated in Figure S10, from which strong EPR signals of DMPO/·O2- adduct were clearly observed in the presence of g-C3N4-tri-s-acid, verifying the existence of ·O2-. Meanwhile, the signal increased with UV irradiation in the presence of g-C3N4-tri-s-acid, showing the significance of UV light, which was in accordance with the phenomenon in Figure S11. It can be seen that CO after the reaction under the UV irradiation made lacmus liquid red, while there was no variation in lacmus liquid in the absence of UV. However, there were nearly no signals related to ·O2- without g-C3N4-tri-s-acid. When CO gas molecules passed through the CTL system in the presence of UV, the EPR signals decreased dramatically, which indicated the involvement of ·O2- in the CO oxidation process. The effect of surface oxygen functional groups of the material on promotion of the CO oxidation was further evaluated by testing the performances of g-C3N4-tri-s-acid and g-C3N4-tri-s (Figure S12). As expected, the intensities of EPR signals of DMPO/·O2- adduct in g-C3N4-tri-s-acid system remarkably surpassed that of in g-C3N4-tri-s system, indicating more ·O2- radicals were produced in the former system. In view of these observations, we can speculate that ·O2- involved and acted as a major oxidant for CO oxidation. Notably, the enhanced catalytic activity could be attributed to the synergistic effect of UV irradiation and the carboxyl groups on the surface of g-C3N4-tri-s-acid, in which UV irradiation can generate active oxygen species as well as allow CO to be activated and the carboxyl groups can render more CO to be easily adsorbed on the surface of material. This is the reason for the better CTL performance of g-C3N4-tri-s than others. Thus, a possible mechanism of catalytic CO oxidation in this work was proposed as followings (Figure 7). 12


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On one hand, CO molecules were activated with higher energy by UV irradiation, which subsequently combined with the functional carboxyl groups on g-C3N4-tri-s-acid to form the electron-donor compounds via hydrogen-bonding, similarly to the adsorption on the carboxyl groups of graphene oxide.62 On the other hand, the unpaired electrons at the N sites of g-C3N4-tri-s-acid might represent active catalytic sites, which could capture as well as activate O2 via continuous electron transport, reducing O2 to ·O2-. Then ·O2- reacted with the adsorbed CO and generated the intermediates of excited CO2*. Subsequently, strong luminescence emitted when the CO2* returned to the ground state. 4. CONCLUSIONS In this work, a novel UV irradiation-assisted CTL sensor was constructed for CO sensing. Oxygen functionalized g-C3N4 nanomaterials served as an efficient metal-free catalyst for the catalytic oxidization of CO. With the assistance of UV irradiation, the detection sensitivity toward CO was dramatically enhanced. The enhanced catalytic activity might be attributed to the synergistic effect of UV irradiation and the oxygen functional groups of g-C3N4-tri-s-acid. The UV-CTL system is green, sensitive, stable, and low cost, which possesses great potential for gas sensing and monitoring.

ASSOCIATED CONTENT Supporting Information Schematic diagram of CO detection (Scheme 1); XRD patterns of g-C3N4-tri-s and g-C3N4-s (Figure S1); XPS spectra of g-C3N4-tri-s and g-C3N4-s (Figure S2); In-situ DRIFTS of CO adsorbed on g-C3N4-tri-s and g-C3N4-s (Figure S3); The comparison CTL performance of different materials towards CO. (Figure S4) FT-IR spectra of g-C3N4-tri-s-acid and g-C3N4-tri-s (Figure S5); XPS spectra of g-C3N4-tri-s-acid (Figure S6); SEM images of g-C3N4-tri-s, g-C3N4-s and g-C3N4-tri-s-acid (Figure S7); The XPS of g-C3N4-tri-s-acid before and after reaction with CO (Figure S8); The application for the artificial sample analysis (Figure S9); EPR spectrums of DMPO spin trapping adducts of ·O2- in the UV-assisted CTL system (Figure S10); The variation 13



of lacmus liquid in the absence or presence of UV after the reaction of CO (Figure S11) and EPR spectrums of DMPO spin trapping adducts of ·O2- in the UV-assisted CTL system (Figure S12). AUTHOR INFORMATION Corresponding authors Email: [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 21575093) 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.

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Figure 1. CTL signals obtained by different systems with three parallel measurements. (a) UV background; (b) UV with CO background; (c) without UV on g-C3N4-s; (d) without UV on g-C3N4-tri-s; (e) with UV on g-C3N4-s; (f) with UV on g-C3N4-tri-s. Flow rate: 100 mL min-1, temperature: 187 °C, concentration: 0.31 µg mL-1.

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Figure

2.

The

calculations

carried

out

by

employing DFT

methods in the Dmol3 software of the Materials studio package to investigate the electron density on the two materials of g-C3N4-tri-s and g-C3N4-s (a) electrophilic Fukui function; (b) nucleophilic Fukui function.

19



Figure 3. The comparison CTL performance of different materials towards CO. Flow rate: 100 mL min-1, temperature: 187 °C, concentration: 0.31 µg mL-1.

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Figure 4. (a) XRD pattern of g-C3N4-tri-s-acid; (b) FT-IR spectra of g-C3N4-tri-s-acid; (c) SEM image of g-C3N4-tri-s-acid; (d) TEM image of g-C3N4-tri-s-acid.

21



Figure 5. (a) CTL spectra emission on g-C3N4-tri-s-acid. Flow rate: 100 mL min-1, temperature: 187 °C, concentration: 0.31 µg mL-1. (b) Temperature dependence of the CTL intensity and S/N. Wavelength: 460 nm, flow rate: 100 mL min-1, concentration: 0.31 µg mL-1. (c) Flow rate dependence of the CTL intensity and S/N. Wavelength: 460 nm, temperature: 187 °C, concentration: 0.31 µg mL-1. (d) Temporal profiles of the CTL emission to CO sensor based on g-C3N4-tri-s-acid at three different concentrations: 0.25 µg mL-1,0.31 µg mL-1 as well as 1.88 µg mL-1. Origin software was used to smooth the profiles. Error bars stand for ±SD (standard deviation).

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Figure 6. (a) Typical results of the reproducibility of CTL responses to CO with a range of different concentrations. (b) The calibration curve between the average CTL intensity and CO concentration at optimal working conditions. Wavelength: 460 nm, flow rate: 100 mL min-1, working temperature: 187 °C. (c) Selectivity of CO sensor on g-C3N4-tri-s-acid. The concentration of 0.31 µg mL-1 was CO, accompanying with a battery of materials (the concentration were also about 100 µg mL-1) (d) The stability of CO sensor based on g-C3N4-tri-s-acid.Wavelength: 460 nm, flow rate: 100 mL min-1, working temperature: 187 °C, the concentration of 0.31 µg mL-1 was CO. Error bars stand for ±SD (standard deviation).

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Figure 7. Schematic representation of CO oxidation mechanism on the functional g-C3N4.

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For TOC only

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UV-Assisted Cataluminescent Sensor for Carbon Monoxide Based on

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