RESEARCH PAPER

Triazine-based graphitic carbon nitride: controllable synthesisand enhanced cataluminescent sensing for formic acid

Wei Zhu1& Hongjie Song1

& Yi Lv1,2

Received: 26 June 2018 /Revised: 10 August 2018 /Accepted: 6 September 2018 /Published online: 26 September 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractA novel preparation method of triazine-based graphitic carbon nitride (g-C3N4) and its application in the cataluminescence (CTL)sensing system is proposed in the present work. Netty triazine-based g-C3N4 were synthesized via a solid and mild strategy,utilizing the copolymerization interaction between guanidine hydrochloride and trimesic acid. The chemical structure, morphol-ogy, optical property, and cataluminescence (CTL) sensing characteristics were investigated in detail. Control experiments werecarried out to investigate the CTL sensing characteristics of g-C3N4 towards formic acid, which showed the prepared triazine-based g-C3N4 possessed a superior catalytic activity than that of tri-s-triazine. Meanwhile, the prepared g-C3N4 also showedcommendable catalytic oxidization selectivity towards formic acid. In view of the advantageous features of low cost, environ-ment friendliness, and long-term stability, triazine-based g-C3N4 was chosen as a highly efficient material to design a CTL sensorfor formic acid.

Keywords Graphitic carbon nitride (g-C3N4) . Cataluminescence (CTL) . Volatile organic compounds (VOCs) . Formic acid

Introduction

Graphitic carbon nitrides (g-C3N4) are a kind of two-dimensional (2D) polymeric material which consists exclu-sively of covalently linked sp2-hybridized nitrogen and carbonatoms [1, 2]. Similar to graphene, g-C3N4 has attracted a largenumber of scientific interests due to its unique structure andmorphology. Great efforts have been devoted to exploring thevarious synthesis routes and structures of g-C3N4 for numer-ous applications since the 1830s [3–5]. Graphitic carbon ni-trides are typically synthesized by direct condensation of nu-merous nitrogen-rich and oxygen-free compounds involvingpre-bonded C–N core structures (such as heptazine derivatives

and triazine) through various thermal treatments. Cyanamide[1, 6], dicyandiamide [7–9], melamine [10, 11], urea [12],thiourea [13, 14], guanidinium chloride [15], and guanidinethiocyanate [16] were usually chosen as precursors for thepreparation of g-C3N4. However, the obtained materials wereusually bulk g-C3N4, which had limitations for directly appli-cations in many fields due to the ordinary properties and poordispersity. Controllable synthesis has great potential to enableg-C3N4 with special structures, properties, and functionalities,which have been of particular interest in recent years.Meanwhile, the advantageous features of g-C3N4 (stablephysicochemical properties, high catalytic activities, sim-ple composition, facile synthesis, and low cost) havetherefore stimulated the exploration on applications of g-C3N4 in a wide range. Especially, lots of literatures havereported that g-C3N4 is an attractive platform for develop-ing gas sensors [17–31].

Volatile organic compounds (VOCs) are a major kind ofindoor and industrial air pollutants, which have serious nega-tive effects on human health and the environment. Formicacid, the simplest organic acid, is one of the importantVOCs that have been extensively used in many areas suchas medicine, pesticide, organic synthesis, and tannery industry[32]. Besides, formic acid is an effective hydrogen carrier forhydrogen production [33–35]. Formic acid is highly pungent

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s00216-018-1368-0) contains supplementarymaterial, which is available to authorized users.

* Hongjie Songsonghj@scu.edu.cn

1 Key Laboratory of Green Chemistry & Technology, Ministry ofEducation, College of Chemistry, Sichuan University,Chengdu 610064, Sichuan, China

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

Analytical and Bioanalytical Chemistry (2018) 410:7499–7509https://doi.org/10.1007/s00216-018-1368-0

and corrosive; the inhalation or direct contact of formic acidvapor could hurt the skin, eyes, and lungs, while the inhalationof higher concentrations may result in chemical pneumonitisand even death. The detection of formic acid is of great im-portance in industrial, medical, and environmental settings[36–40]. Sensitive and selective determination of formic acidcan protect worker health, monitor air quality, diagnose healthconditions, limit corrosion of metal components by formicacid, and facilitate the adoption of formic acid as a hydrogencarrier for energy storage. While many works have been doneon detecting formic acid with gas chromatography [41–44], aninexpensive, real-time, sensitive, and selective sensor forformic acid vapor has been less explored. Therefore, it is dra-matically urgent to seek a low-cost, portable, fast, and highlyefficient gas sensor for formic acid vapor.

Cataluminescence (CTL) refers to one class of chemilumi-nescence (CL) that is emitted during the heterogeneous cata-lytic oxidation that happens on a gas–solid interface in anatmosphere containing oxygen [45–47]. CTL has been devel-oped for decades as a relatively novel strategy for VOC gassensors; many efforts have been devoted to CTL-related re-searches and great achievements have been made [48–64].Sensing material is the most critical part of the CTL-basedsensors; exploration on highly efficient catalyst (as sensingmaterials) is always one of the most difficult challenges forthe development of CTL-based sensors. Graphitic carbon ni-trides were found to catalyze the oxidation process of VOCssuch as alkanes, olefins, and alcohols [65], which means g-C3N4 could be utilized as a metal-free, robust, and low-costcatalyst in the field of CTL sensors. Novel morphology of g-C3N4 with abundant porous micro/nanostructures could beobtained via controllable synthesis or modification, whichwould endow g-C3N4 highly improved catalytic activity[66–68] and adsorption capacity [69, 70] for gas molecules.In view of this, g-C3N4 may have a promising outlook forfabricating low-cost, environmentally friendly, highly effi-cient CTL-based sensors. Therefore, the CTL performanceof g-C3N4 was investigated in this paper.

Herein, netty triazine-based g-C3N4 were controlled syn-thesized via the simple copolymerization of guanidine hydro-chloride and trimesic acid. Trimesic acid was chosen to be apromising precursor material which could convert theestablished condensation pathway of g-C3N4 from tri-s-triazine to triazine, tailoring the structure and photoelectricproperties of g-C3N4 (Scheme 1). The novel nanostructuresand netty morphology with abundant porous micro/nanostructures made the obtained triazine-based g-C3N4

promising catalytic materials in CTL sensors. A series of ex-periments were carried out to understand the CTL property oftriazine-based g-C3N4 towards formic acid. In view of thephenomenon that strong CTL emission could be generatedbecause of the excellently selective catalytic oxidization offormic acid on the surface of g-C3N4, we further designed a

novel CTL sensor for formic acid. The CTL response behav-iors, sensing mechanism, analytical performance, selectivity,long-term stability, and so on also were investigated anddiscussed in detail.

Experimental section

Chemical reagents and materials

Guanidine hydrochloride was purchased from ChengduChemical Regent Co. Ltd. Trimesic acid was obtained fromChengdu Best Reagent Co., Ltd. Ethanol was purchasedfrom Chengdu Jinshan Chemical Reagent Co. Ltd. All thereagents above were analytical grade and directly used with-out further purification. Formic acid (≥ 88.0%) was pur-chased from Kelong Chemical Reagent Company(Chengdu, China). Deionized water with conductivity of18.24 MΩ cm−1 was from a water purification system(ULUPURE, Chengdu, China).

Synthesis of graphitic carbon nitride

In a typical synthesis of carbon nitride, a quartz tube of 25mminner diameter and 1000 mm length was used as a reactionchamber. The well-proportioned mixture of guanidine hydro-chloride (6 g) and trimesic acid (1.2 g) was obtained with agiven mass ratio of guanidine hydrochloride to trimesic acid(5:1). After stirring, the resultant solids were put into an alu-mina crucible and placed in the middle region of the quartztube. In order to react completely, the initial materials wereheated in an electric furnace with a rate of 5 °C min−1 up to350 °C, which was maintained for 3.5 h under ambient atmo-spheric conditions. Subsequently, the quartz tube was cooledto ambient temperature. The product was ground into powder

Scheme 1 Schematic illustration of graphitic carbon nitride synthesisroute

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and washed several times with ultrapure water and ethanolalternately. After drying at 80 °C, the carbon nitride materiallabeled as CN-5 was obtained for further characterization andanalytical application. A series of carbon nitrides were pre-pared with different mass ratios of guanidine hydrochlorideto trimesic acid (20:1; 10:1; 5:1) under the same condition,labeled as CN-20, CN-10, and CN-5 respectively. For com-parison, guanidine hydrochloride (6 g) was heated at 350 °Cfor 3.5 h with the absence of trimesic acid under the samecondition; the product was white powder, labeled as CN-0,which was different with g-C3N4 reported by literatures; then,we prepared tri-s-triazine-based carbon nitride (labeled as p-CN) by direct pyrolysis of 6 g guanidine hydrochloride under450 °C for 3.5 h.

Instruments and methods

The surface morphology of as-prepared g-C3N4 was exam-ined using a JSM-5900LV scanning electron microscope(SEM, JEOL). Transmission electron microscopy (TEM)was acquired on a JEM-2100F microscope with a field emis-sive gun, operated at 200 kV. The atomic force microscopy(AFM) images were obtained using an Agilent 5500 atomicforce microscope under tapping mode to characterize thethickness of the prepared materials. XRD patterns were car-ried out using an X’ Pert Pro X-ray diffractometer (Philips)with Cu Kα radiation (λ = 1.5406 Å, 40 kV, 30 mA). X-rayphotoelectron spectroscopy (XPS) was studied on an XSAM800 electron spectrometer (Kratos) with monochromatic AlKα radiation, and elemental analysis was carried out onFlash EA1112. Each data was parallel at least twice and theaverage values were obtained via measuring two kinds ofsample batches. Fourier transform infrared (FT-IR) spectrawere recorded on a Nicolet IS10 FTIR spectrometer(Thermo) with KBr discs. Ultraviolet visible (UV–Vis) diffusereflection spectra were investigated using a UV-2550 UV–Visspectrophotometer (Shimadzu), and the photoluminescenceemission spectra were tested on an F-7000 fluorescence spec-trophotometer (Hitachi). Electron paramagnetic resonance(EPR) spectra were obtained, utilizing a JES FA200 X-bandelectron paramagnetic resonance spectrometer (JEOL). Thespecific surface area was measured with the Brunauer–Emmett–Teller (BET) equation using a Quadrasorb SI auto-mated surface area and pore size analyzer (MicromeriticsGemini VII 2390). The 13C NMR spectra were recordedon a Bruker AVII 600 (600 MHz) Fourier transformNMR spectrometer device. A gas chromatograph–massspectrophotometer (GCMS-QP2010 Plus, Shimadzu) wasused to test the oxidation reaction product of formic acid,and a gas chromatograph (SCION 456, Techcomp) with aflame ionization detector was for the contrast experimentsduring the practical samples analysis.

Cataluminescence sensing measurements

The cataluminescence sensing characteristics of the as-prepared g-C3N4 for formic acid were carried out under aCTL sensing system as described in our previous work [20].The CTL reaction cell connected to a voltage controller washomemade. Briefly, about 0.05 g g-C3N4 was coated as asensing layer on a ceramic tube which could be heated byadjusting the voltage controller, and the ceramic tube wasinserted into a quartz tube (i.d. = 10 mm and length =100 mm) to form the reaction cell. The air, chosen as thecarrier gas, was mixed with formic acid vapor and then flowedthrough a quartz tube, where formic acid was oxidized bychemisorbed oxygen on g-C3N4 at the proper temperature,and the the consequent CTL emission was recorded by aBPCL ultra-weak luminescence analyzer (BPCL-II, Instituteof Biophysics, Academia Sinica). The data integration timewas set at 0.1 s per spectrum and a work voltage of 800 V.The air flow rate was monitored with a flowmeter, and thecatalytic temperature was controlled by transforming the volt-age of a ceramic rod. The resultant gases were collected at theoptimum condition from the CTL sensor by the DMF solventthat was embedded in a cold trap (dry ice), and then wereanalyzed using the Shimadzu QP2010 GC/MS system(Shimadzu Technologies).

Results and discussion

Characterization of g-C3N4

The product CN-0 was white powder (Fig. 1a), while g-C3N4

reported by literatures were usually pale yellow, which meansthe failure of formation of g-C3N4 in CN-0. In contrast, CN-5shows a yellow fluffy morphology (Fig. 1a), which directlydemonstrates the important role of trimesic acid in the processof forming g-C3N4. The morphology of as-prepared g-C3N4

was investigated via SEM. Interestingly, CN-5 displays anordered network crossing structure (Fig. 1c) rather than thetypical layered platelet-like morphology of p-CN (Fig. 1b)or common morphology of g-CN reported in literatures.After ultrasonic treatment, the TEM (Fig. 1d) image exhibitsthe irregular stacking of nanosheets. AFM (see ElectronicSupplementary Material (ESM) Fig. S1) was also performedto identify the secondary assembling of CN-5. The micro-graph revealed that the CN-5 seems to be exfoliated into ho-mogeneous nanosheets via ultrasonic treatment. Four siteswere selected randomly, showing an average thickness ofaround 0.76 nm, less than that of the two layers of carbonnitride reported [71], which further elucidates that CN-5 fea-tured netty textures are assembled with an ultrathin nanosheet.

The coupled structural changes of the involved phase tran-sitions throughout condensation were indicated according to

Triazine-based graphitic carbon nitride: controllable synthesis and enhanced cataluminescent sensing for... 7501

XRD patterns of different products from various synthesisexperiments mentioned above. When the content of trimesicacid increases, the characteristic index peaks of carbon nitrideare gradually formed. As shown in Fig. 2a, the XRD pattern ofCN-5 has two distinct diffraction peaks: the intense diffractionpeak at 2θ = 26.5° that corresponds to an increased interlayerd002 spacing of 0.336 nm, associatedwith the effect of trimesicacid, which is larger than that of p-CN (0.323 nm). The minordiffraction peak, derived from in-plane stacked arrangement,appears around 2θ = 16.3° (0.544 nm); it is close to the exper-imental or simulated diffraction peak of g-C3N4 constitutedfrom s-triazine structure units [72, 73], and far away from thatof tri-s-triazine-based p-CN (2θ = 13.4°, d = 0.661 nm), indi-cating CN-5 possesses s-triazine structure units instead of tri-

s-triazine structure units. The generation of triazine-based g-C3N4 was further confirmed by the FT-IR spectra (Fig. 2b) ofthe samples prepared under different conditions. The absorp-tion peaks at 3160 and 3340 cm−1 of CN-5 are related to thestretching modes of N–H. Additionally, the feature-distinctivestretching mode of C–N heterocycles located at 1200 to1620 cm−1 were observed, alongside the vibration peak at812 cm−1, which also clearly demonstrates the presence ofthe s-triazine units in prepared g-C3N4 [74, 75].

The surface chemical states of the prepared g-C3N4 wereinvestigated by X-ray photoelectron spectroscopy (XPS). Thefull survey scan spectrum (ESM Fig. S2a) further proves theexistence of C 1s and N 1s, along with a weak O 1s peakwithout other impurities. The C 1s spectrum (ESM Fig. S2b)

Fig. 1 a Photographs of CN-5(yellow) and CN-0 (white); bSEM image of p-CN; c SEMimage of CN-5; d TEM image ofCN-5

Fig. 2 XRD patterns (a) and FT-IR spectra (b) of g-C3N4 preparedfrom guanidine hydrochlorideand trimesic acid with variousmass ratios or reactions atdifferent reaction temperatures

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was fitted to two main peaks with binding energies centeringat 287.4 and 284.6 eV, originated from sp2 C atoms in theintegrated carbonaceous environment and sp2-bonded C inN-containing aromatic cycles, respectively. Furthermore, theN 1s spectrum (ESM Fig. S2c) was deconvoluted into threenitrogen species located at binding energies of 398.5, 399.1,and 400.6 eV, which are attributed to the aromatic N atomconnected to two C atoms (C=N–C), C–N–C/H–N–(C)2,and N-(C)3 groups, respectively. Elemental analysis (ESMTable S1) displays CN-5 had a C/N atom ratio of 0.96, higherthan that of P-CN (C/N = 0.72), which proves that aromaticcarbon has been inserted into the layered structure of carbonnitride via copolymerization reaction. This additional carbonwas further confirmed by the solid-state 13C NMR spectra(Fig. 3). In the spectrum, the typical peaks at 13C chemicalshifts of 150 to 175 ppm were detected, corresponding to thebasic building blocks of g-C3N4. Interestingly, new carbonspecies in CN-5 networks can be clearly observed; the broadpeak with the chemical shift centered at 196 ppm suggests theresiduals of carbonyl in aromatic carbon. The intense reso-nance showing a chemical shift of 130 ppm is mainly attrib-uted to the carbon in the phenylene ring [76]. The result de-scribed above agrees reasonably that CN-5 have been graftedwith the aromatic carbon via copolymerization.

The photoluminescence (PL) spectroscopy and UV–Visdiffuse reflectance spectroscopy (DRS) were measured tostudy the optical and optoelectronic properties of the obtainedg-C3N4. Figure 4a shows that both of p-CN and CN-5 featureda broad range of fluorescence emission, and the fluorescenceemission peak is red-shifted from 445 nm associated with tri-s-triazine-based p-CN to 512 nm assigned to CN-5 under330 nm ultraviolet light excitation, which demonstrates theobvious difference of their structure and morphology.Compared to p-CN with the feature of narrow ultraviolet lightabsorption, solid CN-5 has intensely broad absorption in the

UV region with the absorption peak extended into the redregion near 700 nm (Fig. 4b). It means that the CN-5 has awide range of light harvesting, which can be presumably as-cribed to the replacement of nitrogen atom with carbon atomdue to the introduction of trimesic acid.

Cataluminescent sensing for formic acid

The CTL sensing performance of the as-prepared carbon ni-tride was evaluated by the response of the proposed g-C3N4-based CTL system to formic acid with three different concen-trations. As can be seen from Fig. 5a, the dynamic CTL inten-sity increases with the increasing of formic acid concentration,but the response profiles were similar to each other. In addi-tion, the present CTL sensing system has a response time closeto 10 s and a recovery time near to 150 s, which were slowerthan most of the CTL sensor towards VOCs reported previ-ously because the catalytic oxidation reaction of formic acidby carbon nitride was a slow progress. For comparison, theCTL responses of as-prepared netty triazine-based g-C3N4

and p-CN to formic acid with equal concentration under thesame CTL test conditions were investigated (Fig. 5b).Although p-CN is showing a response to formic acid, CN-5displays a greatly enhanced and steady CTL property withmuch higher response intensity and superior sensitivity undersame experimental conditions.

In order to explain the enhanced CTL property of CN-5, theBrunauer–Emmet–Teller (BET) specific surface area and poresize distributions of two materials were characterized by ni-trogen adsorption–desorption isotherm measurements andshown in Fig. 6a. Though the pore size distribution of thetwo materials (insert of Fig. 6a) is of certain difference, itmay be no contribution to the discrepant CTL intensity be-tween them for their tiny surface area of 4.13, and 5.32m2 g−1,respectively. The electron paramagnetic resonance (EPR)spectra (Fig. 6b) of CN-5 and p-CN show a slightly asymmet-ric Lorentzian line at g values of 2.00186 and 2.00221, sug-gestive of a singly occupied orbital in the ground state, whichpossibly makes g-C3N4 to supply electron for oxygen andpromotes the oxidation of formic acid [77]. Meanwhile, theCN-5 presents a stronger EPR signal compared to p-CN, in-dicating triazine-based CN-5 extends the delocalization of theπ-conjugated system by incorporating trimesic acid, whichcan efficiently stimulate exciton dissociation to produce freeelectrons and holes for relevant redox reactions [78]. Thecharacteristic chemical composition and structure morpholo-gy of CN-5 may be responsible for the enhanced CTL re-sponse to formic acid. With this in mind, triazine-based CN-5 as CTL sensing material for formic acid detection was in-vestigated further in this work.

In order to investigate the response mechanism of the pro-posed CTL sensor for formic acid, a GC/MS experiment wasperformed to explore the reaction products and speculate the

Fig. 3 Solid-state 13C NMR for blank (p-CN) and obtained (CN-5) g-C3N4 samples

Triazine-based graphitic carbon nitride: controllable synthesis and enhanced cataluminescent sensing for... 7503

luminescence mechanism during the catalytic reaction offormic acid. Abundant carbon dioxide (CO2) has been detect-ed in the reaction product after catalytic reaction of formicacid; meanwhile, cataluminescence emission could not be ob-served when replacing clean air with high-purity nitrogen.These results demonstrate that oxygen is necessary for thecatalytic reaction of formic acid and CO2 is the main productof the catalytic reaction during which a CTL emission is pro-duced. In view of the broadband emission of excited state CO2

(CO2*) from 340 to 650 nm [79–81], it is likely that CTLradiation is from CO2* intermediates generated during thecatalytic oxidation reaction of formic acid with thechemisorbed oxygen on CN-5.

Several factors that always have important influence onthe detection sensitivity of CTL sensors were investigatedfor acquiring highly effective analytical performance of theproposed CTL sensor. The signal to noise ratio (S/N) of the

CTL sensor was studied under a series of detection wave-lengths from 460 to 620 nm acquired using cutoff filters(ESM Fig. S3). It is clear that the S/N of the CTL sensorwith CN-5 as sensing material reaches the maximum under535 nm. Therefore, 535 nm is preferred for quantitativedetection. In the catalytic process, temperature is one ofthe most critical factors. The sensor based on CN-5 wastested towards 271.1 μg mL−1 formic acid at different tem-peratures ranging from 210 to 300 °C with a flow rate of100 mL min−1. As Fig. S4 (see ESM) showed, the CTLsignal is obviously heightening with the temperature in-creasing, but 253 °C was chosen as the optimal temperatureon account of the higher signal-to-noise ratio (S/N). Theflow rate of carrier gas also played a significant role inCTL reaction. Figure S5 (see ESM) disclosed that the CTLintensity of 271.1 μg mL−1 formic acid reached the maxi-mum at a flow rate of 100 mL min−1, when the reaction

Fig. 5 a Typical CTL response spectra of formic acid at differentconcentrations (air flow rate, 100 mL min−1; catalytic reactiontemperature, 253 °C); b response curves of the CTL sensor based on

CN-5 and p-CN to formic acid under the same tested condition (air flowrate, 100 mL min−1; catalytic reaction temperature, 253 °C)

Fig. 4 The photoluminescence (PL) spectroscopy (a) and UV–Vis diffuse reflectance spectroscopy (DRS) (b) of CN-5 and p-CN

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temperature was 253 °C. It had been demonstrated that asuitable flow rate could lead to a faster cycle of determina-tion owning to the formic acid molecules contacting withoxygen rapidly and sufficiently at the flow rate. However,the CTL response descended with a further increasing of theair flow rate, which might be caused by the insufficientreaction time. Therefore, the optimal air flow rate was100 mL min−1, which was used in the followingexperiments.

The analytical characteristics of the CTL sensor based onprepared netty carbon nitride had been evaluated under opti-mal conditions. Figure 7a clearly exhibits that the sensor has alinear CTL response to formic acid in the concentration rangeof 43.38~216.88 μg mL−1; the regression equation is I =2.6646C + 37.8565, with the correlation coefficient (R) of0.9997 (insert of Fig. 7a), where I is the average relativeCTL intensity in three replicate tests at the same concentrationlevel andC is the concentration of formic acid vapor. The limit

of detection is 28.58 μg mL−1 (S/N = 3). The relative standarddeviations (R.S.D.) of the signal intensity at different formicacid concentrations shown in Figs. 5 and 7a are all less than5%, which implies the good repeatability of this recommend-ed CTL sensor.

The selectivity of gas sensors is also another vital param-eter for practical applications. Hence, several common for-eign substances, including ethanol, methanol, n-propanol,formaldehyde, acetaldehyde, acetone, propionaldehyde,butone, ethylacetate, benzene, carbon tetrachloride, chloro-form, acetic acid, hydrogen sulfide, and carbon disulfide, athigher concentrations than formic acid, were investigatedunder the same CTL detecting conditions. From Fig. 7b, itcan be found that the as-prepared netty triazine-based car-bon nitride has a special CTL response towards formic acidwith excellent selectivity, indicating that the obtained car-bon nitride is a good candidate sensing material for CTLdetecting formic acid.

Fig. 6 a N2 adsorption–desorption isotherms of CN-5 and p-CN (insets: pore size distributions calculated by the BJH method). b EPR spectra of CN-5and p-CN in the dark

Fig. 7 a Typical relative CTL spectra versus formic acid with a series ofdifferent concentrations (the inset is the calibration curve and the linearregression equation). b The selectivity towards formic acid of the

established CTL sensor based on CN-5 (air flow rate, 100 mL min−1;catalytic reaction temperature, 253 °C)

Triazine-based graphitic carbon nitride: controllable synthesis and enhanced cataluminescent sensing for... 7505

The XRD characterization of triazine-based g-C3N4 afterthe CTL reaction process was conducted to study the dura-bility of this sensing material. The XRD pattern (ESM Fig.S6a) of triazine-based g-C3N4 after the CTL reaction pro-cess shows that the material retains its original structureafter the CTL sensing experiments. The long-term stabilityof the designed sensor based on g-C3N4 was examined byworking continuously at 253 °C for about 50 h over 7 dayswith 51.35 μg mL−1 of formic acid passing through thesensing material (ESM Fig. S6b). We found that the CTLintensity remained stable over 7 days, with the relative stan-dard deviations (R.S.D.) of CTL signal intensity recordedevery hour of less than 5%. These results demonstrate thatthe proposed CTL sensor is a recyclable sensing system forformic acid detection.

In addition, the application of the proposed system wasinvestigated with practical samples determination. Three sam-ples were detected simultaneously by the proposed CTL sen-sor and the well-established method (gas chromatography,GC). The developed CTL method was validated with theGC method for formic acid determination, and data compari-son is shown in Table 1. The results from this CTL methodwere in accordance with those obtained by the GC method,which exhibits that the designed CTL sensor has relativelygood precision and feasibility.

Conclusions

In summary, we prepared netty g-C3N4 via the thermal co-polymerization of guanidine hydrochloride and trimesic ac-id under a mild reaction condition. Trimesic acid wasproved to be a promising precursor material which couldconvert the established condensation pathway of g-C3N4

from tri-s-triazine to triazine, tailoring the structure andphotoelectric properties of g-C3N4. A series of experimentsindicated that triazine-based g-C3N4 can be served as anefficient metal-free catalyst for the catalytic oxidization offormic acid. The proposed CTL sensor with metal-free cat-alyst was reported for the first time to detect formic acid,which will inspire great interest in the field of CTL sensingfor formic acid. In view of the features of low cost, environ-ment friendliness, and long-term stability, the proposedCTL sensor with triazine-based g-C3N4 as sensing materialshas potential applications for detecting formic acid.

Acknowledgements The authors gratefully acknowledge the financialsupport from the National Natural Science Foundation of China [Nos.21405107 and 21575093] for this project. The authors also appreciatethe Comprehensive Training Platform of Specialized Laboratory andAnalytical & Testing Center at Sichuan University for characterizationanalysis.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflicts ofinterest.

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Table 1 The result contrast ofpractical samples detection withthe designed CTL sensor and GCmethod

Sample Concentration (μg mL−1)

(CTL sensor)

Concentration (μg mL−1)

(GC method)

Relative error (%)

Sample 1 31.26 ± 1.25 30.12 ± 2.04 3.7

Sample 2 34.31 ± 1.44 32.86 ± 2.13 4.4

Sample 3 42.78 ± 1.89 41.03 ± 2.38 4.2

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