The effect of excitation light source and humidity to photocatalytic activity of g-C₃N₄ nanosheets for NO removal

Nội dung text: The effect of excitation light source and humidity to photocatalytic activity of g-C₃N₄ nanosheets for NO removal

1. Science & Technology Development Journal, 24(2):1924-1932 Open Access Full Text Article Research Article The effect of excitation light source and humidity to photocatalytic activity of g-C3N4 nanosheets for NO removal Tran Hoang The Vinh1,2, Huynh Cam Tu1,2, Pham Van Viet1,2,* ABSTRACT Introduction: Photocatalysis using nanostructured semiconductors is the potential strategy to solve the problem of environmental pollution. Besides traditional semiconductor materials, the Use your smartphone to scan this novel polymeric metal-free semiconductor g-C3N4 has emerged as a potential substitute mate- QR code and download this article rial because of its many outstanding features. Methods: This study successfully synthesized two dimensions (2D)-structured g-C3N4 nanosheets by a simple thermal-exfoliation method with an- nealing route at 2 oC/min. Firstly, melamine was placed in a ceramic crucible with cover and then o undergone the annealing route at 550 C for 2 h to develop into the g-C3N4 bulk. Then the as- o synthesized g-C3N4 bulk was further annealed without the cover at 550 C for 2 h to form the final product, g-C3N4 nanosheets. Results: The results of XRD patterns and FTIR spectra show two typ- ical diffraction peaks and chemical bonds that characterize3 theg-C N4 matrix. The TEM images demonstrated that the as-prepared g-C3N4 possesses 2D-structured material, including several singly exfoliated sheets with a width of around several hundred nanometers. The photocatalytic NO removal efficiency of g-C3N4 nanosheets is highest at 48.27% under 30 min solar irradiation at 70% humidity. Meanwhile, the NO2 conversion yield is very low, only 9.44%, much smaller than the NO 1 − Faculty of Materials Science and decomposition efficiency to form3 NO ion products. The results of trapping tests indicated that Technology, University of Science, the hole plays the most critical role in the photocatalytic process of g-C3N4 nanosheets. Especially, VNU-HCM, 227 Nguyen Van Cu Street, the photocatalytic NO removal efficiency still achieves 45.03% after the recycling test. Moreover, District 5, Ho Chi Minh City, 700000, Vietnam all characteristic peaks and chemical bonds in material remain even undergoing fifth times reuse as the results of XRD and FTIR. Conclusion: From various modern analytic characterization meth- 2 Vietnam National University-Ho Chi ods and photocatalytic investigation, we can conclude that g-C3N4 nanosheets are very stable and Minh City, Linh Trung Ward, Thu Duc possible to apply in practical applications to decompose NO gas at atmospheric conditions. District, Ho Chi Minh City, 700000, Key words: g-C3N4 nanosheets, NO removal, solar irradiation, thermal-exfoliation method Vietnam Correspondence Pham Van Viet, Faculty of Materials ysis 5. Therein, the photocatalytic method using the Science and Technology, University of INTRODUCTION Science, VNU-HCM, 227 Nguyen Van Cu In recent years, the accelerating pace of industri- nanostructured semiconductor material has emerged Street, District 5, Ho Chi Minh City, rapidly as an inexpensive, efficient, and environ- 700000, Vietnam alization, modernization, and population explosion has harmed the environment due to the emissions of mentally friendly method that can decompose con- Vietnam National University-Ho Chi Minh taminants at high concentrations 5–7. For instance, City, Linh Trung Ward, Thu Duc District, greenhouse gases such as NO, NO2, SO2, CO, CO2 Ho Chi Minh City, 700000, Vietnam from means of transportation, industrial zones, fac- TiO2, ZnO, SnO2 are traditional semiconductors that Email: pvviet@hcmus.edu.vn tories, etc. 1. Among them, NO is one of the most have been widely studied in the photocatalytic field for many decades. Still, most of them are limited History toxic gases emitted mainly from vehicles and partly • Received: 2021-02-22 from natural phenomena such as volcanic explosions, by a few factors such as large bandgap, small spe- • Accepted: 2021-05-10 thunderstorms 2,3. Moreover, the high concentra- cific surface area, complicated synthesis method that • Published: 2021-05-13 tion of NO gas in the atmosphere will negatively im- have restricted the applicability of the photocatalytic 5,8,9 DOI : 10.32508/stdj.v24i2.2521 pact the environment causing smog formation, pho- method . Recently, the novel material graphitic tochemical smog, and reaction with O3 to puncture carbon nitride (g-C3N4) has been widely studied and the ozone layer 4. Especially, NO gas will directly af- known as a polymeric metal-free semiconductor for fect human health, causing diseases of the liver, respi- use in photocatalytic fields such as wastewater treat- Copyright ratory system, blood vessels, etc. 1,4,5. ment, water-splitting, and pollutant gas treatment 10. © VNU-HCM Press. This is an open- To solve this problem, many research groups world- Thanks to the superior properties of the large spe- access article distributed under the wide have spent a lot of effort in researching to re- cific surface area and narrow bandgap of about 2.7 terms of the Creative Commons move NO gas from the air by methods such as electro- eV, the photocatalytic ability of the g-C N is sig- Attribution 4.0 International license. 3 4 static air purification, chemical oxidation, wet collec- nificantly improved 10. Especially, the morphology tors, physical treatment, biological, and photocatal- of g-C3N4 in 2D-structured nanosheets can optimize Cite this article : Vinh T H T, Tu H C, Viet P V. The effect of excitation light source and humidity to photocatalytic activity of g-C3N4 nanosheets for NO removal . Sci. Tech. Dev. J.; 24(2):1924-1932. 1924
2. Science & Technology Development Journal, 24(2):1924-1932 o the effective area of the photocatalyst that facilitates 550 C for 2 h to develop into the g-C3N4 bulk. Then the reactions between the incident light and the sur- the as-synthesized g-C3N4 bulk was further annealed face of the photocatalyst. In addition, the fabrica- without the cover at 550 oC for 2 h to form the fi- tion of 2D material is feasible because the C-N bond nal product, g-C3N4 nanosheets. The ramping rate of o to form g-C3N4 sheets is the covalent bond that is the annealing route is 2 C/min. The whole material much stronger than the van der Waals weak phys- synthesis process was illustrated in detail, as shown in ical bond between the sheets 10,11. There are many Scheme 1. ways to synthesize g-C3N4 nanosheets, such as phys- ical method, chemical method with acid or base sol- Characterizations of materials vents, ultrasonic exfoliation, etc. However, most of The various modern analytical techniques have been these methods require expensive facilities and are applied, such as X-ray diffraction (XRD, using a not environmentally friendly because of using acid Bruker D8, Advance 5005 with Cu Kα radiation (k and base solvents 11. To overcome this problem, = 0.154064 nm)), Fourier transforms infrared (FTIR) the thermal-exfoliation process is an optimal strategy spectroscopy (Jasco V- 4700 spectrometer), transmis- to form g-C3N4 nanosheets while ensuring environ- sion electron microscopes (TEM, JEM 2100, JEOL, mental safety factors. Japan), and UV-Vis diffuse reflectance spectroscopy In this study, we synthesize g-C3N4 nanosheets by (DRS, JASCO-V550) to investigate the properties of a simple thermal-exfoliation method. First, these materials. Therein, phase composition, chemical modern analytical methods, such as X-ray diffrac- bonds, morphology, and optical properties of the ma- tion (XRD), Fourier transform infrared (FTIR) spec- terials are surveyed by XRD, FTIR, TEM, and DRS, re- troscopy, transmission electron microscopes (TEM), spectively. Furthermore, for evaluating the photocat- and UV-Vis diffuse reflectance spectroscopy (DRS) alytic activity of g-C3N4 nanosheets for NO removal, are used to investigate the properties of the material. we conducted the measurement procedure as our pre- Then, the g-C3N4 nanosheets are evaluated for the vious study under different excitation light sources 12 photocatalytic NO removal ability at 500 ppb (part per listed as shown in Table 1 . In addition, the calcula- billion) of concentration under various conditions of tion formula of photocatalytic NO removal efficiency, irradiation and humidity to find the optimal condi- NO2 conversion, NO oxidation efficiency into green tion to evaluate the applicability in Vietnam. In addi- products, and kinetic reaction rate k value was explic- 8,12 tion, the trapping tests are conducted to elucidate the itly presented in our previous works . Moreover, main factor that contributes to the photocatalytic pro- we also experimented with different humidity condi- tions (70%, 40%, and 20%) to deeply investigate the cess of g-C3N4 nanosheets. Moreover, we also con- duct recycling experiments to evaluate the practical photocatalytic activity of g-C3N4 nanosheets. In ad- applicability of the materials. Finally, a photocatalytic dition, the trapping experiments were also performed in the same procedure as the photocatalytic activity mechanism based on the g-C3N4 nanosheets is pro- posed and described concretely from the results ob- experiment but with the presence of some trappers + − • − tained. such as KI (h ), K2Cr2O7 (e ), and BQ ( O2 ) to clarify key factors in the photocatalytic process. MATERIALS-METHODS RESULTS Chemicals and materials Figure 1a clearly shows two strong peaks located at o o All chemicals were analytical grade reagents and 13.2 and 27.4 of the g-C3N4 nanosheets pattern. In used as received without further purification. Figure 1b, FTIR spectra reveal a marked change in − Melamine (C3H6N6, 99.99%), potassium iodide the region from 1700 to 500 cm 1 after undergoing (KI, 99.99%), potassium dichromate (K2Cr2O7, a thermal-exfoliation process of melamine precursor. 99.99%), p-benzoquinone (BQ, C6H4O2, 99.99%), TEM images (Figure 2a-b) with different magnifica- and deionized water (DI) is extracted from the Puris tions showed that the morphology of the g-C3N4 is Evo-UP Water System. a 2D-structured material including several singly ex- foliated sheets with a width of around several hun- The synthesis procedure of the materials dred nanometers. In addition, we further measure The g-C3N4 nanosheets were fabricated by the and compare the bandgap of g-C3N4 material using straightforward thermal-exfoliation method. De- the UV-Vis DRS method.Figure 3 indicated that the tailly, 1 g melamine was placed in a ceramic crucible bandgap of g-C3N4 bulk and g-C3N4 nanosheets is with cover and then undergone the annealing route at 2.65 eV and 2.77 eV, respectively. The results clearly 1925
3. Science & Technology Development Journal, 24(2):1924-1932 Table 1: Detailed parameters of the excitation light sources Notation of exciting light sources Manufacturer Power (W) Wavelength (nm) Solar-OSRAM OSRAM, Germany 300 280 – 750 Vis-OSRAM OSRAM, Germany 300 380 – 750 Solar-ABET ABET, USA 150 300 – 750 UV-OSRAM OSRAM, Germany 18 254 Figure 1: X-ray diffraction (XRD) pattern (a) and Fourier transforms infrared spectroscopy (FTIR) spectrum (b) of g-C3N4 nanosheets. Figure 2: Transmission electron microscopes (TEM) images of g-C3N4 nanosheets at 200 nm (a) and 100 nm scale (b) in the same shooting position. 1926
4. Science & Technology Development Journal, 24(2):1924-1932 Scheme 1: The synthesis process of g-C3N4 nanosheets through a straightforward thermal-exfoliation method by using melamine as the precursor. Figure 3: UV-Vis diffuse reflectance spectroscopy (DRS) spectra (a) and Tauc plots (b) ofg-C3N4 bulk and g-C3N4 nanosheets. show that g-C3N4 in nanosheets form has a wider tabulated key photocatalytic parameters to compare bandgap than g-C3N4 in bulk form. more visually and clearly with recent studies in Ta- Figure 4a shows the results of the photocatalytic ac- ble 2. As a result, we can easily see that the perfor- tivity of g-C3N4 nanosheets for NO removal under mance of g-C3N4 nanosheets with a single compo- various excitation light sources at a humidity of 70%, nent is quite superior to that of the material combi- consistent with the actual conditions in Vietnam. The nations previously studied for NO degradation. Af- results showed that the photocatalytic NO removal ef- ter identifying that the photocatalytic efficiency of g- ficiency of g-C3N4 nanosheets is the highest at 48.27% C3N4 nanosheets is the highest under the solar irra- for the case of Solar-OSRAM. Meanwhile, the pho- diation, we continue to investigate how the humid- tocatalytic NO removal efficiency of the remaining ity in the atmosphere will affect the photocatalytic cases is very low at 37.52%, 8.27%, and 7.26% for Vis- process under the solar irradiation, as shown in Fig- OSRAM, Solar-ABET, and UV-OSRAM, respectively. ure 4c. The results showed that after reducing hu- In addition, the NO2 conversion yield of the Solar- OSRAM case is very low at 9.44%, which is approx- midity to 40% and 20%, the photocatalytic efficiency imately 4.1 times smaller than the NO removal ef- was decreased linearly by 37.52% and 34.18%, respec- − ficiency to green product (NO3 ). Meanwhile, the tively. Besides, the NO2 conversion yield was also in- conversion yield of other cases is very high compared creased significantly by 11.74% and 13.37%, respec- − to NO removal to NO3 (Figure 4b). Moreover, we tively (Figure 4d). 1927
5. Science & Technology Development Journal, 24(2):1924-1932 Figure 4: Photocatalytic NO removal efficiency (a), NO removal efficiency, andNO2 conversion yield (b) in different excitation light sources at 70% humidity, photocatalytic NO removal efficiency (c), NOremoval efficiency, and NO2 conversion yield (d) in different humidity conditions under solar irradiation ofg-C3N4 nanosheets. Table 2: A comparison of photocatalysts for the removal of NO Photocatalyst NO conc. Exciting light Synthesis method Efficiency Ref. (ppb) sources (%) SnO2/PANI 500 OSRAM-300 Hydrothermal 14.43 13 W 14 C-N-S-TiO2 400 Halogen Hydrothermal 25 lamp-300W Ag/ZnO 400 Xenon-300W Microwave-assisted 55 15 one-pot g-C3N4 500 OSRAM-300 thermal-exfoliation 48.27 This study W 1928
6. Science & Technology Development Journal, 24(2):1924-1932 To identify the main factor in the photocatalytic pro- nanosheet, as shown in the results of the TEM images. cess, we conducted photocatalytic trapping tests un- Therefore, the morphology of g-C3N4 is nanosheets der solar irradiation conditions at 70% humidity with with a large effective area that will be favorable for the presence of KI, K2Cr2O7, and BQ to trap factors the photocatalytic process. From the results of mod- + − . − h , e , and O2 , respectively (Figure 5a). The pho- ern analytical methods mentioned above (Figures 1 tocatalytic NO removal efficiency was significantly and 2), we can conclude that g-C3N4 nanosheets ma- declined by 4.56%, 17.31%, and 35.09% in the exis- terial has been successfully synthesized by a simple tence of KI, K2Cr2O7, and BQ, respectively. In ad- thermal-exfoliation method. dition, we also calculate the reaction kinetic k value Figure 3 presents the DRS profile of g-C3N4 bulk and to investigate the photocatalytic activity of g-C3N4 g-C3N4 nanosheets. This result is perfectly reasonable nanosheets quantitatively. As expected, the results as there will be a color change from the bright yellow of Figure 5b indicate that the k value of the addi- of the bulk to the white color of the sheets. More- − tion of KI is 0.004 min 1, which is much lower than over, the previous publications also showed that when −1 another case (0.123 min /pure g-C3N4 nanosheets; the size of the material is reduced, it leads to a quan- −1 −1 0.062 min /g-C3N4 + K2Cr2O7; 0.101 min / g- tum confinement effect that causes the bandgap struc- 18,19 C3N4 + BQ). Moreover, we also investigated the ma- ture to increase . Consequently, the enhancement terial’s durability for practical application by con- in the redox ability of the material by increasing the ducting a reusable experiment 5 times of photocat- bandgap and thereby facilitating the subsequent pho- alyst. Figure 5c shows that the photocatalytic NO tocatalytic reactions. removal efficiency of the g-C3N4 nanosheets is still In Figure 4a-b, the results of the photocatalytic NO very high at 45.05% after recycling tests. Figure 5d removal efficiency of 3g-C N4 nanosheets are high- presents that the NO2 conversion yield increases lin- est under solar irradiation at 70% humidity condi- early from 9.44% to 16.74%, corresponding to the first tion. This result can be explained as due to the high to fifth reuse. In addition, we also measure XRD and power of Osram, 300 W that can provide enough pho- FTIR of material after recycling tests (Figure 5e-f). ton energy to activate the maximum photocatalytic Surprisingly, all the chemical bonds in the g-C3N4 reactions of g-C3N4 nanosheets. In addition, g-C3N4 nanosheets still remain after the fifth times reuse and nanosheets have a relatively narrow bandgap of about 20 the two diffraction peaks characterize the (100) and 2.7 eV . Consequently, g-C3N4 nanosheets can har- (002) planes of g-C3N4 are also preserved. vest light in both visible and UV regions leading to the photocatalytic NO removal efficiency in the case DISCUSSION Solar-OSRAM is the highest. Furthermore, from the Figure 1a shows two typical peaks located at 13.2o and results of Figure 4c-d, it can be confirmed that the hu- 27.4o corresponding to (100) and (002) planes which midity in the air directly influences the photocatalytic characterize for in-plane repeated tri-s-triazine units efficiency of the material. This phenomenon can be 16 and triazine aromatic systems in g-C3N4 structure . explained by the fact that in the photocatalytic pro- In Figure 1b, FTIR spectra showed that after under- cess, the photogenerated hole (h+) will oxidize the ad- going a simple annealing route, melamine precursor sorbed H2O molecules on the photocatalyst surface to • formed more chemical bonds in the range from 1700 form free radical OH that contributes to the photo- − to 1200 cm 1 that characterizes for stretching vibra- catalytic reactions 5. Therefore, when the humidity is tion of C-N heterocycles of g-C3N4 nanosheets struc- reduced, the number of H2O molecules adsorbed on ture 17. In addition, the single peak centered at 810 the photocatalyst surface is low, leading to a decrease −1 • cm in the FTIR spectrum of g-C3N4 reveals the in the amount of OH radicals. Consequently, the bending vibrations of the triazine units, which is one photocatalytic efficiency of the material is decreased 17 of the typical bonds of the g-C3N4 matrix . drastically. TEM images (Figure 2a-b) showed that the morphol- Figure 5a indicated that h+ plays an essential role ogy of the g-C3N4 is a 2D-structured material, includ- in the photocatalytic process, and this result is com- ing several singly exfoliated sheets. The construction pletely consistent with the results of Figure 4c-d. On − • − of the 2D-structured nanosheets is due to the forma- the other hand, e and O2 also play relative roles tion of strong C-N covalent bonds after undergoing an in photocatalytic reactions but not as dominant as h+. annealing route to form the nanosheets. Continuing In addition, we also calculate the reaction kinetic k the annealing treatment process, the bond between value to quantiatively investigate the photocatalytic the nanosheets is the weak physical bond of van der activity of g-C3N4 nanosheets. The reaction kinetic Waals, so they can be easily exfoliated to create a single k value is lowest in the case of KI. This means that 1929
7. Science & Technology Development Journal, 24(2):1924-1932 Figure 5: Trapping tests (a), L-H Fit lines (b), recycling test (c), NO removal efficiency and NO2 conversion yield (d), FTIR spectra (e), and XRD patterns after recycling test of g-C3N4 nanosheets. by trapping h+, the photocatalytic reaction rate has adsorbed O2 and H2O molecules on the photocata- been reduced significantly, or almost none happened. lyst surface (Equations (2) and (3)). The h+ oxidizes • − From the results of Figure 5a-b, we can solidly con- H2O molecules to OH radicals. Meanwhile, e will + • − firm that h plays a key role in the photocatalytic pro- reduce O2 molecules to O2 radicals. These free rad- cess. The results in Figure 5c-d can be explained due icals with high redox activity will decompose NO gas − to the light-shielding effect that reduces the reaction into NO3 , which is a less toxic product than the area between the photocatalyst g-C3N4 nanosheets original NO gas (Equations (4), (5) and (6)). and incident light during the photocatalytic process. ( ) − + After each recycling test, the NO gas is decomposed g − C3N4 → g − C3N4 e + h (1) − into NO3 ions, which will gather on the surface of the photocatalyst, causing the light-shielding effect that prohibits the interaction between the photocata- + → . + lyst and the light, so decreases the photocatalytic ef- H2O + h HO + H (2) ficiency 5. From the results of Figure 5c-f, we can confirm that the g-C3N4 nanosheets have very high durability and can be put into practical application − . − O2 + e → O (3) for photocatalytic NO removal at a high concentra- 2 tion level with many times reuse. From the analyzed results above, we propose a pho- tocatalytic mechanism model for the 2D-structured . → − O2 + NO NO3 (4) g-C3N4 nanosheets under solar light at 70% humid- ity by the following equations (Figure 6). When sun- light is irradiated with an incident wavelength greater − . + than the energy of the bandgap, electrons (e ) will OH + NO → NO2 + H (5) move from the valence band (VB) to the conduc- tion band (CB), leaving a hole (h+)(Equation (1)). − Therefore,+ h will be abundant in VB, and e will − − + . → + be rich in CB. Then, a few e and h will migrate OH + NO2 NO3 + H (6) to the surface of the photocatalyst and react with the 1930
8. Science & Technology Development Journal, 24(2):1924-1932 Figure 6: The proposed photocatalytic mechanism of g-C3N4 nanosheets for NO removal under solar irra- diation. CONCLUSIONS 2D: two dimensions In brief, we have successfully synthesized 2D- XRD: X-ray diffraction FTIR: Fourier transforms infrared spectroscopy structured g-C3N4 nanosheets by undergoing a sim- ple thermal-exfoliation method. This is evidenced TEM: Transmission electron microscopes by the results of XRD, FTIR, TEM, and DRS results. DRS: UV-Vis diffuse reflectance spectroscopy The as-synthesized g-C3N4 nanosheets achieve a very VB: valence band high photocatalytic NO removal at 48.27% under so- CB:conduction band + lar irradiation at 70% humidity thanks to the bene- h : hole − fits of large specific surface area and narrow bandgap e : electron of the material. Moreover, the NO2 conversion yield is very low, only 9.44%, compared to 38.83% of the ACKNOWLEDGEMENTS − decomposition efficiency NO to NO3 ions. In ad- This research is funded by Vietnam National Foun- dition, we also indicated that the h+ plays the most dation for Science and Technology Development critical role in the photocatalytic reaction by trap- (NAFOSTED) under grant number 103.02-2019.343. ping tests. Besides, the photocatalytic NO removal efficiency still reaches 45.03% after fifth times reuse. AUTHORS CONTRIBUTION These results of XRD pattern and FTIR spectrum of Tran Hoang The Vinh: Investigation, Writing - Orig- photocatalyst after recycling tests still show all the inal draft preparation; characteristic diffraction peaks and typical chemical Huynh Cam Tu: Formal analysis; bonds in the g-C3N4 nanosheets original state of the Pham Van Viet: Writing — Review & Editing, Super- photocatalyst before recycling test. From the analysis vision, Funding acquisition. results above, we can confirm that g-C3N4 nanosheets possess a very high practical application potential for CONFLICT OF INTEREST decomposing NO gas at high concentrations under The authors declare that they have no known compet- solar irradiation. ing financial interests or personal relationships that ABBREVIATIONS could have appeared to influence the work reported in this paper. g-C3N4: graphitic carbon nitride 1931
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