- Email: [email protected]
MnV2O6 heterojunction photocatalyst for the removal of methylene blue and indigo carmine
Journal Pre-proofs Research paper A Novel g-C3N4/MnV2O6 Heterojunction Photocatalyst for the Removal of Methylene Blue and Indigo Carmine M. Nithya, S. Vidhya, Keerthi PII: DOI: Reference:
S0009-2614(19)30813-9 https://doi.org/10.1016/j.cplett.2019.136832 CPLETT 136832
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
5 September 2019 1 October 2019 7 October 2019
Please cite this article as: M. Nithya, S. Vidhya, Keerthi, A Novel g-C3N4/MnV2O6 Heterojunction Photocatalyst for the Removal of Methylene Blue and Indigo Carmine, Chemical Physics Letters (2019), doi: https://doi.org/ 10.1016/j.cplett.2019.136832
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
A Novel g-C3N4/MnV2O6 Heterojunction Photocatalyst for the Removal of Methylene Blue and Indigo Carmine M. Nithya, S. Vidhya, Keerthi* *Department of Chemistry, CEG Campus, Anna University, Chennai-25, India ABSTRACT Composite of g-C3N4/MnV2O6 heterostructure were synthesized by facile hydrothermal method. The obtained photocatalysts with different ratios were characterized using X-ray diffraction (XRD), Scanning electron microscope (SEM), Fourier transform infrared spectrophotometer (FTIR), UV-Visible Diffuse reflectance spectroscopy (UV-Vis DRS) and X ray photoelectron spectroscopy (XPS). Among all composites 1:1 g-C3N4/MnV2O6 exhibited best photodegradation performance than pure g-C3N4 and MnV2O6 for Methylene blue and Indigo carmine dyes degradation. It showed 95% and 94% degradation for Methylene blue and Indigo carmine dyes respectively. The scavenging studies showed that among the oxidants formed during photocatalysis, hole played a major role in methylene blue degradation. Keywords: Heterojunction, Photodegradation, MnV2O6, Indigo Carmine,
Corresponding author mail id: [email protected]
1. INTRODUCTION Water pollution created by organic pollutants has attracted worldwide attention due to its serious threat to the environment [1]. Main sources of water contamination include waste water discharge from industries, agricultural activities, municipal wastewater, environmental and global changes, but most important pollutant is considered to be the textile industry in India. Textile industry is one of the primary sectors which generate more amounts of effluents containing toxic dye and organic compounds [2]. Textile industry wastewater containing various types of dye is considered to be potential carcinogens and is dangerous to the human health and environment. Several physical, chemical, and biological techniques are generally applied to remove the organic pollutants from wastewater, but are not economic and efficient. Recently, AOPs using semiconductor based heterogeneous photocatalysis has been proven to be an efficient green technique for remediation of environmental pollution by utilizing solar light to remove water pollutants [3-5]. So far, enormous research has deliberated on photocatalytic processes using titanium dioxide (TiO2) as a conventional photocatalyst. But the practical application of TiO2 is greatly restricted by its poor solar energy conversion efficiency due to its wide band gap of approximately 3.2 eV [6]. Therefore, urgent need to find out efficient photocatalyst having broad visible-light absorption, separation of photogenerated charge carriers and high stability with reusability should be addressed. The narrow-band gap (2.7 eV) of two-dimensional (2D) graphitic carbon nitride (g-C3N4) semiconductor can be harnessed for the degradation of organic pollutants under visible light irradiation. At the same time, the advantages of nontoxicity, easy to synthesize, inexpensive and sustainable nature, high thermal and chemical stability, makes g-C3N4 as a suitable candidate for solar-energy conversion [7-10]. Even though g-C3N4 has versatile properties, the major disadvantage is the fast charge recombination which can be improved by metal doping, Heterojunction and Z scheme formation [11-14]. Metal vanadates, such as lanthanide vanadate, aluminium vanadate and bismuth vanadate exhibit good catalytic activities for the treatment of organic compounds [15-17]. Among these metal vanadates, Manganese Vanadate has attracted great research interest for the photocatalytic degradation of organic pollutants owing to its large specific surface area, unique electron properties, reduction ability, high stability and narrow band gap making it a good photocatalyst 2
to degrade organic pollutants by absorbing visible light [19-23]. Nowadays, heterojunctions are studied for more efficient dye degradation application [24-26]. Therefore to achieve higher photocatalytic performance p-type MnV2O6 was combined with n-type g-C3N4 to form p-n heterojunction which could be made into efficient photocatalyst compare to the individual photocatalysts. 2. Experimental 2.1 Materials Manganese nitrate heptahydrate (Mn(NO3)3.6H2O) was purchased from Sigma-Aldrich and Melamine, Ammonium metavanadate (NH4VO3), ethanol and distilled water were purchased from Sisco Research Laboratories (SRL) Private Limited, India. The entire chemicals purchased were used without any extra purification. 2.2 Preparation of g-C3N4 10 g of melamine is weighed and is transferred into a crucible with lid and it was heated to 520°C and kept for 2 h in a muffle furnace with heating rate of 8°C/min. After cooling, the obtained light yellow colored sample was ground into powder. 2.3 Preparation of MnV2O6 The calculated amount of manganese nitrate heptahydrate was weighed and then dissolved in 32ml of distilled water by stirring. The calculated amount of ammonium metavanadate was weighed and then dissolved in 32 ml of distilled water separately by stirring at 80°C. To the manganese nitrate solution, the above solution was added drop wise and stirred for 30 minutes. Then above solution was transferred into a 100 ml Teflon lined autoclave and maintained at 200°C for 8 hours. The autoclave was cooled then returned to room temperature naturally. Finally the products were washed several times with water and ethanol to get black colored product. Finally, the products were dried in hot air oven at 80 °C for overnight. 2.4 Preparation of g-C3N4/MnV2O6 composite
3
The desired quantity of g-C3N4 was dispersed in 32ml of water and this solution was ultrasonicated for 30mins. Then calculated amount of manganese nitrate heptahydrate was weighed and then it was added to the above solution, which was dissolved by stirring. The calculated amount of ammonium metavanadate is weighed and then dissolved in 32ml of distilled water by stirring at 80°C. To the g-C3N4 solution the above solution was added drop wise and stirred for 30 minutes. Above solution was transferred into a 100 ml Teflon lined autoclave and maintained at 200°C for 8 hours then cooled to room temperature naturally to get brown color product. The precipitates collected by centrifugation were washed several times with water and absolute ethanol. Finally, the products were dried in hot air oven at 80°C for 12 h. Different ratio of samples with various amount of g-C3N4 and MnV2O6 (1:0.5, 1:1, 1:1.5) were synthesized following the above procedure. 2.5 Characterization of Photocatalyst To observe the formation and properties of prepared g-C3N4/MnV2O6 photocatalyst, several analyses were conducted. X-ray Diffraction spectra of g-C3N4, MnV2O6 and gC3N4/MnV2O6 photocatalysts were recorded in D8 advance Bruker instrument with Cu Kα radiation (kα =1.5418 Å) with scanning step width of 0.02°, scanning rate of 10°/minute and range of 10-90°. Fourier transform infrared (FTIR) spectra of photocatalysts were recorded in 400-4000cm-1 range by using ABB MB 3000 instrument using KBr pellets technique. UV Visible diffuse Reflectance spectra of photocatalysts were recorded in 200-2500cm-1 in JASCO UV-Visible spectrophotometer. The SEM images of the prepared composite photocatalyst were analyzed using scanning electron microscopy (VEGA 3 TESCAN, SEM). X-Ray photoelectron spectroscopy (XPS) was carried out using a PHI 5000 VERSAPROBE II SCANNING ESCA MICROPROBE system with a monochromatic Al Kα source. The Photoluminescence (PL) spectra were provided using a spectrofluorometer (HORIBA Fluoromax plus CP-011) at room temperature with an excitation wavelength of 360 nm 2.6 Evaluation of photocatalytic activity The photocatalytic activity of prepared samples was calculated by using MB and IC as a model pollutant under visible light irradiation. In this photocatalytic process 300W tungsten halogen lamp used as visible light source. For the photocatalytic experiment, 100mg of the 4
prepared catalyst was added into 100 mL of MB or IC aqueous solutions (10 mg/L) in a Pryrex glass reactor with a cooling water jacket. Prior to illumination, the suspensions were stirred for 1h in the dark to attain adsorption-desorption equilibrium at room temperature. A small amount of sample was collected at regular time interval and centrifuged to remove the photocatalyst powders (4000rpm, 10min). The concentration of MB or IC aqueous solution was analyzed using Elico SL159 UV-Vis spectrophotometer at maximum absorption (664 nm for MB and 612nm for IC) at room temperature. The stability of the 1:1 g-C3N4/MnV2O6 composite catalyst was evaluated by reusing the catalyst for three runs for the decomposition of methylene blue under the same conditions. After each run, the catalyst was separated by centrifugation procedure, washed with water and ethanol then dried in oven at 80°C. The chemical oxygen demand (COD) was analyzed by the open reflux potassium dichromate method. 2.7 Scavenging experiment To find the major active species in MB degradation, scavenging experiments were carried out by using benzoquinone (BQ, a superoxide anion radical scavenger, .O2-) Ammoniumoxalate (AO hole scavenger) and isopropanal (IPA .OH radical scavenger). Similar to photocatalytic tests, 10mM of scavengers were added, stirred for 1 h in the dark to attain adsorption and desorption equilibrium between the catalyst and the pollutant. After light irradiation, the suspensions were collected by centrifugation at regular time intervals and then the MB concentration was measured using UV-Vis spectrophotometer. 3. RESULTS AND DISCUSSIONS 3.1 CHARACTERISATION OF PHOTOCATALYST 3.1.1 XRD analysis The crystalline nature of the heterostructured photocatalyst was analyzed by XRD and the results are shown in Fig. 1. The characteristic position of g-C3N4 with two peaks results from graphite structure and tri-s-triazine units which is analogous to the C3N4 [27]. The strongest peak at 27.3° is due to the stacking of the conjugated aromatic system, corresponding to the 002 crystal face and as well as a peak at 13.2°, resulting from the periodic arrangement of the condensed tri-s-triazine units in the sheets, is indexed as (001), corresponding to JCPDS card no. 5
87-1526. The diffraction at 2θ value, 27, 29, 30, 32.5, 39 and 43 corresponds to the diffraction plane 110, 202, 201, 111, 311 and 003 respectively to monoclinic phase of MnV2O6 [JCPDS card-350139]. It was demonstrated that the synthesized composite have crystalline properties of both MnV2O6 and g-C3N4. 3.1.2 FTIR Analysis The FTIR measurement was carried with wavenumber range from 400-4000cm-1. Fig. 2 shows the FTIR response for g-C3N4, MnV2O6 and 1:1 g-C3N4/MnV2O6 respectively. In detail, for g-C3N4 the sharp absorption peak at 809cm-1 corresponds to the typical triazine ring. The peaks at 1640, 1556cm-1 are ascribed to stretching modes of derived repeating units of triazine. Furthermore, the peaks at 1333 and 1242 cm-1 are assigned to stretching modes of the C-N and C-C and broad band around 3200cm-1 is the stretching vibration of the N-H bond. FTIR spectrum of MnV2O6 shows peaks in the region of from 500 to 700 cm-1 corresponding to the symmetric and asymmetric stretching band of V-O-V bonds. The bands at 515cm-1 can be allocated to the stretching mode of Mn-O modes [26]. The FTIR spectra of 1:1 g-C3N4/MnV2O6 composite maintained main characteristic peaks of g-C3N4, and it contains major MnV2O6 peaks. This indicates both g-C3N4 and MnV2O6 are embraced in the composite. 3.1.3 UV DRS Analysis The
solid
UV-Vis
diffuse
reflectance
spectrum
of
g-C3N4,
MnV2O6
and
1:1g-C3N4/MnV2O6 composite is shown in Fig. 3. All photocatalyst exhibited significant absorption in the visible light region. The absorption edge of pristine g-C3N4 is around 445 nm, and MnV2O6 shows broad absorption range of about 600-700nm. The 1:1g-C3N4/MnV2O6 heterostructures exhibit much broader absorption band compared to pure g-C3N4 throughout the entire visible light region. This observation is important, as it indicates that g-C3N4/MnV2O6 composite can be photoexcited under visible-light irradiation to generate more electron-hole pairs and enhance the photocatalytic performance. 3.1.4 SEM analysis The morphology of the synthesised 1:1 g-C3N4/MnV2O6 was characterised using SEM and is shown in Fig. 4. The images show sheet like structure, layer arrangements and some small 6
rods dispersed over the sheets. The layer structure corresponds to the g-C3N4 and small rods to MnV2O6. It was observed that the products were enclosed of both MnV2O6 and g-C3N4 material by showing heterogeneous structure. 3.1.5. XPS analysis XPS analysis of 1:1 g-C3N4/MnV2O6 is shown in Fig. 5. The Fig. 5a shows survey spectrum and it demonstrates the elemental composition of Carbon, Nitrogen, Manganese, Vanadium and Oxygen. Four peaks at 284.6, 285.5 288.2 and 289.7 eV appeared for deconvoluted C 1s spectra (Fig. 5b) which corresponds to graphitic C=C or the cyano-group, adventitious carbon, C-NH2 species and N − C − N coordination in the graphitic carbon nitride respectively [28]. Fig. 5c shows two peaks at 399.1 and 400.69 eV for N 1s due to sp2 N in the triazine rings and bridging nitrogen atoms respectively [29]. The XPS spectra of Mn2+ ion and Mn3+ are adjacent and cannot to be distinguished. Fig. 5d displays the peak for Mn 2p3/2 at 642.16 eV attributed to Mn2+ existence in the composite [30]. Fig. 5e displayed V 2p high resolution XPS spectra, in which two peaks at 517.19 eV and 524.39 eV corresponds to V 2p3/2 and V 2p1/2 respectively wihich is assigned to V5+ ions. The O 1s spectra (Fig. 5f) shows two peaks at 530.3ev and 532.12eV thus indicating the presence of oxygen species like lattice oxygen and hydroxyl moiety at the surface of the nanocomposite. The XPS results further confirmed the successful construction of g-C3N4/MnV2O6 heterostructure. To determine the photocatalytic efficiency, the separation of photoexcited charge carriers is a key factor and thus was investigated by PL spectra measurements. As shown in Fig. 6, around 420 nm, a broad PL band for g-C3N4 was observed, which was attributed to the recombination of the electron and hole with emission energy equal to its bandgap energy [31]. Significant decrease in the PL intensity of 1:1g-C3N4/MnV2O6 was observed. The weaker PL emission intensity suggested the decline in the recombination of the photoexcited charge carriers as a result of the enhanced migration of electron-hole pairs. 3.2
Photocatalytic activity The photocatalytic degradation efficiency of g-C3N4, MnV2O6 and g-C3N4/MnV2O6
composites was evaluated by the degradation of MB and IC under visible light irradiation. Fig. 7b showed the absorption spectrum of MB with different irradiation time for 7
1:1 g-C3N4/MnV2O6 composite. The degradation efficiency of all catalysts was calculated using the following equation. Removal % =
C0 ― Ct C0
x 100
(1)
Where C0 is the MB initial concentration at adsorption equilibrium and Ct is the concentration of MB at time t. Fig. 7a shows the photocatalytic activities of all the synthesized materials. The MB degradation for pure g-C3N4 and MnV2O6 was 47% and 37% respectively. Among them 1:1 g-C3N4/MnV2O6 shows higher degradation efficiency and 98% of MB was degraded within 210 min of irradiation. Further the photocatalytic degradation efficiencies of the composites decreased with the increase in the ratio, which may be due to the surface blocking of MnV2O6 in g-C3N4. Hence, it is clear that the optimum ratio of the composite is 1:1 for the efficient removal of MB. Furthermore to understand the reaction kinetics of the photocatalytic degradation of MB (Fig. 8a), the rate constant k, was calculated from the following equation and the process followed first order reaction: ln(C0/Ct) = kt
(2)
Where, C0 and Ct are the concentrations of the MB solution at time 0 and t, respectively. The rate constant obtained from degradation of methylene blue are given below in the table 1 and this indicates the reaction proceeds via pseudo first order kinetics. The rate constant followed the sequence 1:1> 1:1.5 > 1:0.5 > g-C3N4>MnV2O6 Among all composites, 1:1 ratio composite exhibits highest k value than that of other catalysts. The rate constant (k) of 1:1 g-C3N4/MnV2O6 photocatalyst was 10 times and 9 times higher than that of pure MnV2O6 and g-C3N4 respectively. These results indicate that the combination of MnV2O6 and g-C3N4 is promoting the photocatalytic performance for MB degradation under visible light region. 3.3 Scavenging Studies 8
To study the photocatalytic process and mechanism, the roles of three common active oxidant species, hydroxyl radical (•OH), hole (h+) and superoxide radical (•O2−) are studied (Fig. 8b). Herein, AO, IPA and BQ are used as the scavengers for h+, •OH and •O2−, respectively. The degradation percentage decreases with the addition of AO which shows that h+ plays the major role in the degradation of MB whereas in the case of the IPA and BQ they equally contribute less compared to AO to the degradation of MB dye. This shows that there is a significant enhancement of photocatalytic activity and was attributed to the improved charge separation efficiency due to the electron transfer between the MnV2O6 and g-C3N4. In addition Indigo carmine degradation by g-C3N4, MnV2O6 and g-C3N4/MnV2O6 composites was evaluated under visible light irradiation. Fig. 9b showed the degradation efficiency of 1:1 g-C3N4/MnV2O6. The IC degradation for pure g-C3N4 and MnV2O6 was 68% and 53% respectively but the 1:1 g-C3N4/MnV2O6 composites shows higher degradation performance of 94% than the bare materials. This also confirms that the composites are efficient catalyst for dye degradation applications. The COD analysis is commonly used to measure the oxidizable organic waste present in the wastewater. In the present work COD analysis was done to confirm the mineralization efficiency of the catalysts. Before and after the photocatalysis process COD of the dye solutions was determined by open reflux method. The composite showed reasonable COD removal efficiency for both MB and IC dyes (70% and 73% respectively), than the bare materials (Fig. 9a). These results indicate that the removal of dyes from the solution by photocatalysis process is through degradation rather than decolourization. Chemical stability of the photocatalyst and recycling capacity of photocatalyst is an important property for catalysts. The cyclic study of 1:1 g-C3N4/MnV2O6 composite was evaluated by recycling the photocatalytic degradation of MB experiments under similar conditions. Fig. 9c shows the photocatalytic efficiency of composites after three times of recycling test, indicating that the sample possesses good stability. Comparison of photocatalytic performance of different heterojunction with this work is shown in Table 2. Table 2 describes light sources, type of dye, time taken to degrade and the removal efficiency by different catalysts. The current study of g-C3N4/MnV2O6 heterostructure 9
using visible light as source and time taken to dye degradation is comparable to other reported works. Some materials are showing higher performance, but the cost of the material is higher compared to the present material and has advantage of degrading both anionic and cationic dyes. 4. CONCLUSION The heterostructured photocatalysts composed of MnV2O6 and g-C3N4 have been prepared via a simple hydrothermal method. The formation of a heterojunction structure and strong interface interaction between MnV2O6 and g-C3N4 were characterized by XRD, FTIR, SEM, and UV-DRS. The photocatalytic performance of the synthesized heterostructures was evaluated by degradation of methylene blue and indigo carmine. The heterojunction formation could efficiently enhance the photoexcited charge transfer and suppress the recombination of electrons and holes. Among the entire photocatalysts 1:1 g-C3N4/MnV2O6 exhibited enhanced photocatalytic activity than pure MnV2O6 and g-C3N4. The reaction rate constant k in photocatalytic degradation of MB was found to be about 9 times and 10 times higher than pure g-C3N4 and MnV2O6 respectively. The COD removal efficiency was also superior for the composite material. Moreover, g-C3N4/MnV2O6 heterojunction photocatalysts exhibited good photocatalytic stability and reusability, which are very important for its practical applications. Acknowledgement This work was funded by UGC-Startup-BSR (Project No. F.30-368/2017) and the authors acknowledge the same. The first author acknowledges Anna University for the Anna Centenary Research Fellowship (ACRF)
10
REFERENCES 1. R. P. Schwarzenbach, B. I. Escher, K. Fenner,; T. B. Hofstetter, C. A. Johnson, U. Von Gunten, B. Wehrli, The Challenge of Micropollutants in Aquatic Systems, Science 313 (2006) 1072−1077. 2. K. Singh, S. Arora, Removal of synthetic textile dyes from wastewaters: A critical review on present treatment technologies. Crit. Rev. Environ. Sci. Technol., 41, (2011) 807−878. 3. H. Wang, X.Z. Yuan, Y. Wu, G.M. Zeng, H.R. Dong, X.H. Chen, L.J. Leng, Z.B. Wu, L.J. Peng, In situ synthesis of In2S3@ MIL-125 (Ti) core-shell microparticle for the removal of tetracycline from wastewater by integrated adsorption and visiblelightdriven photocatalysis, Appl. Catal. B: Environ. 186 (2016) 19–29. 4. L. Xu, C. Srinivasakannan, J.H. Peng, L.B. Zhang, D. Zhang, Synthesis of Cu-CuO nanocomposite in microreactor and its application to photocatalytic degradation, J. Alloy. Compd. 695 (2017) 263–269. 5. H. Wang, X.Z. Yuan, Y. Wu, G.M. Zeng, X.H. Chen, L.J. Leng, Z.B. Wu, L.B. Jiang, H. Li, Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr (VI) reduction, J. Hazard. Mater. 286 (2015) 187–194. 6. R.A. Torres, J.I. Nieto, E. Combet, C. Pétrier, C. Pulgarin, Influence of TiO2 concentration on the synergistic effect between photocatalysis and high-frequency ultrasound for organic pollutant mineralization in water, Appl. Catal. B: Environ. (2008) 80 168–175. 7. Aiwu, Chudong, Recent advances of graphitic carbon nitride-based structures and applications in catalyst, sensing, imaging, and LEDs, Nanomicro Lett. 9 (2017) 47. 8. W. Jiuqing, X. Jun, C. Xiaobo, L. Xin, A review on g-C3N4-based photocatalysts, Applied Surface Science, 391 (2017) 72–123. 9. H. Yia, D. Huanga, L. Qina, G. Zenga, C. Lai, M. Cheng, S. Yea, B. Song, X. Rena, X. Guoa, Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production, Applied Catalysis B: Environmental 239 (2018) 408-424.
11
10. Y. Yanga, C. Zhanga, D. Huanga, G. Zenga, J. Huanga, C. Laia, C. Zhoua, W. Wanga, H. Guoa, W. Xuea, R. Denga, M. Chenga, W. Xionga, Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation, Applied Catalysis B: Environmental 245 (2019) 87-99 11. T. Jesty, K.S Ambili, S. Radhika, Synthesis of Sm3+-doped graphitic carbon nitride nanosheets for the photocatalytic degradation of organic pollutants under sunlight, Catalysis Today, 310 (2017) 11-18. 12. C. Jiayi, X. Xinyan, W. Yi, Y. Zhihao, Ag nanoparticles decorated WO3/g-C3N4 2D/2D heterostructure with enhanced photocatalytic activity for organic pollutants degradation, Applied Surface Science, 467, (2019) 1000–1010 13. Ravichandran, Sindhuja, Fabrication of cost effective g-C3N4+Ag activated ZnO photocatalyst in thin film form for enhanced visible light responsive dye degradation, Materials Chemistry and Physics, 221 (2019) 203–215 14. B. Abdullah, T. Muhammad, A. S. A. Nor, Well-designed ZnV2O6/g-C3N4 2D/2D nanosheets heterojunction with faster charges separation via p-CN as mediator towards enhanced photocatalytic reduction of CO2 to fuels, Applied Catalysis B: Environmental, 242 (2019) 312–326. 15. N. Dhachapally, V. N. Kalevaru, A. Bruckner, Metal vanadate catalysts for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine, Appl Catal A., 443 (2012) 111118. 16. Y. Shen, M. Huang, Y. Huang, The synthesis of bismuth vanadate powders and their photocatalytic properties under visible light irradiation. J Alloys Compounds. 496 (2010) 87-292. 17. Y. Hu, F. Luo, F. Dong, Design synthesis and photocatalytic activity of a novel lilaclike silver vanadate hybrid solid based on dicyclic rings of [V4O12]4 with {Ag7}7C cluster. Chem Commun. 47 (2011) 761-763. 18. J. Ren, W. Wang, M. Sheng, Photocatytic activity of silver vanadate with onedimensional structure under fluorescent light. J Hazard Mater. 183 (2010) 950-953.
12
19. Z. Brandon, G. Elijah, A. M. Paul, A small bandgap semiconductor, p-type MnV2O6, active for photocatalytic hydrogen and oxygen production, Dalton Trans. 46 (2017) 10657–10664. 20. Z. Shaoyan, H. Ruisheng, L. Lei, W. Dianyong, Hydrothermal synthesis of MnV2O6 nanobelts and its application in lithium-ion battery, Materials Letters, 124 (2014) 57– 60. 21. M.L. Hneda, J. B. M. Da Cunha, M. A. Gusmao, S.R. Oliveira Neto, J. RodriguezCarvajal, O. Isnard, Low-dimensional magnetic properties of orthorhombic MnV2O6:A nonstandard structure stabilized at high pressure, Physical review, 95 (2017). 22. P. Hongrui, D. Jie, W. Ning, L.Guicun, Facile synthesis of MnV2O6 nanostructures with controlled morphologies, Materials Letters, 63 (2009) 1404–1406. 23. H. Yi, L. Qina, D. Huanga, G. Zenga, C. Laia, X. Liua, B. Lia, H. Wanga, C. Zhoua, F. Huanga, S. Liua, X. Guoa,
Nano-structured bismuth tungstate with controlled
morphology: Fabrication, modification, environmental application and mechanism insight, Chemical Engineering Journal 358 (2019) 480-496 24. Y. Bai, T. Chen, P. Wang, L. Wang, L. Ye, X. Shi, W. Bai, Size-dependent role of gold in g-C3N4/BiOBr/Au system for photocatalytic CO2 Reduction and Dye Degradation. Sol. Energy Mater. Sol. Cells, 157 (2016) 406. 25. L. Shen, Z. Xing, J. Zou, Z. Li, X. Wu, Y. Zhang, Q. Zhu, S. Yang, W. Zhou, Black TiO2 Nanobelts/g-C3N4 Nanosheets Laminated Heterojunctions with Efficient VisibleLight-Driven Photocatalytic Performance. Sci. Rep. 7 (2017) 41978. 26. S. Rajendra Prasad, S. Srikantaswamy, M.R. Jagadish, Abhilash, M.B. Nayan, SolarLight-Induced Photocatalytic Properties of Novel MnV2O6/TiO2 Nanocomposite, Journal of Environmental Science, Computer Science and Engineering & Technology, Section A; 7 (2014) 383-391. 27. S. Tonda, S. Kumar, S. Kandula, V. Shanker, Fe-doped and Mediated Graphitic Carbon Nitride Nanosheets for Enhanced Photocatalytic Performance under Natural Sunlight. J. Mater. Chem. A, 2 (2014) 6772. 28. S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Adv. Mater., 25 (2013) 2452-2456. 13
29. K. Dai, L. Lu, C. Liang, G. Zhu, Q. Liu, L. Geng, J. He, A high efficient graphiticC3N4/BiOI/graphene oxide ternary nanocomposite heterostructured photocatalyst with graphene oxide as electron transport buffer material. Dalton Trans., 44 (2015) 79037910. 30. Y. Liua, Y. Zhanga, J. Dua, W. Yua, Y. Qian, Synthesis and characterization of singlecrystal MnV2O6 nanobelts, Journal of Crystal Growth 291 (2006) 320-324. 31. P. Suyana, K. R. Sneha, B. N. Nair, V. Karunakaran, A. P. Mohamed, K. G. K. Warrier, U. S. A. Hareesh, Facile one potsynthetic approach for C3N4-ZnS composite interfaces as heterojunctions for sunlight-induced multifunctional photocatalytic applications. RSC Adv. 6 (2016) 17800−17809. 32. M. Jian, S. Yanbiao, J. Huang, L. Wang, H. She Jinhui Tong, B. Su, and Q. Wang, Synthesis of Flowerlike g-C3N4/BiOBr with Enhanced Visible Light Photocatalytic Activity for Dye Degradation, Eur. J. Inorg. Chem. (2018) 1834–1841 33. P.S. Konstas, I. Konstantinou, D. Petrakis, T. Albanis, Synthesis, Characterization of gC3N4/SrTiO3 Heterojunctions and Photocatalytic Activity for Organic Pollutants Degradation, Catalysts 8 (2018) 554. 34. J. Singh, Pooja Kumari, S. Basu, Degradation of toxic industrial dyes using SnO2/gC3N4 nanocomposites: Role of mass ratio on photocatalytic activity, Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 136–143. 35. D. Monga, S. Basu, Enhanced photocatalytic degradation of industrial dye by gC3N4/TiO2 nanocomposite: Role of shape of TiO2, Advanced Powder Technology 30 (2019) 1089–109
14
Figure Captions Fig. 1. XRD patterns of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 2. FT-IR spectrum of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 3 UV-Visible DRS spectra of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 4. SEM images of 1:1 g-C3N4/MnV2O6 composite photocatalyst Fig. 5. a) Survey spectrum, High resolution XPS patterns (b) C 1s, (c) N 1s, (d) Mn 2p, (e) V 2p and (f) O 1s of 1:1 g-C3N4/MnV2O6 composite Fig. 6. PL spectra of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 7 a) Photocatalytic degradation of MB in the presence of various composites b)Absorption spectrum of MB with time in the presence 1:1 g-C3N4/MnV2O6 Fig. 8. a) Kinetic fit for the degradation of MB with the as-prepared catalysts, b) Active radical species trapping experiments for the photocatalytic degradation of MB. Fig. 9. a) COD removal efficiency of synthesized catalyst for MB and IC degrdadation b) IC removal efficiency for synthesized catalysts c) Reusability of 1:1 g-C3N4/MnV2O6 photocatalyst toward visible light driven photocatalytic degradation of MB and IC
15
Figures
Figure 1
16
Figure 2
17
Figure 3
18
Figure 4
19
Figure 5
20
Figure 6
21
Figure 7
22
Figure 8
23
Figure 9
24
Table 1. Rate constant value for different photocatalysts in MB degradation Catalysts
Rate constant values
R2
(min-1) g-C3N4
0.0025
0.96
MnV2O6
0.0021
0.98
1:0.5 ratio
0.0116
0.96
1:1 ratio
0.0216
0.99
1:1.5 ratio
0.0075
0.98
Table 1. Comparison of various photocatalysts for dye degradation Catalyst used
Light source
Dye
Time and Removal reference percentage
g-C3N4/BiOBr
500W Xe
MO,
90mins-98%
RhB
120mins-80%
[32] [33]
g-C3N4/SrTiO3
2.2kW Xe
MB
180-95%
SnO2/g-C3N4
65W CFL lamp
RhB
90mins-98.7%
RbX
80mins-93.7%
[34]
g-C3N4/TiO2
65W CFL lamp
RhB
60mins-96.7%
[35]
MnV2O6/TiO2
Sun light
MB
240mins-90%
[26]
g-C3N4/MnV2O6
500W Tungsten Halogen
MB
210mins-95%
This work
IC
60mins-94%
25
Graphical abstract
26
Highlights: 1. A novel g-C3N4/MnV2O6 heterostructure photocatalyst was synthesized by facile hydrothermal method. 2. The composite material exhibit higher photocatalytic performance than the bare materials. 3. The heterostructure could prevent the recombination effect. 4. The heterostructure proved to be an efficient photocatalyst for anionic and cationic dye degradation.
27
S0009-2614(19)30813-9 https://doi.org/10.1016/j.cplett.2019.136832 CPLETT 136832
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
5 September 2019 1 October 2019 7 October 2019
Please cite this article as: M. Nithya, S. Vidhya, Keerthi, A Novel g-C3N4/MnV2O6 Heterojunction Photocatalyst for the Removal of Methylene Blue and Indigo Carmine, Chemical Physics Letters (2019), doi: https://doi.org/ 10.1016/j.cplett.2019.136832
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
A Novel g-C3N4/MnV2O6 Heterojunction Photocatalyst for the Removal of Methylene Blue and Indigo Carmine M. Nithya, S. Vidhya, Keerthi* *Department of Chemistry, CEG Campus, Anna University, Chennai-25, India ABSTRACT Composite of g-C3N4/MnV2O6 heterostructure were synthesized by facile hydrothermal method. The obtained photocatalysts with different ratios were characterized using X-ray diffraction (XRD), Scanning electron microscope (SEM), Fourier transform infrared spectrophotometer (FTIR), UV-Visible Diffuse reflectance spectroscopy (UV-Vis DRS) and X ray photoelectron spectroscopy (XPS). Among all composites 1:1 g-C3N4/MnV2O6 exhibited best photodegradation performance than pure g-C3N4 and MnV2O6 for Methylene blue and Indigo carmine dyes degradation. It showed 95% and 94% degradation for Methylene blue and Indigo carmine dyes respectively. The scavenging studies showed that among the oxidants formed during photocatalysis, hole played a major role in methylene blue degradation. Keywords: Heterojunction, Photodegradation, MnV2O6, Indigo Carmine,
Corresponding author mail id: [email protected]
1. INTRODUCTION Water pollution created by organic pollutants has attracted worldwide attention due to its serious threat to the environment [1]. Main sources of water contamination include waste water discharge from industries, agricultural activities, municipal wastewater, environmental and global changes, but most important pollutant is considered to be the textile industry in India. Textile industry is one of the primary sectors which generate more amounts of effluents containing toxic dye and organic compounds [2]. Textile industry wastewater containing various types of dye is considered to be potential carcinogens and is dangerous to the human health and environment. Several physical, chemical, and biological techniques are generally applied to remove the organic pollutants from wastewater, but are not economic and efficient. Recently, AOPs using semiconductor based heterogeneous photocatalysis has been proven to be an efficient green technique for remediation of environmental pollution by utilizing solar light to remove water pollutants [3-5]. So far, enormous research has deliberated on photocatalytic processes using titanium dioxide (TiO2) as a conventional photocatalyst. But the practical application of TiO2 is greatly restricted by its poor solar energy conversion efficiency due to its wide band gap of approximately 3.2 eV [6]. Therefore, urgent need to find out efficient photocatalyst having broad visible-light absorption, separation of photogenerated charge carriers and high stability with reusability should be addressed. The narrow-band gap (2.7 eV) of two-dimensional (2D) graphitic carbon nitride (g-C3N4) semiconductor can be harnessed for the degradation of organic pollutants under visible light irradiation. At the same time, the advantages of nontoxicity, easy to synthesize, inexpensive and sustainable nature, high thermal and chemical stability, makes g-C3N4 as a suitable candidate for solar-energy conversion [7-10]. Even though g-C3N4 has versatile properties, the major disadvantage is the fast charge recombination which can be improved by metal doping, Heterojunction and Z scheme formation [11-14]. Metal vanadates, such as lanthanide vanadate, aluminium vanadate and bismuth vanadate exhibit good catalytic activities for the treatment of organic compounds [15-17]. Among these metal vanadates, Manganese Vanadate has attracted great research interest for the photocatalytic degradation of organic pollutants owing to its large specific surface area, unique electron properties, reduction ability, high stability and narrow band gap making it a good photocatalyst 2
to degrade organic pollutants by absorbing visible light [19-23]. Nowadays, heterojunctions are studied for more efficient dye degradation application [24-26]. Therefore to achieve higher photocatalytic performance p-type MnV2O6 was combined with n-type g-C3N4 to form p-n heterojunction which could be made into efficient photocatalyst compare to the individual photocatalysts. 2. Experimental 2.1 Materials Manganese nitrate heptahydrate (Mn(NO3)3.6H2O) was purchased from Sigma-Aldrich and Melamine, Ammonium metavanadate (NH4VO3), ethanol and distilled water were purchased from Sisco Research Laboratories (SRL) Private Limited, India. The entire chemicals purchased were used without any extra purification. 2.2 Preparation of g-C3N4 10 g of melamine is weighed and is transferred into a crucible with lid and it was heated to 520°C and kept for 2 h in a muffle furnace with heating rate of 8°C/min. After cooling, the obtained light yellow colored sample was ground into powder. 2.3 Preparation of MnV2O6 The calculated amount of manganese nitrate heptahydrate was weighed and then dissolved in 32ml of distilled water by stirring. The calculated amount of ammonium metavanadate was weighed and then dissolved in 32 ml of distilled water separately by stirring at 80°C. To the manganese nitrate solution, the above solution was added drop wise and stirred for 30 minutes. Then above solution was transferred into a 100 ml Teflon lined autoclave and maintained at 200°C for 8 hours. The autoclave was cooled then returned to room temperature naturally. Finally the products were washed several times with water and ethanol to get black colored product. Finally, the products were dried in hot air oven at 80 °C for overnight. 2.4 Preparation of g-C3N4/MnV2O6 composite
3
The desired quantity of g-C3N4 was dispersed in 32ml of water and this solution was ultrasonicated for 30mins. Then calculated amount of manganese nitrate heptahydrate was weighed and then it was added to the above solution, which was dissolved by stirring. The calculated amount of ammonium metavanadate is weighed and then dissolved in 32ml of distilled water by stirring at 80°C. To the g-C3N4 solution the above solution was added drop wise and stirred for 30 minutes. Above solution was transferred into a 100 ml Teflon lined autoclave and maintained at 200°C for 8 hours then cooled to room temperature naturally to get brown color product. The precipitates collected by centrifugation were washed several times with water and absolute ethanol. Finally, the products were dried in hot air oven at 80°C for 12 h. Different ratio of samples with various amount of g-C3N4 and MnV2O6 (1:0.5, 1:1, 1:1.5) were synthesized following the above procedure. 2.5 Characterization of Photocatalyst To observe the formation and properties of prepared g-C3N4/MnV2O6 photocatalyst, several analyses were conducted. X-ray Diffraction spectra of g-C3N4, MnV2O6 and gC3N4/MnV2O6 photocatalysts were recorded in D8 advance Bruker instrument with Cu Kα radiation (kα =1.5418 Å) with scanning step width of 0.02°, scanning rate of 10°/minute and range of 10-90°. Fourier transform infrared (FTIR) spectra of photocatalysts were recorded in 400-4000cm-1 range by using ABB MB 3000 instrument using KBr pellets technique. UV Visible diffuse Reflectance spectra of photocatalysts were recorded in 200-2500cm-1 in JASCO UV-Visible spectrophotometer. The SEM images of the prepared composite photocatalyst were analyzed using scanning electron microscopy (VEGA 3 TESCAN, SEM). X-Ray photoelectron spectroscopy (XPS) was carried out using a PHI 5000 VERSAPROBE II SCANNING ESCA MICROPROBE system with a monochromatic Al Kα source. The Photoluminescence (PL) spectra were provided using a spectrofluorometer (HORIBA Fluoromax plus CP-011) at room temperature with an excitation wavelength of 360 nm 2.6 Evaluation of photocatalytic activity The photocatalytic activity of prepared samples was calculated by using MB and IC as a model pollutant under visible light irradiation. In this photocatalytic process 300W tungsten halogen lamp used as visible light source. For the photocatalytic experiment, 100mg of the 4
prepared catalyst was added into 100 mL of MB or IC aqueous solutions (10 mg/L) in a Pryrex glass reactor with a cooling water jacket. Prior to illumination, the suspensions were stirred for 1h in the dark to attain adsorption-desorption equilibrium at room temperature. A small amount of sample was collected at regular time interval and centrifuged to remove the photocatalyst powders (4000rpm, 10min). The concentration of MB or IC aqueous solution was analyzed using Elico SL159 UV-Vis spectrophotometer at maximum absorption (664 nm for MB and 612nm for IC) at room temperature. The stability of the 1:1 g-C3N4/MnV2O6 composite catalyst was evaluated by reusing the catalyst for three runs for the decomposition of methylene blue under the same conditions. After each run, the catalyst was separated by centrifugation procedure, washed with water and ethanol then dried in oven at 80°C. The chemical oxygen demand (COD) was analyzed by the open reflux potassium dichromate method. 2.7 Scavenging experiment To find the major active species in MB degradation, scavenging experiments were carried out by using benzoquinone (BQ, a superoxide anion radical scavenger, .O2-) Ammoniumoxalate (AO hole scavenger) and isopropanal (IPA .OH radical scavenger). Similar to photocatalytic tests, 10mM of scavengers were added, stirred for 1 h in the dark to attain adsorption and desorption equilibrium between the catalyst and the pollutant. After light irradiation, the suspensions were collected by centrifugation at regular time intervals and then the MB concentration was measured using UV-Vis spectrophotometer. 3. RESULTS AND DISCUSSIONS 3.1 CHARACTERISATION OF PHOTOCATALYST 3.1.1 XRD analysis The crystalline nature of the heterostructured photocatalyst was analyzed by XRD and the results are shown in Fig. 1. The characteristic position of g-C3N4 with two peaks results from graphite structure and tri-s-triazine units which is analogous to the C3N4 [27]. The strongest peak at 27.3° is due to the stacking of the conjugated aromatic system, corresponding to the 002 crystal face and as well as a peak at 13.2°, resulting from the periodic arrangement of the condensed tri-s-triazine units in the sheets, is indexed as (001), corresponding to JCPDS card no. 5
87-1526. The diffraction at 2θ value, 27, 29, 30, 32.5, 39 and 43 corresponds to the diffraction plane 110, 202, 201, 111, 311 and 003 respectively to monoclinic phase of MnV2O6 [JCPDS card-350139]. It was demonstrated that the synthesized composite have crystalline properties of both MnV2O6 and g-C3N4. 3.1.2 FTIR Analysis The FTIR measurement was carried with wavenumber range from 400-4000cm-1. Fig. 2 shows the FTIR response for g-C3N4, MnV2O6 and 1:1 g-C3N4/MnV2O6 respectively. In detail, for g-C3N4 the sharp absorption peak at 809cm-1 corresponds to the typical triazine ring. The peaks at 1640, 1556cm-1 are ascribed to stretching modes of derived repeating units of triazine. Furthermore, the peaks at 1333 and 1242 cm-1 are assigned to stretching modes of the C-N and C-C and broad band around 3200cm-1 is the stretching vibration of the N-H bond. FTIR spectrum of MnV2O6 shows peaks in the region of from 500 to 700 cm-1 corresponding to the symmetric and asymmetric stretching band of V-O-V bonds. The bands at 515cm-1 can be allocated to the stretching mode of Mn-O modes [26]. The FTIR spectra of 1:1 g-C3N4/MnV2O6 composite maintained main characteristic peaks of g-C3N4, and it contains major MnV2O6 peaks. This indicates both g-C3N4 and MnV2O6 are embraced in the composite. 3.1.3 UV DRS Analysis The
solid
UV-Vis
diffuse
reflectance
spectrum
of
g-C3N4,
MnV2O6
and
1:1g-C3N4/MnV2O6 composite is shown in Fig. 3. All photocatalyst exhibited significant absorption in the visible light region. The absorption edge of pristine g-C3N4 is around 445 nm, and MnV2O6 shows broad absorption range of about 600-700nm. The 1:1g-C3N4/MnV2O6 heterostructures exhibit much broader absorption band compared to pure g-C3N4 throughout the entire visible light region. This observation is important, as it indicates that g-C3N4/MnV2O6 composite can be photoexcited under visible-light irradiation to generate more electron-hole pairs and enhance the photocatalytic performance. 3.1.4 SEM analysis The morphology of the synthesised 1:1 g-C3N4/MnV2O6 was characterised using SEM and is shown in Fig. 4. The images show sheet like structure, layer arrangements and some small 6
rods dispersed over the sheets. The layer structure corresponds to the g-C3N4 and small rods to MnV2O6. It was observed that the products were enclosed of both MnV2O6 and g-C3N4 material by showing heterogeneous structure. 3.1.5. XPS analysis XPS analysis of 1:1 g-C3N4/MnV2O6 is shown in Fig. 5. The Fig. 5a shows survey spectrum and it demonstrates the elemental composition of Carbon, Nitrogen, Manganese, Vanadium and Oxygen. Four peaks at 284.6, 285.5 288.2 and 289.7 eV appeared for deconvoluted C 1s spectra (Fig. 5b) which corresponds to graphitic C=C or the cyano-group, adventitious carbon, C-NH2 species and N − C − N coordination in the graphitic carbon nitride respectively [28]. Fig. 5c shows two peaks at 399.1 and 400.69 eV for N 1s due to sp2 N in the triazine rings and bridging nitrogen atoms respectively [29]. The XPS spectra of Mn2+ ion and Mn3+ are adjacent and cannot to be distinguished. Fig. 5d displays the peak for Mn 2p3/2 at 642.16 eV attributed to Mn2+ existence in the composite [30]. Fig. 5e displayed V 2p high resolution XPS spectra, in which two peaks at 517.19 eV and 524.39 eV corresponds to V 2p3/2 and V 2p1/2 respectively wihich is assigned to V5+ ions. The O 1s spectra (Fig. 5f) shows two peaks at 530.3ev and 532.12eV thus indicating the presence of oxygen species like lattice oxygen and hydroxyl moiety at the surface of the nanocomposite. The XPS results further confirmed the successful construction of g-C3N4/MnV2O6 heterostructure. To determine the photocatalytic efficiency, the separation of photoexcited charge carriers is a key factor and thus was investigated by PL spectra measurements. As shown in Fig. 6, around 420 nm, a broad PL band for g-C3N4 was observed, which was attributed to the recombination of the electron and hole with emission energy equal to its bandgap energy [31]. Significant decrease in the PL intensity of 1:1g-C3N4/MnV2O6 was observed. The weaker PL emission intensity suggested the decline in the recombination of the photoexcited charge carriers as a result of the enhanced migration of electron-hole pairs. 3.2
Photocatalytic activity The photocatalytic degradation efficiency of g-C3N4, MnV2O6 and g-C3N4/MnV2O6
composites was evaluated by the degradation of MB and IC under visible light irradiation. Fig. 7b showed the absorption spectrum of MB with different irradiation time for 7
1:1 g-C3N4/MnV2O6 composite. The degradation efficiency of all catalysts was calculated using the following equation. Removal % =
C0 ― Ct C0
x 100
(1)
Where C0 is the MB initial concentration at adsorption equilibrium and Ct is the concentration of MB at time t. Fig. 7a shows the photocatalytic activities of all the synthesized materials. The MB degradation for pure g-C3N4 and MnV2O6 was 47% and 37% respectively. Among them 1:1 g-C3N4/MnV2O6 shows higher degradation efficiency and 98% of MB was degraded within 210 min of irradiation. Further the photocatalytic degradation efficiencies of the composites decreased with the increase in the ratio, which may be due to the surface blocking of MnV2O6 in g-C3N4. Hence, it is clear that the optimum ratio of the composite is 1:1 for the efficient removal of MB. Furthermore to understand the reaction kinetics of the photocatalytic degradation of MB (Fig. 8a), the rate constant k, was calculated from the following equation and the process followed first order reaction: ln(C0/Ct) = kt
(2)
Where, C0 and Ct are the concentrations of the MB solution at time 0 and t, respectively. The rate constant obtained from degradation of methylene blue are given below in the table 1 and this indicates the reaction proceeds via pseudo first order kinetics. The rate constant followed the sequence 1:1> 1:1.5 > 1:0.5 > g-C3N4>MnV2O6 Among all composites, 1:1 ratio composite exhibits highest k value than that of other catalysts. The rate constant (k) of 1:1 g-C3N4/MnV2O6 photocatalyst was 10 times and 9 times higher than that of pure MnV2O6 and g-C3N4 respectively. These results indicate that the combination of MnV2O6 and g-C3N4 is promoting the photocatalytic performance for MB degradation under visible light region. 3.3 Scavenging Studies 8
To study the photocatalytic process and mechanism, the roles of three common active oxidant species, hydroxyl radical (•OH), hole (h+) and superoxide radical (•O2−) are studied (Fig. 8b). Herein, AO, IPA and BQ are used as the scavengers for h+, •OH and •O2−, respectively. The degradation percentage decreases with the addition of AO which shows that h+ plays the major role in the degradation of MB whereas in the case of the IPA and BQ they equally contribute less compared to AO to the degradation of MB dye. This shows that there is a significant enhancement of photocatalytic activity and was attributed to the improved charge separation efficiency due to the electron transfer between the MnV2O6 and g-C3N4. In addition Indigo carmine degradation by g-C3N4, MnV2O6 and g-C3N4/MnV2O6 composites was evaluated under visible light irradiation. Fig. 9b showed the degradation efficiency of 1:1 g-C3N4/MnV2O6. The IC degradation for pure g-C3N4 and MnV2O6 was 68% and 53% respectively but the 1:1 g-C3N4/MnV2O6 composites shows higher degradation performance of 94% than the bare materials. This also confirms that the composites are efficient catalyst for dye degradation applications. The COD analysis is commonly used to measure the oxidizable organic waste present in the wastewater. In the present work COD analysis was done to confirm the mineralization efficiency of the catalysts. Before and after the photocatalysis process COD of the dye solutions was determined by open reflux method. The composite showed reasonable COD removal efficiency for both MB and IC dyes (70% and 73% respectively), than the bare materials (Fig. 9a). These results indicate that the removal of dyes from the solution by photocatalysis process is through degradation rather than decolourization. Chemical stability of the photocatalyst and recycling capacity of photocatalyst is an important property for catalysts. The cyclic study of 1:1 g-C3N4/MnV2O6 composite was evaluated by recycling the photocatalytic degradation of MB experiments under similar conditions. Fig. 9c shows the photocatalytic efficiency of composites after three times of recycling test, indicating that the sample possesses good stability. Comparison of photocatalytic performance of different heterojunction with this work is shown in Table 2. Table 2 describes light sources, type of dye, time taken to degrade and the removal efficiency by different catalysts. The current study of g-C3N4/MnV2O6 heterostructure 9
using visible light as source and time taken to dye degradation is comparable to other reported works. Some materials are showing higher performance, but the cost of the material is higher compared to the present material and has advantage of degrading both anionic and cationic dyes. 4. CONCLUSION The heterostructured photocatalysts composed of MnV2O6 and g-C3N4 have been prepared via a simple hydrothermal method. The formation of a heterojunction structure and strong interface interaction between MnV2O6 and g-C3N4 were characterized by XRD, FTIR, SEM, and UV-DRS. The photocatalytic performance of the synthesized heterostructures was evaluated by degradation of methylene blue and indigo carmine. The heterojunction formation could efficiently enhance the photoexcited charge transfer and suppress the recombination of electrons and holes. Among the entire photocatalysts 1:1 g-C3N4/MnV2O6 exhibited enhanced photocatalytic activity than pure MnV2O6 and g-C3N4. The reaction rate constant k in photocatalytic degradation of MB was found to be about 9 times and 10 times higher than pure g-C3N4 and MnV2O6 respectively. The COD removal efficiency was also superior for the composite material. Moreover, g-C3N4/MnV2O6 heterojunction photocatalysts exhibited good photocatalytic stability and reusability, which are very important for its practical applications. Acknowledgement This work was funded by UGC-Startup-BSR (Project No. F.30-368/2017) and the authors acknowledge the same. The first author acknowledges Anna University for the Anna Centenary Research Fellowship (ACRF)
10
REFERENCES 1. R. P. Schwarzenbach, B. I. Escher, K. Fenner,; T. B. Hofstetter, C. A. Johnson, U. Von Gunten, B. Wehrli, The Challenge of Micropollutants in Aquatic Systems, Science 313 (2006) 1072−1077. 2. K. Singh, S. Arora, Removal of synthetic textile dyes from wastewaters: A critical review on present treatment technologies. Crit. Rev. Environ. Sci. Technol., 41, (2011) 807−878. 3. H. Wang, X.Z. Yuan, Y. Wu, G.M. Zeng, H.R. Dong, X.H. Chen, L.J. Leng, Z.B. Wu, L.J. Peng, In situ synthesis of In2S3@ MIL-125 (Ti) core-shell microparticle for the removal of tetracycline from wastewater by integrated adsorption and visiblelightdriven photocatalysis, Appl. Catal. B: Environ. 186 (2016) 19–29. 4. L. Xu, C. Srinivasakannan, J.H. Peng, L.B. Zhang, D. Zhang, Synthesis of Cu-CuO nanocomposite in microreactor and its application to photocatalytic degradation, J. Alloy. Compd. 695 (2017) 263–269. 5. H. Wang, X.Z. Yuan, Y. Wu, G.M. Zeng, X.H. Chen, L.J. Leng, Z.B. Wu, L.B. Jiang, H. Li, Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr (VI) reduction, J. Hazard. Mater. 286 (2015) 187–194. 6. R.A. Torres, J.I. Nieto, E. Combet, C. Pétrier, C. Pulgarin, Influence of TiO2 concentration on the synergistic effect between photocatalysis and high-frequency ultrasound for organic pollutant mineralization in water, Appl. Catal. B: Environ. (2008) 80 168–175. 7. Aiwu, Chudong, Recent advances of graphitic carbon nitride-based structures and applications in catalyst, sensing, imaging, and LEDs, Nanomicro Lett. 9 (2017) 47. 8. W. Jiuqing, X. Jun, C. Xiaobo, L. Xin, A review on g-C3N4-based photocatalysts, Applied Surface Science, 391 (2017) 72–123. 9. H. Yia, D. Huanga, L. Qina, G. Zenga, C. Lai, M. Cheng, S. Yea, B. Song, X. Rena, X. Guoa, Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production, Applied Catalysis B: Environmental 239 (2018) 408-424.
11
10. Y. Yanga, C. Zhanga, D. Huanga, G. Zenga, J. Huanga, C. Laia, C. Zhoua, W. Wanga, H. Guoa, W. Xuea, R. Denga, M. Chenga, W. Xionga, Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation, Applied Catalysis B: Environmental 245 (2019) 87-99 11. T. Jesty, K.S Ambili, S. Radhika, Synthesis of Sm3+-doped graphitic carbon nitride nanosheets for the photocatalytic degradation of organic pollutants under sunlight, Catalysis Today, 310 (2017) 11-18. 12. C. Jiayi, X. Xinyan, W. Yi, Y. Zhihao, Ag nanoparticles decorated WO3/g-C3N4 2D/2D heterostructure with enhanced photocatalytic activity for organic pollutants degradation, Applied Surface Science, 467, (2019) 1000–1010 13. Ravichandran, Sindhuja, Fabrication of cost effective g-C3N4+Ag activated ZnO photocatalyst in thin film form for enhanced visible light responsive dye degradation, Materials Chemistry and Physics, 221 (2019) 203–215 14. B. Abdullah, T. Muhammad, A. S. A. Nor, Well-designed ZnV2O6/g-C3N4 2D/2D nanosheets heterojunction with faster charges separation via p-CN as mediator towards enhanced photocatalytic reduction of CO2 to fuels, Applied Catalysis B: Environmental, 242 (2019) 312–326. 15. N. Dhachapally, V. N. Kalevaru, A. Bruckner, Metal vanadate catalysts for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine, Appl Catal A., 443 (2012) 111118. 16. Y. Shen, M. Huang, Y. Huang, The synthesis of bismuth vanadate powders and their photocatalytic properties under visible light irradiation. J Alloys Compounds. 496 (2010) 87-292. 17. Y. Hu, F. Luo, F. Dong, Design synthesis and photocatalytic activity of a novel lilaclike silver vanadate hybrid solid based on dicyclic rings of [V4O12]4 with {Ag7}7C cluster. Chem Commun. 47 (2011) 761-763. 18. J. Ren, W. Wang, M. Sheng, Photocatytic activity of silver vanadate with onedimensional structure under fluorescent light. J Hazard Mater. 183 (2010) 950-953.
12
19. Z. Brandon, G. Elijah, A. M. Paul, A small bandgap semiconductor, p-type MnV2O6, active for photocatalytic hydrogen and oxygen production, Dalton Trans. 46 (2017) 10657–10664. 20. Z. Shaoyan, H. Ruisheng, L. Lei, W. Dianyong, Hydrothermal synthesis of MnV2O6 nanobelts and its application in lithium-ion battery, Materials Letters, 124 (2014) 57– 60. 21. M.L. Hneda, J. B. M. Da Cunha, M. A. Gusmao, S.R. Oliveira Neto, J. RodriguezCarvajal, O. Isnard, Low-dimensional magnetic properties of orthorhombic MnV2O6:A nonstandard structure stabilized at high pressure, Physical review, 95 (2017). 22. P. Hongrui, D. Jie, W. Ning, L.Guicun, Facile synthesis of MnV2O6 nanostructures with controlled morphologies, Materials Letters, 63 (2009) 1404–1406. 23. H. Yi, L. Qina, D. Huanga, G. Zenga, C. Laia, X. Liua, B. Lia, H. Wanga, C. Zhoua, F. Huanga, S. Liua, X. Guoa,
Nano-structured bismuth tungstate with controlled
morphology: Fabrication, modification, environmental application and mechanism insight, Chemical Engineering Journal 358 (2019) 480-496 24. Y. Bai, T. Chen, P. Wang, L. Wang, L. Ye, X. Shi, W. Bai, Size-dependent role of gold in g-C3N4/BiOBr/Au system for photocatalytic CO2 Reduction and Dye Degradation. Sol. Energy Mater. Sol. Cells, 157 (2016) 406. 25. L. Shen, Z. Xing, J. Zou, Z. Li, X. Wu, Y. Zhang, Q. Zhu, S. Yang, W. Zhou, Black TiO2 Nanobelts/g-C3N4 Nanosheets Laminated Heterojunctions with Efficient VisibleLight-Driven Photocatalytic Performance. Sci. Rep. 7 (2017) 41978. 26. S. Rajendra Prasad, S. Srikantaswamy, M.R. Jagadish, Abhilash, M.B. Nayan, SolarLight-Induced Photocatalytic Properties of Novel MnV2O6/TiO2 Nanocomposite, Journal of Environmental Science, Computer Science and Engineering & Technology, Section A; 7 (2014) 383-391. 27. S. Tonda, S. Kumar, S. Kandula, V. Shanker, Fe-doped and Mediated Graphitic Carbon Nitride Nanosheets for Enhanced Photocatalytic Performance under Natural Sunlight. J. Mater. Chem. A, 2 (2014) 6772. 28. S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Adv. Mater., 25 (2013) 2452-2456. 13
29. K. Dai, L. Lu, C. Liang, G. Zhu, Q. Liu, L. Geng, J. He, A high efficient graphiticC3N4/BiOI/graphene oxide ternary nanocomposite heterostructured photocatalyst with graphene oxide as electron transport buffer material. Dalton Trans., 44 (2015) 79037910. 30. Y. Liua, Y. Zhanga, J. Dua, W. Yua, Y. Qian, Synthesis and characterization of singlecrystal MnV2O6 nanobelts, Journal of Crystal Growth 291 (2006) 320-324. 31. P. Suyana, K. R. Sneha, B. N. Nair, V. Karunakaran, A. P. Mohamed, K. G. K. Warrier, U. S. A. Hareesh, Facile one potsynthetic approach for C3N4-ZnS composite interfaces as heterojunctions for sunlight-induced multifunctional photocatalytic applications. RSC Adv. 6 (2016) 17800−17809. 32. M. Jian, S. Yanbiao, J. Huang, L. Wang, H. She Jinhui Tong, B. Su, and Q. Wang, Synthesis of Flowerlike g-C3N4/BiOBr with Enhanced Visible Light Photocatalytic Activity for Dye Degradation, Eur. J. Inorg. Chem. (2018) 1834–1841 33. P.S. Konstas, I. Konstantinou, D. Petrakis, T. Albanis, Synthesis, Characterization of gC3N4/SrTiO3 Heterojunctions and Photocatalytic Activity for Organic Pollutants Degradation, Catalysts 8 (2018) 554. 34. J. Singh, Pooja Kumari, S. Basu, Degradation of toxic industrial dyes using SnO2/gC3N4 nanocomposites: Role of mass ratio on photocatalytic activity, Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 136–143. 35. D. Monga, S. Basu, Enhanced photocatalytic degradation of industrial dye by gC3N4/TiO2 nanocomposite: Role of shape of TiO2, Advanced Powder Technology 30 (2019) 1089–109
14
Figure Captions Fig. 1. XRD patterns of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 2. FT-IR spectrum of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 3 UV-Visible DRS spectra of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 4. SEM images of 1:1 g-C3N4/MnV2O6 composite photocatalyst Fig. 5. a) Survey spectrum, High resolution XPS patterns (b) C 1s, (c) N 1s, (d) Mn 2p, (e) V 2p and (f) O 1s of 1:1 g-C3N4/MnV2O6 composite Fig. 6. PL spectra of pure and g-C3N4/MnV2O6 composite photocatalysts Fig. 7 a) Photocatalytic degradation of MB in the presence of various composites b)Absorption spectrum of MB with time in the presence 1:1 g-C3N4/MnV2O6 Fig. 8. a) Kinetic fit for the degradation of MB with the as-prepared catalysts, b) Active radical species trapping experiments for the photocatalytic degradation of MB. Fig. 9. a) COD removal efficiency of synthesized catalyst for MB and IC degrdadation b) IC removal efficiency for synthesized catalysts c) Reusability of 1:1 g-C3N4/MnV2O6 photocatalyst toward visible light driven photocatalytic degradation of MB and IC
15
Figures
Figure 1
16
Figure 2
17
Figure 3
18
Figure 4
19
Figure 5
20
Figure 6
21
Figure 7
22
Figure 8
23
Figure 9
24
Table 1. Rate constant value for different photocatalysts in MB degradation Catalysts
Rate constant values
R2
(min-1) g-C3N4
0.0025
0.96
MnV2O6
0.0021
0.98
1:0.5 ratio
0.0116
0.96
1:1 ratio
0.0216
0.99
1:1.5 ratio
0.0075
0.98
Table 1. Comparison of various photocatalysts for dye degradation Catalyst used
Light source
Dye
Time and Removal reference percentage
g-C3N4/BiOBr
500W Xe
MO,
90mins-98%
RhB
120mins-80%
[32] [33]
g-C3N4/SrTiO3
2.2kW Xe
MB
180-95%
SnO2/g-C3N4
65W CFL lamp
RhB
90mins-98.7%
RbX
80mins-93.7%
[34]
g-C3N4/TiO2
65W CFL lamp
RhB
60mins-96.7%
[35]
MnV2O6/TiO2
Sun light
MB
240mins-90%
[26]
g-C3N4/MnV2O6
500W Tungsten Halogen
MB
210mins-95%
This work
IC
60mins-94%
25
Graphical abstract
26
Highlights: 1. A novel g-C3N4/MnV2O6 heterostructure photocatalyst was synthesized by facile hydrothermal method. 2. The composite material exhibit higher photocatalytic performance than the bare materials. 3. The heterostructure could prevent the recombination effect. 4. The heterostructure proved to be an efficient photocatalyst for anionic and cationic dye degradation.
27