Potassium-doped g-C3N4 enables efficient visible-light-driven dye degradation

doi:10.21203/rs.3.rs-2102494/v1

Download PDF

Research Article

Potassium-doped g-C3N4 enables efficient visible-light-driven dye degradation

https://doi.org/10.21203/rs.3.rs-2102494/v1

This work is licensed under a CC BY 4.0 License

Journal Publication

published 28 Mar, 2023

Read the published version in Environmental Science and Pollution Research →

You are reading this latest preprint version

Elements doping is recognized as an efficient method to boost the photocatalytic performance of photocatalysts. Here, a new potassium ion-doped precursor, potassium sorbate, was employed in melamine configuration during calcination process to prepare the potassium doped g-C₃N₄ (KCN). By various characterization techniques and electrochemical measurements, the doping of K in g-C₃N₄ can efficiently modify the band structure to enhance the light absorption, greatly increase the conductivity to accelerate charge transfer and photo-generated carriers separation, ultimately achieving an excellent photodegradation of the organic pollutant (methylene blue, MB). These results have demonstrated that the approach of potassium incorporation in g-C₃N₄ has potential in fabricating high-performance photocatalysts for organic pollutants removal.

g-C3N4

Potassium incorporation

Photocatalyst

Methylene blue

Degradation

K incorporated g-C₃N₄ was successfully fabricated with a new precursor.
KCN exhibited improved conductivity and photo-generated carriers separation.
KCN showed an improved photocatalytic activity in degrading MB.

In the past few decades, the fast development of industry and extensive use of fossil fuels have caused serious harm to our environment, especially the irreversible water pollution by various organic compounds, inorganic toxic and heavy metal pollutants (McGlade and Ekins, 2015; Schrope, 2011). Photocatalytic technique has been recognized as an effective environmental detoxification approach, since it can convert the non-biodegradable and toxic materials into CO₂, H₂O and other harmless salts (Li and Li, 2002; Zhang et al., 2018). Hence, seeking and developing high-performance photocatalysts are of significant importance.

Recently, various semiconductors have been reported and utilized for water purification, such as the metal-based photocatalysts TiO₂ (Ye et al., 2017), CdS (Lin et al., 2021), ZnO (Rokhsat and Akhavan, 2016). Nevertheless, these materials always present wide band gap, greatly limits their practical application (Wang et al., 2010). Other photocatalysts, like Cu_2 − xSe (Liu et al., 2019), ZnIn₂S₄ (Mao et al., 2021), also have been widely researched, whereas their slow photo-generate charges-holes separation rate still hinder their further development. Graphitic carbon nitride (g-C₃N₄), a metal-free photocatalyst, has been intensively studied in water splitting (Cheng et al., 2018), water purification (Akulinkin et al., 2021; Yan et al., 2009), since it was first reported in 2009. g-C₃N₄ is composed by sp²-hybridized C and N forms a π-conjugated system, favoring the adjustment of nanosheet-based materials in electronics and optoelectronics (Cui et al., 2011; Yang et al., 2015). Additionally, g-C₃N₄ presents suitable energy gap of 2.7 eV, indicating it can absorb and utilize visible light with wavelengths < 470 nm (Li et al., 2019). To further expand the visible-light absorption range of g-C₃N₄, and depress the photo-generate carriers recombination, a great deal strategies are developed, such as doping metal ions (Xu et al., 2022; Yan et al., 2018), doping non-metallic elements (Bellardita et al., 2018), introducing defects or vacancy (Cao et al., 2019a), and building composites or heterojunctions (Ali et al., 2022; Zhao et al., 2018), etc. Among these strategies, the elements doping can change the band structure and expand the light absorption range, and increase the conductivity to boost the charge transfer, thus boosting carriers’ separation and increasing the reaction kinetics (An and Onishi, 2015; Wang et al., 2020).

Herein, a potassium-doped g-C₃N₄ (KCN) has been prepared via calcining a mixture of melamine and potassium sorbate. The effects of K doping on phase structure, chemical composition, and band gap are identified via various characterizations. Then, methylene blue (MB) is selected as a model organic pollutant to explore the photocatalytic performance under the visible light irradiation. As expected, the KCN delivers a significant improvement in photo-degrading MB, and the O₂^•− is confirmed as the main reactive radical for MB removal.

The chemicals, materials preparation, characterizations and photocatalytic degradation experiments are illustrated in the Supporting Information in detail.

The structure information of CN and KCN was identified via the XRD, as shown in Fig. 1a, all the diffraction signals of CN and KCN are well matched with the standard pattern of JCPDS 87-1526 (Wang et al., 2020), and peaks located at 12.9° and 27.8° are ascribed to the (100) and (002) plane, respectively. Notably, the characteristic peak of the (002) plane is slightly shifted to lower angle, indicating the interlayer distance of KCN is enlarged, demonstrating that the small sized K atoms enter into the CN structure (Fig. 1b). Furthermore, the weakened (002) peak intensity can be corresponded to the incomplete polymerization induced by the K incorporation during the condensation process. In addition, the chemical structure information of CN and KCN were analyzed by FT-IR, as displayed in Fig. 1c, the characteristic signals at 1200–1650 and 815 cm^− 1 are related to the aromatic heterocyclic rings stretching and the heterocyclic ring condensation of triazine unit in g-C₃N₄ (Li et al., 2007; Zhou et al., 2016). And the apparent peaks of –OH and uncondensed ammonium groups (NH) are observed in 3000–4000 cm^− 1 (Bai et al., 2013). Impressively, there is no K-related vibration found in the FT-IR spectra, indicating the introduction of K atoms in CN will not affect the internal chemical deposition.

The XPS spectrum of CN and KCN are further fitted to understand the chemical valence states and element composition. As depicted in Fig. 1d, a pair of apparent K 2p peaks can be fitted in K 2p XPS single, whereas there is no K 2p single in CN sample, indicating the successful introduction of K. Besides, the C 1s fine spectra of CN and KCN (Fig. 1e) can be divided into two obvious peaks, the peak located in 284.5 eV and 284.5 eV are ascribed to the sp² hybridization of C–C and sp² hybridization of the carbon (N–C = N) (Li et al., 2014). And in the Fig. 1f, three peaks located at 400.8, 399.0, 398.5 eV in N 1s spectrum can be attributed to N–H of the uncondensed amino groups, tertiary nitrogen in N–(C)₃ group, C–N = C group (Gu et al., 2015; Lin and Wang, 2013). Note that the intensity ratio of sp² N (C–N = C) /sp³ N (N–(C)₃) in KCN is 3.95, which is much lower than that in CN (5.79), implying the introduction of K atoms in KCN will cause more structural distortion and defects, as sp² N becomes sp³ N when the C–N plane is distorted (Wang et al., 2020). As presented by SEM and energy dispersive spectroscopy (EDS) images, the KCN demonstrates the typical morphology of g-C₃N₄ composed of highly condensed sheets (Fig. 2a). And the EDS mapping demonstrates that the C, N, K elements are uniformly distributed on the surface of KCN (Fig. 2b-d), further confirming the K atoms are successfully inserted into the g-C₃N₄.

The optical properties of CN and KCN have been investigated by DRS, PL and photo-generate current measurements. As shown in Fig. 3a, CN sample presents a visible light absorption and the edge is located at 478 nm, whereas the KCN shows an apparent shift to larger absorption wavelength at 492 nm, suggesting the electronic structure and band gap are significantly modified in KCN. Further, the energy band gap of CN and KCN are calculated via the Tauc/David–Mott model to be 2.8 eV and 2.7 eV, respectively (Ding et al., 2018), demonstrating that the photocatalytic performance of KCN can be greatly improved via increasing the visible light absorption. Besides, the separation rate of photo-generated electrons/holes in CN and KCN are studied by PL measurement, as depicted in Fig. 3b, the peak at 450 nm can be related to the direct recombination of holes and electrons. And the CN sample displays the lower intensity when comparing with the KCN, indicating the K doping can efficiently depress the recombination of holes and electrons. In addition, the charge separation efficiency can be further confirmed via the photoelectrochemical measurements. As depicted in the photo-generate current response single (conduct at the open circuit potential), it can be found that the photocurrent density of KCN is much higher than that of pure CN, almost two times improved (Fig. 3c). And the EIS plots of CN and KCN are recorded in Fig. 3d, it’s obvious to see that the arc radius of KCN is smaller than that of CN, these results are consistent with photocurrent response and PL results, all implying the K doping in KCN can enhance the visible light absorption range and accelerate the photo-generated carriers transfer, thus improving the photocatalytic activity.

To verify the photocatalytic activity of KCN and CN, the degrading methylene blue dye (MB) efficiency under the visible light were compared. To achieve adsorption-desorption equilibration, the configurations with catalysts and pollutants were stirred for 30 mins in the dark to achieve adsorption-desorption equilibration. As shown in Fig. 4a, the utilization of CN brought 61% removal in 60 mins. In a sharp contrast, the photodegradation of MB was greatly improved in KCN, which achieved a superior removal efficiency of 99% after 60 mins. Furthermore, the photodegradation results were fitted with the fist-order kinetic in Fig. 4b, and rate constant of MB degradation in KCN was about 3.6 times that of CN. To further investigated the positive effect of K incorporation in CN, the KCN with different K content were also conducted the photocatalytic activity. As displayed in Fig. S1, the KCN with 0.1 g potassium sorbate precursor also shows a little enhancement in degradation, whereas the excessive K doping leads to the degradation of catalytic performance in KCN-0.5, which is ascribed to the excessive doping will cause the lattice collapse and affect their performance. After the degradation, the used catalyst was collected and washed, and then reused to check the cycling stability of KCN. In Fig. 4c, the MB removal by KCN could still be maintained above 95% after 5 cycles, indicating an excellent stability of KCN photocatalyst. In addition, to reveal photocatalytic mechanism and reactive species in the degradation system, the ESR spectrums in different configurations were conducted. In ESR spectrum of Fig. 4d, the O₂^•− intensity in KCN was much stronger than that of CN, means that there were more O₂^•− generated during the light irradiation, which can be related to the improved conductivity and enhanced photo-generated carriers separation rate. Therefore, more electrons will react with the dissolved O₂ to form more O₂^•− in KCN system (Cao et al., 2019b), thus accelerating the degradation of MB.

In summary, to increase the visible-light utilization and the separation of photo-generated carriers in the g-C₃N₄, potassium incorporating g-C₃N₄ (KCN) is proposed to promote effective MB degradation. KCN was fabricated by a facile thermal condensation, the K doping in g-C₃N₄ was identified via XRD, XPS, and EDS mapping. The characterizations and electrochemical measurements revealed that the K incorporation in g-C₃N₄ could not only alter the band structure to enhance the visible light absorption, but also improve the conductivity to accelerate the separation of photo-generated electrons/holes, thus achieving a superior MB degradation efficiency, and the degradation kinetics reached 0.119 min^− 1. This work presented a new potassium ion-doped precursor to synthesize high-performance photocatalyst, thus offing a new pathway for designing and developing high-efficiency photocatalysts.

Ethics approval

Ethical approval was not required for this research.

Author Contributions

All authors contributed to the study conception and design. Lianxin Yuan: Methodology, Conceptualization, Investigation, Writing - original draft. Weixuan Liu: Writing - review & editing, Resources. Wanting Zhang: Conceptualization, Resources, Visualization.

Funding

No funds, grants, or other support was received.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Data availability

The full dataset for the current study is available from the corresponding author on reasonable request.

Akulinkin, A., Bolgaru, K. & Reger, A.: (2021), Facile synthesis of porous g-C₃N₄/β-SiAlON material with visible light photocatalytic activity, Mater. Lett. 305, 130788.
Ali, M. K. M., Modwi, A., Idriss, H., Aldaghri, O., Ismail, M. & Ibnaouf, K. H.: (2022), Detoxification of Pb (II) from aquatic media via CaMgO₂@g-C₃N₄ nanocomposite, Mater. Lett. 322, 132501.
An, L. & Onishi, H.: (2015), Electron–Hole Recombination Controlled by Metal Doping Sites in NaTaO₃ Photocatalysts, ACS Catal. 5, 3196-3206.
Bai, X., Wang, L., Zong, R. & Zhu, Y.: (2013), Photocatalytic activity enhanced via g-C₃N₄ nanoplates to nanorods, J. Phys. Chem. C 117, 9952-9961.
Bellardita, M., García-López, E. I., Marcì, G., Krivtsov, I., García, J. R. & Palmisano, L.: (2018), Selective photocatalytic oxidation of aromatic alcohols in water by using P-doped g-C₃N₄, Appl. Catal. B: Environ. 220, 222-233.
Cao, J., Nie, W., Huang, L., Ding, Y., Lv, K. & Tang, H.: (2019a), Photocatalytic activation of sulfite by nitrogen vacancy modified graphitic carbon nitride for efficient degradation of carbamazepine, Appl. Catal. B: Environ. 241, 18-27.
Cao, J., Pan, C., Ding, Y., Li, W., Lv, K. & Tang, H.: (2019b), Constructing nitrogen vacancy introduced g-C₃N₄ p-n homojunction for enhanced photocatalytic activity, J. Environ. Chem. Eng. 7, 102984.
Cheng, J., Hu, Z., Lv, K., Wu, X., Li, Q., Li, Y., Li, X. & Sun, J.: (2018), Drastic promoting the visible photoreactivity of layered carbon nitride by polymerization of dicyandiamide at high pressure, Appl. Catal. B: Environ. 232, 330-339.
Cui, Y., Zhang, J., Zhang, G., Huang, J., Liu, P., Antonietti, M. & Wang, X.: (2011), Synthesis of bulk and nanoporous carbon nitride polymers from ammonium thiocyanate for photocatalytic hydrogen evolution, J. Mater. Chem. 21, 13032-13039.
Ding, J., Xu, W., Wan, H., Yuan, D., Chen, C., Wang, L., Guan, G. & Dai, W.-L.: (2018), Nitrogen vacancy engineered graphitic C₃N₄-based polymers for photocatalytic oxidation of aromatic alcohols to aldehydes, Appl. Catal. B: Environ. 221, 626-634.
Gu, Q., Liao, Y., Yin, L., Long, J., Wang, X. & Xue, C.: (2015), Template-free synthesis of porous graphitic carbon nitride microspheres for enhanced photocatalytic hydrogen generation with high stability, Appl. Catal. B: Environ. 165, 503-510.
Li, F. B. & Li, X. Z.: (2002), The enhancement of photodegradation efficiency using Pt–TiO2 catalyst, Chemosphere 48, 1103-1111.
Li, J., Cao, C. & Zhu, H.: (2007), Synthesis and characterization of graphite-like carbon nitride nanobelts and nanotubes, Nanotechnology 18, 115605.
Li, K., Yan, L., Zeng, Z., Luo, S., Luo, X., Liu, X., Guo, H. & Guo, Y.: (2014), Fabrication of H₃PW₁₂O₄₀-doped carbon nitride nanotubes by one-step hydrothermal treatment strategy and their efficient visible-light photocatalytic activity toward representative aqueous persistent organic pollutants degradation, Appl. Catal. B: Environ. 156-157, 141-152.
Li, X., Wang, B., Huang, Y., Di, J., Xia, J., Zhu, W. & Li, H.: (2019), Boosting photocatalytic degradation of RhB via interfacial electronic effects between Fe-based ionic liquid and g-C₃N₄, Green Energy Environ. 4, 198-206.
Lin, Y., Pan, D. & Luo, H.: (2021), Hollow direct Z‒Scheme CdS/BiVO₄ composite with boosted photocatalytic performance for RhB degradation and hydrogen production, Mater. Sci. Semicon. Proc. 121, 105453.
Lin, Z. & Wang, X.: (2013), Nanostructure engineering and doping of conjugated carbon nitride semiconductors for hydrogen photosynthesis, Angew. Chem. Int. Ed. 52, 1735-1738.
Liu, Y., Shen, S., Zhang, J., Zhong, W. & Huang, X.: (2019), Cu_2−xSe/CdS composite photocatalyst with enhanced visible light photocatalysis activity, Appl. Surf. Sci. 478, 762-769.
Mao, S., Shi, J.-W., Sun, G., Ma, D., He, C., Pu, Z., Song, K. & Cheng, Y.: (2021), Au nanodots@thiol-UiO66@ZnIn₂S₄ nanosheets with significantly enhanced visible-light photocatalytic H₂ evolution: The effect of different Au positions on the transfer of electron-hole pairs, Appl. Catal. B: Environ. 282, 119550.
McGlade, C. & Ekins, P.: (2015), The geographical distribution of fossil fuels unused when limiting global warming to 2 °C, Nature 517, 187-190.
Rokhsat, E. & Akhavan, O.: (2016), Improving the photocatalytic activity of graphene oxide/ZnO nanorod films by UV irradiation, Appl. Surf. Sci. 371, 590-595.
Schrope, M.: (2011), Oil spill: Deep wounds, Nature 472, 152-154.
Wang, C., Zhang, H., Li, F. & Zhu, L.: (2010), Degradation and mineralization of bisphenol A by mesoporous Bi₂WO₆ under simulated solar light irradiation, Environ. Sci. Technol. 44, 6843-6848.
Wang, S., Zhan, J., Chen, K., Ali, A., Zeng, L., Zhao, H., Hu, W., Zhu, L. & Xu, X.: (2020), Potassium-doped g-C₃N₄ achieving efficient visible-light-driven CO₂ reduction, ACS Sustain. Chem. Eng. 8, 8214-8222.
Xu, K., Zhang, Q., Xu, W., Kang, X. & Wang, L.: (2022), Er-doping g-C₃N₄/boron carbon nitride quantum dots composites under visible light irradiation for tetracycline degradation, Mater. Lett. 311, 131489.
Yan, S. C., Li, Z. S. & Zou, Z. G.: (2009), Photodegradation Performance of g-C₃N₄ Fabricated by Directly Heating Melamine, Langmuir 25, 10397-10401.
Yan, X., Jia, Z., Che, H., Chen, S., Hu, P., Wang, J. & Wang, L.: (2018), A selective ion replacement strategy for the synthesis of copper doped carbon nitride nanotubes with improved photocatalytic hydrogen evolution, Appl. Catal. B: Environ. 234, 19-25.
Yang, Z., Zhang, Y. & Schnepp, Z.: (2015), Soft and hard templating of graphitic carbon nitride, J. Mater. Chem. A 3, 14081-14092.
Ye, J., Liu, J., Li, C., Zhou, P., Wu, S. & Ou, H.: (2017), Heterogeneous photocatalysis of tris(2-chloroethyl) phosphate by UV/TiO₂: Degradation products and impacts on bacterial proteome, Water Res. 124, 29-38.
Zhang, Y., Zhou, J., Cai, W., Zhou, J. & Li, Z.: (2018), Enhanced photocatalytic performance and degradation pathway of Rhodamine B over hierarchical double-shelled zinc nickel oxide hollow sphere heterojunction, Appl. Surf. Sci. 430, 549-560.
Zhao, J., Li, N., Yu, R., Zhao, Z. & Nan, J.: (2018), Magnetic field enhanced denitrification in nitrate and ammonia contaminated water under 3D/2D Mn₂O₃/g-C₃N₄ photocatalysis, Chem. Eng. J. 349, 530-538.
Zhou, Y., Zhang, L., Huang, W., Kong, Q., Fan, X., Wang, M. & Shi, J.: (2016), N-doped graphitic carbon-incorporated g-C₃N₄ for remarkably enhanced photocatalytic H₂ evolution under visible light, Carbon 99, 111-117.

4SupportingInformation.docx

Download PDF

Journal Publication

published 28 Mar, 2023

Read the published version in Environmental Science and Pollution Research →

Reviewers agreed at journal
16 Nov, 2022
Reviewers invited by journal
10 Nov, 2022
Editor invited by journal
12 Oct, 2022
Editor assigned by journal
03 Oct, 2022
First submitted to journal
28 Sep, 2022

You are reading this latest preprint version

Potassium-doped g-C3N4 enables efficient visible-light-driven dye degradation

Status:

Journal Publication

Version 1

Abstract

Figures

Highlights

Introduction

Experimental

Results And Discussion

Conclusion

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1