Abstract
In this study, graphitic carbon nitride (g-C3N4) and niobium pentoxide nanofibers (Nb2O5 NFs) heterojunction was prepared by means of a direct electrospinning approach combined with calcination process. The characterizations confirmed a well-defined morphology of the g-C3N4/Nb2O5 heterojunction in which Nb2O5 NFs were tightly attached onto g-C3N4 nanosheets. Compared to pure g-C3N4 and Nb2O5 NFs, the as-prepared g-C3N4/Nb2O5 heterojunction exhibited remarkably enhanced photocatalytic activity for degradation of rhodamine B and phenol under visible light irradiation. The enhanced catalytic activity was attributed predominantly to the synergistic effect between g-C3N4 sheets and Nb2O5 NFs, which promoted the transferring of carriers and prohibited their recombination, confirmed by the measurement of transient photocurrent responses and photoluminescence spectra. In addition, the active species trapping experiments indicated that superoxide radical anion (·O2–) and hole (h+) were the major active species contributing to the photocatalytic process. With its high efficacy and ease of preparation, g-C3N4/Nb2O5 heterojunction has great potentials for applications in treatment of organic pollutants and conversion of solar energy.
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Introduction
In recent years, water pollution caused by textile dyes and other organic pollutants has made serious damage to the ecosystem and human health as they are toxic, mutagenic, and mostly non biodegradable1,2. For the sustainable development of human being, there is urgent demand to remove water contamination. Traditionally, physical, chemical, and biological wastewater treatment processes are used but generally have several disadvantage such as high cost, low degradation efficiency, etc.3. Recently, as an ideal “green strategy” to deal with increasing environmental issues, metal oxide semiconductor based photocatalysis has drawn great attention because of their versatile properties4,5. In photocatalysis process, when the semiconductors are illuminated by photons with energy higher than their band gap, active charges are generated to cause photocatalytic reactions toward pollutant degradation6. Nowadays, semiconductor materials such as TiO2, ZnO, Nb2O5, CeO2, BiOI, graphene, g-C3N4 and their heterojunction composites like Nb2O5/TiO2, BiOI/TiO2, g-C3N4/TiO2, Nb2O5/ZnO and CeO2/Nb2O5 have been used to overcome the water pollution issues7,8,9,10,11,12,13,14,15,16,17,18,19,20. Among these semiconductor materials, Nb2O5, a promising traditional semiconductor material with a band gap of ca. 3.2 eV, has been widely used in a variety of fields, for instance, electrode materials, catalysis, photodecomposition of water and especially photodedegradation of harmful organic pollutants in water, due to its outstanding advantages of thermodynamic stability, nontoxicity and relatively high photocatalytic activity21. However, the rapid recombination of photogenerated charges hindered the practical application of the pure Nb2O5, similar to other traditional semiconductor photocatalysts22,23. With the purpose of facilitating the separation of photoinduced charge, novel Nb2O5-based composites which are suitable for catalysis of pollutant degradation should be constructed. Consequently, researches found that Nb2O5-based heterojunctions with other materials, such as metal (Ag, Au, Pt, etc.), metal oxide (TiO2, NiO, Ag2O, Fe2O3, etc.) and graphene is a prominent method24,25,26,27,28,29,30. Particularly, Nb2O5 heterojunctions coupled with visible-light-responsive semiconductors are recognized as the most effective photocatalysts for wastewater treatment because of the internal electric field, which can suppress the recombination of photogenerated charge and effectively improve mutual transfer of photogenerated charge in the heterojunctions, thus ultimately enhance the photocatalytic activity. In addition, visible-light-responsive semiconductor coupling Nb2O5 can also improve the light absorption capacity to a significant extent31.
g-C3N4, a two dimensional polymeric semiconductor, has attracted much attention in recently years due to its unique delocalized conjugated π structure formed by sp2 hybridization of C and N atoms which offers a rapid photoinduced charge separation in the electron transfer process32. In particular, g-C3N4 can activate molecular oxygen and produce superoxide radicals, which effectively enhances the activities for photocatalytic reactions. Furthermore, g-C3N4 shows massive prospects for application of photocatalytic degradation due to its advantages of narrow band gap, low cost, eco-friendliness and excellent optical and thermal properties33,34. Unfortunately, the photocatalytic activity of pure g-C3N4 is also usually restricted because of the fast recombination of photogenerated electron/hole pairs. In order to improve its efficiency in photocatalytic processes, strategies such as coupling g-C3N4 with TiO2, CeO2, BiVO4 have been proposed, which can not only effectively reduce the photoinduced electron–hole recombination rate, but also form intermediate energy levels in the forbidden band of metal oxide owing to the matching band structure of g-C3N4 and metal oxide35,36,37. So far, many g-C3N4/Nb2O5 heterojunction photocatalysts have been successfully prepared to enhance photocatalytic activity for pollutants degradation due to the well match of band gap edges between Nb2O5 and g-C3N4, which facilitates the charge carrier separation and thus improves the photocatalytic performance. Carvalho et al. reported that g-C3N4/Nb2O5 heterojunction photocatalysts, assembled as Nb2O5 nanoparticles decorating the g-C3N4 surface via hydrothermal process, exhibited remarkable enhanced photocatalytic activity in the degradation of methylene blue and rhodamine B dyes38. Silva et al. reported that g-C3N4/Nb2O5 heterostructures, assembled as Nb2O5 nanospheres decorating the g-C3N4 surface by a sonochemical method, showed high activity for dye and drug pollutants degradation under visible light irradiation39. Hong et al. reported Nb2O5/g-C3N4 heterojunctions prepared by a simple heating method showed significantly enhanced photocatalytic activity in the degradation of tetracycline hydrochloride40. As shown in these reports, the g-C3N4/Nb2O5 heterojunction had excellent photocatalytic performance and remarkable optoelectronic characteristics for degrading organic pollutants in wastewater compared with individual g-C3N4 and pure Nb2O5. Nevertheless, most of these studies were based on powder-form Nb2O5, which was prepared by complicated means such as solvothermal method, chemical precipitation method, et al. These conventional preparation methods may lead to agglomeration of Nb2O5 nanoparticles and then reduce the photocatalytic activity. Hence, it is of great interest to develop a facile and practical method for effective preparation of the evenly nanostructured g-C3N4/Nb2O5 heterojunction with large specific surface area and improved photocatalytic activity.
To date, one-dimensional semiconductor metal oxide nanofibers fabricated through electrospinning have been greatly attractive due to the advantages of high surface areas and large surface-to-volume ratio, which can provide quick charge transfer channels and more active sites41,42,43,44. More significantly, the electrospun nanofibers with a rather high surface area will be optimal carriers for fabricating heterojunction photocatalysts and then provide more active reaction sites for interaction with pollutants45. Meanwhile, the electrospun nanofibers with nonwoven web structure results in an easy separation and recovery from fluid in photocatalytic process46,47. On the basis of above, constructing heterojunctions of electrospun Nb2O5 nanofibers coupled with g-C3N4 would be expected as promising composite photocatalyst for practical applications.
In the present work, we developed a g-C3N4/Nb2O5 nanofibers heterojunction via a simple electrospinning technique, which exhibited a photocatalytic activity superior to the pure g-C3N4 and electrospun Nb2O5 NFs. Meanwhile, the as-prepared heterojunction could be recovered easily by filtration without reducing the photocatalytic activity. Moreover, a possible degradation mechanism is also proposed based on the detailed structural analysis of the heterojunction.
Experimental
Synthesis of g-C3N4/Nb2O5 heterojunction
In a typical procedure, g-C3N4/Nb2O5 heterojunction was synthesized by electrospinning. First, 10 g of CH6ClN3 was heated in an open crucible in static air at a heating rate of 10 °C/min to 600 °C and kept at that temperature for 4 h. The product was collected and ground into powder in an agate mortar to obtain the g-C3N4. Subsequently, 0.25 g of NbCl5 and 0.025 g of g-C3N4 were added into 2.65 mL of N,N-dimethylformamide (DMF) and ultrasonicated for 1 h. Then 0.35 g of polyvinylpyrrolidone (PVP) was dissolved in the mixture. After magnetic stirring for 8 h, the precursor solution of g-C3N4/NbCl5/PVP composite was afforded and transferred into a 5 mL plastic syringe with a 25 gauge stainless steel needle for electrospinning. In this electrospinning experiment, the collector was positioned 20 cm away from the tip of needle and the applied direct voltage between the collector and the needle tip was ~ 18 kV, the precursor solution flow rate was 0.25 mL/h. Then, the collected precursor nanofibers were calcined in muffle furnace at 600 °C for 1 h in air with a heating rate of 10 °C/min to obtain g-C3N4/Nb2O5 heterojunction. For comparison, pure Nb2O5 nanofibers was prepared under the same condition without adding g-C3N4.
Characterizations and photocatalytic experiment
Supporting Materials showed their details.
Results and discussion
Characterization
The Fourier Transform infrared (FTIR) spectra of as prepared samples were shown in Fig. 1. In the FTIR spectrum of Nb2O5 NFs, the peaks at 827 cm−1 and 665 cm−1 were assigned to Nb–O–Nb and Nb=O bonds of Nb2O5, respectively48. For g-C3N4, the broad adsorption band centered at 3188 cm−1 and the peak at 1637 cm−1 belong to the stretching vibration mode of terminal NH groups and the C=N stretching vibration modes, respectively. The peaks around at 1411 cm−1, 1317 cm−1 and 1238 cm−1 were attributed to aromatic C–N stretching. The unique absorption peak at approximately 806 cm−1 was related to the s-triazine ring modes49. As expected, all of the major characteristic absorption peaks of g-C3N4 and Nb2O5 were present in the spectrum of g-C3N4/Nb2O5 heterojunction, suggesting that composite samples contained g-C3N4.
FTIR spectrum of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction.
Figure 2 depicted the X-ray diffraction (XRD) patterns of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction. As shown in Fig. 2a, the quintessential characteristic diffraction peaks of pure g-C3N4 at 13.4° and 27.6° associated to the (100) and (002) planes of the graphite-like structure of C3N4, respectively50. The XRD pattern revealed that the diffraction peaks of Nb2O5 NFs correspond to the orthorhombic phase (standard JCPDS card 30-0873). It is noticeable that the synthesized g-C3N4/Nb2O5 heterojunction exhibited similar pattern to Nb2O5 NFs. The characteristic peak of g-C3N4 (002) closed to orthorhombic (180) peak was not extrusive in the pattern of g-C3N4/Nb2O5 heterojunction owing to the comparatively low dosage of g-C3N4. Moreover, compared with Nb2O5 NFs, the diffraction peaks of Nb2O5 in the heterojunction became weaker and shifted slightly to a smaller value diffraction angle (Fig. 2b) with the addition of g-C3N4, which may be caused by the interaction between g-C3N4 and Nb2O5 in the heterojunction. Similar phenomenon was also reported in an earlier literature51.
(a) XRD patterns of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction and (b) enlarge of 2 theta scale at 26°–40° region of Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction.
Fig. S1 showed the thermogravimetric analysis (TGA) curves of g-C3N4 and g-C3N4/Nb2O5 heterojunction. The thermal profile of g-C3N4 indicated that the as-prepared material was stable in air flow below 600 °C, and heating to 700 °C resulted in no residue of the material being observable52. The weight of the g-C3N4/Nb2O5 heterojunction decreased rapidly in the temperature range 600–700 °C, owing to the demoposition of g-C3N4 occurred in this temperature range. Hence, the g-C3N4 content for the g-C3N4/Nb2O5 heterostructure could be estimated to be 9.2 wt% neglecting the amount of surface-bound water.
The morphology of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction were observed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). It was found that the Nb2O5 NFs represent 1D nanofiber morphology, which had an average diameter of 150 nm with random orientation (Fig. S2a). The g-C3N4 sample displayed an irregular sheet like structure (Fig. S2b). It was perceptible on the SEM image of g-C3N4/Nb2O5 heterojunction (Fig. 3a) that two semiconductors, Nb2O5 nanofibers and g-C3N4, directly and sufficiently contacted through mixed eletrospinning process and calcination. Such line/area contact increased porosity-related characteristics of the heterojunction and resulted in less agglomeration. In particular, such heterojunction formed by nanofiber and nanosheet can enhance the transport of charges and reduce the recombination probability of photoexcited charge carriers, ultimately further improve the photodegradation efficiency53. TEM image (Fig. 3b) further revealed that the g-C3N4/Nb2O5 heterojunction was composed of homogeneous, long and narrow Nb2O5 NFs that were in direct contact with g-C3N4 nanosheets. In addition, the selected area electron diffraction (SAED) pattern (inset of Fig. 3b) depicted broad and strong spots that was assigned to the essentially 1D characteristic of the long nanofibers, indicating the highly single crystallinity of the orthorhombic-phase Nb2O554,55, which was in accordance with the XRD analysis results of Nb2O5. No diffraction spots/rings of g-C3N4 were detected in the SAED spectrum, indicating that g-C3N4 was amorphous in the g-C3N4/Nb2O5 heterojunction56. Figure 3c exhibited that Nb2O5 NFs with a distribution of diameters ~ 150 nm were attached onto the surfaces of g-C3N4 nanosheets, forming g-C3N4/Nb2O5 heterojunction structure. Moreover, the high resolution TEM image (Fig. 3d) revealed the lattice fringe with d-spacing of approximately 0.245 nm, which was in agreement with the (181) spacing of orthorhombic Nb2O557. With these, a heterojunction formation between Nb2O5 and g-C3N4 was confirmed. This heterojunction would promote charge transfer between Nb2O5 and g-C3N4 and separation of photogenerated electron–hole pairs, both of which would enhance the photocatalytic activity.
(a) SEM, (b,c) TEM and (d) high resolution TEM images of g-C3N4/Nb2O5 heterojunction. The inset of (b) is SAED pattern of g-C3N4/Nb2O5 heterojunction.
The X-ray photoelectron spectroscopy (XPS) survey spectrum and high resolution spectrum of g-C3N4/Nb2O5 heterojunction were performed to illuminate the surface composition and the chemical environment. As shown in Fig. 4a, the survey XPS spectrum exhibited that the g-C3N4/Nb2O5 heterojunction not only contained Nb, O elements related to the Nb2O5 phase, but also contained C, N elements related to the g-C3N4 phase. Correspondingly, the C 1 s high-resolution spectra (Fig. 4b) was divided into three fitted peaks at 284.78 eV, 286.28 eV and 288.43 eV. The peak at 284.78 eV corresponded to sp2-hybridized carbon atoms of the carbon standard used to calibrate the binding energies. The peaks at 286.28 eV and 288.43 eV was attributed to the C–N–C and N–C=N backbones coordination in the triazine rings of g-C3N4, respectively58. The N 1s high-resolution spectra (Fig. 4c) was deconvoluted into two fitted peaks at 398.93 eV and 401.13 eV, which could be assigned to N sp2-bonded to C (N-sp2C) and tertiary nitrogen groups (N-(C)3) of g-C3N4, respectively59. The Nb 3d spectrum (Fig. 4d) exhibited two significant peaks at around 207.43 and 210.13 eV, corresponding to 3d5/2 and 3d3/2 states of Nb5+, respectively60. The O 1s XPS spectrum (Fig. 4e) could be fitted to two peaks centered at around 530.28 and 531.63 eV, which were assigned to the Nb–O bond and adsorbed oxygen, respectively. Notably, compared with those of pure Nb2O5 NFs, the Nb 3d and O 1s peaks of g-C3N4/Nb2O5 heterojunction shifted slightly to higher energies, evidencing the intense interaction between the Nb2O5 and g-C3N4 that resulted from the formation of effective heterojunction. These phenomena are in good agreement with previous report53.
(a) XPS survey spectrum and high resolution XPS spectra of (b) C 1s, (c) N 1s, (d) Nb 3d and (e) O 1s in g-C3N4/Nb2O5 heterojunction.
Figure 5 showed N2 absorption–desorption isotherms and pore size distribution curves of Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction. As illustrated in Fig. 5a, both the isotherms belonged to type IV isotherm possessed obvious type H3 hysteresis loop at relative higher p/p0, suggesting the existence of slit-shaped pores due to the aggregation of nanoparticles61. The BET specific surface areas of Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction are 30.09 m2/g and 36.18 m2/g, respectively. In addition, Fig. 5b displays the pore-size distributions of Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction estimated with the BJH method. As seen from the spectra, two characteristic diameters of mesopores (2–50 nm) are located primarily at 3.7 and 10.6 nm for pure Nb2O5 NFs, 3.7 and 7.1 nm for g-C3N4/Nb2O5 heterojunction, respectively. The increased BET specific surface area and decreased pore size were likely caused by the incorporation of g-C3N4, which was fundamental in enhancing the photodegradation activity of electrospun g-C3N4/Nb2O5 heterojunction.
(a) N2 sorption isotherm and (b) pore size distribution of Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction.
The UV–Vis absorption spectra of as-prepared samples were depicted in Fig. 6a. It is obvious that the g-C3N4 has absorption in the visible region, which is in good agreement with the narrow band gap. However, the Nb2O5 NFs only absorbs UV light, consistent with wide band gap. Compared to Nb2O5 NFs, the absorption edge of g-C3N4/Nb2O5 heterojunction shifts to the visible light region with the absorbance edge between those of Nb2O5 NFs and g-C3N4. It can be concluded that the combination of g-C3N4 and Nb2O5 may enhance the visible light absorption of the sample and thus improve their catalytic activity in the visible region35. In addition, the bandgap energy of samples were obtained from Tauc plot by extrapolation to the photon energy-axis. As illustrated in Fig. 6b, the band gap values of the g-C3N4 and Nb2O5 NFs were estimated to be approximately 2.7 and 3.2 eV, respectively, consistent with those described for these phases in the literature31,62. In addition, the band gap of g-C3N4/Nb2O5 heterojunction was found to be 2.84 eV. This suggested that the synthetized g-C3N4/Nb2O5 heterojunction had smaller band gap than that of pure Nb2O5 NFs, which was the precondition of effective photocatalytic activity in visible region. Besides, Mott–Schottky plots (Fig. S3) of the Nb2O5 NFs and g-C3N4 were collected to define their conduction band potential (ECB). The derived ECB of Nb2O5 NFs and g-C3N4 were estimated to be − 0.59 and − 0.98 eV, respectively63. Thus, the valence band potential (EVB) of them were calculated to be 2.61 and 1.72 eV, respectively.
(a) UV–vis diffused reflectance spectra and (b) Tauc plot graph of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction.
Photocatalysis
The photocatalytic activities of the synthetized samples were investigated by decomposing rhodamine B (RhB) under visible light irradiation. As shown in Fig. 7a, compared to Nb2O5 NFs and g-C3N4, g-C3N4/Nb2O5 heterojunction possessed the highest photocatalytic degradation rate owing to the existence of synergistic effect between g-C3N4 and Nb2O5. After 120 min irradiation, the photodegradation efficiency of RhB for g-C3N4/Nb2O5 heterojunction was 98.1%. The superior photocatalytic activity of g-C3N4/Nb2O5 heterojunction over RhB could also be confirmed from kinetics experiment. On the base of the rate equation of − ln(C/C0) = kt, k representing the degradation rate constant, the kinetic curves for the RhB photodegradation over three photocatalysts were obtained, as shown in Fig. 7b. It was obviously that the k value of g-C3N4/Nb2O5 heterojunction is larger than that of Nb2O5 NFs and g-C3N4. In addition, phenol, a colorless organic pollutant model, was selected to further evaluate the photocatalytic activity of g-C3N4/Nb2O5 heterojunction. It can be found that the g-C3N4/Nb2O5 heterojunction also displayed the highest photocatalytic activity for phenol degradation. It was observed that essentially complete degradation of phenol occured over 120 min for g-C3N4/Nb2O5 heterojunction (Fig. 7c). The degradation rate constant of phenol over different catalysts were also calculated by the above rate equation. It is found that g-C3N4/Nb2O5 heterojunction still achieved the highest apparent rate constant among all these samples (Fig. 7d). Meanwhile, the total organic carbon (TOC) removal rate of g-C3N4/Nb2O5 heterojunction sample for degradation of RhB and phenol were shown in Fig. S4. It can be seen that the TOC of both solutions decreased as the reaction time increased and reached almost to zero indicating that mineralization of the pollutants occur during the photocatalytic reaction64. Table 1 presented the photocatalytic degradation rate of various g-C3N4/Nb2O5 composites, which are comparable to as-synthesized g-C3N4/Nb2O5 heterojunction, is exhibited remarkable degradation rate.
The degradation efficiency of (a) RhB and (b) phenol, kinetics curves for (c) RhB and (d) phenol over g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction under visible light irradiation.
Photocatalysis mechanism
Investigating the lifetime of photoexcited charge carriers is fundamental to understand the photocatalysis mechanism. The photocurrent responses of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction were undertaken to manifest the separation efficiency of the electron–hole pairs. As shown in Fig. 8a, g-C3N4/Nb2O5 heterojunction exhibited the strongest photocurrent density in comparison with that of the single component, which reflected the reduced interface resistance and the enhanced separation and migration efficiency of the photoinduced electron–hole pairs in g-C3N4/Nb2O5 heterojunction65. This was also consistent with the photoluminescence (PL) spectroscopy results. Based on Fig. 8b, it can be seen that the peak intensities gradually decrease in the order of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction, indicating the g-C3N4/Nb2O5 heterojunction possessing the lowest electron–hole recombination efficiency. It can be concluded that the recombination of photogenerated charge carriers were inhibited effectively due to the heterojunction construction of g-C3N4 and Nb2O566.
(a) Transient photocurrent response and (b) PL spectra of g-C3N4, Nb2O5 NFs and g-C3N4/Nb2O5 heterojunction.
On the other side, hydroxyl radical (·OH), hole (h+) and superoxide radical anion (·O2–) as major active species play key roles in photocatalytic reaction. To elucidate the mechanism of g-C3N4/Nb2O5 heterojunction, free radicals trapping experiments were performed to confirm the active species contributing to RhB and phenol photodegradation. In these experiments, isopropanol (IPA), triethanolamine (TEA) and p-benzoquinone (BZQ) acted as the scavenger for ·OH, h+ and ·O2–, respectively. Figure 9 revealed the photocatalytic efficiencies of RhB and phenol with g-C3N4/Nb2O5 heterojunction after adding various scavengers. The reaction rate constant showed almost unchanged in the presence of IPA, while the addition of TEA and BZQ led to an obviously decrease of the reaction rate constant. In view of the above results, it can be proposed that ·O2– is the main active species and h+ also play important role in g-C3N4/Nb2O5 heterojunction for RhB and phenol degradation.
Effects of different scavengers on RhB and phenol photodegradation over the g-C3N4/Nb2O5 heterojunction.
Based on the above analysis and experiments, the mechanism was proposed and schematically illustrated in the Fig. 10. It was well understood that the distinction of CB edge potentials between semiconductors strongly promoted the electrons transfer at the heterojunction interface and reduced the recombination of carriers, ultimately improved the photodegradation activity of heterojunction67. When g-C3N4/Nb2O5 heterojunction was exposed to irradiation, g-C3N4 component was easy to be excited by visible light due to its narrow band gap. And the photogenerated electrons were easy to transfer from CB of g-C3N4 to that of Nb2O5 because the CB edge potential of g-C3N4 (− 0.98 eV) was more negative than that of Nb2O5 (− 0.59 eV). The photogenerated electrons could react with dissolved O2 near the Nb2O5 NFs to form the main active specie of ·O2–, which was mostly responsible for degrading pollutants. Similarly, the VB edge potential of g-C3N4 (1.72 eV) was also more negative than that of Nb2O5 (2.61 eV), and the photogenerated h+ transfer to VB of g-C3N4 from VB of Nb2O5 through the tight heterojunction interface. The residual h+ on VB of Nb2O5 could oxidize organic molecules to photodegradation products directly. Moreover, the multi-point connected Nb2O5 nanofiber and porous g-C3N4 were also helpful to promote the electron migration.
Schematic diagram of photogenerated charges in g-C3N4/Nb2O5 heterojunction.
Reusability of g-C3N4/Nb2O5 heterojunction
To investigate the stability and reusability of the g-C3N4/Nb2O5 heterojunction, recycling test of the photodegradation were performed. After each photodegradation cycle, the catalyst was filtered from the solution and dried at 80 °C for further use. As depicted in Fig. 11a, the g-C3N4/Nb2O5 heterojunction showed no obvious reduction of photocatalytic activity after four cycles for both RhB and phenol, demonstrating its excellent stability and reusability for the multiple times for organic pollutants degradation. And the XRD patterns of the g-C3N4/Nb2O5 heterojunction before and after the reaction were similar (Fig. 11b). From a typical SEM image of g-C3N4/Nb2O5 heterojunction (inset of Fig. 11b) after photocatalytic reaction, it can be clearly seen that the Nb2O5 NFs is still tightly attached with g-C3N4 nanosheet, which indicates that the morphology of g-C3N4/Nb2O5 heterojunction remains unchanged after photocatalytic reaction.
(a) The reusability of g-C3N4/Nb2O5 heterojunction: photodegradation of RhB and phenol. (b) XRD patterns of g-C3N4/Nb2O5 heterojunction before and after photodegradation, and a typical SEM image (inset) of g-C3N4/Nb2O5 heterojunction after the photodegradation.
Conclusions
In summary, the g-C3N4/Nb2O5 heterojunction was successfully synthesized using a simple and facile electrospinning-calcination process and displayed excellent photocatalytic activity for RhB and phenol degradation. Particularly, the as-prepared g-C3N4/Nb2O5 heterojunction exhibited higher photocatalytic activities towards the photodegradation of RhB and phenol under visible irradiation, compared to the pure g-C3N4 and Nb2O5 phases. The low bandgap energy (2.84 eV) as well as the synergistic effect between Nb2O5 NFs and g-C3N4 in the g-C3N4/Nb2O5 heterojunction enhanced the photocatalytic activity, which were beneficial to a rapid photoinduced charge separation in the electron transfer process and a slow charge recombination. In addition, both superoxide radical anion (·O2–) and hole (h+) were the major oxidative species for RhB and phenol degradation over the g-C3N4/Nb2O5 heterojunction photocatalyst. This work may provide a promising future of applying photocatalyst to solving dye pollutant problems and solar energy conversion.
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Acknowledgements
This research was funded by the Qinglan Project of Jiangsu Province, the Advanced Research and Training Program for Academic Leader Teacher in Higher Vocational Colleges of Jiangsu Province (2021GRFX046), the Science and Technology Program of Nantong, China (JC2021165, JC2021063).
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L.W. prepared photocatalysts and wrote the main manuscript text. Y.L. designed catalyst and measured the chemical structure of catalyst. P.H. performed manuscript revision and editing.
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Wang, L., Li, Y. & Han, P. Electrospinning preparation of g-C3N4/Nb2O5 nanofibers heterojunction for enhanced photocatalytic degradation of organic pollutants in water. Sci Rep 11, 22950 (2021). https://doi.org/10.1038/s41598-021-02161-x
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DOI: https://doi.org/10.1038/s41598-021-02161-x
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