Electrospinning preparation of g-C3N4/Nb2O5 nanofibers heterojunction for enhanced photocatalytic degradation of organic pollutants in water

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.

effectively improve mutual transfer of photogenerated charge in the heterojunctions, thus ultimately enhance the photocatalytic activity. In addition, visible-light-responsive semiconductor coupling Nb 2 O 5 can also improve the light absorption capacity to a significant extent 31 . g-C 3 N 4 , a two dimensional polymeric semiconductor, has attracted much attention in recently years due to its unique delocalized conjugated π structure formed by sp 2 hybridization of C and N atoms which offers a rapid photoinduced charge separation in the electron transfer process 32 . In particular, g-C 3 N 4 can activate molecular oxygen and produce superoxide radicals, which effectively enhances the activities for photocatalytic reactions. Furthermore, g-C 3 N 4 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 properties 33,34 . Unfortunately, the photocatalytic activity of pure g-C 3 N 4 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-C 3 N 4 with TiO 2 , CeO 2 , BiVO 4 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-C 3 N 4 and metal oxide [35][36][37] . So far, many g-C 3 N 4 / Nb 2 O 5 heterojunction photocatalysts have been successfully prepared to enhance photocatalytic activity for pollutants degradation due to the well match of band gap edges between Nb 2 O 5 and g-C 3 N 4 , which facilitates the charge carrier separation and thus improves the photocatalytic performance. Carvalho et al. reported that g-C 3 N 4 /Nb 2 O 5 heterojunction photocatalysts, assembled as Nb 2 O 5 nanoparticles decorating the g-C 3 N 4 surface via hydrothermal process, exhibited remarkable enhanced photocatalytic activity in the degradation of methylene blue and rhodamine B dyes 38 . Silva et al. reported that g-C 3 N 4 /Nb 2 O 5 heterostructures, assembled as Nb 2 O 5 nanospheres decorating the g-C 3 N 4 surface by a sonochemical method, showed high activity for dye and drug pollutants degradation under visible light irradiation 39 . Hong et al. reported Nb 2 O 5 /g-C 3 N 4 heterojunctions prepared by a simple heating method showed significantly enhanced photocatalytic activity in the degradation of tetracycline hydrochloride 40 . As shown in these reports, the g-C 3 N 4 /Nb 2 O 5 heterojunction had excellent photocatalytic performance and remarkable optoelectronic characteristics for degrading organic pollutants in wastewater compared with individual g-C 3 N 4 and pure Nb 2 O 5 . Nevertheless, most of these studies were based on powder-form Nb 2 O 5 , which was prepared by complicated means such as solvothermal method, chemical precipitation method, et al. These conventional preparation methods may lead to agglomeration of Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 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 sites [41][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 pollutants 45 . Meanwhile, the electrospun nanofibers with nonwoven web structure results in an easy separation and recovery from fluid in photocatalytic process 46,47 . On the basis of above, constructing heterojunctions of electrospun Nb 2 O 5 nanofibers coupled with g-C 3 N 4 would be expected as promising composite photocatalyst for practical applications.
In the present work, we developed a g-C 3 N 4 /Nb 2 O 5 nanofibers heterojunction via a simple electrospinning technique, which exhibited a photocatalytic activity superior to the pure g-C 3 N 4 and electrospun Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction. In a typical procedure, g-C 3 N 4 /Nb 2 O 5 heterojunction was synthesized by electrospinning. First, 10 g of CH 6 ClN 3 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-C 3 N 4 . Subsequently, 0.25 g of NbCl 5 and 0.025 g of g-C 3 N 4 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-C 3 N 4 /NbCl 5 / 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-C 3 N 4 /Nb 2 O 5 heterojunction. For comparison, pure Nb 2 O 5 nanofibers was prepared under the same condition without adding g-C 3 N 4 .
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 www.nature.com/scientificreports/ modes 49 . As expected, all of the major characteristic absorption peaks of g-C 3 N 4 and Nb 2 O 5 were present in the spectrum of g-C 3 N 4 /Nb 2 O 5 heterojunction, suggesting that composite samples contained g-C 3 N 4 . Figure 2 depicted the X-ray diffraction (XRD) patterns of g-C 3 N 4 , Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 heterojunction. As shown in Fig. 2a, the quintessential characteristic diffraction peaks of pure g-C 3 N 4 at 13.4° and 27.6° associated to the (100) and (002) planes of the graphite-like structure of C 3 N 4, respectively 50 . The XRD pattern revealed that the diffraction peaks of Nb 2 O 5 NFs correspond to the orthorhombic phase (standard JCPDS card 30-0873). It is noticeable that the synthesized g-C 3 N 4 /Nb 2 O 5 heterojunction exhibited similar pattern to Nb 2 O 5 NFs. The characteristic peak of g-C 3 N 4 (002) closed to orthorhombic (180) peak was not extrusive in the pattern of g-C 3 N 4 /Nb 2 O 5 heterojunction owing to the comparatively low dosage of g-C 3 N 4 . Moreover, compared with Nb 2 O 5 NFs, the diffraction peaks of Nb 2 O 5 in the heterojunction became weaker and shifted slightly to a smaller value diffraction angle ( Fig. 2b) with the addition of g-C 3 N 4 , which may be caused by the interaction between g-C 3 N 4 and Nb 2 O 5 in the heterojunction. Similar phenomenon was also reported in an earlier literature 51 . Fig. S1 showed the thermogravimetric analysis (TGA) curves of g-C 3 N 4 and g-C 3 N 4 /Nb 2 O 5 heterojunction. The thermal profile of g-C 3 N 4 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 observable 52 . The weight of the g-C 3 N 4 /Nb 2 O 5 heterojunction decreased rapidly in the temperature range 600-700 °C, owing to the demoposition of g-C 3 N 4 occurred in this temperature range. Hence, the g-C 3 N 4 content for the g-C 3 N 4 /Nb 2 O 5 heterostructure could be estimated to be 9.2 wt% neglecting the amount of surface-bound water.
The morphology of g-C 3 N 4 , Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 heterojunction were observed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). It was found that the Nb 2 O 5 NFs represent 1D nanofiber morphology, which had an average diameter of 150 nm with random orientation (Fig. S2a). The  www.nature.com/scientificreports/ g-C 3 N 4 sample displayed an irregular sheet like structure (Fig. S2b). It was perceptible on the SEM image of g-C 3 N 4 /Nb 2 O 5 heterojunction (Fig. 3a) that two semiconductors, Nb 2 O 5 nanofibers and g-C 3 N 4 , 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 efficiency 53 . TEM image (Fig. 3b) further revealed that the g-C 3 N 4 /Nb 2 O 5 heterojunction was composed of homogeneous, long and narrow Nb 2 O 5 NFs that were in direct contact with g-C 3 N 4 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 orthorhombicphase Nb 2 O 5 54,55 , which was in accordance with the XRD analysis results of Nb 2 O 5 . No diffraction spots/rings of g-C 3 N 4 were detected in the SAED spectrum, indicating that g-C 3 N 4 was amorphous in the g-C 3 N 4 /Nb 2 O 5 heterojunction 56 . Figure 3c exhibited that Nb 2 O 5 NFs with a distribution of diameters ~ 150 nm were attached onto the surfaces of g-C 3 N 4 nanosheets, forming g-C 3 N 4 /Nb 2 O 5 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 Nb 2 O 5 57 . With these, a heterojunction formation between Nb 2 O 5 and g-C 3 N 4 was confirmed. This heterojunction would promote charge transfer between Nb 2 O 5 and g-C 3 N 4 and separation of photogenerated electron-hole pairs, both of which would enhance the photocatalytic activity.
The X-ray photoelectron spectroscopy (XPS) survey spectrum and high resolution spectrum of g-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction not only contained Nb, O elements related to the Nb 2 O 5 phase, but also contained C, N elements related to the g-C 3 N 4 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 sp 2 -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-C 3 N 4 , respectively 58 . 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 sp 2 -bonded to C (N-sp 2 C) and tertiary nitrogen groups (N-(C) 3 ) of g-C 3 N 4 , respectively 59 . The Nb 3d spectrum (Fig. 4d) exhibited two significant peaks at around 207.43 and 210.13 eV, corresponding to 3d 5/2 and 3d 3/2 states of Nb 5+ , respectively 60 . The O 1s XPS spectrum (Fig. 4e) could be fitted to two peaks centered at around 530.28 and www.nature.com/scientificreports/ 531.63 eV, which were assigned to the Nb-O bond and adsorbed oxygen, respectively. Notably, compared with those of pure Nb 2 O 5 NFs, the Nb 3d and O 1s peaks of g-C 3 N 4 /Nb 2 O 5 heterojunction shifted slightly to higher energies, evidencing the intense interaction between the Nb 2 O 5 and g-C 3 N 4 that resulted from the formation of effective heterojunction. These phenomena are in good agreement with previous report 53 . Figure 5 showed N 2 absorption-desorption isotherms and pore size distribution curves of Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 heterojunction. As illustrated in Fig. 5a, both the isotherms belonged to type IV isotherm possessed obvious type H 3 hysteresis loop at relative higher p/p 0 , suggesting the existence of slit-shaped pores due to the aggregation of nanoparticles 61 . The BET specific surface areas of Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 heterojunction are 30.09 m 2 /g and 36.18 m 2 /g, respectively. In addition, Fig. 5b displays the pore-size distributions of Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 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 Nb 2 O 5 NFs, 3.7 and 7.1 nm for g-C 3 N 4 /Nb 2 O 5 heterojunction, respectively. The increased BET specific surface area and decreased pore size were likely caused by the incorporation of g-C 3 N 4 , which was fundamental in enhancing the photodegradation activity of electrospun g-C 3 N 4 /Nb 2 O 5 heterojunction.
The UV-Vis absorption spectra of as-prepared samples were depicted in Fig. 6a. It is obvious that the g-C 3 N 4 has absorption in the visible region, which is in good agreement with the narrow band gap. However, the Nb 2 O 5 NFs only absorbs UV light, consistent with wide band gap. Compared to Nb 2 O 5 NFs, the absorption edge of g-C 3 N 4 /Nb 2 O 5 heterojunction shifts to the visible light region with the absorbance edge between those of Nb 2 O 5 NFs and g-C 3 N 4 . It can be concluded that the combination of g-C 3 N 4 and Nb 2 O 5 may enhance the visible light absorption of the sample and thus improve their catalytic activity in the visible region 35 . In addition, the bandgap  www.nature.com/scientificreports/ 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-C 3 N 4 and Nb 2 O 5 NFs were estimated to be approximately 2.7 and 3.2 eV, respectively, consistent with those described for these phases in the literature 31,62 . In addition, the band gap of g-C 3 N 4 /Nb 2 O 5 heterojunction was found to be 2.84 eV. This suggested that the synthetized g-C 3 N 4 /Nb 2 O 5 heterojunction had smaller band gap than that of pure Nb 2 O 5 NFs, which was the precondition of effective photocatalytic activity in visible region. Besides, Mott-Schottky plots (Fig. S3) of the Nb 2 O 5 NFs and g-C 3 N 4 were collected to define their conduction band potential (E CB ). The derived E CB of Nb 2 O 5 NFs and g-C 3 N 4 were estimated to be − 0.59 and − 0.98 eV, respectively 63 . Thus, the valence band potential (E VB ) of them were calculated to be 2.61 and 1.72 eV, respectively.
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 Nb 2 O 5 NFs and g-C 3 N 4 , g-C 3 N 4 /Nb 2 O 5 heterojunction possessed the highest photocatalytic degradation rate owing to the existence of synergistic effect between g-C 3 N 4 and Nb 2 O 5 . After 120 min irradiation, the photodegradation efficiency of RhB for g-C 3 N 4 /Nb 2 O 5 heterojunction was 98.1%. The superior photocatalytic activity of g-C 3 N 4 /Nb 2 O 5 heterojunction over RhB could also be confirmed from kinetics experiment. On the base of the rate equation of − ln(C/C 0 ) = 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-C 3 N 4 /Nb 2 O 5 heterojunction is larger than that of Nb 2 O 5 NFs and g-C 3 N 4 . In addition, phenol, a colorless organic pollutant model, was selected to further evaluate the photocatalytic activity of g-C 3 N 4 /Nb 2 O 5 heterojunction. It can be found that the g-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction still achieved the highest apparent rate constant among all these samples (Fig. 7d). Meanwhile, the total organic carbon (TOC) removal rate of g-C 3 N 4 / Nb 2 O 5 heterojunction sample for degradation of RhB and phenol were shown in Fig. S4. It can be seen that www.nature.com/scientificreports/ 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 reaction 64 . Table 1 presented the photocatalytic degradation rate of various g-C 3 N 4 /Nb 2 O 5 composites, which are comparable to as-synthesized g-C 3 N 4 /Nb 2 O 5 heterojunction, is exhibited remarkable degradation rate.
Photocatalysis mechanism. Investigating the lifetime of photoexcited charge carriers is fundamental to understand the photocatalysis mechanism. The photocurrent responses of g-C 3 N 4 , Nb 2 O 5 NFs and g-C 3 N 4 / Nb 2 O 5 heterojunction were undertaken to manifest the separation efficiency of the electron-hole pairs. As shown in Fig. 8a, g-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction 65 . 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-C 3 N 4 , Nb 2 O 5 NFs and g-C 3 N 4 /Nb 2 O 5 heterojunction, indicating the g-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 and Nb 2 O 5 66 . On the other side, hydroxyl radical (·OH), hole (h + ) and superoxide radical anion (·O 2 -) as major active species play key roles in photocatalytic reaction. To elucidate the mechanism of g-C 3 N 4 /Nb 2 O 5 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 ·O 2 -, respectively. Figure 9 revealed the photocatalytic efficiencies of RhB and phenol with g-C 3 N 4 /Nb 2 O 5 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 ·O 2 is the main active species and h + also play important role in g-C 3 N 4 /Nb 2 O 5 heterojunction for RhB and phenol degradation.
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  www.nature.com/scientificreports/ promoted the electrons transfer at the heterojunction interface and reduced the recombination of carriers, ultimately improved the photodegradation activity of heterojunction 67 . When g-C 3 N 4 /Nb 2 O 5 heterojunction was exposed to irradiation, g-C 3 N 4 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-C 3 N 4 to that of Nb 2 O 5 because the CB edge potential of g-C 3 N 4 (− 0.98 eV) was more negative than that of Nb 2 O 5 (− 0.59 eV). The photogenerated electrons could react with dissolved O 2 near the Nb 2 O 5 NFs to form the main active specie of ·O 2 -, which was mostly responsible for degrading pollutants. Similarly, the VB edge potential of g-C 3 N 4 (1.72 eV) was also more negative than that of Nb 2 O 5 (2.61 eV), and the photogenerated h + transfer to VB of g-C 3 N 4 from VB of Nb 2 O 5 through the tight heterojunction interface. The residual h + on VB of Nb 2 O 5 could oxidize organic molecules to photodegradation products directly. Moreover, the multi-point connected Nb 2 O 5 nanofiber and porous g-C 3 N 4 were also helpful to promote the electron migration.
Reusability of g-C 3 N 4 /Nb 2 O 5 heterojunction. To investigate the stability and reusability of the g-C 3 N 4 / Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction before and after the reaction were similar (Fig. 11b). From a typical SEM image of g-C 3 N 4 /Nb 2 O 5 heterojunction (inset of Fig. 11b) after photocatalytic reaction, it can be clearly seen that the Nb 2 O 5 NFs is still tightly attached with g-C 3 N 4 nanosheet, which indicates that the morphology of g-C 3 N 4 /Nb 2 O 5 heterojunction remains unchanged after photocatalytic reaction.

Conclusions
In summary, the g-C 3 N 4 /Nb 2 O 5 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-C 3 N 4 /Nb 2 O 5 heterojunction exhibited higher photocatalytic activities towards the photodegradation of RhB and phenol under visible irradiation, compared to the pure g-C 3 N 4 and Nb 2 O 5 phases. The low bandgap energy (2.84 eV) as well as the synergistic effect between Nb 2 O 5 NFs and g-C 3 N 4 in the g-C 3 N 4 / Nb 2 O 5 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 (·O 2 -) and hole (h + ) were the major oxidative species for RhB and phenol degradation over the g-C 3 N 4 / Nb 2 O 5 heterojunction photocatalyst. This work may provide a promising future of applying photocatalyst to solving dye pollutant problems and solar energy conversion.