Construction of g-C3N4-mNb2O5 Composites with Enhanced Visible Light Photocatalytic Activity

Abstract

A series of composites consisting of g-C₃N₄ sheet and mesoporous Nb₂O₅ (mNb₂O₅) microsphere were fabricated by in situ hydrolysis deposition of NbCl₅ onto g-C₃N₄ sheet followed by solvothermal treatment. The samples were characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). The photocatalytic activity of the composites was studied by degradation of rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) in aqueous solution under visible light irradiation (λ > 420 nm). Compared with g-C₃N₄ and mNb₂O₅, g-C₃N₄-mNb₂O₅ composites have higher photocatalytic activity due to synergistic effect between g-C₃N₄ and mNb₂O₅. Among these composites, 4% g-C₃N₄-mNb₂O₅ has the highest efficiency and good recyclability for degradation of both RhB and TC-HCl.

1. Introduction

Organic dyes and antibiotics are two types of important products which are widely used in textile and pharmaceutical industries, respectively. The direct discharge of these chemical compounds along with sewage to environment would be seriously harmful to ecosystem and human health. It is, therefore, desirable to explore efficient ways to remove them from water [1,2,3]. In the past decades, the methods of biological treatment, physical adsorption and chemical transformation have been employed in removal of the organic pollutants [1,4,5,6,7]. Among them, much attention has been focused on photocatalytic degradation of organic pollutants over semiconductors such as TiO₂ [7,8], WO₃ [9], ZnO [10], MoS₂ [11], etc. due to their ability to oxidize organics through a redox process with low cost. Nevertheless, the majority of the above semiconductors still suffer from some problems such as limited absorption of visible light, difficult recycling of the catalyst, rapid recombination of photogenerated electrons, holes pairs, etc. n-Type transition metal oxide Nb₂O₅ attracts much interest in photocatalytic reactions due to its high chemical stability, water tolerance, and nontoxicity, although the light absorption of Nb₂O₅ with a wide band gap of ~3.4 eV limits its application only in the UV region [12,13,14]. Therefore, many efforts have been made to improve the absorption ability of Nb₂O₅ in the visible region, such as combination with other semiconductor material [15], ion doping [16] and deposition of noble metal [17].

Recently, graphitic carbon nitride (g-C₃N₄) has been widely used in photocatalytic reactions such as degradation of pollutants, hydrogen generation and selective oxidation of alcohols under visible light due to its excellent chemical/thermal stability, small band gap of 2.7 eV and low cost [18,19,20]. However, pure g-C₃N₄ shows low photocatalytic efficiency owing to fast recombination of photogenerated electrons and holes and small specific surface area [21,22]. It was reported that the composites of g-C₃N₄ and other semiconductors, such as TiO₂ [23,24,25], WO₃ [26], MoS₂ [27], and ZnO [28], could produce a certain number of heterojunction sites which is beneficial to promote electron-hole separation and restrain the recombination efficiently. In addition, some efforts have been paid to the Nb₂O₅/g-C₃N₄ composite which exhibits photocatalytic activity in degradation of tetracycline hydrochloride (TC-HCl) [29], methylene blue (MB) and rhodamine B (RhB) under UV and visible light [30], and production of H₂ under visible light [31]. However, it is still a challenge to prepare novel Nb₂O₅/g-C₃N₄ composites with strong interaction and high dispersion between mesoporous Nb₂O₅ microsphere and g-C₃N₄ layer which could be used as efficient photocatalysts.

Herein, a series of g-C₃N₄-mNb₂O₅ composites, prepared by in situ hydrolysis deposition of NbCl₅ onto g-C₃N₄ sheet followed by solvothermal treatment, have been used as photocatalysts in degradation of RhB and TC-HCl and characterized in details. The results indicate that 4% g-C₃N₄-mNb₂O₅ exhibits low photoluminescence (PL) intensity and narrow band gap which account for its high catalytic activity.

2. Materials and Methods

2.1. Materials

Niobium chloride (NbCl₅) was purchased from Strem Chemicals, Inc (Newburyport, MA, USA). Melamine (99%), absolute ethanol (analytical grade) and tetracycline hydrochloride (TC-HCl, 96%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Rhodamine B (RhB) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pluronic 123 (PEG-PPG-PEG, P123) was purchased from Sigma-Aldrich, Co. (ST. Louis, MO, USA). All the reagents were used without further purification.

2.2. Preparation

The bulk g-C₃N₄ was prepared by heating 2.50 g of melamine in an alumina crucible with a cover in air at 550 °C with ramp rate of 2 °C/min and maintained at 550 °C for 4 h [32]. After being cooled down to room temperature, the obtained product was ground into powder.

The g-C₃N₄-mNb₂O₅ composites were prepared through in situ hydrolysis of NbCl₅ onto g-C₃N₄ and then solvothermal treatment. Typically, 0.99 g of P123 was added into 20 g of absolute ethanol with vigorous stirring until complete dissolution of P123. Bulk g-C₃N₄ powder was dispersed into absolute ethanol and the suspension was sonicated for 1 h. Then, 1.49 g of NbCl₅ was added into a pear-shaped flask under N₂ atmosphere in a glovebox. The solution of P123 and the suspension of g-C₃N₄ were added and the mixture was stirred for 30 min. After that, 0.50 mL of distilled water was pumped into the flask with a rate of 0.0167 mL/min under stirring. After stirring for another 30 min, the mixture was transferred into a 40 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After being cooled down to room temperature, the precipitate was obtained by filtration and washing by ethanol for four times. The solid was dried at 30 °C overnight in a drying oven under vacuum and then transferred to a tube furnace to be calcined at 400 °C for 3 h with a rate of 2 °C/min. The g-C₃N₄-mNb₂O₅ composites prepared with the g-C₃N₄ weight ratios of 1%, 4%, 10%, 20% and 50% were denoted as 1% g-C₃N₄-mNb₂O₅, 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅, 20% g-C₃N₄-mNb₂O₅, 50% g-C₃N₄-mNb₂O₅, respectively. The pure mNb₂O₅ was prepared by the same procedure without adding g-C₃N₄. In addition, the corresponding mechanic mixture of g-C₃N₄ and mNb₂O₅ with the same weight ratio of 4% g-C₃N₄-mNb₂O₅ was prepared by simple grinding and was denoted as 4% g-C₃N₄/mNb₂O₅.

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance diffractometer (Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15418 nm) operated at 40 kV and 40 mA in the 2θ range of 5–70°. The Fourier transform infrared (FT-IR) spectra were recorded on a Thermo Fisher Nicolet iS10 instrument (Waltham, MA, USA) with KBr pellet from 4000 to 400 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was measured by a Perkin Elmer PHI 5000C spectroscope (Waltham, MA, USA). The spectra were recorded with Mg Kα line as the excitation source (hν = 1253.6 eV) at 14 kV and 20 mA. UV-vis diffuse reflectance spectroscopy (DRS) was performed on a Perkin Elmer Lambda 650 spectrophotometer (Waltham, MA, USA) using BaSO₄ as reference. The transmission electron microscopic (TEM) images were obtained using a FEI Tecnai G² F20 S-Twin field emission transmission electron microscope (Hillsboro, Oregon, USA) with an accelerating voltage of 200 kV and a JEOL JEM-2011 transmission electron microscope with an accelerating voltage of 200 kV. The samples were prepared by dropping the ethanol suspension of samples onto the copper grid. The N₂ adsorption-desorption measurements were carried out at 77.3 K on a Micromeritics Tristar II 3020 analytical system. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Pore size distributions were obtained from analysis of the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) model. Photoluminescence (PL) spectra were obtained on an Edinburgh Instruments FLS-980 spectrometer (Edinburgh, UK) with the excitation wavelength of 300 nm.

2.4. Photocatalytic Experiments

The photocatalytic activity of the as-prepared catalysts was performed by photocatalytic degradation of 15 mg/L RhB aqueous solution and 40 mg/L TC-HCl aqueous solution under visible light. The photocatalytic degradation tests were carried out in an instrument (CEL-HXF300, Beijing China Education Au-light Co., Ltd., Beijing, China) with a cylindrical glass reactor and a condensate water circulation equipment (Figure S1). The visible light was provided by a Xe lamp (14 V, 15 A) with a 420 nm cut-off filter (Figure S2). In a typical photocatalytic degradation of RhB or TC-HCl experiment, 30 mg of catalyst was added into 70 mL of RhB aqueous solution or 100 mL of TC-HCl aqueous solution. The suspension was firstly stirred in dark at room temperature to reach adsorption/desorption equilibrium. Then, the reaction was irradiated under visible light (λ > 420 nm) and the solution was vigorously stirred with air bubbling. During the process, 3 mL of suspension was taken at intervals of 20 min (for RhB) or intervals of 5 min (for TC-HCl). All suspensions were centrifuged to remove the catalyst and the concentration of the RhB and TC-HCl were analyzed by UV-vis spectrophotometer with the scanning ranges of 300–700 nm for RhB and 200–600 nm for TC-HCl. To be reused for next reaction cycle, the separated catalyst by centrifugation was washed by water and ethanol three times and then dried at 30 °C in a drying oven under vacuum.

3. Results and Discussion

3.1. Characterization of the Catalysts

3.1.1. XRD

XRD was employed to analyze mNb₂O₅, g-C₃N₄ and g-C₃N₄-mNb₂O₅ composites with different contents of g-C₃N₄ and the results are shown in Figure 1. For mNb₂O₅, the broad diffraction peak around 23.7° corresponds to partial crystallized Nb₂O₅ (JCPDS number: 19-0862), which results from the relatively low calcination temperature of 400 °C. The main diffractions of g-C₃N₄ are at 12.8° and 27.1°, which are assigned to (100) plane due to in-plane tris-s-triazine frameworks and (002) plane due to interlayer stacking of aromatic systems, respectively. The latter reflects interlayer stacking of aromatic systems and the calculated interlayer distance is 0.338 nm [33,34,35]. In the composites, the diffractions of mNb₂O₅ are retained and the characteristic (002) diffraction of g-C₃N₄ is observed when the content of g-C₃N₄ increases to 20% and 50%.

Figure 1. XRD patterns for mNb2O5, g-C3N4 and g-C3N4-mNb2O5 composites with different contents of g-C3N4.

3.1.2. FT-IR

Figure 2 shows the FT-IR spectra of mNb₂O₅, g-C₃N₄ and g-C₃N₄-mNb₂O₅ composites. For the mNb₂O₅ sample, a broad band around 3382 cm⁻¹ is attributed to vibration of O-H of the water molecules adsorbed on the sample [27,30]. The broad band at 615 cm⁻¹ is ascribed to Nb–O–Nb angular vibration [36]. As for g-C₃N₄, the broad band ranging from 3000 to 3500 cm⁻¹ is due to the stretching vibration of N–H groups in g-C₃N₄ and O–H of adsorbed water [25]. Other characteristic bands arisen from the typical stretching modes of C=N and C–N in heterocycles of g-C₃N₄ are at 1237, 1317, 1405, 1461, 1574 and 1640 cm⁻¹. Additionally, the band at 803 cm⁻¹ of g-C₃N₄ is assigned to the breathing mode of the triazine units [33,34]. For g-C₃N₄-mNb₂O₅ composites, all the characteristic bands of mNb₂O₅ and g-C₃N₄ exist except 1% g-C₃N₄-mNb₂O₅ due to small amount of g-C₃N₄, which confirm the existence of two components in the composites. With the increasing of g-C₃N₄, the band at 3382 cm⁻¹ of mNb₂O₅ slightly shifts to smaller wavenumber for g-C₃N₄-mNb₂O₅ composites, which reveals the vibration of N-H. Moreover, the band of g-C₃N₄ at 1237 cm⁻¹ slightly shifts to 1244 cm⁻¹ for all g-C₃N₄-mNb₂O₅ composites except 1% g-C₃N₄-mNb₂O₅ (Figure 2b), revealing the interaction between g-C₃N₄ and mNb₂O₅.

Figure 2. (a) FT-IR spectra of mNb2O5, g-C3N4 and g-C3N4-mNb2O5 composites with different contents of g-C3N4. (a, mNb2O5; b, 1% g-C3N4-mNb2O5; c, 4% g-C3N4-mNb2O5; d, 10% g-C3N4-mNb2O5; e, 20% g-C3N4-mNb2O5; f, 50% g-C3N4-mNb2O5; g, g-C3N4); and (b) enlarged FT-IR spectra corresponding to rectangle region from (a).

3.1.3. TEM

The prepared samples were investigated by field emission transmission electron microscope. Figure 3 shows the TEM images of g-C₃N₄, mNb₂O₅ and 4% g-C₃N₄-mNb₂O₅. The mNb₂O₅ sample is mesoporous materials with pore size of 3–5 nm and its lattice fringe spacing is 0.395 nm (Figure 3a). Figure 3b shows that g-C₃N₄ is veil-like with lamellar structure. In Figure 3c, we could measure that the interlayer distance g-C₃N₄ is 0.339 nm, which is in accordance with XRD result. For the 4% g-C₃N₄-mNb₂O₅ sample, it can be observed that mNb₂O₅ microspheres are deposited on the surface of g-C₃N₄ sheet in Figure 3d,e. Moreover, the corresponding EDS of Figure 3e indicates that the sample consists of C, N, O and Nb elements (Figure 3f). The relevant element contents are shown in Table S1.

Figure 3. TEM images of: (a) mNb2O5; (b,c) g-C3N4; and (d,e) 4% g-C3N4-mNb2O5; (f) EDS analysis of 4% g-C3N4-mNb2O5 corresponding to (e).

3.1.4. N₂ Adsorption-Desorption

Figure 4 shows the nitrogen adsorption-desorption isotherms of g-C₃N₄, mNb₂O₅, 1% g-C₃N₄-mNb₂O₅, 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅ and 50% g-C₃N₄-mNb₂O₅. The isotherm of g-C₃N₄ belongs to the type IV isotherm with the H4 type hysteresis loop. All isotherms of mNb₂O₅, 1% g-C₃N₄-mNb₂O₅, 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅ and 50% g-C₃N₄-mNb₂O₅ are ascribed to the type V isotherm with the H3 type hysteresis loop, indicating the existence of mesoporous structure. The BET specific surface areas of g-C₃N₄ and mNb₂O₅ are 11.2 and 138 m²/g, respectively. The surface area of the composites follows the sequence: 4% g-C₃N₄-mNb₂O₅ (190 m²/g) > 10% g-C₃N₄-mNb₂O₅ (182 m²/g) > 1% g-C₃N₄-mNb₂O₅ (152 m²/g) > 50% g-C₃N₄-mNb₂O₅ (136 m²/g). Among them, 4% g-C₃N₄-mNb₂O₅ exhibits the largest surface area, which is 1.4 times larger than that of mNb₂O₅ and 17 times larger than that of g-C₃N₄. It may originate from that, after ultrasonication of g-C₃N₄, the interlayer distance of g-C₃N₄ increases and the addition of g-C₃N₄ leads to partial disaggregation of mNb₂O₅ [37,38]. When the content of g-C₃N₄ is 50%, high content of g-C₃N₄ in the composites results in g-C₃N₄ stacking together, thus 50% g-C₃N₄-mNb₂O₅ sample shows almost the same specific surface as mNb₂O₅.

Figure 4. Nitrogen adsorption–desorption isotherms of g-C3N4, mNb2O5, 1% g-C3N4-mNb2O5, 4% g-C3N4-mNb2O5, 10% g-C3N4-mNb2O5 and 50% g-C3N4-mNb2O5.

3.1.5. UV-vis DRS

UV-vis diffuse reflectance spectroscopy (DRS) was performed to estimate the band gap of the catalysts, which is important to determine if the catalysts can be excited in the visible-light region [39]. Figure 5a shows the UV-vis diffuse reflectance spectra of mNb₂O₅, g-C₃N₄ and g-C₃N₄-mNb₂O₅ composites with different content of g-C₃N₄. Due to the high band energy value of mNb₂O₅ (3.34 eV), mNb₂O₅ only absorbs ultraviolet light with its fundamental absorption edge near 371 nm. However, g-C₃N₄ possesses a broad peak in the visible region with an absorption edge at ca. 458 nm. For g-C₃N₄-mNb₂O₅ composites, the absorption edge exhibits an obvious red shift relative to pristine mNb₂O₅, indicating the composites could absorb more visible light than mNb₂O₅. The band gap energy (Eg) of the samples was determined by UV-vis DRS with the Tauc model according to the following equation:

αhυ = A(hυ − E_g)^n/2

where α, h, υ and A corresponds to absorption coefficient, Planck constant, light frequency and a constant, respectively, and the constant n depends on whether the transition is direct (n = 1) or indirect (n = 4) [40]. For both g-C₃N₄ and mNb₂O₅, the values of n are 1 [41,42]. Thus, the band gap values of 1% g-C₃N₄-mNb₂O₅, 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅, 20% g-C₃N₄-mNb₂O₅ and 50% g-C₃N₄-mNb₂O₅ are estimated as 3.34, 3.10, 3.19, 3.15 and 2.88 eV, respectively (Figure 5b). The narrowed band gaps of 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅ and 20% g-C₃N₄-mNb₂O₅ results from the formation of heterostructure between g-C₃N₄ and mNb₂O₅ [29]. It implies that 4% g-C₃N₄-mNb₂O₅ can harvest more visible light, which is beneficial to improve the visible-light photocatalytic activity of catalysts. Interestingly, the band gap of 50% g-C₃N₄-mNb₂O₅ is lower than that of other composites, due to high content of g-C₃N₄ in 50% g-C₃N₄-mNb₂O₅.

Figure 5. (a) UV-vis diffuse reflectance spectra; and (b) plots of the (αhυ)2 vs. (hυ) of mNb2O5, g-C3N4, and g-C3N4-mNb2O5 composites with different content of g-C3N4.

3.1.6. XPS

XPS was used to characterize the surface chemical composition and elemental valence states of the samples. As shown in Figure 6a, C and N elements are detected from g-C₃N₄ and Nb and O elements are detected from mNb₂O₅. As for the spectrum of 4% g-C₃N₄-mNb₂O₅, a very weak N 1s peak is ascribed to small content of g-C₃N₄. Figure 6b presents the high resolution XPS spectra of Nb 3d for 4% g-C₃N₄-mNb₂O₅ and mNb₂O₅. For mNb₂O₅, the signals of Nb 3d_5/2 and 3d_3/2 locate at 207.0 and 209.6 eV, respectively, whereas the signals of Nb 3d_5/2 and 3d_3/2 shift slightly (ΔBE ~ 0.4 eV) to a higher binding energy for 4% g-C₃N₄-mNb₂O₅. The upshift may be attributed to band bending.

Figure 6. (a) XPS survey spectra of g-C3N4, mNb2O5 and 4% g-C3N4-mNb2O5; and (b) Nb 3d spectra for mNb2O5 and 4% g-C3N4-mNb2O5.

3.1.7. PL Spectroscopy

To test the generation and recombination efficiency of photogenerated electrons and holes in semiconductors, PL spectroscopy is often conducted. The PL spectra of mNb₂O₅, 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅, 20% g-C₃N₄-mNb₂O₅, 50% g-C₃N₄-mNb₂O₅ and g-C₃N₄ with an excitation wavelength of 300 nm are shown in Figure 7. It can be observed that mNb₂O₅ has a strong emission peak at 474 nm and g-C₃N₄ has a strong emission peak at 472 nm. Compared to mNb₂O₅, the PL emission intensity of g-C₃N₄-mNb₂O₅ samples is significantly reduced, suggesting that the g-C₃N₄-mNb₂O₅ composite has a lower recombination rate of photogenerated electrons and holes than mNb₂O₅. Among the composites, the 4% g-C₃N₄-mNb₂O₅ sample shows the lowest emission intensity at the similar emission peak position which means the lowest recombination efficiency of photogenerated electrons and holes [27,43,44]. Combined with the UV-vis DRS analysis result that 4% g-C₃N₄-mNb₂O₅ has the lowest band gap, 4% g-C₃N₄-mNb₂O₅ demonstrates best potential in photocatalytic degradation reactions among the catalysts studied in this work.

Figure 7. PL spectra of mNb2O5, 4% g-C3N4-mNb2O5, 10% g-C3N4-mNb2O5, 20% g-C3N4-mNb2O5, 50% g-C3N4-mNb2O5 and g-C3N4.

3.2. Visible-Light Photocatalytic Performance and Stability of the Catalysts

The photocatalytic test of mNb₂O₅, g-C₃N₄ and g-C₃N₄-mNb₂O₅ composites with different content of g-C₃N₄ in degradation of RhB and TC-HCl were carried out under visible light irradiation (λ > 420 nm).

3.2.1. Photodegradation of RhB under Visible Light Irradiation

The photocatalytic degradation results are shown in Figure 8a. The standard curve of the absorbance intensity (A) vs. concentration (C) of RhB solution is shown in Figure S4a. The RhB suspension was firstly stirred in dark for 2 h at room temperature to reach adsorption/desorption equilibrium (Figure S3a) [45,46]. No evident decrease of RhB concentration is observed in the absence of catalyst. Therefore, the direct photolysis of RhB could be ignored. Pure mNb₂O₅ and g-C₃N₄ show low photocatalytic activities with only about 27.3% and 25.4% of RhB being degraded after 180 min of irradiation, respectively. The g-C₃N₄-mNb₂O₅ composites show higher degradation efficiency than mNb₂O₅ and g-C₃N₄. For the catalysts containing different content of g-C₃N₄, the sample 4% g-C₃N₄-mNb₂O₅ exhibits the highest degradation efficiency for RhB (97.5%) within 180 min, which is 3.6 and 3.9 times higher than that of mNb₂O₅ and g-C₃N₄, respectively. Furthermore, the corresponding mechanic mixture 4% g-C₃N₄/mNb₂O₅ shows 51% of degradation efficiency for RhB, which is much smaller than that of 4% g-C₃N₄-mNb₂O₅ composite. Therefore, the high degradation efficiency of 4% g-C₃N₄-mNb₂O₅ is ascribed to the formation of heterostructure between g-C₃N₄ and mNb₂O₅, as found in the UV-vis DRS and PL studies.

Figure 8. (a) Photolysis of RhB and photocatalytic activity over as-prepared photocatalysts for RhB; and (b) recyclability for the photodegradation of RhB in the presence of 4% g-C3N4-mNb2O5 under visible light irradiation.

In addition to photocatalytic efficiency, stability and recyclability of the catalysts are also important for application of the catalysts. The 4% g-C₃N₄-mNb₂O₅ sample for photodegradation of RhB was further tested for five cycles. As shown in Figure 8b, after five cycles, the high efficiency for photocatalytic degradation of RhB by 4% g-C₃N₄-mNb₂O₅ is maintained, demonstrating good reusability and stability of 4% g-C₃N₄-mNb₂O₅.

3.2.2. Photodegradation of TC-HCl under Visible Light Irradiation

The photocatalytic degradation of TC-HCl results are shown in Figure 9a. The standard curve of the absorbance intensity (A) vs. concentration (C) of TC-HCl solution can be seen in Figure S4b. The TC-HCl suspension was firstly stirred in dark for 1 h at room temperature to reach adsorption/desorption equilibrium (Figure S3b) [45,46]. No evident decrease of TC-HCl concentration is observed in the absence of catalyst. The pure mNb₂O₅ and g-C₃N₄ show low photocatalytic efficiency in degradation of TC-HCl with about 49.7% and 5.3% after 60 min of irradiation, respectively. The degradation efficiency of g-C₃N₄-mNb₂O₅ composites within 60 min could be listed as the following order: 4% g-C₃N₄-mNb₂O₅ > 10% g-C₃N₄-mNb₂O₅ ≈ 20% g-C₃N₄-mNb₂O₅ > 50% g-C₃N₄-mNb₂O₅ > 1% g-C₃N₄-mNb₂O₅. All g-C₃N₄-mNb₂O₅ composites display higher degradation efficiency than that of mNb₂O₅ and g-C₃N₄. For comparison, the corresponding mechanic mixture 4% g-C₃N₄/mNb₂O₅ shows its degradation efficiency of 52%. Overall, 4% g-C₃N₄-mNb₂O₅ is the optimal catalyst, for which the degradation efficiency of TC-HCl in 60 min is 75.7%. After 30-min experimental run, for 4% g-C₃N₄-mNb₂O₅, 10% g-C₃N₄-mNb₂O₅ and 20% g-C₃N₄-mNb₂O₅, degradation reaction of TC-HCl solution has approached the endpoint. Further degradation is hard to proceed so that the degradation rate is slow and the change of absorbance change is not evident. An appropriate content of g-C₃N₄ leads to this composite with large surface area, narrow band gap and low PL intensity, which favors the titled photocatalytic reactions [41,44,47].

Figure 9. (a) Photolysis of TC-HCl and photocatalytic activity over as-prepared photocatalysts for TC-HCl; and (b) recyclability for the photodegradation of TC-HCl in the presence of 4% g-C3N4-mNb2O5 under visible light irradiation.

The stability and recyclability of 4% g-C₃N₄-mNb₂O₅ catalysts for photocatalytic degradation of TC-HCl were also tested for five cycles. As shown in Figure 9b, after five cycles, photodegradation efficiency of 4% g-C₃N₄-mNb₂O₅ for TC-HCl is almost unchanged. Thus, 4% g-C₃N₄-mNb₂O₅ sample could maintain good stability and it is able to be reused in the photodegradation reaction of TC-HCl as well as in photodegradation reaction of RhB.

In other words, in two photocatalytic reactions of RhB solution and TC-HCl solution under visible-light irradiation, g-C₃N₄-mNb₂O₅ composites showed enhanced photocatalytic activity than mNb₂O₅. The 4% g-C₃N₄-mNb₂O₅ sample shows the highest photocatalytic activity. The other photocatalytic performance of Nb₂O₅/g-C₃N₄ composites were also tested by Ribeiro et al., Shi et al., and Li et al. [29,30,31]. Compared to the reported work, our work focus on the stability of catalysts, which is greatly distinct. The strategy based on the in situ hydrolysis can efficiently avoid the aggregation of Nb₂O₅, thus further facilitating the full exertion of catalyst activity. Besides, the interlayer of g-C₃N₄ sheets can be enlarged by the existence of Nb₂O₅, increasing the surface area of catalyst. The stability of our catalyst is superior to many reported literatures when evaluated in the photocatalytic of RhB and TC-HCl, which can be attributed to highly uniformity and tightly anchoring of Nb₂O₅ on g-C₃N₄ sheets.

4. Conclusions

In summary, we have prepared a series of g-C₃N₄-mNb₂O₅ composites by in situ NbCl₅ hydrolysis, deposition and solvothermal treatment. It is found that the g-C₃N₄-mNb₂O₅ composites exhibit lower PL intensity than pure mNb₂O₅ and g-C₃N₄ and narrower band gap than pure mNb₂O₅, leading to a significant enhancement of photocatalytic activity with excellent stability and recyclability for degradation of RhB and TC-HCl. Among the composites, 4% g-C₃N₄-mNb₂O₅ sample shows the highest photocatalytic activity under visible-light irradiation (λ > 420 nm), which is ascribed to the strong interaction between g-C₃N₄ and Nb₂O₅, its smallest band gap and lowest PL intensity. Based on the above characterization and photocatalytic results, the excellent photocatalytic activity of 4% g-C₃N₄-mNb₂O₅ can be attributed to combined effects between g-C₃N₄ and mNb₂O₅ as follows: (1) the induction of g-C₃N₄ into mNb₂O₅ leads to extension the absorption into visible light region; (2) the recombination of photogenerated electrons and holes is inhibited to some extent; (3) its highest surface area among all composites leads to more active sites; and (4) interaction exists between mNb₂O₅ and g-C₃N₄, as observed in XPS and FT-IR. These also account for the remarkable stability and recyclability of the catalyst 4% g-C₃N₄-mNb₂O₅ in the degradation reaction.

Highlights

1. g-C3N4-mNb2O5 composites were obtained by in situ hydrolysis deposition and solvothermal treatment.

2. Under the visible light irradiation, 4% g-C3N4-mNb2O5 exhibited the highest photocatalytic activities to degrade rhodamine B (97.5% degradation within 180 min) and tetracycline hydrochloride (75.7% degradation within 60 min) solution with excellent stability and recyclability.

3. The g-C3N4-mNb2O5 composites showed narrower band gap, lower PL intensity and larger surface area than pure mNb2O5 and g-C3N4, leading to a significant enhancement of their photocatalytic activity.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/8/6/427/s1, Figure S1: A schematic diagram of photocatalytic equipment, Figure S2: Emission spectrum of Xe lamp with 420 nm filter, Figure S3: Influence of adsorption process on: (a) RhB solution (15 mg/L, 70 mL); and (b) TC-HCl solution (40 mg/L, 100 mL) containing 30 mg 4% g-C3N4-mNb2O5 powder under dark, Figure S4: Absorption changes of: (a) RhB solution (15 mg/L, 70 mL); and (b) TC-HCl solution (40 mg/L, 100 mL) containing 30 mg 4% g-C3N4-mNb2O5 powder under visible light irradiation, Figure S5: Standard curves of absorbance intensity vs. concentration of: (a) RhB solution; and (b) TC-HCl solution, Table S1: The elements content of 4% g-C3N4-mNb2O5 by EDS analysis.

Author Contributions

M.W. managed all the experiments and initial writing; H.W., Y.R., C.W. and Z.W. contributed greatly to data analysis, discussion and manuscript preparation. B.Y. and H.H. supervised and managed the project.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21673046, 21473036, 91645201 and 21371035).

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Barbosa, M.O.; Moreira, N.F.F.; Ribeiro, A.R.; Pereira, M.F.R.; Silva, A.M.T. Occurrence and removal of organic micropollutants: An overview of the watch list of EU Decision 2015/495. Water Res. 2016, 94, 257–279.

2. Zhang, Q.Q.; Ying, G.G.; Pan, C.G.; Liu, Y.S.; Zhao, J.L. Comprehensive evaluation of antibiotics emission and fate in the river basins of China: Source analysis, multimedia modeling, and linkage to bacterial resistance. Environ. Sci. Technol. 2015, 49, 6772–6782.

3. Molla, A.; Sahu, M.; Hussain, S. Synthesis of tunable band gap semiconductor nickel sulphide nanoparticles: Rapid and round the clock degradation of organic dyes. Sci. Rep. 2016, 6, 1–11.

4. Gao, P.; Munir, M.; Xagoraraki, I. Correlation of tetracycline and sulfonamide antibiotics with corresponding resistance genes and resistant bacteria in a conventional municipal wastewater treatment plant. Sci. Total Environ. 2012, 421, 173–183.

5. Lv, Z.Y.; Liu, X.J.; Jia, B.; Wang, H.; Wu, Y.; Lu, Z.P. Development of a novel high-entropy alloy with eminent efficiency of degrading azo dye solutions. Sci. Rep. 2016, 6, 1–11.

6. Ahmed, M.B.; Zhou, J.L.; Ngo, H.H.; Guo, W.S. Adsorptive removal of antibiotics from water and wastewater: Progress and challenges. Sci. Total Environ. 2015, 532, 112–126.

7. Gaya, U.I.; Abdullah, A.H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J. Photochem. Photobiol. C 2008, 9, 1–12.

8. Sorathiya, K.; Mishra, B.; Kalarikkal, A.; Reddy, K.P.; Gopinath, C.S.; Khushalani, D. Enhancement in rate of photocatalysis upon catalyst recycling. Sci. Rep. 2016, 6, 1–9.

9. Szilagyi, I.M.; Forizs, B.; Rosseler, O.; Szegedi, A.; Nemeth, P.; Kiraly, P.; Tarkanyi, G.; Vajna, B.; Varga-Josepovits, K.; Laszlo, K.; et al. WO3 photocatalysts: Influence of structure and composition. J. Catal. 2012, 294, 119–127.

10. Daneshvar, N.; Salari, D.; Khataee, A.R. Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2. J. Photochem. Photobiol. A 2004, 162, 317–322.

11. Zhou, Z.H.; Lin, Y.L.; Zhang, P.A.; Ashalley, E.; Shafa, M.; Li, H.D.; Wu, J.; Wang, Z.M. Hydrothermal fabrication of porous MoS2 and its visible light photocatalytic properties. Mater. Lett. 2014, 131, 122–124.

12. Zhao, Y.; Eley, C.; Hu, J.P.; Foord, J.S.; Ye, L.; He, H.Y.; Tsang, S.C.E. Shape-dependent acidity and photocatalytic activity of Nb2O5 nanocrystals with an active TT (001) surface. Angew. Chem. Int. Ed. 2012, 51, 3846–3849.

13. Liu, H.; Gao, N.; Liao, M.Y.; Fang, X.S. Hexagonal-like Nb2O5 nanoplates-based photodetectors and photocatalyst with high performances. Sci. Rep. 2015, 5, 1–9.

14. Zhao, W.L.; Zhao, W.; Zhu, G.L.; Lin, T.Q.; Xu, F.F.; Huang, F.Q. Black Nb2O5 nanorods with improved solar absorption and enhanced photocatalytic activity. Dalton Trans. 2016, 45, 3888–3894.

15. Oliveira, L.C.A.; Oliveira, H.S.; Mayrink, G.; Mansur, H.S.; Mansur, A.A.P.; Moreira, R.L. One-pot synthesis of CdS@Nb2O5 core shell nanostructures with enhanced photocatalytic activity. Appl. Catal. B 2014, 152, 403–412.

16. Hu, C.Y.; Teoh, W.Y.; Ji, S.L.; Ye, C.H.; Iwase, A. In situ metal doping during modified anodization synthesis of Nb2O5 with enhanced photoelectrochemical water splitting. AIChE J. 2016, 62, 352–358.

17. Zhang, H.L.; Lin, Q.Y.; Ning, S.B.; Zhou, Y.G.; Lin, H.X.; Long, J.L.; Zhang, Z.Z.; Wang, X.X. One-step synthesis of mesoporous Pt-Nb2O5 nanocomposites with enhanced photocatalytic hydrogen production activity. RSC Adv. 2016, 6, 96809–96815.

18. Zhao, Z.W.; Sun, Y.J.; Dong, F. Graphitic carbon nitride based nanocomposites: A review. Nanoscale 2015, 7, 15–37.

19. Su, F.Z.; Mathew, S.C.; Lipner, G.; Fu, X.Z.; Antonietti, M.; Blechert, S.; Wang, X.C. mpg-C3N4-catalyzed selective oxidation of alcohols using O2 and visible Light. J. Am. Chem. Soc. 2010, 132, 16299–16301.

20. Yang, X.L.; Qian, F.F.; Zou, G.J.; Li, M.L.; Lu, J.R.; Li, Y.M.; Bao, M.T. Facile fabrication of acidified g-C3N4/g-C3N4 hybrids with enhanced photocatalysis performance under visible light irradiation. Appl. Catal. B 2016, 193, 22–35.

21. Wang, X.C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76–80.

22. Wang, X.C.; Maeda, K.; Chen, X.F.; Takanabe, K.; Domen, K.; Hou, Y.D.; Fu, X.Z.; Antonietti, M. Polymer semiconductors for artificial photosynthesis: Hydrogen evolution by mesoporous graphitic carbon nitride with visible Light. J. Am. Chem. Soc. 2009, 131, 1680–1681.

23. Yan, J.Q.; Wu, H.; Chen, H.; Zhang, Y.X.; Zhang, F.X.; Liu, S.F. Fabrication of TiO2/C3N4 heterostructure for enhanced photocatalytic Z-scheme overall water splitting. Appl. Catal. B 2016, 191, 130–137.

24. Tong, Z.W.; Yang, D.; Xiao, T.X.; Tian, Y.; Jiang, Z.Y. Biomimetic fabrication of g-C3N4/TiO2 nanosheets with enhanced photocatalytic activity toward organic pollutant degradation. Chem. Eng. J. 2015, 260, 117–125.

25. Jiang, Y.H.; Li, F.; Liu, Y.; Hong, Y.Z.; Liu, P.P.; Ni, L. Construction of TiO2 hollow nanosphere/g-C3N4 composites with superior visible-light photocatalytic activity and mechanism insight. J. Ind. Eng. Chem. 2016, 41, 130–140.

26. Li, Y.G.; Wei, X.L.; Yan, X.Y.; Cai, J.T.; Zhou, A.N.; Yang, M.R.; Liu, K.Q. Construction of inorganic-organic 2D/2D WO3/g-C3N4 nanosheet arrays toward efficient photoelectrochemical splitting of natural seawater. Phys. Chem. Chem. Phys. 2016, 18, 10255–10261.

27. Wang, X.; Hong, M.Z.; Zhang, F.W.; Zhuang, Z.Y.; Yu, Y. Recyclable nanoscale zero valent iron doped g-C3N4/MoS2 for efficient photocatalysis of RhB and Cr(VI) driven by visible light. ACS Sustain. Chem. Eng. 2016, 4, 4055–4062.

28. Wang, J.; Xia, Y.; Zhao, H.Y.; Wang, G.F.; Xiang, L.; Xu, J.L.; Komarneni, S. Oxygen defects-mediated Z-scheme charge separation in g-C3N4/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution. Appl. Catal. B 2017, 206, 406–416.

29. Hong, Y.Z.; Li, C.S.; Zhang, G.Y.; Meng, Y.D.; Yin, B.X.; Zhao, Y.; Shi, W.D. Efficient and stable Nb2O5 modified g-C3N4 photocatalyst for removal of antibiotic pollutant. Chem. Eng. J. 2016, 299, 74–84.

30. Carvalho, K.T.G.; Nogueira, A.E.; Lopes, O.F.; Byzynski, G.; Ribeiro, C. Synthesis of g-C3N4/Nb2O5 heterostructures and their application in the removal of organic pollutants under visible and ultraviolet irradiation. Ceram. Int. 2017, 43, 3521–3530.

31. Huang, Q.Z.; Wang, J.C.; Wang, P.P.; Yao, H.C.; Li, Z.J. In-situ growth of mesoporous Nb2O5 microspheres on g-C3N4 nanosheets for enhanced photocatalytic H2 evolution under visible light irradiation. Int. J. Hydrogen Energy 2017, 42, 6683–6694.

32. Niu, P.; Zhang, L.L.; Liu, G.; Cheng, H.M. Graphene-like carbon nitride nanosheets for improved photocatalytic activities. Adv. Funct. Mater. 2012, 22, 4763–4770.

33. Zheng, Y.; Lin, L.H.; Ye, X.J.; Guo, F.S.; Wang, X.C. Helical graphitic carbon nitrides with photocatalytic and optical activities. Angew. Chem. Int. Ed. 2014, 53, 11926–11930.

34. Yan, S.C.; Li, Z.S.; Zou, Z.G. Photodegradation performance of g-C3N4 fabricated by directly heating melamine. Langmuir 2009, 25, 10397–10401.

35. Dong, F.; Wu, L.W.; Sun, Y.J.; Fu, M.; Wu, Z.B.; Lee, S.C. Efficient synthesis of polymeric g-C3N4 layered materials as novel efficient visible light driven photocatalysts. J. Mater. Chem. 2011, 21, 15171–15174.

36. Yue, Z.K.; Chu, D.M.; Huang, H.; Huang, J.; Yang, P.; Du, Y.K.; Zhu, M.S.; Lu, C. A novel heterogeneous hybrid by incorporation of Nb2O5 microspheres and reduced graphene oxide for photocatalytic H2 evolution under visible light irradiation. RSC Adv. 2015, 5, 47117–47124.

37. Ding, J.; Liu, Q.Q.; Zhang, Z.Y.; Liu, X.; Zhao, J.Q.; Cheng, S.B.; Zong, B.N.; Dai, W.L. Carbon nitride nanosheets decorated with WO3 nanorods: Ultrasonic-assisted facile synthesis and catalytic application in the green manufacture of dialdehydes. Appl. Catal. B 2015, 165, 511–518.

38. Yang, S.B.; Gong, Y.J.; Zhang, J.S.; Zhan, L.; Ma, L.L.; Fang, Z.Y.; Vajtai, R.; Wang, X.C.; Ajayan, P.M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light. Adv. Mater. 2013, 25, 2452–2456.

39. Aguirre, M.E.; Zhou, R.X.; Eugene, A.J.; Guzman, M.I.; Grela, M.A. Cu2O/TiO2 heterostructures for CO2 reduction through a direct Z-scheme: Protecting Cu2O from photocorrosion. Appl. Catal. B 2017, 217, 485–493.

40. Butler, M.A. Photoelectrolysis and physical-properties of semiconducting electrode WO3. J. Appl. Phys. 1977, 48, 1914–1920.

41. Shang, Y.Y.; Chen, X.; Liu, W.W.; Tan, P.F.; Chen, H.Y.; Wu, L.D.; Ma, C.; Xiong, X.; Pan, J. Photocorrosion inhibition and high-efficiency photoactivity of porous g-C3N4/Ag2CrO4 composites by simple microemulsion-assisted co-precipitation method. Appl. Catal. B 2017, 204, 78–88.

42. Lopes, O.F.; Paris, E.C.; Ribeiro, C. Synthesis of Nb2O5 nanoparticles through the oxidant peroxide method applied to organic pollutant photodegradation: A mechanistic study. Appl. Catal. B 2014, 144, 800–808.

43. Liu, B.S.; Zhao, X.J.; Zhao, Q.N.; He, X.; Feng, J.Y. Effect of heat treatment on the UV-vis-NIR and PL spectra of TiO2 films. J. Electron Spectrosc. Relat. Phenom. 2005, 148, 158–163.

44. Xiang, Q.J.; Lv, K.L.; Yu, J.G. Pivotal role of fluorine in enhanced photocatalytic activity of anatase TiO2 nanosheets with dominant (001) facets for the photocatalytic degradation of acetone in air. Appl. Catal. B 2010, 96, 557–564.

45. Zhou, R.X.; Guzman, M.I. CO2 Reduction under Periodic Illumination of ZnS. J. Phys. Chem. C 2014, 118, 11649–11656.

46. Zhou, R.X.; Guzman, M.I. Photocatalytic Reduction of Fumarate to Succinate on ZnS Mineral Surfaces. J. Phys. Chem. C 2016, 120, 7349–7357.

47. Liu, D.; Fernandez, Y.; Ola, O.; Mackintosh, S.; Maroto-Valer, M.; Parlett, C.M.A.; Lee, A.F.; Wu, J.C.S. On the impact of Cu dispersion on CO2 photoreduction over Cu/TiO2. Catal. Commun. 2012, 25, 78–82.