Synthesis and photocatalytic activity of a bentonite/g-C₃N₄ composite

Abstract

Novel bentonite/g-C₃N₄ composite photocatalysts were synthesized through a conventional calcination method and systematically characterized by thermogravimetric analysis (TG), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and photocurrent–time measurement (PT). The optimal sample bentonite/g-C₃N₄ (5.4%) composite shows an enhanced photocurrent value (about 8 times as high as that of g-C₃N₄) under visible light irradiation and high efficiency for the degradation of methylene blue (the photoreaction kinetics constant value is about 2.5 times that of g-C₃N₄) under visible light. Results show the improved photoactivity is mainly attributed to the electrostatic interaction between g-C₃N₄ and negatively charged bentonite, this leads to the efficient migration of the photogenerated electrons and holes of g-C₃N₄. This study reports an inexpensive and environmentally friendly photocatalyst for pollution degradation and prospective photoelectric materials.

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1 Introduction

Due to the incoming energy and environmental problems, photocatalysis is an attractive but very challenging process to convert solar energy into chemical energy. One of the central tasks to be solved before its practical application is to develop highly efficient, sustainable, visible-light-responsive photocatalytic materials.

Recently, Wang et al. reported that a metal-free polymeric photocatalyst, graphitic carbon nitride (g-C₃N₄), showed a good photocatalytic performance for hydrogen or oxygen production via water splitting under visible light irradiation.¹ The metal free g-C₃N₄ photocatalyst possesses very high thermal and chemical stability as well as interesting electronic properties, which make it valuable for photocatalytic applications. However, the photocatalytic efficiency of bare g-C₃N₄ is limited due to the high recombination rate of photogenerated electron hole pairs.^2,3 To resolve this problem, efforts have been made to improve the photocatalytic performance of g-C₃N₄, for example, by controlling shape and size,^4,5 introducing heteroatoms (e.g. Au,⁶ Ag,⁷ Zn,⁸ B–F,⁹ S,¹⁰ P² and CNT¹¹) and coupling with other semiconductors (e.g. ZnO,¹² WO₃,^13,14 BiPO₄,¹⁵ YVO₄,¹⁶ BiOBr,¹⁷ AgX¹⁸ and TaON¹⁹). Therefore, it is significant to search for efficient materials to modify g-C₃N₄.

Being cheap, abundant, non-toxic and environmental friendly, bentonite clay represents an attractive substrate for immobilization of variety of photocatalysts. The basic structural unit of bentonite is composed of two tetrahedrally coordinated sheets of silicon ions surrounding a sandwiched and octahedrally coordinated sheet of aluminum ions. The isomorphous substitution of Al³⁺ for Si⁴⁺ in the tetrahedral layer and Mg²⁺ or Zn²⁺ for Al³⁺ in the octahedral layer results in a net negative charge on the clay surface.^20,21 Compared with other clays, it has excellent adsorption properties and possesses sorption sites available within its interlayer space as well as on the outer surface and edges.²⁰ The above factors make bentonite a good adsorbent for the removal of dye in aqueous solutions.^22–24 Furthermore, studies have confirmed that bentonite is a good matrix for synthesizing composite photocatalysts with higher photocatalytic activity, such as ZnO/bentonite,²⁵ TiO₂/bentonite,²⁶ MoS₂/bentonite,²⁷ BiVO₄/bentonite,²⁸ etc. These composites all showed more reactivities towards the degradation of organic pollutants due to proper dispersion of nanosized photocatalyst in a solid supports thus generating distinct reaction sites. Moreover, all as-prepared bentonite-composites show much higher S_BET values and improved adsorption capacity, which led to better photocatalytic efficiency compared to bare nanoparticles and less amount of photocatalyst required for the degradation of organic pollutants. As a result, combining semiconductor photocatalysts with bentonite may increase the photocatalytic efficiency by extending the visible light absorption and increasing the charge separation. However, the photocatalytic effect of bentonite combining with g-C₃N₄ has not been discussed, and the photocatalytic mechanism needed to be further investigated.

It is well known that both g-C₃N₄ and bentonite clay are layered materials,^29,30 so the interface of the two layered compounds may be tight, which allow the prompt migration of light-induced charge, thus resulting in an efficient photocatalytic reaction.³¹ Further more, the composite structure of bentonite/g-C₃N₄ with a large surface area can be prepared readily thus generating more distinct reactions,²⁵ which is beneficial to the increasing of photocatalytic activity. Remarkably, both bentonite and g-C₃N₄ do not contain heavy metal ion and all elements are rich in the soil. Above factors are favorable for bentonite/g-C₃N₄ as a potential “green” photocatalyst in photodegradation pollutants application.

Herein, a facile calcination route was proposed to synthesize the novel visible-light-responsive bentonite/g-C₃N₄ composite. In this work the combining process of bentonite with g-C₃N₄ was studied. The photodegradation experiments were performed on methylene blue (MB) and the possible mechanisms of enhancement of photocatalytic activity were also investigated.

2 Experimental

2.1 Preparation of photocatalysts

All starting regents were purchased from sinopharm company sources and used without further purification. Distilled water was used throughout.

Fig. 1 shows schematic illustration of the fabrication of bentonite/g-C₃N₄ composites. Synthesis of bentonite/g-C₃N₄: first, in order to obtain well dispersed bentonite sheets, three different amounts of bentonite (0.05 g, 0.10 g, 0.15 g) was dispersed in 50 mL distilled water with 30 minutes continuous ultrasonic at 80 °C. Subsequently 3 g dicyanamide was added to the bentonite solutions respectively with stirring at 25 °C overnight. Second, the mixture solution was dried at 90 °C for 24 h, during this process part of the bentonite sheets maybe aggregate and accompany with the precipitation of dicyanamide crystals. The resulting dried mixtures were heated at a rate of 20 °C min⁻¹ to reach a temperature to 520 °C and kept at this temperature for another 5 h in a muffle furnace with a semiclosed system. The obtained products are grayish yellow, which were obtained from three reactant systems (0.05 g bentonite + 3 g dicyanamide, 0.10 g bentonite + 3 g dicyanamide, 0.15 g bentonite + 3 g dicyanamide). According to thermogravimetric analysis (TG) results, the weight contents of bentonite in bentonite/g-C₃N₄ composites were estimated to be 2.2%, 5.4% and 13.2%, respectively. As a result, the bentonite/g-C₃N₄ composites were named as bentonite/g-C₃N₄ (2.2%), bentonite/g-C₃N₄ (5.4%) and bentonite/g-C₃N₄ (13.2%).

Fig. 1 Schematic illustration of the fabrication of bentonite/g-C₃N₄ composites.

Bulk g-C₃N₄ was prepared as follows: 5 g dicyanamide was directly heated to 520 °C at a rate of 20 °C min⁻¹ and then kept at this temperature for another 4 h.

Based on the TG result of bentonite/g-C₃N₄ (5.4%) composite, bentonite and g-C₃N₄ mechanical mixture were prepared by mixing bentonite (0.1 g) with g-C₃N₄ (1.76 g), which was denoted as bentonite/g-C₃N₄ mixture.

2.2 Characterization of photocatalysts

The crystalline phases of bentonite/g-C₃N₄ composites were analyzed by XRD using Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5418 Å) within the range of 2θ = 5–80°. The morphologies and structure of the obtained samples were examined by TEM (H-600-II, Hitachi). FT-IR spectra of samples were recorded on a Nicolet Avatar-370 spectrometer at room temperature. Ultraviolet visible (UV-vis) diffuse reflection spectra were measured using a UV-vis spectrophotometer (Shimadzu UV-2450, Japan) in the range of 200 nm to 800 nm. BaSO₄ was used as the reflectance standard material. XPS analysis was performed on an ESCA Lab MKII X-ray photo-electron spectrometer using the Mg Kα radiation. TG analysis was done on STA-449C Jupiter (NETZSCH Corporation, Germany). The experiment temperature ranged from 25 °C to 790 °C at a constant heating rate of 10 °C min⁻¹ under air condition. The nitrogen adsorption–desorption isotherms at 77 K were investigated using a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, USA). PL spectra of the catalyst were measured on the QuantaMaster & TimeMaster Spectrofluorometer, QuantaMaster™ 40 (Photon Technology International, Inc.). The samples were excited at wavelength of 325 nm to measure the emission spectrum.

2.3 Photocurrent–time measurement

Photocurrent measurements were performed on an electro-chemical analyzer (CHI660D, CHI Shanghai, Inc.) in a standard three-electrode configuration with a Pt wire as the counter electrode and Ag/AgCl (in saturated KCl) as a reference electrode. Irradiation proceeded with a Xe arc lamp. Na₂SO₄ (0.1 M) aqueous solution was used as the electrolyte. The typical working electrode was prepared as follows: the 5 mg ground sample was mixed with 1 mL distilled water solution to make slurry. The slurry was then dispersed onto an ITO glass electrode with 1 cm × 1 cm area and the electrode was dried at 50 °C for 8 h.

2.4 Photocatalytic activity

The photocatalytic activity of the samples was evaluated by the degradation of MB dye under visible light irradiation. An aqueous solution of MB (100 mL, 20 mg L⁻¹) was placed in a glass, and 100 mg photocatalyst was added. Photocatalytic activity of the sample was evaluated under a 300 W Xe lamp with a 400 nm cutoff filter. Prior to irradiation, the suspensions were magnetically stirred in the dark for about 60 min to ensure the establishment of an adsorption–desorption equilibrium between the photocatalysts and MB dye. At certain time intervals, 3 mL aliquots were sampled and centrifuged to remove the photocatalyst particles. Then the filtrates were analyzed by recording variations of the absorption band maximum (663 nm) in the UV-vis spectra of MB by using an UV-vis spectrophotometer (UV-2450 Shimadzu). The photocatalytic degradation efficiency was obtained by the concentration decrease of MB.

3 Results and discussion

3.1 XRD analysis

Fig. 2 shows the powder XRD patterns of bentonite, g-C₃N₄ and various bentonite/g-C₃N₄ composites. As can be seen from Fig. 2A, for g-C₃N₄, two pronounced peaks can be indexed to (100) and (002) diffraction planes of the graphite-like carbon nitride, this corresponds to the interlayer stacking and the characteristic interlayer stacking peak of aromatic systems, respectively.^1,32 For the bentonite clay, the peaks marked by (M) are the characteristic of the montmorillonite in the bentonite sample. The other peaks correspond to the impurities of feldspar, quartz and Fe(Ca)CO₃.³³ In the case of bentonite/g-C₃N₄ composites, crystalline bentonite is not obviously observed in the bentonite/g-C₃N₄ photocatalysts with low bentonite loadings. However, in the enlarged Fig. 2B from 25° to 30°, the as-prepared bentonite/g-C₃N₄ composites with higher bentonite loadings (>5.4%) exhibited a crystalline bentonite peak at 26.66°, the peak intensity increased with the increase of bentonite loading. This result suggested that the bentonite/g-C₃N₄ composites are constituted by g-C₃N₄ and bentonite.

Fig. 2 (A) XRD pattern of bentonite, g-C₃N₄ and bentonite/g-C₃N₄ composite, (B) enlarged XRD pattern of bentonite, g-C₃N₄ and bentonite/g-C₃N₄ composites from 25° to 30°.

3.2 BET analysis

In order to investigate the surface property changing of the bentonite/g-C₃N₄ composites after introducing bentonite into g-C₃N₄, the S_BET dates were obtained. The S_BET of the bentonite/g-C₃N₄ (2.2%), bentonite/g-C₃N₄ (5.4%) and bentonite/g-C₃N₄ (13.2%) composites are 18.3 m² g⁻¹, 19.6 m² g⁻¹ and 24.1 m² g⁻¹, respectively, which are much higher than that of pure g-C₃N₄ (5.9 m² g⁻¹). In view of the large surface area bentonite, the S_BET increases with the increasing weight content of bentonite. Similarly, in BiVO₄/bentonite²⁸ and TiO₂/bentonite³⁴ system, after BiVO₄ and TiO₂ were loaded on the bentonite clay, increased S_BET of the composites were obtained. This fact implies that the introduction of bentonite is favorable for the formation of high specific surface areas bentonite/g-C₃N₄ composite.

3.3 TEM analysis

Fig. 3 shows the size and morphology of g-C₃N₄, bentonite and bentonite/g-C₃N₄ composites. It can be seen that g-C₃N₄ sample has thick layered structures (Fig. 3A) and bentonite also has sheet like morphology (Fig. 3B). In contrast to g-C₃N₄, it can be clearly observed that bentonite/g-C₃N₄ (2.2%) and (5.4%) composites are composed of thin and curved layered structure (Fig. 3C–F). In the enlarged TEM (Fig. 3D), the bentonite layers can be observed clearly, suggesting bentonite layers combining well with g-C₃N₄ layers. After introducing more bentonite into g-C₃N₄, the bentonite/g-C₃N₄ (13.2%) composite can be observed with abundant fluffy sheets (Fig. 3G and H) and parts of the sample shows dark grey color, which may be due to some aggregating products. The fluffy sheets are in accordance with the enlarged surface area in BET analysis.

Fig. 3 TEM images of (A) g-C₃N₄, (B) bentonite, (C and D) bentonite/g-C₃N₄ (2.2%) composite, (E and F) bentonite/g-C₃N₄ (5.4%) composite and (G and H) bentonite/g-C₃N₄ (13.2%) composite.

3.4 TG analysis

Fig. 4 shows the TG curves of the bentonite/g-C₃N₄ composites. It can be observed that bentonite shows slight weight loss from 25 °C to 790 °C, which is associated with residual water and dehydroxylation of the aluminosilicate.³⁵ As for bentonite/g-C₃N₄ composites, they have no obvious difference in the thermal stability. The initial decomposition temperature of bentonite/g-C₃N₄ composites is about at 530 °C, which is lower than that of g-C₃N₄ at 550 °C. The reason may be attributed to the existence of fluffy sheets of g-C₃N₄ in the composites (Fig. 3C–H). However, the complete decomposition temperature of bentonite/g-C₃N₄ composites is about at 730 °C, which is higher than that of g-C₃N₄ at 670 °C. The reason is due to the presence of bentonite nanolayers, which can act as barriers maximizing the heat insulation.^36,37

Fig. 4 TG curves for heating bentonite, g-C₃N₄ and bentonite/g-C₃N₄ composites from 25 °C to 790 °C at a heating rate of 10 °C min⁻¹.

3.5 XPS analysis

XPS measurements were performed to determine the valence states of various species. Fig. 5A shows the survey scan XPS spectra of g-C₃N₄ and the bentonite/g-C₃N₄ (5.4%) composite. The result indicates the presence of Al, Si, C, N and O in the bentonite/g-C₃N₄ (5.4%) composite. High resolution spectra of C 1s and N 1s are shown in Fig. 5B and C. The C 1s peak positions in g-C₃N₄ are at 288.1 eV and 284.5 eV, respectively. The binding energy at 288.1 eV is assigned to a C–N–C coordination.³⁸ The other C 1s peak at 284.5 eV is attributed to the adventitious carbon on the surface of g-C₃N₄.³⁹ Fig. 5C shows the N 1s binding energy of g-C₃N₄ at 398.7 eV, which is assigned to sp²-hybridized nitrogen (C=N–C).^38,40 In contrast to g-C₃N₄, the C 1s and N 1s binding energies of bentonite/g-C₃N₄ (5.4%) composite show minor negative shifts to 288.0 eV and 398.6 eV, respectively. This result suggests that there exists some interaction between bentonite and g-C₃N₄. Therefore, with the combination of the XRD, TEM and XPS investigation, the results confirmed that there were both bentonite and g-C₃N₄ species in the composite.

Fig. 5 XPS spectra of g-C₃N₄ and bentonite/g-C₃N₄ (5.4%) composite: (A) survey spectrum, (B) C 1s and (C) N 1s.

3.6 FT-IR analysis

Fig. 6 shows the FT-IR spectra of bentonite, g-C₃N₄, and bentonite/g-C₃N₄ composites. As can be seen in Fig. 6, several bands in the 1200–1650 cm⁻¹ regions (1241, 1322, 1405, 1561 and 1642 cm⁻¹) are associated with the typical stretching modes of CN heterocycles.⁴¹ The peaks at 1642 cm⁻¹ and 1561 cm⁻¹ belonged to the C=N stretching vibrations and s-triazine ring modes,^42,43 while the peaks at 1241 cm⁻¹, 1322 cm⁻¹ and 1405 cm⁻¹ to the aromatic C–N strechings.^3,43 For bentonite sample, the band at 794 cm⁻¹ confirms the presence of quartz in the sample. The band at 691 cm⁻¹ corresponds to the deformation modes of the Si–O bond.³³ The bands at 522 cm⁻¹ and 1044 cm⁻¹ correspond to Al–O–Si bending vibrations and Si–O bending vibrations,^33,44 respectively. In the case of bentonite/g-C₃N₄ composites, the bands of g-C₃N₄ can be observed clearly, whereas the typical bands of bentonite can not be obviously observed, which is attributed to the low content of bentonite and existence of g-C₃N₄.

Fig. 6 FT-IR spectra of bentonite, g-C₃N₄, and bentonite/g-C₃N₄ composites.

3.7 DRS analysis

Fig. 7 shows the UV-vis DRS spectra of pure g-C₃N₄ and bentonite/g-C₃N₄ composites. In Fig. 7A, the pure g-C₃N₄ has an absorption edge at about 460 nm, which can be assigned to the intrinsic band gap of g-C₃N₄ (2.70 eV).¹ The absorption edges of bentonite/g-C₃N₄ composites shift significantly to longer wavelengths compared with the pure g-C₃N₄, indicating that there may be some interaction between bentonite and g-C₃N₄. In previous BET and TEM analysis, the bentonite/g-C₃N₄ composites show enlarged surface area and much fluffy layer structure. In view of the enlarged surface area and the partly imperfect polymer-like structure of g-C₃N₄,⁴⁵ the g-C₃N₄ in the as-obtained bentonite/g-C₃N₄ composites might have more defects compared with the pure g-C₃N₄, which leads to the enhanced light harvesting in the visible light range.^46,47

Fig. 7 (A) UV-vis spectra of g-C₃N₄ and bentonite/g-C₃N₄ samples, (B) plot of (αhν)^1/2 versus energy (hν) for the band gap energy of g-C₃N₄ and bentonite/g-C₃N₄ composites.

The band gap energy (E_g) can be estimated from the intercept of tangents to the plots of (αhν)^1/2 vs. photon energy. As shown in Fig. 7B, the calculated E_g value of pure g-C₃N₄ is 2.70 eV, in the case of bentonite/g-C₃N₄ composites, the E_g values of bentonite/g-C₃N₄ (2.2%, 5.4%, 13.2%) composites are 2.63 eV, 2.59 eV and 2.55 eV respectively, which are lower than that of pure g-C₃N₄. Therefore, the visible light responses of bentonite/g-C₃N₄ composites are significantly improved.

3.8 Photocatalytic activity

Fig. 8A shows the photocatalytic activity of g-C₃N₄ and bentonite/g-C₃N₄ samples for the degradation of MB (20 mg L⁻¹) under visible light. When there is no photocatalyst involved with visible light irradiation, the organic contaminant MB is very low, indicating that MB self-degradation was negligible. In the presence of various bentonite/g-C₃N₄ composites the MB undergoes pronounced decomposition under visible light irradiation. Significantly, the bentonite/g-C₃N₄ (5.4%) induced 90% MB degradation within 3 h under visible light irradiation, which is much higher than that of g-C₃N₄ (62%) and bentonite/g-C₃N₄ mixture (66%). Fig. 8B shows the corresponding first-order kinetics plot by the equation of ln(C₀/C) = kt, where C₀ and C are the MB concentrations in solution at times 0 and t, respectively, and k is the apparent first-order rate constant. It can be seen from Fig. 8B, the k values of pure g-C₃N₄, bentonite/g-C₃N₄ mixture and bentonite/g-C₃N₄ (2.2%, 5.4%, 13.2%) composites were 0.30 h⁻¹, 0.34 h⁻¹, 0.39 h⁻¹, 0.74 h⁻¹ and 0.67 h⁻¹ respectively. In the bentonite/g-C₃N₄ composite system, the rate constant of the bentonite/g-C₃N₄ (5.4%) photocatalyst is up to 2.5 times as high as that of the pure g-C₃N₄. However, further increasing the proportion of bentonite, the degradation rate shows a slight decrease though it remains higher than that of g-C₃N₄. The decrease in the activity of samples with a heavy loading of bentonite is likely due to the shading effect, which can seriously block the absorption of the incident light by g-C₃N₄, In MoS₂/mpg-CN system, the excess doping of MoS₂ led to the agglomeration of MoS₂ nanoparticles on the surface of mpg-CN, which could decrease the catalytic activity.³¹ Therefore, excess loading of bentonite could prevent g-C₃N₄ from absorbing the visible light and then lead to a deceasing of photocatalytic activity.

Fig. 8 (A) Photocatalytic degradation efficiency of MB by g-C₃N₄ and bentonite/g-C₃N₄ composites, (B) kinetic fit for the degradation of MB with g-C₃N₄ and bentonite/g-C₃N₄ composites, (C) adsorption activity and rate constants of (a) g-C₃N₄, (b) bentonite/g-C₃N₄ mixture, (c) bentonite/g-C₃N₄ (2.2%), (d) bentonite/g-C₃N₄ (5.4%) and (e) bentonite/g-C₃N₄ (13.2%) composite, (D) absorption spectral changes of MB under visible light: bentonite/g-C₃N₄ (5.4%) as photocatalyst.

Fig. 9 Photoluminescence (PL) spectra of g-C₃N₄ and bentonite/g-C₃N₄ composites.

Because adsorption plays an important role in most photocatalytic reactions, the comparison of the adsorption capacity and rate constants of all catalysts during the degradation process was investigated (Fig. 8C). Compared with pure g-C₃N₄, bentonite/g-C₃N₄ mixture has the superior adsorption capacity; almost 56% of MB molecules were adsorbed on the mixture sample, which is much higher than that of g-C₃N₄ (20%). However, rate constant of the bentonite/g-C₃N₄ mixture (0.34 h⁻¹) only increased a little compared with that of g-C₃N₄ (0.30 h⁻¹).

The adsorption capacity of bentonite/g-C₃N₄ composites is not superior to that of bentonite/g-C₃N₄ mixture, whereas the rate constants are much higher than that of bentonite/g-C₃N₄ mixture. Remarkably, bentonite/g-C₃N₄ (5.4%) composite shows the optimal rate constant (0.74 h⁻¹), which is around 2.2 times that of the bentonite/g-C₃N₄ mixture (0.34 h⁻¹). From these data, it is obvious that the high degradation activity of bentonite/g-C₃N₄ composites is not mainly attributed to the adsorption capability any more. The enhanced photocatalytic activity can be ascribed to the significant differences in the interface of the samples, as the hybrid composites show a closely contacted interface (shown in Fig. 3), whereas only a diffuse interface can be formed after a simple mechanically mixing. Fig. 8D shows temporal evolution of the spectral changes during photocatalytic degradation of MB over bentonite/g-C₃N₄ (5.4%) composite. It can be seen that with the increase of irradiation time in bentonite/g-C₃N₄ (5.4%) composite aqueous suspension, the intensity of the absorption peak at 663 nm decreased drastically within 3 h. The bentonite/g-C₃N₄ (5.4%) composite shows high photocatalytic activity under the experimental condition.

3.9 PL analysis

In the previous studies, PL analysis was used to reveal the efficiency of charge carrier trapping, transfer, and separation and to investigate the fate of photogenerated electrons and holes in semiconductors, because the PL emission results from the recombination of free charge carriers.^48,49 Herein, we present the PL measurement for pure g-C₃N₄ and the bentonite/g-C₃N₄ composites. As can be seen in Fig. 9, for the pure g-C₃N₄, one main emission peak appears at about 460 nm, which is attributed to the inner band gap of g-C₃N₄ (2.70 eV).¹ After introducing bentonite into g-C₃N₄, the intensities of the PL signal for the bentonite/g-C₃N₄ composites are lower than that of g-C₃N₄, indicating that the composites have a lower recombination rate of electrons and holes under visible-light irradiation.

Fig. 10 Photocurrent profiles of bentonite, g-C₃N₄ and bentonite/g-C₃N₄ (5.4%) composite.

3.10 Photocurrent–time analysis

The transient photocurrent responses of g-C₃N₄ and g-C₃N₄/bentonite (5.4%) composite electrodes were recorded under visible light irradiation. Fig. 10 shows the I–t curves for the aforementioned samples with two on–off cycles of intermittent irradiation. The photocurrent value of bentonite is very small and can be ignored. The photocurrent value of g-C₃N₄ and g-C₃N₄/bentonite (5.4%) composite rapidly decreases to zero as soon as the irradiation of light turned off and the photocurrent come back to a constant value when the light is on again, which was reproducible. Significantly, the photocurrent of the g-C₃N₄/bentonite (5.4%) electrode was about 8 times as high as that of the pure g-C₃N₄ electrode. The enhancement of photocurrent of g-C₃N₄/bentonite (5.4%) composite indicated there was not only a strong interaction between bentonite and g-C₃N₄ but also more efficient carrier separation at the g-C₃N₄ and bentonite interfaces.^12,48

Fig. 11 Proposed mechanism for the photodegradation of MB on bentonite/g-C₃N₄ (5.4%) composite.

3.11 Mechanism of enhancement of photocatalytic activity

As discussed above, four main reasons for the increase in the photocatalytic efficiency of coupled g-C₃N₄/bentonite composites are: (i) the absorption edges of bentonite/g-C₃N₄ composites shift significantly to longer wavelengths compared with the pure g-C₃N₄, which indicates the composites can be excited by more visible light photons; (ii) the enlarged BET surface of bentonite/g-C₃N₄ composites, the increased surface area offers more surface active sites for adsorption and photocatalytic reaction;^50,51 (iii) the improved adsorptive activity of bentonite/g-C₃N₄ composites compared with that of g-C₃N₄, the improved adsorptivity for the MB molecules could accelerate the photodegradation reaction;^51,52 (iv) the electrostatic interaction, the negatively charged bentonite can promote the immigration of electrons and holes, thus suppresses the charge recombination. The mechanism for the enhanced photocatalytic activity is illustrated in Fig. 11. As can be seen in Fig. 11, although bentonite has an overall neutral charge, it has excess negative charges on its crystal lattice due to the isomorphous substitution. This kind of negative charges is compensated for by exchangeable cations.²³ When bentonite is placed in water, the exchangeable cations, such as sodium ions, diffuse away from the solid surface, leaving the bentonite surface with negative charges.^23,53 Considering the fact that bentonite is known as a very good electrical insulator,⁵⁴ bentonite can not be excited and only g-C₃N₄ can be activated, the electron–holes of g-C₃N₄ have no opportunity to migrate to bentonite and should still present on g-C₃N₄ surface. The excited electrons and holes of g-C₃N₄ should be driven to migrate efficiently due to electrostatic interaction between g-C₃N₄ and the negatively charged bentonite. Thus, the charge recombination could be easily suppressed, leaving more charge carriers and enhancing the photocatalytic activity. Similarly, in other insulator–semiconductor system, such as h-BN/TiO₂, owing to the electrostatic interaction, the negatively charged h-BN can promote the immigration of h⁺ from the bulk of TiO₂ to its surface and consequently improves the photocatalytic activity.⁵⁵ Driven by the calcinations process, the layers of bentonite and g-C₃N₄ are splitted into many fluffy and thin sheets and a tight contact of bentonite with g-C₃N₄ can be achieved (shown in Fig. 3), which can strengthen the electrostatic interaction between them.

Therefore, the high photoactivity of bentonite/g-C₃N₄ composite arises from the synergistic effects between bentonite and g-C₃N₄ interfaces. Further study is necessary to clarify the mechanism and the synthesis route can be applied to other bentonite/semiconductor photocatalysts.

4 Conclusions

A novel bentonite/g-C₃N₄ composite was successfully synthesized via a calcination method. XPS, DRS, PL and photocurrent–time analyses reveal that the obtained bentonite/g-C₃N₄ composites are a hybridization photocatalyst. Bentonite/g-C₃N₄ composites exhibited higher photocatalytic activity than the pure g-C₃N₄ in the degradation of MB (20 mg L⁻¹). The bentonite/g-C₃N₄ (5.4%) composite induced 90% MB degradation within 3 h under visible light irradiation, which is much higher than that of pure g-C₃N₄ (62%). The significant enhancement in the photocatalytic performance of bentonite/g-C₃N₄ composites is ascribed not only to its high surface area, adsorptivity and enhanced absorptivity in visible light but also to the electrostatic interaction between g-C₃N₄ and negatively charged bentonite, this can promote the efficient migration of the photogenerated electrons and holes of g-C₃N₄ and consequently improves the photocatalytic activity.

Acknowledgements

The authors appreciate the financial support of this work from the National Nature Science Foundation of China (no. 21177050, 21007021, 21076099 and 21175061) Society Development Fund of Zhenjiang (SH2012020, SH2011011), Postdoctoral Foundation of China (2012M521014) and Foundation of Jiangsu University (04JDG048).

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