Synthesis and properties of visible light responsive g-C₃N₄/Bi₂O₂CO₃ layered heterojunction nanocomposites

Abstract

Visible light responsive g-C₃N₄/Bi₂O₂CO₃ layered heterojunction nanocomposites were prepared by two methods, self-assembly and chemical precipitation. X-ray diffraction (XRD), UV-Vis spectroscopy, N₂ adsorption, Scanning electron microscopy (SEM), electrochemical impedance spectra (EIS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the prepared catalysts. The results indicated that the preparation method does not influence the crystal phase, morphology and optical property of the obtained nanocomposites, but affects the interaction strength between g-C₃N₄ and Bi₂O₂CO₃, leading to an obvious difference in the separation rate of photogenerated electrons and holes. The activities were tested in photocatalytic rhodamine B (RhB) and phenol degradation under visible light. The g-C₃N₄/Bi₂O₂CO₃ nanocomposite with a stronger interaction showed the better activity. The ˙O₂⁻ radicals are responsible for the degradation of RhB. No obvious decrease in activity was observed for g-C₃N₄/Bi₂O₂CO₃ nanocomposites prepared by the self-assembly method after three cycles. The significant enhancement of the photocatalytic activity was attributed to the high charge-separation efficiency due to the hybrid effect of RhB, Bi₂O₂CO₃ and g-C₃N₄. The self assembly method assisted by CTAB leads to tight decoration of Bi₂O₂CO₃ nanoparticles on g-C₃N₄ sheets facilitating intimate contact and formation of a stable heterojunction.

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Introduction

In recent years, the design of efficient, simple, and sustainable photocatalysts has received much attention owing to their potential value in addressing the worldwide energy shortage and environmental purification.^1,2 However, the most widely used photocatalyst, TiO₂, has limited practical applications due to its low solar energy conversion efficiency and the high recombination of photogenerated electron–hole pairs.^3,4 Therefore, it is very urgent to develop efficient visible-light-active photocatalysts.

Very recently, graphitic carbon nitride (g-C₃N₄), a metal-free, stable and inexpensive polymeric semiconductor, has received increasing attention.⁵ As a “sustainable” photocatalyst, g-C₃N₄ has many advantages, such as good thermal and chemical stability, metal-free and tunable electronic structure. Moreover, the moderate band gap energy (2.7 eV) make it can utilize visible light directly. These unique properties make g-C₃N₄ a valuable material in various potential applications, such as energy conversion,⁶ organic synthesis,⁷ pollutants treatment,⁸ hydrogen production⁹ and carbon dioxide storage.¹⁰ The synthesis of g-C₃N₄ involves the pyrolysis of nitrogen-rich organic precursors during which condensation of the C–N bond occurs to form two dimensional (2D) tri-s-triazine (melem) sheets connected via tertiary amines.¹¹ This 2D morphology can provide a good plane for contact with other semiconductors, resulting in the formation of 2D nanojunctions, such as g-C₃N₄/rGO,¹² g-C₃N₄/SnS₂,¹³ g-C₃N₄/BiIO₄,¹⁴ g-C₃N₄/MoO₃ (ref. 15) and g-C₃N₄/Bi₂WO₆ (ref. 16) etc. Furthermore, the layered 2D nanojunctions could facilitate the photo-generated charge transfer across the intimate interface, which is expected to improve the photocatalytic performance compared with individual photocatalysts.

Bismuth subcarbonate (Bi₂O₂CO₃) as a novel material has triggered great attention due to its potential applications in many fields such as pharmacy, medicine and photocatalysis.^17–19 However, the pure Bi₂O₂CO₃ with a wide band gap (∼3.3 eV) only can be excited by ultraviolet light (<5% fraction of solar light), which restricts its practical applications. Therefore, much efforts have been made to resolve this problem, such as graphene-wrapping,²⁰ non-metal-doping²¹ and nanojunction-constructing²² etc. However, high cost and rigorous reaction conditions restrain their large-scale applications. Thus, it is highly desirable to develop novel and efficient strategies to improve the visible light photocatalytic performance of Bi₂O₂CO₃.

It is the heterostructure that dominates the behaviors of charge carriers, such as the transportation direction, separation distance and recombination rate etc.²³ Therefore, the rational design of a heterojunction with rapid transfer and separation efficiency and studying the effect of a heterostructure on the behavior of charge carriers are important and desirable for the exploitation of high-effective photocatalysts. Zhang et al. prepared novel g-C₃N₄/(BiO)₂CO₃ organic-inorganic nanojunctioned photocatalysts by in situ depositing (BiO)₂CO₃ nanoflakes onto the surface of g-C₃N₄ nanosheets.²⁴ They suggested that the enhanced photocatalytic activity can be mainly ascribed to the well-matched band structures, dye photosensitization and efficient crystal facets coupling interaction between g-C₃N₄ {002} and (BiO)₂CO₃ {002}. Xiong et al. prepared flower-like g-C₃N₄/Bi₂O₂CO₃ microspheres with a high adsorption ability were fabricated via a facile method. They suggested that the matching of the energy levels of the dye, Bi₂O₂CO₃ and g-C₃N₄ facilitates the transfer of photogenerated electrons and holes, leading to superior photocatalytic activity.²⁵ In this work, we designed novel g-C₃N₄/Bi₂O₂CO₃ nanojunction photocatalysts by two methods, self-assembly and chemical precipitation. Their photocatalytic activities were compared each other. Due to the well-matched band energies and effective charge separation, the g-C₃N₄/Bi₂O₂CO₃ nanojunction showed significant enhancement in photocatalytic removal of RhB and phenol compared with the individual g-C₃N₄ and Bi₂O₂CO₃. A possible photocatalytic mechanism is proposed.

Experimental

Preparation and characterization

3 g melamine was finely ground in a mortar and annealed at 520 °C for 2 h (at a rate of 5 °C min⁻¹). After it was cooled to room temperature, the yellow powder was obtained and denoted as g-C₃N₄. Two typical preparations of g-C₃N₄/Bi₂O₂CO₃ nanocomposites were as follows. A simple schematic representation of the preparative route is shown in Fig. S1.†

Method 1 is reported by previous literature.²⁴ 1.712 g Bi(NO₃)₃·5H₂O was dissolved in 70 mL of aqueous solution containing 2.8 mL nitric acid (pH 1) and stirred for 1 h. 0.9 g g-C₃N₄ (mass ratio g-C₃N₄ : Bi₂O₂CO₃ = 1 : 1) was added to above solution and the mixture was ultrasonicated for 30 min to obtain a suspension. Concentrated NH₃·H₂O (10 mL) was added dropwise into above suspension and the white precipitation was formed (pH 10). Then, the suspension was pumped air stream at a flow rate of 0.5 L min⁻¹ for 10 h (pH 8). The precipitates were filtrated, washed and then dried at 80 °C overnight. The final products were denoted as BiOC(1)–CN. The neat Bi₂O₂CO₃, which denoted as BiOC(1) was obtained according to the method 1 but in the absence of g-C₃N₄.

Method 2 is reported by previous literature.²⁵ 0.5 g CTAB (hexadecyl trimethyl ammonium bromide) and 3 g Na₂CO₃ were added to 90 mL distilled water (pH 11), followed by the addition of 0.9 g g-C₃N₄. The mixture was treated with ultrasonication for 1 h. Then a mixture of 1.712 g Bi(NO₃)₃·5H₂O and 10 mL HNO₃ (1 mol L⁻¹) was added dropwise under vigorous stirring, and white precipitation was formed (pH 9). After stirring for another 2 h, the solid was obtained by filtration, washed with distilled water and ethanol, and finally dried at 80 °C overnight. The product was denoted as BiOC(2)–CN. The neat Bi₂O₂CO₃, which denoted as BiOC(2) was obtained according to the method 2 but in the absence of g-C₃N₄. For comparison, the mechanical mixture of g-C₃N₄ and Bi₂O₂CO₃ with the same mass ratio was prepared. g-C₃N₄ and Bi₂O₂CO₃ were dispersed in 20 mL deionized water under stirring. The obtained suspension was heated to 100 °C to remove the water. The solid product was dry at 80 °C in oven and denoted as BiOC–CN.

XRD patterns of the prepared samples were recorded on a Rigaku D/max-2400 instrument using Cu-Kα radiation (λ = 1.54 Å). The scan rate, step size, voltage, and current was 0.05° min⁻¹, 0.01°, 40 kV, and 30 mA, respectively. Nitrogen adsorption was measured at −196 °C on a Micromeritics 2010 analyzer. All the samples were degassed at 393 K before the measurement. BET surface area (S_BET) was calculated according to the adsorption isotherm. UV-Vis spectroscopy measurement was carried out on a JASCO V-550 model UV-Vis spectrophotometer, using BaSO₄ as the reflectance sample. The morphology of prepared catalyst was observed by using a scanning electron microscope (SEM, JSM 5600LV, JEOL Ltd.). Elemental analysis was performed with a vario EL cube from Elementar Analysensysteme GmbH. XPS measurements were conducted on a Thermo Escalab 250 XPS system with Al Kα radiation as the exciting source. The binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. Curve fits were made using CasaXPS, while the relative sensitivity factors and asymmetry functions were taken from the PHI ESCA handbook. Electrochemical impedance spectra (EIS) made from these as-made materials were measured via an EIS spectrometer (EC-Lab SP-150, BioLogic Science Instruments) in a three-electrode cell by applying 10 mV alternative signal versus the reference electrode (SCE) over the frequency range of 1 MHz to 100 mHz. The cyclic voltammograms were measured in 0.1 M KCl solution containing 2.5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1) as a redox probe with the scanning rate of 20 mV s⁻¹ in the same three electrode cell as EIS measurement.

Photocatalytic reaction

RhB and phenol were selected as the model compound to evaluate the photocatalytic performance of the prepared g-C₃N₄ based catalysts in an aqueous solution under visible light irradiation. The inner diameter and height of the glass reactor are 10 and 16 cm. The distance from the bottom of lamp to the reactor is 5 cm. 0.05 g catalyst was dispersed in 200 mL aqueous solution of RhB or phenol (10 ppm) in an ultrasound generator for 10 min. The suspension was transferred into a self-designed glass reactor, and stirred for 30 min in darkness to achieve the adsorption equilibrium. In the photoreaction under visible light irradiation, the suspension was exposed to a 250 W high-pressure sodium lamp with main emission in the range of 400–800 nm, and air was bubbled at 130 mL min⁻¹ through the solution. The UV light portion of sodium lamp was filtered by 0.5 M NaNO₂ solution. All runs were conducted at ambient pressure and 30 °C. At given time intervals, 4 mL suspension was taken and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentrations of RhB or phenol before and after reaction were measured by UV-Vis spectrophotometer at a wavelength of 550 or 270 nm.

Results and discussion

Fig. 1 shows the XRD patterns of the as-synthesized g-C₃N₄, Bi₂O₂CO₃, BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites. Two distinct diffraction peaks located at 13.2 and 27.6° in g-C₃N₄ is attributed to the (100) and (002) crystal facets, respectively.^26,27 All the diffraction peaks for Bi₂O₂CO₃ can be highly indexed to pure tetragonal Bi₂O₂CO₃ (JCPDS 41-1488).²⁸ No other diffraction peak is detected, hinting that the Bi₂O₂CO₃ with high purity is synthesized. These typical diffraction peaks of g-C₃N₄ and Bi₂O₂CO₃ can also be observed in BiOC(1)–CN and BiOC(2)–CN, which demonstrates that Bi₂O₂CO₃ have been successfully combined with g-C₃N₄ nanosheets. Besides, it is noted that the diffraction intensities of g-C₃N₄ in BiOC(1)–CN and BiOC(2)–CN are much lower than that of mechanical mixture BiOC–CN. This is probably due to that g-C₃N₄ is coated by Bi₂O₂CO₃ in g-C₃N₄/Bi₂O₂CO₃ nanocomposites. The enlarged figure with diffraction angle 20–35 degree are shown in Fig. S2.† It indicates that no obvious difference in peak position is observed among prepared catalysts. BiOC(1) shows slight higher peak intensity and narrower FWHM of (013) reflection than that of BiOC(2), indicating the particle size of BiOC(1) is slight bigger than that of BiOC(2). This is probably due to that the addition of CTAB in method 2 can inhibit the nucleation, leading to the formation of smaller (BiO)₂CO₃ crystalline grains. Such finer particles can tightly decorate and assemble on g-C₃N₄ sheets resulting in intimate contact in the coupled systems.²⁹ The C/N ratio obtained by elemental analysis are 0.67, 0.69 and 0.68 for g-C₃N₄, BiOC(1)–CN and BiOC(2)–CN. These values are slight lower than the theoretical value (0.75) which is probably due to the incomplete condensation.³⁰

Fig. 1 XRD patterns of synthesized g-C₃N₄, BiOC(1), BiOC(2), BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites.

The morphologies of the representative samples are examined by using SEM analysis (Fig. 2). Fig. 2a indicates as-prepared g-C₃N₄ exhibit smooth layer structure that is similar to its analogue graphite. The particle size of g-C₃N₄ is 2–5 μm. Bi₂O₂CO₃ has a flower-like structure assembled from dozens of nanosheets (Fig. 2b). For BiOC–CN, the bulk g-C₃N₄ and flower-like Bi₂O₂CO₃ are observed obviously (Fig. 2c). In the case of BiOC(1)–CN and BiOC(2)–CN, flower-like Bi₂O₂CO₃ can not be observed. The Bi₂O₂CO₃ nanosheets attach on the g-C₃N₄ surface to assemble g-C₃N₄/Bi₂O₂CO₃ nanocomposites, as shown in Fig. 2d–f. The particle size of Bi₂O₂CO₃ is 200–500 nm. The elemental mapping images of BiOC(2)–CN indicates that Bi, O and N elements are homogenously distributed in the whole catalyst particles (Fig. 2g–i). This hints that the g-C₃N₄ is fully coated by the Bi₂O₂CO₃ nanosheets.

Fig. 2 SEM images of synthesized g-C₃N₄ (a), BiOC(2) (b), BiOC–CN (c), BiOC(1)–CN (d), BiOC(2)–CN (e and f) and elemental mapping images of BiOC(2)–CN (g–i).

The light absorption property of as-prepared catalysts is studied by UV-Vis spectra. As shown in Fig. 3, BiOC(1) and BiOC(2) show significant light absorption in the UV region. The absorption band edge of g-C₃N₄ is ∼450 nm, originating from charge transfer response of g-C₃N₄ from the VB populated by N 2p orbital to the CB formed by C 2p orbital.⁵ For g-C₃N₄/Bi₂O₂CO₃ nanocomposites, the UV light absorption decreases, whereas the absorption ability in the region of 350–450 nm increases obviously compared with Bi₂O₂CO₃. It is noted that obvious difference in absorption curve between two g-C₃N₄/Bi₂O₂CO₃ nanocomposites and mechanical mixture BiOC–CN is shown in Fig. 3. This is probably due to the existence of interaction between g-C₃N₄ and Bi₂O₂CO₃ in g-C₃N₄/Bi₂O₂CO₃ nanocomposites. No essential difference is observed between two g-C₃N₄/Bi₂O₂CO₃ nanocomposites, indicating the preparation method does not influence the optical property of prepared g-C₃N₄/Bi₂O₂CO₃ nanocomposites. The band gaps of g-C₃N₄ and Bi₂O₂CO₃ estimated from the intercept of the tangents to the plots of (αhν)^1/2 vs. photo-energy are 2.73 and 3.25 eV, respectively.

Fig. 3 UV-Vis absorbance spectra of g-C₃N₄, Bi₂O₂CO₃, BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites.

In order to understand the band structure of g-C₃N₄/Bi₂O₂CO₃ nanojunctions, the positions of conduction band (CB) edge and valence band (VB) edge are calculated by a theoretical method. The conduction band edge (E_CB) of a semiconductor at the point of zero charge (pH_ZPC) can be determined by the equation E_CB = X − E − 1/2E_g, where X is the absolute electronegativity of the semiconductor; E is the energy of free electrons on the hydrogen scale (∼4.5 eV); E_g is the band gap energy.³¹ The calculated results indicates that the CB positions of g-C₃N₄ and Bi₂O₂CO₃ locate at −1.08 and +0.42 eV. Combined with band gap energy, the obtained VB positions of g-C₃N₄ and Bi₂O₂CO₃ are +1.65 and +3.67 eV. The band structures of the two components are well-matched with each other. This fact indicates that the successful construction of 2D g-C₃N₄/Bi₂O₂CO₃ nanojunctions could provide great potential in optimizing the band structures and designing novel photocatalysts with desired optical properties. The photo-generated electrons can transfer from g-C₃N₄ to Bi₂O₂CO₃ easily, which results in efficient separation and transport of photo-induced electrons and holes.

In order to identify the chemical status of the elements in synthesized catalysts, g-C₃N₄, Bi₂O₂CO₃, BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites are characterized by XP spectra. In N 1s region (Fig. 4a), the main N 1s peak at a binding energy of 398.5 eV can be assigned to sp² hybridized nitrogen (C=N–C), thus confirming the presence of sp² bonded graphitic carbon nitride. The peak at higher binding energy 400.5 eV is attributed to tertiary nitrogen (N–(C)₃) groups.³² No obvious difference is observed among three catalysts. In C 1s region (Fig. 4b), two components are located at 284.6 and 288.5 eV for g-C₃N₄. The sharp peak around 284.6 eV is attributed to the pure graphitic species in the CN matrix. The peak with binding energy of 288.5 eV indicates the presence of sp² C atoms bonded to aliphatic amine (–NH₂ or –NH–) in the aromatic rings.³³ In the case of BiOC(1)–CN and BiOC(2)–CN, the similar curves as g-C₃N₄ are observed but a slight shift to lower binding energy at ca. 288 eV. This is probably due to the overlapping of sp² C atoms in g-C₃N₄ and carbonate in Bi₂O₂CO₃.^19,34 In Fig. 4c, the peaks for Bi₂O₂CO₃ located at 158.2 and 163.4 eV are assigned to Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺, respectively.^19,35 In the case of BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites, distinct binding energy shifts are observed. We investigated the properties of WO₃/g-C₃N₄ nanocomposites in our previous work.³⁶ It is found that the binding energy difference, ΔE, between individual WO₃ and composite in the region of W 4f could reflect the strength of interaction between g-C₃N₄ and WO₃.³⁶ In this work, ΔE in the region of Bi 4f increases in the order: BiOC–CN < BiOC(1)–CN < BiOC(2)–CN, indicating that the interaction between g-C₃N₄ and Bi₂O₂CO₃ in BiOC(2)–CN is probably stronger than that in BiOC(1)–CN. This is probably due to that the addition of CTAB in method 2 can inhibit the nucleation, leading to the formation of smaller (BiO)₂CO₃ crystalline grains. Such smaller (BiO)₂CO₃ can assemble with g-C₃N₄ hard, leading to the stronger interaction compared to BiOC(1)–CN. This stronger interaction in BiOC(2)–CN can form more heterojunction which cause the more effective separation of photogenerated electron–hole pairs.

Fig. 4 XPS of synthesized g-C₃N₄, BiOC(2), BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites in the region of N 1s (a), C 1s (b) and Bi 4f (c).

Electrochemical impedance spectroscopy (EIS) is a very useful tool to characterize the charge-carrier migration, thus was used to further confirm the interfacial charge transfer effect of as-prepared g-C₃N₄/Bi₂O₂CO₃ nanocomposites. Fig. 5 shows the EIS Nyquist plots of the as-prepared catalysts. Obviously, g-C₃N₄/Bi₂O₂CO₃ nanocomposites show much decreased arc radius compared with neat g-C₃N₄ and Bi₂O₂CO₃. The reduced arc radius indicates diminished resistance of working electrodes, suggesting a decrease in the solid state interface layer resistance and the charge transfer resistance across the solid–liquid junction on the surface by forming heterojunction between g-C₃N₄ and Bi₂O₂CO₃.³⁷ Since the radius of the arc on the EIS spectra reflects the migration rate occurring at the surface, it suggests that a more effective separation of photogenerated electron–hole pairs and a faster interfacial charge transfer occurs on BiOC(2)–CN surface under this condition.³⁸

Fig. 5 EIS spectra of synthesized g-C₃N₄, BiOC(1), BiOC(2), BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites.

The photocatalytic activity of the as-prepared samples is studied for the degradation of RhB under visible light irradiation (Fig. 6). Control experiment results indicate that the RhB degradation performance could be ignored in the absence of either irradiation or photocatalyst, indicating that RhB is degraded via photocatalytic process. The initial pH of the reaction solution is 7 and not change during the reaction process. Obviously, the adsorption ability influences the photocatalytic activity. The higher adsorption ability of the RhB molecules, more RhB molecules can be degraded simultaneously, leading to the higher photocatalytic activity of catalyst. The influence of pH value on the photocatalytic RhB degradation over BiOC(2)–CN is also investigated (Fig. S3†). The pH value is adjusted by the HCl and NH₄OH. It can be seen that the pH value strongly influences the adsorption ability of RhB molecules. The lower pH value, the higher adsorption ability of RhB molecules, leading to the better photocatalytic activity. Despite being strong in visible light absorption, less than 40% of RhB is degraded in 120 min over g-C₃N₄ because of the high electron–hole recombination rate. It is noted that, Bi₂O₂CO₃ with a wide band gap (3.25 eV) shows higher activity than that of g-C₃N₄ under visible light irradiation. g-C₃N₄/Bi₂O₂CO₃ nanocomposites show clearly higher activities than that of individual g-C₃N₄, Bi₂O₂CO₃ and BiOC–CN. Furthermore, for BiOC(2)–CN, RhB is degraded completely within 90 min, which activity is much higher than BiOC(1)–CN. The rate constant of BiOC(2)–CN is 0.03181 min⁻¹, which is 7.4, 4.9 and 6.2 times higher than that of neat g-C₃N₄, BiOC(1) and BiOC(2) (Table S1†). Considering that no essential difference in crystal phase, morphology, optical property and adsorption ability is observed between two g-C₃N₄/Bi₂O₂CO₃ nanocomposites, the higher activity of BiOC(2)–CN is probably due to the more effective separation of photogenerated electron–hole pairs.

Fig. 6 Photocatalytic performances of g-C₃N₄, BiOC(1), BiOC(2), BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites in the degradation of RhB under visible light irradiation.

The influence of mass ratio of g-C₃N₄ to Bi₂O₂CO₃ in prepared composite on the photocatalytic RhB degradation is shown in Fig. S4.† It can be seen that the g-C₃N₄/Bi₂O₂CO₃ composites show only slight higher activity than that of neat g-C₃N₄ and Bi₂O₂CO₃ except BiOC(2)–CN, which the mass ratio g-C₃N₄ : Bi₂O₂CO₃ = 1 : 1. We test the BET surface areas (S_BET) and found the S_BET of g-C₃N₄ and Bi₂O₂CO₃ is close (11.2 and 14.5 m² g⁻¹). Therefore, it is deduced that when the mass ratio g-C₃N₄ : Bi₂O₂CO₃ = 1 : 1, two components have the approximate S_BET. They can contact with each other as much as possible, leading to the formation of the maximum area of the heterojunction. Thus, it is reasonable that BiOC(2)–CN shows much higher activity than that of g-C₃N₄/Bi₂O₂CO₃ composites with other mass ratio of g-C₃N₄ to Bi₂O₂CO₃.

Fig. S5† shows the photocatalytic performances of g-C₃N₄, BiOC(1), BiOC(2), BiOC–CN and g-C₃N₄/Bi₂O₂CO₃ nanocomposites in the degradation of phenol under visible light irradiation. The initial pH of the reaction solution is 7 and not change during the reaction process. The activity trend is similar to that of RhB degradation except that Bi₂O₂CO₃ exhibits a very low activity. It is reasonable because Bi₂O₂CO₃ can not be excited by visible light. Considering that RhB with E₀ = −1.42 V (vs. NHE) demonstrates the highest absorption in the visible region at wavelengths of 552 nm. The RhB degradation activity over Bi₂O₂CO₃ under visible light should be attributed to the indirect dye photosensitization process. The rate constant of BiOC(2)–CN is 0.00389 min⁻¹, which is 4.6 times higher than that of neat g-C₃N₄ (Table S1†).

To confirm whether the indirect dye photosensitization process is existed, the wavelength-dependent RhB degradation rates of g-C₃N₄, BiOC(2) and BiOC(2)–CN are evaluated (Fig. 7). The desired wavelength of light is selected by band pass interference filters (RAYAN TECHNOLOGY CO,. LTD.). It is demonstrated that the RhB degradation rates of three catalysts are decreased when more excitation lights with short wavelengths are cut off. When the light wavelength shorter than 450 nm is cut off, 15–30% RhB is degraded. However, when the 550 nm filter is used, which only RhB molecules are excited and both g-C₃N₄ and Bi₂O₂CO₃ have no response in this region, still 5–20% RhB is degraded. This hints that indirect dye photosensitization process is existed in not only Bi₂O₂CO₃ but all the catalytic systems. It is noted that the RhB degradation rate of Bi₂O₂CO₃ is more than 10% when the 550 nm filter is used, much higher than that of g-C₃N₄ (∼5%). This indicates that, compared with g-C₃N₄, indirect dye photosensitization process is occurred much easier over Bi₂O₂CO₃ surface. Due to the large offset of energy level (∼1.84 eV) at the RhB/Bi₂O₂CO₃ interfaces, photogenerated electrons can move more rapidly from RhB to Bi₂O₂CO₃ in a thermodynamically favorable manner.

Fig. 7 Wavelength-dependent RhB degradation rate of g-C₃N₄, BiOC(2) and BiOC(2)–CN.

To understand the reaction mechanism, it is important to identify the active species that participate in the photocatalytic processes. The active species generated during the reaction process are identified by hole and free radical trapping experiment. In this investigation, EDTA-2Na, tert-butyl alcohol (t-BuOH) and 1,4-benzoquinone (BQ) are used as the hole (h⁺), hydroxyl radical (˙OH) and superoxide radical (˙O₂⁻) scavenger, respectively.³⁹ Fig. 8 shows the influence of various scavengers on the visible light photocatalytic activity of g-C₃N₄, Bi₂O₂CO₃ and BiOC(2)–CN. For g-C₃N₄, the photodegradation rate of RhB decreased slightly after the addition of t-BuOH, indicating that hydroxyl radicals are not the main active species in current photocatalytic systems. When BQ is added, the degradation of RhB is inhibited sharply, indicating ˙O₂⁻ is the main active species in current photocatalytic systems. This is consistent with our previous results.^40,41 In the presence of EDTA-2Na, the RhB degradation rate is increased obviously, which is different from previous result.³⁹ Zhang et al. prepared C₃N₄/Bi₅Nb₃O₁₅ heterojunction catalyst for 4-CP photodegradation.³⁹ The photodegradation rate was decelerated significantly after the addition of EDTA-2Na, indicating the direct photogenerated holes oxidation happened. In our investigation, although the redox potential of RhB is reported to be 1.43 V,⁴² which is higher than the VB of g-C₃N₄, the direct photogenerated holes oxidation does not happen. On the contrary, the addition of EDTA-2Na to trap the h⁺ can promote the separation rate of e⁻/h⁺ pairs, leading to the increased photocatalytic performance. For Bi₂O₂CO₃, the similar trend is obtained, indicating the main oxidative species is ˙O₂⁻. This active species should be produced by the adsorbed O₂ which traps the electrons transferred from the excited RhB molecule (RhB*) through the indirect dye photosensitization process. In the case of BiOC(2)–CN, the similar trend is obtained, indicating coupling two materials does not change the reaction mechanism.

Fig. 8 Influence of various scavengers on the visible light photocatalytic activity of g-C₃N₄, BiOC(2) and BiOC(2)–CN.

Combined with above results, a mechanism for electron–hole separation and transportation at the g-C₃N₄/Bi₂O₂CO₃ interfaces is proposed (Fig. 9). The redox potentials of RhB and RhB* are 0.95 V and −1.42 V (vs. NHE).⁴³ Under visible light irradiation, RhB and g-C₃N₄ can be excited simultaneously (step 1). Excited state RhB molecules adsorbed on the catalyst surface can transfer their photogenerated electrons to the CB of g-C₃N₄ and Bi₂O₂CO₃. Simultaneously, the holes in the VB of g-C₃N₄ can transfer to the RhB molecules (step 2). This causes the increased separation rate of photogenerated electrons and holes. The electrons in the CB of g-C₃N₄ are trapped by the molecule oxygen to produce ˙O₂⁻ radicals for further degradation of RhB⁺˙ (step 3). Furthermore, some electrons in the CB of g-C₃N₄ move quickly into the CB of Bi₂O₂CO₃ because of the large offset of energy level (∼1.5 eV), making charge separation efficiently and reducing the probability of electron–hole recombination (step 4). The electrons in the CB of Bi₂O₂CO₃ are also trapped by the molecule oxygen to produce ˙O₂⁻ radicals which is responsible for the degradation of RhB⁺˙ (step 5). Hence, the significant enhancement of photocatalytic activity is attributed to the high charge-separation efficiency due to the hybrid effect of RhB, Bi₂O₂CO₃ and g-C₃N₄.

Fig. 9 Mechanism of the photocatalytic degradation of RhB over g-C₃N₄/Bi₂O₂CO₃ nanocomposite under visible light irradiation.

Fig. 10 shows photocatalytic stabilities of BiOC(1)–CN and BiOC(2)–CN. No obvious decrease in activity is observed for BiOC(2)–CN after three cycles, hinting its good stability. However, for BiOC(1)–CN, the activity decreases obviously. The activity of 3rd reused BiOC(1)–CN is 66%, which is close to the mechanical mixture BiOC–CN (63%). This is probably attributed to the different interaction strength between g-C₃N₄ and Bi₂O₂CO₃. The stronger interaction exists between g-C₃N₄ and Bi₂O₂CO₃, leading to a better structural stability of BiOC(2)–CN. No obvious difference in XPS peak intensity and position is observed for fresh and reused BiOC(2)–CN in the region of Bi 4f. This indicated that the chemical state of Bi is not changed in reused BiOC(2)–CN. However, compared with fresh BiOC(1)–CN, it is noted that the binding energy of reused BiOC(1)–CN shifts obviously to lower value. This is probably due to the poor interaction between g-C₃N₄ and Bi₂O₂CO₃ which causes the structural damage of BiOC(1)–CN after reuse. The Bi concentrations are measured by elemental analysis. The values are 40 wt% and 39 wt% for fresh and reused BiOC(1)–CN and 41 wt% and 40 wt% for fresh and reused BiOC(2)–CN. It is hinted that the decreased activity of reused BiOC(1)–CN is not caused by the loss of Bi₂O₂CO₃. The XRD results of recycled catalysts are also investigated (Fig. S6†). Obviously, the XRD pattern of reused BiOC(2)–CN is almost unchanged compared with fresh catalyst. However, for reused BiOC(1)–CN, the XRD pattern is obviously different from fresh catalyst. It looks like the mechanical mixture BiOC–CN in the Fig. 1. This confirms the result that the poor photocatalytic stability of BiOC(1)–CN is due to the poor interaction between g-C₃N₄ and Bi₂O₂CO₃ which causes the structural damage of BiOC(1)–CN after reuse, leading to the decreased separation efficiency of electron–hole pairs.

Fig. 10 The photocatalytic stabilities of RhB degradation over BiOC(1)–CN and BiOC(2)–CN.

Conclusions

Visible light responsive g-C₃N₄/Bi₂O₂CO₃ layered heterojunction nanocomposites are prepared by two methods, self-assembly and chemical precipitation. SEM result shows that the g-C₃N₄ is fully coated by the Bi₂O₂CO₃ nanosheets which is favorable for the formation of heterojunction and facilitates interfacial charge transfer. The preparation method does not influence the crystal phase, morphology and optical property of obtained nanocomposites, but affect the interaction strength between g-C₃N₄ and Bi₂O₂CO₃, leading to the obvious difference in separation rate of photogenerated electrons and holes. The indirect dye photosensitization process exists in the current photocatalytic system. The significantly enhanced photocatalytic activity of g-C₃N₄/Bi₂O₂CO₃ nanocomposites is attributed to the high charge-separation efficiency due to the hybrid effect of RhB, Bi₂O₂CO₃ and g-C₃N₄. The self assembly method assisted by CTAB leads to tight decoration of Bi₂O₂CO₃ nanoparticles on g-C₃N₄ sheets facilitating intimate contact and formation of stable heterojunction. Because of the stronger interaction between g-C₃N₄ and Bi₂O₂CO₃, BiOC(2)–CN exhibits the more effective separation of photogenerated electron–hole pairs and stable chemical structure, thus shows the higher photocatalytic activity and stability than that of BiOC(1)–CN.

Acknowledgements

The authors are grateful for the financial supports from the National Natural Science Foundation of China (no. 21103077), Program for New Century Excellent Talents in University (no. NCET-11-1011), the Natural Science Foundation of Liaoning Province (no. 20092080), Education Department of Liaoning Province (no. L2013150) and Tianjin Research Program of Application Foundation and Advanced Technology (13JCYBJC41600).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04189a

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