Fabrication of porous g-C₃N₄/Ag/Fe₂O₃ composites with enhanced visible light photocatalysis performance

Abstract

Porous graphitic carbon nitride (pg-C₃N₄) synthetized by pyrolysis of urea was hybridized with Ag-doped Fe₂O₃ to form a visible-light-driven photocatalyst pg-C₃N₄/Ag/Fe₂O₃ via a simple chemical adsorption method. The obtained pg-C₃N₄/Ag/Fe₂O₃ composites with different Ag/Fe₂O₃ content were characterized in terms of composition, morphology, and optical properties by XRD, EDS, FT-IR, TEM and UV-vis DRS. The photocatalytic activities were evaluated by degradation of Rhodamine B (RhB) as a representative organic pollutant under visible light irradiation. The results showed Ag/Fe₂O₃ (5 wt%) modified pg-C₃N₄ exhibited excellent photocatalytic activity in the degradation of RhB and the degradation rate was 2.74 times higher than that of pure pg-C₃N₄. The heterostructured coupling of pg-C₃N₄ with a novel metal, Ag, and semiconductor, Fe₂O₃, in the aspect of energy level matching would effectively improve visible-light absorption capability and facilitate photogenerated electron–hole pairs separation, synergistically accounting for the enhancement of photocatalytic activity. A possible photocatalytic mechanism was also tentatively proposed.

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Introduction

Organic dyes are extensively used in many industry fields and their release into the environment causes water contamination. The presence of organic dyes in aqueous systems is harmful to human health and living ecosystems due to the non-biodegradability, toxicity, and carcinogenic nature of the dyes.¹ So the removal of such toxic organic pollutants in wastewater is a highly essential issue. Semiconductor photocatalysis appears to be a green and efficient technology to deal with water purifying in comparison with other conventional techniques such as physical, chemical, and biological methods.^2–5

Recently, graphitic carbon nitride (g-C₃N₄), a promising polymeric metal-free photocatalyst, has attracted increasing attention due to its unique two-dimensional structure, excellent chemical stability and tunable electronic structure.⁶ g-C₃N₄ has been well employed in the photocatalysis field such as degradation of organic pollutants and hydrogen production by water splitting.^7–10 However, low visible light utilization efficiency, fast recombination of photogenerated electron–hole pairs and small specific surface area of pure g-C₃N₄ still limit its photocatalytic activity. Some efforts have been devoted to enhance the visible light photocatalytic performance of g-C₃N₄, such as doping,^11–13 nanostructuring,^14,15 cocatalyzing,^16,17 copolymerization^18,19 and so forth. In particular, the fabrication of heterostructured composites by combining g-C₃N₄ with other materials, such as semiconductors (TiO₂, ZnO, CdS) with matched energy levels,^20–22 noble metals (Au, Ag, Pd),^23–25 and carbon materials (rGO, CNT, C₆₀)^26–28 can effectively enhance photocatalytic activity by improving the optical adsorption ability and/or promoting photogenerated electron–hole pairs separation. In addition, it is reported that porous structure introduced in g-C₃N₄ by soft or hard templates would make it possess increased surface area and provide more active sites for adsorption and photocatalytic reaction,^29–31 which are beneficial to the enhancement of photocatalytic efficiency. Although porous g-C₃N₄ (denoted as pg-C₃N₄) has showed enhanced visible light photocatalysis compared to bulk g-C₃N₄, there are still some shortages that need to be overcomed. Constructing pg-C₃N₄ based heterostructured photocatalysts is a promising way to further improve the photocatalytic activity of pg-C₃N₄. Hematite (α-Fe₂O₃), as a semiconductor with a band gap of 2.2 eV, is widely used in photocatalysis due to its low cost, non-toxicity and high resistance to corrosion. And recent studies have revealed that the photocatalytic performances of g-C₃N₄ in the degradation of organic pollutants can be improved by forming Fe₂O₃/g-C₃N₄ heterostructures.^32–34 Besides, not only g-C₃N₄ but also α-Fe₂O₃ can act as an idea support material for noble metals³⁵ and the surface plasmon resonance (SPR) effect of the deposited noble metals can facilitate visible-light harvesting and induce a strong localized electric field, which may enhance the activities of the semiconductors for photocatalytic reactions.^36–38

Herein, for the first time, a new type of hybrid architecture, pg-C₃N₄/Ag/Fe₂O₃ composites have been fabricated as an effective visible-light-driven photocatalyst. The photocatalytic performances of a series of pg-C₃N₄/Ag/Fe₂O₃ composites with different mass ratios of Ag/Fe₂O₃ were investigated by photodegradating a typical organic dye RhB under visible light irradiation. The results demonstrated that the obtained pg-C₃N₄ based composites with a optimal content of Ag/Fe₂O₃ exhibited enhanced photocatalytic activity compared to pure pg-C₃N₄. Furthermore, the corresponding photocatalytic mechanism is proposed and discussed.

Experimental section

Synthesis of pg-C₃N₄

The pg-C₃N₄ power is achieved by polymerization of precursor urea according to the literature.³⁹ In a typical procedure, 25 g urea was placed into a crucible and dried at 80 °C for 12 h, then the covered crucible with urea was put in a muffle furnace and heated at 550 °C for 3 h to complete the reaction. Gas bubbles produced during the reaction could be used as a soft template to introduce porous structure in the sample.⁴⁰ The synthesized powder washed with nitric acid (0.1 M) and distilled water, finally dried at 80 °C for 12 h to obtain the light yellow-colored product.

Synthesis of Ag-doped Fe₂O₃ (Ag/Fe₂O₃)

Ag-doped Fe₂O₃ was prepared through a hydrothermal method. 2 g of PVP was dissolved in 30 mL of deionized water. The PVP solution was mixed with a 10 mL Fe₂(NO₃)₃·9H₂O (0.135 M) aqueous solution and stirred for 20 min. Then, 10 mL of AgNO₃ (0.1 M) queous solution was added dropwise to the mixed solution and stirred for another 20 min. After that, 2.4 mL of NH₃·H₂O (25%) was slowly added into above solution to form a mixture. The mixture was stirred at room temperature for 1 h and then transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 160 °C for 3 h. The precipitate was filtered, washed with distilled water and ethanol, dried in air at 60 °C overnight to get Ag/Fe₂O₃ with a calculated mass ratio of about 1 : 2.

Synthesis of pg-C₃N₄/Ag/Fe₂O₃ composites

pg-C₃N₄/Ag/Fe₂O₃ composites were prepared through a chemical adsorption method. Generally, 0.2 g of pg-C₃N₄ and the calculated amount of Ag/Fe₂O₃ were dispersed in 50 mL of methanol with ultrasonication for 30 min, then the suspension stirred and heated at 60 °C to evaporate the methanol, and the solid product was obtained after dried at 80 °C for 12 h. pg-C₃N₄/Ag/Fe₂O₃ composites with different mass ratios 2 wt%, 5 wt% and 10 wt% of Ag/Fe₂O₃ were prepared and are denoted in this manuscript as pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2, pg-C₃N₄/Ag/Fe₂O₃-3, respectively.

Sample characterizations

X-ray diffraction (XRD) measurements were performed on a SmartLab XRD spectrometer (Rigaku) with Cu Kα radiation in the range of 10–80° (2θ). Energy dispersive X-ray spectroscopy (EDS) was used to analyze the composition of samples. Fourier transform infrared (FT-IR) spectra were recorded on a BRUKER-ALPHA FT-IR spectrometer. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were taken with a JEM-2100 high-resolution transmission electron microscope. The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP 2020 (Micromertics USA). UV-vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV-vis spectrophotometer (UV-3600, Shimadzu) with an integrating sphere attachment. PL spectra were measured at room temperature on a Shimadzu RF-5301 fluorescence spectrophotometer with 385 nm excitation wavelength.

Photocatalytic activity test

The photocatalytic activities of the as-prepared samples were evaluated by photodegradation of RhB under visible light irradiation. The photocatalytic reaction was carried out at room temperature in a Pyrex reactor with reflux water to keep its temperature constant. In the experiment, the photocatalyst (25 mg) was dispersed in 100 mL of RhB aqueous solution (5 mg L⁻¹) with ultrasonication for 10 min. Prior to irradiation, the mixed suspension was magnetically stirred for 60 min in the dark to achieve absorption–desorption equilibrium. Subsequently, the above suspension was irradiated by a 500 W xenon lamp (Philips) with a 420 nm cutoff filter, which was employed for the light source. At 30 min intervals, 3 mL suspension was sampled and filtered through a 0.45 μm PTFE syringe filter to get clear liquid. The quantitative determination of RhB in the supernatant was then monitored by measuring its intensity of the absorption peak at 552 nm with a UV-vis spectrophotometer (UV-3600; Shimadzu).

Results and discussion

Structure and morphology

The XRD patterns of the as-prepared pg-C₃N₄, Ag/Fe₂O₃ and a series of pg-C₃N₄/Ag/Fe₂O₃ composites are shown in Fig. 1. As can be seen, the pure pg-C₃N₄ sample shows two characteristic diffraction peaks at 27.2° and 12.9°, which can be indexed to the (002) and (100) planes of the graphite-like carbon nitride.¹³ The XRD pattern of Ag/Fe₂O₃ exhibits diffraction peaks corresponding to both metallic Ag and α-Fe₂O₃ (hematite), in well agreement with the literature values of JCPDS no. 65-2871 and JCPDS no. 33-0664,^41,42 respectively, and no other impurity peaks were observed. Moreover, the diffraction peaks of pg-C₃N₄, metallic Ag and α-Fe₂O₃ are all appeared in the curves of pg-C₃N₄/Ag/Fe₂O₃ composites, indicating that the three components were successfully hybridized. In addition, it should be noted that the relative intensity of Ag/Fe₂O₃ diffraction peaks in pg-C₃N₄/Ag/Fe₂O₃ composites became stronger with an increase of Ag/Fe₂O₃ content. The EDS pattern (Fig. 1(b)) further demonstrated that the pg-C₃N₄/Ag/Fe₂O₃ sample was composed of C, N, O, Ag and Fe atoms.

Fig. 1 XRD patterns of (a) pg-C₃N₄ (1), Ag/Fe₂O₃ (2), pg-C₃N₄/Ag/Fe₂O₃-1 (3), pg-C₃N₄/Ag/Fe₂O₃-2 (4) and pg-C₃N₄/Ag/Fe₂O₃-3 (5), and (b) EDS spectrum of pg-C₃N₄/Ag/Fe₂O₃-2 composites.

Fig. 2 depicts the FTIR spectra of pure pg-C₃N₄ and a series of pg-C₃N₄/Ag/Fe₂O₃ composites. In the spectrum of pure pg-C₃N₄, three main absorption regions can be observed clearly. The broad peak at 3000–3500 cm⁻¹ can be assigned to the stretching vibration of N–H and the stretching vibration of O–H of the physically adsorbed water.⁴³ The strong band of 1200–1700 cm⁻¹ presents the characteristic peaks of the typical stretching vibration of C–N heterocycles. And the peak at 806 cm⁻¹ corresponds to the characteristic breathing mode of triazine units.⁹ Furthermore, the FTIR spectra of pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 composites represent the overlap of the spectra of both pg-C₃N₄ and Fe₂O₃, whose absorption band at 545 cm⁻¹ assigned to the characteristic stretching vibrations of the Fe–O bond in hematite particles.⁴⁴ Nevertheless, the characteristic peak of Fe₂O₃ is not detected in pg-C₃N₄/Ag/Fe₂O₃-1 sample due to its low loading content.

Fig. 2 FTIR spectra of prepared pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3.

The detailed morphologies of and microstructures of pg-C₃N₄, Ag/Fe₂O₃ and pg-C₃N₄/Ag/Fe₂O₃ composites were investigated by TEM and HRTEM, and demonstrated in Fig. 3. As shown in Fig. 3(a), pg-C₃N₄ shows a two-dimensional structure consisting of small flat sheets with wrinkles and irregular shape and a typical porous morphology of the sample is also exhibited, which was highly similar to previously reported results.⁴⁰ The TEM image (Fig. 3(b)) of Ag/Fe₂O₃ displays that Ag nanoparticles with a diameter of 10–25 nm deposited on the surfaces of larger Fe₂O₃ nanoparticles. Besides, when coupling Ag/Fe₂O₃ with pg-C₃N₄, close interfacial connections would be formed among the three components of pg-C₃N₄, Ag, and Fe₂O₃ in the composites, as shown in Fig. 3(c). Furthermore, the interplanar distances of 0.235 and 0.25 nm measured out in the HRTEM image (Fig. 3(d)) can be indexed to the lattice spacing of the Ag (111) and Fe₂O₃ (110) planes,⁴⁵ respectively. All of the above observations indicated that the heterostructured pg-C₃N₄/Ag/Fe₂O₃ was indeed formed.

Fig. 3 Typical TEM images of (a) pg-C₃N₄, (b) Ag/Fe₂O₃, (c) pg-C₃N₄/Ag/Fe₂O₃-2 and HRTEM (d) image of pg-C₃N₄/Ag/Fe₂O₃-2 composites.

Nitrogen adsorption–desorption isotherm is measured in order to investigate the porous nature and specific surface area of the sample. As shown in Fig. 4, both of the pg-C₃N₄ and pg-C₃N₄/Ag/Fe₂O₃-2 samples exhibit a type IV with a H3 hysteresis loop according to the IUPAC classification, reflecting the presence of a mesoporous structure within the samples.²⁹ The parameters obtained from nitrogen desorption isotherms of different samples are given in Table 1. The synthesized pg-C₃N₄ shows a BET surface area of 83.50 m² g⁻¹, a single-point total pore volume of 0.47 cm³ g⁻¹ and an average pore size of 24.97 nm. The BET surface area, the single-point total pore volume and the mean pore size for pg-C₃N₄/Ag/Fe₂O₃-2 sample came out to be 54.05 m² g⁻¹, 0.34 cm³ g⁻¹ and 22.65 nm, respectively. Compared with pure pg-C₃N₄, the pg-C₃N₄/Ag/Fe₂O₃-2 sample exhibit a decrease in BET surface areas and pore volumes due to the incorporation of Ag/Fe₂O₃ might load in the porous structure.

Fig. 4 N₂ adsorption–desorption isotherms of the as-prepared pg-C₃N₄ and pg-C₃N₄/Ag/Fe₂O₃-2 composites.

Table 1 Parameters obtained from the nitrogen desorption isotherm experiments

Sample	Surface area m^2 g^−1	Pore volume cm^3 g^−1	Mean pore size nm
Pg-C3N4	83.50	0.47	24.97
Pg-C3N4/Ag/Fe2O3-2	54.05	0.34	22.65

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The optical absorption plays an important role in the photocatalysis, especially in the visible-light-induced photodegradation of contaminants as visible light constitutes a large fraction of solar energy.⁴⁶ Fig. 5 shows the optical absorption properties of pure pg-C₃N₄ and a series of pg-C₃N₄/Ag/Fe₂O₃ composites in visible light region measured by UV/Vis DRS. As can be seen, pure pg-C₃N₄ shows a steep adsorption edge located at 460 nm and an absorption band lower than 460 nm, resulting in a low visible light utilization efficiency. Whereas the pg-C₃N₄/Ag/Fe₂O₃ composites exhibit significant enhancement on visible light absorption capacities and the absorption intensities increased with an increase in the content of Ag/Fe₂O₃. Meanwhile, the appearance of SPR absorption of Ag at around 537 nm which is attributed to the dipole resonance peak of large Ag nanoparticles clearly proves the existence of large Ag nanoparticles in the composites.⁴⁷ The results demonstrate that the pg-C₃N₄/Ag/Fe₂O₃ composites achieve more efficient utilization of visible light and therefore can be expected to show improved visible-light-driven photocatalytic performance.

Fig. 5 UV/Vis diffuse reflectance spectra of pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 composites.

PL spectral analysis is applied to investigate the recombination rate of photogenerated electron–hole pairs. It was generally believed that a lower PL emission intensity is an indication of a lower recombination of photogenerated electron–hole pairs. The PL spectra of pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 composites under the excitation wavelength of 385 nm are shown in Fig. 6. As can be seen, all of the samples have the same wide emission peak at about 460 nm, which is ascribed to the band-gap recombination of photoexcited electron–hole pairs in g-C₃N₄. Moreover, the PL intensity of the samples follows an order of pg-C₃N₄ > pg-C₃N₄/Ag/Fe₂O₃-1 > pg-C₃N₄/Ag/Fe₂O₃-3 > pg-C₃N₄/Ag/Fe₂O₃-2. Such order demonstrated that the recombination rate of photogenerated electron–holes first decreased with an increase of Ag/Fe₂O₃ content in the composites, and then increased when in incorporation with excess amount of Ag/Fe₂O₃, which may act as recombination centers to cover the active sites on the pg-C₃N₄ surface and thus lower the charge separation efficiency.^33,34

Fig. 6 PL emission spectra of pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 composites.

Photocatalytic performance

Fig. 7(a) inset shows the RhB adsorption capacities of pg-C₃N₄ and a series of pg-C₃N₄/Ag/Fe₂O₃ composites. All of the photocatalysts exhibited excellent adsorption capacities of about 58–60% for RhB after adsorption–desorption equilibrium reached in the dark. The presence of excellent adsorption capacities is probably resulted from the porous structure within the samples. Fig. 7(a) shows the visible-light-driven photocatalytic degradation of RhB monitored according to the concentration change versus time for the samples. The blank experiment demonstrates that the direct photolysis of RhB can be ignored since the target pollutant is only slightly degraded without photocatalyst in the reaction. It is observed that pg-C₃N₄/Ag/Fe₂O₃-2 composites exhibited remarkable photocatalytic performance among all of the samples. In the photocatalytic reaction, the RhB degradation rate for pg-C₃N₄/Ag/Fe₂O₃-2 have reached 66.8% after the first 10 min visible light irradiation, while pure pg-C₃N₄ can only approach 34.5% in the same time. Further, almost 100% of RhB could be degraded by pg-C₃N₄/Ag/Fe₂O₃-2 photocatalyst after irradiated for 50 min. Generally, the degradation efficiency displayed an order of pg-C₃N₄/Ag/Fe₂O₃-2 > pg-C₃N₄/Ag/Fe₂O₃-1 > pg-C₃N₄/Ag/Fe₂O₃-3 ≈ pg-C₃N₄, indicating the photocatalytic activity increased with increasing the Ag/Fe₂O₃ content in the composites at first, however, the followed over-loading of Ag/Fe₂O₃ would have a negative effect on the photocatalytic activity, which also has been confirmed in the PL part. Consequently, a suitable loading amount of Ag/Fe₂O₃ in the composites is significant for effectively enhancing the photocatalytic activity. Furthermore, to quantitatively investigate the reaction kinetics of RhB degradation by the as-prepared pg-C₃N₄ based photocatalysts, we plotted the experimental data in Fig. 7(b) according to the pseudo-first-order kinetic model as expressed by eqn (1), which is generally used for photocatalytic reactions of degrading organic pollutants when C_o is within the millimolar concentration range.

ln(C_o/C) = kt (1)

where C_o is the initial concentration of RhB before irradiation, C is the concentration after irradiation time of t and k is the kinetic rate constant. The kinetic rate constants of pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 samples are calculated to be 0.0327 min⁻¹, 0.0432 min⁻¹, 0.0895 min⁻¹ and 0.0341 min⁻¹, respectively. Obviously, pg-C₃N₄/Ag/Fe₂O₃-2 sample shows the highest rate constant among all of the samples, giving a 2.74 times higher rate constant of RhB degradation than pure pg-C₃N₄. Therefore, the as-prepared pg-C₃N₄/Ag/Fe₂O₃-2 composites with a suitable Ag/Fe₂O₃ loading amount of 5 wt% can be regard as an effective photocatalyst for RhB degradation under visible light irradiation. The probable reason for the enhanced photocatalytic activity will be discussed in the following proposed photocatalytic mechanism section in detail. Moreover, the photocatalytic performance of the as-prepared pg-C₃N₄/Ag/Fe₂O₃ with optimal Ag/Fe₂O₃ content is obviously better than the previously reported Fe₂O₃/g-C₃N₄ composites, as shown in Table 2. In addition, repeated degradation reactions were performed to determine the stability of pg-C₃N₄/Ag/Fe₂O₃-2 photocatalyst, as shown in Fig. 8. It is observed that the pg-C₃N₄/Ag/Fe₂O₃-2 sample exhibits a slight decline rather than any significant loss of activity after four consecutive reaction cycles, where the photocatalytic efficiency reduces only 5.9%, indicating good stability of this photocatalyst.

Fig. 7 (a) Photocatalytic activities and adsorption capacities (Inset), and (b) kinetics of the as-prepared pg-C₃N₄, pg-C₃N₄/Ag/Fe₂O₃-1, pg-C₃N₄/Ag/Fe₂O₃-2 and pg-C₃N₄/Ag/Fe₂O₃-3 composites for degradation of RhB under visible light irradiation.

Table 2 RhB photodegradation performance comparisons between previously reported Fe₂O₃/g-C₃N₄ and the as-prepared pg-C₃N₄/Ag/Fe₂O₃ composites

Photocatalyst	Photocatalyst concn (mg mL^−1)	Initial RhB concn (mg L^−1)	Experiment condition	Degradation rate	Ref.
Fe2O3/g-C3N4	0.25	10	250 W high-pressure sodium lamp provides visible light	After 120 min of irradiation: 85%	33
Fe2O3/g-C3N4	0.83	5	500 W Xenon lamp (λ > 420 nm)	After 160 min of irradiation: almost 100%	34
Pg-C3N4/Ag/Fe2O3	0.25	5	500 W Xenon lamp (λ > 420 nm)	After 50 min of irradiation: almost 100%	This paper

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Fig. 8 Four photocatalytic degradation cycles of RhB by pg-C₃N₄/Ag/Fe₂O₃-2 under visible light irradiation.

Proposed mechanism for enhanced photocatalytic activity

On the basis of the above results, a possible photocatalytic mechanism of the as-prepared pg-C₃N₄/Ag/Fe₂O₃ composites under visible light irradiation was proposed. Fig. 9 shows a schematic for electron–hole separation and transportation at the pg-C₃N₄/Ag/Fe₂O₃ photocatalyst interface. Under visible light irradiation, pg-C₃N₄ and Fe₂O₃ both could be excited to produce electrons (e⁻) and holes (h⁺) due to the narrow band gap of 2.69 eV and 2.2 eV, respectively. At the same time, the incident visible light was absorbed by the surface plasmon resonance (SPR) of Ag nanoparticles. The SPR effect of Ag can not only enhance visible light adsorption, but also cause intense local electromagnetic fields, which would speed up the formation rate of holes and electrons within the semiconductors.^24,25,48,49 Additionally, the SPR-excitation of Ag generated hot electron–hole pairs, with the hot electrons transiently occupying empty states in the Ag conduction band (CB) above the Fermi energy. As the Fermi level for Ag is about 0.4 eV, and the CB values of pg-C₃N₄ and Fe₂O₃ are −1.13 eV and +0.28 eV, respectively, the hot electrons generated by the SPR can transfer from Ag to the CB of the nearby pg-C₃N₄ and Fe₂O₃.⁴² Since E_CB of pg-C₃N₄ (−1.12 eV) was more negative than that of Fe₂O₃ (+0.28 eV), and E_VB of Fe₂O₃ (+2.48 eV) was more positive than that of pg-C₃N₄ (+1.57 eV),³⁴ the photogenerated electrons in pg-C₃N₄ had a tendency to diffuse to the CB of Fe₂O₃ via the interface, while the photogenerated holes in Fe₂O₃ had an opposite transfer direction and they injected into the VB of pg-C₃N₄. This caused an efficient separation of photogenerated electrons and holes and the lifetime of the excited electrons and holes could be prolonged in the transfer process. Meanwhile, some of the photogenerated electrons in pg-C₃N₄ migrated to the surface and can react with dissolved oxygen molecules O₂ to generate active species ˙O₂⁻ owning to E_CB of pg-C₃N₄ (−1.12 eV) is more negative than the redox potential of O₂/˙O₂⁻ (−0.33 eV), and ˙OH are then formed by the reaction of ˙O₂⁻ with H₂O (2˙O₂⁻ + 2H₂O → 2˙OH + 2OH⁻ + O₂). However, the holes transferred from Fe₂O₃ to the VB of pg-C₃N₄ and photogenerated holes in the VB of pg-C₃N₄ could not oxidize H₂O to produce active species ˙OH due to the E_VB value of pg-C₃N₄(+1.57 eV) were lower than the redox potential of ˙OH/H₂O (+2.68 eV).⁵⁰ Hence, the photogenerated holes would react with the target pollutant directly since RhB owns relatively low redox potential of 1.43 eV.⁵¹ Moreover, since the potential for multi-electron reduction of O₂ is more positive (O₂ + 2H⁺ + 2e⁻ = H₂O₂ (aq), +0.682 eV) than the CB value of Fe₂O₃, the electrons transferred from pg-C₃N₄ to the CB of Fe₂O₃ and photogenerated electrons in Fe₂O₃ theoretically would react with O₂ to produce H₂O₂, which is known to be a strong oxidant and also can further react with electrons to form ˙OH (H₂O₂ + e⁻ → ˙OH + OH⁻).^52,53 Besides, some photogenerated holes left in the VB of Fe₂O₃ could react with OH⁻ to produce active species ˙OH due to E_VB of Fe₂O₃ (+2.48 eV) is more positive than the potential of ˙OH/ OH⁻ (+1.99 eV).³³ Generally, photoinduced active species including h⁺, ˙OH and ˙O₂⁻ radicals are expected to be involved in the photocatalysis process and play an important role in the degradation of the pollutant.⁵⁰ In order to distinguish the roles of the active species, the trapping experiment with scavenger investigation was performed. Here, t-BuOH, p-benzoquinone and EDTA were used as the scavengers of ˙OH, ˙O₂⁻ and h⁺, respectively. Fig. 10 shows the degradation rates of RhB by pg-C₃N₄/Ag/Fe₂O₃ photocatalyst in the conditions of adding various scavengers. As can be seen, the photodegradation rate of RhB by pg-C₃N₄/Ag/Fe₂O₃ was almost 100% without scavengers after 50 min visible light irradiation. When t-BuOH (1 mM) was added into reaction solution, the photocatalytic efficiency was moderately affected and the degradation rate was reduced to 82.8%. However, the photocatalytic efficiency exhibited a significant decrease with the addition of p-benzoquinone (1 mM) or EDTA (1 mM), and the degradation rates of RhB were reduced to 30.4% and 11.6% after the same irradiation time, respectively. Obviously, the results suggested that ˙O₂⁻ and h⁺ are the main active species in the current pg-C₃N₄/Ag/Fe₂O₃ photocatalytic degradation system. According to all of the above characterization, test and discussion, it is concluded that the combination of Ag/Fe₂O₃ with pg-C₃N₄ would improve the visible light utilization efficiency, facilitate the separation of photogenerated electron–hole pairs and promote the interfacial electron transfer process, resulting in the enhanced photocatalytic activity. Nevertheless, over loading Ag/Fe₂O₃ in the composites may cover the active sites on the pg-C₃N₄ surface and create recombination centers to increase the recombination rate of photogenerated electron–holes, which is disadvantageous to the photocatalytic activity.

Fig. 9 Proposed photocatalytic mechanism for degradation of RhB by pg-C₃N₄/Ag/Fe₂O₃ composites under visible light irradiation.

Fig. 10 Effects of a series of scavengers on the degradation efficiency of RhB by pg-C₃N₄/Ag/Fe₂O₃ photocatalyst.

Conclusions

In summary, we have successfully fabricated a novel visible-light-driven photocatalyst pg-C₃N₄/Ag/Fe₂O₃, which consisted of two semiconductors with different frameworks but matched energy levels and doped novel metal nanoparticles. The obtained pg-C₃N₄/Ag/Fe₂O₃ composites with an appropriate Ag/Fe₂O₃ content of 5 wt% exhibit remarkable photocatalysis enhancement in the degradation of RhB compared to pure pg-C₃N₄ under visible light irradiation. The SPR effect of Ag nanoparticles and the suitable heterostructure formed among the three components of pg-C₃N₄, Ag and Fe₂O₃ via interfacial connections with energy level matching could improve the visible light harvesting ability as well as charge separation efficiency, synergistically facilitating the photocatalysis process. In all, we believe that the pg-C₃N₄/Ag/Fe₂O₃ composites are expected to be an effective and stable photocatalytic material in the applications of environmental remediation and even hydrogen production by efficiently utilizing solar energy.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21376051, 21306023, 21106017), Natural Science Foundation of Jiangsu (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100) and the Fundamental Research Funds for the Central Universities (3207045301).

References

1. O. Kerkez and I. Boz, Chem. Eng. Commun., 2015, 202, 534–541.
2. S. Kaur and V. Singh, J. Hazard. Mater., 2007, 141, 230–236.
3. B. N. Lee, W. D. Liaw and J. C. Lou, Environ. Eng. Sci., 1999, 16, 165–175.
4. M. A. Rauf and S. S. Ashraf, Chem. Eng. J., 2009, 151, 10–18.
5. K. Soutsas, V. Karayannis, I. Poulios, A. Riga, K. Ntampegliotis, X. Spiliotis and G. Papapolymerou, Desalination, 2010, 250, 345–350.
6. Z. W. Zhao, Y. J. Sun and F. Dong, Nanoscale, 2015, 7, 15–37.
7. X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu and M. Antonietti, J. Am. Chem. Soc., 2009, 131, 1680–1681.
8. J. G. Yu, S. H. Wang, B. Cheng, Z. Lin and F. Huang, Catal. Sci. Technol., 2013, 3, 1782–1789.
9. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397–10401.
10. Y. J. Cui, J. H. Huang, X. Z. Fu and X. C. Wang, Catal. Sci. Technol., 2012, 2, 1396–1402.
11. X. H. Li, J. S. Zhang, X. F. Chen, A. Fischer, A. Thomas, M. Antonietti and X. C. Wang, Chem. Mater., 2011, 23, 4344–4348.
12. G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2010, 132, 11642–11648.
13. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2010, 26, 3894–3901.
14. X. C. Wang, S. Blechert and M. Antonietti, ACS Catal., 2012, 2, 1596–1606.
15. J. H. Sun, J. S. Zhang, M. W. Zhang, M. Antonietti, X. Z. Fu and X. C. Wang, Nat. Commun., 2012, 3, 1139.
16. Y. J. Wang, R. Shi, J. Lin and Y. F. Zhu, Energy Environ. Sci., 2011, 4, 2922–2929.
17. Y. Liu, G. Chen, C. Zhou, Y. D. Hu, D. G. Fu, J. Liu and Q. Wang, J. Hazard. Mater., 2011, 190, 75–80.
18. J. S. Zhang, G. G. Zhang, X. F. Chen, S. Lin, L. Mohlmann, G. Dolega, G. Lipner, M. Antonietti, S. Blechert and X. C. Wang, Angew. Chem., Int. Ed., 2012, 51, 3183–3187.
19. J. S. Zhang, X. F. Chen, K. Takanabe, K. Maeda, K. Domen, J. D. Epping, X. Z. Fu, M. Antonietti and X. C. Wang, Angew. Chem., Int. Ed., 2010, 49, 441–444.
20. X. S. Zhou, B. Jin, L. D. Li, F. Peng, H. J. Wang, H. Yu and Y. P. Fang, J. Mater. Chem., 2012, 22, 17900–17905.
21. J. X. Sun, Y. P. Yuan, L. G. Qiu, X. Jiang, A. J. Xie, Y. H. Shen and J. F. Zhu, Dalton Trans., 2012, 6756–6763.
22. J. Fu, B. B. Chang, Y. L. Tian, F. N. Xi and X. P. Dong, J. Mater. Chem. A, 2013, 1, 3083–3090.
23. N. Y. Cheng, J. Q. Tian, Q. Liu, C. J. Ge, A. H. Qusti, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, ACS Appl. Mater. Interfaces, 2013, 5, 6815–6819.
24. X. J. Bai, R. L. Zong, C. X. Li, D. Liu, Y. F. Liu and Y. F. Zhu, Appl. Catal., B, 2014, 147, 82–91.
25. C. Chang, Y. Fu, M. Hu, C. Y. Wang, G. Q. Shan and L. Y. Zhu, Appl. Catal., B, 2013, 142, 553–560.
26. Y. B. Li, H. M. Zhang, P. R. Liu, D. Wang, Y. Li and H. J. Zhao, Small, 2013, 9, 3336–3344.
27. Y. G. Xu, H. Xu, L. Wang, J. Yan, H. M. Li, Y. H. Song, L. Y. Huang and G. B. Cai, Dalton Trans., 2013, 7604–7613.
28. B. Chai, X. Liao, F. K. Song and H. Zhou, Dalton Trans., 2014, 982–989.
29. D. L. Jiang, L. L. Chen, J. J. Zhu, M. Chen, W. D. Shi and J. M. Xie, Dalton Trans., 2013, 15726–15734.
30. M. Zhang, J. Xu, R. L. Zong and Y. F. Zhu, Appl. Catal., B, 2014, 147, 229–235.
31. J. C. Shen, H. Yang, Q. H. Shen, Y. Feng and Q. F. Cai, CrystEngComm, 2014, 16, 1868–1872.
32. J. Theerthagiri, R. A. Senthil, A. Priya, J. Madhavan, R. J. V. Michael and M. Ashokkumar, RSC Adv., 2014, 4, 38222–38229.
33. S. Z. Hu, R. R. Jin, G. Lu, D. Liu and J. Z. Gui, RSC Adv., 2014, 4, 24863–24869.
34. S. Ye, L. G. Qiu, Y. P. Yuan, Y. J. Zhu, J. Xia and J. F. Zhu, J. Mater. Chem. A, 2013, 1, 3008–3015.
35. J. Zhang, X. H. Liu, X. Z. Guo, S. H. Wu and S. R. Wang, Chem.–Eur. J., 2010, 16, 8108–8116.
36. S. Linic, P. Christopher and D. B. Ingram, Nat. Mater., 2011, 10, 911–921.
37. Z. W. Seh, S. H. Liu, M. Low, S. Y. Zhang, Z. L. Liu, A. Mlayah and M. Y. Han, Adv. Mater., 2012, 24, 2310–2314.
38. Y. C. Pu, G. M. Wang, K. D. Chang, Y. C. Ling, Y. K. Lin, B. C. Fitzmorris, C. M. Liu, X. H. Lu, Y. X. Tong, J. Z. Zhang, Y. J. Hsu and Y. Li, Nano Lett., 2013, 13, 3817–3823.
39. J. H. Liu, T. K. Zhang, Z. C. Wang, G. Dawson and W. Chen, J. Mater. Chem., 2011, 21, 14398–14401.
40. Y. W. Zhang, J. H. Liu, G. Wu and W. Chen, Nanoscale, 2012, 4, 5300–5303.
41. J. S. Jang, K. Y. Yoon, X. Y. Xiao, F. R. F. Fan and A. J. Bard, Chem. Mater., 2009, 21, 4803–4810.
42. L. L. Sun, W. Wu, S. L. Yang, J. Zhou, M. Q. Hong, X. H. Xiao, F. Ren and C. Z. Jiang, ACS Appl. Mater. Interfaces, 2014, 6, 1113–1124.
43. M. Xu, L. Han and S. J. Dong, ACS Appl. Mater. Interfaces, 2013, 5, 12533–12540.
44. B. Zhao, Y. Wang, H. Guo, J. Wang, Y. He, Z. Jiao and M. Wu, Mater. Sci., 2007, 25, 1143–1148.
45. X. J. Liu, Z. Chang, L. Luo, X. D. Lei, J. F. Liu and X. M. Sun, J. Mater. Chem., 2012, 22, 7232–7238.
46. Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
47. W. Y. Gao, M. Q. Wang, C. X. Ran, X. Yao, H. H. Yang, J. Liu, D. L. He and J. B. Bai, Nanoscale, 2014, 6, 5498–5508.
48. Y. X. Yang, Y. N. Guo, F. Y. Liu, X. Yuan, Y. H. Guo, S. Q. Zhang, W. Guo and M. X. Huo, Appl. Catal., B, 2013, 142, 828–837.
49. X. H. Li, X. C. Wang and M. Antonietti, Chem. Sci., 2012, 3, 2170–2174.
50. H. T. Ren, S. Y. Jia, Y. Wu, S. H. Wu, T. H. Zhang and X. Han, Ind. Eng. Chem. Res., 2014, 53, 17645–17653.
51. T. Shen, Z. G. Zhao, Q. Yu and H. J. Xu, J. Photochem. Photobiol., A, 1989, 47, 203–212.
52. K. Katsumata, R. Motoyoshi, N. Matsushita and K. Okada, J. Hazard. Mater., 2013, 260, 475–482.
53. C. S. Pan and Y. F. Zhu, Environ. Sci. Technol., 2010, 44, 5570–5574.

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Footnote

† J. J. Xue and S. S. Ma contributed equally to this work.

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