Heterostructured g-C₃N₄/Ag–TiO₂ composites with efficient photocatalytic performance under visible-light irradiation

Abstract

g-C₃N₄/Ag–TiO₂ composites with close interfacial contact among Ag, TiO₂ and g-C₃N₄ were facilely fabricated and thoroughly characterized. As the deposited Ag nanoparticles played important roles as an electron-conduction bridge and in the surface plasmon resonance effect, Ag modification feasibly improved the separation efficiency for photoinduced electron–hole pairs and enhanced visible-light response. Furthermore, due to the further enhanced separation for photogenerated charges resulting from the existing build-in electric field of the heterojunction and the superposed light response from the hybridization of TiO₂ and g-C₃N₄, g-C₃N₄/Ag–TiO₂ composites exhibited remarkably improved photocatalytic activities for degrading rhodamine B dye compared with pristine TiO₂ and g-C₃N₄ and single-component modified photocatalysts. Additionally, the g-C₃N₄/Ag–TiO₂ catalyst retained excellent stability even after five recycles.

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1. Introduction

Nowadays, the photocatalysis technique is considered to be one of the most promising techniques for solving the global energy crisis and environmental pollution. TiO₂ has become the most extensively researched photocatalyst due to its low cost, nontoxicity and long-term stability. Fujishima and Honda first reported the decomposition of water on illuminated TiO₂ electrodes in 1972.¹ However, its large band gap energy of 3.2 eV and the high recombination rate of electron–hole pairs seriously restricted the practical application of TiO₂.^2–7 Therefore, it is of great significance to develop novel visible-light-responsive photocatalysts by enhancing light harvesting and reducing the recombination rates of the photoinduced electron–hole pairs.

Recently, graphitic carbon nitride (g-C₃N₄) as a metal-free semiconductor with suitable band gap to absorb the visible light and the unique properties has attracted attentions in photocatalytic performance for H₂ generation or organic pollutant degradation.^8–14 It has been reported that the photocatalytic activity of the semiconductor could be improved by g-C₃N₄ modification.^15,16 Ge et al.⁹ developed novel visible-light induced g-C₃N₄/Bi₂WO₆ composite photocatalyst, which exhibited significantly enhanced photocatalytic performance in degrading methyl orange. Pan et al.¹⁰ fabricated core/shell structured C₃N₄/BiPO₄ photocatalyst via a facile ultrasonic dispersion method. The optimum photocatalytic activity for degradation of methylene blue (MB) over C₃N₄/BiPO₄ was almost 4.5 times as high as that of reference catalyst P25 and 2.5 times of BiPO₄. Sridharan et al.¹¹ prepared g-C₃N₄/TiO₂ composite through a thermal transformation methodology and investigated the photocatalytic activity of MB and Cr(VI) ions under visible light. Moreover, it was well recognized that Ag modification was a feasible method to improve the separation efficiency for photoinduced electron–hole pairs due to the excellent conductivity and strong electron trapping ability.¹⁷ Besides, the surface plasmon resonance (SPR) effect of Ag enhanced visible-light response of semiconductor, contributing to the utilization of solar energy.¹⁸ Therefore, TiO₂ modified by Ag and g-C₃N₄ would combine peculiar characteristics of the three-component composites and seemed to be an ideal photocatalyst compared with single-component modified TiO₂.

In this contribution, the heterostructured g-C₃N₄/Ag–TiO₂ composites were facilely fabricated via physical mixing followed by calcination method. Close interfacial contact among Ag, TiO₂ and g-C₃N₄ made the g-C₃N₄/Ag–TiO₂ composites display the reduced recombination efficiency for photoinduced electron–hole pairs and enhanced visible-light absorbance. The synergistic positive effects contributed to g-C₃N₄/Ag–TiO₂ composites better activity for degrading the model pollutant of Rhodamine B (RhB) under visible-light irradiation than the bare g-C₃N₄ and TiO₂ and single-component modified photocatalysts. In addition, g-C₃N₄/Ag–TiO₂ catalyst retained excellent stability even after five recycles. Besides, the tentative photodegradation process over g-C₃N₄/Ag–TiO₂ composites was proposed based on the experimental results.

2. Experimental

2.1 The preparation of the photocatalysts

g-C₃N₄ was prepared by heating 5 g melamine at 600 °C for 2 h with the heating rate of 2 °C min⁻¹ in a muffle furnace.¹⁹ After being naturally cooled to room temperature, faint yellow g-C₃N₄ was obtained.

Ag-loaded TiO₂ (Ag–TiO₂) was prepared through photodeposition procedure.²⁰ 0.5 g TiO₂ (P25) was dispersed in 100 mL deionized water and 3 mL 0.05 M AgNO₃ solution were added slowly under stirring, then the mixture was irradiated under 300 W Xe-lamp for 3 h with continuous magnetic stirring. After centrifugation, the sample was washed with water and dried at 80 °C overnight to obtain 3 wt% Ag–TiO₂. For comparison, Ag loaded g-C₃N₄ (Ag–C₃N₄) was prepared according to the similar photodeposition procedure.

g-C₃N₄/Ag–TiO₂ composites were prepared as follows: g-C₃N₄ and Ag–TiO₂ with different mass ratios were dispersed in 100 mL distilled water to obtain uniform solution by ultrasonic treatment for 3 h, then the mixture was dried at 60 °C for 12 h and heated at 400 °C for 1 h with the heating rate of 2 °C min⁻¹. A series of g-C₃N₄/Ag–TiO₂ composites with mass ratios of g-C₃N₄ to Ag–TiO₂ from 1 : 9 to 9 : 1 were synthesized and denoted as g-C₃N₄/Ag–TiO₂-X. For example, g-C₃N₄/Ag–TiO₂-10 represents the mass percentage of g-C₃N₄ in the g-C₃N₄/Ag–TiO₂ composites is 10%.

2.2 Characterization

X-ray diffraction (XRD) measurements were carried out on Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (40 kV, 30 mA) for phase identification. X-ray photoelectron spectroscopy (XPS) measurements were recorded on Phi Quantera spectrometer with Al Kα radiation (hν = 1486.6 eV). Fourier transform infrared spectroscopy (FT-IR) was recorded in transmission mode from 4000 to 400 cm⁻¹ on Perkin Elmer 100 FTIR spectrometer using KBr discs. The morphology and chemical compositions of the composites were examined by transmission electron microscopy (TEM, Tecnai G2 Spirit) and scanning electron microscopy (SEM, HITACHI SU8000) with energy dispersive X-ray spectroscopy (EDS) mapping. Nitrogen adsorption–desorption isotherms were measured at −196 °C on ASAP 2020 volumetric analyzer. UV-vis diffuse reflectance spectra (DRS) were measured on Perkin Elmer Lambda 750 UV-vis spectrometer. Photoluminescence spectra (PL) were obtained using Perkin Elmer LS55 at room temperature with the excitation wavelength of 325 nm.

2.3 Evaluation of photocatalytic performance

The photocatalytic performance of the as-prepared g-C₃N₄/Ag–TiO₂ composites were examined by degrading RhB dye. The visible light was provided by 300 W Xe-lamp with a 400 nm cut-off filter. 50 mg photocatalyst was added into 50 mL 5 mg L⁻¹ RhB aqueous solution, prior to the light irradiation, the dispersion was kept in dark for 30 min to reach the adsorption–desorption equilibrium under magnetic stirring. The distance between the surface of the suspension and the light source was about 20 cm. Solutions were collected every 5 min irradiation and centrifuged to remove the catalyst then analyzed on UV-vis spectrometer. For comparison, the reactions were carried out in the presence of g-C₃N₄, P25, Ag–TiO₂ and Ag–C₃N₄. The degradation efficiency was calculated using C/C₀, wherein C was the concentration of the remaining dye solution at each irradiated time, and C₀ was the initial concentration.

3. Results and discussion

3.1 Structure, morphology and spectra characterizations of g-C₃N₄/Ag–TiO₂ composites

The XRD patterns of g-C₃N₄/Ag–TiO₂ composites were shown in Fig. 1. The peaks at 25.3°, 37.8°, 47.9°, 53.8°, 55.1°, 62.7°, 68.7°, 70.3° and 75.0° were attributed to (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes of anatase TiO₂.²¹ For g-C₃N₄, the peaks at 27.4° and 13.1° were corresponded to (002) and (100) planes of hexagonal g-C₃N₄ (JCPDS card no. 87-1526), and respectively attributed to the graphite-like stacking and in-plane structural repeating motifs of the conjugated aromatic units.²² It was obvious that both TiO₂ and g-C₃N₄ were observed in the composites, and the peak intensities at 27.4° became stronger with the increasing contents of g-C₃N₄ in the composites. Nevertheless, there appeared no peaks ascribed to Ag, which might be ascribed to its lower content or its high dispersity.²³

Fig. 1 XRD patterns of (a) g-C₃N₄, (b) g-C₃N₄/Ag–TiO₂-90, (c) g-C₃N₄/Ag–TiO₂-70, (d) g-C₃N₄/Ag–TiO₂-50, (e) g-C₃N₄/Ag–TiO₂-30, (f) g-C₃N₄/Ag–TiO₂-10, (g) TiO₂ and (h) Ag–TiO₂.

Fig. 2 showed the XPS spectra of g-C₃N₄/Ag–TiO₂-50 composites. In C 1s spectrum, three deconvoluted peaks appeared at 284.3, 286.0 and 287.8 eV. The peak at 284.3 eV was assigned to C–C and the adventitious carbon, and the peaks at 286.0 and 287.8 eV were ascribed to C–N–C and the C–(N)₃ coming from g-C₃N₄.²⁴ For the N 1s spectrum, three peaks at 397.9, 398.6 and 399.9 eV were separately belonged to sp²-bonded N to two carbon atoms (C=N–C), tertiary nitrogen (N–(C)₃) and amino functional group with a hydrogen atom (N–H).^25,26 O 1s spectrum was fitted into two peaks of Ti–O bond at 529.4 eV and O–H bond at 530.4 eV.^27,28 Ti 2p spectrum showed two symmetric peaks at 458.2 and 463.9 eV, which ascribed to electron binding energies of Ti 2p_3/2 and 2p_1/2.^27,28 As shown in Fig. 2E, the peaks at 367.0 and 373.1 eV were attributed to Ag 3d_3/2 and Ag 3d_5/2,^29,30 which confirmed the existence of Ag in the composites.

Fig. 2 XPS spectra of g-C₃N₄/Ag–TiO₂-50, (A) C 1s, (B) N 1s, (C) O 1s, (D) Ti 2p and (E) Ag 3d.

Fig. 3 depicted the FT-IR spectra of the bare g-C₃N₄, TiO₂, and g-C₃N₄/Ag–TiO₂-50 composites. For the bare g-C₃N₄, there were three main absorption regions to be observed clearly. The peaks at 1640, 1247, 1325 and 1408 cm⁻¹ were associated with the stretching vibrations of CN heterocycles.^15,31 The peak at 808 cm⁻¹ was attributed to the characteristic breathing modes of triazine units.¹⁵ The broad peak at 3150–3500 cm⁻¹ was assigned to the stretching vibration modes of terminal N–H and O–H.³² In the case of TiO₂, a broad absorption band at 500–700 cm⁻¹ was regarded as the characteristic peak of TiO₂.³³ The FT-IR spectra of the g-C₃N₄/Ag–TiO₂-50 composites represented the characteristic peaks of both g-C₃N₄ and TiO₂, indicative of their coexistence in the composites.

Fig. 3 FT-IR spectra of (a) g-C₃N₄, (b) g-C₃N₄/Ag–TiO₂-50 and (c) TiO₂.

Fig. 4 showed the TEM images of the bare g-C₃N₄, TiO₂ and g-C₃N₄/Ag–TiO₂-50 composites. The pristine g-C₃N₄ displayed aggregation of 2D lamellar structure, and TiO₂ presented as agglomerates of small spherical-like particles. For g-C₃N₄/Ag–TiO₂-50 composites in Fig. 4C, it was observed that TiO₂ and Ag nanoparticles highly dispersed on the g-C₃N₄ surface. Additionally, HRTEM image of g-C₃N₄/Ag–TiO₂-50 composites (Fig. 4D) clearly revealed the interface between g-C₃N₄ and Ag–TiO₂, and an discerned interplanar spacing of 0.35 nm conforms to the (101) crystal plane of TiO₂ while 0.23 nm matches with the (111) plane of Ag.³⁴ Therefore, the close interfacial contact could facilitate the photogenerated electron transfers from g-C₃N₄ to Ag or TiO₂ nanoparticles. Hence, it was reasonable that an improved photocatalytic activity was expected for the as-prepared g-C₃N₄/Ag–TiO₂ composites.

Fig. 4 TEM images of (A) bare g-C₃N₄, (B) TiO₂, (C) g-C₃N₄/Ag–TiO₂-50 composites and (D) HRTEM image of g-C₃N₄/Ag–TiO₂-50 composites.

Fig. 5 SEM image of g-C₃N₄/Ag–TiO₂-50 and corresponding EDS mapping images for rectangle area.

The SEM image of g-C₃N₄/Ag-TiO₂-50 and its EDS elemental mappings (Fig. 5) further indicated that g-C₃N₄/Ag–TiO₂-50 composites were composed of C, N, Ti, O and Ag elements, which were evenly dispersed. Thus the composites coupled with g-C₃N₄, Ag and TiO₂ were successfully fabricated.

Nitrogen adsorption–desorption analysis in Fig. 6 was used to determine BET surface areas of the catalysts, the surface areas of the as-prepared samples were 35.77 m² g⁻¹, 26.45 m² g⁻¹, 27.19 m² g⁻¹, 18.07 m² g⁻¹, 10.97 m² g⁻¹ and 4.99 m² g⁻¹ for g-C₃N₄/Ag–TiO₂-10, g-C₃N₄/Ag–TiO₂-30, g-C₃N₄/Ag–TiO₂-50, g-C₃N₄/Ag–TiO₂-70, g-C₃N₄/Ag–TiO₂-90 and bare g-C₃N₄.

Fig. 6 Nitrogen adsorption–desorption isotherms of (a) g-C₃N₄/Ag–TiO₂-10, (b) g-C₃N₄/Ag–TiO₂-30, (c) g-C₃N₄/Ag–TiO₂-50, (d) g-C₃N₄/Ag–TiO₂-70, (e) g-C₃N₄/Ag–TiO₂-90 and (f) g-C₃N₄.

The absorbance properties of the as-prepared samples were measured using UV-vis DRS in Fig. 7. A sharp basal absorption edge for TiO₂ appeared at 393 nm, corresponding to the electron transition of bandgap at 3.2 eV. Ag–TiO₂ showed obvious light absorption in the visible-light region, which was ascribed to the SPR effect of the loaded Ag nanoparticles.³⁵ The main absorption edge of g-C₃N₄ occurred at 473 nm due to its narrowed bandgap. Noticeably, g-C₃N₄/Ag–TiO₂-50 composites displayed significantly enhanced visible-light absorption due to the loaded Ag and the superposed absorption from g-C₃N₄ and TiO₂. The bandgap energies (E_g) of bare TiO₂ particle, g-C₃N₄, Ag–TiO₂ and g-C₃N₄/(Ag–TiO₂)-50 composites were calculated to be 3.16 eV (TiO₂), 2.62 eV (g-C₃N₄), 2.95 eV (Ag–TiO₂) and 2.57 eV (g-C₃N₄/Ag–TiO₂) according to E_g = 1240/λ_g equation.³³ And based on the empirical equations of E⁰_VB = χ − E^C − 1/2 E_g and E_CB = E_VB − E_g,³⁶ the potentials of VB and CB of g-C₃N₄ were calculated at −1.105 eV and 1.515 eV, and those of TiO₂ were −0.295 eV and 2.865 eV. After the light illuminated, the photoinduced electrons were generated on g-C₃N₄ and easily transferred from CB of g-C₃N₄ to the CB of TiO₂ via the interface. Namely, the CB of TiO₂ could act as a sink for photogenerated electrons, benefitting the separation of electrons and holes.

Fig. 7 UV-vis diffuse reflectance absorption spectra of (a) bare TiO₂, (b) Ag–TiO₂, (c) g-C₃N₄ and (d) g-C₃N₄/Ag–TiO₂-50.

3.2 Photocatalytic activity for degrading RhB

The photocatalytic activity of degrading RhB was shown in Fig. 8A, when g-C₃N₄ and P25 were added, RhB concentration gradually decreased and the degradation efficiencies were 86% and 40% within 30 min. By contrast, the photocatalytic activities over Ag–TiO₂ and Ag–C₃N₄ were enhanced by Ag modifications. With the co-modifications of Ag and g-C₃N₄, the photocatalytic activities of g-C₃N₄/Ag–TiO₂ were remarkably improved. The activities were in the order of g-C₃N₄/Ag–TiO₂-10 < g-C₃N₄/Ag–TiO₂-70 < g-C₃N₄/Ag–TiO₂-90 < g-C₃N₄/Ag–TiO₂-30 < g-C₃N₄/Ag–TiO₂-50. In the case of g-C₃N₄/Ag–TiO₂-10 composites, photocatalytic activity was low resulted from the reduced visible-light harvesting due to the low content of g-C₃N₄. With more contents of g-C₃N₄ at mass ratios of g-C₃N₄/Ag–TiO₂ 7 : 1 and 9 : 1, although the absorptions to visible light were enhanced, the high recombination rates of photoinduced charges because of low contents of active separation centers acted by TiO₂ and Ag led to the decreased activities. Thus the optimum catalytic performance was obtained in the g-C₃N₄/Ag–TiO₂-50 composites, after 30 min visible-light irradiation, the removal efficiency of RhB was improved to 96%. To have a better understanding of the reaction kinetics of RhB degradation catalyzed by various catalysts, the relationships between ln(C₀/C) and irradiation time were depicted in Fig. 8B. The linear relationships suggested that the photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. The kinetic constants were 0.0983 min⁻¹ for g-C₃N₄/Ag–TiO₂-50, 0.0391 min⁻¹ over g-C₃N₄/TiO₂ (mass ratio 1 : 1), 0.0156 min⁻¹ over TiO₂, 0.0603 min⁻¹ over g-C₃N₄, 0.0445 min⁻¹ over Ag–TiO₂, and 0.0653 min⁻¹ for Ag–C₃N₄. The g-C₃N₄/Ag–TiO₂-50 composites exhibited the highest rate constant, which was almost 6.30 and 1.63 times higher than pure TiO₂ and pure g-C₃N₄, obviously displaying the positive effects of Ag and g-C₃N₄.

Fig. 8 (A) Photodegradation activities and (B) first-order kinetic plots for the photodegradation of RhB; (s1) g-C₃N₄/Ag–TiO₂-10, (s2) g-C₃N₄/Ag–TiO₂-30, (s3) g-C₃N₄/Ag–TiO₂-50, (s4) g-C₃N₄/Ag–TiO₂-70, (s5) g-C₃N₄/Ag–TiO₂-90, (a) P25, (b) g-C₃N₄/TiO₂, (c) Ag–TiO₂, (d) g-C₃N₄ and (e) Ag–C₃N₄.

From the viewpoint of practical applications, the stability of photocatalyst is a very important factor. As the representative sample, g-C₃N₄/Ag–TiO₂-50 composites were selected to evaluate the reusability. As shown in Fig. 9A, the photocatalytic activity of g-C₃N₄/Ag–TiO₂-50 composites still kept at 90% of fresh catalyst even after five recycles, which suggested that g-C₃N₄/Ag–TiO₂ composites possessed excellent stability. In contrast, the physical mixing of g-C₃N₄ and Ag–TiO₂ without calcination showed poor stability even in the second run (Fig. 9B), attribute to the lost contact interaction between g-C₃N₄ and Ag–TiO₂. In addition, the XRD patterns and FT-IR of g-C₃N₄/Ag–TiO₂-50 composites before and after five photodegradation runs were investigated and shown in Fig. 9C and D. After five recycles, the structure of the spent g-C₃N₄/Ag–TiO₂-50 composites was the same to the fresh catalyst, indicative of the structure stability during the photoreaction process.

Fig. 9 Circulating runs in the photodegradation of RhB (A) over g-C₃N₄/Ag–TiO₂-50 composites, (B) over physical mixture of g-C₃N₄ with Ag–TiO₂ without calcination, (C) XRD and (D) FT-IR patterns of g-C₃N₄/Ag–TiO₂-50 composites before and after reuses for the photodegradation of RhB.

3.3 Possible photodegradation process

Since photoluminescence emission spectrum arises from the recombination of excited electrons and holes, thus, PL technique is useful for disclosing the migration, transfer and recombination processes of the photogenerated electron–hole pairs in the semiconductors. The lower PL intensity is a general indication of the lower recombination of electron–hole pairs. Fig. 10 showed the PL spectra of g-C₃N₄, Ag–C₃N₄ and g-C₃N₄/Ag–TiO₂-50 composites with the excitation wavelength of 325 nm. Pristine g-C₃N₄ exhibited much stronger intensity than g-C₃N₄/Ag–TiO₂-50 composites. For g-C₃N₄/Ag–TiO₂-50 composites, the lower intensity meant that the loaded Ag and existed heterojunction facilitated charge transfers among Ag, TiO₂ and g-C₃N₄, leading to the decreased recombination probability of the electrons-holes and the improved photocatalytic activity.

Fig. 10 PL spectra of g-C₃N₄, Ag–C₃N₄ and g-C₃N₄/Ag–TiO₂-50 composites.

In order to investigate the main reactive species for the photodegradation of the RhB, the addition effects of radical scavengers were examined to discuss the reaction mechanism (Fig. 11). Here, methanol was used to quench ˙OH in the solution,³⁷ EDTA was h⁺ scavenger,³⁸ p-benzoquinone (p-BQ) was ˙O₂⁻ quencher,³⁹ and dimethyl sulfoxide (DMSO) was e⁻ scavenger.⁴⁰ The additions of methanol, EDTA and DMSO led to the remarkable decreases in degradation activities, which implied that ˙OH, h⁺ and e⁻ were the active reactive species. The addition of p-BQ made the degradation of RhB almost inhibit, suggesting that ˙O₂⁻ was the main reactive species, which was consistent with the previous report.⁴¹

Fig. 11 Effects of adding different radical scavengers on the degradation of RhB over g-C₃N₄/Ag–TiO₂-50 composites. (a) no scavenger, (b) with 10 mM methanol, (c) with 5 mM DMSO, (d) with 1 mM EDTA, and (e) with 1 mM p-BQ.

Based on the above results, a possible photodegradation mechanism over g-C₃N₄/Ag–TiO₂ composites were proposed in Fig. 12. During the photocatalytic oxidation process, g-C₃N₄ first was excited to form photogenerated electron–hole pairs under visible-light irradiation. The CB and VB edge potentials of g-C₃N₄ were at −1.105 and +1.515 eV, more negative than those of TiO₂ at −0.295 and +2.865 eV. Subsequently, the photogenerated electrons transferred from CB of g-C₃N₄ to TiO₂ or trapped by Ag nanoparticles. These electrons further reacted with O₂ adsorbed on the catalyst surface to generate ˙O₂⁻, and ˙O₂⁻ radicals combined with H₂O to further transformed to ˙OH. At the same time, h⁺ in the VB of g-C₃N₄ was less positive than the standard redox potential of ˙OH/H₂O (2.68 eV vs. SHE),⁴² indicating that the photogenerated holes on the g-C₃N₄ could not oxidize H₂O into ˙OH, but directly involved into the degradation of pollutant. And h⁺ in the VB of TiO₂ is more positive than the standard redox potential of ˙OH/H₂O, so they could combine with H₂O to produce active ˙OH. These reactive radical species of ˙O₂⁻, ˙OH, h⁺ and e⁻ were so reactive that they could efficiently react with the organic pollutant to generate the degradation products. In the photocatalytic reaction process, the recombination rates of the electrons and holes were efficiently prevented and the absorption to the visible light was significantly enhanced, thus resulting in the enhanced photocatalytic activity over g-C₃N₄/Ag–TiO₂ composites.

Fig. 12 Possible photodegradation process over g-C₃N₄/Ag–TiO₂ composites under visible-light irradiation.

4. Conclusion

In summary, g-C₃N₄/Ag–TiO₂ photocatalyst was facilely synthesized and exhibited enhanced visible-light-driven activity for degrading RhB. And g-C₃N₄/Ag–TiO₂ with mass ratio ofg-C₃N₄ to Ag–TiO₂ at 1 : 1 showed the optimal photodegradation efficiency and excellent stability. The interfacial contact existed in the g-C₃N₄/Ag–TiO₂ composites contribute to retarding the charge recombinations, moreover, the deposited Ag nanoparticles played an important role as electron reservoir for the efficient separation of electrons-holes. Besides, the enhanced absorption to the visible light coming from the Ag SPR effect and superpose absorption from g-C₃N₄ coupled with TiO₂ also assisted the improvement of catalytic activity. Therefore, g-C₃N₄/Ag–TiO₂ composites may be potential photocatalyst candidate applied in environmental pollutants removal.

Acknowledgements

We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21373069), Science Foundation of Harbin City (NJ20140037), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2015DX08) and the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. 201327).

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