Facile fabrication of a mpg-C₃N₄/TiO₂ heterojunction photocatalyst with enhanced visible light photoactivity toward organic pollutant degradation

Abstract

A facile hard template approach has been developed to prepare mpg-C₃N₄/TiO₂ composites using SiO₂ nanoparticles as a hard template and cyanamide as a precursor. The samples have been well characterized using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). The results demonstrate that TiO₂ nanoparticles sized 5–10 nm were distributed on the surface of mpg-C₃N₄ to form the mpg-C₃N₄/TiO₂ heterojunction photocatalyst. In this way, the mpg-C₃N₄/TiO₂ heterojunction catalyst has a porous structure and large surface area, which increases the contact area for pollutants. Rhodamine B (RhB) was used as a representative organic pollutant to evaluate the photocatalytic activity of the samples under visible light irradiation. The results show that the as-prepared mpg-C₃N₄/TiO₂ heterojunction catalyst has a significantly enhanced visible light photocatalytic activity compared to pure mpg-C₃N₄ and that the degradation rate was 1.6 times higher than that of pure mpg-C₃N₄. The increased photocatalytic activity of the mpg-C₃N₄/TiO₂ heterojunction catalyst can be attributed to the formation of the heterojunction between mpg-C₃N₄ and TiO₂, which suppresses the recombination of photoinduced electron–hole pairs. Furthermore, based on systematic characterization and discussion, a possible photocatalytic mechanism for the excellent photocatalytic performance was proposed.

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Introduction

In recent decades, environmental problems such as organic pollutants and toxic water pollutants produced by some industries have become more and more harmful to human health.^1–3 Photocatalysis is highly expected to be a useful green technology for solving many current environmental and energy issues through its efficiency and broad applicability.⁴ With the steadily and fast growing fields of nanoscience and nanotechnology, various nanostructured semiconductor materials such as metal oxides, nitrides and sulfides have been exploited as photocatalysts for photocatalytic degradation of organic pollutants.⁵ Notably, TiO₂ is widely studied due to its favorable optical and electronic properties, long-term stability, low cost, and nontoxicity. However, with a bandgap of 3.2 eV, TiO₂ could only utilize radiation with a wavelength less than 390 nm, which only takes up 5% of the solar light. The narrow optical absorption range and the highly efficient recombination of photogenerated electrons and holes are the main factors responsible for the low energy efficiency.⁶ Therefore, lots of efforts have been devoted to extend the absorption edge of TiO₂, such as the doping of TiO₂ with metallic or nonmetallic elements to increase the visible light absorbance,^7–9 and coupling with other semiconductors to increase the separation efficiency of photogenerated electron–hole pairs during photocatalysis.^10,11 Considering that most light sources include plenty of visible light, coupling a wide-bandgap semiconductor with a small-bandgap inorganic semiconductor as a visible light sensitizer is a key method for enhancing the photocatalytic energy efficiency to develop new photocatalysts with good photogenerated charge separation as well as a wide response wavelength range.

Recently, graphitic carbon nitride (g-C₃N₄), a polymeric metal-free semiconductor with a band gap of about 2.70 eV, has been a focus of the photocatalytic fields such as water splitting and/or organic pollutant decomposition owing to its unique two-dimensional structure, excellent chemical stability and tunable electronic structure.^12–14 However, the photocatalytic efficiency of g-C₃N₄ is still limited due to the high recombination rate of the photogenerated electron–hole pairs. Several methods have been developed to improve the photocatalytic performance of g-C₃N₄, for instance, dye sensitization, transition metal doping and coupling with a semiconductor.^15–20 Specifically, g-C₃N₄ has a highest occupied molecular orbital (HOMO) which is located at −1.12 eV and is more negative than the conduction band (CB) of the common wide bandgap semiconductor photocatalysts such as ZnO and TiO₂, which is favorable for forming heterojunctions with the wide bandgap semiconductors and thus for extending their visible light response.²¹ In particular, the fabrication of heterostructured composites by combining g-C₃N₄ with other semiconductors can not only restrict efficiently the recombination of photogenerated charge carriers, but can also endow the composites with novel characteristics or some enhanced properties from the synergistic effects or antagonistic effects.^22,23 In addition, previous reports indicate that the porous structure introduced in g-C₃N₄ by soft or hard templates would make it possess an increased surface area and provide more active sites for adsorption and photocatalytic reactions, which are also beneficial to the enhancement of photocatalytic efficiency.^24–26

In this paper, we demonstrate a facile hard template method to synthesize mpg-C₃N₄/TiO₂ heterostructures. Herein, mesoporous g-C₃N₄ (mpg-C₃N₄) with an open crystalline pore wall and a large surface area was used as a support to disperse TiO₂ nanoparticles. The mesoporous structure can in principle enhance the light harvesting ability and the reactant adsorption capability of the material due to its large surface area and multiple scattering effects. The photocatalytic activity of the mpg-C₃N₄/TiO₂ heterostructures was carefully investigated using the degradation of Rhodamine B (RhB) dye under visible light irradiation. The results indicate that the as-prepared mpg-C₃N₄/TiO₂ heterostructures exhibit a much higher visible light photocatalytic activity than that of bare mpg-C₃N₄ or TiO₂ nanoparticles, which is attributed to the improvement in the separation efficiency of the photogenerated electron–hole pairs in the mpg-C₃N₄/TiO₂ composites. Meanwhile, we also propose a possible mechanism for the enhanced activity of the mpg-C₃N₄/TiO₂ composites on account of the experimental results.

Experimental section

Materials

Cyanamide (CN-NH₂, 50%), a 40% dispersion of SiO₂ particles (12 nm, Ludox HS40), titanium oxide (TiO₂, 5–10 nm) and ammonium bifluoride (NH₄HF₂) were purchased from Aladdin Chemical Regent Co., Ltd (Shanghai, China). All the reagents in this study are analytically pure and were used without further purification.

Preparation of the mpg-C₃N₄/TiO₂ heterostructures

In a typical procedure, 8.0 g of cyanamide (50% solution) was added dropwise to 24 g of a 40% dispersion of 12 nm SiO₂ particles, which were used as a hard template, and then 0.5 g of TiO₂ powder was added into the above solution with ultrasonication. The mixture was heated at 80 °C with stirring to evaporate the water. The resulting white powder was then heated at a rate of 2.3 °C min⁻¹ over 4 h to reach a temperature of 550 °C, and then kept at this temperature for another 4 h. The brown-yellow product was treated with 4 M NH₄HF₂ for two days to remove the silica template. The powder was then centrifuged and washed with distilled water and ethanol several times. Finally the mpg-C₃N₄/TiO₂ was dried at 70 °C under vacuum overnight. Meanwhile, a pure mpg-C₃N₄ sample was synthesised without the addition of TiO₂ according to the previous report.²⁷

Sample characterization

X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (SmartLab, Rigaku) using Cu Kα radiation in the angle range of 10–80° (2θ). Then energy-dispersive X-ray spectroscopy (EDS) was used to analyze the elements of the as-prepared samples. The FT-IR spectra (KBr disc) of the as-prepared samples were recorded on a BRUKER-ALPHA FT-IR spectrometer. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained with a JEM-2100 high-resolution transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP 2020 (Micromertics USA). UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on UV-vis spectrometer (UV-3650, Shimadzu) with an integrating sphere attachment. Here, BaSO₄ was used as the reflectance standard material. The PL spectra were measured at room temperature on a Shimadzu RF-5301 fluorescence spectrophotometer with a 325 nm excitation wavelength.

Photocatalytic activity test

The photocatalytic reactions were carried out at room temperature in a Pyrex reactor with refluxing water to keep the temperature constant. A 500 W Xe lamp (Philips) was used as a simulated solar light source (UV-visible light), and a 420 nm cutoff filter was mounted on the lamp to eliminate infrared irradiation. Rhodamine B (RhB) with a concentration of 10 mg L⁻¹ was used as the contaminant. In order to obtain an optimally dispersed system and reach complete adsorption–desorption equilibration, 100 mg of photocatalyst powder was dispersed in 100 mL of reaction solution by ultrasonicating for 10 min, and then the suspension was magnetically stirred in the dark for 1 h. Subsequently, the above suspension was irradiated using the 500 W xenon lamp. At regular intervals, a 3 mL portion of the suspension was sampled and filtered through a 0.45 μm PTFE syringe filter to get a clear liquid. Quantitative determination of the RhB in the supernatant was then performed by measuring the intensity of the absorption peak at 553 nm with a UV-vis spectrophotometer (UV-3600; Shimadzu).

Results and discussion

Structure and morphology

Fig. 1 shows the XRD patterns of the bare mpg-C₃N₄, TiO₂ and the mpg-C₃N₄/TiO₂ heterojunctions. As can be seen, the bare mpg-C₃N₄ shows two pronounced diffraction peaks of the (100) and (002) planes at 2θ ≈ 13.1° and 27.2°, which can be ascribed to the characteristic inter-layer structural packing and the interplanar stacking peaks of the aromatic systems, respectively.²⁸ In the pure TiO₂ sample, the strong diffraction peaks at 2θ = 25.34°, 37.82°, 48.08°, 53.94°, 55.10°, 62.78° and 68.80° can be assigned to the (101), (004), (200), (105), (211), (204) and (116) crystal planes of pure TiO₂ with an anatase phase (JCPDS 21-1272), respectively.²⁹ The mpg-C₃N₄/TiO₂ heterojunction samples exhibit diffraction peaks corresponding to both mpg-C₃N₄ and TiO₂, reflecting the presence of two phases, and no other impurity peaks were observed. In addition, EDS analysis was employed to investigate the composition of the heterojunctions (in Fig. 1b), where the appeared peaks confirmed that the product was only composed of C, N, O and Ti elements.

Fig. 1 XRD patterns of (a) mpg-C₃N₄, TiO₂ and the mpg-C₃N₄/TiO₂ heterojunctions, and an EDS spectrum of (b) the mpg-C₃N₄/TiO₂.

The morphologies of the mpg-C₃N₄, pure TiO₂ and mpg-C₃N₄/TiO₂ heterojunctions were investigated using transmission electron microscopy (TEM) and high resolution TEM (HRTEM), as shown in Fig. 2. As shown in Fig. 2a, mpg-C₃N₄ displays a 2D lamellar structure and many spherical pores with a mean diameter of about 12 nm are observed on the surface. These pores exactly reflect the geometric properties of the original template of SiO₂ particles with a mean diameter of 12 nm. Fig. 2b displays the irregular spheres of the TiO₂ particles with diameters in the range of 5–10 nm. For the mpg-C₃N₄/TiO₂ heterojunctions (Fig. 2c), the TiO₂ nanoparticles are found embedded in the mpg-C₃N₄ lamellar structure. The corresponding HRTEM image which is shown in Fig. 2d clearly reveals the close interfacial connections between mpg-C₃N₄ and the TiO₂ nanoparticles. By measuring the lattice fringes in the HRTEM image, the resolved interplanar distance of 0.35 nm agrees well with the lattice spacing of the (101) plane of anatase TiO₂. These observations suggest the formation of a heterojunction between TiO₂ and mpg-C₃N₄ which would be an ideal system to achieve improved electron–hole separation via a smooth charge transfer process.

Fig. 2 Typical TEM images of (a) mpg-C₃N₄, (b) pure TiO₂ and (c) the mpg-C₃N₄/TiO₂ heterojunctions, and an HRTEM image (d) of the as-prepared mpg-C₃N₄/TiO₂ heterojunctions.

In order to investigate the interactions between the TiO₂ and mpg-C₃N₄ in the as-prepared mpg-C₃N₄/TiO₂ heterojunctions, FTIR spectra were obtained and are presented in Fig. 3. The pure mpg-C₃N₄ shows characteristic IR peaks similar to those in previous results.^18,30 The broad peak at 3000–3600 cm⁻¹ is contributed by N–H stretching;³¹ the strong band at 1240–1640 cm⁻¹ is attributed to the typical stretching vibrations of C–N bonds and C@N heterocycles;^32,33 and the band near 808 cm⁻¹ can be attributed to the characteristic bending mode of triazine units.³⁴ It can be seen that a band around 500–700 cm⁻¹ is clearly shown in the FTIR spectrum of TiO₂, which corresponds to the Ti–O–Ti stretching vibration.³⁵ The major characteristic peaks of TiO₂ and mpg-C₃N₄ almost all appear in the mpg-C₃N₄/TiO₂ sample, further indicating the formation of mpg-C₃N₄/TiO₂ heterojunctions.

Fig. 3 FTIR spectra of the as-prepared mpg-C₃N₄, TiO₂ and mpg-C₃N₄/TiO₂ heterojunctions.

XPS measurements were carried out to obtain information on the oxidation state and surface chemical composition of the mpg-C₃N₄/TiO₂ heterojunction sample. The survey spectrum in Fig. 4a shows peaks of Ti, O, C, and N for the mpg-C₃N₄/TiO₂ composite. Fig. 4b shows a high resolution spectrum of the Ti 2p transition. The Ti 2p peak is composed of spin doublets and the Ti 2p peak has two main components as determined by the fitting of the curve. The values of the binding energies (BE) obtained for the Ti 2p_3/2 peaks of 455.6 eV and 457.1 eV verify the presence of Ti–N and an intermediate phase, respectively.³⁶ Accordingly Ti 2p_1/2 peaks were observed at 461.4 eV and 462.8 eV, respectively. The intermediate phase could be attributed to Ti₂O₃ or oxynitrides,³⁷ although their existence is still debated.^38,39 The XPS results together with those from the TEM and FTIR studies clearly reveal the formation of a composite material with chemically bound interfaces between mpg-C₃N₄ and TiO₂.

Fig. 4 (a) XPS survey scan of the mpg-C₃N₄/TiO₂ heterojunctions, and the corresponding Ti 2p high-resolution spectrum (b).

In order to obtain more information on the porous structures, the as-prepared mpg-C₃N₄/TiO₂ heterojunction sample was examined using N₂ sorption analysis. The N₂ adsorption–desorption isotherms of the sample are illustrated in Fig. 5. It can be observed that the mpg-C₃N₄/TiO₂ heterojunction sample has a type IV isotherm and type H3 hysteresis loop according to the IUPAC classifications, which indicates a mesoporous structure of the sample.⁴⁰ This is almost consistent with the TEM observations. Based on the desorption curve, the Brunauer–Emmett–Teller (BET) surface area of the porous mpg-C₃N₄/TiO₂ sample was calculated to be 106.42 m² g⁻¹, the total pore volume was 0.3105 cm³ g⁻¹, and the average pore diameter was 10.4 nm.

Fig. 5 N₂ adsorption–desorption isotherms of the as-prepared mpg-C₃N₄/TiO₂ heterojunctions.

Fig. 6a shows UV-vis diffuse reflectance spectra of the mpg-C₃N₄, TiO₂ and mpg-C₃N₄/TiO₂ heterojunction samples. For comparison, the absorption spectra of all the samples were obtained under identical conditions. As illustrated in Fig. 6a, for pure TiO₂ the absorption occurs at wavelengths shorter than 390 nm, consistent with the intrinsic band gap of anatase TiO₂ (about 3.2 eV), while the absorption intensity of the pure mpg-C₃N₄ rises rapidly at about 450 nm, in good accordance with the band gap of g-C₃N₄ (2.7 eV).⁴¹ Compared with pure TiO₂, the mpg-C₃N₄/TiO₂ heterojunctions show an additional absorption in the region of visible light, attributed to the existence of mpg-C₃N₄. Fig. 6b illustrates the PL emission spectra of the as-prepared samples obtained using an excitation wavelength of 325 nm. PL spectral analysis was applied to investigate the recombination rate of the photogenerated electron–hole pairs. It is generally believed that a lower PL emission intensity is an indication of a lower recombination of photogenerated electron–hole pairs. As can be seen, pure mpg-C₃N₄ has a strong peak around 460 nm in the PL spectrum, which can be attributed to the band–band PL phenomenon, i.e. the emission light energy is approximately equal to the bandgap energy of g-C₃N₄. Once the TiO₂ nanoparticles were anchored on the mpg-C₃N₄ nanosheets, the emission intensity of the PL spectrum decreased significantly, suggesting that the mpg-C₃N₄/TiO₂ heterojunctions have a lower recombination rate of photo-generated electron–hole pairs than mpg-C₃N₄. This result shows a good agreement with those of other heterojunction semiconductors.^22,23

Fig. 6 Optical properties of mpg-C₃N₄, TiO₂ and the mpg-C₃N₄/TiO₂ heterojunctions. (a) UV-vis diffuse reflectance spectra and (b) PL emission spectra obtained using an excitation wavelength of 325 nm.

Photocatalytic activity

The Fig. 7a inset shows the RhB adsorption capacities of the samples, for the adsorption process in the dark. The absorbance values of the RhB decreased by 31.6%, 35.7% and 36.8% due to absorption of RhB molecules by the TiO₂, mpg-C₃N₄ and mpg-C₃N₄/TiO₂ samples, respectively. The photocatalytic activity of the mpg-C₃N₄, TiO₂ and mpg-C₃N₄/TiO₂ heterojunctions was examined using visible-light degradation of RhB molecules in aqueous solution, where C is the concentration of RhB remaining in the solution after the irradiation time t, and C_o is the initial concentration of RhB. The results are shown in Fig. 7a. The blank experiment demonstrates that decomposition of RhB is inappreciable within the test period in the absence of photocatalysts, suggesting that the photolysis of RhB is negligible. As can be seen from Fig. 7a, the pure TiO₂ and mpg-C₃N₄ sample displayed a certain photocatalytic efficiency of 21% and 88% after visible-light irradiation for 100 min, respectively. As expected, the mpg-C₃N₄/TiO₂ heterojunctions exhibited higher photocatalytic activity than the pure mpg-C₃N₄ sample, and provided 96.5% degradation of the RhB under visible light irradiation. Fig. 7b reveals that the photocatalytic degradation of RhB on the different catalysts fits pseudo-first-order kinetics, ln(C_o/C) = kt, where C is the concentration of RhB at time t, C_o is the initial concentration of the RhB solution, and the slope k is the apparent reaction rate constant. The results imply that the k value for the mpg-C₃N₄/TiO₂ sample is higher than those of the pure mpg-C₃N₄ and TiO₂. It can be found that the k value of the mpg-C₃N₄/TiO₂ sample (0.0335 min⁻¹) is about 1.6 times that of mpg-C₃N₄ (0.0214 min⁻¹) and about 14 times that of TiO₂ (0.0024 min⁻¹). These results indicate that the formation of the heterojunction structure of mpg-C₃N₄/TiO₂ could greatly enhance the photocatalytic efficiency of singular mpg-C₃N₄ and TiO₂. For evaluating the recyclability of the mpg-C₃N₄/TiO₂ sample, additional experiments were carried out to degrade RhB under visible light four times in a cycle (Fig. 8). The photocatalytic activity of the mpg-C₃N₄/TiO₂ heterojunction sample for RhB degradation decreased from 96.5% to 88.5% after four cycling runs, which demonstrates that the mpg-C₃N₄/TiO₂ heterojunction sample has a high stability in the photocatalytic process under visible light irradiation.

Fig. 7 Photocatalytic activities (a), adsorption capacities (inset), and kinetics (b) of the mpg-C₃N₄, TiO₂ and mpg-C₃N₄/TiO₂ heterojunctions for degradation of RhB under visible light irradiation.

Fig. 8 Four cycles of photocatalytic degradation of RhB by the mpg-C₃N₄/TiO₂ heterojunctions under visible light irradiation.

Proposed mechanism for the enhanced photocatalytic activity

On the basis of the above results and discussion, a photocatalytic mechanism for the reaction under visible light irradiation was tentatively proposed and is schematically illustrated in Fig. 9. When the as-prepared mpg-C₃N₄/TiO₂ heterojunctions are irradiated with visible light, the electrons get promoted from the valence band (VB) to the conduction band (CB) in mpg-C₃N₄, providing a route for the electronic transition. According to previous studies, the CB and VB edge potentials of mpg-C₃N₄ were at −1.12 and 1.57 eV, respectively.²¹ The CB and VB edge potentials of TiO₂ were at −0.29 and 2.91 eV, respectively.⁴² The CB edge potential of mpg-C₃N₄ is more negative than that of TiO₂, allowing the excited electron on the surface of mpg-C₃N₄ to transfer easily to TiO₂ via the well-built heterojunction. The recombination of the photogenerated charge is inhibited and the photocatalytic activities are enhanced effectively for TiO₂ which provides a site for electron translation. Then, the electrons stored in the CB of TiO₂ are trapped by the air near the surface of the TiO₂ to form reactive ˙O₂⁻. Besides, the E_VB (g-C₃N₄, +1.57 eV vs. SHE) values were lower than the standard redox potential of ˙OH/H₂O (+2.68 eV vs. SHE), indicating that the photogenerated holes on mpg-C₃N₄ could not oxidize H₂O to the active species ˙OH.^43,44 Therefore, h⁺ on the VB of mpg-C₃N₄ would react with RhB directly. On the other hand, as the electrons in the CB of TiO₂ (−0.29 eV) are located at more negative potential positions than the O₂/˙O₂⁻ couple (−0.046 eV), electrons stored in the CB of TiO₂ are trapped by O₂ dissolved in the solution near the surface of the TiO₂ to generate reactive superoxide radical ions (˙O₂⁻), while the holes in the VB of mpg-C₃N₄ directly oxidize the pollutants.⁴⁵ Furthermore, the previously mentioned characterization details indicate that coupling TiO₂ with mpg-C₃N₄ produces a well-contacted solid–solid heterojunction interface between the mpg-C₃N₄ and TiO₂ semiconductor particles, which promotes effective interfacial electron transfer and separation of the photogenerated charge carriers, significantly enhancing photocatalytic activity.⁴⁶

Fig. 9 Proposed photocatalytic mechanism for the degradation of RhB over the mpg-C₃N₄/TiO₂ heterojunctions under visible light irradiation.

Conclusions

In summary, we have successfully fabricated mpg-C₃N₄/TiO₂ heterojunctions using SiO₂ nanoparticles as a hard template and cyanamide as a precursor via a facile hard template approach. The obtained mpg-C₃N₄/TiO₂ heterojunctions exhibited enhanced photocatalytic activity toward RhB degradation under visible light irradiation compared to that of pure mpg-C₃N₄ and TiO₂ nanoparticles. This enhancement of the photocatalytic activity has been demonstrated to be due to the high separation and easy transfer of photogenerated electron–hole pairs at the heterojunction interfaces derived from the match in energy levels between the TiO₂ and mpg-C₃N₄. Thus, the mpg-C₃N₄/TiO₂ heterojunctions can be a promising candidate for efficient visible light driven photocatalytic systems for organic pollution residue removal.

Acknowledgements

The authors are grateful for the financial support of the National Natural Science Foundation of China (grant no. 21376051, 21306023, 21106017), the Natural Science Foundation of Jiangsu (grant no. BK20131288), the 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, T15192014, KYLX_0161).

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Footnote

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

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