Facile synthesis of sheet-like N–TiO₂/g-C₃N₄ heterojunctions with highly enhanced and stable visible-light photocatalytic activities

Abstract

Sheet-like N–TiO₂/g-C₃N₄ heterojunctions with well-controlled structures as high-efficiency visible-light photocatalysts were synthesized by direct co-calcination of preformed N–TiO₂ nanoparticles and g-C₃N₄ nanosheets. The composition, structure, morphology, and optical absorption properties of the as-prepared g-C₃N₄ nanosheets and N–TiO₂/g-C₃N₄ heterojunctions were thoroughly investigated with XRD, FT-IR, SEM, TEM, XPS, and UV-vis DRS, respectively. The visible-light photocatalytic activity of the heterojunctions with different N–TiO₂/g-C₃N₄ mass ratios was evaluated by photodegradation of rhodamine B dye and the highest enhancement was attained at a mass ratio of 1 : 3, which was 19 and 5.3 times as high as that of the individual N–TiO₂ nanoparticles and g-C₃N₄ nanosheets, respectively. The photocatalytic activity of the composites originating from nanosheets is 11 times as high as that of the bulk g-C₃N₄/N–TiO₂ heterojunctions. Moreover, the sheet-like N–TiO₂/g-C₃N₄ photocatalyst exhibits excellent stability against repeated use and could still retain over 98.8% of the initial activity after seven cycles. Such a good visible-light photocatalytic performance is mainly due to the effective structure and chemical composition provided by the present method, which endow the as-prepared heterojunctions with exceptionally high specific surface area (>80 m² g⁻¹) as well as efficient charge transfer at the heterojunction interfaces. The charge separation mechanism during the photodegradation process was also studied and a possible photocatalytic mechanism was proposed.

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1. Introduction

Since the discovery of photoinduced water-splitting on TiO₂ electrodes by Fujishima and Honda,¹ semiconductor-based photocatalysis has received considerable interest for enabling pollutant decomposition,^2,3 and renewable fuel synthesis.^4,5 TiO₂-based materials have been considered as the most promising candidate in terms of catalytic activity and chemical stability.⁶ Unfortunately, the wide band gap of TiO₂ (∼3.2 eV for anatase phase) restricts its efficiency in photoelectric conversion under solar light.⁵ Therefore, a lot of efforts have been devoted in recent years to the development of photocatalysts with desirable high reactivity in visible light.^7–10 However, the major challenge is to develop a highly efficient, low-cost, and robust photocatalyst that can satisfy industrial requirements.

Graphitic carbon nitride (g-C₃N₄) has drawn much attention due to its absorption in visible light, corrosion resistance to acid and alkali and easy adjustment of the structure and performance.¹¹ The admirable properties of g-C₃N₄ make it widely used in the field of decomposition of contaminants,¹² water-splitting,¹³ and photosynthesis.¹⁴ Still, the photocatalytic activity of g-C₃N₄ is relatively low owing to the high recombination rate of photo-generated electron–hole pairs. Improvements of g-C₃N₄ photocatalytic activity have been made, for example, by chemical doping with nonmetal or metal elements,^15,16 designing mesoporous structures,¹⁷ building heterojunctions,¹⁸ and so on.^19,20 To date, the formation of heterojunctions by coupling g-C₃N₄ to another semiconductor has proven to be an effective way to the enhancement of photocatalytic activity.²¹ It can lead to enhanced quantum yields by decreasing the recombination rate of the photogenerated electron–hole pairs and inducing a synergistic effect including efficient charge separation and upgrading of photostability, thereby compensating the disadvantages of the individual components.²² The intense interest in two-dimensional g-C₃N₄ is driven by large specific area and abundant active sites.²³ The excellent value of g-C₃N₄ nanosheets is significantly favorable for application as a photocatalyst.

It is remarkable that many heterojunctions are formed by combining ultraviolet-light photocatalysts with g-C₃N₄, such as TiO₂,²⁴ ZnO,²⁵ and BiPO₄.²⁶ Only the g-C₃N₄ could be excited in visible light. To get most utilization of the light source, some visible-light photocatalysts were coupled with g-C₃N₄ to form composites.^27–29 N–TiO₂, which exhibits visible-light photocatalytic activity owing to the narrow band-gap induced by the hybrid of the N2p state with O2p states,⁸ is one of the most interesting semiconductor for coupling g-C₃N₄ with advantages such as low cost, high stability and activity and most importantly it possesses a band structure that matches well with the energy levels of g-C₃N₄. Therefore, the formation of N–TiO₂/g-C₃N₄ heterojunctions are supposed to be quite attractive as visible-light photocatalytic materials. However, until now, methods for the preparation of N–TiO₂/g-C₃N₄ heterojunctions are still very limited in the literature. Generally, N–TiO₂/g-C₃N₄ heterojunctions were reported by heating the mixture of the hydrolysis product of TiCl₄ and C₃N₄,³⁰ or an in situ microwave-assisted approach.³¹ Yet, the aforementioned N–TiO₂/g-C₃N₄ composite photocatalysts only exhibited ordinary improvement of photocatalytic activity and poor stability. The main reason is due to the lack of structural control over the individual components as well as the overall composites. Therefore, the development of powerful and less complicated preparation methods, in-depth illumination of morphology/structure related electrons/holes transfer mechanism and further improvement of the visible-light driven photocatalytic performance on both activity and stability are substantially demanded.

Herein, we report on the facile synthesis of sheet-like N–TiO₂/g-C₃N₄ heterojuctions with highly enhanced and stable visible-light photocatalytic activities. The heterojunctions with a well-defined sheet-like morphology were prepared by direct co-calcination of pre-synthesized N–TiO₂ nanoparticles and g-C₃N₄ nanosheets, which ensures an effective structure/morphology control over individual components as well as an exceptionally high specific surface area. The visible-light photocatalytic activity and the stability/reusability of the N–TiO₂/g-C₃N₄ heterojunctions were evaluated by photodegradation of RhB dye. The possible photocatalytic degradation mechanism of the N–TiO₂/g-C₃N₄ heterojunctions was also investigated.

2. Experimental section

2.1 Chemicals

Titanium tetrachloride (TiCl₄, 99.9%), ammonium thiocyanate (NH₄SCN, 98.5%), ammonium hydroxide (25–28% NH₃·H₂O in water) and rhodamine B (RhB) were all purchased from Sinopharm Chemical Co. Ltd (China) and used without further purification.

2.2 Synthesis

Synthesis of N–TiO₂ nanoparticles. Excessive NH₃·H₂O was added to an aqueous solution of TiCl₄ (3.5 M, diluted with water in an ice-water bath) and a white precipitate immediately formed, which was collected by filtration, dried under 70 °C and subsequently calcined at 400 °C for 2 h in a muffle furnace, with a heating rate of 5 °C min⁻¹.

g-C₃N₄ nanosheets and pristine g-C₃N₄. g-C₃N₄ nanosheets were synthesized by thermal polycondensation of NH₄SCN. Typically, 6 g of NH₄SCN was put into a crucible with a half-open cover and calcined at 550 °C for 4 h in a muffle furnace, with a heating rate of 10 °C min⁻¹. The resulting light yellow product was collected and ground into powders for further use. The prinstine g-C₃N₄ was synthesized in a well-closed crucible and heated at the same calcining condition. The g-C₃N₄ nanosheets and pristine g-C₃N₄ were labeled as g-C₃N₄ and p-g-C₃N₄.

N–TiO₂/g-C₃N₄ and N–TiO₂/p-g-C₃N₄ heterojunctions. The as-prepared g-C₃N₄ nanosheets were mixed with various mass ratios and ground to form a uniform mixture before co-calcination in a muffle furnace at 400 °C for 2 h, with a heating rate of 1 °C min⁻¹. Then, the resultant composites were collected and grounded for further characterization. The N–TiO₂/p-g-C₃N₄ heterojunctions were prepared by the same method.

2.3 Characterizations

Transmission electron microscopy (TEM), high resolution TEM (HR-TEM) and scanning electron microscopy (SEM) images were taken from a JEOL JEM-1011, a JEOL JEM-2010 and a Hitachi S-4800 electron microscope, respectively. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max 2200PC diffractometer with a graphite-monochromatized Cu Kα radiation source (λ = 0.15418 nm). Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet 5DX FT-IR spectrophotometer using KBr pellets. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI-5300 ESCA spectrometer with a 35.75 eV pass energy and an AlKα line excitation source. The core levels were calibrated according to the C1s binding energy (BE) of 284.6 eV. Thermal gravimetric analysis (TGA) was carried out with a Mettler Toledo TGA/SDTA 851E analyzer at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C under air atmosphere. Nitrogen adsorption–desorption isotherms at 77 K, Brunauer–Emmett–Teller (BET) surface areas and Barrett–Joyner–Halenda (BJH) pore size distributions were measured and calculated with a Quantachrome QuadraSorb SI analyzer and the samples were vacuum-degassed at 100 °C for 12 h before measurements. Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) were recorded on an Agilent Cary 100 UV-vis spectrophotometer coupled to an integrating sphere with BaSO₄ as reference. Photoluminescence (PL) spectra were recorded on an Edinburgh FI/FSTCSPC 920 spectrophotometer under an excitation wavelength of 400 nm. The element analysis was performed on the Elemental Analyzer (Vario EL III, Elementar Analysensysteme) under the high temperature combustion (ca. 1100 °C furnace temperature) in an oxygen-rich environment for determining the CHNS contents of the samples. The O contents of graphitic carbon nitride were obtained by deducting the CHNS contents from the total weight.

2.4 Photocatalytic activity testing

The photocatalytic activities of the as-prepared N–TiO₂/g-C₃N₄ composites were evaluated by photodegradation of RhB dye. An 800 W xenon lamp with a UV cut-off filter (emission range >420 nm, maximum emission at ∼470 nm) is loaded in the empty chamber of an annular quartz tube, with a circulating water jacket surrounding it to keep the temperature at 25 °C. 0.050 g of the catalyst was dispersed in a 50 mL 20 ppm RhB aqueous solution by ultrasonication for 20 min. In order to attain adsorption equilibrium, the suspension was first stirred continuously in dark for 30 min before irradiated under the xenon lamp. 6.0 mL of the suspension was taken out from the reaction flask every 5 min and centrifuged to remove the catalyst. The concentration of the remaining RhB was determined by its maximum absorbance at 554 nm (λ_max) measured with a Perkin-Elmer Lambda-35 UV-vis spectrometer. Blank (without catalysts) and contrast experiments were performed following identical procedures.

3. Results and discussion

3.1 Structure and composition of g-C₃N₄ sheets and bulk p-g-C₃N₄

The morphology and microstructures of the g-C₃N₄ nanosheets and the p-g-C₃N₄ were thoroughly studied using SEM and TEM. A typically aggregated morphology with a large size and wrinkles is found in Fig. 1a and c. The surface of the sample is very rough. The essential stacked lamellar structure of g-C₃N₄ can be obtained from the SEM image. As shown in Fig. 1b and d, the g-C₃N₄ nanosheets appear as loose nanosheets with a thickness of 8–15 nm rather than condensed solid agglomerates. The sheets tend to bend and form ragged edges as a result of minimizing their surface energy.³² Fig. 1b shows the typical TEM image of an isolated piece of g-C₃N₄ nanosheets, which displays a two-dimensional sheet-like structure with a lateral scale of several micrometers. The FT-IR spectra (Fig. S1a†) of g-C₃N₄ and p-g-C₃N₄ are almost the same. These bands between 1700 cm⁻¹ and 1250 cm⁻¹ are ascribed to the C=N and C–N stretching vibration.³³ The peak at 810 cm⁻¹ is associated with breathing mode of s-triazine.³⁴ The IR spectra suggest the conjugated triazine ring forms in the compounds after being calcined.

Fig. 1 TEM images of p-g-C₃N₄ (a) and g-C₃N₄ (b) and typical FE-SEM images of p-g-C₃N₄ (c) and g-C₃N₄ (d).

In order to confirm the surface composition and chemical state of the p-g-C₃N₄ and g-C₃N₄ nanosheets, the samples were analyzed by XPS technique. The XPS spectra of C1s, N1s and O1s are shown in Fig. 2a–c. For the C1s spectrum (Fig. 2a), the peak at 284.6 eV is typically attributed to graphitic carbon or external carbon contamination³⁵ and the other peak at 287.7 eV can be assigned to the C sp² atoms (N–C=N) in triazine rings.³⁶ The N1s features show a broad peak extending from 395 to 403 eV as shown in Fig. 2b. The main N1s peak at 398.3 eV in g-C₃N₄ can be assigned to sp² N (C=N–C) in triazine rings and the peak at 400.2 eV can be attributed to the bridging tertiary nitrogen N–(C)₃.^35,36 The C1s and N1s spectra of XPS are coincided with the two samples, indicating the formation of indistinctive triazine ring structure. The p-g-C₃N₄ shows O1s core level at 532.9 eV (Fig. 2c), ascribed to adsorbed water.³¹ No additional signal related to the C–O or N–O bond is observed. For the g-C₃N₄ nanosheets, the other two core levels at 531.6 eV and 534.0 eV are obviously detected on the doped sample. These signals could be attributed to the formation of N–C–O species and the adsorbed O₂, respectively.³⁷ It implies that the oxygen could be directly bonding with sp²-hybridized carbon atom in the g-C₃N₄ matrix. Fig. S1b† shows the XRD patterns of the two samples. The main peak of the p-g-C₃N₄ was detected at around 27.3°, while the peaks of g-C₃N₄ shifts to 27.5° corresponding to a decrease in the interplanar stacking distance from 0.326 to 0.324 nm. The change is probably due to the introduction of oxygen into the graphitic structure.³⁷

3NH₄SCN → 2NH₃ + C₃N₄ + 3H₂S (1)

Fig. 2 XPS spectra (a–c) of g-C₃N₄ and p-g-C₃N₄.

As previous reports, the g-C₃N₄ can be obtained by the self-polymerization of NH₄SCN according to the eqn (1). The formation processes are shown in Scheme S1a.†³⁸ The present oxygen in the experiment can probably participate in the reaction and be doped into the g-C₃N₄ matrix (Scheme S1b†). According to the former report, the nanosheets of g-C₃N₄ could be obtained in the process of the thermal oxidation etching.³² In the experimental condition, sufficient oxygen contents can also thermally oxidate the matrix to form sheet-like morphology. As a result, the BET surface area of the g-C₃N₄ nanosheets increases up to 42.4 m² g⁻¹, which is approximately 2 times as high as that of the bulk p-g-C₃N₄ (22.0 m² g⁻¹) (Fig. S2†). The hierarchical structures with mesopores are expected to provide more active sites.

Fig. 3 UV-visible diffuse reflectance spectra (a) and fluorescence emission spectra (b) of g-C₃N₄ and p-g-C₃N₄. The inset is the plots of (αhν)² versus photon energy (hν).

As above discussion, the crystalline structure of g-C₃N₄ nanosheets basically corresponds with bulk p-g-C₃N₄. The electronic band structures of the nanosheets were studied by diffuse reflectance spectra (DRS) and fluorescence emission spectra. The UV-visible DRS in Fig. 3a show a little blue shift of the intrinsic absorption edge compared with bluk-p-g-C₃N₄. The derived band-gaps from the plots of the (αhν)ⁿ versus the photo energy of the light adsorbed (according to the Kubelka–Munk equation, n = 2 for the direct band-gap¹⁹) become larger due to the well-known quantum confinement.³² The result is further confirmed by the blue shift of the fluorescence emission spectra in Fig. 3b. Also, there is a significant decrease in the PL intensity of the g-C₃N₄ nanosheets, compared to p-g-C₃N₄. It suggests that the recombination of electron–hole pairs might be effectively inhibited on the g-C₃N₄ nanosheets.

Fig. 4 Powder XRD patterns (a) and FT-IR spectra (b) of g-C₃N₄, N–TiO₂ and N–TiO₂/g-C₃N₄ composites with different mass ratios.

To evaluate the thermostability of the g-C₃N₄ nanosheets, the TG-DSC experiment was performed at a heating rate of 10 °C min⁻¹ (Fig. S3†). The g-C₃N₄ nanosheets become unstable when the heating temperature is above 500 °C and there is an endothermal peak centered at 688 °C, which can be attributed to the decomposition of g-C₃N₄.³⁹ From room temperature to 500 °C, there is no clear change for the g-C₃N₄ nanosheets. Thus, the g-C₃N₄ is relatively stable when the calcined temperature is below 500 °C. As prepared, the N content of N–TiO₂, the O content of g-C₃N₄ nanosheets and the O content of pristine g-C₃N₄ agglomeration are 0.469%, 6.96% and 2.08%, respectively. After calcinations at 400 °C, the corresponding contents are changed to 0.435%, 7.45% and 2.22%. The changes of the N and O contents in the samples are very small. The high thermostability of nanosheets makes the formation of heterojunctions with N–TiO₂ possible by direct co-calcination.

3.2 Morphology and microstructures of the heterojunctions of N–TiO₂/g-C₃N₄

The structure and composition of the as-prepared samples was characterized by XRD and FT-IR. Powder XRD patterns of N–TiO₂/g-C₃N₄ composites with different mass ratios together with the diffraction patterns of pristine N–TiO₂ and g-C₃N₄ were shown in Fig. 4a. For the pure N–TiO₂ sample, the diffraction peaks are indexed to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of the anatase phase (JCPDS no. 21-1272). The patterns of the N–TiO₂/g-C₃N₄ composites show no other diffraction peaks except that of N–TiO₂ and g-C₃N₄, indicating the coexistence of N–TiO₂ and g-C₃N₄, and no impurities are formed during the fabrication of the composites. Furthermore, with a decreasing of the N–TiO₂/g-C₃N₄ mass ratio from 1 : 1 to 1 : 5 the intensity of the typical diffraction peaks of the g-C₃N₄ gradually increases, indicating an increase of the g-C₃N₄ content in the as-formed composite. Fig. 4b shows the FT-IR spectra of pure N–TiO₂, pure g-C₃N₄ and N–TiO₂/g-C₃N₄ composites with mass ratios of 1 : 1, 1 : 2, 1 : 3 and 1 : 5. For the pure N–TiO₂, the most obvious absorption at 454 cm⁻¹ corresponds to the Ti–O–Ti vibration.⁴⁰ Besides, the observed broad peak around 3500 cm⁻¹ and the peak at 1661 cm⁻¹ belong to the O–H stretching vibrations. For the heterojunctions, the characteristic bands between 1200 and 1640 cm⁻¹ and the peak at 810 cm⁻¹ are attributed to the typical stretching vibration of C–N heterocycles, and the breathing mode of triazine unites. The FT-IR spectra of the N–TiO₂/g-C₃N₄ composites represent the overlap of the spectra of both N–TiO₂ and g-C₃N₄, indicating composite formation does not change the structure and chemical skeleton of both components.

To investigate the surface composition and chemical state as well as the interaction between N–TiO₂ and g-C₃N₄, the as-prepared composite was also analyzed using XPS technique. The XPS survey spectra (Fig. S4†) of the as-prepared N–TiO₂/g-C₃N₄ (1 : 3) composite displays no other elements than Ti, C, N, and O. The high resolution XPS spectra of Ti2p, C1s, N1s and O1s for the N–TiO₂/g-C₃N₄ (1 : 3) composite are displayed in Fig. 5. The Ti2p_3/2 and Ti2p_1/2 core level appear at 458.7 and 464.0 eV, respectively, which is very close to the binding energy value of N–TiO₂. The inset shows the N1s spectrum for N–TiO₂. The weak peak at 399.0 eV can be attributed to the N atoms located at the interstitial sites of the TiO₂ lattice, such as Ti–N–O or Ti–O–N,⁴¹ and its low intensity indicates the trace amount of nitrogen in N–TiO₂. The C1s and N1s spectra of the composite are similar with the g-C₃N₄ nanosheet. It implies the structure of g-C₃N₄ was retained during the formation of heterojunctions. In the O1s spectrum, the peak centered at 529.4 eV is associated with O²⁻ in N–TiO₂,⁴² and those at 531.7 and 533.1 eV is ascribed to N–C–O species or the absorbed H₂O on the surface of photocatalysts.^37,38 The higher binding energy is attributed to the heterojunctions of N–TiO₂ and g-C₃N₄ nanosheets.

Fig. 5 High resolution XPS spectra of Ti2p (a), C1s (b), N1s (c) and O1s (d) for the N–TiO₂/g-C₃N₄ (1 : 3) composite. The inset shows the N1s spectra of N–TiO₂.

The morphology of the pure N–TiO₂ and the N–TiO₂/g-C₃N₄ composite with the mass ratio of 1 : 3 were studied using SEM, TEM, HR-TEM and SAED. As shown in Fig. 6a and b, the as-prepared N–TiO₂ is well-defined particles in good dispersity with sizes ranging from 10 to 30 nm. The HR-TEM image (Fig. 6c) reveals the lattice fringes of an individual nanoparticle and the lattice spacing are measured to be 0.239 and 0.356 nm, corresponding to the (004) and (101) planes of anatase TiO₂, respectively. The selected area electron diffraction (SAED) pattern in the inset of Fig. 6c are made up of distinct diffraction rings, indicating the high crystallinity of the nanoparticles. By mixing and co-calcination of the as-obtained N–TiO₂ nanoparticles and g-C₃N₄ nanosheets the N–TiO₂/g-C₃N₄ composites are formed. As can be seen from Fig. 6d, the g-C₃N₄ still maintains the sheet-like structure although the surface became a little rougher. The TEM image of an isolating composite sheet in Fig. 6e shows clearly darker N–TiO₂ nanoparticles scattered on the g-C₃N₄ nanosheet, as denoted by the arrows. The interface between the two components was further revealed by HR-TEM image in Fig. 6f, which shows the N–TiO₂ nanoparticle with a lattice spacing of 0.232 nm that corresponds to the (112) plane of the anatase phase were tightly contacted to the surface of the g-C₃N₄ nanosheet. Besides, the size of N–TiO₂ nanoparticles in the composite stays almost the same as that of the pristine ones. All of the above results imply that intimate heterojunctions between N–TiO₂ and g-C₃N₄ were indeed formed via co-calcination of the pre-synthesized nanoparticles and nanosheets. Moreover, the sheet-like structure of the N–TiO₂/g-C₃N₄ heterojunctions is also beneficial for charge transfer compared with condensed agglomerates.

Fig. 6 SEM (a and d), TEM (b and e) and HR-TEM (c and f) images of N–TiO₂ (a–c), N–TiO₂/g-C₃N₄ (1 : 3) composite (d–f). Insets show corresponding SAED patterns.

3.3 Optical properties and specific surface area

The optical properties and the band gap energies of the as-prepared samples were investigated by UV-vis DRS measurement. Fig. S5a† shows the UV-vis DRS of N–TiO₂ nanoparticles, g-C₃N₄ nanosheets and the N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions, respectively. The absorption edge of the N–TiO₂ nanoparticles is above the wavelength of 420 nm. Compared with anatase TiO₂, such a red shift toward the visible light range is attributed to the nitrogen doping effect. The g-C₃N₄ nanosheets display absorption around 460 nm. For the N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions, the absorption edge is very close to the g-C₃N₄ nanosheets. Accordingly, the plots of (αhν)² versus photon energy (hν) are shown in Fig. S5b.† The corresponding band gap energies are estimated to be 3.10, 2.75 and 2.73 eV for N–TiO₂, g-C₃N₄ and the N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions, respectively. The resulting narrow band gap of the as-prepared N–TiO₂/g-C₃N₄ heterojunctions can lead to enhanced visible-light absorption, and consequently higher photocatalytic activities under visible-light irradiation.

The porosity and the specific surface area of the samples, which are important for photocatalytic performances, were carefully examined by nitrogen adsorption–desorption measurements (Fig. S6†). As a comparison, g-C₃N₄ nanosheets after being calcined at 400 °C and a mechanically mixed N–TiO₂/g-C₃N₄ (1 : 3) composite (without calcination) were also characterized. All the samples display typical type IV nitrogen isotherms, indicating the existence of mesoporous structures. The N–TiO₂ nanoparticles show the H1 type hysteresis loops with a large pore volume, a wide pore size distribution and a most probable pore size of 11 nm, which are evaluated with the Barrett–Joyner–Halenda (BJH) method. These pores are derived from the irregular aggregation of the nanoparticles. All other samples present H4 type hysteresis loops. The g-C₃N₄ nanosheets, both before and after being calcined at 400 °C, display negligible total pore volumes. The as-prepared N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions present a wide pore size distribution and a very small total pore volume, which is a little bit larger than the mechanically mixed N–TiO₂/g-C₃N₄ (1 : 3) composite. The corresponding Barrett–Emmett–Teller specific surface area (S_BET) for all the samples are listed in Table 1. As expected, the N–TiO₂ nanoparticles show a relatively high S_BET of 69.68 m² g⁻¹ as a result of the small size of the particles. The S_BET of the g-C₃N₄ nanosheets increases significantly from 42.40 to 98.72 m² g⁻¹ after being calcined at 400 °C, which can be attributed to the thermal oxide etching of g-C₃N₄.²⁷ The mechanically mixed N–TiO₂/g-C₃N₄ (1 : 3) composite presents a S_BET of 52.45 m² g⁻¹, whereas the N–TiO₂/g-C₃N₄ heterojunctions display much higher S_BET values, which are 74.29, 84.69, 81.83 and 82.22 m² g⁻¹ for the 1 : 1, 1 : 2, 1 : 3 and 1 : 5 mass ratio samples, respectively. These results indicate that the present co-calcination synthesis endows the N–TiO₂/g-C₃N₄ heterojunctions with a large porosity as well as an exceptionally high specific surface area, which is superior to the reported N–TiO₂/g-C₃N₄ heterojunctions, and would provide more reaction sites that are favorable for photocatalytic applications.

Table 1 Specific surface area S_BET and reaction rate constant k of various samples for photodegradation of RhB

Sample	Weight ratio	SBET (m^2 g^−1)	k (min^−1)
N–TiO2	—	69.68	0.00881
g-C3N4	—	42.40	0.0328
g-C3N4 (400 °C calcined)	—	98.72	0.0366
N–TiO2/g-C3N4 (mechanically mixed)	1 : 3	52.45	0.0283
N–TiO2/g-C3N4 heterojunctions	1 : 1	74.29	0.0513
	1 : 2	84.69	0.128
	1 : 3	81.83	0.173
	1 : 5	82.22	0.102

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3.4 Photocatalytic activity

The photocatalytic activities of the as-prepared N–TiO₂/g-C₃N₄ heterojunctions with different mass ratios were evaluated by visible-light induced photocatalytic degradation of RhB. For comparison, the photocatalytic activities of the pure N–TiO₂, the pure g-C₃N₄, the pure g-C₃N₄ after being calcined at 400 °C and the mechanically mixed N–TiO₂/g-C₃N₄ (1 : 3) composite were also tested under the same conditions. The characteristic absorption peak of RhB at 554 nm was employed to determine the degradation degree. Fig. 7 demonstrates the evolution of C/C₀ (wherein C₀ is the initial concentration of RhB and C represents the concentration at reaction time t) with the visible-light irradiation time in the presence of different photocatalysts. The blank experiment indicates that the direct photolysis of RhB is almost negligible in the absence of photocatalyst, which excludes the possibility of photolysis in the present system and therefore the degradation of RhB is indeed resulted from photocatalytic reactions. The pure N–TiO₂ nanoparticles show a very poor and the lowest photocatalytic activity for degradation of RhB among all these samples. The pure g-C₃N₄, both before and after being calcined at 400 °C, and the mechanically mixed N–TiO₂/g-C₃N₄ composite display a very close degradation efficiency toward RhB, among which the mechanically mixed N–TiO₂/g-C₃N₄ composite shows the lowest activity. Compared to the above control samples, the as-synthesized heterojunctions with different mass radio of 1 : 1, 1 : 2, 1 : 3 and 1 : 5 all exhibit much higher photocatalytic activities. Specifically, the photocatalytic degradation efficiency of the heterojunctions increases as the g-C₃N₄ portion increase from 1 : 1 to 1 : 3, yet decreases at the weight radio of 1 : 5. These results strongly suggest that the intimate junctions between N–TiO₂ and g-C₃N₄ indeed bring a synergistic effect that is crucial to the enhancement of photocatalytic activities, and that a suitable mass ratio and effective formation of heterojunctions in the composites play an important role for the improvement of photocatalytic performance. In this case, the N–TiO₂/g-C₃N₄ heterojunctions show the optimum photocatalytic property at the mass radio of 1 : 3, which is probably due to the formation of the most proper amount of heterojunctions in the composite. The photocatalytic degradation process of RhB in the presence of the g-C₃N₄/N–TiO₂ (1 : 3) heterojunctions is presented in Fig. S7.† As can be seen, the adsorption equilibrium of RhB onto the photocatalyst could be reached within the first 30 min and about 40% of the total amount of RhB was adsorbed. Once the visible light was turned on, the intensity of the absorption peak of RhB at 554 nm decreased rapidly and almost disappeared after 30 min. The gradual blue shift of the maximum absorption indicates the formation of intermediate products, which were also completely degraded at the end of the degradation process.

Fig. 7 Visible-light photocatalytic degradation efficiency of RhB in the presence of N–TiO₂, g-C₃N₄, g-C₃N₄ after being calcined at 400 °C, a mechanically mixed N–TiO₂/g-C₃N₄ (1 : 3) composite and N–TiO₂/g-C₃N₄ heterojunctions with different mass ratios.

To compare the photocatalytic efficiency of these samples quantitatively, the reaction kinetics of the RhB degradation were fitted by a first-order kinetic model as expressed by eqn (2):

ln(C₀/C) = kt, (2)

where k is the rate constant (min⁻¹). Fitting plots of the first-order kinetic model for the photocatalysis kinetic data were provided in Fig. S8.† The k values of all the above samples for RhB degradation were calculated and drawn in Fig. 8. As has been discussed, the N–TiO₂/g-C₃N₄ heterojunctions with a mass ratio of 1 : 3 show the highest rate constant (0.173 min⁻¹), which is 19 and 5.3 times as high as that of the individual N–TiO₂ nanoparticles (0.00881 min⁻¹) and g-C₃N₄ nanosheets (0.0328 min⁻¹), respectively. Such a significant enhancement of the photocatalytic efficiency of g-C₃N₄ nanosheets is 2.0 times higher than pristine bulk g-C₃N₄ (0.0111 min⁻¹). At the same time, the heterojunctions of p-g-C₃N₄ with N–TiO₂ are also prepared using the same method. The optimal photocatalytic activity of heterojunctions (1 : 3) is improved by 0.44 times compared with p-g-C₃N₄. The rate constant of sheet-like N–TiO₂/g-C₃N₄ heterojunctions is about 11 times as high as that of the composites of p-g-C₃N₄ and N–TiO₂ (0.0159 min⁻¹). Therefore, the as-prepared sheet-like N–TiO₂/g-C₃N₄ heterojunctions can be regarded as a very competitive visible-light photocatalyst.

Fig. 8 Rate constants (k) of RhB degradation over N–TiO₂, g-C₃N₄ nanosheets and their heterojunctions (a) and N–TiO₂, p-g-C₃N₄ and their heterojunctions (b).

3.5 Stability of the N–TiO₂/g-C₃N₄ photocatalysts

The stability and recycle/reusability of the photocatalysts were evaluated over multiple cycles of RhB degradation and photocatalyst regeneration with the N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions as the representative sample. In every cycle experiment, the used photocatalyst was regenerated by centrifuging and drying at 60 °C for 12 h. As shown in Fig. 9, the as-prepared N–TiO₂/g-C₃N₄ heterojunctions have demonstrated excellent photocatalytic stability, maintaining nearly the same high level of reactivity after seven cycles. The degraded proportion of RhB over the photocatalyst was still up to 98.8% at the end of the 7th cycle. In addition, the composition and microstructure of the recycled composite were characterized by XRD, FT-IR and SEM. The XRD pattern and FT-IR spectra of the photocatalyst after seven cycles are almost the same as those of the freshly prepared sample (Fig. S9†). Moreover, the regenerated photocatalyst still appears the light yellow color without any visible change. The SEM image (Fig. S10†) shows that the sheet-like morphology of the photocatalyst remained. These results verify that the stability and recycle/reusability of the as-prepared N–TiO₂/g-C₃N₄ heterojunctions are good enough for applications in practical photocatalytic degradations.

Fig. 9 Cycling runs (a) and degraded amount (b) for the photocatalytic degradation of RhB over N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions under visible-light irradiation.

3.6 Possible photocatalytic mechanisms of N–TiO₂/g-C₃N₄ heterojunctions

To investigate the fate of electron–hole pairs within the as-prepared heterojunctions, photoluminescence (PL) emission has been carried out. For comparison, the PL of the pure g-C₃N₄ nanosheets was also measured. As shown in Fig. 10, the main emission peak for pure g-C₃N₄ is centered at 450 nm, which is approximately equal to the band gap energy of pure g-C₃N₄, whereas the as-prepared N–TiO₂/g-C₃N₄ heterojunctions with different mass ratios all show significant quenching of the PL peaks. As the intensity of the PL peaks is closely related to the recombination of the electron–hole pairs, this result further indicates an efficient charge transfer within the N–TiO₂/g-C₃N₄ heterojunctions and thus an improvement in the separation efficiency of the light-stimulated carriers. That is to say, the photogenerated electron–hole pairs can efficiently transfer at the interface of N–TiO₂/g-C₃N₄ heterojunctions, resulting in higher photocatalytic activity under visible light irradiation.

In order to elucidate the photocatalytic oxidation process of RhB in the presence of N–TiO₂/g-C₃N₄, radical and hole trapping experiments are designed by introducing different scavengers to the degradation process. Fig. 11 displays the effect of different scavengers on the RhB degradation in the presence of the N–TiO₂/g-C₃N₄ (1 : 3) heterojunctions under visible light irradiation for 20 min. The less percentage of RhB is converted, the more important role the active species play in the reaction. Superoxide radical (˙O₂⁻), hole (h⁺), electron (e⁻) and hydroxyl radical (˙OH) are explored in the reaction. The scavengers used in the experiments are benzoquinone (BQ, 2 mM, the ˙O₂⁻ scavenger),⁴³ disodium ethylenediaminetetraacetate (EDTA, 2 mM, the h⁺ scavenger),⁴⁴ AgNO₃ (2 mM, the e⁻ scavenger)⁴⁵ and isopropanol (IPA, 2 mM, the ˙OH scavenger).²⁵ As shown in Fig. 11, without any scavenger addition the conversion percentage is 99.1%. The addition of IPA and AgNO₃ both lead to non-significant change, suggesting that ˙OH and e⁻ are not the main reactive species involved in the RhB photocatalytic oxidation process. However, upon the addition of BQ and EDTA, the conversion percentage of RhB is greatly suppressed to 59% and 45%, respectively. This result indicates that ˙O₂⁻ and h⁺ are the major species involved in the photocatalytic oxidation process.

Fig. 10 PL spectra of g-C₃N₄ and N–TiO₂/g-C₃N₄ heterojunctions with different mass ratios.

Fig. 11 Conversion percentage of RhB in the presence of N–TiO₂/g-C₃N₄ photocatalyst with different scavengers.

Based on the above results, a possible photocatalytic mechanism of the as-prepared N–TiO₂/g-C₃N₄ heterojunctions under visible light irradiation was proposed and illustrated in Fig. 12. In this study, the band gap energy of g-C₃N₄ and N–TiO₂ are determined to be 2.75 and 3.10 eV, respectively, suggesting the two semiconductors have well-matched energy band structure for the formation of heterojunctions. Both the N–TiO₂ and g-C₃N₄ have the ability to absorb visible light, resulting in the excitation of electrons from valence band (VB) to the conductor band (CB). As the CB potential of g-C₃N₄ is more negative than that of N–TiO₂,³¹ the photogenerated electrons can directly inject into the CB of N–TiO₂ via the interfaces, while the holes on the N–TiO₂ surface can migrate to the VB of g-C₃N₄ in the same manner. Such transference can efficiently suppress the recombination of electrons and holes. The untransferred electrons on the CB of g-C₃N₄ and electrons on the CB of N–TiO₂ can capture the O₂ to generated ˙O₂⁻ radicals easily, which is one of the main active species for oxidizing RhB. On the other hand, the g-C₃N₄ with congregated holes can be charged with the segments of RhB and fall back to ground state according to the above results, thus also playing an important part in the degradation of RhB. These two paths present the most likely photocatalytic mechanism of the degradation of RhB over the as-prepared N–TiO₂/g-C₃N₄ photocatalysts under visible-light irradiation. Compared with individual components, the formation of N–TiO₂/g-C₃N₄ heterojunctions induces more electrons to congregate on the surface of N–TiO₂ and more holes on the surface of g-C₃N₄, thus leading to the overall enhancement of photocatalytic activity.

Fig. 12 Schematic diagrams for the possible photocatalytic mechanism of the N–TiO₂/g-C₃N₄ heterojunction under visible light irradiation. Band energy diagrams for N–TiO₂ and g-C₃N₄.

4. Conclusions

Sheet-like N–TiO₂/g-C₃N₄ heterojunctions with different mass ratios were synthesized by direct co-calcination of separately formed N–TiO₂ nanoparticles and g-C₃N₄ nanosheets. Such a processing method not only ensures the successful formation of N–TiO₂/g-C₃N₄ heterojunctions but also allows the structure/morphology control over individual components as well as the overall composite. Compared to individual components, the as-prepared N–TiO₂/g-C₃N₄ heterojunctions have exhibited highly enhanced photocatalytic activity toward the degradation of RhB under visible-light irradiation, which is mainly attributed to the synergetic effects between N–TiO₂ nanoparticles and g-C₃N₄ nanosheets. In the degradation of RhB, ˙O₂⁻ and h⁺ have played a major role whereas the contribution from ˙OH and e⁻ can be negligible. The highly enhanced visible-light photocatalytic activity and its excellent stability surpass existing N–TiO₂/g-C₃N₄ composite systems obtained via other synthetic routes mainly due to the effective structural control over the individual components as well as the overall composites provided by the present method, which endow the as-prepared heterojunctions with exceptionally high specific surface area (>80 m² g⁻¹) and efficient charge transfer at the heterojunction interfaces. This work shows that structural control over heterojunctions via designing synthetic strategy can bring about improved photocatalytic performance and provides a facile and powerful method for fabricating highly efficient and stable heterostructured photocatalysts for environmental remediation.

Acknowledgements

This work is supported by the Taishan Scholars Climbing Program of Shandong Province, Natural Science Foundation of China (NSFC 21271118, 21303095), Independent Innovation Foundation of Shandong University (2013TB001). The author W. Li acknowledges Dr Y. Lai for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available: X-ray photoelectron spectra, UV-vis diffuse reflectance spectra, FT-IR spectra, TG-DSC thermograms, nitrogen adsorption–desorption isotherms of different photocatalysts, evolution of UV-vis spectra of RhB in the presence of photocatalysts, XRD, FT-IR and SEM of the recycled photocatalyst. See DOI: 10.1039/c5ra04100g

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