Facile stir-dried preparation of g-C₃N₄/TiO₂ homogeneous composites with enhanced photocatalytic activity

Abstract

g-C₃N₄/TiO₂ composites with homogeneous well-combined structures were prepared by a simple stir-dried method, using dicyandiamide (DICY) and tetrabutyl orthotitanate (TBOT) as the precursors, followed by high-temperature calcination. Various characterization techniques including XRD, FTIR, nitrogen adsorption–desorption, SEM and XPS confirm the formation of an interconnected structure between g-C₃N₄ and TiO₂ in the composites. g-C₃N₄/TiO₂ composites exhibit much higher photocatalytic activity than that of pure g-C₃N₄ and TiO₂ in the degradation of methylene blue under visible light. In particular, the CT-5 composite prepared with DICY/TBOT at a mass ratio of 5 : 1, exhibited a photodegradation activity that is about 3.8 times that of TiO₂ and 2.9 times that of pure g-C₃N₄. The homogeneous g-C₃N₄/TiO₂ composite CT-5 can be repetitively used without significant loss of activity. The strong synergistic effect between g-C₃N₄ and TiO₂ achieved by this preparation method greatly improves the separation efficiency of photo-generated electrons and holes, thus offering enhanced photocatalytic performances.

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1. Introduction

Titania has been the most popular photocatalyst in photocatalysis since 1972 when Fujishima and Honda discovered the TiO₂ photoelectrode.¹ It has been widely used in generating H₂ from water and in controlling water pollution discharged from industry, such as the reduction of heavy metal ions^2,3 and the degradation of organic dyes.^4–6 However, the biggest drawback of TiO₂ is its large band gap (3.2 and 3.0 eV for anatase and rutile, respectively) and thus can only harvest UV light, which makes up merely 3–5% of solar light, thereby restricting practical applications of titania.⁷ Therefore, much effort has been devoted to make TiO₂ absorb visible light. In recent years, various methods for TiO₂ modification have been attempted, including doping non-metal elements^6,8–10 and constructing composites with other semiconductors.^11–13 Besides these modifications, metal deposition^14–16 has also been extensively explored, and plasmonic metals, such as Au, Ag, and Cu which absorb visible light owing to their surface plasmon resonance effect, are employed as a sink of photo-induced electrons to enhance charge separation efficiency.^17,18 Though progress has been made, the cost of the metals is expensive and their enhancing efficiency is still in question.

Polymeric graphitic carbon nitride (g-C₃N₄) is the most stable material of all the carbon nitride allotropes under ambient conditions.^19,20 Besides, unlike the photocatalysts of sulphide and oxynitride semiconductor, the g-C₃N₄ is stable under light irradiation in water solution as well as strong acid or base solutions.^21,22 Furthermore, its band gap energy is only 2.58–2.89 eV.²³ The above-mentioned advantages make g-C₃N₄ a promising photocatalyst under visible light in various solutions. Some reports have focused on the applications of g-C₃N₄ in water splitting^24–26 and organic pollutant degradation;^27,28 Nonetheless, the high recombination rate of photo-generated electron–hole pairs results in a low photocatalytic activity of g-C₃N₄. To overcome this problem, coupling with TiO₂ is a feasible method because the interfacial connection between g-C₃N₄ and TiO₂ can make the electrons transfer easily from g-C₃N₄ (CB) to TiO₂ (CB).²⁹ Sridharan et al. prepared g-C₃N₄/TiO₂ through a thermal transformation method, and the improved photocatalytic activity was attributed to the formation of a synergistic heterojunction and the combination of both g-C₃N₄ and doped TiO₂ to absorb visible light more effectively.³⁰ A hybrid g-C₃N₄/TiO₂ has been synthesized by a ball milling method, and the enhanced photocatalytic activities were due to the hybrid structure.³¹ The growth of g-C₃N₄ on mesoporous TiO₂ spheres with well-controlled structures has been achieved by melt-infiltrating dicyandiamide, and the separation efficiency between photo-generated electrons and holes was enhanced significantly.³² Ma and co-workers prepared g-C₃N₄/TiO₂ via a simple one-step calcination method using commercial P25 and melamine as the precursors, and they found the interfacial interaction of g-C₃N₄ and TiO₂ was crucial to the efficiency of NO photocatalytic oxidation.²⁹ Although the prepared g-C₃N₄/TiO₂ composites have performed good photocatalytic activities, there is still a challenge to readily synthesize such a structure with well interconnected g-C₃N₄ and TiO₂ to reduce the migration barrier of charge carriers.

In this work, g-C₃N₄/TiO₂ homogeneous composites have been synthesized by a simple and effective stir-dried method. The TiO₂ precursor tetrabutyl orthotitanate (TBOT) and the g-C₃N₄ precursor dicyandiamide (DICY) were dissolved in ethanol and experienced the stir-drying to form a well-blended mixture. Thus-formed g-C₃N₄/TiO₂ composites were obtained by calcining the mixture in a muffle furnace. Moreover, g-C₃N₄/TiO₂ composites with different DICY/TBOT mass ratios were studied systematically, and the resulting composites exhibited higher photoactivity than that of the pure g-C₃N₄ and TiO₂ for the degradation of methylene blue under visible light.

2. Materials and methods

2.1 Chemicals

Dicyandiamide (DICY, 99%) was obtained from Adamas, China. Tetrabutyl orthotitanate (TBOT) was purchased from Shanghai Lingfeng Chemical, China. Methylene Blue (MB) was obtained from Acros, New Jersey, USA. Anhydrous ethanol was purchased from Sinopharm Chemical, China. All chemicals were used as received without further treatment.

2.2 Preparation of homogeneous g-C₃N₄/TiO₂

A series of g-C₃N₄/TiO₂ mixtures were prepared. In a typical synthesis, 0.2–2 g of TBOT was dissolved in 50 mL of anhydrous ethanol and stirred for 10 min. 2 g of DICY was added into the TBOT solution and ultrasonicated for 10 min, and then stirred for another 2 h to ensure their thorough dissolving and mixing. The mixture solution was undergone alcoholysis and stir-dried by evaporation at 65 °C oil bath. The resulting slurry was further dried at 60 °C overnight. The mixture solids were then ground and calcined at 500 °C for 1 h. g-C₃N₄/TiO₂ composites were obtained and marked as CT-0, CT-1, CT-5 and CT-10 when DICY/TBOT mass ratios were of 0 : 1, 1 : 1, 5 : 1 and 10 : 1, respectively. For comparison, pure g-C₃N₄ was obtained by calcining 2 g of DICY under the same calcination condition.

2.3 Characterizations

The phase structure of the TiO₂-based photocatalysts were examined by X-ray diffraction (XRD) using Rigaku MiniFlex II with Cu Kα radiation (λ = 0.1542 nm) at 40 kV. Nitrogen adsorption–desorption analysis was conducted using a Micromeritics ASAP 2020 at 77 K. Each sample was degassed at 120 °C for 120 min previous to analysis. The specific surface areas (S_BET) were calculated by the Brunauer–Emmett–Teller (BET) method. Fourier transform infrared spectra (FTIR) was recorded by a FTIR spectrophotometer (Thermo Electron Nicolet-360, USA) using the KBr wafer technique. The morphology of the samples was observed by scanning electron microscopy (SEM) utilizing a MAGELLAN 400 (FEI, USA) with an operating voltage of 5 kV, and elemental mapping images were obtained by using Energy-Dispersive X-ray Spectroscopy (EDX) on MAGELLAN 400 microscope. X-ray photoelectron spectroscopy (XPS) (AXIS UltraDLD, Japan) was employed to determine surface electronic states. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The UV-Vis diffuse reflectance spectra (DRS) of the samples over a range of 200–800 nm were recorded by UV-2600 (Shimadzu, Japan) spectrophotometer with a BaSO₄ reference. Room temperature photoluminescence (PL) spectra was recorded on a spectrofluorometer (FluoroMax-4, HORIBA Jobin Yvon) using a Xe lamp as the excitation source.

2.4 Photocatalytic activity test

Methylene blue (MB) was used as a probe dye to evaluate the photocatalytic activity of the g-C₃N₄/TiO₂ composites under a 300 W xenon lamp (CEL-HXF300, light intensity: 83 mW cm⁻²). To analyze the photocatalytic activity of g-C₃N₄/TiO₂ composites under visible light, a UV cut-off filter (λ > 420 nm) was used. Typically, 40 mg of the as-prepared photocatalyst was dispersed in a 40 mL of MB aqueous solution (10 mg L⁻¹) under constant stirring. Prior to illumination, the solution was kept in dark for 60 min and stirred to reach an adsorption–desorption equilibrium. At given intervals, the supernatant liquid was obtained by filtration with 0.22 μm filter and examined using a UNICO WFZ UV-2802s spectrophotometer at the characteristic wavelength of 664 nm. To evaluate the stability of photocatalyst, the photocatalyst was collected by centrifugation, washing and drying after the photocatalytic reaction for the recycling test. The degradation rates were calculated according to the following equation:where ν_s (mg L⁻¹ min⁻¹) is the degradation rate of samples to MB. C₀ (mg L⁻¹) and C_t (mg L⁻¹) stand for the concentrations of dye at initial and time t (min).

3. Results and discussion

3.1 Preparation of g-C₃N₄/TiO₂ composite

XRD patterns of g-C₃N₄, CT-10, CT-5, CT-1, CT-0 are shown in Fig. 1. The diffraction peak of pure g-C₃N₄ at 2θ 27.4° represents the conjugated aromatic system, which is indexed to (002) graphitic material.³³ An obvious g-C₃N₄ characteristic peak is shown in CT-10. A relatively weak peak of g-C₃N₄ is observed in CT-5 (inset in Fig. 1), showing that the presence of g-C₃N₄. However, no obvious diffraction peak of g-C₃N₄ is found in CT-1, probably due to the very small amount of DICY used. CT-0 shows several characteristic peaks at 2θ 25.3°, 37.8°, 48.0°, 53.9° and 55.0°, which are attributed to (101), (004), (200), (105) and (211) crystal planes of anatase TiO₂, respectively.^34,35 For CT-1, CT-5 and CT-10, TiO₂ characteristic peaks are found in all samples. It is worthy to note the diffraction peak of the (101) face shifts toward lower angles (25.1°) for sample CT-5 and CT-10, and the d-spacing of (101) face changes from 0.3520 to 0.3548 nm according to the Bragg equation, probably because some N ions are doped into TiO₂ crystal.³⁶

Fig. 1 XRD patterns of g-C₃N₄, CT-10, CT-5, CT-1, CT-0 and magnified CT-5 (inset).

FTIR spectra were recorded to show chemical structures of g-C₃N₄, CT-10, CT-5, CT-1 and CT-0. As presented in Fig. 2, characteristic peaks of g-C₃N₄ and pure anatase TiO₂ (CT-0) are consistent to those in literature.^37–39 For g-C₃N₄, several adsorption peaks in the region of 1200–1650 cm⁻¹ are attributed to the typical stretching modes of g-C₃N₄ heterocycles. Among them, the adsorption peaks at 1255, 1328, 1417, 1544 cm⁻¹ are assigned to aromatic C–N stretching, and the peaks at 1635 cm⁻¹ are ascribed to C–N stretching.^40,41 In addition, the peak at 808 cm⁻¹ is ascribed to characteristic breathing mode of triazine unites.⁴² A wide adsorption region of 3000–3600 cm⁻¹ (e.g. 3184, 3444 cm⁻¹) are assigned to N–H stretching vibration of remainder NH₂ group attached to the sp² hybridized carbon and O–H stretching concerned with the adsorbed water.^43,44 In the spectrum of CT-0, a wide band from 400 to 800 cm⁻¹ is ascribed to Ti–O–Ti stretching vibration mode.^45,46 All characteristic adsorption peaks of g-C₃N₄ and anatase TiO₂ appear obviously in CT-10 and CT-5 spectra, demonstrating a successful combination of g-C₃N₄ and TiO₂.⁴⁷ As expected, the spectrum of CT-1 is very similar to that of CT-0 that confirms the low amount of g-C₃N₄ in CT-1.

Fig. 2 FTIR spectra of g-C₃N₄, CT-10, CT-5, CT-1 and CT-0.

The morphologies of pristine g-C₃N₄, pure anatase TiO₂ and g-C₃N₄/TiO₂ composites were investigated, and SEM images of g-C₃N₄, CT-0, CT-5 and CT-10 are shown in Fig. 3. Pristine g-C₃N₄ presents a bulk structure composed of sub-micrometer particles, and pure anatase TiO₂ CT-0 shows spherical-like particles with sizes of 20–200 nm. CT-5 and CT-10 also show a bulk structure, whereas no spherical-like particles were observed. Such structures could partially confirm that TiO₂ and g-C₃N₄ are interconnected. In order to further prove the homogenous structure of g-C₃N₄/TiO₂ composites, CT-5 was selected to conduct the EDX mapping with the results give in Fig. 4, which shows that the sample CT-5 contains elements of N, O and Ti. It is clearly shown that element N, O and Ti are well dispersed in the sample, indicating g-C₃N₄ and TiO₂ form a homogeneous structure. The homogeneous structure is beneficial to efficient charge carrier separation in photocatalysis.

Fig. 3 SEM images of the g-C₃N₄ (a), CT-0 (b), CT-5 (c) and CT-10 (d).

Fig. 4 EDX mapping of N, O and Ti element of the CT-5.

BET surface areas and pore volumes of the samples were tested by nitrogen adsorption–desorption, and the nitrogen adsorption–desorption isotherms of g-C₃N₄, CT-0 and CT-5 are displayed in Fig. 5. The amount of nitrogen adsorption is very low for g-C₃N₄ and CT-0. g-C₃N₄ has a BET surface area of 14 m² g⁻¹ that is similar to the previous reports,^48,49 indicating the formation of bulk structure of g-C₃N₄. Pure anatase TiO₂ CT-0 also shows a low BET surface area of 14 m² g⁻¹. By incorporation of g-C₃N₄, CT-1 has a BET surface area of 29 m² g⁻¹. For CT-5, its nitrogen adsorption capacity has a dramatic increase, and its BET surface area increases to 78 m² g⁻¹. Pore size distribution indicates CT-5 has a peak pore size of 3.8 nm (inset in Fig. 5), which should arise from the intraparticle voids. The pore volume of CT-5 increases to 0.147 cm³ g⁻¹ that is higher than those of g-C₃N₄ and CT-0 (0.026–0.034 cm³ g⁻¹). It is believed that the suitable amount of g-C₃N₄ favors a good dispersion of g-C₃N₄ and TiO₂ to form a homogeneous structure. However, excess amount of g-C₃N₄ in CT-10 tends to self-aggregation, leading to a low surface area (49 m² g⁻¹) and a low pore volume (0.098 cm³ g⁻¹). A high surface area and pore volume benefit the enhancement of photocatalytic performance.

Fig. 5 N₂ adsorption–desorption isotherms of g-C₃N₄, CT-0 and CT-5, and pore size distribution of CT-5 (inset).

XPS analyses were carried out to investigate the surface chemical compositions of the samples and the oxidation state of key elements of C, N, and Ti. XPS survey spectra of g-C₃N₄, CT-0 and CT-5 are given in Fig. 6a, showing the element C, N, Ti and O are observed. The common C 1s signal about 284.8 eV is set for calibration. N and Ti elements are obviously observed in CT-5, implying the presence of g-C₃N₄ and TiO₂ in the composite, which is in accordance with the results demonstrated by the XRD patterns and the FTIR spectra. Fig. 6b–d show the high-resolution XPS spectra of C 1s, N 1s and Ti 2p. In the C 1s XPS spectrum of g-C₃N₄, there are two peaks (Fig. 6b). The peak at about 284.8 eV is ascribed to the adventitious carbon, and the peak at about 288.2 eV is ascribed to C–N or C–(N)₃ groups.⁵⁰ For the CT-5 sample, the C 1s peaks are similar to those of g-C₃N₄. However, the peaks of C–N or C–(N)₃ groups shift 0.4 eV towards the higher binding energy, probably due to the hybridization of g-C₃N₄ and TiO₂. The N 1s spectrum of g-C₃N₄ and CT-5 are shown in Fig. 6c and two peaks can be seen. Peaks at about 398.5 and 403.9 eV are attributed to the C–N=C groups and charging effects.^51,52 By comparison, the two peaks of CT-5 in the N 1s spectrum shift 0.3 and 0.2, respectively, which is due to the chemical environment change arising from the close interaction between g-C₃N₄ and TiO₂.⁵³ The Ti 2p spectra of CT-0 and CT-5 are shown in Fig. 6d, two peaks at ca. 458.4 and 464.1 eV for CT-0 sample are assigned to Ti 2p_3/2 and Ti 2p_1/2, suggesting the existence of Ti(IV).⁵⁴ For the CT-5 sample, the binding energy of Ti 2p_3/2 is the same as that of CT-0, and the slight shift for binding energy of Ti 2p_1/2 is ascribed to the interaction between g-C₃N₄ and TiO₂.

Fig. 6 XPS spectra of g-C₃N₄, CT-5 and CT-0 (a), high resolution XPS spectra of C 1s (b), N 1s (c) and Ti 2p (d).

3.2 Photocatalytic performance

The photocatalytic performances of g-C₃N₄, CT-0, CT-1, CT-5 and CT-10 were investigated by the degradation of MB. CT-5 exhibits the best photocatalytic performance under visible light irradiation as compared to other photocatalysts (Fig. 7a), about 3.8 times higher as to CT-0 and 2.9 times to the pure g-C₃N₄. The photoactivity of CT-10 is better than that of CT-1. These results demonstrate that g-C₃N₄/TiO₂ composites (CT-1, CT-5 and CT-10) offer much better degradation performance than that of pure g-C₃N₄ and pure anatase TiO₂ CT-0. Pure g-C₃N₄ shows a little higher photocatalytic activity than CT-0, owing to its small band gap, but the high recombination rate of photo electrons and holes limits its photocatalytic activity. The above results demonstrate that the combination of g-C₃N₄ and TiO₂ not only enormously improves light harvest, but also increases the survival time of photo-induced electrons. The photodegradation efficiency of CT-5 under Xe light without UV cut-off filter is shown in Fig. 7b, in which CT-5 could degrade 94% of MB in 2.5 h, but only 67% of MB could be decomposed under visible light irradiation in 3 h, demonstrating the generality of the synthetic effect of g-C₃N₄/TiO₂ composites.

Fig. 7 Photodegradation of MB using different photocatalysts under visible light (λ > 420 nm, light intensity 83 mW cm⁻²) (a) and photodegradation of MB over CT-5 under visible light (λ > 420 nm, light intensity 83 mW cm⁻²) and Xe light without UV cut-off filter (light intensity 83 mW cm⁻²) (b).

In Fig. 8, the UV-Vis spectra of g-C₃N₄ and g-C₃N₄/TiO₂ composites show that the main absorption edge of the pure g-C₃N₄ occurs at ca. 450 nm, comparing with CT-0 in the wavelength of 400–800 nm. The absorption threshold values of the g-C₃N₄/TiO₂ composites are extended up to the visible light region (500 nm) obviously, which confirms that more visible light can be harvested by g-C₃N₄/TiO₂ composites.

Fig. 8 UV-Vis diffuse reflection spectra of various samples: g-C₃N₄, CT-0, CT-1, CT-5 and CT-10.

Photoluminescence (PL) spectra of the photocatalysts is shown in Fig. 9. The PL intensity of CT-5 and CT-10 are significantly reduced in comparison with pure g-C₃N₄,⁴⁸ indicating that the electron–hole recombination on the surface of these photocatalysts is largely inhibited, to generate more photo-induced electrons and holes to participate in the photocatalytic reaction. At the same time, the survival time of photoelectrons is increased greatly, which is crucial to the photocatalytic efficiency. This is because the interfacial interaction between g-C₃N₄ and TiO₂ can prevent the recombination of photo-generated charge effectively. The strong intensity of CT-1 is probably due to small amount of g-C₃N₄ in TiO₂, leading to poor interfacial interaction between g-C₃N₄ and TiO₂. Therefore, the above results, including BET surface areas, UV-Vis diffuse reflection spectra and PL spectra, are consistent to their photodegradation performance of MB.

Fig. 9 PL spectra of various photocatalysts: g-C₃N₄, CT-0, CT-1, CT-5 and CT-10.

To evaluate the reusability of CT-5, the experiments were carried out on MB degradation under visible light irradiation for three times at the same conditions. The photocatalysts were recycled by centrifugation, washed with water three times and then dried. To make sure the amount of recycled catalysts was equal to that of the first-time use, several batches of reactions were carried out under equal conditions and then combined the recycled photocatalysts together. The experimental results are shown in Fig. 10, no significant decrease of activity is observed. The slight decrease should relate to the intermediates that were strongly adsorbed on the surface of the as-prepared photocatalysts, which might decline the electron transfer velocity and the adsorption capacity.⁵⁵ The reusability test indicates that CT-5 has good stability in MB photodegradation under visible light irradiation.

Fig. 10 Recycling test of CT-5 on photodegradation of MB (λ > 420 nm, light intensity: 83 mW cm⁻²).

3.3 Photocatalytic mechanism of homogeneous g-C₃N₄/TiO₂

The above-mentioned results have confirmed that g-C₃N₄/TiO₂ composite CT-5 exhibits much better photocatalytic efficiency than the pure g-C₃N₄ and TiO₂ (CT-0). The better photocatalytic performance is mainly associated with two factors: (1) the capacity of harvesting light and (2) the separation efficiency of the photoelectrons and holes. For the first factor of light absorption, TiO₂ has little adsorption of visible light; on the contrary, g-C₃N₄ shows a wide adsorption edge (Fig. 8). Therefore, the composite CT-5 could harvest the visible light greatly since the two substances combined together. For the second factor of charger separation, as shown in Scheme 1, when the visible light irradiates, photo-generated electrons (e⁻) transfer from the valence band (VB) to the conduction band (CB) of g-C₃N₄, leaving the holes (h⁺) in the VB. The different energy levels of g-C₃N₄ and TiO₂ drive the electrons transfer from the CB of g-C₃N₄ to that of TiO₂, thus increasing the survival time of the electrons and separating electrons and holes effectively. The homogeneously interconnected interfacial structure between g-C₃N₄ and TiO₂ provides a closely-matched path for the free transfer of electrons, compared with the physical mixture of g-C₃N₄ and TiO₂.³² The discussions above are called the synergistic effects between g-C₃N₄ and TiO₂. The electrons accumulated in the CB of TiO₂ are believed to reduce the oxygen (O₂) to active oxygen radicals (˙O²⁻), and the electron-deficient holes in the VB of g-C₃N₄ can oxidize the hydroxyl (OH⁻) to hydroxyl radicals (˙OH). Both ˙O²⁻ and ˙OH are capable to decompose MB effectively.³²

Scheme 1 Proposed mechanism of charge transfer at the interface between g-C₃N₄ and TiO₂.

4. Conclusions

Facile preparation of g-C₃N₄/TiO₂ homogeneous composites with well-combined structures has been achieved by stir-dried evaporation and high temperature calcination. The resulting g-C₃N₄/TiO₂ composites exhibited much higher photocatalytic activity than that of pure g-C₃N₄ and TiO₂ in the degradation of methylene blue under visible light due to the strong synergistic effect between g-C₃N₄ and TiO₂. The CT-5 composite performed the best photocatalytic activity which was nearly 3.8 times to the TiO₂ and 2.9 times compared to the pure g-C₃N₄. In addition, the CT-5 composite showed outstanding photocatalytic activity under Xe light, good stability and recyclability. Overall, this work provides a simple and effective path to prepare photocatalysts with fine hybrid structures and may inspire peers to design novel photocatalytic nanomaterials to enhance the efficiency of solar energy utilization for such as environmental remediation and clean chemicals synthesis.

Acknowledgements

The authors are grateful for the financial support of Natural Science Key Project of the Jiangsu Higher Education Institutions (15KJA220001), Jiangsu Specially-Appointed Professor Program and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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