Template free synthesis of graphitic carbon nitride/titania mesoflowers

Abstract

An efficient solar active catalyst is in immense demand for water splitting, waste water treatment and environmental remediation applications due to the growing energy crisis. The key to improving the activity of these materials is to increase the surface area, use a dopant and reduce the charge carrier recombination. This growing concern can be addressed by graphitic carbon nitride (g-C₃N₄) which has excellent visible light activity. In the present work a mesoporous flower like arrangement of carbon nitride–titania nanocomposites obtained through a solvothermal method without the use of any templates at ambient temperature is reported. A detailed investigation on the formation of these mesoflowers has been studied. The properties of the nanocomposite such as efficient absorption in the visible regime and delayed recombination of charge carriers make it an apt material for solar light photocatalysis for the degradation of organic contaminants. The high surface area of 147 m² g⁻¹ of the mesoflowers has been utilized for heavy metal removal such as Cr(VI) with a sorption percentage of 81% within 2 minutes of contact time in simulated water samples. This report paves the way for a new class of mesoflower nanocomposites for various environmental applications.

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Introduction

Researchers around the globe are in search of a versatile material for various applications such as in energy conversion, batteries, hydrogen production and water treatment. Numerous metal oxide nanocomposites including titania, ceria and zinc oxide have been explored throughout the years to tailor its structure so as to develop new properties. Research has been focussed on the nitrogen doping of graphene and titania to enhance its visible light activity for various energy efficient technologies.^1,2 Carbonaceous nanomaterials have gained great deal of interest such as carbon nanotubes, graphene, boron nitride due to its promising properties and enhanced surface area. The development of mesostructures for energy efficient application is addressed here. A facile method of synthesis of titania–carbon nitride mesostructures have been developed in this manuscript. Carbon nitride has a low band gap of 2.7 eV and excellent visible light activity.³ Graphitic carbon nitride (g-C₃N₄) is considered to be the most stable allotrope of carbon nitride.⁴ The commonly used precursors for chemical synthesis of g-C₃N₄ are nitrogen-rich compounds containing pre-bonded C–N core structures, such as triazine and heptazine derivatives, cyanamide, dicyanamide, urea, melamine, etc. Several methods have been employed to synthesize g-C₃N₄ which include liquid phase pulsed laser ablation of graphite target in the presence of ammonia,⁵ pyrolysis of urea,⁶ melamine^7,8 and liquid phase exfoliation of bulk g-C₃N₄.⁹ Nanoporous graphitic carbon nitride has been synthesized using Triton X-100 and ionic liquids as templates through the self-polymerization reaction of dicyandiamide.¹⁰ All these methods require either high temperature or presence of a template for the synthesis of g-C₃N₄. Being a metal free photocatalyst g-C₃N₄ has emerged as a promising candidate for various applications such as hydrogen evolution,¹¹ artificial photosynthesis, photocatalysis,¹² fuel cells,¹³ and heavy metal removal.¹⁴ Titania being an environmental friendly photocatalyst has gained a lot of attention over a decade for decontamination,¹⁵ degradation of organic contaminants,¹⁶ hydrogen evolution,¹⁷ solar cells, etc. Studies are in progress to improve the solar activity of TiO₂ using metals, non metals, carbon nanostructures, etc. Recently, TiO₂–carbon nitride composites have gained significant interest due to the enhancement in the visible light activity. Zou et al. reported the synthesis of mesoporous TiO₂ spheres which convert urea to carbon nitride at 300 °C.¹⁸ Recently, Zhou et al. has reported the synthesis of a series of composites of g-C₃N₄ and TiO₂ with in situ doping of nitrogen in TiO₂ by calcination with Ti(SO₄)₂ and urea as precursors for the photoreduction of CO₂ under simulated light radiation.¹⁹ Reports on the development of carbon nitride–TiO₂ nanocomposites are very limited in literature. As discussed earlier several steps are followed for the synthesis of these nanocomposites in literature using high temperature treatment, templates, nitrogen doping, etc. The synthesis route we present here is a one pot synthesis without the use of any templates. The sample obtained can be used as such without any further treatment.

The urge for developing an advanced material to satisfy the energy and environmental needs is addressed here. It is in this context, the synthetic approach we report here for the synthesis of g-C₃N₄–TiO₂ mesoflowers leading to the large-scale production of 3D architectures is significant. Graphitic carbon nitride is synthesized from melamine. TiO₂ and g-C₃N₄ is assembled into flower like morphology by a simple solvothermal method. The resulting 3D mesoflower structures provided a high surface area of ∼147 m² g⁻¹, and efficient photocatalytic activity under sunlight. Consequently, the 3D mesoflower composite shows excellent heavy metal adsorption rate and enhanced photocatalytic degradation of organic dye under solar light irradiation. A simple and efficient synthetic protocol for carbon nitride–TiO₂ mesoflower (g-C₃N₄–TiO₂) is developed in this work.

Experimental

Materials

All chemicals used were analytical-grade reagents without further purification. Melamine (99%, Sigma Aldrich), titanium(IV) isopropoxide (TTIP) purity 98% (Fischer scientific), acetic acid (Merck), tetraethyl orthosilicate (TEOS, Aldrich, 98%), ammonia solution 25% (Merck), 1,5-diphenylcarbazide (Merck) and potassium dichromate (Fischer scientific) were used in the experiments.

Synthesis of g-C₃N₄

g-C₃N₄ was prepared by calcining 1 g of melamine taken in a crucible covered with aluminum foil in a muffle furnace at 520 °C for 5 h.⁷ The white powder changes to light yellow colored powder with a yield of ∼350 mg.

Synthesis of g-C₃N₄/TiO₂ nanocomposites

g-C₃N₄/TiO₂ nanocomposite was prepared by one pot solvothermal method. Briefly, 0.1 wt% of g-C₃N₄ was added into 50 mL of concentrated acetic acid (Merck) and sonicated for 5 minutes. Then 750 μL of TTIP was added drop wise into the mixture under stirring. The mixture was stirred vigorously at room temperature for 30 minutes, then transferred into a Teflon beaker and sealed in an autoclave reactor. The reactor was heated in a muffle furnace at a temperature of 160 °C for 7 hours. Once the reactor cools down the product was collected by centrifuging the reaction mixture at 8000 rpm in water, washed thrice and dried at 60 °C. The obtained solids were denoted as g-C₃N₄–TiO₂. The reaction was carried for different time intervals to study the evolution of these structures. The effect of concentration of g-C₃N₄ was also studied by varying its concentration. The samples calcined at 400 °C for 3 hours showed no change in the mesoflower structure and hence all the characterization was done for the material without any calcination.

Characterization

FE-SEM images were taken using a Hitachi SU6600 field emission gun scanning electron microscope (FE-SEM). Specimens were mounted on black carbon tape. TEM images were taken using Hitachi, H-7650 electron microscope working at an accelerating voltage of 80 kV. XRD measurements were conducted on a Rigaku Miniflex 600 X-ray diffractometer using a monochromatized Cu Kα radiation source (30 kV, 10 mA) with a wavelength of 0.1542 nm and analyzed in the range 10° ≤ 2θ ≤ 80°. UV-Vis absorption spectra were measured using a Shimadzu UV-1800 UV-Visible spectrophotometer. Functional group analysis was done using FTIR (Jasco FTIR 4108). The specific surface area was measured using BET surface area measurement (Micromeritics Gemini 2375 Surface area analyser) via multi point method after degassing the powder in flowing N₂ at 200 °C for 2 h. The photoluminescence (PL) spectra were obtained by using fluorescence spectrometer (LS 55 Perkin Elmer).

Chromium adsorption

The sorption of Cr(VI) was investigated by batch experiments under ambient conditions. To 10 ppm of Cr(VI) solution, 1 g L⁻¹ adsorbent was added. The reaction was done at a pH of 5. The solution was mixed by magnetic stirring. Then, 1 mL of the solution was centrifuged at 8000 rpm for 3 min and supernatant was collected at regular time intervals. The spectrophotometric estimation of Cr(VI) species was done using diphenylcarbazide as a complexation reagent.²⁰ Cr(VI) form a colored compound Cr(III)-diphenyl dicarbazone which has a maximum absorption at 540 nm. The concentrations of the samples were determined by carrying out identical measurements to those of the standard using the calibration graph.

Photocatalytic degradation of Rhodamine B

In a typical process, 100 μL aqueous solution of Rhd B (1 mM) was added to 20 mL suspension of photocatalyst (0.1 mg mL⁻¹) in water. The photocatalyst suspension was ultrasonicated for 5 minutes for better dispersion before adding the dye. The reaction vessel was exposed to sunlight with measured light intensity between 70 000 and 80 000 luxes. Light intensity was measured using a Lutron light meter in lux. At regular time intervals, 3 mL of the photoreacted solution was withdrawn and analyzed by the variations of the absorption maximum of Rhd B (553 nm) in the UV-Visible spectrum.

Results and discussion

Synthesis of g-C₃N₄–TiO₂ mesoflowers

Graphitic carbon nitride was synthesized by the calcination of melamine. These nanosheets were further used for the synthesis of the nanocomposites. g-C₃N₄–TiO₂ mesoflowers were synthesized by a facile solvothermal method at ambient temperature. Morphology of the nanocomposites was studied using FE-SEM and TEM. The sheet like nature of g-C₃N₄ was confirmed from the FE-SEM image (Fig. 1a). The FE-SEM images confirmed the formation of g-C₃N₄ as well as mesoflowers (Fig. 1b–d). It could be observed that the mesoflowers have a uniform size distribution with an average size of <2 μm (Fig. 1b). The TEM images in Fig. 1d shows the magnified region of a petal. In order to elucidate the mechanism of formation of these structures, role of various parameters such as solvents, precursors, weight ratio of g-C₃N₄ were investigated.

Fig. 1 FE-SEM images of (a) graphitic carbon nitride (b) g-C₃N₄–TiO₂ mesoflowers (c) magnified region of a mesoflower (d) TEM images of g-C₃N₄–TiO₂, a magnified region of the petal (scale bar 100 nm) and mesoflower (scale bar 500 nm).

First a series of control experiments by varying the reaction time was carried out. The structural evolution of the flower like structures was studied using FE-SEM. Fig. 2 show the FE-SEM images of the samples synthesized at different reaction time. Initially the reaction was carried out for 30 minutes and it was found that g-C₃N₄ remained as sheets and titania as very small nanoparticles. When the reaction was done for 4 hours, evolution of the flower structures initiated though they were not well defined. Whereas, when the reaction time was extended to 7 hours, perfect flower like structures with petals originating from the centre were observed in SEM images. The mesoflowers were approximately of 2 μm with defined petal like structures arranged in a flower like pattern. Aggregation of structures and thereby alteration of flower like morphology were observed when the reaction time prolonged to 12 h. From EDS (Table S1, ESI†) a qualitative analysis of the formation of TiO₂ was done. The weight percentage of TiO₂ was found to increase as the reaction time increases.

Fig. 2 FE-SEM images of g-C₃N₄–TiO₂ at different time intervals.

The concentration of g-C₃N₄ also had a major role in the formation of the flower structures. To study the effect of concentration of g-C₃N₄ on the structure formation 0.025, 0.05, 0.1, 0.2 wt% of g-C₃N₄ were used as the precursor in the reaction without changing the concentration of titania precursor. SEM images of the products are shown in Fig. 3. On addition of 0.025 wt% g-C₃N₄ lead to the formation of less dense flowers as evident from the SEM image. The concentrations, 0.1 and 0.05 wt% yielded almost similar flower like morphology but the evolution of petals was prominent in 0.1 wt%. There is a possibility of g-C₃N₄ wrapping over titania mesoflowers also. When the amount of g-C₃N₄ increased to 0.2 wt% the morphology was not well defined and the resulting structures were very dense. The reaction without the addition of g-C₃N₄ was performed which lead to the formation of rice grain like TiO₂ (ESI, Fig. S1†).

Fig. 3 FE-SEM images of g-C₃N₄–TiO₂ with varying weight ratios of g-C₃N₄.

The effect of other parameters such as solvent, precursor and route of synthesis was also investigated. In order to study the role of acetic acid in the evolution of flower structure, ammonia was used as the solvent under the same reaction conditions. It was found that the flower morphology was not obtained showing that acetic acid is essential for the formation of flower morphology (Fig. 4a). In order to study whether acetic acid medium itself facilitated the formation of bare g-C₃N₄ flowers, the reaction was carried out in acetic acid alone in the absence of titanium tetraisopropoxide. From Fig. 4b it was found that no flower structures were formed either. In order to investigate the role of titania, tetraethyl orthosilicate was introduced instead of TTIP under same conditions using acetic acid as solvent. It led to the formation of silica nanoparticles on the surface of g-C₃N₄. It was found that the flower like structures were not formed (Fig. 4c). The precursor for the synthesis of g-C₃N₄, melamine was used in the synthesis instead of g-C₃N₄ directly under the same conditions and the obtained product was calcined at 520 °C for 5 h. From the FE-SEM analysis, it was found that flower structures of larger sizes were obtained but the exact petal-like morphology was not obtained (Fig. 4d). Different routes of synthesis were investigated to find out their effect on the formation of mesoflowers. Initially sol–gel method of synthesis was adopted where in TTIP and g-C₃N₄ was allowed to react in an ethanol water mixture (1 : 1) for 3 h at 60 °C. The nanocomposites were washed and collected through centrifugation. The SEM images show the distribution of spherical shaped TiO₂ nanoparticles on the surface of g-C₃N₄. It was evident that the flower shaped structures were not formed in this route of synthesis (Fig. 4e). Then sonochemical route of synthesis was also investigated using 1 : 2 weight ratio of g-C₃N₄ : TiO_{2(rice grain shaped)}. The mixture was dispersed in water and sonicated for 1 h. This route of synthesis also did not yield flower-like morphology. It can be concluded that the route of synthesis also had a prominent role in the formation of these mesostructures (Fig. 4f). These experiments suggest that the flower structure formation is mediated through the formation of titanium–acetic acid complex²¹ leading to the formation of g-C₃N₄–TiO₂ composite. In order to elucidate the evolution of the g-C₃N₄ mesoflower structures in the presence of TiO₂ further characterization were done.

Fig. 4 FE-SEM images of g-C₃N₄ (a) reaction with TTIP in ammonia solution (b) in acetic acid alone, solvothermal synthesis using (c) TEOS as precursor (d) melamine as the precursor for g-C₃N₄, using different routes of synthesis of g-C₃N₄–TiO₂ (e) sol–gel route (f) sonochemical route.

Characterization of g-C₃N₄–TiO₂ mesoflowers

In order to elucidate the structural evolution of the g-C₃N₄–TiO₂ mesoflower structures XRD analysis was performed. Fig. 5 shows the XRD patterns of g-C₃N₄ and g-C₃N₄–TiO₂ mesoflowers. The XRD pattern of g-C₃N₄ nanosheets exhibited a small peak at 13.1° which is indexed to the characteristic in-planar structural packing (100) and a prominent peak at 27.4° assigned to inter-planar stacking peaks of the aromatic systems in graphite-like g-C₃N₄ (002).^9,22 The interplanar spacing d₍₀₀₂₎ of 3.254 Å indicated that aromatic units in g-C₃N₄ are packed tighter than the packing in graphene units (3.61 Å).²³ The XRD patterns of the materials synthesised at different reaction time were analyzed. In the samples after the reaction with TTIP, intensity of the major peak at 27.4° has significantly reduced and the peaks of TiO₂ evolved. Upon 30 minutes of reaction time, the peaks corresponding to anatase phase have slightly evolved but not prominent. As the reaction time increased the peaks of anatase TiO₂ become prominent and the intensity of g-C₃N₄ (002) diminished. The size of TiO₂ nanoparticles calculated from XRD patterns using Scherrer equation was 5.5 nm. This correlated well with the TEM images (Fig. 1d) where formation of TiO₂ nanoparticles of 5–10 nm were observed which self-assemble to form the petal like morphology. It was observed that the low angle peak at 13.1° is less prominent in flowers which in turn confirm the formation of g-C₃N₄.²⁴ The characteristic peaks (2θ) of anatase TiO₂ was well resolved at 25.2°, 37.6°, 47.8°, 53.9°, 62.3°, and 75.0° for the samples synthesised with reaction time 4 h, 7 h and 12 h.

Fig. 5 XRD patterns of g-C₃N₄ and g-C₃N₄–TiO₂ at different time intervals. ‘*’ shows the peaks due to carbon nitride and ‘A’ corresponds to the anatase phase of titania.

The chemical characterization of the nanocomposite was studied using FTIR spectroscopy. The FTIR spectra confirmed the existence of graphite like sp² bonding in g-C₃N₄ (Fig. 6a). In the spectrum of g-C₃N₄, the peaks between 1200–1650 cm⁻¹ corresponds to the C–N and C=N stretching in carbon nitride which are associated with the skeletal stretching vibrations of s-triazine/tri-s-triazine rings. These peaks were observed in bare g-C₃N₄ as well as TiO₂ incorporated samples. The peaks were sharper in the composite probably due to the weak coupling with the skeletal vibrations. The peak at 1566 cm⁻¹ correspond to the stretching mode of g-C₃N₄ heterocycle was shifted to 1530 cm⁻¹ on incorporation of TiO₂ indicating that TiO₂ forms bond with the triazine ring. Additionally, on reaction with TTIP for 2 h a peak at 1024 cm⁻¹ evolved which is attributed to the C–O bond stretching. The broad peaks between 3000 and 3600 cm⁻¹ in g-C₃N₄ is due to the N–H stretching and O–H stretching due to the adsorbed water in between the g-C₃N₄ layers. It was found to be broader on incorporation of TiO₂ due to O–H stretching. The sharp peak at 809 cm⁻¹ was attributed to the out of plane ring bending modes of s-triazine rings which is in accordance with previous studies.^7,19 This peak is found to broaden upon incorporation of TiO₂. The broad peak at low frequency (500–700 cm⁻¹) was attributed to the stretching modes of Ti–O in the spectrum after 30 minute reaction time which is in accordance with the XRD patterns. It was observed that the molecular structure of g-C₃N₄ did not alter due to the incorporation of titania.

Fig. 6 (a) FTIR spectra of g-C₃N₄ and g-C₃N₄–TiO₂ at different reaction time (b) UV-Vis absorption spectra of g-C₃N₄ and g-C₃N₄–TiO₂ (c) photoluminescence spectra of g-C₃N₄ and g-C₃N₄–TiO₂ at different reaction time.

The UV-Vis absorption spectrum of g-C₃N₄ exhibited an absorption band centered at 235–250 nm which is attributed to π → π* electronic transition in the aromatic 1,3,5-triazine compounds and sharp absorption ∼300 nm may be attributed to the n–π* electronic transitions involving lone pairs of nitrogen atoms in the aromatic heterocyclic ring systems. The absorption edge of g-C₃N₄–TiO₂ around 310 nm is due to the incorporated TiO₂ (Fig. 6b).

To understand the extent of generation, separation, and recombination rate of the photogenerated charge carriers of g-C₃N₄–TiO₂ mesoflower, the room temperature photoluminescence (PL) was analyzed (Fig. 6c). It was observed that the bare g-C₃N₄ exhibited intense photoluminescence intensity, which reduces upon 30 minutes of reaction with TTIP. Further, on increasing the reaction time the PL intensity enormously reduced. This gradual decrease confirms that on incorporation of TiO₂ the recombination rate of the charge carriers have been considerably reduced which is a favorable factor for the photocatalytic activity.

Surface area analysis is an important criterion for any material in energy and environmental application. The surface area, pore volumes, and average pore sizes were determined from Brunauer–Emmett–Teller (BET) isotherms. Since the flower like morphology was well evolved upon 7 h of the reaction time, this sample was further characterized for its porous nature. Fig. 7a shows the nitrogen adsorption and desorption isotherms of g-C₃N₄–TiO₂ after 7 h of reaction time. The isotherms showed characteristic H3 type loop which are usually seen for aggregates of plate-like particles or adsorbents containing slit-shaped pores.²⁵ The BET surface area and pore volume (Fig. 7b) are 147 m² g⁻¹ and 0.001722 cm³ g⁻¹, respectively.

Fig. 7 (a) Nitrogen adsorption and desorption isotherms (b) pore diameter distribution curve of g-C₃N₄–TiO₂ (7 h).

Environmental application of g-C₃N₄–TiO₂ mesoflowers

The g-C₃N₄–TiO₂ flower structures due to its enhanced surface area and excellent electron–hole recombination delay they were evaluated for its potential in photocatalytic degradation of organic pollutants under solar radiation and adsorption of heavy metals such as Cr(VI).

Adsorption of heavy metals – Cr(VI)

In order to evaluate the sorption property of g-C₃N₄–TiO₂ mesoflowers for the removal of heavy metal ions, the sorption processes of hexavalent chromium was studied. Sorption of Cr(VI) on g-C₃N₄–TiO₂ as a function of contact time is shown in Fig. 8a. A rapid adsorption of 81% was achieved in the first 2 minutes of stirring time after which equilibrium was attained. It was observed that the large surface area of the meso structures aided in the rapid removal of Cr(VI) within 2 minutes. In order to investigate the sorption kinetics in detail, the experimental data were fitted by the pseudo-second-order rate equation (Fig. 8b). From the linear plot of t/q_t versus t, the value of k₂ calculated from the slope and intercept is 0.0006 g mg⁻¹ min⁻¹. The regression coefficients (R² [Cr(VI)] = 0.9989) suggests that the kinetics of Cr(VI) on g-C₃N₄–TiO₂ follow the pseudo-second-order model. ESI Fig. S2† shows the variation of q_t with time. In order to understand the sorption mechanism better, the experimental data were simulated by Langmuir and Freundlich isotherms (ESI, Fig. S3†). The sorption capacity at equilibrium time q_e (mg g⁻¹) is compared with the experimental and calculated values as shown in Fig. 8c. It was observed that adsorption followed Freundlich model and it showed best fit with the experimental values. All the parameters and regression coefficients obtained from the equilibrium plots of Langmuir and Freundlich isotherms models are listed in Table 1. From the isotherms it is observed that Freundlich model suits better. This result indicates the heterogeneous adsorption of Cr(VI) on g-C₃N₄–TiO₂ mesoflowers suggesting the efficient use of this structure in the heavy metal removal from contaminated water.

Fig. 8 (a) Sorption of Cr(VI) on g-C₃N₄–TiO₂ as a function of contact time (b) pseudo second-order kinetic plot fitting of t/q_t versus t (c) comparison of experimental q_e with calculated q_e from Langmuir and Freundlich isotherm model.

Table 1 Regression coefficients and constants of isotherm models of adsorption of Cr(VI)

Isotherm model	KL [L mg^−1]	KF [mg g^−1 (L mg^−1)^1/n]	R^2	RL	1/n
Langmuir	3.25	—	0.976	0.01	—
Freundlich	—	6.48	0.988	—	0.324

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Photocatalytic activity of g-C₃N₄–TiO₂ mesoflowers

The photocatalytic performance of g-C₃N₄–TiO₂ mesoflowers was evaluated using a cationic dye, Rhodamine B as a model dye pollutant under direct sunlight. Rhodamine B has an absorption maximum at 553 nm, which is directly proportional to its concentration. The absorption at λ_max is monitored and the degradation efficiency of the nanocomposite was evaluated using eqn (1).where A₀ represents the initial absorbance, and A_t represents the absorbance after a reaction time (t).

The UV-Vis absorption spectra showing the degradation profile of the dye under solar radiation is shown in Fig. 9a. The enhanced surface area of mesoflowers enabled better adsorption of the dye molecules on the surface of the mesoflowers. There is possibility of wrapping of g-C₃N₄ over the titania mesoflowers which in turn aided the solar light activity to the nanocomposite. The reduced charge carrier recombination enhanced the photocatalytic activity of the composite under solar radiation. The kinetics of degradation of the dye was studied. It was observed that the reaction is first order with a rate constant of 49.73 × 10⁻³ min⁻¹ (Fig. 9b). It was observed that under solar irradiation the degradation efficiency was found to be 100% within 50 minutes of exposure (Fig. 9c). A comparison table on the photocatalytic degradation of dyes using carbon nitride/titania nanocomposites as well as carbon nitride and titania is shown in Table 2. The degradation efficiency obtained in this work is higher than the previous literature wherein high power lamps and high amount of catalyst has been used in the degradation process. The mesoflower structures showed good solar photocatalysis for the degradation of organic contaminants such as dyes which can be further extended to other organic pollutants such as pesticides, phenols, etc.

Fig. 9 (a) UV-Vis absorption spectra of photocatalytic degradation of Rhodamine B under solar radiation (b) the kinetic plot of degradation (c) photocatalytic degradation efficiency of g-C₃N₄–TiO₂ mesoflowers over Rhodamine B under solar radiation.

Table 2 A comparison table on the photocatalytic degradation of dyes using carbon nitride/titania nanocomposites

No.	Material	Light source/lamp	Surface area m^2 g^−1	Catalyst conc. (mg mL^−1)	Pollutant selected	Time	Ref.
1	SiO2/C3N4	12 W LED @ 420 nm	70	1	Rhodamine B	120 min	26
2	g-C3N4/SmVO4	350 W Xe	51	1	Rhodamine B	2 h	8
3	g-C3N4/TiO2 (B)	108 W	—	—	Sulforhodamine B	>5 h	27
4	Ag/g-C3N4	500 W Xe	—	3	Methyl orange	3 h	28
5	Porous g-C3N4	500 W Xe	5.4–60	0.5	Methylene blue	—	29
6	N-doped TiO2	500 W Xe	171.7	1	Methylene blue	100 min	30
7	g-C3N4–TiO2	100 W halogen	10.67	0.125	Methylene blue	100 min	31
8	Graphene-like C3N4 with 300 μL H2O2	300 W Xe	32.54	0.5	Methylene blue	25 min	32
9	g-C3N4/TiO2 mesoflowers	Solar radiation	147	0.1	Rhodamine B	50 min	This work

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Conclusions

In summary, we have developed a facile solvothermal route of synthesis of mesoporous flower-like arrangement of g-C₃N₄ and titania. A detailed investigation on the formation of the flower-like structures is discussed in this work. A possible mechanism of formation of mesoflowers is discussed. The formation of flower structures strongly depended on the concentration of g-C₃N₄, reaction time and the solvent used. Owing to their high surface area (147 m² g⁻¹) they prove to be an apt candidate for removal of heavy metals with a sorption percentage of 81% within 2 minutes. The sorption processes for Cr(VI) followed the pseudo second-order kinetics, and the sorption isotherm was simulated well by the Freundlich model. They also have proved to be an efficient solar active photocatalyst. These properties of the mesostructures make them promising materials for wide range of energy and environmental applications such as heavy metal removal, photocatalysis, hydrogen evolution under solar radiation, lithium ion batteries and fuel cells. The synthetic protocol can be extended to the more general fabrication of 3D composites of carbon nitride with appropriate metal or metal oxide nanomaterials for the specific applications.

Acknowledgements

The authors thank Department of Science and Technology and Department of Biotechnology, India for the financial support to the nanoscience research laboratory. The authors thank Mrs Sabna V and Dr S. Bhuvaneshwari for their valuable discussions in the adsorption experiments. Department of Physics, Calicut University is greatly acknowledged for the XRD facility. We thank Dr C. Arunkumar and Mr Rahul Soman, Department of Chemistry, NITC for providing the fluorescence measurement facility.

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Footnote

† Electronic supplementary information (ESI) available: TEM images of TiO₂, amount of Cr(VI) adsorbed on g-C₃N₄–TiO₂ at various time intervals and initial concentration, table showing the EDS analysis of g-C₃N₄–TiO₂ at different reaction time intervals. See DOI: 10.1039/c5ra14547c

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