Growth of g-C3N4 Layer on Commercial TiO2 for Enhanced Visible Light Photocatalytic Activity

Novel visible light photocatalytic graphitic carbon nitride/TiO 2 (g-C 3 N 4 /TiO 2 ) composite samples were synthesized by heating mixtures of melamine and commercial TiO 2 (TO) at different weight ratios. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electronmicroscopy (TEM), photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR), and UV-visible diffused reflectance spectroscopy (UV-vis DRS). Characterization confirms formation of nanocomposites of g-C 3 N 4 /TiO 2 . At the optimized precursor weight ratio (melamine : TiO2 = 2.5), the samples exhibited highest adsorption capacity and visible light photocatalytic activity, measured by degradation of methylene blue (MB). Under visible light irradiation, the excited electrons on the surface of g-C 3 N 4 transfer easily to the conduction band (CB) of TiO 2 via the well-built heterojunction. The g-C 3 N 4 /TiO 2 nanocomposites exhibit enhanced visible light catalytic activity due to increased visible light adsorption and effective separation of photogenerated electron-hole pairs. These g-C 3 N 4 /TiO 2 nanocomposites could find broad applicability in environmental protection due to their excellent visible light photocatalytic property and facile, cost-effective preparation process.


Introduction
Visible light photocatalysis has attracted much attention due to its extensive application in the fields of hydrogen generation [1,2] and environmental protection [3,4].TiO 2 has been proved to be a competent photocatalyst for environmental applications; however, due to the wide band gap of anatase TiO 2 (3.2 eV), it cannot be excited under visible light irradiation ( > 400 nm) [5].Therefore, there is much interest in developing visible light-responsive TiO 2 with catalytic activity high enough for practical applications.
Recently, graphite-like carbon nitride (g-C 3 N 4 ) has drawn wide attention for its potential application in the fields of catalytic support, gas storage, drug delivery, and optical and electronic materials [6] and as a metal-free, type semiconductor with tri-s-triazine units [7].The band gap of g-C 3 N 4 is 2.73 eV, imbuing a response wavelength of up to 450 nm, creating enormous potential in the photocatalysis field.Research has focused on synthesizing the g-C 3 N 4 with several precursors [8][9][10][11].Ma et al. reported a strategy for enhancing the photoactivity of g-C 3 N 4 via doping of nonmetal elements [9].
Recently, Yan and Yang reported a TiO 2 -g-C 3 N 4 composite material for photocatalytic H 2 evolution under visible light irradiation [2].The TiO 2 -g-C 3 N 4 composites were prepared by thermal treatment of the mixture of TiO 2 and g-C 3 N 4 .Yang et al. reported that N-doped TiO 2 /g-C 3 N 4 composites with enhanced daylight photocatalytic properties were prepared by heating titanium tetrachloride ethanol solutions with C 3 N 4 [10].However, commercial g-C 3 N 4 is not readily available and easily obtainable; inexpensive precursors for g-C 3 N 4 need to be used requiring tedious preparation which may prevent large-scale application.Fu et al. reported a solid-state approach to synthesizing g-C 3 N 4 coated TiO 2 nanocomposites from urea and commercial TiO 2 precursors [12]; however, the preparation conditions and proposed mechanism for enhancing photocatalytic activity of g-C 3 N 4 /TiO 2 require further research.
In the present work, all raw materials were purchased from commercial sources.The g-C 3 N 4 /TiO 2 composite 2 Journal of Nanomaterials photocatalysts were prepared by a facile solid-phase method of directly heating melamine and commercial TiO 2 mixtures.In the process of melamine pyrolysis, sublimation and thermal condensation of melamine occurred simultaneously in temperature range of 297-390 ∘ C [13] and triazine-ringbased radicals and NHx were formed as a result of the decomposition [11].Based on XPS, FTIR, and TEM results, it was confirmed that g-C 3 N 4 existed in prepared composites.Photocatalytic efficiency of the as-prepared g-C 3 N 4 coated TiO 2 nanocomposites was determined by degradation of methylene blue (MB) dye under visible light irradiation.

Experimental
2.1.Synthesis of Photocatalysts.g-C 3 N 4 -TiO 2 photocatalysts with various mass ratios ( melamine :  TO = 1.5, 2.5, 3.5, and 4.5, denoted TO-M-1.5, 2.5, 3.5, and 4.5, resp.) were prepared as follows. 2 g of TiO 2 (TO) and Xg of melamine were combined in an alumina crucible and stirred with deionized water at room temperature.The mixture was dried at 60 ∘ C for 1 h and then annealed to 520 ∘ C in a covered muffle furnace for 5 h.Finally, the sample was cooled to room temperature to yield the g-C 3 N 4 -TiO 2 photocatalyst.The g-C 3 N 4 photocatalyst was also synthesized through pyrolysis of melamine, via direct heating of melamine at 520 ∘ C for 4 h.For comparison, TO-520-5 and M-520-5 were prepared analogously in the absence of melamine and TiO 2 , respectively.Finally, precursors with the same mass ratios ( melamine :  TO = 2.5) were prepared at temperatures of 320 ∘ C, 420 ∘ C, 520 ∘ C, and 620 ∘ C to yield TO-M-320, TO-M-420, TO-M-520, and TO-M-620 for 5 h, respectively.

Materials Characterization.
Comprehensive structural characterization of as-prepared samples was undertaken.The crystal structures of samples were determined by an X-ray diffractometer with Cu-K radiation (XRD: modelD/max RA, Rigaku Co., Japan).The surface area and porosity of the samples were estimated by measuring the nitrogen adsorption-desorption isotherms on a Quantachrome Autosorb-1 system.FTIR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets.XPS spectra of the products were collected on a photoelectron spectrometer (Kratos Co., UK).The morphology of samples was characterized by TEM (JEM-2010).UVvis diffuse reflection spectra were taken on a scan UVvis spectrophotometer (UV-vis DRS: UV-2550, Shimadzu, Japan).Photoluminescence spectra of samples were obtained by using a fluorescence spectrometer (HITACHI, F-7000) at room temperature.

Evaluation of Photocatalytic Activity.
Methylene blue (MB) oxidation reactions were performed on a photochemical reactor (BL-GHX) using a 500 W Xe lamp fitted with a thimble containing NaNO 2 solution (1 mol/L) to block UV light ( < 400 nm).Additionally, immediate heat removal from the lamp was achieved via water flow through the inner thimble of the annular quartz tube.0.02 g of catalyst was dispersed in MB aqueous solution (50 mL, 5 mg/L) with stirring for 40 min without irradiation until adsorptiondesorption equilibrium was reached.During the irradiation, 5 mL aliquots were taken from the reaction cell at hourly intervals for subsequent dye concentration analysis following centrifugation.The degradation rate of MB was evaluated via the intensity of UV-vis absorption spectra at 665 nm.

Results and Discussion
3.1.XRD. Figure 1 depicts the XRD patterns of the asprepared, mass ratio differentiated photocatalysts alongside g-C 3 N 4 and TiO 2 .The g-C 3 N 4 has two main diffraction peaks at 13.1 ∘ and 27.5 ∘ as a result of a polymeric layered structure [14].The characteristic peaks at 27.5 ∘ and 13.1 ∘ correspond to the stacking of the conjugated aromatic system and the periodic arrangement of the condensed tri-s-triazine units of g-C 3 N 4 , respectively.The peak intensity at 27.5 ∘ corresponding to g-C 3 N 4 increases with increasing weight ratio of melamine, which confirms that g-C 3 N 4 has been produced in the as-prepared samples and the yield of it also gradually increased.For all g-C 3 N 4 /TiO 2 composites and TiO 2 , characteristic anatase TiO 2 peaks with similar position, intensities and widths were detected.This implies that the phase structure and the crystallite size of TiO 2 particles in as-prepared samples are conserved.

Visible Light Catalytic Activity.
The degradation of MB is carried out under visible light ( > 400 nm) illumination in aqueous sample suspensions.the 5 hours of visible light photocatalytic degradation, TO-M-2.5 exhibits the highest visible light catalytic activity of all compounds including TiO 2 (TO) and g-C 3 N 4 .The degradation rate  (%) can be calculated as where  0 is the initial concentration of dye and  is the time dependent concentration.Degradation rate of MB induced by TO-M-2.5 is about 91%.It is surmisable that when combined TiO 2 and g-C 3 N 4 produce a synergistic effect and effectively enhance the visible light photocatalytic efficiency.The photocatalytic activity of the samples prepared at different temperatures is shown in Figure 2(b).The variation in degradation observed (Figure 2(b)) suggests that the calcination temperature is also a key factor with regard to photocatalytic activity.

Microstructure Analysis.
The BET surface areas and porous structures of TiO 2 and TO-M-2.5 were investigated by nitrogen adsorption/desorption.In Figure 3, the two samples show a type IV adsorption isotherm with a H 2 hysteresis loop in the range (/ 0 ) of 0.6-1.0.The surface area and pore volume (Table 1) of pure TiO 2 are 81.63 m 2 /g and 0.221 cm 2 /g, higher than those of the TO-M-2.5 (50.57m 2 /g, 0.154 cm 2 /g).Compared with TiO 2 , the specific surface area of TO-M-2.5 is reduced, potentially due to the presence of g-C 3 N 4 layers on the TiO 2 surface.However, the surface area of g-C 3 N 4 /TiO 2 is not a positive factor.The enhanced visible light activity of g-C 3 N 4 /TiO 2 should be ascribed to the enhanced visible light adsorption and dispersion of TiO 2 resulting in increased
The absorption peak at ca. 1329 cm −1 and ca.1635 cm −1 can be attributed to C-N and C=N stretching modes, respectively [15,16].Additionally, the wide band between 3100 cm −1 and 3400 cm −1 corresponds to the N-H and O-H stretching vibration [14,17].These characteristic peaks were largely absent in the FTIR patterns of TO-M-1.5, which probably results from the low content of g-C 3 N 4 in the sample.the sp 3 C-N bond [15,16,18], respectively.The N 1s XPS spectrum of TO-M-2.5 can be deconvoluted into two peaks at 400.1 eV and 398.6 eV (Figure 5(b)), which were attributed to the binding energy of N-(C) 3 and C-N-C, respectively.These C 1s and N 1s binding energies were consistent with previously reported XPS data [8,9], which further confirms that our as-prepared samples contain the graphite-like carbon nitride.The O 1s XPS spectrum (Figure 5(c)) included two peaks at 529.8 eV and 531.6 eV, which were closely related with C-O functional groups and surface hydroxyl groups [19].
The adsorbed water molecule was a major cause of surface hydroxyl groups benefitting capture of photogenerated holes and inhibition of electron-hole recombination.The peaks located at 458.7 and 464.4 eV in the Ti 2p spectra correspond to the Ti 2p 3/2 and Ti 2p 1/2 (Figure 5(d)).There were no Ti-C or Ti-N bond peaks, which implied that C and N elements do not enter the lattice of TiO 2 .By the analysis of XPS, it can be ensured that the g-C 3 N 4 coated TiO 2 composites were prepared by heating mixture of melamine and TiO 2 .
3.6.TEM. Figure 6(a) shows the TEM images of pure TiO 2 , consisting of nanoparticles with ca. 10 nm radius, consistent with the size of TiO 2 from microstructure analysis (Table 1).Figure 6  shown that TiO 2 is well distributed on a layered structure.
The XPS and FTIR have confirmed the existence of g-C 3 N 4 .
As the g-C 3 N 4 is a semicrystallized material, it is hard to obtain the clear lattice fringe.However, by comparison of the morphology of g-C 3 N 4 (Figure 6(c)) and TO-M-2.5, it can be concluded that the secondary phase in TO-M-2.5 is g-C 3 N 4 .In Figure 6(d), g-C 3 N 4 /TiO 2 (TO-M-2.5)displayed a connection between TiO 2 and g-C 3 N 4 , indicating that the formation of heterojunction was possible.Visible light absorption of composites is enhanced by the presence of g-C 3 N 4 ; however, it cannot be simply concluded that visible light absorption is consistent with photoactivity as many factors influence catalytic activity under visible light [20].Enhanced activity is due to the presence of the TiO 2 , which can promote effective interfacial electron transfer [21] and conduce efficient space separation of photogenerated electron-hole pairs of g-C 3 N 4 .The band gaps of TiO 2 and g-C 3 N 4 are showed in Figure 7(b), which are 3.2 eV and 2.7 eV, respectively.

UV-Vis
3.8.Fluorescence Analysis.Figure 8 shows the photoluminescence spectra of as-prepared samples.Excluding TiO 2 and M-TO-1.5, the as-prepared composites and g-C 3 N 4 exhibit similar profiles with a broad emission band from 350 to 600 nm under an excitation wavelength of 330 nm, with intensity correlated to g-C 3 N 4 content.These spectra imply that PL intensities of as-prepared composites are strongly dependant on recombination of electron-hole pairs in g-C 3 N 4 .The PL quenching was observed in g-C 3 N 4 /TiO 2 nanocomposites as the content of TiO 2 increases, probably due to the charge transfer occurring from g-C 3 N 4 to TiO 2 [11,15].

Proposed Photocatalytic Mechanism
The g-C 3 N 4 is formed by sp 2 hybridization between C and N atoms, which depicts the  conjugate structure.Presence of  g-C 3 N 4 benefits the dispersion of TiO 2 resulting in increased adsorption capacity.When the as-prepared composite is irradiated with visible light, the electrons get promoted from the valence band (VB) to the conduction band (CB) in g-C 3 N 4 (Figure 9), providing a route for electronic transition.The CB edge potential of g-C 3 N 4 is more negative than that of TiO 2 allowing the excited electron on the surface of g-C 3 N 4 to transfer easily to TiO 2 via the well-built heterojunction [22,23] (Figure 6(d)).The recombination of photogenerated charge was inhibited and photocatalytic activities were enhanced effectively [24] for TiO 2 which provides a site for electron translation.The previously mentioned characterization indicates that coupling TiO 2 with g-C 3 N 4 produces a well-contacted solid-solid heterojunction interface between g-C 3 N 4 and TiO 2 semiconductor particles, which promotes the effective interfacial electron transfer   [21,25] and separation of photogenerated charge carriers, significantly enhancing photocatalytic activity.

Conclusions
g-C 3 N 4 /TiO 2 nanocomposite samples were simplistically synthesized by thermal treatment of mixtures of industrial TiO 2 and melamine at different weight ratios.It was observed that TiO 2 particles were distributed on the layered g-C 3 N 4 formed in situ from melamine.TO-M-2.5, the g-C 3 N 4 /TiO 2 composite fabricated with a precursor ratio ( melamine :  TO ) of 2.5, exhibited the highest visible light catalytic activity.The introduction of g-C 3 N 4 increased visible light adsorption of the g-C 3 N 4 /TiO 2 nanocomposite.The enhanced visible light catalytic activity arises from the heterojunction between g-C 3 N 4 and TiO 2 which effectively enhances separation of photogenerated electron-hole pairs.The novel g-C 3 N 4 /TiO 2 nanocomposites prepared by this facile approach could have broad application in environmental protection.

4 Figure 8 :
Figure 8: PL spectra of samples fabricated with varying ratios of precursors, TiO 2, and g-C 3 N 4 .

Figure 9 :
Figure 9: Proposed mechanism of MB photodegradation on the g-C 3 N 4 /TiO 2 composites under visible light irradiation.

Table 1 :
Specific surface area, pore volume, and average pore radius.