Journal of Nanomaterials

Volume 2014 (2014), Article ID 869094, 8 pages

http://dx.doi.org/10.1155/2014/869094

Research Article

Growth of g-C ₃N ₄ Layer on Commercial TiO ₂ for Enhanced Visible Light Photocatalytic Activity

Min Fu, Jiazhen Liao, Fan Dong, Hongmei Li, and Hongyan Liu

Chongqing Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China

Received 17 January 2014; Accepted 2 March 2014; Published 30 March 2014

Academic Editor: Haiqiang Wang

Copyright © 2014 Min Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Novel visible light photocatalytic graphitic carbon nitride/TiO₂ (g-C₃N₄/TiO₂) composite samples were synthesized by heating mixtures of melamine and commercial TiO₂(TO) at different weight ratios. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (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-C₃N₄/TiO₂. At the optimized precursor weight ratio (), 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-C₃N₄ transfer easily to the conduction band (CB) of TiO₂ via the well-built heterojunction. The g-C₃N₄/TiO₂ nanocomposites exhibit enhanced visible light catalytic activity due to increased visible light adsorption and effective separation of photogenerated electron-hole pairs. These g-C₃N₄/TiO₂ nanocomposites could find broad applicability in environmental protection due to their excellent visible light photocatalytic property and facile, cost-effective preparation process.

1. 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]. TiO₂ has been proved to be a competent photocatalyst for environmental applications; however, due to the wide band gap of anatase TiO₂ (3.2 eV), it cannot be excited under visible light irradiation ( nm) [5]. Therefore, there is much interest in developing visible light-responsive TiO₂ with catalytic activity high enough for practical applications.

Recently, graphite-like carbon nitride (g-C₃N₄) 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-C₃N₄ 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-C₃N₄ with several precursors [8–11]. Ma et al. reported a strategy for enhancing the photoactivity of g-C₃N₄ via doping of nonmetal elements [9].

Recently, Yan and Yang reported a TiO₂-g-C₃N₄ composite material for photocatalytic H₂ evolution under visible light irradiation [2]. The TiO₂-g-C₃N₄ composites were prepared by thermal treatment of the mixture of TiO₂ and g-C₃N₄. Yang et al. reported that N-doped TiO₂/g-C₃N₄ composites with enhanced daylight photocatalytic properties were prepared by heating titanium tetrachloride ethanol solutions with C₃N₄ [10]. However, commercial g-C₃N₄ is not readily available and easily obtainable; inexpensive precursors for g-C₃N₄ need to be used requiring tedious preparation which may prevent large-scale application. Fu et al. reported a solid-state approach to synthesizing g-C₃N₄ coated TiO₂ nanocomposites from urea and commercial TiO₂ precursors [12]; however, the preparation conditions and proposed mechanism for enhancing photocatalytic activity of g-C₃N₄/TiO₂ require further research.

In the present work, all raw materials were purchased from commercial sources. The g-C₃N₄/TiO₂ composite photocatalysts were prepared by a facile solid-phase method of directly heating melamine and commercial TiO₂ 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-ring-based radicals and NHx were formed as a result of the decomposition [11]. Based on XPS, FTIR, and TEM results, it was confirmed that g-C₃N₄ existed in prepared composites. Photocatalytic efficiency of the as-prepared g-C₃N₄ coated TiO₂ nanocomposites was determined by degradation of methylene blue (MB) dye under visible light irradiation.

2. Experimental

2.1. Synthesis of Photocatalysts

g-C₃N₄-TiO₂ photocatalysts with various mass ratios ( :  = 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 TiO₂ (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-C₃N₄-TiO₂ photocatalyst. The g-C₃N₄ 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 TiO₂, respectively. Finally, precursors with the same mass ratios ( :  = 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.

2.2. 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-Kα 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). UV-vis diffuse reflection spectra were taken on a scan UV-vis 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.

2.3. 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 NaNO₂ solution (1 mol/L) to block UV light ( 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 adsorption-desorption 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.

3. Results and Discussion

3.1. XRD

Figure 1 depicts the XRD patterns of the as-prepared, mass ratio differentiated photocatalysts alongside g-C₃N₄ and TiO₂. The g-C₃N₄ 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-C₃N₄, respectively. The peak intensity at 27.5° corresponding to g-C₃N₄ increases with increasing weight ratio of melamine, which confirms that g-C₃N₄ has been produced in the as-prepared samples and the yield of it also gradually increased. For all g-C₃N₄/TiO₂ composites and TiO₂, characteristic anatase TiO₂ peaks with similar position, intensities and widths were detected. This implies that the phase structure and the crystallite size of TiO₂ particles in as-prepared samples are conserved.

Figure 1: XRD patterns for g-C₃N₄/TiO₂ composites fabricated from varying ratios of precursors, g-C₃N₄, and TiO₂.

3.2. Visible Light Catalytic Activity

The degradation of MB is carried out under visible light ( nm) illumination in aqueous sample suspensions. Figure 2(a) shows degradation of MB in the presence of samples fabricated from varying precursor ratios ( :  = 1.5, 2.5, 3.5, and 4.5). Contrastingly, TO-520-5 and M-520-5 were prepared from single precursors by heating TiO₂ and melamine, respectively, under analogous reaction conditions. The photocatalytic activity of g-C₃N₄/TiO₂ nanocomposites first increases and then decreases with the increasing melamine additive. The excess g-C₃N₄ prevented pollutants from being in contact with TiO₂. As shown in Figure 2(a), during adsorption and the 5 hours of visible light photocatalytic degradation, TO-M-2.5 exhibits the highest visible light catalytic activity of all compounds including TiO₂ (TO) and g-C₃N₄. The degradation rate (%) can be calculated as

where 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 TiO₂ and g-C₃N₄ produce a synergistic effect and effectively enhance the visible light photocatalytic efficiency.

(a)

(b)

Figure 2: Evolution of MB with photodegradation time for g-C₃N₄, TiO₂ (TO), M-520-5, TO-520-5, and composite photocatalysts (TO-M-1.5, -2.5, -3.5, and -4.5) (a). Evolution of MB with photodegradation time for composite photocatalysts prepared by different temperatures (TO-M-320, -420, -520, and -620) (b).

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.

3.3. Microstructure Analysis

The BET surface areas and porous structures of TiO₂ 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 H₂ hysteresis loop in the range (/) of 0.6–1.0. The surface area and pore volume (Table 1) of pure TiO₂ are 81.63 m²/g and 0.221 cm²/g, higher than those of the TO-M-2.5 (50.57 m²/g, 0.154 cm²/g). Compared with TiO₂, the specific surface area of TO-M-2.5 is reduced, potentially due to the presence of g-C₃N₄ layers on the TiO₂ surface. However, the surface area of g-C₃N₄/TiO₂ is not a positive factor. The enhanced visible light activity of g-C₃N₄/TiO₂ should be ascribed to the enhanced visible light adsorption and dispersion of TiO₂ resulting in increased adsorption capacity due to the presence of g-C₃N₄ (Figures 6(c) and 7).

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

Sample
Specific surface area (m²/g)
Pore volume (cm²/g)
Average pore radius (nm)

TiO₂
81.63
0.221
5.41

TO-M-2.5
50.57
0.154
6.09

M-520-5
26.09
0.161
12.35

(a)

(b)

Figure 3: Adsorption-desorption isotherm of TiO₂ and TO-M-2.5 (a) and pore size distribution (b).

3.4. FTIR

The FTIR spectra (Figure 4) of TO-M-2.5, TO-M-3.5, and TO-M-4.5 were similar to that of g-C₃N₄, particularly in three regions: 805 cm⁻¹, 1200~1650 cm⁻¹, and 3100 cm⁻¹~3400 cm⁻¹. The peaks at 805 cm⁻¹ and 1200~1650 cm⁻¹, in the spectra of TO-M-2.5, TO-M-3.5, and TO-M-4.5, are assigned to the characteristic stretching modes of g-C₃N₄, indicating the existence of g-C₃N₄. The band at 805 cm⁻¹ corresponds to the characteristic breathing mode of the triazine units [8, 13]. 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-C₃N₄ in the sample.

(a)

(b)

Figure 4: FTIR spectra for g-C₃N₄/TiO₂ composites fabricated from varying precursors ratios ( :  = 1.5, 2.5, 3.5, and 4.5), g-C₃N₄, and TiO₂ ranging from 400 to 2000 (a) and from 2000 to 4000 (b).

3.5. XPS

The elemental chemical states of TO-M-2.5 were detected by XPS as shown in Figure 5 and were consistent with the FTIR results. The C 1s peaks (Figure 5(a)) at 284.8 eV and 288.1 eV were assigned to the C–C bond [15, 18] and the sp³ 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 2p3/2 and Ti 2p1/2 (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 TiO₂. By the analysis of XPS, it can be ensured that the g-C₃N₄ coated TiO₂ composites were prepared by heating mixture of melamine and TiO₂.

(a)

(b)

(c)

(d)

Figure 5: N 1s (a), C 1s (b), O 1s (c), and Ti 2p (d) XPS spectra of TO-M-2.5.

(a)

(b)

(c)

(d)

(e)

Figure 6: TEM images of pure TiO₂ (a), g-C₃N₄ (b), and TO-M-2.5 (c, d, and e).

(a)

(b)

Figure 7: UV-vis DRS (a) of g-C₃N₄, TiO₂, TO-M-4.5, TO-M-3.5, and TO-M-2.5 and image of band gap of TiO₂ and g-C₃N₄ (b).

3.6. TEM

Figure 6(a) shows the TEM images of pure TiO₂, consisting of nanoparticles with ca. 10 nm radius, consistent with the size of TiO₂ from microstructure analysis (Table 1). Figure 6(d) shows clear lattice fringes for the identification of crystallographic spacing of ca. 0.35 nm, matching well with an anatase TiO₂ (101) plane. The TEM and HRTEM images of TO-M-2.5 samples (Figures 6(b), 6(d), and 6(e)) also indicate that partial surface of TiO₂ was covered with a secondary phase and in Figures 6(b) and 6(d), it is clearly shown that TiO₂ is well distributed on a layered structure. The XPS and FTIR have confirmed the existence of g-C₃N₄. As the g-C₃N₄ is a semicrystallized material, it is hard to obtain the clear lattice fringe. However, by comparison of the morphology of g-C₃N₄ (Figure 6(c)) and TO-M-2.5, it can be concluded that the secondary phase in TO-M-2.5 is g-C₃N₄. In Figure 6(d), g-C₃N₄/TiO₂ (TO-M-2.5) displayed a connection between TiO₂ and g-C₃N₄, indicating that the formation of heterojunction was possible.

3.7. UV-Vis

Figure 7(a) shows the UV-vis DRS spectra of pure TiO₂, g-C₃N₄, and g-C₃N₄/TiO₂ nanocomposites. Visible light absorption of composites is enhanced by the presence of g-C₃N₄; 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 TiO₂, which can promote effective interfacial electron transfer [21] and conduce efficient space separation of photogenerated electron-hole pairs of g-C₃N₄. The band gaps of TiO₂ and g-C₃N₄ are showed in Figure 7(b), which are 3.2 eV and 2.7 eV, respectively.

3.8. Fluorescence Analysis

Figure 8 shows the photoluminescence spectra of as-prepared samples. Excluding TiO₂ and M-TO-1.5, the as-prepared composites and g-C₃N₄ 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-C₃N₄ content. These spectra imply that PL intensities of as-prepared composites are strongly dependant on recombination of electron-hole pairs in g-C₃N₄. The PL quenching was observed in g-C₃N₄/TiO₂ nanocomposites as the content of TiO₂ increases, probably due to the charge transfer occurring from g-C₃N₄ to TiO₂ [11, 15].

Figure 8: PL spectra of samples fabricated with varying ratios of precursors, TiO2, and g-C₃N₄.

4. Proposed Photocatalytic Mechanism

The g-C₃N₄ is formed by sp² hybridization between C and N atoms, which depicts the π conjugate structure. Presence of g-C₃N₄ benefits the dispersion of TiO₂ 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-C₃N₄ (Figure 9), providing a route for electronic transition. The CB edge potential of g-C₃N₄ is more negative than that of TiO₂ allowing the excited electron on the surface of g-C₃N₄ to transfer easily to TiO₂ 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 TiO₂ which provides a site for electron translation. The previously mentioned characterization indicates that coupling TiO₂ with g-C₃N₄ produces a well-contacted solid-solid heterojunction interface between g-C₃N₄ and TiO₂ semiconductor particles, which promotes the effective interfacial electron transfer [21, 25] and separation of photogenerated charge carriers, significantly enhancing photocatalytic activity.

Figure 9: Proposed mechanism of MB photodegradation on the g-C₃N₄/TiO₂ composites under visible light irradiation.

5. Conclusions

g-C₃N₄/TiO₂ nanocomposite samples were simplistically synthesized by thermal treatment of mixtures of industrial TiO₂ and melamine at different weight ratios. It was observed that TiO₂ particles were distributed on the layered g-C₃N₄ formed in situ from melamine. TO-M-2.5, the g-C₃N₄/TiO₂ composite fabricated with a precursor ratio ( : ) of 2.5, exhibited the highest visible light catalytic activity. The introduction of g-C₃N₄ increased visible light adsorption of the g-C₃N₄/TiO₂ nanocomposite. The enhanced visible light catalytic activity arises from the heterojunction between g-C₃N₄ and TiO₂ which effectively enhances separation of photogenerated electron-hole pairs. The novel g-C₃N₄/TiO₂ nanocomposites prepared by this facile approach could have broad application in environmental protection.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research is financially supported by the Science and Technology Project of CQ CSTC (stc2013yykfB50008, cstc2013jcyjA20018), the Science and Technology Project from Chongqing Education Commission (KJ120713, KJ130725), and the Innovative Research Team Development Program in University of Chongqing (KJTD201314).

References

W. Haiqiang, W. Zhongbiao, and L. Yue, “A simple two-step template approach for preparing carbon-doped mesoporous hollow microspheres,” Journal of Physical Chemistry C, vol. 113, no. 30, pp. 13317–13324, 2009.

H. Yan and H. Yang, “ composite materials for photocatalytic evolution under visible light irradiation,” Journal of Alloys and Compounds, vol. 509, no. 4, pp. L26–L29, 2011.

L. Wang, X. Fu, Y. Han, et al., “Preparation, characterization, and photocatalytic activity of /ZnO nanocomposites,” Journal of Nanomaterials, vol. 2013, Article ID 321459, 6 pages, 2013.

C. Xue, Q. Zhang, J. Li, et al., “High photocatalytic activity of functional particles with core-shell structure,” Journal of Nanomaterials, vol. 2013, Article ID 762423, 8 pages, 2013.

H. Wang, S. Zhou, L. Xiao, Y. Wang, Y. Liu, and Z. Wu, “Titania nanotubes—a unique photocatalyst and adsorbent for elemental mercury removal,” Catalysis Today, vol. 175, no. 1, pp. 202–208, 2011.

D. C. Cameron, “Optical and electronic properties of carbon nitride,” Surface and Coatings Technology, vol. 169-170, pp. 245–250, 2003.

X. Wang, K. Maeda, A. Thomas et al., “A metal-free polymeric photocatalyst for hydrogen production from water under visible light,” Nature Materials, vol. 8, no. 1, pp. 76–80, 2009.

F. Dong, L. Wu, Y. Sun, M. Fu, Z. Wu, and S. C. Lee, “Efficient synthesis of polymeric g- layered materials as novel efficient visible light driven photocatalysts,” Journal of Materials Chemistry, vol. 21, no. 39, pp. 15171–15174, 2011.

X. Ma, Y. Lv, J. Xu, et al., “A strategy of enhancing the photoactivity of g- via doping of nonmetal elements: a first-principles study,” The Journal of Physical Chemistry, vol. 116, no. 44, pp. 23485–23493, 2012.

N. Yang, G. Li, W. Wang, X. Yang, and W. F. Zhang, “Photophysical and enhanced daylight photocatalytic properties of N-doped composites,” Journal of Physics and Chemistry of Solids, vol. 72, no. 11, pp. 1319–1324, 2011.

X. Li, J. Zhang, L. Shen et al., “Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine,” Applied Physics A: Materials Science and Processing, vol. 94, no. 2, pp. 387–392, 2009.

M. Fu, J. Pi, F. Dong, Q. Duan, and H. Guo, “A cost-effective solid-state approach to synthesize g-C₃N₄ coated TiO₂ nanocomposites with enhanced visible light photocatalytic activity,” International Journal of Photoenergy, vol. 2013, Article ID 158496, 7 pages, 2013.

S. C. Yan, Z. S. Li, and Z. G. Zou, “Photodegradation performance of g- fabricated by directly heating melamine,” Langmuir, vol. 25, no. 17, pp. 10397–10401, 2009.

C. Miranda, H. Mansilla, et al., “Improved photocatalytic activity of g- composites prepared by a simple impregnation method,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 253, pp. 16–21, 2013.

Q. Lv, C. Cao, C. Li et al., “Formation of crystalline carbon nitride powder by a mild solvothermal method,” Journal of Materials Chemistry, vol. 13, no. 6, pp. 1241–1243, 2003.

Y. Qiu and L. Gao, “Chemical synthesis of turbostratic carbon nitride, containing C-N crystallites, at atmospheric pressure,” Chemical Communications, vol. 9, no. 18, pp. 2378–2379, 2003.

C. Li, X. Yang, B. Yang, Y. Yan, and Y. Qian, “Synthesis and characterization of nitrogen-rich graphitic carbon nitride,” Materials Chemistry and Physics, vol. 103, no. 2-3, pp. 427–432, 2007.

X. Zhou, B. Jin, L. Li, et al., “A carbon nitride/ nanotube array heterojunction visible-light photocatalyst: synthesis, characterization, and photoelectrochemical properties,” Journal of Materials Chemistry, vol. 22, pp. 17900–17905, 2012.

C. Li, X. Yang, B. Yang, Y. Yan, and Y. Qian, “Synthesis and characterization of nitrogen-rich graphitic carbon nitride,” Materials Chemistry and Physics, vol. 103, no. 2-3, pp. 427–432, 2007.

Q. Y. Li, J. W. Zhang, Z. S. Jin, et al., “A novel with a large amount of bulk intrinsic defects-visible-light-responded photocatalytic activity,” Chinese Science Bulletin, vol. 11, no. 58, pp. 1007–1013, 2013.

G. Li and K. A. Gray, “The solid-solid interface: explaining the high and unique photocatalytic reactivity of -based nanocomposite materials,” Chemical Physics, vol. 339, no. 1–3, pp. 173–187, 2007.

S. C. Yan, S. B. Lv, Z. S. Li, and Z. G. Zou, “Organic-inorganic composite photocatalyst of g- and TaON with improved visible light photocatalytic activities,” Dalton Transactions, vol. 39, no. 6, pp. 1488–1491, 2010.

J. Yua, S. Wang, J. Low, and W. Xiao, “Enhanced photocatalytic performance of direct Z-scheme g- photocatalyst for decomposition of formaldehyde in air,” Physical Chemistry Chemical Physics, vol. 15, no. 39, pp. 16883–16890, 2013.

K. Sridharan, E. Jang, and T. J. Park, “Novel visible light active graphitic composite photocatalyst: synergistic synthesis, growth and photocatalytic treatment of hazardous pollutants,” Applied Catalysis B: Environmental, vol. 142-143, pp. 718–728, 2013.

K. Kondo, N. Murakami, C. Ye, et al., “Development of highly efficient sulfur-doped photocatalysts hybridized with graphitic carbon nitride,” Applied Catalysis B: Environmental, vol. 142-143, pp. 362–367, 2013.