International Journal of Photoenergy

Volume 2013 (2013), Article ID 158496, 7 pages

http://dx.doi.org/10.1155/2013/158496

Research Article

A Cost-Effective Solid-State Approach to Synthesize g-C₃N₄ Coated TiO₂ Nanocomposites with Enhanced Visible Light Photocatalytic Activity

Min Fu, Junmin Pi, Fan Dong, Qiuyan Duan, and Huan Guo

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

Received 22 June 2013; Accepted 2 September 2013

Academic Editor: Guisheng Li

Copyright © 2013 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 graphitic carbon nitride (g-C₃N₄) coated TiO₂ nanocomposites were prepared by a facile and cost-effective solid-state method by thermal treatment of the mixture of urea and commercial TiO₂. Because the C₃N₄ was dispersed and coated on the TiO₂ nanoparticles, the as-prepared g-C₃N₄/TiO₂ nanocomposites showed enhanced absorption and photocatalytic properties in visible light region. The as-prepared g-C₃N₄ coated TiO₂ nanocomposites under 450°C exhibited efficient visible light photocatalytic activity for degradation of aqueous MB due to the increased visible light absorption and enhanced MB adsorption. The g-C₃N₄ coated TiO₂ nanocomposites would have wide applications in both environmental remediation and solar energy conversion.

1. Introduction

Visible light photocatalysis has attracted the worldwide attention due to its potential application in environmental remediation and solar energy conversion [1–7]. The photocatalyst TiO₂, however, can only utilize the ultraviolet light (about 5% of natural solar light) because of its wide band gap (ca. 3.2 eV for anatase TiO₂). During the past 40 years, many efforts have been devoted to enhance the visible light photocatalytic activity of TiO₂, including metal doping [8–10], nonmetal doping [11–14], surface modification [15], and heterojunction construction [16–19].

In recent years, polymeric g-C₃N₄ materials have attracted much attention because of their similarity to graphene. Zhang et al. reported that the polymeric g-C₃N₄ semiconductors exhibit high photocatalytic performance for water splitting under visible light irradiation [20]. Dong and coworkes reported that polymeric g-C₃N₄ layered materials as novel efficient visible light photocatalyst, which can be synthesized facilely by directly heating urea or thiourea [21, 22].

Very recently, Zhou et al. reported a g-C₃N₄/TiO₂ nanotube array heterojunction with excellent visible light photocatalytic activity [17]. Zhao et al. reported g-C₃N₄/TiO₂ hybrids with wide absorption wavelength and effective photogenerated charge separation [18]. However, the precursors for g-C₃N₄ (dicyandiamide and melamine) are poisonous and detrimental to the environment. The preparation processes were relatively tedious, which may prevent large-scale application [17, 18].

In the present work, g-C₃N₄/TiO₂ nanocomposites were prepared by a facile and cost-effective solid-state method using urea and commercial TiO₂ as precursors. It was interesting to find that g-C₃N₄ was in situ coated on the surface of TiO₂. The precursors (urea and commercial TiO₂) are low cost and easily available. The as-prepared g-C₃N₄ coated TiO₂ nanocomposites exhibited enhanced photocatalytic activity under visible light irradiation.

2. Experimental

2.1. Synthesis

The g-C₃N₄ coated TiO₂ nanocomposites were prepared by a facile and cost-effective solid-state method. In a typical synthesis, 2 g TiO₂ and 6 g urea were immersed in 10 mL H₂O and dried at 60°C to completely remove the water. The mixtures were put into an alumina crucible with a cover, and then heated to a certain temperature in the range of 400 and 600°C in a muffle furnace for 1 h at a heating rate of 15°C min⁻¹. The final samples were collected for use without further treatment.

2.2. Characterization

The crystal phases of the sample were analyzed by X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Rigaku Co., Japan). The morphology and structure of the samples were examined by transmission electron microscopy (TEM: JEM-2010, Japan). The UV-vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV-Vis spectrophotometer (UV-Vis DRS: UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly, using BaSO₄ as reflectance sample. The spectra were recorded at room temperature in air range from 250 to 800 nm. X-ray photoelectron spectroscopy with Al Kα X-rays (h ν = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. The nitrogen adsorption-desorption isotherms were determined by the BET method (BET-BJH: ASAP 2020, USA), from which the surface area, pore volume, and average pore diameter were calculated by using the BJH method. All the samples were degassed at 200°C prior to measurements.

2.3. Evaluation of Photocatalytic Activity

Photocatalytic activity of g-C₃N₄/TiO₂ for MB photodegradation was evaluated in a quartz glass reactor. 0.05 g of N-TiO₂ was dispersed in MB aqueous solution (50 mL, 5 mg/L). The light irradiation system contains a 500 W Xe lamp with a jacket filled with flowing and thermostated aqueous NaNO₂ solution (1 M) between the lamp and the reaction chamber as a filter to block UV light ( nm) and eliminate the temperature effect. The suspension was first allowed to reach adsorption-desorption equilibrium with continuous stirring for 60 min in the dark prior to irradiation. The degradation rate of MB was evaluated using the UV-Vis absorption spectra to measure the peak value of a maximum absorption of MB solution. During the irradiation, 5 mL of suspension was continually taken from the reaction cell at given time intervals for subsequent dye concentration analysis after centrifugation. The MB solution shows a similar pH value at 6.8, which does not affect the light absorption of MB. The maximum absorption of MB is at wavelength of 665 nm. The degradation rate η (%) can be calculated as

where is the initial concentration of MB considering MB adsorption on the catalyst and is the revised concentration after irradiation.

3. Results and Discussion

Figure 1 shows the XRD patterns of the as-prepared g-C₃N₄ coated TiO₂ nanocomposites at different temperatures. The peaks of all the samples can be indexed to the anatase phase of TiO₂ (JCPDS file No. 21-1272). It can be seen that the peak intensity increases gradually under higher treatment temperature, which indicates that the crystal sizes of TiO₂ nanocomposites increase under higher treatment temperature. No typical peaks of g-C₃N₄ can be found for all the samples due to the fact that g-C₃N₄ with layered structures on the surface of TiO₂ is ultrathin (Figure 2) and the crystallinity is low [22].

Figure 1: XRD patterns of the C₃N₄/TiO₂ nanocomposites obtained under different temperatures.

(a)

(b)

Figure 2: TEM images of pure TiO₂ (a) and g-C₃N₄/TiO₂ nanocomposites sample obtained under 450°C (b).

The morphology of pure TiO₂ and g-C₃N₄/TiO₂ nanocomposites were observed by TEM. As shown in Figure 2, both samples contain a number of monodispersed nanoparticles of TiO₂ with a size of about 11 nm. The intra-aggregation of particles could form the mesoporous structure [23]. It can be seen from Figure 2(b) that the ultrathin g-C₃N₄ with layered structures are dispersed and coated on the surface of TiO₂ particles, which is consistent with absence of the peaks of g-C₃N₄ in XRD (Figure 1).

The FT-IR spectra of pure TiO₂ and g-C₃N₄ coated TiO₂ nanocomposites are shown in Figure 3(a). The absorption band around 400–800 cm⁻¹ is attributed to Ti–O bonds [23]. Several bands in the range of 1100–1650 cm⁻¹ correspond to the typical stretching vibration of CN heterocycles in g-C₃N₄. The characteristic vibration mode of triazine units can also be found at 801 cm⁻¹ [22]. The peak at 1630 cm⁻¹ is associated with the stretching vibration of water molecules for both samples, including molecular water and hydroxyl groups [23]. The FT-IR spectra further confirm the existence of g-C₃N₄ on the surface of TiO₂.

(a)

(b)

Figure 3: FTIR spectra of pure TiO₂ and g-C₃N₄/TiO₂ nanocomposites (a) and TG-DSC for heating the mixture of TiO₂ and urea (b).

The TG and DSC thermograms (Figure 3(b)) show that there are several phase transformations during heating. An endothermal peak at 135°C is the melting point of urea. The peak at 242°C indicates the reaction of urea into melamine. The weight loss during the two stages decreases rapidly by 36.1%. The sharp peak at 367°C implies that the thermal condensation of melamine into g-C₃N₄ occurred in this temperature range. The weight loss in this stage is about 26.6%. The further weight loss of 4.8% with endothermal peak at 520°C can be attributed to the decomposition of g-C₃N₄. The TG-DSC result implies that g-C₃N₄ can be in situ formed on the surface of TiO₂, which is consistent with Figure 2(b).

The C1s spectra in Figure 4(a) show that two main carbon species with binding energies of 284.9 and 288.1 eV, corresponding to C–C and C–N–C, respectively. Three binding energies in N1s region (Figure 4(b)) can be observed, which can be indexed to C–N–C (398.8 eV), N–(C)₃ (400.1 eV), and N–H groups (401.2 eV), respectively. The binding energy at 529.7 and 533.0 eV can be ascribed to Ti–O, surface hydroxyl groups, and adsorbed molecular water (Figures 4(c) and 4(d)) [22]. The XPS results are consistent with the FT-IR spectra. XPS results also indicate that no peak for Ti–C or Ti–N bond can be observed, which implies that there is no chemical bond connection between g-C₃N₄ and TiO₂.

(a)

(b)

(c)

(d)

Figure 4: XPS spectra of the as-prepared C₃N₄ coated TiO₂ nanocomposite under 450°C.

The nitrogen adsorption-desorption isotherms of pure TiO₂ and g-C₃N₄/TiO₂ nanocomposites obtained under 450°C are shown in Figure 5(a). The two samples show a type IV adsorption isotherm with a H₂ hysteresis loop in the range () of 0.6–1.0, which indicates the presence of mesopores. The surface areas and pore volume of pure TiO₂ are 78 m²/g and 0.281 cm³/g, higher than those of g-C₃N₄/TiO₂ nanocomposites (48 m²/g and 0.216 cm³/g). The pore size distribution curve (Figure 5(b)) indicates that the large mesopores of pure TiO₂ and g-C₃N₄/TiO₂ nanocomposites are about 37 and 48 nm, respectively. The presence of large mesopores can be ascribed to the aggregation of TiO₂ particles. It can be observed that the g-C₃N₄/TiO₂ nanocomposites have small mesopores of around 13.6 nm (inset in Figure 5(b)), which originates from the presence of layered g-C₃N₄ on the TiO₂ surface. The small mesopore is advantageous for enhancing the adsorption for reactant.

(a)

(b)

Figure 5: BET-BJH of the pure TiO₂ and C₃N₄ coated TiO₂ nanocomposite obtained under 450°C.

Figure 6 shows the UV-Vis DRS spectra of pure TiO₂ and the as-prepared g-C₃N₄/TiO₂ nanocomposites. It is clear that the visible light absorption of g-C₃N₄/TiO₂ nanocomposites is enhanced with increased treatment temperatures until 450°C. Then the visible light absorption decreases when the temperature is higher than 450°C. This fact implies that the as-prepared g-C₃N₄ coated TiO₂ nanocomposites under 450°C may exhibit excellent visible light photocatalytic activity. However, the decrease of visible light absorption intensity of coated TiO₂ nanocomposites under higher treatment temperature can be attributed to the decomposition of g-C₃N₄.

Figure 6: UV-Vis DRS of the pure TiO₂ and g-C₃N₄/TiO₂ samples obtained under different temperatures.

Figure 7 shows the adsorption and photocatalytic activity of pure TiO₂ and g-C₃N₄/TiO₂ nanocomposites for removal of MB. It can be seen that the g-C₃N₄/TiO₂ nanocomposites obtained under 450°C exhibit the highest adsorption capacity, which may be ascribed to presence of layered g-C₃N₄ and small mesopores of the nanocomposites sample. The photocatalytic activities of g-C₃N₄/TiO₂ nanocomposites first increase and then decrease with the increased treatment temperature. Pure TiO₂ shows low visible light activity due to its large band gap. The observed slight visible light activity for the pure TiO₂ sample can be ascribed to the photosensitization effect of the MB as MB can absorb visible light [18]. During the visible light irradiation, the part of MB was self-decomposed due to the photosensitization. When TiO₂ was coated by g-C₃N₄, all the nanocomposite samples show decent visible light activity. Under visible light irradiation, g-C₃N₄ with a band gap of 2.7 eV could be excited and the photogenerated electrons could transfer from the conduction band (CB) of g-C₃N₄ to the CB of TiO₂ [17, 18, 24]. The holes in the valence band (VB) of g-C₃N₄ and electrons on the CB of TiO₂ could initiate the following degradation reactions. The as-prepared g-C₃N₄/TiO₂ nanocomposites under 450°C exhibit the highest photocatalytic activity under visible light irradiation. Considering the fact that the surface area of g-C₃N₄/TiO₂ nanocomposites (48 m²/g) is lower than that of pure TiO₂ (78 m²/g), 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 because of the presence of g-C₃N₄ (Figures 2(b) and 6) and the improved MB adsorption because of the small mesopores of the nanocomposites sample (Figure 5(b)). As the precursors (urea and commercial TiO₂) are cheap and preparation method is very simple, the as-prepared g-C₃N₄ coated TiO₂ nanocomposites are ready for large-scale applications in environmental pollution control and solar energy conversion [25].

Figure 7: Adsorption and photocatalytic activity of the pure TiO₂ and g-C₃N₄/TiO₂ samples obtained under different temperatures for removal of MB.

4. Conclusion

The g-C₃N₄/TiO₂ nanocomposites were synthesized by a cost-effective solid-state approach by thermal treatment of the mixture of urea and commercial TiO₂. It was found that the surface of TiO₂ particles was coated by the in situ formed thin layered g-C₃N₄ from urea. The adsorption capacity and visible light photocatalytic activity were significantly enhanced. Under the optimized treatment temperature of 450°C, the g-C₃N₄/TiO₂ nanocomposites exhibited highest adsorption capacity and visible light photocatalytic activity toward removal of MB. The enhanced adsorption capacity can be ascribed to the presence of g-C₃N₄ and small mesopores. The enhanced visible photocatalytic activity originated from the increased visible light adsorption and small mesopores of the nanocomposites sample. The novel g-C₃N₄ coated TiO₂ nanocomposites prepared by the cost-effective solid-state approach would find wide application in environmental remediation.

Conflict of Interests

The authors declare no conflict of interests.

Acknowledgments

This research is financially supported by the Key Project from CQ CSTC (cstc2013yykfB50008), the Science and Technology Project from Chongqing Education Commission (KJZH11214, KJ120713, KJTD201314, KJTD201020, KJ130725, and KJ090727), the Key Discipline Development Project of CTBU (1252001), the National Natural Science Foundation of China (51108487), and the Natural Science Foundation Project of CQ CSTC (cstc2012jjA20014, CSTC2010BB0260).

References

J. Zhang, J. Sun, K. Maeda et al., “Sulfur-mediated synthesis of carbon nitride: band-gap engineering and improved functions for photocatalysis,” Energy and Environmental Science, vol. 4, no. 3, pp. 675–678, 2011.

Y. J. Cui, Z. X. Ding, X. Z. Fu, and X. C. Wang, “Construction of conjugated carbon nitride nanoarchitectures in solution at low temperatures for photoredox catalysis,” Angewandte Chemie—International Edition, vol. 51, no. 47, pp. 11814–11818, 2012.

Z. Z. Lin and X. C. Wang, “Nanostructure engineering and doping of conjugated carbon nitride semiconductors for hydrogen photosynthesis,” Angewandte Chemie—International Edition, vol. 52, no. 6, pp. 1735–1738, 2013.

Q. Y. Chen and D. Ma, “Preparation of nanostructured Cu₂SnS₃ photocatalysts by solvothermal method,” International Journal of Photoenergy, vol. 2013, Article ID 593420, 5 pages, 2013.

N. Todorova, T. Giannakopoulou, G. Romanos, T. Vaimakis, J. Yu, and C. Trapalis, “Preparation of fluorine-doped TiO₂ photocatalysts with controlled crystalline structure,” International Journal of Photoenergy, vol. 2008, Article ID 534038, 9 pages, 2008.

Q. Xiang, J. Yu, and M. Jaroniec, “Graphene-based semiconductor photocatalysts,” Chemical Society Reviews, vol. 41, no. 2, pp. 782–796, 2012.

X. Li, P. Zhang, L. Jin, T. Shao, Z. Li, and J. Cao, “Efficient photocatalytic decomposition of perfluorooctanoic acid by indium oxide and its mechanism,” Environmental Science and Technology, vol. 46, no. 10, pp. 5528–5534, 2012.

Z. Wu, Z. Sheng, H. Wang, and Y. Liu, “Relationship between Pd oxidation states on TiO₂ and the photocatalytic oxidation behaviors of nitric oxide,” Chemosphere, vol. 77, no. 2, pp. 264–268, 2009.

M. Fu, Y. Li, S. Wu, P. Lu, J. Liu, and F. Dong, “Sol-gel preparation and enhanced photocatalytic performance of Cu-doped ZnO nanoparticles,” Applied Surface Science, vol. 258, no. 4, pp. 1587–1591, 2011.

X. W. Cheng, X. J. Yu, B. Y. Li, L. Yan, Z. P. Xing, and J. J. Li, “Enhanced visible light activity and mechanism of TiO₂ codoped with molybdenum and nitrogen,” Materials Science and Engineering B, vol. 178, no. 7, pp. 425–430, 2013.

Z. H. Ai, Z. T. Gao, K. Su, W. K. Ho, and L. Z. Zhang, “Aerosol flow synthesis of N, Si-codoped TiO₂ hollow microspheres with enhanced visible-light driven photocatalytic performance,” Catalysis Communications, vol. 29, no. 5, pp. 189–193, 2012.

F. Dong, W. Zhao, and Z. Wu, “Characterization and photocatalytic activities of C, N and S co-doped TiO₂ with 1D nanostructure prepared by the nano-confinement effect,” Nanotechnology, vol. 19, no. 36, Article ID 365607, 2008.

P. Periyat, D. E. McCormack, S. J. Hinder, and S. C. Pillai, “One-pot synthesis of anionic (nitrogen) and cationic (sulfur) codoped high-temperature stable, visible light active, anatase photocatalysts,” Journal of Physical Chemistry C, vol. 113, no. 8, pp. 3246–3253, 2009.

H. Wang, Z. B. Wu, and Y. Liu, “A simple two-step template approach for preparing carbon-doped mesoporous TiO₂ hollow microspheres,” Journal of Physical Chemistry C, vol. 113, no. 30, pp. 13317–13324, 2009.

F. Chen, W. Zou, W. Qu, and J. Zhang, “Photocatalytic performance of a visible light TiO₂ photocatalyst prepared by a surface chemical modification process,” Catalysis Communications, vol. 10, no. 11, pp. 1510–1513, 2009.

F. Dong, Y. J. Sun, and M. Fu, “Enhanced visible light photocatalytic activity of V₂O₅ cluster modified N-doped TiO₂ for degradation of toluene in air,” International Journal of Photoenergy, vol. 2012, Article ID 569716, 10 pages, 2012.

X. S. Zhou, B. Jin, L. D. Li et al., “A carbon nitride/TiO₂ nanotube array heterojunction visible-light photocatalyst: synthesis, characterization, and photoelectrochemical properties,” Journal of Materials Chemistry, vol. 22, no. 34, pp. 17900–179055, 2012.

S. S. Zhao, S. Chen, H. T. Yu, and X. Quan, “g-C₃N₄/TiO₂ hybrid photocatalyst with wide absorption wavelength range and effective photogenerated charge separation,” Separation and Purification Technology, vol. 99, no. 8, pp. 50–54, 2012.

J. G. Yu, S. H. Wang, B. Cheng, Z. Lin, and F. Huang, “Noble metal-free Ni(OH)₂-g-C₃N₄ composite photocatalyst with enhanced visible-light photocatalytic H₂-production activity,” Catalysis Science Technology, vol. 3, no. 7, pp. 1782–1789, 2013.

J. S. Zhang, M. W. Zhang, R. Q. Sun, and X. C. Wang, “A facile band alignment of polymeric carbon nitride semiconductors to construct isotype heterojunctions,” Angewandte Chemie—International Edition, vol. 51, no. 40, pp. 10145–10149, 2012.

F. Dong, L. Wu, Y. Sun, M. Fu, Z. Wu, and S. C. Lee, “Efficient synthesis of polymeric g-C₃N₄ layered materials as novel efficient visible light driven photocatalysts,” Journal of Materials Chemistry, vol. 21, no. 39, pp. 15171–15174, 2011.

F. Dong, Y. J. Sun, L. W. Wu, M. Fu, and Z. B. Wu, “Facile transformation of low cost thiourea into nitrogen-rich graphitic carbon nitride nanocatalyst with high visible light photocatalytic performance,” Catalysis Science and Technology, vol. 2, no. 7, pp. 1332–1335, 2012.

F. Dong, S. Guo, H. Wang, X. Li, and Z. Wu, “Enhancement of the visible light photocatalytic activity of C-doped TiO₂ nanomaterials prepared by a green synthetic approach,” Journal of Physical Chemistry C, vol. 115, no. 27, pp. 13285–13292, 2011.

S. K. Choi, S. Kim, S. K. Lim, and H. Park, “Photocatalytic comparison of TiO₂ nanoparticles and electrospun TiO₂ nanofibers: effects of mesoporosity and interparticle charge transfer,” Journal of Physical Chemistry C, vol. 114, no. 39, pp. 16475–16480, 2010.

W. D. Zhang, Q. Zhang, F. Dong, and Z. W. Zhao, “The multiple effects of precursors on the properties of polymeric carbon nitride,” International Journal of Photoenergy, vol. 2013, Article ID 685038, 9 pages, 2013.