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 (hν = 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].
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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.
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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.
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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.
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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 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].
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).