Graphitic g-C₃N₄-WO₃ Composite: Synthesis and Photocatalytic Properties

Abstract

Graphitic g-$C_3N_4-WO_3$ composite was synthesized simply by decomposing melamine in the presence of $WO_3$ at $500^{\circ}C$. The obtained material was characterized by XRD, SEM, IR and XPS. The results showed that the as-prepared composite exhibits orthorhombic $WO_3$ phase coated by g-$C_3N_4$ and the g-$C_3N_4$ decomposed completely with N-doped $WO_3$ remaining at elevated calcination temperatures. The photocatalytic activity of the composite was evaluated by the photodegradation of methylene blue under visible light. An enhancement in photocatalytic activity for the graphitic g-$C_3N_4-WO_3$ composite compared to the conventional nitrogen-doped $WO_3$ was observed, which can be attributed to the presence of g-$C_3N_4$ in the material.

Introduction

Photodegradation of organic pollutants using semicon-ductors as heterogeneous catalysts has attracted much atten-tion of researchers because of its efficiency and promises of economy.1,2 Among the semiconductor materials, TiO2 and ZnO have been widely investigated.3,4 However, their techno-logical application seems limited because they only work under ultraviolet (UV) light. As an alternative semiconductor, WO3 with a band gap of ca. 2.7 eV can be capable of harvest-ing the blue part of the solar spectrum,5 and is photostable in acid media, which makes it useful in photocatalysis for wastewater treatment in the presence of organic acids.6 Therefore, tungsten oxide WO3 is of particular interest in photocatalysts in the past decades.7 However, it has some drawbacks in practical application because of its high solubility in water6 and poor photodegradation of organic compounds under O2 condition due to its conduction band edge with a position unfavorable for single-electron reduc-tion of O2.8 In order to overcome these limitations, a number of modifications such as size-controlling,9,10 noble metal deposition11,12 coupling with other semiconductors13-15 have been tried. For reducing the solubility in water, some strate-gies have been developed to prepare tungsten oxides in composites with carbon.16

Recently, graphitic carbon nitride (g-C3N4) has attracted much attention for application in photocatalytic water splitt-ing and degradation of organic pollutants under visible light.17,18 This material has some advantages such as band gap of 2.7 eV, production ability in large scale, non-toxicity. However, the pure g-C3N4 exhibits high recombination rate of its photogenerated electron-hole pair.19 To solve these drawbacks, several strategies have been applied to modify g-C3N4 such as preparation of g-C3N4 in mesostructure20 and combination of g-C3N4 with other materials by doping or grafting.21-37 The obtained materials showed an improve-ment in photocatalytic performance.

More recently, g-C3N4-WO3 composites with enhanced photocatalytic activity have been reported.38-40 However, in those works, the composites were prepared by mixing WO3 and g-C3N4 in the two separate forms. In this work, g-C3N4-WO3 composite was synthesized directly in which g-C3N4 formed in the presence of WO3. The presence of g-C3N4 in the composite and its effect in photocatalytic activity of the material were demonstrated.

 

Experimental

Chemicals. Tungsten trioxide (WO3), melamine (C3H6N6) and methylene blue (C16H18N3S) were purchased from Sigma Aldrich. All the chemicals were of reagent grade and used without further purification.

Synthesis. The samples were prepared by a simple method using commercial crystalline WO3 powder and melamine as raw materials. First, a mixture of WO3 and melamine with a weight ratio of 1:3 was well mixed and grinded with mortar. Then, the homogeneously mixed precursor was dried in an oven at 60 °C overnight. The as-prepared precursors were then transferred into alumina crucible, sealed with aluminum foil and calcined at different temperatures, 500, 600 and 700 °C, for 1 h under air atmosphere. The resulting samples were denoted as WM-500, WM-600 and W-700, corresponding to the heat-treatment temperatures, 500, 600 and 700 °C, respec-tively.

Characterization. Powder X-ray diffraction (PXRD) patterns were acquired using a Bruker diffractometer (D/max 2200), with Ni-filtered Cu Kα radiation (λ = 1.5418 Å) from the powder samples, which were placed on a glass substrate. The morphology and the size of the synthesized samples were characterized by scanning electron micro-scopy (SEM, JEOL JSM-600F). Infrared (IR) spectra for the samples were recorded on a Thermo Nicolet spectrometer. Diffuse-reflectance UV-vis spectra were investigated on a Sinco S-4100 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALab spectrometer (Thermo VG, U.K.) with monochromated Al-Kα radiation.

Photocatalytic Activity. In order to evaluate photocata-lytic activity, methylene blue (MB), C16H18N3S was selected as an organic pollutant. Into a 50 mg/L MB solution of 50 mL, the prepared sample of 0.05 g was dispersed under stirring and then the solution was kept in dark condition for 2 h. After then, the solution was irradiated by visible light source from a fluoresecent lamp with a filter cutting UV rays. The degradation of MB was monitored by taking the suspension at the irradiation time intervals, 1 h. Each the suspension was centrifuged to separate the catalyst from the MB solution. Subsequently, the degradation rate was cal-culated as a function of irradiation time from absorbance change at a wavelength of 664 nm using a UV-vis spectro-photometer (Jenway 6800).

 

Results and Discussion

Characterization of Catalysts. The X-ray diffraction patterns of the samples in Figure 1 show that as-prepared samples from thermal treatment of the mixture of WO3 and melamine at 500 °C, 600 °C and 700 °C contain mainly WO3 in orthorhombic phase. However, it is worth to note that a board peak with 2θ value at about 13°, which can be attributed to g-C3N438,39 for WM-500. The formation of g-C3N4 when heating melamine at temperature of about 500 °C was reported in several papers.39,40 Meanwhile, this peak is not observable for the WM-600 and WM-700, implying that the further decomposition of g-C3N4 with remaining WO3 phase occurs at elevated temperatures. This matches with the results in the papers.39,40

Figure 1.XRD patterns of WM-500 (a), WM-600 (b) and WM-700 (c).

Morphology of the samples was investigated by SEM. From Figure 2(d), it can be observed that WO3 precursor exhibits the smooth particles with size of approximately 100-200 nm. The particle size nearly maintains for WM-500, WM-600 and WM-700. However, the treated samples exhibit a certain degree of particle agglomeration. This may be caused by the decomposition of melamine on surface of WO3.

To further clarify the presence of g-C3N4, the materials were also characterized by IR and the results were presented in Figure 3. All materials have a board peak with strong intensity at about 820 cm−1 which is assigned to stretching vibration of the W-O-W.38,39 In addition to this peak, the spectrum for WM-500 has some additional peaks which are characteristic of g-C3N4. Indeed, peaks attributable to the vibration of the CN heterocycles can be clearly observed in the range of 1200–1650 cm−1.38,39 the characteristic breathing mode of the triazine units and the stretching vibration modes of NH were also observed at respectively around 809 cm−1 and 3160 cm−1.38,39 This observation supports strongly the presence of the g-C3N4 in WM-500.

Optical absorptions of WO3, WM-500, WM-600 and WM-700 were investigated by UV-vis diffuse reflectance spectra. Figure 4 shows that all samples exhibited absorbance in the visible light region. The absorption edges of the WM-500 and WO3 were located around 550 and 460 nm, respectively, while the absorption edge of WM-600 and WM-700 occurr-ed at 480 nm. This indicates a change in optical property of the materials during the thermal treatment with melamine.

Figure 2.SEM images of WM-500 (a), WM-600 (b), WM-700 (c) and WO3(d).

Figure 3.IR spectra of WM-500 (a), WM-600 (b), WM-700 (c) and WO3 (d).

Figure 4.UV-vis spectra of WM-500, WM-600, WM-700 and WO3.

The high-resolution XPS surface probe technique can further confirm the local structure of the g-C3N4 in the materials (Fig. 5). Figure 5(a) shows the presence of the peak at 284.7 eV corresponding to the referenced C 1s for the three materials. Besides this peak, WM-500 exhibits the most intense peak at 288.0 eV which may be attributed to the carbon in the C-N-C configurations of g-C3N4.41,42

Figure 5(b) presents a difference in position and intensity of N 1s peaks for the three samples. Generally, it can be seen that content of N on surface of WM-500 is significantly higher than that of WM-600 and WM-700 samples. For WM-500, the most intense peak at 398.7 eV is assigned to sp2 hybridized aromatic nitrogen atoms bonded to carbon atoms (C-N=C).43 The weak peak at 400.3 eV for WM-600 and WM-700 samples may come from W-N bond in the N-doped WO3.44 The XPS of the three samples in the O 1s binding energy regions were shown in Figure 5(c). A relatively low content of oxygen in WM-500 was observed.

Figure 5.XPS spectra of the materials: (a), (b), (c) are corre-sponding to WM-500, WM-600, WM-700, respectively, for all (a), (b), (c) and (d).

The intense peak at 530.7 eV for WM-600 and WM-700 samples can be attributed to O-W in WO3.44 Figure 5(d) depicts W 4f XPS of the samples. The relatively low content of tungsten in WM-500 was also observed. The binding energy values of W4f7/2 and W4f5/2 in the WM-600 and WM-700 are observed at 35.2 eV and 37.3 eV, which are slightly lower than those for pure WO3 (35.5 eV and 35.7 eV, respectively). Such a shift may be attributed to the presence of N-W in WM-600 and WM-700.38 The obtained results show clearly that in WM-500, WO3 is attached by a thin layer of g-C3N4 on the surface to form g-C3N4-WO3 composite; and for the samples treated 600 °C and 700 °C, instead of disappearing g-C3N4, the presence of doping N in WO3 were obtained. This may affect significantly their cata-lytic activity.

Photocatalytic Test. The photocatalytic activity of the samples was determined by the degradation of methylene blue in water under visible-light. Figure 6 shows the variation of methylene blue concentration (C/C0) with irradiation time on the three catalysts. It was reported that methylene blue can be easily absorbed on many materials and long time is required to reach adsorption equilibrium. Therefore, the adsorption of methylene blue on the materials by stirring in the dark for two hours was carried out. As seen in Figure 6, all materials can act as photocatalysts in the degradation of methylene blue under visible light. However, a difference in catalytic performance of the catalysts can be observed. On the WM-500 sample, the decrease in C/Co is much faster than for the other materials. On the WM-600 and WM-700 samples, the decrease in C/Co is slower under visible light. The good performance in photocatalytic activity for WM-500 may be due to the presence of g-C3N4 in this material. Graphitic g-C3N4 playing a key role in enhancement of photocatalytic activity for several composites was reported in the documents.38-40 The enhanced photocatalytic activity of g-C3N4-WO3 composites was discussed in several reports.38,39 It can be seen in Figure 7 that the valence and conduction band potential of the pure g-C3N4 are 1.57 eV and -1.13 eV, respectively.45 These values for pure WO3 are corresponding to 3.43 eV and 0.75 eV.37 The band gaps of g-C3N4 and WO3 are 2.70 eV and 2.68 eV, respectively. Therefore, the pure g-C3N4 and WO3 can produce photogenerated electron–hole pairs under visible light irradiation. In g-C3N4-WO3 com-posite, the photogenerated electrons on the conduction band of the g-C3N4 can directly inject into the conduction band of WO3 and the photogenerated holes on the valence band of WO3 can directly transfer to the valence band of g-C3N4. This phenomenon can lead to a significant decrease in the electron–hole recombination. As mentioned above, the pure g-C3N4 exhibits high recombination rate of its photogene-rated electron-hole pair.19 Hence, this may contribute to the enhancement of photocatalytic reactivity.

Figure 6.Photocatalytic activity of WM-500, WM-600, WM-700 toward the degradation of methylene blue.

To clarify further role of g-C3N4, we prepared WM-400 sample by heating the mixture of WO3 and melamine with a weight ratio of 1:3 at 400 °C. The photocatalytic activity in degradation of methylene blue for this sample was insigni-ficant (not shown). This may be attributed to no formation of g-C3N4 at this temperature. The preparation of g-C3N4 from melamine at temperature of 500 °C or higher was report-ed. 36,39,40

Figure 7.Proposed mechanism for the photodegradation of methylene blue on g-C3N4-WO3 composite.

 

Conclusion

A composite photocatalyst, WM-500, based on g-C3N4 coated WO3 was synthesized successfully by thermally decomposing melamine in the presence of WO3 at 500 °C The elevated calcination temperatures at 600 °C and 700 °C led to decomposition of the g-C3N4 and formation of N-doped WO3. The composite demonstrates a high activity for degradation of methylene blue in aqueous solution under visible light irradiation. The presence of g-C3N4 is believed to be beneficial for enhancement in photocatalytic activity of the g-C3N4-WO3 composite.