Facile transformation of low cost thiourea into nitrogen-rich graphitic carbon nitride nanocatalyst with high visible light photocatalytic performance

Abstract

Polymeric nitrogen-rich graphitic carbon nitride (g-C₃N₄-T) nanocatalyst was synthesized for the first time with a facile approach by directly treating low cost thiourea in air. The g-C₃N₄-T demonstrated much higher visible light photocatalytic activity than that of g-C₃N₄ obtained from toxic dicyandiamide, C-doped TiO₂ and P25.

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Challenges of environmental pollution and energy shortage faced by human beings have raised awareness of a potential global crisis. Among the various technologies to tackle the challenges, semiconductor photocatalysis has emerged as one of the most promising technologies because it can effectively utilize the abundant energy of either natural sunlight or artificial indoor illumination.¹ The mostly applied traditional TiO₂ photocatalyst can only utilize UV light (4–5% of the total solar energy) due to its wide band gap (3.0–3.2 eV).² In recent years, intensive efforts have been devoted to modifying TiO₂, including nonmetal doping, hetero-junction construction and quantum dots or dye sensitization in order to make it visible light active.³ Some complex metal oxides as new visible light photocatalytic materials including tungstates, niobates, tantalates, vandates and carbonates have been successfully developed recently.⁴ A number of sulfides, nitrides and oxynitrides have also been synthesized as alternative materials for efficient visible light photocatalysis.⁵

Very recently, novel metal-free polymeric graphitic carbon nitride (g-C₃N₄) prepared from cyanamide was identified as a high performance photocatalyst for the production of hydrogen from water and degradation of organic pollutants under visible light irradiation.⁶ The g-C₃N₄ with a narrow band gap (2.7 eV) could utilize visible light directly without modification. The search for suitable precursors for better synthesis of g-C₃N₄ is a key issue. In general, the commonly used precursors are nitrogen-rich compounds containing pre-bonded triple or double C–N core structure, such as cyanamide, dicyandiamide, trithiocyanuric acid, melamine, triazine and heptazine derivatives.⁷ Some of the precursor compounds are difficult to obtain and expensive, and some of them are unstable and even highly explosive. Zou et al. transformed urea into g-C₃N₄ over mesoporous TiO₂ spheres followed by HF etching.⁸ The precursor urea, a nitrogen-rich compound, is cheap and easily available.⁸ However, such a process to prepare g-C₃N₄ with the assistance of pre-synthesized TiO₂ is complicated and use of HF is not environmentally benign.

Thus, it is highly desirable to develop a facile and environmental benign approach to synthesize g-C₃N₄ from cheap and nontoxic precursors for large scale applications.^3b We have recently developed an efficient synthesis method for g-C₃N₄ by directly heating urea without employing additional assistance.⁹ There is, however, still a challenge to explore other suitable green precursors to synthesize g-C₃N₄. The present work reports the first facile synthesis of polymeric nitrogen-rich g-C₃N₄ nanostructure by direct treating thiourea at 550 °C for 2 h, where thiourea is a common and low cost raw material in chemical industry substituting the widely used toxic or unstable precursors. The as-prepared ordered g-C₃N₄ from thiourea exhibited high visible light photocatalytic activity for degradation of organic pollutant compared with g-C₃N₄ prepared from toxic dicyandiamide. The high activity can be ascribed to the increased visible light absorbance and promoted charge separation. As this method is facile, economic and environmental benign, it is highly attractive and ready for large scale environmental and energetic applications.

The XRD patterns of the samples prepared from thiourea and dicyandiamide (Fig. 1a) can be indexed to g-C₃N₄. The strong interplanar stacking peak of aromatic systems at around 27.5° is indexed to (002) peak.^6a The peak at 13.0° corresponding to in-plane ordering of tri-s-triazine units with a period of 0.675 nm which form 1D melon strands.⁹Fig. 1b further implies that the characteristic (002) peak shifts from 27.50 for g-C₃N₄-T to 27.75 for g-C₃N₄-D, indicating the average interlayer distance of g-C₃N₄-T (d = 0.326 nm) is slightly larger than that of g-C₃N₄-D (d = 0.321 nm). The (002) peak of g-C₃N₄-T is wider and lower than g-C₃N₄-D. These results suggests that g-C₃N₄-T sample has less degree of condensation and smaller domain size.^6b

Fig. 1 XRD pattern of g-C₃N₄-T and g-C₃N₄-D (a) and enlarged view of (002) peak (b).

Wang et al. prepared g-C₃N₄ photocatalyst from cyanamide.^6a The triple C–N bond in cyanamide played a key role in the formation of g-C₃N₄.^6,7 In the present work, the chemical structure of thiourea doesn’t contain a triple C–N bond, which implies that the formation mechanism of g-C₃N₄ from thiourea is totally different from that of cyanamide. However, the fundamental chemical reactions involving the formation mechanism of g-C₃N₄ from thiourea is rather complicated. A systematic investigation is currently underway to elucidate the chemical mechanism and it will be reported in future work.

The detailed discussion on XPS can be found in ESI (Fig. S1†). Fig. S1d† shows that the atomic ratio of hydroxyl groups to the total three kinds of oxygen contributions is 42.4% for g-C₃N₄-T, much higher than that of g-C₃N₄-D (23.3%). The increase in surface hydroxyl content is advantageous for trapping more photo-generated holes and thus preventing electron hole recombination.¹⁰ The atomic ratios of C : N : O of g-C₃N₄-T and g-C₃N₄-D was determined to be 1 : 1.40 :  0.021 and 1 : 1.37 : 0.021 from XPS, respectively. The nitrogen content is higher than the stoichiometry in C₃N₄, suggesting the g-C₃N₄-T and g-C₃N₄-D samples are rich in nitrogen. The excess nitrogen can be attributed to uncondensed amino groups during the condensation of thiourea.^7h

The SEM image of g-C₃N₄-D in Fig. S2a† shows that some irregular stacks consisting of layers and particles are formed. The high magnification in Fig. S2b† indicates that some nanoparticles are attached on the layers with a thickness of 10 nm. For the g-C₃N₄-T sample, large amounts of layers with different sizes are stacked together (Fig. S3†). The curved layers are smooth and interconnected with pores of different sizes (Fig. 2a). The thickness of the layers is about 15 nm. The structural morphology was further investigated by TEM. Fig. S4a and S4b† show that g-C₃N₄-D are composed of numerous randomly organized nanosheets and nanoparticles with irregular shape. The nanosheets are very thin and transparent to the electron beam. The inset in Fig. S4a† shows the corresponding SAED pattern with clear diffraction rings. Observing carefully at the TEM images for g-C₃N₄-T (Fig. 2b), one can see the smooth and flat layers with ordered structure. The ordered non-crystalline structure goes consistently with the XRD pattern. The SAED in Fig. 2b indicates that diffraction rings of g-C₃N₄-T are not as distinct as that of g-C₃N₄-D, consistent with the widened (002) peak in Fig. 1b.

Fig. 2 SEM (a) and TEM images of g-C₃N₄-T (b), inset in (b) shows SAED image.

Fig. S5a† shows that the isotherms of g-C₃N₄-D are close to type IV and the isotherms of g-C₃N₄-T can be classified as type II. The corresponding pore-size distribution curves (Fig. S5b†) indicate the presence of mesopores for the two g-C₃N₄ samples, which can be ascribed to the aggregation of nanosheets or particles. The mesoporous structure is proved to facilitate the transportation of reactants and products and to favor the harvesting of photo-energy (See detailed discussion following Fig. S5†).¹¹

The UV-vis DRS of g-C₃N₄ obtained from thiourea and dicyandiamide are shown in Fig. 3a. The g-C₃N₄-T exhibits stronger visible light absorption in the range of 450 to 700 nm than that of g-C₃N₄-D. The band gap energy (E_g) estimated from the intercept of the tangents to the plots of (αhν)^1/2vs. photon energy are 2.46 and 2.58 eV for g-C₃N₄-T and g-C₃N₄-D, respectively (Fig. 3b), which are smaller than the literature value (2.7 eV). The enhanced visible light absorbance and reduced band gap of the g-C₃N₄-T obtained from thiourea can be attributed to the enriched nitrogen content, which modified the band structure. Generally, the photocatalytic activity of a semiconductor is proportional to (I_αΦ)ⁿ, where I_α is the photo numbers absorbed by photocatalyst per second and Φ is the efficiency of the band gap transition.⁹ The increased visible light absorbance could contribute to the increase of I_αΦ, thus enhancing the photocatalytic activity.

Fig. 3 UV-vis DRS (a) and plots of (αhν)^1/2vs. photon energy (b) of g-C₃N₄-T and g-C₃N₄-D.

Fig. S6† shows the room temperature PL spectra of g-C₃N₄-T and g-C₃N₄-D. The PL intensity of g-C₃N₄-D is much higher than that of g-C₃N₄-T, implying that the separation of photogenerated electrons and holes is enhanced on g-C₃N₄-T. It was found that 2D nanostructures favored the transfer of electrons and holes on the crystal surface and promoted the charge separation.¹² Thus, the 2D layered nanostructure (Fig. 2a) and increased surface hydroxyl content (Fig. S1d†) of the g-C₃N₄-T facilitate the separation of charge carriers, thus contributing to the lower PL intensity.

The precursors employed to synthesize g-C₃N₄ semiconductor photocatalyst usually contain triple or double C–N bonds. Wang et al. prepared g-C₃N₄ by condensation of trithiocyanuric acid which exhibited a moderate rate of photocatalytic H₂ evolution.^7c Li et al. fabricated g-C₃N₄ by directly heating helamine and the as-prepared g-C₃N₄ showed good performance in photodegradation of organic pollutant.^7d Domen synthesized a carbon nitride structure for visible-light catalysis by copolymerization of dicyandiamide and barbituric acid.^7e The results indicate that g-C₃N₄ is an excellent visible light driven photocatalyst.

Fig. 4a shows adsorption property and visible light photocatalytic performance of g-C₃N₄-T, g-C₃N₄-D, C-doped TiO₂ and P25 for degradation of contaminant RhB. Visible photocatalytic degradation of organic dyes was well documented. The adsorption–desorption equilibrium was reached within 60 min. The amounts of RhB adsorbed by the g-C₃N₄-T with low S_BET is highest, indicating that g-C₃N₄-T has a strong power for RhB adsorption. When there is no photocatalyst involved with visible light irradiation, the RhB is stable, indicating that self-degradation was negligible. The RhB undergoes pronounced obvious decomposition in the presence of photocatalysts under visible light irradiation. P25 exhibits decent visible light activity with a removal ratio of 18.6% due to the existence of the rutile phase. The band gap of rutile TiO₂ is 3.0 eV and can be excited by visible light with wavelengths shorter than 413 nm. Although the C-doped TiO₂ has a higher S_BET and a larger pore volume than that of g-C₃N₄-T and g-C₃N₄-D, the visible light activities of g-C₃N₄ samples are even higher than that of the high-performance C-doped TiO₂ (Table S1†), which implies that g-C₃N₄ is an efficient photocatalyst for degradation of organic pollutants.^6a,9 Furthermore, the g-C₃N₄-T prepared from thiourea exhibits much more efficient photocatalytic activity (removal ratio of 70.9% and initial rate constant of 0.0031 min⁻¹) than that of g-C₃N₄ prepared from dicyandiamide (removal ratio of 49.3% and rate constant of 0.0018 min⁻¹). The enhanced visible light absorbance (Fig. 3a) and promoted charge separation (Fig. S6†) of g-C₃N₄-T directly contributed to the outstanding visible light photocatalytic activity.^10,11 These facts also suggest that thiourea is a better precursor than toxic dicyandiamide.

Fig. 4 The adsorption property and visible light photocatalytic performance of g-C₃N₄-T, g-C₃N₄-D and C-doped TiO₂ (a) and repeated photocatalytic activities (b) for degradation of RhB.

To test the stability of g-C₃N₄-T on photocatalytic RhB degradation, multiple runs of photocatalysis are carried out (Fig. 4b). It is interesting to find that the novel g-C₃N₄-T could maintain efficient and durable visible light photocatalytic activities after five cycles of repeated runs with no obvious deactivation. The results suggest that g-C₃N₄-T photocatalyst is both stable and efficient, which is important for practical applications. For the first time, thiourea was facilely transformed to g-C₃N₄ polymer materials with high visible light activity and stability. The as-prepared g-C₃N₄ materials can also be applied in solar energy conversion and fine chemicals synthesis.^6a,7e As the low-cost thiourea precursor is easily available and the synthesis method is simple, it is extremely attractive and valuable for large scale environmental and energetic applications.

Polymeric nitrogen-rich g-C₃N₄ layered nanostructure was synthesized with a facile and environmental benign approach by directly treating low-cost thiourea in air at 550 °C. Thiourea is a better precursor for the synthesis of g-C₃N₄ polymer than toxic dicyandiamide. The curved thin smooth layers of g-C₃N₄-T were interconnected, leading to the formation of mesoporous structure. The as-prepared polymeric g-C₃N₄ from thiourea exhibited high photocatalytic activity and stability under visible light irradiation, exceeding that of g-C₃N₄-D, C-doped TiO₂ and P25. The enhanced visible light absorbance due to enriched nitrogen and promoted charge separation due to the 2D layered nanostructure and abundant surface hydroxyl groups are responsible for the outstanding photocatalytic performance of g-C₃N₄-T. This work demonstrates a highly valuable facile method to synthesize high-performance g-C₃N₄ polymeric photocatalysts from easy-available thiourea for large scale environmental and energetic applications.

Acknowledgements

This research is supported by the NSFC (51108487), “863 Program” of China (2010AA064905), the Program for Talented Young Teachers (Chongqing, 2011), Chongqing Education Commission (KJTD201020, KJZH11214) and the key discipline development project of CTBU (1252001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Fig. S1–S6. Further discussions on XPS results in Fig. S1 and pore structure in Fig. S4. See DOI: 10.1039/c2cy20049j

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