Nanoporous photocatalysts developed through heat-driven stacking of graphitic carbon nitride nanosheets

Abstract

Nanoporous graphitic carbon nitride was prepared through direct heat treatment of guanidinium cyanurate at 550–600 °C in an air atmosphere. The BET surface area of the resulting materials can reach 200 m² g⁻¹. High porosity was developed through a heat-driven stacking of g-C₃N₄ nanosheets. The mechanism was revealed in detail through TEM and N₂ adsorption measurements. Large-size g-C₃N₄ nanosheets are formed at 550 °C and stacked in a state similar to randomly creased paper slips. Further increase of treatment temperature to 600 °C results in curling and fragmentation of g-C₃N₄ nanosheets, which build up a highly porous matrix. Nanoporous graphitic carbon nitride with higher surface area exhibits better optical properties and has enhanced photocatalytic activity. The nanoporous g-C₃N₄ shows high photocatalytic activity for the decomposition of rhodamine (RhB) in an aqueous solution.

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1. Introduction

Photocatalysis has shown its importance in energy and environmental applications since the discovery of water photolysis over a TiO₂ electrode in 1972.¹ There are many types of photocatalysts such as metal oxides, sulfides, nitrides and phosphides.² Many efforts have been put into the development of efficient, stable, low-cost and environmentally benign photocatalysts. Graphitic carbon nitride (g-C₃N₄) is a metal-free organic semiconductor and active for H₂ evolution from water under visible light irradiation in the presence of a sacrificial donor.³ Graphitic-C₃N₄ has many special properties such as inexpensive precursors, facile preparation, non-toxicity, excellent stability and unique band structure.⁴

Graphitic-C₃N₄ is synthesized by condensation of precursors containing carbon and nitrogen, such as cyanamide, dicyanamide or melamine.⁵ Many strategies have been employed to enhance its catalytic performance, such as doping with heteroatoms,⁶ coupling with other semiconductors,⁷ coupling with carbon materials or polymers,⁸ surface modification,⁹ copolymerization¹⁰ and texture modification.¹¹

Two-dimensional nanosheets are expected to possess exceptional optical, electronic and mechanical properties and show application potential in sensors, electronics, catalysis, and energy conversion.¹² Graphitic-C₃N₄ is constituted by two-dimensional tri-s-triazine units connected with planar amino groups. Each layer is connected by weak van der Waals force, which is similar to graphite. Inspired by graphite and graphene, g-C₃N₄ nanosheets are synthesized to further develop its applications.¹³ Graphitic-C₃N₄ nanosheets of around 2 nm were prepared through thermal oxidation etching of bulk g-C₃N₄ in air.^13a Chemical exfoliation by H₂SO₄ is another effective method to synthesize g-C₃N₄ nanosheets.^13b Under controlled conditions, single layer g-C₃N₄ nanosheets with large S_BET and enhanced photogenerated charge carrier transport were obtained. Ultrasonic assistant liquid exfoliation has been commonly employed for the fabrication of inorganic nanosheets from their parent layered materials.¹⁴ Ultrathin g-C₃N₄ nanosheets have been successfully synthesized through this cost-effective route.^13c–k In addition to photocatalyst, the two-dimensional nanosheets prepared by liquid exfoliation also show promising applications in bioimaging, electrochemiluminescence detection, disinfection, metal ions detection and drug nanocarriers because of the unique optical and electronic properties. However, liquid exfoliation is time consuming and not suitable for large-scale applications.

Recently, urea, an oxygen-containing precursor, was used for the synthesis of g-C₃N₄.¹⁵ Gas bubbles stemmed from oxygen result in g-C₃N₄ with platelet-like or porous morphology. Other precursors with heteroatom can also be used for the synthesis of g-C₃N₄ with or without the assistant of templates.¹⁶ Owing to the peculiar nanostructures, large S_BET and enhanced optical and electronic properties, most of as-prepared g-C₃N₄ nanostructures exhibit enhanced catalytic activity.

In this work, guanidinium cyanurate (GC) was used as a novel precursor. Nanostructured g-C₃N₄ (CNG) was easily prepared through GC self-condensation under heat treatment during 450–600 °C in air atmosphere without the assistance of any template. The resultant materials had a stacking microstructure of g-C₃N₄ nanosheets. Self-condensation temperature is the key parameter to determine the stacking model of g-C₃N₄. Three typical stacking models were observed with the increase of treatment temperature. The BET surface area of as-synthesized materials at 600 °C nearly reaches 200 m² g⁻¹. Heat-driven stacking mechanism was detailedly revealed through TEM and N₂ adsorption measurement. The catalytic performance of g-C₃N₄ nanosheets was evaluated by rhodamine (RhB) degradation under light irritation, and compared with that of samples and prepared from melamine (CNM) and urea (CNU) under the same conditions.

2. Experimental

2.1 Preparation of catalysts

All the reagents were analytical-grade and used as received. Guanidinium cyanurate (GC) was synthesized according to the literature.¹⁷ CNG were synthesized by directly heating of GC in air. Typically, 5 g of GC was placed in a covered crucible and then heated to a certain temperature in muffle furnace for 4 h with a heating rate of 10 °C min⁻¹. For comparison, CNM and CNU were also synthesized from melamine and urea respectively at 550 °C under the same heating conditions.

2.2 Characterization and methods

The powder X-ray diffraction (XRD) measurements were performed with Rigaku Rotaflex diffractometer equipped with Cu Kα radiation source (40 kV, 200 mA; λ = 1.54056 Å). Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 by using KBr pellets. X-ray photoelectron spectroscopy (XPS) dates were obtained on a Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 instrument. The transmission electron microscopy (TEM) images were obtained on a JEOL JSM-2100 instrument with an accelerating voltage of 200 kV. The nitrogen adsorption–desorption experiments were performed at 77.3 K by a Quanta-chrome Autosorb Automated Gas Sorption System (Quantachrome Corporation). Before experiments, the samples were dried at 350 °C under vacuum for 6 h. The UV-visible absorption spectra were obtained using a UV-visible spectrometer (Shimadzu UV-2600) equipped with an integrating sphere assembly. BaSO₄ was used as reference. Photoluminescence spectra (PL) were recorded with a Hitachi F-4500 fluorescence spectrometer with excitation at 350 nm.

2.3 Photocatalytic activity test

The photocatalytic activities of the prepared g-C₃N₄ were evaluated by the degradation of 10 mg L⁻¹ RhB at room temperature under light irradiation. The light was provided by a 500 W xenon lamp. Typically, 150 mL of RhB aqueous solution and 20 mg g-C₃N₄ were mixed in a 250 mL quartz reactor. Prior to irradiation, the suspension was stirred in the dark for 1 h to ensure adsorption–desorption equilibrium. About 4 mL of suspension was sampled at 10 min interval. After centrifugation, the upper clear liquid was analyzed by UV-visible absorption spectra. The maximum absorption was recorded at 553 nm and used for evaluating the concentration of RhB.

The degradation rate and rate constant k are calculated by eqn (1) and (2) respectively as follows:

Degradation rate = (C₀ – C)/C₀ × 100% (1)

ln(C/C₀) = −kt (2)

where, C₀ is the adsorption–desorption equilibrium concentration of RhB and C is the concentration of RhB at reaction time of t.

3. Results and discussion

3.1 The development of g-C₃N₄ through self-condensation of guanidinium cyanurate

Fig. 1a shows powder X-ray diffraction (XRD) patterns of g-C₃N₄ from melamine (CNM) and guanidinium cyanurate (CNG). There are two typical peaks: one at 13.10° and another at 27.48°. The former is ascribed to tri-s-triazine units (100). The later belongs to interlayer-stacking structures (002). The intensity of CNG's (002) peak is markedly lower than that of CNM. This indicates that g-C₃N₄ nanosheets are stacked in a very loose pattern. Graphitic-C₃N₄ nanosheets from liquid exfoliation^13b,d show the same character. The as-synthesized CNG has the same FT-IR spectrum as CNM (Fig. 1b). The bands of 1200–1650 cm⁻¹ are ascribed to the typical stretching modes of C–N heterocycles. The band at 814 cm⁻¹ is ascribed to the breathing mode of triazine units. The bands between 3200 and 3500 cm⁻¹ are ascribed to the N–H stretching modes. It is clearly demonstrated that g-C₃N₄ can be obtained through self-condensation of guanidinium cyanurate and g-C₃N₄ nanosheets show a loose stacking model.

Fig. 1 (a) Powder X-ray diffraction (XRD) patterns of g-C₃N₄ from melamine (CNM) and guanidinium cyanurate (CNG). (b) FT-IR spectra of CNM and CNG. (c) Powder XRD patterns of g-C₃N₄ nanosheets under different treatment temperature. (d) Enlarged XRD peaks at 25–30°.

The development of g-C₃N₄ nanosheets under different treatment temperature was characterized by powder XRD. The sample treated at 400 °C shows two intense peaks at 10.58° and 27.78° that indicates the low polymerization degree (Fig. 1c). When the condensation temperature increased to 450 °C, two characteristic diffraction peaks (100) and (002) at 13.25° and 27.15° are detected, which suggest the incipient formation of g-C₃N₄. With the increasing heating temperature, the (100) peak tends to being weaker. This is caused by the decrease in thickness of g-C₃N₄ nanosheets.^13a,c The treatment temperature directly influences the interlayer-stacking structure. As shown in Fig. 1d, when treatment temperature increased from 500 °C to 550 °C, the (002) peak shifts from 27.15° to 27.48°. This indicates that layer-stacking structures are developed above 550 °C.

The chemical composition of CNG prepared at 550 °C was further characterized by XPS. XPS survey spectrum (Fig. 2a) shows that the main elements on the surface of are C, N. There is a small amount of oxygen, which is mainly attributed to the adsorbed water and carbon dioxide. The high-resolution XPS spectra of C1s can be fitted into two peaks. The peak centered at 288.1 eV is attributed to tri-s-triazine units in lattice, and the peak centered at 284.9 eV is attributed to the pure carbon stemmed from the combustion of nitrogen in air atmosphere.^11c,13a The content of pure carbon is increased with the increasing heating temperature because nitrogen tends to be burned easily at a high temperature. The content of pure carbon is 35% when the heating temperature is 450 °C, and it increases to 55% when the heating temperature is 550 °C (Fig. 2b and d). Fitting of N1s envelope results in identification of two peaks: one at 398.6 eV and another at 400.1 eV. The former is indexed to tri-s-triazine units in lattice and the latter is attributed to nitrogen atoms at edge of g-C₃N₄ nanosheets.

Fig. 2 (a) XPS survey spectrum of CNG prepared at 550 °C. (b) C1s XPS peaks of CNG prepared at 550 °C. (c) N1s XPS peaks of CNG prepared at 550 °C. (d) C1s XPS peaks of CNG prepared at 450 °C.

3.2 Porosity properties and heat-driven stacking of g-C₃N₄ nanosheets

Nitrogen adsorption–desorption measurements are preformed to investigate porosity properties of GC-derived g-C₃N₄ prepared at different temperature. Fig. 3a shows isotherm curves of the samples prepared at 450 °C, 500 °C, 550 °C and 600 °C. With the increase of treatment temperature, there is a marked change in porosity of the as-synthesized materials. The samples prepared at 450 °C and 500 °C have poor porosity. But the solids treated under 550 °C and 600 °C show improved porosity. Hysteresis loops appear when P/P₀ > 0.7, which indicates the occurrence of open pore matrixes. As revealed by BET surface area calculated from the isotherm curves (Fig. 3b), the values are below 50 m² g⁻¹ for the samples at 450 °C and 500 °C and sharply increase to 133.8 m² g⁻¹ when temperature reaches 550 °C. Very high BET surface area of 194.7 m² g⁻¹ is achieved under treatment temperature of 600 °C. Pore volume also exhibits a markedly increasing curve with treatment temperature (Fig. 3c). However, average pore size (calculated based on BJH desorption model) shows an unexpected fluctuation with the increase of treatment temperature (Fig. 3d). The value sharply increases to 8.45 nm at 550 °C and then decreases to 1.53 nm at 600 °C. There may be a microstructure transform mechanism for this heat-driven development of porosity.

Fig. 3 (a) Nitrogen adsorption–desorption isotherm curves of CNGs prepare at different temperature. (b) BET surface area of CNG, CNU, and CNM. (c) Pore volume of CNGs prepare at different temperature. (d) Average pore size CNGs prepare at different temperature.

TEM measurements are performed to investigate this heat-driven microstructure transform mechanism. The materials prepared at 450 °C have microstructures of thick stacking layers (Fig. 4a). When treated at 500 °C, sub-structures are developed in the stacking layers. Honeycombed layers are observed (Fig. 4b). When treatment temperature is increased to 550 °C, very thin g-C₃N₄ nanosheets are formed and their sizes reach 1.0 μm × 1.0 μm (Fig. 4c and d). Large g-C₃N₄ nanosheets are stacked in a state similar to randomly creased paper slips. These microstructures lead to high porosity of the bulk materials, which has been demonstrated by nitrogen adsorption–desorption measurements. BET surface area values of the samples at 450 °C and 500 °C are below 50 m² g⁻¹, but when temperature reaches 550 °C, it sharply increases to 133.8 m² g⁻¹. As revealed by Fig. 3c, g-C₃N₄ nanosheets show large two-dimension size and pore systems formed by a randomly stacking and creasing model have relatively large average pore size. The calculated pore size is 8.45 nm, but the value is approximately 1.8 nm for the materials treated at 500 °C (Fig. 3d). Fig. 4d shows that g-C₃N₄ nanosheets have one to three layers and their thickness is in the range from 0.6 nm to 1.5 nm. Further increase of treatment temperature to 600 °C results in curl and fragmentation of g-C₃N₄ nanosheets (Fig. 4e and f). The curled and fragmented g-C₃N₄ nanosheets build up a highly porous matrix, which has BET surface area of 194.7 m² g⁻¹. It is inevitable that curl and fragmentation of g-C₃N₄ nanosheets directly lead to decrease of average pore size. The value is 1.53 nm at 600 °C. It is clearly revealed that heat-driven transform of g-C₃N₄ nanosheets' microstructures determines the development of porosity properties.

Fig. 4 TEM images of CNG at 450 °C (a), 500 °C (b), 550 °C (c and d), and 600 °C (e and f).

The morphological structures of the CNG prepared at different treatment temperatures are further characterized by scanning electron microscope (SEM). As shown in Fig. 5a and b, the materials prepared at 450 °C and 500 °C consist of thick layer-like structures that are packed in a random model. After being treated at 550 °C, uniform g-C₃N₄ nanosheets are achieved and their thicknesses are sharply reduced (Fig. 5c). This is in accordance with the information revealed by TEM images (Fig. 4c and d). Further treatment at 600 °C lead to curl and fragmentation of g-C₃N₄ nanosheets (Fig. 5d). This trend is also demonstrated by TEM measurements (Fig. 4e and f). Treatment at high temperature in air directly leads to over-oxidation of g-C₃N₄ nanosheets and then fragments those microstructures. With the decrease thickness of g-C₃N₄ nanosheets under high temperature, the edges of nanosheets tend to curl and then reduce the large surface energy in nanometre scale.

Fig. 5 SEM images of CNG at 450 °C (a), 500 °C (b), 550 °C (c), and 600 °C (d).

3.3 The optical properties of porous g-C₃N₄

The UV-visible absorption spectra are used to investigate the optical absorption of g-C₃N₄ nanosheets. Fig. 6a shows that both CNM and CNG have strong UV and visible light absorption. The absorption edge of CNG shows an obvious blue shift from that of CNM. The band gap is 2.74 eV for CNM and the value increases to 2.87 eV for CNG (Fig. 6b). This trend has been found in other g-C₃N₄ nanosheets and nanoparticles.^13a,c,d,18 With the increase of heating temperature from 450 to 600 °C, the absorption edge of CNG exhibits a continual blue shift. The optical absorption of the samples is further improved.¹⁹

Fig. 6 (a) UV-visible absorption spectra. (b) Plots of (ahv)² versus energy (hv) of CNM and CNG. (c) PL spectra of CNG obtained at different temperatures.

The photoluminescence (PL) spectra of CNG were also recorded. As presented in Fig. 6c, all samples exhibit a broad emission peak. The emission peak shows a slight blue shift from 449 nm to 470 nm when heating temperature increases from 450 °C to 600 °C. PL intensity decreases gradually with the increase of heating temperature. This is attributed to the enhanced quantum confinement effect and the increased surface carbon content.^8b,20 The thickness of g-C₃N₄ nanosheets decreases sharply with the increase of treatment temperature. Fragmentation and stacking are gradually developed and attributed to the blue shift of PL spectra. The decrease of PL intensity implies that efficiency for the separation of charge carriers is improved,²¹ and beneficial for the photocatalytic performance.

3.4 The photocatalytic activity and stability of porous g-C₃N₄

The photocatalytic activity of porous g-C₃N₄ nanosheets is evaluated by the degradation of RhB under light irritation. The catalysts prepared from melamine (CNM) and urea (CNU) are tested under the same reaction conditions. Before reaction, the reaction mixtures are stirred in dark for 1 h to ensure that the adsorption equilibrium is established. The adsorption capacity of the g-C₃N₄ increases with their S_BET surface area and the amount is less than 15%. The degradation rate is calculated after the effect of adsorption has been eliminated. Direct degradation of RhB was negligible without the presence of any catalyst.²² As shown in Fig. 7a, RhB can be degraded gradually under light irritation after CNM is added. The degradation rate is 22% after 60 min reaction. The catalytic activity of CNG and CNU are much higher than that of CNM. For CNU, 84% of RhB is degraded. CNG prepared at 550 °C can degrade 98% of RhB. The high photocatalytic activity of CNG is related to its large BET surface area. As shown in Fig. 3b, the S_BET of CNM and CNU are 8.0 m² g⁻¹ and 62.8 m² g⁻¹. The values are below 50 m² g⁻¹ for the samples at 450 °C and 500 °C and sharply increase to 133.8 m² g⁻¹ when temperature reaches 550 °C. Very high BET surface area of 194.7 m² g⁻¹ is achieved under treatment temperature of 600 °C. The catalysts with high porosity show improved photoactivity. High porosity provides more available catalytic active sites and benefits for diffusion of the reactants. Another important factor related to photocatalytic activity is improvement of optical properties with the development of g-C₃N₄ nanosheets with the increase of heating temperature. With the increase of heating temperature from 450 °C to 600 °C, the optical absorption of the samples is further improved. PL intensity decreases gradually with the increase of heating temperature. The decrease of PL intensity implies that efficiency for the separation of charge carriers is improved, and beneficial for the photocatalytic performance.

Fig. 7 (a) Photocatalytic activity for RhB degradation. (b) Reaction rate constants of RhB degradation over different catalysts. (c) Recyclability of CNG obtained at 550 °C for RhB degradation.

Reaction rate constants of RhB degradation over different catalysts are calculated based on the first-order model (Fig. 7b). The degradation rate constant for CNM is 0.0041 min⁻¹ and it increases to 0.031 for CNU. The largest rate constant of 0.055 min⁻¹ is achieved when the reaction is catalyzed by CNG prepared at 600 °C. The value is 13.41 and 1.78 times as high as those of CNM and CNU. The high stability is a special advantage of g-C₃N₄ as photocatalysts.^4e,23 A five-run recycling test of RhB degradation is performed over CNG prepared at 550. The catalysts are separated by centrifugation and directly recycled into the next batch. The degradation rate of RhB has not decreased significantly during the five-run test. It dropped slightly from 98% in the first run to 91% in the fifth run. Graphitic-C₃N₄ nanosheets show excellent stability and recyclability for catalyzing RhB degradation.

4. Conclusions

In summary, nanoporous g-C₃N₄ materials have been successfully prepared through self-condensation of guanidinium cyanurate (GC) under heat treatment during 550–600 °C in air atmosphere without the assistance of any template. Porosity of these materials is developed through heat-driven stacking of g-C₃N₄ nanosheets. The mechanism is detailedly revealed through TEM and N₂ adsorption measurement. The BET surface area of as-synthesized materials at 600 °C nearly reaches 200 m² g⁻¹. The photocatalytic activity of g-C₃N₄ nanosheets is evaluated by rhodamine (RhB) degradation under light irritation, and compared with that of samples and prepared from melamine (CNM) and urea (CNU) under the same conditions. The samples with higher surface area exhibit better optical properties and have enhanced photocatalytic activity.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21304101, 21101161, 21174148).

Notes and references

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