Fabrication of carbon nitride nanotubes by a simple water-induced morphological transformation process and their efficient visible-light photocatalytic activity

Abstract

Carbon nitride nanotubes (C₃N₄ NTs) were synthesized based on the nanosheets roll-up mechanism by a simple water-induced morphological transformation process using graphitic carbon nitride (g-C₃N₄) as a precursor. Water was used as the phase-transfer reagent, making the preparation process environmentally friendly. The visible-light photocatalytic activity of the as-prepared C₃N₄ NTs significantly increased compared to bulk g-C₃N₄ and g-C₃N₄ nanosheets toward rhodamine B degradation and hydrogen evolution from water-splitting. This result can be attributed to the high photogenerated carrier transfer efficiency, excellent mass transfer capability, sufficient active sites, and enhanced light utilization efficiency of C₃N₄ NTs.

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1. Introduction

The visible-light photocatalytic activity of g-C₃N₄ for hydrogen production was discovered by Wang et al.¹ Since then, the application of g-C₃N₄ as an efficient organic semiconductor photocatalyst or chemical catalyst for hydrogen evolution, organic pollutant degradation, and oxidation reaction has been widely developed.^2,3 The g-C₃N₄ can be prepared through the polycondensation of various organic precursors, such as urea, cyanamide, and melamine, at high temperatures and exhibits a graphite-like interlayer structure.^4–6 High-temperature calcination determines the layered bulk structure of g-C₃N₄. The bulk structure limits the catalytic activity of g-C₃N₄ because of the small number of active sites, rapid recombination of photogenerated electrons and holes, and high mass transfer resistance. The fabrication of g-C₃N₄-based heterostructured photocatalysts is an effective strategy to further improve the photocatalytic performance of bulk g-C₃N₄. However, the interaction of bulk g-C₃N₄ and the dopant through the interfacial reaction in these heterostructured photocatalysts only is limited to the exposed surface of bulk g-C₃N₄. Adjusting the morphology of bulk g-C₃N₄ is an important strategy to improve its photocatalytic performance. Yang et al.⁷ fabricated g-C₃N₄ nanosheets using a liquid exfoliation method to enhance the hydrogen evolution activity of bulk g-C₃N₄ under visible-light irradiation. Tubular nanostructure photocatalytic materials (e.g., TiO₂ nanotubes) with open mesoporous morphology can efficiently transfer the photogenerated carriers along the 1D path, whereas their exposed inner and outer surface can lead to a high surface photocatalytic reaction rate.^8,9 Therefore, the development of C₃N₄ NTs photocatalysts has gained increasing attention. However, controlling the morphology of bulk g-C₃N₄ to form C₃N₄ NTs during the high-temperature polycondensation process of organic precursors is difficult because of its layered structural tendency. Some scholars attempted to prepare C₃N₄ NTs using catalytic self-assembly, templating, or chemical vapor deposition method.^10–12 However, the aforementioned processes suffer from some drawbacks, including harsh preparation conditions, operational complexity, toxicity, and high cost, which limit the large-scale preparation of C₃N₄ NTs.

In the present study, C₃N₄ NTs were successfully demonstrated for the first time through a simple water-induced morphological transformation process from g-C₃N₄ nanosheets, and g-C₃N₄ nanosheets were obtained by ultrasonic exfoliation of bulk g-C₃N₄. Photocatalytic tests showed that the visible-light photocatalytic activity of C₃N₄ NTs significantly increased compared with that of bulk g-C₃N₄ and g-C₃N₄ nanosheets toward rhodamine B degradation and hydrogen evolution from water-splitting. Water was used as the “phase-transfer reagent” in this study, making the preparation process environmental friendly.

2. Experimental

2.1. Preparation of C₃N₄ NTs

C₃N₄ NTs were fabricated from g-C₃N₄ nanosheets through water-induced morphological transformation process, and g-C₃N₄ nanosheets were obtained by ultrasonic exfoliation of bulk g-C₃N₄. In a typical synthesis, 5 g of melamine was calcined in air at 550 °C for 2 h, and the heat rate is 10 °C min⁻¹ in the heating process. After exfoliating the bulk g-C₃N₄ powder by a 500 W ultrasonic crasher for 1 h in water, the dry g-C₃N₄ nanosheets powder was heated at 350 °C for 10 min (heating rate is 10 °C min⁻¹). Then, the hot g-C₃N₄ nanosheets powder was rapidly transferred to 500 mL cool water. Finally, the C₃N₄ NTs sample was obtained by centrifugation and dried.

2.2. Characterizations

Field emission scanning electron microscopy (FESEM) images were recorded using a JEOL JSM-6700F field emission scanning electron microscope. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were recorded on a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were obtained using a Panalytical X'Pert PRO diffractometer via Cu Kα radiation. Nitrogen gas porosimetry measurements were performed on a Quantachrome NOVA 2000e surface area and porosity analyzer after the samples were outgassed under a vacuum at 70 °C for 20 min and 150 °C for 6 h. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 FTIR apparatus. UV-visible/diffuse reflection spectra (UV-vis/DRS) were conducted using a Lambda 750S UV-vis-NIR spectrometer. Photoluminescence (PL) measurements were carried out on a HITACHI F-7000 fluorescence spectrophotometer. The incident photon to current conversion efficiency (IPCE) was measured with a QE/IPCE Measurement Kit (Newport, USA) in the wavelength range of 300–750 nm.

2.3. Adsorption and photocatalytic tests

A PLS-SXE300 Xe lamp served as the light source, and the output wavelength λ > 320 nm. The visible light irradiation was obtained by removing the UV irradiation from the lamp using a 400 nm cut filter, which can control the output wavelength λ > 400 nm. For the rhodamine B adsorption and degradation, the catalyst amount is 100 mg, the initial concentration of rhodamine B is 10 mg L⁻¹, and the initial volume is 100 mL. Changes in the rhodamine B concentrations were analyzed using an UNICO UV-2000 spectrophotometer at λ = 554 nm. For the hydrogen evolution reaction conditions: 100 mg of catalyst loaded with 3 wt% of Pt co-catalyst; 100 mL of H₂O containing 10 vol% triethylamine. The generated hydrogen was in situ analyzed with a GC 7890-II TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to the gas circulating line with argon carrier.

2.4. Photocurrent measurements

The working electrode was prepared on the rectangle titanium (Ti) sheets (size 10 mm × 50 mm, thickness 140 μm, purity > 99.6%), which was cleaned by sonication in water and alcohol for 10 min, respectively. The cleaned Ti sheets were chemically etched in a mixture of HF, HNO₃ and H₂O for 30 s (HF–HNO₃–H₂O = 1 : 4 : 5; v/v/v) followed by rinsing with distilled water and kept in alcohol. The catalyst powder (10 mg) was mixed with alcohol (2 mL) under sonication for 30 min to get slurry. The obtained slurry was used for spin-coating on the Ti sheet at an initial spin rate of 500 rpm for 9 s and then 2000 rpm for 10 s. After air drying, the prepared working electrode was heated at 60 °C for 6 h in air to improve the adhesion of the catalyst. The cooled Ti sheets were washed with water for three times. Finally, the sheets were dried at 60 °C for 24 h.

Photocurrent measurements were carried out using the conventional three electrode setup connected to an electrochemical station (CHI 630E, Shanghai Chenhua, China). In this electrochemical system, the prepared catalyst/Ti sheet was used as the working electrode; a Pt wire was used as the counter electrode and a Ag/AgCl electrode (saturated KCl) was used as the reference electrode. The electrolyte is 0.01 mol L⁻¹ Na₂SO₄ aqueous solution (100 mL). A 300 W Xe lamp served as a light source. The measurements were carried out at a constant potential of +1.0 V to the working electrode.

3. Results and discussion

3.1. Fabrication of C₃N₄ NTs by water-induced morphological transformation process

The fabrication of C₃N₄ NTs through water-induced morphological transformation is based on the nanosheets roll-up mechanism.¹³ Some scholars suggested that the tubular nanostructure reduces the charge imbalance caused by unsaturated or dangling bonds at the edges of the intermediate structure.¹⁴ Other studies justified nanotubes formation through an asymmetric chemical environment or mechanical stress.¹⁵ In the current preparation system, C₃N₄ NTs were formed through the crimping of g-C₃N₄ nanosheets after the sudden hot–cool process driven by the generated thermal stress. As shown in Fig. 1, the morphological transformation process was monitored via FESEM. First, bulk g-C₃N₄ was exfoliated to form g-C₃N₄ nanosheets with less than five layers after ultrasonic crashing for 1 h in water (Fig. 1a and b). The atomic spacing of the g-C₃N₄ nanosheets was then increased when the temperature increased from room temperature to 350 °C. In the subsequent cold water quenching process, the surface cooling rate of the g-C₃N₄ nanosheets was higher than the inner cooling rate. Therefore, the surface atoms in the g-C₃N₄ nanosheets rapidly returned to the initial position than the inner atoms. As a result, the thermal stress was generated in the g-C₃N₄ nanosheets, and the g-C₃N₄ nanosheets demonstrated inner stretching and surface compression. Finally, the g-C₃N₄ nanosheets formed a plastic deformation and then curled to form C₃N₄ NTs when the thermal stress exceeded the yield limit of the g-C₃N₄ nanosheets (Fig. 1c and d). In the C₃N₄ NTs preparation process, water was selected as the phase-transfer reagent because of its poor thermal conductivity. The poor thermal conductivity of water caused a relatively stable temperature difference at the interface of the g-C₃N₄ nanosheets and water, which resulted in a continuous thermal stress in the g-C₃N₄ nanosheets. Therefore, the morphological transformation process can occur under continuous thermal stress.

Fig. 1 Fabrication mechanism of C₃N₄ NTs and corresponding FESEM images of the morphological evolution from bulk layered structure into tubular nanostructure: bulk g-C₃N₄ (a), exfoliated g-C₃N₄ nanosheets (b), half rolled-up C₃N₄ NTs (c), and completely rolled-up C₃N₄ NTs (d).

3.2. Characterization

3.2.1. Morphology. The representative TEM images of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs are shown in Fig. 2. As shown in Fig. 2a, bulk g-C₃N₄ exhibits a graphite-like layered structure corresponding to the FESEM observation presented in Fig. 1a. The SAED pattern of bulk g-C₃N₄ further confirms its layered structure, and the diffraction ring corresponds to the diffraction of (002) lattice plane of layered structure (inserts in Fig. 2a). As shown in Fig. 2b, bulk g-C₃N₄ is successfully exfoliated to form g-C₃N₄ nanosheets with less than five layers after ultrasonic crashing. A number of hollow tubular nanostructures with different sizes are clearly shown in Fig. 2c and d. This result indicates that C₃N₄ NTs are successfully prepared by water-induced morphological transformation method from g-C₃N₄ nanosheets. For the g-C₃N₄ nanosheets and C₃N₄ NTs samples, the diffraction intensity of (002) lattice plane is weakened due to the decrease of layered structure (insert in Fig. 2b and c). Two individual C₃N₄ NTs view across the connected edges are shown in Fig. 2e and f, respectively. The gaps on the wall confirm that the C₃N₄ NTs are obtained by g-C₃N₄ nanosheets crispation. Fig. 2g–i show three individual C₃N₄ NTs view from the opposite visual angle of the gap on the wall. The inner and outer diameters of C₃N₄ NTs shown in Fig. 2g are approximately 150 and 300 nm, respectively, which are larger than that shown in Fig. 2h and i. This result suggests that the size of each individual C₃N₄ NTs cannot be controlled but determined by the size of the curled g-C₃N₄ nanosheets unit.

Fig. 2 TEM images of bulk g-C₃N₄ (a), g-C₃N₄ nanosheets (b), C₃N₄ NTs (c and d), and individual C₃N₄ NTs in different viewing directions (e–i). SAED patterns of bulk g-C₃N₄ (insert a), g-C₃N₄ nanosheets (insert b), and C₃N₄ NTs (insert c).

3.2.2. Phase structure, textural property, and chemical structure. The phase structures of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs are characterized by XRD analysis. As shown in Fig. 3a, bulk g-C₃N₄ shows a characteristic (002) interlayer-stacking peak at 27.5°, which corresponds to an interlayer distance of d = 0.33 nm. The (100) peak at 12.6° represents the in-plane structural packing motif with a period of 0.675 nm. The diffraction intensity of the (002) peaks in the prepared g-C₃N₄ nanosheets and C₃N₄ NTs is weakened, demonstrating that the layered structural units are decreased. All the tested materials exhibit type IV isotherms with H3 hysteresis loops, regardless of the difference in their morphologies (Fig. 3b). The result indicates the mesoporosity of the materials that is constructed by the secondary accumulation of the structural unit in the bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs materials. From the pore-size distribution curves it is found that all of the tested materials exhibit a narrow peak centered at 4.1 nm, which is attributed to the released NH₃ that act as soft-templates during the course of melamine polycondensation (inserts in Fig. 3b). The broad pore size distribution peak centered at 6.7 nm in the g-C₃N₄ nanosheets sample is due to the void space between the formed nanosheets. For the C₃N₄ NTs sample, the broad pore size distribution peak centered at 5.9 nm originates from the open end of C₃N₄ NTs and the void space between tubular nanostructures. The BET surface area of g-C₃N₄ nanosheets is larger than that of bulk g-C₃N₄, and the BET surface area of C₃N₄ NTs is larger than that of g-C₃N₄ nanosheets. This result implies that bulk g-C₃N₄ and g-C₃N₄ nanosheets were successfully exfoliated step by step during the ultrasonic crash and water-induced morphological transformation process. Information on the structures of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs is obtained by FTIR measurements. As shown in Fig. 3c, a series of peaks within the range of 1700 cm⁻¹ to 1000 cm⁻¹ can be attributed to the stretching modes of C–N and C=N in the CN heterocycles. The sharp peak at 806.7 cm⁻¹ is the typical bending vibration of s-triazine units. The broad absorption peaks located in the range of 3400 cm⁻¹ to 2800 cm⁻¹ originate from the stretching vibrational modes of the primary (−NH₂) and secondary (=N–H) amines associated with uncondensed amino groups. The FTIR spectra of g-C₃N₄ nanosheets and C₃N₄ NTs are apparently similar to that of bulk g-C₃N₄, indicating that g-C₃N₄ nanosheets and C₃N₄ NTs maintain the same chemical structure as bulk g-C₃N₄. However, g-C₃N₄ nanosheets and C₃N₄ NTs demonstrate stronger FTIR modes than bulk g-C₃N₄ because of their more exposed surface functional groups. In addition, from the above XRD, BET surface area, and FTIR results it can be found that C₃N₄ NTs exhibit weaker XRD intensity, larger BET surface area, and stronger FTIR mode than g-C₃N₄ nanosheets. This result suggests that g-C₃N₄ nanosheets are exfoliated once again during the water-induced morphological transformation process.

Fig. 3 XRD patterns (a), N₂ sorption isotherms (b) and pore size distribution profiles (insert), and FTIR spectra (c) of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs.

3.2.3. Optical and electronic properties. The light absorption properties of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs are studied by UV-vis/DRS. The results shown in Fig. 4a indicate that all the tested materials show a typical semiconductor absorption in the 200 nm to 450 nm region, which originates from the charge-transfer response from valence band (VB) populated by N 2p orbit to the conduction band (CB) formed by C 2p orbit. The light absorption abilities of g-C₃N₄ nanosheets and C₃N₄ NTs are higher than that of bulk g-C₃N₄, because nanostructure is more beneficial than bulk structure for multiple reflections of incident light. The photogenerated carriers transfer efficiency of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs is studied by PL measurement. As shown in Fig. 4b, all the tested materials exhibit luminescence in a broad range (410 nm to 550 nm) and centered at ca. 450 nm under room temperature with an excitation wavelength of 330 nm. This finding suggests that the photoinduced electrons and holes generate and recombine within the semiconductors. The PL intensity of C₃N₄ NTs significantly decreases compared with that of bulk g-C₃N₄ and g-C₃N₄ nanosheets. This finding suggests that the electrons and holes recombination rate in the C₃N₄ NTs sample have decelerated. This condition originates from the efficient transportation of the photoexcited carriers at the C₃N₄ NTs surface.

Fig. 4 UV-vis/DRS (a) and PL spectra (b) of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs.

The photoelectric conversion efficiency of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs is studied by photocurrent and IPCE measurements (Fig. 5). As shown in Fig. 5a, sharp increases in the photocurrent responses are observed in all the tested working electrodes after pulse Xe lamp irradiation. The generated photocurrents are reproducible and stable during the five intermittent on–off irradiation cycles. The prompt increase in photocurrent response from light-off to light-on state is mainly ascribed to the fast separation and transportation of the photogenerated electron on the surface of the working electrodes. The C₃N₄ NTs exhibit a higher photocurrent response than bulk g-C₃N₄ and g-C₃N₄ nanosheets under the same conditions, indicating that tubular nanostructure can effectively delay the recombination rate of the photogenerated carriers. The IPCE measurements were conducted by directly coating the powder sample on the conductive surface of two pieces of overlapped FTO (fluorine-doped tin oxide) conductive glass. The result shows that C₃N₄ NTs have a higher IPCE value compared with bulk g-C₃N₄ and g-C₃N₄ nanosheets, implying that tubular nanostructure can effectively reduce the probability of electron–hole pair recombination by improving photogenerated carriers transfer capability, thereby prolonging the charge lifespan.

Fig. 5 Photocurrent responses (a) and IPCE measurements (b) of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs.

3.3. Adsorption kinetics, degradation kinetics, and hydrogen evolution activity

The mass transfer capacity of as-prepared materials is evaluated by adsorption kinetics study. The dynamic curves for the adsorption of rhodamine B on bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs were shown in Fig. 6a. As shown in Fig. 6a, C₃N₄ NTs exhibit the highest adsorption amount and fastest adsorption rate among the tested materials. This result fully proves the excellent mass transfer capability of C₃N₄ NTs. The mass transfer capability has significant effect on the photocatalytic activity of as-prepared materials, therefore, C₃N₄ NTs sample is expected to exhibit high photocatalytic activity toward rhodamine B degradation and hydrogen evolution reaction. The photocatalytic activities of bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs are evaluated by aqueous rhodamine B degradation and hydrogen evolution from water-splitting. The degradation kinetics curves of rhodamine B show that the degradation rate of C₃N₄ NTs sample is the fastest among the tested materials (Fig. 6b). Therefore, the K value (first-order rate constant) of C₃N₄ NTs sample is the largest among the tested materials. The photocatalytic activities of as-prepared materials for hydrogen evolution from water-splitting are evaluated in a 10 vol% triethylamine aqueous solution under λ > 400 nm visible-light irradiation in the presence of 3 wt% Pt nanoparticles co-catalyst. As shown in Fig. 6c, C₃N₄ NTs sample has the highest photocatalytic hydrogen evolution activity among the tested materials, and the hydrogen evolution efficiency of C₃N₄ NTs is 28.5 μmol h⁻¹. Basing on the characterization results, we can attribute the superior visible-light photocatalytic activity of C₃N₄ NTs to their unique tubular nanostructure with high photogenerated carriers transfer efficiency, excellent mass transfer capability, sufficient active sites, and enhanced light utilization efficiency.

Fig. 6 Kinetics curves of adsorption (a) and degradation (b) toward aqueous rhodamine B and hydrogen evolution activity (c) over bulk g-C₃N₄, g-C₃N₄ nanosheets, and C₃N₄ NTs photocatalysts.

4. Conclusions

C₃N₄ NTs were successfully prepared based on the nanosheets roll-up mechanism via a simple water-induced morphological transformation process. The enhanced photocatalytic activity of as-prepared C₃N₄ NTs toward rhodamine B degradation and hydrogen evolution from water-splitting can be attributed to their unique tubular nanostructure with high photogenerated carriers transfer efficiency, excellent mass transfer capability, sufficient active sites, and enhanced light utilization efficiency. In addition to their valuable application as a photocatalyst, the as-prepared C₃N₄ NTs may provide broad applications in the fields of photoelectrochemistry, electronics, sensors, and energy storage.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51208248, 21366024, 21165013, 51468043, 51238001, 51278092); Science and Technology Major Bidding Project of Jiangxi Province, China (no. Gan Ke Fa Ji Zi [2010] 217); Youth Science Foundation of Jiangxi Province, China (20114BAB213015); Natural Science Foundation of Jiangxi Provincial Department of Education, China (GJJ12456, GJJ14515).

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