Infrared ray assisted microwave synthesis: a convenient method for large-scale production of graphitic carbon nitride with outstanding nitrogen photofixation ability

Abstract

Nitrogen fixation is the second most important chemical process in nature next to photosynthesis. Both the energy consumption and raw material costs are high for the conventional artificial nitrogen fixation technology, the Haber–Bosch process. Here, we report a convenient infrared ray assisted microwave method for synthesizing graphitic carbon nitride (g-C₃N₄) with outstanding nitrogen photofixation ability under visible light. XRD, N₂ adsorption, UV-vis, SEM, TEM, TPD, EPR, PL and photocurrent measurements were used to characterize the prepared catalysts. The results indicate that microwave treatment can form many irregular pores in the as-prepared g-C₃N₄, which cause an increase in the surface area and promote the separation rate of electrons and holes. More importantly, microwave treatment causes the formation of many nitrogen vacancies in the as-prepared g-C₃N₄. These nitrogen vacancies not only serve as active sites to adsorb and activate N₂ molecules but also promote interfacial charge transfer from catalysts to N₂ molecules, thus significantly improving the nitrogen photofixation ability. The higher nitrogen vacancies concentration of g-C₃N₄ prepared by infrared ray assisted microwave treatment causes more chemical adsorption sites, leading to a higher nitrogen photofixation performance. Moreover, the present process is a convenient method for large-scale production of g-C₃N₄ which is significantly important for practical applications.

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Introduction

Nitrogen is an essential building element of plants and animals. Because the bond energy of the N≡N triple bond is too high, molecular nitrogen cannot be utilized by the organism directly. Most plants can take up nitrogen in the form of ammonium ion (NH₄⁺). Thus nitrogen fixation is the second most important chemical process in nature next to photosynthesis. However, the natural nitrogen fixation does not satisfy the increased demand of modern agriculture.^1,2 Artificial nitrogen fixation is carried out through the Haber–Bosch process, in which hydrogen gas reacts with nitrogen gas to yield ammonia in the presence of catalysts under high pressure and temperature. Both the energy consumption and raw material costs are high for this process. Therefore, artificial nitrogen fixation under milder conditions is of considerable significance from the perspectives of cost and environmental protection.

In 1977, Schrauzer et al. first reported that N₂ can be reduced to NH₃ over Fe-doped TiO₂ under UV light.³ Since then, because of the advantages such as green cleaning, mild conditions, low power consumption and low cost, photocatalytic nitrogen fixation technology is considered to be the best alternative to the Haber–Bosch process. Many Ti-based metal oxides and composite catalysts have been reported successively.^4–8 However, because of the poor visible light absorption caused by the wide band gap energy, the nitrogen fixation ability of the Ti-based metal oxides and composite catalysts is still low under visible light. Thus, in recent years, many novel nitrogen-photofixation systems are reported successively.^9–12

Recently, graphitic carbon nitride (g-C₃N₄) has been widely applied in a variety of fields, including photocatalysis,^13–15 fuel cells,¹⁶ organic synthesis¹⁷ and gas storage.^18,19 In general, g-C₃N₄ is synthesized by calcination raw materials at 500–550 °C for several hours. During this process, because of the temperature gradient, the surface temperature of the raw materials is much higher than that in the bulk. This results in that the polycondensation occur on the surface of raw materials with producing harmful gases. At the same time, the polycondensation has not yet occurred in the bulk of raw materials because of the not satisfied temperature. This leads to high energy consumption, large emission of harmful gas and low catalyst yield.

In 2014, Yuan et al. reported a rapid synthetic strategy named “microwave-assisted heating synthesis”.²⁰ The microwave-assisted processes involve energy transfer from microwave to the microwave-absorber materials which induces strong heating in minutes. When the microwave energy is absorbed by the raw material, the molecules are orderly arrangement in the electromagnetic field of the microwave. Then the high frequency reciprocating motion occurs inside the molecules of raw materials, causes the frequent collisions between molecules, leading to the generation of a lot of frictional heat. Under this heating method, the raw material is rapidly heated without the presence of temperature gradient. For microwave method, the microwave absorbing material is required to heat conduction. However, the heat transfer performance between solids is uneven and unsatisfactory.

In recent years, infrared heating has been widely applied in many fields, such as material synthesis, food hygiene, medical insurance and solar battery. Because the frequency of infrared ray and molecular vibration is close, resonance effects will greatly increase the amplitude of molecular vibration, thus improve the molecular kinetic energy. More importantly, this method does not need the heat transfer medium. Thus, in this work, infrared ray assisted microwave method is used to prepare g-C₃N₄ catalysts. Characterization results indicate that many nitrogen vacancies are formed in the as-prepared g-C₃N₄ catalysts. Li et al. discovered that the introduction of oxygen vacancies in BiOBr photocatalyst could activate N₂ and promote interfacial electron transfer, thus significantly improving the nitrogen photofixation ability.²¹ We hypothesize that nitrogen vacancies may be more effective than oxygen vacancies for nitrogen photofixation because nitrogen vacancies have the same shape and size as the nitrogen atoms in N₂ molecules. N₂ can be adsorbed and activated more easily on nitrogen vacancies. Thus the nitrogen photofixation performance under visible light was tested to evaluate the performance of the as-prepared catalysts. From the standpoint of energy saving and cost reduction, NH₄⁺ production rate per kilowatt of energy consumption (r(NH₄⁺)/kW) and NH₄⁺ production rate per gram raw material (r(NH₄⁺)/g_material) are used to compare three catalysts prepared by calcination, microwave and infrared ray assisted microwave method.

Experimental

Preparation and characterization

In a typical process, 6 g of thiourea was grounded for 30 min in a mortar and then transferred to an alumina crucible (25 mL). This crucible was then put into another alumina crucible (200 mL), buried with the CuO powder (microwave absorbing material), and treated by infrared ray assisted microwave for 20–40 min in a normal microwave oven (G70D20CN1P-D2, Galanz). The obtained catalysts were denoted as IM-CN(x), where x stands for the treated time of infrared ray assisted microwave (min). When only microwave (15–30 min) was used to synthesize g-C₃N₄ following the same procedure mentioned above, the obtained catalysts were denoted as M-CN(x), where x stands for treated time of microwave (min). For comparison, thiourea was heated at 520 °C for 2 h at the rate of 5 °C min⁻¹, and denoted as CN₅₂₀.

The XRD patterns of the prepared samples were recorded on a Rigaku D/max-2400 instrument using Cu-Kα radiation (λ = 1.54 Å). The scan rate, step size, voltage and current were 0.05° min⁻¹, 0.01°, 40 kV and 30 mA, respectively. UV-vis spectroscopy was carried out on a JASCO V-550 model UV-vis spectrophotometer using BaSO₄ as the reflectance sample. The morphologies of prepared catalyst were observed by using a scanning electron microscope (SEM, JSM 5600LV, JEOL Ltd.). TEM images were taken on a Philips Tecnai G220 model microscope. Nitrogen adsorption was measured at −196 °C on a Micromeritics 2010 analyser. All the samples were degassed at 393 K prior to the measurement. The BET surface area (S_BET) was calculated based on the adsorption isotherm. Elemental analysis was performed with a vario EL cube from Elementar Analysensysteme GmbH. Electron paramagnetic resonance (EPR) spectrum was monitored using a digital X-band spectrometer (EMX-220, Bruker, USA) equipped with a Bruker ER 4121VT temperature controller within the temperature range 113–273 K. The XPS measurements were performed on a Thermo Escalab 250 XPS system with Al Kα radiation as the excitation source. The binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. Temperature Programmed Desorption (TPD) studies were performed using a CHEMBET-3000 (Quantachrome, U.S.A.) instrument in the temperature range of 313 to 1073 K. The photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (FP-6300) using a Xe lamp as the excitation source. The Mott–Schottky plots were obtained using an electrochemical analyzer (LK2006A, Lanlike) using a three-electrode cell. The photocurrents were measured using an electrochemical analyzer (CHI 618C Instruments) equipped with a rectangular-shaped quartz reactor (20 × 40 × 50 mm) using a standard three-electrode system. The prepared sample film was used as the working electrode, a Pt flake was used as the counter electrode, and Ag/AgCl was used as the reference electrode. A 500 W Xe lamp was used to irradiate the working electrode from the back side. The light intensity on the working electrode was 120 mW cm⁻². In addition, a mechanical shutter was used to minimize the exposure of the sample to light. A 1.0 M Na₂SO₄ solution was used as the electrolyte. The applied potential was 0.00 V vs. Ag/AgCl. All the measurements were performed at room temperature (298 K).

Photocatalytic reaction

The nitrogen photofixation property was evaluated according to previous literature.⁸ The nitrogen photofixation experiments were performed in a double-walled quartz reactor in air. For these experiments, 0.2 g of photocatalyst was added to a 500 mL 0.789 g L⁻¹ ethanol as a hole scavenger.⁸ The suspension was dispersed using an ultrasonicator for 10 min. During the photoreaction under visible light irradiation, the suspension was exposed to a 250 W high-pressure sodium lamp with main emission in the range of 400 to 800 nm, and N₂ was bubbled at 100 mL min⁻¹ through the solution. The UV light portion of the sodium lamp was filtered by a 0.5 M NaNO₂ solution. All runs were conducted at ambient pressure and 30 °C. At given time intervals, 5 mL aliquots of the suspension were collected and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentration of ammonia was measured using the Nessler's reagent spectrophotometry method (JB7478-87) with a UV-2450 spectrophotometer (Shimadzu, Japan).^8,21

RhB was selected as the model compound to evaluate the photocatalytic performance of the prepared g-C₃N₄ in aqueous solution under visible light irradiation. For this purpose, 0.05 g of catalyst was dispersed in 200 mL of an aqueous solution of RhB (10 ppm) in an ultrasonicator for 10 min. The suspension was transferred into a lab-designed glass reactor and stirred for 30 min in darkness to achieve adsorption equilibrium. The photoreaction under visible light was performed using the same light source as that used for nitrogen photofixation. The concentrations of RhB before and after the reaction were measured using a UV-vis spectrophotometer at a wavelength of 550 nm.

Results and discussion

Fig. 1 shows the nitrogen photofixation performance over the as-prepared catalysts under visible light. The results of the control experiment indicate that the NH₄⁺ production rate can be ignored in the absence of irradiation, N₂ or photocatalyst, indicating that nitrogen photofixation occurs via a photocatalytic process (Fig. S1†). In Fig. 1a, CN₅₂₀ shows the NH₄⁺ production rate (r(NH₄⁺)) of 1.02 mg L⁻¹ h⁻¹ g_cat⁻¹. For M-CN(20), r(NH₄⁺) increases to 2.0 mg L⁻¹ h⁻¹ g_cat⁻¹. It is noted that the r(NH₄⁺) of IM-CN(30) further increases to 5.1 mg L⁻¹ h⁻¹ g_cat⁻¹, which is 5-fold and 2.5-fold higher than those of CN₅₂₀ and M-CN(20), respectively. The Fig. 1b shows the photocatalytic stabilities of IM-CN(30). No obvious decrease in nitrogen photofixation ability is observed after 20 h, hinting its good stability. Fig. 1c and d shows the NH₄⁺ production rate per kilowatt of energy consumption (r(NH₄⁺)/kW) and NH₄⁺ production rate per gram raw material (r(NH₄⁺)/g_material) of as-prepared catalysts. The r(NH₄⁺)/kW and r(NH₄⁺)/g_material for CN₅₂₀ are 0.2 and 0.32 mg L⁻¹ h⁻¹ g_cat⁻¹. For M-CN(20), these values increase to 5.5 and 0.6 mg L⁻¹ h⁻¹ g_cat⁻¹. For IM-CN(30), these values further increase to 12.5 and 4.0 mg L⁻¹ h⁻¹ g_cat⁻¹. These results hints that the infrared ray assisted microwave treatment is an effective method to further improve the nitrogen photofixation performance of catalysts from the standpoint of energy saving and cost reduction. In addition, the addition of AgNO₃ as electron scavenger sharply suppresses the nitrogen photofixation ability of IM-CN(30) (Fig. S2a†), indicating the main active species are the photogenerated electrons. Using aprotic solvents (DMF and DMSO) instead of water shows that no NH₄⁺ is generated, confirming the necessity of H₂O as the proton source for the nitrogen photofixation (Fig. S2b†). H₂ production is a possible competitive reaction. Thus the photocatalytic H₂ production experiment is performed according to previous work.²² The result shown in Table S1† indicates that the H₂ production ability of as-prepared catalysts is negligible, which is probably due to the absence of a proper co-catalyst.

Fig. 1 The NH₄⁺ production rate (r(NH₄⁺)) (a), nitrogen photofixation stability (b), NH₄⁺ production rate per kilowatt of energy consumption (r(NH₄⁺)/kW) (c) and NH₄⁺ production rate per gram raw material (r(NH₄⁺)/g_material) (d) over as-prepared catalysts under visible light.

The XRD patterns of as-prepared catalysts under various conditions are shown in Fig. 2. CN₅₂₀ shows the typical characteristic peaks of g-C₃N₄ located at 13.1 and 27.5°. The characteristic peaks of g-C₃N₄ appear after 15 min (20 min) microwave (infrared ray assisted microwave) treatment. This indicates the graphitic carbon nitride can be formed in a very short time by microwave method. However, some impurity peaks still exist after this short treated time. Further extending the microwave time leads to the formation of g-C₃N₄ materials with perfect crystal structure. 30 min is needed for forming the g-C₃N₄ structure under infrared ray assisted microwave treatment, which is longer than that of single microwave treatment (20 min). This is because the input power of single microwave (1 kW h⁻¹) is higher than that of infrared ray assisted microwave treatment (0.8 kW h⁻¹). It is noted that, compared with CN₅₂₀, a 0.3° shift to lower 2θ value is observed for both microwave and infrared ray assisted microwave treated g-C₃N₄ materials. This is probably due to that microwave treatment causes some crystal lattice defects in g-C₃N₄. The C/N ratio for CN₅₂₀ is 0.74 obtained by elemental analysis, close to the theoretical values. For M-CN(20) and IM-CN(30), the C/N ratio is 0.78 and 0.82, respectively. Combine with the XRD results, it is deduced that the crystal lattice defects in g-C₃N₄ should be the nitrogen vacancies. The higher C/N ratio for IM-CN(30) causes the higher nitrogen vacancies concentration compared with M-CN(20). Why infrared ray irradiation can influence the structure of g-C₃N₄ material is not clear till now. We propose that the infrared ray assisted heating method could change the polycondensation degree of raw material, leading to the formation of more nitrogen vacancies. Besides, no sulfur element is found in the catalysts, which is consistent with previous result.²³

Fig. 2 XRD patterns of as-prepared catalysts under microwave (a) and infrared ray assisted microwave (b) treatment.

The morphologies of the representative samples were examined by SEM analysis (Fig. 3). The result in Fig. 3a indicates that CN₅₂₀ displays layer structure that is similar to the analogue graphite. For M-CN(20) and IM-CN(30) (Fig. 3b and c), layer structures are also observed, accompanying with the formation of many irregular pores. No obvious morphological difference between M-CN(20) and IM-CN(30) is observed. It is deduced from the SEM results that the microwave treatment but not infrared radiation has a significant influence on the morphology of g-C₃N₄, which most likely affects its photocatalytic performance. The typical HRTEM image of IM-CN(30) exhibits the clear lattice fringe (Fig. 3d). The measured lattice spacings are 0.330, very close to the (0 0 2) crystal face of graphitic carbon nitride.^24–26 This confirms that the graphitic carbon nitride has been successfully synthesized by this infrared ray assisted microwave method.

Fig. 3 SEM images of as-prepared CN₅₂₀ (a), M-CN(20) (b), IM-CN(30) (c) and HRTEM image of IM-CN(30) (d).

Generally, a catalyst with high specific surface area (S_BET) is significant to the enhancement of catalytic performance.²⁷ The nitrogen adsorption and desorption isotherms of CN₅₂₀, M-CN(20) and IM-CN(30) were measured (Fig. S3†). The isotherms of as-prepared catalysts are of classical type IV, suggesting the presence of mesopores (2–50 nm). Because of the agglomeration, CN₅₂₀ shows a low S_BET of 9.4 m² g⁻¹. The S_BET of M-CN(20) and IM-CN(30) are 45.6 and 47.2 m² g⁻¹, much higher than that of CN₅₂₀. This is probably due to the formation of many pores after microwave treatment which is consistent with the SEM results. In order to eliminate the influence of surface area on the N₂ photofixation ability, the g-C₃N₄ catalyst with high surface area was prepared according to previous work,²⁸ and denoted as MCN₅₄₀. The C/N ratio for MCN₅₄₀ is 0.74 obtained by ICP. The N₂ adsorption result shows that the surface area of MCN₅₄₀ is 44.6 m² g⁻¹, close to that of M-CN(20) and IM-CN(30). However, the NH₄⁺ production rate of MCN₅₄₀ is only 1.44 mg L⁻¹ h⁻¹ g_cat⁻¹, much lower than that of M-CN(20) and IM-CN(30). This hints that the increased surface area of microwave treated g-C₃N₄ catalysts is not the main factor which improves the N₂ photofixation ability.

Fig. 4a compares the UV-vis spectra of the as-prepared CN₅₂₀, M-CN(20) and IM-CN(30). All the catalysts show typical semiconductor absorption. The band gap energy calculated based on the method of Oregan and Gratzel indicates that the value for CN₅₂₀ is 2.75 eV.²⁹ M-CN(20) has an absorption edge at ∼465 nm, corresponding to a band gap of 2.67 eV. For IM-CN(30), the absorption edge further shifts to 472 nm, corresponding to a band gap of 2.62 eV. Considering the absence of sulfur element in the catalysts, this decreased band gap energy of microwave treated g-C₃N₄ catalysts is probably due to the presence of nitrogen vacancies, leading to changes in the electronic structures and optical properties. It is noted that the absorption tail in the whole visible light region is observed in the spectra of M-CN(20) and IM-CN(30) but not CN₅₂₀. This absorption tail should be due to the formation of nitrogen vacancies in microwave treated g-C₃N₄ catalysts.³⁰ The CB potential of as prepared CN₅₂₀, M-CN(20) and IM-CN(30) using Mott–Schottky plots was measured and shown in Fig. 4b. The E_CB is −1.16, −1.12 and −1.1 V for CN₅₂₀, M-CN(20) and IM-CN(30), respectively. Combine with the band gap results obtained from UV-vis spectra, the E_VB of CN₅₂₀, M-CN(20) and IM-CN(30) located at +1.59, +1.55 and +1.52 V, respectively. The photogenerated electrons on CN₅₂₀ should have the stronger reduction capability than that of M-CN(20) and IM-CN(30). Thus it is deduced that the influence of nitrogen vacancies on the band structure of as-prepared carbon nitride is not the main factor to enhance its photocatalytic nitrogen fixation activity.

Fig. 4 UV-vis spectra (a), Mott–Schottky plots (b), EPR spectra (c) of as-prepared catalysts and the mass spectra of the indophenol prepared from different atmosphere (d).

EPR can provide direct information on monitoring various behaviors of native defects, such as oxygen and nitrogen vacancies.^31,32 As shown in Fig. 4c, CN₅₂₀ shows no peaks, suggesting that no localized unpaired electrons present in the CN₅₂₀. However, for IM-CN(30) and M-CN(20), a resonance signal at g = 2.0031 is observed, which confirms the presence of nitrogen vacancies. In order to further investigate the nitrogen source of NH₄⁺, the N₂ photofixation ability of IM-CN(30) under ¹⁵N isotope-labeled N₂ (purity > 98%) was performed. The produced ¹⁵NH₄⁺ reacts with phenolic and hypochlorite to form ¹⁵N labeled indophenol, which was analyzed by LC-MS. A strong ¹⁵N labeled indophenol anion mass spectroscopy signal presented at 199 m/z in LC-MS studies (Fig. 4d). It is note that the intensity of this signal was obviously higher than that of the ¹⁴N : ¹⁵N natural abundance ratio. This observation further confirmed that N₂ was the source of generated ammonium ion in this N₂ photofixation process.

The surface chemical compositions of the as-prepared g–C₃N₄–based catalysts were characterized using XPS. In Fig. 5a (N 1s region), the two contributions of CN₅₂₀ located at 398.5 and 400.2 eV are assigned to the sp²-hybridized aromatic nitrogen atoms bonded to carbon atoms (C–N=C) and nitrogen atoms bonded to three carbon atoms (N–C₃).³³ For M-CN(20) and IM-CN(30), no obvious difference in peak position is observed. However, the peak area ratio of (N–C₃)/(C–N=C) decreases from 0.27 for CN₅₂₀ to 0.25 for M-CN(20) and 0.24 for IM-CN(30), clearly indicating that nitrogen vacancies are primarily located at the tertiary nitrogen lattice sites. For the C 1s region (Fig. 5b), the three contributions located at 284.6, 286 and 288.4 eV for CN₅₂₀ are attributed to C–C bonds, which originated from sp² C atoms bonded to N in an aromatic ring (N–C=N); C=N or C≡N, which could be attributed to defect-containing sp²-hybridized carbon atoms present in graphitic domains; and pure graphitic sites in a CN matrix.^34,35 Note that, in addition to the three C 1s peaks mentioned above, a new peak at a high binding energy of 290 eV appears in M-CN(20) and IM-CN(30). This peak is attributed to the two-coordinated carbon (N–C–N) formed by the disappearance of three-coordinated nitrogen, thereby confirming the generation of nitrogen vacancies. In addition, for IM-CN(30), the area of this peak is larger than that of M-CN(20), confirming the higher nitrogen vacancies concentration in IM-CN(30).

Fig. 5 XPS spectra of CN₅₂₀, M-CN(20) and IM-CN(30) in the region of N 1s (a) and C 1s (b).

The photocatalytic performance of RhB degradation over CN₅₂₀, M-CN(20) and IM-CN(30) was also evaluated under visible light (Fig. S4†). The reaction rate constant k was obtained by assuming that the reaction followed first-order kinetics (Fig. S4,† inset).³⁶ Prior to turning the light on, the RhB adsorption ability of M-CN(20) and IM-CN(30) is improved compared with CN₅₂₀, which is probably due to the larger S_BET. Due to a high recombination rate for electrons and holes, CN₅₂₀ exhibits a low degradation rate (∼60%) and reaction rate constant (0.009 min⁻¹). For M-CN(20) and IM-CN(30), the degradation rates sharply improve to 85% and 94%, probably due to the increased S_BET and decreased band gap energy. The rate constants for M-CN(20) and IM-CN(30) are 0.0144 and 0.0236 min⁻¹, which is 1.6-fold and 2.6-fold greater than that of CN₅₂₀ respectively. It is noted that the r(NH₄⁺) for M-CN(20) and IM-CN(30) is 2-fold and 5-fold higher than that of CN₅₂₀, which growth rate is obvious higher than that of RhB degradation performance. Thus, it is deduced that the enhanced nitrogen photofixation ability is not only due to the increased S_BET and decreased band gap energy but also due to some other reasons.

Because the chemical adsorption sites are considered to be reaction centers capable of activating N₂, chemisorption is an essential step in photocatalytic N₂ fixation. TPD investigations were performed to understand N₂ chemisorption on the surface of the as-prepared catalysts. In Fig. 6, the N₂-TPD results on CN₅₂₀, M-CN(20) and IM-CN(30) are compared. Two adsorbed N₂ species in M-CN(20) and IM-CN(30) and only one adsorbed N₂ species in CN₅₂₀ are observed. The peak at ∼120 °C is related to physical adsorption. The peak at 260 °C, which is related to the strong chemisorption species of N₂, is observed for M-CN(20) and IM-CN(30) but not for CN₅₂₀. This result indicates that nitrogen vacancies could introduce many chemical adsorption sites on the surface of M-CN(20) and IM-CN(30). Because chemisorption is generally associated with activation, these chemical adsorption sites will activate N₂ for nitrogen photofixation. Thus the higher nitrogen vacancies concentration of IM-CN(30) causes the more chemical adsorption sites, leading to the higher nitrogen photofixation performance.

Fig. 6 The N₂-TPD of as-prepared CN₅₂₀, M-CN(20) and IM-CN(30).

It is deduced from XPS result that the nitrogen vacancies are located at the three-coordinated nitrogen. As shown in Fig. 7, there are two possible positions for nitrogen vacancies. To further confirm that N₂ is activated by nitrogen vacancies, density functional theory (DFT) simulations were employed to investigate the interaction between a N₂ molecule and the g-C₃N₄ with two nitrogen vacancies location (Fig. 7). The optimized results indicated that the adsorption energy is −166.2 kJ mol⁻¹ for position 1, confirming the chemisorption occurs. When the N₂ molecule adsorbs on the nitrogen vacancies, an σ bond between the N₂ molecule and the nearest C atom is formed, confirming N₂ is mainly adsorbed on the nitrogen vacancies. The N≡N bond is prolonged from 1.107 Å to 1.242 Å, proving that nitrogen vacancies can activate N₂. However, only physical adsorption with the adsorption energy of −2.4 kJ mol⁻¹ occurs when N₂ is adsorbed on position 2. The N≡N bond length is also not changed for this situation. Thus it is deduced the nitrogen vacancies is probably located at position 1.

Fig. 7 The optimal N₂ adsorption models on nitrogen vacancies.

In general, at a lower PL intensity, the separation rate of the photogenerated electron–hole pairs is higher.^37,38 In Fig. 8a, the PL intensity follows the sequence IM-CN(30) < M-CN(20) < CN₅₂₀. Compared with CN₅₂₀, the higher surface areas of M-CN(20) and IM-CN(30) results in a shorter migration distance, which is favourable for charge transfer from the bulk to the surface of the g-C₃N₄ material and lead to a higher separation rate. Besides that, IM-CN(30) shows the higher separation rate than that of M-CN(20) though they have the comparable S_BET. This is probably due to that IM-CN(30) possesses more nitrogen vacancies which could act as electron trappers to improve carrier separation. Fig. 8b compares the PL intensity of CN₅₂₀, M-CN(20) and IM-CN(30) under Ar and N₂ atmospheres. Under N₂ atmosphere, the PL intensities of M-CN(20) and IM-CN(30) are reduced compared with that under an Ar atmosphere. This hints that nitrogen vacancies may promote photogenerated electron transfer from these catalysts to adsorbed N₂, thus improve the carrier separation. The different reduction level should be related to the different concentration of nitrogen vacancies. By contrast, no difference is observed for CN₅₂₀ between the PL intensities under N₂ and Ar atmospheres, confirming this point of view.

Fig. 8 PL spectra under N₂ atmospheres (a) and the comparison of PL intensity of as-prepared CN₅₂₀, M-CN(20) and IM-CN(30) under Ar and N₂ atmospheres (b).

Fig. 9a shows the photocurrent responses of CN₅₂₀, M-CN(20) and IM-CN(30) under N₂ atmospheres. Because of the higher separation rate of electrons and holes, M-CN(20) and IM-CN(30) shows the higher photocurrent than that of CN₅₂₀. Note that the photocurrent generated on the CN₅₂₀ electrode remains unchanged with irradiation time. However, the photocurrent densities of M-CN(20) and IM-CN(30) gradually decrease at the beginning and then remain stable. This photocurrent decay is probably due to the competition between N₂ and FTO glass for trapped electrons. Li and his co-workers reported that oxygen vacancies can trap electrons and promote interfacial charge transfer from BiOBr nanosheets to N₂.²¹ We hypothesize that nitrogen vacancies have a similar effect. The photogenerated electrons that arrived at the surface of M-CN(20) and IM-CN(30) are trapped by the nitrogen vacancies, and then transferred immediately from the catalysts to the adsorbed N₂, causing the photocurrent decay.

Fig. 9 Photocurrent responses of as-prepared catalysts under N₂ or Ar atmospheres.

In Fig. 9b, the photocurrents of CN₅₂₀ under N₂ and Ar atmosphere are almost the same. Whereas, the photocurrent of IM-CN(30) does not decay under Ar atmosphere. This result confirms the fast electron transfer process from the IM-CN(30) to the adsorbed N₂. In summary, the possible nitrogen photofixation process over microwave treated g-C₃N₄ materials is as follows. First of all, N₂ molecules are chemisorbed on the nitrogen vacancies of catalyst. When catalyst is excitated by the irradiation, the formed photogenerated-electrons are trapped by the nitrogen vacancies. Those photogenerated-electrons are transferred immediately from the catalyst to the adsorbed N₂. Because the bonding orbitals of N₂ molecule are occupied by four electrons, this photogenerated-electron has to occupy the anti-bonding orbitals, leading to the nitrogen activation. The activated N₂ molecule reacts with H⁺ in water to form NH₃, and then it finally forms NH₄⁺. During this process, nitrogen vacancies not only serve as active sites to adsorb and activate N₂ molecule but also promote interfacial charge transfer from g-C₃N₄ to N₂ molecules, thereby significantly improving the nitrogen photofixation ability.

In order to compare the nitrogen photofixation ability with Ti-based catalysts, the Fe–TiO₂, Fe₂Ti₂O₇ and Ru–TiO₂ were prepared according to the previous reports.^4,5,8 The BiOBr catalyst with oxygen vacancies reported by Li was also prepared.²¹ The nitrogen photofixation abilities of these catalysts are shown in Fig. 10. Obviously, the activities of Fe–TiO₂ and Fe₂Ti₂O₇ are much lower than that of IM-CN(30). BiOBr also shows the lower N₂ photofixation activity than that of IM-CN(30). The precious metal Ru doped TiO₂ shows the comparable nitrogen photofixation ability to that of IM-CN(30). However, considering the high price of precious metal, IM-CN(30) is the best candidate among these catalysts.

Fig. 10 The comparison of nitrogen photofixation ability of Fe–TiO₂, Fe₂Ti₂O₇, Ru–TiO₂, BiOBr and IM-CN(30) under visible light.

Conclusions

A convenient infrared ray assisted microwave method for synthesizing graphitic carbon nitride (g-C₃N₄) with outstanding nitrogen photofixation ability under visible light is reported. Microwave treatment can form many irregular pores in the as-prepared g-C₃N₄, which causes the increased surface area and promoted separation rate of electrons and holes. Moreover, microwave treatment causes the formation of many nitrogen vacancies in the as-prepared g-C₃N₄. These nitrogen vacancies not only serve as active sites to adsorb and activate N₂ molecules but also promote interfacial charge transfer from catalysts to N₂ molecules, thus significantly improving the nitrogen photofixation ability. The higher nitrogen vacancies concentration of g-C₃N₄ prepared by infrared ray assisted microwave treatment causes the more chemical adsorption sites, leading to the higher nitrogen photofixation performance. More importantly, the present process is a convenient method for large-scale production of g-C₃N₄ which is significantly important for the practical application.

Acknowledgements

This work was supported by Education Department of Liaoning Province (No. L2014145) and Environmental Science and Engineering Innovation Team of Liaoning Shihua University ([2014]-11).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08817a

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