Synthesis of Porous Carbon Nitride Nanobelts for Efficient Photocatalytic Reduction of CO₂

Abstract

Sustainable conversion of CO₂ to fuels using solar energy is highly attractive for fuel production. This work focuses on the synthesis of porous graphitic carbon nitride nanobelt catalyst (PN-g-C₃N₄) and its capability of photocatalytic CO₂ reduction. The surface area increased from 6.5 m²·g⁻¹ (graphitic carbon nitride, g-C₃N₄) to 32.94 m²·g⁻¹ (PN-g-C₃N₄). C≡N groups and vacant N_2C were introduced on the surface. PN-g-C₃N₄ possessed higher absorbability of visible light and excellent photocatalytic activity, which was 5.7 and 6.3 times of g-C₃N₄ under visible light and simulated sunlight illumination, respectively. The enhanced photocatalytic activity may be owing to the porous nanobelt structure, enhanced absorbability of visible light, and surface vacant N-sites. It is expected that PN-g-C₃N₄ would be a promising candidate for CO₂ photocatalytic conversion.

1. Introduction

Significantly increased CO₂ concentration in the atmosphere has caused problems such as air pollution and global warming in the past decades, posing a serious threat to our future generations [1,2]. In order to alleviate these issues, innovative and sustainable technologies are needed to effectively capture and convert CO₂. Sustainable conversion of CO₂ to high value-added products not only helps to reduce the content of CO₂ in the atmosphere but also promotes the carbon cycle [3,4]. However, due to the lack of efficient, stable, and selective catalysts, the research on CO₂ photoreduction is still progressing slowly.

Carbon nitride, a low-cost, thermally stable, and nontoxic material with a band gap of 2.7 eV and well-matched requirements of various redox reactions, shows excellent photocatalytic activity [5,6]. However, both low surface area and separation efficiency of photogenerated electron-hole pairs can depress the photoactivity of g-C₃N₄ [7,8]. Various methods have been developed to solve these problems, such as nonmetal element doping [9], metal oxide or hydroxides [10,11], ion doping [12,13], noble metal deposition [14,15], controlling morphology [16,17], and loading on carriers [18,19]. Previous research has demonstrated the great potential of g-C3N4 in photocatalytic fields, including carbon dioxide reduction, water splitting, organic pollutant degradation, and organic transformations [5,20].

In bulk g-C₃N₄, the stacked 2D single layers are held together in place by weak van der Waals forces of attraction [21]. Aiming to provide more reaction sites, exfoliation has been of particular interest for modifying g-C₃N₄ in recent years. g-C₃N₄ is treated with thermal exfoliation [20], concentrated acid treatment [22], and ultrasonic exfoliation [23] to synthesize nanoribbon or nanobelt samples. In this study, we used melamine as the precursor and a mixed solution of distilled water and ethylene glycol as the solvent. Melamine molecularly dissolved in the mixed aqueous solution at elevated temperature and polymerized into porous carbon nitride nanobelt (PN-g-C₃N₄), as displayed in Figure 1. The thin layer may be formed due to the polyols introduced into the interlayer [24] in the hydrothermal process and ultrasonic-induced exfoliation [25] in the washing stage of supramolecular precursor. Meanwhile, during thermal calcination process the released gas and volume shrinkage of precursor would create many pores on the layers, finally producing porous few-layer C₃N₄. The morphologies, microstructures, and physicochemical properties of the photocatalyst were studied. CO₂ was used as the raw material to evaluate PN-g-C₃N₄ photocatalytic performance under visible light and simulated sunlight. The obtained PN-g-C₃N₄ was confirmed to be an efficient photocatalyst in the conversion of CO₂.

2. Results

2.1. SEM Analysis

Figure 2 presents the morphologies and micro-structures of PN-g-C₃N₄ and g-C₃N₄ samples. The as-obtained PN-g-C₃N₄ nanobelts are characterized with thickness of 30–80 nm with a lateral size of micrometers. It can be seen that PN-g-C₃N₄ has loose nanobelt structures with pores in its framework (Figure 2a,b). The existence of a large number of edges and pores in the obtained porous nanobelt structures is extremely important for improving the photochemical and catalytic performance of carbon nitride. On the contrary, g-C₃N₄ has bulk structure (Figure 2c,d), which is formed by lamellar structures stacking with each other.

The nitrogen adsorption–desorption isotherms of the samples are presented in Figure 3a. Both g-C₃N₄ and PN-g-C₃N₄ exhibited a type IV isotherm with a hysteresis loop at P/P_o = 0.6–1.0. The pore sizes in PN-g-C₃N₄ are about 3–20 nm, which is attributed to the pores formed in the porous nanobelt structures (Figure 3b). The BET surface areas of g-C₃N₄ and PN-g-C₃N₄ were calculated to be 6.5 m²·g⁻¹, and 32.94 m²·g⁻¹, respectively. The specific surface area of PN-g-C₃N₄ increases greatly due to its porous nanobelt structures, which is beneficial for the exposure of more active catalytic sites.

2.2. IR and UV-Vis DRS Analysis

The FTIR spectrum for g-C₃N₄ and PN-g-C₃N₄ (Figure 4) showed a peak at 807 cm⁻¹ typical for the out-of-plane bending mode of heptazine rings, whilst peaks locked between 800 and 1800 cm⁻¹ originated from N-C=N heterorings [26]. The peak at 3000–3500 cm⁻¹ corresponded to N-H stretching vibrations. For the PN-g-C₃N₄ samples, a new peak centered at 2173 cm⁻¹ is found in the spectrum, which is assigned to an asymmetric stretching vibration of C≡N triple bond. The other change was the decrease in the intensity of the N-H stretching peaks between 3000 and 3300 cm⁻¹. The results suggest the synthesis of PN-g-C₃N₄ decreases the concentration of N-H groups and introduces C≡N groups. The existence of C≡N groups in PN-g-C₃N₄ is supposed to increase the electron delocalization and adjust band structures, beneficial for visible-light absorption and photon-generated carrier separation [27].

The optical absorption properties of the photocatalyst have a great effect on the photocatalytic performance. In order to investigate the optical absorption properties of the samples, diffuse reflectance absorption spectra were recorded on UV-Vis system. The optical absorption spectra of g-C₃N₄ and PN-g-C₃N₄ are displayed in Figure 5a. The absorption edge of g-C₃N₄ was at around 460 nm. However, the absorption spectrum of PN-g-C₃N₄ extends to the more visible light region from 420 nm to 800 nm. The results demonstrate that PN-g-C₃N₄ has enhanced optical adsorption of the visible light, which ascribe to large number of edges and pores in the obtained porous nanobelt structures. The band gaps of g-C₃N₄ and PN-g-C₃N₄ are presented in Figure 5b. The band gap of PN-g-C₃N₄ (2.50 eV) is lower than that of g-C₃N₄ (2.68 eV). These changes are related to quantum confinement effect, due to excitation into the lower energy defect states [28]. The relatively low band gap of PN-g-C₃N₄ allows it to absorb a good number of photons in the visible domain of the solar spectrum, which is the most important for an effective photocatalyst.

2.3. XRD and XPS Analysis

The XRD pattern for pristine g-C₃N₄ (Figure 6a) showed two characteristic peaks at 13.0° and 27.4°, which can be assigned to the (100) and (002) crystal planes of g-C₃N₄, representing in-plane packing and interfacial stacking of g-C₃N₄ sheets, respectively [29]. The peak at 27.4° of PN-g-C₃N₄ is weaker and wider, suggesting that the interlayer structure of g-C₃N₄ has been weakened, which agrees well with the changes in the micro-morphology.

The survey XPS spectra of g-C₃N₄ and PN-g-C₃N₄ samples are shown in Figure 6b. The XPS data showed a decrease in the N/C ratio from 1.29 (g-C₃N₄) to 1.20 (PN-g-C₃N₄) on the surface, suggesting the introduction of surface N defects. High-resolution XPS peaks of C1s spectra of the g-C₃N₄ sample in Figure 6c are deconvoluted into three peaks for C-C (285.0 eV), C-N (286.5 eV), and N-C=N (288.5 eV) bonds. Moreover, the high-resolution XPS peaks of N1s spectrum (Figure 6d) is deconvoluted into four peaks. The first peak at 398.9 eV represents the C=N-C bond. The other peaks at 400.3, 401.5, and 404.8 eV belongs to N-(C)₃, C-N-H, and π excitation bonds, respectively (Figure 6d). Compared to g-C₃N₄, PN-g-C₃N₄ showed a slight shift in all the peaks of C1s (0.1–0.2 eV) and N1s (0.1–0.3 eV) spectra, which may be caused by the defects in the carbon nitride network. Interestingly, the intensity of C-C peaks of PN-g-C₃N₄ slightly increased. The peak area ratios between C-C and N-C-N of C1s spectra were calculated to be 0.18 and 0.25 for g-C₃N₄ and PN-g-C₃N₄ samples, respectively. Similarly, the peak area ratios between C=N-C and N-(C)₃ peaks in the N1s spectra were determined to be 4.5 (g-C₃N₄) and 3.6 (PN-g-C₃N₄), respectively. It is strong evidence that C=N-C vacancies are formed on the surface of PN-g-C₃N₄, which can act as entrapping points for charges, yielding longer lifetimes for the charge carrier photoexcitons [30].

2.4. Photocatalytic Performance

The photocatalytic activities of as-prepared g-C₃N₄ and PN-g-C₃N₄ are shown in Figure 7. In the range of 420–800 nm, simulated sunlight has the similar profile with visible light. However, simulated sunlight has energy distribution at UV zone (360–420 nm) and NIR zone (820 nm) while these parts of visible light are cut off. Remarkably, the sample PN-g-C3N4 exhibits an excellent CO evolution rate (29.8 μmol·h⁻¹·g⁻¹), which is about 5.7 times that of g-C₃N₄ (5.2 μmol·h⁻¹·g⁻¹) under the visible light (Figure 7a). The CO evolution rate under simulated sunlight catalyzed by PN-g-C₃N₄ is 52.6 μmol·h⁻¹·g⁻¹, which is about 6.3 times that of g-C₃N₄ (8.3 μmol·h⁻¹·g⁻¹). The results confirmed porous nanobelt structures of PN-g-C₃N₄ can extremely enhance the specific surface area and provide more space for mass transfer and reaction, which in turn improves the photocatalytic activity of the samples. The superior activities of PN-g-C₃N₄ can also be attributed to enhanced visible-light absorption and N defects.

As presented in Figure 7b, the yield of CO is stable without any significant deactivation after five cycles (15 h illumination), which indicates high photostability of PN-g-C₃N₄ for the CO₂ reduction. It is worth noting that no other gas products such as CH₃OH or CH₄ generated by PN-g-C₃N₄ were detected by gas chromatography.

Electrochemical tests were performed in a three-electrode cell with a g-C₃N₄-coated working electrode to further understand the dynamics of electron transfer at the PN-g-C₃N₄ surface. Figure 8a shows the current of the electrochemical cell with pulsed light excitation. Under visible light illumination, both g-C₃N₄ and PN-g-C₃N₄ generated significant photocurrent, implying efficient photogeneration of charge carriers in both materials that is then transferred to the working electrode. Furthermore, the PN-g-C₃N₄ showed higher photocurrent intensity than that of g-C₃N₄, suggesting the higher separation rate of photogenerated charge carriers in the PN-g-C₃N₄. Additionally, the photocurrent can reproducibly increase and recover in every on-off cycle of irradiation, demonstrating the high stability in practical applications. The photogenerated electrons and holes are likely separated more efficiently in PN-g-C₃N₄ than in g-C₃N₄. To test this, photoluminescence (PL) measurements were performed to study the separation of photogenerated electrons and holes in g-C₃N₄ and PN-g-C₃N₄. Figure 8b displays the PL spectra of the two samples under 380 nm excitation at room temperature. The strong emission peak of g-C₃N₄ around 465 nm was derived from the direct band transition. By contrast, the PL intensity of PN-g-C₃N₄ was 65% lower, indicating the higher efficiency in separation of the photogenerated charge carriers. Furthermore, the morphology change from multi-layer structure (g-C₃N₄) to thin nanobelts (PN-g-C₃N₄) would shorten the distance for the photogenerated electrons to reach the surface, thus facilitating the charge separation.

The VB XPS spectra (Figure 8c) shows that the band gap of g-C₃N₄ and PN-g-C₃N₄ between the valence band (VB) and Fermi level (E_f) are 2.38 and 2.25 eV [31], respectively. The Mott–Schottky plot (Figure 8d) of g-C₃N₄ and PN-g-C₃N₄ illustrates that the flat band potentials are −0.80 and −0.69 V, versus the saturated calomel electrode (SCE). The Fermi levels of g-C₃N₄ and PN-g-C₃N₄ are −0.58 and −0.47 V (vs. NHE) [32]. Therefore, the CB and VB potentials of g-C₃N₄ can be calculated to −0.88 and 1.80 eV, respectively, while the CB and VB potentials of PN-g-C₃N₄ were equal to −0.72 and 1.78 eV, respectively [33]. The potential position change between g-C₃N₄ and PN-g-C₃N₄ is shown in Figure 9, and the band gap structures and charge migration of g-C₃N₄ and PN-g-C₃N₄ are illustrated.

2.5. Possible Mechanism

The possible reaction mechanism is discussed for photocatalytic CO2 reduction with water into CO over PN-g-C₃N₄ as depicted in Figure 10. Generally, the photocatalytic CO₂ reduction reaction involves the following three steps: (i) CO₂ adsorption and activation; (ii) photo-produced charge carriers’ excitation and transfer to the catalyst surface; and (iii) photocatalytic reaction [34]. Upon illumination with light, the photocatalyst generated electrons (e^–) in the CB and holes (h⁺) in the VB, as shown in Equation (1). Further, the e^– are exploited to reduce CO₂ to its radical (CO₂^•−), as shown in Equation (2) [7]. The water (H₂O) oxidation arises at VB of the catalyst to produce the energetic protons (H⁺) and oxygen, as shown in Equation (3). The CO₂^•−, H⁺, and e^– further boosted the rate CO generation, as shown in Equation (4). In the present investigation, CO was developed, which involves an 2e^–/2H⁺ reduction process [35], as shown in Equation (5). In semiconductors, the numerous e^– and H⁺ transfer by proton-coupled electron transfer mechanism is feasible for multi e^– reduction reaction.

PN-g-C₃N₄ + hυ → PN-g-C₃N₄^* + h⁺+ e⁻

(1)

CO₂ + e⁻ + hυ → CO₂^•−

(2)

H₂O + 2h⁺ + hυ → (½)O₂ + 2H⁺

(3)

CO₂^•− + 2H⁺ + e⁻ + hυ → CO + H₂O

(4)

CO₂ + 2H⁺ + 2e⁻ + hυ → CO + H₂O

(5)

3. Materials and Methods

3.1. Materials

The chemical reagents used for the synthesis of PN-g-C₃N₄ were commercially available reagents. Melamine, hydrochloric acid, and ethylene glycol were purchased from Sigma-Aldrich. All the chemicals were used as received without further purification.

3.2. Synthesis of Catalysts

The PN-g-C₃N₄ was synthesized using a simple one-pot hydrothermal method. First, in a typical synthesis procedure, melamine (2 g, 99%) was dissolved in the mixture of distilled water (40 mL) and ethylene glycol (20 mL, 99%) to make a clear solution at 60 °C. Then, 2.4 mL concentrated hydrochloric acid (36.5%) was added into 60 mL of this solution by stirring for 10 min. Then, the mixed solution was transferred into a Teflon-lined autoclave and heated at 150 °C for 12 h. The mixture was filtered to remove the solvent and the precipitate was washed several times with ethanol and deionized water under ultrasonication, followed by drying overnight at 60 °C in vacuum oven. The resulting solid was heated at 600 °C for 2 h with a heating rate of 3 °C·min⁻¹. The g-C₃N₄ was synthesized by directly heating melamine at 500 °C for 2 h with a heating rate of 3 °C·min⁻¹.

3.3. Characterization

XRD spectra were recorded on a Bruker D8 Advance diffractometer (Cu Ka radiation). The IR spectra were collected with a Thermo Nicolet iS50 FTIR spectrometer, equipped with an attenuated total reflection (ATR) setup. Diffuse reflectance absorption spectra were recorded on a Varian Cary 4E UV-Vis system equipped with a Labsphere diffuse reflectance accessory. X-ray Photoelectron Spectroscopy (XPS) experiments were performed on Thermo ESCALAB 250 using monochromatized Al Kα at hυ = 1486.6 eV. Bandgap energy (Eg) of the g-C₃N₄ and PN-g-C₃N₄ samples was calculated according to the formula below:

(αhv)^1/n = C(hυ − Eg)

where α, υ, and C are the absorption coefficient, light frequency, and a constant, respectively. The parameter n is a pure number corresponding to different electronic transitions (n = 2 or 1/2 for indirect-allowed or direct-allowed transitions, respectively.

3.4. Photoactivity Meaasurements

The photocatalytic CO₂ reduction test was performed using a batch process under visible light with a 300 W Xenon lamp. In addition, a 420 nm cutoff filter was used to prevent the UV light and Am1.5 filter was used to simulate solar spectral. In this experiment, as-prepared photocatalyst (10 mg) was ultrasonically dispersed in 10 mL of deionized water using a 50 mL round-bottom quartz photo-reactor. Then, the reactor was tightly closed with a silicone rubber septum and the solution was saturated with CO₂ gas for 30 min before the light illumination. After illumination, the gaseous product such as CO was analyzed by gas chromatography.

4. Conclusions

In summary, porous g-C₃N₄ nanobelts were synthesized via a facile hydrothermal method. The obtained PN-g-C₃N₄ had abundant pores and edges, high specific surface areas, and possessed C≡N groups and vacant N on the surface, which dramatically improved the photocatalytic performance. This catalyst displayed enhanced optical absorption in the visible range. It efficiently and selectively catalyzed CO₂ reduction to CO under both visible light and simulated sunlight illumination. Enhanced visible light absorption and the existence of vacant N-sites on the surface also contributed to the photocatalytic activity of PN-g-C₃N₄. The successful synthesis of PN-g-C₃N₄ opens up a new way to improve the photochemical performance of carbon nitride-based catalyst.