Catalytic activity of porous carbon nitride regulated by polyoxometalates under visible light

Abstract

A series of porous carbon nitrides modified by different polyoxometalates (POMs) were prepared by the ultrasonic method. POMs were assembled on the surface of mpg-C₃N₄ via electrostatic attraction. The catalyst has visible light degradation activity for phenol (λ > 420 nm). mpg-C₃N₄ modified by H₄SiW₁₂O₄₀ with a mass ratio of 1 : 5 showed the highest catalytic activity, which was 3.5 times higher than that of mpg-C₃N₄. As an electron acceptor, polyoxometalate can capture the photoelectron of C₃N₄, which can promote the separation of photocharge and improve the photocatalytic activity. ESR also confirmed that the superoxide radicals play a major role in degradation. The results show that the charge separation efficiency and catalytic activity can be enhanced by polyacids.

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Introduction

Graphitic carbon nitride (g-C₃N₄) is the first visible light response material discovered in the field of photocatalysis. Its crystal structure is very simple. The valence band is made up of N 2p orbitals, and the conduction band is made up of C 2p orbitals. Due to its unique electronic structure, suitable bandgap width, high thermal and chemical stability, environmental friendliness, low cost, etc., it has wide application prospects in the fields of catalysis, environmental governance, sensors, drug carriers, and other fields.^1–3 Although carbon nitride (g-C₃N₄) has attracted considerable attention in the field of photocatalysis, pure g-C₃N₄ has many disadvantages: (1) due to its special lamellar structure, the photocatalytic degradation efficiency of pollutants is not high; (2) the specific surface area is small; and (3) the visible light utilization is less.⁴ These shortcomings limit its large-scale applications in environmental purification. Therefore, it is necessary to modify C₃N₄ by extending the visible light absorption of g-C₃N₄ and reducing the photogenerated electron–hole recombination rate.^4–6 Various C₃N₄/semiconductor complexes have been reported, including C₃N₄/TiO₂, C₃N₄/ZnO, C₃N₄/CdS, and C₃N₄/precious metal, all of which significantly improve the photocatalytic activity of C₃N₄.^7,8

The pore structure of a catalyst has a certain influence on its performance. Porous g-C₃N₄ (mpg-C₃N₄) has a larger specific surface area than pure g-C₃N₄. Apart from this, it has abundant catalytic active sites and also exhibits the crystallization with open hole, which is conducive to the diffusion of the mpg-C₃N₄ nanostructure, improving the ability to capture light and helping other photocatalytic materials to grow on its surface. Therefore, on the basis of mpg-C₃N₄, the catalytic effect of g-C₃N₄ can be greatly improved by the compound method. It is the most common way to improve the photocatalytic activity to improve the poor separation efficiency of carriers by means of heteroatom binding, semiconductor composite, and supporting Pt, Au, and other precious metal ions.^9–11

Polyoxometalates (POMs) after the electronic reversible redox process become a more complete structure, which means that more acids can be used for electronic “shallow well” capture and storage; at the same time, under the condition of appropriate release, they also exhibit electronic capture and have strong electronic storage capacity, due to which more acids as electron acceptors can improve the semiconductor optical carrier separation.^12–15 Using this property, it is expected to realize the effective separation of semiconductor photocarriers and improve the photocatalytic efficiency. Professor Zhangguangjin loaded multi-walled carbon nanotubes with Au nanoparticles wrapped in an acid (Au NPs@POM-CNT).¹⁶ The H₃PW₁₂O₄₀-doped C₃N₄ nanotubes prepared by Professor Luo Shenglian exhibit excellent visible light degradation of methyl orange and organic pollutants.¹⁷

The catalytic mechanism shows that the photoelectrons generated by C₃N₄ are transferred to the LUMO orbital of PW₁₂ under the excitation of visible light, and the photogenerated holes stay in the valence band of C₃N₄. The transferred electrons and holes generate free radicals with high redox activity and react with the adsorbed substrate molecules, respectively.^18–20

In this study, a series of porous g-C₃N₄ complexes were designed and synthesized using POMs, and a variety of acids were used to regulate the photocatalytic activity of C₃N₄. The polyacids include H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiW₁₂O₄₀, and H₄SiMo₁₂O₄₀. The regulation of the photocatalytic activity of C₃N₄ without polyacids was discussed.

Results and discussion

Controllable construction of C₃N₄-POM composite structure

Because mpg-C₃N₄ has a large volume and layered structure, in order to make full contact between the polyacid and mpg-C₃N₄, ultrasound was first used to peel it off. The acid was then combined with the stripped mpg-C₃N₄ through electrostatic attraction and hydrogen bonding. In order to enhance the bonding of polyoxoanions with C₃N₄, the composite was annealed at 200 °C. POM-modified C₃N₄ was successfully prepared by the above-mentioned steps.

The XRD results shown in Fig. 1a indicate that mpg-C₃N₄ has two characteristic absorption peaks at 13.1° and 27.0°, corresponding to the C₃N₄ (JCPD-1526) crystal planes of (100) and (002). The absorption peak at 27.3° corresponds to the accumulation direction between the (002) crystal plane of g-C₃N₄ and the CN aromatic plane. There is no characteristic diffraction peak of the polyacid, indicating that the compound amount is very small and the crystallinity of polyanions is poor. The diffraction peak at 27.3° corresponds to the (002) crystal plane of the mpg-C₃N₄ thin layer.²¹ After compounding with polyanions, the peaks shift slightly. This is because the electrostatic attraction and hydrogen bonding between multiple acids and mpg-C₃N₄ reduces the spacing between the layers of C₃N₄, indicating that multiple acids have compounded to the surface of C₃N₄. Polyacids can act as electron acceptors to capture the photogenerated electrons of C₃N₄, thus improving the photocatalytic activity of C₃N₄.

Fig. 1 (a) The XRD patterns of C₃N₄ and POM/C₃N₄; (b) the UV-vis spectra of C₃N₄, POM and POM/C₃N₄.

The spectral absorption properties of the complex were determined by a UV-vis diffuse reflectometer (DRS), as shown in Fig. 1b. The absorption band edge of both mpg-C₃N₄ and g-C₃N₄ was 450 nm, which was consistent with the band width of g-C₃N₄ reported in the literature (about 2.7 eV). The absorption band edges of SiW₁₂ and PW₁₂ were at 400 nm and 420 nm, and the absorption band edges of SiMo₁₂ and PMo₁₂ were at 500 nm and 550 nm, respectively. The optical absorption range of the products of SiW₁₂, PW₁₂, SiMo₁₂, and mpg-C₃N₄ in the visible region was the same as that of mpg-C₃N₄, indicating that there was no effect on the spectral absorption of C₃N₄. The absorption intensity of PMo₁₂/C₃N₄ in the visible region was slightly enhanced.

The morphology of SiW₁₂/C₃N₄ was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The SEM (Fig. 3) and TEM (Fig. S2†) images show that the prepared CN has a porous structure. In Fig. 2, we can observe that many polyacid particles are distributed on the surface of C₃N₄, and their size is 1 nm. The SEM images (Fig. 3 and S3†) show that g-C₃N₄ has a distinct layered structure with a smooth surface and no other components; pores of different sizes are distributed on the surface of mpg-C₃N₄. The EDAX map (Fig. S3†) and the related element distribution indicated that C, Si, and W existed in the complex, and the polyacid mass fraction in SiW₁₂/C₃N₄-3 was about 2%. The XPS results also showed that the polyacid successfully modified the surface of C₃N₄. The BET adsorption curve (Fig. S4†) shows that there is a hysteresis loop. Combined with the SEM images, we can observe that there are holes of about 100 nm; thus, the materials are basically between macroporous and mesoporous ones, with a specific surface area of 41.899 m² g⁻¹.

Fig. 2 HRTEM image of SiW₁₂/C₃N₄-3.

Fig. 3 SEM images of (a) g-C₃N₄ and (b) mpg-C₃N₄.

Catalytic activity by the composite structure

The phenol degradation under visible light was evaluated to reveal the photocatalytic activity of the composite structure. Blank experiments showed that phenol basically did not degrade without the catalyst. Under visible light irradiation, the activity of POMs/mpg-C₃N₄ was better than that of bulk g-C₃N₄ and mpg-C₃N₄. When only g-C₃N₄ was used as the catalyst, the phenol degradation was about 13.4%, that for mpg-C₃N₄ was 26.2%, and the degradation efficiency of POMs/mpg-C₃N₄ improved, indicating that the introduction of a polyacid improved the photocatalytic activity. In a composite structure, the addition of multiple acids reduces the composite center of carriers and accelerates the separation of photogenerated electrons and holes. The order of the catalytic efficiency of different polyacids is SiW₁₂/C₃N₄ > PW₁₂/C₃N₄ > PMo₁₂/C₃N₄ > SiMo₁₂/C₃N₄. SiW₁₂/C₃N₄ has the best activity, which is 6.8 times that of g-C₃N₄ and 3.5 times that of mpg-C₃N₄. With the increase in the SiW₁₂ content, the activity of the complexes also increased but when the ratio of SiW₁₂ increased to 1 : 5, the activity was the highest, and the catalytic activity of SiW₁₂ no longer increased. It is known that polymetallic oxalate is an excellent electron capture center, which can speed up the separation of photogenerated carriers with an appropriate concentration. If too many polyanions are loaded, it will become the recombination center, which reduces the photocatalytic performance of the composite. Therefore, the catalytic activity of SiW₁₂/C₃N₄-4 and SiW₁₂/C₃N₄-5 decreased. Cyclic experiments were conducted to investigate the stability of the complex using SiW₁₂/C₃N₄-3 as an example. It was observed that after three cycles, the degradation rates of phenol were 90.3%, 82.4%, and 70.2%. The catalytic activity of g-C₃N₄ and mpg-C₃N₄ decreased by 44.5% and 58.5%, respectively, after three cycles. The photodegradation activity after the two subsequent cycling tests did not decrease significantly compared with that for the first test, indicating that SiW₁₂/C₃N₄ had good stability (Fig. 4 and 5).

Fig. 4 Recycle experiment on the SiW₁₂/C₃N₄-3 activity for degradation of phenol under visible light.

Fig. 5 Photocatalytic activity of phenol under visible light.

Mechanism of enhancing the photocatalytic activity of the composite structure

(1) Photoluminescence spectroscopy and electrochemical impedance spectroscopy. The separation, migration, and recombination of photocarriers in the compound photocatalyst were studied by the PL technique. As shown in Fig. 6, mpg-C₃N₄ exhibits the strongest intensity in the fluorescence emission spectrum at 460 nm; as the modified SiW₁₂ content increases, the fluorescence emission intensity of the complex decreases gradually, suggesting that SiW₁₂ as electronic photoproduction by physical capture C₃N₄, which accelerates the separation of the carriers and reduces the recombination probability of the photogenerated electrons and holes. The fluorescence emission intensities of SiW₁₂/C₃N₄-3, SiMo₁₂/C₃N₄, PW₁₂/C₃N₄ and PMo₁₂/C₃N₄ are all lower than that of mpg-C₃N₄, indicating that these polyacids can act as electron receptors to capture the photogenerated electrons of C₃N₄ and reduce their fluorescence intensity. SiMo₁₂/C₃N₄ > SiW₁₂/C₃N₄ > PMo₁₂/C₃N₄ > PW₁₂/C₃N₄, indicating that the ability to capture electrons for the four polyacids is from high to low: PW₁₂ > PMo₁₂ > SiW₁₂ > SiMo₁₂. In the composite structure, the addition of multiple acids reduces the composite center of the carriers and accelerates the separation of the photogenerated electrons and holes. SiW₁₂/C₃N₄ > PMo₁₂/C₃N₄ > PW₁₂/C₃N₄, indicating that the ability to capture electrons for the four polyacids ranges from high to low: PW₁₂ > PMo₁₂ > SiW₁₂ > SiMo₁₂. In the composite structure, the addition of multiple acids reduces the composite center of carriers and accelerates the separation of the photogenerated electrons and holes.

Fig. 6 The photoluminescence (PL) spectra of POM/C₃N₄.

The surface composition and the chemical state of the complex were characterized by XPS. C, N, O, Si, and W exist in SiW₁₂/C₃N₄-3. Fig. 7 shows that the binding energies of C 1s are 288.3 and 284.8 eV. The peak at 288.3 eV corresponds to the sp² carbon (C–N=C), and the peak at 284.8 eV corresponds to the graphitized carbon. The peak at 399 eV corresponds to ternary N–(C)₃ or H–N–(C)₂, and the peak at 400.1 eV is assigned to the aromatic ring. The O1s spectrum can be divided into three peaks at 530.2 eV, 530.9 eV, and 532 eV. The binding energy of 101.8 eV corresponds to the Si–O bond, which is consistent with the standard value of Si 2p. The W 4f spectrum shows that the binding energies are 35.8 eV and 37.7 eV, indicating that the W element is at the highest valence, i.e., W⁶⁺, which is consistent with the previous report.²²

Fig. 7 The XPS spectra of SiW₁₂/C₃N₄-3.

(2) Photocatalytic mechanism. The interface charge separation efficiency was studied by electrochemical impedance spectroscopy (EIS). As can be seen from Fig. 8a, the arc radius in the SiMo₁₂/C₃N₄ impedance spectrum is significantly smaller than that for other catalysts with multiple acids and C₃N₄ composites and also smaller than that of pure C₃N₄ with light irradiation. The results showed that the composite catalyst could increase the photocatalytic activity and increase the separation and transfer of photocatalytic active charge at the interface.²³

Fig. 8 (a) EIS Nynquist plot under visible light irradiation (λ_vis > 20 nm) [Na₂SO₄ = 0.1 M]. (b) ESR of superoxide radical test in DMSO (100 mM) as radical trapper under visible light irradiation (λ_vis > 420 nm). (c) ESR of hydroxyl radical test in an aqueous solution under visible light (λ_vis > 420 nm).

A free radical capture experiment was used to study the catalytic mechanism. The free radical capture agents included triethanolamine (TEOA), isopropanol (IPA), and p-benzoquinone (BQ), which were used to capture the photosynthetic holes (h⁺), hydroxyl radicals (˙OH), and superoxide radicals (˙O₂⁻) of the reactive species, respectively. The concentrations of TEOA, IPA, and BQ were 1.0 mmol L⁻¹, 10 mmol L⁻¹ and 1.0 mmol L⁻¹, respectively. Before the photocatalytic test, the capture agent was added to the phenol solution, and the more serious the degradation rate, the more important the relative species captured by the capture agent for the photocatalytic process. The addition of BQ can inhibit the photocatalytic reaction, while isopropanol and triethanolamine have almost no effect on phenol degradation, indicating that superoxide free radicals (˙O₂⁻) were the main active species in the photocatalytic degradation reaction (Fig. S5†).

In order to further elucidate the mechanism of photocatalytic degradation, ESR was used to detect the main oxidizing substances during the photocatalytic degradation. As shown in Fig. 8b, there was no obvious hydroxyl superoxide radical (·O₂⁻) formation in the DMSO solution without light. After exposure to visible light, both C₃N₄ and SiMo₁₂/C₃N₄ produce ·OOH (·O₂⁻ receiving proton H⁺ in DMSO generate ·OOH), H₄SiW₁₂O₄₀ mainly produces hydrogen radical after illumination (·H). Fig. 8c showed that under the light condition, both C₃N₄ and SiMo₁₂/C₃N₄ produced ·OH in the aqueous solution, while H₄SiW₁₂O₄₀ had no ·OH signal peak. Combined with the free radical capture experiment, the results show that the superoxide radical (˙O₂⁻) was the main active material in the photocatalytic degradation.^24,25

Experimental

Preparation

(1) mpg-C₃N₄. First, 300 nm CaCO₃ was placed at 550 °C for 12 h; the surface organic layer was removed and then, 0.5 g cyanamide (DCDA) and 2 g 300 nm CaCO₃ were mixed according to the mass ratio of 1 : 4. Subsequently, 30 mL water was added and the dry powder was concentrated at 100 °C; then, it was kept at 3.1 °C min⁻¹ to 400 °C in nitrogen for 2 hours, 1 mol L⁻¹ hydrochloric acid for 2 hours to remove CaCO₃, and at 4.4 °C min⁻¹ to 400 °C until 550 °C for 2 hours.

(2) H₄SiMo₁₂O₄₀. We added 37 mL concentrated HNO₃ to 120 mL 1 M Na₂MoO₄ solution and then, 50 mL 0.2 M Na₂SiO₃ solution was added dropwise to it. Next, we added 40 mL concentrated HCl, cooled to room temperature, and extracted with 80 mL ether, followed by the release of the ether layer. Subsequently, 40 mL water was added, and it was then dried in vacuum to obtain yellow crystals.

(3) POMs/mpg-C₃N₄. First, 0.5 g mpg-C₃N₄ powder was added into 20 mL methanol, and the lamellar mpg-C₃N₄ was evenly dispersed by ultrasound for 14 h. Different polyacids, namely, H₃PW₁₂O₄₀·xH₂O, H₃PMo₁₂O₄₀·xH₂O, H₄SiW₁₂O₄₀·26H₂O, and H₄SiMo₁₂O₄₀ with a certain mass were dissolved in 10 mL methanol and were added to the mpg-C₃N₄ suspension for 4 h ultrasonication. Subsequently, it was washed with methanol, dried at 60 °C, and then kept at 200 °C for 4 h in a muffle furnace. The products were named as PW₁₂/C₃N₄, PMo₁₂/C₃N₄, SiW₁₂/C₃N₄ and SiMo₁₂/C₃N₄. H₄SiW₁₂O₄₀·26H₂O and mpg-C₃N₄ were prepared according to different mass ratios of 1 : 50, 1 : 10, 1 : 5, 3 : 10, and 6 : 10. The products were named as SiW₁₂/C₃N₄-1, SiW₁₂/C₃N₄-2, SiW₁₂/C₃N₄-3, SiW₁₂/C₃N₄-4, and SiW₁₂/C₃N₄-5.

Characterizations

An HT7700 TEM transmission electron microscope, HITACHI Regulus8100 scanning electron microscope and JEM-2011F high-resolution electron microscope (HRTEM) were used to characterize the product morphology. UV-3010 (Shimadzu) UV-visible solid diffuse reflection (DRS) was used to characterize the spectral properties of the products. The XPS data of the product were characterized by the PHI Quantera SXMTM system. The fluorescence spectra were measured with a Varian Cary Eclipse500 fluorescence photometer, and the excitation wavelength was 320 nm. The photocurrents were measured on the CHI 660B electrochemical system. The active species of phenol degradation can be detected by ESR (JEOL JES-FA200 ESR Spectrometer) using dimethyl sulfoxide (DMSO) as a free radical trapping agent.

Photocatalytic experiments

The 350 W Xe lamp was used as the visible light source, the 420 nm filter was used to filter the ultraviolet light, and the XPA-7 photochemical reaction instrument was used. Before illumination, 50 mg catalyst was mixed with 50 mL 1 × 10⁻⁵ mol L⁻¹ phenol solution, and the dark equilibrium was maintained for 1 h, thereby reaching the adsorption–desorption equilibrium on the catalyst surface. After light exposure, the reaction solution was taken at an interval of 1 h. The concentration of phenol was analysed by Shimadzu LC-20A high-performance liquid chromatography (HPLC) with a Venusil XBP-C18 and a UV detector at 270 nm. The mobile phase consisted of methanol and water (volume ratio: 55/45) at a flow rate of 1 mL min⁻¹.

A series of mpg-C₃N₄ complexes modified by polyacids (POMs = H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiW₁₂O₄₀, and H₄SiMo₁₂O₄₀) were prepared. The results of XRD, TEM, XPS, and EDAX confirmed that a small amount of a polyacid was modified on the surface of mpg-C₃N₄ by electrostatic interaction and hydrogen bonding. Photocatalysis demonstrated the complex's high visible light degradation of phenol (λ > 420 nm). The content and type of a polyacid can regulate the photocatalytic activity of the product. The 1 : 5 SiW₁₂-modified mpg-C₃N₄ had the highest catalytic activity. The catalytic mechanism indicates that the polyacid as an electron acceptor can capture the photogenerated electrons of C₃N₄ and promote the separation of photogenerated carriers, leading to improved photocatalytic activity. This work has great significance for the study of polyacids to improve the photocatalytic activity of semiconductors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by National Nature Science Foundation of China (21401131).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra10441k

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