Facile synthesis of in situ phosphorus-doped g-C₃N₄ with enhanced visible light photocatalytic property for NO purification

Abstract

A novel phosphorus doped g-C₃N₄ with typical optical property has been synthesized using phosphonitrilic chloride trimer and thiourea as precursors through a thermal copolymerization route. The samples were characterized by XRD, XPS, TEM, BET-BJH, UV-vis, DRS and PL. Their photocatalytic activities were evaluated by the removal of NO at the indoor air level under visible light irradiation. The typical optical performance and enhanced photocatalytic activity have been investigated in detail. The present work could provide new insight into the appropriate selection of the precursor for photocatalytic materials.

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Photocatalysis is a cost-effective and environment-friendly technology that has been employed in many fields, such as water splitting, contaminant purification, solar cells, etc.¹ However, the photoactivity of the catalyst is one of the key limitations in its large scale application. Graphitic carbon nitride (g-C₃N₄) has attracted increasing attention in the energy and environmental fields because it is metal free, easily synthesized, and offers desirable qualities including excellent chemical and thermal stability, a suitable band gap and intrinsic texture and unique electronic structure. However, g-C₃N₄ also suffers from many disadvantages which largely prevent its practical application, including low quantum efficiency, small BET surface areas, low visible-light utilization and poor photocatalytic performance. Therefore, it is highly desirable to explore novel strategies to enhance the visible-light photocatalytic performance in g-C₃N₄.²

Recently, many modified methods have been developed to deal with those drawbacks, like metal (e.g. Fe, Li, K) or non-metal (e.g. S, B, P) doping, construction of heterojunctions (e.g. g-C₃N₄/(BiO)₂CO₃, g-C₃N₄/MoS₂), surface plasmonic response (e.g. g-C₃N₄/Ag, g-C₃N₄/Bi) and micro-structural engineering (e.g. nanosphere, nanoflower, mesoporous).³ Wang et al. used a nanostructured silica template to fabricate highly stable, mono-disperse, hollow g-C₃N₄ nanospheres with controlled shell thicknesses as a light-harvesting platform for catalysing hydrogen evolution under visible light irradiation, and achieved a 7.5% overall apparent quantum yield. Kang et al. prepared carbon dots–C₃N₄ nanocomposites (CDots) by heating a mixture of urea and carbon dots solution. The incorporation of CDots into the g-C₃N₄ matrix is beneficial for improving the ultraviolet-visible light absorbance and photocatalytic efficiency. Contrary to traditional metal doping, non-metal doping is more suitable to the geometric structure of g-C₃N₄, which features nitrogen triangles having six lone-pair electrons.⁴ Generally, phosphorus modification has also been recognized as a promising strategy to fine-tune the intrinsic texture and electronic structure due to its influence on the conformation and connectivity of g-C₃N₄.³ Shi et al. prepared brand new P-doped g-C₃N₄ via a thermally induced copolymerization of hexachlorocyclotriphosphazene and guanidinium hydrochloride. The as-prepared P-doped g-C₃N₄ samples showed enhanced photocatalytic activity for hydrogen evolution and rhodamine B degradation under visible light irradiation. Yuan et al. reported that mesoporous phosphorus-doped g-C₃N₄ with nanostructured flowers was synthesized by a template-free phosphonic-mediated route and exhibited superior performance in photocatalytic hydrogen evolution. Xue et al. prepared red phosphorus/g-C₃N₄ heterojunctions by annealing a mixture of red phosphor and g-C₃N₄ in inert atmosphere. The resulting material showed considerable improvement in photocatalytic activity for H₂ production and CO₂ conversion into CH₄ in comparison to pure g-C₃N₄. Furthermore, the absorption band edge of P modified g-C₃N₄ apparently red shifted in comparison to pure g-C₃N₄ in the previous reports. It should be noted that these application studies have examined only the energy conversion and organic dyes purification.⁵ Although P modified g-C₃N₄ has been reported, little work has been focused on the photocatalytic removal of nitric oxide (NOx) and fabrication of novel P-doped g-C₃N₄ with blue-shift optical properties.

In this work, we demonstrate a novel P-doped g-C₃N₄ photocatalyst with typical optical properties, which was prepared by directly heating the mixture of thiourea and Cl₆N₃P₃ in air. Compared with the pure g-C₃N₄, the absorption of band edge of as-prepared P-doped g-C₃N₄ samples could not only show blue shift, but also exhibit significant enhancement in photocatalytic removal of NO. The typical optical properties and improved photocatalytic mechanism of P-doped g-C₃N₄ samples have been investigated in detail. This work provides a new insight into the design and synthesis of P-doped g-C₃N₄ materials with typical optical properties for practical application.

Fig. 1 shows the XRD patterns of the as-obtained samples. The strongest diffraction (002) peaks at 27.6° can be indexed to the inter-planar stacking peaks of conjugated aromatic systems; the small angle (100) peaks at 13.0° correspond to the in-plane tri-s-triazine units. No additional peaks can be detected in the XRD patterns of the CN–P-X samples.⁶ It is noteworthy that both the (002) and (100) peak intensities decreased with increased phosphorus content. This indicates that the crystal growth of g-C₃N₄ can be inhibited by the introduction of phosphorus, which could be ascribed to the decrease in correlation length of particle size in the phosphorus doped g-C₃N₄ network structure. Furthermore, this phenomenon could produce more secondary nanoparticles.

Fig. 1 XRD patterns of CN, CN–P-3, CN–P-5 and CN–P-10.

XPS measurements were carried out to confirm the surface chemical compositions of the CN–P-5 sample. The XPS survey spectrum (Fig. 2a) illustrates that CN–P-5 is composed of C, N, P and O; thus, a low concentration of P has been successfully doped into the g-C₃N₄ matrix. The corresponding surface atomic concentration is summarized in Table 1. Fig. 2b shows three evident peaks of C1s spectra at 284.8, 288.1 and 289.2 eV. The C peaks at 284.8 eV can be indexed to adventitious carbon species. The C peaks at 288.1 eV are identified as the sp²-hybridized carbon in the trizine rings (N–C=N). The peaks located at 289.2 eV in the C1s spectrum can be correlated to the O–C=O. The N1s spectrum (Fig. 2c) can be divided into four peaks located at 398.5, 399.6, 400.7 and 404.2 eV. The N peak at 398.5 eV is assigned to sp²-hybridized aromatic nitrogen in carbon-containing trizine rings (C=N–C), whereas the peak at 399.6 eV corresponds to the tertiary N bonded to carbon atoms in the form of H–N–(C)2 or N–(C)3. In addition, the N peaks at 400.7 and 404.2 eV are attributed to the residual amino groups (N–H) and the π-excitations, respectively. Fig. 2d displays the P2p spectra. The P peaks at 133.2 and 134 eV are attributable to the P–N and P=N bonds, respectively. Furthermore, the P–N bond has lower energy than P=N bond. However, it is reported that the binding energy of P–N species (133.5 eV) is higher than P–C coordination (131.5–132.5 eV). Therefore, the results indicate that P atoms could replace C atoms in the C–N and C=N framework of g-C₃N₄. The P atoms most probably replace the corner or bay C atoms in the doped g-C₃N₄ framework.^3,5 Besides, the P peak at 135.1 eV can be indexed to the P=O bond, which can be ascribed to the reaction between P species and O₂ during the polymerization process at high temperature.⁵ The O1s peak (Fig. 2e) at 534.6 eV is assignable to the adsorbed H₂O on the surface of the CN–P-5 sample, while the peak at 533.1 eV can be attributed to the combined effects of double bonded oxygen (=O) in the C=O and P=O groups. The peak located at 530.9 eV can be assigned to the single bonded oxygen (–O–) in the C–O group.⁷ No evident peak shifts can be found in the C1s, N1s, P2p and O1s spectra of CN–P-5 as compared with pure CN, suggesting that P has been successfully doped into the CN matrix with no apparent effect on the chemical state of CN–P-5.⁸ However, the valence band (VB) maxima of CN–P-5 is revealed to be 1.61 eV (Fig. 2f), which is more positive than that of the pure CN (1.58 eV).⁹ The relative down-shift VB-edge potential could exhibit much stronger oxidation ability, which could work to promote the photocatalytic oxidation reaction.

Fig. 2 (a) XPS survey spectrum and XPS spectra, (b) C1s, (c) N1s, (d) P2p, (e) O1s and (f) valence band of CN–P-5.

Table 1 Atomic surface compositions of CN–P-5 as obtained with XPS

Samples	Atomic compositions (%)
	C	N	P	O
CN–P-5	11.15	75.91	2.45	10.5

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The morphologies and microstructures of CN and CN–P-5 were observed by TEM. As shown in Fig. 3, both CN and CN–P-5 are composed of a large number of soft layered nanosheets. In addition, there are abundant pore structures on the surface of CN–P-5 nanosheets as shown in Fig. 3d, demonstrating that Cl₆N₃P₃ favours the generation of pore structures during thermal polymerization. The enriched pore structures can promote mass transfer and provide many more active sites for the photochemical reaction.

Fig. 3 TEM images of CN (a and b) and CN–P-5 (c and d).

Fig. 4 shows the N₂ adsorption–desorption isotherms and the corresponding pore-size distribution curves of the as-obtained samples. The N₂ adsorption–desorption isotherms in Fig. 4a reveal that all the samples are of type IV (BDDT classification), suggesting the existence of mesopores. Hysteresis loops with type H3 display the formation of slit-like pores arising from aggregation of the sheet-like particles.¹⁰ Fig. 4b shows that the average pore size increases with increasing Cl₆N₃P₃ content. The BET specific surface areas and total pore volume of the samples are shown in Table 2. It can be seen that both S_BET and V_p increase with the addition of Cl₆N₃P₃, which could be due to the release of the gaseous Cl₆N₃P₃ during copolymerization and the formation of more secondary nanoparticles.

Fig. 4 N₂ adsorption–desorption isotherms (a) and the corresponding pore-size distribution curves (b) of CN, CN–P-3, CN–P-5 and CN–P-10.

Table 2 The BET surface areas (S_BET) and total pore volume (V_p) of CN, CN–P-3, CN–P-5 and CN–P-10

Samples	SBET (m^2 g^−1)	Vp (cm^3 g^−1)
CN	13.49	0.11
CN–P-3	22.17	0.15
CN–P-5	23.80	0.17
CN–P-10	30.33	0.21

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The typical optical properties of as-synthesized CN and CN–P-X were investigated by UV-vis DRS and PL spectra. As shown in Fig. 5a, all the samples show light absorption in the visible region. However, compared with CN, the absorption band edges of the CN–P-X samples exhibit significant blue-shifts with the introduction of phosphorus, indicating an increase in band gap energy.¹¹ Fig. 5b shows that the PL intensity of CN–P-5 is certainly higher than that of CN. These phenomena are not consistent with the previous reports of phosphorus doped g-C₃N₄.¹² The typical optical properties of UV-vis DRS and PL could be caused by electron relocalization and quantum size effect (Fig. 6).

Fig. 5 UV-vis DRS (a) and PL spectra (b) of the as-synthesized samples.

Fig. 6 Photocatalytic activity (a) and Arrhenius rate constants (b) of CN, CN–P-3, CN–P-5 and CN–P-10 for the removal of NO in air under visible light irradiation (λ > 420 nm).

In order to demonstrate the intrinsic activity of the as-synthesized samples under visible light irradiation, their photocatalytic properties were evaluated by removal of NO in the gas phase. NO can be transformed to HNO₂ and HNO₃ after the photocatalytic oxidation reaction with the photo-generated reactive radicals. When the reaction reached equilibrium in 30 min of visible light irradiation, the photocatalytic activity was kept constant.⁹

After 30 min of irradiation, the NO removal ratios of CN, CN–P-3, CN–P-5 and CN–P-10 were 28.7%, 37.7%, 42.3 and 37.5%, respectively. The removal ratios of CN–P-X first increased and then decreased, indicating that the excess phosphorus doping could act as recombination centers of photogenerated electron–hole pairs. Moreover, the rate constant of CN–P-5 is 0.1224 min⁻¹, which is 1.43 times higher than that of CN (0.086 min⁻¹). The enhancement of the photocatalytic activity of CN–P-X can be ascribed to the following factors.¹³ First, the relatively large S_BET and V_p of CN–P-X could promote mass transfer and provide more active sites for the photocatalytic reaction. Second, the P doped g-C₃N₄ is favorable for the separation and transfer of the photo-generated electrons and holes, which contributes to the increase in the quantum yield and promotes the photocatalytic activity. Eventually, the down-shift VB-edge potential of CN–P-5 showed much stronger oxidation ability and improved photocatalytic efficiency.

Phosphorus doped g-C₃N₄ with typical optical properties was synthesized using Cl₆N₃P₃ as the phosphorus source and thiourea as the g-C₃N₄ precursor. The introduction of an appropriate concentration of phosphorus can decrease the degree of crystallinity, increase the band gap energy, and improve the S_BET and V_p. Moreover, the optical properties of CN–P-5 have been changed, causing not only a blue-shift in light absorption, but also an enhancement in the PL intensity. Moreover, the down-shift VB-edge of CN–P-5 could exhibit much stronger oxidation ability, which ensures enhanced photocatalytic NO removal. These results indicate that different precursors and phosphorus sources have crucial effects on the physicochemical properties of P doped g-C₃N₄ photocatalysts.

Acknowledgements

This research is financially supported by the China Postdoctoral Science Foundation funded project (No. 2016M592642), Project from Chongqing Education Commission (KJ1600305), Chongqing Basic Science and Advanced Technology Research (cstc2016jcyjAX0003), the Start-up Foundation for Doctors of Chongqing Normal University (No. 15XLB010, 15XLB014), and the National Natural Science Foundation of China (No. 51478070, 51108487).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization. See DOI: 10.1039/c6ra18349b

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