A simple and efficient method to prepare a phosphorus modified g-C₃N₄ visible light photocatalyst

Abstract

Phosphorus doped g-C₃N₄ was prepared by a simple method using dicyandiamide monomer and diammonium hydrogen phosphate as precursors. X-ray diffraction (XRD), UV-Vis spectroscopy, Fourier transform infrared spectra (FT-IR), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) and photocurrent measurement were used to characterize the prepared catalysts. The results indicated that the introduction of phosphorus inhibited the crystal growth of graphitic carbon nitride, decreased the band gap energy and increased the separation efficiency of photogenerated electrons and holes. The P doping site is influenced by the phosphorus source. Interstitial P doping is more effective in improving the photocatalytic activity of Rhodamine B (RhB) degradation compared with substitutional P doping. A possible mechanism was proposed.

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Introduction

Photocatalytic oxidation of aqueous organic pollutants into CO₂, water and other nonhazardous compounds is one of the environmentally friendly manners to remove organic pollutants from water.¹ One of the primary challenges in practical application of this technique is to design a robust, cheap and stable photocatalyst that exhibits excellent visible-light absorption as well as efficient separation and transportation of the photogenerated holes and electrons. However, the wide band gap energy and low quantum efficiency are still the “bottleneck” of this technology to meet the requirement of practical application. In the past decades, TiO₂ is the most common used photocatalyst. However, TiO₂ performs rather poorly in the processes associated with solar photocatalysis because of its wide band gap.² For this purpose, the modified TiO₂ and TiO₂-based nanocomposite photocatalysts have been developed to improve the visible light absorption ability and promote the separation of holes and electrons pairs.^3–9 N doping into TiO₂ has been reported to be one of the most effective approaches to enhance the visible light activity.³ However, Chen et al.¹⁰ and Hu et al.¹¹ reported that the photocatalytic activity of N doped TiO₂ was not stable. The lattice-nitrogen was oxidized easily by photogenerated holes during the degradation reaction, leading to the decrease of activity. Therefore, it is still highly desirable to develop new photocatalyst with enhanced activity under visible light.

Recently, graphitic carbon nitride (g-C₃N₄) has attracted a great deal of interest in photocatalytic applications. Carbon nitride can exist in five phases, α, β, cubic, graphitic and pseudocubic, among which graphitic phase is regarded as the most stable structure. The heptazine ring structure and high condensation degree make it possess many advantages, such as good chemical stability, appropriate band gap energy and unique electronic structure. In addition, g-C₃N₄ is metal-free and easily-synthesized via one-step polymerization of cheap raw materials. All of these make g-C₃N₄ to be the best candidate for photocatalytic application. However, g-C₃N₄ also suffers from many disadvantages, such as the low carrier mobility, low visible-light utilization efficiency, high recombination rate and small BET surface area. Therefore, modification of g-C₃N₄ with other metal or non-metal element was developed in recent years.^12–18

It is known that g-C₃N₄ contains nitrogen triangles having six lone-pair electrons, which are available for nonmetal doping. Therefore, nonmetal doping of g-C₃N₄ seems to be a feasible way to deal with those drawbacks. Yan et al. prepared boron-doped g-C₃N₄ by heating the mixture of melamine and boron oxide.¹² They suggested that boron doping increased the photocatalytic activity of RhB degradation due to the improvement of dye adsorption and light absorption of catalyst. Zhang et al. used elemental sulfur (S₈) as the solvent and mediator to promote the photocatalytic performance of prepared g-C₃N₄.¹³ This sulfur-flux-mediated polymerization route of carbon nitride improved its intrinsic texture, morphology, optical and electronic properties, thus increased the photocatalytic water reduction performance. Generally speaking, conjugated polymer doped with higher periodic elements tends to show narrower band gap. For example, polythiophene shows a decreased band gap than that of polypyrrole.¹⁹ Therefore, phosphorus atom is expected to be the ideal dopant to tune the texture and electronic structure of g-C₃N₄. Zhang et al. first synthesized phosphorus doped g-C₃N₄ using dicyandiamide and ionic liquid [Bmim]PF₆ as raw material.¹⁴ They found the electrical conductivity and photocurrent were significantly improved after phosphorus doping. After that, Zhang et al. synthesized the phosphorus doped g-C₃N₄ using the same raw material.¹⁵ The obtained phosphorus doped g-C₃N₄ exhibited obvious improved photocatalytic activities for RhB and methyl orange degradation under visible light. However, ionic liquid [Bmim]PF₆ is not an optimal phosphorus source because of its high price and complicated synthesis process. It is known that the doping site can significantly influence the properties of prepared catalysts. There are many literatures concerning the effect of N doping site on properties and photocatalytic performance of prepared TiO₂ based catalysts. N dopant can enter the TiO₂ matrix by either direct substitution of an oxygen site or by stationing itself interstitially. The difference in doping site could cause the different oxygen vacancy content and band gap structure, thus leading to the difference in activity. Although many metal and non-metal element doped g-C₃N₄ catalysts were reported, few of those literatures investigated the effect of doping site on the photocatalytic activity of prepared g-C₃N₄ based catalysts. Hence, in this work, a simple method to synthesize phosphorus doped g-C₃N₄ using diammonium hydrogen phosphate as phosphorus source is reported. The photocatalytic activities were evaluated in the photocatalytic degradation of RhB under visible light. For comparison, [Bmim]PF₆ was also used as the phosphorus source to investigate the effect of P doping site on the structural property, optical property and photocatalytic performance of prepared g-C₃N₄ based catalysts. The possible reaction mechanism is proposed.

Experimental

Preparation and characterization

In a typical experiment, 3 g dicyandiamide was dissolved into 15 ml deionized water under stirring. Then desired amount of (NH₄)₂HPO₄ was added. The obtained solution was heated to 100 °C under stirring to remove the water. The solid product was dry at 100 °C in oven, followed by milling and annealing at 520 °C for 2 h (at a rate of 5 °C min⁻¹). The prepared catalyst was denoted as P(x%)-CN, where x% stands for the mass percent of (NH₄)₂HPO₄ to dicyandiamide. For comparison, g-C₃N₄ was prepared following the same procedure as in the synthesis of P(x%)-CN but in the absence of (NH₄)₂HPO₄. When ionic liquid [Bmim]PF₆ was used to replace (NH₄)₂HPO₄ (with the same P quality as (NH₄)₂HPO₄) following the same procedure as in the synthesis of P(6%)-CN, the product is denoted as IL-P(6%)-CN.

XRD patterns of the prepared TiO₂ samples were recorded on a Rigaku D/max-2400 instrument using Cu-Kα radiation (λ = 1.54 Å). UV-Vis spectroscopy measurement was carried out on a JASCO V-550 model UV-Vis spectrophotometer, using BaSO₄ as the reflectance sample. Fourier transform infrared spectra (FT-IR) were obtained on a Nicolet 20DXB FT-IR spectrometer. Elemental analysis was performed with a vario EL cube from Elementar Analysensysteme GmbH. XPS measurements were conducted on a Thermo Escalab 250 XPS system with Al Kα radiation as the exciting source. The binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. Photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (FP-6300) using a Xe lamp as excitation source. Working electrodes were prepared as follows: 0.3 g of sample was ground with 0.7 g of ethanol to make a slurry, then coated on an indium–tin oxide glass by the doctor blade method. Photocurrents were measured by electrochemical analyzer (CHI 618C Instruments) in a standard three electrode system which the prepared sample film was used as the working electrodes, a Pt flake as the counter electrode, and Ag/AgCl as the reference electrode. A 500 W Xe-lamp was used to irradiate the working electrode from the back side. A 1.0 M Na₂SO₄ solution was used as the electrolyte.

Photocatalytic reaction

RhB was selected as model compound to evaluate the photocatalytic performance of the prepared g-C₃N₄ based catalysts in an aqueous solution under visible light irradiation. 0.05 g catalyst was dispersed in 200 ml aqueous solution of RhB (10 ppm) in an ultrasound generator for 10 min. The suspension was transferred into a self-designed glass reactor, and stirred for 30 min in darkness to achieve the adsorption equilibrium. In 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 400–800 nm, and air was bubbled at 130 ml min⁻¹ through the solution. The UV light portion of sodium lamp was filtered by 0.5 M NaNO₂ solution. All runs were conducted at ambient pressure and 30 °C. At given time intervals, 4 ml suspension was taken and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentrations of RhB before and after reaction were measured by means of a UV-Vis spectrophotometer at a wavelength of 550 nm.

Results and discussion

XRD analysis is used to investigate the phase structure of prepared photocatalysts. The typical (0 0 2) interlayer-stacking peak at 27.3° corresponds to an interlayer distance of d = 0.32 nm for g-C₃N₄, while the peak at 13.1° represents in-plane structural packing motif (1 0 0) with a period of 0.675 nm.^20,21 No peak for phosphorus species was observed in all the phosphorus-doped g-C₃N₄ materials. It is noted that the P(x%)-CN showed an obviously decreased peak intensities with increasing the phosphorus content, indicating that the crystal growth of graphitic carbon nitride is inhibited by introduction of phosphorus. This phenomenon is consistent with previous results.^15,16,22 Besides, compared with g-C₃N₄, a slight shift toward a higher 2θ value is observed for IL-P(6%)-CN but not in the patterns of P(x%)-CN catalysts. This is probably due to the different P doping site caused by the different P sources. Because of the radius difference, the lattice distortion of IL-P(6%)-CN should occur when the substitutional doping happen. In the case of P(x%)-CN, P is probably doped into interstitial site which exhibited no influence on the crystal lattice of g-C₃N₄ (Fig. 1).

Fig. 1 XRD patterns of synthesized g-C₃N₄, P(x%)-CN and IL-P(6%)-CN.

To characterize the specific surface area of g-C₃N₄ based catalysts, the nitrogen adsorption and desorption isotherms were measured (Fig. 2). The isotherm of both g-C₃N₄ and P(6%)-CN are of classical type IV, suggesting the presence of mesopores. The hysteresis loop in the low pressure range (0.4 < P/P₀ < 0.8) is associated with the intra-aggregated pores. The high-pressure hysteresis loop (0.8 < P/P₀ < 1) is related to the larger pores formed by the accumulation of secondary particles. The BET specific surface areas (S_BET) of g-C₃N₄, P(2%)-CN, P(4%)-CN, P(6%)-CN, P(8%)-CN, P(10%)-CN and IL-P(6%)-CN are calculated to be 10.9, 12.4, 14.8, 16.5, 17.7, 18.6 and 16.9 m² g⁻¹. P(6%)-CN and IL-P(6%)-CN exhibit the comparable BET specific surface area. The S_BET increases with increasing the P content. This increased S_BET could be probably due to that P doping inhibited the crystal growth of graphitic carbon nitride, leading to the formation of more secondary particles. The large S_BET can promote adsorption, desorption and diffusion of reactants and products, which is favorable to the photocatalytic performance.

Fig. 2 N₂ adsorption–desorption isotherm of g-C₃N₄ and P(6%)-CN.

FT-IR spectra shown in Fig. 3 can provide plentiful structural information concerning of synthesized g-C₃N₄, P(6%)-CN and IL-P(6%)-CN. For g-C₃N₄, a series of peaks in the range from 1200 to 1600 cm⁻¹ are attributed to the typical stretching modes of CN heterocycles, while the sharp peak located at 810 cm⁻¹ is assigned to the bending vibration of heptazine rings, which indicating the synthesized g-C₃N₄ is composed of heptazine units. The broad absorption band around 3200 cm⁻¹ is originated from the stretching vibration of N–H bond, associated with uncondensed aminogroups.¹² For P(6%)-CN and IL-P(6%)-CN, all the characteristic vibrational peaks of g-C₃N₄ are observed, suggesting that the structure of g-C₃N₄ is not changed after phosphorus doping. Zhang et al.¹⁵ synthesized P doped g-C₃N₄ using [Bmim]PF₆ as phosphorus source. The vibrations of P-related group were not observed in their investigation. They attributed it to the low phosphorus content or its vibration was overlapped by that of C–N bond. In this investigation, no P-related group was observed in IL-P(6%)-CN, which is consistent with the previous results.^14,15 However, a peak centered at ca. 950 cm⁻¹ is clearly observed in the spectra of P(6%)-CN, which is assigned to the P–N stretching mode. This is probably due to that the different phosphorus source causes the different P doping site.

Fig. 3 FT-IR spectra of synthesized g-C₃N₄, P(6%)-CN and IL-P(6%)-CN.

The light absorption property of as-prepared g-C₃N₄, P(x%)-CN and IL-P(6%)-CN catalysts is studied by UV-Vis spectra, and the results are shown in Fig. 4. Pure g-C₃N₄ shows a typical semiconductor absorption, originating from charge transfer response of g-C₃N₄ from the VB populated by N 2p orbitals to the CB formed by C 2p orbitals.²⁰ For P(x%)-CN, the obvious red shifts of absorption band were observed with increasing the phosphorus content, indicating phosphorus doping changed the optical property of synthesized g-C₃N₄, thus decreased the band gap energy. The band gap energy, which is calculated according to the method of Oregan and Gratzel, is 2.7 eV for g-C₃N₄. This is consistent with the previous results.^23–25 In the case of IL-P(6%)-CN, P(2%)-CN, P(4%)-CN, P(6%)-CN and P(10%)-CN, this value decreased to 2.68, 2.66, 2.65, 2.63 and 2.60 eV. This result is consistent with the academic calculation result of electronic structure of phosphorus doped g-C₃N₄ using first-principles reported by Ma et al.²⁶ They found a isolated P 3p state is localized just 0.15 eV below the bottom of the CB of the host g-C₃N₄, leading to the decreased band gap energy. For IL-P(6%)-CN, the absorption band was only slight changed compared with g-C₃N₄. This obvious difference in optical property between IL-P(6%)-CN and P(6%)-CN was probably due to the difference in P doping site. The possible reason is discussed (see below).

Fig. 4 UV-Vis diffuse reflectance spectra of g-C₃N₄, P(x%)-CN and IL-P(6%)-CN.

XPS were conducted to confirm the structure of g-C₃N₄ and the chemical state of phosphorus. In Fig. 5a and b, the spectra of g-C₃N₄ in both N 1s and C 1s region can be fitted with three contributions. For N 1s region (Fig. 5a), three contributions located at 398.2, 399.2 and 400.3 eV were assigned to the sp² hybridized aromatic nitrogen atoms bonded to carbon atoms (C–N=C), tertiary nitrogen N–(C)₃ groups linking structural motif or amino groups carrying hydrogen ((C)₂–N–H) inconnection with structural defects and incomplete condensation, and nitrogen atoms bonded three carbon atoms in the aromatic cycles.^27,28 In Fig. 5b, three components located at 284.6, 285.8 and 287.6 eV for g-C₃N₄ were attributed to the C–C bond which originated from sp² C atoms bonded to N in an aromatic ring (N–C=N), C=N or C≡N which could be ascribed to defect-containing sp² hybridized carbon atoms present in graphitic domains and pure graphitic sites in a CN matrix.^29–31 It is shown in Fig. 5 that the spectra of P(6%)-CN and IL-P(6%)-CN in N 1s and C 1s regions also can be fitted with three contributions. However, the obvious shifts to higher binding energies were observed for P(6%)-CN and IL-P(6%)-CN in N 1s and C 1s regions compared with that of g-C₃N₄. This is probably due to the change of chemical environment after phosphorus doping. However, no obvious difference in binding energies between P(6%)-CN and IL-P(6%)-CN in N 1s and C 1s regions was observed, indicating the difference in P doping site did not influence the chemical environment of C and N atoms in g-C₃N₄.

Fig. 5 XP spectra of synthesized g-C₃N₄, P(6%)-CN and IL-P(6%)-CN in the region of N 1s (a), C 1s (b) and P 2p (c).

Fig. 5c showed that the binding energies of P(6%)-CN and IL-P(6%)-CN in P 2p region were located at 133.4 and 133.7 eV. It is reported that the binding energy of P 2p in P–N and P–C bonds was 133.5 eV and 131.5–132.5 eV, respectively.¹⁴ Thus, it is deduced that phosphorus doped into g-C₃N₄ lattice in the formation of P–N bond. This binding energy difference, 0.3 eV, was probably due to the different chemical environment of P atoms in two catalysts. Zhang et al. suggested that the phosphorus heteroatoms most probably replace the corner or bay carbon to form P–N bond in the doped C₃N₄ framework, as shown in Fig. 6a.¹⁴ Ma et al. calculated the dopant formation energies and electronic properties of S and P doped g-C₃N₄ systems using first-principles density functional theory.²⁶ They suggested that S atom preferentially substitutes for the edge N atom of g-C₃N₄, whereas the P atom preferentially situates the interstitial sites of in-planar of g-C₃N₄, as shown in Fig. 6b. Obviously, in Fig. 6, P atoms bond to three N atoms when it replaces the carbon atoms. Whereas, only two P–N bonds formed when the interstitial doping occurs. Because the electronegativity of N atom is higher than that of P atom, more P–N bonds causes the lower electron density of P atoms, leading to the higher binding energy of IL-P(6%)-CN. Therefore, summarizes the obtained results above, it is deduced that P doping site is influenced by the phosphorus source. Substitutional doping occurs using [Bmim]PF₆ as phosphorus source, whereas interstitial doping occurs when (NH₄)₂HPO₄ is used. In addition, elemental analyse results revealed that the C/N ratio for g-C₃N₄ is 0.72, obviously higher than that of IL-P(6%)-CN (0.69). This is probably due to that partial C atoms were replaced by P atoms. However, for P(6%)-CN, this value is 0.71, similar to that of g-C₃N₄. This result confirms our point of view. Besides, the P concentrations in prepared catalysts were also measured by elemental analyse. The results indicated that the molar ratio of P element to g-C₃N₄ were 0.03, 0.056, 0.08, 0.11, 0.14 and 0.082 for P(2%)-CN, P(4%)-CN, P(6%)-CN, P(8%)-CN, P(10%)-CN and IL-P(6%)-CN. These values were very close to the theoretical value, indicating that P element did not lose during the preparation process.

Fig. 6 The possible doping sites of P atoms in g-C₃N₄.

Fig. 7 shows the room temperature PL spectra of g-C₃N₄, P(x%)-CN and IL-P(6%)-CN under the excitation wavelength of 320 nm. For g-C₃N₄, the broad PL band is around 465 nm, which can be attributed to the band–band PL phenomenon with the energy of light approximately equal to the band gap of g-C₃N₄ calculated by UV-Vis spectra.³⁰ Such band–band PL signal is attributed to excitonic PL, which mainly results from the n–π* electronic transitions involving lone pairs of nitrogen atoms in g-C₃N₄.³² In the case of P doped g-C₃N₄, the shape of the curves is similar to that of g-C₃N₄, whereas the peak intensities significantly decrease. Considering that the PL emission results from the free charge carrier recombination, the decreased peak intensity indicates that P doped g-C₃N₄ exhibits lower electrons–holes recombination rate compared with g-C₃N₄. The previous calculation results show that phosphorus doping can increase the dispersion of the contour distribution of HOMO and LUMO, which improves the carrier mobility.²⁶ The noncoplanar HOMO and LUMO favors the separation of photogenerated e⁻/h⁺ pairs and thus decreases the recombination rate. Obviously, the PL intensity of IL-P(6%)-CN is higher than that of P(6%)-CN. This indicated that P doping site could influence the separation rate of photogenerated e⁻/h⁺ pairs. Besides, the PL intensity of P(8%)-CN is stronger than that of P(6%)-CN, indicating the excess doping phosphorus could act as recombination centers for excited electrons and holes.

Fig. 7 Photoluminescence emission spectra of g-C₃N₄, P(x%)-CN and IL-P(6%)-CN.

The generation of electrons–holes pairs as well as their separation, migration and capture by the reactive species is regarded as the basic process for the photocatalysis. Photoelectrochemistry test is a powerful tool to monitor these complicated processes. Fig. 8 shows the photocurrent responses of g-C₃N₄, P(6%)-CN and IL-P(6%)-CN. Obviously, the sharp increased photocurrent responses appeared for three catalysts once the pulse Xe lamp was applied. Besides, the photocurrent responses did not decay with increasing the illuminated time, indicating the prepared catalysts could provide stable quantity of electrons and holes during the irradiation. The photocurrent value of P(6%)-CN was much higher than that of g-C₃N₄ and IL-P(6%)-CN, which can be attributed to the more efficient separation of photogenerated electron–hole pairs.

Fig. 8 The photocurrent responses of g-C₃N₄, P(6%)-CN and IL-P(6%)-CN.

Fig. 9a shows the photocatalytic performances of g-C₃N₄, P(x%)-CN and IL-P(6%)-CN in the degradation of RhB under visible light irradiation. Before the light on, the RhB concentrations of P doped g-C₃N₄ were slightly lower than that of g-C₃N₄. This is probably due to that phosphorus doping changed the surface property of synthesized carbon nitride, leading to the improved RhB adsorption ability. Control experiments (not shown here) indicated that the RhB degradation rate can be ignored in the absence of either irradiation or photocatalyst, indicating that RhB was degraded via photocatalytic process. P(x%)-CN showed obviously higher activities than g-C₃N₄. The degradation rate increased with increasing x% to the maximum of 6%, beyond which the activity of the catalysts began to decrease. This trend is exactly opposite to the PL intensity order. Among the synthesized phosphorus doped g-C₃N₄ catalysts, no essential difference in optical property is shown in UV-Vis spectra, therefore the photocatalytic performance of P(x%)-CN is mainly determined by electrons–holes separation efficiency. When the P content was beyond the optimal value, the recombination of photogenerated electrons and holes increased, leading to the decreased activities of P(8%)-CN and P(10%)-CN. Besides, P(6%)-CN exhibited much higher activity than that of IL-P(6%)-CN. Considering that, the P concentration of P(6%)-CN and IL-P(6%)-CN which measured by elemental analyse is very close (molar ratio P/g-C₃N₄ = 0.08 and 0.082), the difference in photocatalytic performance between them should be not caused by the P concentration but by the different doping site. Interstitial P doping is more effective to improve the photocatalytic activity compared with substitutional P doping.

Fig. 9 Photocatalytic performances of g-C₃N₄, P(x%)-CN and IL-P(6%)-CN (a) and plot of −ln(C/C₀) against reaction time (b) in the degradation of RhB under visible light irradiation.

The reaction rate constant k was obtained by assuming that the reaction followed first order kinetics.³³ In a batch reactor, the performance equation is as follows: −ln(C/C₀) = kt where C₀ and C represent the concentrations of RhB dye before and after photocatalytic degradation, respectively. If a linear relationship is established when −ln(C/C₀) is plotted against t (reaction time), the rate constant k can be obtained from the slope of the line. Fig. 9b displays the plot of −ln(C/C₀) against reaction time. The calculated results indicated that the rate constant k was 0.0115, 0.0169, 0.0243, 0.0466, 0.0277, 0.0222 and 0.0258 min⁻¹ for g-C₃N₄, P(2%)-CN, P(4%)-CN, P(6%)-CN, P(8%)-CN, P(10%)-CN and IL-P(6%)-CN respectively. P(6%)-CN exhibited the highest rate constant which is 4 and 1.8 times higher than that of g-C₃N₄ and IL-P(6%)-CN. This increased photocatalytic performance should be attributed to the synergistic effect of decreased band gap energy which utilize visible light more efficiently, improved electrons–holes separation efficiency and increased RhB adsorption ability which caused by phosphorus doping.

To check the catalytic stability of P doped g-C₃N₄, the photocatalytic performance of P(6%)-CN was investigated in three circles (Fig. 10a). No obvious decrease in activity was observed after three circles, indicating that prepared P doped g-C₃N₄ is stable under visible-light irradiation. Moreover, the P element concentrations of fresh and reused P(6%)-CN which measured by elemental analyse were 0.08 and 0.081. No essential difference was observed between them, indicating the structure of P(6%)-CN is stable. Fig. 10b shows the comparison of XP spectra of fresh and reused P(6%)-CN in the region of P 2p. The binding energy is not changed after three circles, indicating that the chemical state of P element is stable.

Fig. 10 The catalytic stability test of P(6%)-CN (a) and comparison of XP spectra of fresh and reused P(6%)-CN in the region of P 2p (b).

In order to clarify the mechanism on the improved activity of phosphorus doped g-C₃N₄ in depth, the active species generated during the reaction process are identified by hole and free radical trapping experiment. In this investigation, EDTA-2Na, tert-butyl alcohol (t-BuOH) and 1,4-benzoquinone (BQ) are used as the hole (h⁺) scavenger, hydroxyl radical (˙OH) scavenger and superoxide radical (˙O²⁻) scavenger, respectively.³⁴ Fig. 11 shows the influence of various scavengers on the visible light photocatalytic activity of g-C₃N₄, P(6%)-CN and IL-P(6%)-CN. For g-C₃N₄, the photodegradation rate of RhB decreased slightly after the addition of t-BuOH, indicating that hydroxyl radicals are not the main active species in current photocatalytic systems. When BQ was added, the degradation of RhB was inhibited sharply, indicating ˙O₂⁻ is the main active species in current photocatalytic systems. It is reported that the CB and VB of g-C₃N₄ were −1.12 V and +1.57 V, respectively.²¹ Whereas, the redox potentials for ˙OH/OH⁻ and O₂/˙O₂⁻ were determined to be +1.99 V and −0.33 V.³⁵ Therefore, the reduction potential of CB electrons in g-C₃N₄ was negative enough to reduce O₂ to form ˙O₂⁻, whereas the VB holes in g-C₃N₄ were not positive enough to generate ˙OH. This is consistent with our experiment results. In the presence of EDTA-2Na, the RhB degradation rate is increased obviously, which is different from previous result.³⁴ Zhang et al. prepared C₃N₄/Bi₅Nb₃O₁₅ heterojunction catalyst for 4-CP photodegradation.³⁴ The photodegradation rate was decelerated significantly after the addition of EDTA-2Na, indicating the direct photogenerated holes oxidation happened. In our investigation, although the redox potential of RhB is reported to be 1.43 V,³⁶ which is higher than the VB of g-C₃N₄, the direct photogenerated holes oxidation did not happen. On the contrary, the addition of EDTA-2Na to trap the h⁺ can promote the separation rate of e⁻/h⁺ pairs, leading to the increased photocatalytic performance. For P(6%)-CN and IL-P(6%)-CN, the similar trend was obtained, indicating the main oxidative species is the same as that of g-C₃N₄. The difference in phosphorus doping site does not change the reaction mechanism.

Fig. 11 Influence of various scavengers on the visible light photocatalytic activity of g-C₃N₄, P(6%)-CN and IL-P(6%)-CN.

Conclusions

Phosphorus doped g-C₃N₄ was prepared conveniently by a simple method using dicyandiamide monomer and diammonium hydrogen phosphate as precursors. The introduction of phosphorus inhibited the crystal growth of graphitic carbon nitride, decreased the band gap energy and increased the separation efficiency of photogenerated electrons and holes. Phosphorus atoms doped into g-C₃N₄ lattice in the formation of P–N bond. P doping site is influenced by the phosphorus source. Substitutional doping occurs using [Bmim]PF₆ as phosphorus source, whereas interstitial doping occurs when (NH₄)₂HPO₄ is used. The results of photocatalytic RhB degradation indicated that the rate constant of P(6%)-CN was 0.0466 min⁻¹, which is 4 and 1.8 times higher than that of g-C₃N₄ and IL-P(6%)-CN. Moreover, P(6%)-CN exhibited stable catalytic activity and chemical structure. Interstitial P doping is more effective to improve the photocatalytic activity compared with substitutional P doping. The superoxide radical ˙O₂⁻ is the main active species for RhB degradation in both P(6%)-CN and g-C₃N₄ systems. The difference in phosphorus doping site does not change the reaction mechanism.

Acknowledgements

This work was supported by Program for New Century Excellent Talents in University (no. NCET-11-1011), National Natural Science Foundation of China (no. 41071317, 30972418, 21103077), National Key Technology R & D Programme of China (no. 2007BAC16B07, 2012ZX07505-001), the Natural Science Foundation of Liaoning Province (no. 20092080), and the Natural Science Foundation of Liaoning Shihua University (no. 2011XJJ-021).

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