Coaddition of Phosphorus and Proton to Graphitic Carbon Nitride for

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Co-addition of Phosphorus and Proton to Graphitic Carbon Nitride for Synergistically Enhanced Visible Light Photocatalytic Degradation and Hydrogen Evolution Moqing Wu, Tong Ding, Jinmeng Cai, Yating Wang, Hui Xian, Hao Zhang, Ye Tian, Tianyong Zhang, and Xingang Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Co-addition of Phosphorus and Proton to Graphitic Carbon Nitride for Synergistically Enhanced Visible Light Photocatalytic Degradation and Hydrogen Evolution Moqing Wu, a Tong Ding, a Jinmeng Cai, a Yating Wang, a Hui Xian, b Hao Zhang, a Ye Tian, a Tianyong Zhang, a and Xingang Li*a a

Collaborative Innovation Center for Chemical Science & Engineering, Tianjin Key

Laboratory of Applied Catalysis Science & Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China b

School of Continuing Education, Tianjin Polytechnic University, Tianjin, 300387, P.

R. China

*

Corresponding author

Email: [email protected] Address: No. 92 Weijin Road, Nankai District, Tianjin, 300072, P. R. China

1

ACS Paragon Plus Environment

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Abstract: Graphitic carbon nitride (g-C3N4) attracts enormous attention in photocatalysis owing to its special structure and properties. The insufficient light absorption and fast charge carrier recombination limit its further photocatalytic application. Herein, we report a facile approach to fabrication of the g-C3N4 modified simultaneously with phosphorus and proton by directly heating the mixture of urea phosphate (UP) and urea in air. The incorporation of the phosphorus atoms in g-C3N4 can significantly decrease the band gap, leading to the enhanced light absorption efficiency. Furthermore, UP can also introduce the protons to the structure of g-C3N4 from protonation. The protons can inhibit the recombination of the charge carriers and improve their utilization. The synergistic effect of the phosphorus doping and protonation in g-C3N4 results in the superior visible-light photocatalytic performances for both degradation of Rhodamine B (RhB) and H2 evolution from water splitting. We believe that our findings have a broad applicability to design efficient and novel g-C3N4 based photocatalysts. Keywords: g-C3N4; phosphorus; proton; synergistic effect; photodegradation; H2 evolution

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Introduction Photocatalysis is one of the most promising techniques to solve the problems of global energy shortage and environmental pollution.1 Recently, g-C3N4 has emerged as a hopeful metal-free visible-light photocatalyst owing to its nontoxicity, abundance, stability, and chemical tenability.2-4 However, the photocatalytic performance of g-C3N4 is still restricted by the limited utilization of solar energy (λ 400 nm) was used as the visible light source. 50 mg of the sample was dispersed into the aqueous solution of RhB (100 mL, 5 mg L-1). Then, the suspension was magnetically stirred in dark for 0.5 h before the irradiation to reach adsorption-desorption equilibrium between the photocatalyst and RhB. During the irradiation, 5 mL of suspension was continually taken from the reaction cell at a given time interval. After the reaction, we centrifuged the obtained suspensions to remove the sample. The absorbance of the after-photoreacted solutions was taken by a UV-vis spectrophotometer (Lambda 750S, Perkin-Elmer), and distilled water was used as the reference. The maximum absorption was recorded at 553 nm and used for evaluating the concentration of RhB. The degradation rate of RhB over the different samples was calculated according to the following expression: Degradation rate = (C0 - C) / C0 × 100% = (A0 - A) / A0 × 100%

(1)

Where C0 is the adsorption-desorption equilibrium concentration of RhB, and C is the concentration of RhB after different irradiation period. A0 and A represent the initial and the changed absorbances of RhB at 553 nm, respectively. We used photocatalytic H2 evolution from water splitting as the other model reaction to evaluate the photocatalytic activity of the samples. The measurement was carried out in the equipment of Labsolar-III AG system supplied by Beijing Perfectlight Technology Co., Ltd. In a typical procedure, 30 mg of samples was 7



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dispersed into 100 mL of an aqueous solution of methanol (1:1 by volume), while 1 wt % Pt was loaded on the surface of the samples by an in situ photodeposition method. The irradiation light source was a 300 W Xe lamp with a 420 nm cut-off filter (λ > 420 nm). The generated H2 was detected per 20 min by an online gas chromatography (GC) using a thermal conductivity detector (TCD). The apparent quantum efficiencies of the powdered samples were determined using a 300 W Xe lamp equipped with a 420 nm bandpass filter. 100 mg of the sample was dispersed into 100 mL of methanol aqueous solution (1:1 by volume). The irradiation intensity measured using a PM100D power meter is 2.07 mW cm-2, and the irradiation area is 26.4 cm2. The AQE of the samples was calculated using equation (2).

=

× 100%

(2)

where M is the amount of hydrogen molecules, NA is the Avogadro’s constant, h is Planck constant, c is the light velocity, I is the intensity of the light, A is the irradiation area, t is the reaction time, and λ is the wavelength of light. Photoelectrochemical measurements The

photoelectrochemical

analysis

was

performed

on

a

CHI

6043E

electrochemistry workstation (Chenhua Instrument). 4 mg of the as-prepared samples was uniformly mixed with 0.8 mL H2O, 0.2 mL ethanol and 4 µL Nafion, and ultrasonically treated for 5 min to form a homogeneous ink. Thereafter, 20 µL ink was sprayed onto 1 cm2 fluorine-doped tin oxide (FTO) glass via drop-coating technique. We used a standard three-electrode system consisting of the coated FTO glass as the

8


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work electrode, the Ag/AgCl electrode as the reference electrode, and the Pt foil as the counter electrode. While 0.5 M Na2SO4 aqueous solution was used as electrolyte. Chronoamperometry tests were operated at 0.5 V under chopped light irradiation with light on/off cycles of 60 s. A 300 W Xe lamp with a 400 nm cut-off filter (λ > 400 nm) was used as the irradiation source. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 0.01 Hz to 100 kHz. Mott-Schottky plots were taken at a frequency of 1000 Hz with a bias potential that ranged from 1.0 V to -1.0 V (vs Ag/AgCl).

Results and discussion Morphology structures and chemical properties Fig. 1A shows the XRD patterns of the samples. In Fig. 1A, all of the samples retain the original g-C3N4 network after phosphorus doping and protonation, with the typical (100) in-plane structural packing diffraction peak at the 2θ angle of around 13.3o and (002) interlayer-stacking diffraction peak at the 2θ angle of around 27.9o.26,27 That is, the value of the (002) diffraction angle is directly related to the interlayer spacing of the carbon nitride. With the increasing amount of UP added to the raw materials, a slight increase and then decrease of the 2θ angle value of (002) diffraction peak can be observed (the inset in Fig. 1A). The slight change of the interlayer distance suggests that phosphorus and protons can be incorporated into the crystal structure of g-C3N4. When the amount of UP in the raw materials is 0.5 or 2 wt %, or when 2 wt % NP is added, the addition of phosphorus and protons leads to 9



the structural compaction. As a result, the interlayer distance of g-C3N4 decreases.28,29 Therefore, the (002) diffraction peaks of 0.5UP-CN, 2UP-CN and 2NP-CN shift to high angles, as compared with CN. For 4UP-CN and 6UP-CN, with the decrease of the pH value of the raw materials caused by the increased UP amount, bridging portions between the tri-s-triazine monomers in the structure of the resulting g-C3N4 are partially destroyed, leading to the out-of-plane distortions of tri-s-triazine monomers.30,31 So, the (002) diffraction peaks of them shift to lower angles compared with 2UP-CN. These results are consistent with previous studies,26,28-31 and can be further confirmed by subsequent characterizations. Fig. 1B and C visually show the schematic illustrations for the proposed frameworks of the normal g-C3N4 and the g-C3N4 with excessive protons, respectively.

Fig. 1 (A) XRD patterns of nUP-CN (n=0.5, 2, 4, 6), 2NP-CN and CN. (B) and (C) Schematic illustrations for frameworks of the normal g-C3N4 and the g-C3N4 with excessive protons, respectively. Gray, blue and pink spheres represent the C, N and H atoms, respectively. 10


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We obtained the pore structure and Brunauer-Emmett-Teller (BET) surface area of the samples from the N2 adsorption-desorption measurements. Fig. S1 shows the N2 adsorption-desorption isotherms of the samples. The isotherms for all of the samples show type IV curve, indicating their mesoporosity.32 Furthermore, the typical H3 hysteresis loops in both isotherms indicate the presence of slit-like mesopores, which are caused by the aggregation of plate-like particles.33,34 The BET surface areas, pore volumes and average pore diameters were calculated and summarized in Table S1. The BET surface area of 2UP-CN is 28.7 m2 g-1, the similar value to CN (29.0 m2 g-1), 0.5UP-CN (29.0 m2 g-1) and 2NP-CN (28.2 m2 g-1). That is, a small amount of phosphorus and protons does not change the specific surface area and pore structure of the samples. However, for 4UP-CN and 6UP-CN, the BET surface area increases, and the pore diameter reduces significantly. It suggests that excessive incorporation of UP in the raw materials will cause the fragmentation of the structure of g-C3N4 in the samples. Fig. 2 shows the TEM images of the samples to characterize their morphologies. The sample of CN clearly shows a two-dimensional porous lamellar-like morphology with several stacking layers in Fig. 2A. In Fig. 2B, C and F, 0.5UP-CN, 2UP-CN and 2NP-CN have the similar two-dimensional morphology with CN in Fig. 2A. Fig. 2D and E show that the layered structures of 4UP-CN and 6UP-CN are partially damaged. This morphology change further indicates that the addition of the excessive amount of UP will damage the original structure of g-C3N4. This phenomenon is in good agreement with the result and inference of XRD in Fig. 1 and surface area in Table S1. 11



Additionally, in Fig. 2C and F, we can observe that the structure of 2UP-CN is slightly broken compared with 2NP-CN. Probably, the damage of structure is due to the breakage of the bridging portions between the tri-s-triazine monomers caused by the excessive proton incorporation. Fig. S2 shows the scanning transmission electron microscopy (STEM) image of 2UP-CN. The corresponding elements of carbon, nitrogen and phosphorus homogeneously distribute in 2UP-CN. It indicates that the carbon nitride structure is uniform and phosphorus is homogenously doped in g-C3N4.21,35

Fig. 2 TEM images of (A) CN, (B) 0.5UP -CN, (C) 2UP- CN, (D) 4UP-CN, (E) 6UP-CN and (F) 2NP-CN sample.

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Fig. 3 (A) XPS survey spectra and high-resolution XPS spectra of (B) P 2p, (C) C 1s, and (D) N 1s core levels of the samples.

Fig. 3 shows the XPS spectra of the samples to determine their elemental chemical states. In Fig. 3A, their survey spectra are similar, except that the weak signals of P are observed in the phosphorus-doped samples. The P 2p binding energy (B.E.) peaks of the phosphorus-doped samples in Fig. 3B are centered at 133.0 eV. It is typical for a P-N coordination, indicating that P substitutes C in g-C3N4 framework to form P-N bonds.18,36 The C 1s spectra of the samples in Fig. 3C present two remarkable B.E. 13



peaks centered at 284.6 and 288.2 eV, corresponding to carbon impurities species (C=C species) and carbon atoms bonded to their neighboring three nitrogen atoms (N−C=N) in the g-C3N4 lattice, respectively.37-39 Table S2 gives the superficial proportion of carbon species for all of the samples. 4UP-CN and 6UP-CN possess the larger proportion of C=C species, indicating that over-addition of UP makes the polymerization defective and generates more carbon impurities. Typically, g-C3N4 gives three N 1s B.E. peaks at 398.2, 399.8 and 400.8 eV, which originate from sp2 hybridized nitrogen (C=N−C), sp3 hybridized nitrogen tertiary nitrogen (N−(C)3), and amino functional groups (C−N−H), respectively.30,36,40 In Fig. 3D, the B.E. peaks of C=N−C in g-C3N4 on the modified samples slightly shift towards higher energies. The shift of 2UP-CN is more evident than that of 2NP-CN, which results from simultaneous modification of g-C3N4 with phosphorus and proton.26,38 After calculating the ratio of N−(C)3 to C−N−H in all of the samples in Table S3, we find that nUP-CN have more amino functional groups than CN and 2NP-CN. It indicates the generation of more defects and terminal groups in g-C3N4 during protonation.30 The O 1s spectrum of CN in Fig. S3 shows a single B.E. peak centered at 531.5 eV, possibly due to the formation of C=O moieties.41 Compared with CN, the modified samples exhibit another B.E. peak at 533.0 eV, corresponding to oxygen species in phosphate.42 We further determine the approximate composition of the samples by elemental analysis. In Table S4，the atomic ratio of C to N has decreased with the introduction of NP or UP. It is due to the substitution of doped phosphorus for the carbon in the 14


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framework. Furthermore, the increased hydrogen content of 2UP-CN and 6UP-CN compared with CN and 2NP-CN gives direct evidence of protonation. It is in line with the results of N1s XPS spectra. In addition, we measured the solid-state 1H NMR spectra and zeta potentials of the samples to further demonstrate protonation. Fig. S4 shows that the signal of solid-state 1H NMR is very weak and all samples show a broad peak between 2 and 14 ppm. We can only compare their peak areas, and find that the hydrogen content in 2UP-CN is indeed higher than that in CN and 2NP-CN. In Fig. 4, the zeta potential value of CN is measured to be -21.6 mV when dispersed in deionized water. The surface charge property of 2NP-CN is almost the same as that of CN. But compared with CN, the surface charge properties of nUP-CN are obviously changed, and the zeta potential increased significantly with the gradual introduction of UP. It should be attributed to the protonation of g-C3N4 caused by the addition of UP.43,44

Fig. 4 Zeta potentials of the samples.

Fig. S5 shows the FT-IR spectra to determine the functional groups of the samples. The IR band at 807 cm-1 belongs to the characteristic breathing mode of tri-s-triazine 15



unit.35,45 The IR bands at 1245 and 1320 cm-1 correspond to stretching vibration of the connected units of C−N(−C)−C (full condensation) or C−NH−C (partial condensation). The bands locating in the 1370–1650 cm-1 region belong to the typical stretching modes of aromatic C−N heterocycles.35,45,46 The broad band at 3000-3500 cm1 belongs to uncondensed terminal amino groups (−NH2 or =NH groups).30,38 The FT-IR results well confirm the presence of the g-C3N4 structure in all of the samples. Additionally, we can observe a weak IR band at 952 cm-1, corresponding to the stretching mode of P−N,18 only in the spectrum of 6UP-CN. It is probably because of the high phosphorus content in 6UP-CN, compared with other samples. Photocatalytic performance

Fig. 5 (A) The photocatalytic degradation of RhB over the different samples under visible light irradiation and (B) the kinetic data drawing by ln(C0/C) = kapp t.

Fig. 5 shows the photocatalytic activities of the as-prepared samples for the photodegradation of RhB under visible light irradiation (λ > 400 nm). In Fig. 5A, the nUP-CN samples had the higher photodegradation activities of RhB than CN. When the addition amount of UP equals to 2 wt %, the photodegradation efficiency reaches 16


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the highest catalytic activity. The activity of the samples begins to drop when the addition amount of UP exceeds 2 wt %. It is due to the excessive introduction of UP. There are too many defects in 4UP-CN and 6UP-CN, and the structure of g-C3N4 is partially destroyed. 2UP-CN is able to completely decolor RhB in 10 min irradiation, while CN only decolors 42% of RhB in the same period. The photodegradation activity of 2NP-CN is lower than that of 2UP-CN, but is higher than that of other samples. The photodegradation of RhB in the aqueous suspension containing the as-prepared sample under visible light follows the pseudo-first-order kinetics.25 Fig. 5B shows the kinetic plots calculated by the apparent first-order linear transform equation:47,48 ln (C0/C) = kapp t

(3)

Here, kapp is the apparent pseudo-first-order rate constant (min-1), C0 is the initial RhB concentration (mg L-1), and C is the RhB concentration in aqueous solution at the time

t (mg L-1). Table S5 lists the kapp and the linear fitting correlation coefficient R for the samples, which are determined by the kinetic plots. Among these samples 2UP-CN (kapp = 0.346 min-1) exhibits the highest photodegradation activity, whose apparent pseudo-first-order rate constant is 4.8 times to CN (kapp = 0.072 min-1). In Fig. S6, RhB can be completely decomposed in 10 min under visible light in the presence of 2UP-CN. In the absence of 2UP-CN, RhB has little change after 10 min under visible light irradiation.

17



Fig. 6 Hydrogen evolution of 2UP-CN, 2NP-CN and CN under visible light irradiation (λ > 420 nm) with 1 wt % Pt.

Basing on the results of the RhB photodegradation test, we selected the best-active sample 2UP-CN and two comparative samples, 2NP-CN and CN, for more in-depth characterization and analysis, to illuminate the effect of the phosphorus doping and protonation. Fig. 6 shows the results of H2 evolution experiments under visible light irradiation (λ > 420 nm), and the variations of H2 yield with the increasing time were plotted for CN, 2UP-CN and 2NP-CN. 2UP-CN exhibits the highest hydrogen production rate of 1013.3 µmol g-1 h-1, which is nearly four times than that of CN and twice than that of 2NP-CN. The calculated apparent quantum efficiency (AQE) of 2UP-CN is 3.5 % at 420 nm. It is much higher compared with CN (0.28 %) and 2NP-CN (1.1 %). We summarize the photocatalytic H2 evolution rate for the g-C3N4-based photocatalysts in Table S6. 2UP-CN in our work has a good photocatalytic activity for hydrogen production. We also investigated the stability of 2UP-CN by cycling the catalytic experiments, and 2UP-CN showed the high 18


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photocatalytic stability in Fig. S7. Optical properties and electronic band structure

Fig. 7 (A) UV-vis DRS and (B) the corresponding Kubelka-Munk transformed diffuse reflectance spectra of the samples.

The optical properties of CN, 2UP-CN and 2NP-CN were investigated by UV-vis DRS, and the results are shown in Fig. 7A. Absorption edges of 2UP-CN and 2NP-CN slightly shift to the longer wavelengths compared with CN. The band gap energy (E) for each sample is estimated by the Tauc equation.49,50,51

αhν = C1(hν − E)2

(4)

Where α is the optical absorption coefficient, C1 is the absorption constant, and hν is the photon energy. Fig. 7B shows the plots of (αhν)1/2 versus hν for the samples. By extending the vertical segment to the hν axis, the band gap (E) can be obtained.52 In Fig. 7B, the optical band gap energies shift to the lower energies for the phosphorus-doped g-C3N4 samples. 2NP-CN and 2UP-CN have the similar band gap about 2.59 eV, which is much less than that of CN (2.81 eV). Such phenomenon 19



suggests that the phosphorus-doped g-C3N4 can absorb more visible light. It is in favor of the improvement of the photocatalytic activity.8,53,54 Additionally, the similar band gap of 2UP-CN and 2NP-CN suggests that protonation of g-C3N4 has little effect on the band gap. The main factor causing the difference between 2UP-CN and 2NP-CN in the catalytic activity is not the ability of light absorption. Fig. 8A shows the valence band XPS spectra of CN, 2UP-CN and 2NP-CN. Compared with the spectrum of CN, we can observe a shift of the relative position of the valence band to the direction of high energies for both 2UP-CN and 2NP-CN. It should be attributed to the doping of the P atoms into the g-C3N4 lattice. The binding energy of 2UP-CN and 2NP-CN had little difference, which is entirely consistent with the result of the UV-vis spectra in Fig. 7. It indicates that the phosphorus, rather than proton, results in the change of the band structure of g-C3N4.

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Fig. 8 (A) VB XPS of CN, 2UP-CN, 2NP-CN. (B) Mott-Schottky plots of CN, 2UP-CN, 2NP-CN. (C) Mott-Schottky plots of CN measured at three different frequencies and (D) Proposed energy band diagram of CN and phosphorus-doped g-C3N4 (2UP-CN and 2NP-CN).

Fig. 8B shows the Mott-Schottky plot of CN, 2NP-CN, and 2UP-CN. We can observe the positive slopes in the Mott-Schottky plots of the samples, as expected for n-type semiconductors.55 Importantly, by comparing the Mott-Schottky plots of CN, 2NP-CN, and 2UP-CN, we can see that the P-doped samples show substantially smaller slopes. It suggests an increase of donor densities due to the P dopant. In Fig. 8C, the flat-band potential (Efb) of CN is determined to be -0.899 V vs. Ag/AgCl by 21



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the plots of three frequencies. The potential measured according to Ag/AgCl could be converted into the normal hydrogen electrode (NHE) potential through the subsequent equation.56 Efb(vs.NHE) = Efb(pH=0,

vs. Ag/AgCl)

+ EAgCl + 0.059 × pH

(5)

Where EAgCl equals to 0.197 V, and the pH value of the electrolyte is around 6.8. Therefore, the calculated flat-band position of CN is -0.301 V vs. NHE (pH= 0). As the Efb is about 0.3 V below the conduction band minimum (CBM) for undoped n-type semiconductors,57 the CBM of CN is -0.601 V vs. NHE. Combining the above characterization results of VB XPS and UV-vis spectra, we calculated the band structure of CN and the phosphorus-doped g-C3N4, including 2UP-CN and 2NP-CN. Fig. 8D shows the proposed energy band diagram. Compared with CN, the phosphorus-doped g-C3N4 samples have a narrower band gap and a lower valence band (2.399 V vs. NHE). These changes are beneficial for improving the photocatalytic activity. 2NP-CN and 2UP-CN have similar band structures, but the photocatalytic activity of 2UP-CN is much better than that of 2NP-CN, whether for degradation or hydrogen production. We suppose that the addition of protons does not change the band structure of the sample, but plays a critical role for the improvement of the photocatalytic activity. Charge-carrier separation and transport To further elucidate how the additive proton in g-C3N4 promotes photocatalytic degradation and water splitting, we implemented the steady-state and transient-state photoluminescence spectroscopic measurements to determine the course of the 22


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photogenerated carriers in the samples. Fig. 9A shows the PL spectra of CN, 2UP-CN and 2NP-CN. The emission peak of CN appears at about 460 nm. It arises from the band-band PL phenomenon because the energy of light approximately equals to the band gap energy of g-C3N4.58 For 2UP-CN and 2NP-CN, the positions of the emission peak in the PL spectra had a slight red-shift compared with CN, because of the narrower band gap of the phosphorus-doped g-C3N4 samples, as indicated in the results of UV-vis spectra in Fig. 7.54 Furthermore, compared with CN, 2NP-CN shows a weaker intensity of PL spectrum, which suggests that phosphorus doping inhibits the recombination of photogenerated carriers. It is worth noting that 2UP-CN exhibits the much weaker intensity of PL spectrum compared with 2NP-CN. It indicates that the introduction of protons inhibits the recombination of the photogenerated electrons and holes in 2UP-CN greatly.59,60 In conclusion, the separation of the photogenerated carriers in the g-C3N4 modified simultaneously with phosphorus and proton is more efficient than that in the unmodified or only phosphorus-doped samples.

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Fig. 9 (A) PL emission spectra and (B) the transient-state fluorescence spectra monitored at 450 nm of 2UP-CN, 2NP-CN and CN. Both samples were excited with 340 nm light from a pulsed laser.

Samples CN 2NP-CN 2UP-CN

τ1 (ns) 1.884 1.897 2.376

Rel1 (%) 76.9 75.2 73.0

τ2 (ns) 7.484 7.392 10.637

Rel2 (%) 23.1 24.8 27.0

τ (ns) 3.178 3.258 4.603

Adj. R-Square 0.999 0.999 0.999

Table 1 Kinetic constants extracted from the photoluminescence decay profiles of CN, 2UP-CN and 2NP-CN in Fig. 9B.

Fig. 9B shows the photoluminescence decay kinetics of the samples investigated by the transient-state fluorescence spectra. 2NP-CN has analogous decay kinetics with CN. However, 2UP-CN shows the slower decay kinetics, corresponding to a slower charge recombination of the excited state.61 In addition, the decay curves of the samples obviously deviate from the single-exponential decay.62,63 Thus, the PL life-times of the samples were calculated by fitting the transient-state fluorescence decay curves with the following quadratic exponential fitting equation:64 24


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= + exp(− ) + exp(− )

(6)

Where y0, A1, and A2 are constants and obtained after fitting the decay curve. Fitting the decay spectrum shows a fast ( ) and a slow ( ) decay component with the different percentages. The fast decay time (

) results from trapping the

photogenerated electrons at the conduction band into the trap sites. The slow decay time ( ) presumably results from the recombination of the electrons at the trap sites with the photogenerated holes at the valence band.65 According to the above fitting results, the intensity-average lifetime ( ) was calculated and presented to make an overall comparison of the PL lifetimes of the samples by equation (7):

=

!

(7)

!

All of the fitting PL decay data were summarized and listed in Table 1. Compared with CN, the PL lifetime of 2NP-CN is almost no improvement and the slow decay time ( ) of 2NP-CN is even a little shortened (7.484 ns → 7.392 ns) due to the decrease in band gap caused by phosphorus doping. In particular, compared with CN and 2NP-CN, 2UP-CN shows the prolonged fast decay time ( ), slow decay time ( ) and intensity-average PL lifetime ( ). In detail,

of

1.897 ns and

of 7.392 ns in

2NP-CN increase up to 2.736 ns and 10.637 ns in 2UP-CN, respectively. The intensity-average PL lifetime ( ) increases from 3.258 ns of 2NP-CN to 4.603 ns of 2UP-CN. Furthermore, the percentage of the charge carriers with the longer lifetime increases from 24.8% in 2NP-CN to 27.0% in 2UP-CN. These data suggest that protonation of g-C3N4 effectively elongates the lifetime of the charge carriers.

25



Fig. 10 (A) Chronoamperometry tests and (B) EIS Nyquist plots of electrochemical impedance of 2UP-CN, 2NP-CN and CN.

Fig. 10A show the photocurrents of the samples under visible light irradiation to gain deeper insight into the charge transfer behaviors. The transient photocurrents of the samples were measured during the repeated ON/OFF illumination cycles at 0.8 V. On each illumination, all of the samples exhibit the prompt and reproducible photocurrent responses. When the irradiation is interrupted, the photocurrent rapidly drops, and the photocurrent reverts to the original value as soon as light is switched on again. In Fig. 10A, the photocurrents of the phosphorus-doped samples are larger than that of CN. This phenomenon demonstrates that the increase of the density of the photogenerated electron-hole pairs in the phosphorus-doped samples is mainly due to the improved band structure caused by phosphorus doping.28 In addition, compared with 2NP-CN, the photocurrent of 2UP-CN increased. It suggests that protonation improves the separation efficiency of photogenerated carriers of g-C3N4.66 The function of the protons is further revealed by the electrochemical impedance spectroscopies (EIS). Fig. 10B shows the Nyquist plots of the samples. The arc radius 26


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of 2NP-CN is shortened compared with CN. That is, phosphorus doping has little effect on the resistance of the samples. It is worth noting that 2UP-CN shows a much smaller arc radius compared with CN and 2NP-CN. It indicates that the protonated sample has a lower resistance,67 resulting from the more efficient separation of charge carriers.68

Fig. 11 Schematic illustrations for frameworks of 2UP-CN. Gray, blue, pink and yellow spheres represent the C, N, H and P atoms, respectively.

The

decreased

intensity

of

photoluminescence,

prolonged

lifetime

of

photongenerated carriers and lowered resistance of 2UP-CN are apparently related to the improved charge localization on the carbon nitride matrix, which is believed to promote the heterogeneous photocatalysis.30,59 We have made a reasonable speculation on the mechanism of the efficient separation of charge carriers. Fig. 11 shows the schematic illustrations for the frameworks of 2UP-CN. The additive 27



protons in the framework act as a bridge, and establish a new path of electron transport. Thus, the chance of carrier recombination is greatly reduced, and the efficiency of charge separation is improved, as well. In combination with the characterization and analysis of the front part, the doped phosphorus could occupy carbon sites. And the doping of phosphorus could reduce the band gap and increase the utilization of visible light, subsequently generating more photogenerated carriers. The synergistic effect of simultaneous phosphorus doping and protonation of g-C3N4 makes our sample of 2UP-CN exhibit the high photocatalytic performance.

Conclusion In summary, we have successfully synthesized the on-step modified g-C3N4 with phosphorus and proton by directly heating the mixture of urea and UP in air. Among all of the samples, 2UP-CN shows the highest photocatalytic performance, not only in degradation of RhB but also in H2 evolution under visible light irradiation. Our results show that the doped phosphorus can substitute carbon sites in the framework of g-C3N4 in our as-prepared samples. The substitution of carbon with phosphorus reduces the band gap of the samples by significantly lowering the conduction band position of g-C3N4, thereby increasing the utilization of visible light. The additive proton in g-C3N4 can establish a new path of electron transport to inhibit the recombination of the carriers, and then make the photogenerated carriers have a longer lifetime and higher utilization efficiency. However, the excessive addition of protons will destroy the bridging portions in the framework of g-C3N4. The 28


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synergistically enhanced efficiency of light absorption and charge separation leads to the superior performance of the g-C3N4 modified simultaneously with phosphorus and proton. Prospectively, we expect that our findings can offer useful insights for development of efficient photocatalysts toward practical solar energy systems.

Associated content Supporting Information: N2 adsorption–desorption isotherms, STEM and the corresponding element mapping images of the 2UP-CN sample, Solid-state 1H NMR spectra, FT-IR spectra, photodegradation of RhB under visible light without catalyst and in the presence of 2UP-CN, stability tests of 2UP-CN, BET surface area and pore structure, the peak areas and relative proportions of C species in the high resolution C1s spectra, the peak areas and ratios of N species in the high resolution N1s spectra, the relative proportions of elemental contents determined by elemental analysis, the kapp and the linear fitting correlation coefficient R, the comparison of photocatalytic H2 evolution rate over reported g-C3N4-based photocatalysts.

Author information Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgment We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 21676182), the 973 program (Grant No. 2014CB932403), and

29



the Natural Science Foundation of Tianjin, PR China (Grant No. 15JCZDJC37400, 15JCYBJC23000). Authors are also grateful to the Program of Introducing Talents of Disciplines to China Universities (B06006). Thanks for the contribution of Prof. M. Meng in this work.

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The synergistically enhanced efficiency of light absorption and charge separation and transfer leads to the superior performance of the phosphorus and protons co-modified g-C3N4.

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Coaddition of Phosphorus and Proton to Graphitic Carbon Nitride for

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