Facile in situ reductive synthesis of both nitrogen deficient and protonated g-C3N4 nanosheets for the synergistic enhancement of visible-light H2 evolution

A new strategy is reported here to synthesize both nitrogen deficient and protonated graphitic carbon nitride (g-C3N4) nanosheets by the conjoint use of NH4Cl as a dynamic gas template together with hypophosphorous acid (H3PO2) as a doping agent. The NH4Cl treatment allows for the scalable production of protonated g-C3N4 nanosheets. With the corresponding co-addition of H3PO2, nitrogen vacancies, accompanied by both additional protons and interstitially-doped phosphorus, are introduced into the g-C3N4 framework, and the electronic bandgap of g-C3N4 nanosheets as well as their optical properties and hydrogen-production performance can be precisely tuned by careful adjustment of the H3PO2 treatment. This conjoint approach thereby results in improved visible-light absorption, enhanced charge-carrier separation and a high H2 evolution rate of 881.7 μmol h−1 achieved over the H3PO2 doped g-C3N4 nanosheets with a corresponding apparent quantum yield (AQY) of 40.4% (at 420 nm). We illustrate that the synergistic H3PO2 doping modifies the layered g-C3N4 materials by introducing nitrogen vacancies as well as protonating them, leading to significant photocatalytic H2 evolution enhancements, while the g-C3N4 materials doped with phosphoric acid (H3PO4) are simply protonated further, revealing the varied doping effects of phosphorus having different (but accessible) valence states.


Sample characterization
The morphologies of the g-C 3 N 4 samples were characterized by the Hitachi S4800 field emission scanning electron microscope (SEM) operating at 3.0 kV and JEOL-2100F transmission electron microscope (TEM). Energy dispersive X-ray analysis (EDX) of the representative samples were also conducted on a scanning transmission electron microscope (STEM, JEM-2100F, JEOL, Japan). N 2 adsorption-desorption isotherms and pore size distributions were obtained at -196 °C with an ASAP2020 Plus HD88 apparatus (Micromeritics, USA), and then the specific areas were calculated using the Brunauer-Emmett-Teller (BET) method. The X-ray diffraction patterns of the samples were collected using a Bruker D8 Focus Diffractometer with Cu Kα as the radiation source. Fourier transform infrared spectroscopy (FTIR) was obtained by a Bruker Tensor-II spectrometer over a range of 4000-450 cm -1 with a resolution of 1 cm -1 . X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250XI (Thermal Scientific, USA) X-ray photoelectron spectrometer. Electron paramagnetic resonance (EPR) experiments were conducted on a Bruker A300 spectrometer at room temperature. The organic elemental analysis (OEA) of C, N, O and H was performed on Elementar Vario MACRO cube. 13 C, 31 P and 1 H solid-state nuclear magnetic resonance (NMR) spectra were recorded by the Bruker AVIII 600 NMR spectrometer. For the zeta potential measurements, 10 mg of each sample was homogeneously dispersed in 200 mL deionized water using strong ultrasonication and then the zeta potentials were measured at 25 °C with a zeta potential analyzer (Malvern zetasizernano, UK). Shimadzu UV2600 UV-vis spectrometer was used to record the UV-vis absorption spectrums of the samples with BaSO 4 as reference. The photoluminescence data excited with 325 nm light source were collected on an Edinburgh Instruments FLS 920 luminescence spectrometer. Time-resolved photoluminescence decay spectra were collected on Edinburgh Instruments FLS 1000 spectrometer with an excitation wavelength of 340 nm.

Transient photocurrent measurement
To obtain the photocurrent responses of the samples, working electrodes with g-C 3 N 4 coated on the Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2020 transparent FTO conductive glass were prepared. Typically, 20 mg sample was dispersed in 20 mL of 50mg/mL naphthalenol/ethanol solution with strong ultrasonication. The dispersed g-C 3 N 4 solution was evenly dropped on the conductive side of the FTO glass with a 1×1 cm area and then the electrode was dried in 80 °C oven. The as prepared working electrodes was tested in 1 mol/L Na 2 SO 4 aqueous solution and excited by the visible-light (λ≥400nm) irradiation pulse with 20 seconds width, the photocurrent response was recorded by the CHI 660E electrochemical workstation.

Photocatalytic H 2 evolution and AQY measurement
The photocatalytic H 2 evolution ability of each sample was evaluated with the RTK-Solar non-vacuum photocatalytic H 2 evolution testing system (RTKINS Ltd. Wuhan, China, Fig. S1). This novel H 2 evolution testing system works under atmosphere and the generated H 2 volume can be directly measured by the standard microflow cell with a calibrated volume of 33.7μL, it means that the volume of each bubble flowing through the microflow cell is 33.7 μL. Then, the number of bubbles flowing through the microflow cell will be counted by a photoelectric counter and recorded on the workstation installed on a computer, subsequently obtaining the volume of the generated H 2 . Thus, with the system temperature being maintained constant using a waterbath, the amount of the generated H 2 can be calculated by the ideal gas equation of state PV=nRT. Fig. S1 shows the composing parts this testing system. The whole reactor is surrounded by cooling water, thereby ensuring the system temperature maintained constant. The photocatalytic H 2 generation rate of each sample was measured with 1.5 wt% of Pt loading. The detailed operating procedures were as following: 1) 15mg sample was homogeneously dispersed in 80mL of 20 vol.% TEOA solution using strong ultrasonication; 2) 1.2 g of 0.5 mg/mL H 2 PtCl 6 solution (stabilized with 5 wt.% HCl ) was added to the above catalyst solution and then transferred to the specially designed H 2 evolution reactor (reactor volume=120 mL; 3) the reactor was tightly sealed and purged using high purity N 2 for 30mins to remove the oxygen in the solution and system; 4) Stop N 2 purging and ensure that there was no gas leakage in the system; 5) Start visible-light irradiation (λ≥400nm) using the 300 W Xe lamp (Aulight, China) with a 400 nm cutoff filter and the reactor temperature was kept at 20 ℃ using a constant temperature waterbath. 6) After irradiation for 1 hour to insure that the Pt photoreduction loading process was completed, start the RTK-Solar workstation to record the volume of generated H 2 . For the H 2 evolution experiments conducted with K 2 HPO 4 addition, the K 2 HPO 4 solid was premixed with 20 vol.% TEOA solution. For all experiments, the reactor was wrapped with foil to reduce light leakage (as shown in the inset of Fig. S1). Then, the molar amount of the generated H 2 (V mol ) can be calculated according to Equation S1 and S2: Here, V represents the volume of the generated H 2 , mL; n represents the H 2 molar amount, mmol. The AQY of the 0.8-P2-CN sample at 420 nm was measured with the Xe lamp equipped an 420±10 nm monochromatic filter, and all the other details were the same with those in the H 2 evolution experiments. The light intensity was measured with the same procedures in our previous work [1] and the calculated light intensity was 2.93 mW/cm 2 . The calculated light irradiation area was 33.18 cm 2 .

Energy changes (∆E) calculation for removing a lattice N atom in the g-C 3 N 4 basic melon framework with PH 3
The energy changes (∆E) for removing a lattice N atom from N1, N2, N3 and N4 were calculated as Equation S4 and S5. The reduction reactions using H 2 and PH 3 as reduction agents could be illustrated as Equation S6 and S7.
The energy changes for removal of one N atom at N1, N2, N3 and N4 positions and terminating the dangling carbon atoms in H 2 atmosphere were taken from literature [2] and as follows:

Accuracy verification of the RTK-Solar H 2 evolution system
The verification contained two steps. Firstly, two H 2 evolution experiments (using 1.5 wt.% Pt loaded 0.8-P2-CN as catalysts) in the 20 vol.% TEOA and 20 vol.% TEOA+32 mmol K 2 HPO 4 mixture solutions, respectively. The operating procedures were the similar with those in the previous manuscript. In each experiment, the reaction system was purged by high purity N 2 (99.99%) and 4 mL helium was added as internal standard. The reactor was tightly sealed and irradiated under visible light (λ≧400 nm) for 120 mins. After reaction, the reactor was connected to a Hiden HPR 20 gas chromatograph/mass spectrometer system and the gases were carefully checked. As shown in Fig. S6, the mass spectrums indicated that only H 2 was detected excluding the purging gas N 2 and internal standard Helium. Thus, the volume measured in the RTK-Solar system can be directly regarded as the volume of photocatalytic evolved H 2 . Secondly, to verify the accuracy of the H 2 evolution results obtained by the RTK-Solar system, parallel experiments were conducted with the reactor being connected to a closed gas-circulation system and the generated H 2 was determined by a calibrated gas chromatography equipped TCD detector (Clarus 580, PerkinElmer, Helium as carrier gas). All the experimental conditions were the same with those in the RTK-Solar system. The system was purged by high purity N 2 (99.99%) and 5 mL Argon was injected into the system as internal standard. The 1.5 wt.% Pt loaded B-CN, G-CN, 1.6-P1-CN and 0.8-P2-CN were tested in the 20 vol.% TEOA solution and the gas sample was analyzed for every 1 hour. Table S2 and Fig. S7 presented the summarized gas chromatography original data and curves for the H 2 evolution tests, respectively. The full original gas chromatography curves can be found with the following link: https://www.dropbox.com/s/4k5j46w5rdkuejp/GC%20original%20data%20of%20the%20supplemente d%20H2%20evolution%20tests.pdf?dl=0 The generated H 2 amount determined by the gas chromatography method can be calculated by the following equation: According to the summarized gas chromatograph data, the evolved H 2 amount can be calculated. Then, H 2 evolution vs time plot can be also illustrated in Fig. S7. Based on the linear fitting results, the gas chromatography determined H 2 evolution rates over the 1.5 wt.% Pt loaded B-CN, G-CN, 1.6-P1-CN and 0.8-P2-CN in the 20 vol.% TEOA solution were 42.8, 106.6, 140.1 and 261.6 μmol·h -1 , respectively. Thus, the ratios of the H 2 evolution rates determined by gas chromatography and RTK-Solar system were located in a narrow range of 0.972~1.076. It means that the RTK-Solar measured H 2 evolution rate is very close to that determined by the widely accepted gas chromatography method. In the other word, this novel RTK-Solar H 2 evolution system has adequate accuracy to measure the amount of the generated gases. Combining with the mass spectrums and gas chromatography data, the H 2 evolution rates determined using the novel RTK-Solar H 2 evolution system should be reliable. In addition, the Effect of photocatalyst amount on the H 2 evolution was explored and the results were presented in Fig. S9 and the results indicated that it was suitable to fix the photocatalyst usage at 15 mg for each experiment.

Density-Functional-Theory (DFT) calculations
Electronic band structure of B-CN and other g-C 3 N 4 samples were carried out using general-gradientapproximation-Perdew-Burke-Ernzerhof (PBEsol) and projector augmented-wave (PAW) method [4] as implemented in the Vienna Ab initio simulation package (VASP) [5] . The energy cutoff is 500 eV. Here 3×4×2-centered k-point grids were used to integrate the Brillouin zone. A two dimensional (2D) melon sheet was constructed to represent the incomplete polycondensation of melon, with a large vacuum space of 24 Å to separate two neighboring 2D melon sheets. A cubic cell was adopted for the carbon nitrides materials with experimental lattice constants of a = 16.7 Å and b = 12.4 Å [6]. The representative cell units are presented as in Fig. S6. For better understanding, basic atoms composing the g-C 3 N 4 "melon" structure ( Fig. S6A), the proposed groups and vacancies were clearly marked. As seen in Fig. S6B, the carboxyl groups which originated from the N1 or N2 nitrogen atoms (referring to Fig. 3A in the manuscript) removal and heptazine ring opening, are located at the apex of g-C 3 N 4 "melon" structure. While, Fig. S6C presents the other possible nitrogen vacancy which is originated from N3 nitrogen atoms loss.    As seen in Fig. S10, the FTIR spectra indicated that the H 3 PO 2 doped 0.8-P2-CN nanosheets could well conserve the cyano group signal after photocatalytic H 2 evolution. The elemental mapping shown in Fig. S11 also revealed that the compositions of the g-C 3 N 4 could maintain at the same level as before H 2 evolution experiments. The XPS spectra in Fig. S12 indicate that C and N are kept the same after reaction. There is a small 133.8 eV peak shift towards lower binding energy in the P2p spectra of the used 0.8-P2-CN nanosheets, which should be attributed to the interaction of the doped phosphorus with deposited Pt nanoparticles. This coincides with the interesting phenomenon observed in the elemental mapping, in which the distribution of Pt is highly consistent with that of the doped phosphorus ( Fig. S11B and S11C). In brief, the surface properties comparison between the used and fresh 0.8-P2-CN indicate that there are no obvious changes which may induce adverse effect on the performance of the samples. Thus, the stability of 0.8-P2-CN could be further evidenced. The H 2 evolution rate increases with the increasing amount of 0.8-P2-CN in the range of 0~15 mg, and the H 2 evolution rate at 15 mg catalyst was very close to that of 25 mg 0.8-P2-CN. While, with further addition of g-C 3 N 4 , the H 2 evolution rate will slightly go down. It indicates that it is suitable to fix the photocatalyst usage at 15 mg.  Tables   Table S1. Summarized XPS data for the as prepared g-C 3 N 4 samples. C1s, N1s and O1s binding energies for the selected carbon, nitrogen and oxygen species, respectively, and N/C and O/C ratios determined from the quantitative analysis were provided: