Rational Design of Carbon/Potassium Poly(heptazine imide) Heterojunction for Enhanced Photocatalytic H 2 and H 2 O 2 Evolution

We present the rational design of carbon/potassium poly(heptazine imide) (KPHI) heterostructures via one-step salt-melt assisted condensation for efficient photocatalysis. Hybridizing KPHI with an adenine-derived carbonaceous material (Ad-carbon) displayed an outstanding...

3 g of citric acid and 1 g of urea were mixed in a 8 mL of deionised H 2 O in a beaker and vigorously stirred until a transparent solution was obtained.Then the solution was placed inside a microwave oven and irradiated at 600 W for 10 min.The resulting solid product was then dried in an oven at 80 °C for 10 h to remove residual small molecules.The crude CD suspension was purified in a centrifuge at 8000 rpm for 1 h to remove large particles or agglomerates.The final brown aqueous solution was washed with a mixture of methanol and dichloromethane in ratios of 1:2 and 2:1.As a last step, the solution was dried to obtain the CD as a solid. 2 Synthesis of 4-hydroxy-1H-pyrrolo [3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT): 2 g of citric acid and 3 g of urea were added to a large beaker and stirred at 120°C in an oil bath until everything is molten.The reaction mixture was reacted further at 120°C.It turned brown and increasingly solid.When it became completely solid, the product was ground and stirred for 24 h at 120°C in an oil bath.

Synthesis of KPHI and carbon material/KPHI (CM/KPHI) hybrids:
Potassium poly(heptazine imide) (KPHI) was synthesized according to a previously reported study. 32.5 g of 5-aminotetrazole and 12.5 g of KCl/LiCl eutectic (0.55/0.45 ratio) were placed in a steel ball mill vessel.The mixture was then ground at operational frequency of 25 Hz for 5 min.The resulting white powder was transferred into a porcelain crucible covered with a lid, and placed in a furnace.The temperature inside the furnace was increased to 600 °C within 4 h under a flow of N 2 gas (4 L min −1 ) and maintained this temperature for another 4 h.Subsequently, the furnace was allowed to naturally cool down to room temperature.The melt from the crucible was then transferred into a beaker with 300 mL deionized H 2 O and was stirred at room temperature overnight.Then the product was vacuumfiltered and washed extensively with H 2 O by centrifugation (5 x 2mL, 13 500 min −1 , 3 min), and dried in a vacuum oven at 60 °C for 3 h.The preparation for CM/KPHI hybrids follows the same protocol except that the carbon materials were added along with 5-aminotetrazole and the eutectic salt template.

Materials Characterizations
X-ray diffraction was conducted with a Rigaku SmartLab (Japan, Cu K, 0.154 nm).
UV-vis absorption spectra were acquired using Shimadzu UV 2600 in diffuse reflectance mode.FTIR spectroscopy was performed using a Thermo Scientific Nicolet iD5 spectrometer with the attenuated total reflection sampling technique.Steady-state photoluminescence (PL) spectra were measured using a Jasco FP-8300 fluorescence spectrometer at an excitation wavelength of 365 nm.Time-resolved PL (TRPL) spectra were recorded on fluorescence lifetime spectrometer (FluoTime 250, PicoQuant) equipped with PDL 800-D picosecond pulsed diode laser drive.Elemental combustion analysis was performed with a vario MICRO cube CHNOS elemental analyzer (Elementar Analysensysteme GmbH).Inductively coupled plasma mass spectrometry (ICP-MS) was performed with a PerkinElmer ICP-OES Optima 8000.Transmission electron microscopy (TEM) images were taken using JEOL JEM F200 and a double Cs corrected JEOL JEM-ARM200F operated at 80kV and equipped with a cold-field emission gun and high-angle silicon drift Energy Dispersive X-ray (EDX) detector (Jeol JED-2300 100 mm 2 , Japan) (solid angle up to 0.98 steradians with a detection area of 100 mm 2 ).Annular Dark Field Scanning Transmission Electron Microscopy (ADF -STEM) images were collected at a probe convergence semi-angle of 25 mrad.The morphologies of the samples were observed using a scanning electron microscope (SEM) (Zeiss LEO 1550-Gemini) equipped with an EDX (Oxford Instruments X-MAX).Nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Quadrasorb SI apparatus.The samples were degassed at 150 °C under vacuum (0.5 Torr) for 20 h prior to each measurement.The specific surface area of each material was calculated from the adsorption branch data (P/P 0 < 0.3) using the Brunauer−Emmett−Teller (BET) method.

Photocatalytic H 2 Evolution
Sacrificial photocatalytic H 2 evolution experiments were performed in a closed system equipped with a pressure detector to monitor the pressure build-up of the gases evolving during photocatalytic reactions.White LED (50 W, λ > 420 nm), purple LED (50 W, λ = 410 nm) and a green LED (50 W, λ = 535 nm) were used as light source for the photocatalytic evaluation.
A total volume of 38 mL was used and the temperature during the reaction was maintained at 295 K by a water circulator unit.Typically, 50 mg of photocatalyst powder was dispersed in a 38 mL mixture of DI H 2 O and TEOA with a volume ratio of 9:1, which was degassed beforehand to remove the dissolved O 2 in the solution.The reactor was then illuminated from the side for 6 h. 3 wt% of Pt cocatalyst was nominally photodeposited onto the photocatalysts using a K 2 PtCl 4 precursor.Finally, the amount of the evolved gas was calculated after 5 h (1st h was excluded) of irradiation according to the Clausius-Clapeyron relation (PV = nRT).

Photocatalytic H 2 O 2 Production
Sacrificial photocatalytic H 2 O 2 production experiments were performed in a 4 mL vial reactor.First, 5 mg of catalyst was dispersed in 2 mL of a 3.5% w/w glycerin aqueous solution then O 2 gas was bubbled for 1 min.The reactor was then irradiated under stirring using two purple LED lamps (50 W each, λ = 410 nm) for 1 h.The suspension was then centrifuged at 10 000 rpm for 10 min to separate the catalysts from the solution.The generated H 2 O 2 was quantified spectrophotometrically following the titanium oxalate method previously reported in the literature. 4Basically, a 10 g L -1 solution of K 2 [TiO(C 2 O 4 ) 2 ]•2H 2 O was prepared using 450 mL of water and 50 mL of sulfuric acid to avoid the complex precipitation.Subsequently, 3 mL of this reagent was mixed with 1 mL of the supernatant from the photocatalytic experiment.The resulting solutions, properly diluted when needed, were analyzed using the UV-vis spectrometer, collecting absorbance values at 400 nm.A calibration curve was made with external samples of known H 2 O 2 concentrations between 0-10 mmol L -1 with a linear analytical response (R 2 = 0.99996).

Apparent Quantum Yield (AQY) Estimation
The AQY was measured using a monochromatic visible light (410 ± 1.0 nm).The AQY was obtained by the following equation: where is the production rate of H 2 or H 2 O 2 molecules (mol s −1 ) after the 1 st hour of

Transient Photocurrent Response, Electrochemical Impedance Spectroscopy (EIS), and Mott-Schottky Analysis
The photocurrent response was measured (0.3 V vs. ref) under 100 mW cm -2 white LED illumination in 0.2 M aqueous Na 2 SO 4 solution using a Gamry Interface 1010E potentiostat.
For EIS, the same electrodes were used as described above and the measurements were done in a frequency range of 10 kHz to 1 Hz.The data were fitted to a full semicircle using ZView software.Mott-Schottky measurements were performed in a Biologic MPG-2 system at 10 kHz.

Figure
Figure S1.(A) XRD patterns and (B) FTIR spectra of Ad-carbon, CD and HPPT.

Figure
Figure S7.(A) XRD patterns and (B) FTIR spectra of 0.3Ad/KPHI before and after H 2 evolution recyclability tests.

O 2 Figure
Figure S12.Schematic illustration of photocatalytic H 2 and H 2 O 2 evolution over the 0.3Ad/KPHI hybrid photocatalyst system.

Figure S14 .
Figure S14.Additional BF-and HAADF-STEM images showing various random areas of 3Ad/KPHI hybrid sample (Ad-carbon concentration is increased by 10x to easily locate the heterostructure).

Table S1 .
BET specific surface area and pore diameter size of CM, KPHI and CM/KPHI

Table S4 .
Comparison of H 2 O 2 production rates of different carbon nitride-based photocatalysts reported in the literature

Table S5 .
PL lifetime values of KPHI and 0.3Ad/KPHI in aqueous suspension under different experimental conditions.