Electrophoretic deposition of supramolecular complexes for the formation of carbon nitride films

The large-scale fabrication of polymeric carbon nitride (CN) films with tunable thickness, composition and photoelectrochemical properties is reported.


Supramolecular precursor synthesis
A cyanuric acid-melamine (CM) supramolecular complex was prepared by mixing equal amounts of CA and M (4 mmol) in 30 mL water. The mixture was shaken for 2 h, centrifuged and dried for 24 h at 60 °C in a vacuum oven, resulting in a CM powder.

Supramolecular-films preparation
CM powder was ground for 45 min using Fritsch Pulverisette 7 planetary ball mill (3 mm ZrO 2 balls, dry milling), washed with water, centrifuged, and dried for 24 hours at 60 °C in a vacuum oven. Next, 50 mg of ball-milled CM powder was dispersed in 5 mL of toluene to form a stable colloidal suspension (10 mg mL -1 ). The obtained suspension was used as the deposition medium for forming CM films on top of FTO by electrophoretic deposition (EPD). The EPD setup consisted of two FTOs dipped into the deposition medium in a parallel capacitor configuration, i.e., the FTO electrodes in the suspension were connected to a DC voltage using an ENDURO power supply. A constant voltage of 300 V was applied during the deposition for 2 s, 5 s, 15 s, 30 s, 45 s, 60 s, 120 s, and 180 s. S-3 Each CM x (x = 2, 5, 15, 30, 45, 60, 120, and 180 s) electrode was placed in a glass tube (16 mm diameter × 100 mm length) along with 1.0 g melamine as a CN precursor powder for the vapor deposition. The tube was purged with N 2 for several seconds and covered tightly with an Al foil. The electrodes in each tube were calcined under N 2 atmosphere (constant flow rate of 120 mL min -1 ), to 550 °C with a heating ramp of 5 °C min -1 and kept for 4 h, resulting in CN-CM x M electrodes. In the same calcination conditions, CM 120 electrode with CM powder (1.0 g), which was used as a CN precursor powder instead of melamine, was prepared, forming CN-CM 120 CM electrode, and CM 60 electrodes with different amounts of melamine powder were prepared, forming CN-CM 60 M y electrodes (y = 0.2, 0.4, 0.6 and 0.8 g).

Characterization
FTIR spectra were obtained by using a Thermo Scientific Nicolet iS5 in the 650-4000 cm -1 range using a diamond iD7 ATR. UV-vis spectra were acquired using a Cary 100 spectrophotometer equipped with a DRA (integrating sphere), in transmittance (T) and reflectance (R) modes, while the Abs(%) has calculated according to 100% -T(%) -R(%). Photoluminescence spectra were measured by using Edinburgh instruments FLS920P Fluorimeter with an excitation wavelength of  ex = 380 nm. Digital photos under UV (365 nm) illumination were taken using a TLC viewing cabinet Vilber-Lourmat CN-6. X-ray diffraction patterns (XRD) were recorded on a PANalytical's Empyrean Diffractometer equipped with a position sensitive detector X'Celerator. The data was collected for 2θ ranging from 5° to 60°, with a scanning time of ~7 min using Cu Kα radiation (λ = 1.54178 Å, 40 kV, 30 mA). XPS data was collected by using an X-ray photoelectron spectrometer (Thermo Fisher ESCALAB 250) ultrahigh vacuum (1×10 -9 bar) with an Al Kα X-ray source and a monochromator. The X-ray beam size was 500 μm and survey spectra was recorded with a pass energy (PE) of 150 eV and high energy resolution spectra were recorded with a PE of 20 eV. All XPS spectra peaks were shifted relative to the C 1s peak, positioned at 284.8 eV, to correct for charging effects. The XPS results were analyzed by using the AVANTGE software. For measuring film thickness, the electrodes were scratched in three different areas on top of the same film, using a needle (1.20 mm diameter). Thickness profile was obtained by using a 3D laser microscope (LEXT OLS5000), under low magnification (×10), via focusing on a specific scan area around the scratches. The roughness from both sides of the scratch (300 S-4 × 200 µm) was averaged by the software, then the distance between the lowest part (FTO level), to the averaged top part of the coating (to the left and right of the scratch) has been calculated. Scanning electron microscopy (SEM) images were recorded on an FEI Verios 460L high resolution SEM, operated at 3.0 kV, and equipped with a FEG source. To avoid charging effects, the samples were coated with 10 nm of sputtered gold (for CN precursors) or a carbon (for CN).

Photoelectrochemical measurements
Photoelectrochemical analysis was performed using a three-electrode system coupled to RHE under one-sun illumination (power density of 100 mW cm -2 ), provided by a solar simulator (Newport, OPS-A500, 300 W Xe arc lamp, equipped with an air mass AM 1.5G and water filters) and calibrated using a power meter (Newport, 919P thermopile detector).
The electrolyte was purged with N 2 for 15 min, followed by linear sweep voltammetry (LSV) measurements in the dark and under 1 sun illumination, at a scan rate of 10 mV s -1 .
Mott-Schottky measurements were performed in 1 M Na 2 SO 4 at a 1.0 kHz frequency.
Incident photon-to-current conversion efficiency (IPCE) values at different wavelengths were calculated from the following equation: Where J is the photocurrent density (J KOH is the photocurrent obtained in 0.1 M KOH aqueous solution, while J TEOA is the photocurrent obtained in 0.1 M KOH aqueous solution S-5 containing 10% (v/v) TEOA; λ is the wavelength of the incident monochromatic light (400, 420, 450, and 480 nm); I is the light power density. Incident monochromatic light of different wavelengths was obtained by inserting a corresponding band-pass filter (Newport 10BBPF10-400, 10BBPF10-420, 10BBPF10-450, and 10BBPF10-480) between the solar simulator and the PEC cell.
The amount of photogenerated H 2 in the reactor headspace was analyzed using a gas chromatograph (Agilent 7820 GC system) equipped with a thermal conductivity detector (TCD).
Faraday efficiency (FE) was calculated using the following equation: Where m is the number of moles of gas actually produced; n is the number of electrons in the electrochemical reaction; F is the Faraday constant; I is photocurrent; t is reaction time.
This equation represents the ratio between the actual hydrogen gas evolution rate and calculated one from measuring the generated photocurrent. S-6