Efficient Hole Trapping in Carbon Dot/Oxygen‐Modified Carbon Nitride Heterojunction Photocatalysts for Enhanced Methanol Production from CO2 under Neutral Conditions

Abstract Artificial photosynthesis of alcohols from CO2 is still unsatisfactory owing to the rapid charge relaxation compared to the sluggish photoreactions and the oxidation of alcohol products. Here, we demonstrate that CO2 is reduced to methanol with 100 % selectivity using water as the only electron donor on a carbon nitride‐like polymer (FAT) decorated with carbon dots. The quantum efficiency of 5.9 % (λ=420 nm) is 300 % higher than the previously reported carbon nitride junction. Using transient absorption spectroscopy, we observed that holes in FAT could be extracted by the carbon dots with nearly 75 % efficiency before they become unreactive by trapping. Extraction of holes resulted in a greater density of photoelectrons, indicative of reduced recombination of shorter‐lived reactive electrons. This work offers a strategy to promote photocatalysis by increasing the amount of reactive photogenerated charges via structure engineering and extraction before energy losses by deep trapping.

The FAT polymer was synthesised from formic acid (897 μL, 23.8 mmol) and DCDA (2 g, 23.8 mmol) dissolved in 40 mL DI water, as described in our previous report. [1] The solution was set to heat at 130 o C for 6 h before drying and calcination in a lidded crucible by a muffle furnace (ramp rate: 2 o C / min, 500 o C for 4 h).
CD/FAT was synthesized from CD (100-300 mg 5-15% w.t.) and FAT (2 g) suspended in 10 mL DMF. After drying at 60 o C for 10 h, the sample was annealed in the same furnace on a ceramic plate (ramp rate: 10 o C/ min, 500 o C for 4 h), together with pure FAT and CN (to keep consistency). Reference samples involving CD/CN were synthesised from DCDA using the same parameters. [2] DI water, 0.1 M NaOH and HCl were used to wash the produced powders adequately.

Material characterisation
Powder X-Ray Diffraction (PXRD) measurements were taken using a StadiP diffractometer from Stoe company, a voltage of 40 kV, at 30 mA, using a Cu source with Kα1 = 1.540562 Å and Kα2 = 1.544398 Å. (Company: Stoe. Diffractometer: StadiP. Cu X-ray tube run at 40kV 30mA Capillary transmission geometry. Pre-sample Ge (111) monochromator selects K alpha 1 only. Sample rotated in the beam. Dectris "Mythen 1k" silicon strip detector covering 18 deg 2 theta.) Diffuse reflectance spectra were obtained on a Shimadzu UV-Vis 2550 spectrophotometer fitted with an integrating sphere. A standard barium sulphate powder was used as a reference. Absorption spectra were calculated from the reflection measurements via the Kubelka-Munk transformation. ATR-FTIR spectroscopy was collected using a Perkin-Elmer 1605 FT-IR spectrometer in the wavenumber range 500 -4000 cm -1 with a resolution of 0.5 cm -1 . Raman spectroscopic measurements were performed on a Renishaw InVia Raman Microscope, using a 325 nm excitation laser and a wavenumber range 100-2000 cm -1 . XPS measurements were obtained on a Thermoscientific XPS K-alpha surface analysis machine using an Al source. The results of etched samples were carried out on the same XPS equipment. The XPS analysis was performed using CasaXPS software. The high-resolution transmission electron microscopic (HR-TEM) images were taken by a Titan Themis at 120 kV accelerating voltage.

Photocatalytic analysis
Before the photocatalytic reduction of CO2, 10 mg photocatalyst and 10 mL water were added into a septumsealed borosilicate glass reactor with a volume of 140 mL. Then, the reactor was purged for 20 min with CO2 before the start of the photoreduction experiment. A 300 W Xe lamp (Newport) was utilised as a light source, and the light output power was measured by a Newport 918-D calibrated photodetector. During the reaction, the products were analysed by GC (Varian GC-450) with a thermal conductivity detector (TCD, connected to a molecular sieve column) and a flame ionisation detector (FID, connected to a CP-SIL 5CB capillary column) containing a methanizer equipment. Ar gas was used as the GC carrier gas. The CH3OH oxidation conditions: 10 ml H2O, 0.12 µmol MeOH, 10 mg CD/FAT photocatalyst, 300 W Xenon lamp irradiation with 420 nm longpass filter in 1 bar Argon atmosphere.

Calculation of internal quantum efficiency
The internal quantum yields for CD/FAT photocatalyst was measured using the same experimental setup as the photocatalysis measurement, with a bandpass filter (λ=420 nm, 500 nm or 600 nm).
The internal quantum yields are defined by the following equation: Two electrons are consumed by per CO molecule evolved, and six electrons are consumed by per CH3OH molecule evolved according to reaction (3) or (4).
As a result, the internal quantum efficiency can be estimated by the equation: where: NCO is the amount of CO, NMeOH is the amount of CH3OH, NA is the Avogadro's number, Ha is the average intensity of absorbed light, obtained by the subtraction of the transmitted intensity from the incident intensity. A is the irradiation area (12 cm 2 ), h is the Planck's constant, c is the speed of light, λ is the wavelength of the incident light, t is the time.
The quantum yield report here is the internal quantum yield, which was calculated based on the absorbed light density (by measurement of the light intensities in front of and behind the reactor containing 10 mg CD/FAT in 10 ml water). For example, the absorbed light intensity at 420 nm was 161.2 µW/cm 2 , and 0.242 µmol methanol was collected in 1 hour (3600 s). No CO was produced which leads to NCO = 0. =5.9% (7) We are also interested in the activity at a short wavelength and have measured the quantum efficiency under the light irradiation of a 365 nm LED (Thorlabs M365LP1). In 5 hours, 7.15 micromoles of methanol have been produced and we calculate an internal quantum efficiency of 18.6%, another remarkable value for a metal-free system.

Transient absorption spectroscopy
TAS data were acquired on home-built setups as described previously. [3] Samples were purged with argon to remove oxygen. 355 nm or 600 nm laser excitation was generated from an Nd:YAG laser (OPOTEK Opolette 355 II, 7 ns pulse width). The 355 nm excitation fluence was set to 460 µJ/cm 2 The probe light was generated from a quartz halogen lamp (Bentham IL1). Long pass filters (Comar Instruments) were placed between the lamp and sample to minimise short-wavelength irradiation of the sample. A 5 cm path length cuvette filled with DI water was also placed in the beam path as an IR filter to avoid heating effects. A long pass filter positioned between the sample and a monochromator was used to block the scattered laser light.
The probe wavelength was selected by the monochromator, and the light relayed to a Si photodiode detector (Hamamatsu S3071). Data on the sub-ms timescale were conditioned by an electronic amplifier box (Costronics) and recorded on an oscilloscope. Data on the ms timescale were simultaneously recorded on a National Instruments DAQ card. Acquisitions were triggered by a photodiode (Thorlabs DET10A) exposed to laser scatter. Data from at least 32 laser pulses were acquired and processed using software written in the Labview environment (Austin Consultants) to obtain kinetic traces. The linearly spaced data (ca. 35k points) was reduced to logarithmically spaced data (ca. 160 points) with averaging to reduce the noise.