Surface-Specific Modification of Graphitic Carbon Nitride by Plasma for Enhanced Durability and Selectivity of Photocatalytic CO2 Reduction with a Supramolecular Photocatalyst

Photocatalytic CO2 reduction is in high demand for sustainable energy management. Hybrid photocatalysts combining semiconductors with supramolecular photocatalysts represent a powerful strategy for constructing visible-light-driven CO2 reduction systems with strong oxidation power. Here, we demonstrate the novel effects of plasma surface modification of graphitic carbon nitride (C3N4), which is an organic semiconductor, to achieve better affinity and electron transfer at the interface of a hybrid photocatalyst consisting of C3N4 and a Ru(II)–Ru(II) binuclear complex (RuRu′). This plasma treatment enabled the “surface-specific” introduction of oxygen functional groups via the formation of a carbon layer, which worked as active sites for adsorbing metal-complex molecules with methyl phosphonic-acid anchoring groups onto the plasma-modified surface of C3N4. Upon photocatalytic CO2 reduction with the hybrid under visible-light irradiation, the plasma-surface-modified C3N4 with RuRu′ enhanced the durability of HCOOH production by three times compared to that achieved when using a nonmodified system. The high selectivity of HCOOH production against byproduct evolution (H2 and CO) was improved, and the turnover number of HCOOH production based on the RuRu′ used reached 50 000, which is the highest among the metal-complex/semiconductor hybrid systems reported thus far. The improved activity is mainly attributed to the promotion of electron transfer from C3N4 to RuRu′ under light irradiation via the accumulation of electrons trapped in deep defect sites on the plasma-modified surface of C3N4.

(a) I-V characteristics and (b) optical emission spectrum of the plasma. The inset image in (b) is a photograph of the plasma. The plasma was generated between tungsten electrodes by applying the pulsed voltage shown in Figure S1(a).

Figure S2
TG signals of C 3 N 4 before and after plasma treatment. Measurements were conducted in air with a 5 ℃ min −1 ramp rate.

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Figure S3 N 2 adsorption-desorption isotherms of C 3 N 4 (a) before and (b) after plasma treatment.

Figure S4
N 1s XPS spectra of C 3 N 4 before and after plasma treatment.

Figure S5
Adsorption amount of Ru(PS) on C 3 N 4 before and after plasma treatment. The dashed line indicates the total amount of Ru(PS) added to the solution for the adsorption process. Note that the adsorption density and surface coverage in Figure 4 were calculated using the BET surface area assuming uniform adsorption. For the calculation of surface coverage, the short molecular radius of Ru(PS), 0.6 nm, was utilized for the size of the Ru(PS) on C 3 N 4 , based on MM2 molecular mechanics program calculations. 34 S-6

Figure S7
Spectrum of light irradiation used for photocatalytic CO 2 reduction, with 410 nm and 460 nm centered-LEDs. Kubelka-Munk conversed diffuse reflectance absorption spectra of C 3 N 4 and plasma-C 3 N 4 and absorption spectrum of RuRu' measured in MeCN are also displayed. The 410-nm centered-LED was used for photoexcitation of C 3 N 4 by bandgap absorption, while the 460-nm centered-LED was used for photoexcitation of the photosensitizer unit of RuRu' by 1 MLCT absorption.

Figure S9
(a) XANES spectra, (b) Ag K-edge EXAFS spectra and (c) FT of EXAFS spectra of RuRu' (0.5 μmol g −1 )/Ag (1.25wt%)/C 3 N 4 before and after light irradiation. The spectra of Ag foil and Ag 2 O are also shown as a reference. Ag in the hybrid photocatalyst can be assigned as Ag 2 O with some contribution of Ag 0 , although other Ag species should contribute to the structure because the absorption edge structure of RuRu'/Ag/C 3 N 4 cannot be reproduced perfectly by the superposition of the reference spectra of Ag and Ag 2 O. 65 Anyway, the structure of Ag in the hybrid photocatalyst was maintained after the photocatalytic reaction Figure S10 Ag 3d XPS spectra of RuRu'(0.5 μmol g −1 )/Ag(1.25wt%)/C 3 N 4 before and after the 132 h light irradiation.

Figure S11
UV-vis absorption spectra of 10 mM NaOH aqueous solution containing RuRu' detached from RuRu'/Ag/plasma-C 3 N 4 powders before and after the 132-h light photocatalytic reaction.

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Figure S13 ESR spectra of (a) C 3 N 4 and plasma-C 3 N 4 measured in the dark (dashed lines) or under red light ( ex > 550 nm) irradiation (solid lines) and (b) Ag/C 3 N 4 and Ag/plasma-C 3 N 4 measured in the dark (dashed lines) or under UV-vis light (750 nm  ex > 360 nm) irradiation (solid lines). In Figure 12a, the ESR spectrum of C 3 N 4 (black dashed line) is overlapped by that of C 3 N 4 under red light irradiation (blue solid line). Single Lorentzian peak was observed at g = 2.0042 for Ag/C 3 N 4 and at g = 2.0048 for Ag/plasma-C 3 N 4 , both of which are very similar to that of C 3 N 4 and plasma-C 3 N 4 , respectively.

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Figure S15 Cyclic voltammogram of 0.5 mM Ru(cat) in DMA/TEOA (v:v = 4:1) containing 0.1 M of Et 4 NBF 4 as an electrolyte. WE: glassy carbon (3 mm), RE: Ag/AgNO 3 , CE: Pt, Scan rate: 100 mV s -1 . The reduction potential of Ru(cat) was estimated from the peak potential of the first reduction wave measured in an Ar-bubbled solution. The increase in current measured in a CO 2 -bubbled solution indicates the electrochemical reduction of CO 2 .

Figure S16
Mott-Schottky plots C 3 N 4 (a) before and (b) after plasma surface modification, measured in DMA containing 0.1 M Et 4 NBF 4 as an electrolyte. WE: FTO electrode on which C 3 N 4 was deposited, RE: Ag/AgNO 3 , CE: Pt. Flat band potentials of C 3 N 4 before and after the plasma treatment were estimated to be −1.69 V and −1.64 V (vs Ag/AgNO 3 ), respectively. The conduction band potentials of the n-type semiconductor are empirically known to be located 0.1-0.3 V more negative than the flat-band potential,