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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction Ryo Kuriki, Chandana Sampath Kumara Ranasinghe, Yasuomi Yamazaki, Akira Yamakata, Osamu Ishitani, and Kazuhiko Maeda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03996 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 3, 2018
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The Journal of Physical Chemistry
Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction Ryo Kuriki,1,2 Chandana Sampath Kumara Ranasinghe,3 Yasuomi Yamazaki,1,# Akira Yamakata,3 Osamu Ishitani,1 and Kazuhiko Maeda*1 1
Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan. 2
Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan.
3
Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan.
#
Current address: Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijojikitamachi, Musashino-shi, Tokyo 180-8633, Japan.
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ABSTRACT. We have previously developed photocatalytic CO2 reduction systems using graphitic carbon nitride (g-C3N4) and a Ru(II) mononuclear complex (e.g. trans(Cl)–[RuII{4,4’(H2PO3)2bpy}2(CO)2Cl2] bpy=2,2’-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C3N4 and electron-transfer process to RuP, here we examined the photophysical properties of gC3N4 as well as mesoporous g-C3N4 (mpg-C3N4) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C3N4 (as well as mpg-C3N4) has at least three emissive excited states with different lifetimes (g-C3N4; 1.3 ± 0.4 ns, 3.9 ± 0.9 ns, and 15 ± 4 at 269 nm photoexcitation) in aqueous suspension. These excited states are not quenched upon addition of a hole scavenger (e.g. disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though photochemical electron-transfer processes from/to g-C3N4 has been experimentally confirmed by photocatalytic reactions. On the other hand, TR-IR spectroscopy clearly indicated that mobile electrons photogenerated in mpg-C3N4, which are shallowly trapped and/or free electron in the conduction band, are able to move into RuP with a timescale of a few picoseconds. These results suggest that main emission centers and reaction sites (including charge-transfer interfaces) are separately located in the C3N4 materials, and that electron transfer from C3N4 to RuP progresses through less- or non-luminescent sites, in which mobile electrons exist with a certain lifetime.
Introduction
CO2
reduction
to
produce
energy-rich
compounds
using
visible-light-responsive
heterogeneous photocatalysts is a promising means to address global warming and depletion of
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The Journal of Physical Chemistry
fossil resources.1–5 Various semiconducting materials have been reported to construct photocatalytic systems for various reactions, including CO2 reduction, since the first report by Fujishima and Honda who demonstrated photoelectrochemical water splitting using rutile TiO2 single crystal as an anode in 1970s.6–10 To achieve efficient solar energy conversion, it is necessary to develop a photocatalyst that can work under visible light. However, a satisfactory result has not been achieved to date. Graphitic carbon nitride (g-C3N4) and its variants have attracted considerable attention as visible-light-responsive semiconductor photocatalysts for various reactions including water splitting, organic synthesis, the degradation of harmful compounds, etc.11–18 Our group demonstrated that mesoporous g-C3N4 (mpg-C3N4) was capable of photocatalyzing CO2 reduction to give formate by coupling with Ru(II) diimine carbonyl complexes as catalysts under visible light (λ > 400 nm), as confirmed by isotope tracer experiment with 13CO2.19 Since then, effects of the structure of the Ru complex and C3N4 on photocatalytic CO2 reduction were examined so as to maximize photocatalytic activity and stability.19–23 A high apparent quantum yield (AQY) of 5.7% at 400 nm and a catalytic turnover number (TON) of greater than 1000 (based on the loaded Ru complex) were obtained using mpg-C3N4 modified with trans(Cl)– Ru{4,4’-(H2PO3)2bpy}(CO)2Cl2 (RuP, bpy=2,2’-bipyridine), demonstrating the highest value among hybrid photocatalysts consisting of a mononuclear catalytic complex and a visible-lightabsorbing semiconductor.21 In these hybrid systems, electron transfer from the excited states of C3N4 to the loaded Ru(II) complex catalyst is necessary to accomplish CO2 reduction reaction. Therefore, investigating excited-states dynamics of C3N4 and electron injection process should be important. Excited state dynamics of C3N4 has been studied by means of time-resolved photoluminescence and/or time-
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resolved infrared absorption (TR-IR) spectroscopy, which are useful methods to understand the fundamental photophysical processes and how charge trapping dictates photocatalytic activity in a semiconductor material.16,24–34 Very recently, Godin et al. examined the relationship between excited state dynamics of C3N4 and its abilities for photocatalytic H2 evolution.24 In addition, Walsh et al. also examined the relationship between photophysical properties of C3N4 and its abilities for photocatalytic CO2 reduction, which is constructed with C3N4 and catalytic– [Co(bpy)n]2+.27 Yamanaka et al. investigated the excited-state dynamics of nitrogen-doped Ta2O5 adsorbed with [Ru(dcbpy)2(CO)2]2+ dcbpy=4,4’-dicarboxy-2,2’-bipyridine,28 which has been reported to be an active hybrid photocatalyst for CO2 reduction to formate under visible light.35 According to the study by time-resolved emission spectroscopy, it is claimed that ultrafast electron transfer from the shallow defect sites of nitrogen-doped Ta2O5 to the adsorbed Ru complex occurred with a time constant of 12±1 ps. Despite the recent progress on spectroscopic studies, photodynamics of Ru-complex/C3N4 hybrid has not been clarified. In this work, we conducted time-resolved emission/absorption spectroscopy to investigate the excited-state dynamics of C3N4 and electron-transfer processes from C3N4 to Ru(II) complexes. First, we report photophysical properties of g-C3N4 (also mpg-C3N4) in solution in the presence or absence of an electron donor (e.g. disodium dihydrogen ethylenediamine tetraacetate dehydrate (EDTA·2Na) or acceptor (RuP).21,25 Second, we study photoexcited electron transfer from mpg-C3N4 to the loaded RuP by means of time-resolved visible-to-mid IR absorption spectroscopy. Our results suggest that main emission centers and reaction sites (including charge-transfer interface) are independently located in g-C3N4 (also in mpg-C3N4), and that the mpg-C3N4-to-RuP electron transfer occurs with a timescale of a few picoseconds through nonluminescent sites by utilizing mobile electrons photogenerated in mpg-C3N4.
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Scheme 1. Metal complex and carbon nitride hybrid photocatalysts for CO2 reduction.19–23 mpg-C3N4
e– V.B.
Electron Injection
e– C.B.
RuP
h+ Formate
CO2
Experimental Section Materials All reagents were reagent-grade quality and were used without further purification except for DMA and TEOA. DMA was dried over molecular sieves 4A, and distilled under reduced pressure (10–20 Torr). TEOA was distilled under reduced pressure (
