Surface Modification of Graphitic Carbon Nitride by Plasma in Hydroquinone Solution for Enhanced Selectivity and Durability of Visible Light CO₂ Reduction with a Ru(II)-Ru(II) Supramolecular Photocatalyst

For a sustainable energy management, effective conversion of CO₂ into energy-rich chemicals is highly demanded as an alternative to fossil fuels and nuclear energy. Photocatalytic CO₂ reduction using a semiconductor [1] or a metal complex [2] has been vigorously investigated because this approach can potentially harvest solar energy for CO₂ conversion. Recently, hybrid photocatalytic systems combining a supramolecular photocatalyst and a semiconductor photocatalyst have been developed for realizing highly efficient CO₂ reduction with using visible light [3, 4]. For example, the hybrid of mesoporous graphitic carbon nitride (C₃N₄) and binuclear ruthenium(II) complex (<b>RuRu’</b>) demonstrated selective reduction of CO₂ to formic acid (HCOOH) under visible light irradiation (<i>λ</i>_ex &gt; 400 nm) and exhibited a very high durability with a turnover number over 33000 with respect to the amount of <b>RuRu’</b>, which was the highest among the metal-complex/semiconductor hybrid systems reported to date [4]. In this system, <b>RuRu’</b>, which consists of a Ru(II) catalytic unit for CO₂ reduction and a Ru(II) redox photosensitizer unit for initiating photochemical electron transfer, was adsorbed onto mesoporous C₃N₄ via phosphonic acid anchoring groups of<b> RuRu’</b>, where C₃N₄ donates electrons to <b>RuRu’</b> under visible light irradiation. However, poor functional groups on C₃N₄ which can interact with the phosphonic acid anchoring group hinders interfacial affinity of <b>RuRu’</b> onto C₃N₄, which might prevent potential development for better photocatalytic system.<br/>To gain better interfacial affinity, surface modification of a semiconductor should have a great potential. In this study, as the first example of this concept, C₃N₄ nanosheet was modified with a non-equilibrium plasma in hydroquinone-containing aqueous solution [5]. The plasma treatment modified the surface of C₃N₄ selectively without influencing on the bulk properties of C₃N₄ such as crystal structure, bandgap energy, and chemical structure. By X-ray photoelectron spectroscopy and UV−vis diffuse reflectance absorption spectroscopy, deposition of oxygen-rich carbon layer based on sp² bonding structure was tentatively identified. Furthermore, the adsorption density of ruthenium complex with phosphonic acid anchoring groups onto C₃N₄ was improved by approximately three times, with the surface coverage of ~50%. Therefore, the interfacial affinity was improved by the surface modification of C₃N₄. The influence of the surface modification on photocatalytic CO₂ reduction by hybrid photocatalytic system was investigated in the case of the aforementioned well-developed hybrid system with C₃N₄ and <b>RuRu’</b> [4]. The hybrid particles of C₃N₄ and <b>RuRu’</b> were dispersed in <i>N</i>,<i>N</i>-dimethylacetamide/triethanolamine mixed solution with 4:1 volume ratio with CO₂ bubbling, and photocatalytic reactions were performed by irradiating visible light. The surface modification improved the selectivity of HCOOH production and durability up to approximately 2 times. This result reveals positive effect of the surface modification on the photocatalytic performance. In this talk, we will discuss the influence of the surface modification onto the photocatalytic activity in more detail.<br/>[1] J. Ran, M. Jaroniec, S-Z. and Qiao, <i>Adv. Mater.</i> <b>30</b>, 1704649 (2018).<br/>[2] Y. Kuramochi, O. Ishitani, and H. Ishida, <i>Coord. Chem. Rev.</i> <b>373</b>, 333 (2018).<br/>[3] A. Nakada, H. Kumagai, M. Robert, O. Ishitani, and K. Maeda, <i>Acc. Mater. Res.</i> <b>2</b>, 458 (2021).<br/>[4] R. Kuriki, H. Matsunaga, T. Nakashima, K. Wada, A. Yamakata, O. Ishitani, and K. Maeda, <i>J. Am. Chem. Soc.</i> <b>138</b>, 5159 (2016).<br/>[5] N. Sakakibara, K. Inoue, S. Takahashi, T. Goto, T. Ito, K. Akada, J. Miyawaki, Y Hakuta, K. Terashima, and Y. Harada, <i>Phys. Chem. Chem. Phys.</i> <b>23</b>, 10468 (2021).