Interface Energetics and Chemical Modification of Graphitic Carbon Nitride Film

Carbon neutral is now required to secure our future quality of life. Reduction of amount of carbon dioxide as well as production of CO₂-free energy resources by utilizing renewable energy such as sunlight have thus been targets of intensive researches worldwide. Photocatalytic hydrogen evolution is one of technologies to produce molecular hydrogen for energy use via water splitting by free charges generated in semiconducting materials. Organic semiconductor and its junction with either dissimilar organic or inorganic semiconductor exhibit photocatalytic behaviors under visible-light irradiation [1]. Among organic photocatalysts, graphitic carbon nitride (g-CN) has attracted considerable attentions owing to facile preparation, visible light absorption (optical gap = 2.7 eV), and high chemical stability [2]. Applications of g-CN cover photocatalytic hydrogen evolution and CO₂ reduction, and photodegradation of organic molecules [3], in addition to a fluorescent material for light emitting diode [4] and moisture-sensitive actuator [5]. Surface of g-CN is crucial in a photocatalytic reaction because free charges generated react with adsorbates. An interface with a dissimilar material facilitates exciton dissociation and/or recombination of counter charges in Z-scheme reaction. Behavior of free charges is governed by energies of frontier orbitals at surface and interface. Thus, we addressed energetics of g-CN and its interface with a co-catalyst by utilizing a well-oriented g-CN film. We determined the HOMO and LUMO levels of melon, a zig-zag polymer of heptazine units, via direct and inverse photoemission spectroscopies. Their energetic positions were found to be favorable for water splitting [6]. Valence and core level photoemission spectra clarified that Z-scheme photocatalytic reaction is possible at the interface between melon and MoO_x because of a huge built-in-potential caused by charge transfer from melon to MoO_x [7]. In our talk, after briefly reviewing these findings, we will present that a commercial glass slide used as a substrate introduces cyanamide moiety into a g-CN film [8]. Reaction mechanism, basic properties of the modified g-CN film and effect of the chemical modification on photocatalytic activity in the film form will be discussed.<br/><b>References</b><br/>[1]Kosco <i>et al</i>., <i>Adv. Energy Mater</i>., <b>10</b>, 2001935 (2020)<br/>[2]Wang <i>et al</i>., <i>Nat. Mater</i>., <b>8</b>, 76 (2009)<br/>[3]Ong <i>et al</i>., <i>Chem. Rev</i>., <b>116</b>, 7159 (2016)<br/>[4]He <i>et al</i>., <i>Mater</i>. <i>Today</i>, <b>22</b>, 1369 (2018)<br/>[5]Arazoe <i>et al</i>., <i>Nat. Mater</i>., <b>15</b>, 1084 (2016)<br/>[6]Akaike <i>et al</i>., <i>Chem. Mater</i>., <b>30</b>, 2341 (2018)<br/>[7]Kawase <i>et al</i>., <i>Appl. Catal</i>. <i>B</i>, <b>273</b>, 119068 (2020)<br/>[8]in preparation.