Time-resolved Optical Spectroscopy to Accelerate the Development of Particulate Carbon Nitride (CN_x) Photocatalysts

Polymeric photocatalysts made of Earth-abundant elements have been extensively developed over the past decade to take advantage of their synthetic tunability.¹ Within this family, carbon nitrides (CN_x) are emerging as exciting photocatalysts because of their high photocatalytic performance combined with good stability and facile synthesis. However, significant gaps remain in our knowledge of the photophysical properties of these organic polymeric materials. Determining the pathways and mechanism of photoinduced processes will greatly aid our efforts to engineer better CN_x photocatalysts for solar fuel production.²<br/>Time-resolved spectroscopies, particularly transient absorption spectroscopy (TAS), enable us to determine the nature of short lived photoexcited states and determine the kinetics of competing processes. We utilize TAS to shed light on leading strategies followed in the pursuit of improved photocatalytic activities: 1) morphological modifications, 2) heterojunction formation, and 3) CN_x precursor blends.<br/>CN_x possesses a 3D morphology resulting from π–π stacking of 2D sheets composed of condensed heptazine units, which gives insoluble particles with low surface area. Nanostructured CN_x can show improved efficiency by allowing for fine-tuning of properties such as length, diameter, and polymer backbone conformation. However, there are contradictory reports in the literature the on the relationship between surface area and photocatalytic efficiency. To address this question, we devised a method to tune the morphology of CN_x after polymerization occurred, thus limiting the potential for uncontrolled chemical changes during the high temperature polymerization step. Using water-soluble CN_x nanosheets as the building block, we observed unique CN_x morphologies using mesoporous silica as the template.³ Our spectroscopic investigations identified that residual CN_x nanosheets caused charge trapping and the loss of photocatalytic activity, possibly counteracting any gains from reducing the size of features.<br/>Forming a heterojunction is a common strategy to promote charge separation and charge carrier lifetime to improve the efficiency of CN_x photocatalysts, though the distinct molecular interface formed are not well-understood.⁴ A highlighted example is the case if organic-organic CN_x heterojunctions with carbon nanodots (CDs). Despite the similarity in material compositions, different CN_x/CD heterojunctions show that the flow of charges is dependant on the type of CDs incorporated.⁵ Segregation of opposite charges across the two different materials leads to impressive chemical selectivity methanol formation from CO₂.<br/>Incorporation of other small molecules into the framework of CN_x in a copolymerization process provides a handle to regulate the intrinsic framework of CN_x and optimize electronic band structures, optical absorption, and the overall photocatalytic performance of CN_x.⁶ We carried out the systematic synthesis of CN_x prepared through copolymerization at various precursor ratios. Our optical spectroscopy and material characterization investigations highlight the complex interconnections between chemical structure, electronic structure, physical properties, photophysics, and photoactivity.<br/> <br/>(1) Wang, Y.; Vogel, A.; et al. <i>Nat. Energy</i> <b>2019</b>, <i>4</i> (9), 746.<br/>(2) Godin, R.; Wang, Y.; Zwijnenburg, M. A.; Tang, J.; Durrant, J. R. <i>J. Am. Chem. Soc.</i> <b>2017</b>, <i>139</i> (14), 5216.<br/>(3) Pankratz, J.; Mitchell, E.; Godin, R. <i>Nanoscale</i> <b>2022</b>, <i>14</i> (37), 13580.<br/>(4) Mitchell, E.; Law, A.; Godin, R. <i>Journal of Photochemistry and Photobiology C: Photochemistry Reviews</i> <b>2021</b>, <i>49</i>, 100453.<br/>(5) Wang, Y.; Liu, X.; et al. <i>Nat Commun</i> <b>2020</b>, <i>11</i> (1), 2531.<br/>(6) Wang, S.; Zhang, J.; Li, B.; Sun, H.; Wang, S. <i>Energy Fuels</i> <b>2021</b>, <i>35</i> (8), 6504.