Linking Photocatalytic Hydrogen Evolution Performance of Carbon Nitrides to Excited State Dynamics via Bulk and Single-Particle Spectroscopic Methods

Given rising global energy needs, developing sustainable routes to efficiently produce clean fuels has become imperative. Solar-driven photocatalytic hydrogen production using carbon nitride (CNx) has gained a lot of research traction, but these materials suffer inefficiencies in solar energy conversion. Additionally, the large variations in activity across similarly designed CNx materials remain poorly understood, convoluted by their highly disordered nature. Here, we establish a framework to combine multiple spectroscopic characterization methods such as UV–vis spectroscopy, FT-IR, time-resolved photoluminescence, and transient absorption (TA) methods to study CNx materials derived from 5 different precursors, gaining insights on emissive state dynamics and non-emissive trapping phenomena of these samples. We employed multivariate analyses including linear regression, principal component analysis, and Pearson correlation on the data set to identify the key spectroscopic state features which best explain the photocatalytic performance across CNx materials. A key distinction in this study is the use of single-particle TA measurements (SP TA) to include the effects of particle-to-particle heterogeneity in charge carrier dynamics. Our analyses identified three features which have the highest importance in dictating hydrogen evolution reaction (HER) efficiency: (a) the trapped charge half-lives in the μs–s regime, (b) the fraction of free and reactive charges, and (c) the particle-to-particle heterogeneity in half-lives of trapped charge carriers. Notably, this effect of particle-to-particle heterogeneity on determining the overall performance is inaccessible from ensemble measurements alone, highlighting the importance of insights that can be gained from advanced spatial spectroscopic methods. The links between the identified key parameters indicate that studying non-emissive charge trapping phenomena in the μs–s time scale and incorporating single-particle excited-state insights are important for understanding the broad discrepancies in HER performances arising from subtle electronic differences in disordered polymeric CNx. These results hence establish such spectroscopic techniques for guiding the rational design of high-efficiency CNx photocatalysts.