Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Abstract

Polymeric carbon nitride (PCN)-based materials have opened new research avenues in the photocatalytic field. The understanding of parameters underlying the optical properties of PCNs is critical to improve their performance. Herein, the optical properties of PCNs are investigated with a combination of Time-Dependent Density Functional Theory (TD-DFT) calculations and laboratory characterizations, including Variable Temperature (VT)-XRD, VT-UV-Vis and VT-IR techniques, on samples prepared using a broad range of synthesis temperatures. Upon increasing the synthesis temperature from 390 to 600°C, the optical gap E opt g (eV) narrows from 2.92 to 2.67, and on further increasing the synthesis temperature to 700°C, widens to 3.03 and another absorption at ∼2.2 eV is observed. The TD-DFT calculations show that E opt g of PCNs converges rapidly with heptazine oligomer size, indicating localized photophysics behavior and justifying a molecular approach. Calculations exploring geometrical variations upon increasing the temperature at which the optical properties were determined revealed that the net effect of these variations were limited, suggesting a vibrational origin of the observed decrease in the gap. The absorption at ∼2.2 eV is tentatively attributed to fully polymerized graphitic PCN oligomers as suggested by the excitation wavelength-dependent photoluminescence emission behavior. The results provide novel insights into the optical properties of PCNs, providing directions for the development of the next generation of PCN-based photocatalysts with optimal light harvesting abilities.

Combining machine learning and high-throughput experimentation to discover photocatalytically active organic molecules

“…Figure 1d shows a statistical summary of the photocatalytic hydrogen evolution rates (HERs) of the dataset. In comparison with two benchmark conjugated polymers PCN 29 and CTF-1 30 , synthesised in-house and measured under exactly the same conditions, 63 molecules showed HERs higher than for PCN (2.2 mol h -1 ), and 6 molecules surpassed the HER for CTF-1 (17.0 mol h -1 ). The highest HER among our molecular photocatalysts (ID153; see Figure 2b for structure) was 28.3 mol h -1 (5660 mol g -1 h -1 ), which is comparable to the highest HER (~6000 mol g -1 h -1 ) measured for the 175 conjugated polymers using the same experimental setup using a more design-led approach.…”

“…d, Statistical summary of the photocatalytic hydrogen evolution performance of the candidate molecular catalysts in the library. The hydrogen evolution rate (HER) was classified against two conjugated polymers as a benchmark: carbon nitride PCN 29 (2.2 mol/h) and a covalent triazine framework CTF-1 30 (17.0 mol/h).…”

Non-Monotonic Trends of Hydrogen Adsorption on Single Atom Doped g-C3N4

“…Also, it is worth noting that developing an adsorption phase diagram can be a rational strategy for evaluating competing adsorption among multiple adsorbates with varying chemical potential and pressure and for understanding the selectivity of the reaction that is completed by HER [40,41]. Experimentally, multiple HER active sites might be detected using state-of-the-art techniques [42,43], since the polycondensation reaction could be terminated at the intermediate step with a non-negligible hydrogen content, as indicated from previous combined theoretical and experimental studies [44,45]. Nevertheless, since this brief report mainly discusses the unusual hydrogen adsorption trends, we do not include further results on the g-C 3 N 4 substrate.…”

Harnessing the Potential of Graphitic Carbon Nitride for Optoelectronic Applications

“…The routes to g-CN have been examined in many reviews such as those by Volokh et al, [5] Zhao et al, [6] Ghosh and Pal, [12] Ong et al, [10] Kessler et al, [23] Xu and Shalom, [26] Bian et al, [27] and Barrio et al [31] The common precursors and methods are summarized in Figure 3 and Table 1 but are not described in detail to avoid a lengthy discussion. In general, bulk CN is synthesized by the polycondensation of nitrogen-rich precursors such as melamine, [32][33][34][35][36][37][38][39] urea, [40][41][42][43][44] cyanamide, [45] dicyandiamide, [46][47][48][49][50][51][52] and benzoguanamine. [53][54][55][56] Direct thermal polycondensation (calcination) is the most popular of the synthetic methods because of its simplicity and cost efficiency.…”