Conductivity Mechanism in Ionic 2D Carbon Nitrides: From Hydrated Ion Motion to Enhanced Photocatalysis

Abstract

Carbon nitrides are among the most studied materials for photocatalysis; however, limitations arise from inefficient charge separation and transport within the material. Here, this aspect is addressed in the 2D carbon nitride poly(heptazine imide) (PHI) by investigating the influence of various counterions, such as M = Li+, Na+, K+, Cs+, Ba2+, NH4+, and tetramethyl ammonium, on the material's conductivity and photocatalytic activity. These ions in the PHI pores affect the stacking of the 2D layers, which further influences the predominantly ionic conductivity in M‐PHI. Na‐containing PHI outperforms the other M‐PHIs in various relative humidity (RH) environments (0–42%RH) in terms of conductivity, likely due to pore‐channel geometry and size of the (hydrated) ion. With increasing RH, the ionic conductivity increases by 4–5 orders of magnitude (for Na‐PHI up to 10‐5 S cm‐1 at 42%RH). At the same time, the highest photocatalytic hydrogen evolution rate is observed for Na‐PHI, which is mirrored by increased photogenerated charge‐carrier lifetimes, pointing to efficient charge‐carrier stabilization by, e.g., mobile ions. These results indicate that also ionic conductivity is an important parameter that can influence the photocatalytic activity. Besides, RH‐dependent ionic conductivity is of high interest for separators, membranes, or sensors.

Photomemristive sensing via charge storage in 2D carbon nitrides

“…Potassium polyheptazine imide (K-PHI), a recently discovered 2D carbon nitride, 26,36,37 has remarkable optoelectronic and optoionic properties 38 that are derived from its dual functionality of light harvesting and charge storage. 24,28,39,40 This versatile toolkit has led to the design of novel responsive and/or charge storing concepts, such as ''dark photocatalysis'' 23 and a solar battery anode. 28 K-PHI's large bandgap of B2.7 eV (see Fig.…”

Photocharging of Semiconductor Materials: Database, Quantitative Data Analysis, and Application in Organic Synthesis

“…[43] Cation containing PHIs demonstrate at least two orders of magnitude higher ion conductivity compared to electron conductivity. [44] Due to the diameter of hydrated Na + ion (≈0.72 nm) that matches perfectly the pore diameter (≈0.76 nm) in PHI, this material demonstrates roughly one order of magnitude higher conductivity, 2.5 ± 1.0 × 10 −6 S cm −1 at 42% relative humidity, compared to K-PHI and Li-PHI.…”

“…In cation containing PHIs, diffusion coefficients for ion motion determined via the Nernst-Einstein equation are (50 ± 7) × 10 3 nm 2 s −1 for Na-PHI and (5 ± 0.7) × 10 3 nm 2 s −1 for K-PHI and Li-PHI suggesting directional ion transport through channels (Figure 3). [44] Several orders of magnitude higher diffusivity of ions via micropores compared to slow intercalation of H + and Li + into the bulk of semiconductor nanoparticles explains enhanced rate of H 2 evolution under continuous illumination [44,133,134] and fast electron extraction in PHI-based photoanodes [135] -alkali metal ions assist electron transport through the carbon nitride network.…”

Photocharging of Materials

“…Microporous negatively charged network of poly(heptazine imide)s is beneficial for ion transport in the bulk of the material upon photocharging. [127] From this standpoint, negatively charged covalent organic frameworks (COFs) are promising materials for photocharging as well. COFs constructed from γ-cyclodextrine, negatively charged spiroborate linkers and various counterions, such as Li + , dimethylammonium, piperazinium, possess high Li + conductivity of up to 2.7 mS cm -1 at 30 °C.…”