Photocatalytic water splitting by 2D polymer with out-of-plane carrier flow

Solar-driven water splitting with semiconductor particulates offers a sustainable pathway for hydrogen production^1,2. Two-dimensional (2D) π-conjugated polymers have emerged as promising photocatalysts owing to their cost effectiveness and optoelectronic tunability^3,4. However, photoexcited states in polymers are largely confined within π-conjugated 2D planes, making charge carriers vulnerable to recombination. Despite widespread modification of the electronic structure to enhance in-plane charge separation, long-term experimental efforts continue to highlight a persistent bottleneck in quantum efficiency⁵. To maximize charge utilization efficiency, the main challenge lies in inducing out-of-plane carrier migration, namely, fostering carrier flow through van der Waals-bonded layers. Here, using polymeric carbon nitride crystals as model systems, we demonstrate that out-of-plane carrier transport can be activated over surprisingly long distances (about 200 nm) by applying lateral or vertical internal electric fields by means of encapsulating nanofilms on different polymer facets. The lateral and vertical electric fields boost apparent quantum efficiency for overall water splitting to 53.4% and 82.1%, respectively. Our study introduces a strategy for transitioning from intrinsic 2D-confined excited states into kinetic-driven 3D spatially separated states and paves the way for maximizing energy conversion by polymer photocatalysis.