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Here, we effectively layered economically viable pyrolytic carbon nanotubes (p-CNTs) as solid-state mediators to accelerate the charge carrier transfer between hematite (α-Fe2O3) and boron-doped graphitic carbon nitride (B-C3N4). This synergistic combination leads to higher photoelectrochemical water splitting performance with a photoanodic current density of 2.85 mA cm−2, which is a 4.1-fold enhancement compared to pristine α-Fe2O3 and the O2 evolution rate detected was 22.70 μmol h−1 cm−2 with a Faraday efficiency of ∼98% at 1.7 VRHE. Mott–Schottky analysis confirms the highest donor density of 55.7 × 1019 cm−3 for the α-Fe2O3/B-C3N4/p-CNT photoanode, compared to α-Fe2O3 and α-Fe2O3/B-C3N4. Superstructuring the B-C3N4 and p-CNT onto pristine α-Fe2O3 enhances the charge separation and transfer efficiencies, and moreover mitigates recombination losses. DFT calculations suggest the type II charge transfer mechanism switched to an enhanced Z-scheme type by simple deposition of p-CNT on the α-Fe2O3/B-C3N4 heterojunction. Achieving such cost-effective, highly efficient hematite-based photoanodes offers an opportunity to fabricate tandem photoelectrochemical devices for low-cost solar fuel production. [ABSTRACT FROM AUTHOR]

Understanding the pivotal role of surface co‐catalysts is paramount in the strategic design of forthcoming photoelectrodes. However, the nuanced impacts of co‐catalysts remain elusive, particularly in promoting the water oxidation reaction on hematite, especially in connection to surface states denoted as S1 (higher energy) and S2 (lower energy). For this purpose, we tailored two isomorphous hematite nanoarrays with a thin layer of amorphous copper oxide (CuOx), composed of a blend of Cu(I) and Cu(II) species, via a soft electrodeposition technique. Remarkably, we discovered that in pristine hematite (α‐Fe2O3), the S2 state played a pivotal role in activating the CuOx ad‐layer for water oxidation. At lower external biases (approximately 0.9–1.1 VRHE), CuOx served as charge reservoir in equilibrium with the S2 state. Notably, beyond 1.1 VRHE, where the high‐energy holes of the S1 state became available, CuOx was activated indirectly through the equilibrium with the S2 state, and a pronounced enhancement in photocurrent was observed. Conversely, in the case of Ti‐doped hematite (Ti : α‐Fe2O3) devoid of the S2 state, the presence of CuOx resulted in a decline in charge transfer efficiency. Instead of facilitating water oxidation, CuOx adversely affected the S1 surface sites and reduced the charge carrier density in Ti : α‐Fe2O3. [ABSTRACT FROM AUTHOR]

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid–liquid interface via molecular catalyst design. Specifically, the surface of a p‐Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda‐SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda‐SO3H)2, which contains an electron‐withdrawing −SO3H group in the second coordination sphere, elevates the flat‐band potential of the polished p‐Si photocathode and nanoporous p‐Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band‐edge engineering by molecular catalysts could be an important design consideration for semiconductor–catalyst hybrids toward PEC H2 production. [ABSTRACT FROM AUTHOR]

For photocatalytic CO2 reduction, traditional amorphous polymeric carbon nitride (PCN) has suffered from fast photoexcited electron‐hole recombination and low specific surface area, resulting in low photocatalytic activity. Herein, starting from thermally polymerized PCN, cayanamide groups (─CN) functionalized crystalline carbon nitride (CCN) self‐supporting aerogels are obtained through a molten‐salt and self‐assembly two‐step strategy, which realized efficient photocatalytic CO2 reduction into CO as the main reduction products and O2 as the oxidation product, with CO evolution rate and selectivity reaching 25.7 µmol g−1 h−1 and 93.8%, respectively. It is revealed that the self‐supporting porous aerogel structure can enhance the mass transport of reactants and products, and increase the light absorption ability via multiple photon reflection. With charge carrier separation greatly enhanced in CCN aerogels with high crystallinity as revealed by charge carrier dynamics investigations, theoretical calculations and in situ spectral characterizations evidence that the introduced ─CN groups would act as the active sites for photoreduction of CO2 into CO, with energy barrier greatly reduced for the formation of COOH* intermediates, the rate‐determining step for CO2─to─CO reduction. This work demonstrates a novel and controllable strategy to develop semiconducting polymers with crystal, molecular and morphological structures synergistically modulated for highly efficient and selective photocatalytic CO2 reduction. [ABSTRACT FROM AUTHOR]