Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

Synergy of P doping and crystallinity modulation in carbon nitride for enhancing photocatalytic uranyl reduction

Doping modification is a useful way to promote the catalytic activity of carbon nitride (CN). However, most doped CNs have lower structural symmetry and several edge defects, which hinder the transfer of charge carriers. This work reports a P-doped crystalline carbon nitride (crystalline PCN) for the efficient photoreduction of uranyl. The thermal polymerization and salt post-treatment convert the amorphous PCN into crystalline PCN. Compared to the pristine CN, the crystalline PCN has over 1620 % higher activity for uranyl (U(VI)) reduction, reaching a 97.8 % reduction rate in 60 min. Furthermore, the 2-PCN shows excellent stability and a U(VI) removal efficiency >85.7 % in the pH range of 5-8. Characterization analysis reveal that both the P doping and crystalline modulation do not obviously change their morphology, light absorption property and energy band structure, but markedly promote the delocalization of electrons around the doped P atoms, thereby severely inhibit direct electron-hole recombination. Thus, the more efficient separation of charge carriers generates more reactive specials to participate in the photocatalytic uranyl reduction reaction. This study demonstrates a dual-modification strategy for the rational synthesis of highly active metal-free CN-based photocatalysts for uranyl reduction.

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

Water-soluble ionic carbon nitride as unconventional stabilizer for highly catalytically active ultrafine gold nanoparticles

Ultrafine metal nanoparticles (NPs) hold promise for applications in many fields, including catalysis. However, ultrasmall NPs are typically prone to aggregation, which often leads to performance losses, such as severe deactivation in catalysis. Conventional stabilization strategies (e.g., immobilization, embedding, or surface modification by capping agents) are typically only partly effective and often lead to loss of catalytic activity. Herein, a novel type of stabilizers based on water-soluble ionic (K+ and Na+ containing) polymeric carbon nitride (i.e., K,Na-poly(heptazine imide) = K,Na-PHI) is reported that enables effective stabilization of highly catalytically active ultrafine (size of ∼2-3 nm) gold NPs. Experimental and theoretical comparative studies using different structural units of K,Na-PHI (i.e., cyanurate, melonate, cyamelurate) indicate that the presence of functionalized heptazine moieties is crucial for the synthesis and stabilization of small Au NPs. The K,Na-PHI-stabilized Au NPs exhibit remarkable dispersibility and outstanding stability even in solutions of high ionic strength, which is ascribed to more effective charge delocalization in the large heptazine units, resulting in more effective electrostatic stabilization of Au NPs. The outstanding catalytic performance of Au NPs stabilized by K,Na-PHI is demonstrated using the selective reduction of 4-nitrophenol to 4-aminophenol by NaBH4 as a model reaction, in which they outperform even the benchmark "naked" Au NPs electrostatically stabilized by excess NaBH4. This work thus establishes ionic carbon nitrides (PHI) as alternative capping agents enabling effective stabilization without compromising surface catalysis, and opens up a route for further developments in utilizing PHI-based stabilizers for the synthesis of high-performance nanocatalysts.

Carbon Nitrides from Supramolecular Crystals: From Single Atoms to Heterojunctions and Advanced Photoelectrodes

Carbon nitride materials (CN) have become one of the most studied photocatalysts within the last 15 years. While CN absorbs visible light, its low porosity and fast electron-hole recombination hinder its photoelectric performance and have motivated the research in the modification of its physical and chemical properties (such as energy band structure, porosity, or chemical composition) by different means. In this Concept we review the utilization of supramolecular crystals as CN precursors to tailor its properties. We elaborate on the features needed in a supramolecular crystal to serve as CN precursor, we delve on the influence of metal-free crystals in the morphology and porosity of the resulting materials and then discuss the formation of single atoms and heterojunctions when employing a metal-organic crystal. We finally discuss the performance of CN photoanodes derived from crystals and highlight the current standing challenges in the field.

Operando IR Optical Control of Localized Charge Carriers in BiVO4 Photoanodes

In photoelectrochemical cells (PECs) the photon-to-current conversion efficiency is often governed by carrier transport. Most metal oxides used in PECs exhibit thermally activated transport due to charge localization via the formation of polarons or the interaction with defects. This impacts catalysis by restricting the charge accumulation and extraction. To overcome this transport bottleneck nanostructuring, selective doping and photothermal treatments have been employed. Here we demonstrate an alternative approach capable of directly activating localized carriers in bismuth vanadate (BiVO4). We show that IR photons can optically excite localized charges, modulate their kinetics, and enhance the PEC current. Moreover, we track carriers bound to oxygen vacancies and expose their ∼10 ns charge localization, followed by ∼60 μs transport-assisted trapping. Critically, we demonstrate that localization is strongly dependent on the electric field within the device. While optical modulation has still a limited impact on overall PEC performance, we argue it offers a path to control devices on demand and uncover defect-related photophysics.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations

Carbon nitride (CN_x ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CN_x photoelectrodes has remained low. Here, cyanamide-functionalized CN_x (^NCN CN_x ) was co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the ^NCN CN_x , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al₂ O₃ |ITO : ^NCN CN_x film displays a benchmark performance for CN_x -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm^-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CN_x in fundamental and applied photoelectrochemical (PEC) studies.

Ultrafast anisotropic exciton dynamics in a water-soluble ionic carbon nitride photocatalyst

Ultrafast transient absorption anisotropy spectroscopy (TAA) reveals the orientational dynamics of light-induced excitations in a water soluble poly(heptazine imide). The results provide insights into the fast charge transfer processes in the material.