Water‐Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi‐Homogeneous Photocatalysis

Abstract

Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

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

“…For the details on synthesis, characterization, experimental setup and conditions for PEC measurements see Supporting Information. Briefly, the PHI solution was synthesized followed the methodology reported by Krivtsov et al [16] A mixture of 0.2 g NaOH, 0.56 g KOH, and 1.5 g melamine was treated for 2 h at 330°C in a lid-covered crucible. The solid reaction product was dissolved and dialyzed to obtain the PHI solution.…”

“…Recently, we introduced a bottom-up approach for the synthesis of water-soluble PHI in the form of colloidal nanoparticles (size~10 nm), owing their solubility to the surface cyamelurate groups. [16] Since smaller heptazine-derived species such as cyameluric acid or its salts, although being catalytically relevant sites in PHI materials, do not show any significant photocatalytic activity on their own, [17c] the prepared water-soluble PHI colloids might be considered as the dimensionally smallest photocatalytically active PCN units reported so far. This is one of the key prerequisites for their successful use as a precursor for fabrication of homogeneous, stable and highly photoactive layers on conductive FTO surfaces, in contrast to conventional approaches (Figure 1).…”

“…We have recently developed a novel synthetic procedure yielding fully water-soluble and stable nanoparticles (~10 nm size) of alkali metal poly(heptazine imide) (PHI), [16] a promising member of the PCN family. [17] As our synthetic strategy avoids the use of aggressive exfoliating or oxidizing agents that might impair the material's structural integrity, the water-soluble PHI (more precisely potassium and sodium ion/containing K,Na-PHI) exhibited excellent photocatalytic activity in alcohol oxidation and concurrent H 2 O 2 production without any apparent deactivation.…”

“…[17] As our synthetic strategy avoids the use of aggressive exfoliating or oxidizing agents that might impair the material's structural integrity, the water-soluble PHI (more precisely potassium and sodium ion/containing K,Na-PHI) exhibited excellent photocatalytic activity in alcohol oxidation and concurrent H 2 O 2 production without any apparent deactivation. [16] Notably, as the presence of hydrophilic oxygenated (mainly cyamelurate) moieties on the nanoparticle surface is highly beneficial in view of effective attachment to the surface of conductive substrates, such as fluorine-doped tin oxide (FTO), and since the water-soluble PHI can be gelled, [16] we anticipated that our water-soluble carbon nitride might be a promising candidate for fabrication of high-quality photoelectrode via solÀ gel processing (Figure 1). Herein, we demonstrate, for the first time, the fabrication of binder-free, mechanically robust and photoelectrochemically stable PCN photoanodes via an aqueous solÀ gel technique with watersoluble PHI using various and versatile protocols based on electrodeposition, doctor-blading, and spray pyrolysis.…”

Emerging Concepts in Carbon Nitride Organic Photocatalysis

“…Krivtsov, Beranev, and co-workers introduced 'quasi-homogeneous photocatalysis' concept, according to which heptazine-based polymeric carbon nitrides nanoparticles with size 10 � 3 nm are used as water-soluble photocatalyst, but at the same time can be recovered by simple modulation of the ionic strength of the medium. [130] Mazzanti, Savateev and coworkers, employed exceptional properties of K-PHI to form stable colloid solution in polar solvents, namely highly negative zeta-potential of À 40 mV, in the flow photoreactor with the design similar to that used in homogeneous photocatalysis (Figure 9f). [99a] At the same time, the average diameter of K-PHI particles is 100 nm, enabling simple recovery of the photocatalyst from the reaction mixture via centrifugation.…”