Investigation of the Photocatalytic Hydrogen Production of Semiconductor Nanocrystal‐Based Hydrogels

Abstract

Destabilization of a ligand‐stabilized semiconductor nanocrystal solution with an oxidizing agent can lead to a macroscopic highly porous self‐supporting nanocrystal network entitled hydrogel, with good accessibility to the surface. The previously reported charge carrier delocalization beyond a single nanocrystal building block in such gels can extend the charge carrier mobility and make a photocatalytic reaction more probable. The synthesis of ligand‐stabilized nanocrystals with specific physicochemical properties is possible, thanks to the advances in colloid chemistry made in the last decades. Combining the properties of these nanocrystals with the advantages of nanocrystal‐based hydrogels will lead to novel materials with optimized photocatalytic properties. This work demonstrates that CdSe quantum dots, CdS nanorods, and CdSe/CdS dot‐in‐rod‐shaped nanorods as nanocrystal‐based hydrogels can exhibit a much higher hydrogen production rate compared to their ligand‐stabilized nanocrystal solutions. The gel synthesis through controlled destabilization by ligand oxidation preserves the high surface‐to‐volume ratio, ensures the accessible surface area even in hole‐trapping solutions and facilitates photocatalytic hydrogen production without a co‐catalyst. Especially with such self‐supporting networks of nanocrystals, the problem of colloidal (in)stability in photocatalysis is circumvented. X‐ray photoelectron spectroscopy and photoelectrochemical measurements reveal the advantageous properties of the 3D networks for application in photocatalytic hydrogen production.

Self‐Assembly of Semiconductor Nanoplatelets into Stacks Directly in Aqueous Solution

“…This new direct self-assembly of CdSe-NPL-stacks in aqueous media opens up multiple processes and methods, such as gelation, [16][17][18][19] photocatalysis, [21] sensing, [24] ink-jet printing [20] and photoelectrochemistry, [13] for future applications.…”

“…Therefore, the present work focuses on a direct selfassembly of NPL-stacks in aqueous solution (without polymer encapsulation) to open up the possible applications and methods, such as various gelation methods, [16][17][18][19] inkjet printing, [20] photocatalysis, [21] photo-electrochemistry, [12,13,22] photodetection [23] and sensing, [24] light emitting diodes, [25] and many more. For this approach CdSe NPLs are first transferred to aqueous solution via ligand exchange to 11-mercaptoundecanoic acid.…”

Recent Advances in Functional Nanoparticle Assemblies

“…Combining this capability with the high porosity and free reactive surface makes them highly efficient photocatalysts compared to pure NP. [169][170][171] Generally, to achieve the maximum efficiency in photooxidation or photoreduction, the right choice of the appropriate hole or electron scavenger, or cocatalyst is crucial. [208,209] Numerous works on pure NPs are known in the literature, [210][211][212] which can be the basis and inspiration to develop highly efficient photocatalysts based on semiconductor NP-based gel networks.…”

“…Here, it was shown that the hydrogels show a significantly higher efficiency in hydrogen evolution compared to the nongelated NPs too (Figure 9b). [ 170 ] In both examples, the higher efficiency of the gels compared to nongelated NPs can be explained by the gelation process itself as well as the high porosity of the gel structure. To transfer the NP into its gel structure, a ligand exchange with short‐chain ligands is carried out beforehand, which shortens the distance from the surrounding medium to the NP surface, thus enabling better charge carrier transfer.…”

“…), and a mixture of Na 2 S/Na 2 SO 3 (Sulfid/Sulfit). Adapted with permission [170]. Copyright 2023, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.…”