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. 2021 Dec 6;27(68):16904-16911.
doi: 10.1002/chem.202102032. Epub 2021 Sep 23.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

Giuseppina La Ganga  1 , Fausto Puntoriero  1 , Enza Fazio  2 , Mirco Natali  3 , Francesco Nastasi  1 , Antonio Santoro  1 , Maurilio Galletta  1 , Sebastiano Campagna  1
Affiliations

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

Giuseppina La Ganga et al. Chemistry. .

Abstract

The luminophore Ru(bpy)2 (dcbpy)2+ (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 ⊂NS-RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 ⊂NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 ⊂NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)3 2+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7×104 s-1 ) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 ⊂NS-RuCh system is behind the improved photochemical quantum yield of IrO2 ⊂NS-RuCh.

Keywords: artificial photosynthesis; electron transfer; photochemical water oxidation; photochemistry; ruthenium.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Representation of a chitosan repetitive unit.
Scheme 1
Scheme 1
Schematic representation of the synthesis of RuCh. Inset: Photo of RuCh powder under UV light.
Figure 2
Figure 2
1H NMR spectrum of RuCh in D2O (CD3COOD, 1 % v/v). The 1H NMR spectrum of low‐molecular‐weight chitosan in D2O (CD3COOD, 1 % v/v) is shown in the inset. Spectra were recorded at 40 °C.
Scheme 2
Scheme 2
A cartoon representation of NS‐RuCh synthesis by using the RuCh polymer.
Figure 3
Figure 3
Absorption and emission spectra of NS‐RuCh in phosphate buffer (20 mM, pH 7).
Figure 4
Figure 4
DLS analysis of IrO2 nanoparticles (blue), NS‐RuCh (orange) and IrO2⊂NS‐RuCh (green). Note that the x‐axis is drawn on a logarithmic scale.
Figure 5
Figure 5
SEM analysis of IrO2⊂NS−Ru at different scales.
Figure 6
Figure 6
Absorption spectra of NS‐RuCh (solid line) and IrO2⊂NS‐RuCh (dashed line) in phosphate buffer (20 mM, pH 7). The inset shows a zoomed area (500–800 nm) of the spectra.
Figure 7
Figure 7
Oxygen evolution in the IrO2⊂NS−Ru/Na2S2O8 (red) and NS‐RuCh/IrO2/Na2S2O8 systems (black).
Figure 8
Figure 8
Flash photolysis experiments: λ ex=355 nm; Na2S2O8 (1×10−2 M). IrO2⊂NS−Ru (red) and NS‐RuCh/IrO2 (5×10−5) systems (black).
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