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Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

ABSTRACT: The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO₄) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm^-2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm^-2) of BiVO₄. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.

SUBMITTER: Lee MG

PROVIDER: S-EPMC6193156 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

Lee Mi Gyoung MG Jin Kyoungsuk K Kwon Ki Chang KC Sohn Woonbae W Park Hoonkee H Choi Kyoung Soon KS Go Yoo Kyung YK Seo Hongmin H Hong Jung Sug JS Nam Ki Tae KT Jang Ho Won HW

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 20180806 10

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO<sub>4</sub>) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm<sup>-2</sup> is achieved. It is close to 85% of t ...[more]

PMID: 30356939

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Project description:Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO2, polyoxometalate (POM) water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (1), and both known photosensitizer [Ru(bpy)2(H4dpbpy)]2+ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)2(H2dpbpy)](H22). Both triads, containing catalyst 1, and catalyst-free dyads, produce O2 with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H22 absorbs light more strongly than P2, and increases O2 quantum yields by up to 270%. TiO2-2 based photoelectrodes are also more stable to desorption of active species than TiO2-P2: losses of catalyst 1 are halved when pH > TiO2 point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O2 are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO2, and is also consistent with the 4th1-to-dye electron transfer limiting performance rather than catalyst TOFmax. Transient absorption reveals that TiO2-2-1 has similar 1st electron transfer dynamics to TiO2-P2-1, with rapid (ps timescale) formation of long-lived TiO2(e-)-2-1(h+) charge separated states, and demonstrates that metallation of the crown ether groups (Na+/Mg2+) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO2-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO2 can oxidize water without a catalyst.

Dataset Information

Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

Publications

Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

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