Photoinduced H₂ Evolution by Hexaniobate Sheets Grafted with Metal Ions

The Fate of Photogenerated Carriers

authored by

Barbara N. Nunes, Detlef W. Bahnemann, Antonio Otavio T. Patrocinio

Abstract

Layered niobates are well-known photocatalysts for H2 evolution with a rich surface chemistry. Their photoactivity, however, is limited by their wide band gap energy (â 3.5 eV) and partial deactivation due to surface poisoning by photogenerated H2O2. In this way, a surface modification method able to induce novel electronic processes without changing the bulk properties can improve their performance. In this work, the surface of exfoliated hexaniobate (K4-xHxNb6O17) layers was modified by grafting with metal ions such as Co(II) and Fe(III) and their photocatalytic properties were fully investigated. Morphological characterization showed that grafting ions are attached to the niobate surface forming amorphous clusters. These species induce an additional absorption feature in the UV-A region, which is attributed to an interfacial charge transfer from the niobate valence band to the metal ion centers. Enhanced UV-driven photoactivity was observed for 0.1% grafted samples, especially for those modified with Co(II) ions, while smaller H2 evolution rates are observed as the concentration of the grafting ions increases. When Pt was added to the photocatalyst, the H2 evolution rate for the 0.1% Co-grafted sample in plain water was 70% higher than that observed for the nongrafted Pt-hexaniobate. Full characterization by electron paramagnetic resonance, transient absorption spectroscopy, and photoelectrochemical measurements reveals that grafted ions can work as both electron and hole acceptors. In the presence of Pt as a preferential electron acceptor, Co(II) ions act as hole acceptors forming Co(III) centers, favoring the formation of OHâ radicals from water and avoiding surface poisoning. At higher Co(II) concentrations and in the absence of Pt clusters, electrons are trapped at the Co centers, decreasing the H2 evolution rate. Thus, grafted Co(II) ions act as active redox shuttles in the hexaniobate sheets, contributing to more efficient charge separation.

Organisation(s)

Institute of Technical Chemistry

External Organisation(s)

Universidade Federal de Uberlandia
Saint Petersburg State University

Type

Article

Journal

ACS Applied Energy Materials

Volume

4

Pages

3681-3692

No. of pages

12

Publication date

26.04.2021

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Chemical Engineering (miscellaneous), Energy Engineering and Power Technology, Electrochemistry, Electrical and Electronic Engineering, Materials Chemistry

Sustainable Development Goals

SDG 7 - Affordable and Clean Energy

Electronic version(s)

https://doi.org/10.1021/acsaem.1c00128 (Access: Closed)