TY - JOUR

T1 - Photoinduced H2 Evolution by Hexaniobate Sheets Grafted with Metal Ions

T2 - The Fate of Photogenerated Carriers

AU - Nunes, Barbara N.

AU - Bahnemann, Detlef W.

AU - Patrocinio, Antonio Otavio T.

N1 - Funding Information: This work was supported by Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, PPM-00220-17), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, 406392/2018-8 and 310303/2018-4), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). D.W.B. acknowledges financial support from Saint Petersburg State University (Research Grant 39054581). B.N.N. gratefully acknowledges the financial support from CAPES, Brazil, from the CAPES/DAAD/CNPQ (15/2017) program, grant number 88887.161403/2017-00. A.O.T.P. is thankful to the Alexander von Humboldt Foundation for the equipment subsidy grant. The authors also thank Carsten Günnemann for helping with the TAS analysis, Prof. Osmando Lopes, Heinrich Hartmann and Astrid Besmehn from the Central Institute for Engineering, Electronics and Analytics (ZEA-3), Forschungszentrum Jülich GmbH for the XPS measurements, Institute of Geology, Leibniz University Hannover for ICP–OES measurements, Laboratorium für Nano- und Quantenengineering (LNQE) for TEM equipment, and Grupo de Materiais Inorgânicos do Triângulo (GMIT), a research group supported by FAPEMIG (APQ-00330-14).

PY - 2021/4/26

Y1 - 2021/4/26

N2 - 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.

AB - 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.

KW - 2D structures

KW - niobium oxides

KW - photocatalysis

KW - surface modification

KW - water splitting

UR - http://www.scopus.com/inward/record.url?scp=85104917873&partnerID=8YFLogxK

U2 - 10.1021/acsaem.1c00128

DO - 10.1021/acsaem.1c00128

M3 - Article

AN - SCOPUS:85104917873

VL - 4

SP - 3681

EP - 3692

JO - ACS Applied Energy Materials

JF - ACS Applied Energy Materials

IS - 4

ER -