Electronic origin of hydrogen storage in MOF-covered palladium nanocubes investigated by synchrotron X-rays

Yanna Chen,Hirokazu Kobayashi,Hiroshi Kitagawa,Chulho Song,Osami Sakata,Michihisa Koyama,Loku Singgappulige Rosantha Kumara,Yusuke Nanba,Anli Yang,Guangqin Li

doi:10.1038/s42004-018-0058-3

Abstract

Hybrid materials composed of metal nanoparticles and metal-organic frameworks have attracted attention for various applications because of the synergistic functionality between their constituent materials. Interfacial interaction is expected however the mechanism remains ambiguous. Here we report the valence bands of palladium nanocubes covered by copper(II) 1, 3, 5-benzenetricarboxylate (HKUST-1), denoted as Pd@HKUST-1, and the charge transfer from the palladium nanocubes to HKUST-1 at the Pd/HKUST-1 interface is investigated quantitatively. Interfacial density of states are different from those of internal constituents and imply that the Cu–O group in HKUST-1 acts as a charge accepter. The role of Cu–O group in charge transfer behaviour is also observed experimentally. Finally, we reveal the charge transfer mechanism from the Pd 4d bands to the Cu 3d (4sp) – O 2p hybridization bands of HKUST-1 at the Pd/HKUST-1 interface, which explains the enhanced hydrogen storage capacity in Pd@HKUST-1.

Highlights

Hybrid materials composed of metal nanoparticles and metal-organic frameworks have attracted attention for various applications because of the synergistic functionality between their constituent materials
The findings show that the hydrogen storage properties of Pd@HKUST-1 can be explained through the electronic structure near the Fermi level, which arises from an interfacial charge transfer
Interfacial charge transfer from Pd 4d to Cu 3d in Pd@HKUST-1 investigated by hard X-rays photoelectron spectroscopy (HAXPES)

Summary

Introduction

Hybrid materials composed of metal nanoparticles and metal-organic frameworks have attracted attention for various applications because of the synergistic functionality between their constituent materials. The charge transfer behaviour of palladium (Pd) nanocubes covered by copper(II) 1, 3, 5- benzenetricarboxylate (HKUST-1), denoted by Pd@HKUST-1, has been shown to improve its hydrogen storage properties[5,6]. As a representative metal/MOF hybrid material, Pd@HKUST-1 is an important example for studying the interaction between the constituent materials linked with improved hydrogen storage, aiding the design of other functional hybrid materials through rational modification of their electronic structures. In studying the electronic structure of Pd@HKUST-1 near the Fermi level based on charge transfer, it is important to investigate the valence bands of the transition metals, palladium and copper, in bulk materials and at Pd/HKUST-1 interface. The findings show that the hydrogen storage properties of Pd@HKUST-1 can be explained through the electronic structure near the Fermi level, which arises from an interfacial charge transfer

Methods

Results

Conclusion