Kyushu Synchrotron Light Research Center（ Form: No.5）

Experiment Report for Prefectural Beamline
Proposal no. : 1 7 1 1 1 1 5 SBeamline no.: BL12
Report date : 03/22/2018

Electron ic states of Mg, Sn -d oped Ga 2 O 3 thin fil ms on
ultra-s moo th sapphire su bstrates using a NiO seed layer
Yanna Chen 1,2 , Osami Sakata 1,21Synchrotron X-ray Station at SPring-8, Research Network and Facility Services
Division, National Institute for Materials Science (NIMS), 1 -1-1 Kouto, Sayo-cho,
Sayo-gun, Hyogo 679-5148, Japan 2 Synchrotron X-ray Group, Research Center for
Advanced Measurement and Characterization, NIMS, 1 -1-1 Kouto, Sayo-cho,
Sayo-gun, Hyogo 679-5148, Japan※１Long term users of Advanced Research User Proposal s are requested to submit thisreport corresponding to each research term (I), (II), or (III), specifying each researchterm at the end of the Report Title.

※2

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１ ． Summary（ Note: Please include conclusions）
Hybrid materials composed of metal nanoparticles and metal -organic frameworks (MOFs) haveattracted attention for various applications because of the synergistic functionality between
their constituent materials. However, the charge transfer mechanism is still ambiguous. Here,
we studied Pd nanocubes supported by copper(II) 1, 3, 5-benzenetricarboxylate (HKUST -1),
denoted Pd@HKUST-1, with the significantly enhanced hydrogen storage capacity. We
investigated Cu L edge and O K edge and the role of oxygen in charge transfer behavior was
also confirmed experimentally. This result is beneficial to reveal a charge transfer mechanism
from the Pd bands to the Cu-O hybridization bands of HKUST-1 at the Pd/HKUST -1 interface.

２．Purpose of experiment and backgroundIn the previous work, we used hard X-ray photoelectron spectroscopy (HAXPES) at SPring-8 to study thevalence band of Pd@HKUST-1 consisting of Pd nanocubes and HKUST-1 MOF compared to Pd nanocubes orHKUST-1. After HAXPES measurements, Cu in HKUST-1 shows feature of Cu2+ ions. While inPd@HKUST-1, Cu shows features of both Cu2+ and Cu+ ions in HKUST-1. This may be related to oxygen
bands. We also used a density of states (DOS) calculation to predict that oxygen also takes part in the chargetransfer behavior. To obtain the clear relation between copper and oxygen, we measured the NEXAFS of theCu L edge and O K edge. Because Cu is directly binding to O atoms, here we chose CuO and Cu2O asreferences.
３．Experimental（Note: Description of sample，method of experiment and analysis, etc.）
The NEXAFS data of Pd@HKUST-1 and HKUST-1 were recorded at BL12, SAGA Light Source. Both a total
electron yield and a fluorescence yield were collected. The total electron yield mode can probe the materials
with the order of several nanometers beneficial for the study of nanomaterials. While the probing depth for
fluorescence measurements of the order of 100 nm for the fluorescence X-rays, used for the bulk references.
The overall resolution is about 0.1 eV.
４．Results and Discussions
To reveal the clear relation between copper and oxygen, we measured the NEXAFS of the Cu L edge and O Kedge for Pd@HKUST-1 and HKUST-1. The Cu L edge was obtained while Cu 2p electrons absorbed thephoton energy and then excited into unoccupied 3d states. At the O K edge, the 1s electrons were excited toempty 2p states. In Fig. 1(a), the Cu L3 and L2 peaks are shifted to the higher energy comparing withHKUST-1, which have the same peak position with CuO (Fig.1 (b)). That is consistent with the previous reportof HKUST-1 with bivalent Cu2+.2 The similarity to CuO with the typical octahedron structure results from thelocal atomic structure around Cu inside of HKUST-1. The Cu and O atoms construct two connected distortedoctahedral. While HKUST-1 is covered to Pd nanocube in the structure, only the interfacial Cu2+ reduced toCu+. There are coexist of Cu2+ and Cu+ in Pd@HKUST-1, leading to the peak position between the peak ofCu2+ and that of Cu+. In Fig. 1 (c), both spectra contains two regions, the sharp π* region and the broad σ*region. When we consider the local atomic structure around O, the π* region originates from electrons excitedfrom O 2p hybridized to Cu 3d and the σ* region arises from electrons excited from O 2p hybridized to Cu sp
and also C sp. Based on the previous research,3-6 the peaks were distinguished and marks in gray arrows for thepeaks related to C sp and in red arrows for those related to Cu 4sp. Comparing with the HKUST-1 spectrum,the peak intensity of Pd@HKUST-1 was increased clearly. This suggests that there are more electrons on theoxygen bands. The O K edge of Pd@HKUST-1 was shifted to higher energy side around 532 eV and 545 eV
marked in red arrow, which is related to the Cu 3d - O 2p hybridization and Cu 4sp - O 2p hybridization,respectively. Comparing with the CuO and Cu2O references (Fig. 1 (d)), the peak position of HKUST-1 O K
edge is different from that of CuO because O connects to not only Cu but also C. The energy value of the firstpeak shifts by 0.4 eV to the higher energy side, much less than the copper oxide bulk. This is because of theamount of the interfacial oxygen binding to Cu. That means, inside of Pd@HKUST-1, interfacial HKUST-1
accepts electrons from Pd 4d, and Cu-O hybridization bands are filled. In case of atomic structure ofHKUST-1, there are probably two kinds of atomic connection at the interface, Pd-Cu-O and Pd-O-Cu. Thismeans there are Cu-O features and/or Pd-O features may appear in a NEXAFS spectra. Then we also measured
the O K edge of the standard PdO reference, as the same as the calculated result.7 The O K edge NEXAFS
spectra of Pd@HKUST-1 shows only the features of Cu-O, without the features of Pd-O. All the NEXAFS
spectra confirmed the involvement of oxygen in the charge transfer behavior, consistent with the calculated
results from HAXPES and predicted by the DOS calculation above.

Fig. 1 (a) Cu L edge and (c) O K edge NEXAFS spectra of HKUST-1 and Pd@HKUST-1.
(b) Cu L edge and (d) O K edge NEXAFS spectra of CuO and Cu2O.
５．Future issues
Understanding the link between the charge transfer behavior and chemical or physical properties will allow as
to construct new hybrid materials by engineering their electronic structure. This will drive progress in thecreation of new functional materials for hydrogen storage, catalysis and other application.
６．References[1] Chui, S. S.-Y., Lo, S. M.-F., Charmant, J. P., Orpen, A. G. & Williams, I. D. A chemically functionalizable
nanoporous material [Cu3(TMA)2(H2O)3]n. Science 283, 1148–1150 (1999).
[2] Duke, A. S. et al. Active sites in copper-based metal–organic frameworks: Understanding sub- strate
dynamics, redox processes, and valence-band structure. J. Phys. Chem. C 119, 27457– 27466 (2015).
[3] Chen, J. G. NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and otherinterstitial compounds. Surf. Sci. Rep. 30, 1–152 (1997).
[4] Xu, J. etal. X-ray absorption spectra of graphene and graphene oxide by full-potential multiple scattering
calculations with self-consistent charge density. Physical Review B 92, 125408 (2015).
[5] Li, C. et al. The organic-moiety-dominated li+ intercalation/deintercalation mechanism of a cobalt-based
metal–organic framework. Journal of Materials Chemistry A 4, 16245–16251 (2016).
[6] Han, R. et al. Toward interfacing organic semiconductors with ferromagnetic transition metal substrates:
enhanced stability via carboxylate anchoring. Chemical Communications 52, 9805–9808 (2016).
[7] Mogi, M. et al. Near-edge x-ray absorption fine structure of PdO at O K-edge. Jpn. J. Appl. Phys. 44, 4057
(2005).
７．Publications, patents（Note: Typical deliverables related to this proposal．）
Not yet.
８．Keywords（Note: 2-3 words about samples and experimental methods．
）
Metal organic framework, Electronic strucute, Oxygen
９．About the publication of research results
Please delete either item ① or item ② indicated below, according to a user’s choice of publications as
described in ※2. Also, please fill in an expected date of publication of a refereed journal article or an
expected date of submission of a SAGA-LS Research Report. For example, deadline of both publications
corresponding to the proposals performed in the fiscal 2017 is by the end of fiscal 2019 in both ① and②.
① Publication of a refereed journal article(the date of publication：8. 2018～12. 2019）