Photon Factory Activity Report 2022 #40 (2023)
BL-9A, BL-12C/2021G143, 2021G584

Hydrothermal synthesis of non-stoichiometric Co–Ga oxide nanoparticles
Bo XIE1, Chiya NUMAKO2, Takashi NAKA3, and Seiichi TAKAMI1,*
1
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
2
Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
3
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
We synthesized non-stoichiometric Co–Ga nanoparticles with controllable Co/Ga molar ratio by using supercriticalhydrothermal synthesis. By applying X-ray absorption fine structure analysis at Co K-edge, non-stoichiometric Co–
Ga NPs were found to have large numbers of cobalt atoms occupying octahedral sites compared to CoGa2O4. Owing
to the high octahedral occupancy, their superparamagnetic behavior at room temperature was observed for the firsttime.
1 Introduction
Mixed metal oxides with a spinel-type crystal
structure (Fig. 1) have attracted considerable interest
owing to their remarkable electrical, optical, magnetic,
and catalytic properties. Spinel-type Ga-based metaloxide, CoGa2O4 has been considered for applications inceramic pigments, electrocatalysts, and cathodes andanodes for various types of capacitors.

A(II)B(III)2O42 Experiment
Preparation of Co–Ga NPs. The batch-type
supercritical hydrothermal synthesis was conducted asshown in Fig. 2. The precursor solution was prepared bydissolving cobalt (II) nitrate hexahydrate and gallium(III) nitrate n-hydrate with a concentration of 0.050
mol/L and 0.10 mol/L, respectively (Co/Ga = 0.5) in purewater (2.0 mL). To this aqueous solution, 1.0 mol/L
NaOH solution were added to adjust pH to 7, 9, and 11.
The precursor solutions including solid precipitates (1.5mL) were then transferred to a pressure-resistant
Hastelloy reactor (inner volume of 5.0 mL) and put in a
furnace with constant temperature of 400 °C. Thereactor was taken out after 10 min. The reaction wasterminated by submerging the reactor into a cold-waterbath. Solid and liquid products were collected andseparated for further analysis.

Fig. 1: Crystal structure of spinel-type metal oxides [1].

Co(NO3)2(0.025, 0.050 mol/L )Material properties of the spinel compounds could befurther enhanced by producing their counterpart with
nonstoichiometric composition and nanosize. However,trials on the production of non-stoichiometric spineltype cobalt gallate nanoparticles (Co–Ga NPs) andstudies on their properties have been rarely studied.
In this work, we report the supercriticalhydrothermal synthesis of Co–Ga NPs at 400 °C for 10
min without performing the calcination process. Spineltype Co–Ga NPs with broad range of Co/Ga molar ratios
were synthesized by controlling the pH value of theprecursor solutions. By applying X-ray absorption finestructure (XAFS) analysis at Co K-edge, products were
found to have large octahedral cobalt occupancy
compared to calcined CoGa2O4, and this resulted in a
new magnetic behavior at room temperature.

Ga(NO3)3(0.10 mol/L )1.0 M NaOH

(1.5 mL)pH ≈ 3 (pH 7, 9, 11)

400 ℃,10 min.

Fig. 2: Experimental procedure.
After centrifugation and decantation, the solid
products were analyzed by X-ray diffraction (XRD),
transmission electron microscopy (TEM), andinductively coupled plasma atomic emission
spectroscopy (ICP-AES) to confirm crystalline phase,morphology and composition (Co/Ga molar ratio) of theproducts.
XAFS analysis. Ga K-edge and Co K-edge XAFS
spectra were measured with the transmission mode at
BL-9A, Photon Factory, KEK, Japan. Athena was used
for the data processing of the X-ray absorption near-edge
structure (XANES) and extended X-ray absorption finestructure (EXAFS) analyses [2].

Photon Factory Activity Report 2022 #40 (2023)(b)

CoGa2O4 (ICSD: 172183)

k χ(k) (Å )−332468100−1044

115004022

1110(c)468

Wavenumber k (Å ),,,pH= 9 Co/Ga=0.563

2−1Wavenumber k (Å )pH = 7

Intensity (-)−3

pH= 7 Co/Ga=0.613

113(a)

(b)

(a)

3

3 Results and Discussion
XRD patterns and TEM images showing thecrystalline phase and morphology of the solid products
are summarized in Fig. 3. The diffraction patterns forproducts prepared at pH 7–11 were indexed based on anICSD card (ICSD: 172183) to single-phase spinel-typecobalt gallium metal oxides (CoGa2O4) with Fd-3m spacegroup. TEM images are showing synthesized spinel-type
cobalt gallate metal oxides were nanoparticles withoctahedron morphology. To investigate the compositionof the spinel-type Co–Ga NPs, solid products at each pH
were dissolved in a mixture of nitric acid and hydrogen
peroxide with a volume ratio of 1:1, and Co/Ga molar
ratio in the resultant solution was measured by ICP-AES.
In this work, the Co/Ga molar ratio was defined as thecomposition of the products at each pH condition, andwas summarized in Fig. 3(b). Co/Ga molar ratios at pH
7–11 were nonstoichiometric composition, and werelarger than that in a stoichiometric CoGa2O4 preparedfrom traditional calcination method (Co/Ga = 0.50).
Therefore, prepared spinel-type Co–Ga NPs may havesite distributions of metal atoms that have not beenrealized by the traditional calcination method.
Because there are more cobalt atoms existing in theunit cell of the prepared non-stoichiometric Co–Ga NPs
compared to CoGa2O4, we evaluated the site distributionof metal atoms. In detail, the metal occupancy of Coatoms at octahedral and tetrahedral sites for each pH
condition was quantitatively evaluated by consideringthat the measured k3-weighted χ(k) oscillation at the CoK-edge of the products is a linear combination of thoseof IV(Co)VI[Ga2]O4 (normal spinel) and IV(Ga)VI[CoGa]O4(inverse spinel) [3]. The k3-weighted χ(k) oscillations ofthe normal spinel and the inverse spinel at the Co Kedge were theoretically calculated by Larch [5], asshown in Fig. 4(a) and Fig. 4(b). Meanwhile, Fig. 4(c) isshowing the fitting results. The occupancies of Ga andCo calculated from the previous linear combination of
the prepared non-stoichiometric Co–Ga NPs aresummarized in Fig. 5. As a result, cobalt atoms aremainly occupying octahedral sites, and extremely
surpassed cobalt octahedral occupancy in a calcined
CoGa2O4 (line with cyan color). Moreover, because Co
atoms are the only magnetic metal atoms in the unit cellof spinel-type cobalt gallate metal oxides.
Nonstoichiometric Co–Ga NPs prepared in this workwith large cobalt octahedral occupancy might exhibit
different magnetic behavior compared to CoGa2O4.

k χ(k) (Å )Linear combination of k3-weighted χ(k) oscillation.
Site distribution of cobalt atoms of the products wasdetermined by using EXAFS data. To determine Co
occupancy at octahedral and tetrahedral sites, a linear
combination of k3-weighted χ(k) oscillation of Co–Ga
NPs at Co K-edge was conducted for each pH condition
while using theoretically calculated k3-weighted χ(k)oscillations of IV(Co)VI[Ga2]O4 and IV(Ga)VI[CoGa]O4 [3].
This calculation was enlightened by the concept
reported by Harada et al [4].
Magnetism investigation. Alternating current (AC)and direct current (DC) magnetization measurements
were conducted using the magnetic properties
measurement system (MPMS-XL; Quantum Design).

Original and fit data (pH 7)
Original and fit data (pH 9)
Original and fit data (pH 11)

2−3k χ(k) (Å )R : 0.7838

20 nm

2

R : 0.7881

3

pH = 92

R : 0.8411

pH= 11 Co/Ga=0.765
pH = 1102468

10−1

Wavenumber k (Å )102030405060702θ (deg.)

Fig. 3: (a) XRD patterns and (b) TEM images ofsolid products produced at 400 °C at pH 7–11.

Fig. 4: Theoretically calculated and measured
k3-weighted χ(k) oscillation at Co K-edge of(a) IV(Co)VI[Ga2]O4, (b) IV(Ga)VI[CoGa]O4, and(c) linear combination fitting results [3].

10

Photon Factory Activity Report 2022 #40 (2023)

Ga, Co occupancy [-]Occupancies of Co and Ga
in tetrahedral site and octahedral site (-)

Tetrahedral site

1.0

Octahedral siteoccupancy of Ga
Ga in CoGa2O4 (ICSD: 172183)
occupancy of Co
Co in CoGa2O4 (ICSD: 172183)

1.0

0.8

0.8

0.6

0.6

0.40.40.20.20

pH=7

pH=9occupancy of Ga
Ga in CoGa2O4 (ICSD: 172183)
occupancy of Co
Co in CoGa2O4 (ICSD: 172183)

pH=110

pH=7

pH=9

pH=11

Acknowledgement
This work was supported by JSPS KAKENHI Grant
Numbers 17H06467, 20H02514. XAFS measurements
were conducted under the approval of the Photon
Factory Program Advisory Committee (Proposal No.
2021G143, 2021G584). We thank Mr. Itakura (KojiItakura), Division for Medical Research Engineering,
Graduate School of Medicine, Nagoya University, forgiving advice on TEM measurements.

pH value of precursor solutions (-)

pH value of the precursor [-]Fig. 5: Site distribution of Ga, Co atoms intetrahedral and octahedral sites of the unit cell of
the spinel-type nonstoichiometric Co–Ga NPs
prepared in this work.
The M–H curves of the Co–Ga NPs measured at room
temperature are shown in Fig. 6. Notably, the magnetic
behavior of our products at room temperature wasdifferent from calcined CoGa2O4 with stoichiometriccomposition [6]. The magnetism was considered to be
the combination of partially paramagnetic and partiallyferromagnetic (superparamagnetic) behaviors. We
consider this because the high cobalt octahedral
occupancies at pH 7–11 were larger than 0.390 [7], andconsequently super-exchange interaction of JBB
percolated through the nanocrystal. In this way, amagnetic domain develops macroscopically in thecrystals and ferromagnetism was induced. In summary,superparamagnetic behavior of spinel-type Co–Ga NPs
at room temperature was observed for the first time withthe help of non-stoichiometry and nanosizing in thepresent technique.
pH = 7
pH = 9pH = 11

1.2

Prepared nonstoichiometric
Co–Ga NPsMg (emu/g)

0.8
0.4
0calcined CoGa
calcinedCoGa
2O
24O4(1000℃)(1000℃)[7][7]-0.4-0.8-1.2-20 x103-1001020

H (Oe)

Fig. 6: M–H curves (measured at 300 K) ofnon-stoichiometric Co–Ga NPs produced at 400 °C
using precursor solutions with pH 7–11.

References[1] K. Momma and F. Izumi, J. Appl. Cryst. 44, 1272
(2011).
[2] B. Ravel and M. Newville, J. Synchrotron Radiat. 12,537 (2005).
[3] B. Xie, C. Numako, T. Naka, S. Takami, Cryst. GrowthDes. 23, 2511 (2023).
[4] M. Harada, M. Kuwa, R. Sato, T. Teranishi, S.
Maenosono, S., ACS Appl. Nano Mater. 3, 8389 (2020).
[5] M. Newville and M. Larch, J. Phys.: Conf. Ser. 430,012007 (2013).
[6] S. Mathur, C. Cavelius, H. Shen, Z. Anorg. Allg. Chem.
635, 2106 (2009).
[7] F. Scholl and K. Binder, Z. Phys. B - Condens. Matter39, 239 (1980).
* takami.seiichi@material.nagoya-u.ac.jp