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Bo Xie, Chiya Numako, [Takashi Naka](https://orcid.org/0000-0002-0645-6952), Seiichi Takami

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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Crystal Growth & Design, copyright © 2023 The Authors. Published by American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acs.cgd.2c01435.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Supercritical Hydrothermal Synthesis of Spinel Type Cobalt Gallate Nanoparticles with Controlled  Crystal Defects and Their Magnetic Properties](https://mdr.nims.go.jp/datasets/d3eda5e9-2682-41f0-a891-4bd258b5d7c8)

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xieboThis is the manuscript before the peer review of Crystal Growth & DesignFor published manuscript with the newest data,please refer to DOI: doi.org/10.1021/acs.cgd.2c01435 1 Supercritical Hydrothermal Synthesis of 1 Spinel-Type Non-stoichiometric Cobalt Gallate 2 Nanoparticles and Their Magnetic Properties 3 Bo Xiea, Chiya Numakob, Takashi Nakac, Seiichi Takamia* 4 aDepartment of Materials Process Engineering, Graduate School of Engineering, Nagoya 5 University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 6 bDepartment of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, 7 Inage-ku, Chiba 263-8522, Japan 8 cNational Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, 9 Japan 10  11  12  13  14  15  16 Page 1 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 2 ABSTRACT: The spinel-type metal oxide nanoparticles are known to exhibit various properties, 17 which can be further enhanced by the synthesis of non-stoichiometric nanoparticles. In this work, 18 we report the production of non-stoichiometric cobalt gallate nanoparticles (Co–Ga NPs) with 19 larger inversion parameter that resulted in enhanced magnetic properties compared to previous 20 studies. The synthesis of non-stoichiometric nanoparticles was realized by supercritical 21 hydrothermal process, and without performing the calcination at higher temperature which 22 tended to synthesize thermodynamically stable stoichiometric products. We synthesized Co–Ga 23 NPs with controllable Co/Ga molar ratio, morphology, and particle size at different pH values of 24 the precursor solution. By applying Rietveld refinement, non-stoichiometric Co–Ga NPs were 25 found to have more Co2+ occupying tetrahedral and octahedral sites compared to CoGa2O4, and 26 their superparamagnetic behavior with enhanced spontaneous magnetization at room temperature 27 owing to enhanced and percolated JAA, JBB, and JAB interactions was observed for the first time. 28 We also proposed a possible formation mechanism of non-stoichiometric Co–Ga NPs from 29 cobalt gallium nitrate layered double hydrates in the precursor solutions.    30 Page 2 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 3 1. INTRODUCTION 31 Mixed metal oxides with a spinel-type crystal structure have attracted considerable interest 32 owing to their remarkable electrical, optical, magnetic, and catalytic properties.1 Those properties 33 can be widely applied in fields including but not limited to electrocatalysts,2-4 gas sensors,5-7 34 electrodes of Li- or Mg-ion rechargeable batteries,8, 9 and photocatalysts.10, 11 In gas sensor 35 applications, spinel-type ZnFe2O4 exhibits high sensitivity, selectivity, and fast response to Cl2 36 gas owing to its n-type semiconductor behavior.5 Since tetrahedral and octahedral sites within the 37 spinel-type crystal structure can accommodate metal ions with different ionic radii, spinel-type 38 ZnCo2O4 can maintain a stable crystal structure during the extraction and insertion of Mg2+ as a 39 cathode material in Mg-ion rechargeable batteries with improved cyclability.9 Meanwhile, 40 manifold compositions of spinel compounds can enable the use of compounds like FeCo2O4 and 41 CoFe2O4, in energy storage and conversion systems, since transition metal oxides can hold 42 several oxidation states that favor rapid redox reactions.12 The manipulation of the spinel-type 43 crystal structure such as the implementation of rare-earth metal atoms or oxygen vacancies also 44 allows spinel-type metal oxides to be used as persistent luminescence phosphors and for carbon 45 dioxide decomposition.13-15 46 The diverse properties of spinel-type metal oxides arise from the combination of divalent and 47 trivalent metal elements, represented by A and B in the empirical formula AB2O4, respectively. 48 Spinel-type metal oxides can further be divided into normal spinel (IV(A)VI[B2]O4), inverse spinel 49 (IV(B)VI[AB]O4), and partially inverse spinel structures. Moreover, synthesis conditions, heat 50 treatment, or chemical environment can play a crucial role in the distribution of A and B cations, 51 further dictating the properties such as magnetic behavior, catalytic activity, and conductivity.16, 52 17 53 Page 3 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 4 As a prospective candidate in power devices, Ga2O3 is a semiconducting oxide with a wide 54 bandgap of approximately 4.9 eV.18 It shows polymorphism exhibiting α, β, γ, δ, ε, and a 55 recently reported κ forms.19, 20 Moreover, owing to its electronic properties, optical properties, as 56 well as significant chemical and thermal stability, it finds applications in catalysts, phosphors, 57 and electroluminescent devices, gas sensors.21, 22 As one of the previously introduced polymorphs, 58 spinel-type γ-Ga2O3 exhibits intrinsic Ga vacancies in both tetrahedral and octahedral sites, as 59 reported by Playford et al.20 Therefore, γ-Ga2O3 can be an efficient host accommodating and 60 replacing a transition metal or rare-earth metal element, which can further broaden its application 61 scope. In recent years, spinel-type Ga-based metal oxides such as FeGa2O4, CuGa2O4, CdGa2O4, 62 and ZnGa2O4 have been studied for use as magnetic materials, gas sensors, photocatalysts, and 63 transparent conductors.23-27 As a spinel-type Ga-based metal oxide, CoGa2O4 has been considered 64 for applications in ceramic pigments, p-type conductors,28 gas sensors,29 electrocatalysts,30, 31 and 65 cathodes and anodes for various types of capacitors32-35 owing to its superior conductivity, 66 electrochemical activity, and energy storage capability. It has a partially inverse structure, as 67 shown in Figure 1, where divalent and trivalent cations, i.e., Co2+ and Ga3+ occupy both 68 tetrahedral and octahedral sites. The empirical formula of CoGa2O4 is IV(Co1-xGax)VI[CoxGa2-x]O4, 69 where x represents the degree of inversion showing the fraction of divalent cations in the 70 octahedral site.  71  72 Page 4 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 5  73 Figure 1. Crystal structure of a partially inverse CoGa2O4 created by VESTA.36 74 There are newly emerged demands of producing non-stoichiometric spinel-type metal oxides 75 with nanosize for the purpose of improved properties. Actually, photocatalytic properties of 76 spinel compounds were improved by decreasing particle size,37 and nanosizing affected the 77 magnetic and dielectric properties of materials used in gas sensing.38 In addition, 78 non-stoichiometric spinel compounds also possess the potential for exhibiting different 79 properties that have not been seen in their stoichiometric counterpart. For example, Gokul et al. 80 produced non-stoichiometric cobalt ferrite oxides and discovered field-dependent magnetizations 81 that are different to either CoFe2O4 or Fe3O4 at 300 K.39  82 For the production of CoGa2O4, traditional synthesis methods usually require a precursor 83 produced from electrodeposition,32 chemical vapor deposition,40 electrospinning,29 or simple 84 hydrothermal or solvothermal treatment,31, 34, 35, 41 followed by calcination conducted at 600–85 Page 5 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 6 1000 °C for several hours. Such calcination process favors the production of thermodynamically 86 stable phase of CoGa2O4, however, it may cause the agglomeration of products. For the synthesis 87 without the application of the calcination process, Playford et al. employed the solvothermal 88 method at a synthesis temperature of 240 °C for six days by making use of a mixture containing 89 metallic gallium and cobalt nitrate hexahydrate and successfully produced stoichiometric 90 CoGa2O4 with high crystallinity and a size range of 20–40 nm.17 However, trials on the 91 production of non-stoichiometric spinel-type cobalt gallate nanoparticles (Co–Ga NPs) and 92 studies on their properties have been rarely studied. 93 In this work, we report the supercritical hydrothermal synthesis of Co–Ga NPs at 400 °C for 10 94 min without performing the calcination process. Contrary to previous studies, which either 95 calcinated a hydrothermally or solvothermally prepared precursor31, 34, 35, 41 or treated the 96 precursor in a solvothermal environment for a long time,17 we produced Co–Ga NPs using a 97 short synthesis time without performing calcination. As a result, spinel-type non-stoichiometric 98 Co–Ga NPs with a broad range of Co/Ga molar ratios were hydrothermally synthesized using 99 supercritical water by controlling the pH value of the precursor solution. Owing to different Co2+ 100 occupancies within tetrahedral and octahedral sites in those non-stoichiometric Co–Ga NPs 101 compared to CoGa2O4, enhanced magnetic behavior at room temperature was observed. In 102 addition, we also propose a possible formation mechanism of non-stoichiometric Co–Ga NPs 103 produced by present technique. 104   105 Page 6 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 7 2. EXPERIMENTAL SECTION 106 2.1. Materials. Cobalt (II) nitrate hexahydrate (Co(NO3)2•6H2O, 99.5%), gallium (III) nitrate 107 n-hydrate (Ga(NO3)3•nH2O, n = 7–9, 99.9%), nitric acid (HNO3, weight fraction in the range of 108 60%–61%), hydrochloric acid (HCl, weight fraction in the range of 35.0%–37.0%), hydrogen 109 peroxide (H2O2, 30.0%–35.5%), cobalt (II, III) oxide (Co3O4, practical grade), and β-gallium 110 oxides (β-Ga2O3, 99.99%) were purchased from FUJIFILM Wako Pure Chemical Co. Sodium 111 hydroxide (NaOH, purity over 97.0%) was purchased from Kishida Chemical Co., Ltd., and they 112 were used as received without further purification.  113 2.2. Preparation of Co–Ga NPs. The precursor solution was prepared by dissolving cobalt (II) 114 nitrate hexahydrate and gallium (III) nitrate n-hydrate with a concentration of 0.050 mol/L and 115 0.10 mol/L, respectively (Co/Ga = 0.5) in pure water (2.0 ml). To this aqueous solution (pH 116 around 2–3), 0.56 mL, 0.70 mL, 0.77 mL, and 0.98 mL of 1.0 mol/L NaOH solution was added 117 to adjust pH to 5, 7, 9, and 11. Consequently, the obtained precursor solutions had pale pink 118 precipitates, and the added volume of NaOH was recorded for each condition. The precursor 119 solutions including solid precipitates (1.5 mL) were then transferred to a pressure-resistant 120 Hastelloy reactor (inner volume of 5.0 mL) and put in a furnace whose temperature was 121 maintained at 400 °C in advance. After 10 min, the reactor was taken out and submerged into a 122 cold-water bath at room temperature to terminate the reaction. Solid and liquid products were 123 collected and separated for further analysis. 124 2.3. Product collection and purification. After performing centrifugation and decantation 125 three times using ion-exchanged water, the solid products were analyzed by scanning electron 126 microscopy (SEM), and the samples of the solid products were freeze-dried (FDS-1000+7E-C, 127 Tokyo Rikakikai Co., Ltd.) for X-ray diffraction (XRD) analysis. Meanwhile, the supernatants of 128 Page 7 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 8 the liquid products were analyzed by inductively coupled plasma-atomic emission spectroscopy 129 (ICP-AES) analysis.  130 2.4. XRD analysis. Freeze-dried solid products were put on a silicon sample holder and 131 analyzed by AERIS (PANalytical) with CuKα radiation operating at 40 kV and 15 mA. 132 Diffraction patterns were recorded in the range of 10º < 2θ < 70º. 133 2.5. SEM analysis. The samples were observed by JEOL JSM-7500FA with an accelerating 134 voltage of 15 kV. To evaluate the product size, Feret diameter was measured for at least 400 135 particles for one synthesis condition. Meanwhile, element composition was investigated by 136 applying energy-dispersive X-ray spectroscopy (EDS).  137 2.6. ICP-AES analysis. ICP-AES analysis was performed using SPS7800 (Seiko Instruments) 138 to investigate the concentrations of metal elements in the supernatants of the reactant solutions 139 after supercritical hydrothermal synthesis, as well as the aqueous solutions of the precursors. 140 Standard solutions for ICP-AES analysis were purchased from FUJIFILM Wako Chemicals Co. 141 Moreover, ICP-AES analysis was applied to the measurement of the concentrations of various 142 metal elements in the produced solid products. The produced solid products were dissolved in a 143 mixture of nitric acid and hydrogen peroxide with a volume ratio of 1:1. For solid particles 144 which were hard to dissolve, hydrochloric acid was added.  145 2.7. X-ray absorption fine structure (XAFS) analysis. To evaluate the chemical conditions 146 of Ga and Co in the solid product, Ga K-edge and Co K-edge XAFS spectra were measured with 147 the transmission mode at BL-9A, Photon Factory, KEK, Japan. Every product was mixed with a 148 BN reagent in an agate mortar, made into a disk with 10 mm φ, and put into a polyethylene bag. 149 REX200042 software (RIGAKU) was used for the data processing of the X-ray absorption near 150 edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses.  151 Page 8 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 9 2.8. Rietveld refinement. Rietveld refinement for crystal structure refinement was conducted 152 using RIETAN-FP developed by Izumi et al.43 The crystal information file of CoGa2O4 with an 153 ICSD number of 172183 was used and standardized before Rietveld refinement (Table 1). A split 154 pseudo-Voigt function of Toraya was used for a profile function,44 and the conjugate-direction 155 method was used as the least-squares method. Basic parameters such as shift parameters for 156 profile function and asymmetry parameters, as well as decay parameters, were refined to obtain 157 good fitting, followed by the refinement of atom occupancy. The atom displacement parameter 158 was fixed, and linear constraints were determined based on the Co/Ga molar ratios measured by 159 ICP-AES. 160 2.9. Magnetic measurements. Alternating current (AC) and direct current (DC) 161 magnetization measurements were conducted using the magnetic properties measurement system 162 (MPMS-XL; Quantum Design). The samples loaded in a gelatin capsule were cast with paraffin 163 to prevent nanoparticle rotation under a magnetic field.  164  165 Table 1. Standardized crystal structure of CoGa2O4 166 Atom x y z Site Occupancy Co1 0.37500 0.37500 0.37500 8b 0.425 Ga1 0.37500 0.37500 0.37500 8b 0.575 Co2 0.00000 0.00000 0.00000 16c 0.288 Ga2 0.00000 0.00000 0.00000 16c 0.712 O 0.24158 0.24158 0.24158 32e 1.000   167 Page 9 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 10 3. RESULTS AND DISCUSSION 168 3.1. Effect of precursor solution pH on composition and Co2+ site distribution in Co–Ga 169 NPs.  170 XRD diffraction patterns of the produced solid particles are shown in Figure 2. Relatively 171 broad diffraction patterns revealed that the particles were nanocrystals. The diffraction patterns 172 for pH 5–11 were indexed based on an ICSD card (ICSD: 172183) of single-phase spinel-type 173 cobalt gallium metal oxides (CoGa2O4) with a cubic F d -3 m space group and were found to 174 shift toward higher angles. In the solid product prepared without NaOH, the peak attributable to 175 the 003 reflection of α-Co (OH)2 reported by Liu et al.45 was labeled with a circle, and peaks 176 attributable to the unidentified contamination were labeled with triangles. Moreover, the reduced 177 full width at half-maximum of the (113) peak indicated an increased crystallite size of the 178 produced Co–Ga NPs as the pH value increased. The average crystallite sizes of the nanocrystals 179 were calculated using Scherrer’s equation as summarized in Table 2. 180  181 Page 10 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 11  182 Figure 2. X-ray diffraction patterns of solid products produced at 400 °C under various pH 183 values of precursor solutions. 184  185  Intensity (-)70605040302010 2θ (deg.)022044113111003 No NaOH added CoGa2O4 (ICSD: 172183) pH = 11 pH = 9pH = 7 pH = 5       : α-Co(OH)2 : unidentified Page 11 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 12 Table 2. Average crystallite sizes of Co–Ga NPs produced at 400 °C from precursor solutions 186 with various pH values calculated using Scherrer’s equation 187 pH of precursor solutions Calculated average crystallite size (nm) No NaOH added 13.1 5 14.2 7 15.0 9 22.8 11 28.5  188 The morphologies of the Co–Ga NPs were characterized by SEM (Figure 3). In general, 189 spherical particles were observed in low-pH conditions, and an increase in the number of cubic 190 nanoparticles was observed at pH 9 and pH 11. The average particle size was in the range of 19 191 nm to 58 nm with the increased pH value. A great increase in the average particle size was 192 obtained at pH 11 owing to the decreased supersaturation after substantially adding NaOH. 193 Because CoGa2O4 nanoparticles agglomerate easily,41 the average particle sizes measured from 194 SEM images were larger than those calculated from Scherrer’s equation.  195 Page 12 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 13  196 Figure 3. Morphologies and measured average particle sizes of Co–Ga NPs produced at 400 °C 197 under various pH values of precursor solutions (Dav: average particle diameter, CV: coefficient of 198 variation). 199 Before the exploration of the Co/Ga molar ratios of the prepared Co–Ga NPs using ICP-AES, 200 EDS analysis was performed. EDS results showed non-negligible amounts of Ni and Cr when no 201 NaOH was added and at pH 5, which were due to the dissolution of metal elements from the 202 reactor. Therefore, we performed ICP-AES measurements for Co and Ga along with Ni, Cr, and 203 Fe. ICP-AES results are shown in Figure 4. The concentrations of Ni and Cr, as well as Fe, 204 decreased significantly to nearly zero with the increased pH, indicating that higher pH values, i.e., 205 pH 7, pH 9, and pH 11 are suitable for the synthesis of Co–Ga NPs with nearly no contamination. 206 Figure 4 also shows Co/Ga molar ratios. At high pH conditions, the Co/Ga molar ratios of the 207 produced Co–Ga NPs exceeded the stoichiometric composition of Co/Ga = 0.5. 208 pH= 5 Dav = 22 nmCV : 38.4%CV : 38.9% CV : 24.6%CV : 37.5%pH= 7 Dav = 19 nmpH= 11 Dav = 58 nmpH= 9 Dav = 29 nm100 nmPage 13 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 14  209 Figure 4. M/Ga (M = Co, Ni, Cr, and Fe) molar ratios of Co–Ga NPs produced at 400 °C under 210 various pH values of precursor solutions measured by ICP-AES. 211 Because there are more cobalt atoms in the prepared non-stoichiometric Co–Ga NPs compared 212 to CoGa2O4, Rietveld refinement using RIETAN-FP43 was applied to investigate the detailed site 213 distribution of metal atoms. First, Ga and Co K-edge XANES analyses were performed to 214 evaluate the valance states of prepared Co–Ga NPs in comparison to purchased references and 215 spinel-type CoGa2O4 calcinated at 1300 °C for 24 hours by the solid-state reaction method using 216 mixed powders of CoO and Ga2O3. Then, the compositions of the prepared Co–Ga NPs were 217 calculated based on the ICP-AES and valance results. Figure 5 shows the obtained spectra. The 218 results confirmed that Co2+ and Ga3+ were the main valances for the NPs produced at pH 7–11. 219 0.80.60.40.20.0 Molar ratios of various transition metal elements to gallium (-)119753 pH value of precursor solution (-) Co/Ga   Ni/Ga   Cr/Ga Fe/Gastoichiometric value in CoGa2O4 Page 14 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 15 However, the peaks greatly shifted to the right for the NPs produced at pH 5, which could 220 indicate a large portion of Co3+ ions. Therefore, Rietveld refinements were not conducted for pH 221 5 because of the contamination issues and the complicated valance states, and “No NaOH added” 222 for the reason of a secondary crystalline phase. Meanwhile, the number of the oxygen atom was 223 fixed at 4 in the final given composition while keeping charge neutrality. Moreover, because 224 there is almost no oxygen vacancy within the spinel-type γ-Ga2O3,20 the oxygen vacancy of the 225 produced Co–Ga NPs was not considered. Thus, the total number of metal atoms in a unit cell 226 was considered to be extremely close to 24 during the refinement. An example of the Rietveld 227 refinement of the diffraction pattern of Co–Ga NPs with a composition of Co1.09Ga1.94O4 at pH 9 228 and 400 °C is presented in Figure 6, and all results of the refinement along with the calculated 229 composition based on the ICP-AES results are listed in Table 3. For each condition, the 230 calculated versus measured diffraction patterns showed good agreement. Surprisingly, the 231 inversion parameters of the produced Co–Ga NPs were relatively higher compared to those 232 reported by Melot et al.,46 Ikeda et al.47 as well as Naka et al.,48 which were 0.630, 0575, and 233 0.664, respectively. 234 Page 15 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 16  235 Figure 5. (a) Ga and (b) Co K-edge X-ray absorption near edge structure spectra of 236 non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of precursor 237 solutions. 238  239 (a) (b) Normalized absorption coefficient104001039010380103701036010350 Photon energy (eV) Beta-Ga2O3 pH 11 pH 9 pH 7 CoGa2O4 (1300 ℃, 24 h) pH 5 No NaOH added Ga K-edge77507740773077207710 CoGa2O4 (1300 ℃, 24 h) pH 7 pH 5 No NaOH added Co3O4 pH 9 pH 11 Co K-edge Normalized absorption coefficient104001039010380103701036010350 Photon energy (eV) Beta-Ga2O3 pH 11 pH 9 pH 7 CoGa2O4 (1300 ℃, 24 h) pH 5 No NaOH added Ga K-edgePage 16 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 17  240 Figure 6. Example (pH 9, 400 °C) of Rietveld refinement using RIETAN-FP in this study.  241  242 Table 3. Rietveld refinement results of non-stoichiometric Co–Ga NPs produced at 400 °C using 243 precursor solutions with pH 7–11 244 Atom Site Occupation Crystal size (a=b=c) Inversion parameter GOF pH 11, Co1.35Ga1.77O4    Co1 8b (Tet.) 0.4299 8.327 0.8676 2.049 Ga1 8b 0.5731 Co2 16c (Oct.) 0.4338 Ga2 16c 0.5647 O 32e 0.9628 pH 9, Co1.09Ga1.94O4    Co1 8b 0.5018 8.334 0.5774 2.840 Ga1 8b 0.5308 Co2 16c 0.2887 Ga2 16c 0.6946 O 32e 0.9898 pH 7, Co1.16Ga1.89O4    Co1 8b 0.4438 8.331 0.6972 2.196 Ga1 8b 0.5598 Co2 16c 0.3486 Ga2 16c 0.6495 O 32e 0.9823 Page 17 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 18 The occupancies of Ga and Co calculated from Rietveld refinement in either tetrahedral or 245 octahedral sites of the prepared spinel-type Co–Ga NPs are summarized in Figure 7. The 246 occupancies of Ga atoms in both tetrahedral and octahedral sites at each pH value were lower 247 than those in CoGa2O4, and the occupancy of Co atoms slightly exceeded that in CoGa2O4 in 248 tetrahedral sites, and an even more increase was observed in octahedral sites at pH 7 and pH 11. 249  250 Figure 7. Occupancies of Ga and Co in tetrahedral and octahedral sites derived from Rietveld 251 refinement results of non-stoichiometric Co–Ga NPs produced at 400 °C using precursor 252 solutions with pH 7–11. 253  The results of the radial structure functions of Ga and Co atoms derived from the EXAFS 254 analysis of Co–Ga NPs prepared at various pH values, as well as CoGa2O4 produced by the 255 solid-state reaction method are summarized in Figure 8. Prepared non-stoichiometric Co–Ga NPs 256 are showing typical shape of the radial structure function of a spinel compound when compared 257 tetrahedral site octahedral site1.00.80.60.40.20 Occupancies of Co and Ga in tetrahedral site and octahedral site (-)pH=7 pH=9 pH=11 pH value of precursor solutions (-) occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 1.00.80.60.40.20.0pH=7 pH=9 pH=11 occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 1.00.80.60.40.20 Occupancies of Co and Ga in tetrahedral site and octahedral site (-)pH=7 pH=9 pH=11 pH value of precursor solutions (-) occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 Page 18 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 19 with CoGa2O4. The peak around 1.5 Å is attributable to Ga–O and Co–O interactions, and two 258 intensive peaks ranging from 2.5 to 3.5 Å are attributable to Ga–Ga and Ga–Co interactions 259 (Figure 8 (a)), and Co–Co and Co–Ga interactions (Figure 8 (b)).  260 Figure 8. Radial structure function of (a) Ga and (b) Co derived from the extended X-ray 261 absorption fine structure analysis of non-stoichiometric Co–Ga NPs produced at 400 °C under 262 various pH values of precursor solutions. 263 In Figure 8 (a), regarding the radial structure function of Ga, peaks around 2.5 Å and 3.0 Å 264 refer to Ga–Ga and Ga–Co interactions in octahedral and tetrahedral sites respectively. The 265 shape of the peaks at “No NaOH added” is closer to γ-Ga2O3 owing to similar occupancy of Ga 266 in octahedral and tetrahedral sites,20 and the shape at pH 5 became closer to a spinel compound. 267 Meanwhile, non-stoichiometric Co–Ga NPs prepared at pH 7, 9, and 11 have similar intensities 268 |F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)|F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)Co–OOct. –Oct. Tet. –Tet. Oct. –Oct. Ga–OTet. –Tet. (a) (b)Ga–Ga, Ga–Co Co–Co, Co–Ga|F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)Page 19 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 20 at tetrahedral site (3.0–3.5 Å) due to comparable degree of crystallinity and Ga occupancy, but 269 lower at octahedral site (2.5–3.0 Å) when compared to CoGa2O4 due to decreased Ga atoms. In 270 Fig. 8 (b), the radial structure function of Co, the intensities of all three peaks in the range of 271 1.5–3.5 Å at pH 11 greatly decreased, which might be due to the decreased crystallinity of the 272 local environment around Co atoms in a non-stoichiometric spinel compound. 273 In summary, spinel-type non-stoichiometric Co–Ga NPs with the Co/Ga molar ratio larger than 274 0.5 were successfully prepared at pH 7–11. They were found to hold more cobalt atoms within 275 the octahedral site, but lower crystallinity when compared to CoGa2O4. 276  277 3.2. Magnetic properties of the produced non-stoichiometric Co–Ga NPs.  278 Since cobalt ions is the only magnetic ion, their site distribution could affect super-exchange 279 interactions between Co2+ ions, that is, tetrahedral–tetrahedral exchange interaction JAA, 280 octahedral–octahedral exchange interaction JBB, and tetrahedral–octahedral exchange interaction 281 JAB. Therefore, non-stoichiometric Co–Ga NPs with more Co2+ within the octahedral site 282 prepared by present technique might exhibit different magnetic behavior compared to CoGa2O4. 283 Regarding the magnetic behavior of CoGa2O4, density functional calculations conducted by 284 Rafiq et al. revealed the ferromagnetic behavior of spinel-type CoGa2O4, assuming that Co ions 285 completely occupy the tetrahedral sites.49 However, in the Co–Ga NPs produced in this work, 286 Co2+ occupied both tetrahedral and octahedral sites. Fiorani et al.50 revealed an antiferromagnetic 287 order of CoGa2O4 with TN = 10 K, produced from the solid-state reaction method. Similarly, 288 Mathur et al.40 investigated the magnetic properties of CoGa2O4 NPs produced from the sol–gel 289 method, and the M–H curve measured at room temperature showed a typical paramagnetic 290 behavior, i.e., no spontaneous magnetization corresponding with an intercept on the Y axis (see 291 Page 20 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 21 our results in Figure 9). In our work, compared to CoGa2O4, the occupancies of the magnetic 292 Co2+ ion in tetrahedral (0.443(8), 0.501(8), 0.429(9)) and octahedral (0.348(6), 0.288(7), 293 0.433(8)) sites of the non-stoichiometric Co–Ga NPs produced at pH 7, 9 and 11 were closer to 294 or even higher than the corresponding percolation thresholds, which were reported to be 0.429(3), 295 0.390(3), and 0.227(3) for JAA, JBB, and JAB, respectively.51 Therefore, JAA, JBB, and JAB 296 interactions are considered to be enhanced and percolate through the crystal structures of the 297 products in this work.  298 The M–H curves of the Co–Ga NPs measured at room temperature are shown in Figure 9. 299 Notably, a closed hysteresis loop was obtained, and the magnetic behavior was considered to be 300 the combination of partially paramagnetic and partially ferromagnetic (superparamagnetic) 301 behaviors. In fact, as can be seen in Figure 9, Mg did not saturate at high fields as expected for 302 ferromagnets, and it increased linearly with respect to H. The latter is a typical feature observed 303 in paramagnets. At all pH conditions, Co–Ga NPs with higher Co/Ga molar ratios exhibited 304 larger spontaneous magnetization M0 (intercepts at H = 0 of the extrapolated line from the linear 305 part of Mg(H) curve in Figure 9). Since JAA, JBB, and JAB interactions are enhanced and percolated 306 through the nanocrystal, a magnetic domain develops macroscopically in the crystals as a result. 307 Notably, Co–Ga NPs prepared at pH 11, which had the highest Co/Ga ratio and Co occupancies 308 in tetrahedral and octahedral sites, exhibited the largest spontaneous magnetization at room 309 temperature. For Co–Ga NPs prepared at “No NaOH added” and pH 5 with relatively smaller 310 Co/Ga molar ratios, M0 is comparable with those for others. Plausibly, the magnetic 311 contaminations Ni and Cr are situated in the cation sites in the spinel structure and contribute to 312 the formation of magnetic domains. In summary, we speculate that the observed ferromagnetism 313 at room temperature is induced by a cationic configurational deviation from that of a normal 314 Page 21 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 22 spinel52 and the effects of crystal morphology such as large surface-to-volume ratio, surface 315 anisotropy,53 and valence transition observed with reduced crystal size in metal oxides could also 316 be considered.54 317 Figure 9. M–H curves (measured at room temperature, 300 K) of non-stoichiometric Co–Ga NPs 318 produced at 400 °C under various pH values of precursor solutions. 319 To further investigate the magnetic properties of our products, M–T curves measured at 20 kOe 320 and 50 kOe are summarized in Figure 10 (a) and (b). Based on Eq. 1, which was established for 321 H above 15 kOe, both susceptibility χ and M0 were calculated as a function of temperature (from 322 2 K to 300 K), as depicted in Figure 10 (c) and (d). Herein, χ is a differential susceptibility 323 defined as χ(T) = [Mg(50 kOe) - Mg(20 kOe)]/(50 kOe – 20 kOe).  324 1-1.5-1.0-0.500.51.01.5Mg (emu/g)-40 x103 -20 0 20 40H (Oe) No NaOH added  pH = 5 pH = 7 pH = 9 pH = 11spontaneous magnetization at pH 11 (intercept on Y axis)Page 22 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 23 M(H) = M0 + χ(H)H  (Eq.1) 325 From Figure 10 (c), χ(T) curves exhibit anomalous cusp and kink below 50 K, which are 326 expected to occur at magnetic transitions. To identify magnetic transitions, we measured AC and 327 DC susceptibilities under small magnetic fields. As a result, a spin-glass phase transition was 328 indicated at spin-glass phase transition temperature (TSG) below 10 K at every pH condition, 329 along with ferri- or ferromagnetic transitions temperature (Tm) below 50 K at “No NaOH added” 330 and pH 5 exclusively. Moreover, in the AC susceptibility, a frequency-dependent peak was 331 found to shift to high temperature with higher frequency. In addition, a cusp at TSG in DC 332 susceptibility measured after zero-field cooling (ZFC) was also observed for each pH condition 333 (see supporting information Figures S1 and S2). Those are typical in spin-glass transition, and 334 transition temperatures were summarized (see supporting information Table S1). Figure 10 (d) 335 shows the reciprocal magnetic susceptibility χ-1 as a function of temperature, from which 336 deviations from a modified Curie–Weiss law shown as Eq. 2 were observed above 50 K. Herein, 337 C is the Curie constant, and θ is the Curie–Weiss temperature. We experimentally obtained the 338 effective paramagnetic moment p = 3.74 μB for pH 9 deduced from the experimental value C = 339 1.749(5), which is smaller than but comparable to 4.96 μB for the stoichiometric bulk sample, 340 while the Curie–Weiss temperature θ = −28.2 K is considerably higher than that of the bulk 341 (−49.9 K).48 342 χ = C/(T - θ)  (Eq. 2) 343 Figure 10 (d) also shows the enhancement of the Curie–Weiss temperature θ corresponding to 344 the development of interactions between Co2+. This can be attributed to the strengthened 345 interactions between the increased number of Co2+ ions in octahedral sites with the neighboring 346 Co2+ ions.48 To summarize, with the help of enhanced and percolated JAA, JBB, and JAB 347 Page 23 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 24 interactions owing to large number of cobalt atoms within tetrahedral and octahedral sites, 348 superparamagnetic behavior of non-stoichiometric Co–Ga NPs was observed for the first time. 349 Both effective paramagnetic moment and Curie–Weiss temperature were comparable to the 350 stoichiometric bulk sample. 351  352 Figure 10. M–T curves measured at (a) 20 kOe and (b) 50 kOe and (c) χ, m0 vs T plots and (d) 353 χ−1 vs T plots of non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of 354 precursor solutions. 355 12108642Mg (emu/g)4 5 6 7102 3 4 5 6 71002 3T (K)No NaOH added pH = 5pH = 7pH = 9pH = 11  H = 20 kOe2015105Mg (emu/g)4 5 6 7102 3 4 5 6 71002 3T (K)No NaOH added pH = 5pH = 7pH = 9pH = 11 H = 50 kOe250200150100500χ (10-6 emu/g)300250200150100500T (K)543210M0  (emu/g), No NaOH added, pH = 5, pH = 7, pH = 9, pH = 1150403020100 χ-1 (103 g/emu)3002001000-100T (K) No NaOH added   pH = 5 pH = 7 pH = 9 pH = 11 Curie–Weiss temperature θ(a)(b)(c)(d)Page 24 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 25 3.3. Formation mechanism of non-stoichiometric Co–Ga NPs.  356 To reveal the formation mechanism of non-stoichiometric Co–Ga NPs with high Co/Ga molar 357 ratio, we investigated the crystal phase and the composition (Co/Ga molar ratio) of solid 358 precipitates in the precursor solutions. In addition, solid precipitates were collected on a silicon 359 sample holder for XRD analysis without centrifugation. However, no solid precipitate was 360 observed in the aqueous solution when no NaOH was added. All XRD diffraction patterns of the 361 solid precipitates are shown in Figure 11.  362  363  Intensity (-)70605040302010 2θ (deg.) 003 006 009 015 Co-Ga-CO32- layered double hydroxides  reported by Radha et al. α-Co(OH)2 reported by Liu et al. 003 006 pH = 5 110  113 015 012 111  022 113 044 002 022  224 024 001 011 pH = 7 pH = 9 pH = 11 : unidentifiedCoGa2O4 (ICSD: 172183)β-Co(OH)2 (ICSD: 88940)Ga(OH)3 (ICSD: 252677)Page 25 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 26 Figure 11. X-ray diffraction (XRD) patterns of solid precipitates in precursor solutions at 364 various pH values. (The XRD patterns of Co–Ga-CO32- LDHs and α-Co (OH)2 are plotted based 365 on research papers of Radha et al.55 and Liu et al.45, respectively.) 366 Comparing with the diffraction pattern of Co–Ga-CO32- LDHs reported by Radha et al.,55 we 367 attributed the observed diffraction pattern of the solid precipitate obtained from the precursor 368 solutions at pH 11 to cobalt gallium nitrate layered double hydroxides (Co–Ga-NO3- LDHs). 369 Herein, cobalt gallium LDHs consist of positively charged brucite-like layers including 370 interconnected octahedral sites occupied by divalent (M2+ = Co2+, Mg2+, and Fe2+) and trivalent 371 (M3+= Ga3+, Al3+, and Fe3+) metal ions, along with interlayer anions (An- = NO3− or CO32−) to 372 achieve a neutral environment. A general formula for this type of structure is 373 [M2+1-xM3+x(OH)2]x+[An-x/n•mH2O], and typically, the molar ratio of M3+/(M3+ + M2+) was reported 374 to be a value between 0.2 and 0.4 by Wang et al.56 When it comes to the connection between Co–375 Ga-NO3- LDHs and the generation of a spinel-type CoGa2O4, Cook et al. conducted the 376 solvothermal synthesis of spinel-type CoGa2O4 using gallium metal and cobalt(II) nitrate 377 hexahydrate as starting materials and revealed that the LDH structure can act as a metastable 378 phase, followed by redissolution or amorphization before the final formation of spinel-type 379 CoGa2O4.57 Similarly, the diffraction patterns of Co–Ga-NO3- LDHs were observed at pH 7 as 380 well as pH 9. The remaining diffraction patterns (labeled by a circle) at pH 5, pH 7, as well as 381 pH 9, were compared with two notable polymorphs of cobalt hydroxide, i.e., β-Co (OH)2 and 382 α-Co (OH)2 reported by Liu et al. 45 and gallium hydroxide for further identification. However, 383 they did not match any of those compounds. Therefore, we consider them as an intermediate, 384 which may have disappeared during hydrothermal synthesis.  385 Page 26 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 27 Then, we conducted ICP-AES analysis to measure the chemical composition in aqueous 386 solution of the precursor solutions at room temperature. Figure 12 shows the detailed molar 387 fractions of Co and Ga (Y axis = 1.0 means no metal element in the solid precipitate). As 388 reported by Hassan Mohamed et al., Co hydrolyzes to Co(OH)+, and consequently, the 389 precipitation of Co(OH)2 initiates when the pH value of the aqueous solution exceeded 7.58 390 Similar results were reported by Sheha et al. and Dakroury et al.59, 60 On the other hand, Ga is an 391 amphoteric metallic element, which dissolves in aqueous solutions at either low or high pH.61 In 392 our measurements, the proportion of Co (marked as pink circles) in the aqueous solution 393 continuously decreased with an escalating pH value, which is a consequence of the precipitation 394 of Co ions from pH 5 to pH 11. Meanwhile, the proportion of Ga in the aqueous solution 395 (marked as black squares) started to increase from pH 7, which is considered the redissolution 396 process of solid Ga at high pH values.  397  398 Figure 12. Molar fractions of cobalt and gallium in supernatant of precursor solutions at various 399 pH values. 400 1.00.80.60.40.20.0 Molar fraction of the metal elements in the aqueous solution at room temperature (-)119753 pH value of precursor solutions (-) Co2+ in supernatant Ga3+ in supernatant Page 27 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 28 Next, we evaluated the actual composition (Co/Ga molar ratio) of the solid precipitate in the 401 precursor solution at each condition at room temperature. For a better understanding, the 402 calculation method is shown in Figure 13, where M stands for the number of moles, and C 403 represents the concentration (mol/L) obtained from ICP-AES analysis.  404  405 Figure 13. Schematic depicting the calculation of chemical composition in the solid precipitate. 406 Figure 14 shows comparison between calculated Co/Ga molar ratio in the solid precipitate, and 407 that in the final products. The Co/Ga molar ratio of the solid precipitate at room temperature 408 keeps increasing from pH 5 to pH 11 and that of the Co–Ga-NO3- LDH obtained at pH 11 was 409 1.24, which almost resembles the proposed reasonable ratio.56 Notably, this increasing tendency 410 of Co/Ga molar ratio in the solid precipitate at room temperature is similar to that observed in the 411 final products. 412 OH-metal ionsAqueous solutionSolid precipitate!("#, %&'()%)*+*') = !("#, )- ./#0' %&'(1&2#& 2#01*)#-2) − $ "#, +312#12 2#01*)#- ×&!(4+, %&'()%)*+*') = !(4+, )- ./#0' %&'(1&2#& 2#01*)#-2) − $ 4+, +312#12 2#01*)#- ×&Volume (V) was measured for each condition.Page 28 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 29  413 Figure 14. Co/Ga molar ratios in the solid precipitate and produced non-stoichiometric Co–Ga 414 NPs. 415 Based on the results, we propose a possible formation mechanism in the present technique as 416 follows: The solid precipitate containing Co–Ga-NO3- LDHs could form non-stoichiometric Co–417 Ga NPs during supercritical hydrothermal synthesis in a mechanism similar to that reported by 418 Cook et al.57, while maintaining large Co/Ga molar ratio owing to the absence of calcination at 419 high temperature. However, Co2+ and Ga3+ in the supernatant of the precursor solutions could 420 react with the solid precipitate during hydrothermal synthesis, especially at pH 5 and pH 11 (See 421 Co2+, Ga3+ in supernatant of reactant solutions in supporting information Figures S3). Therefore, 422 the manipulation of the pH value of the precursor solution in the present technique can affect the 423 1.21.00.80.60.40.20.0 Co/Ga in the solid precipitate and produced Co–Ga NPs (-)119753　pH value of the precursor solutions (-)  Co/Ga in solid precipitate  Co/Ga in produced Co–Ga NPs  Page 29 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 30 solubilities of Co and Ga at room temperature, and further give effect on the Co/Ga molar ratios 424 in the final solid products.  425   426 Page 30 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 31 4. CONCLUSIONS 427 We demonstrated the synthesis of spinel-type non-stoichiometric cobalt gallate nanoparticles 428 (Co–Ga NPs) using fast supercritical hydrothermal synthesis without performing the calcination 429 process. Non-stoichiometric Co–Ga NPs with controllable Co/Ga molar ratios were produced by 430 manipulating the precursor solutions pH. At pH 7 and pH 11, the prepared Co–Ga NPs exhibited 431 large inversion parameters and Co/Ga molar ratios. Produced Co–Ga NPs exhibited a 432 combination of paramagnetic and ferromagnetic (superparamagnetic) behaviors at room 433 temperature, and those prepared at pH 11 with the highest inversion parameter showed the 434 largest spontaneous magnetization M0 due to percolated JAA, JBB, and JAB interactions. The 435 formation mechanism likely involves solid precipitates containing Co–Ga-NO3- LDHs or their 436 intermediates in the precursor solutions, which play a key role in the generation of Co–Ga NPs, 437 and we found a tight relationship between Co/Ga molar ratio in the solid precipitate at room 438 temperature and that in the final solid products. Generally, present technique has demonstrated 439 great potentials as effective synthesis method for producing Ga-based spinel-type metal oxides 440 with non-stoichiometric composition that exhibit enhanced properties.  441   442 Page 31 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 32 ASSOCIATED CONTENT 443 Supporting Information 444 Additional experimental details, including (a) AC susceptibility at various frequencies, DC 445 susceptibility measured after the ZFC of Co–Ga NPs at 400 °C prepared from precursor solutions 446 with pH 5 and 7. (b) Summary of the spin-glass transition temperature Tsg, ferri- or ferromagnetic 447 transition temperature Tm, and error of Tsg of Co–Ga NPs. (c) Molar fractions of cobalt and 448 gallium in supernatant of reactant solutions after supercritical hydrothermal synthesis at various 449 pH values (PDF). 450  451 AUTHOR INFORMATION 452 Corresponding Author 453 *Email: takami.seiichi@material.nagoya-u.ac.jp 454 Author Contribution 455 Bo Xie : conceptualization, formal analysis, investigation (material production, XRD, SEM, 456 ICP-AES, XAFS measurements), validation, visualization, writing – original draft, writing – 457 review & editing.  458 Chiya Numako : formal analysis, investigation (XAFS measurements), resources, validation, 459 writing – review & editing.  460 Takashi Naka : formal analysis, investigation (magnetization measurements), resources, 461 validation, writing – review & editing.  462 Page 32 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 33 Seiichi Takami : conceptualization, funding acquisition, investigation (XAFS measurements), 463 formal analysis, project administration, resources, supervision, validation, writing – review & 464 editing.  465 Notes 466 The authors declare no competing financial interest. 467 ACKNOWLEDGMENT 468 This work was supported by JSPS KAKENHI Grant Numbers 17H06467, 20H02514. 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Jpn. 1989, 62 640 (12), 3823–3827. 641 Page 42 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 1 All graphics for : 1 Supercritical Hydrothermal Synthesis of 2 Spinel-Type Non-stoichiometric Cobalt Gallate 3 Nanoparticles and Their Magnetic Properties 4 Bo Xiea, Chiya Numakob, Takashi Nakac, Seiichi Takamia* 5 aDepartment of Materials Process Engineering, Graduate School of Engineering, Nagoya 6 University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 7 bDepartment of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, 8 Inage-ku, Chiba 263-8522, Japan 9 cNational Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, 10 Japan 11  12 Page 43 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 2   13 Figure 1. Crystal structure of a partially inverse CoGa2O4 created by VESTA. Page 44 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 3   14 Figure 2. X-ray diffraction patterns of solid products produced at 400 °C under various pH values of precursor solutions.  Intensity (-)70605040302010 2θ (deg.)022044113111003 No NaOH added CoGa2O4 (ICSD: 172183) pH = 11 pH = 9pH = 7 pH = 5       : α-Co(OH)2 : unidentified Page 45 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 4   15 pH= 5 Dav = 22 nmCV : 38.4%CV : 38.9% CV : 24.6%CV : 37.5%pH= 7 Dav = 19 nmpH= 11 Dav = 58 nmpH= 9 Dav = 29 nm100 nmFigure 3. Morphologies and measured average particle sizes of Co–Ga NPs produced at 400 °C under various pH values of precursor solutions (Dav: average particle diameter, CV: coefficient of variation). Page 46 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 5   16 0.80.60.40.20.0 Molar ratios of various transition metal elements to gallium (-)119753 pH value of precursor solution (-) Co/Ga   Ni/Ga   Cr/Ga Fe/Gastoichiometric value in CoGa2O4 Figure 4. M/Ga (M = Co, Ni, Cr, and Fe) molar ratios of Co–Ga NPs produced at 400 °C under various pH values of precursor solutions measured by ICP-AES.  Page 47 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 6  17   18 (a) (b) Normalized absorption coefficient104001039010380103701036010350 Photon energy (eV) Beta-Ga2O3 pH 11 pH 9 pH 7 CoGa2O4 (1300 ℃, 24 h) pH 5 No NaOH added Ga K-edge77507740773077207710 CoGa2O4 (1300 ℃, 24 h) pH 7 pH 5 No NaOH added Co3O4 pH 9 pH 11 Co K-edge Normalized absorption coefficient104001039010380103701036010350 Photon energy (eV) Beta-Ga2O3 pH 11 pH 9 pH 7 CoGa2O4 (1300 ℃, 24 h) pH 5 No NaOH added Ga K-edgeFigure 5. (a) Ga and (b) Co K-edge X-ray absorption near edge structure spectra of non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of precursor solutions. Page 48 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 7   19   20 Figure 6. Example (pH 9, 400 °C) of Rietveld refinement using RIETAN-FP in this study. Page 49 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 8  21   22 tetrahedral site octahedral site1.00.80.60.40.20 Occupancies of Co and Ga in tetrahedral site and octahedral site (-)pH=7 pH=9 pH=11 pH value of precursor solutions (-) occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 1.00.80.60.40.20.0pH=7 pH=9 pH=11 occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 1.00.80.60.40.20 Occupancies of Co and Ga in tetrahedral site and octahedral site (-)pH=7 pH=9 pH=11 pH value of precursor solutions (-) occupancy of Ga Ga in CoGa2O4  occupancy of Co  Co in CoGa2O4 Figure 7. Occupancies of Ga and Co in tetrahedral and octahedral sites derived from Rietveld refinement results of non-stoichiometric Co–Ga NPs produced at 400 °C using precursor solutions with pH 7–11.  Page 50 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 9  23  24  25  26  27  28  29  30 |F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)|F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)Co–OOct. –Oct. Tet. –Tet. Oct. –Oct. Ga–OTet. –Tet. (a) (b)Ga–Ga, Ga–Co Co–Co, Co–Ga|F(R)|543210R (Å) No NaOH added pH 5   pH 7  pH 9   pH 11 CoGa2O4 (1300 ℃, 24 h)Figure 8. Radial structure function of (a) Ga and (b) Co derived from the extended X-ray absorption fine structure analysis of non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of precursor solutions. Page 51 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 10  31  32  33  34  35  36 1-1.5-1.0-0.500.51.01.5Mg (emu/g)-40 x103 -20 0 20 40H (Oe) No NaOH added  pH = 5 pH = 7 pH = 9 pH = 11spontaneous magnetization at pH 11 (intercept on Y axis)Figure 9. M–H curves (measured at room temperature, 300 K) of non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of precursor solutions.  Page 52 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 11  37  38  39  40  41  42  43 12108642Mg (emu/g)4 5 6 7102 3 4 5 6 71002 3T (K)No NaOH added pH = 5pH = 7pH = 9pH = 11  H = 20 kOe2015105Mg (emu/g)4 5 6 7102 3 4 5 6 71002 3T (K)No NaOH added pH = 5pH = 7pH = 9pH = 11 H = 50 kOe250200150100500χ (10-6 emu/g)300250200150100500T (K)543210M0  (emu/g), No NaOH added, pH = 5, pH = 7, pH = 9, pH = 1150403020100 χ-1 (103 g/emu)3002001000-100T (K) No NaOH added   pH = 5 pH = 7 pH = 9 pH = 11 Curie–Weiss temperature θ(a)(b)(c)(d)Figure 10. M–T curves measured at (a) 20 kOe and (b) 50 kOe and (c) χ, m0 vs T plots and (d) χ−1 vs T plots of non-stoichiometric Co–Ga NPs produced at 400 °C under various pH values of precursor solutions. Page 53 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 12   44  Intensity (-)70605040302010 2θ (deg.) 003 006 009 015 Co-Ga-CO32- layered double hydroxides  reported by Radha et al. α-Co(OH)2 reported by Liu et al. 003 006 pH = 5 110  113 015 012 111  022 113 044 002 022  224 024 001 011 pH = 7 pH = 9 pH = 11 : unidentifiedCoGa2O4 (ICSD: 172183)β-Co(OH)2 (ICSD: 88940)Ga(OH)3 (ICSD: 252677)Figure 11. X-ray diffraction (XRD) patterns of solid precipitates in precursor solutions at various pH values. (The XRD patterns of Co–Ga-CO32- LDHs and α-Co (OH)2 are plotted based on research papers of Radha et al.55 and Liu et al.45, respectively.) Page 54 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 13  45  46  47  48  49 Figure 12. Molar fractions of cobalt and gallium in supernatant of precursor solutions at various pH values. 1.00.80.60.40.20.0 Molar fraction of the metal elements in the aqueous solution at room temperature (-)119753 pH value of precursor solutions (-) Co2+ in supernatant Ga3+ in supernatant Page 55 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 14   50 OH-metal ionsAqueous solutionSolid precipitate!("#, %&'()%)*+*') = !("#, )- ./#0' %&'(1&2#& 2#01*)#-2) − $ "#, +312#12 2#01*)#- ×&!(4+, %&'()%)*+*') = !(4+, )- ./#0' %&'(1&2#& 2#01*)#-2) − $ 4+, +312#12 2#01*)#- ×&Volume (V) was measured for each condition.Figure 13. Schematic depicting the calculation of chemical composition in the solid precipitate. Page 56 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 15  51  52  53  54  55 1.21.00.80.60.40.20.0 Co/Ga in the solid precipitate and produced Co–Ga NPs (-)119753　pH value of the precursor solutions (-)  Co/Ga in solid precipitate  Co/Ga in produced Co–Ga NPs  Figure 14. Co/Ga molar ratios in the solid precipitate and produced non-stoichiometric Co–Ga NPs. Page 57 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 16  56  57  58   59 Figure S1. (a) AC susceptibility at various frequencies, (b) DC susceptibility measured after zero-field cooling (ZFC) of the Co-Ga NPs at 400 ℃ prepared from precursor solutions with pH 5. Page 58 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 17  60   61 Figure S2. (a) AC susceptibility at various frequencies, (b) DC susceptibility measured after zero-field cooling (ZFC) of the Co-Ga NPs at 400 ℃ prepared from precursor solutions with pH 7. Page 59 of 60ACS Paragon Plus EnvironmentCrystal Growth & Design123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 18  62 1.00.80.60.40.20.0 Molar fraction of the metal elements in the aqueous solution at room temperature (-)119753 pH value of precursor solutions (-) Co2+ in supernatent Ga3+ in supernatentFigure S3. Molar fractions of cobalt and gallium in supernatant of reactant solutions after supercritical hydrothermal synthesis at various pH values..  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