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

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[Color-Controlled Nonstoichiometric Spinel-Type Cobalt Gallate Nanopigments Prepared by Supercritical Hydrothermal Synthesis](https://mdr.nims.go.jp/datasets/c1404bed-6e6c-4e9a-b611-9c5b8dd46456)

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Color-controlled nonstoichiometric spinel-type cobalt gallate nanopigments prepared by supercritical hydrothermal synthesisDaltonTransactionsPAPERCite this: Dalton Trans., 2023, 52,16285Received 20th September 2023,Accepted 13th October 2023DOI: 10.1039/d3dt03086ersc.li/daltonColor-controlled nonstoichiometric spinel-typecobalt gallate nanopigments prepared bysupercritical hydrothermal synthesis†Bo Xie, a Chiya Numako, b Takashi Naka c and Seiichi Takami *aSpinel-type inorganic pigments with intensive color and chemical/thermal stability are showing extensiveapplications that could be further broadened by color manipulation and improvement of the material pro-perties through nanosizing. In this study, we report the supercritical hydrothermal synthesis of nonstoi-chiometric spinel-type cobalt gallate nanoparticles (Co–Ga NPs) with controlled color. Without the con-ventional calcination procedure, NPs with greenish-blue, blue, and yellowish-green colors were syn-thesized from precursor solutions at pH 7, 9, and 11, respectively, with a low Co/Ga molar ratio of 0.25.X-ray diffraction, scanning/transmission electron microscopy, and inductively coupled plasma-atomicemission spectroscopy methods suggest that the products were spinel-type cobalt gallate NPs with highcrystallinity and a nonstoichiometric composition. Based on an X-ray absorption fine structure investi-gation, the prepared nonstoichiometric Co–Ga NPs were found to have different cationic configurationsfrom stoichiometric CoGa2O4 produced by a solid-state reaction during calcination. Meanwhile, thedegrees of distortions at tetrahedral and octahedral sites in the NPs were evaluated by Raman spec-troscopy. In particular, nonstoichiometric Co–Ga NPs with a blue color were prepared without calcinationfor the first time and were found to have lower tetrahedral cobalt occupancy but comparable octahedralcobalt occupancy and larger polyhedral distortions at tetrahedral sites when compared to calcinedCoGa2O4. We also discuss strategies that could realize Co–Ga NPs with a more brilliant blue color usingthe present technique based on an investigation of the growth process.1. IntroductionInorganic pigments with a spinel-type crystal structure showgreat utilization value owing to their high thermal/chemicalstability, high mechanical resistance,1 good compatibility withorganic modifiers and optical absorption performance.2,3Spinel-type pigments exhibit a wide range of intrinsic colorswith a high intensity by accommodating various types of tran-sition metal cations, including Fe2+, Co2+, Cu2+, Fe3+, andCr3+.4 Therefore, color manipulation can be realized by dopingwith transition or rare-earth metal ions.5–7 Other methods forcolor manipulation include the use of capping agents,8 use ofdifferent polymorphs of the starting material,9 and control ofthe calcination temperature.10 A change in the calcinationtemperature was thought to realize different cationic configur-ations at tetrahedral and octahedral sites in the spinel struc-ture, which could control the color of the products.11–13Compared to the normal spinel (IV(A)VI[B2]O4) and inversespinel (IV(B)VI[AB]O4) structures, CoGa2O4 has a partiallyinverse structure expressed as IV(Co1−xGax)VI[CoxGa2−x]O4 (x:inversion parameter), as shown in Fig. 1. The inversion para-meter of intensive blue CoGa2O4 calcined at 1300 °C for 24 hwas estimated to be 0.664.15 When using different calcinationtemperatures, Mathur et al. successfully produced CoGa2O4with dark green, grass green, and blue colors by calciningintermediate species at 400–500 °C, 600–800 °C, and1000–1200 °C, respectively.16 The emergence of the blue colorat 1000–1200 °C was considered the result of an increasednumber of tetrahedrally coordinated Co2+ ions (Co2+–O4) anddeoxidation of Co3+ to Co2+ during the temperature increase,similar to CoAl2O4.17 However, blue CoGa2O4 nanoparticleshave not been synthesized without calcination.†Electronic supplementary information (ESI) available: (a) Temperature insidethe reactor, (b) results of Williamson–Hall plot, (c) discussions on size effect, (d)TEM-EDS results at pH 7 and pH 9, (e) conversions of cobalt and gallium andtheir percentages in aqueous phase at R.T., (f ) band gap calculations, (g)Rietveld refinement results, (h) results of peak fitting of Raman spectra, and (i)digital images of the nonstoichiometric Co–Ga NPs prepared at 400 °C, pH 9and calcined CoGa2O4. See DOI: https://doi.org/10.1039/d3dt03086eaDepartment of Materials Process Engineering, Graduate School of Engineering,Nagoya University, Nagoya 464-8603, Japan.E-mail: xie.bo.d7@s.mail.nagoya-u.ac.jp, takami.seiichi@material.nagoya-u.ac.jpbDepartment of Chemistry, Graduate School of Science, Chiba University, Chiba 263-8522, JapancResearch Center for Materials Nanoarchitectonics, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0047, JapanThis journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 16285–16296 | 16285Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttp://rsc.li/daltonhttp://orcid.org/0000-0002-5254-1461http://orcid.org/0000-0002-4120-3109http://orcid.org/0000-0002-0645-6952http://orcid.org/0000-0002-3834-981Xhttps://doi.org/10.1039/d3dt03086ehttps://doi.org/10.1039/d3dt03086ehttp://crossmark.crossref.org/dialog/?doi=10.1039/d3dt03086e&domain=pdf&date_stamp=2023-11-11http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086ehttps://pubs.rsc.org/en/journals/journal/DThttps://pubs.rsc.org/en/journals/journal/DT?issueid=DT052044Apart from color manipulation, inorganic pigments withimproved material properties could be realized by producingtheir nanosized counterparts. In the field of painting, nano-scale ceramic pigments could show improved brilliance withthe help of a higher surface coverage and a higher number ofreflection points, as well as enhanced mechanical strengthowing to better dispersibility with binders.18 Meanwhile, nano-scale pigments also show negligible visible light scattering,and therefore, applications related to high transparency areexpected.19,20 Furthermore, nanopigments could also exhibitenhanced dispersibility in aqueous/organic solutions afterencapsulation and surface modification.21–23 In regard to thetraditional preparation of CoGa2O4, calcination at 600–1200 °Cfavors the production of CoGa2O4 with high crystallinity, andcolor manipulation from green to blue is easily achievable.However, the high synthesis temperature and long calcinationtime could result in not only unexpected agglomeration thatmakes nanosizing harder but also large energy consumption.To our knowledge, there are still no studies that have reportedthe synthesis of spinel-type metal oxides with different colorsat a fixed low temperature of approximately 300–500 °Cwithout calcination, which could favor the production of metaloxides with a small crystallite size, suppress agglomerationand serve the purpose of energy savings.In this work, we studied the supercritical hydrothermal syn-thesis of spinel-type cobalt gallate nanoparticles (Co–Ga NPs)with controlled color. Supercritical hydrothermal synthesiswas conducted at 400 °C without additional calcination in air.As seen in CoAl2O4,24,25 octahedrally coordinated Co3+ ions(Co3+–O6) could be generated from the oxidization process ofCo2+ to Co3+ at 400–600 °C, and they could hamper colormanipulation due to the exhibition of an intense greencolor.17,26,27 Therefore, we conducted supercritical hydro-thermal synthesis at 400 °C for 10 min, and we consider thatthe rapid synthesis time of 10 min could alleviate the previousoxidization process. Moreover, we also used a precursor solu-tion with a low Co/Ga molar ratio (0.25) to further reduce thegeneration of Co3+ during the synthesis. To achieve colormanipulation, we changed the pH of the precursor solution torealize different cationic configurations, as reported in our pre-vious work.28 We investigated the effect of the precursor solu-tion pH on the coloration, crystalline phase, composition, cat-ionic configurations and degrees of polyhedral distortions attetrahedral and octahedral sites of the products. Moreover, wealso discuss the growth process of prepared Co–Ga NPs bycomparing the local structure around cobalt atoms in the solidproducts obtained at 200 °C, 300 °C, and 400 °C.2. Experimental procedure2.1. Preparation of cobalt gallate NPsThe supercritical hydrothermal synthesis of Co–Ga NPs wasperformed as follows. Cobalt(II) nitrate hexahydrate(Co(NO3)2·6H2O, 99.5%) and gallium(III) nitrate n-hydrate(Ga(NO3)3·nH2O, n = 7–9, 99.9%) were purchased fromFUJIFILM Wako Pure Chemical Co. and used without furtherpurification. Co(NO3)2·6H2O and Ga(NO3)3·nH2O were dis-solved in pure water at concentrations of 0.025 mol L−1 and0.10 mol L−1, respectively. For pH control, an aqueous solutionof 1.0 mol L−1 NaOH was prepared using sodium hydroxide(NaOH, Kishida Chemical Co., Ltd, purity over 97.0%) andadded dropwise to adjust the pH of the reactant solution to 7,9, and 11. The added volume of NaOH for each condition wasrecorded, and solid precipitates with a pale pink color wereobserved at this moment. Prepared precursor solutions(1.5 mL) were poured into a pressure-resistant Hastelloy C-276reactor (inner volume of 5.0 mL). After being tightly capped,the reactor was placed in an electric furnace with a preheatedtemperature of 400 °C. After a synthesis time of 10 min, thereaction was terminated by removing the reactor and submer-ging it into a cold water bath. Temperatures inside the reactorduring the heating and cooling process were recorded (seeFig. S1 in the ESI†).2.2. Product collection and purificationAfter synthesis, solid products were collected from the reactantsolutions by centrifugation (4 °C, 12 000 rpm, 20 min) anddecantation and were freeze-dried (FDS-1000, Tokyo RikakikaiCo., Ltd) for X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and transmission electron microscopy(TEM) analyses.2.3. Product characterizationThe crystalline phase of the freeze-dried solid products wasanalyzed by AERIS (PANalytical) with Cu-Kα radiation operat-ing at 40 kV and 15 mA. XRD patterns were recorded in therange of 10° < 2θ < 70°. For SEM observation, a JSM-7610F(JEOL Ltd) with an accelerating voltage of 15 kV was used. ForTEM observation, a JEM-1400 Plus (JEOL Ltd) was used, andenergy-dispersive X-ray spectroscopy (EDS) was conducted toconfirm the elemental composition of the products.Fig. 1 Crystal structure of a partially inverse CoGa2O4(IV(Co1−xGax)VI[CoxGa2−x]O4) created by VESTA.14Paper Dalton Transactions16286 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086eThe Co/Ga molar ratios in the solid products weremeasured using inductively coupled plasma-atomic emissionspectroscopy (ICP-AES) analysis. The solid products were dis-solved in a mixed solution of nitric acid and hydrogen per-oxide in a volume ratio of 1 : 1. For solid products that werehard to dissolve, hydrochloric acid was added. Nitric acid(HNO3, weight fraction in the range of 60–61%), hydrochloricacid (HCl, weight fraction in the range of 35.0–37.0%), hydro-gen peroxide (H2O2, 30.0–35.5%), and standard solutions forICP-AES analysis were purchased from FUJIFILM Wako PureChemical Co. and used as received without any further purifi-cation. Next, an SPS7800 (Seiko Instruments) was used tomeasure the Co and Ga concentrations in the resultantaqueous solution. Additionally, we performed ICP-AES analysisto measure Ni, Cr and Fe along with Co and Ga in the solidproducts when taking the issue of contamination from thereactor into consideration. Moreover, the concentrations ofunreacted Co and Ga in the supernatant of the reactant solu-tions after synthesis were also measured by ICP-AES analysis,based on which the conversion of Co and Ga for each pH con-dition was calculated.To investigate the main valence states of metal ions andtheir cationic configurations in the solid products, Ga K-edgeand Co K-edge X-ray absorption fine structure (XAFS) spectrawere measured at BL-9A and BL-12C, Photon Factory, KEK,Japan, using the transmission method. Data processing of theX-ray absorption near-edge structure (XANES) region andextended X-ray absorption fine structure (EXAFS) region in theXAFS spectra was carried out with a data processing program,Athena.29 In particular, the absorption edges of the productswere determined as the photon energies at the maximum ofthe first derivative of the normalized μ(E) in the XANESregion.30 The cationic configurations of the products weredetermined by using a linear combination of k3-weighted χ(k)oscillations, and they were used as the initial cationic configur-ations in Rietveld refinement using RIETAN-FP.31 Specifically,the metal occupancy of Co atoms at tetrahedral and octahedralsites for each pH condition was quantitatively evaluated byconsidering that the measured k3-weighted χ(k) oscillations atthe Co K-edge of the products are a linear combination ofthose of IV(Co)VI[Ga2]O4 (normal spinel) and IV(Ga)VI[CoGa]O4(inverse spinel).28 The k3-weighted χ(k) oscillations ofIV(Co)VI[Ga2]O4 and IV(Ga)VI[CoGa]O4 at the Co K-edge weretheoretically calculated by Larch using all possible scatteringpaths, an amplitude reduction factor of 0.70, and a thermaland static disorder parameter σ2 of 0.01.32The diffuse reflectance spectra in the UV–Vis region of thesolid products were measured by a UV–Vis spectrophotometer(V-650, JASCO Co.) using an integrating sphere (SIV-767). Themeasurement range was 200–900 nm, and the scanning speedwas 100 nm min−1. A color compass MFA (AT System Co., Ltd)with a spectrometer (C12880MA, Hamamatsu Photonics K. K.,spectral response range of 340–850 nm) was used to measurethe CIE XYZ parameters of the products. Then, the CIE XYZparameters were converted to sRGB and RGB parameters usinga gamma correction of 2.4.The degree of distortions in the solid products was evalu-ated by Raman spectroscopy. Raman spectra were collectedusing inVia Reflex (RENISHAW). The excitation was achievedby using 532 nm green laser emission (J150GS, KYOCERA SOCCo.) The laser power output and irradiation time on thesample were set to 1%–5% and 30–300 s, respectively.3. Results and discussion3.1. Effects of precursor solution pH on the coloration,crystalline phase, and composition of solid productsThe coloration of the solid products prepared at each pH bythe present technique was investigated first. Fig. 2 showsdigital images of the solid products synthesized at each pH,together with their corresponding RGB parameters convertedfrom measured XYZ parameters in the CIE 1931 XYZ colorspace. The colors obtained at pH 7, pH 9, and pH 11 weregreenish-blue, blue, and yellowish-green, respectively.Therefore, solid products with tunable coloration were success-fully synthesized at a fixed temperature of 400 °C by supercriti-cal hydrothermal synthesis.To confirm whether the coloration change was due to thedifference in the crystal structure, we investigated the crystal-line phase of the products. Fig. 3 shows the measured XRDpatterns of the products. All of them agreed with that ofspinel-type CoGa2O4 (ICSD: 172183, Fd3̄m), and slight right-ward shifts were observed for all three pH conditions. The crys-tallite sizes were evaluated by the Williamson–Hall equation,as shown in Fig. S2 (ESI†), and the calculated crystallite sizesare summarized in Fig. 3. They were in the range of 10 nm to53 nm, which corresponded to the obtained narrowed fullwidth at half maximum (FWHM) values of the XRD patterns atpH 11. As shown in Fig. 3, there was a large difference in sizebetween NPs at pH 11 with a green color (53 nm) and those atpH 9 with a blue color (10 nm). To confirm whether the sizeeffect of the NPs could result in the changing color, we com-pared Co–Ga NPs prepared at pH 9 in this study to those pre-pared at pH 7 using a precursor solution with a high Co/Gamolar ratio (Co/Ga = 0.50) in our previous study.28 They havecomparable crystallite sizes (this study: 10 nm; previous study:13 nm) but a large difference in coloration. Therefore, we con-sider that the color difference between NPs prepared atdifferent pH in this study may not be caused by the size effect(see Fig. S3 in ESI†).The crystallite sizes of the Co–Ga NPs were further con-firmed by SEM and TEM observations. Fig. 4 shows SEM,TEM, and TEM-EDS images of the prepared Co–Ga NPs. TheSEM and TEM images indicated particle sizes comparable tothe calculated crystallite sizes and an octahedral shape of theCo–Ga NPs prepared at all pH conditions. Meanwhile, theTEM-EDS images showed a uniform element distribution ofGa and Co in the prepared Co–Ga NPs (Fig. 4(c) and Fig. S4 inESI†). As shown in Fig. 3 and 4, a larger crystallite size wasobtained at higher pH conditions, which can be explained bythe formation mechanism reported in our previous work.28 AsDalton Transactions PaperThis journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 16285–16296 | 16287Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086epreviously reported, the solid precipitates with a pale pinkcolor in the precursor solutions are a layered double hydrox-ide-like material. In this layered double hydroxide structure,both cobalt and gallium ions are in octahedral coordination.Meanwhile, these solid precipitates could act as a startingmaterial and finally form spinel-type cobalt gallate metaloxides during the temperature increase in supercritical hydro-thermal synthesis. Based on the previous discussion, we con-sider that the solid precipitates could show high stability in ahigh-pH environment, such as pH 11. Thus, the reaction rateof the formation from the layered double hydroxide-like start-ing material to the spinel-type cobalt gallate may decrease atpH 11, resulting in an increased crystallite size owing toimproved crystal growth.ICP-AES analysis was conducted to investigate the compo-sition of the Co–Ga NPs and the possibility of contamination.As shown in Fig. 5, the prepared Co–Ga NPs had nonstoichio-metric compositions, and the Co/Ga composition increasedfrom 0.282 to 0.728 with increasing pH. The Co/Ga molarratios of the NPs prepared at pH 9 and pH 11 greatly surpassedthose in the precursor solutions (Co/Ga = 0.25), especially theCo/Ga molar ratio at pH 11, which exceeded that in stoichio-metric CoGa2O4 (Co/Ga = 0.50). To explain the increasing non-stoichiometric compositions, we focused on the conversions ofcobalt and gallium and their percentages in the aqueousphase of the reactant solutions at room temperature (seeFig. S5 in ESI†). As shown in Fig. S5,† large amounts ofgallium were found to dissolve in the aqueous phase of thereactant solutions, especially at pH 11, and the percentageshowed the highest value of 0.931. Compared to gallium,cobalt remained in the solid phase of the reactant solutions atroom temperature, and the conversions were close to 1.0 at allpH conditions. Therefore, we suppose that gallium may preferto dissolve in the aqueous phase and attempt to remainunreacted during synthesis at higher pH. As a result, thedecreasing conversion of gallium contributed to the increasingCo/Ga composition from pH 7–11. Meanwhile, contaminationwas barely detected except at pH 7, where a nonnegligibleamount of Ni contamination was detected. These Ni2+ ionscould exist at the octahedral site and result in an unexpectedgreen hue, as shown in Fig. 2.33To investigate the main valence states of Ga and Co in theprepared nonstoichiometric Co–Ga NPs, normalized μ(E)values were calculated from the XAFS spectra in the XANESregion. Fig. 6(a) and (b) show the normalized μ(E), andFig. 6(c) and (d) show the first derivative of the normalizedμ(E) of our products, commercially available references, andCoGa2O4 produced by the solid-state reaction method using amixed powder of CoO and Ga2O3 at 1300 °C for 24 hours. Themaximum peak of the first derivative of the normalized μ(E) islabeled with a black circle, at which the photon energy wasdetermined as the absorption edge, as previously introduced.The Ga–K absorption edges of the Co–Ga NPs prepared at pH7, 9, and 11 were 10370.7 eV, 10370.4 eV, and 10370.4 eV,respectively, which were almost the same as those of β-Ga2O3Fig. 2 (a) Digital images and calculated corresponding RGB parameters (γ = 2.4) of the solid products produced at 400 °C using precursor solutionswith pH 7–11 and (b) measured XYZ parameters based on the CIE 1931 XYZ color space.Fig. 3 XRD patterns of the solid products produced at 400 °C usingprecursor solutions with pH 7–11.Paper Dalton Transactions16288 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086eand CoGa2O4 (10370.4 eV) (Fig. 6(c)). Meanwhile, the Co–Kabsorption edges of the Co–Ga NPs prepared at pH 7, 9, and11 were 7718.52 eV, 7718.53 eV, and 7718.59 eV, respectively,which were close to those of CoO and CoGa2O4 (7718.66 eVand 7718.53 eV) (Fig. 6(d)). Therefore, Ga3+ and Co2+ are con-sidered the main valence states of the prepared nonstoichio-metric Co–Ga NPs. Nevertheless, to further investigate thechemical condition of metal atoms in the products, photonenergies at the maximum of the white lines in the normalizedμ(E) measured at the Ga and Co–K edge of the products werealso compared to corresponding references (Fig. 6(a) and (b)).The maximum value of the white line for each pH condition atthe Ga K-edge was close to those of β-Ga2O3 and CoGa2O4(10375.4 eV), but that at the Co K-edge was slightly larger thanthose of CoO and CoGa2O4 (7723.80 eV and 7724.47 eV).Therefore, there could be a small amount of Co3+ existing inFig. 4 (a) SEM images and (b) TEM images of Co–Ga NPs produced at 400 °C using precursor solutions with pH 7–11, and (c) TEM-EDS resultsshowing the element distribution of the NPs produced at pH 11.Fig. 5 M/Ga (M = Co, Ni, Cr, and Fe) molar ratios of Co–Ga NPs pro-duced at 400 °C using precursor solutions with pH 7–11 measured byICP-AES.Fig. 6 The normalized μ(E) at the (a) Ga K-edge and (b) Co K-edge, firstderivative of the normalized μ(E) at the (c) Ga K-edge and (d) Co K-edgein the XANES region of nonstoichiometric Co–Ga NPs produced at400 °C using precursor solutions with pH 7–11 and corresponding refer-ences (@KEK PF BL-9A; the maximum of the while line of the normalizedμ(E), and that of the first derivative of the normalized μ(E) are labeledwith a black circle).Dalton Transactions PaperThis journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 16285–16296 | 16289Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086ethe Co–Ga NPs produced at pH 7 and pH 9 and more at pH 11.In summary, color-controlled nonstoichiometric Co–Ga NPswith little Co3+ were successfully synthesized by supercriticalhydrothermal synthesis at a fixed low temperature of 400 °Cwithout calcination when using precursor solutions with a lowCo/Ga molar ratio of 0.25 and pH 7–11.3.2. Investigations of the UV–Vis absorption spectra ofprepared nonstoichiometric Co–Ga NPsNonstoichiometric spinel-type Co–Ga NPs with Ga3+ and Co2+as the main valences and greenish-blue, blue, and yellowish-green colors were prepared at pH 7, 9, and 11, respectively. Tostudy the relationship between their nonstoichiometric com-position (Co/Ga molar ratio) and coloration, we explored thecoloration mechanism based on investigations of the UV–Visregion absorptions and cationic configurations. In thissection, we measured the diffuse reflectance spectra of thenonstoichiometric Co–Ga NPs. The Kubelka–Munk absorbancein the UV–Vis region is shown in Fig. 7.At pH 7 and pH 9, where products with blue hues wereobtained, evident triplet absorption was observed at 560 nm,600 nm, and 650 nm. We consider that the spin-allowed d–dtransition of ν1 [4A2(F) →4T1(P)] of Co2+–O4 could result in thetriplet absorption band based on reports by Llusar et al.34These three transitions could give rise to blue coloration owingto absorptions at 560 nm (green region), 600 (yellow-orangeregion), and 650 nm (red region).34 In addition, another broadabsorption peak was observed below approximately 400 nm.We consider that the charge transfer between the oxygenanion and Co2+ at the tetrahedral site (O2−–Co2+ (Tet.)) couldcontribute to this absorption.35 When compared to pH 7 andpH 9, the UV–Vis absorption spectrum at pH 11 showed amore intensive absorption at 400–500 nm and an area withlarger absorption at approximately 700 nm. We consider thatthe intensive absorption at 400–500 nm at pH 11 could also bedue to the charge transfer of O2−–Co2+ (Tet.), similar to pH 7and pH 9 but with a larger intensity. We will discuss ourassumptions on this significant increase in the absorbanceintensity at 400–500 nm later in section 3.4. In regard to thearea with a large absorption at approximately 700 nm, we con-sider that the charge transfer between the oxygen anion andCo3+ at the octahedral site (O2−–Co3+ (Oct.)) is contributablebased on previous studies.35,36 Note that three absorptionpeaks relating to ν1 [4A2(F) →4T1(P)] at pH 11 were not evident,which might be because of less Co2+–O4 compared to pH 7and pH 9. The band gaps of the prepared Co–Ga NPs were eval-uated by Tauc plot (see Fig. S6 in the ESI†). The band gaps atpH 7 and pH 9 were approximately 3.31 eV, which could con-tribute to the absorption at approximately 375 nm. Meanwhile,NPs prepared at pH 11 with the highest Co/Ga molar ratio(Co/Ga = 0.728) showed a much lower band gap of 2.54 eV.This band gap could cause absorption at approximately488 nm at pH 11.Based on previous investigations, we assume that therecould be more Co2+–O4 in the Co–Ga NPs prepared at pH 7and pH 9 than at pH 11 because of the exhibition of blue colorand the evident triplet absorption of ν1 at 560–650 nm.Meanwhile, there could be larger numbers of Co3+–O6 in theNPs prepared at pH 11 than at pH 7 and pH 9 because of theintensive charge transfer absorption at approximately 700 nm.Generally, the coloration change from pH 7–11 is consideredto be due to the change in the cationic configuration. The cat-ionic configuration will be quantitatively determined insection 3.3. Moreover, the effect of the polyhedral distortionsat the tetrahedral and octahedral sites on the coloration willbe discussed later in section 3.4.3.3. Investigations of the cationic configurations of preparednonstoichiometric Co–Ga NPsTo confirm our predictions on the cationic configuration insection 3.2, we quantitatively determined the occupancies ofcobalt and gallium atoms at tetrahedral and octahedral sitesin the products based on the XAFS measurement and Rietveldrefinement, as shown in Fig. 8. Specifically, Fig. 8(a) shows themeasured k3-weighted χ(k) oscillations at the Co K-edge of theprepared Co–Ga NPs. The cationic configurations at the tetra-hedral and octahedral sites were determined by fitting thesek3-weighted χ(k) curves. Fig. 8(b) shows the linear combinationwhen using k3-weighted χ(k) oscillations that were theoreticallycalculated for IV(Co)VI[Ga2]O4 (normal spinel) andIV(Ga)VI[CoGa]O4 (inverse spinel) arrangements. Fig. 8(b) alsoshows the corresponding coefficient of determination (R2factor) for each pH condition. Next, theoretically determinedcationic configurations were used as the initial cationic con-figuration required in the Rietveld refinement to calculate theXRD patterns of the products. The calculated XRD patternswere compared with the observed patterns for each pH con-dition to examine the reliability of the calculated cationic con-Fig. 7 UV–Vis absorption spectra converted from diffuse reflectancespectra of nonstoichiometric Co–Ga NPs produced at 400 °C using pre-cursor solutions with pH 7–11.Paper Dalton Transactions16290 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086efigurations. As a result, the R2 factor in Fig. 8(b) indicates agood fit of the linear combination for each pH, and the calcu-lated XRD patterns fit the observed patterns in the Rietveldrefinement well, as shown in Fig. 8(c) and Fig. S7 (ESI†). Insummary, the cationic configurations of the prepared nonstoi-chiometric Co–Ga NPs were quantitatively determined bylinear fitting of XAFS results using the Co K-edge k3-weightedχ(k) oscillations of IV(Co)VI[Ga2]O4 and IV(Ga)VI[CoGa]O4arrangements and were confirmed to have high reliability byRietveld refinement using XRD results.The theoretically determined cationic configurations andRietveld refinement results of the prepared nonstoichiometricCo–Ga NPs and those of the calcined stoichiometric CoGa2O4with a bright blue color determined by Naka et al.15 are sum-marized in Table S1 (ESI†). The cationic configurations aredepicted in Fig. 9 for a better comparison.Unlike what we assumed from the UV–Vis absorptionspectra, cobalt ions in the products mainly occupy the octa-hedral site even at pH 7 and pH 9, at which point productswith a blue hue were obtained. These results show that ourproducts were highly inverse spinel compounds. When focus-ing on the blue and black lines representing the Co and Gaoccupancies in the calcined CoGa2O4, our nonstoichiometricproducts have a completely different cationic configuration.That is, the cobalt occupancies are lower at the tetrahedral sitebut comparable or much higher at the octahedral site.As shown in Fig. 9, nonstoichiometric Co–Ga NPs producedat pH 11 with a yellowish-green color were found to have thelargest octahedral cobalt occupancy (approximately 0.603).Notably, the octahedral cobalt occupancy at pH 11 greatly sur-passed that in inverse CoGa2O4 (0.50). One of the possiblereasons is that a part of the Co ions existed as Co3+ at the octa-hedral site. To verify this, we focused on the photon energiesat the maximum of the XAFS white lines in the normalizedμ(E) measured at the Co–K edge (see Fig. 6(b)). The photonenergy at pH 11 was larger than those at pH 7 and pH 9, indi-cating the presence of more Co3+ ions. Therefore, we considerthat Co3+ ions may replace Ga3+ ions at the octahedral site asCo3+–O6 at pH 11. This resulted in a large octahedral cobaltoccupancy of 0.603 at pH 11 and an area with a large absorp-tion at approximately 700 nm owing to the charge transfer ofO2−–Co3+ (Oct.). Meanwhile, because the number of octahedralsites is 2 times that of tetrahedral sites in a spinel structure,this large octahedral cobalt occupancy resulted in a high Co/Ga molar ratio for NPs produced at pH 11, as shown in Fig. 5.The products obtained at pH 7 and pH 9, with an octahedralcobalt occupancy comparable to that of the calcined CoGa2O4,exhibited a blue hue. However, due to the lower tetrahedralcobalt occupancy, they showed a Co/Ga molar ratio lower thanthe stoichiometric value of 0.50.To explain why the cobalt occupancies of the prepared Co–Ga NPs were lower at the tetrahedral site but comparable ormuch higher at the octahedral site when compared to the cal-cined CoGa2O4, we focus again on the gallium ions in theaqueous phase of the reactant solutions, as mentioned in ourprevious discussion on the increasing nonstoichiometric com-positions of the products in section 3.2. We suppose thatgallium ions may prefer to dissolve in the aqueous phaseduring the initial temperature increase and be stable in tetra-hedral coordination, such as Ga(OH)4−. With a further temp-Fig. 8 (a) Measured k3-weighted χ(k) oscillations in the EXAFS region at the Co K-edge of nonstoichiometric Co–Ga NPs produced at 400 °C usingprecursor solutions with pH 7–11, (b) linear combination fitting results, and (c) result of Rietveld refinement using RIETAN-FP (pH 9, 400 °C).Fig. 9 Occupancies of Ga and Co at the tetrahedral and octahedralsites of nonstoichiometric Co–Ga NPs produced at 400 °C using pre-cursor solutions with pH 7–11 derived from the linear combination ofk3-weighted χ(k) oscillations at the Co K-edge (black and blue linesshow the Ga and Co occupancies in stoichiometric CoGa2O4 producedfrom a solid-state reaction15).Dalton Transactions PaperThis journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 16285–16296 | 16291Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086eerature increase, some of the Ga(OH)4− species becomeunstable and may prefer to incorporate into the tetrahedralsite with ease. Currently, many cobalt ions are considered toremain in the octahedral coordination because of the rapidreaction from the layered double hydroxide-like material to thespinel phase. Previous assumptions could result in a largecobalt octahedral occupancy (highly inverse spinel com-pounds) at all pH conditions.3.4. Investigations of the degree of distortions at tetrahedraland octahedral sites of prepared nonstoichiometric Co–GaNPsWhen transition metal ions exist in distorted polyhedrons withless symmetric geometries compared to regular octahedral ortetrahedral geometries, they may have different absorption inthe visible region, i.e., produce different colors.37 In this study,the polyhedral distortions in the prepared NPs may be inducedby the Jahn–Teller effect of the transition metal ions (Co2+,Co3+) and may also be due to the synthesis conditions.38 Thesepolyhedral distortions may affect the absorptions of Co2+–O4and Co2+/3+–O6 in the prepared Co–Ga NPs and further affectthe coloration of the NPs. Therefore, we simply evaluated thedegrees of the polyhedral distortions at tetrahedral and octa-hedral sites in the spinel lattice by using Raman spectroscopy.Raman spectra in the range of 150–850 nm of the preparedNPs and reference of calcined CoGa2O4 are shown inFig. 10(a). Because CoGa2O4 belongs to the Fd3̄m space group,we theoretically considered five Raman active modes (A1g + Eg+ 3F2g) and assigned them to the observed intensive peaks ofcalcined CoGa2O4.39 Notably, the presence of an extra peakmarked with an asterisk (A*1g) was considered to be due to thedegree of inversion.39,40 Compared to the calcined CoGa2O4,the Raman spectra of the prepared Co–Ga NPs generally showbroader peaks, and peaks such as Eg and F2g (2) were notevident. In particular, the increase in the breadth of theobserved peaks in our products was considered the result ofpolyhedral distortions within the spinel lattice.41 In Fig. 10(a),Raman spectra of our NPs also showed peak shifts comparedto the calcined CoGa2O4. We consider that polyhedral distor-tions could affect the bond lengths of Co–O and Ga–O andresult in a shift in the peak center. In this study, we focusedon the full width at half maximum (FWHM), that is, thebreadth of the peaks of F2g (1) and A*1g in the Raman spectra,to investigate tetrahedral distortions and the FWHM of A1g toinvestigate octahedral distortions of the NPs. Multiple peakfitting was conducted for the calcined CoGa2O4 and each pHcondition, as shown in Fig. 10(b–e). The FWHM results as wellas the peak location of each fitted peak are summarized inTable S2 (ESI†). Notably, the peak of F2g (3) was also intense inour products. However, this peak is considered to be associ-ated with vibrations of oxygen atoms at both tetrahedral andoctahedral sites and was not discussed in this study.42According to reports by Bouchard and Gambardella, the A1gpeak is characteristic of vibrations involving the motion ofoxygen atoms at octahedral sites, and the F2g (1) peak is associ-ated with the motion of oxygen atoms at tetrahedral sites.41Previous reports are on the study of a normal spinel ofCoAl2O4, and we referred to those results in our study ofCoGa2O4. However, the prepared Co–Ga NPs and the calcinedCoGa2O4 are spinel compounds with inversions, and they havethe additional characteristic peak of A*1g. We assigned A*1g tostretching vibrations involving oxygen atoms at tetrahedralsites.39 When focusing on the FWHM of the peaks of F2g (1),A1g, and A*1g, NPs prepared at pH 9 showed the largest FWHMFig. 10 (a) Raman spectra of nonstoichiometric Co–Ga NPs produced at 400 °C using precursor solutions with pH 7–11 and calcined CoGa2O4(1300 °C, 24 h) and their (b–e) peak fitting results.Paper Dalton Transactions16292 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086evalue at F2g (1) and A*1g and those at pH 11 showed the largestFWHM value at A1g (see Table S2 in ESI†). Therefore, therecould be large polyhedral distortions existing at the tetrahedralsite at pH 9 and at the octahedral site at pH 11. Nevertheless,NPs prepared at pH 7 also showed a considerable degree of dis-tortion at the tetrahedral site when focusing on the largeFWHM at A*1g.Next, we correlated the degree of polyhedral distortions tothe UV–Vis absorption spectra of the prepared Co–Ga NPs. AtpH 7 and pH 9, the large degrees of polyhedral distortions atthe tetrahedral site might decrease the crystal field strength(10Dq) and may further intensify the triplet absorption bandof Co2+–O4, as reported by Raj and Rao.43 As a result, evidenttriplet absorption at 560–650 nm and a blue hue were observedfrom the NPs at pH 7 and pH 9 despite the low tetrahedralcobalt occupancy. At pH 11, where there could be more Co2+/3+–O6, we consider that the large degree of distortions at theoctahedral site might alleviate the effect of Laporte–forbiddentransitions of the octahedral complexes (Co2+/3+–O6) due toasymmetric octahedrons.17 In our work, octahedral distortionsmight intensify the transitions of ν2 [1A1g → 1T2g] of Co3+–O6.This ν2 transition was observed at 430 nm in Co3O4 spinelcompounds, which have a high content of Co3+–O6 similar tothe NPs at pH 11 in this work.44 We also consider that thelarge absorption at 400–500 nm might also contain intensifiedabsorptions of transitions ν3 [4T1g(F) →2A1g(G)] and ν4 [4T1g(F)→ 4T1g(P)] of Co2+–O6. They were observed at 470 nm and510 nm in an alumina-supported cobalt system.45 In summary,all those intensified possible absorptions together with theabsorption at 488 nm induced by the band gap absorption atpH 11 might finally cause a significant increase in the absor-bance intensity at 400–500 nm when compared to pH 7 andpH 9 and result in an intensive green hue in the NPs preparedat pH 11.In summary, color-controlled nonstoichiometric Co–Ga NPsprepared by the present technique were found to havedifferent cationic configurations at tetrahedral and octahedralsites compared to the calcined stoichiometric CoGa2O4.Prepared NPs were also found to have considerable degrees ofpolyhedral distortions. Based on previous discussions of themain valences, cationic configurations, and degrees of distor-tions at the tetrahedral and octahedral sites, the relationshipbetween the prepared Co–Ga NPs and their colorations is sum-marized in Table 1.3.5. Growth process of prepared nonstoichiometric Co–GaNPsDurable inorganic blue color materials have continuouslyattracted attention from human beings for a long time.46,47 Inthis work, we successfully prepared nonstoichiometric Co–GaNPs with high crystallinity and a blue hue at a fixed low temp-erature of 400 °C without calcination but with less brilliancethan calcined stoichiometric CoGa2O4, as shown in Fig. S8(ESI†). We consider this result to be due to a lower tetrahedralcobalt occupancy that could result in weaker triplet absorptionof the previously introduced spin-allowed transition of Co2+–O4. To find synthesis conditions that could allow more Co2+–O4, i.e., realizing a more brilliant blue color, we investigatedthe growth process in the present technique. Specifically, weconducted synthesis at 200 °C and 300 °C using the same pre-cursor solution (Co/Ga = 0.25) and measured the XAFS spectraat the Co K-edge of the solid particles prepared at each pH con-dition. Next, Fourier transforms of the k3-weighted χ(k) oscil-lations in the EXAFS region of the products were derived. Weinvestigated the growth process and discussed the strategy fora more brilliant blue color using the present technique bycomparing the local structures (Fourier transforms) of thesolid products produced at 200 °C, 300 °C, and 400 °C withthat of calcined CoGa2O4.As shown in Fig. 11, the Fourier transforms of the k3-weighted χ(k) oscillations of most solid products and the cal-cined CoGa2O4 generally showed an intensive peak at 1.5 Åand two intensive peaks ranging from 2.2 to 3.5 Å. Thesepeaks correspond to Co–O interactions and interactionsbetween the metal atoms of MOct.–MOct. (2.2–3.0 Å) and MTet.–MOct./Tet. (3.0–3.5 Å), respectively. When focusing on the cal-Table 1 Relationship between the nonstoichiometric composition (Co/Ga) of Co–Ga NPs produced at 400 °C using precursor solutions with pH7–11 and their colorationpH 7 pH 9 pH 11Composition (Co/Ga molar ratio) 0.282 0.423 0.728Cobalt valence Co2+ as the main valence Co2+ as the mainvalenceCo2+ and Co3+Tetrahedral cobalt occupancy (0.336 forcalcined CoGa2O415)0.115 (<0.336) 0.129 (<0.336) 0.0540 (<0.336)Octahedral cobalt occupancy (0.332 forcalcined CoGa2O415)0.273 (≈0.332) 0.382 (≈0.332) 0.603 (>0.332)Number of octahedrally coordinated Co3+ions (Co3+–O6)Small Small LargeDistortions at tetrahedral sites Large than calcined CoGa2O4 (whenfocusing on A*1g)The largest Smaller than calcined CoGa2O4(when focusing on A*1g)Distortions at octahedral sites Larger than calcined CoGa2O4 Larger than calcinedCoGa2O4The largestContamination Nonnegligible Ni2+ ions Almost none Almost noneColoration Greenish-blue Blue Yellowish-greenDalton Transactions PaperThis journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 16285–16296 | 16293Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086ecined CoGa2O4 (dotted line) with a brilliant blue color, there isnearly no difference in height between the two intense peaksranging from 2.2 to 3.5 Å. Therefore, to prepare Co–Ga NPsexhibiting an intense blue color, the difference between thetwo peaks from 2.2 to 3.5 Å needs to be as small as possible.This difference was smaller at pH 7, larger at pH 9, andmuch larger at pH 11. This result indicates that cobalt ions showhigher selectivity for the octahedral site during the growthprocess at higher pH. At pH 9, the differences between the twopeaks at 2.2–3.5 Å significantly decreased with increasing syn-thesis temperature. This indicates the possibility of producingCo–Ga NPs with a more brilliant blue color when using a highersynthesis temperature in the present technique, even for an alka-line environment of pH 9. Unlike pH 9, the difference betweenthe two peaks changed only slightly at a high synthesis tempera-ture when using the precursor solution of pH 11. We considerthis result to probably be due to an extremely high octahedral siteselectivity of cobalt ions at pH 11. Therefore, Co–Ga NPs with amore brilliant blue coloration could be realized in an acidic syn-thesis environment or in an alkaline synthesis environment (pH< 11) when using a synthesis temperature higher than 400 °C inthe present technique.4. ConclusionsIn this research, color-controlled nonstoichiometric spinel-type cobalt gallate nanopigments were realized by a rapidsupercritical hydrothermal synthesis at 400 °C for 10 minwithout calcination when using a precursor solution with alow Co/Ga molar ratio (0.25) and different pH values. To sum-marize, the prepared Co–Ga NPs exhibited a higher Co/Gamolar ratio (0.282–0.728), a larger amount of Co3+, a largeroctahedral cobalt occupancy (0.273–0.603), and a larger degreeof octahedral distortions as the precursor solution pHincreased. Generally, the color manipulation from greenish-blue to yellowish-green is probably due to an increasingamount of Co2+/3+–O6 and an increasing degree of distortionsat the octahedral site in the spinel structure at higher pH. Inparticular, nonstoichiometric Co–Ga NPs with a blue color (R:80, G: 150, B: 185) were successfully synthesized without calci-nation for the first time and were found to have a large degreeof tetrahedral distortions. Additionally, investigations of thegrowth process of Co–Ga NPs during temperature increaseindicated the possibility of producing Co–Ga NPs with a morebrilliant blue color when using a higher synthesis temperaturein the present technique. Our synthesis method has meritssuch as a lower synthesis temperature, a shorter synthesistime, use of inexpensive metal nitrate as raw materials, andless use of cobalt compared to traditional calcinationmethods. We believe that the present technique could lead toenergy-efficient and environmentally friendly production ofcoloristic cobalt gallate nanopigments in the future.Author contributionsBo Xie: Conceptualization, formal analysis, investigation(material production and XRD, SEM, ICP-AES, Raman spec-troscopy and XAFS measurements), validation, visualization,writing – original draft, writing – review & editing. ChiyaNumako: Formal analysis, investigation (XAFS measurements),resources, validation, writing – review & editing. Takashi Naka:Formal analysis, investigation (material production of calcinedCoGa2O4), resources, validation, writing – review & editing.Seiichi Takami: Conceptualization, funding acquisition, inves-tigation (XAFS measurements), formal analysis, project admin-istration, resources, supervision, validation, writing – review &editing.Conflicts of interestThe authors declare no competing financial interests.Fig. 11 Fourier transforms of the k3-weighted χ(k) oscillations in the EXAFS region at the Co K-edge (@KEK PF BL-12C) of stoichiometric CoGa2O4prepared by calcination at 1300 °C for 24 h (dotted line) and solid products prepared at 200 °C, 300 °C, and 400 °C using precursor solutions with(a) pH 7, (b) pH 9, and (c) pH 11.Paper Dalton Transactions16294 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086eAcknowledgementsThis work was supported by JSPS KAKENHI Grant Numbers20H02514 and 23H01752 and was partially supported by theWorld Premier International Research Center Initiative (WPI),MEXT, Japan. XAFS measurements were conducted under theapproval of the Photon Factory Program Advisory Committee(Proposal No. 2021G143, 2021G584). We acknowledge theDivision for Medical Research Engineering, Graduate Schoolof Medicine, Nagoya University for usage of the JSM-7610F andJEM-1400 Plus and corresponding technical support from MrKoji Itakura.References1 V. D’Ippolito, G. B. Andreozzi, U. Hålenius, H. Skogby,K. Hametner and D. Günther, Phys. Chem. Miner., 2015, 42,431.2 B. Serment, M. Gaudon, A. Demourgues, A. Noël, G. Fleury,E. Cloutet, G. 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Subramanian, ACS Omega, 2019,4, 22114.Paper Dalton Transactions16296 | Dalton Trans., 2023, 52, 16285–16296 This journal is © The Royal Society of Chemistry 2023Open Access Article. Published on 15 October 2023. Downloaded on 12/7/2023 4:32:37 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3dt03086e Button 1: