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## Creator

[Takayuki Nakane](https://orcid.org/0000-0003-0282-169X), [Takashi Naka](https://orcid.org/0000-0002-0645-6952), Minako Nakayama, [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781)

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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10853-023-08866-w[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Direct bottom-up synthesis of ZnAl2O4 nanoparticle via organic ligand dissolution method](https://mdr.nims.go.jp/datasets/e6c9e3df-d8ae-4e0b-82c0-7f4862ebf8b8)

## Fulltext

Direct Bottom-Up Synthesis of ZnAl2O4 Nanoparticle via Organic Ligand Dissolution Method  Takayuki Nakane1, Takashi Naka2, Minako Nakayama2 and Tetsuo Uchikoshi1  1. National Institute for Materials Science, Research Center for Electronic and Optical Materials, 1-2-1, Sengen, Tsukuba, 305-0047 Ibaraki, Japan 2. National Institute for Materials Science, Research Center for Materials Nanoarchitectonics (MANA), 1-1, Namiki, Tsukuba, 305-0044 Ibaraki, Japan  This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://dx.doi.org/10.1007/s10853-023-08866-w  Corresponding Author: Takayuki Nakane NAKANE.Takayuki@nims.go.jp   https://www.springernature.com/gp/open-research/policies/accepted-manuscript-terms   Abstract    This study improves the hydrothermal synthesis of ZnAl2O4 to realize direct bottom-up chemical synthesis from a liquid precursor solution. The liquid solution was prepared by the organic ligand dissolution (OLD) method. Synthesized ZnAl2O4 was identified as a single phase comprising ZnAl2O4 with organic components speculated to work as surfactants that aid in size stabilization. The synthesized ZnAl2O4 product was observed to be in nanoparticle form, exhibiting a wide-bandgap attribute to the quantum size effect. The growth rate of ZnAl2O4 nanoparticles in the proposed method is low, and a series of results revealed the phase formation process of synthesized ZnAl2O4. This formation process seems common for chemically synthesized ZnAl2O4 and indicates the importance of compositional analysis in the study for chemical synthesis of this material. The application of the OLD method enables us to synthesize ZnAl2O4 across a wide pH range (3 ~ 11), and it is applicable to the continuous synthesis using a flow-type reaction system of hydrothermal reaction. Moreover, our technique is basically applicable to the synthesis of other spinel oxide. These characteristics of the OLD method are expected to extensively improve the investigation of chemical processing of spinel oxides.  Keywords  Hydrothermal Synthesis, Chemical Synthesis, Nanoparticle, ZnAl2O4, Spinel     Introduction ZnAl2O4 is an important spinel oxide. This material consists of ubiquitous, inexpensive, and nontoxic elements that have low environmental loads, and it shows excellent chemical and thermal stability. From the viewpoint of the electric structure, a wide band-gap of over 3.8 eV gives some attractive characteristics [1,2]. Thus, this material is being actively investigated for its optical properties [3-5], its application to sensing devices [6,7], and its catalytic performance [8-10]. In particular, the surface state of this material is similar to that of ZnO or γ-Al2O3, hence this material itself also shows an excellent catalytic properties. For instance, the photocatalytic properties of ZnAl2O4 have been utilized for the decomposition of dye [8,11-13] or some pollutants [14,15]. The catalytic performance of ZnAl2O4 has also been reported in various applications, including the decomposition of the steam during the ethanol reforming [9], the combustion of soot in a NO2/O2 atmosphere [10], and the combustion of iso-butane [11]. These pioneering reports for ZnAl2O4 stimulated us to further investigate its potential for industrial application. From this viewpoint, the development of a chemical processing technique for ZnAl2O4 is an important issue, since the cost of production will be a key in its utilization. In general, the synthesis temperatures of aluminum compounds are relatively high, because aluminum and/or oxides require high energy to react with the other materials. This is one reason why it is difficult to establish a chemical synthesis technique for ZnAl2O4. Chemical synthesis of ZnAl2O4 is generally conducted by a technique based on the sol-gel method [9,13-18] or the Pechini method [19]. The former chemically prepares a   layered-double-hydroxide (LDH) precursor involving Zn2+ and Al3+ by mixing these salts in solution. The later dissolves Zn2+ and Al3+ salts in water separately, and the resulting solutions are mixed [7]. These chemical synthesis techniques relatively reduces the heating temperature compared to that in the solid-state reaction and help to improve the homogeneity of the product. However, almost all techniques use water-insoluble precipitants or suspensions as precursors [7-22], and thus we cannot expect size homogeneity based on the bottom-up creation of nano crystalline. Moreover, these chemically prepared precursors often require after-sintering in the furnace, similar to a solid-state reaction, and the temperature is higher than 500 oC [7-14,17,18,21]. This kind of synthesis route was improved recently by the application of microwave heating techniques [11,20,21]. These techniques do not require heating to such high temperature, but they still require water-insoluble precursors. Water-insoluble precursors are not convenient, however, especially in industrial applications entailing mass synthesis. Thus, a direct bottom-up synthesis technique from a liquid precursor solution is desired for the fabrication of ZnAl2O4. In addition, a chemical synthesis technique for ceramic oxides like ZnAl2O4 commonly requires alkaline solution, because oxide produced from acidic solution often involves numerous deficiencies. Therefore, preparing a liquid precursor solution that can be used in alkaline region is considered a meaningful challenge for promoting the industrial application of ZnAl2O4. Both Zn and Al contain some water-soluble compounds, but their coexistence in alkaline solution is not easy to achieve. Both Zn and Al are known as amphoteric elements, but they can solve as metal ions in   acidic solution and as complex ions in alkaline one. This difference decreases the solubility of these ions in the neutral pH region. In fact, water-soluble compounds of Zn2+ and Al3+ basically decrease the pH when they are solved in water. However, pH tuning of the solution from the acidic to the alkaline one is quite difficult and yields a lot of precipitation of hydroxides. Hence, almost all studies for the chemical synthesis of ZnAl2O4 used precipitant such as LDH, which is obtained in alkaline solution, as a precursor [7-22]. To address this worrisome issue, we were inspired to use organic acid salt. The addition of organic acid salt decreases the pH of water, thus the solubility of Zn2+ and that of Al3+ are speculated to be high in this solution. Even so, we can expect that both Zn2+ and Al3+ dissolve in this solution as complex ions with ligands originated in the organic acid. These complex ions are expected to be stable in alkaline solution even though the pH of the solution is increased by adding some bases. That is, the addition of organic acid salt to a solution of Zn2+ and Al3+ is considered to provide a liquid precursor solution usable across a wide pH range. Based on this concept, we here successfully synthesized ZnAl2O4 by applying a hydrothermal technique using a liquid precursor solution of organic acid with Zn2+ and Al3+ salts. This preparing technique of a liquid precursor solution is named as organic-ligand dissolution (OLD) method. This technique is basically applicable to the synthesis of other spinel oxides. Therefore, we believe that the OLD method can extensively contribute to the investigations into the application for chemical processing of the spinel oxides.    Experimental  ZnAl2O4 samples were synthesized in nanoparticle form by applying a hydrothermal technique using a liquid precursor solution. First, Zn(CH3COO)2･2H2O (99.9 %: FUJIFILM Wako Pure Chemical: Japan), Al2O(CH3COO)3･nH2O (14.8 ~ 18.0 % as Al: Kanto Chemical: Japan) and citric acid (98.0 %: FUJIFILM Wako Pure Chemical: Japan) as the organic components of the OLD method were dissolved in distilled water and mixed for over 3 days. This long mixing process made a liquid precursor solution from the translucent to the transparent. The mixing ratio was w : 1 : 2 of Zn(CH3COO)2･6H2O, Al2O(CH3COO)3･nH2O and citric acid with the concentration of the expected product (ZnwAl2O4) controlled at 0.05 M. Second, the pH of this transparent precursor solution was adjusted by adding ammonia solution (28%: Kanto Chemical, Japan), and its value was checked by using pH indicators (MN92118, MN92120 and MN92125: Macherey-Nagel: Germany). The amount of ammonia solution was controlled in the range of 0 ~ 4 ml as appropriate. Third, hydrothermal synthesis was performed for this pH-tuned precursor solution in a PTFE vessel including 15% carbon fiber (HUTc-25: SAN-AI Kagaku, Japan) enclosed in a stainless steel container (HU-25: SAN-AI Kagaku: Japan). This container was set in a furnace at a constant heating temperature in the range of 200 ~ 260 °C, and the pressure in the PTFE vessel was controlled to 10 MPa. The container was quenched in a water bath after hydrothermal synthesis for 2 ~ 24 hours. Fourth, synthesized ZnAl2O4 nanoparticles in the vessel were refined in the solution with 1ml of H2SO4 and 40 ml   of distilled water twice to remove excess ZnO impurity by a combination of centrifugation and decantation. Fifth, the refined nanoparticles were washed in distilled water twice by the same procedure used above for refinement. The obtained product ZnAl2O4 nanoparticles were dried under vacuum. As a reference, this study used ZnAl2O4 powder fabricated by a solid-state reaction. The fabrication and the characterization procedure of this powder are described elsewhere [23].  The phase contents, lattice constants, and average crystalline sizes of these nanoparticles were examined using a powder X-ray diffractometer (XRD, mini-Flex: Rigaku: Japan). The crystalline sizes were estimated from the full width at half maximum (FWHM) of the diffraction peaks by employing the Scherer equation [24,25]. The composition of the product ZnAl2O4 nanoparticles was evaluated using an inductively coupled plasma optical emission spectrometer (ICP-OES, 720-ES ICP-OES: Agilent Technologies: USA). The carbon amount in the product ZnAl2O4 nanoparticles was measured by the combustion infrared absorption method (CS444-LC: LECO Japan: Japan). On the other hand, optical absorbance for each sample was measured using a powder diffuse reflectance ultraviolet-visible spectroscopy (UV-Vis, V-650: JASCO Global: Japan) to discuss the bandgap of the ZnAl2O4 nanoparticles and by using a fourier transform infrared spectroscopy (FT-IR, FT/IR-6200: JASCO Global: Japan) for the samples in KBr pellets to obtain information on the residual organic components in the sample. For discussing these optical spectra, UV-VIS and FT-IR spectra were converted into the absorption spectra using the Kubelka-Munk equation [26]. Additionally, we simulated UV-VIS spectra using CASTEP [27,28]   for further comparison. The crystal structural data of ZnAl2O4 for CASTEP were previously obtained for the reference sample by using synchrotron XRD and Rietveld analysis [23,29,30]. For the calculation, we used OTFG ultrasoft as the pseudopotential, a plane wave basis set with a cutoff energy of 571.4 eV, and 555 as the K-point set, and we performed a full cell optimization. On the other hand, the morphology of the ZnAl2O4 nanoparticles was observed by transmission electron microscopy (TEM, JEM-2000F: Japan Electron: Japan) with an accelerating voltage of 200 kV.  Results and Discussion  ZnAl2O4 nanoparticles were successfully fabricated by hydrothermal synthesis via the OLD method. Typical XRD patterns of ZnAl2O4 nanoparticle are shown in Figure 1. The figure compares the effect of the heating time for the precursor solution of w = 1.0 and pH = 10.5. The precursor solution was a transparent liquid without precipitants. That is, this figure demonstrates the success of our proposed synthesis route for ZnAl2O4 from a liquid precursor solution usable in the alkaline region. In this case, heating temperature is an important parameter for obtaining the ZnAl2O4 phase as the product, since the XRD pattern of the sample heated at 200 oC is assigned as boehmite. On the other hand, the FWHM of each XRD pattern implies that long duration of hydrothermal synthesis promotes the growth of the particle size. As the next step, the influence of the pH of the precursor solution (w = 1.0) on the product was verified by reducing the amount of ammonia solution. Figure 2 shows the results. The precursor solutions were all   transparent liquid without precipitants, indicating that liquid precursor solution of the OLD method is usable across a wide pH range (3.0 ~ 11). Interestingly, the XRD pattern of ZnAl2O4 shows sharp peaks in the case of the product from the precursor of pH = 11. These peaks seem to originate in the large particle size of ZnAl2O4, since excess ammonium water suppresses the citric acid working as the surfactant to the generated nanoparticles. Figure 3 focuses on the 440 peak of the XRD patterns plotted in Figure 2. We can find differences in FWHM between the products made from the acidic precursor (pH < 3.5), from the neutral one (5 < pH < 10), and from the alkaline one (pH > 10.5).  10 20 30 40 50 60 702θ / degreesIntensity(A.U.)Figure 124 h100 h220400311422 51144033112 h2 h200 oC, 20h* Boehmite** ****Fig. 1  Heating time dependence of the XRD patterns of ZnAl2O4 products synthesized at 260 oC in a hydrothermal atmosphere for the precursor of w = 1.00 and pH  10.5. For comparison, the bottom data in green line shows XRD pattern of the product heated at 200 oC for 20 hours.   Figure 4 (a) plots the carbon amounts of ZnAl2O4 nanoparticles shown in Figure 2 against the pH of the 10 20 30 40 50 60 702θ / degreesIntensity(A.U.)Figure 2PH  10.5PH  11PH  9.5PH  7PH  5PH  3.5PH  3220400311422 511 44033160 62 64 66 68 70Figure 32θ (degrees)PH  3PH  3.5PH  5440PH  7PH  9.5PH  10.5Intensity(A.U.)Fig. 3   Comparison of the FWHM of the 440 peak in the XRD pattern of product ZnAl2O4 nanoparticles to reveal the influence of the precursor pH with w = 1.00. The hydrothermal synthesis conditions were all 260 oC for 24 hours. Fig. 2     Influence of pH on the XRD pattern of product ZnAl2O4 nanoparticles made from precursor with w = 1.00. The hydrothermal synthesis conditions in all cases were 260 oC for 24 hours.   precursor solution. This result is consistent with the speculation about citric acid working as the surfactant. Increasing the pH of the precursor solution indicates the sum titration of citric acid by ammonium water, so, it is considered to decrease the coupling activity as the surfactant. Thus, increasing the pH results in growth in both the size and surface area of ZnAl2O4 nanoparticles in the region of pH < 7. Probably, the coupling activity of citric acid as the surfactant saturates (or deteriorates) in the region of pH > 7, and the amount of the surfactant does not correspond to the increase in the surface area of ZnAl2O4 nanoparticles.  Although the mechanism underlying this relationship between pH and the carbon amount in ZnAl2O4 nanoparticles remains to be clarified, above discussion implies that the pH of the liquid precursor solution 2 3 4 5 6 7 8 9 10 11 120.700.750.800.850.900.951.000.60.91.21.51.82.12.4Figure 4pHCarbonamount (mass%)x(a)(b)Fig. 4    Dependence of (a) carbon amount and (b) relative Zn amount expressed as x against Al amount defined as 2 on pH of the precursor with w = 1.00 for the product ZnAl2O4 nanoparticles synthesized at 260 oC for 24 hours.   influences the quality of the product ZnAl2O4 nanoparticles. This point is further verified by the result of ICP-OES. Experimental data of ICP-OES indicate the change in the molar ratio of Zn and Al in ZnAl2O4 nanoparticles made from a precursor with different pH values. Here, the elemental ratios (i.e., Al/Zn) of all results were higher than 2 (i.e., they were Al-rich) even though the reference data measured for the ZnAl2O4 fabricated by a solid-state reaction shows reasonable value (i.e.,  2). Considering the phase stability of the spinal structure and the charge valance of ZnAl2O4, it is difficult to imagine an extremely deficient composition such as Zn0.5Al1.15O2.225. Consequently, we fix the Al content to 2.0 and calculate the x of ZnxAl2O3+x/2 composition from the raw Al/Zn data obtained by ICP-OES measurement. Figure 4 (b) plots the x value against the pH of the precursor solution. Figures 4 (a) and (b) show similar curvatures. These results at least indicate the incompleteness of the product nanoparticle as ZnAl2O4 with respect to the chemical composition, and the neutral region of pH  7 seems to be the best for obtaining typical ZnAl2O4. Therefore, we verify the influence of the mixing process’s w parameter, which defines the ZnwAl2O4 composition in the precursor solution, on the x in the resulting product nanoparticle’s ZnxAl2O3+x/2 composition. Here, the pH of the precursor solution was fixed at pH  7. Figure 5 plots the variation of x in the nanoparticle made from the precursor solution at pH  7 with different compositions (1.00 ≤ w ≤ 2.50) and compares the impact of hydrothermal synthesis duration on the final product’s x value. The figure indicates that hydrothermal synthesis using the precursor of the nominal initial composition (i.e., w = 1.0) tends to yield Zn-deficient ZnAl2O4 nanoparticles. Increasing the w value can modify this, but excess   preparation (w ≥ 1.75) does not promise to overcome the Zn deficiency. To control the chemical composition of the final product ZnAl2O4 nanop  articles, the important parameter is the duration of the hydrothermal synthesis rather than the w of the precursor solution. Figure 6 plots the x of the nanoparticles made from the precursor solution with pH  7 under different synthesis times. The result directly indicates the importance of the duration of hydrothermal synthesis via the OLD method; it should be longer than 1000 minutes ( 16.7 h) for obtaining ZnAl2O4 nanoparticles with the appropriate chemical composition. This means that ZnxAl2O3+2/x with a deficient Zn2+ is incorporated during the hydrothermal process. Then, Figure 1 shows that low-temperature synthesis yields boehmite as the product powder. Furthermore, our background experiments reveal that our synthesis condition cannot yield γ-Al2O3 (spinel structure) in the case of the precursor solution without Zn salt; i.e., the existence of Zn2+ in the precursor solution is 1.00 1.25 1.50 1.75 2.00 2.25 2.500.40.60.81.01.2Figure 5w6 h24 hx2 hFig. 5   Relationship between the relative Zn amount (expressed as x) against the Al amount (defined as 2) in the prepared composition (expressed as w) of the product ZnAl2O4 nanoparticle. Hydrothermal synthesis was performed for the same precursor with w = 2.00 and pH  7, with heating at 250 oC. The results were differentiated and represented using different plotting colors to indicate the varying heating times. Green, red, and blue represent the product heated for 2, 6, and 24 hours, respectively.   important for the formation of the spinel oxide under the condition of our hydrothermal synthesis. From these data, we speculated that the phase formation process of ZnAl2O4 nanoparticles by hydrothermal synthesis via the OLD method proceeds as follows.  AlOOH (boehmite)  ZnxAl2O3+2/x (deficient spinel)  ZnAl2O4   Of course, this speculation simplifies the compositional change, and the real process is thought to be more complicated. For instance, we should mind the possibility of (Zn,Al)OOH. A deficiency of Al3+ at each process has not yet been excluded as a possibility. In addition, we should also mind the occurrence of the site exchange phenomenon between Zn2+ and Al3+ in the case of the spinel aluminate composition. However, our speculation is essentially similar to the actual phase formation process of ZnAl2O4 nanoparticles during hydrothermal synthesis using the OLD method. Figure 6Time (min)x100 200 600 1000 20000.70.80.91.01.1Fig. 6   Heating time dependence of the relative Zn amount expressed as x of the product ZnAl2O4 nanoparticle made from the precursor with w = 2.00 and pH  7. Hydrothermal synthesis was performed at 250 oC.   In the next step, we compared the UV-Vis spectra of our product ZnAl2O4 nanoparticles with those of a sintered reference sample. Additionally, we simulated ZnAl2O4 spectra using CASTEP [27,28] for further comparison. Figure 7 compares these three spectra. There are some differences between the spectra of the sintered reference sample and that of the simulation, but they are essentially similar. On the other hand, the spectra of our ZnAl2O4 nanoparticle looks quite different from the other spectra. Figure 8 shows TEM images of this sample. The ZnAl2O4 nanoparticles exhibited strong agglomeration, so that identification of each nanoparticle was difficult. However, we inferred from this figure that our ZnAl2O4 nanoparticle is a square-shaped nanocrystalline (see the dashed orange line) with an average particle size of about 4~8 nm (at least less than 10 nm). The particle size of this sample is calculated as 6.47 nm from the FWHM of the XRD pattern. This is consistent with the particles observed in Figure 8. A size of less than 10 nm is small Fig. 7   UV-Vis spectrum of ZnAl2O4 nanoparticle synthesized at 250 oC for 20 hours from the precursor with w = 2.00 and pH  7. Data are compared with the UV-Vis spectrum of sintered ZnAl2O4 powder heated at 1300 oC as well as with the simulated results obtained by CASTEP and the spectrum of citric acid. 200 300 400 500 600 700 800 900Figure 7Wavelength (nm)SimulationZnAl2O4 nanoparticleCitric acidSintered ZnAl2O4KM (A.U.)  enough to induce the quantum size effect. Therefore, UV-Vis spectrum is considered to shift to a higher energy region in the case of ZnAl2O4 nanoparticles than in the case of the sintered sample. We speculated that the origin of the peak indicated by the blue arrow in Figure 7 is the same as that of the peak indicated by the green arrow. On the other hand, the broad absorption band shown in the visible light region is thought to originate from the organic component introduced by the OLD method. The UV-Vis spectrum of citric acid is also plotted in Figure 7. It indicates that the organic component of our ZnAl2O4 nanoparticle is not simple citric acid or a related material. The FT-IR spectra are also compared in Figure 9 for the ZnAl2O4 nanoparticles, the sintered reference sample, and citric acid to obtain information about the organic Fig. 8   TEM images of ZnAl2O4 nanoparticle synthesized at 250 oC for 20 hour from the precursor with w = 2.00 and pH  7. Figure 820 nm10 nm70 nm  component of our sample. Figure 9 also indicates that the organic component of our ZnAl2O4 nanoparticles is not simple citric acid or a related material. The OLD method uses citric acid (C6H8O7; C(OH)(CH2COOH)2COOH) as the organic component and ammonium water as the pH adjuster. Moreover, our hydrothermal process, similar to the typical chemical synthesis, applies high temperature and involves Zn2+ ion in the precursor solution. This Zn2+ ion has the potential to work as the catalyst to yields amide compound from carboxylic acid in ammonia solution. The FT-IR spectrum of the ZnAl2O4 nanoparticle shown in Figure 9 can be com pared to that of L-glutamine (C5H10N2O3; H2N-CO-(C2H4-CHNH2-COOH), which has a similar molecular structure to citric acid with an amide. The characteristic absorption bands originating in the organic component in our ZnAl2O4 nanoparticles are observed around 1000 ~ 1100 cm-1, 1500 ~ 1700 cm-1, and 2500 ~ 3700 cm-1. Then, the band around 2500 ~ 3700 cm-1 is considered a typical Fig. 9   FT-IR spectrum of ZnAl2O4 nanoparticle synthesized at 250 oC for 20 hours from the precursor with w = 2.00 and pH  7. Data are compared with thr FT-IR spectra of sintered ZnAl2O4 powder heated at 1300 oC, of L-glutamine, and of citric acid. Figure 9KM (A.U.)Wavenumber (cm-1)4000 2000 1000 600 400L-glutamineZnAl2O4 nanoparticleCitric acidSintered ZnAl2O4  one originating in the stretching mode of O-H including absorbed H2O. On the other hand, we cannot assign the band around 1000 ~ 1100 cm-1, unfortunately. However, the peak position of the band around 1500 ~ 1700 cm-1 (the blue area in Figure 9) is apparently similar to that of L-glutamine rather than to that of citric acid. This is important, since it is a typical difference in the stretching mode of C=O between carboxylic acid (COOH) and amide (CO-NH2). This result impressed upon us that the citric acid added as the organic component in the OLD method reacts with the ammonium ion of the pH adjuster and works as the surfactant to control the size of the product ZnAl2O4 nanoparticles. Unfortunately, determination of the molecular structure of this organic component exceeds the focus of this study. However, this result is quite interesting from the viewpoint of the material design for organic-inorganic hybrid nano-materials. Finally, we comment on the crystal structure of the ZnAl2O4 nanoparticles produced by hydrothermal synthesis via the OLD method. Figures 1 and 2 show that the 400 peak of the XRD pattern is higher than the 331 peak for all our ZnAl2O4 nanoparticles. This is expected trend for ZnAl2O4 having large amount of site exchange phenomenon between Zn2+ at tetrahedral site and Al3+ at octahedral site within the spinel structure [22]. This trend can also be identified from the FT-IR spectra shown in Figure 9. In the FT-IR spectrum, the site exchange phenomenon of spinel aluminates decrease the intensity of the absorption band around 480 ~ 540 cm-1 (the green area in Figure 9) [23,31]. Figure 9 clearly shows this trend. In spinel materials, the site exchange phenomenon is difficult to prevent, but ZnAl2O4 stands out as an exceptional phase with a remarkably low occurrence (less than 1%) of such exchanges. This understanding is derived   from investigations conducted using samples fabricated through solid-state reaction methods. Thus, fabrication of ZnAl2O4 with a significant amount of site exchange would be practically impossible through a solid-state reaction route. This study produced it easily through a chemical synthesis route. The site exchange phenomenon is considered one approach to enhance catalytic performance in the case of transition metal aluminates expressed as MAl2O4 (M: transition metal), since it increases the amount of Al3+ in tetrahedral coordination [32,33]. That is, the results of this study are interesting from the perspective of applying metal aluminates as catalysts in various applications.   Summary ZnAl2O4 has been actively investigated in some application fields, but a direct bottom-up synthesis route from a liquid precursor solution has not been proposed until now. This study investigated a hydrothermal technique for synthesizing ZnAl2O4 and proposed the OLD method as a novel synthesis route using a transparent liquid precursor solution. The advantages of the OLD method are that it enables the direct bottom-up synthesis of ZnAl2O4 and that it is usable across a wide pH range (3.0 ~ 11). These advantages are important for discussing the application of nanoparticles synthesized by hydrothermal technique. For example, if we apply the OLD method, we will be able to synthesize ZnAl2O4 continuously by applying the flow-type hydrothermal synthesis system [34,35].   Synthesized ZnAl2O4 was determined to be a single phase based on XRD analysis, even for the sample synthesized over a period of 2 hours. The product ZnAl2O4 is thought to be in the nanoparticle form, and the particle size was increased by elongation of the heating time. However, the growth rate of the ZnAl2O4 nanoparticle was not high in the case of hydrothermal synthesis via our OLD method. The average particle size was basically smaller than 10 nm even though the sample was synthesized for 20 hours. This small size suggested the potential for inducing the quantum size effect. In fact, evaluation of UV-Vis spectra suggested the appearance of this effect, and the absorption energy of the bandgap seemed to increase. We attribute the slow growth rate to the existence of organic components in the ZnAl2O4 nanoparticle. The origin of the components is the organic acid of the OLD method, and it is speculated to work as the surfactant at the phase formation process. The present findings are highly intriguing from the perspective of application studies involving nanoparticles and the design of organic-inorganic hybrid nanomaterials. On the other hand, ICP-OES measurement reveals the importance of a long heating time (over 1000 minutes) to obtain ZnAl2O4 without cation deficiency (i.e., Zn : Al = 1 : 2). Additionally, the structural analysis conducted using XRD and FT-IR results reveals that the ZnAl2O4 nanoparticles synthesized through the hydrothermal technique using the OLD method exhibit a substantial occurrence of site exchange between Zn2+ and Al3+ within the spinel structure. These results provide crucial data for the future studies of ZnAl2O4 nanoparticles.   Our OLD method is basically applicable to the synthesis of other spinel oxides. Thus, we believe the OLD method can extensively improve investigations into chemical processing for spinel oxides. Further investigation is required.  Acknowledgements We acknowledge Dr. H. Isago of the National Institute for Materials Science for his meaningful discussion about the chemical synthesis of complex materials. Compositional analysis using ICP-OES in this study was supported by the NIMS Surface and Bulk Analysis Unit. We are deeply grateful to Dr. Iwanade, Dr. Kato, Dr. Yamaguchi and Ms. M. Hayashi for their technical support. TEM observation was supported by the NIMS Electron Microscopy Unit. Finally, this work was partially supported by a KAKENHI Grant-in-Aid for Challenging Exploratory Research (16K13999).  References [1]  Sampath SK, Kanhere DG, Pandey R (1999) Electronic structure of spinel oxides: zinc aluminate and zinc gallate. J Phys Cond Matter 11: 3635-3644. doi: 10.1088/0953-8984/11/18/301 [2]  Dixit H, Tandon N, Cottenier S, Saniz R, Lamoen D, Partoens B Saeybroeck VV, Waroquier M (2011) Electronic structure and band gap of zinc spinel oxides beyond LDA: ZnAl2O4, ZnGa2O4 and ZnIn2O4. 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