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Nasrat Hannah Shudin, [Ryuto Eguchi](https://orcid.org/0009-0003-2859-6934), Takeshi Fujita, Tomoharu Tokunaga, [Ayako Hashimoto](https://orcid.org/0000-0002-1985-7667), [Hideki Abe](https://orcid.org/0000-0002-8392-7586)

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[Phase textures of metal–oxide nanocomposites self-orchestrated by atomic diffusions through precursor alloys](https://mdr.nims.go.jp/datasets/8a3008fd-7bcb-438a-8ad7-bc8a7caa5763)

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D3CP05157A.pdfARTICLEPlease do not adjust marginsPlease do not adjust marginsReceived 00th January 20xx,Accepted 00th January 20xxDOI: 10.1039/x0xx00000xPhase Textures of Metal-Oxide Nanocomposites Self-Orchestrated by Atomic Diffusions through Precursor AlloysNasrat Hannah Shudin, a, b Ryuto Eguchi, a, c Takeshi Fujita,*d Tomoharu Tokunaga, e Ayako Hashimoto a and Hideki Abe *a, bMetal-oxide nanocomposites (MON) are of pivotal importance as electrode materials yet lack a guiding principle to tune their phase texture. Here we report that the phase texture of MON can be tuned at the nanoscale by controlling the nanophase separation of precursor alloys. In-situ transmission electron microscopy (in-situ TEM) has demonstrated that a MON material of platinum (Pt) and cerium oxide (CeO2) is obtained through promoted nanophase separation of a Pt5Ce precursor alloy in an atmosphere containing oxygen (O2) and carbon monoxide (CO). The Pt-CeO2 MON material comprised an alternating stack of nanometre-thick layers of Pt and CeO2 in different phase textures ranging from lamellae to mazes, depending on the O2 fraction in the atmosphere. Mathematical simulations have demonstrated that the phase texture of MON originates from a balance in the atomic diffusions across the alloy precursor, which is controllable by the O2 fraction, temperature, and composition of precursor alloys.1. IntroductionMetal-oxide nanocomposites (MON) are of confocal interest in industries and day-to-day life due to their prominent functionalities as electrodes of batteries and/or fuel cells.[1-3] MON is usually materialized by assembling nano-scaled building blocks of metal and oxides through bottom-up approaches including mechanical mixing, sol-gel processes, or coprecipitation methods.[4-10] Indeed, nano-scaled particles of nickel (Ni) and yttria-stabilized zirconia (YSZ) materials are mixed with high-speed ball mills, pelletized, or spread over electrolyte membranes, and finally calcined at high temperatures to yield anodes for solid-oxide fuel cells (SOFCs).[11] The MON materials obtained in such bottom-up approaches however often suffer from poor transport performances that lead to increased power loss of the fuel cells and/or batteries. Each of the phases of MON is composed of a closed heterointerface between the different phases that terminates the flow of currents to increase the inherent impedance. Recently, a new class of MON has been materialized through a different approach than the conventional assembly of nano-scaled building blocks.[12-14] It was demonstrated that MON having extended nanotextures can spontaneously emerge out of precursor alloys that consist of highly oxyphilic metals such as early d-metals and lanthanides.[15-17] When subjected to an atmosphere containing oxygen (O2), such precursor alloy is dissociated into metal and oxides as the result that the oxyphilic metal species are selectively converted into oxides. The infusion of oxygen atoms from the surface into the bulk of the precursor alloy develops a phase-connected MON comprising a nanometre-thick, interconnected network of metal and oxide (i.e., nanophase separation).[16] The interconnected metal-oxide network in MON leads to superior transport performances due to the spatially connected pathways of metal- and oxide phases for electronic- and ionic transports, respectively.[18-20] The phase-connected MON materials are, despite the inherent priority of spatial connectedness of materials phases, still precluded from wide use in batteries or fuel cells due to the lack of a guiding principle to tune their phase texture toward maximized performances.Herein, we report that a hybrid approach consisting of microscopic observations and mathematical simulations allows an understanding and even predictions of the mode of phase textures of MON. In-situ transmission electron microscopy (in-situ TEM) has demonstrated that a layer-by-layer, nanometre-thick (2 through 10 nm) lamella texture of platinum (Pt) and cerium oxide (CeO2) spontaneously emerges out of a precursor alloy of Pt5Ce when subjected to an atmosphere consisting of O2 and carbon monoxide (CO) at temperatures higher than 500 oC. The lower the O2 fraction concerning the CO concentration, the more sluggish the texture formation and the more folded the phase texture that ranges from lamellae to mazes. Mathematical simulations based on a linearized reaction-diffusion model have shown that either a maze- or lamella pattern tends to emerge when a.National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-00443, Japan.b.Graduate School of Science and Engineering, Saitama University, Shimo-Okubo 255, Saitama 338-8570, Japan.c. Tsukuba University, 1-chome-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan.d.Kohchi University of Technology, Kami, Kochi, Japan.e. Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000xPage 1 of 5 Physical Chemistry Chemical PhysicsPhysicalChemistryChemicalPhysicsAcceptedManuscriptOpen Access Article. Published on 23 April 2024. Downloaded on 4/23/2024 12:49:57 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDOI: 10.1039/D3CP05157AARTICLE Journal Name2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xxPlease do not adjust marginsPlease do not adjust marginsthe atomic diffusivity of one of the metal species is in the order of 1/50 or 4/50 compared to that of the counterpart metal species, respectively. This simulation result supports a picture that the atomic diffusion of Ce, which is much sluggish compared to that of Pt across the oxidating Pt5Ce alloy, plays a predominant role in the emergence of Pt-CeO2 MON. This work opens up a route towards a tuned phase texture of MON by controlling the atomic diffusions of the constituent elements of precursor alloys through temperature, alloy composition, and/or the composition of the oxidative atmosphere. 2. Experimental SectionPt5Ce alloy was obtained by melting ingots of Pt- and Ce metals at a mole ratio of Pt:Ce = 5:1 using an arc-torch in Argon atmosphere to avoid oxidation. The alloy ingot was crushed with a mortar and sieved to collect particles with an average diameter of 50 μm. The Pt5Ce powder was then cut with a field-ion bombardment (FIB) apparatus down to a thickness of 80 nm. In-situ TEM observation was performed on the thinly cut sample of Pt5Ce using JEM-1000K RS TEM (JEOL) at Nagoya University, which was equipped with a specially designed atmosphere-controllable cell. The sample was exposed to a reactant gas consisting of CO and/or O2 at 200 Pa while the sample temperature was set at 500 0C throughout the experiments. A. Turing proposed a mathematical model that mutual reactions between two diffusing components can lead to spatially uneven patterns (i.e., the Reaction-Diffusion (RD) model). [21-22] The RD model has been extensively applied to different systems including patterns on the skin of animals, an embryotic emergence of organs, and non-biological systems such as nanophase separation of high-tensile alloys. [23-25] The RD model is generally described as a set of non-linear, time-dependent differential equations of two chemical species of uand v: ∂∂ = ( , ) + Δ∂∂ = ( , ) + Δeq. 1This set of equations can be simplified in the form of a set of linear equations within a rational boundary condition to describe the pattern formation of nanophase-separated alloy systems as follows:(∂∂∂∂ ) = ( )( ) + ( oo )(ΔΔ ) + ( )            eq. 2where each of the parameters of , , , and  is regarded as constants in the vicinity of the origin of the (u, v) plane. See SI for the derivative of the linearized equation.Numerical simulations were performed in cubic simulation boxes of 100 x 100 grids using a homemade software that was coded with C. The process of visualization of the simulation images from the mathematical model is described in detail in SI Fig. S1. An array of complex variables was executed to generate different 2-dimensional patterns. A periodic boundary condition was applied to the whole of the simulating area. Both u and v were assumed to depend on time (t) and coordinates (r), and monotonously depend on the concentrations of the chemical species. The coefficients of av, and bu in eq. 2 denote a reaction of the two metal species comprising the precursor alloy: Pt- and Ce atoms, respectively. 3. Experimental ResultsIn-situ TEM has demonstrated that the Pt5Ce precursor alloy is separated into different phases when subjected to an oxidative atmosphere, leading to the formation of intricate patterns consisting of different nanophases of Pt metal and Figure 1: Snapshots of in-situ TEM images where lamellae and maze patternswere observed after Pt5Ce alloy was subjected to a) O2 and b) CO: O2; 2:1 environment. Wavefront of phase separation propagation indicated with white broken line. Close view of pattern formed as shown in (a.4) and (b.4) showing lamellae and maze respectively, Page 2 of 5Physical Chemistry Chemical PhysicsPhysicalChemistryChemicalPhysicsAcceptedManuscriptOpen Access Article. Published on 23 April 2024. Downloaded on 4/23/2024 12:49:57 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDOI: 10.1039/D3CP05157AJournal Name  ARTICLEThis journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3Please do not adjust marginsPlease do not adjust marginscerium oxide (CeO2) (see supplementary videos S2(i) and S2(ii) in SI). Ce metal in Pt5Ce is oxidized to develop a CeO2phase due to its high oxygen affinity, driving the formation of intricate patterns consisting of Pt metal- and CeO2 phases. This phenomenon was only observed at temperature 500 0C and higher, no phase separation was observed at temperature lower than 500 0C due to lack of energy to initiate oxidation. See supplementary video S2(iii) in SI.A set of experiments was conducted to investigate the nanophase separation of the Pt5Ce alloy, one in a gas mixture of CO and O2 at partial pressures of 200 Pa and 100 Pa, and the other at partial pressures of 0 Pa and 200 Pa.  As can be seen from the snapshots of in-situ TEM in Fig.1(a.2), lamella pattern emerged when the Pt5Ce alloy was exposed to a pure O2 atmosphere. Continuous supply of O2 further promoted the nanophase separation that led to a spatially ordered lamella pattern at an emergence rate of 20 nm/sec (Fig. 1(a.3)). In the gas mixture of CO and O2, an entangled, maze-like pattern emerged instead of the lamella pattern (Fig. 1(b.2)). CO here acts as an inhibitor and constraints the oxidation rate by intercepting some of the O species on the surface. [20] By increasing the CO concentration atmosphere, more O species were intercepted which results in a more sluggish, maze-like pattern. Note that the emergence rate of the maze pattern was much lower than that of the lamella pattern: an aliquot of 120 sec was necessary to accomplish the pattern formation, whereas the lamella pattern required only 5 sec (Fig. 1(a.2) and (b.2)). We observed the same trend in simulation where a longer iteration time required to generate maze pattern compared to lamella pattern (SI Fig. S3). eq. 3( ) = ( ― ―+ + )                Computational simulations were conducted on variablevalues of the off-diagonal terms of (au, av) and the diffusion coefficients of (Du, Dv), setting the other parameters constant (SI, Fig. S4 and S5). First, we have noticed that the off-diagonal terms in the equation determine the nature of the “reaction” between the chemical species. When both av and bu are the same in sign, the concentrations of the chemical species monotonously either diverge or converge with increasing time, being accompanied by no pattern formation. Only in the event the off-diagonal terms are different in sign (eq. 3), spatially uneven patterns can spontaneously emerge out of a uniform medium due to a competition between the diffusions of mutually reacting chemical species. We further computed the patterning evolution by considering the different parameters at the same time (Fig. 2). Figure 2a corresponds to av = 0.125 and Du = 0.02. With increasing av from 0.125 while fixing Du, the pattern evolves from isolated spots to a lamella pattern that consists of a parallel array of stripes at av = 0.150 (Fig. 2b). The warm-colored region diminishes with the increase in av (Fig. 2c). As seen on the first column of figure 2 that corresponds to av = 0.125, the spotty phase becomes more highly distributed with the decrease in Du (Fig. 2a, 2d, 2g). Another trend is recognized in the second column of figure 2 that corresponds to av = 0.15. The lamella pattern (Fig. 2b) is folded (Fig. 2e) with the decrease in Du to develop a maze pattern when Du = 0.002 (Fig. 2h).  A similar trend is recognized for the third column of figure 2 that corresponds to av = 0.20, where the parallel array of broken stripes (Fig. 2c) is folded with decreasing Du values (Fig. 2f) to finally reach a maze pattern (Fig. 2i). The results of computational simulation demonstrate that spatially connected patterns are generated in a limited range of av and Du, where Du (0.02 to 0.002) is always 10 times or smaller than Dv (0.5). It is acknowledged that Ce atoms diffuse much more slowly than Pt atoms across a given Pt-rich alloy matrix. [26] We assign Du and Dv to the diffusion constants of Ce and Pt, and accordingly, the variables of u and v to the local concentrations of Ce and Pt, respectively. The driving force of the nanophase separation of the Pt-Ce alloy precursor is most likely the exothermic oxidation of Ce, which leads to positive feedback of the Ce concentration (u) to both the Ce- and Pt concentrations (v) through the positive parameters of au and av, respectively. Note that the feedback from the Pt concentration to the Ce- and Pt concentrations are negligibly small (bv = 0.0) or if any, negative (bu = -0.05), showing that Pt plays the role Figure 2: Patterns obtained from simulation by triggering the reaction and diffusion terms. av value was set to (0.125, 0.15 and 0.2) and Du value was set to (0.002, 0.008, 0.02). All the other parameters were set to constant.Figure 3: simulation images a, b) showing stripes and maze pattern which comparable to the experimental In-Situ TEM. Du = 0.02 and Du = 0.002 andsnapshot images of in-situ TEM taken at different conditions c) in pure O2 and d) CO-O2 atmospheres.Page 3 of 5 Physical Chemistry Chemical PhysicsPhysicalChemistryChemicalPhysicsAcceptedManuscriptOpen Access Article. Published on 23 April 2024. Downloaded on 4/23/2024 12:49:57 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDOI: 10.1039/D3CP05157AARTICLE Journal Name4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xxPlease do not adjust marginsPlease do not adjust marginsof an inhibitor for the nanophase separation of Pt-Ce alloy precursor. Indeed, an increased Pt concentration can lower the concentration of the activator for the phase separation, namely, Ce. The diffusion of the fast-diffusing inhibitor, Pt, determines the distance between the two adjacent Pt phases that are separated by the activator, i.e., the Ce phase. Figure 3 shows a similarity between the simulated patterns and the in-situ TEM images. The computational simulation has demonstrated that either lamella- or maze pattern is generated when the diffusivity ratio for the different chemical species is Du/Dv = 0.02/0.50 = 1/25 (Fig. 2b) or 0.002/0.50 = 1/250 (Fig. 2h). Note that the lamella- and maze patterns emerged in the atmosphere of pure O2 gas and a mixture gas of CO-O2, respectively. See SI Fig. S6 for Scanning Transmission Electron Microscope (STEM) image of lamella pattern formed over a micrometre-sized area. The Ce oxidation not only drives the nanophase separation but also can lead to a rise in the local temperature of the precursor alloy at the propagation front of the Pt-CeO2 MON. Considering the reducing nature of CO, it is rational to presume that the rise in the local temperature at the propagating front can be mitigated in the CO-O2 atmosphere, leading to slow atomic diffusions. This presumption is supported by the experimental fact that the emergence rate of the maze pattern, 3 nm s-1, was much slower than that of the stripe pattern, 20 nms-1 (Fig.1(b.3 and a.3)). The diffusivity ratio steeply increases with lowering temperature according to a widely acknowledged formula: , where / ∝ exp ( ― ( ― )), , and  correspond to the activation energy for the diffusions of Ce, Pt, and temperature, respectively ( ―). We interpret the dramatic change in the mode of > 0nanophase separation from the lamella- to maze patterns as a result of the increased diffusivity ratio, which is further attributed to the mitigated Ce oxidation in the CO-O2 atmosphere. ConclusionsThrough a hybrid approach of computational simulations and in-situ transmission electron observations, we have successfully shed light to the emergence of metal-oxide nanocomposites (MON) from precursor alloys via nanophase separation. The in-situ TEM observation has allowed not only to monitor the emergence of MON but also to quantify the emergence rate of the MON with different phase textures. The simulation has shown that the phase texture of the MON such as a lamella- or maze pattern is predominantly determined by the ratio in the atomic diffusivity of the constituent atoms of the precursor alloy. The computational-experimental hybrid approach reported in this work has opened up a route to tailor the MON material by controlling the diffusivity of the constituent atoms of precursor alloys possibly through tuned temperature, alloy compositions, and/or atmospheres that promote nanophase separation of precursor alloys.Author ContributionsAll authors contributed equally to the study concept and experimental design.Conflicts of interestThe authors declare no conflict of interest.AcknowledgementsThis work has been supported by Japan Society for the Promotion of Science (JSPS) through a grant # 22H01799. We acknowledge support from the Advanced Characterization Nanotechnology Platform of the High Voltage Electron Microscope Laboratory of Nagoya University. We finally acknowledge Prof. Nagayama of Hokkaido University for permitting to conduct mathematical simulations using the computational program he has developed.References1 Jeon.S, Seo. J, Shin. J, Lee. S, Seo. H, Lee. S, Tsvetkov. N, Kim. J, An. J and Jung. W, Chemical Engineering Journal, 2023, 455 140611.2 Li. M., Hou. J, Fan. Y, Xi. X, Fu. X, Luo. J, ELectrochimica Acta, 2022, 426, 140771.3 Ghouri. Z.K, Barakat. N.A.M, Kim. H.Y, Park. M, Khalil. K.A, El-Newehy. M.H and Al-Deyab. S.S, Arabian Journal of Chemistry, 2016, 9, 219-228. 4 Alanazi. H, AlHaddad. M and Shawky. A, Material Research Bulletin, 2023, 164, 1122485 Yakovlev. I, Tikhov.S, Gerasimov. E, Kardash.T, Valeev. K, Salanov. A, Chesalov. Y, Lapina. O, Lomovskii. O and Dudina. D, Materials, 2023, 16, 1074.6 Mendes.P, Song. Y, Ma. W, Gani. T, Lim. K, Kawi. S and Kozlov. 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This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDOI: 10.1039/D3CP05157AJournal Name  ARTICLEThis journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5Please do not adjust marginsPlease do not adjust margins22 Menou. L, Luo. C and Zwicker. D, J. R. Soc. Interface, 2023, 20: 2023024423 Kondo. S, Watanabe. M and Miyazawa. S, Phil. Trans. R. Soc. A 379, 2021, 2020027424 Heemskerk. I, Development Biology 460, 2020, 86-9825 Wen. Y, Abe. H, Mitsuishi. K and Hashimoto. A, Nanoscale, 2021, 13, 18987-1899526 Malacrida. P, Escudero-Escribano. M, Verdaguer-Casadevall. A, Stephens. E. L and Chorkendorff. I, J. Mater. Chem. A, 2014, 2, 4234Page 5 of 5 Physical Chemistry Chemical PhysicsPhysicalChemistryChemicalPhysicsAcceptedManuscriptOpen Access Article. Published on 23 April 2024. Downloaded on 4/23/2024 12:49:57 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDOI: 10.1039/D3CP05157A