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[Gen Hayase](https://orcid.org/0000-0003-1970-6129)

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[Hollow skeletal macroporous aluminum hydroxide monoliths and their calcined derivatives with high diffuse reflectance](https://mdr.nims.go.jp/datasets/ac252dc1-b641-4eef-980e-246d9d7e613d)

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Hollow skeletal macroporous aluminum hydroxide monoliths and their calcined derivatives with high diffuse reflectanceReceived: 6 June 2025 Revised: 7 August 2025 Accepted: 8 September 2025DOI: 10.1111/jace.70276SPEC IAL I S SUE ART ICLEHollow skeletal macroporous aluminum hydroxidemonoliths and their calcined derivatives with high diffusereflectanceGen HayaseResearch Center for Electronic andOptical Materials, National Institute forMaterials Science, Tsukuba, JapanCorrespondenceGen Hayase, Research Center forElectronic and Optical Materials, NationalInstitute for Materials Science, 1-1Namiki, Tsukuba 305-0044, Japan.Email: gen@aerogel.jpA previous version of this manuscript hasbeen deposited on a preprint server(https://doi.org/10.26434/chemrxiv-2025-3nz7d-v2)AbstractMacroporous aluminum hydroxide monoliths with continuous hollow skele-tons were fabricated through a scalable, template-free sol‒gel synthesis frommetal‒salt precursors. Tuning the concentration of propylene oxide, whichserves as a proton scavenger, yielded crack-free monoliths several centimetersin diameter. Scanning electron microscopy revealed a three-dimensional porousnetwork composed of an interconnected nanoplatelet shell. The as-preparedsample exhibited a Brunauer–Emmett–Teller-specific surface area of 429 m2 g‒1.Calcination at 500◦C converted the framework into γ-alumina while preservingthe hollow architecture and a high specific surface area of 314 m2 g‒1. Subse-quent heating to 1250◦C produced α-alumina while preserving the monolithicshape and continuity of the skeletal framework; however, the hollow structurecollapsed, and the surface area decreased significantly. Optical measurementsshowed that aluminum hydroxide and α-aluminamonoliths both exhibited hightotal diffuse reflectance across the ultraviolet–visible–near-infrared (UV–Vis–NIR) range (400–1200 nm), comparable to Spectralon. The α-alumina retainedover 90% reflectance up to 2500 nm and demonstrated excellent thermal stabil-ity. These results suggest that the fabricated monoliths could be used as diffusereflectance reference materials.KEYWORDSalumina, optical materials/properties, porous materials, sol‒gel1 INTRODUCTIONMacroporous monoliths possess continuous three-dimensional skeletal networks and hierarchically orderedpores. They have been widely applied in fields such ascatalyst supports, adsorbents, sensors, and separationmembranes.1 Compared to particulate porous materials,This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in anymedium, provided the original work is properly cited and is not used for commercial purposes.© 2025 The Author(s). Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of The American Ceramic Society.these monoliths exhibit superior mechanical strengthand fluid transport properties, making them partic-ularly attractive as media for chemical reactions andseparations. In addition to the continuous macroporesat the microscopic level, incorporating mesopores andmicropores within the skeleton can further optimizereactivity and mass-transport characteristics. Sol–gelJ Am Ceram Soc. 2026;109:e70276. wileyonlinelibrary.com/journal/jace 1 of 10https://doi.org/10.1111/jace.70276https://orcid.org/0000-0003-1970-6129mailto:gen@aerogel.jphttps://doi.org/10.26434/chemrxiv-2025-3nz7d-v2https://doi.org/10.26434/chemrxiv-2025-3nz7d-v2http://creativecommons.org/licenses/by-nc/4.0/https://wileyonlinelibrary.com/journal/jacehttps://doi.org/10.1111/jace.70276http://crossmark.crossref.org/dialog/?doi=10.1111%2Fjace.70276&domain=pdf&date_stamp=2025-09-262 of 10 HAYASEmethods are among the most widely employed fabricationstrategies for producing such macroporous monoliths.In ceramic processing, the sol–gel method involvesthe hydrolysis and condensation of metal alkoxides ormetal oxide precursors to form a gel. This gel is thendried and calcined to produce a porous solid. Sol–gelprocesses involving phase separation allow for controlover macropore formation during gelation, making themsuitable for producing centimeter-scale monoliths.2,3 Theresulting uniform porous structures have been exploredfor practical applications as separation media andcatalysts.Numerous reports exist on hierarchical monoliths withmesopores or micropores embedded within the skeleton.However, reports onmonolithswhose skeletal frameworksare intrinsically hollow are scarce. In contrast, hollowceramic materials in particulate form have been exten-sively studied. Common fabricationmethods include hard-template and soft-template approaches.4 These methodshave also been successfully applied to aluminum hydrox-ide [Al(OH)3] and aluminum oxide (Al2O3) systems.5 Inthe hard-template approach, a precursor is coated ontosurfaces of templates, such as polystyrene beads. Thetemplate is then removed by calcination to obtain thehollow structure.6 The soft-template approach uses sur-factants or micelles in microemulsions to produce hollownanoparticles.5,7–9 In boehmite (AlOOH)-based systems,hydrothermal treatment allows for the formation of hol-low particles through dissolution and reprecipitation.10–12These hollow particles have been investigated for poten-tial applications in adsorption, drug delivery, catalysis, andoptical materials.10,13–16One of the few reported approaches for forming hol-low structures within porous monoliths is atomic layerdeposition (ALD).17,18 In this method, ultrathin filmsare deposited onto a template surface. Subsequent tem-plate removal yields hollow architecture. However, theslow deposition rate, high equipment cost, and limitedscalability of ALD restrict its practical applicability, espe-cially for large-area or high-volume production. Addition-ally, increasing the thickness of the resulting structuresposes a significant challenge from the perspective of gasdiffusion.In this study, I developed a scalable route for centimeter-scale macroporous monoliths possessing hollow skeletonsby exploiting phase separation during ametal‒salt-derivedsol–gel process. With this approach, the hollow structureformed along the phase-separated interfaces. Sequen-tial calcination of the aluminum hydroxide monolithyielded γ-alumina and, subsequently, α-alumina. Sinceno hard templates are needed, the process is simplifiedand scalable. Moreover, using metal salts offers practicaladvantages, such as reducing the cost of raw materials,enabling milder reaction conditions, and facilitating solu-tion preparation. Previously, Tokudome et al. reportedon the fabrication and structural control of macrop-orous aluminamonoliths usingmetal salts. However, theirmethod was limited to 10 mm-scale samples and didnot yield hollow structures.19,20 In the present work, Iovercame this limitation by establishing a reproducibleprocedure that yields crack-free monoliths up to 50 mm indiameter.The optical properties of the resulting hollow porousmonoliths and their potential applications were alsoinvestigated. The hollow structure enhances light scat-tering, which is expected to provide high light diffu-sion performance.21–23 Additionally, upon calcination, α-alumina loses hydroxyl groups and is expected to exhibithigher reflectance in the near-infrared (NIR) region thanconventional materials, such as barium sulfate. This paperdiscusses the development of these hollow monolithicstructures and their potential use as light-diffusive reflec-tive materials.2 EXPERIMENTAL ANDMETHODS2.1 MaterialsAluminum chloride hexahydrate (AlCl3⋅6H2O, >97.0%),propylene oxide (PO, >98.0%), and ethanol (>99.5%) wereobtained from Kanto Chemical Co., Inc. Polyethyleneoxide (PEO; Mw ∼1 000 000, containing 200–500 ppmbutylated hydroxytoluene as inhibitor) was purchasedfrom Merck. All reagents were used as received.2.2 Sample preparationAluminumhydroxide porousmonolithswere reproduciblyprepared by a sol–gel process as follows. A stock solutionof water and ethanol (volume ratio 4:6) was pre-mixedand used as the solvent throughout the experiments. In atypical preparation, 10 mL of the water–ethanol mixtureand 5.0 g of AlCl3⋅6H2O were combined in a glass tube,to which 0–0.08 g of PEO was optionally added depend-ing on the composition. Subsequently, PO in the rangeof 4.0–8.0 mL was added dropwise while maintaining thereaction mixture in a water bath at room temperature(∼22◦C). The mixture was stirred for 1 min before beingtransferred into a sealed mold. The sol was then heated at40◦C for 24 h to induce gelation and aging. The gelationtimes for representative PO amounts were approximately20 min for 4.0 mL, 3 min for 6.0 mL, and 2 min for 8.0 mL.After aging, the resulting gel was demolded, washed byimmersion in ethanol, and subjected to evaporative drying 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseHAYASE 3 of 10to obtain monolithic xerogels. The samples were denotedas AMx-y, where x and y represent the amounts (in mLand g, respectively) of PO and PEO used in the initialformulation. Various compositions with different amountsof PO and PEO were explored in preliminary trials todetermine suitable conditions for fabricating crack-freemonoliths. Only selected formulations were characterizedin detail. Some of the aluminum hydroxide monolithswere calcined in air using an electric furnace (ROP-001H, AS ONE Corp.) at 500◦C for 2 h or at 1250◦C for2 h.2.3 CharacterizationThe microstructure of the samples was examined usinga scanning electron microscope (SEM) TM3000 (HitachiHigh-Tech Corp., Japan) and field emission scanning elec-tron microscopes (FE-SEMs) SU8000 and S-4800 (HitachiHigh-Tech Corp.). Bulk density was calculated from thevolume and mass of samples cut into cubes. The relativeerror of the density determination, estimated from fiveindependent measurements, was ≤3%. Porosity was calcu-lated using the bulk density obtained from measurementsand the true density values reported in the literature.24Total reflectance in the ultraviolet–visible–near-infrared(UV–Vis–NIR) region was measured using a spectropho-tometerV-770 (JASCOCorp.) equippedwith an integratingsphere unit (ISN-923). A diffusive reflectance standard(Spectralon SRS-99-010, Labsphere, Inc.) was employedas the reference in all measurements. Mid-infrared (MIR)reflectance measurements were performed using a 10 cmgold-coated integrating sphere in conjunctionwith Fouriertransform infrared spectrometer Nicolet iS20 (ThermoFisher Scientific Inc.), with a gold diffuse reflector used asthe reference.Nitrogen gas adsorption and desorption measurementswere carried out using BELSORP-max (MicrotracBELCorp.). Before measurement, each sample was degassedunder vacuum at 110◦C for 12 h. The specific surface areawas determined based on the Brunauer–Emmett–Teller(BET) method.Thermogravimetry analysis (TGA) was performed usingTG–DTA2000SR (Netzsch GmbH). The measurementswere conducted under a flow of high-purity air at 150 mLmin‒1, with a heating rate of 10◦Cmin‒1. An aluminumpanserved as the sample container, while α-alumina was usedas the reference material. X-ray diffraction (XRD) mea-surements were conducted using MiniFlex600 (RigakuCorp.) with Cu Kα radiation. Prior to measurement, thepre-dried samples were finely ground into a powder.3 RESULTS AND DISCUSSION3.1 Fabrication of hollow-structuredmonolithsIn this study, the method reported by Tokudome et al. wasrevisited with the aim of fabricating disk-shaped macro-porous monoliths with a diameter of 25 mm (≈1 in).19,20While monoliths with diameters exceeding 10 mm couldbe formed using the same precursor composition as in pre-vious studies, cracks developed during the drying process,making it difficult to obtain intact monoliths. Conse-quently, a modification was made to the composition ofthe reaction system, resulting in the fabrication of largermonoliths that were free of cracks. In this process, a sta-ble sol–gel system was identified, consisting of aluminumchloride hexahydrate (5.0 g) as a precursor and PO as a pro-ton scavenger in a 10 mL water–ethanol mixture (volumeratio 4:6).25,26 The structural formation and characteristicsof this system are discussed in detail below.Figure 1 presents SEM images illustrating how themorphology of the samples varies with PEO content. Insol–gel-derived macroporous monoliths, the employmentof phase separation inducers constitutes a well-establishedstrategy for the control of macropore morphology. In linewith this approach, PEO was added as a phase separationinducer in this study. As a first step, its effect was examinedin a series of samples containing 4.0 mL of PO, desig-nated as AM4-y. Composition-dependent morphologicalchanges were observed as the PEO content increased from0 to 0.08 g. This structural change reproduces what hasbeen reported in previous studies,19,20 and a compositionAM4-0.04 exhibiting a bicontinuous structure was alsoconfirmed. However, all samples in this system developedcracks when scaled above 10mm,making them unsuitablefor large monolith fabrication.To address this size limitation, the PO content wasincreased to 6.0 mL (AM6-y system). Increasing the POamount accelerated the pH rise and shortened the gela-tion time. These alterations resulted in the formation ofwet gels that exhibited slightly diminished fragility. Inthese systems, crack-free monoliths with larger diameterswere successfully fabricated, even after evaporative drying.While structural changes were observed upon the addi-tion of PEO, these changes were less pronounced thanthose observed in the AM4-y system. The experimentalprocess was successful in yielding crack-free monolithswith centimeter-scale diameters for all compositions. Thediameter of the samples produced reached up to 50 mmwithout the incorporation of PEO (larger diameters werenot explored). 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License4 of 10 HAYASEF IGURE 1 Scanning electron microscopy (SEM) images of aluminum hydroxide macroporous monoliths AMx-y.F IGURE 2 (A) Scanning electron microscopy (SEM) image of aluminum hydroxide monolith AM6-0. (B) Field emission scanningelectron microscopy (FE-SEM) image of a fractured skeleton of AM6-0. A continuous hollow cavity network can be observed within theskeleton, and similar features were observed at multiple locations during imaging.When the PO content was increased to 8.0 mL (AM8-y system), the system produced aggregates of spheri-cal particles. The structural alterations resulting fromthe incorporation of PEO were minimal. When the POexceeded 10 mL, gelation occurred within seconds afterPO addition, which precluded sufficient stirring and pre-vented uniform monolith formation. In contrast, whenthe PO content was reduced below 3.0 mL, only whiteprecipitates formed, and no monolithic gelation wasobserved.A SEM analysis of the AM6-0 specimen revealed a con-tinuous skeletal structure with highly contrasted regionson the surface (Figure 2). These regions, which exhibited ahigh degree of contrast, were initially presumed to be imag-ing artifacts. However, high-magnification FE-SEM obser-vations (Figure 2B) confirmed that they correspondedto a structure consisting of aggregates of nanoplatelet-like domains with internal cavities. Although the XRDpattern (Figure 3B) shows broad features characteristicof an amorphous or poorly crystalline phase, the pres-ence of nanoplatelet morphology suggests some degreeof local order. To gain deeper insight into the crystal-lographic nature of these domains, advanced techniquessuch as high-resolution transmission electron microscopy 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseHAYASE 5 of 10F IGURE 3 (A) Photograph of the mold used for gel preparation, the aluminum hydroxide monolith AM6-0, and the sample calcined at1250◦C. Although shrinkage occurs during aging, drying, and calcination, the monolithic shape is maintained. (B) X-ray diffraction (XRD)patterns of AM6-0 and its calcined samples. The material transitions from amorphous or nanocrystalline aluminum hydroxide to γ-aluminaand eventually to α-alumina. (C) Thermogravimetric‒differential thermal analysis (TG‒DTA) curve of the as-prepared monolith (AM6-0)measured in air up to 500◦C.TABLE 1 Physical properties of aluminum hydroxide monolith AM6-0 and its calcined samples.CalcinationtemperatureBulk density(g cm−3) Porosity (%)BET specific surfacearea (m2 g−1)As prepared 0.152 93.8 429500◦C 0.174 95.2 3141250◦C 0.246 93.8 <10Abbreviation: BET, Brunauer–Emmett–Teller.or synchrotron-based analysis may be useful in futurestudies. Themonolithmanifested a chain-likemorphologycomposed of interconnected spherical particles, form-ing a continuous hollow framework. Although AM8-0also exhibited nanoplatelet shell structures, the hollowinteriors were isolated and did not form a continuousnetwork.The AM6-0monoliths were further calcined at tempera-tures up to 1250◦C.Despite some shrinkage, themonolithicstructure remained intact at all temperatures (Figure 3A).The bulk density increased with calcination, while theporosity did not decrease significantly, which is an impor-tant characteristic of this material (Table 1). TGA andXRD confirmed that the as-prepared material consistedof amorphous or nanocrystalline aluminum hydroxide.After heat treatment at 500◦C, the sample exhibited amass loss of approximately 35%, as determined by com-paring the sample weights before and after heating, whichis consistent with dehydration and the formation of γ-alumina. Although this transformation was supported bythe XRD results, a distinct thermal event associated withdehydration was not clearly observed in the TGA curve(Figure 3B,C). The transformation into α-alumina at tem-peratures above 1200◦Cwas confirmedbyXRD, as theTGAmeasurement was limited to 500◦C. These phase transi-tions caused shrinkage and microstructural changes dueto dehydration and recrystallization of the nanoplateletskeleton. At 500◦C, the hollowmorphology was preserved,and SEM images showed narrow necks between particles(Figure 4). At 1250◦C, the outer walls of the hollow struc- 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License6 of 10 HAYASEF IGURE 4 Field emission scanning electron microscopy (FE-SEM) images of AM6-0 and its calcined samples. The hollow structure ispreserved after calcination at 500◦C, whereas pores appear in the shell at 1250◦C, resulting in the loss of the continuous hollow structure.F IGURE 5 Nitrogen adsorption–desorption isotherms of (A) AM6-0 and (B) the sample calcined at 500◦C. Reliable data could not beobtained for the 1250◦C-calcined sample due to the collapse of pores and a significant decrease in surface area.tures collapsed, resulting in discontinuity. Nevertheless,skeletal continuity was retained.Nitrogen adsorption–desorption measurementsrevealed that both the as-prepared and the 500◦C-calcined samples exhibited type IV isotherms with H3hysteresis loops (Figure 5),27,28 which is consistent withthe nanoplatelet-based aluminum hydroxide structureobserved in SEM. The BET specific surface areas were 429and 314 m2 g‒1 for the as-prepared and 500◦C-calcinedsamples, respectively. The decrease in specific surface 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseHAYASE 7 of 10area and the narrowing of the hysteresis loop suggeststhat some mesopores collapsed during dehydration.In contrast, the 1250◦C-calcined sample exhibited asignificant reduction in surface area due to the forma-tion of the α-phase, and reproducible isotherms couldnot be obtained. FE-SEM images confirmed structuraldensification, evidenced by a smooth skeleton surfacedevoid of mesopores; however, the monolithic form andoverall continuity of the skeletal framework remainedintact.Although the formation mechanism of the hollowframework remains to be clarified, the following reactionpathway is proposed. In the presence of chloride ions, POundergoes a ring-opening reaction that consumes protonsand produces byproducts such as 1-chloro-2-propanol.25,26This raises the pH, inducing the precipitation of aluminumhydroxide.19,20 These early byproducts are miscible withthe ethanol–water solvent mixture. As the reaction pro-ceeds, further condensation of these and related speciesis expected to yield more hydrophobic oligomers. Theresulting decrease in solubility may induce phase separa-tion, forming distinct domains within the gel. Aluminumhydroxide may preferentially precipitate at the interfaceof these domains, which could contribute to the forma-tion of the hollow skeletal structure.29 The formation ofnanoplatelet aluminum hydroxide under pH 4–6 condi-tions has been reported previously,30–34 which aligns withthe reaction conditions used in this study. These processeslikely lead to the formation of a three-phase structure con-sisting of a solvent phase, an aluminum hydroxide shell,and a PO-derived phase. This yields a hollow structureupon washing and drying. I also found that insufficientwashing of AM6-0 before calcination at 500◦C led to black-ening of the sample. This blackeningwas attributed to ther-mal decomposition of residual PO-derived species withinthe aluminumhydroxide shell under oxygen-deficient con-ditions. To directly verify this hypothesis and extend itto other metal oxide systems, future studies will involvedetailed structural and compositional analyses using syn-chrotron radiation and cryo-Raman microscopy in futurework. This synthetic approach can be extended to othermetal oxides (e.g., iron oxides)26,35 and oxide–polymerhybrid composites.363.2 Optical properties of thehollow-structured monolithsHollow particles are known to exhibit high light-scatteringperformance by inducing multiple scattering eventsthrough refractive index inhomogeneities created byinternal voids.22 When the particle size approachesthe wavelength of visible light, Mie scattering becomeshighly efficient.37 Hollow particles composed of materialssuch as silica have been widely used as light-diffusingcomponents, particularly in white coatings and reflectivecomposites.21–23,36 These systems take advantage of theinternal voids and low absorption characteristics of silica(refractive index ∼1.46 at 550 nm)24 to achieve highreflectance across the visible spectrum. Alumina has asignificantly higher refractive index (∼1.76 at 550 nm forα-alumina) as well as superior thermal and chemicalstability, making it an attractive candidate for broadbandand high-temperature optical applications.38 While densealumina plates have been used as diffuse reflectors,systematic studies focusing on hollow-structured aluminaparticles or monoliths for light-scattering applicationsare scarce. To the best of my knowledge, porous aluminamonoliths with internally hollow skeletal frameworkshave not been investigated in detail for their broadbandreflectance behavior, particularly in the MIR region. Thisstudy addresses that gap by evaluating the total diffusereflectance of such alumina monoliths across the UV toMIR spectral regions.Motivated by these considerations, I investigated thetotal diffuse reflectance of the as-prepared hollow alu-minum hydroxide monoliths and their calcined coun-terparts to evaluate their potential use as light-diffusivereflective materials. Figure 6 shows the total reflectancespectra of the uncalcined aluminum hydroxide and the α-alumina monoliths calcined at 1250◦C, measured acrossthe near-UV, Vis, NIR, and MIR regions. Spectralon, awidely used standard for diffuse reflectance, was employedas the reference material. Both monoliths exhibited totaldiffuse reflectance equivalent to or higher than that ofSpectralon in the wavelength range of 400‒1200 nm. Spec-tralon is a widely used reflectance standard composedof sintered polytetrafluoroethylene. The material exhibitsextremely low absorption and strong multiple scatteringacross a broad spectral range. High reflectance in the MIRregion is attributed to the absence of strong vibrationalabsorption bands, unlike hydroxyl-containing or inorganicmaterials. Consistent with this, the aluminum hydrox-ide monolith showed slightly reduced reflectance in theNIR region due to absorption by surface hydroxyl groups.After calcination and transformation to the α-aluminaphase, however, the material exhibited consistently highreflectance above 90%, showing clearly improved perfor-mance in the MIR region compared to the uncalcinedsample.These results suggest that the alumina monoliths devel-oped in this study can be used as diffuse reflectance stan-dards like Spectralon. These materials can be synthesizedvia a simple, low-cost, sol–gel-based process. Additionally,α-alumina maintains structural stability under extremethermal conditions exceeding 2000◦C. These materials 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License8 of 10 HAYASEF IGURE 6 (A) Near-ultraviolet–visible–near-infrared (UV–Vis–NIR) and (B) mid-infrared (MIR) total reflectance spectra of thealuminum hydroxide monolith AM6-0 and its sample calcined at 1250◦C.could be used as durable white reflectance standards inhigh-temperature or infrared environments where Spec-tralon is not suitable. They could also be used in spectro-scopic instruments or environmental sensing systems thatrequire high-performance optical reference materials.4 CONCLUSIONCentimeter-scale macroporous aluminum hydroxidemonoliths with continuous hollow skeletal frameworkswere fabricated by combining metal‒salt precursorswith phase-separation-assisted sol–gel processing. Therepresentative sample AM6-0 had a bulk density of0.152 g cm‒3, corresponding to a porosity of 93.8%. Thenanoplatelet-based shells formed a contiguous hollownetwork through bead-like junctions. The as-preparedsample exhibited a BET specific surface area of 429 m2 g‒1.Calcination at 500◦C dehydrated the framework intoγ-alumina, preserving the architecture and yielding aspecific surface area of 314 m2 g‒1. Subsequent calcinationat 1250◦C produced α-alumina while preserving themono-lithic form and the continuity of the skeletal framework;however, the hollow structure collapsed. The hollowmonolith displayed total diffuse reflectance comparableto Spectralon over 400–1200 nm. The reflectance of theuncalcined material decreased in the MIR due to hydroxylabsorption. However, the α-alumina monolith retainedover 90% diffuse reflectance in the NIR region up to2500 nm.The aluminum hydroxide monoliths and their calcinedalumina derivatives can be synthesized via a simple, cost-effective process and offer a versatile platform for a rangeof applications. Potential uses include thermal insulation,white optical substrates, and durable reflectance standardsfor high-temperature spectroscopy. Their continuous hol-low skeletons and interconnected pore networks enablefunctions such as adsorption, catalysis, and flow-throughreaction media, comparable to those of conventional hier-archical porous materials. The fabrication of centimeter-scale monoliths with continuously hollow skeletons hasnot been previously reported. This approach may also beapplicable to other compositionally similar oxide systems,offering a potential route to hollow monolithic materialswith tunable structures.ACKNOWLEDGMENTSThe author thanks Dr. Satoshi Ishii (National Institute forMaterials Science) for helping with the MIR reflectancemeasurements. The author also acknowledges Dr. YasuakiTokudome (Osaka Metropolitan University) for kindlyaddressing questions regarding his previous publications.This work was supported by “Advanced Research Infras-tructure forMaterials andNanotechnology in Japan” of theMinistry of Education, Culture, Sports, Science and Tech-nology (MEXT) (proposal numbers JPMXP1223NM5285,JPMXP1224NM5101 and JPMXP1225NM5072).CONFL ICT OF INTEREST STATEMENTThe author declares no conflicts of interest.ORCIDGenHayase https://orcid.org/0000-0003-1970-6129REFERENCES1. Feinle A, Elsaesser MS, Hüsing N. Sol‒gel synthesis of mono-lithic materials with hierarchical porosity. Chem Soc Rev.2016;45(12):3377–99. https://doi.org/10.1039/c5cs00710k 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. 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Hollowskeletal macroporous aluminum hydroxidemonoliths and their calcined derivatives with highdiffuse reflectance. J Am Ceram Soc.2026;109:e70276. https://doi.org/10.1111/jace.70276 15512916, 2026, 1, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70276 by National Institute For, Wiley Online Library on [01/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://doi.org/10.1021/acs.cgd.9b00468https://doi.org/10.1021/acs.inorgchem.1c01111https://doi.org/10.1021/acs.inorgchem.1c01111https://doi.org/10.1039/d0ce00114ghttps://doi.org/10.1021/acs.cgd.7b01400https://doi.org/10.1021/acs.cgd.7b01400https://doi.org/10.1021/acsanm.8b01969https://doi.org/10.1002/adma.201501130https://doi.org/10.1002/adma.201501130https://doi.org/10.1021/acs.langmuir.5b04063https://doi.org/10.1039/d2cc03768hhttps://doi.org/10.1039/d2cc03768hhttps://doi.org/10.1111/jace.70276 Hollow skeletal macroporous aluminum hydroxide monoliths and their calcined derivatives with high diffuse reflectance Abstract 1 | INTRODUCTION 2 | EXPERIMENTAL AND METHODS 2.1 | Materials 2.2 | Sample preparation 2.3 | Characterization 3 | RESULTS AND DISCUSSION 3.1 | Fabrication of hollow-structured monoliths 3.2 | Optical properties of the hollow-structured monoliths 4 | CONCLUSION ACKNOWLEDGMENTS CONFLICT OF INTEREST STATEMENT ORCID REFERENCES