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[Hiroshi Fudouzi](https://orcid.org/0000-0003-1442-4667)

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[Structural color coatings for ceramics and glass surface by silica opal films](https://mdr.nims.go.jp/datasets/0ddbf91e-2f0e-4250-ba5e-0b1e1fc46c99)

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Structural color coatings for ceramics and glass surface by silica opal filmsFULL PAPERStructural color coatings for ceramics and glass surfaceby silica opal filmsHiroshi Fudouzi1,³1National Institute for Materials Science, 1–2–1 Sengen, Tsukuba, Ibaraki 305–0047, JapanStructural colors derived from colloidal crystals are gaining attention as new color materials that are non-toxicand minimize environmental pollution. Vivid structural colors were formed by coating glass and ceramics with acolloidal crystal (opal) thin film composed of silica particles. This paper focuses on reporting this thin filmformation process. A milky-white suspension of silica particles (particle size 290 nm) precisely arranged on thesubstrate surface exhibited vivid red structural color after drying. The dip coating is not limited to flat substrateslike glazed ceramics or glass; it is versatile and can be applied to curved surfaces, uneven surfaces, and evenrough, unglazed ceramic surfaces. Optical evaluation utilized reflectance spectra. By employing silica opal filmswith varying particle sizes, diverse colors spanning the entire visible spectrum from blue to red were achieved.Furthermore, through heat treatment and modification to create a hydrophobic surface, stable structural coloremission was enabled, preventing loss of color due to abrasion or wetting.Key-words : Silica colloid, Colloidal crystal, Structural color, Opal film, Bragg’s diffraction, Hydrophobic, Glaze[Received February 16, 2026; Accepted March 6, 2026; Published online April 2, 2026]1. IntroductionStructural color is a widely observed phenomenon innature and living organisms.1) Rather than originating frompigments or dyes, this coloration comes from nanostruc-tures. Humans have valued structural color in jewelry andother valuable materials since ancient times. Examples ofstructural color in nature include a jewel beetle, a peacock’sfeather, the pearl and opal. In contrast, examples of artificialstructural colors in glass and ceramics include ancientRoman iridescent glass2) and Tenmoku tea bowls.3) Theseartifacts are also captivated modern people with theirunique hues of structural color, which are distinct frompigment-based colors. In recent years, research on struc-tural color materials using biomimetic approaches has beenactively pursued.4) Additionally, research on structuralcolor and optical sensors using multilayer films of one-dimensional photonic crystals via liquid-phase processeshas flourished.5–7) The main materials used are organicpolymers and soft materials. However, a high refractiveindex is necessary for excellent photonic properties. Thus,for example, studies have investigated forming multilayerfilms of inorganic substances via liquid-phase processes.8)One of important advantages of structural colors is that theyavoid harmful pigment components, making them promis-ing as safe, environmentally friendly color materials froman engineering perspective.Silica particle aggregates exist in two types (amorphousand colloidal crystals), each exhibiting distinct structuralcolor characteristics. P. Chi et al. reported the creation ofcolored glazes featuring amorphous photonic crystal struc-tures that exhibit diverse coloration without any coloringagents.9) Research on structural colors, including the syn-thesis of glazes mimicking tenmoku glaze and the eluci-dation of their coloration mechanisms, has attracted sig-nificant attention. Amorphous photonic crystal structuralcolor glazes,10,11) which exhibit non-iridescent color inde-pendent of viewing angle, are sensitive to firing conditionsdue to their phase-separated structure with short-rangeorder. These are structural colors originating from amor-phous photonic crystal structures.12,13) In contrast, the lat-ter exhibits angle-dependent structural colors due to Braggdiffraction of visible light by colloidal crystals.14,15) Thisstructural color mechanism originates from photonic crys-tal function by three-dimensional colloidal crystal arrays(opal or inverse opal structures).This type of structural color of inorganic colloidalcrystals also has been potential applications for a structuralcolor coating in ceramics or glass wares. For the pioneerworks for alternative pigments, A. Stein et al. reported inthe field of research using harmless, low-environmentalimpact zirconium oxide as a pigment substitute to achievestructural colors with colloidal crystals.16) They fabricatedinverse opal structures of zirconia using colloidal crystalscomposed of polymer colloidal particles as templates. Byselecting the particle size of the polymer particles, theresearchers reported success in producing powder pig-ments ranging from purple to red.17) Subsequent reportsindicated the existence of structural color pigments using³ Corresponding author: H. Fudouzi; E-mail: FUDOUZI.Hiroshi@nims.go.jpJournal of the Ceramic Society of Japan 134 [5] 375-382 2026DOI https://doi.org/10.2109/jcersj2.26016 JCS-Japan©2026 The Ceramic Society of Japan 375This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.26016https://creativecommons.org/licenses/by/4.0/silica instead of ZrO2. By strategically manipulating theperiodicity of the granular inverse opal’s porous structure,the development of three monochromatic pigments; red,green, and blue was achieved. Mixing these pigmentsenabled the creation of mixed-color pigments such as cyan(blue and green combination), yellow (green and red com-bination), and magenta (red and blue combination).Colloidal suspension as an ink for structural color coat-ing are anticipated wide field industrial color decorations.For example, coating a car shape surface,18) coating silicacolloids on conductive surface by electrophoresis deposi-tion.19) Beyond interior decoration, the structural color ofopal is expected to find applications in automotive exteriorcoatings and interior finishes. In recent the core challengein structural color applications lies in the formulation ofcolloidal inks or paint suspensions. As alternative pigmentmaterials, numerous research groups20–23) are advancingstudies on polymer or silica colloidal suspensions as novelcolored, non-toxic, environmentally friendly pigments ordyes.The academic research on color decoration for glass andceramics is scarce. As one of the few examples, Fukazawaet al.24) attempted to apply colloidal crystals as structuralcolor ink onto ceramics and glass. They used alkoxide sili-con as monodisperse colloidal particles, incorporated poly-mer as an additive, and adopted a water-alcohol solution asthe ink solvent. This ink was applied with a brush to formpatterns on ceramic and glass containers. Ultimately, ther-mal treatment at 500 °C burned off the polymer, forminga porous periodic structure that exhibits structural color.Although not explicitly stated in the paper, it is presumedthat a silica inverse opal structure was formed. Recently,Ono reported on the formation of structural color bycoating silica particle suspensions onto ceramic and glasscontainer surfaces.25) To fix the silica opals arranged forpractical applications, silicon alkoxide was added as a pre-cursor to the suspension ink and fixed via heat treatment.This ink is easy to use in the conventional ceramic pro-cess. In a different approach, Kohoutek et al. formed asingle-particle film of silica colloids using the Langmuir–Blodgett method and transferred it onto a glass spheresurface.26) Decorative applications on glass spheres areanticipated to utilize structural color as an interior designelement.This paper proposes a simple and convenient techniquefor imparting structural color to surfaces in glass andceramic. This technique deposits opal thin films onto targetsurfaces via a liquid-phase process originating from amonodisperse silica colloid suspension. A dip-coatingmethod using polystyrene particles has already been devel-oped for biomimetic research on novel structural colormaterials.27) This study focuses on silica particles, aimingto establish a practical process for imparting structuralcolor to glass and ceramics. As competing technologies,engineering techniques for dry processes to impart struc-tural color to ceramic and glass surfaces are well-established. For example, one method involves multi-layer coating via Physical Vapor Deposition (PVD) undervacuum conditions.28,29) This technology is a color deco-ration technique derived from dielectric mirror manufac-turing. Compared to these existing dry coating processes,the proposed wet coating process holds potential for reduc-ing large manufacturing costs since it does not requireexpensive vacuum coating equipment.2. Experimental section2.1 Chemical & materialsSilica colloid suspensions (Silbol EX series, Diametersize from 210 to 310 nm dispersed in water, 30wt% con-centration) were obtained from Fuji Chem., Co. (Osaka,Japan). The other chemicals purchased from as follows anduse in no purification. Ethanol (99.5%, Kishida ChemicalCo., Ltd., Osaka, Japan), polydimethylsiloxane, PDMSfluid (KF-96, 100cs, Shin-Etsu Chemical Co., Ltd., Tokyo,Japan) and Hexane and Isopropanol (Kanto Chemical, Inc.,Tokyo, Japan). Silica opal thin films were applied to a blackglass plate (Shade #13 filter plate, 55mm © 110mm, t =3mm) obtained from Trusco Nakayama Co., Tokyo, Japan.Small black ceramic and tile products (obtained fromKusaba Chemical Co., Ltd., Tajimi, Japan), either unglazedor with a glaze layer, were used as test pieces for the coat-ing experiment.2.2 Coating procedureAs illustrated in Fig. 1, the process is comprised ofthree steps, coating opal film, heat fixing deposited silicasand surface modification. The dip coating targets is slowlywithdrawing from a suspension of silica colloidal particlesdispersed in an ethanol aqueous solution. The dip coaters(Homemade equipment using an actuator with steppingmotor) are shown in supplementary Fig. S1. The suspen-sion was prepared by diluting a silica particle suspensiondispersed in water with ethanol at a 1:3 weight ratio. Thesuspension concentration was approximately 7.5wt%.The substrate lifting speed was extremely slow, approx-imately from 0.1 to 0.6¯m/s. The suspension formed aliquid film on the substrate surface and, during evapora-tion, self-assembled into a densely packed colloidal crystal(opal) thin film. This lifted silica opal was merely depos-ited, and its arrangement could be easily disrupted byscratching or similar actions. Therefore, it was fixed byheat treatment in an electric furnace (FO511, Yamato Sci-entific Co., Ltd., Tokyo, Japan). The heat treatment tem-perature varied depending on the substrate: 1000 °C for 5 hfor ceramic substrates and 400 °C for 5 h for black glassplates. Subsequently, to prevent structural color changescaused by water infilling among silica particles in the opalfilm, the silica opal surface was modified water repellencyhas been imparted by a bake-on method described in thetechnical note.30) A coating liquid composed of PDMSfluid, dissolved in hexane, was diluted to a specific per-centage at 4.8wt.%, and the substrate was drawn off at acontrolled velocity of 0.4 millimeters per second. The topsurface was modified to a hydrophobic state through a heattreatment process at 300 °C for a duration of 5min. Addi-tionally, to remove any excess PDMS liquid remaining onFudouzi: Structural color coatings for ceramics and glass surface by silica opal filmsJCS-Japan376the opal film, the substrate was cleaned with isopropanol.The opal film surface characteristics changed from hydro-philic to hydrophobic as the surface of the silanol groupwas substituted with a methyl group as illustrated inFig. 1. This treatment keeps the structural color from waterwetting.2.3 Characteristic evaluationDigital camera (EXILIM EX-F1, CASIO Computer Co.,Ltd., Tokyo, Japan). Reflection photo image and moviewere obtained under using a flat high brightness LEDwhite light (Color temperature: 5000K, Shinkosha Co.,Tokyo, Japan) uniform irradiation. To minimize the influ-ence of angle dependence in structural color, photographswere taken using a homemade coaxial illumination system(As supplementary shown in Fig. S2). The microstructureof the colloidal photonic crystal films was observed usinga Scanning Electron Microscope, SEM (JSM-6500F,JEOL Ltd., Tokyo, Japan).The reflectance spectra of the silica opal films wererecorded using a miniature fiber optic spectrometer (OceanOptics, USB2000+, Dunedin, FL, USA). The incidentlight was perpendicular to the samples (to measure specu-lar reflectance, the probe was oriented at 90° to the samplein a reflectance probe holder) at a local spot (less than2mm in diameter) by HL-2000 tungsten halogen lightsource. The fiber has a core diameter of 200¯m, a numer-ical aperture of 0.22, and is banded with six illuminationfibers around a single read fiber.The fixing effect of heat treatment on silica opal isevaluated by assessing its wear resistance using a cottonswab subjected to a load and moved horizontally back andforth. A simple reciprocating wear tester (MMS-1, ASONE Co., Tokyo, Japan) was used to the glass plate.Surface hydrophilic and hydrophobic property was eval-uated by the contact angle of a drop of pure water. Acontact angle meter (Simage Standard 100, 1.5M pixelCCD camera, Excimer Inc., Yokohama, Japan) was usedand analysis by a half-angle method. Assuming the shapeof the drop of water dropped on the sample is a perfectcircle, the contact angle can be found from the radius andheight of the circle. Here we obtained the contact angle ascalculated by finding the angle of the line connecting theleft and right endpoints of the drop to the vertex, relative tothe solid surface, and doubling this as described later.3. Results and discussion3.1 Opal films coating on ceramic test piecesAs illustrated in Fig. 2, silica particles with a diameterof 290 nm were coated onto the surfaces of commerciallyavailable ceramic products, thereby producing a red struc-tural color. In Fig. 2(A), the photo shows a glazed squareplate being elevated at a very low speed from a silicasuspension. When photographed with coaxial illuminationfrom directly above, a red structural color is observedCBDAFig. 2. A process for forming a red structural color film on ablack ceramic plate by coating at a slow speed from a colloidalsilica suspension. The silica with 290 nm diameter solutionshowed white milky color and the opal film showed red structuralcolor.Fig. 1. A conceptual procedure for stable structural color of silica colloidal crystal (opal) films on ceramic orglass surfaces. Heat treatment improved bonding silica colloids to the substrate without loss of structural colorand, hydrophobic modification to protect change or lost structural color by wetting water on the surface.Journal of the Ceramic Society of Japan 134 [5] 375-382 2026 JCS-Japan377[Fig. 2(B)]. Conversely, when observed from a tilting per-spective, the structural color transitions to green, as illus-trated in Fig. 2(C). This angle dependence of the structuralcolor is a phenomenon common to colloidal crystals (opalthin films). In Fig. 2(D), silica opal was applied to thesurface of an unglazed, unfinished round plate. The surfaceis uneven and not flat, however a red structural color isobserved. When viewed at tilting angle, the structural colorchanged to green. The red structural color exhibited isinfluenced by particle concentration and lifting speed. Thecoating film exhibits a change in its structural color from apale red state to an opaque state as it transitions from a thinto a thick state. Additionally, the presence of thick coatingfilms was observed to undergo a process of peeling anddetachment from the ceramic surface. The most suitablecoating conditions were estimated to be a particle concen-tration of approximately 10wt% and a withdraw speedranging from 0.1 to 0.6¯m/s.One of the issues in this suspension process is theextremely slow the withdraw speed for applied to engi-neering use. The authors have previously reported a fastercoating process in their existing research.31) We have de-monstrated that increasing the particle concentration of thesuspension by one order of magnitude enables colloidalcrystals to form films in a short time. However, in thisstudy, priority was given to the crystallinity of the colloidalcrystals, so accelerating the film formation speed remainsa future challenge.Another challenge in silica opal film coating is the un-intended formation of colloidal crystal lines perpendicularto the substrate during substrate lift-off from the silicasuspension. As the mechanism behind this phenomenonremains unclear, this paper presents it as a technical issueto be resolved in Fig. S3 as the supplementary informa-tion. This tendency to form silica aggregated lines wereobserved not across the entire substrate but specifically atthe start of withdraw. Even under identical deposition con-ditions, locally formed lines are sometimes observed whileothers are not. It is highly probable that silica particleaggregates in the suspension adhere to the substrate andserve as growth nucleation sites. Future work in produc-tion must clarify the cause of this phenomenon and estab-lish suppression methods.3.2 Structural color design by silica sizeThe structural color of silica opal is contingent upon theparticle size of its constituent colloidal particles. Figure 3shows the structural colors observed on the black glassplate as the particle size (diameter) was varied from 210to 310 nanometers by 10 nm steps. The structural colorsexhibited a wide spectrum, ranging from blue to red. Thecoloration of silica colloidal crystals is attributable toBragg’s diffraction in visible wavelength spectrum color.From a theory, it is well known that the structural colors ofcolloidal crystals depend on the particle size. One peak ofBragg’s diffraction (appearing from 400 to 700 nm) causesstructural color in the visible wavelengths below. Here ­ isdiffraction peak position, d111 is an interspace of cubicclosely packing (ccp) layers and neff is average refractiveindex as expressed in Eq. (1).14) In addition, ª, D, np, nm,Vp, Vm are corresponding to incident angle, silica particlediameter, refractive index of silica, refractive index of air,volume of particle, volume of air, respectively.­ ¼ 2d111ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðn2eff � sin2 ªÞqð1Þd111 ¼ffiffiffi23rD ð2Þn2eff ¼ n2pVp þ n2mVm ¼ 0:74n2p þ 0:26n2m ð3ÞHere ª = 0, np = 1.4 and nm = 1. From Eqs. (1), (2)and (3), we obtain following a relationship between thepeak position (wavelength) and the silica particle size(diameter). The reflection peak wavelength, as calculatedtheoretically, is expressed in Eq. (4).­ ¼ 2:136D ð4ÞFigure 4 shows the superimposed reflection spectra ofthe three structural colors (blue, green, and red). Within thevisible light range of from 350 to 750 nm, one large reflec-tion peak and several smaller peaks (called fringes) areobserved for each color. The fringes result from thin-filminterference and cause structural coloration. However,their intensity is lower compared to the Bragg diffractionpeak. Therefore, the primary structural color visible canbe described as a nearly monochromatic color originatingfrom the single, intense Bragg diffraction reflection peak.The particle sizes were 210 nm for blue silica particles,250 nm for green, and 290 nm for red. The difference inparticle size required to generate the three structural colorswas only 40 nm. As shown in Eq. (4), a positive correla-tion exists between particle size and the peak wavelengthof Bragg diffraction. Therefore, the relationship betweenparticle size and peak wavelength was measured for thestructural colors shown in Fig. 3.Figure 5 demonstrates the relationship between struc-tural color and reflection peaks wavelength for particles ofFig. 3. Diverse spectrum types of structural colors exhibited bythin films of colloidal crystals composed of silica particles withvarying diameters on black glass plates. The white numbersindicate the diameter of the silica colloids (in nm).Fudouzi: Structural color coatings for ceramics and glass surface by silica opal filmsJCS-Japan378various sizes, as illustrated in Fig. 3. The slope of the lineis 2.132 and R2 is 0.998. The measurement results dem-onstrated a high correlation between particle size and theobserved wavelength, yielding results that closely approx-imated the calculated value of 2.136 derived from theBragg equation with Snell’s law [Eq. (4)]. As illustrated inFig. 5, the graph encompasses a substantial portion of thevisible spectrum region between 440 and 660 nm. Theseresults demonstrate that structural colors can be producedin diverse hues solely by only selecting the particle size ofsilica particles.3.3 Heating opal film on ceramics and blackglassSimply forming silica opal results only in the depositionof silica particles on the ceramic surface. These depositedparticles can be easily scraped off the surface with lightscratching. Therefore, fixing the particles is critically im-portant from an engineering perspective. One effectivemethod for this is a fixation process using heat treatment(the second stage shown in Fig. 1). Figure 6 shows theresult of heat treating the silica opal film on a glazed tiletest specimen at 1000 °C for 5 h. A slight color change wasobserved before and after heating, with the structural colorshifting slightly from red to orange [Fig. 6(A)]. Thischange in structural color can also be confirmed by com-paring the reflectance spectra before and after heating. Theheat treatment caused a peak shift (8.3 nm) toward thelower wavelength side. From the Eq. (1), d111 (the inter-layer spacing in the ccp layer of the silica opal film) likelydecreased due to thermal shrinkage. The optimization ofheat treatment conditions is imperative for effective parti-cle fixation. Furthermore, Figs. 6(C) and 6(D) show SEMimages of the microstructure of the silica opal film surfaceafter heat treatment. The (111) planes of the CCP structuremaintained their orientation even after heat treatment.The efficacy of the heat treatment process in adheringsilica opal film to the substrate can be determined throughthe implementation of an abrasion test. Due to constraintsimposed by the equipment, it was not possible to con-duct abrasion testing in this instance for the ceramic plate[Fig. 6(A)]. However, a rudimentary evaluation revealedno occurrence of silica particle detachment, even undermanual manipulation with a finger. The quantitative eval-uation of abrasion resistance remains a future task. Incontrast, the thermal treatment effect on silica opal film ona flat glass plate was evaluated by a simple reciprocatingwear tester shown in Fig. 7. The force applied to the cot-ton rod was 0.98N, with a reciprocating speed of 33 rpmand a stroke of 22mm. Test specimen B, which was silicaopal without heat treatment, showed silica particles peel-ing off after one cotton swab reciprocation, exposing theunderlying glass substrate. Specimens C, D, and E corre-spond to heat treatment conditions of 350 °C for 5 h,Fig. 4. Structural colors with different particle sizes and theirreflection spectra. Reflection spectra for silica particles at sizes of210, 250, and 290 nm.Fig. 5. Relationship between silica particle size and Bragg’sdiffraction peak wavelength. Red circles are data and the redstraight line indicates a linear approximation using least squarescut.Fig. 6. Heat treatment for fixing silica particles. A) Photo afterheating, B) Reflectance spectrum before and after heating treat-ment, C) and D) SEM images microstructures of surface of thesilica opal of 290 nm film after heating.Journal of the Ceramic Society of Japan 134 [5] 375-382 2026 JCS-Japan379400 °C for 5 h, and 450 °C for 5 h, respectively. Test spe-cimen C exhibited scratches exposing the substrate after10 cycles. The specimens D and E showed improved wearresistance. Specimen D exhibited weak scratches after 100cycles. Specimen E, however, exhibited scratches after1000 cycles. It was found that the fixation strength of silicaparticles increases with rising heat treatment temperature.However, heat treatment at 450 °C caused partial loss ofthe structural color of silica opal. This relates to the heatresistance of the glass substrate and indicates an upperlimit to the heat treatment temperature. For the black glasssubstrates used in this experiment, heat treatment at 400 °Cfor 5 h was concluded to be the optimal condition balanc-ing both wear resistance and structural color retention.3.4 Surface modificationsAnother challenge with the structural color of silica opalis the phenomenon where the structural color disappearswhen the surface is wetted with water shown in Fig. 8.The silica opal film, composed of 290 nm silica particlesfollowing heat treatment (400 °C for 5 h), exhibits a redstructural color Fig. 8(A). The structural color of the filmis known to disappear upon its wetting by water, shown inFig. 8(B). However, upon drying, the original red struc-tural color recovers, as original condition. The observedphenomena are believed to originate from Bragg diffrac-tion peaks in the reflection spectrum Figs. 8(C) and 8(D),respectively. In dry condition, the diffraction peak was 624nm and its reflectance was 51.8%. In addition, many smallpeaks due to fringe. In contrast, in wet condition, the dif-fraction peak shifted to 668.1 nm (near NIR wavelength)and the reflectance was 12.2%. This peak shift and reduc-ing intensity origin colorless of silica opal film. When thesilica opal was exposed to water, the peak demonstrated asignificant decrease in intensity. Following the evaporationof the liquid film and subsequent drying, the reflectionpeak underwent a recovery to its initial state.The disappearance of structural color is thought to beprimarily caused by the transition of the interstitial spacein silica opals from air (1.00) to water (1.33). Conse-quently, the average refractive index in Eq. (3) increasesfrom 1.308 to 1.375. The peak shift calculated fromEq. (4) based on the peak shift amount was 31.6 nm.Although slightly smaller than the measured value (43.7nm), main reason can be qualitatively explained by watermolecules filling the gaps between silica particles, shiftingthe peak from the visible (red) to the near-NIR region, aquantitative explanation for the reducing peak intensityremains unclear. However, Fig. 8 suggested that prevent-ing wetting can serve as one countermeasure to achieve thedisappearance of structural color. Inspired by the knowl-edge of technical note,30) a water-repellent layer wasformed on the surface of the silica opal (corresponding tothe third step in Fig. 1).Figure 9 shows the contact angle measurements arecontingent upon the surface condition of silica opal. Thehydrophobic treatment described in this paper was carriedout using the bake-method outlined in the technical notesof the silicone oil manufacturer. Figures 9(A), 9(B) and9(C) are Photos taken from directly above to measurecontact angles on surfaces with different surface conditions.Figure 9(D) shows the contact angle measurement setup.The contact angle was calculated from the measured valuesobtained from this photograph using Eq. (5). Here ªc iscontact angle of a drop of pure water on silica opal film. h, rare high and radius of water drop shown in Fig. 8(D).ªc ¼ 2 arctan h=r ð5ÞFig. 7. Wear resistance evaluation for silica opal film on blackglass after heating. A) a simple reciprocating wear tester. Scratchmarks caused by a cotton swab, B) No heating, C) 350 °C-5 h,D) 400 °C-5 h and E) 450 °C-5 h.Fig. 8. The phenomenon of reversible structural color changes,precipitated by pure water wetting of silica opal film (after heat-ing 400 °C-5 h). A) Red structural color (Dry film condition) andB) Almost colorless (Wet film condition). The reflectance spectraC and D correspond to the photos A and B, respectively.Fudouzi: Structural color coatings for ceramics and glass surface by silica opal filmsJCS-Japan380The contact angles for the three surface states shownin Fig. 1 were summarized in Table 1. Here silica opalconsists of 250 nm in diameter. In case of C, the contactangles indicate over 100 degrees. According to the tech-nical note,30) the silica surface suggested modified froma hydrophilic surface due to silanol groups to a highlyhydrophobic surface with methyl groups substituted.Figure 10 shows the water-repellent effect of the hy-drophobic surface of silica opal in pure water. The testspecimen was prepared by depositing a 290 nm diameterfilm on a black glass substrate, fixing it by heat treat-ment at 400 °C for 5 h, and then treating the silica particlesurface with a hydrophobic coating using the silicon oilbaking method.30) Figure 10(A) shows the silica opal in itsinitial state exhibiting a red structural color when dry.Figure 10(B), on the other hand, shows the substrate im-mersed in pure water. No significant color change is ob-served. Figure 10(C) compares the reflection spectra in thedry and wet states. The peak wavelength shifted 6.7 nmtoward the longer wavelength side during the transitionfrom the dry to the wet state. Conversely, the peak inten-sity decreased by 15.8% due to wetting. Comparing theseresults with the hydrophilic surface in Fig. 8(D) clearlydemonstrates the surface modification effect. Hydrophobictreatment of the silica opal layer is effective in suppressingthe disappearance of structural color caused by water.Regarding the baking process of silicone oil that replacessilanol groups on the surface of silica opal with methylgroups, further detailed investigation, including its mech-anism, is necessary.4. ConclusionThe color decorating process by dip coating from asilica colloid with uniform particle size suspension to formcolloidal crystal films. The result with high quality col-loidal crystal is silica opal film with vivid structural colordue to Bragg’s diffraction. The following substrates werecoated: black glass plates, unglazed and glazed ceramicdishes, and black tile with glaze. The opal film coating’sversatility extends beyond flat and smooth surfaces, as itcan also be applied to curved surfaces and even the unevensurfaces of unglazed ceramics.Additionally, it was determined that the structural colorscan be selected by altering the size of the silica particles.This study confirmed that the optical properties of thecolor-producing layer can be controlled using silica par-ticles, enabling color control of the spectrum from blue(particle size 210 nm) to red (particle size 310 nm).Finally, to stabilize the arranged silica opal films, par-ticle fixation was achieved through a thermal treatmentprocess, and the surface layer was rendered hydrophobic toprevent structural color loss due to wetting. This demon-strated the potential to form stable structural color filmson ceramic and glass substrates. These results indicate thepossibility of replacing existing pigments that are harmfulor have significant environmental impacts.Acknowledgements The author thanks Ms. Kaori Teruifor her technical support on experiments. This work has beenfinancially supported by the Okura Kazuchika MemorialFoundation in 2024 Research Grant. The ceramic productswere obtained from Kusaba Chemical Co., Ltd., Tajimi, Japan.In addition, a part of this work (Taking SEM image photos byJSM-6500F) was supported by the “Advanced Research Infra-structure for Materials and Nanotechnology in Japan (ARIM)”of the Ministry of Education, Culture, Sports, Science andTechnology (MEXT). Proposal Number JPMXP1225NM5378.References1) S. Kinoshita and S. Yoshioka, in “Structural Colors inBiological Systems”, Ed. by S. Kinoshita and S.Fig. 9. Changes of contact angles by surface change. A) A dropof water on 250 nm silica opal film, B) After heating the opalfilm, C) A drop of water formed on a hydrophobic surface. A con-tact angle was obtained by a half-angle method as shown in D).Table 1. Silica opal films with different treatmentsSurface conditionsof silica opal filmsContactangle, ªcA As coated silica opal film 19.3°B After heating, 400 °C for 5 h 11.1°C Hydrophobic, 300 °C for 5min 123.1°ABDryWet● Dry●WetCFig. 10. The hydrophobic silica opal surface effect on struc-tural color of silica opal films. A) Photography on dry condition,B) Photography on wet condition, and C) Spectrum comparisonof dry and wet conditions.Journal of the Ceramic Society of Japan 134 [5] 375-382 2026 JCS-Japan381Yoshioka, Osaka University Press (2005) pp. 3–26.2) G. Guidetti, R. Zanini, G. Franceschin, M. Moglianetti,T. Kim, N. Cohan, L. Chan, J. Treadgold, A. Travigliaand F. G. Omenetto, P. Natl. Acad. Sci. USA 120,e2311583120 (2023).3) C. Y. Chiang, H. F. Greer, R. Liu and W. Zhou, Ceram.Int. 42, 7506 (2016).4) A. Saito, Sci. Technol. Adv. Mat. 12, 064709 (2011).5) Y. Qi and S. Zhang, Structural color due to self-assembly, in “Functional Materials from Colloidal Self-Assembly”, Ed. by G. Zhao and Q. Yan, Wiley-VCHGmbH (2022) pp. 183–235.6) E. Palo, M. A. Papachatzakis, A. Abdelmagid, H.Qureshi, M. Kumar, M. Salomäki and K. S. Daskalakis,J. Phys. Chem. C 127, 14255 (2023).7) W. Feng, J. Paika and L. J. Guo, Mater. Chem. Front. 8,3474 (2024).8) T. Yasuda and T. Takeuchi, J. Ceram. Soc. Jpn. 131,330 (2023).9) J. Zhu, P. Shi, F. Wang, L. Dong and T. Zhao, J. Ceram.Soc. Jpn. 124, 229 (2016).10) P. Shi, F. Wang, J. Zhu, H. Yang, Y. Wang, Y. Fang,B. Zhang and J. Wang, J. Eur. Ceram. Soc. 38, 2228(2018).11) B. Zhang, Y. Wu, Z. Jin, H. Ning, F. Wang, H. Luo, J.Zhu, C. Yang and P. Shi, J. Ceram. Soc. Jpn. 132, 438(2024).12) Y. Takeoka, J. Mater. Chem. 22, 23299 (2012).13) G. Topçu, T. Güner and M. M. Demir, Photonic.Nanostruct. 29, 22 (2018).14) H. Fudouzi, J. Colloid Interf. Sci. 275, 277 (2004).15) H. Fudouzi, Adv. Powder Technol. 20, 502 (2009).16) R. C. Schroden, M. Al-Daous, C. F. Blanford and A.Stein, Chem. Mater. 14, 3305 (2002).17) D. P. Josephson, M. Miller and A. Stein, Z. Anorg. Allg.Chem. 640, 655 (2014).18) M. Ishii, R&D Review of Toyota CRDL 45, 17 (2014).19) K. Katagiri, Y. Tanaka, K. Uemura, K. Inumaru, T. Sekiand Y. Takeoka, NPG Asia Mater. 9, e355 (2017).20) M. Iwata, M. Teshima, T. Seki, S. Yoshioka and Y.Takeoka, Adv. Mater. 29, 1605050 (2017).21) M. Kohri, Sci. Technol. Adv. Mat. 21, 833 (2020).22) C. Park, K. Koh and U. Jeong, Sci. Rep.-UK 5, 8340(2015).23) R. Ohnuki, M. Sakai, Y. Takeoka and S. Yohioka,Langmuir 36, 5579 (2020).24) N. Fukazawa and R. Jin, Hyoumen Gijutsu 61, 754(2010) [in Japanese].25) Y. Ono, J. Asian Ceram. Soc. 8, 578 (2020).26) T. Kohoutek, M. Parchine, M. Bardosova and M. E.Pemble, Colloid. Surface. A 593, 124625 (2020).27) H. Fudouzi and T. Hariyama, J. Jpn. Soc. Colour Mater.93, 149 (2020) [in Japanese].28) M. Vorobyova, F. Biffoli, W. Giurlani, S. M. Martinuzzi,M. Linser, A. Caneschi and M. Innocenti, Materials 16,4919 (2023).29) S. Niyomsoan, W. Grant, D. L. Olson and B. Mishra,Thin Solid Films 415, 187 (2002).30) Technical data sheets, Silicone fluids KF-96 perform-ance test results, Shin-Etsu Chemical Co., Ltd, https://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdf(2014).31) G. T. H. Tran, M. Koike, T. Uchikoshi and H. Fudouzi,Langmuir 36, 10683 (2020).Fudouzi: Structural color coatings for ceramics and glass surface by silica opal filmsJCS-Japan382https://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdfhttps://www.shinetsusilicone-global.com/catalog/pdf/kf96_e.pdf