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Journal of Wood Chemistry and Technology
ISSN: 0277-3813 (Print) 1532-2319 (Online) Journal homepage: https://www.tandfonline.com/loi/lwct20
Structurally Colored Wood Composite with Reflective Heat Insulation and Hydrophobicity Yingying Li, Likun Gao, Yingtao Liu & Jian Li To cite this article: Yingying Li, Likun Gao, Yingtao Liu & Jian Li (2019): Structurally Colored Wood Composite with Reflective Heat Insulation and Hydrophobicity, Journal of Wood Chemistry and Technology, DOI: 10.1080/02773813.2019.1652325 To link to this article: https://doi.org/10.1080/02773813.2019.1652325
Published online: 13 Aug 2019.
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Journal of Wood Chemistry and Technology, 0: 1–10, 2019 Copyright # Taylor & Francis Group, LLC ISSN: 0277-3813 print/1532-2319 online DOI: 10.1080/02773813.2019.1652325
STRUCTURALLY COLORED WOOD COMPOSITE WITH REFLECTIVE HEAT INSULATION AND HYDROPHOBICITY Yingying Li1,2, Likun Gao1,2, Yingtao Liu1, and Jian Li1,2 1
College of Material Science and Engineering, Northeast Forestry University, Harbin, China Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, Harbin, China 2
Structural colors are environmentally beneficial as they originate from the physical structure of the material, and they cannot be imitated by chemical dyes and are free from photo-bleaching. In this study, structurally colored wood was prepared by building a structurally colored film based on a wood substrate. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transforminfrared (FT-IR). The angel-dependent optical effects were confirmed by comparing the CIE chromatic coordinates over a range of viewing angles. Through octadecyltrichlorosilane (OTS) treatment, the sample surface was made hydrophobic and had a water contact angle of 139 , which broadens the range of applications for the material. Thermal gravimetric analysis results showed that the thermal stability of the modified structurally colored wood (MCW) was much better than that of pristine wood. The reflectance spectra and the model room test results demonstrated that the MCW possesses the reflective heat insulation ability. The unique and promising properties of the MCW could potentially be applied in buildings, furniture, and for energy conservation. KEYWORDS. Structural colors, wood composite, energy saving, hydrophobic performance
manner.[9–11] Structural colors have many properties that differ from those of pigmentary colors and bioluminescence.[1,12] The structural colors are environmentally friendly because they originate from the physical structure of the material. Furthermore, structural colors cannot be imitated by chemical dyes or pigments and are free from photobleaching; hence, they are usually more durable than conventional chemical pigments and dyes.[13] Therefore, structurally colored materials are potential candidates to replace dyes and pigments in many applications.[14] As a kind of structurally colored material, mica-titania exhibits a pearl-shine effect due to the angle-dependent optical effects that arise from alternating transparent layers with
INTRODUCTION Nature has been the inspiration for many important human inventions. The unique, brilliant colors in the natural world have attracted a significant amount of research interest.[1–3] To the best of our knowledge, an organism’s color is produced either through pigments, bioluminescence, or structurally.[4–6] The observed colors in a pigment are caused by their absorption of certain spectral wavelengths. Bioluminescence originates from chemical reactions in certain organisms that contain photophores.[7,8] Structural colors are ubiquitous in nature, they result from periodic self-organized microstructures, which interact with light to produce brilliant colors in a complex
Address correspondence to Jian Li, College of Material Science and Engineering, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lwct. 1
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different refractive indices.[15,16] Mica-titania pigments are typically produced by the deposition of TiO2 on mica from an aqueous suspension, followed by a calcination process.[17,18] In general, the higher refractive index of rutile allows for a better color effect to be achieved when TiO2 is coated on mica.[19] Therefore, rutile mica-titania has attracted intensive attention in the field of fillers. Furthermore, rutile mica-titania has also attracted significant attention in the fields of decoration, energy conservation, and as a durable coating because of its excellent reflective heat insulation, weather resistance, and thermal stability. Wood has been used by humans since prehistoric times due to their attractive features, such as good mechanical properties, low thermal expansion, low weight, esthetics, excellent formability, and sustainability.[20,21] It is commonly known that wood is a natural composite composed of cellulose, hemicellulose, and lignin.[22] A network of these three basic components and hollow tubes construct the multi-scale porous structure, which also determines the penetrability, accessibility, and reactivity of wood-based materials.[23–25] As a result, its unique composition and subtle hierarchical structures make wood an ideal template to combine with functional particles. This results in the possibility for it in meeting future market requirement and compete with other advanced materials.[26] Several studies have reported woodbased materials with additional thermochromic or photochromic properties, but the color of the samples all originate from chemical dyes.[27,28] To the best of our knowledge, there have been no reports on structurally colored solid wood materials. In this article, we demonstrate a simple and facile process for fabricating structurally colored wood. The phase composition, morphology, optical properties, thermal stability, and hydrophobic performance were investigated. The structurally colored wood displayed not only a beautiful decorative function but was also an ideal candidate for use as an insulative energy
STRUCTURALLY COLORED WOOD COMPOSITE
saving material. The modified structurally colored wood (MCW) can be economically beneficial because they could reduce the need for air-conditioning by reflecting excess solar heat, thus reducing energy consumption and greenhouse gas emissions. EXPERIMENTAL Materials For this study, mica was used as the substrate and was supplied by Sanbao Pearl Luster Mica Tech CO. LTD, China. Hydrochloric acid (HCl), stannic chloride pentahydrate (SnCl45H2O), sodium hydroxide (NaOH), titanium tetrachloride (TiCl4), and polyvinyl alcohol (PVA) (alcoholysis 98.0–99.0%, model 1750 ± 50) were purchased from Kaitong Chemical Reagent Co. Ltd. (Tian jian, China). Octadecyltrichlorosilane (OTS) (95.0%) was used for modification of the surface hydrophobicity and was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Larch, Birch, and Manchurian ashes were obtained from Harbin, China, which reprocessed into wood slices (radial section) with a size of 250 mm 250 mm 15 mm. The wood samples were oven-dried (24 h, 103 ± 2 C) to constant weight after ultrasonically rinsing in deionized water. All chemicals were used as received without further purification. Synthesis of Rutile Titania-Mica The preparation of the titania-mica pigment was carried out in the following manner. The 10 wt% mica was uniformly dispersed in distilled water. The solution was then maintained at 70 C and the pH value was adjusted to 2.0 with diluted hydrochloric acid under stirring. Then, SnCl4 solution was added in a drop wise manner while the pH was held constant through the simultaneous addition of NaOH. Next, a 1.5 M TiCl4 solution was introduced into the above suspension at a constant speed of 1.0 ml/min. The pH value of the slurry was kept constant by
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FIGURE 1. Schematic illustration of the fabrication of the modified structurally colored wood (MCW).
the simultaneous addition of NaOH solution. The samples were collected by centrifugation, washed with deionized water several times, and dried at 70 C for more than 24 h in vacuum environment. Finally, the titania-mica pigment was obtained by calcination in a muffle furnace at 850 C for 1 h. Synthesis of the Structurally Colored Wood The wood samples were ultrasonically rinsed in deionized water, acetone, and ethanol for 10 min each, and then oven-dried (24 h, 103 ± 2 C) to a constant weight after. A 4 wt% PVA solution was prepared at 75 C for 2 h with magnetic stirring. Next, 0.3 wt% of titania-mica pigment was added to the above solution and stirred for 15 min. Next, a 1.0 ml solution was dropped onto the wood surface and then transferred to a vacuum environment for 10 min in order to make sure the solution fully contacted with the wood substance. Finally, the treated samples were placed in a 30 C oven until completely dry. Unless explicitly stated, the substances of the samples were Larch wood. Preparation of modified structurally colored wood (MCW) As shown in Figure 1, the formation of the hydrophobic structurally colored wood was accomplished by building a selfassembled OTS monolayer. First, the as-prepared samples were submerged into 25 ml of a 1.0% OTS/chloroform solution at room
temperature for 1 h, followed by rinsing with chloroform. The prepared samples were then dried at room temperature for 3 h to obtain the hydrophobic structurally colored wood. Unless explicitly stated, the substances of the samples were Larch wood. Characterization A scanning electron microscope (SEM) (Quanta200; FEI, Hillsboro, OR) was used to analyze the surface morphology. For this, the samples were glued onto a specific holder and coated with gold to ensure conductivity. The phase of the as-prepared sample was checked by X-ray diffraction (XRD) (Rigaku, and D/MAX 2200) measurements operated ˚ ). using Cu target radiation (k ¼ 1.54 A Attenuated total reflectance Fourier transform-infrared spectra (FT-IR) (Nicolet Nexus 670 FT-IR) were recorded in the range of 400–4000 cm1 with a resolution of 4 cm1. Reflectance spectra of these samples were analyzed with the UV-Vis spectrophotometer (UV-Vis DRS) (TU-1901, China) equipped with an integrating sphere. The thermal stabilities of the samples were investigated using a thermal gravimetric analyzer (TGA) (TA, Q600), and the experiments were performed in the temperature range of 25–700 C at a heating rate of 10 C/min under a nitrogen atmosphere. An OCA 40 contact angle system (Dataphysics, Germany) was used to measure the water contact angles (WCAs). For this, a 5 ll droplet of deionized water
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respectively. The rooms were sealed during the testing process. A solar simulator (XES40S2-CE, Lamp L150SS, Japan) with a wavelength range of 300–1100 nm was used to simulate natural light. Two thermoelectric couples were employed to monitor the temperature change inside the two respective rooms. RESULTS AND DISCUSSION XRD Analyses
FIGURE 2. X-ray diffraction patterns of (a) rutile titania-mica, (b) pristine wood, and modified structurally colored wood (MCW).
was injected onto the sample surface at room temperature. Color Test The samples at different observation angles were obtained by using a digital camera, and these records were imported to a computer. The Commission Internationale de L’Eclairage (CIE) L, a, and b values were measured by using the Adobe Photoshop CS6 software installed in the computer.[27] The total color difference is represented by DE, which was calculated using the following equations: DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðL2 L1 Þ2 þ ða2 a1 Þ2 þ ðb2 b1 Þ2
(1)
The L1, a1 and b1 are the color parameters of untreated wood; L2, a2 and b2 are the color parameters of treated samples at different observation angles. The color parameters were measured at five different points on each sample, and the final value for color parameters were obtained as an average of these five measurements.[28] Environment Simulation Test A model house room was constructed using 250 mm 250 mm 15 mm wood planks; the roofs of the two rooms were made of MCW and pristine wood,
XRD was carried out to determine the crystalline structure of the samples. The XRD patterns of rutile titania-mica, pristine wood, and MSW are shown in Figure 2. In Figure 2(a), we see that the XRD patterns exhibit strong diffraction peaks at 27 , 36 and 55 , indicating the presence of TiO2 in the rutile phase (JCPDS No. 21-1276). The characteristic diffraction peaks of KMg3(AlSi3O10)F2 were also observed. The results prove that rutile titania-mica was successfully synthesized and is consistent with values in literature.[29,30] In Figure 2(b), the diffraction peaks centered at 16.1 and 22.2 corresponded to the (101) and (002) diffraction planes of cellulose in wood. As shown in Figure 2(c), characteristic peaks belonging to wood, TiO2, and KMg3(AlSi3O10)F2 also appeared. This confirmed that the rutile titania-mica was successfully anchored onto the surface of wood substance. FT-IR Spectra Figure 3 shows the FT-IR spectra of pristine wood and MCM. As shown in Figure 3(a), the broad peak at 3332 cm1 was assigned to the OH group. The peaks at 1371 cm1 and 1024 cm1 were ascribed to the CH bending vibration and the CO stretching vibration, respectively, which correspond to cellulose and hemicellulose. In Figure 3(b) we see that the intensity of the peak corresponding to the OH group decreased dramatically, which indicates a reduction in hydrophilicity of the MCW. The character peaks associated to cellulose and
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FIGURE 3. Fourier transform-infrared spectra of (a) pristine wood, and (b) modified structurally colored wood (MCW).
hemicellulose were preserved, verifying that the basic structure of the wood was unaffected by the modification. The peaks at 792 cm1 and 686 cm1 were assigned to SiO-Al and confirmed the successful immobilization of the titania-mica particles, which is consistent with the XRD results. The peaks at 1461 cm1 and 1051 cm1 are ascribed to the methylene deformation vibration and SiO-Si, respectively. The results indicated the condensation reaction of the silane, which demonstrate that the OTS film was formed. The results confirm that the hydrophobic coating was successfully applied onto the wood surface. Surface Morphologies The micromorphology of the samples was observed by SEM. The morphology of the broad, smooth, and flaky rutile titania-mica is shown in Figure 4(a). In Figure 4(b), we see that the TiO2 deposited homogenously onto the mica surface. Figure 4(c) shows the multilevel microstructure and multi-scale porous architecture of the pristine Larch wood, the wood cell lumens were very trimly and free of other foreign material. From Figure 4(d), it can be clearly seen that the rutile titania-mica particles are dispersed on the surface of the wood substrate while lying flat. The results suggest that the rutile titania-mica particles were successfully attached to the wood
FIGURE 4. Scanning electron microscopy images of rutile titania-mica at (a) low and (b) high magnification, pristine wood (c), structurally colored wood (d), and the modified structurally colored wood (MCW) at (e) low and (f) high magnification.
surface and are consistent with the XRD results. With the OTS treatment, molecular layers were formed by self-assembly of longchain hydrophobic alkyls (Figure 4(e, f)), forming a hydrophobic film on the surface of the sample. Optical Properties In order to investigate the color characteristics of the samples, the CIE Lab color space values of the samples were measured; where L is the lightness; a ¼ red (þ), green (); and b ¼ yellow (þ), blue (). The total color difference is represented by DE. It is worth mention that a higher DE value indicates a more significant colors effect.[31,32] The L, a, b, and DE values of the MSW samples with different substrates at an
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TABLE 1. Color characters of the samples at an observed angle of 10 . Wood species Larch Birch Manchurian ash
L
a
b
DE
12.0 6.0 24.7
14.0 1.3 5.0
61.0 4.3 23.7
63.7 7.5 34.5
TABLE 2. Color characters of the untreated Larch wood and modified structurally colored wood (MCW) at different observed angle. Larch sample Untreated wood MCW at 10 MCW at 110
L
a
b
65.0 77.0 68.3
17.7 3.6 18.0
36.7 24.3 16
observed angle of 10 are given at Table 1. When the sample was made of Larch wood, the green and blue had the maximum values, as shown in the Table 1. This led to the highest DE value and produced obvious color effects. The results prove that the type of wood species used for the substrate has a direct impact on the color effect of the samples. Among substrates made of Larch, Birch, and Manchurian, it was observed that Larch has the most significant color effect due to its darker texture. In addition, color characters of the untreated Larch wood and modified structurally colored wood (MCW) at different observed angle were also given at Table 2. In Figure 5 we can see the angel-dependent optical effects by comparing the CIE chromatic coordinates. The color of pristine wood is yellow, and the apparent color of MCW varies from blue to red purple on changing the observation angles. Wettability Analyses The wettability of the samples was evaluated by measuring the water contact angle on the sample surface (the sample exhibits hydrophilicity when the water contact angle is less than 90 ). The water contact angle on pristine wood, structurally colored wood, and MSW are shown in Figure 6. The pristine wood surface in Figure 7(a) demonstrates hydrophilicity with a water contact angle of
FIGURE 5. The a and b values of pristine wood and modified structurally colored wood (MCW) at different observation angles.
FIGURE 6. Contact angles of droplets of water on the surface of (a) pristine wood (b) structurally colored wood, and (c) modified structurally colored wood (MCW).
83.5 . And the water contact angle subsequently reduces to 43.0 after 15 s, which proved the hydrophillic nature of pristine wood. As shown in Figure 6(b), the structurally colored wood possesses a hydrophobic surface with a water contact angle of 100 , the contact angle reduces to 89 after 15 s, which can be mainly ascribed to the hydroxyl groups from PVA and the wood substrate. During OTS treatment, the hydroxyl groups produced by OTS hydrolysis react with the surface hydroxyl groups of PVA and the wood substrate; in this manner, long-chain
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FIGURE 7. UV-Vis diffuse reflectance spectra of the (a) rutile titania-mica, (b) modified structurally colored wood (MCW), and (c) pristine wood.
FIGURE 8. The temperature of the model rooms with different irradiation times.
hydrophobic alkyls is introduced on the sample surface. This result in a remarkable reduction in the wettability of the sample, and the MCW shows a hydrophobic surface with a water contact angle of 139 , and the contact angle was maintained at 134 after 15 s (Figure 7(c)). The results suggested that the surface property of wood sample is successfully changed from hydrophilic to hydrophobic by the OTS treatment. Reflective Heat Insulative Ability Figure 7 shows the reflectance spectra of the mica titanium pigments, pristine wood, and MCW. From Figure 7(a), we can see that the rutile titania-mica shows a high reflectance across the whole visible region. In addition, the reflectance of pristine wood increased in the region of 400–900 nm and reached 50% at 900 nm (Figure 7(c)). As shown in Figure 7(b), the reflectance of MSW was generally higher than that of pristine wood and reached 91% at 900 nm, which indicated the radiant barrier ability of MSW. Furthermore, in order to verify the heat shielding performance of the samples in practical application, a comparative experiment between MCW and the pristine wood was carried out with an environment simulation test. As shown in Figure 8, after continuous radiation for 15 min, the temperature of the
FIGURE 9. TG curves of (a) rutile titania-mica, (b) modified structurally colored wood, and (c) pristine wood.
model room with the MCW roof was about 10 C lower than that of the model room with the pristine wood roof. This demonstrates that a part of the solar radiation was blocked by the MCW roof. The results indicate that MCW is an ideal material for smart thermoregulation because of its reflective insulating ability. Thermal Gravimetric Analysis The thermal stability of the samples was evaluated using thermal gravimetric analysis. Figure 9 shows the TG curves of rutile titaniamica, MCW, and pristine wood. The thermogravimetric trace in Figure 9(a) shows that
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STRUCTURALLY COLORED WOOD COMPOSITE
almost no degradation of the rutile titaniamica occurs, which demonstrates that the rutile titania-mica has excellent thermal stability. As seen in Figure 9(c), from 40 to 100 C, the pristine wood exhibited a small weight loss, which was due to loss of moisture and the matter highly volatile. The main pyrolysis process occurred in the range of 200–380 C, which can be attributed to the decomposition of lignin, hemicellulose, and cellulose.[33] At 700 C, a 89% loss in total mass was observed. After modification of the pristine wood with the composite coating (Figure 9(b)), the MCW displays similar thermal decomposition behavior to the pristine wood with a lower mass loss (total mass loss for the MCW is 78%). We see that the protective coating on the MCW results in it having a lower total mass loss than pristine wood during the pyrolysis process. This suggests that the thermal stability of MCW is much better than that of pristine wood. CONCLUSION Structurally colored wood was fabricated by building a structurally colored film on a wood-based substrate. The XRD, FT-IR, and SEM results confirmed that rutile titania-mica was immobilized on the wood substance. The angel-dependent optical effects of the samples were determined by comparing the CIE chromatic coordinates. The CIE Lab values indicated that the substrate made from Larch had the most significant color effect. The wettability analysis confirmed that a hydrophobic film was successful formed on the wood surface and had a water contact angle of 139 . The reflectance spectra and environmental simulation test results demonstrated that MCW has good reflective insulation properties. Finally, the thermal gravimetric analysis showed that the thermal stability of the MCW was much better than that of pristine wood. FUNDING The authors gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities
(Grant 2572018AB16), the National Natural Science Foundation of China (31470584), the Overseas Expertise Introduction Project for Discipline Innovation, 111Project (No. B08016), and the Fundamental Research Funds for the Central Universities (Grant 2572017AB08). CONFLICT OF INTEREST The authors declared that they have no conflict of interest. ORCID Jian Li http://orcid.org/0000-00031962-2494 REFERENCES [1] Kinoshita, S.; Yoshioka, S.; Miyazaki, J. Physics of Structural Colors. Rep. Prog. Phys. 2008, 71, 076401. [2] Ziobro, G. C. Origin and Nature of Kraft Colour: 2 the Role of Bleaching in the Formation of the Extraction Stage Effluent Colour. J. Wood Chem. Technol. 1990, 10, 151–168. [3] Diao, Y. Y.; Liu, X. Y.; Toh, G. W.; Shi, L.; Zi, J. Multiple Structural Coloring of SilkFibroin Photonic Crystals and HumidityResponsive Color Sensing. Adv. Funct. Mater. 2013, 23, 5373–5380. [4] Wei, P.; Zhou, M.; Pan, L.; Xie, J.; Chen, J.; Wang, Y. Suitability of Printing Materials for Heat-Induced Inkless EcoPrinting. J. Wood Chem. Technol. 2016, 36, 129–139. [5] Dumanli, A. G.; Savin, T. Recent Advances in the Biomimicry of Structural Colours. Chem. Soc. Rev. 2016, 45, 6698–6724. DOI: 10.1039/c6cs00129g. [6] Liu, H.; Yang, S.; Ni, Y. Dye Stability in the Presence of Hydrogen Peroxide and Its Implication for Using Dye in the HYP Manufacturing Process. J. Wood Chem. Technol. 2009, 29, 1–10. [7] Yang, Y.; Shao, Q.; Deng, R.; Wang, C.; Teng, X.; Cheng, K.; Cheng, Z.; Huang,
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ISSN: 0277-3813 (Print) 1532-2319 (Online) Journal homepage: https://www.tandfonline.com/loi/lwct20
Structurally Colored Wood Composite with Reflective Heat Insulation and Hydrophobicity Yingying Li, Likun Gao, Yingtao Liu & Jian Li To cite this article: Yingying Li, Likun Gao, Yingtao Liu & Jian Li (2019): Structurally Colored Wood Composite with Reflective Heat Insulation and Hydrophobicity, Journal of Wood Chemistry and Technology, DOI: 10.1080/02773813.2019.1652325 To link to this article: https://doi.org/10.1080/02773813.2019.1652325
Published online: 13 Aug 2019.
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Journal of Wood Chemistry and Technology, 0: 1–10, 2019 Copyright # Taylor & Francis Group, LLC ISSN: 0277-3813 print/1532-2319 online DOI: 10.1080/02773813.2019.1652325
STRUCTURALLY COLORED WOOD COMPOSITE WITH REFLECTIVE HEAT INSULATION AND HYDROPHOBICITY Yingying Li1,2, Likun Gao1,2, Yingtao Liu1, and Jian Li1,2 1
College of Material Science and Engineering, Northeast Forestry University, Harbin, China Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, Harbin, China 2
Structural colors are environmentally beneficial as they originate from the physical structure of the material, and they cannot be imitated by chemical dyes and are free from photo-bleaching. In this study, structurally colored wood was prepared by building a structurally colored film based on a wood substrate. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transforminfrared (FT-IR). The angel-dependent optical effects were confirmed by comparing the CIE chromatic coordinates over a range of viewing angles. Through octadecyltrichlorosilane (OTS) treatment, the sample surface was made hydrophobic and had a water contact angle of 139 , which broadens the range of applications for the material. Thermal gravimetric analysis results showed that the thermal stability of the modified structurally colored wood (MCW) was much better than that of pristine wood. The reflectance spectra and the model room test results demonstrated that the MCW possesses the reflective heat insulation ability. The unique and promising properties of the MCW could potentially be applied in buildings, furniture, and for energy conservation. KEYWORDS. Structural colors, wood composite, energy saving, hydrophobic performance
manner.[9–11] Structural colors have many properties that differ from those of pigmentary colors and bioluminescence.[1,12] The structural colors are environmentally friendly because they originate from the physical structure of the material. Furthermore, structural colors cannot be imitated by chemical dyes or pigments and are free from photobleaching; hence, they are usually more durable than conventional chemical pigments and dyes.[13] Therefore, structurally colored materials are potential candidates to replace dyes and pigments in many applications.[14] As a kind of structurally colored material, mica-titania exhibits a pearl-shine effect due to the angle-dependent optical effects that arise from alternating transparent layers with
INTRODUCTION Nature has been the inspiration for many important human inventions. The unique, brilliant colors in the natural world have attracted a significant amount of research interest.[1–3] To the best of our knowledge, an organism’s color is produced either through pigments, bioluminescence, or structurally.[4–6] The observed colors in a pigment are caused by their absorption of certain spectral wavelengths. Bioluminescence originates from chemical reactions in certain organisms that contain photophores.[7,8] Structural colors are ubiquitous in nature, they result from periodic self-organized microstructures, which interact with light to produce brilliant colors in a complex
Address correspondence to Jian Li, College of Material Science and Engineering, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lwct. 1
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different refractive indices.[15,16] Mica-titania pigments are typically produced by the deposition of TiO2 on mica from an aqueous suspension, followed by a calcination process.[17,18] In general, the higher refractive index of rutile allows for a better color effect to be achieved when TiO2 is coated on mica.[19] Therefore, rutile mica-titania has attracted intensive attention in the field of fillers. Furthermore, rutile mica-titania has also attracted significant attention in the fields of decoration, energy conservation, and as a durable coating because of its excellent reflective heat insulation, weather resistance, and thermal stability. Wood has been used by humans since prehistoric times due to their attractive features, such as good mechanical properties, low thermal expansion, low weight, esthetics, excellent formability, and sustainability.[20,21] It is commonly known that wood is a natural composite composed of cellulose, hemicellulose, and lignin.[22] A network of these three basic components and hollow tubes construct the multi-scale porous structure, which also determines the penetrability, accessibility, and reactivity of wood-based materials.[23–25] As a result, its unique composition and subtle hierarchical structures make wood an ideal template to combine with functional particles. This results in the possibility for it in meeting future market requirement and compete with other advanced materials.[26] Several studies have reported woodbased materials with additional thermochromic or photochromic properties, but the color of the samples all originate from chemical dyes.[27,28] To the best of our knowledge, there have been no reports on structurally colored solid wood materials. In this article, we demonstrate a simple and facile process for fabricating structurally colored wood. The phase composition, morphology, optical properties, thermal stability, and hydrophobic performance were investigated. The structurally colored wood displayed not only a beautiful decorative function but was also an ideal candidate for use as an insulative energy
STRUCTURALLY COLORED WOOD COMPOSITE
saving material. The modified structurally colored wood (MCW) can be economically beneficial because they could reduce the need for air-conditioning by reflecting excess solar heat, thus reducing energy consumption and greenhouse gas emissions. EXPERIMENTAL Materials For this study, mica was used as the substrate and was supplied by Sanbao Pearl Luster Mica Tech CO. LTD, China. Hydrochloric acid (HCl), stannic chloride pentahydrate (SnCl45H2O), sodium hydroxide (NaOH), titanium tetrachloride (TiCl4), and polyvinyl alcohol (PVA) (alcoholysis 98.0–99.0%, model 1750 ± 50) were purchased from Kaitong Chemical Reagent Co. Ltd. (Tian jian, China). Octadecyltrichlorosilane (OTS) (95.0%) was used for modification of the surface hydrophobicity and was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Larch, Birch, and Manchurian ashes were obtained from Harbin, China, which reprocessed into wood slices (radial section) with a size of 250 mm 250 mm 15 mm. The wood samples were oven-dried (24 h, 103 ± 2 C) to constant weight after ultrasonically rinsing in deionized water. All chemicals were used as received without further purification. Synthesis of Rutile Titania-Mica The preparation of the titania-mica pigment was carried out in the following manner. The 10 wt% mica was uniformly dispersed in distilled water. The solution was then maintained at 70 C and the pH value was adjusted to 2.0 with diluted hydrochloric acid under stirring. Then, SnCl4 solution was added in a drop wise manner while the pH was held constant through the simultaneous addition of NaOH. Next, a 1.5 M TiCl4 solution was introduced into the above suspension at a constant speed of 1.0 ml/min. The pH value of the slurry was kept constant by
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FIGURE 1. Schematic illustration of the fabrication of the modified structurally colored wood (MCW).
the simultaneous addition of NaOH solution. The samples were collected by centrifugation, washed with deionized water several times, and dried at 70 C for more than 24 h in vacuum environment. Finally, the titania-mica pigment was obtained by calcination in a muffle furnace at 850 C for 1 h. Synthesis of the Structurally Colored Wood The wood samples were ultrasonically rinsed in deionized water, acetone, and ethanol for 10 min each, and then oven-dried (24 h, 103 ± 2 C) to a constant weight after. A 4 wt% PVA solution was prepared at 75 C for 2 h with magnetic stirring. Next, 0.3 wt% of titania-mica pigment was added to the above solution and stirred for 15 min. Next, a 1.0 ml solution was dropped onto the wood surface and then transferred to a vacuum environment for 10 min in order to make sure the solution fully contacted with the wood substance. Finally, the treated samples were placed in a 30 C oven until completely dry. Unless explicitly stated, the substances of the samples were Larch wood. Preparation of modified structurally colored wood (MCW) As shown in Figure 1, the formation of the hydrophobic structurally colored wood was accomplished by building a selfassembled OTS monolayer. First, the as-prepared samples were submerged into 25 ml of a 1.0% OTS/chloroform solution at room
temperature for 1 h, followed by rinsing with chloroform. The prepared samples were then dried at room temperature for 3 h to obtain the hydrophobic structurally colored wood. Unless explicitly stated, the substances of the samples were Larch wood. Characterization A scanning electron microscope (SEM) (Quanta200; FEI, Hillsboro, OR) was used to analyze the surface morphology. For this, the samples were glued onto a specific holder and coated with gold to ensure conductivity. The phase of the as-prepared sample was checked by X-ray diffraction (XRD) (Rigaku, and D/MAX 2200) measurements operated ˚ ). using Cu target radiation (k ¼ 1.54 A Attenuated total reflectance Fourier transform-infrared spectra (FT-IR) (Nicolet Nexus 670 FT-IR) were recorded in the range of 400–4000 cm1 with a resolution of 4 cm1. Reflectance spectra of these samples were analyzed with the UV-Vis spectrophotometer (UV-Vis DRS) (TU-1901, China) equipped with an integrating sphere. The thermal stabilities of the samples were investigated using a thermal gravimetric analyzer (TGA) (TA, Q600), and the experiments were performed in the temperature range of 25–700 C at a heating rate of 10 C/min under a nitrogen atmosphere. An OCA 40 contact angle system (Dataphysics, Germany) was used to measure the water contact angles (WCAs). For this, a 5 ll droplet of deionized water
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respectively. The rooms were sealed during the testing process. A solar simulator (XES40S2-CE, Lamp L150SS, Japan) with a wavelength range of 300–1100 nm was used to simulate natural light. Two thermoelectric couples were employed to monitor the temperature change inside the two respective rooms. RESULTS AND DISCUSSION XRD Analyses
FIGURE 2. X-ray diffraction patterns of (a) rutile titania-mica, (b) pristine wood, and modified structurally colored wood (MCW).
was injected onto the sample surface at room temperature. Color Test The samples at different observation angles were obtained by using a digital camera, and these records were imported to a computer. The Commission Internationale de L’Eclairage (CIE) L, a, and b values were measured by using the Adobe Photoshop CS6 software installed in the computer.[27] The total color difference is represented by DE, which was calculated using the following equations: DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðL2 L1 Þ2 þ ða2 a1 Þ2 þ ðb2 b1 Þ2
(1)
The L1, a1 and b1 are the color parameters of untreated wood; L2, a2 and b2 are the color parameters of treated samples at different observation angles. The color parameters were measured at five different points on each sample, and the final value for color parameters were obtained as an average of these five measurements.[28] Environment Simulation Test A model house room was constructed using 250 mm 250 mm 15 mm wood planks; the roofs of the two rooms were made of MCW and pristine wood,
XRD was carried out to determine the crystalline structure of the samples. The XRD patterns of rutile titania-mica, pristine wood, and MSW are shown in Figure 2. In Figure 2(a), we see that the XRD patterns exhibit strong diffraction peaks at 27 , 36 and 55 , indicating the presence of TiO2 in the rutile phase (JCPDS No. 21-1276). The characteristic diffraction peaks of KMg3(AlSi3O10)F2 were also observed. The results prove that rutile titania-mica was successfully synthesized and is consistent with values in literature.[29,30] In Figure 2(b), the diffraction peaks centered at 16.1 and 22.2 corresponded to the (101) and (002) diffraction planes of cellulose in wood. As shown in Figure 2(c), characteristic peaks belonging to wood, TiO2, and KMg3(AlSi3O10)F2 also appeared. This confirmed that the rutile titania-mica was successfully anchored onto the surface of wood substance. FT-IR Spectra Figure 3 shows the FT-IR spectra of pristine wood and MCM. As shown in Figure 3(a), the broad peak at 3332 cm1 was assigned to the OH group. The peaks at 1371 cm1 and 1024 cm1 were ascribed to the CH bending vibration and the CO stretching vibration, respectively, which correspond to cellulose and hemicellulose. In Figure 3(b) we see that the intensity of the peak corresponding to the OH group decreased dramatically, which indicates a reduction in hydrophilicity of the MCW. The character peaks associated to cellulose and
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FIGURE 3. Fourier transform-infrared spectra of (a) pristine wood, and (b) modified structurally colored wood (MCW).
hemicellulose were preserved, verifying that the basic structure of the wood was unaffected by the modification. The peaks at 792 cm1 and 686 cm1 were assigned to SiO-Al and confirmed the successful immobilization of the titania-mica particles, which is consistent with the XRD results. The peaks at 1461 cm1 and 1051 cm1 are ascribed to the methylene deformation vibration and SiO-Si, respectively. The results indicated the condensation reaction of the silane, which demonstrate that the OTS film was formed. The results confirm that the hydrophobic coating was successfully applied onto the wood surface. Surface Morphologies The micromorphology of the samples was observed by SEM. The morphology of the broad, smooth, and flaky rutile titania-mica is shown in Figure 4(a). In Figure 4(b), we see that the TiO2 deposited homogenously onto the mica surface. Figure 4(c) shows the multilevel microstructure and multi-scale porous architecture of the pristine Larch wood, the wood cell lumens were very trimly and free of other foreign material. From Figure 4(d), it can be clearly seen that the rutile titania-mica particles are dispersed on the surface of the wood substrate while lying flat. The results suggest that the rutile titania-mica particles were successfully attached to the wood
FIGURE 4. Scanning electron microscopy images of rutile titania-mica at (a) low and (b) high magnification, pristine wood (c), structurally colored wood (d), and the modified structurally colored wood (MCW) at (e) low and (f) high magnification.
surface and are consistent with the XRD results. With the OTS treatment, molecular layers were formed by self-assembly of longchain hydrophobic alkyls (Figure 4(e, f)), forming a hydrophobic film on the surface of the sample. Optical Properties In order to investigate the color characteristics of the samples, the CIE Lab color space values of the samples were measured; where L is the lightness; a ¼ red (þ), green (); and b ¼ yellow (þ), blue (). The total color difference is represented by DE. It is worth mention that a higher DE value indicates a more significant colors effect.[31,32] The L, a, b, and DE values of the MSW samples with different substrates at an
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TABLE 1. Color characters of the samples at an observed angle of 10 . Wood species Larch Birch Manchurian ash
L
a
b
DE
12.0 6.0 24.7
14.0 1.3 5.0
61.0 4.3 23.7
63.7 7.5 34.5
TABLE 2. Color characters of the untreated Larch wood and modified structurally colored wood (MCW) at different observed angle. Larch sample Untreated wood MCW at 10 MCW at 110
L
a
b
65.0 77.0 68.3
17.7 3.6 18.0
36.7 24.3 16
observed angle of 10 are given at Table 1. When the sample was made of Larch wood, the green and blue had the maximum values, as shown in the Table 1. This led to the highest DE value and produced obvious color effects. The results prove that the type of wood species used for the substrate has a direct impact on the color effect of the samples. Among substrates made of Larch, Birch, and Manchurian, it was observed that Larch has the most significant color effect due to its darker texture. In addition, color characters of the untreated Larch wood and modified structurally colored wood (MCW) at different observed angle were also given at Table 2. In Figure 5 we can see the angel-dependent optical effects by comparing the CIE chromatic coordinates. The color of pristine wood is yellow, and the apparent color of MCW varies from blue to red purple on changing the observation angles. Wettability Analyses The wettability of the samples was evaluated by measuring the water contact angle on the sample surface (the sample exhibits hydrophilicity when the water contact angle is less than 90 ). The water contact angle on pristine wood, structurally colored wood, and MSW are shown in Figure 6. The pristine wood surface in Figure 7(a) demonstrates hydrophilicity with a water contact angle of
FIGURE 5. The a and b values of pristine wood and modified structurally colored wood (MCW) at different observation angles.
FIGURE 6. Contact angles of droplets of water on the surface of (a) pristine wood (b) structurally colored wood, and (c) modified structurally colored wood (MCW).
83.5 . And the water contact angle subsequently reduces to 43.0 after 15 s, which proved the hydrophillic nature of pristine wood. As shown in Figure 6(b), the structurally colored wood possesses a hydrophobic surface with a water contact angle of 100 , the contact angle reduces to 89 after 15 s, which can be mainly ascribed to the hydroxyl groups from PVA and the wood substrate. During OTS treatment, the hydroxyl groups produced by OTS hydrolysis react with the surface hydroxyl groups of PVA and the wood substrate; in this manner, long-chain
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FIGURE 7. UV-Vis diffuse reflectance spectra of the (a) rutile titania-mica, (b) modified structurally colored wood (MCW), and (c) pristine wood.
FIGURE 8. The temperature of the model rooms with different irradiation times.
hydrophobic alkyls is introduced on the sample surface. This result in a remarkable reduction in the wettability of the sample, and the MCW shows a hydrophobic surface with a water contact angle of 139 , and the contact angle was maintained at 134 after 15 s (Figure 7(c)). The results suggested that the surface property of wood sample is successfully changed from hydrophilic to hydrophobic by the OTS treatment. Reflective Heat Insulative Ability Figure 7 shows the reflectance spectra of the mica titanium pigments, pristine wood, and MCW. From Figure 7(a), we can see that the rutile titania-mica shows a high reflectance across the whole visible region. In addition, the reflectance of pristine wood increased in the region of 400–900 nm and reached 50% at 900 nm (Figure 7(c)). As shown in Figure 7(b), the reflectance of MSW was generally higher than that of pristine wood and reached 91% at 900 nm, which indicated the radiant barrier ability of MSW. Furthermore, in order to verify the heat shielding performance of the samples in practical application, a comparative experiment between MCW and the pristine wood was carried out with an environment simulation test. As shown in Figure 8, after continuous radiation for 15 min, the temperature of the
FIGURE 9. TG curves of (a) rutile titania-mica, (b) modified structurally colored wood, and (c) pristine wood.
model room with the MCW roof was about 10 C lower than that of the model room with the pristine wood roof. This demonstrates that a part of the solar radiation was blocked by the MCW roof. The results indicate that MCW is an ideal material for smart thermoregulation because of its reflective insulating ability. Thermal Gravimetric Analysis The thermal stability of the samples was evaluated using thermal gravimetric analysis. Figure 9 shows the TG curves of rutile titaniamica, MCW, and pristine wood. The thermogravimetric trace in Figure 9(a) shows that
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almost no degradation of the rutile titaniamica occurs, which demonstrates that the rutile titania-mica has excellent thermal stability. As seen in Figure 9(c), from 40 to 100 C, the pristine wood exhibited a small weight loss, which was due to loss of moisture and the matter highly volatile. The main pyrolysis process occurred in the range of 200–380 C, which can be attributed to the decomposition of lignin, hemicellulose, and cellulose.[33] At 700 C, a 89% loss in total mass was observed. After modification of the pristine wood with the composite coating (Figure 9(b)), the MCW displays similar thermal decomposition behavior to the pristine wood with a lower mass loss (total mass loss for the MCW is 78%). We see that the protective coating on the MCW results in it having a lower total mass loss than pristine wood during the pyrolysis process. This suggests that the thermal stability of MCW is much better than that of pristine wood. CONCLUSION Structurally colored wood was fabricated by building a structurally colored film on a wood-based substrate. The XRD, FT-IR, and SEM results confirmed that rutile titania-mica was immobilized on the wood substance. The angel-dependent optical effects of the samples were determined by comparing the CIE chromatic coordinates. The CIE Lab values indicated that the substrate made from Larch had the most significant color effect. The wettability analysis confirmed that a hydrophobic film was successful formed on the wood surface and had a water contact angle of 139 . The reflectance spectra and environmental simulation test results demonstrated that MCW has good reflective insulation properties. Finally, the thermal gravimetric analysis showed that the thermal stability of the MCW was much better than that of pristine wood. FUNDING The authors gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities
(Grant 2572018AB16), the National Natural Science Foundation of China (31470584), the Overseas Expertise Introduction Project for Discipline Innovation, 111Project (No. B08016), and the Fundamental Research Funds for the Central Universities (Grant 2572017AB08). CONFLICT OF INTEREST The authors declared that they have no conflict of interest. ORCID Jian Li http://orcid.org/0000-00031962-2494 REFERENCES [1] Kinoshita, S.; Yoshioka, S.; Miyazaki, J. Physics of Structural Colors. Rep. Prog. Phys. 2008, 71, 076401. [2] Ziobro, G. C. Origin and Nature of Kraft Colour: 2 the Role of Bleaching in the Formation of the Extraction Stage Effluent Colour. J. Wood Chem. Technol. 1990, 10, 151–168. [3] Diao, Y. Y.; Liu, X. Y.; Toh, G. W.; Shi, L.; Zi, J. Multiple Structural Coloring of SilkFibroin Photonic Crystals and HumidityResponsive Color Sensing. Adv. Funct. Mater. 2013, 23, 5373–5380. [4] Wei, P.; Zhou, M.; Pan, L.; Xie, J.; Chen, J.; Wang, Y. Suitability of Printing Materials for Heat-Induced Inkless EcoPrinting. J. Wood Chem. Technol. 2016, 36, 129–139. [5] Dumanli, A. G.; Savin, T. Recent Advances in the Biomimicry of Structural Colours. Chem. Soc. Rev. 2016, 45, 6698–6724. DOI: 10.1039/c6cs00129g. [6] Liu, H.; Yang, S.; Ni, Y. Dye Stability in the Presence of Hydrogen Peroxide and Its Implication for Using Dye in the HYP Manufacturing Process. J. Wood Chem. Technol. 2009, 29, 1–10. [7] Yang, Y.; Shao, Q.; Deng, R.; Wang, C.; Teng, X.; Cheng, K.; Cheng, Z.; Huang,
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