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Scalable aesthetic transparent wood for energy efficient buildings

Scalable aesthetic transparent wood for energy efficient buildings ARTICLE https://doi.org/10.1038/s41467-020-17513-w OPEN Scalable aesthetic transparent wood for energy efficient buildings 1,6 1,2,6 3,4 5 1 1 1 Ruiyu Mi , Chaoji Chen , Tobias Keplinger , Yong Pei , Shuaiming He , Dapeng Liu , Jianguo Li , 1 1 5 3,4 1,2 Jiaqi Dai , Emily Hitz , Bao Yang , Ingo Burgert & Liangbing Hu Nowadays, energy-saving building materials are important for reducing indoor energy con- sumption by enabling better thermal insulation, promoting effective sunlight harvesting and offering comfortable indoor lighting. Here, we demonstrate a novel scalable aesthetic transparent wood (called aesthetic wood hereafter) with combined aesthetic features (e.g. intact wood patterns), excellent optical properties (an average transmittance of ~ 80% and a −1 −1 haze of ~ 93%), good UV-blocking ability, and low thermal conductivity (0.24 W m K ) based on a process of spatially selective delignification and epoxy infiltration. Moreover, the rapid fabrication process and mechanical robustness (a high longitudinal tensile strength of −3 91.95 MPa and toughness of 2.73 MJ m ) of the aesthetic wood facilitate good scale-up capability (320 mm × 170 mm × 0.6 mm) while saving large amounts of time and energy. The aesthetic wood holds great potential in energy-efficient building applications, such as glass ceilings, rooftops, transparent decorations, and indoor panels. 1 2 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA. Center for Materials Innovation, University of 3 4 Maryland, College Park, MD 20742, USA. Wood Materials Science, ETH Zürich, Stefano-Franscini-Platz 3, CH-8093 Zürich, Switzerland. Wood Technology, Cellulose & Wood Materials, EMPA, CH-8600 Dubendorf, Switzerland. Department of Mechanical Engineering, University of Maryland, 6 ✉ College Park, MD 20742, USA. These authors contributed equally: Ruiyu Mi, Chaoji Chen. email: binghu@umd.edu NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w o date, the development of green, energy-saving materials softwood pine disk): the EW is usually wider, weaker, more has been a prevailing research topic from the perspective of porous and lighter in color than the LW. In their microstructure, Tsustainability in response to the rapidly growing burden of EW cells have a relatively larger lumen diameter and thinner cell 1,2 18 energy consumption and environmental pollution . Natural walls compared to LW . By profiting from the special structural materials such as wood and its derivatives have been regarded as organization of Douglas fir wood, aesthetic wood not only one of the most important alternatives in green and energy- inherited the original aesthetic from the wood itself but also efficient buildings due to the abundance, renewability, low cost, possessed favorable optical, and mechanical properties. Moreover, 3,4 and sustainability of source materials . One latest trend in the efficient delignification process made it possible to realize the wood-based building material fabrication is the recently devel- large-scale production of transparent wood with little time and oped transparent wood composite, which integrates the aniso- energy consumption. tropic hierarchical wood structure with optical, mechanical, and Figure 1b demonstrates the fabrication process of aesthetic 5–7 thermal properties . Numerous merits are endowed to the wood-R: natural wood is obtained by the industry-adopted cross- transparent wood composites including light weight, high section cutting method with the annual growth rings visible due optical transmittance, tunable haze, low thermal conductivity to the sharp microstructure difference between EW and LW 8–10 compared to glass and excellent mechanical robustness . (Fig. 1b–d). After spatially selective delignification using our Additionally, transparent wood composites can harvest sunlight efficient method, the EW area has become almost completely effectively due to the light guiding effect, which is meaningful white due to the light scattering and the vast removal of light for energy saving and comfortable indoor lighting. With these absorbers (i.e., lignin and some extractives) while the LW area integrated advantages, the transparent wood composites have preserves partial lignin. Afterwards, the refractive-index-matched emerged as the promising engineering components (e.g., rooftops, polymer/epoxy was filled into the wood backbone to prepare the windows, and transparent decorations) in green energy-efficient aesthetic wood based on this special structure. The scanning 1,9 buildings . electronic microscope (SEM) image in Fig. 1e specifically reveals Current approaches for fabricating transparent wood compo- the maintaining dense structure after fully impregnating polymer, sites were generally based on a complete (or nearly complete) particularly in the LW area. Aesthetic wood-R was fabricated by delignification process, that is, removing most of light absorbing this approach, which possesses not only the preserved wood 11–13 materials (lignin and extractives) or chromophoric compo- patterns, but a high average transparency (80% at 600 nm, the nents with lignin remaining about 80% . However, the intensive UMD logo can be seen clearly behind the aesthetic wood-R, see chemical treatment can severely break down the original wood Fig. 1f). This work provides new horizons and more potentials for structure (e.g., the cell wall is partly degraded, and the growth green buildings and other construction applications, which is ring patterns become less visible) to ensure efficient impossible to achieve with regular glass. polymer infiltration. In addition, these previous works generally focused on the morphology and anisotropy of optical, mechan- 13,15,16 ical, and thermal properties , yet alternating structures, the Morphological and chemical characterizations of aesthetic natural aesthetics of wood’s original annual growth patterns, wood. Softwood (gymnosperms) generally relies on tracheids to 19,20 and scalable manufacturing via efficient process are rarely transport water, like pine, and Douglas fir . The structural discussed. differences between different wood species of softwood and In this work, we develop an aesthetic transparent wood hardwood can lead to different aesthetic results. Here, Douglas fir (denoted as aesthetic wood) by spatial-selectively removing lignin was chosen as a proof-of-concept demonstration, which has a of native wood material to make wood transparent and preserve pronounced contrast of both color and density between EW and its natural patterns simultaneously. Softwood (e.g., Douglas fir) is LW and exhibits a unique aesthetic with obvious wood chosen as the proof-of-concept demonstration due to the pro- patterns. The mesoporous structure of Douglas fir is shown in nounced structural contrast between its low-density earlywood Fig. 2a. There is a rather sharp boundary between EW and LW to (EW) and high-density latewood (LW). In a short 2 h chemical a very indistinct separation in Douglas fir. The EW is more treatment, natural wood is selectively delignified to preserve its porous with much thinner cell walls (1.4~2.6 μm) than the LW original growth ring patterns. The refractive-index-matched (5~10 μm) (Fig. 2b, c). The distribution of the wood tracheids epoxy is then infiltrated into nanoscale framework to make the as shown in Fig. 2d (hollow tube-like structure) with a lumen wood transparent with preserved wood patterns. Consequently, diameter range of 20–80 μm and 5–35 μm involving EW and the novel concept of aesthetic wood in this work is demonstrated LW, respectively (Fig. 2e). As a result, the different pore-size for the first time possessing integrated excellent functions of distributions are usually indicative of different densities of optical transparency, UV-blocking, thermal insulation, mechan- EW and LW. ical strength, scalability, and aesthetics. We anticipate such In a typical experiment, a wood block with dimensions of multifunctional aesthetic wood will hold great potential in 60 mm × 60 mm × 2 mm was applied for the procedure analysis modern green buildings. of delignification treatment. Briefly, a simple delignification process using the acidic NaClO method was employed to partial remove the colored components (mainly lignin, along with Results extractives) from the bulk wood. As shown in Fig. 2f, the Fabrication of aesthetic wood. We demonstrate two types of evolution of the wood’s macroscopic color indicates the removal aesthetic wood based on the periodicity and anisotropy of natural of the color compounds presented at the surface. Specifically, the wood (Fig. 1a): one type with aligned microchannels perpendi- weight loss at various points during the delignification process cular to the wood plane is defined as aesthetic wood-R while (0–10 h) was recorded by a balance in the dry states (dried at another type with channels parallel to the wood plane is aesthetic 100 °C for 48 h, Fig. 2g), which is mainly ascribed to the removal wood-L. Natural softwood presents the intrinsic aesthetic prop- of lignin and a little extractive during the process. After 2 h of erties of the annual growth ring patterns with alternating struc- treatment, spatially selective delignification can be realized: the tures at macroscopic and microscopic scales . From the macro EW has become almost completely white whereas the LW kept perspective, the rings are developed by the alternating formation the pattern well owing to the residual lignin and colored of EW in spring and LW in summer (Supplementary Fig. 1, components. The main contributor to spatially selective 2 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE Periodicity: annual growth rings Alternating structure with natural aesthetics Earlywood and Latewood Anisotropy: aligned channels Efficient lignin removal Light guiding Natural wood b c d 100µm 1% @600nm Selective Polymer delignification infiltration Aesthetic wood e f Earlywood Latewood 100µm 80% @600nm Fig. 1 Fabrication, microstructure and appearance of aesthetic wood. a An indication of the design which combines the periodicity (annual growth rings) with anisotropy (aligned channels) of wood to realize a new kind of transparent wood composite. b Schematic to display the procedures for fabricating aesthetic wood (aesthetic wood-R) from natural wood with vertically aligned cells and annual growth rings after fast spatially selective delignification and polymer infiltration. c, e The cross-sectional SEM images of natural wood and dense aesthetic wood-R microstructures after polymer filling (there is a sharp boundary between EW and LW). d, f Photos to show a large piece of aesthetic wood-R (86 mm × 86 mm × 2 mm) with preserved wood patterns and high average transparency (80% at 600 nm) derived from Douglas fir. delignification is the inherent structural difference between EW hardwood possessing bimodal pores and uniform solution and LW, which accordingly leads to a faster solution diffusion in diffusion (Supplementary Fig. 6). EW than in LW (Supplementary Fig. 2). The weight loss was Raman spectroscopy imaging in combination with vertex 13.5 wt% after 2 h of treatment (Fig. 2g). After selective component analysis (VCA), a multivariate data analysis method, delignification, the nano- and macro-features of original wood were employed to assess the distribution of lignin in the wood were essentially preserved as well (Supplementary Fig. 3), scaffold after selective delignification . The cell wall component including the wood patterns to show the nature of aesthetics. for EW and LW in natural wood and delignified wood, Note that it took much more time (e.g. 10 h) for LW to be respectively are shown in Fig. 2h. The corresponding Raman completely transformed white with a corresponding weight loss of spectra are demonstrated in Fig. 2i, especially the characteristic −1 −1 ~35 wt%. Moreover, the integrity of the delignified wood structure bands of lignin component locate at 1598 cm , 1656 cm and −1 cannot be maintained upon long treatment times resulting in poor 1269 cm (a marker band of the aryl-OH and aryl-OCH in 25,26 mechanical properties (treated 9–10 h) (Fig. 2f) owing to the guaiacyl (G) units in lignin) ascribing to aromatic C=C distinct density difference (Supplementary Fig. 4) between the EW stretching, coniferyl alcohol C=C, C=O stretching, and C–H −3 −3 (284.6 kg m ) and LW (846 kg m ), and the complete removal banding of C=C, aromatic C=C stretching, respectively. As 14,22,23 of lignin, which also acts as binder among the wood cells . expected, contrasting with the EW and LW in natural CWs, the Note that the choice of wood species is vital to the successful representative lignin bands almost disappear in EW cell walls fabrication of aesthetic wood. Although both hardwood and after delignification while they are still shown in LW cell walls. −1 softwood are in principal suitable, hardwood possesses a Meanwhile, respective cellulose peaks, for example at 1095 cm significantly different structure consisting of vessels and fibers (C–O–C stretching vibrations), remain unchanged after NaClO (Supplementary Fig. 5), while softwood mainly consists of treatment . These results give strong evidence to support that tracheids . Basswood, a type of hardwood, for example, has most of the lignin in the EW has been removed while a small uniform cell wall thickness of around 1.3~2.9 μm (Supplementary proportion of the lignin in the LW remains. This phenomenon Fig. 5c), much thinner than the cell wall thickness of LW in leads to the retained aesthetic wood patterns in the final aesthetic Douglas fir. Meanwhile, the vessel channels exhibit larger lumen wood products. diameters than narrow tracheids (Supplementary Fig. 5d), with bimodal pore-size distribution (Supplementary Fig. 5e). Conse- quently, owing to the almost synchronous reaction process in Scalability and the mechanical properties of aesthetic wood. basswood of the EW and LW, there are almost no apparent Following the same procedure, we then constructed the aesthetic wood patterns preserved after a couple of hours’ treatments wood-L with straight patterns created by the quarter slicing (Supplementary Fig. 5f). The same result occurred in balsa wood cutting strategy (Fig. 3a). The efficient spatially selective (another type of hardwood), confirming that aesthetic wood with delignification process not only endows excellent structural patterns is nearly impossible to fabricate from diffuse-porous integrity but also facilitates the large-scale production of aesthetic NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w a b d Tracheid ~3.8µm 20µm 20 30 40 50 60 70 80 Diameter (µm) c 25 ~15.7µm 15 200µm 20µm 50µm 0 510 15 20 25 30 Diameter (µm) 0h 0.5h 1h 2h f g Our process White wood 4h 6h 9h 10h 02468 10 Break Break Reaction time (h) EW LW hi Lignin Delignified LW Delignified EW 10µm 10µm 0 Natural LW Cellulose Natural EW 10µm 10µm 400 800 1200 1600 –1 Wavenumber (cm ) Fig. 2 Morphological and chemical characterizations of aesthetic wood. a The SEM image of Douglas fir to show its mesoporous structure. b, c Magnified SEM images of EW and LW to present the differences in microstructural lumina. d The aligned micro-sized channels with tracheids. e The pore diameter distributions of EW and LW in the natural Douglas fir. f Photo comparison of color and pattern changes in wood templates during lignin removal process in the laboratory (0–10 h). g The weight loss behavior as a function of delignification process time. Error bars represent standard deviation. h Cell wall components of the EW and LW in natural wood (non-treated reference) and delignified wood cell wall (CW) after VCA. i The corresponding Raman spectra in (h). wood-L. In Fig. 3b, we demonstrate the ability to fabricate a direction after successful infiltration. Zoomed in from the top sample size of 320 mm × 170 mm × 0.6 mm, which is significantly view, although the lumen of LW is much smaller than that of larger than all reported transparent woods using delignified wood EW, they are all densely packed (Fig. 3d, e). Additionally, the 1,9–11,15,16,28 as the framework (Supplementary Table 1) . Large- identical channels and apertures are fully filled with polymer scale manufacturing has been regarded as one of the major (Fig. 3f) which acts as a glue to create strong interaction challenges for transparent wood manufacturing and indus- between the cellulosic cell wall and polymer itself. Raman trialization. Our work points to a potential route towards spectroscopy imaging was further performed to identify the addressing the manufacturing challenge of transparent wood (e.g., distribution of the impregnating polymer in the obtained large scale with a short processing time). However, it is worth wood cell including cell corner (CC), compound middle mentioning that thicker aesthetic wood yet without compromis- lamella (CML) (Fig. 3h), cellwall(CW)(Fig. 3i) and lumen ing its aesthetic features and other properties is preferred in order (Fig. 3j). According to the corresponding Raman spectrum in to provide better load-bearing properties in building applications, Fig. 3k, the strong-signal peaks within lumina indicate the bond −1 which should be considered in future research. stretching of epoxy: 640 cm (aromatic C–H out-of-plane −1 The straight-line patterns display the traditional symmetric deformation), 1001 cm (polyamidoamine adduct, amino −1 29 aesthetic (the background is an A4 paper). In the meantime, the groups) and 1608 cm (aromatic ring breathing mode) . obtained aesthetic wood-L (with a thickness of 0.6 mm) is Polymer signals can also be detected in the CML/CC and CW, optically transparent (Supplementary Fig. 7), with a total suggesting that polymer has been well-infiltrated into the wood transmittance of 87% and optical haze of 65% at 600 nm. To cells, forming robust interfaces with cellulose in the delignified identify the compatibility between epoxy and treated wood wood scaffold. scaffold, SEM was applied to illustrate the detailed micro- From the perspective of construction materials, the mechan- structures. Figure 3c shows that aesthetic wood displays ical properties are equally important . The hierarchical cellular massive aligned dense microchannels along the wood growth structure of wood leads to unique anisotropic mechanical 4 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications Delignified wood Natural wood Weight (%) Raman intensity (A.U.) Counts (%) Counts (%) Aligned nanofiber NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE a b EW LW Quarter slicing cutting d fg 20µm 200µm 20µm 100µm 500nm zx Lumen CC/CML CW Cellulose Pol. 10µm CML/CC Lignin Polymer 10µm CW Lumen 10µm 400 600 800 1000 1200 1400 1600 –1 Wavenumber (cm ) Fig. 3 Scalability of aesthetic wood. a Schematic of quarter slicing cutting to obtain the wood veneer with straight-line patterns. b A large-scale aesthetic wood assembled by L-wood veneer (demonstrated for sample size of 320 mm × 170 mm × 0.6 mm). c SEM image of the preserved whole wood microstructure after filling with polymer. d–e Zoomed-in SEM images to show the EW and LW well-defined lumina full of polymer. f–g The detailed SEM image of the aligned micro-sized channels and the aligned cellulose nanofibers on the corresponding cell wall. h–j VCA of wood cells in obtained aesthetic wood. k Corresponding Raman spectra. features. As shown in Supplementary Fig. 8a, aesthetic wood-R aesthetic wood-L after tensile tests show that the polymer is exhibits dramatically improved tensile strength over natural R- fully filled in the middle of the wood backbone and connects the wood (21.56 MPa vs. 6.24 MPa), while aesthetic wood-L separated wood fibers. The anisotropic mechanical properties of possesses a higher tensile strength of 91.95 MPa due to the aesthetic wood can be attributed to the aligned cellulose synergy between the wood matrix and filling polymer. The nanofibers in the cell wall, giving rise to high strength along toughness of aesthetic wood-R and aesthetic wood-L are 0.523 the fiber direction yet relatively low strength perpendicular to −3 −3 8 MJ m and 2.73 MJ m , respectively (Supplementary Fig. 8b), the tracheid direction (Fig. 3g) . enhanced contrasting with natural wood, yet yielding the remarkably mechanical anisotropy. For energy efficient build- Optical properties and patterns design of aesthetic wood. ings, both high tensile strength and high toughness are greatly Previous works have demonstrated that by tuning the starting advantageous for load-bearing functions .Moreover, to 31,32 wood materials or the chemical processing parameters , further reveal the details of the fracture behavior of natural some blurred wood patterns can be maintained in the final wood and aesthetic wood, the fractured surface after tensile products. It remains challenging to achieve clear and designable measurement of each type was characterized by SEM (Supple- aesthetic patterns in transparent wood with integrated advan- mentary Fig. 8c–f). The porous lumen structure and aligned tageous features such as a high optical transmittance, UV- microchannels in the natural wood are visible after fracture. blocking capability, low thermal conductivity and high The cross-sectional SEM images of aesthetic wood-R and mechanical strength. Our aesthetic wood, demonstrated for the NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 5 Raman intensity (A.U.) ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w UV VIS ab 100 100 80 80 Absoption Transmittance 1 7 EW 1’ 7’ LW Reflectance 0 0 1 1' 2 2' 3 3' 4 4' 5 5' 6 6' 7 7' 8 8' 200 300 400 500 600 700 800 Location Wavelength (nm) One-layer pattern Stack two layers Two-layer pattern Fig. 4 Optical properties and patterns design of aesthetic wood. a The transmittance in the EW and LW of obtained aesthetic wood (The locations marked 1–8 represent the EW areas while 1′–8′ represent the LW areas). b As-prepared aesthetic wood exhibits excellent UV-blocking performance: high absorption in 200–400 nm, high transmittance at 600 nm and low reflectance. c The latticed aesthetic patterns can be obtained by stacking two layers of aesthetic wood. first time, features a combination of these abovementioned 200–400 nm, a high average transparency (80%) at 600 nm, and advantages. Such combined advantages are desirable for energy alow reflectance at the visible wavelengths (Fig. 4b). efficient building applications, particularly in pattern ceiling. Subsequently, more types of patterns can be realized by Owing to the inhomogeneous distribution of lignin and cell stacking multiple layers of aesthetic wood. For example, various structures between EW and LW in the assembled aesthetic lattice patterns can be designed by stacking two layers of aesthetic wood, the transmittance is not uniform. We chose 8 locations wood rotated at an angle relative to each other. Based on the high in EW and LW areas and measured the transmittance, transmittance and intrinsic aesthetic, this capability can enable respectively (Fig. 4a). As initially conceived, the LW area the potential application on patterned ceilings (Fig. 4c). More- exhibits lower transmittance (Average ≈ 68%) than in EW area over, the abundance of patterns can be further increased by (Average ≈ 86%). Despite the difference of transmittance developing aesthetic wood using other wood species (mainly between LW and EW, the remaining pattern only slightly softwoods) through this universal fabrication method. Here, decreases the overall average transmittance of the aesthetic another type of esthetic wood was also fabricated using pine wood in the visible light region. (Supplementary Fig. 10). Double-layer pine aesthetic wood and Furthermore, the favorable UV-blocking performance is one-layer pine aesthetic wood with one-layer Douglas fir aesthetic specially expected in aesthetic wood. Solar radiation reaching wood both show various aesthetic patterns (Supplementary the earth surface constitutes infrared radiation, visible light, Fig. 10b–d). and ultraviolet (UV) radiation, in which UV is made up of three Furthermore, we evaluated the weathering stability of aesthetic bands UVA (320–400 nm), UVB (275–320 nm) and UVC wood-R and -L by exposing the materials outdoors for 3 weeks (200–275 nm), the smaller wavelength has the stronger and measuring the optical properties, including the transmittance energy . UVC (possessing the highest energy) is filtered by and haze (Supplementary Fig. 11a, b). Compared with the the ozone layer. UV is invisible to the human eye but can original aesthetic wood-R, the transmittance of the outdoor- provide a hazard and damage to many materials including exposed aesthetic wood-R decreased slightly while the haze furniture and interior displays .Particularly,3.5%UVB and increased from ~93% to ~98% in the wavelength range of 96.5% UVA will reach the earth’ssurface on asummerday. 400–800 nm. Moreover, the same trends in the transmittance and Here, the aesthetic wood with tunable UV-blocking perfor- haze occurred in esthetic wood-L as well. Additionally, we mance over a wide range from 200 to 400 nm was fabricated compared the mechanical properties between the samples before successfully. The sample (2-mm thick) was treated for various and after outdoor exposure (Supplementary Fig. 11c, d). There treatment times. Under 2 h, the aesthetic wood was able to was no significant degradation in the strength of the aesthetic shield almost 100% of the UVC and UVB spectra and most of wood. These results indicate the aesthetic wood’s strong short- the UVA spectrum. If the reaction is prolonged to 9 h, the UVA term weathering stability, which suggests the material’s potential blocking was remarkably decreased along with an increase in for practical applications. However, there may be durability the transmittance from 47% to 85% at 600 nm (Supplementary concerns for long-term outdoor operation, which requires further Fig. 9). The excellent UV-blocking properties are ascribed to exploration in future studies. the existence of phenylpropane structures and phenolic hydroxyl groups in the lignin molecules with UV absorption ability. Consequently, the aesthetic wood treated for 2 h Light guiding and anti-glare effect of aesthetic wood.The exhibited a good UV absorption performance at the range of aesthetic wood also demonstrates excellent optical management 6 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications Transmittance (%) NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE capability including the anti-glare effect and light guiding, which Thermal insulation properties of aesthetic wood. The aesthetic are of great significance for transparent ceiling applications. The wood with good mechanical and optical performances can find esthetic wood largely scatters the forward light, leading to a high potential applications as a patterned ceiling in a museum or optical haze of ~93% (Supplementary Fig. 12a). SEM image of gallery where its aesthetics can be showcased and can potentially the fabricated aesthetic wood demonstrates inherited aligned replace glass ceiling (Fig. 5a, b). Simultaneously, aesthetic wood microstructure (Supplementary Fig. 12b). With the refractive- can also improve energy efficiency due to its excellent thermal 9,36 index-matched polymer (e.g., epoxy) in wood lumina, light can insulation properties compared to glass . As revealed in Fig. 5c, −1 propagate along the microchannels, which function as lossy d, aesthetic wood exhibits a thermal conductivity of 0.24 W m 9 −1 waveguides . Moreover, in order to demonstrate the optical K in the radial direction (perpendicular to the wood growth management of the aesthetic wood used as pattern ceiling with direction) which is a lower thermal conductivity than that in the high transparency and high haze, model houses designed with axial direction. The heat transferred in the radial direction is glass and aesthetic wood ceiling are compared in Supplementary restrained owing to the larger phonon scattering effect than in the Fig. 12c. The white light source created by a solar simulator was axial direction (along the growth direction), which exhibits a −1 −19 applied in this design model. In order to verify that the uniform thermal conductivity of around 0.41 W m K . For compar- indoor light distribution can be observed by using the aesthetic ison, the isotropic thermal conductivity of common window glass −1 −1 wood ceiling, we collected the light intensities of eight points in is 1 W m K , making aesthetic wood highly desirable from a the designed house model via a calibrated Si detector from thermal insulation perspective. The anisotropic thermal transport Thorlabs, respectively. As the results revealed in Supplementary of aesthetic wood combined with low thermal conductivities is Fig. 12d, in the glass model house, the maximum light intensity favorable for energy-efficient buildings. The superior thermal −2 (56.8 mW cm ) is about 17 times higher than the minimum insulation to glass positions our developed esthetic wood to be a −2 light intensity (3.4 mW cm ), leading to the non-uniform illu- potential candidate for energy-efficient building materials. mination. On the contrary, the diffused light distribution is To further illustrate the insulating effect of aesthetic wood ceilings much more uniform through the aesthetic wood ceiling because applied in energy efficient buildings, we conducted a comparative there is no obvious light intensity decrease between the brightest evaluation in which a simplified house model for nighttime was −2 −2 spot (48.2 mW cm ) and the darkest spot (20.9 mW cm ). used to calculate the indoor temperature change when single-pane Note that its high haze is the main reason, which changes the glass ceilings are replaced with single-pane aesthetic wood ceilings. path of light propagation to avoid the appearance of strong glare The house is assumed to have a floor area of 10 m × 10 m= 100 m as well . Therefore, the aesthetic wood ceiling not only provides and a 45° rooftop. The related parameters of the assumed us a different experience of visual beauty and comfort but also house are described in Supplementary Table 3. The R-values and enhances energy efficiency for indoor lighting owing to its high U-factors in Supplementary Table 3 are based on recommendations haze and anti-glare effects compared with a glass ceiling. Our of the Department of Energy (https://www.homedepot.com/c/ab/ aesthetic wood shows excellent performance in terms of its insulation-r-valuechart/9ba683603be9fa5395fab9091a9131f, https:// optical properties, mechanical strength, thermal insulation, UV- www.energy.gov/energysaver/design/windows-doors-and-skylights/ blocking, and aesthetic function, all of which make it stand out doors) . The total U-factors of the ceilings were calculated from previously reported transparent wood materials (Supple- using the WINDOW 7.7 algorithm developed by Lawrence Berkeley mentary Table 2). National Laboratory (LBNL) (Supplementary Fig. 13a). Based on a Aesthetic wood ceiling b Glass ceiling Diffusing light Direct light cd 45°C 40°C 1.0 Al Block Al Block 0.8 0.6 AW-axial AW-radial 0.4 Al Block Al Block 0.2 0.0 Glass AW-axial AW-radial 26°C 21°C Fig. 5 Light guiding effect and thermal insulation properties of aesthetic wood. a-b The schematic scene shows the light distribution and aesthetic appeal inside a building via applying the aesthetic wood (abbreviated as AW in the d) ceiling comparing with the glass ceiling. c IR images of aesthetic wood with temperature distributions in the axial (heat transfer direction is parallel to the aligned wood microchannels) and radial (heat transfer direction is perpendicular to the aligned wood microchannels) directions. d Thermal conductivities of glass , axial and radial direction of our aesthetic wood (AW). Error bars represent standard deviation. NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 7 Thermal conductivity –1 –1 (W m K ) ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w energy balance, the heat loss should be equal to the indoor heating infiltrating into the delignified wood scaffold. The aesthetic wood was fabricated after a 24 h solidification. power P when the indoor temperature T is higher than the h in outdoor temperature T , thuswehaveequation: out Characterizations. The morphologies of the wood samples were characterized by X Tescan XEIA FEG SEM. The transmittance (T), haze and reflectance (R) were measured via the UV–vis Spectrometer Lambda 35 (PerkinElmer, USA) equipped ðT  T Þ þ U A þ U A ¼ P : ð1Þ in out 5 5 6 6 h Rvalue with an integrating sphere. The absorbance spectra (A) was defined based on the i¼1 transmittance (T) and reflectance (R)(A = 1 − T − R). The aesthetic wood ceiling and glass ceiling with a dimension of 60 mm × 60 mm × 2 mm are employed for We can write it as the house models to test the light guiding effect. Thereinto, a Xenon lamp of the solar simulator from Newport was applied as the white light source with an illu- ΔT ¼ P R; ð2Þ mination area of 5 cm in diameter. The samples surfaces were polished with a microtome (Leica, Germany) for Raman spectroscopy measurement, which was where the temperature difference ΔT ¼ T  T (°C) and the in out performed with a confocal Raman microscope (Renishaw inVia, Wotton-under- 1 −1 absolute thermal resistance R ¼ (°C W ) 4 Edge, England) using a 785 nm laser and a water immersion objective (Nikon, þU A þU A 60×). The integration time 2 s and a step size of 600 nm were used in the mea- 5 5 6 6 Rvalue i¼1 surement. Cosmic ray removal and baseline correction of the spectra were per- When single-pane glass ceilings are replaced with single-pane formed in the software Wire 3.2. For the VCA the mapping data were exported into aesthetic wood ceilings under the same heating power P , R and CytoSpec (commercially available MatLab based software). The mechanical per- formance was assessed by a tensile tester (Instron) and three specimens (with a ΔT will change accordingly. Supplementary Fig. 13b shows the length of 90 mm and a width of 60 mm) were used to obtain the average values. relative change of ΔT (%) when single-pane glass ceilings are The Steady State Laser-Infrared Camera Thermal Conductivity Characterization replaced with single-pane aesthetic wood ceilings with different System was used to test the thermal conductivities. As demonstrated in Fig. 5, the thicknesses. If ΔT is 30 °C in a balanced state when glass ceilings sandwich structure where consisted of one sample (1 cm × 1 cm × 2 mm) in the middle of two Al blocks. The corresponding temperature contribution was are used, the change of indoor temperature (the outdoor recorded by the IR camera. temperature does not change) when aesthetic wood ceilings are used should be approximately +2.43 °C for the 6-mm-thick Data availability aesthetic wood-L and +0.81 °C for the 2 mm-thick aesthetic The data that support the findings of this study are available from the corresponding wood-L. In cases when the indoor temperature is lower than the author upon reasonable request. outdoor temperature (summer), the indoor temperature would decrease in a similar manner. The above results indicate that the Received: 7 November 2019; Accepted: 24 June 2020; aesthetic wood with integrated advantageous mechanical, optical, thermal and aesthetic features holds promise for sustainable energy-efficient buildings . Discussion References In this work, we demonstrated an aesthetic transparent wood 1. Wang, X. et al. 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Revealing changes in molecular composition of plant cell walls created the 3D illustrations. R.M. and S.H. performed optical measurements. L.H., R.M., on the micron-level by Raman mapping and vertex component analysis and C.C. collectively wrote the paper. E.H. contributed to editing of the manuscript. All (VCA). Front. plant Sci. 5, 306 (2014). authors commented on the final manuscript. 25. Keplinger, T. et al. A versatile strategy for grafting polymers to wood cell walls. Acta Biomaterialia 11, 256–263 (2015). 26. Bock, P. & Gierlinger, N. Infrared and Raman spectra of lignin substructures: Competing interests coniferyl alcohol, abietin, and coniferyl aldehyde. J. Raman Spectrosc. 50, L.H., M.R. and C.C. are inventors on a patent application related to this work (University 778–792 (2019). of Maryland Invention Disclosure PS-2019-110, filed on July 10, 2020). All other 27. Gierlinger, N. & Schwanninger, M. 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Study on the properties of partially transparent wood under and the other, anonymous, reviewer(s) for their contribution to the peer review of different delignification processes. Polymers 12, 661–675 (2020). this work. 31. Wu, Y. et al. Study on the colorimetry properties of transparent wood prepared from six wood species. ACS omega 5, 1782–1788 (2020). Reprints and permission information is available at http://www.nature.com/reprints 32. Magrini, T. et al. Transparent and tough bulk composites inspired by nacre. Nat. Commun. 10, 2794 (2019). Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in 33. Zhang, X., Liu, W., Yang, D. & Qiu, X. Biomimetic supertough and strong published maps and institutional affiliations. biodegradable polymeric materials with improved thermal properties and excellent UV-blocking performance. Adv. Funct. Mater. 29, 1806912 (2019). Open Access This article is licensed under a Creative Commons 34. Boye, C., Preusser, F. & Schaeffer, T. UV-blocking window films for ues in Attribution 4.0 International License, which permits use, sharing, museums-revisited. WAAC Newsl. 32,13–18 (2010). 35. Bellia, L., Cesarano, A., Iuliano, G. F., & Spada, G. Daylight glare: a review of adaptation, distribution and reproduction in any medium or format, as long as you give discomfort indexes, in Visual Quality and Energy Efficiency in Indoor Lighting: appropriate credit to the original author(s) and the source, provide a link to the Creative Today for Tomorrow Rome. (2008). Commons license, and indicate if changes were made. The images or other third party 36. Li, Y. et al. Towards centimeter thick transparent wood through interface material in this article are included in the article’s Creative Commons license, unless manipulation. J. Mater. Chem. A 6, 1094 (2017). indicated otherwise in a credit line to the material. If material is not included in the 37. U.S. Department of Energy. Guide to Energy-Efficient Windows. Energy article’s Creative Commons license and your intended use is not permitted by statutory Efficiency & Renewable Energy 2010. regulation or exceeds the permitted use, you will need to obtain permission directly from 38. Curcija, D. C., Zhu, L., Czarnecki, S., Mitchell, R. D., Kohler, C., BERKELEY the copyright holder. To view a copy of this license, visit http://creativecommons.org/ LAB WINDOW. 2015. licenses/by/4.0/. 39. Chen, C. et al. Structure–property–function relationships of natural and engineered wood. Nat. Rev. Mater. https://doi.org/10.1038/s41578- 020-0195-z (2020). © The Author(s) 2020 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 9 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Scalable aesthetic transparent wood for energy efficient buildings

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ARTICLE https://doi.org/10.1038/s41467-020-17513-w OPEN Scalable aesthetic transparent wood for energy efficient buildings 1,6 1,2,6 3,4 5 1 1 1 Ruiyu Mi , Chaoji Chen , Tobias Keplinger , Yong Pei , Shuaiming He , Dapeng Liu , Jianguo Li , 1 1 5 3,4 1,2 Jiaqi Dai , Emily Hitz , Bao Yang , Ingo Burgert & Liangbing Hu Nowadays, energy-saving building materials are important for reducing indoor energy con- sumption by enabling better thermal insulation, promoting effective sunlight harvesting and offering comfortable indoor lighting. Here, we demonstrate a novel scalable aesthetic transparent wood (called aesthetic wood hereafter) with combined aesthetic features (e.g. intact wood patterns), excellent optical properties (an average transmittance of ~ 80% and a −1 −1 haze of ~ 93%), good UV-blocking ability, and low thermal conductivity (0.24 W m K ) based on a process of spatially selective delignification and epoxy infiltration. Moreover, the rapid fabrication process and mechanical robustness (a high longitudinal tensile strength of −3 91.95 MPa and toughness of 2.73 MJ m ) of the aesthetic wood facilitate good scale-up capability (320 mm × 170 mm × 0.6 mm) while saving large amounts of time and energy. The aesthetic wood holds great potential in energy-efficient building applications, such as glass ceilings, rooftops, transparent decorations, and indoor panels. 1 2 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA. Center for Materials Innovation, University of 3 4 Maryland, College Park, MD 20742, USA. Wood Materials Science, ETH Zürich, Stefano-Franscini-Platz 3, CH-8093 Zürich, Switzerland. Wood Technology, Cellulose & Wood Materials, EMPA, CH-8600 Dubendorf, Switzerland. Department of Mechanical Engineering, University of Maryland, 6 ✉ College Park, MD 20742, USA. These authors contributed equally: Ruiyu Mi, Chaoji Chen. email: binghu@umd.edu NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w o date, the development of green, energy-saving materials softwood pine disk): the EW is usually wider, weaker, more has been a prevailing research topic from the perspective of porous and lighter in color than the LW. In their microstructure, Tsustainability in response to the rapidly growing burden of EW cells have a relatively larger lumen diameter and thinner cell 1,2 18 energy consumption and environmental pollution . Natural walls compared to LW . By profiting from the special structural materials such as wood and its derivatives have been regarded as organization of Douglas fir wood, aesthetic wood not only one of the most important alternatives in green and energy- inherited the original aesthetic from the wood itself but also efficient buildings due to the abundance, renewability, low cost, possessed favorable optical, and mechanical properties. Moreover, 3,4 and sustainability of source materials . One latest trend in the efficient delignification process made it possible to realize the wood-based building material fabrication is the recently devel- large-scale production of transparent wood with little time and oped transparent wood composite, which integrates the aniso- energy consumption. tropic hierarchical wood structure with optical, mechanical, and Figure 1b demonstrates the fabrication process of aesthetic 5–7 thermal properties . Numerous merits are endowed to the wood-R: natural wood is obtained by the industry-adopted cross- transparent wood composites including light weight, high section cutting method with the annual growth rings visible due optical transmittance, tunable haze, low thermal conductivity to the sharp microstructure difference between EW and LW 8–10 compared to glass and excellent mechanical robustness . (Fig. 1b–d). After spatially selective delignification using our Additionally, transparent wood composites can harvest sunlight efficient method, the EW area has become almost completely effectively due to the light guiding effect, which is meaningful white due to the light scattering and the vast removal of light for energy saving and comfortable indoor lighting. With these absorbers (i.e., lignin and some extractives) while the LW area integrated advantages, the transparent wood composites have preserves partial lignin. Afterwards, the refractive-index-matched emerged as the promising engineering components (e.g., rooftops, polymer/epoxy was filled into the wood backbone to prepare the windows, and transparent decorations) in green energy-efficient aesthetic wood based on this special structure. The scanning 1,9 buildings . electronic microscope (SEM) image in Fig. 1e specifically reveals Current approaches for fabricating transparent wood compo- the maintaining dense structure after fully impregnating polymer, sites were generally based on a complete (or nearly complete) particularly in the LW area. Aesthetic wood-R was fabricated by delignification process, that is, removing most of light absorbing this approach, which possesses not only the preserved wood 11–13 materials (lignin and extractives) or chromophoric compo- patterns, but a high average transparency (80% at 600 nm, the nents with lignin remaining about 80% . However, the intensive UMD logo can be seen clearly behind the aesthetic wood-R, see chemical treatment can severely break down the original wood Fig. 1f). This work provides new horizons and more potentials for structure (e.g., the cell wall is partly degraded, and the growth green buildings and other construction applications, which is ring patterns become less visible) to ensure efficient impossible to achieve with regular glass. polymer infiltration. In addition, these previous works generally focused on the morphology and anisotropy of optical, mechan- 13,15,16 ical, and thermal properties , yet alternating structures, the Morphological and chemical characterizations of aesthetic natural aesthetics of wood’s original annual growth patterns, wood. Softwood (gymnosperms) generally relies on tracheids to 19,20 and scalable manufacturing via efficient process are rarely transport water, like pine, and Douglas fir . The structural discussed. differences between different wood species of softwood and In this work, we develop an aesthetic transparent wood hardwood can lead to different aesthetic results. Here, Douglas fir (denoted as aesthetic wood) by spatial-selectively removing lignin was chosen as a proof-of-concept demonstration, which has a of native wood material to make wood transparent and preserve pronounced contrast of both color and density between EW and its natural patterns simultaneously. Softwood (e.g., Douglas fir) is LW and exhibits a unique aesthetic with obvious wood chosen as the proof-of-concept demonstration due to the pro- patterns. The mesoporous structure of Douglas fir is shown in nounced structural contrast between its low-density earlywood Fig. 2a. There is a rather sharp boundary between EW and LW to (EW) and high-density latewood (LW). In a short 2 h chemical a very indistinct separation in Douglas fir. The EW is more treatment, natural wood is selectively delignified to preserve its porous with much thinner cell walls (1.4~2.6 μm) than the LW original growth ring patterns. The refractive-index-matched (5~10 μm) (Fig. 2b, c). The distribution of the wood tracheids epoxy is then infiltrated into nanoscale framework to make the as shown in Fig. 2d (hollow tube-like structure) with a lumen wood transparent with preserved wood patterns. Consequently, diameter range of 20–80 μm and 5–35 μm involving EW and the novel concept of aesthetic wood in this work is demonstrated LW, respectively (Fig. 2e). As a result, the different pore-size for the first time possessing integrated excellent functions of distributions are usually indicative of different densities of optical transparency, UV-blocking, thermal insulation, mechan- EW and LW. ical strength, scalability, and aesthetics. We anticipate such In a typical experiment, a wood block with dimensions of multifunctional aesthetic wood will hold great potential in 60 mm × 60 mm × 2 mm was applied for the procedure analysis modern green buildings. of delignification treatment. Briefly, a simple delignification process using the acidic NaClO method was employed to partial remove the colored components (mainly lignin, along with Results extractives) from the bulk wood. As shown in Fig. 2f, the Fabrication of aesthetic wood. We demonstrate two types of evolution of the wood’s macroscopic color indicates the removal aesthetic wood based on the periodicity and anisotropy of natural of the color compounds presented at the surface. Specifically, the wood (Fig. 1a): one type with aligned microchannels perpendi- weight loss at various points during the delignification process cular to the wood plane is defined as aesthetic wood-R while (0–10 h) was recorded by a balance in the dry states (dried at another type with channels parallel to the wood plane is aesthetic 100 °C for 48 h, Fig. 2g), which is mainly ascribed to the removal wood-L. Natural softwood presents the intrinsic aesthetic prop- of lignin and a little extractive during the process. After 2 h of erties of the annual growth ring patterns with alternating struc- treatment, spatially selective delignification can be realized: the tures at macroscopic and microscopic scales . From the macro EW has become almost completely white whereas the LW kept perspective, the rings are developed by the alternating formation the pattern well owing to the residual lignin and colored of EW in spring and LW in summer (Supplementary Fig. 1, components. The main contributor to spatially selective 2 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE Periodicity: annual growth rings Alternating structure with natural aesthetics Earlywood and Latewood Anisotropy: aligned channels Efficient lignin removal Light guiding Natural wood b c d 100µm 1% @600nm Selective Polymer delignification infiltration Aesthetic wood e f Earlywood Latewood 100µm 80% @600nm Fig. 1 Fabrication, microstructure and appearance of aesthetic wood. a An indication of the design which combines the periodicity (annual growth rings) with anisotropy (aligned channels) of wood to realize a new kind of transparent wood composite. b Schematic to display the procedures for fabricating aesthetic wood (aesthetic wood-R) from natural wood with vertically aligned cells and annual growth rings after fast spatially selective delignification and polymer infiltration. c, e The cross-sectional SEM images of natural wood and dense aesthetic wood-R microstructures after polymer filling (there is a sharp boundary between EW and LW). d, f Photos to show a large piece of aesthetic wood-R (86 mm × 86 mm × 2 mm) with preserved wood patterns and high average transparency (80% at 600 nm) derived from Douglas fir. delignification is the inherent structural difference between EW hardwood possessing bimodal pores and uniform solution and LW, which accordingly leads to a faster solution diffusion in diffusion (Supplementary Fig. 6). EW than in LW (Supplementary Fig. 2). The weight loss was Raman spectroscopy imaging in combination with vertex 13.5 wt% after 2 h of treatment (Fig. 2g). After selective component analysis (VCA), a multivariate data analysis method, delignification, the nano- and macro-features of original wood were employed to assess the distribution of lignin in the wood were essentially preserved as well (Supplementary Fig. 3), scaffold after selective delignification . The cell wall component including the wood patterns to show the nature of aesthetics. for EW and LW in natural wood and delignified wood, Note that it took much more time (e.g. 10 h) for LW to be respectively are shown in Fig. 2h. The corresponding Raman completely transformed white with a corresponding weight loss of spectra are demonstrated in Fig. 2i, especially the characteristic −1 −1 ~35 wt%. Moreover, the integrity of the delignified wood structure bands of lignin component locate at 1598 cm , 1656 cm and −1 cannot be maintained upon long treatment times resulting in poor 1269 cm (a marker band of the aryl-OH and aryl-OCH in 25,26 mechanical properties (treated 9–10 h) (Fig. 2f) owing to the guaiacyl (G) units in lignin) ascribing to aromatic C=C distinct density difference (Supplementary Fig. 4) between the EW stretching, coniferyl alcohol C=C, C=O stretching, and C–H −3 −3 (284.6 kg m ) and LW (846 kg m ), and the complete removal banding of C=C, aromatic C=C stretching, respectively. As 14,22,23 of lignin, which also acts as binder among the wood cells . expected, contrasting with the EW and LW in natural CWs, the Note that the choice of wood species is vital to the successful representative lignin bands almost disappear in EW cell walls fabrication of aesthetic wood. Although both hardwood and after delignification while they are still shown in LW cell walls. −1 softwood are in principal suitable, hardwood possesses a Meanwhile, respective cellulose peaks, for example at 1095 cm significantly different structure consisting of vessels and fibers (C–O–C stretching vibrations), remain unchanged after NaClO (Supplementary Fig. 5), while softwood mainly consists of treatment . These results give strong evidence to support that tracheids . Basswood, a type of hardwood, for example, has most of the lignin in the EW has been removed while a small uniform cell wall thickness of around 1.3~2.9 μm (Supplementary proportion of the lignin in the LW remains. This phenomenon Fig. 5c), much thinner than the cell wall thickness of LW in leads to the retained aesthetic wood patterns in the final aesthetic Douglas fir. Meanwhile, the vessel channels exhibit larger lumen wood products. diameters than narrow tracheids (Supplementary Fig. 5d), with bimodal pore-size distribution (Supplementary Fig. 5e). Conse- quently, owing to the almost synchronous reaction process in Scalability and the mechanical properties of aesthetic wood. basswood of the EW and LW, there are almost no apparent Following the same procedure, we then constructed the aesthetic wood patterns preserved after a couple of hours’ treatments wood-L with straight patterns created by the quarter slicing (Supplementary Fig. 5f). The same result occurred in balsa wood cutting strategy (Fig. 3a). The efficient spatially selective (another type of hardwood), confirming that aesthetic wood with delignification process not only endows excellent structural patterns is nearly impossible to fabricate from diffuse-porous integrity but also facilitates the large-scale production of aesthetic NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w a b d Tracheid ~3.8µm 20µm 20 30 40 50 60 70 80 Diameter (µm) c 25 ~15.7µm 15 200µm 20µm 50µm 0 510 15 20 25 30 Diameter (µm) 0h 0.5h 1h 2h f g Our process White wood 4h 6h 9h 10h 02468 10 Break Break Reaction time (h) EW LW hi Lignin Delignified LW Delignified EW 10µm 10µm 0 Natural LW Cellulose Natural EW 10µm 10µm 400 800 1200 1600 –1 Wavenumber (cm ) Fig. 2 Morphological and chemical characterizations of aesthetic wood. a The SEM image of Douglas fir to show its mesoporous structure. b, c Magnified SEM images of EW and LW to present the differences in microstructural lumina. d The aligned micro-sized channels with tracheids. e The pore diameter distributions of EW and LW in the natural Douglas fir. f Photo comparison of color and pattern changes in wood templates during lignin removal process in the laboratory (0–10 h). g The weight loss behavior as a function of delignification process time. Error bars represent standard deviation. h Cell wall components of the EW and LW in natural wood (non-treated reference) and delignified wood cell wall (CW) after VCA. i The corresponding Raman spectra in (h). wood-L. In Fig. 3b, we demonstrate the ability to fabricate a direction after successful infiltration. Zoomed in from the top sample size of 320 mm × 170 mm × 0.6 mm, which is significantly view, although the lumen of LW is much smaller than that of larger than all reported transparent woods using delignified wood EW, they are all densely packed (Fig. 3d, e). Additionally, the 1,9–11,15,16,28 as the framework (Supplementary Table 1) . Large- identical channels and apertures are fully filled with polymer scale manufacturing has been regarded as one of the major (Fig. 3f) which acts as a glue to create strong interaction challenges for transparent wood manufacturing and indus- between the cellulosic cell wall and polymer itself. Raman trialization. Our work points to a potential route towards spectroscopy imaging was further performed to identify the addressing the manufacturing challenge of transparent wood (e.g., distribution of the impregnating polymer in the obtained large scale with a short processing time). However, it is worth wood cell including cell corner (CC), compound middle mentioning that thicker aesthetic wood yet without compromis- lamella (CML) (Fig. 3h), cellwall(CW)(Fig. 3i) and lumen ing its aesthetic features and other properties is preferred in order (Fig. 3j). According to the corresponding Raman spectrum in to provide better load-bearing properties in building applications, Fig. 3k, the strong-signal peaks within lumina indicate the bond −1 which should be considered in future research. stretching of epoxy: 640 cm (aromatic C–H out-of-plane −1 The straight-line patterns display the traditional symmetric deformation), 1001 cm (polyamidoamine adduct, amino −1 29 aesthetic (the background is an A4 paper). In the meantime, the groups) and 1608 cm (aromatic ring breathing mode) . obtained aesthetic wood-L (with a thickness of 0.6 mm) is Polymer signals can also be detected in the CML/CC and CW, optically transparent (Supplementary Fig. 7), with a total suggesting that polymer has been well-infiltrated into the wood transmittance of 87% and optical haze of 65% at 600 nm. To cells, forming robust interfaces with cellulose in the delignified identify the compatibility between epoxy and treated wood wood scaffold. scaffold, SEM was applied to illustrate the detailed micro- From the perspective of construction materials, the mechan- structures. Figure 3c shows that aesthetic wood displays ical properties are equally important . The hierarchical cellular massive aligned dense microchannels along the wood growth structure of wood leads to unique anisotropic mechanical 4 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications Delignified wood Natural wood Weight (%) Raman intensity (A.U.) Counts (%) Counts (%) Aligned nanofiber NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE a b EW LW Quarter slicing cutting d fg 20µm 200µm 20µm 100µm 500nm zx Lumen CC/CML CW Cellulose Pol. 10µm CML/CC Lignin Polymer 10µm CW Lumen 10µm 400 600 800 1000 1200 1400 1600 –1 Wavenumber (cm ) Fig. 3 Scalability of aesthetic wood. a Schematic of quarter slicing cutting to obtain the wood veneer with straight-line patterns. b A large-scale aesthetic wood assembled by L-wood veneer (demonstrated for sample size of 320 mm × 170 mm × 0.6 mm). c SEM image of the preserved whole wood microstructure after filling with polymer. d–e Zoomed-in SEM images to show the EW and LW well-defined lumina full of polymer. f–g The detailed SEM image of the aligned micro-sized channels and the aligned cellulose nanofibers on the corresponding cell wall. h–j VCA of wood cells in obtained aesthetic wood. k Corresponding Raman spectra. features. As shown in Supplementary Fig. 8a, aesthetic wood-R aesthetic wood-L after tensile tests show that the polymer is exhibits dramatically improved tensile strength over natural R- fully filled in the middle of the wood backbone and connects the wood (21.56 MPa vs. 6.24 MPa), while aesthetic wood-L separated wood fibers. The anisotropic mechanical properties of possesses a higher tensile strength of 91.95 MPa due to the aesthetic wood can be attributed to the aligned cellulose synergy between the wood matrix and filling polymer. The nanofibers in the cell wall, giving rise to high strength along toughness of aesthetic wood-R and aesthetic wood-L are 0.523 the fiber direction yet relatively low strength perpendicular to −3 −3 8 MJ m and 2.73 MJ m , respectively (Supplementary Fig. 8b), the tracheid direction (Fig. 3g) . enhanced contrasting with natural wood, yet yielding the remarkably mechanical anisotropy. For energy efficient build- Optical properties and patterns design of aesthetic wood. ings, both high tensile strength and high toughness are greatly Previous works have demonstrated that by tuning the starting advantageous for load-bearing functions .Moreover, to 31,32 wood materials or the chemical processing parameters , further reveal the details of the fracture behavior of natural some blurred wood patterns can be maintained in the final wood and aesthetic wood, the fractured surface after tensile products. It remains challenging to achieve clear and designable measurement of each type was characterized by SEM (Supple- aesthetic patterns in transparent wood with integrated advan- mentary Fig. 8c–f). The porous lumen structure and aligned tageous features such as a high optical transmittance, UV- microchannels in the natural wood are visible after fracture. blocking capability, low thermal conductivity and high The cross-sectional SEM images of aesthetic wood-R and mechanical strength. Our aesthetic wood, demonstrated for the NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 5 Raman intensity (A.U.) ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w UV VIS ab 100 100 80 80 Absoption Transmittance 1 7 EW 1’ 7’ LW Reflectance 0 0 1 1' 2 2' 3 3' 4 4' 5 5' 6 6' 7 7' 8 8' 200 300 400 500 600 700 800 Location Wavelength (nm) One-layer pattern Stack two layers Two-layer pattern Fig. 4 Optical properties and patterns design of aesthetic wood. a The transmittance in the EW and LW of obtained aesthetic wood (The locations marked 1–8 represent the EW areas while 1′–8′ represent the LW areas). b As-prepared aesthetic wood exhibits excellent UV-blocking performance: high absorption in 200–400 nm, high transmittance at 600 nm and low reflectance. c The latticed aesthetic patterns can be obtained by stacking two layers of aesthetic wood. first time, features a combination of these abovementioned 200–400 nm, a high average transparency (80%) at 600 nm, and advantages. Such combined advantages are desirable for energy alow reflectance at the visible wavelengths (Fig. 4b). efficient building applications, particularly in pattern ceiling. Subsequently, more types of patterns can be realized by Owing to the inhomogeneous distribution of lignin and cell stacking multiple layers of aesthetic wood. For example, various structures between EW and LW in the assembled aesthetic lattice patterns can be designed by stacking two layers of aesthetic wood, the transmittance is not uniform. We chose 8 locations wood rotated at an angle relative to each other. Based on the high in EW and LW areas and measured the transmittance, transmittance and intrinsic aesthetic, this capability can enable respectively (Fig. 4a). As initially conceived, the LW area the potential application on patterned ceilings (Fig. 4c). More- exhibits lower transmittance (Average ≈ 68%) than in EW area over, the abundance of patterns can be further increased by (Average ≈ 86%). Despite the difference of transmittance developing aesthetic wood using other wood species (mainly between LW and EW, the remaining pattern only slightly softwoods) through this universal fabrication method. Here, decreases the overall average transmittance of the aesthetic another type of esthetic wood was also fabricated using pine wood in the visible light region. (Supplementary Fig. 10). Double-layer pine aesthetic wood and Furthermore, the favorable UV-blocking performance is one-layer pine aesthetic wood with one-layer Douglas fir aesthetic specially expected in aesthetic wood. Solar radiation reaching wood both show various aesthetic patterns (Supplementary the earth surface constitutes infrared radiation, visible light, Fig. 10b–d). and ultraviolet (UV) radiation, in which UV is made up of three Furthermore, we evaluated the weathering stability of aesthetic bands UVA (320–400 nm), UVB (275–320 nm) and UVC wood-R and -L by exposing the materials outdoors for 3 weeks (200–275 nm), the smaller wavelength has the stronger and measuring the optical properties, including the transmittance energy . UVC (possessing the highest energy) is filtered by and haze (Supplementary Fig. 11a, b). Compared with the the ozone layer. UV is invisible to the human eye but can original aesthetic wood-R, the transmittance of the outdoor- provide a hazard and damage to many materials including exposed aesthetic wood-R decreased slightly while the haze furniture and interior displays .Particularly,3.5%UVB and increased from ~93% to ~98% in the wavelength range of 96.5% UVA will reach the earth’ssurface on asummerday. 400–800 nm. Moreover, the same trends in the transmittance and Here, the aesthetic wood with tunable UV-blocking perfor- haze occurred in esthetic wood-L as well. Additionally, we mance over a wide range from 200 to 400 nm was fabricated compared the mechanical properties between the samples before successfully. The sample (2-mm thick) was treated for various and after outdoor exposure (Supplementary Fig. 11c, d). There treatment times. Under 2 h, the aesthetic wood was able to was no significant degradation in the strength of the aesthetic shield almost 100% of the UVC and UVB spectra and most of wood. These results indicate the aesthetic wood’s strong short- the UVA spectrum. If the reaction is prolonged to 9 h, the UVA term weathering stability, which suggests the material’s potential blocking was remarkably decreased along with an increase in for practical applications. However, there may be durability the transmittance from 47% to 85% at 600 nm (Supplementary concerns for long-term outdoor operation, which requires further Fig. 9). The excellent UV-blocking properties are ascribed to exploration in future studies. the existence of phenylpropane structures and phenolic hydroxyl groups in the lignin molecules with UV absorption ability. Consequently, the aesthetic wood treated for 2 h Light guiding and anti-glare effect of aesthetic wood.The exhibited a good UV absorption performance at the range of aesthetic wood also demonstrates excellent optical management 6 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications Transmittance (%) NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w ARTICLE capability including the anti-glare effect and light guiding, which Thermal insulation properties of aesthetic wood. The aesthetic are of great significance for transparent ceiling applications. The wood with good mechanical and optical performances can find esthetic wood largely scatters the forward light, leading to a high potential applications as a patterned ceiling in a museum or optical haze of ~93% (Supplementary Fig. 12a). SEM image of gallery where its aesthetics can be showcased and can potentially the fabricated aesthetic wood demonstrates inherited aligned replace glass ceiling (Fig. 5a, b). Simultaneously, aesthetic wood microstructure (Supplementary Fig. 12b). With the refractive- can also improve energy efficiency due to its excellent thermal 9,36 index-matched polymer (e.g., epoxy) in wood lumina, light can insulation properties compared to glass . As revealed in Fig. 5c, −1 propagate along the microchannels, which function as lossy d, aesthetic wood exhibits a thermal conductivity of 0.24 W m 9 −1 waveguides . Moreover, in order to demonstrate the optical K in the radial direction (perpendicular to the wood growth management of the aesthetic wood used as pattern ceiling with direction) which is a lower thermal conductivity than that in the high transparency and high haze, model houses designed with axial direction. The heat transferred in the radial direction is glass and aesthetic wood ceiling are compared in Supplementary restrained owing to the larger phonon scattering effect than in the Fig. 12c. The white light source created by a solar simulator was axial direction (along the growth direction), which exhibits a −1 −19 applied in this design model. In order to verify that the uniform thermal conductivity of around 0.41 W m K . For compar- indoor light distribution can be observed by using the aesthetic ison, the isotropic thermal conductivity of common window glass −1 −1 wood ceiling, we collected the light intensities of eight points in is 1 W m K , making aesthetic wood highly desirable from a the designed house model via a calibrated Si detector from thermal insulation perspective. The anisotropic thermal transport Thorlabs, respectively. As the results revealed in Supplementary of aesthetic wood combined with low thermal conductivities is Fig. 12d, in the glass model house, the maximum light intensity favorable for energy-efficient buildings. The superior thermal −2 (56.8 mW cm ) is about 17 times higher than the minimum insulation to glass positions our developed esthetic wood to be a −2 light intensity (3.4 mW cm ), leading to the non-uniform illu- potential candidate for energy-efficient building materials. mination. On the contrary, the diffused light distribution is To further illustrate the insulating effect of aesthetic wood ceilings much more uniform through the aesthetic wood ceiling because applied in energy efficient buildings, we conducted a comparative there is no obvious light intensity decrease between the brightest evaluation in which a simplified house model for nighttime was −2 −2 spot (48.2 mW cm ) and the darkest spot (20.9 mW cm ). used to calculate the indoor temperature change when single-pane Note that its high haze is the main reason, which changes the glass ceilings are replaced with single-pane aesthetic wood ceilings. path of light propagation to avoid the appearance of strong glare The house is assumed to have a floor area of 10 m × 10 m= 100 m as well . Therefore, the aesthetic wood ceiling not only provides and a 45° rooftop. The related parameters of the assumed us a different experience of visual beauty and comfort but also house are described in Supplementary Table 3. The R-values and enhances energy efficiency for indoor lighting owing to its high U-factors in Supplementary Table 3 are based on recommendations haze and anti-glare effects compared with a glass ceiling. Our of the Department of Energy (https://www.homedepot.com/c/ab/ aesthetic wood shows excellent performance in terms of its insulation-r-valuechart/9ba683603be9fa5395fab9091a9131f, https:// optical properties, mechanical strength, thermal insulation, UV- www.energy.gov/energysaver/design/windows-doors-and-skylights/ blocking, and aesthetic function, all of which make it stand out doors) . The total U-factors of the ceilings were calculated from previously reported transparent wood materials (Supple- using the WINDOW 7.7 algorithm developed by Lawrence Berkeley mentary Table 2). National Laboratory (LBNL) (Supplementary Fig. 13a). Based on a Aesthetic wood ceiling b Glass ceiling Diffusing light Direct light cd 45°C 40°C 1.0 Al Block Al Block 0.8 0.6 AW-axial AW-radial 0.4 Al Block Al Block 0.2 0.0 Glass AW-axial AW-radial 26°C 21°C Fig. 5 Light guiding effect and thermal insulation properties of aesthetic wood. a-b The schematic scene shows the light distribution and aesthetic appeal inside a building via applying the aesthetic wood (abbreviated as AW in the d) ceiling comparing with the glass ceiling. c IR images of aesthetic wood with temperature distributions in the axial (heat transfer direction is parallel to the aligned wood microchannels) and radial (heat transfer direction is perpendicular to the aligned wood microchannels) directions. d Thermal conductivities of glass , axial and radial direction of our aesthetic wood (AW). Error bars represent standard deviation. NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 7 Thermal conductivity –1 –1 (W m K ) ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17513-w energy balance, the heat loss should be equal to the indoor heating infiltrating into the delignified wood scaffold. The aesthetic wood was fabricated after a 24 h solidification. power P when the indoor temperature T is higher than the h in outdoor temperature T , thuswehaveequation: out Characterizations. The morphologies of the wood samples were characterized by X Tescan XEIA FEG SEM. The transmittance (T), haze and reflectance (R) were measured via the UV–vis Spectrometer Lambda 35 (PerkinElmer, USA) equipped ðT  T Þ þ U A þ U A ¼ P : ð1Þ in out 5 5 6 6 h Rvalue with an integrating sphere. The absorbance spectra (A) was defined based on the i¼1 transmittance (T) and reflectance (R)(A = 1 − T − R). The aesthetic wood ceiling and glass ceiling with a dimension of 60 mm × 60 mm × 2 mm are employed for We can write it as the house models to test the light guiding effect. Thereinto, a Xenon lamp of the solar simulator from Newport was applied as the white light source with an illu- ΔT ¼ P R; ð2Þ mination area of 5 cm in diameter. The samples surfaces were polished with a microtome (Leica, Germany) for Raman spectroscopy measurement, which was where the temperature difference ΔT ¼ T  T (°C) and the in out performed with a confocal Raman microscope (Renishaw inVia, Wotton-under- 1 −1 absolute thermal resistance R ¼ (°C W ) 4 Edge, England) using a 785 nm laser and a water immersion objective (Nikon, þU A þU A 60×). The integration time 2 s and a step size of 600 nm were used in the mea- 5 5 6 6 Rvalue i¼1 surement. Cosmic ray removal and baseline correction of the spectra were per- When single-pane glass ceilings are replaced with single-pane formed in the software Wire 3.2. For the VCA the mapping data were exported into aesthetic wood ceilings under the same heating power P , R and CytoSpec (commercially available MatLab based software). The mechanical per- formance was assessed by a tensile tester (Instron) and three specimens (with a ΔT will change accordingly. Supplementary Fig. 13b shows the length of 90 mm and a width of 60 mm) were used to obtain the average values. relative change of ΔT (%) when single-pane glass ceilings are The Steady State Laser-Infrared Camera Thermal Conductivity Characterization replaced with single-pane aesthetic wood ceilings with different System was used to test the thermal conductivities. As demonstrated in Fig. 5, the thicknesses. If ΔT is 30 °C in a balanced state when glass ceilings sandwich structure where consisted of one sample (1 cm × 1 cm × 2 mm) in the middle of two Al blocks. The corresponding temperature contribution was are used, the change of indoor temperature (the outdoor recorded by the IR camera. temperature does not change) when aesthetic wood ceilings are used should be approximately +2.43 °C for the 6-mm-thick Data availability aesthetic wood-L and +0.81 °C for the 2 mm-thick aesthetic The data that support the findings of this study are available from the corresponding wood-L. In cases when the indoor temperature is lower than the author upon reasonable request. outdoor temperature (summer), the indoor temperature would decrease in a similar manner. The above results indicate that the Received: 7 November 2019; Accepted: 24 June 2020; aesthetic wood with integrated advantageous mechanical, optical, thermal and aesthetic features holds promise for sustainable energy-efficient buildings . Discussion References In this work, we demonstrated an aesthetic transparent wood 1. Wang, X. et al. 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Boye, C., Preusser, F. & Schaeffer, T. UV-blocking window films for ues in Attribution 4.0 International License, which permits use, sharing, museums-revisited. WAAC Newsl. 32,13–18 (2010). 35. Bellia, L., Cesarano, A., Iuliano, G. F., & Spada, G. Daylight glare: a review of adaptation, distribution and reproduction in any medium or format, as long as you give discomfort indexes, in Visual Quality and Energy Efficiency in Indoor Lighting: appropriate credit to the original author(s) and the source, provide a link to the Creative Today for Tomorrow Rome. (2008). Commons license, and indicate if changes were made. The images or other third party 36. Li, Y. et al. Towards centimeter thick transparent wood through interface material in this article are included in the article’s Creative Commons license, unless manipulation. J. Mater. Chem. A 6, 1094 (2017). indicated otherwise in a credit line to the material. If material is not included in the 37. U.S. Department of Energy. Guide to Energy-Efficient Windows. Energy article’s Creative Commons license and your intended use is not permitted by statutory Efficiency & Renewable Energy 2010. regulation or exceeds the permitted use, you will need to obtain permission directly from 38. Curcija, D. C., Zhu, L., Czarnecki, S., Mitchell, R. D., Kohler, C., BERKELEY the copyright holder. To view a copy of this license, visit http://creativecommons.org/ LAB WINDOW. 2015. licenses/by/4.0/. 39. Chen, C. et al. Structure–property–function relationships of natural and engineered wood. Nat. Rev. Mater. https://doi.org/10.1038/s41578- 020-0195-z (2020). © The Author(s) 2020 NATURE COMMUNICATIONS | (2020) 11:3836 | https://doi.org/10.1038/s41467-020-17513-w | www.nature.com/naturecommunications 9

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