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emanating from the energy use of buildings are also a major concern. Consequently, there is growing inter- est in the construction of energy-efficient buildings that incorporate renewable, safe, and cheap building materials. The production of glass is an expensive and environmentally unfriendly process that emits 25,000 metric tons of CO 2 annually (EIA 2016; Li et al. 2019b). The high intrinsic thermal conductiv- ity (~1 W m −1 K −1 ) of glass windows in buildings causes significant heat loss (Qiu et al. 2019; Mi et al. 2020b). Glass also poses safety risks as it shatters upon impact (Li et al. 2016b; Mi et al. 2020b). Some limitations of natural wood are due to its anisotropy, low conductivity, moisture expansion and drying, shrinkage and non-transparency caused by its low optical transmittance. Therefore, there is an urgent need to explore a more sustainable alter- native to glass, such as transparent composites pro- duced from wood, referred to as engineered transpar- ent wood (ETW) composites. The ETW composites possess advantages such as high mechanical perfor- mance of wood along with optical properties such as transmittance and haze. Although other options such as internal and external blinds, concrete with optical fibre, transparent thermal barriers, angular selective shading systems, coatings, films, and multi-pane win- dows have been developed, the problem of high cost is still a major drawback (Li et al. 2018c; Mi et al. 2020b). ETW is cheap, sustainable, renewable, light- weight (low density = 1200 kg m −3 ), and has a high fracture toughness which alleviates safety issues asso- ciated with the brittleness of glass (Li et al. 2016b, 2017a; Wang et al. 2018). Li et al. (2017a) showed that ETW had stress at break of approximately 100 MPa, which was comparable to that of glass (116 MPa). The strain at failure of ETW was 2.18% which was much higher than glass (0.19%). The low optical transmittance of wood is due to three main factors: (1) light scattering at the inter- faces between the cell wall tissue with a refractive index (RI) around 1.56 and the empty lumen pore space (RI of 1.0) in wood cells (e.g. cells such as tracheids, wood fibres, and vessels) and the pres- ence of lignin (Li et al. 2019b; Wu et al. 2019a; Chen et al. 2020; Bisht et al. 2021); (2) light scat- tering due to the mismatch of refractive indices of wood components such as cellulose and hemi- celluloses (both with RI of 1.53) and lignin with RI of 1.61 (Li et al. 2018a); and (3) the strong
light-absorption of lignin chromophoric groups that accounts for 80–95% of the light absorption (Li et al. 2018a). In contrast, cellulose and hemi- celluloses are optically colourless. In producing ETW, it is necessary to reduce light absorption by removing or modifying lignin and to minimise light scattering at the air/cell wall interface to achieve transparency (Wu et al. 2019a). Conven- tionally, the production of ETW entails delignifi- cation of the wood followed by infiltration of the wood ultrastructure with a refractive index-matched polymer to reduce light scattering and refractive index, suppress light reflection and increase trans- parency (Montanari et al. 2019). The delignified wood has a unique mesoporous cell wall structure and a high specific area (Montanari et al. 2019). The infiltrated polymer improves the mechanical strength of delignified wood by glueing wood cellu- lose nanofibers together (Zhu et al. 2016a). Studies have shown that ETW has excellent optical proper- ties: transmittance > 80%, high haze > 70%, high thermal insulation (thermal conductivity less than 0.23Wm −1 K −1 ) (Mi et al. 2020b), unique hierarchi- cal structure, good loadbearing performance with tough failure behaviour (no shattering) and high ductility.(Yu et al. 2017; Yaddanapudi et al. 2017; Li et al. 2018a). In addition, ETW is lightweight, environmentally safe and mechanically stronger than unmodified wood (Wu et al. 2020). However, ETW is susceptible to weathering and photodeg- radation due to exposure to natural outdoor condi- tions such as sunlight, rain, temperature, humidity and pollutants (Bisht et al. 2021). This may cause discolouration, decrease in light transmittance, chemical degradation, microstructure breakdown and loss of mechanical properties, thereby limiting its applications (Bisht et al. 2021). For example, at a wavelength of 550 nm and UV irradiation exposure time of 0 h, 50 h, 100 h and 250 h, the light trans- mittance was 83%, 75%, 71% and 60%, respectively. Applications of ETW range from optical compo- nents to building materials, solar cells, and mag- netic materials to luminescent and decorative mate- rials (Fig. 1) (Gan et al. 2017a, 2017b; Yu et al. 2017; Bi et al. 2018; Li et al. 2019c; Wang et al. 2019a; Wu et al. 2020; Zhang et al. 2020; Xia et al. 2021). Montanari et al. (2019) reported successful use of ETW for thermal energy storage due to its
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