UV-protection of wood surfaces by controlled morphology fine-tuning of ZnO nanostructures

Published Date
Published Online: 2016-01-07 | DOI: https://doi.org/10.1515/hf-2015-0185

Author
Huizhang Guo

• Corresponding author
• Wood Materials Science, Institute for Building Materials, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich (Switzerland)
• Applied Wood Materials, Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf (Switzerland)
• Email
• Other articles by this author:
• De Gruyter OnlineGoogle Scholar

 / Peter Fuchs

 / Etienne Cabane

 / Benjamin Michen

 / Harald Hagendorfer

 / Yaroslav E. Romanyuk

 / Ingo Burgert

Abstract

One of the most significant limitations for a wider utilisation of the renewable and CO2-storing resource wood is its low ultraviolet (UV) light stability. The protection of the wood surface without altering its aesthetic appeal requires an optically transparent but UV protective coating which should be strongly attached to the rough and inhomogeneous substrate. For this purpose, ZnO nanostructures were deposited onto the wood surface via a chemical bath deposition process. The morphology of crystalline ZnO was controlled by aluminium nitrate or ammonium citrate in the growth step resulting in nanorod arrays or platelet structures, respectively. Detailed structural, chemical and mechanical characterisations as well as accelerated weathering exposure revealed the effective performance of the platelet structure, which formed a dense and thin ZnO coating on spruce. The total colour change (ΔE in the CIE system) was calculated to be 20.5 for unmodified wood, while it was about three for the modified samples after 4 weeks accelerated weathering test. Moreover, the ZnO coating also suppressed crack initiation and propagation indicating a substantial increase in durability.

Keywords: chemical bath deposition; CIE L*a*b* system; colour change upon UV irradiation; surface coating by dense ZnO layer; UV protection of wood; weathering of wood; wood surface; zinc oxide (ZnO); ZnO nanorods; ZnO platelets

Introduction

Wood is an excellent and widely used sustainable material for construction and furniture, but it is prone to degradation under UV exposure. The UV-induced degradation leads to colour change and surface alteration, and even to surface destruction in presence of water, which strongly affects the reliability of wood in various applications. Among the three main wood constituents (cellulose, hemicelluloses and lignin), the aromatic polymer lignin is the best UV absorber (Deka et al. 2008). As an example, the absorption spectrum of lignin from Norway spruce (Picea abies (L.) Karst.) displays a maximum absorption at 280 nm with a tail extending to ~350 nm (Sander and Koch 2001). As a result, in the case of unprotected wood surfaces, UV light from the solar spectrum induces lignin degradation via photo-oxidation. The natural polymers undergo bond cleavage and hydrogen abstraction resulting in the formation of radicals or peroxides which finally decompose and generate the coloured and hydrophilic by-products (chromophores) (Müller et al. 2003). Lignin is fragmented into low molecular weight species (such as water soluble polyphenols), which can be washed away by rain in outdoor applications. This exposes the underlying cell wall structure and leads to further degradation of the surface, which is then more prone to fungal attack (Hernandez and Evans 2015). Hence, the photochemical reactions result in a significant surface alteration of wood, with a loss of gloss and surface roughening (George et al. 2005; Zahri et al. 2007; Liu et al. 2015).

To address this challenge and reduce the photo-degradation of wood, different techniques to protect wood surfaces have been developed, including chemical modification of the wood polymers, surface treatments with additives (organic and inorganic UV absorbers, radical scavengers), and finishing (varnishes and opaque coatings). Chemical modification of wood such as acetylation and silylation (Plackett et al. 1992) bind chemicals to the cell wall polymers which can stabilise the wood colour (Rowell 2006; Baur and Easteal 2013; Lahtela and Kärki 2014). For example, Baur and Easteal (2013) used α- or γ-methacryloxysilanes and γ-epoxysilane to suppress free radical generation under UV exposure. Sander and Koch (2001) applied acetic anhydride and a 2-step hydrothermal treatment to modify Norway spruce samples, which retarded the UV-degradation of lignin. In another study, acetylated wood exhibited enhanced colour stabilisation when compared to their non-acetylated counterpart for the first 28 days in the UV chamber, but thereafter, the stabilisation could not be retained and discolouration began (Hon 1995). Surface modifications by silylation, thermal and copper ethanolamine treatment altered the colour of the wood specimens (Deka et al. 2008; Srinivas and Pandey 2012).

Nowadays, there is an increasing attention dedicated to wood modification with inorganic components (Sun et al. 2010; Mahr et al. 2013; Lu et al. 2014). For example, SiO2 nanoparticles with hydrolyzed hexadecyltrimethoxysilane or Al(OH)3 micro/nanospheres were applied to make wood surface hydrophobic (Wang et al. 2013) or fire resistance (Wang et al. 2014). Coating wood with metallic Ag nanoparticles and fluoroalkyl silane resulted in conductive and hydrophobic wood surfaces (Gao et al. 2016). A few studies report wood surface coatings based on inorganic nanoparticles acting as UV absorbers (Rao et al. 2016). Generally, these nanoparticles are semiconductor materials which absorb UV light via electron transition between conduction band and valence band but are transparent to visible light (Aloui et al. 2007; Auclair et al. 2011; Fu et al. 2015). For instance, southern pine specimens were vacuum impregnated by nano-ZnO dispersions for UV protection (Clausen et al. 2010). TiO2nanoparticles achieved by a sol-gel process were deposited onto wood surfaces via dip-coating and subsequent annealing to block UV irradiation (Rassam et al. 2012). The main drawback of such treatments is the leaching of added particles over time, lowering the protection efficiency, and necessitating regular application of new layers. Facing this challenge, a mixture of inorganic UV absorbers in a transparent organic coating to obtain highly durable and efficient UV protection coatings was reported recently (Auclair et al. 2011). ZnO-based acrylate coatings were applied to impregnate wood to reduce yellowing under UV exposure (Weichelt et al. 2010). Salla et al. (2012) dispersed ZnO nanoparticles in maleic anhydride modified polypropylene or polyurethane to prepare UV barrier coatings. The prospective use of nanofibrillated cellulose as carrier of UV absorbers for wood coating was described by Grüneberger et al. (2014a,b). Following a similar approach, TiO2 nanoparticles incorporated into water-based acrylic systems, and CeO2 based nano-composite (Veronovski et al. 2013; Auffan et al. 2014) were applied onto wood surfaces for a long-term resistance against weathering. However, the self-degradation of the applied polymers caused by sunlight exposure limits their long-term performance in photo-stabilizing wood.

In-situ coating of the substrate with a ZnO layer via a liquid phase route, which has been originally developed for solar cells or transparent conductive electrodes (Law et al. 2005; Greene et al. 2006; Hoang et al. 2013; Hagendorfer et al. 2014) would be a facile and promising method for wood UV protection. Yu et al. (2010) grew nanorods on wood surface via a two-step process, consisting of the generation of a seed layer on the wood surface followed by further particle growth. Sun et al. (2012) developed a one-pot hydrothermal method to grow highly ordered ZnO nanorod arrays onto the wood surface, which improved UV resistance. Aligned ZnO nanorods were fabricated on wood surfaces via a similar process by Liu et al. (2013). However, the morphology and the crystallinity of ZnO nanostructures on the wood surface is difficult to control, and the interaction between the inorganic coating and wood surface might not be sufficiently tight with regard to weathering stability. Further, ZnO nanorod arrays can show a strong scattering of visible light (Law et al. 2005, 2006), which diminishes the aesthetic appearance of the wood surfaces.

In this article, we report on increasing the UV-stability of wood by a coating that almost retains the natural aesthetic appearance, through the growth of an inorganic ZnO layer onto the rough and inhomogeneous wood surface. The surface of Norway spruce was coated by a conformal ZnO layer via chemical bath deposition (CBD), with ZnO powder and ammonium hydroxide as precursors in aqueous solution. CBD is a known soft chemical method, which is promising for the preparation of metal oxide nanomaterials with controllable morphologies and sizes, as well as allows for rather low temperature and ambient reaction condition (Niederberger et al. 2004; Hoang et al. 2013). It was demonstrated that the c-axis growth of ZnO crystal can be suppressed by a structure directing agent (Tian et al. 2003) to achieve platelet structures that result in a dense and highly crystalline ZnO film on the wood surface. The performance of the material will be tested by surface analysis and weathering. The interface between wood and ZnO should also be carefully studied by means of electron microscopy.

Materials and methods

Results and discussion

Conclusions

Acknowledgments

References

• Aloui, F., Ahajji, A., Irmouli, Y., George, B., Charrier, B., Merlin, A. (2007) Inorganic UV absorbers for the photostabilisation of wood-clearcoating systems: comparison with organic UV absorbers. Appl. Surf. Sci. 253:3737–3745. Google Scholar
• Auclair, N., Riedl, B., Blanchard, V., Blanchet, P. (2011) Improvement of photoprotection of wood coatings by using inorganic nanoparticles as ultraviolet absorbers. For. Prod. J. 61:20–27. Google Scholar
• Auffan, M., Masion, A., Labille, J., Diot, M.A., Liu, W., Olivi, L., Proux, O., Ziarelli, F., Chaurand, P., Geantet, C., Bottero, J.Y., Rose, J. (2014) Long-term aging of a CeO2 based nanocomposite used for wood protection. Environ. Pollut. 188:1–7. Google Scholar
• Avirah, R.R., Jyothish, K., Ramaiah, D. (2008) Infrared absorbing croconaine dyes: synthesis and metal ion binding properties. J. Org. Chem. 73:274–279. Google Scholar
• Baur, S.I., Easteal, A.J. (2013) Improved photoprotection of wood by chemical modification with silanes: NMR and ESR studies. Poly. Adv. Technol. 24:97–103. Google Scholar
• Cho, S., Jang, J.-W., Jung, S.-H., Lee, B.R., Oh, E., Lee, K.-H. (2009) Precursor effects of citric acid and citrates on ZnO crystal formation. Langmuir 25:3825–3831. Google Scholar
• Clausen, C.A., Green, F., Kartal, S.N. (2010) Weatherability and leach resistance of wood impregnated with nano-zinc oxide. Nanoscale Res. Lett. 5:1464–1467. Google Scholar
• Danilov, V.V., Ravdel, A.A., Lutsik, V.P. (1976) Study of the solubility of zinc oxide and hydroxide in aqueous solutions of ammonia. Zh. Organich. Khim. 46:976–981. Google Scholar
• Deka, M., Humar, M., Rep, G., Kricej, B., Sentjurc, M.S., Petric, M. (2008) Effects of UV light irradiation on colour stability of thermally modified, copper ethanolamine treated and non-modified wood: EPR and DRIFT spectroscopic studies. Wood Sci. Technol. 42:5–20. Google Scholar
• Fu, Y., Fu, W., Liu, Y., Zhang, G., Liu, Y., Yu, H. (2015) Comparison of ZnO nanorod array coatings on wood and their UV prevention effects obtained by microwave-assisted hydrothermal and conventional hydrothermal synthesis. Holzforschung 69:1009–1014. Google Scholar
• Gao, L., Lu Y., Li, J., Sun, Q. (2016) Superhydrophobic conductive wood with oil repellency obtained by coating with silver nanoparticles modified by fluoroalkyl silane. Holzforschung 70:63–68. Google Scholar
• George, B., Suttie, E., Merlin, A., Deglise, X. (2005) Photodegradation and photostabilisation of wood – the state of the art. Polym. Degrad. Stabil. 88:268–274. Google Scholar
• Greene, L.E., Yuhas, B.D., Law, M., Zitoun, D.,Yang, P. (2006) Solution-grown zinc oxide nanowires. Inorg. Chem. 45: 7535–7543. Google Scholar
• Grüneberger, F., Künniger, T., Zimmermann, T., Arnold, M. (2014a) Nanofibrillated cellulose in wood coatings: mechanical properties of free composite films. J. Mater. Sci. 49: 6437–6448. Google Scholar
• Grüneberger, F., Künniger, T., Zimmermann, T., Arnold, M. (2014b) Rheology of nanofibrillated cellulose/acrylate systems for coating applications. Cellulose 21:1313–1326. Google Scholar
• Hagendorfer, H., Lienau, K., Nishiwaki, S., Fella, C.M., Kranz, L., Uhl, A.R., Jaeger, D., Luo, L., Gretener, C., Buecheler, S., Romanyuk, Y.E., Tiwari, A.N. (2014) Highly transparent and conductive ZnO: Al thin films from a low temperature aqueous solution approach. Adv. Mater. 26:632–636. Google Scholar
• Hernandez, V.A., Evans, P.D. (2015) Technical note: melanization of the wood-staining fungus Aureobasidium pullulans in response to UV radiation. Wood Fiber Sci. 47:120–124. Google Scholar
• Hoang, S., Berglund, S.P., Fullon, R.R., Minter, R.L., Mullins, C.B. (2013) Chemical bath deposition of vertically aligned TiO2 nanoplatelet arrays for solar energy conversion applications. J. Mater. Chem. A 1:4307–4315. Google Scholar
• Hon, D.N.S. (1995) Stabilization of wood color: is acetylation blocking effective? Wood Fiber Sci. 27:360–367. Google Scholar
• Joo, J., Chow, B.Y., Prakash, M., Boyden, E.S., Jacobson, J.M. (2011) Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis. Nat. Mater. 10:596–601. Google Scholar
• Katepetch, C., Rujiravanit, R., Tamura, H. (2013) Formation of nanocrystalline ZnO particles into bacterial cellulose pellicle by ultrasonic-assisted in situ synthesis. Cellulose 20:1275–1292. Google Scholar
• Lahtela, V., Kärki, T. (2014) Improving the UV and water-resistance properties of Scots pine (Pinus sylvestris) with impregnation modifiers. Eur. J. Wood Prod. 72:445–452. Google Scholar
• Law, M., Greene, L.E., Johnson, J.C., Saykally, R., Yang, P. (2005) Nanowire dye-sensitized solar cells. Nat. Mater. 4:455–459. Google Scholar
• Law, M., Greene, L.E., Radenovic, A., Kuykendall, T., Liphardt, J., Yang, P.D. (2006) ZnO-Al2O3 and ZnO-TiO2 core-shell nanowire dye-sensitized solar cells. J. Phys. Chem. B 110:22652–22663. Google Scholar
• Leary, G. (1968) Photochemical production of quinoid structures in wood. Nature 217:672–673. Google Scholar
• Lebo, S.E., Lonsky, W.F.W., Mcdonough, T.J., Medvecz, P.J., Dimmel, D.R. (1990) The occurrence and light-induced formation of ortho-quinonoid lignin structures in white spruce refiner mechanical pulp. J. Pulp Pap. Sci. 16:J139–J143. Google Scholar
• Liu, Y., Fu, Y., Yu, H., Liu, Y. (2013) Process of in situ forming well-aligned zinc oxide nanorod arrays on wood substrate using a two-step bottom-up method. J. Colloid Interface Sci. 407:116–121. Google Scholar
• Liu, Y., Shao, L., Gao, J., Guo, H., Chen, Y., Cheng, Q., Via, B.K. (2015) Surface photo-discoloration and degradation of dyed wood veneer exposed to different wavelengths of artificial light. Appl. Surf. Sci. 331:353–361. Google Scholar
• Lu, Y., Xiao, S., Gao, R., Li, J., Sun, Q. (2014) Improved weathering performance and wettability of wood protected by CeO2 coating deposited onto the surface. Holzforschung 68:345–351. Google Scholar
• Mahr, M.S., Hübert, T., Stephan, I., Bücker, M., Militz, H. (2013) Reducing copper leaching from treated wood by sol-gel derived TiO2 and SiO2 depositions. Holzforschung 67:429–435. Google Scholar
• Müller, U., Rätzsch, M., Schwanninger, M., Steiner, M., Zöbl, H. (2003) Yellowing and IR-changes of spruce wood as result of UV-irradiation. J. Photoch. Photobio. B 69:97–105. Google Scholar
• Niederberger, M., Pinna, N., Polleux, J., Antonietti, M. (2004) A general soft-chemistry route to perovskites and related materials: synthesis of BaTiO3, BaZrO3, and LiNbO3 nanoparticles. Angew. Chem. Int. Ed. 43:2270–2273. Google Scholar
• Owen, N.L., Thomas, D.W. (1989) Infrared studies of “hard” and “soft” woods. Appl. Spectrosc. 43:451–455. Google Scholar
• Plackett, D.V., Dunningham, E.A., Singh, A.P. (1992) Weathering of chemically modified wood. Holz Roh. Werkst. 50:135–140. Google Scholar
• Rao, X., Liu, Y., Fu, Y., Liu, Y., Yu, H. (2016) Formation and properties of polyelectrolytes/TiO2 composite coating on wood surfaces through layer-by-layer assembly method. Holzforschung 70:361–367. Google Scholar
• Rasmussen, J.S., Barsberg, S., Felby, C. (2014) Complex between lignin and a Ti-based coupling agent. Holzforschung 68: 541–548. Google Scholar
• Rassam, G., Abdi, Y., Abdi, A. (2012) Deposition of TiO2 nano-particles on wood surfaces for UV and moisture protection. J. Exp. Nanosci. 7:468–476. Google Scholar
• Rosu, D., Teaca, C.-A., Bodirlau, R., Rosu, L. (2010) FTIR and color change of the modified wood as a result of artificial light irradiation. J. Photoch. Photobio. B 99:144–149. Google Scholar
• Rowell, R.M. (2006) Chemical modification of wood: a short review. Wood Mater. Sci. Eng. 1:29–33. Google Scholar
• Salla, J., Pandey, K.K., Srinivas, K. (2012) Improvement of UV resistance of wood surfaces by using ZnO nanoparticles. Poly. Degrad. Stabil. 97:592–596. Google Scholar
• Sander, C., Koch, G. (2001) Effects of acetylation and hydrothermal treatment on lignin as revealed by cellular UV-spectroscopy in Norway spruce (Picea abies [L.] Karst.). Holzforschung 55:193–198. Google Scholar
• Srinivas, K., Pandey, K.K. (2012) Photodegradation of thermally modified wood. J. Photochem. Photobio. B 117:140–145. Google Scholar
• Sun, Q.F., Yu, H., Liu, Y., Li, J., Lu, Y., Hunt, J.F. (2010) Improvement of water resistance and dimensional stability of wood through titanium dioxide coating. Holzforschung 64:757–761. Google Scholar
• Sun, Q.F., Lu, Y., Zhang, H.M., Yang, D.J., Wang, Y., Xu, J.S., Tu, J.C., Liu, Y.X., Li, J. (2012) Improved UV resistance in wood through the hydrothermal growth of highly ordered ZnO nanorod arrays. J. Mater. Sci. 47:4457–4462. Google Scholar
• Tian, Z.R., Voigt, J.A., Liu, J., Mchenzie, B., Mcdermott, M.J., Rodriguez, M.A., Konishi, H., Xu, H. (2003) Complex and oriented ZnO nanostructures. Nat. Mater. 2:821–826. Google Scholar
• Veronovski, N., Verhovsek, D., Godnjavec, J. (2013) The influence of surface-treated nano-TiO2 (rutile) incorporation in water-based acrylic coatings on wood protectin. Wood Sci. Technol. 47:317–328. Google Scholar
• Wang, X., Chai, Y., Liu, J. (2013) Formation of highly hydrophobic wood surfaces using silica nanoparticles modified with long-chain alkylsilane. Holzforschung 67:667–672. Google Scholar
• Wang, N., Fu, Y., Liu, Y., Yu, H., Liu, Y. (2014) Synthesis of aluminum hydroxide thin coating and its influence on the thermomechanical and fire-resistant properties of wood. Holzforschung 68:781–789. Google Scholar
• Weichelt, F., Emmler, R., Flyunt, R., Beyer, E., Buchmeiser, M.R., Beyer, M. (2010) ZnO-based UV nanocomposites for wood coatings in outdoor applications. Macromol. Mater. Eng. 295:130–136. Google Scholar
• Yu, Y., Jiang, Z., Wang, G., Song, Y. (2010) Growth of ZnO nanofilms on wood with improved photostability. Holzforschung 64:385–390. Google Scholar
• Zahri, S., Belloncle, C., Charrier, F., Pardon, P., Qquideau, S., Charrier, B. (2007) UV light impact on ellagitannins and wood surface colour of European oak (Quercus petraea and Quercus robur). Appl. Surf. Sci. 253:4985–4989. Google Scholar

About the article

Corresponding authors: Huizhang Guo and Ingo Burgert, Wood Materials Science, Institute for Building Materials, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich (Switzerland); and Applied Wood Materials, Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf (Switzerland), e-mail:  (H. Guo),  (I. Burgert)

---

Received: 2015-08-25

Accepted: 2015-11-24

Published Online: 2016-01-07

Published in Print: 2016-08-01

---

Citation Information: Holzforschung, ISSN (Online) 1437-434X, ISSN (Print) 0018-3830, DOI: https://doi.org/10.1515/hf-2015-0185.

Export Citation

©2016 Walter de Gruyter GmbH, Berlin/Boston. 

For further details logon website :
https://www.degruyter.com/view/j/hfsg.2016.70.issue-8/hf-2015-0185/hf-2015-0185.xml?format=INT