Highly Efficient UV Protection of the Biomaterial Wood by A

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Highly efficient UV protection of the biomaterial wood by a transparent TiO2/Ce xerogel Huizhang Guo, Daniel Klose, Yuhui Hou, Gunnar Jeschke, and Ingo Burgert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12574 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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ACS Applied Materials & Interfaces

Highly Efficient UV Protection of the Biomaterial Wood by A Transparent TiO2/Ce Xerogel Huizhang Guo,*1,2 Daniel Klose,3 Yuhui Hou,4 Gunnar Jeschke,3 Ingo Burgert*1,2 1, Wood Materials Science, Institute for Building Materials, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich, Switzerland 2, Applied Wood Materials, Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland 3, Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland 4, Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan *Corresponding author: [email protected], [email protected] Abstract Titanium dioxide is most widely used in sunscreens because of its strong ultraviolet (UV) light absorbing capabilities and its resistance to discoloration under UV exposure. However, when deposited as a thin film, the high refractive index of titanium dioxide typically results in whiteness and opacity, which limits the use of titanium dioxide for material surfaces, for which long-term natural appearance is of high relevance. Since the whitish appearance is due to the strong light scattering and reflection on the interface of oxide particles and air, one can increase the transparency of TiO2 coatings by forming a continuous TiO2 layer. The purpose of the present article is twofold. Firstly, we show that, in the presence of cerium ammonium nitrate, titanium dioxide can be turned from a white powder into a TiO2/Ce xerogel via a facile bottom-up fabrication process. Secondly, we demonstrate that the transparent TiO2/Ce xerogel can diminish surface deterioration induced by UV light and preserve the natural appearance of the highly abundant biomaterial wood. Furthermore, EPR spectroscopy revealed that the TiO2/Ce xerogel coating suppresses free radical generation on wood surfaces upon UV irradiation. Our research expands the applicability of the protective effect of titanium dioxide to coatings for natural engineering materials, which will become increasingly important in future bioeconomies. Introduction There is a growing demand to utilize renewable biomaterials, such as wood, to design and fabricate aesthetically appealing and eco-friendly products. However, most of the biomaterials are affected by degradation of organic compounds upon UV irradiation. For instance, wood materials that are exposed to sunlight undergo bond cleavage and hydrogen abstraction mainly of the natural polymer lignin, resulting in the formation of radicals or peroxides which finally leads to discoloration, as well as an increase in 1 ACS Paragon Plus Environment


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hydrophilicity and fungi attack.1 More generally, UV light from the solar spectrum is responsible for the photo degradation of many materials exposed outdoors, which causes millions of dollars of material damage every year.2 Hence, it is important to develop a new generation of UV protective coatings that are almost transparent and highly durable. UV blocking coatings for wood and plastic preservation can be prepared by dispersing semiconductor nanoparticles such as TiO2, ZnO and CeO2 in organic binders.3 For example, surface-treated nano-titanium dioxide (TiO2) particles were incorporated into water-based acrylic systems for wood protection4 and TiO2 nanoparticles were added to a water-based varnish to protect tropical woods from weathering.5 Moreover, a polyelectrolytes/TiO2 composite coating was fabricated on a wood surface by layer-by-layer assembly for wood preservation.6 However, the poor durability of the polymer matrix in outdoor exposure is still an issue, and demands too short maintenance intervals. Hence, there is an increasing research effort to modify natural materials with all-inorganic coatings for UV protection. Abidi et al. modified cotton fabric with titania nanosols, which exhibited a very good protection against UV radiation.7 Xiao et al. modified silk fibers with a nanoscale titania coating using atomic layer deposition (ALD), which resulted in an increased UV absorbance, less yellowing, and enhanced mechanical properties.8 Wang et al. coated the surface of cotton textiles with ZnO@SiO2 nanorods in order to obtain superhydrophobic and UV blocking properties.9, 10 CeO2 particles were attached to the surface of cotton fabrics.11 However, the fabrication of a completely inorganic TiO2 coating on the uneven wood surface has not been reported yet. TiO2 with a band gap of 3.05 eV is an excellent candidate for UV protection because it does not absorb visible light but strongly absorbs UV light.12, 13 However, due to the strong visible light scattering and the very high refractive index (2.76~2.55), TiO2 cannot be used as a coating that provides UV protection while also preserving the aesthetic appeal of the substrate.14 Because whitening is due to the strong light scattering and reflection on the interface of individual oxide particles and air, one can increase the transparency by generating a continuous TiO2 layer. Transparent semiconductor coatings have been prepared by physical deposition such as magnetic sputtering deposition,15-17 or plasma deposition.18 These technologies are commonly highly energy demanding, require high vacuum, and thermally stable substrates, which makes them expensive and impedes their applicability for soft materials. Herein, we propose a novel and facile approach for the fabrication of transparent TiO2 xerogel at room temperature and atmospheric pressure. Wood, one of the most abundant renewable biomaterials that is widely used in building construction, building envelopes, furniture, and indoor decoration, was utilized as an object of study. However, the versatile procedure can be easily adapted to various other materials. Cerium(IV) ammonium nitrate was used as a stabilizing agent, which inhibited the rapid hydrolysis of titanium isopropoxide induced by moisture from ambient air. The strong oxidation property of Ce(IV) 2 ACS Paragon Plus Environment

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makes it bind to the oxygen atom, which serves as the connection center for the growth of the Ti-O network into a large scale transparent TiO2/Ce xerogel. The transparent TiO2/Ce xerogels were studied by different characterization techniques including XPS (X-ray photoelectron spectroscopy), UV-Vis (Ultraviolet-visible spectroscopy), and XRD (X-ray Powder Diffraction) as well as regarding surface discoloration under UV exposure. The results demonstrate that the TiO2/Ce xerogel formed a layer following the topography of the wood surface, which not only protected the wood against discoloration and degradation upon UV irradiation but also preserved its natural appearance. Experimental section Chemicals: Cerium(IV) Ammonium Nitrate (purum p.a., ≥98% (T), (NH)2Ce(NO3)6, CAN), Titanium(IV) isopropoxide (97%, TTIP), 4-hydroxy tempo (97 %) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were purchased from Sigma-Aldrich (Buchs, Switzerland). Isopropanol (≥99.5%) was purchased from Thommen-Furler AG (Rueti, Switzerland). Synthesis of transparent TiO2/Ce xerogel: CAN was dissolved in isopropanol by sonication to form a clear orange solution. The concentration of CAN was varied between 0.5 mM to 10 mM. TIPP was added with a volume ratio of 1:10 to the isopropanol solvent, causing a color change from orange to light yellow. The resulted solution is named as precursor solution. After magnetic stirring at room temperature for half an hour, the precursor solution (200 mL) was transferred into a poly(propylene) plastic box with a dimension of 17 cm × 13 cm × 5 cm in length × width × height, respectively, covered by a lid. The plastic box was stored at room temperature in ambient conditions. Transparent TiO2/Ce xerogel was formed in about 2 weeks as the isopropanol evaporated. Wood surface modification: Spruce wood (Picea abies) was cut into samples of a dimension of 50 mm × 50 mm × 5 mm in longitudinal × radial × tangential directions, respectively. The modification process consists of two steps: step 1, the samples were treated with the aqueous solution of CAN with a concentration of 1.5 mM for 24 h, and then dried at 65 oC for 6 h; step 2, three wood samples were placed into the plastic box containing 100 mL of precursor solution for 24 h followed by a dip-coating process to increase the thickness of TiO2/Ce xerogel on the surface of wood. The dip-coating process included a 5 min dipping into the precursor solution followed by 20 min drying at an ambient condition which was repeated up to 6 times. Control samples were only modified by step 2. Color change under UV exposure: The UV protection efficiency of the transparent TiO2/Ce coating was assessed by accelerated UV irradiation in a UV curing chamber (UVACUBE 400, Hoenle Group, Graefelfing, Germany). The UV irradiation intensity is 1.87 W/(m2 nm) at 340 nm (UV-A), and 2.43 W/(m2 nm) at 313 nm (UV-B). Four weeks’ UV exposure was carried out with interruptions for color 3 ACS Paragon Plus Environment


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measurements. Color evaluation of the wood samples before and after UV exposure was determined in the CIE 1976 (L*a*b*) color space by the Chroma Mater CR-200 (KONICA MINOLTA). L* represents the lightness from black (0) to white (100), while a* and b* are the chromaticity indices, where +a* is the red, -a* is the green, and +b* is yellow, -b* is the blue direction. The total color change is defined as ΔE = √ΔL + Δa + Δb . Color measurements were performed on three repeated samples (10 measuring points for each), and then the average value was calculated. Spectroscopic Characterization: The transparent TiO2/Ce xerogel, as well as the modified and unmodified wood, were analyzed by X-ray diffraction (XRD) in Bragg-Brentano mode and in θ-2θ geometry with Cu-Kɑ radiation (PANalytical, Almelo, the Netherlands). Optical properties of the TiO2/Ce xerogel were studied on a UV-Vis spectrophotometer (LAMBDA 850, Perkin Elmer, Waltham, USA). Xray photoelectron spectra (XPS) were acquired on a JPC-9010MC instrument (JEOL, Akishima, Tokyo, Japan) at a pass energy of 10 eV using Mg Kα as an X-ray source, 260 scans were performed on each sample. Surface topographies were analyzed by scanning electron microscopy (SEM, Quanta 200F, FEI, Hillsboro, USA). Wood cross sections were prepared by broad ion beam milling (BIB 4000, Tokyo, Japan). Electron Paramagnetic Resonance (EPR) experiments were performed on slices of spruce with a thickness of 0.9 mm at X-band, ca. 9.6 GHz, on a Bruker EMX spectrometer (Bruker BioSpin, Rheinstetten, Germany), equipped with a TE102 resonator (Bruker ER4102ST) with a 10x23 mm optical window covered by a microwave-containing grid that allows for 50 % transmittance. A low-pressure nitrogen gas flow through the resonator was used to stabilize the temperature without excluding ambient oxygen. Continuous wave (CW) EPR spectra were detected under non-saturating conditions with 0.2 mW incident microwave power using a 100 kHz magnetic field modulation with an amplitude of 0.3 mT, a conversion time of 81.92 ms and typically three scans were accumulated. For g-factor determination, the magnetic field was calibrated using DPPH. For in situ EPR experiments, samples were UV-irradiated inside the resonator using an OmniCure S2000 UV mercury lamp with a nominal output power of 200 W. The lamp’s optical fiber was centered in front of the EPR resonator at a distance of 2.5 cm from the sample. To record kinetics, the conversion time was reduced to 20.48 ms to facilitate better time resolution. The radical concentration was determined by double-integration of the first-derivative cw EPR spectra, each preceded by a first-order polynomial baseline correction. Integral values were calibrated using 4hydroxy tempo in toluene (1 mM, 10 µl) dried onto a wood sample covered by a layer of 20 µl polystyrene (600 mg/ml in toluene) as a reference. Results and discussion Hydrolysis of titanium isopropoxide (TTIP) has been widely employed in the sol-gel synthesis of TiO2based materials in the form of powder or thin films.19 We found that the concentration of CAN plays a key 4 ACS Paragon Plus Environment

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role in the formation of transparent xerogel, as the only white powder is generated in the absence of CAN. Upon using the isopropanol solution of the CAN with a concentration of 0.5 mM, the reaction of TTIP resulted in a xerogel. The UV-Vis spectrum of the xerogel (Figure 1a, black line) demonstrates that almost no light with a wavelength lower than 370 nm can pass the xerogel, while the visible light can still pass through it. The relative low visible light transmittance is due to the powder generated on the surface of the xerogel which causes light scattering and in consequence only a semi-transparent xerogel (Figure 1b, left image).

Figure 1. a) UV-Vis spectra of the freestanding TiO2/Ce xerogel with a thickness of around 1 mm fabricated in the isopropanol solution with a CAN concentration of 0.5 mM (black), 1.5 mM (red), 5 mM (green) and 10 mM (blue). b) Photograph of the free-standing TiO2/Ce xerogel prepared with the isopropanol solution with a CAN concentration of 0.5 mM, 1.5 mM, 5 mM and 10 mM from left to right. TiO2 Particle generation on the surface of the xerogel could be eliminated by increasing the concentration of CAN in the isopropanol solution to 1.5 mM or 5 mM, which improved the transparency in the visible light range (Figure 1a, red line, and green line; Figure 1b, 2nd and 3rd sample). However, with increasing cerium concentration the color of the transparent xerogel turned more and more to yellow. Interestingly, the transparency decreased again, when the concentration was further increased to 10 mM due to the deposition of the particles on the xerogel surface (Figure 1a, blue line, Figure 1b, right side). Consequently, for the further investigations xerogels prepared on the basis of the 1.5 mM CAN isopropanol solution were used as the best compromise between visible light transparency and UV filtering unless noted otherwise. 5 ACS Paragon Plus Environment


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To probe for crystallinity of the xerogel, it was ground into powder and then measured by XRD. No diffraction assigned to titanium dioxide or cerium oxide was observed in the XRD pattern, which revealed that the TiO2/Ce xerogel was amorphous (Figure 2, line (a)). The amorphous xerogel could be transformed to anatase by 2 hours of heat treatment at 600oC (Figure 2, line (b)). The diffraction peaks observed at 25.4, 37.8, 48.0, 54.0, 55.2 degrees in Figure 2 line b are assigned to the (101), (004), (200), (105) and (211) planes of anatase TiO2. However, no diffraction peaks assigned to cerium oxide could be detected in Figure 2, line b, which indicates that the cerium atoms had a homogeneous distribution in the TiO2 matrix. The TiO2/Ce xerogel deposited on the wood surface was amorphous and did not show any characteristic diffraction peaks of TiO2 crystals in the XRD spectrum (Figure 2, line (c)). However, the dense coating suppressed the diffraction intensity of cellulose crystals of wood when compared to the XRD spectrum of unmodified wood (Figure 2, line (d)).

Figure 2. XRD patterns of the TiO2/Ce xerogel (a, red line), the same xerogel after heat treatment at 600 o

C for 2 hours (b, black line), TiO2/Ce xerogel (c, purple line) deposited on the wood surface and the

unmodified wood (d, blue line). To resolve the interaction between the additive Ce and TiO2 bulk, an XPS study was performed on the xerogel fabricated using 10 mM CAN isopropanol solution (for a significant Ce signal) as well as the one after annealing. Analysis by XPS in Figure 3a shows that the Ti 2p spectrum of the TiO2/Ce xerogel displayed a couple of peaks which were assigned to Ti 2p3/2 state and Ti 2p1/2 state, respectively.20 The peak positions are located at the same binding energy with or without heat treatment (Figure 3b). This revealed that titanium and oxygen in the TiO2/Ce xerogel adopted the same bonding as that of anatase TiO2.21 Figure 3c and d display the XPS spectra of Ce 3d of the xerogel with and without heat treatment. The observed peaks can be assigned as v, v’ and v’’ for Ce 3d5/2, while the corresponding peaks labeled as u and u’ are assigned to Ce 3d3/2. The results indicate that 3+ is the dominant oxidation state. The v’’ peak 6 ACS Paragon Plus Environment

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revealed the formation of Ce(III)-O-Ti species in the xerogel.22 There is no signal detected in the region between 915 eV and 920 eV where the characteristic satellite peaks region of Ce4+ are expected.23 This further proves that all of the Ce4+ ions of the CAN solution had been reduced to Ce3+. The Ce3+ is in a chemically stable condition inside the TiO2 matrix and cannot be oxidized to Ce4+ even under thermal treatment at 600 oC in ambient air (Figure 3d).

Figure 3. Ti 2s XPS spectra of the TiO2/Ce xerogel prepared in 10 mM CAN isopropanol solution (a) and the same xerogel after thermal treatment at 600 oC for 2 hours (b); Ce 3d XPS spectra of the TiO2/Ce xerogel prepared in 10 mM CAN isopropanol solution (c) and the same xerogel after thermal treatment at 600 oC for 2 hours (d). The ionic radius of Ce3+ is 0.103 nm, which is much larger than that of Ti4+ (0.064 nm). Hence, it is difficult for the Ce3+ ions to enter the TiO2 lattice and form a stable solid-state solution24, 25 which makes Cerium atoms remain at the surface of TiO2 grains. CAN is a strong one-electron oxidizing agent, indicated by the fading color of the isopropanol solution of the CAN from orange to a pale yellow upon the addition of TTIP. CAN reacted with TTIP to form Ce-O-Ti species at the interface. In other words, the additive Ce(IV) acts as an intermediary to bond the TiO2 grains together to form a continuous structure which is crucial for its transparency. Photochemical reactions induced by UV radiation result in a significant surface alteration of wood such as yellowing/darkening and surface roughening, affecting its aesthetic appeal and durability upon weathering. Hence, there has been a widespread interest in preventing the photodegradation of wood. To demonstrate 7 ACS Paragon Plus Environment


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the UV protection property of the transparent TiO2/Ce xerogel, the gel prepared in a solution of isopropanol with 1.5 mM CAN was coated onto the surface of the spruce wood. Figure 4a displays the radial/longitudinal surface of the wood that was used as the main exposure surface. The SEM image acquired from the earlywood region shows the roughness of the wood surface, based on cut-open cells with cell wall ridges and lumen cavities. The TiO2/Ce xerogel formed a microsheet coating on the wood surface with fracture in between due to the fast drying process, as illustrated in Figure 4c. The thickness of the coating after five cycles of dip-coating was determined by SEM imaging with a cross-sectional view (Figure 4d), and was measured to be around 7 µm.

Figure 4. a) Schematic illustration of the wood sample preparation with the radical section as the main exposure surface; b) SEM image acquired from spruce earlywood; c) SEM image of the TiO2/Ce xerogelcoated wood acquired from a region with similar wood surface topography as the red square shown in b; d) SEM image acquired from the cross-section of the coated wood. The surface modification only marginally affected the natural appearance of the surface (Figure 5a and b), with a minor color change, which could be quantified by color measurements of the modified and unmodified wood. The modified sample had an increment of 1.34 for lightness (∆L*), while a reduction of 0.99 and 0.34 for redness (∆a*) and yellowness (∆b*), respectively. The coating resulted in a total color change of 2.42, which shows that the surface treatment retained the natural appearance of the wood. 8 ACS Paragon Plus Environment

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Figure 5. Photograph of natural wood (a), cerium oxide and TiO2/Ce xerogel-coated wood (b), unmodified wood after 4 weeks of UV exposure (c) and the modified wood after 4 weeks of exposure (d). The UV protection efficiency of the transparent TiO2/Ce xerogel can be assessed by the color measurement after UV exposure. The occurrence of color changes on the surface of wood samples is a result of the increasing number of chromophores produced during irradiation, due to UV-induced photooxidation of lignin.1 Four weeks of UV irradiation with interruptions for color measurement was conducted. The comparison of natural and modified wood samples without and with UV treatment in Figure 5 already illustrate the strong protection against discoloration by the almost transparent coating. In Figure 6 this effect was quantified on the basis of color measurements. The results show that the unmodified wood had a total color change of 19.70 in the end of the UV exposure test (Figure 6d, black line). In contrast, the ∆E value of the wood only coated by the TiO2/Ce xerogel was considerably smaller, with a value of 7.15. The results reveal that the TiO2/Ce xerogel not only preserved the natural appearance of wood materials but also efficiently protected the surface from UV exposure. It was interesting to notice that the yellowness index (b*) of the TiO2/Ce xerogel modified sample increased up to 25.18 in the first 100 hours of exposure and then dropped down gradually to 20.85 towards the end of the UV exposure test (Figure 5c, blue line). This phenomenon may be due to photocatalytic effects caused by direct contact between the wood surface and the TiO2/Ce after absorbing UV.26, 27 Hence, the UV protection performance was further improved by adding a cerium oxide layer between the wood surface and the xerogel. Prior to the coating with xerogel, a pre-treatment of the wood sample by an aqueous solution of the CAN was carried out. The color stability of the wood sample with 9 ACS Paragon Plus Environment


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CAN pre-treatment and TiO2/Ce xerogel modification (Ce_TiO2/Ce) is displayed in red in Figure 6, showing that this additional modification further reduced the color change upon UV irradiation. As a result, the total color change of Ce_TiO2/Ce was about 3.86, which was even smaller than that of the wood only modified by TiO2/Ce xerogel.

Figure 6. Dependence of lightness index (L*) (a), redness index (a*) (b), yellowness (b*) (c) and total color change (∆E) (d) of the wood materials versus the irradiation time. (●) the spruce wood modified with the pre-treatment by the aqueous solution of the CAN, and then the TiO2/Ce xerogel; (▲) the spruce wood modified by the TiO2/Ce xerogel; (■) unmodified spruce wood. To understand the effect of the thickness of the TiO2/Ce coating on the UV protection performance, wood samples with different numbers of cycles of dip-coating were irradiated by UV for one week and then the total color changes were assessed. As illustrated in Table 1, the sample that was only treated with the aqueous solution of CAN exhibited almost no UV protection and showed a total color change of 15.88. However, the sample with an additional soaking treatment in the titanium precursor solution and one cycle of dip coating displayed a significant improvement in preventing color change upon UV exposure and showed a total color change of 5.77. The total color change could be further improved by increasing the thickness of the xerogel on the wood surface. After six dip-coating cycles, the total color change was suppressed to 2.88.

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Table 1. Total color changes (∆E) of the wood samples with different cycles of dip-coating after one week of UV exposure.

Cycle of Dip-coating ∆E

CAN Pretreatment 0 15.88

CAN Pretreatment and TiO2/Ce 1 2 3 4 5 6 5.77 4.22 4.45 3.83 3.34 2.88

This reveals that an efficient UV protection can be achieved based on the synergistic effect of a cerium oxide pre-coating and a TiO2/Ce xerogel with a sufficient thickness. The later acts as a UV cutoff filter, while the pre-coating functions as an antioxidant, which suppresses the photocatalytic effect caused by the TiO2/Ce layer when absorbing UV light. For unraveling the specific surface interactions that result in the substantial wood surface protection further studies were conducted by XPS and EPR. The XPS analysis on the wood surface only treated by the aqueous solution of CAN is shown in Figure 7. The binding energy related to both, Ce(IV) oxide and Ce(III) oxide can be observed in the spectrum. The strong oxidation property of CAN may let it bind to the wood surface via a redox reaction, and form Ce2O3, while CeO2 could be formed through deprotonation of cerium ions in a tetravalent state (such as Ce(OH)22+) in aqueous solution.28 The cerium oxide layer on the wood surface can act as an antioxidant due to the surface 3+/4+ valency switch. The same effect results in anti-inflammatory, anti-aging properties in experiments on living cells.26 Xu and Qu also drew a similar conclusion by stating that cerium oxide nanoparticles can be a potentially antioxidant towards almost all noxious intracellular reactive oxygen species.29

Figure 7. Ce3d spectrum acquired from the wood surface after treatment with an aqueous solution of 5 mM CAN.



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In wood, UV light induces the generation of free radicals, primarily different C-centered lignin radicals,30 many of which will interact with oxygen to produce hydroperoxide impurities. The hydrogen peroxide impurities are easily decomposed to produce chromophoric groups such as carbonyl and carboxyl groups,31-33 which indicates that the generation of free radicals is directly responsible for the wood yellowing/darkening. Therefore, irradiation experiments together with radical concentration detection can help to understand the UV protection effect of the transparent TiO2/Ce xerogel.


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Figure 8. a) EPR spectra of the unmodified spruce wood before (red) and after (black) UV irradiation; b) EPR spectra of the Ce_TiO2/Ce modified spruce wood; c) in situ radical concentrations tracked during and after 2 min of UV irradiation. Different levels of EPR signal intensity, which is directly proportional to free radical concentration, were obtained from the wood. From the EPR signal shown in Figure 8a, a g-factor of 2.0046 can be calculated for natural wood, which has been attributed to phenoxy/semiquinone radicals generated from lignin.30, 34,35 Being naturally present in wood, these radicals are long-lived and hence stable or sterically shielded. After two minutes of UV irradiation, the EPR signal intensity of untreated spruce increased significantly without shift or change of the lineshape. Quantification of the radicals by integration over the EPR line (see Materials & Methods) revealed a 7.3-fold increase in radical amount from 0.16 to 1.16 nmol for an illuminated surface area of approximately 0.64 cm2. This indicates a strong formation of further Ccentered semiquinone radicals, with a lifetime well beyond the illumination period. Other species, such as peroxy radicals are typically short-lived due to their chemical reactivity and therefore might not accumulate to detectable concentrations. After the coating of the spruce surface and before UV exposure, the radical concentration remained the same within experimental errors as without coating. However, the radical concentration observed upon irradiation was significantly reduced by the coating; here irradiation led to a 3.6-fold increase (from 0.14 to 0.46 nmol for an illuminated surface area of 0.64 cm2) after 2 min of UV irradiation (Figure 8b). The in situ kinetics of radical buildup and their stability after irradiation are shown in Figure 8c. The concentration of radicals increased rapidly upon UV irradiation indicating the onset of radical-mediated photodegradation of the wood. The concentration remained stable (to about 90 % within 16 hours) once the irradiation was turned off. The lower radical concentration after the coating is due to the partial absorbance of the UV light by the TiO2/Ce xerogel coating. Additional radical species within the coating were not observed. It is important to note that a very strong UV source was used (much stronger than the common light source of the UV chamber used in Figures 5 and 6), in order to significantly increase the radical concentration. Hence, this very intense UV could only be partly filtered out by the surface coating. Conclusion In summary, we addressed the typical changes of whiteness and opacity in TiO2 coatings via a facile chemical based sol-gel process. Because of the unique optical and UV spectroscopic properties of the TiO2/Ce xerogel, it holds potential applications in materials preservation, for which the long-term natural appearance is of high relevance. Our results show that transparent TiO2/Ce xerogel can be fabricated with the addition of CAN into the precursor solution. The xerogel can filter out almost all the UV light while keeping transparency in the visible light range. The UV cutoff window of the fabricated xerogel 13 ACS Paragon Plus Environment


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experienced a red shift as the concentration of CAN increased. We were able to strongly reduce the UV degradation of wood while preserving its natural aesthetic appearance. Since the coating process is carried out at room temperature under atmospheric pressure, it can be applied to thermally sensitive materials and is easily scalable as well. According to current trends favoring transparent UV protection coatings that preserve the aesthetic appearance of the substrate, the transparent TiO2/Ce xerogel could find application in biomaterials modification, building material protection, and cultural heritage preservation. To upscale the technique for large-scale application, spraying or painting processes could be used alternatively, which should result in coatings with similar performances compared to the dip-coating process. The freestanding transparent TiO2/Ce holds potential to be used as UV cutoff optical lenses. Acknowledgement The authors gratefully acknowledge financial support by Swiss Commission for Technology and Innovation (CTI, No. 2-69704-15) and EIT Climate KIC in the framework of BTA (No. 2-73708-17). Reference 1. Guo, H.; Fuchs, P.; Cabane, E.; Michen, B.; Hagendorfer, H.; Romanyuk, Y. E.; Burgert, I., UV-Protection of Wood Surfaces by Controlled Morphology Fine-Tuning of ZnO Nanostructures. Holzforschung 2016, 70, 699-708. 2. Lee, S.-W.; Kim, S.; Bae, S.; Cho, K.; Chung, T.; Mundt, L. E.; Lee, S.; Park, S.; Park, H.; Schubert, M. C.; Glunz, S. W.; Ko, Y.; Jun, Y.; Kang, Y.; Lee, H.-S.; Kim, D., UV Degradation and Recovery of Perovskite Solar Cells. Sci. Rep. 2016, 6, 38150. 3. Bertrand, F.; German, S.-A.; Anwar, A.; Irune, V.; Gemma, B.; Yolanda, R. D. M.; Lennart, B., Dispersion and Surface Functionalization of Oxide Nanoparticles for Transparent Photocatalytic and Sunscreens. Sci. Technol. Adv. Mater. 2013, 14, 023001. 4. Veronovski, N.; Verhovsek, D.; Godnjavec, J., The Influence of Surface-Treated Nano-TiO2 (Rutile) Incorporation in Water-Based Acrylic Coatings on Wood Protection. Wood Sci. Technol. 2013, 47, 317-328. 5. Moya, R.; Rodríguez-Zúñiga, A.; Vega-Baudrit, J.; Puente-Urbina, A., Effects of Adding TiO2 Nanoparticles to A Water-Based Varnish for Wood Applied to Nine Tropical Woods of Costa Rica Exposed to Natural and Accelerated Weathering. J. Coat. Technol. Res. 2017, 14, 141-152. 6. Rao, X.; Liu, Y.; Fu, Y.; Liu, Y.; Yu, H., Formation and Properties of Polyelectrolytes/TiO2 Composite Coating on Wood Surfaces through Layer-by-Layer Assembly Method. Holzforschung 2016, 70, 361-367. 7. Abidi, N.; Cabrales, L.; Hequet, E., Functionalization of a Cotton Fabric Surface with Titania Nanosols: Applications for Self-Cleaning and UV-Protection Properties. ACS Appl. Mater. Interfaces 2009, 1, 2141-2146. 8. Xiao, X.; Liu, X.; Chen, F.; Fang, D.; Zhang, C.; Xia, L.; Xu, W., Highly Anti-UV Properties of Silk Fiber with Uniform and Conformal Nanoscale TiO2 Coatings via Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2015, 7, 21326-21333. 9. Wang, L.; Zhang, X.; Li, B.; Sun, P.; Yang, J.; Xu, H.; Liu, Y., Superhydrophobic and Ultraviolet-Blocking Cotton Textiles. ACS Appl. Mater. Interfaces 2011, 3, 1277-1281. 10. Guo, H.; Fuchs, P.; Casdorff, K.; Michen, B.; Chanana, M.; Hagendorfer, H.; Romanyuk, Y. E.; Burgert, I., Bio‐Inspired Superhydrophobic and Omniphobic Wood Surfaces. Adv. Mater. Interfaces 2017, 4, 1600289. 11. Duan, W.; Xie, A.; Shen, Y.; Wang, X.; Wang, F.; Zhang, Y.; Li, J., Fabrication of Superhydrophobic Cotton Fabrics with UV Protection Based on CeO2 Particles. Ind. Eng. Chem. Res. 2011, 50, 4441-4445. 12. Montazer, M.; Seifollahzadeh, S., Enhanced Self-Cleaning, Antibacterial and UV Protection Properties of Nano TiO2 Treated Textile through Enzymatic Pretreatment. Photochem. Photobiol. 2011, 87, 877-883. 13. Montazer, M.; Pakdel, E., Reducing Photoyellowing of Wool Using Nano TiO2. Photochem. Photobiol. 2010, 86, 255-260. 14. Winkler, J., Titanium dioxide. Vincentz Network: Hannover, 2003.


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TOC

Highly efficient UV Protection of the biomaterial wood by a transparent TiO2/Ce Xerogel Huizhang Guo, Daniel Klose, Yuhui Hou, Gunnar Jeschke, Ingo Burgert

Cerium(IV) ammonium nitrate was used as a stabilizing agent to fabricate a transparent TiO2/Ce xerogel, which protects the biomaterial wood from deterioration induced by UV irradiation with almost no effect on its aesthetic appearance. It demonstrates the applicability of the protective effect of titanium dioxide coatings for natural engineering materials, which will become increasingly important in future bioeconomies.


Highly Efficient UV Protection of the Biomaterial Wood by A

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