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UV protection of wood surfaces by graphitic carbon nitride nanosheets
Accepted Manuscript Full Length Article UV protection of wood surfaces by graphitic carbon nitride nanosheets Bingnan Yuan, Xiaodi Ji, Tat Thang Nguyen, Zhanhua Huang, Minghui Guo PII: DOI: Reference:
S0169-4332(18)33033-2 https://doi.org/10.1016/j.apsusc.2018.10.251 APSUSC 40819
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 August 2018 24 October 2018 29 October 2018
Please cite this article as: B. Yuan, X. Ji, T. Thang Nguyen, Z. Huang, M. Guo, UV protection of wood surfaces by graphitic carbon nitride nanosheets, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.10.251
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
UV protection of wood surfaces by graphitic carbon nitride nanosheets Bingnan Yuan a, Xiaodi Ji a, Tat Thang Nguyen a,b, Zhanhua Huang a, Minghui Guo a,* a
Key Lab of Bio-based Material Science and Technology (Ministry of Education), College of Material Science
and Engineering, Northeast Forestry University, Harbin 150040, PR China b Vietnam
National University of Forestry, Hanoi 156220, Viet Nam.
*Corresponding
author. Fax: 86 13903653748. E-mail address: [email protected]
Abstract: To protect the environmentally friendly and renewable wood materials from the photodegradation caused by the ultraviolet (UV) light, different kinds of UV absorbers are typically applied to form a protective layer on the wood surface. However, most common UV absorbers are metallic oxides whose synthesis and use are not environmentally benign due to the heavy metal presence. In this work, graphitic carbon nitride (g-CN) nanosheets were chosen as a novel non-toxic, green and low-cost UV absorber. It was successfully applied to the wood surface using simple hydrothermal method. Experimental results confirmed presence of g-CN nanosheet coating on the wood surface. This coating was strongly bound to the wood surface by forming hydrogen bonds with underlying lignin and cellulose. After 15-day accelerated weathering test, the surface of g-CN coated wood (CW) did not show any visible discoloration, and the total color change (ΔE*) was less than 10%. g-CN nanosheets also enhanced wood thermal stability. Our research introduced a new UV absorber for wood products with efficient mechanism of UV protection. Resulting novel CW materials have good potential application prospects as functional wood products. Keywords: wood surface; UV protection; g-C3N4; coatings 1. Introduction Wood as a kind of renewable and environment-friendly natural polymer material is getting 1
more and more interests from people. It is widely used as a construction material for buildings and furniture because of its unique inherent properties, such as strong mechanical properties, good formability, high strength-to-weight ratio and natural texture structure. However, like most biomaterials, wood is easily degraded by the UV irradiation. The main component of wood, lignin, absorbs UV light, which causes free-radical induced degradation of wood. Surface discoloration is the first sign of such decomposition, which followed by a mechanical failure [1-4]. To solve this problem, biomaterials coatings with UV absorbers are often used. Variety of UV absorbers have been successfully developed and commercialized. Most of them are semiconductors with strong absorption in the UV region such as TiO2, Fe3O4, ZnO, and CeO2 as well as their mixed oxides [5-10]. However, these oxide UV absorbers also consist of heavy metals which maybe cause the heavy metal ion pollutions. g-CN is as a metal-free conjugated semiconductor photocatalyst and was first reported in relationship to water photolysis in 2008 [11]. Prepared by the polycondensation of organic precursors, bulk g-CN consists of graphene-like two-dimensional laminated structure. The metalfree g-CN have a visible-light driven bandgap, proper band edges, biosecurity and excellent chemical stability [12, 13]. Because of combination of these unique properties, photocatalytic potential of g-CN has stimulated a lot of research interest [14]. However, g-CN is rarely used as a UV absorber even though it has the same light absorption ability. Recently, Li et al. [15] synthesized g-CN as a component in the Z-scheme heterostructure in the UV isolating coatings. Therefore, we believe that using g-CN as UV absorber might provide a simple, low cost, non-toxic and green way to protect the wood surface. This work reports for the first-time coatings with g-CN as a new metal-free UV absorber to 2
protect wood substrate from the UV radiation. Heterogeneous interface structure was fabricated using hydrogen bonding between g-CN coating and wood surface by a simple hydrothermal method. The coated wood surface was subjected to the 15-day accelerated UV irradiation test. Protection efficiency was evaluated by the discoloration of the sample surface. Our results demonstrated that coatings with g-CN provided good protection to the underlying wood surfaces against UV light. 2. Experimental 2.1. Materials Wood samples of 20×20×2 mm in size were cut from the sapwood sections of poplar wood (Populus ussuriensis Kom from Jiaozuo state forest farm in Henan province). Freshly-cut pieces were ultrasonically cleaned in deionized water for 30 min and dried under vacuum at 60℃ for 4 h then we got the original wood (OW). Melamine (C3N3(NH2)3, 99.5%) and sulfuric acid (H2SO4, 98 wt%) were purchased from Tianjin Guangfu Fine Chemical Research Institute and used without any further purification. Dialysis hoses (MW: 12000-14000) were purchased from Biosharp Science and Technology Ltd. 2.2. Preparation of g-CN coated wood Experimental procedure for the OW preparation is displayed in Fig. 1. First, bulk g-CN was synthesized by directly heating melamine according to a previous report[16]. Melamine powder was placed into a semi-closed quartz crucible with a lid and heated to 550℃ for 2 h, at a heating rate of 10℃/min. The obtained yellow agglomerates were ground to powder. Second, g-CN nanosheets were prepared by the chemical exfoliation method [17]. 1 g of the bulk g-CN powder was mixed with 100 mL of H2SO4 in a 200 mL flask and stirred vigorously for 12 h at room 3
temperature. Then, the mixture was slowly poured into a dialysis hoses and placed in a water bath, which was refreshed frequently until the pH of the solution was neutral. The resulting suspension was centrifuged at 4000 rpm to remove unexfoliated g-CN and dried at 60℃ for 12 h. Wood was covered with the g-CN-based coating by a simple hydrothermal method. 100 mg of the g-CN nanosheet powder was dispersed in 100 mL of deionized water and then was ultrasonically homogenized (using JY98-ⅢDN, Ningbo Scientz Biotechnology Co., China) at 800 W in ice-water bath for 30 min. The obtained g-CN aqueous suspension was transferred into a Teflon-lined stainless-steel autoclave together with the wood samples. The hydrothermal reaction was performed at 120℃ for 2 h, after which the wood samples were rinsed with deionized water 3 times to remove unreacted g-CN nanosheets and then dried at 60℃ for 24 h. Finally, we got the gCN coated wood (CW).
Fig. 1. Schematics of the preparation procedure of the g-CN coated wood. 2.3. Characterization g-CN nanosheets were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), results of which are shown in Fig. S1~S3. Surface morphologies of wood samples were studied using field emission scanning electron 4
microscopy (FE-SEM, JSM-7500F, JEOL, Japan) operating at 5 kV coupled with energy dispersive X-ray spectrometer (EDX). Samples for FE-SEM samples were coated with a thin layer of gold. Thickness and three-dimensional morphologies of the coated wood samples were measured obtained by atomic force microscope (AFM, Seiko-SPA400, Japan). Chemical composition was characterized using Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, USA). X-ray diffraction (XRD, D/max 2200, Rigaku, Japan) with Cu Kα radiation (λ=0.15406 nm) was performed at 40 kV and 30 mA in the 2θ range from 10 to 40° with a 5 °/min scan rate. Heat stability of the samples was investigated by thermal analysis (using TA, Q600 analyzer) from 25 to 800℃ with a 10 ℃/min heating rate under nitrogen atmosphere. The UV-vis absorption spectra were collected using TU-1950 equipment (Beijing Purking General Instrument CO. LTD) at room temperature. 2.4. Accelerated weathering tests UV protection efficiency of the g-CN coatings was assessed by the accelerated UV irradiation test using Accelerated Weathering Tester (Q-PANEL Lab Products, QUV/SPRAY, USA). UV irradiation intensity was 0.77 W/m2 and the temperature was controlled at 60℃ by a circulating water bath system. The samples were irradiated for 15 days. Discolorations were measured at 3-day intervals using a CIELAB system implemented by a portable spectrophotometer (NF333, Mekaster, Beijing). CIELAB color scale was used. Three coordinates (L*, a* and b*) were recorded for each sample: L* represents the lightness of the color from black (0) to blank/white (100), a* demonstrates the color component from green (-60) to red (+60), and b* shows the color component from blue (-60) to yellow (+60). Δ L*, Δ a*, and Δ b* values were calculated as a difference between final and initial values. The total color change was defined as 5
Δ𝐸 ∗ = Δ𝐿 ∗ 2 + Δ𝑎 ∗ 2 + Δ𝑏 ∗ 2. For each sample, color measurements were performed on 3 different areas. The average value was then taken and reported throughout the manuscript. 3. Results and discussion 3.1. Morphologies and chemical compositions SEM of the longitudinal section of OW is shown in Fig. 2a. Oval-shaped pits were uniformly distributed on the smooth wood surface and no other materials and featured were observed. Compared with the OW, it can be clearly seen that after the hydrothermal treatment of the OW, multilayers of carbon nitride nanosheets were observed on the CW surface and all oval-shaped pits were completely covered (see Fig. 2b). Additionally, individual g-CN particles ~1 μm in size were observed on the CW surface (see Fig. S1). Comparison of EDX results of OW and CW confirmed presence of g-CN on the OW since nitrogen content increased from 0 (in the OW) to 10.2 wt% (in CW). In order to clearly distinguish g-CN coating from the wood substrate, we mapped elements distribution of the sample cross-section (shown in Fig. 2d). We can see a layer corresponding to the g-CN coating since OW did not contain any nitrogen and CW did (see Fig. 2e). 3D morphology of CW (shown in Fig. 2f) demonstrated a rough surface with ~2 μm top coating, which agrees with the results obtained in Fig. 2d.
6
Fig. 2. Top view SEM images of the OW (a) and CW (b) and their EDX spectra (c). Cross sectional SEM view image of CW (d) and EDX mapping (e). 3D AFM image of CW (f).
X-ray diffraction spectrum of the g-CN sample (see Fig. 3a) shows a broad peak at ~27.3°, which is characteristic for the interlayer stacking of conjugated aromatic systems, indexed to the (002) crystal plane [18]. Peaks at 14.8, 22.5 and 35° belong to the (101), (002),and (040) crystal planes of the cellulose in wood, respectively [7, 19]. Diffraction patterns of CW demonstrated peaks similar to the OW ones as well as peaks belonging to the g-CN phase, which is yet another confirmation of successful application of the g-CN coatings on the wood surface. Broad bands located between 3100 and 3400 cm-1 in the FTIR spectrum of g-C3N4 are because of the N-H stretching vibrations. Broad but less intense band in the OW spectrum in the same frequency region corresponds to the OH stretching vibrations. Bands located in the 12201630 cm-1 range of the FTIR spectrum of g-CN are assigned to the C-N stretching, typical for tris-triazine structures [20]. The sharp peak at around 810 cm-1 is originated from heptazine ring system [21]. The locations of bands in the CW spectrum are almost the same as in the g-CN spectrum, because g-CN coating covered most of the OW functional surface groups. Relatively week absorption peaks in CW are probably due to the low content of the g-CN. FTIR results agree 7
well with the XRD analysis.
Fig. 3. XRD patterns (a) and FTIR spectra (b) of OW, CW, and g-C3N4, respectively.
3.2. Protective properties UV-Vis spectra of CW showed only 10% of the incident light reflected between 400~260 nm which means that 90% of UVA (400~315 nm UV region) and UVB (315~280 nm region) was absorbed by the g-CN coatings. Much more light was reflected in the 280~200 nm region, indicating poor UV protection in this region. However, wavelengths in the 280~100 nm range correspond to the UVC radiation, which is completely absorbed by the ozone layer and atmosphere. Also, the visible spectrum (400~800 nm region) would hardly cause damage to the wood. Thus, the wood surface could be well protected from UV degradation when g-CN coating is applied.
8
Fig. 4. UV-vis diffuse reflection spectra of CW
Fig. 5 shows photographs of OW and CW exposed to the UV light. After 15 days of UV exposure, OW color changed significantly from off white to light yellow. However, CW did not show any significant visible discoloration.
Fig. 5. Photographs of OW and CW exposed to the UV irradiation for 0, 3, 6, 9, 12 and 15 days.
CIELAB color scales for samples at different stages of UV aging are shown in Fig. 6. Lightness index (L*) of CW increased comparing to the one of OW, which indicates that the wood samples became darker. Change of the color indices followed the same trends for both OW and CW but the values changed more significantly for OW than for CW. Values of redness (a*) and yellowness (b*) indices increase with the exposure time increase and they follow the same trends for both OW and CW. The total color change (ΔE*) for CW was much smaller comparing to OW (see Fig. 6d). Smaller ΔE* values suggest less lignin damage. These results demonstrated effectiveness of the g-CN coatings for wood protection again UV damage and discoloration.
9
Fig. 6. Lightness (L*) (a), redness (a*) (b) and yellowness (b*) (c) indices as well as the total color change (ΔE*) (d) of OW and CW samples as function of the aging time.
Fig. 7 shows TG and DTG curves of OW and CW. The slight weight loss observed for both samples before 120℃ is associated with the absorbed water evaporation (~5 wt%). Overall thermal decomposition process of OW could be described as follows. Accelerated weight loss at 200~290℃ is mainly attributed to the thermal degradation of the hemicellulose. Second accelerating weight loss event starts at ~320ºC and reaches its maximum at 360℃. This can be attributed to the cellulose degradation. Since lignin is the most thermal stabile component in the wood, its slow degradation keeps happening almost constantly among other faster ones. TG and DTG curves of CW shifted to higher temperatures comparing to the OW curves, which indicates better thermal stability of CW. Thermal stability of the g-CN coating is a main contributor to the thermal stability of CW, since g-CN starts to decompose at 550℃ (see Fig. 7).
10
Fig. 7. TG (a) and DTG (b) curves of OW and CW
Base on the above results, we proposed a possible mechanism of the g-CN coating formation and its protection of the wood surface (see Fig. 8). Each nitrogen atom in tri-s-triazine structure of g-CN has a lone pair of electrons. These lone electron pairs form hydrogen bonds with the OH groups from the wood during the hydrothermal reaction forming layers of the g-CN coating. Most of the incident light was either absorbed or reflected by the g-CN. The following mechanism can be proposed based on all experimental results described above. Photon energy is absorbed by the g-CN electrons in the valance band, after which electrons jump to the conduction band forming free electrons with super-reduction and holes with super-oxidation states at the same time. Because of the high recombination rate of charge carriers in g-CN, these high reactive free electrons and holes would quickly recombine before reacting with wood, helping to avoid the damage to the wood. Recombination of free electrons and holes is accompanied by the energy release either by light or heat.
Fig. 8. Schematics of the mechanisms of hybrid composite formation and its UV resistance 11
4. Conclusions We successfully prepared g-CN-coated wood using simple hydrothermal method. g-CN nanosheets form hydrogen bonds with the wood surface. g-CN protects the treated wood from the UV radiation by absorbing the energy and dissipating it as heat. Accelerated weathering test lasted for 15 days showed no visible discolorations of the wood with the g-CN coating and the total color change (ΔE*) was less than 10%. g-CN nanosheets also contributed to the thermal stability of the wood. Our work offers an alternative UV absorber for natural material protection, and it is nontoxic, metal free and has a long service life. g-CN coated wood can be used as a novel functional wood products for construction purposes.
Acknowledgements The authors gratefully acknowledge financial support by The National Key Research and Development Program of China (No.2017YFD0600204) and Special Project for Double FirstClass—Cultivation of Innovative Talents (No.000/41113102). In addition, Bingnan Yuan wishes to thank Cheer Chen whose songs accompany him get through those dark nights. References [1] F. Aloui, A. Ahajji, Y. Irmouli, B. George, B. Charrier, A. Merlin, Inorganic UV absorbers for the photostabilisation of wood-clearcoating systems: Comparison with organic UV absorbers, Applied Surface Science, 253 (2007) 3737-3745. [2] P. Hayoz, W. Peter, D. Rogez, A new innovative stabilization method for the protection of natural wood, Progress in Organic Coatings, 48 (2003) 297-309. [3] C. Schaller, D. Rogez, New approaches in wood coating stabilization, Journal of Coatings Technology and Research, 4 (2007) 401-409. [4] D. Rosu, C.A. Teaca, R. Bodirlau, L. Rosu, FTIR and color change of the modified wood as a result of artificial light irradiation, Journal of photochemistry and photobiology. B, Biology, 99 (2010) 144-149. [5] H. Guo, D. Klose, Y. Hou, G. Jeschke, I. Burgert, Highly Efficient UV Protection of the Biomaterial Wood by A Transparent TiO2/Ce Xerogel, ACS applied materials & interfaces, 9 (2017) 39040-39047. [6] W. Gan, Y. Liu, L. Gao, X. Zhan, J. Li, Magnetic property, thermal stability, UV-resistance, and 12
moisture absorption behavior of magnetic wood composites, Polymer Composites, 38 (2017) 16461654. [7] Y. Dong, Y. Yan, H. Ma, S. Zhang, J. Li, C. Xia, S.Q. Shi, L. Cai, In-Situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-Resistance Properties, Journal of Materials Science & Technology, 33 (2017) 266-270. [8] H. Guo, P. Fuchs, E. Cabane, B. Michen, H. Hagendorfer, Y.E. Romanyuk, I. Burgert, UV-protection of wood surfaces by controlled morphology fine-tuning of ZnO nanostructures, Holzforschung, 70 (2016). [9] M. Auffan, A. Masion, J. Labille, M.A. Diot, W. Liu, L. Olivi, O. Proux, F. Ziarelli, P. Chaurand, C. Geantet, J.Y. Bottero, J. Rose, Long-term aging of a CeO(2) based nanocomposite used for wood protection, Environmental pollution, 188 (2014) 1-7. [10] F. Cheng, S.M. Sajedin, S.M. Kelly, A.F. Lee, A. Kornherr, UV-stable paper coated with APTESmodified P25 TiO(2) nanoparticles, Carbohydrate polymers, 114 (2014) 246-252. [11] X. Wang, K. Maeda, A. Thomas, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nature materials, (2008) 271-275. [12] K.C. Christoforidis, T. Montini, E. Bontempi, S. Zafeiratos, J.J.D. Jaén, P. Fornasiero, Synthesis and photocatalytic application of visible-light active β -Fe 2 O 3 /g-C 3 N 4 hybrid nanocomposites, Applied Catalysis B: Environmental, 187 (2016) 171-180. [13] P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities, Advanced Functional Materials, 22 (2012) 4763-4770. [14] J. Wen, J. Xie, X. Chen, X. Li, A review on g-C 3 N 4 -based photocatalysts, Applied Surface Science, 391 (2017) 72-123. [15] Y. Li, X. Wu, J. Li, K. Wang, G. Zhang, Z-scheme g-C 3 N 4 @Cs x WO 3 heterostructure as smart window coating for UV isolating, Vis penetrating, NIR shielding and full spectrum photocatalytic decomposing VOCs, Applied Catalysis B: Environmental, 229 (2018) 218-226. [16] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir the Acs Journal of Surfaces & Colloids, 25 (2009) 10397-10401. [17] J. Zhang, M. Zhang, L. Lin, X. Wang, Sol processing of conjugated carbon nitride powders for thinfilm fabrication, Angewandte Chemie, 54 (2015) 6297-6301. [18] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P.M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv Mater, 25 (2013) 2452-2456. [19] C. Wan, Y. Jiao, J. Li, In situ deposition of graphene nanosheets on wood surface by one-pot hydrothermal method for enhanced UV-resistant ability, Applied Surface Science, 347 (2015) 891-897. [20] Y. Bu, Z. Chen, J. Yu, W. Li, A novel application of g-C3N4 thin film in photoelectrochemical anticorrosion, Electrochimica Acta, 88 (2013) 294-300. [21] B.V. Lotsch, M. Doblinger, J. Sehnert, L. Seyfarth, J. Senker, O. Oeckler, W. Schnick, Unmasking melon by a complementary approach employing electron diffraction, solid-state NMR spectroscopy, and theoretical calculations-structural characterization of a carbon nitride polymer, Chemistry, 13 (2007) 4969-4980.
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1. g-CN were used as non-toxic, green and low-cost UV absorber for wood protection. 2. g-CN was successfully coated on the wood surface by forming hydrogen bonds. 3. After aging test, the total color change of the treated wood was less than 10%.
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S0169-4332(18)33033-2 https://doi.org/10.1016/j.apsusc.2018.10.251 APSUSC 40819
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 August 2018 24 October 2018 29 October 2018
Please cite this article as: B. Yuan, X. Ji, T. Thang Nguyen, Z. Huang, M. Guo, UV protection of wood surfaces by graphitic carbon nitride nanosheets, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.10.251
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
UV protection of wood surfaces by graphitic carbon nitride nanosheets Bingnan Yuan a, Xiaodi Ji a, Tat Thang Nguyen a,b, Zhanhua Huang a, Minghui Guo a,* a
Key Lab of Bio-based Material Science and Technology (Ministry of Education), College of Material Science
and Engineering, Northeast Forestry University, Harbin 150040, PR China b Vietnam
National University of Forestry, Hanoi 156220, Viet Nam.
*Corresponding
author. Fax: 86 13903653748. E-mail address: [email protected]
Abstract: To protect the environmentally friendly and renewable wood materials from the photodegradation caused by the ultraviolet (UV) light, different kinds of UV absorbers are typically applied to form a protective layer on the wood surface. However, most common UV absorbers are metallic oxides whose synthesis and use are not environmentally benign due to the heavy metal presence. In this work, graphitic carbon nitride (g-CN) nanosheets were chosen as a novel non-toxic, green and low-cost UV absorber. It was successfully applied to the wood surface using simple hydrothermal method. Experimental results confirmed presence of g-CN nanosheet coating on the wood surface. This coating was strongly bound to the wood surface by forming hydrogen bonds with underlying lignin and cellulose. After 15-day accelerated weathering test, the surface of g-CN coated wood (CW) did not show any visible discoloration, and the total color change (ΔE*) was less than 10%. g-CN nanosheets also enhanced wood thermal stability. Our research introduced a new UV absorber for wood products with efficient mechanism of UV protection. Resulting novel CW materials have good potential application prospects as functional wood products. Keywords: wood surface; UV protection; g-C3N4; coatings 1. Introduction Wood as a kind of renewable and environment-friendly natural polymer material is getting 1
more and more interests from people. It is widely used as a construction material for buildings and furniture because of its unique inherent properties, such as strong mechanical properties, good formability, high strength-to-weight ratio and natural texture structure. However, like most biomaterials, wood is easily degraded by the UV irradiation. The main component of wood, lignin, absorbs UV light, which causes free-radical induced degradation of wood. Surface discoloration is the first sign of such decomposition, which followed by a mechanical failure [1-4]. To solve this problem, biomaterials coatings with UV absorbers are often used. Variety of UV absorbers have been successfully developed and commercialized. Most of them are semiconductors with strong absorption in the UV region such as TiO2, Fe3O4, ZnO, and CeO2 as well as their mixed oxides [5-10]. However, these oxide UV absorbers also consist of heavy metals which maybe cause the heavy metal ion pollutions. g-CN is as a metal-free conjugated semiconductor photocatalyst and was first reported in relationship to water photolysis in 2008 [11]. Prepared by the polycondensation of organic precursors, bulk g-CN consists of graphene-like two-dimensional laminated structure. The metalfree g-CN have a visible-light driven bandgap, proper band edges, biosecurity and excellent chemical stability [12, 13]. Because of combination of these unique properties, photocatalytic potential of g-CN has stimulated a lot of research interest [14]. However, g-CN is rarely used as a UV absorber even though it has the same light absorption ability. Recently, Li et al. [15] synthesized g-CN as a component in the Z-scheme heterostructure in the UV isolating coatings. Therefore, we believe that using g-CN as UV absorber might provide a simple, low cost, non-toxic and green way to protect the wood surface. This work reports for the first-time coatings with g-CN as a new metal-free UV absorber to 2
protect wood substrate from the UV radiation. Heterogeneous interface structure was fabricated using hydrogen bonding between g-CN coating and wood surface by a simple hydrothermal method. The coated wood surface was subjected to the 15-day accelerated UV irradiation test. Protection efficiency was evaluated by the discoloration of the sample surface. Our results demonstrated that coatings with g-CN provided good protection to the underlying wood surfaces against UV light. 2. Experimental 2.1. Materials Wood samples of 20×20×2 mm in size were cut from the sapwood sections of poplar wood (Populus ussuriensis Kom from Jiaozuo state forest farm in Henan province). Freshly-cut pieces were ultrasonically cleaned in deionized water for 30 min and dried under vacuum at 60℃ for 4 h then we got the original wood (OW). Melamine (C3N3(NH2)3, 99.5%) and sulfuric acid (H2SO4, 98 wt%) were purchased from Tianjin Guangfu Fine Chemical Research Institute and used without any further purification. Dialysis hoses (MW: 12000-14000) were purchased from Biosharp Science and Technology Ltd. 2.2. Preparation of g-CN coated wood Experimental procedure for the OW preparation is displayed in Fig. 1. First, bulk g-CN was synthesized by directly heating melamine according to a previous report[16]. Melamine powder was placed into a semi-closed quartz crucible with a lid and heated to 550℃ for 2 h, at a heating rate of 10℃/min. The obtained yellow agglomerates were ground to powder. Second, g-CN nanosheets were prepared by the chemical exfoliation method [17]. 1 g of the bulk g-CN powder was mixed with 100 mL of H2SO4 in a 200 mL flask and stirred vigorously for 12 h at room 3
temperature. Then, the mixture was slowly poured into a dialysis hoses and placed in a water bath, which was refreshed frequently until the pH of the solution was neutral. The resulting suspension was centrifuged at 4000 rpm to remove unexfoliated g-CN and dried at 60℃ for 12 h. Wood was covered with the g-CN-based coating by a simple hydrothermal method. 100 mg of the g-CN nanosheet powder was dispersed in 100 mL of deionized water and then was ultrasonically homogenized (using JY98-ⅢDN, Ningbo Scientz Biotechnology Co., China) at 800 W in ice-water bath for 30 min. The obtained g-CN aqueous suspension was transferred into a Teflon-lined stainless-steel autoclave together with the wood samples. The hydrothermal reaction was performed at 120℃ for 2 h, after which the wood samples were rinsed with deionized water 3 times to remove unreacted g-CN nanosheets and then dried at 60℃ for 24 h. Finally, we got the gCN coated wood (CW).
Fig. 1. Schematics of the preparation procedure of the g-CN coated wood. 2.3. Characterization g-CN nanosheets were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), results of which are shown in Fig. S1~S3. Surface morphologies of wood samples were studied using field emission scanning electron 4
microscopy (FE-SEM, JSM-7500F, JEOL, Japan) operating at 5 kV coupled with energy dispersive X-ray spectrometer (EDX). Samples for FE-SEM samples were coated with a thin layer of gold. Thickness and three-dimensional morphologies of the coated wood samples were measured obtained by atomic force microscope (AFM, Seiko-SPA400, Japan). Chemical composition was characterized using Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, USA). X-ray diffraction (XRD, D/max 2200, Rigaku, Japan) with Cu Kα radiation (λ=0.15406 nm) was performed at 40 kV and 30 mA in the 2θ range from 10 to 40° with a 5 °/min scan rate. Heat stability of the samples was investigated by thermal analysis (using TA, Q600 analyzer) from 25 to 800℃ with a 10 ℃/min heating rate under nitrogen atmosphere. The UV-vis absorption spectra were collected using TU-1950 equipment (Beijing Purking General Instrument CO. LTD) at room temperature. 2.4. Accelerated weathering tests UV protection efficiency of the g-CN coatings was assessed by the accelerated UV irradiation test using Accelerated Weathering Tester (Q-PANEL Lab Products, QUV/SPRAY, USA). UV irradiation intensity was 0.77 W/m2 and the temperature was controlled at 60℃ by a circulating water bath system. The samples were irradiated for 15 days. Discolorations were measured at 3-day intervals using a CIELAB system implemented by a portable spectrophotometer (NF333, Mekaster, Beijing). CIELAB color scale was used. Three coordinates (L*, a* and b*) were recorded for each sample: L* represents the lightness of the color from black (0) to blank/white (100), a* demonstrates the color component from green (-60) to red (+60), and b* shows the color component from blue (-60) to yellow (+60). Δ L*, Δ a*, and Δ b* values were calculated as a difference between final and initial values. The total color change was defined as 5
Δ𝐸 ∗ = Δ𝐿 ∗ 2 + Δ𝑎 ∗ 2 + Δ𝑏 ∗ 2. For each sample, color measurements were performed on 3 different areas. The average value was then taken and reported throughout the manuscript. 3. Results and discussion 3.1. Morphologies and chemical compositions SEM of the longitudinal section of OW is shown in Fig. 2a. Oval-shaped pits were uniformly distributed on the smooth wood surface and no other materials and featured were observed. Compared with the OW, it can be clearly seen that after the hydrothermal treatment of the OW, multilayers of carbon nitride nanosheets were observed on the CW surface and all oval-shaped pits were completely covered (see Fig. 2b). Additionally, individual g-CN particles ~1 μm in size were observed on the CW surface (see Fig. S1). Comparison of EDX results of OW and CW confirmed presence of g-CN on the OW since nitrogen content increased from 0 (in the OW) to 10.2 wt% (in CW). In order to clearly distinguish g-CN coating from the wood substrate, we mapped elements distribution of the sample cross-section (shown in Fig. 2d). We can see a layer corresponding to the g-CN coating since OW did not contain any nitrogen and CW did (see Fig. 2e). 3D morphology of CW (shown in Fig. 2f) demonstrated a rough surface with ~2 μm top coating, which agrees with the results obtained in Fig. 2d.
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Fig. 2. Top view SEM images of the OW (a) and CW (b) and their EDX spectra (c). Cross sectional SEM view image of CW (d) and EDX mapping (e). 3D AFM image of CW (f).
X-ray diffraction spectrum of the g-CN sample (see Fig. 3a) shows a broad peak at ~27.3°, which is characteristic for the interlayer stacking of conjugated aromatic systems, indexed to the (002) crystal plane [18]. Peaks at 14.8, 22.5 and 35° belong to the (101), (002),and (040) crystal planes of the cellulose in wood, respectively [7, 19]. Diffraction patterns of CW demonstrated peaks similar to the OW ones as well as peaks belonging to the g-CN phase, which is yet another confirmation of successful application of the g-CN coatings on the wood surface. Broad bands located between 3100 and 3400 cm-1 in the FTIR spectrum of g-C3N4 are because of the N-H stretching vibrations. Broad but less intense band in the OW spectrum in the same frequency region corresponds to the OH stretching vibrations. Bands located in the 12201630 cm-1 range of the FTIR spectrum of g-CN are assigned to the C-N stretching, typical for tris-triazine structures [20]. The sharp peak at around 810 cm-1 is originated from heptazine ring system [21]. The locations of bands in the CW spectrum are almost the same as in the g-CN spectrum, because g-CN coating covered most of the OW functional surface groups. Relatively week absorption peaks in CW are probably due to the low content of the g-CN. FTIR results agree 7
well with the XRD analysis.
Fig. 3. XRD patterns (a) and FTIR spectra (b) of OW, CW, and g-C3N4, respectively.
3.2. Protective properties UV-Vis spectra of CW showed only 10% of the incident light reflected between 400~260 nm which means that 90% of UVA (400~315 nm UV region) and UVB (315~280 nm region) was absorbed by the g-CN coatings. Much more light was reflected in the 280~200 nm region, indicating poor UV protection in this region. However, wavelengths in the 280~100 nm range correspond to the UVC radiation, which is completely absorbed by the ozone layer and atmosphere. Also, the visible spectrum (400~800 nm region) would hardly cause damage to the wood. Thus, the wood surface could be well protected from UV degradation when g-CN coating is applied.
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Fig. 4. UV-vis diffuse reflection spectra of CW
Fig. 5 shows photographs of OW and CW exposed to the UV light. After 15 days of UV exposure, OW color changed significantly from off white to light yellow. However, CW did not show any significant visible discoloration.
Fig. 5. Photographs of OW and CW exposed to the UV irradiation for 0, 3, 6, 9, 12 and 15 days.
CIELAB color scales for samples at different stages of UV aging are shown in Fig. 6. Lightness index (L*) of CW increased comparing to the one of OW, which indicates that the wood samples became darker. Change of the color indices followed the same trends for both OW and CW but the values changed more significantly for OW than for CW. Values of redness (a*) and yellowness (b*) indices increase with the exposure time increase and they follow the same trends for both OW and CW. The total color change (ΔE*) for CW was much smaller comparing to OW (see Fig. 6d). Smaller ΔE* values suggest less lignin damage. These results demonstrated effectiveness of the g-CN coatings for wood protection again UV damage and discoloration.
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Fig. 6. Lightness (L*) (a), redness (a*) (b) and yellowness (b*) (c) indices as well as the total color change (ΔE*) (d) of OW and CW samples as function of the aging time.
Fig. 7 shows TG and DTG curves of OW and CW. The slight weight loss observed for both samples before 120℃ is associated with the absorbed water evaporation (~5 wt%). Overall thermal decomposition process of OW could be described as follows. Accelerated weight loss at 200~290℃ is mainly attributed to the thermal degradation of the hemicellulose. Second accelerating weight loss event starts at ~320ºC and reaches its maximum at 360℃. This can be attributed to the cellulose degradation. Since lignin is the most thermal stabile component in the wood, its slow degradation keeps happening almost constantly among other faster ones. TG and DTG curves of CW shifted to higher temperatures comparing to the OW curves, which indicates better thermal stability of CW. Thermal stability of the g-CN coating is a main contributor to the thermal stability of CW, since g-CN starts to decompose at 550℃ (see Fig. 7).
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Fig. 7. TG (a) and DTG (b) curves of OW and CW
Base on the above results, we proposed a possible mechanism of the g-CN coating formation and its protection of the wood surface (see Fig. 8). Each nitrogen atom in tri-s-triazine structure of g-CN has a lone pair of electrons. These lone electron pairs form hydrogen bonds with the OH groups from the wood during the hydrothermal reaction forming layers of the g-CN coating. Most of the incident light was either absorbed or reflected by the g-CN. The following mechanism can be proposed based on all experimental results described above. Photon energy is absorbed by the g-CN electrons in the valance band, after which electrons jump to the conduction band forming free electrons with super-reduction and holes with super-oxidation states at the same time. Because of the high recombination rate of charge carriers in g-CN, these high reactive free electrons and holes would quickly recombine before reacting with wood, helping to avoid the damage to the wood. Recombination of free electrons and holes is accompanied by the energy release either by light or heat.
Fig. 8. Schematics of the mechanisms of hybrid composite formation and its UV resistance 11
4. Conclusions We successfully prepared g-CN-coated wood using simple hydrothermal method. g-CN nanosheets form hydrogen bonds with the wood surface. g-CN protects the treated wood from the UV radiation by absorbing the energy and dissipating it as heat. Accelerated weathering test lasted for 15 days showed no visible discolorations of the wood with the g-CN coating and the total color change (ΔE*) was less than 10%. g-CN nanosheets also contributed to the thermal stability of the wood. Our work offers an alternative UV absorber for natural material protection, and it is nontoxic, metal free and has a long service life. g-CN coated wood can be used as a novel functional wood products for construction purposes.
Acknowledgements The authors gratefully acknowledge financial support by The National Key Research and Development Program of China (No.2017YFD0600204) and Special Project for Double FirstClass—Cultivation of Innovative Talents (No.000/41113102). In addition, Bingnan Yuan wishes to thank Cheer Chen whose songs accompany him get through those dark nights. References [1] F. Aloui, A. Ahajji, Y. Irmouli, B. George, B. Charrier, A. Merlin, Inorganic UV absorbers for the photostabilisation of wood-clearcoating systems: Comparison with organic UV absorbers, Applied Surface Science, 253 (2007) 3737-3745. [2] P. Hayoz, W. Peter, D. Rogez, A new innovative stabilization method for the protection of natural wood, Progress in Organic Coatings, 48 (2003) 297-309. [3] C. Schaller, D. Rogez, New approaches in wood coating stabilization, Journal of Coatings Technology and Research, 4 (2007) 401-409. [4] D. Rosu, C.A. Teaca, R. Bodirlau, L. Rosu, FTIR and color change of the modified wood as a result of artificial light irradiation, Journal of photochemistry and photobiology. B, Biology, 99 (2010) 144-149. [5] H. Guo, D. Klose, Y. Hou, G. Jeschke, I. Burgert, Highly Efficient UV Protection of the Biomaterial Wood by A Transparent TiO2/Ce Xerogel, ACS applied materials & interfaces, 9 (2017) 39040-39047. [6] W. Gan, Y. Liu, L. Gao, X. Zhan, J. Li, Magnetic property, thermal stability, UV-resistance, and 12
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1. g-CN were used as non-toxic, green and low-cost UV absorber for wood protection. 2. g-CN was successfully coated on the wood surface by forming hydrogen bonds. 3. After aging test, the total color change of the treated wood was less than 10%.
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