- Email: [email protected]
In-Situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-Resistance Properties
Accepted Manuscript Title: In-Situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-Resistance Properties Author: Youming Dong, Yutao Yan, Huandi Ma, Shifeng Zhang, Jianzhang Li, Changlei Xia, Sheldon Q. Shi, Liping Cai PII: DOI: Reference:
S1005-0302(16)30019-6 http://dx.doi.org/doi: 10.1016/j.jmst.2016.03.018 JMST 680
To appear in:
Journal of Materials Science & Technology
Received date: Revised date: Accepted date:
20-10-2015 21-12-2015 27-1-2016
Please cite this article as: Youming Dong, Yutao Yan, Huandi Ma, Shifeng Zhang, Jianzhang Li, Changlei Xia, Sheldon Q. Shi, Liping Cai, In-Situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-Resistance Properties, Journal of Materials Science & Technology (2016), http://dx.doi.org/doi: 10.1016/j.jmst.2016.03.018. 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.
In-situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-resistance Properties Youming Dong1, Yutao Yan1, Huandi Ma2, Shifeng Zhang1,*, Jianzhang Li1,*, Changlei Xia3, Sheldon Q. Shi3, Liping Cai3 1
MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry
University, Beijing 100083, China 2
College of Biological Science and Technology, Beijing Forestry University, Beijing
100083, China 3
Department of Mechanical and Energy Engineering, University of North Texas, 3940
N Elm St., Suite F101, Denton, TX 76207, USA *Corresponding author. Tel./Fax: +86 10 62336072. E-mail
addresses:
[email protected]
(Shifeng
Zhang),
[email protected] (Jianzhang Li). [Received 20 October 2015; Received in revised form 21 December 2015; Accepted 27 January 2016]
Highlights
Wood/ZnO hybrid composites were prepared by a facile process.
ZnO nanoflowers with nanosheets were formed in wood cell walls.
ZnO nanoparticles endowed wood with the antibacterial property and UV-resistance.
1
Page 1 of 17
Hybrid wood materials have attracted considerable attention because they have combined advantages of both wood and inorganic compounds. This work investigated the microstructural morphology, thermal stability, ultraviolet (UV) stability, and antibacterial property of composites made from wood/ZnO hybrid materials through a facile
in-situ
chemosynthesis
methods.
The
X-ray
diffraction
(XRD)
and
thermogravimetric analysis (TGA) results indicated that the synthesized ZnO particles had an average grain size of about 10.8 nm. The scanning electron microscopy (SEM) observations showed that ZnO nanoflowers self-assembled with nanosheets were presented in wood cell lumens and increased with increasing Zn2+ concentrations. ZnO nanoparticles were also generated in the wood cell wall, which was confirmed by the results of energy-dispersive spectroscopy (EDS). The TGA tests also indicated that the thermal stability of wood/ZnO hybrid materials was improved after the formation of ZnO inorganic particles. Finally, the results of antibacterial efficacy tests and UV resistance tests revealed that ZnO nanoparticles showed a promising future as antimicrobial agents against Escherichia coli (E. coli) and UV resistance agents for wood protection. Key words: Hybrid wood material; Nanoflowers; Zinc oxide; Antibacterial property; UV-resistance
1. Introduction Organic-inorganic hybrid composites have attracted considerable attention recently, since they couple the advantages of both organic polymers and those of inorganic 2
Page 2 of 17
compounds, and may exhibit unexpected properties different from their original components[1,2]. In the last decade, many approaches have been conducted to create high-performance or multi-functional organic-inorganic hybrid composites to be widely used as the catalyst, coating, separation devices, solar cells, and sensors[3–7]. Inspired by the good performance of these composites, researchers have investigated the preparation and characterization of the hybrid wood materials. The consolidation of wood cell walls with a mineral phase at the nanostructural level can result in a highly desirable material combination and versatile properties[3,8,9]. Accordingly, a large variety of multi-functional wood-inorganic composites have been developed in recent years. For example, Oka group have made plenty of work on the fabrication and characterization of magnetic wood by the combination of wood and magnetic nanoparticles[10,11]. This wood-inorganic composite has potential applications on the electromagnetic wave absorption and heating materials[12]. In this field, lots of inorganic materials have been used for the preparation of wood-inorganic materials, such as SiO2, TiO2, ZnO, CuO, CaCO3, and so on. Particularly, ZnO is a very interesting multifunctional material. It has been considered for the applications in sensors, solar cells, thin film transistors, photocatalysis, and optoelectronic devices[13,14]. ZnO is also beneficial to the fabrication of functional composites with wood materials, due to its resistance to decay ultraviolet light and other properties[15,16]. Various approaches have been used to prepare wood-inorganic materials. Impregnating wood with nanoparticles is a typical approach to functionalize wood. Devi et al. confirmed that vacuum impregnation of ZnO nanoparticles into wood could 3
Page 3 of 17
significantly improve ultraviolet (UV) resistance of the composite, thermal stability, and mechanical properties of wood[17]. However, this process is highly challengeable due to the difficulty in dispersion of nanoparticles throughout the wood. As a result, the nanoparticles should be modified with organic surface active agents or couple agents and impregnated with other monomers or resins[18]. Although the properties of modified wood are significantly improved, the procedures are complicated accordingly. To address the issue, in-situ chemosynthesis method at room temperature is a facile fabrication approach to synthesizing nanoparticles within the wood’s structure. Merk et al.[3] reported a systematic and eco-friendly preparation of the wood/CaCO3 materials using aqueous salt solutions at ambient temperature and the nanoparticles could deposit in the cell walls. In addition, wood can be used as a template for self-assembled nanoparticles, affecting the nanostructure of generated particles[19]. Therefore, this study explored the preparation of wood/ZnO hybrid materials using in-situ chemosynthesis at room temperature and the microstructural morphology, thermal stability, UV stability, and antibacterial property of the hybrid materials were evaluated. 2. Experimental 2.1. Preparation of wood/ZnO hybrid materials Poplar wood samples (populus tomentosa Carr.) were cut into a size of 30 mm (tangential) × 10 mm (radial) × 100 mm (longitudinal) and oven-dried at 105 °C until a constant weight was obtained. The preparation of wood/ZnO hybrid materials was implemented as follows. First, Zn(NO3)·6H2O was dissolved in deionized water in 4
Page 4 of 17
concentrations of 0.2, 0.6, and 1.0 mol/L. The dried wood samples were submerged in the Zn(NO3) solution under a vacuum (ca. 0.095 MPa) for 30 min, and soaked under atmospheric pressure for 2 h. After impregnation, excessive solution on the wood surface was cleared up with deionized water and rapidly immersed in the NaOH solution (0.4 mol/L) for 48 h at room temperature. Next, wood samples were washed with deionized water until they reached a neutral pH value. Finally, the samples were oven-dried at 60 °C for 24 h to obtain wood/ZnO hybrid materials. 2.2. Characterization 2.2.1. XRD analysis The X-ray diffraction (XRD) pattern was recorded on a Bruker D8 Advance diffractrometer (Germany) with CuKα radiation. The apparatus parameters were set as follows: CuKα radiation with a graphite monochromator, voltage 40 kV, electric current 40 mA and 2θ scan range from 5° to 85° with a scanning speed of 2°/min. 2.2.2. Thermal stability The thermal stability of the samples was evaluated by thermogravimetric analysis TGA (Q50, TA Instruments, USA) at a constant heating rate of 5 °C/min from room temperature to 600 °C under a flowing nitrogen atmosphere. 2.2.3. Microstructural morphology To observe the microstructural morphology of materials, the interior portions of cross and longitudinal sections in untreated and treated samples were cut with a sliding microtome (REM-710, Yamato, Tokyo, Japan), mounted on conductive adhesives, and coated with gold-sputter, followed by scanning electron microscopy (SEM, Hitachi 5
Page 5 of 17
S-4800 N, Japan) observation using a voltage of 5 kV. In addition, energy dispersive X-ray spectroscopy (EDS, EX-350, Horiba Scientific, Japan) was employed to analyze the distribution of zinc in samples. 2.2.4. Antibacterial properties tests Tests on the antibacterial properties of wood/ZnO composites were carried out for Escherichia coli (E. coli) according to the previous research[20]. Briefly, the bacterial suspension (3.7 × 106 cfu/ml) was applied uniformly on the surface of a nutrient agar plate before placing the prepared samples on the plate (4 per plate). The plates were incubated at 37 °C for 24 h, after the average diameter of the inhibition zone surrounding the sample was measured with a ruler with a resolution up to 1 mm. 2.2.5. UV resistance tests The UV resistance of the samples was carried out using a UV chamber (metal halide lamp with a spectrum of 200–500 nm; power of 500 W; Advanced Research Co., China). The wood samples with a dimension of 30 mm × 10 mm × 100 mm were placed at even distances for six weeks of exposure period. Then the effect of the UV resistance was analyzed by examining surface color. The color difference was calculated before and after the test. The surface color of the specimens was measured with a color measurement instrument (Dataflash 110 Datacolor, USA) according to the CIELAB color system, where L* is a measure of lightness, a* is the chroma from green to red, and b* is the chroma from blue to yellow[21]. The color difference is defined by the following equation: (1) 6
Page 6 of 17
where ΔE* is the color difference, ΔL* is the lightness difference, and Δa* and Δb* are the chroma differences. Δ means the differences between the initial and final parameters of samples after UV irradiation. L*, a*, and b* are the average values of five locations on each sample. 3. Results and Discussion 3.1. Structural analysis Fig. 1 displays the XRD patterns of untreated wood and wood/ZnO hybrid materials. All samples exhibit characteristic peaks at around 15.8°, 22.1°, and 34.5° corresponding to the (101), (002), and (040) planes of cellulose, respectively[22]. When the wood was treated with ZnO, some peaks which can be indexed to wurtzite-structured ZnO (ZnO: ICSD 29272) appeared. The peak intensities slightly increased with increasing concentration of Zn2+. No characteristic peaks were observed for other impurities, such as Zn(OH)2. The strong and sharp diffraction peaks suggested that the products were highly crystalline. The average grain size of ZnO was estimated by Scherrer’s equation: D=Kλ/(βcosθ)
(2)
where D is the average diameter of ZnO nanoparticles, λ represents the X-ray wavelength (0.15418 nm), K refers to the Scherrer constant (0.89), β is the full width of the peak at half maximum (FWHM), and θ represents the Bragg diffraction angle. The average diameter of ZnO nanoparticles in wood/ZnO hybrid materials was calculated using the peaks of (1 0 0), (0 0 2), and (1 0 1). Therefore, the average grain size of ZnO was estimated approximately to be 10.8 nm. 7
Page 7 of 17
3.2. Morphology analysis Fig. 2 displays the morphologies of the cross and longitudinal sections of untreated wood and wood/ZnO composites. Compared with the untreated wood (Fig. 2(g)), numerous substances are apparently deposited in the cell lumens, middle lamella, and cell corners of ZnO treated wood samples. In addition, the quantity of the substances is increased with increasing Zn2+ concentration, especially deposited to the cell lumens. These substances are aggregated ZnO particles. Moreover, EDS was employed to analyze the distribution of zinc element in samples. In Fig. 2(i), the prominent signals are shown on the wood cell walls uniformly, indicating that the ZnO nanoparticles are generated in the wood cell walls. From the high-magnification image (Fig. 2(h)), it shows that there are numerous flowerlike aggregates with multi-leaves, and almost all of them have similar morphology. The diameters of ZnO nanoflowers are less than 8 μm. Many approaches were reported to synthesize ZnO nanoflowers with nanosheets and many of them depended on the surfactants, templates, structure-directing solvent, or high temperature[23–25]. Recently, Huang et al.[26] prepared flowerlike ZnO nanostructure with nanosheets on a large scale through a very simple solution method at proximate room temperatures. The diameters and structures of the synthesized ZnO nanoflowers in this study coincided with the results of Huang’s. The results showed that wood can be considered as a natural template for synthesizing nanostructures. The anisotropic and hierarchical structures of wood can affect the architecture of nanoparticles. 3.3. Thermal stability The TG and dTG curves of the untreated wood and wood/ZnO hybrid materials are 8
Page 8 of 17
shown in Fig. 3. The measurements performed on all samples showed a similar course below 100 °C, which could be related to the removal of the residual water in samples. There was no apparent decomposition on the TG curves below 200 °C for all treated wood samples. This result also revealed that there was no Zn(OH)2 impurities, since Zn(OH)2 decomposes in the range of 110–140 °C[27], which coincided with the result of XRD analysis. The thermal degradation profiles revealed that most degradation actions occurred between 160 and 400 °C. The two stages of the thermal degradation of the untreated wood were clearly visible. At stage one (160–370 °C), the decomposition rate reached two maximums, at 300 °C and 341 °C (Fig. 3(b)). These maximums in literature were assigned to the maximum decomposition of hemicellulose and cellulose[28]. The weight loss of this stage reached 77.93%. At stage two (above 370 °C), all the components of wood degraded gradually leading to aromatization and carbonization. Lignin was the most difficult one to be decomposed and slowly decomposed in both stages[29]. At last, the weight residue was about 12.67%. From the dTG curves (Fig. 3(b)), the untreated wood shows that the first decomposition shoulder peak is at about 300.27 °C and the major second decomposition peak at about 341.45 °C. For wood/ZnO hybrid materials, the dTG curves present strong and sharp peaks at a minimum of 319.32, 330.66, and 335.59 °C for the samples with 0.2, 0.6, and 1.0 mol/L Zn2+ aqueous solution, respectively, corresponding to the weight losses shown in the TG curves. Since the combination of ZnO and wood, the residue weight ratio of samples was improved from 12.67% to 26.11%. It can be reasoned that inorganic particles in hybrid wood material could improve its thermal stability by fostering the self-insulating 9
Page 9 of 17
char formation of wood and inhibiting heat transfer[30]. 3.4. Antibacterial efficacy and UV Stability Fig. 4 shows the results of a zone inhibition test against E. coli. The zone of inhibition is the area in which the bacterial growth is stopped due to bacteriostatic effect of the composites and it measures the inhibitory effect of composites towards a particular microorganism[31]. In Fig. 4, in contrast to untreated samples, wood samples treated with 0.6 mol/L and 1.0 mol/L Zn2+ solution have clear inhibition zones, indicating a bactericidal effect against E. coli. In addition, the width of the inhibition zone appears to increase as the amount of ZnO nanoparticles increases. This indicated that ZnO in the wood played an important role on the antibacterial efficacy of composites. The antibacterial activity of ZnO is considered to be the generation of hydrogen peroxide (H2O2) from its surface and it acts on gram-positive bacteria more strongly than on gram-negative bacteria[32]. In addition, it was found that the antibacterial efficiency of ZnO increased with decreasing particle size and increasing concentration[33,34]. Further studies on the antibacterial effectiveness to another type of bacterium on wood/ZnO hybrid materials are in progress. The color changes of wood/ZnO hybrid materials after the UV irradiation are presented in Fig. 5. The 0 value of the concentration refers to the value of color changes of the untreated samples. It was apparent that the values of L*, a*, and b* for the untreated sample changed greatly after the UV irradiation, especially for the changes in values of L* and b*, which reached –8.98 and 17.41, respectively. The negative lightness stability (ΔL*) values occurred because the surface became darker, and the 10
Page 10 of 17
positive values of Δb* showed an increase of yellow color[35]. Markedly, these changes decreased gradually with increasing concentration of Zn2+. Especially, the values of Δb* decreased greatly, which could be due to the white color of ZnO. Additionally, the total color difference (ΔE*) of wood samples was decreased with increasing concentration of Zn2+. Compare to the untreated sample, the ΔE* of the sample treated with 1.0 mol/L Zn2+ was reduced by 67.0%. This result indicated that the ZnO could improve the UV resistance of wood, which was due to the strong ultraviolet absorption of ZnO[36]. 4. Conclusion Wood/ZnO
hybrid
materials
were
prepared
successfully
by
the
in-situ
chemosynthesis method at room temperature. Zn2+ played an important role in the control of the preparation of ZnO nanostructure. XRD and TGA results revealed that there were no Zn(OH)2 impurities. The average grain size of ZnO was about 10.8 nm. Findings indicated that ZnO inorganic particles can be used to improve the thermal stability and UV resistance of hybrid wood materials. It was also indicated that the antibacterial property depended on the amount of loaded ZnO nanoparticles. In addition, it was shown that wood/ZnO hybrid materials have a promising future for the antibacterial efficacy on E. coli. This study conducted an important implication for the wood products industry. Acknowledgments The authors are very grateful for financial support from the Special Fund for Forestry Research in the Public Interest (No. 201404502). References 11
Page 11 of 17
[1] N. Guigo, A. Mija, R. Zavaglia, L. Vincent, N. Sbirrazzuoli, Polym. Degrad. Stab. 94 (2009) 908–913. [2] Y. Chujo, Curr. Opin. Solid State Mater. Sci. 1 (1996) 806–811. [3] V. Merk, M. Chanana, T. Keplinger, S. Gaan, I. Burgert, Green Chem. 17 (2015) 1423–1428. [4] H. Rahimi, R. Mozaffarinia, A.H. Najafabadi, J. Mater. Sci. Technol. 29 (2013) 603–608. [5] A.P. Wight, M.E. Davis, Chem. Rev. 102 (2002) 3589–3614. [6] J. Pyun, K. Matyjaszewski, Chem. Mater. 13 (2001) 3436–3448. [7] C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559–3592. [8] S.N. Kartal, E. Terzi, B. Woodward, C.A. Clausen, S.T. Lebow, Holzforschung 68 (2014) 831–837. [9] C. Qin, W. Zhang, Mater. Lett. 89 (2012) 101–103. [10] H. Oka, A. Hojo, H. Osada, Y. Namizaki, H. Taniuchi, J. Magn. Magn. Mater. 272 (2004) 2332–2334. [11] H. Oka, A. Hojo, K. Seki, T. Takashiba, J. Magn. Magn. Mater. 239 (2002) 617–619. [12] H. Oka, K. Tanaka, H. Osada, K. Kubota, F.P. Dawson, J. Appl. Phys. 105 (2009) 1–3. [13] J. Iqbal, N. Safdar, T. Jan, M. Ismail, S. Hussain, A. Mahmood, S. Shahzad, Q. Mansoor , J. Mater. Sci. Technol. 31 (2015) 300–304. 12
Page 12 of 17
[14] Y. Wang, W. Tang, L. Zhang, J. Mater. Sci. Technol. 31 (2015) 175–181. [15] C.A. Clausen, F. Green, S.N. Kartal, Nanoscale Res. Lett. 5 (2010) 1464–1467. [16] G. Mantanis, E. Terzi, S.N. Kartal, A. Papadopoulos, Int. Biodeterior. Biodegrad. 90 (2014) 140–144. [17] R.R. Devi, T.K. Maji, Ind. Eng. Chem. Res. 51 (2012) 3870–3880. [18] A. Hazarika, T.K. Maji, Defence Sci. J. 64 (2014) 262–272. [19] V. Merk, M. Chanana, N. Gierlinger, A.M. Hirt, I. Burgert, ACS Appl. Mater. Interf. 6 (2014) 9760–9767. [20] X. Lin, F. Wang, S. Kuga, T. Endo, M. Wu, D. Wu, Y. Huang, Cellulose 21 (2014) 2489–2496. [21] D. Aydemir, G. Gunduz, S. Ozden, Color Res. Appl. 37 (2012) 148–153. [22] Y. Dong, Y. Yan, S. Zhang, J. Li, BioResources 9 (2014) 6028–6040. [23] Y. Zhang, Y. Liu, L. Wu, H. Li, L. Han, B. Wang, E. Xie, Appl. Surf. Sci. 255 (2009) 4801–4805. [24] A.L. Pan, R.C. Yu, S.S. Xie, Z.B. Zhang, C.Q. Jin, B.S. Zou, J. Cryst. Growth 282 (2005) 165–172. [25] W. Zhao, X.Y. Song, Z.L. Yin, C.H. Fan, G.Z. Chen, S.X. Sun, Mater. Res. Bull. 43 (2008) 3171–3176. [26] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Sens. Actuat. B: Chem. 146 (2010) 206–212. [27] S.V. Nistor, D. Ghica, M. Stefan, I. Vlaicu, J.N. Barascu, C. Bartha, J. Alloy. Compd. 548 (2013) 222–227. 13
Page 13 of 17
[28] L. Gasparovic, Z. Korenova, L. Jelemensky, Chem. Pap. 64 (2010) 174–181. [29] A.N. Shebani, A.J. van Reenen, M. Meincken, Thermochim. Acta 471 (2008) 43–50. [30] T.R. Hull, A. Witkowski, L. Hollingbery, Polym. Degrad. Stab. 96 (2011) 1462–1469. [31] R.R. Gandhi, S. Gowri, J. Suresh, M. Sundrarajan, J. Mater. Sci. Technol. 29 (2013) 533–538. [32] J. Sawai, S. Shoji, H. Igarashi, A. Hashimoto, T. Kokugan, M. Shimizu, H. Kojima, J. Ferment. Bioeng. 86 (1998) 521–522. [33] O. Yamamoto, Int. J. Inorg. Mater. 3 (2001) 643–646. [34] S. Nair, A. Sasidharan, V.D. Rani, D. Menon, S. Nair, K. Manzoor, S. Raina, J. Mater. Sci. Mater. Med. 20 (2009) 235–241. [35] A. Temiz, N. Terziev, M. Eikenes, J. Hafrenb, Appl. Surf. Sci. 253 (2007) 5355–5362. [36] M. Li, G. Li, J. Jiang, Z. Zhang, X. Dai, K. Mai, J. Mater. Sci. Technol. 31 (2015) 331–339.
Figure captions Fig. 1 XRD patterns of untreated wood and wood/ZnO hybrid materials. Fig. 2 SEM observations of untreated and treated wood samples: cross section of wood treated with 0.2 mol/L Zn2+ solution (a), 0.6 mol/L Zn2+ solution (b), and 1.0 mol/L Zn2+ solution (c); longitudinal section wood treated with 0.2 mol/L Zn2+ solution (d), 0.6 14
Page 14 of 17
mol/L Zn2+ solution (e), and 1.0 mol/L Zn2+ solution (f); cross section of untreated wood (g); high-magnification image of aggregation of ZnO (h); zinc distribution obtained by SEM-EDS (i). Fig. 3 TG and dTG curves of untreated wood and wood/ZnO hybrid materials. Fig. 4 Images of agar plates and the inhibition zone for E. coli. Fig. 5 Color changes of wood samples treated with different concentrations of Zn2+.
Fig. 1
15
Page 15 of 17
Fig. 2
Fig. 3
16
Page 16 of 17
Fig. 4
Fig. 5
17
Page 17 of 17
S1005-0302(16)30019-6 http://dx.doi.org/doi: 10.1016/j.jmst.2016.03.018 JMST 680
To appear in:
Journal of Materials Science & Technology
Received date: Revised date: Accepted date:
20-10-2015 21-12-2015 27-1-2016
Please cite this article as: Youming Dong, Yutao Yan, Huandi Ma, Shifeng Zhang, Jianzhang Li, Changlei Xia, Sheldon Q. Shi, Liping Cai, In-Situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-Resistance Properties, Journal of Materials Science & Technology (2016), http://dx.doi.org/doi: 10.1016/j.jmst.2016.03.018. 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.
In-situ Chemosynthesis of ZnO Nanoparticles to Endow Wood with Antibacterial and UV-resistance Properties Youming Dong1, Yutao Yan1, Huandi Ma2, Shifeng Zhang1,*, Jianzhang Li1,*, Changlei Xia3, Sheldon Q. Shi3, Liping Cai3 1
MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry
University, Beijing 100083, China 2
College of Biological Science and Technology, Beijing Forestry University, Beijing
100083, China 3
Department of Mechanical and Energy Engineering, University of North Texas, 3940
N Elm St., Suite F101, Denton, TX 76207, USA *Corresponding author. Tel./Fax: +86 10 62336072. E-mail
addresses:
[email protected]
(Shifeng
Zhang),
[email protected] (Jianzhang Li). [Received 20 October 2015; Received in revised form 21 December 2015; Accepted 27 January 2016]
Highlights
Wood/ZnO hybrid composites were prepared by a facile process.
ZnO nanoflowers with nanosheets were formed in wood cell walls.
ZnO nanoparticles endowed wood with the antibacterial property and UV-resistance.
1
Page 1 of 17
Hybrid wood materials have attracted considerable attention because they have combined advantages of both wood and inorganic compounds. This work investigated the microstructural morphology, thermal stability, ultraviolet (UV) stability, and antibacterial property of composites made from wood/ZnO hybrid materials through a facile
in-situ
chemosynthesis
methods.
The
X-ray
diffraction
(XRD)
and
thermogravimetric analysis (TGA) results indicated that the synthesized ZnO particles had an average grain size of about 10.8 nm. The scanning electron microscopy (SEM) observations showed that ZnO nanoflowers self-assembled with nanosheets were presented in wood cell lumens and increased with increasing Zn2+ concentrations. ZnO nanoparticles were also generated in the wood cell wall, which was confirmed by the results of energy-dispersive spectroscopy (EDS). The TGA tests also indicated that the thermal stability of wood/ZnO hybrid materials was improved after the formation of ZnO inorganic particles. Finally, the results of antibacterial efficacy tests and UV resistance tests revealed that ZnO nanoparticles showed a promising future as antimicrobial agents against Escherichia coli (E. coli) and UV resistance agents for wood protection. Key words: Hybrid wood material; Nanoflowers; Zinc oxide; Antibacterial property; UV-resistance
1. Introduction Organic-inorganic hybrid composites have attracted considerable attention recently, since they couple the advantages of both organic polymers and those of inorganic 2
Page 2 of 17
compounds, and may exhibit unexpected properties different from their original components[1,2]. In the last decade, many approaches have been conducted to create high-performance or multi-functional organic-inorganic hybrid composites to be widely used as the catalyst, coating, separation devices, solar cells, and sensors[3–7]. Inspired by the good performance of these composites, researchers have investigated the preparation and characterization of the hybrid wood materials. The consolidation of wood cell walls with a mineral phase at the nanostructural level can result in a highly desirable material combination and versatile properties[3,8,9]. Accordingly, a large variety of multi-functional wood-inorganic composites have been developed in recent years. For example, Oka group have made plenty of work on the fabrication and characterization of magnetic wood by the combination of wood and magnetic nanoparticles[10,11]. This wood-inorganic composite has potential applications on the electromagnetic wave absorption and heating materials[12]. In this field, lots of inorganic materials have been used for the preparation of wood-inorganic materials, such as SiO2, TiO2, ZnO, CuO, CaCO3, and so on. Particularly, ZnO is a very interesting multifunctional material. It has been considered for the applications in sensors, solar cells, thin film transistors, photocatalysis, and optoelectronic devices[13,14]. ZnO is also beneficial to the fabrication of functional composites with wood materials, due to its resistance to decay ultraviolet light and other properties[15,16]. Various approaches have been used to prepare wood-inorganic materials. Impregnating wood with nanoparticles is a typical approach to functionalize wood. Devi et al. confirmed that vacuum impregnation of ZnO nanoparticles into wood could 3
Page 3 of 17
significantly improve ultraviolet (UV) resistance of the composite, thermal stability, and mechanical properties of wood[17]. However, this process is highly challengeable due to the difficulty in dispersion of nanoparticles throughout the wood. As a result, the nanoparticles should be modified with organic surface active agents or couple agents and impregnated with other monomers or resins[18]. Although the properties of modified wood are significantly improved, the procedures are complicated accordingly. To address the issue, in-situ chemosynthesis method at room temperature is a facile fabrication approach to synthesizing nanoparticles within the wood’s structure. Merk et al.[3] reported a systematic and eco-friendly preparation of the wood/CaCO3 materials using aqueous salt solutions at ambient temperature and the nanoparticles could deposit in the cell walls. In addition, wood can be used as a template for self-assembled nanoparticles, affecting the nanostructure of generated particles[19]. Therefore, this study explored the preparation of wood/ZnO hybrid materials using in-situ chemosynthesis at room temperature and the microstructural morphology, thermal stability, UV stability, and antibacterial property of the hybrid materials were evaluated. 2. Experimental 2.1. Preparation of wood/ZnO hybrid materials Poplar wood samples (populus tomentosa Carr.) were cut into a size of 30 mm (tangential) × 10 mm (radial) × 100 mm (longitudinal) and oven-dried at 105 °C until a constant weight was obtained. The preparation of wood/ZnO hybrid materials was implemented as follows. First, Zn(NO3)·6H2O was dissolved in deionized water in 4
Page 4 of 17
concentrations of 0.2, 0.6, and 1.0 mol/L. The dried wood samples were submerged in the Zn(NO3) solution under a vacuum (ca. 0.095 MPa) for 30 min, and soaked under atmospheric pressure for 2 h. After impregnation, excessive solution on the wood surface was cleared up with deionized water and rapidly immersed in the NaOH solution (0.4 mol/L) for 48 h at room temperature. Next, wood samples were washed with deionized water until they reached a neutral pH value. Finally, the samples were oven-dried at 60 °C for 24 h to obtain wood/ZnO hybrid materials. 2.2. Characterization 2.2.1. XRD analysis The X-ray diffraction (XRD) pattern was recorded on a Bruker D8 Advance diffractrometer (Germany) with CuKα radiation. The apparatus parameters were set as follows: CuKα radiation with a graphite monochromator, voltage 40 kV, electric current 40 mA and 2θ scan range from 5° to 85° with a scanning speed of 2°/min. 2.2.2. Thermal stability The thermal stability of the samples was evaluated by thermogravimetric analysis TGA (Q50, TA Instruments, USA) at a constant heating rate of 5 °C/min from room temperature to 600 °C under a flowing nitrogen atmosphere. 2.2.3. Microstructural morphology To observe the microstructural morphology of materials, the interior portions of cross and longitudinal sections in untreated and treated samples were cut with a sliding microtome (REM-710, Yamato, Tokyo, Japan), mounted on conductive adhesives, and coated with gold-sputter, followed by scanning electron microscopy (SEM, Hitachi 5
Page 5 of 17
S-4800 N, Japan) observation using a voltage of 5 kV. In addition, energy dispersive X-ray spectroscopy (EDS, EX-350, Horiba Scientific, Japan) was employed to analyze the distribution of zinc in samples. 2.2.4. Antibacterial properties tests Tests on the antibacterial properties of wood/ZnO composites were carried out for Escherichia coli (E. coli) according to the previous research[20]. Briefly, the bacterial suspension (3.7 × 106 cfu/ml) was applied uniformly on the surface of a nutrient agar plate before placing the prepared samples on the plate (4 per plate). The plates were incubated at 37 °C for 24 h, after the average diameter of the inhibition zone surrounding the sample was measured with a ruler with a resolution up to 1 mm. 2.2.5. UV resistance tests The UV resistance of the samples was carried out using a UV chamber (metal halide lamp with a spectrum of 200–500 nm; power of 500 W; Advanced Research Co., China). The wood samples with a dimension of 30 mm × 10 mm × 100 mm were placed at even distances for six weeks of exposure period. Then the effect of the UV resistance was analyzed by examining surface color. The color difference was calculated before and after the test. The surface color of the specimens was measured with a color measurement instrument (Dataflash 110 Datacolor, USA) according to the CIELAB color system, where L* is a measure of lightness, a* is the chroma from green to red, and b* is the chroma from blue to yellow[21]. The color difference is defined by the following equation: (1) 6
Page 6 of 17
where ΔE* is the color difference, ΔL* is the lightness difference, and Δa* and Δb* are the chroma differences. Δ means the differences between the initial and final parameters of samples after UV irradiation. L*, a*, and b* are the average values of five locations on each sample. 3. Results and Discussion 3.1. Structural analysis Fig. 1 displays the XRD patterns of untreated wood and wood/ZnO hybrid materials. All samples exhibit characteristic peaks at around 15.8°, 22.1°, and 34.5° corresponding to the (101), (002), and (040) planes of cellulose, respectively[22]. When the wood was treated with ZnO, some peaks which can be indexed to wurtzite-structured ZnO (ZnO: ICSD 29272) appeared. The peak intensities slightly increased with increasing concentration of Zn2+. No characteristic peaks were observed for other impurities, such as Zn(OH)2. The strong and sharp diffraction peaks suggested that the products were highly crystalline. The average grain size of ZnO was estimated by Scherrer’s equation: D=Kλ/(βcosθ)
(2)
where D is the average diameter of ZnO nanoparticles, λ represents the X-ray wavelength (0.15418 nm), K refers to the Scherrer constant (0.89), β is the full width of the peak at half maximum (FWHM), and θ represents the Bragg diffraction angle. The average diameter of ZnO nanoparticles in wood/ZnO hybrid materials was calculated using the peaks of (1 0 0), (0 0 2), and (1 0 1). Therefore, the average grain size of ZnO was estimated approximately to be 10.8 nm. 7
Page 7 of 17
3.2. Morphology analysis Fig. 2 displays the morphologies of the cross and longitudinal sections of untreated wood and wood/ZnO composites. Compared with the untreated wood (Fig. 2(g)), numerous substances are apparently deposited in the cell lumens, middle lamella, and cell corners of ZnO treated wood samples. In addition, the quantity of the substances is increased with increasing Zn2+ concentration, especially deposited to the cell lumens. These substances are aggregated ZnO particles. Moreover, EDS was employed to analyze the distribution of zinc element in samples. In Fig. 2(i), the prominent signals are shown on the wood cell walls uniformly, indicating that the ZnO nanoparticles are generated in the wood cell walls. From the high-magnification image (Fig. 2(h)), it shows that there are numerous flowerlike aggregates with multi-leaves, and almost all of them have similar morphology. The diameters of ZnO nanoflowers are less than 8 μm. Many approaches were reported to synthesize ZnO nanoflowers with nanosheets and many of them depended on the surfactants, templates, structure-directing solvent, or high temperature[23–25]. Recently, Huang et al.[26] prepared flowerlike ZnO nanostructure with nanosheets on a large scale through a very simple solution method at proximate room temperatures. The diameters and structures of the synthesized ZnO nanoflowers in this study coincided with the results of Huang’s. The results showed that wood can be considered as a natural template for synthesizing nanostructures. The anisotropic and hierarchical structures of wood can affect the architecture of nanoparticles. 3.3. Thermal stability The TG and dTG curves of the untreated wood and wood/ZnO hybrid materials are 8
Page 8 of 17
shown in Fig. 3. The measurements performed on all samples showed a similar course below 100 °C, which could be related to the removal of the residual water in samples. There was no apparent decomposition on the TG curves below 200 °C for all treated wood samples. This result also revealed that there was no Zn(OH)2 impurities, since Zn(OH)2 decomposes in the range of 110–140 °C[27], which coincided with the result of XRD analysis. The thermal degradation profiles revealed that most degradation actions occurred between 160 and 400 °C. The two stages of the thermal degradation of the untreated wood were clearly visible. At stage one (160–370 °C), the decomposition rate reached two maximums, at 300 °C and 341 °C (Fig. 3(b)). These maximums in literature were assigned to the maximum decomposition of hemicellulose and cellulose[28]. The weight loss of this stage reached 77.93%. At stage two (above 370 °C), all the components of wood degraded gradually leading to aromatization and carbonization. Lignin was the most difficult one to be decomposed and slowly decomposed in both stages[29]. At last, the weight residue was about 12.67%. From the dTG curves (Fig. 3(b)), the untreated wood shows that the first decomposition shoulder peak is at about 300.27 °C and the major second decomposition peak at about 341.45 °C. For wood/ZnO hybrid materials, the dTG curves present strong and sharp peaks at a minimum of 319.32, 330.66, and 335.59 °C for the samples with 0.2, 0.6, and 1.0 mol/L Zn2+ aqueous solution, respectively, corresponding to the weight losses shown in the TG curves. Since the combination of ZnO and wood, the residue weight ratio of samples was improved from 12.67% to 26.11%. It can be reasoned that inorganic particles in hybrid wood material could improve its thermal stability by fostering the self-insulating 9
Page 9 of 17
char formation of wood and inhibiting heat transfer[30]. 3.4. Antibacterial efficacy and UV Stability Fig. 4 shows the results of a zone inhibition test against E. coli. The zone of inhibition is the area in which the bacterial growth is stopped due to bacteriostatic effect of the composites and it measures the inhibitory effect of composites towards a particular microorganism[31]. In Fig. 4, in contrast to untreated samples, wood samples treated with 0.6 mol/L and 1.0 mol/L Zn2+ solution have clear inhibition zones, indicating a bactericidal effect against E. coli. In addition, the width of the inhibition zone appears to increase as the amount of ZnO nanoparticles increases. This indicated that ZnO in the wood played an important role on the antibacterial efficacy of composites. The antibacterial activity of ZnO is considered to be the generation of hydrogen peroxide (H2O2) from its surface and it acts on gram-positive bacteria more strongly than on gram-negative bacteria[32]. In addition, it was found that the antibacterial efficiency of ZnO increased with decreasing particle size and increasing concentration[33,34]. Further studies on the antibacterial effectiveness to another type of bacterium on wood/ZnO hybrid materials are in progress. The color changes of wood/ZnO hybrid materials after the UV irradiation are presented in Fig. 5. The 0 value of the concentration refers to the value of color changes of the untreated samples. It was apparent that the values of L*, a*, and b* for the untreated sample changed greatly after the UV irradiation, especially for the changes in values of L* and b*, which reached –8.98 and 17.41, respectively. The negative lightness stability (ΔL*) values occurred because the surface became darker, and the 10
Page 10 of 17
positive values of Δb* showed an increase of yellow color[35]. Markedly, these changes decreased gradually with increasing concentration of Zn2+. Especially, the values of Δb* decreased greatly, which could be due to the white color of ZnO. Additionally, the total color difference (ΔE*) of wood samples was decreased with increasing concentration of Zn2+. Compare to the untreated sample, the ΔE* of the sample treated with 1.0 mol/L Zn2+ was reduced by 67.0%. This result indicated that the ZnO could improve the UV resistance of wood, which was due to the strong ultraviolet absorption of ZnO[36]. 4. Conclusion Wood/ZnO
hybrid
materials
were
prepared
successfully
by
the
in-situ
chemosynthesis method at room temperature. Zn2+ played an important role in the control of the preparation of ZnO nanostructure. XRD and TGA results revealed that there were no Zn(OH)2 impurities. The average grain size of ZnO was about 10.8 nm. Findings indicated that ZnO inorganic particles can be used to improve the thermal stability and UV resistance of hybrid wood materials. It was also indicated that the antibacterial property depended on the amount of loaded ZnO nanoparticles. In addition, it was shown that wood/ZnO hybrid materials have a promising future for the antibacterial efficacy on E. coli. This study conducted an important implication for the wood products industry. Acknowledgments The authors are very grateful for financial support from the Special Fund for Forestry Research in the Public Interest (No. 201404502). References 11
Page 11 of 17
[1] N. Guigo, A. Mija, R. Zavaglia, L. Vincent, N. Sbirrazzuoli, Polym. Degrad. Stab. 94 (2009) 908–913. [2] Y. Chujo, Curr. Opin. Solid State Mater. Sci. 1 (1996) 806–811. [3] V. Merk, M. Chanana, T. Keplinger, S. Gaan, I. Burgert, Green Chem. 17 (2015) 1423–1428. [4] H. Rahimi, R. Mozaffarinia, A.H. Najafabadi, J. Mater. Sci. Technol. 29 (2013) 603–608. [5] A.P. Wight, M.E. Davis, Chem. Rev. 102 (2002) 3589–3614. [6] J. Pyun, K. Matyjaszewski, Chem. Mater. 13 (2001) 3436–3448. [7] C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559–3592. [8] S.N. Kartal, E. Terzi, B. Woodward, C.A. Clausen, S.T. Lebow, Holzforschung 68 (2014) 831–837. [9] C. Qin, W. Zhang, Mater. Lett. 89 (2012) 101–103. [10] H. Oka, A. Hojo, H. Osada, Y. Namizaki, H. Taniuchi, J. Magn. Magn. Mater. 272 (2004) 2332–2334. [11] H. Oka, A. Hojo, K. Seki, T. Takashiba, J. Magn. Magn. Mater. 239 (2002) 617–619. [12] H. Oka, K. Tanaka, H. Osada, K. Kubota, F.P. Dawson, J. Appl. Phys. 105 (2009) 1–3. [13] J. Iqbal, N. Safdar, T. Jan, M. Ismail, S. Hussain, A. Mahmood, S. Shahzad, Q. Mansoor , J. Mater. Sci. Technol. 31 (2015) 300–304. 12
Page 12 of 17
[14] Y. Wang, W. Tang, L. Zhang, J. Mater. Sci. Technol. 31 (2015) 175–181. [15] C.A. Clausen, F. Green, S.N. Kartal, Nanoscale Res. Lett. 5 (2010) 1464–1467. [16] G. Mantanis, E. Terzi, S.N. Kartal, A. Papadopoulos, Int. Biodeterior. Biodegrad. 90 (2014) 140–144. [17] R.R. Devi, T.K. Maji, Ind. Eng. Chem. Res. 51 (2012) 3870–3880. [18] A. Hazarika, T.K. Maji, Defence Sci. J. 64 (2014) 262–272. [19] V. Merk, M. Chanana, N. Gierlinger, A.M. Hirt, I. Burgert, ACS Appl. Mater. Interf. 6 (2014) 9760–9767. [20] X. Lin, F. Wang, S. Kuga, T. Endo, M. Wu, D. Wu, Y. Huang, Cellulose 21 (2014) 2489–2496. [21] D. Aydemir, G. Gunduz, S. Ozden, Color Res. Appl. 37 (2012) 148–153. [22] Y. Dong, Y. Yan, S. Zhang, J. Li, BioResources 9 (2014) 6028–6040. [23] Y. Zhang, Y. Liu, L. Wu, H. Li, L. Han, B. Wang, E. Xie, Appl. Surf. Sci. 255 (2009) 4801–4805. [24] A.L. Pan, R.C. Yu, S.S. Xie, Z.B. Zhang, C.Q. Jin, B.S. Zou, J. Cryst. Growth 282 (2005) 165–172. [25] W. Zhao, X.Y. Song, Z.L. Yin, C.H. Fan, G.Z. Chen, S.X. Sun, Mater. Res. Bull. 43 (2008) 3171–3176. [26] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Sens. Actuat. B: Chem. 146 (2010) 206–212. [27] S.V. Nistor, D. Ghica, M. Stefan, I. Vlaicu, J.N. Barascu, C. Bartha, J. Alloy. Compd. 548 (2013) 222–227. 13
Page 13 of 17
[28] L. Gasparovic, Z. Korenova, L. Jelemensky, Chem. Pap. 64 (2010) 174–181. [29] A.N. Shebani, A.J. van Reenen, M. Meincken, Thermochim. Acta 471 (2008) 43–50. [30] T.R. Hull, A. Witkowski, L. Hollingbery, Polym. Degrad. Stab. 96 (2011) 1462–1469. [31] R.R. Gandhi, S. Gowri, J. Suresh, M. Sundrarajan, J. Mater. Sci. Technol. 29 (2013) 533–538. [32] J. Sawai, S. Shoji, H. Igarashi, A. Hashimoto, T. Kokugan, M. Shimizu, H. Kojima, J. Ferment. Bioeng. 86 (1998) 521–522. [33] O. Yamamoto, Int. J. Inorg. Mater. 3 (2001) 643–646. [34] S. Nair, A. Sasidharan, V.D. Rani, D. Menon, S. Nair, K. Manzoor, S. Raina, J. Mater. Sci. Mater. Med. 20 (2009) 235–241. [35] A. Temiz, N. Terziev, M. Eikenes, J. Hafrenb, Appl. Surf. Sci. 253 (2007) 5355–5362. [36] M. Li, G. Li, J. Jiang, Z. Zhang, X. Dai, K. Mai, J. Mater. Sci. Technol. 31 (2015) 331–339.
Figure captions Fig. 1 XRD patterns of untreated wood and wood/ZnO hybrid materials. Fig. 2 SEM observations of untreated and treated wood samples: cross section of wood treated with 0.2 mol/L Zn2+ solution (a), 0.6 mol/L Zn2+ solution (b), and 1.0 mol/L Zn2+ solution (c); longitudinal section wood treated with 0.2 mol/L Zn2+ solution (d), 0.6 14
Page 14 of 17
mol/L Zn2+ solution (e), and 1.0 mol/L Zn2+ solution (f); cross section of untreated wood (g); high-magnification image of aggregation of ZnO (h); zinc distribution obtained by SEM-EDS (i). Fig. 3 TG and dTG curves of untreated wood and wood/ZnO hybrid materials. Fig. 4 Images of agar plates and the inhibition zone for E. coli. Fig. 5 Color changes of wood samples treated with different concentrations of Zn2+.
Fig. 1
15
Page 15 of 17
Fig. 2
Fig. 3
16
Page 16 of 17
Fig. 4
Fig. 5
17
Page 17 of 17