A simple, one-step hydrothermal approach to durable and robust superparamagnetic, superhydrophobic and electromagnetic wave-absorbing wood

In this work, lamellar MnFe2O4 was successfully planted on a wood surface through the association of hydrogen bonds via the one-pot hydrothermal method. Simultaneously, the fluoroalkylsilane (FAS-17) on the surface of the MnFe2O4 layer formed long-chain or network macromolecules through a poly-condensation process and provided a lower surface energy on the wood surface. The MnFe2O4/wood composite (FMW) presented superior superparamagnetism, superhydrophobicity and electromagnetic wave absorption performance. The results indicated a saturation magnetization of the FMW with excellent superparamagnetism of 28.24 emu·g−1. The minimum value of reflection loss of the FMW reached −8.29 dB at 16.39 GHz with a thickness of 3 mm. Even after mechanical impact and exposure to corrosive liquids, the FMW still maintained a superior superhydrophobicity performance.

Scientific RepoRts | 6:35549 | DOI: 10.1038/srep35549 caused by the hydrophilicity of the wood surface. Therefore, to overcome the wood's defects and prolong the wood's service life, the preparation of superhydrophobic wood is necessary 23,24 . In general, a superhydrophobic wood surface can be prepared by two approaches: forming a layer of inorganic particles on the wood surface to create a rough structure or chemically modifying a rough surface with a low surface free energy (such as FAS-17) 25,26 . Most of the preparation methods for these materials are complicated and require at least two or three steps for the formation of the superhydrophobic surface. In this work, we employed a simple one-step hydrothermal method for the growth of MnFe 2 O 4 on a wood surface, and the as-prepared MnFe 2 O 4 /wood composite exhibited superior superparamagnetism, superhydrophobic and electromagnetic wave absorption properties. In addition, the chemical and mechanical stability of superhydrophobic surfaces are very important to the performance of the material and have received broad interest. Thus, we tested the superhydrophobicity of the wood surface by sand abrasion and a test of resistance against corrosive liquids, which indicated a good chemical and mechanical stability of the wood surface when facing environmental erosion. Figure 1 shows the X-ray diffraction (XRD) patterns of the wood and the MnFe 2 O 4 /wood composite. As shown in Fig. 1a,b, strong diffraction peaks at 16.0° and 22.5° appear in the wood and the FMW spectra, which originated from the crystalline region of the cellulose in the wood 27 . Seven crystalline peaks were observed at 2θ ° of 17 28 . Therefore, MnFe 2 O 4 had been successfully grown on the wood surface. Figure 2 shows the surface morphologies of the wood and the FMW. As shown in Fig. 2a, the microstructure of the wood in a longitudinal section and the inner surface of the lumen and the vessels were observed on the wood surface. Figure 2b shows the surface of the FMW with a very low magnification. After the one-step hydrothermal process, the MnFe 2 O 4 was packed tightly on the wood surface, masking the vessels and other details. The magnified SEM image of the FMW revealed that the MnFe 2 O 4 layer was composed of a number of lamellas with different sizes (Fig. 2c,d). Therefore, these results indicate that the lamella MnFe 2 O 4 had been successfully coated onto the wood surface. Figure 3 shows the typical FT-IR spectra of the wood and the FMW. The peaks at 3450-3400 cm −1 were attributed to the -OH stretching absorption bands arising from the hydroxyl groups of the wood, which shifted to lower wavenumbers. This result indicates the formation of a strong interaction between the hydroxyl groups of the wood surface and the MnFe 2 O 4 through hydrogen bonds 29 . Then, as shown in Fig. 3a, the peaks at 2921 cm −1 , 2852 cm −1 and 1740 cm −1 were ascribed to the stretching vibrations of -CH 3 , -CH 2 and C= O, respectively 30 . These peaks in the FMW spectra (Fig. 3b) had significantly decreased, which might have occurred by the hydrolysis of fatty acids through the alkaline hydrothermal process. More importantly, the peaks at 1202 cm −1 and 1332 cm −1 were assigned to the C-F stretching vibration, which corresponds to the FAS-17 that is incorporated into the MnFe 2 O 4 surface 31 . In addition, a strong adsorption peak appeared at 576 cm −1 , which was attributed to the intrinsic vibrations of the manganese ferrite (Fe-O or Mn-O) 32 . Therefore, the analysis of the FT-IR spectra of the FMW exhibited the existence of FAS-17 and hydrogen bonds between the wood surface and the MnFe 2 O 4 .

Results
X-ray photoelectron spectroscopy (XPS) was employed as a potent technique to analyze the chemical structure characteristics and surface modification of the FMW after the hydrothermal process. The Fe 2p peak (Fig. 4b) was a doublet with a spin-orbit separation of 13.7 eV, and the Fe 3+ cation had a well-defined structure at 8 eV after the Fe 2p 3/2 peak (711.58 eV). However, this peak in the Fe 3 O 4 was smeared and almost equal in proportion because of the presence of both Fe 2+ and Fe 3+ 33 . As shown in Fig. 4c, the Mn 2p spectra consisted of spin-orbit-split 2p 3/2 and 2p 1/2 with a separation of 11.7 eV, which was approaching the spin-orbit separation (11.6 eV) of the MnO 34 . In addition, the satellite peak at 647.08 eV was approximately 5 eV distant from the 2p 1/2 peak of the Mn 2p spectra, which is the typical Mn 2+ ion behaviour 35 . Therefore, these results provided powerful evidence of the presence of Mn 2+ and Fe 3+ in the FMW surface.
The O 1s region ( Fig. 4d) was dominated by three components centred at 530.18, 531.78 and 533.08 eV, arising from the photoelectrons ejected from the oxygen 1 s orbital. The dominant peak at 533.08 eV was attributed to the Fe-O and Mn-O from the MnFe 2 O 4 layer 36 . The peak at 531.78 eV included a carbon-oxygen double bond (such as lignin), -OH groups (such as adsorbed water, hydroxyl groups from the wood or hydrogen bonds between the wood surface and MnFe 2 O 4 ) and a Si-O bond (FAS-17 layer) 31 . Finally, the C-H bond at 533.08 eV was provided by the FAS-17 and the wood substrate.
The C 1 s spectrum (  31 . The presence of the CF 3 -CF 2 , CF 2 -CF 2 , CF 2 -CH 2 and C-Si in the spectrum provided powerful evidence that the hydrolytic FAS-17 had been incorporated into the MnFe 2 O 4 surface and formed the hydrophobic layer through the self-assembly process. The other peaks  originated from the wood surface (cellulose, hemicellulose and lignin), and a small amount of the hydrocarbon originated from the XPS instrument itself 37 . Figure 4f shows the high-resolution XPS F 1 s spectra of the FMW surface. The peak at 693 eV was attributed to the F 1 s from the FAS-17 layer. In summary, all of the XPS results demonstrated that the FMW sample consisted of C, O, Mn, Fe, F and Si elements with an anomic ratio of Fe:Mn at 2.35:1 and proved the presence of the MnFe 2 O 4 and the hydrophobic layer (FAS-17 layer) on the wood surface. Figure 5 shows the magnetization-hysteresis (M-H) curves of the FMW. The magnetic property of the MnFe 2 O 4 /wood composite was analyzed by room temperature VSM with an applied field of − 10 KOe ≤ H ≤ 10 KOe. The value of the saturation magnetization (Ms) of the FMW was 28.08 emu·g −1 . The lower right corner inset shows that the remnant magnetization (Mr) and coercivity field were 1.2 emu·g −1 and 25 Oe, respectively. These results show that the FMW had superior superparamagnetic properties, small hysteresis loops and low coercivity 38 . Therefore, the MnFe 2 O 4 /wood composite showed superparamagnetism via a one-pot hydrothermal process with low temperature.
The mechanism of the formation of the MnFe 2 O 4 can be expressed by reaction Equations (1)   According to previous results, the possible mechanism can be described as follows (Fig. 6). First, when the ammonia solution was added into the Fe 3+    To provide a more visible demonstration of the superhydrophobic performance, four types of corrosive liquids were placed on the surface of the wood and FMW, including water, acid, salt, and alkali droplets. Figure 7a shows that all of the liquid profiles on the wood surface were approximately zero, which proved the hydrophilicity of the pristine wood. In contrast, the surface of the FMW was highly repellent to the water and other liquids, and the droplets exhibited spherical shapes on the sample surface (Fig. 7b). Figure 7b shows the contact angle values and profiles of several typical liquids. The CA value of the water droplet on the FMW surface was approximately 156°, and all of the CA values of the acid, salt, and alkali droplets on the sample surface were over 150° because the high surface concentration of -CF 3 and -CF 2 from the FAS-17 layer provided a low surface energy on the FMW surface, which is conducive for superior superhydrophobic performance.
The superhydrophobic property of the FMW was further evaluated by a sand abrasion experiment. In this test, sand grains (10 g) with an average diameter of approximately 100 μ m were flowed to the FMW surface from a height of 30 cm, and the sand grains repeatedly impacted the FMW surface 10 times (100 g of sand). As shown in Fig. 8b, it is intuitive to show that the values of the contact angles of water (WCA) and the saturation magnetization (Ms) of the FMW gradually decreased with increases in the experiment frequency. The WCA images and Ms curves of the FMW with its values were recorded at 0 g, 10 g, 20 g, 40 g, 60 g and 100 g, shown in Fig. 8c-h, respectively. Before the experiment, the WCA of the FMW was 156°. After 10 cycles of the sands abrasion test, the WCA of the FMW was 149.1°. In addition, the value of the Ms of the FMW had only decreased by 1.61 emu g −1 , indicating its stable performance for superhydrophobicity and superparamagnetism. Therefore, these results indicate that the samples have excellent durability magnetism and mechanical stability when confronted with external abrasion, and the FMW might have great potential in a wide range of applications.
To evaluate the durability of the superhydrophobic wood surface for mild acids and strong bases, the as-prepared FMW samples were placed into a NaCl solution of 0.1 mol/L (Fig. 9a) HCl solution with a pH of 1 (Fig. 9b) and a NaOH solution with a pH of 14 (Fig. 9c) for 4 h at 50 °C. In contrast to the baseline condition, the WCA of the FMW surface displayed only a small change, and the water contact angles were larger than 150°. These results showed that all of the samples maintained their superhydrophobic properties after the acid and base corrosion tests and had good durability towards acidic (152°) and basic (151°) liquids. In addition, the salt solution corrosion test demonstrated that Cl and Na ions had almost no effect on the superhydrophobic performance under the same conditions (Fig. 9a). Figure 9b,c show that the acid resistance of the FMW was appreciably stronger than the base resistance. Therefore, this superhydrophobic MnFe 2 O 4 /wood composite might have applications in outdoor building materials to protect wood substrates from acid rain erosion by the environment.
To investigate the electromagnetic absorption property of the wood and the FMW, the reflection loss (RL) values were calculated by the transmission line theory 39 .
where Z in is the input impedance of the samples, Z 0 is the impedance of air, ε r is the complex permittivity, μ r is the complex permeability, d is the thickness of the samples, c is the velocity of light and f is the microwave frequency. The values of the reflection loss (RL) of the wood and the FMW are exhibited by three-dimensional and colour-filling pattern images in the frequency range of 2-18 GHz in Fig. 10. Figure 10a   surface via the polycondensation process provided a low surface energy to achieve superhydrophobicity. After a one-step hydrothermal reaction, the value of the saturation magnetization of the FMW reached 28.08 emu·g −1 and presented excellent superparamagnetism properties. The value of the water contact angles of FMW reached 156°, and the FMW maintained its superhydrophobic properties even after sand abrasion and exposure to corrosive liquids. In addition, the value of the minimum reflection loss of the FMW reached − 8.29 dB at 16.39 GHz with a thickness of 3 mm, and the absorption bandwidth improved. These findings suggest that the MnFe 2 O 4 / wood composite has potential applications in electromagnetic wave-absorbing materials, electromagnetic shielding and outdoor building materials.

Methods
Materials. All chemicals were supplied by Shanghai Boyle Chemical Co. Ltd (Shanghai. China). and used without further purification. The wood slices were cut to dimensions of 10 mm (length) × 10 mm (width) × 10 mm (height), and then the slices were ultrasonically rinsed with deionized water for 30 minutes and dried at 80 °C in a vacuum.