The surface morphology and chemical composition of cellulose, cellulose film, AgNWs, and AgNWs/cellulose hybrid film were characterized using SEM and energy dispersive spectroscopy (EDS). As shown in Fig. 1A, as-prepared sawdust cellulose shows a three-dimensional linear network structure and the single cellulose exhibit slender thread-like with an average diameter of 15 µm. The one-dimensional fiber structures of sawdust cellulose can enhance the flexibility and mechanical properties of the hybrid film. From Fig. 1B, it is shown that cellulose film with a microporous structure constructed by microfine cellulose and other fillers, the microporous structure could improve the transmittance of the hybrid film. Also, Fig. 1C shows as-prepared AgNWs with an average diameter ofཞ50 nm and a high aspect ratio ofཞ150 are synthesized, the high aspect ratio can modify the mechanical properties of the hybrid film. As can be seen in Fig. 1D, it shows that the AgNWs/cellulose hybrid film was successfully prepared, because it can be seen that the AgNWs are cross-linked and dispersed in the cellulose grid. Furthermore, the well hydrophilic properties of cellulose could make the AgNWs uniformly distributed on the cellulose film, and the cross-linked AgNWs facilitate the transmission of electrons that increase the electric heating performance and heat preservation of the hybrid film. And from Fig. S1, it is indicated that C, N, O, Ag elements exist on the hybrid film, respectively, which strongly supports the composition of the AgNWs/cellulose hybrid film.
The crystal structures of AgNWs and AgNWs/cellulose hybrid film were characterized using XRD, as shown in Fig. 2. It can be seen from Fig. 2A that the characteristic diffraction peaks of cellulose and AgNWs are detected in the XRD pattern, implying the successful preparation of AgNWs/cellulose hybrid film. The AgNWs were also examined by XRD as shown in Fig. 2Aa. The typical characteristic peaks located at 2θ of 38.12°, 44.28°, 64.43°, and 77.47°are attributed to the (111), (200), (220), and (222) crystalline planes of AgNWs crystals and these characteristic peaks are consistent with the reported literature (Yin et al. 2020). The sharp and intense XRD diffraction peaks of AgNWs show their highly crystalline characteristics. Figure 2Ab shows the peak of celluloseⅡ at 2θ of 19.48°and 22.59°, in addition to the characteristic peak of AgNWs detected, suggesting the AgNWs/cellulose hybrid film was successfully prepared (French et al. 2014). However, the diffraction intensities of AgNWs were slightly decreased in AgNWs/cellulose hybrid film, implying that AgNWs were successfully included in the interior hybrid film.
The FT-IR spectra are very helpful to investigate the surface functional groups and molecular interaction of hybrid film, and results are shown in Fig. 2B. The AgNWs themselves did not contain any organic functional groups (Chen et al. 2020), so they could only rely on a small amount of PVP on the surface of AgNWs to study the possible interaction between cellulose and AgNWs. From Fig. 2Ba, The FT-IR spectrum of the cellulose film shows the characteristic stretching vibration modes of -C-O, -CH2, and -CH at 1092 cm− 1, 919 cm− 1, and 838 cm− 1, respectively. Additionally, the cellulose films exhibit the typical peaks at 3355 cm− 1 and 2913 cm− 1, corresponding to the stretching vibration -OH and –CH2 groups. Figure 2Bb shows the FT-IR spectrum of AgNWs/cellulose hybrid film, indicating that all the above characteristic peaks are present in the sample of hybrid film. Moreover, as shown in the Fig. 2Bb, characteristic absorption bands at 1667 cm− 1, which can be assigned to the stretching vibration peaks of -C = O, derived from the ingredient PVP from AgNWs (Ma et al. 2019). It can be seen that the characteristic peaks of cellulose and related to the composition of the AgNWs all appeared on the hybrid film, proving that the AgNWs/cellulose hybrid film was successfully constructed.
XPS measurement was conducted to ensure the composition of the AgNWs/cellulose hybrid film. The survey XPS spectra of the as-prepared cellulose film and AgNWs/cellulose hybrid film are displayed in Fig. 3. The full XPS spectrums of cellulose film and AgNWs/cellulose hybrid film are shown in Fig. S2, which are consistent with corresponding EDS results. The peaks centered at the binding energies of 284.0, 399.1, 531.0, and 368.1 eV are attributed to C1s, N1s, O1s and Ag3d. As shown in Fig. 3Aa, for the cellulose film, the high-resolution C1s spectrum can be divided into two peaks with binding energies centered at 284.1 and 285.9 eV corresponding to the C-C and the C-O-C, respectively. The high-resolution C1s spectrum of the hybrid film is similar to that of the cellulose film, except that the C-O-C binding energy centered at 285.5 eV is lower than that of the cellulose film, which may be caused by the addition of AgNWs, as can be shown on Fig. 3Ab. Furthermore, the high-resolution N1s and O1s spectrums both display one characteristic peak at 399.4 and 531.3 eV corresponding to the N-H and the C = O, (Fig. 3B and C). For the Ag3d spectrum, two peaks at binding energies of 366.7 and 372.6 eV correspond to Ag3d5/2 and Ag3d3/2, testifying the AgNWs/cellulose hybrid film was fabricated. There are no other impurity peaks such as Ag+, and Ag2+ were detected, implying that the surface of the hybrid film was not oxidized after it was exposed toan open environment for some time. The analysis results between XPS and XRD are consistent, indicating that the AgNWs were successfully loaded on the cellulose film and the material was successfully constructed.
For applications, functional agricultural film materials should possess essential properties like flexibility, durability, thermal stability, and ability to thermal insulation properties. Therefore, the mechanical properties, thermal stabilities of AgNWs/cellulose hybrid film were investigated and the relevant results are represented in Fig. 4. For a better comparison, the mechanical properties of cellulose film and AgNWs/cellulose hybrid film were tested, and stress-strain curves of samples are shown in Fig. 4A. Both cellulose film and AgNWs/cellulose hybrid film show a trend of gradually increasing and then decreasing, the tensile strength and elongation at break of cellulose film and AgNWs/cellulose hybrid film are 0.36 MPa, 0.45 MPa, and 1.5%, respectively. Ultimately, under the same elongation, the tensile strength of the cellulose film is greater than that of the hybrid film, the result can be ascribed to the rigidity of AgNWs loaded on hybrid film and it also proves that the hybrid film material loaded with AgNWs was successfully formed.
To investigate the thermal stability of as-obtained film material, the thermal decomposition of cellulose films and the hybrid film was evaluated from room temperature to 600 ℃. The TGA curves of cellulose films and hybrid film are shown in Fig. 4B. Notably, the obvious slight weight loss inspected for the two samples in the initial stage (below 200 ℃) was possibly ascribed to the volatilization of free water adsorbed and low molecular weight compounds in samples. As the temperature continues to rise, from 200 to 400 ℃, the quality of the samples dropped rapidly, which was caused by the structural degradation of films. Then, the weight loss rate of the samples slowed down. Finally, due to decomposition of carbonaceous matter or serious damage to the specimens, cellulose film reached the full complete decomposition station at around 550 ℃ with the residue of 3%, and hybrid film reached the full complete decomposition station at around 550 ℃ with the residue of 10%. Overall, compared with cellulose film, the thermal stability of hybrid film was enhanced by introducing nanowires into the cellulose matrix. The addition of AgNWs is beneficial for enhancing the thermal stability of hybrid film, which may be attributed to the uniform dispersion of AgNWs in the hybrid film, forming an effective barrier to the diffusion of volatile or decomposable components in the product and hindering the release of these components of the hybrid film.
Light transmittance was assessed by UV-Vis as part of film characterization and also for further influence in the agricultural applications of hybrid film. Therefore, UV-Vis Spectrophotometer was carried to investigate the transmittance of cellulose film and hybrid film, as shown in Fig. 5. In the visible light range of 400–800 nm, the transmittance of the cellulose film reaches about 80% on average, demonstrating that the cellulose film is transparent. Comparing with cellulose film, the as-prepared hybrid film obtains a result of nearly 55% transmission at a wavelength of 400–800 nm, the transmittance of the hybrid film is significantly reduced. The transmittance of cellulose film is higher than the AgNWs/cellulose hybrid film, because the addition of cross-linked AgNWs has become the barrier to the penetration of light, which the result indicated hybrid film was successfully synthesized. Although the addition of AgNWs reduces the transmittance of the hybrid film, the film with 55% of the transmittance can still allow to entering most of the light sources, providing a certain amount of heat, and playing the role of growth and heat preservation.
AgNWs have high infrared reflectivity and low infrared emissivity making it an ideal material for enhancing the radiant heat insulation ability of the hybrid film. Therefore, to study the heat insulation properties of the hybrid film, the thermal image of the heating plate with the cellulose film and the hybrid film attached were studied, and the result is shown in Fig. 6. Before imaging, the cellulose film and the AgNWs/cellulose hybrid film shall be placed on the surface of a heated plate that is uniformly heated, as shown in Fig. 6A. Both specimens are in thermal equilibrium on the heating plate which temperature is set at 40°C and atmosphere temperature of 12.0°C. It is observed from Fig. 6B that the surface temperature of cellulose film (right) and AgNWs/cellulose hybrid film (left) are 34.33 and 31.26°C as detected through an IR camera, respectively. Compared with cellulose film, the surface temperature of the hybrid was lower, indicating the infrared thermal insulation properties of the hybrid film.
Facing extreme environments, plants rely on lowering the rate of heat dissipation to maintain a warm space that is not enough to withstand the sharp down temperature brought by the cold wave. Therefore, it is necessary to provide heat from the outside to protect plants from frostbite for on-demand agricultural thermal management applications. As a derivative of silver, AgNWs also possess excellent conductivity, loading in hybrid film, which can take the ability to conduct electricity to the hybrid film. As shown in Fig. S3, due to the loading of AgNWs, the hybrid film differs from the cellulose film and plastic film. It can be seen that the hybrid film has a certain resistance, while the cellulose film and the plastic film have infinite resistance examined by multimeter and are insulating materials. The temperature profiles of the AgNWs/cellulose hybrid film plotted against heating time are shown in Fig. 7, which proves the hybrid film loaded AgNWs heats up quickly in a short time, providing Joule heat. The initial temperature of the film is 17.8°C monitored by thermocouple, the current passed through the film to generate heat with a fixed voltage of 3 V. And the temperature is detected once in 30 s, an infrared image is taken, the detected temperatures are 22.5, 27.2, 33.6, 38.2, 43.5°C at 30 s, 60 s, 120 s, and 150 s, respectively, in Fig. 7a-f. The hybrid film exhibits excellent electrical heating properties, and the surface temperature of the material rises 25.7°C in just 150 s, indicating that the hybrid film can rapidly heat up to meet the temperature requirements at a low voltage.
Since the hybrid film composition containing AgNWs, the antibacterial properties were evaluated for hybrid film modified with AgNWs and cellulose film against Escherichia coli and Staphylococcus aureus bacteria. As shown in Fig. 8A and B, Escherichia coli on cellulose film without AgNWs were found (right), on the contrary, no bacteria existed around the hybrid film modified by AgNWs (left), forming a circular antibacterial zone. Similar situation, Staphylococcus aureus on cellulose film (right) unmodified by AgNWs were found, and no bacteria existed around the hybrid film modified by AgNWs (left), also forming a circular antibacterial zone, showing in Fig. 8C and D. Clearly, the AgNWs/cellulose hybrid film showed both bacteriostatic and bactericidal action against bacteria, which can be attributed to the addition of AgNWs destroying the living environment and state of bacteria. Therefore, the AgNWs/cellulose hybrid cellulose film not only can achieve the effect of double insulation but also can resist harmful bacteria from the outside world, the insulation and antibacterial process was shown in Scheme 2.