3.1 Preparation of ZnO/HNTs and ZnO/HNTs/PDMS@Cotton
The preparation process of the ZnO/HNTs and ZnO/HNTs/PDMS@Cotton was schematically shown in Fig. 1. First, the Zn2+ ionized by Zn(NO3)2.6H2O in the aqueous solution attracts water molecules to forme the solvent unit Zn(H2O)2. The solvent unit releases H+ in an alkaline environment to maintain the coordination number to generate the precursor Zn(OH)2. Zn(OH)2 combined with Al-OH and Si-OH groups on the inner and outer walls of HNTs nanotubes by hydrogen bonding. The active sites on the inner and outer surfaces of the HNTs nanotubes can play a role in dispersing the precursors and prevent the precursors from agglomerating. Subsequently, the precursor-loaded HNTs nanotubes were calcined at a high temperature of 450°C to obtain ZnO/HNTs nanocomposites. Then, the pristine cotton fabric was dipped into micro/nano hierarchical ZnO/HNTs hybrid particles dispersion to obtain ZnO/HNTs@Cotton. The ZnO/HNTs hybrid particles dispersion can endow fibers desirable roughness, which could generate micro/nano hierarchical structure combined with the natural morphology of the cellulose fiber. Finally, the ZnO/HNTs@Cotton was modified by PDMS to obtain superhydrophobic cotton fabric,which can lowered the surface tension of ZnO/HNTs@Cotton.
3.2 Characterization of ZnO/HNTs and superhydrophobic cotton fabrics
The surface morphology of HNTs, ZnO/HNTs, PDMS@Cotton and ZnO/HNTs/PDMS@Cotton were observed by SEM. As illustrated in Fig. 2A, HNTs showed a homogeneously nanorod-like structure with a length and a diameter range of 0.7-1.5 µm and 30-50 nm, respectively. Compared to nanorod-like structures of the HNTs, the ZnO/HNTs hybrid particles exhibited unique rod-dot micro/nano hierarchical structures when the ZnO nanoparticles were introduced on surfaces of HNTs (Fig. 2C, D). PDMS@Cotton fabric with relative smooth surface, but there were still special folds and grooves in the longitudinal direction (Fig. 2E, F). In contrast, the ZnO/HNTs/PDMS@Cotton became much rougher, with particulate protrusions compacted on the cotton fabric surface (Fig. 2G). Moreover, the higher magnification SEM image of ZnO/HNTs/PDMS@Cotton shown in Fig. 2H revealed that ZnO/HNTs coating was covered with a thin layer of PDMS coating.
For further investigation of coated cotton fabric, the chemical components of the untreated cotton fabric, PDMS@Cotton, and ZnO/HNTs/PDMS@Cotton were characterized by EDS and FTIR. As shown in Fig. 2J, the C, O, Si and Zn elements were distributed on the surface of the coated fabric, suggesting the successful coating of ZnO/HNTs and PDMS on coated fabric surface and ZnO/HNTs/PDMS@Cotton was successfully prepared. In addition, the uniform distribution of C, O, Zn and Si elements indicated that ZnO/HNTs hybrid particles and PDMS were evenly covered on cotton surface (Fig. 2I).
As shown in Fig. 3A, in FT-IR spectrum of pristine cotton fabric, it could be clearly observed from the infrared spectrum that a strong and broad absorption peak appeared at 3332 cm−1, which was the characteristic peak of the stretching vibration of the hydroxyl group in the cellulose macromolecule of the pristine cotton fabric. Two characteristic absorption peaks can be observed at 2893 cm−1 and 1310 cm−1, namely -CH3 stretching vibration and bending vibration absorption. The strongest absorption peak at 1027 cm−1 originated from the flexural vibration of the hydroxyl group (-OH) in the cotton cellulose macromolecule and the C-O-C stretching vibration absorption. When the cotton fabric was coated with PDMS, new peaks at 2967 cm−1, 1258 cm−1, and 783 cm−1 occurred, which were assigned to –CH3 stretching vibration, Si–C bend vibration and Si–O–Si symmetrical stretching vibration, respectively, which were due to a lot of -CH3 groups and Si–O–Si groups of PDMS. The appearance of these characteristic peaks indicated that PDMS had been successfully grafted onto cotton fabrics. Compared with PDMS@Cotton, there were no obvious new bands from FT-IR spectra of ZnO/HNTs/PDMS@Cotton.
The thermal stability of the ZnO, ZnO/HNTs, pristine cotton fabric, PDMS@Cotton and ZnO/HNTs/PDMS@Cotton were investigated by thermo gravimetric analysis. As shown in Fig. 3B, there was 11% weight loss at temperatures below 350°C in the case of the pristine cotton fabric, and the weight loss was 13% for PDMS@Cotton and 10% for ZnO/HNTs/PDMS@Cotton, respectively. The weight loss of the samples from room temperature to 350 ℃ was small, mainly due to the water volatilization and impurities decomposition which absorbed on cotton fabric surface. From 350℃ to 420℃, the weight loss of pristine cotton fabric was 78.5% because of the decomposition of cellulose. Untreated cotton fabric showed residual weight rate of about 0.01% after being heated to 700℃ because of the residual groups (such as -CH2-, etc.) decomposition. It was indicated that there was little pristine fabric left. In contrast, the remaining weight percentage of PDMS@Cotton was 2.3%, which was higher than 0.3% of pristine cotton fabric. This was mainly caused by the incomplete decomposition of PDMS at high temperature. However, for ZnO/HNTs/PDMS@Cotton, the residual weight percentage reached to 9.1%, showed the improved thermal stability. As mentioned above, ZnO/HNTs hybrid particles had excellent thermal stability. Thereby, TG result of ZnO/HNTs/PDMS@Cotton revealed the successful incorporation of ZnO/HNTs hybrid particles and PDMS on the cotton fabric surface.
3.3 Wettability of superhydrophobic cotton fabrics
Surface roughness and chemical composition are two key factors for preparation of superhydrophobic surface. Therefore, the controlled variable method was adopted to study the wettability of coated cotton fabric by changing the content of Zn(NO3)2.6H2O and PDMS. As displayed in Fig. 4A, the HNTs/PDMS@Cotton exhibited hydrophobicity with a water contact angle of 143.9±1.2°. This result indicating that the only HNTs and PDMS were unable to endow pristine cotton fabric superhydrophobicity. With the mass fraction of Zn(NO3)2.6H2O increasesed to 2 wt. %, the ZnO/HNTs/PDMS@Cotton displayed the superhydrophobicity with a WCA of 162.5±1°. This result can be explained that the cotton fabric surface was composed of hierarchical micro/nano structures with higher roughness after modified by ZnO/HNTs hybrid particles. However, the superhydrophobicity of ZnO/HNTs@Cotton was negatively affected by the further increase of Zn(NO3)2.6H2O mass fraction (6 wt. %), the WCA was 150.2±0.7°. This may be caused by the slight disappearance of the micro/nano hierarchical structure of ZnO/HNTs/PDMS@Cotton due to excessive Zn(NO3)2.6H2O, resulting in a decrease in roughness. Therefore, Zn(NO3)2.6H2O with mass fraction of 2 wt. % was the optimal choice for subsequent experiments.
Similarly, in Fig. 4B, for only the ZnO/HNTs hybrid particles were employed to the pristine cotton fabric, the ZnO/HNTs@Cotton was easily wetted due to the inherent hydrophilic properties of the ZnO/HNTs hybrid particles and cellulose. The WCA of ZnO/HNTs@Cotton increased dramatically from 0° to 148.1±1° even if only 5% of PDMS was introduced. This was because the introduction of PDMS lowered the surface tension of ZnO/HNTs@Cotton. As the PDMS mass fraction was up to 15%, the WCA of ZnO/HNTs@Cotton increased to 162.5±1° and the rolling angle dropped to 4°. However, the WCA of ZnO/HNTs@Cotton decreased to 152.4±0.8° when the mass fraction of PDMS reached 25%. This was because excessive PDMS completely cover ZnO/HNTs hybrid particles hierarchical micro/nano structure, which made the surface of ZnO/HNTs/PDMS@Cotton too smooth to construct roughness.
To further evaluate superhydrophobic property of the coated cotton fabric, the Cassie-Baxter model was used to explain the influence of hierarchical rough surface on wettability. According to Cassie-Baxter’s equation, the apparent contact angle on the surface(θ*) is calculated as
where θ presents the Young’s contact angle that is obtained on the slippy surface with the same composition, r is the actual wetted area divided by the projected wetted area of the surface, f is the fraction of the projected area of the solid surface contacting the liquid.
In Equation (1), r is greater than 1, f is less than 1, and both are positive. The θ < 90° is due to the natural hydrophilic properties of cotton fabric. Therefore, if f is small enough, θ*can be greater than 150° according to equation (1). One strategy to minimize the f value is to increase the surface roughness of the material, such as flower-like structure, and micro/nano re-entrant structure. As shown in Fig. 5A, after HNTs and PDMS coating, HNTs/PDMS@Cotton showed hydrophobicity with a WCA of 143.9°. Compared with the microrod-like HNTs stacked coating, the roughness factor of ZnO nanoparticles has increased, showing a certain degree of superhydrophobicity with a water contact angle of 152.4° (Fig. 5B). In this paper, a micro-/nano hierarchical re-entrant structure was constructed by depositing ZnO nanoparticles onto the rod-like HNTs surfaces. As displayed in Fig. 5C, the deposition of ZnO nanoparticles further increases the surface roughness of ZnO/HNTs hybrid particles, resulting in a significant reduction in the liquid/solid contact area. The coated cotton fabric displayed excellent superhydrophobicity with a water contact angle of 162.5° due to could trap the more air layer.
The surface wettability of superhydrophobic cotton fabrics were analyzed by evaluating their static/dynamic wettability performances and antifouling ability. As shown in Fig. 6A, different liquid droplets including blue colored water, juice, and cola were dropped on pristine cotton fabric and ZnO/HNTs/PDMS@Cotton. These aqueous droplets collapsed on the untread cotton fabric due to its hydrophilic nature. In contrast, the ZnO/HNTs/PDMS@Cotton could hold on all liquid droplets in a spherical shape without any penetration. When pristine cotton fabric and ZnO/HNTs/PDMS@Cotton were immersed in water, photographs of cotton fabrics were shown in Fig. 6B. The untreated cotton fabric promptly sank to the bottom of water. However, ZnO/HNTs/PDMS@Cotton can float on water without any wetting. As illustrated in Fig. 6C, when the fixed ZnO/HNTs/PDMS@Cotton was immersed in water, a sliver mirror surface was observed due to the existence of an air cushion surrounded by the coating. Additionally, a jet of red water could easily bounce off from the coated cotton fabric surface without leaving a trace. In contrast, a jet of red colored water easily wet on pristine cotton fabric (Fig. 6D, E).
3.4 Durability evaluation of the superhydrophobic cotton fabrics
In practical applications, most of superhydrophobic coatings easily lose their superhydrophobicity under harsh conditions, such as mechanical abrasion and UV irradiation. As illustrated in Fig. 7A, after 2000 abrasion cycles, the WCA of ZnO/HNTs/PDMS@Cotton showed only slight change, still higher than 150°. This result indicated that ZnO/HNTs hybrid particles with silicone elastomer PDMS coating still maintained excellent superhydrophilicity after mechanical abrasion although ZnO/HNTs hybrid particles of the fiber surface was partially destroyed (Fig. 7B). This was attributed to PDMS, which not only lowered the surface energy of the coating, but also formed a dense film on the surface of the cotton fabric coating and bonded the ZnO/HNTs hybrid particles tightly on the surface of the cotton fiber to enhance the mechanical durability of superhydrophobic cotton fabric. In addition, as presented in Fig. 7C the WCAs of superhydrophobic cotton fabric remained around 150° after 150W UV irradiation for 90h, showing good UV-durability. From Fig. 7D, typical micro/nano hierarchical structure was retained on the microscale fiber surface of ZnO/HNTs/PDMS@Cotton after UV irradiation for 90h. This was mainly due to the anti-ultraviolet properties of ZnO/HNTs hybrid particles and the strong bond energy of the -Si-O-Si- bond contained in the PDMS layer, which made PDMS resistant to UV decomposition Moreover, the chemical stability of the coated cotton fabric was also evaluated. The influence of acid and alkali on the wettability of the superhydrophobic cotton fabric was tested by immersing in different pH value solutions ranging from 1 to 14 for 1 h. As displayed in Fig. 7E, the WCA of ZnO/HNTs/PDMS@Cotton after dipping in solution with pH value varying from 1 to14 still kept higher than 151°. As shown in Fig. 7F, there were still hierarchical rough morphology on ZnO/HNTs/PDMS@Cotton fiber surface. These results indicated that the prepared cotton fabric showed great potential in arousing consumer interest and large-scale industrial applications.
3.5 Self-cleaning antifouling performance
Self-cleaning behavior refers to the phenomenon that various contaminants on solid surfaces can be eliminated under natural circumstance. To investigate self-cleaning of the cotton fabric samples prepared in this study, a layer of methyl orange powder was used as contaminant. When a continuous process of water droplets was dropped onto ZnO/HNTs/PDMS@Cotton surface, they quickly rolled off and picked up the dissolved methyl blue powder without adhering to it (Fig. 8A). Consequently, the preferable superhydrophobic self-cleaning ability of the ZnO/HNTs/PDMS@Cotton was confirmed.
Moreover, to demonstrate the excellent antifouling ability of the superhydrophobic cotton fabric, liquid pollutants experiments were carried out. As presented in Fig. 8B, the ZnO/HNTs/PDMS@Cotton was immersed in red colored aqueous solutions and then taken out. The superhydrophobic cotton fabric still maintained dry without any residue due to the presence of a layer of air between the surface and the liquid, which protects the cotton fabric from wetting.
3.6 Photocatalytic performance
To investigate the photocatalytic degradation of samples, photodegradation experimental toward MB was carried out. Fig. 9A displayed the photodegradation efficiencies of ZnO, ZnO/HNTs, ZnO/HNTs@PDMS, and ZnO/HNTs/PDMS@Cotton toward MB under UV-vis irradiation. Fig. 9B given the UV–vis absorption spectra of MB in ZnO/HNTs/PDMS@Cotton photocatalytic process. As can be seen in Fig. 9A, the ZnO photodegradation efficiencies reached 98% after UV-vis irradiation about 1.5 hours. While the absorption bands of MB solution in the presence of the ZnO/HNTs hybrid particles decreased significantly and 96.7% photodegradation efficiencies was obtained after UV-vis irradiation for 0.5 hours. The excellent photocatalytic performance of the ZnO/HNTs hybrid particles may be ascribed the active sites on the inner and outer surfaces of HNTs hinder the agglomeration of precursors to form smaller-sized ZnO nanoparticles and HNTs support as adsorbent. Compared with ZnO/HNTs photocatalyst, ZnO/HNTs@PDMS and ZnO/HNTs/PDMS@Cotton takes longer time to achieve the similar photocatalytic efficiencies. Interestingly, ZnO/HNTs@PDMS mostly floated on the surface of methyl blue dilution because of its excellent superhydrophobicity. Undoubtedly, this great superhydrophobicity of ZnO/HNTs@PDMS negatively affected its degradation effect in this experiment. When ZnO/HNTs@PDMS was coated on cotton fabric, its photocatalytic efficiencies were reduced. This was because the contact area between ZnO/HNTs/PDMS@Cotton and methylene blue was smaller than that of ZnO/HNTs@PDMS. As can be seen in Fig. 9B, in 0-3 h, the degradation rate of ZnO/HNTs/PDMS@Cotton to MB gradually decreases. This can be attributed to the fact that in the range of 0-3h, as the degradation reaction progresses, the MB concentration in the system gradually decreased, resulting in the gradually decreased of the contact between ZnO/HNTs/PDMS@Cotton and MB. When the degradation reaction progressed to 3h, the degradation rate of ZnO/HNTs/PDMS@Cotton to MB gradually increased. This can be explained by the fact that as the degradation reaction progresses, the hydrophobicity of ZnO/HNTs/PDMS@Cotton was getting worse and the contact surface with MB gradually becames larger, and this effect was greater than the reduction of MB concentration in the system on the degradation reaction.
3.7 Oil-water separation of the superhydrophobic cotton fabrics
The ZnO/HNTs/PDMS@Cotton possessed both superhydrophobic and superoleophilic characteristics and had a certain attraction for the separation of oil/water mixtures. Herein, dichloromethane was used to measure the oil/water separation ability of the ZnO/HNTs/PDMS@Cotton. Fig. 10A shown the separation processes of dichloromethane by the as-prepared cotton fabric. As expected, while the dichloromethane/water mixture was poured into the filter apparatus, yellow dichloromethane could permeate through the superhydrophobic cotton fabric filter membrane instantaneously due to gravity, leaving only the water above the filter. After 10 cycles of separation, the superhydrophobic cotton fabric still exhibited high separation efficiency, and the separation efficiency and flux of dichloromethane/water mixture was up to 94.6% and 636 Lm−2s−1, indicating excellent recyclability (Fig. 10B). As displayed in 10C, with the increase of oil/water separation cycles, WCA of ZnO/HNTs/PDMS@Cotton decreased slightly but remained above 150°. These results above mentioned demonstrated that ZnO/HNTs/PDMS@Cotton showed wide application prospects in the field of oil/ water separation.
3.8 UV-shielding property of the superhydrophobic cotton fabrics
Figure 10 shows the ultraviolet transmittance spectra and UPF values of pristine cotton fabric, PDMS@Cotton, and ZnO/HNTs/PDMS@Cotton. From Fig. 11, pristine cotton fabric and PDMS@Cotton displayed similar transmittances curves and didn’t show UV shielding performance. In comparison, pristine cotton fabric and PDMS@Cotton, ZnO/HNTs/PDMS@Cotton exhibited high UV shielding effect and UPF value significantly improved to 1643.28 and T(UVA) and T(UVB) were only 0.3% and 0.05%, respectively. This result was attributed to strong ultraviolet-absorption, light scattering and frequent light reflection of ZnO nanoparticles in ZnO/HNTs hybrid particles coated on cotton fabric (Momen et al. 2012).