3.1 Characterization of cotton/pDA/MnO2
The morphology of the pristine cotton, pDA coated cotton, and cotton/pDA/MnO2 were detected by a scanning electron microscope. It can be seen from Fig. 3a that untreated pristine cotton exhibits a smooth surface, and the ribbon-like profile of cotton fibers are clearly observed. In Fig. 3b, the deposition and polymerization of dopamine has resulted in a rough surface of the fibers. The evenly distributed nanoparticles can be observed from Fig. 3c, and this is due to the incorporation of MnO2 into the deposited pDA film on cotton fibers. In the SEM image captured under a high magnification (Fig. 3d), MnO2 NPs are densely deposited on the surface of the cotton fibers without obvious aggregations. After the in situ growth process, the color of the cotton/pDA sample shifted from brown to black due to the loading of MnO2 NPs (Insets of Fig. 3).
The EDX mapping (Fig. 3e) and spectrum (Fig. 3f) of the cotton/pDA/MnO2 show four peaks for carbon (C), oxygen (O), manganese (Mn) and potassium (K) elements with the weight percentage of 22.26%, 54.78%, 18.91% and 4.05%, respectively. The high content of Mn element indicates that large amounts of MnO2 NPs have been anchored onto the fabric surface. The dense loading of MnO2 NPs is due to the abundant phenolic hydroxyl groups of pDA through chelation effects, which promotes the uniform growth of MnO2 NPs in the pDA film. These SEM and EDS results demonstrate that MnO2 NPs have been uniformly dispersed on the surface of cotton fabrics.
The chemical structures of the pristine and nanocomposite deposited cotton fabrics were characterized by FTIR spectroscopy in the spectral ranges of 400 to 4000 cm−1. The FT-IR spectra are shown in Fig. 4a. It can be observed that pristine cotton exhibits an obvious wide peak at 3330 cm−1 arising from –OH stretching vibration in hydroxyl groups. The peak at 2900 cm−1 corresponds to –CH bending absorption bands in methylene groups. The peaks at 1720 cm−1 and 1435 cm−1 are assigned to C=O stretching vibration and –CH2 bending vibration, respectively. The peaks at 1330 cm−1 and 1030 cm−1 are resulted from O–H in-plane deformation vibration and C–O stretching vibration, respectively (Ran et al. 2021; Cheng et al. 2021). Similar characteristic peaks are appeared in the FTIR pattern of cotton/pDA. Besides, the extra peak at 1660 cm−1 is due to the stretching vibration effects of benzene ring groups, which can be attributed to the polymerization of dopamine on cotton surface (Ran et al. 2019). In addition, the intensity of the absorption peak at 3330 cm−1 is stronger than the original cotton, which is mainly due to the increase of reactive OH groups on the fiber surface after modification with dopamine. After MnO2 is anchored on the surface of cotton fibers, the pristine characteristic peaks of cotton have been obviously weakened, and the characteristic peak for MnO2 appears. For example, the peak (725 cm−1) detected below 800 cm−1 is attributed to metal-oxygen (Mn–O) stretching (Li et al. 2011). These results have confirmed the successful modification of cotton substrate through pDA templating and MnO2 nanocomposite deposition.
The crystalline structure of the different cotton samples was investigated by XRD (Fig. 4b-c). In Fig. 4b, the pristine cotton exhibits four obvious diffraction peaks at 2θ angle of 14.1o, 16.5o, 22.5o and 34.2o, which are attributed to the crystal faces (1-10), (110), (200), and (004) of cellulose Iβ, respectively (French et al. 2013; French 2014; Cheng et al. 2018). The cotton/pDA fabrics present a similar XRD pattern to pristine cotton, suggesting the negligible effects of the pDA layer on the crystalline structure of cotton. According to reported studies, this is mainly due to the amorphous structure of pDA (Wang et al. 2018; Cheng et al. 2020). With respect to cotton/pDA/MnO2 (Fig. 4c), the characteristic peak intensity of pristine cotton phase is decreased, which is possibly due to the growing of MnO2 NPs on the surface of cotton fabrics. Furthermore, four extra diffraction peaks at 2θ = 12.1°, 24.8°, 36.6° and 65.4° corresponding to the characteristic peaks (001), (002), (006) and (119) crystal planes of δ-MnO2 (JCPDS No. 18-0802), respectively can be observed (Ma et al. 2019; Zhang et al. 2019). The XRD results proved the presence of δ-MnO2 NPs on the surface of cotton fabrics.
The surface chemical state of the as-prepared samples was further investigated by XPS. The XPS wide scan spectrum of cotton/pDA/MnO2 proved the presence of C, O, Mn, and K elements on the cotton/pDA/MnO2 (Fig. 5a). The K element is from the reactant KMnO4, which is in good agreement with EDX results. The high resolution C1s spectrum in Fig. 5b can be deconvoluted into three peaks at 288.1 eV, 286.2 eV and 284.8 eV corresponding to the C=O, C-O/C-H, C-C bonds, respectively (Qi et al. 2021). Mn 2p XPS core level spectrum is shown in Fig. 5c, in which two peaks at 653.7 eV and 642.2 eV are attributed to Mn2p 1/2 and Mn2p 3/2 of Mn4+, respectively (Miao et al. 2021). Additionally, the spin-energy separation of 11.5 eV has agreed well with previously measured spectra of MnO2 (Lu et al. 2017; Shi et al. 2018). The O1s spectrum can be fitted into three peaks at 533.7 eV, 531.5 eV, 529.7 eV, which are attributed to C-O/C=O, Mn-O-H, and Mn-O-Mn, respectively (Fig. 5d) (Wang et al. 2016). XPS analysis indicated that MnO2 NPs were successfully coated on the cotton fabric surface by in-situ growth.
3.2 Thermal properties
The thermal properties of cotton, cotton/pDA, and cotton/pDA/MnO2 were characterized by TG analysis, as shown in Fig. 6. It can be observed that the residual weight of all samples exhibited a slight decrease in the initial stage (as shown the TG curves in Fig. 6a), which can be attributed to the water evaporation phenomenon. With the heating temperature of increasing to higher than 400°C, the residual weight decreased rapidly with only 10% residue at 700°C. Compared with the pristine cotton, the cotton/pDA shows a similar thermal decomposition behavior due to the amorphous structure of pDA. The residual weight of the cotton/pDA is slightly higher than original cotton. After loading of MnO2 NPs, however, the TG curve is totally different from either cotton or cotton/pDA. The residual weight of cotton/pDA/MnO2 is 40.8% higher than cotton/pDA, and it has proven the excellent thermal stability of cotton/pDA/MnO2. The enhanced thermal stability can be attributed to the enhanced carbonization and invigoration effect of cellulose chain as a result of MnO2 NPs deposition.
According to DTG curves (Fig. 6b), the maximum thermal degradation temperature (DTGmax) of cotton fabric and cotton/pDA is very similar (355°C and 354°C, respectively). This is mainly because the amorphous structure of pDA has little influence on the thermal properties of cotton fibers. For cotton/pDA/MnO2 sample, the DTG curve has two main weight loss stages occurring over 200 ℃. The first DTGmax representing the decomposition of polymers was detected at 233 ℃, which was much lower than the original cotton sample. This is mainly due to the existence of MnO2 NPs, as the catalytic nanoparticles has reduced the activation energy and accelerated the depolymerization reaction. The second DTGmax was at 378 ℃, which was related to phase transition from MnO2 to Mn3O4 by the losing of oxygen from MnO2 lattice (JCPDS No. 24-0734) (Yang et al. 2021).
3.3 HCHO adsorption property
The HCHO adsorption test of the cotton/pDA/MnO2 was carried out in a sealed quartz reactor. In the presence of the as-prepared fabrics, the concentration of HCHO decreased from 22.7 ppm to 0 (Fig. 7a). The drop of HCHO concentration is due to the complete absorption of the HCHO by the coated MnO2 NPs. The experiment was repeated again after the sample had been illuminated by UV lamp for 3h, and it was found that HCHO could be completely adsorbed in about 20 min after three cycles. This result indicates that the cotton/pDA/MnO2 has good stability for adsorbing HCHO, and the sample can be recycled.
As shown in Fig. 7b, due to the higher concentration of HCHO, the removal efficiency of HCHO gradually increased and the adsorption rate of cotton/pDA/MnO2 was faster at the initial stage. With the removal of HCHO, the adsorption rate became slower, and finally the HCHO was completely adsorbed in about 20 min.
In order to evaluate the effect of air circulation on the adsorption of HCHO by the cotton/pDA/MnO2, the parameter of air flowing rate was introduced by turning on a fan mounted on the floor of the reactor. As shown in Fig. 7c, the concentration of HCHO from the 1 mL liquid formaldehyde after gasification was slightly less than 22.7 ppm after adding the fan. This phenomenon is due to the accelerated movement of HCHO molecules that leads to the backflow of HCHO. Obviously, the rate of HCHO adsorption by cotton/pDA/MnO2 is faster after adding a fan, as the HCHO molecules will move faster to promote absorption.
MnO2 has been reported as a catalyst for decomposing HCHO. Herein, the efficiency of cotton/pDA/MnO2 photocatalytic degradation of HCHO under the condition of sunlight and ultraviolet lamp was evaluated by the yield of CO2 (Fig. 7d). At the initial stage, the concentration of CO2 was about 1000 ppm due to the presence of CO2 in the air. Under the irradiation of sunlight and UV lamp, the concentration of CO2 was increased with the irradiation time, and finally reached the maximum threshold of the detector (5500 ppm). In details, the concentration of CO2 under sunlight irradiation rose faster and took less time to reach the maximum value than that under UV irradiation. The high solar intensity and temperature (2021.06.24, 3:00 p.m., 36°C, Wuhan, China) of sunlight accelerated the decomposition of HCHO molecules, thus leading to the high efficiency of photocatalytic decomposition of HCHO. On the other hand, the concentration of CO2 under UV irritation was slow at first and then increased sharply. The temperature in the closed environment was low at the beginning of UV irradiation, resulting in a relatively slow decomposition rate of HCHO molecules. After irradiation for a certain time, the temperature rose and the decomposition of HCHO molecules was accelerated. Overall, cotton/pDA/MnO2 demonstrated its potential of being used as a catalyst to decompose the adsorbed HCHO into CO2 under the irradiation of either sunlight or UV lamp.
3.4 Proposed reaction mechanism
A possible reaction mechanism of HCHO removal on the cotton/pDA/MnO2 is proposed here as shown in Fig. 8. Firstly, the surface of MnO2 catalyst is oxidized by O2 in the air to form surface adsorbed oxygen, which then oxidizes HCHO as-adsorbed on the catalyst surface to methylene dioxygen (DOM) (as shown in Eq. ①) (Lin et al. 2019; Wang et al. 2020). Then, the DOM will be rapidly transformed into formate (HCOO-) species on the cotton/pDA/MnO2 (as shown in Eq. ②) (Huang et al. 2021). Finally, the formate species decompose into CO, which further oxidizes into CO2 (as shown in Eq. ③) (Sun et al. 2019; Ye et al. 2020). The excellent HCHO oxidation performance of the cotton/pDA/MnO2 catalyst is mainly due to the following two aspects. First, cotton fabrics have a large specific surface area and porous structure, which is conducive to HCHO diffusion to catalyst surface. Besides, poly(dopamine) is structurally similar to natural melanin pigment, which can absorb sunlight and facilitate the decomposition of formate species into CO2. In summary, the cotton/pDA/MnO2 can adsorb HCHO, and followed by the reutilization.