Conductive and self-cleaning composite membranes from corn husk nanofiber embedded with inorganic fillers (TiO₂, CaO, and eggshell) by sol–gel and casting processes for smart membrane applications

2.1 Materials and methods

Corn husk fiber was collected from the local market in Bangkok. Commercial TiO₂, CaO, and sodium hydroxide (NaOH) were purchased from Ajax Finechem in Bangkok, Thailand. Eggshells were collected from the local cafeteria in Bangkok, Thailand. The eggshells were classified using an eggshell classifier to separate them from the eggshell membrane. Then, the eggshells were ground using a rapid mill for 1 h to obtain eggshell powder. PVA was purchased from Chemipan Co., Ltd. in Bangkok, Thailand. Uncoated paper was used as a reference sample in this study.

2.2 Instruments

2.3 Preparation of corn husk nanofiber combined with inorganic fillers as a 3D biocomposite membrane

The corn husk was cleaned and dried at room temperature for 2–3 days, as shown in Figure 1a and b. The dried corn husk had a light-pale yellow color and was ground using a rapid mill to obtain finely ground corn peel fiber, as shown in Figure 1c. The corn husk fiber was then mixed with water at a ratio of 10:1, and then, NaOH (10 wt%) was added. The mixture was heated at 90°C. The choice of treatment in a 10 wt% alkaline NaOH aqueous solution was based on the efficient cleaning of the corn husk to obtain cleaned cellulose nanofiber. This treatment aimed to eliminate undesired accumulated residues, reduce stiffness, and remove noncellulosic components such as lignin, fatty acids, and resins, resulting in more accessible cellulose fibers. In addition, the Na⁺ ions bonded within the cellulose fiber nanostructure to form metal alkoxide, facilitating further sol–gel processes, as shown in Scheme 1. The resulting corn husk nanofiber treated with NaOH exhibited a dark brown color and had a soft and fine texture. The treated corn husk nanofiber was thoroughly washed with tap water multiple times to remove the NaOH solution. Subsequently, the obtained corn husk nanofiber was utilized as a raw material to create a biocomposite membrane, as illustrated in Figure 2.

Figure 1

(a) Corn husk, (b) dried corn husk, and (c) ground corn husk fiber.

Figure 2

(a) Corn husk fiber, (b) sieve the corn-husk plate (32 × 23 cm) using aluminum wire mesh no. 180 for the formation of a 3D composite membrane, (c) preparation of a corn husk membrane, (d) formula (1), (e) formula (5), and (f) formula (7).

Scheme 1

Chemical structures of corn husk nanofiber with/without the addition of additives and PVA.

Figure 3

FTIR spectra of the following raw materials: commercial uncoated white paper (80 g), corn husk nanofiber, CaO, TiO₂, and eggshell powder.

The 3D biocomposite membrane samples were prepared based on the formulas listed in Table 1. The corn husk nanofiber served as the raw material for creating the 3D biocomposite membrane through sol–gel and casting processes. A square sieve was employed to control membrane thickness (80 g), as depicted in Figure 2b. A total of seven formulas were prepared, varying in the types of fillers used, including commercial CaO, commercial TiO₂, and eggshell powder acting as a source of CaCO₃. NaOH was utilized for the treatment of corn husk, removing substances other than cellulose. In addition, NaOH served as an alkaline catalyst, facilitating the transformation from sol to gel in two main reactions: hydrolysis and condensation. The casting process was employed as an innovative method for the preparation of the biocomposite membrane. For all formulas, a 5 wt% solution of PVA was used as a binder to form the biocomposite membrane. Formula (1) served as the reference sample.

3.1 Characteristics and physical properties of a 3D biocomposite membrane composited of corn husk nanofiber and inorganic fillers

The physical properties, including bulk density, true density, water absorption, contact angle values at initial time (t _0 s) and at t _12,000 s, of the biocomposite membrane samples were tabulated in Table 2. The uncoated white paper (reference sample, 80 g) exhibited bulk density, true density, water absorption, contact angle (t _0 s), and contact angle (t _12,000 s) values of 0.22 ± 0.03 g·cm⁻³, 0.50 ± 0.08 g·cm⁻³, 54.79 ± 10.34%, 9.42 ± 0.88°, and 1.25 ± 0.21°, respectively. After treating the corn husk nanofiber with 10 wt% NaOH, the resulting bulk density, true density, water absorption, contact angle at (t _0 s), and contact angle (t _12,000 s) values were 0.09 ± 0.03 g·cm⁻³, 0.48 ± 0.10 g·cm⁻³, 80.81 ± 4.05%, 9.77 ± 0.88°, and 1.25 ± 0.21°, respectively. The morphology of the corn husk, including fiber length (1.70 ± 0.2 mm), fiber diameter (21.89 ± 3.0 mm), cell wall thickness (7.63 ± 2.30 μm), and lumen width (6.63 ± 3.50 μm), was consistent with the results reported by Fagbemi et al. [56]. The corn husk nanofiber treated with 10 wt% NaOH and supplemented with 5 wt% PVA (formula (1)) exhibited bulk density, true density, water absorption, contact angle (t _0 s), and contact angle (t _12,000 s) values of 0.07 ± 0.00 g·cm⁻³, 0.48 ± 0.10 g·cm⁻³, 64.12 ± 4.91%, 10.20 ± 0.47°, and 2.00 ± 0.94°, respectively. The use of NaOH served two functions: as a basic catalyst and as an activator for the corn husk nanofiber, enabling the chemical reaction during the sol–gel and casting processes. This resulted in a biocomposite membrane with desirable characteristics such as purification, low cost, antimicrobial properties, and good mechanical and electrical properties. The addition of PVA as a binder increased viscosity, aiding in gelation and crystal growth, leading to an increased contact angle, decreased water absorption, and reduced bulk density. The precursor sol of biocomposite membrane underwent activation with a basic catalyst, dispersing in suspension through chemical reactions to transform and crosslink into a gel and solid membrane. This involved processes such as hydrolysis, polycondensation, gelation, aging, drying, densification, and crystallization. Among the various processes, the casting process was selected for this study due to its affordability, compatibility with room temperature conditions, suitable for thin and thick films, and versatility in shaping products depending on the size and shape of molds. The prepared biocomposite membrane exhibited a thickness comparable to commercial uncoated paper with a mesh size of 80 g, while having lower bulk and true density. Formula (1) with the addition of 5 wt% PVA showed lower water absorption compared to the corn husk nanofiber treated with 10 wt% NaOH without PVA and no filler. Wang et al. conducted a study on the preparation of smart food packaging using bio-based nanomaterials or biopolymers such as polylactic acid, polyhydroxyalkanoates, polycarprolactone, PVA, reinforced with nanofiber such as paper, vegetable fiber, flax, hemp, rice straw, bamboo fiber, and corn husk, incorporated with nano clay and ZnO using conventional plastic processes such as compression molding and injection molding [42]. They found that their food packaging exhibited high performance in mechanical properties, gas barrier properties, antimicrobial properties, food preservation, and extended shelf-life [42]. Therefore, the prepared 3D biocomposite membrane, supplemented with 5 wt% PVA and inorganic fillers, has the potential to be a suitable candidate for smart food packaging and other applications such as coating, encapsulation, wound dressing, and goods transportation. The addition of TiO₂ to the biocomposite membrane resulted in characteristics consistent with biodegradation, hydrophobicity, self-cleaning, and improved gas penetration, and barrier properties, as observed in other studies [43,44]. Furthermore, the inclusion of inorganic fillers influenced the bulk and true density of the biocomposite membrane depending on the microstructures and density values of the chosen fillers.

The FTIR spectra of raw materials, including corn husk nanofiber, CaO, TiO₂, and eggshell powder, were measured and compared for the preparation of the 3D biocomposite membrane, as shown in Figure 3. The FTIR spectra of CaO and eggshell powder exhibited broadband peaks corresponding to chemical functional groups such as C═O, O–H, and Ca–O at wavenumber 1421.32, 3689.42, and 556.15 cm⁻¹, respectively. These findings are consistent with the FTIR results reported by Hossain et al. [57]. CH₂ and ═CH bending were observed in the wavenumber range of 600–800 cm⁻¹, while a small peak at wavenumber 2,890 cm⁻¹ indicated C–H stretching. The chemical functional groups of Ti–O, Ca–O, and Ca–C–O appeared at low wavenumbers within the range of 500–700 cm⁻¹ [57,58,59,60]. The FTIR spectra of the corn husk fiber composited with inorganic fillers were compared to the FTIR spectrum of commercial uncoated white paper, which served as a reference (Figure 4). In addition, the obtained FTIR spectrum of the corn husk nanofiber was consistent with the FTIR result reported by Zheng et al. [25].

Figure 4

A comparison of FTIR spectra was conducted for the following samples: commercial uncoated white paper (80 g), corn husk nanofiber, and corn husk nanofiber composited with CaO, TiO₂, and eggshell powder.

The X-ray peak patterns of the raw materials, commercial uncoated white paper (80 g), and corn husk nanofiber combined with fillers (TiO₂, CaO, and eggshell powder) via the sol–gel process were characterized and compared to the standard XRD peak patterns from the JCPDS database, as shown in Figure 5. The XRD pattern of commercial uncoated white paper exhibited semicrystalline structures similar to those observed in the XRD patterns of the corn husk nanofiber combined with fillers (TiO₂, CaO, and eggshell powder). The XRD peak patterns of TiO₂ displayed a tetragonal crystalline structure, specifically the anatase phase, in agreement with the JCPDS file no. 00-004-0477 for the (hkl): (101), (200), and (105) planes. Thus, TiO₂ was selected as an additive suitable for UV absorption and antimicrobial applications, aligning with Eqs. (1)–(7). The XRD peak pattern of commercial CaO corresponded to the JCPDS file no. 01–078–0649, representing a cubic structure for the (200), (220), and (111) planes. In addition, the eggshell consumption comprised CaO mixed with calcium hydroxide (Ca(OH)₂) consistent with the JCPDS file nos. 01-078-0649 and 01-084-1264, respectively. The crystalline XRD peaks in the biocomposite membranes with added fillers (TiO₂, CaO, and eggshell powder) exhibited a peak position at 2Ɵ similar to the XRD patterns of the corresponding inorganic filler raw materials (TiO₂, CaO, and eggshell powder). Moreover, the obtained XRD peak pattern of the corn husk nanofiber was consistent with the XRD result reported by Chen et al. [28]. The XRD peak pattern of eggshell appeared at 2Ɵ = 17.5°, 28.8°, 34.0°, 37.3°, 50.5°, and 54.5°, in line with the XRD results reported by Ayodeji et al. [61]. The obtained XRD pattern of CaO was consistent with the XRD result reported by El-Sherif et al. [62] and Hossain et al. [57]. Therefore, the addition of inorganic filler particles as a dispersed phase or a reinforcement phase into the corn husk nanofiber matrix showed potential to enhance the physical, mechanical, and electrical properties of ceramic-polymer composites [42].

Figure 5

The comparison of XRD peak patterns for the following samples: commercial uncoated white paper (80 g), corn husk nanofiber, and corn husk nanofiber composited with CaO, TiO₂, and eggshell powder.

SEM micrographs of the biocellulose nanofiber and membrane at magnifications of 500 and 5,000× are shown in Figure 6. The SEM micrograph of the commercial uncoated white paper revealed long, flat fiber overlapping randomly, as depicted in Figure 6a. On the other hand, the SEM micrograph of the corn husk fiber displayed interconnected hollow long fibers, as shown in Figure 6b, which were composed of nanofibers with diameters of up to 50 nm. Furthermore, the SEM micrographs of the biocomposite membrane with added TiO₂ and eggshell powder exhibited different microstructures and surface compositions depending on the types and properties of the inorganic fillers, as shown in Figure 6c and d, respectively. In particular, the SEM micrograph of the biocomposite membrane with added eggshell powder revealed a remarkable hierarchical structure of nano papillae, resulting in a contact angle of water droplets on its surface exceeding 90°, as shown in Figure 6e and f. Generally, materials exhibiting hydrophilic behavior have a contact angle in the range of 0°–90°, while materials displaying hydrophobic behavior have contact angles in the range of 90°–150°.

Figure 6

SEM micrographs and contact angles of the biocellulose fiber and membrane at magnification 500×. The images include (a) commercial uncoated white paper (80 g), (b) corn husk fiber, (c) formula (5) (TiO₂), (d) formula (7) (eggshell), (e) formula (7) at a magnification of 5,000×, and (f) contact angle of water droplet on the sample surface.

The contact angle measurement of commercial uncoated white paper, corn peel fiber with/without the addition of PVA, was analyzed to study the hydrophobic and hydrophilic behaviors from the initial time (t _0 s) to t _12,000 s, as shown in Figure 7, with the data tabulated in Table 2. NaOH was used to enhance the hydrophilic properties of the corn husk nanofiber, as the Na⁺ ions aided in achieving good water solubility and bonding with the corn husk nanofiber through gelation and crosslink via hydrolysis and condensation processes. This finding is consistent with the contact angle results reported by Debnath et al. [63]. Consequently, the contact angle value of the corn husk nanofiber treated with NaOH was equivalent to that of the commercial uncoated white paper throughout the duration from t _0s to t _12,000 s. Moreover, the addition of PVA as a polymer binder facilitated gelation formation and rapid crystal growth, resulting in increased hydrophobicity and decreased water absorption of the treated corn husk nanofiber [42]. The contact angle of corn husk nanofiber films with PVA (formula (1)) and without PVA (no filler) is 10.20° ± 0.47° and 9.77° ± 0.88°, respectively. The addition of fillers (TiO₂, CaO, and eggshell powder) significantly influenced the contact angle, increasing it by 5–10 orders of magnitude in formula (1). The prepared biocomposite membrane exhibited favorable characteristics suitable for further applications. Eggshell powder demonstrated excellent performance as a biofiller when mixed with PVA for composite films, as it formed strong hydrogen bonding interaction between eggshell and PVA due to the presence of a collagen network in its composition. The contact angle values of formulas (6) and (7) with the addition of 1 and 5 wt% eggshell powder are greater than 90°. As a result, the biopolymer film obtained showed a high degree of hydrophobic behavior due to the morphology and structure of eggshell, enhancing the thermal stability of the composite films [64]. The behavior of liquid droplets and the contact angle on the surface of biocomposite membrane depends on the surface roughness and chemical composition, which influence wettability and spreading. In general, wettability follows Young’s equation, which is based on the balance of tensions among the liquid, gas, and solid phases at the three-phase boundary [64].

Figure 7

A comparison of the avg. contact angle values for commercial uncoated white paper (80 g), corn husk nanofiber treated with NaOH, and corn husk nanofiber treated with NaOH-added PVA. The values are expressed as the avg. ± SD, with three samples.

3.2 Mechanical properties of corn husk nanofiber composited membrane with inorganic fillers in a 3D composite structure

The mechanical properties of corn husk nanofiber composites with inorganic fillers such as CaO, TiO₂, and eggshell were tabulated in Table 3. When corn husk nanofiber was combined with 1 wt% of CaO, TiO₂, and eggshell, the composite exhibited higher values of mechanical stress, Young’s modulus, and tensile strength compared to commercial uncoated white paper and the formula (1) without any fillers. Among the composites, the addition of 1 wt% eggshell (formula (6)) resulted in the highest mechanical stress, Young’s modulus, and tensile strength, measuring 5.00 ± 0.41, 10.00 ± 0.01, and 8.85 ± 2.32 MPa, respectively. In a study by Liu et al., intelligent starch/PVA films with antimicrobial activity for food packaging applications exhibited a tensile strength of 4.51 ± 0.14 MPa and a percentage of elongation of 11.39 ± 0.12% [65]. The addition of 1 wt% inorganic filler in each formula resulted in higher mechanical properties compared to adding 5 wt% filler, as the behavior of the added inorganic filler can differentially distribute, intercalate, and bond within the corn husk nanofiber matrix. Formula (1) without any filler exhibited lower mechanical stress, Young’s modulus, and tensile strength, measuring 2.68 ± 0.83, 2.52 ± 0.90, and 2.75 ± 0.77 MPa, respectively. Furthermore, commercial white paper exhibited high mechanical stress, Young’s modulus, and tensile strength, measuring 12.67 ± 2.51, 12.75 ± 0.87, and 9.25 ± 2.08 MPa, respectively. The treatment of lignin in the corn husk fiber with NaOH solution influenced the mechanical properties of corn husk nanofiber and the strength of intermolecular bonding within its structure, as shown in Scheme 1. The concentration of NaOH solution used for treatment affected the characteristics and physical properties of corn husk nanofiber, such as color, softness, size, and purity, depending on the remaining lignin content. In addition, other factors reported by Pirsa et al. [66,67], such as degree of polymerization, molecular weight, intermolecular bonds, and types and pH of solvent, can also influence the mechanical properties of biopolymer films. Stronger intermolecular bonds contribute to higher structural strength and improved mechanical properties [68]. Thakur et al. reported in 2015 that reinforcing particulate nanomaterials, such as SiO₂, carbon black, oxides, CaCO₃, and fibers, used as additives in polymer mixtures can enhance the thermal, mechanical, and electrical properties of food and beverage packaging [69].

Therefore, the addition of inorganic fillers (TiO₂, CaO, and eggshell), particularly eggshell, proved beneficial in improving the mechanical properties of corn husk nanofiber composites in accordance with the mixture’s rule, consistent with the results reported by Azman et al. [44]. Furthermore, Nicolik et al. reported in 2021 the efficiency of incorporating metal oxide nanoparticles such as TiO₂, ZnO, Fe₃O₄, and MgO into biopolymers and bio-based degradable composite materials like chitosan and cellulose nanofiber through casting and electrospinning processes to produce smart (active and/or intelligent) packaging [70]. They found that chitosan and cellulose nanofiber-based smart packaging films exhibited favorable mechanical and thermal properties [70].

3.3 Electrical properties of corn husk nanofiber composited with inorganic filler to prepare a 3D composite membrane

The electrical properties, including electrical conductivity, dielectric constant, and dielectric loss, were measured, and the average (avg.) data ± standard deviation (SD) of each formula from three samples was reported using an impedance analyzer. The measurements were taken at frequencies of 5 × 10², 1 × 10⁵, and 1 × 10⁶Hz at a room temperature of 27°C. The results are tabulated in Table 4. The electrical conductivity of the raw materials, eggshell (CaCO₃), CaO, and TiO₂ were found to be 5.1 × 10⁻⁶–2.7 × 10⁻⁴, 10⁻⁸, and (2.3–2.7) × 10⁻⁶S·cm⁻¹, respectively, as reported in our previous study [51]. The addition of 5 wt% fillers (TiO₂, CaO, and eggshell powder) to the corn husk nanofiber matrix increased the highest dielectric constant. This increase was attributed to the mobilization and polarization of charges in the electrical field, as shown in Schemes 2 and 3, which is consistent with previous results [71,72]. Formula (7) with the addition of 5 wt% eggshell power shows electrical conductivity, dielectric constant, and dielectric loss measured at 500 Hz equal to 1.986 × 10³ (Ω·m)⁻¹, 3.300 ± 0.508, and 0.67 ± 0.30, respectively. On the other hand, the addition of 5 wt% CaO into biocomposite films resulted in the highest electrical conductivity (2.111 × 10³ (Ω·m)⁻¹), dielectric loss (0.68 ± 0.12), and dielectric constant (1.612 ± 0.319) measured at 500 Hz and room temperature (27°C). The addition of 5 wt% eggshell powder also increased the highest electrical conductivity due to the orientation polarization or dipolar polarization of Ca²⁺ and CO₃ ²⁻ ions when subjected to an electrical field, as shown in Scheme 4. The types of ions from the inorganic filler charges (Ti⁴⁺, O²⁻, Ca²⁺, and CO 3 2 − ) in each formula added to the corn husk nanofiber affected the increase in electrical conductivity and dielectric constant differently, in accordance with the results reported by Pirsa and Asadi [73] and Pirsa et al. [66]. These studies have highlighted the importance of cations added to biopolymer made of natural cellulose, as they play a crucial role in the properties of the resulting biocomposite polymer. Such biocomposite polymers exhibit antimicrobial properties [43,74], good antioxidant properties [75], easy biodegradation [43,44,76], effective inhibition of oxygen [77], flexibility and fat resistance [78], as well as chemical sensor characteristics [79–81]. Treating the corn husk fiber with a NaOH solution was found to enhance the electrical properties (electrical conductivity and dielectric constant) of the biocomposite membrane. This enhancement is attributed to the interfacial-space charge polarization of Na⁺ and OH⁻ ions, as illustrated in Scheme 5 [66,82]. The dielectric constant and polarization standard values of the materials were measured at an electrical frequency of 1 MHz and a temperature of 27°C, as shown in Scheme 6. These standard values were used to compare the electrical properties of the prepared biocomposite membranes. Furthermore, Pirsa reported that the addition of nanoparticle fillers is desirable for increasing optical properties and electrical conductivity [83]. Nanoparticles, which are elongated within cellulose fibers or nanotubes, are essential for the food industry in producing smart packaging that functions as sensors and detectors by measuring external conditions [84–87]. According to the standard values of polarization shown in Scheme 6, commercial uncoated white paper and corn husk nanofiber exhibited an orderly arrangement polarization known as dipolar orientation [66,82]. On the other hand, all inorganic fillers such as TiO₂, CaO, and eggshell powder (CaCO₃ source) exhibited electronic-ionic-interfacial space charge polarization [66,82]. Therefore, the addition of inorganic fillers to corn husk nanofiber to produce 3D-composite membranes can induce polarization, making them suitable for a variety of electrical and mechanical applications, such as smart food packaging, drug delivery, batteries, photovoltaics, and alternative energy [82,88,89]. In addition, the obtained 3D composite membrane exhibited low dielectric loss, making it suitable for applications across various electrical frequencies.

Scheme 2

Model the relationship between polarization and electrical conductivity of corn husk nanofiber combines with PVA and composite materials of titanium oxide (TiO₂) and eggshell powder (CaCO₃) [72,80].

Scheme 3

Prototype the behavior of dielectric materials when an electric field is applied between electrodes: (a) in the absence of any dielectric material and (b) with dielectric material, either with or without the addition of fillers [72,80].

Scheme 4

Examine the relationship between dielectric constant and polarization as a function of electric frequency [72,80].

Scheme 5

Types of polarization observed in materials: (a) electronic polarization, (b) atomic or ionic polarization, (c) high-frequency dipolar polarization or orientation polarization, (d) low-frequency dipolar polarization in linear dielectrics and glasses, (e) interfacial-space charge, and (f) interfacial-space charge at heterogeneities [69,72,76,80].

Scheme 6

The standard values of dielectric constant and polarization for materials at 27°C and an electrical frequency of 1 MHz [69,72,76,80].