3.1. Morphology Analysis
Addition of ZnO-NPs on BNC results in a membrane morphology, as depicted in Fig. 1. It shows a smooth morphology with small porous (Fig. 1A). After being added by ZnO-NPs 2.5%wt into BNC membrane, the surface becomes rough (Fig. 1B) because an amount of ZnO-NPs is deposited on surface. The addition of ZnO-NPs 5.0%wt exhibits small aggregation of ZnO-NPs, so the morphology becomes more rough (Fig. 1C) because ZnO-NPs have a tendency to adhere to neighboring particles and subsequently aggregate [25]. The introduction of 5.0wt.% of ZnO-NPs leads to a fibrillated surface morphology in BCN nanocomposite membrane (Fig. 1D). Besides ZnO-NPs deposited on the surface, ZnO-NPs are also mixed with BNC in internal membrane and more ZnO-NPs agglomeration is observed in the membrane. Even, more ZnO-NPs have good interaction with cellulose through hydrogen bonds [26], but high interaction ZnO-NPs with cellulose has an impact on membrane morphology.
3.2. Functional Group Analysis
The FTIR test was performed on different cellulose fiber samples to investigate the structural differences before and after reinforcement. The findings from these tests are depicted in Fig. 2. The infrared transmittance spectra were recorded for a BNC membrane (shown in blue curve) and for BNC functionalized with ZnO-NPs at 2.5wt.%, 5.0wt.% and 7.5wt.% different weight percentages (shown in red, black, and green curves). The wavenumber range analyzed was from 400 to 4000 cm− 1.
The characteristic transmittance bands for BNC were identified in two main wave number regions: the "fingerprint" region spanning 400–1650 cm− 1 and the O–H and C–H stretching in 2700–3600 cm− 1 range [27]. Incorporating ZnO-NPs enhances transmittance, and it also reveals the reducing C-H bonds stretching from cellulose at a wavenumber of 2897 cm− 1 [28]. The broadband in 3300–3500 cm− 1 region, represents the OH-stretching vibration, which provides valuable information about hydrogen bonding. O-H stretching transmittance enhancement at a wavenumber of 3400 cm− 1 indicates that ZnO-NPs make the membrane more hydrophobic. A similar phenomenon was also reported by other researchers [29][30]. The spectra in "fingerprint" region for both cellulose samples appeared complex. The wavenumber range of 1520 − 1400 cm− 1 displays the broadening of cellulose peak as a result of bending in functional groups such as CH2, C-H, and C-O in cellulose [31]. Comparing the IR spectra of the reinforced BNC with the original BNC membrane, some noticeable changes were detected. As shown in Fig. 2, the peak of 1487 cm− 1 shifts to 1477 cm− 1 after being added by ZnO-NPs. IR wavenumber around 750 cm− 1 is characteristic of triclinic (Iα) [32]. This wavenumber shifts to around 720 cm− 1 after incorporating ZnO-NPs into the membrane. The characteristic absorption band of ZnO is responsible for broad absorption peak observed in FTIR spectra of pure ZnO, ranging from 414 to 1339 cm− 1 [33].
3.3. Crystalline Analysis
Figure 3 illustrates the diffraction pattern of nanocomposite. The diffractogram of BNC membrane displays 3 main peaks observed at insert specific values or positions of peaks at 14.4°, 16.9°, 22.8°, and 34.1° corresponding to crystal plane of [110], [10], [200], and [004] [34][35], respectively, which indicates cellulose Iβ. BNC membrane shows a distinct peak in the diffraction pattern at angles of 15.2°, 31.6°, and 36.8°, which serves as a clear indication of the presence of ZnO-NPs [36][25]. These peaks in 2θ range of 10°-60° were almost the same, though sharp diffraction peaks increased with increasing ZnO-NPs content. The ZnO-NPs in the BNC membrane are very weak, which could be attributed to ZnO-NPs growing on the surface of BNC membrane, which makes it harder to collect the diffraction data of cellulose during XRD test. When a high level of ZnO-NPs was used, the presence of a larger number of self-agglomerated nanoparticles caused the crystalline character to become clearer.
The crystalline degree and index of BNC membrane are 78.92% and 73.30%, respectively (Table 1). The addition of ZnO-NPs of 2.5%wt increases the crystalline degree and also the crystalline index of membrane by 84.30% and 81.37%, respectively. However, the introduction of higher amounts of ZnO-NPs (5.0wt.% and 7.5wt.%t) leads to a slight reduction in the crystalline properties, specifically the crystalline degree and crystalline index by 83.56%, 80.33%, and 82.45%, 78.72%, respectively. The crystallite size of BNC/ZnO-NPs nanocomposite membranes is 9.95, 11.89, 12.20, and 13.74 nm for the membrane of BNC, BNC with ZnO-NPs of 2.5wt.%, 5.0wt.% and 7.5wt.% respectively. As can be seen upon doping of ZnO-NPs into the crystallite size increased, which might be due to incorporation ZnO-NPs into the BNC element because ZnO-NPs have higher crystallite size (11.84–26.2 nm), depending on their synthesis route[37][38][39] Overall, these observations showed that addition of ZnO-NPs caused an increase in the overall crystallinity of BNC membrane, which can be effective on the final membrane properties.
Table 1
Crystalline properties of BNC/ZnO-NPs nanocomposite membrane
| Intensity (a.u.) | Crystalline parameter |
| I22.5 | I18 | CI | %Cr | L (nm) |
Control | 2076.80 | 554.60 | 73.30 | 78.92 | 9.95 |
ZnO-NPs 2.5wt% | 2952.20 | 550.00 | 81.37 | 84.30 | 11.89 |
ZnO-NPs 5.0wt% | 2994.40 | 589.00 | 80.33 | 83.56 | 12.20 |
ZnO-NPs 7.5wt% | 2459.50 | 523.40 | 78.72 | 82.45 | 13.74 |
3.4. BET Analysis
Figure 4 illustrates the adsorption-desorption curve of membrane samples, which is employed for the analysis of pore size distribution, surface area of membrane, and the influence of ZnO-NPs addition. Nitrogen gas adsorption is the technique used for this purpose. The hysteresis loop observed within the relative pressure range of 0.5–1.0 (Fig. 5a) confirms the mesoporous nature of BNC membrane [40]. For the BNC/ZnO-NPs nanocomposite membrane, the relative pressure range is 0.45–1.0 (Fig. 4b, 4c, 4d), indicating a well-developed mesoporous structure with a classical type IV(A) isotherm and H3 type hysteresis, according to IUPAC classification. The addition of ZnO-NPs results in type IV(A) isotherm with H2(b) type hysteresis, which is linked to materials lacking pores or having large pores, making it less significant for pore size analysis [41].
The surface area and pore size data are presented in Table 2. The BET surface area of BNC membrane reduces after adding ZnO-NPs. It indicates that ZnO-NPs alter the membrane's pore structure and surface chemistry of membrane, leading to changes in its adsorption properties and surface area. The pores size of membranes is determined to be mesoporous, with pore sizes below 12 nm. The size of pores in the BNC membrane decreases as a result of ZnO-NPs (2.5wt.% and 5.0wt.%) coming together, leading to improved connections between existing pores. Additionally, the ZnO-NPs may serve as spacers or templates during membrane formation, creating gaps or spaces between the BNC fibrils during manufacturing process. It has been observed in various studies that incorporating ZnO-NPs into the membrane formation tends to enhance membrane pore size [42] [43]. Although the average pore sizes are larger than 0.3–0.5 nm in molecular sieve membranes, these are still well below the pore size of 10 nm-1µm for polymeric membranes used in membrane distillation systems [44].
Table 2
BET results for pore size distribution and specific surface area
Membrane | Pore Volume (cm³/g) | Pore diameter (nm) | BET surface area (m²/g) |
BNC | 0.059590 | 5.7634 | 36.9605 |
BNC/ZnO-NPs 2.5%wt | 0.023730 | 8.7842 | 2.9168 |
BNC/ZnO-NPs 5.0%wt | 0.043293 | 8.8236 | 8.0436 |
BNC/ZnO-NPs 7.5%wt | 0.075349 | 11.8070 | 13.7783 |
3.5. Antibacterial Characteristic
The antibacterial activity of BNC/ZnO-NPs nanocomposite was assessed against E. coli and S. aureus. In this method, the test disk is positioned on the surface of the inoculated test medium. The disks become moist as they soak up water from the agar medium, which in turn triggers the antimicrobial agent's release into the nearby agar medium. This process creates a gradient of antibacterial concentration in the surrounding agar. Positive control samples showed clear zones of inhibition, and the zone inhibition sizes for E. coli and S. aureus were measured at 17 mm and 18.9 mm, respectively (Fig. 5(AI)). The addition of ZnO-NPs in BNC exhibited blur halo was present in concentrations ZnO-NPs of 5.0wt.% and 7.5wt.%. It indicates that the nanocomposite BNC/ZnO-NPs membrane has antibacterial activity against S.aureus lower than the membrane with gentamicin (as control) because gentamicin is an antibiotic with strength and broad-spectrum antibacterial activity. Unfortunately, for E. coli, no antibacterial activity was found at nanocomposite BNC/ZnO-NPs (Fig. 5) (AII, AIII, AIV)).
Some researchers report similar results for no antibacterial activity of ZnO-NPs for gram-negative bacteria (E. coli[42] [45]). The antibacterial activity of ZnO is dependent on concentration. The low concentration of ZnO has no effect on antibacterial activity [46]. The particle size of ZnO was directly correlated with antibacterial activity, where a lower size has better antibacterial properties [47]. It is correlated with increased specific surface area that enhanced particle surface reactivity [48]. As the BET results (Table 2), nanocomposite with higher ZnO-NPs contents increases the surface area, but indicates a blurred halo rise that indicates ZnO-NPs in BNC have weak reactivity to kill the gram-positive bacteria.
3.6. Mechanical Properties
The intermolecular force of polymers and their stiffness plays a significant role in determining the mechanical characteristics of a polymer [49]. Figure 6 illustrates the results of the tensile test conducted on membrane. The BNC membrane exhibits a tensile strength of 240.19 ± 24.74 MPa. However, the introduction of ZnO-NPs into the membrane at different weight percentages (2.5wt.%, 5.0wt.% and 7.5wt.%) resulted in reduced tensile strength values of 230.23 ± 28.3 MPa, 181.72 ± 2.06 MPa, and 162.31 ± 35.6 MPa, respectively. According to one-way ANOVA analysis, the incorporation of ZnO-NPs with various concentrations has a significant impact on the tensile strength (n = 4, Pvalue of 0.0028, significance level of 5.0%). However, after conducting post hoc analysis using Tukey LSD methods, it was found that there is a significant difference in the tensile strength of BNC membrane compared to BNC with ZnO-NPs content of 5wt.% and 7.5wt.%. This means that the tensile strength of BNC membrane differs significantly from the strength observed in nanocomposites containing 5.0wt.% and 7.5wt.% of ZnO-NPs. The chemical interaction between ZnO-NPs and BNC takes place through the OH groups of cellulose, leading to the formation of a secondary interaction involving hydrogen between the oxygen of ZnO-NPs and the hydroxyl group on the cellulose [21]. However, when the ZnO-NPs content is high, it causes significant agglomeration, resulting in more damage to membrane, as depicted in Fig. 1D. The agglomerated ZnO-NPs are unable to penetrate the BNC network and are found outside the membrane structure. As a consequence, they do not effectively bind to cellulose or participate in hydrogen bonding, acting more like impurities rather than contributing to the mechanical properties of membrane. In fact, they even cause a reduction in the strength of membrane. This decrease in strength was proved by the tensile strength of BNC membrane compared to BNC/ZnO-NPs membrane, which showed a reduction of 32%. These findings align with prior research indicating that higher concentrations of ZnO can lead to reduced strength in nanocellulose films [50].