Jute stick extract assisted hydrothermal synthesis of zinc oxide nanoflakes and their enhanced photocatalytic and antibacterial efficacy

2.1 Materials

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), sodium hydroxide (NaOH), and methylene blue (MB) are all analytical grades and are used without further purification. All the chemicals were purchased from Sigma Aldrich. The jute sticks were collected from the remote village of Harriaghope in the Jessore district of Bangladesh. Distilled water (DW) collected from Active Fine Chemicals was used for the whole experiment.

2.2 Collection of jute extract

The gathered dried jute sticks were broken up into small pieces and processed through a blender into a powder. 1.50 g of the jute sticks powder was added to 75 mL of DW water in a Teflon tube. The Teflon tube, along with the mixture, was then placed into a sealed hydrothermal reactor and heat treatment was performed at 140 °C for 3 h in an electric furnace. The heat-treated aqueous solution of jute powder was filtered, and the extracted filtrate was stored in the refrigerator for further analysis (Mrabet et al., 2017).

2.3 Nano zinc oxide synthesis

Based on the literature (Abinaya et al., 2016) with little modification, for the synthesis of ZnO NFs (Scheme 1A), 0.40 g of Zn(CH₃COO)₂·2H₂O was mixed with 20 mL of DW with vigorous stirring for 15 min to get a homogeneous solution. 0.36 g of NaOH was also mixed with 20 mL of DW for a homogeneous solution. Then the homogeneous solution of NaOH was dropwise added to the solution of Zn(CH₃COO)₂·2H₂O under continuous stirring conditions. The mixture was then vigorously stirred for 1 h, and next transfered in a teflon tube and sealed in a hydrothermal reactor for heat treatment at 170 °C for 5 h in an electric oven. After 5 h of reaction, we took out the reactor and cooled it in room temperature water. After cooling, the synthesized NFs were several times washed with water. Finally the washed NFs were dried in an oven at 120 °C for 6 h and used for further analysis.

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Scheme 1

Schematic representation for the step-wise hydrothermal synthesis of (A) ZnO NFs and (B) J–ZnO NFs.

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Similarly, jute extract assisted product (J–ZnO NFs) were synthesized (Scheme 1B); while 0.40 g of Zn(CH₃COO)₂·2H₂O was mixed with 20 mL of aqueous jute extract with vigorous stirring for 15 min to get a homogeneous solution. With the help of vigorous stirring for 15 min, 0.36 g of NaOH was also mixed with 20 mL of DW to produce a homogeneous solution. Then the homogeneous solution of NaOH was added dropwise into the solution of Zn(CH₃COO)₂·2H₂O under continuous stirring. The mixture was then vigorously stirred for 1 h. The mixture was then transfered in the Teflon tube and sealed in a hydrothermal reactor and heated in an electric furnace for 5 h at 170 °C. The reactor was cooled to room temperature water after 5 h of reaction. After cooling, DW water was used to repeatedly wash the produced (J–ZnO NFs). The cleaned J–ZnO NFs was then used for additional examination after being dried in an oven for 6 h at 120 °C.

2.4 Characterization of the synthesized products

For jute stick extract, ZnO NFs and J–ZnO NFs, UV–vis spectra were recorded in the range of 200–800 nm with a resolution of 0.5 nm using a SHIMADZU UV-1800 double-beam spectrophotometer (Shimadzu Corporation, Kyoto, Japan). UV–vis spectra were acquired using 3 mL of all sample contents in a quartz cuvette with a 1 cm path length. As a baseline adjustment, distilled water was used for UV–vis analysis. X-ray diffraction (XRD, Rigaku Miniflex-II diffractometer) was used to determine the crystallinity of the produced NFs. The diffractometer was set at a wavelength of 0.15416 nm, a current of 10 mA, and a voltage of 30 kV. A field emission scanning electron microscope was used to examine the surface morphology of the produced nanoparticles (FESEM, TESCAN-LYRA-3, Czech Republic). Transmission electron microscopy (TEM) (JEM-2011; JEOL) was used to obtain the TEM images, and the selected area electron diffraction (SAED) pattern was taken using the same instrument. Thermogravimetric analysis (TGA) was used with a Mettler Toledo (TGA1 star system) analyzer to evaluate the weight loss of the products as a function of temperature in order to determine their stability. In order to study the possibility of chemical interaction between ZnO and jute stick extract as well as functional groups of the jute stick extract on the surface of the ZnO NFs, Fourier transformation infrared (FTIR) spectra of KBr pellet were collected using a JASCO FT/IR680 spectrometer. To determine the elemental makeup of the generated J–ZnO NFs, X-ray photoelectron spectroscopy (XPS) with an Al-K-alpha monochromatic X-ray source was used (ESCALAB-250Xi XPS-Microprobe, Thermo-Scientific, USA). For the BE (binding energy) calibration, the 284 eV BE of the C 1s peak was employed.

2.5 Antivacterials assay

2.6 Degradation of MB dye under UV light irradiation

Under the influence of UV light, ZnO NFs and J–ZnO NFs photocatalytic degradation capabilities were assessed against MB dye. To a 100 mL aqueous solution containing 10 ppm MB, 50 mg of catalyst was added. The suspension solution was magnetically stirred for one hour in complete darkness prior to irradiation in order to achieve adsorption–desorption equilibrium. In a beaker with a magnetic stirrer and three UV tube lights (20 W of each), the photocatalytic experiment was carried out at room temperature. Light wasn't allowed to enter the reaction medium since a box was covered in aluminum foil. The beaker containing solution was exposed to UV light while it was being stirred at room temperature. The solution was then exposed to radiation for 8 h, and 5 mL of the sample was obtained at regular intervals of 30 min to 5 h for degradation analysis. The catalyst was then removed right away by centrifuging the sample for 15 min at 10,100 rpm. A SHIMADZU UV-1800 double-beam spectrophotometer (Shimadzu Corporation, Kyoto, Japan) was used to measure the deterioration of the derived solutions with a resolution of 0.5 nm between 200 and 800 nm. Then the percent of photocatalytic degradation was calculated based on Eq. (1) (Balcha et al., 2016).

3.1 UV–vis spectra analysis

The structural characterisation of NFs can be assessed by UV–vis spectroscopy. Visual inspection serves as the primary sign of NFs creation, and this change is then measured by UV light at various wavelengths (200 to 800 nm) to find the surface plasmon resonance (SPR). Following the addition of zinc acetate as a precursor and hydrothermal treatment for ZnO NFs in this work, the color of the jute extract was altered to turbid white. After that, UV–vis spectroscopy was used to confirm yet another primary indicator of the formation of ZnO and J–ZnO NFs. A noticeable SPR signal for ZnO NFs occurred at 373 nm, confirming the preparation of ZnO NFs (Sharmila et al., 2019, Chakraborty et al., 2020).

The J–ZnO NFs absorbance spectra were observed at 368 nm, which was characterized spectra for J–ZnO NFs based on previous literature (Sharmila et al., 2019, Chakraborty et al., 2020). Fig. 1 shows the UV–vis spectrum for jute extract (a), ZnO NFs (c), and J–ZnO NFs (b). We also calculated the band gap energy of ZnO NFs and J-ZnO NFs using tau Eq. (2) and it was 3.27 eV and 3.15 eV for ZnO NFs and J-ZnO NFs respectively. Where Eg is the band gap energy, hν is the photon energy, A is a constant, and α refers to the absorption coefficient (Chithra, Sathya and Pushpanathan, 2015).

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Fig. 1

UV–vis spectrum of (a) jute extract, (c) synthesized product without jute extract (ZnO NFs), and (b) synthesized product with jute extract (J–ZnO NFs).

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The small change in peak position and band gap energy between ZnO NFs and J-ZnO NFs indicated jute stick extract played an important role in the formation of J-ZnO NFs. Furthermore, the UV–vis spectrum shoulder demonstrates that the produced NFs are efficient against both UVB (290–320 nm) and UVA (320–400 nm) radiations. The biological activity of ZnO nanostructures is also influenced by the UV light action (Ansari et al., 2020).

3.2 XRD analysis

Fig. 2 displays the powder XRD patterns of as-synthesised ZnO NFs and J–ZnO NFs. The ZnO NFs diffraction peaks were observed at 31.6°, 34.3°, 36.2°, 47.5°, 56.6°, 62.8°, 67.8°, 69.1°, 72.46°, and 76.9°, which correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), (2 0 1), (0 0 4), and (2 0 2) planes, respectively. All of the diffraction peaks may also be clearly linked to the hexagonal wurtzite structure of ZnO NFs, which has the P63mc (1 8 6) space group and is generated as a pure phase with highly crystalline peaks. The J-ZnO NFs showed a comparable crystal plane, indicating ZnO NFs was created (Saoud et al., 2015, Alshamsi et al., 2018, Kwon and Kim, 2020, Prasad et al., 2019). Additionally, the highly intense peaks in the XRD spectrum of the J–ZnO NFs demonstrate their highly crystalline nature. We also calculate the average crystallite size of the synthesized ZnO NFs and J-ZnO NFs using Debye–Scherer equation (3) (Hossain et al., 2018, Nayem et al., 2020).

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Fig. 2

XRD patterns of the product; (a) without jute extract (ZnO) and (b) with jute extract (J–ZnO).

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3.3 Surface morphology analysis

The surface morphology of prepared ZnO NFs and J–ZnO NFs was examined by FESEM analysis at various magnifications, and the findings are shown in Fig. 3. The synthesized ZnO (both type with presence of jute extract or without jute extract) exhibits nanoflakes (NFs) type morphology. The average thickness of the prepared NFs was 42.6 nm and 28.7 nm for ZnO NFs and J-ZnO NFs respectively. Whereas, a clear difference in shape can be seen in the FESEM micrographs of the ZnO NFs and J–ZnO NFs. ZnO NFs exhibited irregular morphology while J–ZnO NFs exhibited regular morphology with flower like shape. This demonstrates that the presence of the jute stick extract during hydrothermal treatment for the preparation of J–ZnO NFs had an important role in the nucleation process, resulting in a more defined pattern and less agglomeration. The micrograph hence confirms that the presence of jute stick extract acted as a growth template for flower-like J–ZnO NFs (Soto-Robles et al., 2019, Yang et al., 2016, Lefatshe et al., 2017). The formation of collectively flower like J–ZnO NFs is due to electrostatic interactions between polar biomolecules of jute stick extract and Zn²⁺ ions (Yang et al., 2016). This NFs-like structure of the ZnO is important for photocatalytic applications as this structure allows light to easily pass through.

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Fig. 3

FESEM images of the product synthesized without jute stick extract (ZnO NFs) at different magnifications; (A), (B) and of the product synthesized with jute stick extract (J–ZnO NFs) at different magnifications; (C) and (D).

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TEM analysis was carried out for further morphological and particle shape nature investigation of the J–ZnO NFs. In the collective flakes-like structure with an outer diameter of about 100 nm, the TEM image (Fig. 4A) shows the needle shape. A high-resolution TEM picture (HRTEM) of the J–ZnO NFs with an interplanar spacing of less than 0.25 nm is shown in Fig. 4B, demonstrating their high crystalline nature. In agreement with the XRD study of the ZnO hexagonal structure, the selected area electron diffraction (SAED) pattern (Fig. 4C) reveals that the produced J–ZnO NFs crystallized as single crystals with high crystallinity (Kim and Leem, 2021, Greene et al., 2006).

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Fig. 4

(A) TEM image, and (B) HRTEM image, and (C) SAED pattern of the J–ZnO NFs.

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3.4 FTIR spectra analysis

In order to pinpoint the potential biomolecules responsible for the production of J–ZnO NFs, FTIR analysis was done. As can be seen in Fig. 5, the 400–4000 cm⁻¹ range was used to record the FTIR spectra of jute stick extract, J–ZnO NFs, and ZnO NFs. Jute stick extract's spectra (Fig. 5(a)) shows two broad peaks at wavelengths 3436 cm⁻¹ and 2905 cm⁻¹ that correspond to the -O—H stretching vibrations and the -C—H stretching vibration of aromatic alkane (El-Belely et al., 2021, Vijayakumar et al., 2016). The peaks observed at 2324 cm⁻¹ due to the presence of CO₂ (Wang et al., 2019). A broad and intense band at 1410 cm⁻¹ depicts the presence of —C—C stretching of the aliphatic groups. The peak at 1628 cm⁻¹ was due to —C⚌C (stretching) of alkane di-substituted. The peak at 1070 cm⁻¹ is assigned to the C—O stretching vibration (El-Belely et al., 2021, Vijayakumar et al., 2016). The spectrum of J–ZnO NFs (Fig. 5c) and ZnO NFs (Fig. 5b) contained a band that was the distinctive signature of the Zn—O bond at 468 cm⁻¹ and 517 cm⁻¹, proving that the substance was in fact zinc oxide. With the exception of a few, all of these peaks were visible in the spectrum of J–ZnO NFs (Fig. 5(c)); however, peaks at 2905 cm⁻¹, 1628 cm⁻¹, and 1410 cm⁻¹ were absent from the spectrum of ZnO NFs (Fig. 5b). The current FTIR data corroborates with the FTIR spectra of ZnO NFs produced from different plant extracts, revealing biomolecules from jute stick extract werepresent on J–ZnO NFs surface (El-Belely et al., 2021, Vijayakumar et al., 2016, Zheng et al., 2015, Varadavenkatesan et al., 2019, Rupa et al., 2018, Bekele et al., 2021, Alayande et al., 2019). This implies that biological molecules of jute stick extract may act as hydrolyzing agents for preparing well organized ZnO NFs. From literature, survey we can assume that the biomolecules might be cellulose (Hospodarova, Singovszka and Stevulova, 2018, Kunusa et al., 2018).

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Fig. 5

FTIR spectra of (a) jute extract, (b) ZnO NFs, (c) J–ZnO NFs.

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3.5 Surface composition determination by XPS analysis

Using the XPS method depicted in Fig. 6, the chemical makeup and compositional insight of the produced compounds were evaluated. The survey spectrum of the J–ZnO NFs, shown in Fig. 6A, contains all the predicted elements, including Zn 2p, O 1s, and C 1s. O 1s (531 eV), C 1s (286 eV), Zn 3d (11 eV), Zn 3p (89 eV), Zn 3 s (141 eV), Zn_LMM (499 eV), Zn 2p1/2 (1022 eV), and Zn 2p3/2 are the primary XPS peaks in the survey spectrum (1045 eV). These components show the existence of Zn and O atoms alongside C in the J–ZnO NFs' XPS survey spectrum. This led to the conclusion that the produced J–ZnO NFs were chemically pure with some jute stick extract-derived carbon doping. In order to comprehend the oxidation states of these Zn and O elements in the sample, the deconvoluted high-resolution XPS spectra of the Zn 2p (Fig. 6B), O 1s (Fig. 6C), and C 1s (Fig. 6D) were taken. The accentuated peaks were found at 1021.5 and 1045.3 eV, which correspond to the Zn 2p1/2 and Zn 2p3/2, respectively, in Fig. 6B, which shows the deconvolution XPS spectra for Zn 2p. The presence of Zn 2p is highlighted, and the spin–orbit interaction of the Zn 2p components is confirmed. The substantially stronger peak at 1021.5 eV demonstrated that Zn exists in an entirely oxidized state with no neutral nature (Qu et al., 2019). Fig. 6C shows the high-resolution O 1s deconvoluted XPS spectrum, which demonstrates the occurrence of a peak at 531.5 eV, which is mapped to the O 1s. This could be ascribed to the oxygen bound within the wurtzite hexagonal phase of J–ZnO NFs (Qu et al., 2019). The deconvolution of the O1s (Fig. 6C) in the wurtzite ZnO structure is ascribed due to the O₂^– ions mainly and displays three significant peaks centered at 531.5 eV (i) 532.6 eV (ii), and 533.1 eV (iii). The oxygen atoms coupled with Zn atoms are mostly responsible for the peak at 531.5 eV. The signal at 532.6 eV most likely relates to oxygen adsorbed on the sample surface. Water molecules on the surface are most likely responsible for the peak at 533.1 eV. According to the literature, they are caused by surface imperfections and chemisorbed oxygen (Diallo et al., 2015, Bhirud et al., 2012). Additionally, the intense deconvoluted peaks of C 1s in Fig. 6D at 283.27, 284.76, 286.34, and 288.8 eV might be attributed to the sp³ carbon atoms (C—C), sp² carbon atoms (C⚌C), —C—OH, and —O—C⚌O species, confirming the hydrocarbon composition formed in the medium. This may be because ZnO NFs made from cellulose (jute stick extract) have carbon on its surface, which was also validated by FTIR spectroscopy (Yang et al., 2016, Geetha et al., 2016, Khan et al., 2019, Diallo et al., 2015, Soto-Robles et al., 2019, Al Marzouqi et al., 2019, El-Belely et al., 2021, Luo et al., 2014, Kayan et al., 2021).

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Fig. 6

XPS (A) survey spectrum and high-resolution spectrum of (B) Zn 2p, (C) O 1s, and (D) C 1s for the J–ZnO NFs.

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3.6 Possible mechanism of J–ZnO NFs formation

From above characterization data, it can say that cellulose from jute stick extract plays an important role in the formation of J–ZnO NFs. From UV–vis analysis, it was observed that cellulose from jute stick extract shifted the peak position and responsible for the lower band gap energy indicating cellulose has an impact on size and shape of the synthesized product. From FESEM it was seen that J–ZnO NFs was forming flower-like shape which indicated cellulose of jute stick extract facilitated the growth of ZnO NFs on cellulose surface to get a flower-like shape. TEM image also clearly confirmed the well organized growth of ZnO NFs on cellulose surface. The XPS and FTIR data confirmed the presence of cellulose on the surface of J–ZnO NFs. The TGA (Fig. S1) also confirmed that J–ZnO NFs were more stable than ZnO NFs and modification of cellulose with J–ZnO NFs. Hence based on characterization data a possible mechanism for the formation of collectively flowers like J–ZnO NFs could be demonstrated. Initially, NaOH converted zinc acetate into Zn(OH)₂. Then OH⁻ of alkali again converted Zn(OH)₂ into Zn(OH)₄²⁻ which was then electrostatically attached by cellulose of jute stick extract (positive ion of zinc and negative ion of cellulose due to alkali treatment), resulting in the formation of well organized ZnO NFs on the template of cellulose. In this process NFs of ZnO acts like nuclei on the surface of cellulose template of jute stick extract resulting collectively flower like J–ZnO NFs. The proposed mechanism is shown through the reactions below.

3.7 Degradation of MB dye under UV light irradiation

To study the photocatalytic performance of pure ZnO NFs and J–ZnO NFs under UV light irradiation, MB is used as a target pollutant. It is seen that the intensity of the peak at 664 nm decreases gradually with continuous exposure to irradiation, and the presence of ZnO NFs under UV light (Zhang et al., 2006). Fig. 7A and 7C display the degradation of MB dye using ZnO NFs and J–ZnO NFs. It was found that J–ZnO NFs display stronger photocatalytic performance than that ZnO NFs. The degradation efficiency of ZnO NFs and J–ZnO NFs was 79 and 89 % for 5 h which was obtained from Eq. (1) and shown in Fig. 7B and 7D.

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Fig. 7

UV–vis spectrum of MB degradation with (A) ZnO NFs and (C) J–ZnO NFs at different times. The efficiency of MB for (B) ZnO NFs and (D) J–ZnO NFs. (E) The kinetic plot of MB degradation.

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The pseudo-first-order reaction kinetics represented by the following Eq. (7) governs the photocatalytic degradation of MB under UV irradiation (Chen et al., 2019):

J–ZnO NFs were substantially more active than ZnO NFs. The reason for this is because cellulose from jute stick extract add a lot of surface area, increasing the amount of active sites per square meter and boosting reactivity. Because J–ZnO NFs in the nanocomposite have smaller thickness than ZnO NFs, cellulose can be used to control the creation of new photocatalysts. After 5 h of exposure to radiation in the presence of the J–ZnO NFs, approximately 89 % of the dye had been broken down, indicating high photocatalytic activity that may be caused by the strong electronic interaction between the ZnO and cellulose (Lefatshe et al., 2017). The longer-time degradation of MB using J–ZnO NFs is shown in Fig. 8A. MB dye was 98 % degraded after 480 min under UV light as shown in (Fig. 8B).

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Fig. 8

(A) Longer-time degradation of MB using J–ZnO NFs under UV light. Inset shows the image of the color of the MB solution before and after 480 min of degradation. (B) The efficiency of MB after 480 min.

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The mechanism of photocatalytic activity can be described as follows: ZnO NFs are utilised as the electron and hole providers during the photodegradation of MB. Electrons (e) from the valence band (VB) are stimulated to the conduction band (CB) and produce holes (h) in the valence band when ZnO NFs are exposed to UV light with an energy equal to or higher than the bandgap (Türkyılmaz et al., 2017, Alshamsi et al., 2018, Georgekutty et al., 2008). Before exposure, a forbidden band separates VB and CB; when energy from UV light is absorbed, VB electrons in J–ZnO NFs will be stimulated to the VB, and CB will become charged. Because of this, these free e/h will attempt to leave the catalyst surface and interact with the H₂O solvent and the dissolved molecular oxygen, respectively, to create a variety of reactive oxygen species, such as hydroxyl radicals (—OH), superoxide radicals (O₂^°), hydrogen peroxide (H₂O₂), or hydroperoxyl radicals (OOH^°) (Türkyılmaz et al., 2017, Alshamsi et al., 2018, Georgekutty et al., 2008). The MB solution would cause the photogenerated electrons to react with the dissolved O₂ molecules and produce superoxide anion radicals (O₂^°) while the holes would react with H₂O and produce hydroxyl radicals (OH^°), both of which are potent oxidizing agents that would cause the dye to degrade (Türkyılmaz et al., 2017, Alshamsi et al., 2018, Georgekutty et al., 2008). For the enhanced degradation of J-ZnO NFs, we can assume that the electrochemical properties of J-ZnO NFs is increased due to polar functional groups of cellulose from jute stick extract resulting excellent photocatalytic activity (Yang et al., 2016). A possible MB dye degradation mechanism is shown in Fig. 9.

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Fig. 9

Photocatalytic degradation mechanism of MB against J–ZnO NFs.

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3.8 Antibacterial activity assessment

In this study, the disc diffusion method was used to examine the bactericidal activity of jute stick extract-mediated crystalline J–ZnO NFs against E. coli. In comparison to pure ZnO NFs, the antibacterial activity of produced J–ZnO NFs samples was tested against microorganisms. At a concentration of 1 mg/mL, all produced ZnO NFs samples significantly inhibited E. coli growth at 19 mm for J–ZnO NFs (Fig. 10B) and 17 mm for ZnO NFs (Fig. 10A). It was thought that the lipid peroxidation process caused by the production of reactive oxygen species from ZnO NFs was what caused the inhibitory impact. It is also believed that peroxide damages the bacterial cell membrane, causing intracellular contents to leak out and ultimately leading to the death of the bacteria cell. Furthermore, the toxicity in bacterial cells has been linked to the free zinc ion from ZnO NFs. For J-ZnO NFs, it can say that strong interaction between cellulose of jute stick extract and ZnO NFs is responsible for the more reactive oxygen species resulting higher zone of inhibition of J-ZnO NFs than ZnO NFs (Yuvakkumar et al., 2014, Khan et al., 2020). The ZnO nanostructure dispersion enhanced by cellulose polymer, resulting in a greater surface area. It is also hypothesized that microorganisms have a negative charge whereas metal oxides have a positive charge, resulting in a “electromagnetic” attraction between the bacterium and the treated surface (Lefatshe et al., 2017).

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Fig. 10

Antibacterial activity towards the E. coli pathogen by (A) ZnO NFs and (B) J–ZnO NFs.

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The comparative study of antibacterial activity of J-ZnO NFs against E. Coli with other reported works are summarized in Table 1.