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Bioinspired Fibrous Architectures Based on ZnO Templated by Eggshell Membranes

Written By

Nicoleta Preda, Marcela Socol, Andreea Costas and Irina Zgura

Submitted: 30 January 2024 Reviewed: 09 March 2024 Published: 08 June 2024

DOI: 10.5772/intechopen.1005214

Zinc Oxide Nanoparticles - Fundamentals and Applications IntechOpen
Zinc Oxide Nanoparticles - Fundamentals and Applications Edited by Ana Rovisco

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Zinc Oxide Nanoparticles - Fundamentals and Applications [Working Title]

Dr. Ana Rovisco and Dr. Ana Pimentel

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Abstract

ZnO-based nanostructures emerge as promising materials due to their potential applications in fields including electronic devices, photodetectors, photocatalysts, biocides, etc. The bio-template-mediated synthesis is a straightforward approach for obtaining inorganic or hybrid organic/inorganic materials with tailored morphologies and functional properties. Eco-friendly waste, eggshell membrane (ESM) is an ideal bio-template for the development of 3D hierarchical porous architectures due to its specific 3D interlaced fiber protein network structure. Therefore, this chapter is focused on the ESM-mediated synthesis of 3D fibrous architectures based on ZnO, the ESM organic network being functionalized with inorganic nanostructures or replicated into an inorganic one as follows: i) coated with ZnO layer by RF magnetron sputtering, (ii) covered with ZnO by electroless deposition and (iii) replicated into ZnO web by biomorphic mineralization. The obtained ZnO shows wurtzite structure, band-gap value and emission bands typical for this semiconductor. The electrical properties of the ZnO fiber webs were measured using interdigitated metallic electrodes patterned substrates. The ESM conversion from a bio-waste into new value-added nanomaterials is very attractive from the sustainability and recycle waste perspective, the ZnO-based fibrous architectures featured by a large specific surface area having potential applications in water purification, photocatalysis or chemical sensors areas.

Keywords

  • ZnO
  • eggshell membranes
  • organic/inorganic fibers
  • inorganic fibers
  • fibrous architectures

1. Introduction

Over the last decades, zinc oxide (ZnO) has been extensively and intensively assessed attracting an enormous interest from both science and industry. Thus, ZnO micro/nanostructures emerge as a powerful class of advanced functional materials [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20], which can find applications in various domains ranging from electronics, energy, and environmental protection to healthcare. Recently, a large number of comprehensive review papers provide synopses on the recent achievements attained in the preparation and properties of ZnO nanomaterials and their use in electronic devices [8], UV photodetectors [9], photovoltaic devices [10], gas sensors [11], nonlinear optical materials [12], biosensors [13, 14], photocatalysis [15, 16, 17], antibacterial agents [18, 19], surface coating with special wetting properties [20], and so on. ZnO is an outstanding material due to its properties [21, 22] such as wide bandgap (~3.3 eV in bulk), high exciton binding energy (~60 meV), high thermal conductivity (50 W/mK), ultraviolet light absorption, high transmittance of visible light, strong luminescence (even at room temperature), mechanical stability at room temperature, good biocompatibility, biodegradability and low toxicity. Moreover, the ZnO characteristics, such as abundance, environmental friendly, and a vast family of 0D, 1D, 2D, and 3D morphologies, recommend the ZnO-based materials for mass production with low manufacturing costs, which can subsequently broaden their use in different industrial settings. Furthermore, ZnO micro/nanostructures can be relative easily synthesized in large quantities by different approaches [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22] including physical techniques, conventional chemical methods, or green synthesis routes. Hence, in previous papers, we reported on the preparation of various ZnO micro/nanostructures (particle, rod, wire, core-shell, flower, or snowflake) by thermal oxidation in air [23, 24, 25, 26], chemical precipitation [27, 28, 29], aqueous solution growth [26] and chemical bath deposition [30] for potential applications in field effect transistors [26], photovoltaic cells [27], chemical sensors [30], photocatalysts [24, 28], antibacterial agents [28], and surfaces with special wetting properties (superhydrophobic, no-loss liquid transportation, anticorrosion, or self-cleaning) [23, 25, 30]. Finally, ZnO is an ideal system for exploring the morphology-properties-applications relationship being well-known that the size, shape, and surface chemistry of the semiconducting structures can strongly influence their properties, these being further responsible for their applications.

Eggshell membrane (ESM) is an affordable, inexpensive, easy-handling, and renewable biomaterial, which, for a long time, was considered just a common industrial and domestic waste. Nevertheless, owing to its unique three-dimensional (3D) interlaced fibrous protein network structure, ESM presents very useful properties [31, 32, 33] such as flexibility, high porosity, high absorption capacity, large surface area, good permeability, good mechanical strength, good thermal stability, biocompatibility, and biodegradability. As a consequence, in the last decade, ESM has been in the spotlight gaining a huge popularity in its conversion from an abundant daily waste into a source for developing new value-added materials using bioinspired strategies. It must be mentioned that the ESM biopolymeric network is formed by fibers containing 80–85% proteins in their composition (10% collagens types I, V, and X and 70–75% other proteins and glycoproteins) [31]. Accordingly, the ESM surface contains numerous functional chemical groups (amine, amides, carboxylic) that play a major role in its functionalization or replication. Thus, in different forms (pristine, modified, or carbonized), the ESM has been used as (i) component in triboelectric nanogenerators [34], fuel cells [35], supercapacitors [36], lithium-ion battery [37]; (ii) sorbent of water contaminants, such as dye molecules [38] or heavy metals [39]); and (iii) immobilization platform for biosensors [40]. Biomaterial with an antibacterial and anti-inflammatory activity, the ESM porous structure can favor the diffusion of gas and water molecules assisting the attachment and proliferation of the cells. In this way, ESM can control the moisture content of a wound, provides an antibacterial barrier, and helps the covering of the wound preventing and reducing the bacterial growth. Therefore, ESM is an excellent biocompatible material for wound healing of skin injuries [41] and tissue engineering [42]. Furthermore, the ESM fiber-interwoven architecture makes this biowaste an ideal bio-template for designing 3D hierarchical porous architectures. Hence, the ESM network was successfully functionalized with conducting polymers [43, 44, 45] or semiconducting nanostructures [46, 47, 48] and replicated into a semiconducting one [49, 50, 51], the new materials having potential applications in areas, including supercapacitors [44], fuel cells [45], actuators [43], antibacterial agents [46], photocatalysts [49], and water treatment [47, 50].

In this context, the aim of this chapter is to highlight the way in which functional hybrid ESM/ZnO webs and ZnO fiber webs can be developed by combining ESM as bio-template and ZnO preparation approaches such as radio frequency (RF) magnetron sputtering, electroless deposition, and biomorphic mineralization. In the following, we summarized the major attribute of each ZnO preparation methods involved in our work.

  1. The radio frequency magnetron sputtering can be regarded as a “clean” environmental friendly preparation approach being a solution-free route (does not involve hazardous precursors and does not generate harmful by-products), which uses only solid sputtering targets in the deposition of the metal oxide layer [46, 52, 53]. Versatile, high-yield, and low-temperature vapor deposition technique, this preparation path can be employed to obtain nanostructured layers on a wide range of substrates, the deposited films being usually polycrystalline, uniform, with a good adhesion on the surface intended to be covered.

  2. The electroless deposition can be considered an easily approach, which uses accessible raw precursors (metal salt and reducing agent), low process temperature, ambient pressure processing, and simple equipment (the chemical redox reactions take place in aqueous solutions) [47, 54, 55]. This synthesis method is based on the growth of the metal oxide structures only on a metal-catalyzed surface, the surface intended to be coated being firstly modified with a catalyst metal, such as palladium or gold. Hence, the electroless deposition allows the coating of organic nonconductive substrates with plain or 3D shapes by a uniform and adherent metal oxide structured layer.

  3. The biomorphic mineralization is a straightforward approach for obtaining metal oxide materials with tailored architectures and properties by selecting an appropriate bio-template, this preparation path using low-cost reagents (metal salt precursors), inexpensive equipment and involving just a simple immersion of a bio-template into precursor solution and a subsequent calcination [49, 50, 51, 56, 57, 58, 59, 60]. In the case of this method, nature is an inexhaustible source of organic templates (leaves [56, 57], wood [58], butterfly wings [59], eggshell membranes [50, 51], etc.), each of them with an innate architecture that a priori can be perfectly replicated into an inorganic one containing metal oxide particles as building blocks.

The properties of the prepared samples (pristine ESM, ZnO-functionalized ESM, and ZnO replica of ESM) were evaluated from the structural, optical, and morphological point of view employing X-ray diffraction (XRD), reflectance, photoluminescence, field-emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The elemental composition and vibrational properties of some samples were investigated by energy dispersive X-ray analysis (EDX) and Fourier-transform infrared spectroscopy (FTIR). The wetting properties of the samples were also tested. Furthermore, a possible pathway to integrate the bioinspired ZnO fiber webs in the electronic devices is evaluated by using interdigitated metallic electrodes as substrates during the ESM removal by calcination treatment. The main advantage of this approach is represented by the fact that the semiconducting fibers form bridges over the neighboring grid structures providing electrical paths between them. In this way, the electrical circuit is closed without any additional lithographical contacting steps.

The information from this chapter provides new insights concerning the development of the bioinspired 3D fibrous architectures based on ZnO templated by eggshell membrane. In the same time, from the waste valorization perspective, a biowaste is converted into new value-added ZnO-based nanomaterials [61]. Furthermore, owing to their high surface area, these bioinspired architectures with a well-defined morphology can emerge as key materials for applications in gas sensing, water treatment, photocatalysis, etc.

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2. Eggshell membrane mediated synthesis of 3D fibrous architectures based on ZnO

The eggshell membranes were obtained from commercial fresh hen eggs bought from local supermarkets using the following steps: (i) the eggs were gently broken, (ii) the eggshells were rinsed with distilled water in order to eliminate the yolk and the albumen, (iii) the eggshell membranes were manually stripped from eggshells and washed many times with distilled water for several days in order to remove any residual albumen traces (Figure 1a), (iv) the wet eggshell membranes were drying for a few days in air (ambient conditions) by fixing them mechanically between two overlapping stainless-steel frames for avoiding their warp (Figure 1b), and (v) the dried eggshell membranes were taken off from the metal supports and used as bio-template in the functionalization or replication processes. The FESEM images of the ESM reveal the distinctive 3D hierarchical porous architecture (Figure 1c) formed by interpenetrating fibers with diameters in the micrometer range (Figure 1d).

Figure 1.

Optical images of ESM during washing in water (a) and drying in air (b). FESEM images of ESM at two magnifications (c, d).

The preparation of ZnO-based architectures using ESM as bio-template was carried in accordance to the procedures described in the references [46, 47, 49] as follows:

  1. Radio frequency (RF) magnetron sputtering: The native ESM was coated with the ZnO layer using a ZnO sputtering target as source, and the process was carried in argon atmosphere using the following experimental parameters: 3 h — deposition time, 5.4 × 10−3 mbar — pressure in the chamber, and 100 W – power applied on the magnetron.

  2. Electroless deposition: The native ESM was firstly coated with a catalytic gold layer by magnetron sputtering, and the metalized ESM was subsequently immersed at 70°C for 2 h into an aqueous deposition solution containing zinc nitrate (0.07 M Zn(NO3)2 6H2O) and dimethylamineborane (0.01 M (CH3)2NHBH3), the growth of ZnO structures taking place only on the activated surface via the redox reactions.

  3. Biomorphic mineralization: The native ESM was immersed for 24 h at room temperature into an aqueous solution containing 0.3 M Zn(NO3)2 6H2O, the impregnated ESM being further placed on Si/SiO2 substrates, dried in air for 12 h at ambient conditions and subsequently calcined at 550°C for 3 h.

The following will be given some particularities of the electroless deposition and biomorphic mineralization processes. Hence, according to references [55, 62], the chemical reactions involved in the ZnO electroless deposition are:

  1. Zn(NO3)2 → Zn2+ + 2NO3−

  2. 2(CH3)2NHBH3 +4H2O → 2HBO2 + 2(CH3)2NH2+ + 5H2 + 2 e−

  3. NO3− + H2O + 2e− → NO2− + 2HO−

  4. Zn2+ + 2HO− → Zn(OH)2

  5. Zn(OH)2 → ZnO + H2O

Thus, in the presence of the Au catalyst, the dimethylamineborane is oxidized releasing electrons that further reduce nitrate ions to nitrite ions. In this way, the pH increases in the vicinity of the metalized surface, and the hydroxyl ions react with the zinc ions and lead to the formation of zinc hydroxide that subsequently is thermal dehydrated to ZnO. For this reason, the ZnO growth occurs mainly on the catalytic surface, the electroless deposition offering an excellent site-selectively deposition on complex structures such that of ESM, namely a 3D fibrous architecture.

As regards the replication of the ESM into ZnO, the functional chemical groups from the ESM surface [31] can assist the binding of zinc ions. The FESEM image (Figure 2a) and the corresponding EDX mapping (Figure 2b) of the ESM impregnated with zinc ions show the following chemical elements: (i) C, O, and N (ESM typical elements), (ii) Au (a thin layer of metal being deposited on the ESM prior to its morphological assessment), and (iii) Zn (zinc ions adsorbed on the ESM). The Zn distribution on the ESM surface confirms the ESM control through its functional groups on the uniform adsorption of the zinc ions during the immersion.

Figure 2.

FESEM (a) and EDX mapping (b) images of the ESM impregnated with zinc (used in the biomorphic mineralization).

The vibrational fingerprints of these functional chemical groups presented in the ESM protein fibers can be identified in the FTIR spectrum (Figure 3), the assignment of the absorption bands being made based on the literature data [32]: (i) stretching vibrations of the O-H and N-H groups at 3310 cm−1; (ii) asymmetric stretching vibrations of the C–H bond from =C–H and =CH2 groups at 3084 cm−1, 2966 cm−1 and 2878 cm−1; (iii) amide I, amide II, and amide III groups from the ESM protein structure at 1686 cm−1, 1560 cm−1 and 1240 cm−1 linked to vibrations of the C=O bond and the deformation vibrations of the C-N and N-H bonds; (iv) stretching vibrations of C=C at 1452 cm−1; (v) asymmetric and symmetric stretching of the -COO from the ESM protein structure at 1400 cm−1; (vi) vibration modes of C-O at 1082 cm−1; and (vii) vibration modes of C-S at 662 cm−1.

Figure 3.

FTIR spectrum of raw ESM.

Concerning the ESM thermal behavior during the calcination step involved in the biomorphic mineralization, according to the literature [63], the thermal decomposition of ESM is a multistep process: the first stage of decomposition starts very early at ~60°C due to the thermal denaturation of collagen and finishes at ~120°C; the second stage of decomposition takes place in the range between 250 and 450°C as a consequence of the thermal degradation of collagen.

Optical and FESEM images at low magnification of the investigated samples are presented in Figure 4 being labeled as follows: pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), and ZnO-functionalized ESM by electroless deposition (P2) and ZnO replica of ESM by biomorphic mineralization (P3). It can be noticed that the color of the samples varies from the white-ivory (P0) to beige (P1), brown (P2), and white-gray (P3). Also, it must be emphasized that the distinctive 3D framework of the ESM bio-template is well preserved during the functionalization process by RF magnetron sputtering and electroless deposition or is perfectly inherited during the replication process by biomorphic mineralization.

Figure 4.

Optical images and FESEM images of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

The structural information of the investigated samples was obtained by XRD analysis, while their optical properties were evaluated by diffuse reflectance and photoluminescence. Thus, XRD patterns, the reflectance spectra, the representation of the Kubelka–Munk function used for estimating the value of the band gap, and the photoluminescence spectra of the P0–P3 samples are presented in Figures 5–8, respectively.

Figure 5.

XRD patterns of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3). The star, circle, and rhombus symbols are linked to the peaks attributed to ZnO, Au, and Cu, respectively.

Figure 6.

Reflectance spectra of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

Figure 7.

Representation of Kubelka–Munk function employed to estimate the bad gap value of the ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

Figure 8.

Photoluminescence spectra of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

In Figure 5, the XRD pattern of P0 sample discloses the amorphous character of ESM owed to its composition based on amine, amides, and carboxylic compounds. The XRD patterns of P1–P3 samples reveal the peaks at 2θ: 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, and 62.8° assigned to (100), (002), (101), (102), (110), and (103) planes of the hexagonal wurtzite ZnO structure (ICDD 00-035-1451). Besides of these diffraction peaks, the XRD pattern of P2 sample exhibits peaks at 2θ: 38.2°, 44.4°, and 64.5° indexed to (111), (200), and (220) planes of the face-centered cubic Au structure (ICDD 03-065-2870), the metal playing the catalytic role in the ZnO electroless deposition. Because the P2 sample was fixed with a Cu tape on the substrate, the XRD pattern of this sample discloses also the peaks at 2θ: 43.3° and 50.4° associated to (111) and (200) planes of the face-centered cubic Cu structure (ICDD 00-004-0836).

In Figure 6, the reflectance spectrum of P0 sample shows a decrease at ~330 nm, in agreement to that reported for ESM [64]. In the reflectance spectra of P1–P3 samples, a strong decrease can be noticed at about 380–420 nm associated to the band-to-band transition in ZnO [22, 65].

In Figure 7, based on the reflectance data and using the Kubelka–Munk function, the band-gap value of ZnO was evaluated at approximately 3.1 eV (P1), 3.3 eV (P2), and 3.24 eV (P3) by plotting [F(R)*E]2 vs. photon energy (E), where F(R) = (1 – R)2/2R and R was the measured diffuse reflectance.

In Figure 8, the photoluminescence spectrum of P0 sample shows a broad emission band with two peaks at ~430 and ~470 nm, similar to those reported for ESM [66]. The photoluminescence spectra of P1 and P2 samples are dominated by the ESM emission, only a weak shoulder at ~550 nm (~2.25 eV) due to the ZnO emission being barely visible in the case of P2 sample. In the photoluminescence spectrum of P3 sample, it can be clearly noted the ZnO emission: (i) a weak band in the UV range centered at ~380 nm (~3.3 eV) linked to the band-band transition originating from the recombination of the free excitons and (ii) an intense and broadband in the visible domain centered at ~510 nm (~2.4 eV) related to various defects such as oxygen vacancy, zinc vacancy, interstitial oxygen, interstitial zinc, etc. [67].

The morphology of the investigated samples was evaluated by FESEM technique. The FESEM images acquired at different magnifications and in cross-sectional mode of the P0–P3 samples are given in Figures 9–11, respectively. In Figure 9 (left side at lower magnification), the FESEM image of P0 sample reveals a 3D porous interwoven fibrous network structure consisting in a cross-linked smooth fiber with diameter sizes in the micrometer range and pores of several micrometers. The FESEM images of P1–P3 samples illustrate that this specific hierarchical porous configuration is successfully preserved or replicated during the functionalization of replication, respectively. Hence, the experimental parameters involved in the RF magnetron sputtering, electroless deposition, or biomorphic mineralization processes are adequately chosen for avoiding the embedding of the ESM fibers into a metal oxide thick layer. In Figure 9 (right side at medium magnification), the FESEM images of P1–P3 samples disclose the following details: (i) RF magnetron sputtering leads to the deposition of a continuous and uniform ZnO structured film on the surfaces of the ESM fibers, (ii) electroless deposition results in a completely and uniformly covered of the ESM fibers with densely packed twin well faceted hexagonal prisms, and (iii) biomorphic mineralization directs an accurate replication process in which the inorganic replica inherit perfectly the original 3D interconnected fiber skeleton of the organic template.

Figure 9.

FESEM images (at two magnifications) of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

Figure 10.

FESEM images at higher magnification of ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

Figure 11.

Cross-sectional FESEM images of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2), and ZnO replica of ESM by biomorphic mineralization (P3).

The FESEM images at higher magnification from Figure 10 disclose the following details concerning the structure and the size of ZnO: (i) P1 sample — the ZnO film consists in nanoparticles with a relatively homogeneous size at ~20 nm, (ii) P2 sample — the ZnO prisms have sizes varying from 400 nm to 1 μm, and (iii) P3 sample — the ZnO fibers are built from particles with a relatively homogeneous size distribution in the range of ~10–30 nm. In addition, the cross-sectional FESEM images from Figure 11 emphasize interesting features such as (i) P1 and P2 samples — a hybrid structure based on ESM fiber organic core and ZnO inorganic shell, (ii) P1 sample — the thickness of the ZnO shell is of ~300 nm, (iii) P2 sample — a site-selectively growth of ZnO hexagonal prims only on the Au catalyst layer, and (iv) P3 sample — the assembly of ZnO nanoparticles as building blocks in the ZnO fibers. Hence, the mechanism involved in the ZnO growth by electroless deposition can be described as follows: (i) the nucleation process begins from the catalyst layer, (ii) the catalytic reaction leads to the formation of the ZnO nuclei, and (iii) when the activated surface of the fibers is entirely covered by the initial ZnO nuclei, these become the growth sites for the hexagonal prism structures. In the case of the biomorphic mineralization, the mechanism can take place as follows: (i) during the immersion in the zinc nitrate aqueous solution, the ESM governs the uniform adsorption of the zinc ions and (ii) during the calcination, the zinc-occupied positions become incipient centers of nucleation and initiate the growth of ZnO crystallites, the process being accompanied by the ESM burnout, and in this way, ESM directing the assembly of the ZnO crystallites in its particular architecture.

The topography of the investigated samples was analyzed by AFM technique. The AFM images of P0–P3 samples from Figure 12 show that there is no distortion in the fibrous architecture after functionalization or replication processes.

Figure 12.

AFM images of pristine ESM (P0), ZnO-functionalized ESM by RF magnetron sputtering (P1), ZnO-functionalized ESM by electroless deposition (P2) and ZnO replica of ESM by biomorphic mineralization (P3).

Thus, in comparison to the P0 sample, P1–P3 samples show a lowering in the roughness due to (i) the uniform coverage of the surface of the fibers with the compact nanostructured ZnO layer by RF magnetron sputtering, (ii) the growth of ZnO hexagonal prisms by electroless deposition, and (iii) the assembly of the ZnO nanoparticles into the fiber resulting in an accurate inorganic replica of the organic bio-template by biomorphic mineralization.

The behavior of raw ESM or ZnO-functionalized ESM in the presence of water is a key parameter for potential applications in water purification, photocatalysis, antibacterial areas, etc. Thus, when a water droplet is placed on the surface of the pristine ESM, a hydrophobic behavior (water contact angle of ~122°) is noted immediately after the water droplet placement (Figure 13a), similar to the results previously reported for ESM [68]. In this case, the water droplet does not penetrate into the membrane due to the air entrapped in its 3D porous structure. But this hindrance disappears very quickly (~1min) and a penetration of the water droplet into membrane occurs (Figure 13b). The same behavior is observed for ZnO-functionalized ESM by electroless deposition but the penetration time of the water droplet increasing at ~3–4 min, the presence of the ZnO structures on the surface of the ESM fibers being responsible for the decrease of the space between them. Consequently, after the functionalization, the hybrid structure conserves the ESM characteristic regarding the water absorption.

Figure 13.

The optical images of a water droplet immediately on its placement on the surface of the pristine ESM (a) and after its penetration into ESM (b).

In order to evaluate the potential integration of ZnO fiber webs in the electronic devices, sensors, etc., electrical measurements were carried on samples prepared as follows: the ESM impregnated with zinc ions were placed on Si/SiO2 substrates patterned with interdigitated metallic electrodes, dried in air, and subsequently calcined. The fabrication of the interdigitated metallic electrodes combines conventional photolithography and RF magnetron sputtering deposition. Hence, a photoresist was spin-coated on the Si/SiO2 wafers, the obtained layer was exposed to UV light through a mask, thermally treated, and after a developing procedure, the Ti (10 nm)/Pt (200 nm) thin layer was deposited. The Ti layer is required for the improvement of the adhesion of the Pt layer to the Si/SiO2 substrates. Finally, after the photoresist removal in a solvent, the interdigitated Ti/Pt electrodes were obtained. As can be seen in the FESESM image of the ZnO fiber webs prepared on Si/SiO2 patterned with interdigitated metallic electrodes, the semiconducting fibers formed bridges between the neighboring grid structures being electrically self-contacted (Figure 14a). Hence, the I–V characteristic of this sample recorded at room temperature in a two-point configuration when the bias was swept from –1 V to +1 V reveals a symmetrical nonlinear behavior (Figure 14b), indicating a Schottky contact between Pt and ZnO [69]. As consequence, the Pt/ZnO/Pt structure can be considered a system consisting of two back-to-back Schottky diodes, its ideality factor value being estimated at ~1.1 from the semilogarithmic representation of the I–V characteristic (Figure 14c). Because the ZnO fibers connect the interdigitated metallic electrodes, the transport of charges takes place by percolating through the semiconducting junctions until it reaches to the metallic electrodes [70].

Figure 14.

FESEM image (a), I-V characteristic (b), and semilogarithmic representation of the I-V characteristic (c) of the 3D fibrous networks based on ZnO.

Therefore, this approach that combines the biomorphic mineralization and substrates patterned with interdigitated metallic electrodes offers the opportunity to benefit in applications of both high surface-to-volume ratio and electrically self-contacted features.

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3. Conclusions

Eggshell membrane, a naturally occurring biopolymer with a unique architecture consisting in 3D porous interwoven fibrous protein network, was used as bio-template for developing ZnO-based nanomaterials using relatively simple and inexpensive preparation approaches such as radio frequency (RF) magnetron sputtering, electroless deposition, and biomorphic mineralization. The specific 3D fibrous web of the ESM was successfully preserved during its functionalization with ZnO by RF magnetron sputtering and electroless deposition or perfectly inhered by its ZnO replica obtained by biomorphic mineralization. Thus, FESEM images disclose that the ESM fibers were coated with a continuous and uniform metal oxide layer formed by ZnO nanoparticles (RF magnetron sputtering), covered with ZnO hexagonal prisms (electroless deposition), or replicated into metal oxide fibers containing ZnO nanoparticles as building blocks (biomorphic mineralization). The experimental parameters involved in the functionalization processes were properly chosen for avoiding the embedding of the ESM fibers into a metal oxide thick layer, the hybrid organic core/inorganic shell structure being emphasized by the cross-sectional FESEM images. The XRD patterns and reflectance spectra confirm the presence of ZnO in the functionalized or replicated samples while the photoluminescence spectra clearly evidence ZnO only in the replicated one. The wettability investigation shows that after functionalization procedures, the hybrid structures maintain the ESM characteristic regarding the water absorption. By combining the biomorphic mineralization method and the substrates patterned with interdigitated metallic electrodes, the electrical properties of the self-contacted ZnO fiber webs were investigated, the transport of charges taking place through the formed junctions over the interdigitated metallic electrodes. Thus, electrical paths between the neighboring grid structures are provided without any additional contacting steps.

Consequently, such bio-template mediated synthesis pathway based on the conversion of an abundant biowaste, such as ESM, into a source for obtaining new value-added nanomaterials can be very attractive from the environmental sustainability and recycle waste perspective, the ZnO-based fibrous architectures having potential applications in different areas (photocatalysis, water purification, electronic devices, sensors, etc.).

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Acknowledgments

This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS - UEFISCDI, project number PN-III-P4-PCE-2021-1131, within PNCDI III.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Nicoleta Preda, Marcela Socol, Andreea Costas and Irina Zgura

Submitted: 30 January 2024 Reviewed: 09 March 2024 Published: 08 June 2024

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.