https://orcid.org/0000-0002-7429-238X
Correction
25 Mar 2025: Bernal-Sánchez LJ, Vázquez-Olmos AR, Sato-Berrú RY, Mata-Zamora E, Rivera M, et al. (2025) Correction: Multifunctional hybrid films made from CoT3OTx4 and CoFeT2OT4 nanoparticles inside a poly 3-hydroxybutyrate matrix and study of their impact in methyl orange photodegradation. PLOS ONE 20(3): e0320895. https://doi.org/10.1371/journal.pone.0320895 View correction
Figures
Abstract
This work aims to produce hybrid materials with potential applications in dye photodegradation. Therefore, hybrid films were obtained by incorporating cobalt (II, III) oxide (Co3O4) or cobalt ferrite (CoFe2O4) nanoparticles (NPs) with 18 ± 1.6 nm and 26 ± 1.3 nm, respectively, into a poly 3-hydroxybutyrate (P3HB) polymeric matrix. The Co3O4@P3HB and CoFe2O4@P3HB hybrid films were fabricated by solvent casting in a ratio of 85 mg to 15 mg (P3HB-NPs). Different spectroscopic and microscopy techniques characterized the Co3O4 and CoFe2O4 NPs and the P3HB, Co3O4@P3HB and CoFe2O4@P3HB films. The optical band gap for Co3O4 and CoFe2O4 NPs was estimated from their diffuse reflectance spectra (DRS) around 2.5 eV. X-ray diffraction (XRD) of the hybrid films revealed that the nanometric sizes of the Co3O4 and CoFe2O4 nanoparticles incorporated into the P3HB are preserved. The magnetic hysteresis curve of CoFe2O4 nanoparticles and CoFe2O4@P3HB film showed a ferromagnetic behaviour at 300 K. Transmission electron microscopy (TEM) confirmed the formation of nanocrystals, and scanning electron microscopy (SEM) provided evidence for the successful incorporation of the NPs into the P3HB matrix. The surface roughness and hydrophilicity of the hybrid films are increased compared to the P3HB film. The impact of the nanoparticles and the hybrid films on the photodegradation of methyl orange (MO) in its acidic form was studied. The photodegradation tests were carried out by direct sunlight exposure. The CoFe2O4@P3HB hybrid film achieved 85% photodegradation efficiency of a methyl orange solution of 20 ppm after 15 minutes of exposure to sunlight. After 30 minutes of exposure to sunlight, the nanoparticles and the hybrid films reached about 90% of the MO degradation. The results suggest that combining nanoparticles with the polymer significantly enhances photodegradation compared to isolated nanoparticles.
Citation: Bernal-Sánchez LJ, Vázquez-Olmos AR, Sato-Berrú RY, Mata-Zamora E, Rivera M, Garibay-Febles V (2024) Multifunctional hybrid films made from CoT3OTx4 and CoFeT2OT4 nanoparticles inside a poly 3-hydroxybutyrate matrix and study of their impact in methyl orange photodegradation. PLoS ONE 19(10): e0312611. https://doi.org/10.1371/journal.pone.0312611
Editor: Amir Elzwawy, National Research Centre, EGYPT
Received: June 27, 2024; Accepted: October 10, 2024; Published: October 31, 2024
Copyright: © 2024 Bernal-Sánchez et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the manuscript.
Funding: LJBS, CONAHCyT, 885692, funded the PhD Scholarship. ARVO, UNAM-PAPIIT project IN113423 funded this research. The funders had no role in the study design, data collection and analysis, publication decision, or manuscript preparation.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Over the past three decades, extensive research has been conducted to obtain nanostructured materials of several kinds. The primary chemical and physical synthesis methods produce nanostructured powders or colloidal dispersions [1–3]. However, in many cases, the applications of these nanomaterials are enhanced when dispersed in an organic or inorganic matrix that does not interfere with their properties or even improve them [4]. In this sense, several attempts have been made to integrate inorganic NPs into a polymeric matrix, forming hybrid materials (NPs@polymer) that interact through molecular or nanometric scale interactions [5,6]. These compounds have emerged as a promising alternative to exploring and improving the applications of metallic oxide nanoparticles due to their cost-effectiveness, photoconductivity of the electrons, low density, and other outstanding properties [4,6,7]. In this work, multifunctional hybrid materials were prepared by incorporating cobalt (II, III) oxide (Co3O4) and cobalt ferrite (CoFe2O4) nanoparticles into a poly 3-hydroxybutyrate (P3HB) matrix to evaluate their potential in dye photodegradation. The Co3O4 NPs are an antiferromagnetic p-type semiconductor with a narrow optical band gap that varies from the range of 1.45 to 2.7 eV [8]. This compound is a mixed-valence oxide [Co(II)Co(III)2O4] with a normal spinel crystal structure based on an array of cubic close-packed oxide ions, in which Co(II) ions occupy the tetrahedral 8a sites, and Co(III) ions occupy the octahedral 16d sites [9]. This cobalt oxide has many industry applications, including biomedical technology [10], anode materials for rechargeable Li-ion batteries [11], energy device storage [12], and especially as an effective catalyst [13]. Otherwise, CoFe2O4 is also a semiconductor with a narrow bandgap ranging from 0.11 to 2.6 eV [14]. This ferrite has an inverse spinel-type structure, where the tetrahedral positions are occupied by trivalent cations (B). In contrast, the divalent anions (A) and trivalent cations (B) occupy the octahedral positions in a spatial arrangement [B]Td[AB]ohO4 [15]. The particle size, morphology and surface effects of said ferrite determine its applications in different areas [16], which are related to magnetic, magneto-optic, electrical, chemical, electrochemical, thermal, photoelectrochemical, thermoacoustic, and adsorption properties [17], including its use as a catalyst for removing pollutants in an aqueous medium [18–23].
On the other hand, biopolymers offer a promising alternative to conventional polymers, which are linked to fossil resource consumption, microplastic formation, non-degradability, and limited end-of-life options. Among these biopolymers, poly (3-hydroxybutyrate) (P3HB) emerges as a promising option [24]. This is a thermoplastic and isotactic polymer with a tensile strength like polyethylene. It is insoluble in water and highly crystalline, with a glass transition temperature of 4°C, a melting temperature of 180°C, and a degradation temperature of around 270°C [25]. Due to these properties, it has potential applications in several fields, such as biodegradable packaging [26,27], controlled drug release [28], and synthetic prostheses [29]. Therefore, combining P3HB with metal oxide nanoparticles can create hybrid films, incorporating the nanoparticles’ chemical and physical properties and the polymer’s mechanical properties [30]. Despite the extensive research on metallic oxide NPs and biopolymers, limited studies exist on creating multifunctional hybrid materials employing these molecules [31–38]. Moreover, it has been reported that around 280,000 tons of synthetic dyes are released annually through industrial discharge [39]. These dyes are resistant to biodegradation and can harm aquatic life and photosynthesis. Among these dyes, Methyl orange (MO) is widely used in the cosmetic, pharmaceutical, and textile industries. It is also known to cause skin diseases, respiratory tract infections, and eye irritation [39]. Therefore, removing and decolorizing these harmful dyes from water resources is crucial. Even though the degradation of MO has been studied extensively, conventional wastewater treatment plants still need to find ways to eliminate this molecule, considering its effects on the environment and human health [40–42]. This study represents the first successful endeavor to produce hybrid films using Co3O4 and CoFe2O4 NPs and the poly (3-hydroxybutyrate) (P3HB) polymer. The NPs and hybrid films obtained were also investigated for their effectiveness in the photodegradation of MO in its acid form.
Materials and methods
Materials
All chemical reagents were purchased from Sigma-Aldrich and were used as received without further purification. Cobalt (II) chloride hexahydrate (CoCl2•6H2O) (purity = 98%), iron (III) chloride hexahydrate (FeCl3•6H2O) (purity≥98%), sodium hydroxide (NaOH) (purity≥98%), and acetone (CO(CH3)2) (99.5%). Poly (R)- 3-hydroxybutyric acid (P3HB) and anhydrous chloroform (purity≥99%). Methyl Orange ACS reagent and linseed oil.
Nanoparticles synthesis
The Co3O4 NPs were obtained from 1x10-3 moles (0.24 g) of cobalt (II) chloride hexahydrate (CoCl2•6H2O) grounded in an agate mortar with 2x10-3 moles (0.08 g) of sodium hydroxide previously milled. The mixture was milled for 30 minutes until the powder remained unchanged. The obtained product was a dark green Co(OH)2 powder, which was washed four times with cold distilled water and then twice with acetone. In each case, the product was separated by centrifugation at 3500 rpm for 10 minutes. Finally, it was air-dried at room temperature and heated to 600°C for 2 hours. The resulting product was a black powder made up of Co3O4 nanoparticles. The general chemical reaction is as follows:
Furthermore, CoFe2O4 NPs were synthesized following the previous procedure from 1x10-3 moles (0.24 g) of cobalt (II) chloride hexahydrate (CoCl2•6H2O) and 2x10-3 moles (0.54 g) of iron (III) chloride hexahydrate (FeCl3•6H2O) which was milled with 1x10-3 moles (0.32 g) of sodium hydroxide (NaOH). A dark brown powder was obtained, washed, air-dried, and then heated at 800°C for 2 hours. Finally, a black powder formed by CoFe2O4 NPs was obtained. The general chemical reaction is presented below, and the general procedure to get the nanoparticles is depicted in Fig 1.
(a) Milling the precursor metallic salts with sodium hydroxide for 30 minutes. (b) The product is washed, air-dried, and then calcinated for 2 hours.
Hybrid film synthesis
The hybrid films were obtained by solvent-casting method (Fig 2). First, 85 mg of the finely poly (3-hydroxybutyrate) (P3HB) was dissolved in 15 mL of chloroform at 80°C under constant stirring for 10 minutes (Fig 2A). Then, in an ultrasonic bath, 15 mg of the previously synthesized Co3O4 or CoFe2O4 NPs were dispersed in 10 mL of chloroform for 10 minutes (Fig 2B). After dissolving the P3HB and dispersing the NPs, both phases were mixed in a beaker at 80°C while constantly stirring. A film was formed when around 90% of the solvent evaporated. This film was dissolved in 25 mL of chloroform and added 0.05 mL of linseed oil, which acts as a plasticizer. The mixture was heated at 80°C and constantly stirred to produce the final hybrid film (Fig 2C).
(a) P3HB is dissolved in chloroform. (b) The NPs are dispersed in chloroform and added to the P3HB solution. (c) Hybrid film forms after the first film is redissolved.
Instruments for nanoparticles and hybrid films characterization.
The X-ray diffraction patterns were performed with Cu Kα radiation (λ = 1.5406 Å) in a diffractometer Empyrean PIXel 1D Malvern Panalytical; diffraction intensity was measured between 10° and 70°, with 2θ step of 0.01°, for 0.25 s per point. The average crystal size (D) of the NPs was estimated from their diffractograms by the Debye-Scherrer formula, D = Kλ/βcosθ where K is the shape factor equal to 0.9, λ is the Cu Kα radiation, β is the full width at half maximum intensity of selected peaks (FWHM), and θ is the Bragg angle. Transmission electron micrographs were obtained with a (TEM) FEI Tecnai F20 microscope, operating at 200 KV S/TEM with an X-TWIN lens and a high brightness field emission electron gun (FEG). Raman spectra of 100 to 900 cm−1 were acquired in a Nicolet Almega XR dispersive Raman spectrometer and detected by a CCD camera at 25 s and a resolution of ~4 cm−1. The excitation beam was an Nd:YVO4 532 nm laser and the incident power on the sample was ~3 mW. Furthermore, 1 mg of NPs with 100 mg of potassium bromide (KBr) was pressed into a pellet. The pellet was adjusted and analyzed with a Nicolet Nexus 670 Fourier Transformed Infrared (FTIR) spectroscopy from 4000 to 400 cm-1 with a resolution of 4 cm-1. Ultraviolet-visible (UV–Vis) absorption spectra of the powdered samples were obtained by the diffuse reflectance technique with an Ocean Optics USB2000 miniature fiber-optic spectrometer. The nanoparticles’ optical band gap energy was determined by Tauc’s plot, obtained from their corresponding Diffuse Reflectance Spectra (DRS). According to Tauc’s equation [43] for a direct bandgap:
(1)
where α(2.303 cm-1) is the absorption coefficient, hυ is the photon energy,
Eg the bandgap energy (eV), and A is constant depending on the type of transition. When αhυ = 0,Eg = hυ. The bandgap energy is determined by plotting (αhυ)2 versus hυ and finding the intercept on the hυ axis by extrapolating the plot to(αhυ)2 = 0. The magnetization curves of the CoFe2O4 NPs were obtained at room temperature in a Vibrating Sample Magnetometer (VSM) Quantum Desing MPMS3 with an applied field of 20 kOe.
Scanning Electron micrographs were obtained with Philips equipment ESEM XL 30 at 5 kV to characterize the films. Raman spectra of 200 to 3500 cm−1 were acquired with a WItec Alpha 300 XR dispersive Raman spectrometer and detected in a 180° backscatter configuration. The excitation beam was an Nd: YVO4 532 nm laser and the incident power on the sample was ~1 mW. The topographic profiles and roughness parameters were performed in an Atomic Force VEECO NANOSCOPE IV+Multimode MMAFM/ST microscope. The tapping mode was employed with a scanning rate of 1 Hz and a scan resolution of 256 lines. The scan area was set at 50 μm x 50 μm. We used a Tap300GD-G 300 kHz, 40 N/m tip. All images were processed with the software NanoScope Analysis v1.40. Water drops (30 μL) were applied to three areas of the film’s surface with Pocket Goniometer Model PG-1 for contact angle measurement.
Photodegradation assays.
Methyl Orange solutions were prepared at pH 3 with concentrations ranging from 20 to 2 mg/L (20-2ppm). The UV-Vis spectra of these solutions were recorded, and the band at 507.40 nm corresponding to the quinoid structure (N-N-H) of the MO in its acid form was used as a reference to create a calibration curve, as shown in Fig 3.
(a) Methyl Orange (MO) structure in its acid form, (b) UV-Vis spectra of MO from 2 to 20 mg/L, (c) Table of the MO concentration vs absorbance and (d) calibration curve.
A stock solution of MO (20 mg/L) was prepared in darkness and adjusted to pH 3.0 by adding 0.1M of hydrochloric acid (HCl). Then, 15 mg of Co3O4 or CoFe2O4 NPs were added to 30 mL of the MO solution, and the mixture was exposed to sunlight under constant stirring. For 2 hours, 1 mL of the mixture was collected every 5 minutes during the experiment. The same procedure was followed with the Co3O4@P3HB and CoFe2O4@P3HB hybrid films. Once the experiment was completed, the corresponding dispersions were stored in amber bottles to prevent direct exposure to light. All the experiments were repeated three times to ensure reproducibility. The percentage of dye photodegradation can be estimated using the following expression [44].
C0 (mg/L) is the dye concentration at the initial stage, and C (mg/L) is the dye concentration at specific time intervals upon sunlight illumination. Additionally, the films were subjected to a recycling process after using the hybrid films for the first time (cycle 1). Subsequently, they were washed three times with distilled water to remove the adsorbed dye on its surface. Then, it was kept in an oven at 60°C for 24 hours to eliminate water molecules and reused for the photodegradation of methyl orange. The same procedure was repeated three times. The kinetic model of the fabricated nanoparticles and hybrid films was thoroughly analyzed with an equation to study the relationship between time (t) in minutes as the x-axis and In(C0/C) as the y-axis [44]. The methyl orange photodegradation rate constant (k) (min-1) was calculated by Eq (3).
Results and discussion
Characterization of the Co3O4 and CoFe2O4 nanoparticles
The powder X-ray diffraction patterns of Co3O4 and CoFe2O4 NPs are shown in Fig 4. For cobalt (II, III) oxide NPs, all reflections in the XRD patterns (Fig 4A) can be attributed to the face-centered cubic structure, with space group Fd-3m and crystal lattice parameters a = 8.083 Å. According to the Joint Committee on Powder Diffraction Standards (JCPDS) 42–1467 card [45], the main characteristics peaks at 19.2° (111), 31.4° (220), 37.0° (311), 38.7° (222), 45.0° (400), 56.0° (422), 59.5° (511), 65.4° (440) are observed. On the other hand, the XRD pattern of the CoFe2O4 NPs (Fig 4B) corresponds to inverse spinel-type structure and cubic phase, with the same space group as de cobalt oxide, and crystal lattice parameters a = 8.4 Å according to the JCPDS 22–1086 card [46], in this case, all the reflections appear at 18.5° (111), 30.3° (220), 33.3° (103), 35.7° (311), 37.3° (222), 43.3° (400), 49.6° (331), 53.7° (422), 57.2° (511), 62.8° (440), 64.2° (531). In both systems, no additional peaks owning to other phases are observed. Scherrer’s equation estimated the average crystallite sizes of 18 ± 1.6 nm and 26 ± 1.3 nm for Co3O4 and CoFe2O4 NPs, respectively. Figs 5 and 6 show representative TEM micrographs of each oxide, confirming the presence of nanocrystals with close dimensions to those determined from their corresponding X-ray diffraction patterns. Representative images of Co3O4 particles with cuboid shapes are shown in Fig 5A. A zoom image of a cuboid measuring 12 nm in length, 10 nm in width, and 9.5 nm in height dimensions is depicted in Fig 5B. Moreover, Fig 6A and 6B show representative TEM images of a cube and a cuboid shape of CoFe2O4 measuring approximately 24 nm and 22.5 nm, respectively.
(a) Co3O4 and (b) CoFe2O4.
(a) Cuboid shapes and (b) zoom image of a cuboid.
(a) Cube and (b) cuboid shapes.
On the other hand, Raman scattering spectra of the nanostructured powder of Co3O4 and CoFe2O4 NPs (Fig 7) confirms the structure and composition of the phases. Raman spectroscopy is an effective technique for identifying lattice phenomena exhibiting vibrational mode variations. For normal spinel where Co2+ and Co3+ ions occupy the sites tetrahedral and octahedral, respectively. This compound presents a symmetry group
. According to group theory, the following irreducible representation is predicted:
Being five modes active in Raman. Raman spectrum of the Co3O4 NPs (Fig 7A) shows five characteristic bands in the 100 to 800 cm-1 regions. The signal with a maximum of 665 cm-1 was assigned to the A1g mode, characteristic of the octahedral sites. In contrast, the signal at 468 cm-1 corresponding to the Eg mode and the signals with a maximum at 190, 508, and 605 cm-1 associated with the three modes F2g are related to the combined vibrations of the tetrahedral site and octahedral oxygen motions [47]. Similar behavior has been observed for CoFe2O4 NPs (Fig 7B). In this case, six main signals are shown. The peak at 680 cm-1 (A1g (1)) has been assigned to the symmetric stretching of Fe-O4 tetrahedral bonds, proving the inverse-spinel structure. The signals at 302 cm-1 (Eg) and 560 cm-1 (F2g (3)) are related to the symmetric and asymmetric bending of oxygen concerning Fe (or Co), respectively. The peak at 464 cm-1 (F2g (2)) is assigned to the asymmetric stretching of the Fe(Co)-O bond, whereas the signal at 188 cm-1 (F2g (1)) is attributable to the translational movement of the whole tetrahedron. Furthermore, an additional signal at 612 cm-1 (A1g (2)) corresponds to the stretching vibrations of the Fe-O and M-O bonds in tetrahedral sites is observed [48]. Besides, the FT-IR spectra confirm the formation of the spinel structure for both the Co3O4 and CoFe2O4 nanoparticles. For cobalt oxide (Fig 7C), two characteristic signals are observed below 1000 cm-1 associated with α-Co3 O4spinel. The first band at 695 cm-1 was assigned to the bridging vibration of νCo-O mode at the octahedral and tetrahedral sites. In contrast, the peak at 604 cm-1 corresponds to the νCo-O stretching vibration mode at the octahedral sites. Also, a low-intensity band is observed around 3500 cm-1 due to water adsorption [49]. Besides, the spectrum for cobalt ferrite (Fig 7D) shows a band located at 589 cm-1 assigned to the νFe-O mode of the tetrahedral sites of the inverse spinel. Another signal at 399 cm-1 is associated with νCo-O and νFe-O vibrations in the octahedral sites [50].
Raman spectra of (a) Co3O4 NPs and (b) CoFe2O4 NPs. FT-IR spectra of (c) Co3O4 NPs and (d) CoFe2O4 NPs.
The Diffuse Reflectance Spectra (DRS) corresponding to Co3O4 and CoFe2O4 NPs are shown in Fig 8A and 8B, respectively. The sample’s optical band gap was determined by extrapolating the (αhv)2 (eVcm-1)2 versus hυ. The absorption energy corresponding to the band gap (Eg) at α = 0 is 2.4 eV for Co3O4 NPs and 2.5 eV for CoFe2O4 NPs when the value of hυ is extrapolated. A semiconductor metal oxide-mediated photodegradation involves exciting electrons from the valence band to the conduction band, generating electron-hole pairs. The electron and hole then interact with O2 and H2O to produce oxidative radicals that break down organic compounds. In this scenario, the Co3O4 and CoFe2O4 NPs have an energy band gap (Eg) that allows them to be active under visible light exposure [8,51].
For (a) Co3O4 NPs and (b) CoFe2O4 NPs.
Moreover, the hysteresis curve of the CoFe2O4 NPs (Fig 9A) was obtained from -60 to +60 KOe at 300 K. A characteristic ferrimagnetic behaviour was observed, with a saturation magnetization (Ms) of 0.49 emu, a remanence magnetization (Mr) of 0.20 emu and a coercive field (Hc) of 1188 Oe, as depicted in Fig 9B. The magnetization values observed are lower than those of cobalt ferrite in its bulk form and other nanostructures of these inverse spinel [51,52]. However, it is important to note that the magnetic properties depend not only on the size and shape of the nanoparticles but also on the synthesis method used in its preparation [52]. The hysteresis curve of the CoFe2O4@P3HB hybrid film (Fig 10A) was obtained under the same conditions used to get the CoFe2O4 NPs loop. A weak ferrimagnetic behavior was observed, with a Ms of 0.017 emu, a Mr of 0.007 emu, and a Hc of 1142 Oe. The magnetic response decreases in this case because the nanoparticles are incorporated into a diamagnetic polymer. Furthermore, once embedded in the polymer matrix, the distance between the nanoparticles increases, reducing their magnetic coupling. However, the magnetic coercivity is similar in both cases, indicating that the nanoparticles do not lose their magnetic force once inside the film. Although several magnetic parameters are reduced in the hybrid film compared to the isolated NPs, the CoFe2O4@P3HB film’s ability to magnetize is remarkable, as shown in Fig 10B.
(a) Hysteresis curve (M-H) corresponding to CoFe2O4 NPs and (b) zoom image of the hysteresis loop.
(a) Hysteresis curve (M-H) corresponding to CoFe2O4@P3HB hybrid film and (b) zoom image of the hysteresis loop, including an image of the hybrid film attracted by magnets.
Characterization of the films
All films were characterized by Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The roughness and hydrophilicity of the films were evaluated by atomic force microscopy (AFM) and through contact angle measurement, respectively. The results obtained are presented below.
Characterization of the P3HB film
The X-ray diffraction pattern of P3HB film is presented in Fig 11A. The diffraction peaks at 13.5°, 17.2°, 20.2°, 21.6°, 22.7°, 25.7°, 27.3°, and 28.5° were assigned to the (020), (110), (021), (101), (111), (130), (040) and (002) planes, respectively, according to those reported in the (International Center for Diffraction Data) crystallographic card ICDD 49–2212 and corresponds to those reported by Rincón-Granados et al. [37]. Also, the Raman spectrum of P3HB film (Fig 11B) was obtained, and the Raman active modes due to the biopolymer were observed. The thermal treatment during the film synthesis process produces the reorganization of the polymeric chains; therefore, the Raman signals in the range from 2860 to 3000 cm-1 corresponding to the group’s (νC-H), (νsCH2), (νsCH3), and (νasCH2) appear slightly displaced. These bands are characteristic of the crystalline state of the polymer. The band at 1730 cm-1 is assigned to the vibration of the symmetric νsC = O group, and the asymmetrical vibration of this group νasC = O appears at 1650 cm-1. The band at 1447 cm-1 corresponds to the asymmetrical bending of methyl and methylene groups (δCH2, δCH3). Additionally, the bands between 1224–1360 cm-1 are associated with the bending of methylene groups (δCH), the wagging of methylene groups (wCH2) and the characteristic band of the helical configuration of the crystalline state. The band at 1097 cm-1 corresponds to the asymmetrical stretching vibration of the ester group (νasC-O-C). The signal at 1062 cm-1 is identified as the vibration of the symmetric νsC-CH3 group. The bands in the range from 870 cm-1 to 945 cm-1 are due to the stretching vibration of carbonated chains (νC-C), and the band at 840 cm-1 is a consequence of the stretching vibration of the νC-COO group. Finally, the bands between 200–800 cm-1 could be attributed to the out-of-plane bending of carbonyl groups (γC = O). These last bands are associated with the interactions between the polymer chains in their arrangement film.
(a) X-ray diffraction pattern, including the P3HB film image and (b) its corresponding Raman spectrum.
Characterization of the Co3O4@P3HB and CoFe2O4@P3HB hybrid films
The Co3O4@P3HB and CoFe2O4@P3HB hybrid films’ Raman spectra are shown in Fig 12A and 12B. There are not some noticeable changes when comparing the hybrid films to the P3HB film. The intensity of the polymer bands in the hybrid films remains unchanged because the amount of polymer is much higher than that of the NPs (15% NPs-85% P3HB w/w). However, the analysis of the hybrid films’ Raman spectra suggests that the interaction with the NPs occurs through the carbonyl and carboxyl groups of P3HB since the region from 840 to 200 cm-1 is the most affected in the hybrid films. Although there is evidence of interaction between the polymeric matrix and nanoparticles, XRD proved to be a more sensitive technique for analyzing these hybrid films.
Raman spectra of (a) Co3O4@P3HB and (b) CoFe2O4@P3HB films.
Fig 13A and 13B show the XRD patterns of Co3O4@P3HB and CoFe2O4@P3HB hybrid films, respectively. The characteristic diffraction peaks of the P3HB and the Co3O4 (JCPDS 42–1467) or CoFe2O4 (JCPDS 22–1086 cards) are observed, which confirms that the NPs were successfully included in the polymeric matrix.
X-ray diffraction patterns of (a) Co3O4@P3HB and (b) CoFe2O4@P3HB. A representative image of the corresponding hybrid films is included.
Furthermore, Scherrer’s equation estimated the average crystallite size of each oxide included in the hybrid film of 19 ± 1 nm for Co3O4@P3HB film (Fig 13A) and 27 ± 0.8 nm for CoFe2O4@P3HB film (Fig 13B). These sizes closely match those obtained from the XRD patterns of the crystallites of each oxide before being incorporated into the polymer matrix (18 ± 1.6 nm for Co3O4 and 26 ± 1.3 nm for CoFe2O4). Therefore, the experimental processing used to create the hybrid films did not affect the size of the NPs in the polymeric matrix.
On the other hand, SEM micrographs of the P3HB film and the NPs@P3HB hybrid films are presented in Fig 14. The first image (Fig 14A) shows the surface of P3HB films obtained by the solvent casting method. This surface has irregular pores of different sizes, possibly caused by the removal of the volatile solvent. Conversely, the surface of the hybrid films (Fig 14B and 14C) reveals a rough surface with no pores, suggesting that the presence of the nanoparticles provides a significant change in the surface. From a physicochemical point of view, slower kinetic solvent evaporation in the films containing the NPs could be the reason for the reductions in surface pore roughness.
(a) P3HB film, (b) Co3O4@P3HB, and (c) CoFe2O4@P3HB hybrid films.
The thickness of all the films was measured using a micro vernier at three different points on three films of each system studied. The P3HB film without NPs has a thickness of 551 ± 25 μm, while the Co3O4@P3HB and CoFe2O4@P3HB hybrid films have thicknesses of 261 ± 15 μm and 362 ± 19 μm, respectively. The method used to obtain the films is simple and demonstrates good reproducibility. The hybrid films are thinner than the P3HB film without NPs, partly because the latter has 15% less polymer by weight. In addition, the sizes of the NPs are different, with CoFe2O4 NPs being the largest (27 nm), which could also influence the thickness of the films.
On the other hand, the films’ surface roughness was analyzed using atomic force microscopy (AFM). Fig 15 shows the 2D and 3D images of the AFM height mapping, which illustrate the variations in surface topography between the hybrid films and the P3HB film. The root mean square (RMS) roughness was determined by averaging the measurements from three different lines on the surface of each sample. The RMS values were 18 ± 0.7 nm for P3HB, 26 ± 0.8 nm for Co3O4@P3HB, and 24 ± 0.6 nm for CoFe2O4@P3HB. These results suggest that adding Co3O4 or CoFe2O4 nanoparticles to a P3HB matrix results in a similar increase in the surface roughness of the hybrid films. The hybrid films show more consistent roughness across the surface, as observed in the 3D image and confirmed by the standard deviation values of the RMS measurements. Therefore, the roughness induced by nanoparticles in the P3HB matrix mainly occurs at the nanometer level, as expected for these hybrid materials.
(a) P3HB film, (b) Co3O4@P3HB hybrid film, and (c) CoFe2O4@P3HB hybrid film.
The hydrophilicity of the films was determined using the contact angle technique. Table 1 presents the average contact angle measurements for three films of each system. The contact angle for the P3HB film was 80° ± 2, while for the Co3O4@P3HB and CoFe2O4@P3HB hybrid films, it was 53° ± 2 and 43° ± 2, respectively. Representative photographs of the water droplet shape used for measuring the contact angles of the films are shown in Fig 16.
(a) P3HB film, (b) Co3O4@P3HB film and (c) CoFe2O4@P3HB film.
Photodegradation studies
The study of sunlight-induced degradation of methyl orange (MO) in its acid form was conducted, employing the Co3O4 and CoFe2O4 NPs and Co3O4@P3HB and CoFe2O4@P3HB hybrid films previously obtained. In Figs 17 and 18, the behavior of the absorption band at 507.40 nm over time is observed. This band is associated with MO’s quinoid structure (N-N-H) in its acidic form. Fig 17A and 17B show the photodegradation induced by the Co3O4 NPs and the Co3O4@P3HB hybrid film, while Fig 18A and 18B show the same for CoFe2O4 NPs and CoFe2O4@P3HB, respectively. Both systems successfully promote the photodegradation of MO when they are directly exposed to sunlight.
In the presence of (a) Co3O4 NPs, (b) Co3O4@P3HB hybrid film. (a) and (b) Were exposed directly to sunlight.
In the presence of (a) CoFe2O4 NPs, (b) CoFe2O4@P3HB hybrid film. (a) and (b) Were exposed directly to sunlight.
On the other hand, Fig 19 illustrates the change in MO concentration (C/C0) over time exposed to sunlight. Fig 19A displays the results for the Co3O4 NPs and Co3O4@P3HB film, while Fig 19B depicts the results obtained with CoFe2O4 NPs and CoFe2O4@P3HB film. In both cases, MO degradation was achieved in less than 30 minutes.
(a) Co3O4 NPs and Co3O4@P3HB hybrid film. (b) CoFe2O4 NPs and CoFe2O4@P3HB hybrid film.
Although better results were obtained with the isolated nanoparticles due to a larger surface contact area, incorporating the nanoparticles into a polymeric matrix offers broader potential applications. Fig 20 shows the colour changes observed in each system of nanoparticles and hybrid films over time.
(a) Co3O4 NPs, (b) Co3O4@P3HB hybrid film, (c) CoFe2O4 NPs, and (d) CoFe2O4@P3HB hybrid film.
In addition, the degradation efficiency of MO was evaluated using Eq (2). Table 2 presents the degradation efficiency (%) of methyl orange (MO) under sunlight exposure. The experiment involved 15 mg of Co3O4 or CoFe2O4 NPs dispersed in 30 mL of 20 mg/L of MO solution at pH 3. Additionally, the table contains data on the degradation efficiency under the same conditions for the Co3O4@P3HB and CoFe2O4@P3HB hybrid films obtained from 15 mg of nanoparticles dispersed in 85 mg of P3HB. All experiments were performed in triplicate.
The results in Table 2 show that using isolated nanoparticles leads to 50% photodegradation efficiency for Co3O4 NPs and 55% for CoFe2O4 NPs after 15 minutes of sunlight exposure. However, the photodegradation significantly increased when the hybrid films were used under the same conditions, reaching 75% for the Co3O4@P3HB film and 85% for the CoFe2O4@P3HB film at 15 minutes. After 25 minutes of sunlight exposure, the photodegradation of the isolated nanoparticles and the hybrid films was very similar, reaching 94% and around 90%, respectively. After 30 minutes, no changes were observed in either system. These results represent a significant finding because the hybrid films have a lower concentration of nanoparticles in contact with the MO solution than the isolated nanoparticles, which have a larger contact area.
However, a valid comparison can only be made between NPs and hybrid films fabricated with NPs of the same chemical composition. In this sense, as shown in Fig 21A and 21B, the Co3O4@P3HB hybrid film was 25% more efficient than Co3O4 NPs, and the CoFe2O4@P3HB film was 35% more efficient than CoFe2O4 NPs after 25 minutes of exposure to sunlight. Besides, the higher hydrophilicity of the CoFe2O4@P3HB film could contribute to its improved initial response.
Photodegradation efficiency (%) of Co3O4 and CoFe2O4 nanoparticles, and Co3O4@P3HB and CoFe2O4@P3HB hybrid films after sunlight exposure (a) 15 minutes and (b) 25 minutes.
These results suggest that the P3HB polymer also acts synergistically with the NPs to promote the photodegradation of MO. This would explain why, over time, both NP systems and hybrid films reach similar photodegradation efficiencies. Under the same conditions evaluated, the P3HB film without nanoparticles showed a maximum photodegradation efficiency of 10% after 30 minutes of sunlight exposure and remained unchanged (not shown here).
Furthermore, Fig 22 presents the photodegradation efficiency (%) of the MO by the hybrid films after 25 minutes of exposure to sunlight and three cycles of use. It was observed that the photodegradation efficiency decreased from 89% to 79% for the Co3O4@P3HB film and from 90% to 81% for the CoFe2O4@P3HB film after the third cycle of use. This decrease of around 10% in efficiency could be attributed to the loss of some active sites after each use.
The photodegradation rate constants in min-1 for all the systems after 15 min of sunlight exposure (Fig 23) were estimated to be 0.046 ± 0.002 and 0.091 ± 0.004 for Co3O4 NPs and Co3O4@P3HB film. For CoFe2O4 NPs and CoFe2O4@P3HB film, 0.053 ± 0.003 and 0.126 ± 0.006, respectively. According to these results, the hybrid films show the highest rate of photodegradation. These findings are consistent with previous studies suggesting that the reaction rate of dye photodegradation can be explained by a model with pseudo-first-order kinetic rate constants [44].
The degradation of the methyl orange molecule in its acid form employing metal oxide nanoparticles (Co3O4 or CoFe2O4 NPs) incorporated in a polymeric matrix (P3HB) under solar light can be achieved through the following equations:
This process (Fig 24) is made possible by the absorption of photons by the cobalt oxide or the cobalt ferrite on the surface of the hybrid films. These create holes in the valence band and excite the electrons in the conduction band. The holes then react with H2O to form •OH and H+. Meanwhile, the electrons in the conduction band react with dissolved oxygen to form superoxide radicals. The superoxide radicals can further react with H2O to form perhydroxyl radicals. Hydroxyl radicals, superoxide, and perhydroxyl compounds effectively degrade methyl orange in its acid form [44].
Finally, the CoFe2O4@P3HB hybrid film exhibits a ferrimagnetic behaviour at room temperature and a better photodegradation rate at 15 minutes, making it the most attractive material for future applications since it could be removed from a medium through a magnet. Future research might evaluate different concentrations of nanoparticles within the P3HB polymeric matrix to achieve 100% photodegradation of the MO. Furthermore, it could be extended to assess the response of such films to different pHs.
This work aimed to create two hybrid films using a biopolymer and nanoparticles of a mixed oxide of cobalt (II, III) and cobalt ferrite through a simple and reproducible method. This motivates future research to produce hybrid films with other polymers and with different metal oxide nanoparticles.
Conclusions
The successful incorporation of Co3O4 and CoFe2O4 nanoparticles in a P3HB polymeric matrix has led to hybrid materials that significantly enhance the photodegradation of methyl orange in its acid form. Incorporating nanoparticles into the polymeric matrix increases the hydrophilicity and the roughness of the film surfaces, favouring the interaction with the MO solution. Photodegradation measurements have revealed that combining the nanoparticles with the polymer significantly improves the degradation performance compared to isolated nanoparticles. The CoFe2O4@P3HB film, in particular, has exhibited the best photodegradation performance, achieving a photodegradation efficiency of 85% in only 15 minutes of exposure to sunlight and a photodegradation rate of 0.126 min-1. These findings inspire the potential use of CoFe2O4@P3HB hybrid films in addressing various environmental challenges, given their promising biodegradability, magnetic response, cost-effectiveness, and high photodegradation activity.
Acknowledgments
We want to thank Mr. José Arturo Martinez for the hysteresis curves, Dr. Jesús M. Rivera for the contact angle measurements, Dr. Marco Antonio Valadez for the AFM analysis, and Dra. Elsi Violeta Mejía Uriarte for the film’s thickness measurements.
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