Evaluation of Photocatalytic Activity in Water Pollutants and Cytotoxic Response of α-Fe2O3 Nanoparticles.

α-Fe2O3 samples were manufactured by means of the polymeric precursor method. The powders were sintered and calcined at temperatures of 300-700 °C for 2 h, respectively. In the X-ray diffraction results, the formation of the rhombohedral phase without secondary phases was exhibited. The size of the particle increased after calcination at 700 °C, exhibiting a slightly more irregular morphology for the samples calcined with the addition of NH4OH in the synthesis process. From the field-emission scanning electron microscopy measurements, the particle size was determined, showing a smaller size for the samples without NH4OH in the synthesis process. The samples calcined at 600 °C had a size of 100 nm, with the sizes for lower temperatures being smaller. The size of the nanoparticle agglomerates was largest for the samples with NH4OH; however, the zeta potential was slightly lower over time for these samples. The phase study of the α-Fe2O3 nanoparticles was confirmed by means of Raman spectroscopy, without additional bands of another crystal structure. In addition, the synthesized nanoparticles exhibited good photocatalytic activity in the degradation of rhodamine B (RhB) and atrazine (ATZ) within 40 min, with a maximum degradation of 59% for ATZ and 40% for rhodamine. The best responses in the degradation were for the samples without the addition of NH4OH in the synthesis process and in proportions lower than 0.1 g. The cytotoxic effects of the nanoparticles obtained at 600 °C were evaluated in apical cells of onion roots. The results are promising for future applications because no changes were observed in the mitosis of the cells.

Introduction

Iron oxide nanostructures constitute a very important class of materials used in a variety of fields, including catalytic applications.[1−4] From the industrial point of view, the main advantages are that the materials are prepared in a nontoxic and economical way and have different crystalline structures that possess unique properties.[5] The functionality of the materials can be improved when their crystal sizes are limited to the nanoscale and the morphology is controlled in order to produce a high surface area.[1,6] Iron oxide nanoparticles (NPs) are the most popular magnetic NPs used in biomedical applications, due to their low cost, low toxicity, and unique magnetic properties.[6,7] Iron oxide has been used in the cosmetic industry, as well as TiO2, ZrO2, ZnO, CeO2, and α-Fe2O3, which can act as sun protection factors or pigments in cosmetic products.[6,8]

Iron oxide NPs have attracted attention because of their unique properties, such as their superparamagnetism and surface–volume ratio.[9] Various physical, chemical, and biological methods have been adopted in order to synthesize NPs.[5,9] Iron oxides exhibit great potential in the field of life sciences, such as biomedicine, agriculture, and environmental remediation.[10] The decomposition/degradation of organic dyes has been studied extensively in the literature, using visible light in the catalytic process. For this purpose, various materials have been studied, among them iron oxide,[11] hematite has also been used as a biomarker in Parkinson’s disease.[12−14]

Nanostructured materials with unique physical and chemical properties are useful for the detection of pesticides, especially in surface and ground water.[15] These NPs are also promising tools for strategies for the elimination and degradation of pesticides in order to remedy environmental pollution in different areas.[16] Some papers in the literature describe the use of iron-based compounds for the degradation of pesticides, nevertheless the cytotoxic effects of these NPs have not been evaluated appropriately.[17−19] The nanomaterials mentioned seem promising; however, most have not yet been studied in real-world applications.[9,20] Therefore, research efforts are still needed in this area, focused on the creation of profitable and rapid devices in the field of detection and degradation in contaminated areas.[18,21] The surface activity of NPs plays a critical role in the catalytic reactions used to degrade pesticides, resulting in the formation of benign products.[22] All particle systems in aqueous medium carry an electrical charge that can be positive, negative, or neutral. In the NPs that derive from the dissociation of an acid group, they produce a negative charge on the surface, while the dissociation of a basic group of NPs produces a surface with a positive charge.[23]

In this article, the synthesis of NPs of an iron oxide n-type semiconductor with a band gap of 2.1 eV[24] and antiferromagnetic properties was studied. A comparative study was carried out by means of the polymeric precursor method[25−27] with sintered samples and without the addition of ammonium hydroxide in the synthesis process. The samples were calcined at different temperatures. The synthesized powder was characterized via powder X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FEG–SEM), and zeta potential and UV–vis spectrophotometry in the degradation of rhodamine B (RhB) and atrazine (ATZ). A study of the effects of the cytotoxicity of NPs of hematite in the roots of Allium schoenoprasum cepa was also carried out.

Hematite NPs have also been demonstrated to be safe for use in biomedical applications.[28,29] For this reason, it is important to study the cytotoxic effects of this type of nanomaterials. In the literature, there are several investigations where the root tips of several plant species have been used for the study of induced chromosomal aberrations. Its importance lies in the fact that the root meristems contain a high proportion of cells in mitosis, which facilitates the incidence of toxic contaminants in the cells. The degree of alteration of the roots will depend on the nature and toxicity of the substances and the time they remain exposed.[30,31] This present study demonstrates the important role played by the synthesis, temperature, size of agglomerate, zeta potential, and the behavior of the vibrational modes in the material in improving the catalytic performance. Therefore, we provide a study where environmentally friendly α-Fe2O33 NPs were synthesized via the polymeric precursor method for water remediation. α-Fe2O3 particulate displayed efficient photocatalytic response for RhB and ATZ degradation in 40 min. No alterations were exhibited in the mitosis stage in the α-Fe2O3 cytotoxic assay on Allium cepa cells, suggesting a noncytotoxic character for the evaluated concentration.

Characterization

For the characterization of iron oxide result of the synthesis Figure , XRD (Shimadzu XRD 6000) at 30 mA with Cu K radiation at (1 °C min–1), dynamic light scattering (DLS), and SEM–FEG (JEOL-JSM-6701F) were the techniques used. The Raman spectra were recorded using a triple-grating spectrometer in the subtractive mode (Jobin Yvon, T64000), equipped with a N2-cooled charge-coupled device detection system. The 632 nm line of an argon ion laser was used for excitation.

Figure 1

Scheme of the synthesis procedure of α-Fe2O3 NPs, (A) samples without the addition of ammonia and (B) with the addition of NH4OH in the process.

Photodegradation Using the Hematite NPs

For the photodegradation of RhB and the pesticide ATZ, a study of the photocatalytic properties of the samples was done in the following way: the aqueous solutions were prepared with RhB 5 mg L–1 (water), and then, in a different beaker, 50 mL of this solution of rhodamine and water was added along with the photocatalyst. The system was maintained at room temperature under constant stirring and illuminated by four UVC lamps (Philips TUV, 15 W, maximum emission of 254 nm). In the case of ATZ, 2.5 mg L–1 was used. The photodegradation of the dye and the pesticide was monitored by absorbance measurements, performed on a UV–vis spectrometer (Shimadzu, uv-1601pc) at different times of exposure of the solution containing the photocatalyst to the light.

Cytotoxicity Assay

Common onion, A. cepa, seeds were used for the cytotoxicity analysis. Initially, the seeds were placed in Petri dishes in contact with water for germination. After growth to 1.0–2.0 cm length, the seeds were exposed to 2.5 mg L–1 hematite particles for 24 h at room temperature. After the contact time, the roots were exposed to Carnoy’s solution, 3:1 (v/v) ethanol/acetic acid, for 24 h. In the next stage, acid hydrolysis, HCl (1 N), was carried out for 9 min at 60 °C, in order to perform the fixing of the dye in the cells. Then, the roots were washed with water and remained for 2 h in the dark in contact with the Schiff’s reagent. With the aid of a microscope, Nova 107, the meristematic system of A. cepa cells was observed.

Results and Discussion

The structural changes caused by thermal treatment of the prepared samples were studied via XRD. Figure A shows the XRD spectrum of α-Fe2O3 NPs prepared at different calcination temperatures for samples prepared without the addition of ammonium hydroxide. The characteristic diffraction peaks of the samples coincide with the standard pattern of the rhombohedral phase of α-Fe2O3. The peaks of the XRD correspond to the lattice planes (012), (104), (110), (113), (024), (116), (018), (214), and (300). The diffraction peaks of the samples are clear for the samples annealed at 500° and 600 °C, which indicates that the α-Fe2O3 NPs are in a better state of crystallinity compared to 300 °C.

Figure 2

(A) XRD pattern of hematite (α-Fe2O3) NPs, without the addition of ammonium hydroxide at different calcination temperatures (300, 400, 500, and 600 °C). (B) NPs, with the addition of ammonium hydroxide at different calcination temperatures (300, 400, 500, and 600 °C) and (C) XRD pattern of hematite, at a temperature of 700 °C of calcination; (a) without ammonium hydroxide, (b) with addition of NH4OH.

In Figure B, XRD patterns can be seen for the powders with the addition of ammonium hydroxide, which confirm the crystal structure of the rhombohedral hematite type. The XRD pattern of the precursor, shown in Figure B for 300 °C, does not exhibit all the peaks, due to its low crystal order nature, in comparison to Figure A. However, for higher temperatures, the crystallinity of the material increases at the same rate as the samples without any addition in the process. The narrow, sharp peaks indicate that the hematite products are highly crystalline, implying that a high purity of synthesized hematite particles is obtained by using this synthesis method.[32,33] The α-Fe2O3 structure of the samples synthesized with and without the addition of ammonia at 700 °C was confirmed. The XRD patterns of the samples are shown in Figure C. The peaks are narrow and sharp, suggesting that the samples at 700 °C are highly crystalline, respective of the process of synthesis.

The diffraction peaks are affected by the crystallite size, and for that reason we used the Scherrer equation[34,35]Dc = Kλ/β cos θ where K is the so-called form factor (0.9), λ is the wavelength (0.15418 nm, Cu Kα), β is the full width at half maximum (fwhm), and θ is the diffraction angle. Therefore, in order to compare our results with the literature, we used the plane (104) to calculate the size of the α-Fe2O3 crystallite. The results obtained in Table agree with the data in the literature for obtaining the hematite structure by different methods of synthesis. α-Fe2O3 crystallite sizes between 15.3 and 27.28 nm were obtained via the hydrothermal method.[36] Using the chemical precipitation method, the authors demonstrated that the particle sizes increased with an increase in the precursor concentration (21–82 nm).[34] Using the same chemical method, α-Fe2O3 samples were calcined, and crystallite sizes of 24 nm (500 °C) and 31 nm (700 °C) were obtained.[37] Therefore, it is evident that our results are comparable with those reported in the literature. Correlating the values from Table with the Fourier transform infrared (FTIR) results (Supporting Information), a band of approximately 1500 cm–1 for samples at 300 °C suggests the presence of residual carbon from the synthesis method. This fact indicates that the material is not yet completely crystalline, and consequently the definition of the particle boundary is not yet perfect. Upon complete material crystallization (400 °C), the crystallites decrease, and with increasing temperature, a tendency for increasing values is observed, due to the improvement in mass transport.

Table 1

Size of the Crystallite Diameters Estimated from the Diffraction Peaks by the Scherrer Equation Formulated for the Different Samples

calcination temperature (°C)	(a) diameter size (nm) (without NH4OH)	(b) diameter size (nm) (with NH4OH)
300	27.2	24.6
400	21.4	19.6
500	20.6	21.7
600	25.3	22.7
700	32.4	25.2

FEG–SEM was used for the characterization of the synthesized samples. Figure shows the FEG–SEM image of the samples of the iron oxide. In Figure a,b, the precalcined sample obtained during the synthesis process can be seen without additional annealing. In Figure c,d, samples precalcined at 300 °C are shown. In the precalcined samples without the addition of ammonia (Figure c), a well-defined grain formation can be seen; however, in the samples with NH4OH (Figure d), it can be seen that the samples present low crystallinity, with blocks of agglomerates. This indicates that NH4OH prejudices the formation of particles in the precalcined ones.

Figure 3

FEG–SEM, figures on the right, with the addition of NH4OH and figures on the left, without addition of NH4OH. (a,b) Precalcined samples; and (c,d) samples calcined at 300 °C.

The iron oxide NPs sintered via the polymer precursor method are shown in Figure . The images of the samples on the left are without the addition of ammonium hydroxide (NH4OH) and on the right with the addition of NH4OH (NH3) in the process. The annealing temperatures for the samples are from 400 to 700 °C. It was found that the calcination temperature had little effect on the particle size up to 600 °C. Figure shows iron oxide NP images of FEG–SEM in the form of rice grains with small necks. Nonetheless, the particles are more regular for temperatures of 400 to 600 °C. For 700 °C, the particles have an irregular, elongated shape, this being more uniform for the samples without NH4OH in the process. As can be seen, the samples calcined at 700 °C exhibited considerable growth in comparison with the other calcinations. This indicates that starting at 700 °C, the calcination favors an increase in the particle size in both cases. According to Lassoued et al.,[12] NP size depends on the concentration of the precursor used in the synthesis of hematite. The particle size increased with a rise in the concentration of iron chloride hexahydrate.

Figure 4

FEG–SEM, figures on the right, with the addition of NH4OH. Figures left, without addition of NH4OH. (a,b) samples annealed at 400 °C; (c,d) samples annealed at 500 °C; (e,f) samples annealed at 600 °C, and (g,h), respectively, at 700 °C.

In Figure , by way of example, the particle size for the samples at 600 °C was calculated because starting at this temperature the size seems to be larger for higher temperatures, and the particle size seems smaller for temperatures lower than 600 °C. ImageJ software was used for particle analysis. A count of 100 particles was performed, in which the length of each particle was found because these particles have an elongated shape, as seen in the SEM figures. For the sample with the addition of NH4OH, the particle size was 139 ± 14 nm, while for the sample of Figure b without the addition of ammonium hydroxide it was 100 ± 23 nm. This result indicates that the particle size is smaller for the sample without NH4OH, and in addition, the distribution of particles is a little better for this sample compared to the sample in Figure a.

Figure 5

Particle size distribution histogram and figure (a) sample with annealing at 600 °C with the addition of NH4OH in the process; (b) sample calcined at 600 °C without NH4OH in the synthesis.

Particle size control is very important for the photocatalysis process. In the literature, it is reported that NPs of controlled size showed high catalytic activity such as Fe3O4,[38] in addition to being an effective catalyst material for the degradation of organic pollutants.[39] Fe3O4 NPs have also been used as the solid support for the development of magnetically recoverable catalytic systems.[40] When comparing the morphology/size of Figure a,b in the sample with ammonia, it is difficult to calculate with a greater degree of precision its particle size compared to the particles at 600 °C obtained without ammonia because the latter have a little more regular and defined form, and also their particle size is smaller. This property of its morphology and size is associated with increased photocatalytic activity because of its surface area and possibly its morphology’s influencing the cytotoxicity results. Through analyses of the samples heat-treated at 600 °C in Figure a,b, a decrease of particle size distributions was observed without NH4OH addition. This fact can be explained by the elimination of gaseous NH4 during the polyester pyrolysis with treatment at 300 °C for obtaining the precalcined samples (precursor). The elimination of NH4 produces a more porous precursor than without NH4 addition, with a greater and consequently more reactive surface area. In the crystallization stage, the reactivity favors mass transport, increasing the particle size diameters, during crystallization temperatures from 400 to 700 °C.

In Figure A, the measurement for the DLS of the particles is shown. The measurement obtained by DLS can only be a particle agglomerate in water and not the individual particles that were observed via FEG–SEM. The diameter of the agglomerates increases when the calcination temperature increases in Figure A(a), especially for samples with the addition of NH4OH in the synthesis. For the samples without the addition of ammonium hydroxide, the diameter of the agglomerate remains practically constant. In zeta potential, results show agglomeration size results similar to the literature.[23] Measurements of the potential in relation to the time and diameter of the agglomerates were studied by Michael et al.[23] for TiO2, Fe2O3, Al2O3, ZnO, and CeO2. They found that the size of the NP agglomerates increased in α-Fe2O3 over time and with an increase in the zeta potential. Comparable values in the literature of the zeta potential were observed for the sample with NH4OH heat treated at 300 °C, with values for γ-Fe2O3 at pH 10 found in the literature,[41] which represented an important characteristic for obtaining iron oxide nanochains.[42] In Figure B, the zeta potential can be seen as a function of the calcination temperature. The zeta potential increases when the calcination temperature increases, being higher for the samples without the addition of NH4OH. In the literature, some authors say that factors such as pH can influence the zeta potential of different NPs.[23] Other factors to be taken into account when measuring the zeta potential are the material under study, the suspension medium, the agglomeration of particles, and external factors such as the flow of hydrogen ions (H+) in the solution. The zeta potential represents the charge of a NP. However, as seen in Figure A, it is not a real measure of the individual particle surface charge. We can say that the conditions under which the potential is measured are very important and that there are many variables that can influence the final results. Because the purpose of this work is the application of α-Fe2O3 particles in aqueous media for photocatalytic processes, the evaluations of the agglomerate formation and surface potential in water are extremely important for understanding the material behavior.

Figure 6

(a) Particle size agglomeration (nm) as a function of the calcination temperature; (b) zeta potential values of samples calcined and different temperatures. As a dispersant, water at 25 °C was used, in a time of 20 min for all samples. In figures (A,B), (a) samples with NH4OH and (b) samples without ammonia in the synthesis are shown.

By means of the DLS measurements, large agglomerate sizes were observed despite the high loading of the zeta measurements. It could be expected that with this zeta charge, the agglomeration of the particles would be low. However, the method of polymer precursors that involves calcination stages for the formation of the phase of interest, causing that agglomeration/aggregation to occur. This is due to the diffusion process that results in the mass transport of the particles for the crystallization stage. An advantage of this method is obtaining pure phases, having byproducts CO2 in gaseous form and water, that is, without residues to discard and/or treatment. However, the agglomeration/aggregation of particles occurs in heat treatment.

Figure shows the Raman spectra for α-Fe2O3 synthesized at different annealing temperatures 300, 400, 500, 600, and 700 °C at room temperature using 632 nm excitation wavelengths. The spectra show seven phonons in particular, five related to the Eg modes and two related to the A1g mode. In the Raman spectroscopy results, no other iron oxide was detected. In Figure a, the Raman spectra can be seen for the samples calcined from 300 to 700 °C, samples without the addition of NH4OH in their synthesis. The sample at 300 °C exhibits the main bands of α-Fe2O3; however, they are wide bands with low intensity, and are indicative of the low crystallinity material in the sample. For the other samples, the bands are narrower and of a higher intensity indicative of a more crystalline material. In these spectra, the seven typical phonons of hematite can be seen without any contamination of any other oxide product of iron. In Figure b, the Raman spectra of iron oxide with NH4OH in the synthesis process are shown. As can be seen in the spectra of the samples calcined from 300 to 700 °C, the Raman spectrum for the 300 °C sample is practically amorphous, with three very broad bands. In these samples, the Raman bands A1g (2) and Eg (3) are practically not exhibited; the band Eg (3) is covered by the Eg (2). However, the bands are smaller and less intense compared to the Raman spectra of Figure a. In addition, the widths of the large lines and changes in the wave number of the Raman peaks could be due to the effects of phonon confinement in the nanocrystals.

Figure 7

(a) Raman spectra for samples without addition of NH4OH at different calcination temperatures. (b) Raman spectra for samples with addition of NH4OH at different calcination temperatures.

Figure 9

For every 500 mL of water was added 5 mg of RhB. In 20 mL of the previous fluid is added 0.01 and 0.05 g of the sample for the photocatalytic study of rhodamine. (a) Calcined samples at 500 °C with and without NH4OH. (b) Samples at 600 °C with and without NH4OH in the process.

Using the PeakFit program, the peak position and fwhm was found. The analysis was carried out modeling a set of Lorentzian spectral functions. As an example, we took only two Raman bands, which appear in Figure , but the position of the Raman peaks and the fwhm was found for all modes. Figure shows the position of the phonons A1g[43] and Eg;[44] furthermore, there is also the fwhm for the different samples calcined at different temperatures, where Figure a shows the samples without ammonium hydroxide and 8b with NH4OH. It can be seen that the change in the position is greater for the samples with the addition of NH4OH in the process; however, for the other samples, the temperature only changes the position of the Raman modes a little. Compared the fwhm value of the different samples, for Figure b, this value is higher. Generally, these data are related to the stresses and the crystallinity within the sample. The peak shifted and the fwhm increased; this may also be indicative of the particle size in Figure b. Therefore, a majority of published studies refer to Raman dependence and particle size by means of the phonon confinement model. The optical phonon bands show changes with the particle size.[45,46] In other words, comparing (a) and (b) of Figure , we can say that in sample (b), the structural changes are greater, possibly because they exhibit more amorphous material or very irregular changes in their particles.

Figure 8

(a) Samples without NH4OH and (b) samples with NH4OH. Position of the Raman peak as a function of the annealing temperature for the phonons A1g (1) and Eg (4). The numbers in parentheses equals fwhm (cm–1) of each Raman peak.

The degradation process of rhodamine can be seen in Figure . As a catalyst agent, samples of 500 and 600 °C (0.01 and 0.05 g/L) were used, with and without the addition of NH4OH. In particular, RhB is an amphoteric dye, although it is also classified as basic due to its positive total charge. It exhibits strong fluorescence.[47−53]Figure a shows the degradation of RhB for samples with and without the addition of ammonia at 500 °C. For the sample at 500 °C without NH4OH and 0.05 g, the degradation of rhodamine is greater compared to the samples with pH = 7 and concentrations of 0.01 and 0.05 g/L of hematite. In Figure b, the degradation of RhB can be seen for the samples calcined at 600 °C. As can be seen, the sample that responds better to the degradation was the sample without the addition of NH4OH at a concentration of 0.05 g/L. Otherwise, the best samples with respect to the degradation of ATZ and rhodamine were the samples without the addition of ammonium hydroxide. The behavior for the samples with the addition of NH4OH is not the best. This behavior could be related to the particle size, which began to increase with the calcination temperature, and therefore possibly influenced its effectiveness in the degradation of rhodamine. In Figure a the degradation of rhodamine with the samples calcined at 500 °C and a time of 120 min, the degradation was 28%. In the case of Figure b, for the samples at 600 °C and time of 100 min the degradation was 40%.

In Figure , the effect of the amount of the catalyst on ATZ degradation can be seen. To avoid sedimentation in the process of degradation of ATZ, the samples were kept under constant stirring. The degradation rate of ATZ depends on the concentration of the photocatalyst. In Figure a, we have the sample of hematite at 400 °C without the addition of NH4OH in the process. By increasing the concentration of the catalyst, the number of molecules adsorbed increases, increasing the rate of degradation. However, the change is not significant for the concentrations of 0.01 and 0.05 g/L. In Figure b, the catalyst isα-Fe2O3 without NH4OH. 0.01, 0.05, and 0.1 g of the sample at 500 °C were used for these tests. With this sample, the degradation was much better compared to the sample at 400 °C. However, the turbidity of the suspension increased; therefore, the photodegradation was less effective.[54] In Figure c, we have a comparison of the sample with ammonia and without it in the synthesis process for temperatures at 500 and 600 °C. The amount of the α-Fe2O3 sample that was used for the degradation was 0.03 g in all four cases. As seen in Figure c, the samples that responded better to the degradation during a period of 40 min were the samples without addition of ammonium hydroxide in the process. The degradation of ATZ in the conditions already described was as follows: for NPs at 400 °C of the 27%, for samples at 500 °C of the 59% and finally for samples at 600 °C was the 35%.

Figure 10

For every 500 mL of water was added 2.5 mg of ATZ. In 20 mL of the previous fluid is added of the sample for the photocatalytic. Effect of dosages of α-Fe2O3, α-Fe2O3–NH4OH, in degradation of ATZ. (a) Sample at 400 °C—α-Fe2O3, (0.01 and 0.05 g); (b) sample at 500 °C—α-Fe2O3, (0.01, 0.05, and 0.1 g); (c) samples at 500 and 600 °C, α-Fe2O3, α-Fe2O3–NH4OH using 0.03 g of sample for the assay.

Photocatalysis assays under conditions similar to the present investigation can be found in the literature with different nanoparticulate oxide photocatalysts. Mourao et al.[55] synthesized TiO2 NPs using the hydrothermal method at different temperatures (125–250 °C) and exposed them from 1 to 4 h to a photocatalytic process. As a result, the TiO2 sample that showed the best performance for methylene blue was synthesized at 250 °C for 2 h, degrading about 99% of the dye in 60 min. The authors correlated this response to the highest TiO2 crystallinity obtained under the synthesis conditions, demonstrating the importance of this parameter in the photocatalytic process. Lopes et al.[56] obtained nanoparticulate Nb2O5 with an average size between 31 and 35 nm via a hydrothermal route at 125 °C for 12 h. In the RhB photocatalytic application, Nb2O5 promoted a 50% degradation of the dye in 300 min, being superior to TiO2 (P25) under the same conditions. This result was correlated with the better availability of hydroxyl radicals for pollutant degradation from the Nb2O5 catalyst. Oliveira et al.[57] obtained N-doped ZnO NPs smaller than 50 nm from the addition of urea to the sol–gel synthesis. In photocatalytic activity against RhB dye, ZnO/N exhibited a degradation of about 80% in 120 min, being superior to pure ZnO, with an efficacy of 20%. de Castro et al.[58] elaborated a TiO2/WO3 heterostructure, varying the TiO2 concentration from 10 to 90% in the composition. In the degradation of RhB dye, it was observed that a 50% ratio (w/w) was the heterostructure with the best result in relation to the dye, degrading about 99% after 200 min. TiO2/WO3 performance was higher when compared to isolated WO3 and TiO2, with values of 25 and 65%, respectively, being attributed to changes in the donor–receptor electronic levels of pure materials. Regarding hematite, Yang et al.[59] obtained NPs via the hydrothermal method for RhB degradation in the presence of H2O2 with exposure to the visible region. The result was that hematite NPs with a size of 100–150 nm, in the presence of 0.3 mL of H2O2 (30% v/v) under 180 min visible exposure, allowed complete dye degradation. In the present investigation, 80–120 nm hematite NPs in the UV–vis region without the presence of H2O2 allowed the degradation of 40% RhB dye and 35% ATZ pesticide after only 40 min of exposure. Thus, the obtained result demonstrates that the hematite NPs exhibit significant and competitive photocatalytic activity compared to the other oxides evaluated in the literature.

Higher photocatalytic activity was associated with a smaller particle size and greater specific surface area. Nevertheless, when looking at Figures and 10, both for ATZ and for rhodamine in the various assays that were performed in the laboratory, the best response obtained was for samples at 600 °C, and especially for the sample without the addition of ammonia. This response is possibly related to the particle size crystallinity because the samples at 500 °C were slightly smaller but less crystalline. A similar behavior was observed by Mourao et al.[55] for TiO2 NPs. It was observed in the present investigation that is possible to obtain better results for photocatalytic activity with samples heat treated at temperatures lower than 700 °C; with larger particle sizes, the samples practically did not respond. This indicates that particle size and crystallinity are essential for photocatalytic activity in α-Fe2O3 samples obtained via the polymeric precursor method.

In the assay of cytotoxicity in the roots of A. cepa, the hematite particles (at 600 °C) were used at a concentration of 2.5 × 10–3 g/mL, due to the better photocatalysis conditions that was observed, Figure b (0.05 g in 20 mL). It is possible to verify through the images, Figure a–c, that the NPs did not cause changes during the mitosis of the cells. In Figure a–c, it the dividing cells during the prophase, metaphase, and anaphase steps are shown, respectively. In addition, abnormalities regarding mutations and/or changes in mitotic behavior expected for cell division are not seen. Batista-Gallep et al.[60] observed that at low concentrations of Fe2O3 NPs (1.96 × 109 NPs/mL), the toxicity effect on A. cepa was lower when compared to high concentrations (39 × 109 NPs/mL). Kaygisiz and Cigerci[61] found that in particles smaller than 100 nm, independent of the concentration present, the hematite particles exhibited chromosomal alterations in the mitotic division. Also previous results in a commercial Madin–Darby canine kidney cell with hematite NPs have shown that size and morphology influence cytotoxicity.[62] The authors demonstrate that the degree of internalization in the cells was favored by the stick-shaped NPs and not the spherical ones, due to the greater surface area. In other studies,[63] hematite has also been successfully used in the human lung fibroblast cell line (MRC5), with low levels of cytotoxicity. Thus, the result found in the present investigation may be due to the concentration used in the test, 2.5 × 10–3 g/mL, as well as the particle size, mostly around 100–200 nm, presenting a low oxidative stress effect in the cells and consequently lower toxicity due to the low effect with respect to the nanometric size (<100 nm). The NPs used for the cytotoxicity assay were from the sample at 600 °C without ammonia, with a crystallite size of 25.3 nm, due to their better response in photocatalysis. From the results of photocatalysis, Figure , and cytotoxicity, Figure , the hematite NPs obtained with a polymeric precursor exhibited promising results for the degradation of pollutants in an aqueous medium, with a low harmful effect on the ecosystem.

Figure 11

Images of meristematic cells of A. cepa (a) prophase, (b) metaphase, and (c) anaphase step during cell division (mitosis).

Conclusions

The synthesis procedure that was carried out is low-cost, and it was possible to obtain samples with an orthorhombic crystalline structure of α-Fe2O3. The particle size obtained in this study was lower for the calcined samples without the NH4OH addition in the synthesis process, and for temperatures lower than 600 °C, the particle size was lower than 110 nm. Raman spectroscopy verified the synthesis of the hematite, finding the seven Raman phonons (two A1g and five Eg), without additional bands, indicating a pure structure for the samples. By means of XRD analysis, it was also verified that the samples with better crystallinity were the samples free of NH4OH in the synthesis. In α-Fe2O3 photocatalytic investigation toward rhodamine and ATZ degradation, the best responses were found for the samples without NH4OH in the synthesis process, with the efficacy of, respectively, 40 and 59% in 40 min.

The α-Fe2O3 particles showed no cytotoxicity to A. cepa cells for samples calcined at 600 °C with a concentration of 2.5 × 10–3 g/mL, constituting a promising biocompatible alternative with an adequate degradation rate for the rhodamine and ATZ pollutants. Once, that toxicity is a limiting factor for the application of nanoparticulate materials in photodegradation systems, α-Fe2O3 particles obtained by the polymeric precursor method are excellent candidates in these kinds applications.

Experimental Details

Synthesis Procedure of the α-Fe2O3 Nanocomposite

To obtain the samples, the polymeric precursor method was used,[32] which is based on the completion of metal cations by a hydroxycarboxylic acid, in this case citric acid. The completion process occurs when salts and citric acid are mixed in aqueous solution.[32,33,35] The chemicals used were: iron(III) nitrate 9-hydrate, (Fe(NO)3O) (98%, Mallinckrodt), ethylene glycol (EG), HOCH2CH2OH (≥99.0%, Merck), anhydrous citric acid, C6H8O7, ((99%) Dinâmica), ammonium hydroxide, and NH4OH (28–30%, Synth). Deionized water was obtained from the Milli-Q Water Purification System. EG was added to this solution, keeping the temperature constant until the water is removed and a polymer resin is formed. Afterward, the samples were subjected to a thermal treatment in order to eliminate organic matter and crystallize the material of interest (α-Fe2O3). In this investigation, the iron oxide NPs were sintered in the following way: 21.7 g of citric acid, Milli-Q (200 mL), was added to water with constant stirring at 90 °C. The polymeric precursor solution was prepared using 15.2 g of iron(III) nitrate, which was added under constant stirring for half an hour at a temperature of 90 °C. After total dissolution, 13 mL of EG was added to the solution, with a (mass) proportion of citric acid/EG of 60:40 w/w. At the end of the reaction, a resin was obtained. During the preparation, Figure , method A was without the addition of ammonium hydroxide (NH4OH) and method B with the addition of NH4OH as a pH controller agent. In method B, the citric acid (AC) and the iron(III) nitrate 9-hydrate in a molar ratio of 3:1 (AC/Fe) were solubilized in water under magnetic stirring. In this stage, the pH was varied to 7 with NH4OH addition for comparing with the synthesis without pH adjust. The EG was added to the reaction medium in a ratio of 60% AC to 40% EG and heated at 90 °C under magnetic stirring for obtaining a polyester. The polyester was calcined in a muffle furnace (3000 EDG) at 300 °C for 2 h, under a heating rate of 10 °C min–1, producing a precalcined material. The precalcined samples were de-agglomerated using a mortar and pistil and heat treated from 300 to 700 °C for 2 h for the crystallization process of the two methods (A and B).