Effects of MnO2/In2O3 thin films on photocatalytic degradation 17 alpha-ethynylestradiol and methylene blue in water

In recent years, there has been a growing concern about some substances in the aquatic environment such as methylene blue dye (MB) and 17α-ethynylestradiol (EE2) hormone, mainly from waste textile and pharmaceutical industries, respectively. This waste, which is not effectively treated, generates large amounts of contaminated effluents. Textile industries, which use dyes mainly in their dyeing and finishing processes, generate a lot of effluent contamination by dyes. The 17α-ethynylestradiol hormone which is used in different contraceptive pills and in hormone replacement therapy, is the main substance found in the aquatic environment because of its high resistance to biodegradation. It is known that conventional water treatment processes, which involve the steps of coagulation, flocculation and sedimentation among others, have been reported to be ineffective in the removal of dyes and hormones. The heterogeneous photocatalysis has attracted great interest from many research groups around the world due to its potential application as a pollutant degradation method. For this purpose, the present work evaluated the performance of MnO2/In2O3 thin films in the degradation process of the blue methylene and 17α-ethynylestradiol in aqueous solutions. The degradation processes were monitored by UV–Vis spectroscopy and HPLC chromatography. From these analyzes, the concentration of the solutions was evaluated and it was possible to infer that the MnO2/In2O3 thin films significantly promoted a degradation of 89% of 17α-ethynylestradiol, however, the degradation of the methylene blue was not efficient.


Introduction
Concern about environmental preservation is a subject of growing debate, as environmental pollution from toxic organic contaminants in wastewater poses a serious threat to humanity and to living organisms well-being [1,2].Even in small quantities, some pollutants may have major negative effects in the environment in which they are inserted.This is the case of textile dyes and synthetic hormones present in domestic sewage [1].Such components can kill aquatic life and even interfere in the endocrine and reproductive system of fish and humans [1], causing problems such as high proportions of hermaphrodite fish, decreased egg numbers and sperm production, decreased gamete quality and reduced fertility.In addition to behavioral changes in humans, these effects include a reduction in the amount of sperm, an increasing incidence of breast cancer, testicular cancer, prostate cancer and endometriosis in the case of synthetic hormones [3].Regarding the methylene blue dye, the pollution of water bodies with these compounds causes, in addition to visual pollution, changes in biological cycles affecting mainly the photosynthesis process.In humans, studies have shown that some classes of dyes may lead to skin and airways irritation and, when ingested, they may produce substances with carcinogenic and mutagenic properties [4].Therefore, finding means to reduce the quantity of pollutants in the water is extremely important to preserve this ecosystem and to reduce possible diseases resulting from this condition.Several techniques, such as coagulation, flocculation, sedimentation, among others, were already employed in the degradation of these substances contained in the aquatic environment, but did not show to be effective [3,5], as these processes occurred at a much slower rate compared to the photocatalytic oxidation via semiconductors [6].Therefore, the photocatalysis has attracted great interest from many groups of researchers around the world, because this process in semiconductors is one of the most promising methods for water purification.The Semiconductor photocatalytic activity of involves interactions of the photoexcited charge carriers leading to the generation of reactive oxygen species, which result in the degradation of toxic pollutants in water [2].Recent advancement in shape and size-controlled synthesis of metal oxide semiconductor nanostructures has inspired applications in diverse areas including environmental remediation, water purification and disinfection [7].The metal oxide semiconductors are particularly useful as photocatalysts due to a favorable combination of electron structure, light absorption properties, charge transport characteristics and excited state lives [8].
Most studies on photocatalysis focus on the development of powdered nanostructured photocatalysts.For practical applications in which the reuse over a long period of time is required, powder photocatalysts need to be recovered from the dispersions after each cycle.Thus, researchers began using nanostructured coatings on different substrates such as photocatalysts to overcome this panorama [9], considering that the thin film structure has a relatively well-established morphology and a viable commercial production [10].For the production of thin films with photocatalytic properties, the use of precursors based on semiconductors presented great potential [11].At the moment, some oxides that can be highlighted for the preparation of thin films, are: ZnO, TiO 2 , SnO 2 e In 2 O 3 [12].These oxides have some advantages such as bandwidth, transparency, durability and they do not cause damage to the environment [11].Indium oxide has been widely used as a semiconductor because it has good properties regarding transparency and conductivity [13,14].Manganese oxides, for example, have gained prominence in studies because of their low cost, natural abundance, specific capacitance and for being benign to the environment [15,16].
In this work, the photocatalytic properties of thin films formed from MnO 2 and In 2 O 3 were investigated.As the In 2 O 3 and MnO 2 present good properties, it was studied the junction of these two semiconductors, not yet studied together by other researchers, to be used as photocatalysts.Based on this property, the performance of these films was evaluated regarding the degradation of the methylene blue dye and 17α-ethynylestradiol hormone.Therefore, the deposition method used was the spin coating, which allows the nanocatalyst to be evenly deposited on the substrate [17,18].

Experimental
2.1 MnO 2 , In 2 O 3 , Mn/In/Mn/In and In/Mn/In/Mn thin films synthesis It was used the complex polymerization method [8] to produce the precursors MnO 2 and In 2 O 3 nanocatalysts, with citric acid, ethylene glycol, indium nitrate and manganese nitrate as reagents, all of them stoichiometrically calculated.The synthesis of the MnO 2 resin initially occurred from the reaction between the manganese nitrate and citric acid.These two reactants were added to 60 ml of water and heated at 70 °C under stirring.Then, ethylene glycol was added to promote polymerization by the polyesterification reaction among the reactants.After this procedure, the solution was maintained at 70 °C and under stirring until a viscous resin was obtained.The same procedure was used to obtain the resin based on indium oxide, replacing the manganese nitrate for indium nitrate.Finally, the viscosity of both resins was adjusted to 20 cPs.Before the thin films deposition, the substrate must be subjected to a preparation in order to provide a better adhesion of the resin to the substrate.The substrate used was n-type silicon and it had 0.7 cm 2 .It was washed with the aid of an EXTRAN.
After the entire cleaning process, the substrate was covered with resin and subjected to a spin speed and spin time of 990 rpm for 3 s and 7000 rpm for 30 s respectively in a spin coating.Subsequently, with the deposited layer, the substrate was placed in a heating plate preheated at 60 °C for the solute evaporation.The thin films were then crystallized at 500 and 600 °C for 2 h at a rate of 1 °C/min.This procedure was performed with each layer of deposited resin, resulting in four layers in each film, varying the order of deposition of the resins, as shown in Fig. 1.

Characterization
The crystal structures of the sample powders were characterized using a Shimadzu XRD-7000 diffractometer with already defined parameters, such as low angle of incidence of the beam and the use of CuKα radiation.
The photocatalytic activity of the prepared photocatalysts was investigated through the degradation of EE2 and the degradation of the MB.The photocatalytic degradation experiments were conducted via an UV-Vis spectrophotometer, model UV-2600.
For photoluminescent emission measurements at room temperature, slits were used in the Jarrel-Ash Monospec 27 thermal monochromator with a width of 200 nm, as well as a Hamatsu R446 photomultiplier coupled to a controlled lockin acquisition system SR-530 by a microcomputer.

Results and discussion
Figure 2a shows the standard XRD of the In 2 O 3 and MnO 2 calcined at 600 °C, Fig. 2b and c shows the standard XRD of the In/Mn/In/Mn and Mn/In/Mn/In thin films respectively, calcined at 500 and 600 °C.The In 2 O 3 diffraction peaks were indexed according to ICSDS card No. 98-001-4188, from which a cubic structure can be identified.The diffraction peaks of MnO 2 were indexed according to ICSDS card No. 98-008-2205, which also has a cubic structure.As shown in Fig. 2a, the major diffraction peaks of the photocatalyst In 2 O 3 were recorded at approximately 31° and 36° (2θ) which are respectively consistent with (222) and (004); the diffraction peak located at approximately 33° (2θ) corresponds to the crystal plane (222) of the MnO 2 photocatalyst.It was observed that the intensity of the peak (222) of In 2 O 3 in Fig. 2b was more intense than in Fig. 2c, because the outermost layer was In 2 O 3 .In the same way, the peak (222) of Mn 2 O 3 was more intense in Fig. 2c than in Fig. 2b, because the outermost layer was Mn 2 O 3 .
It was shown that the MnO 2 structure (pyrolusite) for manganese oxide can be transformed into Mn 2 O 3 (bixbyite).According to Mansoor, the MnO 2 phase is decomposed into others as the temperature rises above 500 °C and from the atmosphere into which the material was subjected [19].In his work, Downs [20] also obtained the Mn 2 O 3 , from the MnO 2 , after calcination at 500 and 600 °C.The thin films Mn/In/Mn/In and In/Mn/In/Mn, prepared at different crystallization temperatures, did not reveal any other additional phases.This indicates that there is no chemical interaction between In 2 O 3 and Mn 2 O 3 at the mentioned temperatures.It was only observed the presence of Si, which refers to the substrate used.600 °C, respectively.The cross-sectional view of the thin films clearly shows a porous morphology.This result is in agreement with the results reported by Hunge et al. [21] in which porous sheet like thin films were obtained.According to Hunge, the dissimilar morphologies of the films may be associated to the structure of the network and the defects generated during the deposition, such defects interfere in the photocatalytic property [21].Table 1 shows the film thickness values.
The degradation curves were formed by UV-Visible spectroscopy for methylene blue and by HPLC analysis for ethynylestradiol.The photo-degradation of methylene blue and ethynylestradiol was used to study the degradation of the In 2 O 3 , Mn 2 O 3 , Mn/In/Mn/In and In/Mn/In/Mn thin films by UV radiation, the concentration of dye and hormone as a function of the irradiation time is shown in Figs. 4 and 5, respectively.In the photo-degradation process, the active species, including the superoxide radical (•O 2 ), the hydroxyl radicals (OH) and the holes (h + ) with powerful oxidation capacity will be produced in the photocatalytic degradation process (see Fig. 6) [22].
The photodegradation efficiency of MB was calculated from the following equation: The relative values of the change in the methylene blue concentration were used to study the degradation of the In/ Mn/In/Mn and Mn/In/Mn/In thin films by UV radiation, the concentration of the dye as a function of the irradiation time is shown in Fig. 4.During this process, it was observed that the concentration of methylene blue decreases when exposed to UV radiation.It is possible to observe in Fig. 4 and Table 2 that this decrease was predominant in the In/ Mn/In/Mn films, mainly in the films calcined at 500 °C, due to their characteristics, such as high surface area and large pores, favoring a better reaction (fast kinetics) with the dye molecules.Thus, the reduction of catalytic activity, as the calcination temperature increases, is explained by the fact that the surface area of the catalyst decreases, thus decreasing the amount of free sites to act in the reaction [23].According to Li, the surface area, the photoabsorption capacity and separation and transport rate of electrons and photogenerated holes are the main factors that affect the catalytic activity of a photocatalyst [23].
The influence of photocatalytic activity on the hormone due to the action of the thin films studied by HPLC analysis is seen in Fig. 5 and Table 2.According to these analyzes, it is possible to identify that the solution was highly influenced by the photocatalytic activity as the compounds decreased about of 80% of their concentration when exposed to the system.After 300 min the In 2 O 3 film resulted in a degradation of 85%, the Mn 2 O 3 in a degradation of 82% and the combined film resulted in a degradation of 89%, showing that this mixture of layers improved the photocatalytic activity of these films.According to Jiang [24], EE2 molecules can be adsorbed by the carbon of water and transferred to the center of the decomposition in photocatalysts, to obtain a higher mass transfer rate, therefore a higher degradation removal ratio of EE2 can be achieved [24].Tran et al. [25] proposed that the degradation of EE2 (1)  is a surface reaction that can be promoted by increasing the number of surface reaction sites.Consequently, the organic pollutants are first adsorbed onto the surface of the oxide, forming a precursor complex.The electrons were transferred within this complex, followed by the release of oxidation products [25].
The possibility of recovery and reuse of photocatalysts is important for technological application in largescale processes [26], since it can contribute significantly to reduce the steps of the operation saving time [27].
A recycling efficiency test of these photocatalysts was performed in order to evaluate the reuse of the photocatalysts In 2 O 3 , Mn 2 O 3 , In/Mn/In/Mn and Mn/In/Mn/In in the photocatalysis of the methylene blue dye.The use of cycles, as well as the maintenance of high photocatalytic activity, is a fundamental issue for the long-term use in the catalyst practical applications.Therefore, two criteria are required: (i) the stability of the catalyst to maintain its high activity over the time, and (ii) the ease with which the catalyst can be recycled from the solution Fig. 4 Effect of crystallization temperature of the thin films Mn/In/Mn/In and In/Mn/In/Mn under UV irradiation and photodegradation of methylene blue: a 500 °C and b 600 °C [28].Figures 7, 8, 9 and 10 show that the thin films used as photocatalysts were reused under identical conditions 3 times during each cycle of 350 min.Figures 7 and 8 show the results of the reuse of the thin films (pure) and Figs. 9 and 10 show the results of the reuse of the thin films (combined), under identical conditions 3 times during each 300 min cycle.
In Fig. 7 it is possible to observe that In 2 O 3 films calcined at 600 °C presented a better efficiency in the dye degradation, and it is also possible to reuse it, with a yield of 35% still in the third cycle of exposure to UV and visible wave lengths.In Fig. 8 it is observed that Mn 2 O 3 films calcined at 600 °C have an extremely better yield than those calcined at 500 °C, both in relation to the film reuse and to the dye degradation of the presenting over 40% of the third cycle.
In Figs. 9 and 10 it is shown the reuse of the films composed of Mn 2 O 3 and In 2 O 3 with inversion in the order of the oxides deposition.In these graphs, it is possible to observe that the combined films have higher degradation efficiency when calcined at 500 °C.In addition, it is also possible to infer from the graphical observation that the order of deposition does not significantly interfere with the pattern of the curves formed.However, the films with the first layer of In 2 O 3 and the last one of Mn 2 O 3 present better performance in the degradation of the methylene blue dye and reuse with a yield of 35% in the third cycle.
Figure 11 shows the photoluminescence (PL) spectra for samples calcined at 500 and 600 °C.The photoluminescence spectra of the thin films calcined at 500 and 600 °C were performed from an excitation wavelength of 350 nm at room temperature.From the spectrum, bands between 350 and 600 nm can be detected.The photoluminescence emission is a consequence of the direct recombination of  the electrons and photogenerated holes to semiconductors, so that the PL spectra can reflect the separation efficiency of the photogenerated charge carriers [29].According to Xinmei [30], a greatest number of defects may contribute to the increase of photocatalytic activity.On the other hand, a lower PL intensity implies in a lower recombination rate of e − /h + , allowing more electrons and holes to participate in the oxidation and reduction reactions, thus improving the photocatalytic performance.This same behavior is observed in the In/Mn/In/Mn films and the Mn/In/Mn/In films, which have preferentially photocatalytic activity and a lower PL intensity.In pure films this behavior is not observed, most probably due to a smaller recombination.From the photoluminescence spectra, it was possible to calculate the chromaticity coordinates of the samples.As it can be observed in Fig. 12, all the samples showed coordinates in the blue region, which is important to the photocatalytic activity, since they are related to the presence of oxygen vacancies, which can trap electrons, avoiding the recombination of charges [31].

Conclusion
In this work, the photocatalysts In 2 O 3 , Mn 2 O 3 , In/Mn/ In/Mn and Mn/In/Mn/In, were successfully synthesized to study their efficiency in relation to the degradation of EE2 and MB.It was possible to observe that the combined films did not show good efficiency in the dye degradation.The films Mn/In/Mn/In, presented a reuse with less than 10% yield in the third cycle.The results in this study suggest that the degradation of the hormone ethynylestradiol was highly influenced by the photocatalytic activity when the compounds decreased about 80% of their concentration and exposed to the system.The combined film resulted in a degradation of 89.63%, leading to the conclusion that this mixture of the layers improved the photocatalytic activity of these films for the degradation of ethynylestradiol hormone.

Fig. 1
Fig. 1 Deposition order of the resins in the substrate

Figure 3
Figure 3 obtained by MEV-FEG shows the cross section of the thin film Mn/In/Mn/In on the Si substrate, with four layers and thermally treated at temperatures of 500 and 600 °C.The structure of the film is composed of porous nanosheets with thicknesses of approximately 303.6 and 279.3 nm for the Mn/In/Mn/In films crystallized at 500 and

Fig. 2 a
Fig. 2 a X-ray diffraction for thin films In 2 O 3 and Mn 2 O 3 calcined at 600 °C, b X-ray diffraction for thin films In/Mn/In/Mn calcined at 500 and 600 °C and c X-ray diffraction for thin films Mn/In/Mn/In calcined at 500 and 600 °C

Fig. 5 Fig. 6
Fig. 5 Degradation of EE2 in water induced by the thin films In 2 O 3 , Mn 2 O 3 and In/Mn/In/Mn

Fig. 7 Fig. 8 Fig. 9
Fig. 7 Photocatalytic activity of the thin film In 2 O 3 for the degradation of the methylene blue dye with 3 cycles used

Fig. 10
Fig. 10 Photocatalytic activity of the thin film Mn/In/Mn/In for the degradation of the methylene blue dye with 3 cycles used

Table 1
Thickness of the thin films In 2 O 3 , Mn 2 O 3 , Mn/In/Mn/In and In/Mn/In/Mn crystallized at 500 and 600 °C

Table 2
Photocatalytic efficiency for the degradation of MB and 17 alpha-ethynylestradiol Degradation of In 2 O 3 , Mn 2 O 3, In/Mn/In/Mn and Mn/In/Mn/In thin films