Ferrites, modified with silver nanoparticles, for photocatalytic degradation of malachite green in aqueous solutions

Catalysis Today xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Review

Ferrites, modified with silver nanoparticles, for photocatalytic degradation of malachite green in aqueous solutions M. Tsvetkov, J. Zaharieva, M. Milanova

⁎

Department of Inorganic Chemistry, Faculty of Chemistry and Pharmacy, University of Sofia “St. Kliment Ohridski”, 1, J. Bourchier, 1164 Sofia, Bulgaria

A R T I C LE I N FO

A B S T R A C T

Keywords: Ferrites Silver nanoparticles Photocatalysis Malachite green

Ferrites of the type MFe2O4, where M = Zn, Co, and Mg, were prepared via a sol-gel procedure using propylene oxide as a gelating agent. Silver nanoparticles with an average size of 6 nm were prepared in the presence of the ferrite powders in suspension using raffinose as a reducing agent in a basic water solution. Nine samples of the ferrite/silver nanoparticles nanocomposites were prepared in this way with different Ag content. The composites obtained were characterized by XRD, TEM, and UV/Vis spectroscopy. The size distribution of the particles was determined using the data from TEM. By TEM-EDAX, the location of the silver nanoparticles on the ferrite surfaces was determined. Using UV/Vis spectra, the band gap energies for the pure ferrites and the nanocomposites were calculated; the calculated band gap energies were influenced by the presence of the silver nanoparticles. The photocatalytic activity of the silver nanoparticles/ferrite nanocomposites for decomposition of Malachite Green in model water solutions under UV and visible light irradiation was determined. The ZnFe2O4/Ag-NPs nanocomposites were found to be active under visible light irradiation, while the MgFe2O4/ silver nanoparticles composites especially active more than the pure MgFe2O4. The presence of silver nanoparticles did not influence the very low photocatalytic activity of CoFe2O4.

1. Introduction Spinel ferrites with general formula MFe2O4 (M = Fe(II), Ni(II), Co (II), Zn(II), etc.) are among the many materials that have been tested as photocatalysts for degradation of different organic water pollutants under light irradiation [1–7]. They have an electronic band structure that is well suited for visible light-induced catalysis: the energy of the forbidden zone for ZnFe2O4 has been reported to be 1.92 eV [5,8], 2.18 eV for MgFe2O4 [5], while for CoFe2O4 a value of 2.7 eV has been reported [9,10]. However, their photocatalytic activity remains low due to the bad charge-carrier separation process [11]. To remedy this, they have often been combined to form composites in order to apply them as photocatalysts [5] for photodegradation of different pollutants, such as Codoped NiFe2O4/carbon nanotube composites for Rhodamine B removal [12], NiFe2O4/carbon nanotube nanocomposite for degradation of phenol [13], etc. Malachite green is an organic water pollutant, known to be harmful for living creatures because of its potential carcinogenicity, mutagenicity and teratogenicity in mammals [14]. Ferrites are not often used for photodegradation of malachite green, although some ferrites showed activity for the degradation of malachite green under UV light [1,15]. ⁎

Composites of ferrites with silver nanoparticles (Ag-NPs) have been prepared and tested for different activities [16–20] such as CoFe2O4/ Ag-NPs for antibacterial activity [21], ZnFeO/Ag as photocatalysts for degradation of methylene blue [18], and ZnFe2O4/Ag clusters [19] as well as Ag/Fe3O4 composites [20,22] for antibacterial and catalytic activity. In the examples mentioned, silver nanoparticles have been prepared in the presence of the surface of a solid sample. For the preparation of silver nanoparticles, chemical reductions have been applied using reducing agents such as hydrazine [23], sodium citrate [23–25], ascorbic acid [24], borohydride [24], alcohols [25,26], even tea leaf extracts [27] or a combination of two reducing agents [25,28]. Some agents can act as both a reducing agent and as stabilizing agent. These include polysaccharides, which are attractive for their good biodegradation and biocompatibility, used in green-chemistry processes for the preparation of metallic nanoparticles [29–31], for instance D-(+) raffinose [30], used in the work presented here. In order to provide a solution for the environmental problem of water pollution, nanocomposites of the type semiconductor/metal nanoparticles were prepared in order to test them for photocatalytic water purification. The photocatalytic properties of ferrite/silver nanoparticles nanocomposites for the degradation of malachite green in water solution are presented here.

Corresponding author. E-mail addresses: [email protected], [email protected]fia.bg (M. Milanova).

https://doi.org/10.1016/j.cattod.2019.07.052 Received 2 January 2019; Received in revised form 28 May 2019; Accepted 29 July 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: M. Tsvetkov, J. Zaharieva and M. Milanova, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.052

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

Fig. 1. Synthetic procedure for the ferrites (a) and for modification of the ferrites with silver nanoparticles (b).

2. Experimental

Table 1 Symbols of the samples and the amount of Ag, mass. % in the samples. Composites MFO/Ag-NPs

Samples/Symbols

Ag, mass. %

CoFe2O4/Ag-NPs

CoFO/Ag0.04 CoFO/Ag0.06 CoFO/Ag0.32 ZnFO/Ag0.19 ZnFO/Ag0.79 ZnFO/Ag0.69 MgFO/Ag0.17 MgFO/Ag0.37 MgFO/Ag0.69

0.04 0.06 0.32 0.19 0.79 0.697 0.17 0.37 0.692

ZnFe2O4/Ag-NPs

MgFe2O4/Ag-NPs

2.1. Synthesis of ferrites MFe2O4 (M = Mg(II), Co(II), Zn(II)) were prepared by a sol-gel technique using 1,2- propylene oxide (PO) as gelating agent. The propylene oxide was used in order to create hydroxyl groups on the hydrated [M(H2O)]2+ cations and to induce a polycondensation reaction between Fe3+ and M2+ ions (M = Zn, Mg, Co). The gel formation is almost immediate; other gelation agents sometimes need days or even weeks to form a gel. The typical procedure (Fig. 1, a) includes dissolving appropriate molar ratios of MgCl2.6H2O, Zn(NO3)2.6H2O, CoCl2.6H2O and Fe(NO3)3.9H2O in order to obtain approx. 1 g of sample in 95% ethanol. After 30 min of homogenization, the propylene oxide was added (molar ratio EtOH:PO = 4:1). After the fast sol formation, it was stirred for an additional 30 min. The formed sols were kept at 105 °C for 24 h to dry, followed by annealing for 5 h at 550 °C.

Fig. 2. XRD pattern of the MFO/Ag-NP’s (M = Mg, Zn, Co) nanocomposites. 2

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

The temperature of 550 °C was chosen to ensure the complete decomposition of the powdered precursor to form MFe2O4 (M = Zn, Mg, Co), and is based on our previous TG/DTA investigations on the thermal behavior of the precursor [32]. The as prepared powdered ferrites were further used for modification with silver nanoparticles (Fig. 1, b).

Table 2 Phase composition, cell parameters and crystallite size of the samples. Sample

Phase composition

Cell parameters, Å

Average crystallite size, nm

CoFO/Ag0.06 ZnFO/Ag0.79 MgFO/Ag0.37

CoFe2O4/Ag-NPs ZnFe2O4/Ag-NPs MgFe2O4/Fe2O3/ Ag-NPs

a = 8.406 ± 0.003 a = 8.395 ± 0.006 MgFe2O4 a = 8.379 ± 0.002 Fe2O3 a = 5.041 ± 0.007 c = 13.746 ± 0.003

31 ± 1 22 ± 2 MgFe2O4 25 ± 2 Fe2O3 27 ± 2

2.2. Preparation of silver nanoparticles and MFe2O4/Ag-NPs nanocomposites Silver nanoparticles were produced [30] by adding raffinose solution and AgNO3 solution (0.1 M) to distilled water, followed by homogenization by ultrasound for 15 min before addition of sodium hydroxide solution (0.1 M). At this stage, the liquid slowly turned light brown, which was taken as an indication for silver nanoparticles formation. The reaction mixture was further sonicated at 30 °C for 45 min and stirred for 24 h, and a pink-red colloidal solution was obtained. The final liquid was then centrifuged to separate the particles, which were dried at 60 °C overnight. The colour of the remaining liquid was light brown. MFe2O4/Ag-NPs composites were prepared by suspending the individual ferrites in water followed by ultrasonication for 30 min (Fig. 1, b). Silver nanoparticles were then produced as described above, this time in the presence of the ferrite particles. Composites with different amounts of Ag nanoparticles were obtained by varying the volume of 0.1 M AgNO3 solution. The calculated amount of Ag-NPs, assuming 100% adsorption, was 0.38, 0.77 and 1.15 mass. %. The actual amount of Ag in the composites was determined by atomic absorption spectroscopy and was much lower due to the adsorption capacity of the ferrites. The data and the symbols of the samples tested are presented in Table 1. The samples are described as MFO/Ag-NPs (M = Mg, Zn, Co) when the amount of silver nanoparticles is not mentioned.

Fig. 3. UV/Vis absorption spectra of supernatant after adsorption of silver nanoparticles on the surface of MgFe2O4 (1), ZnFe2O4 (2) and CoFe2O4 (3) from bottom to the top, respectively. The spectrum of the initial silver nanoparticles colloidal solution is added for comparison (4).

2.3. Methods for characterization of the samples X-Ray Diffraction to determine the crystal structure of the materials was performed using a PANalytical Empyrean X-ray diffractometer in the 2θ range of 15°-90° using CuKα radiation (λ = 0.15405 nm), steps of 0.01 °C and 20 s exposure time at each step. The average crystallite size was calculated using Scherrer’s equation D = kλ/B cosθ, where D is the average diameter in nm, k is the shape factor (k = 0.9), B is the broadening of the diffraction line measured at half of its maximum intensity in radians, λ is the X-ray wavelength and θ is the Bragg’s diffraction angle. The average crystallite sizes were estimated with full profile Rietveld analysis using PANalytical X'Pert HighScore Plus software. UV-VIS absorption spectroscopy - an Evolution 300 UV–vis spectrometer (Thermo Scientific) was used for measuring the absorption of the samples in the range 200–900 nm. Band gap energies were calculated from the UV–vis absorption spectra in the range from 200 to 400 nm. The UV–Vis data were analyzed for the relation between the optical band gap, absorption coefficient and energy (hν) of the incident photon for near edge optical absorption in semiconductors. The band gap energy was calculated from the measured curves by fits according to Tauc’s equation [33] αhν = A(hν − Eg)n/2, where A is a constant independent of hν, Eg is the semiconductor band gap and n depends on the type of transition. The value used for n was 1, reflecting a direct transition. The well-known approach for semiconductor band gap energy determination from the intersection of linear fits of (αhν)1/n versus hv on the x-axis was used, where n can be 1/2 and 2 for direct and indirect band gap, respectively. Transmission electron microscopy (TЕM): a JEOL JEM 2100 microscope was used at 200 kV and up to 100k magnification for characterization of the morphology of the samples.

Fig. 4. UV–vis absorption of the powdered ferrites, MgFe2O4 (1), ZnFe2O4 (2), CoFe2O4 (3). Table 3 Band gap energy values, Eg, eV. Samples

Eg, eV

Samples

Eg, eV

Samples

Eg, eV

CoFe2O4 CoFO/Ag0.04 CoFO/Ag0.06 CoFO/Ag0.32

1.22 1.26 1.20 1.26

ZnFe2O4 ZnFO/Ag0.19 ZnFO/Ag0.79 ZnFO/Ag0.69

1.83 1.93 1.91 1.83

MgFe2O4 MgFO/Ag0.17 MgFO/Ag0.37 MgFO/Ag0.69

2.16 2.02 2.04 2.00

3

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

Fig. 5. TEM images of the nanocomposites ZnFO/Ag-NPs (a, b) at different magnifications. The arrows are showing the silver nanoparticles on the surface of the ferrite.

period (in order to establish the equilibrium of the sorption process), the system was illuminated by a UV-lamp (Sylvania 18 W BLB T8, emission in the 345–400 nm region with a maximum at 365 nm), situated at 9.5 cm distance above the slurry (illumination intensity 0.5 W m-2), or by a halogen lamp (Kanlux S.А., J-300W 118 MM, correlated colour temperature 2710 K, 300 W, polychromatic, 300–800 nm, maximum at 600 nm), situated at 20 cm distance above the slurry. The slurry was continuously stirred using a magnetic stirrer (400 min−1) while bubbling with air (45 L h−1) to ensure that the concentration of oxygen in the liquid is constant at close to saturation at all times. The initial pH of the solutions was between 5.8 and 5.9. Periodically, a 5 mL aliquot was taken from the solution and filtered through a 0.20 μm Minisart filter to remove the catalyst. The dye concentration was determined spectrophotometrically by the band at 620 nm. The data obtained were plotted in coordinates (C/C0)/t and -ln(C/C0)/t (where C0 is the concentration after the” dark” period, and C is the concentration after t min irradiation), and apparent rate constants of the degradation process were determined assuming pseudo first-order kinetics (the oxygen concentration is assumed to be constant at close to saturation). The sorption capacity was calculated as the ratio (C00–C0)/C00, where C00 is the starting solution concentration (before the” dark” period). The MG degradation at moment t is determined by the formula: degradation, % = (Ао – Аt)/Ао х 100, where Ао is the initial absorption of the MG solution at t =0 min, and Аt is the absorption at t min. Experiments using visible light were repeated in triplicate, while UV experiments were done only once for each sample.

Fig. 6. The size distribution of silver nanoparticles in ZnFO /Ag-NPs, based on TEM. Table 4 Photocatalytic and sorption activity of the composites under UV light. The sorption and degradation were calculated as presented in the experimental part 2.4. Sample

MgFе2O4 MgFO/Ag0.17 MgFO/Ag0.37 MgFO/Ag0.69 ZnFе2O4 ZnFO/Ag0.19 ZnFO/Ag0.79 ZnFO/Ag0.69 CоFе2O4 CoFO/Ag0.04 CoFO/Ag0.06 CoFO/Ag0.32

Rate constant, x10−3 min-1 0-150 min

0-60 min

60-150 min

2.6 4.6 7.9 8.1 8.2 5.0 5.5 7.8 2.1 2.7 2.2 2.3

4.1 3.4 8.0 9.4 9.6 6.7 6.4 8.5 3.1 3.8 3 3.4

0.8 6.1 8.3 6.5 7.3 5.3 3.8 6.5 1.2 1.5 1.3 1.1

Sorption,%

Degradation, %

3. Results and discussion 88 92 87 87 79 83 78 82

28 53 67 70 70 46 54 66

22 25 22

31 27 27

3.1. Characterization of the samples 3.1.1. Phase homogeneity The phase composition and the cell parameters and crystallite size of the oxide phases in the samples were determined using the X-ray diffraction results (Fig. 2). In Table 2, the data for the samples MFO/ AgNPs (M = Mg, Zn, Co) modified with silver nanoparticles are presented. The average crystallite size of the Ag nanoparticles was found to be 6 ± 1 nm for all samples. The CoFe2O4 and ZnFe2O4 samples obtained (Fig. 1, a) were phase homogeneous while the MgFe2O4 phase contained 31% Fe2O3 (Fig. 2). In spite of all efforts, homogeneous phase MgFe2O4 without a Fe2O3 phase was not obtained by the synthetic procedure applied. A different synthetic procedure was not attempted as even if it was successful, the properties of the resulting catalyst depend on the preparation method and no comparison among the samples can be made. The presence of silver nanoparticles in the composite samples MFO/Ag-NPs (M = Mg, Co, Zn) was proved by XRD and is pointed out on the diffractogram

2.4. Photocatalytic tests The photocatalytic tests were performed in slurry of 1 g catalyst L−1, using a 10-5 M aqueous solution of Malachite Green oxalate (MG), (Chroma GmbH) as a model pollutant. The equipment and the procedure applied were as used by us in [1,2,34,35]. After a 60-min” dark” 4

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

Fig. 7. Photocatalytic activity of ZnFO/Ag-NPs (a) and MgFO/Ag-NPs (b) under UV irradiation.

into the visible light region shorter than 700 nm, which implies the possibility of photocatalytic activity of these materials under visible light irradiation. The amount of light absorbed by any photocatalyst depends on the optical band gap energy (difference between valence band and conduction band). The UV/vis absorption spectra were used to calculate the band gap energy of the samples. The values obtained for the band gap energy for the pure ferrites and the nanocomposites based on them are included in Table 3, showing that the samples are narrow band gap semiconductors. The figures showing the calculated values are included in the supplementary materials (Figures S1-S4). The value for the pure ZnFe2O4, 1.83 eV, is lower than the value of 1.92 eV reported in the literature [36]; for MgFe2O4 it is close to 2.18 eV [5].

Table 5 Catalytic activity of ZnFO/Ag-NPs under visible light irradiation. Sample

ZnFе2O4 ZnFO/Ag0.19 ZnFO/Ag0.79 ZnFO/Ag0.69

Rate constant, x10−3 min-1 0-150 min

0 -60 min

60-150 min

6.2 9.6 7.1 9.5

4.9 10.5 6.3 10.2

7.1 7.6 8.1 8.6

Sorption,%

Degradation, %

84 85 87 85

60 73 66 72

3.1.4. Morphology of the samples TEM was used to examine the morphology, particle size, and particle size distribution of the samples. TEM images show the nanocomposites particles (Fig. 5 а, b). The images are at different magnification (80k, 40k, scale 50 nm, 200 nm). Data for the particle size distribution are obtained for silver nanoparticles in ZnFO/Ag-NPs (Fig. 6) based on TEM images. 3.2. Photocatalytic properties 3.2.1. MFe2O4/Ag-NPs under UV-light illumination The data for the catalytic activity of the Ag NPs-modified ferrites ZnFe2O4 and MgFe2O4 under UV light irradiation are presented in Table 4 and in Fig. 7. From Fig. 7 it is clear that photolysis (disappearance of malachite green in the absence of the catalyst) is insignificant compared to the rate of disappearance with catalyst present. The rate constants for the interval 0–60 min and for 60–150 min were calculated; a tendency for a slight decrease of the rate constant in the interval 60–150 min is observed for all catalysts (Table 4). Apparently, during the photocatalysis process the surface of the catalysts is changing, possibly due to strong adsorption of some MG decomposition products that block catalytic sites. In the interval 0–60 min, pure ZnFe2O4 is more active than pure MgFe2O4, with rate constants 8.2 × 10−3 and 2.6 × 10−3 min-1, respectively. In the interval 60–150 min, MgFe2O4 has hardly any activity, probably because of its high adsorption capacity. The modification with Ag-NPs apparently has a stronger influence on MgFe2O4, leading to much higher rate constants in comparison with the pure ferrite. The highest values for the rate constant for the photocatalytic process were found for the samples MgFO/Ag0.69 and MgFO/Ag0.37: 8.1 × 10−3 and 7.9 × 10−3 min-1, respectively. The difference in activity for these two samples is insignificant, and probably indicates that

Fig. 8. Catalytic activity of ZnFе2O4 and ZnFO/Ag-NPs under visible light irradiation.

(Fig. 2). 3.1.2. Adsorption of silver nanoparticles on the surface of the ferrites The adsorption of silver nanoparticles on the ferrite surface during preparation of the ferrite/silver nanoparticles nanocomposites was followed by colorimetric analysis of the supernatant liquid after the adsorption step. In Fig. 3, the UV–vis spectra of the initial silver nanoparticle liquid and the supernatant liquids after adsorption on the ferrites are shown. It can be seen that the silver nanoparticles have different degrees of adsorption on MgFe2O4, ZnFe2O4 and CoFe2O4. This result is in coincidence with the data from the Table 1, part 2.2. 3.1.3. Optical properties and band gap energies of the prepared catalysts The absorption of UV/Vis light by the powdered samples was recorded (Fig. 4). The samples exhibited photoabsorption from UV light 5

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

Fig. 9. Mechanism of Malachite green degradation under light irradiation in the presence of the MFO/Ag-NPs nanocomposites (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

the photocatalytic activity of the pure ferrites and the nanocomposites, it can be concluded that the activity does not depend only on the band gap energy. (ii) The low photocatalytic activity of the cobalt ferrite and the nanocomposites CFO/Ag-NPs is an issue, as the band gap is narrower than the other ferrites investigated. It is reported that the photocatalytic activity of photocatalysts is affected by crystal size [37], the method for preparation determining the size and by that the activity [38]. The change in the optical and the photocatalytic properties can be attributed to a change in the population of the Co2+ at tetrahedral and octahedral sites [39]. The possible reason could be the excited electrons not being energetic enough and cannot form the reactive oxygen species that cause the oxidation of malachite green. (iii) The photocatalytic decomposition of the malachite green in the presence of the nanocomposites MFO/Ag-NP’s under irradiation of light is presented in Fig. 9. The generation of the electron/hole e−/ h+ pairs enabling the oxidation and reduction process is shown. In aqueous solution, a hydroxide ion OH- reacts with h+ to form a hydroxyl radical that enables photocatalytic oxidation of malachite green. The photoinduced electrons react with O2 to yield O2∙ radicals, which react with H+ ions to produce HOO∙. The latter decomposes to ∙OH, which degrades MG [5,40]. In the Fig. 9, the AgNP-‘s are shown playing a crucial role to trap the electrons from the conduction band and thus avoid e-/h + recombination.

a further increase of the silver nanoparticle content will not further improve the activity of MgFe2O4. The silver nanoparticles were most beneficial for MgFeO/Ag-NPs, increasing the activity in comparison with the pure MgFe2O4. For ZnFO/Ag-NPs, the opposite was observed: the addition of silver nanoparticles caused a decrease of the activity in comparison with the pure ZnFe2O4 sample. The samples MgFO/Ag0.17 and ZnFO/Ag0.19 as well as MgFO/ Ag0.69 and ZnFO/Ag0.69 have similar amount of Ag on the surface and comparatively close values for the rate constants, 4.6 × 10−3 vs. 5.0 × 10−3 and 8.1 × 10−3 vs. 7.8 × 10−3 min-1. The pure CoFe2O4 as well as the CFO/Ag-NPs composites were tested but the photocatalytic activity detected was close to the one without a catalyst i.e. photolysis, in spite of the low value for the energy of the band gap (1.2 eV, Table 3). The low activity of CoFe2O4 is quite likely because of its very narrow band gap. The excited electrons may not be energetic enough and cannot form the reactive oxygen species that cause the oxidation of malachite green. At the same time the narrow band gap favours the e-/h + recombination. Apparently Ag-NPs cannot influence it, taking into account the lower adsorption of the pure CoFe2O4. 3.2.2. MFe2O4/Ag NPs under visible light-illumination In Table 5 and Fig. 8 data for the catalytic activity under visible light-illumination of ZnFe2O4, both pure and modified with AgNPs are given. The values of the rate constants for ZnFe2O4 modified with silver nanoparticles are higher than for the pure ZnFe2O4. There is no clear tendency observed in the dependence on the quantity of silver nanoparticles. The values of 9.5–9.6 × 10−3 min-1 are among the highest observed. Only ZnFe2O4 modified with silver nanoparticles was tested under both UV and visible light illumination, so only for these samples a comparison can be made. The pure is more active under UV than under visible light illumination. However, the addition of silver nanoparticles is beneficial for the activity of ZnFe2O4 under visible light. The reason for the different activity of ZnFe2O4 could be a matter of combination between the irradiation energy and the electron/hole production. That ZnFe2O4 shows activity in visible light means that sunlight can be used for illumination in practical applications. The MgFO/Ag-NPs samples did not show any photocatalytic activity under visible light.

The results show that this mechanism is more likely to occur for the MgFe2O4 modified with Ag-NPs, where their presence has shown to be beneficial. 4. Conclusions Nanosized ferrites with formula MFe2O4 (M = Mg, Co, Zn) were successfully prepared by a sol-gel technique. The synthesized ferrites were analyzed and their purity and phase composition were proved. The ferrites show good stoichiometric composition, except that MgFe2O4 showed the presence of a hematite phase. The as prepared ferrites were successfully modified with silver nanoparticles, Ag-NPs, by a nanoprecipitation technique and between 50–60 % of them were adsorbed on the ferrite surfaces. The grain sizes of the prepared nanocomposites were found to be in the order of 4–8 nanometers. The photocatalytic activity of the ZnFO/Ag-NPs nanocomposites for photodegradation of Malachite Green solution (10−5 M) under UV- and

3.2.3. Some issues to be taken into account (i) The nanocomposites have values for the band gap energy close to that for the pure ferrites investigated. Considering the difference in 6

Catalysis Today xxx (xxxx) xxx–xxx

M. Tsvetkov, et al.

Visible light illumination was determined. The Ag-NPs lead to a significant increase of the photocatalytic activity of MgFe2O4, but it does not affect positively the photocatalytic activity of ZnFe2O4 under UV-light irradiation. CoFe2O4 did not show significant photocatalytic activity, neither pure nor as a nanocomposite with Ag-NPs. The results are in agreement with the narrower band gap of CoFe2O4, allowing fast recombination between the holes and the electrons. A possible reason for the low photocatalytic activity of CoFe2O4 could be its inverse spinel structure.

527–539. [15] L. Zhang, M. Su, Na Liu, X. Zhou, P. Kang, Water Sci. Technol. (2009) 2563–2569, https://doi.org/10.2166/wst.2009.497. [16] M. Kooti, S. Saiahi, H. Motamedi, J. Magn. Magn. Mater. 333 (2013) 138–143. [17] M.K. Satheeshkumar, E.R. Kumar, Ch. Srinivas, N. Suriyanarayanand, M. Deepty, C.L. Prajapat, T.V.C. Rao, D.L. Sastry, J. Magn. Magn. Mater. 469 (2019) 691–697, https://doi.org/10.1016/j.jmmm.2018.09.039. [18] A. Mahmood, S.M. Ramay, Y.S. Al-Zaghayer, M. Imran, S. Atiq, M.S. Al-Johani, J. Alloys Compounds 614 (2014) 436–442, https://doi.org/10.1016/j.jallcom.2014. 06.055. [19] X. Cao, L. Gub, X. Lan, C. Zhao, D. Yao, W. Sheng, Mater. Chem. Phys. 106 (2007) 175–180. [20] U. Kurtan, M. Amir, A. Baykal, Appl. Surf. Sci. 363 (2016) 66–73, https://doi.org/ 10.1016/j.apsusc.2015.11.214. [21] D. Gingasu, I. Mindru, L. Patron, G. Marinescu, S. Preda, J.M. Calderon-Moreno, P. Osiceanu, S. Somacescu, N. Stanica, M. Popa, C. Saviu, M.C. Chifiriuc, Ceramics Intern. 43 (2017) 3284–3291, https://doi.org/10.1016/j.ceramint.2016.11.161]. [22] A. Amarjargal, L.D. Tijing, I.-T. Im, C.S. Kim, Chem. Eng. J. 226 (2013) 243–254, https://doi.org/10.1016/j.cej.2013.04.054. [23] M.G. Guzmán, J. Dille, S. Godet, Int. J. Chem. Biomolecular Eng. 2 (3) (2009) 104–111. [24] S. Piñero, S. Camero, S. Blanco, IOP Conf. Series: Journal of Physics: Conf. Series 786 (2017) 012020, , https://doi.org/10.1088/1742-6596/786/1/012020. [25] N.G. Bastús, F. Merkoçi, J. Piella, V. Puntes, Chem. Mater. 26 (9) (2014) 2836–2846, https://doi.org/10.1021/cm500316k. [26] B. Lee, S. Koo, J. Ind. Eng. Chem. 18 (2012) 1191–1195, https://doi.org/10.1016/j. jiec.2012.01.009. [27] S. Onitsuka, T. Hamada, H. Okamura, Colloids Surf. B Biointerfaces 173 (2019) 242–248, https://doi.org/10.1016/j.colsurfb.2018.09.055. [28] K. Ranoszek-Soliwoda, E. Tomaszewska, E. Socha, P. Krzyczmonik, A. Ignaczak, P. Orlowski, M. Krzyzowska, G. Celichowski, J. Grobelny, J. Nanopart. Res. 19 (8) (2017) 273, https://doi.org/10.1007/s11051-017-3973-9. [29] C. Wang, X. Gaо, Z. Chen, Y. Chen, H. Chen, Polymers 9 (12) (2017) 689, https:// doi.org/10.3390/polym9120689. [30] M. Chichova, M. Shkodrova, P. Vasileva, K. Kirilova, D. Doncheva-Stoimenova, J. Nanopart. Res. 16 (2) (2014) 2243, https://doi.org/10.1007/s11051-014-2243-3. [31] J. Yang, S. Han, H. Zheng, H. Dong, J. Liu, Carbohydr. Polym. 123 (2015) 53–66, https://doi.org/10.1016/j.carbpol.2015.01.029. [32] M. Tsvetkov, M. Milanova, I. Ivanova, D. Neov, Z. Cherkezova-Zheleva, J. Zaharieva, M. Abrashev, J. Mol. Struct. 1179 (2019) 233–241, https://doi.org/ 10.1016/j.molstruc.2018.07.083. [33] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627–637. [34] K. Zaharieva, V. Rives, M. Tsvetkov, Z. Cherkezova-Zheleva, B. Kunev, R. Trujillano, Mater. Chem. Phys. 160 (2015) 271–278. [35] R. Kralchevska, M. Milanova, D. Hristov, A. Pintar, D. Todorovsky, Mater. Res. Bull. 47 (2012) 2165–2177. [36] M.A. Valenzuela, P. Bosch, J. Jimenez-Becerrill, O. Quiroz, A.I. Paez, J. Photochem. Photobiol. A: Chem. 148 (2002) 177–182. [37] M. Shang, W.Z. Wang, S.M. Sun, L. Zhou, L. Zhang, J. Phys. Chem. C 112 (2008) 10407–10411. [38] Y. Sun, W. Wang, L. Zhang, S. Sun, E. Gao, Mater. Lett. 98 (2013) 124–127. [39] D. Sharma, N. Khare, AIP Adv. 6 (2016) 085005, https://doi.org/10.1063/1. 4960989. [40] R. Sharma, S. Singhal, Physica B 414 (2013) 83–90.

Acknowledgements The financial support of the Sofia University Fund for Scientific Investigations via Project 80-10-227 and of the Operational Program "Science and Education for Smart Growth" by the Project BG05M2OP001-1.002-0019” Clean technologies for a sustainable environment - waters, wastes, energy for a circular economy” is highly appreciated. References [1] M.P. Tsvetkov, K.L. Zaharieva, Z.P. Cherkezova-Zheleva, M.M. Milanova, I.G. Mitov, Bulg. Chem. Comm. 47 (2015) 354–359. [2] M. Tzvetkov, M. Milanova, Z. Cherkezova-Zheleva, I. Spassova, E. Valcheva, J. Zaharieva, I. Mitov, Acta Chimica Slov. 64 (2017) 299–311, https://doi.org/10. 17344/acsi.2016.3049. [3] L. Zhang, M. Su, N. Liu, X. Zhou, P. Kang, Water Science & Technology, WST, 60.10 (2009) 2563–2569, https://doi.org/10.2166/wst.2009.497. [4] A. Arimi, L. Megatif, L.I. Granone, R. Dillert, D.W. Bahnemann, J. Photochem. Photobiol. A: Chemistry 366 (2018) 118–126, https://doi.org/10.1016/j. jphotochem.2018.03.014. [5] E. Casbeer, V.K. Sharma, X.-Zh. Li, Sep. Purif. Technol. 87 (2012) 1–14. [6] Y. Diao, Z. Yan, M. Guo, X. Wang, J. Hazard. Mater. 344 (2018) 829–838, https:// doi.org/10.1016/j.jhazmat.2017.11.029. [7] S.K. Rashmi, H.S.B. Naik, H. Jayadevappa, R. Viswanath, S.B. Patil, M.M. Naik, Materials Science Eng.: B 225 (2017) 86–97, https://doi.org/10.1016/j.mseb.2017. 08.012. [8] M. Su, Ch. He, V.K. Sharma, M.A. Asi, D. Xia, X.- Li, H. Deng, Y. Xiong, J. Hazard. Mater. 211–212 (2012) 95–103. [9] S. Saadi, A. Bouguelia, M. Trari, Renew. Energy 31 (2006) 2245–2256. [10] R.S. Gaikwad, S.-Y. Chae, R.S. Mane, S.-H. Han, O.-S. Joo, Int. J. Electrochem. ID 729141 (2011) 6 pages, https://doi.org/10.4061/2011/729141. [11] E. Skliri, J. Miao, J. Xie, G. Liu, T. Salim, B. Liu, Q. Zhang, G.S. Armatas, Appl. Catal. B 227 (2018) 330–339, https://doi.org/10.1016/j.apcatb.2018.01.045. [12] C. Singh, S. Bansal, V. Kumar, S. Singhal, Ceramics Intern. 41 (3) (2015) 3595–3604, https://doi.org/10.1016/j.ceramint.2014.11.021. [13] P. Xiong, Y. Fu, L. Wang, X. Wang, Chem. Eng. J. 195-196 (2012) 149–157. [14] E. Sudova, J. Machova, Z. Svobodova, T. Vesely, Veterinarni Med. 52 (12) (2007)

7