ZnFe2O4

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Potential Applications of Magnetic #-AgVO3/ZnFe2O4 Nanocomposites in Dyes Photocatalytic Degradation and Catalytic Thermal Decomposition of Ammonium Perchlorate Reza Abazari, and Ali Reza Mahjoub Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03727 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 1, 2017

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Industrial & Engineering Chemistry Research

Potential Applications of Magnetic β-AgVO3/ZnFe2O4 Nanocomposites in Dyes Photocatalytic Degradation and Catalytic Thermal Decomposition of Ammonium Perchlorate

Reza Abazari, Ali Reza Mahjoub,* Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran

1 ACS Paragon Plus Environment


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ABSTRACT The present study has developed a straightforward and efficient route for the synthesis of quite spherical β-AgVO3/ZnFe2O4 nanocomposites under ambient conditions in the absence of any cosurfactants. FE-SEM, PXRD, EDAX, XRF, FT-IR, UV-DRS, PL, BET, and DLS techniques have been used for proof of the formation and characterization of the structure, size, surface morphology, and phase composition of the suggested nanocomposites. The prepared nanocomposites have been used as both a catalyst and a photocatalyst. Results of the DSC and UV-visible spectroscopy analyses show that, due to their synergistic effects, our synthesized nanocomposites can significantly increase the catalytic thermal decomposition of the ammonium perchlorate (AP) and photocatalytic degradation of the methylene blue (MB), compared to their constituent nanoparticles (i.e. β-AgVO3 and ZnFe2O4). KEYWORDS: Emulsion nanoreactors, β-AgVO3/ZnFe2O4 nanocomposites, Ammonium perchlorate, Photocatalytic degradation, Magnetic separation


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1.

INTRODUCTION

Process industries have proved to discharge vast quantities of water pollutants (e.g., hazardous organic compounds), which are usually non-degradable and biologically harmful.1, 2 In view of this, much attention has already been paid to the generation of green energy and elimination of the environmental pollution using photo-catalytic agents.3-5 Photocatalytic activities are determined by the shape, band gap energy, crystal phase, surface to volume ratio, and other intrinsic characteristics of the photocatalysts.6, 7 TiO2 and ZnO are two appropriate photocatalysts with such advantages as eco-friendliness, availability, nontoxicity, cost-effectiveness, high photocatalytic activity, and chemical stability in aqueous solution under light illumination. Nevertheless, due to their large band gap (TiO2: anatase 3.2 eV; rutile 3.0 eV and ZnO: 3.37 eV), these substances can only adsorb UV irradiation.8-11 Great importance has thus been attached to the preparation of the photocatalysts with maximum efficiency under visible light irradiation.12, 13 The most common oxidizer in the composite solid propellants is ammonium perchlorate. The combustion properties of the propellants is affected by their thermal decomposition characteristics. In general, the combustion behavior of the solid propellants, and particularly their combustion rate, are significantly affected by the activation energy, reaction rate, and thermal decomposition temperature of the AP.14-17 In this regard, different additives have been employed to improve the AP decomposition. It is worth mentioning that the additives characteristics are determined by their chemical composition as well as their structure, morphology, size, phase, and size distribution.18-20 Binary transition metal oxides are generally regarded as proper materials. Using these materials, the disadvantages of the simple oxides can effectively be tackled. Besides, if different metal oxides are suitably combined, then high specific capacity, good cycling stability, and



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excellent rate performance will be obtained.21,

22

As an example, great importance has been

attached to the spinel metal ferrites (MFe2O4), which are particularly applicable for the fuel cells, electro-chromic windows, gas sensors, water splitters, optical devices, batteries, solar cells, catalyst and photocatalysts. Accordingly, different researchers have focused on the study of ZnFe2O4 NPs as a suitable material with large surface area, low toxicity, high chemical and thermal stability, magnetic properties, physical flexibility, availability, and eco-benignity.23-25 Furthermore, ZnFe2O4 NPs seem to offer considerable potentials for applications in semiconductor photocatalysis, super capacitor transformers, biosensor devices, medical applications, and loading coils.26, 27 On the other hand, due to the promising effects of β-AgVO3, research has also attempted to synthesize composites of the said material with the other metal oxides.28, 29 Besides, the visible light-driven photocatalyst β-AgVO3 demonstrates a narrow band gap and a highly-dispersed valence band, which are due to the unique hybridization of the valence bands of V 3d, O 2p, and Ag 4d orbitals in β-AgVO3.30-32 It has been suggested that the synergistic combination of β-AgVO3 and ZnFe2O4 is a desired catalytic system. As far as we know, no previous attempt has been made to study ZnFe2O4 NP, and especially its composites with β-AgVO3, as a catalyst and as a photocatalyst for AP thermal decomposition and MB dye degradation, respectively. As reported in the literature, in order to increase the catalytic efficiency, it is first required to control the morphology. Different routes such as hydrothermal,33 Sonochemistry,34 sol–gel,35 coprecipitation36 etc. are proposed for the synthesis of the transition metal oxides nanostructures. Some of these approaches cannot afford to control the shape and size distribution of the transition metal oxide nanostructures. Besides, some are particularly inappropriate for the largescale production. On the other hand, expensive ultrasonic equipment and large quantities of


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organic solvents are usually needed for the synthesis of the transition metal oxide nanostructures. Consequently, it is of the highest importance to develop straightforward, cost-efficient, environmentally friendly, and nontoxic protocols for synthesizing transition metal oxide nanostructures. Accordingly, in order to control the size and composition, it is appropriate to prepare nanostructures using emulsion nanoreactors at a low formation temperature.37-39 Based on a straightforward protocol, the present study has made use of the magnetic βAgVO3/ZnFe2O4 nanocomposites as a catalyst and a photocatalyst for AP thermal decomposition and MB dye degradation, respectively. To the best of our knowledge, no previous attempt has been made to investigate this novel nanocomposite as a catalyst and a photocatalyst for the AP thermal decomposition and MB dye photodegradation, respectively. We have provided a viable photocatalyst, which is appropriate for various, and especially industrial, applications. Also, as opposed to the two-step decomposition procedure recorded in the literature, the decomposition is here performed just in one single step, and a 120 ℃-reduction in the decomposition temperature has been observed. As far as we know, our observed drop in the temperature is the maximum value for the decrease in the temperature of ammonium perchlorate exothermic peak. Accordingly, our prepared nanocomposites seem to be highly promising for use in the composite solid rocket propellants.

2.

EXPERIMENTAL SECTION

2.1.

Materials



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The following chemicals were used in the synthesis procedure: Ammonium metavanadate (NH4VO3 99.99%, Aldrich), silver nitrate (AgNO3 99%, Alderich), zinc nitrate hexahydrate (Zn(NO3)2. 6H2O 98%, Alderich), and iron(III) chloride tetrahydrate (FeCl3. 4H2O 99%, Merck) as precursor salts; ammonia solution (NH4OH 25%, Merck) as a reducing agent; cyclohexane (≥99.9%, Merck) as the oil phase; dioctyl sulfosuccinate sodium salt (aerosol–OT, AOT 98%, Aldrich) as a surfactant; ethanol (≥99.9%, Merck) as a washing agent and solvent; and water used in the experiments was deionized (DI) and doubly distilled prior to use as the aqueous phase in emulsion system. Cationic dye of methylthioninium chloride (methylene blue) was purchased from Merck. We purchased analytical reagent grade NH4ClO4 powder having the average size of particles of 80 to 100 mm from Fluka. Furthermore, the non-solvent methyl isobutyl ketone (MIBK, >99%) was provided from Merck.

2.2.

Characterization

The surface morphology, size, crystal structure and elemental composition of β-AgVO3 and ZnFe2O4 nanoparticles and β-AgVO3/ZnFe2O4 nanocomposites were characterized using field emission scanning electron microscope (FE–SEM), transmission electron microscopy (TEM), energy dispersive analysis of X–ray (EDAX), X–ray diffraction (XRD), UV–Visible absorption spectroscopy, and Fourier transform infrared spectroscopy (FT–IR). A Hitachi S–1460 field emission scanning electron microscope at an accelerating voltage of 15 kV was employed to obtain the FE–SEM images. On the other hand, a Philips device, model EM–208S, with the accelerating voltage of 100 kV was used to conduct TEM measurements of β-AgVO3/ZnFe2O4 nanocomposites. For estimation of the size distribution and average size of the particles, more than 100 particles from different points of the grid were utilized. Moreover, the JEOL JSM–


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6380LV scanning electron microscope was used to obtain EDAX spectrum. For all of the samples, using a Philips diffractometer (Model TM–1800), XRD analyses were performed. Using a nickel filtered Cu–Kα radiation source, X–ray (λ = 1.542 Å) was achieved. Besides, using a proportional counter detector at 4◦/min scan rate, measurement of the scattered radiation was carried out. The scanning angle was in the 20°-90° range with 40-kV voltage, for which a potential current of 30 mA was taken into account. For all samples, via a Camspec M330 UV– Vis spectrometer (200–800 nm), the measurement of the UV–Vis absorption spectra was carried out in a quartz cell. Using KBr disks on a Shimadzu FT–IR model Prestige 21 spectrometer, the FT–IR spectra (in 400-4000 cm-1 wavenumbers) were provided. Using the Nova Station A instrument at 77 K N2, the measurement of the absorption-desorption isotherms was performed. Based on the Brunauer-Emmet-Teller (BET) method, calculation of the specific surface areas was done. Besides, through the Barrett-Joyner-Halenda (BJH) method and via the isotherm desorption branch, the pore-size distribution was estimated. Also, for measuring the magnetic properties, a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan) was used at ambience temperature. The Agilent-5975C mass spectrometer with 70 eV ionization potential was utilized for the mass spectrometry. For the powdered samples, a Perkin-Elmer LS-5 luminescence spectrometer operating at the excitation wavelength of 480 nm at RT was employed to record the photoluminescence (PL) spectra.

2. 3. Preparation of the β-AgVO3 and ZnFe2O4 nanoparticles The synthesis route for the highly dispersed β-AgVO3 nanoparticles under the reverse micelle method is represented in Figure 1. Three emulsions with dissimilar aqueous phases containing 0.23 mmol AgNO3, 0.23 mmol NH4VO3, and 0.46 mmol NH4OH have been synthesized. After



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that, emulsion I (with AgNO3 as great as 0.23 mmol) and emulsion II (with NH4VO3 as great as 0.23 mmol) were mixed. This mixture is hereafter named as emulsion A. On the other hand, emulsion III (containing 0.46 mmol NH4OH) was mechanically agitated for approximately 20 min, and then it was mixed with emulsion A. In order to obtain the precursor β-AgVO3, the reaction mixture was stirred quickly at room temperature for three hours. Then, ethanol was added to the beaker. After phase separation, the solution was washed for four minutes two times. The centrifuge of the resultant mixture was done so that β-AgVO3 precursor can be produced. The mixture was then dried at 70 ℃. The yield was then crystallized through calcination at 450 ℃. The heating rate was observed to be equal to 2℃/min. Also, natural cooling was employed. Complete crystallization of the β-AgVO3 nanoparticles into pure phase at 450 ℃ was considered while no intermediary phases were formed. Finally, using EDAX, FE–SEM, FT–IR, BET, PL, UV-DRS, and XRD analyses, the prepared β-AgVO3 nanoparticles were characterized. This preparation process can also be used for ZnFe2O4 nanoparticles. However, in the case of ZnFe2O4 nanoparticles, instead of AgVO3 and NH4VO3 as precursors, Zn(NO3)2. 6H2O and FeCl3. 4H2O with a molar ratio of 1:2 are used.


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Figure 1. Schematic illustration of preparation of the β-AgVO3 and ZnFe2O4 nanoparticles synthesized in reverse micelle system.

2.4.

Preparation of the β-AgVO3/ZnFe2O4 nanocomposites

Through a new method, first an emulsion having the precursor of ZnFe2O4 nanoparticles was synthesized (solution I). Then, in a separate beaker, an emulsion containing AgNO3 and NH4VO3 was produced (solution II). The produced emulsion from phase II was added to the solution obtained from phase I. The resultant mixture was stirred for 2 h at room temperature for a better homogeneity and uniformity of the solution. The remainder of the synthesis processes is similar to the synthesis process of the β-AgVO3 nanoparticles, as described in the previous section.

2.5.

Photocatalytic activity measurement 9 ACS Paragon Plus Environment


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Through the MB dye degradation, the photocatalytic characteristics of the β-AgVO3 and ZnFe2O4 nanoparticles and those of the β-AgVO3/ZnFe2O4 nanocomposites were examined. The MB dye was photo-catalytically degraded in a 50 mL beaker with 30 mL of the MB solution (60 mg L-1) and 24 mg of the photocatalyst at room temperature. Using ultrasound, the dispersal of the suspension was carried out for fifteen minutes, and then the suspension was stirred magnetically for 20 min before irradiation so that the MB dye adsorption/desorption equilibrium on the surface of the catalyst can be attained. In order to separate the catalyst, a suspension sample of about 2 mL was centrifuged at fixed time intervals. Besides, using UV–vis spectrophotometer at the 670 nm wavelength, the supernatant absorbance was analyzed.

2.6.

Preparation of Cu-Cr/AP composites

In order to examine the catalytic properties of β-AgVO3 and ZnFe2O4 nanoparticles and βAgVO3/ZnFe2O4 nanocomposites, the ammonium perchlorate (AP) was utilized as a target substance. Using solvent/nonsolvent protocol, the as-prepared nanostructures, nanostructures, and NH4ClO4 (for preparation of the composites) were completely mixed so that their catalytic behaviors can properly be estimated.40 In this context, nanostructures are expected not to be agglomerated while they are further dispersed. This in turn leads to the improvement of both activity and stability of the catalysts. The present study has made use of MIBK and water as nonsolvent and solvent, respectively. First, using ultrasound, the dispersal of the nanostructures in the 40-mL MIBK was considered. Furthermore, in order to produce a saturated solution, the AP was then dissolved into water at 75 ℃. Subsequently, this saturated AP solution was added dropwise into the nanostructures so that a composite of β-AgVO3/AP, ZnFe2O4/AP, and βAgVO3/ZnFe2O4/AP can be produced. In this work, a weight ratio of 97:3 is held for the mixture


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of AP and nanostructures. In order to deposit the AP completely on the nanostructure surface, the reaction was maintained for some minutes. Using 100 mL MIBK as the non-solvent material, the filtration and washing of these coated composites were performed two times. The said particles were finally dried at room temperature.

2.7.

Catalytic activity tests

The catalytic activity of the as-prepared nanostructures was defined in terms of their capability for the thermal decomposition of AP. Note that AP is often used as a common oxidizer in the composite solid propellants. Through a STA-780 thermal analysis system with 10 ℃ min-1 heating

rate,

the

thermal

decompositions

of

β-AgVO3/AP,

ZnFe2O4/AP,

and

β-

AgVO3/ZnFe2O4/AP at temperatures from 50 to 500 ℃ were analyzed using the simultaneous thermogravimetry–differential scanning calorimetry (TGA–DSC) under the air flow. The mass of the used sample was around 24 mg.

3.

RESULTS AND DISCUSSION

3.1. Characterization of β-AgVO3/ZnFe2O4 nanocomposites The crystallinity and phase structure of the β-AgVO3 and ZnFe2O4 nanoparticles, as well as those of the β-AgVO3/ZnFe2O4 nanocomposites were explored via the XRD image analysis, as demonstrated in Figure 2. The analysis results show that in all samples, for the nanoparticles βAgVO3 and ZnFe2O4, the characteristic peak is seen. For instance, for the nanoparticles βAgVO3, all of the diffraction peaks are properly indexed to the monoclinic structure, in which the lattice constants are a = 17.87 Å, b = 3.580 Å and c = 8.036 Å and are in good agreement with the lattice constants in the recorded literature (JCPDS: 29-1154). Furthermore, as regards



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the other impurity phases, no diffraction peaks were observed, which is also indicative of the high purity of the synthesized product. In addition, the observed narrowness of the diffraction peaks shows the highly crystalline nature of the produced product. Moreover, with regard to the ZnFe2O4 nanoparticles, for the spinel structure (220), (311), (400), (422), (511) and (440), the corresponding prominent planes are well indexed to the single-phase cubic spinel structure characteristic peaks (JCPDS card no. 22-1012), the results of which are presented in Figure 2. Due to the fact that in the XRD analysis, no further crystalline phases were observed, it can thus be shown that, as it was expected, our obtained nanoparticles were pure ZnFe2O4. With regard to the nanocomposites β-AgVO3/ZnFe2O4, the diffraction peaks were observed to be, both in their peak position and their relative intensity, congruent with the standard spectrum (β-AgVO3: JCPDS no. 29-1154; and ZnFe2O4: JCPDS no. 22-1012). Consequently, in the nanocomposites of β-AgVO3/ZnFe2O4, formation of both β-AgVO3 and ZnFe2O4 nanoparticles were in situ; however, the crystal structure of the β-AgVO3 and ZnFe2O4 nanoparticles did not vary in the composites. Also, due to nonexistence of any other diffraction peaks, the composites are thus thoroughly comprised of the nanoparticles of β-AgVO3 and ZnFe2O4. Based on Scherrer formula, from the full width of the half maximum of the diffraction peak, the mean crystallite sizes of β-AgVO3, ZnFe2O4, and β-AgVO3/ZnFe2O4 were obtained. Using Debye–Scherrer equation d = Kλ/(βcosθ), the values of around 18.33 nm (a), 31.58 nm (b), and 47.5 nm (c) were thus obtained for the average crystallite size of β-AgVO3, ZnFe2O4, and β-AgVO3/ZnFe2O4, respectively. It is worth mentioning that, for further verification of the combination mode of the suggested binary compound, other analyses, along with the XRD, must also be taken into account.


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Figure 2. XRD patterns of the β-AgVO3 (a), ZnFe2O4 (b), and β-AgVO3/ZnFe2O4 nanocomposites (c).

In view of the aforementioned, for reexamination of the β-AgVO3 impact on the nanoparticles of ZnFe2O4, FT-IR spectroscopy was taken into account, the results of which are presented in Figure 3. FT-IR spectroscopy demonstrates a high sensitivity to the vibrations in the metal– oxygen bonds and yields vital information about the structural integrity of the catalysts. Besides, this technique can afford to analyze and estimate the purity of the compound. Concerning βAgVO3/ZnFe2O4, the FT-IR spectrum is shown in Figure 3. Generally, for M-O bonds, modes of the stretching vibration are shown by a region below 1000 cm−1, which can well demonstrate the metal oxide formation. The existence of a band at 919 cm-1 can be ascribed to the symmetric stretching vibrations of VO3, whereas the 846 and 710 cm−1 bands are because of the antisymmetric stretching vibrations of VO3. The symmetric stretching mode of V-O-V units is in relation to a band at 511 cm−1. On the other hand, due to the overtone band, a band can also be seen at 1394 cm−1.41 Figure 3(b) represents the FT-IR spectra of the ferrite samples of ZnFe2O4.



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Due to the nitrate existence, another band is observed at approximately 1380 cm−1. Moreover, As a result of the stretching vibration of tetrahedral Zn2+ (Zn-O mode), a band also appears at ∼564 cm−1. Besides, at ∼436 cm−1, a band exists, which is associated with the stretching vibration of the octahedral Fe3+ (Fe-O mode).42 In addition, since the water was adsorbed on the surfaces of the nanoparticles under study, three peaks also appeared at 3438, 1631 and 1381 cm−1.43 During the preparation process of the suggested nanocomposite, we did not observe any organic species. Therefore, FT-IR spectra prove to match with the XRD image analysis. That is to say, when the XRD and FT-IR results are combined, the successful preparation of the β-AgVO3/ZnFe2O4 nanocomposites can better be understood.

Figure 3. FTIR spectra of the β-AgVO3 (a), ZnFe2O4 (b), and β-AgVO3/ZnFe2O4 nanocomposites (c).

As shown in Figure 4, the morphologies of pure β-AgVO3, ZnFe2O4, and their composite nanostructures were characterized using FE-SEM technique. Figures 4a and b demonstrate the FE-SEM

analyses

of

β-AgVO3

and

ZnFe2O4

nanoparticles

and

β-AgVO3/ZnFe2O4

nanocomposites, respectively. These figures demonstrate the existence of some sphere-shaped β-


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AgVO3 and ZnFe2O4 nanoparticles with a mean particle size of 20 and 35 nm, respectively. Furthermore, examination of β-AgVO3 and ZnFe2O4 surface morphology verifies the formation of some excellently uniformed and highly monodispersed nanoparticles. As previously mentioned in Introduction, using emulsion nanoreactors for the preparation of the nanostructures, the size and composition of the suggested structures can appropriately be controlled. Figure 4c demonstrates that β-AgVO3/ZnFe2O4 nanocomposites are large in size and nearly spherical in shape. However, these nanocomposites appear to be a little less uniform than both β-AgVO3 and ZnFe2O4 nanoparticles. Because β-AgVO3 + ZnFe2O4 nanoparticles are formed, the average particle size of the nanocomposite under study increases. Using TEM analysis, morphology of the β-AgVO3/ZnFe2O4 nanocomposites were further characterized (Figure 4d). Results of the TEM analysis also matched with those of the FE-SEM analysis.



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Figure 4. FFE-SEM images of the β-AgVO3 (a), ZnFe2O4 (b), and β-AgVO3/ZnFe2O4 nanocomposites (c); TEM image of the β-AgVO3/ZnFe2O4 nanocomposites (d).

In order to verify the presence of metal elemental, the EDAX technique was exploited. Figure 5 represents the chemical composition of β-AgVO3/ZnFe2O4, of which the results are congruent with the XRD image analysis results. Furthermore, in Figure 5, the peaks are shown to be related to the Ag, V, Zn, Fe, and O elements, hence verifying the presence of the β-AgVO3 + ZnFe2O4 nanoparticles in the synthesized nanocomposites. Due to the nonexistence of the sulfur and 16 ACS Paragon Plus Environment

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sodium atoms, the aerosol-OT and sodium salts are not observed anymore. Further peaks in these spectra are caused by Au element, which serves as the sample’s sputter coating in EDAX stage. In view of this, using our synthesis route, some highly crystalline, pure-phased, and welldispersed β-AgVO3/ZnFe2O4 nanocomposites are obtained. Besides, based on the DLS analysis, the characterization of the size distribution of the particles was done. As shown in the inset of Figure 5c, the size distribution of the β-AgVO3/ZnFe2O4 nanoparticles is in congruence with the results of the XRD, FE-SEM, and TEM analyses.

Figure 5. EDAX analysis of the β-AgVO3 (a), ZnFe2O4 (b), and β-AgVO3/ZnFe2O4 nanocomposites (c); and DLS analysis of the β-AgVO3/ZnFe2O4 nanocomposites (inset).



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In order to determine the volume, pore size, and surface area of the BET, the adsorption isotherms of nitrogen on the surfaces of β-AgVO3 and ZnFe2O4 NPs and β-AgVO3/ZnFe2O4 nanocomposites were analyzed at an increasing relative pressure. In this context, the values of 5.69 m2 g-1 (β-AgVO3) and 12.75 m2 g-1 (ZnFe2O4) were obtained for the BET surface area of βAgVO3 and ZnFe2O4 NPs, respectively. Concerning the surface areas of β-AgVO3/ZnFe2O4 nanocomposites, however, a value of 15.87 m2 g-1 was obtained. From the BJH pore-size distribution curve, a sharp peak at 13.52 Å was seen for β-AgVO3 nanoparticles. For ZnFe2O4 nanoparticles, a sharp peak existed at 24.92 Å. Besides, concerning β-AgVO3/ZnFe2O4 nanocomposites, the presence of the inter-particle voids with pore-size distribution can be understood due to a sharp peak observed at 27.26 Å. In this regard, the dye molecule adsorption and photocatalytic activity are improved since β-AgVO3/ZnFe2O4 nanocomposites have shown a more specific surface area and pore volume.44 As is known, the photo-catalytic magnetic separability is influenced by the magnetic features of the photocatalysts. Consequently, the ZnFe2O4 nanoparticle size and quantity in a composite material generally determine its magnetic behavior. As shown in Figure 6a, a value of 63.63 emu/g can be observed for the saturated magnetization of ZnFe2O4 nanoparticles at 8000 Oe. This value is lower than the Fe3O4 bulk value, which is 98 emu/g.45 The said value (i.e., 63.63 emu/g) can also be explained by the magnetic moments disorder of the atoms at the surface layer, i.e., the spin glass effect, in nanoparticles.46,

47

Figure 6b also shows the magnetic

properties of the β-AgVO3/ZnFe2O4 nanocomposites. As the figure shows, a value of 28.92 emu/g can be seen at 8000 Oe for the saturation magnetization of the nanocomposite. Since nonmagnetic β-AgVO3 particles exist together with the magnetic ZnFe2O4 nanoparticles, a reduction in the magnetic properties of β-AgVO3/ZnFe2O4 nanocomposites can be seen. Figure 6


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demonstrates that because β-AgVO3/ZnFe2O4 nanocomposites have sufficient saturation magnetization, they can be separated effectively from the solution after decontamination.

Figure 6. VSM pattern of the ZnFe2O4 (a), and β-AgVO3/ZnFe2O4 nanocomposites (b), and rapid separation of the β-AgVO3/ZnFe2O4 nanocomposites (inset).

3.2. Catalytic activity In the next stage, the DSC analysis has been used to explore the thermal characteristics and coating behavior of the prepared samples. As mentioned before, the rate of the reaction, the activation energy, and the thermal decomposition temperature of AP are in close association with the solid propellants performance. Accordingly, the final decomposition temperature is influenced by such key parameters as the AP-catalysts combination procedure, the catalytic amount, the type of the catalysts, and the catalytic particle sizes. Figure 7 shows the curve of the thermal decomposition temperature for pure AP and AP in the presence of the considered nanocatalysts with a weight percentage of 3%. As it can be observed, the endothermic peak related to the thermal decomposition of AP in the presence of the ZnFe2O4 19 ACS Paragon Plus Environment


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NPs has a slight, if any, change. However, the exothermic peaks related to the low-temperature decomposition and high temperature decomposition of AP in the presence of these nanocatalysts show significant changes and moved to the lower temperatures. Therefore, these catalysts are highly capable of speeding up the AP thermal decomposition procedure. In general, the thermal decomposition of the pure AP occurs in three steps. However, the added catalysts to the AP are effective only in the exothermic stages – depending on the type of the catalyst involved, the amount of the decrease in the temperature may change (in this regard, the temperature minimization is a challenge which needs to be further answered by the researchers). On the other hand, these catalysts are inefficient in the first step, which is endothermic and related to the AP phase transition from the orthorhombic to the cubic phase. As shown in this figure, for pure AP, three distinctive peaks are observed. The first peak (at 236 ℃ in proportion to the increase in the temperature) is endothermic and related to the phase transition. In general, the catalysts have no effect on the first peak. The second and the third peaks are exothermic. The first exothermic peak (334 ℃) is associated with the formation of the intermediate products, whereas the second exothermic peak (432 ℃) shows the completion of the AP decomposition. By the addition of β-AgVO3 and ZnFe2O4 nanoparticles to AP, the first exothermic peak moves from 334 to 323 and 310 ℃ and the second exothermic peak changes from 432 to 402 and 367 ℃, respectively. However, with the addition of β-AgVO3/ZnFe2O4 nanocomposites, the reduction in the temperature of the second exothermic peak is to the extent that this peak merges with the first exothermic peak, and thus the peak emerges in the temperature limit of about 339 ℃. As a result, after the addition of the 3% weight percentage of the nanocatalysts ZnFe2O4 and β-AgVO3/ZnFe2O4, the thermal decomposition temperatures decrease to 65 ℃ and 93 ℃, respectively, compared to the pure AP. Furthermore, values of the


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heat released (∆H) for pure AP, AP+ZnFe2O4, and AP+β-AgVO3/ZnFe2O4 have been obtained to be 764.8, 1169, and 1487.3 Jg−1, respectively. In the presence of the catalysts, a reduction occurs in the heat released since these catalysts improve the electron transfer, which may be due to the filled 3d orbitals that help in the electron transfer procedure. In other words, due to the existence of a positive hole in the metal oxides which are the semiconductors of type P, they can easily accept an electron from ClO4-.

Figure 7. DSC curves for pure AP and for AP mixed with additives of β-AgVO3, ZnFe2O4 and βAgVO3/ZnFe2O4 nanocomposites.

In the next stage, different amounts of 5% and 10% of the β-AgVO3/ZnFe2O4 nanocomposites were used for the AP decomposition (Figure 8). The significant point in this context is related to the β-AgVO3/ZnFe2O4 nanocomposites with the weight percentage of 5, in which the two peaks are completely merged into one unique peak that appears at 312 ℃. This peak shows a 120 ℃ reduction, compared to the pure AP. To the best of our knowledge, this value is the maximum reduction observed in the temperature of the exothermic peak. Moreover, this value has the most 21 ACS Paragon Plus Environment


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heat released (1598.9 Jg−1). The heat released of the pure AP was observed to be equal to 765.3 Jg−1. By the addition of 5% of the suggested nanocatalysts, the heat released grew 2 times; however, with further increases in the nanocomposites to 10%, no reduction in the catalytic activity was seen and the heat released reached 1492.6 Jg−1 and the temperature of the high temperature decomposition decreased as large as 87 ℃. Consequently, based on our experimental results, it can be suggested that due to the synergistic effect of the constituent metal oxides (i.e. β-AgVO3 and ZnFe2O4) in β-AgVO3/ZnFe2O4 nanocomposites, both the lowtemperature and the high-temperature exothermic processes of AP decomposition have significantly been improved. In general, electron transfer procedure in the presence of AgVO3 can increase the catalytic activity of AgVO3/ZnFe2O4 nanocomposites, which can be explained by the synergistic effects involved. The prepared nanocomposites in the present study can, therefore, offer considerable potentials for further uses in the composite solid propellants for the rocket propulsions.

Figure 8. DSC curves for AP mixed with additives of the 3%, 5%, and 10% of the βAgVO3/ZnFe2O4 nanocomposites. 22 ACS Paragon Plus Environment

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Through the application of these as-synthesized β-AgVO3/ZnFe2O4 nanocomposites, electrons are accelerated, which in turn result in expediting the two controlled decomposition procedures: the electron transfer from ClO4- to NH4+ and the transformation from O2 to superoxide ion (O2-), as shown in the Figure 9. First, the accelerated electrons created through the suggested nanocomposites cause NH4+ and ClO4- to be transformed into NH3 and HClO4. After that, O2 is synthesized from HClO4 and is then transformed into superoxide ion (O2-) at a faster speed in the catalytic presence of β-AgVO3/ZnFe2O4 nanocomposites, which cause the electrons to flow freely. NH3 decomposition is facilitated by the superoxide ions as well as the other products of HClO4, which then complete the AP thermal decomposition.

Figure 9. Illustration of catalytic thermal decomposition process of AP by β-AgVO3/ZnFe2O4 nanocomposites.

In order to have a better understanding of chemical reactions in the process of thermal decomposition, evolution of gases can be helpful. Therefore, through the mass spectroscopy, the AP thermal decomposition with the catalytic activity of the β-AgVO3/ZnFe2O4 nanocomposites 23 ACS Paragon Plus Environment


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was investigated. Using mass spectroscopy, the mass intensities of molecules produced in the thermal decomposition of AP + β-AgVO3/ZnFe2O4 nanocomposites were characterized. It has been observed that the decomposition of AP was completely achieved at a narrow range of temperature with HCl, NH3, N2, NO, N2O, and H2O as the decomposition products. In the presence of β-AgVO3/ZnFe2O4, the AP decomposition products such as HCl and NH3 were nearly at the same time detected.

3.3. Photocatalytic activity Investigation of the optical absorption of the prepared samples before exploring the photocatalytic properties is of great importance due to the association of the UV–Vis absorption edge with the catalytic semiconductor’s energy band.48, 49 Consequently, due to the importance attached to the optical absorption features and since the prepared nanocomposites band gap has an electronic nature for solar energy photocatalytic reaction, in this work the variations in the optical characteristics of β-AgVO3, ZnFe2O4, and β-AgVO3/ZnFe2O4 nanocomposites have been taken into account. In Figure 10, UV–Vis diffusive reflectance spectra of the as prepared samples is presented. This figure indicates that a proper absorbance exists in the entire region of light. Besides, it can be seen that the absorptivity substantially increases in the wavelengths from 400 to 700 nm. A broad absorption at about 482 nm can be observed for the absorption edge of the βAgVO3 nanoparticles. However, in the case of the ZnFe2O4 nanoparticles, a broad absorption at about 610 nm is seen. With regard to the β-AgVO3/ZnFe2O4 nanocomposites, a red-shift to the region of the visible light is observed for the absorption edge and this red-shift exists up to 665 nm. Using the following equation, the energy band gap can be calculated (Eq. (1)). As reported in the literature, the energy band gap and λonset relationship would be written as follows:50


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E =

1239.8 1 λ

Here, Eg signifies the band gap energy (eV), while the absorption edge wavelength (nm) in the spectrum is shown by λ. Concerning our prepared β-AgVO3 nanoparticles, the corresponding energy band gap is equal to 2.57 eV using Eq. (1). This finding is in close affinity to the results in the recorded literature.51 Furthermore, a value of 2.03 eV has been obtained for the Eg of ZnFe2O4 nanoparticles, which is slightly greater than the Eg of ZnFe2O4 in the work of Hou,52 this slight difference might be due to the fact that our synthesized ZnFe2O4 nanoparticles have an irregular structure. The band-gap adsorption edge of β-AgVO3/ZnFe2O4 nanocomposites demonstrates a red-shift to 665 nm. Besides, compared to β-AgVO3 and ZnFe2O4 nanoparticles, it shows an increase in the absorption spectra intensity in the region of the visible light. Based on this red shift, a reduction in the composite band gap can be understood. Moreover, it indicates the existence of the size quantization effect.53 Also, compared to the β-AgVO3 and ZnFe2O4 nanoparticles, the aforesaid variation in the β-AgVO3/ZnFe2O4 energy band gap (1.86 eV) shows that an electronic interaction exists between β-AgVO3 and ZnFe2O4 NPs. Under visible light illumination, β-AgVO3/ZnFe2O4 nanocomposites can thus be assumed as a superior photocatalyst.



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Figure 10. UV−Vis diffuse reflectance spectra of β-AgVO3 (a), ZnFe2O4 (b), and βAgVO3/ZnFe2O4 nanocomposites (c).

For the disclosure of the transfer, migration, and separation efficiency of the photo-generated charge carriers in the semiconductor materials, the photoluminescence (PL) spectra is applied.54 Accordingly, as Figure 11 shows, the PL spectrometry has been conducted for the β-AgVO3, ZnFe2O4 nanoparticles and β-AgVO3/ZnFe2O4 nanocomposites so that the defects and other impurity states of the system can be recognized. Using a 480 nm excitation wavelength, the prepared samples have been excited. For all samples, a broader visible emission band was obtained at 715 nm, which can be ascribed to recombination of the electrons entrapped deeply in the oxygen vacancies with the photo-generated holes.55 If the PL intensity is lower, then the charge carriers recombination is also lower. In this context, a greater number of electrons and holes are involved in the reactions of oxidation and reduction, which in turn leads to the increased photocatalytic properties. The order of the emission intensity for the PL spectra samples is as follows: β-AgVO3> ZnFe2O4 > β-AgVO3/ZnFe2O4. Therefore, compared to the pure β-AgVO3 and ZnFe2O4 nanoparticles, β-AgVO3/ZnFe2O4 nanocomposites have a low 26 ACS Paragon Plus Environment

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relative intensity. This indicates the efficient inhibition by the β-AgVO3/ZnFe2O4 nanocomposites from the electron-hole recombination. This can be explained by the synergetic effects between β-AgVO3 and ZnFe2O4 NPs as evidenced by the effective charge transfer and interaction between these nanoparticles. This behavior is caused by the emergence of novel electronic levels between the conduction and the valence bands and is also likely to be owing to the increased intrinsic defects.56 In this view, the β-AgVO3/ZnFe2O4 nanocomposites with appropriate proportions prove to be highly efficient in inhibiting the recombination of the electrons and the holes, which is essential for the increase in the MB dye degradation.

Figure 11. Photoluminescence spectra of β-AgVO3 (a), ZnFe2O4 (b), and β-AgVO3/ZnFe2O4 nanocomposites (c).

In this work, the photocatalytic activity of the β-AgVO3/ZnFe2O4 nanocomposites has been investigated. Figure 12 shows the photocatalytic behaviors of the β-AgVO3, ZnFe2O4 nanoparticles and β-AgVO3/ZnFe2O4 nanocomposites with the molar ratio of 1:1 under the visible light irradiation for 60 min for the photocatalytic degradation of the MB dye. First, for



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demonstration of the role of the catalysts, a reaction was performed in the absence of the said catalysts. As depicted in Figure 12a, the photocatalytic efficiency in the absence of the catalysts shows no enhancement and even involves some partial fluctuations. In the next stage, a concentration of 20 ppm of the pollutant in the presence of 0.04 g of the catalysts was taken into account. As indicated by Figure 12b and c, by mere reliance on the commercial P25 and βAgVO3 nanoparticles, no significant improvement occurs in the photocatalytic activity, in comparison with a non-catalytic system. When the photocatalytic reaction of ZnFe2O4 nanoparticles was studied (Figure 12d), some quite unfavorable results were obtained, and even after the passage of 60 min from the beginning of the reaction, the solution color was not transparent yet and a blue dye could even be observed with the naked eye. Using UV-Vis spectroscopy, percentage of MB dye photocatalytic degradation was found to be about 72%. Obviously, the remaining percentage has still had very harmful effects on the environment. In the next stage, the magnetic β-AgVO3/ZnFe2O4 nanocomposites with the molar ratio of 1:1 were used. As Figure 12e shows, the photocatalytic degradation nearly reaches its final completion, and after 60 min from the start of the reaction, the MB dye degradation is 95%.


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Figure 12. Photocatalytic efficiency of the MB dye degradation percentage curves under ultraviolet light irradiation: (a) without photocatalyst, (b) β-AgVO3 nanoparticles, (c) commercial P25 TiO2 powder, (d) ZnFe2O4 nanoparticles and (e) β-AgVO3/ZnFe2O4 nanocomposites.

In step next, the impact of varying the molar ratios of β-AgVO3 and ZnFe2O4 nanoparticles (2:1, 1:1, and 1:2) in terms of the MB dye photocatalytic degradation irradiated by the visible light for 60 min is investigated. The supreme photocatalytic degradation in the βAgVO3/ZnFe2O4 nanocomposites has been observed to be 97%, which is associated with a 1:1 molar ratio of β-AgVO3 and ZnFe2O4 nanoparticles. After the photocatalytic degradation was performed for these nanocomposites, in the molar ratio of 2:1 and 1:2 of this nanocomposite, the MB dye degradation was 87% and 56%, respectively. In this regard, compared to the 1:1 molar ratio of β-AgVO3 and ZnFe2O4 nanoparticles, a reduction has happened in the MB dye degradation – especially in the molar ratio of 2:1. Therefore, when a molar ratio of 1:1 is taken into account, the best performance between these two metal oxides is achieved.



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With regard to the industrial applications of the catalysts, principal importance is given to the facility of the magnetic separation and recyclability. Therefore, in the next stage, the recycling capability of the β-AgVO3/ZnFe2O4 nanocomposites with the molar ratio of 1:1 were explored. As shown in Figure 13, the extraordinary recovery capability of this catalyst is to such an extent that, even after 6 periods of the test procedure, no considerable change has occurred in its catalytic activity. One main reason for this is that the suggested catalyst can be easily collected by a magnet and in each cycle of the catalytic procedure, losing of a portion of the catalytic amount is improbable and the original structure does not change during the catalytic activity. βAgVO3/ZnFe2O4 nanocomposites are thus hoped to create a positive impact on the photocatalytic degradation of other organic pollutants due to their promising applications in different industrial and commercial fields.

Figure 13. Recycle and reuse of β-AgVO3/ZnFe2O4 nanocomposites for MB dye photocatalytic degradation.

Amount of the catalyst plays a key role in the photodegradation of the organic pollutants, and particularly the dyes photocatalytic degradation. In this regard, as shown in Figure 14, this work 30 ACS Paragon Plus Environment

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is an attempt to study the effect of various catalytic volumes of β-AgVO3/ZnFe2O4 nanocomposites having a 1:1 molar ratio on the photo-catalytic degradation of 30 mL of the MB dye. Moreover, It has been observed that after the passing of thirty minutes from the beginning of the reaction, with the increase in the amount of β-AgVO3/ZnFe2O4 nanocomposites from 24 to 48 mg, an increase also occurs in the efficiency of the degradation of the MB dye from 80% to 87% (Figure 14). This increase can be explained by the fact that, with the increase in the catalytic amount, the amount of the photogenerated electron-hole pairs simultaneously increases. According to the recorded literature, when the amount of the photocatalysts goes up, some proper photocatalysts with more active sites are produced,57 and subsequently both the adsorbed photons quantity and the molecular dyes amount increase. However, in the case of increasing the amount of the β-AgVO3/ZnFe2O4 nanocomposites from 48 mg to 72 mg, the efficiency of the MB dye degradation reduces to 75% – as compared to the efficiency of degradation for 24 mg which is 80%. A possible reason behind this issue is that with the increase in the amount of the photocatalyst, the light penetration might be inhibited.58 In other words, the aforesaid reduction in the degradation efficiency can be explained by the fact that, as the catalytic amount goes up, both the steric and the scattering effects are imposed on the use of the light in the presence of the suggested catalytic particles. Moreover, when the amount of the catalyst is reduced from 24 mg to 12 mg, the MB dye degradation drastically varies, and resultantly the degradation efficiency decreases from 80% to 46%, which demonstrates the significance of the amount of the catalyst in this reaction. Consequently, in our considered case, with regard to the degradation of the MB dye, a value of 48 mg has been obtained for the amount of the β-AgVO3/ZnFe2O4 nanocomposites (as the optimal sample).



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Figure 14. Effects of amount of β-AgVO3/ZnFe2O4 nanocomposites with the molar ratio of 1:1 in the MB dye photocatalytic degradation for 30 min.

Based on our previous works as well as the other reports in the literature, the pH of the solution is a determining factor that can affect the absorption capacity of the adsorbents.59, 60 Obviously, there are differences between the impacts of the anionic and the cationic dyes at various pH levels. Thus, by the start of the reaction, in order to optimize the conditions of the photocatalytic reaction, numerous pH levels were utilized in a way that a more proper efficiency would be achieved. In view of this, other conditions of the experiment are kept fixed. Besides, in order to study the impact of the pH on the rate of the photocatalytic degradation, the initial values of the pH for the MB dye have been changed from 5 to 9, the results of which are shown in Figure 15. Assuming that all of the other variables are fixed, after thirty minutes from the beginning of the reaction, the photocatlytic degradation of the MB dye in the presence of the β-AgVO3/ZnFe2O4 nanocomposites at the pH of 9 amounts to 98%. To put it differently, compared to the acidic solutions, alkaline solutions show a superior efficiency. With regard to the MB dye as a cationic agent, a strong absorption and higher rate of degradation can be observed at the higher pH levels due to the electrostatic interactions between the cationic MB dye and β-AgVO3/ZnFe2O4 32 ACS Paragon Plus Environment

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nanocomposites negative surface. On the other hand, at the acidic pH levels, the absorption between the photocatalytic surface and the cationic MB is weak, and therefore the rate of the degradation as well as the reaction speed decreases. Consequently, at the acidic levels of pH, a reduction occurs in the degradation efficiency, which leads to the variation in the adsorption of the MB dye on the surface and the rate of the reaction. Accordingly, it can be suggested that pH plays a key role in degrading the dyes.

Figure 15. The Effect of pH values on the degradation of MB dye by β-AgVO3/ZnFe2O4 nanocomposites under UV-Visible light irradiation for 30 min at pH=5 to 9.

4.

CONCLUSION

Using emulsion nanoreactors, the present study has introduced a green and facile route for the mild and cost-efficient preparation of the non-agglomerated and pure phase β-AgVO3/ZnFe2O4 nanocomposites (~10nm) at ambient conditions. The procedure can easily be scaled up for the large scale production. Moreover, the reaction has been performed in the absence of any toxic agents without high pressure and high energy reaction conditions. According to our obtained results, these nanocomposites are quite spherical, crystalline, and homogenously-distributed. 33 ACS Paragon Plus Environment


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Furthermore, these spherical β-AgVO3/ZnFe2O4 nanocomposites are highly efficient in the catalytic thermal decomposition of AP. Addition of β-AgVO3/ZnFe2O4 nanocomposites in the AP has resulted in a 56-84 ℃ decrease in the decomposition temperature as well as an increase in the apparent heat released by 516-608 J g-1, compared to the pure AP thermal decomposition. The synthesized nanocomposites outperform both β-AgVO3 and ZnFe2O4 NPs in terms of the photocatalytic behavior for the photodegradation of the MB dye under visible light illumination. Moreover, after thirty minutes from the beginning of the photo-catalytic reaction, 9 mg and 48 mg have been obtained for the optimal pH value and β-AgVO3/ZnFe2O4 nanocomposites quantity, respectively. The suggested nanocomposites can be used as an efficient photocatalytic or catalytic material for different industrial applications.

AUTHOR INFORMATION Corresponding Author * Tel.: +98 2182883442; fax. +98 2182883455. E-mail address: [email protected] (A. R. Mahjoub). [email protected] (R. Abazari).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Tarbiat Modares University. The authors are grateful for the financial support.


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