Turning materials from bulk into nanostructure can have interesting outcome which is substantial changes in their physical properties. From structural point of view, it has been demonstrated that bulk material which have molecular structure of AB2O4 show a normal spinel-type structure which is a very famous structure and various ions can flexibly incorporate into their crystal structure. In this structure, atoms A(divalent cations) and B (trivalent cations) take up one-eighths of tetrahedral and half of octahedral sites, respectively. These atoms are distributed to fcc lattice constituted by the O2- ions . Undergoing some procedures such as mixed spinel or inverse spinel structure, may alter their crystal structure in a way that B3+ occupies both the tetrahedral and the octahedral sites and the A2+ ions occupy octahedral ones. Moreover, in materials with general formula of (A1_x Bx) [Ax B2_x] O4, mixed spinel structures, A and B atoms can take up both octahedral and tetrahedral sites . Among numerous materials with spinel structure, NiMn2O4 has gained much attention, Because catalytic activities and electrical resistance of the NiMn2O4 are highly dependent on the temperature [3, 4]. This feature has motivated researchers in synthesis of NiMn2O4. However, in most methods of synthesis, prolong time and high temperature which are the necessary steps to sinter, make those procedures inappropriate for production of this compound [5, 6].
J. M. A. Almeida and co-worker synthesized NiMn2O4 by gelatin, as an organic precursor and the average crystalline size of nanoscaled NiMn2O4 was found to be in the range of 14-44 nm. Also, they discovered that both unit cell parameters and average crystallite size are positively correlated with synthesis temperature and magnetic transition occurred in 110 K . In this work, we synthesized NiMn2O4 nanostructure by new approach. Besides, several experiments were performed in order to investigate the effect of different solvents and capping agents such as valine, asparagine and arginine on the morphology, particle size, and crystal structure of final products. The photocatalytic activities of NiMn2O4 catalysts were studied for the degradation of methyl orange under ultraviolet light [9-16].
MATERIALS AND METHODS
Synthesis of NiMn2O4 nanoparticles
NiMn2O4 nanoparticles were prepared using sol-gel method. First, capping agents solution was added dropwise into the nickel chloride solution under magnetic stirrer and then manganese nitrate solution was added into the above solution (with molar ratio of Mn:Ni = 2:1). The pH of solution was regulated to 4-5. The gained green gel was dried at temperature 100 for 5 h. The as-obtained products were dried at 100 ˚C under vacuum for 3 h. Next, these products were calcined at 700 ˚C and 800 ˚C for 2 h. The preparation conditions of NiMn2O4 nanoparticles have been depicted in Table 1.
The photocatalytic activity of nanoscaled NiMn2O4 was investigated by rhodamine B (Rh B), and phenol red (Ph R) solution. The degradation reaction was conducted in a quartz photocatalytic reactor. The photocatalytic degradation was conducted with 5 × 10−5 M of solutions containing 0.05 g of nanoparticles. Then in order to reach adsorption equilibrium the mixture was aerated for 30 min. Then, the mixture was placed in the photoreactor in which the vessel was nearly 30 cm away from the UV source. A 400W ultraviolet light lamp was used as a visible light source, and the experiments were performed at room temperature. Furthermore, the pH of the solutions were adjusted to 3. The mixture was aliquoted in periodic intervals throughout the irradiation, and before the analysis of mixture with the UV–Vis spectrometer, it was centrifuged. The percentage of dyes degradation was appraised as the following formula.
Where C0 and Ct are the absorbance value of solution at 0 and t min, respectively.
Materials and physical measurements
Mn(NO3)3·۹H2O, NiCl2, valine, asparagine and arginine was purchased from Merck and all the chemicals were used as received without further purifications. Room temperature magnetic properties were investigated using an alternating gradient force magnetometer (AGFM) device, (made by Meghnatis Daghigh Kavir Company, Iran) in an applied magnetic field sweeping between ± 10000 Oe. XRD patterns were recorded by a Philips, X-ray diffractometer using Ni-filtered Cu Kα radiation. SEM images were obtained using a LEO instrument model 1455VP. Prior to taking images, the samples were coated by a very thin layer of Pt (using a BAL-TEC SCD 005 sputter coater) to make the sample surface conductor and prevent charge accumulation, and obtaining a better contrast. An S-10 4100 Scinco UV-Vis scanning spectrometer was used to obtain the electronic spectra.
RESULTS AND DISCUSSION
A lot of interest has recently expressed toward controlling the particle size and shape of the nanostructures by manipulating reaction parameters [17–23]. As a result, several experiments were performed to verify the influences of the presence of natural polymers on the size and morphology of the nanostructures.
Fig 1a-c represents as-prepared NiMn2O4 nanoparticles with glycerol, 2-propanol, and water in the presence of valin as a capping agent respectively. As this figure demonstrates, by changing the solvent from glycerol (Fig. 1a) to propanol (Fig. 1b), and then to water (Fig. 1c) the size of final products was reduced. The effect of amino acids such as asparagine and arginine on the morphology and particle size of NiMn2O4 nanoparticles has been investigated in Fig. 2 a and b. As shown in the figure, increasing steric hinderance in amino acids leads to smaller particle size (Fig. 3).
The crystalline structure and phase of NiMn2O4 nanoparticles were verified by the XRD analysis. Fig. 4 and Fig. 5 illustrtes the XRD patterns of products prepared at 700 ˚C and 800 ˚C respectively. The peak intensity of NiMn2O4 increases through enhancing the calcinations temperature, and the broadening of peaks decreases owing to growth of the nanoparticles.
The crystallite size of the products (D) can be computed from XRD patterns by Scherrer’s equation :
Where λ is the wavelength of incident X-rays, FWHM is full width at half maximum and θ is the position of the maximum of the diffraction peak. The average crystallite size of the products was evaluated about 9/3 and 7/1 nm, correspondingly.
EDS analysis proves presence of Ni, Mn, and O elements without any impurity (Fig. 6). The magnetization measurements as a function of the magnetic field were evaluated at 300 K. Also, the hysteric curve with nearly saturated nature at high fields has been shown in Fig. 7. The magnetic property of NiMn2O4 nanoparticles was obtained at room temperature. VSM analysis shows that NiMn2O4 nanoparticles have ferromagnetic properties. The saturation magnetization and coercivity are found 0.3 emu/g.
It is famous that the band-gap of the nanostructures materials has a major role in utilizing photocatalytic applications. The diffuse reflectance spectroscopy (DRS) of NiMn2O4 nanoparticles has been depicted in Fig. 8. The band-gap of the samples was calculated by the following equation (3) :
Where Eg is the optical band gap of the material, hʋ is the photon energy, A is a material constant, α is the amount of the absorbance, and n is constant that depends on the type of the electronic transition . The energy gap of the samples (Eg) was obtained by extrapolating the linear portion of the plots of (αhʋ)2 curve in return hʋ to the energy axis. The band-gap (Eg) of NiMn2O4nanoparticles were 3 eV.
The degradation rate of two different dyes such as rhodamine B (Rh B), and phenol red (Ph R) as organic pollutants was verified by NiMn2O4 nanoparticles under ultra violet. It is obvious that in Fig. 9 the photocatalytic activity of nanocomposite in decolouration of Rh B is higher than others as Photocatalytic efficience for Rh B and Ph R are 98%, and 65%, correspondingly.
Moreover, a photocatalytic test as a blank for the investigation of photacatalytic activity of NiMn2O4 nanoparticles was conducted. In this test degradation rate for rhodamine B (Rh B), and phenol red (Ph R) was obtained 6% and 4% respectively. As illustrated, all three patterns confirm the presence of NiMn2O4 nanoparticles in the products.
The suggested procedure of photocatalytic degradation of Rh B can be as following:
The composition procedure of contaminants for the NiMn2O4 nanoparticles has been depicted in Fig. 10. These consequences show a high degree of competence of the as-produced NiMn2O4 nanoparticles (sample No. 5) to be employed as an appropriate, new, and favorable type of photocatalyst under ultraviolet light for elimination of cationic contaminants. Moreover, applying of polymers as capping gents for production of nanocomposites is the creativity of this effort. Moreover, changing the capping agent resulted in production of a fine grain size and very homogenous and sphere-like NiMn2O4 nanoparticles. Interestingly, to the best of our knowledge, in the literature, there is no report on the study of photocatalytic activity of NiMn2O4 nanoparticles. Further, photocatalytic activity of final products can be influenced by various factors such as grain size of NiMn2O4 nanoparticles and kind of pollutant. The outcomes demonstrated that the as-prepared NiMn2O4 nanoparticles show considerable possibility to be used as a proper, useful, and innovative kind of photocatalyst under visible light to erase cationic contaminants. Respecting the production of NiMn2O4 nanoparticles, one can simply understand the facileness, convenience, and originality of the approach stated above [26-28]. In comparison to other similar works illustrated in Table 2, our method is simple, has low cost, and scale-up route. Besides, to the best of author’s knowledge, it is the first time that CuMn2O4 nanoparticles was synthesized in the presence of amino acids as the capping agent and used for degradation of rhodamine B, and phenol red under ultraviolet light.
NiMn2O4 nanoparticles is prepared to remove dyes from water. Furthermore, the evaluated band gap of NiMn2O4 nanoparticles proves that it can be employed as a photocatalyst. In order to investigate the effect of several capping agents on the morphology and particle size of final products several tests were conducted in the presence of valine, asparagine and arginine. Applying nanocrystalline NiMn2O4 as the photocatalyst, causes maximum of 98 % for degradation of Rh B after 80 min irradiation under UV light. This result suggests that as-obtained nanocrystalline NiMn2O4 as favorable material has high potential to be used for photocatalytic applications under ultraviolet light. Furthermore, high purity of the as-prepared NiMn2O4 nanoparticles was proved by EDS and XRD analyses.
Authors are grateful to council of University of Kashan for providing financial support to undertake this work.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.