Document Type : Reasearch Paper
Authors
1 Department of Chemistry, Semnan University, Semnan, Iran.
2 Department of physics, Damghan University, Semnan, Iran.
Abstract
Keywords
Main Subjects
INTRODUCTION
AB2O4 Spinel-type metal oxides are applied in various fields due to their various technological applications and properties. There are two types of cations in a normal spinel structure, A2+ and B3+, which A2+ ions occupy tetrahedral sites which are surrounded by oxygen and B3+ ions occupy octahedral sites which have been surrounded by six oxygens [1, 2]. Copper chromate has been attended due to distinguished electrical, catalytic, magnetic and optical properties in recent years; for example, it is used as versatile catalyst due to its stable structure for oxidation of CO which is widely used in space launch vehicles [3] and hydrogenation process catalyst in oil industries [4, 5]. Also, this chromate is a visible light active photocatalyst with band gap energy of 1.4ev. So, this p-type semiconductor has an extensive range of usages as photocatalyst because according to Shockley-Quiser limit, materials have the most visible light absorption of solar light irradiation in this rang, so this band gap makes it highly activated [6-8].
The most common methods which have been used for synthesizing copper chromate are: hydrothermal [4], co-precipitation [9, 10], sol-gel [11], thermal decomposition [12], solid state [7] and so on. Among these methods, hydrothermal is one of the best methods to fabricate verity morphologies. Also, this method is high efficiency and cost-effective method [13-15]. In this research, the hydrothermal method has been performed because the recent studies show that the photodegradation process by means of CuCr2O4 has a direct relationship with its morphology [16]. Mn0.5Cu0.5Cr2O4 is chromate that Mn+2 and Cu2+ occupy tetrahedral sites contemporaneously. This chromate has not been synthesized yet; so, its applications haven’t been assessed before.
In this work, we investigate the effect of Mn2+-substituted in spinel structure of CuCr2O4. At first step, a pure phase of Mn0.5Cu0.5Cr2O4 was synthesized successfully via hydrothermal method. In the next step, it was tried to assess the photodegradation activity of Mn0.5Cu0.5Cr2O4 for irradiation a cationic dye (malachite green). Among water pollutant, toxic dye molecules like Malachite green (4-{[4-(dimethylamino) phenyl] (phenyl) methylidene}-N’Ndimethylcyclohexa-25-dien-1-iminium chloride) (MG) which are harmful to the environment, has got more attention these days. The hazardous compound has been widely used in textile industry for the dyeing wool, food coloring and ceramics, so that 1-15% of this dye is distributed during dyeing process and enter to the water [17-19]. So, the removal process of this compound is important from industrial waste water. There are various methods for elimination a variety ranges of pollutants and toxic dyes from contaminating water; one of these processes is chemical oxidation technology which is used in certain semiconductor photocatalyst process and the solar radiation.
One of the most important effective factors on photocatalytic performance is the existence of a powerful source of energy for the excitement of electrons from valence band to conduction band which can create electron-hole pair. This energy should be either equal with or stronger than semiconductor band gap-which is a distance between the conduction band and valance band. The solar light detaches an electron from the photocatalyst surface and creates a hole. This hole can react with and creates active radicals. As a matter of fact, the progress of these processes regularly depends on production of and reactive radicals like OH• that is capable of oxidation organic contaminants. In this study is tried to produce more reactive radicals with assistantship H2O2 for improving the photodegradation process [20, 21]. The photocatalyst mechanism of MG degradation has been shown in Fig. 1.
The solar energy is an eco-friendly source of energy that exists on earth surface. This energy contains 46% visible light, 8% ultraviolet light and the 49.4% as infrared radiation, so finding a suitable photocatalyst with the ability of absorbing sun light or having appropriate band gap is important. Another significant factor in the photodegradation process is the size of nanoparticles. So that, electron-hole pair operation is done by absorption of radiation on nanoparticles surface before they recombine with the surface adsorbed species. In smaller particles, the transition of an electron to the surface is easier and need shorter time duration [22, 23].
In this study, the chromates were synthesized by hydrothermal method and the obtained materials were investigated by various methods like FESEM, FTIR, TGA and XRD. At the end the photocatalyst activity of synthesized material were assessed and compared.
EXPERIMENTAL
Materials and methods
All chemical materials were of analytical grade and used without further purification. Phase identifications were performed on a powder X-ray diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation in the range 2θ = 10-90° and the XRD data was analyzed by using X’Pert package and Fullprof program [24]. The morphology of the obtained materials was examined with a field emission scanning electron microscope (Hitachi FE-SEM model S-4160). Fourier-transform infrared spectroscopy (FTIR)was recorded on a Tensor 27 (Bruker Corporation, Germany) in the range 400-4000 cm-1. The thermogravimetric analysis (TGA) has been recorded by STA PT 1600 thermal analysis performed between 25-900°C with the 5°C/min constant rate of heating under an air atmosphere in alumina pan. For controlling photodegradation of Malachite green is used UV–vis diffuse reflectance spectra which were recorded by UV-Visible spectra (Shimadzu UV-1650 PC).
Synthesis of CuCr2O4 and Mn0.5Cu0.5Cr2O4
In this method, firstly 1 mmol Cu(NO3)2 dissolved in 25 mL of distilled water by 15 min stirring. Then, 2 mmol Cr(NO3)2 was added to the solution and stirred for 15 min again. After that, the solution of 25mL of NaOH (2 M) was added dropwise to achieve pH = 14. In the end, the solution was transferred to the 100 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment at 180°C for 11 h. When the reaction was completed, it was cooled to room temperature. The obtained CuCr2O4 was washed with deionized water three times to remove impurity and achieve nurture pH. The prepared powder was washed with distilled water and dried at 60 °C for 60 min under normal atmospheric conditions. The obtained powder was added to platinum crucibles and heated in a furnace maintained at 700°C for 5 h. For synthesizing Mn0.5Cu0.5Cr2O4 was used the same method, but 0.5 mmol Cu(NO3)2 , 0.5 mmol of Mn(CH3COO)2 and 2 mmol Cr(NO3)2 were used as initial materials. After solving material, the solution of 25 mL of NaOH (2 M) was added dropwise to receive pH = 14 and heated as same as synthesizing procedure of CuCr2O4.
Photocatalytic study measurements
Photocatalytic activity of synthesized Mn0.5Cu0.5Cr2O4 was performed in 100 mL of 10 mg/L Malachite green. The wavelength of the visible part of solar light is between 390 nm to 590 nm so for simulation of this part was used 5 FPL 36W 70LM. The solution was stirred in darkness at room temperature for 45 min for the establishment of adsorption/ desorption between dyes and catalyst. After this time, various amount of H2O2 was added to the solution, then 5 mL of suspension was taken as sample after certain interval time and centrifuged (EBA 20 Hettich) with 3500 rpm for removing photocatalyst particles then the absorbance of degraded dye was recorded by spectrophotometer in the range of 200-800 nm, using 1cm quartz cell. Also, it should be considered that Malachite Green has high deionization at alkaline pH without the presence of photocatalyst, so the effect of pH was ignored [25].
The efficiency of catalyst performance was calculated commonly by the following equation:
(1)
Where, Ci is the initial concentration of dye (mg L-1) and Cf is residual dye concentration after certain periods (mg L-1).
RESULT AND DISCUSSION
PXRD analysis
The XRD diffraction pattern of synthesized CuCr2O4 and Mn0.5Cu0.5Cr2O4 are presented in Fig. 2a and 2b. The investigation of XRD data has been done by X’Pert High Score package and Fullprof program [24]. Identification of crystal structure has been studied by using the X’pert package that confirms the formation of single-phase Mn0.5Cu0.5Cr2O4. The results demonstrate that all the diffraction peaks of both CuCr2O4 and Mn0.5Cu0.5Cr2O4 can be quite well indexed in tetragonal structure (space group I 41/a m d) and the best fit with the least difference is carried out. Complete adaptability and perfect performance in Rietveld refinement require the best amount of initial values and type of space group, which are taken from X’Pert package. The refined lattice parameters, cell volume, specific surface area, density and porosity are summarized in Table 1.
The crystallite size of particles can be calculated by Scherrer’s equation:
(2)
In this equation, D show particle size, λ is X-ray wavelength (0.154 nm) and β is broadening at half the maximum intensity of the peak. The largest peak is located at 45.46°. The obtained average crystallite size of synthesized Mn0.5Cu0.5Cr2O4 nanoparticle is 53 nm.
Another significant factor which is investigated by XRD analysis is Specific Surface Area (SSA). SSA or surface area per unit volume as a material property affects the physical features and chemical reactivity. This factor can be evaluated by measurement of XRD density and particle size and can be calculated according to equation [26]:
(3)
Where d is the average size of particle and 𝜌xrd is the density of synthesized particles that can be estimated by XRD measurement as following relation:
(4)
Where Z is the number of atoms in the unit cell (For tetragonal system Z= 4), M is the molecular weight of the samples, N is the Avogadro’s constant number, a and c are the lattice parameter.
Also, porosity (P) is another factor that can determine the active surface area. This parameter can be calculated by evaluating experimental density by compressing synthesized material and make it as pallet and its difference with theoretical density as following equations:
(5)
×100 (6)
Where m is mass of packed powder, r and h are radius and thickness of created pallet. All these parameters are presented in Table 1.
Surface morphology
FE-SEM analysis was used for studying morphology and the size of synthesized materials. Fig. 3 and 4 (a: 2μm, b: 1μm, c: 500nm, d: 200nm) depict various magnificent of synthesized chromates. As could be seen, Fig. 3 reveals that small size particles of CuCr2O4 uniformly distributed and closely packed and the average size of particles is about 30 nm, also Fig. 4 shows multi-dimensions surface particles of Mn0.5Cu0.5Cr2O4. The size of the particles is between 30-90 nm. Also, Fig. 5 shows 3D surface topology of Mn0.5Cu0.5Cr2O4 which was done by Image J software. This image can prove the high surface area of obtained material. The 3D surface morphology is an important factor that can indicate porosity of nanostructures as well and indicates some small bumps to prove high-efficient photocatalyst activity. In recent years, several researches have been done to synthesize semiconductor nanostructures with tunable size and morphology with high surface to volume ratio to effectively degrade contaminant in the environment; synthesized Mn0.5Cu0.5Cr2O4 is one of those materials. Also, for better comparison should be considered that SSA of P25 is 50 [27].
Fourier transforms infrared spectroscopy (FT-IR) analysis
Figs. 6(a) and 6(b) illustrate the FT-IR spectra of as-prepared CuCr2O4 and Mn0.5Cu0.5Cr2O4 respectively; there are two sharp peaks at 520 cm-1 and 615 cm-1, which are attributed to stretching vibration mode of Cr-O-Cu, Cr-O and Mn-O. Mn-O stretching vibration mode is overlaid with Cr-O stretching vibration position [28, 29]. Hence high intensity of these peaks refer to the overlay of them with vibration mode of Cr-O-Mn and Cr-O-Cu and confirm synthesis Cr2O4-2 spinel structure [30]. Also, a peak in the FTIR spectrum at 904 cm-1 is attributed to bond bending of H2O.
Thermogravimetric analysis (TGA) analysis
Thermogravimetric analysis (TGA) is a technique which determines thermal decomposition behavior of material by gaining weight as a result of increasing temperature and differential thermal analysis (DTA) is a technique usually complete TGA method and is used for calculating heat capacity variations that associated with phase transitions. Fig. 7 shows the TGA curve of as-prepared Mn0.5Cu0.5Cr2O4 from room temperature to 900°C. The sharp fall at the plot at the beginning up to 130°C belong to the departure of residual water. After that, the sample heated up to 900°C the curve continues with the normal slope which includes no weight loss that emphasizes on the nonexistence of organic material and forming pure crystalline tetragonal phase. After 600° C, the TG curve becomes flat and crystallized Mn0.5Cu0.5Cr2O4 and only 9.6% weight loss occurs in the temperature range 200-900°C.
UV-vis diffuse reflectance analysis
The band gap energy (Eg) of Mn0.5Cu0.5Cr2O4 was determined by Tauc model (equation (7)) which is used in semiconductor material commonly.
(7)
Which α is linear absorption coefficient of the material, h is Plank’s constant, ʋ is photon’s frequency, A is proportionally constant. Eg is the band gap energy. The value of exponent (r) depends on forbidden or allowed electron transition that causes to create 4 stats: directly allowed transition (r = 1/2), directly forbidden transition (r = 3/2), indirectly allowed transition (r = 2), indirectly forbidden transition (r = 3). The value of Direct band gap energy can be calculated by using the (αhυ)2 versus hυ plot and extrapolating a straight line on a curve with x-axis [31]. This plot is shown in Fig. 8. The value of band gap for this new chromate is estimated 1.37 eV that almost equal with previous reported of Cr2O4-2 [32, 33].
Photocatalytic Activity for Degradation of MG in Water
Effect of initial catalyst concentration
In this part the connection between the initial concentration of Mn0.5Cu0.5Cr2O4 and decolorization of malachite green are showed in various ranges of catalyst in Fig. 9-a and the rate of degradation is illustrated in Fig. 9-b. As it is obvious, the rate of degradation was increased with adding amount of catalyst to a certain amount of catalyst -up to 0.03g- after that, the rate of degradation became stable. That it can be a result of preparation saturated solution.
Reaction kinetics is a part of the chemical reaction which is assessed to give useful data about the speed of reaction rates and the mechanisms of the reactions. In the photocatalytic process, the rate law is used to predict the reaction rate. In a simple chemical reaction, A→B+C, the rate of each part can be evaluated by = = . It shows that the decreasing rate of the reactant, lead to an increasing rate of the products reaction also, demonstrates that rate depends on the concentration of the reactants and the rate constant has the unit of 1/s. A total way to demonstrate the reaction rate is to use the power law: rate= = k[C]n, where k is the rate constant and n is the order of the reaction [34, 35].
Theoretically form of rate is:
(8)
This relationship may rely more on the concentration of one particular reactant, and the resulting rate law may include some, all, or none of the reactant species involved in the reaction. So the theoretically rate law can be expressed as equation 8, in which is the initial concentration of catalyst, m is reaction order of catalyst concentration, is dye concentration and n is reaction order which is depended on dye concentration. In this photocatalytic process the reaction rate is pseudo-first order reaction it means that the catalyst concentration is constant (in this experiment is 0.01 %) the rate of reaction is depended on dye concentration, so the experimental rate constant is:
(9)
So, the reaction is the pseudo-first order reaction as follows:
(10)
Fig. 9-b, demonstrates kinetic study of the initial concentration of photodegradation reaction of Mn0.5Cu0.5Cr2O4, this data can provide applied and significant information regarding the mechanism of photodegradation and reaction process [36]. Photodegradation of MG in various concentrations 0.01g, 0.02g, 0.03g and 0.04 are 0.3752, 0.3499, 0.8202 and 0.8289 min-1
Effect of H2O2
In this part, the relation between the initial amount of H2O2 as a parameter with degradation ability and decolorization rate of malachite green is shown in Fig. 10(a). This part is done to investigate the role of H2O2 on photodegradation. This diagram which is a pseudo-first-order kinetic curve, establish the effect of initial H2O2 on photodegradation. As can see in Fig. 10(a), increase in initial H2O2, lead to decrease in dye concentration, slightly. The positive effect of H2O2 may have two reasons, first photolysis of H2O2 to •OH– which is one of the dye degradation accelerators- second reason may refer to the oxidative ability of H2O2, Fig. 10-b determine the pseudo-first order rate constants of CuCr2O4 for photodegradation of MG in various concentrations.
Effect of reactive oxidative species
In this part, the influence of initial reactive species is evaluated by scavenger experiments that are assessed to identify reactive species in the photodegradation process, respectively. The effect of scavengers on the degradation of MG is shown in Fig. 11. Adding 10mM Azide is used to scavenge , that cause to increase the degradation rate slightly in target dye that is referred to the formation of superoxide radical. h+ is another important parameter in photocatalytic degradation, for carrying out its effect, oxalate ion was added to quench h+. As it is obvious, it has decreased the rate of photodegradation on this process. Also, isopropanol was added for evaluating •OH on photodegradation. Here, •OH could facilitate photocatalytic degradation as well as .
Fig. 12 demonstrate and compare the photodegradation ability of CuCr2O4 and Mn0.5Cu0.5Cr2O4. This comparison shows that the photocatalytic activity of Mn0.5Cu0.5Cr2O4 is better than CuCr2O4. This result has a direct relationship with morphology and surface area which is one important factor in photodegradation process; it seems that agglomeration morphology of CuCr2O4 leads to create lower activated surface so that, has less photodegradation ability. In the other side, Mn0.5Cu0.5Cr2O4 sample has lower total electronegativity than CuCr2O4 that it can effect on band gap, so easier electron transition between the conduction band and the valence band [37].
Reusability of photocatalyst
At the last part, the reusability of Mn0.5Cu0.5Cr2O4 was investigated in three cycles in Fig. 13. Reusability of the catalysts is one of the most important factors which make them useful and economical catalysts. So, checking the recyclability is an essential test [38]. Higher photocatalytic activity of Mn0.5Cu0.5Cr2O4 leads to higher recyclability which is demonstrated in Fig. 13. The yield of the product approximately decreases by less than 8 percent after three successive cycles. This fact indicates that the possibility of photochemical activity of the catalyst is more than adsorption as result of activated surface. If the degradation had been done by adsorption process, the reusability of catalyst would have decreased rapidly as a result of obstruction of active sites.
Table 2 compare the efficiency of the obtained new synthesized spinel as a photocatalyst and other reported metal oxides in decolorization of MG for more illustration. As it is seen, the ability of degradation by considering time, pH and amount of catalyst for Mn0.5Cu0.5Cr2O4 nanoparticle in comparison with other reported catalysts is undeniable.
CONCLUSION
In this study, the pure phase of new spinel structure, Mn0.5Cu0.5Cr2O4, has been synthesized via hydrothermal method. The band gap energy of obtained new material was evaluated 1.37eV which is almost the most effective visible light absorption. The photocatalytic performance of synthesized material for photodegradation of MG was investigated and demonstrated excellent dye photodegradation activity of as-prepared Mn0.5Cu0.5Cr2O4 that was about 100% after 40min. This issue refers to the appropriate band gap and high specific surface area. Also, the reusability of the synthesized catalyst was investigated and indicated that only 8 percent of the degradation decreased after three successive cycles that this issue clarify priority of photocatalyst activity than adsorption.
ACKNOWLEDGMENT
This work was financially supported by Chemistry department, Semnan University of Iran and the author gratefully thank to their cooperation.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.
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