INTRODUCTION:
Water treatment and prevision of safe potable water are tasks that most developing countries struggle to undertake. Waste water from textile, paper and some other industries contain residual dyes, which are not readily biodegradable. Organic dyes are major pollutants of waste water. Increasing the availability of procedures to control or eliminate this type of pollution is a challenge [1]. One of the most promising idea to remove colored pollutants from waste water is Nanophotocatalysis. Photocatalysis works on photo generation of electrons (e-) and holes (h+) when subjected to UV light [2] and their migration of (e-/h+ pairs) to the catalyst surface, resulting in the chemical reaction with adsorbed pollutants and oxygen from air. Photocatalytic degradation is a non-toxic, inexpensive and highly reactive method [3]. Visible–light-driven photocatalysts have attracted much attention in the effective utilization of solar energy in environmental purification, water splitting for hydrogen generation and organic synthesis. The low efficiency for utilization of visible light and the fast recombination between the photogenerated electrons and holes are often two major limiting factors which obstacle the improvement of photocatalytic activity of TiO2 [4]. TiO2 has been the most widely studied and used in many applications because of its strong oxidizing abilities, super hydrophilicity, chemical stability, long durability, non toxicity and low cost [5]. In order to adjust the wider energy band structure of TiO2 [6,7], some modified methods were adopted, e.g. doping with metal [8-10] or non-metal element [11-13], co-doping with different metal ions [14,15], as well as coupling with low-energy band semiconductor, [16] etc. Now a day’s molybdate photocatalysts have attracted a great interest owing to their outstanding photocatalytic activity and the simple synthetic protocol [17-21]. Recently metal molybdates such as NiMoO4, Na2Ni(MoO4)2, metal molybadte incorporated TiO2 (MxMox Ti1-x O6) (M=Ni, Cu, Zn), CdMoO4, PbMoO4, NiMoO4 doped Bi2Ti4O11, Bi2MoO6[18], Bi2MoO9[22], Bi2Mo3O12[23], NaM(MoO4)2 (M=Y, Bi) [24, 25], Fe2(MoO4)3[26], Ag2MoO4[27] these molybdates are shown an excellent visible light responsive photocatalytic activity for degradation of dyes in water due to excellent structural and optical properties. Among them the catalytic activity of nickel molybdates is large and concerns different rections:hydrocarbonsdestilation as hydrodesulfuration and hydrodenitrogenation, water-gas shift, vapour reforming steps, hydrogenolysis, n-butane cracking reactions, oxidative dehydrogenation of olifins and photocatalyst.
Several methods have been taken to synthesize this nickel molybdate as sonochemical, hydrothermal, sol-gel, chemical precipitation, chemical solution decomposition (CSD), microwave synthesis, conventional, facile vaccum filtration method, modified Pechini method and combustion synthesis. However no reports on the coprecipitation synthesis of Cu, Fe-codoped NiMoO4 nanoflakes have been published by our knowledge. Co-precipitation technique can be described as a facile, low cost and useful route for the preparation of the photocatalyst with a high purify in nanometric scale. Recently, simultaneous doping of two kinds of atoms (co-doping) into semiconductor materials has attracted considerable interest, as it could result in a higher photocatalytic activity and special charactyeristics compared with single element doping into semiconductor oxides. It has been known that the, advantage of the doping of metal ions in semiconductor particles is the temporary trapping of the photogenerated charge carriers by the dopant and the inhibition of their recombination during migration from inside of the material to the surface or the enchanced association of the functionalized organic pollutants to the doping ion surface sites . Ping Yang [28] et al found that TiO2 co-doped with Fe3+ and Eu3+ ions could lead to superior photocatalysis. A pronounced synergistic effect in catalytic activity was noted. The facts indicate that introducing Cu and Fe metals into nanoflakes of NiMoO4 particles will improve the photocatalytic effect. It is notable that the majority of studies have not reported on photodegradation of methylene blue under UV-visible irradiation using Co-doped NiMoO4. The influence of the crystallite size, the catalyst and dye concentration is investigated by using Methylene blue (MB) as model pollutant. The possible mechanisms of the optical absorption property and photocatalytic activity in the Fe and Cu doped NiMoO4 were discussed.
EXPERIMENTAL SECTION:
Materials
NiCl2, (NH4)6Mo7O24.4H2O, Cu(NO3)2.3H2O, FeCl3 and ethanol were purchased from Merck chemiscals, India and applied without further purification. Double distilled water was used throughout the photodegradation experiments.
Preparation of Fe-Cu-NiMoO4
Fe-Cu-NiMoO4nanospheres were synthesized by simple co-precipitation method. For a typical synthesis, an aqueous solution of NiCl2 was added to an equal volume of (NH4)6Mo7O24.4H2Osolution under constant magnetic stirring. To the above solution, Cu(NO3)2.3H2O, and FeCl3 solution was added individually and combined by the precipitate obtained was filtered and washed with ethanol and distilled water and dried in an oven at 120 °C for 1h followed by calcination at 500°C for 1h. NiMoO4 was prepared without addition of dopants by the same procedure.
Measurement of Photocatatic activity
Photocatalytic expriements were carriedout in an immersion type photoractor. 300 ml of methylene blue (MB) with an initial concentration of 5 µM was taken in a cylindrical glass vessel which was surrounded by a circulating water jacket to cool the lamp. Air was bubbled contiously into the aliquot by an air pumb, in order to provide a constant source of dissolved oxygen. Before irradiation the reaction mixture was stirred in dark for 30 min to achieve the adsorption desorption equlibrium between the catalyst and dye molecules. A 300 W Xe arc lamp with an Ultra violet cut off filter was used as the visible light or radiation, 5 ml of aliquot was collected at regular time intervals. Then the samples were centrifuge to remove the photocatalyst and the filterate was analysed by UV-spectrophotometer at λmax equal to 669 nm. The photodegratations percentage was (MB) calculated by the formula giiven below.
Photodegradation(%)
= 
x 100-----------(1)
Where ,
Co is the concentration of (MB) before irradiation time andC is the concentration of (MB) after a certain irradiation time.
Characterization
The nanoparticles were characterized by the following methods. The UV- visible diffuse reflectance spectra were obtained for dry-pressed disk sample using JASCO V-550 double beam spectrophotometer with PMT detector equipped with an integrating sphere assembly, using BaSO4 as a reference sample. The spectra were recorded at room temperature ranging from 200 nm to 800 nm. Fourier Transformation Infrared Spectrum (FT-IR) of synthesized sample was obtained using JASCOFT/IR 4200 with KBr – nanoparticles mixture in the form of pellets. The structure and phase of the samples were determined by X-Ray and powder diffraction with Cu Kα radiation at 25o C using PANI XPERT PRO X – RAY and the structural assignments were made with reference to the standard JCPDS powder diffraction files. Scanning Electron Microscope (SEM) of the nanoparticles was taken by a JM – 6701F – 6701 instrument in both secondary and backscattered electron modes. The elemental analysis was detected by an energy dispersive X-Ray spectroscopy (EDX) attachment to the SEM.
RESULT AND DISCUSSION:
UV-Vis-DRS
The UV-vis diffuse reflectance spectra of as-prepared NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4 are shown in Fig. 1. (a). The samples showed intense absorption in a wide wavelength range from UV to visible light with absorption tail extending into IR region [29]. It was clear that the absorption spectrum of red shifted compared to undoped and single doped the direct band gap (Eg) of nanoparticles was determined using the direct transition equation:
αhυ = Eg (hυ- Eg) ˝----------(2)
Where α is a optical absorption co-efficient, hυ is the photon energy, Eg is the direct band gap and Ed is the constant. By plotting (α hυ) ˝ as a function of photon energy and extrapolating the linear portion of the curve absorption equal to zero, the band gap can be found. Tauc plot of NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4 are shown in Fig. 1.(b-e). The band gap (Eg) of Fe-Cu-NiMoO4 nanoparticles was found to decrease (1.26 eV) when compared to NiMoO4 (3. 20 eV), Fe-NiMoO4 (2.6 eV) and Cu-NiMoO4 (2.12 eV).
NiMoO4
Fig. 1 (a) UV-vis-DRS of NiMoO4,
Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4
Fig. 1.(b-e) Tauc plot of NiMoO4, Fe- NiMoO4, Cu- NiMoO4,Fe-Cu-NiMoO4
FT-IR Spectral analysis
Fig. 2 illustrates the FT-IR spectra for NiMoO4, Fe-NiMoO4, Cu- NiMoO4 andFe-Cu-NiMoO4 synthesized via co-precipitation method. The presence of large bands at 3470cm-1 and 1622 cm-1, which could be associated to the stretching and flexing modes of the O-H linkages from the water molecules adsorbed in the sample surfaces. The bands at 962 and 882 cm-1can be assigned to the symmetric and anti-symmetric stretching of the Mo=O linkage and the band at 492 cm-1could be associated to torsions of the Mo-O-Mo attachment. The bands at 808 and 706 cm-1however are assigned to the vibrations of the Mo-O-Ni. The results show that the band located at 952 cm-1is associated to the symmetric stretching mode of Mo-O linkage [30-33].
Fig. 2. FTIR Spectrum of NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4
X-ray diffraction (XRD)
The XRD patterns of the samples are shown in Fig.3. The diffraction peak of NiMoO4,Fe-NiMoO4,Cu-NiMoO4 and Fe-Cu-NiMoO4 nanoparticles can be indexed well with monoclinic NiMoO4(JCPDS No:160291). The diffraction peaks at 2θ of 23.3,25.6,27.2,33.1,33.7,35.4,38.9 and 39.6 no other patterns can be indicate the formation of NiMoO4 observed, indicating high purity of the as prepared Fe-Cu-NiMoO4nanoparticles. Furthermore, the diffraction peaks of all the samples are sharp and intense, indicating a good crystalline nature. The average crystallite sizes of the prepared nanoparticles were calculated using Debye-scherrer equation:
Where, D is the crystalline size, λ is the wavelength of x-ray(λ = 1.54 Ĺ), β is the value of FWHM which is expressed in radians and θ is NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4 was found to be 36.3nm, 30.21 nm, 28.21 nm and 24.36 nm respectively [34].
Fig. 3. XRD pattern of NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4
SEM and EDX
The morphological structure of NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4 was investigated by SEM, which is shown in the Fig. 4 (a-d) shows the uniform flakes with plate like structure. Fig. 4 (d) shows the SEM image of Fe-Cu-NiMoO4 which is more agglomerated further confirms that doping with Fe and Cu has change the morphology of the NiMoO4 [35]. Table 1 show the EDX data of NiMoO4, Fe-NiMoO4, Cu- NiMoO4 and Fe-Cu-NiMoO4 taken at different locations contains the expected elements (Fe, Cu, Ni, Mo and O) with no other impurity elements.
Fig. 4.(a-d) Tauc plot of NiMoO4, Fe- NiMoO4, Cu- NiMoO4and Fe-Cu-NiMoO4
Table-1
|
Compound |
Element |
Weight % |
Atomic % |
keV |
|
NiMoO4 |
Ni |
6.26 |
3.09 |
7.41 |
|
Mo |
41.03 |
36.71 |
2.29 |
|
|
O |
52.71 |
60.20 |
0.525 |
|
|
Fe-NiMoO4 |
Ni |
4.48 |
2.00 |
7.41 |
|
Mo |
48.22 |
13.18 |
2.29 |
|
|
O |
46.64 |
84.51 |
0.54 |
|
|
Fe |
4.48 |
0.31 |
0.71 |
|
|
Cu-NiMoO4 |
Ni |
5.74 |
3.04 |
7.41 |
|
Mo |
57.21 |
18.58 |
2.29 |
|
|
O |
36.14 |
77.95 |
0.525 |
|
|
Cu |
0.91 |
0.43 |
0.93 |
|
|
Fe-Cu-NiMoO4 |
Ni |
4.94 |
2.15 |
7.41 |
|
Mo |
47.21 |
12.56 |
2.29 |
|
|
O |
42.86 |
84.50 |
0.525 |
|
|
Fe |
1.02 |
0.37 |
0.71 |
|
|
Cu |
3.97 |
0.42 |
0.93 |
Photocatalytic activity
Photocatalytic degradation of MB dye was performed to evaluate the photocatalytic activities of as-prepared Fe-Cu-NiMoO4 nanocomposites. It is commonly accepted that the photocatalytic performance of catalysts depends essentially on various factors, including mainly crystalline, particle size and surface area, along with its absorption properties. First, without illumination of light, the MB solution was accomplished adsorption equilibrium after 30 min, for all the prepared nanocomposites, more than 80% of the initial MB molecules were remained in the solution. It is worth noting that Fe-Cu-NiMoO4sample has the highest adsorption capacity. The decomposition efficiency of MB at a dye concentration of 10 µM with a catalyst dosage of 0.14 g/L under visible light illumination was determined by measuring the absorption spectra after 3 h. Comparative experiments for photocatalytic degradation of MB over Fe-Cu-NiMoO4nanocomposites showed enhanced photocatalytic activities than pure NiMoO4, Fe-NiMoO4, Cu-NiMoO4andFe-Cu-NiMoO4.Photodegradation efficiency of 61% was achieved using NiMoO4, 75% was achieved for Fe-NiMoO4 and 82% was achieved for Cu-NiMoO4 increased to 93% using Fe-Cu-NiMoO4respectively. The increase in photocatalytic activity is likely related to the synergistic effect between NiMoO4 and metals. Fig. 5 showsPhotodegradation curve of MB using NiMoO4, Fe-NiMoO4, Cu-NiMoO4 and Fe-Cu-NiMoO4. The mechanism of charge separation in codoped NiMoO4 is similar; the interfacial charge transfer is notably different. In a codoped semiconductor system, the particles are in contact with each other and the holes and electrons are available for oxidation or reduction reactions on the surface of different particles. The possible mechanism is given in Fig. 6.
Fig. 5. Photodegradation efficiency of NiMoO4, Fe-NiMoO4, Cu-NiMoO4 and Fe-Cu-NiMoO4
Fig. 6. Mechanism of Fe-Cu-NiMoO4
Effect of catalyst concentration
In order to optimize the best catalytic system for the degradation of MB, experiments were carried out by varying the concentrate of Fe-Cu-NiMoO4from 0.05 g/L to 0.125g/L with a constant dye concentration of MB (15µM). In this case, an increase of degradation was observed with the corresponding increase in the catalyst concentration. However, when the degradation experiment was carried out without photocatalyst, only 6.7% of initial concentration of the dye was degraded even after a same duration of exposure to visible light. The percentage of photodegradation of MB increases with the increase of catalyst concentration 0.05 g/L to 0.10 g/L and a further increase of catalyst concentration leads to decrease in the Photodegradation percentage. This may be attributed to the fact that as the catalyst concentration is increased, the exposed surface area also increases. A further increase in catalyst concentration 0.125 g/L leads to decrease in photodegradation. This is because with an increase of catalyst concentration total active surface area increases. Consequently the generation of reactive species required for MB degradation also increased. At the same time the suspension becomes more turbid at high photocatalyst concentration, which hinders the penetration of light. Hence the photo activated volume of the suspension decreases. Fig. 7 shows Effect of catalyst concentration on the photodegradation MB.
Fig. 7. Effect of Fe-Cu-NiMoO4 dosage on the Photodegradation of MB
Effect of initial dye concentration
The effect of initial MB concentration on the photodegradation is investigated at three initial dye concentrations of 1, 2 and 3 µM as a function of irradiation time in the presence of 0.14 g/L of Fe-Cu-NiMoO4at pH 10. The results are shown in Fig. 8. The photodegradation percentage of MB are found to be 48, 98.05 and 74% respectively for 12, 15 and 18.2 mg/L. Fig. 8 has revealed that the photodegradation efficiency decreases with increase in MB concentration. The possible explanation for this behavior is that at higher concentration of the dye, the path length of the photons entering the solution decreases with direct consequence on the electron–hole formation. At the same time, a significant amount of light may be absorbed by the dye molecules than the photocatalyst at higher MB concentration. Thus, the formation of hydroxyl free radicals at the catalytic surface is hindered [36].
Fig. 8. Effect of MB concentration on its Photodegradation
Effect of initial pH
Generally, the pH of the solution is an important parameter in the photocatalytic processes, since it not only plays an important role in the characteristics of dye, but also determines the surface charge properties of NiMoO4, the size of aggregates formed, the charge of dye molecules, adsorption of dye onto NiMoO4 surface and the concentration of hydroxyl radicals. Hence, the photodecolorization of MB using Fe-Cu-NiMoO4was studied in pH range 6-12 and the results are shown in Fig. 9. It was found that increase of solution pH from 6 to 10 increased the decolonization efficiency.
Kinetics of degradation of methylene blue
PhotodecolorizationIn all the photodecolorization experiments, the reaction followed first order kinetics. Plots of −ln[C/C0] versus time showed linear relationship where C is the concentration of eosin-Y remaining in the solution at irradiation time of t, and C0 is the initial concentration at t = 0min.Firstorderrateconstantswere evaluated fromtheslopesof −ln[C/C0] versus time plots (Fig. 10). The observed rate constant for the photodecolorization of MB in the presence of Fe-Cu-NiMoO4is 1.196×10−2 S−1, which is significantly higher than that observed for bare NiMoO4 (1.28 ×10−3 S−1) as well as Fe-NiMoO4 (9.01 ×10−3 S−1) and Cu-NiMoO4 (6.21 ×10−3 S−1) and without catalyst (3.45 ×10-3 S-1) [37].
Fig. 9. Effect of pH on Photodegradation of MB
Fig. 10. Kinetic plot of –ln(C/Co) versus irradiation time for the Photodegradation of MB
CONCLUSION:
In summary, Fe-Cu-NiMoO4 nanoflakes have been synthesized through coprecipitation method. Fe-Cu-NiMoO4 nanoflakes displayed excellent photocatalytic performance and that provided a good electron acceptor favoring the transfer of photo generated electron from the conduction band to valence band. When 0.14 g/L of nanoflakes was used the degradation of MB exceeds 98% under visible light irradiation for 180 min with a dye concentration of 10 µM. The kinetics of MB photodegradation on the prepared nanoflakes follows pseudo-first order model. The Fe-Cu-NiMoO4 nanoflakes protocol is a simple, fast and efficient, less toxic, low-cost and suitable photocatalyst for remediation of organic dyes in wastewater.