Photocatalysts are determined as materials which decompose detrimental substances under the sun lights containing UV rays [1-3]. Since photocatalysts became a viable alternative for pollution control, researchers have worked to improve their reaction rate and photocatalytic activity. Due to their possible applications in solar energy conversion and environmental purification, semiconducting oxide photocatalysts have received a lot of attention in recent years [4-9]. When water comes into contact with a photocatalyst, it creates hydroxyl radicals (•OH) and superoxide (•O2), which are scavenger radicals. These scavenger radicals then destroy organic contaminants in a nonselective manner, degrading them to smaller, less toxic compounds [10-12].
ZnO is known as one of the most familiar photocatalyst. ZnO-based binary and multiple heterojunction/nanocomposite photocatalysts are gaining a lot of attention due to their potential for solving global energy supply crises and pollutant degradation [13-17]. ZnO has unique optical properties, which helps ZnO to maintain photocatalytic activity at low recombination rates. The optical properties of photocatalyst plays a key role in degradation of pollutants. The photocatalytic application of ZnO nanostructures suffer from different limitations. Till now, various methods have been applied for improvement photocatalytic activity of ZnO nanoparticles, such as doping, and preparing ZnO-based nanocomposites [18-21]. M. Chitra and coworkers measured band gap of ZnO, tin oxide (SnO2) and vanadium oxide (V2O5), their binary and ternary combinations were prepared via hydrothermal route. The unique hierarchical nanostructures of the zinc–tin–vanadium oxide (ZTV) nanocomposite with a larger surface area provided the active surface sites required for ethanol adsorption. ZTV’s band gap is calculated to be 1.97 eV. The competing effects of high free carrier concentration and Burstein-Moss shift may be responsible for the band gap narrowing observed in ZTV . Kayode Adesina Adegoke and co-authors prepared ZnO/CdS nanocomposites through two steps via hydrothermal and photochemical method. They found that the composite photodegraded Rhodamine B (RhB) dye azo dye with an efficacy of 85% after 30 minutes under UV irradiation. They reported that CdS/ZnO nanocomposites has better photocatalytic performance than ZnO nanorod . Reda M. Mohamed et al. prepared Ag2O–ZnO Heterojunctions and applied for photodegradation of tetracycline. They showed that The mesoporous 1.5% Ag2O–ZnO nanocomposite shows the very best degradation rate among all prepared samples with different ratio, and it had been twenty three and eight orders of magnitudes bigger than those of pristine ZnO NPs and P-25, respectively .
In this work, ZnO, CuMoO4, and ZnO/CuMoO4 were prepared via simple and novel method. The prepared samples were characterized via XRD, FTIR, SEM, TEM, and UV-Vis analysis. Then the obtained samples were applied for photodegradation of methylene blue as a water pollutant and compared their efficiency.
MATERIALS AND METHODS
Apparatus and chemicals
Nicolet Magna-550 spectrometer in KBr pellets was connected for recording Fourier change infrared (FT-IR) spectra. Morphological properties of products were examined through filtering electron microscopy (SEM) that gotten on LEO-1455VP prepared with an vitality dispersive X-ray spectroscopy. For in-depth studying of morphological structure, Philips EM208S transmission electron magnifying lens was utilized. X-ray diffraction (XRD) patterns investigation was done by a Philips-X’pertpro, X-ray diffractometer utilizing Ni-filtered Cu Kα radiation. The complete chemicals utilized in this research were of analytical grade: Zinc (Ⅱ) nitrate (Zn(NO3)2.6H2O), sodium hydroxide (NaOH), CuSO4, , Polyvinylpyrrolidone (PVP), (NH4)6Mo7O24.H2O, sodium dodecyl sulfate (SDS) from Merck.
Synthesis of ZnO nanoparticle
5M Zn(NO3)2.6H2O was dissolved in 30 ml deionized water. In other beaker, PVP was dissolved in deionized water. The PVP solution was added to Zn2+ solution under stirring. Then NaOH solution was added dropwise to obtained solution under stirring until pH=11. After 30 minutes, the solution was transferred to Teflon lined stainless autoclave and heated at 130 °C for 4 h. The as-obtained white precipitate was centrifuged and washed with ethanol and water and dried in 60°C for 12 h.
Synthesis of CuMoO4 nanoparticle
CuMoO4 nanoparticles were prepared via co-precipitation method. 3M CuSO4 and sodium dodecyl sulfate (SDS) were dissolved in deionized water. In other beaker, aqueous solution of (NH4)6Mo7O24.H2O was prepared. Mo-based solution was added dropwise to Cu-based solution. The solution was heated to 65 °C for 2 h. The obtained greenish precipitate washed and dried in vacuum at 60 °C.
Synthesis of ZnO/CuMoO4 nanoparticle
The prepared ZnO and CuMoO4 nanoparticles were dispersed in ethanol and sonicated for 6 h. Prepared nanocomposites centrifuged and dried at 60 °C for 12 h.
The degradation of methylene blue in aqueous solution was monitored under UV light to determine the photocatalytic activity of prepared samples. In a quartz photocatalytic reactor, the degradation process was carried out. With a 20 ppm dye solution and 0.02 g of samples, photocatalytic degradation was performed. The mixture was then put in a photoreactor under UV light and stirred for 20 minutes in the dark to ensure that the dye molecules on the nanostructures surface were in the proper adsorption-desorption equilibrium needed to serve as an effective photocatalyst. Air was blown into the vessel through a pump to keep the solution oxygen-saturated in the reaction. Then, using 5 minutes of centrifugation at 12,000 rpm, nanoparticles were isolated from the 5cc samples taken from the degraded solution at different time intervals. A UV-vis spectrophotometer was used to calculate the dye concentration.
RESULTS AND DISCUSSION
X-ray diffraction (XRD) was used to describe the phase structure and crystallinity of the prepared products. The XRD pattern of prepared samples is shown in Fig. 1. XRD analysis of a prepared ZnO NPs confirms formation of ZnO (JCPDS: 01-080-0075), CuMoO4 (JCPDS: 31-0449), and ZnO/CuMoO4 nanostructures without any impurity. Scherrer equation , Dc = Kλ/βCosθ, was utilized for crystalline calculation, where β is the width of the observed diffraction peak at its half maximum intensity (FWHM), K is the shape factor, which takes a value of about 0.9, and λ is the X-ray wavelength (CuKα radiation, equals to 0.154 nm). According to Scherrer equation, crystalline size of of ZnO, CuMoO4, and ZnO/CuMoO4 were determined 42, 53, and 48 nm respectively.
Fig. 2 shows FT-IR spectrum of the ZnO, CuMoO4, and ZnO/CuMoO4. For ZnO case, the broad peak from 3100 to 300 cm-1 can be attributed to the stretching vibration of the hydrogen bonded OH groups of the surface-adsorbed water. The peak at 570 cm-1 is related to the Zn–O bond stretching mode. No other absorption peaks, indicating the purity of the ZnO. For the CuMoO4, the presence of peaks in 1200-1400 cm-1 can be related to C-C bond which is related to unwashed SDS. The peak at 800-100 can be assigned to Mo-O-Mo stretching mode. The peaks at CuMoO4, and ZnO/CuMoO4 are confirmed formation metal-oxygen (Zn-O, Mo-O, and Cu-O) bonds.
Scanning electron microscope was applied for morphological investigation of prepared products. Fig. 3a and Fig. 3b presents SEM image of as-prepared ZnO nanoparticles at two magnifications. As well as shown, homogenous nanoparticles with average particle size of 140 nm is formed. For CuMoO4, average particle size was increased to 190 nm (Fig. 3c and Fig. 3d). Fig. 4a and Fig. 4b provides SEM images of ZnO/CuMoO4 nanocomposites. It is clear that particle size of ZnO/CuMoO4 nanocomposites is smaller than ZnO, and CuMoO4 nanoparticles. It can be concluded that applied ultrasonic for preparation of ZnO/CuMoO4 separate agglomerated particles and provide smaller particle size.
Fig. 5 shows UV-Vis absorption spectrum of prepared samples. For ZnO NPs, The observable absorption peak at 330 nm asserts the formation of ZnO NPs by the hydrothermal method. In comparison with previously reported papers, there was a considerable blue-shift in the absorption of the prepared ZnO NPs rather than the bulk ZnO. It is clear that there was no impurity-related peak in the spectrum which reveals that synthesized ZnO NPs possess high purity. For CuMoO4 nanoparticles, the absorption spectrum shows a high absorption peak at 268 nm, which is related to Ligand to Metal Charge Transfer (LMCT) from 2p orbitals of oxygen to 4d orbitals of molybdenum inside the MoO42- anion.
Recently, azo-dyes included water become a major concern in environmental fields. The prepared nanostructures were applied for photocatalytic investigation. It is concluded from the optical properties which prepared samples can be acted as a good photocatalyst. The present study utilize ZnO, CuMoO4, and ZnO/CuMoO4 for photocatalytic degradation of Rhodamine B and Methylene blue as water pollutants under UV irradiation. Fig. 6 shows photocatalytic performance of samples for removal of methylene B from water. Results show that ZnO/CuMoO4 nanocomposites degraded 92% of methylene blue after 70 minutes. For ZnO and CuMoO4, the efficiency was measured 74 and 66% respectively. The same results was repeated for photocatalytic degradation of Rhodamine B. As well as shown in Fig. 7, 84% Rhodamine B was degraded after 70 minutes UV irradiation. The photocatalytic efficiency was measured 59, and 73% for CuMoO4 and ZnO respectively. The optical properties of ZnO/CuMoO4 nanocomposites confirms the charge transfer from CuMoO4 to ZnO. So, recombination of electron-hole possibility in CuMoO4 becomes small and create holes in valance band react with OH groups on the surface of nanocomposites and cause to produce highly reactive OH• radicals. As-produced radicals react with Rhodamine B and Methylene blue and degraded. In overall, results introduce high potentially ZnO/CuMoO4 nanocomposites as a photocatalytic degradant of water pollutants.
In summary, the ZnO, CuMoO4 and ZnO/CuMoO4 nanostructures were prepared via hydrothermal, co-precipitation, and ultrasonic- assisted routes respectively. The prepared samples were characterized via XRD, FTIR, FESEM, and UV-Vis analysis. The attractive optical properties of prepared samples lead to application of products in photocatalytic process. So, prepared nanostructures were applied for photocatalytic degradation of azo dyes as water pollutants. It is found that prepared ZnO/CuMoO4 nanocomposites has a high potential for removal Methylene blue and Rhodamine B from water. The results revealed that 92 and 84% of methylene blue and Rhodamine B were degraded after 70 minutes respectively.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.