Water purification is one of the most significant topics in environmental science [1–7] and synthetic dyes are the major pollutant groups of wastewater [8–11]. Even in low quantities, presence of dyes can occasion critical environmental problems, for instance, growth of aquatic bacteria can be prevented by the interference of influence of sun light in to water by organic dye molecules [8–11]. Hence, much attempt has been made to decline the concentration of organic dyes in the wastewater [8–15]. Utilize of photocatalysts has been evaluated as one of the most promising approaches of eliminating organic compounds from water [16, 20–24].
Dye is an essential chemical utilized in variouse industries such as those involved in generating fabric, food, furniture and paint, representing a significant threat to the environment because of its toxicity and potentially carcinogenic nature [23- 34].
In current years, molybdates are materials that have involved the benefit of many researchers because of their broad potential to industrial application involving optic fiber, humidity sensor, catalysts, scintillation detector, solid-state lasers, photoluminescent devices, microwave applications, and so on. Molybdates are significant luminescent materials with scheelite-type tetragonal structure, membership I41/a space group with two formula units per primitive cell. Every of X atoms (X=Mo) is surrounded by four equivalent O atoms composing the [XO4]2 tetrahedral configuration and every divalent metal shares corners with eight adjacent O atoms of [XO4]2 tetrahedrons [35–39].
In this investigation, a novel sonochemical way was offered to prepare CdMoO4 nanostructures. Besides, the morphology of CdMoO4 nanostructures can be regulated by adjusting the processing parameters.
The reaction proceeded in an open system at atmospheric pressure. No other additives were used. This approach has obvious benefits like simple experimental set-up, easy handling, short reaction time, environmental friendliness and the procedure is very easy, attracting and novel by focusing large amount of sonochemical in to the solutions to prepare pure products. To the best of our knowledge, no such studies have ever been reported.
MATERIALS AND METHODS
Materials and experiments
All the chemicals used in our experiments were of analytical grade, were purchased from Merck and used as received without further purification. A multiwave ultrasonic generator (sonicator UP400-A; TOP Sonics , Iran), equipped with a converter/transducer and titanium oscillator (horn), 25 mm in diameter, operating at 20kHz with a maximum power output of 400W, was used for the ultrasonic irradiation. The ultrasonic generator was automatically adjusted as needed. The XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni- filtered Cu Kα radiation. SEM images were obtained on Philips XL- 30 ESEM equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy (FT-IR) was recorded with Shimadzu Varian 4300 spectrophotometer in KBr pellets. EDS analysis was obtained on Philips EM208.
Synthesis of Cd(Sal)2 complex
[Cd(Sal)2] was prepared as follows: cadmium(II)nitrate [Cd(NO3)2.4H2O], 2mmol, was dissolved in 25 ml distilled water, a solution of salicylaldehyde, 4mmol ,dissolved in the same volume of ethanol was drop wise added to the above solution under magnetic stirring. After addition of all reagents, the mixture was refluxed for about 6h.The precipitate was collected and then washed with ethanol and dried in vacuum oven in 60 °C.
Preparation of CdMoO4 nanostructures (Photocatalyst preparation)
CdMoO4 nanostructures were synthesized by simple sonochemical approach. In a typical process, an aqueous solution of Cd(Sal)2 in the presence of various surfactants, such as cetyl tri methyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and poly vinyl pyrrolidone(PVP) was mixed with Na2MoO4.2H2O aqueous solution and the solution under low frequency sonochemical at room temperature for 20 min. The yellow precipitate was centrifuged, washed out with distilled water and methanol for three times and dried under vacuum at 60 °C. The effects of surfactant and power source on the morphology and the particle sizes of CdMoO4 nanostructures were considered.
The photocatalytic activities of the samples were distinguished by the degradation of aqueous methylene blue (MB) under UV light. About 5 mg of the sample was first inserted to a reactor that contained 5*10-6 M of aqueous MB. The suspension was transferred in to a self-designed glass reactor, and stirred in darkness to achieve the adsorption equilibrium. In the research of photo- degradation by UV light, a 125W high pressure mercury lamp with a water cooling cylindrical jacket was employed. The concentration of MB was checked on the basis of its UV–visible absorption peak at 664 nm.
RESULTS AND DISCUSSIONS
Fig. 1 reveals FT-IR spectra of Cd(Sal)2 precursor and CdMoO4 nanostructures. Vibrations of CdMoO4 are categorized in to the internal and external modes . The first belongs to the vibration inside [MoO4]2- units. Their centers of masses are stationary. The second is called lattice phonon mode which corresponds to the motion of Cd2+ cations and the rigid molecular units [20–22]. In the case [Cd(Sal)2] (Fig. 1a), the peaks at 1520 and 1635cm-1 belong to the C-O stretching vibrations (ʋC-O) and the peak at 1430cm-1 belongs to C-C stretching vibrations (ʋC–C) of the salicylaldehyde. Free salicylaldehyde ʋC-O appears at 1685 and 1665cm-1, and ʋC–C appears at 1495 and 1385cm-1. Upon complex formation, these stretching vibrations shifted to lower regions. Fig. 1b exhibits FT-IR spectra of CdMoO4 nanostructures acquired in methanol as solvent and CTAB as surfactant. The spectra indicate a band of Mo-O stretching vibration in MoO42- tetrahedrons  at 795–930cm-1. It is one of the internal modes illustrate as anti-symmetric stretching vibrations  of the three products. Weak Mo-O bending vibrations  were also discerned at about 405 cm-1 for CdMoO4 nanostructures. The broad absorption band nearby 3330cm-1 in Fig. 1b is assigned to the stretching vibrations of absorption water. Absorption peaks at 460–945 cm-1 are because of Cd–O band, there are no absorption bands nearby this range in salicylaldehyde.
X-ray diffraction patterns
The crystal structure and the composition of the as-prepared products were exhibited by XRD. The XRD patterns of the cadmium molybdates has been presented in Fig. 2. All peaks in Fig. 2, correspond to the reflections of octahedral phase of CdMoO4 which are in a good accord with the reported data (JCPDS: 85-0888). No considerable diffractions of other phases can be found in the figure, exhibiting that a pure CdMoO4 phase has been formed.
EDX analysis measurement was applied to investigate the chemical composition and purity of as-synthesized CdMoO4 nanostructures. The EDX pattern of CdMoO4 in Fig. 3 exhibits that the only elements which existed were Cd, Mo, and O. No peak of any impurity was detected, indicating the high purity of the product.
Fig. 4 reveals the (αhυ)2 vs hυ curve of CdMoO4 nanostructures which were estimated by their UV–visible absorbance utilizing the equation designed by Wood and Tauc revealed Eq. (1)  below.
where α is the absorbance, h the Planck constant, υ the photon frequency, Eg the energy gap, and n is the pure numbers associated with the various types of electronic transitions. For n=1/2, 2, 3/2 and 3, the transitions are directly allowed, indirectly allowed, directly forbidden, and indirectly forbidden, respectively. Each energy gap was specified by extrapolation of each linear portion of the curves to α=0. In the current consideration, the CdMoO4 offers directly allowed electronic transition (n=1/2) , and the energy gaps of CdMoO4 nanostructures is 3.8 eV.
Photocatalytic decolorization of MB dye
The photocatalytic activity of the CdMoO4 nanostructures was evaluated by monitoring the degradation of methylene blue (MB) in an aqueous solution, under irradiation with UV light (Fig. 5). Without light or nanostructures, nearly no MB was breakdown after 60min, revealing that the contribution of self-degradation was negligible. However, CdMoO4 nanostructures exhibited high photocatalytic activity. The heterogeneous photocatalytic processes including many steps, such as diffusion, adsorption and reaction, convenient distribution of the pore is beneficial to diffusion of reactants and products, which prefer the photocatalytic reaction. In this paper, the improved photocatalytic activity may be ascribed to convenient sharing of the pore, high hydroxyl content and high separation rate of photo induced charge carriers .
The morphology of the samples was considered by SEM images. Fig. 6 exhibits SEM images of the samples prepared at 30 °C, for 15 min with different surfactant via sonochemical approach. SEM images of the samples attained in the presence of CTAB, PVP and SDS have been illustrated in Fig. 6a, b, and c, respectively. The SEM images which revealed sphere-like nanostructures were organized in the presence of the surfactants. In the presence of PVP as surfactants, more regular sphere-like nanostructures were organized.
To investigate the effect of sonication power on the morphology of the products, the reaction was carried out in the presence of three different powers in water, 30 °C and 20 min SEM images of the samples were attained in the power of 200, 300 and 400W have been exhibited in Fig. 7a, b, and c, respectively. The SEM images displayed sphere-like nanostructures were formed in the three different powers without presence of any surfactant. In the power of 400W, sphere-like nanostructure sizes were increased.
In brief, sphere-like cadmium molybdate nanostructures have been successfully prepared from Cd(Sal)2 and Na2MoO4.2H2O utilizing a sonochemical approach, under low temperature and pressure. By regulating the surfactant and power source, we could gain cadmium molybdate nanostructures morphology. To the best of our knowledge, it is the first time that Cd(Sal)2 is utilized as Cd source for the synthesis of cadmium molybdate nanostructures by sonochemical process. The prepared cadmium molybdate nanostructures can be used in some analytical applications such as removal of methylene blue dye, since the % uptake was found to be >90% within 60 min.
Author is grateful to the Farhangian University for supporting this work.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.