In situ solid phase microwave assisted synthesis of silver / nickel substituted phosphomolybdate nanoparticles using organic driving agent

A solid state microwave assisted technique was used for facile and eco-friendly synthesis of silver/nickel substituted salt of phosphomolybdate nanoparticles. This solid phase reaction was promoted using urea as organic driving agent under microwave treatment with the power of 360 W for 7 min. The structural and morphological characterizations revealed the formation of mixed silver/nickel ammonium salt of phosphomolybdate with Keggin-type structure and uniform nano-sized particulate morphology with the average size about 41 nm. The experimental strong point of this strategy is the use of a facile, one-pot and solid state method for synthesis of nanomaterials at short time without any acidic conditions. As a result, this method can be proposed as an instrumental way to fabricate various macromolecules of polyoxometalate in nanoscale.


Introduction
Polyoxometalates (POMs) with a Keggin structure are a well-known class of mixed metal oxides due to their special properties and potential applications in materials science, gas sensing, bioscience, medicine, catalysis [1][2][3][4][5].Although these polyanion compounds are introduced as the effective materials in various applications, their wide practical uses have been hindered by their low structural stability, multi-stage complex synthesis methods and poor reproducibility.The substitution and incorporation of different counter cations in these molecules and production of their salts is one of the solutions to overcome the mentioned weaknesses, which yields a versatile manner in these compounds [6][7][8].In fact, the nature of cationic unit effects on the chemical and physical properties of POMs.Therefore, the design of POM molecules can be developed based on the different kinds of substituted counter cations [7][8][9].Due to fact-growing demand for the application of such compounds, introduction of a simple, safe, low cost and high efficient synthesis strategy has favored.The most of the described synthesis methods in literature for the preparation of POMs require to severe acidic experimental conditions and complex equipment [10][11][12][13][14].Among the reported methods solid state microwave assisted technique is an interesting method.It is a simple, cost effective, environmental friendly and rapid method to synthesize materials in micro and nano scales.In this technique, the heating energy resulting from the reactant-microwave interactions leads to prepare the product at short reaction time.In addition, the use of effective organic reagents can accelerate the reaction and create the suitable conditions to produce nano-sized architectures [15].The use of organic driving agents such as urea promote the synthesis reaction and also, act as capping agent for the trapping of metal units in specific orientations to provide an appropriate morphology of product [16,17].To the best of our knowledge, the preparation of mixed ammonium salts of phosphomolybdate polyoxoanion in nano-sized particles by solid state microwave assisted strategy using urea as driving agent has not been yet reported.The present work reports a facile and rapid microwave assisted strategy for synthesis of the substituted Ag + /Ni 2+ ammonium salts of phosphomybdate nanoparticles.The product was characterized by FT-IR, XRD, EDX and SEM techniques.

Materials and synthesis method
All chemicals were purchased from Merck Co. and used without further purification.To prepare product, the stoichiometric amounts of metal nitrate salts were mixed to each other in the presence of urea and ammonium nitrate as driving agents in solid phase.The mixture was transferred into a domestic microwave oven with a power of 360 W for 7 min.The yellow colored powder was obtained and analyzed by FT-IR, XRD, SEM and EDX techniques.

Characterizations
The X-ray diffraction (XRD) patterns were recorded by a STOE powder diffraction system using CuKα radiation (wavelength, λ=1.54060Å).Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu-8400S spectrometer in the range of 400-4000 cm −1 using KBr pellets.Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy analysis (EDX) were taken on VEGA\TESCAN S360 with gold coating.

Results and Discussion
Fig. 1 indicates FT-IR spectrum of the prepared mixed ammonium polyoxoanion after solid state microwave heating reaction using urea with a power of 360 W for 7 min.The appeared peaks especially in low wavenumber range of 1100-500 cm -1 indicate the formation of ammonium molybdophosphate compound with a Keggin-type structure [18].The observed peaks at 1062, 962, 864 and 788 cm -1 are attributed to the asymmetric stretching vibrations of P-Oa, Mo-Od, Mo-Ob-Mo, Mo-Oc-Mo, respectively.The marked oxygen atoms of Oa, Ob, Oc and Od refer to the constitutive oxygen atoms of the Keggin structure; Oa indicates the O atom shared between PO4 tetrahedron and a trimetallic group, Ob is the oxygen atom linking two trimetallic groups, Oc is the O atom linking two MoO6 octahedral group inside a trimetallic unit and Od refers to the terminal oxygen atom of this structure.The characteristic band at 595 cm -1 can be related to the P-O bending vibration frequency [7,18].The peaks at 1413, 1625 and 3207 cm -1 are assigned to the vibration frequencies of ammonium ions and H2O molecules, respectively [7,19].Since it is observed no significant difference between NH4PMo12 FT-IR spectrum (shown in the inset of Fig. 1.) and that of XPMo12 salt, it can be concluded that the primary Keggin structure has been remained and only the ammonium ions have been partially substituted by X ions.The elemental analysis determined by EDX (shown in Fig. 3) revealed the presence of Mo, N, P, Ag and Ni elements in the prepared product.Based on the EDX and ICP results, the molecular formula of product was obtained as (NH4)1.4Ag0.8Ni0.4PMo12O40.4H2O.using urea as the organic driving agent in the power of 360 W for 7 min.These images revealed a micro-sized spherical particulate morphology for synthesized PMo12 that it goes to the nano-scale with average particle size of 41 nm by incorporating the partial amounts of other counter cations to its network.It is proposed that the employed urea in addition to burning as fuel can play an important role in the arrangement of building blocks of compound due to its composition and structural nature.

Conclusions
In a summary, we successfully used a simple and rapid method for synthesis of mixed ammonium salt of PMo12 substituted by silver and nickel counter cations using microwave heating treatment in solid phase.With the attention to the inherent nature of the employed counter ions, this group of the synthesized mixed metal oxides can be introduced as the favorable candidates for catalysis and gas sensing works.The experimental strong point of this strategy is the simplicity and rapid synthesis route in solid phase of target compound from initial materials without using complex method or materials.It can be a facile process to produce the various species of mixed polyoxoanions in nanoscale for the wide spectrum of applications.

Fig. 4
Fig. 4 presents the SEM images of the prepared samples of PMo12 (a), and AgNiPMo12 (b-c)

Fig. 4 .
Fig. 4. SEM images of the prepared PMo12 (a) and AgNiPMo12 (b-c) using urea as organic