In recent years, many researchers have shown interest in vanadium-based metal oxides for their electronic technological applications such as lithium rechargeable battery [1, 2] due to their layered nature, excellent kinetics, and heterogeneous catalytic feature [3, 4]. Besides, many vanadates contain crystal water which exists as OH groups on the external and internal surfaces. This enhances the reversible capacity, because the extra oxygen in the crystal water facilitates the formation of “Li−O” type binding in the low voltage reduction process [5, 6]. The most important advantage of these vanadates is their lower formation energy compared to other oxide compounds, thus rapidly opening up to synthesis with colloidal chemistry, while at the same time preventing agglomeration of nanoparticles at higher reaction temperatures. The methods used to prepare these materials are mostly the hydrothermal [1, 2, 4] based techniques requiring high temperature and pressure environment, though the amount of product obtained with these techniques is quite small.
Despite the great advantages of precipitation method including simplicity, high reaction rates, and cost-effective synthetic procedure for preparing of vanadate nanostructures, not many works have been performed using this technique [3, 7]. Recently, several novel facile methods have been proposed to synthesize transition metal vanadates under an ambient environment [8, 9].
We have recently explored a new approach for preparing nanovanadate compounds at room temperature [10, 11]. This approach is based on the increase of solubility of NH4VO3 in water by adding NaOH. The general equations for the synthesis are as follow:
NH4VO3 +3NaOHNa3VO4 + NH3 + 2H2O (1)
Na3VO4 + M(NO3)3 .nH2O MVO4+ 3NaNO3 +nH2O (2.1)
2Na3VO4 + 3M(NO3)2 .nH2O M3(OH)2V2O7 + 6NaNO3 +(3n-1)H2O (2.2)
Herein, we have selected cobalt, nickel, copper and zinc as metal owing to their wide range of applications, thus M3(OH)2V2O7·nH2O (M; Co, Ni, Cu, Zn) compounds were synthesized according to Equation 2.2 since their ion valences are 2+.
Volborthite (Cu3(OH)2V2O7.2H2O) has an interesting crystalline structure that is of porous framework  as shown in Fig. 1, so it is a potential candidate for many applications in different areas. In addition, as a metastable phase, it can be used as an active starting material for preparing other new advanced mixed oxides based on vanadium and copper. The Cu3(OH)2V2O7.2H2O nanoparticles were successfully synthesized via hydrothermal method . Their magnetic and electrochemical properties as a positive electrode of lithium ion battery were studied by Ni et al [1, 13]. Zhang et al. [7, 14] also performed the cyclic voltammograms of the electrode materials made from the as-prepared Cu3(OH)2V2O7.2H2O and reported that nanowires, nanoflakes, and nanoparticles exhibited high discharge capacities and ideal shaped discharge curves. Melghit et al.  prepared a pure copper pyrovanadate by mixing copper monoxide Cu2O with a solution of vanadium oxide xerogel, V2O5.2H2O, for a long time. Besides, different copper-organic complexes such as [Cu(en)2]SO4, [Cu (TETA)] SO4 were used as Cu precursor in a precipitation method and then the synthesized copper pyrovanadate was added to the polymer solutions (Poly vinyl alcohol, poly styrene) to investigate its influence on the flame retardancy, by Arani et al. .
Martyite or zinc pyrovanadate, (Zn3(OH)2V2O7·٢H2O) was first prepared under hydrothermal conditions at 438 K in 1997  . It consists of alternated Zn–O layer and V–O layer with channels in V–O layer . Water molecules fill the large cavities; they are attached by hydrogen bonds to hydroxyl groups present in the structure (Fig. 1). It has the potential to be used as a negative material for the lithium ion battery due to the channel structure in the V-O layer . In addition, zinc pyrovanadate exhibits efficient UV-light-driven photocatalytic activity for the degradation of methylene blue [3, 4, 19, 20].
Recently, well-crystallized Zn3(OH)2V2O7.nH2O nanosheets were successfully synthesized by Ni et. al and their optical, magnetic and photoluminescence properties were reported in a series of papers [2, 21-23]. However, studies on nano- and micro-scale Zn3(OH)2V2O7.nH2O particles are limited [24, 25].
To the best of our knowledge, experimental report about a simple and rapid chemical precipitation synthesis of Cu3(OH)2V2O7.nH2O and Zn3(OH)2V2O7·nH2O without surfactant or template has been rare by now. Moreover there is only one report  for hydrothermal synthesis of Co3(OH)2V2O7.nH2O and no report available for Ni3(OH)2V2O7.nH2O.
In this study, M3(OH)2V2O7·nH2O ( M : Co, Ni, Cu, Zn) nanostructures were synthesized by a novel simple and economic precipitation route which will offer great opportunities for the scale-up preparation of novel type metal vanadate nanostructures. In addition to examining the effect of the metal type in the study, the reactant concentration effect on the structural properties of Co compounds was investigated. Remarkable changes were observed in the crystallinity, particle shape and size of samples with changing metal type. Moreover, the crystallinity of Co samples was improved by increasing the reactant concentration.
All the chemicals were of analytical grade and used as received without further purification. Cobalt nitrate (Co(NO3)2.6H2O),nickel nitrate (Ni(NO3)2 .6H2O), copper nitrate (Cu(NO3)2.3H2O), zinc nitrate (Zn(NO3)2.4H2O), sodium hydroxide (NaOH) and ammonium metavanadate (NH4VO3) were purchased from Merck.
In a typical synthesis, 30 mmol NaOH and 10 mmol NH4VO3 were added to 20 ml of water and the solution was mixed for 10 min to prepare Na3VO4 aqueous solution. The metal solution was also prepared in 20 ml water using 15 mmol of metal nitrate. The solutions were mixed together suddenly in a reaction vessel at 2000 rpm for 30 minutes. Finally, the precipitate was formed at room temperature. After the samples were filtered, they were washed with distilled water for five times and ethanol for one times to remove the impurities completely and then dried at 70 oC for one day. The pH values of the precipitate solutions were measured at around 8.28 for all metals. No pH adjustment was made during the reaction. To increase pH, the initial concentration of NaOH was increased and pH was measured after the reaction was completed.
To investigate the concentration effect in Co samples, while the OH/VO4/Co ratio was kept constant at 6/2/3, the ionic quantities were increased from 30/10/15 (mmol) to 100/33,33/50 (mmol). Thus, eight samples with different concentrations were synthesized and labeled as Co1 (30/10/15 (mmol)), Co2 (40/13.33/20 (mmol)), Co3 (50/16.67/25 (mmol)), Co4 (60/20/30 (mmol)), Co5 (70/23.33/35 (mmol)), Co6 (80/26.67/40 (mmol)), Co7 (90/30/45 (mmol)), and Co8 (100/33.33/50 (mmol)).
The crystallinity and phase studies were carried out using Rigaku Rint 2200 X-ray diffraction (XRD) analyzer with Cu Ka radiation (1.54059 Å) at 30 mA and 40 kV. The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded using Perkin Elmer 65 model FT-IR spectrometer in the range of 4000–600 cm-1. High resolution transmission electron microscopy (HRTEM) investigations were performed on JEOL-2100 HRTEM operating at 200 kV (LaB6 filament). Images were taken by Gatan Model 794 Slow Scan CCD Camera and also by Gatan Model 833 Orius SC200D CCD camera. Carbon support film coated copper TEM grids (Electron Microscopy Sciences, CF200-Cu, 200 mesh) were used. Additionally, powder samples were coated with 3 nm Au/Pd and visualized under high vacuum with a FEI Quanta 200 FEG scanning electron microscope (SEM) equipped with ETD detector. Thermo-gravimetric analysis (TGA) was carried out using powder samples (∼10mg) with a heating rate of 10 oC/min in air atmosphere up to 600 oC using a Perkin-Elmer TG-DTA analyzer. UV-visible diffuse reflectance measurements were performed by T80 UV/Vis spectrometer (PG Instruments). Elemental analysis was performed by Rikagu ZSX Premius model X-ray fluorescence (XRF). Magnetizations of samples were measured by using a Vibrating Sample Magnetometer (VSM) in Physical Property Measurement System (PPMS, Quantum Design).
RESULTS AND DISCUSSION
Effect of metal type
M3(OH)2V2O7.nH2O nanostructures were successfully synthesized using Co, Ni, Cu, Zn transition metals. As can be seen in XRD graphics of Fig. 2, the Cu and Zn compounds are more crystalline than the ones of Co and Ni. Moreover Ni compound has a completely amorphous phase.
As pointed at Table 1, these transition metals have a strong tendency to complex with the NH3 molecules released in the reaction, shown in Equation 1. Such complexes may prevent vanadate ion collapse or may interfere with nuclei growth by cage effects, thus affecting the crystallinity of the samples. Comparing the tendency for complex formation of ions, Ni and Co are much more willing than Zn and Cu, because they generate a coordination sphere with six ammonia ligands. When a further comparison is made, considering the constants in Table 1, the most desirable ion is nickel [Ni(NH3)6,Kf : 2x10 8] and the most reluctant ion is zinc [Zn(NH3)4 2+,Kf : 7.8x108 ], which is compatible with the order of crystallinity of the products: Zn> Cu> Co> Ni.
Volborthite, Cu3V2O7(OH)2·nH2O, adopts monoclinic symmetry due to the significant Jahn–Teller distortion of one of the two symmetrically non-equivalent CuO6 octahedral. In Fig. 2(c), all the diffraction peaks can be readily indexed to the pure phase of Cu3V2O7(OH)2·nH2O with the monoclinic structure [JCPDS-ICDD Card No. 80-1170]. Strong and sharp peeks suggest that the as-synthesized products are well crystallized.
The XRD pattern of the as-prepared Zn3(OH)2V2O7·nH2O product is shown in Fig. 2(d). All diffraction peaks are indexed in hexagonal unit cell similar to the reported unit cell for zinc pyrovanadate (JCPDS card No. 50-0570) [2, 3, 21-23].
The sizes and morphologies of the products were characterized by HRTEM and SEM. Fig. 3 shows the HRTEM images of the products in a typical procedure. When Co and Ni were used in the synthesis of the products, very small (~9 nm) and irregularly shaped nanoparticles were obtained. In a previous study, Co3V2O7(OH)2·nH2O nanosheets with larger sizes (200 nm to 2 μm) were synthesized by a hydrothermal method but again with irregular shapes . Unfortunately, for nickel, we did not find a study in the literature for comparison . As mentioned earlier, the reason for the small size may be the high complexing effect of the ammonium ions with Ni and Co and as a result, poor crystallization. Another reason might be due to a higher coordination number that provide a better cage effect.
From Fig. 3 and Fig. 4, we can see that the shapes of Cu3V2O7(OH)2·nH2O and Zn3V2O7(OH)2·nH2O are nanoflake or nanoplate in accordance to the literature . Fig. 4 (b, d) is a low magnification that exhibits a large quantity of nanoplates for Cu and Zn compounds. While the thickness of Cu3V2O7(OH)2·nH2O nanoplates is about 25 nm, the thickness of Zn3V2O7(OH)2·nH2O nanoplates is less, about 20 nm. Both have an average diameter of less than 500 nm. High and low magnification SEM images of Zn3V2O7 (OH)2·nH2O in Fig. 4 (e, f) clearly reveal 3D structure of nanoplates and their tendency to stick to each other (aggregation).
For further clarification of the composition of the samples, their thermal behaviors are investigated by means of TG measurement in air atmosphere at a heating rate of 10 K min−1. The TGA curves at Fig. 5 exhibit the weight losses between 25 oC and 600 oC due to the elimination of water molecules (adsorbed or crystal), hydroxyl groups and the phase transformations [2, 13, 15, 25]. A general reaction during the thermal process would likely be as follows:
M3(OH)2V2O7 . nH2O + heat M3V2O8 + (n+1) H2O (M= Co, Ni, Cu, Zn) (3)
According to the elimination temperatures listed in Table 1, Co and Ni samples in the amorphous form release crystal waters at lower temperatures compared to Cu and Zn samples when exposed to heat. However, their phase formation temperatures are higher than those of Cu and Zn. By using total weight loss and Equation 3, n values can be deduced as shown in Table 2.
The mol number (2.8) of water found in the Zn3(OH)2V2O7 .nH2O and Co3(OH)2V2O7. nH2O compounds was higher than that reported in the literature [4, 24, 26]. This is probably due to the fact that the crystals are arranged in abundant water. However, Cu3(OH)2V2O7.nH2O included only 2 moles of water as in previous reports [7, 14-16], because its crystal lattice and vacancies do not allow more water molecules to penetrate.
FT-IR spectrums of the samples are depicted in the wavelength region of 600-4000 cm-1 in Fig. 6. While the spectral shapes of the samples are very similar, the locations and intensities of the peaks are different due to use of different metals. Several intense absorption peaks in 600–950 cm-1 region correspond to V–O–M (Co, Ni, Cu, Zn), V-O-V symmetric and asymmetric vibrations. Moreover, the peaks around 3500 cm-1 and 1620 cm-1 are attributed to the symmetric stretching vibration and bending vibration of H–O–H in H2O molecules, respectively. The other strong and broad peaks around 3200 cm-1 belong to the OH group in the framework. The absorption peaks of Ni3(OH)2V2O7. 5.6 H2O in 3000–3500 cm-1 region is stronger than the others because of the presence of more water molecules and amorphous structure. Obviously, all identifying functional groups from IR spectrums are consistent with the previous reports [1, 3, 15, 24].
Fig. 7 shows UV-Vis reflection spectrum which is a useful tool for determining optical band gap in semiconductor materials. At all samples, it displayed a high reflection in the region from 400 to 800 nm. And then, the reflection decreased sharply in the range from 300 to 400 nm. The optical band gap (Eg) was calculated using the following equation: , where is the photo energy, k is a constant relative to the material, (R: reflectance) and n is either 2 for direct transition or ½ for indirect transition. The band gaps of the samples were deduced by plotting (F(R) · hν)2 vs. hν as shown in Fig. 7. These are 3.62 (Co), 3.77 (Ni), 3.22, 3.80, 4.34 (Cu) and 3.63 (Zn) eV, which are consistent with the literature values [4, 9, 20, 26] .
Effect of concentration at Co and Ni samples
In order to obtain more crystalline nickel hydroxy vanadate nanoparticles, we firstly increased the concentration of reactants by maintaining the other parameters. Then, NaOH concentration was increased from 60 to 100 mmol at 60/20/30 (mmol) of reactant concentration, so, the final pH values measured also increased from 8.27 to 12.5, whereas for Ni, we could not get a good crystal product at all. Thus, XRF analysis was done for Ni compound with amorphous structure. According to the XRF analysis values in Fig. 8, the Ni/V ratio in the compound is 3/2. Moreover, Ni3(OH)2V2O7.nH2O was calcined at 600 °C for 3 hours for testing the crystallite composition of Ni nanoparticles and then the XRD analysis was performed as shown in Fig. 8.
Those diffraction peaks are in accordance with Ni3V2O8 which is in agreement with JCPDS, No 74-1485 . Therefore, this structure confirms the thermal procedure at Eq.3, and the estimated compound formulation is found as Ni3(OH)2V2O7 .5.6 H2O.
In contrast to nickel, good crystalline cobalt hydroxy vanadate nanoparticles would be obtained by increasing the concentration of reactants. The XRD patterns in Fig. 9 show that the peaks of Co6, Co7 and Co8 samples are considerably stronger and sharper than those of Co at Fig. 2 (a), indicating more crystalline phases.
Karpenkoite, Co3(OH)2V2O7.nH2O, is the cobalt analogue of Zn3(OH)2V2O7.nH2O  and its synthetic analogue . In particular, the stoichiometry of both minerals is identical (effective ionic radii are 0.74 Å and 0.75 Å for Zn2+ and Co2+, respectively) and their crystal morphology and physical properties are very close. Therefore, the diffraction peaks in Fig. 2(a) and Fig. 9 are in accordance with hexagonal phase of Zn3(OH)2V2O7.nH2O as also reported previously .
The effect of reactant concentrations on the structural properties of cobalt compounds (Co1-Co8) were also investigated by FT-IR, as shown in Fig. 10. As the concentration increased, the OH vibration band (around 3200 cm -1) in the crystal framework became stronger and more prominent, as a result of the increase in crystallinity. Interestingly, other bands growing stronger with increasing crystallinity are 1410 cm-1 and 1620 cm -1, attributed to the absorption of NH4+ and H2O molecules on the vanadate compounds, respectively [29, 30]. The V2O74- ion in the Co3(OH)2V2O7. 2.8H2O is composed of two tetrahedral units (O3V-O-VO3)4- formed by the bridgical use of oxygen. The absorption band at 883 cm-1, corresponding to the symmetric stretching of terminal VO3 units, shifted to 875 cm-1 and became stronger. Moreover, the antisymmetric stretching of bridging V-O-V units at 777 cm-1 also shifted to 763 cm -1 and expanded by overlapping the absorption band (the symmetric stretching of VOV) at 650 cm-1 . Thus, as the crystallization increases, the characteristic vibrations become stronger and more pronounced as previously reported .
Fig .11 shows UV-visible diffuse reflectance measurements of Co1 and Co7 samples and the graphics of (F(R) . )2 to . The calculated band gaps are 3.61 and 3.57 eV for Co1 and Co7, respectively. According to these results, it can be said that there is no significant effect of the reactant concentration on the optical properties of the Co samples.
Fig. 12a- b shows the M(H) curves of Co1, Co4, Co6 and Co8 samples measured in an applied magnetic field range from -5 T and +5 T at 5 K and 300 K temperatures, respectively. The magnetic hysteresis loops indicate ferromagnetic (FM) coupling for the samples at 5 K temperature. From Figs. 12a-b, the reduction in magnetization by increasing Co concentration is due to increasing Co2+ clusters with antiferromagnetically coupled moments, in good agreement with earlier reports [32, 33]. In contrast to low temperature experiment, the magnetization response to applied magnetic field becomes linear at room temperature that indicates samples are paramagnetic in nature (Fig. 12b). In addition to field dependence of magnetization experiments, we performed temperature dependence of magnetization, M(T) measurements by sweeping temperature from 2 to 300 K for field-cooled (FC) and zero-field-cooled (ZFC) conditions under 100 Oe external magnetic field (Fig. 13). The ZFC and FC curves of samples nearly follow the same path that the magnetization first decreases sharply with increasing temperature (2-25 K) and stays almost zero magnetization even after further temperature (25-300 K). This region of curves is assigned to a typical paramagnetic behavior.
In order to compare the magnetic coupling degree of Co1, Co4, Co6 and Co8 samples, we plot the inverse susceptibility 1 as a function of T as shown in Fig. 14. The Curie-Weiss law can be applied to this Fig. in the temperature range of 25 to 200 K because the data in this part could be fitted to a straight line. These straight lines cut the temperature axis corresponding to Curie-Weiss temperature, θ. Since the sign and magnitude of θ are related to the strength of coupling between magnetic ions in the lattice [34, 35], we first determined the Curie-Weiss temperature using the following relation;
where C is the Curie constant. The θ values obtained from the Fig. 14 are -44.20, -48.74, -49.65 and -82.37 for Co1, Co4, Co6 and Co8 samples, respectively. Negative θ values indicate that the samples have antiferromagnetic interaction . It is interesting to point out that the magnitude of θ values increase with increasing Co concentration. It means that Co8 sample has a stronger antiferromagnetic interaction than Co1. Further magnetic analysis has been performed by the effective magnetic moment of the samples calculated using the following relation:
where NA, M and kB are the Avogadro number, molecular weights of the samples and Boltzmann constant, respectively. The effective magnetic moment of Co1, Co4, Co6 and Co8 samples are found to be 3.75, 3.42, 3.40 and 2.72µB, respectively. The trend of effective magnetic moment as a function of Co amount is consisted with the results of hysteresis measurements.
We demonstrated that the recently new approach proposed for the preparation of LnVO4 nanoparticles at room temperature is also successful for the preparation of M3(OH)2V2O7·nH2O (M : Co, Ni, Cu, Zn) nanostructures. Reaction conditions were mild and reaction process was simple and feasible, offering great opportunities for the scale-up preparation of metal vanadates with different morphologies. This study also shows how the metal type can affect the properties of the final products. For example, Ni3(OH)2V2O7.nH2O is always amorphous and includes very small (9 nm) nanoparticles with irregular shape. Co3(OH)2V2O7.nH2O nanoparticles also are very small and not good crystalline but it is possible to obtain better crystals by increasing the reactant concentrations. The best crystalline phase was observed for zinc and copper compounds. These results revealed a relation between the crystallinity and the tendency of these metals to complex with the NH3 molecules, for the first time. The shapes of Cu3V2O7(OH)2·nH2O and Zn3V2O7(OH)2·nH2O are nanoflake or nanoplate and both have an average diameter of less than 500 nm. Mol numbers of water calculating from TGA curves also changed depending on metal type. The compound formulas were found to be Co3(OH)2V2O7.2.8H2O, Ni3(OH)2V2O7. 5.6H2O, Cu3(OH)2V2O7.2H2O and Zn3(OH)2V2O7.2.8H2O. In the magnetic experiments, all Co samples show similar trend in both temperature and external field dependence magnetizations. Verified by the Curie-Weiss law, the magnetic coupling is getting weaker when Co amount increased in the structure..
This work was supported by Balikesir University Research Project, Grant no. BAP 2015/60.
CONFLICT OF INTEREST
The author declared to no conflict of interest.