Nickel ortho-arsenate (As2Ni3O8) is composed of the two As5+ and Ni2+ cations. The compound has two crystal structure including orthorhombic and monoclinic structures. When the compound crystallizes in the orthorhombic crystal structure, the space group and lattice parameters of the target are Cmca and a = 5.943, b = 11.263, and c = 8.164 Å, respectively. The space group and lattice parameters of the compound crystallized in the monoclinic crystal system are P121/c1 and a = 5.764, b = 9.559, and c = 10.194 Å, respectively. As2Ni3O8 contains NiO and As2O5 in its crystal system. NiO is interested in its electrical, magnetic and catalytic properties. Nickel (II) oxide is used in several applications such as catalyst production, electrochromic films, fuel cell electrodes, gas sensors, and etc. [1-5]. As2O3 is used in some fields including wood reservoir, and production of arsenic organic materials for various applications [6-7]. There is only one method for the synthesis of As2Ni3O8. The hydrothermal method has been used for the synthesis of the single crystal of the target. Ni3(AsO4)2.8H2O was used as raw material in this method .
In the present study, a facile solid- state route is applied for the first time to synthesize nanostructured As2Ni3O8 powders using As2O3, Ni(CH3COO)2.2H2O and Ni(NO3)2.6H2O at 850 and 950 ᵒC for 8 h. Besides, there is no report on the studying the optical properties of the obtained materials. So, the direct optical band gaps using UV-Vis spectra are calculated and related to the reaction temperature and crystallite sizes of the obtained targets. FTIR analysis is also studied.
All chemicals were of analytical grade, obtained from commercial sources, and used without further purification. Phase identifications were performed on a powder X-ray diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation. The morphology of the obtained materials was examined with a field emission scanning electron microscope (Hitachi FE-SEM model S-٤١٦٠). The elemental analyses of the obtained materials were examined with a Philips XL٣٠ scanning electron microscope (Philips, Amsterdam, Netherlands). The surface area and pore volume and average nanoparticles size were calculated using the Brunauer-Emmett-Teller (BET) equation. Pore size distributions, pore volume and pore surface area were calculated by the Barrett-Joyner-Halenda (BJH) method. BET surface areas were acquired on a Beckman Coulter SA3100 Surface Area Analyzer. Absorption spectra were recorded on an Analytik Jena Specord 40 (Analytik Jena AG Analytical Instrumentation, Jena, Germany). FTIR spectrum was recorded on a Tensor 27 spectrometer (Bruker Corporation, Germany).
Synthesis of As2Ni3O8 nanomaterials
In a typical synthetic solid state experiment, 0.١٩٨ g (1 mmol) of As2O3 (MW = ١٩٧.8 gmol−1) and 0.297 g (1 mmol) of Ni (NO3)2.6H2O (MW = 290.7 gmol−1) (S1) or 0.248 g (1 mmol) of Ni (CH3COO)2.2H2O (MW = 248.0 gmol−1) (S2) were mixed in a mortar and ground until a nearly homogeneous powder was obtained. The obtained powder was added to a 25 mL crucible and treated thermally in one step at 850 ᵒC for 8 h. When the reaction was completed, the crucible was cooled normally in the furnace to the room temperature. The obtained powder was collected for further analyses.
For the synthesis of S3 and S4, the synthesis procedures explained for S1 and S2, respectively, were used. In this case, the reaction temperature is 950 ᵒC.
RESULTS AND DISCUSSIONS
The phase composition of S1, S2, S3, and S4 were examined by powder X-ray diffraction technique. Fig. 1(a-d) shows the PXRD patterns of the obtained materials in the 2θ range 10-90° as well as the structural analyses performed by the FullProf program. The structural analyses were performed employing profile matching with constant scale factors. Red lines are the observed intensities; the black ones are the calculated data; the blue ones are the difference: Yobs-Ycalc; and the Bragg reflections positions are indicated by blue and red bars for monoclinic phase of As2Ni3O8  and As2O3 [6, 7], respectively. The patterns have well fitted with monoclinic structure. The results showed that the patterns had a main As2Ni3O8 crystal structure with space group P121/c1 .
Quantitative phase analysis was investigated with direct comparison method. In this method, we compared the experimental line intensity of the impurity phase (As2O3) from the mixture to a line from the main phase (As2Ni3O8) in the mixture. For this purpose, we chose the peaks with the highest intensity for each phase at about 26.8˚ and 37.6˚ for As2O3 and As2Ni3O8 (S1-S3) and 35.2˚ for As2Ni3O8 (S4), respectively. The phase comparison values are summarized in Table 1. Table 1 shows that the purity of the obtained As2Ni3O8 is decreased at the two reaction temperatures 850 and 950 ᵒC when nickel acetate is used as raw material. However, the data show that increasing the reaction temperature introduces a compound with higher purity only up to 3%. Besides, when nickel nitrate is used, the material with 100 % purity is obtained at the two reaction temperatures. Besides, the counts values are included in Table 1. The data show that the crystal counts values of the targets are related considerably to the raw material type. However, the data show that the obtained materials are crystallized better at lower reaction temperature. The rietveld parameters are also included in Table 1. The data show the goodness of the refinements. It is obvious that χ2 value is smaller when the refinement is performed for S1 and S3. Besides, it is clear that the value for S1 is smaller than that of S3. It indicates that crystal growth amount is an important factor on the refinement goodness. Also, the other important factor is crystallite size. As will be mentioned in Table 2, the dislocation density and strain values are increased by decreasing the crystallite size. So, the rietveld refinement will be performed well when the crystallite size is increased.
Table 2 and (and graphically Fig. 2) shows the crystallite size data, dislocation density, strain, interplanar spacing (d) and cell parameters data for S1 to S4. The crystallite size data of the obtained nanomaterials is calculated by Scherrer equation (1):
In this equation, D is the entire thickness of the crystalline sample, λ is the X-ray diffraction wavelength (0.154 nm), K is the Scherrer constant (0.9), B1/2 of FWHM is the full width at half its maximum intensity and θ is the half diffraction angle at which the peak is located. The crystallite size data show that raw material type plays an important role to produce particles in nanometer size scale. The data included in Table 1 show that the crystallite sizes are decreased when nickel acetate is used as raw material. However, when the reaction temperature is increased, the value is increased when nickel nitrate is used as nickel source. Decreasing the D value for S4 is maybe due to the faster decomposition of the organic part of nickel acetate. This can introduce a smaller particle size compared to that for S2.
The value of the dislocation density (δ (lines/m2) ×1014) which is related to the number of defects in the crystal was calculated from the average values of the grain size (D) by the relationship given below:
The strain (å×١٠-3) values were determined with the use of the following formula:
The variation in the strain as a function of raw materials type and reaction temperature is included in Table 1. Increase in the strain with increasing the reaction temperature is probably due to the retrograde in the degree of crystallite of the obtained target. This is because of the deficiency of the crystals when the crystal sizes become smaller. So, the data in Table 1 indicate that the raw material type influences the crystallite size, strain and dislocation density values.
The interplanar spacing values are calculated using equations 4 and 5:
The highest intensity peak at 2θ ≈ 37.50 ᵒ, the (h k l) value of (0 4 0), and β = 92.93 ᵒ are used in the above equation.
As we know, sin (92.93) is about 1, cos (92.93) is -0.051. So,
And for S4, with using the peak at 2θ=35.26, the (h k l) value is (004). So
Besides, the unit cell volume can be obtained from the bellow formula:
Where a and c are the lattice parameters and V is the cell volume.
The cell parameters values were calculated and refined by the rietveld analysis. The data show that the increasing/decreasing trend for parameters a and c is in reverse with parameter b. Because the trend for parameters a and c is in accordance with that for D values, so it is suggested that there is a preference in crystal growth to the a.c plane when the reaction time is increased to 950 ᵒC. Fig. 2 shows the summary data of the obtained crystallographic data schematically. The data show that the changing trends of the data are similar to each other’s, except for parameter b.
Fig. 3 shows the FESEM images of the obtained nanomaterials. The images show that the morphology of the targets is sponge. It is clear that the sponge size of the two materials is homogeneous. However, the data show that the particle size of S1 is smaller than that for S3. The increasing the particle size is in accordance with the trend of the changing the crystallite size described in the PXRD analysis section. The images show that the particle sizes are about 50 - 70 nm for S1 and 100 - 150 nm for S3.
The synthesized powders were characterized by their surface area, average pore size, and average pore volume. Prior to N2-physical adsorption measurement, the samples were degassed at 150 °C for 120 min in the nitrogen atmosphere. So, the specific surface area (SBET) of the obtained materials was determined with adsorption-desorption isotherms of N2 at 77 K. The surface area, pore volume and average pore diameter of the synthesized materials are summarized in Table 3. From Table 3, it can be seen that the average surface area and pore volumes are about 4.669 m2 g-1 and 0.0089 cm3 g-1 for S1, 3.251 and 0.0065 cm3 g-1 for S3, respectively. The data summarized in Table 4 shows that the specific surface area of pores of S1 is larger than that of S3 and the pore width of S3 is larger than that of S1. So, the investigated results of BET and BJH measurements suggest that the surface area of S1 is larger than that of S2. This observation is in good accordance with the FESEM images.
The direct and indirect optical band gap energies of the As2Ni3O8 nanomaterials obtained from UV-Vis absorption spectra are shown in Fig. 4. According to the results of Pascual et al. , the relation between the absorption coefficient and incident photon energy can be written as (αhν)n = A(hν - Eg), where A is a constant and Eg is the direct band gap energy if n=2 and indirect band gap energy if n=0.5. The Band gap energies were evaluated from extrapolating the linear part of the curve to the energy axis. The direct optical band gaps were 2.50, 2.65, 2.30 and 2.80 eV for S1, S2, S3, and S4, respectively. The data show that the band gap energy values for the pure materials (S1 and S3) are smaller than those for the impure materials (S2 and S4). This can be due to the composite nature (As2O3and As2Ni3O8) of the obtained materials when nickel acetate is used as raw material. Besides, the value for S3 is smaller than S1. This can be due to the larger crystallite sizes of S3 compared to S1. This is due to the affect of D (nm) value in the calculation of the band gap energies.
Fig. 5 show the FTIR spectrum of S3. There are some peaks at around 476, 518, 703, 754, 788, 803 and 1394 cm-1. The strong bands at 476 and 518 cm-1 are assigned to the As–Obridging stretches. The band at about 754 cm-1 is attributed to the symmetric As–Oterminal stretches. The band at around 803 cm-1 is assigned to doubly degenerate stretching vibrations of As-O bonds [10-13].
In this work, As2Ni3O8 nanomaterial was synthesized via a facile solid state method. Two reaction parameter including raw material type and reaction temperature were investigated for the synthesis of the targets. PXRD patterns and structural analyses done by the FullProf program employing profile matching showed that nickel source in the reaction mixture had a main influence on the crystal size, growth and purity of the obtained targets. In the present work, we found that nickel nitrate created a highly crystalline and pure As2Ni3O8 structures. However nickel acetate created the targets with lower purity and crystal phase growth, it produced that the samples with smaller crystallite sizes. Reaction temperature changing showed that the parameter affected the crystal growth amount of the obtained material. In this case, increasing the reaction temperature decreased the crystal growth of the target. It showed that the crystal phase of the sample was unstable at the higher temperature than 850 ᵒC. Also, increasing the parameter couldn’t improve the purity of the targets considerably when nickel acetate was used. FESEM images showed the obtained materials had sponge morphology. The data showed that increasing the reaction temperature could only decrease the particle size of the target and didn’t change the morphology of the samples. Also, the direct optical band gaps were calculated and related to the reaction temperature. It was found that the band gaps were decreased with increasing the reaction temperature from 850 °C to 950 °C.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.