Pure and Gd3+, Tb3+ and Ho3+-doped As2Ni3O8: A new visible light induced photocatalyst for the photodegradation of malachite green water pollutant

INTRODUCTION

Nickel ortho-arsenate (As₂Ni₃O₈) is composed of two As⁵⁺ and Ni²⁺ cations. The compound has two crystal structures including orthorhombic and monoclinic structures. When the compound crystallizes in orthorhombic crystal system, the space group and lattice parameters are Cmca and a = 5.943, b = 11.263, and c = 8.164 Å, respectively. The space group and lattice parameters of 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 for 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]. Hydrothermal [8] and solid state [9] methods have been reported for the synthesis of As₂Ni₃O₈ material.

A photocatalytic application for removing MG from aqueous waste is reported in the present work. MG is classified in the dyestuff industry as a triarylamine dye and used in pigment industry. MG is used as an antifungal and anti-protozoan agent in fisheries and aquaculture industry [10]. However, MG causes mutagenic, carcinogenic, and teratogenic effects to living organisms [11]. MG causes skin irritation, blurred vision or cause interference. MG inhalation may cause irritation to the respiratory tract, and in large quantities can cause tissue damage and inflammation of kidney [12,13]. So, we decided to study the performance of the semiconductor nanomaterial for the degradation of MG. Several metal oxides and sulfides have been used for the degradation of MG under different conditions summarized in ref [14].

In the present study, a facile solid state rout is applied for the first time to synthesize Gd³⁺, Tb³⁺ and Ho³⁺-doped As₂Ni₃O₈ powders using As₂O₃, Ni(NO₃)_2.6H₂O, Gd₂O₃, Tb₂O₃ and Ho₂O₃ at 850 ᵒC for 8 h. To the best of our knowledge, there is no report on the synthesis of As₂Ni₃O₈ doped nanomaterials in the literature. Rietveld analysis was used to find the experimental crystallographic data of the as-prepared targets. The morphology of the obtained targets was studied by FESEM. The photocatalytic application of the synthesized doped As₂Ni₃O₈ nanomaterials was investigated for the first time for the degradation of MG under direct visible light irradiation. Experimental design method was used to optimize the factors affecting the degradation process.

MATERIALS AND METHODS

General remarks

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 by a field emission scanning electron microscope (Hitachi FE-SEM model S-4160). The elemental analyses of the obtained materials were examined with a Philips XL30 scanning electron microscope (Philips, Amsterdam, Netherlands). The concentration of MG was determined at 612 nm using a Shimadzu UV-visible1650 PC spectrophotometer. A BEL PHS-3BW pH-meter with a combined glass-Ag/AgCl electrode was used to adjust the solution pH value. A digital Lux meter model GM 1010 was used to measure the light intensity used in the photocatalytic process.

To prepare 70 ppm of MG solution, 70 mg of MG powder was dissolved in 1000 mL of distilled water. The pH value of the obtained solution was 4. To increase and decrease the pH value, 0.01 M solutions of NaOH and HCl, respectively, were used. In a typical experiment, certain amount (g) of the as-synthesized pure As₂Ni₃O₈ photocatalyst was added into 100 mL of MG solution and sonicated for 10 min in a dark room to establish an adsorption/desorption equilibrium between MG molecules and the surface of the photocatalyst. Afterwards, certain volume (mL) of H₂O₂ was added into the mixture solution followed by further magnetic stirring under visible light irradiation. When the designed time (min) was elapsed, the solution was drawn out and the photocatalyst was separated by centrifugation in order to measure the absorption spectrum of MG and calculate the MG concentration by UV-Vis spectroscopy. The photodegradation percentage of MG was calculated by the following formula:

 (1)

where, A₀ and A_t represent the initial absorbance of MG at 616 nm and the absorbance at time t, respectively.

Synthesis of Gd³⁺, Tb³⁺ and Ho³⁺ As₂Ni₃O₈ nanomaterials

The synthesis rout is according to our previously reported work [9]. In a typical synthetic solid state experiment, 0.198 g (1 mmol) of As₂O₃ (MW = 197.8 gmol⁻¹) and 0.297 g (1 mmol) of Ni(NO₃)₂.6H₂O (MW = 290.7 gmol⁻¹) 0.05 mmol of Gd₂O₃ (S₁) or Tb₂O₃ (S₂) or Ho₂O₃ (S₃) were mixed in a mortar and ground until a nearly homogeneous powder was obtained. The obtained powder was added into 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.

RESULTS AND DISCUSSIONS

Characterization

The phase composition of S₁, S₂ and S₃ 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: Y_obs-Y_calc; and the Bragg reflections positions are indicated by blue bars for monoclinic phase of As₂Ni₃O₈ [9]. The results showed that the patterns had a main As₂Ni₃O₈ crystal structure with the space group P12₁/c1 [8,9]. The ionic radii of the component ions are as follow: Ni²⁺ (CN=6): 0.69 Å, As⁵⁺ (CN=4): 0.34 Å, As⁵⁺ (CN=6): 0.46 Å, Gd³⁺ (CN=6): 0.94, Tb³⁺ (CN=6): 0.92, Ho³⁺ (CN=6): 0.91 Å [15].

Quantitative phase analysis was investigated with direct comparison method [16]. In this method, we compared the experimental line intensity of the impurity phases SrCO₃ for S₁ and S₃, and SrCO₃ and As₂O₃ for S₂ from the mixture to a line from the main phase (As₂Ni₃O₈) in the mixture. For this purpose, we chose the peaks with highest intensity for each phase at about 26.8˚, 29.8 and 37.6˚ for As₂O₃, SrCO₃ and As₂Ni₃O₈, respectively. The phase comparison values are summarized in Table 1. Table 1 shows that the purity and crystal phase growth of the samples are decreased more when Tb³⁺ is doped into the crystal system. As will be mentioned in Table 2, the dislocation density and strain values are increased by decreasing the crystallite size value.

The cell parameters values were calculated and refined by the rietveld analysis. According to the lattice volume data, it is clear that introducing the lanthanide ions into the crystal system has decreased the lattice size. However, the decreasing for Tb³⁺ doped material is lower than the other samples.

Table 2 shows the crystallite size data, dislocation density, strain and interplanar spacing (d) data for S₁ to S₃. The crystallite size data of the obtained nanomaterials is calculated by Scherrer equation (1):

 (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), B_1/2 of FWHM is the full width at half of its maximum intensity and θ is the half diffraction angle at which the peak is located. The crystallite size data show that the dopant type plays an important role to produce particles in nanometer size scale. The data included in Table 2 show that the crystallite sizes are decreased more when Tb₂O₃ is used as the dopant source.

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 crystallite size (D) by the relationship given below [17]:

It was found that the dislocation density was increased when the dopant ions were introduced into the crystal system. However, the data show that doping Tb³⁺ ion into the crystal cavity increases more the dislocation density due to the decreasing the crystallite sizes of the materials. The strain ε (10^–3) values were determined with by the following formula:

The variation in the strain as a function of the dopant ion type is included in Table 2. Increase in the strain value with changing the dopant ion is probably due to the retrograde in the degree of the crystallite of the obtained target. However, when Tb³⁺ is doped into the crystal system, the strain value is increased more.

The interplanar spacing data of the obtained monoclinic samples were also calculated and compared to the observed data.

The highest intensity peak with the (h k l) value of (004) at 2θ=35.26 was used in the above equation.

As we know, sin (92.9) is about 0.999. So,

Besides, the unit cell volume can be obtained from the below formula:

Where a, b and c are the lattice parameters and V is the cell volume.

Morphology analysis

Fig. 2 shows the FESEM images of the obtained nanomaterials. The images show that the morphology of the targets is sponge. However, the data indicate that the sponge structure of the obtained doped nanomaterials is different from the pure Ni₃As₂O₈ [9]. This difference is due to doping the lanthanide ions into the crystal system. It is obvious from the images that when Gd³⁺ and Tb³⁺ are doped into the crystal system, some particles are observed on the surface of the sponge. But this observation is not seen for Ho³⁺ doped material. The particle size on the surface of the sponge is about 40 nm.

Fig. 3a - c illustrates the EDX analyses for the samples doped theoretically with 0.05 mmol of the lanthanide dopants ions into the crystal system which verify the doping and the compositional analysis of Gd³⁺ or Tb³⁺ and/or Ho³⁺ in Ni₃As₂O₈. The peaks corresponded to Gd or Tb or Ho and (Ni, As and O) atoms present in the samples are labeled. The respective energy positions and the specific X-ray lines from various elements are also indicated. The A% values of the dopants in the obtained samples and investigating the capacity of the sample to accept the ions in the crystal systems is reported. The A% values are 0.82, 0.79 and 0.78 for Gd³⁺, Tb³⁺ and Ho³⁺ doped nanomaterials. The data reveal that when a heavier lanthanide ion is included in the crystal system, the capacity of the crystal system to accept the ions is decreased.

Photocatalytic activity

Experimental design for achieving optimal conditions in MG degradation process

Experimental design approach is attracting attention in the literature for exploring the optimal level of factors affecting chemical reactions. One of the most common designs is full factorial design. Response surface methodology (RSM) is a mathematical and statistical method, which analyzes experimental design by applying an empirical model. The adequacy of the applied model is checked using analysis of variance (ANOVA) which needs some replicate experiments. The response is the measured degradation yield (Y%). Different possible combinations of these factors are designed which are collected in table 3. The observed data of the factorial design was fitted to a linear response model. The below equation shows the relation between the factors and the yield, Y%, based on the first order model:

Y (%) = 55.35 + 19.65 × Catalyst + 13.99 × H₂O₂ + 5.24 × Time

Prior to the ANOVA analysis, low and high factor levels were coded to -1 and +1, respectively (Table 4). As could be seen from the ANOVA results listed in table 5, the p-value of the regression is smaller than 0.05. This indicates that the model is significant at a high level of confidence (95%). The p-value probability of lack of fit is also greater than 0.05, which confirms the significance of the model. Also the coefficient of determination (the R-square, adjusted–R-square) was used to express the quality of fit of polynomial model equation. In this case, R² value of variation fitting for Y% = 94 indicated a high degree of correlation between the response and the independent factors (R² = 0.97). The high value of adjusted regression coefficient (R²-adj = 0.90) was also another index for the high significance of the proposed model. This means that the difference between the experimental and predicted responses is negligible. The dispersal of residuals is also shown in Fig. 4. As it is evident in this figure, the data points obtained consistently appear on a straight trend line, demonstrating that there is no obvious dispersal. It is clear that the residuals are in the range of -3 to +3 confirming more the reliability of the model.

To illustrate the effects of the factors in the above models, the two and three–dimensional (3D) response surface plot of the response (when the time was fixed at optimal level and the other two were allowed to vary) is shown in Fig. 5. The figure represents the 3D and counter plots related to the interaction of AB.

Fig. 6 shows the optimum values of the parameters involving in the MG degradation process obtained by the mentioned model. The data show that the optimum values for catalyst, H₂O₂ and time are 60 mg, 0.10 mL and 90 min, respectively.

A three-level CCD (Central Composite Design) with three factors (catalyst (A), H₂O₂ (B) and time (C)) was used to investigate the effects of the factors. The condition of 20 experiments designed by CCD accompanied to dye degradation percentage (response (R%)) are also given in Table 3. As shown in table 4, the independent variables (H₂O₂ volume (A), catalyst amount (B) and stirring time (C)) are given the coded form (-α, -1, 0, +1, +α).

Fig. 7 shows the dye degradation graphs for the obtained materials at the optimum conditions. Fig. 7 a shows the effect of solution pH on the degradation yield. It is clear that by increasing the pH value, the degradation of dye is increased. The data reveal data the existance of H⁺ ion decreases the degradation yield. But, when the concentration of H⁺ is decreased or the concentration of OH^- is increased, the degradation yield is increased considerably. Fig. 7b shows the reusability test for the as-prepared catalyst in the photodegradation process. It indicates that the degradation is high until run 4. Fig. 7c presents the dye solution volume influence on the photodegradation yield. The data indicate that when the dye volume is increased more up to 140 mL, the yield is still desirable. However, when the solution volume is increased more, the degrataion yield is decreased considerably. It is due to the decreasing the adsorption of dye on the catalyst; so the process is continued slowly at the desired time. Fig. 7d shows the effect of the dye concentration influence on the degradation yield. According to the data, it is clear that with increasing the dye concentration to 80 ppm, the degradation yield is decreased by about 20 %. The observation indicates that increasing the dye concentration has a main effect on the retrograding the degradation yield and the process is related to the concentration of the dye, considerably.

We present a copmarison study to show the merit of the present work with the other reported photocatalytic degradation researches (table 6).

CONCLUSION

In this work, Gd³⁺, Tb³⁺ and Ho³⁺-doped Ni₃As₂O₈ nanomaterial was synthesized via a facile solid state method. PXRD patterns and structural analyses done by the FullProf program employing profile matching. The rietveld analyses showed that the obtained materials were crystallized well in monoclinic crystal structure with the space group P12₁/c1. FESEM images showed the obtained materials had sponge morphology. The data showed that doping the lanthanid ions into the crystal system affect on the morphology of the obtained nanomaterials. The photocatalytic degradation data showed that Ni₃As₂O₈ had excellent efficiency for the removal of MG from aqueous solution. It was found that the optimum condition was 0.1 mL H₂O₂, 60 mg catalyst and 90 min for the removal of 100 mL of 70 ppm MG solution. The degradation yield in these conditions was 94 % for pure Ni₃As₂O₈.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.