Novel synthesis of Dy₂Ce₂O₇ nanostructures via a facile combustion route

Abstract

Dy₂Ce₂O₇ nanostructures were synthesized by the facile salt-assisted combustion method, with Dy(NO₃)₃·5H₂O and (NH₄)₂Ce(NO₃)₆ as dysprosium and cerium sources and various amino acids such as L-alanine, phenylalanine, glycine, valine and serine as fuel and capping agents in the presence of sodium chloride as a dispersing agent. Various amino acids as fuel in the presence of sodium chloride as a dispersing agent were employed to fabricate Dy₂Ce₂O₇ nanostructures for the first time. The as-prepared Dy₂Ce₂O₇ nanostructures were analyzed by energy dispersive X-ray microanalysis (EDX), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), Fourier transform infrared (FT-IR) spectroscopy, UV-vis diffuse reflectance spectroscopy and X-ray diffraction (XRD). According to the morphological investigations of the as-synthesized products, it was found that shape and size of the Dy₂Ce₂O₇ nanostructures are controlled by changing the amino acid type factor. The photocatalytic behavior of as-synthesized Dy₂Ce₂O₇ nanostructures was also evaluated by degradation of methyl orange dye as a water pollutant under UV light.

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Introduction

Dy₂Ce₂O₇ belongs to the family of rare earth-doped cerium oxide materials. Since rare earth-doped cerium oxide materials have excellent and substantial electrical, optical, mechanical and catalytic characteristics, they have become one of the most attractive and important materials for oxygen sensors, photocatalysts, thermal barrier coatings, solid oxide fuel cells (SOFC) and catalyst carriers.^1–7 So far, solid state reaction, citrate auto ignition, combustion and carbonate coprecipitation^8–14 have been reported to prepare rare earth-doped cerium oxide. It has been reported that the shape and grain size have main impact on the behavior of the nanostructured materials, and therefore they play the considerable and important role in the final utilizations of nanostructured materials. Thus, various preparation processes have been reporting to control the shape and grain size of nanostructured materials.^15–21 Until now, very limited numbers synthesis methods for controlling the morphology and particle size of earth-doped cerium oxide nanostructures have been suggested.^22–24 Of the different ways of fabrication of nanostructured materials, the combustion process is well-known as a reliable, cost-effective and facile approach to control the shape and size of nanostructured materials. This method includes a self-sustained reaction in homogeneous solution of various oxidizers (e.g., metal nitrates) and fuels (e.g., glycine).

This work describes a simple salt-assisted combustion way to synthesize Dy₂Ce₂O₇ nanostructures with the aid of Dy(NO₃)₃·5H₂O and (NH₄)₂Ce(NO₃)₆ as dysprosium and cerium sources and various amino acids such as L-alanine, phenylalanine, glycine, valine and serine as fuel and capping agent in presence of sodium chloride as dispersing agent. To our knowledge, it is the first time that various amino acids as fuel in presence of sodium chloride as dispersing agent are employed for the preparation of Dy₂Ce₂O₇ nanostructures and the influence of amino acid type factor on the shape and grain size of the Dy₂Ce₂O₇ nanostructures by the facile combustion approach are studied.

Experimental

Materials

All the chemicals applied for the synthesis of nanostructured Dy₂Ce₂O₇ including Dy(NO₃)₃·5H₂O, (NH₄)₂Ce(NO₃)₆, sodium chloride, L-alanine, phenylalanine, glycine, valine and serine were purchased from Merck Company and were applied as received without additional purification. GC-2550TG (Teif Gostar Faraz Company, Iran) were used for all chemical analyses.

Synthesis of Dy₂Ce₂O₇ nanostructures

Dy₂Ce₂O₇ nanostructures were synthesized by facile salt-assisted combustion way. In a typical experiment, 4 mmol of phenylalanine was dissolved in distilled water. Then 1 mmol of Dy(NO₃)₃·5H₂O, 1 mmol of (NH₄)₂Ce(NO₃)₆ and stoichiometric amount of sodium chloride were added to amino acid solution in turn and the prepared solution was kept constantly stirred at 60 °C for 1 h. Evaporation of the resulting solution lead to the preparation of loose powders which was dried at 110 °C. The prepared powders were calcined at 700 °C for 4 h (sample no. 5). The as-calcined sample was washed with hot distilled water and methanol to remove salt (sodium chloride). It was dried at 80 °C. Schematic diagram of the preparation of the Dy₂Ce₂O₇ nanostructures is demonstrated in Scheme 1. The effect of the amino acid type factor on the grain size and shape of the Dy₂Ce₂O₇ nanostructures were investigated (Table 1).

Scheme 1 Schematic diagram of the synthesis of the nanostructured Dy₂Ce₂O₇.

Table 1 The synthesis conditions of the Dy₂Ce₂O₇ prepared in this investigation

Sample no.	Amino acid type	Dispersing agent	Figure of FESEM images
1	Glycine	NaCl	4a
2	L-Alanine	NaCl	4b
3	Serine	NaCl	4c
4	Valine	NaCl	4d
5	Phenylalanine	NaCl	4e
6	Phenylalanine	—	7a
7	—	NaCl	7b

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Characterization

The EDS analysis of the as-obtained nanostructured Dy₂Ce₂O₇ was carried out by applying a Philips XL30 microscope. Powder X-ray diffraction (XRD) pattern of the as-obtained nanostructured Dy₂Ce₂O₇ was recorded by applying a diffractometer of Philips Company with X'PertPro monochromatized Cu Kα radiation (λ = 1.54 Å). Thermogravimetric-differential thermal analysis (TG-DTA) of the as-prepared sample was perform by applying a thermal gravimetric analysis instrument (Shimadzu TGA-50H) with a flow rate of 20.0 ml min⁻¹ and a heating rate of 10 °C min⁻¹. The UV-vis diffuse reflectance spectrum of the as-synthesized nanostructured Dy₂Ce₂O₇ was obtained on a UV-vis spectrophotometer (Shimadzu, UV-2550, Japan). Transmission electron microscope (TEM) images of as-prepared nanostructured Dy₂Ce₂O₇ were taken on a JEM-2100 with an accelerating voltage of 200 kV. Fourier transform infrared spectra of as-produced products were obtained on a Shimadzu Varian 4300 spectrophotometer in KBr pellets in the 400–4000 cm⁻¹ range. The particle sizes of the samples were estimated by using Digimizer software on FE-SEM images. FESEM images of nanostructured Dy₂Ce₂O₇ were visualized by applying a Tescan mira3 field emission scanning electron microscope (FESEM).

Photocatalytic test

The photocatalytic performance of as-prepared nanostructured Dy₂Ce₂O₇ was studied by employing of an anionic dye (methyl orange) solution. 50 ml of solution comprising the 0.04 g of the nanostructured Dy₂Ce₂O₇ and 0.001 g of the anionic dye in the quartz reactor was applied to evaluate the photocatalytic performance. This mixture was aerated and stirred (at 500 rpm) continuously for 1/2 h to reach adsorption–desorption equilibrium. Later, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV source of 400 W mercury lamps, and was irradiated at 365 nm. The quartz vessel and light source were placed inside a black box equipped with a fan to hinder UV leakage. The photocatalytic test was performed at room temperature. Aliquots of the mixture (3 ml) were taken at definite interval of times during the irradiation, and after centrifugation they were analyzed by a UV-vis spectrometer. The dye photodegradation percentage was estimated as follow:where A_t and A₀ are the absorbance value of dye solution at t and 0 min by a UV-vis spectrometer, respectively.

Results and discussion

The infrared spectra of the as-prepared powders (before calcination stage) and as-synthesized nanostructured Dy₂Ce₂O₇ (sample no. 5) are indicated in Fig. 1a and b, respectively. The band appears at 1375 and 787 cm⁻¹ in the Fig. 1a are attributable to the bending vibration of ionic CO₃²⁻, which prepared by the hydrolysis of the amino acid (phenylalanine).²⁵ The bands located at 1415 and 1055 cm⁻¹ corresponding to the several vibration modes of the NH₂ and CN groups,²⁵ which illustrates the presence of the free amino acid (phenylalanine) in the as-prepared powders (before calcination stage). The weak band appears at 2933 cm⁻¹ in the Fig. 1a may correspond to (C–H) stretching vibration of the free amino acid (phenylalanine).²⁶ The bands appear at 3431 and 1627 cm⁻¹ in the FT-IR spectrum of the as-obtained nanostructured Dy₂Ce₂O₇ (Fig. 1b) are corresponding to the v(OH) stretching and bending vibrations of physisorbed water molecules.²⁰ The characteristic bands of the nanostructured Dy₂Ce₂O₇ located at 1025 and 497 cm⁻¹ (ref. 27 and 28) (Fig. 1b).

Fig. 1 FT-IR spectra of the as-prepared powders (before calcination stage) (a) and as-formed nanostructured Dy₂Ce₂O₇ (sample no. 5) (b).

Thermal gravimetric (TGA) and differential thermal analyses (DTA) were employed to investigate the thermal stability of as-prepared powders (sample no. 5 before calcination stage). The TGA/DTA curve of as-obtained powders (sample no. 5 before calcination stage) is indicated in Fig. 2. Fig. 2 reveals that there are three mass loss stages. The happened weight loss in the 25–245 °C (indicating 4% weight loss) may be related to the vaporization of the surface moisture. Two occurred exothermic stages at the 250–700 °C (showing 60% weight loss) correspond to the auto-ignition stages and illustrate the combustion synthesis and the deletion of the residual combustible carbon, and the formation of nanostructured Dy₂Ce₂O₇.

Fig. 2 TGA/DTA curve of as-formed powders (sample no. 5 before calcination stage).

To examine the chemical purity of as-prepared nanostructured Dy₂Ce₂O₇ (sample no. 5), the EDS analysis was performed. Fig. 3a reveals that the sample no. 5 is composed of Dy, Ce and O elements.

Fig. 3 EDS (a) and XRD (b) patterns of the nanostructured Dy₂Ce₂O₇ (sample no. 5).

XRD analysis, which is very beneficial technique for determination of crystalline structure and mean crystallite size, was applied to examine the as-obtained nanostructured Dy₂Ce₂O₇ (sample no. 5). All the diffraction peaks in Fig. 3b are well-matched to pure fluorite (standard) Dy₂Ce₂O₇.⁸ No impurities are indicated in this pattern. The mean crystallite size of nanostructured Dy₂Ce₂O₇ (sample no. 5) determined by applying the Scherrer formula¹⁹ is 14 nm.

FESEM technique was employed to examine the influence of amino acid type factor on the shape and grain size of the Dy₂Ce₂O₇ nanostructures. To investigate the influence of the amino acid type factor on the grain size and shape of Dy₂Ce₂O₇, the five various amino acids as fuel including glycine, L-alanine, serine, valine and phenylalanine were applied (sample no. 1–5). Fig. 4a–e reveals FESEM images of sample no. 1–5 obtained in the presence of the glycine, L-alanine, serine, valine and phenylalanine, respectively. As demonstrated in Fig. 4a–e, by applying five various amino acids, the nanoparticles are formed. Among these prepared samples, sample no. 5 shows very uniform sphere-like nanoparticles with small grain size (Fig. 4e). Among these applied amino acid sorts (Scheme 2), phenylalanine has the highest steric hindrance influence. Phenylalanine can act as both fuel and capping agent by impeding from the aggregation of the prepared nanoparticles (Fig. 4e). In addition, FESEM images reveal that the enhancement in the steric hindrance influence leads to the size becomes small (Fig. 4a–e). It seems that the nucleation to be occurred rather than the particle growth by enhancing the steric hindrance influence. From the FESEM results, it can be deduced that phenylalanine was the best amino acid for pure Dy₂Ce₂O₇ nanostructures with very homogeneous spherical shape and small grain size (Fig. 4e). Typical histograms of the particle sizes for the sample no. 1–5 are depicted in Fig. 5a–e, respectively. By comparing the particle size distribution of the samples, it is found that by utilizing phenylalanine in the presence of sodium chloride, uniform sphere-like Dy₂Ce₂O₇ nanoparticles with small grain size forms.

Fig. 4 FESEM images of the sample no. 1, 2, 3, 4 and 5 synthesized in presence of glycine (a), L-alanine (b), serine (c), valine (d) and phenylalanine (e).

Scheme 2 Five various amino acids applied as fuel.

Fig. 5 (a–e) Particle size distribution of sample no. 1–5, respectively.

In order to elucidate the detailed morphology and grain size of the synthesized Dy₂Ce₂O₇ nanostructures in the presence of the phenylalanine (sample no. 5), TEM analysis was carried out. The TEM images (Fig. 6a–c) reveal that quasi-spherical Dy₂Ce₂O₇ nanoparticles with diameter from 20 to 50 nm are agglomerated. The high-resolution TEM (HRTEM) images of a single nanoparticle in Fig. 6d and e show that the nanoparticle is highly crystalline. The marked lattice fringes correspond to (222) plane in cubic phase of Dy₂Ce₂O₇ with a d-spacing of 0.156 nm.

Fig. 6 TEM (a–c) and HRTEM (d and e) images of the nanostructured Dy₂Ce₂O₇ (sample no. 5).

To examine the effect of the sodium chloride on the grain size and shape, the sample no. 6 was prepared in the absence of sodium chloride. Fig. 7a indicates FESEM image of the sample no. 6. It is noteworthy that high agglomerated particle-like structures were prepared. The sodium chloride as dispersing agent precipitates and melts on the surface of the formed nanoparticles and prevents from the sintering of the obtained nanoparticles.²⁹ It is apparent that the utilizing sodium chloride in presence of phenylalanine brings about that pure fluorite Dy₂Ce₂O₇ nanostructures with homogeneous sphere-like shape are prepared (Fig. 3b and 4e).

Fig. 7 FESEM images of the samples formed in absence of the sodium chloride (a) and phenylalanine (b).

In continuation, the influence of the phenylalanine on the grain size and morphology was examined (Fig. 7b). To study the effect of this factor, on reaction was carried out in absence of the phenylalanine (sample no. 7). The sample no. 7 reveals the high agglomerated particles/bulk structures (Fig. 7b). The phenylalanine as fuel with high steric hindrance influence plays a capping agent role. It seems that phenylalanine with high steric hindrance influence causes nucleation occurrence rather than the growth of particle. It can be clearly seen that utilizing phenylalanine in presence of sodium chloride as a dispersing agent causes to obtain very homogeneous spherical nanoparticles (Fig. 4e). Hence, the advantage of utilizing phenylalanine is that it results in very homogeneous spherical Dy₂Ce₂O₇ nanostructures preparation.

It has been shown that the band gap (E_g) has a substantial impact on the determining the features of the nanometer-scale compounds applied as photocatalyst and is frequently calculated from the UV-vis diffuse reflectance data. Fig. 8a reveals the UV-vis diffuse reflectance spectrum of the as-formed nanostructured Dy₂Ce₂O₇ (sample no. 5). In this spectrum, the absorption band is seen at 352 nm. The E_g quantity can be estimated based on the UV-vis diffuse reflectance data by utilizing Tauc's equation.²⁰ The E_g quantity of the nanostructured Dy₂Ce₂O₇ was calculated by extrapolating the linear section of the plot of (αhν)² against hν to the energy axis (inset in Fig. 8a). The E_g quantity of the as-synthesized Dy₂Ce₂O₇ calculated to be 3.28 eV. According to the determined E_g quantity, as-obtained Dy₂Ce₂O₇ sample may be applied as the photocatalyst.

Fig. 8 UV-vis diffuse reflectance spectrum of the as-prepared nanostructured Dy₂Ce₂O₇ (sample no. 5) (a), photocatalytic anionic dye (methyl orange) degradation of sample no. 1 and 5 (b) and reaction mechanism of methyl orange dye photodegradation over Dy₂Ce₂O₇ under UV light irradiation (c) (inset: the curve of (αhν)² against hν).

The effect of morphology on photocatalytic behaviour was examined by monitoring the photodegradation of methyl orange (anionic dye) as water pollution in an aqueous solution over as-obtained sample no. 1 (synthesized in presence of glycine) and 5 (prepared in presence of phenylalanine) with different morphology under UV light illumination (Fig. 8). Fig. 8b reveals the obtained results. No methyl orange dye was practically broken down after 60 min without applying UV light illumination or Dy₂Ce₂O₇. This observation revealed that the contribution of self-degradation was insignificant. The proposed mechanism of the photocatalytic degradation of the methyl orange dye can be assumed as:

Dy₂Ce₂O₇ + hν → Dy₂Ce₂O₇* + e⁻ + h⁺

h⁺ + H₂O → OH˙

e⁻ + O₂ →˙O₂⁻˙

OH˙ + O₂⁻˙+ methyl orange dye → degradation products

Utilizing photocatalytic calculations by eqn (1), the methyl orange dye degradation was about 42 and 80% over sample no. 1 and 5, respectively, after 60 min illumination of UV light, and Dy₂Ce₂O₇ with uniform sphere-like shape and small grain size illustrated high photocatalytic activity. This obtained results reveal that as-obtained nanostructured Dy₂Ce₂O₇ (sample no. 5) have high potential to be utilized as suitable and favorable material for photocatalytic usages under illumination of UV light. The heterogeneous photocatalytic processes have diffusion, adsorption and reaction stages. It is generally accepted that the favorable distribution of the pore has advantageous and key impact on the diffusion of the reactants and products, and thus effects on the photocatalytic behavior. In this investigation, the enhanced photocatalytic activity of sample no. 5 with uniform sphere-like shape and small grain size may be owing to favorable distribution of the pore, high hydroxyl quantity and high separation rate of charge carriers³⁰ (Fig. 8c).

Conclusions

A facile salt-assisted combustion way has been developed to synthesize pure fluorite Dy₂Ce₂O₇ nanostructures by applying Dy(NO₃)₃·5H₂O and (NH₄)₂Ce(NO₃)₆ as dysprosium and cerium sources and various amino acids such as L-alanine, phenylalanine, glycine, valine and serine as fuel and capping agent in presence of sodium chloride as dispersing agent. To the best of our knowledge, it is the first time that various amino acids as fuel in presence of sodium chloride as dispersing agent are employed for the preparation of the Dy₂Ce₂O₇ nanostructures. This study reveals that the phenylalanine in presence of sodium chloride is an extremely good selection for homogeneous spherical Dy₂Ce₂O₇ nanostructures synthesis. By changing the amino acid type factor, we could prepare Dy₂Ce₂O₇ nanostructures with different shapes and grain sizes. The as-synthesized nanostructured Dy₂Ce₂O₇ may be applied as suitable and favorable material for photocatalytic usages under illumination of UV light such as deletion of methyl orange as anionic dye, since the methyl orange photodegradation percentage was found to be 80 after 60 min. This method provides a new, quick and simple process for the preparation of nanostructured Dy₂Ce₂O₇.

Acknowledgements

The authors are grateful to University of Kashan for supporting this work by Grant No. (159271/20).

Notes and references

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