New sodium dodecyl sulfate-assisted preparation of Nd₂O₃ nanostructures via a simple route

Abstract

The effect of the anionic shape- and size-controller on the morphology and size of Nd₂O₃ prepared by a new simple approach was studied. Nanostructured Nd₂O₃ was synthesized by heat treatment in air at 900 °C for 5 h, employing powder, which was prepared by a solvent-free solid-state reaction from [NdL(NO₃)₂]NO₃ (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound), as a new precursor, in the presence of sodium dodecyl sulfate (SDS) as shape- and size-controller. The structural, morphological and optical properties of the as-obtained nanostructured Nd₂O₃ were characterized by thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX), UV-vis diffuse reflectance spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy. The obtained results demonstrated that the amount of SDS plays a key role in the morphology and particle size control of the neodymium oxide. Furthermore, the catalytic properties of the as-prepared nanostructures were evaluated by the photocatalytic degradation of the erythrosine (anionic dye) as water contaminant.

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Introduction

In recent years, the preparation and characterization of nanostructured materials has drawn much attention owing to their potential applications in various fields as well as excellent and unique characteristics.^1–6 Neodymium oxide (Nd₂O₃) is well known as one of the most interesting rare earth metal oxides because of its specific, excellent and unique optical and electrical properties as well as several applications in thin films, luminescent and thermoluminescent materials, advanced materials, protective coatings, photonics and catalysts.^7–12 So far, neodymium oxide has been prepared by the gel combustion,¹³ microemulsion,¹⁴ hydrothermal,¹⁵ thermal decomposition,¹⁶ solution-coprecipitation¹⁷ and hydrogen plasma-metal reaction.¹⁸ The development of a reliable and simple way for preparation of nanostructured Nd₂O₃ is of great importance to the potential investigation of its characteristics. It has been reported that particle size and shape have key impact on the characteristics of nanoscale materials.^19,20 So, exploring desirable and appropriate ways to produce nanostructured Nd₂O₃ and controlling its morphology and particle size seems necessary and noteworthy.

The thermal decomposition of the organometallic molecular way is generally accepted as cost effective, versatile, convenient, applicable, reliable and simple synthetic technique. This way does not need special apparatus, complicated approaches and severe reaction conditions, which can restrict the large-scale production of the nanomaterials. Furthermore, this procedure provides effective and desirable method to the production of pure nanostructured materials with uniform morphology.

Herein, we present a novel simple way for the synthesis of pure nanostructured Nd₂O₃ via the thermal treatment of powder, which is prepared by a solvent-free solid-state reaction from [NdL(NO₃)₂]NO₃ (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound), as new precursor, in presence of sodium dodecyl sulfate (SDS) as shape- and size-controller. Furthermore, the influence of the amount of the SDS on the particle size and shape of Nd₂O₃ via a facile thermal decomposition way is studied.

Experimental

Materials

In this investigation, all the chemical reagents with analytical grade including sodium dodecyl sulfate (SDS), methanol, ethyl acetate, neodymium nitrate (Nd(NO₃)₃·6H₂O), 2-hydroxy-1-naphthaldehyde, ethylenediamine, and chloroform, were bought from Merck Co. and were employed as received.

Preparation of bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound (L)

In order to synthesize of Schiff base compound (L), 0.16 mol of 2-hydroxy-1-naphthaldehyde dissolved in methanol (80 ml) was added drop-wise to an ethanediamine solution (0.08 mol) in 80 ml of methanol. The mixture was refluxed for 3 h. The yellow precipitate was separated by filtration, washed and air-dried. It was then recrystallized from methanol. The as-synthesized compound was characterized utilizing FT-IR and ¹H NMR techniques.

Preparation of the neodymium precursor

In order to synthesize of the neodymium precursor, 1 mmol neodymium nitrate was dissolved in 20 ml ethyl acetate and then was drop-wise to a solution of 1 mmol compound L in 20 ml chloroform. The mixture was stirred for 2 h. Then, the yellow precipitate was filtered, washed with ethyl acetate and chloroform for three times, and after that air-dried for 24 h at room temperature. The resulting neodymium precursor was characterized utilizing FT-IR technique.

Preparation of Nd₂O₃ micro/nanostructures

Neodymium precursor (NP) and SDS were mixed and ground for 15 min in an agate mortar at room temperature in a NP to SDS molar ratio of 1 : 0.5, 1 : 1, 1 : 2, 1 : 3, 1 : 4, 1 : 5 and 1 : 6. The identification of the resulting powders was based on Fourier transform (FT-IR) spectrometer. Nd₂O₃ micro/nanostructures were synthesized by subjecting 0.12 g of the as-obtained powders to heat treatment at (900 °C) in the air. An average temperature increase of 30 °C is recorded every minute, before the temperature reached 900 °C, and after keeping the thermal treatment at 900 °C for 5 h, it was allowed to cool to room temperature. Schematic diagram of preparation of Nd₂O₃ micro/nanostructures is depicted in Scheme 1. To investigate the influence of calcination temperature, the thermal treatment process of the as-obtained powder (1 : 1 molar ratio of NP to SDS) was carried out in the 700, 800 and 900 °C. In order to examine the effect of SDS, a blank test was performed. In the blank test, 0.12 g of NP was subjected to heat treatment at (900 °C) in the air. To investigate the influence of L, one experiment was carried out without L (blank test 1). The preparation conditions of all products were demonstrated in Table 1. The as-obtained nanostructured Nd₂O₃ was characterized by TEM, XRD, EDS, UV-vis, FESEM, FT-IR analyses.

Scheme 1 Schematic diagram of the preparation of neodymium oxide micro/nanostructures.

Table 1 Preparation conditions of Nd₂O₃ micro/nanostructures

Sample no.	Neodymium source	Molar ratio of neodymium source/SDS	Calcination temperature (°C)	Figure of FESEM images
a Blank test, in the absence of SDS.b Blank test 1.
1	Nd(III) complex	1 : 1	700	—
2	Nd(III) complex	1 : 1	800	—
3	Nd(III) complex	1 : 1	900	Fig. 6b
4	Nd(III) complex	1 : 0.5	900	Fig. 6a
5	Nd(III) complex	1 : 2	900	Fig. 6c
6	Nd(III) complex	1 : 3	900	Fig. 7a
7	Nd(III) complex	1 : 4	900	Fig. 7b
8	Nd(III) complex	1 : 5	900	Fig. 7c
9	Nd(III) complex	1 : 6	900	Fig. 8a and b
10^a	Nd(III) complex	—	900	Fig. 10a
11^b	Nd(NO3)3·6H2O	1 : 1	900	Fig. 10b

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Characterization

The electronic spectra of the nanostructured Nd₂O₃ were obtained on a Scinco UV-vis scanning spectrometer (Model S-4100). Powder X-ray diffraction (XRD) patterns of products were collected with a diffractometer of Philips Company with X'PertPro monochromatized Cu Kα radiation (l = 1.54 Å). The energy dispersive spectrometry (EDS) analysis was performed by XL30, Philips microscope. Thermogravimetric-differential thermal analysis (TG-DTA) was carried out by 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. Fourier transform infrared spectra were recorded by utilizing a Magna-IR, spectrometer 550 Nicolet in KBr pellets in the range of 400–4000 cm⁻¹. The microscopic morphology of Nd₂O₃ was visualized by a Tescan mira3 field emission scanning electron microscope (FESEM). Transmission electron microscope (TEM) images of nanostructured Nd₂O₃ were taken by a JEM-2100 with an accelerating voltage of 200 kV equipped with a high resolution CCD camera.

Photocatalytic test

The photocatalytic activity of as-prepared nanostructured Nd₂O₃ (sample no. 3) was studied by monitoring the photodegradation of erythrosine as anionic dye in an aqueous solution. A quartz photocatalytic reactor was employed to carry out the photodegradation reaction. The photocatalytic degradation was performed by employing 0.003 g of erythrosine solution including 0.05 g of as-obtained nanostructured Nd₂O₃ at room temperature. For reaching adsorption equilibrium, this mixture was aerated for 30 min. 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. In order to hinder UV leakage, the light source and quartz vessel were placed inside a black box that equipped with a fan. Aliquots of the mixture were taken at definite interval of times during the illumination, and after centrifugation they were analyzed by a UV-vis spectrometer. The erythrosine photocatalytic degradation percentage was calculated as follow:where C_t and C₀ are the concentration of erythrosine at t and 0 min, respectively.

Results and discussion

As described before, in this study the bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound (L) was firstly synthesized, to be employed in the preparation of nanostructured Nd₂O₃. For ensuring an adequate purity of the obtained (L), FT-IR and ¹H NMR analyses were performed. The FT-IR spectrum of the Schiff base compound is illustrated in Fig. 1a. Two absorption bands located at 1644 and 1359 cm⁻¹ can be ascribed to the ν(C=N) of the azomethines and ν(Ar–O) of the phenolic hydroxyl substituent, respectively. Fig. 2a reveals the ¹H NMR spectrum of the prepared (L) compound. ¹H NMR data of the as-synthesized Schiff base compound (DMSO, δ, ppm): 2.3 (s, 4H, 2 × CH₂), 6.7 (s, 1H, CH aromatic), 7.4 (s, 1H, CH aromatic), 7.6 (s, 1H, CH aromatic), 7.66 (s, 1H, CH aromatic), 7.69 (s, 1H, CH aromatic), 8.29 (s, 1H, CH aromatic), 9.06 (s, 1H, CH), and 14.04 (s, 1H, OH). As observed, the results of the FT-IR and ¹H NMR analyses illustrated the high purity of the as-prepared Schiff base compound.

Fig. 1 FT-IR spectra of (a) Schiff base compound, (b) neodymium precursor, (c) neodymium precursor in presence of SDS and (d) nanostructured Nd₂O₃ obtained from 1 : 1 molar ratio of SDS to neodymium precursor by thermal treatment at 900 °C for 5 h.

Fig. 2 ¹H NMR spectra of the (a) Schiff base compound (L), (b) NP and (c) the UV-vis absorption spectra of the Schiff base compound (L) and NP.

Fig. 1b–d exhibits the infrared spectra of as-synthesized NP, NP in presence of SDS and nanostructured Nd₂O₃, respectively. Two absorption bands seen at 1634 and 1353 cm⁻¹ can be ascribed to the ν(C=N) of the azomethines and ν(Ar–O) of the phenolic hydroxyl substituent, respectively. The FT-IR spectrum of NP illustrates the (L) compound characteristic peaks with several different shifts due to the complex formation. The FT-IR spectrum of the NP reveals (Fig. 1b) an increase in the C–O stretching frequency (10 cm⁻¹) compared to the free (L) Schiff base compound. This demonstrates that the coordination to the neodymium ion happens via the oxygen atoms of the hydroxyl benzene of the as-synthesized (L) Schiff base compound. This was additional indicated by employing the absorption band seen at 470 cm⁻¹ (Fig. 1b) which may be related to ν(Nd–O) vibration.

In comparison with the free (L) compound, the ν(C=N) band in the NP shifted to 1634 cm⁻¹. This seen blue shift (10 cm⁻¹) indicates a stronger double bond characteristic of the iminic bonds as well as contribution of the azomethines nitrogen atoms in the coordination.¹⁶ This result was additional confirmed by employing the absorption band seen at 500 cm⁻¹ (Fig. 1b) which might be related to ν(Nd–N) vibration. The different coordination modes of the nitrate groups to the neodymium metal were detected by the differences in the two seen bands |ν₄ − ν₁|. Several peaks seen at 1479 cm⁻¹ (ν₁), 1032 cm⁻¹ (ν₂), 843 cm⁻¹ (ν₃) and 1311 cm⁻¹ (ν₄) in the FT-IR spectrum of the NP (Fig. 1b) can be ascribed to the coordinated nitrate ions. The difference between ν₄ and ν₁ is nearly 168 cm⁻¹ illustrating that the coordinated NO₃− ions in the as-prepared NP may play a bidentate compound role.¹⁸ In the spectrum of the as-obtained NP, the (ν_o) free nitrate seen at 1382 cm⁻¹. In the spectrum of the as-obtained powder (NP in presence of SDS), the characteristic peaks of SDS seen at 2920, 2852, 1220 and 1076 cm⁻¹ (Fig. 1c). In the FT-IR spectrum of the as-synthesized nanostructured Nd₂O₃, the peaks observed at 3441 and 1645 cm⁻¹ ascribed to the v(OH) stretching and bending vibrations, respectively, which illustrates the presence of physisorbed water molecules linked to as-synthesized nanostructured Nd₂O₃.²¹ The characteristic bands of the Nd₂O₃ seen at 619 and 512 cm⁻¹ (Fig. 1d).^{22 1}H NMR analysis results shown in Fig. 2a and b confirms the formation of the NP.

The UV-vis absorption spectra of the Schiff base compound (L) and NP were performed in DMF solvent (Fig. 2c). The values of the maximum absorption wavelength (λ_max) and the absorbance are listed in Table 2. The absorption spectrum of the Schiff base compound (L) shows four absorption peaks located at λ_max 361, 381, 400 and 426 nm. The two peaks locating at low energy side can be ascribed to n → π* transitions of conjugation between the lone pair of electrons of p orbital of N-atom in C=N group and a conjugated π bond of phenyl and naphthyl rings. The two peaks locating at higher energy arise from π → π* transitions within the phenyl and naphthyl rings and the π → π* of the C=N group. UV-vis spectrum of the NP is different from that of Schiff base compound (L) demonstrating the coordination of the Schiff base compound (L) to the Nd(III) ions. Upon the coordination, the two higher energy absorption peaks of the NP are shifted to longer wavelength region (red shift) compared to those of the free Schiff base compound (L). The two lower energy peaks in the free Schiff base compound (L) are overlapped in a single peak and shifted to shorter wavelength (blue shift). This shift can be ascribed to the donation lone pair of electrons from the Schiff base compound N-atoms to the Nd(III) ions (Nd → Ln).²³ As demonstrated, FT-IR, ¹H NMR and UV-vis analysis results confirm L and NP preparation.

Table 2 The UV-vis absorption peaks (λ_max) and absorbance (Abs.) of Schiff base compound (L) and NP in DMF solution

Compound	λmax (nm)	Abs.	Band assignments
L	361	1.529	π → π*
	400	1.933	π → π*
	426	1.687	π → π*
	381	1.873	n → π*
NP	412	2.169	π → π*
	425	1.943	n → π*
	381	2.000	π → π*

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In order to study the thermal observation of the as-prepared NP, thermal gravimetric technique (TGA) was employed. Fig. 3 reveals the TGA curve of the as-obtained NP. As illustrated in Fig. 3, the weight loss steps are two. The first weight losses takes place in the temperature range 200–380 °C, and displays 27.73% weight loss, which corresponds to the loss of two and one nitrate species from the inner and the outer coordination sphere of the as-synthesized NP, respectively. The second step takes place at the temperature range 380–850 °C (illustrating 42% weight loss), which may be attributed to the loss of the (L) compound from the inner coordination sphere of the as-prepared NP, and the preparation of Nd₂O₃.

Fig. 3 TGA of the neodymium precursor.

XRD technique, which is generally accepted as applicable technique for determination of the crystal structure, was utilized to investigate the composition and purity of the as-synthesized Nd₂O₃ (Fig. 4 and Table 3). The XRD pattern of products obtained at 700, 800 and 900 °C for 5 h (sample nos 1–3) are illustrated in Fig. 4a–c. XRD results demonstrate that the products prepared through calcination at 700 and 800 °C consist of the cubic Nd₂O₃ that the pure hexagonal Nd₂O₃ has not been synthesized (Fig. 4a and b). By increasing the temperature, pure hexagonal Nd₂O₃ has been obtained. All diffraction peaks observed in the Fig. 4c can be readily indexed to the pure hexagonal phase of Nd₂O₃ (JCPDS card file no. 74-2139). Utilizing Scherrer formula,²⁴ the crystallite size of the obtained Nd₂O₃ from the XRD results was calculated to be about 32 nm. Therefore, pure hexagonal Nd₂O₃ can be synthesized by calcining the prepared powder (1 : 1 molar ratio of NP to SDS) at 900 °C.

Fig. 4 XRD pattern of products obtained by thermal treatment of powder in 1 : 1 molar ratio of SDS to NP at (a) 700, (b) 800 and (c) 900 °C for 5 h.

Table 3 Preparation conditions and corresponding products

Sample no.	Calcination temperature (°C)	Composition of the products (XRD result)
1	700	Hexagonal Nd2O3 + cubic Nd2O3
2	800	Hexagonal Nd2O3 + cubic Nd2O3
3	900	Hexagonal Nd2O3

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The chemical composition and purity of the as-synthesized nanostructured Nd₂O₃ (sample no. 3) were additional examined by EDS technique. The illustrated bands in Fig. 5 related to Nd and O. So, the prepared XRD and EDS results demonstrate that hexagonal nanostructured Nd₂O₃ with high level of purity was successfully produced by the present way.

Fig. 5 EDS pattern of nanostructured Nd₂O₃ obtained from 1 : 1 molar ratio of SDS to NP by thermal treatment at 900 °C for 5 h.

As mentioned before, in this research, nanostructured Nd₂O₃ was synthesized by employing [NdL(NO₃)₂]NO₃ (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound), as NP in presence of SDS as capping agent via a new simple way. In order to examine the effect of the amount of SDS on the size and morphology of the Nd₂O₃, FESEM images of obtained Nd₂O₃ micro/nanostructures with SDS : NP molar ratio of 0.5 : 1, 1 : 1, 2 : 1, 3 : 1, 4 : 1, 5 : 1 and 6 : 1 (Table 1) were taken and illustrated in Fig. 6, 7 and 8, respectively. It can be seen that by employing molar ratio of 0.5 : 1 (SDS to NP), the nanoparticles were not form well (Fig. 6a). Therefore, in order to reduce the amount of aggregate and contribute to the formation of nanostructured Nd₂O₃ greater amount of SDS was added. Fig. 6b illustrated that uniform spherical nanoparticles are obtained in the presence of 1 mmol of SDS. When 2 : 1, 3 : 1 and 4 : 1 molar ratio of SDS : NP were employed, nanostructures composed of nanobundles and rod-like nanostructures as well as nanobundles and nanorods were obtained (Fig. 6c, 7a and b). By increasing the mmol of SDS from 2 to 3, the amount of the rod-like nanostructures increased.

Fig. 6 FESEM images of the products prepared from (a) 0.5 : 1, (b) 1 : 1, and (c) 2 : 1 molar ratios of SDS to NP, by thermal treatment at 900 °C for 5 h.

Fig. 7 FESEM images of the samples obtained from (a) 3 : 1, (b) 4 : 1, and (c) 5 : 1 molar ratios of SDS to NP, by thermal treatment at 900 °C for 5 h.

When 5 and 6 mmol of SDS were employed, corrugated-like and scales-like nanostructures were obtained (Fig. 7c, 8a and b). It seems that when the amount of SDS from 0.5 to 1 mmol increases, the uniform and sphere-like Nd₂O₃ nanoparticles forms and also the particle size decreases with decreasing NP to SDS molar ratio (Scheme 2b). It seems that when the amount of the SDS increases, the aggregation between Nd₂O₃ nanoparticles decreases and also nucleation to be taken place rather than the particle growth owing to the steric hindrance influence of SDS. By increasing the mmol of SDS from 2 to 6, the SDS as capping agent can provide the different reaction interfaces for NP (Scheme 2a) and induce the nanoparticles to assemble in the definite directions, and so rod-like nanostructures, nanobundles, nanorods, corrugated-like and scales-like nanostructures were prepared, respectively. Therefore the molar ratio of SDS : NP has a significant impact on the morphology and particle size control of the Nd₂O₃.

Fig. 8 FESEM images of the product prepared from 6 : 1 molar ratio of SDS to NP, by thermal treatment at 900 °C for 5 h.

Scheme 2 Schematic diagram showing (a) the effect of the amount of SDS on the reaction interfaces for NP and (b) the formation of neodymium oxide products at various conditions.

In order to investigate the detailed particle size and morphology of the as-prepared nanostructured Nd₂O₃, TEM technique was utilized. The typical TEM images of the nanostructured Nd₂O₃ in a molar ratio 1 : 1 (SDS to NP) is illustrated in Fig. 9a–d. The TEM images of the Nd₂O₃ demonstrate that nanoparticles with quasi-sphere-like morphology are sintered together. Furthermore, the images indicate that as-obtained Nd₂O₃ nanoparticles have diameter in the range of 20–55 nm.

Fig. 9 TEM images of nanostructured Nd₂O₃ obtained from 1 : 1 molar ratio of SDS to NP, by thermal treatment at 900 °C for 5 h.

To examine the effect of the SDS on the morphology of the Nd₂O₃, sample no. 10 was synthesized as blank product without utilizing any SDS. Fig. 10a reveals FESEM image of sample no. 10. It can be observed that in the absence of SDS, high agglomerated particle-like and bulk structures were obtained. This result indicates that employing SDS with high steric hindrance effect led to synthesize nanostructured Nd₂O₃. SDS as capping agent limits the size of the Nd₂O₃ nanoparticles and protects them from additional aggregation.

Fig. 10 FESEM images of Nd₂O₃ prepared in the absence of (a) SDS and (b and c) Schiff base compound.

In order to investigate the effect of the neodymium source on the shape of the Nd₂O₃, sample no. 11 was prepared as blank 1 product with employing Nd(NO₃)₃·6H₂O in the presence of the SDS. FESEM image of the sample no. 9 is illustrated in Fig. 10b and c. It is noteworthy that bulk structures were formed. The bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine compound with high steric hindrance influence in [NdL(NO₃)₂]NO₃, can act as a co-capping agent. Obviously, employing [NdL(NO₃)₂]NO₃ as neodymium source in the presence of the SDS results in nanostructured Nd₂O₃ preparation (Fig. 6–8). Therefore, a privilege of employing [NdL(NO₃)₂]NO₃ in the presence of the SDS is that it cause to formed nanostructured Nd₂O₃.

It has been reported that the band gap (E_g) has a main impact on the determining the nanostructured materials characteristics utilized in photocatalytic applications and is oftentimes calculated from the UV-vis diffuse reflectance data. The UV-vis diffuse reflectance spectrum of the nanostructured Nd₂O₃ in a molar ratio 1 : 1 (SDS to NP) demonstrated in Fig. 11a. In the UV-vis diffuse reflectance spectrum of the as-prepared nanostructured Nd₂O₃ (Fig. 11a), the eight absorption bands (located at 434, 470, 532, 594, 612, 686, 756 and 800 nm) are ascribed to charge transition from ⁴I_9/2 to ²G_9/2 + ²D_3/2 + ⁴G_11/2 + ²K_15/2, ⁴G_9/2 + ⁴G_7/2 + ²K_13/2, ⁴G_5/2 + ²G_7/2, ²H_11/2, ⁴F_9/2, ⁴F_7/2 + ⁴S_3/2, ⁴F_5/2 + ²H_9/2 and ⁴F_3/2, respectively.²² The E_g can be determined based on the UV-vis diffuse reflectance data employing Tauc's equation.^25,26 The energy gap of the nanostructured Nd₂O₃ has been calculated by extrapolating the linear portion of the plot of (αhν)² against hν to the zero^27,28 (Fig. 11b). The E_g amount of the as-synthesized nanostructured Nd₂O₃ calculated to be 5.12 eV, which illustrated a blue shift compared with the reported band gap amount of Nd₂O₃ in previous literatures²⁹ which this blue shift is attributable to decrease in the grain size which cause alteration in particle energy levels and increase the band gap amount.

Fig. 11 UV-vis diffuse reflectance spectrum (a) and plot to determine the band gap (b) of the nanostructured Nd₂O₃ obtained from 1 : 1 molar ratio of SDS to NP, by thermal treatment at 900 °C for 5 h.

The influence of morphology on photocatalytic activity was studied by monitoring the photodegradation of the erythrosine as anionic dye in an aqueous solution over as-prepared sample nos 3 (Nd₂O₃ obtained from 1 : 1 molar ratio of SDS to NP) and 10 (prepared in the absence of Schiff base compound) with different morphology under UV light illumination (Fig. 12 and Scheme 3). No erythrosine was practically broken down after 120 min without using UV light illumination or Nd₂O₃. This observation indicated that the contribution of self-degradation was insignificant. The possible mechanism of the photodegradation of the anionic dye may be summarized as follows:

Nd₂O₃ + hν → Nd₂O^*₃ + e⁻ + h⁺

h⁺ + H₂O → OH˙

e⁻ + O₂ → ·O₂⁻˙

OH˙ + O₂⁻˙ + anionic dye → photodegradation products

Fig. 12 Photocatalytic erythrosine degradation of sample nos 3 and 10 under UV light.

Scheme 3 Reaction mechanism of erythrosine photodegradation over Nd₂O₃ under UV light illumination.

According to photocatalytic calculations by eqn (1), the erythrosine degradation was about 78 and 56% over sample nos 3 and 10, respectively, after 120 min illumination of UV light, and Nd₂O₃ with uniform spherical morphology demonstrated high photocatalytic activity. It is well known that the heterogeneous photocatalytic processes comprise the diffusion, adsorption and reaction steps, and the desirable distribution of the pore is helpful and advantageous to diffusion of reactants and products, which prefer the photocatalytic reaction. In this research, the enhanced photocatalytic activity of sample no. 3 with uniform spherical morphology can be related to desirable distribution of the pore, high hydroxyl amount and high separation rate of charge carriers.³⁰

In this study, we described a new way to prepare nanostructured Nd₂O₃ with the aid of [NdL(NO₃)₂]NO₃ (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound), as neodymium source, in presence of sodium dodecyl sulfate (SDS) as capping agent. The novelty of this research compared to other reports is that for the preparation of Nd₂O₃, [NdL(NO₃)₂]NO₃ as new neodymium source in presence of SDS were used. The bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine compound with high steric hindrance influence in [NdL(NO₃)₂]NO₃, can act as a co-capping agent. In Table 4 some of the precursors utilized in the other works are compared with the present investigation. According to these results, by employing the [NdL(NO₃)₂]NO₃ in presence of SDS, Nd₂O₃ was synthesized successfully with the smallest size and narrowest size distributions.

Table 4 Characterization comparison of Nd₂O₃ with other works

Method	Precursors	Morphology, size (nm)	Calcination temperature (°C)	Ref.
Thermal treatment	[NdL(NO3)2]NO3 (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine), SDS	Sphere-like, 20–55 nm	900	This work
Thermal treatment	Neodymium tartrate	Nearly spherical, 200–3000 nm	800	31
Thermal decomposition	Nd(NO3)3·5.2HO, Na2C2O4, PEG400 N-butanol, n-octane, Nd(NO3)3	Irregular shapes, 300–1000 nm	950	13
Microemulsion	Nd(NO3)3, CTAB (needed stirring for 24 h)	Spherical, 40–80 nm	900	12
Combustion	Nd(NO3)3, oxalyl dihydrazide	Spherical, 20–100 nm	900	32

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Conclusions

This work describe a new facile way for the large-scale preparation of nanostructured Nd₂O₃ with different morphologies via the thermal treatment of the powder, which was obtained by a solvent-free solid-state reaction from [NdL(NO₃)₂]NO₃ (L = bis-(2-hydroxy-1-naphthaldehyde)-ethanediamine Schiff base compound), as neodymium source, in presence of sodium dodecyl sulfate (SDS) as capping agent. Employing of [NdL(NO₃)₂]NO₃ as a neodymium source in presence of SDS is the novelty of this research. When as-synthesized nanostructured Nd₂O₃ was employed as photocatalyst, the percentage of erythrosine photodegradation was about 78 after 120 min illumination of UV light. This result suggests that as-obtained nanostructured Nd₂O₃ as desirable material has high potential to be employed for photocatalytic applications under UV light. High purity of the as-synthesized Nd₂O₃ was confirmed by EDS, XRD and FT-IR techniques. The optical characteristics of as-formed Nd₂O₃ were also investigated.

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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