1 Introduction

Materials at the nanometer scale have been studied for decades because of their unique properties arising from the large fraction of atoms residing on the surface, and also from the finite number of atoms in each crystalline core. Especially, because of the increasing need for high area density storage, the synthesis and characterization nanocrystals have been extensively investigated [1–7]. Recently, extensive researches on the environmental problems, especially photocatalytic applications have been performed. The decolorization of hazardous organic dyes such as rhodamine B (RhB), methylen blue (MB), and methyl orange are very important, because their discharge from industrial applications causes severe adverse effects to the environment and greatly increases the water pollution and hence effectively disturbs the ecosystem [8–11]. Furthermore studies on thermophysical properties, including thermal expansion and heat capacity of Ln2TiO5 (e.g. Dy2TiO5, Gd2TiO5, Eu2TiO5), demonstrate their advantages of thermal stability with low swelling at high temperature (up to 1500 K), desired for control rod materials [12, 13]. In addition, the nanocluster form of yttrium titanate found its application in oxide-dispersion-strengthened steel when it was dispersed in ferritic alloy. This alloy is considered as the most promising structural material for advanced nuclear systems due to its greatly enhanced creep resistance at high temperature [14, 15] and exceptional radiation resistance originating from stabilized oxide nanoclusters. However, the structure and chemistry of the nanocluster oxides have not been fully understood, remaining one of the challenging scientific issues in nuclear materials research [16, 17]. In this report, for the first time, we had presented the preparation of Nd2TiO5 nanoparticles by sol–gel method in the presence of alanine, leucine, and histidine without adding external surfactant. A simple approach for Nd2TiO5 nanoparticles synthesis by utilizing natural template permits the reaction to proceed usually in milder conditions. Although existing chemical approaches have effectively produced well defined Nd2TiO5 nanoparticles, these processes are generally costly and include the employ of toxic chemicals. The photocatalytic degradation was investigated using methyl orange (MO) under ultraviolet light irradiation [18–26].

2 Experimental

2.1 Characterization

X-ray diffraction (XRD) pattern was recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10 < 2θ < 80. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. The electronic spectra were obtained on a Scinco UV–Vis scanning spectrometer (Model S-10 4100). The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope. The magnetic measurement of sample was carried out in a vibrating sample magnetometer (VSM; Meghnatis Daghigh Kavir Co.; Kashan Kavir; Iran) at room temperature.

2.2 Synthesis of Nd2TiO5 nanoparticles

The amino acids, neodymium salt, ethanol, and tetra-n-butyl titanate (TNBT) were purchased from Merck Company and used without further purification. At first, a stoichiometric amount of TNBT and neodymium nitrate (1:1) was separately dissolved in ethanol in two A and B beakers, respectively. Then, amino acid as capping agent was dissolved in ethanol and added to the A solution under constant stirring. Afterwards, the neodymium nitrate solution was mixed with solution under stirring at room temperature. Subsequently, the final mixed solution was kept stirring to form a gel at 90 °C. Finally, the obtained product was calcinated at different temperatures for 3 h in a conventional furnace in air atmosphere, and calcination at different temperatures was carried out (Table 1).

Table 1 The preparation conditions of the Nd2TiO5 nanoparticles
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2.3 Photocatalytic experimental

The methyl orange (MO) photodegradation was examined as a model reaction to evaluate the photocatalytic activities of the Nd2TiO5 nanoparticles under ultraviolet light irradiations. The photocatalytic degradation was performed with 150 mL solution of methyl orange (0.0005 g) containing 0.1 g of Nd2TiO5. This mixture was aerated for 30 min to reach adsorption equilibrium. Later, in order to perform photocatalytic tests, the mixture was placed inside the photoreactor in which the vessel was 15 cm away from the ultraviolet source of 400 W mercury lamps at room temperature. Aliquots of the mixture were taken at definite interval of times during the irradiation (60 min) and after centrifugation they were analyzed by a UV–Vis spectrometer. The methyl orange (MO) degradation percentage was calculated as:

$${\text{Degradation rate (}}{\% } ) { = } \frac{{{\text{A}}_{0} - {\text{A}}}}{{{\text{A}}_{0} }} \times 100$$
(1)

where A0 and A are the absorbance value of solution at A0 and A min, respectively.

3 Results and discussion

Crystalline structure and phase purity of as-prepared product has been determined using XRD. The XRD pattern of as-prepared Nd2TiO5 (sample 1) is shown in Fig. 1. Based on the Fig. 1, the diffraction peaks observed can be indexed to pure orthorhombic phase of Nd2TiO5 (a = 10.7251 Å, b = 11.3407 Å, and c = 3.8457 Å) with space group of Pnam and JCPDS no. 33-0944. No diffraction peaks from other species could be detected, which indicates the obtained sample is pure. From XRD data, the crystallite diameter (Dc) of Nd2TiO5 nanoparticles (sample 1) was calculated to be 16 nm using the Scherer equation: Dc = Kλ/ β cosθ Scherer equationwhere β is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor, which usually takes a value of about 0.9, and λ is the wavelength of X-ray source used in XRD. In the third millennium, current studies show that different type of capping agents such as ionic, polymeric; etc. play a fundamental role in synthesis procedures [27–38]. Moreover, capping agents are essential materials for preparation of many disperse systems such as solid/liquid dispersions (usually referred to as suspensions); therefore, in this research we examined the effect of capping agents such as alanine, leucine, and histidine on the morphology and particle size of final products. Figure 2a–c shows the SEM images of the sample 1–3, respectively. In the presence of capping agents (alanine and leucine), the product mainly consists of spherical shape nanoparticles with average particle size 40–45 nm, as shown in Fig. 2a, b. Besides, in the presence of histidine as the capping agents the particle size of products were increased and products become agglomerate, as shown in Fig. 2c. Therefore, amino acids cause to increase the particle size of final products. Therefore, amino acids cause to increase the particle size of final products. Since in this study we used amino acids; therefore, the EDS analysis measurement was used to investigate the chemical composition and purity of Nd2TiO5 nanoparticles (sample 1), as shown in Fig. 3. According to the Fig. 3, the product consists of Ni, Ti, and O elements. Furthermore, neither N nor C signals were detected in the EDS spectrum, which means that the product is pure and free of any amino acids or impurity. The VSM magnetic measurement spectrum for the Nd2TiO5 (Fig. 4) shows the magnetic properties of Nd2TiO5 nanoparticles calcined at 900 °C. The Nd2TiO5 nanoparticle (sample 1) exhibits ferromagnetic behavior at room temperature with a saturation magnetization of 1.35 emu/g. The diffused reflectance spectrum of the as-prepared Nd2TiO5 nanoparticles is shown in Fig. 5. The fundamental absorption edge in most semiconductors follows the exponential law. Using the absorption data the band gap was estimated by Tauc’s relationship:

$$\upalpha =\upalpha_{0} ({\text{h}}\upnu - {\text{Eg}}){\text{n}}/{\text{h}}\upnu$$
(2)

where α is absorption coefficient, hν is the photon energy, α0 and h are the constants, Eg is the optical band gap of the material, and n depends on the type of electronic transition and can be any value between ½ and 3. The energy gap of the Nd2TiO5 nanoparticles (sample 1) is determined by extrapolating the linear portion of the plots of (αhν)1/2 against hν to the energy axis, as shown in Fig. 5. The Eg value is calculated as 3.0 eV for the Nd2TiO5 nanoparticles. Photodegradation of methyl orange (MO) solution under UV light illumination was employed to evaluate the photocatalyst properties of the as-synthesized Nd2TiO5 nanoparticles (Fig. 6). No methyl orange was practically broken down after 60 min in the absence of Nd2TiO5 nanoparticles. The proposed mechanisms of the photocatalytic degradation of the methyl orange with the aid of Nd2TiO5 nanoparticles can be assumed as:

$${\text{Nd}}_{2} {\text{TiO}}_{5} + {\text{h}}\upnu \to {\text{Nd}}_{2} {\text{TiO}}_{5}^{{ \cdot }} + {\text{e}}^{ - } + {\text{h}}^{ + }$$
(3)
$${\text{h}}^{ + } + {\text{H}}_{2} {\text{O}} \to {\text{OH}}^{{ \cdot }}$$
(4)
$${\text{e}}^{ - } + {\text{O}}_{2} \to {\text{O}}_{2}^{{{ \cdot } - }}$$
(5)
$${\text{OH}}^{{ \cdot }} + {\text{O}}_{2}^{{{ \cdot } - }} + {\text{methyl orange}} \to {\text{Degradation products}}$$
(6)

Utilizing photocatalytic calculations by Eq. (1), the methyl orange degradation was about 63 % after 60 min illumination of UV light in the presence of Nd2TiO5 nanoparticles (sample 1). Besides, the whole mechanism is shown in Scheme 1.

Fig. 1
figure 1

XRD pattern of Nd2TiO5 nanoparticles calcined at 900 °C (sample 1)

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Fig. 2
figure 2

SEM images of Nd2TiO5 nanoparticles calcined at 900 °C a sample 1, b sample 2 and c sample 3

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Fig. 3
figure 3

EDS pattern of Nd2TiO5 nanoparticles calcined at 900 °C (sample 1)

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Fig. 4
figure 4

VSM curve of Nd2TiO5 nanoparticles calcined at 900 °C (sample 1)

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Fig. 5
figure 5

DRS pattern of Nd2TiO5 nanoparticles calcined at 900 °C (sample 1)

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Fig. 6
figure 6

Photocatalytic methyl orange degradation of Nd2TiO5 nanoparticles (sample 1) under ultraviolet light

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Scheme 1
scheme 1

Reaction mechanism of methyl orange photodegradation over Nd2TiO5 nanoparticles under UV light irradiation

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4 Conclusions

In this work, Nd2TiO5 nanoparticles were successfully synthesized by green method in an aqueous solution. EDS and XRD results proved high purity of the as-prepared Nd2TiO5 nanoparticles. In order to investigate the effect of polymeric surfactants on the morphology and particle size of final products several tests were performed in the presence of alanine, leucine, and histidine. Applying nanocrystalline Nd2TiO5 as the photocatalyst causes maximum 63 % degradation of methyl orange after 60 min irradiation of UV light. This result suggests that as-obtained nanocrystalline Nd2TiO5 as favorable material has high potential to be used for photocatalytic applications under UV light.