1 Introduction

In recent years, the preparation of low-dimensional nanostructures has been intensively pursued because of their useful applications in various areas [1–11]. Global industrialization (such as textile, refineries, leather, paper, chemical, and plastic industries) has used different types of dyes resulted in the release of large amounts of toxic compounds into environment [12–17]. Generally, 30–40 % of these dyes remain in the waste waters. Additionally, presence of these dyes diminishes the photosynthesis and causes many serious health problems for humanity. To overcome these problems, the waste water from those industries must be treated before their discharge. Various physical and chemical methods have been used for color removal from waste waters. One of these methods is semiconductor photocatalysis and it has proven to be an effective in treating waste water pollution since it is an environmentally friendly, low-cost, and sustainable treatment methodology [18–22]. The sol gel method of preparing oxide powders generally involves polymerization via hydrolysis and condensation of an alkoxide, gelation, and heat treatment under suitable conditions. In recent years Bi2M4O9 (M = Al3+, Ga3+, Fe3+) compounds with mullite-type structures [23] have obtained considerable technical interest as potential candidates for applications as oxygen-ion conductors or mixed ionic–electronic conductors (MIEC). Common structural items are chains of edge-shared MO6 octahedra running parallel to the orthorhombic c-axis linked by (M2O7) dimers of MO4 tetrahedra especially, Bi2Al4O9 does not show any phase transition until 1080 °C and has a high chemical stability in hydrogen atmospheres [24, 25], a necessary feature for the application in fuel cells. Tutov and Echerlin described the possibility of orthorhombic bismuth aluminate preparation in 1965 [26–32], as well as defined the synthesis conditions, temperature and melting character of the compound. Among the wet chemical routes, sol–gel technique has been used widely because it has the advantage of producing pure, ultrafine powders at low temperatures, High surface area and pore size distribution [33, 34]. In this report, for the first time, we had presented the preparation of Bi2Al4O9 nanoparticles by novel sol–gel method at 800 °C in the presence of starch without adding external surfactant, capping agent or template. A green approach for Bi2Al4O9 nanoparticles synthesis by utilizing natural template permits the reaction to proceed usually in milder conditions. Although existing chemical approaches have effectively produced well defined Bi2Al4O9 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.

2 Experimental

2.1 Characterization

Bismuth nitrate pentahydrate Bi(NO3)3.5H2O, aluminium nitrate nonahydrate (Al(NO3) .3 9H2O), were purchased from Merck Company and used without further purification. X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10 < 2θ < 80. The electronic spectra of the bismuth aluminate 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. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. The magnetic measurement of samples were carried out in a vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir Co.; Kashan Kavir; Iran) at room temperature.

2.2 Synthesis of Bi2Al4O9 nanoparticles

At first, 3.41 g of Bi(NO3)3.5H2O was dissolved in 50 mL of distilled water. Then, 8.7 of starch was subsequently added to the above solution under magnetic stirring at 70 °C for 30 min. Afterwards, 5.28 g of Al(NO3) .3 9H2O was dissolved in 50 mL of distilled water and was added to the above solution under magnetic stirring. A solution was obtained and further heated at 100 °C for 1 h to remove excess water. During continued heating at 120 °C for 1 h, the solution became more and more viscous to become a gel. Finally, the obtained product was calcinated at 800 °C for 2 h in a conventional furnace in air atmosphere and then cooled it to room temperature.

2.3 Photocatalytic experimental

The methyl orange (MO) photodegradation was examined as a model reaction to evaluate the photocatalytic activities of the bismuth aluminate nanoparticles. The photocatalytic experiments were performed under an irradiation ultraviolet light. The photocatalytic activity of nanocrystalline Bi2Al4O9 obtained was studied by the degradation of methyl orange solution as a target pollutant. The photocatalytic degradation was performed with 150 mL solution of methyl orange (0.0005 g) containing 0.05 g of Bi2Al4O9. This mixture was aerated for 30 min to reach adsorption equilibrium. Later, the mixture was placed inside the photoreactor in which the vessel was 15 cm away from the visible source of 400 W mercury lamps. The photocatalytic test was performed at room temperature. Aliquots of the mixture were taken at definite interval of times during the irradiation, and after centrifugation they were analyzed by a UV–vis spectrometer. The methyl orange (MO) degradation percentage was calculated as:

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

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

3 Results and discussion

Figure 1 shows a typical XRD pattern (10° < 2θ < 80°) of Bi2Al4O9 nanoparticles. Based on the Fig. 1, the diffraction peaks can be indexed to pure orthorhombic phase of Bi2Al4O9 (space group Pbam, JCPDS No. 25-1048). No other crystalline phases were detected. From XRD data, the crystallite diameter (Dc) of Bi2Al4O9 nanoparticles was calculated to be 40 nm using the Scherer equation:

$${\text{D}}_{c} = {\text{ K}}\lambda /\beta \cos \theta \quad {\text{Scherer equation}}$$

where β is the breadth of the observed diffraction line at its half intensity maximum (400), 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. The morphology of the nanoparticles was investigated using SEM which demonstrates uniform nanoparticles with spherical shape homogenously distributed all over the sample, as it could be clearly observed in Fig. 2. The Bi2Al4O9 nanoparticles with particle size of about 50–55 nm were observed. The EDS analysis measurement was used to investigate the chemical composition and purity of Bi2Al4O9 nanoparticles. According to the Fig. 3, the product consists of Bi, Al, and O elements. Furthermore, neither N nor C signals were detected in the EDS spectrum, which means the product is pure and free of any surfactant or impurity. The VSM magnetic measurements for the Bi2Al4O9 Fig. 4 show the magnetic properties of nanoparticles calcined at 800 °C. The nanoparticles exhibit ferromagnetic behaviour at room temperature, with a saturation magnetization of .007 emu/g and a coercivity of 75 Oe. The room temperature UV–vis absorption spectra of Bi2Al4O9 nanoparticles were also measured in the range of 300–600 nm. Figure 5a shows the diffuse reflection absorption spectra of the Bi2Al4O9 nanoparticles calcinled at 800 °C. The figure indicates that the Bi2Al4O9 nanoparticles shows absorption maxima at 373 nm, the direct optical band gap estimated from the absorption spectra for the Bi2Al4O9 nanoparticles is shown in Fig. 5b. An optical band gap is obtained by plotting (αhν)2 versus hν where α is the absorption coefficient and hν is photon energy. Extrapolation of the linear portion at (αhν)2 = 0 gives the band gaps of 2.8 eV for perovskite Bi2Al4O9 material. Photodegradation of methyl orange under UV light irradiation (Fig. 6 a–c) was employed to evaluate the photocatalytic activity of the as-synthesized Bi2Al4O9. No methyl orange was practically broken down after 60 min without using UV light irradiation or nanocrystalline Bi2Al4O9. This observation indicated that the contribution of self-degradation was insignificant. The probable mechanism of the photocatalytic degradation of methyl orange can be summarized as follows:

$${\text{Bi}}_{2} {\text{Al}}_{4} {\text{O}}_{9} + \, {h\nu } \, \to {\text{Bi}}_{2} {\text{Al}}_{4} {\text{O}}_{9}^{ \cdot } + {\text{ e}}^{ - } + {\text{ h}}^{ + }$$
(1)
$${\text{h}}^{ + } + {\text{ H}}_{2} {\text{O }} \to {\text{ OH}}^{ \cdot }$$
(2)
$${\text{e}}^{ - } + {\text{ O}}_{2} \to {\text{ O}}_{2}^{ \cdot - }$$
(3)
$${\text{OH}}^{ \cdot } + {\text{O}}_{2}^{ \cdot - } + {\text{ methyl orange }} \to {\text{ Degradation products}}$$
(4)

Using photocatalytic calculations by Eq. (1), the methyl orange degradation was about 65 % after 60 min irradiation of UV light, and nanocrystalline Bi2Al4O9 presented high photocatalytic activity (Fig. 6a). The spectrofluorimetric time-scans of methyl orange solution illuminated at 510 nm with nanocrystalline Bi2Al4O9 are depicted in Fig. 6b. Figure 6b shows continuous removal of methyl orange on the Bi2Al4O9 under UV light irradiation. It is generally accepted that the heterogeneous photocatalytic processes comprise various steps (diffusion, adsorption, reaction, and etc.), and suitable distribution of the pore in the catalyst surface is effective and useful to diffusion of reactants and products, which prefer the photocatalytic reaction. In this investigation, the enhanced photocatalytic activity can be related to appropriate distribution of the pore in the sponge-like nanocrystalline Bi2Al4O9 surface, high hydroxyl amount and high separation rate of charge carriers 24 (Fig. 6c). Furthermore, this route is facile to operate and very suitable for industrial production of Bi2Al4O9 nanoparticles. In addition, this process can be versatile to easily synthesize other titanium based perovskite oxides. The synthesis pathway of Bi2Al4O9 nanoparticles is shown in Scheme 1.

Fig. 1
figure 1

XRD pattern of Bi2Al4O9 nanoparticles calcined at 800 °C

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Fig. 2
figure 2

SEM image of Bi2Al4O9 nanoparticles calcined at 800 °C

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Fig. 3
figure 3

EDS pattern of Bi2Al4O9 nanoparticles calcined at 800 °C

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Fig. 4
figure 4

VSM curves of Bi2Al4O9 nanoparticles calcined at 800 °C

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Fig. 5
figure 5

a UV–vis absorption spectra of prepared Bi2Al4O9 nanoparticles for 120 min at calcination temperature of 800 °C and b plot to determine the direct band gap of Bi2Al4O9

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Fig. 6
figure 6

a photocatalytic methyl orange degradation of Bi2Al4O9 nanoparticles under ultraviolet light b fluorescence spectral time scan of methyl orange illuminated at 510 nm with Bi2Al4O9 nanoparticles and c reaction mechanism of methyl orange photodegradation over Bi2Al4O9 nanoparticles under ultraviolet light irradiation

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Scheme.1
scheme 1

Schematic diagram illustrating the formation of Bi2Al4O9 nanoparticles

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4 Conclusions

In this work, bismuth aluminate nanoparticles were successfully synthesized by a novel sol–gel method at 800 °C for 120 min. The stages of the formation of Bi2Al4O9, as well as the characterization of the resulting compounds were done using X-ray diffraction and energy dispersive X-ray spectroscopy. The products were analyzed by scanning electron microscopy (SEM), and ultraviolet–visible (UV–Vis) spectroscopy to be round, about 50–55 nm in size and Eg = 2.80 eV. VSM analyzes indicates a ferromagnetic behavior for the synthesized nanoparticles. When as-prepared nanocrystalline bismuth aluminate was utilized as photocatalyst, the percentage of methyl orange degradation was about 65 % after 60 min irradiation of ultraviolet light.