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 of semiconductor nanocrystals have been extensively investigated [1–12]. Metal tungstates presents a general formula (AWO4) can be divided in two great groups: Scheelite (A = Ca, Sr, Ba and Pb) [13] and Wolframite (A = Cd, Mg, Mn, Zn and Cu) [14, 15]. In particular, zinc tungstate (ZnWO4) crystals only exhibit a wolframite-type monoclinic structure at high pressure [16]. At room temperature and low pressure the ZnWO4 crystal is an important member of transition metal tungstates which exhibits a triclinic structure. The electronic properties related to pure ZnWO4 crystals and thin films have been reported in different fields such as: photoluminescence [17], magnetic [18], multiferroic [19], electrical transport [20], photoelectrochemical water splitting [21–23], photovoltaic electrochemical [24], visible and solar-assisted water splitting [25, 26] and photoanode for solar water oxidation [27, 28], photocatalytic (PC) degradation under visible light of eosin yellow dye [29]. Regarding obtaining ZnWO4 crystals different synthesis method have been employed by researchers at around world such as oxide mixture or solid state reaction, flux growth technique, czochralski process, and melting at a high temperature [30–32]. However, these preparation methods require high temperatures, long processing times and sophisticated equipment with high maintenance costs as well as the formation of deleterious phases. Hence, in this manuscript, a simple was performed to synthesize and characterize of ZnWO4 nanoparticles in distilled water as solvent. In addition, the effects of ethylene glycol (EG), propylene glycol (PG), and sodium dodecyl benzene sulfonates (SDBS) as the surfactants were investigated on the morphology and particle size of ZnWO4 nanoparticles. Moreover, the photocatalytic degradation was investigated using methyl orange (MO) under ultraviolet light irradiation to study the photocatalytic activity of as-prepared nanoparticles.

2 Experimental

2.1 Materials

Methylene orange was of analytical reagent grade quality used without further purification. Other chemicals were commercial products of analytical grade or reagent-grade. All the solutions were prepared with distilled water.

2.2 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 samples were carried out in a vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir Co.; Kashan Kavir; Iran) at room temperature in an applied magnetic field sweeping between ±10,000 Oe.

2.3 Synthesis of ZnWO4 nanoparticles

Materials and characterization: All chemical reagents in this research were of analytical grade and used without any further purification. In typical synthesis procedure, (1 gr) of Na2WO4·2H2O and (0.90 gr) of Zn(NO3)2·6H2O were dissolved in 10 ml of distilled water under stirring in two separate beakers (A and B). Then, 0.22 g of SDBS as the surfactant was added to the beaker B under constant stirring. Finally, the white precipitate was filtered and washed three times with distilled water. The final product was dried at 60 °C and then calcined at 500 °C for 120 min in a conventional furnace in air atmosphere. Reaction conditions are listed in Table 1.

Table 1 The preparation conditions of the zinc tungstate nanoparticles
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2.4 Photocatalytic experimental

The methyl orange (MO) photodegradation was examined as a model reaction to evaluate the photocatalytic activities of the ZnWO4 nanoparticles under ultraviolet light irradiations. The photocatalytic degradation was performed with 50 mL solution of methyl orange (0.0005 g) containing 0.1 g of ZnWO4. 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 and after centrifugation they were analyzed by a UV–vis spectrometer. The methyl orange (MO) degradation percentage was calculated as:

$${\text{Degradation}}\;{\text{rate}}\left( \% \right) = 100\left( {{\text{C}}_{0} - {\text{C}}_{\text{t}} } \right)/{\text{C}}_{0}$$
(1)

where Ct and C0 are the obtained absorbance value of the methyl orange solution at t and 0 min by a UV–vis spectrometer, 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 ZnWO4 (sample 1) is shown in Fig. 1. Based on the Fig. 1, the diffraction peaks observed can be indexed to pure monoclinic phase of ZnWO4 with space group of P2/c and JCPDS no. 73-0554. No diffraction peaks from other species could be detected, which indicates the obtained sample is pure. From XRD data, the crystallite diameter (Dc) of ZnWO4 nanoparticles (sample 1) was calculated to be 13 nm using the Scherer equation:

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

where 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 surfactants such as ionic, polymeric; etc. play a fundamental role in synthesis procedures [33–35]. Moreover, surfactants 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 three surfactants such as ethylene glycol (EG), propylene glycol (PG), and sodium dodecyl benzene sulfonates (SDBS) on the morphology and particle size of final products. Figure 2a–c shows the SEM images of ZnWO4 nanoparticles in the presence (EG), (PG), and (SDBS) as the surfactant accordance with sample 1–3, respectively. According to the Fig. 2a, product mainly consists of spherical shape nanoparticles with average particle size 25–30 nm. Furthermore, in the presence of EG and PG as surfactant products have smaller size than SDBS as the surfactant. The purity of nanocrystalline product was also confirmed by EDS analysis (Fig. 3). According to Fig. 3, the sample no. 1 is composed of Zn, W and O elements. Furthermore, no impurity peaks are seen, which indicates a high level of purity in the sample. The VSM magnetic measurement spectrum for the ZnWO4 (Fig. 4) shows the magnetic properties of ZnWO4 nanoparticles calcined at 500 °C. The ZnWO4 nanoparticle (sample 1) exhibits ferromagnetic behavior at room temperature with a saturation magnetization of 0.004 emu/g and a coercivity of 80 Oe. The optical band gap (Eg) has been estimated using Tuac’s equations:

$$\left( {\upalpha{\text{h}}\upupsilon} \right)^{n} = {\text{A}}\left( {{\text{h}}\upupsilon{-}{\text{Eg}}} \right)$$
(2)

where hν is the photon energy, α is the absorption coefficient, A is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The graph of (αhν)2 versus hν was plotted by extrapolating the linear portion of the curve to horizontal axis in which (αhν)2 = 0 (Fig. 5). The curve indicates that the value of the direct band gap (Eg) is about 3.05 eV. The band gap values suggest that ZnWO4 nanoparticle is a semiconductor. Also, this value of band gap is in the same range as that of highly efficient photovoltaic materials. Photodegradation of methyl orange (MO) solution under UV light illumination was employed to evaluate the photocatalyst properties of the as-synthesized ZnWO4 nanoparticles (Fig. 6). No methyl orange was practically broken down after 80 min in the absence of as-prepared ZnWO4 nanoparticles. The proposed mechanisms of the photocatalytic degradation of the methyl orange with the aid of ZnWO4 nanoparticles can be assumed as:

$${\text{ZnWO}}_{4} + {\text{h}} \nu \rightarrow {\text{ZnWO}}_{4}^{ \cdot } + e^{ - } + h^{ + }$$
(3)
$${\text{h}}^{ + } + {\text{H}}_{2} {\text{O}} \rightarrow {\text{OH}}^{ \cdot }$$
(4)
$${\text{e}}^{ - } + {\text{O}}_{2} \rightarrow {\text{O}}_{2}^{ \cdot - }$$
(5)
$${\text{OH}}^{ \cdot } + {\text{O}}_{2}^{ \cdot - } + {\text{methyl}}\;{\text{orange}} \rightarrow {\text{Degradation}}\;{\text{products}}$$
(6)
Fig. 1
figure 1

XRD pattern of ZnWO4 nanoparticles (sample 1)

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Fig. 2
figure 2

SEM images of ZnWO4 nanoparticles a sample 1, b sample 2, c sample 3

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Fig. 3
figure 3

EDS pattern of ZnWO4 nanoparticles (sample 1)

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Fig. 4
figure 4

VSM curve of ZnWO4 nanoparticles (sample 1)

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Fig. 5
figure 5

UV–Vis pattern of ZnWO4 nanoparticles (sample 1)

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Fig. 6
figure 6

Photocatalytic methyl orange degradation of ZnWO4 nanoparticles (sample 1) under ultraviolet light

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Utilizing photocatalytic calculations by Eq. (1), the methyl orange degradation was about 82 % after 80 min illumination of UV light in the presence of samples 1. This obtained result demonstrates that as-prepared ZnWO4 nanoparticles have high potential to be applied as favorable and appropriate material for photocatalytic applications under illumination of UV light. The heterogeneous photocatalytic processes have diffusion, adsorption and reaction steps. It has been shown that the desirable distribution of the pore has effective and important impact on the diffusion of the reactants and products, and therefore effects on the photocatalytic activity. It seems that the enhanced photocatalytic activity of the as-obtained nanoparticles ZnWO4 can be owing to desirable and appropriate distribution of the pore, high hydroxyl amount and high separation rate of charge carriers (Scheme 1). Furthermore, this route is facile to operate and very suitable for industrial production of ZnWO4 nanoparticles.

Scheme 1
scheme 1

Reaction mechanism of methyl orange photodegradation over ZnWO4 nanoparticles under UV light irradiation

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4 Conclusions

In this work, ZnWO4 nanoparticles were successfully synthesized by a simple precipitation method in an aqueous solution. EDS and XRD results proved high purity of the as-prepared ZnWO4 nanoparticles. In order to investigate the effect of surfactants on the morphology and particle size of final products several tests were performed in the presence of ethylene glycol (EG), propylene glycol (PG), and sodium dodecyl benzene sulfonates (SDBS). Applying nanocrystalline ZnWO4 as the photocatalyst causes maximum 82 % degradation of methyl orange after 80 min irradiation of UV light. This result suggests that as-obtained nanocrystalline ZnWO4 as favorable material has high potential to be used for photocatalytic applications under UV light.