1 Introduction

Semiconductors have many highly structurally sensitive properties that are necessary for technological applications like photocatalytic activity. The basis of searching for effective photocatalysts is the understanding of the nature of photocatalysis and its relevant influencing factors. It is documented that the activity of a photocatalystis closely related to surface area, morphology, crystal structure, and electronic Structure as well. The surface are and morphology can be taken as external factors since they can be well controlled by experimental conditions, while crystal electronic structure are intrinsic nature of the photocatalytic activities [1–11]. Recently, a remarkable attentions have been given on application wolframite-type tungstates of transition metals (MWO4; A = Mn, Fe, Cu, Ni, Pb, Zn, earth rare elements) for photocatalytic degradation of organic compounds [12–14]. These materials are used as photocatalyst, photoluminescence, photovoltaic, super capacitors, conventional oxidation catalysts, magnetic materials multiferroics, molecular precursors to metal tungstates, optical ibers, scintillators, and etc. The previous investigations have shown that the tungsten oxide (WO3) is an active photocatalyst material for oxygen evolution, while it does not hydrogen evolution neither. Therefore, various eforts have been performed to obtain a series of materials based on AWO3 and AWO4 as power full photocatalyst compounds. In this way the resulted materials exhibited interesting technological properties such as ionic photoluminescence, ferro elasticity and conductivity [15–18]. However, the synthesize of AWO4 may carried out by various methods such as sol–gel, solution combustion synthesis, solid-state reaction, co-precipitation, and solvothermal and hydrothermal synthesis [19–21]. It seems that the co-precipitation method includes various advantageous such as economic, simple, faster, soft chemical synthetic methodology, needs to low calcination temperature, and does not require special working conditions [22]. Hence, in this manuscript, a simple precipitation method was performed to synthesize and characterize of CoWO4 nanoparticles in distilled water as solvent. In addition, the effects of different capping agents such as valine, glycine, and alanine were investigated as capping agents on the morphology and particle size of cobalt tungstate nanostructures. 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 Characterization

All the chemicals used in this method were of analytical grade and used as-received without any 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. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. Spectroscopy analysis (UV–Vis) was carried out using shimadzu UV–Vis scanning UV–Vis diffuse reflectance spectrometer. 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.

2.2 Synthesis of CoWO4 nanoparticles

CoWO4 nanoparticles were prepared by simple coprecipitation method. In a typical procedure, an aqueous solution of cobalt (II) nitrate hexahydrate in the presence of different capping agent, such as valine, glycine, and alanine was mixed with sodium tungstate dihydrate (Na2WO4⋅2H2O) aqueous solution and the solution was heated up to 70 °C for 15 min. The green precipitate was centrifuged, washed out with distilled water and methanol for three times and dried under vacuum 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 (Fig. 1).

Table 1 The preparation conditions of the CoWO4 nanoparticles
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Fig. 1
figure 1

SEM image of CoWO4 nanoparticles calcined at 500 °C a sample 1, b sample 2 and c sample 3

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2.3 Photocatalytic experimental

The photo-degradation of methyl orange (MO) in water solutions under ultraviolet light was evaluated to access the photocatalytic activity of CoWO4 nanoparticles. The experiments were performed using a 50 mL solution of methyl orange also containing 0.1 g of CoWO4 nanoparticles after 30 min of aeration. The solution was next transferred to a photo-reactor. The reaction vessel was placed 15 cm away from the 400 W ultraviolet source (mercury lamps) and was kept at ambient temperature. Both before the onset of the reaction and at 10 min intervals of its beginning aliquots of the mixture were taken and their MO contents were determined through UV–Vis spectrometry. Based on the obtained data the degradation percentages of MO were calculated using the below equation:

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

In which A0 and A represent the absorbance of the solution before the onset of the reaction and at each interval.

3 Results and discussion

In the third millennium, current studies show that different type of surfactants such as ionic, polymeric; etc play a fundamental role in synthesis procedures [23–34]. Moreover, polymeric 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 capping agents such as valine, glycine, and alanine on the morphology and particle size of final products. Figure 2a–c shows the SEM images of the sample 1–3, respectively. According to the Fig. 2 a, b, product mainly consists of spherical shape nanoparticles with average particle size 50–80 nm. Furthermore, in the presence of valine and glycine as capping agent products have smaller size than alanine as the capping agent.

Fig. 2
figure 2

XRD pattern of CoWO4 nanoparticles calcined at 500 °C (sample 1)

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The powder X-ray difraction technique was used to evaluation of crystal structure and phase purity of CoWO4 nanoparticles and the data has been shown in Fig. 2. The XRD peaks demonstrated a well match with the pure monoclinic phase of CoWO4 nanoparticles with space group of P2/a and JCPDS no. 15–0867. Also no additional peaks were observed because of purity of samples. By the Debye–Scherrer approximation the crystallite size of CoWO4 nanoparticles were calculated from XRD data as 14 nm.

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

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. The morphology and shape of CoWO4 nanoparticles were investigated by SEM method and the results conirmed that the particles are packed with an average size of 50–80 nm (See Fig. 1a–c). Also the results demonstrated that the particle size that obtained by SEM is larger than obtained by XRD because of the SEM shows the sum of many crystallites. The purity of nanocrystalline product was also confirmed by EDS analysis (Fig. 3). According to Fig. 3, the sample no. 1 is composed of Co, W and O elements. Furthermore, no impurity peaks are seen, which indicates a high level of purity in the sample. The VSM magnetic measurements for the cobalt tungstate oxide (Fig. 4) show the magnetic properties of nanoparticles calcined at 500 °C. The CoWO4 nanoparticles exhibit paramagnetic behaviour at room temperature, with a saturation magnetization of 0.13 emu/g. Figure 5 shows the (αhʋ)2 vs hy curve of CoWO4 nanoparticles which were calculated from their UV–Vis absorbance using the equation proposed by Wood and Tauc exhibited the equation below.

Fig. 3
figure 3

EDS pattern of CoWO4 nanoparticles calcined at 500 °C (sample 1)

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Fig. 4
figure 4

VSM curves of CoWO4 nanoparticles calcined at 500 °C (sample 1)

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Fig. 5
figure 5

DRS pattern of CoWO4 nanoparticles calcined at 500 °C (sample 1)

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$$\alpha \text{h}\upsilon \text{ }=\text{ }{{\left( \text{h}\nu -\text{Eg} \right)}^{\text{n}}}$$
(2)

where a is the absorbance, h the Planck constant, y the photon frequency, Eg the energy gap, and n the pure numbers associated with the different types of electronic transitions. For n = 1/2, 2, 3/2 and 3, the transitions are directly allowed, indirectly allowed, directly forbidden, and indirectly forbidden, respectively. Each energy gap was determined by extrapolation of each linear portion of the curves to a = 0. In the present research, the CoWO4 presents directly allowed electronic transition (n = 1/2) and the energy gaps of CoWO4 nanoparticles is 3.1 eV. Photodegradation of methyl orange (MO) as water contaminant under UV light illumination was employed to evaluate the properties of the as-synthesized CoWO4 nanoparticles. Figure 6 exhibits the obtained result. No methyl orange was practically broken down after 70 min without employing UV light illumination or as-prepared nanoparticles CoWO4. This observation illustrated that the contribution of self-degradation was insignificant. The proposed mechanism of the photocatalytic degradation of the methyl orange can be assumed as:

Fig. 6
figure 6

Photocatalytic methyl orange degradation of CoWO4 nanoparticles under ultraviolet light

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$$\text{CoW}{{\text{O}}_{\text{4}}}+\text{ h}\nu \text{ }\to \text{ CoW}{{\text{O}}_{\text{4}}}^{\bullet }+\text{ }{{\text{e}}^{-}}+\text{ }{{\text{h}}^{+}}$$
(3)
$${{\text{h}}^{+}}+\text{ }{{\text{H}}_{\text{2}}}\text{O }\to \text{ O}{{\text{H}}^{\bullet }}$$
(4)
$${{\text{e}}^{-}}+\text{ }{{\text{O}}_{\text{2}}}\to \text{ }{{\text{O}}_{\text{2}}}^{\bullet -}$$
(5)
$$\text{O}{{\text{H}}^{\bullet }}+{{\text{O}}_{\text{2}}}^{\bullet -}+\text{ methyl orange }\to \text{ Degradation products}$$
(6)

Utilizing photocatalytic calculation by Eq. (1), the methyl orange degradation was about 82% after 70 min illumination of UV light in the presence of samples 1. This obtained result demonstrates that as-prepared CoWO4 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 CoWO4 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 CoWO4 nanoparticles [35–42].

Scheme 1
scheme 1

Reaction mechanism of methyl orange photodegradation over CoWO4 nanoparticles under UV light irradiation

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4 Conclusions

A strong photocatalyst CoWO4 nanoparticles was synthesized by a co-precipitation method. Several amino acid such as valine, glycine, and alanine were used as capping agent in order to obtained various morphology and size of CoWO4 nanoparticles. The VSM results demonstrated that all type of CoWO4 nanoparticles has a remarkable paramagnetic property. Also, the MO degradation by employing of photocatalytic effect of CoWO4 nanoparticles was about 81% after 80 min irradiation of UV light. The results indicated that the CoWO4 nanoparticles are interesting materials for photocatalytic applications under UV light to degradations of organic materials.