1 Introduction

Today, environmental pollution is a global issue, especially water pollution is a key problem. The textile industry is one of the most polluted industrial sectors and also a large amount of water for consumers. Textile wastewater usually includes of dyes, surfactants, minerals, heavy spiritual ions, detergents, electrolytes, solvents and stubborn compounds. Dye is the main source in textile wastewater, it affects the color of wastewater and reduces the transmission of light through expansion [1,2,3,4]. Water pollution is an acute and chronic effect on human health, affecting many different systems. Then, it is urgent to solve the problem of water pollution. No classified treatment of the organic pollution caused by treating wastewater indiscriminately is a worldwide environmental problem, which is very harmful to human health [5]. Therefore, it is an essential to find out an environment-friendly green technology which has a potential to improve environment. Recent days, different technical approaches such as ion exchange, electrochemical reactions, oxidation, adsorption and reduction have been referred for the removal of these pollutants from the industrial waste water [6]. In these methods, the photocatalytic degradation of organic pollutants by semiconductors is being paid more and more attention, which is the preferred method for the degradation of organic waste because it is efficient and cheap. In semiconductor photocatalysis, when material is exposed to photo radiation electron pair. Electrons enter the conduction band by creating holes in the valence band and reacting with pollutants through oxidation and reduction. Therefore, photocatalysts must be photosensitive, chemically stable, absorbing the maximum solar radiation, non-toxic and economical [7].

The significant energy percentage of the solar radiation (43%) corresponds to visible light. As a result, researchers have recently been interested in photocatalysts ultraviolet–visible–light–driven [8]. The development of new and more effective ultraviolet visible light driven photocatalyst has been still an important topic of solar energy utilization. CuWO4 is a promising material for solar oxidation that has received attention. Moreover, the degradation of contaminants in aqueous solution by CuWO4 nanochains has not been studied. The monoclinic structure of the wolframite type is common to the tungstate consisting of transition metals. The transition mental belongs to the fourth period of the periodic table (ZnWO4 [9], NiWO4 [10], CoWO4 [11], FeWO4 [12], and MnWO4 [13]. The only exception is CuWO4, which crystallizes at room temperature with a triclinic structure [14, 15]. In addition, the CuWO4 crystals with triclinic structure is affected by second-order Jahn–Teller (SOJT) effect. CuWO4 is a n-type semiconductor with an indirect band gap of 2.25 eV [16]. The band gap strongly depends on the structure of WO6 and CuO6 octahedra and how they are coupled [17, 18]. Research has shown that traditional binary metal oxide semiconductor has good photocatalytic stability. In addition, binary metallic oxide has the great potential in this aspect. The Cu (3d 2x 2−y ) and O (2p) hybridization orbit feature can effectively reduce the band gap of the material and extend absorption range. Moreover, the method of increasing the valence band in the formation of the ternary mental oxide is better than the reducing the location of semiconductor conduction band in order to reduce the band gap. Tungsten ions have a formal + 6 valence state (5d0 electron configuration), forming partial covalent bonds and six oxygen atoms. As a result, the octahedron of WO6 is distorted by the second-order SOJT effect. There configuration improves the absorption capacity of visible light and photocatalytic stability [19,20,21,22,23].

Due to the toxicity of MB, water and water waste have attracted significant attention in the green environment. In this contribution, we use the CuWO4 to investigate the photocatalytic degradation of organic pollutants MB performance in water solution by UV–Vis. The photocatalytic materials are including the nanoparticles and nanochains assembled from nanoparticles, the two type samples were synthesized though a solvothermal method. Additionally, the mechanism of photocatalytic degradation was analyzed by kinetic and photogenic electron hole.

2 Experimental

2.1 Materials

Copper (II) chloride dihydrate (CuCl2·2H2O), tungstate (VI) sodium dehydrate (Na2WO4·2H2O), copper (II) nitratetrihydrat (Cu(NO3)2·3H2O), sodium bromide (NaBr), polyvinyl pyrrolidone (PVP), trisodium citrate (C6H5O7Na3·2H2O), methylene blue (MB), ethylene glycol (EG) and ethanol were all obtained from Chuan Dong Ltd. All of these reagents were analytical grade and were used without further purification. Deionized (DI) water (18.2 MΩ cm) was used in the whole experiment.

2.2 Preparation of CuWO4 nanochains

CuWO4 nanochains was synthesized via solvothermal method as follows. Firstly, 1 mmol of Cu(NO3)2·3H2O and 1 mmol Na2WO4·2H2O were dissolved into a mixture of 15 mL of ethylene glycol and 5 mL of DI water. 5 mmol of C6H5O7Na3·2H2O and 1 mmol of NaBr were added into the as-prepared mixture solution under magnetic stirring. Then the green solution was magnetically stirred for 60 min at room temperature completely dissolved to a homogeneous solution and was transferred into a glass bottle. That were both transferred into 50 mL Teflon lined stainless steel autoclave and maintained at 170 °C for 12 h. After cooling to the room temperature, the resulting brick-red sediments were washed with anhydrous ethanol and DI water for several times to remove the surfactant and then dried at 60 °C in air for 8 h. After drying, the solid powders were annealed at 500 °C for 3.5 h at the heating rate of 2 °C/min under air atmosphere.

2.3 Preparation of CuWO4 nanoparticles

For the facile solvothermal method synthesis of CuWO4 nanoparticles, the standard synthesis process is as follows: firstly, 8 mmol of Na2WO4·2H2O and 8 mmol of CuCl2·2H2O were dissolved in 160 ml pure solvent of ethylene glycol. Then the blue solution was magnetically stirred at room temperature for 60 min and a certain amount of PVP was added into the above as-prepared solution. The above mixture was then stirred for 30 min, and the homogeneous blue-white precursor solution was transferred into the polytetrafluoroethylene lined stainless steel autoclave and heated at 170 °C for 12 h. After cooling to room temperature, the products were washed with DI water and hydrous ethanol several times, respectively, and then dried at 60 °C in air for 8 h. Finally, the powders were sintered in air at 500 °C for 3.5 h at a heating rate of 2 °C/min.

2.4 Characterization instruments

The powder XRD system (PXRD, Shimadzu ZD-3AX) equipped with Cu Kɑ radiation source (λ = 1.5406 Å) were studied in order to identify the crystallography of as-synthesized nanomaterials, 2 θ rang of 10°–70° with 5° and a step time of 1 min is used to collect the data. Fourier transform infrared (FTIR) spectra were performed using a Nicolet 6700 FTIR spectrophotometer in the range of 400–4000 cm−1 by a KBr pellet method with ratio of 1:100 for samples to KBr. The Brunauer–Emmett–Teller (BET) surface area was measured by the nitrogen adsorption isotherms measurements (Micromeritics ASAP 2020). The surface morphology was analyzed by scanning electron microscopy (FE-SEM, Nova 400 Nano-SEM). The UV–Vis absorption spectrophotometer (TU-1810SPC) was used to study the optical properties of all the samples in the spectral range of 200–800 nm.

2.5 Photocatalytic measurements

Photocatalytic activities of the CuWO4 nanoparticles and nanochains were evaluated by the photocatalytic degradation of MB neutral solutions. Experiments were conducted as follows: firstly, 10 mg of the MB were dispersed in 100 mL DI water and then put it in a 1000 mL volumetric flask, the 100 mL MB aqueous solution was taken out and placed in the beaker and covered with a black cloth. Subsequently, 40 mg of photocatalyst was added to 10 mg/L MB aqueous solution under ultrasonication for 10 min at room temperature until the homogeneous solution was formed. Before lighting, the mixed suspension was stirred in the dark for 60 min to achieve the adsorption–desorption equilibrium between the photocatalyst and MB. Then the stirred suspension was illuminated by a 300 W Xenon lamp (PLS-SXE300UV) as light source. At 10 min irradiation time intervals, after irradiation, 2 mL of suspensions were collected, the concentration of the reaction solution was determined by ultraviolet visible spectrophotometer and the absorption peak was measured at 664 nm. The total reaction time was 90 min, and the UV–Vis adsorption spectrum was recorded in the region of 200–800 nm.

The degradation efficiency (%) was calculated on the basis of the Eq. 1 as follow:

$$ {\text{degradation}}\, \left( \% \right) = \frac{{{\text{C}}_{0} - {\text{C}}}}{{{\text{C}}_{0} }} \times 100\% $$
(1)

where C0 is the initial concentration of MB after adsorption, and the desorption equilibrium is achieved before irradiation. C is the concentration of pollutants in different time intervals after photocatalytic reaction. In the absence of a catalyst, a blank test was conducted to assess the role of catalyst in the degradation process.

The efficiency of this degradation and the kinetic rate of MB dye degradation was computed in the presence of the synthesized CuWO4 nanoparticles and nanochains assembled from nanoparticles.

3 Results and discussion

3.1 Characterization of the synthesized CuWO4 with nanochains and nanoparticles

The typical XRD patterns of CuWO4 with nanochains and nanoparticles morphologies were synthesized via solvothermal method as shown in Fig. 1a. It is obvious that the powder diffraction pattern could be assigned to the pure phase of CuWO4 (JCPDS No. 72-0616), which are consistent with the pure CuWO4 phase (ICSD card No.16009) [24]. According to this figure, there are several peaks at 2 θ of 19 [100], 22.9 [110], 23.6 [0−11], 24.1 [011], 26.9 [101], 28.7 [−1−11], 30.1 [111], 36.4 [021], 38.6 [−120]. All the peaks in the XRD spectra are completely matched with the pure CuWO4 phase, and there are no obvious peaks resulting from impurities, such as copper oxide or tungstic oxide were discovered, proving that the triclinic phase is CuWO4. In order to confirm the triclinic structure of CuWO4 crystals, the structure of periodic or ordered CuWO4 crystal was refined by means of Rietveld method [25]. The refinement of the CuWO4 crystal is shown in Fig. 1b. The structure refinement data (Table 1) shows that the CuWO4 crystal has a triclinic structure with a point group (Ci), space group (P \( \bar{1} \), No.16009) and a molecular unit of two unit cell (Z = 2) [26]. The atomic position get from the Rietveld refinement data are illustrated in Table 1. The model describes the configuration of O–W–O and O–Cu–O bonds between [WO6] and [CuO6] clusters as a distorted octahedral structure. These clusters are made up of octahedron-type polyhedrons with twelve-sides, eight-faces and six-vertices, the distorted clusters play an important role in the formation of oxidant groups ·OH and ·O2− radicals [27].

Fig. 1
figure 1

a PXRD patterns of CuWO4 crystals with different morphologies and b schematic illustration of a triclinic CuWO4 unit cell indicating the distorted octahedral CuO6 and WO6 clusters

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Table 1 The refined data from Rietveld are the lattice parameters, unit cell volume, atomic coordinates and site occupation of CuWO4 crystal
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The CuWO4 nanochains and nanoparticles were further analyzed by FT-IR spectra. As is shown in Fig. 2a, b, the absorption peaks near 3424 cm−1 is called the antisymmetric/symmetric vibration of the free water molecules adsorbed on the surface of the prepared material, and the peak of the 1632 cm−1 is the OH expansion vibration of the free hydrogen bond hydroxyl radical. Seven peaks (at 480, 561, 627, 723, 808, 912 and 1385 cm−1) can be observed, which can be attributed to the tensile band and vibration of the CuWO4 [28]. In the WO4 structure, the infrared absorption peak at 480, 627 and 912 cm−1 might be due to the tensile mode of W–O. The small band at 808 cm−1 verified the stretching mode of the W–O-W bond. In the scope of 800–700 cm−1, the features of the belt are Cu–O stretching vibration [29, 30]. Besides, we compared the variation in surface area of the CuWO4 nanochains and nanoparticles. As is shown in Fig. 3a, b, the nitrogen adsorption–desorption isotherm shows that the Brunauer–Emmett–Teller (BET) surface area of CuWO4 nanochains is 4.742 m2/g, CuWO4 nanoparticles is 10.832 m2/g.

Fig. 2
figure 2

FTIR spectrum of CuWO4 nanocrystals a nanochains and b nanoparticles (the band of 808 and 723 cm−1 corresponds to the stretching vibration of W–O–W and Cu–O, respectively)

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Fig. 3
figure 3

Nitrogen adsorption—desorption isotherm a nanochains and b nanoparticles

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The morphology of the as-prepared CuWO4 nanochains and nanoparticles were characterized by SEM microcopy, as shown in Fig. 4a, b, the magnified image is inserted in the lower left corner. From SEM images it is revealed that the surface morphology, interestingly, Fig. 4a shows that the CuWO4 nanochains microcrystals are self-assembled by aggregated nanoparticles and the size of CuWO4 nanoparticles (Fig. 4b) is in the range of 20–70 nm.

Fig. 4
figure 4

SEM images of CuWO4 samples a nanochains and b nanoparticles

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3.2 Photocatalytic activity and spectral characteristics

Photocatalytic activity of products under UV–Vis light irradiation was evaluated by photocatalytic degradation method, and the adsorption kinetics and adsorption equilibrium of CuWO4 suspension were studied. In the dark, the adsorption equilibrium was established only for an hour. The maximum coverage of the surface was found to depend on the surface active sites, the chance of atomic collisions, and ultraviolet light exposure. The kinetic of adsorption is described by pseudo second order rate law. Figure 5a illustrates the absorption spectra of the MB solution in the absence of the catalyst taken at different degradation times under ultraviolet visible light irradiation. It can be seen that the autocatalysis of MB is negligible without CuWO4 catalyst. Figure 5b, c are the temporal evolution of spectra by the 40 mg CuWO4 nanochains and nanoparticles, which represent the photo degradation of 10 mg/L 100 mL MB under UV–Vis light illumination. Obviously, the absorption peaks for both samples at 664 nm corresponding to MB gradually become weaker, which is confirmed by the faded color of MB solution (Fig. 5d). Moreover, for CuWO4 nanoparticles, the time-dependent spectra exhibit lower peak intensity than that of CuWO4 nanochains, implying its better photo degradation activity of MB. In Fig. 5d, the curves of C/C0 versus irradiation time and time-dependent photographs of MB solution are shown. For the CuWO4 nanochains photocatalytic system, about 49% of MB was decomposed after 90 min irradiation. It is worth noting that for the photocatalytic system of CuWO4 nanoparticles, about 86% of MB was decomposed after irradiation 90 min.

Fig. 5
figure 5

Absorption spectra of the MB solutions with different degradation time under UV–Vis irradiation for a blank; b CuWO4 nanochains; c CuWO4 nanoparticles; d Curves of C/C0 versus irradiation time and time-dependent photographs of MB solution

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The physicochemical process of adsorption involves the transfer of MB dyes from liquid phase to the nanomaterial catalyst. In addition, the evaluation of reaction kinetics is the basis for the comparison of catalytic performance, kinetic analysis can be used to prove the effectiveness of the proposed mechanism. The study of the kinetics of the absorption process at different time periods provides a way of thinking for the mechanism of adsorption. For a quantitative understanding of the reaction kinetics of MB degradation, the kinetic curve of photocatalytic degradation has been studied by the simplified Langmuir–Hinshlwood (L–H) kinetics, which is well confirmed on the low concentration of photocatalyst [31,32,33,34]. The formula is as follows [35]:

$$ \ln (C_{0} /C) = Kt $$
(2)

where t is the irradiation time (90 min), C0/C is the concentration of adsorption–desorption equilibrium after different time intervals, and K is the apparent rate constant (min−1), which is derived from the gradient of the ln(C0/C) diagram relative to the time (t). The results show that in Fig. 6, it is clear that all the curves of ln(C0/C) versus time (t) are almost linear, the experimental data were fitted to Langmuir, and the R2 (correlation coefficient) value is as high as 0.9. The rate constants of photocatalytic degradation of MB by blank, nanochains and nanoparticles were 0.00137 min−1, 0.00596 min−1 and 0.02226 min−1, respectively. Degradation efficiency under above conditions was found to follow the order of blank < nanochains < nanoparticles. The higher degradation efficiency of CuWO4 nanoparticles may be due to its tremendous properties such as larger surface area, excellent water solubility and high absorbance [36]. The catalyst plays an important role in the whole process of degradation, the morphology of nanoparticles is the best because the nanochains are so heavy that they tend to settle, reducing the likelihood of collisions between molecules.

Fig. 6
figure 6

First-order kinetics data for the UV–Vis photo-degradation of aqueous MB over nanostructured CuWO4

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3.3 Possible mechanism

In principle, CuWO4 nanoparticles were obtained by the reaction between Cu2+ and WO42− ions as described in Eqs. (3–5):

$$ 2{\text{Na}}^{ + } \left( {\text{aq}} \right) + {\text{WO}}_{4}^{2 - } \left( {\text{aq}} \right) + 2{\text{H}}_{2} {\text{O}} + {\text{Cu}}^{2 + } \left( {\text{aq}} \right) + 2{\text{Cl}}^{ - } \left( {\text{aq}} \right) + 2{\text{H}}_{2} {\text{O}} + \left( {{\text{CH}}_{2} {\text{OH}}} \right)_{2} + {\text{PVP}} {\mathop{\longrightarrow}\limits_{ {25\,^\circ {\text{C}}} }^{ {20\,{\text{h}}} }} $$
$$ {\text{CuWO}}_{4} \left( {\text{amorphous}} \right) + 2{\text{Na}}^{ + } \left( {\text{aq}} \right) + 2{\text{Cl}}^{ - } \left( {\text{aq}} \right) + 4{\text{H}}_{2} {\text{O}} + \left( {{\text{CH}}_{2} {\text{OH}}} \right)_{2} + {\text{PVP}} $$
(3)
$$ {\text{CuWO}}_{4} \left( {\text{amorphous}} \right) {\mathop{\longrightarrow}\limits_{ {170\,^\circ {\text{C}}} }^{ {12\,{\text{h}}} }} {\text{CuWO}}_{4} \left( {\text{amorphous}} \right) $$
(4)
$$ {\text{CuWO}}_{4} \left( {\text{amorphous}} \right) {\mathop{\longrightarrow}\limits_{ {500\,^\circ {\text{C}}} }^{ {3.5\,{\text{h}}} }} {\text{CuWO}}_{4} \left( {\text{crystalline}} \right) $$
(5)

Based on the above results, the formation process and growth mechanism of CuWO4 nanochains assembled from nanoparticles are proposed, as shown in Scheme 1. From two aspects of thermodynamics and dynamics, in the beginning, when the saturation of the target solution exceeds the critical value, many tiny CuWO4 crystals nucleate in the solution. With the proceeding of the reaction, when the nucleation size is larger than the critical size, the change value of Gibbs’s free energy is greater than the surface energy, and the particles of different sizes are formed in the solution. The surface energy can be adjusted to change the growth rate, the surface free energy of larger particles is smaller compared to that of smaller particles, which increase at a smaller cost. However, according to a typical Ostwald ripening process, the interaction between the growing particles is stronger than the surface active agent [37]. Subsequently, the CuWO4 particles spread and gather together to form tiny CuWO4 nanosheets, because of the magnetic dipole–dipole interaction, these tiny sheets grow into larger nanochains that contribute to the assembly process, that only these main building blocks are linked together rather than results in specific structure directly [38, 39].

Scheme 1
scheme 1

Schematic representation of the formation process for the CuWO4 nanochains

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In the presence of nanostructured CuWO4 catalyst, the photocatalytic degradation rate of MB is much higher than that of compared to others. It can be deduced that the degradation reaction has changed considerably during the decolorization of organic dye pollutants. As we all know, the photocatalytic activity mainly comes from the photo-induced electrons and holes, and electrons and holes can effectively separate [40, 41]. The electrons and holes produced by photocatalyst have strong redox ability. However, they usually do not react directly with the organic dyes. On the contrary, some active species (such as ·OH and ·O2−) are formed through the reaction of H2O or O2 by charge and absorption [42, 43]. It is well learned that holes (h+), hydroxyl radicals (·OH) and superoxide anion radical (·O2−) are regarded as the reactive species of photocatalytic reaction [44]. When the CuWO4 samples were irradiated by UV–Vis light with the energy of higher or equal than it is band gap energy (hυ ≥ Eg), the photo generated electrons were induced in the conduction band, and holes were induced in the valence band and water oxidation reduction potentials. The specific schematic for degradation of MB over CuWO4 photocatalyst was shown in Scheme 2 and the mechanism of photocatalysis for degradation of MB is proposed as follows:

$$ {\text{CuWO}}_{4} + {\text{hv}} \to {\text{CuWO}}_{4} \left( {{\text{e}}^{ - }_{\text{CB}} + {\text{h}}^{ + }_{\text{VB}} } \right) $$
(6)
$$ {\text{e}}^{ - }_{\text{CB}} + {\text{O}}_{2} \to \cdot {\text{O}}_{2}^{ - } $$
(7)
$$ {\text{h}}^{ + }_{\text{VB}} + {\text{H}}_{2} {\text{O}} \to {\text{H}}^{ + } + {\text{OH}} \cdot $$
(8)
$$ {\text{h}}^{ + }_{\text{VB}} + {\text{OH}}^{ - } \to {\text{OH}} \cdot $$
(9)
$$ {\text{H}}_{2} {\text{O}} + \cdot {\text{O}}_{2}^{ - } \to {\text{O}}_{2} {\text{H}} \cdot \, + \,{\text{OH}}^{ - } $$
(10)
$$ 2{\text{O}}_{2} {\text{H}} \cdot \to {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}_{2} $$
(11)
$$ {\text{H}}_{2} {\text{O}}_{2} \to 2{\text{OH}} \cdot $$
(12)
$$ {\text{OH}} \cdot + {\text{MB}} \to {\text{degradation}}\;{\text{products}} $$
(13)
$$ \cdot {\text{O}}_{2}^{ - } + {\text{MB}} \to {\text{degradation}}\;{\text{products}} $$
(14)
Scheme 2
scheme 2

Reaction mechanism of MB photo-degradation over CuWO4 under UV–Vis light irradiation

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The results demonstrated that the CuWO4 nanomaterials may have high photocatalytic activity for remove of organic compounds from waste water under irradiation of UV–Vis light. Besides, on the basis of mechanism, OH·, O2H·, ·O2− and H2O2 play an important role in degradation process, if H2O2 is added into the aqueous solution of MB, it can promote the separation of excited carriers under illumination. In addition, the smaller particles can ensure better contact between atoms and a greater chance of collision, which is conducive to the reaction and the separation of photo generated electron–hole pairs. This proposal has given directions for future work and improvements.

4 Conclusions

In summary, the photocatalytic activity of CuWO4 was studied to assess their ability to depurate water streams efficiently from organic pollutants, such as MB. Here, CuWO4 nanoparticles have been synthesized through the solvothermal method, the nanoparticles sample has better degradation MB ability than nanochains. 86% MB were degraded by employing nanoparticles as photocatalyst after 90 min under irradiation of UV–Vis light, while the nanochains sample only degraded by 49%. In addition, it is demonstrated that the shape and size of materials have an impact on the photocatalytic activity of CuWO4, which follows the first order kinetic mechanism. By adjusting their size from a kinetic and thermodynamic perspective, the band gap of the material can be adjusted, the smaller the nanoparticles, the higher the frequency of absorption wavelengths. Our results indicated that the CuWO4 nanomaterials are potential materials for photocatalytic degradations of MB.