Optimization and detailed stability study on coupling of CdMoO4 into BaWO4 for enhanced photodegradation and removal of organic contaminant

Mohammad Eghbali-Arani; Saeid Pourmasoud; Farhad Ahmadi; Mehdi Rahimi-Nasrabadi; Vahid Ameri; Ali Sobhani-Nasab

doi:10.1016/j.arabjc.2018.05.007

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Original article

13 (

); 2425-2438

doi:

10.1016/j.arabjc.2018.05.007

Optimization and detailed stability study on coupling of CdMoO₄ into BaWO₄ for enhanced photodegradation and removal of organic contaminant

Mohammad Eghbali-Arani^{^a,⁎}, Saeid Pourmasoud^{^a}, Farhad Ahmadi^{^b}, Mehdi Rahimi-Nasrabadi^{^c,^d}, Vahid Ameri^{^e}, Ali Sobhani-Nasab^{^f,⁎}

a Department of Physics, University of Kashan, Kashan, Iran

b Department of Medicinal Chemistry, School of Pharmacy-International Campus, Iran University of Medical Sciences, Tehran, Iran

c Chemical Injuries Research Center, Baqiyatallah University of Medical Sciences, Tehran, Iran

d Department of Chemistry, Faculty of Pharmacy, Baqiyatallah University of Medical Sciences, Tehran, Iran

e Department of Physics, Faculty of Science, University of Hormozgan, Bandar-Abbas, Iran

f Young Researchers and Elites Club, Arak Branch, Islamic Azad University, Arak, Iran

⁎Corresponding authors. m.eghbali@kashanu.ac.ir (Mohammad Eghbali-Arani), Ali.sobhaninasab@gmail.com (Ali Sobhani-Nasab)

Received: 2018-2-21, Accepted: 2018-5-16, Published: 2018-01-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

BaWO₄ nanoparticles and BaWO₄/CdMoO₄ nanocomposites were synthesized by eco-friendly method at low temperature. The effects of various capping agents such as glucose, fructose, lactose, and starch on the morphology and particle size of BaWO₄/CdMoO₄ nanocomposites were investigated. The nanocluster was obtained with glucose and Ba²⁺ to capping agent molar ratio of (1:2). BaWO₄/CdMoO₄ nanocomposites were analyzed through techniques including, XRD, FT-IR, VSM, TEM, PL, FESEM, UV–vis, and EDX. According to the BET results, in the presence of glucose as the capping agent, surface area increased from 13.781 m₂/g to 122.22 m₂/g. Furthermore, adding CdMoO₄ nanoparticles to BaWO₄ causes optical properties and photocatalytic activity of BaWO₄ to improve. Moreover, the effects of several factors such as BaWO₄/CdMoO₄ nanocomposites concentration and its particle size and difference dyes on the photocatalytic performance of BaWO₄/CdMoO₄ nanocomposites were studied under visible light.

Keywords

BaWO4/CdMoO4

Visible light

Nanocomposites

Eco-friendly method

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1 Introduction

Dye is one of the most important chemicals applied in many industries such as paint, food and furniture. However, it can be considered as a real threat to the environment. Synthetic dyes are main pollutant of wastewater. Up to now, extensive research has been carried out to decrease concentration of organic dyes in wastewater (Zhong et al., 2012; Zhu et al., 2018a; Dutta et al., 2009; Ozer et al., 2007; Fan et al., 2009 ). Several methods have been developed to synthesize photocatalysts including reverse-microemulsion process (Mi et al., 2009); solvothermal synthesis (Zhang et al., 2006), microwave assisted citrated complex (Ryu et al., 2005), combustion synthesis (Xia et al., 2001), solid-state reaction (Pupp et al., 1969), complex capping agentization (de Azevedo Marques et al., 2006), molten salt route (Afanasiev, 2007), and electrochemical method (Sun et al., 2011). Nowadays, tungstates and molybdates have received much attention due to their application in many fields involving humidity sensors, catalysts, solid states lasers, microwave applications, photoluminescent devices, and so on. They have scheelite-type tetragonal structure. Each X atom (X = Mo and W) is surrounded by four equivalent O atoms consisting the [XO₄]² tetrahedral configuration and every divalent metal shares corners with eight oxygen atoms of [XO₄]² tetrahedrons (Kongsinlark et al., 2012; Joshi et al., 2014). BaWO₄ has been extensively used as catalysts with good efficiency for the photodegredation of organic pollutants. Despite the fact that BaWO₄ is a well-known photocatalyst, it has two important problems. Firstly, this semiconductor is active under UV (Wei et al., 2010; Sathishkumar et al., 2011). Secondly, fast recombination and disappearance of photogenerated electron-hole (e-h) pairs in BaWO₄ can reduce the effective degredation of pollutants (Ahmadi et al., 2018). Therefore, a tremendous effort has been made to tackle these problems. There are two significant methods to enhance the photocatalytic efficiency of BaWO₄. One refers to the coupling of two semiconductors and the other is to add metal ion dopants as modifier into the matrix of BaWO₄ (Safardoust-Hojaghan and Salavati-Niasari, 2017). The aim of this study is to synthesize BaWO₄/CdMoO₄ nanocomposites at low temperature by the co-precipitation process and investigate its photocatalytic properties of final products under visible light. In addition to this, the effects of solution pH and photocatalyst loading were considered as well. To the best of our knowledge, this is a first report on the preparation of BaWO₄/CdMoO₄ nanocomposites. Using glucose, fructose, lactose, and starch with the Ba²⁺ to capping agent molar ratios of (1:1) and (1:2), can be considered as the novelty of the preparation of BaWO₄/CdMoO₄ nanocomposites. As the obtained nanocomposite was studied by scanning electron microscopy, Fourier transforms infrared spectroscopy, vibrating sample magnetometer, X-ray diffraction, Brunauer–Emmett–Teller (BET), UV–visible absorption, and X ray energy dispersive spectroscopy.

2 Experimental

2.1

2.1 Synthesis of BaWO₄ nanoparticles

At first, 1 mmol of Ba(NO₃)₂.4H₂O was dissolved in 20 ml of distilled water, which we called it solution A. Then, 1 mmol of Na₂WO₄·H₂O and 3 mmol of capping agent were dissolved in 30 mL of distilled water and mixed together to form solution B. Afterwards, solutions A and B were mixed under constant stirring for 45 min at room temperature. Finally, the precipitation was centrifuged and dried at room temperature.

2.2

2.2 Synthesis of BaWO₄/CdMoO₄ nanocomposite

BaWO₄/CdMoO₄ nanocomposites were prepared by co-precipitation method. Ba(NO₃)₂·4H₂O and green capping agents with different molar ratios were dissolved in 30 ml of distilled water (solution A). The solution B was formed by adding 1 mmol of Na₂WO₄·H₂O to 30 mL of distilled water. Then, solution A was added into solution B under vigorous stirring to make solution C. Subsequently, two solutions containing 1 mmol of Cd(CH₃CO₂)₂ and (NH₄)₆Mo₇O₂₄.4H₂O was added to solution C under constant stirring. Finally, the obtained precipitation was washed three times with distilled water and dried out at 25 °C for 3 h. Synthesis pathway of the BaWO₄/CdMoO₄ nanocomposites is shown in Scheme 1. All reaction conditions are listed in Table 1.

Schematic the effects of capping agents and their different ratio on the size of final products.

Table 1 The preparation conditions of the BaWO₄ nanostructures and BaWO₄/CdMoO₄ nanocomposites.

Sample no	Capping agents	Molar ratio (M: Capping agents)	Temperature °C	Product	Debye–Scherrer crystallite size (nm)	Figure of SEM images
1	Glucose	(1:2)	25	BaWO₄	21.1	–
2	–	–	25	BaWO₄/CdMoO₄	36.7	Fig. 3a
3	Glucose	(1:1)	25	BaWO₄/CdMoO₄	31.5	Fig. 3b
4	Fructose	(1:1)	25	BaWO₄/CdMoO₄	32.7	Fig. 3c
5	Lactose	(1:1)	25	BaWO₄/CdMoO₄	29.8	Fig. 3d
6	Starch	(1:1)	25	BaWO₄/CdMoO₄	30.6	Fig. 3e
7	Glucose	(1:2)	25	BaWO₄/CdMoO₄	24.6	Fig. 4a
8	Fructose	(1:2)	25	BaWO₄/CdMoO₄	25.8	Fig. 4b
9	Lactose	(1:2)	25	BaWO₄/CdMoO₄	25.6	Fig. 4c
10	Starch	(1:2)	25	BaWO₄/CdMoO₄	25.3	Fig. 4d

2.3

2.3 Materials and physical measurements

All of the chemicals used in synthesis of BaWO₄/CdMoO₄ nanocomposites including Ba(NO₃)₂·4H₂O, Na₂WO₄·H₂O, Cd(CH₃CO₂)₂, (NH₄)₆Mo₇O₂₄.4H₂O, glucose, fructose, lactose, and starch were purchased from Merck Company and didn’t purify any more. The XRD patterns were recorded by a Philips-X’pertpro, X-ray diffractometer using Ni-filtered Cu Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on Nicolet Magna- 550 spectrometer in KBr pellets. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. The EDX analysis with 20 kV accelerated voltage was done. Transmission electron microscopy (TEM) image was found by a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. The diffused reflectance UV–visible spectrums (DRS) of the samples, were recorded by the V-670 UV–Vis Spectrophotometer (Jasco). The magnetic measurements 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.

2.4

2.4 Photocatalytic measurements

The photocatalytic ability of BaWO₄/CdMoO₄ nanocomposites in the presence of varied dyes was measured. The degradation reaction was conducted in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 5 × 10⁻⁵ M of solutions containing 0.05 g of nanocomposites. The mixture was aerated for 30 min, and it was positioned in the photoreactor in which the vessel was 20 cm away from the light source (a 400 W Osram lamp). The mixture was aliquoted in periodic intervals during the irradiation, and previous to be analyzed with the UV–Vis spectrometer, it was centrifuged. The percentage of dyes degradation was evaluated through the following formula:

(1)

Degradation rate (%) = \frac{100 (C_{0} - C_{t})}{C_{0}}

where C₀ and C_t are the absorbance value of solution at 0 and t min, respectively.

3 Results and discussion

XRD patterns of BaWO₄ nanostructures are shown in Fig. 1a. The spectrum of bare BaWO₄ sample shows a series of diffraction peaks at the position of 26.32° ((112) line), 32.18° ((002) line), 43.18° ((200) line), and 54.63° ((222) line) which is in good agreement with the standard JCPDS file of BaWO₄ tetragonal phase (space group I41/a, JCPDS No. 85-0588). The XRD patterns of BaWO₄/CdMoO₄ nanocomposites with different parameters such as molar ratio and capping agent have been displayed in Fig. 1a–j, correspondingly. As our results suggest, all prepared nanocomposites are pure and have two phases. The first is the phase of BaWO₄ and CdMoO₄ with crystal structure of tetragonal (JCPDS 85-0588) and space group of I41/amd and the second is tetragonal (JCPDS 85-0888) with space group of I41/a, respectively. From XRD data and Scherrer equation the crystallite diameter (Dc) of BaWO₄ nanostructures and BaWO₄/CdMoO₄ nanocomposites, samples 1–10, calculate to be 21.1, 36.7, 31.5, 32.7, 29.8, 30.6, 24.6, 25.8, 25.6 and 25.3 nm, respectively (Javidan et al., 2015).

(2)

Dc = \frac{K λ}{β \cos θ}

XRD pattern of BaWO4 obtained in the presence of (a) glucose, (b) BaWO4/CdMoO4 without capping agent, and BaWO4/CdMoO4 obtained with the Ba2+ to capping agent molar ratio of (1:1) in the presence of (c) glucose (d) fructose, (e) lactose and (f) starch, BaWO4/CdMoO4 obtained with the Ba2+ to capping agent molar ratio of (1:2) in the presence of (g) glucose (h) fructose, (i) lactose and (j) starch.

In which β is the breadth of the noticed diffraction line at its half intensity maximum, K is the so-called shape factor, which commonly takes a value of approximately 0.9, and λ is the wavelength of X-ray source applied in XRD. Therefore, one can simply find out that the steric hindrance of a capping agent increases with the decrease in the size of nanocomposites.

The purity of BaWO₄/CdMoO₄ nanocomposites was confirmed by EDS technique. As illustrated in Fig. 2 (sample No. 7), the BaWO₄/CdMoO₄ nanocomposites are composed of Ba, W, Cd, Mo, and O elements. Furthermore, no impurity peaks were observed which indicates high purity level of as-prepared BaWO₄/CdMoO₄ nanocomposites.

EDS pattern of BaWO4/CdMoO4 nanocomposites obtained with the Ba2+ to capping agent molar ratio of (1:2) in the presence of glucose (Sample No. 6).

The reaction mechanism with capping agent can be proposed as follow:

(3)

7 {Na}_{2} {WO}_{4} \cdot 2 H_{2} O + (capping agent) \to 14 Na + + 7 {WO}_{4}^{2 -} + (capping agent) + 14 H_{2} O

(4)

7 Ba {({NO}_{3})}_{2} \cdot 4 H_{2} O \to 7 {Ba}^{2 +} + 14 {NO}_{3}^{-} + 28 H_{2} O

(5)

7 {Ba}^{2 +} + 7 W O_{4}^{2 -} \to 7 {BaWO}_{4}

(6)

14 {Na}^{+} + 14 {NO}^{3 -} \to 14 {NH}_{4} {NO}_{3}

(7)

7 Cd {({CH}_{3} COO)}_{2} \cdot 2 H_{2} O \to 7 {Cd}^{2 +} + 14 {CH}_{3} {COO}^{-} + 14 H_{2} O

(8)

{({NH}_{4})}_{6} {Mo}_{7} O_{24} \cdot 4 H_{2} O + (capping agent) \to 6 {NH}_{4}^{+} + 7 {MoO}_{4}^{2 -} + (capping agent) + 8 H +

(9)

14 {CH}_{3} {COO}^{-} + 8 H^{+} + 6 {NH}_{4}^{+} \to 14 {CH}_{3} COOH

(10)

7 {Cd}^{2 +} + 7 {MoO}_{4}^{2 -} \to 7 {CdMoO}_{4}

(11)

7 {Na}_{2} {WO}_{4} \cdot 2 H_{2} O + 7 Ba {({NO}_{3})}_{2} \cdot 4 H_{2} O + 7 Cd {({CH}_{3} COO)}_{2} \cdot 2 H_{2} O + {({NH}_{4})}_{6} {Mo}_{7} O_{24} \cdot 4 H 2 O + (capping agent) \to 7 {BaWO}_{4} / {CdMoO}_{4} + 14 {NH}_{4} {NO}_{3} + 14 {CH}_{3} COOH + (capping agent) + 60 H_{2} O

Green capping agents and surfactants used in different nanoparticles synthesis procedure can cause de-agglomerate particles by reducing condensation reaction in liquid phase (Zinatloo-Ajabshir et al., 2017; Hosseinpour-Mashkani et al., 2016; Sobhani-Nasab et al., 2016 ). The effects of difference molar ratios of carbohydrates as green capping agents to metal (Ba²⁺) on the size and morphology of the final products were investigated, as shown in Schematic 2. A blank test in the absence of capping agent was performed and as a result, micrometer nanocomposites were produced, as shown in Fig. 3a. Next, the different kinds of capping agents with the Ba²⁺ to capping agent molar ratio of (1:1) were employed. Furthermore, the prepared nanocomposites in the presence of glucose as a capping agent, the prepared nanoclusters were mixed with sphere-like nanoparticles. Also, the size of nanoparticles became smaller as we changed the type of capping agent (from glucose to fructose, lactose, and starch, respectively Fig. 3(b)–(e). Increasing capping agent, glucose, to Ba²⁺ molar ratio from (1:1) to (1:2) causes uniform nanoclusters with appropriate size distribution to produce. Moreover, changing glucose to other capping gents causes morphology of products to change from nanoclusters to sphere-like nanoparticles Fig. 4a–d).

SEM images of BaWO4/CdMoO4 nanocomposites obtained (a) without capping agents and with the Ba2+ to capping agent molar ratio of (1:1) in the presence of four different capping agents (b) glucose, (c) fructose, (d) lactose, and (e) starch.

SEM images of BaWO4/CdMoO4 nanocomposites obtained with the Ba2+ to capping agent molar ratio of (1:2) in the presence of four different capping agents (a) glucose, (b) fructose, (c) lactose, and (d) starch.

Transmission electron microscopy (TEM) technique was used to further examine the morphology of nanocomposites. Two typical samples of as-prepared nanocomposites in the presence of glucose and starch with the Ba²⁺ to capping agent molar ratio of (1:2) in different magnification have been depicted in Fig. 5. The as-prepared nanoclusters with the size of nearly 50–70 nm in the presence of glucose Fig. 5(a) and sphere-like nanocomposites prepared in the presence of starch have been shown in Fig. 5b, respectively.

TEM image of BaWO4/CdMoO4 nanocomposites obtained with the Ba2+ to capping agent molar ratio of (1:2) in the presence of (a) glucose, and (b) starch.

Fig. 6a and b shows the FT-IR spectra of the BaWO₄ nanoparticles (sample No. 1) and BaWO₄/CdMoO₄ nanocomposites (sample No. 7), respectively. The characterization peaks in the BaWO₄ nanoparticles spectrum are 835 cm⁻¹ which is attributed to vibration of atoms in tetrahedral oxygen environment; W–O at BaWO₄ and 824.58 cm⁻¹ which is related to vibration of Ba atoms in the octahedral oxygen environment in nanosized BaWO₄ nanoparticles. Moreover, according to Fig. 6b, the FT-IR spectrum of BaWO₄/CdMoO₄ nanocomposites, bands at 876 cm⁻¹ are related to the vibration of W-O at BaWO₄ (de Azevedo Marques et al., 2006). Besides, the characteristic peaks at 654 and 579 cm⁻¹ are belong to O-Mo-O bending mode and Cd-O stretching modes, respectively. Moreover, the band located in the region 3200–3700 cm⁻¹ could be attributed to the v(OH) stretching vibration of physisorbed water molecules (Sobhani-Nasab et al., 2017).

FT-IR spectra of (a) BaWO4 nanostructures (Sample No. 1) and (b) BaWO4/CdMoO4 nanocomposites (Sample No. 7).

The magnetic property of BaWO₄/CdMoO₄ nanocomposites at 300 K is shown Fig. 7. VSM data demonstrate that as-synthesized BaWO₄/CdMoO₄ nanocomposites have ferromagnetic properties, and the correspondent magnetization value is nearly 0.042 emu/g.

VSM curve of BaWO4/CdMoO4 nanocomposites (Sample No 6).

The thermal characteristics of BaWO₄ nanoparticles and BaWO₄/CdMoO₄ nanocomposites in temperature range of 27–850 °C were examined by thermo gravimetric (TG) analysis. The graphs have been presented in Fig. 8. The TG curves of BaWO₄ and BaWO₄/CdMoO₄ nanoparticles exhibit two stages of weight loss in the temperature between 30–200 °C and 200–550 °C as well. The first stage of weight loss could be associated with water molecules, which have been physically absorbed on the surface of catalysts. The next one, could be linked with the removal of chemisorbed water molecules. The organic resides trapped inside the pores on the surface of catalyst were released in the weight loss observed between 200–550 °C. An ove 550 °C, no considerable weight loss is seen for two samples, suggesting that phase transition and crystallization do not take place in this temperature region. These findings reveal that both catalysts have more stability.

Thermogravimetric profiles of BaWO4 nanostructures and BaWO4/CdMoO4 nanocomposites.

Fig. 9a and b shows the adsorption/desorption isotherm and BJH plot of as-made BaWO₄/CdMoO₄ nanocomposites with and without glucose to evaluate their pore volume and surface area. Fig. 9b reveals broad pore size distribution with maximum around pores of 64 nm diameter. The total pore volume and average pore diameter for this sample were measured to be 0.06164 cm³/g and 21.54 nm, respectively. Moreover, the BET analysis was found to be 13.781 m2/g specific surface areas. The surface of the prepared composition in the presence of glucose increased, whereas its size decreased as shown in Fig. 9. Based on the IUPAC classifications, the N₂ adsorption/desorption isotherm belongs to type IV isotherms with H1-type hysteresis loops, that is the substantial features for ordered mesophorous materials. The pore of size distribution for BaWO₄/CdMoO₄ nanocomposites has been depicted in Fig. 9b, which demonstrates broad pore size distribution with maximum around pores of 10.57 nm diameter. The total pore volume and mean pore diameter for this sample was computed approximately 0.3136 cm³/g and 11.431 nm, respectively, and the BET analysis was found to be 122.22 m²/g specific surface areas. Hence, one can conclude that the use of capping agent in the synthesis of nanoparticles can lead to decreased size and increased surface area of nanoparticles Table 2).

BET pattern of BaWO4/CdMoO4 nanocomposites obtained (a,b) without capping agents and (c,d) in the presence of glucose.

Sample no	Debye–Scherrer crystallite size (nm)	Capping agents	Total pore volume	Mean pore diameter	Specific surface areas
BaWO₄/CdMoO₄	36.7	–	0.06164	21.54	13.781
BaWO₄/CdMoO₄	24.6	Glucose	0.3136	11.431	122.22

The optical properties of the BaWO₄/CdMoO₄ nanocomposites were characterized by photoluminescence (PL) spectroscopy. Room temperature PL spectrum of as-obtained samples excited at 350 nm is presented in Fig. 10. The PL spectrum of the BaWO₄ shows a blue maximum emission around 521 nm as shown in Fig. 10. The PL spectrum of BaWO₄/CdMoO₄ nanocomposites is around 537 nm as shown in Fig. 10. The red-shift observed in the BaWO₄/CdMoO₄ nanocomposites compared to BaWO₄, could be due to the possible presence of the CdMoO₄. Another interesting effect in PL spectra is the enhancement in the emission for the BaWO₄/CdMoO₄ nanocomposites, which can be associated with an inter band connection between the interface of BaWO₄ and CdMoO₄, where CdMoO₄ confines the photo generated electron–hole pairs to the BaWO₄ interface which is modified by the quantum confinement effect. This, leads to the passivation of non-radiative transitions, thus enhancing the luminescence intensity. Thus, the nanocomposites interface can be directly monitored by PL.

Room temperature PL spectra of BaWO4 nanostructures and BaWO4/CdMoO4 nanocomposites.

The optical property of nanocomposites, which is related to the electronic structure, can be considered as an important factor to specify the photocatalytic properties. The UV–Vis diffuse reflectance spectra of BaWO₄ and BaWO₄/CdMoO₄ nanocomposites have been illustrated in Fig. 11 a. The absorption edge for all the samples falls in the visible region (k > 250 nm) which indicates that the entire sample is active at UV-Visible light, with enhanced absorption property observed for BaWO₄/CdMoO₄ photocatalyst. The band gaps of the samples were determined from the [αhν]^1/2 versus photon energy (hν) plot as shown in Fig. 11b. It could be noted that each spectrum has two slopes. Therefore extrapolating the linear region of the curve gives the band gap values for BaWO₄ and BaWO₄/CdMoO₄ as 3.00 eV, and 2.85 eV, respectively. As there is no doping between the metal oxides, the existence of two band gaps in BaWO₄/CdMoO₄ could be related to the vast difference in the conduction band energy of the metal oxides.

UV–vis diffuse reflectance spectrum (a) of the as prepared BaWO4 nanostructures and BaWO4/CdMoO4 nanocomposites (sample No. 1) and (b) Tauc plot pattern BaWO4 nanostructures and BaWO4/CdMoO4 nanocomposites (sample No. 7).

Recently nanoparticles and nanocomposites are frequently being used as catalysts (Yu et al., 2017; Wang and Zeng, 2017; Zhu et al., 2018b,c,2017a; Pastrana-Martínez et al., 2014; Chen et al., 2018; Wang et al., 2018; Li et al., 2018 ). Photocatalytic activities of BaWO₄ and BaWO₄/CdMoO₄ nanocomposites have been investigated under visible light irradiation. In Fig. 12 we have shown photocatalytic reactions for Rh B in solution with various pH levels. As one can expect, the photocatalytic activity was improved with increase in the value of pH. It has been proposed that the impact of solution pH on the photocatalytic degredation is a complex subject, which is related to the adsorption characteristics of substrate onto photocatalyst surface and reaction mechanism. Based on the principle of heterogeneous photocatalysis, the concentration of HO^• ions can be considered as a crucial factor for the generation of HO^• radicals. Hence, at higher pH value, the formation of hydroxyl ion was preferred. However, it should be emphasized that the lifetime of HO^• radicals is very short and photocatalytic reactions can only take place at or near the surface of photocatalyst. Thank to the non-ion property of Rh B, the neutral medium seems to be advantageous for the absorption of Rh B. Consequently, considering the combined influence of generation of OH and the interaction between surface of photocatalyst and Rh B, the photocatalytic degradation of Rh B was highly effective in neutral medium (Sobhani-Nasab et al., 2015).

The influence of the various pH on the Rh B destruction percentage (samples No. 7).

Based on Fig. 13a, BaWO₄ (sample No. 1) can destruct only 60.3% of Rh B pollutant, whereas destruction of this pollutant in BaWO₄/CdMoO₄ nanocomposites (sample No. 2) can peak at 98%. Therefore, BaWO₄ had substantial influence on photocatalytic activity of CdMoO₄. Interestingly, BaWO₄ nanoparticles can contribute to a red shift in nanocomposites, which caused reduce in band gap and more adsorption in visible light. Moreover, it is suggested that, adding CdMoO₄ may give rise to appropriate distribution of BaWO₄, facileness of the absorption of Rh B pollutant, and its transition to the active sites on BaWO₄, and further separation of charge carrier as well. Therefore, it can cause the enhancement in destruction efficiency of BaWO₄ to Rh B pollutant. Thus, introducing CdMoO₄ can enhance effectiveness of BaWO₄ (Huang et al., 2014; Zinatloo-Ajabshir et al., 2017).

(a) The photocatalytic behavior of BaWO4 nanostructures (sample No. 1) and BaWO4/CdMoO4 nanocomposites (sample No. 2), (b) the influence of the dosage of the BaWO4/CdMoO4 nanocomposites on the Rh B destruction percentage (samples No. 6), (c) the photocatalytic behavior of BaWO4/CdMoO4 nanocomposites (sample No. 7) on decomposition of various contaminants (cationic and anionic types), and (d) effects of the reuse of the BaWO4/CdMoO4 nanocomposites on the degradation efficiency of RhB (sample No. 7).

Fig. 13a demonstrates effects of various capping agents on the photocatalytic behavior of BaWO₄/CdMoO₄ nanocomposites (samples 6–9). Additionally, compounds with the Ba²⁺ to capping agent molar ratio (1:2), have higher photocatalytic degradation. Moreover, fabricated nanocomposites with the capping agent of glucose (sample No. 6) and degradation rate of 99%, demonstrated the most appropriate outcome for the photocatalytic test.

Blank test, Fig. 13a, was carried out in the absence of catalyst to examine the stability of Rh B under visible light. The efficiency of blank test was 4%, which showed a comparatively good stability. The effect of BaWO₄/CdMoO₄ nanocomposites concentration on the Rh B destruction has been shown in Fig. 13b. Based on Fig. 13b, the quantities of samples are positively correlated with the percentage of destruction. The destruction rate for 50, 100, and 150 mg of nanocomposites (sample No 7) were 80, 94, and 98%, respectively. Thanks to an insignificant difference between photocatalytic activity of 100 and 150 mg of samples, only 4 percent, and tangible dissimilarity of the second and third samples for this behavior, 15 percent, the appropriate and optimum value for BaWO₄/CdMoO₄ nanocomposites, were chosen to be 100 mg.

The photocatalytic degradation of four various dyes, rhodamine B (Rh B), 2-naphthol (Na), methylene blue (MB), phenol red (Ph R), and eosin Y (EY) in the presence of BaWO₄/CdMoO₄ nanocomposites was examined under visible irradiation Fig. 13c). Looking at this figure, one can find out that photocatalytic activity of nanocomposites in decolouration of Rh B is more than other dyes. The photocatalytic efficiencies of Rh B, Na, MB, Ph R and EY are 99%, 88%, 85%, 79%, and 71%, respectively.

In addition, repeated degradation reactions have been carried out to determine the stability of the BaWO₄/CdMoO₄ nanocomposites. As illustrated in Fig. 9d, the BaWO₄/CdMoO₄ nanocomposites (sample No. 7) did not demonstrate any substantial loss of activity after five consecutive reaction cycles, suggesting the high stability of catalyst. The degradation efficiency decreased to 80% after six cycles. The results of photocatalysis survey clearly disclosed that BaWO₄/CdMoO₄ nanocomposites may be applied as a useful and successful photocatalyst under visible light.

The visible light induced photocatalytic activities of bare BaWO₄ and BaWO₄/CdMoO₄ were evaluated via the photocatalytic Rh B degradation. Fig. 9 shows the time-dependent profiles of Rh B degradation in the presence of our catalysts under visible light irradiation. In these figures, all the plots of (ln(Ct/C0) = k_appt) versus time (C is the Rh B concentration at time t and C₀ is the initial Rh B concentration) are mostly linear, proving that the kinetics of photocatalytic Rh B decomposition can be described by the pseudo-first-order Langmuir-Hinshelwood kinetic model. Table 3 presents the apparent first-order rate constant (k) of MB degradation for our catalysts under visible light. It can be seen that bare BaWO4/CdMoO4 sample only operated under UVA light (k = 1.556 h⁻¹) and showed very low activity BaWO4 under visible Table 3.

Sample no	Debye–Scherrer crystallite size (nm)	Band edge (nm)	Band gap values (eV)	Rate constant of Rh B degradation (h⁻¹)	λ maximum emission
BaWO₄	21.1	472	3.00	0.0934	521
BaWO₄/CdMoO₄	24.6	498	2.85	0.2765	537

These results demonstrated a high degree of the BaWO₄/CdMoO₄ nanocomposites (sample No. 7) to be employed as a favorable, suitable, and new type of photocatalyst under visible light for elimination of cationic contaminants. The composition procedure of contaminants for the BaWO₄/CdMoO₄ nanocomposites has been shown in Scheme 2. Once the surface of catalyst is illuminated with the light energy higher than its band gap energy, the holes (h⁺) in the valence band and an electron (e⁻) in the conduction band of BaWO₄/CdMoO₄ nanocomposites (Eq. (12)) can be formed. The holes (h⁺) either operate as an oxidizing agent and directly oxidize the pollutant or react with water to form hydroxyl radicals (Eq. (17)). Simultaneously, the electrons (e⁻) in the conduction band acts as a reducing agent and reduce the oxygen adsorbed on the surface of BaWO₄/CdMoO₄ nanocomposites (Eq. (17)). The probable mechanism for the photodegradation of dye is presented as following (Zinatloo-Ajabshir and Salavati-Niasari, 2017; Aoudjita and Martinsc, 2018; Zhu et al., 2017b ).

(12)

BaWO₄/CdMoO₄ + h⁺ → BaWO₄/CdMoO₄^• + e_CB⁻ + h_VB⁺

(13)

e_{CB}^{-} + h_{VB}^{+} \to heat (unproductive)

(14)

h_VB⁺ + dye → dye^•+ → oxidative degradation of dye

(15)

h_VB⁺ + H₂O → H⁺ + HO^•

(16)

h_VB⁺ + HO⁻ → HO^•

(17)

O₂ + e_CB⁻ → O₂^•−

(18)

O^{2 •} + 2H₂O → HOO^• + 2HO⁻

(19)

2HOO^• + H₂O + e_CB⁻ → HOO^• + 2HO⁻

(20)

2HOO^• + H₂O + e_CB⁻ → H₂O₂ + HO⁻

(21)

H_{2} O_{2} + e_{CB}^{-} \to OH + HO

(22)

H₂O₂ + h⁺ → 2HO^•

(23)

HO^• + dye → dye degradation

(24)

O^{2 •−} + dye → dye degradation

Schematic illustration of charge transfer in a coupled semiconductor system (BaWO4/CdMoO4 Nanocomposites).

4 Conclusions

The BaWO₄ nanoparticles and BaWO₄/CdMoO₄ nanocomposites were prepared using green method. To the best of our knowledge, it is the first time that BaWO₄/CdMoO₄ nanocomposites are prepared via carbohydrate sugars as a green capping agent. The influence of various carbohydrate sugars including glucose, lactose, fructose, and starch on the shape and size of final products were investigated. Influences of different parameters such as type of dye and light visible on photocatalytic ability of samples were studied. The highest and lowest percentages of degradation of dyes were obtained for Rh B and EY dyes with 99% and 60.3%, respectively. According to the photocatalytic results, one can suggest the high potential of final products for the photocatalytic applications under visible light in degradation of Rh B dye.

Acknowledgment

Authors are grateful to council of University of Kashan and University of Hormozgan for providing financial support to undertake this work. This work was supported by research council of University of Kashan – Iran by Grant Agreement, No. 682151/9.

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Introduction

Experimental

Synthesis of BaWO4 nanoparticles
Synthesis of BaWO4/CdMoO4 nanocomposite
Materials and physical measurements
Photocatalytic measurements

Results and discussion

Conclusions

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[1] Afanasiev P., . Molten salt synthesis of barium molybdate and tungstate microcrystals. Mater. Lett.. 2007;61:4622-4626.
[Google Scholar]

[2] Ahmadi F., Rahimi-Nasrabadi M., Behpour M., Ganjali M.R., . A simple process for the preparation of photocatalytically active bismuth aluminate nanoparticles. J. Mater. Sci.: Mater. Electron.. 2018;29:146-152.
[Google Scholar]

[3] Aoudjita L., Martinsc P.M., Madjenea D.F., Petrovykhe D.Y., Lanceros-Mendez S., . Photocatalytic reusable membranes for the effective degradation of tartrazine with a solar photoreactor. J. Hazard. Mater.. 2018;344:408-416.
[Google Scholar]

[4] Chen P., Dong F., Ran M., Li J., . Synergistic photo-thermal catalytic NO purification of MnO_x/g-C₃N₄: enhanced performance and reaction mechanism. Chin. J. Catal.. 2018;39:619-629.
[Google Scholar]

[5] de Azevedo Marques A.P., de Melo D.M.A., Paskocimas C.A., Pizani P.S., Joya M.R., Leite E.R., Longo E., . Photoluminescent BaMoO4 nanopowders prepared by complex capping agentization method (CPM) J. Solid State Chem.. 2006;179:671-678.
[Google Scholar]

[6] Dutta S., Parsons S.A., Bhattacharjee C., Jarvis P., Datta S., Bandyopadhyay S., . Kinetic study of adsorption and photo-decolorization of Reactive Red 198 on TiO2 surface. Chem. Eng. J.. 2009;155:674-679.
[Google Scholar]

[7] Fan T.X., Chow S.K., Zhang D., . Biomorphic mineralization: from biology to materials. Prog. Mater. Sci.. 2009;54:542-659.
[Google Scholar]

[8] Hosseinpour-Mashkani S.M., Sobhani-Nasab A., Mehrzad M., . Controlling the synthesis SrMoO₄ nanostructures and investigation its photocatalyst application. J. Mater. Sci.: Mater. Electron.. 2016;27:5758-5763.
[Google Scholar]

[9] Huang S.T., Lee W.W., Chang J.L., Huang W.S., Chou S.Y., Chen C.C., . Hydrothermal synthesis of SrTiO₃ nanocubes: characterization, photocatalytic activities, and degradation pathway. J. Taiwan Inst. Chem. Eng.. 2014;45:1927-1936.
[Google Scholar]

[10] Javidan A., Ramezani M., Sobhani-Nasab A., Hosseinpour-Mashkani S.M., . Synthesis, characterization, and magnetic property of monoferrite BaFe₂O₄ nanoparticles with aid of a novel precursor. J. Mater. Sci.: Mater. Electron.. 2015;26:3813-3818.
[Google Scholar]

[11] Joshi R., Kumar P., Gaur A., . Structural, optical and ferroelectric properties of V doped ZnO. Appl. Nano Sci.. 2014;4:531-536.
[Google Scholar]

[12] Kongsinlark A., Rempel G.L., Prasassarakich P., . Synthesis of monodispersed polyisoprene–silica nanoparticles via differential microemulsion capping agentization and mechanical properties of polyisoprene nanocomposite. Chem. Eng. J.. 2012;193:215-226.
[Google Scholar]

[13] Li X., Zhang W., Cui W., Sun Y., Jiang G., Zhang Y., Huang H., Dong F., . Bismuth spheres assembled on graphene oxide: Directional charge transfer enhances plasmonic photocatalysis and in situ DRIFTS studies. Appl. Catal. B. 2018;221:482-489.
[Google Scholar]

[14] Mi Y., Huang Z.Y., Hu F.L., Li X.Y., . Room temperature reverse-microemulsion synthesis and photoluminescence properties of uniform BaMoO4 submicro-octahedra. Mater. Lett.. 2009;63:742-744.
[Google Scholar]

[15] Ozer D., Dursun G.B., Ozer A., . ethylene blue adsorption from aqueous solution by dehydrated peanut hull. J. Hazard. Mater.. 2007;144:171-179.
[Google Scholar]

[16] Pastrana-Martínez L.M., Morales-Torres S., Likodimos V., Falaras P., Figueiredo J.L., Faria J.L., Silva A.M.T., . Role of oxygen functionalities on the synthesis of photocatalytically active graphene–TiO₂ composites. Appl. Catal. B. 2014;158:329-340.
[Google Scholar]

[17] Pupp C., Yamdagni R., Porter R.F., . Mass spectrometric study of the evaporation of BaMoO4 and BaWO4. J. Inorg. Nucl. Chem.. 1969;31:2021-2029.
[Google Scholar]

[18] Ryu J.H., Yoon J.W., Lim C.S., Shim K.B., . Microwave-assisted synthesis of barium molybdate by a citrate complex method and oriented aggregation. Mater. Res. Bull.. 2005;40:1468-1476.
[Google Scholar]

[19] Safardoust-Hojaghan H., Salavati-Niasari M., . Degradation of methylene blue as a pollutant with N-doped grapheme quantum dot/titanium dioxide nanocomposite. J. Clean. Prod.. 2017;148:31e36.
[Google Scholar]

[20] Sathishkumar P., Sweena R., Sambandam Anandan W.U., . Synthesis of CuO-ZnO nanophotocatalyst for visible light assisted degradation of a textile dye in aqueous solution. Chem. Eng. J.. 2011;171:136-140.
[Google Scholar]

[21] Sobhani-Nasab A., Rangraz-Jeddy M., Avanes A., Salavati-Niasari M., . Novel sol–gel method for synthesis of PbTiO₃ and its light harvesting applications. J. Mater. Sci.: Mater. Electron.. 2015;26:9552-9560.
[Google Scholar]

[22] Sobhani-Nasab A., Maddahfar M., Hosseinpour-Mashkani S.M., . Ce(MoO₄)₂ nanostructures: synthesis, characterization, and its photocatalyst application through the ultrasonic method. J. Mol. Liq.. 2016;216:1-5.
[Google Scholar]

[23] Sobhani-Nasab A., Zahraei Z., Akbari M., Maddahfar M., Hosseinpour-Mashkani S.M., . Synthesis, characterization, and antibacterial activities of ZnLaFe₂O₄/NiTiO₃ nanocomposite. J. Mol. Struct.. 2017;1139:430-435.
[Google Scholar]

[24] Sun Y., Ma J., Fang J., Gao C., Liu Z., . Effect of MgO/CaO ratio on the microstructure of cordieritebased glass-ceramic glazes for floor tiles. Ceram. Int.. 2011;31:683-686.
[Google Scholar]

[25] Wang H., Sun Y., Jiang G., Zhang Y., Huang H., Wu Z., Lee S.C., Dong F., . Unraveling the mechanisms of visible light photocatalytic NO purification on earth-abundant insulator-based core-shell heterojunctions. Environ. Sci. Technol.. 2018;52:1479-1487.
[Google Scholar]

[26] Wang C., Zeng W., . New insights into multi-hierarchical nanostructures with size-controllable blocking units for their gas sensing performance. J. Mater. Sci.: Mater. Electron.. 2017;28:10847-10852.
[Google Scholar]

[27] Wei S., Chen Y., Ma Y., . Fabrication of CuO/ZnO composite films with cathodic co-electrodeposition and their photocatalytic performance. J. Mol. Catal. A Chem.. 2010;331:112-116.
[Google Scholar]

[28] Xia C.T., Fuenzalida V.M., Zarate R.A., . Electrochemical preparation of crystallized Ba1-xSr_xMoO₄ solid-solution films at room-temperature. J. Alloy Compd.. 2001;316:250-255.
[Google Scholar]

[29] Yu Y., Xia Y., Zeng W., Liu R., . Synthesis of multiple networked NiO nanostructures for enhanced gas sensing performance. Mater. Lett.. 2017;206:80-83.
[Google Scholar]

[30] Zhang C., Shen E.H., Wang E.B., Kang Z.H., Gao L., Hu C.W., Xu L., . One-step solvothermal synthesis of high ordered BaWO4 and BaMoO4 nanostructures. Mater. Chem. Phys.. 2006;96:240-243.
[Google Scholar]

[31] Zhong J.B., Li J.Z., Feng F.M., Lu Y., Zeng J., Hu W., Tang Z., . Improved photocatalytic performance of SiO2–TiO2 prepared with the assistance of SDBS. J. Mol. Catal. A: Chem.. 2012;357:101-105.
[Google Scholar]

[32] Zhu S., Li L., Liu J., Wang H., Wang T., Zhang Y., Zhang L., Ruoff R.S., Dong F., . Structural directed growth of ultrathin parallel birnessite on β-MnO₂ for high-performance asymmetric supercapacitors. ACS Nano. 2018;12:1033-1042.
[Google Scholar]

[33] Zhu L., Li Y., Zeng W., . Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties. Appl. Surf. Sci.. 2018;427:281-287.
[Google Scholar]

[34] Zhu S., Lu L., Zhao Z., Wang T., Liu X., Zhang H., Dong F., Zhang Y., . Mesoporous Ni-doped δ-Bi₂O₃ microspheres for enhanced solar-driven photocatalysis: a combined experimental and theoretical investigation. J. Phys. Chem. C. 2017;121:9394-9401.
[Google Scholar]

[35] Zhu L., Zeng W., Ye H., Li Y., . Volatile organic compound sensing based on coral rock-like ZnO. Mater. Res. Bull.. 2018;100:259-264.
[Google Scholar]

[36] Zhu Y., Zheng G., Dai Z., Zhang L., Ma Y., . Photocatalytic and luminescent properties of SrMoO₄ phosphors prepared via hydrothermal method with different stirring speeds. J. Mater. Sci. Technol.. 2017;33:23-29.
[Google Scholar]

[37] Zinatloo-Ajabshir S., Salavati-Niasari M., Zinatloo-Ajabshir Z., . Facile size-controlled preparation of highly photocatalytically active praseodymium zirconate nanostructures for degradation and removal of organic pollutants. Sep. Purif. Technol.. 2017;177:110-120.
[Google Scholar]

[38] Zinatloo-Ajabshir S., Mortazavi-Derazkola S., Salavati-Niasari M., . Sonochemical synthesis, characterization and photodegradation of organic pollutant over Nd₂O₃ nanostructures prepared via a new simple route. Separ. Purificat. Technol.. 2017;178:138-146.
[Google Scholar]

[39] Zinatloo-Ajabshir S., Salavati-Niasari M., . Photo-catalytic degradation of erythrosine and eriochrome black T dyes using Nd₂Zr₂O₇ nanostructures prepared by a modified Pechini approach. Sep. Purif. Technol.. 2017;179:77-85.
[Google Scholar]

Optimization and detailed stability study on coupling of CdMoO₄ into BaWO₄ for enhanced photodegradation and removal of organic contaminant