Soft chemical synthesis and characterization of BaWO4 nanoparticles for photocatalytic removal of Rhodamine B present in water sample
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In recent years, the use of metal oxides as photocatalysts for degradation of organic substances has attracted the attention of the scientific community. Metal oxide nanoparticles have been studied due to their novel optical, electronic, magnetic, thermal and potential applications as catalysts, gas sensors, photo-electronic devices, etc. In this research work, we report a simple, soft chemical route for synthesizing BaWO4 nanoparticles using cheap chemicals such as barium nitrate (precursor salt) and sodium tungstate (precipitating agent). The final product was dried at room temperature overnight and calcined at 400 °C and 800 °C for 2 h to get phase-pure product. The prepared nanoparticles (as prepared and heat-treated samples) were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and UV–Vis spectroscopy techniques. Photocatalytic degradation characteristics of Rhodamine B in water using BaWO4 nanoparticles were studied and reported.
KeywordsBaWO4 nanoparticles Soft chemical method Characterization Photocatalytic studies
Water pollution is one of the worldwide problems nowadays and this can directly affect the health of living organisms. Because of the industrialization, effluents from most of the industries are discharged directly or indirectly into water sources without treating the harmful or dangerous compounds present in it and this may lead to water pollution. There are wide varieties of water pollutants available, which include waste chemicals, waste organic matter, presence of harmful pathogens, etc. Water pollution is a burning global problem; hence development of suitable eco-friendly treatment procedures is a mandatory requirement at present . One of the most harmful pollutants present in industrial waste water is organic dyes. Organic dyes are used for various industrial applications such as paper, leather, cosmetics, drugs, electronics, plastics, textiles, etc. From among these, it was reported that the textile industry alone utilizes 80 % of the synthetic dyes for printing purpose . Most of the dyes have non-biodegradable compounds . Recently, researchers have developed methods for the treatment of waste water especially for the removal of dyes using techniques based on chemical, physical and biological means . However, these treatment methods are not suitable for large scale due to their high cost. Therefore, alternative treatment methods, which are financially viable and green-chemical in nature, are required by the industrial sectors. Photocatalysis technology is one of the best water treatment technologies, since it is an economically viable and environment-friendly technique for the purification of waste water; it removes all kinds of organic and inorganic pollutants and contaminants present in waste water .
Barium tungstate (BaWO4) is the heaviest member of the family of the alkaline earth tungstates. Like many other ABX4 type compounds, BaWO4 crystallizes at ambient conditions in the tetragonal scheelite-type structure (space group [SG]: I41/a, No. 88, Z = 4) . BaWO4 is extensively investigated because of its good electrical conductivity, magnetic and photoluminescence properties . As one of the most reactive alkaline earth tungstates, BaWO4 based materials play an important role in wide variety of technological applications as light emitting diodes , humidity sensors , optic filters , scintillator detectors , photocatalysts , microwave dielectrics , phosphors  and solid state lasers . Recently, many studies have been reported on the preparation and characterization of metal tungstates using various preparation methods such as Czochralski process , precipitation method , hydrothermal synthesis , solid-state reaction , pulsed laser deposition method , electrochemical process , molten salt synthesis , polymeric precursor method , solvothermal synthesis , sonochemical route  and DNA-templated synthesis .
It was found that, Rhodamine B is a most important basic dye of the xanthene class. It is highly water soluble and is widely used as a colorant in textile industry, food stuffs and is a well-known water tracer fluorescent. It is harmful to human beings and animals, and causes irritation of the skin, eyes and respiratory tract. The carcinogenicity, reproductive and developmental toxicity, neurotoxicity and chronic toxicity of Rhodamine B toward humans and animals have been experimentally proven. Also, it was found that Rhodamine B cannot be effectively removed by biological treatment processes due to the slow kinetics reaction .
The first part of this research work has been focused on the synthesis of BaWO4 nanoparticles by simple soft chemical route and systematic characterization of these materials in order to explore their structural, microstructural, elemental, chemical and surface properties. The second part has been dealt with the study of photocatalytic properties of BaWO4 to degrade organic dye (Rhodamine B) present in water under UV-light irradiation. The obtained results are discussed and presented in this research article.
The analytical grade chemicals such as Barium Nitrate (99.8 % purity, Merck, India), Sodium Tungstate (99.0 % purity, Merck, India), Rhodamine B (>95.0 % purity, Sigma-Aldrich, India) and Ethanol (99.0 % purity, Merck, India) were used in this study. These materials were used as received without any further purification. All reactions were carried out with deionized water.
Preparation of BaWO4 nanoparticles
The crystallographic properties of BaWO4 were examined by X-ray diffraction (Shimadzu XRD6000) using Cu Kα (λ = 0.154059 nm) radiation with a nickel filter and a power of 40 kV × 30 mA. The intensity data were collected at 25 °C over a 2θ range of 10–90° with a scan rate of 10° min−1. The FTIR spectra of the BaWO4 were examined by Fourier transform infrared spectrometer (Shimadzu spectrophotometer) using KBr pellet technique in the range from 2,000 to 400 cm−1 (spectral resolution was 4 cm−1 and number of scans was 20). The morphology, particle size and elemental compositions of the prepared material were studied by scanning electron microscope (SEM JEOL JSM-6610) equipped with an energy dispersive X-ray (EDAX) spectrophotometer and operated at 20 kV. Absorbance spectra of the catalyst were obtained by UV–Visible spectrophotometer (Shimadzu 1800). The samples were loaded into a quartz experimental set-up and the spectrum was recorded in the range 200–600 nm using absorbance method. The photoluminescence spectral analysis was examined by spectroflurometer (JASCO) at room temperature.
Results and discussion
Characterization of BaWO4 nanoparticles
The crystallographic parameters of the BaWO4 nanoparticles
Unit cell lattice parameter ‘a and c’ (Å)
Unit cell volume (Å)3
Theoretical density (g/cc)
Crystallite size (nm)
BaWO4 (JCPDS No. 85-0588)
Tetragonal body centered
a = 5.613
c = 12.720
As prepared sample
Tetragonal body centered
a = 5.590
c = 12.637
Calcined at 400 oC
Tetragonal body centered
a = 5.594
c = 12.657
Calcined at 800 oC
Tetragonal body centered
a = 5.598
c = 12.687
Fourier transform infrared spectroscopy
In Eq. 9, all four vibrational modes are Raman active but only the F2(ν3) and F2(ν4) modes are IR active . Therefore, a strong W–O stretching in [WO4]2− tetrahedrons was detected at 822–824 cm−1. Also, a weak W–O bending was found in the range 438–629 cm−1 . The obtained results are in accordance with the reported data.
Scanning electron microscopy
The atomic weight percentage elemental composition of BaWO4 nanoparticles by EDAX analysis
Percentage of chemical composition
Calcined at 400 oC
Calcined at 800 oC
From the mechanism, it was found that reactions can be split into fragments.
In the first step, the Rhodamine B dye can absorb photons from light source and may be excited to singlet state. By losing some energy through inter-system crossing, the Rhodamine B dye can be converted into triplet state. On the other hand, BaWO4 absorbs photon, and one electron from its conduction band is transferred to valence band, generating a hole. This hole may be responsible for bleaching of Rhodamine B dye. This hole may abstract an electron from OH− ion and free radical OH* is generated. This free radical abstracts an electron from conjugated and weaker site of the Rhodamine B dye. As a result Rhodamine B dye is broken down into fragments. Scavenger study has proved the participation of free radical in the reaction. Finally, various degraded products such as NO2, CO2, H2O, etc. may take place in the process.
The BaWO4 nanoparticles were prepared by the simple, low-temperature route; furthermore, they were characterized by the XRD, FTIR, SEM, EDAX and UV–visible spectroscopy techniques. The XRD patterns show that the prepared samples are of tetragonal-type structure. No impurity phase has been observed in XRD. FTIR spectra confirmed the presence of M–O bond in the product. The SEM studies confirmed the presence of granular-like grains in the samples. The EDAX data confirmed the presence of corresponding elements in the samples. The band gap data obtained on the sample based on absorbance spectra studies are similar to the reported data. It was found that among the samples studied, the as-prepared BaWO4 is more effective in degrading the Rhodamine B dye present in the water sample in presence of UV light at the wave length of 554 nm at normal room temperature. Hence, BaWO4 nanoparticles are suggested as a potential candidate to remove organic pollutants present in water by simple photocatalysis at room temperature.
ASN has guided MMJS to carry out this research study. MMJS has carried out the experiments and he has written the raw manuscript. ASN has edited and refined the manuscript towards publication. Both authors have read and approved the final manuscript.
The authors are grateful to the DST Nano Mission, Government of India, New Delhi, for its financial assistance to carry out the research work. The authors are also thankful to the management of Karunya University for their support and encouragement to carry out and publish this research work.
Conflict of interest
The authors declare that they have no conflict of interest.
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