Degradation of organic dyes using spray deposited nanocrystalline stratified WO3/TiO2 photoelectrodes under sunlight illumination

doi:10.1016/j.optmat.2017.12.044

Optical Materials

Volume 76, February 2018, Pages 260-270

https://doi.org/10.1016/j.optmat.2017.12.044 Get rights and content

Highlights

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Enhanced photoelectrocatalytic activity of sprayed deposited stratified WO₃/TiO₂ thin films.
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Photoelectrocatalytic degradation of rhodamine B and reactive red 152 dye.
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Reaction kinetics and degradation of pollutants by COD measurement.

Abstract

The need to utilize TiO₂ based metal oxide hetero nanostructures for the degradation of environmental pollutants like Rhodamine B and reactive red 152 from the wastewater using stratified WO₃/TiO₂ catalyst under sunlight illumination. WO₃, TiO₂ and stratified WO₃/TiO₂ catalysts were prepared by a spray pyrolysis method. It was found that the stratified WO₃/TiO₂ heterostructure has high crystallinity, no mixed phase formation occurs, strong optical absorption in the visible region of the solar spectrum, and large surface area. The photocatalytic activity was tested for degradation of Rhodamine B (Rh B) and reactive red 152 in an aqueous medium. TiO₂ layer in stratified WO₃/TiO₂ catalyst helps to extend its absorption spectrum in the solar light region. Rh B and Reactive red 152is eliminated up to 98 and 94% within the 30 and 40 min respectively at optimum experimental condition by stratified WO₃/TiO₂. Moreover, stratified WO₃/TiO₂ photoelectrode has good stability and reusability than individual TiO₂ and WO₃ thin film in the degradation of Rh B and reactive red 152. The photoelectrocatalytic experimental results indicate that stratified WO₃/TiO₂ photoelectrode is a promising material for dye removal.

Introduction

Nowadays semiconductor-assisted photocatalytic oxidation of organic pollutants has attracted much attention because it is an economic and eco-friendly solution for the remediation of an environmental pollutant like dyes [1]. As there are several applications of organic dyes in different industries such as textile, paper, pigment, food, cosmetic, and drug manufacturing. Dyes discarded from industries, mostly cause water pollution and pose threat to public hygiene, health, and environment. A large quantity of reactive dyes (30%) is wasted during the dyeing process and dumped into water sources without any active treatment. In general, textile wastewater contains high concentrations of organic compounds, heavy metals, high chemical oxygen demand (COD), high pH and has a strong colour [2], [3]. Since some of the dyes and their metabolites are toxic, carcinogenic, and hence the wide use of organic dyes and their hazardous discarding leads to serious environmental problems. Therefore, the elimination of dyes from the wastewater is of vital significance [4]. In this perspective, the advanced oxidation processes (AOP's) are technologies that offer a good route in the treatment of the wastewater containing recalcitrant organic pollutants like dyes, organic acids etc. [5]. Heterogeneous photocatalysis comes under AOP's in which a source of appropriate light and a semiconductor material as catalysts are required to promote a chemical reaction by means of the creation of electron-hole pairs. Due to its ambient condition it is a useful method for the removal of organic pollutants from wastewater [6], [7]. Among these Rhodamine B (RhB) have been recognized as, a highly water-soluble azoic dye and it is extensively used for fluorescent labeling and food coloring due to its fastness, low cost. However, it was also found to be toxic and carcinogenic in multiple feeding tests of rats and mice. It is also affecting the human respiratory system, skin, and brain [8]. In the past three decades, lot of studies have shown that TiO₂ is widely used photocatalyst, material, due to it is stable, inexpensive and non-toxic in nature. However, main drawback of TiO₂ is that it absorbs only 4% photons from the solar spectrum, which limits the effective application of TiO₂ under solar irradiation On the other hand, in order to further improve the photocatalytic efficiency of TiO₂, coupling it with another suitable semiconductor material is one of the most imperative tasks for the application of heterogeneous photocatalysis in the future [9]. Umar et al. conducted review on high-energy (001)-faceted anatase TiO₂ nanostructures and their application in photocatalysis as well as a review of any attempts to modify their electrical, optical and photocatalytic properties via doping [10]. Javaid et al. samarium (Sm) supported on tin oxide–titanium oxide (SnO₂/TiO₂) nanoparticles (Sm/SnO₂–TiO₂) were synthesized by sol–gel, ultrasonic and hydrothermal method and studied the effect of the optical band gap and particle size on the catalytic properties of Sm/SnO₂–TiO₂ nanoparticles [11]. Many different combinations (heterogeneous semiconductor systems) have been investigated, such as CdS/TiO₂, TiO₂/SnO₂, TiO₂/ZnO and TiO₂/WO₃ [12]. Among these WO₃/TiO₂ combinations is better due to the presence of Lewis and Bronsted acidic sites (W⁶⁺ species) which adsorb a greater amount of OH or H₂O, and hence helps generates more number of OH radicals to degrade organic pollutants. Also the conduction band edge of WO₃ is located at a more positive potential than the one of TiO₂. Therefore, WO₃ can act as a sink for the photogenerated electrons [13]. Therefore, electrons are injected from the conduction band TiO₂ to the conduction band of WO₃, while holes transfer between valence bands occurs in the opposite direction and recombination of photo generated charge carriers is reduced, which further helps to improve the photocatalytic efficiency [14]. Bosko Grbic et al. prepared TiO₂/WO₃ photocatalytic composite coatings prepared by spray pyrolysis and studied the photocatalytic degradation of methyl orange [15]. He et al. prepared WO₃/TiO₂ prepared by plasma electrolytic oxidation and observed that 85% degradation of RhB in 100 min [16]. The WO₃/TiO₂ composite prepared using different methods such as ultrasound-assisted [9], plasma electrolytic oxidation [17], dip coating [18], RF magnetron sputtering [8]. In comparison with the reported methods for forming WO₃/TiO₂ films, our spray pyrolysis process exhibits significant advantages such as easy control of preparation conditions, non-selectivity for substrate and low cost [19]. Even though few studies have been conducted on preparation WO₃/TiO₂ films, to the best of our knowledge, no report on the synergistic effect of light absorption and the photo generated charge carries transport in spray deposited stratified WO₃/TiO₂ photoelectrode for the degradation of RhB and reactive red 152 under natural sunlight illumination.

In this paper, we reported on the preparation of stratified WO₃/TiO₂ photoelectrodes by two step spray pyrolysis method. In the first step, a homogenous WO₃ thin film was deposited on FTO substrate. Afterwards, TiO₂ nanosheets were deposited onto the WO₃, as a result, stratified WO₃/TiO₂ photoanodes exhibited strong light absorbance in the whole visible region of solar spectrum. The synergism in light absorption and charge transport in the stratified WO₃/TiO₂ films was used for the photoelectrochemical degradation of Rhodamine B and reactive red 152 under natural sunlight illumination. RhB and reactive red 152 has been successfully degraded using stratified WO₃/TiO₂ photoelectrodes, without the use of external catalyst like HClO₄, NaH₂PO₄ etc. and got maximum degradation efficiency. The results demonstrated that the spray deposited stratified WO₃/TiO₂ photoelectrode exhibited a higher photocatalytic activity than the single WO₃ and TiO₂ sample under visible light irradiation. A mechanism of photodegradation and improvement in photocatalytic activity of stratified WO₃/TiO₂ was also discussed.

Section snippets

Materials

The tungsten metal powder (Sigma Aldrich), absolute ethanol and hydrogen peroxide (H₂O₂) (30% W/V, Thomas Baker) were analytical grade and used as received. Titanylacetylacetonate (TiAcAc) (C₁₀H₁₄O₅Ti) (AR grade, 99.9% pure, Merk made, Germany) and used without any further purification.

Preparation of stratified WO₃/TiO₂ photoelectrodes

The spray pyrolysis method has been adopted for the synthesis of WO₃, TiO₂, and stratified WO₃/TiO₂ thin film photoelectrodes on glass and Fluorine doped tin oxide (FTO) glass substrates of size 10 × 10 × 0.125 cm

Structural studies

The XRD analysis of WO₃, TiO₂ and stratified WO₃/TiO₂ thin films indicates that all the films are polycrystalline in nature and TiO₂ layer has little influence sprayed WO₃/TiO₂ thin films (Fig. 1 (a)). The XRD pattern of WO₃ shows the preferred orientations along (002), (001) and (220) agree well with the monoclinic crystal structure (JCPDS card No. 00-005-0364). The phase structure and crystallinity of TiO₂ play important roles in photocatalytic activity, as only anatase phase of TiO₂ shows

Photoelectrocatalytic degradation of rhodamine B and reactive red 152

To explore the superiority of photoelectrocatalytic degradation, a runs of degradation experiments were achieved on a three-electrode configuration photoelectrochemical cell (PEC) with and without sunlight illumination and photoelectrodes. Fig. S4, S5 and Fig. 8 exhibit the photoelectrocatalytic activities of WO₃, TiO₂ and stratified WO₃/TiO₂ for the degradation of Rh B and reactive red 152, respectively. The dark adsorption reaction was conducted on the PEC reactor to study the effect of

Photostability and reusability

The photostability and reusability of pure WO₃, TiO₂ and stratified WO₃/TiO₂ thin film catalysts are evaluated. After experiment, the solution further studied for Atomic Absorption AAS study to evaluate loss of ions in solutions. Atomic absorption spectra are used for detection of W, Ti ions in the RhB and Reactive red 152 solution. It is found that 0.041, and 0.081 μgL⁻¹ which show 4and 8 %for pure WO₃ and 0.021 and 0.033 μgL⁻¹ which shows 2 and 3% for TiO₂film is dissolved during

Conclusions

The WO₃, TiO₂ and stratified WO₃/TiO₂ thin films have been successfully synthesized by the two step spray pyrolysis method. The influence of TiO₂ layers onto the structural, optical and morphological properties of deposited thin films has been investigated. FE-SEM and EDAX elemental mapping analysis reveals the formation of the stratified WO₃/TiO₂ structures with a TiO₂ layer covered on the surface of WO₃ films. A layer of TiO₂ on pre-deposited WO₃ shifts absorption edge of TiO₂ into the

Acknowledgements

This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189), Republic of Korea. One of the author YMH is thankful to gs3:Science and Engineering Research Board, New Delhi, for the financial support and awarding National Postdoctoral Fellowship (N-PDF) award F. No. PDF/2017/000691.

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Degradation of organic dyes using spray deposited nanocrystalline stratified WO3/TiO2 photoelectrodes under sunlight illumination

Highlights

Abstract

Introduction

Section snippets

Materials

Preparation of stratified WO3/TiO2 photoelectrodes

Structural studies

Photoelectrocatalytic degradation of rhodamine B and reactive red 152

Photostability and reusability

Conclusions

Acknowledgements

Appl. Catal. B

Trends Anal. Chem.

Water Sci.

J. Environ. Manag.

J. Environ. Manag.

J. Chromatogr. B

Opt. Mater.

Superlattice. Microst.

Surf. Coating. Technol.

Ultrason. Sonochem.

J. Electroanal. Chem.

Surf. Coating. Technol.

Mater. Chem. Phys.

J. Alloys Compd.

Thin Solid Films

Ceram. Int.

J. Photochem. Photobiol., B

J. Ind. Eng. Chem.

J. Ind. Eng. Chem.

Appl. Surf. Sci.

Ceram. Int.

Physica E

Mater. Res. Bull.

Ultrason. Sonochem.

Appl. Surf. Sci.

J. Phys. Chem. Solid.

Degradation of organic dyes using spray deposited nanocrystalline stratified WO₃/TiO₂ photoelectrodes under sunlight illumination

Preparation of stratified WO₃/TiO₂ photoelectrodes