Iridium nanoparticles anchored WO3 nanocubes as an efficient photocatalyst for removal of refractory contaminants (crystal violet and methylene blue)

Chemical Physics Letters 745 (2020) 137285

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Research paper

Iridium nanoparticles anchored WO3 nanocubes as an efficient photocatalyst for removal of refractory contaminants (crystal violet and methylene blue) M. Dhanalakshmia,b, S. Lakshmi Prabavathib, K. Saravanakumarb, B. Filip Jonesb, V. Muthurajb, a b

T

⁎

Department of Chemistry, V. V. Vanniaperumal College for Women (Autonomous), Virudhunagar 626 001, Tamil Nadu, India Department of Chemistry, V. H. N. Senthikumara Nadar College (Autonomous), Virudhunagar 626 001, Tamil Nadu, India

H I GH L IG H T S

loaded WO nanocomposites were synthesized via hydrothermal process. • Iridium Ir NPs doped WO nanocomposites could efficiently preventing the electrons-holes pair recombination. • The (3%) catalyst exhibited better photocatalytic activity towards the degradation of CV and MB. • Ir/WO • The main active species OH and h are generated in the reaction. 3

3

3

%

+

A R T I C LE I N FO

A B S T R A C T

Keywords: Ir/WO3 nanocomposite Photo-catalytic process Degradation Crystal violet and methylene blue Visible light

A series of novel visible light driven Iridium loaded WO3 (Ir/WO3) nanocomposites were successfully synthesized via single step hydrothermal process. The Ir/WO3 nanocomposites were systematically characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM) and UV–vis diffused reflectance spectra (UV–vis DRS) techniques. The WO3 nanocparticles were showed nanocubes-like structure and Ir NPs were anchored on surface of WO3 which was confirmed by SEM and TEM analysis. Optical result indicated that the absorption band edge is positioned in the visible region with band gap energy of WO3, Ir/WO3 (1%), Ir/ WO3 (3%) and Ir/WO3 (5%) was 2.87, 2.8, 2.6 and 2.5 eV, respectively. From EDX and XPS analysis, Ir/WO3 nanocomposite contains Ir, W and O elements only. The Ir/WO3 nanocatalysts have been utilized for the photodegradation of crystal violet (CV) and methylene blue (MB) organic pollutants under the visible light irradiation. Compared to pure WO3 and other Ir/WO3 nanocomposites, 3% loaded Ir/WO3 catalyst exhibited superior photocatalytic efficiency towards the degradation of CV and MB. The enhancement of photocatalytic activity was mainly due to the presence of Ir nanoparticles on the surface of WO3 nanocubes preventing the photogeneration electrons-holes pair recombination and the synergistic bonding interaction between Ir NPs and WO3 nanocubes. Various factors such as Ir loading concentration, effect of catalyst, effect of pollutants and stability of the photocatalyst were explained. Furthermore, the possible photocatalytic mechanism of Ir/WO3 catalyst was determined by radical trapping studies. This novel Ir/WO3 nanocomposite proved to an efficient tool catalyst for the organic contaminants abatements.

1. Introduction Environmental hazardous have gained considerable attention in entire world over the last decade, because the toxic chemicals and organic contaminates were affect directly or indirectly our human life. Several traditional methods were employed to eliminate the organic pollutants which present in the environment. In the present scenario, visible light photocatalyst have attracted worldwide because of their

⁎

excellent activity and important techniques for removal of toxic organic contamination from water [1,2]. Recently, semiconductor based photocatalyst was promising technology for tackle the above issue, for the reason that semiconductor photocatalyst has green, in expensive and efficient method. It used solar energy to generate electron-hole pairs to drive the photocatalysis process for degradation of organic contaminations without adds any other chemicals. Nevertheless, commonly used semiconductor photocatalyst (TiO2 and ZnO) has poor

Corresponding author. E-mail address: [email protected] (V. Muthuraj).

https://doi.org/10.1016/j.cplett.2020.137285 Received 5 December 2019; Received in revised form 4 February 2020; Accepted 26 February 2020 Available online 27 February 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved.

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Table 1 The physico-chemical properties and structure of two organic dye contaminants. Properties

Methylene Blue

Crystal Violet

Chemical formula Molecular weight λmax Stability Structure

C16H18ClN3S 319.85 gmol−1 664 Soluble in water

C25N3H30Cl 407.97 gmol−1 586 Soluble in water

WO3 significantly enhanced the photocatalytic activity for the degradation methyl orange under visible light [14]. M. Torabi Merajin et al synthesized a visible-light-driven Fe doped WO3 nanostructures by one step co-precipitation method for oxidation of n-pentane. They observed that Fe doped WO3 nnostructure has high photocatalytic activity due to the large surface area and narrowing particle size distribution [15]. W. Mu et al. reported Nb doped WO3 nanomaterials were fabricated by simple hydrothermal method for degradation of MB under visible light. They reported that photocatalytic activity of WO3 was enhanced after the doping of Nb which may due to the decrease the band gap energy [16]. Therefore metal ions doping play a crucial role for photocatalytic activity due to enhance the charge separation under visible light. Recently, iridium metal has received more attention since they exhibit high catalytic activity because it can be used for oxygen species on the surface at low potential [17]. V. M. M. Flores et al. constructed a visible light driven Ir doped TiO2 brookite photocatalyst by hydrothermal microwave-assisted process. Under visible light irradiation, the Ir doped TiO2 photocatalyst shows higher photocatalytic efficiency for degradation of acetaldehyde and toluene [18]. X. Z. Zhao and his co workers fabricated Ir, C, N doped TiO2 nanoparticles via wet chemical method and investigate the photocatalytic efficiency for the evolution of H2. The enhanced the photocatalytic efficiency was ascribed to the effectively separate photogenerate charge carriers [19]. C. Castaneda and his co-workers developed Ir/CeO2 photocatalyst was synthesized via simple precipitation method. The photocatalytic activity of Ir/CeO2 photocatalyst was evaluated by reduction of 4-nitrophenol under UV irradiation. The photocatalytic activity was improved due to the more active sites were present in the photocatalyst [20]. Nevertheless, Ir and WO3 nanocomposite have been studied as photocatalyst for first time. In the present work, Ir doped WO3 was synthesized by simple one step hydrothermal method. With the help of through exploration of the optical properties and composites structure, the origin of the photocatalytic activity of Ir/WO3 nanocomposite were discussed. The synthesized Ir/WO3 nanocomposites catalytic activity was investigated by degradation of MB and CV dyes under visible light. Furthermore, the photocatalytic activity possible mechanism was proposed and discussed.

Fig. 1. X-ray diffraction patterns of pure WO3 and Ir/WO3 nanocomposites.

photocatalytic activity under visible light illuminations because of their wide band gap. Hence, the development of highly effective visible light semiconductor photocatalyst and environmentally friendly catalyst is greatly desirable, since solar light contain above 40% energy of visible light [3,4]. In contrast, tungsten oxide (WO3) is a well known photocatalyst materials due to their narrow band gap (2.6–3 eV) energy, stability in acid, small size, high surface energy, good thermal stability and large surface area [5,6]. It has applied in variety of applications such as gas sensor, photocatalyst, elctrochromic devices and biological activities [7–10]. WO3 has high absorption capacity besides that high photocatalytic activity owing to it’s negatively charge surface. Regrettably, single semiconductor photocatalyst alone is very difficult to separate the photocatalytic generation of electron-hole pairs because strong coulombic force and WO3 has excited by blue and near ultraviolet region of solar spectrum [11,12]. Consequently, the narrowing band gap of WO3 can effectively be reduced and improved the photocatalytic performance of WO3 by the doping of various metal ions. Doping of some suitable metal ions with WO3 nanomaterials is one of the best way solve this problem [13]. S. V. Mohite et al. have reported that Yb doped 2

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Fig. 2. (a) XPS survey spectrum of Ir/WO3 (3%) nanocomposite, (b) High resolution W4f spectrum, (c) High resolution O1 spectrum and (d) High resolution Ir4f spectrum.

2. Experimental section

2.3. Material characterization

2.1. Materials and methods

The powder X-ray diffraction (XRD) spectra were used to analyze the chemical structure of the samples. XRD were carried out utilizing a PANalytical X’pert Pro out fitted with a Cu radiation source. Fourier transform infrared spectra (FT-IR) were measured by using Shimadzu FT-IR 3000. The morphology of the sample were analyzed on a scanning electron microscopy (SEM, VEGA3 TESCAN model) equipped with an energy dispersive spectroscopy (EDX, Bruker Nano GmbH, X 50 Flash Detector (Model-5010)) detector and transmission electron microscopy (TEM, PHILIPS CM 200 model) The X-ray photoelectron spectroscopy of the sample were recorded on XPS KAlpha surface analysis, Thermo Fisher Scientific, U.K. All the binding energy was allusion to the C 1s peak at 284.6 eV which are adventitious carbon signals resulting from the surface of the catalyst. Optical properties of the sample were obtained by DRS UV–visible spectrophotometer (UV240, Shimadzu) using BaSO4 as a reference.

All the chemicals such as sodium tungstate, iridium chloride, hydrochloric acid, crystal violet and methylene blue were purchased from Merck Sigma-Aldrich (India) and used without any further purification. The structure and physicochemical properties of methylene blue and crystal violet were listed in Table 1. All the reaction solutions were prepared in double deionized (DD) water.

2.2. Ir/WO3 catalyst preparation Ir/WO3 nanocomposites were synthesized using the following method; 3 g of Na2WO4 was dissolved in 40 mL of DD water followed by the addition of 5 mL of concentrated HCl, and the resulting mixture was allowed to stirring for 30 min to form clear solution. The calculated amount of IrCl3 was added into the above solution. Then, 1 M of NaBH4 solution was added into the above mixture of solution and then solutions was holding for another 1 h stirring condition. The final solution was collected and transferred to a Teflon-lined autoclave and kept at 180 °C for 12 h an oven. The obtained solid samples were centrifuged, washed with water and ethanol for elimination of volatile foreign matter then dried at 60 °C. Finally, Ir/WO3 nanocomposites were obtained by calcination at 500 °C for 3 h under air atmosphere.

2.4. Photocatalytic activity for dye degradation In the photodegradation process, the photocatalytic performance of synthesized WO3 and Ir/WO3 nanocomposites were tested through the degradation of methylene blue (MB) and crystal violet (CV) under (150 mW/cm−2) tungsten lamp was used to provide the visible light. In this experiments, the photocatalyst (50 mg) was added into 100 mL organic dye solutions (MB = 10 mg/L and CV = 10 mg/L) at first. Before irradiation, the mixture was magnetically stirred in the dark for 30 min to make the sample mixed with the organic dye fully to achieve 3

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Fig. 3. (a, b) SEM image of WO3 and (c, d) Ir/WO3 (3%) nanocomposite.

Fig. 4. EDX spectrum of (a) WO3 and (b) Ir/WO3 (3%) nanocomposite.

were used in the reaction. The photodegradation of the dye solution was monitored for the detection of ROS. Here, 0.01 M of scavengers was used for the trapping experiment. For the stability tests, the used photocatalyst were separated from the reaction solution mixture centrifugation at 5000 rpm, washed with DD water and dried at 80 °C. The recycle tests were also carried out under the same experimental parameters. To compensate the loss of the catalysts during the washing process, a constant amount of the photocatalyst concentration was maintained in each cycle test.

adsorption–desorption equilibrium at room temperature. Moreover, 5 mL of suspension was extorted in every time interval and then supernatant dye solution was obtained by centrifugation and analyzed the concentration of MB and CV dye solutions by using the UV–vis spectra at 664 nm and 586 nm, respectively. The photocatalytic effectiveness of the sample for degradation of dye solutions were calculated by using the following equation:

C⎞ Photodegradation efficiency(%) = ⎛1 − × 100% C 0⎠ ⎝ ⎜

⎟

3. Results and discussion

where C0 is the initial concentration of dye solutions and C is the concentration of dye solutions at the time t. The reactive oxidative species (ROS) involved in the photocatalytic reaction which was found by the radicals trapping experiment. The different scavengers viz 2propyl alcohol (IPA), benzoquinone (BQ) and ammonium oxalate (AO)

3.1. XRD analysis XRD pattern of the pure and Ir loaded/doped WO3 nanomaterials 4

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Fig. 5. TEM images of (a, b) Ir/WO3 (3%) nanocomposite, (c) HR-TEM and their corresponding (d) SAED pattern.

Fig. 2b displays two typical peaks centered at 35.8 and 37.9 eV were assigned to W 4f7/2 and W 4f5/2 for W6+ oxidation states in WO3 composite [22,23]. The O 1s high resolution spectra exhibits the binding energy was 530.5 eV which ascribed to the lattice oxygen (O2–) in the WO3 nanocomposite (Fig. 2c) [24]. The high resolution XPS spectrum of Ir covers the region of 63.2 and 62.7 eV corresponding to the Ir 4f spectra of metallic Ir, which occurs from spin-orbit coupling of Ir 4f7/2 and Ir 4f5/2, respectively (Fig. 2d) [25]. Thus, this results were confirms the co-existence of Ir and WO3 in the Ir/WO3 nanocomposite.

were shows only diffraction features WO3, which displayed in Fig. 1. Herein, the diffraction peaks at 2θ of 23.09°, 23.56°, 24.33°, 33.24°, 34.14° and 55.87° were correspond to the (0 0 2), (0 2 0), (2 0 0), (0 2 2), (2 0 2) and (1 4 2) crystallographic planes of WO3, which is good agreement with the standard JCPDS Card No: 43-1035. For Ir doped WO3, the main peaks were still remained, but Ir peaks were not observed in all the doped WO3 nanocomposites which may be explained by small amounts of Ir introduction and high dispersion in the composite samples. The WO3 peak intensity was gradually decreased with increasing the Ir loading content, suggesting that crystal growth of WO3 was hindered. The average crystalline size of the synthesized Ir loaded/ doped WO3 nanocomposites were calculated by using Debye-Scherrer’s equation [21].

D=

3.3. SEM, EDX and TEM analysis SEM is used to primarily characterize the morphology of the synthesized WO3 and Ir/WO3 (3%) nanocomposite, which was demonstrated in Fig. 3. It can be observed that the WO3 shows like nanocubes morphology with smooth surface and the nanocubes are combined with each other Fig. 3 (a and b). As seen in Fig. 3 (c and d), after introduction of Ir on WO3, the smooth surface of WO3 was quite rough and the Ir NPs were anchored on the surface of WO3. These indicate that Ir was successfully doped on the surface of WO3 nanocomposite. Elementary compositions of WO3 and Ir/WO3 photocatalyst were investigated by EDX spectrum which is shown Fig. 4. According to Fig. 4a, the EDX result observed that WO3 composite existence of W and O elements only. It is evident that the EDX spectrum of Ir/WO3 nanocomposite reveals that Ir, W and O elements only detected, indicating that the nanocomposite consisted of Ir and WO3 semiconductor (Fig. 4b). To further investigate the details of the morphology, TEM images of Ir/WO3 nanocomposite was taken and the images were shown in Fig. 5.

kλ βCosθ

where, λ is the X-Ray wave length, D is the crystallite size, θ is the diffraction angle, β is the full width at half maximum (FWHM) of the diffraction peaks and k is the Scherrer’s constant. The calculated average crystalline size was around 43, 39, 35 and 37 nm for WO3, Ir/ WO3 (1%), Ir/WO3 (3%) and Ir/WO3 (5%) respectively. 3.2. XPS analysis To exemplify the chemical structure and composition of Ir/WO3 nanocomposite were analyzed by using XPS spectroscopy. The XPS survey spectrum was revealed that Ir/WO3 contains Ir, W and O elements only which shown in Fig. 2. The W 4f high resolution spectrum 5

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Fig. 7. UV–vis adsorption spectra (a) Removal efficiencies of CV and (b) MB dye solutions under visible light irradiation.

Fig. 6. (a) UV–vis diffuse reflectance spectra of synthesized neat WO3 and Ir/ WO3 nanocomposites, and (b) Estimated band gap energy.

the synthesized samples was calculated from these absorption spectra by using Tauc’s equation [27].

From the Fig. 5 (a, b) can be seen that, Ir/WO3 nanocomposite were nanocubes like structure and Ir NPs were anchored on surface of WO3 nanocubes which good agreement with SEM image. It clearly shows that the nanoparticles were not aggregated they are closely attached each other. Fig. 5c shows the HR-TEM image of Ir/WO3 nanocomposite, indicating the lattice spacing of WO3 were determined to the (0 0 2) planes. The selected area electron diffraction (SAED) pattern of Ir/WO3 was used to identify the crystalline nature of sample with observing the bright spots. Fig. 5d clearly observed bright clear spots, which indicate that Ir/WO3 nanocomposite has high crystalline nature.

(α hν )\; = \;A\;(hν − −Eg ) p / 2 where hν is photon energy, α is the absorption coefficient, A is the constant, and Eg is optical bandgap energy. Here, p depends on the features of transition (n = 1 for direct transition and n = 4 for indirect transition). The optical band gap energy of photocatalysts were calculated from (αhν)2 vs hν plot by extrapolating the linear portion to hν axis. The calculated band gap value of WO3, Ir/WO3 (1%), Ir/WO3 (3%) and Ir/WO3 (5%) was 2.87, 2.8, 2.6 and 2.5 eV, respectively (Fig. 6b). Therefore, the visible light absorption of WO3 nanocomposite was significantly increased by Ir doping, and thus enhanced the photocatalytic activities than pure WO3 nanomaterials.

3.4. Optical properties The optical property of synthesized semiconducting materials is regarded as main factor for controlling the photocatalytic reaction process. Optical absorption property of the synthesized pure WO3 and Ir/WO3 (1%, 3% and 5%) materials were scrutinized by UV–vis DRS which is demonstrated in Fig. 6. It can be seen that a strong absorption band at 490 nm is observed for the pure WO3 material. As shown in Fig. 6a, the absorption intensity was increased and an edge is extended as increase of Ir content. As compared with pure WO3 the absorption edge of Ir/WO3 was red shift and the typical absorption edge of Ir/WO3 (1%, 3% and 5%) composite was nearly about at 500, 550 and 600 nm, respectively. The broad and strong absorption in the visible light region of Ir/WO3 nanocomposite is assigned to the local electric field enhancement of surface plasma resonance effect [26]. The band gap of all

3.5. Photocatalytic activity The photocatalytic performance of pure WO3 and Ir/WO3 nanocomposites were assessed by the photocatalytic degradation of CV and MB in aqueous solutions under visible light radiance (λ > 400 nm) as shown in Fig. 7. All the UV absorption peaks of the CV and MB gradually decreased with increasing irradiation time and almost disappear in 50 and 60 min, respectively. The order of photocatalytic degradation of CV and MB under visible light can be summarized as follows, Ir/WO3 (3%) > Ir/WO3 (5%), > Ir/WO3 (1%), > pure WO3 > without catalyst is given Fig. 8. It is found that, Ir/WO3 (3%) catalyst exhibits the 6

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Fig. 8. Effects of Ir loading content in the Ir/WO3 catalysts on the degradation of (a) CV and (c) MB under visible light irradiation. Kinetic fit for the degradation of (b) CV and (d) MB.

50 mg catalyst dosage of Ir/WO3 (3%) was used further parameter based studies. In order to further determine the photocatalytic activity Ir/WO3 (3%) by different initial concentration of organic dye pollutant solutions is main factor for affect the degradation rate. Fig. 9 (c, d) shows effect of the different dye concentration (10–30 mg/L) on the photocatalytic degradation of dye solutions by keeping catalyst dosage 50 mg. The trend of degradation performance was decreased as the dye concentration increased (10–30 mg/L). At low dye concentration (10 mg/L), the photodegradation efficiency was increased. It was unstable that high concentration of dye solution was absorbed more number of dye molecules on the surface of the catalyst, which leads to decrease the active sites of the catalyst [30]. Thus this result confirms that the higher catalytic efficiency of Ir/WO3 (3%) was obtained as lower initial dye concentration (10 mg/L). The radical scavenger study was executed to understand the enhanced photocatalytic activity via trapping the active species in the process of photodegradation and underlying photodegradation mechanism. Here the different scavengers such as ammonium oxalate (AO), benzoquinone (BQ) and iso-propanol (IPA) were used for trapping of holes (h+), super oxide (O2%−) and hydroxyl radical (%OH), respectively [31]. Fig. 10 displays the photocatalytic degradation of MB with different scavengers in the presence of Ir/WO3 (3%). When BQ scavenger was added to the reaction system, the photocatalytic activity of Ir/WO3 (3%) was slightly inhibited. The introduction of IPA into the reaction system the photocatalytic activity of Ir/WO3 (3%)

highest photocatalytic efficiency, which can degrade 99% of CV and 97% of MB under the same reaction conditions. The enhanced photocatalytic activity of Ir/WO3 (3%) can be ascribed to the perfect interaction of Ir NPs and WO3 that significantly increased the separation charge carriers. When the loading level of Ir was over the 3%, the photocatalytic rate was decreased which due to the excess of Ir may act as recombination center for photogenerated e¯-h+ pairs. Moreover, controlled experiments (absence of catalyst) indicated that no degradation of CV and MB dye solution was observed under visible light. In this results were further confirmed by the pseudo-first-order linear transform. To determine the optimum catalyst loading and initial concentration of dye solution are main factor for the photodegradation of organic dye solutions. In order to investigate the effect of photocatalyst dosage Ir/WO3 (3%), different experiments were demeanor as illustrated in Fig. 9 (a, b). It is shown that the degradation efficiency was increased at catalyst dosage gradually increased as 10–50 mg. This is due to the fact that, the number of active sites increases on the surface of the catalyst when the photocatalyst dosage increased (10–50 mg). Thus it has increased the hydroxyl radical, resulting in rapid degradation of dye solutions (MB and CV). However, further increased dosage (70 mg) of photocatalyst result decrease the photodegradation efficiency compared to 50 mg. The decreased degradation performance was mainly due to the light penetration through the dye solution may be diminished and the unfavorable light scattering caused by the hindrance to the photons to reach the surface of the substrate [28,29]. Therefore, 7

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Fig. 9. Effect of catalyst dosages (a) CV, (b) MB dye solutions and the effect initial dye concentrations of (c) CV, (d) MB solutions under visible light irradiations.

Fig. 10. Influence of various scavengers on the photodegradation of CV over Ir/ WO3 (3%) nanocomposite under visible light irradiation.

Fig. 11. Photocatalytic degradation of CV solution at the existence of Ir/WO3 (3%) catalyst under visible light irradiation from the first to fifth cycles.

nanocomposite was highly decreased. It is worth to mention that in the degradation of MB, the photogenerated %OH radicals play a major role. The addition of AO into the reaction system caused a high decrease in the photocatalytic activity. This can be assigned to the effect h+ on the photocatalytic activity. It clearly concludes that %OH and h+ are the dominant reactive species in the degradation of MB when Ir/WO3

nanocomposite was used as photocatalyst. In addition, the recycling reactions were executed on most effective catalyst to appraise the stability of the photocatalyst. The recycling experiment of dyes degradation was carried out at the same experimental conditions (50 mg of catalyst, 10 mg/L of dye solutions). Fig. 11a shows the durability of Ir/WO3 photocatalyst for five cyclic 8

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Table 2 Comparison study for the degradation of CV and MB dye. Photocatalysts

Pollutant

Weight

Pollutant Concentrations

Light source

% Degradation

Time (min)

Reference

TiO2 SiO2@α-Fe2O3 α-Fe2O3 Ir/WO3 CeO2–TiO2 BiOxCly/BiOmIn BiVO4/FeVO4 Ir/WO3

MB MB MB MB CV CV CV CV

20 m 40 mg 0.1 g 50 mg 0.15 gm 10 ppm 0.1 g 50 mg

10 mg/L 5 ppm 10 mg/L 10 mg/L 5 ppm 10 ppm 10 mg/L 10 mg/L

UV–vis light Visible light Visible light Visible light Visible light Visible light Visible light Visible light

99 96 89 97 100 99.5 99.1 99

150 100 150 60 60 12 h 60 60

[32] [33] [34] This Work [35] [36] [37] This Work

CRediT authorship contribution statement

runs, were carried out the photocatalytic experiment for 50 min. After five successive runs the catalyst activity was slightly deactivation, illustrating that a little loss of catalyst in the cyclic experiment. Therefore, Ir/WO3 composite can be regarded as a good stability. This result indicated Ir/WO3 (3%) photocatalyst has excellent stability and good potential value in environmental purification. The degradation of organic dyes has been reported by several researchers which were compared to synthesized Ir/WO3 (3%) photocatalyst as tabulated in Table 2.

M. Dhanalakshmi: Conceptualization, Investigation, Writing original draft. S. Lakshmi Prabavathi: Investigation, Data curation. K. Saravanakumar: Validation, Writing - review & editing. B. Filip Jones: Validation. V. Muthuraj: Supervision, Writing - review & editing.

3.6. Photocatalytic mechanism

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Competing Interest

The possible photocatalytic mechanism for the degradation process and the reactions are given below the equation: − + WO3 + hυ → WO3 (eCB + hVB )

(1)

− − (WO3 eCB + Ir) → WO3 + Ir(eCB )

(2)

− Ir(eCB ) + O2 → O∙− 2

(3)

Acknowledgements We gratefully acknowledge to the College Managing Board, The Head of the Institution and Head of the Department (Chemistry), VHNSN College (Autonomous) for providing necessary research facilities for research. References

O2

%−

HO%2 %

+ H2O →

HO%2

%−

+ OH

(4)

+ H2O → OH + H2O2

(5)

%

OH + h+ + Organic pollutants → Mineralized product

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(6)

Upon, the visible light illumination of WO3 nanomaterials has been generates the charge carriers, the electrons in the valance band (VB) could be excited to the conduction band (CB) and the same number of holes present in VB [38,39]. The photoelectrons at the CB of WO3 can transfer to the Ir nanoparticles. The photogenerated electrons in the Ir nanoparticles could react with O2 to produce super oxide radical O2%− [40]. The O2%− can react with water to form the HO%2, it can be readily react with water to produce hydroxyl radical (HO%) [41]. In the end of the reaction HO% and h+ were able to degrade organic pollutants to mineralized product. 4. Conclusions In summary, a novel Ir/WO3 photocatalysts were successfully fabricated by facile hydrothermal method. XRD, XPS, SEM-EDX and TEM were used for pure WO3, and Ir/WO3 characterization. The effective interaction and formation of Ir loaded on WO3 was confirmed by XRD and HR-TEM studies. The Ir/WO3 (3%) catalyst showed much better photocatalytic effect than that of the pure WO3 and other Ir/WO3 nanocomposite. The enhanced photocatalytic efficiency can be ascribed to the synergistic effects of Ir NPs and WO3 that significantly suppressed the recombination of e¯-h+ pairs and greatly promoted the separation of photoexcited charge carriers. The trapping experiments revealed that % OH and h+ are played the crucial role in the photocatalytic activity. Finally, we believe that Ir/WO3 nanocomposites are a potentially active candidate for the treatments of organics polluted wastewater. 9

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[13]

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