Iridium doped ZnO nanocomposites: Synergistic effect induced photocatalytic degradation of methylene blue and crystal violet

Journal Pre-proofs Iridium Doped ZnO Nanocomposites: Synergistic Effect Induced Photocatalytic Degradation of Methylene Blue and Crystal Violet M. Dhanalakshmi, K. Saravanakumar, S. Lakshmi Prabavathi, V. Muthuraj PII: DOI: Reference:

S1387-7003(19)30801-9 https://doi.org/10.1016/j.inoche.2019.107601 INOCHE 107601

To appear in:

Inorganic Chemistry Communications

Received Date: Revised Date: Accepted Date:

5 August 2019 17 September 2019 10 October 2019

Please cite this article as: M. Dhanalakshmi, K. Saravanakumar, S. Lakshmi Prabavathi, V. Muthuraj, Iridium Doped ZnO Nanocomposites: Synergistic Effect Induced Photocatalytic Degradation of Methylene Blue and Crystal Violet, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/j.inoche.2019.107601

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Iridium Doped ZnO Nanocomposites: Synergistic Effect Induced Photocatalytic Degradation of Methylene Blue and Crystal Violet M. Dhanalakshmi1, 2, K. Saravanakumar2, S. Lakshmi Prabavathi2, V. Muthuraj2,* 1Department

of Chemistry, V. V. Vanniaperumal College for Women (Autonomous),

Virudhunagar-626 001, Tamil Nadu, India 2Department

of Chemistry, V. H. N. Senthikumara Nadar College (Autonomous), Virudhunagar-

626 001, Tamil Nadu, India Abstract A series of novel iridium (Ir) nanoparticles doped ZnO nanocomposites were constructed via one-pot simple hydrothermal strategy and used as a visible light driven photocatalyst for the degradation of Methylene Blue (MB) and Crystal Violet (CV). The synthesized pure ZnO and Ir loaded ZnO nanocomposites were systematically characterized by crystal structure, structural morphology, elemental and optical properties. The photocatalytic performance of Ir doped ZnO (2%) nanocomposite is much higher than that of pure ZnO and other Ir doped ZnO nanocomposites and more encouragingly, 10 mg/L of MB can be completely removed within 50 min of irradiation. The Ir doped in the lattice of ZnO can act as the electron trapping sites, which effectively improve the charge carrier separation. The enhanced photocatalytic activity of Ir doped ZnO (2%) composite is mainly attributed to the synergistic interaction between ZnO and Ir NPs, which could not only enhance the light absorption range, but also accelerate photo-induced interfacial charge transfer during the photocatalytic processes. Influence factors such as initial dye concentrations and catalyst doses were investigated. The generation of reactive oxidative species (ROS) such as •OH,

h+ and O2•− was also been demonstrated. Moreover, this study also paves a new vista for

promising applications in environmental water purity and energy harvesting. 1

Keywords: Synergistic effect, Iridium, Photocatalyst, Dye solutions, Visible light irradiation. *Corresponding author: Dr. V. Muthuraj E-Mail address: [email protected] : [email protected] 1. Introduction In this year water pollution is one of most worldwide problem of our society because many of the industry are ejecting the wastewater into rivers. Industry wastewater containing large amount of organic dye compounds such as methyl orange, rhodamine B, congo red, crystal violet and methyl blue and inorganic compounds, and these dyes are comes from textile industry, food industry, leather industry and etc., [1, 2]. Most of these organic dyes are present in water at low concentration, causes many problem to human and animals due to their mistrusted carcinogenic and mutagenic [3-5]. The complex structures and chemical stability of organic dyes were affecting the degradation process [6, 7]. The organic dye pollutants were removed from water by using several techniques, they are biological degradation [8, 9], chemical oxidation [10], ultrasound degradation, advanced oxidation processes (AOPs), reverse osmosis, coagulation and flocculation [11, 12]. AOPs are one of the cost effective and highly recommendable techniques for completely degradation of the dye hazardous to simpler and non-toxic inorganic compound [13, 14]. In recent years, heterogeneous photocatalyst has been widely studied for the environment remediation. ZnO is one of the mostly used semiconductor photocatalyst which has the advantages of high efficiency, low cost, non-toxicity, photochemical stability and high luminescence property [15-17]. It has wide band gap semiconductor with the band gap value is 3.4 eV and the large excitation energy of ~60 meV [18]. Different kinds of nanostructured ZnO photocatalyst used to degradation of organic pollutants, such as nanorods, nanobelts, nanoplate and hollow sphere [1922]. The drawback of ZnO photocatalyst was activated only UV light irradiation because of its 2

large band gap energy value. Another drawback of this photocatalyst is its photo instability in aqueous solution due to the photocorrosion with UV light irradiation, which leads to decrease the photocatalyst activity, rapid recombination rate of photogenerated charge carriers and very poor responsible for visible light [23]. The enhancement in the photocatalytic activity of ZnO can be adequately done by doping or coupling with metals/non metals and forming heterojuctions with semiconductor metal oxides. The doping of noble metals (such as Ag, Au, Pd and Pt, etc.) with semiconductor has been receiving considerable attention due to improve the charge transfer by trapping the photo induced charge carriers and enhance the photocatalytic activity [24, 25]. Besides, among the noble metals, Pt-group metals (i.e., Pt, Pd, Rh, Ru, and Ir), Iridium is having some unique properties like low surface coverage, sintering resistance, charge injection properties and non-toxic effects to the environmental [26]. In this work, we report a series of Ir loaded ZnO nanoparticles were prepared with facile one-pot hydrothermal method. The structural, morphology, optical properties and photocatalytic activity of as prepared ZnO and Ir loaded/doped ZnO nanocomposites were systematically investigated and evaluated by multi-technologies. The effect of catalyst weight, concentration and catalyst stability were also been discussed. Meanwhile, the photocatalytic mechanism were discussed by radical trapping experiments and proposed in detail.

2. Materials and methods 2.1. Chemicals

3

All the chemicals, zinc acetate, iridium chloride, sodium hydroxide and Methylene blue were of analytical reagent (A.R) grade and used without any further purification. All the solutions were prepared in deionized water. 2.2. Synthesis of Ir doped ZnO photocatalyst The pure ZnO and Ir doped ZnO nanocomposites were successfully synthesized by onestep hydrothermal strategy. In a typical method, 1.83 g of zinc acetate and certain amount of iridium chloride was dissolved in 50 mL of double distilled water under constant stirring condition. Then, 0.5 g of sodium hydroxide was introduced drop wise into above solution under constant stirring for 30 mins. The whole suspension was poured into 100 ml of Teflon lined stainless steel autoclave and heated hydrothermally 160 °C for 8 h. After naturally cooling down to room temperature, the obtained precipitates were washed carefully with water and ethanol and then dried at 60 °C for overnight in an oven. Finally, Ir doped ZnO nanocomposites were obtained by calcinations at 500 °C for 2h. The pure ZnO was constructed through the same procedure except inclusion of iridium chloride. 2.3. Characterization technique Crystallographic structure and phase composition of the synthesized nanocomposite was estimated by X-ray diffract meter (XRD; PANalytical X’pert Pro.) in the ranging 2 from 10-80° with using mono chromatized Cu Ka ( = 1.54178 Å) radiation under 40 kV and 100 mA. The surface morphology and microstructure of the nanomaterials were investigated using scanning electron microscopy (SEM, VEGA3 TESCAN model) and transmission electron microscopy (PHILIPS CM 200 model). Energy dispersive spectrometer (EDX – Bruker Nano GmbH, X 50 flash Detector (model-5010)) is used to evaluate the element present in the samples. The structural 4

information was carried out using Fourier Transform Infrared Spectroscopy (Shimadzu FT-IR 3000) using KBr as diluents. UV-vis diffuse reflection spectra (UV-DRS mode) (UV-2400, Shimadzu) was used to determine the absorbance ranges of the samples. Specific surface area, pore volume and pore sizes were restrained with liquid Nitrogen using a (Micromeritics ASAP-2020) Porosimeter instrument by the Brunauer-Emmett-Teller (BET) and the pore size distribution plots were estimated from Barret-Joyner-Halenda (BJH) method. 2.4. Evaluation of photocatalytic performance The photocatalytic efficiency of the Ir doped ZnO nanocomposites and undoped pure ZnO was appraised by towards the photodegradation of organic dye solutions under visible light irradiation. A tungsten incandescent lamp (λ> 400 nm) as a light source and the intensity of the visible light is 150 mW/cm-2. In typical photocatalytic experiments, 50 mg of the Ir doped ZnO photocatalyst was strewed to the 100 mL of dye solution in 150 mL reaction vessel. Before light irradiation, the MB solution mixed with the photocatalysts was stirred for 30 min in dark condition to reach the adsorption-desorption equilibrium. The temperature of the reaction system was maintained at room temperature by providing cooling water during the whole process. The absorption wavelength of MB and CV dye solution was observed at λ = 664 and λ = 586 nm, respectively. At certain time intervals 3 ml of dye solution were collected and measure the absorption peaks at nm using UV-vis spectrometer (Shimadzu-2600). The photocatalytic degradation efficiency was calculated by the following equation,

(

𝑫𝒆𝒈𝒓𝒂𝒅𝒂𝒕𝒊𝒐𝒏 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 (%) = 𝟏 ―

)

(

)

𝑪 𝑨 𝑿 𝟏𝟎𝟎 = 𝟏 ― 𝑿 𝟏𝟎𝟎 𝑪𝟎 𝑨𝟎

5

Where C0 is the concentration of the dye solution, C is the concentration of dye solution after a given time of the reaction; A0 and A is the absorbance intensity of the dye solution after the dark and after irradiation reaction, respectively. The reactive oxidative species involved in the photocatalytic reaction was found by the radicals trapping experiment by using different scavengers viz 2-propyl alcohol (IPA), benzoquinone (BQ) and ammonium oxalate (AO). The degradation of the organic dye solution was monitored for the detection of active species. 0.01 M of scavengers was used for the trapping experiment in the present investigation. The used photocatalyst were separated from the reaction solution mixture after the photocatalytic degradation reactions by centrifugation at 5000 rpm, washed with de-ionized water for several times and dried at 80 °C for the reusability tests. The recycle experiments were also carried out under the same experimental conditions. To compensate the loss of the catalysts during the washing process, a constant amount of the photocatalyst concentration was maintained in each cycle test and several degradation tests were carried out simultaneously at each recycle tests and enough catalysts were collected. 3. Results and discussion 3.1. X-ray spectra analysis Figure 1, shows the powder X-ray pattern of bare ZnO and different concentration of Ir doped ZnO nanocomposite; produce the information about crystallinity and crystal phase of the prepared sample. All the diffraction peaks of synthesized bare ZnO nanoparticles 100% matching with standard JCPDS [card no (89-0511)] and it has been indexed as hexagonal wurtzite structure. The intensity of the all diffraction peaks are high, it indicates that the obtained ZnO nanoparticles have high crystallinity nature. IZO1, IZO2 and IZO3 revealed that Ir doped ZnO, the diffraction 6

peaks are all in good agreement with the bare ZnO peaks. Ir doped ZnO nanocomposite diffraction angle was slightly shifted to higher Bragg angle and the intensity of the peaks is decreased, which is due to the broadening of peaks and increased the FWHM value. The diffraction peak broadening and peak shift were shown on enlarged version of the XRD spectra 35° to 38° in Figure (1 b). It suggesting that, the Ir was successfully doped on the surface of ZnO nanoparticles. No other additional characteristic peaks were observed in this spectrum, which further confirms the bare ZnO and Ir doped ZnO nanocomposite was high quality and pure. The average crystallite size of the bare ZnO nanoparticles and different concentration Ir doped/loaded ZnO nanocomposite was calculated by X-ray line broadening method using Dye-Scherrer’s equation [27],

𝒌 D =  𝜷𝑪𝒐𝒔𝜽 where D is crystallite size, k is the Scherrer’s constant,  is the wavelength of the incident X-ray radiation used (Cu Kα radiation- 1.5408 Å),  is the full width half maximum (FWHM) of the diffraction peaks and  is the diffraction angle in degree. The obtained average crystallite sizes of 40, 34, 30 and 25 nm were pure ZnO, IZO1, IZO2 and IZO3 nanocomposites which was calculated by using (101) reflection plane respectively. The crystallinity of the ZnO was reduced after doping of Ir, may be produced some strain or stress on the ZnO lattice by Ir.

3.2. XPS analysis

7

The surface composition and chemical state of as-synthesized sample were investigated by X-ray photoelectron spectroscopy (XPS), which demonstrated in Figure 2. As Figure 2a shows the survey spectrum of Ir doped ZnO nanocomposite, it revealed that the presence of Ir, Zn and O elements. Figure 2(b-d) displayed that high resolution XPS spectra of Ir, Zn and O elements. The strong two bands observed at 62 eV and 65.1 eV in the XPS spectrum of Ir 4f were assigned to Ir 4f7/2 and Ir 4f5/2 respectively, which illustrated in Figure 2b [28]. As could be seen in Fig. 2c, the binding energies of Zn 2p3/2 and Zn 2p1/2 located at 1021.7 and 1044.8 eV respectively, in the Ir doped ZnO material, which good agreement with binding energy of ZnO [29, 30]. For O 1s spectra binding energy was observed at 531 eV, which corresponds to the O bonded to Zn atoms in Ir doped ZnO nanocomposite [31]. 3.3. Morphological analysis The structural morphology of the pure ZnO and Ir doped ZnO nanocomposite were analyzed via scanning electron microscopy (SEM) were shown in Figure 3. The SEM images of ZnO exhibited the irregular spherical like structure morphology and agglomerated with each other particles (Figure 3 a, b). After the loading of Ir, the Ir NPs are highly decorated on the rough surface of bare ZnO nanoparticles and the aggregation phenomena observed between the Ir and ZnO (Figure 3 c, d). Transmission electron microscopy (TEM) was employed to investigate the morphological and crystal structure of Ir doped ZnO nanocomposite. The TEM images of Ir doped ZnO nanocomposite in Figure 4 a-c showed that quasi-spherical particle morphology with dimension in the range of 30-50 nm. Ir NPs were closely dispersion over the surface of the ZnO nanomaterial. Furthermore, the corresponding selected area electron diffraction (SAED) pattern confirmed the formation of polycrystalline nature of Ir doped ZnO nanocomposite and shown in

8

Figure 4d. The SAED rings are sharp and clear which mainly suggests that the crystalline grains could be tiny and scatter evenly on the surface. The energy dispersive X-ray analysis (EDX) was carried out to confirm the elemental presence and proportion of Ir doped ZnO nanocomposite. EDX analysis depicts the presence of Zn, O and Ir in the as-prepared Ir doped ZnO which was displayed in Figure 5. In addition, EDX elemental mapping images were also revealed the existence of Zn, O and Ir in Ir loaded ZnO nanocomposite with an even distribution of Ir NPs in Ir doped ZnO nanocomposite. The morphological studies of SEM, TEM and EDX with elemental mappings confirmed the successful construction of Ir loaded ZnO nanocomposite. 3.4. Optical studies In photocatalytic performance, optical properties are considered to be main indicator. So pure ZnO and Ir loaded ZnO nanocomposites were investigated by UV-vis diffused reflectance absorption spectrum and displayed in Figure 6. As shown in Figure 6, the pristine ZnO observed the absorption range at 405 nm and the light absorption ability has been increased from UV to visible light range by the doping of Ir nanoparticles [32]. In detail, the Ir doped ZnO nanocomposites have obvious red shift and the absorption edge is 500-700 nm. This result should be ascribed to the increasing the loading of Ir concentration absorption wave length suggested that Ir doped ZnO nanocomposite more absorb the visible light and produce more number of photogenerated e--h+ pairs, which should be favorable for the photocatalytic reaction. The bandgap energies of synthesized pure ZnO and Ir loaded ZnO nanocomposites were determined by using Tauc’s equation [33]. (αhν) = A (hν–Eg)p/2 9

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 values of pure ZnO, IZO1, IZO2 and IZO3 nanocomposites were 3.19, 3.15, 3.06 and 2.94 eV, respectively. To investigate the influence of doped Ir with ZnO nanoparticles, migration and trapping of photo-generated carrier on the photoluminescence spectroscopy studies were performed, through which the recombination of photogenerated electron-hole pairs [34]. The room temperature PL spectra of ZnO and different concentration of Ir doped ZnO nanocomposite measured the excitation wavelength, which illustrated in Figure 7. The PL intensity of Ir doped ZnO was greatly decreased as compared with that of pure ZnO nanocomposite. It reveals that the Ir doped ZnO nanocomposite higher photocatalytic activity than bare ZnO, because of Ir doped ZnO nanocomposite effectively suppressed the photogenerated electron-hole (e−-h+) pair recombination [35]. IZO2 nanocomposite shows the higher photocatalytic activity compared to the other IZO catalysts and Pure ZnO which mainly due to the IZO2 was weaker intensity of the emission peaks. 3.5. Nitrogen adsorption-desorption analysis The textural properties of pure ZnO, and (2%) Ir doped ZnO nanocomposite was inferred adsorption–desorption isotherms with their pore size spreading were exposed in Figure 8. Figure 8 (inset) illustrates the dispersal of pore size values of the ZnO and Ir doped ZnO, which was presented by the using of BJH method. The Brunauer-Emmett-Teller specific surface area (SBET) and pore volume of the pristine ZnO is about 14.2 m2/g and 0.037 cm3/g, respectively. When the Ir doped on the ZnO nanoparticles, the SBET of the Ir doped ZnO nanoocomposite (SBET = 9.3 m2/g, 10

pore volume = 0.037 cm3/g) become smaller which is slightly lower than the pure ZnO. The decrease in the surface area might be due to the self-restacking and pore blockage effects between Ir NPs and ZnO. Nevertheless, the positive effects of Ir NPs (sensitizing the catalyst, decrease the e--h+ recombination rate, higher active sites, etc.) far outweigh its disadvantage of lowering the surface area. 3.6. Photocatalytic activity The photocatalytic activity of the pure ZnO and Ir doped ZnO nanocomposites were examined for methylene blue (MB) and crystal violet (CV) organic dye pollutants degradation under visible light irradiation. Time dependent UV-vis absorption spectrum of the MB and CV dye degradation are shown in Figure 9. When the visible light irradiated on the photocatalytic materials, the strong absorption peak intensity at 650 nm is gradually decreased as the irradiation time increases time due to the complete degradation of MB after 50 min. From Figure 8, the IZO2 nanocomposite exhibited the outstanding photocatalytic efficiency under visible light illumination, which significantly higher than the pure ZnO and other Ir loaded ZnO photocatalysts. In order to perform a blank experiment, there is no degradation observed in the absence of light as well as in the absence of catalyst and the rate of degradation or mineralization process is not noticeable. The photocatalytic activities of pure ZnO with different percentages of Ir were estimated by the degradation of organic dye pollutants under visible light irradiation which is illuminated in Figure 10. The degradation efficiency of MB and CV dye solutions for pristine ZnO was 46%, 40% within 50 and 60 min of visible light irradiation, respectively. As expected, the photocatalytic performance is enhanced after doping of Ir into ZnO. Obviously, IZO2 exhibits the superior catalytic activity compared to other Ir loaded ZnO nanocomposites. Note that the photocatalytic 11

activity enhanced when the content of Ir increased from 1 to 2, but further increment causes decrement in photocatalytic efficiency. When the doping percentage of Ir is over 2%, aggregation of Ir NPs would gradually emerge and the separation efficiency of photogenerated charge carriers decreased which could restrict the photocatalytic performance. Another reason for that, it would increase the internal defects of ZnO crystals which could be act as a charge carrier’s recombination centres. With a lower doping level of Ir, the former positive effect dominates resulting in enhanced photocatalytic efficiency increases. However, the excessive doping amount can provide more defects leading to the decrease of photocatalytic performance of ZnO. These results indicate the suitable amount of Ir is important parameters for photocatalytic process. The amount of catalyst dosage added will affect the photocatalytic activity on degradation of dyes under visible light irradiation. The catalyst dosage varied from 10 to 75 mg at constant dye concentration which are shown in Figure 11 (a, b). The photocatalytic activity ability was increased with increased the amount of photocatalyst dosage at 10 to 50 mg. In this case 50 mg of the photocatalyst has maximum photocatalytic ability compared to the other amount of photocatalyst dosage might lower efficiency. Future increased the catalyst weight 50 mg to 75 mg the catalytic activity was decreased may also tend to the catalyst prevents light penetrations [36]. Hence, 50 mg of the photocatalyst was a sufficient amount for the photocatalytic degradation of dyes. The effect of the dye concentration on photocatalytic degradation was evaluated by the dye concentration vary from 10, 20 and 30 mg/L by keeping the remaining parameters such as photocatalyst weight and pH of the solution are constant and the result illustrated in Figure 11 (c, d). The dye concentration increased from 10 to 30 mg/L the rate of photodegradation efficiency was decreased. However, 10 mg/L above concentration the rate of photodegradation efficiency was decreased. This result indicated that, at high concentration screening the light intensity by the 12

dye solution to reach the fewer photons on the surface of the Ir doped ZnO photocatalyst. Hence, reduce the rate of degradation due to the generation of electron-hole pairs was gradually decreased [37]. Therefore, 10 mg/L is optimized concentration of the dye solution the Ir doped ZnO photocatalyst was effective and efficient. In order to detect the ROS during the MB dye degradation process under visible light illuminations, different scavengers such as 2-prapanol (IPA), benzoquinone (BQ) and ammonium oxalate were employed as typical •OH, O2•− and h+, respectively [38, 39]. As shown Figure 12, the degradation rate of MB was dramatically quenched in the presence of BQ, indicating that O2•− radical was the dominating species for the photodegradation. Notably, in the presence of IPA, moderate level of degradation efficiency was declined. Additionally, the AO showed little effect on the photodegradation MB, indicating that h+ was negligible in the pathway MB degradation process. From this results, O2•− and •OH are the major reactive species, and h+ plays a minor role in the photocatalytic reaction compare with others. The stability and reusability of photocatalyst was vital factor for their practical application [40]. The stability of the Ir doped ZnO nanocomposite photocatalyst was examined by photocatalytic degradation of dyes under visible light irradiation, and the result shown in Figure 13a. It shows that the photocatalytic activity efficiency first to fifth cycle, the catalyst have stable and the degradation efficiency was slightly decreased during the catalytic degradation reactions. The XRD pattern of the photocatalyst don’t changed after fifth cycle runs which displayed in Figure 13b. This indicated that the crystal structure of the Ir doped ZnO photocatalyst did not change after the photocatalytic degradation of organic dye solutions, suggesting a better reusability and stability of Ir doped ZnO photocatalyst. Hence the Ir doped ZnO nanocomposite photocatalyst have more stable under visible light irradiation and the ability to reuse after fifth cycle. 13

4. Conclusions In summary, bare ZnO and Ir loaded ZnO nanocomposites were synthesized by using a simple hydrothermal method as a novel photocatalyst for the dye abatements. The synthesized Ir doped ZnO nanocomposites and ZnO characterized using a range of physico-chemical characterization techniques. The influence of ZnO with different Ir loading ratio to the photocatalytic activity has been studied detailed by the degradation of MB and CV under visible light illumination. The results revealed that the 2% of Ir loaded ZnO photocatalyst have an excellently photocatalytic efficiency comparing with ZnO and Ir doped ZnO nanocomposites. The improved photocatalytic performance was mainly attributed to the synergistic effect between the Ir NPs and ZnO, and the Ir NPs act as an electron sinks which can decreases the recombination rate of charge carriers. The trapping study suggested that O2•− and •OH could have played the key role in degradation of MB over Ir doped ZnO catalyst.

Acknowledgements We gratefully acknowledge to the College Managing Board, The Principal and Head of the Department (Chemistry), V. H. N. Senthikumara Nadar College (Autonomous) for providing necessary research facilities. References

14

1. Na Tian, Hongwei Huang, Yihe Zhang, Mixed-calcination synthesis of CdWO4/g-C3N4 heterojunction with enhanced visible-light-driven photocatalytic activity, Appl. Surf. Sci., 358 (2015) 343-349. 2. Weilong Shi, Feng Guo, Jibin Chen, Guangbo Che, Xue Lin, Hydrothermal Synthesis of InVO4/Graphitic Carbon Nitride Heterojunctions and excellent visible-light-driven photocatalytic performance for rhodamine B, J. Alloys Compd., 612 (2014) 143-148. 3. K. Saravanakumar, V. Muthuraj, M. Jeyaraj, The design of novel visible light driven Ag/CdO as smart nanocomposite for photodegradation of different dye contaminants, Spectrochim. Acta A., 188 (2018) 291–300. 4. H. H. Hammud, A. Shmait, N. Hourani, Removal of Malachite Green from water using hydrothermally carbonized pine needles, RSC Adv. 5 (2015) 7909–7920. 5. V.K. Gupta, R. Kumar, A. Nayak, T.A. Saleh, M.A. Barakat, Adsorptive removal of dyes from aqueous solution onto carbon nanotubes: a review, J. Colloid Interf. Sci., 193–194 (2013) 24–34. 6. K. Saravanakumar, V. Muthuraj, Fabrication of sphere like plasmonic Ag/SnO2 photocatalyst for the degradation of phenol, Optik., 131 (2017) 754–763. 7. B. Neppolian, H. C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, Solar/UV-induced photocatalytic degradation of three commercial textile dyes, J. Hazard. Mater. 89 (2002) 303–317. 8. S. Kuppusamy, M. Sethurajan, M. Kadarkarai, R. Aruliah, Biodecolourization of textile dyes by novel, indigenous Pseudomonas stutzeri MN1 and Acinetobacter baumannii MN3, J. Environ. Chem. Eng., 5 (2017) 716-724.

15

9. A. Das, S. Mishra, Removal of textile dye reactive green-19 using bacterial consortium: Process optimization using response surface methodology and kinetics study, J. Environ. Chem. Eng., 5 (2017) 612-627. 10. V. M. Vasconcelos, F. L. Migliorini, J. R. Steter, M. R. Baldan, M. R. de Vasconcelos Lanza, Electrochemical oxidation of RB-19 dye using a highly BDD/Ti: Proposed pathway and toxicity, J. Environ. Chem. Eng., 4 (2016) 3900-3909. 11. İ. Arslan, I. A. Balcioǧlu, D. W. Bahnemann, Advanced chemical oxidation of reactive dyes in simulated dyehouse effluents by ferrioxalate-Fenton/UV-A and TiO2/UV-A processes, Dyes Pigm., 47 (2000) 207-218. 12. J. H. Choi, W. S. Shin, S. H. Lee, D. J. Joo, J. D. Lee, S. J. Choi, L. S. Park, Application of synthetic polyamine flocculants for dye wastewater treatment, Sep. Sci. Technol., 36 (2001) 2945–2958. 13. C. Vidya, M. N. Chandra Prabha, M. A. L. Antony Raj, Green mediated synthesis of Zinc oxide nanoparticles for the Photocatalytic degradation of Rose Bengal dye, Environmental Nanotechnology, Monitoring and Management, 6 (2016) 134-138. 14. S. Kaur, S. Sharma, A. Umar, S. Singh, S. K. Mehta, S. K. Kansal, Solar light driven enhanced photocatalytic degradation of brilliant green dye based on ZnS quantum dots, Superlattice Microst., 103 (2017) 365-375. 15. P. Li, Z. Wei, T. Wu, Q. Peng, Y. Li, Au-ZnO hybrid nanopyramids and their photocatalytic properties, J. Am. Chem. Soc. 133 (2011) 5660–5663. 16. S. Ma, J. Xue, Y. Zhou, Z. Zhang, Photochemical synthesis of ZnO/Ag2O heterostructures with enhanced ultraviolet and visible photocatalytic activity, J Mater Chem A., 2 (2014) 7272-7280.

16

17. R. D. C. Soltani, A. Rezaee, M. Safari, A. R. Khataee, B. Karimi, Photocatalytic degradation of formaldehyde in aqueous solution using ZnO nanoparticles immobilized on glass plates, Desalin. Water Treat. 53 (2013) 1613-1620. 18. Y. K. Mishra, G. Modi, V. Cretu, V. Postica, O. Lupan, T. Reimer, I. Paulowicz,

V.

Hrkac, W. Benecke, L. Kienle, R. Adelung, Direct growth of freestanding ZnO tetrapod networks for multifunctional applications in photocatalysis, UV photodetection, and gas sensing, ACS Appl. Mater. Inter. 7 (2015) 14303–14316. 19. A. Hui, J. Ma, J. Liu, Y. Bao, J. Zhang, Morphological evolution of Fe doped sea urchinshaped ZnO nanoparticles with enhanced photocatalytic activity, J. Alloy. Compd. 696 (2017) 639–647. 20. K. Ravichandran, R. Mohan, B. Sakthivel, S. Varadharajaperumal, P. Devendran,

T.

Alagesan, K. Pandian, Enhancing the photocatalytic efficiency of sprayed ZnO thin films through double doping (Sn + F) and annealing under different ambiences, Appl. Surface Sci. 321 (2014) 310–317. 21. Maryam Shekofteh-Gohari, Aziz Habibi-Yangjeh, Masoud Abitorabi, Afsar Rouhi, Magnetically separable nanocomposites based on ZnO and their applications in photocatalytic processes: A review, Critical Reviews in Environmental Science and Technology, 48 (2018) 806-857. 22. Mahsa Pirhashemi, Aziz Habibi-Yangjeh, Shima Rahim Pouran, Review on the criteria anticipated for the fabrication of highly efficient ZnO-based visible-light-driven photocatalysts, J Ind Eng Chem, 62 (2018) 1-25.

17

23. Mitra Mousavi, Aziz Habibi-Yangjeh, Shima Rahim Pouran, Review on magnetically separable graphitic carbon nitride-based nanocomposites as promising visible-light-driven photocatalysts, J Mater Sci: Mater Electron., 29 (2018) 1719-1747. 24. K. Saravanakumar, V. Muthuraj, S. Vadivel, Constructing novel Ag nanoparticlesanchored on MnO2 nanowires as an efficient visible light driven photocatalyst, RSC Adv. 6 (2016) 61357–61366. 25. S. Karunamoorthy, M. Velluchamy, Design and synthesis of bandgap tailored porous Ag/NiO nanocomposite: an effective visible light active photocatalyst for degradation of organic pollutants, J Mater Sci: Mater Electron., 29 (2018) 20367–20382. 26. M. Dhanalakshmi, K. Saravanakumar, S. Lakshmi Prabavathi, M. Abinaya, V. Muthuraj, Fabrication of novel surface plasmon resonance induced visible light driven iridium decorated SnO2 nanorods for degradation of organic contaminants. J. Alloys Compd. 763, (2018) 512–524. 27. K. Saravanakumar, S. Muthupoongodi, V. Muthuraj, A novel n-CeO2/n-CdO heterojunction nanocomposite for enhanced photodegradation of organic pollutants under visible light irradiation, J Rare Earths 37 (2019) 853-860. 28. M. R. Ayats, G. Garcıa, J. L. Galante, M. A. Pena, M. V. Martınez-Huerta, Electrocatalytic stability of Ti based-supported Pt3Ir nanoparticles for unitized regenerative fuel cells, Int J Hydrogen Energy., 39(10) (2014) 5477-5484. 29. Somayeh Zarezadeh, Aziz Habibi-Yangjeh, Mitra Mousavi, BiOBr and AgBr co-modified ZnO photocatalyst: A novel nanocomposite with p-n-n heterojunctions for highly effective photocatalytic removal of organic contaminants, J. Photochem. Photobiol. A., 379 (2019) 11-23.

18

30. Somayeh Zarezadeh, Aziz Habibi-Yangjeh, Mitra Mousavi, Fabrication of novel ZnO/BiOBr/C-Dots nanocomposites with considerable photocatalytic performances in removal of organic pollutants under visible light, Adv Powder Technol, 30 (2019) 11971209. 31. Mahsa Pirhashemi, Aziz Habibi-Yangjeh, Facile fabrication of novel ZnO/CoMoO4 nanocomposites: Highly efficient visible-light-responsive photocatalysts in degradations of different contaminants, J. Photochem. Photobiol. A., 363 (2018) 31-43. 32. Mahsa Pirhashemi, Aziz Habibi-Yangjeh, Ultrasonic-assisted preparation of plasmonic ZnO/Ag/Ag2WO4 nanocomposites with high visible-light photocatalytic performance for degradation of organic pollutants, J Colloid Interface Sci., 491 (2017) 216-229. 33. K. Saravanakumar, M. Mymoon Ramjan, P. Suresh, V. Muthuraj, Fabrication of highly efcient visible light driven Ag/CeO2 photocatalyst for degradation of organic pollutants. J. Alloys Compd. 664, 149–160 (2016). 34. K. Fujihara, S. Izumi, T. Ohno, M. Matsumura, Time-resolved photoluminescence of particulate TiO2 photocatalysts suspended in aqueous solutions, J. Photochem. Photobiol. A., 132 (2000) 99–104. 35. Mahsa Pirhashemi, Aziz Habibi-Yangjeh, ZnO/NiWO4/Ag2CrO4 nanocomposites with pn-n heterojunctions: highly improved activity for degradations of water contaminants under visible light, Sep Purif Technol., 193 (2018) 69-80. 36. P.S. Kumar, M. Selvakumar, S.G. Babu, S.K. Jaganathan, S. Karuthapandian,

S.

Chattopadhyay, Novel CuO/chitosan nanocomposite thin film: facile hand-picking recoverable, efficient and reusable heterogeneous photocatalyst, RSC Adv. 5 (2015) 57493–57501.

19

37. K. Saravanakumar, R. Karthik, S. M Chen, J. V. Kumar, K. Prakash, V. Muthuraj. Construction of novel Pd/CeO2/g-C3N4 nanocomposites as efficient visible-light photocatalysts for hexavalent chromium detoxification. J Colloid Interface Sci. 504 (2017) 514-526. 38. I. Nath, J. Chakraborty, P. M. Heynderickx, F. Verpoort, Engineered synthesis of hierarchical porous organic polymers for visible light and natural sunlight induced rapid degradation of azo, thiazine and fluorescein based dyes in a unique mechanistic pathway, Appl Catal B-Environ., 227 (2018) 102–113. 39. S. L. Prabavathi, K. Saravanakumar, G. Mamba, V. Muthuraj, 1D/2D MnWO4 nanorods anchored on g-C3N4 nanosheets for enhanced photocatalytic degradation ofloxacin under visible light irradiation, Colloids Surf A., 581 (2019) 123845. 40. K. Saravanakumar, P. Senthil Kumar, J. Vinoth Kumar, S. Karuthapandian, Robert Philip, V. Muthuraj, Controlled Synthesis of Plate Like Structured MoO3 and Visible Light Induced Degradation of Rhodamine B Dye Solution, Energy and Environment Focus, 5 (2016) 50-57.

Figure captions

20

Figure 1: XRD patterns of as-prepared pure ZnO and Ir doped ZnO nanocomposite with different Ir loading concentration (1, 3 and 5%) in the range of 10–70o (a) and 35–38o (b).

21

Figure 2: The XPS of as-prepared (2%) Ir doped ZnO: (a) survey spectrum and high-resolution spectra for: (b) Ir 4f, (c) Zn 2p, and (d) O 1s.

22

Figure 3: SEM images of (a, b) pristine ZnO and (c, d) (2%) Ir doped ZnO nanocomposite

23

Figure 4: TEM images of (2%) Ir doped ZnO nanocomposite with different magnifications (a200, b-100 and c-50 nm) and the corresponding SAED pattern

24

Figure 5: EDX spectra and elemental mapping of (2%) Ir doped ZnO nanocomposite. The color of EDX mapping red (Zinc), green (Oxygen) and blue (Iridium)

25

Figure 6: (a) The light absorbance (in term of Kubelka-Munk equivalent absorbance units) of the pure ZnO and Ir doped ZnO nanocomposites (b) (αhν)2 versus hν plots of pure ZnO and Ir doped ZnO nanocomposites.

26

Figure 7: Photoluminescence spectra of as prepared pure ZnO and Ir doped ZnO nanocomposites.

27

Figure 8: Nitrogen adsorption-desorption curves of (a) pure ZnO and (b) 2% Ir doped ZnO nanocomposite, Inset shows the BJH pore size distribution curves.

28

Figure 9: UV spectra of (a) MB and (b) CV dye solutions during photocatalytic reaction in the presence of (2%) Ir doped ZnO nanocomposite photocatalyst

29

Figure 10: Visible light driven photocatalytic degradation of MB (a) and CV (b) dye solution upon different Ir doped ZnO nanocomposites and ZnO photocatalyst

30

Figure 11: Influence of catalyst loading on the performance of IZO2 nanocomposite photocatalyst (a) MB and (b) CV, Influence of initial dye concentration (c) MB and (d) CV

31

Figure 12: Role of different scavengers on the photodegradation efficiency of MB catalyzed by IZO2 after 50 min of reaction

32

Figure 13: Reusability test of IZO2 nanocomposite for degradation of MB under visible light irradiation

33

Graphical abstract

34

Highlights 

A novel Ir doped ZnO photocatalysts were successfully constructed via one-pot hydrothermal method.



Ir doped ZnO nanocomposites exhibited superior photocatalytic activity endowed by synergistic effect.



Ir nanoparticles were played a major role in charge carrier separations.

Conflict of interest The authors declare that, there is no conflict of interest.

35