Optical Materials

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Optical Materials 107 (2020) 110000

Available online 13 May 2020

Highly efficient and Reusable ZnO microflower photocatalyst on stainless steel mesh under UV – Vis and natural sunlight

Monoj Kumar Singha

^a^,^*

, Aniket Patra

^b

aDept. of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, India

bElectrical and Communication Engineering, Indian Institute of Science, Bangalore, India

A R T I C L E I N F O Keywords:

Ultrasonic spray pyrolysis

Single-stepZnO microflowerdeposition Methylene blue degradation Highly efficient and repeatability pH medium

Photocatalysis mechanism

A B S T R A C T

The photocatalytic process is used to remove organic contaminants, dyes, industrial sewage from wastewater before reusing it for human consumption again. Here a simple cost-effective ultrasonic spray pyrolysis (USP) technique is used to deposit ZnO microflower on glass and stainless steel (SS) mesh in a single step. X-Ray Diffraction (XRD) and Raman spectroscopy indicate the polycrystalline wurtzite nature of ZnO. Field emission scanning electron microscope (FESEM) shows the ZnO microflower structure. PL and XPS studies show the oxygen defect in the films. Photocatalytic degradation results show that methylene blue (MB) dye with 1 �10 ⁵M concentration can be degraded around 97.94% and 85% in 120min and 210 min with a degradation rate of 0.03035 min ¹and 0.00917 min ¹respectively using ZnO deposited on SS mesh and glass substrate in both UV–Vis and sunlight condition. ZnO deposited on SS mesh shows high repeatability performance to degrade the MB and it can degrade 96% of MB at 10th cycle. Degradation rate at base medium is higher than acidic medium.

A plausible photocatalysis mechanism is also explained.

1. Introduction

Substantial quantities of dyes are processed per annum, which is utilized in diverse industries including leather, cosmetic, textile, paper, pharmaceutical, etc. As of now, more than a lakh of mercenary dyes are there, and rough approximation suggests 70K tones of the same are manufactured per year, with a loss of about 15% during the dyeing operation [1]. In nature, almost all of these dyes are toxic and even an ample amount of dyes prompt some hazardous problems to human health and aquatic life [2]. Worldwide around 70% of industrial effluent is released without proper treatment; as a consequence by 2025, it is guessed that 50% of humanity will entice freshwater crises [3]. These harmful elements (dyes) need to remove from the water before using it again for human consumption. Especially organic contaminants from textile industries discharge many non-biodegradable dyes into the aquatic environment. So, the removal of organic contaminants like dyes is one of the major research in the present day. Several processes like chemical precipitation/separation, coagulation, ion exchange, catalytic, molecular sieves, adsorption, membrane filtration, reverse osmosis, chlorination, ozonation, advance oxidation processes are used to remove the dyes [4–6]. Photocatalytic process is one of the techniques in which

chemical transformation rate is promoted by the photocatalyst and dyes can be easily degraded from the water.Various semiconductor oxides like TiO2, ZnO, Fe2O3, WO3, CuO and CTS, CZTS are used for visible-light response and UV-response photocatalytic experiments [7–14]. The efficiency of photocatalytic activity not only depends on proper bandgap and well defined electronic configuration of these materials but also depends on light absorption capability, porous structures and charge transfer properties, etc. Although TiO₂shows efficient photocatalytic activity and has been tremendously studied for application in degradation of dyes, it has recently been reported that, in comparison, ZnO has higher degradation ability for organic contaminants due to its higher absorbance of light and used in larger spectrum region even in visible region for photocatalytic activity [15,16]. ZnO, a wideband semiconductor having bandgap of 3.37eV, has many applications like solar cells, gas sensors, field emission, piezoelectric generator, UV photodiode [17–24]. ZnO, as thin film or nanopowder, is used to deposit or synthesize in various methods like sputtering, atomic layer deposition, pulse layer deposition, MOCVD, SILAR method, electrodeposition, spray pyrolysis, thermal evaporation, sol-gel, and chemical precipitation [25–31]. Among these spray pyrolysis is a simple method to deposit thin film on the substrate or synthesis of nanoparticles.

* Corresponding author.

E-mail addresses: [email protected], [email protected] (M.K. Singha), [email protected] (A. Patra).

Contents lists available at ScienceDirect

journal homepage: http://www.elsevier.com/locate/optmat

https://doi.org/10.1016/j.optmat.2020.110000

Received 6 February 2020; Received in revised form 25 April 2020; Accepted 8 May 2020

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Optical Materials 107 (2020) 110000 It has been observed that a tremendous amount of work has been

done on ZnO nanopowders as photocatalyst to degrade dyes. But removing these nanoparticles after the photocatalysis process from the solution is extremely difficult which leaves it sterile to be used for such applications. To overcome this problem people have used different techniques like mixing with magnetic particles with nanopowders, fil- trations. But these processes increase the cost in the degradation process and it is also time-consuming to reuse as photocatalyst for the second time. So, another possible method is to use thin film technology to deposit the films and degrades the dyes. But thin films have limitations of slow degradation rate due to less surface area. In order to enhance the reaction surface area and resultantly dye degradation rates of the films, an effort has been made to modify the surface anatomy/structure of the thin films. These modified shapes and structures of the thin film can improve the dye degradation rates. Shapes like nanorods, pencil shape, flowers are useful for the degradation process. Or by changing the substrates like mesh type will increase the surface area which subse- quently enhance the degradation rate [32]. There is contradictory views on effect of film thickness for photocatalytic activity. Some report sug- gest that increase of film thickness improves the photocatalytic activity whereas some researcher showed that film thickness has no effect on photocatalytic activity [33–35].P. Jongnavakit et al. have used sol gel derived ZnO thin film on glass substrate for degradation of Methylene Blue (MB) [36]. R. Ahumada-Lazoet. al have deposited ZnO using RF sputtering system to degrade the organic G dyes [37]. Wang et al. have electrochemically deposited ZnO on stainless steel (SS) mesh to degrade the dyes [38]. A. Abdel et al. have used evaporation followed by air annealing to grow ZnO thin films for degradation of 2,4,6-Trichlorophe- nol (TCP) [39]. All of these techniques used to grow or deposit ZnO are either complex in nature, less yielding, time-consuming or not cost-effective. Very fewer works were also carried out in UV–Vis light or in natural sunlight environment for dye degradation, though sunlight has almost ~45% visible spectrum. Therefore, in this paper we have

deposited ZnO thin films by a facile ultrasonic spray pyrolysis technique.

Here we have used an ultrasonic spray pyrolysis to deposit ZnO on the glass and SS mesh substrate. Microflower like shapes were grown on both substrates in a single step deposition method. Flowers shape thin film has more surface to volume ration compare to other nanostructured based thin film. 1 � 10 ⁵M MB solution was tested to perform the degradation test under UV–Vis light irradiation and natural sunlight.

Methylene Blue is one of the dye which is heavily used and it is toxic to aquatic life. It has side effects like breathing problems, headaches, vomiting to the human body.We have compared the ZnO thin films on a glass substrate and SS mesh substrate for photocatalytic activity.

Different loading of active materials, repeatability, dye degradation under different pH environment and their mechanism are well studied in this paper.

2. Materials

All chemicals (Zinc acetate dehydrate, MB were procured from Sigma Aldrich and Himedia respectively) were of analytical reagent grade and used without further purification. pH 4 and 9 solutions were purchased from Thomas Beaker. Benzoquinone (BQ), isopropyl alcohol (IPA) and disodium ethylenediaminetetraacetate (EDTA-2Na) were ob- tained from SRL chemicals. The aqueous solution of zinc acetate was prepared using Millipore deionized water (18.2 Ω-cm). The SS Mesh (300 mesh) was purchased from the hardware store. Glass substrates (micro slide) were from Blue dart. In the first stage, the SS Meshes were put into diluted hydrochloric acid for the 60s to remove the rust and other contaminant, then it was cleaned with ultrasonication in acetone and IPA for 10 min respectively, finally rinsed with deionized water.

Glass slides are cut into 1.5 �2.5 cm²and cleaned in chromic acid, followed by ultrasonication in DI water, acetone, and IPA respectively.

2.1. Experimental details

ZnO was deposited on the SS mesh and glass slide by using a homemade automatic ultrasonic spray pyrolysis system with 1.7 MHz frequency. Before the deposition of ZnO, the SS substrates were again cleaned in acetone under ultra-sonication for 10 min followed by ultrasonication cleaning of IPA and dried in a hot air oven at 80 ^�C for 6 h.

After drying the substrate, it was kept on a hotplate for 450 ^�C for spray deposition of ZnO. Spray solution was prepared in 120 ml of Millipore DI water with 0.1 M of zinc acetate as the source of ZnO. The distance between the substrate and nozzle was kept 1.5 cm and the total spray duration was 125 min. After spray deposition, substrates were annealed on the hotplate for 45 min at 450 ^�C temperature. After that SS mesh samples were cut into 3 �1.2 cm²for 3 pieces and used for further characterizations and photocatalytic experiments.

2.2. Characterization and analysis

The surface morphologies and microstructure of ZnO on SS were characterized by a field emission scanning electron microscope (ULTRA 55 FESEM, Zeiss). The ultraviolet and visible (UV–Vis) absorption spectrum was measured by Specord S-600 UV–Vis spectrophotometer.

To investigate the phase formation of ZnO, X-Ray Diffraction (XRD) was carried out using the X’pert Pro MPD XRD system (PAN analytical) with nickel-filtered CuKα (λ ¼0.15404 nm, with a step size of 0.02). The characteristics vibrational modes and oxygen vacancy of the ZnO substrate were analyzed using Raman Spectroscopy and PL spectroscopy (Labram Horiba). Binding energy is measured using AXIS ULTRA XPS system.

2.3. Photocatalysis tests

Photocatalytic experiment was performed using a photocatalytic setup purchased from LELSIL India. MB dye in aqueous solution was Fig. 1. (a) XRD of ZnO deposited on (a) SS mesh and,(b) the glass substrate.

M.K. Singha and A. Patra

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used for the photocatalytic experiment in a 250 ml quartz beaker. 60 ml of MB aqueous solution of 1 �10 ⁵M concentration was used for the degradation test at different time. 3 pieces of ZnO SS mesh substrate (dimension 1.2 �2.5 cm²) were immersed into the solution for the test.

First, the strips were magnetically stirred for 30 min under a dark environment to reach the full adsorption state. Thereafter the magnetically stirred solution contains three strips were exposed to radiation of 125W UV–Vis lamp at room temperature. Then MB solutions were collected at fixed interval time to measure the absorbance spectra by the UV–Vis spectrophotometer. Similarly, 3 pieces of the glass substrate (1.5 �2.5 cm²) were immersed in the same concentration and volume of MB to compare the results with SS mesh substrates. Similarly, experiments were performed under the natural sunlight environment.

Different sample loadings (2 pieces and 3 pieces) were also tested for dye degradation. Samples were not only tested at pH7 but it was also tested in acidic (pH 4) and base medium (pH 9) to check their compatibility in degradation performance at different pH level.

It was found that the peak intensity of the MB is at 664 nm. So, at a different time interval, the peak intensity at 664 nm was considered for the degradation rate measurement. Degradation rate is given by the ratio of C_tand C₀where C₀is the equilibrium concentration before light irradiation and Ct is the concentration after a certain time interval.

Again Ct/C0 is equivalent to At/A0 according to Beer-Lambert law.

Where A₀is the Absorbance value before irradiation at 654 nm and A_tis the absorbance value at 664 nm after a certain interval of time.

3. Results and discussion 3.1. XRD studies

The structural properties of ZnO are studied using XRD. Fig. 1 shows the XRD pattern of the ZnO nanostructure deposited on SS mesh and

glass substrate. Both the patterns are well-matched with JCPDS refer- ence pattern: 00-036-1451. It shows that the pattern is in hexagonal Wurtzite structures with a space group of P63mc. In the figure, strong and sharp XRD peaks indicated that the ZnO were highly crystalline.

Additional peaks in case of ZnO deposited on SS mesh areobserved (Fig. 1a) at 44.05⁰, 50.93⁰, 75.65⁰which can be attributed to the (111), (200),(220) crystal planes of SS [40]. XRD results evince that no other impurities are found in both ZnO samples. The crystalline sizes of the ZnO are calculated by Scherer equation which is given as:

D¼ 0:9λ

βcosθ (1)

Where D, λ, β and θ are crystallite size, X-ray wavelength, full width half maxima (FWHM) of the peak and Bragg diffraction angle. Crystallite size is found to be 37.72 nm for the ZnO on SS mesh.

Fig. 2 shows the FESEM images of SS mesh and ZnO deposited on glass substrate and SS Mesh substrates respectively. Fig. 2 (a) shows the microstructure of ZnO thin film deposited on the glass substrate.

Thickness of the film deposited on glass substrate is found to be ~1.4 μm from the cross sectional SEM micrograph (not shown in here). From Fig. 2 (b) we can calculate the gap between the woven wires in bare SS substrate which is approximate ~ 42 μm and radius of single SS string is around 20 μm. A mesh with dimension 1.2 �2.5 cm²has around 11.049 cm²surface area which is almost three time higher than glass substrate having same dimensions. Fig. 2c and d show that the ZnO is very well deposited on the top side of SS mesh and Fig. 2e shows the bottom side of ZnO deposited SS mesh which indicates that SS mesh is roughly coated with ZnO from all direction. Thus it allows more surface area to participate in photocatalytic activity. Fig. 2f shows the microflower structure of the ZnO films.

Fig. 3a and (b) show the Raman spectra of the ZnO on SS mesh and glass substrate. The peaks are observed at 335 cm ¹, 384 cm ¹, 439 cm ¹and 573 cm ¹and 1150 cm ¹. Among the peaks, peak at 439 cm ¹ is more prominent which is related to E2 (high) mode of ZnO Wurtzite structure and it appears due to vibration of oxygen atoms. Peaks at 384 Fig. 2.FESEM image of (a) ZnO thin film deposited on glass substrate, (b) bare

SS mesh, (c) and (d) ZnO deposited on top side of mesh, (e) ZnO deposited on backside of SS mesh and (f) flower-like ZnO pores structure on the SS mesh.

Fig. 3.Raman spectra of ZnO deposited on (a) SS meshand (b) glass substrate.

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Optical Materials 107 (2020) 110000

cm ¹is transverse optical Raman mode of ZnO (A1 (T0) mode).The peak at 573 cm ¹refers to the A1 (L0) mode which says that C axis of Wurtzite structure is parallel to the substrate. Again it is also found that some peak at 584 cm ¹which is also the E1 (L0) mode of ZnO and it appears when the C axis is perpendicular to substrate. Peaks at 335 cm ¹and 1150 cm ¹appear due to multiphonon and second-order vibration process [41].

3.2. PL studies

Fig. 4 shows the room temperature PL of the ZnO on SS mesh. It was excited with higher energy (325 nm laser) and thus band edge excited emission and deep level emissions are observed at 381 nm and around 520 nm respectively. Narrow UV peak emissions are focused on 381 nm and a high visible emission ranging from 400 nm to 618 nm. The deconvolution of the Gaussian spectrum for the increased intensity of samples is shown in the figure. Four deconvoluted peaks are found at 381, 404, 527 and 618 nm respectively. Peaks at the visible range are broad which indicates the presence of a large number of defect states.

There is also another peak available which is found to be at 404 nm. This is a violet peak which is attributed to the transition of an electron from Zni [42]. As per literature, there is the presence of a partially filled electron state below the CB i.e. presence of the sub-CBM localized state.

A peak at 381 nm indicates the ultraviolet emission and it is near band edge (NBE) emission which is attributing to recombination of free ex- citons [43]. For further analysis we have taken the Gaussian deconvolution of the entire emission spectrum. Interestingly two peaks are observed during the deconvolution at the visible range. Visible peaks are

found at 527 nm and 618 nm respectively which are assigned to Green luminescence (GL) and orange-red luminesce (ORL). ORL luminesce is attributed to oxygen interstitials (Oi). There are many theories and hy- potheses related to GL like vacancies in oxygen, vacancies in zinc and donor-acceptor pairs [44,45].

As per literature microflower shape has a higher surface area and has the largest surface to volume ratio which generate the surface oxygen vacancies [46]. During the photocatalysis process, oxygen defects act as acceptor which can trap the photogenerated electron. Though this process is temporary it reduces electron-hole recombination which in turn increases the degradation efficiency and hence acts as a good photocatalytic substance [46].

3.3. XPS studies

Fig. 5 shows the XPS spectra of the ZnO films taken in a wide spectrum in the range of 0–1200 eV Zn2p spectrum has two peaks found to be at 1022.4 eV and 1045.5 eV respectively. These peaks are assigned to Zn2p_3/2and Zn2p_1/2respectively. Binding energy difference between two Zn2p is found to be 23.1eV which are matching with literature report and states that Zn has þ2 oxidation states [47]. And Fig. 5c shows the core level and its deconvoluted XPS spectra of O1s peaks of the films.

Deconvolution spectrum has two peaks found to be at 529.9 and 531.4 eV respectively. A peak at 529.9 eV represents the Wurtzite structure and it is attributed to O² ions in the Zn–O bonding. The peak at 531.4 eV is known for oxygen deficiency in the ZnO matrix. As the O1s peak fitted with two peaks, so it is possible to have found more adsorbed oxygen molecules in the O vacancy sites [45]. Wide spectrum of the ZnO is shown in Fig. 5a. Peaks around 10.82 eV, 89.43 eV and 139.30 are attributed to Zn3d, Zn3p3/2 and Zn3s1/2 respectively. C1s peak was found around 285.90eV. Some peaks were found between 500 and 600 eV (except O1s peak around ~530 eV). These peaks are from Zn (LMM) [48,49].

3.4. MB degradation under UV–Vis light

Fig. 6a and Fig. 6b show the UV–vis absorbance spectra of Methylene blue for a time period of 0–120 min during the photocatalytic activity of ZnO deposited on glass substrate and SS mesh. It clearly shows in both case that with increasing of time, absorbance spectra at 664 nm de- creases which implies the degradation of MB. Around 97% of MB is degraded by ZnO on SS at 120 min under UV irradiation. Fig. 6c which is the pie chart of MB degradation using ZnO on SS indicates that almost all (96%) of the MB dyes were broken down in an initial 75 min. Fig. 6d shows the first-order kinetics of MB degradation. First-order kinetics is given by the equation:

ln

�Ct

C0

�

¼ Kt (3)

Fig. 4. Pl of ZnO deposited on SSmesh and its deconvoluted peaks.

Fig. 5.XPS spectra of (a) wide spectrum of ZnO deposited on SS mesh, (b) Zn and (c) O and its deconvoluted peaks.

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Where k is the degradation rate constant and t is the time. Ct and Co are the dye concentration at time t and time 0. The value of k is found to be 0.03035 min ¹and the adj. The R²value is found to be 0.99015 for the ZnO deposited on SS mesh. The value of k for ZnO deposited on glass is found to be 0.00917 min ¹and the adj. The R²value is found to be 0.99816. In Fig. 6d both plots have almost linear fit with R²value as unity. Degradation mechanism can be explained as follows.

When photon energy from UV light with or greater than the bandgap of ZnO irradiate on ZnO, it will generate electron and hole pairs in conduction and valence band respectively. Generated electrons at the conduction band generate reactive oxygen radicals whereas hole at valence band generates hydroxyl radicals. These radicals then react with dyes and degrade into CO2 and other less harmful byproducts. The probable reaction mechanism is given below:

ZnOþhν→e_CB þh^þ_VB (4)

e_CBþ O2→^:O₂ (5)

O₂þH2O→H2O2þ OH (6)

h^þ_VBþH2O→H^þþ^:OH (7)

OH þ dye→dyedegradation (8)

The comparison study of SS mesh coated ZnO with ZnO thin film coated on the glass substrate was done using almost the same dimension samples with the same deposition condition. It shows that the thin film on the glass substrate takes 210 min to degrade the MB of the same concentration (~90%) and its degradation rate is as 0.009 min ¹. The reason for the higher degradation rate on ZnO coated SS meshes are changed in effective surface area and surface morphology. As discussed earlier ZnO deposited on SS mesh has a more effective surface area than ZnO on a glass substrate which in term increase the photocatalytic activity.Our report is also matching with other literature also. It is said that

increasing surface area compares to exposed polar facets (002) of ZnO is more important in photocatalytic activity [50]. Due to microgap structures in the SS mesh, MB in water can react from all the sides of the SS mesh whereas water on the surface of ZnO on glass takes parts in the reaction mechanism. So microflower in ZnO has more surface area.

Besides oxygen defects increase the photocatalytic activity in the degradation of MB [51]. Oxygen vacancies used to capture generated electron under light illumination. As it captures the electron, it will reduce the recombination rate of photoelectron and holes which increase the photocatalytic activity. These are the reasons for the faster degradation of the MB using SS mesh.

3.5. Repeatability studies under UV–Vis light

ZnO nanostructures on SS mesh were used to test the repeatability performance whether the ZnO flower on SS is good enough to perform the same photocatalysis at a different time but same conditions. Total ten cycles of experiments performed in 5 days interval and each day two experiments were conducted one after another. On the first day and after first experiment, we have cleaned the substrate with water and then again immersed it into new fresh solutions of 1 �10 ⁵M MB for another photodegradation experiment. The next day (after 24 h later) again we performed the same experiments and found the good repeatability.

Fig. 7a shows the photodegradation studies of 4 cycles with keeping equal degradation rate. Fig. 7b shows the bar diagram of a total of 10 cycles of the repeated experiments and it clearly shows that the ZnO on SS is effective even after 10th cycle. Fig. 7c and d shows the SEM images of the samples after the 10th cycle experiment. It shows that there is no structural/morphological change. It implies that samples are not photo corroded and highly stable even after the 10th cycle of use. Fig. 7e shows the efficiency of the photocatalytic (with the inset image of the dye solution in different time intervals) after the 10th cycle of the experiment. So ZnO on SS mesh is a good option from an economic point of view to degrade the dyes.

Fig. 6.(a) Photocatalysis degradation of MB using ZnO thin film deposited on glass substrate, (b) Photocatalysis degradation of MB using ZnO thinfilm deposited on SS mesh, (c) MB degradation pie chart as a function of time(d) Photocatalysis degradation rate and degradation versus time interval degradation (inset).

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Fig. 7.(a) Repeatability taste of ZnO on SS mesh for the 4th cycle, (b). Bar diagram of repeatability test up to 10th cycle, (c) and (d). FESEM images ZnO after 10th cycle photocatalytic test (recycling), (e). Photocatalysis degradation of MB using ZnO thin film deposited on SS mesh at 10th cycle.

Fig. 8. Effect of sample loading (a) degradation percentage and (b) degradation rate.

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3.6. Effect of sample loading

MB degradation under different loading (using 2 pieces and 3 pieces of the sample respectively) was done under UV–Vis and natural sunlight environment. It was found that there is a negligible difference when the experiment was carried out in UV–Vis or natural sunlight environment.

For all the cases concentration of the sample was kept constant. But under sunlight condition, no magnetic stirrer was used. When 3 samples were used, the degradation was ~97% under 2 h whereas it was 81%

and 91% when 2 samples were used and the duration of irradiation was 2hrs and 3 h respectively. Fig. 8 shows the degradation percentage using 2 and 3 samples respectively. Increasing the no of samples increase the active surface area which in turn increase the reactive species for the reaction.

3.7. Effect of pH

In general wastewater from the chemical, pharmaceutical industries are not neutral. Effluent from these industries are either acidic or alkalic in nature. ZnO is a well-known amphoteric substance. So its degradation will be different at different pH of the solution. Here we have measured the degradation percentage and degradation rate under different pH using 2 numbers of ZnO samples under UV–Vis environment. Three different pH solutions are taken: pH4, pH 7 and pH9. Fig. 9 shows the degradation percentage of MB at different pH solutions under 2hr UV–Vis irradiation using 2 samples. It is found that the reaction rate and

% of degradation increase with the pH. In acidic medium reaction rate is less whereas at alkali medium it is higher. Alkali medium has excess OH- anions that reacts with holes and generates OH radicals. These excess OH radicals then enhance the dye degradation process. So degradation efficiency is increased when the medium is alkali whereas degradation efficiency reduced in acidic medium. So degradation efficiency is more in pH9>pH7>pH4.

3.8. Degradation under natural sunlight

We have performed the MB degradation under the natural sunlight environment. Fig. 10 shows the degradation behavior of MB under natural sunlight. In the inset of the figure shows the absorbance spectra of the MB at different times. As predicted, absorbance peaks at 664.5 nm are decreasing with respect to time. Another figure shows the first-order degradation rate of MB using 3 samples under 2 h sunlight illumination.

It was found that the degradation rate is 0.02576 min ¹and it was found that around ~96% of Mb degraded at that time. Degradation rate is highly linear with adj. R-square value is 0.99948.

3.9. Mechanism

To know the active species involved in the photocatalysis activity, we have studied the in-depth mechanism behind it. Different scavengers like benzoquinone (BQ), isopropyl alcohol (IPA) and disodium ethylenediaminetetraacetate (EDTA-2Na) were used for oxygen (.O2-) scavenger, hydroxyl (-OH) scavenger and hole (hþ) scavenger respectively.

Fig. 11 shows the degradation efficiency of MB under 2 h of sunlight illumination using 2 pieces of ZnO mesh samples. Degradation efficiency under BQ, IPA, EDTA-2Na and No scavenger solutions are 40.22, 47.51, 63.38 and 81.25% respectively. Hence superoxide radical and super- hydroxide radicals are the main reactive species for the sunlight driven photocatalytic activity which is also matching with other literature.

3.10. Repeatability studies under sunlight

Repeatability studies under natural sunlight were performed in the department of Instrumentation & applied physics, Bangalore, India on 2nd, 3rd, April 5, 2019 and May 10, 2019 using 3 samples and 60 ml of Fig. 9. Effect of pH in MB degradation.

Fig 10.Degradation of MB under natural sunlight using 3 pieces of samples.

Fig. 11.degradation % of MB under different scavengers.

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MB solution with a concentration of 1 �10 ⁵M. Experiment time was between 11.00 a.m. and 2.15 p.m. There was no fixed timing of the experiment. Some experiments start at 11 a.m. whereas some experiments started at 12.15 p.m. Fig. 12 shows the repeatability studies of the MB degradation on different days. It is found from the figure that the MB degradation is almost constant irrespective of experimental days. There is a very negligible difference between the degradation percentageson different days. Minimum degradation was found to be ~95.5% whereas maximum degradation is 98% under 120 min of sunlight irradiation.

This minor difference may come from the different intensities of sunlight in a day. Under the natural sunlight environment, ZnO microflower coated SS mesh can act as a highly photocatalytic substrate and it can efficiently degrade MB with having highly reusability performance.

4. Conclusions

We have successfully synthesized ZnO flower-like structure on SS mesh substrates and glass substrates by using low-cost ultrasonic spray pyrolysis system. XRD and Raman studies show that grown ZnO is polycrystalline in nature with Wurtzite nature of ZnO. FESEM images show that grown ZnO is formed as a microflower shape and it can be deposited all the surfaces. It has a more active surface area to reacts with MB for the degradation process. As grown ZnO on SS mesh has a high degradation rate of MB than ZnO thin film grown on the glass substrate.

SS mesh structure also shows very good and high repeatability activities of degradation of MB (97%). Oxygen defects in the film observed from XPS and PL studies are reasons for fast degradation process. Different loadings of the sample were tested for MB degradationin both UV–Vis and natural sunlight conditions. It shows irrespective of the source, it is highly reusable with a faster degradation rate. MB degradation using spray deposited ZnO work better in base medium than acidic medium.

Mechanism studies show that superoxide radicals and hydroxyl radicals are the main species for MB dye degradation in UV–Vis and sunlight environment. Present study shows that photocatalytic activity using ZnO on SS mesh increases due to increased surface roughness. Even after 10th repeatability studies it does not photo corroded and highly stable.

Declaration of competing interest

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.

CRediT authorship contribution statement

Monoj Kumar Singha: Conceptualization, Methodology, Investiga- tion, Experimentation, Writing - original draft, Writing - review &

editing. Aniket Patra: Experimentation, Writing - original draft, Writing - review & editing.

Acknowledgements

Authors would like to acknowledge MNCF of CENSE (Centre for Nano Science and Engineering), IISc for providing the FESEM and RAMAN characterization facility.

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