Enhanced photocatalytic activity in ZnO nanoparticles developed using novel Lepidagathis ananthapuramensis leaf extract

The present study focuses on the green synthesis of zinc oxide nanoparticles (ZnO NPs) using a novel Lepidagathis ananthapuramensis (LA) leaf extract and a systematic study on the photocatalytic degradation of methylene blue (MB) dye. The structural, thermal, morphological, optical, and surface area analysis of prepared ZnO NPs were examined using X-ray diffraction (XRD), UV-visible spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, thermogravimetric analysis (TGA), field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDAX) and high-resolution transmission electron microscopy (HR-TEM). The LA stabilised ZnO NPs produced NPs with diverse morphologies, low band gap and cost-effective high yield of production. A systematic study has been carried out to determine the crystallinity and crystallite size of ZnO NPs based on the concentration of Zn(NO3)2 precursor, concentration of LA leaf extract, calcination temperature and calcination time. The crystallinity and crystallite size of ZnO NPs were evaluated based on the XRD technique. The photocatalytic activity of ZnO NPs was thoroughly investigated for the degradation of MB dye based on various physicochemical parameters such as reaction time, concentration of catalyst, concentration of precursors, concentration of LA extract, concentration of MB, calcination temperature and calcination time. These systematic photocatalytic studies followed green protocols and provided an excellent photocatalytic efficiency result of 96–98.5% towards the decomposition of MB. Hence, this material can work as a potential candidate for waste water treatment by also degrading other toxic dyes.


Introduction
Over the last decade, various metal oxides such as ZnO, 1-3 CuO, 4 NiO, 5 CoO, 6 Fe 2 O 3 , 7 MnO 2 , 8 TiO 2 , 9,10 SnO 2 , 11 MgO, 12 etc have been synthesised for numerous applications, among which photocatalytic applications and waste water treatment have been explored enormously. Among various metal oxides, ZnO and TiO 2 NPs are studied the most due to their non-toxic nature, cost effectiveness, chemical stability and inertness, ecofriendliness, potential photocatalytic efficiency, etc. 13,14 . Even though both are effectively used, ZnO NPs are generally a better candidate for photocatalytic degradation of pollutants because of their high stability, non-toxicity, 15 single oxidation state, natural abundance, suitable band gap 16 and better biocompatibility. 17 ZnO is a wide bandgap (3.37 eV) 18 semiconductor which acts as an excellent photocatalyst for the decomposition of water pollutants. 19 One of the top most priorities of the present-day research is for clean water in order to retain the ecosystem. Rapid growth of textile industries is one of the major causes for the water contamination. Dyes and its effluents from textile, food, and printing industries are the major pollutant released in water bodies. Generally, dyes are highly toxic and nonbiodegradable because of its complex nature, causing serious threat to the ecosystem. methylene blue (MB) is a widely used dye in textile industries, which is extremely carcinogenic and toxic. 20 Presently, more than 800 000 tonnes of dye are produced every year and its waste contents are released into water bodies by the industries. 21 This is undoubtedly an alarming situation which needs to be addressed urgently. Various conventional methods such as coagulation, adsorption, ultraltration, ozonisation etc. are available, but they deal with the removal of dyes, effective in destroying some of the dyes only and need post treatment of secondary waste materials. 22 So, semiconductor photocatalysis 23 involving advanced oxidation process (AOP) has gained immense attention due to its effectiveness in degradation of the toxic dyes without generating any harmful products. Dye degradation using semiconductor photocatalyst is more environmentally friendly method, which can be a possible contender in waste water treatment.
The major worldwide challenge is to synthesise these metal oxide NPs in a cost effective, eco-friendly and sustainable manner. There are various conventional methods available such as physical methods (ball milling, solid state, physical vapor deposition etc.), chemical methods (sol-gel, coprecipitation, solution combustion, hydrothermal etc.) etc for synthesising metal oxide NPs. [1][2][3][4][5][6][7][8][9][10][11][12] But these methods fail to address the problems related to toxicity, cost, waste production and environmental contamination. In order to overcome them, today's researchers have been immensely focusing on the green chemistry route for synthesizing metal oxides. Green approach towards the synthesis of metal oxide NPs is more eco-friendly, 24 economically viable, socially accepted, safer, sustainable to environment etc. and account to increase in the yields of production. 25 Synthesis of metal oxide NPs through greener route involves various biological components such as bacteria, fungi, algae, leaves, root etc. Among the available greener methods, the use of leaf extracts is the best way as it involves simplest, less investment, shorter time period, highly efficient, non-pathogenic 26 and feasible process to prepare metal oxide at larger scale relative to bacteria, fungi and algae mediated synthesis. Various leaf extract such as Cayratia pedata 27 Raphanus sativus var. Longipinnatus 28 , Tectona grandis (L.) 29 , Cassia fstula, Melia azedarach 30 etc. were used as the stabilising agent for the synthesis of ZnO NPs. Therefore, in this regard, we have focused on synthesis of ZnO NPs from the Lepidagathis ananthapuramensis (LA) leaf extract, which is a highly effective green approach.
LA is a new species recently discovered in the Ananthapuram region of Kasaragod district, Kerala. 31 It belongs to the Acanthaceae family, whose advanced studies related to medicinal or any other applications need to be done. The use of this LA leaf is a green mediated route for the synthesis of ZnO NPs. The leaf extract plays a crucial role in the synthesis process as it contains phytochemicals. These phytochemicals act as effective capping, stabilizing and reducing agents in the synthesis process. The phytochemicals present in the LA leaf extract are shown in the supplementary le, † which has been evaluated based on the method described in. 32,33 The phytochemicals present in the LA leaf extract like alkaloids, avonoids, terpenoids, glycosides, saponins, tannins, cardiac glycosides, reducing sugars, quinones, phenols etc. facilitate the production of ZnO NPs to give a high yield. In this one pot synthesis of ZnO NPs, the functional groups present in these phytochemicals play a binary role i.e., they act as effective reducing and capping agents. The use of LA extract for the synthesis of ZnO NPs is a novel approach which can provide diverse nature of particles with the effectiveness in controlling its size and shape. Moreover, this leaf extract can provide better results related to other properties of ZnO NPs which has been clearly discussed in the entire work. Also, particles and crystallites size, shape, surface area and band gap are highly affected by the type of precursor, concentration of precursor, calcinations temperature, doping etc. 34,35 Focusing on the eco-friendly approach, the present study emphasised on the novel LA leaf mediated synthesis of ZnO NPs and its capability of degrading the MB dye using solar driven photocatalytic process. The great band gap tuning, varied morphologies, outstanding stability, etc. of ZnO NPs enable them to achieve good photocatalytic outcomes.

Synthesis of LA leaf extract
LA leaves were collected, washed with distilled water, dried under shade for 1 week and hot air oven dried for couple of days. Then the leaves were grinded to ne powder using mixing grinder. This ne powder was used as a stock powder for future prospects.
10 g of ne powder of LA leaves were added to 250 mL distilled water taken in a 500 mL conical ask. The LA solution was stirred continuously for 2 hours at 80°C. The solution was then cooled down to room temperature and ltered using a cotton cloth. The ltrate was used as the LA extract for the synthesis of ZnO NPs and stored in the refrigerator for future uses.

Synthesis of ZnO NPs using LA extract
Firstly, the synthesis of ZnO was done using 25 mL 0.1 M Zn(NO 3 ) 2 $6H 2 O and 5 mL LA extract. Both the solutions were mixed well and stirred continuously at 60°C for 4 hours in a 100 mL conical ask using a hot magnetic stirrer. As the reaction proceeds, the colour changes from green to dark green. The nal product obtained was oven dried and calcined at 500°C for 2 hours. The product calcined at 500°C was grinded well using a mortar and pestle. Then this calcined sample was used for the characterisation to conrm the formation of ZnO NPs.
Further property study of ZnO NPs was done with respect to concentration of Zn(NO 3 ) 2 $6H 2 O, concentrations of LA extract, calcination temperatures and calcination hours following the same procedure mentioned earlier. (Table 1) The following  table shows the different parameter used for the property  studies and their corresponding sample name which is mentioned in their characterisation plots (Fig. 1).
The crystallite size and crystallinity percentage are calculated using the following equations: where l is the wavelength of X-ray used i.e., 0.154 nm, b is the full width at half maximum, q is the Bragg's angle. All the parameters' units are used in radian and the obtained crystallite size is calculated in radian only.

Photocatalytic set-up
The photocatalytic studies were done using ZnO NPs as a semiconductor photocatalyst and MB dye as the model dye.
The photocatalytic activity of ZnO NPs was analysed based on the observation of degradation (decolouration) of MB dye. The photocatalytic efficiency is calculated using the following ZnO-4 4 23 ZnO- 6 6 Sample names (ZnO-0.10, ZnO-1 : 5, ZnO-500 and ZnO-2) are the same material. These sample names are given as common code ZnO-LA, reected in the characterisation plots. The solution then stirred continuously under sunlight. At a regular interval of 15 minutes, 5 mL of reaction mixture is taken out using a micropipette by switching of the stirring process. Up to 90 minutes, 6 such aliquots were taken and each mixture was centrifuged at 10 000 rpm for 10 minutes. Each mixture was then analysed using the UV-visible spectroscopy over the wavelength range of 200-800 nm, in order to check the absorbance values. The change in absorbance values is the guidance towards the MB degradation studies.
The similar procedure was followed for the systematic study of photocatalysis based on various parameters i.e., inuence of sunlight and no sunlight (dark reactions), catalyst loading, effect of concentrations of Zn(NO 3 ) 2 $6H 2 O precursor, LA extract and MB dye, effect of calcinations temperatures and effect of calcination hours. Most of the photocatalytic activity of ZnO NPs based on various parameters has been compared with ZnO-LA and 5 mg ZnO photocatalyst is used for the photocatalytic studies.

Characterisation techniques
The structural characterisation was done based on XRD analysis with the help of PAN analytical X'Pert Pro diffractometer using the wavelength of Cu-Ka (l = 1.54060Å) radiation. The materials were slow scanned in the range of 20-80°with the step size of 0.02. The optical properties were studied with the help of UV-1800 Shimadzu spectrophotometer. The characteristic structural information of ZnO was analysed using LabRam HR evolution Raman spectrometer having a 633 nm laser (Horiba scientic, France). FT-IR spectroscopy was performed using PerkinElmer 100 spectrometer (USA) in the scan range of 4000-400 cm −1 . The electronic states were analysed using Thermo Scientic K-Alpha XPS instrument having monochromatic Al Ka radiation, E photon = 1486.6 eV. TGA was employed to check the chemical and thermal stability using TA Q500 Instrument (USA) in the range of 0-800°C at 5°min −1 heating rate under nitrogen atmosphere. The BET was employed to check the surface area using Microtrac BEL instrument (BELPREP vacII, JAPAN). The surface and elemental composition were studied using FE-SEM/EDAX analysis with the help of Hitachi-S520 (Oxford link ISISSEM Model), Japan.

Results and discussions
3.1 Possible mechanism of synthesising ZnO NPs from LA leaf template The importance of leaf extract for synthesising ZnO NPs is because of the availability of phytochemicals present in leaf extract. The phytochemicals contain different types of functional groups like ketone, aldehyde, hydroxyl, carboxylic acid etc. which helps in the formation of ZnO NPs involving chemical reduction and oxidation process. The present LA extract contain alkaloids, avonoids, phenols, quinones etc, as mentioned in ESI † le. These phytochemicals will act as effective capping agents as well as reducing-oxidising agents in the synthesis process. The exact mechanism of forming ZnO NPs from leaf extract is not yet understood completely, several research works are being performed to study the exact mechanism. (Fig. 2) The possible mechanism of forming ZnO NPs via greener way involves two steps as mentioned in: 35 1. Formation of Zn(OH) 2 from the corresponding Zn( 2. Calcination of the subsequent reaction product into ZnO NPs. Zn 2+ ions present in the zinc electrolytic solution [Zn(NO 3 ) 2 $6H 2 O solution]is reduced to the stabilised compound by chelating to the phytochemicals such as avonoids and polyphenols, present in the leaf extract during the reaction. The anionic part of zinc salt will be eliminated as corresponding acid by taking proton from the phytochemicals. The water molecules will try to donate its proton to the corresponding phytochemicals. As a result, the hydroxyl group present in the reaction mixture (biomolecule or in solution) will try to make bond with stabilised metal species leading to the generation of zinc hydroxide (Zn(OH) 2 ). All these processes occur simultaneously. The obtained solution is oven-dried overnight and a yellow-orange pre-product was obtained which was calcined to eliminate the hydroxides and the impurities to nally obtain ZnO NPs.

Analysis of synthesized ZnO NPs
3.2.1 XRD analysis. The XRD analysis was employed to know the structural formation and phase purity of ZnO NPs as well as to know about the crystallinity and crystallite size. The sample was analysed over the scan angle from 20°to 80°with the step size of 0.02. The obtained XRD data was rened using the Full Prof soware. Fig. 3 represents the Rietveld renement of ZnO and the rened parameters are shown in Table 2 It was observed that the synthesized ZnO NPs have the 81.40% crystallinity with the average crystallite size of 28.12 nm. This indicates that the sample is highly crystalline possessing nano-crystallites. Further study of XRD analysis with respect to effect of concentrations of precursor and LA extract, effect of calcinations temperatures and calcinations hours are discussed later.
3.2.2 UV-visible analysis. The UV-visible spectroscopy was employed to know the absorption property and thereby evaluate the band gap of ZnO NPs. The sample was analysed in the range 200-800 nm and the band gap is calculated by the Tauc method. The obtained absorption spectrum is shown in Fig. 4 along with the Tauc plot. The maximum absorbance is observed at 372 nm which can be due to the intrinsic semiconductor band gap transition having the surface plasmon resonance as well as quantum size effect phenomena. The characteristic narrow peak obtained at 372 nm is in agreement with the earlier reports. 36,37 The optical band gap obtained for ZnO NPs is   around 3.01 eV which is slightly lower than the previously reported or standard band gap of ZnO (3.10 eV-3.37 eV). 38,39 Generally, the band gap reduces with respect to particle size, as the particle size increases ultimately the band gap decreases as a consequence of quantum size effect. But it is not always true that the band gap depends only on particle size factor. The decrement in the band gap of ZnO NPs can also be attributed to various factors as well, such as synthesis process, morphology, 40 lattice strain, grain size, defect and roughness of the surface. 41 It is difficult to judge the actual factors that has changed the optical gap, but it can be either one or combination of former factors mentioned. Moreover, the appearance of sharp intense peak along with no other small peaks in the absorption spectrum indicates highly pure nature of synthesised ZnO NPs.

Raman analysis.
The Raman spectroscopy was performed in order to get the conrmation of characteristic hexagonal crystalline structure of ZnO NPs. The sample was analysed over the range of 0-800 cm −1 which is the nger print region of ZnO NPs. The Raman spectrum depicted in Fig. 5 shows the characteristic peaks of hexagonal wurtzite structure of ZnO NPs which corresponds to C 6v point group whose irreducible representation is expressed as A 1 + 2B 1 + E 1 + 2E 2 . Among this, polar modes A 1 and E 1 are Raman and infrared active whereas E 2 is also a polar mode which is Raman active but infrared inactive and B 1 mode is both Raman and infrared inactive. 42 Moreover, A 1 and E 1 modes are divided into two modes i.e., transverse optical (TO) phonons and longitudinal optical (LO) phonons. E 2 consists of low frequency mode (E 2 low) and high frequency mode (E 2 high). The intense peak at 99.3 cm −1 belongs to E 2 low phonon mode which is due to the vibrations of Zn lattice and the dominant peak at 438.1 cm −1 belongs to E 2 high frequency mode which is because of the lattice oxygen vibrations. The small broad peak at 332 cm −1 corresponds to multi-phonon mode (E 2M ). This broad peak is also known as second-order mode caused by the multi-phonon process (31) (combined vibrations associated with the ZnO lattice). The small intense peaks at 378.1 cm −1 , 412 cm −1 and 583.5 cm −1 are attributed to A 1 (TO), E 1 (LO) and E 1 low phonon modes. Generally, these peaks arise due to the structural defect present in the ZnO lattice (31,29). More the deciency, more the peaks are affected (especially E 1 (LO) being affected strongly 43 ) causing a change in the peak intensity. Sometimes, peaks are also generated at 275, 510 and 643 cm −1 which are extra phonon modes arise due to the defects. 44,45 In the present case, the intensity of these peaks are very small suggesting that synthesised ZnO NPs are having negligible defects. These results suggest that ZnO NPs are of high purity nature having fewer defects.
3.2.4 XPS analysis. The XPS analysis of ZnO NPs was carried out to explore the elemental compositions, defects and oxidation states of Zn element present in the material. All the binding energies (B.E.) measured were corrected with respect to C 1s peak positioned at 284.6 eV. The wide scan spectrum manifested in Fig. 6 clearly ensured that no foreign materials are present on the surface of the nanomaterials. The highresolution scan of Zn 2p and O 1s spectra are depicted in the Fig. 6 along with the deconvolution. The slow scan of Zn 2p spectrum shows two characteristic strong peaks located at  1021.9 eV and 1045.05 eV, which belong to the binding energies of Zn 2p 3/2 and Zn 2p 1/2 respectively. The difference between the B.E. of Zn 2p 3/2 and Zn 2p 1/2 peaks is 23.15 eV, which is a characteristic spin-orbit splitting binding energy of ZnO. The deconvolution of Zn 2p spectrum shows that both the peaks are tted with one peak only. This suggests that Zn existed as Zn 2+ ions instead of metallic Zn particularly. Similarly, the slow scan of O 1s spectrum is deconvoluted into two subpeaks positioned at 529.93 eV and 531.59 eV, which are attributed to the lattice oxygen (Zn-O) and oxygen of hydroxyl group adsorbed on ZnO NPs i.e., moisture adsorption (Zn-OH). If the deconvolution of O 1s spectrum has resulted in 3 sub peaks, surface defects will be present. This result clearly suggests that chance of defect generation in the sample may be less since the O 1s spectrum is tted with two peaks only. All these results are in well agreement with the previous reports. 46 The main conclusion from the XPS analysis is that synthesised ZnO NPs are free from impurities involving fewer defects. 3.2.6 TGA analysis. The TGA characterisation was carried out in order to determine the thermal and chemical stability of synthesised ZnO NPs. The sample was analysed aer one year of preparation in the range of 30°C to 800°C at a ramp rate of 5°C min −1 . The obtained thermal plot of ZnO NPs is shown in Fig. 8. It is observed that the overall percentage of degradation of the ZnO NPs is 1.17% which can be neglected. The overall 1.17% decomposition of ZnO NPs may be due to the elimination of adsorbed moisture from the nanoparticles surface and removal of some impurities. The main observation from the Fig. 7 FT-IR spectrum of ZnO NPs and LA leaf extract. Fig. 8 Temperature variable thermal study of ZnO NPs. TGA analysis is that the material is highly stable for prolonged duration.
3.2.7 BET analysis. The BET analysis was performed to know about the surface area, pore diameter and pore volume of the sample. The N 2 adsorption-desorption isotherm along with Barrett-Joyner-Halenda (BJH) plot are depicted in Fig. 9. The N 2 adsorption-desorption isotherm shows that the isotherm is of type 4 involving H 3 hysteresis loop noticed in the p/p 0 (relative pressure) range of 0.84-0.98. This result indicates that the ZnO NPs is a mesoporous material showing slit shaped pores. 52 The total surface area obtained using BET method is 1.44 m 2 g −1 . This extremely low surface area can be due to the agglomeration of particle and the measured pore size and pore volume is 2.21 nm and 0.0067 cm 3 g −1 respectively, which signies ZnO NPs as a mesoporous material.

FE-SEM & EDAX analysis.
The surface morphology of ZnO NPs was taken out using FE-SEM analysis and the elemental analysis was carried out using EDAX technique. The obtained FE-SEM micro images are shown in the Fig. 10 along with the elemental spectrum of ZnO. The micro images of FE-  SEM clearly showed that some of the ZnO particles are in hexagonal morphology (also depicted in SEM image- Fig. S1 †), but most of the particles are agglomerated 27 which resulted in the stacking morphology (marked with yellow circle). Sometimes, the agglomeration of the particles resulted in the stacking morphology looked like umbrella type (mushroom form), ower type, honey comb cage like morphologies. The microimages also indicated that most of the particles are in micrometre range due to agglomeration. The existence of nanoparticles was conrmed from HRTEM analysis (Fig. S2 †histogram plot). Hence the synthesised ZnO NPs are mostly agglomerated showing stacking morphology.
The elemental analysis of ZnO NPs showed four absorption peaks pertaining to Zn and O with the weight percentage of 80 and 20. There are no additional peaks observed indicating that the synthesised ZnO NPs is of high purity.
3.2.9 TEM and HR-TEM analysis. TEM and HR-TEM was employed to study the morphology and particle size of ZnO NPs and the selected area electron diffraction (SAED) was done to conrm the crystal planes of ZnO NPs. Fig. 11 displays the TEM images of ZnO NPs in different magnications and Fig. S2 † represent the histogram, SAED and lattice fringe pattern of ZnO NPs. It is clearly observed that ZnO NPs were formed with different morphologies like rod, hexagonal, and spherical shapes. The examination of crystal lattice planes, d-spacing, and particle size distribution were performed using Image J soware. The average particle size of ZnO NPs was around 10-15 nm (displayed in histogram), which has been evaluated based on the selection of 100 NPs (mostly spherical type particles) from different TEM images. It is also noticed that the length and breadth of an individual rod type particle is around 29 nm and 12 nm respectively. Similarly, the particle size of an individual hexagonal shaped ZnO NPs (seen in the HR-TEM image) is 14.25 nm. So, it is clear that the formed ZnO NPs are of polydisperse nature (also conrmed from dynamic laser scattering (DLS) measurement mentioned in Fig. S3 †). HR-TEM images clearly showed lattice fringes without any defects indicating high degree of crystallinity. The calculated interplanar dspacing of neighbouring lattice fringes is approximately 1.89-1.91Å, 1.473Å and 1.305Å for (012), (013) and (004)  Code: 98-008-1255. There are no additional peaks indicating that there are no impurities or any secondary (mixed) phases as the concentrations are varied. It is noticed that the intensity of each diffraction plane is enhancing but the (011) peak is of great interest due to its intense and sharpest nature. Degree of crystallinity and crystallite size were calculated with respect to (011) plane. It is clearly observed that as the concentration of precursors is increased, the crystallinity percentage and crystallite size are also increasing up to a particular concentration. The crystallinity percentage increased to 87.10% for 0.2 M concentration, aer that crystallinity percentage almost remains the same. Similarly, the crystallite size increased up to 52.3 nm for 0.7 M concentration. The increase in crystallinity percentage is reected in the (011) plane as it became more intense and sharper as the concentration is increased. Moreover, from Fig. S4, † there are slight variations in Bragg's angle, FWHM and peak intensity for (011) plane as the concentration is varied. The increase in crystallite size with the increase in the precursor concentrations can be due to the agglomeration of crystallites as a result of higher concentration. Also, the rate of conversation of Zn(NO 3 ) 2 $6H 2 O to Zn(OH) 2 increases as the collision (reduction and oxidation process) of highly concentrated electrolytes with phytochemicals increases. Moreover, there are chances of defect generation (oxygen vacancies (O v ), zinc interstitials (Zn i ) etc.) as a result of agglomeration of crystallites. However, there is no direct relationship between crystallinity and crystallite size. Hence, the highest crystallinity and crystallite size is obtained for 0.2 M and 0.7 M, which means that long range order of atoms, is attained for 0.2 M with crystallite size around 35.12 nm. Therefore, in order to get good crystallinity and lowest crystallite size, concentration of precursors should be optimised. In the present study, 0.1 M Zn(NO 3 ) 2 $6H 2 O is used as the optimised concentration for further study as it has 81.40% crystallinity with the crystallite size of 28.12 nm.
3.3.2 Effect of concentration of LA extract. The effect of concentration of LA extract on the degree of crystallinity and crystallite size is explored by keeping the calcinations temperature, calcinations hours and concentration of precursor constant. The obtained XRD proles are represented in Fig. 13 along with the bar graphs showing crystallinity percentage and crystallite size. It is interesting to say that XRD patterns showed generation of Zn(OH) 2 impurity phase along with the pure ZnO as the volume of LA extract is increased. The generation of Zn(OH) 2 impurity phase is increased with the increase in the LA content. The obtained impurity phase is in well agreement with earlier reported results. 53,54 The obtained ZnO crystalline peaks are in well concordance with the ICSD ref. Code: 98-008-1255.
The broadness of the highly intense (011) peak can be seen as the concentration of LA is increased, as represented in Fig. 13 and S5. † The broadness of this peak signies the formation of the small crystallites due to the large FWHM value (Scherrer's equation). This is clearly reected in the crystallite size also, as the concentration of LA extract is increased, crystallite size simultaneously decreased. The lowest crystallite size is obtained for ZnO-5 : 5 i.e., 14.72 nm. The decrement in the crystallite size may be due to the increase in the concentration of stabilising  agent (effective capping), which will prevent the particles (crystallites) to agglomerate. 55 So, the smaller crystallite size can be obtained for ZnO NPs involving more LA content in the synthesis process. But increase in the concentration of LA extract generates other impurity phases also. It is also observed that the degree of crystallinity is rst increasing slightly and then decreasing as the concentration of LA content is increased. This can be due to the formation of Zn(OH) 2 impurity phase. The obtained results are in well agreement with earlier report. 56 Hence, the degree of crystallinity and size of the crystallites are signicantly inuenced with the amount of LA extract used in the synthesis process.
3.3.3 Effect of calcination temperatures. Fig. 16 (Le) represents the XRD proles of ZnONPs calcined at different temperatures i.e.,500°C, 600°C, 700°C, 800°C, 900°C and 1000°C. Fig. 14 (Right) shows the crystallite size and crystallinity percentage pattern with respect to different calcinations temperatures. All the samples calcined at different temperatures show single phase hexagonal crystalline structure without forming any intermediate phases (ICSD ref. code: 98-008-1255). From the XRD pattern, it is clearly observed that the intensity of each peak increases with the increase in the calcinations temperatures. The slight increment in the peak intensity of (011) plane is depicted in closure picture of Fig. S4. † These results clearly indicate that the degree of crystallinity is increasing with the increase in the calcinations temperature. The highest crystallinity percentage is 90.04% which is obtained for ZnO-1000. Moreover, there is a slight variation in the Bragg angle for (011) plane (Fig. S3 †) as the calcinations temperatures is varied, suggesting that the calcinations temperature would affect the atoms' orientation in lattice planes. The higher calcinations temperature will provide more energy for the atoms to diffuse and occupy at particular site in the crystal lattice. 57 The change in Bragg angle of (011) plane can be related to stress factor as discussed in. 58 Similarly, the crystallite size showed increasing trend as the calcinations temperature is increased. But the crystallite size increased more from 500°C to 700°C and then slight increment from 700°C to 1000°C. The more increment in the crystallite size from 500°C to 700°C can be due to the coalescence process. 47,48 This process can cause major crystal and grain growth enhancing surface roughness and porosity. The later slight increment in the crystallite size from 700°C to 1000°C can be due to the higher mobility of atoms leading them to  agglomerate. Sometimes, random motion of the atoms follows dispersion and diffusion as well. As a result, all the factors compete with each other in order to determine the crystallite size. Generally, the increase in the crystallite size can be related to the merging of particles as the consequence of thermal calcination. Hence, the calcinations temperature plays a vital role in tuning the crystallite size and degree of crystallinity.
3.3.4 Effect of calcinations hours. The XRD analysis was carried out for ZnO NPs synthesised at different calcinations hours (2, 4 and 6 hours) and the obtained XRD pattern along with crystallinity and crystallite size results are depicted in Fig. 15. All the samples; ZnO-2 h, ZnO-4 h and ZnO-6 h showed single phase hexagonal wurtzite crystalline structure (ICSD ref. Code: 98-008-1255). There are no additional peaks for all the samples, which means that there are no foreign particles or any secondary (mixed) phases as the calcinations hour is changed. The calculated crystallinity percentage and crystallite size are 81.40%,83.90% and 86.01%, 28.12 nm, 34.01 nm and 40.50 nm respectively. These results clearly specify that the crystallinity is slightly increasing with the increase in the calcinations hours. The enhancement in the degree of crystallinity is shown in Fig. S4 † as there is a sequential increment in the peak intensity of (011) plane. Similarly, the crystallite size is systematically increasing with the increase in the calcinations hours. This can be due to the agglomeration of crystallites as more calcinations time facilitates the crystal growth. These results are in good agreement with the previously reported literature pertaining to effect of calcinations time. 59 So, the calcinations hours are slightly affecting the degree of the crystallinity but having a signicant effect on crystallite size.
The main observation from the XRD results is that the degree of crystallinity and crystallite size can be tuned based on the concentration of precursors, leaf extract, calcinations temperatures and calcinations hours. The appropriate sample can be used for the effective decomposition of MB dye based on their properties.

Photocatalytic studies
3.4.1 Mechanism of photodegradation of MB dye. The photodegradation of MB dye using ZnO photocatalyst follows radical generation mechanism. The radicals generated in the solution are highly reactive and attacks the dye which results in less harmful products (carbon dioxide and water). The possible way of radical generation with the help of ZnO photocatalyst is clearly illustrated in the Fig. 16. When the sunlight is irradiated on the MB dye solution containing milligram of ZnO catalyst, electrons (e − ) and holes (h + ) are produced in the conduction and valence bands of photocatalyst. 61 This e − and h + act as charge carriers for the generation of radicals. The e − in the valence band reacts with atmospheric or adsorbed oxygen to give O 2 c − (oxygen radical anion or superoxide anion) and h + in the valence band reacts with water molecules to give H + (proton) and OHc (hydroxyl radical). Similarly, the h + also reacts with adsorbed hydroxyl anion (OH − ) to give OH .62 These radicals may react further to give corresponding reactive oxygen species radicals such as HOOc (peroxide radical), OHc and OH − (hydroxyl anion). Generally, the oxygen radical anion and hydroxyl radical attack and degrade the dye molecules by undergoing reduction and oxidation process with the dye molecules. But other radicals generated as the consequence of further reactions of O 2 c − also helps in the degradation process. So, the prime reason for the dye decomposition is the radical's generation (O 2 c − , OHc, HOOc,cOH − ). Even though it is the prime factor for dye degradation but there are some other secondary factors also in which the dye decomposition depends. They are; reaction time, catalyst loading, precursor concentration, pH conditions, oxidants, calcinations temperature, calcinations hours, doping etc. In the present study, reaction time, catalyst Fig. 16 Mechanism of photocatalytic performance of ZnO NPs using MB dye. 60 loading, precursor concentration, calcinations temperature and calcinations hours are the parameters taken into consideration which have given the excellent result without including the external factors such as pH conditions, oxidants, doping. The prime reason for not including these external factors is the concept of focusing on green chemistry. The detailed study on photocatalytic activity of ZnO NPs for the dye degradation is explored by focusing on various parameters. Hence, if all the parameters deciding the dye degradation is optimised, water pollution can be minimised using this catalyst.
3.4.2 Effect of ZnO photocatalyst loading. The photocatalytic potency of ZnO catalyst based on catalyst loading for 10 −5 M MB dye solution was studied for 5 mg and 10 mg photocatalyst calcined at 500°C and the photocatalytic plots are shown in Fig. 17. The photocatalytic efficiency calculated for 5 mg and 10 mg is 84.10% and 95.01%. It is clearly observed that the percentage of degradation of MB dye is increased as the amount of catalyst is increased. This can be due to more adsorption of photons by the catalyst surface resulting in a greater number of e-h pairs which can generate more radicals. Larger amount of catalyst can generate more active sites available for adsorption of dye molecules as well as adsorption of OH − . All these factors will favour the radical generation which can degrade the dye efficiently. But as the amount of catalyst is  increased beyond a limit, there are some other factors which can decrease the photocatalytic efficiency of ZnO NPs. The possible reason is that high loading of catalyst results in increased turbidity of solution which leads to lesser penetration of solar rays into the solution. Moreover, increase in the amount of catalyst causes overcrowding of adsorbent species results in overlapping of active sites for adsorption. 63 Also, it can cause collision between the active adsorbed species with the ZnO without any adsorption taken place resulting in the deactivation of ZnO catalyst. Hence, catalyst loading to degrade the dye should be optimised in such a way that the highest photocatalytic efficiency is achieved.
3.4.3 Effect of concentrations of MB dye. The photodegradation of MB dye by varying the MB dye concentration from 10 −5 M to 10 −4 M is studied without changing the ZnO loading. The photocatalytic results obtained are shown in Fig. S11. † It is noticed that as the concentration of dye is increased, the photocatalytic activity of 5 mg ZnO photocatalyst is decreased. This can be due to the following reasons; rstly, as the concentration of dye is increased, the adsorption of dye molecules on the active sites of photocatalyst would ultimately increase. So, the deactivation of ZnO semiconductor will be more as the irradiation of photocatalyst with solar light will be less. As a result, the generation of e-h pairs will be very less.
Moreover, more adsorption of dyes molecules decreases the adsorption of OH − which suggests that the rate formation of formation of OHc will be very less. 64 Secondly, increase in the concentration of dye decreases the penetration of solar rays into the reaction mixture. 20 Ultimately, the generation of radicals decreases. Hence, the particular amount of ZnO will degrade only the specic or lesser concentration of dye.  3.4.6 Effect of calcinations temperatures. The photocatalytic degradation of MB dye is analysed with respect to ZnO-500, ZnO-700 and ZnO-900 samples and the obtained photocatalytic results are depicted in Fig. 20. The photocatalytic activity percentage calculated for ZnO-500, ZnO-700 and ZnO-900 is 84.10%, 96.02% and 90.06%. It is observed that the photocatalytic efficiency increases as the calcinations temperatures is increased from 500°C to 700°C and then decreases when the calcinations temperature is further increased to 900°C . The initial increment in the photocatalytic efficiency can be due to reduction in the band gap (as represented in Fig. S6 †). The increase in calcinations temperature may generate more defects as well as results in agglomeration of particles. These defects reduce the band gap by introducing impurity level above the valence band as discussed earlier. Moreover, the agglomeration of particles reduces the band gap because of the principle of quantum size effect. Hence, both factors; generation of defect and particle agglomeration are narrowing the band gap which improves the recombination process and thereby improving the photocatalytic degradation activity. The later decrement in the photocatalytic activity can be due to the particle agglomeration, even though the band gap is reducing. The dominance of one factor over the other will reect in the photocatalytic results. The particle agglomeration may be dominant over the band gap reducing factor which would have lowered the photocatalytic efficiency percentage. Moreover, increase in the calcinations temperature may favour sintering process also, which will deactivate the photocatalyst by decreasing the surface area, pores and thus eliminating the catalytic sites of the ZnO photocatalyst. 67 So, the photocatalytic degradation of MB dye greatly depends on the calcinations temperature but doesn't follow general trends. There are several other factors which decide the degradation percentage along with the calcinations temperature. Hence, the optimised calcinations temperature should be considered for the complete degradation of MB dye.
3.4.7 Effect of calcinations hours. The photocatalytic activity of ZnO NPs catalyst on percentage degradation of MB dye is examined with respect to 2, 4 and 6 hours of calcinations and the obtained photocatalytic plots of ZnO-2, ZnO-4, and ZnO-6 are depicted in Fig. 21. The photocatalytic potency percentage calculated ZnO-2, ZnO-4, and ZnO-6 are 84.10%, 84.70% and 86.05% respectively. These results signify that increase in the calcinations hours doesn't produce any signicant change on the decomposition of MB dye. The small increment in the catalytic efficiency may be due to good crystallinity resulting in more generation of e-h pairs as a result of more adsorption of photons. Even though a small increment in photocatalytic activity is observed, it was not signicant. Hence, the tuning of calcinations hours does not play any signicant role towards the photocatalytic degradation of MB dye.
A comparative study of photocatalytic efficiency of ZnO NPs of current work with the previous results is represented in Table 3.

Conclusion
The green approach towards the synthesis of ZnO NPs using a novel LA leaf extract offered a simple, stable, economically viable, less invested, sustainable and eco-friendly method. The systematic study of ZnO NPs showed diverse morphologies, low band gap (2.81-3.01 eV) with increase in calcinations temperatures, and generation of Zn(OH) 2 with the increase in LA concentration. The crystallinity percentage and crystallite size were signicantly affected with the variation in concentration of precursor, concentration of LA leaf extract, calcinations temperatures and calcinations hours which played a crucial role in the activity of ZnO NPs as a photocatalyst. The mentioned synthesis process can be employed in industrial scale so that waste minimisation can be increased as well as enhancing simple high yield production. To be employed in industrial scale, all the synthesis parameters should be optimised in such a way that ZnO NPs can be accumulated in a larger amount. The photocatalytic application of ZnO NPs showed an excellent result by degrading the MB dye completely. The dye degradation study was done systematically based on various physicochemical parameters which guided towards the efficient photocatalytic result, 96-98.5% (5 mg degrading 10 ppm MB dye completely). Hence, ZnO NPs photocatalyst synthesised through systematic green chemistry protocol can be used effectively for degrading other toxic dyes and can be employed in large-scale for the effective treatment of waste water.