A critical review on novel eco-friendly green approach to synthesize zinc oxide nanoparticles for photocatalytic degradation of water pollutants

ABSTRACT Green synthesis of nanoparticles (NPs) is a revolutionary approach in terms of cost-effectiveness as well as minimization of environmental pollution by using perilousness-free bio-sources (different parts of plant, microorganism, plant-based waste). The different phytochemicals present in bio-sources play the role of potential reducing, stabilizing and capping agent during the synthesis of nanoparticles. An impetus in the rise of zinc oxide (ZnO) semiconductor has been perceived over the past decade as advanced nanomaterial to ameliorate the photocatalytic characteristics. Specifically, the use of ZnO is regarded as quite promising, not only because of the choice of green material but also owing to their alluring characteristics like high surface area to volume ratio, varying morphology etc. This review article represents several literature reports revealing the photocatalytic activities of green synthesized ZnO, prepared by using different green sources in degrading various organic pollutants which are present in wastewater. The method of derivation of phytochemicals during green synthesis approach of ZnO formation and the proposed mechanism of ZnO nanoparticles formation by interaction of those phytochemicals has been discussed thoroughly in this critical review. Numerous scientific investigations have been performed which elucidate on the controlling factors of ZnO (band gap, surface defects, surface morphology) and influence the photocatalytic activity. A summarization of such important reports has been synopsised in this article. The optimized parameters influencing the effective green synthesis of ZnO nanoparticles e.g. pH, temperature, metal precursor concentration and bio extract concentration have been concisely discussed with some published reports. Finally, a conclusion and future prospectus of the pioneering research arena are emphasized. Graphical Abstract


Introduction
Water is one of the elementary needs in terms of sustainability of life for all living organisms.Among the entire amount of earth's surface water, only 0.03% is fit for human consumption.The basic sources of fresh water are mainly lakes, ponds and rivers etc. Owing to the tremendous increase in population and global augmentation of industrialization by leaps and bounds such resources of fresh water are being polluted day by day.Moreover, ground and surface water become contaminated hugely by different effluents coming out of various industries as well as domestic activities [1].Various inorganic and organic pollutants and heavy metals are adversely decrementing the quality of surface and ground water resources.The leachates produced from different agricultural lands carries a large number of pesticides, herbicides, insecticides, fertilisers, which are applied basically to increase the quality of crop.Such chemically contaminated water has many perilous effects on human health and may cause serious diseases such as diarrhea, hepatitis dysentery, typhoid, skin rash, etc.The protracted consumption is highly dangerous to human health [2].The various organic compounds used in textile industries to colour different textiles also cause severe health issues and hazardous environmental effects.The additional dye materials ultimately get discarded into environment and cause consequential environmental pollution [3].Moreover, a plethora of water borne diseases like hemorrhage, dermatitis, ulceration of skin, nausea, mucous membrane, perforation of nasal septum and severe irritation of respiratory tract occurs due to the presence of such untreated dyes [4,5].In recent days, the pharmaceutical substances are proven to be the emerging pollutants in water bodies [6].The wastes emerging from pharmaceutical industries and hospitals have many toxic traits on the environment [7].Few of these compounds may also cause sustainable, irreversible changes to the micro-organism's genome [8].
Therefore, treatment of wastewater has become an urgent requisite and a crucial challenge.There are several well-known conventional wastewater treatment techniques such as disinfection, flotation, chemical oxidation, coagulation, flocculation, crystallization, micro-and ultra-filtration, membrane separation, ion exchange, reverse osmosis, adsorption etc.But, these aforementioned conventional physical and physico-chemical methods are undesirable because these can only transform the non-biodegradable part into sludge resulting in the generation of secondary waste products, which further demand for the expensive disposing steps [9].Besides, total elimination of the organic contaminants is very difficult by these techniques.Hence, only the transformation of the molecules takes place from one phase to another [10].Therefore, these methods possess some limitations as for example formation of detrimental toxic volatiles products by incineration [11] such as dioxane, furans, sulfur dioxide and hydrochloric acid which has perilous effect on the environment; prolonged treatment time for biological treatment resulting in foul smell, fly nuisance and production of huge amount of sludge to be disposed [10,12,13].Therefore, new advanced methodologies should be implemented for treatment of waste water.In recent era, advanced oxidation processes (AOP) have gained much attention among scientific communities as an efficient and alternative approach for treating the recalcitrant pollutants in aqueous bodies [14].The term 'Advanced' in AOP is based on the in-situ emergence of strong hydroxyl radicals (•OH) in ample amount to oxidize the hazardous noxious organic pollutants by ambushing them non-selectively and coherently.Thus, AOP is regarded as feasible and beneficial for complete mineralization of organic molecules and converts those into non-toxic CO 2 and H 2 O under atmospheric conditions [10].Generally, AOP is classified as two types, e.g.homogenous and heterogeneous photocatalytic oxidation.Herein, heterogeneous photocatalytic AOP possess some merit over homogeneous photocatalytic AOP because of non-toxic, cheap, easy separation and recyclable property of photocatalyst which ameliorate the overall efficiency of this process.Moreover, heterogeneous AOP involves the usage of semiconductor nanomaterials, which can act as photo-catalysts.Recently, several transition metal oxides and its stabilized composite have been gained popularity as an efficient photocatalyst.In this context, Dhiman et al., synthesized Rhodamine B dye -doped TiO 2 -SiO 2 core-shell composite microspheres, which exhibited about 93.2% K 2 Cr 2 O 7 degradation efficiency within 180 min under UV exposure [15].Therefore, among various semiconductor nanomaterial ZnO is chosen as potential and suitable photocatalyst because of their chemical stabilization, cost-effectiveness, easy synthesis with varying morphology (nanorod, nanoflower etc) [16,17].Basically, ZnO is an n-type semiconductor with a wide band gap of 3.37 eV and strong exciton binding energy of 60 mV at room temperature [18].Therefore, it absorbs only in the UV region that constitutes only a very small fraction (5%) of solar photons [19].As a result, the photocatalytic activity of ZnO is diminished to some extent as the rate of electrons (e − ) and holes ((h + ) recombination in excited ZnO becomes quite high.In spite of having this disadvantage ZnO has been proved to be an ameliorated photocatalyst [20].OH − ions are formed during oxidation of polluted water mostly prone to be adsorbed onto positively charged (0001)-Zn surface, which further can react with h + [21].As a result, sufficient amount of powerful oxidant •OH and HOO• radicals are generated.These radicals effectively degenerate organic pollutants in wastewater [22].On the other hand, surface defects (oxygen vacancies and interstitial Zn) present on surface of ZnO can accelerate the formation of h + and e − .Hence, further absorption of OH − ions and atmospheric O 2 is facilitated.Thus, it acts as the driving force of producing reactive species ̟ OH radical.Apart from this, ZnO possess a variety of surface morphologies, high surface area to volume ratio that intensify its photocatalytic activity.Thus, ZnO is proved to be an important material in degrading perilous organic environmental pollutants through photocatalysis [23,24].Variety of physical, chemical, hydrothermal [25] methods employed for the genesis of ZnO NPs are quite exorbitant and virulent to the environment.Wherefore, an eco-friendly, innocuous and reasonable method should be anticipated for the evolution of ZnO NPs that may function like an appropriate substitute to the regular chemical synthesis methods.In this framework, green synthesis is an arising sector committed to the development and advancement of nanoparticle formation in an effective, non-hazardous, and environmental-benign manner during the last decades.Different parts of plant such as leaf, flower, fruit, root, stem, bark etc. along with microorganisms, plant-based waste materials are used as green materials [26], which are rich in phytochemicals such as alkaloids, glycosides, flavonoids, tannins, terpenoids, phenols, saponins, alkaloids, proteins, vitamins etc.Thus, different plant extracts play a vital role as a reduction and capping or stabilization agent for the facile genesis of ZnO NPs.The mechanism of how the green sources helps for the formation of ZnO NPs is discussed briefly.At first, the phytochemicals are bound to the surface of Zn 2+ ions resulting the formation of zinc aqua complex.Then, the reduction reaction of Zn 2+ and chelation of Zn 2+ by phytochemicals occurs sequentially under the optimum reaction condition.Finally, ZnO NPs is formed in sufficient amount in medium [27].In addition, the facet-specific binding of the phytochemical regulates the size and shape of the produced ZnO NPs [28], which ultimately help to facilitate the photocatalytic activity of ZnO NPs.There are several literature reports revealing the photocatalytic activity of ZnO NPs synthesized by green methods.
A. Raja et al., (2018), depicted the synthesis of spherical shaped ZnO NPs via green route with 36.82 nm crystallite size using Tabernaemontana Divaricate leaf extract [29].Different phytochemicals such as terpenoids, flavonoids, phenolic acid, steroids, phenyl propanoids and enzymes stabilize the ZnO NPs.Moreover, the photocatalytic activity of methylene blue (MB) dye was tested by employing these synthesised ZnO NPs under sunlight.Almost 100% degradation of MB in 90 min was obtained.Sekar Rajkumar et al., (2019), fabricated the ZnO NPs by employing the aqueous extract derived from dry onion peels (Allium cepa L), which is regarded as waste material [30].The principal phytochemical i.e. flavonoid present in brown onion peel functions as the factor of reduction and capping throughout the formation ZnO NPs having hexagonal shape with an average size about 35 nm.The percentage of Crystal violet and methylene blue degradation by using ZnO NPs was 74.82 and 94.04 under sunlight, respectively.
The novelty of this critical review is a comprehensive overview on a broad range of numerous cost-effective green sources used for the preparation of ZnO NPs emphasizing on the photocatalytic activity of those green synthesized ZnO NPs towards different organic pollutants present in the aquatic environment.A proposed mechanism on ZnO NPs formation by various important phytochemicals has been elucidated with a pictorial view.Apart from that, different controlling factors, like band gap, surface defects, surface morphology of ZnO NPs, which have crucial role towards the removal efficiencies are explored here with some published literature.Moreover, other external influencing factors as for example pH of medium, temperature, metal salt solution concentration etc. have been deliberated, which can pave the way for wastewater remediation by using ZnO NPs.This critical review could be useful to researchers engaged in waste water treatment of various organic contaminants by employing cost-effective and eco-friendly route.

Green synthesis of ZnO from different source
There are different sources for green synthesis of ZnO for instance plants, microbes (Virus, bacteria, fungi, algae etc) and other plant-based waste products etc.These have been demonstrated as safe, efficient and cost-effective green sources.ZnO NPs formation via parts of plant is comparatively a convenient and method [31].Wide variety of these sources are briefly discussed in this section along with this below diagram (Figure 1).

Plant source and herbs
The synthesis routes via plants do not demand for any complicated protocols or preparation methodologies.Moreover, the synthetic process by using plants is relatively lucid for high-scale production of nanoparticles without the use of complex equipments.The plant components like leaves, root, stem, fruits, flowers, stems etc. are generally being employed in the green synthesis process of ZnO NPs.Different plants falling into Lamiaceae family like Anisochilus carnosus, Plectranthus amboinicusand Vitex negundo have been investigated thoroughly [32].Flowers of Anchusa italic,Jacaranda mimosifolia, fruits of Artocarpus gomezianus, leaves of Plectranthus amboinicus, Tamarindus indica, Pongamia pinnata, Parthenium hysterophorous etc., shoots of Sedum Alfred, stem bark of Boswellia ovalifoliolata etc., are some of the examples of plant parts, which are used in the green synthesis of ZnO NPs [33].The plant extract secretes few phytochemicals which function as both the factors of reduction and capping or stabilization; for example, genesis of ZnO NPs from the leaf extract of Conyza canadensis the phytochemicals containing hydroxyl group (O-H) and carbonyl side groups help to stabilize the ZnO NPs as well as control the size and morphology.Hence, the percentage degradation of 94.5 in 45 minutes and 85.3 in 20 min reaction time was achieved for Methyl Orange and Methylene Blue dye, respectively, by using this synthesized ZnO NPs [34].

Microorganism as source
Microorganisms, such as bacteria, fungus, have gained popularity for evolution of nanoparticles because of the comparative lucidity in handling and manipulating genetically [35].Generally, bacteria possess few organic functional groups, which can reduce Zn 2+ ions [36].Gram positive bacteria such as Rhodococcus pyridinivorans, [37] Gram negative bacteria Serratia ureilytica [38] are some examples where ZnO NPs were biosynthesized using those microbes as capping and reducing agents.However, screening of microbes and careful monitoring of culture is a time-consuming process.A pathogenic bacterium, B. licheniformis, was used for synthesis of ZnO NPs.These ZnO nanoflowers showed excellent degradation efficiency towards degradation of Methylene Blue dye [39].Owing to the high rate of production, easier downstream processing and cost-effectiveness, extracellular synthesis of ZnO NPs by employing fungus is highly preferred [36].Fungal strains are more superior than bacteria due to high tolerance power and better bioaccumulation characteristics in case of metals [40].Aspergillus species, Candida albicans fungi [41], had been largely used for ZnO NPs synthesis and NPs synthesized from fungal strains were mostly found to be spherical in shape.

Algae as a source
One of the renewable resources from the marine environment.Algae are categorised based on the pigments present in them such as green (Chlorophytes), brown (Pheophytes) and red (Rhodophytes) seaweed [42].The nucleation and growth of nanoparticles get faster because of the existence of negative charge on the surface of algal cells.Apart from this macroalgae of seaweed possess crucial bioactive compounds such as polysaccharides as well as amines, sulphates, carboxyl and hydroxyl functional groups [43].These functional groups form chelation with Zn 2+ by adsorption and finally cleavage of Zn-seaweed chelate occurs to form ZnO NPs [44].Pandimurugan & Thambidurai (2016) synthesized ZnO NPs capped with brown seaweed of Padina tetrastromatica using four various zinc sources precursors of, e.g.zinc chloride, zinc acetate, zinc sulphate and zinc nitrate by precipitation route [45].The highest degradation efficiency of 90.4% and 98.5% was obtained for ZnO NPs prepared from zinc nitrate precursor during photocatalytic degradation of Drimarene Turquoise Blue S-G and Methylene Blue dye, respectively, in the presence of solar radiation.

Other plant/biogenicbased waste source
Now-a-days use of biomass waste for evolving nanoparticles has attained quite a large research attention.An ample quantity of biodegradable food wastes including fruit pulp, fruit rind, as well as agricultural wastes such as fruit peel, rice bran, sorghum bran, wheat bran, corn cob etc. has been employed as green sources for efficient fabrication of ZnO NPs and applied it towards wastewater remediation purpose [46,47].These waste products are resourced with phenolic compounds, pectin and lignin etc. that function as factor of reduction, stabilization and template agent for the facile synthesis of ZnO nanoparticles.In this aspect Singh et al., (2017), synthesized flower-shaped ZnO nanoparticle from watermelon rind (WR), which is commonly discarded as agricultural waste [48].This WR is enriched with pectin, citrulline, cellulose, proteins, carotenoids, alkaloids, flavonoids, saponins, which act as potential reducing factor for ZnO NPs synthesis.Hence, they found 89% removal efficiency for photocatalytic degradation of methylene blue dye by ZnO nanoflower within 150 min time of reaction.

Different steps for derivation of phytochemicals
Plants and vegetables carry some active biological compounds such as isoflavones, flavonoids, xanthophyll, anthocyanins, carotenoids, organo-sulfides etc., which play a vital role on the synthesis of nanomaterials.In order to derive those various phytochemicals, different crucial steps are adopted that includes extraction, isolation, purification and characterization of the compound [49].
Extraction is basically the step involved to bring the phytochemicals from green source to solution.In this step, the different parts of the plants are properly washed with distilled water to remove the debris and unwanted contamination.After that the cleaned plant parts are dried and cut into smaller pieces or grinded into the form of powder.These grinded powders are boiled in a suitable solvent such as water, ethanol, etc. to make it corroborate of the efficacious wrenching out without causing any damage to the phytochemicals.Herein, choosing of solvent and setting of boiling temperature is very important for extraction of phytochemicals without any structural damage [49].Jafarirad et al. (2016) have wrenched out the natural factors from Rosa caninafruit pulp to produce ZnO NPs [50].At first, the fruit flesh had made dried at normal standard room temperature followed by weighing and then grounded to convert into powder.After that it was mixed with deionized water for 24 h.Then, the prepared mixture was heated at 50°C for around 15 min.As a result, color change of the mixture occurs which is a evidence of the successful extraction.Other developed extraction methods like microwave-assisted extraction, ultrasonic-assisted extraction, supercritical fluid extraction and solid phase micro-extraction have advantages in terms of lower solvent consumption, emission of undesirable and insoluble substances at faster rate from the extract.Jafarirad et al. ( 2016) compared the conventional heating and microwave irradiation [50].It was observed that the rate of MI is faster than CH.Rajabi et al., 2017 have derived the phytochemicals from an aqueous extract of Suaeda aegyptiaca by executing microwave-accompanied method of extraction [51].In this method the plant was first reduced into powder form by grinding and then dried followed by mixing with double distilled water.Later, it was irradiated under different power of microwave irradiation for 15 min in microwave apparatus.After the extraction of phytochemicals, the isolation procedure is performed through filtration to get rid of undesirable contaminants.Finally, the characterization process is done by using several chromatographic and spectroscopic analytical techniques like high pressure liquid chromatography, liquid chromatography-mass spectrometer, Fourier transform infrared spectroscopy, Raman spectroscopy etc. to confirm the presence of functional groups and actual compound [52].

Proposed mechanism of ZnO nanoparticles formation via green approach
Phytochemicals present in bio sources have emerged as a significant compound, which play the pivotal role for formation of nanoparticles in a green approach.Several scientists intensify the significance of various functional groups present in bioactive compounds that persuade the evolution of nanoparticles [53].Researchers are pre-eminently focused on O-H, C = C and C-H groups that are broadly present in secondary metabolites such as terpenoids, flavonoids or alkaloids.In this section, a brief overview based on a mechanism for formation of ZnO NPs has been highlighted with a pictorial representation (Figure 2).
The formation mechanism of ZnO NPs in a green synthesis is based on some series of phenomenon depending on the nature of phytochemicals present in bio-source.They are (i) binding of phytochemicals to the surface of Zn 2+ ; (ii) formation of zinc aqua complex; (iii) Reduction of Zn 2+ (iv) Chelation of Zn 2+ by biomolecules forming Zn 2+ -biomolecule complex; (v) Formation of ZnO NPs under the optimized conditions (pH, calcination temperature etc) in solution.Senthilkumar et al., (2017), suggested the ZnO NPs evolution mechanism by deploying the leaf extract ofTectona grandis L [54].Here, a large number of -OH groups present in phenol and flavonoids are accountable for zinc nitrate decomposition into ZnO NPs.Initially, phenols and flavonoids in the aqueous leaf extract bind the surface of zinc in precursor to activate the formation of nanoparticles.Król et al., (2019), elucidated a probable mechanism of ZnO NPs formation by Medicago sativa L. leaf extract [55].In this investigation, the formation of Zn 2+ -aqua complex occurs as Zn(II) complexes are kinetically labile.After that phenolics, favonoids, ascorbic acid, reducing sugar, starch etc. there in majorly plant extract can prompt Zn 2+ reduction and regulate the size of produced nanoparticles.On account of the specific coordination chemistry of zinc, their aqua complexes are capable of exchanging their water molecules when chelating to the biomolecules which function as ligand.As a result, Zn 2+-chelate complex has been formed in solution [56].Król et al., (2019), reported that carboxyl and amid-I groups derived from alfalfa proteins binds with Zn 2+ as proteins are the major ligands for the zinc ions (Zn 2+ ).[55] Finally, the Zn 2+ -biomolecule chelate complex prevailing in solution undergoes feasible reaction under optimized condition.As an ultimate outcome, ZnO NPs are formed in sufficient amount and ready for further usage.

Photocatalytic activities of green synthesized ZnO nanoparticle
A clear perception regarding photocatalytic mechanism of ZnO NPs with a suitable diagram has been represented in this section prior to discussion of some reported work on photocatalytic activities of ZnO NPs in degrading hazardous contaminants available in wastewater.
In general, the photocatalytic activity of nanomaterial is dependent on absorption of light, transportation of charge and separation on catalyst surface.In the first instance, light of acceptable wavelength (UV light) is absorbed by ZnO resulting in creation of electron-hole pairs.These electron-hole pairs then seek to get spatially separated in the volume of ZnO before getting transferred to reduction-oxidation (redox) active species across the interface [57].The electron evolved from the valence band of ZnO photocatalyst gets induced to the conduction band generating electron-hole pairs.After that, the photogenerated electron-hole charge carriers get transported towards the surface.The highly oxidative holes which are present in the valence band get trapped by the water molecules, adsorbed at the surface produce hydroxyl radicals ( • OH).Electrons are scavenged by the adsorbed molecular oxygen to generate reactive superoxide radical anion (O 2

•-)
.Meanwhile, the valence band holes react with water and produce super hydroxyl radicals, ( • OH − ).Further, The O 2 •-radical anions react with water molecules and generate strong oxidants like • OH and HOO • radicals.[57] These radicals efficaciously take part in degrading those organic pollutants, which are present in aqueous media [58].Following series of reactions illustrate the process analogous to the degradation of organic pollutants governed by photoelectrons.

Reactions
Photoexcitation: Furthermore, the boosted sensitization effect between the dyes and the ZnO semiconductor may also ameliorate the degradation efficiency.In this route, the chromophore groups present in the dye molecule absorbs photons from light source causing the electron on the highest occupied molecular orbital (HOMO) of dye molecules to excited to the lowest unoccupied molecular orbitals (LUMO).[59] This excited dye molecule then releases electrons which are then injected to the conduction band of the semiconductor leaving cationic dye radical.The adsorbed molecular oxygen would capture the electrons in the conduction band of semiconductor to degrade the dye molecule [60].The mechanistic approach of photocatalytic oxidation by ZnO nanoparticles process is represented in Figure 3.
ZnO NPs derived from biological origin has efficacy to show remarkable photocatalytic degradation of numerous dyes, such as methyl orange, methylene blue etc., and other contaminants.The photocatalytic activity of ZnO NPs established by some reported work has been discussed in table S1.

Plant extract
Several plant components e.g.stem, root, callus, flower, fruit, seed, peel, leaves and latex etc. are used in the process of green genesis of ZnO NPs with tuned shapes and sizes.Basically, the metal ions reduction rate, plants or different parts of plants had been noticed to be quite rapid in comparison with that employing micro-organism.On top of that, a certain regulation may be obtained on the shape and size of the nanoparticles formulated by using plant extract through changing the reaction conditions [62].Different types of sources of plant extract are discussed below.

Leaf extract
Leaves are supreme and limelight part of plant which in turn eventually nourish and sustain all land animals by photosynthesis.Many leaf extracts are delineated in literature for ZnO NPs genesis along with their photocatalytic activities.Hassan et al., (2015), prepared ZnO NPs from Coriandrumsativum leaf extract by using Zn acetate precursor and studied the catalytic degradation of anthracene, a polycyclic aromatic hydrocarbon pollutant found in wastewater under UV light irradiation using synthesized ZnO NPs [1].Various standard tests showed the occurrence of several phytochemicals in leaf extract of Coriandrumsativum that facilitates the evolution of ZnO NPs possessing the size range from 9 to 18 nm.The experiment depicts that the percentage of anthracene omission for green synthesized ZnO NPs and chemically synthesized ZnO NPs was 96% and 31%, respectively, in 240 min reaction time.The optimum photocatalytic degradation of 100 μgL −1 anthracene was carried out in presence of 1000 μg L −1 ZnO NP, at atmospheric temperature of 25°C at pH 7.This result emphasizes the better effectiveness of green synthesized ZnO NPs over chemically synthesized ZnO NPs towards water remediation.
Furthermore, Saraswathi et al., (2016), explored the degradation of Methyl orange (MO) using Lagerstroemia speciosa leaf extract mediated synthesized ZnO under solar light exposure [63].It contains a disparity of phytochemicals such as alkaloids, glycosides, flavonoids, tannins, terpenoids, phenols, saponins, alkaloids, proteins, vitamins, etc.The synthesised ZnO NPs were highly stable and in hexagonal phase having the average particle size around 40 nm.Upto 93.5% degradation of methyl orange was achieved by the synthesised ZnO NPs within the reaction time range of 2 h.After photocatalytic experiment, the mineralisation study was also carried out in order to examine the efficiency of mineralisation by estimating the COD of the MO solution in pre-and post-photocatalytic treatment.The COD values were prominently decreased from 5600 mg/L to 374 mg/L after 100 min of solar radiation, which implies the occurrence of sufficient degradation of Methyl Orange.
Gawade et al., (2017), synthesized ZnO NPs that are spherical in shape, having size ranging from 15 nm to 25 nm by using aqueous leaf extract obtained from Calotropis procera.[64] This leaf extract contains several phytochemicals such as hydroxyl groups, aldehydes, amines, ketones, carboxylic acid, aldehydes, amines, terpenoids, phenolic compounds, which function as factors of stabilization and reduction.Hence, the photocatalytic degradation of Methyl Orange (MO) dye was found to be around 81% in 100 min under UV light irradiation using this biogenic ZnO NPs.The efficiency of photodegradation of MO at its maximum was found at the optimum catalyst loading of 1.5 g/ dm 3 .
Rathnasamy et al., (2017), studied the photodegradation of methylene blue dye by utilizing hexagonal wurtzite ZnO NPs prepared from Carica papaya leaf extracts [65].The complete degradation (about 100%) of methylene blue dye was attained in 180 min time of reaction under UV light irradiation.The presence of important phytochemicals such as ketone, hydroxyl groups, saponins, tannins, cardiac glycoside, alkaloid in the leaf extract of Carica papaya played a vital role towards synthesis of potential ZnO NPs, which showed its enhanced photocatalytic activities.

Flower extract
Flower of a plant is contemplated as most beautiful parts of plant and responsible for reproduction.Numerous flower extracts were used for synthesis of ZnO NPs, which are briefly discussed below.
Bharati, R. & Suresh, S., (2016), prepared ZnO NPs via green route by using Butea monosperma Palash flower powder (PFP), which functions as suitable factor for reduction and stabilization [66].Different characterization studies showed that the green synthesized nanoparticles possess angular and irregular shape with an average particle size of 17.6 nm and overall particle size ranging from 3 nm to 40 nm.The optimum photocatalytic degradation of phenol was found to be around 62% at 4 h time reaction and after 6 h the concentration of phenol was reduced to 35.8 mg/L from 100 mg/L.Soto-Robles et al., (2019), evaluated the photocatalytic activity of ZnO NPs synthesised by using Hibiscus sabdariffa flower extract in a greener approach [67].X-ray diffraction (XRD) analysis presented that the produced nanoparticles had hexagonal crystalline structure (Wurzite) with the size of particle ranging from 8 nm to 30 nm.Characterisation studies also showed the presence of different phytochemicals like phenolic, flavonoids and several compounds in the flower extract that may prompt zinc salts reduction and stabilization along with controlling the size of the ZnO nanoparticle.The photocatalytic degradation percentage of methylene Blue dye was found to be 97 in 150 min by using this green synthesized ZnO NPs.

Fruit Extract
According to Botany, a fruit is a structure that holds the seeds in a flower bearing plant, which is conceived from the ovary in the wake of flowering.Numerous literature reports concerning the synthesis of ZnO NPs from fruit extract is gaining popular till date.Some of them are discussed in this section.
Chakraborty et al., (2020), inspected the bio genesis of ZnO NPs by employing star fruit (Averrhoe carrambola) extract and applied it towards photo-degradation of Congo Red (CR) dye, a carcinogenic dye mostly used in textile industry [68].The oxalate group of oxalic acid present in star fruit as phytochemical helps to reduce the size of bulk ZnO to nanoflakes ZnO having the size of about 20 nm effectually.Therefore, the study of photodegradation accomplished on CR dye revealed good efficiency of photocatalysis by the synthesized ZnO with a capacity of adsorption as 285 mg g −1 and a degradation percentage of 93% in 150 min under UV light illumination.Golmohammadi et al., (2019), synthesized ZnO NPs using jujube fruit extract and unraveled its photocatalytic degradation efficiency of organic dyes named methylene blue (MB) and eriochrome black-T (ECBT) under direct sunlight.[69] The presence of flavonoids biomolecule in jujube fruit extract imparts a superior factor of reduction for the green genesis of ZnO NPs.The degradation efficiencies using ZnO NPs was about 92% and 86% within 5 h for MB and ECBT, respectively.Fowsiya et al., (2016), investigated the photodegradation of Congo red (CR) dye using flower shaped ZnO, which is synthesized through microwave expedited extraction of Carissa edulis fruit [70].The flavonoids, phenolic and terpenoid compounds present in Carissa edulis fruit extract were responsible for the preparation of ZnO NPs.Consequently, 97% of CR dye had been degraded at maximum in the presence of ZnO NPs after 135 min time of reaction.Rana et al., (2015) studied the green synthesis of ZnO spherical shaped nanoparticles using aqueous fruits extract of Terminalia chebula and investigated the photocatalytic degradation of rhodamine B.About 70% photodegradation was achieved after 5 h of irradiation [71].

Peel extract
Apart from the fruits, the fruit peels have also been used for the genesis of ZnO nanoparticles.
Karnan, T. & Selvakumar, S.A.S., (2016), prepared ZnO NPs with the use of extract derived from the rambutan (Nephelium lappaceum L.) fruit peel and applied it towards photodegradation of methyl orange (MO) dye [72].Various types of phytochemicals such as ellagic acid, corilagin, geranin and ellagitannins etc. help to stabilize and restrict the growth of ZnO NPs beyond a certain range.Moreover, around 83.99% removal efficacy was obtained at 120 min time of UV light illumination at pH 7.01.Apart from photodegradation experiment COD experiment was also carried out to determine the mineralization efficiency, which was decreased noticeably from 6162 mg/l to 481 mg/l after the UV illumination for 120 min.This indicated the satisfactory degradation of MO by synthesised ZnO NPs.Aminuzzaman et al., (2018), triggered ZnO NPs with 23.56 nm crystallite size employing biogenic route from the pericarp of Garcinia mangostana (G.mangostana) fruit.Pericarp of G. mangostana was enriched with numerous phytochemicals e.g.phenol, flavonoid, xenthone and anthocyanin compounds, which act as the potential reducing and capping agent [73].The photocatalytic characteristics of the ZnO NPs was examined by conducting the degradation of Malachite Green dye, mostly used in aquaculture industry as a fungicide and parasiticide under solar irradiation.Finally, about 99% of MG was degraded within 180 min of irradiation.

Seed extract
Extracts derived from seeds have been deployed outstandingly for the genesis of metal nanoparticles, but researches devoted to this field are very less in number.Synthesis of metal oxide nanoparticles.Kavyashree et al., (2015) synthesized porous ZnO NPs with morphological diversification followed by a facile green approach using Kalonji (Nigella sativa) seed extract as fuel and showed excellent performance in degrading methylene blue under the irradiation of both UV and sunlight [74].The different phytochemicals available in the seed extract such as alkaloids, flavonoids, tannins and phenolic compounds etc. help to cap and stabilize the ZnO NPs having average crystallite size of 34 nm.It was observed that the photocatalytic activity was about 90% for flower shaped ZnO at an optimum catalyst dose of 500 mgL −1 .
Moghaddas et al., (2019), fabricated ZnO NPs through biosynthesis by using the Quince seed Mucilage (Cydonia oblonga) as the stabilising agent [75].Various characterization results confirmed the presence of pure ZnO NPs possess fairly uniform crystals with average size of the particles around 25 nm.Herein, the mucilage derived from quince seed is responsible for nucleation of ZnO NPs followed by adsorption of more Zn ions on the nanosurface.Biomolecules, for example, arabinose, xylose, mannose, galactose etc., were present in the extracts.It has been noted that the green synthesized ZnO NPs could degrade more than 80% of Methylene Blue dye by synthesized ZnO NPs within 2 h reaction time under UV light exposure.

Latex extract
In general, latex is a milk like fluid which may be found in around 10% of all flower bearing plants also known as angiosperms.Latex can be defined as a complex emulsion that primarily consists of proteins, alkaloids, starches, sugars, oils, tannins, resins and gums, which get coagulated while exposed in air.
Sudheer Kumar et al., (2018) investigated the degradation of Methylene blue, a recalcitrant dye through photo-catalysis, coming from different textile, printing and tannery industries under UV light exposure using highly efficient ZnO NPs synthesised from the latex of E. tirucalli plant [76].Several phytochemicals e.g.alkaloids, flavonoids, phenols, tannins, etc. stabilising present in the latex extract was able to form ZnO NPs with controlled morphology and size.Therefore, 96% degradation efficiency of ZnO NPs was achieved within 120 min of reaction time while degrading Methylene blue dye through photocatalysis at pH 12.
Sharma S. C., (2016), carried out the green synthesis of ZnO having different morphologies through bio-route with the use of Carica papaya milk (CPM) latex and applied it in degrading Alizarin Red-S (ARS) dye [77].Pre-eminently, the latex obtained from papaya is a fluid, thixotropic in nature having a milk like form.It comprises of phytochemicals such as phenolic compounds, alkaloids, flavones, nutritional factors like proteins, vitamins etc.These phytochemicals function as prospective stabilizing factors in the evolution of ZnO NPs.The ZnO morphology has been tuned by varying the latex concentration among which ZnO nano-flower was predominant exhibiting maximum photocatalytic activity.The ZnO nano-flower degraded almost 99% of the ARS dye solution in 90 min under UV irradiation.Moreover, the total oxygen carbon (TOC) was evaluated as a mineralization index to verify the ARS degradation.The maximum removal percentage of TOC was found out to be around 81% in an irradiation time period of 90 min.It indicates that the maximum of the ARS dye had been mineralized in the course of photocatalysis and it was quite significant in case of its industrial applications to terminate secondary pollution.

Stem extract
The stem is the portion of the plant that sits above the ground and performs a variety of functions, which are important for various biochemical processes including photosynthesis.Some literature reports involving the green synthesis of ZnO NPs from stem extract along with its photocatalytic activity are highlighted in this section.
Nahi et al., (2020), have synthesized ZnO assimilated nanocellulose from the stem extract of Tinospora Cordifolia plant and Sugar bagasse for the purpose of photodegradation of Enrofloxacin (EF), an antibiotic which is broadly utilised in the veterinary medicines for the treatment of animals from bacterial infection [78].Various characterization studies showed that the flower-shaped ZnO NPs were enriched with important phytochemicals which play a role as reducing and capping agent during formation of ZnO nanoparticles.Hence, 97% of degradation efficiency has been obtained in a time period of 120 min of reaction under solar radiation.

Root extract
Roots are very vital underground constituent of all vascular plants.This is a part of the plant, basically accountable for anchoring it deep into the ground and taking in the intrinsic mineral elements, nutrients and water from the soil.Roots also store food.Many scientists employed root extract for synthesis of ZnO NPs, which are noted in this section.Chen et al., (2019), approached for biogenic synthesis of highly stable and sphericalshaped ZnO NPs composite by using the root extract derived from Scutellaria baicalensis (S. baicalensis) [79].The existence of flavonoid phytochemical is liable for stabilizing the ZnO NPs.ZnO NPs was capable to degrade Methylene Blue dye efficiently.Ostensibly as a result almost 98.6% of degradation within 210 min under UV light illumination was attained.

Plant gum extract
Primarily plant gums are polysaccharides which are experiencing substantial recognition in sustainable nanofabrication because of their green nature, abundance and costefficient feasibility.These are also being used as stabiliszng/dispersing agents, a matrix for nanocomposites, and for surface modifications of nanostructured materials.[80] Suganya et al., (2018), studied the degradation of trypan blue (TB) organic dye using ZnO NPs prepared via Neem (Azadirachta indica) gum-assisted sol-gel process under the exposure of UV radiation [81].Herein, neem gum plays a crucial role as an effective chelating agent and cross-linking agent for Zn 2+ ions, which facilitates the uniform distribution of metal ions throughout the gum matrix.The highest degradation percentage of 97% at 180 min was conquered from the photocatalytic degradation experiment of TB.Similarly, Saeid Taghavi Fardood et al., (2017) depicted the formation of hexagonal wurtzite ZnO nanoparticles using an arabic gum as a bio-polymeric template the sol-gel method evaluated the photocatalytic activity of ZnO-NPs in degrading direct blue 129 dye (DB129) under visible light irradiation [82].The catalyst removed almost 95% of the DB129 in 105 min.

Marine algae extract
The green fabrication of nanoparticles by using marine substances has been done and attained remarkable significance because of their profusion and protocol of ecofriendliness [83].In this regard, a broad variation of seaweeds have been inspected for the nanoparticle synthesis.Nevertheless, it is worth mentioning that restricted researches have been executed and delineated in the literature in this specific case of the ZnO NPs synthesis.Ishwaryaa et al., (2018), demonstrated the green synthesis of ZnO NPs with average crystallite size of 10-50 nm using Ulva lactuca seaweed extract [84].The sea lettuce, U. lactuca, is edible green seaweed largely to be present in marine environments worldwide contains alcohol and phenol groups, which can act as stabilizing and capping agent due to formation of ZnO NPs.The sponge-like asymmetrical-shaped ZnO NPs showed around 90.4% photocatalytic degradation efficiency for Methylene blue dye under sunlight within a period of 120 min at optimum pH 7. Rajaboopath S. & Thambidurai S., (2018), investigated Padina gymnospora algae extract-mediated facile synthesis of Ag-ZnO nanoparticles having an average crystallite size of 20-40 nm by the chemical co-precipitation method [85].Hence, the algae extract possesses water-soluble biomolecules polysaccharides, protein, polyphenols, fibers and minerals acting as capping agent during synthesis of nanoparticle.Moreover, photodegradation and decolourization performances of the catalyst nanoparticles were performed by both methylene Blue and Reactive Blue 198 dyes under the direct sunlight irradiations.Thus, 85 and 95% degradation were obtained for MB and RB198 dyes, respectively, within the irradiation period of 30 min exposure.Finally, the chemical oxygen demand values at different time interval endorse the mineralization of 69% and 73% achieved using MB and RB198 dyes.

Bio-waste extract
Different waste coming from plant part (fruit rind, rice bran, wheat bran etc.) or others agro-industrial sources can be used for nanoparticle synthesis.Some of them are discussed below.In this regard, a huge amount of date fruit wastes (about 1.5 million tons) is produced by the date palm industries every year at various stages of processing the ultimate fruit into valuable products like syrups and sugars are taken into consideration [86].Waste date pulp is a dry biomass which is produced after the extraction of syrup from the fruit accounting for two-thirds of the bio-wastes composition of the date syrup processing industries and are being thrown away without having any value.Hence, Rambabu et al., (2020), reported date pulp waste (DPW) valorization from Phoenix dactylifera for effective synthesis of ZnO NPs and embarked it towards photocatalytic degradation of methylene blue (MB) and eosion yellow (EY) dyes concerning solid waste minimization and renewal [86].The incredible number of flavonoids, quinones, carotenoids, sterols, phenolics and anthocyanins present in this DPW makes it as a marvelous stabilizing agent for ZnO NPs synthesis.Moreover, degradation efficiency of ZnO NPs about 90% was obtained at the end of 180 min for MB and EY, respectively.Fatimah (2018) looked for the effective green synthesis strategy of ZnO NPs fabrication using rice bran powder, discarded material as template.Herein, crucial phytochemical i.e. xylan act as potential templating agent, which restrict the overgrowth of ZnO NPs during its formation.Therefore, the photodegradation experiment was carried out on bromo phenol blue by employing synthesized ZnO NPs with particle size of 17 nm as photocatalyst.As a result, about 95% degradation was achieved at 180 min time of reaction.

Other biogenic extract
Apart from the aforementioned substances, different other resources such as rhizome, bacteria and fungi are also being used for the synthesis of ZnO NPs.Nevertheless, no widespread researches have been assigned in that field.The biomolecules e.g.anthocyanins, flavonoids, dianthrones, stilbenes, anthraglycosides, chromone glycosides, polyphenols, organic acids, essential oil, and vitamins present within rhizome extract enhance the stability of ZnO NPs and also act as reducing agent during formation of nanoparticle.At this point, Nemati et al., (2018), used rhizome extract derived from Rheum turkestanicum plant in order to synthesize hexagonal-shaped ZnO NPs with a size of 17-20 nm [87].The photocatalytic assessment was successfully carried out on methylene blue under UV light exposure.It indicated that ZnO-NPs can degrade 100% of the dye in 60 minutes.Jain et al., (2014), employed green synthesis route for facile fabrication of ZnO NPs using the extracellular protein derived from soil-borne fungus Aspergillus sp [88].Different characterization studies showed that the synthesized ZnO NPs having the size ranging from 80 nm to 120 nm were quasi-spherical, symmetrical, poly-disperse and distributed in a well manner with no aggregation.Moreover, the individual ZnO NPs was coated by protein molecules, which act as stabilizing agents during formation of ZnO NPs.The highly stable ZnO NPs showed 90% degradation efficiency achieved in 30 min towards methylene blue dye exposure under UV irradiation.The result was more superior to the commercially produced ZnO nanoparticles that showed around 40% degradation of MB dye in 30 min.The remarkable photocatalytic execution is pre-eminently owing to the occurrence of surface proteins, which works as an potent host for methylene blue dye and expedites the dye absorption.Prasad et al., (2019) represented the green synthesis of ZnO Nps with the use of Abelmoschus esculentus (okra) mucilage extract, which is prepared in the absence of any solvent [89,.The mucilage was proved to contain proteins, carbohydrates, polysaccharides, minerals, polyphenols and flavanol derivatives.The excellent metal-chelating property of this mucilage ultimately results an effective binding of zinc ions from an aqueous medium and formation of ZnO NPs.Therefore, the photocatalytic activity of the prepared ZnO NPs having wurtzite shape was performed with methylene blue (MB), rhodamine B (RhB), methyl orange (MO) and Congo red (CR) in as target dyes under UV light.Due to negative surface charge of ZnO, it removed MB and RhB efficiently.The degradation efficiency for MB was 95% after 60 min of reaction and 100% for RhB after 50 min of reaction.

Controlling factor of ZnO for enhanced degradation efficiency
The origination of oxidative hydroxyl radical is quite vital in degrading the pollutants happened to be present in aqueous media.Nevertheless, the heterogeneous catalyst also has iquite essential effects in the process of degradation.The presence of suitable heterogeneous photon energy utilizing catalyst in the media of reaction intensifies the generation of higher quantity of hydroxyl radicals and also comes up with the surface for undergoing process of degradation methodically.This is how in that case formatting of a photocatalyst exhibits a presiding role.The different controlling factors, such as band gap energy, carrier transport, crystallinity, surface area and chemical stability, for the designing photocatalytic materials have been discussed below.

Band gap
The formation of carriers of charge (hole, electron) is the utmost essential criteria for fruitful photocatalytic reactions, which has been already discussed in previous section.When the photocatalyst is illuminated by the energetic photons, electron from the valence band jumps to the energy interval present in the semiconductor and reaches to the conduction band.This energy interval or bandgap energy (Eg) is a vital structural characteristic of a photocatalyst [2].Smaller energy gap indicates easier expenditure of low energy and photo activation.However, the band gap energy of ZnO is 3.37 eV, which is quite high.So, from this perception, the efficient photocatalytic activity of pure ZnO becomes sequestrated.Hence, narrow band gap photocatalyst is desirable and also research is in progress for preparation of solar light harvesting as well as narrow band gap photocatalyst.Conversely, a major drawback of very narrow bandgap of photocatalyst is that fast recombination of photogenerated charge carriers may get developed [118].Therefore, a certain balance may be implemented between those two cryptic phenomena for pure ZnO by varying the external factors e.g.bio-extract concentration, pH, temperature etc. during green synthesis of ZnO.Besides, doping of ZnO with different metal, non-metal, metal-oxide etc. can reduce or increase its band gap and make it as potential photocatalyst.In this context, Rana et al., 2015 synthesized several Magnesium (Mg) doped ZnO (Mg x Zn 1−x O (0≤x≤0.20)) by polymeric precursor method.In this investigation, the band gap energy increases from 3.14 to 3.44 eV as the concentration of Mg increases up to 16 mole %.This is due to the participation of 3s orbital of Mg to the conduction band along with 4 s orbital of Zn.As the 3s orbital of Mg occupies higher energy level as compared to the 4s orbital of Zn so it moves the conduction band upward, resulting the wide band gap [119].On the contrary, Rana et al., 2015 reported the reduction of band gap upon doping of ZnO nanostructure with cadmium (Cd) of higher concentration (Zn 1-x Cd x O: 0 < x < 0.03).Herein Zn 1-x Cd x O alloys showed red-shift with high concentration of Cd doping [120].
Luque et al., 2018, observed the efficiencies of three ZnO photocatalyst (denoted as M1, M2 and M3) which was green synthesized using different concentrations of Citrus sinensis peel extract for the degradation purpose of methylene blue under UV light illumination [121].The band gaps of M1, M2 and M3 were 2.92, 2.93 and 2.85, respectively.Herein, the photocatalytic activity for M2 was 83% of MB degradation within 120 min; highest percentage degradation than that of M1 and M3.

Surface defects and oxygen vacancies
ZnO is usually n-type semiconductor with local defects such as vacancies of oxygen and zinc.Lattice defects like:(a) zinc vacancies, V Zn (b) zinc on interstitial sites, Zn i (c) oxygen on interstitial sites, O i and (d) the oxygen vacancies may be present in three different charged states (Vo++, Vo+, V O ) [37].Defect works as a centres of recombination for hole and electron [2].Therefore, this is quintessential to maintain less number of defect in photocatalytic material for an intensified performance of photocatalytic degradation.Nevertheless, several experiments and theory have showcased that O vacancies are deep donors, whereas Zn interstitials are too mobile to be stable at the standard room temperature [122].Therefore, a broad band emission ranging from 400 to 650 nm can be accredited to ZnO surface defects, where oxygen vacancies were highly pre-eminent.The superior photocatalytic activity of ZnO has been observed to be analogous with the presence of oxygen vacancy and Zn located in interstitial sites [123].Either the formation of OH − groups or the formation of oxygen vacancies would be the energetically favourable process for stabilization of ZnO nanostructure.These oxygen vacancies on the surface of polar ZnO performs an indispensable role in O-O bond cleavage of atmospheric O 2 as they act as the active centers at the interface between catalyst surface and organic pollutant [24].Thereby the oxygen vacancies can facilitate the adsorption of O 2 , which further interact with the photo-induced electrons and producing O 2 .
These radical groups are chemically active and promote the oxidation of menace organic pollutants present in wastewater.Thus, the photocatalytic activity of ZnO is ameliorated.Manuela Stan et al., (2015) explored the biological synthesis of ZnO NPs using extracts of Allium sativum (garlic), Allium cepa (onion)and Petroselinum crispum (parsley), which were denoted as ZnO(g), ZnO(o) and ZnO(p), respectively [124].The highest intensity for the signal due to Zn vacancy (g = 2.003) and to oxygen vacancy (g = 1.96) was achieved for ZnO (g) and ZnO(o), respectively, from EPR characterization.This suggests the presence of an increased number of defects (Zn vacancy or oxygen vacancy) in these samples as compared with ZnO and ZnO(p).This phenomenon is well suited from the photocatalytic degradation test of methylene blue under UV light irradiation.Herein, the order of photocatalytic activities of ZnO nanoparticles prepared using different plant extracts can be ordered as follows: ZnO(g)> ZnO(o)> ZnO(p)> ZnO within 180 min time of reaction.Thus, higher the defects present in ZnO higher will be the tendency to trap the produced electrons and finally inhibiting the recombination of electron-hole pairs.Hence, the presence of defects will lead to high degree of reactive oxygen species generation [125].

Surface morphology
Surface morphology of nanomaterial is another vital factor for accomplishing effective photocatalytic activities.ZnO can exhibit a wide variety of surface morphological shape such as nanoflower, nanorod, nanobud, nanoneedle, nanowire etc of various dimensions [126].Generally, 1D ZnO include nanorods, nanofibres, nanowires, nanotubes and nanoneedles [127].On the other hand, ZnO nanostructures in 2D and 3D are nanosheets and nanoflowers, respectively [128].Different surface morphology of ZnO can be attributed owing to the fact of distribution in particle size and presence of surface defects.Kajbafvala et al., 2012 examined the effect of various surface morphologies of ZnO nanostructure on photodegradation of methylene blue under UV irradiation for 4 h [129].Hence, 78% and 17% of MB degradation was achieved by ZnO nanosphere and ZnO nanoflower, respectively.This result was attributed to the fact of higher BET surface area, more porous structure of ZnO nanosphere (98 m 2 /g) compared to ZnO nanoflower (22.9 m 2 /g).In addition, bonding of small particle constituents of the nanostructures are flat form in the flower-like ZnO, which makes it less porous than ZnO nanosphere.Sharma, S. C., (2016), demonstrated the morphological and size variation of ZnO NPs by varying the carica papaya milk (CPM) extract [77].At 10, 20, 25, 30 ml of CPM extract assynthesized ZnO NPs possess hexagonal pyramid, hexagonal shapes with open tips, nanobuds with open tips and nanoflower with a large number of hexagonal flakes, respectively.Furthermore, the photocatalytic activity of the ZnO nanostructure increased from 66% (prismatic tip) to highest activity 99% (nano-flower) and nano-buds exhibited second lowest activity (70%) during photo degradation of Alizarin Red-S dye under UVA irradiation.This might be explained as high concentration of oxygen vacancies present in ZnO nanoflower can trap the electrons, leading to the holes free to diffuse to the ZnO surface where oxidation of organic species may take place.Consequently, effectual content of oxygen vacancies will boost the photocatalytic process by separating the electron-hole pairs efficaciously.Thus, ZnO nanoflower with smallest particle size distribution was proved to be best green photo-catalyst among another nanostructure.Madan et al., (2016), employed green synthesis strategy for fabrication of ZnO NPs from neem (Azadirachta indica) extract as fuel [130].In this case, different surface morphologies of ZnO NPs such as Plates, Bullets, Flower, Prismatic tip, closed pine cone: with different concentrations (2, 4, 6, 10, 15, 20 ml) of neem plant extract in the reaction mixture were obtained fruitfully.

Different factors affecting the green synthesis of ZnO nanoparticles
Optimization of different parameters of synthesis is quite indispensable to ascertain the pre-eminent conditions of reaction for the synthesis of ZnO nanoparticles to intensify the superior production and come up with the foremost activity for ZnO.A minor disparity in the parameters during the synthesis process may upshot in various performances.

Effect of pH
The reaction mixture pH contributes a very crucial role in ZnO NPs via green synthesis strategy.Therefore, the synthesis of ZnO NPs by varying the pH of solution medium can be explained from two different aspect i.e. morphology and band gap energy.
Basically, the electrical charges of biomolecules present in biogenic sources can be changed by pH, which may cast important influence in the growing process of the nanoparticles along with their capabilities of reduction and capping [131] as with the increase in the pH of solution the higher amount of OH − ions are generated.As a result, OH − ions are highly attracted to the positively charged Zn 2+ ions, which helped in the formation of strong Zn-O bond.Subsequently, the size of nanoparticles are reduced at higher pH.Wang et al., (2008), showed that various ZnO morphologies like rods and flowers might be customized by modifying the basic nature of the solution.[132] In this investigation, 4 chloro phenol (4-CP), a recalcitrant pollutant found in wastewater was almost completely eliminated by ZnO nano-flowers synthesized at higher pH while ZnO nano rods synthesised at lower pH show 80% degradation of 4-CP after illuminated under UV light for 120 min.Hence, in a nutshell, it can be inferred that the solution pH in green synthesis is accomplished to regulate the size and shape of ZnO nanoparticles.Furthermore, the energy of band gap of ZnO rises with the increase in the pH values because of particle size is reduced at higher pH as mentioned in previous discussion.When the band gap is small, it accompanies with the formation of particles having larger size when on the contrary larger band gap concur with nanoparticles with smaller size.This happens owing to the confinement of electron which is known as the effect of quantum size [133].This is a phenomenon in which the electrons present in conduction band and holes in valence band are confined into a quantum dimension.This may enlarge the energy gap present in between these bands.With a decrement in the size of the nanoparticles, the energy levels come to be discrete on account of the smaller dimension of confinement.Subsequently, this expands the band gap between those bands followed by an increment in the band gap energy.In this context, Abdullah et al., (2020), obtained the band gap energy of 2.95, 3.00, 3.05, 3.08, and 3.14 eV for ZnO NPs biosynthesized from Musa acuminata peel extract at pH 8, 9, 10, 11, and 12 respectively [134].In this case, the evaluated crystallite size of ZnO was found to be 79.9 nm, 66.6 nm, 40.0 nm, 33.3 nm, and 30.7 nm for pH 8 to pH 12, respectively.This phenomenon can also be explained on the basis of quantum size effect.Herein, the photocatalytic experiment on methylene blue degradation was performed using all ZnO prepared at pH 8, 9, 10, 11 and 12.Under UV light, the efficiency of degradation at its lowest was shown by ZnO formed at pH 8 with 82.4%.On the other hand the h,ighest degradation efficiency value of 96.8% has been derived in case of ZnO NPs synthesized at pH 12.

Effect of temperature
Temperature is another crucial factor which influences the synthesis of ZnO nanoparticles through green route.The effect of temperature in the synthesis process of nanoparticle can be explained in terms of collision frequency of nanoparticles and stability of phytochemicals.The reaction medium temperature has a big influence on the nature of the ZnO nanoparticle formed.In general, nanoparticles having larger size are synthesized at higher temperature because agglomeration of nanoparticles occurs predominantly.At high temperature, the frequency of collision is higher owing to the high reaction kinetics.Hence, the propensity of collision and binding with each other by the atoms prevails to be higher generating particles larger in size.In additional temperature regulates the process of nucleation in nanoparticle preparation.The process of nucleation is slow when the temperature is low, hence nuclei require enough time to flourish into a explicit and specific structure and size [134].Secondly, the structure of phytochemicals present in bio-extract is deformed at higher temperature resulting poor stability.Moreover, this leads to poor reduction tendency of metal ions and uncontrolled as well as fast aggregation of nanoparticles.[135] Abdullah et al., 2020, showed the influence of temperature on green synthesis of ZnO NPs, which have been prepared from Musa acuminata peel extract [134].Hence, the synthesis temperature was altered from 50°C to 80°C.60°C has been found to be the optimum temperature, which could be accredited in the generation of particles with smaller size at this temperature.Again, the maximum degradation efficiency of 98.13% was obtained for ZnO NPs synthesized at 60°C during photodegradation of methylene blue under UV exposure.

Metal precursor concentration
Precursor concentration remarkably influences the formation of nanoparticles.In general, degradation efficiency becomes lower at lower concentration of precursor and gradually increases with increasing the precursor concentration upto a certain limit.On the other hand, the degradation efficiency decreases above this limit as the concentration of metal ions has been put up far off a threshold value, it holds back the depletion of the precursor material; hence, a decrement in the number of particles synthesized is observed.Besides, by increasing the precursor concentration, larger particle size would be produced due to higher availability of metal ions.This results in agglomeration and aggregation of nanoparticles due to high attraction between the atoms [134].Abdullah et al., (2020), investigated the influence of Zn 2+ precursor concentration ranging from 0.005 to 0.03 M on green synthesis of ZnO NPs utilizingMusa acuminata peel extract [134].Subsequently the photocatalytic degradation experiment has been executed for methylene blue dye in the presence of all ZnO photo-catalyst synthesized at different Zn 2+ concentration.Therefore, the optimum concentration of zinc acetate dehydrate has been observed to be 0.02 M, which exhibited the highest efficiency of degradation of 96.83% among the other.

Bio-extract concentration
Green synthesis of different nanoparticles is significantly affected by the alteration in the concentration of used bio-extract.The size as well as the morphology of the nano particles are regulated by changing the amount of bio-extract.The increase in the quantity of bio-extract above a particular range may result in the nanoparticle agglomeration.As a result, the no of active sites to adsorb the pollutant fall to be insufficient for efficacious degradation reaction, lowering the efficiency of the process.C.A. Soto-Robles et al., 2018, observed the influence of Hibiscus sabdariffa extract concentration on the synthesis of ZnO NPs through green route and tested their photocatalytic activity by employing methylene blue dye in the presence of UV light illumination [67].Herein, extracts of Hibiscus sabdariffa were prepared in the percentage of weight-volume such as 1%, 4% and 8%, in the aqueous medium and these samples have been indicated as ZnO-J1, ZnO-J4 and ZnO-J8 with crystallite size of 38.63, 9.05, and 8.71 nm, respectively.It has been perceived that the sample of ZnO-J8 showed the highest photocatalytic activity (95%), in comparison with the ZnO-J4 and ZnO-J1 samples (91% and 87%, respectively), through out a reaction time of 90 minutes.It can be accredited to the fact of the average particle size and the availability of sufficient phytochemicals required for formation of ZnO NPs.But the degradation efficiency gradually dropped above this concentration of Hibiscus sabdariffa extract [136].Therefore, the ZnO nanostructures synthesized from 0.5 ml of pomegranate juice showed highest methylene blue degradation efficiency of 95% after 60 min reaction.Hence, the degradation efficiency decreases with increasing the pomegranate juice concentration.Basically, the occupancy of 0.5 mL of pomegranate juice, ZnO NPs chiefly comprises of nanoparticles which are homogenous in nature covered by structures as nanorods.Increasing the molar ratio of pomegranate juice to 1 and 1.5 mL has been followed by the agglomeration of nanoparticles.Further increase in the molar ratio of pomegranate juice (1.5-2 mL) aggravated the agglomeration of nanorods more.Similarly, Sharma, S.C., (2016), demonstrated the morphological and size variation of ZnO NPs by varying the carica papa milk (CPM) extract [77].At 10, 20, 25, 30 ml of CPM extract as-synthesized ZnO NPs possess hexagonal pyramid, hexagonal shapes with open tips, nanobuds with open tips and nanoflower having large number of hexagonal flakes respectively.Again, by the same token, the photocatalytic activity of the ZnO nanoparticles got raised from 66% (prismatic tip) to its highest activity as 99% (nano-flower) and nano-buds showed the second lowest activity (70%) in the process of photo degradation of Alizarin Red-S dye under UV irradiation.Thus, ZnO nanoflower with smallest particle size distribution was proved to be best green photo-catalyst among other nanostructures.

Conclusion
This critical review provides a clean, non-toxic, relatively cost-effective and environmentally benign green chemistry approaches for synthesis of ZnO NPs from different biogenic sources.ZnO nanomaterial during last decade have gained demand as a propitious substance to sustainably reveal photocatalytic activities.In this review, different biogenic sources e.g.plant, microorganism, waste material and the underlying rationale behind their development have been discussed.The important phytochemicals present in those biogenic sources acts as the reducing, capping and also as a stabilizing factor during the synthesis of ZnO NPs.General method for derivation of those phytochemicals along with a probable mechanism of ZnO NPs formation has been elucidated with relevant schemes.The generation of strong hydroxyl radicals (•OH) in adequate proportion is the driving force for the oxidization of those intractable noxious organic pollutants available in the concerning wastewater.We also have executed the ideas on the intrinsic characteristics such as Band Gap Energy, Surface defects, surface morphology of ZnO, which improve the potentiality of ZnO as a photocatalyst.Numerous researchers have devoted to the research work regarding photocatalytic degradation of hazardous water pollutants by employing green synthesized ZnO.Therefore, some literature reports highlighting the ameliorated degradation efficiency of ZnO during photocatalytic degradation of water pollutants has been discussed in this review.The feasibility of ZnO NPs to behave as potential photocatalyst is regulated by few of the external factors like the concentrations of bio-extract and precursor temperature and pH which are discussed here.Hence, the green route for the production of ZnO NPs and its application for photocatalytic degradation of water pollutants is the promising and emerging research arena.

Future prospectus
Despite the significant progress as well as increased use which is made so far in the development of ZnO nanomaterial, its progress in research still suffers from some hindrance.There is a research gap concerning the green synthesis of ZnO NPs using microorganism, animal-based waste source as only plant-based source has been hugely focused.From minimization of environmental pollution, renewable resources and economical perspective bio waste sources should be emphasized hugely for synthesis purpose of ZnO NPs.Aside from these limitations, there is also the paucity of a comprehensive understanding on the meticulous mechanisms for ZnO NPs formation.Most of the reports regarding mechanisms for ZnO NPs formation is solely based on hypothesis and no direct evidence is not available till date.Advanced equipment should be evolved also to explicate the inclusive mechanism of reaction and precise reaction pathways system to solve the purpose.Last but not least, green synthesis of ZnO nanoparticle and its application towards water remediation should be implemented in laboratory scale as well as in industrial scale.

Figure 1 .
Figure 1.Schematic diagram of different sources for synthesis of ZnO nanoparticles.

Figure 2 .
Figure 2. Suggested mechanism for the evolution of ZnO nanoparticles.