Facile Green Synthesis of ZnO NPs and Plasmonic Ag-Supported ZnO Nanocomposite for Photocatalytic Degradation of Methylene Blue

: Removing organic pollutants, textile dyes, and pharmaceutical wastes from the water bodies has become an essential requirement for a safe environment. Therefore, the present study aimed to prepare semiconductor zinc oxide nanoparticles (ZnO NPs) and plasmonic Ag-supported ZnO nanocomposite (ZnO–Ag) using an environmentally friendly bio-approach as an alternative to hazardous synthesis approaches. ZnO NPs and ZnO–Ag nanocomposite were characterized by using UV–Vis diffuse reﬂectance spectroscopy (UV–DRS) (the Ag-supported ZnO nanocomposite exhibited an absorption band between 450–550 nm, attributed to the Ag NPs surface plasmon resonance (SPR)), Photoluminescence (PL) spectral investigation, which revealed the PL emission intensity of ZnO–Ag NPs was lower than pure ZnO NPs, describing an extended electron-hole pair (e–h+) lifespan of photogenerated charge carriers, Fourier transform infrared spectroscopy (FTIR), FT-Raman, and X-ray diffraction (XRD) analyses were deduced. In addition, energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) were performed and further ascertained the successful biosynthesis and thermally stable ZnO Nps and ZnO–Ag nanocomposite. The as-prepared ZnO–Ag nanocomposite displayed increased photocatalytic characteristics due to the decline in the bandgap energy from 3.02 eV (ZnO NPs) to 2.90 eV (ZnO–Ag nanocomposite). The photocatalytic activity of the developed nanocomposite for the degradation of methylene blue (MB) dye, a primary textile industry released water-pollutant, was conducted under UV light irradiation. Meanwhile, the maximum % degradation of MB dye molecules was attained by 98.0 % after 60 min exposure of UV-light irradiation. Increased photocatalytic activity of ZnO–Ag nanocomposites and a faster rate of MB degradation were achieved by the deposition of plasmonic Ag NPs and the surface plasmon resonance (SPR) effect possessed by Ag NPs. The primary oxidative route that resulted in MB degradation was the production of hydroxyl radicals (OH • ). The SPR effect of the photocatalyst induced the synergistic enhancement of the optical response and separation of the photo-induced charge carriers. The combined study gives comprehensive information and directions for future research on noble metal-modiﬁed nanocatalysts for direct applications in the photocatalytic degradation of textile and organic wastes in water.


Introduction
Metal nanocomposites or metal oxide nanocomposites are a major part of nanotechnology that have extraordinary properties owing to their nanoscale size relative to their surface volume [1].Metal nanocomposites stimulate extensive scientific research to discover novel optical, surface plasmon resonance, electrical, magnetic, and catalytic properties [2].These nanocomposites play an essential role in developing various applications such as medicine [3][4][5], sensors [6][7][8], renewable energies [9,10], cosmetology [11][12][13], catalysis [14,15], optoelectronics [16,17], pharmacology [18][19][20], control of environmental pollution [21], drug delivery systems [22], surface disinfection, antimicrobial applications [23][24][25], and many others that all benefit human life [26].Numerous material properties can be altered by the addition of materials with complementary qualities [27].Mixed metal oxides are attractive due to their targeted action [28].They provide a facility for fine-tuning the properties of individual components and show a great potential to be used in the fields of photocatalysis, to increase the efficacy of antimicrobial drugs, and to reveal of new capabilities [29].Among metal oxide nanocomposites, ZnO-Ag NPs have attracted attention because each component in the composite offers a unique feature with the target of achieving new bifunctional or synergistic effects [30].ZnO is an excellent and inexpensive 3D support for heterogeneous catalysis because it has higher charge carrier mobility and a longer electron life than other metal oxides, making it more reliable for mild catalytic and antibacterial behavior.Moreover, adding Ag NPs can dramatically boost photocatalytic activity by extending the lifespan of electron-hole pairs and gaining a longer retention time [31].
Among the functional oxides, the n-type ZnO semiconductor NPs retain a large bandgap with enhanced exciton binding energy of 60 meV.Moreover, based on their low toxicity profile and high chemical stability, ZnO NPs are suitable candidates for UV screening applications.The observed wide bandgap of 3.37 eV at 300 K of ZnO NPs is found to limit photodegradation reactions under normal conditions because of the rapid electron and photo-excited hole pair recombination in the presence of visible light radiations.On this premise, several studies have focused on tailoring ZnO structure and morphology to solve this problem and meet future energy demands [32].Nowadays, academic and industrial scientists have a zeal for achieving semiconductor-assisted dopant nanophotocatalysts [33][34][35][36][37].In general, the primary focus of research related to semiconductor dopant nanocatalysts is on their potential for electron transfer in visible light with efficient degradation of recalcitrant chemicals in their aqueous media [38][39][40][41].ZnO as a nanocatalyst has been extensively scrutinized among semiconductor-doped photocatalysts because of its high surface area and optimum photosensitivity [42][43][44][45].However, it has been reported that ZnO doping with a noble metal such as Ag or Au limits the electron-hole pair recombination [46].Thus, the formation of the Schottky junction was apparent in the nano-assembly of ZnO with Ag/Au.Such metal-doped catalysts are also referred to as plasmonic photocatalysts, where silver (Ag) as a dopant has gained much attention [47,48].However, doping of Ag accelerates the surface plasmon resonance (SPR) effect under visible light radiations [49,50].The increased SPR effect accentuates the uniform dispersion and adsorption of Ag NPs onto the surface of p-type ZnO [51].In addition, many potential procedures have recently been adopted to develop tri-metallic nano-hybrid materials.As such, to yield hybrid ZnO-Ag NPs, methods including sol-gel [52], co-precipitation [53], hydrothermal [54], solvothermal [55], photoreduction [56], and microwave-assisted solution combustion are encompassed [57].Biosynthesis has received the most attention from these methods, owing to sustainable biocompatible solutions for renewable energy and environmental rehabilitation [45].In addition, using a surfactant mediating route, ZnO-Au and ZnO-Ag nanocomposites were developed and were utilized as catalysts in remediation of dye molecules [42].In the present study, we used a green synthetic approach, as this method is biocompatible with low toxicity, easy to reproduce high yield of NPs, economical, and biodegradable, while using phytochemicals as a source of reducing and capping agents [46,47].This method optimizes resources and transforms phytochemicals (reduc-ing/stabilizing agents) for the biosynthesis of dopant metal oxide photocatalysts.Hence, green synthetically yielded ZnO-Ag NPs can be used more effectively as nanocatalysts if they are morphologically tuned [32,58].Meanwhile, the morphology of nanocomposites has an important role in photo degradation because non-spherical nanocomposites possess enhanced physical and chemical properties thus inheriting lower values of dielectric constant, higher transmittance, and enhanced catalytic activity and possessing good antibacterial efficiency behind the changing crystalline structure, surface electronic structure, and morphology [59,60].Moreover, the exceptional features of non-spherical nanocomposites, such as anisotropic degradation, along and spatially tuned particle morphology, entertain them to achieve improved charge carrier transport, generation, and combined reactions and too are responsible for the efficient photodegradation upon electron-hole pair recombination [61,62].
It is worth mentioning, here, that the semiconductor metal oxides are being widely used because of their unique chemical and physical properties.In general, the main three features observed behind their excellent photocatalyst activity are rapid electronic transitions, enhanced separation between photo-induced reaction species, and the establishment of an easy carrier transport throughout a photo-induced reaction [63].However, the accomplished semiconductor metal oxides having wider bandgaps did not acquire the optimum photocatalytic efficiency [64,65].Moreover, both academic and industrial researchers are using various strategies to further enhance the photocatalytic activities of such semiconductor metal oxides.These strategies are understood to developed composite formation, metal-metal oxide heterojunctions, doping, co-doping, and metal substitution.These nano-assemblies are applied to attain a suitable bandgap to enhance the photocatalytic degradation efficacies behind the suppression of charge recombination entities.In view of practical application, the use of ZnO NPs as a catalyst in the photodegradation of organic dyes under UV light irradiation has been confronted by two significant challenges: (i) limited spectrum range at λ max < 387 nm; and (ii) the recombination of electron-hole in a photodegradation process at comparatively lower quantum yield [56,66].The present study incorporates the biocompatible green synthetic approach to achieve ZnO-Ag NPs.During this study, we believed that the obtained green synthesized ZnO-Ag hybrid NPs could enhance photocatalytic degradation due to the synergetic effect between Ag and ZnO on the ZnO-Ag surface.In general, green synthesis being biocompatible approach has various positive aspects in comparison with conventional physical and chemical methods.Such nanocomposites are easy to reproduce, eco-friendly and biodegradable in nature, and relatively economical; nanocomposites are preferred to toxic chemicals which are expensive to produce.However, the plasmon resonance accelerates the production rate of photogenerated charge carriers; it prevents the rapid recombination of photogenerated excitons, leading to the enhanced generation of hydroxyl radicals (-OH) on the ZnO-Ag surface [67].This results in the enhanced photodegradation of MB molecules in the visible light spectrum.Thus, in terms of interpretations from structural and spectral properties, including spectral characterizations, doping Ag to ZnO NPs leads to enhanced photocatalysis which attributes to two distinct factors: (i) the transfer of plasmon-induced electrons from Ag to ZnO NPs facilitates the reduction of oxygen by CB electrons and bypasses imitating the steps of the degradation process; (ii) the SPR of Ag NPs significantly increases the electric field at the Ag and ZnO interface regions, thereby increasing the rate of photogenerated charge carriers and improving the overall photo-decomposition performance of ZnO NPs [68].These nanoparticles are proven to possess less cytotoxicity and high photocatalytic activity in degrading organic dye under natural sunlight [45,69,70].In the present work, we report a facile, eco-friendly, and green synthetic method for preparing ZnO and ZnO-Ag NPs using Carthamus tinctorius L. (Safflower) extract.

Chemicals and Reagents
Metal precursor salts of zinc nitrate hexahydrate ([Zn(NO 3 ) 2 .6H 2 O] with vendor molecular purity of 99.0%), silver nitrate (AgNO 3 of 99.0% purity), and methylene blue (C 16 H 18 ClN 3 S with vendor-mentioned purity of 99.0%) were purchased from Sigma Aldrich, MO, USA.In addition, other highly pure chemicals and reagents used in the present study were also purchased from Sigma Aldrich, MO, USA.

Preparation of ZnO and Plasmonic Ag-Supported ZnO Nanocomposite
The collected plant leaves of Carthamus tinctorius L. with tiny soil particles and dust adhered to their surfaces were washed thoroughly with deionized water.Then the leaves were sun-dried, ground, and pulverized to a fine powder.After that, ground 20 g of the dried leaf powder was mixed in 250 mL of distilled water and the mixture was boiled between 90 and 100 • C for 20 min to obtain an aqueous plant extract.The reaction mixture was filtered and the filtrate as an extract of C. tinctorius was consumed in the biogenesis of Ag-supported ZnO nanocomposite.In this typical synthetic process, we mixed 50 mL of 0.5 M zinc nitrate solution with 50 mL of the extract under continuous stirring at 60 • C for 4 h [54,55].The color change and the precipitation were monitored, and the completion of the reaction was attained until a complete yellowish precipitate was formed.The reaction mixture was cooled at room temperature, and the acquired precipitate was centrifuged at 4000 rpm for 30 min.The developed material was washed with ethanol and deionized water, dried at 120 • C for 6 h, and finally calcined at 500 • C for 4 h to acquire the material in powder form as ZnO NPs.For the preparation of Ag-supported ZnO nanocomposite, 5 g of as-prepared ZnO NPs were dispersed in deionized water, and for complete dispersion, ultrasonication was operated for 5 min.To this solution, 50 mL (0.1 M) of silver nitrate solution was added under continuous stirring for 5 min.Freshly prepared 50 mL of plant extract was added under constant stirring at 50 • C to the silver nitrate solution.The extract solution acts as a reduction agent to reduce Ag + to Ag 0 within 30 min [56].The formation of a stable ZnO-Ag nanocomposite is deduced from the color change from colorless to dark brown with the adsorption of Ag onto the surface of ZnO NPs [57].Besides, the plant extract plays the role of stabilizing agent in Ag-supported ZnO nanocomposite.ZnO and ZnO-Ag NPs were consecutively washed, recrystallized, and dried before characterization.Further, the synthetic procedures are systematically proposed to prepare ZnO NPs and ZnO-Ag nanocomposite, as depicted in Figure 1.

Characterization
The as-prepared ZnO NPs and Ag-supported ZnO nanocomposite were characterized for crystal structure information by recording the X-ray diffraction (XRD) patterns on the diffractometer (D8, Advance, Bruker, Germany) using the CuKα (1.542Å) beam as the radiation source in the range of 20-80 (2θ diffraction angle).The active role of functional groups for the reduction and surface capping of ZnO and Ag-supported ZnO nanocomposite was measured by Fourier transform infrared (FTIR) spectroscopy using a Bruker Alpha spectrometer in the range of 4000-400 cm −1 .The surface morphology,

Characterization
The as-prepared ZnO NPs and Ag-supported ZnO nanocomposite were characterized for crystal structure information by recording the X-ray diffraction (XRD) patterns on the diffractometer (D8, Advance, Bruker, Germany) using the CuKα (1.542Å) beam as the radiation source in the range of 20-80 (2θ diffraction angle).The active role of functional groups for the reduction and surface capping of ZnO and Ag-supported ZnO nanocomposite was measured by Fourier transform infrared (FTIR) spectroscopy using a Bruker Alpha spectrometer in the range of 4000-400 cm −1 .The surface morphology, size, and elemental composition of ZnO and Ag-supported ZnO nanocomposite were measured by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energydispersive X-ray spectroscopy (EDX).The optical properties of ZnO and Ag-supported ZnO nanocomposite were comprehended by UV-Visible diffuse reflectance spectra (UV-DRS) and photo-luminance (PL) measurements using a Perkin Elmer spectrophotometer and Xe lamp utility at 330 nm (excitation wavelength), respectively.The thermal stability of ZnO and Ag-ZnO NPs in the temperature range between 25-800 • C was performed by thermogravimetry (TGA, Perkin Elmer).The Raman scattering measurements of ZnO NPs and ZnO-Ag nanocomposite were carried out on a Raman microscope in the spectral range of 50 to 1200 cm −1 at room temperature.The surface area, pore size, and pore volume of the ZnO NPs and Ag-supported ZnO nanocomposite were measured on Quantachrome (autosorb in-MP/XR).

Photocatalytic Experiments
The photocatalytic performance of ZnO NPs and Ag-supported ZnO nanocomposite was performed against cationic toxic dye under ultraviolet radiation as a light source using 400 W (UV lamp).The experiment was set up in a photochemical reactor, using 30 mg of biogenic nanoparticles dissolved to 50 mL each of 10 ppm methylene blue (MB) inside a quartz reaction vessel installed with the outer cooling jacket to attain a constant temperature.Meanwhile, before the photodegradation procedure, the reaction mixture was stirred for 30 min in the dark to attain an equilibrium solution inside the photochemical reactor (Figure 2).Moreover, in the reaction mixture solution, the pH was adjusted either by adding 0.050 M NaOH or 0.050 M H 2 SO 4 .A known volume (3 mL) of the reaction mixture was withdrawn at specific time intervals (10 min), followed by centrifugation to separate the photocatalyst from the solution and, at the same time, absorption of the dye solution was measured using a spectrophotometer in the wavelength range of 200-800 nm.In addition, different experimental parameters were optimized for different experiments, such as concentration of dye, irradiation time, pH of the reaction mixture, reusability performance of the photocatalyst, and catalytic load.The photocatalytic degradation of dye was determined by the following Equation (1): where, C 0 and C t , are referred to as the initial concentration and final concentration of MB after adding the photocatalyst.
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Photocatalytic Experiments
The photocatalytic performance of ZnO NPs and Ag-supported ZnO nanocomposite was performed against cationic toxic dye under ultraviolet radiation as a light source using 400 W (UV lamp).The experiment was set up in a photochemical reactor, using 30 mg of biogenic nanoparticles dissolved to 50 mL each of 10 ppm methylene blue (MB) inside a quartz reaction vessel installed with the outer cooling jacket to attain a constant temperature.Meanwhile, before the photodegradation procedure, the reaction mixture was stirred for 30 min in the dark to attain an equilibrium solution inside the photochemical reactor (Figure 2).Moreover, in the reaction mixture solution, the pH was adjusted either by adding 0.050 M NaOH or 0.050 M H2SO4.A known volume (3 mL) of the reaction mixture was withdrawn at specific time intervals (10 min), followed by centrifugation to separate the photocatalyst from the solution and, at the same time, absorption of the dye solution was measured using a spectrophotometer in the wavelength range of 200-800 nm.In addition, different experimental parameters were optimized for different experiments, such as concentration of dye, irradiation time, pH of the reaction mixture, reusability performance of the photocatalyst, and catalytic load.The photocatalytic degradation of dye was determined by the following Equation (1): % deg 100 where, C0 and Ct, are referred to as the initial concentration and final concentration of MB after adding the photocatalyst.

Spectroscopic Analysis
Safflower (C.tinctorius) has several medicinal properties because of its rich bioactive phytochemical composition which includes flavonoids, alkaloids, phenolic acids, and many natural pigments [71,72].The highly water-soluble glycosylated flavonoids, including hydroxy-safflower yellow A, safflower yellow B, and anhydrous-safflower B, are the most prominent pigments in dried flower petals [73,74].Moreover, the most prime phytochemicals present in C. tinctorius extract along their chemical structures are depicted in Figure 3.

Spectroscopic Analysis
Safflower (C.tinctorius) has several medicinal properties because of its rich bioactive phytochemical composition which includes flavonoids, alkaloids, phenolic acids, and many natural pigments [71,72].The highly water-soluble glycosylated flavonoids, including hydroxy-safflower yellow A, safflower yellow B, and anhydrous-safflower B, are the most prominent pigments in dried flower petals [73,74].Moreover, the most prime phytochemicals present in C. tinctorius extract along their chemical structures are depicted in Figure 3.The UV-DRS experiments were performed to investigate light absorption characteristics and the Ag-doping effect on optical properties, including bandgap energy of ZnO NPs and Ag-ZnO NPs [62].ZnO is a wide-bandgap semiconductor with good activity in the ultraviolet (UV) region, which limits its photocatalytic performance.The support of metallic nanocrystals including silver, platinum, and gold on the surface of ZnO enhances the optical properties and exhibits enhanced photocatalytic performance in the visible range [63].The UV-DRS spectra of ZnO NPs exhibited a prominent absorption peak at 360 nm in the UV region (Figure 3a), (Figure 4a), and the reflectance spectra exhibited a sharp increase at a wavelength of 375 nm (Figure 3b) (Figure 4b) which related to the direct bandgap of ZnO NPs due to the electron transition from the valance band (VB) to the conduction band (CB) [ [75].Ag-supported ZnO nanocomposite exhibited an absorption band between 450-550 nm that attributed to the Ag NPs surface plasmon resonance (SPR) [76], displayed enhanced visible light absorption, and reduced the loss of light energy due to the strong light scattering induced by interaction of incident light and Ag NPs on the surface of the ZnO NPs [66,67].The observed UV-DRS results indicated a slight redshift in ZnO-Ag NP in comparison to pure ZnO NPs ascertained more energy harvest and possessed versatile applicability in photocatalysis.The UV-DRS experiments were performed to investigate light absorption characteristics and the Ag-doping effect on optical properties, including bandgap energy of ZnO NPs and Ag-ZnO NPs [62].ZnO is a wide-bandgap semiconductor with good activity in the ultraviolet (UV) region, which limits its photocatalytic performance.The support of metallic nanocrystals including silver, platinum, and gold on the surface of ZnO enhances the optical properties and exhibits enhanced photocatalytic performance in the visible range [63].The UV-DRS spectra of ZnO NPs exhibited a prominent absorption peak at 360 nm in the UV region (Figure 4a), and the reflectance spectra exhibited a sharp increase at a wavelength of 375 nm (Figure 4b) which related to the direct bandgap of ZnO NPs due to the electron transition from the valance band (VB) to the conduction band (CB) [75].Ag-supported ZnO nanocomposite exhibited an absorption band between 450-550 nm that attributed to the Ag NPs surface plasmon resonance (SPR) [76], displayed enhanced visible light absorption, and reduced the loss of light energy due to the strong light scattering induced by interaction of incident light and Ag NPs on the surface of the ZnO NPs [66,67].The observed UV-DRS results indicated a slight redshift in ZnO-Ag NP in comparison to pure ZnO NPs ascertained more energy harvest and possessed versatile applicability in photocatalysis.Moreover, the Ag-supported ZnO nanocomposite showed a typical broad absorption after the bandgap transition in the visible region caused by the SPR characteristics of silver, thus confirming the successful deposition of Ag NPs on the surface of ZnO NPs [77].This is owing to the strong interaction between electrons in the d-orbital of Ag and s-and p-orbitals of the host ZnO.In pure ZnO, the renormalization effect is induced by the exchange of sp-d band electrons of ZnO with localized d-electrons of Ag+ [78].This further confirmed that the ZnO-Ag NPs have significant applications in photocatalysis due to the efficient utilization of visible light.Figure 4c shows the bandgap of 3.0 eV and 2.93 eV for ZnO NPs and ZnO-Ag NPs, respectively.The results showed that Ag-doping on ZnO NPs shifted the bandgap of ZnO from 3.02 eV to 2.90 eV which may be attributed to the availability of oxygen vacancies that may easily move electrons from the VB to CB [79].
the exchange of sp-d band electrons of ZnO with localized d-electrons of Ag+ [78].This further confirmed that the ZnO-Ag NPs have significant applications in photocatalysis due to the efficient utilization of visible light.Figure 4c shows the bandgap of 3.0 eV and 2.93 eV for ZnO NPs and ZnO-Ag NPs, respectively.The results showed that Ag-doping on ZnO NPs shifted the bandgap of ZnO from 3.02 eV to 2.90 eV which may be attributed to the availability of oxygen vacancies that may easily move electrons from the VB to CB [79].
Room temperature photo-luminance (PL) spectral analysis was deduced after the migration of charge carriers to interpret the possible recombination efficiency of ZnO NPs and ZnO-Ag NPs.The photocatalytic performance efficiency is closely related to the photo-luminance intensity and the recombination rate of photogenerated electrons and holes (e --h + ) and inversely related to the photocatalytic efficiency of semi-conduction nanomaterials.Figure 4d describes the PL spectra of ZnO NPs and ZnO-Ag NPs at room temperature at an excitation wavelength (λexc) of 350 nm.The PL emission intensity of ZnO-Ag NPs was lower than pure ZnO NPs, suggesting the phenomenon of Stern-Volmer quenching and confirming that the Ag-doping quenched the fluorescence from ZnO NPs and extended the electron-hole pair (e --h + ) lifespan of photogenerated charge carriers [80].However, increased PL emission effects upon doping Ag NPs onto the surface of ZnO NPs were favorable to charge separation and inhibition of electron-hole recombination.In the presence of visible light, the inhibition of electron-hole combination was felt by anchoring Ag NPs to the surface of ZnO, thereby enhancing the PL emissions.Room temperature photo-luminance (PL) spectral analysis was deduced after the migration of charge carriers to interpret the possible recombination efficiency of ZnO NPs and ZnO-Ag NPs.The photocatalytic performance efficiency is closely related to the photo-luminance intensity and the recombination rate of photogenerated electrons and holes (e --h + ) and inversely related to the photocatalytic efficiency of semi-conduction nanomaterials.Figure 4d describes the PL spectra of ZnO NPs and ZnO-Ag NPs at room temperature at an excitation wavelength (λ exc ) of 350 nm.The PL emission intensity of ZnO-Ag NPs was lower than pure ZnO NPs, suggesting the phenomenon of Stern-Volmer quenching and confirming that the Ag-doping quenched the fluorescence from ZnO NPs and extended the electron-hole pair (e --h + ) lifespan of photogenerated charge carriers [80].However, increased PL emission effects upon doping Ag NPs onto the surface of ZnO NPs were favorable to charge separation and inhibition of electron-hole recombination.In the presence of visible light, the inhibition of electron-hole combination was felt by anchoring Ag NPs to the surface of ZnO, thereby enhancing the PL emissions.

Characterization of ZnO NPs and ZnO-Ag Nanocomposite
FTIR spectroscopy is a steadfast and sensitive spectroscopic technique for determining and detecting significant functional groups of biomolecules on the surface of the synthesized material.The presence of phytochemicals is deduced upon the presence of respective functional groups involved after biosynthesis of ZnO NPs and Ag-supported ZnO nanocomposite from an aqueous extract of C. tinctorius.The FTIR spectral analysis was accomplished as shown in Figure 5.The major peak intensities of C. tinctorius extract were observed at 3373.3 cm −1 , 2931.4 cm −1 , 2888.4 cm −1 , 2348.2 cm −1 , 1732.6 cm −1 , 1649.1 cm −1 , 1444.7 cm −1 , 1361.2 cm −1 , 995.4 cm −1 , 929.7 cm −1 , 712.4 cm −1 , and 578.6 cm −1 , as shown in Figure 5.The appearance of such functional group peaks is due to the presence of a rich source of phytochemical composition in C. tinctorius extract.Moreover, the observed peak intensity at 3373.3 cm −1 was attributed to the presence of phenolic (-OH) stretching vibration.The peak intensity at 2931.4 cm −1 corresponded to (-C-H) stretching vibration due to alkanes.A comparatively lower intensity peak at 2888.4 cm −1 was assigned to (=C-H) stretching vibration modes.In addition, a peak at 2348.2 cm −1 was the corresponding aldehyde peak due to C= stretching vibration.Moreover, the attained peak at 1732.6 cm −1 behind carbonyl (C=O) stretching vibrations (1649.1 cm −1 and 1361.2 cm −1 ) was attributed to N-H bending vibrations in amines and due to carbon-nitrogen stretching vibrations (-C-N) of amides, respectively.However, the peak at 1444.7 cm −1 was associated with (C=C) aromatic ring vibrations.
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Characterization of ZnO NPs and ZnO-Ag Nanocomposite
FTIR spectroscopy is a steadfast and sensitive spectroscopic technique for determining and detecting significant functional groups of biomolecules on the surface of the synthesized material.The presence of phytochemicals is deduced upon the presence of respective functional groups involved after biosynthesis of ZnO NPs and Ag-supported ZnO nanocomposite from an aqueous extract of C. tinctorius.The FTIR spectral analysis was accomplished as shown in Figure 5.The major peak intensities of C. tinctorius extract were observed at 3373.3 cm −1 , 2931.4 cm −1 , 2888.4 cm −1 , 2348.2 cm −1 , 1732.6 cm −1 , 1649.1 cm −1 , 1444.7 cm −1 , 1361.2 cm −1 , 995.4 cm −1 , 929.7 cm −1 , 712.4 cm −1 , and 578.6 cm −1 , as shown in Figure 5.The appearance of such functional group peaks is due to the presence of a rich source of phytochemical composition in C. tinctorius extract.Moreover, the observed peak intensity at 3373.3 cm −1 was attributed to the presence of phenolic (-OH) stretching vibration.The peak intensity at 2931.4 cm −1 corresponded to (-C-H) stretching vibration due to alkanes.A comparatively lower intensity peak at 2888.4 cm −1 was assigned to (=C-H) stretching vibration modes.In addition, a peak at 2348.2 cm −1 was the corresponding aldehyde peak due to C= stretching vibration.Moreover, the attained peak at 1732.6 cm −1 behind carbonyl (C=O) stretching vibrations (1649.1 cm −1 and 1361.2 cm −1 ) was attributed to N-H bending vibrations in amines and due to carbon-nitrogen stretching vibrations (-C-N) of amides, respectively.However, the peak at 1444.7 cm −1 was associated with (C=C) aromatic ring vibrations.
Meanwhile, the observed peaks at 995.4 cm −1 and 929.7 cm −1 were assigned to the presence of polyphenols.Peak intensity at 712.4 cm −1 was believed after the stretching vibration of C-Cl bonds.The peak intensity at 578.6 cm −1 was attributed to the presence of bond vibrations of C-H and C-N.The observed peaks of different phytochemicals further ascertained their role as reducing and stabilizing agents in the biosynthesis of ZnO NPs and ZnO-Ag nanocomposite.Moreover, the observed shift of peak intensities in ZnO NPs and ZnO-Ag nanocomposite towards lower (blue) and higher (red) wavelengths, respectively, further assured the biosynthesis of both ZnO NPs and ZnO-Ag nanocomposite.Moreover, the stretching modes corresponding to the metal-oxygen bond observed behind broad absorption bands of 400-600 cm −1 further determined the formation of Zn-O NPs and Ag-ZnO ZnO-Ag nanocomposite.Raman spectroscopy is an indispensable method for identifying structural defects and crystal structure perfection.To further investigate the ZnO NPs and Ag-supported ZnO nanocomposite, Raman spectral analysis was performed at room temperature, as shown in Figure 5b.The peaks observed at 97 cm −1 and 429 cm −1 corresponded to E2(low) further assured the biosynthesis of both ZnO NPs and ZnO-Ag nanocomposite.Moreover, the stretching modes corresponding to the metal-oxygen bond observed behind broad absorption bands of 400-600 cm −1 further determined the formation of Zn-O NPs and ZnO-Ag nanocomposite.
Raman spectroscopy is an indispensable method for identifying structural defects and crystal structure perfection.To further investigate the ZnO NPs and Ag-supported ZnO nanocomposite, Raman spectral analysis was performed at room temperature, as shown in Figure 5b.The peaks observed at 97 cm −1 and 429 cm −1 corresponded to E 2 (low) and E 2 (high) fundamental phonon modes of ZnO with hexagonal wurtzite structure.These two phonon modes E 2 (low) and E 2 (high) of Raman spectra are characteristics of perfect ZnO crystals and these modes broaden Ag-support on the ZnO NPs [4,18,72].Concurrently, the intensity of E 2 (low) and E 2 (high) phonon modes is strongly related to the ZnO crystal quality [4,18,72].On Ag doping on the surface of ZnO NPs, the intensity of E 2 (low) and E 2 (high) phonon modes increased, which signifies that Ag doping improves the crystal quality of ZnO NPs.The peaks at 576 cm −1 and 377 cm −1 corresponded to A 1 (LO) and A 1 (TO), respectively, and showed peak intensity stimulation on Ag-doping over the surface of ZnO NP.The peak assigned to A 1 (LO) phonon mode initiates due to the presence of oxygen vacancies and Zn interstitial in ZnO NPs [4,18,72].This result is mediated via the defects caused by Ag segregation and Ag deposition at the crystallite interface.The peak at 1132 cm −1 corresponding to the multiple phonon process of E 1 (LO) originates due to the lattice vibration.For ZnO NPs and ZnO-Ag nanocomposite, the peak that appeared at 326 cm −1 was assigned to E 2 (high)-E 2 (low) multiphonon scattering.The peaks at 241 cm −1 and 396 cm −1 corresponding to LVM(Zn-Ag) and LVM(Ag-O) were clear in ZnO-Ag nanocomposite due to the sufficient doping of Ag on the surface of ZnO NPs.
The powder XRD analysis was performed to deduce the crystallinity, phase, crystallite size, and composition of both ZnO NPs and Ag-ZnO nanocomposite.The observed XRD patterns of ZnO and ZnO-Ag nanocomposites are shown in Figure 6.In ZnO NPs at an angle (2θ), the observed diffraction peaks at 31.83  101), ( 102), ( 110), ( 103), (200), ( 112), (201), (004), and (202), respectively, attributing to the hexagonal wurtzite structure in accordance to ZnO (JCPDS card no.36-1451) [81].The purity of obtained crystal was further established as no other impurity peaks were detected in XRD spectra.Powder XRD analysis of ZnO-Ag nanocomposites was also performed to identify the phase change and confirm the deposition of the Ag NPs on the surface of ZnO NPs.The XRD pattern of ZnO-Ag nanocomposites depicted in Figure 6 shows additional peaks at a diffraction angle (2θ), i.e., 38.16 • , 44.33 • , and 64.51 • with corresponding lattice planes at (111), (200), and (220), respectively, attributing to the presence of Ag NPs on the surface of ZnO NPs [82].The intense peaks of Ag NPs confirm the significant growth of Ag NPs on the surface of ZnO NPs.The superb crystal growth with well-preserved wurtzite phase ZnO NP was confirmed by the lack of peaks in the XRD data corresponding to any impurities.The Rietveld XRD patterns of ZnO-Ag nanocomposite showed the presence of ZnO NPs (46%) in the hexagonal crystal system and Ag NPs (54%) in the cubic crystal structure.The XRD analysis after the deposition of Ag NPs on ZnO NPs shows no change in the wurtzite phase ZnO NP and confirmed the formation of ZnO-Ag nanocomposites.The crystallite size of the ZnO NPs and ZnO-Ag nanocomposites was estimated using Debye Scherrer's equation [64,83]: where D is the crystallite size, λ is X-ray wavelength and equal to λ = 1.5406Å, β is full width at half maximum of peak, and θ is diffraction angle.The surface morphology and elemental composition of ZnO NPs and ZnO-Ag nanocomposite were investigated using SEM and EDX analysis, respectively, as shown in Figure 7a,d.The SEM images showed agglomerated spherical mushrooms-like masses in ZnO NPs; however, little decrease in agglomeration was observed on Ag deposition on the surface of ZnO NPs.The results from this Figure 7c,d confirmed the presence of the elements carbon (C), oxygen (O), and zinc (Zn), with peaks appearing approximately at 0.02 keV for C and 0.5 keV for O, respectively; whereas Zn appears at 1 keV, 8.6 keV, and 9.5 keV, with Ag element appearing at 3 keV.There is no proof that other elements can be seen in EDX spectra.Based on the elemental composition of pure ZnO NPs, the nanoparticles were composed of Zn (71.31%), O (26.56%), and C (2.11%), and the high amount of Zn and O confirms the purity of the as-prepared ZnO NPs [75].In the case of ZnO-Ag nanocomposite, the Ag peak confirms the formation of ZnO-Ag nanocomposite with Zn (53.46 %), O (28.75%), Ag (14.91%), and C (2.88%) [28].At the same time TEM analysis of The structural parameters of ZnO NPs and ZnO-Ag nanocomposites calculated from XRD patterns are presented in Table 1.The surface morphology and elemental composition of ZnO NPs and ZnO-Ag nanocomposite were investigated using SEM and EDX analysis, respectively, as shown in Figure 7a,d.The SEM images showed agglomerated spherical mushrooms-like masses in ZnO NPs; however, little decrease in agglomeration was observed on Ag deposition on the surface of ZnO NPs.The results from this Figure 7c,d confirmed the presence of the elements carbon (C), oxygen (O), and zinc (Zn), with peaks appearing approximately at 0.02 keV for C and 0.5 keV for O, respectively; whereas Zn appears at 1 keV, 8.6 keV, and 9.5 keV, with Ag element appearing at 3 keV.There is no proof that other elements can be seen in EDX spectra.Based on the elemental composition of pure ZnO NPs, the nanoparticles were composed of Zn (71.31%), O (26.56%), and C (2.11%), and the high amount of Zn and O confirms the purity of the as-prepared ZnO NPs [75].In the case of ZnO-Ag nanocomposite, the Ag peak confirms the formation of ZnO-Ag nanocomposite with Zn (53.46 %), O (28.75%), Ag (14.91%), and C (2.88%) [28].At the same time TEM analysis of ZnO NPs and ZnO-Ag nanocomposite was carried out to further identify the morphology of ZnO NPs and the existence of Ag NPs on the surface of ZnO NPs as depicted in Figure 7e,f.It is clear from the TEM analysis that the Ag NPs were deposited on the ZnO surface (Figure 7f).The close perusal of Figure 7e shows spherical along with some irregular-edged clumping aggregated mass of ZnO NPs.The adsorption of the spherical mass of Ag NPs was clearly visible on the surface of ZnO NPs' spherical mass along an irregular edge in Ag-supported ZnO nanocomposite, as shown in Figure 7f.The average particle size of ZnO-Ag, ZnO NPs, and Ag NPs was calculated by ImageJ software and was found to be ~22 nm, ~10 nm, and ~11 nm, respectively.Transmission electron micrographs from Figure 7e,f confirm that silver nanoparticles ranging in the average size of 10.32 nm were dispersed over the surface of ZnO NPs.Similar interpretations of surface morphology were attained in BET analysis; the larger particle size with enhanced surface area were observed for ZnO-Ag NPs in comparison with pure ZnO and Ag NPs.Thus, the overall results of EDX, SEM, TEM, and BET analyses of elemental composition and obtained surface morphology limit the successful biogenesis of ZnO NPs and ZnO-Ag nanocomposite, respectively.
Water 2023, 15, x FOR PEER REVIEW 13 of 26 ZnO NPs and ZnO-Ag nanocomposite was carried out to further identify the morphology of ZnO NPs and the existence of Ag NPs on the surface of ZnO NPs as depicted in Figure 7e,f.It is clear from the TEM analysis that the Ag NPs were deposited on the ZnO surface (Figure 7f).The close perusal of Figure 7e shows spherical along with some irregular-edged clumping aggregated mass of ZnO NPs.The adsorption of the spherical mass of Ag NPs was clearly visible on the surface of ZnO NPs' spherical mass along an irregular edge in Ag-supported ZnO nanocomposite, as shown in Figure 7f.The average particle size of ZnO-Ag, ZnO NPs, and Ag NPs was calculated by ImageJ software and was found to be ~22 nm, ~10 nm, and ~11 nm, respectively.Transmission electron micrographs from Figure 7e,f confirm that silver nanoparticles ranging in the average size of 10.32 nm were dispersed over the surface of ZnO NPs.Similar interpretations of surface morphology were attained in BET analysis; the larger particle size with enhanced surface area were observed for ZnO-Ag NPs in comparison with pure ZnO and Ag NPs.Thus, the overall results of EDX, SEM, TEM, and BET analyses of elemental composition and obtained surface morphology limit the successful biogenesis of ZnO NPs and ZnO-Ag nanocomposite, respectively.To investigate the specific surface areas and the porous nature of the ZnO and Agsupported ZnO nanocomposite, Brunauer-Emmett-Teller (BET) gas sorption measurements were performed.The nitrogen adsorption-desorption isotherms, pore volume, and pore size distribution plots of the ZnO NPs and ZnO-Ag nanocomposite are shown in Figure 8a,b.The as-prepared ZnO NPs and ZnO-Ag nanocomposite displayed type IV isotherm with an apparent hysteresis loop at relative pressure, indicating that mesopores predominated the prepared samples.The pore size and cumulative pore volume of ZnO NPs and ZnO-Ag nanocomposite were evaluated by using the density functional theory (DFT) model (Figure 8c,d).In addition, the BET surface area, pore size, and cumulative pore volume of ZnO NPs and ZnO-Ag nanocomposite are summarized in Table 2.The specific surface area of the Ag-supported ZnO nanocomposite was higher than the pure ZnO NPs.The increase of surface area and pore volume in case of ZnO-Ag nanocomposite sample is possibly due to formation of less agglomerated lower size particles (SEM and TEM analyses).The SEM images clearly showed the presence of void spaces in between the agglomerates.To investigate the specific surface areas and the porous nature of the ZnO and Ag-supported ZnO nanocomposite, Brunauer-Emmett-Teller (BET) gas sorption measurements were performed.The nitrogen adsorption-desorption isotherms, pore volume, and pore size distribution plots of the ZnO NPs and ZnO-Ag nanocomposite are shown in Figure 8a,b.The as-prepared ZnO NPs and ZnO-Ag nanocomposite displayed type IV isotherm with an apparent hysteresis loop at relative pressure, indicating that mesopores predominated the prepared samples.The pore size and cumulative pore volume of ZnO NPs and ZnO-Ag nanocomposite were evaluated by using the density functional theory (DFT) model (Figure 8c,d).In addition, the BET surface area, pore size, and cumulative pore volume of ZnO NPs and ZnO-Ag nanocomposite are summarized in Table 2.The specific surface area of the Ag-supported ZnO nanocomposite was higher than the pure ZnO NPs.The increase of surface area and pore volume in case of ZnO-Ag nanocomposite sample is possibly due to formation of less agglomerated lower size particles (SEM and TEM analyses).The SEM images clearly showed the presence of void spaces in between the agglomerates.

Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was attained to determine the thermal stability of ZnO NPs and Ag-ZnO nanocomposite after monitoring the weight loss of volatile components of the arranged samples.The TGA thermograms were obtained for pure ZnO NPs and Ag-supported ZnO nanocomposite (5 wt %) as shown in Figure 9.The observed different TGA regions with % weight loss are displayed in Figure 9.The first region of ZnO NPs was observed at up to 200 • C with weight loss of 25.98 % due to evaporation of moisture and disintegration of volatile organic compounds, and further ascertained from the peak intensities of DTG as observed at 81 • C and 182 • C, respectively.Moreover, the observed first region of the TGA curved up to 200 • C for Ag-ZnO nanocomposite with DTA intensities at 81 • C and 188 • C, respectively, were attributed to the evaporation of surrounding moisture on the surface of Ag-ZnO nanocomposite.The region of TGA between 200-700 • C of pure ZnO NPs with DTA sharp peak intensity at 595 • C was assigned to the thermal decomposition volatile functional groups of various phytochemicals, which act as reducing or stabilizing agents in the biogenesis of ZnO NPs with weight loss of 16.53 %.However, a weight loss of 12.93 % was observed in the TGA curve between 200-500 • C, attributed to lower molecular weight phytochemicals.However, the observed TGA cure in the region between 500-700 • C with DTA peak intensity at 595 • C was believed to be from the thermal decomposition of higher molecular weight phytochemicals serving as capping or stabilizing agents in the biosynthesis of Ag-ZnO nanocomposite.The overall results of the TGA/DTA analysis conform the thermally stable biosynthesis of ZnO and ZnO-Ag to an overall % weight loss of 42.51 % and 32.49 %, respectively.
Water 2023, 15, x FOR PEER REVIEW 15 of 26

Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was attained to determine the thermal stability of ZnO NPs and Ag-ZnO nanocomposite after monitoring the weight loss of volatile components of the arranged samples.The TGA thermograms were obtained for pure ZnO NPs and Ag-supported ZnO nanocomposite (5 wt %) as shown in Figure 9.The observed different TGA regions with % weight loss are displayed in Figure 9.The first region of ZnO NPs was observed at up to 200 °C with weight loss of 25.98 % due to evaporation of moisture and disintegration of volatile organic compounds, and further ascertained from the peak intensities of DTG as observed at 81 °C and 182 °C, respectively.Moreover, the observed first region of the TGA curved up to 200 °C for Ag-ZnO nanocomposite with DTA intensities at 81 °C and 188 °C, respectively, were attributed to the evaporation of surrounding moisture on the surface of Ag-ZnO nanocomposite.The region of TGA between 200-700 °C of pure ZnO NPs with DTA sharp peak intensity at 595 °C was assigned to the thermal decomposition volatile functional groups of various phytochemicals, which act as reducing or stabilizing agents in the biogenesis of ZnO NPs with weight loss of 16.53 %.However, a weight loss of 12.93 % was observed in the TGA curve between 200-500 °C, attributed to lower molecular weight phytochemicals.However, the observed TGA cure in the region between 500-700 °C with DTA peak intensity at 595 °C was believed to be from the thermal decomposition of higher molecular weight phytochemicals serving as capping or stabilizing agents in the biosynthesis of Ag-ZnO nanocomposite.The overall results of the TGA/DTA analysis conform the thermally stable biosynthesis of ZnO and ZnO-Ag to an overall % weight loss of 42.51 % and 32.49 %, respectively.

Photocatalytic Activity of ZnO-Ag Nanocomposite for MB dye Degradation
The ZnO-Ag nanocomposite was utilized to explore the potential efficiency against photodegradation of MB dye molecules in the presence of UV-light irradiation.Many metal-deposited nanomaterials are used after the enhanced photocatalytic degradation of dye molecules [6,[84][85][86][87].Moreover, such heterogeneous photocatalysts are also involved in the detoxification of air pollutants [88,89].In general, the photocatalytic activity of biogenic nanoparticles is related to the recombination of the electron-hole pairs in reaction media.In addition, it also depends on other factors such as size, surface area, radiation source, etc. [90][91][92][93].However, bare ZnO NPs possess high bandgap energy and as such are not used as a potential visible light catalyst [42,94,95].Thus, the Ag-supported surfaces of ZnO NPs are potentially used as a novel heterogeneous catalyst within visible light radiations.Meanwhile, during this study, significant emission of λmax at 365 nm was utilized as a visible light supply.The tungsten lamp (60 W) was used to produce a continuous light spectrum between 400-800 nm to explore photocatalysis.

Photocatalytic Activity of ZnO-Ag Nanocomposite for MB dye Degradation
The ZnO-Ag nanocomposite was utilized to explore the potential efficiency against photodegradation of MB dye molecules in the presence of UV-light irradiation.Many metal-deposited nanomaterials are used after the enhanced photocatalytic degradation of dye molecules [6,[84][85][86][87].Moreover, such heterogeneous photocatalysts are also involved in the detoxification of air pollutants [88,89].In general, the photocatalytic activity of biogenic nanoparticles is related to the recombination of the electron-hole pairs in reaction media.In addition, it also depends on other factors such as size, surface area, radiation source, etc. [90][91][92][93].However, bare ZnO NPs possess high bandgap energy and as such are not used as a potential visible light catalyst [42,94,95].Thus, the Ag-supported surfaces of ZnO NPs are potentially used as a novel heterogeneous catalyst within visible light radiations.Meanwhile, during this study, significant emission of λ max at 365 nm was utilized as a visible light supply.The tungsten lamp (60 W) was used to produce a continuous light spectrum between 400-800 nm to explore photocatalysis.The photodegradation of MB molecules was physically observed from the color change (blue to colorless).In addition, it was comprehended from the continuous decrease in peak intensity at λ max = 670 nm with increasing time intervals.The maximum % degradation of MB dye molecules was attained by 98.0 % after 60 min exposure of UV-light irradiation which was further ascribed from observed minimum or constant peak intensity as depicted in Figure 10a.Meanwhile, the percentage of dye degradation against time at different time intervals was calculated from obtained data, as shown in Figure 10b.The increased efficiency of using ZnO-Ag as a nanocatalyst for MB dye degradation was expected behind the Schottky barrier [96][97][98].However, in bare ZnO NPs, the recombination of electron-hole pairs occurred within nanoseconds and after the limited photocatalytic efficiency.Accordingly, introducing Ag with ZnO NPs as an interface is capable of producing the Schottky barrier effect which limits the recombination of the electron-hole pair with enhanced photocatalytic efficacy [99,100].The difference in work functions of ZnO and Ag-supported ZnO nanocatalyst is behind the accelerated electron transfer.It is worth mentioning that the enhanced electron transfer was achieved from doping Ag to the surface of ZnO as metal nanoparticles which act as electron sinks or traps onto the surface of ZnO nanoparticles.Moreover, in a photolytic reaction with an enriched reaction, oxidative species (ROS) cause an increase in surface traps.In particular, the electrons are captured from ZnO catalytic surface and are utilized via Ag-supported nanocomposite in the degradation of MB dye molecules [91,101].Recently, Bugra et.al chemically synthesized 1% Ag over ZnO NPs as photocatalyst and have achieved 91% degradation of MB within 2.5 min [102].Similarly, Stanley et.al, have reported chemically synthesized Ag-ZnO nanocomposite as photocatalyst for degradation of different dye molecules with maximum degradation efficiency for MB with 98.51 % within 60 min [103].Thus, the biogenic ZnO-Ag NPs developed by green synthesis reported here is worth applying as a photocatalyst besides its eco-friendly and biocompatible nature in degradation of MB within 60 min.The photodegradation of MB molecules was physically observed from the color change (blue to colorless).In addition, it was comprehended from the continuous decrease in peak intensity at λmax = 670 nm with increasing time intervals.The maximum % degradation of MB dye molecules was attained by 98.0 % after 60 min exposure of UV-light irradiation which was further ascribed from observed minimum or constant peak intensity as depicted in Figure 10a.Meanwhile, the percentage of dye degradation against time at different time intervals was calculated from obtained data, as shown in Figure 10b.The increased efficiency of using ZnO-Ag as a nanocatalyst for MB dye degradation was expected behind the Schottky barrier [96][97][98].However, in bare ZnO NPs, the recombination of electron-hole pairs occurred within nanoseconds and after the limited photocatalytic efficiency.Accordingly, introducing Ag with ZnO NPs as an interface is capable of producing the Schottky barrier effect which limits the recombination of the electron-hole pair with enhanced photocatalytic efficacy [99,100].The difference in work functions of ZnO and Ag-supported ZnO nanocatalyst is behind the accelerated electron transfer.It is worth mentioning that the enhanced electron transfer was achieved from doping Ag to the surface of ZnO as metal nanoparticles which act as electron sinks or traps onto the surface of ZnO nanoparticles.Moreover, in a photolytic reaction with an enriched reaction, oxidative species (ROS) cause an increase in surface traps.In particular, the electrons are captured from ZnO catalytic surface and are utilized via Ag-supported nanocomposite in the degradation of MB dye molecules [91,101].Recently, Bugra et.al chemically synthesized 1% Ag over ZnO NPs as photocatalyst and have achieved 91% degradation of MB within 2.5 min [102].Similarly, Stanley et.al, have reported chemically synthesized Ag-ZnO nanocomposite as photocatalyst for degradation of different dye molecules with maximum degradation efficiency for MB with 98.51 % within 60 min [103].Thus, the biogenic ZnO-Ag NPs developed by green synthesis reported here is worth applying as a photocatalyst besides its eco-friendly and biocompatible nature in degradation of MB within 60 min.

Effect of Catalyst Lodging
The photocatalytic properties associated with the ZnO-Ag nanocomposite are inferred from the photocatalytic degradation of dye and MB molecules.Moreover, the varying concentrations of photocatalysts ranging from 10 mg to 50 mg in 20 ppm of MB molecules maintained at pH 8 were studied under UV light irradiation.In general, the photodegradation of dye molecules is observed via color change/vanishing of color in the presence of UV-light irradiations behind the conversion of dye carbon molecules to less toxic biodegradable chemical species, including CO2, O2 -, or H2O molecules [52,104,105].Further, to comprehend the photocatalysis of MB molecules, we adopted

Effect of Catalyst Lodging
The photocatalytic properties associated with the ZnO-Ag nanocomposite are inferred from the photocatalytic degradation of dye and MB molecules.Moreover, the varying concentrations of photocatalysts ranging from 10 mg to 50 mg in 20 ppm of MB molecules maintained at pH 8 were studied under UV light irradiation.In general, the photodegradation of dye molecules is observed via color change/vanishing of color in the presence of UV-light irradiations behind the conversion of dye carbon molecules to less toxic biodegradable chemical species, including CO 2 , O 2 − , or H 2 O molecules [52,104,105].Further, to comprehend the photocatalysis of MB molecules, we adopted pseudo-first-order kinetics as per the Hinshelwood kinetic model [106][107][108].Meanwhile, the photodegradation kinetic curve of C t /C o against time (min) was attained for varying catalytic doses of 10 mg-50 mg, as shown in Figure 11a.The comparatively higher photodegradation of MB dye molecules was observed at a higher numerical value of catalytic dose, i.e., 50 mg.The catalytic dose was an optimum dose for photodegradation of MB molecules; meanwhile, the availability of maximum active sites is expected with maximum catalytic efficiency via rate constant, k = 0.01704 min −1 and R 2 = 0.99117.In addition, the acquired results were dose-dependent with an increase in catalytic dose and photocatalytic efficiency of ZnO-Ag nanocomposite as photocatalyst was attained for the degradation of MB molecules.However, the enhanced catalytic properties of ZnO-Ag nanocomposite were behind the acquired optical absorption capability or optical bandgap, the minimum surface-to-volume ratio, and the associated photogenerated charge carriers.Attained suitable surface-to-volume ratio and applicability in a range of small bandgaps as an effective photogenerated precursor for the generation of charge carriers were the prime factors leading to an increase in the photodegradation process when ZnO-Ag nanocomposite was used as a photocatalyst for degradation of MB dye molecules.Moreover, during this study, we observed a relatively higher catalytic dose of 50 mg as an optimum catalytic dose with higher mobility and maximum generation of photo-carriers from bulk to the surface, initiating maximum adsorption of dye molecules onto the active sites of ZnO-Ag nanocomposite with comparatively higher catalytic efficiency.
pseudo-first-order kinetics as per the Hinshelwood kinetic model [106][107][108].Meanwhile, the photodegradation kinetic curve of Ct/Co against time (min) was attained for varying catalytic doses of 10 mg-50 mg, as shown in Figure 11a.The comparatively higher photodegradation of MB dye molecules was observed at a higher numerical value of catalytic dose, i.e., 50 mg.The catalytic dose was an optimum dose for photodegradation of MB molecules; meanwhile, the availability of maximum active sites is expected with maximum catalytic efficiency via rate constant, k = 0.01704 min −1 and R2 = 0.99117.In addition, the acquired results were dose-dependent with an increase in catalytic dose and photocatalytic efficiency of ZnO-Ag nanocomposite as photocatalyst was attained for the degradation of MB molecules.However, the enhanced catalytic properties of ZnO-Ag nanocomposite were behind the acquired optical absorption capability or optical bandgap, the minimum surface-to-volume ratio, and the associated photogenerated charge carriers.Attained suitable surface-to-volume ratio and applicability in a range of small bandgaps as an effective photogenerated precursor for the generation of charge carriers were the prime factors leading to an increase in the photodegradation process when ZnO-Ag nanocomposite was used as a photocatalyst for degradation of MB dye molecules.Moreover, during this study, we observed a relatively higher catalytic dose of 50 mg as an optimum catalytic dose with higher mobility and maximum generation of photo-carriers from bulk to the surface, initiating maximum adsorption of dye molecules onto the active sites of ZnO-Ag nanocomposite with comparatively higher catalytic efficiency.

Effect of Dye Concentration
The accountability of initial MB dye concentration with optimum photodegradation efficiency using ZnO-Ag nanocomposite as a photocatalyst was ascertained from the kinetics in accordance with the Langmuir-Hinshelwood kinetic pseudo-first-order kinetic model.The plot of C t /C o against time (min) was attained at a constant catalytic dose Water 2023, 15, 384 16 of 24 (50 mg) with varying dye concentrations (20-60 ppm) maintained at pH 8 for 60 min irradiation time, as depicted in Figure 11b.The experimental data were used to calculate the degradation efficiency for various dye concentrations at a constant catalytic dose.The maximum photodegradation of MB dye was achieved at 50 mg of ZnO-Ag nanocomposite and was achieved at 20 ppm dye concentration.However, keeping the constant catalytic concentration (50 mg) with MB concentration (20-60 ppm) in varying ranges decreased after the probabilities of binding interactions of the nanocatalyst with the dye molecules onto their active sites.Furthermore, this fact can be well understood with constant catalytic concentration, and the maximum available sites are occupied at comparatively lower dye concentrations.Moreover, an increase of dye molecules leads to an inhibition effect because of catalytic poisoning and sedimentation with the observed numerical values of % degradation efficiencies and rate constant (k) values.During this study, we established the optimum dye concentration of 20 ppm in the presence of a catalytic dose of 50 mg under UV light.

Effect of pH Dependency
The pH dependency on photodegradation of MB molecules was studied in the presence of catalyst ZnO-Ag in the range between pH 2-pH 12.The obtained data were interpreted using kinetic pseudo-first-order (Langmuir-Hinshelwood model) via graph C o /C t against time (min), as shown in Figure 11c.The study was established in UV light with an optimum dye concentration of 20 ppm and catalytic (ZnO-Ag nanocomposite) optimum concentration of 50 mg.However, the study correlates degradation efficiency towards MB dye molecules, where the maximum (%) degradation efficiency of MB molecules was observed at pH 8 followed by pH 10 and pH 12, with minimum dye degradation at acidic pH media, i.e., at pH 2. In this study, we observed the photodegradation of MB dye molecules via ZnO-Ag nanocomposite following the order as pH 8 < pH 10 < pH 12 < pH 6 < pH 4 < pH 2. We observed an increase in numerical values of pH fastening the photodegradation of MB molecules on the active surface of ZnO-Ag nanocomposite up to pH ranging from pH 2-pH 8 followed by a decrease in % degradation efficacies at pH 10 and pH 12.Moreover, the nanocatalyst ZnO-Ag nanocomposite possesses both positive and negative ends with its potential to dissolve in acidic and alkaline environments behind its amphoteric nature.In general, the pH dependency is related to the surface characteristics of nanocatalysts.Thus, the % degradation efficiency of dye molecules via nanocatalysts is limited upon changing numerical values of pH, depending on the active surface charge properties of such catalysts.However, an increase in pH values and the overall negative charge on the surface of ZnO-Ag nanocomposite is enhanced by the adsorption of OH-ions with an increased formation of hydroxyl radicals.However, at numerical lower values of pH 8, MB dye molecules preferentially occupied nanocatalyst ZnO-Ag nanocomposite.Moreover, with an increase in pH values, the chances of generating a higher number of -OH • ions increased with enhanced hydroxyl radicles, though maximum degradation of MB was observed at pH 8.However, with further increases in pH values, such as pH 10 and pH 12, the electrostatic repulsion between MB anion and the positive surface charge of ZnO-Ag nanocomposites increased.Conversely, at comparatively higher numerical pH values, the possible slow diffusion of surface generated OH • towards the double layer against the MB low concentrations led to a decline in % degradation of MB molecules compared to the direct charge transfer taking place at optimum pH 8. Based on electrostatic repulsion and direct charge transfer, we observed the highest decolorization efficacy at pH 8, and decolorization abilities greatly decreased at pH 10 and pH 12.

The Recyclability Tests
The test of recyclability of synthesized ZnO-Ag as a nano-photocatalyst was attained after its applicability in the degradation of MB dye molecules.With this, we deduced the recyclability test to assure the stability and reusability of synthesized ZnO-Ag nanocomposite in UV light as a nanocatalyst for degrading MB dye molecules.As a result, the photodegradation of MB molecules was attained, and complete degradation was achieved within 60 min exposure time in each cycle.After every cycle of exposure, the nanocatalyst was collected upon centrifugation, washed with absolute alcohol followed by washing with deionized water, and dried overnight at 70 • C in an oven.The collected nanocatalyst was added to a freshly prepared MB dye reaction solution to proceed to the next cycle.After four consecutive cycles, the degradation of MB molecules was observed in the presence of UV-Visible light by 75-98 % within the time intervals of 60 min exposure via ZnO-Ag nanocomposite as a nanocatalyst, as shown in Figure 11d.The obtained results of recyclability further emphasize the potential use of ZnO-Ag as a nanocatalyst in the photodegradation of MB dye molecules.However, the observed decreased efficiency of ZnO-Ag as a nanocatalyst after four successive cycles was expected because of the loss of the sample (ZnO-Ag nanocomposite) during each cycle in the collection and centrifugation process.Overall, the result of recyclability ascertained the use of biogenic ZnO-Ag as a potential catalyst in the photodegradation of MB molecules and possessed a photocatalytic window to be utilized in the photodegradation of other thiazine dye molecules.In addition, we also compared our results with the previous studies, shown in Table 3. ), hydroxyl radicals ( • OH), and photogenerated holes (h + ) are typically the main active species for the breakdown of organic contaminants in water.The role of active species in the photodegradation process is evaluated by adding the appropriate scavengers to the suspensions of the MB degradation in the presence of ZnO-Ag.This method was used to investigate the producing ability of active species.Ammonium oxalate (AO) is used to trap holes (h + ), benzoquinone (BQ) is used to trap oxygen (O 2 ), and isopropanol (IPA) is used to trap OH.According to the findings, the secondary contributors in the photodegradation process are the h+ and O 2 species, with the OH radical acting as the main active species.The majority of MB degradation is inhibited by the addition of IPA, which causes a 73% drop in degradation efficiency.Figure 12 displays the outcomes of the photocatalytic MB degradation with the addition of various scavengers.
ZnO-Ag.This method was used to investigate the producing ability of active species.Ammonium oxalate (AO) is used to trap holes (h + ), benzoquinone (BQ) is used to trap oxygen (O2), and isopropanol (IPA) is used to trap OH.According to the findings, the secondary contributors in the photodegradation process are the h+ and O2 species, with the OH radical acting as the main active species.The majority of MB degradation is inhibited by the addition of IPA, which causes a 73% drop in degradation efficiency.The possible catalytic photoassisted degradation of MB dye molecules was expected upon the generation of ROS such as superoxide radical anion (O2 •-), hydroxyl radical ( • OH), electron (e -), and holes (h + ) in a photolytic reaction as depicted in Figure 13.In a photocatalytic reaction in the presence of light radiations, electrons on VB are excited to CB leaving behind holes in VB.The excited electrons of CB are responsible for the reduction reaction onto the surface of the catalyst and holes of CB diffuse from the photocatalytic surface are behind the oxidation reaction.The reduction and oxidation reactions together are involved in the photodegradation of dye molecules [118,119].Hereby, the possible various reactions in a stepwise manner are represented after the photodegradation of MB molecules via ZnO-Ag nanocomposite as: As a result, the Ag-supported semiconductor photocatalyst (ZnO-Ag nanocomposite) was expected to undergo excitation in the presence of UV-Visible light radiation at comparatively lower energy than the bandgap energy of ZnO.Moreover, undergoing excitation, electron (e -) transfer from the VB to CB leaves behind holes (h + ) in the VB.The possible catalytic photoassisted degradation of MB dye molecules was expected upon the generation of ROS such as superoxide radical anion (O 2 •− ), hydroxyl radical ( • OH), electron (e − ), and holes (h + ) in a photolytic reaction as depicted in Figure 13.In a photocatalytic reaction in the presence of light radiations, electrons on VB are excited to CB leaving behind holes in VB.The excited electrons of CB are responsible for the reduction reaction onto the surface of the catalyst and holes of CB diffuse from the photocatalytic surface are behind the oxidation reaction.The reduction and oxidation reactions together are involved in the photodegradation of dye molecules [118,119].Hereby, the possible various reactions in a stepwise manner are represented after the photodegradation of MB molecules via ZnO-Ag nanocomposite as: During this study, the nanocatalyst (ZnO-Ag nanocomposite), upon UV-Visible light irradiation, generates the e -in the CB and h + in their VB, as per Eqn. 1.The as-generated e -in CB after reaction with oxygen molecule forms O2 •-, then this photogenerated O2

Conclusions
A simple green synthesis approach was used to create ZnO NPs and plasmonic Ag-supported ZnO nanocomposite with excellent photocatalytic dye degradation efficiency.A technique for making a very stable metal/semiconductor nanocomposite is As a result, the Ag-supported semiconductor photocatalyst (ZnO-Ag nanocomposite) was expected to undergo excitation in the presence of UV-Visible light radiation at comparatively lower energy than the bandgap energy of ZnO.Moreover, undergoing excitation, electron (e − ) transfer from the VB to CB leaves behind holes (h + ) in the VB.During this study, the nanocatalyst (ZnO-Ag nanocomposite), upon UV-Visible light irradiation, generates the e − in the CB and h + in their VB, as per Equation (1).The as-generated e − in CB after reaction with oxygen molecule forms O 2 •− , then this photogenerated O 2 •− further reacts with water molecules in a mixed reaction medium which are converted into • HO 2 as per Equations ( 2) and (4).In addition, the left behind h + of VB gets adsorbed onto the water molecules with the generation of corresponding • OH radicals, as illustrated in Equation (2).Overall, the photogenerated ROS are considered responsible for the photodegradation of MB molecules via ZnO-Ag nanocomposite, and Equations ( 1) to ( 6) demonstrate the stepwise manner of the photocatalytic reduction process.

Conclusions
A simple green synthesis approach was used to create ZnO NPs and plasmonic Agsupported ZnO nanocomposite with excellent photocatalytic dye degradation efficiency.A technique for making a very stable metal/semiconductor nanocomposite is provided by producing Ag NPs on the surface of ZnO during the formation of Ag-supported ZnO nanocomposite utilizing the Carthamus tinctorius aqueous extract.When creating nanomaterials, the polyphenols in the C. tinctorius extract act as both reducing and capping/stabilizing agents.FTIR, XRD, SEM, TEM, EDS, PL, and UV-Vis spectroscopy were among the methods used to characterize the as-prepared ZnO NPs and ZnO-Ag nanocomposite.The deposition of Ag NPs on the surface of ZnO was confirmed by XRD analysis, SEM-EDX, and TEM micrographs.By observing the photocatalytic decolorization of MB in ambient reaction circumstances, it was possible to determine the photocatalytic performance of the ZnO-Ag nanocomposite as an as-synthesized nanocatalyst under UV light irradiation.Our tests revealed that a composite made of ZnO and Ag was produced under simple green reaction conditions which had a sizable amount of catalytic activity.A proposed photocatalytic system using a ZnO-Ag nanocomposite catalyst was made.It was shown that the photocatalytic activity primarily causes •OH radicals to contribute to deterioration.Additionally, it was shown that ZnO-Ag is stable enough to be recycled multiple times.The excellent photocatalytic efficacy of ZnO and Ag-supported ZnO nanocomposite makes them potentially valuable materials for wastewater treatment.

3 Figure 1 .
Figure 1.The proposed method for the preparation of ZnO NPs and ZnO-Ag nanocomposite.

Figure 1 .
Figure 1.The proposed method for the preparation of ZnO NPs and ZnO-Ag nanocomposite.

Figure 5 .
Figure 5. (a) FTIR spectra of Safflower, ZnO NPs, and ZnO-Ag nanocomposite, (b) Raman spectra of ZnO NPs and ZnO-Ag nanocomposite.Meanwhile, the observed peaks at 995.4 cm −1 and 929.7 cm −1 were assigned to the presence of polyphenols.Peak intensity at 712.4 cm −1 was believed after the stretching vibration of C-Cl bonds.The peak intensity at 578.6 cm −1 was attributed to the presence of bond vibrations of C-H and C-N.The observed peaks of different phytochemicals further ascertained their role as reducing and stabilizing agents in the biosynthesis of ZnO NPs and ZnO-Ag nanocomposite.Moreover, the observed shift of peak intensities in ZnO NPs and ZnO-Ag nanocomposite towards lower (blue) and higher (red) wavelengths, respectively,

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Figure 10 .
Figure 10.(a) UV-light absorbance against wavelength (nm) of MB molecules via ZnO-Ag nanocomposite and (b) the percentage degradation of MB molecules via ZnO-Ag nanocomposite against time (min) at different intervals.

Figure 10 .
Figure 10.(a) UV-light absorbance against wavelength (nm) of MB molecules via ZnO-Ag nanocomposite and (b) the percentage degradation of MB molecules via ZnO-Ag nanocomposite against time (min) at different intervals.

Figure 12 .
Figure 12.Effect of scavengers on the photocatalytic degradation of MB dye.

Figure 12 .
Figure 12.Effect of scavengers on the photocatalytic degradation of MB dye.

3 Figure 13 .
Figure 13.The possible mechanism of photocatalytic degradation of MB dye using ZnO-Ag nanocomposite under UV light irradiation.

Figure 13 .
Figure 13.The possible mechanism of photocatalytic degradation of MB dye using ZnO-Ag nanocomposite under UV light irradiation.

Table 1 .
X-ray diffraction parameters of ZnO NPs and ZnO-Ag nanocomposite.

Table 1 .
X-ray diffraction parameters of ZnO NPs and ZnO-Ag nanocomposite.

Table 2 .
BET surface area, pore size, and pore volumes of ZnO NPs, and ZnO-Ag nanocomposite.

Table 2 .
BET surface area, pore size, and pore volumes of ZnO NPs, and ZnO-Ag nanocomposite.

Table 3 .
Literature survey concerned with ZnO-Ag nanocomposite in photodegradation of organic dyes and pharmaceuticals.