Aloe-inspired eco-friendly synthesis of Ag/ZnO heterostructures: boosting photocatalytic potential

The current research focuses on the development of Ag–ZnO heterostructures through a “bottom-up” approach involving the assembly and extraction of Aloe barbadensis Miller gel. These heterostructures composed of metals/semiconductor oxide display distinct and notable optical, electrical, magnetic, and chemical properties that are not found in single constituents and also exhibit photocatalytic applications. These synthesized heterostructures were characterized by XRD, FTIR, SEM, and UV–visible spectroscopy. The high peak intensity of the Ag/ZnO composite shows the high crystallinity. The presence of Ag–O, Zn–O, and O–H bonding is verified using FTIR analysis. SEM analysis indicated the formation of spherical shapes of Ag/ZnO heterostructures. The Zn, O, and Ag elements are further confirmed by EDX analysis. Ag–ZnO heterostructures exhibited excellent photocatalytic activity and stability against the degradation of tubantin red 8BL dye under visible light irradiation.

Silver and zinc oxide nanostructures are the most widely used metallic and metallic oxide nanostructures.Zinc oxide nanostructures have gained significant attention in research due to their increased industrial potential as well as their high piezoelectric properties, large binding energy, and low toxicity.Additionally, owing to their exceptional anti-oxidant activity, zinc oxide nanostructures are among the most widespread inorganic nanostructures.Zinc oxide nanostructures, an n-type semiconductor with a direct band gap of 3.37 eV 1 , have been generally utilized in water decontamination photocatalysis owing to their notable photocatalytic features such as their physically and chemically stable structure, low affordability, biocompatibility, strong oxidizing ability, high photosensitivity, and high availability 2 .On the other hand, due to electron-hole recombination, which results in reduced active species on the photocatalyst surface, the wide band gap in ZnO led to non-optimal photocatalytic activity 3 .Therefore, to improve ZnO photocatalytic activity, it is essential to prevent electron-hole recombination through appropriate modifications.Additionally, they offer potential for environmental and biological applications because of their non-toxic nature.ZnO nanostructures are widely used in several applicable areas, including energy storage 4 , dye-sensitized solar cells 5 , nanosensors 6 , optoelectronic devices 7 , nano-electronics 8 , gas sensors 9 , photocatalysts 10 , spintronics 11 , and biomedical treatment 12 .ZnO nanostructures have been employed as semiconductors in the fabrication of microelectronics and pollutant degradation.ZnO nanostructures are also suitable for UV screening applications due to their greater chemical stability and low toxicity 13 .
Silver nanostructures are attracting much interest due to their carrier properties, which include anti-microbial 14 , anti-inflammatory 15 , anticancer drugs 16 , and antioxidant activities 17 .Unique biological, chemical, and physical properties of silver nanostructures among metallic nanostructures are advantageous, reserving their potentiality in industrial applications.Ag nanostructures have an exceptional wide-spectrum of anti-fungal, anti-bacterial, and anti-oxidant activities that are commonly used in several electronic devices such as washing machines, televisions, and refrigerators, as well as for water and air purification, food packaging, clothing, cosmetics, and medicine [18][19][20] .Moreover, the morphological features of synthesized Ag nanostructures are significantly affected by their anti-oxidant activity 21,22 .In order to exploit the anti-oxidant activity in the fabrication of innovative products, a variety of plant extracts have been utilized to fabricate Ag nanostructures in various shapes

Materials and methods
Analytical-grade chemicals and precursors needed in experiments are purchased from Sigma Aldrich Company and used without further purification because all chemicals are 99% pure.Among these chemicals are powdered forms of silver nitrate (AgNO 3 ), zinc nitrate hexahydrate Zn (NO 3 ) 2 .6H 2 O, and sodium hydroxide (NaOH).To synthesize the Ag/ZnO nanocomposites as starting materials, deionized water, ethanol, and fresh Aloe vera leaves have been utilized.Ag/ZnO composites/nanocomposites were synthesized via ex-situ techniques.

Gel extract preparation
Aloe vera fresh leaves were collected from plants and washed completely with water.Then, we dried them in the air for about twenty minutes to prepare the aqueous extract of Aloe vera leaves.The gel was isolated from the leaves.400 g of gel was divided into chunks and crushed with the help of a pestle and mortar.The gel was blended with a comparable quantity of distilled water and dried for 40 min at 85 °C in an oven.The prepared extract was stored in a refrigerator at room temperature for further experiments and then filtered using filter paper.

Synthesis of silver (Ag) nanostructures
An aqueous solution of AgNO 3 was prepared by mixing 0.5 M of AgNO 3 in 40 mL of distilled water, and the solution was magnetically stirred for 30 min at 85 °C.40 mL of plant extract, Aloe vera, was gradually added to the AgNO 3 solution, and the above mixture was kept under magnetic stirring for about 1 h.Then, sonicate the abovementioned mixture for 2 h.The final solution was then transferred into a Teflon-lined, sealed vessel of 100 ml capacity under 150 °C conditions for 6 h.The centrifugation technique is used to separate a dark grey precipitation that was continually rinsed with ethanol and deionized water after being dried in an oven for 12 h at 80 °C.

Synthesis of zinc oxide (ZnO) nanostructures
To prepare zinc oxide nanostructures, 0.5 M zinc nitrate hexahydrate Zn (NO 3 ) 2 .6H 2 O was dispersed in 20 mL of deionized water and continued on magnetic stirring at 85 °C for 20 min.We added 20 mL of plant extract, Aloe vera, to the above mixture.Through this procedure, a solution of 0.5 M NaOH in 20 mL of deionized water was made.We further added the NaOH solution drop-wise to the above mixture and left it on magnetic stirring for 1 h.The abovementioned solution was sonicated for 1 h.The solution was transferred to a 100 mL coated Teflonlined jar and heated at 150 °C for 6 h.The pale-white precipitation was gathered via centrifugation, repetitively scrubbed with ethanol and deionized water, and formerly dried for 12 h in an oven at 80 °C.

Ex-situ synthesis of Ag/ZnO heterostructures
Ag/ZnO heterostructures have been synthesized using an ex-situ approach.In this approach, previously-made nanostructures of ZnO and Ag have been utilized.Initially, in order to prepare the Ag/ZnO heterostructures with a molar ratio of 1:1, 0.2 g of ZnO was mixed in 20 mL of ethanol, and 0.2 g of Ag was mixed in 20 mL of ethanol, and both mixtures were combined together for 30 min with continuous stirring.Then add a drop-wise Ag solution to the ZnO solution and maintain magnetic stirring for 1 h.We then sonicated the aforementioned mixture for 1 h.Later on, the mixture was transferred into the centrifuge tube to gather the particles.Then it was dried in an oven for 6 h at about 80 °C.A dark grey precipitation was formed after grinding. www.nature.com/scientificreports/

Photocatalytic activity
The photocatalytic activity was investigated using the Tubantin red 8BL dye.The study was carried out in the presence of a sun simulator named Abet Technologies Sunlight TM solar simulator.Prior to being employed in subsequent degradation experiments, each of these catalysts (ZnO and Ag/ZnO) was disinfected, implementing a range of cleaning and centrifugation methods.In order to estimate the catalyst's reusability and stability, three cycles were performed.The following equation is used to examine the degree of degradation: the pseudo-firstorder process is a catalytic decolorization of dye solution 37,38 .
Here C 0 is the initial concentration, and here is the concentration after a particular amount of time.The deg- radation was calculated with the help of 50 mL of aqueous Tubantin red 8BL dye (10 ppm) for 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min in the light and for 30 min in the dark, respectively.All the samples were achieved with 0.015 g of the catalyst's standard pH.The photocatalytic breakdown mechanism was utilized to determine the absorption peak characteristics of tubantin red 8BL dye.In the absence of catalyst, there was particularly no degradation.

XRD Analysis
X-ray diffraction was used to analyze the crystalline nature of the prepared heterostructures.Figure 1 illustrates the XRD pattern of prepared Ag/ZnO heterostructures using Aloe vera gel extract.Sharp diffraction patterns were observed at 2 θ = 31 • , 34 • , 36 • , 47 • , 56 • , 67 • , and 69 • , which were ascribed to diffraction planes having indices (100), ( 002), ( 101), ( 102), ( 110), (112), and (201), respectively, that resemble the hexagonal crystal structure of ZnO nanostructures (JCPDS-files: PDF#36-1451).No other impurity peaks were found in the diffraction pattern, showing that all the precursors have been totally transformed into nanostructures, and therefore the resulting ZnO nanostructures are extremely crystalline in nature.The XRD pattern shows that only a single phase exists in ZnO nanostructures with the space group (P63mc).The crystalline character of the prepared specimen presents the sharp and broad nature of diffraction patterns.The comparatively higher intensity (101) peak implies a preferred alignment and anisotropic growth of crystallites.It is obvious from the results of XRD analysis that there were characteristic diffraction patterns with a distinct value of sharp peaks at 2 θ = 38 • and 43 • attained, which correspond to the diffraction forms of ( 111) and ( 200) orientations.Based on standard diffraction data card no.(PDF#04-0783), some additional peaks occurred at 62 • and 77 • were compatible with diffraction planes (220) and (311), respectively.The examined Bragg's reflection correlating to (111) and ( 200) may be recorded the prediction of the face-centered cubic crystal structure of Ag nanostructures.This pattern affirmed that there are no additional phases other than the FCC structure of metallic Ag.The confirmation of phase purity and synthesis of elemental silver were clearly investigated for consistency with reference data 39 .Furthermore, the presence of Ag in the ZnO lattice is verified by two additional peaks that have been identified as (111) and (200).Similar findings have been reported for Ag/ZnO heterostructures [39][40][41][42][43][44][45] .

FTIR analysis
The purity of samples and identification of functional groups were observed in the range of 500-4000 cm −1 using FTIR analysis.Figure 2 depicts the FTIR spectra of Ag nanostructures, ZnO nanostructures, and Ag/ZnO heterostructures synthesized using Aloe vera gel.The strong absorption band at 3200-3500 cm −1 correlates with the overlaying of the stretching vibrations of O-H in the phenolic group and N-H in the amine group 46 .With the transformation of Ag + ions into Ag and Zn +2 ions into ZnO, the O-H groups in phenols behave as reducing agents 46 .The peaks in the range of 2700-cm −1 are also linked with amide groups, which contain amino acids

SEM analysis
The morphological properties of prepared nanostructures were investigated via the SEM technique.The growth mechanism, shape, and surface morphology of Ag nanostructures, ZnO nanostructures, and Ag/ZnO heterostructures are illustrated in Fig. 3a-c.It is examined that Aloe vera gel extract acts as capping and reducing agents, leading to the fabrication of Ag/ZnO heterostructures in various shapes.The randomly oriented and densely packed spherical shapes of silver nanostructures are clearly observed using SEM analysis in Fig. 3a.The collected clusters that are distributed over the surface with a huge random empty space were displayed in the SEM image 49 .The nanostructure agglomeration shows that they were in direct contact and indicates the stability of the formation of Ag.The majority of nanostructures exhibit a spherical shape, though some nanostructures demonstrate cubic shapes that are not precisely determined.The synthesized ZnO nanostructure's morphology and the micrographs are presented in Fig. 3b.The formation of uniformly dispersed ZnO nanosheets with definite particle boundaries was clearly observed in Fig. 3b.The 2D nanosheets are interconnected with each other to form a flower configuration.The micrographs indicated the three-dimensional nanoflower-like structures of ZnO nanostructures.Regarding the Aloe vera gel extract, more homogeneity of nanoflowers was examined in ZnO nanostructures 50 .The SEM image revealed that the shape of ZnO nanostructures is somehow nano needle-like morphology 51 .Figure 3c depicts the SEM analysis of Ag/ZnO heterostructures.The micrograph indicates that the morphology of heterostructures is irregular and non-uniform in shape.The arrangement of particles looks like randomness in the form of clusters.This morphology implies that the composition might not be homogenous and the synthesized procedure might not have been uniformly optimized.It is observed that some spherical shapes are present due to Ag nanostructures on the surface of ZnO 40 .However, a small amount of Ag nanostructures appears to be comparatively tiny in size, whereas a huge amount of Ag agglomerates at the surface of porous and dispersed ZnO 39 .The formation of some irregular nanosheets was due to the presence of Ag nanostructures covering the surface of ZnO nanostructures.This agglomeration is generated because of the densification produced by the small space among the particles.The particles in Ag/ZnO heterostructures are conspicuously more agglomerated, with the creation of tiny particles over bigger clusters, which might be attributed to the fabrication of Ag nanostructures 44,52 .Furthermore, different morphologies might be attributed to the gel extract of the bioactive components.The presence of different shapes of nanostructures could be produced due to the bio-component nature of the extract 53,54 .Figure 3d represented the average particle size distribution of prepared nanostructures.The average size was 160.3 nm for Ag nanoparticles, 258.4 nm for ZnO, and 303.9 nm for Ag/ZnO.From this, we can clearly confirm the fact that average particle size was increased in composite as compared to Ag and ZnO.www.nature.com/scientificreports/

EDX analysis
The most effective characterization technique for identifying the synthesis of Ag, ZnO, and Ag/ZnO nanostructures is EDX analysis.EDX measurements may also be employed to investigate the elemental composition.Figure 4a-c describes the EDX spectrum of Ag, ZnO, and Ag/ZnO heterostructures using Aloe vera gel.The fabrication of Ag nanostructures was confirmed by the strong peak for metallic silver at 3 keV observed in the EDX spectrum in Fig. 4a.The existence of silver elements is represented by a line in the EDX plot.Therefore, the formation of Ag nanostructures from Aloe vera gel can be confirmed.The total composition of silver is 87.0%(wt%), which is not found in previous literature.In order to observe the chemical composition of ZnO nanostructures, the graph depicts the two sharp and strong peaks associated with zinc and one distinct peak of the oxygen element.Figure 4b reveals the significant peaks of zinc and oxygen, which verify that these are the two major components in nanostructures.The produced sample contains no impurities of other elements, which strongly confirmed the formation of high-purity ZnO nanostructures.The EDX spectrum of Ag/ZnO heterostructures indicates the specific peaks of Zn, O, and Ag.The distributions of elements in the analysis presented peaks of Zn, O, and Ag with atomic percentages of 21.07%, 70.50%, and 6.82%, respectively.While weight percentages are taken into consideration, the exposed peaks of 41.75% for Zn, 34.19% for O, and 22.32% for Ag are represented in Fig. 4c.For zinc, there were two prominent signals near 1.5 keV and 8.5 keV.The only significant peak of the oxygen element was generated at 0.6 keV, whereas in the case of silver, two moderate peaks were observed at approximately 0.2 keV and 3.0 keV.Some minor peaks of C and Cl were examined in the EDX spectra.These elements were obtained from the biomolecules that were bound to the surfaces of Ag nanostructures and Ag/ ZnO heterostructures in Aloe vera gel 45,55 .

UV analysis
The optical absorption properties of ZnO, and Ag/ZnO heterostructures prepared via Aloe vera gel extract in the range of 200-800 nm were examined.The equation representing the Tauc relationship 56 was utilized for calculating the prepared samples optical band gap values: In this equation, "E g " signifies the optical bandgap energy, "h" is the Plank's constant, " ν " represents frequency, " α " symbolizes the absorption coefficient and "A" is a constant that depends on transition probability 57,58 .Here, n relies on electrons transition and the value of n = 1/2 belongs to direct electrons transition with a direct optical band gap. Figure 5a,b exhibit a plot between αhν 2 and E g that determines the optical band gap energy of prepared nanocomposites.This Tauc plot indicated a band gap of 3.15 eV and 2.96 eV for ZnO and Ag/ZnO heterostructures, respectively.The decrease in energy band gap in Ag/ZnO heterostructures was observed when Ag is embedded into the ZnO.This might be due to n-type conductivity of metallic ions.The oxygen vacancies generated in ZnO, which may capture impurities and possible chemical reactions originated among ZnO and Ag surface could also be involved in narrowing the energy band gap.This contributes the more oxygen vacancies, whereas high electron density was produced in ZnO.Ag + replaces the Zn 2+ sites, leading to increasing electron densities and vacancies due to variation in ionic radius which lowers the Fermi level and brings about narrowing the band gap.The connection between degradation rate ( C/C 0 ) and irradiation time (min) is explained in Fig. 7b.The degradation rate of Ag/ZnO (1:1) heterostructures are 0.5593, 0.5245, 0.4898, 0.4608, 0.4085 and 0.3535 and ZnO nanostructures are 0.8869, 0.8291, 0.7899, 0.7507, 0.7072, and 0.6551 respectively after 10, 20, 30, 40, 50 and 60 min in the presence of light.Likewise after 30 min under dark conditions, the degradation rates of pure and nanocomposites are 0.9116 and 0.57338 respectively.Figure 7c examines the graph of Ln ( C/C 0 ) values over time for the Ag/ZnO (1:1) are 0.5791, 0.6427, 0.7063, 0.7768, 0.8895 and 1.0354 and for the ZnO nanostructures are 0.1114, 0.1868, 0.2313, 0.2854, 0.3418 and 0.4195 respectively.The rate constant values of Ag/ZnO (1:1) heterostructures and ZnO nanostructures are 0.013804 and 0.006477 min −1 accordingly, demonstrating the pseudo-1st order kinetics and has significant photocatalytic activity.The stability of Ag/ZnO (1:1) and ZnO synthesized heterostructures has been confirmed using three cycles as represented in Fig. 7d.The degradation characteristics and stability of generated heterostructures were obviously visible after being subjected to solar radiation after 60 min.

Conclusions
In the current work, Ag/ZnO heterostructures were successfully synthesized using the hydrothermal method via Aloe vera gel extract.The Ag/ZnO heterostructures are preferentially oriented along the (111) plane, which was verified by using XRD analysis.FTIR measurements exhibited that the absorption bands demonstrate the formation of H-bonds and their interaction with O-H functional groups in Ag/ZnO heterostructures.The SEM analysis reveals the existence of both ZnO and Ag in spherical shapes that could be produced due to the biocomponent nature of the gel extract.The EDX spectrum designates the weight percentages of Zn, O, and Ag as 41.75%, 34.19%, and 22.32%, respectively.The energy band gap (E g ) observed in Ag/ZnO heterostructures is 2.96 eV.Ag/ZnO exhibited a maximum photodegradation efficiency of 64.55%, demonstrating that it is a promising material for excellent photocatalytic degradation.The aforementioned findings imply that the synthesized