Visible light assisted surface plasmon resonance triggered Ag/ZnO nanocomposites: synthesis and performance towards degradation of indigo carmine dye

Water pollution caused by organic compounds, generated from different industries, has gained attention worldwide today. In this regard, significant efforts have been made for a suitable dye degradation technology. Zinc oxide (ZnO)–based photocatalysts are considered novel materials to degrade organic effluents in contaminated water. The facile synthesis of Ag/ZnO nanocomposites and its application for the enhanced degradation of indigo carmine (IC) dye under visible light irradiation is reported in this paper. The prepared photocatalysts were characterized using various analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron (XPS) spectroscopy, FTIR, Raman, impedance study, UV–Vis, and photoluminescence (PL). Prepared Ag/ZnO nanocomposites were tested for degradation of IC dye in visible light. The degradation efficiency of IC dye was found to be 95.71% in 120 min, with a rate constant of 0.02021 min−1. This improved photocatalytic activity of Ag/ZnO nanocomposites was mainly due to the absorption of visible light caused by surface plasmon resonance (SPR) derived from Ag nanoparticles (NPs) and electron–hole separation. Radical trapping experiments suggest that holes (h+) and superoxide radical (O2•–) are the key factors in photocatalytic IC dye degradation.


Introduction
With the rising global population and widespread industrial growth, fossil fuel consumption is rapidly increasing (Guo et al. 2019).As a result, serious environmental pollution (caused by toxic agents and industrial waste) often contains considerable amount of various organic dye pollutants (Janbandhu et al. 2022;Sukhadeve et al. 2022a).These poured chemicals into the groundwater reservoirs cause irreversible damage to human health and the natural environment.As a consequence, it appears that the elimination of organic dyes from wastewater is extraordinarily important (Lellis et al. 2019).Semiconductor heterogeneous photocatalysis has become a hot topic that sparked interest all around the world, particularly in dye treatment.Over the last decades, various photocatalysts, such as TiO 2 (Sukhadeve et al. 2022b), SnO 2 (Elango and Roopan 2016), ZnO (Gao et al. 2013), and CdS (Janbandhu et al. 2019a), have been used to degrade water contaminants.Among them, ZnO semiconductor is considered one of the most promising photocatalysts in the industry as well as for the research community due to its potential applications for the decomposition of many toxic and non-biodegradable organic pollutants.Such catalyst, due to its incredible features like low cost, higher chemical stability, non-toxicity, better sensing behavior, and high photocatalytic activity, is a suitable candidate for photocatalysis (Chaudhary et al. 2013;Saravanan et al. 2015).However, ZnO is generally regarded as a non-viable material for practical applications because of its two major limitations.The first particularly limiting factor is the larger bandgap of ZnO (~ 3.37 eV) which limits its response to UV light only.The second one is the fast recombination of photoexcited charge carriers (Zhang et al. 2017).So, it is important to extend the absorption spectrum of ZnO to include more visible light for boosting photocatalytic efficiency.To overcome these drawbacks, doping with metal or non-metal is considered a useful tool to enhance the photocatalytic activity of ZnO (Georgekutty et al. 2008).Doping a semiconductor entails introducing impurities or other elements to modify its characteristics and improve its photocatalytic activity (Lee et al. 2016).
Most recent numerous studies performed by various researchers mainly focused on noble metals such as Pd (Kumari et al. 2020), Pt (Jaramillo-Páez et al. 2018), Au (Bueno-Alejo et al. 2021), and Ag (Zhang et al. 2014), to improve the photocatalytic activity of ZnO (Li et al. 2018b).Of these, Ag-sensitized ZnO nanomaterials received very much attention because of two reasons: (i) reduction in the recombination rate of photogenerated electron-hole pairs at the metal-semiconductor interface due to the generation of the Schottky barrier, and (ii) improvement of visible light absorption performance caused by the surface plasmon resonance (SPR), which consequently improves the photocatalytic efficiency (Raji et al. 2018).Compared with other noble metals, silver got more attention due to its high electrical and thermal conductivity, antibacterial characteristics, costeffectiveness, and non-toxicity (Zhang et al. 2017).Although Ag/ZnO nanocomposites have been widely studied for photocatalytic properties, there are very few reports on Ag/ZnO nanomaterials for photocatalytic degradation of indigo carmine (IC) dye under the illumination of visible light.
This work reports the synthesis of Ag/ZnO nanocomposites for photodegradation of IC dye under visible light.The prepared nanoparticles (NPs) have been characterized by various techniques, and optimized composition was tested for the degradation of IC dye in visible light.

Materials
Zinc nitrate hexahydrate (Zn (NO 3 ) 2 .6H 2 O) (Sigma Aldrich), silver nitrate (AgNO 3 ) (Sigma Aldrich), and sodium hydroxide (NaOH) (Merck, Germany) were used as precursors in this experimental work, and the commercial indigo carmine (IC) dye produced by Alfa Aesar was used as a pollutant dye and was used as supplied, and its characteristics are depicted in Table 1.Double-distilled water (DDW) was used throughout the experiment.All the chemicals were used without further purification.

Preparation of ZnO and Ag/ZnO nanocomposites
The preparation of ZnO and Ag-doped ZnO nanomaterials was carried out using the chemical co-precipitation method (Fig. 1).In a typical synthesis, Zn (NO 3 ) 2 .6H 2 O and NaOH were used as the starting materials.Forty milliliter aqueous solution of Zn (NO 3 ) 2 .6H 2 O (0.1 M) with AgNO 3 (0, 1, 3, and 5 mol%) (solution A) was prepared under mild stirring at 60 °C, and 20 mL of sodium hydroxide (1 M) aqueous solution (solution B) was added dropwise in solution A with constant stirring until pH ~ 12 was reached.The obtained solution was stirred magnetically for 2 h to complete homogenization.After that, the resultant solution was kept overnight in order to settle down the NPs.The resultant products were centrifuged and washed multiple times using DDW and ethanol.Finally, the materials were dried overnight at 60 °C in a hot-air oven, and ZnO NPs were obtained by heat treatment at 500 °C for 2 h with a ramp rate of 10 °C min −1 .Hereafter, bare ZnO and Ag/ZnO  samples with different mole percentages (1 mol%, 3 mol%, and 5 mol%) were referred to as AgZ-0, AgZ-1, AgZ-3, and AgZ-5 respectively.

Catalysts characterization
The structural properties of the prepared materials were recorded by an X-ray diffractometer (Rigaku SmartLab SE) in the 2θ range of 20-80° using Cu as a target (Cu K α = 1.5406Å) at 45 kV and 40 mA.The morphologies of bare ZnO and Ag (concentration = 1, 3, 5 mol%) doped ZnO were observed by a scanning electron microscope (SEM, ZEISS EVO18) and high-resolution transmission electron microscopy (HRTEM, TEM; Thermo Fisher, Talos F200S G2).X-ray photoelectron spectroscopy (XPS) data was obtained from PHI 5000 Versaprobe III spectrometer.FTIR spectra were recorded on a Thermo Fisher Scientific iS50 spectrometer.The Raman spectra of prepared nanomaterials were analyzed by a Raman spectrometer (NOST, HEDA-URSM4/5/7).The charge transport properties of Ag/ZnO nanocomposites were studied using Novocontrol Alpha-A impedance analyzer.The data of UV-Visible spectra were analyzed through a UV-Vis NIR spectrophotometer (JASCO, V-670).The spectra of photoluminescence (PL) were provided by JASCO FP-8300 spectrofluorometer.

Detection of reactive species
Various scavenger tests were performed in the same way as activity evaluations in order to scrutinize the active species responsible for photodegradation of IC.

Photocatalytic setup and procedure
IC was utilized as a pollutant to estimate the photocatalytic activity of Ag/ZnO nanoparticles in a closed instrument.The photocatalytic studies were calculated under visible light illumination induced by lamps.A custom-built photoreactor system is made up of iron, and degradation experiments were performed inside this reaction chamber.In the inner wall of the reactor, 12 fluorescent lamps of 100 W each were vertically installed (Fig. 2).For maintaining the temperature of the reactor, two exhaust air circulating fans are provided at the exterior walls.
The degradation of IC dye (10 ppm concentration) was studied by taking 100 mL of dye in a glass beaker.The 0.1 g catalyst was dispersed in the 100 mL of aqueous IC dye solution.Before photocatalysis, the suspension was stirred constantly at 500 rpm using a magnetic stirrer in the dark for 30 min for attaining the adsorption-desorption mechanism between the dye and catalyst.Later, the photodegradation efficiency was measured under visible light illumination by putting suspension in the custom-built reaction system.In the end, 3 mL of the suspension was collected after appropriate time intervals in order to measure IC concentration.The solution was centrifuged for 5 min at 2500 rpm and was analyzed using a UV-Vis spectrophotometer.The degradation of IC dye was obtained by observing a decrease in the absorption characteristic band intensity at 610 nm.The percentage degradation was calculated using Eq.(1) (Janbandhu et al. 2018): where C 0 and C correspond to the concentration of IC dye solution at time 0 (after dark adsorption) and t respectively.

XRD analysis
Figure 3a illustrates the XRD patterns of samples AgZ-0, AgZ-1, AgZ-3, and AgZ-5.The majority of the diffraction peaks in Fig. 3a can ascribe to zinc oxide (JCPDS, 01-076-0704), whereas the peaks marked with # can be assigned to Ag (JCPDS, 01-087-0717).All diffraction peaks in the XRD patterns reveal the presence of hexagonal wurtzite phase of ZnO (space group: P 6 3 mc, (1) Fig. 2 Schematic layout of the systematically designed photoreactor system a = 3.2530 Å, c = 5.2130 Å) with preferential growth along (101) crystal plane.No additional peaks were observed in the case of 1% Ag doping due to its low concentration and high dispersity (Chelli and Golder 2018).However, some additional peaks were observed for the samples with Ag concentration (≥ 3%) and are indexed as ( 111), (200), and (220) crystal planes of cubic silver (Sun et al. 2012).It was observed that the position of (101) peak slightly shifts towards higher 2θ values with the increasing Ag doping concentration (Fig. 3b).As a result of the difference in the ionic radii of Zn 2+ (0.074 nm) and Ag + (0.122 nm), stress is produced at the boundaries and edges of the host lattice during the growth and formation process (Raji et al. 2018).The mean crystallite sizes of the nanoparticles were calculated from the XRD data with the Scherrer equation: where D hkl is the crystallite size, λ = 1.5406Å, and β and θ indicate the wavelength, full width at half maximum (FWHM), and the Bragg angle respectively.The calculated values of the crystallite size are depicted in Table 2. From this table, it is observed that the crystallite size increases with increase in Ag content.This increase in crystallite size was due to the segregation of Ag species on the grain boundaries of ZnO crystallites, or an insignificant amount of Ag atoms would have incorporated

SEM and TEM analysis
SEM was used to examine the morphology of Ag/ZnO nanocomposites.Figure 4 shows SEM micrographs of (a) AgZ-0, (b) AgZ-1, (c) AgZ-3, and (d) AgZ-5 respectively.From the SEM micrographs, irregular and non-uniform size of NPs was observed.The low-resolution SEM images suggest that the NPs are agglomerated with each other, and the size of these NPs was increased with the increase in doping concentration.HRTEM measurements were conducted to investigate the particle size and morphological characteristics of AgZ-5. Figure 5a shows the high-magnification TEM images of AgZ-5, which exhibits nanoparticle-like morphology.The observed nanoparticles were spherical in nature with hexagonal shape.The corresponding SAED pattern (Fig. 5b) of Ag/ZnO heterostructures exhibits reflections (002), ( 101), ( 102), (103), and (112) corresponding to the hexagonal wurtzite phase of ZnO particles along with the (111) diffraction lines of Ag. Figure 5c depicts the HRTEM image of the AgZ-5, which shows 0.21 nm lattice fringes belong to plane (111) of fcc Ag at the edge portion, and 0.19 nm fringes belong to plane (102) of hexagonal-type wurtzite ZnO at the edge portion, confirming crystallinity of Ag and ZnO components at the edge portion.The particle size of AgZ-5 was determined by combining several crystallites and was fitted with Gaussian distribution (Fig. 5d).It appeared that particle size was greater than crystallite size.The standard ImageJ software was used to estimate the size of AgZ-5 nanoparticles, and the average particle size was found to be 29.82nm.
Energy-dispersive spectroscopy (EDS) is a common scientific technique for analyzing the elemental composition of a specimen.In order to analyze the formation of pure nanocomposite without other impurities, EDS with a HRTEM   was used, and it revealed peaks only associated with Zn, Ag, and O (Fig. 6).The observed atomic percentages of Zn, O, and Ag in nanocomposite were 48.46%, 51.23%, and 0.29% respectively.

XPS analysis
The elemental and chemical composition of the as-prepared AgZ-5 sample was investigated by XPS analysis.
The full-scan XPS spectra of AgZ-5 sample are shown in Fig. 7a.In the survey scan spectrum, Zn, C, Ag, and O peaks were observed.The carbon present in the sample might be caused by the hydrocarbon from the XPS instrument itself (Xin et al. 2018).No additional impurity peaks were observed, which is in agreement with XRD and EDS results.FTIR spectroscopic studies FTIR spectra of the prepared nanocomposites were recorded in the range of 400-4000 cm −1 , and it is given in Fig. 8a.A significant vibration band in the FTIR spectrum is assigned to the characteristic stretching mode of the ZnO bond ranging from 400 to 500 cm −1 (Gayathri1 et al. 2015).A similar spectra profile can be observed for all Ag/ZnO samples with different band positions due to addition of Ag content in ZnO.In FTIR spectra, there is a broad peak at ~ 3434 cm −1 (stretching) and ~ 1330 to ~ 1670 cm −1 (bending) which confirms the presence of hydroxyl residues due to stretching.These bands are due to the stretching mode of the O-H group (Nagaraju et al. 2017).

Raman spectroscopy
Raman spectroscopy is a very useful tool to find the structural disorder and defects in the prepared samples.For Raman spectroscopy, group theory predicts that wurtzite ZnO features the following characteristic optical phonon modes: where A 1 and E 1 modes are polar, and these are split into transverse (TO) and longitudinal optical (LO) phonons (Zhang et al. 2009).The phonon modes of Raman active (A 1 , E 1 , and E 2 ) and infrared active (A 1 and E 1 ) were observed.(Lupan et al. 2010).But the B 1 modes are infrared and Raman inactive and are normally silent modes (Sánchez Zeferino et al. 2011).
The Raman spectra of all four samples are shown in Fig. 8b.The Raman spectra of all the nanocomposites were taken in the frequency range of 50 to 800 cm −1 .The dominant peaks of pristine ZnO at ~ 99 and ~ 436 cm −1 are associated with the vibration of Zn and O atoms in the ZnO lattice, respectively.These peaks are attributed to the low and high E 2 mode (E 2L and E 2H ) of nonpolar optical phonons while the peaks around 380 and 574 cm −1 correspond to A 1 (TO) and A 1 (LO) fundamental modes of hexagonal ZnO respectively.The Raman bands at ~ 406 cm −1 reflecting the strength of the polar lattice bonds are assigned to the E 1 (TO) modes, while the peak at about ~ 330 cm −1 corresponds to the multiphonon scattering mode (Udayabhaskar et al. 2015;Sawant et al. 2018).

Impedance study
The impedance studies of the prepared materials have been recorded.The respective Nyquist plots are shown in Fig. 9 along with the equivalent circuit.An equivalent circuit contains a series resistance (R s ), a charge transfer resistance (R ct ), and a capacitor (Dridi et al. 2018).The electrochemical charge transfer resistances of the prepared materials were found to decrease with the addition of Ag and were minimal for the AgZ-5 sample.As a result, the spatial separation and transport of photogenerated e − − h + pairs are maximum in AgZ-5.

Absorption study
Figure 10a demonstrates the UV-Vis DRS absorption spectra of the ZnO containing various proportions of silver.The absorption spectra of Ag/ZnO NPs show enhanced absorbance in the visible region as compared to bare ZnO; this is most likely owing to a strong interfacial electron coupling between Ag NPs and ZnO.The prepared Ag/ZnO nanoparticles can efficiently utilize light for organic pollutant photodegradation because of the broad absorption in the visible range (Raji et al. 2018).This increased absorbance is due to the surface plasmon resonance (SPR) of Ag NPs.
The optical band gap (E g ) of the prepared photocatalysts was estimated using Tauc's equation as follows (Sukhadeve et al. 2021): where α, absorption coefficient; hν, energy of photon; A, constant; E g , optical energy band gap; and n depends on the transition (n = 1/2, 2 corresponding to allowed direct and allowed indirect transitions respectively) (Janbandhu et al. 2019b).
The band gap of all samples was estimated by plotting the (αhν) 2 and photon energy (hν).The bandgap was obtained by extrapolating the tangential line of the x-axis intercept from the linear region of the plot (Fig. 10b) and is shown in Table 1.It is observed from this table that the energy band gap of Ag/ZnO NPs decreases with an increase in Ag content.This reduction in energy band gap may be due to the increase in crystallite size.

Photoluminescence measurement
Photoluminescence spectroscopy is a well-known technique to study electronic structure, impurities, and recombination rate of free carriers of semiconductor materials.Figure 11 depicts the PL spectra recorded at room temperature for bare ZnO and Ag/ZnO nanoparticles with the excitation wavelength of 325 nm.A number of peaks were observed in the 335-650 nm range spanning both UV and visible regions.The narrow emission in the UV range is due to free exciton recombination, although emission in the visible region was due to the defects like oxygen vacancies and zinc interstitials.From Fig. 11, PL intensity was found to be decreased with an increase in Ag doping in ZnO.The doping of Ag in ZnO acts as a trap for photogenerated e − which reduces the recombination of e − − h + pairs resulting in lower PL intensity as compared to bare ZnO (Chitradevi et al. 2019).The PL intensity for AgZ-5 is found to be the lowest among all due to lower recombination of photogenerated e − − h + pairs, and therefore, it can be used for photocatalytic performance (Sarma and Sarma 2017;Sukhadeve et al. 2021).The results observed from PL are also in good agreement with the impedance study.

Photocatalytic degradation activity of IC dye
The effectiveness of Ag/ZnO nanocomposites for the IC dye degradation was studied under visible light illumination.It was observed that the photocatalytic activity was improved as the Ag concentration increased.The intensity of the absorption peaks was gradually decreased after every time interval (Fig. 12a).The decrease in dye concentration under light illumination indicates that Ag/ZnO nanocomposites are a promising group of photocatalysts for IC dye degradation (Fig. 12b).
To get more quantitative insight, the kinetic study was done for the photocatalytic activity of Ag/ZnO nanocomposites.The pseudo-first-order law gives rate constant of photocatalytic reaction (Chen et al. 2017;Raza et al. 2019) and is depicted in Table 3.
where k is the reaction rate constant, and t is irradiation time.
Figure 12c shows a plot of − ln(C/C 0 ) vs. the irradiation time, which suggests that the degradation of IC by Ag/ZnO nanocomposites follows pseudo-first-order kinetics.The (4) 1n C C 0 = kt calculated rate constants (k) for all samples are as follows: AgZ-0 (0.00524 min −1 ), AgZ-1 (0.00683 min −1 ), AgZ-3 (0.00747 min −1 ), and AgZ-5 (0.02021 min −1 ).From Table 3, it is observed that the rate constant of AgZ-5 was significantly higher than that of AgZ-0, which indicates that the photocatalytic reaction was faster in the AgZ-5 sample.In addition, the degradation efficiency was calculated for all the prepared samples.It was observed that the nanocomposites AgZ-0, AgZ-1, and AgZ-3 have a moderate effect on degradation efficiency of IC, and the percentage degradation efficiency of these photocatalysts was 72.20%, 75.20%, and 79.09%, respectively.In contrast, nanocomposite AgZ-5 had a considerable effect on the photodegradation of IC and gave a degradation efficiency of 95.71% during the span of 120 min (Fig. 12d).Consequently, among all the samples, the AgZ-5 sample had the greatest rate constant and also had the highest photocatalytic effectiveness of 95.71% degradation towards IC dye.
For the prepared samples, efficiency for IC dye degradation and rate constant (k) are calculated and are shown in  Table 3.The half-life period of the first-order reaction is the time required for 50% completion of reaction.For the prepared samples, half-life was calculated by using the following equation: The calculated half-life period for the samples is depicted in Table 3. From this table, it is observed that half-life period is found to be minimum (34.29 min −1 ) for AgZ-5 sample.
In addition, the observed results of AgZ-5 are compared with the available literature (Table 4), and the found prepared sample is more efficient for IC dye degradation.

Possible photocatalytic mechanism and trapping studies
Bare ZnO showed limited photodegradation under the visible light irradiation, whereas Ag/ZnO nanocomposites showed significantly higher degradation as the Ag concentration was increased.The limited visible-light photocatalytic activity of bare ZnO was due to the presence of oxygen vacancies, while the enhanced photocatalytic activity mechanism of Ag/ZnO towards IC dye degradation under visible light irradiation can be understood as follows: The photocatalytic activity of Ag/ZnO nanocomposites was enhanced significantly because the doping of Ag in ZnO leads to the formation of Schottky barrier at the surface of Ag and ZnO.This Schottky barrier was developed due to the (5) Half − lifeperiod = In(2) k difference in the work functions of Ag (~ 4.2 eV) and bare ZnO nanoparticles (~ 5.3 eV) (Raji et al. 2018).However, due to the strong electron oscillation by surface plasmon resonance (SPR) excitation, it has been observed that electrons can migrate from Ag to the conduction band of ZnO.Generally, electrons are transported from a substance with a lower work function to one with a higher work function until they reach an equilibrium point for the formation of fermi level (Liu et al. 2015) where χ is the absolute electronegativity, E g is the band gap energy of the semiconductor, and E VB and E CB are the VB and CB edge potentials respectively.Moreover, E e ( 6)  To better understand the underlying mechanism involved in the degradation process, radical capture experiments were performed.The benzoquinone (BQ) was adopted to scavenge superoxide radical (O 2 • − ), ammonium oxalate (AO) was selected to trap h + , potassium dichromate (PD) was introduced to trap e − , and isopropanol (IPA) was adopted to quench the hydroxyl radical (OH•).Figure 13b shows the rate of IC degradation over photocatalysts in the presence of scavengers.The addition of AO and BQ drastically

Conclusions
In summary, we have prepared Ag/ZnO nanocomposites by a simple co-precipitation method.XRD, SEM, HRTEM-EDS, UV-Visible, FTIR, Raman, impedance spectroscopy, and photoluminescence analysis were used to characterize the prepared photocatalysts.The UV-Vis study reveals that the SPR effect of Ag nanoparticles causes increased absorbance in the visible region.With the increase of Ag doping concentration, the photocatalytic efficiency was improved, and also the rate constant is higher than that of bare ZnO.Our experimental results suggest that the 5 mol% Ag/ZnO photocatalyst shows better results compared to other prepared photocatalysts.More specifically, the photocatalytic efficiency of the 5 mol% Ag/ZnO was 95.71% in a 120-min reaction time.Consequently, the prepared photocatalyst materials are suitable for the degradation of IC dye.It has been revealed that the improved photocatalytic efficiency was due to the enhanced charge transfer between ZnO and Ag and their synergistic effect.Also, the quencher study analysis indicates that the h + and O 2 • − radical play an important role to degrade the IC dye.The improved photocatalytic activity of the developed photocatalyst shows that it is effective under visible light irradiation and could be useful for organic pollutant degradation.methodology, and formal analysis.Gaurav K. Sukhadeve: resources, writing -review and editing.Rupesh S. Gedam: conceptualization, investigation, resources, writing -review and editing, and supervision.

Fig. 1
Fig. 1 Schematic illustration of the reaction process for the synthesis of Ag/ZnO nanocomposites of ZnO(Georgekutty et al. 2008;Chauhan et al. 2020).These results support the shifting of (101) diffraction peak towards higher 2θ value.

Fig. 4 Fig. 5 a
Fig. 4 SEM micrographs of a AgZ-0, b AgZ-1, c AgZ-3, and d AgZ-5 Figure7bshows the two symmetrical peaks of Zn 2p 1/2 and 2p 3/2 at 1045 eV and 1022 eV respectively.The difference between two bonding energies (~ 23 eV) confirms + 2 oxidation state of Zn, and these peaks are due to the presence of Zn-O bonds(Lu et al. 2019).The XPS spectra (Fig.7c) of Ag 3d show valence state of Ag species in ZnO.As seen in Fig.7c, two sets of peaks Ag 3d 5/2 (367.3 eV) and Ag 3d 3/2 (373.3 eV) are the characteristics of metallic silver.The difference of ~ 6 eV between these two peaks indicates the main silver species that exists in ZnO is metallic Ag 0(Patil et al. 2016;Xin et al. 2018;Ahmad et al. 2019).These obtained high-resolution XPS results confirm the formation of Ag/ZnO nanocomposites.

Fig. 8 aFig. 9
Fig. 8 a FTIR spectra and b Raman spectra of all samples

Fig. 10 a
Fig. 10 a Absorbance spectra of prepared samples.b Band gap calculations of prepared samples using Tauc's plot

Fig. 12 a
Fig. 12 a Absorption spectra of IC dye under visible light irradiation for AgZ-5 sample, b concentration changes of IC dye as the function of illumination time, c the first-order kinetics of IC photocatalytic degradation, and d degradation efficiency plot for IC dye represents the energy of free electrons which is ~ 4.5 eV on the hydrogen scale and χ is the geometric mean of absolute Mulliken electronegativity having a value of ~ 5.4 eV for ZnO semiconductor(Janbandhu et al. 2019a).The value of E CB and E VB was calculated and found to be − 0.27 eV and 2.85 eV, respectively.Here, the CB of Ag/ZnO nanocomposites has a lower redox potential than O 2 /O 2 • − (− 0.33) eV, which indicates that O 2 will not get easily reduced to O 2 • − (Nosaka and Nosaka 2017), but the zero-valent silver metal (Ag 0 ) present in Ag/ZnO exhibits plasmon resonance that may have been coupled with an electron of ZnO, leading to the reduction of O 2 to O 2 • −(Li et al. 2018a).But holes have enough positive redox potential because the oxidation potential of H 2 O/OH• − is 2.27 eV, so holes oxidize surface hydroxyl ions into OH• radicals in the valence band(Liu et al. 2019).On the basis of these explanations, we have proposed a possible mechanism for the degradation of IC dye by Ag/ZnO nanocomposites, which is shown in Fig.13a.The corresponding reaction mechanism is demonstrated as follows (Eqs.7-12): Ag∕ZnO + h → Ag∕ZnO(e _ + h + ) (8) e − + O 2 ⋯ → O 2 • − (9) Ag + + e _ (CB) → Ag (10) h + + OH _ → OH (11) OH • +ICdye → degradationproducts (12) O 2 • _ − +ICdye → degradationproductsreduces the rate of IC removal.Based on the above trapping experiments, it was found that the h + species were mostly responsible for the degradation of IC dye.The O 2 • − have a secondary role in the degradation, whereas the contribution of hydroxyl radicals is negligible.

Table 1
Characteristics of IC

Table 2
Variation of crystallite size and band gap for the synthesized Ag/ZnO nanocomposites

Table 3
. Furthermore, the Schottky barrier generated at the Ag/ZnO interface can inhibit electron transmission from Ag to ZnO.The migrated electrons are scavenged by adsorbed oxygen to form highly oxidative species such as O 2 • − .As a result, these reactive O 2 • − cause IC to degrade.Meanwhile, Ag on the ZnO surface may also function as an electron scavenger by transferring photo-excited electrons from the oxygen vacancy (V O ••) defect level of ZnO to the E f of Ag.Further information on Ag/ZnO semiconductors with photocatalytic potential can be understood by VB and CB potential analyses.The CB and VB edge potentials can be calculated by Mulliken's electronegativity (Eqs.6 and 6a) at the normal hydrogen electrode potential (E e , NHE = 4.5 eV), and the band gap value (E g ):

Table 4
Comparison of our results with the other photocatalysts