3.1. XRD analysis
Figure 3(a) illustrates the XRD patterns of samples AgZ-0, AgZ-1, AgZ-3, and AgZ-5. The majority of the diffraction peaks in Fig. 3 (a) can ascribe to zinc oxide (JCPDS: 01-076-0704), whereas the peaks marked with as # 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: P63mc, 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. 3(b)). As a result of the difference in the ionic radii of Zn2+ (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:
$${D_{hkl}}=\frac{{K\lambda }}{{\beta \operatorname{Cos} \theta }}$$
2
where \({D_{hkl}}\)is the crystallite size, where \(\lambda\)= 1.5406 Å, \(\beta\)and \(\theta\) indicates the wavelength, full width at half maximum (FWHM), and the Bragg angle respectively.
The calculated values of 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 into the lattice of ZnO (Georgekutty et al. 2008; Chauhan et al. 2020). These results support the shifting of (101) diffraction peak towards higher 2θ value.
Table 2
Variation of crystallite size and band gap for the synthesized Ag/ZnO nanocomposites.
Sample ID | Ag concentration (mole%) | Average crystallite size (nm) | Band gap (eV) |
AgZ-0 | 0 | 26.21 | 3.17 |
AgZ-1 | 1 | 26.36 | 3.16 |
AgZ-3 | 3 | 27.43 | 3.13 |
AgZ-5 | 5 | 27.83 | 3.12 |
Figure 3
Table 2
3.2. 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.
Figure 4
HRTEM measurements were conducted to investigate the particle size and morphological characteristics of AgZ-5. Figure 5(a) shows the high magnification TEM images of AgZ-5, which exhibits nanoparticles like morphology. The observed nanoparticles were spherical in nature with hexagonal shape. The corresponding SAED pattern (Fig. 5(b)) of Ag/ZnO heterostructures exhibit reflections (002), (101), (102), (103), (112) corresponding to the hexagonal wurtzite phase of ZnO particles along with the (111) diffraction lines of Ag. Figure 5(c) 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 belonging 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. 5(d)). 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 29.82 nm.
Figure 5
EDS is a common scientific technique for analyzing the elemental composition of a specimen. In order to analyze formation of pure nanocomposite without other impurities, energy dispersive spectroscopy (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.
Figure 6
3.3. FTIR Spectroscopic studies
FTIR spectra of the prepared nanocomposites was recorded in the range of 400–4000 cm− 1, and it is given in Fig. 7(a). A significant vibration band in the FTIR spectrum is assigned to the characteristic stretching mode of the ZnO bond ranging from 400 cm− 1 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 cm− 1 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).
Figure 7
3.4. 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:
Γ = 1A1 + 2B1 + 1E1 + 2E2
where A1 and E1 modes are polar and these split into transverse (TO) and longitudinal optical (LO) phonons (Zhang et al. 2009). The phonon modes of Raman active (A1, E1, and E2) and infrared active (A1 and E1) were observed. (Lupan et al. 2010). But the B1 modes are infrared and Raman inactive and are normally silent modes (Sánchez Zeferino et al. 2011).
The Raman spectra of all four samples was shown in Fig. 7(b). 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 E2 mode (E2L and E2H) of nonpolar optical phonons while the peaks around 380 and 574 cm− 1 correspond to A1(TO) and A1(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 E1(TO) modes. While the peak at about ~ 330 cm− 1 corresponds to multiphonon scattering mode (Udayabhaskar et al. 2015; Sawant et al. 2018).
3.5. Impedance study
The impedance studies of the prepared materials have been recorded. The respective Nyquist plots are shown in Fig. 8 along with the equivalent circuit. An equivalent circuit contains a series resistance (Rs), a charge transfer resistance (Rct), 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 was minimal for the AgZ-5 sample. As a result, the spatial separation and transport of photogenerated e−-h+ pairs is maximum in AgZ-5.
Figure 8
3.6. Absorption study
Figure 9(a) 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.
Figure 9
The optical band gap (Eg) of the prepared photocatalysts was estimated using Tauc's equation as follows (Sukhadeve et al. 2021):
αhv = A (hν - Eg)n (3)
where α - absorption coefficient, hν – energy of photon, A - constant, Eg - optical energy band gap, n - depends on the transition (n = 1/2, 2 corresponding to allowed direct, 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 x-axis intercept from the linear region of the plot (Fig. 9(b)) and shown in Table 1. It is observed from this table that, 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.
3.7. Photoluminescence measurement
Photoluminescence spectroscopy is a well known technique to study electronic structure, impurities, and recombination rate of free carriers of semiconductor materials. Figure 10 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, zinc interstitials, etc. From Fig. 10, 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 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.
Figure 10
3.8. Photocatalytic degradation activity of IC dye
The effectiveness of Ag/ ZnO nanocomposites for the IC dye degradation were 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. 11(a)). The decrease in dye concentration under light illumination indicates that Ag/ZnO nanocomposites are a promising group of photocatalysts for IC dye degradation (Fig. 11(b)).
Figure 11
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.
Table 3
Photocatalytic degradation of IC dye.
Photocatalyst | Dye | Degradation time (min) | % Degradation | k (min− 1) | T1/2 (min− 1) | Adj-R2 |
AgZ-0 | IC | 120 | 72.20 | 0.00857 | 80.88 | 0.9698 |
AgZ-1 | IC | 120 | 75.20 | 0.00900 | 77.02 | 0.9376 |
AgZ-3 | IC | 120 | 79.09 | 0.00953 | 72.73 | 0.9399 |
AgZ-5 | IC | 120 | 95.71 | 0.01957 | 35.42 | 0.9299 |
\(\ln \frac{C}{{{C_0}}}=\) kt (4)
where k is the reaction rate constant, and t is irradiation time.
Figure 11(c) shows a plot of -ln(C/C0) vs. the irradiation time, which suggests that the degradation of IC by Ag/ZnO nanocomposites follows pseudo-first order kinetics. The calculated rate constants (k) for all samples are as follows: AgZ-0 (0.00857 min− 1), AgZ-1 (0.00900 min− 1), AgZ-3 (0.00953 min− 1) and, AgZ-5 (0.01957 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 gives a degradation efficiency of 95.71% during the span of 120 minutes (Fig. 11(d)). Consequently, among all the samples 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) were calculated and shown in Table 3. The half life period of first order reaction is the time required for 50% completion of reaction. For the prepared samples, half life was calculated by using following equation:
Half life period = \(\frac{{ln\left( 2 \right)}}{k}\) (5)
The calculated half life period for samples is depicted in Table 3. From this table, it is observed that half life period is found to be minimum (35.42 min− 1) for AgZ-5 sample.
Table 3
In addition, the observed results of AgZ-5 are compared with the available literature (Table 4) and found the prepared sample is more efficient for IC dye degradation.
Table 4
Comparison of our results with the other photocatalysts.
S. No. | Method | Material | Pollutant (Concentration) | Source | Catalyst | Time (min) | Efficiency | References |
1 | SILAR | ZnO: Ag 5wt% | MB (10 mg/L) | UV light | 50 mg | 180 min | 75.8% | [37] |
2. | Chemical precipitation method | Ce–ZnO | MO (5 mg/L) | High-pressure mercury lamp (250 W) | 500 mg | 240 min | 89.5% | [38] |
3. | Electrospinning method | Fe–ZnO | MB (10 mg/L) | Mercury lamp | 400 mg | 360 min | 88% | [39] |
4. | Sol gel method | Ag-doped ZnO | MB (10 mg/L) | Visible light | NM | 140 min | 45.1% | [40] |
5. | Sol–gel spin coating technique | Ag/ZnO NPs nanocomposite thin films | IC (6.6 mg/L) | Visible light | NM | 441 min | NM | [41] |
6. | Spray pyrolysis | ZnO/Bi2O3 heterostructures thin films | IC (5 mg/L) | Visible light | NM | 720 min | NM | [42] |
7. | Chemical bath deposition method | ZnO nanorod ZnO nanoflower | IC (10 mg/L) IC (10 mg/L) | Visible light Visible light | NM | 720 min | ∼14% 9%. | [43] [43] |
8. | Sol-gel | Ag/GO/ZnO | MB (15 mg/L) | 100 W UV lamp | 300 mg | 180 min | 97.5% | [44] |
9. | Co-precipitation method | Ag/ZnO nanocomposites | IC (10 mg/L) | Visible light | 100 mg | 120 min | 95.71% | Present study |
* NM- not mentioned |
Table 4
3.9. 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 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). 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 O2•−. As a result, these reactive O2•− 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 (VO••) defect level of ZnO to the Ef 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 Mullikens electronegativity equations (6) and (7) at the normal hydrogen electrode potential (Ee, NHE = 4.5 eV), and the band gap value (Eg):
E(CB) = χ - Ee − 0.5 Eg (6)
E(VB) = E(CB) + Eg (7)
Where χ is the absolute electronegativity, Eg is the band gap energy of the semiconductor, and EVB and ECB are the VB and CB edge potentials respectively. Moreover, Ee 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 ECB and EVB were 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 O2/ O2•− (-0.33) eV, which indicates that O2 cannot be reduced to O2•− (Nosaka and Nosaka 2017). But holes have enough positive redox potential because the oxidation potential of H2O/ 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 and which is shown in Fig. 12 (a).
Figure 12
The corresponding reaction mechanism is demonstrated as follows (equations 8 to 13):
Ag/ZnO + hν → Ag/ZnO (e− + h+) (8)
e− + O2 → O2•− (9)
Ag+ + e− (CB) → Ag (10)
h+ + OH− → OH• (11)
OH• + IC dye → degradation products (12)
O2•− + IC dye → degradation products (13)
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 (O2•−), 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 12(b) shows the rate of IC degradation over photocatalysts in the presence of scavengers. The addition of AO and BQ drastically reduces the rate of IC removal. Based on the above trapping experiments, it was found that the h+ species were mostly responsible for degradation of IC dye. The O2•− have a secondary role in the degradation, whereas the contribution of hydroxyl radicals is negligible.