Ag Doped ZnO Thin Films Synthesized by Spray Coating Technique for Methylene Blue Photodegradation under UV Irradiation

Silver (Ag) substituted ZnO thin films were successfully deposited onto glass substrates by spray coating technique. Structure, morphology, and optical properties were evaluated by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and UVVis spectrophotometer, respectively. XRD spectra had polycrystalline wurtzite structure; SEM images showed that thin films had different surface morphology at different Ag doping concentration. From transmittance spectra, thin films transparency decreased as well as band gap energy along with increase of Ag doping concentration.Methylene blue (MB) solution was used as a pollutant in the photodegradation studies. Under UV light irradiation, the optimal Ag doping is 25%, with 83% of the decolorizing efficiencies after 3 h irradiation time and apparent constant (kapp) about 9.69 × 10 min.


Introduction
Indonesia has high number of textile industries which usually emit effluents directly into the environment.The effluents contain dyes, oils, and other chemical substances that are so harmful to the environment.The current treatment of textile industrial waste water requires high amount of chemicals and longer treatment time.Therefore an advance technology for treating waste water before being discharge into the environment is required.One of the technologies is by using a photocatalytic treatment to decompose organic pollutants including dyes and other harmless compounds like CO 2 and H 2 O [1].One of the semiconductors that now gain the attention of researchers as a photocatalyst materials is Zinc oxide (ZnO) [2].
ZnO is a direct band gap semiconductor and is found to be useful in various applications such as photocatalysts, electronics, and cosmetics [1].However, for photocatalytic application, it is desirable that ZnO should absorb not only UV but also visible light due to the fact that visible light accounts for 45% of energy in the solar radiation, while UV light accounts for less than 10%.Addition of cations (transition metals) and anions (nitrogen, sulfur, carbon, or boron) into the semiconductor has been extensively studied to enhance the activity of photocatalyst [3,4].
Addition of metal into ZnO using appropriate dopant and method can improve their applications performance.As an example in the photocatalytic application, ZnO can be added with Ag elements to improve efficiency.The Ag ions can act as acceptors in ZnO and may change Zn ions.Moreover, the addition of Ag ions can also reduce the energy gap in semiconductors so it can increase the absorption of light wavelength [5][6][7].
There are various methods that can be used to create a ZnO thin film such as pulsed laser deposition (PLD) [8], spray pyrolysis [9], metal oxide chemical vapor deposition (MOCVD) [10], and sol gel [11].Sol gel is one of the most widely used methods because it is easy to prepare and of low cost.In this study, we made Ag substituted ZnO thin films by sol gel method.In order to test the photocatalytic activity, we used methylene blue (MB) as a pollutant and photodegradation conducted for 3 h under UV light irradiation.The objective of this paper is to evaluate the substitution high 2 International Journal of Chemical Engineering concentration Ag into ZnO and to evaluate its application for methylene blue (MB) degradation.
The structural and morphological characterizations were carried out using Shimadzu Maxima 7000 X-ray diffractometer with CuK radiation (1.5405 Å) and scanning electron microscopy (SEM) JEOL-JSM 6510 LA, respectively.The optical study was carried out using UV-Vis spectrophotometer (Shimadzu 1240 SA).
The photocatalytic test was performed in an aqueous solution using methylene blue ( = 664 nm) as the probe molecule.The samples were irradiated with UV (A) light (10 W UV tube, Aquaco).The Ag-ZnO films were horizontally fixed in the middle of beaker.The dye solution (40 mL) and samples were irradiated with light source placed horizontally above the beaker.The photodegradation of the dye was followed by measuring the absorption spectra at regular interval (every 30 min) using a UV-Vis spectrophotometer (Shimadzu 1240 SA).
MB degradation efficiency () was calculated according to where  0 represents the initial concentration and  represents the concentration after  minutes of photocatalysis.

X-Ray Diffractogram (XRD) Analysis.
The changes in the structure and phase identification of Ag-ZnO thin films were conducted by XRD analysis.The diffracting angle (2) was varied between 10 ∘ and 90 ∘ and the spectra were recorded for all the samples which are shown in Figure   The interplanar spacing  ℎ values of given Miller indices ℎ, , and  of Ag doped ZnO thin films were also calculated by using Bragg equation as follows [12]: where  is the order of diffraction (usually  = 1) and  is the X-ray wavelength.The plane spacing is related to the lattice constants ,  and the Miller indices in ZnO hexagonal structure by the following equation [12]: Both lattice parameters a and c for the hexagonal closedpacked (HCP) are calculated via (002) and (101) orientations [12].Table 1 summarizes also the calculated values of interplanar spacing  ℎ of Ag doped ZnO thin films.
It can be seen from the results that the value of c in the ZnO layer is greater than the standard value of   ZnO powder (0.5205 nm) [13].Increasing value of c is caused by a radius of Ag + (1.26 Å) which is greater than Zn 2+ (0.74 Å) when Ag is on the Zn site [13].An increase in the lattice parameter can be also caused by interstitial incorporation of Ag ions into the lattice or Ag ion substitution of the Zn ions [13].Table 2 depicts the grain size D values that were estimated from (002) diffraction lines of ZnO free and Ag doped ZnO thin films using Scherrer formula [12]: where  = 0.90 is Scherrer constant,  1/2 is the width at half-maximum, and  = 1.54 Å is the wavelength of CuK radiation.

Scanning Electron Microscopy (SEM) Analysis.
The surface morphological study of the synthesized thin films was carried out using scanning electron microscopy (SEM).SEM images reveal that the films surface are different in each Ag concentration.At ZnO we found that the surface has thread-like morphology.ZA5 morphology does not have uniform surface, while in ZA10 a lot of particles in uniform size are clearly shown.ZA15 shows that the film has nonuniform surface.Thread-like morphology is shown in ZA20.The same result was observed by Tarwal and Patil [14].At ZA25, the film surface is not uniform just like ZA5 and ZA15.Shidpour et al. [15] showed that the different surface morphology determined the outcome of photocatalytic activity, which is indicated by nanowire-like morphology as having higher photocatalytic activity than nanoporous.

UV-Vis Spectrophotometer Analysis.
Figure 3 shows the absorbance spectra as a function of wavelength for Ag-ZnO thin films deposited on glass substrates prepared by sol gel spray coating.It can be seen that increase of Ag concentrations makes slightly shifts of absorption edge to higher wavelengths.In addition, the nearly flat spectra are shown in the higher wavelength area.This is due to the larger amount of Ag, increasing the substitution of Zn ion by Ag ions.
Figure 4 shows the transmittance spectra for Ag-ZnO thin films.The transmittance spectra are formed by interference of light at the interface between the films and the substrate.Transmittance spectra of Ag-ZnO films in the visible region decrease from 90 to 30% as the Ag percentage increases.This decrease in the transmittance value of the Ag-ZnO thin films may be due to the grain boundary scattering.It is also caused by surface plasmon resonance (SPR) related phenomenon of Ag nanoparticles which can absorb the visible light.
Absorption coefficient and band gap for the direct band gap semiconductor obey the following relationship: where  is the absorption coefficient, ℎV is the photon energy,  is a constant, and   is the optical band gap.The optical band gap   can be obtained by extrapolating the linear part of the curve to (ℎV) 2 = 0 if one plots (ℎV) 2 ∼ ℎV.The related curves for our samples are shown in Figure 5.The optical band gap energy for Ag-ZnO thin films is found to be in the range of 2.98 eV to 3.12 eV.The band gap narrowed with

Photocatalytic Study.
The photocatalytic process occurs when the ZnO material is irradiated by light that its energy is greater than the gap energy of ZnO.Electrons (e − ) and holes (h + ) are formed in the conduction band and valence band, respectively.Then, holes will react with water to form OH * , while the electrons form superoxide (O 2 * − ) and subsequently form other reactive oxygen species (H 2 O 2 and OH * ).
Holes and OH * are highly reactive toward organic pollutants if they are in contact.The C-C and C-H bonds of MB adsorbed on the surface of Ag-ZnO thin films can be destroyed by the oxidizing power of OH * radicals leading to CO 2 and H 2 O production [16].Thin films were immersed in 40 mL MB (10 ppm) and irradiated with UV (A) for 3 h.The photodegradation of the MB under UV (A) light irradiation was determined by measuring absorption spectra using UV/Vis spectrophotometer.Figure 6 shows the relationship between difference of concentration (  / 0 ) and irradiation time.
It can be seen that the photocatalytic degradation of MB by Ag-ZnO thin films is up to 83% after 3 h, while the MB without photocatalyst shows flat line about 18%.This result informs that, without photocatalysts, MB does not significantly change.The addition of Ag on ZnO can improve the photocatalytic activity due to the active hydroxyl radical formation on the surface of ZnO [17].On the other hand, there was also an influence of surface plasmon resonance (SPR) effect that can increase the photocatalytic activity by the increase of optical absorbance and charge carrier separation under UV irradiation [17].
Langmuir-Hinshelwood model describes the kinetic of photocatalytic degradation of MB and the photocatalytic degradation rate () is given by the following equation [16]: where / is the degradation rate (mgL −1 min −1 ),  is the dye concentration (mgL −1 ) at the irradiation time (),   is the rate constant (min −1 gL −1 ), and  dye is the adsorption coefficient of the dye (Lmg −1 ).Equation ( 7) is simplified into the first-order kinetic model with an apparent first-order rate constant  app (min −1 ) and at low concentrations ( dye  ≪ 1) [16]: The constants  app of MB are determined from the slopes of the linear plots ln(  / 0 ) against the time () (Table 3).Figure 7 shows that ZA25 has highest apparent first-order rate constant ( app ) about 9.69 × 10 −3 min −1 and the correlation coefficient about 0.95.These results are consistent with the results of the band gap ZA25, which is smaller than the other layers.The smaller gaps of energy band will facilitate the electrons movement from the valence to the conduction band, thus increasing the photocatalytic process.

Conclusion
Ag-ZnO thin films have been successfully synthesized onto glass substrates by using sol gel spray coating technique.From X-ray diffraction analysis, the films have polycrystalline spectra with wurtzite structure.The SEM images showed different morphology on each Ag doping concentration.The surface morphology for the sample ZnO and ZA20 shows thread-like morphology.Optical properties of Ag-ZnO thin films show that transparency decreases along with increase of Ag content.The absorbance spectra of thin films are slightly shifted to higher wavelengths and ZA25 has lowest energy gap about 2.98 eV.The photocatalytic degradation of MB solution using Ag-ZnO thin films was investigated under UV light irradiation.Sample ZA25 has the highest efficiency on photodegradation of MB.

Figure 1 :
Figure 1: X-ray pattern of Ag-ZnO thin films prepared by thermal spray coating at 450 ∘ C.

Fig- ures 2
(a)-2(f) show the SEM images with ×10.000 magnification of all samples deposited at 450 ∘ C onto glass substrates.

Figure 6 :
Figure 6: Degradation under UV irradiation with various films.

Table 2 :
FWHM and grain size of Ag-ZnO sprayed thin films.

Table 3 :
Apparent rate constants of methylene blue degradations.