EFFECT OF SILVER NANOPARTICLES SILAR CYCLE ON TiO 2 NANOPARTICLES THIN FILM: OPTICAL AND STRUCTURAL STUDY

Titanium dioxide (TiO 2 ) has gained a lot of research interests due to its applicability in electronic materials, energy, environment, health & medicine, catalysis etc as a result of its high permittivity, refractive index, efficiency, low-cost chemical inertness, eco-friendliness, photocatalytic activity, photostability and ability of decomposing a wide variety of organic compounds. In this study, the effect of silver nanoparticles (AgNPs) deposited through Successive Ionic Layer Adsorption and Reaction (SILAR) on the optical, structural and morphological properties of TiO 2 was explored systematically. The investigation was achieved via a combined effect of UV-vis spectroscopy, Scanning Electron Microscope (SEM) and X-ray Diffractometer (XRD) characterizing tools. As illustrated from the SEM micrographs, introduction of AgNPs result to enhanced nucleation and films growth with presence of shining surface which can be seen to contribute to good photon management through enhanced light scattering. The XRD results showed that, the presence of AgNPs on TiO 2 results to peaks corresponding to that of the TiO 2 crystallographic planes with no silver peaks detected due to its low concentration in the nanocomposite which shows that it was just homogeneously distributed on the surface of the TiO 2 nanoparticles. The UV-Vis results show a red shift to higher wavelength, showing an increase in visible light absorption which can be ascribed to the strong field effect of the Localized Surface Plasmon Resonance (LSPR). There was a decrease in band gap edge with introduction of AgNPs which indicated an increase in the optical conductivity of the AgNPs modified film.


INTRODUCTION
The wide band gap energy of titanium dioxide prevents it from being active under ultraviolet (UV) light [1][2][3]. The solar spectrum is composed of 3-5 % UV light and around 40% visible (Vis) light [4]. As such, the photocatalytic properties of TiO 2 observed under sunlight irradiation is less active. There is need for modification of TiO 2 to improve its photocatalytic efficiency under sunlight visible irradiation. Several works have been done to improve TiO 2 so as to render it active in the visible light (400-800 nm) [5][6][7]. These include: (a) sensitization of TiO 2 with photosensitizers to absorb light in the visible region [8][9][10][11][12], (b) doping TiO 2 with metallic nanoparticles (MNPs) [13,14], and (c) doping TiO 2 with non-metals [15]. Presently, most researchers focus their attention on the MNPs approach by incorporating the metallic nanoparticles to the dense surface of TiO 2 to increase the photoactive wavelength range and enhance the photocatalytic activity under UV irradiation [5][6][7]13]. Among the doped metals (Ag, Au, Cu, Pt, Nb, Al, B, S, N, Zn, etc), noble metals have attracted significant attentions because of their enhanced properties through Surface Plasmon Resonance (SPR) effect [5,6]. These noble metals serve as scattering centers and sub-wavelength antennas in which the confined electromagnetic energy based on LSPR may greatly improve the absorption factor of the active medium surrounding the NPs [5,13].
Some current reports have shown that metal nanoparticles, such as Ag nanoparticles, augment the catalytic movement in the visible region of the electromagnetic spectrum [16,17]. The results disclosed that the existence of AgNPs is accountable for increasing the photocatalytic activity of TiO2 in the visible region of sunlight [18].
There are some contradictory results that also reported showing the decreased activity of silver modified titania [12,19]. This may be due to their preparation method, nature of organic molecules, photoreaction medium, or the metal content and its dispersion. Even though there are many studies show the photocatalytic activity of silver doped titania [20][21][22], the exact mechanism and the role of silver is under debate. In this paper we reported a systematic study of photons initiated photocatalytic activity of screen-printed titania with silver nanoparticles. We discussed the effect on the optical and structural properties and the results show that the introduction of silver nanoparticle in the TiO2 enhances the optical-response and contributes to the band gap narrowing and also enhances nucleation and grain growth.

Preparation of the Cationic Precursor
The AgNPs was prepared by dissolving 0.05 g of silver nitrate in 30 ml of deionized water. Ammonium hydroxide (NH 4 OH) was added dropwise. When small amount of NH 4 OH was added, Brownish precipitate appears which resulted in silver (I) oxide (Ag 2 O (s) ) formation: When excess NH 4 OH was added to silver (I) oxide (Ag 2 O (s) ), colorless solution of diammine silver complex ion was formed: The equation that describes the ionic state of the complex system is given as:

Deposition of TiO2
The TiO 2 was used as received at its analytical grade level. The TiO 2 was deposited using screen printing technique where a 120 mesh device was used to directly screen print Ti-Nanoxide D/SP on four glass slides. They were dried at 150 o C then annealed at 450 o C.

Deposition of AgNPs using SILAR procedure
The glass slide with TiO 2 was immersed in 3 2 [ ( ) ] Ag NH  solution for 2 minutes which describes the adsorption process, then rinsed with distilled water for 1 minute to remove excess adsorbed ions from the diffusion layer, and thereafter, it was then transferred to the tin chloride solution for 2 minutes which describes the reduction process. At this point the film turns brownish due to the reduction from Ag+ to Ag, it was then rinsed in distilled water for 1 minute to remove excess unreacted specie. This process is called one (1) SILAR cycle and is as shown in Figure 1 below. The process was repeated for two and three SILAR cycles.

Characterization and Measurement
Optical study of pure TiO 2 and TiO 2 with 1, 2 and 3 SILAR cycles of AgNPs were recorded on Axiom Medicals (UV752 UV-Vis-NIR spectrophotometer). Scanning Electron Microscopy (SEM) images were obtained using Phenom pro-X at an acceleration voltage of 10 kV. Structural analysis of the TiO 2 film was performed using X-ray diffractometer (Rigaku D, Max 2500).  This AgNPs can broaden the active area of TiO 2 through the introduction of localized surface plasmon resonance (LSPR) effect. When 1 SILAR cycle of AgNPs is introduced, a relatively symmetric distribution of shining particles were observed which can be speculated to contribute to good photon management through enhanced light scattering. The result demonstrates a sample size in the range of 25-55 nm as obtained from Isolution image analyzer with a smooth surface with no aggregated islands. Increasing the number of SILAR cycle from 2 to 3 cycles resulted to formation of significant islands with inhomogeneous distribution of nanoparticles leading to lesser transmittance of light.

X-ray Diffractometer (XRD)
The XRD pattern of TiO 2 , TiO 2 with 1, 2 and 3 SILAR cycles are as shown in Figure 3a-d. This result clearly shows that the TiO 2 nanoparticles were preferentially oriented with (101) face [23]. The anatase phase of TiO 2 is preferred over rutile and brookite for photocatalytic degradation of organic compounds [6,24]. The Fermi level of anatase is ~0.1 eV which is considered higher than that of the rutile phase and as such preferred as a photoelectrode material due to its generation of higher photovoltage over the rutile [25].
After depositing AgNPs on the TiO 2 nanoparticles surface, the observed peaks at the 2 theta angles correspond to that of the TiO 2 crystallographic planes. The peaks due to AgNPs was not detected due to its low concentration which shows that it was just homogeneously distributed on the surface of the TiO 2 nanoparticles [6]. Hence, the peaks are identical for TiO 2 and TiO 2 with 1, 2 and 3 SILAR cycles of AgNPs which shows that TiO 2 was effectively passivated with the AgNPs.

Optical study
The optical absorbance, transmittance, and reflectance bands of pure TiO 2 and TiO 2 with 1, 2 and 3 SILAR cycles of AgNPs are shown in Figure 4a-c. The measurement was taken at room temperature over the wavelength range of 200-1200 nm.

Figure 4 (a) absorbance, (b) transmittance and (c) reflectance against Wavelength
As shown in Figure 4a, the pure TiO 2 can be seen to be absorbing in the UV region with peak around 317 nm which can be attributed to its high refractive index. This property displayed suggests that, modification of the TiO 2 is desirable to make it activated in the visible region. Furthermore, Figure 4a also shows a red-shift towards higher wavelength after introducing AgNPs. This enhanced behavior can be seen to arise significantly through the introduction of LSPR from the deposited AgNPs. Figure 4(b) presents typical transmittance spectra, which was obtained from the absorbance plot using equation (7).
where T is transmittance and A is absorbance. There was a relatively constant transmittance in the visible to infrared region from 580 nm for TiO 2 and TiO 2 with 1 SILAR cycle of AgNPs. For the 2 SILAR cycle, the constant transmittance was observed from 650 nm. In TiO 2 /3cycles nanocomposite, we observed variation in transmittance from the wavelength range of ~739 nm to ~ 1025 nm. It was observed generally, that transmittance intensity was inversely proportional to increase in the AgNPs SILAR cycle. For the reflectance properties, the reflectance was obtained from Equation (8).
As observed in the micrographs, there was presence of valleys and peaks in the profile with cycle increase. The red shift and the increase in optical path length is attributed to the non-uniform aggregation of the dispersed AgNPs forming the clusters [26] and the effect of interstitial dominance due to introduction of localized defect states during film formation [18].
The refractive index, Extinction coefficient and optical conductivity against photon energy (hv) is as shown in Figure 5a-c. The refractive index ( n ) shows the frequencies within a range in which films are not absorbing strongly [27]. The refractive index is crucial in characterizing photonic materials and it was estimated using equation (9), where R is the reflectance.
The optical behavior of refractive index against photon energy of TiO 2 and TiO 2 with 1, 2 and 3 SILAR cycles of AgNPs is shown in Figure 5a. The results obtained show that the refractive index decreases with the increasing photon energy slightly before maintaining a constant value after 2.3 eV. The observation that the refractive index decreases with increasing photon energy is attributed to the dispersion of light at various interstitial layers present in the composite films. Figure 5b shows the extinction coefficient versus photon energy as calculated from equation (11), where  is the coefficient of absorption obtained from equation (10) and  is the wavelength.
The extinction coefficient is referred to the measure of the fractional light loss as a result of scattering and absorption per unit distance of the medium of penetration [27].
It is seen from Figure 5b that the extinction coefficient of all AgNPs modified TiO 2 increases with the increase in the photon energy up to 2.5 eV before it starts decreasing to form a valley at 3.4 eV. This decrease indicates that the fractional loss of light as a result of scattering and absorbance decreases between the range of 2.5 and 3.4 eV. It can also be noted from the figure that the value of extinction coefficient maintains a constant value from ~3.7 to 4.9 eV.
The Optical Conductivity was calculated from equation (12), where  is the optical conductivity,  is the coefficient of absorption, n is the refractive index and c is the speed of light with magnitude of 8 3×10 m/s. The variation of optical conductivity with the incident photon energy is shown in Figure 5c. The rise in optical conductivity when the energies of the photon is increasing can be attributed to high absorbance of the films in regions with higher photon energies and also due to increase in absorption coefficient. This can also be seen to arise from increased in density of localized states in the boundaries due to the rise in new defect states [28]. The energy bandgap has been evaluated from the absorption spectra. The photocatalytic ability of TiO 2 photocatalyst under visible light is determined by lower bandgap energy ( g E ) or absorption in visible light. The energy bandgap ( g E ) was determined based on the wavelength of maximum absorption (λ) according to equation (13). 1240 here, the maximum absorption wavelength is obtained from the absorption wavelength data.
The g E was obtained in the formula describes as Tauc plot [29]. versus hv plot to hv = 0. Figure 6a-d show the optical band gap of TiO 2 and TiO 2 loaded with 1, 2 and 3 SILAR cycle of AgNPs. As shown from the results, increase in number of SILAR cycles of AgNPs leads to decrease in band gap of the film. A reduction in the band gap demonstrates an increase in the optical conductivity of the material, this is because the electrons will require less energy to cross the fermi energy level of the material [7]. this shows that, a reduction in the band gap energy of the modified samples will yield a better electron transport capability to the conduction band of the pure TiO 2 . The band gap of TiO 2 , TiO 2 /Ag, TiO 2 /2Ag, and TiO 2 /3Ag nanocomposites are: 3.23 eV, 2.27 eV, 1.88 eV and 1.74 eV.

CONCLUSION
The optical, structural and morphological properties of TiO 2 and TiO 2 with 1, 2, and 3 SILAR cycles were investigated using the combined effect of UV-vis spectroscopy, Scanning Electron Microscope and X-ray Diffractometer. The results obtained show that TiO 2 was significantly enhanced to absorb photons at higher wavelength with silver nanoparticles inclusion which is attributed to localized surface plasmon resornance phenomenon from AgNPs. The band gap of TiO 2 was dramatically reduced from 3.