Photocatalytic Activity of Ag-TiO 2 Composites Deposited by Photoreduction under UV Irradiation

In this work, we synthesized Ag nanoparticles on TiO2 thin films deposited on soda lime glass substrates. Ag nanoparticles were synthesized by photoreduction under UV irradiation silver nitrate solution. X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) measurements were used for physicochemical characterization. The structural study showed that all samples were polycrystalline, main phases were anatase and rutile, and no additional signals were detected after surface modification. Raman spectroscopy suggested that silver aggregates deposited on the TiO2 films could exhibit the surface plasmon resonance (SPR) phenomenon; XPS and SEM analysis confirmed TiO2 film morphological modification after photoreduction process. Photocatalytic degradation of methylene blue (MB) was studied under UV irradiation in aqueous solution, and, besides, pseudo-first-order model was used to obtain kinetic information about photocatalytic degradation. Results indicated that Ag-TiO2 showed an important increase in photocatalytic activity under UV (from 20% to 35%); finally, Ag-TiO2 thin films had kapp value 2.4× 10± 0.003min of 1.8 times greater than the kapp value 1.3× 10± 0.0004min of TiO2 thin films.


Introduction
Nowadays, titanium dioxide (TiO 2 ) is one of the most important semiconductors in photocatalytic applications because of its prominent properties (e.g., it is harmful, stable, resistant to photocorrosion, inexpensive, and has high photocatalytic properties in the degradation of different pollutants) [1,2].However, despite all their characteristics, TiO 2 has three main drawbacks: (a) fast recombination rate of photogenerated electron-hole pair, (b) low quantum yield in the photocatalytic reactions in aqueous solutions, and (c) a high bandgap value (3.2 eV, photocatalytic active only under UV irradiation) [3][4][5].Due to energy transfer that must be efficient during electron-hole pair separation, fast recombination of the photogenerated charge pair (e − /h + ) after activation of semiconductor is an important drawback for using titanium dioxide (TiO 2 ) as a photocatalyst, because it inhibits redox processes and reduces strongly photocatalytic efficiency [6][7][8].Two strategies have been explored to solve this drawback: (a) TiO 2 synthesis methods can be modified and optimized and (b) the TiO 2 surface can be modified.TiO 2 surface modification by transition metal deposition (e.g., silver, platinum, ruthenium, and palladium) has been reported to decrease the recombination process by forming heterostructures and new interfaces that improve semiconductor photocatalytic properties [9][10][11].Noble metal cocatalysts can improve photocatalytic properties of semiconductor because they act as electron traps near to conduction bands and/or they improve surface electron excitation by surface plasmonic resonance [12][13][14].Nalbandian et al. fabricated Ag-TiO 2 nanofibers by electrospinning, and they showed that the presence of the Ag cocatalyst enhanced reactivity [15].Yilmaz et al. deposited photochemically Pd nanoparticles on TiO 2 nanorods, and they corroborated that electron transfer between Pd and TiO 2 was improved after modification process [16].Currently, different reports showed advances for the fabrication of Ag/TiO 2 nanoheterostructure catalysts (e.g., Ag-modified TiO 2 nanoparticles, nanowires, and nanorods) [17,18]; however, several practical problems arise from the use of powder during the photocatalytic process: (a) separation of insoluble catalyst from suspension is harder, (b) particles in suspensions create aggregates especially at high concentrations, and (c) suspensions are difficult to apply on continuous flow systems.The TiO 2 thin film is an efficient alternative to solve these drawbacks.
In this contribution, we evaluate the photocatalytic activity of Ag-TiO 2 composite thin films in the degradation of methylene blue as a potential alternative in photocatalysis.

Experimental
2.1.Photocatalyst Synthesis.TiO 2 films were deposited on soda lime glass (SLG) by doctor blade method.TiO2-Degussa powder (P25) was used as reagent, and after this process, 1.5 μm TiO 2 thin film thickness was obtained [19].Silver particle deposition on TiO 2 films was carried out by chemical photoreduction: first, TiO 2 films were immersed in an aqueous silver nitrate solution (8.0 × 10 −2 M) for 30 and 60 minutes; after the photoreduction process, TiO 2 film colors changed from white to grayish brown.

Photocatalyst Characterization.
We study thin film physicochemical properties through measurements of Raman spectroscopy, X-ray diffraction (XRD), and scanning electronic microscopy (SEM); furthermore, the thickness of the thin films was measured through a Veeco Dektak 150 profilometer.Raman spectra were recorded on DRX Raman microscope with laser at 680 nm; SEM images were carried out using a Cambridge S360 microscope operating at acceleration voltage of 20 kV with a scanned area 11 μm × 10 μm, and XRD X-ray diffraction patterns of the samples were recorded in Shimadzu 6000 diffractometer with a source of CuKα radiation (λ = 0 15418 nm) in a range diffraction angle 2θ between 20 °and 70 °.Finally, XPS measurements were performed on X-ray photoelectronic spectrometer (NAP-XPS; brand Specs) with a PHOIBOS 150 1D-DLD analyzer, using a monochromatic source of Al-K α (1486.7 eV, 13 kV, 100 W) with energy from the 90 eV for the general spectra and 20 eV for the high-resolution spectra.The step was 1 eV for the general spectra and 0.1 eV for the high spectra.We performed 20 measurement cycles for the highresolution spectra and 3 for the general spectra.
2.3.Photocatalytic Activity.Thin films were immersed in blue methylene solution (10 ppm was used as target solution), and prior to irradiation, the system was magnetically stirred in the dark by 1 hour to ensure the equilibrium of dye adsorption-desorption on thin film surface.The system was irradiated by UV lamp with an emission maximum of 260 nm and power 30 W during 180 minutes.All the tests of catalytic reactivity were carried out in aerobic conditions (magnetically stirred: 150 rpm); the irradiated surface area was 3.0 cm 2 , and the volume of the suspension was 0.015 l. 1) shows a general scheme of photoreactor.We chose methylene blue as the model compound because of, since 2010, International Organization for Standardization (ISO) published standard: 10678:2010, namely, the "Determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue" [20].Finally, the concentration of dye was determined by spectrophotometry at λ = 665 nm for using calibration curve (correlation coefficient R = 0 998).

Results
3.1.SEM Analysis.SEM and EDS were used for morphological compound characterization.Figure 2 shows SEM images for both TiO 2 and Ag-TiO 2 thin films.Figures 2(a) and 2(b) show that the microaggregates compose TiO 2 films: microaggregates have a size range around 50 nm; these results are according to the typical morphology of TiO 2 -Degussa P25 [21].Figures 2(c) and 2(d) show the morphological results to Ag-TiO 2 thin films; the SEM image allows to differentiate the deposited silver particles.Figure 2 shows the photoreduced particles agglomerated reaching sizes of the order of 200 nm.The EDS analysis showed that the agglomerates observed in the surface of TiO 2 corresponded to silver particles (Figures 2(e)).

Raman Assay.
Figure 3 shows the Raman spectrum for TiO 2 films and the Ag-TiO 2 composite thin films deposited after 30 minutes of UV light irradiation.TiO 2 films showed characteristic vibration Raman mode signals to anatase phase: signals located at 144 cm −1 , 398 cm −1 , and 520 cm −1 are assigned to E 1g vibration mode, for the signal located at 639 cm −1 , it is attributed to 398 cm −1 E 3g vibration mode, and results are according to other reports [22].Raman   International Journal of Photoenergy that some noble metal nanoparticles (gold, silver, palladium, and platinum) can generate an intense surface plasmon resonance when adsorbed onto semiconductors; finally, SPR process could affect optical, electrical, and photocatalytic properties of TiO 2 [26,27].these reflections can be associated to the planes (110) and (220) of the rutile crystalline phase (JCPDS #021-1276).The presence of the rutile is due to the composition of the material used as TiO 2 source (Degussa P25).Furthermore, after Ag photoreduction on TiO 2 surface, the intensity of main signal (located at 2θ = 25 4 °) decreases indicating that the anchoring process did not change the characteristic crystal surface structure of TiO 2 ; these results are according to SEM assay [28].The signals at 574.0 eV, 374.0 eV, and 368.0 eV correspond to the electronic transitions of Ag 3p 3/2 , Ag 3d 3/2 , and Ag 3d 3/2 , respectively; Figure 5(c) plots the HRXPS spectrum of Ag 3d 5/2 and Ag 3d 3/2 double peaks, which are centered at 367.8 and 373.8 eV, respectively; and the splitting of the 3d doublet was 6.0 eV.This binding energy indicated that silver was of metallic nature.These results are according to other reports [29][30][31].Figure 5(b) shows the peaks at 458.6 and 464.3 eV; these signals correspond to Ti 2p 3/2 and Ti 2p 1/2 indicating the formation of Ti 4+ in TiO 2 [32][33][34].Figure 5(a) also shows an important signal at 530.2 eV, and this corresponds to the phototransition O1s; finally, the signal located at 285 eV corresponds to electronic transition of C 1s.This signal is typical of atmospheric CO 2 absorbed on the surface of the simple.

XRD Assay.
We performed quantitative assay of Ag-TiO 2 thin films for using XPS survey spectra shown in Figure 5; it mainly consists in determination of the relative concentration of respective surface atoms like Ti, O, C, and Ag as follows: where I i corresponds to relative intensity height of the O 1s , Ti 2p , Ag 3d , and C 1s core-level line peaks and ASF i is the atomic sensitivity factors related to the height of specific peaks [35][36][37].Table 1 summarizes the corresponding partial concentration of specific elements in the Ag-TiO 2 thin films.
3.5.Photocatalytic Assay. Figure 6 shows the C t /C o (methylene blue) as a function of time under UV irradiation, in the presence of TiO 2 and Ag-TiO 2 thin films for 180 min.Figure 6 shows similar profile to behavior observed for the photocatalytic degradation of different dyes; previous studies showed that photocatalytic degradation rate of textile dyes in heterogeneous photocatalytic oxidation systems under UV light illumination followed Langmuir-Hinshelwood (L-H); the Langmuir-Hinshelwood expression that explains the kinetics of heterogeneous catalytic systems is given by [38][39][40][41][42][43][44] where r is the rate of dye mineralization, k t is the rate constant, C is the methylene blue concentration, and K is the adsorption coefficient.Equation ( 1) can be solved explicitly for t by using discrete changes in [MB] from the initial concentration to a zero reference point; however, when the concentration of substrate is in the scale of μmoles, an apparent first-order model can be assumed; in our case, Integration of (2): where t represents time in minutes, k app (k t K) is the apparent reaction rate constant (min −1 ), and C t is the  4 International Journal of Photoenergy concentration of MB at each time.(L-H) kinetics is commonly used to explain the kinetics of the heterogeneous catalytic processes; the assumption of a pseudo-first-order kinetic model was used in several studies to characterize the effect of different experimental conditions [45][46][47].Figure 6 shows that when the MB solution was exposed to UV radiation on TiO 2 thin films, a small reduction in the concentration of MB was observed (near 20%).These results are according to expected because TiO 2 thin films are photocatalytic active under this kind of radiation.Visible is a typical behavior for TiO 2 due to their well-known wide bandgap value [48].Furthermore, Figure 6 shows that Ag-TiO 2 thin films showed an important increase in the photocatalytic activity under UV (near 35%); these results could be due to the silver aggregates deposited on the TiO 2 surface that can accumulate charge and accept electrons inside the semiconductor.The accumulation of electrons can reduce the carrier's recombination increasing generation of reactive oxygen species, and besides, Ag particle on TiO 2 can change and a collective oscillation can be generated, which is known as localized surface plasmon resonance (SPR) [49]; according to other reports, silver particles exhibit collective oscillation of their electrons in the conduction band, and it is suggested that the association between the semiconductor and the metal can increase photocatalytic activity, which coincides with the results obtained in this work [50].Pseudo-first-order model was applied to kinetic data of Figure 6.The k app values for each test were determined from the slope of the linear fitting of ln C t /C o vs time (Figure 7).Ag-TiO 2 thin films had a k app value 2.4 × 10 −3 min −1 ; this value was 1.8 times greater than the k app value 1.3 × 10 −4 min −1 of TiO 2 thin films.k app values confirmed that the photoreduction process was effective and demonstrated that this method could be used as an alternative to increasing the photocatalytic activity of TiO 2 under UV light irradiation.We reported in a previous work that Ag-TiO 2 thin films had antimicrobial activity against Staphylococcus aureus; same material demonstrated both photocatalytic and antimicrobial activities [51].Reactive oxygen species generated in advance oxidation process are endogenous, highly reactive, oxygen molecules, and these species can originate oxidation reactions of many organic and biological compounds (e.g., singlet oxygen and oxygen free radicals such as hydroxyl radical, superoxide anion, hydroperoxyl (HOO ⋅ ), or other similar radicals); our results corroborate high reactivity of these chemical species [52].

Conclusions
In this work, we deposited Ag microaggregates on TiO 2 thin films.SEM characterization demonstrated microaggregate formation on TiO 2 surface, and besides, Raman spectroscopy indicated that surface modification generated SPR process; furthermore, XRD assay indicated that all thin films were crystalline and XPS assay indicates that silver was of metallic nature after photoreduction on TiO 2 thin films.Photocatalytic activity of Ag-TiO 2 thin films was higher than that of TiO 2 thin films; kinetic results showed that the k app for

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

3. 4 .
XPS Assay.XPS measurements were conducted to verify the surface components and valence states of Ag-TiO 2 thin film.Figure 5(a) shows profile XPS spectrum to Ag-TiO 2 thin film.The profile of the sample shows the typical signals of binding energy corresponding to Ag, Ti, O, and C elements.

Figure 3 :
Figure 3: Raman spectrum to TiO 2 thin films and Ag-TiO 2 composite thin films.

Figure 5 :
Figure 5: (a) XPS spectrum to Ag-TiO 2 thin films, (b) HRXPS Ti 2p spectrum to Ag-TiO 2 thin films, and (c) HRXPS Ag 3d spectrum to Ag-TiO 2 thin films.All binding energies of the XPS spectra are calibrated with reference to the C1s peak at 285 eV.

Figure 6 :Figure 7 :
Figure 6: C t /C o vs time under UV light irradiation, where C t refers to the concentration at time t of MB and C o refers to initial concentration.

Table 1 :
Partial concentration for elements of Ag-TiO 2 thin films.