Silver Nanoparticles Influence on Photocatalytic Activity of Hybrid Materials Based on TiO 2 P 25

1Department of Bioresources and Polymer Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania 2Laboratory of Water Chemistry, Faculty of Sciences, University of Lomé, Lomé, Togo 3National Research and Development Institute for Chemistry and Petrochemistry (ICECHIM), 202 Independence Street, District 6, CP 35-174, 060021 Bucharest, Romania 4National Institute for Research and Development in Microtechnologies (IMT-Bucharest), 126 A, Erou Iancu Nicolae Street, P.O. Box 38-160, 023573 Bucharest, Romania


Introduction
One of the most omnipresent problems affecting people throughout the world is inadequate access to clean water and sanitation.Problems with water are expected to increase over the next decades, with water insufficiency occurring globally, even in regions currently assigned as water-rich [1,2].TiO 2 is the most commonly used oxide semiconductor in numerous fields (white pigments, dyes, and coatings, cosmetics, food, bleaching products, and solar cells) [3] which proved to be an ideal photocatalyst due to its ability to completely mineralize the target pollutants from wastewaters into H 2 O and CO 2 [4].Although TiO 2 is abundant, cost-effective, environmentally friendly, and photochemically stable under severe conditions [5], it has several disadvantages.One of these is related to the wide bandgap energy of 3.0 eV for rutile, respectively, and 3.2 eV for anatase which limits its applications, making possible the utilization of only 5% of the solar spectrum [1].Furthermore, the high dispersion in water that causes sedimentation difficulties and sensitivity to the recombination of photoinduced electrons and holes also decreases its photocatalytic activity.Nevertheless, many studies have been reported using TiO 2 as photocatalyst for wastewater treatment against organic pollutants and pathogens [6][7][8].The photocatalytic tests use TiO 2 as powder suspension or as thin films immobilized on different substrates.The major drawback of the TiO 2 powder dispersed in suspension systems is represented by the filtering expensive process necessary for the TiO 2 recovering [4,9,10].Degussa P25 is a product commercially available in the market and represents one of the most used forms of TiO 2 composed of 70-80% anatase and 20-30% rutile crystallites with an average particle size of 25 nm.Degussa P25 is commonly used as a standard reference photocatalyst in order to compare the photocatalytic activity with other photocatalysts [4].The TiO 2 photodegradation mechanism can be explained by the promotion of an electron in the conduction band (CB) under UV light resulting in a "hole" in the valence band (VB) and the formation of electron-hole (e − -h + ) pairs.After losing the electron, the hole tries to recover the electron deficiency from the conduction band (recombination) or from the water adsorbed on the material's surface.The interaction with water leads to the formation of hydroxyl radicals (HO • ) which are able to oxidize any organic compound and finally to CO 2 and water [1,7].The electron from the CB has no hole to initiate the recombination process; thus the most rapid alternative to reduce is to form the superoxide radical anion ( • O 2 − ) by reducing the oxygen adsorbed at the TiO 2 surface.The superoxide anion reacts with water to form more HO • [7,11].
The performance of TiO 2 based photocatalysts can be enhanced by doping with noble metals (Au, Ag, and Pt) [12] or several semiconductor compounds (In 2 O 3 , Cu 2 S, CdTe, and NiO) [13][14][15] that improve the sensitivity of TiO 2 to visible light.Recent studies revealed that silver and silver nanoparticles (AgNPs) improve the photocatalytic activity of TiO 2 [16,17] inducing an efficient surface plasmon resonance effect under sunlight preventing the recombination of e − -h + pairs responsible for the decreasing process of the photocatalytic activity of TiO 2 [18,19].
Therefore, the TiO 2 /AgNPs hybrid materials can be activated for water disinfection and photodegradation by both UV and visible light based on three possible routes (Scheme 1) that associate the photocatalytic properties of TiO 2 (route I) with the antimicrobial and photocatalytic characteristics of AgNPs (route II) [17,20] and the interaction between TiO 2 and Ag (route III).
In this study, two methods were employed to obtain TiO 2 films doped with AgNPs with enhanced photoactivity.The first approach involved the synthesis of AgNPs solutions which were then used for realization of TiO 2 P25 dispersions in order to obtain films on a quartz tube followed by annealing.The size distribution influence on the enhancement of the photocatalytic activity was investigated.The second approach was aimed at simplifying the doping procedure.Thus, the manufacturing process consisted in the generation of AgNPs directly on the surface of TiO 2 P25 films and sinterization.The photodegradation activity of the films was investigated using methyl orange (MO) as target organic compound and TiO 2 as reference.

TiO 2 P25 Ex Situ AgNPs Film Realization. The TiO 2 P25
and TiO 2 P25/AgNPs films realization steps consist in the obtaining of stable dispersions, dip-coating of the quartz tube, and annealing at 500 ∘ C for 2 h.
3 g of TiO 2 P25 was well dispersed in 60 mL of AgNPs solution (A, B, or C) using an ultrasonic processor.The pH of the dispersion was adjusted to 4 using concentrated acetic acid.
For the reference TiO 2 P25 film ultrapure water was used for the realization of the dispersion and acetic acid for pH control.Prior to film deposition, the quartz tube was hydrophilized by immersion in a 5% NaOH solution overnight followed by rinsing with ultrapure water.

TiO 2 P25
AgNPs on Surface Generation.After obtaining by dip coating the TiO 2 P25 film on the quartz tube, the film was dried in oven at 80 ∘ C for 2 h and then immersed in solution A containing AgNO 3 (0.221 mmol/L) for 8 minutes after which it was introduced in solution B containing trisodium citrate (0.775 mmol/L) and NaBH 4 (0.0126 mmol/L).The obtained yellowish film is then sinterized in an oven at 500 ∘ C for 2 h.

Characterization.
UV-VIS absorption spectra were recorded with an Able Jasco V-550 spectrophotometer; for the solid samples, reflection spectra were registered using an integrating sphere at normal incidence, a band width of 1 nm, and a scanning speed of 1000 nm min −1 .
The particle size measurement, through dynamic light scattering (DLS), was realized using a Nani ZS device (redbadge).For the morphological characterization of the obtained TiO 2 and TiO 2 AgNPs films scanning electron microscopy analyses were performed using a FE-SEM (field emissionscanning electron microscope) RAITH e Line at 10 kV acceleration voltage.The AgNPs morphology was investigated by TEM using a high resolution transmission electron FEI Tecnai F30 S-Twin microscope.A drop of the AgNPs suspension was mounted on a holey carbon film copper grid allowing the solvent to evaporate at room temperature.For the photodegradation studies, 200 mL MO with a concentration of 0.5 mol/L was utilized.

Results
In order to obtain hybrid nanomaterials with photocatalytic activity, the first step was to synthesize different AgNPs and characterize their behavior.In Figure 1 is presented the color variation of the solutions from light yellow to brown-orange, due to the different size distribution and concentration of the AgNPs.
The first characterization of the AgNPs solutions consisted in UV-VIS analysis.The UV-VIS spectra presented in Figure 2 revealed a difference in absorbance and wavelength value.The wavelength value of the maximum absorption band is red-shifted between the samples which can be correlated with an increase of larger size particle content [21,22].Thus, sample A has the lowest values of both size and absorbance of AgNPs, followed by samples B and C, respectively.DLS analysis was performed on the three solutions in order to obtain the mean diameter and size distribution of the AgNPs (Figure 3).For a more accurate size characterization, intensity and number of weighted mean diameters and distributions from DLS technique were considered.Although the whole range of diameters is shown in the intensity-weighted distribution (-distribution), the proportionality to the sixth power of diameter of particle () ( ∝  6 ) underestimates the small particles, which are only very weakly weighted [22,23].
In the case of sample A, the DLS analysis revealed almost monodisperse AgNPs with a particle size range under 20 nm.From the DLS data, the weighted average diameter for sample A was estimated to be 12.6 nm.Sample B revealed two types of nanoparticles: the first presenting a weighted average diameter 13.6 nm while the second was calculated to be 83.6 nm.For sample C, the size polydispersity and weighted average diameter increased to around 24 nm with a small domain of larger particles.Thus, the main difference between the three samples is the increase in polydispersity and particle size from A to C, in the order A < B < C, respectively, and the decrease in content of particles less than 20 nm.In order to confirm the particles size, TEM analyses have been performed.In Figure 4 it can be noticed that the DLS data are sustained by TEM images.
The next step of this study was directed to the synthesis and investigation of the hybrid material (TiO 2 P25/AgNPs films).The images from Figure 5 present the color variation of TiO 2 P25, respectively, the hybrid TiO 2 P25/AgNPs films after the deposition and annealing process on a quartz tube.The color of the hybrid material can be explained by the addition of the reflected wavelength domain of TiO 2 P25 and AgNPs.
In order to characterize this color modification, the films obtained using TiO 2 P25 and the three AgNPs solutions (A, B, and C) were analyzed by UV-VIS (Figure 6).The obtained TiO 2 P25/AgNPs hybrid films registered different reflection bands compared with the reference (TiO 2 P25 film).The absorption bands are blue-shifted, while the AgNPs size distribution decreases (see inset).This can be explained by the overlaying of the adsorption bands of TiO 2 P25 film and different AgNPs particle size distribution.Thus, the registered signals are in good correlation with the results registered by DLS analysis (Figure 1) and UV-VIS analysis (Figure 1) of the initial AgNPs solutions.
The concentration of TiO 2 P25 is much higher than that of the AgNPs; therefore the spectroscopic influence of silver nanoparticles is rather minor on the UV-VIS absorption properties of the films; however, all the samples present absorption at a lower wavelength compared to the initial TiO 2 P25.Thus, the hybrid material is photoactive for a larger wavelength domain.
The photocatalytic activity of the hybrid nanocomposite films was determined using the experimental set-up presented in Scheme 2 (details on the procedure are presented in Section 2.4).The most important element of our experiment was the reactor equipped with the quartz tube and illuminated with an UV lamp.
The kinetics of photocatalytic degradation and the percentage of MO photodegraded are highly dependent on AgNPs size.The highest photocatalytic activity was registered for the nanoparticles with the smallest size and narrower particle size distribution (sample A) (see DLS analysis, Figure 3).Furthermore, in all three cases involving the hybrid materials, although the size of AgNPs nanoparticles increases, the photocatalytic activity registered superior values compared with the TiO 2 P25.Thus, after 150 min, the photodegradation percentage reaches 33%.The degradation curves are linear confirming a first order reaction degradation kinetics mechanism which allowed the utilization of the Langmuir-Hinshelwood model [24,25] for determination of the apparent reaction constant (Figure 7).All the samples have presented adsorption characteristics similar to TiO 2 P25 for the "in the dark" study.
The analysis of the photodegradation results proved that the materials based on AgNPs improve the photoactivity of the TiO 2 films.The introduction of AgNPs in the TiO 2 P25 suspension increased the photodegradation of MO in all cases, regardless of the AgNPs size distribution.
For this reason, the next approach of this study aimed to increase the contact surface between TiO 2 P25 and AgNPs, simplifying the manufacturing process.Thus, in order to validate this method, AgNPs were generated directly on the surface of TiO 2 P25 film and the photoactivity of the obtained hybrid material was analyzed using MO.
In Figure 8, the SEM images of the TiO 2 P25 and for the new hybrid material are presented comparatively.
The SEM analysis reveals that AgNPs generated on the TiO 2 P25 surface form aggregates in different areas (Figure 7(b)) (red highlighted).Although their formation was preferential, the contact area with MO is enlarged and should register higher efficiency values.
The evolution of the decomposition process of the dye is presented in Figure 8.For a better investigation of the hybrid film, the results were compared with a simple TiO 2 P25 and the generated AgNPs/TiO 2 P25 film in the dark.From Figure 9, the film is very stable in the dark and the MO was not decomposed.The hybrid film resulting from the generation of AgNPs directly on TiO 2 P25 registered a photodegradation capacity of MO of 45% after 150 min.
This approach of reducing silver on the surface of TiO 2 P25 increased the direct contact of AgNPs with the contaminated medium which determined a higher efficiency of   the degradation process, sustaining also the possible photodegradation mechanism of the dye.

Conclusions
The aim of the present study consisted in the obtaining of hybrid material films, using TiO 2 P25 and AgNPs.In the first case (mixing TiO 2 P25 with AgNPs solutions), the size distribution of the AgNPs proved to be a key element in the photodegradation activity of the materials; the main difference between the three samples is the increase in polydispersity and particle size from A to C, in the order A < B < C, respectively, and the decrease in content of particles less than 20 nm.For the first case, the degradation of MO is about 33% after 150 min under UV irradiation.
The second approach involved the generation of AgNPs on the surface of the TiO 2 P25 film.In this case, the contact surface of AgNp with MO is higher and the photodegradation activity increased to 45% after 150 min.

Scheme 2 :
Scheme 2: The experimental set-up used for photocatalytic degradation.

2. 4 .
Photodegradation Procedure.The photodegradation measurements were analyzed using a modified commercial set-up comprised of (a picture of the set-up is presented in Scheme 2):(i) microdosing pump Masterflex-Unipan 335 A with a flow fixed at 350 mL/h, (ii) stainless steel jacket (21 cm) with a quartz tube core (used for photoactive material deposition) (the space between the jacket and the tube (180 cm 3 ) was used for the dye solution circulation), (iii) UV-C lamp introduced inside the quartz tube ( = 253.7 nm, power 4 W), (iv) collecting flask used as starting and end point for the dye solution, (v) silicon tubing connections used for connecting the collecting flask, pump, and reactor.

Figure 3 :
Figure 3: DLS analysis of A, B, and C AgNPs samples.

Figure 4 :
Figure 4: TEM images for the samples A, B, and C.

Figure 7 :
Figure 7: (a) Photodegradation kinetics; (b) percentage of decomposed MO versus maximum absorption wavelength value of the AgNPs solution; (c) values of apparent decomposition constant.