Performance of Ag-TiO 2 Photocatalysts towards the Photocatalytic Disinfection ofWater under Interior-Lighting and Solar-Simulated Light Irradiations

This paper reports the characterization and photoactivity of Ag-TiO2 materials using different amounts of silver during the hydrothermal synthesis. Photocatalysts were characterized by means of TEM, XPS, XRD, DRS, and N2 sorption isotherms to determine the textural properties. The photocatalyst’s configuration was observed to be as anatase-brookite mixed phase particles with Ag partially oxidized aggregates on the TiO2 surface, which increased visible light absorption of the material. Moreover, photoproduction of singlet oxygen was followed by EPR analysis under visible light irradiations following the formation of TEMPOL. Such photoproduction was totally decreased by using the singlet oxygen scavenger DABCO. Photocatalysts were tested towards the photocatalytic disinfection of water suing a solar light simulator and an interior-light irradiation setup. Results evidenced an increase in the photooxidative effect of TiO2, while dark processes evidenced that part of the inactivation process is due to the Ag-TiO2 surface bactericidal effect and possible lixiviated Ag+.


Introduction
The design of TiO 2 -based photoactive materials has been a major research topic during the last two decades [1][2][3].TiO 2 , as a semiconductor, is activated under UV irradiation promoting charges that lead to the formation of reactive oxygen species when they interact with H 2 O and O 2 from the reaction media.Nowadays, recombination of the photogenerated charges [4,5] and visible (vis) light response [4] are two drawbacks still to surpass in order to achieve efficient photocatalysts to be used under solar light or interiorlighting irradiations.
Water disinfection using TiO 2 had attracted research interest since the oxidative attack of photoproduced ROS may cause total inactivation of bacteria [6,7] without generating harmful byproducts [8].In order to increase its photooxidative effect, towards bacteria inactivation, TiO 2 has been modified with Ag, which increases vis light absorption [9][10][11] and has been proposed to decrease recombination [12], on TiO 2 thin films [12] and powdered materials [13].New insights have proposed electronic transitions arising from vis-excited Ag aggregates to the TiO 2 conduction band promoting its photobactericidal effect [14], as it has been observed on plasmonic photocatalysis [10].Furthermore, Ag-containing antimicrobial devices are designed due to the well-known Ag bacteriostatic activity [15].
Ag-modified TiO 2 photocatalysts have been synthesized using the hydrothermal method since this is a relatively simple route to load TiO 2 nanoparticles with Ag [16,17], with a wide range of optimum silver content from 0.1 to 6.5% wt correspondent to the major increase in photoactivity [11,18,19].Ag has been proposed to be as aggregates located in crystals borders of TiO 2 particles [20,21].Since Ag + and Ti 4+ ionic radii are 1.16 [22] and 0.64 Å [23], respectively, it is not expected that Ag may be embedded into the TiO 2 structure.
The aim of this work is to present the performance of Ag-TiO 2 photocatalyst, obtained using the hydrothermal synthesis route, under different light irradiations setups such as solar-simulated and interior-lighting lamps with different irradiation powers, towards the photocatalytic inactivation of E. coli in water.Moreover, materials were characterized using TEM, XPS, DRS, and N 2 adsorption-desorption isotherms in order to elucidate properties affecting the photoactivity.Results show an effective modification of TiO 2 with enhanced visible light absorption capacity as well as photoactivity.

Experimental
2.1.Photocatalyst Synthesis.Photocatalysts were hydrothermal synthesized as follows: 3.8 mL of titanium butoxide (99.9%,Fluka) was added dropwise to 19 mL of isopropanol (Suprasolv.grade reagent, Merck).Then, an adequate amount of AgNO 3 (Extra pure, Merck) was diluted in 2 mL of water (pH = 1.5) adjusted with HNO 3 (65%, Merck).Ag aqueous solution was added dropwise to the isopropoxideisopropanol solution with an appropriate concentration to obtain Ag nominal weight percentages (% wt), of 0.5, 2, 4, and 8. Formed gel was steam-pressure-treated in autoclave during 3 h at 120 • C and ∼144 KPa.TiO 2 was obtained as described without addition of AgNO 3 .The obtained wet crystals were grounded in mortar, dried in oven at 60 • C for 12 h, and kept in dark to avoid surface oxidation.Samples were labeled as Ag(x)-TiO 2 , x = Ag % wt.

Photocatalyst Characterization.
Fresh Ag(x)-TiO 2 and TiO 2 samples were XPS-analyzed.To follow Ag states after a typical photocatalytic test a Ag(2)TiO 2 suspension, without E. coli, was submitted 2 h to solar-simulated light irradiation, filtered and dried in oven at 60 • C, and then XPSanalyzed.Analyses were carried out using an AXIS-NOVA photoelectron-spectrometer (Kratos-analytical, Manchester, UK) equipped with a monochromatic AlKα (hv = 1486.6eV) anode.Electrostatic-charge effect was overcompensated by means of the low-energy electron source working in combination with a magnetic immersion lens.C1s line at 284.8 eV was used as calibration reference.Spectra were decomposed using the CasaXPS program (Casa Software Ltd., UK) with a Gaussian/Lorentzian (70/30) product function after subtraction of a Shirley baseline.Assignment of Ag and Ti states was restricted using peak distances of 6 eV and 5.3 eV for Ag [12] and Ti [24], respectively.
X-ray diffraction (XRD) patterns were collected using a DMax-IIIB Rigaku system operated at room temperature, 40 kV and 80 mA with monochromatic Cu-Kα radiation.The average crystallite size (d XRD ) was calculated using the Scherrer equation applied to anatase peaks at 2θ = 25.2 • on samples' diffractograms.Al 2 O 3 corundum oxide was used to quantify crystalline phases for bare TiO 2 and the highest Ag loading in order to identify possible changes in phase composition and crystallinity due to Ag loading for modification.
Diffusive reflectance spectroscopy (DRS) was done using a UV-2401PC Shimadzu spectrophotometer with an ISR240A integrating sphere accessory.BaSO 4 was used as reference.
Transmission Electron Microscopy (TEM) analysis were performed in a Phillips HRTEM CM 300 (field emission gun, 120 kV) microscope.
Textural analyses of the photocatalysts were performed through N 2 adsorption-desorption isotherms obtained at 77 K in a Nova1200 equipment of Quantachrome.Before the analysis 0.15-0.2g samples were degasified during 12 h at 373 K under 10 −5 mmHg vacuum.Specific surface area (S BET ) and average pore diameter (d p ) were obtained by means of BET and BJH methods, respectively.
Reactive scavenging of ROS by electron paramagnetic resonance spectroscopy (EPR) was done using 2 mL of 0.3 g/L Ag(2)-TiO 2 suspension and 5×10 −2 mol/L of 2,2,6,6-tetramethyl-4-piperidinol (TMP-OH, 99%, Fluka), with spectral parameters: D H = 1.58 G, a N = 16.9G, and g = 2.0066, and was prepared in ultrapure H 2 O, as well as, in D 2 O (99.9% atomic purity, Aldrich).1,4-diazabicyclo [2,2,2] octane (DABCO, 99%, Fluka) was used as a singlet oxygen quencher [25].Suspensions were kept in test tubes and ultrasound treated in a water bath with a frequency of 40 kHz for 1 min prior to illumination. 1 mL aliquot of the suspension was transferred into a 5 mL Pyrex beaker and exposed to illumination under constant magnetic stirring with a white light halogen spot source of 150 W from OSRAM reference Gx5.3 (93638) emitting vis.∼7 μL aliquots of the illuminated suspensions were transferred into glass capillary tubes 0.7 mm ID and 0.87 mm OD, from VitroCom, NJ, USA.Tubes were sealed on both ends with Cha-Seal tube-sealing compound (Medex International, Inc., USA).To maximize sample volume in the active zone of the ESR cavity, assemblies of seven packed capillaries were positioned in a wider quartz capillary (standard ESR tube, 2.9 mm ID and 4 mm OD, Wilmad-LabGlass, Vineland, NJ, USA).Such setup resulted in ∼65 μL sample volume in the active zone of the ESR cavity, thus markedly improving sensitivity of measurements [26].
Experiments were performed using an ESR300 spectrometer (Bruker-BioSpin-GmbH) at room temperature, equipped with standard-rectangular mode TE 102 cavity.Routinely, for each experimental point five scan field-swept spectra were recorded with instrumental parameters: microwave frequency: 9.38 GHz, microwave power: 2.0 mW, sweep width: 120 G, modulation frequency: 100 kHz, modulation amplitude: 0.5 G, receiver gain: 4 × 10 −4 , time constant: 20.48 ms, conversion time: 40.96 ms, and time per single scan: 41.9 s.Acquired EPR traces correspond to the second derivative of the sample's paramagnetic absorption.

Photocatalytic Activity.
Photobactericidal activity was measured by sampling E.coli strain ATCC 11229 from Pyrex photoreactors with a 50 mL of a 0.1 g/L TiO 2 suspension.Before experiments, bacteria were inoculated into Luria Bertani growth media (1% wt, tryptone from Oxoid, 0.5% wt yeast extract from Oxoid, and 1% wt NaCl from Merck) and grown during 8 h at 37 • C.During the stationary growth phase, bacteria were harvested by centrifugation at 5000 rpm for 10 min at 4 • C. The obtained bacterial pellet was washed three times with saline solution (8 g/L NaCl, 0.8 g/L KCl in Milli-Q water, pH = 7 by addition of HCl or NaOH).A suitable cell concentration (10 7 colony forming units (CFU) per mL) was inoculated to reactors suspensions.Then, photocatalyst was added to each reactor.Suspensions were illuminated during 2 h, and samples (1 mL) were taken at different time intervals.Serial dilutions were performed in saline solution and 10 μL samples were inoculated 4 times in plate count agar (PCA, Merck).The number of colonies was counted after 24 h of incubation at 37 • C.
E. coli + TiO 2 suspensions were kept under magnetic stirring and illuminated under two different lamp setups: (1) 67 lx of white light illumination, with 9.8 W/m 2 of irradiation, using a set of 5 natural sunshine light Phillips lamps (20 W), respectively, with emission between 360 and 700 nm or (2) 250 or 400 W/m 2 of solar-simulated light irradiation using a suntest system model CPS+ from ATLAS, with temperature and irradiation power control, and a Xenon lamp emitting light with wavelengths of 300 y 800 nm, and 5% of the irradiation corresponds to UV-A.The radiant flux was monitored with a Kipp & Zonen (CM3) power meter (Omni instruments Ltd., Dundee, UK) and a Hagner EC1 digital luxmeter.
Bacteria concentration was also followed in dark suspended and stirred Ag-TiO 2 and TiO 2 suspensions to analyze possible induced dead by toxicity of the materials.Moreover, to analyze possible lixiviation of Ag + ions an Ag(2)TiO 2 suspension was stirred under darkness for 10 h and then filtered.Since E. coli is inactivated by Ag + ions in solution it is possible then to determine if lixiviated Ag may decrease bacteria concentration [27].Thus, E. coli was added to the filtered solution and bacteria concentration was determined by intervals of 2 h under darkness.
An additional determination of the E. coli concentration was made in order to analyze the time needed for effective disinfection.In this case a sample of each reactor with Ag-TiO 2 photocatalysts was taken after 1 h of irradiation and kept under darkness for 24 h.After this period the samples were submitted to plating and incubation as already described, and finally, colonies were counted on agar plates.The effective irradiation time (EDT) for inactivation was assigned to the sample with zero bacteria counting after 24 h under darkness and was labeled as EDT 24 .

Photocatalysts Features.
The optical response of the samples has been followed by means of the spectral response on DRS analyses.Figure 1 shows the DRS spectra of the TiO 2and Ag-modified TiO 2 samples.TiO 2 spectrum consists of a wide absorption band below ∼370 nm ascribed to electron transitions from the valence band (VB), to the conduction band (CB) [28].Moreover, an interesting light response was found for Ag-loaded samples.Increase in Ag concentration induces a shift in light absorption to the vis range for wavelengths up to 800 nm.DRS spectrum of the 0.5% Agloaded TiO 2 sample did not show a significant increase in visible absorption capacity of the material, thus results are not presented.In addition, the 8% Ag-loaded sample shows a shoulder-like peak at ∼460 nm which has been proposed for Ag 0 nanoparticles inducing vis light absorption [29].Atoms, ions, and clusters of silver show characteristic absorption peaks with irradiation wavelengths from 190 nm 500 nm in DRS spectra.Bands at 190-230 nm ascribed to 4d 10 -4d 9 5s 1 transitions in Ag + , while those at 250-330 nm attributed to 4d 10 5s 1 -4d 9 5s 1 5p 1 , 4d 10 5s 1 -4d 9 5s 1 6p 1 , or 5s 1 -p 1 transitions in Ag 0 [30], are overlapped by VB-to-CB transitions on TiO 2 as previously observed.In addition, Ag δ+ n clusters (where n is in the range 2-13, and δ = n − 1) with n > 3 present three absorption bands at 360, 460, and 500 nm [31].Bands at 350-380 nm are attributed to Ag clusters of ∼1 nm size, while clusters of 10 nm size and crystallites absorb at 400-500 nm [30,31].
Such shoulder-like peak has been observed for Ag nanoparticles inducing collective oscillations of the conduction band electrons with the incident light [32,33].This effect has also been observed for Au and Pt nanoparticles [34].This electromagnetic resonance is known as a surface plasmon resonance effect which in this case may induce photoactivity of TiO 2 under vis irradiation [35] as it will be discussed.
In contrast, a slight decrease in the slope of the absorption threshold is observed due to Ag loading of TiO 2 .Such decrease is related to a decrease in the energy bandgap.Probably, the increase in N residues coming from the Ag salt precursor, AgNO 3 , may lead to new intrabandgap states promoting electronic transitions of lower energy requirements between N and the TiO 2 conduction band, thus leading to a decrease in the energy gap [36].However, as we have stated, in a previous work, NO 3 − residues in Fe-TiO 2 samples, obtained by the hydrothermal route, do not promote photoactivity under vis light [4].Therefore, DRS spectra observed in Figure 1 suggest that increased absorption due to Ag presence, with a peak at ∼460 nm, is possibly due to the localized surface plasmon resonance of Ag nanoparticles [37,38].
Figure 2 shows the emission spectra of the chosen light setups.The Xenon and interior-lighting lamps have similar distribution spectra between 300 and 800 nm.Both lamps have low emissions under 400 nm in comparison to the emissions in the vis range (400-800 nm).This is supposed to activate TiO 2 alone since its absorption threshold lies at 400 nm International Journal of Photoenergy as observed in Figure 1.However, the major part of both emission spectra suggests that vis irradiation predominates over UV irradiation thus giving the chance to activate surface Ag aggregates.
Figure 3 shows the X-ray diffractograms of samples.Anatase (A) is identified as the primary crystalline phase in both samples with peaks at 2θ of 25.2 • , 37.9 • , 48.2 • , 55.0 • , and 62.6 • .In addition, a slight peak at 30.7 • assigned to brookite (B) is observed for all samples.Al 2 O 3 corundum (C) peaks are observed for TiO 2 and Ag(8)TiO 2 samples' diffractogram since this oxide was used in the quantification of the phase composition of these two samples, while Ag-related phases were not identified in any of the samples due to low concentrations.Ag loading did not cause significant changes in crystallinity since, as previously described, Ag + ion radii is too large to replace Ti 4+ ions in the TiO 2 matrix.Furthermore, on quantitative XRD analysis it was determined that TiO 2 sample is constituted of anatase, brookite, and amorphous in percentages of 38.4,12.2, and 49.2%, respectively, while those for Ag(8)TiO 2 are 39, 9.2, and 51.8%, respectively.As a consequence, since Ag does not change phase structure aggregates may be formed on crystal borders and on the surface of the photocatalyst, thus promoting vis light absorption as discussed in Figure 1.
Table 1 shows the calculated average crystallite size and some textural properties of the samples.A slight decrease in crystallite size was observed due to Ag loading probably due to obstruction to anatase crystallite growth by Ag aggregates located in crystal borders.Fe 3+ modifications on TiO 2 , using the same hydrothermal synthesis, promoted changes on the crystalline structure due to its similarity in sizes with Ti 4+ ions, thus replacing Ti atoms and decreasing crystallite size [4].Therefore, it is possible to suggest that during the synthesis Ag aggregates are deposited on the surface or grain boundaries.
TEM micrographs of the TiO 2 and Ag(2)TiO 2 samples are shown in Figure 4.As observed, the TiO 2 sample is constituted of irregular particles with approximate sizes of 8-10 nm.The difference between d XRD and the observed particle size in TEM analysis is due to agglomeration of crystallites.In addition, the Ag(2)TiO 2 micrograph shows a similar size distribution to that of bare TiO 2 as it was previously suggested in XRD analyses.Moreover, previously suggested Ag nanoparticles were not observed during the analysis probably due to low resolution of equipment.
Textural properties evidence mesoporous materials with high area.Ag loading caused a significant decrease in S BET probably due to deposition of Ag aggregates on the TiO 2 surface thus leading to pore obstruction [39].Figure 5 shows type IV isotherms for TiO 2 and Ag(2)TiO 2 samples.Moreover, the sorption isotherm of the TiO 2 sample demonstrated a hysteresis pattern between types H1 and H2, while that of the Ag(2)TiO 2 sample shows a type H2 hysteresis loop.Type H1 loops are often assigned to agglomerates or compact or spherical particles with uniform size, while type H2 loops are attributed to pore size and shape distributions not well defined [40].Therefore, it is possible to suggest that Ag modification distorted the distribution of pores of the mesoporous structure of TiO 2 due to    Figure 6 shows the Ti 2p and Ag 3d XPS spectra of the fresh and solar-simulated light irradiated Ag(2)TiO 2 sample.The Ti 2p doublet of the fresh Ag-loaded sample is constituted by Ti 2p 3/2 and Ti 2p 1/2 peaks at 458.9 and 464.6 eV, respectively, indicating a predominant state of Ti 4+ [41].Moreover, the Ag 3d spectra of the fresh sample show Ag 3d 5/2 and Ag 3d 3/2 peaks at 368.4 and 374.4 eV, respectively.After peak deconvolution it is observed the presence of two different peaks for Ag3d 5/2 : one at 368.4 eV assigned to Ag + [12,42], while the peak at 368.7 eV is attributed to Ag 0 [12,43].Thus, results suggest the coexistence of Ag 2 O and Ag 0 probably due to the oxidative action of isopropanol on Ag + ions [44] during the synthesis process.Therefore, it is possible to suggest that Ag is deposited on the TiO 2 surface as partially oxidized metallic aggregates.On the other side, the irradiated sample's XPS spectra evidences that the irradiation process shifts signals by ∼1.1 eV for the Ag 3d spectra, while that of Ti 2p is shifted by ∼1 eV.Ag peaks at 367.2 and 373.2 for Ag 3d 5/2 and Ag 3d 3/2 are assigned to Ag 2+ species as suggested by Kaushik [45].In addition, the Ti 2p shift indicates the formation of Ti 3+ species on the TiO 2 surface [46].It has been observed that during irradiation of Ag-TiO 2 samples Ag is oxidized by action of photogenerated holes on the TiO 2 VB [47], therefore it is possible to suggest that Ti 4+ cations, in TiO 2 , are reduced by released electrons during Ag oxidation.
Moreover, Ti 3+ is expected to be rapidly oxidized to Ti 4+ by surrounding oxygen [48].However, in this case the chosen synthesis route leads to formation of anatase-brookite mixed International Journal of Photoenergy phase oxide with ∼50% of amorphous, as early discussed, including organic residues as we have observed in a previous work [4].Such residues may protect surface oxidation of Ti 3+ to Ti 4+ thus allowing its detection in XPS analyses.

Photocatalytic Production of ROS under Vis Irradiation.
Vis-irradiation-induced singlet oxygen ( 1 O 2 ) formation was monitored by EPR. Figure 7 shows the signal obtained after 50 min of vis irradiation of TiO 2 and Ag(2)TiO 2 suspensions in presence of TMP-OH and DABCO. 1 O 2 formation was clearly evidenced for Ag(2)TiO 2 with a characteristic 1 : 1 : 1 triplet signal of TEMPOL [49] as depicted in Figure 7.Moreover, the Ag-modified sample revealed an increase in TEMPOL signal thus evidencing a higher 1 O 2 photoproduction in comparison to TiO 2 .Furthermore, the acquired signal of experiments in presence of DABCO revealed a total decrease in the intensity of TEMPOL signal.Since DABCO is a well-known 1 O 2 scavenger [50] it is possible to confirm its photocatalytic production by action of vis irradiation on Ag(2)TiO 2 suspensions.Therefore, singlet oxygen is promoted by vis-excited Ag aggregates on the TiO 2 's surface, and thus it is possible to expect an increased photobactericidal activity of TiO 2 .

Photocatalytic Disinfection of Water and Postirradiation
Events. Figure 8(a) shows the bactericidal activity of TiO 2 and Ag-TiO 2 samples in darkness.Results evidence zero toxicity of TiO 2 while Ag(2)TiO 2 and Ag(4)TiO 2 samples totally decreased the E. coli population within 180 and 60 min of contact, respectively.Ag ionic species loaded on a substrate's surface have been observed to efficiently inactivate E. coli [41].As a consequence, in our case Ag + species are responsible of the bactericidal activity of the Ag-TiO 2 samples under darkness.Moreover, Figure 8(b) shows the evolution of E. coli added to a filtered solution of a 10 h dark-stirred Ag(2)TiO 2 suspension.After 240 min of stirring the filtered solution and E. coli, it is observed that bacteria population is affected by the aqueous media.This suggests that Ag + ions have been released from the photocatalyst's surface during the dark-stirring process, and after filtration may induce toxicity leading to bacteria death.Then, since the homogeneous bactericidal activity is observed after long times of lixiviation during stirring and the heterogeneous toxicity is observed after 180 min (Figure 8(a)), it seems probable that in-dark contact of the photocatalyst with E. coli the bactericidal activity is promoted by the toxicity of Ag+ ions on the photocatalyst surface, and of course, with time released ions play their bactericide role as observed in Figure 8(b).However, an additional heterogeneous photocatalytic effect has been observed to boost inactivation rates under different irradiation setups as discussed earlier.
On the other side, Figure 9 shows the photodisinfection tests results under 250 and 750 W/m 2 of solar-simulated light irradiation of Ag-TiO 2 samples and their correspondent EDT 24 .Figures 9(a   while the bactericidal effect reduced the population in 60 min (Figure 8).This effect is observed to increase under 400 W/m 2 of irradiation since the total inactivation, by the same sample, is reached after 10 min.Then, the photocatalytic effect decreases the inactivation time by 40 and 50 min, under 250 and 400 W/m 2 , respectively, indicating that the total bactericidal action is a sum of photoinduced processes and photocatalysts toxicity.Furthermore, using irradiation of interior lighting lamps similar effects were observed.Such photocatalytic process can now be correlated to the photoproduction of ROS.As it was previously observed, the spectrum of the Xenon lamp has a ∼50% of irradiance power correspondent to visible light, and even more, the excitation wavelength of the Ag nanoparticles on the TiO 2 surface is emitted by this source (∼470 nm), thus singlet oxygen is produced.A vast majority of cell types, from prokaryotic to mammalian, undergo irreversible damage leading to cell death by exposure to singlet oxygen [51,52], therefore in our case the E. coli inactivation is attributed to 1 O 2 photoproduction.
Moreover, samples of these experiments were kept in dark during 24 h and bacteria concentration was measured to determine the EDT 24 .It is observed that 10 min of solar-simulated light irradiation are required to achieve inactivation International Journal of Photoenergy without regrowth using the Ag(4)TiO 2 sample, even if 10 min of irradiation do not lead to total inactivation as observed in Figure 9(a).On the other hand, the performance of Ag(2)TiO 2 leads to an EDT 24 of 45 min while that of TiO 2 was not reached during the irradiation time.A similar effect was observed with 400 W/m 2 of irradiation as observed in Figure 9(b).However, in this case using TiO 2 and Ag(2)TiO 2 samples the correspondent EDT 24 were 10 and 40 min, respectively, suggesting an increase in the effective oxidative action of the photocatalysts when increasing the irradiation power [53].
Furthermore, Figure 10 shows the results of the photocatalytic water disinfection using the TiO 2 and AgTiO 2 samples under irradiation of 9.8 W/m 2 of natural sunshine irradiation from interior-lighting lamps.Results evidence a fast decay of the E. coli concentration using the Ag(4)TiO 2 sample, while Ag(2)TiO 2 and TiO 2 show lower performances.Such behavior was expected since the light irradiation setup, as previously discussed, has a wide peak of light irradiation at 470 nm which may effectively activate the photocatalyst surface as presented in Figure 2. Nevertheless, the EDT 24 were diminished of course due to the decrease in light irradiation power.
It has been previously discussed that during the photocatalytic inactivation of E. coli it is necessary to achieve a sum of damages to achieve the complete inactivation of bacteria [54].Moreover, Rinc ón and Pulgarin [53] have discussed that even achieving total inactivation during irradiation it is possible to observe regrowth of the bacteria population, after 24 h in dark, indicating that somehow the level of damage needed for absolute death has not been reached.Therefore, in our case we have observed that using Ag modified particles it is possible to achieve a true inactivation in short irradiation periods, as observed after dark periods of 24 h, even without reaching zero concentration of bacteria during irradiation.Such effect, as earlier discussed, is due to action of photoinduced and bactericidal processes.
Finally, to discuss photocatalyst stability it is possible to expect that loosing active silver species on the photocatalyst's surface, during stirring, may lead to a less active surface photocatalysts, and therefore, studies focusing on improving Ag fixing and stability on TiO 2 should be developed.

Conclusions
Ag-TiO 2 photocatalysts, synthesized using the hydrothermal synthesis, lead to Ag aggregates on the TiO 2 surface which increases the visible light response of TiO 2 .In addition, such aggregates are partially oxidized particles that distorted the pore distribution of the samples.This photocatalyst configuration seems to promote photoactivity towards the photocatalytic disinfection of water under solar light and interior-light irradiations.Photoactivity was effectively correlated to the photoproduction of singlet oxygen under visible light irradiation as determined by EPR analyses using DABCO as a singlet oxygen quencher.Moreover, analysis of dark processes evidenced that part of the inactivation process is due to the bactericidal effect of silver in the surface and possible lixiviated Ag + ions.

Figure 2 :
Figure 2: Emission spectra of the Xenon and natural sunshine lamps for interior lighting.

Figure 4 :
Figure 4: TEM micrographs of the (a) TiO 2 and (b) Ag(4)TiO 2 samples.Index (b) is missed in the current phrase.
) and 9(b) show the performance of TiO