Silver-Modified Nano-titania as an Antibacterial Agent and Photocatalyst

Article pubs.acs.org/JPCC

Silver-Modified Nano-titania as an Antibacterial Agent and Photocatalyst D. M. Tobaldi,†,* C. Piccirillo,‡ R. C. Pullar,† A. F. Gualtieri,§ M. P. Seabra,† P. M. L. Castro,‡ and J. A. Labrincha† †

Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal ‡ CBQF/Escola Superior de Biotecnologia, Universidade Católica Portuguesa, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal § Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Via S. Eufemia 19, I-41121 Modena, Italy S Supporting Information *

ABSTRACT: With the increasing demand for nanomaterials, it is essential that they are produced, where possible, by sustainable or “green” synthesis methods, avoiding environmentally harmful processes and solvents, with the aim of reducing the production of hazardous byproducts and wastes and minimizing environmental impact. In this work, Ag-modified titania nanoparticles (NPs) were synthesized via a green aqueous sol−gel method. The products of the synthesis were thermally treated at 450 and 600 °C, and their photocatalytic (in liquid−solid and gas−solid phases) and antibacterial properties were assessed using both UV- and visible-light exposure. The microstructure and phase composition of the prepared samples were also characterized using advanced X-ray powder diffraction methods (whole powder pattern modeling). Results showed that both the amount of Ag and the thermal treatment greatly influenced not only the phase composition and microstructure but also the functional properties of the TiO2. The increasing levels of Ag retarded the anatase-to-rutile phase transition to a greater extent, and 2 mol % was the optimum amount of Ag for methylene blue photodegradation with both UV- and visible-light irradiation. When using a UV-light source, samples showed a much greater antibacterial activity toward Escherichia coli (E. coli; Gramnegative) than methicillin-resistant Staphylococcus aureous (Gram-positive). It was observed that UV light caused a change in the oxidation state of silver, from ionic silver to metallic (Ag+ → Ag0 NPs), this being detrimental for the antibacterial activity. However, under artificial white light irradiation this did not occur and the material kept its excellent antibacterial properties (higher activity than commercial P25); because of this, it could be suitable for use in health care, helping to greatly reduce the spread of Gram-negative type bacteria such as E. coli.

1. INTRODUCTION

green chemistry as: “the utilisation of a set of principles that

Ever-increasing environmental pollution, energy shortages, and emerging penicillin-resistant bacteria have become major global concerns.1,2 For the sustainable development of society, the advance of green technologies for environmental remediation techniques and an alternative clean energy supply is an urgent necessity. At the same time, there is growing concern about the potential health and environmental impacts of the production and use of nanoscale products. Therefore, there is a need for “green” nanosynthesis methods that reduce the amount of hazardous wastes.3−5 Anastas and Warner originally defined

reduces or eliminates the use or generation of hazardous

© 2014 American Chemical Society

substances in the design, manufacture, and application of chemical products”, with the aim of minimizing chemical hazards to health and the environment, reducing waste, and preventing pollution.6 Received: December 7, 2013 Revised: January 16, 2014 Published: February 12, 2014 4751

dx.doi.org/10.1021/jp411997k | J. Phys. Chem. C 2014, 118, 4751−4766

The Journal of Physical Chemistry C

Article

only solvents used are water and isopropyl alcohol, so the process does not result in large amounts of acidic or chlorinated wastes (unlike those using TiCl4), and no protective atmosphere is required. The products of the synthesis were characterized with several analytical techniques, and the photocatalytic (liquid− solid and gas−solid phases) and antibacterial properties were also tested, using both UV- and visible-light sources.

Semiconductor photocatalysis has emerged as one of the most promising green and renewable energy technologies, because it represents an easy way to utilize the energy of either natural sunlight or artificial indoor lighting. Since the discovery of the “Honda−Fujishima” effect,7 titanium dioxide (TiO2) has been the most studied photocatalytic material, due to its stable chemical structure, biocompatibility, and physical, optical, and electrical properties, as well as its inexpensive commercial availability. It is widely used in daily real-world applications, such as a white pigment in paint, food, and cosmetics,8 but also in higher technology applications such as electrochemical electrodes,9 as solar cells,10 and, more recently, as a memristive material.11 Heterogeneous photocatalysis with TiO2 has a wide range of potential and attractive applications, as it can oxidize most organic, and many inorganic, compounds.12−15 This can serve in the decontamination of polluted waters,16 as well as outdoor and indoor air treatments, such as the degradation of volatile organic compounds (VOCs) which cause sick building syndrome (SBS).17,18 Exterior and interior air pollution could be reduced via a “passive” incorporation of such photocatalysts in construction materials.19,20 Because high surface area is an important factor, nanoscale titania is desirable. As well as photocatalytic activity (PCA), TiO2 has antibacterial activity, inactivating/killing bacteria present on the photocatalyst surface, thus making those surfaces self-sterilizing. This is due to the strong oxidizing power of highly reactive free radicals (OH•, H2O2, and O2•) which are generated by TiO2 under UV irradiation; such species may oxidize and eventually eliminate biological molecules. One of the advantages of such selfsterilizing surfaces is that they operate via a simple mechanism, which requires only oxygen and light. They do not need electrical power or chemical reagents, they do not release chemical compounds,21 and titania is nonpoisonous and will not add to environmental pollution.12 Several bacterial strainssuch as Serratia marcescens and Escherichia coli (E. coli; Gram negative), and Microbacterium spp. and Bacillus subtilis22and yeasts23 can be killed via photooxidation processes involving UV radiation and TiO2,24 even in their antibiotic-resistant forms. E. coli, for example, causes diseases of the gastrointestinal, urinary, or central nervous systems in even the most robust human hosts,25 and antibacterial self-sterilizing TiO2-coated surfaces would be a successful tool against these kinds of bacterial strains.26 Of several TiO2 polymorphs, anatase and rutile are the most utilized in photocatalytic applications, anatase being more active than rutile27,28 because of its stronger reducing power,29 lesser electron−hole recombination,30 and better hole-trapping ability.31 Both are wide band gap semiconductors, their energy band gaps (Eg) being 3.2 eV (anatase) and 3.0 eV (rutile).32 This means that TiO2 is able to absorb only 3−5% of the solar spectrum (UV-A), depending on latitude and climate.33 The light absorption of TiO2 can be extended into the visible range by doping with transition metal ions (considered to be quite detrimental)34 and anionic doping.35 More recently, a suggested solution for the energy-harvesting of visible light is the deposition of noble metal nanoparticles (NPs) (i.e., Ag, Au, Cu, and Pt) onto the surface of a semiconductor, to form a metal− semiconductor composite photocatalyst.36 This works because NPs of noble metals are able to absorb visible light, due to their surface plasmon resonance (SPR).37 In particular, for Ag NPs, their long-established and well-known antibacterial properties (oligodynamic effect)38 can be exploited in addition to SPR. In the present work, nano-TiO2 and Ag-modified nano-TiO2 were synthesized via a green aqueous sol−gel method.39 The

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Aqueous titanium(IV) hydroxide sols were made via the controlled hydrolysis of titanium(IV) isopropoxide (Ti−i-pr, Ti(OCH(CH3)2)4), using distilled water diluted in alcohol which was added dropwise over about 40 min. The water/alcohol solution also contains the peptizing acid, HNO3. One part of Ti−i-pr (Aldrich, 97%) was added to four parts of isopropyl alcohol (IPA, propan-2-ol) to make a 20% Ti− i-pr solution. This Ti−i-pr solution was hydrolyzed by the dropwise addition of an excess of water (5:1 water:Ti−i-pr) as a 20% solution in IPA. The acid necessary to peptize the sol (concentrated HNO3, Aldrich, 65%) was also added to this water−IPA solution, in a ratio of Ti4+:acid of 5:1. This water− IPA−acid solution was added dropwise to the Ti−i-pr solution at room temperature (RT), while being stirred. The precipitated mixture was evaporated to a white jelly-like mass on a rotary evaporator, removing the IPA. Distilled water was added to restore the mixture to the original volume; then the gelatinous mass was re-dispersed in a few minutes and the solution evaporated to a dried gel. At this stage, when water was added again, the gel dispersed to form a sol in minutes, and the sol was diluted to a concentration of 1 M Ti4+. To make the 1 and 2 mol % Ag+ doped sols, stoichiometric amounts of silver nitrate (1 M aqueous solution, Aldrich) were added to the sol, when the sol had a 1 M concentration. The aqueous sol−gel synthesis of titania nanoparticles with and without Ag doping, and characterization of the sols, is described in fuller detail in ref 39. The only solvents used are water and IPA, and the IPA can be easily recovered and reused after evaporation from the precipitated mixture. The acid used is the minimum amount required to peptize the sol adequately, and the process does not result in large amounts of acidic or chlorinated wastes, unlike those using TiCl4. Ti−i-pr is not air sensitive, so no protective atmosphere is required. The whole synthesis process takes ∼2 h. We believe that this can justifiably be considered a green process as it is based upon the use of nontoxic water and IPA as solvents, avoiding the use of any toxic, persistent, aromatic, or chlorinated solvents which are essential in many nanosynthesis techniques. A relatively small amount of HNO3 is used, greatly diluted in water so the concentrated form is not required. It is used in the precipitation step as a diluted solution of around 23% concentrated HNO3 in water, which does not fume and does not require ventilation. This is then added to 80 mL of IPA, so the concentration of HNO3 added to the Ti−i-pr solution is a 5.6% solution of concentrated HNO3, which is only 3.6% actual HNO3 in total (as concentrated HNO3 is 65% HNO3). This is then further diluted in the sol, which is typically maintained at a concentration of 1 M Ti, e.g. 0.2 M HNO3 (=12.6 g/L). This is much less acidic than comparable preparation methods based on TiCl4, which produces highly acidic solutions and waste products. In the case of industrial scale-up, both the water and IPA solvents used in the synthesis are removed by evaporation under vacuum, so they could be recycled for reuse in the process, and the acidic waste solvents containing HNO3 could be recycled 4752



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according to the Caglioti et al. relationship.51 Afterward, anatase (SG I41/amd), rutile (SG P42/mnm), and, when present, brookite (SG Pbca) were included in the WPPM modeling. These parameters were refined: backgroundmodeled using a fifth order of the shifted Chebyshev polynomial functionpeak intensities, specimen displacement, and lattice parameters. Crystalline domains were assumed to be spherical and distributed according to a log-normal size distribution. Diffuse reflectance spectroscopy (DRS) was performed with a Shimadzu UV 3100 (JP) spectrometer, and spectra of the samples were acquired in the UV−vis range (250−750 nm), with 0.2 nm in step size and with use of an integrating sphere and a white reference material, both made of BaSO4. The Kubelka− Munk function was applied with the purpose being to convert the diffuse reflectance into the absorption coefficient α:

for the precipitation/peptization step, avoiding unnecessary release into the environment. 2.2. Sample Characterization. X-ray powder diffraction (XRPD) data for the quantitative phase analysis (QPA) were collected using a θ/θ diffractometer, PANalytical X’Pert Pro (NL), equipped with a fast RTMS detector, with Cu Kα radiation (40 kV and 40 mA, 20−80 °2θ range, a virtual step scan of 0.0167 °2θ, and virtual time per step of 50 s). A 0.125° divergence slit, a 0.125° antiscattering slit, 0.04 rad soller slits, and a 15 mm copper mask were mounted in the incident beam pathway. The pathway of the diffracted beam included a Ni filter, soller slits (0.04 rad) ,and an antiscatter blade (5 mm). Full QPA (i.e., determination of both crystalline and amorphous content) were performed using the combined Rietveld−reference intensity ratio (Rietveld−RIR) method, as previously reported by Gualtieri,40 and Gualtieri and Brignoli:41 10 wt % corundum (NIST 676a) was added to the sample and treated as an additional phase in the refinements. In this case, the refined weight fractions of each crystalline phase (Wic) was rescaled with respect to the known weight fraction of added standard (Ws) in order to obtain the real crystalline phase weight fraction (Wi): Wi =

1 ⎡⎛ Ws ⎞ ⎤ ⎢⎜ ⎟Wic ⎥ 1 − Ws ⎢⎣⎝ Wsc ⎠ ⎦⎥

α≈

∑ Wi i

(3)

where K and S are the absorption and scattering coefficients; the reflectance R∞ is equal to Rsample/Rstandard.52 The Eg of the powders was calculated using the differential reflectance method. This method supposes that, plotting the first derivative of reflectance (dR/dλ) versus the wavelength (λ), the maximum value of such a plot corresponds to the band gap of the semiconductor material.53,54 The resulting curves were successfully fitted with a Gaussian function (Origin ProLab, version 8.5.0), and the maximum values were found from the fitting. High-resolution tranmission electron microscopy (HR-TEM) analysis was performed on a JEOL 2200FS (JP) microscope, with a field emission gun, operated at 200 kV and equipped with a energy-dispersive X-ray (EDX) analyzer (Oxford Instruments, Abingdon, U.K.). Samples were prepared by suspending and sonicating the powders in isopropyl alcohol and then placing and evaporating a drop of the suspension on a carbon-coated copper grid. The TEM grid with the Ag-containing sample was irradiated with the same UV-A lamp used for the photocatalytic experiments (see section 3.2.1) for 240 min prior to carrying out HR-TEM analysis. This was done with the aim of detecting any Ag NP (re)formed after such UV-A-light exposure. The specific surface area (SSA) of the prepared samples was evaluated by the Brunauer−Emmett−Teller method (BET; Micromeritics Gemini 2380, Norcross, GA, U.S.), using N2 as the adsorbate gas.

(1)

where Wsc is the refined weight fraction of the internal standard. Knowing the weight fractions of all crystalline phases, the amorphous content (Wa) is calculated using the following equation: Wa = 1 −

(1 − R ∞)2 K = ≡ F(R ∞) S 2R ∞

(2)

For all XRPD measurements, the samples were carefully ground in an agate mortar, reduced to submicrometer size and mounted in aluminum sample holders using the side-loading technique. The Rietveld data analysis was performed using the GSAS software package with its graphical interface EXPGUI.42,43 The starting atomic parameters for anatase and rutile, described in the space groups (SGs) I41/amd and P42/mnm, respectively, were taken from Howard et al.44 The brookite atomic parameters, described in the SG Pbca, were obtained from Meagher and Lager.45 The following parameters were refined: scale factors, zero point, four coefficients of the shifted Chebyshev function to fit the background, unit cell parameters, and profile coefficientsone Gaussian (GW), an angleindependent term, and two Lorentzian terms, LX and LY peak correction for asymmetry. The crystalline domain size distribution, as well as the average domain size of the NPs, was also obtained taking advantage of XRPD datacollected using the same instrument but in the 20− 115 °2θ range, with a virtual step scan of 0.02 °2θ and virtual time per step of 100 sanalyzed via the whole powder pattern modeling (WPPM) method,46,47 through the PM2K software.48 This procedure allows one to extract microstructural information from a diffraction patternrefining model parameters via a nonlinear least-squares routineso as to fit the experimental peaks, with no use of arbitrary analytical functions (i.e., Gaussian, Lorentzian, or Voigtian), the diffraction peak profile being the result of a convolution of instrumental and sample-related physical effects. Hence, the analysis is directly made in terms of physical models of microstructure and/or lattice defects.46,49,50 The instrumental contribution was obtained by modeling a large set of peak profiles from the NIST SRM 660b standard (LaB6),

3. FUNCTIONAL PROPERTIES EVALUATION 3.1. Antibacterial Activity. To test the antibacterial properties of the samples, two bacterial strains were tested: one Gram-positive (methicillin-resistant Staphylococcus aureus, MRSA; ATCC 29213) and one Gram-negative (Escherichia coli, E. coli; NCTC 9001). The protocol for the tests was the same previously used by the same authors; a detailed description can be found in literature.55,56 Briefly, the inocula of both strains were grown in Mueller Hinton agar at 37 °C for 24 h. Liquid cultures of each bacterium with approximate concentrations of 10+7/10+8 CFU/mL were prepared in a 0.85% NaCl solution. A 500 μL aliquot of the culture was pipetted into a sterile Petri dish containing 4.5 mL of 0.85% NaCl solution and a weighed amount of the sample to be tested; the closed Petri dishes were gently shaken (80 rpm). Depending on the experiment, the dishes were either irradiated with UV light (XX-15 BLB UVP lamp; λ = 365 nm; irradiation density of 0.80 mW cm−2), or irradiated with white light (Philips 4753



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TLD 58W/84 fluorescent lamp; spectral emission for 400 nm < λ < 700 nm), or kept in the dark. At regular times, 100 μL aliquots of solution were taken from the plate and appropriate dilutions were performed in 0.85% NaCl solution. The diluted solutions were plated onto plate counting agar (PCA) Petri dishes and incubated at 37 °C. After a 24 h period, the bacteria colonies were counted. Control experiments were performed with bacteria solutions exposed either to UV or white light, in the same conditions. Tests were also done using commercial Aeroxide P25 TiO2 (P25) for comparison. Each experiment was performed in triplicate; bacterial counts were calculated as an average value, with the corresponding standard deviation. 3.2. Photocatalytic Activity. The photocatalytic activity (PCA) of the prepared powders was assessed in liquid−solid and gas−solid phases, by either monitoring the degradation of the organic dye methylene blue (MB, Aldrich), or the abatement of NOx using a chemiluminescence analyzer (AC-30M, Environment S.A, Poissy, France). Commercial P25 photocatalytic powder has been used for comparison in both tests. 3.2.1. Liquid−Solid Phase. The liquid−solid phase tests were performed at room temperature, in a cylindrical photocatalytic reactor (80 mm in diameter) containing an aqueous solution of the dye (0.5 L), at an initial concentration of 5 mg L−1. The concentration of the photocatalyst in the slurry was 0.25 g L−1. In order to mix the solution thoroughly, the slurry was magnetically stirred throughout the reaction; the reactor was covered with a watch glass, so as to avoid the evaporation of the solution. A schematic diagram is shown in the Supporting Information (Figure S1). The reactor was designed following the guidelines of the international standard ISO 10678. A modification was applied concerning the lightning of the reacting systemthis was assured by placing two lamps at either side of the reactor; the distance between the lamps and the reactor was 5 cm. The light sources used were as follows: a germicidal lamp (Philips PL-S 9W, NL), as the UV-light source, and a fluorescent lamp (Philips master PL-S 2P 9W/840, NL), as the visible-light source. The intensity of the radiation reaching the samples was measured with a radiometer (Delta OHM, HD2302.0, IT), and it was found to be around 13 W m−2 in the UV range (315 nm < λ < 400 nm) and ∼50 W m−2 in the visible region (λ > 400 nm). Dyes can be excited by visible-light irradiation, and, as a consequence, they can act as a sensitizer, with electron injection from the photoexcited dye to the photocatalyst.57,58 Hence, that electron transfer may destroy the regular distribution of conjugated bonds within the dye molecule and may cause its decolorization, but not its mineralization.59 Therefore, in order to verify if, under visiblelight irradiation, there was a “real” photocatalytic reaction, or, on the contrary, a decoloration of the dye by photosensitizing effects, total organic carbon (TOC) determination was performed, on selected samples, via a TOC meter (TOC 1200, Thermo Electron Corp., Beverly, MA, U.S.). In the experiments, the photocatalytic degradation of MB was monitored by sampling about 4 mL of the slurry from the reactor, at regular time intervals. Before switching the lamps on, the suspension, with the photocatalyst, was stirred in the dark for 30 min, to account for any absorption of the MB molecules to the surface of the powders. The powders in the samples were separated by centrifugation, and then the MB concentration in the liquid was determined, taking advantage of the Lambert− Beer law, by measuring the absorbance in a spectrometer (Shimadzu UV 3100) at a wavelength of 665 nm, using distilled

water as a reference. The extent of MB photocatalytic degradation, ξ, was evaluated as follows: ξ /% =

C 0 − CS × 100 C0

(4)

where C0 is the initial MB concentration and CS is the concentration after a certain UV/visible irradiation time (the total irradiation time was set at 7 h). Control experiments (photolysis of the MB dye), under direct UV- and visible-light irradiation, were performed prior to testing the PCA of the prepared samples. 3.2.2. Gas−Solid Phase. The reactor employed for gas−solid phase tests operated in continuous conditions, as previously described in detail by Lucas et al.60 This was made of a stainless steel cylinder (35 L in volume); its top was sealed and covered with a glass window, to allow the light to reach the photocatalyst placed inside it. A schematic diagram is shown in the Supporting Information (Figure S2). The light source employed was a solar lamp (Osram Ultra-Vitalux, 300 W), and the distance between that and the photocatalyst was 85 cm; the light intensity reaching the samples, measured with a radiometer (Delta OHM, HD2302.0, IT), was found to be approximately 3.6 W m−2 in the UV-A range and 25 W m−2 in the visible-light range. Samples were prepared in the form of a thin layer of powder, with a constant mass (∼0.1 g), and hence with approximately a constant thickness, in a Petri dish 6 cm in diameter. The tests were performed at 27 ± 1 °C (temperature inside the reactor) with a relative humidity of 31%. These parameterscontrolled by means of a thermocouple that was placed inside the chamber and a humidity sensor placed in the inlet piperemained stable throughout the tests. The outlet concentration of the pollutant gas was measured using a chemiluminescence analyzer (AC-30 M, Environment S.A). After having placed the photocatalyst inside the reactor and covered the glass window, the inlet gas mixture (prepared using gas cylinders containing synthetic air and NOx) was allowed to start flowing until it stabilized at a concentration of 0.5 ppm. Two mass flow controllers were used to prepare such a mixture of air with that concentration of NOx, with a flow rate of 1 L/min. This step is necessary to guarantee the sample saturation, assuring that, during the test, the measurement of NOx is solely due to the photocatalytic process (i.e., no absorption from the sample, nor from the reactor walls).61 Once the desired concentration (0.5 ppmv) was accomplished, the window glass was uncovered, the lamp turned on, and the PCA reaction started. The photocatalytic reaction was presumed to be over when the pollutant concentration reached a minimum and stable level and the sample could no longer keep decomposing any NOx. The photocatalytic efficiency was evaluated as the ratio of the removed concentration of NOx. The conversion rate (%) of the initial NOx concentration was calculated following this equation.62 (NOx conversion)/% =

(NOx )0 − (NOx )s × 100 (NOx )0

(5)

where (NOx)0 and (NOx)S are, respectively, the initial NOx and the NOx concentration (both expressed as ppmv) after a certain irradiation time.

4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction Analysis. The XRPD patterns of the dried gels at 75 °C, and those of the gels calcined at 450 and 600 4754



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Figure 1. XRD patterns of (a) gels dried at 75 °C, (b) gels calcined at 450 °C, and (c) gels calcined at 600 °C. The vertical bars represent the XRD reflections of anatase (orange, JCPDS-PDF card no. 21-1272), rutile (light blue, JCPDS-PDF card no. 21-1276), brookite (dark blue, JCPDS-PDF card no. 29-1360only the three most intense reflections were reported here), and silver (dark gray, JCPDS-PDF card no. 04-0783).

Table 1. Rietveld Agreement Factors and Phase Composition of the Unmodified and Silver-Modified TiO2a agreement factors

phase composition

sample

no. of variables

RF (%)

Rwp (%)

χ

anatase (wt %)

rutile (wt %)

brookite (wt %)

amorphous (wt %)

Ti75 Ag−Ti75 2Ag−Ti75 Ti450 Ag−Ti450 2Ag−Ti450 Ti600 Ag−Ti600 2Ag−Ti600

19 19 15 20 18 17 17 17 20

3.79 3.12 3.14 7.92 4.85 8.69 6.90 6.63 6.25

5.60 5.60 4.56 6.13 5.57 4.77 6.68 5.05 5.66

2.05 1.73 2.28 1.72 1.59 3.20 2.37 1.35 3.51

30.4(3) 44.2(4) 59.4(5) 27.7(3) 38.6(3) 51.9(5) 1.5(1) 6.2(1) 4.4(1)

16.0(2) 17.4(2)

21.0(4) 10.9(3) 14.5(3) 8.7(3) 10.7(3) 14.5(5)

32.6(5) 27.5(5) 26.1(5) 25.8(5) 24.4(4) 25.7(5) 24.0(6) 16.0(9) 19.1(6)

2

2

37.9(3) 26.4(2) 7.8(1) 74.4(6) 77.9(9) 76.5(5)

a The phase composition was calculated from the Rietveld refinements of X-ray diffraction patterns, using the NIST 676a internal standard. There were 4088 observations for every refinement; the number of anatase, rutile, and brookite reflections was 32, 31, and 156, respectively. Values in parentheses are the standard deviations, referred to the last decimal number.

°C, are depicted in Figure 1a−c. The presence of poorly crystalline anatase, rutile, and brookite is already visible at the drying temperature of 75 °C. The introduction of Ag (1 and 2 mol %) in the titania sol delayed the anatase-to-rutile phase transformation (ART) in both the dried sols and calcined gels. No metallic silver could be detected from the XRPD patterns. It

has to be highlighted, however, that the strongest peaks of Ag overlap with those of anatase and/or rutile (see Figure 1a−c; JCPDS-PDF card nos. 21-1272 (anatase), 21-1276 (rutile), and 04-0783 (Ag0)). Moreover, the amount of silver present may be below the detection limit of XRPD; in this regard, Chao et al. were able to detect Ag0 only at concentration ≥ 4 mol %.63 4755



4756

5.6 ± 1.1 4.0 ± 3.4 5.1 ± 0.8 13.4 ± 1.1 12.1 ± 2.9 12.5 ± 0.4 14.2 ± 1.0 17.8 ± 1.3 25.1 ± 1.5 9.8 ± 0.1 7.1 ± 1.0 7.9 ± 0.3 18.4 ± 14.2 19.1 ± 4.8 20.6 ± 4.1 a

Note: ant, rt, and brk, in the “average crystalline domain diameter” columns, stand for anatase, rutile, and brookite, respectively.

0.9025(6) 0.9093(20) 0.9198(12) 0.5332(7) 0.5278(10) 0.5241(12) 0.2959(1) 0.2960(1) 0.2958(1) 0.2959(1) 0.2955(1) 0.2959(1) 0.4594(1) 0.4596(1) 0.4591(1) 0.4593(1) 0.4587(1) 0.4593(1) 0.9509(2) 0.9503(5) 0.9499(1) 0.9525(3) 0.9507(1) 0.9511(1) 0.3786(1) 0.3787(2) 0.3785(1) 0.3780(1) 0.3778(1) 0.3783(1) 1.27 1.09 1.22 1.39 1.40 1.59 6.03 11.73 1.72 4.06 6.20 1.79 7.67 12.82 2.11 5.62 8.65 2.84 Ti450 Ag−Ti450 2Ag−Ti450 Ti600 Ag−Ti600 2Ag−Ti600

c (nm) a = b (nm) sample

Rwp (%)

Rexp (%)

χ2

a = b (nm)

c (nm)

rutile anatase agreement factor

0.5413(7) 0.5437(16) 0.5412(8)

⟨Dbrk⟩ (nm) ⟨Drt⟩ (nm) b (nm) a (nm)

brookite unit cell param

Table 2. WPPM Agreement Factors, Unit Cell Parameters, and Average Crystalline Domain Diameter of the Samples

c (nm)

av crystalline domain diama

QPA data are reported in Table 1, and Supporting Information Figure S3 depicts an example of the graphical output of a Rietveld refinement (sample 2Ag−Ti600). The titania gel dried at 75 °C (Ti-75) had the following composition: 30.4 wt % anatase, 16.0 wt % rutile, 21.0 wt % brookite, and 32.6 wt % amorphous phase. The addition of 1 mol % of Ag in the sol (Ag−Ti75) delayed the ART even at the drying temperature, leading to the formation of more anatase (44.2 wt %) versus 17.4 wt % rutile and 16.2 wt % brookite. Double the amount of Ag (2Ag−Ti75) delayed the ART further, with anatase (59.4 wt %) and brookite (14.5 wt %) being the only crystalline phases present. The thermal treatment of the dried gels at 450 °C led to a general decrease of the anatase, brookite, and amorphous amounts, in favor of rutile. At this temperature, sample Ti-450 is composed of 27.7 wt % anatase, 37.9 wt % rutile, 8.7 wt % brookite, and 25.8 wt % amorphous phase. By contrast, Ag−Ti450 is made of 38.6 wt % anatase, together with a lesser amount of rutile (26.4 wt %), but a slightly greater amount of brookite (10.7 wt %). The modification with 2 mol % Ag delayed the ART to a greater extent, sample 2Ag− Ti450 having the greatest amount of anatase (51.9 wt %), along with 7.8 wt % rutile and 14.5 wt % brookite. Hence, adopting this synthesis method, we can argue that Ag incorporation increased the thermal stability of anatase NPs, hindering the crystallization processa nucleation and growth process64and shifting the ART toward higher temperatures, consistent with the findings of ́ Garcia-Serrano et al.65 A higher thermal treatment (600 °C) promoted the brookite → rutile phase transformation.66 All of the samples are composed of anatase, rutile, and amorphous phase, although it is worth noting that the amorphous contents are still quite high, even with thermal treatment at 600 °C (Table 1). Ti-600 has the lowest amount of anatase (1.5 wt %), but also the highest amount of amorphous phase (24.0 wt %), among the set of samples. Ag−Ti600 has 6.2 wt % anatase, 77.9 wt % rutile, and a lesser amount of amorphous phase (16.0 wt %). By comparison, 2Ag−Ti600 had slightly lower amounts of both anatase (4.4 wt %) and rutile (76.5 wt %) but a greater amount of amorphous phase (19.1 wt %). This agrees well with the work of Chao et al., who reported the change of the ART mechanism with Ag concentration.63 WPPM data are reported in Table 2. Figure S4 in the Supporting Information shows an example of the WPPM graphic output for 2Ag−Ti600, and parts a−e of Figure 2 show the WPPM size distribution of (a) anatase, (b) rutile, and (c) brookite in samples thermally treated at 450 °C and (d) anatase and (e) rutile at 600 °C. As shown in Table 2, the unit cell parameters of anatase, rutile, and brookite are the same (within experimental error) in all samples calcined at 450 °C, independent of the silver content. The same behavior can be observed for the samples fired at 600 °C, as the lattice volume is identical (within errors) for the whole set of samples. These data are consistent with the values of the ionic radii of [6]Ti4+ and [6] Ag+, being equal to 0.61 and 1.15 Å,67 respectively. Because Ag+ is so much larger, it does not enter the titania lattice, but a segregation of Ag at titania grain boundaries happened.68,69 The crystalline domain size from XRPD (Table 2 and Figure 2a−e) shows that, in the samples thermally treated at 450 °C, both Ag modifications led to a slight decrease of anatase, rutile, and brookite average crystalline domain sizes. Among these TiO2 polymorphs, brookite had the smallest average domain size (values ranging from 4.0 to 5.6 nm), while rutile had the greater (values from 12.1 to 13.4 nm). Generally speaking, the modification with 1 mol % Ag (Ag−Ti450) gave titania samples with the smallest average crystalline domain size (Figure 2a−c).

⟨Dant⟩ (nm)

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Figure 2. Size distribution, as obtained from the WPPM modeling, of (a) anatase, (b) rutile, and (c) brookite in the samples thermally treated at 450 °C and of (d) anatase and (e) rutile in the samples thermally treated at 600 °C.

Increasing the calcination temperature to 600 °C led to a common increase of the average domain size, both for anatase and rutile. Considering the crystalline domain size of anatase, all of values lay within the experimental error: that of Ag-modified samples was 19.1 and 20.6 nm (Ag−Ti600 and 2Ag−Ti600, respectively) versus 18.4 nm for Ti-600 (see Figure 2d). For rutile (Figure 2e) Ag insertion led to a greater average domain

size: the greater the Ag concentration, the greater the average domain size (17.8 and 25.1 nm for Ag−Ti600 and 2Ag−Ti600, respectively). Furthermore, the addition of Ag led to an increase of the SSA of the samples, particularly at the lower thermal treatment (Table 3). The estimated particle diameters (diameter in nanometers estimated from D = 6000/(SSA × ρ), where ρ = 4.2 g cm−3 (rutile)), are between 27 nm (2Ag−Ti450) and 34 nm 4757



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1% under both UV- and visible-light irradiation. Under UV-light exposure (Figure 4a), the sample showing the best activity was 2Ag−Ti450, which degraded 80% of the dye in 7 h. With lower silver concentration (Ag−Ti450), MB degradation was less than 70%. The thermal treatment at 600 °C, together with a lowering of the SSA (Table 3) and the increase of the average domain size, also caused a decrease of the PCA; in fact both Ag-containing samples have approximately the same PCA performances (the ξ values were equal to ∼42% for both samples; cf. Figure 5a). The P25 sample had the best activity, degrading all the organic dye in 2 h reaction time. The pseudo-first-order reaction constants calculated from these curves are reported in Figure 4b (R2 > 0.98); these values confirm the differences due to the silver content and the calcination time. It has been reported that, under UV radiation, Ag can be detrimental for the PCA of anatase. As electrons from anatase are accumulated in silver particles, they attract photoholes, becoming recombination centers (because of an electron transfer to the noble metal), thus decreasing the PCA of anatase present in the samples.39,72 Consequently, by analogy with N-doped TiO2,73 even though Ag promotes light absorption in the visible range, the PCA in the UV region is not higher than that of undoped TiO2. Parts c and d of Figure 4 show the PCA of the samples under visible-light exposure, at both thermal treatment (R2 values; for the histograms reported in Figure 4d, are all above 0.98). As with UV irradiation, with visible light the most active samples are those calcined at 450 °C, 2Ag−Ti450 with the higher silver content being the best. It is worth highlighting that, in these conditions, both 2Ag−Ti450 and Ag−Ti450 have a higher activity than the commercial P25 sample43 and 38% MB degradation, respectively, vs 20%after 7 h. The seen discoloration of MB does not necessarily correspond to the oxidation and mineralization of the moleculeit actually exists in a reduced, colorless form of MB (leuco-methylene blue (LMB))74moreover, different degradative routes and inter-

Table 3. Band Gap Energy (Eg) and Specific Surface Area (SSA) of the Prepared Samplesa

a

sample

Eg (eV), dR/dλ

SBET (m2 g−1)

Ti450 Ag−Ti450 2Ag−Ti450 Ti600 Ag−Ti600 2Ag−Ti600

3.06 ± 0.01 3.08 ± 0.01 3.08 ± 0.01 3.03 ± 0.01 3.07 ± 0.01 3.13 ± 0.01

41.9 49.2 52.6 4.0 1.1 4.8

Eg is calculated with the diffuse reflectance dR/dλ method.

(Ti-450) after 450 °C and between 297 nm (2Ag−Ti600) and 1.3 μm (Ag−Ti600) after 600 °C. 4.2. DRS Results. The energy band gaps of the calcined powders were calculated via the differential reflectance method. The Eg of the samples are all slightly shifted to the visible region, with the Ag modification giving the samples a small shift into the UV regionsample 2Ag−Ti600 having the higher blue shift (3.13 eV; 396 nm). This was attributed by Nolan et al.70 to the presence of additional silver, as shown in Table 3. The Egs of the samples were all assigned to rutile, as they are consistent with its expected Eg value (3.02 eV; 411 nm). DRS spectra of samples 2Ag−Ti450 and 2Ag−Ti600 are depicted in Figure 3. Aside from the absorption band belonging to the metal−ligand charge transfer (MLCT) in titania,71 in sample 2Ag−Ti450 a tail can be seen extending into the visible region (approximately in the 450−550 nm range), and in sample 2Ag−Ti600 there is the appearance of a weak plasmonic band, as shown in the inset of Figure 3 (both samples were kept in dark prior to doing the DRS analysis), suggesting the presence of Ag0 NPs. 4.3. Photocatalytic Activity. 4.3.1. Liquid−Solid Phase. PCA results in liquid−solid phase are depicted in Figure 4a−d. The MB photolysis (data not reported in the figure) was considered to be negligible, the ξ values being equal to 3% and

Figure 3. DRS spectra of samples 2Ag−Ti450 and 2Ag−Ti600. 4758



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Figure 4. (a) Photocatalytic activity in the liquid−solid phase of the samples under UV-light exposure, (b) histogram with the apparent pseudo-firstorder constant (k′app) of the reactions under UV-light exposure, (c) photocatalytic activity in the liquid−solid phase of the samples under visible-light exposure, and (d) histogram with the apparent pseudo-first-order constant (k′app) of the reactions under visible-light exposure.

LMB, or of the actual molecule mineralization, TOC analysis was performed. Results of sample P25 showed that, under visiblelight exposure, there was an actual decrease in the organic carbon content (i.e., the normalized organic carbon content measured at the end of the reaction was 74.1%). Hence, by analogy, we believe that the other tested samples also possessed this activity. In any case, with the available data, we cannot exclude a priori potential dye-sensitizing effects (decolorization)together with a “real” photocatalytic reaction.76 The calcination at 600 °C causes a decrease in the photocatalytic activity, as already observed for UV-light irradiation. Despite this, however, 2Ag−Ti600 still had a higher activity than P25 after 7 h (Figure 4d). The presence of Ag NPs, which are segregated at titania grain boundaries, clearly extended the absorption edge of titania into the visible-light range, due to the SPR phenomenon.68,77,78 Hence, we can assert that, with these conditions (synthesis method, thermal treatments, and MB degradation), 2 mol % is the optimum amount of Ag for dye photodegradationusing both UV- and visible-light irradiationat both thermal treatments. Sample 2Ag−Ti450 has a pseudo-first-order constant (k′app) of the reaction under visible-light exposure that is virtually double that of P25 (Figure 4d).

Figure 5. NOx conversion (%) of the samples by using the solar lamp as the light source.

mediates are reported to degrade MB.74,75 With the aim of verifying if the seen discoloration is the result of the formation of 4759



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4.3.2. Gas−Solid Phase. The NOx abatement results are shown in Figure 5. Only the samples thermally treated at 450 °C (those with better results in terms of PCA in the liquid−solid phase) were tested in this work. Samples were shown to be photocatalytically active also in the gas−solid phase regime. Using a solar lamp as the light source, sample 2Ag−Ti450 showed higher activity than Ag−Ti450 (43% and 20% NOx conversion, respectively). However, even in this case they were less active than sample P25 (61% NO x conversion). These results are similar to those observed for the liquid−solid phase under UV-light exposure (see section 4.3.1). The pseudo-first-order kinetic constants for the initial 20 min are shown in Table 4.

dark), and, in the same experiments (sample 2Ag−Ti450), with white light (artificial indoor lighting) as well. The results of the test performed with 1 mg/mL Ag−Ti450 are shown in Figure 6. Considering MRSA, it can be seen that under UV irradiation the sample does not show much activity (Figure 6a); in fact the decrease in the bacteria population due to the sample is comparable to that induced by the UV light alone (values not statistically different). For the tests performed in the dark, however, a significant effect of Ag−Ti450 on the MRSA strain can be seen (Figure 6b) after 2 h. In fact, after 2 h the bacterial population decreased by almost 1 order of magnitude, with an inactivation rate of about 87%. It is interesting to highlight that, without any light irradiation, sample Ag−Ti450 shows better antibacterial properties than the commercial sample P25. For Gram-negative E. coli (Figure 6c), a clear antibacterial activity can be observed. Under UV irradiation, the bacterial population decreased by more than 4 orders of magnitude, corresponding to an activation greater than 99.99%. With no UV irradiation (Figure 6d), the sample also showed good antibacterial properties, as an inactivation of about 99.7% can be seen. As already observed for MRSA, with E. coli the performance of Ag−Ti450 in the dark was better than the commercial P25. An increase in the silver concentration in the samples did not improve the antibacterial properties of the samples toward MRSA. In fact, as shown in Figure 7a,b, the antimicrobial behavior of 1 mg/mL of 2Ag−Ti450 is not significantly different from that of sample Ag−Ti450 described previously. Not even irradiating it with white light increased the antibacterial performance of 2Ag−Ti450 against MRSA, as can be seen in Figure 7c. However, the higher silver concentration in 2Ag−Ti450 had a major effect on the antibacterial behavior toward E. coli. Data for the experiments performed under UV irradiation, in the dark and

Table 4. Initial (20 min) Pseudo-First-Order Kinetic Constants and Relative Correlation Coefficients for the Prepared Samples, in the Case of NOx Degradation in the Gas−Solid Phase sample

k20 × 102 (min−1)

R2

Ag−Ti450 2Ag−Ti450 P25

0.872 ± 0.024 2.115 ± 0.042 4.427 ± 0.194

0.996 0.998 0.990

In all cases, after an initial increase, NOx concentration reached a plateau. This deactivating behavior of TiO2 is due to the NO2 oxidation to HNO3, as the O2− and OH• radicals formed during the photocatalytic reaction reacted with the pollutant gas to produce NO2 and HNO3.79 Ohko et al. reported that HNO3 produced on the surface of TiO2 is likely to act as a physical barrier, hence inhibiting the photocatalytic reaction.80 4.4. Antibacterial Activity. All antibacterial tests were performed against a control with no titania present, and against 1 mg/mL of P25, all with UV irradiation, no irradiation (in the

Figure 6. Antibacterial activity for sample Ag−Ti450: (a) MRSA with UV-light irradiation, (b) MRSA with no light irradiation, (c) E. coli with UV-light irradiation, and (d) E. coli with no light irradiation. Color key: black bars, 0 h; white bars, 1 h; gray bars, 2 h. 4760



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Figure 7. Antibacterial activity for sample 2Ag−Ti450: (a) MRSA with UV-light irradiation, (b) MRSA with no light irradiation, (c) MRSA with whitelight irradiation, (d) E. coli with UV-light irradiation, (e) E. coli with no light irradiation, and (f) E. coli with white-light irradiation. Color key: black bars: 0 h; white bars, 1 h; gray bars, 2 h. The numbers on the x-axes of d−f represent various concentrations of the sample, in milligrams per milliliter.

under white-light irradiation, are shown in Figure 7d−f, respectively, with varying amounts of sample between 0.1 and 1 mg/mL. Under UV light, the antibacterial properties of 1 mg/ mL of 2Ag−Ti450 are comparable with those of 1 mg/mL of the commercial P25 sample; in fact both powders caused an inactivation in the bacterial strain of 99.999% (Figure 7d). Considering the excellent properties shown by this sample, further tests were performed with lower concentrations, specifically 0.1, 0.25, and 0.5 mg/mL. In all cases, a significant antibacterial effect can be seen, as inactivation rates of 93%, 98%, and 99.99% were observed, respectively. With no light irradiation, 2Ag−Ti450 also showed very good antimicrobial properties (Figure 7e); inactivation rates of 75%, 99.97%, 99.99%, and 99.999% were registered for concentrations of 0.1, 0.25, 0.5, and 1 mg/mL respectively. It is interesting that the inactivation observed for the P25 sample was only 93.6%; once again, therefore, the performance of the samples prepared in this work, in this case 2Ag−Ti450, was better than that of the commercial sample even when lower concentrations were used, when these experiments were performed without light irradiation. With a white-light source, an even more powerful action can be seen for 2Ag−Ti450 (Figure 7f). A very large inactivation of the

bacteria of 99.999% was observed for a sample concentration as low as 0.25 mg/mL. Even with a powder concentration of only 0.1 mg/mL, a decrease in the bacterial population of about 3 orders of magnitude was registered. In all cases the activity of 2Ag−Ti450 was superior to that of the commercial P25 sample. Increasing the calcining temperature to 600 °C caused a decrease in the antibacterial properties of the sample, in agreement with what was observed for the photocatalytic activity. As can be seen in Figure 8, sample 2Ag−Ti600 showed no significant activity toward MRSA, either with or without UV irradiation (Figure 8a,b, respectively). Although some effect can be seen toward E. coli, it is much smaller if compared to that observed for sample 2Ag−Ti450, either in the presence or absence of UV light (Figure 8c,d). The antibacterial tests performed on all Ag-modified samples showed a much greater activity toward E. coli (Gram-negative) than MRSA (Gram-positive). Literature reports divergent data about this; in some cases Gram-positive strains seem more susceptible to photocatalytic agents.23,81,82 In other cases, however, the opposite results were observed, with photocatalytic materials being either more active toward Gram-negative microorganisms,83−85 or showing similar activity.86,87 These differences can occur as the antibacterial action of TiO2-based 4761



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Figure 8. Antibacterial activity for sample Ag−Ti600: (a) MRSA with UV-light irradiation, (b) MRSA with no light irradiation, (c) E. coli with UV-light irradiation, and (d) E. coli with no light irradiation. Color key: black bars, 0 h; white bars, 1 h; gray bars, 2 h.

Figure 9. HR-TEM micrographs of sample 2Ag−Ti600irradiated with a UV-A lamp for 240 min prior to the analysisshowing the presence of rounded (re)formed silver NPs clustered around titania.

materials can be affected by several parameters, such as surface morphology/area and, in the case of NPs, average dimensions as well as shape.88 Moreover, often tests were performed using different experimental conditions,83 this making the comparison even more difficult. The mechanisms causing the bacteria killing under light irradiation are not completely understood; the most accepted ones are via damage to the cell membrane and the alteration to the cell morphology caused by the light-generated radical species.85 In this mechanism, the complexity and the density of the cell wall of the microorganisms plays a key role in their resistance to the antibacterial material. Gram-negative strains such as E. coli have a thinner wall in comparison to Gram-positive ones (i.e., MRSA);89 these data can explain the higher antimicrobial efficiency toward Gram-negative bacteria observed here.

Studies performed on this subject, however, suggest different mechanisms for different materials and/or bacterial strains; this can also explain the different results reported in literature. To assess the antibacterial properties of these samples, tests were performed in three different conditionswith UV irradiation, with white light, and without any irradiation. These latter tests were done to verify whether the samples had an intrinsic antibacterial activity, due to their composition and structure, and not due to light activation. Results show that this is definitively the case for both samples calcined at 450 °C (Ag− Ti450 and 2Ag−Ti450) toward Gram-negative E. coli. Such antibacterial behavior is due to the presence of silver, whose antimicrobial properties are well-known.38 It is therefore reasonable for the activity of sample 2Ag−Ti450 to be higher than that of Ag−Ti450. 4762



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As shown in Figure 8, the calcination of the sample at 600 °C caused a decrease in the bactericidal activity; this again can be explained by considering the silver oxidation state. Sample 2Ag− Ti600 was gray in color, indicating the presence of metallic silver NPs. This was also confirmed by the appearance of a weak plasmonic band in the DRS spectrum of sample 2Ag−Ti600 (sample kept in dark), as shown in Figure 3.

This behavior, however, was not observed in the case of MRSA. This could be due to the higher resistance shown by the Gram-positive bacteria, as already mentioned above. It is probable that the silver content, although higher than in Ag− Ti450, does not have an action powerful enough to penetrate the thicker MRSA cell wall. It is likely that an even higher silver concentration in the sample would increase the antibacterial activity also toward this strain; this option, however, was not considered in this study. The main reason is that excessive silver content in TiO2-based samples can have a detrimental effect on their photocatalytic properties.72,90,91 Moreover, due to silver toxicity both to humans and the environment,92 the use of higher silver concentration is not advisable. From Figure 7d,e, it can be seen how in some cases (i.e., 2Ag− Ti450 concentrations equal to 0.25 and 0.5 mg/mL) the samples tested in the dark have a higher antimicrobial activity than those irradiated with UV light. This could be due to the photochromic effect induced by UV irradiation, previously observed by the authors on these samples.68 In fact, the presence of UV light causes a change in the oxidation state of silver NPs, from ionic silver Ag+ to metallic silver Ag0; cf. Figure 9a,b, where rounded (re)formed Ag0 NPs, having approximately ≤15 nm in diameter, are clustered around titania NPs. Such (re)formation of Ag0 NPs is due to the UV-light exposure of the samples, which occurred prior to the HR-TEM analysis.68 This, in turn, causes a change in the color of the sample from whitish−pale yellow to gray. This change in color was indeed observed at the end of the antibacterial tests done with UV-light, confirming that a change in the oxidation state takes place during the test itself. For the tests performed in the dark, on the contrary, no change in color was observed, indicating that silver maintained its ionic state during the whole experiment. The silver reduction causes a decrease in the antibacterial activity; this is in agreement with literature data, which report a higher antibacterial activity for ionic silver than for metallic.87,92 It is interesting to note that this effect can be observed only for the longer irradiation time (2 h). In fact, after only 1 h the efficiency of the irradiated sample is still higher than that of the one in the dark, due to the photocatalytic effect. This indicates that the silver reduction takes place to relatively small extent during the first hour, certainly not enough to influence the antibacterial behavior. With a longer irradiation time, on the other hand, the effect becomes dominant, affecting the antimicrobial properties of the material. Considering the tests with white light, the source used showed emission only in the visible region, between 400 and 700 nm.93 Lamps with similar spectral emission are commonly used in many hospital environments.94 Tests performed under such irradiation can, therefore, be seen as a simulation of the antibacterial efficiency of the material in a healthcare environment, showing their actual potential in the prevention of infections. The results for sample 2Ag−Ti450 (Figure 7f) indicate that it could be very suitable for hospital use, helping to significantly reduce the spread of Gram-negative type bacteria such as E. coli. The higher activity observed with white light is in agreement with the photocatalytic data (see Figure 4c,d), confirming that the presence of silver in the material shifts the absorption toward the visible region, making the material more efficient for this energy range. Moreover, using this light source, no change in the color of the sample was observed; this indicated that silver was in ionic form during the whole test, keeping all its antibacterial properties.

5. CONCLUSION Nanopowders of titania modified with silver were made by a green nanosynthesis technique, based upon environmentally benign aqueous sol−gel techniques. The effects of Ag on the microstructure and phase composition of the prepared samples was characterized using advanced X-ray powder diffraction methods. Due to its large size, silver could not enter the titania lattice and did not modify the titania unit cell parameters. This led to a slight decrease of the anatase, rutile, and brookite average crystalline domain size. Increasing the calcination temperature to 600 °C led to a common increase of the average domain size. On the other hand, the presence of Ag retarded the anatase-to-rutile phase transition (ART)the modification with 2 mol % Ag delaying the ART to a greater extent. As for the functional applications, considering the photocatalytic activity, 2 mol % was the optimum amount of Ag for methylene blue photodegradationusing both UV- and visiblelight irradiationwith both thermal treatment temperatures. In the case of antibacterial activity, when using a UV-light source, samples showed a much greater activity toward E. coli (Gramnegative) than MRSA (Gram-positive). Nonetheless, it has to be highlighted that UV-light causes a change in the oxidation state of silver, from ionic silver to metallic (Ag+ → Ag0 in the form of NPs), this being detrimental for the antibacterial activity. With white-light irradiation, results for sample 2Ag−Ti450 indicated that this could be very suitable for hospital use, helping to greatly reduce noticeably the spread of Gram-negative type bacteria such as E. coli. This sample had an antibacterial behavior much greater than that of P25.

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ASSOCIATED CONTENT

S Supporting Information *

Figures depicting a schematic diagram of the reactors used in the liquid−solid and gas−solid phase photocatalytic tests, the graphic outputs of the Rietveld refinement of sample 2Ag− Ti600 fired at 600 °C and of the WPPM modeling of sample 2Ag−Ti600, and FT-IR spectra of Ti450, 2Ag−Ti450, and 2Ag− Ti600 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +351 234 370 041. E-mail: [email protected]; david@ davidtobaldi.org. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to M. Ferro and RNMEUniversity of Aveiro, FCT Project REDE/1509/RME/2005for HR-TEM analysis. We also acknowledge PEst-C/CTM/LA0011/2013 and PEstOE/EQB/LA0016/2011 programmes. M.P.S. and R.C.P. thank the FCT Ciência2008 programme for supporting this work. C.P. thanks FCT for her research grant (SFRH/BPD/86483/2012). 4763



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