Modification of Titanium(IV) Dioxide with Small Silver Nanoparticles

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Modification of Titanium (IV) Dioxide with Small Silver Nanoparticles : Application in Photocatalysis Ewelina Grabowska, Adriana Zaleska, Sébastien Sorgues, Marinus Kunst, Arnaud Etcheberry, Christophe Colbeau-Justin, and Hynd Remita J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3112183 • Publication Date (Web): 02 Jan 2013 Downloaded from http://pubs.acs.org on January 2, 2013

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The Journal of Physical Chemistry

Modification of Titanium (IV) Dioxide with Small Silver Nanoparticles: Application in Photocatalysis E. Grabowska,1 A. Zaleska,2 S. Sorgues,3 M. Kunst,4 A. Etcheberry,5 C. Colbeau-Justin,3 H. Remita3* 1

Chair of Environmental Engineering, Faculty of Chemistry, University of Gdansk, Poland 2 Department of Chemical Technology, Faculty of Chemistry, Gdansk University of Technology, Poland 3 Laboratoire de Chimie Physique, CNRS UMR 8000, Univ Paris-Sud, Bât. 349, 91405 Orsay, France. E-mail: [email protected] 4 Helmholtz Zentrum Berlin für Materialien und Energie, Institute Solar Fuels and Energy Storage, Glienicker Straße 100, 14109 Berlin, Germany 5 Institut Lavoisier de Versailles, CNRS UMR 8180, 45 Avenue des Etats Unis 78035 Versailles, France

Keywords: Titania, Silver Nanoclusters, Plasmonic Photocatalysis, Radiolysis, Water Treatment.

Abstract The surface of commercial TiO2 compounds (P25 and ST01) has been modified with Ag nanoparticles induced by radiolysis. On P25, the Ag nanoclusters are very small (1-2 nm) and homogeneous in size while on ST01, two populations of nanoparticles are obtained, small nanoparticles (1-2 nm) and larger ones (mean diameter 7 to 12 nm depending on the silver loading). The photocatalytic properties of Ag-modified TiO2 have been studied for phenol photodegradation in aqueous suspensions under UV and visible light. Their electronic properties have been studied by Time Resolved Microwave Conductivity (TRMC) to follow the charge-carrier dynamics. TRMC measurements show that the TiO2 modification (P25 and ST01) with Ag nanoparticles plays a role in charge-carrier separations increasing the activity under UV-light. Indeed, Ag nanoparticles act as electron scavengers, decreasing the charge carrier recombination. TRMC measurements show also that more electrons are produced in the conduction band of P25 under UV-illumination of Ag-modified P25. Surface modification by silver nanoparticles induces also a modification of the absorption properties of the photocatalyst creating an activity under visible light.

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1- Introduction TiO2 is a very efficient photocatalyst due to its strong oxidation capacity, high photochemical and biological stability and low cost. However, titanium dioxide usage has a few shortcomings. Indeed, TiO2 absorbs only 2-3 % of the solar light impinging on the Earth’s surface as it can be excited only under UV irradiation with wavelengths shorter than 400 nm. Moreover, as in most of semiconductors, a high rate of recombination between electrons and holes results into low quantum yield. In order to enhance the photocatalytic activity, modifications of crystalline TiO2 are, in general, involved. 1,2 This last decade, a growing number of studies in the photocatalytic field have focused on the preparation of TiO2 modified with metal nanoparticles exhibiting reactivity under visible light (λ > 400 nm), which should allow the use of a larger part of the solar spectrum or even of poor interior light illumination. Doping TiO2 with nitrogen, sulfur and carbon atoms have resulted in an improvement in visible light absorption and in an enhancement of the photocatalytic activity under visible light.3, 4, 5, 6 However, the obtained photocatalytic activities under UV-light were not much improved by such doping. Modification with noble metal ions, such as platinum, palladium, silver and gold or metal nanoparticles (NPs)1,2,3,7,8 can result in enhancements of the photo-conversion quantum yield and allows the extension of the light absorption of wide band-gap semiconductors to the visible light. Modification of TiO2 with [Pt3(CO)6]n2- clusters or Pt ions can enhance the photoconversion yield by inhibition of the electron hole recombinatio.

9

The metal

nanoparticles often act as electron scavengers improving charge separation within the semiconductor−metal photocatalyst system.10 However, in a few cases, the metal NPs present on the surface can act as recombination centers.11 Another convenient way to probe the electron storage in metal nanoparticles is with its plasmon frequency. 12 It has been shown that the addition of electrons to silver and gold nanoparticles or nanorods causes a blue shift in the absorption spectrum due to the increasing surface plasmon frequency of the electron gas.13,14 Recently, Surface Plasmon Resonance of noble metals (Au, Ag), has been used for activation of wide bandgap semiconductors such as TiO2, 15,16 and CeO2, 17 towards visible light. Different studies demonstrated that a positive relationship exists between Surface Plasmon Resonance intensity and the rate enhancements, and led to a hypothesis that the metallic SPR enhances rates of photocatalytic reactions on nearby semiconductors by transferring energy to the semiconductor and increasing the steady2 ACS Paragon Plus Environment

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state concentration of ‘chemically useful’ energetic charge carriers in the semiconductor.7 The metal nanoparticles can absorb the incident photons through their SPR excitation and electrons can be injected from the metal nanoparticle into the Conduction Band (CB) of the semiconductor. It has been shown that the activity of gold modified titanium dioxide (Au/TiO2) under visible-light irradiation depends on the TiO2 and gold properties, and that photocatalysts with ability of light absorption in a wide wavelength range (broad LSPR peak) showed the highest level of activity because of the utilization of large amount of incident photons.16, 18 Silver nanoparticles are extremely attractive because of their remarkable catalytic activity, 19 , 20 , 21 their size- and shape-dependent optical properties, 22 , 23 and their potential applications in chemical and biological sensing, based on Surface-Enhanced Raman Scattering (SERS),24 Localized Surface Plasmon Resonance (LSPR),25,26 and Metal-Enhanced Fluorescence (MEF).

27 , 28

TiO2 modified with silver nanoparticles show enhanced

photocatalytic activity under UV and visible light and improved anti-bacterial properties.38,39, 29,30,31,32,33,34,35

It has been shown that the deposited Ag-nanoparticles slightly decreased the

extent of adsorption of rhodamine B (RhB), but increased the affinity of the surface to oxygen.38 Silver modification of TiO2 results in an enhancement of the bactericidal activity of TiO2 under UV light by about seventy folds because of the improved microorganism adsorption to the particle surface and lower electron-hole recombination.29,36 Ag NPs show a very intense Localized Surface Plasmon (LSP) absorption band in the near-UV region37 and this is associated with a considerable enhancement of the electric near-field in the vicinity of the Ag NPs. It has been shown that his enhanced near-field could boost the excitation of electron−hole pairs in TiO2 and therefore increase the efficiency of the photocatalysis.36 In general, the preparation of supported metal catalysts from colloidal solutions containing metal nanoparticles is a complex multi-step procedure. In the most commonly applied preparation route, impregnation, the first step involves the interaction of aqueous solutions containing metal precursors with the support, followed by drying, calcination, and reduction to transform the metal precursor into the catalytically active compound. Photoreduction is an efficient way to synthesize nanoparticles directly on semiconductor supports. Different articles report on modifications of TiO2 with metal nanoparticles obtained by photoreduction.16,18,38 ,39 ,40 Radiolysis is a powerful method to synthesize nanoparticles of controlled size and shape41,42,43,44 in solutions and on supports.



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In order to obtain efficient photocatalysts with higher photoactivity and to be able to work under solar irradiation, we modified titanium (IV) dioxide (titania, TiO2) (commercial P25 anatase-rutile and ST01 pure anatase) with silver nanoparticles induced by radiolysis for application in wastewater treatment. Solvent radiolysis induces formation of solvated electrons and radicals which reduce the metal ions homogeneously in the medium leading to a homogeneous nucleation. Small and relatively monodisperse nanoparticles are obtained. Surface modification by silver nanoparticles (NPs) causes improvement in the photocatalytic activity both under UV and visible light irradiation. This surface modification induces an increase of the absorption properties of visible light. Time-Resolved Microwave Conductivity (TRMC) measurements show that, under UV illumination, silver nanoparticles act as efficient electron scavengers, which results in a decrease of charge carrier recombination and an increase of the photocatalytic activity. In case of Ag-P25, electrons arising from the excitation of the SPR are injected into the CB of TiO2. 2- Experimental Materials. Silver perchlorate AgClO4 or Ag2SO4 (of 98 % and 99 % purity respectively) and ethanol (98 % purity) were obtained from Sigma-Aldrich and used as received. N2 gas (purity > 99.995%) was purchased from Air Liquide. TiO2 ST01 having anatase crystal structure was obtained from Ishihara Sangyo, Japan (surface area 300 m2 g-1, particles of a mean size of 7 nm) and P25 was obtained from Evonik (70:30 % anatase:rutile phase mixture with a surface area of 55 ± 15 m2 g-1).

Sample preparation All the photocatalysts were obtained by radiolytic reduction of Ag+ in the TiO2 suspension (0.5; 1 and 2 wt.%). An ethanolic solution containing AgClO4 (2 x 10-3 M) TiO2 (P25 or ST01) in suspension is first sonicated for 3 minutes, degassed with nitrogen and irradiated (under stirring) with a

60

Co panoramic gamma source (dose rate = 2.3 kGy h-1 ). The silver

ions were reduced by the solvated electrons and the alcohol radicals induced by solvent radiolysis and 1 hour 20 min exposure time (3.2 kGy) was necessary to reduce all the silver ions. The modified TiO2 photocatalysts were separated by centrifugation and dried at 60 °C. The modified catalyst were labeled depending on the TiO2 (P25 or ST01) and the loading (see Table 1). The supernatant is completely transparent after deposition and centrifugation,


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indicating that all the Ag NPs were deposited on TiO2, which is also confirmed by TEM observations.

Characterization of modified TiO2 The modified TiO2 samples were examined by transmission electron microscopy (TEM): few drops of the irradiated suspension were deposited on carbon coated copper grids. Transmission Electron Microscopy (TEM) observations were performed with a JEOL JEM 100CX II transmission electron microscope operating at 100 kV. The Diffusion Reflectance Spectra (DRS) of the modified TiO2 samples were obtained using a Cary 5E spectrophotometer equipped with Cary 4/5 diffuse reflection sphere. The baseline was recorded using either BaSO4 or TiO2 as reference. Gemini V (model 2365) was used to measurements of BET surface area of the catalysts. All samples were degassed at 200oC prior to nitrogen adsorption measurements. BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3 The X-ray Photoelectron Spectroscopy (XPS) analysis was performed with a Thermo Electron ESCALAB 220i-XL or a Thermo K-Alpha spectrometers. Sample drops were deposited on an In foils and dried under N2 flow. Either a non-monochromatic or a monochromatic X-ray Al Kα line was used for excitation. The photoelectrons were detected perpendicularly to the support. A constant analyzer energy mode was used with pass energy of 20 eV. Element percentage determination and fitting procedure were performed using the Thermo Avantage Software. The charge-carrier lifetimes in TiO2 after UV illumination were determined by microwave absorption experiments using the Time Resolved Microwave Conductivity method (TRMC).45,46,47,48 The incident microwaves were generated by a Gunn diode of the Ka band (30 GHz). The experiments were performed at 36.8 GHz. Pulsed light source was a Nd:YAG laser providing an IR radiation at λ = 1064 nm. Full width at half-maximum of one pulse was 10 ns, repetition frequency of the pulses was 10 Hz. UV light (355 nm) was obtained by tripling the IR radiation. Visible light (532 nm) was obtained by doubling it. The light energy density received by the sample was 0,8 mJ·cm-2. The principle of TRMC and the experimental set-up have been widely described in a previous paper.45 This technique is based on the measurement of the change of the microwave power reflected by a sample, ∆P(t), induced by its laser pulsed illumination. The relative difference



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∆P(t)/P can be correlated, for small perturbations of conductivity, to the difference of conductivity ∆σ(t) considering the following equation: ∆P (t ) = A∆σ (t ) = Ae∑ ∆ni (t ) µ i P i where ∆ni(t) is the number of excess charge-carriers i at time t and µi their mobility. The sensitivity factor A is independent of time, but depends on different factors such as the microwave frequency or the dielectric constant. Considering that the trapped species have a small mobility which can be neglected, ∆ni is reduced to mobile electrons in the conduction band and holes in the valence band. In the specific case of TiO2, the TRMC signal can be attributed to electrons because their mobility is much larger than that of the holes.49

Photocatalytic tests A phenol solution was used as model wastewater. The photocatalytic activity of Ag-TiO2 powders was estimated by measuring the decomposition rate of phenol (0.21 mmol L-1) in an aqueous solution. Photocatalytic degradation runs were preceded by blind tests in the absence of photocatalyst or illumination. The photocatalytic tests were conducted at constant catalyst loading. Twenty five milliliters suspension containing 125 mg of catalyst was stirred using magnetic stirrer and aerated (5 dm3/h) prior and during the photocatalytic process. Aliquots of 1.0 cm3 of the aqueous suspension were collected at regular time periods during irradiation and filtered through syringe filters (Ø = 0.2 µm) to remove catalyst particles. Phenol concentration was estimated by colorimetric method using UV–Vis spectrophotometer (DU-7, Beckman). The suspension was irradiated using 1000 W Xenon lamp (6271H, Oriel), which emits both UV and visible light. The optical path included water filter and glass filters (GG) to cut off IR and UV irradiation. To limit the irradiation wavelength, the light beam was passed through GG400 filter to cut-off wavelengths shorter than 400 nm. The estimated uncertainty on each point of the curves of photocatalytic degradation is ± 0.1

3- Results and Discussion Radiolytic synthesis High energy radiation (γ-rays, X-rays, electrons or ions beams) of alcohols leads to the formation of free radicals such as solvated electrons (e-s) and alcohol radicals which are strong


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reducing agents. The energy deposition throughout the solution ensures an initial homogeneous distribution of the radiolytic radicals. Silver ions are reduced by solvated electrons: Ag+ + e-s

Ag0

(1)

The radiolytic reduction of silver ions has been studied in methanol.50 The reduction .

potential E0 (HCHO/ CH2OH) is, at least in water, more positive than that of the couple .

Ag+/Ag0, which excludes a direct reduction of free Ag+ by the CH2OH radicals.51 It has been shown that the reduction of free silver ions by alcohol radicals proceeds via the formation of a complex involving the metal ion and the alcohol radical which acts as a ligand. It has been observed in case of 2-propanol:52 .

.

Ag+ + (CH3)2C OH .

[Ag(CH3)2C OH]+ + Ag+

[Ag(CH3)2C OH]+

(7)

Ag2+ + (CH3)2CO + H+

(8)

The same reactions probably occur also with the ethanol radical. However, direct reduction .

by CH2OH of silver cations adsorbed on silver clusters or on TiO2 is also possible (the reduction potentials of Agn+ and Ag+/TiO2 are more positive than the one of Ag+): .

CH3C HOH + Ag+/TiO2 .

CH3C HOH + Agn+

CH3CHO + Ag0/TiO2 + H+

CH3CHO + Agn + H+

(9)

(10)

The radiolytic formation of silver atoms is followed by association of atoms with ions, dimerization reactions and finally by aggregation of these species into clusters of higher nuclearity. The first steps of silver clusters nucleation in water have been studied by pulse radiolysis. 50,53,54,55

Characterization of the photocatalysts All the modified photocatalysts were light pink or pink-violet (see Table 1). TEM observations show small silver nanoparticles homogeneous in size and homogenously distributed on P25. NPs with mean sizes of 1.3 and 1.5 nm were obtained respectively for the metal loading 0.5 % and 1 % and slightly larger silver nanoparticles (mean size of 2.0 nm) were observed for higher loading i.e. 2 % on P25 as shown in Fig. 1a and 1b and in



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supplementary information (Fig. S1a and b). Compared with the other reported studies on Ag/TiO2, the obtained Ag nanoparticles are very small and homogeneous in size.29, 31,39 In case of modified ST01 TiO2, larger Ag nanoparticles were obtained with also a larger distribution in size: Ag NPs with a mean size of 7 nm are obtained for the loadings 0.5 and 1 % and 12 nm for the loading 2 % (see Fig. 1 c and d and and supplementary information

Fig. S1c and d). On ST01 TiO2, very small nanoparticles (size smaller than 2 nm) were also observed (see Fig. S1 d). XPS Analysis of Ag-P25 and Ag-ST01 samples detected peaks from Ti, O, Ag associated to the composites. Ag 3d spectra of the Ag-P25 and Ag-ST01 are shown respectively in Fig. 3a and 3b. For both samples it appears that the Ag absolute signal intensities increase with the metal loading. Fig. 3a and 3b show that the Ag-P25 and ST01 TiO2 composites exhibit for their Ag3d level two reproducible peaks (at around 368.3 and 374.5 eV), which are specific to the 3d5/2 / 3d3/2 spin–orbit splitting of Ag. The Ag 3d5/2 binding energy of peak maximum at 368.3 eV is close to the value reported for metallic Ag.56,57 However the peak wideness are broadened compare to the ones associated to pure clean bare metal for the same recording conditions (Fig. 3c and d). This is an interesting feature which means that additional contributions associated to other chemical Ag environment must be considered. These additional contributions are presented for the Ag 3d5/2 level fittings. Keeping the fwhm values close to the observed ones for bare metal, we must used three contributions with adding two more contributions that surround the principal central peak attributed to the metallic core of Ag- NPs. The interpretation of Ag 3d peaks is rather delicate because the chemical shifts between metallic and oxide phases are very weak. However it is now admitted that oxide contributions generate negative shift. The shift is usually smaller for Ag2O (0.3-0.4 eV) and larger for AgO (0.8-1.0 eV). 58,59,60 In this work the values are close to -0.6 eV, so we can affirm that Ag NPs are partially oxidized and the nature of the phase is probably multiple. The second contribution at higher energy is more difficult in its attribution. It can be only a slight charging effect of a part of the oxide coverage. It can be also more complex and in association with the specific Ag NPs TiO2 particle. Whatever the explanation, we notice that specific triple Ag features can be proposed. In the case of Ag-P25, a part of the Ag atoms are implied in the both additional contribution and they probably correspond to the surface atoms implied either in NPs oxidation as in TiO2 possible interaction; the inner atoms being in the zero oxidation state. Indeed, this is consistent with the small size of the clusters (1.5-2 nm). Considering a cuboctahedral close packed arrangement, these clusters could correspond mainly to 2- or 3-shells nanocrystals 8 ACS Paragon Plus Environment

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composed of 55 or 147 atoms and having respectively a diameter of 1.6 nm and 2.03 nm. 61,62,63

For these clusters, most of the atoms (about 42/55 = 76 % and 92/147= 62 %) are on

the surface. In case of Ag-ST01, despite of the larger size of the silver nanoparticles, a larger signal due to silver oxides or to the interaction of silver with TiO2 is also detected (half of the Ag atoms are implied in both additional contributions). XPS data cannot discriminate the behaviour differences described beneath. The optical properties have been studied in details by Diffuse Reflectance Spectroscopy. Fig. 2 and Fig. S2 (in supplementary information) show the spectra of pure and modified TiO2. It should also be pointed out that the DRS of the modified samples shows a slight shift in the band-gap transition to longer wavelengths for both kinds of surface-modified photocatalysts (P25 and ST01) (see Fig. 2). This shift towards lower energy increases with Ag loading. This effect was previously observed with Pt-modified TiO2.9 This effect can be attributed to a stronger stabilization of the conduction band of TiO2 by the conduction band of the Ag clusters comparing to the stabilization of the valence band. The absorbance in the visible region is always higher for the modified than for pure TiO2. Note that wide absorptions with two maxima at 410 and 540-560 nm are obtained with Ag modified P25 (Fig. 2a and S3a). The latter absorption explains the pink color of the modified TiO2 samples. In the case of Ag-modified ST01, a wide absorption is also observed with a maximum at 410 nm. Silver nanoparticles are known to exhibit a plasmon band with a maximum at around 400 nm in water. This plasmon band is sensitive to the environment and can be shifted depending on the stabilizer or on the substrate. Because of the coupling between Ag nanoparticles and TiO2 support having a high reflective index (the absorption coefficient and refractive index are for anatase phase 90 cm-1 and 2.19 at a wavelength of 380 nm, respectively),64,65 the plasmon band is blue-shifted. The absorption of the modified TiO2 samples in the 540-560 nm region is probably mainly due to small Ag clusters which absorb in this spectral range.66 Indeed, this absorption is more intense for Ag-P25 where a larger number of small Ag nanoclusters were observed compared with Ag-ST01.

Photocatalytic tests Evonik P25 is highly photoactive in phenol degradation under UV-Vis light. After 60 min of irradiation, 33% of phenol was degraded. Under visible light, the same irradiation time resulted in 22% of phenol degradation. Deposition of Ag NPs on P25 resulted in higher photoactivity. Phenol degradation rate was 2.69, 2.28 and 3.14 mmol.dm-3.min-3 for the samples obtained by UV-irradiation of P25 suspension containing 0.5, 1 and 2 wt.% of 9 ACS Paragon Plus Environment


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AgClO4, respectively (Fig. 4). The photocatalytic activity under UV is better with the higher Ag loading e.i. 2 wt.% while under visible light the sample with 1 wt.% loading (Ag-P25(1)) gives the best results. The results obtained with 0.5 and 1 wt.% are quite similar for both illuminations. The Ag-modified P25 samples showed much higher photocatalytic activity that the modified ST01 for phenol degradation. After 60 min of irradiation, 23 % of phenol was degraded. Under visible light, the same irradiation time resulted in 15-17 % of phenol degradation. The Ag loading has not a significant effect on the photocatalytic activity of ST01 under UV light. Under visible light, Ag-ST01(2) exhibits a slightly higher photocatalytic activity than AgST01(1) and Ag-ST01(0.5) (Fig. 5). Under visible light, the photoactivity of the pure compound is normally very low because the illumination energy is below the bandgap. Surface modification with silver nanoparticles induces a shift in the band bap and a modification of the absorption properties of the photocatalyst and particularly an enhancement of the absorption in the visible range creating an activity under visible light.

TRMC signals: The measurements realized under UV illumination show TRMC signals represented on Fig. 6. In this work, the surface modification with Ag NPs shows a strong influence on the chargecarrier decay of P25 and ST01. This influence on the decay can be related to the activity in the case of phenol degradation with UV-light. The Ag-modification accelerates the overall decay for all the TiO2 compounds. As shown previously, the modification by Ag nanoparticles causes an increase in the photocatalytic activity both with P25 and ST01. As explained in the experimental part, the TRMC signal is mainly related to the electron mobility. The decrease of the TRMC signal is then probably due to efficient electron scavenging by the silver nanoparticles deposited on TiO2. It implies a decrease of the charge-carrier recombination that is beneficial to the photoactivity. It has to be noted that the acceleration of the decay is related to the silver loading (Fig. 6). This acceleration is different from our previous observations made with Pt and Pd modified TiO2,8, 9

where a slowdown of the overall decay was observed. Indeed, contrary to metals such as Pt

and Pd which provide an ohmic contact, metals such as Ag and Au exhibit capacitive properties.12 For small nanoparticles (2−5 nm in diameter), these coinage metals are able to store electrons.67,68 Such electron storage induces a shift in the Fermi level. A size-dependent enhancement in photocatalytic degradation has been previously reported for Au modified 10 ACS Paragon Plus Environment

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TiO2.69 The TRMC measurements show that small silver nanoparticles are very efficient in electron scanvenging. Furthermore, the modification with silver nanoparticles increases the initial TRMC signal intensity in the case of Ag-P25, while the contrary effect is observed with Ag-ST01 (Fig. 6a

and b). This indicates that more electrons are produced under UV-illumination in the conduction band of Ag-modified P25. These excess electrons could be due to the electrons injected in the conduction band of TiO2 after excitation of the silver nanoparticles at a wavelength very close to the Surface Plasmon Band (around 400 nm). This behavior is not observed in the case of modified ST01 (Fig. 6b). This could be due to the larger size of the Ag nanoparticles on ST01 or to less efficient Ag-ST01 junction. This can be correlated to the lower photoactivity of Ag-ST01 compared to Ag-P25. At 532 nm in the visible range, there is no light absorption of TiO2 but an excited state of the Ag nanoclusters is reached. Since no TRMC signal is observed at this excitation wavelength, one can conclude that no electron transfer from the Ag nanoclusters to the conduction band of TiO2 occurs.

4- Conclusion Small silver nanoparticles (of 1.3-2.0 nm depending on the loading) homogeneous in size were synthesized on P25 TiO2 by radiolysis. On ST01 TiO2 larger nanoparticles (7-15 nm) were also obtained. The presence of small Ag nanocluscters on TiO2 induces absorption in the visible with a maximum at around 550 nm. Titania surface modification with silver NPs enables in case of P25 the increase of the photocatalytic activity both under UV and visible irradiation. TRMC measurements show that under UV irradiation silver acts as an electron scavenger hindering charge recombination in modified P25 and ST01, and that more electrons are produced under UV-illumination in the conduction band of Ag-modified P25. These excess electrons could be due to the electrons injected in the conduction band of TiO2 after excitation of the silver nanoparticles at a wavelength very close to the Surface Plasmon Band of silver. Under visible light irradiation, a sensitization mechanism should be considered: absorption of light by silver nanoparticles.



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ACKNOWLEDGEMENT The authors thank Patricia Beaunier (Laboratoire de Réactivité de Surface, Université Pierre et Marie Curie) for the TEM observations and Jackie Vigneron (CEFS2, Institut Lavoisier Versailles, Université Versailles Saint Quentin-en-Yvelines) for his assistance for XPS experiments. C’nano - Ile de France is acknowledged for its financial support with the TRMC setup. This research was financially supported by Erasmus Program (Staff Training Mobility) and National Science Center (grant No. N N523 420137).

Supporting Information: Supporting information include additional TEM pictures of Ag-modified TiO2 (P25 and ST01) and diffuse reflectance spectra of pure and modified TiO2 with different silver loadings recorded using TiO2 as reference. This information is available free of charge via the Internet at http://pubs.acs.org.


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Figure captions Table 1: The list of the different prepared samples, their Ag loading, label, colour, porosity and activity for phenol degradation.

Fig. 1. TEM pictures of Ag-modified TiO2 (P25 and ST01) with different silver loadings 0.5 and 2 %: Fig. 1a, b, c and d correspond respectively to the samples Ag-P25 (0.5), Ag-P25 (2), Ag-ST01(0.5), Ag-ST01(2).

Fig. 2. Diffuse reflectance spectra of pure and modified TiO2: (a) P25 and (b) ST01 with different silver loading and recorded respectively using BaSO4 as reference.

Fig. 3: XPS spectra of Ag-TiO2 with different loading and Ag 3d5/2 and 3d3/2 fittings (for the 2 % loading): a, c) Ag-P25 and b, d) Ag-ST01.

Fig. 4. Phenol photodegradation with modified TiO2 (P25 and ST01) with different Ag loading under UV-Vis illumination.

Fig. 5. Phenol photodegradation with modified TiO2 (P25 and ST01) with different Ag loading under visible illumination.

Fig. 6. TRMC signals of pure and Ag modified TiO2: (a-top) P25 and Ag-P25, Inset: a scheme depicting the electron scavenging and transfer on Ag modified TiO2 surface after the absorption of UV photons; and (b-bottom) ST01 and Ag-ST01.



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Sample label

TiO2 matrix

Content of metal precursor [wt. %]

BET surface area [m2/g]

ST01

ST01

0

300 (278*)

Ag-ST01(0.5)

ST01

0.5

242

Ag-ST01(1)

ST01

1

235

Ag-ST01(2)

ST01

2

238

P25 Ag-P25(0.5) Ag-P25(1) Ag-P25(2)

P25 P25 P25 P25

0 0.5 1 2

50 (58*) 49 45 44

*

Page 14 of 23

Sample color

white light pink light pink light pink white pink pink violet

Rate of phenol degradation [µmol dm-3 min1 ] UVVis Vis 0.82 0.25 1.02

0.41

0.58

0.34

1.05

0.57

2.0 2.69 2.28 3.14

0.55 0.33 0.78 0.27

As-measured data

Table 1

Figure 1 14 ACS Paragon Plus Environment

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Figure 2.



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Figure 3


Page 17 of 23

UV-Vis

Phenol concentration, C/Co

1,0

0,8

0,6

0,4

P25 0,5% Ag-P25

0,2

1% Ag-P25 2% Ag-P25 0,0 0

10

20

30

40

50

60

Irradiation time (min)

UV-Vis

1,0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,9

0,8

ST-01 0,7

0,5% Ag-ST-01 1% Ag-ST-01 2% Ag-ST-01

0,6 0

10

20

30

40

50

60


Figure 4.



Vis


1,0

0,9

0,8

P25

Ag-P25(0,5)

10

20

Ag-P25(1)

Ag-P25(2)

0,7

0,6 0

30

40

50

60


Vis

1,0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

0,9

0,8

ST-01 0,7

Ag-ST01(0,5) Ag-ST01(1) Ag-ST01(2)

0,6 0

10

20

30

40

50

60

Irradiation time [min]

Figure 5. 18 ACS Paragon Plus Environment

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Figure 6.a (top) and b (bottom)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

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Table of content


Modification of Titanium(IV) Dioxide with Small Silver Nanoparticles

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