Changes in the Chemical and Structural Properties of Nanocomposite

Sep 15, 2014 - Institut de Sciences des Matériaux de Mulhouse, Université de Haute Alsace, CNRS UMR 7361, 15 rue Jean Starcky, 68057. Mulhouse, Fran...
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Changes in the Chemical and Structural Properties of Nanocomposite Ag:TiO2 Films during Photochromic Transitions Nicolas Crespo-Monteiro,†,‡ Nathalie Destouches,*,†,‡ Thierry Epicier,§ Lavinia Balan,∥ Francis Vocanson,†,‡ Yaya Lefkir,†,‡ and Jean-Yves Michalon†,‡ †

Université de Lyon, F-42023 Saint-Etienne, France Laboratoire Hubert Curien, Université Jean-Monnet, UMR 5516 CNRS, 18 rue Pr. Lauras, F-42000 Saint-Etienne, France § MATEIS, Université de Lyon, INSA-Lyon, UMR 5510 CNRS, 7 avenue Jean Capelle, F-69621 Villeurbanne, France ∥ Institut de Sciences des Matériaux de Mulhouse, Université de Haute Alsace, CNRS UMR 7361, 15 rue Jean Starcky, 68057 Mulhouse, France ‡

ABSTRACT: This study focuses on changes in the oxidation state of silver and in the crystalline phase of TiO2 occurring during the photochromic transitions of amorphous nanocomposite mesoporous Ag:TiO2 films exposed to ultraviolet and visible laser light. The chemical and phase transformations have been investigated by ultraviolet−visible spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, highangle annular dark field scanning transmission electron microscopy, and high-resolution transmission electron microscopy. The results bring to the fore the presence of both silver nanoparticles in face-centered cubic and hexagonal crystalline phases during the photochromic transitions and a large amount of oxidized silver nanoparticles after visible light exposure, along with nucleation of some TiO2 crystals despite the very low intensity used.

1. INTRODUCTION Photochromism on nanocomposite titania films loaded with silver nanoparticles (Ag:TiO2 films) is of great interest for many applications such as rewritable color copy paper, smart glass, multiwavelength optical memory, holographic data storage, or rewritable data carriers.1−5 The reversible changes of color induced by ultraviolet (UV) or visible light result from the tuning of the metal nanoparticle (NP) size distribution within the porous titania matrix. Silver nanoparticles exhibit a localized surface plasmon resonance (LSPR) in the visible range, which gives color to the film and whose spectral features depend on the NP size and the dielectric permittivity of their close environment; the latter can strongly vary in a porous host matrix when the NP size is changed. As shown by the group of Professor T. Tatsuma,1−3 UV light promotes the reduction of silver ions and the growth of silver NPs through the generation of electron−hole pairs by titanium dioxide, whereas visible light leads to NP oxidation and therefore to their size reduction. Under visible exposure, oxidized atoms of silver leave the NPs and stabilize elsewhere within the matrix thanks to an easy electron transfer from the Ag NPs to the TiO2 conduction band and then to trapping centers.6,7 Most experimental demonstrations of such photochromic behavior were reported with silver NPs in contact with crystallized TiO2,1−3 except in our previous studies where we used mesoporous films of amorphous TiO2.5,8,9 We have especially demonstrated that a complete bleaching (disappearance of the LSPR band) could be performed with monochromatic visible laser light5 and that © 2014 American Chemical Society

chromatic information could be stored for a long time on such samples, intentionally erased, and updated.9,10 Photochromic cycles proved that the mesoporous films of amorphous TiO2 are rewritable; however, we did not characterize the film transformations at the nanometer scale during exposure. Here, we propose a deeper insight into the way silver and titanium dioxide evolve during the first UV and visible exposures. Several kinds of techniques are used to characterize the oxidation state of silver and the phases of TiO2 when the optical properties of the films switch. This study shows the localized growth of silver NPs near the film top surface under UV exposure and the presence of two different crystalline phases of metallic silver. It also brings to the fore the coexistence of both reduced and oxidized forms of silver in both photochromic states, which are presented as colored and bleached. We also report the surprising start of crystallization of titania in contact with silver NPs under very low intensity visible laser exposure.

2. EXPERIMENTAL METHODS This study was conducted on mesoporous films of amorphous titania elaborated according to a previously published procedure.8,9,11 A solution of Pluronic P123 copolymer (0.025 mol) in ethanol (28.5 mol) and HCl (0.015 mol) was Received: June 9, 2014 Revised: September 11, 2014 Published: September 15, 2014 24055

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HAADF STEM (parts a and d, respectively, of Figure 2). These particles, grown spontaneously during the 12 h long drying, were identified by HRTEM and electron diffraction as being metallic silver nanoparticles. Such small nanoparticles have a very low theoretical absorption cross-section, typically more than 115 times smaller than that of 10 nm NPs, according to the Mie theory. In addition, due to the low thickness of the film, their presence cannot be brought out by UV−visible spectroscopy. TiO2 is mainly amorphous. High-angle X-ray diffraction, Raman microspectroscopy, and almost all electron diffraction diagrams we recorded did not show any feature related to the presence of a crystalline TiO2 phase, even if a few, but very rare, anatase nanocrystals were found by scanning samples in a transmission electron microscope. 3.2. Film Characteristics after UV Exposure. Silver NPs were then grown photocatalytically by illuminating the films with UV laser light for 2 min. Their growth went with a strong increase in their absorption cross-section in the visible, which gave the films a gray-brown color corresponding to a large LSPR band centered at 470 mn (Figure 1). BFTEM and HAADF STEM imaging modes give both an interesting contrast. In bright field images Bragg diffraction leads to a darker contrast of crystallized Ag NPs as compared to the surrounding mesoporous matrix (amorphous titania). HAADF STEM is not sensitive to diffraction but to the atomic number of atoms, and silver appears much brighter than TiO2 with this mode since high-angle scattering is more efficient for heavier atoms (atomic numbers equal to 8, 22, and 47 for O, Ti, and Ag, respectively). It appears that the growth of silver NPs mainly occurs in the first 100 nm from the film top surface, leading to a larger NP size and a higher NP density in this part (Figure 2b,e). From both bright field TEM and HAADF STEM images of samples prepared as cross sections, the NP size distribution could be estimated: it ranges typically from 5 to 15 nm in the top part, whereas it remains smaller than 5 nm in the rest of the film. Such a localization of the NP growth results from the high absorption coefficient of TiO2 at 244 nm wavelength. The UV laser is rapidly attenuated during its propagation through the film, and electrons released by TiO2, which promote the reduction of silver, are mainly concentrated near the film top surface.12,13 XPS measurements were also performed to better estimate the relative atomic concentration of metallic and oxidized silver in the film at this stage (Figure 3). Two kinds of measurements were carried out, the first one on the film top surface and the second one on the bottom part of the film after the film surface was scratched with a razor blade to take into account a possible change in the oxidation state between surface and volume. The Auger parameters (APs) were also used to clearly identify silver oxidation states and were computed from the experimental (with no charge correction) binding energy of the 3d5/2 component maximum and the kinetic energy of the two most intense peaks in the Auger electron structure. Auger parameters “3d5/2, M4N45N45” and “3d5/2, M5N45N45” are known to be nearly 2 eV higher for Ag in its reduced form than for oxidized silver.14 The experimental results (Table 1) confirm therefore the presence of both states of silver within the UV-treated films with AP values found to be 726.4 and 720.5 for reduced silver and 724.7 and 728.4 for oxidized silver. AgO was considered for fitting the experimental curves, but it has to be noted that such measurements are not accurate enough to clearly distinguish the different oxidation states of silver. The main difference between Ag2O and AgO is the shape of the photoelectron peak

added to a solution of titanium tetraisoproxide (1 mol) in acetylacetone (0.5 mol). Then the films were deposited by dipcoating on cleaned glass slides at a withdrawal rate of 70 mm· min−1. After that, the copolymer was extracted from the film by thermal treatment at 340 °C for 4 h. Finally, a silver precursor was introduced into the TiO2 matrix by soaking the films in a [Ag(NH3)2]+NO3− solution at 1.5 M for 1 h before rinsing with ultrapure water and drying for 12 h at room temperature in the dark. The photochromic cycles were carried out by growing Ag NPs under UV light and bleaching the films under monochromatic visible light. A continuous wave (CW) doubled Ar laser emitting at 244 nm was used to illuminate the samples in UV with an intensity of 38 mW·cm−2 for 2 min. Visible exposures were performed with a CW Ar laser emitting at 488 nm wavelength at 3 W·cm−2 for 10 min. The films were characterized before and after UV exposure and after bleaching. UV−visible spectrophotometric measurements were performed with a PerkinElmer Lambda 900. Bright field transmission electron microscopy (BFTEM), high-angle annular dark field scanning TEM (HAADF STEM), and high-resolution TEM (HRTEM) pictures were achieved with a JEOL 2010F transmission electron microscope, equipped with a fieldemission gun and an Oxford Instruments INCA energydispersive X-ray (EDX) analyzer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Gammadata Scienta (Uppsala, Sweden) SES 200-2 X-ray photoelectron spectrometer under ultrahigh vacuum (P < 10−9 mbar). The spectrometer resolution at the Fermi level is about 0.4 eV. The depth analyzed extends to about 8 nm. The monochromaticized Al Kα source (1486.6 eV) was operated at a power of 420 W (30 mA and 14 kV), and the spectra were acquired at a takeoff angle of 90° (angle between the sample surface and the photoemission direction). During acquisition, the pass energy was set to 500 eV for wide scans and to 100 eV for highresolution spectra. CASAXPS software (Casa Software Ltd., Teignmouth, U.K., www.casaxps.com) was used for all peak fitting procedures, and the areas of each component were modified according to classical Scofield sensitivity factors.

3. RESULTS AND DISCUSSION 3.1. Film Characteristics before Laser Exposure. The initial mesoporous TiO2 films loaded with silver salt are transparent and colorless and do not exhibit clear absorption in the visible range (Figure 1). However, small nanoparticles with a diameter lower than 3 nm are observed by BFTEM and

Figure 1. Absorbance spectra of a Ag:TiO2 film after drying and irradiation at λ = 244 nm (UV) and after different times of exposure at λ = 488 nm. Inset: sample color after irradiation at λ = 244 nm and after bleaching at λ = 488 nm (10 min of exposure). 24056

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Figure 2. BFTEM and HAADF STEM migrographs of Ag:TiO2 films after drying (a, d) and after exposure at λ = 244 nm for 2 min (b, e) and then at λ = 488 nm for 10 min (c, f).

Table 1. Auger Parameters, XPS Ag 3d Fitting Parameters, and Relative Atomic Concentrations Deduced from Measurements Shown in Figure 3 after UV Exposure and after Bleaching UV (surface)

UV (bottom)

bleaching (surface)

Auger Parameters For Ag(0) 3d5/2, M4N45N45 726.9 727.1 726.4 720.7 721.3 720.5 3d5/2, M5N45N45 For Ag−O 3d5/2, M4N45N45 729.4 725.0 724.7 3d5/2, M5N45N45 718.8 719.2 718.4 Binding Energy (eV) Ag 3d5/2−Ag(0) 368.02 367.83 368.41 Ag 3d5/2−Ag−O 367.42 367.23 367.81 Relative Atomic Concentration (%) Ag(0) 84.32 76.70 42.76 Ag−O 15.68 23.30 57.24

Figure 3. XPS detailed regions of (a, c) Ag 3d and (b, d) Ag MNN for films after UV exposure (a, b) and after bleaching (c, d). In blue (red) is shown the contribution of Ag(0) (AgO) to the fitting of the experimental data.

bleaching (bottom)

726.4 720.4 724.7 718.3 368.43 367.84 45.16 54.84

The values show that most of the silver species are reduced after UV exposure, even after scraping the film, which means that most of the silver contained in the whole film is reduced. Interestingly, silver-based NPs with hexagonal symmetry were unambiguously identified from HRTEM diffractograms (Figure 4b) in addition to the expected face-centered cubic (fcc; structure with a unit cell length of 0.407 nm) phase of metallic silver (Figure 4a). Such hexagonal phases were previously identified in the literature: the first hexagonal cell consists of a 4H stacking of dense planes (ABCB/ABCB, with a = 0.288 nm and c = 1.00 nm),16,17 and the second one is based on a shorter 2H stacking (AB/AB, with a = 0.283 nm and c = 0.638 nm).18 In both cases these hexagonal structures are supposed to be metastable forms promoted by a nanosize effect. Such a size confinement would easily be brought up by the nucleation

(not shown here): a single doublet for Ag2O and a complex doublet for AgO.14 The binding energies are also reported in Table 1, but they are less reliable than APs in discriminating reduced silver from oxidized silver. Indeed, they vary with the charge shift occurring on insulating samples during irradiation with X-rays, which is not easy to correct or neutralize. Furthermore, binding energies reported in the literature for Ag(0), AgO, Ag2O, and other oxidized states are rather dispersed and in overlapping ranges.15 The experimental values measured on UV samples (Table 1) are however compatible with what can be found in the literature.15 Fitting the experimental spectra with Gaussian−Lorentzian functions (Figure 3a,b) gives the relative atomic concentrations (Table 1). 24057

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Figure 4. HRTEM analysis of silver-based NPs observed in the volume of Ag:TiO2 films after exposure at λ = 244 nm for 2 min: (a) typical multiple-twinned NPs [the Fourier transform (diffractogram in the inset) of the particle confirms the fcc Ag structure]; (b) another silver particle indexed as a hexagonal 4H lattice (magnification as in (a)). Figure 5. Typical small NP at the edge of the film, consisting in 5-fold twinned domains and identified as silver by EDX and HRTEM imaging.

within the TiO2 matrix nanopores as presently observed (Figure 4b). Finally, to complete the characterization of the sample after UV exposure, TEM work shows that the TiO2 mesoporous matrix remains mainly amorphous for the same reasons as previously discussed. 3.3. Film Characteristics after UV and Visible Exposure. The UV-exposed films were then bleached under monochromatic visible light. Figure 1 shows the disappearance of the LSPR band during exposure, and different characterizations were achieved on the completely bleached films, which are transparent and colorless as the initial ones (Figure 1 inset). As shown by XPS (Figure 3c,d and Table 1), a large part of silver has been oxidized, but nearly half of it remains in the metal form. It is likely that most of the reduced silver came back in the form of small crystalline NPs less than a few nanometers in diameter exhibiting a very low absorption according to the Mie theory (Figure 5). This can be accentuated by the fact that XPS measurements were performed several days after bleaching and that ionic silver generated during visible exposure can be spontaneously reduced and stabilized in the form of very small metallic NPs as before UV exposure. Such a spontaneous recombination of oxidized silver with free electrons has been previously shown on partially bleached samples.8,19 Actually, TEM reveals that the film morphology at the nanoscale changed radically after bleaching. BFTEM micrographs show the presence of a large amount of contrasted particles whose size remains in the range of 1−15 nm as on the UV films; however, BFTEM as well as HAADF TEM micrographs look less contrasted than previously, whatever the observed area (Figure 2c,f). Indeed, this decrease in the contrast is caused by the increased number of crystallized particles, including TiO2 crystallites: we find evidence for anatase, rutile, and brookite phases (as illustrated in Figure 6a,b,d). Others can be indexed as fcc and hexagonal Ag (Figures 5 and 6c), but most frequently in silver oxide with different compositions such as Ag3O4, as in Figure 7. In most studies on photochromic Ag:TiO2 systems, the disappearance of silver NPs after bleaching is supposed to give rise to Ag+ ions with no more details about their stability in the host matrix.1,3 However, authors such as A. Kafizas et al.

Figure 6. HRTEM imaging of Ag and TiO2 crystallites in the Ag:TiO2 film after the final bleaching treatment. Diffractograms b−d correspond respectively to areas labeled B, C, and D in (a).

have also observed the presence of silver oxide after visible illumination of Ag:TiO2 films where silver NPs were in contact 24058

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Figure 7. Typical oxide NP in the Ag:TiO2 film after the final bleaching treatment, indexed as Ag3O4 (monoclinic phase with a = 0.358 nm, b = 0.921 nm, c = 0.568 nm, and β = 106.13°, JCPDS files no. 65-9750). The EDX measurements can be consistently analyzed with an accuracy of better than 2 atom % for each element as a mixed contribution of Ag3O4, TiO2, and SiO2 (the latter compounds arising respectively from the surrounding matrix and a residue of the glass substrate).

homogeneous medium under 3 W·cm−2 visible exposure is insignificant (less than 1 °C). Therefore, we can wonder why such crystallization occurs. First, chemical parameters may enhance the crystallization of the anatase phase. Water can catalyze the phase change, and dopant ions such as bromide and chloride have been shown to promote crystallization leading to anatase TiO2 from 100 °C.22,23 In our case, the presence of silver and adsorbed water in the mesoporosity may be a positive factor for the nucleation of crystalline phases of the titania matrix. The second point concerns the temperature reached in the material. Two factors may have an exalting effect on the temperature increase. The first one is the high density of silver NPs that are concentrated near the film top surface after UV exposure. Energy may accumulate in such high-density NP assemblies and produce a significant increase in the average temperature of the host medium, as this has been proven for water droplets containing gold NPs.24 The second one is the porosity of the TiO2 film. The spatial separation between TiO2 and the surface of silver NPs due to the presence of pores filled with adsorbed water or air whose thermal conductivity is low may result in a loss of thermal contact and in an increase in the local NP temperature. 25 This could explain why TiO 2 nanocrystals seem sometimes to grow around silver metallic nanoparticles (Figure 5a). The local heterogeneity of the system silver NP/air or water gap/TiO2 surrounding medium could also produce significant variations in the local temper-

with crystallized TiO2.20 After bleaching, silver seems therefore to stabilize in the form of small metallic silver and silver oxide nanoparticles. Interestingly, Ag NPs are sometimes surrounded by TiO2 nanocrystals as shown in Figure 6a (in the bottom region, all lattice fringes at about 0.207 nm around the “composite” silver NP are consistent with interplanar distances from TiO2 crystalline phases). Finding TiO2 nanocrystals after bleaching may appear surprising when a visible intensity as low as 3 W· cm−2 is used. We had previously reported crystallization of the amorphous titania matrix for such films, but using a laser intensity more than 1000 times higher.13 Anatase and rutile phases of TiO2 had then been identified by Raman microspectroscopy. This is not the case here, where the density of crystallized TiO2 and the film thickness are likely to be too low to give rise to characteristic peaks on Raman spectra. Such crystallization had been attributed to the heating of silver NPs under visible exposure through plasmon excitation, and a temperature increase of a few hundred degrees had been estimated in situ by simultaneously measuring Stokes and antiStokes Raman peaks of TiO2 being crystallized13 (the laser was used both to crystallize and to excite the Raman signal). TiO2 is usually known to present a progressive heat-induced transition from the amorphous to the anatase phase between 200 and 350 °C.21 However, according to simulations based on the resolution of the heat diffusion equation, the temperature increase for a sphere, from 1 to 15 nm in diameter, in a 24059

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ature at the nanoscale26 and explain why anatase, brookite, and rutile phases are simultaneously obtained.

(9) Nadar, L.; Sayah, R.; Vocanson, F.; Crespo-Monteiro, N.; Boukenter, A.; Sao Joao, S.; Destouches, N. Influence of Reduction Processes on the Colour and Photochromism of Amorphous Mesoporous TiO2 Thin Films Loaded with a Silver Salt. Photochem. Photobiol. Sci. 2011, 10, 1810. (10) Crespo-Monteiro, N.; Destouches, N.; Fournel, T. Updatable Random Texturing of Ag/TiO2 Films for Goods Authentication. Appl. Phys. Express 2012, 5, 075803. (11) Crespo-Monteiro, N.; Destouches, N.; Gamet, E.; Bois, L.; Chassagneux, F.; Nadar, L.; Vocanson, F. Role of Silver Nanoparticles in the Laser-Induced Reversible Colour-Marking and Controlled Crystallization of Mesoporous Titania Films. Proc. SPIE 2011, 80962J. (12) Destouches, N.; Battie, Y.; Crespo-Monteiro, N.; Chassagneux, F.; Bois, L.; Bakhti, S.; Vocanson, F.; Toulhoat, N.; Moncoffre, N.; Epicier, T. Photo-Directed Organization of Silver Nanoparticles in Mesostructured Silica and Titania Films. J. Nanopart. Res. 2013, 15, 1− 10. (13) Crespo-Monteiro, N.; Destouches, N.; Saviot, L.; Reynaud, S.; Epicier, T.; Gamet, E.; Bois, L.; Boukenter, A. One-Step Microstructuring of TiO2 and Ag−TiO2 Films by Continuous Wave Laser Processing in the UV and Visible Ranges. J. Phys. Chem. C 2012, 116, 26857−26864. (14) Ferraria, A. M.; Carapeto, A. P.; Botelho do Rego, A. M. X-ray Photoelectron Spectroscopy: Silver Salts Revisited. Vacuum 2012, 86, 1988−1991. (15) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R. NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, version 3.3; 2003. Web version: http://www.srdata.nist.gov/xps/ (accessed March 2014). (16) Liang, C.; Terabe, K.; Hasegawa, T.; Aono, M. Formation of Metastable Silver Nanowires of Hexagonal Structure and Their Structural Transformation under Electron Beam Irradiation. Jpn. J. Appl. Phys. 2006, 45, 6046. (17) Taneja, P.; Banerjee, R.; Ayyub, P.; Dey, G. K. Observation of a Hexagonal (4H) Phase in Nanocrystalline Silver. Phys. Rev. B 2001, 64, 033405. (18) Chakraborty, I.; Carvalho, D.; Shirodkar, S. N.; Lahiri, S.; Bhattacharyya, S.; Banerjee, R.; Waghmare, U.; Ayyub, P. Novel Hexagonal Polytypes of Silver: Growth, Characterization and FirstPrinciples Calculations. J. Phys.: Condens. Matter 2011, 23, 325401. (19) Matsubara, K.; Kelly, K. L.; Sakai, N.; Tatsuma, T. Effects of Adsorbed Water on Plasmon-Based Dissolution, Redeposition and Resulting Spectral Changes of Ag Nanoparticles on Single-Crystalline TiO2. Phys. Chem. Chem. Phys. 2008, 10, 2263. (20) Kafizas, A.; A Parry, S.; Chadwick, A. V.; Carmalt, C. J.; Parkin, I. P. An EXAFS Study on the Photo-Assisted Growth of Silver Nanoparticles on Titanium Dioxide Thin-Films and the Identification of Their Photochromic States. Phys. Chem. Chem. Phys. 2013, 15, 8254−8263. (21) Lottici, P. P.; Bersani, D.; Braghini, M.; Montenero, A. Raman Scattering Characterization of Gel-Derived Titania Glass. J. Mater. Sci. 1993, 28, 177−183. (22) Goutailler, G.; Guillard, C.; Daniele, S.; Hubert-Pfalzgraf, L. G. Low Temperature and Aqueous Sol−Gel Deposit of Photocatalytic Active Nanoparticulate TiO2. J. Mater. Chem. 2003, 13, 342−346. (23) Yanagisawa, K.; Ovenstone, J. Crystallization of Anatase from Amorphous Titania Using the Hydrothermal Technique: Effects of Starting Material and Temperature. J. Phys. Chem. B 1999, 103, 7781− 7787. (24) Richardson, H. H.; Carlson, M. T.; Tandler, P. J.; Hernandez, P.; Govorov, A. O. Experimental and Theoretical Studies of Light-toHeat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions. Nano Lett. 2009, 9, 1139−1146. (25) Neumann, O.; Urban, A. S.; Day, J.; Lal, S.; Nordlander, P.; Halas, N. J. Solar Vapor Generation Enabled by Nanoparticles. ACS Nano 2013, 7, 42−49.

4. CONCLUSION In summary, TEM and XPS measurements performed after photochromic transformations of mesoporous Ag:TiO2 films show the presence of both metallic silver nanoparticles (in fcc and hexagonal phases) and oxidized silver regardless of the film color. After UV exposure, most of the silver is in the form of metallic NPs 5−15 nm in diameter and highly concentrated near the film top surface, which exhibits an absorption band in the visible range. TiO2 is amorphous. After bleaching, when the film is colorless and transparent, silver is predominantly oxidized, but a part of it also forms crystallized nanoparticles of oxidized silver or very small metallic nanoparticles (typically