Selective UV Reflecting Mirrors Based on Nanoparticle Multilayers

www.afm-journal.de FULL PAPER © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2805 www.MaterialsViews.com wileyonlinelibrary.com J. R. Castro Smirnov , Mauricio E. Calvo , and Hernán Míguez * 1. Introduction Research and development of new materials for selectively protecting against UV light has gained a lot of interest due to the increasing number of applications that make use of this radiation in medicine, [ 1 ] cosmetics, [ 2 ] as well as in a wide range of other industrial processes. [ 3 , 4 ] In general, UV light absorption produces highly reactive free radicals that can induce chemical reactions, resulting in subsequent photodegradation [ 5 , 6 ] and yellowing [ 7 ] of polymers, pigments, and dyes. Also, losses of mechanical strength and impact resistance have been reported when polymers are exposed to this sort of radiation. [ 8 ] Regarding its effect on human tissue, there is a vast and growing literature reporting harmful effects of ultraviolet radiation on the skin, [ 9 ] such as the appearance of cutaneous neoplasm and a variety of photosensitive dermatoses [ 10 ] or skin cancer. [ 11 ] When it comes to protection, completely different approaches are taken depending on the surface to be shielded. In the case of human skin, sun screen lotions containing organic UV absorbers, which typically are compounds containing phenolic groups with intramolecular hydrogen bonds, mixed with TiO 2 or ZnO particles, which contribute to protection with both strong UV light scattering and absorption, are widely spread. [ 12 ] In the case of arbitrary material surfaces, both polymeric and sol-gel coatings embedding one of the abovementioned UV blocking compounds are commonly employed. [ 13 ] In the last few years, visible transparent materials that merge mechanical fl exibility and absorption at certain ultraviolet ranges have been developed by inclusion of the formerly mentioned compounds in polymeric matrices, aiming at creating adaptable coatings for a wide variety of UV sensitive environments. [ 14–19 ] However, radical species produced as a consequence of UV light absorption by the organic guest compounds degrade the polymer host, rendering the coating useless after some exposure time. Alternative approaches to spectrally selective light shielding are based on interference fi lters, Bragg mirrors, or one dimensional photonic crystals (1DPC). [ 20 , 21 ] One of the most appealing properties of this sort of protection, based on strong refl ectance of unwanted frequencies, is that it results from the interference of beams partially refl ected and transmitted at the multi ple interfaces present in the stack. This mechanism does not involve the generation of excited electronic states thus preventing secondary effects. While it is easy to fi nd a large variety of interference fi lters in the visible or infrared region, fabrication of absorption-free UV-refl ecting dielectric mirrors is a diffi cult task because there are few materials that can be shaped as thin fi lms with negligible absorption below 300 nm. For this purpose, dense layers of III-nitride semiconductors (III being aluminium, gallium and boron, principally) [ 22 ] are stacked in order to achieve light confi nement for laser applications in the UV regime. For similar goals, dense SiO 2 /ZrO 2 system prepared by plasma sputtering method combined with III-nitrides in microcavities have been used. [ 23 , 24 ] Effi cient refl ectors for far UV (170–240 nm) have already been realized by single and multiple Fabry-Perot interference fi lters based on a combination of metal layers, typically aluminium, and dielectrics. [ 25–27 ] Polymeric stacks made by dip- or spin-coating are also used to block Selective UV Refl ecting Mirrors Based on Nanoparticle Multilayers A new type of nanostructured selective ultraviolet (UV) refl ecting mirror is presented. Periodic porous multilayers with photonic crystal properties are built by spin-coating-assisted layer-by-layer deposition of colloidal suspensions of nanoparticles of ZrO 2 and SiO 2 (electronic band gap at λ < 220 nm). These optical fi lters are designed to block well-defi ned wavelength ranges of the UVA, UVB, and UVC regions of the electromagnetic spectrum while preserving transparency in the visible. The shielding against those spectral regions arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. In addition, it is shown that the accessible pore network of the as-deposited multilayer allows preparing thin, fl exible, self-standing, transferable, and adaptable selective UV fi lters by polymer infi ltration, without signifi cantly losing refl ectance intensity, i.e., preserving the dielectric contrast. These fi lms offer a degree of protection comparable to that of traditional ones, without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as is the case for organic absorbers, generation of free radicals, all of which are caused by light absorption. DOI: 10. 1002/adfm.201202587 J. R. Castro Smirnov, Dr. M. E. Calvo, Dr. H. Míguez Instituto de Ciencia de Materiales de Sevilla Consejo Superior de Investigaciones Científi cas-Universidad de Sevilla Américo Vespucio 49, Sevilla, 41092, Spain E-mail: [email protected] Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2806 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim obtained by other ZrO 2 sol-gel synthesis tested were proven to yield poor optical quality multilayers. In Figure 1 we show the particle size distribution of both types of colloids used in this work. Particle sizes do not exceed the few tens of nanometers (distribution size maxima located at 8 nm and 25 nm for ZrO 2 and SiO 2 , respectively). This is a strict requirement to minimize diffuse scattering of both UV and visible light. In addition, size distribution of the particles is narrow enough as to allow the formation of smooth interfaces between the layers of SiO 2 and ZrO 2 in the multilayer. Field emission scanning electron microscopy (FESEM) images of the cross sections of the multilayers, obtained detecting secondary and backscattered electrons, are shown in Figure 2 a,b, respectively. Secondary electron image reveals a periodic structure in which the spherical morphology of the bigger SiO 2 particles can be recognized, while the layers made of the smaller ZrO 2 particles appears as an almost continuous layer. A backscattered electron image taken from the same sample region discloses clearly continuous and smooth interfaces between consecutive layers, brighter fringes corresponding to denser ZrO 2 layers. This picture reveals the absence of significant interpenetration that could negatively infl uence the refractive index contrast and hence the intensity of Bragg refl ections from the structure. From these images, we estimate the average thickness of each type of fi lm present in the multilayer, which in this particular case turns out to be around 40 nm for ZrO 2 slabs and 70 nm for SiO 2 ones. FESEM analysis puts in evidence that UV by optical interference effects but, due to the low refractive index contrast existing between them, from 50 to 100 layers are typically needed to achieve signifi cant refl ectance values in a narrow spectral range. [ 28 , 29 ] A recent successful attempt to create an ordered material that refl ects selectively UV radiation near 300 nm was based on the structuring of a block copolymer by collective osmotic shock. [ 30 ] Recently, porous 1DPC consisting of SiO 2 and TiO 2 nanoparticles as building blocks were developed [ 31 ] and rapidly spread as their applications in different fi elds were explored. [ 32 ] Related to the research discussed here, our group recently proved that these optical structures can be infi ltrated with either dissolved polymers [ 33 ] or monomers [ 34 ] to obtain a hybrid 1DPC that can be lifted off the substrate. Although this is a means to attain highly effi cient visible and near infrared mirrors, be they fl exible or rigid, their application to the UV range is hindered by the electronic band gap of TiO 2 ( E g ≈ 3.2 eV), photonic effects being screened by the strong absorption of TiO 2 at wavelengths below 350 nm. Herein we present a series of photonic crystal fi lms capable of effi ciently blocking different UV wavelength ranges comprised between λ = 200 nm and λ = 400 nm. Our arrangement is based on a porous multilayer structure made by the alternated deposition of layers of ZrO 2 nanocrystals of strictly controlled pore size distribution, which we tailor from the synthesis, and SiO 2 nanoparticles. The large electronic band gaps of the materials employed implies that no optical absorption will prevent the observation of the interference effects sought after. Also, the high refractive index contrast between the different types of nanoparticle layers gives rise to intense Bragg refl ections at the targeted wavelengths. In addition, due to the open porosity of the thin layers forming the optical nanostructures, a UV transparent polymer, poly(dimethyl)siloxane (PDMS), could be infi ltrated in the structure to yield self-standing, fl exible, visibly transparent, transferable, and adaptable highly refl ecting fi lters for UV radiation protection that operate only based on interference effects. 2. Results and Discussion 2.1. Microstructural Characteristics The goal to attain visibly transparent UV shields based only on interference effects imposed the use of materials that do not present any optical absorption in that range. The choice of SiO 2 and ZrO 2 nanoparticles as building blocks of the multilayers was then mainly determined by their large electronic band gap, which results in lack of optical absorption from the near infrared down to almost λ = 220 nm. While colloidal suspensions of SiO 2 particles had been previously used to make porous mirrors, in general combined with TiO 2 nanocolloids, [ 31 ] the preparation of ZrO 2 colloids that could be integrated in an optical quality multilayers by dip- or spin-casting required the modifi cation of already existing sol-gel processing techniques. [ 35 ] Peptization in acid medium allows controlling particle size in the desired range, while dialysis provides the necessary surface charge to realize the layer by layer assembly of ZrO 2 particles from methanol/water suspensions. It should be mentioned that particles Figure 1 . Volumetric particle distribution size of colloidal nanoparticles made of ZrO 2 (black triangles) and SiO 2 (grey circles). Figure 2 . FESEM images of the same region of the cross section of a 20 layer SiO 2 /ZrO 2 1DPC deposited onto a silicon substrate, obtained using a) secondary and b) backscattered electron detection. Scale bar is 1 µ m in both fi gures. Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2807 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (390–750 nm), estimated according to standard methods commonly employed to determine light transmittance of solar radiation for glazing in buildings (ISO 9050:2003), which takes into account the photo pic response of the eye, is around 90% in all cases. This is due to the absence of both diffuse scattering, which is a consequence of the strict control over the particle size distribution in the precursor suspensions, and optical absorption. In line or ballistic transmission measurements were also performed and compared to total transmission ones in order to evaluate the amount of light defl ected from the incident beam by effect of imperfections in the sample. Results are presented in Figure S1 in the Supporting Information. From them we can conclude that the density of defects has an almost imperceptible effect on the optical response of the coating. From the fi tting of the refl ectance spectra we can estimate the refractive indexes, n i , and the thickness, t , of the constituent layers using a code based on a transfer matrix formalism. [ 40 ] Layers’ thickness were estimated from SEM images and used as input of the simulations, so that the only free parameters left were the refractive indexes of the individual layers. From this analysis, whose full details can be found in the Supporting Information (Figure S2), we extract n ZrO2 = 1.70 and n SiO2 = 1.32, up to 20 layers can be stacked by spin coated assisted layer by layer deposition without an intermediate thermal treatment, in general necessary for other combinations of metal oxides. [ 36 ] Thus, the particular surface interaction between SiO 2 particles and dialyzed ZrO 2 ones seem to favor the formation of thicker fi lm piles at room temperature, which is in turn convenient for the preparation of fl exible fi lms from them (vide infra). It should be mentioned that nanoparticle fi lms are stable against disassembly in the same dispersion medium from which they were deposited. [ 37 ] Detailed studies on the mechanical stability of colloidal assemblies propose that particles are likely to be held together by bridges of water hydrogen bonded to the surface, which act as a cement for the structure. [ 38 ] 2.2. Optical Properties 2.2.1. One Dimensional Photonic Crystals Periodic stacking of ZrO 2 and SiO 2 layers gives rise to a modulation of the refractive index in one dimension of the space and, therefore, to its behavior as a one dimensional photonic crystal. A distinct optical feature associated to that periodicity is the presence of a maximum in the refl ectance spectra, resulting from the interference of incident light beams partially refl ected and transmitted at the interfaces between layers. The method proposed here allows locating this maximum, also referred to as Bragg peak, at different UV wavelength ranges, as illustrated by the series of total refl ectance ( R T ) spectra plotted in Figure 3 a. These have been measured from a set of porous ZrO 2 /SiO 2 1DPC deposited on a quartz substrate at different fi nal rotation speeds or using different particle concentrations in the precursor suspensions. Complementarily, we measure the total transmittance ( T T ) spectra of the coatings, which are shown in Figure 3 b. Both families of spectra were acquired using an integrating sphere (see Experimental Section). It is clearly observed that samples can be designed to refl ect well-defi ned spectral ranges in different sub-regions of the ultraviolet spectrum, namely, UVA (from 400 nm to 315 nm), UVB (from 315 nm to 280 nm) and part of UVC (from 280 nm to 100 nm). Signifi cant peak intensity (80%) and width ( ∆ λ = 50 nm, 0.68 eV) are attained for the number of unit cells ( N = 10, a unit cell being defi ned as a SiO 2 /ZrO 2 bilayer) herein stacked as a consequence of the high refractive index contrast between the SiO 2 and ZrO 2 layers. We estimated the absorptance as A = 1 – R T – T T (see Figure 3 c) to establish the threshold in which optical losses in the multilayer are signifi cant. Most of the samples absorbs near 50% of incident light at λ = 220 nm (5.63 eV), as a consequence of interband absorption in ZrO 2 layers. It can be observed also that, as the position of the Bragg peak shifts to lower wavelength, some overlap with the absorption starts to be signifi cant, which distorts the refl ectance and the transmittance spectra for λ < 220 nm. The detected fl uctuations in absorptance under 3% at longer wavelengths are due to geometrical restrictions to full solid angle light detection in the integrating sphere. [ 39 ] This analysis reveals that the UV shield effect, plotted as transmittance in Figure 3 b, is totally due to refl ection by the fi lm. In addition, the transparency of the samples in the visible region Figure 3 . a) Total refl ectance, b) total transmittance, and c) absorptance from ZrO 2 /SiO 2 interference fi lms. All fi lms were deposited using the same spin coating protocol ( ω = 6000 rpm and γ = 11 500 rpm/s) but precursor suspensions with different nanoparticle concentrations (expressed as [SiO 2 wt%, ZrO 2 wt%]): [1.0, 2.5] (solid black); [1.5, 2.5] (solid grey); [2.0, 2.5] (short dash dark grey); [2.25, 2.5] (solid dark grey); [2.25, 3.0] (solid light grey); and [2.5, 3.0] (short dash black). Unit cell thicknesses were 75 nm, 80 nm, 95 nm, 105 nm, 110 nm, and 125 nm, respectively. Total number of unit cells was 10 in all cases. Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2808 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim which implies a pore volume fraction of 50% and 25% respectively. Values of 2.25 and 1.43 are taken for the refractive indexes of bulk ZrO 2 and SiO 2 . 2.2.2. 1D Optical Resonators Another type of photonic structure that can be designed with the formerly described nanoparticle fi lms is an optical porous resonator, which is a multilayer system that presents a disruption of the periodicity, realized in our case by insertion of a thicker mid-layer of SiO 2 beads which will behave as an optical cavity. In terms of photon modes, the consequence of symmetry rupture is the generation of a permitted state at photonic band-gap frequencies, whose track is either a dip spectrally located within the refl ectance maximum or the complementary peak in the transmittance minimum. Such features can be readily visualized in Figure S3a,b (Supporting Information). In this sample, the transmission window is designed to be near λ = 310 nm. This type of systems are capable of blocking a wide region of the UV while still transmitting a narrow range of selected wavelengths and can be useful, for instance, to perform a selective photochemical reaction or polymerization of monomers. They may also be of interest for those phototherapy treatments in which irradiation with narrow UVB ranges, typically centered at λ = 311 nm, have long been proven as an effective means to treat skin diseases such as psoriasis [ 41 ] or vitiligo. [ 42 ] From a different perspective, resonant frequencies can be confi ned in the optical cavity, yielding the enhancement of the interaction between radiation and molecules or nanomateriales located in the cavity. [ 39 ] As shown in the previous section, the estimated absorptance ( A = 1 – R T – T T ) of the resonator show that all observed optical characteristics are purely due to interference effects. 2.3. Flexible Photonic Structures 2.3.1. Flexible UV Protecting 1DPCs Since the structures herein presented are porous, they are susceptible to be infi ltrated with polymeric compounds to attain a hybrid coating that can eventually be lifted off and behave as a self-standing selective UV protecting fi lm. In this case, we fi ll the interstitial space with oligomers of PDMS and then trigger the polymerization at a temperature of 110 ° C, as described in previous reports. [ 15 , 34 ] A challenge to successfully achieve our goal is the very small average pore size present in ZrO 2 layers, since that could inhibit the diffusion of the oligomers to the deepest layers of the photonic structure. To overcome this obstacle, we infi ltrate the structure by spin coating, which forces the deposited liquid phase to enter the voids of the porous multilayer. After that, in order to remove the hybrid fi lm from the substrate, we introduce the sample in liquid nitrogen to reach a temperature below that at which the glass transition of PDMS occurs. Once removed from the Dewar and warmed back to room temperature, the 1DPC can be separated from the substrate. In Figure 4 a we show refl ectance spectra for seven fl exible ZrO 2 /SiO 2 photonic crystals of different lattice constant. These experiments were made with samples made of 26 layers to reach close to 100% refl ectance at 300 nm in the as deposited sample (see Figure S4, Supporting Information). The Bragg peak intensity decreases after PDMS infi ltration and lifting off, being then comprised in the range between 60% and 80% in all cases. The decrease in refl ectance is due to the lowering of the refractive index contrast between SiO 2 and ZrO 2 layers upon fi lling of the polymer. These new hybrid polymer/metal oxide samples are mechanically stable and highly fl exible, as the images displayed in Figure 4 b demonstrate. 2.3.2. Stacks of Hybrid Flexible Films The mechanical stability and fl exibility of fi lms of different lattice constant that shield against well-defi ned wavelength regions along the 200–400 nm range, as shown in Figure 4 , open the way to blocking arbitrarily wide spectral regions by piling them up. In Figure 5 we show the refl ectance spectra of a series of fl exible stacks made of an increasing number of multilayers, each one displaying its characteristic Bragg peak at different spectral positions in the UV. Total refl ectance, total transmittance and absorptance for stacks containing 1, 3, 5, and 7 fi lms are shown in Figure 5 . Films were attached sequentially, starting from the one refl ecting the shortest wavelengths up to the one refl ecting closer to the blue region. As the number of photonic crystals increases, a wider spectral range is refl ected, the entire UVC-UVB-UVA range being effi ciently refl ected for the case of 7 stacked fi lms while maintaining a transparency of 60% in the visible region. The observed lowering of transparency with respect to a single deposited multilayer is due to the scattering by irregularities in the interfaces between the PDMS layers put in contact, the magnitude of the effect increasing with the number of fi lms in the stack. Since multilayers are embedded in a thick PDMS matrix of smooth but not fl at surface, interference effects between the different stacks are not detected in the optical properties of the pile. From the point of view of radiation protection, this simple method can be used to selectively shield against selected spectral regions, as well as to mimic or surpass the effect of a conventional UV absorbing material, as shown in what follows. 2.4. Radiation Protection The ability of the SiO 2 /ZrO 2 fl exible photonic crystal fi lms, and stacks of them, to protect from UV light was tested by exposing commercial UV sensitive strips (Control Cure, UV FastCheck Strips, UV process supply Inc.) to the collimated and uniform radiation beam coming out from a solar simulator, employing different combinations of PDMS embedded multilayers to shield them. Detailed analysis of the color changes induced in the strips by effect of the exposure to UV light allows us quantifying the amount of energy they have received from the solar simulator. The performance of the ZrO 2 /SiO 2 interference mirrors as UV shields was compared to that of PDMS fl exible fi lms, in one case embedding TiO 2 nanoparticles, and, in the other, supporting a thin fi lm of benzophenone-3, a widely employed organic UV absorber, also used in sunscreen lotions. [ 12 ] In both cases, UV light blocking occurs only as a consequence of absorption, since the TiO 2 particles employed are too small to give rise to diffuse scattering. UV sensitive strips were placed below different types of UV protecting fi lms and then illuminated at 1 sun light intensity Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2809 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (AM1.5, 1000 W/m 2 ) during the same time. Films tested were: 1) a stack of multilayers that blocks the full UV region (sample ML A ); 2) another Bragg mirror pile that mimics the absorption profi le of TiO 2 (sample ML B ); 3) a PDMS fi lm containing a layer of TiO 2 nanoparticles with the same thickness than the photo nic crystal (approximately 1 µ m); 4) a PDMS fi lm onto which a thin layer of benzophenone-3 was deposited; and 5) a PDMS fi lm, to quantify the effect of receiving this amount of radiation without protection. In Figure 6 a we show the energy per unit area received by the strips in each case. The corresponding transmittance spectra are plotted in Figure 6 b. Remarkably the ability of the fl exible interference mirror tandem ML B to shield against UV rays is as good as that of a similar fi lm of highly absorbing TiO 2 particles, while the performance of ML A surpasses that of the fi lm onto which benzophenone-3 was deposited. This shows that fl exible photo nic materials are suitable substitutes of inorganic or organic UV absorbers traditionally employed to dope polymers to the same end. The expected downshift of the spectral position of the refl ectance peak for oblique angles of light incidence on multilayers, which follows a combination of Bragg and Snell laws, is in our case easily compensated by designing a stack of multilayers for which the longer cut-off wavelength is approximately 30 nm above the lower energy edge of the range to be blocked, since that is the peak shift measured at 45 ° . It should be noted that in order to make use of the protecting character of organic UV absorbers in a fi lm, these compounds must be incorporated within the polymeric matrix, rather than deposited on top as it is done for the comparative study herein presented. Excited benzophenone is known to abstract hydrogen atoms from almost all polymers, thus generating radicals, which can be produced locally at high concentration and give rise to the degradation of the matrix. [ 43 ] Hence, one of the main advantages of integrating the nanostructured inorganic multilayers herein proposed is the absence of any Figure 4 . a) Total refl ectance spectra of individual fl exible hybrid ZrO 2 /SiO 2 photonic crystals. Multilayers displaying Bragg refl ections from the UV to the visible were prepared using the same spin coating parameters but precursor suspensions with different nanoparticle concentrations (expressed as [SiO 2 wt%, ZrO 2 wt%]): [1.5, 2.5] (dash grey); [2.0, 2.5] (solid black); [2.25, 2.5] (solid grey); [2.25, 3.0] (dash black); [2.5, 3.0] (grey); [3.0, 2.5] (dash light grey); and [4.0, 2.5] (solid light grey). b) Photographs of hybrid self-standing PDMS/ZrO 2 /SiO 2 multilayer fi lms. Figure 5 . Total refl ectance (black solid line), total transmittance (grey solid line) and absorptance (estimated as 1 – R T – T T , black dotted line) spectra of different stacks of the fl exible multilayer containing fi lms shown in Figure 4 a. The number of samples piled up is: a) 1, b) 2, c) 3, d) 5, and e) 7. Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2810 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim sort of photodegradation of the polymeric matrix without compromising the achievable degree of UV protection. Also, degradation of the polymer typically results in yellowing of the fi lm, strongly reducing its transparency after some time. 3. Conclusions We have shown a method to prepare both rigid coatings and fl exible self-standing fi lms capable of effi ciently protecting against UV radiation in desired and preselected wavelength ranges. The UV blocking effect arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. We demonstrate that these fi lms may equal or outperform layers of similar thickness made of purely absorbing materials in terms of the degree of radiation protection achieved. These new materials offer a degree of protection comparable to that of traditional ones, but without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as it is the case for organic absorbers, generation of free radicals, all of them caused by light absorption. 4. Experimental Section Preparation of Particle Suspensions : ZrO 2 nanoparticle sols were synthesized using a procedure based on the hydrolysis of zirconium n-propoxide. [ 35 ] A stable colloidal suspension of ZrO 2 nanoparticles was achieved by the slow addition of zirconium n-propoxide (3.7 mL, 70%, Alfa Aesar) in Milli-Q water (50 mL) acidifi ed with HNO 3 (1 mL, 65%, Prolabo) under strong stirring. Immediately after the addition of the zirconium alkoxide a white precipitate is formed. The complete peptization of the solid took place under room temperature for approximately 72 h. Next, the colloidal suspension was dialyzed against pure Milli-Q water in a membrane (MWCO = 10 000, Spectra/Pore) until pH = 3.5. The fi nal dialysis pH was the parameter employed to control aggregation and thus particle size distribution. Finally, water was removed by distillation at reduced pressure and particles were re-suspended in methanol. SiO 2 nanocolloids were purchased from Dupont (LUDOX TMA, Aldrich). Both suspensions were diluted in methanol in concentrations ranging from 3 to 4 wt% for ZrO 2 and from 1 to 4 wt% for SiO 2 particles. Other methods involving the hydrolysis of zirconium alkoxide under the presence of tetralkylammonium salts in aqueous media did not lead to stable colloidal suspensions of ZrO 2 as poly-anion chains of Zr-O could not be broken by the ammonium salts. Steric repulsions due to the coordination of the alkyl chain to the oxide seemed to be insuffi cient to prevent aggregation. Commercial ZrO 2 suspensions were also tested without success. Deposition of Nanoparticle-Based 1D Photonic Crystals : Photonic crystals were built by alternated deposition of ZrO 2 and SiO 2 nanoparticle suspensions, following a generic procedure previously reported. [ 31 ] Suspensions were deposited over quartz slides using a spin coater (Laurell WS-400E-6NPP), in which both the acceleration ramp and the fi nal rotation speed could be precisely determined. Final speed was chosen between nominal value of 4500 and 6000 rpm and acceleration was set at 11 340 rpm s − 1 . Suspensions were spun during 60 s. Sequentially, layers of different types of nanoparticles were deposited following the procedure described above. The process was repeated until the desired number of layers has been deposited. Polymerization within the Multilayer Interstices and Lifting-Off : Elastomeric precursor and curing agent of PDMS (Sylgard 184, Dow Corning) was infi ltrated depositing 0.5 g of the mix on top and then by spin coating (40 s, 700 rpm). After that, samples were maintained at room temperature during 24 h to facilitate the diffusion throughout the pore network. Next, PDMS was polymerized in a stove at 110 ° C for 30 min. In order to lift off the fl exible photonic structures, samples were immersed in liquid nitrogen (–196 ° C). Then samples were allowed to warm up to temperature before being lifted off the substrate. Optical and Structural Characterization : Total refl ectance ( R T ) and total transmittance spectra ( T T ) were obtained using a UV-visible scanning spectrophotometer (SHIMADZU UV-2101PC) attached to an integrating sphere. Spectrophotometer slits were adjusted to measure R T and T T with the same spot size. Absorptance ( A ) was calculated from independent measurements of T T and R T by using A = 1 – ( R T + T T ). Film images were acquired using a SLR digital camera (Pentax Kx). FESEM images of the multilayers fi lms deposited onto silicon were taken by using a microscope Hitachi 5200 operating at 5 kV. Figure 6 . a) Energy density received at the surface of UV sensitive strips covered with different fl exible UV protective fi lms and exposed to a solar simulator (AM 1.5 spectrum, 100 mW/cm 2 ). Height of the bar represents the average value and the black line represents the standard deviation. ML A and ML B stands for different ZrO 2 /SiO 2 fi lm stacks. b) Total transmittance spectra of the protecting fi lms used in Figure 6 a: pure PDMS or naked strip (black short dashed line), TiO 2 nanoparticle fi lm embedded in PDMS (black solid line), benzophenone-3 deposited (light gray solid line) onto PDMS, MLA (black dot dashed line), and MLB (gray solid line). Adv. Funct. Mater. 2013, 23, 2805–2811 FULL PAPER 2811 www.afm-journal.de www.MaterialsViews.com wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Solar Irradiation Experiments : Sample irradiation was carried out using a solar simulator (Sun 2000, Abet Technologies) including a 150 W xenon lamp with an appropriate fi lter for the correct simulation of the AM1.5 solar spectrum. A timer-controlled mechanism opened and closed the shutter. The incident light power was confi rmed to be 1000 W/m 2 (1 sun) using a calibrated silicon solar cell. UV sensitive strips were purchased from UV process Supply Inc. (Control Cure, UV FastCheck Strips). Flexible fi lms were placed on top of the sensitive strips and then the shutter was opened for 45 s. Both naked strips or PDMS fi lm covered ones were used to determine the relationship between energy density received at the strip surface and the color change observed (see Supporting Information, Figure S5). These were quantifi ed by measuring the total refl ectance of the strips in an integrating sphere. In order to ensure the reproducibility of the protection effect, irradiation experiments were repeated three times and the results averaged. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements We thank the Spanish Ministry of Economy and Competitiveness for funding under grants MAT2011-23593 and CONSOLIDER HOPE CSD2007-00007, and Junta de Andalucía for grants FQM3579 and FQM5247. H.M. thanks the European Research Council for the Starting Grant POLIGHT. Received: September 7, 2012 Revised: November 5, 2012 Published online: January 22, 2013 [ 13 ] M. Zayat , P. Garcia-Parejo , D. Levy , Chem. Soc. Rev. 2007 , 36 , 1270 . [ 14 ] M. M. Demir , K. Koynov , U. Akbey , C. Bubeck , I. Park , I. Lieberwirth , G. Wegner , Macromolecules 2007 , 40 , 1089 . [ 15 ] M. E. Calvo , J. R. Castro Smirnov , H. Miguez , J. Polym. 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