The Effect of Ormosil Matrix Composition and Alkaline Earth Metal Doping on the Photochromic Response of Ormosil-Phosphotungstate Films

In this study, polyoxometallate based hybrid photochromic materials were prepared by incorporating phosphotungstate anion, PW12O40, (PW) in hybrid tetraethyl orthosilicate and (3-glycidyloxypropyl)trimethoxysilane TEOS-GPTMS derived organomodified silicates (Ormosil) matrices by sol-gel method and the resulting materials were used to prepare multilayer films by dip-coating method. The effect of alkaline earth metal cations doping and matrix composition (%GPTMS) on the photochromic response of the hybrid films was studied in details. GPTMS, after undergoing ring opening reaction, leads to the formation of chelating sites (diol and ether functionalities) which helps in anchoring of cations, which in turn interacts with phosphotungstate anions and favors their incorporation in the hybrid films. For a fixed concentration of GPTMS, the cation-phosphotungstate electrostatic interaction and hence the photochromic response of the films follow the order Mg < Ca < Sr < Ba, thereby, indicating that larger cations interact more strongly with the heteropolyanions. The presence of these cations and GPTMS concomitantly leads to increased incorporation of phosphotungstic acid hydrate (HPW) in the films, resulting in a significant enhancement of the photochromic response.

Sol-gel synthesis is an easy and versatile strategy for the preparation of such photochromic coatings which allows effective incorporation of photoactive molecules such as POMs in sol-gel derived matrices using mild reaction conditions. 3,23Hybrid organic-inorganic matrices are good candidates for the preparation of such functional materials as they may combine the advantages of both polymeric and ceramic materials. 24,25For instance, we have recently reported the preparation of uniform, adherent and transparent hybrid photochromic and photocatalytic coatings by incorporation of phosphotungstate (PW 12 O 40 ), the Keggin POMs, in organically modified silicate (Ormosils) matrices.The hybrid matrix materials were prepared by sol-gel method using tetraethyl orthosilicate (TEOS) and (3-glycidyloxypropyl)trimethoxysilane Vol. 26, No. 12, 2015   (GPTMS) as matrix-forming precursors. 21,26However, the weak interaction between the hybrid matrix and the Keggin heteropolyanions resulted in lower incorporation of the photoactive species in the films which led to the low photochromic response of these dip-coated films.Such limitations could be overcome by modification of the Ormosil networks with positively charged alkylammonium groups, 21 which are known to interact strongly with POMs. 27Interestingly, a similar effect could be achieved by simple concomitant addition of Zn 2+ cations during sol-gel synthesis, leading to the preparation of highly photochromic Ormosil-polyoxometalate films. 285][36][37] In the present study, we report the preparation of TEOS-GPTMS-phosphotungstic acid hydrate (HPW) hybrid films doped with alkaline-earth divalent cations (Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ ).A systematic study of the role of hybrid matrix composition and the presence of the doping cations in the film assembly (POM incorporation) and photochromic response of the resulting Ormosil-polyoxometallate coatings is presented.We demonstrate how the interaction between the organic functionalities, cations and phosphotungstate polyoxoanions can be exploited to significantly enhance and effectively tune the photochromic response of the hybrid films.From application point of view, such control of film architecture and photochromic response might be of great importance for design of functional photochromic coatings.(Brazil).Ethanol (99.3%) was supplied by QHEMIS (Brazil).All the reagents and solvents were used as received and without further purification.

Material synthesis and film preparation
Hybrid TEOS-GPTMS-HPW films were prepared based on our previously reported sol-gel method for the preparation of Ormosil-phosphotungstate films. 21,26,28e sol-gel derived films were prepared by the addition of phosphotungstic acid and the chosen doping cations to an ethanolic solutions cointaing the silicon alkoxides precursors, TEOS and GPTMS.GPTMS content of the hybrid materials varied from 0 to 80% of total silicon alkoxide molar amount.The compositions of the prepared samples are summarized in Table 1.
The TEOS and GPTMS were firstly dissolved in 25 mL of ethanol in a polypropylene beaker, followed by addition of 25 mL of freshly prepared HPW ethanolic solution under magnetic stirring.In this procedure, HPW plays the additional role of acid catalyst of the hydrolysis and condensation reactions of TEOS and GPTMS.After homogenization, 825 µL of M 2+ Cl 2 0.242 mol L −1 aqueous solution (M 2+ = Mg 2+ , Ca 2+ , Sr 2+ or Ba 2+ ) or deionized water (in the case of undoped samples) was added and the resulting mixture was kept under magnetic stirring for 15 min.The obtained sol was used to deposit thin films on soda-lime glass slides (Bioslide Technologies, USA) by dip-coating method.The glass substrates were cleaned by immersing the glass substrates in a cleaning solution composed of NH 4 OH:H 2 O 2 :H 2 O in 1:1:5 volume ratio at 70 °C for 2 h, followed by rinsing with copious amount of deionized water and drying under a steam of nitrogen.Dip-coating film deposition was carried out using a disc elevator MA-765 Marconi (Piracicaba, SP, Brazil) with ascending and descending speed adjusted at 150 mm min −1 .Each film was prepared by 10 immersion cycles and dried under ambient atmosphere (relative humidity 55 ± 10%, temperature 292 ± 3 K).

Characterization techniques
Fourier transform infrared (FTIR) spectra were collected at transmission mode with a Shimadzu Spectrophotometer (model IRAffinity 1) directly from films deposited on silicon plates.Spectra were recorded, using air as blank reference, from 4000-400 cm −1 , 4 cm −1 of resolution and average of 32 scans.
X-ray diffraction (XRD) was carried out using a Bruker Diffractometer (model D8 Advance) equipped with a high resolution energy-dispersive LINXEYE EXE detector.Grazing incident diffraction was applied to assess the films.X-ray diffraction patterns were collect from 3-50° with an incident angle fixed in 5°, scan step mode of 0.03° min −1 and counting time of 0.6 s per step.Ni-filtered Cu K α radiation (λ = 1.54056Å) were applied at 40 mA and 40 kV.
X-ray fluorescence (XRF) analysis were carried out using a benchmark energy dispersive MiniPal4 (PANalytical) spectrometer equipped with a rhodium tube as X-ray source.All the measurements were acquired after a total measurement time of 840 seconds under He atmosphere.Tungsten and barium X-ray fluorescence intensities were determined by peak fitting of W Lα and Ba Lα using Omnian standardless analysis package (PANalytical).

Photochromism experiments
The photochromic properties of the hybrid films were evaluated by monitoring changes in the visible light electronic absorption spectra after exposure to UV-B light.Irradiation was carried out using a 16S Solar Light Simulator Xe arc lamp (Solar Light Co., USA).The sample-to-lamp distance was 7.3 cm and the UV light spot diameter was 1 cm.The thermal visible radiations (λ > 430 nm) were filtered out by internal filters.Electronic absorption spectra were collected immediatelly after irradiation with a USB 4000 spectrometer (Ocean Optics, FL, U.S.A.) equipped with a P400-2 UV/Vis optical fiber and an LS1 tungsten halogen lamp.Sample films were vertically positioned (perpendicular to the light beam) in a home-made aluminum sample-holder.The spectra of each sample film were measured using the non-irradiated film itself as a blank in order to have the light scattering of the films compensated.As a measure of the photochromic response of the films, the integrated area of the intervalence change transfer (IVCT) absorption band (600-800 nm) was measured from the absorption spectra.The reversibility and stability of the photochromic response of the hybrid films were evaluated by alternatively exposing the film first to UV irradiation for 10 min and then to air/O 2 for 24 h (in dark) for up to 10 times.The electronic absorption spectra of the films after UV illumination (10 min) and after complete bleaching (12 h) were measured during each cycle.

Results and Discussion
Characterization by FTIR analysis FTIR is a powerful technique to study the incorporation of both organic functionalities and HPW in the obtained Ormosil films.Figure 1 presents the FTIR spectra of the undoped films prepared without (Und0) and with the highest content of GPTMS (Und80).The spectra of both samples show a broad band around 3400 cm −1 , assigned to the stretching vibrational mode of −OH groups from silanol (Si-OH) residues.However, -OH groups from adsorbed and entrapped water in the silica matrix and around HPW polyoxoanion may also contribute to this band.Unlike the spectrum of Und0, the spectrum of the Und80 sample shows a set of bands at around 3000-2800 cm −1 .These bands arise from the C-H stretching vibration of the CH 2 groups of the organic modifier, as well the CH 2 and CH 3 groups from entrapped ethanol. 38gnificant differences arise in the region under 1300 cm −1 with the increase in GPTMS content of both the undoped and Ba 2+ -doped samples (Figure 2).Strong absorbance bands observed around 1200-1020 cm −1 region are attributed to Si-O-Si vibrational modes, confirming the formation of silica network.The weaker absorbance bands observed under 850 cm −1 are also related to the vibrational modes of Si-O and Si-O-Si groups, as well Si−OH fragments. 38It is important to note that a new band appears in the spectra of GPTMS-containing samples at around 1190-1140 cm −1 which is assigned to the asymmetric stretching of the C-O bonds.This C-O bond is introduced by the incorporation of GPTMS as organic modifiers groups. 39Another important feature is the appearance of a discrete band in 1200 cm −1 (marked with an asterisk) as we increase GPTMS concentration.This band is related to the C-O-C stretching mode. 40Furthermore, the characteristic band due to the epoxy ring of GPTMS at 1260 cm −1 is not observed.1][42] The possible reactions during the GPTMS ring opening are shown in Figure 3.
Finally, FTIR analysis confirms the presence of HPW in the hybrid films.The Keggin structure of heteropoly acid presents well defined and characteristics bands (marked as ■) at 1079, 979, 897 and 821 cm −1 assigned to the P−O a , W=O d , W-O b -W and W-O c -W vibrational modes, respectively. 43Relative absorbance of these bands is higher for the doped samples, suggesting that addition of doping cations leads to higher incorporation of HPW in the films.Such behavior was further investigated by XRF analysis as discussed later.

Characterization by XRD analysis
Since crystallization of HPW has been previously shown to suppress the photochromic response of HPW based composite materials, 17 structural characterization of the prepared samples by powder XRD was carried out to understand their photochromic behavior.XRD diffractograms, for both the undoped and barium-doped samples (Figure 4) show that the GPTMS concentration strongly influences the dispersion, affinity, and crystallization of the HPW inside the matrix.Distinct diffraction peaks are observed in the diffractograms of pure inorganic SiO 2 samples (0% GPTMS), indicating crystallization of phosphotungstic acid hydrates polymorphs 44,45 and/or their corresponding barium salts. 46,47The diffraction peaks start to disappear upon introduction of GPTMS into the matrix, indicating that the resulting hybrid Ormosil matrix inhibits HPW crystallization, 48 leading to a homogeneous dispersion of the heteropolyanions in the hybrid films.Furthemore, a broad peak at 2θ < 10° is observed in the diffractograms  1).The characteristic bands of Keggin phosphotungstate are marked with (■). of the Ormosil-phosphotungstate films.Such feature in the X-ray diffractograms was already observed for powder samples of Ormosil-phosphotungstate composites and is related to the formation of ordered domains inside the hybrid organosilicate network, as discussed in our previous study. 21

Photochromic behavior of the hybrid films
Photochromic behavior of the hybrid films was studied by following visible light absorption changes upon exposure of the samples to UV radiation.][13] These changes in visible light absorption leads to photoinduced coloration of the films, which acquire a dark bluish color (Figure 5b).The coloration process is reversible and the films bleaches at room temperature due to re-oxidation of polyoxometalate species by oxygen present in air, as demonstrated by photochromic response decay curve (Figure 5c).Furthermore, the coloration/discoloration cycles (Figure 5d) reaveal stable photochromic reponse, with only slight decrease after repeated cycles.
Effect of the hybrid matrix composition and doping cations on the POM content and photochromic response of the hybrid films The impact of the hybrid matrix composition (i.e., %GPTMS) and cation doping on the photochromic response of the films was evaluated by determining the integrated area of IVCT absorption band (600-800 nm). Figure 6 shows the variation of photochromic response of the undoped and Ba 2+ -doped films as function of GPTMS percentage.In both cases, the films prepared without the addition of GPTMS do not present photochromic response.Such behavior is probably related to the crystallization of HPW in the SiO 2 films, as evidenced by XRD analysis.
Upon incorporation of GPTMS at percentages higher than 20%, Ba 2+ -doped and undoped films present contrasting behavior; the photochromic response of the Ba 2+ -doped films enhances while that of the undoped samples slightly diminishes upon increasing the GPTMS content by more than 20% (Figure 6).Ba 2+ -doped sample (Ba80) presented photochromic response up to seven times higher than the most photoactive undoped sample (Und20).Thus, photochromic response of the hybrid TEOS-GPTMS-HPW dip-coated films can be enhanced by addition of Ba 2+ cations and such enhancement can be tuned by changing the GPTMS content (Figure 6).
Similar but less pronounced effects were achieved by doping with other alkaline-earth cations.Figure 7 compares the photochromic response of both undoped and cations-doped (Mg 2+ , Ca 2+ , Sr 2+ or Ba 2+ ) films as function of UV exposure time, fixing the amount of GPTMS at 80% in all cases.The observed photochromic response followed the order: undoped < Mg 2+ < Ca 2+ < Sr 2+ < Ba 2+ .It is evident that larger cations (Sr 2+ , Ba 2+ ) enhance the photochromic response of the films more effectively than smaller ones, suggesting a correlation between ionic size of the dopants and the properties of the resulting material as analyzed below.
Electrostatic interaction between polyoxoanions and cations are known to strongly affect the assembly of POMs-based materials. 34,35,49In the specific case of Ormosil-phosphotungstate nanocomposites, hybrid matrix composition and the presence of Zn 2+ doping cations were shown to influence the incorporation of polyoxometalate species in dip-coated films and consequently their photochromic response. 21,28XRF analysis was employed to evaluate incorporation of phosphotungstate and Ba 2+ in the hybrid films (Figure 8) by monitoring W Lα and Ba Lα fluorescence intensity.Barium incorporation in the Ba 2+ -doped films was found to increase with increase of GPTMS content of the Ormosil matrix (Figure 8a).The tungsten (W) content of the undoped films decreases for GPTMS percentages higher than 20%.On the other hand, the W   content of Ba 2+ -doped sample increases continuously with increase in GPTMS content (Figure 8b), indicating more incorporation of the polyoxometalate species.A similar trend of phosphotungstate incorporation was observed using FTIR spectroscopy (Figure S1 in the Supplementary Information (SI) section).Therefore, the incorporation of both phosphotungstate and cations is clearly influenced by the composition of the hybrid matrix.The increased incorporation of the positively charged species in the films may be related to the presence of GPTMS derived chelating functional groups, such as oligo-ether chains and diols, which can complex the alkaline earth cations. 50,51he chelated cations might then electrostatically attract the PW 12 O 40 3− polyoxoanions, thus acting as nucleating sites for the formation of phosphotungstate-rich domains inside the hybrid matrix.Incorporation of the cations and subsequent adsorption of PW 12 O 40 3− on these nucleation sites seems to be the first step in the formation of such phosphotungstate-rich domains.The initially adsorbed PW 12 O 40 3− polyoxoanions may further interact with other cations in the surrounding, such interaction being dependent on cation size.Again, the newly adsorbed cations may interact with more polyoxoanions in the surrounding.The overall result is an increased GPTMS/cations mediated incorporation of polyoxoanions in the hybrid films, as evidenced by XRF analysis.In the absence of cationic species, polyoxometalate incorporation is not effective because of the weak intermolecular interactions with the Ormosil matrices.
XRF analysis evidenced that the entrapment of phosphotungstate is enhanced by the presence of cations, possibly due to the electrostatic interactions between these species, suggesting it to be the possible reason for the superior photochromic response of cation-doped hybrid films.Such hypothesis is supported by the strong correlation between the photochromic response (Figure 6) and W Lα intensity (Figure 8b) of Ba-doped films.Additionally, a linear correlation (R 2 = 0.99) is observed between photochromic response and W content (as indicated by W Lα intensity) for the films doped with different alkaline earth cations (Figure 9).Since POMs tend to associate more favourably with larger cations 52,53 due to their lower hydration radii and hydration energies, 54,55 cation-phosphotungstate electrostatic interaction should follow the order Mg 2+ < Ca 2+ < Sr 2+ < Ba 2+ .This very same trend is observed in the XRF analysis and photochromism experiments (Figure 9).Therefore, the ion-size dependency observed for POM incorporation and the resulting enhanced photochromic response also fits with the electrostatic interaction based incorporation model.

Conclusions H i g h l y p h o t o c h r o m i c h y b r i d O r m o s i l -
phosphotungstate materials were prepared using sol-gel route and the resulting hybrid materials were immobilized as multilayer films on glass substrate using dip-coating technique.The presence of GPTMS and alkaline earth metal cations leads to a significant enhancement (up to 7 times) of the photochromic response of the films by increasing the amount of phosphotungstate anions incorporated in the films.GPTMS provides the chelating sites for adsorption of cations and the cations in turn interact with phosphotungstate anions in the order: Mg 2+ < Ca 2+ < Sr 2+ < Ba 2+ .The photochromic response follows the same order indicating a correlation between cation size (or cation-phosphotungstate interaction) and photochromic response.The presence of GPTMS seems to prevent crystallization of the HPW leading  to a homogeneous dispersion of the heteropolyanions in the hybrid films.The ion-size dependency observed for phosphotungstate's incorporation and the resulting enhancement of the photochromic response follow the eletrostatic mediated incorporation model and can be extended to other POMs and metal cations.

Figure 3 .
Figure 3. Schematic representation of the ring opening of epoxy groups of GPTMS (adapted from reference 42).
Photochromic behavior of the hybrid films was studied by following visible light absorption changes upon exposure of the samples to UV radiation.Figure5ashows the intensity of the broad absorption bands increases as function of UV exposure time.Such bands indicate the formation of photoreduced phosphotungstate species (e.g., PW 12 O 40 4− and PW 12 O 40 5−) and are assigned to d-d (400-550 nm) and IVCT (600-800 nm) transitions.[11][12][13]These changes in visible light absorption leads to photoinduced coloration of the films, which acquire a dark bluish color (Figure5b).The coloration process is reversible and the films bleaches at room temperature due to re-oxidation

Figure 5 .
Figure 5. Photochromic behavior of the Ba 2+ -doped hybrid films (Ba80): increase in absorptions bands of photoreduced phosphotungstate species as function of UV exposure time (a); digital photograph showing the hybrid film turning blue upon UV-B illumination and colourless upon air oxidation in dark (b); photochromic response decay curve showing the decrease in area of IVCT band (600-800 nm) upon exposure to air/O 2 in dark (c) and reversible and stable photochromic response of the hybrid films (d).The light and dark cycles shown in (d) were performed ex situ by first exposing the film to UV radiation for 10 min and then allowing the color to bleach to complete discoloration during 24 h in dark.

Figure 6 .
Figure 6.Photochromic response of Ba 2+ -doped and undoped films as function of GPTMS percentage.The UV illumination time was 10 min in each case.

Figure 7 .
Figure 7.The photochromic response of the hybrid films (M80) with 80% GPTMS as function of UV exposure time for different alkaline earth cations used as dopants.

Figure 8 .
Figure 8. Increase in the barium (a) and tungsten (b) content of the Ba 2+doped hybrids films as function of GPTMS content.The W content of the undoped film is also shown for comparison in (b).

Figure 9 .
Figure 9. Correlation between the photochromic response and W content (fluorescence intensity) of the hybrid films (M80) for different doping cations.The % of GPTMS is 80% and UV exposure time is 10 min in each case.

Table 1 .
Composition of the sols used in the preparation of hybrid Ormosilphosphotungstate films a Sample codes refer to the type of doping cation added and GPTMS content of the sample (e.g., sample Ba40: Ba 2+ doped sample prepared using 60% TEOS and 40% GPTMS).The Effect of Ormosil Matrix Composition and Alkaline Earth Metal Doping J. Braz.Chem.Soc.2600