Photochromism of Ti(III,IV)/PMMA Opalescent Coatings · Photochromism of Ti(III,IV)/PMMA Opalescent...

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Photochromism of Ti(III,IV)/PMMA Opalescent Coatings Ll. M. Flores Tandy 1 , J. J. Perez Bueno 1, * and Y. Meas Vong 1 1 Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S.C.; Parque Tecnológico Querétaro-Sanfandila, Pedro Escobedo, Qro. México. C.P. 76703; Tel.: (52-442) 2 11 6000; Fax: (52-442) 2 11 6001; *E-mail: [email protected] (www.cideteq.mx). The photochromism of titanium oxide/polymer (PMMA) composites have been investigated. Amorphous titanium oxides were synthesized by sol-gel using different titanium precursors. The solutions were mixed with dissolved polymers. The composites show photoresponse under UV radiation by changing, generally, as a dark brown tonality. This effect was reversible in different time ranges according to exposure time. Nonetheless, when used as a coating for a window, a translucent to white appearance could be obtained by means of titanium oxide concentration. This work presents a photochromic composite of titanium oxide/PMMA at different concentrations, analyzed by reflectance, XPS, and HRTEM. The distributions of micro- or nano-particles, incorporated to increase abrasion resistance, were easily identified and analyzed after UV exposure due to the matrix photochromic change. Keywords: Photochromic, TiO 2 , wear, abrasion resistance, titania, surface modification, electron-hole pair, sol-gel. 1. Introduction Photochromic materials, classified as organics or inorganics, have existed since the end of the XIX century. Photochromic materials change their optical properties when they are exposed to visible or UV light and reverse properties in the darkness. Basically, the phenomenon consists of a reversible change of isolated chemical species between two states with different absorption spectra. A widely known inorganic photochromic material is one used in photochromic lenses, which use metallic halides such as AgBr or AgCl. When UV light illuminates glass, the metallic halide crystals dissociate into metallic silver and halide. This effect causes absorption in the visible range. When removed from the light source, the metallic halide thermally recombines and recovers its original color and glass transmission. The typical optic response for photochromic glasses in colored and discolored states takes 3-4 minutes at room temperature, with an approximate typical optic response of T = 85–50 %. This study presents hybrid materials consisting of titanium oxides obtained by the sol-gel process in a polymeric matrix, the combination of which constitutes a system with color centers that can absorb in a wide range of the visible spectrum, resulting in a brown tonality. The color change is caused by energy in which electron-hole pairs are generated in the system. It is assumed that this propitiates a valence change of titanium oxide/hydroxide in an OH radical environment inside a surrounding polymeric matrix. There is a variety of photochromic materials with quite different physicochemical principles. Organic photochromic materials have excellent color and tonality but, due to great need to improve their characteristic absorption in the visible spectrum that can be a basis for effective coloring, the molecular structure must often contain a delocalized π-electron. Molecules associated with this type of electronic structure, polycyclic aromatic hydrocarbons, pigments, and heterocyclic quinoline azo dyes, are often carcinogenic, penetrating skin and having a high toxicity risk [1]. Major problems faced by photochromic compounds include stability loss when repeatedly exposed to intermittent or continuous radiation in air. This leads to decay in a few days, with a consequent lower response to light [2]. To address these problems, additives or other polymeric materials are added to increase resistance. These are not always compatible, due to photochromic material-matrix interactions. Their photochromic response can be strongly modified by the presence of polar groups, as these may induce complexation, protonation, stiffness and steric hindrance in the matrix [3]. Photochromic materials based on metal-doped titania require very high temperature processes, which imply a high cost. Moreover, if protection is required, they must be incorporated into a polymer matrix with the probability of phase incompatibility and modification of performance of the final material due to titania-matrix interactions [4]. Processes for obtaining photochromic functionalized titania [5] by the sol-gel processor as powders may require esterification, temperature peptization, milling or heat treatment [6]. These processes require a high use of thermal and mechanical energy, in addition to requiring considerable time to be implemented. The processes for obtaining photochromic titania, based on polymeric interactions [7], possess certain limitations. Polymers that allow such effects are conducting polymers with high costs. Also, PVA photochromic materials are very soluble in water and fragile for applications where mechanical properties results are crucial. Current Microscopy Contributions to Advances in Science and Technology (A. Méndez-Vilas, Ed.) © 2012 FORMATEX 1324

Transcript of Photochromism of Ti(III,IV)/PMMA Opalescent Coatings · Photochromism of Ti(III,IV)/PMMA Opalescent...

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Photochromism of Ti(III,IV)/PMMA Opalescent Coatings

Ll. M. Flores Tandy1, J. J. Perez Bueno1,* and Y. Meas Vong1 1Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S.C.; Parque Tecnológico Querétaro-Sanfandila,

Pedro Escobedo, Qro. México. C.P. 76703; Tel.: (52-442) 2 11 6000; Fax: (52-442) 2 11 6001; *E-mail: [email protected] (www.cideteq.mx).

The photochromism of titanium oxide/polymer (PMMA) composites have been investigated. Amorphous titanium oxides were synthesized by sol-gel using different titanium precursors. The solutions were mixed with dissolved polymers. The composites show photoresponse under UV radiation by changing, generally, as a dark brown tonality. This effect was reversible in different time ranges according to exposure time. Nonetheless, when used as a coating for a window, a translucent to white appearance could be obtained by means of titanium oxide concentration. This work presents a photochromic composite of titanium oxide/PMMA at different concentrations, analyzed by reflectance, XPS, and HRTEM. The distributions of micro- or nano-particles, incorporated to increase abrasion resistance, were easily identified and analyzed after UV exposure due to the matrix photochromic change.

Keywords: Photochromic, TiO2, wear, abrasion resistance, titania, surface modification, electron-hole pair, sol-gel.

1. Introduction

Photochromic materials, classified as organics or inorganics, have existed since the end of the XIX century. Photochromic materials change their optical properties when they are exposed to visible or UV light and reverse properties in the darkness. Basically, the phenomenon consists of a reversible change of isolated chemical species between two states with different absorption spectra. A widely known inorganic photochromic material is one used in photochromic lenses, which use metallic halides such as AgBr or AgCl. When UV light illuminates glass, the metallic halide crystals dissociate into metallic silver and halide. This effect causes absorption in the visible range. When removed from the light source, the metallic halide thermally recombines and recovers its original color and glass transmission. The typical optic response for photochromic glasses in colored and discolored states takes 3-4 minutes at room temperature, with an approximate typical optic response of T = 85–50 %. This study presents hybrid materials consisting of titanium oxides obtained by the sol-gel process in a polymeric matrix, the combination of which constitutes a system with color centers that can absorb in a wide range of the visible spectrum, resulting in a brown tonality. The color change is caused by energy in which electron-hole pairs are generated in the system. It is assumed that this propitiates a valence change of titanium oxide/hydroxide in an OH radical environment inside a surrounding polymeric matrix. There is a variety of photochromic materials with quite different physicochemical principles. Organic photochromic materials have excellent color and tonality but, due to great need to improve their characteristic absorption in the visible spectrum that can be a basis for effective coloring, the molecular structure must often contain a delocalized π-electron. Molecules associated with this type of electronic structure, polycyclic aromatic hydrocarbons, pigments, and heterocyclic quinoline azo dyes, are often carcinogenic, penetrating skin and having a high toxicity risk [1]. Major problems faced by photochromic compounds include stability loss when repeatedly exposed to intermittent or continuous radiation in air. This leads to decay in a few days, with a consequent lower response to light [2]. To address these problems, additives or other polymeric materials are added to increase resistance. These are not always compatible, due to photochromic material-matrix interactions. Their photochromic response can be strongly modified by the presence of polar groups, as these may induce complexation, protonation, stiffness and steric hindrance in the matrix [3]. Photochromic materials based on metal-doped titania require very high temperature processes, which imply a high cost. Moreover, if protection is required, they must be incorporated into a polymer matrix with the probability of phase incompatibility and modification of performance of the final material due to titania-matrix interactions [4]. Processes for obtaining photochromic functionalized titania [5] by the sol-gel processor as powders may require esterification, temperature peptization, milling or heat treatment [6]. These processes require a high use of thermal and mechanical energy, in addition to requiring considerable time to be implemented. The processes for obtaining photochromic titania, based on polymeric interactions [7], possess certain limitations. Polymers that allow such effects are conducting polymers with high costs. Also, PVA photochromic materials are very soluble in water and fragile for applications where mechanical properties results are crucial.

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2. Experimental

2.1 Materials

The hybrid solution (HIB1) was prepared using titanium isopropoxide (TIPOT C3H7O4 Ti, 98%, Aldrich) as a titanium hydroxide source (Ti(OH)4) and titanium butoxide (IV) (TBT, C4H9O4Ti, 98% Aldrich). Several polymers and copolymers were included in this study, such as poly(methyl methacrylate) (PMMA P.M.=350,000, Aldrich), polyvinyl Acetate (HIB1-PVAc, mW ~500,000, Aldrich); polyvinyl alcohol (HIB1-PVA, mW ~500,000, Aldrich); hydroxypropyl cellulose (HIB1-HPC, mW ~1,000,000, Aldrich); polycaprolactone (HIB1-PCLA, mW ~80,000, Aldrich); polyethylene glycol (HIB1-PEG20000, mW ~8,000, J.T. Baker); polymethyl methacrylate-co-ethyl methacrylate (HIB1-co-EMA, mW

~39,500, Aldrich); and polymethyl methacrylate-co-methacrylic acid (HIB1-co-AMA) mW ~34,000. The solvents used for this study include diethyl oxalate (Aldrich); tetrahydrofuran (Aldrich); chloroform (Karal); toluene (Alfa-Aesar); methyl formate (Alfa-Aesar), named HIB1-MF; and ethyl (HIB1-EB) butyrate (Alfa-Aesar). Ceramic particles were occasionally added to hybrid materials for anti-abrasion protection properties: (HIB1-γ-Al2O3), γ-Al2O3particles of 0.03 µm (Buehler), and yttria stabilized zirconia (HIB1- YSZ), YSZ. The substrates used were acrylic plates with sandblasting pretreatment. Number 304 stainless steel plates were also used (composition 19 % Cr, 10 % Ni, 0.08 % C, and 2 % Mn). The material used for pickling surface pretreatment was HNO3 60% (Aldrich).

2.2 Sol-gel preparation

A precursor solution of 98% titanium isopropoxide (TIPOT) was mixed with isopropanol, in part to control an exothermic hydrolysis reaction of the titanium precursor. This solution was mixed with distilled water in a ratio of 1:2:40. The obtained dispersions were placed in an ultrasonic bath for 5 min in order to disperse particles. The water/alcohol proportion was 1:4.

2.3 PMMA preparation

PMMA was dissolved in diethyl oxalate, in a w/v proportion of 1:4. The polymer solution solvent was kept in a hot water bath for 180 min at a controlled temperature of 65 °C and with constant stirring. As a result, a transparent polymer-solvent solution was obtained, with an approximate viscosity of 64,000 cp. The other polymers (PVAC, PEG20000, HPC, and PCLA) underwent a similar procedure, but different solvents were tested. PVA was dissolved in distilled water in a proportion of 1:1 at 80°C.

2.4 Hybrid solution preparation: HIB1 (PMMA-TiOOH).

Obtained sol-gel dispersions were directly added to a PMMA solution and vigorously mixed. The hybrid solutions containing 5% of sol-gel titania (named: HIB1)at 10/80 was obtained with an approximate viscosity of 38,000 cp.

2.5 Abrasion resistance solution preparation: HIB1 (PMMA-TiOOH, Al2O3, ZrO2)

Two solutions were prepared by adding γ-Al2O3 and YSZ particles as an anti-abrasion reinforcement for hybrid materials. HIB1 hybrid solutions were mixed with 0.2 g of alumina particles (solution 1) and 0.4 g of YSZ particles (solution 2). These solutions were poured onto 304 stainless steel substrates and dried at room temperature. The coatings were then placed in an oven at 100 °C for 1 h. The optimum coating drying time and temperature were determined, establishing 110 °C and 30 min as needed for proper solvent evaporation and material densification. The coatings obtained with the hybrid solutions were applied by a doctor blade technique with a leveled knife and spatula. After applying the coatings, they were taken to a drying process. The obtained thicknesses ranged between 2 and 26 µm.

2.6 Characterization

High-resolution images were obtained with a high-resolution transmission electron microscope, using a TECNAIG2 F30 microscope. The PMMA-Ti(OH)4 hybrids were dispersed in ethyl alcohol, which was applied to an ultrathin 400 mesh carbon-coated grid. Powder X-ray diffraction (XRD) was used in order to characterize crystallographic phases of TiO2 with a Rigaku D/MAX-2500 diffractometer, using CuKα1 radiation (λ=1.5406). The scanning step size was 0.02° in 2θ with a scanning speed of 1 s per step. UV-vis spectroscopic analyses were essential to gathering information about the absorbance behavior of PMMA-Ti (OH)4 hybrids. UV-vis absorption spectra were obtained using a Cary 100 Scan Varian UV-vis spectrophotometer. XPS measurements were taken with a VG-Scientific Escalab 250 spectrometer. Scanning electron microscope (SEM) analyses were conducted with a JEOL JSM-5400 LV SEM. Reflectance spectra were obtained with an Ocean Optics USB 2000 spectrometer with integrator sphere and optic fiber QTY-1 P200-2-UV/VIS.

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3. Results and Discussion

The properties of Ti02 surfaces were analyzed through X-ray photoelectron spectroscopy (XPS). The representative high resolution of Cls spectrum for PMMA-Ti(OH)4 is shown inFig. 1a. The Cls spectra of PMMA-Ti(OH)4 powder were deconvoluted into a main component and three small components, with peaks lying at 284.5 eV (-CH2- groups), 285.85 eV (C-0 groups), and 288.5 eV (C(0)-0- groups) [8]. The binding energy (eV) values for Ti 3/2p and Ti 1/2p changed two units (Table 1), corresponding, according to some authors, to Ti4+ (~458.8 eV), Ti3+ (~457.1 eV), and TiO─ (~457.7 eV), indicating a titanium valence change [8,9].

a) b)

4100

4200

4300

4400

4500

4600

4700

4800

4900

5000

440450460470480

Cou

nts

/ s (

Res

id. ×

0.2

)

Binding Energy (eV)

Ti 2p 2045 Scans, 7 m 30.4 s, 500µm, CAE 20.0, 0.10 eV

Ti2p 1/2Ti2p 1/2

Ti2p3/2

Ti2p 1/2

c) d)

Fig. 1 XPS spectrum of HIB1-DO: a) C 1s, b) O1s, and c), d) Ti 2p B before and after UV irradiation, respectively. The XPS analysis shows a core-level spectrum of Ti 2p3/2of a prepared hybrid compound (Table 1). This spectrum can be deconvoluted into different curves, which can be ascribed to Ti4p bound to oxygen, with a peak at 458.8 eV, and a Ti ion with a higher negative charge density, with a peak at 457.7 eV. This overlap was consistent with the creation of Ti3+ on the surface upon UV irradiation [9], which was not present in the PMMA-DO-TiO2 sample because no photochromic effects existed. A plausible explanation for this lower-energy peak of Ti is a deprotonation of Ti-OH, resulting in Ti-O─ [10]. The presence of 463.8 and 465.3 eV peaks (Table 1) related to Ti-O bonds, which would indicate a disordered structure of the titanium lattice with the cation in an octahedral coordinated environment. Meanwhile, the 464 eV peak was related to a more ordered lattice. The peak at∼460 eV was associated with pure TiO2 anatase [11].

Table 1 XPS peaks of photochromic hybrids HIB before and after exposure to UV light.

Before UV After UV

Hybrid/ Ti Ti2p3/2 Ti2p 1/2 Ti2p 3/2red

Ti2p 1/2red

Ti2p SAT

Ti2p3/2 Ti2p 1/2 Ti2p B Ti2p 3/2red Ti2p 1/2red

PMMA-DO-Ti(OH)4

458.95 464.91

PMMA-TOL-Ti(OH)4

457.34 463.59 459 464.74 458.05 462.95

PMMA-DO-TiO2 461.47 466.92 459.46 465.21 461.24 PMMA-CHCl3-Ti(OH)4

458.37 464.2 456.96 462.98 460.8 458.65 464.31

0.00E+00

2.00E+03

4.00E+03

6.00E+03

8.00E+03

1.00E+04

1.20E+04

1.40E+04

1.60E+04

1.80E+04

2.00E+04

2.20E+04

2.40E+04

2.60E+04

280290300

Cou

nts

/ s (

Res

id. ×

5)

Binding Energy (eV)

C 1s 2030 Scans, 3 m 0.4 s, 500µm, CAE 20.0, 0.10 eV

C1s A

C1s BC1s C

C1s D

C-C

O-C=O C=O

π -π*

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The UV-Vis absorption spectrum of PMMA/Ti(OH)4 hybrid before and after UV irradiation is shown in Fig. 2. The film shows no significant absorption from 400 to 900 nm before UV irradiation. After exposure to UV light for 20 min, the film turned brown, and a broad intense absorption band was located at 440 nm. This band was attributed to an intensity enhanced by a d-d transition. According to Batal [12], it could indicate transition in the ions Ti4+ to Ti3+. This may arise from a2B2g→2B1g transition of 3d electrons of Ti3+ due to electronic transfer between PMMA and Ti(OH)4.

Fig. 2 Absorbance spectra of a hybrid of PMMA-titanium oxide sol-gel of 5% v/v.

Bandgap values of PMMA-Ti (OH)4 materials prepared with solvents of different flash points were estimated. It was observed that photochromic response was sometimes weak and at other times stronger. The difference in absorption wavelength among nanohybrids indicates a difference in band gap energies, presumably influenced by the type of solvent present in each photochromic hybrid. It is well known that the relationship between absorption coefficient, α, and optical band gap, Eg, for fine particles obeys Tauc’s expression [13-15]: (1) Where m is a parameter that denotes the nature of transition (1/2 for indirect), α0 is the linear absorption coefficient, E=hν is the incident photon energy, and A is a constant that does not depend on E. The band gap energy, Eg, can be thus determined from a Tauc plot of (α0hν) vs. (hν). Since TiOx is a semiconductor with a direct band gap but dipole-forbidden transitions, the value of m can be used as 3/2 [16]. Nonetheless, it can be found from Tauc’s plots for TiO2 films that band-to-band indirect transitions are more probable than direct transitions. Table 2 show extrapolations of parts of linear curves to the energy axis, estimating band gap energy of hybrids obtained with different compositions. The nanohybrid coatings containing AlOOH and Al2O3 had Eg comparable to those of pure anatase TiO2 thin films, in the range 3.20–3.23 eV [17,18]. In the case of PVA-TiO2 hybrids that use TiO2 DegussaTM

particles, the Eg was around 2.9 eV.

Table 2 Band gap values for photochromic hybrid coatings at different UV irradiation times.

Band gap energy at different irradiation times (eV)

Time after exposition to UV (h)

0 1/3 2/3 5/6 1 4/3 5/3 11/6 14

Solvent variation PMMA-CHCl3-Ti(OH)4 3.36 3.28 2.43 2.56 2.72 2.95 PMMA-DB-Ti(OH)4 3.22 2.79 2.19 2.79 2.79 3.16 PMMA-MF-Ti(OH)4 3.32 2.84 2.84 3.03 3.25 PMMA-THF-Ti(OH)4 3.36 3.12 2.6 3.015 3.12 3.31 Polymeric matrix used PVA-H2O-TiO2 2.77 2.74 PEG20000-CHCl3-Ti(OH)4 3.45 3.07 PVAc-DO-Ti(OH)4 2.97 2.8 PMMA-co-EMA-DO-Ti(OH)4 3.3 3.5 2.46 2.84 3.5 3.13 PMMA-co-AMA-DO-Ti(OH)4 3.21 2.94 2.98 2.94 2.94 ------ using AlOOH from different precursors PMMA-DO-Ti(OH)4-AlOOH (TSBAL)

3.3 3.37 3.31 3.144 3.3 ---

PMMA-DO-Ti(OH)4-γAl2O3 ---- 3.18 3.19 3.04 3.18 ---- PMMA-DO-Ti(OH)4-AlOOH (IPAL)

3.33 3.204 3.08 3.08 3.04 -- --

PMMA-DO-Ti(OH)4-SiO2-Al2O3 3.34 3.1 3.18 3.14 3.1 ---

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Fig. 3 shows a PMMA-Ti(OH)4 hybrid coating deposited onto an acrylic substrate (a) before and (b) after 20 min of UV irradiation. The color change produced by UV light can be seen as going from translucent to a dark brown.

a) b)

Fig. 3 a) Translucent PMMA-Ti(OH)4 hybrid coating deposited on an acrylic substrate: a) before, and b) after UV irradiation.

TEM images of PMMA/Ti(OH)4 hybrid particles are shown in Fig. 4. It can be seen that almost all TiOx was amorphous. TiO2 nanoparticles were showing incipient crystalline anatase lattices.

a) b) c)

Fig. 4 a) Sol-gel of a titanium precursor, b) hybrid of PMMA-titanium oxide sol-gel of 5% v/v, and c) the same of 10% v/v.

Fig. 5 presents SEM images of a PMMA-Ti(OH)4 hybrid coating. A non-smooth surface can be observed, which can be attributed to the formation of Ti(OH)4 and PMMA aggregates on the surface. The EDX spectra show the presence of Ti, C, and O, which composes this hybrid coating, and iron (Fe) detected from its substrate.

a1) a2)

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b1) b2)

Fig. 5 a) SEM of a hybrid of PMMA-Ti(OH)4 coating on a stainless steel substrate (500X), and b) corresponding EDX spectra.

XRD analysis (Fig. 6a) confirmed that heating a Ti(OH)4 sol-gel used for hybrid preparation yielded the presence of amorphous TiO2 at 300 °C, anatase TiO2 at 500 °C, and a predominant rutile phase with anatase at 800 °C. Rietveld analysis allowed identification of such structures of approximately 81.1% of rutile and 18.9% of anatase structures. Fig. 6b shows the anatase TiO2 structure with a unit cell included, and Fig. 6c shows the rutile TiO2 structure including a unit cell.

a) b) c)

Fig. 6 a) Diffractograms of TiO2/PMMA hybrids, b) TiO2rutile structure (unit cell included), and c) TiO2 anatase structure (unit cell included).

4. Conclusions

Organic-inorganic hybrid materials composed of sol-gel titanium oxide/organic polymers were obtained. These hybrid materials displayed a photochromic effect upon being exposed to UV light possessing energy just above the TiOx band gap. This was indicative of electron/holes participating in such observed changes, presumably by reduction of TiIV to TiIII at the amorphous titanium oxides. The coatings made with hybrid solutions underwent a change from a whitish to a brownish tonality. The effect was reversible, with a total return to original color in hours, according to intensity of exposure. The photochromic effect was present in hybrid materials of different kinds of solvents and polymers. Nonetheless, not all polymers showed such photochromic effects. The decreasing of sol-gel content in the hybrid matrix increases the translucency and intensifies its original effect. Bandgap calculation results indicate that all obtained hybrids were around TiO2 band gap values. HRTEM images showed incipient formations of nanocrystalline domains in sol-gel TiOx and hybrid solutions.

Acknowledgments: The authors gratefully acknowledge the financial support from the Mexican Council for Science and Technology (CONACyT, Grant CB-2009-01 133157). The first author acknowledges CONACYT for her graduate fellowship. Also, thanks to Dr. Carlos Angeles (IMP), Rosendo López (UAM-I), and Dr. Antonio Loredo (IMP) for their assistance in conducting analyses by HRTEM, UV-vis, and XPS, respectively. The authors are especially grateful to Darlene Garey of the US Peace Corps for her valuable suggestions for this work.

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