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Kinetics of Coloration in Photochromic Tungsten(VI) Oxide/Silicon Oxycarbide/Silica Hybrid Xerogel: An Insight into Cation Self-Diffusion Mechanisms Kenta Adachi, Masataka Tokushige, Kaoru Omata, Suzuko Yamazaki, and Yoshiaki Iwadate ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04115 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Kinetics of Coloration in Photochromic Tungsten(VI) Oxide/Silicon Oxycarbide/Silica Hybrid Xerogel: An Insight into Cation Self-Diffusion Mechanisms

Kenta ADACHI1,2*, Masataka TOKUSHIGE3, Kaoru OMATA4, Suzuko YAMAZAKI1, and Yoshiaki IWADATE5 Notes: The authors declare no competing financial interest

* To whom correspondence should be addressed. E-mail: [email protected]

(1) Department of Chemistry, Graduate School of Sciences & Technology for Innovation, Yamaguchi University, Yamaguchi, 753-8512, Japan. Tel & Fax: +81-83-933-5731. (2) Optics and Energy Research Center, Yamaguchi University (3) Department of Environmental Science & Engineering, Graduate School of Science & Engineering, Yamaguchi University, Yamaguchi, Japan. (4) Department of Biology and Chemistry, Faculty of Science, Yamaguchi University, Yamaguchi, Japan (5) Department of Biology, Graduate School of Sciences & Technology for Innovation, Yamaguchi University, Yamaguchi, Japan. ACS Paragon Plus Environment


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Abstract Silicon oxycarbide/silica composites with well-dispersed tungsten(VI) oxide (WO3) nanoparticles were obtained as transparent hybrid xerogels via an acid-catalyzed sol-gel process (hydrolysis/condensation polymerization) of 3-(triethoxysilyl)propyl methacrylate (TESPMA) and tetraethoxysilane (TEOS). The selfdiffusion mechanism of alkali-metal cations and the kinetics of the photochromic coloration process in the WO3/TESPMA/TEOS hybrid xerogel systems have been systematically investigated. Under continuous UV illumination, a gradual color change (colorless blue) corresponding to the reduction of W6+ into W5+ states in WO3 nanoparticles can be confirmed from the WO3/TESPMA/TEOS hybrid xerogels containing alkalimetal sulfates, although no coloration of the hybrid xerogel without alkali-metal sulfate was observed. The coloration behavior depended exclusively on a variety of alkali-metal cations present in the hybrid xerogel system. Furthermore, a detailed analysis of the self-diffusion mechanism confirmed that the alkali-metal cations electrostatically interact with a layer of unreacted silanol groups on the TESPMA/TEOS matrix surface, and subsequently pass through the interconnected pore network of the hybrid xerogel. More interestingly, in the context of an Arrhenius analysis, we found a good coincidence between the activation energies for alkali-metal cation self-diffusion and UV-induced coloration in the WO3/TESPMA/TEOS hybrid xerogel system containing the corresponding alkali-metal sulfate. It is experimentally obvious that the photochromic properties are dominated by the diffusion process of alkali-metal cations in the WO3/TESPMA/TEOS hybrid xerogel system. Such hybrid materials with cation-controlled photochromic properties will show promising prospects in applications demanding energy-efficient “smart windows” and “smart glasses”.

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1. INTRODUCTION Photochromism describes a phenomenon of material color change in a persistent but reversible manner produced by photochemical induced reactions.1 Photochromic materials have gained extensive interests because of their potential applications in optical switches, optically rewritable data storage, and chemical sensors.2,3 The most important requisites of the materials here are high chromogenic efficiency, short response time, long operating life time, and reduction of energy consumption.4 There are two major types of photochromic materials, inorganic and organic. Organic photochromic materials based on extended π-electron systems such as spiropyrans5, benzochromenes6, and spirooxazines7 are well-known. On the other hand, the photochromic phenomenon in the inorganic systems is also observed in the natural minerals (e.g. hackmanite8). Under the ultraviolet (UV) light, the color change of the metal oxide semiconductors (MOSs) such as tungsten(VI) oxide (WO3)9, molybdenum(VI) oxide (MoO3)10, and vanadium(V) oxide (V2O5)11 means variation in transmittance and/or reflectance change in visible range, which is originated from different electronic absorption bands according to a switching between oxidation and reduction state of material. Generally, MOSs are exhibiting a slower response to potential switching than organic photochromic materials.12 Ever since the first publication by Deb in 1973,13 WO3 is most typical photochromic MOS material and has been the subject of much research investigation to characterize the physical and chemical mechanisms that are responsible for its coloration and this is of fundamental importance to understand the chromogenic behaviors.14,15 In brief, WO3 is an indirect wide band gap semiconductor (~3.2 eV) and can switch reversibly between colorless and blue color by UV light exposure and dark storage,16 which induces the electrons and counter-cations such as proton (H+), lithium (Li+), sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+) to insert into or extract from the crystalline WO3 octahedral sites at the same time.17 Recently, to overcome the weak points of WO3, many studies have been devoted on the nano-hybridization of WO3 materials and various organic and/or inorganic matrices.18-21 It should be noted here that the



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supported WO3 nanomaterials into a matrix network via hydrogen bond and/or chemical bond exhibit dramatically improved photochromism with fast response under UV irradiation, but it is still a matter of debate what is the most relevant parameter in the hybrid system. Indeed, many factors would influence UVinduced coloration, including the disorder and conductivity of the WO3 nanomaterials, the morphology of the matrices and the associated changes in ionic diffusion within that morphology, and ion insertion kinetics. The sol-gel technique is widely used for preparing inorganic and organic/inorganic hybrid materials starting from a chemical solution (sol) acting as the precursor for an integrated network (gel) of discrete particles, which in most case involves in the metal alkoxides of the desired oxide materials.22 The resulting gel is an interconnected rigid network with pores of nano-to-submicrometer dimensions.23 When the gel is dried under ambient conditions, shrinkage of the pores occurs, eventually yielding a monolithic material being termed as “xerogel”. In some xerogels, flexibility of a sol-gel material is desirable to avoid cracking during the drying process.24 The homogeneous xerogels encapsulating nanoparticles can be also successfully fabricated through a “sol-gel” strategy. Encapsulated metal and/or metal oxide nanoparticles play a pivotal role in a number of enormous relevance in optics and electronics, and also as a novel antimicrobial.25 The encapsulation of the nanoparticles within sol-gel architectures would minimize agglomeration due to their very high surface energy which results in rapid decrease in their unique shapedependent properties and functionalities.26 Be inspired by the tailoring sol-gel technique, we develop a novel highly-transparent xerogels encapsulating photochromic nanomaterials to investigate the photochromic response process as well as other kinetic relations. In this study, the photochromic silicon oxycarbide (RSiO3/2, R: organic substituent)/silica (SiO2) hybrid xerogels with well-dispersed WO3 nanoparticles were successfully synthesized via a sol-gel procedure using 3-(triethoxysilyl)propyl methacrylate (TESPMA) and tetraethoxysilane (TEOS) as silicon oxycarbide and silica precursors, respectively. TESPMA was used to avoid excessive shrinking and cracking of the hybrid xerogel. The unique surface and structural features and cation-controlled photochromic nature


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of the WO3/TESPMA/TEOS hybrid xerogels are described, and discussed below. In particular, the effects of the differences in alkali-metal cation diffusion behavior on the coloration kinetics of encapsulated WO3 nanoparticles in the TESPMA/TEOS matrix were studied in detail. The importance of alkali-metal cation for the photochromic effect was evident in the hybrid xerogel system. The results presented here provide an opportunity to address the fundamental studies of photochromic nanoparticles supported in the sol-gel matrices for advanced chromogenic and energy storage/conversion applications.



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2. EXPERIMENTAL 2.1. Materials. Tetraethoxysilane (TEOS), concentrated sulfuric acid (conc. H2SO4), lithium sulfate (Li2SO4), sodium sulfate (Na2SO4), potassium sulfate (K2SO4), and cesium sulfate (Cs2SO4) were purchased from Wako Pure Chemical (Osaka, Japan). 3-(Triethoxysilyl)propyl methacrylate (TESPMA) was obtained from Tokyo Chemical Industry (Tokyo, Japan). Sodium tungstate(VI) dihydrate (Na2WO4·2H2O), concentrated hydrochloric acid (conc. HCl) (Analytical Reagent, Wako Pure Chemical, Osaka, Japan), and dialytic membranes with 3500 dialytic modulus (Spectrum Laboratories, CA, USA) were used to prepare tungsten(VI) oxide (WO3) colloid nanoparticles. These materials were used as received without purification. The water used in this study was first distilled and then passed through a Milli-Q system (Millipore, USA), resulting in the specific resistivity of 18.2 MΩcm.

2.2. Preparation of WO3 Colloid Aqueous Solution. The preparation procedure of WO3 colloid solution is based on the techniques reported previously.27 Conc. HCl (9.7 mL, 0.7 M) was added drop by drop to a Na2WO4 solution (90 mL, 0.027 M) under magnetic stirring. A transparent aqueous colloid solution was obtained, which was then closed in a dialytic membrane pipe and dialyzed in a 1000 mL beaker containing Milli-Q water for a period of 8 hours. The deionized water was periodically replaced until Cl ions could not be detected by ion chromatography analyzer (PIA-1000, Shimadzu, Japan) equipped with an anion-exchange column (Shim-pack IC-A1, Shimadzu, Japan). The concentration of tungsten component in the WO3 colloid aqueous solution was determined by inductively coupled plasma-atomic emission spectrometry (Liberty Series II, Varian, USA). The as-prepared WO3 colloid solutions were stable for at least one month at room temperature, yielding excellent processability, and refrigerated at 4ºC until used in the experiments. The dried WO3 nanoparticles were analyzed by X-ray diffraction (RINT-2500, Rigaku, Japan) with CuKα radiation (40 kV, 100 mA) from


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2θ = 5° to 60° with scan speed of 5°min-1. The XRD pattern was indexed to a WO3·2H2O layered structure in accordance with the JCPDS card No. 18-1419 (see Fig. S1 in Supporting Information). The band gap energy of the as-prepared WO3 colloid nanoparticles was determined as to be 3.22 eV (see Fig. S2 in Supporting Information). From TEM measurement with a JEM-2100 instrument (JEOL, Japan), the resulting products are composed of almost spherical particles with diameters over the range 8-26 nm (see Fig. S3(a) and (c) in Supporting Information).

2.3. Synthesis of WO3/TESPMA/TEOS Hybrid Xerogels. The WO3/TESPMA/TEOS hybrid xerogels were prepared via the sol-gel process. The detailed procedure of synthesis of hybrid xerogel pellets can be described as follows: (i) Dilute sulfuric acid was added drop by drop into the WO3 colloid aqueous solution until pH 2 was reached. (ii) This aqueous solution was stirred at room temperature for 30 min, then mixed with fixed amount of alkali-metal sulfates (a molar ratio of WO3/alkali-metal sulfate = 100000) and stirred for another 30 min. (iii) The obtained acidic WO3 aqueous solution containing alkali-metal sulfates was dropped into the WO3/TESPMA/TEOS mixture, continuing stirring until a homogeneous solution was obtained. Herein, sulfuric acid acted as a catalyst for the hydrolysis/condensation of TESPMA and TEOS. A molar ratio of TESPMA/TEOS/WO3/H2O = 100/20/1/10 was used to obtain a hybrid colloidal solution (sol). (iv) The WO3/TESPMA/TEOS hybrid xerogels were obtained by pouring the sols into plastic molds closed with a hollowed cover to permit the slowly evaporation of the water and ethanol formed as a by-product of hydrolysis. After 7 days at 25ºC, all samples were gelled. The samples were then allowed to dry in a thermostatic chamber (25ºC, 40%RH; SH222, ESPEC, Japan) for 2-3 weeks until no changes in weight were detected.



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2.4. Characterization of Transparent WO3/TESPMA/TEOS Hybrid Xerogels. IR spectra were obtained in an AVATAR 370 spectrometer (Thermo-Nicolet, USA) in KBr tablets, measuring 400-4000 cm-1. An average of 32 scans was obtained with a resolution of 4 cm-1. The solid-state CP/MAS NMR analysis for silicon-29 (29Si CP/MAS NMR) was performed on a JNM-CMX-400 spectrometer (JEOL, Japan) at a resonance frequency of 79.42 MHz, with a pulse width of 45°, and a recycle delay of 100 sec. In all cases, highpower proton decoupling was used, and the chemical shifts were externally referenced to tetramethylsilane. Deconvolution of spectra was performed by using a Gaussian function on an ORIGIN software version 8.0 (Originlab, USA). According to the Glaser's nomenclature,28 the silanes are denoted in this article, using Qm (m= 0, 1, 2, 3, 4) and Tn (n= 0, 1, 2, 3) where m and n refer to the number of siloxane bridges bonded to the silicon atom. The ceramic yield (=[weight of ceramic residue]/[weight of pyrolysis charge]×100) was determined by TGA/DTA analysis. This was carried out in the temperature range of 25-1000°C with temperature ramp of 120°C/min using oxidant atmosphere in a SII Diamond TG/DTA Thermogravimetric/Differential Thermal Analyzer (Perkin Elmer Instruments, USA).

2.5. Diffusion Measurements of Alkali-Metal Cations in WO3/TESPMA/TEOS Hybrid Xerogels. Side-by-side static diffusion cell (Keystone Scientific, Japan) was used as illustrated in Scheme 1(a). Disc-shaped diffusion hybrid xerogel specimens without any alkali-metal sulfates (approximately 1 mm thick and 26 mm diameter) were prepared by the procedure described above. The specimen surfaces were lightly grounded using 600 grit ultrafine silicon carbide sandpapers and then ultrasonically cleaned to remove debris before the disc specimens were mounted in diffusion cells. On completion of each set of diffusion measurements, the exact thicknesses of the hybrid material specimens (L) were determined by means of a micrometer caliper. The mechanical connection between diffusion cell and specimen were sealed using a PTFE sealant. The quality of the seal was checked in a dummy experiment, using an impermeable quartz glass in place of the hybrid specimen, and found to be satisfactory. Side I of the diffusion cell was filled with


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a solution of 0.5 M alkali-metal sulfate aqueous solution, and side II was filled with Milli-Q pure water. The diffusion experiment consisted of measuring the concentration of diffusant in side II of the cell as a function of time. In side I, the concentration was maintained constant for all time. The concentrations of cationic species in side II were monitored by ion chromatography analyzer (PIA-1000, Shimadzu, Japan) equipped with a cation-exchange column (Shim-pack IC-C1, Shimadzu, Japan). The diffusion cell was thermostatted by means of a water jacket at various operative temperatures.

2.6. In-situ Observation on Photochromic Coloration of WO3/TESPMA/TEOS Hybrid Xerogels. The coloration of the WO3/TESPMA/TEOS hybrid xerogels due to UV irradiation was measured by using a homemade optical system. The simplified schematic arrangement is shown in Scheme 1(b). A similar system was used for monitoring the photochromic kinetics of the WO3/cellulose hybrid films under UV irradiation.29 The optical system was constructed based on a black light featuring an emission between 320 and 400 nm (λmax = 366 nm, FL4BLB(4 W)×4, Toshiba, Japan), a synthetic quartz (SQ) optical glass sample holder having dimensions 8×5×0.3 cm3 with transmittance ~95% in 300-1000 nm (Edmund Optics, USA), and a miniature 16-bit CCD-array spectrophotometer (USB 4000, Ocean Optics, USA). For the absorption measurement, collimated light beam of a tungsten halogen light source (20 W; DH-2000, Ocean Optics, USA) through an optical fiber (QP600-2-UV-VIS, Ocean Optics, USA) was illuminated to the hybrid sample placed on the SQ sample holder from downside, and the transmitted light was then collected with the upside condenser lens and introduced to the spectrophotometer through another optical fiber (QP600-2-UVVIS, Ocean Optics, USA). The background absorbance due to the SQ sample holder was subtracted from the total absorbance to get the corrected photochromic spectrum. Measurements were performed in a thermostated chamber (SH-222, ESPEC, Japan) at 25, 40, and 50±1ºC.



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2.7. Other Apparatus The TEM image of WO3 nanoparticles was measured with a JEM-2100 instrument (JEOL, Japan). The TEM sample was prepared by dropping the WO3 colloid sample solution onto a copper grid covered with a carbon film. Each size distribution histogram of WO3 nanoparticles was assessed by averaging the size of 100 particles directly from TEM images. The pH values of the aqueous phase were conducted using a F-14 pH meter (HORIBA, Japan) equipped with a 6366-10D glass electrode.

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3. RESULTS AND DISCUSSION 3.1 Chemical and Structural Characterization of WO3/TESPMA/TEOS Hybrid Xerogels. 3.1.1 Fourier Transform Infrared Analysis. The sol-gel process is based on inorganic polymerization chemistry. Hydrolysis of the alkoxysilane groups is followed by condensation to form the siloxane network.22 Hydrolysis: Si−OR + H2O

Si−OH + R−OH

(1)

Condensation:

2 Si−OH

Si−O−Si + H2O

(2)

In order to analyze the internal siloxane gel structures, the FTIR spectrum of the WO3/TESPMA/TEOS hybrid xerogel, compared with those of the corresponding virgin TESPMA and TEOS samples, were measured using the KBr tablet method. There appear various characteristic vibrations of different functional groups in the xerogels. Figure 1(b) exhibits typical FTIR spectrum of the hybrid xerogel obtained in this work. As references, the FTIR spectra of virgin TESPMA and TEOS (nonhydrolyzed) are also shown. As shown in Fig. 1(b), no intense bands ascribed to non-hydrolyzed TESPMA and TEOS (796, 1080, 1101, and 1176 cm−1 due to ethoxysilane moiety)30 were observed in the collected spectrum of the WO3/TESPMA/TEOS hybrid xerogel. The 804 and 1084 cm−1 peaks are related to the symmetric stretching and asymmetrical stretching of Si–O–Si bonds, respectively. The shoulder at ca. 1200 cm-1 is due to the in-phase movement of Si–O–Si bonds. The hybrid xerogel has a sharp band in the FTIR spectra at 959 cm–1, which corresponds to the Si–OH stretching vibration.31 As seen in Eq.(2), the Si–OH band represents the incomplete condensation of Si–OH groups and is sometimes referred to as a “defect band” within the silica network structure.32 This phenomenon clearly reveals the hydrolysis and condensation of TESPMA and TEOS. Moreover, the absorption peaks at 1713 and 1652 cm−1 are attributed to the C=O symmetric vibration of the methacrylate moieties in TESPMA and the bending vibration ACS Paragon 11/42 Plus Environment


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absorption peak of H–O–H which is derived from the pore water and surface absorbed water, respectively.30 The FTIR result indicates that both organic and inorganic structural units are present in the obtained hybrid xerogel. It is worth stressing that there is no difference between the WO3/TESPMA/TEOS hybrid xerogels containing various alkali-metal sulfates (Li2SO4, Na2SO4, K2SO4, or Cs2SO4, not shown).

3.1.2 Solid-State 29Si CP/MAS NMR Analysis. The solid-state cross polarization/magic angle spinning (CP/MAS) NMR technique for silicon-29 (29Si) can identify the surroundings of silicon atoms in the WO3/TESPMA/TEOS hybrid xerogel. Figure 2 29

Si CP/MAS NMR spectrum of the WO3/TESPMA/TEOS hybrid xerogel. In

displays the solid-state

condensed siloxane species for TEOS, silicon atoms through mono-, di-, tri-, and tetra-substituted siloxane bonds are designated as (SiO)1Si(OR)3 [Q1], (SiO)2Si(OR)2 [Q2], (SiO)3Si(OR)1 [Q3], and (SiO)4Si [Q4], respectively, where R is ethyl group in this study. The chemical shifts of Q1, Q2, Q3, and Q4 are −82, −91, −100, and −110 ppm respectively, and are in good agreement with the literature.28 For TESPMA, mono-, di-, and tri-substituted siloxane bonds are designated as (SiO)1(R’)Si(OR)2 [T1], (SiO)2(R’)Si(OR)1 [T2], and (SiO)3(R’)Si [T3], respectively, where R’ is methacrylate substituent used in this study. The chemical shifts of T1, T2, and T3 are −49, −58, and −68 ppm, respectively, and conform to the literature values.28 From the peak deconvolution by a nonlinear least square method using Gaussian function, the Q’s and T’s integrated peak area ratios (Q1/Q2/Q3/Q4 and T1/T2/T3) of the WO3/TESPMA/TEOS hybrid xerogel were determined to be 2/9/64/25 and 11/27/62, respectively. It can be noticed that the hybrid xerogel prepared in this study contains a large distribution (>60%) of Q3 and T3 species. As seen in Fig. 2, the presence of Q1, Q2, Q3, T1, and T2 species is evidence of incomplete hydrolysis and condensation in the hybrid xerogel system. The results of solid-state

29

Si CP/MAS NMR study are in agreement with the ones of FTIR examinations

described above. In the solid-state 29Si CP/MAS NMR spectra of the WO3/TESPMA/TEOS hybrid xerogels


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containing various alkali-metal sulfates, a clear distinction cannot be made upon the basis of Q and T species’ distribution (not shown).

3.1.3 Thermogravimetric Analysis. To evaluate the thermal behaviors, we carried out the thermogravimetric (TG) analysis of the WO3/TESPMA/TEOS hybrid xerogel as shown in Fig. 3. The TG profile exhibited a three-step weight loss. The first rapid weight loss of approximately 8 % up to 200ºC is attributed to the loss of absorbed water in the hybrid xerogel. The second weight loss around 200-400°C is assigned to further condensation of remaining Si−OH groups leading to Si−O−Si linkages33, and the third weight loss around 400-700°C is likely due to decomposition of the organic groups. Finally, nearly flat TG curves are obtained over 700°C. The ceramic yield of the hybrid xerogel was 78.5%.

3.2 Self-Diffusion of Alkali-Metal Cations in WO3/TESPMA/TEOS Hybrid Xerogel: Influence of Cation Size and Temperature. The diffusion into porous materials such as xerogels can be derived from the solution of Fick’s second law in one dimension34,

 ∂ 2C  ∂C = D  2  , ············································································ (3) ∂t  ∂x  where D is a diffusion coefficient, and C is the concentration of the diffusing species. The large area (26 mm diameter) and slab (1 mm thick) xerogels used in this study would meet the one-dimensional diffusion model. In the WO3/TESPMA/TEOS hybrid xerogel system, the boundary condition is supposed to consist of a uniform initial concentration within the pores of the xerogel equal to that in side II of diffusion cell, and a constant concentration outside the pores corresponding to the alkali-metal sulfate concentration in side I (see Scheme 1(a)), the diffusible process can be determined as,35,36



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 ∞   8 (2n + 1) 2 2    Cside II (t ) = Cside I − Cside I 1 − ∑  exp D π t   , ············· (4) − 2 2 2  4 L  n=0  (2n + 1) π    where Cside II(t) is the time-dependent concentration of diffusing species in side II, Cside I is the concentration of diffusing species in side I, and L is the xerogel sample thickness. Comparative kinetic experiments have been performed to study the effect of alkali-metal cation species and temperature on the self-diffusion rate in the WO3/TESPMA/TEOS hybrid xerogel system. Typical results from diffusion experiments at 25, 40, and 50ºC are shown in Fig. 4. After an initial delay, which means that alkali-metal cation diffusion becomes established across the thickness of the hybrid xerogel specimen, there is a linear increase with time (t) in the alkali-metal cation concentration of the solution in side II of the diffusion cell (Cside II). Incidentally, we confirmed that the cation concentration in side I (Cside I) was remained effectively constant over the period of measurements. These profiles show that the self-diffusion rate increases with temperature, which means that the mobility of the cations in the hybrid xerogel increases with temperature. From the given steady-state diffusion rate (the slope of the rectilinear part of Cside II(t) vs. t plot), the apparent diffusion coefficient (Dapp) can be calculated with the mathematical relation derived from Fick’s solution for the appropriate boundary conditions.37,38

 Cside II (t )  LV  ,····································································· (5) Dapp =   C ⋅t  A  side I  where A is the effective cross-sectional area of the xerogel sample and V the volume of solution in side II. This equation fitted the results quite well, as can be seen from a comparison between the experimental and fitted profiles in Fig. 4. From the diffusion data obtained at different temperatures, the Dapp values for various cations have been calculated using Eq. (5), and summarized in Table S1 (see Supporting Information). Each value is an average of five independent measurements. The maximum deviation for sets of data was approximately 10-15 % of the mean value. The Dapp value inside the WO3/TESPMA/TEOS hybrid xerogel


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decreases in the order of Li+ > Na+ > K+ > Rb+ > Cs+ at any temperature. As shown in Fig. 5, values of ln Dapp against 1/T are plotted according to Arrhenius equation:  − Ea (diffusion )  Dapp = D0 exp  , ························································ (6) RT   where D0 is the pre-exponential diffusion coefficient, Ea(diffusion) is the activation barrier for the diffusion process of the cations in the hybrid xerogel, R is the gas constant (8.31447 J·K-1 mol-1), and T is the absolute temperature. The Ea(diffusion) values has been calculated from the slope of the plots (see Table 1). Compared with the cation self-diffusion in the metal oxide crystal,39-41 the cation transport within the WO3/TESPMA/TEOS hybrid xerogel proceeds at a much higher rate, suggesting that the cation diffusion process occurs via a pore diffusion mechanism rather than a solid state diffusion mechanism.42,43 It also indicates the interaction between cation species and the internal surface sites of the hybrid xerogel. In addition, since the Si−O−W bridge species between WO3 and silicon oxycarbide/silica possess large surface acidity, probably with both Lewis and Brønsted acid sites,44 cations may be tightly adsorbed on the surface of WO3 nanoparticles immobilized within the hybrid xerogel. Moreover, the calculated Ea(diffusion) values increase as the cation radius increases. Interestingly, the order of these (Li+ > Na+ > K+ > Rb+ > Cs+) is the parallel to the one of hydrated radius and hydration energy for alkali metals (see Table 1).45 As will be discussed later in this paper, this may be due to the generally stronger electrostatic interaction of smaller hydrated alkali-metal cations with the binding sites in the WO3/TESPMA/TEOS hybrid xerogel system, thus reducing their mobility in the hybrid xerogel.46

3.3 In situ UV-Vis Absorption Investigation of Photochromic Process of WO3/TESPMA/TEOS Hybrid Xerogel. Figure S4 shows the absorption coefficient α of the WO3/TESPMA/TEOS hybrid xerogel as a function of the incident photon energy hν in a (αhν)0.5 vs. hν plot (see Supporting Information). The linear increase towards higher energies clearly demonstrates an indirect interband transition. From a linear ACS Paragon 15/42 Plus Environment


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extrapolation of the increase at higher energies (dotted line in inset, Fig. S4), the band gap energy of WO3 in the hybrid xerogel was determined as to be 3.28 eV, which is somewhat higher than the band-gap energy of crystalline WO3·2H2O (3.2 eV).13 Because several experimental results indicate that the energy band gap increases with reducing grain size in nanostructured WO3,47 this band gap blue shift can be attributed to the quantum confinement size effect. According to Nedeljkovic’s size rule,48 the shift value corresponds to a mean-crystallite diameter of ca. 14 nm, which closely agrees with the TEM observation of as-prepared WO3 colloid nanoparticles (see Fig. S3). This result indicated that the WO3 colloid nanoparticles were welldispersed (non-agglomerated) in the hybrid xerogel. The

preliminary

experiments

were

carried

out

to

check

the

photochromism

in

the

WO3/TESPMA/TEOS hybrid xerogel system. A photograph of the WO3/TESPMA/TEOS hybrid xerogels containing Li2SO4 is shown in Fig. 6, as a typical example. All as-prepared hybrid xerogels were transparent and colorless with a given hardness, regardless of the type of added alkali-metal sulfate (see Fig. 6, left side). After UV irradiated for a certain time, they changed to blue color (see Fig. 6, right side). The colored hybrid xerogels became colorless in dark room at room temperature overnight. The photochromic reaction of the hybrid xerogels takes place under ambient conditions. Note that the hybrid xerogel without alkali-metal sulfate did not show such the color change. The importance of alkali-metal sulfate for the photochromic effect seems evident in the hybrid xerogel system. Figure 7(a) shows the typical UV-Vis spectral change of the WO3/TESPMA/TEOS hybrid xerogel containing Li2SO4 as a function of the elapse time of UV irradiation. Before irradiation, there was no absorption in visible region. After exposure under UV light, the transparent blue-colored hybrid xerogel had two broad intense absorption bands at ca. 640 and 940 nm, which can be clearly assigned to the W5+W6+ intervalence charge transfer (IVCT), the characteristic absorption band of “heteropolyblues”, indicative of the formation of reduced tungsten species.49 Figure 7(b) proves the good reversibility of the colorationdecoloration cycling of the hybrid xerogel containing Li2SO4. In order to investigate the coloration behavior


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of the WO3/TESPMA/TEOS hybrid xerogels containing alkali-metal sulfate in detail, absorbance change was analyzed using a thermoregulator. The dynamics of typical coloration processes conducted at various temperatures (25, 40, and 50ºC) are illustrated in Fig. 7(b)-(d), in which absorbance value at 640 nm (A640) is plotted against UV irradiation time. For the WO3/TESPMA/TEOS hybrid xerogels containing Li2SO4, a rapid increase of A640 value is obtained, whereas the hybrid xerogel containing Cs2SO4 shows a relatively slow initial coloration rate. Herein, the initial coloration rate (r0color [min-1]) is given by the following equation. 0 rcolor =

dA640 dt

. ·········································································· (7) t =0

The actual r0color values were assessed by differentiating the quadratic equation well-fitted to the observed points (the correlation coefficients R2 > 0.98), and also listed in Table S1 (see Supporting Information). Interestingly, the UV-induced coloration in this hybrid xerogel system containing alkali-metal sulfate is thermally activated, as can be seen from the comparison of Fig. 7(b)-(d) and Table S1. Therefore, the temperature dependence of r0color values would be expressed by an Arrhenius-type exponential function,  − Ea (color)  0 rcolor = A exp   , ······························································ (8) RT   where A is the frequency factor and Ea(color) is the apparent activation energy for the coloration process in this the WO3/TESPMA/TEOS hybrid xerogel system. As demonstrated in Fig. 8, the temperature dependence excellently showed an Arrhenius-type behavior. Ea(color) values were calculated from the slopes of Arrhenius plots derived from in situ spectral studies for each alkali-metal sulfate, and were summarized in Table 1. The coloration activation energies of the WO3/TESPMA/TEOS hybrid xerogel decreased in the following order: Cs2SO4 > K2SO4 > Na2SO4 > Li2SO4. The alkali-metal sulfate with a small cation radius has much smaller activation energy for the UV-induced coloration in the hybrid xerogel system than that with a large one (see Table 1). The size effect of the alkali-metal cation on the initial coloration rate is obvious. Furthermore, Ea(color) values calculated from in-situ spectral data are very similar to Ea(diffusion) ones ACS Paragon 17/42 Plus Environment


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obtained for the corresponding alkali-metal cation in the hybrid xerogel. This finding could lead to better understanding of the inductive effect of alkali metal cations on the photochromic behaviors in the WO3/TESPMA/TEOS hybrid xerogel system.

3.4

Estimated

Mechanism

for

Alkali-Metal

Cation-controlled

Photochromism

in

the

WO3/TESPMA/TEOS Hybrid Xerogel. The characteristic color and absorption spectral changes caused by UV light exposure are quite similar for the WO3/TESPMA/TEOS hybrid xerogels containing various alkali-metal sulfates. However, a deeper kinetic investigation on the photochromic and alkali-metal cation self-diffusion properties of the hybrid xerogels reveals significant differences. The differences are very important in properly understanding and accounting for the WO3 photochromic phenomena in the hybrid xerogel system, and will potentially aid in development of novel semiconductor-based photochromic materials and devices in the future. Using the above results of FTIR,

29

Si CP/MAS NMR, TG, cation self-diffusion, and in-situ UV-

Vis absorption, let us consider the mechanism for alkali metal cation-controlled photochromism in the WO3/TESPMA/TEOS hybrid xerogel. We provide evidence to support the following facts: (i) residual silanol (Si-OH) groups are remaining in the hybrid xerogel (FTIR and

29

Si CP/MAS NMR); (ii) free water

exists in the interconnected pore network of the hybrid xerogel (FTIR and TG); (iii) the apparent diffusion coefficient values calculated in the hybrid xerogel system, less than ones in the solid crystal, are interpreted as transport via a “pore” diffusion mechanism (cation self-diffusion); (iv) the apparent diffusion coefficient and initial coloration rate values depend more strongly on the cation size (cation self-diffusion and in-situ UV-Vis absorption); (v) there is good coincidence between the activation energies for alkali-metal cation self-diffusion and UV-induced coloration in the WO3/TESPMA/TEOS hybrid xerogel system containing the corresponding alkali-metal sulfate (cation self-diffusion and in-situ UV-Vis absorption).


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On the basis of above facts, the possible mechanism for alkali metal cation-controlled photochromism in the WO3/TESPMA/TEOS hybrid xerogel was summarized in Scheme 2. As alluded to earlier section, an important consideration in cation diffusion through porous hybrid xerogels is the surface properties of the matrix. The silicon oxycarbide/silica matrix surface is covered by a layer of acidic silanol (Si−OH) groups and different siloxane groups. To maintain charge neutrality during the diffusion of cations through the interconnected pores, the cations must either accompany the counter anions (in this study case, SO42-) in the pore network filled with water, or be exchanged at silica surface (see Scheme 2). For this reason, the cations present in the hybrid xerogel can have a critical effect on the diffusion behavior. According to the previous binding studies between alkali-metal cations and the ion-exchange resins50-52, the interaction strength of cations to ion-exchange resins is strongly dependent on the cation size. More specifically, assuming the key driving forces being essentially electrostatic, the cation with smallest hydrated cation radius will be able to approach most closely to the negative site (e.g. –COO-, –SO3-) on the surface, and thus will be held most strongly. The order of preference of the alkali-metal cations is Cs+ > Rb+ > K+ > Na+ > Li+ (see Table 1). However, the cations weakly attached to the surface will be eluted first; the elution order in cation exchange can be reversed (Li+ > Na+ > K+ > Rb+ > Cs+). As a result, the hydrated Li+ ion being largest ionic size, has the highest mobility in the ion-exchange resin matrix. On the other hand, the hydrated Cs+ ion being smallest in size has the lowest mobility in the ion-exchange resin matrix. This surface cation-exchange model clearly explains the difference in diffusion behaviors of cations in the WO3/TESPMA/TEOS hybrid xerogel. Additionally, the differences observed in the photochromic properties may also be explained from the dependence of the kinetics of self-diffusion of cations in the WO3/TESPMA/TEOS hybrid xerogel. Generally, the electron-hole pairs are generated in WO3 through exposure to ultraviolet light (hν > the band gap energy (Eg)). Upon the reduction of W6+ ions (transparent) into W5+ ions (colored) by the photogenerated electrons, insertion of the charge-compensating cations into the WO3 frameworks took place through the following process.53



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WO3 + xM+ + xe

6+

Page 20 of 42

5+

MxW1-xWx O3 ························································· (9)

where M+ can be Li+, Na+, K+, Rb+, and Cs+ in this study. The cation-inserted MxW6+1-xW5+xO3 is known as the blue-colored tungsten bronze.54 As can be seen in Eq. (9), cation species plays an important role in the photochromic reaction of WO3. Indeed, the photochromic behavior depends exclusively on a variety of alkali-metal cations present in the WO3/TESPMA/TEOS hybrid xerogel system. Moreover, the activation energy for the UV-induced coloration in the hybrid xerogel system containing cation salt was much the same as the one for the diffusion of corresponding cation (see Table 1). Therefore, it is reasonable to expect that the photochromic properties are dominated by the diffusion process of alkali-metal cations in the WO3/TESPMA/TEOS hybrid xerogel system. However, there are several speculations where the photochromic process according to Eq. (9) is diffusion-controlled, occurring via a pore diffusion mechanism, and the cations adsorbed on the surface of WO3 nanoparticle immobilized within the hybrid xerogel may act more effectively as charge-compensating counterions to trapped electrons as a result of the large acidity effect of Si−O−W linkages. Certainly, these speculations have to be confirmed, and appropriate diffusion and photochromic investigations are under way.


Page 21 of 42

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4. CONCLUTION Highly

transparent

and

photochromic

tungsten

oxide/3-(triethoxysilyl)propyl

methacrylate/tetraethoxysilane (WO3/TESPMA/TEOS) hybrid xerogels have been successfully prepared by the traditional sol-gel process. The as-prepared hybrid xerogels have ion transport path derived from the interconnected pore networks. The cation mobility in the hybrid xerogel was investigated in detail by a diffusion technique employing a side-by-side cell. Ionic diffusion in the hybrid xerogels was found to be greatly dependent on both the temperature and the size of alkali-metal cations. Moreover, the photochromic behavior of the WO3/TESPMA/TEOS hybrid xerogels containing various alkali-metal sulfates was investigated by in situ optical absorption spectroscopy. The initial coloration rate values of the hybrid xerogels also depended strongly on both the temperature and the size of cations. It should be noted that the Arrhenius analysis showed a good coincidence between the activation energies for alkali-metal cation selfdiffusion and UV-induced coloration in the WO3/TESPMA/TEOS hybrid xerogel system containing the corresponding alkali-metal sulfate. Our studies reveal that the impact of the cation diffusion behaviors on the photochromic performance of encapsulated WO3 in the sol-gel matrix is identified as crucial. Although a detailed model of the cation-controlled WO3 photochromic phenomena in the hybrid xerogel has yet to be proposed, present joint evidence favors a mechanism involving cations diffusing as charge-compensating counterions. Transparent WO3-based inorganic/organic hybrid xerogels with controlled photochromic behavior by cation diffusion hold a lot of promise for energy-efficient "smart windows" and large-area information displays. Such a study is already underway.



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ACKNOWLEDGEMENT This study was partially supported by the Scientific Research of the Grant-in-Aid for Young Scientists (B) (No. 24750069) and Scientific Research (C) (No. 16K05817) from Japan Society for the Promotion of Science (JSPS). ICP-AES and TEM measurements documented in this study were performed at the Center for Instrumental Analysis and the Innovation Center, respectively, Yamaguchi University.

SUPPORTING INFORMATION AVAILABLE XRD pattern of dried WO3; UV-Vis absorption spectra of as-prepared WO3 colloid solution and the WO3/TESPMA/TEOS hybrid xerogel; TEM images of the WO3 colloid particles; additional parameters of cation diffusion coefficients and UV-induced initial coloration rates in the WO3/TESPMA/TEOS hybrid xerogel system. This information is available free of charge via the Internet at http://pubs.acs.org.


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[54]

Lampert, C.M. Electrochromic Materials and Devices for Energy Efficient Windows. Sol. Energy Mater., 1984, 11, 1-27. ACS Paragon 26/42 Plus Environment

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[55]

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[56]

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Page 28 of 42

Figure Captions

Figure 1

(a) Chemical structures of alkoxysilane compounds (TEOS and TESPMA) used in this work.

(b) FT-IR spectra of TESPMA, TEOS, and WO3/TESPMA/TEOS hybrid xerogel. For peak assignment, see text.

Figure 2

Typical solid-state

29

Si CP/MAS NMR spectra of the WO3/TESPMA/TEOS hybrid xerogel.

Deconvolution of spectra was performed by using a Gaussian function. For symbol notations (Qm [m = 0, 1, 2, 3, 4] and Tn [n = 0, 1, 2, 3]), see text.

Figure 3

Typical TG analysis curve of the WO3/TESPMA/TEOS hybrid xerogel under oxidant

atmosphere from 25 to 1,000°C with temperature ramp of 120°C/min. For section classification, see text.

Figure 4

Time profiles of the normalized concentration of various alkali-metal cations in side II of side-

by-side diffusion cell during the cation diffusion through the WO3/TESPMA/TEOS hybrid xerogel at (a) 25°C, (b) 40°C, and (c) 50°C.

Figure 5

Plots of logarithmic apparent diffusion coefficients as a function of reciprocal absolute

temperature for various alkali-metal cations in the WO3/TESPMA/TEOS hybrid xerogel. Solid lines were obtained from the analysis with eqn (6).

Figure 6

Appearance of transparent as-prepared (left) and colored (right) WO3/TESPMA/TEOS hybrid

xerogels. (ca. 1 mm thick, ca. 26 mm diameter).

Figure 7

[top] (a) Absorption spectral change of the WO3/TESPMA/TEOS hybrid xerogel containing

Li2SO4 upon UV irradiation at 25°C. The absorption spectra were recorded at 1 sec intervals until the equilibrium was attained. [middle] (b) Reversibility of the WO3/TESPMA/TEOS hybrid xerogel containing Li2SO4 in the coloration-decoloration process at 640 nm. [bottom] Time profiles of the absorbance at 640 nm of the WO3/TESPMA/TEOS hybrid xerogels containing various alkali-metal sulfates during the UV irradiation at (c) 25°C, (d) 40°C, and (e) 50°C.

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Page 29 of 42

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Figure 8


Plots of logarithmic initial coloration rates as a function of reciprocal absolute temperature for

the WO3/TESPMA/TEOS hybrid xerogels containing various alkali-metal sulfates. Solid lines were obtained from the analysis with eqn (8).



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Page 30 of 42

Scheme titles

Scheme 1

Schematic diagram of (a) side-by-side diffusion cell apparatus and (b) experimental setup for

in situ UV-Vis adsorption spectroscopy. SQ is an abbreviation of synthetic quartz.

Scheme 2

Schematic drawing of ionic diffusion model for alkali metal cation-controlled photochromism

in the WO3/TESPMA/TEOS hybrid xerogel system.


Page 31 of 42

Figure 1

(a)

O O

O

O

O

Si O

O

O

Si O O

tetraethoxysilane

3-(triethoxysilyl)propyl methacrylate

[TEOS]

[TESPMA] 1084cm

(b) Absorbance / a.u.

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


-1

1198cm -1

WO 3/TESPMA/TEOS hybrid xerogel 1713cm -1 1652cm -1

959cm

1101cm -1 -1 1080cm -1 1176cm

-1

804cm -1

796cm

TESPMA TEOS

1800

1600

1400

1200

1000 -1

Wavenumber / cm


800

-1


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Page 32 of 42

Figure 2

T1

T2

T3

Q1 Q2 Q3 Q4

×10

-40

-60

-80 -100 Frequency / ppm


-120

Page 33 of 42

Figure 3

Residual weight / wt%

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100 90 80 1st

2nd

3rd

70 200 400 600 800 Temperature / °C


1000


Figure 4

Normalized ion conc. / ×10 -9 Mm-1

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Page 34 of 42

15

(a) : : : : :

10

(b)

(c)

+

Li + Na + K + Rb+ Cs

5

0 0

1000

2000

0

1000

2000

0

Relative Time / min.


1000

2000

Page 35 of 42

Figure 5

-23

ln Dapp

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-24

: : : : :

Li+ + Na + K + Rb + Cs

-25 3.1

3.2

3.3 -3

-1

1/T (×10 K )


3.4


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

Figure 6


Page 36 of 42

Page 37 of 42

0.3

UV irradiation

Absorbance

Figure 7

0.2

1500sec 1110sec 830sec 470sec 250sec 0sec

940nm

(a)

640nm

0.1 0

Absorbance at 640 nm ( A640 )

600 800 Wavelength / nm

1000

(b)

0.2

UV irradiation

Abs. at 640nm ( A640 )

400

0.1

in dark

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


0 0

(c)

0.3

1

2

3

: Li 2SO 4 : Na 2SO 4 : K 2SO 4 : Rb 2SO 4 : Cs 2SO 4 : without salt

7

(d)

8

9

10

(e) 0 r color

0

r color

0 r color

0.2

4 5 6 Cycle index

0.1

0 0

600

1200 0

600

1200 0

Irradiation time / sec


600

1200


Figure 8

-7 ln r 0color

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Page 38 of 42

-8 -9

: : : : :

Li 2SO4 Na 2SO4 K 2SO4 Rb 2SO4 Cs 2SO4

3.1

3.2

3.3 -3

-1

1/T (×10 K )


3.4

Page 39 of 42

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Scheme 1



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

Scheme 2


Page 40 of 42

Page 41 of 42

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Table 1 Cation radii55, hydrated cation radii56, hydration energies56, and apparent activation energies of cation diffusion [Ea(diffusion)] and coloration [Ea(color)] in the WO3/TESPMA/TEOS hybrid xerogel system.

cation radius55 / pm

hydrated cation radius56 / pm

hydration energy56 / kcal mol-1

Li2SO4

76

340

Na2SO4

102

K2SO4

alkali-metal sulfate

Activation energy / kJmol-1 Ea(diffusion)

Ea(color)

-121

20.4±1.7

20.1±1.9

276

-95

24.7±2.2

24.1±2.3

138

232

-76

30.7±1.6

32.6±2.1

Rb2SO4

152

228

-69

35.1±1.9

34.4±2.0

Cs2SO4

167

226

-62

37.7±3.3

38.2±2.3



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Silica Hybrid Xerogel - ACS Publications

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