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Novel silicate glasses in the acceleration of hydrogen diffusion for reducing dopant metal ions
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
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Review
Novel silicate glasses in the acceleration of hydrogen diffusion for reducing dopant metal ions ⁎
Masayuki Nogamia,b, , Xuan Hung Lea, Xuan Quang Vua a b
Laboratory on Optics and Spectroscopy, Duy Tan University, K7/25, Quang Trung, Da Nang, Viet Nam Nagoya Institute of Technology, Showa Nagoya, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen diffusion Glass Reduction Rare-earth Nanoparticle
Controlling valence state of metal ions doped in glasses, in particular with lower valence, has been widely applied for turning the optical properties. Although hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, the development of silicate glasses doped with lower valency metal ions is still a formidable challenge because of low diffusion rate of hydrogen gas in glass. Recently, the authors studied the reaction of metal ions doped in Na2OeAl2O3eSiO2 glass with hydrogen gas and succeeded in the preparation of the glass that exhibited fast hydrogen diffusion and reduced the metal ions to lower valance states. When the metal ions were doped in glasses containing Al2O3 with Al/Na > 1, the hydrogen gas diffusion coefficient was increased more than two orders of magnitude higher than that for glasses with Al/Na < 1 and the reduction of dopants concurrently caused the formation of AleOH bonds in the glass. These completely different reductions, occurred depending on the Al/Na concentration ratio, were discussed to relate with the glass structure and the question whether and how the dopants were reduced in glass was elucidated. The obtained results will help to both understand basic science of hydrogen diffusion process in glass and develop new optical devices.
1. Introduction Recently, doping glass with metal ions, such as rare-earth, transition metal and noble metal ions, has gained interest due to glass's excellent transparency, high dispersive power for dopants, and potential to be formed into various shapes at a low cost [1,2]. Among the rare-earth ions, Eu ions are attractive dopants for light-emitting devices because they exist in trivalent and divalent states in glass, resulting in strong fluorescence in the orange/red and blue wavelength regions, respectively [3]. Doping noble metal ions in nanoparticles has been applied in such photonic fields as phosphors, lasers, optical amplifiers, and sensors. One critical aspect of optimizing device performance is the careful control of the valence states of the dopant metal ions. In oxide glasses, although metal ions with multiple valence states can be stably doped, only higher valency doping leads to doped glasses suitable for practical applications. For example, borate or phosphate glasses doped with lower valency metal ions [4–7] are not appropriate for practical applications as they lack chemical and mechanical durability. The development of silicate glasses doped with lower valency metal ions is still a formidable challenge [6–8]. Special melting and/or secondary
⁎
heating techniques carried out in a reducing atmosphere are needed to incorporate metal ions with lower valence states. One such technique is melting the raw materials together with the reductants such as carbon powders in a reducing atmosphere [9,10], though contamination by the reductants remains a problem. Researchers have also reported on the secondary heat treatment of glass in a reducing atmosphere, in which the metal ions are reduced to lower valence states. However, the reduction is limited to near the surface, because of the low permeation rate of gas molecules in solid glass [11–14]. Thus, the development of silicate glasses doped with lower valency ions would be crucial for the practical implementation of these materials and could enable other glass applications. The authors have studied the reaction of metal ions doped in Na2OeAl2O3eSiO2 glass with hydrogen gas. The Na2OeAl2O3eSiO2 glasses are commercially available and present a challenge in overcoming the aforementioned difficulty. Generally, in this ternary glass system, the Al2O3 content is suppressed to < 10% because of the production temperature limitations. We observed that no metal ions were reduced even when these glasses were subjected to H2 gas at an elevated temperature, as described in Section 3. Recently, we successfully prepared glass that exhibited fast hydrogen diffusion and reduced the
Corresponding author. E-mail address: [email protected] (M. Nogami).
https://doi.org/10.1016/j.jnoncrysol.2018.10.003 Received 21 August 2018; Received in revised form 2 October 2018; Accepted 5 October 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Nogami, M., Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2018.10.003
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with Eu3+ ions were colorless to the naked eye, their optical spectra featured sharp but weak absorption bands at 394 and 465 nm in the visible wavelength region that were assigned to the 7F0 → 7L0 and 7 F0 → 5D0 transitions of the Eu3+ ion, respectively. The presence of Eu3+ ions is also evident from the fluorescence spectra, i.e., strong and sharp FL bands peaking at 570, 590, and 620 nm that were assigned to the 5D0 → 7F0, 7F1, and 7F2 transitions of the Eu3+ ions, respectively; this indicates that the Eu atoms primarily exist in the trivalent state. After the glass samples containing Al2O3 (where Al/Na content ratio > 1) were melted, a weak but broad FL band in the range 400–500 nm was observed in addition to the FL bands of Eu3+ ions. This broad FL band was assigned to the 4f65d → 4f7(8S7/2) transition of Eu2+ ions, indicating that some Eu2+ ions were stably incorporated into the glass. The same phenomenon was also observed for Mn3+ and Cu2+ ions. Therefore, we studied the following reductions; Eu3+ → Eu2+, Mn3+ → Mn2+, and Cu2+ → Cu0.
metal ions to lower valance states. The glass contained Al2O3 with Al/ Na > 1, in which the Eu3+, Mn3+, and Cu2+ ions were reduced to Eu2+, Mn2+, and Cu0, respectively, by heating in H2 gas. The reduction of Cu2+ ions was of interest; doped in the glass with Al/Na > 1, the Cu2+ ions were reduced to Cu atoms that migrated toward the glass surface, on which a film of Cu nanoparticles formed [18]. This one-step synthesis of Cu nanoparticles on a glass substrate has attracted interest in the field of nanoparticle applications. Based on the results of our study, we hypothesized that the Al3+ ions play an essential role in reducing the dopant. However, the H2 gas reaction remained unclear. Here, we expand the study to examine Mn3+ ion-doped glass and systematically discuss the results in the context of the H2 reaction and diffusion. This comprehensive paper will be important in understanding the basic science in gas reactions with glass and in the development of new optical devices.
2. Glass preparation and valence state of the metal ions dopants 3. Hydrogen diffusion-limited reduction of metal ions in glass with Al/Na < 1
Glass samples with compositions of (25-x)Na2O·xAl2O3·75SiO2 and (30-x)Na2O·xAl2O3·70SiO2 (x = 0, 8, 10, 12.5, 15, and 20 mol%) were prepared, in which Eu3+, Mn3+, and Cu2+ ions acted as dopants. The dopant concentration varied from 1 to 5 wt% in terms of oxide. The mixtures of initial raw materials (Na2CO3, Al(OH)3, SiO2, Eu2O3, MnO, and CuO) were melted in a covered corundum (α-Al2O3) crucible at 1500–1650 °C for 6 h in ambient atmosphere, after which they were removed and annealed at the glass transition temperature for 15 min, followed by natural cooling to room temperature in the furnace. The glass samples were cut into 1 × 10 × 15 mm pieces and polished by a slurry of ~1 μm diamond particles. Hereafter abbreviations will be used to refer to the samples; for example, 10N20A1Cu glass refers to 10Na2O·20Al2O3·70SiO2 glass doped with 1 wt% CuO. Reduction in H2 gas was carried out by heating the samples in a fused silica tube furnace under 100% H2 gas flow at ca. 5 ml/min. The valence states of the metal ions were investigated by analyzing the optical absorption, fluorescence (FL), and X-ray absorption nearedge structure spectra of the glass. In silicate glass, the metal ion dopants mainly take high valence states, i.e., Eu3+, Cu2+, and Mn3+, but can vary based on the glass composition. The metal ions are coordinated with oxygen ions, the valence states of which are affected by the affinity of the oxygen atoms surrounding the metal ions. Used as a measurement of the acid–base properties of oxide glass, the affinity of oxygen is related to the electron-donating power of the surrounding oxygen atoms to the metal ion. In the given Na2OeAl2O3eSiO2 glass, the Na+ ions act to increase the basicity, whereas both the Si4+ and Al3+ ions decrease it. A lower basicity of the glass leads to a smaller negative charge on the oxygen atoms, which is beneficial to the lower oxidation state of the metal atom. The glass samples doped with Mn3+ and Cu2+ ions were light brown and blue and exhibited optical absorption bands peaking at approximately 480 and 800 nm, respectively; these were assigned to the 5E → 5T2 transition of Mn3+ and the 2B1g → 2 B2g transition of Cu2+ ions, respectively. Although the glasses doped
Whether the doped metal ions are reduced or not upon heating in H2 gas is dependent on the ionization energies of the dopant ions and the composition of the host glass. For glass with Al/Na < 1 heated in H2 gas, the surface of the Mn3+-doped glass became transparent and the intensity of the 480 nm optical absorption band decreased, indicating the reduction of Mn3+ ions to Mn2+. Similarly, the surface of the Cu2+doped glass turned red and a sharp absorption band at 580 nm appeared. X-ray and electron diffraction spectra clearly showed the precipitation of nanosized Cu crystals in the glass, and the optical absorption band at 580 nm was attributed to the surface plasmon resonance (SPR) signal of Cu nanoparticles confined in the glass matrix. However, in the Eu3+-doped glass, no change was observed in the optical absorption and fluorescence properties; this was true even when the samples were subjected to H2 gas at high temperatures and for long periods of time, indicating that the Eu3+ ions were stable and not reduced. Fig. 1 shows the cross-sectional images of Mn3+ (a and b) and Cu2+ (c and d) ion-doped 20Na2O·10Al2O3·70 SiO2 glass samples (Al/ Na = 0.5) before and after heating in H2 gas; the results for the 10Na2O·15Al2O3·75SiO2 glass (Al/Na = 1.5) doped with Mn3+ ions are shown in Fig. 1 (e and f). The color change marked the sharp interface between the reduced layer (tarnished layer) and the bulk of the glass. The change in the tarnished layer thickness, as measured for samples heated in H2 gas at different temperatures and time periods, is shown in Fig. 2. It is evident that the tarnished layer thickness increased with heating time. Although it was independent of the type of dopants, the time dependence of the tarnished layer thickness varied strongly with the glass compositions. That is, the glass with Al/Na = 1.5 exhibited an order of magnitude greater change in the tarnished layer than the glass with Al/Na = 0.5, the reasons for which will be discussed in Section 6. In the inset of Fig. 2, the tarnished layer thickness is plotted as a
Fig. 1. Photographs of cross section of 20N10A5Mn (a and b) and 20N10A1Cu (c and d), and 10N15A5Mn (e and f) glasses before (a, c, and e) and after (b, d, and f) heating in H2 gas. Heating conditions are 600 °C/5 h (b and d) and 800 °C/2 h (f), respectively. → and ↔ indicate the sample surface and reduced layer, respectively. 2
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Fig. 3. Arrhenius plots of the diffusion coefficients determined from the thickness measurements of tarnishing layer (closed marks) for 20N10A1Cu (A), 20N10A5Mn (B), and 10N15A5Mn (C) glasses. Open marks are of the diffusion coefficients determined from the OH measurements for 10N15A5Mn (W), 10N15A5Eu (X), 10N20A1Cu (Y), and 15N15A1Cu (Z) glasses.
Fig. 2. Dependence of thickness of the reduced layer on the heating period in H2 gas for 20N10A5Mn (triangles), and 20N10A1Cu (squares), and 10N15A5Mn (circles) glasses. The inset shows the square-root of time dependence of the thickness.
function of the square root of the heating time, showing good linearity for both samples. This leads us to believe that the reduction process must be hydrogen diffusion-limited. A similar reaction was reported for oxide glass doped with polyvalent metal ions, such as Fe3+, Ni2+, and V5+ ions, where the H2 gas diffusion kinetics were analyzed using the tarnishing model [14,20,21]. More recently, Smedskjaer, et al. studied the reduction of Fe3+, V5+ and Eu3+/Yb3+ dopants in silicate glass and suggested that the inward diffusion of network modifying cations such as Ca2+/Mg2+ ions was induced by the reduction of metal ions [14,22–25]. However, that process would limit the reduction layer to less than few hundred nanometers from the sample surface since the heating was carried out in a diluted H2 gas atmosphere (1%). Our glasses were treated in a 100% H2 gas atmosphere, resulting in a deep reduction thickness of tens of microns. When the diffused H2 gas reacts with the metal ion dopants, the diffusion equation can be modified as follows [26,27]:
∂C ∂ 2C ∂S =D 2 − ∂t ∂x ∂t
(1)
∂S = RC, ∂t
(2)
determined by the glass compositions but independent of the dopants. Generally, it is known that the H2 gas molecules diffuse through gaps between the ions in a solid, the rate of which is primarily determined by the sizes of the H2 molecules (0.25 nm) and structural voids in the solid. To examine the effect of glass composition on the diffusion coefficient, glass samples with different compositions were prepared in the Na2OeAl2O3eSiO2 system. The diffusion coefficients of the H2 gas in the different samples were determined by using the tarnishing model, and the void content, i.e., the openness of the glass structure, was calculated by considering the glass density and the following ionic radii: Si4+, 0.04 nm; Na+, 0.116 nm; Al3+, 0.053 nm; and O2−, 0.121 nm [28]. The glass structure is thermally relaxed and also influences the diffusion. Therefore, in this study, diffusion coefficients were compared at the glass transition temperature; the effect of thermal relaxation due to compositional differences could be avoided. The results are shown in Fig. 4, where the diffusion coefficients reported for soda-lime silicate glasses are referred for comparison [11–13,29]. In those glasses, alkali and/or alkaline-earth ions occupy the interstitial positions in the SieOeSi network structure, resulting in a closed structure. The openness of the reference glass is lower than 0.62, and the values of our glasses with Al/Na < 1 were also within this range. The highest diffusion coefficient of the low openness glass (0.62) was 10−14 m2/s. This is not sufficient for practical glass applications because it indicates a required 200 day heating period to achieve reduction at the center of a 1 mm thick sample. On the other hand, it is apparent that the 10Na2O·15Al2O3·75SiO2 glass doped with Mn3+ ions exhibited an extremely high diffusion coefficient of 7 × 10−12 m2/s. The openness of the glass was 0.635 and the corresponding diffusion coefficient, ~5 × 10−14 m2/s, was estimated by extrapolating from the linear relation. The obtained diffusion coefficient is more than two orders of magnitude higher than the estimated value, strongly suggesting that in addition to the openness, something else dominates the diffusion of H2 gas and the reduction of the metal ions.
where C and S are the concentrations of the mobile H2 gas molecules and the product, respectively, D is the diffusion coefficient of the H2 molecules in the glass, R is the reaction rate, x is the depth from the sample surface, and t is the time. Since the amount of product (St) is experimentally determined as a function of time, Eqs. (1), (2) are solved as follows [26]:
St 4 = S∞ L
Dt , π
(3)
where S∞ is the amount of product after infinite time and L is the sample thickness. According to the tarnishing model, the tarnished layer thickness, X, can be approximated to be 2×/L = St/S∞. Then,
X=
4Dt . π
(4) 4. Chemistry of Al3+ ions in Na2OeAl2O3eSiO2 glass
Fitting the data shown in Fig. 2 with Eq. (4) allows us to determine the diffusion coefficients that are plotted as functions of reciprocal temperature in Fig. 3. The diffusion coefficients satisfy the Arrhenius equation, D = Do exp.(−Ea/RT); the activation energy, Ea, was approximately 57 kJ/mol. Note that the diffusion coefficients are first
In determining the openness in the Na2OeAl2O3eSiO2 glass samples, the Al3+ ions work to increase the openness because they form an open structure similar to that of Si4+ ions. The introduction of Al2O3 3
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bonding units. These results suggest that the Al3+ ions do not lead to the creation of non-bridging oxygen but form AlO4, where the AlO4 tetrahedra are deformed by the large metal ions. 5. Unusual reduction of dopants in glasses with Al/Na > 1 As shown in Fig. 3, the Mn3+ ion-doped glass with Al/Na > 1 exhibited an extremely high diffusion coefficient when compared with that from the other glass samples. Unusual reduction was observed in the glass samples doped with rare-earth ions such as Ce4+, Eu3+, and Sm3+. Despite heating in H2 at high temperatures and for long periods of time, in glass with Al/Na < 1 that were doped with rare-earth ions, reduction never occurred. On the other hand, when doped in glasses with Al/Na > 1, the rare-earth ions readily reduced to lower valence states. Fig. 6 (a) and (b) show fluorescence spectra of samples 17N8A5Eu and 10N15A5Eu heated in H2 gas at 600 °C. Note that the 10N15A5Eu glass exhibited fluorescence from the Eu2+ ions, whereas the 17N8A5Eu glass only exhibited fluorescence from Eu3+ ions. These results suggest that the valence states of Eu atoms are primarily controlled by selecting the glass composition, which is a great advantage for practical applications in red- and white-emitting devices. Moreover, the reduction of the Eu3+ ions to Eu2+ ions occurred not only on the glass surface but also in the center of the glass and that within a short heating period. Another interesting reduction occurred with Cu2+ ions doped in the glass with Al/Na > 1, where the reduced Cu atoms migrated toward the glass surface, onto which the Cu nanoparticles formed [16]. Fig. 6 (c) shows the scanning electron microscopy image and optical absorption spectrum of the 10N20A1Cu glass sample heated in H2 gas at 400 °C for 10 min. Notice that Cu nanoparticles with uniform size (ca. 50 nm diameter) are homogeneously distributed on the glass surface without aggregation, which is reflected in a sharp SPR band peaking at approximately 580 nm. When compared with chemical and physical deposition methods, this reduction process is unique and represents an advantageous method to deposit metal nanoparticles for optical sensing devices based on SPR. Also, the Cu nanoparticles were not strongly bonded to the substrate and were easily removed by immersion in a dilute acidic solution. Subsequent heating in H2 gas resulted in the formation of nanoparticles on the surface again, indicating a potential Cu nanoparticle synthesis method in which the glass substrate could be used repeatedly. Conversely, for the glasses with Al/Na < 1, the Cu particles only precipitated inside the bulk of the glass but never on the surface.
Fig. 4. Relationship between the diffusion coefficients at the glass transition temperature and openness of glass structure. The diffusion coefficients are shown closed and open marks for determined from the measurements of thickness of tarnishing layer and OH formation, respectively. The points marked as 11, 12, 13, 29 are ones from the references, respectively.
into a Na2OeSiO2 glass eliminates the non-bridging oxygen atoms and simultaneously creates bridging oxygen atoms by forming AlO4 with the SiO4 tetrahedron. The Na+ ions act as charge compensators to form the AlO4 network [30]. Thus, the addition of Al2O3 would create a more open structure. When the Al2O3 content becomes equal to the amount of Na2O, all oxygen atoms are bridging ones. In the (30-x) Na2O·xAl2O3·70SiO2 glass samples, the openness, obtained as 0.593 for glass with x = 0, increased as the Al2O3 content increased and became 0.63 at x = 15 (Al/Na = 1). On the other hand, in glass containing a higher Al/Na ratio (Al/Na > 1) there are not sufficient excess Na+ ions to compensate the charge of the AlO4 tetrahedron. Two different possibilities have been proposed regarding the Al3+ ions bonding [31,32]: (1) the Al3+ ions form AlO6 octahedra as network modifier ions, resulting in decreased openness, or (2) the Al3+ ions act to form an AlO4 tetrahedron. However, it was observed that the degree to which the openness of glass with Al/Na > 1 increases was lower than that for glass with Al/Na < 1; also for Al/Na = 2 at x = 20, the opening is 0.635. Thus, a high diffusion coefficient for (30-x)Na2O·xAl2O3·70SiO2 glass would suggest a critical effect from the chemical bonding, in particular the effect from Al3+ ions, in addition to the openness. The chemical bonding of the Al3+ ions in glass was studied and compared using two-dimensional 27Al 3Q magnetic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy (Fig. 5) [18,19]. In all glass samples with Al/Na < 1, only one 27Al band elongating along an isotropic chemical shift (CS) axis was detected at around 60 ppm, which was assigned to Al atoms in AlO4 tetrahedra. This result agreed with the structural model that is generally accepted for alumino-silicate glasses, i.e., Al3+ ions form AlO4, all oxygen atoms in which are bridging ones accompanied by Na+ ions as charge compensators [33–38]. In contrast, NMR spectra for samples with Al/ Na > 1 varied depending on the glass composition. The 10Na2O·15Al2O3·75SiO2 glass exhibited only one sharp band at 52 ppm and no other difference was observed from the spectra of the glass with Al/Na < 1, indicating that the Al3+ ions do not form an AlO6 unit, but rather form AlO4 units. On the other hand, the 10Na2O·20Al2O3·70SiO2 glass exhibited a small but clear signal at 28 ppm in addition to the band at approximately 60 ppm. The signal at 28 ppm can be assigned to an AlO5 unit. Further interesting is that the spectral band is within the range of AlO4 units elongated along a quadrupole induced shift (QIS) axis in addition to the CS axis, indicating the randomness of the AleO
6. Reduction of metal ions in glasses with Al/Na > 1 As mentioned above, the reduction behavior of metal ions doped in glass with Al/Na > 1 was unique and completely different from that of glass with Al/Na < 1. Analysis using the tarnishing model was not successful for glass with Al/Na > 1, because no color change was observed in the glass for Eu3+- and Eu2+-ion doping. The formation of Cu nanoparticles on the glass surface also does not allow measurement of the reacted layer. Our new finding was that the reduction of the dopants in the glasses with Al/Na > 1 was accompanied with the formation of hydroxide (OH) bonds. Fig. 7 shows Fourier transform infrared (FT-IR) spectra for selected glass samples before and after heating in H2 gas. Before heating in H2, the as-prepared glass exhibited a broad absorption band from 3600 to 2700 cm−1, which corresponds to OH bonding and/or water incorporation during the melting at high temperatures. The absorption band in this wavenumber region is very broad and seems to consist of many overlapping bands, and the position of the absorption bands depends on the strength of the hydrogen bonding in the OH bonds bound with cations in the glass [39,40]. Considering the strength of the hydrogen bond, the absorption band peaking at 3500 cm−1 can be assigned to OH bonds that are hydrogen-bonded with oxide ions bound to 4
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Fig. 5. Two dimensional
27
Al 3Q MAS NMR spectra of as-prepared 20N10A1Cu (a), 12.5N12.5A5Eu (b), 10N15A5Eu (c), and 10N20A1Cu (d) glasses [18,19].
Fig. 6. Fluorescence spectra of 17N8A5Eu (a) and 10N15A5Eu (b) glasses heated in H2 gas at 600 °C for 20 h. (c) is the optical absorption spectrum and SEM image of 10N20A1Cu glass heated in H2 gas at 400 °C for 10 min.
Si4+ and Al3+ ions. However, it is difficult to further decompose the peak, because the hydrogen bonding between the Si4+ and Al3+ ions varies only slightly. On the other hand, the absorption band around 2750 cm−1, which is clearly observed in 20Na2O·10Al2O3·70SiO2 glass (see Fig. 7 (b)), can be assigned to the OH bonds bound with Na+ ions breaking the SieOeSi network. The absorption spectra of glasses heated in H2 gas varied based on the glass composition. The peak intensity decreased upon heating when no dopants were present (Fig. 7 (a)), indicating the removal of OH bonds. This dehydration was similar to that observed in many silicate glasses; upon heating in ambient, vacuum, and reduced atmospheres [41], where the introduced H2 gas did not react with the glass components, dehydration occurred by the reaction of OH + OH = H2O, followed by the diffusion of H2O toward the glass surface. Different trends were observed in the absorption spectra of heated glasses containing dopants. In Fig. 7 (b) the spectra for 20Na2O·10Al2O3·70SiO2 glass doped with Cu2+ ions are shown, where the band at ~2750 cm−1 that is assigned to OH···Na+ bonding decreased in intensity, but no change was observed in the band at ca. 3500 cm−1. Two possibilities can be considered: (1) dehydration only took place in the OH···Na+
bond because of its strong hydrogen bonding, or (2) dehydration took place in all of the OH bonds bound with Si4+, Al3+, and Na+ ions, but the OH bonds around the Si4+ or Al3+ ions were recently formed by the diffused H2 gas, resulting in selectively decreased intensities at ~2750 cm−1. In the 10Na2O·15Al2O3·75SiO2 glass doped with Mn3+ ions, the OH band intensity initially increased (within a few tens of hours) with longer heating times but then began to decline (Fig. 7 (c)). These results suggest that OH bonds were formed by the diffused H2 gas in the glass, but that the amount was so small that they were removed by the dehydration. On the other hand, the spectra of the 10Na2O·20Al2O3·70SiO2 glass doped with Cu2+ ions featured a strong increase in the absorption band intensity upon heating in H2 gas despite the dehydration affect (Fig. 7 (d)). Also, in the Eu3+-doped glass, the absorption spectra confirmed that a large amount of OH bonds only formed in glass with Al/Na > 1, and no OH formation occurred in other glass compositions. Understanding where OH bonds form in the glass upon heating in H2 gas is necessary to elucidate the diffusion and reaction of H2 gas. The FT-IR absorption bands of OH, as shown in Fig. 7, were too broad to be accurately decomposed into component bands due to different chemical 5
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Fig. 7. FT-IR spectra of 10N15A (a), 20N10A1Cu (b), 10N15A5Mn (c), and 10N20A1Cu (d) glasses before and after heating in H2 gas.
spectra [18]. The 1He27Al CPMAS NMR signal was readily detected, while the 1He29Si CPMAS NMR signal was unclear due to a low signalto-noise ratio. Thus, we can conclude that the AlO4 tetrahedra oxidize to form Al − OH bonds, in which the Cu2+ ions reduce to Cu0. If the glass composition satisfies Al/Na < 1, the Al3+ ions form AlO4 by accompanying with Na+ ions that act as charge compensators. The doped metal ions are not surrounded by AlO4 but are positioned to break the Si–O–Si network, resulting in neither reduction nor OH formation during heating in H2 gas. On the other hand, in the glass with Al/Na > 1, excess AlO4 tetrahedra, which are not charge-compensated by Na+ ions, react with H2 gas to form the AleOH bonds that accompany the reduction of dopants. If this hypothesis is true, the reduction dynamics of the dopants can be evaluated by ascertaining OH formation. The change in the integrated OH absorption intensity upon heating in H2 gas, in which the absorption intensity is defined as the absorbance multiplied by the wavenumber in the 3700–2750 cm−1 range, is presented in Fig. 9. It is evident that the change in OH content varies depending on the glass composition. The glass without dopants exhibited continuous dehydration of the OH bonds remaining in the glass. In the Mn3+ ion-doped glass, the OH content initially increased with increasing heating time within a few tens of hours, beyond which it decreased due to dehydration. Therefore, to estimate the intrinsic OH content generated by the reduction, the OH content by the dehydration should be deducted. A great increase in the OH content with time masked the dehydration in heated Cu2+ and Eu3+ ion-doped glasses. The OH content was saturated after heating at high temperature for a long period of time, indicating that the reduction of the 1 mm thick sample was complete and reached its center. The saturated OH content is plotted against the dopant content in Fig. 10. The figure clearly shows a linear relationship between the two values, indicating that the formation of the OH bonds is accompanied by the reduction of the dopants. Furthermore, the OH content for glass doped with Cu2+ ions was twice as large as that for glass containing Eu3+ or Mn3+ ions, which would correspond to 2 OH bonds forming during the reduction of one Cu2+ to Cu0. Cu2+…. (O − Al)2 + H2 = Cu + 2(Al − OH). Eu3+ (or Mn3+)…. (O − Al) + 1/2H2 = Eu2+ (or 2+ Mn ) + Al − OH.
Fig. 8. 27Al (a) and 29Si (b) MAS (black lines) and 1He27Al and 1He29Si CPMAS (red lines) NMR spectra of 10N20A1Cu glasses heated in H2 gas at 600 °C for 1 h [18]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
bonds. 1He27Al and 1He29Si cross polarization magnetic angle spinning (CPMAS) NMR spectra were used, because CPMAS NMR spectra enable the selective detection of the coordination environment around the Al3+ and Si4+ ions present near the H atoms [42]. Fig. 8 shows the CPMAS NMR spectra for the 10Na2O· 20Al2O3·70SiO2 glass doped with Cu2+ ions heated in H2 gas together with the 27Al and 29Si MAS NMR 6
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Fig. 9. Dependence of the integrated intensities of OH band on the heating period in H2 gas for 10N20A1Cu (closed circles), 10N15A5Eu (closed triangles), 10N15A1Eu5Mn (closed squares), 10N15A5Mn (open squares), 20N10A1Cu (open circles), and 10N15A (open triangles) glasses. Left and right scales in vertical axis are of closed and opened marks, respectively.
Fig. 11. Diffusion coefficients (closed circles) of 10N20A1Cu glass in which Cu nanoparticles formed on the surface were removed. Triangles are ones determined by tarnishing model for 20N10A1Cu glass and circles are ones determined from the OH formation for 10N20A1Cu glass.
tube (inner diameter of 5 mm and thickness of 1 mm) using a glass sealant. After achieving a vacuum state in the system, hydrogen gas was loaded on the feed side; the increase in system pressure during H2 glass flow was measured as a function of time. The hydrogen gas permeated through and then out of the glass with a constant time lag, resulting in the increased pressure. The total amount of gas, Q(t), discharged from the glass membrane with thickness, L, for testing time, t, is as follows [43]:
Q(t) =
DC⎛ L2 ⎞ t− L ⎝ 6D ⎠ ⎜
⎟
(5)
where D is the diffusion coefficient and C is the concentration of hydrogen gas dissolved in the glass. Q(t) is related to the increase in pressure, and the diffusion coefficient can be determined by calculating the time axis intercept (Fig. 11). It is clear that after removing the Cu nanoparticles from its surface, the glass exhibits an extremely high diffusion coefficient, for example, ~10−11 m2/s at 200 °C, which is three orders of magnitude and six orders of magnitude higher than that of the glass samples with Al/Na > 1 and with Al/Na < 1, respectively. This glass could be applied to hydrogen separators.
Fig. 10. Relationship between the OH content defined as the absorbance multiplied by the wavenumber in the range of 3700 to 2750 cm−1 and the content of the dopant.
8. Conclusions The diffusion coefficients of the H2 molecules were also determined by fitting the data with Eq. (3). The results are shown in Figs. 3 and 4, where the diffusion coefficients obtained by this method are indicated by open marks. The diffusion coefficients calculated by other methods match our results, indicating that the reduction dynamics can be analyzed by using either method.
We developed new Na2OeAl2O3eSiO2 glasses that exhibited fast hydrogen diffusion and reduced the metal ions to lower valance states. The Al/Na concentration ratio preliminary determines the reduction of the dopants. For Al/Na > 1, there are some AlO4 units which are not charge compensated by Na+ ions interact with the doped metal ions to form AleOeM (M; doped ions). When the glasses are heated in H2 gas, the diffusing hydrogen molecules react to form AleOH bonds while reducing the dopants. The formation of AleOH bonds accelerated the diffusion coefficients of hydrogen by more than two orders of magnitude higher than those without forming AleOH bond. Thus, the development of novel glasses would grant a new properties for practical applications of glasses.
7. Gas diffusion in glass after heating in H2 gas From a series of experiments, we found that the metal ion dopants were selectively reduced by choosing the glass composition. In particular, Cu nanoparticles deposited on the glass surface are attractive for optical sensing device applications owing to their SPR properties. Moreover, as the Cu nanoparticles were not strongly bonded to the glass surface, they were easily removed by immersion in a dilute acidic solution. The obtained glass was expected to exhibit different properties from the original glass. Here, the diffusion properties were measured. The glass sample (~0.1 mm thick) was mounted on a dense alumina
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telecommunication, C. R. Chim. 5 (2002) 815–824. [3] J.W.M. Verwey, G.J. Dirksen, G. Blasse, The luminescence of divalent and trivalent rare earth ions in the crystalline and glass modifications of SrB4O7, J. Phys. Chem. Solids 53 (1992) 367–375. [4] H. Ebendorff-Heidepriem, D. Ehrt, Formation, UV absorption of cerium, europium and terbium ions in different valences in glasses, Opt. Mater. 15 (2000) 7–25. [5] A. Herrmann, S. Fibikar, D. Ehrt, Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses, J. Non-Cryst. Solids 355 (2009) 2093–2101. [6] Z. Lian, J. Wang, Y. Lv, S. Wang, Q. Su, The reduction of Eu3+ to Eu2+ in air and luminescence properties of Eu2+ activated ZnO–B2O3–P2O5 glasses, J. Alloys Compound 430 (2007) 257–261. [7] C. Zhu, Y. Yang, X. Liang, S. Yuan, G. Chen, Composition induced reducing effects on Eu ions in borophosphate glasses, J. Am. Ceram. Soc. 90 (2007) 2984–2986. [8] L. Li, Y. Yang, D. Zhou, Z. Yang, X. Xu, J. Qiu, Influence of the Eu2+ on the silver aggregates formationin Ag+–Na+ ion-exchanged Eu3+-doped sodium-aluminosilicate glasses, J. Am. Ceram. Soc. 97 (2014) 1110–1114. [9] T. Cao, G. Chen, W. Lu, H. Zhou, J. Li, Z. Zhu, Z. You, Y. Wang, C. Tu, Intense red and cyan luminescence in europium doped silicate glasses, J. Non-Cryst. Solids 355 (2009) 2361–2364. [10] Z. Lin, H. Zeng, Y. Yang, X. Liang, G. Chen, L. Sun, The effect of fluorine anions on the luminescent properties of Eu-doped oxyfluoride and aluminosilicate glasses, J. Am. Ceram. Soc. 93 (2010) 3095–3098. [11] J.E. Shelby, J.Jr. Vitko, The reduction of iron in soda-lime-silicate glasses by reaction with hydrogen, J. Non-Cryst. Solids 53 (1982) 155–163. [12] C. Estournes, N. Cornu, J.L. Guille, Reduction of cupper in soda-lime-silicate glass by hydrogen, J. Non-Cryst. Solids 170 (1994) 287–294. [13] W.D. Johnston, A.J. Chelko, Reduction of ions in glass by hydrogen, J. Am. Ceram. Soc. 53 (1970) 295–301. [14] M.M. Smedskjaer, Y.Z. Yue, J. Deubener, H.P. Gunnlaugsson, S. Morup, Modifying glass surface via internal diffusion, J. Non-Cryst. Solids 356 (2010) 290–298. [16] M. Nogami, X.Q. Vu, S. Ohki, K. Deguchi, T. Shimizu, Reduction mechanisms of Cu2+-doped Na2O–Al2O3–SiO2 glasses during heating in H2 gas, J. Phys. Chem. B 122 (2018) 1315–1322. [18] M. Nogami, T.T. Nguyen, Vu X. An, Y. Quang, Y. Matsuoka Kato, One-step fabrication of Cu nanoparticles on silicate glass substrates for surface plasmonic sensors, J. Non-Cryst. Solids 495 (2018) 95–101. [19] M. Nogami, V.X. Quang, T. Nonaka, T. Shimizu, S. Ohki, K. Deguchi, Diffusion and reaction of H2 gas for reducing Eu3+ ions in glasses, J. Phys. Chem. Solids 105 (2017) 54–60. [20] J.E. Shelby, Reaction of hydrogen with hydroxyl-free vitreous silica, J. Appl. Phys. 51 (1981) 2589–2593. [21] J.E. Shelby, J.Jr. Vitko, Hydrogen transport in a machinable glass-ceramics, J. NonCryst. Solids 45 (1981) 83–92. [22] M.M. Smedskjaer, Y.Z. Yue, Redox reaction and inward cationic diffusion in glasses caused by CO and H2 gases, Solid State Ionics 180 (2009) 1121–1124. [23] M.M. Smedskjaer, Y.Z. Yue, Inward cationic diffusion in glass, J. Non-Cryst. Solids 355 (2009) 908–912. [24] M.M. Smedskjaer, J. Wang, Y.Z. Yue, Tunable photoluminescence induced by thermal reduction in rare earth doped glasses, J. Mater. Chem. 21 (2011) 6614–6620.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Review
Novel silicate glasses in the acceleration of hydrogen diffusion for reducing dopant metal ions ⁎
Masayuki Nogamia,b, , Xuan Hung Lea, Xuan Quang Vua a b
Laboratory on Optics and Spectroscopy, Duy Tan University, K7/25, Quang Trung, Da Nang, Viet Nam Nagoya Institute of Technology, Showa Nagoya, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen diffusion Glass Reduction Rare-earth Nanoparticle
Controlling valence state of metal ions doped in glasses, in particular with lower valence, has been widely applied for turning the optical properties. Although hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, the development of silicate glasses doped with lower valency metal ions is still a formidable challenge because of low diffusion rate of hydrogen gas in glass. Recently, the authors studied the reaction of metal ions doped in Na2OeAl2O3eSiO2 glass with hydrogen gas and succeeded in the preparation of the glass that exhibited fast hydrogen diffusion and reduced the metal ions to lower valance states. When the metal ions were doped in glasses containing Al2O3 with Al/Na > 1, the hydrogen gas diffusion coefficient was increased more than two orders of magnitude higher than that for glasses with Al/Na < 1 and the reduction of dopants concurrently caused the formation of AleOH bonds in the glass. These completely different reductions, occurred depending on the Al/Na concentration ratio, were discussed to relate with the glass structure and the question whether and how the dopants were reduced in glass was elucidated. The obtained results will help to both understand basic science of hydrogen diffusion process in glass and develop new optical devices.
1. Introduction Recently, doping glass with metal ions, such as rare-earth, transition metal and noble metal ions, has gained interest due to glass's excellent transparency, high dispersive power for dopants, and potential to be formed into various shapes at a low cost [1,2]. Among the rare-earth ions, Eu ions are attractive dopants for light-emitting devices because they exist in trivalent and divalent states in glass, resulting in strong fluorescence in the orange/red and blue wavelength regions, respectively [3]. Doping noble metal ions in nanoparticles has been applied in such photonic fields as phosphors, lasers, optical amplifiers, and sensors. One critical aspect of optimizing device performance is the careful control of the valence states of the dopant metal ions. In oxide glasses, although metal ions with multiple valence states can be stably doped, only higher valency doping leads to doped glasses suitable for practical applications. For example, borate or phosphate glasses doped with lower valency metal ions [4–7] are not appropriate for practical applications as they lack chemical and mechanical durability. The development of silicate glasses doped with lower valency metal ions is still a formidable challenge [6–8]. Special melting and/or secondary
⁎
heating techniques carried out in a reducing atmosphere are needed to incorporate metal ions with lower valence states. One such technique is melting the raw materials together with the reductants such as carbon powders in a reducing atmosphere [9,10], though contamination by the reductants remains a problem. Researchers have also reported on the secondary heat treatment of glass in a reducing atmosphere, in which the metal ions are reduced to lower valence states. However, the reduction is limited to near the surface, because of the low permeation rate of gas molecules in solid glass [11–14]. Thus, the development of silicate glasses doped with lower valency ions would be crucial for the practical implementation of these materials and could enable other glass applications. The authors have studied the reaction of metal ions doped in Na2OeAl2O3eSiO2 glass with hydrogen gas. The Na2OeAl2O3eSiO2 glasses are commercially available and present a challenge in overcoming the aforementioned difficulty. Generally, in this ternary glass system, the Al2O3 content is suppressed to < 10% because of the production temperature limitations. We observed that no metal ions were reduced even when these glasses were subjected to H2 gas at an elevated temperature, as described in Section 3. Recently, we successfully prepared glass that exhibited fast hydrogen diffusion and reduced the
Corresponding author. E-mail address: [email protected] (M. Nogami).
https://doi.org/10.1016/j.jnoncrysol.2018.10.003 Received 21 August 2018; Received in revised form 2 October 2018; Accepted 5 October 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Nogami, M., Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2018.10.003
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with Eu3+ ions were colorless to the naked eye, their optical spectra featured sharp but weak absorption bands at 394 and 465 nm in the visible wavelength region that were assigned to the 7F0 → 7L0 and 7 F0 → 5D0 transitions of the Eu3+ ion, respectively. The presence of Eu3+ ions is also evident from the fluorescence spectra, i.e., strong and sharp FL bands peaking at 570, 590, and 620 nm that were assigned to the 5D0 → 7F0, 7F1, and 7F2 transitions of the Eu3+ ions, respectively; this indicates that the Eu atoms primarily exist in the trivalent state. After the glass samples containing Al2O3 (where Al/Na content ratio > 1) were melted, a weak but broad FL band in the range 400–500 nm was observed in addition to the FL bands of Eu3+ ions. This broad FL band was assigned to the 4f65d → 4f7(8S7/2) transition of Eu2+ ions, indicating that some Eu2+ ions were stably incorporated into the glass. The same phenomenon was also observed for Mn3+ and Cu2+ ions. Therefore, we studied the following reductions; Eu3+ → Eu2+, Mn3+ → Mn2+, and Cu2+ → Cu0.
metal ions to lower valance states. The glass contained Al2O3 with Al/ Na > 1, in which the Eu3+, Mn3+, and Cu2+ ions were reduced to Eu2+, Mn2+, and Cu0, respectively, by heating in H2 gas. The reduction of Cu2+ ions was of interest; doped in the glass with Al/Na > 1, the Cu2+ ions were reduced to Cu atoms that migrated toward the glass surface, on which a film of Cu nanoparticles formed [18]. This one-step synthesis of Cu nanoparticles on a glass substrate has attracted interest in the field of nanoparticle applications. Based on the results of our study, we hypothesized that the Al3+ ions play an essential role in reducing the dopant. However, the H2 gas reaction remained unclear. Here, we expand the study to examine Mn3+ ion-doped glass and systematically discuss the results in the context of the H2 reaction and diffusion. This comprehensive paper will be important in understanding the basic science in gas reactions with glass and in the development of new optical devices.
2. Glass preparation and valence state of the metal ions dopants 3. Hydrogen diffusion-limited reduction of metal ions in glass with Al/Na < 1
Glass samples with compositions of (25-x)Na2O·xAl2O3·75SiO2 and (30-x)Na2O·xAl2O3·70SiO2 (x = 0, 8, 10, 12.5, 15, and 20 mol%) were prepared, in which Eu3+, Mn3+, and Cu2+ ions acted as dopants. The dopant concentration varied from 1 to 5 wt% in terms of oxide. The mixtures of initial raw materials (Na2CO3, Al(OH)3, SiO2, Eu2O3, MnO, and CuO) were melted in a covered corundum (α-Al2O3) crucible at 1500–1650 °C for 6 h in ambient atmosphere, after which they were removed and annealed at the glass transition temperature for 15 min, followed by natural cooling to room temperature in the furnace. The glass samples were cut into 1 × 10 × 15 mm pieces and polished by a slurry of ~1 μm diamond particles. Hereafter abbreviations will be used to refer to the samples; for example, 10N20A1Cu glass refers to 10Na2O·20Al2O3·70SiO2 glass doped with 1 wt% CuO. Reduction in H2 gas was carried out by heating the samples in a fused silica tube furnace under 100% H2 gas flow at ca. 5 ml/min. The valence states of the metal ions were investigated by analyzing the optical absorption, fluorescence (FL), and X-ray absorption nearedge structure spectra of the glass. In silicate glass, the metal ion dopants mainly take high valence states, i.e., Eu3+, Cu2+, and Mn3+, but can vary based on the glass composition. The metal ions are coordinated with oxygen ions, the valence states of which are affected by the affinity of the oxygen atoms surrounding the metal ions. Used as a measurement of the acid–base properties of oxide glass, the affinity of oxygen is related to the electron-donating power of the surrounding oxygen atoms to the metal ion. In the given Na2OeAl2O3eSiO2 glass, the Na+ ions act to increase the basicity, whereas both the Si4+ and Al3+ ions decrease it. A lower basicity of the glass leads to a smaller negative charge on the oxygen atoms, which is beneficial to the lower oxidation state of the metal atom. The glass samples doped with Mn3+ and Cu2+ ions were light brown and blue and exhibited optical absorption bands peaking at approximately 480 and 800 nm, respectively; these were assigned to the 5E → 5T2 transition of Mn3+ and the 2B1g → 2 B2g transition of Cu2+ ions, respectively. Although the glasses doped
Whether the doped metal ions are reduced or not upon heating in H2 gas is dependent on the ionization energies of the dopant ions and the composition of the host glass. For glass with Al/Na < 1 heated in H2 gas, the surface of the Mn3+-doped glass became transparent and the intensity of the 480 nm optical absorption band decreased, indicating the reduction of Mn3+ ions to Mn2+. Similarly, the surface of the Cu2+doped glass turned red and a sharp absorption band at 580 nm appeared. X-ray and electron diffraction spectra clearly showed the precipitation of nanosized Cu crystals in the glass, and the optical absorption band at 580 nm was attributed to the surface plasmon resonance (SPR) signal of Cu nanoparticles confined in the glass matrix. However, in the Eu3+-doped glass, no change was observed in the optical absorption and fluorescence properties; this was true even when the samples were subjected to H2 gas at high temperatures and for long periods of time, indicating that the Eu3+ ions were stable and not reduced. Fig. 1 shows the cross-sectional images of Mn3+ (a and b) and Cu2+ (c and d) ion-doped 20Na2O·10Al2O3·70 SiO2 glass samples (Al/ Na = 0.5) before and after heating in H2 gas; the results for the 10Na2O·15Al2O3·75SiO2 glass (Al/Na = 1.5) doped with Mn3+ ions are shown in Fig. 1 (e and f). The color change marked the sharp interface between the reduced layer (tarnished layer) and the bulk of the glass. The change in the tarnished layer thickness, as measured for samples heated in H2 gas at different temperatures and time periods, is shown in Fig. 2. It is evident that the tarnished layer thickness increased with heating time. Although it was independent of the type of dopants, the time dependence of the tarnished layer thickness varied strongly with the glass compositions. That is, the glass with Al/Na = 1.5 exhibited an order of magnitude greater change in the tarnished layer than the glass with Al/Na = 0.5, the reasons for which will be discussed in Section 6. In the inset of Fig. 2, the tarnished layer thickness is plotted as a
Fig. 1. Photographs of cross section of 20N10A5Mn (a and b) and 20N10A1Cu (c and d), and 10N15A5Mn (e and f) glasses before (a, c, and e) and after (b, d, and f) heating in H2 gas. Heating conditions are 600 °C/5 h (b and d) and 800 °C/2 h (f), respectively. → and ↔ indicate the sample surface and reduced layer, respectively. 2
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Fig. 3. Arrhenius plots of the diffusion coefficients determined from the thickness measurements of tarnishing layer (closed marks) for 20N10A1Cu (A), 20N10A5Mn (B), and 10N15A5Mn (C) glasses. Open marks are of the diffusion coefficients determined from the OH measurements for 10N15A5Mn (W), 10N15A5Eu (X), 10N20A1Cu (Y), and 15N15A1Cu (Z) glasses.
Fig. 2. Dependence of thickness of the reduced layer on the heating period in H2 gas for 20N10A5Mn (triangles), and 20N10A1Cu (squares), and 10N15A5Mn (circles) glasses. The inset shows the square-root of time dependence of the thickness.
function of the square root of the heating time, showing good linearity for both samples. This leads us to believe that the reduction process must be hydrogen diffusion-limited. A similar reaction was reported for oxide glass doped with polyvalent metal ions, such as Fe3+, Ni2+, and V5+ ions, where the H2 gas diffusion kinetics were analyzed using the tarnishing model [14,20,21]. More recently, Smedskjaer, et al. studied the reduction of Fe3+, V5+ and Eu3+/Yb3+ dopants in silicate glass and suggested that the inward diffusion of network modifying cations such as Ca2+/Mg2+ ions was induced by the reduction of metal ions [14,22–25]. However, that process would limit the reduction layer to less than few hundred nanometers from the sample surface since the heating was carried out in a diluted H2 gas atmosphere (1%). Our glasses were treated in a 100% H2 gas atmosphere, resulting in a deep reduction thickness of tens of microns. When the diffused H2 gas reacts with the metal ion dopants, the diffusion equation can be modified as follows [26,27]:
∂C ∂ 2C ∂S =D 2 − ∂t ∂x ∂t
(1)
∂S = RC, ∂t
(2)
determined by the glass compositions but independent of the dopants. Generally, it is known that the H2 gas molecules diffuse through gaps between the ions in a solid, the rate of which is primarily determined by the sizes of the H2 molecules (0.25 nm) and structural voids in the solid. To examine the effect of glass composition on the diffusion coefficient, glass samples with different compositions were prepared in the Na2OeAl2O3eSiO2 system. The diffusion coefficients of the H2 gas in the different samples were determined by using the tarnishing model, and the void content, i.e., the openness of the glass structure, was calculated by considering the glass density and the following ionic radii: Si4+, 0.04 nm; Na+, 0.116 nm; Al3+, 0.053 nm; and O2−, 0.121 nm [28]. The glass structure is thermally relaxed and also influences the diffusion. Therefore, in this study, diffusion coefficients were compared at the glass transition temperature; the effect of thermal relaxation due to compositional differences could be avoided. The results are shown in Fig. 4, where the diffusion coefficients reported for soda-lime silicate glasses are referred for comparison [11–13,29]. In those glasses, alkali and/or alkaline-earth ions occupy the interstitial positions in the SieOeSi network structure, resulting in a closed structure. The openness of the reference glass is lower than 0.62, and the values of our glasses with Al/Na < 1 were also within this range. The highest diffusion coefficient of the low openness glass (0.62) was 10−14 m2/s. This is not sufficient for practical glass applications because it indicates a required 200 day heating period to achieve reduction at the center of a 1 mm thick sample. On the other hand, it is apparent that the 10Na2O·15Al2O3·75SiO2 glass doped with Mn3+ ions exhibited an extremely high diffusion coefficient of 7 × 10−12 m2/s. The openness of the glass was 0.635 and the corresponding diffusion coefficient, ~5 × 10−14 m2/s, was estimated by extrapolating from the linear relation. The obtained diffusion coefficient is more than two orders of magnitude higher than the estimated value, strongly suggesting that in addition to the openness, something else dominates the diffusion of H2 gas and the reduction of the metal ions.
where C and S are the concentrations of the mobile H2 gas molecules and the product, respectively, D is the diffusion coefficient of the H2 molecules in the glass, R is the reaction rate, x is the depth from the sample surface, and t is the time. Since the amount of product (St) is experimentally determined as a function of time, Eqs. (1), (2) are solved as follows [26]:
St 4 = S∞ L
Dt , π
(3)
where S∞ is the amount of product after infinite time and L is the sample thickness. According to the tarnishing model, the tarnished layer thickness, X, can be approximated to be 2×/L = St/S∞. Then,
X=
4Dt . π
(4) 4. Chemistry of Al3+ ions in Na2OeAl2O3eSiO2 glass
Fitting the data shown in Fig. 2 with Eq. (4) allows us to determine the diffusion coefficients that are plotted as functions of reciprocal temperature in Fig. 3. The diffusion coefficients satisfy the Arrhenius equation, D = Do exp.(−Ea/RT); the activation energy, Ea, was approximately 57 kJ/mol. Note that the diffusion coefficients are first
In determining the openness in the Na2OeAl2O3eSiO2 glass samples, the Al3+ ions work to increase the openness because they form an open structure similar to that of Si4+ ions. The introduction of Al2O3 3
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bonding units. These results suggest that the Al3+ ions do not lead to the creation of non-bridging oxygen but form AlO4, where the AlO4 tetrahedra are deformed by the large metal ions. 5. Unusual reduction of dopants in glasses with Al/Na > 1 As shown in Fig. 3, the Mn3+ ion-doped glass with Al/Na > 1 exhibited an extremely high diffusion coefficient when compared with that from the other glass samples. Unusual reduction was observed in the glass samples doped with rare-earth ions such as Ce4+, Eu3+, and Sm3+. Despite heating in H2 at high temperatures and for long periods of time, in glass with Al/Na < 1 that were doped with rare-earth ions, reduction never occurred. On the other hand, when doped in glasses with Al/Na > 1, the rare-earth ions readily reduced to lower valence states. Fig. 6 (a) and (b) show fluorescence spectra of samples 17N8A5Eu and 10N15A5Eu heated in H2 gas at 600 °C. Note that the 10N15A5Eu glass exhibited fluorescence from the Eu2+ ions, whereas the 17N8A5Eu glass only exhibited fluorescence from Eu3+ ions. These results suggest that the valence states of Eu atoms are primarily controlled by selecting the glass composition, which is a great advantage for practical applications in red- and white-emitting devices. Moreover, the reduction of the Eu3+ ions to Eu2+ ions occurred not only on the glass surface but also in the center of the glass and that within a short heating period. Another interesting reduction occurred with Cu2+ ions doped in the glass with Al/Na > 1, where the reduced Cu atoms migrated toward the glass surface, onto which the Cu nanoparticles formed [16]. Fig. 6 (c) shows the scanning electron microscopy image and optical absorption spectrum of the 10N20A1Cu glass sample heated in H2 gas at 400 °C for 10 min. Notice that Cu nanoparticles with uniform size (ca. 50 nm diameter) are homogeneously distributed on the glass surface without aggregation, which is reflected in a sharp SPR band peaking at approximately 580 nm. When compared with chemical and physical deposition methods, this reduction process is unique and represents an advantageous method to deposit metal nanoparticles for optical sensing devices based on SPR. Also, the Cu nanoparticles were not strongly bonded to the substrate and were easily removed by immersion in a dilute acidic solution. Subsequent heating in H2 gas resulted in the formation of nanoparticles on the surface again, indicating a potential Cu nanoparticle synthesis method in which the glass substrate could be used repeatedly. Conversely, for the glasses with Al/Na < 1, the Cu particles only precipitated inside the bulk of the glass but never on the surface.
Fig. 4. Relationship between the diffusion coefficients at the glass transition temperature and openness of glass structure. The diffusion coefficients are shown closed and open marks for determined from the measurements of thickness of tarnishing layer and OH formation, respectively. The points marked as 11, 12, 13, 29 are ones from the references, respectively.
into a Na2OeSiO2 glass eliminates the non-bridging oxygen atoms and simultaneously creates bridging oxygen atoms by forming AlO4 with the SiO4 tetrahedron. The Na+ ions act as charge compensators to form the AlO4 network [30]. Thus, the addition of Al2O3 would create a more open structure. When the Al2O3 content becomes equal to the amount of Na2O, all oxygen atoms are bridging ones. In the (30-x) Na2O·xAl2O3·70SiO2 glass samples, the openness, obtained as 0.593 for glass with x = 0, increased as the Al2O3 content increased and became 0.63 at x = 15 (Al/Na = 1). On the other hand, in glass containing a higher Al/Na ratio (Al/Na > 1) there are not sufficient excess Na+ ions to compensate the charge of the AlO4 tetrahedron. Two different possibilities have been proposed regarding the Al3+ ions bonding [31,32]: (1) the Al3+ ions form AlO6 octahedra as network modifier ions, resulting in decreased openness, or (2) the Al3+ ions act to form an AlO4 tetrahedron. However, it was observed that the degree to which the openness of glass with Al/Na > 1 increases was lower than that for glass with Al/Na < 1; also for Al/Na = 2 at x = 20, the opening is 0.635. Thus, a high diffusion coefficient for (30-x)Na2O·xAl2O3·70SiO2 glass would suggest a critical effect from the chemical bonding, in particular the effect from Al3+ ions, in addition to the openness. The chemical bonding of the Al3+ ions in glass was studied and compared using two-dimensional 27Al 3Q magnetic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy (Fig. 5) [18,19]. In all glass samples with Al/Na < 1, only one 27Al band elongating along an isotropic chemical shift (CS) axis was detected at around 60 ppm, which was assigned to Al atoms in AlO4 tetrahedra. This result agreed with the structural model that is generally accepted for alumino-silicate glasses, i.e., Al3+ ions form AlO4, all oxygen atoms in which are bridging ones accompanied by Na+ ions as charge compensators [33–38]. In contrast, NMR spectra for samples with Al/ Na > 1 varied depending on the glass composition. The 10Na2O·15Al2O3·75SiO2 glass exhibited only one sharp band at 52 ppm and no other difference was observed from the spectra of the glass with Al/Na < 1, indicating that the Al3+ ions do not form an AlO6 unit, but rather form AlO4 units. On the other hand, the 10Na2O·20Al2O3·70SiO2 glass exhibited a small but clear signal at 28 ppm in addition to the band at approximately 60 ppm. The signal at 28 ppm can be assigned to an AlO5 unit. Further interesting is that the spectral band is within the range of AlO4 units elongated along a quadrupole induced shift (QIS) axis in addition to the CS axis, indicating the randomness of the AleO
6. Reduction of metal ions in glasses with Al/Na > 1 As mentioned above, the reduction behavior of metal ions doped in glass with Al/Na > 1 was unique and completely different from that of glass with Al/Na < 1. Analysis using the tarnishing model was not successful for glass with Al/Na > 1, because no color change was observed in the glass for Eu3+- and Eu2+-ion doping. The formation of Cu nanoparticles on the glass surface also does not allow measurement of the reacted layer. Our new finding was that the reduction of the dopants in the glasses with Al/Na > 1 was accompanied with the formation of hydroxide (OH) bonds. Fig. 7 shows Fourier transform infrared (FT-IR) spectra for selected glass samples before and after heating in H2 gas. Before heating in H2, the as-prepared glass exhibited a broad absorption band from 3600 to 2700 cm−1, which corresponds to OH bonding and/or water incorporation during the melting at high temperatures. The absorption band in this wavenumber region is very broad and seems to consist of many overlapping bands, and the position of the absorption bands depends on the strength of the hydrogen bonding in the OH bonds bound with cations in the glass [39,40]. Considering the strength of the hydrogen bond, the absorption band peaking at 3500 cm−1 can be assigned to OH bonds that are hydrogen-bonded with oxide ions bound to 4
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Fig. 5. Two dimensional
27
Al 3Q MAS NMR spectra of as-prepared 20N10A1Cu (a), 12.5N12.5A5Eu (b), 10N15A5Eu (c), and 10N20A1Cu (d) glasses [18,19].
Fig. 6. Fluorescence spectra of 17N8A5Eu (a) and 10N15A5Eu (b) glasses heated in H2 gas at 600 °C for 20 h. (c) is the optical absorption spectrum and SEM image of 10N20A1Cu glass heated in H2 gas at 400 °C for 10 min.
Si4+ and Al3+ ions. However, it is difficult to further decompose the peak, because the hydrogen bonding between the Si4+ and Al3+ ions varies only slightly. On the other hand, the absorption band around 2750 cm−1, which is clearly observed in 20Na2O·10Al2O3·70SiO2 glass (see Fig. 7 (b)), can be assigned to the OH bonds bound with Na+ ions breaking the SieOeSi network. The absorption spectra of glasses heated in H2 gas varied based on the glass composition. The peak intensity decreased upon heating when no dopants were present (Fig. 7 (a)), indicating the removal of OH bonds. This dehydration was similar to that observed in many silicate glasses; upon heating in ambient, vacuum, and reduced atmospheres [41], where the introduced H2 gas did not react with the glass components, dehydration occurred by the reaction of OH + OH = H2O, followed by the diffusion of H2O toward the glass surface. Different trends were observed in the absorption spectra of heated glasses containing dopants. In Fig. 7 (b) the spectra for 20Na2O·10Al2O3·70SiO2 glass doped with Cu2+ ions are shown, where the band at ~2750 cm−1 that is assigned to OH···Na+ bonding decreased in intensity, but no change was observed in the band at ca. 3500 cm−1. Two possibilities can be considered: (1) dehydration only took place in the OH···Na+
bond because of its strong hydrogen bonding, or (2) dehydration took place in all of the OH bonds bound with Si4+, Al3+, and Na+ ions, but the OH bonds around the Si4+ or Al3+ ions were recently formed by the diffused H2 gas, resulting in selectively decreased intensities at ~2750 cm−1. In the 10Na2O·15Al2O3·75SiO2 glass doped with Mn3+ ions, the OH band intensity initially increased (within a few tens of hours) with longer heating times but then began to decline (Fig. 7 (c)). These results suggest that OH bonds were formed by the diffused H2 gas in the glass, but that the amount was so small that they were removed by the dehydration. On the other hand, the spectra of the 10Na2O·20Al2O3·70SiO2 glass doped with Cu2+ ions featured a strong increase in the absorption band intensity upon heating in H2 gas despite the dehydration affect (Fig. 7 (d)). Also, in the Eu3+-doped glass, the absorption spectra confirmed that a large amount of OH bonds only formed in glass with Al/Na > 1, and no OH formation occurred in other glass compositions. Understanding where OH bonds form in the glass upon heating in H2 gas is necessary to elucidate the diffusion and reaction of H2 gas. The FT-IR absorption bands of OH, as shown in Fig. 7, were too broad to be accurately decomposed into component bands due to different chemical 5
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Fig. 7. FT-IR spectra of 10N15A (a), 20N10A1Cu (b), 10N15A5Mn (c), and 10N20A1Cu (d) glasses before and after heating in H2 gas.
spectra [18]. The 1He27Al CPMAS NMR signal was readily detected, while the 1He29Si CPMAS NMR signal was unclear due to a low signalto-noise ratio. Thus, we can conclude that the AlO4 tetrahedra oxidize to form Al − OH bonds, in which the Cu2+ ions reduce to Cu0. If the glass composition satisfies Al/Na < 1, the Al3+ ions form AlO4 by accompanying with Na+ ions that act as charge compensators. The doped metal ions are not surrounded by AlO4 but are positioned to break the Si–O–Si network, resulting in neither reduction nor OH formation during heating in H2 gas. On the other hand, in the glass with Al/Na > 1, excess AlO4 tetrahedra, which are not charge-compensated by Na+ ions, react with H2 gas to form the AleOH bonds that accompany the reduction of dopants. If this hypothesis is true, the reduction dynamics of the dopants can be evaluated by ascertaining OH formation. The change in the integrated OH absorption intensity upon heating in H2 gas, in which the absorption intensity is defined as the absorbance multiplied by the wavenumber in the 3700–2750 cm−1 range, is presented in Fig. 9. It is evident that the change in OH content varies depending on the glass composition. The glass without dopants exhibited continuous dehydration of the OH bonds remaining in the glass. In the Mn3+ ion-doped glass, the OH content initially increased with increasing heating time within a few tens of hours, beyond which it decreased due to dehydration. Therefore, to estimate the intrinsic OH content generated by the reduction, the OH content by the dehydration should be deducted. A great increase in the OH content with time masked the dehydration in heated Cu2+ and Eu3+ ion-doped glasses. The OH content was saturated after heating at high temperature for a long period of time, indicating that the reduction of the 1 mm thick sample was complete and reached its center. The saturated OH content is plotted against the dopant content in Fig. 10. The figure clearly shows a linear relationship between the two values, indicating that the formation of the OH bonds is accompanied by the reduction of the dopants. Furthermore, the OH content for glass doped with Cu2+ ions was twice as large as that for glass containing Eu3+ or Mn3+ ions, which would correspond to 2 OH bonds forming during the reduction of one Cu2+ to Cu0. Cu2+…. (O − Al)2 + H2 = Cu + 2(Al − OH). Eu3+ (or Mn3+)…. (O − Al) + 1/2H2 = Eu2+ (or 2+ Mn ) + Al − OH.
Fig. 8. 27Al (a) and 29Si (b) MAS (black lines) and 1He27Al and 1He29Si CPMAS (red lines) NMR spectra of 10N20A1Cu glasses heated in H2 gas at 600 °C for 1 h [18]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
bonds. 1He27Al and 1He29Si cross polarization magnetic angle spinning (CPMAS) NMR spectra were used, because CPMAS NMR spectra enable the selective detection of the coordination environment around the Al3+ and Si4+ ions present near the H atoms [42]. Fig. 8 shows the CPMAS NMR spectra for the 10Na2O· 20Al2O3·70SiO2 glass doped with Cu2+ ions heated in H2 gas together with the 27Al and 29Si MAS NMR 6
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Fig. 9. Dependence of the integrated intensities of OH band on the heating period in H2 gas for 10N20A1Cu (closed circles), 10N15A5Eu (closed triangles), 10N15A1Eu5Mn (closed squares), 10N15A5Mn (open squares), 20N10A1Cu (open circles), and 10N15A (open triangles) glasses. Left and right scales in vertical axis are of closed and opened marks, respectively.
Fig. 11. Diffusion coefficients (closed circles) of 10N20A1Cu glass in which Cu nanoparticles formed on the surface were removed. Triangles are ones determined by tarnishing model for 20N10A1Cu glass and circles are ones determined from the OH formation for 10N20A1Cu glass.
tube (inner diameter of 5 mm and thickness of 1 mm) using a glass sealant. After achieving a vacuum state in the system, hydrogen gas was loaded on the feed side; the increase in system pressure during H2 glass flow was measured as a function of time. The hydrogen gas permeated through and then out of the glass with a constant time lag, resulting in the increased pressure. The total amount of gas, Q(t), discharged from the glass membrane with thickness, L, for testing time, t, is as follows [43]:
Q(t) =
DC⎛ L2 ⎞ t− L ⎝ 6D ⎠ ⎜
⎟
(5)
where D is the diffusion coefficient and C is the concentration of hydrogen gas dissolved in the glass. Q(t) is related to the increase in pressure, and the diffusion coefficient can be determined by calculating the time axis intercept (Fig. 11). It is clear that after removing the Cu nanoparticles from its surface, the glass exhibits an extremely high diffusion coefficient, for example, ~10−11 m2/s at 200 °C, which is three orders of magnitude and six orders of magnitude higher than that of the glass samples with Al/Na > 1 and with Al/Na < 1, respectively. This glass could be applied to hydrogen separators.
Fig. 10. Relationship between the OH content defined as the absorbance multiplied by the wavenumber in the range of 3700 to 2750 cm−1 and the content of the dopant.
8. Conclusions The diffusion coefficients of the H2 molecules were also determined by fitting the data with Eq. (3). The results are shown in Figs. 3 and 4, where the diffusion coefficients obtained by this method are indicated by open marks. The diffusion coefficients calculated by other methods match our results, indicating that the reduction dynamics can be analyzed by using either method.
We developed new Na2OeAl2O3eSiO2 glasses that exhibited fast hydrogen diffusion and reduced the metal ions to lower valance states. The Al/Na concentration ratio preliminary determines the reduction of the dopants. For Al/Na > 1, there are some AlO4 units which are not charge compensated by Na+ ions interact with the doped metal ions to form AleOeM (M; doped ions). When the glasses are heated in H2 gas, the diffusing hydrogen molecules react to form AleOH bonds while reducing the dopants. The formation of AleOH bonds accelerated the diffusion coefficients of hydrogen by more than two orders of magnitude higher than those without forming AleOH bond. Thus, the development of novel glasses would grant a new properties for practical applications of glasses.
7. Gas diffusion in glass after heating in H2 gas From a series of experiments, we found that the metal ion dopants were selectively reduced by choosing the glass composition. In particular, Cu nanoparticles deposited on the glass surface are attractive for optical sensing device applications owing to their SPR properties. Moreover, as the Cu nanoparticles were not strongly bonded to the glass surface, they were easily removed by immersion in a dilute acidic solution. The obtained glass was expected to exhibit different properties from the original glass. Here, the diffusion properties were measured. The glass sample (~0.1 mm thick) was mounted on a dense alumina
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