Article pubs.acs.org/JPCC
Structural Characterization of AgI−AgPO3−Ag2WO4 Superionic Conducting Glasses by Advanced Solid-State NMR Techniques Mickael̈ Blais-Roberge,†,‡ Silvia H. Santagneli,*,§ Sandra H. Messaddeq,‡ Maxime Rioux,†,‡ Yannick Ledemi,‡ Hellmut Eckert,∥,⊥ and Younès Messaddeq‡,§ †
Département de Chimie, Université Laval, 1045 av. de la MédecineQuébec, Québec G1V 0A6, Canada Centre d’Optique, Photonique et Laser, Université Laval, 2375 rue la Terrasse, Quebec, Quebec G1V 0A6, Canada § Institute of Chemistry-São Paulo State University-UNESP, CP 355, Araraquara, São Paulo 14801-970, Brazil ∥ Instituto de Física, São Carlos, Universidade de São Paulo, CP 369, São Carlos, São Paulo 13566-590, Brazil ⊥ Institut für Physikalische Chemie, WWU Münster, Corrensstraße 30, D-48149 Münster, Germany ‡
ABSTRACT: Glass samples of composition 40AgI−(60−x)AgPO3− xAg2WO4 (0 ≤ x ≤ 25 mol %) have been prepared by the conventional melt-quenching method. These glasses receive renewed interest due to their ionic conductivity and transparency in the visible range. Because the physical and optical properties of these glasses are highly dependent on composition in this system, a comprehensive structural study has been carried out using Raman spectroscopy and 1D and 2D NMR of the 31P and 109Ag nuclei. With increasing Ag2WO4 content, the network is modified from a 1D Q(2)-like chain structure to a topology in which Q(1) and Q(0) species linked to octahedrally coordinated tungsten species dominate. This structural transformation increases the glass rigidity and stability against hydrolysis reactions. The compositional evolution of the phosphate speciation (in terms of Q(n)mW units) is consistent with maximum tungstate dispersion in glasses with x ≤ 10, while for glasses with higher tungstate content the data are more consistent with a random distribution of P−O−P, P−O−W, and W−O−W linkages. The 109Ag NMR chemical shifts are independent of composition and suggest that mobile silver ions are situated within cluster regions, furnishing a constant mixed iodide/oxide local environment.
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INTRODUCTION Phosphate-based glasses are in widespread current use because of their biocompatibility and well-known ability of metal ion dispersal.1−3 The possibility of altering their physical properties over wide ranges by changing their chemical compositions has resulted in applications of wide diversity such as laser inscription,4,5 bone and tissue repair6 and fast ion conduction.7,8 These glasses can be easily drawn into fibers transparent in the visible region (450 to 900 nm), leading to promising applications in optogenetics and electrophysiology such as brain cell depolarization,9,10 which also relies on their high electrical conductivity.11,12 As previously demonstrated, AgI−AgPO3 glass fibers containing high-valence state metal oxides are suitable for such applications.13−17 Furthermore, the addition of metal oxide to such phosphate glasses serves to increase their resistance against hydrolysis reactions. This effect has been attributed to the interaction of the metal oxide component with nonbridging oxygen atoms (NBOs).1,18−21 With their transparency in the visible region and their high room-temperature electrical conductivity (10−3 to 10−2 S·cm−1) combined with a hydrolytic stability, the ternary AgI−AgPO3− Ag2WO4 glasses and fibers appear well-suited for applications in optogenetic/electrophysiology.22 Despite this interest, the © XXXX American Chemical Society
structural effect of the metal tungstate component has not been understood so far. Here we examine glasses of the composition line 40AgI−(60−x)AgPO3−xAg2WO4, (0 ≤ x ≤ 25 mol %), denominated AIPWx, to discuss compositional trends of glass-transition temperatures (Tg), molar volumes and glass stability on a structural basis, using 31P 1D and 2D solidstate NMR, Raman scattering, as well as 109Ag NMR spectroscopies.
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EXPERIMENTAL SECTION Sample Preparation and Characterization. Glasses with composition 40AgI−(60−x)AgPO3−xAg2WO4 (AIPWx, x = 0, 5, 10, 15, 20, and 25 mol %) were obtained by the meltquenching method as described previously.22 Their vitreous state was confirmed previously by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) on a Netzsch DSC Pegasus 404F3 instrument using sealed Al pans at a heating rate of 10 K/min to 600 °C. Molar volume was calculated with glass densities obtained using the Archimedes’ Received: April 19, 2017 Revised: June 2, 2017
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DOI: 10.1021/acs.jpcc.7b03684 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) DSC traces (Tg values and curve inflections indicated in blue), (b) XRD powder patterns, and (c) Tg values and molar volume measured for the AIPWx glasses.
under the following conditions: 136 data sets over the range of evolution times 0.714 to 97.17 ms, relaxation delay 50 s. Chemical shifts are reported relative to 85% H3PO4 solution. Signal deconvolutions into Gaussian components were done using the DMFIT software package.24 109Ag MAS NMR spectra were measured at 18.62 MHz using π/2 pulses of 8.6 μs length and a recycle delay of 1 s. Chemical shifts are reported relative to an aqueous solution of AgNO3 (1 mol·L−1).
principle using a Mettler Toledo XSE204 analytical balance. Weight-loss experimentation was performed in deionized water at 65 °C during 48 h, followed by 20 h of drying at 80 °C. Tg values have been obtained with the Netzsch data analysis software from the intersection of the tangents, defining the apparent heat-capacity change in the DSC curves (onset points). Thermogravimetric (TG) measurements were performed with a Mettler Toledo TGA 851e thermogravimetric analyzer on 2−4 mg glass pieces in alumina crucibles at a heating rate of 10 K/min from 35 to 400 °C. Raman scattering spectra were obtained on a Renishaw inVia spectrometer coupled to a Leica DM2700 microscope and a Renishaw CCD camera detector. A vertically polarized He−Ne laser with a wavelength of 633 nm was used as light source and samples were exposed for 10 s. A 50× focused laser beam at 50% power was used for all measurements. All further analysis and deconvolution of Raman spectra were achieved using the Wire 4.1 software. Solid-State NMR Spectroscopy. High-resolution solidstate NMR spectra were measured at room temperature using a Bruker Avance III 400WB HD spectrometer, with commercial triple and low-γ resonance 4 mm MAS probes. Typical spinning speeds were 10.0 to 14.0 kHz. 31P MAS NMR measurements were carried out at 162.0 MHz, using π/2 pulses of 3.3 μs length and a recycle delay of 400 s. The scalar-coupling-based 1D-refocused INADEQUATE sequence23 was applied to select only those 31P nuclei involved in the P−O−P connectivity, whereas isolated P species were filtered out. For creating a full P−O−P connectivity map between the various phosphate units, 2D INADEQUATE spectra were recorded at a spinning speed of 14.0 kHz, using 90° pulses (3.3 us length). 128 data sets (64 scans at a relaxation delay of 100 s each) were acquired under rotor-synchronized conditions at t1 increments of 83.34 μs. A double quantum coherence buildup time of 2τ = 8.34 ms was used. While this value is significantly lower than the optimum value of 1/2J ≈ 50 ms, this choice represents a good compromise of DQ excitation efficiency and T2 decay. 1Drefocused INADEQUATE spectra were measured with 2τ = 8.34 ms. Homonuclear 31P J-resolved spectra were obtained
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RESULTS, DATA ANALYSIS, AND INTERPRETATION Glass Properties. As previously explained,22 the glasses’ coloration, transition temperatures (Tg) and stability against hydrolysis can be tuned by adjusting the concentrations of silver tungstate in the glass system. Multiple preparations resulted in glasses with very similar physical properties and spectra, showing the reproducibility of the synthesis method. The DSC traces and Tg values are presented in Figure 1a. Tg values increase from 109 °C for x = 0 to 243 °C for x = 25. While the onsets in some of the DSC curves are relatively poorly defined in some samples, we could rule out by TGA studies that these changes arise from mass losses. Furthermore, as illustrated in Figure 1b, the absence of well-defined diffraction peaks confirms the vitreous state of this material. As expected from the higher molecular weight of WO3, the density of the glasses increases significantly with increasing Ag2WO4 content, in line with observations made for other Wcontaining glasses.25,26 As indicated by Figure 1c, replacement of AgPO3 by Ag2WO4 also leads to a slight increase in molar volume. All of the glasses are stable under ambient conditions of temperature and humidity. To investigate glass stability against hydrolysis degradation, soak tests in water were performed. Figure 2 shows a decrease in hydrolytic weight loss, from ∼7 to as well as an average P−O−W connectivity , which are given by
scenarios. The above-mentioned maximum tungstate dispersion model assumes that each tungstate species replaces four P−O− P linkages by four P−O−W linkages, leading to the prediction that the number of P−O−P linkages is completely exhausted in samples with x = 20. As indicated in Tables 2 and 3, however, we clearly detect a significant amount of P−O−P linkages for this sample, indicating that the maximum dispersion model is not applicable here. Therefore, our results indicate a lower conversion rate for the x = 20 sample, indirectly indicating the formation of some W−O−W linkages. Alternatively, we can consider a statistical connectivity scenario in which , , and (the latter being the average number of W−O−W connectivities) are calculated by the expressions = 0.5 × (2 × [P])2 = 2 × [P] × 4 × [W ] = 0.5 × (4 × [W ])2
In these expressions [P] and [W] are the fractional contributions to the network, calculated by (60 − x)/60 and x/60, respectively, and it is further assumed that each P species contributes two bridging oxygen atoms (as in AgPO3), while each W species contributes four of them (as in Ag2WO4), which is consistent with the overall charge balance provided by the Ag+ ions. As is evident from the comparison with the experimental data, the experimental ratio / is more consistent with the maximum dispersion model in glasses with low x values, whereas for higher x values (x ≥ 15), the random connectivity model appears to be more appropriate. Local Environment of the Silver Ions Studied by 109Ag NMR. The 109Ag MAS NMR spectra are summarized in Figure 9. They reveal a sharp signal near 490 ppm, the location of which is independent of composition. This chemical shift falls within the chemical shift ranges found in other AgI-based glasses,43,52−61 indicating that the silver species are completely ionized. Static experiments reveal a constant line width of 450 Hz at room temperature, indicating that, with the exception of pure AgPO3 glass, the silver ions are in the fast motion regime, where the hopping frequency τ−1 of the Ag+ ions is large compared with the inverse line width in the rigid limit (Δωτ ≪
= 2 × f (Q(2)) + 1 × f (Q(1))
= 3 × f (Q(0)3W) + 2 × f (Q(0)2W) + 1 × [f (Q(0)1W) + f (Q(1)1W)]
These values are summarized in Table 3, where they are compared with the corresponding predictions of two extreme Table 3. Ratios / Calculated by the Maximum Dispersion and the Random Connectivity Scenarios and Comparison with the Experimental Data x
max. disp /
random conn. /
experiment / (±0.2)
5 10 15 20 25
4.9 1.5 0.5 0 0
2.75 1.25 0.75 0.50 0.35
4.40 1.89 0.79 0.35
