Article pubs.acs.org/JPCC
Application of Magnetic Resonance Spectroscopies to the xZnO− (100 − x)NaPO3 Glass System: Glass Network Organization and Effect of Co2+ Doping M. Lahaye,† B. Doumert,‡ B. Revel,§ K. Ben Tayeb,† H. Vezin,† and G. Tricot*,† †
LASIR UMR-CNRS 8516, Université de Lille 1, Villeneuve d’Ascq, France IMMCL, Université de Lille 1, Villeneuve d’Ascq, France § CCM-RMN, Université de Lille 1, Villeneuve d’Ascq, France ‡
S Supporting Information *
ABSTRACT: The xZnO−(100 − x)NaPO3 vitreous system is investigated in this contribution by advanced 1D/2D magnetic resonance spectroscopies. The glass network organization is analyzed in samples containing up to 42.5 mol % of ZnO by standard and correlation solid-state nuclear magnetic resonance: (i) 1D 31P MAS-, DQ-SQ-, and the recently developed DRENAR-NMR sequences are used to describe the local order structure and to quantify the 31P/31P interactions, (ii) the Na+ ions distribution is analyzed by 1D 23Na MAS NMR and 23Na(31P) REDOR NMR experiments, and (iii) 67Zn static NMR experiments are recorded at very high field (21.1 T) to monitor the evolution of the Zn coordination state all along the composition line. Altogether, the set of data offers a detailed analysis of the glass network structure and clarifies the two-domain evolution of the glass transition temperature. While Co2+ doping has been used for decades to reduce the T1 nuclear relaxation times, only sparse information about the optimum CoO amount and its chemical environment is available so far. New data on CoO-doped NaPO3 samples are presented in this contribution based on the first reported continuous wave and pulsed (HYSCORE) electron paramagnetic resonance investigation on that system. limitation of the 2D correlation 31P NMR lies in the long longitudinal relaxation time (T1) of the phosphorus nucleus that leads to unreasonable experimental times (from hours to days). To reduce this parameter and the total recording time, paramagnetic species have been inserted in the glass formulations at very low amounts. Co2+, Mn2+, and Fe3+ ions have shown to be very efficient in reducing T1,9−14 but no clear information about the optimum amount of paramagnetic species and its geometry site, distribution, and chemical environment is available so far. Among all the reported phosphate-based glass compositions, the zinc alkali phosphate (and more precisely zinc sodium phosphate) system has attracted much attention owing to the combination of a very low glass transition temperature (Tg < 400 °C) and good thermal and chemical stability. This system is considered as the best substitute for the lead phosphate system that provided an excellent low-Tg/stability compromise but had to be replaced because of lead toxicity. Numerous papers and patents have thus been devoted to the preparation, properties characterization, and structural analysis of zinc alkali
1. INTRODUCTION Phosphate-based glasses have been extensively studied during recent decades because of their particular properties (low thermal characteristics, high oxide acceptation ability, UV transparency, etc.), which strongly differ from those of classical silicate-based glasses. Phosphate glasses are thus considered as a very efficient alternative to silicate formulations for several technical applications (low-temperature sealing glasses,1,2 anode materials,3,4 laser glass,5 confinement matrix for specific radioactive wastes,6 etc.). It is now well known that the macroscopic properties are governed by the local and medium range organization. Therefore, designing materials with tailored properties requires a clear and precise structural characterization, including both local and intermediate length scale structure. In addition to vibrational spectroscopy, nuclear magnetic resonance (NMR) has been widely applied owing to the good magnetic properties of the 31P nucleus (spin-1/2, high gyromagnetic ratio, 100% natural abundance) and to the valuable information derived from standard 1D NMR analysis about the phosphate unit description (using the Qn speciation, where n is the number of connected phosphate species).7,8 NMR can also be used to analyze the structure beyond the local order because 2D correlation 31P NMR can establish a throughspace or through-bonds interaction scheme.9,11 One major © 2015 American Chemical Society
Received: April 8, 2015 Revised: July 7, 2015 Published: July 7, 2015 17288
DOI: 10.1021/acs.jpcc.5b03377 J. Phys. Chem. C 2015, 119, 17288−17297
Article
The Journal of Physical Chemistry C phosphate-based glasses.15−23 Within the ZnO−Na2O−P2O5 ternary system, a particular composition line (xZnO−(100 − x)NaPO3) has attracted our attention: (i) the starting composition (x = 0, NaPO3) presents a well-known and simple structure composed of infinite chains/rings (PO3− units) compensated by Na+ ions. Insertion of ZnO and its impact on the phosphate network organization, local order, and topology can thus be efficiently investigated; (ii) all the samples present a constant Na/P ratio, and the displacement/ replacement of the Na+ charge compensators of the phosphate network can thus be clearly highlighted; (iii) previous investigations18−20 show a maximum ZnO content of 33 mol %, higher contents leading to crystallized samples. That boundary composition corresponds to an O/P ratio equal to 3.5 that should theoretically produce a dimeric phosphate (P2O74−/Q1) network. It is surprising that this dimeric structure constitutes the end of the glass-forming range of this composition line, and we believe that a higher quenching rate will allow expansion of the glass composition line beyond the dimeric phosphate formulation. Therefore, the ZnO−NaPO3 composition line was investigated in this paper. The samples were prepared with an improved melt−quenching technique to expand the glassforming region and to monitor the structural evolution over a larger composition range. In addition to 1D 31P MAS NMR, 23 Na and 67Zn 1D NMR experiments were recorded to investigate the local order for each cation of the formulations. Then correlation NMR techniques (double quantum-single quantum (DQ-SQ),9,10,24−26 double quantum-based dipolar recoupling effects nuclear alignment reduction (DQ-DRENAR), 2 7 , 2 8 and rotational echo double resonance (REDOR)29,30) were employed to study the 31P/31P and 31 23 P/ Na interactions to analyze the medium range structure and the Na+ charge compensator distribution. Doping by Co2+ (0.02−1 wt % of CoO) was then investigated in specific formulations (x = 0 and 20). Impact of doping on the glass transition temperature and on the 31P nuclear relaxation times (T1, T2′, and T2*) has first been determined to derive an optimum Co2+ amount. Then electron paramagnetic resonance (EPR) techniques were used to describe the local order of Co2+ ions and to investigate the interactions between Co2+ and the 23 Na and 31P elements. The continuous wave (CW) experiments give access to the local symmetry of Co2+ ions by measuring the Landé factor (g), and the chemical environment was probed using advanced pulsed EPR experiments with the hyperfine sublevel correlation spectroscopy (HYSCORE) sequence.31
The glass transition (Tg) temperatures were determined by differential scanning calorimetry (DSC) measurements performed with a DSC SETARAM 131 apparatus. The DSC experiments were conducted on 20 mg of sample with a heating rate of 10 °C/min. 2.2. NMR Characterization. The 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a 9.4 T Bruker spectrometer using a 4 mm probe operating at a spinning frequency (νrot) of 12.5 kHz. The 1D 31P MAS NMR spectra were recorded at 162 MHz with 1.9 μs pulse length, 40 kHz radiofrequency (rf) field strength, 16 accumulations, and a recycle delay (rd) of 120 s. 31 P longitudinal and transverse relaxation times (T1 and T2′) were determined on the Co2+-doped glass series using saturation recovery and spin−echo sequences, respectively, whereas T2* values were directly extracted from the signal linewidth. The 23Na MAS NMR spectra were recorded at 105.8 MHz with 1.4 μs pulse length, 30 kHz rf field strength, 256 accumulations, and 2 s rd. The 23Na/31P dipolar interaction was monitored using the 23Na(31P) REDOR technique with 8−12 kHz νrot and 180° pulses of 10 and 8 μs, for 23Na and 31P, corresponding to rf field strength of 50 and 60 kHz, respectively. Twenty-four experiments, providing 12 REDOR points, were performed on each sample using 32 accumulations and 2 s rd. The initial part of the REDOR curves was fitted with a parabolic function, according to the multispin system approximation27 eq 1 to derive the Van Vleck second moment M2(23Na/31P), that is directly linked to the 23Na/31P dipolar interaction strength and thus to the number of P5+ ions surrounding the Na+ ions. ΔS 4 = M 2 × (nτr)2 S0 3π 2
(1)
The phosphate network organization was also analyzed through the edition of 2D 31P/31P correlation maps recorded with the DQ-SQ NMR sequence. The excitation−reconversion scheme used in our experiments was obtained with the back-toback pulse sequence with a four-rotor period supercycle. The 788 × 24 points were acquired with a short excitation− reconversion time (800 μs) using a rotor-synchronized t1increment of 100 μs, and each slice was recorded with 48 transients separated by a rd of 60 s. Owing to the short excitation−reconversion time used in our experiments, the through-space correlation signals can be reasonably discussed in terms of chemical connectivity. The 31P/31P dipolar interaction was analyzed using the recently developed DQ-DRENAR NMR technique. The experiments were performed at 8−13 kHz νrot with excitation parameters imposed by the C7 scheme conditions (rf field strength = 7νrot). Twenty-four experiments, providing 12 points, were performed on several samples using 16 accumulations and 60 s rd. The initial part of the DRENAR curves (ΔS′/S0 < 0.4) was fitted using a parabolic function eq 2 to extract the dipolar coupling constant ∑b2jk, that can be used to discuss the average 31P/31P dipolar interaction and the number of coupled 31P groups.27,28
2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Characterization. All the glasses were prepared by the standard melt−quenching method. Appropriate mixtures of ZnO and Graham’s salt (NaPO3) were placed in a Pt−Au5 crucible. The mixtures were melted during 30−120 min at 900−1100 °C depending on the composition, before being poured on a brass plate. Higher ZnO content glasses (>30 mol %) required a quicker cooling rate and were thus obtained using small volumes of melt and by dipping the bottom of the Pt−Au5 crucible in water. The melting parameters (time and temperature) were optimized to reduce the P2O5 volatilization ( 25, when previous publications clearly showed only the first domain. The Co2+-doping procedure of the NaPO3 sample produces glasses with increasing blue coloration. Figure 1b shows the evolution of Tg in the Co2+-doped NaPO3 glass series. The results indicate that insertion of Co2+ at such low amounts does not change the glass transition temperature and that this macroscopic property is not significantly altered by the doping procedure. It is noteworthy that similar results have been obtained on the x = 20 sample (see Figure 1 in Supporting Information). 3.2. NMR Characterization. The 1D 31P MAS NMR spectra obtained on the composition line are reported in Figure 2 accompanied by representative simulations obtained with the Gaussian/Lorentzian model available in the dmfit software.35 The NMR parameters (chemical shift (δiso), full width at halfmaximum (fwhm), and the relative proportions (rel prop.) deduced from the deconvolutions) are reported in Table 1. Four different species are observed all along the composition line. At low ZnO contents (0 ≤ x ≤ 5), the spectra are dominated by a signal located at −19.5 ppm, assigned to Q2
Figure 1. Evolution of the glass transition temperature (Tg) in the xZnO−(100 − x)NaPO3 composition line (a) and in the Co2+-doped NaPO3 glass series (b).
Figure 2. 31P MAS NMR experiments acquired at 9.4 T on the xZnO−(100 − x)NaPO3 composition line, accompanied by representative deconvolutions (in dotted lines).
species. At higher ZnO content glasses (7.5 ≤ x ≤25), this signal is progressively replaced by a second signal, at −2 ppm, assigned to the Q1 site. This signal is accompanied by a low intensity resonance attributed to a second Q1 site (denoted as Q1′) owing to its chemical shift values (−7.5 ppm). When ZnO content reaches 30 mol %, a fourth signal, located at 8 ppm, is observed and increases to become a significant contribution. This latter signal has been attributed to Q0 species due to its 17290
DOI: 10.1021/acs.jpcc.5b03377 J. Phys. Chem. C 2015, 119, 17288−17297
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The Journal of Physical Chemistry C Table 1. 31P NMR Parameters Derived from the 1D MASNMR and DQ-DRENAR Experimentsa x 0
5
10
15
20
25
30
35
40
Qn 2
Q Q1′ Q1 Q2 Q1′ Q1 Q2 Q1′ Q1 Q2 Q1′ Q1 Q2 Q1′ Q1 Q2 Q1′ Q1 Q0 Q2 Q1′ Q1 Q0 Q2 Q1′ Q1 Q0 Q2 Q1′ Q1 Q0
δiso (ppm)
fwhm (ppm)
rel prop. (%)
∑b2jk (105 Hz2)
−19.6 −7.4 0.9 −19.4 −7.6 −2.0 −19.4 −7.3 −2.1 −19.3 −7.8 −2.3 −18.8 −7.2 −2.4 −18.1 −7.5 −2.5 8.9 −17.6 −7.8 −2.5 8.8 −17.3 −7.2 −2.4 9.0 −17.3 −7.9 −2.6 9.1
7.4 6.2 5.4 7.6 8.3 7.0 7.9 8.2 7.0 8.0 7.9 7.0 8.6 6.9 6.7 8.7 6.9 6.9 6.2 8.8 5.9 6.8 7.0 8.4 6.7 6.9 7.1 7.7 6.8 7.3 6.9
98.8 0.6 0.6 84.2 3.0 12.8 78.8 3.3 17.8 66.7 4.7 28.6 47.1 8.5 44.4 27.8 7.6 63.7 0.9 15.0 7.5 72.8 4.7 5.3 9.3 72.5 12.9 1.4 5.3 65.6 27.7
21.2 20.3 5.8 20.5 6.2 6.1 2.6 6.2 2.5
NMR parameters (δiso, quadrupolar constant (CQ)) extracted from the spectra deconvolutions are reported in Table 2. No Table 2. 23Na NMR Parameters Derived from the 1D MASNMR and 23Na(31P) REDOR Experimentsa x
δiso (ppm)
CQ (MHz)
M2(23Na/31P) (106 rad2·s−2)
0 5 10 15 20 25 30 35 40 42.5
−5 −4.8 −4.4 −4.4 −4.3 −4.2 −4.1 −4.2 −4.3 −4.3
2.3 2.3 2.3 2.3 2.4 2.4 2.3 2.4 2.4 2.4
3.6 3.5 3.6 3.6 3.7 3.4 3.5 -
a
The chemical shifts (δiso), quadrupolar constants (CQ), and Van Vleck second moments (M2(23Na/31P) are given with error of 0.1 ppm, 0.1 MHz, and 10%, respectively.
significant evolution can be observed all along the composition line in any of the NMR parameters, suggesting that the chemical environment of the Na+ ions is not altered by the ZnO insertion. 67Zn static NMR spectra, obtained on the x = 10, 20, and 40 samples, are sketched in Figure 3b. Very broad signals can be observed without any significant evolution with the ZnO content, suggesting that the coordination state of zinc is not modified in the composition line. Correlation NMR results are summarized in Figure 4. The 23 Na/31P dipolar interactions have been investigated with the REDOR technique. Representative REDOR curves are plotted in Figure 4a, accompanied by a parabolic fit used to determine the Van Vleck second moment. The M2(23Na/31P) values determined for all the glasses using eq 1 are reported in Table 2. The results indicate a constant M2 value of 3.5 × 106 rad2·s−2 all along the composition line. This value, in good agreement with previous publications,22,23,37 suggests that the number of 31 P atoms in the vicinity of Na+ ions does not change and that the chemical environment of Na+ ions is not altered by the insertion of ZnO in the glass structure. The recently developed DQ-DRENAR NMR technique has been used to quantify the 31 P/31P dipolar interaction to determine the number of connected P (n in the Qn notation). All the values determined from eq 2 are gathered in Table 1. The curves, representative of the three main signals observed in the 1D NMR analyses, are reported in Figure 4b, accompanied by parabolic fits used to determine the dipolar coupling constant. The comparison of the obtained values (20.7, 6.1, and 2.6 × 105 Hz2) with data coming from the literature27,28,38 allows determining that the signals at −19.5, −2, and 8 ppm are involved in a 3, 2, and 1 spin-system. These results, based on the 31P homonuclear dipolar interaction, provide unambiguous confirmation of the Q2, Q1, and Q0 assignments previously deduced from the chemical shift values and proves the relevancy of the recently developed DQ-DRENAR sequence. The 2D 31P DQ-SQ NMR method has been applied on the x = 7.5, 12.5, and 30 samples to analyze in detail the phosphate network organization (Figure 5). The 2D maps are accompanied by the 1D 31P MAS NMR spectrum (top projection of the 2D maps) and by the Q2-Q2, Q1-Q2, and Q1-Q1 correlation signals (Figure 5i, ii, and iii, respectively). For the x = 7.5 sample, the 2D maps shows intense Q2-Q2 correlation (Figure 5a-i) and off-diagonal Q1-Q2
a
The chemical shifts (δiso), full width at half maximum (FWHM), relative proportions (rel prop.), and dipolar coupling constant (∑b2jk) are given with errors of ±0.1 ppm, 0.2 ppm, 1%, and 10%, respectively.
“deshielded” chemical shift. The 23Na MAS NMR experiments are reported in Figure 3a accompanied by representative deconvolutions using the Czjzek distribution model.35,36 The
Figure 3. 23Na MAS (a) and 67Zn static (b) NMR experiments acquired on the xZnO−(100 − x)NaPO3 composition line at 9.4 and 21.1 T, respectively. The 23Na NMR analyses are accompanied by representative deconvolutions (dotted lines). 17291
DOI: 10.1021/acs.jpcc.5b03377 J. Phys. Chem. C 2015, 119, 17288−17297
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The Journal of Physical Chemistry C
Figure 4. 23Na(31P) REDOR (a) and 31P DQ-DRENAR (b) experiments acquired at 9.4 T on the xZnO−(100 − x)NaPO3 composition line. Both sets of experiments are accompanied by the parabolic fits (dotted lines) from which the M2 and ∑b2jk have been extracted.
Figure 5. 2D 31P DQ-SQ spectra obtained on the x = 7.5 (a), 12.5 (b), and 30 (c) samples. The 2D maps are accompanied by the horizontal projections showing Q2-Q2 (i), Q1-Q2 (ii), and Q1-Q1 (iii) and their simulations in dotted lines.
(Figure 5a-ii), indicating that the phosphate network is composed of chains of finite length, terminated by Q2-Q1 linkages. In addition to these signals, a low intensity Q1-Q1 correlation (Figure 5a-iii) suggests the presence of some dimers within the glass structure. These dimers are present to a higher extent in the x = 12.5 sample (Figure 5b-iii) as well as the two previously observed Q2-Q2 and Q1-Q2 signals (Figure 5b-i and -ii). The off-diagonal Q1-Q2 signal is more intense in this 2D map, showing that the averaged chain length has been decreased by the ZnO insertion. In high ZnO content glasses (x = 30, Figure 5c), the main structural feature observed in the 2D map is Q1-Q1 (Figure 5c-iii). Only a few Q1-Q2 are present, whereas no trace of Q2-Q2 signal can be detected, confirming that the ZnO introduction decreases the chain length below the tetramer configuration. The relaxation times determined on the Co2+-doped glass series have been reported in Table 3 and sketched in Figure 6. Continuous decrease of the T1 relaxation time can be observed with a significant decrease for low CoO amounts (0.02−0.1). At 1 wt %, T1 values reach 1.9 s whereas the based composition exhibits a T1 of 72 s. Simultaneously, Co2+ insertion induces a decrease of T2′ and T2* relaxation times. If T2′ experiences a moderate decrease (from 4.6 to 3 ms), T2* is significantly affected by the doping, leading to the reported increase of the signal width that strongly restricts the spectrum analysis. It is noteworthy that a similar tendency was observed for the x = 20 sample (see Figure 2 and Table 1 in Supporting Information). 3.3. EPR Characterization. The CW EPR spectra, obtained at 5 K due to the fast relaxation of Co2+ ions,36 of the Co2+-doped NaPO3 samples are displayed in Figure 7. Two
Table 3. Nuclear Relaxation Times (T1, T2′, and T2*) and 31 P NMR Signal Full Width at Half Maximum of the Co2+Doped NaPO3 Glass Seriesa CoO wt %
T1 (s)
T2′ (ms)
T2* (ms)
fwhm (ppm)
0 0.02 0.1 0.5 1
72.0 43.0 16.2 3.8 1.9
4.6 4.5 4.3 3.6 3.0
0.27 0.26 0.25 0.20 0.15
7.2 7.43 7.8 9.82 12.9
a The values are given with errors of ±1 s, 0.2 ms, 0.01 ms, and 0.2 ppm, respectively.
signals can be observed at g = 2.004 and 4.04 corresponding to low-spin (S = 1/2) and high-spin (S = 3/2) electron configuration of Co2+ ions, respectively,39−41 the latter signal being the main contribution in the analysis. All the glass samples exhibit the same g values, indicating that Co2+ ions adopt the same local configuration in the glasses, independently of the Co2+ content. To go further in the description of the nuclear environment of the Co2+ centers, pulsed EPR experiments were performed using the two-dimensional HYSCORE experiment. It should be noted that HYSCORE spectra are composed of two quadrants. The first quadrant (+,−) A > 2νI (with νI the nuclear frequency of the atom I) corresponds to the nuclei with a strong hyperfine coupling A with the unpaired electron of the cobalt, a configuration which 17292
DOI: 10.1021/acs.jpcc.5b03377 J. Phys. Chem. C 2015, 119, 17288−17297
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The Journal of Physical Chemistry C
The HYSCORE spectrum recorded on the 1 wt % doped sample provides different information suggesting that the cobalt ions do not form similar species at a higher amount. As observed in Figure 8c, the (−,+) quadrant is free of any strong couplings and only weak couplings are observed in the (+,+) quadrant through two on-diagonal peaks related to 23Na and 59 Co nuclei with A values of 1.3 and 2.2 MHz, respectively.
4. DISCUSSION 4.1. Glass Network Organization. The modification of the sodium metaphosphate glass structure by the ZnO insertion has been monitored in an extended manner due to the larger composition range obtained in this contribution. The 1D 31P MAS NMR experiments (Figure 2) indicate the presence of three main signals which have been assigned to Q2, Q1, and Q0 based on their chemical shift and dipolar coupling constant ∑b2jk values. The relative proportions between these three species, derived from the spectra deconvolutions, are reported in Figure 9. The simulations also show the presence of a low intensity (
