High Surface Area Mesoporous GaPO4âSiO2 SolâGel Glasses

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High Surface Area Mesoporous GaPO-SiO Sol-Gel Glasses: Structural Investigation by Advanced Solid State NMR Jinjun Ren, Carsten Doerenkamp, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11673 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 7, 2016

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The Journal of Physical Chemistry

High Surface Area Mesoporous GaPO4-SiO2 Sol-Gel Glasses: Structural Investigation by Advanced Solid State NMR

Jinjun Rena,*, Carsten Doerenkampb, Hellmut Eckertb,c,* a

Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China, b Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30, D-48149 Münster, Germany c Instituto de Física de São Carlos, Universidade de São Paulo (USP), C.P. 369, CEP 13560-970, São Carlos, SP, Brazil

ABSTRACT: Mesoporous silica-gallium-phosphate glasses along the composition line, xGaPO4-(1-x)SiO2 (x=0.5, 0.33, 0.20, 0.14, and 0.11, respectively) were prepared via the sol-gel route. This glass-forming range is significantly wider than that accessible by previously reported routes. The glasses exhibit a mesoporous structure with surface areas around 400 m2/g, after calcination of 650℃. The structural evolution from liquid to gel to glass was analyzed by liquid and advanced solid state nuclear magnetic resonance techniques. The NMR results indicate that the glasses consist of GaPO4 and SiO2 nano-domains. With increasing GaPO4 content, the sizes of the GaPO4 domains become larger. Evidence for the connection of both domains at their interfaces by P-O-Si and Ga-O-Si linkages is presented by advanced high-resolution dipolar solid state NMR methods. ACS Paragon Plus Environment

71

Ga,

31

P, and

29

Si


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1. INTRODUCTION Gallium phosphate, gallium silicate and silicate phosphate glasses have been widely investigated for diverse applications, such as bioactive materials1-3, fluorescent materials4-6, and proton conductors7. However, to date, the three-component glass former system Ga2O3-P2O5-SiO2 is rather poorly characterized since the preparation of glasses in this system by melt-quenching methods is seriously impeded by crystallization. At the atomic level, crystalline GaPO4 shows similar structural features as SiO2. Because of this close analogy, the quasi-binary GaPO4-SiO2 system, which is quite remote from the usual glass-formation range, is of great interest from the viewpoint of fundamental study. Sol-gel synthesis has become an attractive alternative to melt-cooling for the preparation of multiple-component oxide glasses. The use of molecular precursors in solution offers the prospect of preparing materials of very high purity and in the form of thin films. In many cases, the compositional regions over which glasses can be prepared can be significantly extended compared to the glass-forming range accessible by melt-cooling. Finally, the moderate processing temperatures of gels into glasses are advantageous from the viewpoint of energy management. Sol-gel methods have become an important tool for the preparation of high power lasers with highly concentrated and well-dispersed rare earth ions.8-11 Furthermore, the sol gel technique offers the preparation of

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materials with high surface area and defined pore size distributions, making the materials interesting for applications in catalysis and medicine.12-16 The incorporation of trivalent cations such as B3+, Al3+, and Ga3+, into the mesoporous SiO2 framework imparts Lewis and/or Bronstedt acidity on them14,17 and numerous organic reactions such as the Claisen rearrangement are catalyzed by micro- and mesoporous silica-gallium oxide mixtures and nanocomposites.18 Recently, Ga3+ has been found to act as a “Trojan horse” for treating hypercalcemia of malignancy19 and to inhibit Pseudomonas aeruginosa growth and biofilm formation, as well as to kill planctonic and biofilm bacteria in vitro.20 We have recently reported sol-gel syntheses of Al2O3-P2O5 and Al2O3-P2O5-SiO2 glasses,21,

22

using aluminum lactate, ammonium

dihydrogen phosphate (NH4H2PO4), and tetraethylorthosilicate (TEOS) as precursors. In this contribution, we report a sol-gel route, based on gallium nitrate, TEOS, and triethyl phosphate, to prepare xGaPO4-(1-x) SiO2 glasses up to x = 0.5. Although Ga3+ has the same charge and similar electronegativity as Al3+, the synthesis of analogous gallium based glasses through sol-gel methods turns out to be completely different, from precursor to the final structure than in the case of the aluminum based glasses. To the best of our knowledge, this is the first time that this glass system has been synthesized. Detailed liquid- and multiple advanced solid-state NMR experiments are carried out to reveal the relevant species



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present in the precursor solutions to GaPO4-SiO2 sols and gels, and to monitor the whole conversion process from the solution state to the solid glassy materials. Furthermore the glasses prepared here exhibit attractive porous properties, which may have potential application in catalysis, drug-delivery, and drug release. To understand the structural features of these glasses we develop and apply a comprehensive solid-state NMR strategy, based on high-resolution 71Ga, 31P, and 29Si spectra and network connectivity studies by advanced dipolar NMR methods.

2. EXPERIMENTAL SECTION 2.1 Sample preparation and characterization. Samples along the composition line xGaPO4-(1-x)SiO2 (0.11, 0.14, 0.20, 0.33 and 0.5) were synthesized using the following procedure. The starting reagents are gallium nitrate (99.9%, Aldrich), tetraethylorthosilicate (TEOS) (99%, ABCR), and triethyl phosphate (98%, Alfa Aesar). The hydrate water content of gallium nitrate was measured by thermogravimetry (35.7 wt.%) and was taken into account when calculating the batch compositions. The synthesis was carried out by dissolving 0.90 ml TEOS in 5 ml isopropanol, followed by addition of the desired quantity of gallium nitrate dissolved in 16 ml distilled water. The pH value of the mixture was adjusted to 1.35 with 2M ammonia solution or 1M nitric acid and controlled within 0.01 units by a pH meter (WTW pH 320, Germany).


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After stirring overnight, the resulting clear solution was spread onto a flat surface and then gelled at 50°C for 7 days, leading to the formation of transparent bulk xerogel. The xerogel was heated first at 100 °C and 200 °C, successively, for 1 day and finally converted into glassy materials by sintering at 650 °C for several hours. The samples with x = 0.5 and 0.33 are opaque and translucent, respectively, which may indicate that these two samples have fully or partially phase separated. The other three compositions led to colorless transparent bulk samples. The amorphous state was proven by X-ray powder diffraction (XRD) using filtered (Ni) Cu Kα radiation in Bragg-Brentano geometry on a PANalytical Empyrean (Netherlands) diffractometer (diffraction angle scanning range 10° ≤ 2θ ≤ 90°).

Differential thermal analysis (DTA) and thermogravimetric

analysis (TGA) were carried out on a NETZSCH STA 409 instrument (NETZSCH Gerätebau GmbH, Selb/Bayern, Germany) using a heating rate of 10 K/min. N2 absorption-desorption isotherms were measured at 77 K on a volumetric adsorption analyzer (Micromeritics ASAP 2010). Prior to analysis, samples weighing about 0.15 g were outgassed at 120 °C for at least 24 h under vacuum until a residual pressure of ≤ 0.6 mm Hg was reached.

The

surface

area

was

determined

based

on

the

Brunauer-Emmett-Teller (BET) equation,23 using nitrogen adsorption data in the relative adsorption range from 0.06 to 0.2. The total pore



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volume, Vp, was obtained from the amount adsorbed at a p/p0 ratio of about 0.99. Mesopore size distributions were obtained using the Barrett-Joyner-Halenda (BJH) method assuming a cylindrical pore model.24 2.2 NMR studies. NMR measurements were carried out at magnetic field strengths of 11.7, 9.4, 7.04 and 4.7 T, respectively.

71

Ga MAS-NMR

spectra were recorded at 152.23 MHz (11.7 T) under the following conditions: pulse length 2.0 µs, nutation frequency 62.5 kHz, spinning speed 25.0 kHz. Chemical shifts are referenced to 1M Ga(NO3)3 solution. 71

Ga{31P} compensated REDOR experiments were conducted at 9.4 T,

using the sequence of Gullion and Schaefer25 modified by Chan and Eckert.26 The optimum pulse length for the decoupling channel was set by maximizing the REDOR difference signal ∆S. In the 71Ga{31P} REDOR experiments, 180° pulse lengths for

31

P and

71

Ga are both 5.0 µs. A

relaxation delay of 0.2 s was used. Data were analyzed in the short evolution time limit, ∆S/S0 ≤ 0.2-0.3, using the parabolic approximation:27, 28

∆

=

( )

(1)

yielding the van-Vleck second moment = ( ) (S =

71

Ga, I =

31

P). This quantity can be used to characterize average dipole-dipole

coupling strengths, independent of the order and geometry of the spin systems.


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31

P liquid and solid state NMR experiments were carried on a Bruker

Avance III-300 spectrometer, using 90° pulses of 3.0 μs length. The recycle delays were 1 s and 90 s, respectively, for liquid and solid state experiments. Solid samples were spun at the magic angle at a rate of 10.0 kHz, within 4 mm rotors. Chemical shifts are externally referenced to 85 % H3PO4. 31P peak deconvolutions were aided by 31P{71Ga} rotational echo adiabatic passage double resonance (REAPDOR) experiments29. To this end, the dephasing of the spins represented by the individual

31

P signal

components was examined at 9.4 T at a fixed dipolar mixing time of 2.0 ms, using the following measurement conditions: The

31

P and

71

Ga

nutation frequencies were 80.0 kHz and 40.0 kHz, respectively. Measurements were done under steady-state conditions with a 40 s recycle delay following a saturation comb at a spinning rate of 12.0 kHz. The length of the 71Ga re-coupling pulse corresponded to 1/3 of the rotor period. The π pulse length on 31P was 5.0 µs. 29

Si MAS-NMR spectra were measured at 39.8 MHz (4.7 T), using

single-pulse excitation by 90° pulses of 5.5 µs length, and relaxation delays of 240 s. Samples were spun at the magic angle at a rate of 8.0 kHz within 4 mm rotors. Chemical shifts are externally referenced to tetramethylsilane. Finally, 29Si{31P} REDOR measurements were done on a Bruker Avance III-500 spectrometer under the following conditions: steady-state conditions with a 90 s recycle delay following a saturation



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comb at spinning rate of 8.0 kHz. The π pulse length on 29Si and 31P are 10.0 and 8.0 µs, respectively. Because of severe signal-to-noise limitations, only a single-point measurement with a dipolar evolution time of 2.75 ms could be accomplished. Dependent on the samples and the evolution time, more than 528 transients have been used to record the REDOR experiments. The connectivity between

31

P nuclei was further checked by 1D

refocused INADEQUATE experiments.30,

31

This technique relies on

double quantum filtering, based on homonuclear J-coupling, to produce correlation signals between nuclei engaged in P-O-P linkages (Q1 and Q2 units), whereas the signals from isolated

31

P nuclei are suppressed

because of the absence of J-coupling for them. The 1D refocused INADEQUATE experiments were done on a Bruker Avance III 300 spectrometer, using spinning rates of 10.0 kHz and π/2 pulse lengths near 3.0 µs. The mixing time was 7.14 ms and the relaxation delay was 40 s, followed by a saturation comb. To investigate the spatial distribution of the phosphorus species

31

P-

DQ-DRENAR32, 33 experiments were conducted on a Bruker Avance III 300 spectrometer, using spinning rates from 8.0 to 12.0 kHz and π/2 pulse lengths near 4.0 µs. A recycle delay of 40 s was used, following a pre-saturation comb. Double-quantum excitation was accomplished by the “back-to-back” BABA-xy16 pulse sequence.34 The normalized


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difference signal intensities (S0-S)/S0 (corresponding to the signal amplitudes without (S0) and with (S) dipolar recoupling effect) were measured as a function of dipolar mixing time NTr. For multi-spin interactions in the limit of short mixing times (∆S/S0 ≤ 0.3-0.5) the normalized difference signal is independent of specific spin system geometries and can be approximated by a simple parabola: ( )

=

!

"∑& $%& '( ) (2)

The curvature of this parabola is the sum of the squares of the dipolar between nuclei k and the observed nuclei j. coupling constants ∑& $%&

This quantity can be used to characterize average homonuclear dipole-dipole coupling strengths, independent of the order and geometry of the spin systems involved. For example, DQ-DRENAR affords a simple distinction between the 31P units Q0 (no P-O-P linkages), Q1 (one P-O-P linkage) and Q2 (two P-O-P linkages).35-37

3. RESULTS, DATA ANALYSIS, AND INTERPRETATION 3.1 Physical Properties. Figure 1(a) shows a typical TGA result obtained for a xerogel (x = 0.20) heated at 100 ℃ for 24 h. The weight loss observed at the temperature below 100 ℃ and from 100 ℃ to 300 ℃ is due to the water and organic residue removal, respectively. They were almost completely removed near 400 ℃. Figure 1(b) shows the TGA trace of the sample ACS Paragon Plus Environment


heated at 650 °C for 5 h. The weight loss of the glass and the endothermic event observed below 400 ℃ are due to desorption of re-absorbed molecular water. No clear thermal event associated with a glass transition temperature (Tg) is observed in the DTA curves, in accordance with similar results obtained for other sol-gel prepared glasses. Figure 2 shows the N2-adsorption-desorption isotherms of selected glass samples sintered at 650 °C, revealing the typical mesoporous structure except for the phase separated x = 0.5 composition.24 5

(a)

(b)

0

95

95

DTA/µv/mg

-5

90 85 80

100

90

-10

85

-15 -20

80

-25

75

TG/Weight%

100

TG/weight/%

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75 -30 200

400

600 800 0 Temperature/ C

1000

1200

200

400

600

800

1000

70 1200

0

Temperature/ C

Figure 1. (a) Thermogravimetric analysis (TGA) traces of xerogel (x = 0.20) heated at 100 °C for 24 h. (b) DTA and TG of 0.2 GaPO4-0.8 SiO2 glass after annealing at 650 ℃ for 5 h.

The adsorption-desorption hysteresis loops indicate that this sample consists of aggregates of plate-like particles (see Supporting information S1).37 The pore radius distributions extracted for the other samples are shown in figure 2f. The main pore characteristics, BET surface area (SBET), pore volume (VP) and average pore radius (rp) of glass samples with different compositions and sintering temperatures are summarized in Table 1. No porosity was found for the x = 0.5 (Ga/Si = 1:1 sample). The BET surface areas of the other samples vary between 341 and 425 m2/g, ACS Paragon Plus Environment

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except for the sample with x=0.33 (Ga/Si = 0.5), which may be partially separated into larger GaPO4 and SiO2 domains and results in relatively large average pore sizes and low BET surface areas. In general the surface areas obtained in this study suggest that these materials can serve as potential catalysts or catalyst supports for high temperature applications. They may also have utility for drug delivery applications. As far as we know, this is the first report on the successful preparation of GaPO4–SiO2 glassy materials, exhibiting high surface area and mesoporous structure.

(a) 160

(c) 240

(b) 160

200

40

0 0.0

3 -1

80

120

Vads / cm g

3 -1

Vads / cm g

Vads / cm g

3 -1

120

80

0.2

0.4 0.6 0.8 Relative pressure p/p0

0 0.0

1.0

120

40

0.2

0.4

0.6

0.8

1.0

0.0

0.2

Relative pressure p/p0

(f)

(e) 240 200

dv/dr(cm /g)

0.30

160

1.0

x= 1/8 1/6 1/4 1/2

0.20

3

Vads / cm g

3 -1

3 -1

160 120


0.25

200

120

80

0.15 0.10 0.05

80

0.00

40 0.0

160

80

40

(d)

Vads / cm g

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0.2

0.4

0.6

0.8

1.0

40 0.0

0.2

Relative pressure p/p0


1.0

2

3

4 5 6 Pore diameter/nm

7

8

Figure 2. Adsorption–desorption isotherms of xGaPO4–(1-x) SiO2 sol–gel glasses annealed at 650 ℃ for 5 h. (a) x = 0.5 (Ga/Si = 1); (b) x = 0.33 (Ga/Si = 1/2); (c) x = 0.20 (Ga/Si =1/4); (d)x = 0.14 (Ga/Si= 1/6); (e) x = 0.11 (Ga/Si = 1/8); (f) pore diameter distributions derived from these data using the BJH method for samples with different Ga/Si ratios.

Table 1. BET Surface Area (SBET), the Total Pore Volume (Vp), and the Average Pore Diameter (rp) of xGaPO4- (1-x)SiO2 Glasses synthesized via the Sol–Gel Method x

SBET(m2/g)(±5% m2/g)

Vp (cm3/g)( ±5% cm3/g)


rp (nm) (±0.15 nm)


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0.5

50.0

0

not detd.

0.33

183.8

0.06

5.3

0.2

367.2

0.24

3.6

0.14

341.4

0.23

3.7

0.11

425.1

0.28

3.6

3.2 Structure evolution during sol → gel → glass conversion monitored by liquid and solid state NMR. Figure S2 (Supporting Materials) shows the

71

Ga and

31

P spectra of the

freshly prepared precursor solutions. The chemical shifts of them ( -0.7 vs Ga(NO3)3 solution and -0.8 ppm vs H3PO4, respectively) indicate the solution-state species [Ga(H2O)6]3+ and triethyl phosphate, respectively.

71

Ga

29

31

Si

P

0

650 C 0

500 C 0

400 C 0

300 C 0

200 C 0

100 C 1600

800

0 ppm

-800

-1600

40

20

0

-20 ppm

-40

-60

-40

-80

-120

-160

-200

ppm

Figure 3. Evolution of the 71Ga (left), 31P (middle), and 29Si (right) MAS-NMR spectra as a function of heat treatment temperature 100 ℃ ×24 hours, 200 ℃ ×24 hours, 300 ℃ ×5 hours, 400 ℃ ×5 hours, 500 ℃ ×5 hours, and 650 ℃ ×5 hours of a sample with the composition 0.2GaPO4-0.8SiO2 after gelled at 50°C for 7 days. The top 31P spectrum is from crystalline GaPO4.

Figure 3 shows the (ex-situ) 71Ga, 31P, and 29Si MAS-NMR spectra of the sample with x = 0.20 during the evolution from xerogel to glass upon heating to different temperatures. Although poorly resolved, the


71

Ga

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spectra suggest the presence of both Ga(IV) and Ga(VI) gallium species. Upon heat treatment up to 400 ℃, the contribution of Ga(VI) gradually decreases, probably owing to the loss of water and/or hydroxyl ligands. Heat treatment at higher temperatures (T > 400 ℃) leads to an apparent increase in the Ga(VI) content and the formation of Ga(V) species as well. An interesting structural evolution is also observed in the spectra. After sample heating at 200 ℃ for 24 h, the

31

31

P NMR

P NMR spectra

have one main peak at about -11.7 ppm with a shoulder at about -23 ppm. The signal at -11.7 ppm, which is close to that of crystalline GaPO4 is attributed to P(4)(4Ga) units, in accordance with previous assignments36,37. As discussed in more detail further below, the shoulder at about -23 ppm is attributed to P(4)(3Ga, 1Si) units (see Figure 4). When the heat treatment temperature is increased to 500℃, the intensity of this signal increases and becomes comparable to that of the P(4)(4Ga) unit, and another signal at about -35 ppm appears, which we attribute to P(4)(2Ga, 2Si) units, based on further arguments discussed below. The 29Si spectra have a main peak at about -110 ppm (Si(4) species) and an obvious shoulder at about -100 ppm assigned to Si(3) species attached to OH groups. With increasing heat treatment temperature, the Si(3) units are gradually removed due to dehydroxylation. The spectroscopic appearance of samples heated at 100 ℃ varies substantially during the course of multiple preparations. Apparently its structure, which still contains many OH- groups and water



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ligands, depends sensitively upon the exact treatment conditions applied. P(4)(4Ga)

P(4)(3Ga)

OGa(IV)

Ga(IV)O

P

P(4)(2Ga) OSi

OGa(IV)

OGa(IV)

Ga(IV)O

P

OGa(IV)

OSi

Ga(VI)O

Ga(IV)

Ga(V)

Ga(VI)

OP

OP

OP

Ga

SiO

PO

OSi

OP

Ga

Ga

OP

OSi

OP

OSi

OSi

PO

Si(4)(1Ga)

Si(4)

Ga(VI)2O

Si

OSi

SiO

Si(3)(1H)

Si

OSi

SiO

Si

OP

OSi

Si(2)(2H)

OSi

OSi

OSi

OSi

OSi

Si

Si(4)(1P)

OSi

OSi

HO

OSi

OGa(VI)

OGa(VI)

OP

PO

P

OSi

OSi

HO

Si

OH

OSi

Figure 4. Sketches of phosphorus (first row), Gallium (second row) and silica (third and fourth rows) units presumed to be present in GaPO4-SiO2 gels and glasses. In the P(n)(mGa) and Si(n)(mGa, mP or (4-n)H) nomenclature the superscript n denotes the total number of bridging oxygen atoms, while the number m describes the number of P-O-Ga, Si-O-Ga, or Si-O-P linkages. The number of non-briding OH groups is given by 4-n.

3.3 Effect of Composition on Glass Structure Figure 5 shows the effect of composition on the


71

Ga,

31

P, and

29

Si

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spectra of the sol-gel prepared glasses heated at 650 °C. Systematic compositional spectroscopic changes are observed for all of these nuclei. To facilitate signal deconvolutions and to aid in peak assignments, a number of advanced dipolar MAS-NMR experiments were carried out, targeted at the selective detection and quantification of hetero- and homonuclear magnetic dipole-dipole interactions, to be related to interatomic network connectivities.

71

Ga

29

31

Si

P

0.11 0.14 0.20 0.33 x=0.50

1600

800

0 ppm

-800

-1600

40

20

0

-20 ppm

-40

-60

-40

-80

-120 ppm

-160

-200

Figure 5. 71Ga, 31P, and 29Si MAS NMR spectra of glasses with the composition of xGaPO4-SiO2 (x=0.5, 0.33, 0.20, 0.14 and 0.11, respectively).

71

Ga MAS-NMR and

71

Ga{31P} REDOR. The

71

Ga NMR spectra

(Figure 5, left) show a dominant signal at a resonance shift of 84 ppm, and two weaker signals located at -6 ppm and -73 ppm for the sample with x=0.5. These signals are again attributed to 4-, 5- and 6-coordinate gallium, respectively. No deconvolution into Ga(IV), Ga(V) and Ga(VI) sites was attempted here because of serious peak overlap. However, the spectra illustrate that the Ga(V) and Ga(VI) signals gradually increase in intensity with decreasing gallium phosphate content. Figure 6 shows the



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results of

71

Ga{31P} REDOR experiments. While the

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71

Ga-31P dipolar

coupling strengths for Ga(IV), Ga(V) and Ga(VI) sites could not be measured separately because of the strong peak overlap, their dephasings in the 31P dipolar field were found to be similar to each other. Thus, the second moments are reported based on the dipolar dephasing of the overall

71

Ga signal. The M2 values extracted in this fashion indicate

strong 71Ga-31P interactions, independent of the glass compositions. Their numerical values are close to, albeit somewhat smaller than the value calculated for crystalline GaPO4 (8.4×106 rad2/s2). In this compound each Ga3+ species is surrounded by four phosphate ions at a distance of 3.089 pm. Thus, the

71

Ga{31P} REDOR results indicate that, independent of

composition, the local environments of the Ga species in these glasses are dominated by phosphate anions. On the other hand, the decreased M2 values found in the glasses, which is ~6.3×106 rad2/s2, close to the M2 of three Ga-O-P in crystalline GaPO4, suggests that the number of Ga-O-P linkages may be somewhat lower than four, which is consistent with the formation of some Ga-O-Si linkages. But it should be noted that, the real distance between Ga and P in these glasses might be a little longer than that in crystalline GaPO4 owing to the looser network structure of the sol-gel glasses.


Page 17 of 35

0.8 x=

0.7

0.50 0.33 0.20 0.11

0.6 0.5 (S0-S')/S0

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


0.4 0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6 NTr/ms

0.8

1.0

Figure 6. 71Ga{31P} REDOR experiment results of glasses with the compositions of xGaPO4-(1-x)SiO2 (x=0.5, 0.33, 0.20, 0.14 and 0.11, respectively). Table 2 71Ga resonance shifts (Including both Isotropic Chemical Shifts and Second Order / Quadrupolar Shifts (± 5 ppm) and M2 (71Ga{31P}) Values from REDOR and ∑. ,-. Values from 31P-31P DQ-DRENAR-BABA-xy16 Obtained for xGaPO4-(1-x)SiO2 Glasses and the Model Compound GaPO4.

x

M2 (71Ga{31P})

71

Ga Resonance Shifts/ppm

/106 rad2/s2 (±15%) 0.5

6.3

84.0a, -5.1b, -72.7c

0.33

6.6

79.8,0.3,-70.7

0.20

5.6

80.5,-3.0, -67.8

0.14

n.m.

79.8,-2.5,-72.7

0.11

5.9

78.6,-2.8,-70.4



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

8.4d

GaPO4 cryst.

Page 18 of 35

n. m.e

a ,b, c 71

Ga resonance shifts of Ga(IV), Ga(V) and Ga(VI), respectively Theoretical values calculated from the crystal structure. e Not Measured. d

31

P MAS-NMR and 31P{71Ga} REAPDOR. Figure 5, middle, shows

the compositional evolution of the 31P MAS-NMR spectra of the glasses. For x = 0.5, the

31

P spectra show a main peak at about -13 ppm and a

shoulder at about -23 ppm. With decreasing GaPO4 content, the signal at -23 ppm increases in relative intensity and an additional signal at about -34 ppm starts to grow. To aid in the signal assignments,

31

P{71Ga}

REAPDOR experiments were conducted on the sample with x=0.2 at mixing time of 2.0 ms, respectively. As illustrated in Figure 7b, the extent of dephasing at 2NTr = 2.0 ms decreases successively from the -13 ppm to the -23 ppm, and to the -35 ppm signal components. This can also be seen by comparing the deconvolutions of the S0 and the S’ signals (see Figures 7c and d). The same behavior was also observed for the x = 1 sample (see Supporting Information). Unfortunately, a quantitative prediction of the REAPDOR effect based on structural scenarios is not possible in the present case. This is due to the fact that the strength of the 71

Ga nuclear electric quadrupolar interaction, which also influences the

REAPDOR effect, cannot be reliably assessed from the present MAS-NMR lineshapes.


71

Ga

Page 19 of 35

(a)

Spin Echo (S0)

(c)

Experimental (4) P (4Ga)

REAPDOR (S') (S0-S')

(4)

P

(3Ga) (4)

P

30

0.35

20

10

0

-10 -20 -30 -40 -50 -60 -70 ppm

30

20

10

0

(2Ga)

-10 -20 -30 -40 -50 -60 -70 ppm

(b) 4

P

0.30

4Ga

4

P

0.25 (S0-S')/S0

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


(d)

3Ga

Experimental (4) P (4Ga)

2

P

2Ga

(4)

0.20

P

0.15

P

(3Ga) (4) (2Ga)

0.10 0.05 0.00 0

1

2

3

4

30

20

10

NTr/ms

0

-10 -20 -30 -40 -50 -60 -70 ppm

Figure 7: 31P{71Ga} REAPDOR results obtained on the glasses with x = 0.2. (a) comparison between the spin Echo, REAPDOR curves and their difference, measured at mixing time of 2.0 ms; (b) dephasing values of 31P from different sites at mixing time of 2.0 ms; (c) and (d) shows the deconvolution of spin echo and REAPDOR curves.

Nevertheless the compositional lineshape trend in Figure 5, together with the REAPDOR data of Figure 7 (showing a successive diminution of the 31

P-71Ga dipolar interaction strength with decreasing resonance frequency)

suggests the assignment of the -13, -23, and -35 ppm signals to P(4)(4Ga), P(4)(3Ga,

1Si)

and P(4)(2Ga,

2Si)

units, respectively (see Figure 4). This

assignment is further supported by 31P-31P 1D refocused INADEQUATE experiments carried out on two representative samples (x=0.33 and 0.11). As detailed in Figure S4 (Supporting Information), no DQ-filtered signal is detected, indicating the absence of P-O-P bond connectivities. The deconvolution of all the 31P MAS-NMR spectra is shown in Figure 8, and



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Page 20 of 35

the lineshape component parameters are summarized in Table 3. Note that the single-pulse spectrum of the x = 0.20 sample differs somewhat from the REDOR S0 signal shown in Figure 7c for the dipolar evolution time of 2.0 ms. We attribute this discrepancy to a T2 difference between the different phosphorus sites, resulting in a relative distortion of the S0 lineshape. Therefore, all the quantitative

31

P MAS-NMR lineshape

deconvolutions were done on the single-pulse spectra of Figure 8. x=0.5

20

0

-20 ppm

-40

-60

20

0

-20 ppm

-40

-60

0

-20 ppm

-40

-60

20

0

-20 ppm

-40

-60

x=0.11

x=0.14

20

x=0.20

x=0.33

0

-20 ppm

-40

-60

20

Figure 8. Deconvolution of 31P spectra of glasses with the compositions of xGaPO4(1-x)SiO2. (x=0.50, 0.33, 0.20, 0.14, and 0.11 respectively). The red, green and blue dashed curves are assigned to P(4)(4Ga), P(4)(3Ga, 1Si) and P(4)(2Ga, 2Si) units respectively.

Table 3. Parameters Used in the Deconvolution of the 31P MAS-NMR Spectra of / xGaPO4-(1-x)SiO2 Glasses and ∑. ,-. Values from 31P-31P DQ-DRENAR-BABA-xy16 Experiments.

x

0.50

0.33

Chemical shift/ppm ±0.5 ppm

Peak width/ppm ±0.5 ppm

Fraction, % ±1.0

-12.7 -22.8 -32.7 -12.8 -24.0

11.8 13.0 13.6 13.6 13.0

69.6 25.4 5.0 57.3 35.1


/ 31 ∑. ,-. ( P)/105

Hz2 (±15%) 1.7a

1.9a

Page 21 of 35

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


0.20

0.14

0.11

GaPO4 cryst.

-33.8 -12.9 -23.9 -34.0 -13.1 -23.9 -34.7 -13.1 -23.9 -35.4 -10.3

13.6 13.6 13.0 13.6 13.8 13.0 13.6 13.8 13.0 13.6

7.6 53.9 36.0 10.1 43.1 42.0 14.9 40.5 41.6 17.9 100

n.m.

n.m.

1.6a, 0.6b 1.4b 1.8b 3.8a(4.8c)

a

Average value over the entire lineshape Values resolved for individual components. c Theoretical values calculated from the crystal structure. b

29

Si MAS-NMR and

Si{31P} REDOR. The 29Si MAS-NMR spectra

29

(see Figure 5, right) are only weakly dependent on GaPO4 content. They have a main peak at about -110 ppm and tail towards higher chemical shift values, suggesting an additional component that may be attributable to Si(4)1Ga and Si(2)2OH, or Si(3)1OH species (see Figure 4). In addition, a low-frequency shoulder at about -114 ppm may indicate the presence of some Si-O-P linkages. Figure 9 shows the

29

Si{31P} REDOR results

obtained on samples with x= 0.20 and 0.33, respectively. Because of severe signal-to-noise limitations, only one data point could be measured, which was chosen to correspond to a dipolar evolution time of 2.75 ms. One can note a clear dephasing effect in the low-frequency region of the signal, which also becomes stronger with increasing GaPO4 content. These results confirm the existence of P-O-Si connectivities. In order to quantitatively analyze the dephasing effect, the

29

Si MAS NMR spectra



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

Page 22 of 35

were deconvolved into the four components listed in Table 4, ascribed to Si(2)(2OH), Si(4)(1Ga)+ Si(3)(1OH), Si(4)(4Si) and Si(4)(3Si, 1P) units, respectively. Figure 9g compares the experimental dephasing effect observed for the signal component near -114 ppm, at a dipolar evolution time of 2.75 ms in the two samples measured. The extent of dephasing values observed for the two samples is very close to that expected for the simulated 29

Si{31P} REDOR difference signal for a 29Si-O-31P two spin system with

a distance of 305 pm. This distance corresponds to the average

29

Si-31P

distance in the model compound Ni2Si(P2O7)2, which contains such Si-O-P linkages.38 Overall, the good agreement of the REDOR dephasing with this predicted value indicates that this site really has only one Si-O-P connectivity, consistent with the assignment to Q4(3Si, 1P) units.

Figures

10 and Table 4 summarize the final 29Si MAS-NMR peak deconvolutions of the single-pulse spectra for all of the samples of the present studies. Based on these deconvolutions, we can deduce the fraction of Si atoms involved in one Si-O-P linkage amounts to 15.2% and 21.6% in glasses with x = 0.2 and 0.33, respectively.


Page 23 of 35

(a)

S0

(b)

(c)

S' S0-S'

-60

-70

-80

-90 -100 -110 -120 -130 -140 -150

-60

-70

-80

ppm

-90 -100 -110 -120 -130 -140 -150

-60

-70

-80

ppm

(d)

(e)

S0

-90 -100 -110 -120 -130 -140 -150

ppm

(f)

S' S0-S'

-60

-70

-80

-90 -100 -110 -120 -130 -140 -150

1.0

-60

-70

-80

-90 -100 -110 -120 -130 -140 -150

-60

-70

-80

ppm

ppm

-90 -100 -110 -120 -130 -140 -150

ppm

(g)

0.8

x=0.33 x=0.2 Simulation

0.6 (S0-S)/S0

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


0.4 0.2 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

NTr/ms

Figure 9. 29Si {31P} REDOR results on samples with x= 0.20 (a-c) and 0.33 (d-f) respectively, using a dipolar evolution time of 2.75 ms. For each composition, (a) and (d) show the spectra without dipolar recoupling (So signal, solid black curve), with 29Si-31P dipolar recoupling (solid red curve) and the REDOR difference signal S0-S’ (dashed black curve). (b) and (e) show the deconvolutions obtained for the spectra without 29Si-31P dipolar recoupling (S0 signal), and (c) and f show the deconvolutions obtained for the REDOR signal S’, in which the contribution from Si4(1P) units is minimized. (g) The comparison between the simulated and experimental 29Si{31P} REDOR results for the signal from Si4(1P) unit represented by red dashed curves contained in the deconvolution spectra. In the simulation, the 29Si-31P distance is assumed to be 305 pm.



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

x=0.50

-60

x=0.33

-80

-100

-120

-140

-160 -60

x=0.20

-80

-100

ppm

-60

-120

-140

-160 -60

-80

-100

ppm

x=0.14

Page 24 of 35

-120

-140

-160

ppm

x=0.11

-80

-100

-120

-140

-160 -60

-80

-100

ppm

-120

-140

-160

ppm

Figure 10. Deconvolution of the 29Si MAS-NMR spectra from glasses with the compositions of xGaPO4-(1-x) SiO2 (x= 0.50, 0.33, 0.20, 0.14, and 0.11, respectively). The magenta, green, blue and red dashed curves correspond to the signals at about -93, -100, -107 and -115 ppm, respectively. Table 4 Parameters used in the Deconvolution of the xGaPO4-(1-x)SiO2 Glasses.

x

0.50

0.33

0.20

0.14

0.11

29

Si MAS-NMR Spectra of

Chemical shift/ppm

Peak width/ppm

Area fraction/%

±0.5 ppm

±0.5 ppm

±1.0

-95.1 -100.4 -107.4 -114.6 -93.1 -99.8 -108.2 -114.3 -92.1 -99.8 -108.0 -114.3 -92.1 -99.8 -108.9 -114.8 -92.1

8.3 8.5 11.2 11.0 8.3 8.5 11.2 11.0 8.3 8.5 11.0 11.2 8.3 8.5 12.4 11.0 8.3

8.7 9.7 52.1 29.5 5.4 14.1 58.9 21.6 6.2 18.5 60.1 15.2 5.7 15.4 66.1 12.8 4.4


Page 25 of 35

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


-99.8 -108.7 -116.0

8.5 11.2 11.0

18.4 66.6 10.6

31

P DQ-DRENAR Results. Finally, the spatial distribution of the

phosphate species has been probed by

31

P DQ-DRENAR experiments.

Results are summarized in Figure 11. Part (a) shows the overall homonuclear dipolar coupling effect of samples with compositions x = 0.5 and 0.11 and the standard crystalline GaPO4 samples, while part (b) shows resolved results for the three distinct phosphorus units. The P(4)(4Ga) units show significantly stronger dipolar coupling strengths than the P(4)(3Ga), and in particular, the P(4)(2Ga) units. The measured summed squares of the dipolar coupling constants ∑& $%& extracted via eq. (2) are listed in Table 4. The ∑& $%& values of the glasses (1.6 – 1.9 × 105 Hz2) are

relatively similar to each other, and this lack of compositional dependence is a clear indication of chemical segregation. On the other hand, these values are only about half of the value measured for crystalline GaPO4 (3.8 × 105 Hz2). In addition, they are also significantly smaller than those previously measured for phase-separated AlPO4-SiO2 glasses prepared via the sol-gel technique (3.2 × 105 Hz2).35 Altogether these results do not agree with a phase separation model into bulk-size SiO2 and GaPO4 microdomains. Consistent with the conclusions from the 31

P and 29Si MAS-NMR and the 71Ga{31P} REDOR results, they indicate

non-negligible interactions between the gallium phosphate and silica ACS Paragon Plus Environment


networks, possibly at the interfaces of nano-sized domains.

(a)

(b)

1.2

(2) 2Ga (1)

1.0

x=0.5 x=0.33 x=0.11

P

3Ga (0)

P

0.8 (S0-S')/S0

0.8

1.2 P

GaPO4

1.0

(S0-S')/S0

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

Page 26 of 35

0.6 0.4 0.2

4Ga

0.6 0.4 0.2

0.0 0.0

0.5

1.0 1.5 NTr/ms

2.0

2.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

NTr/ms

Figure 11. DQ-DRENAR-BABA-xy16 results of the glasses under study. (a) DQ-DRENAR-BABA-xy16 results analyzed from the entire 31P NMR lineshape. (b) DQ-DRENAR-BABA-xy16 results measured separately for the three resolved 31P signal components, measured in a glass with the composition x = 0.11.

4. DISCUSSION As stated above, the dominant presence of P(4)(4Ga) and Si(4)(4Si) units in the

31

P and

29

Si MAS-NMR spectra indicates that GaPO4-SiO2 sol-gel

glasses tend to form segregated systems composed of GaPO4 and SiO2 domains. However, the spectra also indicate that the networks are not entirely separated. Specifically the

29

Si and

31

P MAS NMR spectral

analyses, the 71Ga{31P} and 29Si{31P} REDOR data, and the significantly 31 31 diminished ∑& $%& ( P- P) values in comparison to GaPO4 as extracted

from DRENAR give clear evidence for substantial interactions between the silica and gallium phosphate networks, presumably at the interfaces of their respective nanodomains. This interaction may well arise as a consequence of the structural evolution, signified by the spectroscopic


Page 27 of 35

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


changes as a function of heat treatment temperatures. The number of Si-O-P linkages can be calculated from the fractional area f(Si(4)(3Si,1P)): N (Si-O-P) = f(Si(4)(3Si,1P)) × (1-x). It can also be calculated from the

31

P MAS-NMR spectra using the

analogous expression: N(P-O-Si) = xf(P(4)(3P,1Si) ) + 2xf(P(4)(2P,2Si) ). Table 5 indicates that both numbers are in excellent agreement with each other, confirming the reliability of the spectral deconvolutions in Tables 3 and 4. Table 5 also includes the average coordination numbers q of the phosphorus atoms with Si and Ga, which can be deduced from this analysis, assuming q(P-O-Ga) = 4 – q(P-O-Si). These numbers suggest a consistent trend toward increased network mixing with decreasing GaPO4 content. This result suggests that with decreasing x, the gallium phosphate-like domains become smaller, increasing the interfacial area with the silica component, and hence the relative concentration of the P-O-Si linkages. This conclusion is further supported by the

31

P

DQ-DRENAR result, which suggests some diminution of the strength of the

31

P-31P dipole-dipole interactions at lower GaPO4 contents, which

may well be related to an increase in the relative contribution of the P atoms in the interfacial area towards the DRENAR effect.



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

Table 5: Numbers of N(Si-O-P) and N(P-O-Si) as Deduced from 29Si and 31P MAS-NMR and Average Coordination Numbers of the P Atoms with Si and Ga, Respectively. X N(Si-O-P) N(P-O-Si) q(P-O-Si) q(P-O-Ga) 0.5 0.15 0.18 0.35 3.65 0.33 0.14 0.17 0.50 3.50 0.20 0.12 0.11 0.56 3.44 0.14 0.11 0.10 0.72 3.28 0.11 0.094 0.086 0.77 3.23

Altogether, the extent of GaPO4-SiO2 mixing is significantly more pronounced than that in the compositionally analogous AlPO4-SiO2 sol-gel system, which appears to be macroscopically phase separated by all NMR accounts.21,

35

In the AlPO4-SiO2 sol-gel system, all of the

aluminum is four-coordinated, forming large AlPO4 domains with NMR observables close to those of the bulk AlPO4 glass. In contrast to that system, the GaPO4-SiO2 sol-gel glasses also feature substantial concentrations of higher-coordinated gallium sites (i.e. Ga(V) and Ga(VI)) (see Figure 5). Although the

71

Ga NMR spectra do not allow strict

quantifications of the concentration of these higher-coordinated gallium species, the spectra show a clear increase of the Ga(V) and Ga(VI) concentrations with decreasing x, which is well correlated with the concentrations of the P(4)(3Ga, 1Si) and P(4)(2Ga, 2Si) units. Considering that all of the units that differ from Si(4)(4Si), Si(3)(OH), Si(2)(OH)2, and from the coupled Ga(4)(4P) / P(4)(4Ga) units in bulk GaPO4, require some charge balancing mechanism, we suspect that the formation of Ga(V) and Ga(VI) ACS Paragon Plus Environment

Page 28 of 35

Page 29 of 35

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species may occur as a mechanism of charge balancing anionic Ga(IV) involved in Ga(IV)-O-Si connectivities and/or P(4)(3Ga,1Si) and P(4)(2Ga,2Si) units formed as a consequence of the interfacial interactions between the silica and gallium phosphate nanodomains. To support this hypothesis it would be necessary to probe the spatial correlation between these P(4)(3Ga,1Si) and P(4)(2Ga,2Si) units and the higher-coordinated Ga species, using 2D

31

P{71Ga} heteronuclear correlation NMR.39,40 Efforts in

undertaking challenging experiments of this kind are currently underway in our laboratories.

CONCLUSIONS In summary, we report the first preparation of mesoporous GaPO4-SiO2 sol-gel glasses. Structural examination by multinuclear single and double resonance solid state NMR studies reveals these glasses to be chemically segregated into GaPO4-rich and SiO2-rich domains. Unlike the analogous aluminum phosphate-based glass system, which is macroscopically phase-separated, the present NMR results reveal clear evidence of Si-O-P and (indirectly) Si-O-Ga linkages, which can be quantified by advanced dipolar NMR methods. Most likely these linkages occur in the interfacial regions of the gallium phosphate and silica nanodomains. Based on the compositional evolution of all the spectroscopic results, the nanodomain sizes appear to decrease with decreasing fraction of the gallium phosphate



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

component. Altogether these results indicate the utility of advanced dipolar solid state NMR methodology to characterize chemical nanosegregation processes in glasses and glass ceramics.

ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft through the programme SFB858, the National Nature Science Foundation of China with the granted number NSFC 61475174 , 100 Talents Program of Chinese Academy of Sciences, and Open Fund from State Key Laboratory of Silicate Materials for Architectures （Wuhan University of Technology）. We also acknowledge funding by the DFG, SFB858, and by FAPESP, grant number 2013/07793-6 (CERTEV – Center for Research, Technology and Education in Vitreous Materials).

ASSOCIATED CONTENT Supporting Information Scanning electron micrograph of a representative glass sample, liquid-state NMR data,

31

P{71Ga} REAPDOR data and solid-state 31P

refocused INADEQUATE experiments. This information can be found on the internet at http://pubs.acs.org.


Page 30 of 35

Page 31 of 35

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AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] *E-mail: [email protected]

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