Experimental and Theoretical Investigation of the Structural Role of

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Experimental and Theoretical Investigation of the Structural Role of Titanium Oxide in CaO-PO-TiO Invert Glass 2

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Hirotaka Maeda, Tomoyuki Tamura, and Toshihiro Kasuga J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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The Journal of Physical Chemistry

Experimental and Theoretical Investigation of the Structural Role of Titanium Oxide in CaO-P2O5TiO2 Invert Glass Hirotaka Maedaa*, Tomoyuki Tamurab, c*, Toshihiro Kasugaa a

Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Gokiso-

cho, Showa-ku, Nagoya 466-8555, Japan. b

Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso-cho,

Showa-ku, Nagoya 466-8555, Japan. c

Center for Materials research by Information Integration, National Institute for Materials

Science, Tsukuba 305-0047, Japan.

Manuscript ID: jp-2017-02350v Revised Manuscript

ACS Paragon Plus Environment

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ABSTRACT Understanding the structural role of TiO2 in calcium phosphate invert glasses is key for developing a new glass design for biomedical applications. Experimental and computational analysis methods were used to investigate the impact of TiO2 substitution in these glasses. Spectroscopic analyses indicated that titanium oxide exists as both TiO4 and TiO6 units, leading to the formation of Ti-O-P bonds, in spite of depolymerization of the phosphate chains. Classical molecular dynamics showed that the presence of TiO2 influences the phosphate units and CaO polyhedral structures. The formation of the Ti-O-P bonds caused an increase in the network connectivity of the invert glasses, leading to the improvement of the glass forming ability and wettability. The addition of TiO2 to calcium phosphate invert glasses led to the introduction of bioactivity.


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INTRODUCTION Phosphate-based glasses have been used for various applications, including biomaterials and optical lenses.1,2 The solubility and bioactivity of these glasses play important roles in biomedical applications because they affect how well the glass bonds with human tissue. These glass properties can be tailored to specific applications by modifying the composition of the glass. Phosphate glasses with less than 40 mol% P2O5, classified as invert glasses, are less soluble when in contact with body fluids than the meta- and ultra-phosphate glasses with more than 50 mol%.3 However, the solubility is not only a function of the glass composition. In the case of calcium metaphosphate glasses soaked in simulated body fluids, no apatite formed on the bone surfaces and thus no bonding to the bone occurred. The mechanism for this lack of bioactivity was related to the release of phosphate species into the simulated body fluids, which resulted in a decrease in solution pH.4 In another case of cytotoxicity P2O5-CaO-Na2O glass was reported to be related to the dissolution associated with pH changes.5 In vitro evaluation of the bioactivity of glasses in a CaO-P2O5-TiO2 system with more than 50 mol% CaO showed that apatite forms on the surface of the 60CaO-30P2O5-10TiO2 glass.6 This type of glass maintained a neutral pH in solution after soaking simulated body fluid. In another study, adding TiO2 and Na2O to calcium pyrophosphate invert glasses led to the development of bioactivity.7 These studies suggest that substituting TiO2 into calcium phosphate invert glass may result in bioactivity, since calcium phosphate is a suitable material for biomedical applications and the phosphate species are less soluble in body fluids than meta- and ultra-phosphate glasses.


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Understanding the structural changes in phosphate glasses when the chemical compositions are varied is vital for tailoring their properties for specific biomedical applications. Nano-scale structural characteristics of glasses have been investigated extensively. In the case of invert glass, anionic groups are connected through cations, leading to glass formation. The phosphate invert glasses consist of small phosphate units such as pyrophosphate or orthophosphate groups, but no metaphosphate groups.8 We previously reported that TiO2 substitution in calcium phosphate invert glasses causes crosslinking with the phosphate groups, forming P-O-Ti bonds.9,10 In general, it is widely believed that titania enters as TiO4 tetrahedra or TiO6 octahedra connected to phosphate groups through P-O-Ti bonds, depending on their chemical compositions.11,12 However, the structural impact of TiO2 substitution on the short-, mid-, and long-range bonds of the glass is not clear. Understanding this structural impact is one of the key factors for developing new designs for biomedical applications. Computational analysis with classical and/or first-principles molecular dynamics simulations may be suitable tools for investigating the short- and mid-range structures of these glasses.13,14 A combination of an experimental approach, including spectroscopic methods, with computational analyses has great potential for evaluating the phosphate invert glass structure. To our knowledge, no computational studies of these glasses exist in the literature. In the present work, we investigate the influence of TiO2 substitution in calcium phosphate glass on the glass structure using experimental analysis and classical molecular dynamics simulation.

EXPERIMENTAL METHODS


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Materials A batch mixture of 60CaO-(40-x)P2O5-xTiO2 glass (x=0, 5 and 10 mol%) was prepared using reagent-grade calcium carbonate and phosphoric acid without further purification. The mixture was melted in a Pt crucible at 1500 °C for 30 min in air. The melt was poured onto stainless steel and quenched by iron-pressing to prepare glass plate pieces. In this paper, 60CaO40P2O5, 60CaO-35P2O5-5TiO2 and 60CaO-30P2O5-10TiO2 glasses are denoted as Ti00, Ti05 and Ti10 glasses, respectively. Structural Characterization Density measurements of glasses were conducted on three identical samples by Archimedes’ method. Absolute ethanol was used as a solvent because the glasses are soluble in water. To calculate the density of glass (ρglass), we used the following equation: ρ =

ρ −

where Wair and Wethanol are the masses (g) of the glass in air and ethanol respectively, and ρethanol is the density (g/cm3) of ethanol at ambient temperature. The mass of the glasses in ethanol was measured using a hanging pan. The thermal properties of the glass were measured using a thermogravimetric analyzer (Thermoplus TG8120, Rigaku, Tokyo, Japan) at a rate of 5 oC/min. The glass structure was examined using laser Raman spectroscopy with a 20× objective lens (inVia Raman microscope, Renishaw, Gloucestershire, UK) by a 532 nm excitation laser, and a magic-angle-spinning nuclear magnetic resonance spectrometer (MAS-NMR, JEOL RESONANCE JNM-ECA600II, JEOL, Tokyo, Japan) at a spinning speed of 15 kHz, a delay time of 5 s, and a pulse length of 1.0 µs. The chemical shifts in the 31P MAS-NMR spectra were


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measured relative to 85% H3PO4. X-ray photoelectron spectra (XPS, PHI-5000, Ulvac, Kanagawa, Japan) of O1s in the glass were measured using monochromatized AlKα X-ray irradiation. Ar ion sputtering was carried out for 3 min prior to XPS measurements to clean up the surface. The binding energy was normalized to the C1s energy. The spectra were deconvoluted by the least squares method of a Gaussian function. The static wettability of the glass was characterized using an optical contact angle analyzer (DM-501, Kyowa Interface Science, Saitama, Japan). Molecular Dynamics Simulations Glass models of quenching from the melt were generated through classical molecular dynamics (MD) simulations using our in-house MD code, and contained about 2,000 atoms. The interatomic potentials we used included a short-range Morse function, a repulsive contribution, and a long-range Coulomb interactions with partial ionic charges. The parameters for Ca-O, P-O, Ti-O, and O-O pair interactions were taken from the extensive library of potentials developed by Pedone et al.,15 which have been shown to perform well in MD simulations of phosphosilicate glasses.16,17 We used the Ewald summation method to calculate a Coulomb interaction. We confirmed that the lattice parameters for CaO, P4O10 and TiO2 calculated by our code were in good agreement with those by Pedone et al..15 Periodic cubic cells and the initial configurations were generated by randomly positioning the atoms. MD simulations were performed in an NVT ensemble with a time-step width of 1.2 fs. Velocities of atoms were scaled every 50 steps to keep the temperature constant. Mountjoy et al. used a series of stages for modeling 50CaO-50P2O5; the first three stages were temperature baths (with equilibration) at 6000, 3000, and 1500 K for 40 ps each, and the fourth


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stage was a temperature quench of 60 ps from 1500 to 300 K (quench rate 20 K/ps).18 A similar temperature profile was carried out in our simulations, except the quench rate was 10 K/ps. As reviewed by Pedone,19 the theoretical medium range structure of melt-derived glasses is slightly affected by the cooling procedure, and a cooling rate around 10 K/ps is widely used on most MD studies to obtain reliable structural properties. The system size is inversely proportional to the cooling rate, and we used a periodic cubic cell containing around 2000 atoms, similar to previous studies.15,18,20,21 Some previous simulations employed the system sizes larger than 2000 atoms.16,17,22,23 We also performed another series of simulations for Ti10 glasses whose systems contained 1000 and 4000 atoms, and no significant differences were observed. For these reasons, we believe that a system size of 2000 results in an accurate model.

RESULTS Glass Formation and Experimental Characterization In initial experiments, we were unable to obtain Ti05 in a glassy state with an amorphous structure for any of the 3 glass compositions that were evaluated. Partially crystallized glasses were obtained having compositions Ti00 and Ti10. In subsequent experiments, opaque parts were removed manually for the Ti00 and Ti10 compositions and the densities of the Ti00 and Ti10 glasses were determined to be 2.95±0.04 and 3.17±0.01 g/cm3, respectively. XPS analysis showed that Ti00 and Ti10 glasses have the chemical compositions of 61.3CaO-38.7P2O5 and 58.6CaO-30.7P2O5-10.7TiO2, respectively. The glass density increased with the TiO2 substitution, although the molar weight of the glass decreased when calculated using their


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analyzed chemical compositions. This implies that the presence of TiO2 influences the packing density of the network structure. Both glasses were analyzed using differential thermal analysis (DTA), and the glass transition temperatures (Tg) and crystallization temperatures (Tc; onset of the crystallization) were derived from the DTA curves. The Tg (~655 oC) and Tc (~720 oC) of the Ti10 glass were higher than those (Tg: ~565 oC, Tc: ~615 oC) of the Ti00 glass. The glass-forming ability of the Ti00 and Ti10 glasses was calculated to be 0.060 and 0.076, respectively, by using (Tc-Tg)/Tg (K/K),24 indicating that increasing amounts of TiO2 in the glass composition improved the glassforming ability. The contact angle of the glasses was influenced strongly by the addition of TiO2, because the contact angles for Ti00 and Ti10 glasses were 75.0±4.4° and 54.0±4.9°, respectively. Figure 1 shows Raman spectra of the glasses. In the case of the Ti00 glass, multiple absorption bands describe several modes of bonding. The peaks at ~700 and ~1150 cm-1 are due to symmetric stretching modes of bridging and non-bridging oxygens in the Qp2 unit, respectively. The peaks at ~750 and ~1040 cm-1 are due to symmetric stretching modes of bridging and non-bridging oxygens in the Qp1 unit, respectively, and the ~1120 cm-1 peak is evidence of the P-O stretching chain terminator in the Qp1 unit25. The term Qpn, where n is the number of bridging oxygen atoms attached to neighboring tetrahedra, is used to describe the bonding of PO4 tetrahedra. On the other hand, the Ti00 glass showed new absorption bands with a peak at ~950 cm-1 due to a symmetric stretching mode of non-bridging oxygens in the Qp0 unit,26 and at ~650 and ~900 cm-1 due to Ti-O stretching modes of TiO6 octahedrons and TiO4 tetrahedrons, respectively.27 In addition, peaks at ~990 cm-1 are due to Ti-O-P bonds28 and the absorption bands from Qp1 units. The spectrum for the Ti10 glass was deconvoluted using Gaussian functions and estimated by the relative proportion of the integrated Raman peaks


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related to TiO4 tetrahedra and TiO6 octahedra. The proportion (88%) of the peak due to TiO4 tetrahedra had a much higher intensity than that (12%) due to TiO6 octahedra. In a CaO-MgOSiO2-TiO2 glass system, TiO2 exists as 4- and 6-coordinated Ti4+, and their relative proportions have an influence on the TiO2 content.29 These suggest that the TiO2 substitution into the calcium phosphate invert glass is likely to act as TiO4 tetrahedra.

O-P-O

O-P-O

(Q1)

(Q0)

P-O chain terminator

Intensity (a.u.)

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1

(Q )

P-O-Ti

Ti-O TiO 4 P-O-P 1

(Q ) Ti-O TiO6

O-P-O

(b)

2

(Q ) P-O-P 2

(Q )

(a)

1200

1000 800 Raman shift /cm -1

600

Figure 1. Raman spectra of the glasses with nominal compositions of (a) 60CaO-40P2O5 and (b) 60CaO-30P2O5-10TiO2. Figure 2 shows 31P MAS-NMR spectra of the glasses. For the two peaks evident in the Ti00 glass spectrum, one was at approximately -9 ppm for a Qp1 unit and the other at approximately -25 ppm for a Qp2 unit. When TiO2 was substituted into the glass compositions, the spectrum of the Ti10 glass was dramatically changed and two peaks appeared at approximately 1 ppm for a Qp0 unit and approximately -7 ppm for a Qp1 unit. The spectra were deconvoluted using Gaussian functions into two peaks to extract the contribution of each phosphate unit. In the case of the Ti00 glass, the proportion for the Qp1 unit (58%) was larger


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than that (42%) for the Qp2 unit. The Ti10 glass contained 23% for the Qp0 unit and 77% for the Qp1 unit. This indicates that the TiO2 substitution caused the phosphate chain structure to depolymerize.

Intensity (a. u.)

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(b)

(a)

20

0

-20

-40

Chemical Shift / ppm

Figure 2. 31P MAS-NMR spectra of the glasses with nominal compositions of (a) 60CaO40P2O5 and (b) 60CaO-30P2O5-10TiO2. Figure 3 shows O1s XPS spectra of the glasses. The O1s spectrum of Ti00 glass was deconvoluted into two peaks: ~531.0 eV attributed to P-O-Ca sites and ~532.5 eV attributed to P-O-P sites. These deconvoluted peaks associated with phosphate tetrahedra consist of high and low energy binding peaks that represent the bridging and nonbridging oxygens, respectively, in binary phosphate glasses.30,31 The relative proportions of the integrated XPS peaks for P-O-P and P-O-Ca sites were determined to be 22 and 78%, respectively. The XPS spectrum of the Ti10 glass was deconvoluted by adding a third Gaussian component related to the P-O-Ti sites in order to distinguish the local structure of the glasses. Because of the coordination number of calcium and titanium, the electronic densities of oxygens in P-O-Ti sites were assumed to be


10

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higher than those in P-O-Ca sites. This implies that the lower bonding energy peaks attributed to the P-O-cation sites originate from the P-O-Ti sites. The relative proportions of the integrated XPS peaks for P-O-P, P-O-Ca and P-O-Ti were determined to be 15, 52 and 33%, respectively.

Intensity (a. u.)

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(b)

(a)

525

530

535

540

Binding Energy / eV

Figure 3. O1S XPS spectra of the glasses with nominal compositions of (a) 60CaO40P2O5 and (b) 60CaO-30P2O5-10TiO2. Theoretical Characterization Glass models with three kinds of TiO2 content (0, 5 and 10 mol%) were generated using MD simulations. Although Ti05 glass could not be obtained in our experiment, any glass composition can be modeled through MD simulations. Thus a Ti05 glass model was generated using an atomic density interpolated from experimental data for Ti00 and Ti10 glasses. The analysis of the cation-oxygen X-O (X=P, Ca and Ti) radial distribution functions (RDFs) reveals different features around P, Ca, and Ti, as shown in Fig. 4. The first peaks for P-O and Ca-O were located at 1.51 and 2.38 Å, respectively, in agreement with previous MD results for phosphosilicate


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glasses using the same interatomic potentials.16 The general features around P are unchanged, while RDFs of Ca-O and Ti-O broadened along with the increase in TiO2 content. This means that the coordination environments of Ti and Ca in the Ti10 glass are more disordered than those of P in the glasses. Interestingly, RDFs of Ca-O are almost the same for Ti00 and Ti05 glasses. Table 1 reports the coordination numbers and the distortion indexes for cation-centered polyhedra. The coordination number is calculated using the distance of the first minimum in RDFs as shown in Fig. 4. The distortion index D based on bond lengths is defined as

| − | 1 =

where li is the distance from the central cation to the i-th coordinating atom, and lav is the average bond length. P was located in a tetrahedral coordination, independent of the glass composition, while the coordination numbers of Ca and Ti changed with increasing TiO2 content; those of Ca increased and those of Ti decreased. In our simulations, five-fold Ti ions were the most common. Furthermore, the distortion indexes for both Ca and Ti increased with increasing TiO2 content. These results indicate that TiO2 substitution causes the disorder of cation-centered polyhedra.

Figure 4. (a) P-O, (b) Ca-O and (c) Ti-O pair distribution functions.


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Table 1. Comparison of coordination number distribution (%) and distortion index for simulated models.

P-O

Ca-O

Ti-O

ave

D x10-2

5 3

4.1 4.0

2.0 1.8

1

4.0

2.7

4

5

Ti00 Ti05

95 97

Ti10

99

6

7

8

Ti00

11

50

32

6

6.3

5.5

Ti05

7

40

37

14

6.7

6.2

Ti10

4

23

42

27

7.1

8.5

Ti05

0

53

47

5.5

4.7

Ti10

8

53

38

5.3

5.7

Medium range order in the glass structure can be characterized by considering the Qpn distribution of the network former P in Table 2, as well as the bridging-type distribution X-O-X (X=P, Ca and Ti) in Table 3. We can see a clear dependence of Qpn on the TiO2 content. The phosphate structures were predominantly present as Qp1 and Qp2 units (73%) for the Ti00 glass, and Qp0 and Qp1 units (89%) for the Ti10 glass. Network connectivity (NC), which was estimated as weighted averages of the corresponding Qpn distributions,16 decreased with increasing TiO2 content. This trend is consistent with the significant decrease of the first peak in the P-P RDF from the Ti00 to the Ti10, shown in Fig. 5(a). These results obtained from MD simulations strongly support the scenario that TiO2 substitution causes the phosphate chain structure to depolymerize, and the proposed scenario is also supported by the large chemical shift in Raman and NMR experiments shown in Figs. 1 and 2. The bridging-type distribution shows that more than half of the oxygen ions exist as P-O-Ca. In addition, the proportion of P-O-P and P-O-Ca


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decreased and P-O-Ti increased with an increase in the TiO2 content, which is consistent with the XPS results for O1s shown in Fig. 3. The fraction of Ti-O-X could not be experimentally estimated because of the low Ti content. However, it was clear that more than 80% of Ti ions act as P-O-Ti by the theoretically-derived estimation. The presence of a very small fraction of Ti-OTi and Ti-O-Ca suggests a very low tendency of Ti ions to form clusters without PO4 units.

Figure 5. (a) P-P and (b) Ca-Ca pair distribution functions.

Table 2. Qpn distribution (%) and network connectivity NC estimated from simulated and experimental results.

Ti00

sim

Qp0

Qp1

Qp2

Qp3

Qp4

NC

10

42

32

11

5

1.53

58

42

exp

1.42

Ti05

sim

23

48

20

6

1

1.19

Ti10

sim

48

41

8

3

1

0.67

exp

23

77

0.77


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Table 3. Bridging type distribution of the different species (%) estimated from simulated and experimental results.

P-O-P

P-O-Ca

sim

32.4

67.6

exp

22

78

Ti05

sim

21.9

64.0

12.1

Ti10

sim

12.2

57.7

24.6

exp

15

52

33

Ti00

P-O-Ti

Ca-O-Ca

Ca-O-Ca

Ti-O-Ti

0.4

1.2

0.4

0.4

3.7

1.2

0.0

DISCUSSION The peak position of the Qp1 unit in 31P MAS-NMR spectra shifted to a higher magnetic field when TiO2 was substituted into the glass. The electronegativity of titanium (1.5) is larger than that of calcium (1.0). Adsorption bands and peaks in the Raman and XPS spectra of the Ti10 glass indicate that titanate species are incorporated into the Qp1 unit structure, resulting in formation of P-O-Ti bonds. The depolymerization of the phosphate chain by the TiO2 substitution is expected to cause a decrease in the network connectivity. However, the Ti10 glass appeared to have a higher degree of glass formation than the Ti00 glass. Assuming that only P plays a network-former role in the present glasses, the ideal network connectivity NCideal can be calculated using the following equation32,

=

3P O! " − CaO" − 2TiO " P O! "

where [P2O5], [CaO], and [TiO2] are the molar fractions. Figure 6 shows the NC obtained by the experiments (NCexp) and simulations (NCsim), in comparison with ideal NC (NCideal). For the Ti00


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glass, NCexp and NCsim were estimated to be 1.42 and 1.53, respectively, which is very similar to NCideal=1.50. However, for the Ti05 and Ti10 glasses, the experimental and simulated NCs did not correlate well to NCideal. This suggests phosphate units are not the only network former. It has been reported that invert glasses consist of phosphate units with a short chain structure and modifier cations linked through nonbridging oxygens.33 Since some TiO2 is found with a tetrahedral coordination, Ti plays an intermediate role as a network former, which contributes to the increase in NC. The formation of P-O-Ti bonds causes a large increase in network connectivity of the glass. In fact, the total network connectivity which accounts for network former P and intermediate ion Ti increased to 1.74 for the Ti05 glass and 1.81 for the Ti10 glass. This suggests an increase in packing of the atoms and an improvement in the glass-forming ability.

Figure 6. Experimental and simulated network connectivity (NCexp and NCsim) as a function of Ti content, in comparison with the ideal NCideal.


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XPS analysis and MD simulations showed that the fraction of P-O-Ca sites decreases dramatically when TiO2 is added to the composition, but the fraction of P-O-P sites remains constant, indicating that Ti substitutes for Ca in the P-O-Ca sites. Furthermore, these simulations showed that the TiO2 substitution causes disorder of the CaO polyhedra and the Ca coordination number increases as a result of the formation of Ca-O-Ti. A slight shift to a larger distance can be seen in the Ca-Ca RDF in Fig. 5 (b). Ca2s XPS analysis indicated a peak shift to a lower bonding energy when TiO2 substitution occurred. From these results, we propose that the existence of TiO2 in this glass system should have a strong influence on the CaO polyhedra. When TiO2 substitutes into the CaO-P2O5 system, denser packing of the network modifier, Ca, and greater network connectivity leads to an increase in bond strength of the entire glass, which, in turn, should increase the surface tension of the glass. This would contribute to an improvement of its hydrophilic nature. The wettability of materials influences the behavior of bone-forming cells34, with a higher affinity for cells to form on a hydrophilic surface as opposed to a hydrophobic one. The results of this research have paved the way for improving our ability to tailor glass composition to the properties required for bioactive glasses.

CONCLUSION Understanding the structural role of TiO2 in calcium phosphate glasses is key for developing a new glass design for biomedical applications. The structural role of TiO2 in calcium phosphate invert glasses was investigated using experimental analysis and classical molecular dynamics simulation. Titanium oxides, as TiO4 and TiO6 units, played an intermediate role as a network former due to Ti-O-P bonds. This contributed to an increase in the network connectivity of the


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glass, resulting in the improvement of the glass forming ability and wettability. The substitution of TiO2 caused an interaction between CaO and TiO polyhedral, leading to disorder and a change in the coordination number of the CaO polyhedral in the glass structure. The addition of TiO2 to calcium phosphate invert glasses led to glass properties suitable for bioactivity.

Corresponding Author *HM and TT contributed equally to this work and are the corresponding co-authors. E-mail: [email protected] (HM), [email protected] (TT) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The author declares no competing financial interest.

ACKNOWLEDGMENT We would like to thank Prof. S. Ogata and Mr. S. Sato for their assistances to the molecular dynamics simulations of the glasses. This work was supported in part by JSPS KAKENHI Grant Number 26289238.

REFERENCES


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A Table of Contents Graphic

Intensity (a. u.)

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Ti10

Ti00 525

530 535 540 Binding Energy / eV


24

Experimental and Theoretical Investigation of the Structural Role of

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