Structural effect of cobalt ions added to a

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Journal of Non-Crystalline Solids 481 (2018) 562–567

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Structural effect of cobalt ions added to a borophosphate-based glass system R. Lucacel Ciceo a b c

a,b,⁎

b,c

a

a

T

a,b

, M. Todea , R. Dudric , A. Buhai , V. Simon

Faculty of Physics, Babes-Bolyai University, Cluj-Napoca, Romania Interdisciplinary Research Institute on Bio-Nano-Science, Babes-Bolyai University, Cluj-Napoca, Romania Department of Molecular Sciences, Faculty of Medicine, Iuliu Haţieganu University of Medicine and Pharmacy, Cluj-Napoca, Romania

A R T I C L E I N F O

A B S T R A C T

Keywords: Vitreous Crystallites Local structure Cobalt ions XRD, XPS, FTIR, Raman

A new melt derived glass system consisting of Co2O3 added to 40P2O5·25B2O3·25CaO·10Na2O matrix was investigated in order to evaluate the influence of cobalt ions on the network structure. Multiple techniques, i.e. Xray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the local structure, the nature of the chemical bonding and the surface composition of these samples. The XRD measurements proved the vitreous character of samples with 0 and 3 mol% Co2O3. At higher cobalt oxide content (5, 7 and 10 mol%) the occurrence of a crystalline phase characteristic to Co2P4O12 has been observed. The complementary spectroscopic studies (FTIR and Raman) revealed that the incorporation of Co2O3 enhances the network connectivity and thereby the short range order in samples. Depending on their amount, the cobalt ions differently influence the borophosphate network via the tetraborate/triborate and metaphosphate/pyrophosphate ratios. For the sample containing 10 mol% Co2O3 the FTIR analysis indicates absorptions assigned to vibrations in CoO6 tetrahedra and suggests the presence of six-coordinated cobalt ions in Co3 + valence state. This study has proven that borophosphate based glasses are able to entrap cobalt ions, are sensitive from structural point of view to the amount of Co2O3 and possess interesting structural peculiarities.

1. Introduction Multicomponent glasses are widely studied since their properties can be controlled by changing the ratio of the components. Borophosphate glasses represent one of the most interesting class of glass materials since their properties highly differ of pure borate and phosphate glasses and recommend them for a quite diverse range of applications, from fast ion conductors in solid-state batteries [1], lowmelting glass solders [2], hermetic sealing materials [3,4] to more recently, biomaterials [5–7]. The interest in phosphate based biomaterials comes from their application in bone tissue engineering. In the inorganic phase of the human bone, which is the biological apatite, calcium is present beside phosphorus, so its addition in the compositions considered for bone tissue reconstruction is needed [8–10]. Following recent studies, the boron additions in the phosphate network improves the chemical durability, as well as the thermal and mechanical stability of the glasses [6,7,11]. At the same time, boron itself is a stimulating agent for bone tissue engineering [12–16]. The natrium oxide diminishes the melting temperature of glasses and increases the glass degradability [17–20] so that it is a desirable component of bioglasses.



In order to improve the biological behaviour of bioglasses designed for regenerative medicine applications, a variety of metallic ions such as copper, silver, strontium, gallium, magnesium, zinc and cobalt have been incorporated into different glass compositions [12,21]. Some of these ions stimulate the osteogenesis and angiogenesis process helping the bone formation while some of them confer other properties like antibacterial activity, for example. Low oxygen pressure (hypoxia) plays a vital role in the development and regeneration of skeletal tissue [22–26] and cobalt ions are mimic hypoxia agents, so that their additions in the biomaterial composition is once more motivated [27–29]. In the glass technology the cobalt ions are known to act as nucleating agents that promote the crystallization process. Their influence on the structural characteristics of glasses is influenced by samples composition, synthesis method and parameters. Usually, in vitreous materials cobalt ions exist in two stable valence states, Co2 + and Co3 +. The Co2 + ions are disposed in octahedral (six coordinated) and tetrahedral (four coordinated) structural units while the Co3 + ions are found in octahedral (six coordinated) units, mainly [30]. The relative variation in the concentration of these structural units strongly influences the structure and the properties of the host glass and/or glass-ceramic. This study is focused on the structural effect of cobalt ions

Corresponding author at: Faculty of Physics, Babes-Bolyai University, Cluj-Napoca, Romania. E-mail address: [email protected] (R. Lucacel Ciceo).

https://doi.org/10.1016/j.jnoncrysol.2017.11.050 Received 3 November 2017; Received in revised form 28 November 2017; Accepted 29 November 2017 Available online 05 December 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 481 (2018) 562–567

R. Lucacel Ciceo et al.

incorporation in a new borophosphate based vitreous matrix. The starting composition 40P2O5·25B2O3·25CaO·10Na2O of the host glass matrix was chosen having in mind the potential application of this glass as borophosphate biomaterial [31], but the structural investigation of materials of such a composition may be useful to extend their practical applications as optical devices or rechargeable batteries. The system was synthesized using the classical melt quenching method. The structural changes imposed by the cobalt ions additions were followed first by X-ray diffraction (XRD), then on the basis of two complementary spectroscopic techniques, infrared absorption and Raman scattering, as well as by X-ray photoelectron spectroscopy (XPS). The spectroscopic results have been used to elucidate the connectivity of the various structural groups such as that of phosphorus−oxygen, boron−oxygen, and mixed units, especially after the addition of Co2O3. 2. Experimental procedure 2.1. Samples synthesis Fig. 1. X-ray diffraction patterns of xCo2O3.(100 − x)[40P2O5.25B2O3.25CaO%10Na2O] samples.

The 40P2O5·25B2O3·25CaO·10Na2O based composition with different Co2O3 content, up to 10 mol%, were prepared by conventional melt quenching technique. Appropriate quantities of reagent grade NH3·H2PO4, H3BO3, CaCO3, Na2CO3.10H2O and Co2O3 were mixed in an agate mortar. The xCo2O3.(100 − x)[40P2O5·25B2O3·25CaO·10Na2O] batches (x = 0, 3, 5, 7 and 10 mol%) were melted in air, in sintered corundum crucibles, in an electric furnace at 1200 °C for 15 min. The melts were quickly cooled at room temperature by pouring and stamping between two copper plates. All samples are transparent, colorless for sample with x = 0 and with different shades of blue for the rest of them. The blue shade intensity increases with the cobalt ions addition. Glass frit was ground to powder in a Retsch Planetary ball mills, type PM 100.

3. Results According to X-ray diffraction patterns presented in Fig. 1, the cobalt free sample and that with 3 mol% Co2O3 exhibit vitreous structure. The XRD spectra of samples with x = 5, 7 and 10 mol% Co2O3 point out a crystalline peak centred at 2θ = 28.10 attributed to the Co2P4O12 crystalline phase [32,33]. The intensity of this peak progressively increased when the Co2O6 content increased from 5 to 10 mol%. In Fig. 2 the XPS survey spectra of the obtained glass and glassceramic samples are shown. The survey spectra are quantified in terms of peak intensities and peak positions. All XPS and Auger peaks from the constituent elements of the obtained glasses were clearly identified and marked on the spectra. The results obtained for the atomic concentrations on the outermost layer of samples surface are listed in the Table 1. Usually, the cobalt ions detection in glasses is done using the Co 2p3/2 and Co 2p1/2 doublets observed at about 790 eV and 794 eV. For our samples these peaks were detected as extremely weak, but the peak at 101 eV, assigned to the Co 3s photoelectrons was clearly evidenced. This could be due to Co 2p signal broadening and intensity decreasing noticed in powder samples [34]. We used the Co 3s peak for cobalt in the elemental analyses of the samples. Moreover, increasing the Co2O3

2.2. Characterization methods The structure of the samples was investigated by X-ray diffraction (XRD) using a standard Bruker X D8 Advance diffractometer with a monochromator of graphite for Cu Kα radiation. The XRD patterns were recorded in 2θ range from 10° to 80° with a speed of 2°/min. X-ray photoelectron spectroscopy (XPS) analysis was carried out on finely powdered samples using a SPECS PHOIBOS 150 MCD system equipped with monochromatic Al Kα source (250 W, hν = 1486.6 eV), hemispherical analyser and multichannel detector. The typical vacuum in the analysis chamber during the measurements was in the range of 10− 9–10− 10 mbar. Charge neutralization was used for all samples. The binding energy (BE) scale was charge referenced to the C 1s photoelectron peak at 284.6 eV. The elemental composition on samples surface was obtained by a standard quantitative XPS analysis of survey spectra acquired at pass energy of 100 eV in the binding energy range 0–1200 eV. The atomic concentrations were estimated from the areas of the characteristic photoelectron lines assuming a Shirley type background. High-resolution spectra were obtained using analyser pass energy of 30 eV. The position and full width at half maximum of photoelectron peaks were estimated by spectra deconvolution with Casa XPS (Casa Software Ltd., UK). For Fourier transform infrared (FT-IR) measurements identical amounts of glasses were powdered and mixed with KBr in order to obtain thin pellets containing approximately 1 wt% glass powders. The pellets thickness was about 1.5 mm. The spectra were recorded at room temperature in the 350–4000 cm− 1 range with a 6100 Jasco spectrometer with a maximum resolution of 0.5 cm− 1 and signal/noise ratio 42,000:1. Raman spectra were obtained using multilaser confocal Renishaw InVia Reflex Raman spectrometer (λ = 532 nm). The samples were scanned from 100 to 2000 cm− 1 wavenumber shift at a spectral resolution of 1 cm− 1. The data acquisition time was 40 s and averaging was performed over 10 measurements.

Fig. 2. XPS survey spectra of the investigated xCo2O3.(100 − x)[40P2O5.25B2O3.25CaO %10Na2O] samples.

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Journal of Non-Crystalline Solids 481 (2018) 562–567

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Table 1 Elemental surface composition (at.%) determined from XPS analysis along with the nominal values expected in bulk samples. x [mol %]

Elemental concentration [at.%] XPS data

0 3 5 7 10

Nominal bulk concentrations

O

P

B

Ca

Na

Co

O

P

B

Ca

Na

Co

63.7 61.2 60.8 63.8 59.8

17.6 18.8 18.5 17.8 19.1

11 11.7 11.6 11.2 11.9

5.2 5.5 5.9 4.4 5.5

2.5 2.6 2.7 1.7 2.4

– 0.2 0.4 1.1 1.2

64 63.8 63.7 63.7 63.5

16.5 16 15.7 15.3 14.8

10.3 10 9.8 9.6 9.3

5.2 5 5 4.8 4.6

4.1 4 3.9 3.8 3.7

– 1.2 2 2.9 4.1

content, the XPS peak placed between 60 and 65 eV is shifted to lower binding energy. This peak covers two contributions: one from Co 3p with a XPS signal about 60 eV and a second one from the Na 2s with a XPS signal at 63 eV. As a result, the shift of 60-65 eV XPS signal to lower binding energy can be attributed to the increase of cobalt amount on samples surface. The O 1s core level XPS peaks provide very useful information because the oxide ions in the borophosphate glasses are involved in different types of chemical bonds. Therefore, the O 1s photoelectron spectra of the obtained glasses (Fig. 3) have been deconvoluted using Shirley background and Lorentzian-Gaussian line shapes. The value of the Gaussian-Lorentzian ratio was 30 (GL-30). Based on O 1s peak deconvolution with two contributions, the ratio between the number of non-bridging oxygen (NBO) and the total number of oxygen atoms was determined (see Fig. 3). The lower binding energy peak at 530.8 ± 0.3 eV is associated with the non-bridging oxygen and the higher energy peak at 532.6 ± 0.3 eV with the bridging oxygen (BO). The results of fitting the experimental O1s spectrum (Table 2) show that the BO/NBO ratio increases as the cobalt content increases, excepting the sample containing 7 mol% Co2O3. The deviation from the ascendant trend of BO/NBO ratio for 7 mol% Co2O3 sample may be generated by a different distribution/arrangement of oxygen ions on the surface of this sample. The room temperature infrared absorption spectra are shown in

Table 2 The relative amount of BO and NBO atoms determined from deconvolution of O 1s core level spectra. x [mol %]

0

3

5

7

10

BO % NBO % BO/NBO

72.2 27.8 2.6

75.3 24.7 3.05

76.6 23.4 3.27

69.5 30.5 2.29

83.6 16.4 5.1

Fig. 4. The glass matrix exhibits three absorption bands around 540, 1050, 1195 cm− 1 and two shoulders at ~ 835 respectively 935 cm− 1. According to the literature, (i) the band at 540 cm− 1 can be assigned to the harmonics of δ(O]PeO) [35–37], (ii) the band at 1050 cm− 1 corresponds to both the asymmetric stretching vibration of PO32 − in Q1 units and BeØ bond stretching vibration of BØ4− tetrahedron from tri-, tetra- and penta-borate groups [5–7,35,36] and, (iii) the band at 1195 cm− 1 is ascribed to symmetric stretching of (PO2)− from Q2 units and to BO4 units [5–7]. The weak absorption modes observed around 835 and 935 cm− 1 are attributed to P-O-P asymmetric stretching vibration in borophosphate [37] and to both asymmetric stretching vibration of P2O7− 4 in Q1 and stretching vibration of BO4 units, respectively [36,37]. The addition of cobalt oxide in the 3–7 mol% compositional range, generates a new broad absorption band around 1462 cm− 1 ascribed to Fig. 3. The deconvolution of the O1s spectrum for samples with x = 0, 3, 5 and 10 mol% Co2O3.

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Fig. 4. FTIR spectra of xCo2O3.(100 − x)[40P2O5·25B2O3·25CaO·10Na2O] samples. Fig. 5. Raman spectra of xCo2O3.(100 − x)[40P2O5·25B2O3·25CaO·10Na2O] samples.

stretching vibration of BeOe linkage from BO3 units [5,35–37]. Additionally, the increase in the Co2O3 content leads to a slight increase in the intensity of all bands. For the sample with 10 mol% Co2O3, the FT-IR spectrum is definitely changed: there are two narrow well defined peaks at 548 and 1195 cm− 1 adjacent to another multiple small bands around: 650, 745, 810, 882, 925, 1114, 1280, 1405 and 1462 cm− 1. The peaks at 548 and 1195 cm− 1 were observed in all samples but for 10 mol% their shapes become narrower as a consequence of network structural changes in term of local order degree increases. The other vibrational modes are attributed as follow: (i) around 650 cm− 1 to CoO6 structural units [30], (ii) around 745 cm− 1 to both, the symmetric stretching vibration of PeOeP rings and O3BeOeBO4 bond-bending vibration [7,35,36], (iii) around 882 cm− 1 to asymmetric stretching vibration of P-O-P bond in Q2 units [5,36], (iv) around 1114 cm− 1 to asymmetric stretching vibration of PO32 − in Q1 units [36], (v) around 1280 cm− 1 to both asymmetric stretching vibration of PO2− in Q2 units and asymmetric stretching vibration of BØ3 and BØ2O− [35–37] and (vi) around 1405 cm− 1 to BO3 units [36]. The bands at 810 and 925 cm− 1 are considered to cover the same vibrational modes as the bands from 835 and 935 cm− 1, shifted toward lower wavenumbers with increasing the Co2O3 content. This suggests a weakening of the bonds responsible for these infrared absorption bands. Fig. 5 presents the recorded Raman spectra of the samples. The main band from the cobalt free sample spectrum, around 1125 cm− 1 is due to the asymmetric and symmetric stretching vibration of PO2− from PØ2O2− tetrahedra (Q2 units) linked with boron atom from diborate groups (B4O72 −) [4,7] revealing the formation of borophosphate network via BPO4 groups. Three weak and broad bands are also visible in the spectrum of free cobalt sample: (i) around 627 cm− 1 ascribed to stretching vibration of P-O-B groups [7,38], (ii) around 735 cm− 1 due to the borophosphate rings whose presence is favoured by the composition stoichiometry [38] and (iii) around 1275 cm− 1 assigned to both pyroborate groups (B2O54 −) and asymmetric stretching mode of the two NBO atoms bonded to phosphorous atom in Q2 tetrahedron [36]. The gradual addition of Co2O3 leads to an increase in the intensity of all Raman bands.

rather than the bulk composition which is often most important. In the present study, the elemental composition on the outermost layer of the samples determined by XPS analysis has shown that distribution of the chemical elements is non homogeneous (Table 1). Compared to the concentrations expected in bulk, according to the nominal compositions, one notices that the concentration of cobalt and sodium are much lower, while calcium, phosphorus and boron occur with increased concentrations (Table 1). Both phosphorus and boron are cations of glass network formers and dominate the sample surface, whereas the glass network modifiers could migrate from the surface into inner side of sample the easier the lower the ionic radius is (rCo ~ 0.7 Å, rNa ~ 1.15 Å, rCa ~ 1.2 Å). The higher relative concentration of calcium could indicate its stronger connection to the glass structural units, especially with the phosphorous one. The local order of samples following the 40P2O5·25B2O3·10NaO·25CaO composition is strongly influenced by the Co2O3 incorporation. Cobalt ions are known to act as nucleation centers in the glass systems [30]. Following the XRD data (Fig. 1), the development of the crystalline phase associated to Co2P4O12 started for a concentration of 5 mol% Co2O3. The increase in the cobalt ions quantities leads to an increase in the XRD peak intensity revealing that the cobalt ions act as nucleation centers in the crystallization process. The size of the crystallites was calculated using the Scherrer equation, D = kλ / (βcosθ), where D is particle diameter, k = 0.89 (the Scherrer's constant), λ = 1.5417 Å (wavelength of the Cu Kα radiation), β is the width of line at the half-maximum intensity and θ is the corresponding Bragg angle. It was found that the size of crystallites increased at cobalt oxide additions from 42 ± 2 nm for 5 mol% Co2O3 to 52 ± 2 nm for 5 mol% and 95 ± 2 nm for 10 mol%, respectively. Since the crystallization process includes two distinct process: nucleation and the crystal growth, it becomes very clear that cobalt oxide is responsible for both steps. The crystal structure of Co2P4O12 consists in a three dimensional framework of polyhedral CoO6 linked with P4O12 rings through Co–O–P bonds [39]. Even if the first Co2P4O12 crystallites were detected by XRD in samples containing 5 mol% Co2O3, the occurrence of cobalt ions in CoO6 structural units was proved only in samples with 10 mol%. The infrared absorption band from 650 cm− 1 (Fig. 4) clearly suggests the presence of Co3 + ions in this sample and their distribution in octahedral (six-coordinated) units [30]. The infrared (Fig. 4) and Raman data (Fig. 5) complete well and allow following the structure of the investigated samples. The starting composition 40P2O5·25B2O3·10NaO·25CaO placed the samples between metaphosphate (50 mol% P2O5) and pyrophosphate stoichiometry (33 mol% P2O5). At 50 mol% P2O5, the glass has a structure based on

4. Discussion The surface composition of a material is often very different from the bulk composition. As the surface dictates many important properties of a material designed for potential applications in field of biomaterials or catalysts (and not only these) it is the surface composition 565

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Q2 chains of theoretically infinite length, with the chain length decreasing with the phosphate content, while at 33 mol% P2O5 the glass structure consists of P2O7− 4 (Q1) units. The addition of boron in a methaphosphate network generates modification of the phosphate chains through the intermediate participation of boron atoms causing the structure depolymerisation. It is known that boron atoms are found in the oxide glasses usually in the form of groups making up both BO3 and BO4 structural units, the ration between these two entities being strongly influenced by the composition. In phosphate-rich glasses it was observed that the 4-coordinated boron species are preferred [5]. Following the infrared data we proposed for the glass matrix a mixed network based on phosphate Q1 and borophosphate units. The borophosphate BPO4 unit is made by corner sharing PO4 and BO4 tetrahedra [6], which increases the reticulation of the network. Because of the overlapping of the phosphate and borate bands it was not possible to build a connection model for the glass network. In the 3–7 mol% compositional range, the addition of Co2O3 in the glass matrix composition is reflected in the structure by the formation of BO3 structural units, mainly. These structural changes are described by the shift to the left in the BØ2O− ↔ BØ4− equation that describes the isomerization process between 3- and 4- coordinated boron species. This process is related with the formation of non-bridging (NBO) oxygen atoms and as consequence, with the decreases of BO/NBO ratio. Supplementary: (i) the intensity of the infrared absorption bands increases a little for the 3–7 mol% composition, (ii) the XRD data indicate crystallite starting with 5 mol% and (iii) the XPS data show that with one exception (sample with 7 mol%) the BO/NBO ratio increases with the Co2O3 additions (Table 2). In these terms, we assume that the shift to the left in the equation of isomerization is accompanied by the Q2/Q1 phosphate ratio increases. The sample containing 10 mol% Co2O3, has a complex structure that, most probable, consists in a borophosphate vitreous phase mainly build by Q1 and BO3 units and a crystalline phase that involves CoO6 tetrahedra linked with Q2 units by CoeOeP bond. The Raman data sustain and complete the infrared ones. Based on Raman assignments, the interconnected borophosphate network is well developed, even in the glass matrix. The local structure consists of Q2 and BO4 units, mainly. The increase in the Co2O3 content leads to the progressive increase in the intensity of all Raman bands all over the compositional range (Fig. 5). This is in agreement with the FTIR, XPS and XRD results and indicates ones more that Co2O3 is responsible for the increase of the local ordered degree. No significant changes were observed in the Raman spectra of sample containing 10 mol% comparatively with the other ones (as is happened in case of infrared spectra). By this point of view, the Raman technique proves to be not sensitive enough to the structural changes of the investigated system.

pyrophosphate in metaphosphate. Thus, two opposite processes are associated with Co2O3 addition: the borate lattice tends to be more depolymerised and, on the other hand, the short range order in the phosphate lattice increases. The increase of BO3 units and the decrease in the amount of non-bridging oxygen atoms increases the glass crystallization tendency that was also revealed by XRD analysis. Based on infrared spectroscopy, the Co3 + ions distribution in CoO6 tetrahedra is achieved only in the sample containing 10 mol% Co2O3. Moreover, for this sample, the FTIR spectrum denotes the appearance of Co2P4O12 crystallites. We propose for 10 mol% Co2O3 sample a biphasic structure: a vitreous one mainly built by orthophosphate (Q1) and triborate units and a crystalline one with contribution from CoO6 tetrahedra linked with methaphosphate (Q2) units by CoeOeP bonds. The development of Co2P4O12 crystallites in vitreous borophosphate matrix is a key point for new investigations in order to produce cobalt cyclotetraphosphate, which may be useful not only for dielectrics, surface treatment agents, inorganic ceramic pigment and corrosionproof compositions but also for potential applications for medical application. References [1] A. Magistris, G. Chiodelli, M. Duclot, Silver borophosphate glasses: ion transport, thermal stability and electrochemical behaviour, Solid State Ionics 9–10 (1983) 611–615. [2] J.M. Clinton, W. Coffeen, Low melting glasses in the system B2O3-ZnO-CaO-P2O5, Ceram. Bull. 63 (1984) 1401–1404. [3] R.K. Brow, D.R. Tallant, Structural design of sealing glasses, J. Non-Cryst. Solids 222 (1997) 396–406. [4] L. Koudelka, P. Mošner, Study of the structure and properties of Pb − Zn borophosphate glasses, J. Non-Cryst. Solids 293 −295 (2001) 635–641. [5] A. Saranti, I. Koutselas, M.A. Karakassides, Bioactive glasses in system CaO-B2O3P2O5: preparation, structural study and in vitro evaluation, J. Non-Cryst. Solids 352 (2006) 390–398. [6] D. Carta, D. Qiu, P. Guerry, I. Ahmed, E.A. Abou Neel, J.C. Knowles, M.E. Smith, R.J. Newport, The effect of composition on the structure of sodium borophosphate glasses, J. Non-Cryst. Solids 354 (2008) 3671–3677. [7] J. Massera, Y. Shpotyuk, F. Sabatier, T. Jouan, C. Boussard-Plédel, C. Roiland, B. Bureau, L. Petit, N.G. Boetti, D. Milanese, L. Hupa, Processing and characterization of novel borophosphate glasses and fibers for medical applications, J. NonCryst. Solids 425 (2015) 52–60. [8] M. Epple, E. Bauerlein (Eds.), Handbook of Biomineralization: Medical and Clinical Aspects, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2007, pp. 81–91 (ISBN: 978-3-527-31806-3). [9] S.V. Dorozhkin, Calcium orthophosphates in nature, biology and medicine, Materials 2 (2009) 399–498. [10] W. Habraken, P. Habibovic, M. Epple, M. Bohner, Calcium phosphates in biomedical applications: materials for the future? Mater. Today 2 (19) (2016) 69–87. [11] N. Sharmin, A.J. Parsons, C.D. Rudd, I. Ahmed, Effect of boron oxide addition on fibre drawing, mechanical properties and dissolution behaviour of phosphate-based glass fibres with fixed 40, 45 and 50 mol% P2O5, J. Biomater. Appl. 29 (5) (2014) 639–653. [12] A. Hoppe, N.S. Guldal, A.R. Boccacini, A review of the biological response to ionic dissolution products from bioactive glasses and glass-ceramics, Biomaterials 32 (2011) 2757–2774. [13] F.H. Nielsen, Is boron nutritionally relevant? Nutr. Rev. 66 (4) (2008) 183–191. [14] M. Dzondo-Gadet, R. Mayap-Nzietchueng, K. Hess, P. Nabet, F. Belleville, B. Dousset, Action of boron at the molecular level, Biol. Trace Elem. Res. 85 (1) (2002) 23–33. [15] T. Uysal, A. Ustdal, M.F. Sonmez, F. Ozturk, Stimulation of bone formation by dietary boron in an orthopedically expanded suture in rabbits, Angle Orthod. 79 (5) (2009) 984–990. [16] R.E. Chapin, W.W. Ku, M.A. Kenney, H. McCoy, B. Gladen, R.N. Wine, The effects of dietary boron on bone strength in rats, Fundam. Appl. Toxicol. 35 (2) (1997) 205–215. [17] J.R. Jones, Review of bioactive glass: from hench to hybrids, Acta Biomater. 9 (2013) 4457–4486. [18] K.E. Wallace, R.G. Hill, J.T. Pembroke, C.J. Brown, P.V. Hatton, Influence of sodium oxide content on bioactive glass properties, J. Mater. Sci. Mater. Med. 10 (1999) 697–701. [19] D.S. Brauer, R. Brückner, M. Tylkowski, L. Hupa, Sodium-free mixed alkali bioactive glasses, Biomed. Glass. 2 (2016) 99–110. [20] N.C. Lindfors, I. Koski, J.T. Heikkila, K. Mattila, A.J. Aho, A prospective randomized 14-year follow-up study of bioactive glass and autogenous bone as bone graft substitutes in benign bone tumors, J. Biomed. Mater. Res., Part B 94B (2010) 157–164. [21] S.M. Rabiee, N. Nazparvar, M. Azizian, D. Vashaee, L. Tayebi, Effect of ion substitution on properties of bioactive glasses: a review, Ceram. Int. 41 (2015) 7241–7251.

5. Conclusions The xCo2O3·(100 − x)[40P2O5·25B2O3·25CaO·10Na2O] glass system (0 ≤ x ≤ 10 mol%) was obtained using conventional melt quenching technique. The changes induced by Co2O3 addition in the local structure of the samples strongly depend on cobalt ions content: vitreous samples were obtained only for x = 0 and 3 mol% Co2O3, while for higher Co2O3 contents, 5 ≤ x ≤ 10 mol%, a crystalline phase was detected. The XRD data identified these crystallites as nano-sized Co2P4O12. Their amount and size increase as the Co2O3 content increases. The results obtained by XPS, FTIR and Raman spectroscopies offered details about the connectivity of the structural units that build the glass network of the investigated samples. The local structure of vitreous samples consists in an interconnected lattice mainly built by pyro-, meta-phosphate (Q1, Q2) and tetraborate (BO4) units. The progressive addition of Co2O3 leads to the increase in BO3/BO4 ratio, while the amount of non-bridging oxygen decreases. It is suggested that when the Co2O3 content increases, the transformation of BO4 units in BO3 units takes place simultaneously with the transformation of 566

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