The following chemical precursors have been used without further purification; n-butyl phosphate (1:1 molar ratio of mono OP(OH)2(OBun) and di-butyl phosphate OP(OH)(OBun)2, Alfa Aesar, 98 %), titanium isopropoxide (Ti(OPri)4, Aldrich, 95 %), calcium methoxyethoxide (Ca-methoxyethoxide, ABCR, 20 % in methoxyethanol), sodium methoxide solution (NaOMe, Aldrich, 30 wt% in methanol), 2-methoxyethanol (MeO-EtOH, Aldrich, 99.8 %), and n-dimethyl formamide (n-DMF, Alfa-Aesar, 99.8 %).
Synthesis of the sol
The reaction was started by diluting n-butyl phosphate in MeO-EtOH at the molar ratio of 1:3 and allowing them to react for 10 min (the whole reaction being carried out in a dried vessel). Then Ti(OPri)4 was added to the mixture while it was magnetically stirring. Following that after 1 h stirring Ca-methoxyethoxide was added dropwise into the vessel. Stirring was continued for about 1 h and then in the last stage the vessel was cooled down in an ice-bath before NaOMe and n-DMF were added to the solution. The sol–gel preparation is outlined by the flowchart in Fig. 1 Drying procedure.
For all samples the clear mixture was poured into a glass container and allowed to gel at room temperature. The mixtures turned to gel after about 3 min and were then aged for 3 days at room temperature. Following the drying stage, the temperature was increased to 60 °C and held for 7 days, and then to 120 °C for 4 days before drying at 170 °C for 2 days and 220 °C for 1 h to remove any remaining solvent and to obtain bulk, glassy-like samples (Fig. 2). Table 1 shows the effect of drying procedure in the PCNT glass samples.
The XRD was obtained by a Bruker-D8 Advance Diffractometer (Brüker, UK) on fine glass powders in flat plate geometry using Ni filtered Cu Kα radiation. Data was collected using a Lynx Eye detector with a step size of 0.02° over an angular range of 2θ = 10–100° and a count time of 12 s.
Energy dispersive X-ray spectroscopy (EDX)- Inca 300 (Oxford instrument, UK) was used to determine the exact compositions of the prepared samples. Scanning electron microscopy (SEM)-XL30 (Philips, Netherland) was operated at 20 kV, spot size 5, and working distance of 10 mm to identify the particular elements and their relative proportions with EDX from the scanned area.
Titanium K-edge X-ray absorption near-edge structure (XANES) data was collected using the micro-focus beam line I18 at the Diamond Light source, UK. Data were collected from 80 to 220 eV below and above the edge respectively to allow an accurate background subtraction and all measurements were made at room temperature.
The solid state 31P MAS-NMR spectrum was acquired on a Varian VNMRS-400 spectrometer and was referenced to the resonance of the secondary reference ammonium dihyrogen phosphate (NH4H2PO4) at 0.9 ppm (relative to 85 % H3PO4 solution at 0 ppm). The spectrum was recorded at 161.87 MHz for which glass powders were loaded into a 4 mm (rotor o.d.) magic angle spinning probe. The spectrum was obtained using direct excitation (with a 90° pulse) with a 60 s recycle delay at ambient probe temperature (~25 °C) at a sample spin rate of 10 kHz and between 20 and 88 repetitions were accumulated.
The FTIR spectrum of the glass powders was collected using a Perkin Elmer spectrometer 2000 (USA). Glass powder was scanned at room temperature in the absorbance mode in the range of 4,000–600 cm−1. The obtained data was analysed by the software supplied by Perkin Elmer Co.
Differential thermal analysis of the glass powders was carried out on a Setaram Labsys™ TG–DTA-16 instrument with a heating rate of 20 °C min−1 from 20 to 1,000 °C in a Pt crucible and a purge gas of air. The data was base-line corrected using a blank run and glass transition (Tg) and crystallisation temperature (Tc) were measured.
Ion release study
The obtained sol–gel glasses were ground to form microparticles using a Retsch MM301 milling machine operated at 10 Hz frequency (Retsch, Germany). The microparticles in the size range of 106–150 µm were obtained by passing them through 150 and 106 µm sieves (Endecotts ltd, UK) on a Fritsch Spartan sieve shaker (Fritsch GmbH, Germany). For the ion release study, 1 mg of each of the glass composition chosen were immersed in deionized water for 2 h. The resulting suspensions were centrifuged at 4,600 rpm for 6 min to separate the glass from the solution. The liquid phase was then extracted and the glass subsequently re-suspended in 10 mL dH-2O for a further 4 h. This process was repeated to gain liquid samples following 24 and 48 h total immersion times. Calcium, phosphorous, sodium and titanium in solution were subsequently measured by ICP-MS using a Spectro Mass 2000 Analytical System calibrated across the predicted concentrations range 0–4,000 ppb by dilution of 100 ppm elemental standards. Calcium and titanium standards were obtained as part of a pre-made standard solution (Fluka) whereas phosphorous and sodium standards were created from analytical grade K2HPO4 (Sigma) and NaNO3 (Sigma) salts respectively. Standards were first diluted in dH2O to result in the desired concentration range. Both samples and standards were diluted in 1:1 in 4 % HNO3 (Fluka) and analysed in reference to a blank (2 % HNO3) solution under standard operating conditions (power: 1350 W; Coolant Flow: 15 L min−1: Axillary Flow: 1 L min−1). Results were expressed as cumulative ion release over the full period of the degradation study.