X-ray diffraction was carried out on a X'Pert Pro diffractometer
(PANalytical, USA) equipped with a Cu Kα radiation source
(λ¼0.154 nm). The voltage and the current used were 40 kV and
40 mA, respectively. The diffraction data were collected from
2θ¼1.5–101 in a fixed-time mode with a step interval of 0.011.
The basal spacing of the silicate layer, d, was calculated by means
of the Bragg equation:
λ ¼ 2d sin θ ð7Þ
where θ is the diffraction position and λ is the wavelength.