Study on the structural evolution of SiO₂ extracted from Sidoarjo mud

Sidoarjo mud is known to have a large amount of silica mineral i.e 44.8% with a purity of 88.4 to 95.7%. In this study, the annealing process in extracted silica was carried out for 5 hours to observe the structural changes at 800°C, 1000°C and 1150°C. Thermogravimetric analysis of extracted silica shows that glass temperature transition (Tg) occurs at onset temperature of 800°C and crystalline phase take places at above 1150°C. The results are confirmed by X-Ray Diffraction (XRD). It indicates that there is a transformation phase of amorphous SiO₂ to triclinic crystalline structure with increasing amount of tridymite phase as the temperature increases from 800°C up to 1150°C. The changes in SiO₂ phase structure are also confirmed by FTIR results that show a transmittance shift of Si-O bending band slightly to lower wavenumbers and increasing transmittance intensity of Si-O stretching bonds as the heating treatment temperature is increased. Based on the UV-Vis results, the optical band gap shows no dependencies on heat treatment temperature, whereas the Urbach energy is proportional to the increasing temperature, in which SiO₂ sample heated at 800°C has the lowest Urbach energy or the fewer defects. Urbach energy values of extracted silica are ranging from 7.91-17.33 eV. In accordance with Urbach energy, the DSSC with photoanode containing SiO₂ heated at 800°C shows the best performance with efficiency of 0.077 %.