TiO₂ exists naturally in three crystalline forms: Anatase, rutile, brookite, and TiO₂ (B). These polymorphs exhibit different properties and consequently different photocatalytic performances. This paper aims to clarify the differences between titanium dioxide polymorphs, and the differences in homophase, biphase, and triphase properties in various photocatalytic applications. However, homophase TiO₂ has various disadvantages such as high recombination rates and low adsorption capacity. Meanwhile, TiO₂ heterophase can effectively stimulate electron transfer from one phase to another causing superior photocatalytic performance. Various studies have reported the biphase of polymorph TiO₂ such as anatase/rutile, anatase/brookite, rutile/brookite, and anatase/TiO₂ (B). In addition, this paper also presents the triphase of the TiO₂ polymorph. This review is mainly focused on information regarding the heterophase of the TiO₂ polymorph, fabrication of heterophase synthesis, and its application as a photocatalyst.

Titanium dioxide (TiO₂) has been widely applied as a photocatalyst for wastewater treatment due to its high photocatalytic activity and it can remove various harmful organic pollutants effectively. Under heated system, however, TiO₂ is prone to agglomeration that decrease its abilities as a photocatalyst. In order to overcome the agglomeration and increase its thermal resistance, addition of silica (SiO₂) as supporting material is proposed in this research. Silica or silicon dioxide can be extracted from natural resources such as beach sand. Here, we report the application of a composite photocatalyst of TiO₂/SiO₂ to remove phenolic compounds in wastewater. The photocatalyst was synthesized by adding SiO₂ from beach sand onto TiO₂ through impregnation methods. The results of the X-ray diffraction (XRD) showed that TiO₂ was present in the anatase phase. The highest crystallinity was obtained by TiO₂/SiO₂ ratios of 7:1. SEM results showed that the shape of the particles was spherical. Further characterizations were conducted using Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) analysis, and a particle size analyzer (PSA). By using the optimized condition, 96.05% phenol was degraded by the synthesized photocatalyst of TiO₂/SiO₂, under UV irradiation for 120 min. The efficiency of the TiO₂/SiO₂ is 3.5 times better than commercial TiO₂ P25 for the Langmuir–Hinshelwood first-order kinetic model.

Heavy metals are non-biodegradable and have a high toxicity effect on microorganisms which makes their presence in the environment extremely dangerous. The method of handling heavy metal waste by photocatalysis techniques using TiO₂/SiO₂ composite showed a good performance in reducing harmful pollutants. In this study, SiO₂ from Bengkulu beach sand, Indonesia, was used as a support material for TiO₂ photocatalyst to remove Cr(VI) and Pb(II). SiO₂ was obtained through leaching techniques using NaOH as a solvent. The TiO₂/SiO₂ composite photocatalyst was synthesized using a solvothermal method at 130 °C and then characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and a particle size analyzer (PSA). Based on the XRD diffractogram, the synthesized TiO₂ showed the anatase structure while the SiO₂ showed the amorphous structure. The Ti–O–Si bond is defined in the infrared (IR) spectra, which indicates that the relationship between TiO₂ and SiO₂ is a chemical interaction. The results of SEM and PSA characterizations show agglomerated spherical (round) particles with a mean particle size of 616.9 nm. The TiO₂/SiO₂ composite of 7:1 ratio showed the highest photocatalytic activity after 180 min of ultraviolet (UV) irradiation, with a concentration-decrease percentage of 93.77% and 93.55% for Cr(VI) and Pb(II), respectively.

Extraction of Silica from Bengkulu Beach Sand using Alkali Fusion Method