Development of visible light active platinum, nitrogen co-doped titanium dioxide photocatalyst via dry physical method for photodegradation of methylene blue dye

Titanium dioxide (TiO₂) has widely been studied as a photocatalyst; however, its efficiency is limited by poor visible light absorption and rapid electron-hole recombination. Thus, this research focused on the development of visible light active platinum, nitrogen co-doped TiO₂ (PTN-TiO₂) photocatalyst via dry physical method, involving a calcination and photodeposition procedures. The optimisation study was conducted for nitrogen doped (N-) and platinum doped TiO₂ (Pt-TiO₂) photocatalyst prior preparing the PTN-TiO₂ photocatalyst. The role of N and Pt in the PTN-TiO₂ photocatalyst for the photocatalytic degradation of methylene blue (MB) dye was investigated under different mode preparation, i.e., sequential and reverse modes, labelled as SEQPTN-TiO₂ and REVPTN-TiO₂. The crystallinity, structural, and dopant concentration were determined by X-ray diffraction (XRD), field emission scanning electron microscopy - energy dispersive X-Ray (FESEM-EDX), elemental analyser (EA) and inductively coupled plasma - optical emission spectroscopy (ICP-OES). The visible light photoactivity of the SEQPTN-TiO₂ photocatalyst was 3 and 1.6 times higher than unmodified TiO₂ and REVPTN-TiO₂ photocatalysts, where the photodegradation rate was obtained at about 0.0408 and 0.0777 min‾¹, respectively. The ultraviolet-visible/ diffuse reflectance spectroscopy (UV-Vis/ DRS), high-resolution transmission electron microscopy (HR-TEM) and Brunauer – Emmett – Teller (BET) have revealed that the SEQPTN-TiO₂ photocatalyst showed a narrow bandgap energy of 2.8 eV, good Pt distribution and a higher specific surface area of 57 m² g‾¹. Nevertheless, the photocatalytic activity of the SEQPTN-TiO₂ photocatalyst was obtained slightly lower when comparing to the single N- and Pt-TiO₂ photocatalyst, where it was calculated about (ca.) 0.1236, 0.138 and 0.1308 min‾¹, respectively. A strong signal of Pt4+ ion in the SEQPTN-TiO₂ photocatalyst, evaluated by X-ray photoelectron spectroscopy (XPS) has contributed to lowering the charge transfer efficiency from TiO₂ to the Pt dopant due to an increase in Schottky barrier height. From the Mott-Schottky (M-S) analysis, the flat-band potential for the SEQPTN-TiO2 photocatalyst has shifted to -0.30 V, potentially generating a broad depletion layer. This has influenced the charge carrier concentration, which was determined at 1.54 x 1018 cm‾³ and was the reason for a lowered photoactivity. Therefore, the mechanism reaction effect of the synthesised photocatalyst was determined. This research provides a fundamental understanding of dopant distribution, band structure modification and charge carrier activity in the TiO₂ system. It has the practical potential for wastewater treatment applications in Malaysia, particularly in degrading dye wastewater from textile and batik industries.