Pyridyl anchoring squaraine as a near-infrared dye sensitizer for effective sensitized hydrogen production over a titanium dioxide photocatalyst in water medium

Squaraine dyes are organic dyes having strong and narrow absorption properties in the near-infrared region that are widely used in photovoltaic and biomedical applications. In this work, squaraine dye (SA1) was synthesized as a dye sensitizer for a dye-sensitized photocatalytic system, which was composed of SA1 and Pt-loaded TiO₂ powder photocatalyst (SA1/Pt-TiO₂). The SA1/Pt-TiO₂ system exhibited a good hydrogen production performance within 150 h and an apparent quantum yield of 1.4% under 800 nm monochromatic light irradiation. However, during the photocatalytic reaction, the photocatalytic activity of SA1/Pt-TiO₂ decreased due to photodecomposition. Ultraviolet–visible absorption spectroscopy, ¹H nuclear magnetic resonance spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry measurements were performed to investigate the mechanism of the decomposition of the squaraine moiety of SA1, the decomposition process, and the structure of the decomposed material. The results show that even without the Pt-loaded TiO₂ powder photocatalyst, SA1 undergoes photodissociation, which cleaves the bond between the indoline moiety and the square acid. Graphical abstract: [Figure not available: see fulltext.]