Photoactive layer-by-layer films of cellulose phosphate and titanium dioxide containing phosphotungstic acid

Abstract

A versatile layer-by-layer (LbL) procedure for the preparation of highly dispersed, adherent and porous multilayer films of TiO<SUB>2</SUB> nanoparticles (NPs) and phosphotungstic acid (HPW) on a variety of substrates at room temperature was developed based on the use of cellulose phosphate (CP) as an efficient and non-conventional polyelectrolyte. UV/vis absorption spectroscopy confirmed the linear and regular growth of the films with the number of immersion cycles and a strong adsorption ability of CP towards TiO<SUB>2</SUB> NPs. FTIR spectroscopy showed that HPW binds to the surface of TiO<SUB>2</SUB> through the oxygen atom at the corner of the Keggin structure. XPS results showed that the interaction between TiO<SUB>2</SUB> and CP is through Ti-O-P linkage. A model is proposed for the TiO<SUB>2</SUB>-HPW interaction based on XPS and FTIR results. FEG/SEM study of the surface morphology revealed a porous film structure with a homogenous distribution of the TiO<SUB>2</SUB> NPs induced by CP. HRTEM studies showed that the resulting composite films consist of crystalline anatase and rutile phases and poly-nano-crystalline HPW with a semi-crystalline TiO<SUB>2</SUB>-HPW interface. These CP/TiO<SUB>2</SUB> and CP/TiO<SUB>2</SUB>/HPW LbL films showed good photoactivity against both saturated and unsaturated species, for instance, stearic acid (SA), crystal violet (CV) and methylene blue (MB) under UV irradiation. The CP/HPW films formed on bacterial cellulose (BC) showed good photochromic response which is enhanced in presence of TiO<SUB>2</SUB> due to an interfacial electron transfer from Ti<SUB>2</SUB> to HPW. This simple and environmentally safe method can be used to form coatings on a variety of surfaces with photoactive TiO<SUB>2</SUB> and TiO<SUB>2</SUB>/HPW films.