Fast photodegradation of Orange II azo dye under visible light irradiation using a semiconducting n–p heterojunction of ZnO nanoparticles/polypyrrole as catalyst

Preparation of innovative nano- and microstructured heterojunctions consisting of two semiconductors for highly efficient p–n photocatalysts has been of growing interest due to their advanced applications. n-type ZnO nanoparticles were synthesized by a simple precipitation method from ZnCl₂ and NaOH and coated with semiconducting p-type polypyrrole (PPy) nanospheres by chemical oxidation in the presence of sodium dodecyl sulfate (SDS) as surfactant, obtaining a n-p type ZnO/PPy composite. The composite was characterized by scanning electron microscopy (SEM), FTIR and UV/Vis–NIR spectroscopies, and thermogravimetric analysis (TGA). It was observed that composite consisted of ZnO nanoparticles with rice-like morphology of sizes in the range of 180–600 nm long and between 70 and 227 nm wide, which were well dispersed into a matrix made of agglomerated spherical nanoparticles of PPy. The composite was tested as photocatalyst in the degradation of Orange II azo dye using visible light irradiation at different initial dye concentrations, catalyst loading, initial pH, and in the presence of hole and radical scavengers. Photodegradation efficiencies after 30 min of reaction in the range of 90.9–100% and fast photodegradation rates depending on the initial dye concentration and catalyst load were observed. The best photodegradation efficiency was observed at acidic pH, which was ascribed to an improved electrostatic interaction of dissociated dye molecules with the positively charged surface of photocatalyst. It was demonstrated that ·O2- and other ROS are the main reactive species formed via reaction between O₂ and photogenerated electrons, following a pseudo-first order of reaction.