Strongly coupled Ag/TiO₂ heterojunctions for effective and stable photothermal catalytic reduction of 4-nitrophenol

The development of effective catalysts for the catalytic conversion of the harmful nitrophenol (NP) into the useful aminophenol (AP) has received extensive interest. Herein, we report the easy and large-scale synthesis of strongly coupled Ag/TiO₂ heterojunctions based on the coordinated action of organic components with a multi-kind metal precursor. The heterojunctions were effective and stable catalysts for the photothermal catalytic reduction of 4-NP to 4-AP. In the synthesis, critic acid, ethylene glycol, AgNO₃, and tetrabutyl titanate were dissolved and coordinated in water. Under heating, a precursor gel having a uniform distribution of Ag and Ti was gradually formed. Via calcination in air, the Ti precursor was transformed into TiO₂, accompanied by the reduction of Ag⁺ to Ag nanoparticles. The formation of Ag/TiO₂ composites with intimate interface contact benefited from the uniform distribution of different components in the precursor gel. The Ag/TiO₂ functioned as an effective catalyst for the reduction of 4-NP, exhibiting higher activity than the many reported Ag-based catalysts. The catalytic reaction over Ag/TiO₂ had a small t₀ with good activity and reuse performance. After 10 cycles of reuse, the conversion efficiency exhibited no obvious change. Importantly, the conversion of 4-NP was significantly enhanced under light irradiation provided by a 150-W Xe lamp (the visible light from cutoff have equal function), but ultraviolet light did not promote the conversion. The conversion time was reduced from 620 to 270 s with light irradiation (15 ℃). The reaction rate under light irradiation (0.014 s^-1) was approximately three times higher than that in the dark at 15 ℃ (0.0044 s^-1) and even better than that in the dark at 25 ℃ (0.01 s^-1). A series of experiments indicated that the light irradiation promoted the conversion of 4-NP because of the localized surface plasmon resonance effect of Ag, which generated hot e^- and h⁺ particles and local heating around the particles via their absorption of the light.