TiO<sub>2</sub> thin film photocatalysis has suffered from poor photocatalytic efficiency due to its low surface area-to-volume ratio. The efficiency can be enhanced by narrowing the bandgap, defect engineering or introducing surface plasmonic effect. However, the fabrication process is normally complicated and time consuming. This work offers a simple method to fabricate disordered defect-rich black TiO<sub>2</sub> ultrathin film by atomic layer deposition (ALD). Surface defects of TiO2 have been suggested to play a significant role in the process of photocatalysis. With ALD, the bandgap and surface defects of the material can be controlled effectively through the deposition parameters. Surface plasmonic effects could also be introduced by the deposition of Ag nanoclusters via simple thermal evaporation. Absorption at ~450 nm was significantly enhanced. The overall photocatalytic behavior of composite material is greatly boosted and we observed an excellent efficiency towards the degradation of organic pollutants such as bisphenol A. The mechanism of surface plasmonic enhanced black TiO<sub>2</sub> photocatalysis was studied by in-situ infrared atomic force microscope (IR-AFM) under the illumination of different wavelength. The reaction sites of the composite materials were determined accurately and the working mechanism was discussed.