Two-dimensional TiO_2 nanosheets were synthesized by atomic layer deposition(ALD) on dissolvable sacrificial polymer layer. The photocatalytic performance of free-standing TiO_2 nanosheets prepared with different numbers of ALD cycles(100, 300, 500, and 1000) were investigated by evaluating the degradation rates of methyl orange solutions. It is shown that the photocatalytic activity increases due to Ti~(3+) defect and the locally ordered structures in amorphous TiO_2 nanosheets. The difference in the surface areas of nanosheets may also play a crucial role in the photocatalytic activity. The results obtained in this work can have potential applications in fields like water splitting and dye-sensitized solar cells. Two-dimensional TiO 2 nanosheets were synthesized by atomic layer deposition (ALD) on dissolvable sacrificial polymer layer. The photocatalytic performance of free-standing TiO 2 nanosheets prepared with different numbers of ALD cycles (100, 300, 500, and 1000) were investigated by evaluating the degradation rates of methyl orange solutions. It shows that the photocatalytic activity increases due to Ti ~ (3+) defect and the locally ordered structures in amorphous TiO_2 nanosheets. The difference in the surface areas of nanosheets may also play a crucial role in the photocatalytic activity. The results obtained in this work can have potential applications in fields like water splitting and dye-sensitized solar cells.