A semi-permeable membrane may include a support layer and an active layer in contact with the support layer. The support layer includes a porous structure including a polymer and at least one metal (or metalloid) oxide in the porous structure. In the support layer, the amount of the metal (or metalloid) oxide present in a portion adjacent to the active layer is higher than the amount of the metal (or metalloid) oxide present in a portion farther from the active layer.