The BET theory is based on the physical adsorption of gas molecules on a solid surface and is widely used to calculate the specific surface area of materials. The method involves measuring the amount of a gas, typically nitrogen, adsorbed onto the surface of the photocatalyst at liquid nitrogen temperature. The adsorption data are then fitted to the BET equation: where:  ( A ) is the amount of gas adsorbed,  ( V_m ) is the molar volume of the adsorbed gas layer,  ( C ) is the BET constant,  ( P ) is the equilibrium pressure,  ( P_0 ) is the saturation pressure of the adsorbate at the temperature of adsorption.