A bifunctional plant growth promoter and methods of preparing the plant growth promoter. The plant growth promoter comprises nano carbon black modified with carboxyl groups referred to as “carbon black acid”. The nano carbon black has a structure forming a first functional part of the plant growth promoter and the carboxyl groups form a second functional part of the plant growth promoter. The plant growth promoter has an average particle size in a size range of 5 nm to 200 nm and a mass fraction of carboxyl groups in a mass fraction range of 5% to 25%.
