Quaternary zinc germanium oxynitride (Zn1+xGe)(N2Ox), a solid solution between ZnGeN2 and ZnO with a wurtzite structure, is one of the attractive photocatalysts under visible-light irradiation. In this study, the synthesis of (Zn1+xGe)(N2Ox) nanoparticles was achieved by the nitridation of Zn2GeO4 nanoparticles designed precisely to enhance their photocatalytic NOx decomposition activity under both UV and visible light irradiation. The obtained (Zn1+xGe)(N2Ox) nanoparticles exhibited a high specific surface area and visible light absorption induced by the narrow band gap of ca. 2.6-2.8 eV, both of which are reasons for the enhancement of photocatalytic activity. The oxide precursors with a nanoparticle morphology were prepared by a facile solvothermal method with various volumes of TEA (triethanolamine) as an additive. The relationships of nitridation time and TEA volume in the solvothermal reaction for the synthesis of the precursor with morphology, specific surface area, and photocatalytic NOx decomposition activity of the nitrided samples were investigated. The increase of active sites by the high surface area and the enhanced visible-light absorption ability as well as the defect amounts and states can be largely related to the excellent NOx decomposition activity of (Zn1+xGe)(N2Ox).