A single-step solvent-, catalyst-, and template-free synthesis process to prepare photoluminescent pencils of ZnO either in micro- or in nanosize diameters from a single precursor is demonstrated. The thermolysis of Zn's acetate dihydrate (ZAD) precursor in a closed stainless steel reactor at 700 degrees C under autogenic pressure (6.5 MPa), yielded carbon sphere-decorated ZnO micropencils (ZnO-M's). The ZnO-M's have novel room-temperature photoluminescence (PL) with well-defined emission peaks at the green, yellow, orange, and red regions of the visible spectra while suppressing the blue region. On the contrary, the thermolysis of ZAD in a closed stainless steel reactor at 700 degrees C with released pressure yielded uniformly carbon-coated ZnO nanopencils (ZN's). The coated carbon in ZN's quenches the complete UV-vis PL; however, after annealing ZN's at 600 degrees C/2 h in air, the UV PL is dominant, and the visible PL is suppressed. The carbon coating (partly or completely) on the one-dimensional (1D) ZnO surfaces plays an important role to modify PL properties. The insight into the reaction mechanism was gained through in situ mass spectrometry measurements. The as-prepared ZnO-M's and ZN's have been systematically characterized to determine their morphology, structure, and composition.