To date, wet syntheses of single-crystalline ZnO micro- and nanotubes have been carried out using a two-step indirect approach in which a selective dissolution step is required in order to create the vacant space in the tubular structures. In this work, we develop a direct growth process for preparation of single-crystal ZnO nanotubes and nanorods. We also report that a concave shaped crystal growth front is generally reactive and offers a large surface area for matter deposition during rapid expansion of unidirectional nanomaterials. Depending on the degree of supersaturation of nutrients in solution, the concave growth front can either remain unaltered or undergo a concave-to-convex transformation, leading to the growth of solid nanorods and/or hollow nanotubes. The observed volume inversion should, in principle, also be applicable to the nanoarchitecture of other one-dimensional wurtzite structured nanomaterials, although individual sets of synthesis parameters need to be developed for each target material.