Carbon nanowalls were prepared on silicon substrates by radio frequency magnetron sputtering under an argon-hydrogen mixture, at different hydrogen fluxes and varying the substrate temperature and deposition times. Scanning and transmission electron microscopy showed that carbon films deposited at 510 degrees C are nanostructured polycrystalline carbon nanowalls with crystals of about 3nm inside the flakes. The hydrogen flow induces the growth of nanowalls oriented perpendicularly to the substrate, and the density (amount) of these nanowalls decrease as the hydrogen flow increases. Field emission measurements showed that carbon nanowalls grown in hydrogen have a turn-on voltage of 2.0V/mu m, lower than those grown in pure argon with 4.5V/mu m.