We present the structural and optical properties of (0001)-oriented nanocolumnar films of InN deposited on c-sapphire substrates by radio-frequency reactive sputtering. It is observed that the column density and dimensions are highly dependent on the growth parameters of the buffer layer. We investigate four buffer layers consisting of (i) 30 nm of low-growth-rate InN, (ii) 30 nm of AIN deposited on the unbiased substrate (us), (iii) 30 nm of AIN deposited on the reverse-biased substrate (bs), and (iv) a 60-nm-thick bilayer consisting of 30-nm-thick bs-AIN deposited on top of 30-nm-thick us-AIN. Differences in the layer nucleation process due to the buffer layer induce variations of the column density in the range of (2.516) x 109 cm-2, and of the column diameter in the range of 87-176 nm. Best results in terms of mosaicity are obtained using the bs-AIN buffer layer, which leads to a full width at half-maximum of the InN(0002) rocking curve of 1.2. A residual compressive strain is still present in the nanocolumns. All samples exhibit room temperature photoluminescence emission at 1.6 eV, and an apparent optical band gap at 1.7 eV estimated from linear optical transmittance measurements. (C) 2015 Elsevier B.V. All rights reserved.