This study reports the fabrication of undoped and Al-doped ZnO nanowall structures on ordinary glass substrate by low cost spray pyrolysis technique at low deposition temperature of 350 degrees C. Structural, surface, optical, and photocatalytic studies of these nanostructures have been performed. Surface study confirms the nanowall structure of the fabricated ZnO. Structural study reveals that the fabricated ZnO nanostructures are polycrystalline in nature and crystallinity of the samples depends on the Al doping concentration. Band gap of the ZnO nanostructure has been determined from the transmission spectra. For undoped ZnO, the band gap was obtained 3.27 eV and the band gap decreases up to 3.06 eV with the increase in Al doping concentration up to 4%. Photocatalytic decomposition of methanol has been studied and found that the fabricated ZnO nanowalls can successfully decomposes methanol under UV-visible light irradiation without any surface treatment, and improvement in the photocatalytic activity was obtained after Al doping.