We study the metal-catalyst-free growth of uniform and continuous graphene films on different insulating substrates by microwave plasma-enhanced chemical vapor deposition (PECVD) with a gas mixture of C$2$H$2$, NH$3$, and H$2$ at a relatively low temperature of 700–750 $\mathrm{<mrow\; data-mjx-texclass="ORD">\circ </mrow>}$C. Compared to growth using only C$2$H$2$ and H$2$, the use of NH$3$ during synthesis is found to be beneficial, in transforming vertical graphene nano-walls into a layer-by-layer film, reducing the density of defects, and improving the graphene quality. The effect of different insulating substrates (including Al$2$O$3$ and SiO$2$) on the growth of graphene was studied under different growth temperatures, implying that the growth-temperature window and catalytic effect vary among insulators. The low activation energy barrier of Al$2$O$3$ proves it to be a more suitable substrate for the metal-catalyst-free growth of graphene at low temperature. These directly grown graphene films on insulators can be conveniently integrated into various electronic and optoelectronic applications avoiding any post-growth transfer processes.