In previous reports, organic solvents have usually been used as cosolvents in supercritical fluid applications. In this study, dilute acids were introduced into supercritical CO2 as cosolvents for the first time to improve the adsorption and dispersion of metal precursors on the substrate. Superhighly dispersed Co3O4 nanoparticles were confined in the nanochannels of SBA-15 after deposition and calcination when a small amount of nitric acid was added as the cosolvent. As the concentration of nitric acid increased, more individual small nanoparticles aggregated in adjacent nanochannels through the micropores in the walls of the substrates, which favored the formation of large nanoparticles. Furthermore, only Co3O4 nanoparticles were obtained when nitric acid was added to supercritical CO2−ethanol solution, whereas nanowires were found instead when hydrochloric acid was used. This indicates that H+ favors the adsorption and dispersion of precursors on the substrates and Cl− plays a key role in dictating the nanowire morphology.