There is obviously something wrong with your chart. Look at your ordinate. Why does the absorption exceed 1? The transmittance of 100% indicates total absorption, and the absorption value at this time is 1. Think about the physical meaning of exceeding 1

By the way, is the silica you use crystal or powder? You haven't made it clear about the physical state of matter either. If it is a silicon dioxide crystal, it is completely transparent in the ultraviolet-visible band, and there will be no obvious absorption peak.

Finally, a reference substance is needed to detect the ultraviolet-visible absorption spectrum. In the case of measuring solids, the reference substance may be a solution. But your chart is marked with pH = 8, which means it is a solution. Then the reference should be pure solvent. You need to measure the ultraviolet-visible absorption spectra of samples and reference materials.