It sounds like the silica nanoparticles are primarily adsorbing the methylene blue (MB) dye rather than facilitating its degradation. Silica nanoparticles alone are not inherently photocatalytic. To induce degradation under UV light, consider doping the silica nanoparticles with photocatalytic materials such as titanium dioxide (TiO₂) or zinc oxide (ZnO).These materials can generate reactive oxygen species (ROS) upon irradiation, which would facilitate the breakdown of methylene blue. If you prefer to continue using silica nanoparticles, you can modify their surface to introduce functional groups or metal ions that can participate in photocatalysis.

Thanks for your reply

I modified the surface of silica nanoparticles with stearic acid, but due to its hydrophobicity, it remained submerged on the surface of the dye solution and no degradation occurred. Should I increase the concentration of modified nanoparticles?

IS stearic acid regarded to be photocatalytic? Up to me no so why is it used?

I don't know

I said that maybe by functionalizing the surface of silica nanoparticles, color degradation will happen, not surface absorption.

I think You should do a look at the present literature and published papers on the subject. Then plan the research based on that.

I read the articles and it was very surprising to me that it was reported that silica nanoparticles can destroy the methylene blue dye, if I did this with different concentrations of silica, but no destruction occurred and only surface absorption occurred on the nanoparticles.

For this reason, I request guidance from people who are active in this field.

Silica nano-particles are reported to efficient absorbers for methylene blue?

But just by adsorption not by photocatalytic destruction.