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A report is presented on the preparation of surface‐functionalised nanosilica from silicon tetrachloride (SiCl4, one of the major by‐products in the polycrystalline silicon industry) by in‐situ surface modification in aqueous solution. The approach uses both SiCl4 and sodium metasilicate as the silicon sources to generate nanosilica, whereas the proportion of sodium metasilicate and SiCl4 is adjusted to control the pH value of the reaction system. In this way, desired SiO2 nanoparticles were in‐situ surface‐modified by hexamethyldisilazane as soon as they were generated in the reaction solution, which makes it feasible to prevent the severe aggregation of nanosilica that often happens during the hydrolytic process. The obtained nanosilica with uniform particle size distribution (mean diameter 35–40 nm) shows superhydrophobicity (a water contact angle of 165°) and excellent organic dispersibility. Thanks to the low cost of raw materials as well as the superhydrophobicity and lipophilicity of as‐prepared nanosilica, its application for improving the selective penetration of oil is primarily investigated. The result shows that quartz sand modified by as‐prepared nanosilica can separate diesel oil from water successfully, which could be significant for improving crude oil recovery efficiency.

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