NOVELTY - The method involves providing a silicon oxide film over a substrate, and electrochemically reducing silicon oxide contained in the silicon oxide film in a liquid electrolyte to form a porous film. The silicon oxide liquid or the liquid electrolyte comprises a compound comprising one selected from the group consisting of carbon (C), boron (B), nitrogen (N), aluminum (Al), phosphorus (P), sulfur (S), gallium (Ga), arsenic (As), selenium (Se), indium (In), tin (Sn), antimony (Sb), and tellurium (Te).    USE - Method of preparing a silicon thin film for an electronic device. Uses include but are not limited to a solar cell, a secondary battery, a fuel cell, a water electrolysis cell, a nuclear fuel of a nuclear reactor, a target for producing a radioactive isotope, a catalyst for a chemical reaction, and a sensor.    ADVANTAGE - The method enables preparing the silicon thin film from source materials such as sand, so as to reduce the manufacturing cost extensively with respect to the conventional processes. The method reduces an oxidized silicon element to silicon in a high-temperature molten salt after a coating process.    DETAILED DESCRIPTION - The silicon oxide liquid or the liquid electrolyte comprises a compound comprising one selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), boron (B), aluminum (Al), silicon (Si), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), arsenic (As), selenium (Se), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), tellurium (Te), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), thallium (Tl), lead (Pb), bismuth (Bi), polonium (Po), actinium (Ac), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), and curium (Cm). The method involves providing the silicon oxide liquid comprises mixing silicon oxide with a solvent. The mixing silicon oxide comprises mixing one material selected from the group consisting of sand, glass, quartz, rock, ceramic, silica (SiO2), tetraethoxysilane (TEOS), tetramethoxysilane, a silicon alkoxy, and silicon tetrachloride. The solvent is selected from the group consisting of water, lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, rubidium hydroxide, strontium hydroxide, cesium hydroxide, barium hydroxide, hydrofluoric acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, sodium silicate, ethanol, methanol, benzene, toluene, hexane, pentane, cyclohexane, chloroform, diethyl ether, dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate, acetone, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), and propylene carbonate. The liquid electrolyte is selected from the group consisting of acetonitrile, tetrafluoroborate, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-butylpyridinium chloride, choline chloride, 1-butyl-3-methylimidazolium chloride, dimethylethylphenylammonium bromide, dimethylformamide, dimethyl sulfone, dimethyl sulfoxide, ethylene carbonate, dimethyl carbonate, ethyl-methyl carbonate, ethylene-diaminetetra-acetic acid tetrasodium, ethylene glycol, 1-ethyl-3-methylimidazolium, 1-octyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, 1-propyl-3-methylimidazolium chloride, trihexyl-tetradecyl-phosphonium bis(trifluoromethylsulfonyl)imide, tetrabutylammonium chloride bis(trifluoromethylsulfonyl)imide, tetrahydrofuran, and trimethylphenylammonium chloride.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating a method of electrochemically preparing a thin film.