A method of preparing an adsorbent for carbon dioxide, the method comprising: preparing a first mixture by adding a precursor of a second metal (M 2) to a polar solvent, the second metal (M 2) having a trivalent oxidation number or greater and being selected from at least one of a Group 13 element, a Group 14 element, and a transition element, adding water to the first mixture to prepare a partially-hydrated mixture, preparing a second mixture by adding a precursor of a first metal (M 1) to the partially-hydrated mixture, the first metal (M 1) selected from at least one of an alkali metal and an alkaline-earth metal, adding water to the second mixture to induce hydrolysis and produce a wet gel, and calcining the wet gel to provide a composite metal oxide, the composite metal oxide being mesoporous, wherein the first metal (M 1) and the second metal (M 2) are present at an atomic ratio (M 1 /M 2) of 1/1 to 5/1; wherein preparing the first mixture includes the precursor of the second metal being represented by the following Chemical Formula 2, [Chemical Formula 2] M 2 (R 1) m (R 2) n-m wherein R 1 and R 2 are the same or different and are each independently hydrogen, an acryloyl group, a methacryloyl group, a vinyl group, a C1 to C15 linear or branched alkoxy group, a halogen, or a nitrate group, provided that at least one of R 1 and R 2 is the C1 to C15 linear or branched alkoxy group, the halogen, or the nitrate group, n denotes a valence of M 2, and m is an integer in a range of 0 to n; and wherein preparing a second mixture includes the precursor of the first metal (M 1) including a cation of the first metal and an anion selected from an alkoxide anion OR -, wherein R is a linear or branched C1 to C15 alkyl group, a halide anion X -, wherein X is a halogen, a nitrate anion (NO 3 -), a carbonate anion (CO 3 2-), a sulfate anion (SO 4 2-), a sulfonate anion (SO 3 -), a phosphate anion (PO 4 3-), and an acetate anion (CH 3 COO -). preparing a first mixture by adding a precursor of a second metal (M 2) to a polar solvent, the second metal (M 2) having a trivalent oxidation number or greater and being selected from at least one of a Group 13 element, a Group 14 element, and a transition element, adding water to the first mixture to prepare a partially-hydrated mixture, preparing a second mixture by adding a precursor of a first metal (M 1) to the partially-hydrated mixture, the first metal (M 1) selected from at least one of an alkali metal and an alkaline-earth metal, adding water to the second mixture to induce hydrolysis and produce a wet gel, and calcining the wet gel to provide a composite metal oxide, the composite metal oxide being mesoporous, wherein the first metal (M 1) and the second metal (M 2) are present at an atomic ratio (M 1 /M 2) of 1/1 to 5/1; wherein preparing the first mixture includes the precursor of the second metal being represented by the following Chemical Formula 2, [Chemical Formula 2] M 2 (R 1) m (R 2) n-m wherein R 1 and R 2 are the same or different and are each independently hydrogen, an acryloyl group, a methacryloyl group, a vinyl group, a C1 to C15 linear or branched alkoxy group, a halogen, or a nitrate group, provided that at least one of R 1 and R 2 is the C1 to C15 linear or branched alkoxy group, the halogen, or the nitrate group, n denotes a valence of M 2, and m is an integer in a range of 0 to n; and wherein preparing a second mixture includes the precursor of the first metal (M 1) including a cation of the first metal and an anion selected from an alkoxide anion OR -, wherein R is a linear or branched C1 to C15 alkyl group, a halide anion X -, wherein X is a halogen, a nitrate anion (NO 3 -), a carbonate anion (CO 3 2-), a sulfate anion (SO 4 2-), a sulfonate anion (SO 3 -), a phosphate anion (PO 4 3-), and an acetate anion (CH 3 COO -). R 1 and R 2 are the same or different and are each independently hydrogen, an acryloyl group, a methacryloyl group, a vinyl group, a C1 to C15 linear or branched alkoxy group, a halogen, or a nitrate group, provided that at least one of R 1 and R 2 is the C1 to C15 linear or branched alkoxy group, the halogen, or the nitrate group, n denotes a valence of M 2, and m is an integer in a range of 0 to n; and wherein preparing a second mixture includes the precursor of the first metal (M 1) including a cation of the first metal and an anion selected from an alkoxide anion OR -, wherein R is a linear or branched C1 to C15 alkyl group, a halide anion X -, wherein X is a halogen, a nitrate anion (NO 3 -), a carbonate anion (CO 3 2-), a sulfate anion (SO 4 2-), a sulfonate anion (SO 3 -), a phosphate anion (PO 4 3-), and an acetate anion (CH 3 COO -).