Siloxene is a nanostructured two-dimensional material derived from silicon. Together with silicene and silicane it constitutes the family of silicon nanosheets (Si-NSs), two-dimensional materials with diverse structures and functionalities. Its empirical formula is Si6H3(OH)3 (equivalently Si6O3H6). Siloxene can be structurally derived from layered polysilyne by replacing one half of the hydrogen atoms with OH groups. Two structural variants are recognised, named after their discoverers. Wöhler siloxene (sometimes called Weiss siloxene) consists of a corrugated hexagonal silicon sheet whose two faces are terminated by −H and −OH groups. No oxygen is incorporated into the Si framework. Kautsky siloxene consists of Si6H6 rings connected by Si−O−Si bridges within the layer. The Wöhler form is metastable and slowly converts to the Kautsky form via hydrolysis, with oxygen inserting into Si−Si bonds. Siloxene is a semiconductor with a band gap of about 2.5 eV and exhibits strong photoluminescence, making it a candidate material for optoelectronics, photocatalysis, supercapacitors, and alkali-ion battery anodes.
History Siloxene was discovered by Friedrich Wöhler in 1863 during his investigations of silicon compounds. Wöhler isolated the material from the reaction of calcium disilicide with hydrochloric acid as an unusually colored yellow substance, insoluble in water and most solvents, that bleached on exposure to sunlight. He called the yellow compound Silicon and the bleached product Leucon. The same compound was prepared independently as early as 1900 by Charles Schenck Bradley (1853–1929), a one-time associate of Thomas Edison. Bradley erroneously concluded that the substance was the silicon analog of polyacetylene, (Si2H2)x, and so referred to it as silico-acetylene. Beginning in 1921, Hans Kautsky re-examined the reaction of calcium disilicide with hydrochloric acid and showed that different reaction conditions produced different substances. A white, sometimes greenish, compound obtained from the reaction with alcoholic HCl was named Oxydisilin and, in 1924, renamed siloxene (from Cyclohexasilatrioxen). Kautsky proposed that the material consisted of six-membered silicon rings connected by oxygen bridges. The structure of Wöhler's siloxene was determined by X-ray diffraction by Armin Weiss and co-workers in 1980, showing that the Wöhler preparation produces a different topology from the one Kautsky had proposed. Refined crystallographic studies followed in the 1990s. More recent work has examined siloxene's electronic and optical properties and explored applications in nanotechnology and electronics.
Structure Siloxene is a two-dimensional material. Sheets as thin as 0.7 nm (a single silicon layer) and 1.3 nm (two layers) have been prepared. Two distinct structural variants are known. The two variants interconvert: the Wöhler form is metastable, and slowly hydrolyses to the Kautsky form as oxygen inserts into Si−Si bonds.
Wöhler siloxene
Wöhler siloxene is a corrugated two-dimensional polymer of 1,3,5-trihydroxocyclohexasilane. Each silicon atom is fourfold-coordinated: 3 in-plane Si−Si bonds to neighbouring silicons and 1 out-of-plane Si−H or Si−OH bond. The Si sublattice is the same buckled honeycomb found in silicene and in the (111) layers of α-silicon. One face of each sheet is terminated by hydrogen atoms, and the other by hydroxyl groups, producing an electrostatic dipole that limits close interlayer stacking. Layer ordering is turbostratic. The structure is trigonal in space group P3m1 (No. 156) with one formula unit Si6H3(OH)3 per cell distributed over three primitive cells.
Note that the Si−Si bond length (234 pm) is nearly that of crystalline silicon (235.2 pm). Dahn et al. (1993) reported nearly identical in-plane parameters (a = 3.83 Å, c = 5.90 Å) and confirmed the same topology by powder X-ray diffraction.
Kautsky siloxene
The Kautsky variant has a different in-plane connectivity: discrete Si6 rings linked by Si−O−Si bridges, or one-dimensional Si−Si chains likewise joined by Si−O−Si bridges. Because the material does not form single crystals that can undergo conventional X-ray structure analysis, no refined atomic coordinates are available. The connectivities were measured using Si L2,3 X-ray emission spectroscopy (XES) and infrared spectroscopy.
The overall oxygen content is ≈0.8 O per Si in Kautsky siloxene, compared with ≈0.5 O per Si in Wöhler siloxene. This matches the additional oxygen carried by the Si−O−Si bridges.
Synthesis
Standard route Siloxene is most commonly synthesized from calcium disilicide (CaSi2), a widely used precursor for silicon-based materials. The standard route de-intercalates Ca2+ from CaSi2 by a strong acid. The final structure depends on the acid concentration, temperature and reaction time. CaSi2 powder is mixed with concentrated hydrochloric acid under an inert atmosphere (nitrogen or argon) to prevent unintended oxidation. The mixture is stirred for 2 – 4 days at low temperature, typically close to 0 °C to remove (de-intercalate) Ca2+ ions from the CaSi2 layers. This produces silicon sheets terminated by −OH and −H groups, which is siloxene. The overall reaction is:
3 CaSi2 + 6 HCl + 3 H2O → Si6H3(OH)3 + 3 CaCl2 + 3 H2
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