Oxidation state

Everyday term

In plain words

The charge an atom would carry if every bond it makes were fully ionic – a piece of chemical bookkeeping. In MoS₂ each sulfur counts as −2, so molybdenum is +4; in MoO₃ it is +6. The number says how many outer electrons an atom has given away, and so how many it still has – which decides much of a material’s colour, conductivity and magnetism. It changes when a material is oxidised or reduced, and X-ray methods can measure it.

As the site uses it

Track interlayer anion exchange through the XRD basal spacing; characterise nickel and iron oxidation states by XPS or, better, operando X-ray absorption.

LDHs · Layered double hydroxides

Generally excellent chemical and thermal stability; V₂O₅ and MoO₃ change oxidation state under reducing conditions or intense illumination.

Oxide nanosheets · 2D oxides and oxide nanosheets

Going deeper

Counting every bond as ionic gives each atom an oxidation state – +4 for molybdenum in MoS₂ – and with it the number of d electrons left: paired in MoS₂, unpaired and magnetic in CrI₃. Core levels shift to higher binding energy as an atom loses electrons, which is how X-ray methods read it.

Counting electrons

Assign every bonding electron to the more electronegative partner, and each atom ends up with a whole-number charge: its oxidation state. Sulfur and selenium in a dichalcogenide count as −2, as −1, oxygen as −2, so molybdenum in MoS₂ is +4, chromium in CrI₃ is +3, and molybdenum in MoO₃ is +6.

The count matters because it fixes how many outer electrons a transition metal keeps. Molybdenum has six; as Mo⁴⁺ it keeps two d electrons, which pair up in the split-off d band of -MoS₂ and make it a . Chromium as Cr³⁺ keeps three, unpaired, which is why CrI₃ is magnetic. Vanadium in VSe₂ keeps one, a 3d¹ configuration that confirmed (Wong and colleagues, 2019).

Measuring it

reads the oxidation state from core-level binding energies, which rise as an atom loses electrons: Mo⁶⁺ in an oxide sits at higher binding energy than Mo⁴⁺ in MoS₂, which makes XPS the standard check for oxidation of a dichalcogenide film. X-ray absorption does the same through the position and shape of an element’s absorption edge and, unlike XPS, which needs vacuum and sees only the top few nanometres, works through liquids and gases – on at work.

Operando X-ray absorption on (Ni,Fe) oxyhydroxide catalysts found the iron as Fe³⁺ in sites with unusually short Fe–O bonds, the sites that calculations identify as active in splitting water (Friebel and colleagues, 2015).

When the count is not simple

Oxidation states are bookkeeping; the real charge on an atom is smaller, because bonds share electrons, and where bonding is strongly the bookkeeping can become ambiguous. In RhSeCl, structural and magnetic studies describe rhodium as Rh(III), while X-ray absorption and photoelectron spectra have been read differently – one of the open questions about the compound.

Oxidation states also change in use. Layered oxides such as V₂O₅ and MoO₃ are reduced under reducing conditions or intense illumination, and making MoS₂ from MoO₃ films starts with exactly such a reduction, in hydrogen at 500 °C, before (Lin and colleagues, 2012).

For specialists

A formal charge assigned by treating all bonds as ionic, which fixes the d-electron count of a transition metal: Mo⁴⁺ (d²) in MoS₂, Cr³⁺ (d³) in CrI₃, V⁴⁺ (d¹) in VSe₂. Real is smaller, but the count predicts band filling, magnetism and redox behaviour. It is measured by core-level shifts in XPS and by the position and shape of absorption edges in XAS; operando XAS follows it under reaction conditions, as for the Fe³⁺ sites of (Ni,Fe)OOH catalysts. Where bonding is strongly covalent it can be ambiguous: in RhSeCl, structure and magnetism point to Rh(III), while some spectra have been read otherwise.

Where this comes from

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting Friebel et al. · Journal of the American Chemical Society 137, 1305 (2015) cited by 2,733
Evidence of spin frustration in a vanadium diselenide monolayer magnet Wong et al. · Advanced Materials 31, 1901185 (2019) cited by 152
Wafer-scale MoS₂ thin layers prepared by MoO₃ sulfurization Lin et al. · Nanoscale 4, 6637 (2012)

In the news

The newest items in the site’s news feed that use the term, one from each source.

Preprintnot yet peer reviewed 22 Dec 2025 arXiv

Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction

(2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong , physics, and low- . Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both…

Experiment

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Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

X-ray absorption spectroscopy (XAS)

Shining X-rays of tunable energy on a sample and watching how strongly it absorbs as the energy sweeps past the binding energy of one element’s inner electrons. Each element absorbs at its own energies, so the method looks at one element at a time – the iron in a catalyst, the vanadium in a magnet – and the exact shape of the absorption edge tells its oxidation state and surroundings. The bright, tunable X-rays it needs come from synchrotrons: large rings in which electrons circling close to the speed of light give off intense light, used also for photoemission and diffraction.

Absorption of tunable X-rays at a core-level edge of one element: the near-edge region (XANES) reflects oxidation state, empty states and local symmetry; the extended region (EXAFS) gives bond lengths and coordination; circularly polarised X-rays give X-ray magnetic circular dichroism (XMCD), element-specific spin and orbital moments. It uses synchrotron radiation and works operando. Operando XAS located Fe³⁺ in unusually short-bonded octahedral sites as the active centres of (Ni,Fe)OOH water-oxidation catalysts; XAS and XMCD on monolayer VSe₂ found vanadium 3d¹ and correlated spins without long-range order, against bulk-magnetometry reports of room-temperature ferromagnetism.

Full explanation

Photocatalysis and electrocatalysis

Using light, or an applied voltage, to drive a chemical reaction at a surface – most often splitting water to make hydrogen. Thin materials are natural candidates because nearly every atom is a surface atom, and because their edges are usually far more active than their flat faces.

Surface reactions driven by photons (photocatalysis) or by electrode potential (electrocatalysis), judged by overpotential, Tafel slope and turnover per site rather than by raw current. In MoS₂ the basal plane is close to inert while the edge sites carry the hydrogen-evolution activity – the result that made layered chalcogenides catalyst candidates – so activity scales with edge length, defect density and phase, and any comparison needs an active-site count rather than a geometric area.

Full explanation

Coordination polyhedron

The small shape traced by the atoms that surround a metal atom in a crystal. In many layered materials each metal sits inside an octahedron – six neighbours, three above and three below, turned against each other – or a trigonal prism, where the upper three sit right above the lower three. That one turn can make the same compound a semiconductor or a metal.

The polyhedron formed by an atom’s nearest neighbours; their number is its coordination number. In MX₂ layers the metal is either trigonal prismatic (D3h, as in 2H-MoS₂) or octahedral (D3d, as in 1T-TaS₂, and distorted in 1T′-WTe₂), and the ligand-field splitting of the d levels in each geometry, together with the d-electron count, decides whether the layer is a semiconductor or a metal. Halides and many oxides build layers from MX₆ octahedra sharing edges (CrI₃, RuCl₃) or corners (layered perovskites), and that connectivity sets the lattice – honeycomb, triangular or square – that the metal ions form.

Full explanation

Chemical bond

The glue that holds atoms together. In a covalent bond two atoms share electrons, as within a sheet of graphene. In an ionic bond the atom that pulls harder on electrons – the one with the higher electronegativity – takes them from the other, and the resulting charges attract, as in table salt. In a metal the outer electrons are shared by all the atoms at once. Much weaker van der Waals forces hold molecules and the layers of a layered crystal together. A 2D material is a crystal with strong bonds within its layers and only weak ones between them – which is why it can be peeled.

The attractive interactions that hold atoms in molecules and solids, classified by how electrons are shared: covalent bonds, from overlapping orbitals holding shared electron pairs, strong and directional (sp² carbon, Mo–S in TMDCs); ionic bonds, from electron transfer and electrostatic attraction (halides, oxides); metallic bonding, with electrons delocalised in a partly filled band; and the weak, non-directional van der Waals interaction from correlated charge fluctuations, besides hydrogen bonds. Real bonds are mixtures, characterised by electronegativity differences and charge analyses. Layered crystals combine in-plane bonds of several electronvolts with interlayer binding of only about 20 meV per Å² (some 0.3 J/m²), the criterion high-throughput screens use to identify exfoliable compounds.

Full explanation

Sulfurisation and selenisation

Turning a film of metal or metal oxide into a 2D material by heating it in sulfur or selenium vapour. Instead of growing the crystal atom by atom from a gas, you first lay down a thin film of, say, molybdenum or platinum – easy to do evenly over a whole wafer – and then let sulfur or selenium react with it. The result covers large areas but is usually made of many small crystals. The same trick can swap one layer of atoms for another, which is how Janus layers with two different faces are made.

Conversion of a pre-deposited metal, metal-oxide or dichalcogenide film by reaction with chalcogen vapour or H₂S. It separates thickness control, set by the deposited film, from crystallisation: MoO₃ films reduced in hydrogen at 500 °C and sulfurised at 1000 °C give wafer-scale MoS₂ that is uniform but polycrystalline; selenising platinum films at 400 °C gives PtSe₂ within back-end-of-line thermal budgets; selenising a Pt(111) crystal gives single-crystal monolayer PtSe₂. Controlled partial exchange replaces one chalcogen face of a monolayer, producing Janus MoSSe.

Full explanation

Two-dimensional electron gas (2DEG)

A sheet of electrons free to move in two directions but trapped in the third, so tightly that across it they have only one way to sit. It need not be a thin material: most electron gases live at the boundary between two thick crystals, or under the gate of a silicon transistor, with ordinary bulk material all around. A layer of graphene is the extreme case, where the crystal itself is as thin as its sheet of electrons. Electron gases like these are where the quantum Hall effect was discovered.

Electrons confined in one direction to the lowest subband of a potential well – an inversion layer at the Si/SiO₂ interface, a GaAs/AlGaAs heterojunction, an oxide interface such as LaAlO₃/SrTiO₃ – and free in the other two, with a density of states that does not depend on energy and a density set by gates or doping. Modulation doping, which places the donors away from the electrons, gave GaAs electron gases mobilities above those of comparable bulk GaAs; the quantized Hall resistance was first measured in the electron gas of a silicon MOSFET. Atomically thin crystals such as graphene host two-dimensional electron systems of their own, with no buried interface.

Full explanation

Rashba effect

Some crystals are not the same seen from above and from below, because they sit on a substrate, have a field across them, or are simply built that way. In such a crystal a moving electron feels a magnetic field that depends on which way it is going. Its spin then winds around its direction of motion, which is what lets an electric field steer spins.

A momentum-dependent spin splitting from spin–orbit coupling in a structure without inversion symmetry, H = α(σ × k)·ẑ, locking spin perpendicular to in-plane momentum and winding it around the Fermi contour. The coefficient α follows the potential gradient and the atomic spin–orbit strength, so it is tunable by gating, by the substrate and by the choice of heavy elements; BiTeI splits its bands far enough for the two spin-split Fermi surfaces to be resolved separately.

Full explanation