Bismuth oxyhalides

BiOX (X = Cl, Br, I)

Also called BiOCl, BiOBr, BiOI, BiOX

van der Waals crystal semiconductor

One of the most widely studied families of layered photocatalysts. Swapping chlorine for bromine or iodine moves the band gap from the ultraviolet into the visible, and the layered structure – positively charged bismuth-oxide slabs between negatively charged halide layers – sets up an internal electric field that helps separate the electrons and holes that light creates. Nanosheets that expose particular crystal facets or carry oxygen vacancies have been used to break down pollutants and to turn nitrogen and carbon dioxide into ammonia and carbon monoxide with light. BiOI has also emerged as an air-stable, lead-free absorber for solar cells, and BiOCl has long been used as a pearlescent pigment.

Cell: Square, a = 3.89 Å
Atoms per cell: 6
Bi–O bond: 2.32 Å
Bi–Cl bond: 3.06 Å
Height: 5.21 Å between the outer atom centres

A square net of oxygen with bismuth above and below it – the same [Bi₂O₂]²⁺ slab found in Bi₂O₂Se – capped on each side by a sheet of chlorine. Each bismuth bonds to four oxygens and four chlorines. The electric field inside the layer, pointing from the positive Bi₂O₂ slab towards the chlorine sheets, is often invoked to explain why these compounds separate photogenerated charges well. One layer of bulk BiOCl (Keramidas and colleagues, Zeitschrift für Kristallographie 205, 35, 1993; COD 9008426): a = 3.89 Å, Bi–O 2.32 Å, Bi–Cl 3.06 Å.

Data sheet

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Kind of sheet: van der Waals crystal
Peels off a layered crystal, one sheet at a time
Behaves as: semiconductor
Band gap: Set by the halogen: ~3.2 eV (BiOCl), ~2.6 eV (BiOBr), ~1.8 eV (BiOI)

In colour: this family, 1.8–3.2 eV. In grey: the other 39 families on the catalogue’s chart, each at the middle of its range.
Thickness: ~0.74 nm per BiOCl layer; exfoliated BiOBr monolayers measure ~0.85 nm
Structure: [Bi₂O₂]²⁺ slabs sandwiched between double layers of halide ions, with each bismuth bonded to four oxygen and four halogen atoms; neighbouring halide layers meet across a van der Waals gap (tetragonal, P4/nmm)
Space group: BiOCl: bulk P4/nmm, monolayer P4/nmm
BiOBr: bulk P4/nmm, monolayer P4/nmm
BiOI: bulk P4/nmm, monolayer P4/nmm
Bulk: the Materials Project crystal linked below. Monolayer: the 2DMatPedia layer, as a periodic slab
Stability: Chemically robust and stable in air, one reason these compounds attract interest as lead-free alternatives to halide perovskites. In photocatalysis their surfaces are deliberately modified – halogen removed, oxygen vacancies created – so ‘BiOX’ in that literature often means a defect-engineered, non-stoichiometric material, and the phase can change with pH.
First made: Occurs naturally as the mineral bismoclite (BiOCl); crystal structure redetermined in 1993 (Keramidas and colleagues); ultrathin BiOCl nanosheets with engineered vacancies in 2013 (Guan and colleagues); monolayer BiOBr by liquid exfoliation in 2017 (Yu and colleagues)
Materials Project: BiOCl
BiOBr
BiOI
Computed crystal structure and properties
2DMatPedia: BiOCl
BiOBr
BiOI
Computed monolayer, exfoliated from a bulk crystal of the same compound

Compare with other materials

Key properties

Band gaps of ~3.2, ~2.6 and ~1.8 eV for BiOCl, BiOBr and BiOI – from ultraviolet to visible absorbers
An internal electric field between the [Bi₂O₂]²⁺ slabs and the halide layers, credited with separating photogenerated charges
Facet-dependent photoactivity: BiOCl sheets exposing {001} facets are better at direct photoexcitation under UV, those exposing {010} facets at dye-sensitised reactions in visible light
Defects change with thickness: ultrathin BiOCl nanosheets host bismuth–oxygen–bismuth vacancy associates instead of isolated bismuth vacancies
In sub-3-nm BiOCl sheets the exciton binding energy drops from 137 to 36 meV compared with bulk, and bulk charge separation improves about 50-fold
BiOI films keep their tetragonal phase in air for at least 197 days, and calculations predict tolerance to antisite and vacancy defects

How it is made

Hydrothermal and solvothermal synthesis, for example in ethylene glycol, giving nanoplates and hierarchical microspheres; pH and additives select the exposed facets
Liquid-phase exfoliation with formamide into monolayer BiOBr and BiOI nanosheets, and solution growth of atomically thin BiOCl
Ultrathin BiOBr crystals grown for photodetectors, and BiOI films by chemical vapour transport for solar cells

Uses, and how close they are

Pearlescent pigments in cosmetics (BiOCl)deployed
Photocatalytic degradation of pollutants in water and airlab
Photocatalytic nitrogen fixation and CO₂ reductionlab
Lead-free, air-stable absorbers for solar cells (BiOI)lab
Ultraviolet photodetectors from ultrathin BiOCl and BiOBr crystalslab

Readiness runs lab → prototype → pilot → deployed.

Open problems

How much of the photoactivity comes from the internal electric field, from exposed facets and from vacancies, when a typical synthesis changes all three at once?
Are reported ammonia yields from photocatalytic nitrogen fixation real, given how easily trace ammonia and nitrogen-containing impurities contaminate such measurements?
Can clean exfoliated or vapour-grown flakes separate the intrinsic properties of BiOX from those of defect-rich solution nanosheets?
Why do BiOI solar cells still convert far less light than their absorption allows, and can predicted defect tolerance be turned into efficiency?

Going deeper

Short notes for specialists. Choose a lens in the header and yours comes first.

For theoreticians · your lens

A layered mixed-anion semiconductor whose valence band is built mainly from halogen and oxygen p states and whose conduction band comes from Bi 6p states, so the halogen sets the gap and the valence-band maximum rises from Cl to I. The alternating charged slabs give an intrinsic field across each layer, and exposing van der Waals gaps on the large faces of thin sheets weakens excitonic confinement enough to cut the exciton binding energy several-fold. Heavy bismuth requires spin–orbit coupling, and surface models must decide which termination a facet exposes – cleaving at the van der Waals gap leaves a halogen layer on top.

For experimentalists · your lens

Most BiOX is made in solution, so report facet exposure, thickness distribution and vacancy content – they change activity at least as much as the halogen does. Positron annihilation, EPR and XPS distinguish vacancy types. For photocatalysis, run controls without light and without catalyst, use ¹⁵N₂ isotope labelling before claiming nitrogen fixation and ¹³CO₂ for CO₂ reduction, and quote apparent quantum efficiency rather than rates per gram. Check the phase by XRD after reaction, because bismuth oxyhalides can transform in water.

For engineers · your lens

BiOCl is already a commercial material – as a pearlescent pigment, not a photocatalyst. As a photocatalyst it is cheap, low-toxicity and stable, but, like most powder photocatalysts, it is held back by low quantum efficiency, catalyst recovery from water and poor reproducibility between laboratories. BiOI is interesting as a lead-free, air-stable solar absorber, though its efficiencies remain far below those of silicon or perovskites.

In the research tracks

Growth methods

Liquid-phase and electrochemical exfoliation

Recent news

The newest items tagged BiOCl, BiOBr, BiOI, from the news feed updated 5 Oct 2026.

Journal 23 Sep 2026 Advanced Materials

Directing Near‐Perfect (001) Orientation in Wafer‐Scale BiOI Thin Films via Halide‐Mediated van der Waals Stacking

EngineeringBiOCl, BiOBr, BiOI

Preprintnot yet peer reviewed 3 Sep 2026 arXiv

High-Throughput Computational Discovery of Inverted Resistive Switching in Two-Dimensional Materials

Atomristors, non-volatile resistive switching devices based on two-dimensional (2D) monolayers, are promising building blocks for energy-efficient memory and neuromorphic computing. However, their design remains restricted to a few materials such as MoS2 and h-BN, limiting functional diversity and design flexibility.…

TheoryEngineeringMoS₂ BiOCl, BiOBr, BiOI hBN

Preprintnot yet peer reviewed 12 May 2026 arXiv

A comparative first-principles investigation of bilayer NbOX2 (X=Cl, Br, I) for Photocatalytic water splitting applications

Motivated by our previous work on bulk NbOX2 , where we have reported its high 1dielectric polarisation and finite piezoelectric response, this work extends to its 2D homo bilayer system to explore its potential for photocatalytic water splitting. Herein, density functional theory (DFT) were employed in probing the…

TheoryNbOI₂, CrOCl, TiOCl PtSe₂, PdSe₂, ditellurides BiOCl, BiOBr, BiOI

All 3 items tagged BiOCl, BiOBr, BiOI in the news feed · RSS feed for BiOCl, BiOBr, BiOI

Key references

The crystal structure of BiOClKeramidas et al. · Zeitschrift für Kristallographie 205, 35 (1993)cited by 64doi:10.1524/zkri.1993.205.12.35
Generalized one-pot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X = Cl, Br, I) nanoplate microspheresZhang et al. · Journal of Physical Chemistry C 112, 747 (2008)cited by 1,234doi:10.1021/jp077471t
Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheetsJiang et al. · Journal of the American Chemical Society 134, 4473 (2012)cited by 1,495doi:10.1021/ja210484t
Vacancy associates promoting solar-driven photocatalytic activity of ultrathin bismuth oxychloride nanosheetsGuan et al. · Journal of the American Chemical Society 135, 10411 (2013)cited by 1,286doi:10.1021/ja402956f
Bismuth oxyhalide nanomaterials: layered structures meet photocatalysisLi, Yu & Zhang · Nanoscale 6, 8473 (2014)cited by 906doi:10.1039/C4NR02553A
Efficient visible light nitrogen fixation with BiOBr nanosheets of oxygen vacancies on the exposed {001} facetsLi et al. · Journal of the American Chemical Society 137, 6393 (2015)cited by 1,787doi:10.1021/jacs.5b03105
Strongly enhanced photovoltaic performance and defect physics of air-stable bismuth oxyiodide (BiOI)Hoye et al. · Advanced Materials 29, 1702176 (2017)cited by 199doi:10.1002/adma.201702176
Liquid-phase exfoliation into monolayered BiOBr nanosheets for photocatalytic oxidation and reductionYu et al. · ACS Sustainable Chemistry & Engineering 5, 10499 (2017)cited by 180doi:10.1021/acssuschemeng.7b02508
Van der Waals gap-rich BiOCl atomic layers realizing efficient, pure-water CO₂-to-CO photocatalysisShi et al. · Nature Communications 12, 5923 (2021)cited by 328doi:10.1038/s41467-021-26219-6