Titanium dioxide TiO2

spore: profile
id: titanium-dioxide
spore_version: 0.2
family: oxide-ceramics
name: Titanium dioxide
abbreviation: TiO2
role: white pigment and photocatalyst (anatase and rutile polymorphs)
category: ceramics
type: oxides
summary: |
  Titanium dioxide (TiO2) is a white oxide that occurs in two main crystal forms, anatase and rutile. Rutile is the denser form and the one used as a white pigment: a rutile pigment grade (Chemours Ti-Pure R-706) has a specific gravity of 4.0 and a median particle size of 0.36 µm. Anatase has the wider band gap, 3.23 eV against 3.0 eV for rutile, which is why it absorbs ultraviolet light and drives photocatalysis. Its only space records are NASA ground work: a 1966 Lewis test of TiO2-pigmented coatings for low-temperature space radiators, which stayed stable in vacuum up to 550 K, and a Kennedy Space Center catalyst for cleaning ISS cabin air and water, tested on the ground.
best_for: White, stable thermal-control coatings that keep their emittance in vacuum up to 550 K (NASA Lewis test of a TiO2 paint and tape)
weakest_at: Suspected carcinogen when inhaled as powder (H351, IARC Group 2B), and its photocatalysis needs ultraviolet light, which current reactors make with mercury lamps
identity:
  formula: TiO2
  cas: 13463-67-7
  space_group: I41/amd (anatase), P42/mnm (rutile)
  source: nist-srm1898-msds
  where: 'Section 3, Composition: titanium dioxide, CAS No. 13463-67-7, other designations anatase and rutile; space groups from arXiv paper, Section 3'
  designations:
    - system: Trade name
      code: Ti-Pure R-706
      note: rutile pigment grade, TiO2 93 wt% minimum
      source: chemours-r706
      where: Title, Table 1 Analysis and Physical Properties of Ti-Pure R-706
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - thermal-control
    - habitats-and-suits
properties:
  - benchmark: band_gap
    label: Band gap, anatase (experimental)
    value: 3.23
    unit: eV
    scope: material
    conditions:
      phase: anatase
      basis: experimental value quoted in the arXiv paper, citing Tang et al. 1993
    method: Experimental value quoted in an open preprint on the optical properties of TiO2 polymorphs
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, sentence on band gaps
  - benchmark: band_gap
    label: Band gap, rutile (experimental)
    value: 3
    unit: eV
    scope: material
    conditions:
      phase: rutile
      basis: experimental value quoted in the arXiv paper, citing Diebold 2003
    method: Experimental value quoted in an open preprint on the optical properties of TiO2 polymorphs
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, sentence on band gaps
  - benchmark: band_gap
    label: Band gap, silver-doped TiO2 visible-light catalyst
    value: 2.72
    unit: eV
    scope: material
    conditions:
      material: Ag-doped TiO2 developed at Kennedy Space Center
      basis: narrowed band gap for visible-light response
    method: Value stated in a NASA Kennedy conference paper on the catalyst
    status: ground
    source: ksc-ag-tio2-2016
    where: Abstract, first paragraph
  - benchmark: density
    label: Specific gravity, rutile pigment grade R-706
    value: 4
    unit: g/cm3
    scope: material
    conditions:
      grade: Ti-Pure R-706, rutile TiO2, TiO2 93 wt% minimum
      printed: Specific Gravity 4.0 (no unit printed)
    method: Printed as specific gravity 4.0 in the datasheet table; read as g/cm3 numerically because specific gravity is relative to water
    status: ground
    source: chemours-r706
    where: Table 1, Analysis and Physical Properties of Ti-Pure R-706, Specific Gravity row
  - benchmark: density
    label: Specific gravity, TiO2 nanomaterial powder SRM 1898
    value: 3.8
    unit: g/cm3
    scope: material
    conditions:
      material: NIST SRM 1898 titanium dioxide nanomaterial, dry agglomerated powder of anatase and rutile
      basis: water = 1
    method: Printed as specific gravity (water = 1) 3.8 in the physical properties section of the SDS
    status: ground
    note: A nanocrystalline reference powder, not a pigment grade; the pigment R-706 is 4.0 in the same comparison.
    source: nist-srm1898-msds
    where: Physical and chemical properties section, Specific Gravity (water = 1) row
  - benchmark: melting_point
    label: Melting point of titanium dioxide
    value: 1850
    unit: °C
    scope: material
    conditions:
      material: titanium dioxide, no grade given
      printed: 1850 (3362 ºF)
    method: Value printed in two safety data sheets (1850 °C, 3362 °F)
    status: ground
    note: The Sputtertargets SDS also prints 1,850 °C (3,362 °F).
    source: nist-srm1898-msds
    where: Physical and chemical properties section, Melting point/freezing point row
  - benchmark: max_service_temp
    label: Stable in vacuum, TiO2 paint and tape (coatings 5 and 6)
    value: 550
    unit: K
    scope: material
    conditions:
      form: TiO2 pigment in a polymethyl vinyl siloxane binder, as an air-drying paint (coating 5) and an adhesive-backed tape (coating 6)
      environment: vacuum
    method: Statement in a NASA Lewis test report that both coatings are stable in vacuum up to this temperature
    status: ground
    note: Coating-level limit, not a bulk-material rating. Tape blisters appeared near 450 K during the test.
    source: nasa-1966-coatings
    where: Section on titanium dioxide pigmented coatings (coatings 5 and 6), first paragraph
  - benchmark: total_emittance
    label: Hemispherical total emittance at 400 K, TiO2 paint and tape
    value: 0.94
    unit: '-'
    scope: material
    conditions:
      form: TiO2 pigment in polymethyl vinyl siloxane binder, coatings 5 and 6
      temperature: 400 K
      environment: vacuum
    method: Value read from the text of a NASA Lewis test report, which gives 0.92 at 300 K and 0.89 at 570 K for the same coatings
    status: ground
    source: nasa-1966-coatings
    where: Section on titanium dioxide pigmented coatings (coatings 5 and 6), emittance paragraph
  - benchmark: median_particle_size
    label: Median particle size, rutile pigment grade R-706
    value: 0.36
    unit: µm
    scope: material
    conditions:
      grade: Ti-Pure R-706, rutile TiO2
    method: Printed in the datasheet table
    status: ground
    source: chemours-r706
    where: Table 1, Analysis and Physical Properties of Ti-Pure R-706, Median Particle Size row
  - benchmark: lattice_constant_a
    label: Lattice constant a, anatase
    value: 0.3785
    unit: nm
    scope: material
    conditions:
      phase: anatase, space group I41/amd
      basis: experimental value quoted in the arXiv paper, citing its refs 20 and 21
    method: Experimental lattice constant quoted in an open preprint
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, lattice constants
  - benchmark: lattice_constant_c
    label: Lattice constant c, anatase
    value: 0.9515
    unit: nm
    scope: material
    conditions:
      phase: anatase, space group I41/amd
      basis: experimental value quoted in the arXiv paper, citing its refs 20 and 21
    method: Experimental lattice constant quoted in an open preprint
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, lattice constants
  - benchmark: lattice_constant_a
    label: Lattice constant a, rutile
    value: 0.4593
    unit: nm
    scope: material
    conditions:
      phase: rutile, space group P42/mnm
      basis: experimental value quoted in the arXiv paper, citing its refs 20 and 21
    method: Experimental lattice constant quoted in an open preprint
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, lattice constants
  - benchmark: lattice_constant_c
    label: Lattice constant c, rutile
    value: 0.2959
    unit: nm
    scope: material
    conditions:
      phase: rutile, space group P42/mnm
      basis: experimental value quoted in the arXiv paper, citing its refs 20 and 21
    method: Experimental lattice constant quoted in an open preprint
    status: ground
    source: arxiv-tio2-optical
    where: Section 3 (Results and discussion), first paragraph, lattice constants
corrosion:
  - environment: Vacuum, about 390 to 575 K, TiO2 paint and tape in a polymethyl vinyl siloxane binder
    headline: In vacuum, the TiO2 paint and tape held their emittance through a temperature cycle, but the tape blistered and emittance dropped below 0.90 at 550 K.
    effect: Emittance did not change after temperature cycling to 570 K. Blisters, presumed to be gas trapped in the adhesive, showed under the tape surface from about 450 K. The authors call the coatings marginal for radiators operating at 550 K. The authors say the paint formulation had the least stable solar absorptance of the two; the absorptance values were not entered here because the table could not be matched to the coatings reliably.
    rating: low
    conditions:
      test: emittance and absorptance measured in vacuum on the same specimen, 300 to 575 K
      exposure: temperature cycle to 570 K
      note: lab test of 0.2 mm coatings, not a long-duration flight exposure
    source: nasa-1966-coatings
    where: Titanium dioxide pigmented coatings (coatings 5 and 6) text; Summary; Conclusions
risks:
  hazards:
    - text: The NIST SRM 1898 safety data sheet classifies titanium dioxide nanomaterial powder as Carcinogenicity Category 2 with hazard statement H351, suspected of causing cancer by inhalation.
      ghs:
        - H351
      source: nist-srm1898-msds
      where: Section 2, Hazards Identification, Health Hazard and Hazard Statement rows
    - text: The Sputtertargets safety data sheet for titanium dioxide nanoparticles (1 to 150 nm) also classifies Carc. 2 with H351 at 90 to 100 percent of the substance.
      ghs:
        - H351
      source: sputtertargets-tio2-sds
      where: Section 2.1 and Section 3, hazardous components table
    - text: IARC lists titanium dioxide as Group 2B, possibly carcinogenic to humans. OSHA and NTP do not list it as a carcinogen.
      ghs: []
      source: nist-srm1898-msds
      where: Section 11, Carcinogenicity
  failure_risks:
    - mode: Blistering of the adhesive-backed tape in vacuum
      trigger: Heating an adhesive-backed TiO2 tape in vacuum above about 450 K
      consequence: Blisters formed beneath the tape surface, presumed to be gas trapped in the adhesive, and grew as temperature rose.
      severity: moderate
      source: nasa-1966-coatings
      where: Titanium dioxide pigmented coatings (coatings 5 and 6) text
    - mode: Emittance loss at the top of the service range
      trigger: Operating a TiO2 paint or tape at about 550 K in vacuum
      consequence: Emittance dropped below 0.90 at 550 K, which the authors call marginal for radiators at that temperature.
      severity: moderate
      source: nasa-1966-coatings
      where: Titanium dioxide pigmented coatings (coatings 5 and 6) text, final paragraph
  side_effects:
    - text: Current photocatalytic reactors use mercury-containing vapour lamps to supply the ultraviolet light that anatase needs, and the mercury hinders use in spaceflight.
      affects: Crew safety and reactor design for cabin air and water
      source: ksc-ag-tio2-2016
      where: Introduction
  handling:
    - text: The NIST SRM 1898 safety data sheet gives an OSHA permissible exposure limit of 15 mg/m3 (total particulates) and a NIOSH recommended exposure limit of 0.3 mg/m3 (10 h TWA) for ultrafine titanium dioxide, including engineered nanoscale.
      source: nist-srm1898-msds
      where: Section 8, Exposure Controls and Personal Protection
    - text: Pigment grades are dry powders with a median particle size of 0.36 µm. Control dust, and use a respirator that meets the cited standard where dust is generated.
      source: chemours-r706
      where: Table 1 and Section 8 of the NIST SDS
  mitigations: []
uses:
  - application: thermal-control
    device: White thermal-control coatings for low-temperature space radiators (ground test)
    example: NASA Lewis tested two TiO2-pigmented coatings, a paint and an adhesive-backed tape in a polymethyl vinyl siloxane binder, for low-temperature space radiators. They were chosen because of test data from several space satellite experiments.
    source: nasa-1966-coatings
    where: Summary; Titanium dioxide pigmented coatings (coatings 5 and 6)
  - application: habitats-and-suits
    device: Photocatalytic reactor for ISS cabin air and recovered water (ground bench test)
    example: Kennedy Space Center developed a visible-light-responsive, silver-doped TiO2 catalyst with a band gap of 2.72 eV and tested it in a bench-scale reactor against common ISS cabin VOCs, to remove volatile organics before water processing.
    source: ksc-ag-tio2-2016
    where: Abstract; Introduction
supply:
  - subject: Titanium dioxide pigment (production capacity)
    producers:
      - country: China
        share_percent: 60.6
        year: 2025
      - country: United States
        share_percent: 13.7
        year: 2025
      - country: Mexico
        share_percent: 3.5
        year: 2025
      - country: Japan
        share_percent: 3.3
        year: 2025
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (titanium, used as a white pigment and in metal alloys)
        listed: true
    recycling: 'USGS titanium chapter: consumption data for titanium scrap metal were withheld. The chapter does not give a recycling figure for TiO2 pigment.'
    substitutes:
      - 'USGS: ground calcium carbonate, precipitated calcium carbonate, kaolin and talc compete with TiO2 as a white pigment.'
    source: usgs-mcs2026-titanium
    where: World Sponge Metal Production and Sponge and Pigment Capacity table, pigment capacity column 2025e (world total 9,900,000 t; shares calculated by Space Mushroom from the printed capacities, which are capacity and not output); Substitutes
documents:
  - title: Chemours Ti-Pure R-706 technical datasheet (copy hosted by Knowde)
    type: datasheet
    url: https://media.knowde.com/image/upload/v1733488926/TDS-Ti-PureR-706TitaniumDioxide.pdf20241206-1-fuk500.pdf
    year: 2024
  - title: NIST Standard Reference Material 1898 safety data sheet (titanium dioxide nanomaterial)
    type: sds
    url: https://tsapps.nist.gov/srmext/msds/1898-MSDS.pdf
    year: 2020
  - title: 'Sputtertargets safety data sheet: titanium dioxide (TiO2)'
    type: sds
    url: https://www.sputtertargets.net/wp-content/uploads/2018/05/Titanium-Dioxide-TiO2-sds.pdf
    year: 2017
  - title: 'NASA Lewis: Experimental investigation of total emittance and solar absorptance of several coatings (NTRS 19660011749)'
    type: report
    url: https://ntrs.nasa.gov/citations/19660011749
    year: 1966
  - title: 'Visible-Light-Responsive Photocatalysis: Ag-Doped TiO2 Catalyst Development and Reactor Design Testing (ICES-2016-169, NTRS 20160008969)'
    type: paper
    url: https://ntrs.nasa.gov/citations/20160008969
    year: 2016
  - title: Optical properties of anatase and rutile TiO2 studied by GGA+U (arXiv:1611.08363)
    type: paper
    url: https://arxiv.org/pdf/1611.08363
    year: 2016
  - title: 'USGS Mineral Commodity Summaries 2026: Titanium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-titanium.pdf
    year: 2026
  - title: 'USGS: About the 2025 List of Critical Minerals (final list of 60 minerals)'
    type: report
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
gaps:
  - No flight or vacuum-exposure result for bulk TiO2 was found in a source we could open. The NASA records are ground tests of coatings, and the only space-related catalyst work is a bench test.
  - PubChem's GHS and experimental-property pages returned a browser check and could not be read, so the hazard data rest on two SDSs and the NIST MSDS.
  - Refractive index was not read in a fetched source; sources found in search gave inconsistent figures, so none is entered.
  - Anatase-to-rutile transformation temperature was not read in a fetched source. Search results put it between about 800 and 1000 degC depending on synthesis, which is not enough to enter.
  - Anatase density was not read in a fetched source. Only rutile density (R-706 specific gravity 4.0) and the nanomaterial powder value (3.8) are entered.
  - 'The solar absorptance of the TiO2 paint and tape is not entered: the table in the 1966 report could not be matched to coatings 5 and 6 with confidence.'
  - Atomic weights of titanium and oxygen are not entered because no fetched source printed them.
  - UV, atomic-oxygen and radiation results for TiO2 coatings were not found; the 1972 NASA UV degradation report read covers white coatings but not TiO2 specifically.
  - Crustal abundance of titanium, the EU critical raw materials listing for titanium and the USGS titanium mineral concentrate data were not checked.
  - Band gap, lattice constant and total emittance are not in property-canon.yaml; they are entered under snake_case names.
sources:
  chemours-r706:
    title: Chemours Ti-Pure R-706 technical datasheet, Analysis and Physical Properties table
    id: Ti-Pure R-706 TDS, copy dated 2024-12-06 (file name)
    url: https://media.knowde.com/image/upload/v1733488926/TDS-Ti-PureR-706TitaniumDioxide.pdf20241206-1-fuk500.pdf
    year: 2024
    kind: datasheet
    license: supplier document, linked not reproduced
  nist-srm1898-msds:
    title: NIST Standard Reference Material 1898, Titanium Dioxide Nanomaterial, safety data sheet
    id: NIST SRM 1898, issue date 23 September 2020
    url: https://tsapps.nist.gov/srmext/msds/1898-MSDS.pdf
    year: 2020
    kind: datasheet
    license: public domain (NIST)
  sputtertargets-tio2-sds:
    title: 'Sputtertargets safety data sheet: Titanium Dioxide (TiO2)'
    id: Revision date 09/04/2017
    url: https://www.sputtertargets.net/wp-content/uploads/2018/05/Titanium-Dioxide-TiO2-sds.pdf
    year: 2017
    kind: datasheet
    license: supplier document, linked not reproduced
  nasa-1966-coatings:
    title: Experimental investigation of total emittance and solar absorptance of several coatings (NASA Lewis Research Center)
    id: NTRS 19660011749
    url: https://ntrs.nasa.gov/citations/19660011749
    year: 1966
    kind: report
    license: public domain (NASA)
  ksc-ag-tio2-2016:
    title: 'Visible-Light-Responsive Photocatalysis: Ag-Doped TiO2 Catalyst Development and Reactor Design Testing (46th ICES, Kennedy Space Center)'
    id: ICES-2016-169; NTRS 20160008969
    url: https://ntrs.nasa.gov/citations/20160008969
    year: 2016
    kind: report
    license: public domain (NASA)
  arxiv-tio2-optical:
    title: Optical properties of anatase and rutile TiO2 studied by GGA+U
    id: arXiv:1611.08363
    url: https://arxiv.org/pdf/1611.08363
    year: 2016
    kind: paper
    license: arXiv preprint, linked not reproduced
  usgs-mcs2026-titanium:
    title: Mineral Commodity Summaries 2026, Titanium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-titanium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit2025:
    title: About the 2025 List of Critical Minerals (USGS)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 13
  unreachable: 2
  caveats: 4
  note: 'Read by an AI assistant on 2026-10-26 from the saved text of each source: PubChem and the PMC property table returned browser checks and were not used. The 1966 NASA coating table could not be matched to coatings 5 and 6 with confidence, so no absorptance value is entered. Phase transformation temperature and refractive index were not read in a fetched source.'
story:
  recipe:
    line: TiO2 is a metal oxide. Each formula unit combines one titanium atom with two oxygen atoms. Rutile is the denser of its two crystal forms, and anatase is the form with the wider band gap.
    parts:
      - name: Titanium
        symbol: Ti
        role: The metal centre of the oxide.
      - name: Oxygen
        symbol: O
        role: Two oxygen atoms per formula unit balance the charge of titanium in the oxide.
  risk_groups:
    - title: For people
      level: 1
      verdict: Suspected carcinogen by inhalation
      lead: Two safety data sheets classify titanium dioxide powder as Carcinogenicity Category 2 with H351, suspected of causing cancer by inhalation. The sintered or coated form is a different hazard and is not covered by these records.
      points:
        - label: The powder
          line: The NIST reference-material safety data sheet and a second supplier sheet both carry H351. IARC places titanium dioxide in Group 2B, possibly carcinogenic to humans.
        - label: Exposure limits
          line: The NIST sheet gives an OSHA limit of 15 mg/m3 for total particulates and a NIOSH limit of 0.3 mg/m3 (10 h) for ultrafine titanium dioxide.
    - title: If it fails
      lead: These results come from NASA ground tests of coatings and a bench reactor, not from flight.
      points:
        - label: Blistering
          line: An adhesive-backed TiO2 tape blistered in vacuum once heated above about 450 K, and the authors attribute the blisters to gas trapped in the adhesive.
        - label: Emittance at 550 K
          line: The same coatings held an emittance of 0.94 at 400 K, but it fell below 0.90 at 550 K, which the authors call marginal for radiators at that temperature.
        - label: Mercury lamps
          line: The photocatalysis that needs anatase depends on ultraviolet light, which current reactors supply with mercury-vapour lamps. Mercury in the lamps hinders use in spaceflight.
    - title: Handling and storage
      points:
        - label: Dust
          line: Pigment grades are dry powders. Control dust and use a respirator where it is generated, as the NIST sheet advises.
        - label: Grade
          line: Chemours lists its R-706 rutile grade as at least 93 wt% TiO2 with a median particle size of 0.36 µm, so specify the grade and form for critical parts.
  why: 'Titanium dioxide (TiO2) is a white oxide that comes in two crystal forms, rutile and anatase. Rutile is the denser form and has the higher refractive index of the two. Anatase has a wider band gap (3.23 eV against 3.0 eV for rutile), so it absorbs ultraviolet light more strongly and drives photocatalysis. The records collected here describe a stable, white, high-temperature oxide, and a catalyst that needs ultraviolet light. The space record is thin: a 1966 NASA Lewis test of TiO2-pigmented thermal-control coatings and a Kennedy Space Center catalyst tested on the ground.'
  uses:
    - where: Thermal-control coatings
      line: NASA Lewis tested a paint and an adhesive-backed tape, both TiO2-pigmented in a silicone binder, for low-temperature space radiators. They were chosen because of test data from several space satellite experiments. It was a ground test.
    - where: Air and water on the ISS (ground test)
      line: Kennedy Space Center developed a visible-light-responsive, silver-doped TiO2 catalyst with a band gap of 2.72 eV and tested it in a bench-scale reactor against common ISS cabin volatile organics. The aim is to remove them from recovered water before processing. It has not been shown to fly.
  can_do:
    - label: Stable white coating
      value: Emittance 0.94 at 400 K
      line: Coatings of TiO2 pigment in a silicone binder kept an emittance of about 0.92 to 0.94 between 300 and 400 K in vacuum, and stayed stable in vacuum up to 550 K.
    - label: Visible light catalyst
      value: 2.72 eV band gap
      line: The Kennedy silver-doped catalyst has a narrower band gap than pure anatase (3.23 eV), which lets it respond to visible light rather than only to ultraviolet.
    - label: Structure
      value: Anatase and rutile
      line: Anatase has a wider band gap than rutile, and rutile is denser. Lattice constants for both are printed in the properties above, for a materials reader who needs to tell the two apart.
  compare: The two crystal forms behave differently. Rutile is the denser form and the one used as the white pigment grade R-706, with a specific gravity of 4.0. Anatase has the wider band gap, 3.23 eV against 3.0 eV for rutile, which gives it more ultraviolet activity. The silver-doped catalyst narrows the gap to 2.72 eV so that it responds to visible light.
  compare_rows:
    - label: Specific gravity
      benchmark: density
    - label: Band gap
      benchmark: band_gap
    - label: Stable in vacuum up to
      benchmark: max_service_temp
  handling:
    - Titanium dioxide powder is a suspected carcinogen by inhalation (H351). Keep dust down and use a respirator where it is generated.
    - Specify the grade and form. Pigment grades differ in particle size and purity.
    - Heating a TiO2 tape in vacuum above about 450 K can blister it.
  good_for:
    - Stable white thermal coatings. A TiO2 paint and tape held their emittance through a temperature cycle in a NASA vacuum test.
    - Ultraviolet photocatalysis. Anatase has a band gap of 3.23 eV, which needs ultraviolet light to activate.
  watch_out:
    - Inhalation. The powder is classified H351 (suspected of causing cancer by inhalation) and IARC Group 2B.
    - Mercury lamps. Current photocatalytic reactors use mercury-vapour lamps for their ultraviolet light, which conflicts with spaceflight.
    - Missing space data. No flight or vacuum-exposure result for bulk TiO2 was found in a source we could open.
  verdict: Titanium dioxide is a well-characterised white oxide with a good high-temperature record in NASA ground coating tests. Its space record is thin, and the powder is a suspected carcinogen, so handling and grade choice matter more than the chemistry.