spore: profile
id: copper-oxide
spore_version: 0.2
family: oxide-ceramics
name: Copper oxides (cupric oxide and cuprous oxide)
abbreviation: CuO, Cu2O
role: p-type oxides for solar absorbers and photovoltaic layers, and the copper source in high-temperature superconductor synthesis
category: ceramics
type: oxides
summary: |
  Copper forms two common oxides. Cupric oxide (CuO, tenorite) is monoclinic, has a band gap of 1.35 eV at room temperature, and decomposes at 1326 °C according to PubChem. Cuprous oxide (Cu2O, cuprite) is cubic, has a direct band gap of 2.17 eV that a 2019 study quotes for photovoltaic use, and melts at 1235 °C in PubChem's record. Heating Cu2O in oxygen converts it to CuO from about 250 to 350 °C, which darkens thin films. A 2022 sputtered carbon-doped copper oxide film reached a solar absorptance of 0.8750 and an emittance of 0.254. CuO is also the copper oxide used as a precursor in YBa2Cu3O7 superconductor synthesis, where a 2023 study found it appears in impure samples but can contribute to the target phase. Copper selenide (Cu2Se) is a related copper compound: its superionic transition sits between 372 and 380 K for one sample.
best_for: Solar-absorbing copper oxide films and p-type oxide layers where the material is cheap and the dark or light colour of the film is acceptable
weakest_at: Heat and oxygen. Cu2O turns into CuO above about 250 °C in air or oxygen, which changes its band gap from 2.17 eV to 1.35 eV, and CuO dissolves in dilute acids.
identity:
  formula: CuO (copper(II) oxide, tenorite); Cu2O (copper(I) oxide, cuprite); related Cu2Se (copper(I) selenide)
  cas: 1317-38-0 (CuO); 1317-39-1 (Cu2O)
  crystal_system: monoclinic (CuO); cubic (Cu2O)
  space_group: C2/c (CuO); Pn-3m (Cu2O)
  source: liege-cu2o-2019
  where: 'Experimental section, XRD reference patterns: Cu2O cubic Pn-3m and CuO monoclinic C2/c; CAS numbers from PubChem synonyms for CID 164827 (CuO) and CID 18346908 (Cu2O)'
tags:
  environments: []
  stresses:
    - heat
  applications:
    - thermal-control
    - solar-arrays
    - electronics-and-chips
properties:
  - benchmark: density
    label: Relative density, Cu2O (PubChem)
    value: 6
    unit: '-'
    scope: material
    conditions:
      basis: printed as relative density (water = 1), not converted to g/cm3
    method: Value printed in PubChem's experimental-properties section for copper(I) oxide
    status: ground
    source: pubchem-cu2o
    where: 'Experimental Properties, Density row (printed as ''Relative density (water = 1): 6.0'')'
  - benchmark: melting_point
    label: Decomposition temperature, CuO (PubChem)
    value: 1326
    unit: °C
    scope: material
    conditions:
      behaviour: printed as 'Decomposes at 1326'; CuO decomposes rather than melting
    method: Value printed in PubChem's melting-point section for copper(II) oxide
    status: ground
    note: PubChem's melting-point section also lists 1227 °C for the same compound. The value is kept as printed, with the decomposition statement.
    source: pubchem-cuo
    where: Experimental Properties, Melting Point section, first entry
  - benchmark: melting_point
    label: Melting point, Cu2O (PubChem)
    value: 1235
    unit: °C
    scope: material
    conditions:
      basis: first of two values printed by PubChem; the second is 1232 °C
    method: Value printed in PubChem's melting-point section for copper(I) oxide
    status: ground
    source: pubchem-cu2o
    where: Experimental Properties, Melting Point section
  - benchmark: band_gap
    label: Direct band gap, Cu2O (quoted value)
    value: 2.17
    unit: eV
    scope: material
    conditions:
      type: direct band gap
      context: quoted in the introduction of a thin-film study, without a measurement method for this number
    method: Value stated in the introduction of a 2019 manuscript on Cu2O thin films
    status: ground
    source: liege-cu2o-2019
    where: Introduction, first paragraph (sentence beginning 'photovoltaic applications due to its direct bandgap of 2.17 eV')
  - benchmark: band_gap
    label: Direct band gap, undoped Cu2O nanoparticles (measured)
    value: 2.433
    unit: eV
    scope: material
    conditions:
      form: undoped Cu2O nanoparticles
      method_context: UV-VIS absorption spectroscopy
    method: Value reported as the paper's own UV-VIS result for undoped nanoparticles
    status: ground
    source: rsos-cu2o-2022
    where: Section 3 (UV-VIS results) and Table 1, undoped Cu2O row
  - benchmark: band_gap
    label: Band gap, CuO (cupric oxide phase at room temperature)
    value: 1.35
    unit: eV
    scope: material
    conditions:
      temperature: room temperature
      form: CuO phase formed in Cu2O films
    method: Value quoted in the introduction, which cites an earlier study for it
    status: ground
    note: Quoted from an earlier study, not measured in the manuscript.
    source: liege-cu2o-2019
    where: Introduction, paragraph on CuO formation (sentence beginning 'The cupric oxide phase presents a bandgap of 1.35 eV')
  - benchmark: lattice_constant_a
    label: Lattice constant a, cubic Cu2O (cuprite)
    value: 4.23
    unit: Å
    scope: material
    conditions:
      crystal: cubic cuprite, as described in the paper
      temperature: not stated
    method: Value stated in the XRD section of a 2022 paper on Cu2O nanoparticles
    status: ground
    source: rsos-cu2o-2022
    where: Section 3 (XRD results), first paragraph
  - benchmark: solar_absorptance
    label: Solar absorptance, carbon-doped copper oxide film (DC sputtered)
    value: 0.875
    unit: '-'
    scope: material
    conditions:
      coating: carbon-doped copper oxide thin film, DC sputtered
      reactive_gas: 25% CO2 in the sputtering gas, the ratio the abstract calls most efficient
      substrate: not stated in the abstract
    method: Value printed in the abstract of a 2022 sputtering study; the abstract prints no unit
    status: ground
    source: iraqi-dcsputter-cuo-2022
    where: Abstract (best absorptance of the solar spectrum)
  - benchmark: thermal_emittance
    label: Thermal emittance, carbon-doped copper oxide film (DC sputtered)
    value: 0.254
    unit: '-'
    scope: material
    conditions:
      wavelength_range: 2.5 to 25 μm, from reflectance measurements
      temperature: 373 K
      coating: carbon-doped copper oxide thin film, DC sputtered
    method: Emittance derived from reflectance and printed in the abstract; the abstract prints no unit
    status: ground
    source: iraqi-dcsputter-cuo-2022
    where: Abstract (lowest emittance in the infrared region)
  - benchmark: phase_transition_temperature
    label: Superionic transition temperature, Cu2-xSe (one sample)
    value: 372
    value_high: 380
    bound: range
    unit: K
    scope: material
    conditions:
      phase: beta to alpha transition
      sample: one Cu2-xSe crystal studied in the paper; the composition x depends on the sample
    method: Range printed in the text for the studied sample, from single-crystal diffraction
    status: ground
    note: The introduction gives the same transition as around 400 K, depending on x.
    source: iucrj-cu2se-2017
    where: Section 4 (text giving Tc for this specific sample)
  - benchmark: lattice_constant_a
    label: Lattice constant a, alpha Cu2-xSe at 400 K
    value: 5.8538
    unit: Å
    scope: material
    conditions:
      phase: alpha Cu2-xSe, cubic close-packed selenium with disordered copper
      temperature: 400 K
    method: Value printed in the structure section of a 2017 single-crystal study; the printed standard uncertainty (3) applies to the last digit
    status: ground
    source: iucrj-cu2se-2017
    where: Section 3 (alpha-phase structure text)
  - benchmark: density
    label: Calculated density, alpha Cu2-xSe at 400 K
    value: 6.707
    unit: g/cm3
    scope: material
    conditions:
      phase: alpha Cu2-xSe at 400 K
      basis: calculated from the refined crystal cell
    method: Calculated density printed in the crystallographic table of a 2017 study; it is not a measured density
    status: calculated
    note: Calculated, not measured. PubChem gives no melting point or density for Cu2Se.
    source: iucrj-cu2se-2017
    where: Table 1 (crystallographic data), rho calc row
corrosion:
  - environment: Heating in air or oxygen (Cu2O films)
    headline: Cu2O turns into CuO from about 250 to 350 °C in oxygen, and the change darkens the film.
    effect: The manuscript says high-temperature treatment in oxygen converts Cu2O thin films into CuO, starting at 250 to 350 °C, and that CuO has a band gap of 1.35 eV and is related to darkening of the films. In the film tests, samples annealed at 250, 350 and 450 °C for 30 minutes in air showed CuO peaks by 350 °C in the undoped film, and magnesium doping slowed the conversion.
    rating: moderate
    conditions:
      anneal: 30 minutes in air at 250, 350 and 450 °C
      film: sputter- or spray-deposited Cu2O thin films (undoped and magnesium-doped)
    source: liege-cu2o-2019
    where: Introduction (conversion to CuO, 250 to 350 °C); Experimental (annealing at 250, 350 and 450 °C for 30 minutes); XRD results
  - environment: Dilute acids and ammonium carbonate (CuO)
    headline: CuO dissolves in dilute acids, alkali cyanides and ammonium carbonate solution.
    effect: PubChem's solubility summary says CuO is practically insoluble in water and alcohol, but dissolves in dilute acids, alkali cyanides and ammonium carbonate solution, and slowly in ammonia.
    rating: severe
    conditions:
      form: copper(II) oxide, as summarized by PubChem
    source: pubchem-cuo
    where: Solubility section, first and second entries
  - environment: Acids and ammonium hydroxide (Cu2O)
    headline: Cu2O dissolves in acids and in ammonium hydroxide, but not in water.
    effect: PubChem's solubility summary says Cu2O is insoluble in water and soluble in acids and ammonium hydroxide.
    rating: severe
    conditions:
      form: copper(I) oxide, as summarized by PubChem
    source: pubchem-cu2o
    where: Solubility section, entry beginning 'Soluble in acids and ammonium hydroxide'
risks:
  hazards:
    - text: PubChem's aggregated notifications for CuO list H410 with the Warning signal word, and a second group lists H317, H335 and H370 with the Danger signal word.
      ghs:
        - H317
        - H335
        - H370
        - H400
        - H410
      source: pubchem-cuo
      where: GHS Classification section, notification groups
    - text: PubChem's aggregated notifications for Cu2O list H302, H318, H332, H400 and H410 with the Danger signal word.
      ghs:
        - H302
        - H318
        - H332
        - H400
        - H410
      source: pubchem-cu2o
      where: GHS Classification section, first notification group
    - text: PubChem's aggregated notifications for Cu2Se list H301, H331, H373, H400 and H410 with the Danger signal word, from 213 company reports and 4 ECHA notifications.
      ghs:
        - H301
        - H331
        - H373
        - H400
        - H410
      source: pubchem-cu2se
      where: GHS Classification section
  failure_risks:
    - mode: Cu2O turns into CuO in film service
      trigger: Heating Cu2O films in air or oxygen above about 250 °C
      consequence: The films darken and the band gap drops from the Cu2O value of 2.17 eV to the CuO value of 1.35 eV, which changes the film's optical response.
      severity: moderate
      source: liege-cu2o-2019
      where: Introduction (conversion to CuO and darkening of the films)
    - mode: Structural change at the Cu2Se superionic transition
      trigger: Heating a Cu2-xSe sample through its transition between 372 and 380 K
      consequence: Above the transition the copper sites are disordered, so the crystal changes from the beta to the alpha phase.
      severity: moderate
      source: iucrj-cu2se-2017
      where: Section 3 (alpha-phase structure) and Section 4 (transition temperature)
  side_effects: []
  handling:
    - text: PubChem's precautionary statements for CuO include P260 (do not breathe dust), P261, P264, P270, P271, P273, P280 (wear protective gloves), P302+P352 and P304+P340.
      source: pubchem-cuo
      where: GHS Classification section, precautionary statements
    - text: PubChem's CuO record gives P273 (avoid release to the environment), P391 (collect spillage) and P501 (dispose of contents and container as required).
      source: pubchem-cuo
      where: GHS Classification section, first notification group
    - text: PubChem's Cu2O record lists P260 and P261 for dust and spray, and P280 for protective gloves.
      source: pubchem-cu2o
      where: GHS Classification section, precautionary statements
  mitigations: []
uses:
  - application: solar-arrays
    device: Cu2O p-type absorber layer for photovoltaic cells
    example: A 2019 Cu2O thin-film study says Cu2O is used in photovoltaic applications because of its direct band gap of 2.17 eV and intrinsic p-type conduction, and its heterojunction examples include Cu2O:Na layers.
    source: liege-cu2o-2019
    where: Introduction, first paragraph
  - application: thermal-control
    device: Carbon-doped copper oxide solar-selective absorber film on a metal
    example: A 2022 DC-sputtered film with 25% CO2 in the reactive gas reached a solar absorptance of 0.8750 and a thermal emittance of 0.254, measured from 2.5 to 25 μm at 373 K. The paper is about solar-thermal collectors, not spacecraft surfaces.
    source: iraqi-dcsputter-cuo-2022
    where: Abstract
  - application: electronics-and-chips
    device: YBa2Cu3O7 (YBCO) high-temperature superconductor, from a CuO precursor
    example: A 2023 autonomous synthesis study says the usual Y2O3, CuO and BaCO3 route needs more than 12 h of annealing at 950 °C with regrinding. CuO had a 20% success rate in its precursor tests, and it appeared with other impurity phases in most failed samples.
    source: arxiv-ybco-arrows3
    where: Introduction (YBCO synthesis paragraph) and Results (Fig. 2c text on CuO success rate)
supply:
  - subject: Copper (feedstock for both copper oxides)
    producers:
      - country: Chile
        share_percent: 23
        year: 2025
      - country: Congo (Kinshasa)
        share_percent: 13.9
        year: 2025
      - country: Peru
        share_percent: 11.7
        year: 2025
      - country: China
        share_percent: 7.8
        year: 2025
      - country: Russia
        share_percent: 5.7
        year: 2025
    critical_lists:
      - list: USGS Final 2025 List of Critical Minerals (copper added; Federal Register 90 FR 50494, 7 November 2025)
        listed: true
    recycling: 'USGS: old scrap converted to refined metal provided an estimated 160,000 tons of copper in the United States in 2025, and recovered copper contributed about 30% of U.S. supply.'
    substitutes:
      - 'USGS: aluminum, optical fiber, plastics, titanium and steel substitute for copper in named uses; the chapter does not name a substitute for copper oxide.'
    source: usgs-mcs2026-copper
    where: World Mine and Refinery Production and Reserves table, mine production 2025e (world total 23,000 thousand t, rounded); shares calculated by Space Mushroom against that total (Chile 5,300, Congo 3,200, Peru 2,700, China 1,800, Russia 1,300); Recycling; Substitutes; critical list paragraph. The USGS gives copper metal, not copper oxide production.
documents:
  - title: 'USGS Mineral Commodity Summaries 2026: Copper'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-copper.pdf
    year: 2026
  - title: 'PubChem: Copper(II) oxide (CID 164827), GHS classification, melting point, density and solubility'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/164827
    year: 2026
  - title: 'PubChem: Copper(I) oxide (CID 18346908), GHS classification, melting point, density and solubility'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/18346908
    year: 2026
  - title: 'PubChem: Cuprous selenide (CID 6914519), GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6914519
    year: 2026
  - title: 'Copper oxide nanoparticles doped with lanthanum, magnesium and manganese: optical and structural characterization (Royal Society Open Science, 2022; PMC9667144)'
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC9667144/
    year: 2022
  - title: The Resilience of Cuprous Oxide Under Oxidizing Thermal Treatments Via Magnesium Doping (manuscript jp-2019-00408k.R1, Journal of Physical Chemistry)
    type: paper
    url: https://orbi.uliege.be/bitstream/2268/234487/1/The%20Resilience%20Of%20Cuprous%20Oxide%20-%20PDF%20submission%20Revised.pdf
    year: 2019
  - title: Crystal structure across the beta to alpha phase transition in thermoelectric Cu2-xSe (IUCrJ, 2017; PMC5571810)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC5571810/
    year: 2017
  - title: Autonomous decision making for solid-state synthesis of inorganic materials (arXiv 2304.09353)
    type: paper
    url: https://arxiv.org/pdf/2304.09353
    year: 2023
  - title: DC Sputtering Deposition of Copper Oxide Thin Films Doped with Carbon for Efficient Solar Selective Absorbers (Iraqi Journal of Science, 2022)
    type: paper
    url: https://ijs.uobaghdad.edu.iq/index.php/eijs/article/view/4921
    year: 2022
gaps:
  - Cu2Se thermoelectric figure of merit (zT) and lattice thermal conductivity were not found in a source opened for this profile. The Nano Energy 2012 paper (MIT DSpace) returned a rate-limit response, and the 2025 RSC paper returned HTTP 403.
  - The Cu2Se review in Molecules (MDPI, 2025) returned HTTP 403, so no value from it is used.
  - Copper selenide melting point and measured density were not in PubChem's record (the Cu2Se melting-point and density pages returned no values).
  - PubChem prints a density for CuO (6.315 at 14 °C/4 °C, and 6.4 at 25 °C) without a unit field, so no CuO density is used.
  - Thermal expansion of CuO and Cu2O was not found in a source opened for this profile.
  - Humidity ageing of Cu2O was reported in a 2024 search snippet but the paper was not opened, so no value is used.
  - Catalytic uses of CuO (CO oxidation, methanol reforming) were seen only in search snippets and no source was opened, so no catalysis use is listed.
  - The solar absorber film thickness and substrate are not stated in the 2022 abstract, and no full text was opened.
  - The CuO superconductor precursor papers from 1989 and 1990 (Coppa et al., J. Mater. Res.) are paywalled, so only the 2023 arXiv study is used.
  - The copper oxide solar-thermal use is a laboratory film result; no flight or spacecraft data for copper oxide coatings was found.
  - Exposure limits for copper oxide dust (OSHA or NIOSH) were not found in a source opened for this profile.
  - Atomic weights for copper and oxygen were not in a source opened for this profile, so the recipe parts carry no atomic_weight.
  - The European Union critical raw materials list was not checked for copper.
  - The USGS gives copper metal production, not copper oxide production by country.
  - Copper selenide is covered only as a related section; CuSe (PubChem CID 73980) is a different compound and is not profiled.
  - Selenium supply and selenium hazard data were not opened for this profile.
sources:
  pubchem-cuo:
    title: PubChem compound record, Copper(II) oxide (CID 164827)
    id: CID 164827; CAS 1317-38-0; EC 215-269-1
    url: https://pubchem.ncbi.nlm.nih.gov/compound/164827
    year: 2026
    kind: database
    license: public domain (NCBI)
  pubchem-cu2o:
    title: PubChem compound record, Copper(I) oxide (CID 18346908)
    id: CID 18346908; CAS 1317-39-1
    url: https://pubchem.ncbi.nlm.nih.gov/compound/18346908
    year: 2026
    kind: database
    license: public domain (NCBI)
  pubchem-cu2se:
    title: PubChem compound record, Cuprous selenide (CID 6914519)
    id: CID 6914519; molecular formula Cu2Se
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6914519
    year: 2026
    kind: database
    license: public domain (NCBI)
  usgs-mcs2026-copper:
    title: Mineral Commodity Summaries 2026, Copper
    id: USGS MCS 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-copper.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  rsos-cu2o-2022:
    title: 'Copper oxide nanoparticles doped with lanthanum, magnesium and manganese: optical and structural characterization (Guzman, Tian, Walker, Herrera; Royal Society Open Science 9(11), 220485)'
    id: DOI 10.1098/rsos.220485; PMC9667144
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC9667144/
    year: 2022
    kind: paper
    license: open access (cited, not reproduced)
  liege-cu2o-2019:
    title: The Resilience of Cuprous Oxide Under Oxidizing Thermal Treatments Via Magnesium Doping (Resende et al., Journal of Physical Chemistry, manuscript jp-2019-00408k.R1)
    id: manuscript jp-2019-00408k.R1; Universite de Liege repository 2268/234487
    url: https://orbi.uliege.be/bitstream/2268/234487/1/The%20Resilience%20Of%20Cuprous%20Oxide%20-%20PDF%20submission%20Revised.pdf
    year: 2019
    kind: paper
    license: author manuscript in institutional repository (cited, not reproduced)
  iucrj-cu2se-2017:
    title: Crystal structure across the beta to alpha phase transition in thermoelectric Cu2-xSe (Eikeland et al., IUCrJ 4, 476-485)
    id: doi:10.1107/S2052252517005553; PMC5571810
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC5571810/
    year: 2017
    kind: paper
    license: open access (cited, not reproduced)
  arxiv-ybco-arrows3:
    title: Autonomous decision making for solid-state synthesis of inorganic materials (Szymanski, Nevatia, Bartel, Zeng, Ceder)
    id: arXiv 2304.09353
    url: https://arxiv.org/pdf/2304.09353
    year: 2023
    kind: paper
    license: arXiv preprint (cited, not reproduced)
  iraqi-dcsputter-cuo-2022:
    title: DC Sputtering Deposition of Copper Oxide Thin Films Doped with Carbon for Efficient Solar Selective Absorbers (Abdullah, Humud, Mustafa; Iraqi Journal of Science 63(7), 2924-2934)
    id: Iraqi Journal of Science, vol. 63, no. 7
    url: https://ijs.uobaghdad.edu.iq/index.php/eijs/article/view/4921
    year: 2022
    kind: paper
    license: open access journal (cited, not reproduced)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 12
  unreachable: 3
  caveats: 5
  note: Read by an AI assistant on 2026-10-26 from the pages and PDFs fetched in that run. PubChem values come from its REST data view. The MDPI Cu2Se review and the RSC 2025 Cu2Se paper returned HTTP 403, and the MIT DSpace Cu2Se PDF returned a rate limit, so no value from them is used. The CuO band gap is a value quoted from an earlier study. The Cu2Se transition is for one sample. The copper oxide solar absorber values come from an abstract with no unit printed.
story:
  recipe:
    line: CuO has one copper and one oxygen per formula unit and is monoclinic (space group C2/c). Cu2O has two coppers per oxygen and is cubic (Pn-3m). Cu2O films are annealed in air at 250, 350 and 450 °C in the 2019 study, and heating in oxygen converts them to CuO. Copper selenide (Cu2Se) replaces the oxygen with selenium and has a cubic selenium lattice with disordered copper in its high-temperature alpha phase.
    parts:
      - name: Copper
        symbol: Cu
        role: The metal in both oxides. In Cu2O copper is in the +1 state, and in CuO it is in the +2 state.
      - name: Oxygen
        symbol: O
        role: The anion in both oxides. Heating in oxygen converts Cu2O into CuO.
      - name: Selenium (Cu2Se only)
        symbol: Se
        role: Replaces oxygen in the related copper selenide, where it forms the selenium lattice.
  risk_groups:
    - title: For people
      level: 1
      verdict: Toxic if inhaled or swallowed (Cu2Se)
      lead: PubChem's notifications give the Danger signal word for Cu2O and Cu2Se, and CuO has a Danger group that includes H317, H335 and H370.
      points:
        - label: Cu2Se
          line: Aggregated notifications list H301 (toxic if swallowed), H331 (toxic if inhaled) and H373 (repeated exposure) with the Danger signal word.
        - label: Cu2O
          line: Aggregated notifications list H302, H318 (serious eye damage) and H332, with the Danger signal word.
        - label: CuO
          line: Aggregated notifications include H317 (skin sensitization), H335 (respiratory irritation) and H370 (damage to organs).
    - title: Environment and handling
      lead: Aquatic toxicity is listed for all three compounds, so keep the powders out of drains.
      points:
        - label: Aquatic life
          line: H410 (very toxic to aquatic life with long lasting effects) appears in all three PubChem records.
        - label: Dust
          line: Avoid breathing dust (P260) and wear gloves (P280) for CuO and Cu2O.
    - title: Film service
      points:
        - label: Heat and air
          line: Cu2O films turn into CuO from about 250 to 350 °C, which lowers the band gap from 2.17 eV to 1.35 eV.
        - label: Acids
          line: CuO and Cu2O both dissolve in dilute acids, so acid cleaning steps need care.
  why: Copper oxides are cheap semiconductor oxides, with CuO and Cu2O as the two common forms. Cu2O has a band gap of 2.17 eV that a 2019 study quotes for photovoltaic layers, and CuO has a band gap of 1.35 eV at room temperature. A 2022 carbon-doped copper oxide film reached a solar absorptance of 0.8750, so the material is a candidate for solar-thermal absorbers. CuO is also the copper source used in YBCO superconductor synthesis, where the 2023 study found it appears in impure samples. The main weakness is that Cu2O turns into CuO on heating in air, and that the flight record for copper oxide coatings is not in this profile.
  uses:
    - where: Solar absorber films
      line: A carbon-doped copper oxide film sputtered with 25% CO2 reached a solar absorptance of 0.8750 and an emittance of 0.254 in a 2022 study.
    - where: Photovoltaic p-type layers
      line: Cu2O's 2.17 eV direct band gap and intrinsic p-type conduction make it a candidate absorber, according to a 2019 study.
    - where: Superconductor synthesis
      line: A 2023 autonomous study found that CuO gives YBCO in 20% of its precursor tests, and that the classic route needs more than 12 h at 950 °C.
  can_do:
    - label: Solar absorption
      value: Solar absorptance 0.8750
      line: A 2022 carbon-doped film absorbed 0.8750 of the solar spectrum in the lab, with emittance 0.254.
    - label: Band gap
      value: 2.17 eV for Cu2O
      line: Cu2O has a direct band gap of 2.17 eV, and the 2019 study quotes it for photovoltaic use.
    - label: Superconductor precursor
      value: 20% success rate
      line: CuO contributed to YBCO formation in 20% of the precursor sets tested in 2023.
    - label: Melting
      value: 1235 °C for Cu2O
      line: PubChem gives 1235 °C for Cu2O and 1326 °C with decomposition for CuO.
  compare: The two copper oxides differ in band gap and stability. Cu2O has the wider band gap at 2.17 eV and a melting point of 1235 °C in PubChem, and it turns into CuO on heating in air or oxygen. CuO has the narrower band gap at 1.35 eV and decomposes at 1326 °C. Copper selenide is a different class of compound, with a superionic transition between 372 and 380 K for one sample, so its phase stays put only below that range.
  compare_rows:
    - label: Melting point (Cu2O)
      benchmark: melting_point
    - label: Band gap (Cu2O, quoted)
      benchmark: band_gap
    - label: Density (Cu2O, relative)
      benchmark: density
  handling:
    - Avoid breathing dust, and wear gloves and eye protection when handling CuO and Cu2O powders.
    - Keep powders out of drains and waterways, and collect spills rather than sweeping them dry.
    - Film annealing above 250 °C in air changes the oxide, so set the oven temperature and keep the record.
  good_for:
    - Solar absorber films, where a carbon-doped copper oxide film reached 0.8750 absorptance in a 2022 lab study.
    - Photovoltaic p-type layers that need a 2.17 eV direct band gap, per a 2019 study.
    - Copper precursor for YBCO synthesis, where CuO appears in the 2023 test sets.
  watch_out:
    - 'Heat in air or oxygen: Cu2O becomes CuO from about 250 °C and the film darkens.'
    - The 2022 absorptance is for a lab film with no unit printed and no substrate or thickness stated.
    - The Cu2Se transition values are for one sample, and the Cu2Se thermoelectric data are not in this profile.
    - The aquatic hazard H410 and the PubChem notifications, which differ between records.
  verdict: Copper oxides are cheap, easy to make and useful as solar-absorbing and p-type oxide layers in the lab. Their weak points are the heat-driven change from Cu2O to CuO, the acid solubility, and the thin evidence for flight use, which this profile does not cover.