Compendium

Photocatalysts Deep

22 min read

Photocatalysts Deep

A Tier 2 deep-dive into the materials, mechanisms, and reactions of heterogeneous photocatalysis — the use of semiconductors and molecular hybrids to convert solar photons directly into chemical bonds. From the 1972 Fujishima-Honda TiO₂ water-splitting demonstration through Domen’s 96% quantum-yield SrTiO₃:Al panel (2020) and the first 100 m² outdoor solar-hydrogen demonstrations (2021-2024), photocatalysis has crossed from laboratory curiosity into pilot-scale solar fuels and pollution remediation. This note treats fundamentals (semiconductor band engineering, redox potential alignment, charge separation, surface kinetics), the materials zoo (oxide + nitride + sulfide + chalcogenide semiconductors, MOFs, COFs, perovskite oxides, halide perovskites, carbon nitride, plasmonic + dye-sensitized + Z-scheme architectures, cocatalysts), the canonical reactions (water splitting, CO₂ reduction, N₂ fixation, pollutant degradation, organic synthesis), and the commercial and pilot programs (Toyota CRDL, DICP-CAS, Mitsubishi, Heliogen-adjacent thermochemical, SunHydrogen) shaping the field.

See also

Fundamentals

Band-gap absorption + e-h pair generation

In a semiconductor photocatalyst, photons with energy hν ≥ E_g promote an electron from the valence band (VB) to the conduction band (CB), leaving a positive hole in the VB:

h_ν + semiconductor → e⁻_CB + h⁺_VB

The threshold wavelength: λ_max = 1240 / E_g(eV) nm. For E_g = 3.2 eV (anatase TiO₂), λ_max = 388 nm — only the UV tail (~5% of AM1.5G solar spectrum) drives the reaction. Visible-light photocatalysis requires E_g ≤ 3.0 eV (λ ≥ 413 nm).

Redox potential alignment

For a target redox reaction with potential E_redox vs NHE, the photocatalyst must satisfy:

  • CB minimum E_CB more negative than E_redox(reduction)
  • VB maximum E_VB more positive than E_redox(oxidation)

Standard half-reactions (vs NHE at pH 0):

  • 2 H⁺ + 2 e⁻ → H₂ at E = 0.00 V (pH 0); -0.41 V at pH 7
  • 2 H₂O → O₂ + 4 H⁺ + 4 e⁻ at E = 1.23 V (pH 0); 0.82 V at pH 7
  • CO₂ + 2 e⁻ + 2 H⁺ → CO at -0.53 V (pH 7)
  • CO₂ + 8 e⁻ + 8 H⁺ → CH₄ at -0.24 V (pH 7)
  • N₂ + 6 e⁻ + 6 H⁺ → 2 NH₃ at -0.092 V (pH 7) — equilibrium; kinetic barriers enormous

The Nernst correction: E(pH) = E°(pH 0) - 0.059 · pH per electron. The reversible hydrogen electrode (RHE) absorbs this — E vs RHE is pH-invariant for HER/OER, which is why band diagrams are routinely plotted vs RHE.

Band-edge engineering rules

  • Wider gap → broader redox window but worse spectral overlap.
  • VB position depends mostly on the anion (O 2p ~3 eV below vacuum for most oxides; N 2p higher; S 3p higher still — explains why oxynitrides + sulfides have narrower gaps).
  • CB position depends mostly on the cation (d⁰ transition metals Ti⁴⁺ Nb⁵⁺ Ta⁵⁺ W⁶⁺ Zr⁴⁺; sp-cations Sn⁴⁺ In³⁺).

Charge transport, recombination, and lifetimes

Photogenerated carriers must reach the surface before recombining. Typical lifetimes:

  • TiO₂ anatase: e⁻ lifetime ~ns-µs; trap-limited.
  • Hematite α-Fe₂O₃: hole lifetime < 10 ps (the central problem with hematite).
  • BiVO₄: hole lifetime ~100-200 ps.
  • Halide perovskites CsPbBr₃: > 1 µs (the reason for their photovoltaic prominence).
  • g-C₃N₄: ~ns; long enough for HER cocatalyst transfer.

Strategies to extend lifetime: doping for sub-bandgap states, defect engineering, heterostructure for spatial charge separation, plasmonic hot-carrier injection, and Z-scheme cascading (below).

Schottky barrier + band bending at metal-semiconductor junction

When a metal (workfunction φ_m) contacts a semiconductor (workfunction φ_s), bands bend to align Fermi levels. The barrier height φ_B = φ_m - χ_s (for n-type; Schottky-Mott). Pt (φ_m = 5.65 eV) on TiO₂ (χ = 4.0 eV) gives a ~1.6 eV barrier — Pt nanoparticles on TiO₂ act as e⁻ traps that suppress recombination and catalyze HER. This is why Pt is the gold-standard HER cocatalyst.

Surface reaction steps

The Lindemann-style sequence:

  1. Photon absorption → e⁻ + h⁺
  2. Charge separation + diffusion to surface
  3. Reactant adsorption (often Langmuir-Hinshelwood)
  4. Surface electron/hole transfer to adsorbate
  5. Product desorption + regeneration of active site

Apparent quantum efficiency (AQE) = 2 · n(H₂) · N_A / n(photons) for HER (factor of 2 from 2-electron stoichiometry). Solar-to-hydrogen (STH) efficiency = (ΔG · r_H₂) / (P_sun · A) — accepted commercialization threshold ~10% (US DOE H2 Hub target).

Dye-sensitization

For wide-gap oxides (TiO₂, ZnO, SnO₂) that absorb only UV, a visible-absorbing dye (Ru-polypyridyl N3 / N719 / black dye; Zn-porphyrin; organic D-π-A dyes) is anchored to the surface. The dye absorbs the photon and injects an electron into the oxide CB. The dye-sensitized solar cell (DSSC; Grätzel-O’Regan 1991 Nature 353, 737) is the photovoltaic analog; dye-sensitized photocatalysis follows the same architecture for solar-fuel chemistry.

TiO₂ — the reference photocatalyst

Polymorphs

  • Anatase — tetragonal, a = 3.78 Å, c = 9.51 Å, E_g = 3.20 eV (indirect). Most photocatalytically active polymorph for most reactions.
  • Rutile — tetragonal, a = 4.59 Å, c = 2.96 Å, E_g = 3.02 eV (direct). Higher density and refractive index; ground-state polymorph above ~600 °C.
  • Brookite — orthorhombic, E_g ~3.1 eV. Less studied; sometimes co-occurs with anatase + rutile in nanopowders.

Fujishima-Honda 1972

Akira Fujishima + Kenichi Honda, Nature 238, 37-38 (Jul 1972) — “Electrochemical photolysis of water at a semiconductor electrode.” Single-crystal rutile TiO₂ photoanode + Pt cathode under UV illumination + 0.5 V bias: O₂ evolved at TiO₂, H₂ at Pt. The founding paper of photocatalysis.

Degussa P25 / Evonik AEROXIDE TiO₂ P25

The benchmark commercial photocatalyst (since 1980s, now manufactured by Evonik as AEROXIDE TiO₂ P25). Mixed-phase ~80% anatase + ~20% rutile by mass; flame-pyrolysis (TiCl₄ + O₂ at 1500 K); particle size 21 nm (TEM); BET surface area 50 m²/g. The mixed phase enables anatase-to-rutile interface charge separation (Bickley-Carreño 1991 J Solid State Chem; Hurum-Agrios-Gray-Rajh-Thurnauer 2003 J Phys Chem B). Reference for activity benchmarking in 1000+ papers/year.

Doping strategies

  • N-doping (Asahi-Morikawa-Ohwaki-Aoki-Taga Science 2001, 293, 269-271) — TiO₂:N introduces N 2p sub-bands above O 2p VB; reduces effective gap to ~2.4 eV; some visible-light HER + organic-degradation activity. Synthesis: NH₃ anneal at 500-700 °C.
  • F-doping — F substitutes for O; doesn’t narrow gap but improves crystallinity + reduces electron-trap density.
  • Self-doping (Ti³⁺) — H₂ anneal or chemical reduction creates oxygen vacancies + Ti³⁺ surface states; black TiO₂ (Chen-Liu-Yu-Mao Science 2011, 331, 746-750) absorbs across the visible.
  • Metal cation doping — Fe³⁺, Cr³⁺ — usually deleterious (deep traps); Cu²⁺ helpful for CO₂ reduction selectivity.
  • Anion co-doping (S, N, F combinations) — Liu-Yu-Cheng review Chem Rev 2014, 114, 9559.

Hashimoto-Irie-Fujishima 2005 — photo-induced superhydrophilicity

Wang-Hashimoto-Fujishima Nature 1997, 388, 431-432 — UV-irradiated TiO₂ films become superhydrophilic (water contact angle → 0°) due to surface OH-group restructuring. Foundation for self-cleaning glass (Pilkington Activ, Saint-Gobain Bioclean), anti-fog mirrors, and self-decontaminating tiles (TOTO Hydrotect).

Photo-induced CO₂ reduction

Inoue-Fujishima-Konishi-Honda 1979 Nature 277, 637 — first CO₂ photoreduction on TiO₂; HCOOH, HCHO, CH₃OH detected. Modern work focuses on selectivity (Cu cocatalyst → C2 products; Ni → CH₄; Au → CO) and quantum efficiency.

Architectures

  • Nanopowder — P25, anatase nanoparticles (Sigma-Aldrich, Nanostructured & Amorphous Materials Inc).
  • Anatase nanotubes — anodic oxidation of Ti foil in F⁻-containing electrolyte (Patrik Schmuki, Erlangen; Craig Grimes, Penn State). Ordered arrays 50-500 nm diameter, µm-mm length.
  • Single-crystal facet-engineered — anatase (001) facet has highest reactivity (Yang-Sun-Yang-Smith-Liu-Wang-Cheng-Lu Nature 2008, 453, 638-641 — HF synthesis of (001)-faceted anatase).
  • TiO₂-graphene composites — Zhang-Tang-Wu-Bao 2011 ACS Nano 5, 380.
  • Mesoporous TiO₂ — surfactant-templated; high BET 200-400 m²/g.

Beyond TiO₂ — the oxide photocatalyst zoo

ZnO

Wurtzite (a = 3.25 Å, c = 5.21 Å); E_g = 3.37 eV (direct); n-type; high carrier mobility. Hydrothermal nanorod arrays (Greene-Law-Goldberger-Kim-Johnson-Zhang-Saykally-Yang 2003 Angew Chem Int Ed). Photocatalytic activity comparable to TiO₂ in UV; but photocorrosion in aqueous solution (Zn²⁺ leaching) limits use.

SnO₂

Rutile structure; E_g = 3.6 eV; n-type. Anti-reflective coatings; gas sensors. Photocatalytic use limited by wide gap. CB ~0.5 V more positive than TiO₂ → poor for HER.

WO₃

Monoclinic; E_g = 2.7 eV; n-type. Visible-light absorption in the blue. VB at ~3.1 V vs NHE — excellent for OER but CB at ~0.4 V vs NHE marginal for HER. Used heavily as the OER side of Z-schemes (e.g. WO₃/Pt for water-splitting half-cell).

α-Fe₂O₃ (hematite)

Corundum structure; E_g = 2.1 eV (visible); abundant + cheap. Theoretical STH ~12-15%. Practical efficiency <5% due to (1) ultrashort hole diffusion length 2-4 nm; (2) sluggish OER kinetics; (3) deep traps. Mainstays: nanostructuring (host-scaffold + Si-doping; Grätzel-Tilley 2012 Energy Environ Sci), surface treatments (Co-Pi cocatalyst — Kanan-Nocera 2008 Science 321, 1072), and underlayer hole-conductor stacks.

BiVO₄ (clinobisvanite, monoclinic scheelite)

Kudo-Omori-Kato 1998 J Am Chem Soc 120, 11459 — first BiVO₄ as visible-light water oxidation photocatalyst. E_g = 2.4 eV; VB position +2.4 V vs NHE (great for OER); CB at 0 V (marginal HER). Sayama-Domen-Kato 2003 developed nanocrystalline BiVO₄ + Fe³⁺/Fe²⁺ shuttle. Hole diffusion length ~70 nm; better than hematite. Mo + W doping shifts Fermi level and improves photocurrent (Park-Galstyan-Mullins 2013 ACS Catal). The reference Z-scheme component for visible-light water splitting.

Cu₂O + CuO

Cu₂O: cuprite, E_g = 2.17 eV; p-type. Photocathode for HER. Photocorrosion is the main concern; addressed by NiO or TiO₂ overlayer + Pt-Pd cocatalyst (Paracchino-Mathews-Hisatomi-Stefik-Tilley-Grätzel 2011 Nat Mater 10, 456). CuO: tenorite, E_g = 1.4-1.7 eV; even more photocorrosion-prone.

CdS, CdSe, ZnSe (chalcogenides)

CdS: hexagonal wurtzite or cubic zincblende; E_g = 2.42 eV; HER-competent but photocorrodes (S²⁻ oxidation) — must run with sacrificial hole scavenger (Na₂S/Na₂SO₃, lactic acid, methanol). Modern composites (CdS/MoS₂, CdS/Pt, CdS-quantum-dot-sensitized TiO₂) achieve AQE 60-70% for HER from sacrificial donors.

Niobates + tantalates

  • NaTaO₃ (perovskite, La-doped) — Kato-Asakura-Kudo 2003 J Am Chem Soc 125, 3082; AQE 56% for overall water splitting at 270 nm (UV).
  • NaNbO₃ + KNbO₃ — perovskite niobates with visible-light variants via N-doping.
  • K₄Nb₆O₁₇ — layered niobate; intercalation chemistry.

Bi-based visible-light photocatalysts

  • BiOX (X = Cl, Br, I) — Bi-O-Cl layered structure; E_g 1.7 (BiOI) to 3.4 (BiOCl) eV.
  • Bi₂WO₆, Bi₂MoO₆ — Aurivillius layered; visible-light OER.
  • Bi₂S₃ — narrow gap 1.3 eV.

Perovskite-oxide photocatalysts

SrTiO₃

Cubic perovskite; E_g = 3.2 eV; n-type. Overall water splitting at UV. The dominant platform for Domen-group panel reactor development.

Domen’s 96% AQE breakthrough: Takata-Jiang-Sakata-Nakabayashi-Shibata-Nandal-Seki-Hisatomi-Domen, Nature 581, 411-414 (May 2020) — Al-doped SrTiO₃ (SrTiO₃:Al) with crystal-facet-selective RhCrO_x H₂-evolution cocatalyst and CoOOH O₂-evolution cocatalyst on opposite facets. Apparent quantum efficiency 96% at 350-360 nm; the highest ever reported for any photocatalyst at any wavelength. Mechanism: charge separation enforced by facet selectivity puts oxidation and reduction centers in physically distinct locations, eliminating the dominant back-reaction.

100 m² panel reactor: Nishiyama-Yamada-Nakabayashi-Maehara-Fukumoto-Akiyama-Takata-Domen, Nature 598, 304-307 (Oct 2021) — 100 m² panel array of SrTiO₃:Al / RhCrO_x in shallow water film, outdoor operation in Sakai Japan, STH = 0.76% over months. The first sustained outdoor demonstration of overall water splitting at scale.

BaTaO₂N, LaTiO₂N, TaON (oxynitrides)

Maeda-Domen 2010 J Phys Chem C 114, 7851 — oxynitride perovskites with N 2p mixing into VB give visible-light absorption (E_g 2.0-2.5 eV) while preserving band positions straddling water-splitting potentials. LaTiO₂N + RhCrO_x: AQE ~5% at 410 nm. The promising visible-light path for solar fuels.

BaZrO₃, CaTiO₃, KTaO₃, NaTaO₃, KNbO₃

Wide-gap perovskites (3.5-4.0 eV); UV only; Kato-Kudo + Domen catalogue.

Layered perovskites (Ruddlesden-Popper, Dion-Jacobson, Aurivillius)

K₂La₂Ti₃O₁₀, RbLaTa₂O₇, Bi₂WO₆ — layered structures enable intercalation + reactant access between sheets.

Halide perovskite photocatalysts

CsPbX₃ (X = Cl, Br, I) and methylammonium / formamidinium analogues — see mof-cof-perovskite-catalog for full chemistry. Photocatalytically interesting because of:

  • Long carrier lifetime (>1 µs).
  • Tunable E_g (1.5-3.0 eV).
  • Solution processing.

But: water-instability rules out aqueous water-splitting. Used instead in non-aqueous media — CO₂ reduction in toluene/H₂O microemulsion (Hou-Cao-Liu-Yan-Bian-Mu-Wu-Xu J Am Chem Soc 2017, 139, 5660-5663), HBr photo-decomposition (Wang-Brittman-Ma-Wang-Cao-Yan-Liu Joule 2018), C-N bond formation.

Stability strategies: encapsulation in silica/MOF shells, all-inorganic CsPbX₃ (no MA/FA), 2D layered halide perovskites (BA₂PbX₄), Pb-free Sn or Bi halide analogues.

g-C₃N₄ (graphitic carbon nitride) and N-doped carbon photocatalysts

g-C₃N₄ — the polymeric photocatalyst

Wang-Maeda-Thomas-Takanabe-Xin-Carlsson-Domen-Antonietti, Nature Materials 8, 76-80 (Jan 2009) — “A metal-free polymeric photocatalyst for hydrogen production from water under visible light.” Polymerization of cyanamide / dicyandiamide / melamine / urea at 500-580 °C in air; yields tri-s-triazine (heptazine) polymer; E_g = 2.7 eV; visible-light absorption; HER active with Pt cocatalyst. The first metal-free visible-light photocatalyst with reasonable activity.

g-C₃N₄ properties:

  • Yellow solid; air-stable; thermally stable to ~600 °C.
  • VB ~+1.4 V vs NHE; CB ~-1.3 V vs NHE; straddles HER + OER.
  • Modest visible-light activity in isolation; main use is as the reducing half of Z-schemes or with cocatalysts.

Modifications:

  • Defect engineering — H₂ post-treatment introduces N vacancies; narrows gap.
  • Element doping — B, P, S, F, K all reported.
  • Heterostructuring — g-C₃N₄/TiO₂, g-C₃N₄/BiVO₄, g-C₃N₄/CdS Z-schemes.
  • Exfoliation to nanosheets — surface area 200-300 m²/g.

Markus Antonietti group at MPI Potsdam continues to lead the carbon nitride field; Xinchen Wang group at Fuzhou and Jinhua Ye at NIMS publish prolifically.

N-doped graphene + carbon dots

Lower-activity but tunable; environmental remediation.

MOF + COF photocatalysts

See mof-cof-perovskite-catalog for comprehensive MOF chemistry. Photocatalytically relevant families:

Titanium-based MOFs

  • MIL-125 (Férey-Volkringer-Loiseau 2009 J Am Chem Soc 131, 10857) — Ti₈O₈(OH)₄ clusters + 1,4-BDC linker; E_g ~3.6 eV.
  • MIL-125-NH₂ (NH₂-functionalized BDC) — visible-light absorption (E_g ~2.6 eV) via LMCT (linker-to-metal charge transfer); CO₂ reduction (Fu-Sun-Chen-Liu-Wang-Wang-Yu-Li-Lin-Long Angew Chem Int Ed 2012, 51, 3364).
  • NH₂-UiO-66 + NH₂-UiO-67 (Zr-MOF, Cavka-Jakobsen-Olsbye-Guillou-Lillerud 2008 JACS) — robust Zr clusters; visible-light CO₂ reduction.

Porphyrin-pillared MOFs

  • PCN-222 (Feng-Liu-Yuan-Park-Jiang-Liu-Wang-Yuan-Zhou 2012 Angew) — Zr₆ clusters + tetracarboxyphenyl porphyrin linker; broad visible absorption; photocatalytic CO₂ reduction (Xu-Hu-Li-Yu 2015).
  • MOF-525 (Feng et al. 2012) — Zr + Fe-porphyrin variants.

Cobalt-, iron-MOFs

Co-ZIF-67 + photoactive linker for HER cocatalysis.

Covalent organic frameworks (COFs)

  • TpPa-1, TpPa-2 (Bunck-Dichtel 2013 J Am Chem Soc) — β-ketoenamine linkages; HER under visible light.
  • CTF-1 (covalent triazine framework, Thomas-Fischer-Goettmann-Antonietti 2008 Angew) — triazine network; HER + photoredox.

The MOF/COF photocatalyst field is rapidly growing 2018-2026 but practical STH efficiencies remain <1%.

Z-scheme and heterojunction architectures

Inspired by natural photosynthesis Z-scheme (PSI + PSII), artificial Z-schemes split the redox burden across two photocatalysts with complementary band positions:

Two-photocatalyst + redox mediator Z-scheme

Sayama-Mukasa-Abe-Domen 2001 Chem Commun 2416 — first Pt/SrTiO₃:Rh (HER) + BiVO₄ (OER) Z-scheme using Fe³⁺/Fe²⁺ shuttle in aqueous solution. AQE ~5% at 420 nm. Subsequent variants:

  • Pt/SrTiO₃:Rh + WO₃ + Fe³⁺/Fe²⁺
  • Ru/SrTiO₃:Rh + BiVO₄ + [Co(bpy)₃]³⁺/²⁺
  • BiVO₄ + g-C₃N₄ (direct, no shuttle)
  • BiVO₄ + Ru-doped SrTiO₃ + reduced graphene oxide as solid-state conductor (Iwase-Yoshino-Takayama-Ng-Amal-Kudo 2016 J Am Chem Soc 138, 10260 — first rGO-mediated Z-scheme).

Direct (mediator-free) Z-scheme

Two photocatalysts in direct contact, charge transfer through interface without redox shuttle. Examples: CdS/WO₃, ZnO/CdS, BiVO₄/g-C₃N₄.

Type-II heterojunction

Two semiconductors with staggered band alignment; electrons accumulate in the lower-CB material, holes in the higher-VB material. CdS/TiO₂, BiVO₄/TiO₂ etc. Distinguished from Z-scheme by which material does what.

p-n junction

p-type photocathode + n-type photoanode in tandem (PEC architecture, below).

Plasmonic + hot-carrier injection

Au, Ag, Cu nanoparticle plasmons inject hot electrons into adjacent semiconductor CB; extends absorption into the visible/NIR. Linic-Christopher-Ingram 2011 Nat Mater; Halas-Nordlander group at Rice.

Cocatalysts

The semiconductor light-absorber alone is rarely sufficient — cocatalysts lower kinetic barriers for HER and OER.

HER cocatalysts

  • Pt — gold standard; 0.1-1 wt% Pt nanoparticles via photodeposition or impregnation; expensive but unmatched activity.
  • MoS₂, MoSe₂, WS₂ — 2D layered sulfides; edge sites near as active as Pt (Hinnemann-Moses-Bonde-Jørgensen-Nielsen-Horch-Chorkendorff-Nørskov 2005 JACS; Jaramillo-Jørgensen-Bonde-Nielsen-Horch-Chorkendorff Science 2007).
  • NiO_x, CoO_x, MnO_x — earth-abundant; lower activity than Pt but adequate at scale.
  • CoP, Ni₂P, MoP, FeP — transition-metal phosphides; rising class.
  • MoC, WC — carbides; Pt-like density of states near E_F.
  • Single-atom Pt on TiO₂ — Cheng-Zhang-Sun-Zhang-Liu-Ye-Cheng-Liu J Am Chem Soc 2017 — maximizes atomic utilization.

OER cocatalysts

  • Co-Pi (cobalt phosphate, Kanan-Nocera 2008 Science 321, 1072) — electrodeposited from Co²⁺ + phosphate buffer; the canonical earth-abundant OER catalyst.
  • NiFe(OH)₂ + LDH (layered double hydroxide) — best earth-abundant alkaline OER catalysts.
  • IrO_x, RuO₂ — gold-standard but precious.
  • CoOOH, MnO_x, BiVO₄ surface-grown FeOOH — improved hole-transfer kinetics on hematite + BiVO₄.

Cocatalyst engineering

  • Facet selectivity — Domen’s RhCrO_x / CoOOH split on opposite SrTiO₃:Al facets (Nature 2020).
  • Core-shell architectures — Cr₂O₃ shell over Rh core suppresses back-reaction (Maeda-Teramura-Lu-Saito-Inoue-Domen 2006 Nature 440, 295).
  • Selective photodeposition — UV-driven self-deposition aligns cocatalyst to charge-generating sites.

Reactions

Water splitting

2 H₂O → 2 H₂ + O₂; ΔG° = +237 kJ/mol per H₂; the canonical solar-fuel reaction.

Overall (one-photocatalyst) systems: rare because both half-reactions must align with band edges. SrTiO₃:Al, NaTaO₃:La, GaN:ZnO (Maeda-Teramura-Lu-Takata-Saito-Inoue-Domen 2006 Nature 440, 295). Domen’s 96% AQE SrTiO₃:Al + 100 m² panel reactor STH 0.76% are the high-water marks (2020-2021).

Z-scheme systems: more flexible; the Sayama Pt/SrTiO₃:Rh + BiVO₄ + Fe shuttle remains the textbook example.

Photoelectrochemical (PEC) cells: separated photoanode + photocathode + external wiring; can also include external bias (assisted PEC). Reece-Hamel-Sung-Jarvi-Esswein-Pijpers-Nocera “Wireless artificial leaf” Science 2011, 334, 645. NREL holds tandem PEC STH records (12.4% Kistler 1998; 16.2% Cheng-Wang-Walter-Atwater-Lewis 2018 — tandem III-V PEC).

CO₂ photoreduction

CO₂ + 2 e⁻ + 2 H⁺ → CO (2-electron); + 4 e⁻ → HCHO; + 6 e⁻ → CH₃OH; + 8 e⁻ → CH₄; + 12 e⁻ → C₂H₆ + C₂H₄ + C₂H₅OH.

Selectivity controlled by cocatalyst:

  • Cu, Au → CO + HCOOH
  • Pt, Pd → CH₄
  • Co-porphyrin → CO
  • MOF-Re, MOF-Ni → CO
  • Plasmonic Au/TiO₂ → CH₄

State of the art: ~10-100 µmol·g⁻¹·h⁻¹ for CO, much lower for C2+ products. Selectivity > 90% for single C1 products is achievable; C2+ formation remains a frontier (recent Cu-MOF and dual-atom Cu-Ni systems). Liu-Yu-Jaroniec 2014 Chem Soc Rev + Lewis 2018 Joule reviews.

N₂ photofixation (NRR)

N₂ + 6 e⁻ + 6 H⁺ → 2 NH₃; ΔG° = +33 kJ/mol/NH₃; the (potentially) cheap solar-ammonia path.

Schrauzer-Guth 1977 JACS 99, 7189-7193 — first photocatalytic NH₃ from N₂ on Fe-doped TiO₂. Modest yields (~µmol·g⁻¹·h⁻¹). Hirakawa-Hashimoto-Kuwahara-Yamashita 2017 J Am Chem Soc 139, 10929 — defect-engineered BiOCl with oxygen vacancies binding N₂; visible-light NRR at 1.2 mM NH₃ after 1 h. Bismuth Bi-MOF (Bi-N₂ binding without precious metal) Lu-Liang-Liu 2018 Adv Mater — N₂ activation at Bi sites.

Limitations: most reported NH₃ comes from background contamination (NO_x dust, NH₃ in air, N-containing impurities in reagents). Andersen-Cargnello-Chirik-Jaramillo-Holder-Hellman-Robinson-Suryanarayana et al. 2019 Nature 570, 504 — community protocol for distinguishing genuine photocatalytic NRR from artifacts. Most pre-2019 papers fail the protocol; the field has been forced to retighten standards. See design-green-ammonia-plant for electrocatalytic alternatives.

Photodegradation of pollutants

Dye + organic pollutant + pesticide degradation on TiO₂ + ZnO. The largest commercial application — self-cleaning surfaces (Pilkington Activ glass, TOTO Hydrotect tiles), air purifiers (Daikin), water treatment (UV/TiO₂ slurry reactors for textile + dye wastewater). Methylene blue, rhodamine B, phenol, atrazine are reference pollutants.

Photoredox organic synthesis

Stephenson (Michigan), Yoon (Wisconsin), MacMillan (Princeton, Nobel 2021) — visible-light photoredox catalysis with Ru-polypyridyl, Ir-polypyridyl, or organic photocatalysts (acridinium, eosin, riboflavin) for C-C, C-N, C-X bond formation. Mostly homogeneous but heterogeneous variants (TiO₂-supported Ru, Pt/g-C₃N₄, MOF-Ir) emerging. See medicinal-and-photo-chemistry.

Reactor architectures

Suspended-particle slurry reactor

Powder photocatalyst suspended in water; magnetic stirring; outdoor open-tray operation. Cheap, scalable, but slow gas separation (H₂ + O₂ co-evolve and must be separated downstream — explosion hazard).

Membrane-separated photocatalytic reactor

H₂-permeable Pd or polymer membrane separates the H₂ and O₂ evolution sides — Sayama-Domen and others. Reduces back-reaction.

Photoelectrochemical (PEC) cell

Photoanode (n-type, OER side) + photocathode (p-type, HER side) + external circuit; H₂ and O₂ in separate compartments. Tandem PEC (Cheng-Atwater-Lewis 2018) reaches 16% STH.

Panel reactor

Domen’s flat-panel: photocatalyst powder fixed in shallow water layer (1 mm) under cover glass; gas collected at top. 100 m² Sakai array 2021. Mitsubishi pilot Tokuyama Japan 2024 — 100 m² SrTiO₃:Al panel with H₂/O₂ separation downstream, STH 1.5%.

Fluidized-bed photocatalytic reactor

For air purification (Daikin Streamer, Sharp Plasmacluster — though those use different ROS-generation principles).

Continuous-flow microreactor

Lab-scale; high photon efficiency; used for photoredox organic synthesis.

Commercialization and pilot programs

Toyota Central R&D Laboratories (CRDL)

Mn-PSII bio-inspired water splitting — Najafpour-Renger-Holyńska-Moghaddam-Aro-Carpentier-Nishihara-Eaton-Rye-Shen-Allakhverdiev 2016 Chem Rev 116, 2886 review; Toyota CRDL maintains a long-term program inspired by oxygen-evolving complex (OEC) of Photosystem II.

DICP-CAS (Dalian Institute of Chemical Physics)

Can Li group — 100 m² outdoor demo 2023 with multi-junction PEC + dual cocatalyst architecture. Reported 1.5% STH average.

Mitsubishi + Tokuyama + JCH (Japan)

100 m² SrTiO₃:Al panel demonstration 2024 — direct evolution of project NEDO ARPChem (Artificial Photosynthesis Chemical Process) which ran 2012-2022.

SunHydrogen (formerly HyperSolar, US)

Particulate photocatalyst + PV-tandem reactor; pilot panels announced 2023-2024. Public company (NASDAQ: HYSR); commercial timeline uncertain.

Synhelion + Heliogen (Switzerland + US)

Solar thermochemical CO₂ + H₂O → syngas; not strictly photocatalysis (thermal not photonic activation) but adjacent.

H2Sun, H2Pro, Equatic

Various solar-H₂ start-ups; mix of PEC and electrolysis.

Self-cleaning surfaces (commercial since 2000)

Pilkington Activ self-cleaning glass; Saint-Gobain Bioclean; PPG SunClean; TOTO Hydrotect tiles; LIXIL anti-fouling tiles. ~$1B global market by 2025.

Air purifiers

Daikin Streamer (radical-discharge + TiO₂ filter), Sharp Plasmacluster, Panasonic nanoe — TiO₂ filters for VOC + odor + NOx removal.

Water treatment

Photo-Cat (Purifics, Canada); various textile-effluent installations in India + China.

Solar-to-hydrogen (STH) efficiency

STH = (1.23 V · j_op · η_F) / P_sun

where j_op is operating current density, η_F faradaic efficiency, P_sun = 1000 W/m² AM1.5G.

Records (as of 2026):

SystemSTHYearNotes
InGaP/GaAs tandem PEC19.3%2018 (May-Glaze-Hellstern-Khan-Smith-Atwater-Lewis)Lab-scale single-cell; expensive III-V
AlGaInP/GaInAs/Si triple-junction PEC16.2%2018 (Cheng-Wang-Walter-Atwater-Lewis)
BiVO₄/Si tandem8.1%2015 (Abdi-Han-Smets-Zeman-Dam-van de Krol)
Pt/SrTiO₃:Al panel 100 m²0.76%2021 (Nishiyama-Domen Nature 2021)Sustained outdoor; UV-only
Particulate Z-scheme1.1%2016 (Wang-Hisatomi-Katayama-Minegishi-Domen)
DICP 100 m²~1.5%2023

DOE target: STH ≥ 10% for commercial viability of solar H₂ at <$2/kg.

Characterization

  • UV-vis DRS (Shimadzu UV-3600, PerkinElmer Lambda 950 with integrating sphere) — Tauc plot for E_g determination.
  • Mott-Schottky — flat-band potential + carrier density from 1/C² vs V (Gamry Reference 600, Bio-Logic VMP3).
  • Photoluminescence (PL) — recombination dynamics; time-resolved TR-PL with TCSPC for carrier lifetimes.
  • Transient absorption (TA) — fs-ps transient pump-probe (Helios, Newport) for hot-carrier and charge-transfer kinetics.
  • Steady-state HER/OER — gas chromatography (Agilent 8890, Shimadzu GC-2014) on evolved H₂ + O₂.
  • AQE measurement — monochromator + power meter at specific λ; rigorous papers report AQE(λ) curves matching the absorption spectrum.
  • In-situ XAS — synchrotron oxidation-state monitoring of cocatalysts during operation.
  • Surface photovoltage spectroscopy (SPS) — Kelvin probe + light; band-bending verification.

Suppliers and material ecosystem

TiO₂ photocatalyst-grade powders

  • Evonik AEROXIDE TiO₂ P25 — reference standard; ~$30-80/kg in research quantities; bulk much cheaper.
  • Evonik AEROXIDE TiO₂ P90 — finer (14 nm) variant.
  • Tayca AMT-100, AMT-600 — anatase-pure grades.
  • Cristal Global / Tronox TiPure — pigment-grade rutile; photocatalysis variants.
  • Sachtleben / Venator Hombikat UV100 — anatase nanopowder.
  • Ishihara Sangyo ST-01, ST-21, ST-41 — anatase grades; ST-01 high SA (~300 m²/g).
  • Sigma-Aldrich — anatase nanopowder (637254), rutile (700908), brookite, mixed-phase.

Other oxide photocatalysts

  • ZnO — Strem, US Nanomaterials, American Elements (3N-5N purity)
  • WO₃ — Sigma-Aldrich, Strem; nanopowder + thin-film targets
  • BiVO₄ — research-scale only (Toshima Manufacturing Japan; Sigma)
  • g-C₃N₄ — Chemtrend / synthesized in-house from urea / melamine
  • MXenes — Murata, MXene Inc., Y-Carbon (Drexel licensee)
  • Hematite α-Fe₂O₃ nanopowder — Skyspring, US Research Nanomaterials

Cocatalyst precursors

  • H₂PtCl₆·6H₂O (chloroplatinic acid) — Pt photodeposition; Strem, Sigma, Alfa Aesar
  • RuCl₃ — Strem, Sigma
  • Co(NO₃)₂·6H₂O — for Co-Pi formation
  • Ammonium molybdate — for MoS₂ synthesis
  • Ni(NO₃)₂, FeCl₃ — for NiFe-LDH OER catalyst

PEC cell components

  • FTO-coated glass (fluorine-doped tin oxide; Pilkington TEC-7, TEC-15) — transparent conductive substrate
  • ITO-coated glass — Sigma, Delta Technologies (alternative TCO)
  • Nafion membrane (DuPont N-115, N-117, N-211) — proton exchange in PEC
  • Pt wire counter electrode — Bioanalytical Systems, Pine Research
  • Ag/AgCl + saturated calomel reference electrodes — Bioanalytical Systems, Gamry

Standards and characterization protocols

  • ISO 10678 — Photocatalytic activity of surfaces: methylene blue degradation.
  • ISO 22197-1 through 22197-5 — Air-purification photocatalytic materials (NO removal, acetaldehyde, toluene, formaldehyde, methyl mercaptan).
  • ISO 27447 — Antibacterial activity of semiconducting photocatalysts.
  • JIS R 1701, R 1702, R 1703 — Japanese counterparts for NO_x, acetaldehyde, self-cleaning testing.
  • JIS R 1750 — water-splitting photocatalysts (recently added 2022).
  • Andersen-Hellman protocol (2019 Nature) — community standard for distinguishing real NRR from contamination artifacts.
  • NEDO-ARPChem reporting standards (Japan; used by Domen, Mitsubishi, JCH) — AQE measurement procedure for solar fuels.

The Photocatalysis International Foundation (PIF) maintains certification for self-cleaning surfaces; the Japanese Photocatalysis Industry Association (PIAJ) certifies air-purifier products.

Further reading

  • Fujishima, A + Honda, K — Nature 238, 37-38 (1972). Founding paper.
  • Maeda, K + Domen, K — “Photocatalytic water splitting: recent progress and future challenges,” J Phys Chem Lett 1, 2655-2661 (2010).
  • Hisatomi, T + Kubota, J + Domen, K — “Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting,” Chem Soc Rev 43, 7520-7535 (2014).
  • Wang, Z + Li, C + Domen, K — “Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting,” Chem Soc Rev 48, 2109-2125 (2019).
  • Takata, T et al. — “Photocatalytic water splitting with a quantum efficiency of almost unity,” Nature 581, 411-414 (2020).
  • Nishiyama, H et al. — “Photocatalytic solar hydrogen production from water on a 100-m² scale,” Nature 598, 304-307 (2021).
  • Kudo, A + Miseki, Y — “Heterogeneous photocatalyst materials for water splitting,” Chem Soc Rev 38, 253-278 (2009).
  • Wang, X et al. — “A metal-free polymeric photocatalyst for hydrogen production from water under visible light,” Nat Mater 8, 76-80 (2009).
  • Tachibana, Y + Vayssieres, L + Durrant, J R — “Artificial photosynthesis for solar water-splitting,” Nat Photon 6, 511-518 (2012).
  • White, J L et al. — “Light-driven heterogeneous reduction of carbon dioxide: photocatalysts and photoelectrodes,” Chem Rev 115, 12888-12935 (2015).
  • Schrauzer, G N + Guth, T D — J Am Chem Soc 99, 7189 (1977) — first photocatalytic N₂ fixation.

Adjacent

Graph View