Perovskite Titanates and Related Oxides

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Perovskites have the general formula, ABX3, with SrTiO3 being a prototype. They contain a framework structure containing corner-sharing TiO6 octahedra with the A cation in twelve-coordinate interstices.384,385 Several hundred oxides have this struc

Methanol Oxide
Fig. 6. Twin-compartment photoelectrochemical cell for the photocatalytic generation of H2 from water using electrodeposited p-Cu2O (from Ref. 382). TCO is the transparent conducting oxide substrate and A is an electron donor in the anode compartment.

ture. Table 9 lists the studies that have appeared on SrTiO3 with photoelectrolysis of water as a primary objective. As well as the cubic structure exemplified by SrTiO3, a variety of distorted, non-cubic structures occurs in which the framework of TiO6 octahedra may be twisted. Thus, BaTiO3 is tetragonal at room temperature. Both SrTiO3 and BaTiO3 have energy band gaps around 3.2 eV. With Fe and F doping, the Eg of BaTiO3 has been shrunk from 3.2 eV to ~ 2.8 eV.413 Relative to SrTiO3, studies on BaTiO3 from a photoelectrolysis perspective are much more sparse.413-415

Titanates with tunnel structures have been examined for photoelectrolysis appli-cations.94 Thus, barium tetratitanate (BaTi4O9) has a twin-type tunnel structure in which the TiO6 octahedra are not oriented parallel to one another creating a pentagonal prism space. Alkaline metal hexatitanates (M2Ti6Oi3; M = Na, K, Rb) are Wadsley-Andersson type structures in which TiO6 octahedra share an edge at one level in linear groups of three, giving a tunnel structure with rectangular space. The reader should consult Refs. 91 and 94 for reviews of water photolysis studies using these types of oxides. These materials have been used in powder form in suspensions usually modified with a co-catalyst such as RuO2.416-425

More complex perovskites exist containing two different cations which may occupy either the A or B sites and many of these also have a layered structure. Two

Table 9. Studies on the use of SrTiÜ3 anodes or powders for the photoelectrolysis of water.

Entry

Title of paper

Comments

Reference

number

1

Photoelectrochemical Reactions at SrTiO3 Single Crystal Electrodes.

Cell found to work efficiently even without a pH gradient in the anode and cathode compartments.

386

2

Strontium Titanate Photoelectrodes. Efficient Photoassisted Electrolysis of Water at Zero Applied Potential.

As above but the water photosplitting driven by light only with no external bias. Photoanode stability also confirmed as in the evolution of H2 and O2 in the correct 2:1 stoichiometric ratio.

287

3

Photoelectrolysis of Water in Cells with SrTiO3 Anodes.

Maximum quantum efficiency at zero bias (10% at hv = 3.8 eV) found to be ~an order of magnitude higher than TiO2.

388

4

Photoeffects on Semiconductor Ceramic Electrodes.

Photoresponse of SrTiO3 found to be better than that of BaTiO3. Unlike the use of single crystals in the above studies (Entries 1-3), polycrystalline electrodes with large area were used.

389

5

Surface Photovoltage Experiments on SrTiO3 Electrodes.

The role of surface states in mediating charge transfer between electrode and electrolyte elucidated.

390

6

Photocatalytic and Photoelectro-chemical Hydrogen Production on Strontium Titanate Single Crystals.

Both metal-free and platinized samples studied in aqueous alkaline electrolytes or in the presence of NaOH-coated crystals.

391

7

Photocatalytic Decomposition of Water Vapour on an NiO-SrTiO3 Catalyst.

A series of studies begun with this particular study which uses powdered photocatalyst. See Entries below.

392

8

Visible Light Induced Photo-currents in SrTiO3-LaCrO3 Single-Crystalline Electrodes.

Co-doping of La and Cr shifts photo-response down to 560 nm and strong absorption in the visible range ascribed to Cr3+ Ti4+ charge transfer.

393

9

The Sensitization of SrTiO3 Photoanodes for Visible Light Irradiation.

As in Entry 8 but using the perovskites LaVO3, Sr2CrNbO6 and SrNiNb2Os as dopants.

395

10

The Colouration of Titanates by Transition Metal Ions in View of Solar Energy Applications.

395

11

Evidence of Photodissociation of Water Vapor on Reduced SrTiO3(III) Surfaces in a High Vacuum Environment.

First report of photodecomposition of water adsorbed from the gas phase in high vacuum conditions on metalfree, reduced single crystals.

396

12

Oxygen Evolution Improvement at a Cr-Doped SrTiO3 Photoanode by a Ru-Oxide Coating.

397

Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces 195 Table 9. Continuation.

Entry Title of paper Comments Reference number

Entry Title of paper Comments Reference number

13

Electrochemical Conversion and Storage of Solar Energy

A doped n-SrTiO3 single crystal was combined with a proton-conducting solid electrolyte and a metal hydride allowing for storage of the evolved H2.

Powdered catalysts used and the water photolysis efficiency is found to have a strong pH dependence.

398

14

Water Photolysis by UV Irradiation of Rhodium Loaded Strontium Titanate Catalysts. Relation Between Catalytic Activity and Nature of the Deposit from Combined Photolysis and ESCA Studies.

399

15

Photocatalytic Decomposition of Liquid Water on a NiO-SrTiO3 Catalyst.

As in Entry 7 but for liquid water. Effect of NaOH film (see Entry 6) reproduced for NiO-SrTiO3 powder.

400

16

Study of the Photocatalytic Decomposition of Water Vapour over a NiO-SrTiO3 Catalyst.

Mechanistic aspects probed by using a closed gas circulation system and IR spectroscopy (see Entries 7 and 15).

401

17

Photoelectrolysis of Water under Visible Light with Doped SrTiO3 Electrodes.

Sintered samples used with a variety of dopants (Ru, V, Cr, Ce, Co, Rh).

402

18

Mediation by Surface States of the Electroreduction of Photogenerated H2O2 and O2 on n-SrTiO3 in a Photoelectrochemical Cell.

Back reactions probed and the role of surface states elucidated.

403

20

Photocatalytic Decomposition of Water into H2 and O2 over NiO-SrTiO3 Powder. 1. Structure of the Catalyst.

Mechanism of Photocatalytic Decomposition of Water into H2 and O2 over NiO-SrTiO3.

Nickel metal also found at the interface of NiO and SrTiO3. See also Entries 7, 15 and 16.

HER found to occur on the NiO cocatalyst surface while OER takes place on SrTiO3. See also Entries 7, 15, 16 and 19.

405

21

Water Photolysis over Metallized SrTiO3 Catalysts.

Promoting effect of NaOH not so pronounced as for TiO2.

406

22

Luminescence Spectra from n-TiO2 and n-SrTiO3 Semiconductor Electrodes and Those Doped with Transition-Metal Oxides As Related with Intermediates of the Photooxidation Reaction of Water.

Mechanistic aspects clarified using photo-and electroluminescence measurements.

407

24

Photoinduced Surface Reactions on TiO2 and SrTiO3 Films: Photo-catalytic Oxidation and Photo-induced Hydrophilicity.

Stoichiometric Water Splitting into H2 and O2 using a Mixture of Two Different Photocatalysts and an IO3/I- Shuttle Redox Mediator under Visible Light Irradiation.

A Z-scheme used using a mixture of Pt-WO3 and Pt-SrTiO3 photocatalysts. The latter was co-doped with Cr and Ta.

283 408

Table 9. Continuation.

Entry Title of paper Comments Reference number

Entry Title of paper Comments Reference number

25

Visible-Light-Response and Photo-catalytic Activities of TiO2 and SrTiO3 Photocatalysts Co-doped with Antimony and Chromium.

The band gap of SrTiO3 shrunk to 2.4 eV by co-doping.

409

26

A New Photocatalytic Water Splitting System under Visible Light Irradiation Mimicking a Z-Scheme Mechanism in Photosyn-thesis.

See Entry 23 above.

349

27

Construction of Z-Scheme Type Heterogeneous Photocatalysis Systems for Water Splitting into H2 and O2 under Visible Light Irradiation.

A Pt-SrTiO3 doped with Rh is combined with a BiVO4 photocatalyst.

410

28

Electrochemical Approach to Evaluate the Mechanism of Photo-catalytic Water Splitting on Oxide Photocatalysts.

Cr or Sb co-doped SrTiO3 samples studied amongst others (cf. Table 7, entry i3)

333

29

H2 Evolution from a Aqueous Methanol Solution on SrTiO3 Photocatalysts Co-doped with Chromium and Tantalum Ions under Visible Light Irradiation

411

30

Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation

Mn-, Ru-, Rh- and Ir-doped powder samples studied.

412

31

Nickel and Either Tantalum or Niobium-Co-doped TiO2 and SrTiO3 Photocatalysts with Visible-Light Response for H2 or O2 Evolution from Aqueous Solutions

Co-doping found to afford higher activity for HER compared with Ni alone.

334

main classes of such oxides showing interlamellar activity have been explored for water photolysis:

1. the Dion-Jacobson series of the general formula, AMn-iBnO3n+i (e.g., KCa2Ti3Oio), and

2. the Ruddlesden-Popper series of general formula, A2Mn-iBnO3n+i (e.g., K2La2Ti3Oio).95

Corresponding niobates also exist as discussed below. Noble metal co-catalysts (e.g., Pt) are loaded onto these photocatalysts by photocatalytic deposition from H2PtCl6 (see above). Since the oxide sheets have a net negative charge (that is balanced by the alkali cations), the PtCl62- anions are not intercalated in the host lattice.95 Instead, the Pt sites are formed on the external surfaces of the layered perovs-kite powder.

Table 10. Other ternary oxides with the general formula, ABO3," that have been examined from a water photoelectrolysis perspective.

Entry

Oxide

Energy band gap

Comments

Refer

number

eV

ence^)

1

FeTiO3'

2.16

Unstable with leaching of iron observed

427

during photoelectrolysis.

2

YFeO3

2.58

N-type semiconductor with an indirect

428

optical transition.

3

LuRhO3

~ 2.2

Distorted perovskite structure with p-

429

type semiconductor behavior.

4

BaSnO3

~ 3.0

Estimated to be stable toward photoanod-

65

ic decomposition over a 0.4-14 pH

range.

5

CaTiO3

~ 3.6

--

65

6

KNbO3

~ 3.1

See next Section.

65

7

Bao.8Cao.2TiO3

~ 3.25

--

65

8

KTaO3

~ 3.5

Optical to chemical conversion efficien-

430

cy of ~ 6% reported. See next Section.

9

CdSnO3

1.77

Band-edges not suitably aligned for HER

431

or OER.

10

LaRhO3

1.35

See above.

431

11

NiTiO3c

~ 1.6

N-type semiconductor crystallizing in the

432-434

illmenite structure.

12

LaMnO3

~ 1.1

A p-type semiconductor.

435,436

"Not all the oxides in this compilation have the perovskite structure.

'Other iron titanates: Fe2TiO4 (Eg = 2.12 eV) and Fe2TiO5 (Eg = 2.18 eV) also examined.

'Band gap estimated for the transition from the mid-gap Ni2+ (3 d8) level to the CB. Compound can be regarded as NiO doped TiO2.

"Not all the oxides in this compilation have the perovskite structure.

'Other iron titanates: Fe2TiO4 (Eg = 2.12 eV) and Fe2TiO5 (Eg = 2.18 eV) also examined.

'Band gap estimated for the transition from the mid-gap Ni2+ (3 d8) level to the CB. Compound can be regarded as NiO doped TiO2.

In many of these cases with layered oxides, the H+-exchanged photocatalysts show higher activity toward the HER—a trend rationalized by the easy accessibility of the interlayer space to electron donor species such as methanol.95 Other aspects such as Ni-loading and pillaring of the interlayer spaces have been discussed.95 Another type of layered perovskites have been studied with the generic composition, AnBnO3n+2 (n = 4, 5; A = Ca, Sr, La; B = Nb, Ti).426 Unlike the (100)-oriented structures discussed above, the perovskite slabs in these oxides are oriented parallel to the (110) direction. Thus compounds such as La2Ti2O7 and LatCaTi5On were examined in terms of their efficacy toward water splitting under UV irradiation.426

In closing this Section, a variety of other ternary oxides (besides the SrTiO3 prototype) have been examined over the years. Table 10 contains a representative listing of these compounds.

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