Miscellaneous Multinary Oxides

In this catch-all Section, we mainly discuss the spinel structures with the generic formulas, AB2O4 and A2MO4. The unit cell of the spinel structure is a large cube, eight times (2 X 2 X 2) the size of a typical face-centered cube.385 We also discuss the delafossite-type structure ABO2 in which the A cation is in linear coordination

Table 11. Studies on tantalate and niobate photocatalysts for the splitting of water."

Entry

Compound

A cation(s)

B cation(s)

Comments

References

number

formula

1

Ta2O5

Both crystallized and mesoporous samples studied and in one case, (Ref. 441), NiO co-catalyst was used.

440, 44i

2

ATaO3

Li, Na, K

Excess alkali cation enhances catalytic activity. Co-catalysts not found to be essential although NiO was also used in addition in some studies.

442-446

3

A2M2O7 b

Sr

Have layered perovskite structure. Samples with both Ta and Nb also studied. Strontium niobate compound is ferroelectric at room temperature. In contrast, the tantalum analog is paramagnetic.

426, 447-449

4

ANb2O6

Ni, Co, Zn

-

See text.

438, 450

5

ATa2O6

Mg, Ba, Sr

Orthorhombic structure used with NiO co-catalyst to enhance photocatalyst activity.

442, 45i

6

A2BNbO6 c

Sr

Fe

-

452

7

A3BN32O9

Sr

Fe

-

452

8

A4NbeOi7

K, Rb

Perhaps the most studied of the ni-obates. NiO co-catalyst used in some cases as was aqueous methanol solution. Composites with CdS also studied.

453-457

9

AB2Nb3Oio

K, Rb, Cs

Ca, Sr, Pb

Layered perovskite structure.

458-460

10

A2B2Ti3-xNbxOio

K, Rb, Cs

La

Partial substitution of Ti with Nb leads to a decrease in the negative charge density of the perovskite sheets.

46i

11

A3Ta3Si2Oi3

K

-

Pillared structure with TaO6 pillars linked by Si2O7 ditetrahedral units.

462

12

A2BTa5Oi5

K

Ln

Used with NiO co-catalyst. The Pr and Sm compounds show high activity.

463

13

ATaO4

In

Crystallizes in the monoclinic wolframite-type structure, like the FeNbO4 compound (see text).

464

14

A2Nb4Oii

Cs

-

Structure consists of NbO6 and NbO4 octahedra.

465

'Belongs to the series AnMnO3n+2 with A = Ca, Sr, La and M = Nb or Ti. The SnNb2O7 structure (Entry 3), for example, is the reduced formula of Sr4Nb4Oi4 with n = 4 above. "The Sri.9Fei.iNbO6 compound was also studied here.

and the B cation is in octahedral coordination with oxygen. One way to visualize this structure is parallel arrangement of sheets with edge-shared BO6 octahedral with the A cations occupying the interlayer regions of space. Finally, complex oxides containing V and W are also considered. Table 12 contains a listing of these oxides.

Table 12. Miscellaneous multinary oxides for the photodecomposition of water.

Entry Oxide semicon-number ductor(s)

Energy band gap(s) a, eV

Comments

Reference

1 Cd2SnO4, CdImO4

and Cd2GeO4

ZnFe2O4

BiVO4

Bi2W2O9, Bi2WO6 and Bi3TiNbO9

AgVO3, Ag4V2O7 and Ag3VO4

CuMnO2

PbWO4

CuFeO2

2.12 (indirect), 2.23 (forbidden) and 3.15 (indirect)

1.23

Found to be unsuitable as electrodes 466

in photoelectrolysis cells.

HER observed by visible light 467

irradiation of H2S solution.

Ag+ used as electron scavenger and 468

photocatalytic OER observed.

Structure consists of perovskite slabs 469

interleaved with Bi2O2 layers.

Only Ag3VO4 evolves O2 in aqueous 470

AgNO3 solution (with Ag+ as electron acceptor) under visible light irradiation.

The Sr compound shows much lower 471

activity than the Ba counterpart for HER in aqueous methanol.

Photocatalytic HER observed in H2S 472

medium.

Has tetragonal structure. Used with 473

RuO2 co-catalyst for water photosplitting with a Hg-Xe lamp as radiation source.

Photocatalytic water splitting 71

observed under visible light (cf.Refs.

irradiation. 474,475)

"Values for Eg are listed in the order of appearance of the corresponding oxide compound in column 2.

It is interesting to note that some of the newer studies (e.g., Entry 4, Table 12) are rooted in early investigations dating back to 1981. Thus, Bi2WO6 (as well as Bi4Ti3O12) were examined438 within the context of shrinking Eg values of oxide semiconductors. Both these compounds have Bi2O2 layers, the former with WO4 layers (comprised of corner-shared WO6 octahedra) and the latter with double perovskite layers of composition Bi2Ti3O10. These structures are distorted from pure tetragonal symmetry.

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