Other Binary Oxides

Table 8 contains a compilation of studies on other binary oxides that have been examined for their applicability to drive the photoelectrolysis of water. As cited earlier, general reviews are available on many of the oxides listed in Table 8.32'62'65 Other than TiO2, Fe2O3 and WO3 are two of the most widely studied among the binary oxide semiconductors, and studies on these oxides have continued to appear right up to the time of the writing of this Chapter.

Tungsten oxide shares many of the same attributes with TiO2 in terms of chemical inertness and exceptional photoelectrochemical and chemical stability in aqueous media over a very wide pH range. However, its flat-band potential (Vfb) lies positive of that of TiO2 (anatase) such that spontaneous generation of H2 by the photogenerated electrons in WO3 is not possible. This location of Vfb has been invoked347 for the very high IPCE values observed for the photoinduced OER in terms of the rather slow back electron transfer leading to O2 reduction. A variety of dopants (e.g., F, Mg, Cu) have been tested for WO3341,344,350 and Pt-modified samples have been deployed in a Z-scheme configuration.349 Electron acceptors such as Ag+ 343 and IO3- 349

Table 8. Binary oxides (other than TiO2) that have been considered for the photoelectrolysis of water.

Entry number

Oxide semiconductor

Energy band gap eV

, Comments

Refer-ence(s)

1

WO3

2.5-2.8

This material has been used as single crystals, thin films, powders and in mesoporous/ nanostructured form. Both virgin and doped samples studied.

339-350

2 3

Fe2O3 ZnO

Unstable under irradiation and OER/HER conditions.

208, 351-368 248

4

SnO2

3.5

Sb-doped single crystal samples used. Stable H2 and O2 evolution observed at Pt cathode and SnO2 photoanode respectively.

369, 370

5

NiO

3.47

A p-type semiconductor with indirect gap optical transition.

371, 372

6

CdO

~ 2.3

A n-type semiconductor. Interestingly, RuO2-modified samples reduced the yield of O2 under irradiation.

373

7

PdO

~ 0.8

A p-type semiconductor. Not stable under irradiation in the HER regime.

374

8

Cu2O

2.0-2.2

Claims of water splitting in powder suspensions challenged by others (see text).

375, 376

9

CuO

1.7

Not photoelectrochemically stable.

353, 377

10

Bi2O3

2.8

Both doped and catalytically modified samples studied.

353, 378-380

species have ben used to study the O2 evolution characteristics of the WO3 photoca-talyst under visible light irradiation. As pointed out very early in the history of study of this material,339,381 the lower Eg value of WO3 (relative to TiO2) results in a more substantial utilization of the solar spectrum. This combined with the advances in nanostructured oxide materials will likely sustain interest in WO3 from a photoe-lectrolysis perspective.

The combination of a rather low Eg value, good photoelectrochemical stability and chemical inertness coupled with the abundance of iron on our planet makes Fe2O3 an attractive candidate for the photoelectrolysis of water. Thus it is hardly surprising that this material continues to be intensively studied from this perspective. As with TiO2 and WO3, Fe2O3 (particularly the a-modification) has been examined in single crystal form, as thin films prepared by CVD,351,353 pyrolytic conversion of iron354 and spray pyrolysis,362,364-367 or as sintered pellets from powders.355-360 A variety of dopants have been deployed to modify the host356-359,361,364-367 and remarkably, p-

type semiconductor behavior has been reported356,358,359,365 in addition to the more commonly occurring n-type material. The main handicap with Fe2Ü3 is its rather poor electronic and charge transport characteristics regardless of the method of preparation of the material. Specifically, facile e--h+ recombination, trapping of electrons at defect sites and the poor mobility of holes conspire to result in very low efficiencies for water oxidation. Attempts to circumvent these problems by using unique photoanode configurations (e.g., nanorod arrays363) or compositional tuning (e.g., minimizing sub-stoichiometric phases such as Fe3Ü4, c.f. Refs. 360, 367) are continuing and will undoubtedly contribute to further examinations of this promising material in the future.

By way of contrast, none of the other binary oxides listed in Table 8 appear to hold much promise. While ZnO has enjoyed extensive popularity in the photochemistry community (even comparable to TiÜ2 in the early days prior to ~ 1980), it is rather unstable (at least in the forms synthesized up till now) under illumination and in the ÜER and HER regimes. This problem besets most of the other candidates in Table 8 with the exception of SnÜ2 (whose Eg is too high) and possibly Bi2Ü3. The report375 of photocatalytic water splitting on Cu2Ü powder suspensions (with stability in excess of ~ 1900 h!) has been greeted with skepticism by others376 who have also pointed out that the Cu2Ü band-edges are unlikely to bracket the H+/H2 and Ü2/H2Ü redox levels as required (see Figure 1a and earlier discussion in Section 3 of this Chapter). Üur own studies382 on electrodeposited samples of this oxide have utilized a Ni/NiÜ protective layer, catalyst modification (with e.g., Pt) to drive the HER and the use of optimized electron donors in the anode compartment in a twin-compartment photoelectrochemical cell (Figure 6).382 Under these conditions, spontaneous HER was observed under visible light irradiation of the p-Cu2Ü photocathode. Photoinduced transfer of electrons from p-Cu2Ü to an electron acceptor such as methyl viologen was also demonstrated via in situ spectroscopic monitoring of the blue cation radicals.382 However, the photocurrents generated are only in the |A level necessitating further improvements before assessments of practical viability of Cu2Ü for solar H2 photogeneration.

It is worth noting that some oxides have too low a band gap for optimal solar energy conversion. Palladium oxide in Table 8 exemplifies this trend as does PbÜ2.353 Ün the other hand, PbÜ has an Eg value around 2.8 eV.353 Üther oxides such as CoÜ and Cr2Ü3 (both p-type semiconductors) have been very briefly examined early on in the evolution of this field.353

In closing this Section, comparative studies on binary oxide semiconductors are available62,65,353,383 including one study383 where the electron affinities of several metal oxides (used as anodes in photoelectrolysis cells) were calculated from the atomic electronegativity values of the constituent elements. These electron affinity estimates were correlated with the Vfb values measured for the same oxides in aqueous me-

dia.383

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