Recent Work on TiO on Photosplitting of Water or on the Oxygen Evolution Reaction

Table 7 contains a compilation of studies that have appeared since 1985. Several points are worthy of note here. The vast majority of the entries feature studies on TiO2 powders rather than on electrodes in a photoelectrochemical cell configuration. In this light, the new studies can be regarded as offshoots inspired by the earlier (pre-1985) studies on co-functional photocatalysts and the cyclic cleavage of water.4,30 Second, many of the new studies address two key issues with the earlier systems:

1. non-stoichiometric evolution of H2 and O2, and

2. poor performance stemming from back reactions and electron-hole recombination processes.

With reference to the first point, very little O2 evolution was observed in many cases in studies on TiO2 powder suspensions with reports245,320,324 of stoichiometric H2 and O2 evolution (i.e., in the expected 2:1 ratio) being the exceptions rather than the rule. Initially, this discrepancy was attributed by the community to the photo-induced adsorption of the (evolved) O2 on the TiO2 surface.

The remarkable effect of a NaOH dessicant coating on the TiO2 surface on the efficiency of water photosplitting appears to have radically changed this thinking (see Ref. 92 and references therein). The new results support the deleterious role that Pt islands on the TiO2 play in promoting the reverse reaction, 2 H2+O2 ^ 2 H2O. Interestingly, the irradiation geometry also appears to exert an effect on the extent of back reactions.335 Adsorption of CO on Pt, for example, was also found to inhibit the reverse reaction.336 Subsequent studies on the role of Na2CO3 addition (Ref. 93

Table 6. Representative studies on doping of TiO2 with non-metallic elements.

Entry number

Title of article

Comments

Reference

Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides.

Formation of TiO2-xFx Compounds in Fluorine-Implanted TiO2.

Band Gap Narrowing of Titanium Dioxide by Sulfur Doping.

Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2.

Daylight Photocatalysis by Carbon-Modified Titanium Dioxide.

Carbon-Doped Anatase TiO2 Powders as a Visible-Light Sensitive Photocatalyst

Nitrogen-Concentration Dependence on Photocatalytic Activity of Ti2-xNx Powders.

Visible Light-Induced Degradation of Methylene Blue on S-doped TiO2.

Visible-Light Induced Hydrophi-licity on Nitrogen-Substituted Titanium Dioxide Films.

Spectral Photoresponses of Carbon-Doped TiO2 Film Electrodes.

Photoelectrochemical Study of Nitrogen-Doped Titanium Dioxide for Water Oxidation Metal Ion and N Co-doped TiO2 as a Visible-Light Photocatalyst

Both films and powders considered. 306

Substitutional doping with nitrogen shown to bring about band gap narrowing and also high photocatalytic activity with visible light. Experimental data supported with first-principles calculations.

Fluorine substituted for oxygen sites in the 307

oxide by ion implantation.

Oxidative annealing of TiS2 used. 308

Ab initio calculations also reveal mixing of S 3p states with the valence bond to bring about band gap narrowing.

Combustion of Ti metal in a natural gas 213

flame done to substitute carbon for some of the lattice oxygen sites. The photoca-talysis performance data have been questioned (see Refs. 214-216).

Titanium tetrachloride precursor hydro- 309

lyzed with nitrogen bases to yield (surprisingly) C-doped (instead of N-doped) TiO2. Study oriented toward environmental remediation applicability.

Oxidative annealing of TiC used to afford 310

yellow doped powders. Study focus as in Entry 5.

Samples prepared by annealing anatase 311

TiO2 under NH3 flow at 550-600 oC.

As in Entry 3 (Ref. 308) by the same 312

research group.

Degree of hydrophilicity correlated with 313

the extent of substitution of nitrogen at oxygen sites.

Raman spectra used to identify disordered 314

carbon in the flame-formed samples in addition to lower nonstoichiometric titanium oxides identified by X-ray diffraction.

One of the few studies probing the influ- 315

ence of doping on OER.

Co-doped samples prepared by polyme- 316

rized complex or sol-gel method. Doped N species found to reside at interstitial ^attice^ositionsjnthehost^^^^^^^^^^^^^^^

Table 7. Representative studies that have appeared since 1985 on the photosplitting of water using TiO2.

Table 7. Representative studies that have appeared since 1985 on the photosplitting of water using TiO2.

Entry

Brief outline of study

Refer

Number

ence^)

1

Ferroelectric substrates (poled LiNbO3) were used to support TiO2 films. After platinization of TiO2, water splitting was examined in both liquid and gas phases under Xe arc lamp illumination.

318

2

Both reduced and Pt-modified powder samples were studied in distilled water and in aqueous solutions of HCl, H2SO4, HNO3 and NaOH. Water photodecomposition proceeds moderately in distilled water and in NaOH but is strongly suppressed in acidic aqueous media. The NaOH coating effect mimicks that found by other workers earlier (see Ref. 320 and text).

319

3

Sodium carbonate addition to a Pt/TiO2 suspension in water effective in promoting stoichiometric photodecomposition of water.

321, 322

4

Demonstration of solar H2 and O2 production on NiOx/TiO2 co-catalyst with Na2CO3 or NaOH addition.

323,324

5

A photoelectrolyzer designed with a TiO2 photoanode and a membrane of sulfonated polytetrafluoroethylene as the electrolyte. A quantum efficiency of 0.8 was reported.

325

6

Photochemical splitting of water achieved by combining two photocatalytic reactions on suspended TiO2 particles; namely, the reduction of water to H2 using bromide ions and the oxidation of water using Fe(III) species. High efficiency also observed for the photoassisted OER on TiO2 in the presence of Fe(III) ions.

326, 327

7

Pt- and other catalyst supported TiO2 (P-25) particles studied. Only the HER was observed and stoichiometry H2 and O2 formation was not found. Mechanistic reasons proposed have been challenged by other authors (see text).

328

8

HER observed in semiconductor septum cells using TiO2 or TiO2-In2O3 composites.

106, 107

9

Pure rutile TiO2 phase isolated from commercial samples containing both rutile and anatase by dissolution in HF. The resultant samples studied for their efficacy in driving the photoassisted OER in the presence of Fe(III) species as electron acceptor (see Entry 6 above).

329

10

A Z-scheme system mimicking the plant photosynthesis model developed with Pt-loaded TiO2 for HER and rutile TiO2 for OER. A IO3-/I- shuttle was used as redox mediator.

330

11

Co-doping of TiO2 with Sb and Cr found to evolve O2 from an aqueous AgNO3 solution under visible light irradiation.

331

12

HER observed from a mixed water-acetonitrile medium containing iodide electron donor and dye-sensitized Pt/TiO2 photocatalysts under visible ight irradiation.

332

13

Back-reactions (i.e., O2 reduction and H2 oxidation) studied on both TiO2 or Cr and Sb co-doped TiO2 samples (see Entry 11 above).

333

14

TiO2 nanotube arrays prepared by anodization of Ti foil in a F--containing

210,

electrolyte. Pd-modified photocatalyst samples show an efficiency of

210a-c

4.8% based on photocurrent data for the OER.

15

TiO2 co-doped with Ni and Ta (or Nb) show visible light activity for the OER in aq. AgNO3 and HER in aqueous methanol solution.

334

and Entries 3 and 4 in Table 7) underline the importance of inhibiting back reactions on catalyst-modified TiO2 samples. By the same token, unusual valence states (Ti5+) that have been proposed328 to explain the non-stoichiometric gas evolution have been challenged by other authors.337

Other factors influencing the yield of H2 and O2 in irradiated TiO2 suspensions include the nature of the co-catalyst (see, for example, Entry 4 in Table 7), the crystal form of TiO2, particle size of TiO2, temperature and ambient pressure.92 The reader is referred to Ref. 92 for further details. Other interesting mechanistic aspects of the water photosplitting process on the TiO2 surface such as hydrogen atom spillover have also been discussed.338

An interesting aspect of the new work on TiO2, namely that of combining two photosystems (in a Z-scheme) mimicking plant photosynthesis (see Entries 6 and 10 in Table 7) also has its roots in early work in this field (see, for example, Entry 2 in Table 1). Further elaboration of this strategy is contained in Ref. 96.

Finally, some of the studies considered in Table 7 (Entries 11 and 13) buck the trend mentioned earlier that few of the studies on transition-metal doped TiO2 are oriented toward the water-photosplitting application. These new studies exploit the visible-light sensitization of the doped host material as well as the improved electronic characteristics observed in some cases (particularly the co-doped instance) to enhance the efficiency of the water photosplitting process.

In sum, TiO2 continues to be a veritable workhorse of the photocatalysis and pho-toelectrolysis communities. However, this material to date has not yielded systems for evolving H2 and O2 at the 10% benchmark efficiency level. Studies on TiO2 oriented toward visible light sensitization and efficiency enhancement will undoubtedly continue, at an unabated rate, in the foreseeable future. This is because of the extensive and growing market that already exists for this commodity chemical in a variety of other application areas and because of its excellent chemical attributes such as inertness and stability.

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