3. Drawbacks on photocatalytic activity

There are numerous and challenging processes that need to be realized for photocatalytic evolution of H2 and O2 (Table 4) via a thermodynamically unfavorable reaction (Eq. (3)):

$$\text{H}\_2\text{O} \rightarrow \text{H}\_2 + \text{1/2 O}\_2\\\Delta\text{G}^0 = 237 \text{ kJ/mol} \tag{3}$$

These processes include (i) excitation of the semiconductor photocatalyst with photon having higher energies than the bandgap energy of the material, (ii) transfer of the photo-generated electrons and holes to the reaction sites on the surface, (iii) utilization of these charge carriers in the oxidation/reduction reactions, and (iv) desorption of the products from the surface of the photocatalyst to the liquid/gas medium.

As the timescale of these processes varies, recombination of the electrons and holes in the bulk or on the surface happens more frequently than the rate of the


\* Based on 16,000 μmol O2/g/h O2 evolution rate on Rh0.5Cr1.5O3-doped Ga2O3:Zn upon illumination at 254 nm [41], assuming 10 m<sup>2</sup> /g surface area and 10<sup>15</sup> sites/cm2 site density.

#### Table 4.

(Eg � 2.8 eV) or oxynitrides such as TaON (Eg � 2.4 eV) or Rh-doped SrTiO3 (Eg � 2.4 eV). Examples of these materials and systems can be seen in Table 3. The detailed reviews on two-step photocatalytic water splitting can be found

Schematic diagram for photocatalytic water splitting using a two-step photoexcitation system.

Mediator H2 activity

(μmol/h)

Pt/SrTiO3:Rh BiVO4 Fe3+/Fe2+ 15 7.2 0.4 at 420 nm [77]

Ru/SrTiO3:Rh BiVO4 Fe3+/Fe2+ 88 44 4.2 at 420 nm [81]

O2 activity (μmol/h)

�/I� �16 �8 1 at 420 nm [78]

�/I� �16.5 �8 0.5 at 420 nm [79]

�/I� 52 27 6.3 at 420 nm [80]

�/I� 108 55 6.8 at 420 nm [82]

AQY (%) Reference

There are numerous and challenging processes that need to be realized for photocatalytic evolution of H2 and O2 (Table 4) via a thermodynamically

These processes include (i) excitation of the semiconductor photocatalyst with photon having higher energies than the bandgap energy of the material, (ii) transfer of the photo-generated electrons and holes to the reaction sites on the surface, (iii) utilization of these charge carriers in the oxidation/reduction reactions, and (iv) desorption of the products from the surface of the photocatalyst to the

As the timescale of these processes varies, recombination of the electrons and holes in the bulk or on the surface happens more frequently than the rate of the

H2O ! H2 <sup>þ</sup> <sup>1</sup>=2 O2 <sup>Δ</sup>G<sup>0</sup> <sup>¼</sup> 237 kJ=mol (3)

elsewhere [83].

Figure 2.

Water Chemistry

H2

Ta

Table 3.

photocatalyst

Pt/SrTiO3:Cr/

Pt/MgTa2O6 xNy/ TaON

3. Drawbacks on photocatalytic activity

O2 photocatalyst

Pt/TaON PtOx/WO3 IO3

Pt/ZrO2/TaON PtOx/WO3 IO3

PtOx/WO3 IO3

PtOx/WO3 IO3

Z-scheme-type photocatalysts for water splitting without sacrificial agents.

unfavorable reaction (Eq. (3)):

liquid/gas medium.

182

The processes occurring in photocatalytic water splitting on TiO2 and their timescales [27] and the references therein.

chemical oxidation/reduction reactions. Recombination is therefore considered to be one of the main reasons limiting the photocatalytic activity. Together with the recombination events, realization of back-oxidation reactions (Eq. (4)) on noble metals and the rate-limiting mass transfer events are the major drawbacks in an efficient photocatalytic process:

$$\text{H}\_2 + \text{1/2O}\_2 \rightarrow \text{H}\_2\text{O }\Delta\text{G}^0 = -2\text{37 kJ/mol} \tag{4}$$

Natural photosynthesis yields a much higher rate of O2 evolution (see Table 4) when compared to artificial water splitting due to improved charge carrier and mass transfer events. From this comparison, it is clear that the photocatalytic systems still need to be perfected to compete with the nature's intricate design.

#### 3.1 Charge recombination

Due to the presence of the multiple processes, the overall photocatalytic reactions are extremely complicated. In order to obtain an efficient photocatalytic performance, the photo-generated charges must be transferred to the surface reaction sites as rapidly as possible while preventing recombination or trapping of these charge carriers. It is reported by Leytner and Hupp that 60% of the trapped electron–hole pairs recombine with a timescale of about 25 ns while releasing heat of 154 kJ/mol [85]. As the defects such as vacancies and dislocations are considered as recombination sites, higher crystallinity of the photocatalysts is often aimed to decrease the recombination rates. From diffusion point of view, the shorter distances for the charge carriers to the surface reaction centers are also aimed to prevent the recombination. Shorter pathways are achieved via smaller crystal/particle sizes of the photocatalysts. More than two times of increase in the H2 and O2 evolution rates on Al-doped SrTiO3 photocatalyst (reaching an apparent quantum yield of 56% [45]) as the particle size drops from few micrometers to 200 nm is a direct evidence of the effect of the particle size. Another method for reducing the charge recombination is to make use of phase junctions. One example is the α-βphase junction of Ga2O3, which results in enhanced interfacial charge transfer, charge separation, and therefore enhanced water splitting activity [86]. Loading the photocatalysts with co-catalysts such as noble metals or transition metal oxides to accelerate the reduction/oxidation reactions is a commonly employed method.

These co-catalysts are known to enhance the charge migration from the semiconductor depending on the alignment of the potentials of the semiconductor and the co-catalyst. As these co-catalysts accelerate the desired H2 evolution and O2 evolution reactions, they can also increase the rates of undesired secondary reactions such as hydrogen oxidation or oxygen reduction to water reactions.
