2. Photocatalysis and water splitting

The first report on water splitting via harvesting photon energy is authored by Fujishima and Honda using a photoelectrochemical cell with a TiO2 photoelectrode [7]. Following this first report suggesting the oxidation of water molecule via photogenerated holes on TiO2 surface with the aid of small electrical voltage, photocatalytic water splitting on powder photocatalyst particles is demonstrated by other authors in the late twentieth century [8–15]. Metal-loaded semiconductors (such as Pt/TiO2) are described as "short-circuited photoelectrochemical cells" that provide both the oxidizing centers and the reduction centers on the same catalyst (see Figure 1) [16].

Photocatalytic reactions are initiated by absorption of light having an energy higher than (or equal to) the bandgap of the photocatalysts that consist of semiconductor materials. This bandgap energy should be larger than 1.23 V for overall water oxidation reaction, for which the maximum of the valence band and the minimum of the conduction band should be located at proper potentials for the oxygen and hydrogen evolution reactions to occur. To illustrate, the minimum of

Figure 1.

Schematic representation of photocatalytic water splitting on metal-loaded semiconductor particle systems: (1) light absorption and charge excitation from valence band to conduction band, (2) transfer of the photogenerated electrons and holes to the catalyst surface, (3) surface redox reactions, and (4) charge recombination.

On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235

the conduction band energy level should be located at a more negative potential than 0 V vs. NHE, at pH = 0 for H2 evolution (Eq. (1)), and the maximum of the valence band should be at a more positive potential than 1.23 V vs. NHE at pH = 0 for oxygen evolution reaction (Eq. (2)):

$$\text{4H}^+ + \text{4e}^- \rightarrow \text{2H}\_2\tag{1}$$

$$\text{2H}\_2\text{O} + 4\text{ h}^+ \to 4\text{H}^+ + \text{O}\_2 \tag{2}$$

Following the light absorption, photoexcited electrons are transferred to the conduction band, while a positively charged charge carrier (hole) is generated at the valence band. These charge carriers are then transferred to the catalyst surface (step 2 in Figure 1) to be utilized in surface redox reactions, unless they recombine in the bulk or on the surface (step 4). Ultimately, electrons and holes reduce/oxidize the adsorbed species on the catalyst surface (step 3), the products of which should then be desorbed from the surface to complete the overall process.

#### 2.1 Semiconductors

or 120,000 TW [4]) is comparable to the Earth's yearly energy consumption (reaching 575 EJ/year or 18 TW in 2017). However, the solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is much lower (targeted to be 10%, currently reaching 1% [5]) than that of photovoltaic-assisted electrolysis (reaching 30% [6]) due to the major drawbacks in the one-step photocatalytic water splitting systems. Herein, we firstly introduce photocatalytic water splitting systems and give the major developments in materials such as visible

light utilization and corresponding H2 and O2 production activity values (in Section 2). Then in Section 3, we discuss the causes of the low efficiencies in photocatalytic water splitting systems and the recent approaches in preventing energy efficiency-lowering factors such as inefficient visible light utilization, charge

recombination, back-oxidation reactions, and mass transfer limitations.

generated holes on TiO2 surface with the aid of small electrical voltage,

The first report on water splitting via harvesting photon energy is authored by Fujishima and Honda using a photoelectrochemical cell with a TiO2 photoelectrode [7]. Following this first report suggesting the oxidation of water molecule via photo-

photocatalytic water splitting on powder photocatalyst particles is demonstrated by other authors in the late twentieth century [8–15]. Metal-loaded semiconductors (such as Pt/TiO2) are described as "short-circuited photoelectrochemical cells" that provide both the oxidizing centers and the reduction centers on the same catalyst

Photocatalytic reactions are initiated by absorption of light having an energy higher than (or equal to) the bandgap of the photocatalysts that consist of semiconductor materials. This bandgap energy should be larger than 1.23 V for overall water oxidation reaction, for which the maximum of the valence band and the minimum of the conduction band should be located at proper potentials for the oxygen and hydrogen evolution reactions to occur. To illustrate, the minimum of

Schematic representation of photocatalytic water splitting on metal-loaded semiconductor particle systems: (1) light absorption and charge excitation from valence band to conduction band, (2) transfer of the photogenerated electrons and holes to the catalyst surface, (3) surface redox reactions, and (4) charge recombination.

2. Photocatalysis and water splitting

(see Figure 1) [16].

Water Chemistry

Figure 1.

178

TiO2, having a large bandgap (anatase: 3.2 eV), is the most commonly used photocatalyst due to its photostability, nontoxicity, and high activity (upon UV radiation λ < 387 nm). Following the report on water oxidation reaction [7], various photochemical reaction activities of TiO2 such as carbon dioxide reduction with H2O [17–19], alkene and alkyne hydrogenation [20, 21], CH3Cl oxidation [22], 1 octanol degradation [23], phenol degradation [24], surfactant degradation [25], and more have been reported. Detailed reviews on TiO2-based materials and photocatalytic performances can be found in literature [26–28].

As photostable and active TiO2 is, UV light requirement to activate the large bandgap of TiO2 motivated research for visible light active semiconductors as well as bandgap engineering for TiO2 such as nonmetal ion doping (N [29], C [30], F [31], S [32]). Substitution of lattice oxygen atoms by these anions is reported to shift the valence band level upward and narrow the bandgap to as low as 2.25 eV (�550 nm) with 16.5% N doping [33].

Similar to TiO2, oxides of other transition metals with d<sup>0</sup> (such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+ [34, 35]) and d<sup>10</sup> electronic configurations (such as Ga3+, In3+, Ge4+, Sn4+, and Sb5 [36–38]) are shown to possess large bandgap energies (>3 eV) due to the maximum valence band levels consisting O2p orbitals located near 3 V (vs. NHE at pH = 0). These d<sup>0</sup> and d10 metal oxide catalysts are reported to show remarkable one-step photocatalytic water splitting activity under UV light irradiation [39] reaching 71% quantum yield with photocatalysts such as Al-doped SrTiO3 [40] or Zn-doped Ga2O3 [41]. The H2 and O2 evolution activity under UV radiation and the apparent quantum yields of some of these materials are given in Table 1. The apparent quantum yield is defined as the number of reacted electrons and holes divided by the number of incident photons on the photocatalysts. Table 1 is not intended to cover the whole range of particulate catalysts in literature but rather to give a selection of examples. A wider selection of d0 and d<sup>10</sup> metal oxide particulate catalysts' one-step water oxidation activity and apparent quantum yields can be found in the works of Kudo et al., Chen et al., and Domen et al. [39, 42, 43].

The most remarkable upgrades in the apparent quantum yields are achieved by material engineering such as (i) doping the metal oxides/perovskites with cations having lower valences, (ii) decreasing the crystal sizes to submicron levels, and (iii) loading with H2/O2 evolution co-catalysts.


shifted upward without changing the conduction band potentials. One approach to do this is to use oxynitrides to make use of N2p states that lie at a more negative potential than O2p states. Emerging LaMgTa1xO1+3xN23x [64] and Ga1xZnxN1xOx [65] oxynitrides are representatives of visible light active overall water splitting catalysts. Using N doping, the absorbed light wavelength can be increased up to 500 nm on solid solutions of GaN:ZnO (Ga1xZnxN1xOx) [66] and up to 600 nm on solid solutions of LaTaON2 and LaMg2/3Ta1/3O3

(LaMgTa1xO1+3xN23x) [64]. Other examples include LaScxTa1xO1+2xN22x [67] and CaTaO2N [68] in which La or Ta sites are replaced by Ca and Sc that alters O/N ratios due to charge compensation, which in turn results in valence band energy

Some examples of visible light active photocatalyst and their H2 and O2 evolution activity are given in Table 2. As it can be seen from the table, one-step water splitting quantum yields are quite lower when compared to those of the UVactivated photocatalysts (Table 1). The exceptions to the low activity are reported by Rh2-yCryO3 (Rh 1.0 wt%, Cr 1.5 wt%)-loaded (Ga1xZnx)(N1xOx) photocatalyst [63], multiband InGaN/GaN nanowire arrays [69], and monodisperse 4 nm

An alternative way to cover both oxidation and reduction reactions with semiconductors that could be activated under visible light radiation is to utilize two individual photocatalysts with an electron transfer mediator to obtain two-step excitation known as the two-step water oxidation ("Z-scheme system," see Figure 2). In this system, O2 evolution photocatalysts oxidize the water molecules to O2, while the photo-generated electron is transferred to the mediator to reduce the electron

donating its electron to the H2 evolution photocatalyst. At the same time, the photo-generated electrons in the H2 evolution photocatalyst reduce H+

band maximum/minimum that would enable O2/H2O oxidation and H+

tion. As H2 evolution and O2 evolution reactions are realized at separate

The semiconductors used in this two-step water splitting process should be selected based on the energy levels of their corresponding valence or conduction

photocatalysts, these semiconductors could have bandgap energy values lower than 3 eV that would enable visible light utilization such as Pt- or RuO2-loaded WO3

> H2 activity (μmol/h)

SrTiO3:Rh,Sb IrO2 4.4 1.9 0.1 at 420 nm [71]

CDots-C3N4 2.74 46 16 at 420 nm [70]

BiYWO6 RuO2 2.7 4.1 1.8 0.17 at 420 nm [74] LaMg1/3Ta2/3O2N RhCrOx 22 11 0.18 at 440 nm [75]

H2 and O2 evolution activity of one-step water splitting catalysts under visible light irradiation.

ions). Then, the reduced mediator is oxidized by

O2 activity (μmol/h)

2.8 8.5 3.5 0.3 at 405 nm [72]

2.64 927 460 5.9 at 420 nm [76]

2.22 38 21 12.3 at 400 nm [69]

RuO2 2.5 17 3.2 at 420 nm [73]

s to H2.

AQY (%) Reference

/H2 reduc-

graphite nanoparticle-deposited C3N4 catalysts [70].

On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235

acceptor (such as Fe3+ ions or IO3

Semiconductor Co-

g-C3N4 Pt-

Bi1-xInxV1 xMoxO4

(Zn0.18Ga0.82) (N0.82O0.18)

GaN:Mg/InGaN:

Mg

Table 2.

181

catalyst

CoOx

Rh2 yCryO3

Rh/ Cr2O3 Bandgap (eV)

level shift.

#### Table 1.

H2 and O2 evolution activity of d0 and d<sup>10</sup> metal oxide particulate catalysts under UV light irradiation.

#### 2.2 Co-catalysts

An important addition to the light-harvesting semiconductors is H2 evolution/O2 evolution co-catalysts on the surface. The early co-catalysts that have been widely used included the noble metals and transition metal oxides such as Pt [12, 13], Rh [10], Ru [48], Au [49, 50], and NiOx [11] that mainly promote the hydrogen evolution, and CoOx [51] and Fe [52], Mn [52], RuO2 [53], and IrO2 [54] that accelerate the oxygen evolution. These metals are considered to act as charge carrier sinks that suppress electron–hole pair recombination as well as increasing the reaction kinetics by lowering the activation energy of the redox reactions. Co-catalysts are also known to inhibit photodegradation of the photocatalysts such as oxysulfides and oxynitrides by generated holes due to the effective extraction of these holes by the co-catalysts [55, 56].

Following the works of noble metal co-catalysts, Domen et al. showed water splitting activity on SrTiO3 photocatalyst together with the co-catalyst NiO [57, 58], which became the choice of H2 evolution co-catalyst for many d<sup>0</sup> and d10 metal oxides such as La2Ti2O7:Ba [44], La4CaTi5O17 [59], Rb4Nb6O17 [60], NaTaO3 [46], and Ga2O3:Zn [47]. The photocatalyst stability of NiO-loaded K2La2Ti3O10 is reported to increase by addition of a second co-catalyst, Cr, using a coimpregnation method [61]. Based on the promoting effect of Cr, a systematic study of Cr and various transition metals (such as Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt) on (Ga1-xZnx)(N1-xOx) has been conducted [62], from which core-shell structures of core Rh nanoislands and shell Cr2O3 structures (10–30 nm in size) are found to promote H2 and O2 evolution reactions to significant levels [63].

#### 2.3 Visible light utilization

The alterations to the semiconductors such as doping with low-valence cations, reducing particle sizes to submicron levels and obtaining a high degree of crystallinity help with the overall water splitting activity. However, activation of these photocatalysts uses a narrow portion of the solar spectrum (4%); i.e., the UV light sustains as a problem due to the large bandgap energies of these materials. To enable visible light utilization of the d0 - and d10-type oxide semiconductors and to split water into H2 and O2 via one-step excitation, the valence band levels should be

On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235

shifted upward without changing the conduction band potentials. One approach to do this is to use oxynitrides to make use of N2p states that lie at a more negative potential than O2p states. Emerging LaMgTa1xO1+3xN23x [64] and Ga1xZnxN1xOx [65] oxynitrides are representatives of visible light active overall water splitting catalysts. Using N doping, the absorbed light wavelength can be increased up to 500 nm on solid solutions of GaN:ZnO (Ga1xZnxN1xOx) [66] and up to 600 nm on solid solutions of LaTaON2 and LaMg2/3Ta1/3O3 (LaMgTa1xO1+3xN23x) [64]. Other examples include LaScxTa1xO1+2xN22x [67] and CaTaO2N [68] in which La or Ta sites are replaced by Ca and Sc that alters O/N ratios due to charge compensation, which in turn results in valence band energy level shift.

Some examples of visible light active photocatalyst and their H2 and O2 evolution activity are given in Table 2. As it can be seen from the table, one-step water splitting quantum yields are quite lower when compared to those of the UVactivated photocatalysts (Table 1). The exceptions to the low activity are reported by Rh2-yCryO3 (Rh 1.0 wt%, Cr 1.5 wt%)-loaded (Ga1xZnx)(N1xOx) photocatalyst [63], multiband InGaN/GaN nanowire arrays [69], and monodisperse 4 nm graphite nanoparticle-deposited C3N4 catalysts [70].

An alternative way to cover both oxidation and reduction reactions with semiconductors that could be activated under visible light radiation is to utilize two individual photocatalysts with an electron transfer mediator to obtain two-step excitation known as the two-step water oxidation ("Z-scheme system," see Figure 2). In this system, O2 evolution photocatalysts oxidize the water molecules to O2, while the photo-generated electron is transferred to the mediator to reduce the electron acceptor (such as Fe3+ ions or IO3 ions). Then, the reduced mediator is oxidized by donating its electron to the H2 evolution photocatalyst. At the same time, the photo-generated electrons in the H2 evolution photocatalyst reduce H+ s to H2.

The semiconductors used in this two-step water splitting process should be selected based on the energy levels of their corresponding valence or conduction band maximum/minimum that would enable O2/H2O oxidation and H+ /H2 reduction. As H2 evolution and O2 evolution reactions are realized at separate photocatalysts, these semiconductors could have bandgap energy values lower than 3 eV that would enable visible light utilization such as Pt- or RuO2-loaded WO3


#### Table 2.

2.2 Co-catalysts

Table 1.

\*0.1 g of photocatalyst is used instead of 1 g.

SrTiO3:Al (200–500 nm)

Water Chemistry

Semiconductor Co-catalyst Bandgap

(eV)

H2 activity (μmol/h)

Rh2yCryO3 3.2 1372\* <sup>683</sup>\* 56 at 365 nm [45]

La2Ti2O7:Ba NiOx 3.26 5000 50 [44] SrTiO3:Al Rh2yCryO3 3.2 550\* <sup>280</sup>\* 30 at 300 nm [35]

SrTiO3:Al MoOy/RhCrOx 3.2 1800\* 900\* 69 at 365 nm [40] NaTaO3 NiO 4.0 3390 1580 20 at 270 nm [46] NaTaO3:La NiO 4.1 19,800 9700 56 at 270 nm [34] Ga2O3:Zn NiO 4.4 4100 2200 20 [47] Ga2O3:Zn Rh0.5Cr1.5O3 4.4 32,000 16,000 71 at 254 nm [41]

O2 activity (μmol/h)

AQY (%) Reference

by the co-catalysts [55, 56].

2.3 Visible light utilization

visible light utilization of the d0

180

An important addition to the light-harvesting semiconductors is H2 evolution/O2 evolution co-catalysts on the surface. The early co-catalysts that have been widely used included the noble metals and transition metal oxides such as Pt [12, 13], Rh [10], Ru [48], Au [49, 50], and NiOx [11] that mainly promote the hydrogen evolution, and CoOx [51] and Fe [52], Mn [52], RuO2 [53], and IrO2 [54] that accelerate the oxygen evolution. These metals are considered to act as charge carrier sinks that suppress electron–hole pair recombination as well as increasing the reaction kinetics by lowering the activation energy of the redox reactions. Co-catalysts are also known to inhibit photodegradation of the photocatalysts such as oxysulfides and oxynitrides by generated holes due to the effective extraction of these holes

H2 and O2 evolution activity of d0 and d<sup>10</sup> metal oxide particulate catalysts under UV light irradiation.

Following the works of noble metal co-catalysts, Domen et al. showed water splitting activity on SrTiO3 photocatalyst together with the co-catalyst NiO [57, 58], which became the choice of H2 evolution co-catalyst for many d<sup>0</sup> and d10 metal oxides such as La2Ti2O7:Ba [44], La4CaTi5O17 [59], Rb4Nb6O17 [60], NaTaO3 [46], and Ga2O3:Zn [47]. The photocatalyst stability of NiO-loaded K2La2Ti3O10 is reported to increase by addition of a second co-catalyst, Cr, using a co-

impregnation method [61]. Based on the promoting effect of Cr, a systematic study of Cr and various transition metals (such as Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt) on (Ga1-xZnx)(N1-xOx) has been conducted [62], from which core-shell structures of core Rh nanoislands and shell Cr2O3 structures (10–30 nm in size) are found to

The alterations to the semiconductors such as doping with low-valence cations, reducing particle sizes to submicron levels and obtaining a high degree of crystallinity help with the overall water splitting activity. However, activation of these photocatalysts uses a narrow portion of the solar spectrum (4%); i.e., the UV light sustains as a problem due to the large bandgap energies of these materials. To enable

water into H2 and O2 via one-step excitation, the valence band levels should be


promote H2 and O2 evolution reactions to significant levels [63].

H2 and O2 evolution activity of one-step water splitting catalysts under visible light irradiation.

Figure 2. Schematic diagram for photocatalytic water splitting using a two-step photoexcitation system.


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

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

Observed O2 evolution\* <sup>2</sup>H2<sup>O</sup> <sup>þ</sup> <sup>4</sup>h<sup>þ</sup> ! <sup>O</sup><sup>2</sup> <sup>þ</sup> <sup>4</sup>H<sup>þ</sup> 37 s\* Photosynthesis of H2O oxidation 2H2O þ 4DþTA ! O<sup>2</sup> þ 4H<sup>þ</sup> þ DTA 1.59 ms [84]

Based on 16,000 μmol O2/g/h O2 evolution rate on Rh0.5Cr1.5O3-doped Ga2O3:Zn upon illumination at 254 nm [41],

Process Timescale

Semiconductor !

h<sup>þ</sup> VB ! h<sup>þ</sup> trap

e� CB ! e� trap

CB <sup>þ</sup> <sup>O</sup><sup>2</sup> ! <sup>O</sup><sup>∙</sup>�

tr þ h<sup>þ</sup>

<sup>h</sup><sup>ν</sup> <sup>e</sup>� <sup>þ</sup> <sup>h</sup><sup>þ</sup> fs

tr ! recombination <sup>&</sup>gt;20 ns

<sup>2</sup> 10–100 μs

200 fs 50 ps

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

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.

need to be perfected to compete with the nature's intricate design.

H2 <sup>þ</sup> <sup>1</sup>=2O2 ! H2O <sup>Δ</sup>G<sup>0</sup> ¼ �237 kJ=mol (4)

efficient photocatalytic process:

Light absorption and electron and hole

On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235

Photo-generated electron and hole transfer to the

Recombination of charge carriers e�

Interfacial charge transfer e�

/g surface area and 10<sup>15</sup> sites/cm2 site density.

generation

\*

assuming 10 m<sup>2</sup>

Table 4.

therein.

183

surface and trapping

3.1 Charge recombination

#### Table 3.

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

(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 elsewhere [83].
