Photocatalytic Water-Splitting Reaction from Catalytic and Kinetic Perspectives
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Some particulate semiconductors loaded with nanoparticulate catalysts exhibit photocatalytic activity for the water-splitting reaction. The photocatalysis is distinct from the thermal catalysis because photocatalysis involves photophysical processes in particulate semiconductors. This review article presents a brief introduction to photocatalysis, followed by kinetic aspects of the photocatalytic water-splitting reaction.
KeywordsPhotocatalyst Solar energy Semiconductor Water splitting Hydrogen Hydrogen evolution reaction (HER) Oxygen evolution reaction (OER)
1 Introduction to Photocatalytic Water Splitting
Photocatalytic reactions differ from thermocatalytic reactions in many ways. Photocatalysis involves photophysical processes, which are initiated by photon absorption to generate excited states (new chemical potentials). This process is followed by photochemical or electrochemical redox reactions. These processes involve excited states with finite lifetimes, which determines the efficiency of the system and differentiates photocatalysis from conventional thermal catalytic reactions. Importantly, by utilizing excited states generated from photon energy, reactions that are energetically prohibitive under given reaction conditions (e.g., at room temperature) can be achieved in photocatalytic reactions. That is, some of the photon energy can be harvested as chemical energy as a result of the formation of photocatalytic products. This ability is the principal reason why photocatalysis has attracted growing interest in terms of solar energy conversion technology. Because the solar energy irradiating the surface of the Earth (1.3 × 105 TW) exceeds the current global human energy consumption (1.6 × 101 TW in 2010 ) by approximately four orders of magnitude, efficient photocatalytic solar energy conversion on a large scale should have a significant impact on energy and environmental issues as well as the economy, as described later.
This review article mainly focuses on the reaction kinetics involved in the photocatalytic overall water-splitting reaction. After a general introduction to photocatalytic water splitting, the timescales of the photophysical processes are discussed. Next, the importance of cocatalysts in electrocatalytic reactions is discussed. A list of photocatalysts that are able to split water into hydrogen and oxygen is provided, and literature data on electrocatalytic performance and its correlation with photocatalytic activity are presented. Some unique structures of cocatalysts that effectively suppress unfavorable side reactions, such as water formation from water-splitting products (back reaction), are discussed. The effects of coloading hydrogen evolution catalysts and oxygen evolution catalysts are then described. Furthermore, the effects of light intensity, hydrogen/deuterium isotopes, and reaction temperature (thermal activation energy) on the rates of the photocatalytic water-splitting reaction are reviewed to understand kinetic aspects that are unique to photocatalysis. Finally, the review concludes with some future perspectives.
2 Definition of Photocatalytic Efficiency
Unlike thermal catalytic reactions, photocatalytic rates are not reported per photocatalyst mass used unless the goal is to optimize the performance of a specific photocatalytic reactor. The photocatalytic rates are not proportional to the photocatalyst mass because light absorption reaches saturation at some point. AQY should accordingly be measured when the amount of photocatalyst is sufficient and the incident light is effectively absorbed by the photocatalysts. If the photocatalytic rates increase with an increasing amount of photocatalyst, the measured rates are not a measure of photonic efficiency but simply a reactor-specific reflection of the amount of photocatalyst. Photocatalytic activity, including AQY and the effectiveness of charge separation, must be compared based on the absorbed photons, which must not depend on the amount of photocatalyst present in the photoreactor.
3 Energy Diagram
4 Timescale of Photocatalysis
The dynamics of photoexcited carriers in nanoparticulate TiO2 [10, 11, 12, 13] and CdS [14, 15] have been studied in detail using transient absorption spectroscopy. In the case of TiO2, surface-trapped electrons and holes are generated within 200 fs after photoexcitation . Surface-trapped and bulk electrons equilibrate and relax into deep trap sites with a time constant of a few hundred picoseconds . Photoexcited electrons react with gaseous O2 within 10–100 μs , whereas surface-trapped holes react with methanol, ethanol, and 2-propanol within 300, 1000, and 3000 ps, respectively . It is necessary to load a cocatalyst on a photocatalyst to enhance the H2 evolution reaction (HER). Although the timescale of electron transfer from TiO2 particles to cocatalysts is unknown, electrons reduce water in 10–900 μs on platinized TiO2 , indicating that the electron transfer occurs within this timescale. For comparison, photoexcited electrons in an NaTaO3 photocatalyst migrate to an NiO cocatalyst within 1 μs after excitation . In contrast, photoexcited holes in TiO2 can oxidize water on the timescale of microseconds to seconds [11, 16]. Thus, the surface redox reactions of photocatalytic water splitting take microseconds or longer. Bulk processes, such as light absorption by photocatalysts and charge migration to surface active sites, proceed faster than surface redox reactions. Under weak light irradiation, wherein only a single electron–hole pair is generated in a TiO2 particle, recombination of photoexcited electrons and holes occurs on the microsecond timescale in the absence of effective electron and hole scavengers . It has also been reported that more than 90 % of photoexcited carriers are recombined in 10 ns . Although the rate of charge recombination depends strongly on the physical properties of a material and excitation density, charge recombination clearly competes with the water-splitting reaction and restricts the quantum efficiency for photocatalytic overall water splitting to low values. Photocatalytic reactions proceed efficiently in the presence of appropriate electron or hole scavengers  because such additives rapidly consume the respective photoexcited carrier and effectively prevent charge recombination.
5 Electrocatalytic Hydrogen and Oxygen Evolution Reactions
DFT calculations revealed that there is a constant difference between the adsorption energies of HO* and HOO* regardless of the binding energy of O* . The variation in the overpotential is closely related to the O* adsorption energy, for which Eq. (7) or (8) represents the potential-determining step. Theoretical calculations give the following activity order for the binary oxides considered: Co3O4 ≈ RuO2 > PtO2-rutile phase ≈ RhO2 > IrO2 ≈ PtO2 β-phase (CaCl2) ≈ MnxOy ≈ NiO ≈ RuO2. This trend corresponds well with the experimental findings of Matsumoto and Sato for alkaline conditions . The investigation of photocatalytic and photoelectrochemical methods for oxygen evolution utilizes the oxide forms of Co , Ru , Mn [9, 31], and Ir [31, 32]. The theoretical analysis predicted the following ordering of catalytic activities for the following perovskite-type oxides: SrCoO3 > LaNiO3 > SrNiO3 > SrFeO3 > LaCoO3 > LaFeO3 > LaMnO3. This trend corresponds well with experimental findings by Bockris et al. and Matsumoto et al. [29, 33] under alkaline conditions. More recently, some double perovskite-type oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3–δ  and (Ln0.5Ba0.5)CoO3−δ (Ln=Pr, Sm, Gd and Ho) , were reported as highly active catalysts for oxygen evolution in alkaline conditions, the latter being more active and robust during the reaction. The intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of surface transition metal cations in an oxide. It was concluded that a near-unity occupancy of the eg orbital of surface transition metal ions and high covalency in bonding to oxygen led to the peak OER activity . However, the above two descriptors inevitably suffer from ambiguities when the central ions can have multiple crystal fields, oxidation states, and/or spin states. Subsequently, the computed O p-band center relative to the Fermi level and the derived parameters were suggested as descriptors to screen the OER activity and stability of oxides . Moving the computed O p-band center closer to the Fermi level can increase the OER activity, but the oxide stability during OER is decreased if the computed O p-band center is sufficiently close to the Fermi level. The O p-band of Ba0.5Sr0.5Co0.8Fe0.2O3–δ was overly close to the Fermi level, causing the amorphization of the material during OER. (Pr0.5Ba0.5)CoO3 produced the best activity and durability among the double perovskite-type cobalt oxides examined. The above findings will facilitate the development of efficient cocatalysts for oxygen evolution in photocatalytic systems for water splitting. The redox properties of the cocatalysts may endow the intermediate states with extended lifetimes, enhancing the charge separation.
6 History of Photocatalytic Overall Water Splitting
6.1 UV-Responsive Photocatalysts
A number of studies have found a series of transition metal oxides with a d0 electronic configuration and typical metal oxides with a d10 electronic configuration that are active for photocatalytic water splitting under UV light illumination. TiO2  and SrTiO3 [37, 38] modified with cocatalysts were reported in 1980 as the first reliable materials with photocatalytic overall water-splitting activity. Loading appropriate catalysts is often essential to achieve the overall water-splitting reaction at appreciable rates. For example, the photocatalytic activity of unmodified SrTiO3 for overall water splitting is negligible because of a lack of hydrogen evolution sites . In fact, most oxide photocatalysts exhibit an n-type character, readily accumulating excited holes on their surfaces, and the structure of the oxide surface typically has high OER activity. In this case, the generated excited electrons prefer to stay in the bulk of the semiconductor; as a result, metal nanoparticles can often effectively transport such electrons to the surface by guiding them along the metal–semiconductor interface . Additionally, the oxide surface lacks HER activity. Excellent HER catalysts, such as Pt, also function as good hydrogen evolution sites for photocatalysts; however, in reality, this approach is not effective for photocatalytic overall water splitting because Pt catalyzes the formation of water from hydrogen and oxygen mixtures, even without illumination. In the earliest studies [36, 37], water vapor was used as a reactant to wet the cocatalyst surface and slow the back reactions . Other successful overall water-splitting reactions have used cocatalysts that were less active for water formation, such as NiO .
NaTaO3 doped with La  and Ga2O3 doped with Zn  exhibit the highest known QY water-splitting rates under UV irradiation after successful catalyst loading with NiO and Rh2−yCryO3, respectively. Both NiO and Rh2−yCryO3 cocatalysts improved hydrogen evolution activity, which is essential for achieving overall water splitting. In addition, transient absorption spectroscopy revealed that photoexcited electrons in the conduction band were quenched by the loading of the hydrogen evolution cocatalyst , indicating the successful charge separation of photoexcited electrons and holes by introducing cocatalysts. Similar results were reported for TiO2 modified with Pt . Therefore, cocatalysts can facilitate both charge separation and surface kinetics, especially if an ohmic contact is formed at the photocatalyst-cocatalyst interface to facilitate the flow of electrons into the cocatalyst. Otherwise, the cocatalyst would also collect photoexcited holes and function as a recombination center.
6.2 (Ga1−xZnx)(N1−xOx) Photocatalyst
Certain oxynitride photocatalysts can reproducibly achieve overall water splitting under visible light after modification with cocatalysts, such as Rh2−yCryO3. For example, (Ga1−xZnx)(N1−xOx) and (Zn1+xGe)(N2Ox) loaded with appropriate hydrogen evolution cocatalysts can split water . In particular, (Ga1−xZnx)(N1−xOx) modified with Rh2−yCryO3 has shown the highest AQY to date for overall water splitting using a single photocatalyst under visible light (5.1 % at 410 nm) . For cocatalyst loading, the presence of both Rh and Cr species is essential, with efficiency typically peaking at 1 wt% Rh and 1.5 wt% Cr2O3 . The Rh2−yCryO3 cocatalyst, a mixed oxide of corundum-type Rh2O3 and Cr2O3, was typically 10–30 nm in size, although the composition varied among the particles . An improvement in photocatalytic activity was found by coloading Cr species regardless of the type of metal, suggesting that Cr addition provides some general functionality for overall water splitting.
Photocatalytic activities of (Ga1−xZnx)(N1−xOx) in the presence of sacrificial reagents
10 vol % CH3OH aq
10 vol % CH3OH aq
10 vol % CH3OH aq
10 vol % CH3OH aq
10 mM AgNO3 aq
10 mM AgNO3 aq
10 mM AgNO3 aq
6.3 TaON-Based Photocatalyst
Recently, ZrO2-modified TaON (ZrO2/TaON) was also reported to be active for overall water splitting when coloaded with cocatalysts for both hydrogen and oxygen evolution. This was the first report of overall water splitting by a transition metal oxynitride . TaON is known to generate hydrogen and oxygen under visible light illumination in the presence of methanol and silver cations, respectively. TaON exhibited an acceptable AQY for the sacrificial OER but not the sacrificial HER, even with cocatalyst modifications. In addition, TaON generated only a small amount of hydrogen and no oxygen when it was applied to overall water splitting. These results suggest that the photoexcited electrons did not migrate to cocatalysts effectively because of a high defect density in TaON and/or because photoexcited holes were consumed by the self-oxidation of TaON rather than water oxidation. Therefore, it was necessary to improve the TaON synthesis conditions and the cocatalyst loading methods.
Subsequently, the coloading of a core/shell-type hydrogen evolution cocatalyst and an oxygen evolution cocatalyst was found to enable overall water splitting using ZrO2/TaON . ZrO2/TaON was modified with a RuOx/Cr2O3 core/shell-type hydrogen evolution cocatalyst and then with IrO2 as an oxygen evolution cocatalyst. ZrO2/TaON modified with RuOx/Cr2O3 exhibited some activity for overall water splitting under UV illumination, although the gas evolution rates decreased over time because of the deactivation of the photocatalyst. When IrO2 was coloaded as an oxygen evolution cocatalyst on ZrO2/TaON with RuOx/Cr2O3, overall water splitting proceeded steadily. By optimizing the preparation conditions for the photocatalyst/cocatalyst composite, overall water splitting was achieved, even under visible light irradiation. Coloading with RuOx and IrO2 did not lead to oxygen evolution. These results highlight the importance of activation and stabilization of the photocatalyst by the coloading of hydrogen and oxygen evolution cocatalysts and the suppression of side reactions by the ultrathin chromia layer.
6.4 Doped SrTiO3 Photocatalysts
Overall water splitting was also achieved under visible light using rhodium- and antimony-codoped SrTiO3 (SrTiO3:Rh,Sb) loaded with IrO2, RuO2, or Ru as cocatalysts . Among the three cocatalysts, IrO2 produced the highest activity for overall water splitting. In this photocatalyst, donor levels consisting of trivalent Rh species are excited under visible light. Electrons and holes are generated in the conduction band composed by Ti 3d orbitals and the impurity levels consisting of Rh species. Codoping with Sb5+ ions stabilizes the donor level formed by Rh3+ and enables oxygen evolution on Rh-doped SrTiO3 in the presence of sacrificial reagents. The reactivity of photoexcited holes in impurity levels for the oxidation of water is not typically high because of the low mobility and short lifetimes of the holes. Loading of IrO2 improved the oxygen evolution activity of SrTiO3:Rh,Sb, as confirmed by the sacrificial OER. Interestingly, the loading of IrO2 also improved the photocatalytic activity of the sacrificial HER. Thus, IrO2, a well-known cocatalyst for O2 evolution, enhanced both the hydrogen and oxygen evolution in water splitting using SrTiO3:Rh,Sb. It was suggested that that partially reduced IrO2 worked as an active site for H2 evolution.
7 Kinetic Aspects
7.1 Light Intensity and Cocatalyst Loading Amounts
Correlation of photocatalyst diameter with the number of photons that strike a cross-section of a spherical photocatalyst particle per unit time and the time interval between photons hitting the particle 
Diameter of the spherical photocatalyst/nm
Cross-section of the spherical photocatalyst/cm−2
Number of photons that strike a single photocatalyst particle/s−1
Time interval between photons striking a single photocatalyst particle/μs
2.0 × 10−11
1.8 × 106
5.6 × 10−1
7.9 × 10−11
7.1 × 106
1.4 × 10−1
2.0 × 10−9
1.8 × 108
5.6 × 10−3
7.9 × 10−9
7.1 × 108
1.4 × 10−3
2.0 × 10−7
1.8 × 1010
5.6 × 10−5
The rate of a photocatalytic reaction increases with increasing excitation intensity, although not necessarily in a proportional manner. Some reaction models suggest that the reaction order for light intensity decreases from unity to one half as the light intensity increases . This decrease occurs because the recombination of photoexcited carriers is second-order with respect to carrier concentrations (proportional to both electron and hole concentrations). In contrast, under low light intensities, at which the concentration of photoexcited carriers is negligible with respect to the intrinsic majority carrier concentration, it is reasonable to assume that only the minority carrier concentration depends on the excitation intensity, whereas the majority carrier concentration is constant. As a result, the recombination reaction is approximated as a quasi-first-order reaction with respect to the minority carrier concentration generated by photoexcitation, and the photocatalytic reaction rate becomes proportional to the light intensity. Accordingly, the reaction order for light intensity can be an indirect measure of how many photoexcited carriers exist in photocatalyst particles under illumination.
The activities of most photocatalysts follow the aforementioned light intensity dependency; that is, the reaction order changes from unity to one half as the light intensity increases. This statement means that the AQY decreases monotonically with increasing light intensity. However, in some cases, the AQY of photocatalytic water splitting increases with increasing light intensity under weak excitation conditions. To achieve a high quantum efficiency, it is likely necessary to saturate certain trap states with photoexcited carriers by generating photoexcited carriers at a higher rate than the charge recombination mediated by the trap states to endow the photoexcited carriers with high mobility.
7.2 Hydrogen–Deuterium Isotope Effect
The hydrogen–deuterium (H–D) isotope effect results from reaction processes involving hydrogen (deuterium) atoms at the interface of a photocatalyst and a reaction solution and is expected to reflect the reaction mechanism of the rate-determining step. However, when processes occurring inside a photocatalyst particle have a significant influence on the overall reaction rate, the H–D isotope effect should be small because hydrogen atoms are not involved in the rate-determining step.
H–D isotope effect and apparent activation energy for photocatalytic water splitting using Rh2−yCryO3/(Ga1−xZnx)(N1−xOx) in the presence of sacrificial reagents 
Effect of reaction conditions on the H–D isotope effect and apparent activation energy for photocatalytic water splitting using Rh2−yCryO3/Ga2O3:Zn 
Water-splitting rate/mmol h−1
Rh 0.5–Cr 0.75
Rh 0.5–Cr 0.75
Rh 0.05–Cr 0.075
Rh 0.05–Cr 0.075
7.3 Activation Energy
The effect of reaction temperature on the photocatalytic activity for water splitting was investigated using (Ga1−xZnx)(N1−xOx) and Ga2O3:Zn modified with various cocatalysts. It is natural to expect that the apparent activation energy of photocatalytic water splitting reflects the activation energy of the slowest reaction step and that reaction processes involving the cleavage and formation of chemical bonds have higher activation energies than physical processes, such as charge migration.
The apparent activation energy for photocatalytic water splitting using Rh2−yCryO3/(Ga1−xZnx)(N1−xOx) and Rh2−yCryO3/Ga2O3:Zn was 8 kJ mol−1 from 298 to 323 K [59, 60]. This apparent activation energy is significantly lower than that for the electrochemical HER on Rh electrodes (31 kJ mol−1) . Moreover, the apparent activation energies for D2O splitting, water splitting in the presence of sacrificial reagents, and gas-phase water vapor splitting were highly similar when Rh2-yCryO3/(Ga1-xZnx)(N1-xOx) was used [59, 62]. This observation indicates that the apparent activation energy is associated with processes occurring inside the photocatalyst rather than the formation and dissociation of chemical bonds involving hydrogen or the diffusion, adsorption, and desorption of chemical species. This association likely occurs because only electrons that have successfully escaped recombination with photoexcited holes can contribute to the photocatalytic HER on the surface, whereas the electrons needed to drive hydrogen evolution can be supplied immediately, depending on the potential of the electrode in the electrochemical HER.
The apparent activation energy depends on the type of cocatalyst. For example, loading Ni instead of Rh2−yCryO3 on Ga2O3:Zn increased the apparent activation energy from 8 to 15 kJ mol−1 while lowering the water-splitting rate at room temperature to 40 % . This result suggests that reaction processes involving both the photocatalyst and cocatalysts, such as electron transfer from the photocatalyst to the cocatalyst, contributed to the apparent activation energy. In fact, surface-enhanced infrared spectroscopy under potential control revealed that there was a potential barrier for electron migration at the interface between an n-type GaN single crystal and deposited Pt particles . The slightly higher activation energy of Ni could also be associated with surface electrochemical reactions, considering the relatively low electrochemical activity of Ni. The apparent activation energy for HER on Ni was reported to be 56 kJ mol−1 . However, the apparent activation energy is often considerably lower for photocatalytic reactions than for electrochemical HER using corresponding electrodes because bulk processes have a dominant effect on the apparent activation energy.
When (Ga1−xZnx)(N1−xOx) was modified with RuO2 instead of Rh2−yCryO3, the apparent activation energy decreased from 8 to 0 kJ mol−1 and the water-splitting rate decreased to 40 % . This change in the water-splitting rate cannot be explained by the apparent activation energy alone. Side reactions, such as oxygen reduction, compete with hydrogen evolution on RuO2, whereas Rh2−yCryO3 is selectively active for hydrogen evolution . As a result, the water-splitting rate on RuO2/(Ga1−xZnx)(N1−xOx) decreases drastically in the presence of oxygen. Such competing reactions could significantly complicate the kinetics of photocatalysis.
8 Concluding Remarks
Some examples of powder photocatalysts for successful overall water splitting and their reaction kinetics are overviewed. The photocatalytic reactions using a particulate semiconductor are distinct from thermocatalytic reactions, as the former involves photophysical processes inside semiconductors, which regulates how many charge carriers are available in surface electrochemical redox reactions. Consequently, the kinetic parameters of photocatalytic water splitting, such as the H–D isotope effect and apparent activation energy, can be significantly lower than those for electrochemical water splitting and thermocatalytic reactions. Considering that AQYs lower than 10 % have been reported for water-splitting reactions in visible light regions, a major challenge lies in the efficiency and selectivity of the separation of photoexcited charge carriers generated in visible-light-driven photocatalysts and their transfer to cocatalysts that work as active sites for surface redox reactions. Semiconductor photocatalysts for overall water splitting should be highly crystalline to prevent photoexcited charge carriers from becoming trapped and recombining at defective sites. At the same time, the dimension of photocatalyst particles must be chosen based on the diffusion length of minority carriers so that they can reach the surface active sites before recombination. Thus, it is necessary to balance the crystallinity and dimension of photocatalytic materials with the visible light response. The weight or surface area of a photocatalytic material is not a primary concern unless the photocatalytic reactions involve the cleavage of metal cations and organic pollutants at low concentrations. In such a case, where the rate is proportional to the reactant concentration, the photocatalytic reaction may be regulated by the adsorption of the reactants, and thus, high photonic efficiency cannot be expected.
Cocatalysts loaded on a photocatalyst play key roles in not only charge separation but also electrocatalytic functions. A correlation may be found between the electrocatalytic performances of cocatalyst components and the water-splitting activity of photocatalysts modified with the cocatalysts when sufficient photoexcited carriers are supplied for the surface redox reactions (i.e., the reaction is not limited by the physical process). The loading amount, dispersivity, and size of cocatalyst particles have considerable influence on the photocatalytic activity, suggesting that in addition to the electrocatalytic activity of surface redox reactions, electronic interaction at the cocatalyst-semiconductor interface is crucial. It is expected that the flow of charge carriers can be rectified by the use of Schottky-type junctions. Alternatively, the potential barrier for charge migration from a photocatalyst to a cocatalyst may be minimized by creating ohmic contact at the interface. Additional loading of catalytic components may lead to the development of efficient cocatalysts in which charge separation and charge injection are functionally separated. Coloading of well-designed hydrogen evolution cocatalysts and oxygen evolution catalysts could enhance the charge separation and durability of photocatalytic materials under operation conditions. It is also important to control the selectivity of redox reactions caused by photoexcited charge carriers. The coating of hydrogen evolution catalysts with an ultrathin hydrated chromia layer has been found to effectively improve the reaction selectivity of photoexcited electrons toward the hydrogen evolution reaction because this layer can be penetrated by protons and hydrogen molecules but not by oxygen molecules and certain other electron acceptors. The reaction selectivity could be a substantial problem when the reaction is carried out using water with impurities.
Photocatalytic water splitting under sunlight could contribute to a sustainable society. However, drastic improvements in solar energy conversion efficiencies are still needed. It has been suggested that the solar energy conversion efficiency by photocatalytic water splitting should be 5 % or higher. Photocatalysts should be active for water splitting under irradiation up to 600 nm or even longer wavelengths to achieve a sufficient solar energy conversion efficiency at a reasonable quantum efficiency. This goal requires the development of high-quality semiconductors that are active even under red and deep-red irradiation. To meet this challenge, it is important to understand the kinetic aspects of photocatalytic water splitting and the functionality of cocatalysts.
This work was financially supported by Grant-in-Aids for Specially Promoted Research (No. 23000009) and for Young Scientists (B) (No. 25810112) of the Japan Society for the Promotion of Science (JSPS) and King Abdullah University of Science and Technology. Further financial support came from the International Exchange Program of the A3 Foresight Program of JSPS and Companhia Brasileira de Metalurgia e Mineração (CBMM).
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