Mechanisms of Photocatalytic Molecular Hydrogen and Molecular

Research Article Cite This: ACS Catal. 2018, 8, 2313−2325

pubs.acs.org/acscatalysis

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity Irina Ivanova,† Tarek A. Kandiel,*,‡,§ Young-Jin Cho,∥ Wonyong Choi,∥ and Detlef Bahnemann*,†,⊥ †

Institut fürTechnischeChemie, Leibniz University Hannover, Callinstr. 3, D-30167 Hannover, Germany Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt § Department of Chemistry, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia ∥ Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea ⊥ Laboratory for “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Ulyanovskaya str. 1, Peterhof, Saint-Petersburg, 198504 Russia ‡

S Supporting Information *

ABSTRACT: A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. In the presence of methanol, when the dark injection of an electron into the conduction band of the photocatalyst is possible, the catalytic roles of the investigated cocatalysts toward the formation of H2 gas were found to be decisive, in addition to the charge separation and interfacial electron transfer processes. No overall water splitting into H2 and O2 can be achieved utilizing La-doped NaTaO3 loaded with only one cocatalyst; however, it was found that the loading of La-doped NaTaO3 with two different cocatalysts, i.e. RuO2 and CoO, enables the simultaneous formation of H2 and O2 from pure water. The modification of photocatalyst with two different cocatalysts seems to be essential for enhancing the efficiency of overall photocatalytic splitting. The interfacial electron transfer on the cocatalyst-loaded La-doped NaTaO3 was determined by measuring the cathodic and anodic photocurrents in the presence of Fe2+/Fe3+ electron shuttle. Methanol and bromate were used as electron donors and electron acceptors during the cathodic and anodic photocurrent measurements, respectively. By correlation of the photocurrent with the activity of the investigated cocatalysts, it was concluded that the creation of active sites and subsequent decrease in activation energy for H2 evolution is the main requirement for efficient H2 evolution from the aqueous methanol system, whereas the interaction with the photogenerated holes and the formation of intermediates allowing a multielectron transfer process seems to be an essential step for the water oxidation and O2 evolution. This information appears to be crucial for a rational design of a highly active photocatalyst for overall water splitting under UV−vis illumination. KEYWORDS: sacrificial systems, photocatalysis, cocatalysts, water splitting, La-doped NaTaO3, hydrogen evolution

1. INTRODUCTION

semiconductor photocatalysts and their band gap energies. Moreover, their crystal structure, crystallinity, and particle size play an important role as well.2 Loading of noble metals or metal oxides such as Pt, NiO, and RuO2 is well-known to enhance the photocatalytic activity due

In order to solve the global energy and environmental issues, many technologies have been addressed for the development of highly efficient systems for solar energy conversion and storage. One of them is the photocatalytic conversion of solar energy into the hydrogen fuel via the water splitting process, employing metal oxide semiconductors as photocatalysts.1 The activity of this process is strongly affected by the position of the valence band and the conduction band of the © XXXX American Chemical Society

Received: December 16, 2017 Revised: January 24, 2018

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DOI: 10.1021/acscatal.7b04326 ACS Catal. 2018, 8, 2313−2325

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overall water splitting and in water oxidation and reduction from the sacrificial systems. Especially, no interference of the surrounding light during investigation of their photocatalytic activity has to be taken into account, as in the case of TiO2 photocatalyst. The present work thus addresses the effect of various cocatalysts, including noble metals (Pt, Au, and Rh) and metal oxides (NiO, CuO, CoO, Ag2O, and RuO 2), on the photocatalytic activity of La-doped NaTaO3 toward the overall water-splitting reaction under UV−vis irradiation. The work is further extended to include the role of the aforementioned cocatalysts in H2 and/or O2 evolution from sacrificial systems: i.e., aqueous methanol and silver nitrate solutions. The mechanisms of H2 and/or O2 evolution and the role of each cocatalyst are discussed in detail, and the results have been supported by measuring the efficiency of interfacial electron transfer and/or by analyzing the formed intermediates.

to their ability to reduce the recombination rate of photogenerated charge carriers and of enabling multielectron charge transfer processes.3,4 In the case of the photocatalytic water splitting these loaded materials act as so-called cocatalysts and appear to be essential for the creation of active sites for molecular hydrogen (H 2) and molecular oxygen (O2) evolution. Photocatalytic water splitting into H2 and O2 in the correct stoichiometric ratio employing pure water is still a challenging reaction, however. Sacrificial agents are therefore often employed to evaluate whether a certain photocatalyst satisfies the thermodynamic and kinetic requirements for H2 and/or O2 evolution: i.e., the so-called half-reactions of water splitting. In a photocatalytic system both oxidative and reductive reactions are thought to occur on the same particle and can be described according to the eqs 1−3: SCparticle + 2hν → 2e−CB + 2h+VB

(1)

2e−CB + 2H+aq → H 2(↑)

(2)

2h+VB + H 2O →

0 V vs NHE at pH 0

2. EXPERIMENTAL SECTION 2.1. Materials. Lanthanum-doped NaTaO3 (0.83 wt % of La) powder has been provided by H.C. Starck Company (Goslar, Germany). This material was synthesized by a conventional solid-state reaction described elsewhere.22 Briefly, the starting materials, i.e., wet Ta(OH)5, Na2CO3, and La2(CO3)3·5H2O, were mixed and then dried at 105 °C for 16 h. Afterward, the dry educts were ground and subsequently calcined at 800 °C for 3 h. Finally, the powder was thoroughly ground again. CoO, NiO, CuO, and RuO2 metal oxide cocatalysts were prepared employing aqueous solutions of Co(NO3)2·6H2O (≥98%), Ni(NO3)2·6H2O (≥98.5%), Cu(NO3)2·3H2O (99− 104%), and RuCl3 (99.98%), respectively, while for the preparation of Rh, Pt, and Au noble-metal cocatalysts aqueous solutions of (NH4)3RhCl6 (99.99%), H2PtCl6·6H2O (≥37.50% Pt basis), and HAuCl4·3H2O (≥49.00% Au basis) were respectively used. For enhanced H2 evolution aqueous solutions of methanol (≥99.8%), ethanol, 2-propanol (≥99.7%), butanol (≥99.4%), and 2-methyl-2-propanol (≥99.7%) have respectively been employed, while for enhanced O2 evolution, an aqueous solution of AgNO3 (≥99.0%) has been used. For the determination of H2O2 formation recrystallized 4-hydroxyphenylacetic acid (98%), peroxidase from horseradish (150 U mg−1), H2O2 (30 wt %), and Tris buffer pH 8.8 (prepared from tris(hydroxymethyl)aminomethane (≥99.9%) and HCl (37%)) were employed. All chemicals were purchased from SigmaAldrich and were used as received without further purification. All aqueous solutions were prepared with deionized water from a SARTORIUS ARIUM 611 apparatus (resistivity 18.2 MΩ cm). 2.2. Loading Methods of Cocatalysts. Noble metals were deposited on the surface of the photocatalyst via an in situ photoreduction reaction. Metal loadings of 0.2 or 1 wt % (weight percent of the pure metal) were applied by the addition of appropriate volumes of the aqueous solution containing the desired metal ions to the suspension before the photocatalytic test reaction. The metal oxide cocatalysts were loaded by an impregnation method. The photocatalyst was thoroughly mixed with a precursor species (metal salt solution) of a proper concentration followed by thermal treatment at a given temperature to form a desired cocatalyst. The photocatalysts loaded with different metal salts were then calcined at 266 °C for 1 h in air

1 O2 (↑) + 2H+aq 2

1.23 V vs NHE at pH 0

(3)

Thermodynamically, the overall water splitting reaction is an uphill reaction that is associated with a large positive Gibbs free energy change (eq 4): H 2O + 2hν →

1 O2 (↑) + H 2(↑) 2

G°298 = 237 kJ mol−1 (4)

The photocatalytic H2 production from aqueous alcoholic solutions has been studied extensively.5−13 It is widely agreed that alcohols act as efficient hole scavengers (i.e., electron donors) and undergo an irreversible oxidation on the nanosecond time scale, resulting in a significant enhancement of the photocatalytic H2 production.5,14,15 Recently, the use of alcohols as sacrificial reagents has gained even more interest, because they have been used as model compounds for the photocatalytic degradation of organic pollutants with simultaneous H2 production.16 On the other hand, well-known oxidizing agents such as Ag+ and Fe3+ operate as sacrificial electron scavengers. These oxidizing reagents consume the photogenerated conduction band electrons and thus increase the activity for O2 evolution.17 However, the fact that a photocatalyst is active for both halfreactions (eqs 2 and 3) does not guarantee its photocatalytic activity for the overall water splitting in the absence of sacrificial reagents (eq 4). Therefore, a better understanding of the mechanisms of these half-reactions of water splitting is essential to construct new promising photocatalysts for the overall water splitting in artificial systems. Tantalum-based oxides have shown high activity and stability for the water-splitting process.18−21 Especially, porous sodium tantalate (NaTaO3) doped with lanthanum has demonstrated a high quantum yield of 56% at 270 nm for water splitting after modification with NiO cocatalyst.22 Because of the large band gap of these materials (4 eV), they do not represent any interest from a technical point of view. However, because of their excellent stability and high photocatalytic activity, they are considered to be some of the best candidates to act as model photocatalysts for understanding the role of the cocatalysts in 2314


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2.4. Detection of Photocatalytically Formed H2O2. The photocatalytic formation of H2O2 was determined by a method involving dimerization of p-hydroxyphenylacetic acid in the presence of H2O2 and horseradish peroxidase, yielding the detectible fluorescent product 5,50-dicarboxymethyl-2,2-dihydroxybiphenyl (λex 312 nm, λem 406 nm) according to a method described elsewhere.23,24 For this purpose, aliquots (2 mL) of irradiated La-doped NaTaO3 suspensions were filtered through 0.45 μm filters (Machery-Nagel, Germany) and subsequently added to 0.1 mL of p-hydroxyphenylacetic acid reagent (8 mg of recrystallized p-hydroxyphenylacetic acid, 2 mg of peroxidase, and 50 mL of 0.1 M pH 8.8 Tris buffer). The calibration was performed by standard additions of H2O2 to the mixture and was found to be linear in the concentration ranges of interest. 2.5. Determination of Quantum Yields φ. The photocatalytic activity of pure and modified La-doped NaTaO3 was assessed by the determination of the quantum yield, φ, of H2 and O2 evolution from pure water, aqueous methanol, and silver nitrate solutions by applying previously reported equations (eqs 5−8).25 Accordingly, the quantum yield φ is defined as

as reported elsewhere to form the corresponding metal oxide cocatalysts.22 2.3. Photocatalytic H2 and/or O2 Evolution Tests. The photocatalytic reactions were carried out in a 100 cm3 doublejacketed quartz glass photoreactor that was connected to a quadrupole mass spectrometer (QMS) for gas analysis (Hidden HPR-20). The system was continuously purged with argon as carrier gas; the Ar flow was controlled by a mass flow controller (MFC). This experimental setup, shown in Figure 1, allows an

φ = r /Ia Figure 1. Experimental setup used for photocatalytic H2 and O2 production measurements.

(5)

where Ia = I0Fs

in-line monitoring of the entire course of the reaction with the advantage of simultaneously detecting several gaseous compounds formed during the photocatalytic reaction. In a typical experimental run, 50 mg of the photocatalyst was suspended in 50 mL of pure water, an aqueous alcoholic solution (10 vol %), or an aqueous solution of silver nitrate (0.01 M) by sonication. The initial pH value was in the range of 3.4−4, depending on the nature of the investigated photocatalyst and the system. The suspension was then transferred into the photoreactor and purged with Ar for 30 min to remove dissolved O2. Afterward, the reactor was connected to the mass flow controller and the QMS through metal flanges. To remove the air in the headspace of the reactor, an Ar gas stream with a flow rate of 50 cm3 min−1 was continuously passed through the reactor before irradiation, until no oxygen or nitrogen was detected by the QMS. The inlet flow rate/gas consumption by the QMS was 1 cm3 min−1, and the excess gas was directed toward the exhaust. The sampling rate of the QMS was in the millisecond time range, thus allowing a fast tracking of the reaction. The Ar gas flow rate was subsequently turned down and kept constant at 10 cm3 min−1 during the entire reaction. Before the irradiation was started, the system was left for a further 40 min in the dark to ensure the stabilization of the system background and to monitor the baseline. Afterward, the reactor was irradiated for 5−6 h from the outside using an Osram XBO 1000 W Xe arc lamp in a Müller LAX 1000 lamp housing. During the irradiation time the photocatalytically formed gases were monitored continuously in time intervals of about 30 s. After 4−5 h of irradiation time the lamp was switched off, allowing the system to reach the baseline again. The temperature of the reaction solution was kept constant at 25 °C during the entire experiment by employing a thermostatic bath (Julabo Co.). For the quantitative analysis of H2 and O2 the QMS was calibrated employing standard dilutions of these gases (a mixture of 2% of H2 and 1% of O2 in Ar from Linde Gas Co., Germany).

(6)

r is the photocatalytic H2 or O2 evolution rate. Ia and I0 are the absorbed and incident photon fluxes, respectively. I0 was determined by integration of irradiance of the lamp in the range 250−400 nm. Fs is the integrated absorption fraction of the semiconductor particles over the range from λ1 250 nm to λ2 400 nm: λ

Fs =

∫λ 2 IλTλf f λs dλ 1

λ

∫λ 2 IλTλf dλ

(7)

1

Iλ is the irradiance in the wavelength range dλ, and Tfλ is the transmittance of the media between the light source and the reaction suspension. Since no filter was used and the reactor was made from quartz, Tfλ was assumed to be unity. fsλ is the fraction of light absorbed at the wavelength λ and can be calculated according to s

f λs = 1 − Tλs = 1 − 10−Aλ

Tsλ

(8)

Asλ

where and are the transmittance and absorbance, respectively, of the sample at the wavelength λ. The absorbance Asλ of suspended La-doped NaTaO3 particles in the wavelength range from 250 to 400 nm was measured by means of an UV− vis spectrophotometer (Cary 1000) equipped with an integrating sphere. Figure 2 (left axis) shows the extinction coefficient obtained from the linear regression of the absorbance of the La-doped NaTaO3 suspensions versus concentration plot at different wavelengths (see Figure S1). The irradiance Iλ of the 1000 W Xe arc lamp used for the photocatalytic tests was measured by a radiometer (BW Tek Company), and the obtained spectrum is also shown in Figure 2 (right axis). 2.6. Photocurrent Measurements. Photocurrent measurements were carried out in a conventional three-electrode system connected to a potentiostat (Gamry, Reference 600). A 2315


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Figure 2. Extinction coefficient of suspended La-doped NaTaO3 particles in water (left) and irradiance of a 1000 W Xe arc lamp (right) in the range of 250−400 nm.

coiled Pt wire, a graphite rod, and a Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively. The photocurrent was collected on the inert collector electrode (Pt) immersed in an aqueous suspension of photocatalyst under UV light irradiation (λ >250 nm). The reversible redox couple of Fe3+/Fe2+ was used as an electron shuttle that carries the electron from the photocatalyst particles to the Pt electrode. The pH of the suspension was adjusted to pH 1.8, and the electrolyte solution was continuously purged with Ar during the measurement; a potential bias of +0.7 V (vs Ag/AgCl) was applied to the Pt working electrode. Figure 3. (a) Time course of photocatalytic H2 evolution from pure water employing La-doped NaTaO3 (0.83 wt % La), shown for the sample modified with RuO2 cocatalyst as an example. (b) H2 evolution rates and the corresponding quantum yields for the cocatalysts used in this system. Conditions: catalyst concentration 1 g L−1, 0.2 wt % cocatalyst loadings employed in each photocatalytic test, reactor volume 50 mL, irradiation intensity I250−400 = 41 mW cm−2.

3. RESULTS 3.1. Characterization Results. The surface areas of NaTaO3 and La-doped NaTaO3 powders were calculated from the N2 adsorption/desorption measurements using the BET method. The results indicated that NaTO3 exhibits a BET surface area of 2.4 m2 g−1, whereas the La-doped NaTaO3 has a BET surface area of 5.4 m2 g−1. The increase in the BET surface area as a result of La doping is in good agreement with the observed decrease in the particle size of NaTaO3 after La doping, as revealed from the SEM images (Figure S2). The XRD diffraction patterns of NaTaO3 and La-doped NaTaO3 are presented in Figure S3. All peaks can be indexed to the orthorhombic phase. A close look at the diffraction patterns (Figure S4) indicates that the doping of NaTaO3 with La leads to a slight shift of the diffraction peaks to lower angles, indicating that a part of the La was homogeneously doped into the NaTaO3 lattice, as previously reported by Kudo et al.22 The cocatalyst-loaded NaTaO3 and La-doped NaTaO3 were characterized by HR-TEM. Figures S5−S11 show the TEM images of the photocatalysts loaded with Pt, Rh, Au, Ag2O, (CoO and RuO2), NiO, and CuO, respectively. They ascertain the successful loading of NaTaO3 and La-doped NaTaO3 with a nanosized cocatalyst either from noble metals or from metal oxides. 3.2. Photocatalytic Results. The photocatalytic activities of H2 and O2 evolution have been studied for La-doped NaTaO3 in pure water and in the presence of sacrificial reagents (e.g., methanol and silver nitrate). Different cocatalysts, i.e., noble metals and metal oxides, have been used, with the aim of determining the most efficient cocatalyst for the reduction and oxidation parts of the photocatalytic water-splitting reaction. The role of the sacrificial systems on the photocatalytic H2 and O2 evolution has also been investigated in detail.

3.2.1. Water Splitting. Figure 3a illustrates the time course of the photocatalytic H2 evolution as a typical run of the photocatalytic setup where just pure water was employed. It is clearly seen that the H2 evolution suddenly starts as the light is switched on. It takes 30−40 min until nearly constant H2 evolution rates are reached after the light is switched on. When the H2 evolution rate was nearly constant, the system was irradiated for a further 6 h. After this period of irradiation time the light was switched off and the H2 evolution suddenly decayed, reaching the baseline of the system. In most experimental runs the position of the line at the end of the photocatalytic reaction is usually higher than that at the very beginning within the first 40 min before illumination. Therefore, the line at the very end of each experimental run is considered to be the baseline of the system for all photocatalytic tests performed here. Thus, the H2 evolution rates were determined from the difference between the baseline at the end of each photocatalytic test and the average of the values obtained in the part of the curve with almost constant H2 evolution rates. It is worth mentioning that no H2 was detected in the absence of La-doped NaTaO3 photocatalyst. Figure 3b shows the H2 evolution rates calculated for the employed cocatalyst nanoparticles: i.e., noble metals and metal oxides. It is apparent that, among the noble metals tested, Pt is the most active cocatalyst for H2 evolution from pure water; however, there are no really significant differences in the 2316


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loaded La-doped NaTaO3 photocatalyst was employed in order to achieve the stoichiometric formation of H2 and O2 from pure water. RuO2 cocatalyst was used, as it showed the highest photocatalytic activity for H2 formation, while CoO is a wellknown cocatalyst for improving the oxidative pathway of water splitting: i.e., enhancing the O2 formation from pure water.28,29 Indeed, the simultaneous formation of both H2 and O2 gases is possible when a combination of RuO2 and CoO cocatalysts was employed at the same time (Figure 4). It is worth mentioning

photocatalytic activity among these materials. RuO2, on the other hand, exhibits the highest photocatalytic activity for H2 evolution among the metal oxide cocatalysts. It is, in fact, obvious from the data shown in Figure 3b also that RuO2 and CuO exhibit the highest activities for the photocatalytic H2 evolution among all investigated cocatalysts: i.e., 7 and 5 μmol h−1, respectively. CoO shows a very low activity for H2 evolution, while Ag2O exhibits almost no photocatalytic activity (less than 0.5 μmol h−1 H2 being the detection limit of the QMS). In all of the photocatalytic tests only H2 and no O2 could be detected. This can be explained either by the formation of adsorbed peroxide species at the surface or by an enhanced solubility of the produced O2 in the aqueous photocatalyst suspensions. For instance, with an H2 evolution rate of 5 μmol h−1 (CuO cocatalyst) after 6 h of photocatalytic reaction 0.6 mmol L−1 H2 is evolved for 50 mL of the photocatalyst suspension. Accordingly, the expected amount of evolved O2 is 0.3 mmol L−1. In fact, with solubility values of 1.28 mmol L−1 for O2 and 0.78 mmol L−1 for H2 in water26 a considerably lower fraction of the photogenerated O2 can be expected to enter the gas phase, which was analyzed by QMS in the experimental setup used here. This effect has to be considered, but we should emphasis that, under the current experimental conditions, lower values for the solubility of H2 and O2 are expected as no equilibrium between the evolved gases and aqueous suspension is present. The possible formation of H2O2 during the irradiation was examined in the case of the most efficient cocatalyst: i.e., for Ladoped NaTaO3 modified with RuO2 according to the method described in the Experimental Section. The formation of the fluorescent product was indeed detected, indicating that H2O2 is photocatalytically formed. Assuming that for the formation of one H2O2 molecule two holes are required, the amounts of H2O2 and H2 formed are expected to be similar. However, the detected amount of H2O2 (14 μmol L−1) was 60 times lower than that of H2 (840 μmol L−1) evolved in the same experiment. Hence, only a rather small portion of the photogenerated holes can be accounted for by this hydrogen peroxide formation, while the majority might still lead to (dissolved) O2 formation. The deficiency of detected H2O2 in comparison to the expected value can also be explained by assuming the formation of surface-bound peroxo species which are not sensitive to the horseradish peroxidase. The formation of H2O2 can also occur through the photocatalytic reduction of O2. Probably, this reductive path is more dominant than the holes path. Thus, the photogenerated H2O2 can be rapidly decomposed in situ as soon as it is formed, which might also explain why the detected concentration of H2O2 is very low. The results obtained in this work differ from those obtained by Kudo et al., who reported the stoichiometric water splitting in the case of bare and La-doped NaTaO3 photoatalysts.22,27 A comparison of the photocatalytic performance obtained by Kudo et al. with that observed in the present work is not meaningful, because of the significant differences in the emission spectra of the lamps (mercury lamp vs Xe lamp), in the reactor design (inner irradiation vs outside irradiation), and in the particle size of the photocatalysts employed in both studies. Especially, the differences in the emission spectra of the lamps are expected to play a significant role due to the large band gap of NaTaO3. In order to enhance the simultaneous formation of both gases, i.e., H2 and O2, a new system consisting of two different cocatalysts was designed. For this purpose, a RuO2- and CoO-

Figure 4. Time course of photocatalytic splitting of pure water into H2 and O2 employing La-doped NaTaO3 (0.83 wt % La) loaded with RuO2 and CoO cocatalysts. Conditions: photocatalyst concentration 1 g L−1, cocatalyst loading 0.2 wt %, reactor volume 50 mL, irradiation intensity I250−450 = 41 mW cm−2.

that the RuO2 and CoO are loaded by the impregnation method and thus the probability of interaction/interfaces between CoO and RuO2 cannot be omitted. However, the rate of water splitting was enhanced in the presence of both of them. The obtained rates for H2 and O2 evolution were determined to be 1 and 0.23 μmol h−1, respectively. The O2 evolution rate was found to reach only half of the value expected from the overall water splitting: i.e., 0.5 μmol h−1. Nevertheless, this result gives a strong indication that La-doped NaTaO3 is able to split pure water into H2 and O2. It seems that, for both pathways of the photocatalytic water splitting process, i.e., the reductive (H2 formation) and oxidative (O2 formation) reactions, one cocatalyst for each is the essential prerequisite. 3.2.2. Alcohol Re-forming. In order to determine the most efficient alcohol for photocatalytic H2 evolution from the socalled sacrificial systems, aqueous solutions of methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH), butanol (BuOH), and 2-methyl-2-propanol (t-BuOH) were employed instead of pure water. Figure 5 illustrates the effect of the nature of the employed alcohol on the photocatalytic H2 evolution rate using 0.2 wt % Pt-loaded La-doped NaTaO3 as the photocatalyst. Pt was chosen as a cocatalyst, as it commonly exhibits the highest activity for photocatalytic H2 evolution from aqueous alcohol solutions. A loading of 0.2 wt % was found to be the optimum. It is clearly seen that all employed alcohols enhance the photocatalytic H2 evolution rates to a great extent in comparison with those measured in pure water (Figure 3). However, the Pt-loaded La-doped NaTaO3 exhibits a rather poor photocatalytic activity for H2 evolution from pure water (3 μmol h−1 H2). It is clearly seen that, when methanol and 2propanol are employed as sacrificial reagents, high photocatalytic activities for H2 evolution can be achieved under the experimental conditions used here. The H2 evolution rates are 2317


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in methanol in comparison with that in water: i.e., 3.9 and 0.78 mmol L−1, respectively. Since no equilibrium is present in this system, lower values for H2 solubility in methanol are expected. However, the effect of the H2 solubility in methanol cannot be neglected at this point. Additionally, methanol might slow the kinetics of proton reduction on the Pt surface due to the competitive adsorption of protons and methanol on the Pt surface. Moreover, the coupling of the surface-adsorbed H• radicals on the Pt surface to form H2 gas might be hindered in the presence of an excess of methanol. The most interesting fact deduced from Figure 6 is that the addition of a very small quantity of methanol into the system already suffices for the rapid increase in H2 evolution rate reaching the optimal range for high H2 evolution rates. Thus, the observed behavior indicates that methanol acts as a sacrificial electron donor scavenging holes and preventing electron−hole recombination. According to these results the optimal methanol concentration for the H2 evolution under the current conditions was found to be 2.5 mol L−1. Figure 7a illustrates the typical time course of the photocatalytic H2 evolution for the sample modified with 0.2

Figure 5. Rate of H2 evolution from pure water and from different aqueous alcoholic solutions using La-doped NaTaO3 (0.83 wt % La) as photocatalyst. Conditions: catalyst concentration 1 g L−1, alcoholic solution (2.5 M) and 0.2 wt % Pt employed in each photocatalytic test, reactor volume 50 mL, irradiation intensity I250−400 = 41 mW cm−2.

reduced by almost double when ethanol or 1-butanol is employed. The lowest photocatalytic activity for H2 evolution was observed when 2-methyl-2-propanol was used as the sacrificial reagent. According to these results, methanol has been chosen for further investigations regarding the photocatalytic H2 evolution on La-doped NaTaO3. At first, the effect of the initial concentration of methanol in the suspension on the H2 evolution rate was investigated (Figure 6). The initial methanol concentrations were varied

Figure 6. Rate of H2 evolution from aqueous methanol solution at different concentrations of methanol using La-doped NaTaO3 (0.83 wt % La) photocatalyst. Conditions: catalyst concentration 1 g L−1, 0.2 wt % Pt employed in each photocatalytic test, reactor volume 50 mL, irradiation intensity I250−400 = 41 mW cm−2.

between 0 and 25 mol L−1. Up to a methanol concentration of 4 mol L−1 the course of the curve is similar to a hyperbolic curve. In the saturation region of this curve, the H2 evolution rate reaches 100 μmol h−1. When the methanol concentration is higher than 4 mol L−1, the values for the amount of H2 evolved decrease to 75 μmol h−1 and stay constant without significant changes. This can be explained either by the fact that with increasing methanol concentration in the system the competition between the alcohol and water molecules for adsorption increases or by the difference in dispersion characteristics of the photocatalyst in water and in water/alcohol mixtures, resulting in a slightly lower photocatalytic activity for H2 evolution. Moreover, the photocatalytic H2 evolution at a higher water− methanol ratio can be affected by the enhanced solubility of H2

Figure 7. (a) Time course of H2 evolution in the presence of methanol employing La-doped NaTaO3 (0.83 wt % La) shown for the sample modified with Pt cocatalyst as an example. (b) H2 evolution rates and the corresponding quantum yields for the cocatalysts used in this system. Conditions: catalyst concentration 1 g L−1, aqueous methanol solution (2.5 mol L−1), 0.2 and/or 1 wt % cocatalyst loadings employed in each photocatalytic test, reactor volume 50 mL, irradiation intensity I250−400 = 41 mW cm−2.

wt % Pt cocatalyst in the presence of an aqueous solution of methanol (2.5 mol L−1) as an example. Figure 7b shows the effect of different cocatalysts on the photocatalytic H2 evolution rate and the corresponding quantum yields. The time course of the H2 evolution is similar to that previously described for the 2318


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ACS Catalysis system of pure water. For the present system, however, a shorter time (less than 30 min) is required to reach a constant H2 evolution rate. After this rate was reached, the suspension was irradiated for a further 5 h. Afterward the lamp was switched off and the H2 evolution rate decayed within 10−15 min, reaching the baseline of the system. Considering the last part of the curve i.e., the baseline, it is clearly seen that the H2 evolution rate stays almost constant over the whole period of irradiation time. The H2 evolution rates were determined analogously to the system of pure water. As shown in Figure 7b, noble-metal cocatalysts exhibit higher photocatalytic activities for H2 evolution in comparison to metal oxides. Among the noble-metal cocatalysts Pt and Rh show quite similar photocatalytic activities for H2 evolution, followed by Au with slightly lower photocatalytic activity. Among the metal oxides NiO shows the highest photocatalytic activity for H2 evolution while RuO2 and Ag2O show the lowest activity. Additionally, in the case of noble-metal cocatalysts the effect of the loading amount has been investigated. The results show that higher H2 production rates were obtained with a lower loading amount: i.e., 0.2 wt %. It is known that excess cocatalyst loading can reduce the catalytic activity by hindering the light absorption by the host photocatalyst. 3.2.3. Sacrificial O2 Evolution. To investigate the effect of different cocatalysts on the photocatalytic O2 evolution, silver ions were employed as sacrificial reagents: i.e., electron acceptors. The typical time course of the photocatalytic O2 evolution is shown for the sample modified with 0.2 wt % CoO cocatalyst as an example (Figure 8a). The rates of O2 evolution and the corresponding quantum yields for the different investigated cocatalysts are presented in Figure 8b. By a comparison of the photocatalytic results obtained from the pure water system (Figure 3) with those obtained from the aqueous silver nitrate system (Figure 8), it is apparent that silver ions indeed act as electron acceptors and greatly promote the rate of O2 evolution, whereas no O2 evolution was detected from the water system under the same conditions. It is important also to mention that after the excitation phase is passed the O2 evolution rate continuously decreases with the course of irradiation time. Such a behavior is typical for all photocatalytic tests performed in the presence of silver nitrate. This can be explained either because of the enhanced solubility of O2 or because of the continuous consumption of Ag+ ions with the advancing of irradiation time. It is worth mentioning that the gradual activity loss might also occur due to the continuous loading of the photocatalyst with silver and silver oxide during the photoirradiation. The obtained O2 evolution rates shown in Figure 8b were determined from the difference between the baseline at the end of the photocatalytic run and the average of the values obtained in the middle part of the curve shown in Figure 8a, where the O2 evolution rates are expected to be constant. Figure 8b clearly shows that the highest value for O2 evolution rate is obtained when CoO was employed. The noble-metal cocatalysts exhibit almost the same photocatalytic activity for O2 evolution without any significant changes.

Figure 8. (a) Time course of O2 evolution in the presence of aqueous silver nitrate solution (0.01 mol L−1) employing La-doped NaTaO3 (0.83 wt % La) shown for the sample modified with CoO cocatalyst as an example (b) O2 evolution rates and the corresponding quantum yields for the cocatalysts used in this system. Conditions: catalyst concentration 1 g L−1, 0.2 wt % cocatalyst loadings employed in each photocatalytic test, reactor volume 50 mL, irradiation intensity I250−400 = 41 mW cm−2.

(see Figures 3, 7, and 8). It is worth noting also that such a selectivity can be clearly observed in the cases where pure water and aqueous silver nitrate solution are employed. In contrast, employing a mechanistically more complicated system, i.e. aqueous methanol solution, where several stepwise oxidation reactions occur involving a current-doubling effect etc., the selectivity for H2 evolution cannot be compared with that observed from pure water. Therefore, the selectivity of the system containing methanol will be discussed separately later on. 4.1. Water Splitting and Sacrificial O2 Evolution. Figure 9 summaries schematically the observed selectivity and photocatalytic activity (indicated as best, middle, and worst) of the investigated cocatalysts for H2 and O2 evolution over Ladoped NaTaO3 from pure water and aqueous silver nitrate solution, respectively. The obtained results demonstrate that the cocatalysts, i.e. RuO2 and CuO, with the highest photocatalytic activity for H2 evolution exhibit, however, low photocatalytic activity for O2 evolution. In contrast, CoO cocatalyst shows the highest activity for O2 evolution and the lowest activity for H2 evolution, whereas Ag2O cocatalyst shows negligible activity for H2 evolution and low activity for O2 evolution (see Figures 3 and 8). Cobalt-based catalysts are well-known to oxidize water molecules and to thus catalyze oxygen formation during water splitting.28−32 Moreover, it has been reported that the cobalt-based catalysts deposited on an n-type semiconductor electrode (photoanode) can directly use photogenerated holes

4. DISCUSSION Considering the results obtained from the photocatalytic tests for H2 and O2 evolution from the three different systems, i.e. pure water, aqueous methanol solution, and aqueous silver nitrate solution, it was observed that certain cocatalysts show selective behavior toward either enhanced H2 or O2 evolution 2319


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mechanism. The most commonly used mechanism is proposed to contain the following reaction steps (eqs 9−11), demonstrating, however, only one possible pathway of O2 formation employing Ag+ cations as electron acceptors:36−38 hν ⎯ O2 + 4H+aq 4h+ + H 2O ⎯→

(9)

hν ⎯ 4Ag 0 4e− + 4Ag + ⎯→

(10)

hν ⎯ Ag n n Ag 0 ⎯→

(11)

It is very important to also consider another possible pathway of O2 formation: i.e., through the oxidation reaction of Ag+ ions by the photogenerated holes to form Ag2+, which can further react with water, resulting in Ag2O2 formation (eqs 12 and 13). The subsequent photocatalytic oxidation of these peroxides is then proposed to result in the observed O2 formation (eq 14).39

Figure 9. Schematic illustration of the selectivity and photocatalytic activity of the employed cocatalysts for H2 and O2 evolution over Ladoped NaTaO3 (0.83 wt % La) in the presence of pure water and aqueous silver nitrate solution.

to form O2 without application of an external bias.33,34 Some studies of the dynamics of photogenerated charge carriers showed that the addition of cobalt-based catalysts increases the lifetime of photogenerated holes to a great extent by trapping the holes and not electrons.35 Therefore, we suppose that CoO cocatalyst has ability to prolong the lifetime of the photogenerated holes, resulting in a significant enhancement of the photocatalytic activity for O2 evolution. Noble-metal cocatalysts, i.e. Pt, Rh, and Au, show neither the highest nor the lowest photocatalytic activities for H2 or O2 evolution, which is why they have been classified as middle-activity cocatalysts. Considering the system of pure water, the sequence of the catalytic activity of metal oxide cocatalysts was found to be as follows: RuO2 > CuO > NiO > CoO (Figure 3b). It can be assumed that in the absence of any sacrificial reagents in the system the recombination rate of the photogenerated charge carriers is high. In order to explain the obtained sequence, the chemical properties of these materials have to be considered in more detail. The employed metal oxides have properties of Brønsted acids with the characteristic bond interaction M−O···H, whereby the strength of an acid refers to its ability or tendency to lose a proton. It is well-known that the tendency to lose a proton depends on the oxidation state, size, and electronegativity of the corresponding metal. RuO2 has both the highest oxidation state (IV) and the highest electronegativity (3.5) among the investigated metal oxides. Accordingly, RuO2 is considered to be the strongest Brønsted acid and thus exhibits the highest catalytic activity for H2 evolution in pure water. While CuO, NiO, and CoO exhibit the same oxidation state (II) and similar electronegativities (∼2), the respective cations have different sizes of ionic radii, decreasing in the following sequence: Co > Ni > Cu. Thus, the smaller the ionic radius of the transition-metal ion, the stronger the bond interaction between the metal and the oxygen atom (M−O···H), making the bond between the oxygen and the proton weaker and the proton release easier. Accordingly, the catalytic activity for H2 evolution is expected to be higher in the case of the metal ions with smaller ionic radii, being in a good agreement with the obtained sequence of the catalytic activity for H2 evolution: i.e., CuO > NiO > CoO (see Figure 3b). In the sacrificial O2 evolution system, Ag+ cations are commonly employed as sacrificial electron acceptors and thus it is important to understand their role in the water oxidation

h+ + Ag + → Ag 2 +

(12)

2Ag 2 + + 2H 2O → Ag 2O2 + 4H+aq

(13)

2h+ + Ag 2O2 → 2Ag + + O2

(14)

Assuming that the photooxidation of water occurs according to eqs 9−11, the expected amount of O2 formed from the complete reduction of Ag+ ions employed in this study (i.e., 500 μmol) can be calculated to be 125 μmol of O2. This value is reasonable in comparison to the experimentally detected amount, i.e. 72 ± 4 μmol of O2 for the sample modified with CoO cocatalyst (see Figure 8a), when the solubility of O2 in water is also taken into account. It is worth mentioning that the detected amounts of O2 employing different cocatalysts do not under any circumstances exceed the amount expected from the complete consumption of Ag+ ions. On the basis of the aforementioned facts and dark gray coloration of the suspension when silver nitrate solution is employed, it is very likely that the pathway described in eqs 9−11 take place in the present system. To explain the differences in activity between noble-metal and metal oxide cocatalysts toward H2 and O2 evolution, we should consider that the cocatalyst nanoparticles deposited on the surface of a photocatalyst play mainly two roles in the photocatalytic water splitting reactions: extraction of the photogenerated charge carriers (electrons or holes) from the photocatalyst and creation of active sites decreasing the activation energy for gas evolution. In the pure water system the highest photocatalytic activity for H2 evolution is expected in the case where noble-metal cocatalysts are employed because of their strong ability to extract electrons from the photocatalyst. It is well-known that in the presence of noble-metal deposits on the surface of a semiconductor photocatalyst an efficient charge carrier separation can be achieved by suppressing the electron−hole recombination.40 In particular, these noble-metal islands act as very effective electron traps due to the formation of a Schottky barrier at the respective metal− semiconductor contact.41,42 A serious disadvantage of the employment of noble metals is, however, that they catalyze not only the water splitting but also the undesirable back-reaction: i.e., the water formation from H2 and O2.43 On the other hand, metal oxide nanoparticles exhibiting metallic behavior (such as RuO2 and NiO) are not able to catalyze the back-reaction: i.e., 2320


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TiO2.5,6,16,49 Generally, it is assumed that due to the small dimensions of the methanol molecule its surface coverage is higher in comparison with that of larger alcohol molecules, thus enabling a more efficient interference of the electron/hole recombination and yielding higher amounts of H2.5 Assuming a higher affinity of methanol with holes in comparison with that of the larger alcohol molecules, it might prevent the backreaction of the photocatalytically formed H2 and O2 (eqs 16 and 17): i.e., water formation.

water formation. Therefore, these metal oxides have been utilized in the past as efficient cocatalysts for similar systems using other photocatalysts: e.g., TiO2.17,44−46 The observed lower catalytic activities of noble-metal cocatalysts in comparison with those of metal oxide cocatalysts toward the H2 evolution from pure water can thus be attributed to the undesired back-reaction, i.e, the reaction of H2 with O2 resulting in water formation, which indeed seems to take place on the surface of noble-metal particles. Moreover, in the pure water system, among the deposited metal oxide particles, CuO and RuO2 showed the highest activities for H2 formation. Now the question is whether this activity for H2 formation is based on the ability to extract and transfer electrons from La-doped NaTaO3 to the cocatalyst particles or on the ability to decrease the activation energy for gas evolution. In view of the results for O2 evolution from aqueous silver nitrate solution (Figure 8b) it is clearly seen that the cocatalyst with the lowest photocatalytic activity for H2 evolution, i.e. CoO, exhibits the highest photocatalytic activity for O2 evolution. According to these results, it can be assumed that the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) is responsible for the observed selective behavior for H2 and O2 evolution from pure water and from the aqueous silver nitrate system. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role in the photocatalytic activity of H2 or O2 evolution. The lower photocatalytic activity for O2 evolution in the case of noble-metal cocatalysts in comparison with that of CoO can be explained by the fact that the noble metals exhibit lower ability for hole extraction in comparison with CoO and therefore again take the position of middle-activity cocatalysts among the investigated cocatalysts. It is worth noting that noble-metal cocatalysts take in both cases, i.e. in pure water and in aqueous silver nitrate systems, the position of middle-activity cocatalysts. For that reason, noble-metal cocatalysts can be called “universal” cocatalysts and can be basically applied for catalyzing both H2 and O2 formation. In contrast, Ag2O cocatalyst shows also in the sacrificial system for O2 evolution low photocatalytic activity and therefore is not applicable to enhanced H2 and O2 evolution at all. 4.2. Alcohol Re-forming. The significantly lower photocatalytic activity for H2 evolution from pure water in comparison with that from aqueous alcoholic solutions (see Figures 3 and 7) can be explained by the much lower water oxidation efficiency by the photogenerated holes in comparison with the alcohol oxidation efficiency, as confirmed by Tamaki et al.47 This is evinced by the fact that the endergonic water dehydrogenation (eq 4) is associated with a free energy change which is on average 10 times higher than that of the alcohol dehydrogenation, as shown for example for the case of methanol (eq 15):48

(16)

2H 2O O2 + 4e− ⎯⎯⎯⎯⎯⎯⎯→ 4OH−

(17)

For instance, an adsorption study of different alcohols on an anatase surface (TiO2) in the gas phase also showed that the total amount of adsorbed alcohols decreases with the higher dimensions of alcohol molecules as follows: MeOH = EtOH > 1-PrOH > 2PrOH > 1-BuOH.50 Thus, the obtained order of the photocatalytic H2 evolution activity seems to be related to the adsorption properties of the alcohols. An identical order of reactivity can be predicted by comparing the one-electronoxidation potentials for alcohols vs NHE at pH 7: E°(MeOH) = 0.96 V = E°(EtOH) = 0.96 V < E°(2-PrOH) = 1.14 V.51 Moreover, using transient absorption spectroscopy Tamaki et al. found for the oxidation efficiency of alcohols by trapped holes the order MeOH > EtOH > 2-PrOH, which is also consistent with the other data mentioned above.47 On comparison of the order of different alcohols for the photocatalytic activity for H2 evolution obtained in this work with those obtained from adsorption and kinetic studies, it is clearly seen that the order of ethanol and 2-propanol is exchanged. Normally, the inductive effects of the two methyl groups linked to the α-carbon in the case of 2-propanol should make the abstraction of the hydroxylic hydrogen atom more difficult.49 However, it was found that the photocatalytic activity for H2 evolution of 2-propanol (QY = 25%) is higher than that of ethanol (QY = 15%) for the present photocatalyst. In agreement with the results of this work, most studies published on this topic have shown the same sequence of the photocatalytic activity for H2 evolution: i.e., methanol and butanol followed by 2-methyl-2-propanol. The order of ethanol and 2-propanol in the aforementioned sequence was found to differ from publication to publication.5,11,14,15,49,52 This fact leads to the conclusion that further investigations are required for a better understanding of the origin of the reactivity order of different alcohols, especially with respect to the H2 evolution. When alcohols are employed in the system, in particular methanol, several oxidation reactions accompanied by more complicated mechanisms take place,25 which apparently affects the selectivity and photocatalytic activity of the investigated cocatalysts on the surface of La-doped NaTaO3 to a great extent (see Figure 10). In this system the obvious leader among the employed cocatalysts is the group of noble-metal cocatalysts (Pt, Rh, and Au). All other metal oxide cocatalysts, except RuO2 and Ag2O, exhibit lower photocatalytic activity for H2 evolution without significant differences in their activities (see Figure 7b). Therefore, this group of cocatalysts have been assigned as “others” being middle-activity cocatalysts. RuO2 exhibits the lowest values for H2 evolution and can be therefore classified as the worst cocatalyst together with Ag2O, showing photocatalytic activity close to the detection limit of the instrument.

hν , catalyst CH3OH(l) + H 2O(l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO2 (g) + 3H 2(g) ΔG°298 = 16.1 kJ mol−1

H 2 + 2h+ → 2H+

(15)

The obtained results presented in Figure 5 have shown that the photocatalytic activity for H2 evolution depends on the nature of the alcohol employed in such photocatalytic reactions. Apparently, for the present photocatalyst, methanol is the most efficient electron donor among the investigated alcohols, which is consistent with many other studies performed on 2321


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proton-coupled electron transfer (PCET) from the •CH2OH radical does not take place on the RuO2 surface, thus lowering the catalytic activity of RuO2 to a great extent. Moreover, the extraction of the CB electrons by RuO2 in the presence of methanol also seems to be not very efficient. In addition, it seems that the H2 evolution from aqueous methanol solution is governed more by the ability of the cocatalyst to catalyze the H2 formation rather than the interaction with the photogenerated electrons and holes. This is why RuO2 and CoO cocatalysts, for example, showed much lower activity than noble-metal cocatalysts. This is consistent with the fact that methanol can efficiently react with the photogenerated holes. To evaluate and compare the photoinduced electron transfer on La-doped NaTaO3 photocatalysts, time-profile cathodic photocurrents were measured using Fe2+/Fe3+ as an electron shuttle that transfers electrons from La-doped NaTaO3 photocatalyst particles to a Pt collector electrode in the illuminated photocatalyst suspension according to eqs 18 and 19 (Figure 11). This method has been chosen instead of

Figure 10. Processes involved in the photocatalytic H2 evolution from aqueous methanol solution over semiconductor photocatalyst: (1) photogeneration of the charge carriers e− and h+; (2, 3) surface trapping of the charge carriers; (4) recombination; (5) H2 formation; (6) first oxidation step of methanol; (7) formation of formaldehyde through e− injection into the conduction band of the photocatalyst (current doubling).

The selectivity observed in the system where methanol is present differs from that observed for two other systems discussed above: i.e., pure water and aqueous silver nitrate solution. It is well-known that noble metals exhibit higher values for the work function in comparison with that of metal oxides and subsequently higher Schottky barriers, which can serve as efficient barrera preventing the undesired electron migration back to the semiconductor.53 While the electrons are trapped at the cocatalyst (noble-metal particles) surface, the holes remain at the host photocatalyst and migrate to its free surface, ensuring an efficient charge separation. Anpo and Takeuchi have demonstrated the electron transfer from TiO2 to Pt particles by means of electron spin resonance measurements.54 Moreover, it is believed that Pt nanoparticles lower the water reduction overvoltage, hence accelerating the H2 formation. Supposedly similar electron transfer from La-doped NaTaO3 to Pt particles can occur in the present system as well. In addition to the photogenerated electrons originating from the semiconductor, another fraction of electrons can be injected into the conduction band of the photocatalyst through the methanol oxidation pathway originating from the •CH2OH radical (current-doubling effect; Figure 10, step 7).25 This means that at least the half of the detected amount of H2 gas is formed through the action of holes and not electrons and therefore one photon generating an electron−hole pair is enough to form one H2 gas molecule. Large differences in the photocatalytic activity for H2 evolution between two systems, i.e. in the presence and absence of methanol, can be partially attributed to the so-called current-doubling effect. Thus, such a sacrificial system does not generate any mechanistic information because it is not possible to determine whether the electron transfer from the conduction band or from reduction of organic radicals is rate-limiting or whether the overall efficiency might even be limited by the initial hole transfer to the sacrificial reagent.39 Generally, devices containing even small concentrations of methanol for photocatalytic H2 production should be called methanol reforming and not water-splitting reactions for H2 evolution, as is commonly used in the literature currently. Interestingly, RuO2 demonstrates the highest catalytic activity for H2 evolution from pure water, while in the presence of methanol it exhibits rather poor catalytic activity for H2 evolution (compare Figures 3b and 7b). We assume that the

Figure 11. Fe3+/2+ redox shuttle mediated cathodic photocurrent−time profiles collected on a Pt electrode in UV-light-irradiated suspensions of La-doped NaTaO3 (0.83 wt % La) photocatalyst loaded with different cocatalysts. Experimental conditions: [catalyst] = 1 g L−1, [Fe3+]0 = 0.5 mmol L−1, [LiClO4] = 0.1 mol L−1, [MeOH] = 2.5 mol L−1, pHi 1.8, Pt electrode held at +0.7 V vs Ag/AgCl, continuous Ar purging, λ >250 nm, cocatalyst loadings 0.2 wt %.

immobilization of the photocatalyst on a conductive substrate to keep the experimental conditions comparable to those used for the photocatalytic tests as much as possible and, in addition, to avoid the problems associated with the film fabrication (e.g., mechanical adhesion, thickness control, contamination with chemical additives during the fabrication, etc.). Fe3 + + ecb− → Fe 2 + on La‐doped NaTaO3 photocatalysts Fe2 + → Fe3 + + e−

(18) (19)

on Pt electrode −1

In this experiment, methanol (2.5 mol L ) was added as an electron donor. Under these conditions, the Fe2+/Fe3+mediated current directly originates from the CB electrons while the VB holes are consumed mainly by methanol oxidation. To avoid the probable deposition of Fe3+ species on the photocatalyst particles, the pH value of the investigated suspensions was adjusted to 1.8; however, their initial pH values were in the range of 3.4−4 in the case of the photocatalytic measurements. Since this small difference in pH positively shifts the potential of the proton reduction and the water oxidation as well as the conduction band level and the 2322


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valence band level of La-NaTaO3 by almost the same value, the correlation between the photocatalytic results and the photocurrent can still provide valuable information. As shown in Figure 11 Au(0.2)- or Pt(0.2)-loaded photocatalyst shows a higher cathodic photocurrent in comparison to other metal oxide loaded photocatalysts, which is consistent with the photocatalytic activities of H2 production. On the other hand, RuO2(0.2)- or Ag2O(0.2)-loaded photocatalyst shows the lowest photocurrent among all the tested photocatalysts, which is also consistent with the activity of H2 production. It is noted that the Au(0.2)-loaded sample exhibits higher photocurrent in comparison to the Pt(0.2)-loaded sample, although the Pt-loaded photocatalyst produces more H2 in comparison to the Au(0.2)-loaded sample. However, it should be recognized that the efficiency of interfacial electron transfer (mediated by Fe2+/Fe3+ electron shuttle) does not exactly correlate with the catalytic efficiency of proton reduction to H2. The Fe2+/Fe3+-mediated photocurrent occurs via single-electron transfer (Fe3++ e−→ Fe2+), whereas H2 is produced via two-electron transfer (2H+ + 2e−→ H2) and surface catalysis. It was also noted that the difference in photocurrent generation between Pt(0.2)-loaded and Ag2O(0.2)-loaded photocatalysts is relatively small in comparison with the difference in H2 production. This observation indicates that a significant fraction of photogenerated electrons are not efficiently converted into H2 production in the case of Ag2O(0.2), which supports the idea that the catalytic roles of noble metals or transition-metal oxides are important in H2 production, as well as the charge separation and the interfacial electron transfer on photocatalysts. Anodic photocurrent experiments were also performed, and the results are presented in Figure 12. In this experiment,

(21)

The Ag2O(0.2)-loaded photocatalyst shows relatively higher photocurrent in comparison to any other metal or metal oxide loaded photocatalysts. On the other hand, the CuO(0.2)loaded photocatalyst shows the lowest photocurrent among all tested photocatalysts. It is notable that the CoO(0.2)-loaded sample showed a similar photocurrent in comparison with other catalyst samples and even lower than that of the Ag2O(0.2)-loaded sample, although the CoO(0.2)-loaded catalyst exhibits higher photocatalytic O2 evolution activity than that of any other catalysts. It should be recognized that the efficiency of interfacial electron transfer (mediated by Fe2+/Fe3+ electron shuttle) does not exactly correlate with the catalytic efficiency of water oxidation to O2. Again the Fe2+/Fe3+mediated photocurrent occurs via single-electron transfer (Fe3++ e−→ Fe2+), whereas O2 is produced via four-electron transfer (2H2O → O2 + 4H+ + 4e−) and surface catalysis. This result also indicates that the catalytic roles of noble metals or transition-metal oxides are very important in photocatalytic O2 evolution from water splitting in addition to the charge separation or electron transfer efficiency.

5. CONCLUSIONS The effect of different noble metal (i.e., Pt, Au, and Rh) and metal oxide (i.e., NiO, CuO, CoO, RuO2, and Ag2O) cocatalysts on the photocatalytic H2 and/or O2 evolution over La-doped NaTaO3 has been systematically investigated employing three different systems: i.e., pure water, aqueous methanol solution, and aqueous silver nitrate solution. Different trends have been observed, depending on the type of the cocatalyst and on the investigated systems. For example, RuO2 and CuO cocatalysts showed the highest photocatalytic activity for H2 evolution from pure water; however, they showed low photocatalytic activity for O2 evolution from aqueous silver nitrate solution. In contrast, CoO cocatalyst showed the highest photocatalytic activity for O2 evolution from the latter and the lowest activity for H2 evolution from the former. Noble-metal cocatalysts, i.e., Pt, Rh, and Au, demonstrate neither the best nor the worst photocatalytic activity for both H2 and O2 evolution from pure water and from aqueous silver nitrate solutions, respectively. Ag2O cocatalyst exhibits negligible photocatalytic activity for H2 evolution from water and low activity for O2 evolution from aqueous silver nitrate solution. It was concluded from these results that the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) is the main reason for the observed selective activity for H2 and O2 evolution from pure water and aqueous silver nitrate solution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. When methanol is employed as a sacrificial reducing agent, noble-metal cocatalysts show the highest photocatalytic activity for H2 evolution. Interestingly, while RuO2 showed the highest activity for H2 evolution from pure water, it exhibited the lowest activity for H2 evolution in the presence of methanol. On the basis of these results and on the photoinduced electron transfer measurements it was concluded that the catalytic roles of noble metals and transition-metal oxides toward the production of H2 are crucial in addition to the charge separation and the interfacial electron transfer on the photocatalysts. No overall water splitting into H2 and O2 can be achieved utilizing La-doped NaTaO3 loaded with only one cocatalyst; however, it was found that the loading

Figure 12. Fe2+/Fe3+ redox shuttle mediated anodic photocurrent profiles collected on a Pt electrode in UV-light-irradiated suspensions of La-doped NaTaO3 (0.83 wt % La) photocatalyst loaded with different cocatalysts. Experimental conditions: [catalyst] = 1 g L−1, [Fe2+]0 = 0.5 mmol L−1, [LiClO4] = 0.1 mol L−1, [NaBrO3] = 10 mmol L−1, pHi 1.8, Pt electrode held at +0.7 V vs Ag/AgCl, continuous Ar purging, λ >250 nm, cocatalyst loadings 0.2 wt %.

bromate (BrO3−, 10 mmol L−1) was used as an external electron acceptor. Under these conditions, the Fe2+/Fe3+mediated currents come directly from the VB holes while the CB electrons are consumed mainly by bromate (BrO3−) reduction (eqs 20 and 21). Fe 2 + + h vb+ → Fe3 + at La‐doped NaTaO3 photocatalysts

at Pt electrode

(20) 2323


Research Article

ACS Catalysis

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of La-doped NaTaO3 with two different cocatalysts, i.e. RuO2 and CoO, enable the simultaneous formation of H2 and O2 from pure water. Thus, the modification of a photocatalyst with two different cocatalysts seems essential to enhancing the efficiency of overall photocatalytic water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b04326. Quantum yield calculations and the characterization results of La-doped NaTaO3 photocatalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.A.K.: [email protected]; tarek. [email protected]. *E-mail for D.B.: [email protected]. ORCID

Tarek A. Kandiel: 0000-0003-0120-2408 Wonyong Choi: 0000-0003-1801-9386 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the BMBF (Bundesministerium für Bildung und Forschung), initiative Hycats (No. 033RC1012A), and the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044) funded by the Korea Government (MSIP) through the National Research Foundation (NRF). We thank H. C. Starck for providing the La-doped NaTaO3 materials for the current study. The authors thank Dr. Ralf Dillert and Dr. Amer Hakki for stimulating discussions and for help in the preparation of the manuscript.

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REFERENCES

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