Particulate Photocatalysts for Light-Driven Water Splitting

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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies Qian Wang†,§ and Kazunari Domen*,†,‡ †

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Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Center for Energy & Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japan ABSTRACT: Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply selfsupported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron−hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

CONTENTS 1. Introduction 1.1. Solar Fuels from Water Splitting Using Particulate Semiconductors 1.2. Thermodynamics and Kinetics of Photocatalytic Water Splitting 2. Photon Absorption 2.1. Challenges 2.1.1. The Solar Spectrum and Maximum Solar Light Conversion Efficiency 2.1.2. Semiconductor Materials 2.2. Strategies 2.2.1. Valence Band Potential Engineering and Other Photocatalyst Materials 2.2.2. Doping 2.2.3. Solid Solutions 2.2.4. Dye Sensitization 2.2.5. Localized Surface Plasmon Resonance Effect 2.2.6. Quantum Confinement Effect 2.2.7. Z-Scheme Configurations 3. Charge Separation and Transport 3.1. Challenges 3.1.1. Recombination 3.1.2. Trapping 3.2. Strategies

© XXXX American Chemical Society

3.2.1. Short Charge Carrier Migration Distances 3.2.2. Reduction of Defects 3.2.3. Junctions 3.2.4. Facet Control 4. Surface Chemical Reactions 4.1. Challenges 4.1.1. Side and Back Reactions 4.2. Strategies 4.2.1. Cocatalysts 4.2.2. Surface Modification 5. Conclusion and Perspectives Author Information Corresponding Author ORCID Present Address Notes Biographies Acknowledgments Abbreviations Used References

B B B C C C D F F S U W Z AA AC AD AF AF AF AF

AF AG AH AM AO AP AP AQ AQ AT AT AW AW AW AW AW AW AW AW AX

Special Issue: Nanoparticles in Catalysis Received: March 31, 2019

A

DOI: 10.1021/acs.chemrev.9b00201 Chem. Rev. XXXX, XXX, XXX−XXX

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1. INTRODUCTION Solar energy is the most attractive renewable replacement for fossil fuels because it is plentiful, inexhaustible, and widely distributed. However, due to the diffuse and intermittent nature of solar irradiation, energy harvested from the sun must be efficiently converted into chemical fuels that are storable, transportable, and usable upon demand.1,2 This requirement has motivated the development of sustainable artificial photosynthetic processes, with the aim of mimicking natural photosynthesis to generate fuels from water and carbon dioxide utilizing solar energy (Figure 1).3,4 Solar-to-chemical energy conversion offers a highly useful means of storing solar energy efficiently as sustainable energy carriers with minimal environmental impact. Among the potential solar fuels produced by artificial photosynthesis, hydrogen is one of the most attractive as it offers high energy density and does not generate pollutants during combustion.5,6 Combining solar energy with water (a highly abundant resource) by way of solar water splitting provides a platform for hydrogen production; a schematic of a potential sustainable plant for hydrogen generation and utilization is presented in Figure 2. In this system, water is injected into a solar water splitting module and converted into hydrogen, which is subsequently used in highefficiency power production systems, including internal combustion engines and fuel cells. Thus, water and sunlight are utilized to provide power. In addition, hydrogen can be combined with carbon dioxide captured from the atmosphere or from the emissions of coal or natural gas burning plants to generate methanol and synthetic natural gas or precursors for plastics and fertilizers.7 Sustainable processes such as these would offer numerous advantages in terms of climate change mitigation and compatibility with currently existing infrastructure. One of the most significant challenges associated with realizing such processes is the efficient, cost-effective decomposition of water on the scale required to meet global energy demands (that is, tens of terawatts).8

photocatalytic activity of the powder regardless of size and thus are extensible to the level of square meters and even larger. Despite this potential for scale-up, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Assuming daily AM 1.5G irradiation for 7.6 h (corresponding to 240 W m−2), to reach a hydrogen price of 3.5 USD kg−1, the recent evaluation suggests a photocatalytic reaction system with an STH of 10%, a lifetime of 10 years, an annual depreciation rate of 4%, and an allowable cost of 102 USD m−2.11 Thus, further fundamental investigations regarding the design and development of efficient photocatalysts are vital. 1.2. Thermodynamics and Kinetics of Photocatalytic Water Splitting

Figure 4a presents a diagram of a typical semiconductor particle, loaded with a hydrogen evolution cocatalyst (HEC) and oxygen evolution cocatalyst (OEC) on its surface. Such particles would be applied as a photocatalyst for overall water splitting. The photocatalytic decomposition of water on semiconductor photocatalysts to produce hydrogen and oxygen comprises three sequential processes: (i) the absorption of photons having energy values greater than the semiconductor bandgap to excite electrons from the valence band (VB) to the conduction band (CB), creating electron (e−)/hole (h+) pairs, (ii) the separation of the photoexcited carriers into free carriers and subsequent migration to accumulate at the active sites on the particle surfaces, and (iii) the initiation of redox reactions involving these charges to generate hydrogen and oxygen with the assistance of the cocatalysts. These reactions are summarized below. Hydrogen evolution reaction (HER): in an acidic aqueous solution: 2H+ + 2e− → H 2

in an alkaline aqueous solution: 2H 2O + 2e− → H 2 + 2OH− (1.3)

1.1. Solar Fuels from Water Splitting Using Particulate Semiconductors

Oxygen evolution reaction (OER): in an acidic aqueous solution: 2H 2O → O2 + 4e− + 4H+

Light-driven water splitting can be achieved in relatively simple systems by employing particulate semiconductor materials as photocatalysts, such that both sunlight absorption and catalysis take place on single particles. The estimated average cost of the hydrogen produced by such powder-based photocatalytic devices is in the range of 1.6−3.5 USD kg−1, assuming solarto-hydrogen conversion efficiency (STH) values of 5−10% and a system lifetime of 5 years.9−11 The STH is defined by3,12

(1.4) −

(1.5)

The overall solar energy conversion efficiency, ηtotal, takes into account the efficiencies associated with these three fundamental processes and thus is determined as14,15 ηtotal = ηabsoprtion × ηseparation × ηreaction (1.6) Here, the photon absorption efficiency, ηabsorption, is defined as the fraction of electron−hole pairs excited by the incident photon flux, the separation efficiency, ηseparation, is the fraction of photogenerated charge carriers that separate and migrate to the solid−liquid interface, and the reaction efficiency, ηreaction, is the efficiency of the surface reaction involving the charge carriers at the solid−liquid interface. At pH 0, as illustrated in Figure 4b, the protons are reduced by the electrons in the CB to generate hydrogen. Simultaneously, oxygen is produced via the oxidation of water molecules by photoexcited holes in the VB. Therefore, to achieve the overall water splitting reaction, the CB minimum (CBM) must be more negative than the H+ to H2 reduction potential (0 V relative to normal hydrogen electrode (NHE) at

(mmol H 2 s−1) × (237000 J mol−1) (100 mW cm−2) × area (cm 2) at standard conditions

−

in an alkaline aqueous solution: 4OH → O2 + 4e + 2H 2O

output energy as H 2 energy of incident solar light =

(1.2)

(1.1)

This range meets the United States Department of Energy’s goal of achieving a cost of 2.00−4.00 USD kg−1 H2 by 2020 so as to obtain an economically viable alternative to fossil fuels.9 Importantly, such powder-based systems can be processed into large scale panel facilities while retaining the intrinsic activity of the photocatalyst. As an example, our own group has demonstrated the construction of 1 m2 photocatalytic water splitting panels based on a fixed Al-doped SrTiO3 photocatalyst (Figure 3).13 Such panel reactors maintain the inherent B

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conserve momentum, an electron must undergo a significant change in its momentum to be excited across the bandgap and produce an electron−hole pair in this material.22,23 This occurs because the electron interacts not only with the initial photon to obtain energy but also with a quantized lattice vibration known as a phonon, which either increases or decreases its momentum. Because the electron, photon, and phonon are all involved in this indirect absorption process, it proceeds at a lower probability and at a much slower rate than direct absorption. The absorption of light by a semiconductor particle can be quantified based on the absorption coefficient of the material, α(λ). For a given photon energy, E2 − E1 = hν (as in Figure 5), this coefficient is summed over all possible band transitions from the VB to the CB, as in the equation24

pH 0), while the VB maximum (VBM) must be more positive than the H2O to O2 oxidation potential (+1.23 V relative to NHE at pH 0). Consequently, the minimum theoretical bandgap required to drive the overall water splitting reaction is 1.23 eV, which corresponds to a wavelength of approximately 1000 nm. It is likely that some additional kinetic overpotentials are necessary to initiate and drive the electron transfer process as well as the HER and OER at reasonable rates, which precludes semiconductors with bandgaps smaller than 1.6 eV.4,16 In addition to the band alignment of a photocatalyst relative to the hydrogen and oxygen evolution potentials, photocatalysts must exhibit excellent physicochemical stability in the presence of oxidative and/or reductive photocorrosion processes. For a photocatalyst to be thermodynamically stable, its reductive decomposition potentials and CBM must be more negative than the H+/H2 potential, while its oxidative decomposition potentials and VBM must be more positive than the O2/H2O potential.17 These constraints place severe restrictions on the choice of suitable materials for photocatalytic water splitting. Essentially, these compounds are required to harvest solar radiation efficiently, to exhibit longterm stability, and to support rapid charge migration and surface reactions. The goal of this review is to provide an overview of the fundamentals of photocatalytic overall water splitting, focusing on the design of photocatalysts. Over the past decade, significant progress has been achieved in elucidating the solar water splitting mechanism and developing novel materials to promote this process, and recent research concerning efficient photocatalytic water splitting designs is summarized herein. Several distinct strategies have been explored to meet improved performance requirements, including extending the spectral range of visible light absorption, enhancing photogenerated charge separation and transport, and facilitating the hydrogen and oxygen production reactions. This review presents recent advances in photocatalytic water splitting to illustrate several general approaches that are expected to mitigate the various obstacles involved in achieving practical solar hydrogen production.

α(hν) ∝

∑ P12 g V (E1)gC(E2)

(2.1)

Here, P12 is the probability of the transition of an electron from the initial state E1 to the final state E2, while gV(E1) and gC(E2) are the electron densities in the initial and final states, respectively. The value of α(λ) varies with the intrinsic absorption properties of the photocatalyst, such as its bandgap and band positions, and whether light absorption proceeds directly or indirectly. Because a phonon is involved in indirect bandgap absorption, the absorption coefficient for indirect transitions is relatively small compared to that for direct transitions. As a result, light penetrates more deeply into indirect bandgap semiconductors than direct bandgap semiconductors.24 The absorption depth, α(λ)−1, determined by taking the inverse of α(λ), refers to the distance below which the light intensity is reduced to 1/e of the incident irradiation. This value determines the optimal thickness of a photocatalyst film or suspension or how many particles are required to obtain a suitable degree of photocatalytic efficiency.14,18 It is therefore important to note that the maximum photon absorption of a given photoreactor should be determined using the optimal conditions such that the value is not affected by the amount of photocatalyst used, because otherwise photocatalytic performances reported by different laboratories cannot be compared. Research has demonstrated that the diameter of a particle must be at least 2.3 times α(λ)−1 to ensure 95% absorption of the incident light.25,26 Means of determining α(λ) have been described in detail by Gray et al.24 and Chen et al.14

2. PHOTON ABSORPTION The semiconductor photocatalysis process to convert solar energy into chemical fuels begins with photon absorption. The irradiation of semiconductor particles results in optical excitation in the case that the incident light has an energy equal to or greater than the bandgap of the material, in which case, electrons in the VB are excited to the CB on the femtosecond time scale.18 Charge carriers can also be generated in organic semiconductors, including dyes19,20 and polymers,21 moving from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The rapid relaxation of electrons and holes to the bottom of the CB and the top of the VB, respectively, follows excitation, on a similar time scale.18 Two types of bandgaps arise in semiconductors: direct and indirect. In the case of a direct bandgap, the momentum (wave vector) of an electron in the VBM is equal to that of an electron in the CBM, as shown in Figure 5. Thus, irradiation by a super bandgap photon (i.e., hν > Eg, where Eg is the bandgap energy), an electron is readily promoted from the VB to the CB, with minimal variation in its momentum. In contrast, in an indirect bandgap semiconductor, the states at the minimum of the CB and the maximum of the VB do not have equal momenta. Thus, to

2.1. Challenges

2.1.1. The Solar Spectrum and Maximum Solar Light Conversion Efficiency. The sun emits radiation ranging from X-rays to radio waves, but the most intense solar radiation occurs in the visible light range (Figure 6), such that 43% of the solar energy reaching the Earth’s surface is at visible wavelengths from 400 to 700 nm. Assuming that only ultraviolet (UV) light up to 400 nm is utilized, with a quantum efficiency (QE) of 100%, the STH would be limited to 2% (neglecting entropic losses),27,28 as illustrated in Figure 7. Extending absorption into the visible light region up to 700 and 1000 nm would drastically raise the ideal STH to 25% and 47%, respectively. As such, a QE of approximately 100% is required for a photocatalyst with an absorption edge of 520 nm to attain the target STH of 10%. In contrast, a semiconductor capable of harvesting light up to 650 nm can have a QE of less than 60% while still achieving the target STH. Therefore, in C

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Figure 1. Schematic illustration of natural and artificial photosynthetic systems.

Figure 2. Potential sustainable hydrogen fuel cycle based on photocatalytic water splitting.

plentiful natural resource, and stimulated research into overall water splitting using particulate photocatalysts. In 1977, Schrauzer et al. determined that external energy inputs other than light are not required to split water and demonstrated the production of H2 over TiO2 powder under UV light.30 Since then, many new semiconductor types have been investigated and efforts have been devoted to identifying new photocatalysts. Table 1 summarizes the photocatalysts reported to date for single-step overall water splitting without using sacrificial reagents, of which approximately 80% have wide

order to obtain a high STH, it is crucial to focus on photocatalysts that harvest visible light. 2.1.2. Semiconductor Materials. The pioneering work concerning the decomposition of water into hydrogen and oxygen using heterogeneous photocatalysts was reported by Fujishima and Honda in 1972. Their approach employed a photoelectrochemical (PEC) cell consisting of TiO2 as the photoanode and platinum as the counter electrode under UV irradiation, with an external bias.29 This report revealed the feasibility of generating hydrogen from water, a clean and D

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Figure 3. (a) Schematics and (b) a photograph of a 1 m × 1 m water splitting panel based on SrTiO3:Al powder. Reprinted with permission from ref 13. Copyright 2018 Elsevier.

reaction, differ among these various studies. The gas evolution rates presented herein can be used to find the H2/O2 molar ratios, which are an important means of determining if the process is an overall water-splitting or sacrificial reaction. Several review articles discuss how to report reliable and comparable photocatalytic activity to ensure reproducibility and effective benchmarking and are highly recommended for further reading.12,31−33 Most metal oxide photocatalysts capable of splitting water into H2 and O2 in Table 1 consist of metal cations with either d0 (Ti4+, Zr4+, Nb5+, Ta5+, V5+, W6+ and Ce4+) or d10 (Zn2+, In3+ , Ga 3+ , Ge4+, Sn4+ and Sb5+) configurations. The conduction bands for d0 and d10 metal oxide photocatalysts typically consist of the d and sp orbitals of the metal cations, respectively, while the valence bands are composed of O 2p orbitals, as shown in Figure 8. Alkali metal, alkaline earth metal, and some lanthanide ions usually do not directly contribute to band formation and simply construct the crystal structure (for example, as A site cations in perovskite compounds). Although there is a wide variety of metal oxides with different crystal structures and electronic structures, the key factors affecting water splitting activity are believed to be the symmetry of the metal oxygen octahedral/tetrahedral coordination and the VB and CB structures.34 A number of review articles and book chapters on this subject have been published.3,35−37 Because of the highly positive VB (located at approximately +3 V relative to NHE at pH 0) formed by O 2p orbitals, it is difficult to find an oxide semiconductor photocatalyst that has both a CB sufficiently negative for H2 production and a bandgap sufficiently to absorb visible light (λ > 400 nm). The development of photocatalysts capable of water cleavage and having a narrow bandgap and wide light absorption range is therefore an important aspect of achieving efficient solar hydrogen generation. However, to date, very few reproducible photocatalytic systems have been found to exhibit simultaneous and stoichiometric hydrogen and oxygen generation from pure water under visible light irradiation. In section 2.2, we review the strategies that have been employed to improve the photonic efficiencies of photocatalytic systems. These strategies, which are illustrated in Figure 9, include VB control, doping, solid solution formation, quantum dots (QDs),

Figure 4. (a) Diagram showing the reactions during water splitting on a semiconductor photocatalyst: (i) light absorption, (ii) charge separation and transport, and (iii) redox reactions. (b) Energy diagram for photocatalytic water splitting based on one-step excitation.

bandgaps and thus are only active under UV light. Note that the efficiencies of these materials, as reflected in QE, apparent quantum yield (AQY) values at specific wavelengths (as defined in Table 1) and STH values, could be directly compared in principle, whereas the gas evolution rates cannot. This is because the experimental conditions used to determine gas evolution, including light sources and the scale of the E

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Figure 5. Photon absorption in (a) a direct bandgap semiconductor for an incident photon with energy hν = E2 − E1 > Eg, and (b) an indirect bandgap semiconductor for a photon with energy hν < E2 − E1 and a photon with energy hν > E2 − E1.

The past several decades have seen a surge in the evolution of novel materials for visible-light-driven photocatalytic reactions, including oxides, (oxy)nitrides, (oxy)sulfides, oxyhalides, and chalcopyrites. Some metal elements, such as Ag, Cu, Bi, Pb and Sn, have been found to contribute to the formation of not only conduction band104,105,185−187 but also valence band. As an example, the electron configurations of Ag+,188 Cu+,189,190 Sn2+,191,192 Pb2+,193−195 and Bi3+ (refs 194,196) can construct new VBs appearing at more negative potentials than O 2p orbitals and thus reduce the bandgap. The insertion of transition metals with filled d orbitals or s orbitals can give rise to a narrower bandgap.197 This effect is typified by monoclinic BiVO4 (m-BiVO4), which has a bandgap of 2.4−2.5 eV and has been reported to promote water oxidation to give significant gas evolution rates in the presence of Ag+ as a sacrificial electron acceptor (SEA).198,199 In this compound, the distortion of VO4 tetrahedra and BiO8 dodecahedra results in the hybridization of Bi 6s/O 2p lone pair states at the top of the VB. Electronic structure predictions based on density functional theory (DFT) indicate that the VB consists primarily of O 2p orbitals, while the VBM comprises nonbonding O 2pπ orbitals with a contribution from Bi 6s orbitals.196 Similarly, the VB of SnNb2O6 is composed of Sn 5s and O 2p hybrid orbitals, such that the VBM is more negative than it would be if made solely of O 2p orbitals.191 Consequently, SnNb2O6 has a narrow bandgap of 2.3 eV and exhibits photocatalytic activity during H2 and O2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag+) under visible light irradiation.192 Another approach to VB tuning via the introduction of metal cations is the substitution of alkali ions with Ag+ and Cu+ ions at the interlayers in layered materials, such as RbLaTa2O7,200 K2La2Ti3O10,201 Li2TiO3, and Li2SnO3,202 or at and/or near the surface of bulky materials like NaTaO3, as shown in Figure 10.201 The energy gaps of these metal oxide photocatalysts are narrowed by 1.1−1.5 eV after introducing Ag+ or Cu+ ions because the filled Cu 3d or Ag 4d orbitals form VBs at higher energy levels than those resulting from O 2p orbitals. This technique has also been applied to decrease the bandgaps of metal sulfides, such that the bandgap of ZnGa2S4 (3.4 eV) was reduced to 2.7 eV by cosubstituting Cu+ and Ga3+ at Zn2+ sites.203 In this case, the VBM was raised significantly following substitution with Cu+, while the CBM was slightly lowered by introducing Ga3+. Moreover, due to charge compensation, the cosubstitution of Cu+ together with the high valence Ga3+ ions suppressed the appearance of sulfur defects in the crystal lattice.

Figure 6. AM 1.5 G solar spectrum based on the ASTM G173-03 reference spectrum.

Figure 7. Calculated STH values as functions of photon wavelength for photocatalytic one-step overall water splitting using photocatalysts with various QEs. The associated calculations assumed AM 1.5G solar irradiance. Reprinted with permission from ref 28. Copyright 2010 American Chemical Society.

plasmon resonance, dye-sensitization, and Z-scheme construction. 2.2. Strategies

2.2.1. Valence Band Potential Engineering and Other Photocatalyst Materials. As discussed in section 2.1, obtaining both H2 and O2 production over a metal oxide having a VB consisting of O 2p orbitals requires that the bandgap energy of the oxide exceeds 3 eV, such that it straddles the potentials associated with proton reduction and water oxidation. Tuning the VB potential using the p orbitals of anions or the s orbitals of p-block metal ions has been found to narrow the bandgap successfully. F

DOI: 10.1021/acs.chemrev.9b00201 Chem. Rev. XXXX, XXX, XXX−XXX

G

Pt NiO NiO

3.2 (