Enabling Silicon for Solar-Fuel Production - Chemical Reviews (ACS

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Enabling Silicon for Solar-Fuel Production Ke Sun,† Shaohua Shen,*,‡,§ Yongqi Liang,∥ Paul E. Burrows,§,$ Samuel S. Mao,*,§,$ and Deli Wang*,†,⊥,# †

Department of Electrical and Computer Engineering, ⊥Material Science Program, and #QualComm Institute, University of California at San Diego, La Jolla, California 92093, United States ‡ International Research Center for Renewable Energy, State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China § Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720, United States ∥ Department of Chemistry, Chemical Biological Center, Umeå University, Linnaeus väg, 6 901 87 Umeå, Sweden $ Samuel Mao Institute of New Energy, Science Hall, 1003 Shangbu Road, Shenzhen, 518031, China 4.3.3. Redox Couples 4.3.4. Molecular Complexes 4.3.5. Others 4.4. Summary 5. Solar-Fuel Conversion Systems 5.1. Spontaneous Water Splitting 5.2. Artificial Photosynthesis from CO2 Reduction 5.3. O2 Reduction for H2O2 Generation 6. Conclusions and Prospects Author Information Corresponding Authors Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Properties of Si and the Si/Electrolyte Interface 2.1. Optical Absorption 2.2. Thermodynamics 2.3. Kinetics 2.4. Summary 3. Surface Textures 3.1. Light Absorption and Antireflection 3.2. Si Wet Etching and Subwavelength Structures 3.3. Microstructure Arrays 3.4. Surface-Textured Si Photoelectrodes 4. Modifications of Interfacial Energetics 4.1. Metals, Metal Alloys, Silicides, and 1D/2D Carbon 4.1.1. Noble Metals 4.1.2. Non-Noble Metals 4.1.3. Surface Inversion 4.1.4. Transition-Metal Silicides 4.1.5. Metallic CNT (1D) and Graphene (2D) 4.2. Inorganic Compound Semiconductors 4.2.1. Nondegenerate Semiconductors: Oxides and III−V Compounds 4.2.2. Degenerate Nonelectrocatalytic Semiconductors 4.2.3. Electrocatalytic Late d-Band TransitionMetal Compounds 4.3. Organic Compounds 4.3.1. Polymers 4.3.2. Surface Functionalization © 2014 American Chemical Society

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1. INTRODUCTION After over 150 years of exploiting fossil fuels, nowadays humans are encountering two great global problems: the increasingly serious environmental pollution and the expected depletion of fossil fuels. To maintain our quality of life and continue sustainable development, it is imperative to reduce the excessive use of earth’s nonrenewable energy resources, in particular fossil fuels. Importantly, an alternative energy should be environmentally friendly with minimum or zero greenhouse gas emission, renewable, storable, and economical. Sunlight and seawater offer the ultimate in sustainability among clean energy resources. Together, they form a potential solution to the pending global energy and environmental crisis. Low-cost harvesting, conversion, and storage of solar energy as chemical fuels, in processes akin to natural photosynthesis, are believed to be of great economic and environmental interest. After the pioneering work by Fujishima and Honda in 1972 on solar water splitting using TiO2,1 storage of solar energy in chemical bonds through a photocatalytic or photoelectrochemical process became one of the hottest multidisciplinary research topics.2 During the past few decades, tremendous research efforts have been dedicated to the exploration and development of efficient and stable photocatalytic/electrocatalytic materials, including oxides, sulfides, (oxy)nitrides, III− V compounds, and others,3 all of which have contributed to our

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Received: November 15, 2012 Published: August 1, 2014 8662

dx.doi.org/10.1021/cr300459q | Chem. Rev. 2014, 114, 8662−8719

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Table 1. Milestones for Photocatalytic and Photoelectrochemical Water Splitting materials black TiO2 Rh2−xCrxO3/GaN−ZnO

performance/environment/condition

institute

solar-to-hydrogen power conversion efficiency under AM 1.5: 24% in water−methanol solution quantum yield at 420−440 nm: 5.9% in pure water

S3-capped CdSe with [Co(bdt)2]− complex (bdt, benzene-1,2-dithiolate) CoO nanoparticles

quantum yield 24% at 520 nm light in pH4.5 water

In1−xNixTaO4:NiOy nanoparticles np-junction GaAs/p-GaInP2 Cu2O

0.66% under visible illumination in pure water solar-to-hydrogen power conversion efficiency under 500 mW/cm2: 12.4% quantum yield in the wavelength range of 350−480nm: 40 % at 0 V vs RHE, power conversion efficiency under 0V vs RHE under AM1.5: 9.3%

5% under AM1.5 100 mW/cm2 in neutral water (unbuffered)

ref

LBNL, USA

4

University of Tokyo, Japan University of Rochester, USA University of Houston, USA AIST, Japan NREL, USA EPFL, Switzerland

5 6 7 8 9 10

solar-fuel cell. Enabling Si as a photoelectrode for practical solar-fuel production from water mimics natural photosynthetic systems, where light absorption and catalytic reaction do not happen on the same material. This approach circumvents problems caused by the intrinsic properties of Si, which will be discussed in the next section, minimizes the PV balance-ofsystem (BOS) cost which is about one-third of the total PV system cost, and could provide higher conversion efficiency. Success in this approach requires a fundamental understanding of Si photoelectrochemistry and development of materials and devices optimized for an aqueous electrolyte. Toward these goals, there have been remarkable results on fundamental science, device design and engineering, and the synthesis and screening of cost-effective electrocatalysts.19 This review covers most of the different aspects reported in the literature about silicon photoelectrochemistry. To differentiate from many recent review articles,20 the emphasis of this work is on the fundamental properties of the silicon− electrolyte interface, materials/strategies developed to modify interfacial energetics, and the engineering of Si for practical stand-alone solar-fuel conversion devices. The goals of this review are to benchmark the development of solar-fuel production using Si, to review innovative ideas, and to achieve a broader impact by benefiting not only chemists but also material scientists and chemical engineers. We believe that the joining of forces from different fields will help to bridge the knowledge gap and tackle the challenges of solar-fuel conversion. Electrochemically converting CO2 to chemical fuels efficiently and selectively still remains one of the most promising solutions to reduce greenhouse gas yet the biggest challenges to the solar-fuel community. Although major research efforts have been dedicated to development of lowcost and efficient electrocatalysts, there has been pioneering work coupling catalysts with Si light absorber. In this work, while much of the review focuses on the widely studied watersplitting problem, we also briefly address multielectron CO2 reduction in artificial photosynthetic systems in the later sections.

improved understanding of the process. Some significant milestones in photoelectrode design have been achieved, as detailed in Table 1. Despite this success, the energy conversion efficiency, cost of materials and synthesis, and durability of these materials is still insufficient to achieve the goal of highperformance (>10% solar-fuel conversion efficiency) and longterm (>10 years) solar-fuel production. Alternatively, photocatalytic/photoelectrochemical conversion of CO2 to fuels, such as CH4, CH3OH, and CO, etc., has also demonstrated its effectiveness for the pursuit of solarfuel conversion.11 The pioneering study can be dated back to 1979, when Inoue et al. observed the conversion of CO2 to small amounts of HCOOH, HCHO, CH3OH, and CH4 over semiconductor photocatalysts suspended in water under ultraviolet (UV) irradiation.12 To date, most of the studies on photocatalytic/photoelectrochemical CO2 reduction have been focused on TiO2 mainly due to its high efficiency as a photocatalyst in short wavelengths.13 Although various semiconductors have been developed for CO2 reduction14 and encouraging progress has been achieved, the solar-fuel conversion efficiencies of photocatalytic/photoelectrochemical CO 2 reduction, due to the higher difficulty in both thermodynamics and kinetics, are still several orders of magnitude lower than that of a photocatalytic/photoelectrochemical system for hydrogen generation.13,15 Silicon (Si), the second most abundant element in the earth’s crust, is widely used for photovoltaic applications due to its low cost and narrow band gap (Eg = ∼1.1 eV), which is relatively well matched to the solar spectrum. These properties also render Si a promising material for photoelectrodes in a photoelectrochemical system because a larger number of photons can be absorbed and converted compared to other wider band gap semiconductors. Furthermore, a mature technological infrastructure exists to produce highly controllable structures in Si. Since Candea et al.’s first demonstration of photoelectrolysis of water using Si in 1976,16 it has become one of the most studied materials in the early evolution of the semiconductor photoelectrochemistry field. Solar-fuel production using Si has been realized in two ways. The first approach uses a Si photovoltaic (PV) panel attached to an electrolyzer, typically composed of precious noble-metal catalysts. Although this approach has been proposed and demonstrated for centralized energy production, it may not be economically viable for large-scale H2 generation unless lowcost PV and electrolysis systems can be developed with an improved efficiency.17,18 The other less well developed approach uses Si-based photoelectrodes linked to electrocatalysts in contact with water, a so-called photoelectrolysis or

2. PROPERTIES OF SI AND THE SI/ELECTROLYTE INTERFACE We start by addressing fundamental Si optoelectronics and electrochemistry with an introduction of the important properties of Si and the Si−electrolyte interface (S/E) relevant to the solar-fuel conversion process. This includes topics such as light absorption, thermodynamic limitations, Fermi level pinning due to surface states,21 S/E energetics,22 the barrier height between Si and electrolytes,23 etc. For a broader 8663


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In contrast, GaN, ZnO, and TiO2 exhibit direct but wide energy band gaps due to the low valence band position from the filled N 2p or O 2p states. This limits their absorption to the ultraviolet (UV) region, which is only about 5% of the entire solar spectrum (Figure 2, purple area). Some other

background in semiconductor electrochemistry, readers are referred to books by Memming24 and Morrison.25 The interfacial energetics for p- and n-Si in an electrolyte solution under an equilibrium, flat band condition (Figure 1a) and

Figure 2. Solar AM1.5G solar spectrum (black curve, data from National Renewable Energy Laboratory), calculated maximum photocurrent density (blue curve), and absorption coefficient of water (red curve, data from ref 27) . Band absorptions of the solar spectrum of several commonly studied photoactive semiconductors are highlighted by the shaded areas.

Figure 1. Si band edge positions with respect to water redox levels at flat band condition in pH 0 solution (a), and energy band diagram of a p-type Si at equilibrium (b) and an n-type Si under anodic biases (c).

anodic bias at pH = 0 (the literature standard for measurement) are as shown in Figure 1. Here, Eg is the energy band gap, Ef is the Fermi level or the electrochemical potential, EVB is the potential of the top of the valence band at the interface, ECB is the potential of the bottom of the conduction band, and H+/H2 and H2O/O2 are the electrochemical potentials for proton reduction and water oxidation, respectively. Regardless of the solution, the positions of ECB and EVB of the semiconductor are generally fixed. For p-Si interfacing with the H+/H2 level (Figure 1b), the majority holes are effectively blocked by an energy barrier formed at the Si−electrolyte interface, while the light-excited minority electrons inside the depletion region can be driven by the electric field to the S/E interface. However, these energetic electrons can spontaneously reduce protons for H2 generation only if facile electron donors are present at the counter electrode. Similarly, for n-Si when operating under an anodic bias, the majority of electrons are typically blocked from reaching the electrolyte. Meanwhile, photogenerated holes rise to the EVB and are ideally capable of oxidizing water to obtain O2 (Figure 1c). Electrons excited to the conduction band are driven away from the depletion region to the bulk of the Si and then to the counter electrode for the water reduction reaction, if they can gain additional energy before reaching at the interface for the chemical reaction.

photoactive semiconductors that can absorb visible light have recently attracted a lot research interest, such as GaP, Cu2O, Fe2O3, CdSe, etc. (Figure 2, purple + blue area). The 1.1 eV band gap of crystalline Si allows effective light absorption at wavelengths from the UV to the near-infrared if the challenges of an indirect gap can be addressed (IR, Figure 2, purple + blue + gray area). This results in a higher theoretical photocurrent (Figure 2, blue curve) and a higher conversion efficiency compared to the other aforementioned materials, assuming a unity quantum efficiency (QE). In addition, Si has an acceptable electron mobility, which leads to a wide application as a photoactive material for solar energy conversions. It was reported that water, the working environment of solarfuel cells, showed a nearly perfect transmittance to the UV and visible light but a strong absorption in the IR region (red curve in Figure 2, data from ref 27). The absorption peaks of water are consistent with the dips in the solar spectrum (AM1.5), which is primarily filtered by water vapor, O2, O3, and CO2 in the atmosphere. Considerable IR loss in a 1 cm thick water layer only occurs at wavelengths above 1200 nm, where the absorption coefficient becomes high. In this case, therefore, crystalline Si can still effectively absorb light with wavelengths up to its band edge (1100 nm). Further increasing the thickness of the water layer to 10 cm will increase the light absorbed by the water layer from 36.4% to 98.9% at 970 nm, where the absorption coefficient reaches a local maximum of 0.45 cm−1 at room temperature. In addition, the absorption peaks of liquid water linearly increase with the increasing temperature due to a change in the microscopic structure of water.28 Alternatively, water vapor can be used due to its lower density than liquid phase and thus smaller linear attenuation coefficient. Splitting water using water vapor can be also realized.29 Since these two phases of water share the same mass attenuation coefficient, water vapor has a smaller linear attenuation coefficient due to

2.1. Optical Absorption

The electronic band structure of crystalline Si shows that the band edge absorption is indirect, i.e., the conduction minimum and valence band maximum are located at different positions in wave vector space (in the first Brillouin zone). The indirect band gap complicates the light absorption process and makes the absorption less efficient than in direct band gap materials.26 This leads to a poor absorption coefficient, requiring a long optical pathway (thick film) for effective light absorption. In a thick film, however, electron−hole pairs generated deeper than the minority carrier diffusion length inside the material are unlikely to be collected at the electrodes. 8664


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overpotential losses. It appears that this reaction potential lies way above the Si passivation potential (EhD), suggesting that the passivation reaction (eq 1) is thermodynamically more viable. Therefore, self-oxidation of Si competes with the oxidation of water and needs to be suppressed for an efficient and irreversible water oxidation. Stabilization methods to prevent anodic decomposition of Si were widely studied during the early development of Si photoelectrochemistry, particularly on n-type Si. p-Si passivation under idle conditions (under dark and at zero bias) is also observed and can degrade the photoconversion performance. Proper protection to block the hole transfer to the S/E surface across the barrier through surface defects is also essential for high-performance p-Si photocathodes. Note that the reductive degradation of Si is kinetically difficult at normal conditions. Generally speaking, techniques developed are to alter the surface kinetics to improve the current efficiency for the net interfacial reduction or oxidation of water while suppressing the current efficiency for growth of the insulating oxide.35 On the other hand, thermodynamic problem remains when Si is used in alkaline solution to reduce water, Si dissolution is significant and hydrogen generation can be observed in dark. Therefore, electrochemical passivation of Si photoanode and dissolution of Si photocathode when energetic holes and electrons are present at the Si water interface are one of the major challengs especially for establishing stable and efficient Si based solar fuel systems in alkaline solution. Finally, on the cathode side, unfavorable reduction processes that interfere with the H2 evolution reaction typically include oxygen and carbon dioxide reduction reactions (using CO2 conversion to CH4 as an example, which requires a minimum thermodynamic energy, compared to other CO2 reduction reactions14f)

its smaller density. Therefore, as long as the water layer on the photoelectrode is kept thin, absorption of small band gap semiconductor at long wavelength will not be limited by the water absorption. 2.2. Thermodynamics

The fundamental thermodynamic voltage required for spontaneous water splitting is 1.229 V.30 Considering all the losses, including the reaction overpotentials and the ohmic and transportation losses, a minimal voltage requirement lands in a more realistic range of 1.5−1.6 V, meaning the minimum energy band gap of a semiconductor should be at least 1.6 eV in order to be able to provide 1.229 eV of free energy for water splitting without external energy assistance.31 It appears that Si does not have enough band gap energy for spontaneous splitting of water (1.1 eV for crystalline Si, which can theoretically provide a maximum photovoltage of 0.8 V from a single junction). Although crystalline Si is not able to split water into H2 and O2 simultaneously, it is good enough for the proton reduction half reaction, since the ECB is well above the water reduction level (Figure 1a). Spontaneous H2 generation can only be realized if an electron donor that has a lower redox potential than the EH2O/O2 is added (less redox potential means higher in the energy band diagram, for example, the I3−/I− redox couple). The ECB of Si, however, is too high, which results in a low surface band bending and thus a low driving force for production of H2. Moreover, although Si is an attractive semiconductor with a broad utilization of the solar spectrum, it rapidly forms an insulating oxide in the presence of water and energetic holes at the surface. Technically, oxidation of Si occurs when a Si surface atom meets holes from the valence band that are potential induced to the p-Si|water interface, where the accumulation of majority carriers occurs upon anodic polarization. Oxidation can also occur on the n-Si|water interface, where minority carriers are excited by energetic incident photons.32 This deleterious effect can substantially suppress the photoconversion process due to a thick insulating oxide layer effectively blocking the charge transport and thus passivating the photoelectrode.33 This passivation process can be expressed in eq 1 + + Si + 2H 2O + 4hVB (hν /Va) → SiO2 + 4Haq

(4)

CO2 + 8H+ + 8e− → CH4 + 2H 2O

(5)

Therefore, a saturated solution with inert gases like N2 or Ar to suppress these side reactions and H2 or O2 gas separated gas productions to prevent back reactions, to maintain the Nernstian equilibrium potentials are normally utilized when characterizing photoelectrodes or systems.

(1)

2.3. Kinetics

The reaction kinetics of either the reduction (2-electron) or oxidation (4-electron) of water or the reduction of CO2 (8electrons) are normally multielectron processes, which are typically sluggish on semiconductor surfaces.36 Therefore, these kinetically difficult fuel-forming reactions require completely disabled thermodynamic decomposition reaction pathways as discussed in section 2.2. Moreover, a heterogeneous catalyst to improve the reaction kinetics at the surface37 is typically needed to improve the interfacial electron transfer kinetics, which dramatically increases the charge-transfer rate. This strategy will be examined in section 4. The charge-transfer kinetics of semiconductor electrochemistry under equilibrium or nonequilibrium steady state, both in the dark and under illumination, have been modeled in the literature. Methods include the conventional statistical Gerischer−Marcus model38 and the irreversible Williams− Nozik model.39 Shreve and Lewis extensively compared these two models and found that the conventional photoelectrochemistry model is in accord with all available experimental data, based on their exhaustive review of the literature.40

where hν and Va represent the incident photons and anodic polarization, respectively. This reaction potential (EhD) typically occurs at a potential range from −0.807 to −0.889 V vs reversible hydrogen electrode (RHE) depending on the form of the SiO2.30 In the case of a hydrogen-terminated Si surface, the reaction initially involves hydrogen oxidation and formation of a hydroxide layer prior to bulk oxidation34 (eq 2 uses the Si 111 surface with a single dangling Si bond, as an example) + −Si − H + H 2O + 2hVB (hν /Va) → −Si − OH + 2H+aq

(2)

The reaction rate in a given solution depends on the anodic bias level, the light intensity (or hole density), the stirring rate, and the previous history of the electrodes.33 Now consider the water oxidation reaction 2H 2O + 2h+ → O2 + 4H+

O2 + 4H+ + 4e− → 2H 2O

(3)

This reaction theoretically occurs at 1.229 V (vs RHE) at room temperature and 1 atm pressure, without considering the 8665


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suppressing recombination.56 Both effects lead to an improved photocurrent and photovoltage.40a Interfacial charge-transfer kinetics can also be altered by anodic current supplied to the Si photocathode, formation of insulating oxides, and high incident light intensity under cathodic bias. Either can cause changes in the rates of charge transfer and surface recombination. 43 The SiO x layer (stoichiometry depends on the structure and thickness) formed from the anodic decomposition of Si can significantly block the current when the thickness becomes higher than the tunneling limit (>4 nm). Moreover, surface recombination can also increase when operating a Si photocathode under high-intensity illumination and cathodic bias in acid.57 This causes the penetration of H into the Si up to 100 nm, which results in a drastic change in the properties of the surface layer through irreversible formation of an amorphous hydrogenated layer. This hydrogenation increases the resistivity of the Si due to acceptor atoms being neutralized by the incorporated H atoms.58

Therefore, simplified discussions here on factors that could affect the charge-transfer kinetics of Si photoelectrode will be conducted based on the conventional model. In the conventional model, the charge-transfer kinetics on a bare Si interface without catalysts depend on the change of the surface charge density,41 which is involved in processes such as charge storage at the surface, charge transfer to the solution, and charge recombination. Both the charge-transfer and the recombination rates depend on the illumination intensity and the bias.42 Specifically, the charge recombination rate is also controlled by the concentration of surface states or the surface recombination velocity.43 At an ideal S/E interface, a change in the potential between the bulk semiconductor and the bulk solution mainly results in a change in the potential of the space charge region in the semiconductor instead of the Helmholtz layer or the diffusion double layer in the electrolyte.44 It is estimated that when the density of surface states (due to dangling bonds at the Si surface) becomes larger than 1012 cm−2, the charge density from the surface states becomes larger than the charge density in the depletion region for a moderately doped nondegenerate semiconductor. These interface state charges can provide enough electrons to establish an equilibrium. Thus, at the interface, the Fermi level cannot move freely but is effectively pinned at the surface state. Since the number of atoms on the surface is around 1015 cm−2, we estimate that surface states with a 0.1% surface coverage are sufficient to cause Fermi level pinning. Thus, this barrier becomes independent from the redox level or the electrolyte pH. A space charge region (10−6−10−4 cm) can be developed due to the surface charges caused by electrons in the surface states.45 Also, with the Fermi level pinning at the surface states, the change of potential mainly occurs in the Helmholtz layer instead of in the space charge region in the semiconductor. This fact profoundly affects the charge-transfer kinetics in the surface chemistry and electrochemistry.46 It is worth noting that the surface states are typically uniformly distributed over a range of energies, and the density of localized surface states also affects the potential distribution.45,47 Detailed calculations on an interface free of surface states as well as a practical semiconductor surface with surface states can be found in ref 48. Roughly, the peak density of interface states often occurs near 0.27 V from the conduction band for n-Si and 0.33 V from the valence band for p-Si.49 Moreover, Si is neither a good hydrogen evolution reaction (HER) catalyst nor a good oxygen evolution reaction (OER) catalyst with extremely low exchange current density. Rather, it exhibits slow surface kinetics, which in turn result in large overpotentials for useful current values unless appropriate electrocatalysts are added. The Fermi level at the surface can be unpinned from the surface state level using surface treatments,50 the progress of which will be further discussed later in section 4. These techniques to regain control of the energy barrier at S/E were considered as one of the key focuses to either reduce the surface state density51 (to as low as 1010 cm−2) or reduce the device sensitivity to a high surface state density. Various techniques have been developed such as surface functionalization using monolayer molecules,52 oxidation, converting the surface to inversion,53 heterogeneous p−n junction,54 etc. These techniques also remain important to other photo(electro)catalytic materials.55 The charge-transfer kinetics can be thus improved by improving the barrier height, thus improving the charge separation and depletion width and

2.4. Summary

Competing thermodynamic reactions and slow charge-transfer kinetics on bare Si surfaces in contact with an aqueous solution are critical issues to solve, in addition to the relatively poor light absorption of Si. Surface alteration techniques are needed to overcome these intrinsic disadvantages of Si for solar-fuel production from water. Surface treatment affects cell performance in several ways. Controlling the gross topography modifies the ratio of the absorbed to the reflected light and the local current density. It can also modify the density and distribution of surface states (which, in turn, influence the open-circuit voltage and fill factor by suppressing recombination pathways), the electrochemical reaction rate (which contributes to the apparent internal resistance),59 and the interfacial chargetransfer kinetics (which also affect the reaction rate and the stability when self-decomposition, including reduction or oxidation reactions, becomes competitive).60

3. SURFACE TEXTURES Similar to structures used in solid-state photovoltaic cells, surface textures can be preferentially introduced to the crystalline Si photoelectrode to effectively deal with the poor light absorption.61 Efficient light management from surface textures can be due to the suppressed Fresnel reflection from a smooth transition of refractive indices from the electrolyte to the bulk Si,62 enhanced internal reflection, and intensity multiplication.63 Surface textures could further suppress the incident angle effect and minimize the polarization sensitivity.64 By these means, light absorption can be enhanced and hence the energy conversion efficiency can be boosted. Because of the more efficient light utilization, surface-texturing techniques are believed to offer the potential for cost reduction using less material while maintaining a performance comparable to that of the bulk substrate.65 In addition to light management, surface textures provide enlarged junction areas and more efficient collection of photogenerated minority carriers, which decouple the light absorption and charge separation directions for an improved charge collection efficiency.66 In addition, surface textures fabricated from a bottom-up or top-down method can provide a defect-free crystal with a reduced charge recombination. The increase in the surface area is characterized by the roughness factor, defined as the ratio of the surface area of the textured surface to the surface area projected onto a plane. 8666


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Figure 3. Surface textures on single-crystal Si for reduced reflection and enhanced absorption. (a) Nanopillars from dry etching through a selforganized silica sphere mask. Reprinted with permission from ref 69. Copyright 2008 WILEY-VCH Verbg GmbH & Co. KGaA, Weinheim. (b) Nanotips from plasma etching. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials (ref 72), copyright 2007. (c) Nanopillars from sphere nanoimprinting and RIE etching. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials (ref 77), copyright 2012. (d) Inverted nanopyramid from interference lithography and KOH wet etching. Insets show scanning electron microscopic images of the surface structure. Reprinted with permission from ref 75. Copyright 2012 American Chemical Society.

Introducing surface textures, however, typically reduces the open-circuit voltage because of the reduced splitting of quasiFermi levels at the S/E interface due to recombination.50,66,67 In this section, we will review studies on surface-textured Si for improved light utilization. Techniques to construct subwavelength nanostructures and microscale textures will be discussed. Finally, the effect of texturing on the photoelectrochemical performance will be summarized.

To provide a direct comparison of the profile-dependent absorption, we did a simulation of textured Si substrates based on effective refractive media theory using the transfer matrix method.78 Si wire arrays with two transition profiles of refractive indices were compared, including a step profile, typical of a cylindrical wire array, and a parabolic profile from a cone-shaped array (Figure 3a, insets i and ii). The effective refractive index transition profile from water media (nm = 1.33) to the Si substrate (nSi = 4.015) of these two structures at a wavelength of 570 nm are shown in Figure 4a. The effective refractive index of the cylindrical wire array (nw) remains constant at 1.5985, with sharp transitions at the water/wire and wire/substrate interfaces (red curve in Figure 4a). However, the cone-shaped wire array with a parabolic profile shows (nc) a smoothed transition at both interfaces (green curve in Figure 4b), which is recognized as the main reason for light trapping.79 The mathematical model of nw and nc can be described by the two equations as given below

3.1. Light Absorption and Antireflection

To enhance the absorption of crystalline Si without sacrificing the charge carrier collection efficiency over a thick absorption layer, orthogonalizing the directions of light absorption and charge carrier separation was proposed using a textured surface. In other structures, light is effectively trapped due to a smooth transition of the refractive indices of the media to that of the substrate. Internal reflection can be enhanced, the absorption wavelength range can be broadened, and the sensitivity to both incident angle and light polarization can be minimized. The photoresponse to long wavelengths, where carriers are generated deep in the bulk Si, can be effectively improved. Light trapping provides a significant improvement in the generation of free carriers and thus theoretically a higher photocurrent. If a wire array is used for light trapping, the process greatly depends on the physical geometry and the profile of the array.62 Various profiles were introduced and analyzed62,68 both experimentally and computationally. Examples include a random array, a cylindrical wire array with uniform diameter69 (Figure 3a and 3c), a multisize cylindrical wire array,70 a pyramidal array,71 a cone array with sharp tips72 (Figure 3b), a sphere array,73 a hole array,74 an inverted pyramid array75 (Figure 3d), and a wire array with other controlled sidewall profiles.76

n w = ff × nm + (1 − ff ) × nSi

(6)

nc = nm + (nSi − nm) × (10t 3 − 15t 4 + 6t 5)

(7)

where t = [0, L] and L is the wire length. It is assumed that the Si is immersed in an aqueous electrolyte, all the spaces between the wires are filled with water, and nm of water is wavelength independent. The real and imaginary parts of the refractive index of Si are wavelength dependent, and data used in this simulation were based on a refractive index database from Filmetrics.80 For simplicity, the wavelength-dependent absorption coefficient of Si and the extinction coefficient of water (Figure 2, red curve) were not considered in this set of simulations. The wavelength dependence of reflectance as a function of wire length and 8667


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3.2. Si Wet Etching and Subwavelength Structures

To create a textured surface on a crystalline Si, a standard chemical method has been adopted by the PV industry since 1960. The technique uses a mixture of sodium or potassium hydroxide (NaOH or KOH) with water or isopropyl alcohol (IPA). The resultant etching is anisotropic on single-crystal Si, initially used for MOS transistors. Alkaline anisotropic etching are capable of producing three-dimensional structures in a controllable and reproducible manner. This mechanism is due to the strong dependence of the etch rate on crystal directions. 8 3 The etch rate ratio (selectivity) of ⟨110⟩:⟨100⟩:⟨111⟩ was found to vary from 50:30:1 at 100 °C to about 160:100:1 at room temperature. Morphology and etch rate can be altered by the temperature, solution composition/ concentration, diffusion, dopant concentration, and lateral high index plane etch rate.84 Alkaline wet etching without application of masks yields a typical surface morphology on Si (Figure 5a).85 In order to reach controllable smaller textures,

Figure 4. Comparison of a cylindrical wire array (with a fill factor of 0.5) and a cone array, reflectance versus wire length and incident angle. (a) Transition profiles of the refractive indices from that of water to that of the Si substrate: step profile ((i) red) and parabolic profile ((ii) green). Insets show schematics of these two simulated wire arrays. Length-dependent (b and c) and incident-angle-dependent (d and e) reflectance spectrum over a wavelength range from 250 to 1100 nm.

incident angle of arrays with step and parabolic profiles are shown in Figure 4b/c and 4d/e, respectively. It can be seen that in the cylindrical wire array reflectance is only weakly dependent on wire length, particularly at wavelengths below 400 nm. This is primarily due to the poor absorption of Si in the UV region (Figure 4b). Resonance peaks are shifted with the increasing wire length. The incident angle effect for the cylindrical wire array with a wire length of 1 μm and fill factor of 0.5 is shown in Figure 4d. As the incident angle is increased from parallel (0°) to near perpendicular to the wire axis (80°), the reflectance increases over the entire spectrum. Due to the vertical orientation and considerable area of the flat tip, the reflectance remains highest at normal incidence (0°). A similar result was reported before on microwire arrays.81 For the coneshaped wire array, a broad band suppression of reflectance and minimized resonance over a wide wavelength region is obtained. Reflectance can be further reduced using longer wires. It can be seen that with a wire length above 1 μm the reflectance spectrum does not change with increasing wire length, indicating a minimum length requirement for effective light trapping using this cone-shaped wire array structure (Figure 4c). The incident angle effect on the cone array with a length of 1 μm is also shown in Figure 4e. Under normal incidence (0°), reflectance is effectively suppressed due to the strong coupling of the light at the wire tip. Although the reflectance increases with an increasing incident angle similar to that observed with the cylindrical wire array, a broad band reduction of reflectance is realized using the cone array with a parabolic transition profile of refractive indices, which is consistent with previous results.82

Figure 5. Scanning electron microscopic images of (a) a micropyramid array from an alkaline wet etching of (100) Si (Reprinted from Xi, Z.; Yang, D.; Que, D. Sol. Energy Mater. Sol. Cells 2003, 77, 255 (ref 85), copyright 2003 with permission from Elsevier.) (b) a randomized Si microwire array fabricated from chemical vapor deposition (Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials (ref 81), copyright 2010), (c) a hierarchical microhole array from a patterned dry etching with ClF3 gas (Reprinted from Saito, Y.; Kosuge, T. Sol. Energy Mater. Sol. Cells 2007, 91, 1800 (ref 74), copyright 2007 with permission from Elsevier), and (d) an amorphous Si nanocone array fabricated using a nanosphere lithography and RIE etching (Reprinted with permission from ref 100. Copyright 2008 American Chemical Society).

IPA typically has a higher concentration than the NaOH in the mixed etchant. However, IPA as a solvent is flammable, is a higher percentage of the cost in the IPA/NaOH system, and is hard to dispose of. All these disadvantages have led to a search for IPA-free alternatives. For example, aqueous Na3PO485 and aqueous Na2CO3/NaHCO386 have been examined without addition of IPA. Here, PO43−, CO32+, and HCO3+ ions were believed to play a role similar to IPA in initiating pyramid formation. The etching process was then conducted by the hydroxide ions formed from the dissociated phosphate ions. Nevertheless, Si etching using a solution containing potassium or sodium have some limitations.87 These ions are believed to contaminate the dielectrics in microelectronics as well as in optoelectronic devices, which could potentially degrade their 8668


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etching method. Metal nanoparticles (NPs) were first electrodeposited by dipping a Si substrate in a metal ion-containing HF solution. Then, the metal NP-coated Si substrates were continuously etched in Fe3+−HF solution.103 In this etching scheme, Fe3+ ions work as oxidizing agents for the Si. Metal ions in the etchant can be substituted by H2O2104 and other oxidants.105 A combination of alkaline etching in dilute KOH was used following two-step electroless etching,106 which further improved the antireflection effect due to generation of tapered Si nanowire tips. All of the techniques mentioned above are applicable to the lithography-free, maskless, low-cost surface texturing of Si. In these cases, textures are self-oriented and randomly formed. To define features on a Si surface which have spatial ordering and uniform diameters for designed optical properties, one can use patterning techniques such as lithography, electron-beam lithography, focused ion beam, or nanolithography using various scanning tips to pattern metal masks deposited either by electrochemical or by gas-phase deposition. In addition, one can also use self-assembled polystyrene bead lithography107 or an anodic aluminum oxide (AAO) film as a mask108 or interferometric lithography,75 block copolymer photolithography,109 or a stamping/nanoimprinting method for the same purpose110 Application of these technologies is a trade-off between design flexibility and productivity.

performance and stability.88 For these purposes, other singlechemical solutions have been developed, such as tetramethylammonium hydroxide (TMAH)87 or hydrazine (N2H4) in aqueous solution.89 However, problems with these two etchants remain, such as a high toxicity and cost, as well as lower Si 100/ 111 planes selectivity than alkaline/IPA solution. Since the aforementioned alkaline texturing of Si only works based on the selectivity between 100/110 and 111 planes as well as between Si and SiO2, its application to polycrystalline, nanocrystalline, or amorphous substrates is limited. Laboratory bulk Si processing to create holes and pillars in substrates other than single-crystal Si have been developed (Figure 5d) based on gas-phase dry etching,90 reactive ion etching,69 and ultrafast laser texturing.91 One can also use textured conductive substrates (ZnO:Al or stainless steel), which is typically seen in tandem or multijunction amorphous or nanocrystalline Si solar cells. Although dry etching has been proposed and investigated, wet etching using chemical solutions remains the cost-effective method for mass production. Acidic texturing of Si, particularly polycrystalline Si, has been developed.92 Different from alkaline etching in which electrons are typically needed and H2 is generated at the Si surface, acidic etching needs positive carriers (holes) for the reaction.93 The band-bending situation is now different due to the pH-dependent equilibrium between the Si Fermi levels and the electrolyte redox couples. However, this method is less selective to crystal plans and is thus isotropic and typically gives higher reflectivity and less texturing compared to the alkaline anisotropic method. Typically, various ratios of HF and HNO3 in water were used with and without some acid additives, such as H3PO4,94 H2SO4, NaNO2,95 water-soluble carboxylic acid,96 etc. As discussed in section 2.2, Si photoelectrochemical oxidation is one of the most critical issues to overcome in photoelectrochemical devices in order to ensure a useful operating lifetime for the photoelectrode. However, when this instability-related phenomenon is controlled well, it can be very useful for producing highly ordered surface textures. Porous Si fabricated from photoelectrochemical etching was first introduced in the 1950s, when an electrochemical polishing experiment was carried out at Bell laboratories.97 Electrochemical etching of p-type Si in the dark and n-type Si under illumination in a fluoride-containing electrolyte is typically used to dissolve the Si oxidized by energetic holes, and a porous surface can thereby be formed. In addition, the morphology of the pores can be further controlled by adjusting parameters such as the HF concentration, the pH of the electrolyte, the etching temperature, the substrate dopant concentration, the Si crystal orientation, and the illumination intensity.98 Photoelectrochemical methods are recognized to be the most costeffective methods for controllable fabrication of porous Si for various applications such as reflectors, waveguides, photonic crystals, antireflection coatings, and sensors. An important early review paper about the porous Si-forming mechanism was published by Smith and Collins in 1992 providing a close look at the formation models.99 Metal-assisted chemical etching, termed electroless etching, was proposed by Zhu’s group.101 Large area textured Si can be fabricated from an HF solution containing metal ions such as Ag, Fe, Ni, etc.102 without external anodic bias or illumination. This mechanism entirely relies on a localized microelectrochemical redox reaction. Later, to investigate the fundamental mechanism, researchers from the same group used a two-step

3.3. Microstructure Arrays

Microscale Si wires were developed with a wire diameter similar to the minority carrier diffusion length in the Si in order to achieve efficient carrier collection.111 Light trapping in a Si microwire array was studied experimentally (Figure 5b)81 and computationally112 based on ray optics for wires significantly larger than the wavelength of light. This is quite different from subwavelength nanoscale structures, in which wave optics dominates. Bottom-up synthesis of a Si microwire array relies on the vapor−liquid−solid (VLS) mechanism using chemical vapor deposition (Figure 5b),113 and it can also be obtained through top-down wet etching or dry etching of a planar Si substrate. For example, metal Cu islands were patterned and confined in a thermal Si oxide film (300−500 nm in thickness), which acted as a guide for vertically aligned wire growth. pType doping in Si wires was realized by controlling the BCl3 concentration in the reactive gas incident on a photoinactive (111) p+-Si substrate. Majority carrier concentrations of ∼ 7 × 1017 cm−3 showed an optimum photoelectrochemical energy conversion performance, presumably due to the combination of wire geometry and redox chemical potential leading to optimized junction energetics. As-grown wires were then chemically treated to remove Cu silicide and Cu contaminants near the Si wire surface and native oxides developed during growth. A modified system using a cold-wall process and a rectangular quartz tube (holding a 6 in. wafer) provided predictable growth over a large area,114 showing a potentially commercializable technique. A radial solid-state p−n junction can be formed by thermal diffusion of a solid source, which results in a shallow diffusion profile in Si microwires. A 200 nm junction depth was realized on a planar control sample with a background concentration (ND) of 2.2 × 1016 cm−3, which was more than 1 order of magnitude lower than that of the wire (7 × 1017 cm−3). Theoretically, because of the higher ND, the junction depth should be shallower on the microwire surface considering an identical processing temperature and drive-in time period, 8669


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assuming the wire structure does not significantly disturb the diffusion process. Considering the microwire diameter of 2.8 μm and the thickness of the highly doped emitter (surface inversion), this structure can be treated as a narrow base emitter in a bipolar junction transistor (BJT). This surface-inverted microwire structure has successfully demonstrated outstanding photovoltaic performance.66,115 The as-grown Si microwire array was tested, and a reduced VOC and a lower JSC compared to those from the planar control sample were observed. However, this JSC value was larger considering the low fill factor (∼12.6%) of the wire array with respect to the optical plane. This was primarily because of the efficient light management in the wire and the large surface area.116 Photoelectrodes using p- and n-Si microwires in an organic electrolyte have also been studied in detail. 53b,114,117 Optimizations including a wire pattern for efficient light absorption, wire design for better carrier collection, and mass transport of ions when merged in aqueous electrolyte were also investigated.81,118 Si microwires decorated with high-performance electrocatalysts and heterostructures using a Si microwire array as a platform for solar-fuel production have been reported, which will be discussed separately in section 5. To eliminate the use of high-cost crystalline Si as well as high-cost gas-phase growth facilities, researchers continue to search for cost-effective alternatives to produce Si photoelectrodes. For example, Cho et al. explored the electrodeposition of Si for photoelectrode applications.119 Electrodeposition of Si was realized from a direct electroreduction of SiO2 in a CaCl2 molten salt under a constant bias at a high temperature (up to 850 °C). The p-type photoresponse of the Si was believed to be due to incorporation of Ag metal catalyst during electrodeposition. A comparable photoresponse to that of crystalline p-Si was realized. However, a sharp drop of the photocurrent was noticed due to surface recombination at the Ag, suggesting optimization of the process is needed for future work. Low-temperature deposition (as low as 80 °C) of crystal Si particles (∼500 nm) was recently demonstrated by Gu et al.120 to significantly reduce the energy input for Si electrodeposition, inviting an examination of its optoelectrical and photoelectrochemical properties.

Figure 6. Porous p-Si photocathode showed (a) a reduced quantum yield and photosensitivity with increased porous Si thickness and (b) a decreased spectral response in short and long wavelengths on samples with a thicker porous layer. Reprinted with permission from ref 121. Copyright 1985 The Electrochemical Society.

photoanode was realized in an aqueous electrolyte containing 0.01 M H2SO4 without additional protective coatings or catalyst loading. It was shown that the onset potential for H2 evolution was anodically shifted on the black Si photocathode. This surface area-induced anodic shift was presumably due to the catalytic activity of metal impurities in porous Si and an increased number of chemical reaction sites. A poor spectral response from the nanoporous Si at short wavelengths was likely due to a high surface recombination rate originating from the increased surface area and recombination at surface defects similar to the p-Si case. This loss of blue response suggests that the maximum H2 production rate might actually decrease even further if the effective surface area of the nanostructured electrode is increased. On the other hand, an increased surface area due to the porosity also increased the bubble nucleation density, which further led to an increase in the hydrophilicity of the nanostructured surface.123 This could maximize the gas evolution and minimize the current fluctuation. Although the photoanode remained alive over a much longer period (4−5 h) than the polished n-Si, the stability is still far from that required for practical use. A similar nanoporous structure was recently studied, and consistent conclusions were reported.124 Oh et al. used 20 μm long Si nanowires, also from a chemical etching process and subsequently decorated with Pt nanoparticles.125 They demonstrated an anodic turn-on voltage at −0.42 V vs RHE and 17 mA/cm2 at 0 V vs RHE in H2SO4 and 0.5 M K2SO4 solution (pH = 1). Compared to the performance of the platinized planar Si photocathode, the black Si photocathode showed an earlier onset but a lower light-limited photocurrent. This may suggest a different Pt loading in these systems, which thus caused a higher catalytic activity and a higher shadowing effect, since the electrodeposited Pt nanocatalysts tended to stay on top of the Si nanostructures. A degradation of the

3.4. Surface-Textured Si Photoelectrodes

Anodically etched n- and p-Si with surface textures have been studied as a photoanode and a photocathode by Koshida et al.121 With the increased thickness of the porous layer from 8 to 57 μm, the photosensitivity (defined as the ratio of the saturation current to the light intensity) and the quantum efficiency (obtained at 632 nm laser with a power of 1 W) were both decreased, but they saturated when the porous layer thickness grew beyond 30 μm (Figure 6a). The degraded sensitivity at short wavelengths ( 635 nm with intensity of 38.6 mW/cm2), (●) planar p-Si|Mo3S4 and (left pointing triangle) p-Si microwire|Mo3S4351 (H2-saturated 1 M HClO4 solution under filtered AM1.5G > 635 nm with intensity of 28.3 mW/cm2), (▰) textured p-Si|Mo3S4, and (right pointing triangle) textured p-Si|CoW3S4353 (H2-saturated 1 M HClO4 solution under filtered AM1.5G > 635 nm with intensity of 28.3 mW/cm2). (---) a-Si pin|TiO2|NiMo220 (0.5 M potassium hydrogen phthalate pH 4 aqueous solution with light intensity of 100 mW/cm2). Planar p-Si| Pt, p-Si microwire|Pt, planar pn+-Si|Pt, pn+-Si microwire|Pt116 (H2saturated 1 M H2SO4 solution) are also plotted for comparison. Data derived from corresponding references. 8685


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Table 2. Kinetic Parameters for Some Earth-Abundant Transition-Metal Compound HER Catalysts Reported and Noble-Metal HER Catalysts for Comparison catalysts Pt PtRuIr PtBi NiMoCd MoS2 MoS2 MoS3 MoS2(1T) MoS2/rGO MoS2 MoSe2 MoSe2/C Li−MoS2(1T)b a-MoS3 MoS2:O MoS2/RGO MoSx/NCNT MoSx/PPy Co−S Co−MoS3 Cu2MoS4 MoB MoN NiMoNx/C Co0.6Mo1.4N Mo2C/CNT Mo1Soy FeMo FeP FeS CoCat Co(OH)2 CoWSx CoSe2/C WS2 (1T) Ni2P Ni2P CoP NiVO

b (mV/dec) 30 30 26 55 50 39 43 41 105−120 105−120 59.8 43−47 41 55 41 40 29 93 43 95 59 54.5 35.9 55.2 62.7 78 67 150 140 78 42.1 60 46 100 50 33.8

j0 (mA/cm2)

ξ(mV@10 mA/cm2) −27 −131

1.0 3.6 × 10−4 0.56 10−2.2 5.0 × 10−4 6.7 × 10−4 1.2 × 10−4

−75 −232 −225 −188 < −187 < −150 < −400 < −450 < −250 < −200 −230 < −150 ∼−140 −100 < −50 < −200 −181 −219 ∼−210 >−220 >−250 ←200 −150 ∼−175 −85 ∼−220

2.2 × 10−3 2.0 × 10−3 3.8 × 10−4 2 × 10−4 10−4.7 0.01 0.03 0.56 0.256 5.0 × 10−4 0.04 2 × 10−3 3.6 × 10−2 0.24 0.23 1.4 × 10−2 3.7 × 10−2 0.67 6.6 × 10−4 10−2.5

−547 −186a ∼−250 ∼−130 ∼−200 ∼−100 ∼−250 ∼−75 −118

10−2.25 4.9 × 10−3 ∼10−2 3.3 × 10−2 0.14 2.7 × 10−3

condition

ref

0.5 M H2SO4 (aq) pH = 0.4 H2SO4 (aq) 1 M NaOH (aq) pH = 0.23 H2SO4 (aq) 0.5 M H2SO4 (aq) pH = 7 PBS solution (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 1 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) pH = 7 PBS (aq) pH = 7 PBS (aq) 0.5 M H2SO4 (aq) 1 M KOH (aq) 0.1 M HClO4 (aq) 0.1 M HClO4 (aq) 0.1 M HClO4 (aq) 0.1 M HClO4 (aq) 0.1 M HClO4 (aq) 30 W/O KOH (aq) 0.5 M H2SO4 (aq) 0.1 M PBS pH = 7 (aq) 0.5 M K−Pi (aq) 0.1 M KOH/LiOH (aq) pH 7 PBS (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 0.5 M H2SO4 (aq) 1 M KOH (aq) 0.5 M H2SO4 (aq) 1 M KOH (aq)

331 332 333 334 285 290 308 294 335 297b 297b 297a 296 300 336 304 305 337 143a 308 338 314 309 309 311 312 315 339 316 340 318 320 327 341 295 323 342 343 344

a The value of 186 mV at 10 mA/cm2 is extracted from ref 320 at Co(OH)2-coated Pt (111) surface with a coverage of 40%. bAt 1.1 V vs Li+/Li with Li metal as anode and 1.0 M LiPF6 in 1:1 wt/wt ethylene carbonate/diethyl carbonate (EMD Chemicals) as electrolyte

turnover frequency (TOF) of up to 400 s−1, biomimetic Mncomplex catalysts have been developed for efficient water oxidation.356 Specifically, manganese oxide has attracted a lot of research interest. Various manganese oxides with different stoichiometries (Mn2O3, MnO2, and Mn3O4) and phase (δ-, α-, β-, γ-mixed phase, or amorphous) and synthesized by different methods (electrodeposition, printing, and etc.) have been widely studied and screened in the literature.357 For example, Gorlin et al. presented a MnOx-based electrocatalyst deposited using a constant potential358 or a cyclic scan.359 This catalyst showed bifunctional activity in both ORR and OER directions (Figure 21a). A current density of ∼0.5 mA/cm2 at 1.23 V vs RHE was reached in 0.1 M KOH, approaching those measured using Ru and Ir metal catalysts. For another example, Zaharieva et al. recently reported that controlling the electrodeposition condition could modify the MnOx activity.360 Using a controlled voltage-sweeping technique (MnCat), Zaharieva

was found that Cu-containing MoSx showed a reduced Tafel slope and an improved photocurrent density, while Cocontaining WSx showed an unstable performance (curve designated with a right pointing triangle in Figure 20e).302 Similar findings were also reported by Tran et al.338 Recently, Laursen et al. utilized a sulfurized Mo or W metal as a protective coating on a pn+-Si photocathode.354 Although the MoSx or WSx coating showed inferior catalytic activity compared to high surface area nanocatalysts, this work has demonstrated a one-step method to effectively protect and catalyze the photocathode for water reduction. The solar to H2 efficiencies of above-discussed p-Si photocathodes are calculated based on replotted curves in Figure 20e, and comparative performances are summarized in Table 3. 4.2.3.2. OER Catalysts. 4.2.3.2.1. Mn−OER. Inspired by nature’s Mn-centered (Mn4Ca) molecular complex, which is considered to be the most efficient and stable catalyst with a 8686


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Table 3. Comparative Performance of Si Photocathodes with Different Structuresa photoelectrode structure

JSC (mA·cm−2)

VOC (V)

FF

intensity (mW·cm−2)

efficiency (%)

ref

planar p-Si|Pt p-Si microwire|Pt planar pn+-Si|Pt pn+-Si microwire|Pt planar p-Si|SiO2|Ti|Pt planar p-Si|Au|mono-Pt planar p-Si|SiO2|Rh planar pn+-Si|NiMo pn+-Si microwire|NiMo planar pn+-Si|Ti|MoSx planar p-Si|Mo3S4 pn+-Si microwire|Mo3S4 texture p-Si|Mo3S4 texture p-Si|CoW3S4 p-Si|W2C|Pt planar a-Si pin|TiO2|NiMo

23 7.3 28 15 5.4 17.5 11 22.3 10.3 16.3 7.91 9.28 9.18 NA 18.6 11.12

0.3 0.16 0.56 0.54 0.49 0.24 0.2 0.53 0.49 0.38 0.15 0.15 0.14 −0.06 0.5 0.87

0.3 0.18 0.6 0.71 0.18 0.18 0.11 0.31 0.45 0.47 0.19 0.25 0.16 NA 0.35 0.47

100 100 100 100 100 100 100 100 100 38.6 28.3 28.3 38.6 38.6 100 100

2.1 0.21 9.6 5.8 0.48 0.75 0.24 3.6 2.2 7.56 0.76 1.21 0.53 NA 3.2 5.5

116 116 116 116 138c 138b 145 349b 349b 352 351 351 353 353 355 220

a

Data derived from corresponding references for comparison purposes. bJSC: short-circuit current density at V = 0 vs RHE. VOC: open-circuit voltage when anodic current turns to cathodic current. FF: fill factor.

the same conditions. At an overpotential of 590 mV, the current density reached 1 mA/cm2 (Figure 21b). Identification of the oxidation states was conducted using an electrochemical measurement and X-ray absorption spectroscopy (XAS). It was revealed that the active MnCat from the controlled voltagesweeping technique contained a mixture of Mn(III) and Mn(IV), while inactive MnCat contained mainly Mn(IV). Detailed structural studies also revealed that the presence of diμ-oxo-bridging ([Mn−(μ-O)2−Mn]) in the active MnCat was crucial to its superior catalytic activity, similar to nature’s Mn4Ca complex and other first-row transition-metal catalysts containing Co and Ni. In part, the high activity of MnCat was also attributed to the high concentration of mono-μ-oxo bonds ([Mn−(μ-O)−Mn]), a relatively low concentration of saturated μ3−5-oxo bonds ([Mn3,5−(μ3,5-O)]), and a high availability of coordination sites for water binding. However, the reason for catalytic activity loss in the stability test was still unclear and might result from a slow dissolution of the catalyst. Very recently, Takashima et al. provided insights on the reaction mechanism, providing guidance on developing strategies to further lower the overpotential of MnO2-based OER catalysts.361 They found that effectively suppressing the charge disproportionation of Mn(II) and Mn(IV) from Mn(III) is important to maintain a high activity of MnO2. Surface functionalization using an amine-containing polymer (poly(allylamine hydrochloride), PAH) through N−Mn bonds stabilized the surface Mn(III) species from disproportionation into Mn(II) and Mn(IV) at neutral pH and can further reduce the overpotential (500 mV less) and thus enhance the activity. Important insights about the structure−activity relationships of the Mn-oxide-based OER catalysts prepared from symproportionation deposition−precipitation and incipient wetness impregnation methods were also investigated, drawing similar conclusions.362 Addition of other cations to the Mn oxides has been studied, which is a facile approach to tune the OER catalytic activity. For example, a hydrated CaMn 2 O 4 synthesized from precipitation of a mixed aqueous solvent was introduced by Najafpour et al.363 Incorporation of Ca greatly enhanced the catalytic performance. Interestingly, the hydrate catalysts showed a better activity than the anhydrous ones in their

Figure 21. Mn and Fe oxide-based OER catalysts. (a) MnOx prepared from cyclic electrodeposition (0.1 M KOH solution). Reprinted with permission from ref 359. Copyright 2013 American Chemical Society. (b) MnCat prepared from constant bias electrodeposition, squarewave voltammetry (light blue and green-blue curve, B and A), and cyclic voltammetry (black curve, C) (0.1 M pH 7 phosphate-buffered solution). Reproduced from ref 360 with permission of The Royal Society of Chemistry. (c) FeMoOx from a coprecipitation method (1 M KOH solution). Reprinted from Singh, R. N.; Madhu; Awasthi, R.; Tiwari, S. K. Int. J. Hydrogen Energy 2009, 34, 4693 (ref 367), copyright 2009 with permission from Elsevier. (d) FeCoNiOx from a photochemical metal−organic deposition method (0.1 M KOH solution). Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette, C. P. Science 2013, 340, 60 (ref 241). Reprinted with permission from AAAS.

and co-workers obtained an improved OER catalytic activity compared to the catalysts produced using cycling between two constant potentials358 and in various buffer solutions. At an overpotential of 565 mV, the Faradaic efficiency was close to 100% and the TOF is about 0.01 s−1, which was comparable to Co- (0.017 s−1) and Ni-based (0.01 s−1) electrocatalysts under 8687


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higher than the activity of the Co−Pi catalyst. This film was crack free and provided stable current for more than 50 h, which showed a great potential to protect and catalyze the n-Si photoanode. Supported Co3O4 nanoclusters using mesoporous silica (SBA-15) synthesized using a thermal annealing method was reported by Jiao and Frei.373 The synthesized Co3O4 nanorods with an average diameter of ∼7.6 nm typically contain 14 rods per bundle (a nanorod cluster), and an average rod length of 50 nm. They showed a spinel structure in the SBA-15 scaffold. The estimated TOF of samples with SBA-15/4% Co3O4 in mild conditions (pH = 5.8) was about 1140 s−1, which remains the highest among the reported Co−oxide-based catalysts. Introduction of other cations can also improve the OER catalytic activity. For example, Li et al. presented a Ni-doped Co3O4 nanowire array synthesized from a solution-based method. It showed a considerable OER catalytic activity.374 Mixed-valence states of Co(II/III) and Ni(II/III) were discovered in the doped nanowires. Addition of Ni to the Co3O4 nanowire enhanced the conductivity and surface roughness and increased the active surface sites compared to the pristine Co3O4 nanowires. It was found that the optimum OER activity and stability were realized at a 1:1 ratio of Ni:Co in a NixCo3−xO4 nanowire array by Lu et al.375 To further obtain a high specific surface area, a mixed Ni−Co oxide in the form of an aerogel was prepared.376 The aerogel was synthesized from a Co and Ni chloride gel and dried in a supercritical dryer. The NiCo2O4 aerogel with a specific surface area of ∼134 m2/g, which was 10 times larger than typical NiCo2O4 nanoparticles, showed the lowest overpotential of 184 mV at 100 mA/cm2. This superior OER activity was due to a high porosity and a low charge-transfer resistance associated with the OER process. Recently, Liang et al. introduced Mn into the Co3O4 together with nitrogen-doped graphene (MnCo2O4/N-rmGO, 5 nm particle size).377 This nanocomposite was synthesized by a lowtemperature solution-based method. The Mn-substituted Co3O4 showed a lower Co(III) concentration, while Mn in the hybrid was mainly in the 3+ state. Co(III) and Mn(III) were considered as the OER and ORR active sites, respectively. Addition of Mn led to reduced OER activity compared to the pristine Co3O4.378 However, the MnCo2O4/N-rmGO exhibited a much higher OER activity than pristine MnCo2O4, suggesting a promising strategy for developing novel OER electrocatalysts using graphene. The presence of the nitrogen-doped-graphene effectively reduced the charge-transfer resistance195c (Figure 22a). For another example, Koninck et al. synthesized CuxCo3−xO4 (x = 0 and 1) nanoparticles from a sol−gelbased method.379 Addition of Cu reduced the resistivity of the Co3O4 and most importantly the Co−OH bond strength, both of which led to an enhanced catalytic activity of CuCo2O4 nanoparticles. Research efforts to optimize the first-row transition-metalbased electrocatalysts continue with the hope of outperforming the noble metals or their oxides in order to replace expensive elements in the scaled-up systems. Ternary metal oxides such as Co−Fe−Cr oxide380 and Co−Fe−Ni/Mn oxide381 were also investigated. Recently, Suntivich et al. reported that a quaternary perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) showed a much higher OER activity than the state-of-the-art IrO2 catalysts in an alkaline medium (Figure 22b).382 Instead of using the surface binding energy between metal and OH ions as the activity describer,279,383 a new describer was developed. It

study. Other transition metals were also added to MnOx. For example, Ghany et al. presented a Mn−Mo−Fe oxide using anodic deposition,364 which was used to efficiently electrolyze 0.5 M NaCl solution at pH = 12 with an enhanced OER efficiency and stability. Matsui et al. presented a ternary Mn− Mo−W oxide from anodic deposition.365 They noticed the catalytic activity, reaction selectivity, and durability of the catalysts can be tuned by the electrochemical deposition conditions including chemical composition, deposition temperature, current density, and solution pH. 4.2.3.2.2. Fe−OER. High activity has been noticed using amorphous metal oxide OER catalysts. Smith et al. performed a synthesis of amorphous iron oxide films based on photochemical metal−organic deposition (PMOD) through a UV illumination and annealing processes, a method that can precisely control the composition.241 The film annealed at temperatures below 500 °C showed an amorphous phase and a uniform structure without cracks. Hematite (α-Fe2O3) was noticed at temperatures higher than 500 °C, suggesting a potentially photoactive phase. Electrochemical activity studies on the film annealed at 100 °C revealed that Ni- and Co-doped amorphous Fe2O3 films (Figure 21d) showed comparable OER activities to those of the noble-metal catalysts (e.g., IrO2 and RuO2).366 Dopants can be also introduced in Fe2O3 to tune the activity. For example, Singh et al. introduced a mixture of Mo and Fe oxides through coprecipitation of a mixed aqueous solution.367 It was found that an OER electrocatalyst with a Mo/Fe ratio of 1.0 was able to provide the best activity with the lowest Tafel slope of 35 mV/dec (Figure 21c). To guide an efficient search for dopants in Fe2O3, Liao et al. recently used periodic density functional theory + U calculations to screen the cation doping effect in the Fe2O3 (including Ti, Si, Mn, Co, Ni, and F).368 A predicted “volcano plot” was obtained, where Ni and Co were considered as favorable dopants showing improved OER catalytic activities compared to the pristine Fe2O3, inviting further experimental verification of their findings. 4.2.3.2.3. Co−OER. Theoretical calculations suggested that Co− and Ni−oxides are among the best OER catalysts with an overpotential comparable to that of the best PGM-oxides such as RuO2 and IrO2.279 Nanostructured Co3O4-based OER catalysts containing Co(II) and Co(III) have also been studied extensively.369 For example, Cong et al. showed a photoanode consisting of a Ta3N5 nanotube array decorated with Co3O4 nanoparticles with a comparable OER activity to IrO2-coated arrays.370 In particular, at a high anodic bias this structure showed a three times higher photocurrent than that of the bare Ta3N5. However, Co3O4 showed an inferior activity at zero or a small anodic bias compared to the Co−Pi (a phosphate terminated cobalt oxide OER catalyst, which will be further reviewed later). Improvement of the Co3O4 catalytic activity can be realized by manipulation of the surface orientation and the size and shape of the nanoparticles. Recently, Xiao et al. presented a facile method to control the shape and surface orientation of Co3O4 nanoparticles by varying the concentration of the NaOH and Co(NO3)2·6H2O solution. They measured the resulting effects on electrochemical performances.371 A phase-dependent activity was noticed, which is important to achieve a higher OER catalytic activity. Koza et al. reported an electrodeposited spinel Co3O4 from Co(II) tartrate in an alkaline solution without an additional heat treatment.372 This Co3O4 showed a Tafel slope of 49 mV/dec and an exchange current density of 2.0 × 10−7 mA/cm2, which is 8688


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or Co−Pi. The TOF of 0.1−0.21 s−1 remained the highest among the first-row metal heterogeneous catalysts, showing its great potential as one of the most promising OER catalysts. Another Co-based catalyst recently developed by Gao et al. was CoSe2.395 The CoSe2 nanobelt was synthesized from a hydrothermal-based method by mixing Co(Ac)2 and Na2SeO3 with diethylenetriamine (DETA) and water. Pure CoSe2 was realized by removing amine from DETA at an elevated temperature. The synthesized nanobelts showed a layered nanostructure, formation of which was assisted by the amine groups. Recently, hybrid nanocomposite systems of Mn3O4 and CoSe2 were developed and showed a superior OER catalytic activity (Figure 22d).396 CoSe2 nanobelts decorated with Mn3O4 nanoparticles realized via a heterogeneous nucleation process showed an improved OER activity and better stability than the pristine CoSe2 nanobelt or Mn3O4 nanorods in 0.1 M KOH electrolyte. The cause of the improved activity was confirmed by an X-ray absorption spectroscopic study, which confirmed a positive shift of the binding energy of Co 2p due to electron donation from the Mn3O4. These transferred electrons increased the acidicity of the Mn3O4, which led to an enhanced OER activity through the Lewis acid−base interaction. 4.2.3.2.4. Ni−OER. Ni in the form of (oxy)hydroxides is believed to give the lowest overpotential for a fixed current density compared to Mn, Fe, and Co. This was experimentally verified by Subbaraman.320 This was consistent with observations by Brockis et al. on the transition-metal-terminated perovskites383 in which the Ni-terminated perovskite showed the highest current density at a fixed overpotential. All these results triggered a further investigation and utilization of Nibased OER catalysts. Doping NiO with other transition metals (Fe, Co) could improve its activity. Miller et al., for example, showed that adding Fe into a NiO film improved its catalytic activity (Figure 23a).397 Using a reaction sputtering of Fe and Ni in O2 environment, doped film showed a reduced Tafel slope (95 to 35 mV/dec) and overpotential (447 to 285 mV at 8 mA/ cm2) compared to the pristine NiO. However, addition of Fe significantly degraded the exchange current density from 10−3.8 to 10−7 mA/cm2. Similar observations were also made by Corrigan with a better activity.398 It was also noticed that addition of Fe increased the operating lifetime up to 7000 hrs without significant degradation.397 Recently, Landon et al. presented a detailed spectroscopic study of Ni/Fe mixed oxides and revealed that 10 mol % Fe should give the best OER activity with a Tafel slope as low as 40 mV/dec.399 X-ray absorption studies revealed that the oxidation state of Fe is mainly Fe(III), which indicated the improved activity was not due to the Fe active sites but rather the introduced NiFe2O4 phase. To the best of our knowledge, the most active NiFe− oxide OER catalyst was presented by Merrill et al.344 using cathodic electrodeposition in which NiFe−oxide exhibited an exchange current density of 9.4 × 10−3 mA/cm2 and a Tafel slope of 33.7 mV/dec. It is further clarified that addition of Fe effectively introduced local strain at the Ni−O bond and reduced the average Ni oxidation state. The Ni redox potential, thus shifted to higher potential with a higher Fe concentration, sharing similar features with the aged NiO OER catalyst. Further increasing the Fe concentration, however, reduced the Ni(III) content which degraded the OER activity.400 In another example, doping NiO with Fe, Co, and Cu using cathodic deposition and thermal decomposition was accomplished by Li et al.401 Similarly, Fe doping led to an improved catalytic

Figure 22. Co compound OER catalysts. (a) Polarization curves of a MnCo2O4/nitrogen-doped reduced graphene oxide composite (1 M KOH solution). Reprinted with permission from ref 377. Copyright 2012 American Chemical Society. (b) Tafel behavior of a perovskite BaSrCoFeO OER catalyst compared to the IrO2 OER catalysts measured in 0.1 M KOH. From Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. Science 2011, 334, 1383 (ref 382). Reprinted with permission from AAA Science. (c) Tafel behaviors of cobalt-based catalysts from a 0.1 M pH 7 phosphate solution (Co−Pi), 0.1 M pH 8 methylphosphonate (Co−MePi), and 0.1 M pH 9.2 borate solution (Co−Bi) and measured in the same solution. Reprinted with permission from ref 391. Copyright 2009 American Chemical Society. (d) Polarization curves of the Mn3O4/ CoSe2 composite (0.1 M pH 13 KOH solution, insets show microscopic images of the catalysts.). Reprinted with permission from ref 396. Copyright 2012 American Chemical Society.

was suggested that the near unity occupancy of 3d electrons and a higher covalency of the transition-metal−oxygen bonds would result in a higher OER and ORR activity,384 which is of great assistance to the discovery of new compound materials. The most well-studied Co-based inorganic OER catalyst is Co−PI, which was initially proposed by Nocera’s group in 2008.385 Subsequently, this catalyst has been extensively studied and applied to various photocatalysts, such as Ta3N5,370 BiVO4,386 Fe2O3,387 ZnO,388 TiO2,389 as well as Si,390 to enhance the OER activity and the water oxidation efficiency. The Co−Pi catalyst can be (photo)electrodeposited on the surface of conductive substrates or photoelectrodes in a mixture of phosphate and Co salt in an aqueous electrolyte. The electrolyte plays an important role in tuning the chemical composition, activity, and selectivity (Figure 22c).391 Further insights on the nucleation and growth of Co catalysts during electrodeposition were revealed through an electronkinetic study.392 A Co−Pi OER catalyst on a Si photoanode was demonstrated for efficient water oxidation.390c Optimization of the Si photoanode led to an improved Tafel slope, a minimized ohmic loss,390a and eventually the capability for spontaneous water splitting with assistance from a multijunction Si solar cell.390b Study of the proton−electron transport and transfer led to optimized film thickness with an optimized OER activity393 Ahn and Tilley recently reported another cobalt-based electrocatalyst synthesized from the thermal processing of molecular precursors.394 The metaphosphate Co(PO3)2 exhibited improved catalytic activity compared to that of Co3O4 8689


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deposition, was demonstrated by Nocera’s group and shown to be a good alternative to the previously proposed Co−Pi OER catalyst. Initial studies of nonoptimized coatings on ITO glass showed a catalytic activity of 1 mA/cm2 at an overpotential of 425 mV. Recently, researchers demonstrated an improved performance (100× increase in OER activity) by introducing an anodic activation on the electrodeposited film at a constant current density for a certain amount of time (Figure 23b).187 Xray absorption studies revealed an increased oxidation state (average from +3.16 to +3.6) and a local structure change in the activated Ni−Bi catalyst. Important experimental evidence suggested that the Ni(IV) oxidation state was more active and the γ-NiOOH phase was more efficient than the β-NiOOH phase, which was contrary to the well-accepted theories. A NiFe hydroxide/CNT composite was synthesized through a solvothermal treatment of a mildly oxidized multiwall CNT network decorated with a hydrolyzed nickel/iron (ratio 5:1) salt.408 The CNT network facilitated electron transport and enhanced the OER activity of the complex which led to a high OER activity relative to a commercial Ir catalyst as well as to other control samples (Figure 23c). An X-ray absorption study revealed formation of metal−oxygen−carbon bonds through the carboxyl group, which is believed to be a key to facilitated charge transport between the catalytic sites and the underlying CNT networks. OER activities of other transition-metal compounds were also investigated, such as boride. Osaka et al. prepared transition-metal borides including NixB, CoxB, and FexB through a solid-state reaction by annealing a mixed metal with boron powder.409 They found the metal borides showed a lower Tafel slope, measured in 6 M KOH solution, than that of the metals in the order NixB ≈ CoxB ≫ FexB > NixLaB6 > Co ≈ Fe > Ni (Figure 23d). For another example, Cheng et al. prepared spinel CoxMn3−xO4 by mixing amorphous MnO2 with aqueous CoCl2 solution and treating it with NaBH4/NaOH strong reductants. This yielded CoMnO−B, which showed significantly higher OER activity than the tetragonal CoMn2O4 or cubic Co2MnO4 measured in 0.1 KOH solution.410 It is worth noting that stabilization of earth-abundant metal oxide OER catalysts to operate in acidic environment is essential to couple with currently developed HER catalysts, yet challenging.404,411 Therefore, development and investigation of other transition-metal compounds-based OER catalysts besides oxides could be an alternative and a promising pathway toward this goal. 4.2.3.2.5. Earth-Abundant Metal Compound OER Catalyst-Enabled Si Photoanodes. A MnOx coating on n-Si was demonstrated by Kainthla et al. using electrochemical deposition from a bath containing 3 mM MnCl2, 0.25 mM NH4Cl, and 1.4 M NH3 aqueous solution.420 Mn2O3 of 20 nm thickness was formed by removing water from Mn(OH)2 precipitated on the Pd monolayer-coated Si surface at 250 °C in vacuum for 15 min. Mn remained mainly in Mn(III). The Mn2O3|Pd|n-Si demonstrated an onset potential of 1.285 V vs RHE in 0.2 M NaOH (pH = 13.3), and Mn2O3|Ti showed an overpotential of 800 mV at 1 mA/cm2. This Mn2O3|Pd|n-Si showed a stable photocurrent response of around 1 mA/cm2 for more than 650 h of operation, indicating no mechanical or chemical degradation. Recently, Strandwitz et al. reported an nSi photoanode coated with a MnO thin film for water oxidation.421 A 10−20 nm thick MnO layer was conformally coated on an HF-etched, polished n-Si substrate using ALD) (curve designated with a right pointing triangle in Figure 24).

Figure 23. Polarization curves of Ni compound OER catalysts. (a) Cosputtered Fe-doped NiO measured showing Fe concentrationdependent OER activity (1 M KOH solution). Reprinted with permission from ref 397b. Copyright 1997 The Electrochemical Society. (b) Electrodeposited Ni−Bi catalyst showing the anodization activation (1 M pH 9.2 potassium borate solution). Reprinted with permission from ref 187. Copyright 2012 American Chemical Society. (c) NiFeOOH/CNT composite catalyst (1 M KOH solution). Reprinted with permission from ref 408. Copyright 2013 American Chemical Society. (d) Metal boride catalysts synthesized by sintering the metal and the boron powder (6 M KOH solution). Reprinted from Osaka, T.; Ishibashi, H.; Endo, T.; Yoshida, T. Electrochim. Acta 1981, 26, 339 (ref 409), copyright 1981 with permission from Elsevier.

activity compared to other metal dopants. Lena et al. presented a study of several typical transition-metal oxide-based OER catalysts402 as well as their optical modulations during catalytic reactions,403 including NiOx, CoOx, Co−NiOx, Fe−NiOx, MnOx, FeOx, and IrOx from a solution-based method. Ultrathin films with a thickness of 2−3 nm were studied. Fe-doped NiOx showed the best OER catalytic activity, with a Tafel slope of 30 mV/dec and a current density of 10 mA/cm2 at an overpotential of 336 mV. McCrory et al. reported a standardized protocol to evaluate the electrochemically active surface area, activity, and stability of OER catalysts in different environments.404 It is also demonstrated that NiFeOx showed the lowest overpotential loss of 350 mV for 10 mA/cm2 current density in 1 M NaOH solution. Besides the foreign dopants, the activity of a Ni-based OER catalyst can be improved by electrochemical treatment. Activation and regeneration of an aged oxide film can be realized by applying an anodic bias for a period of time, which was attributed to the recovery of active sites on the surface.405 The film regenerated at a lower anodic bias (1.5 V vs Hg/HgO) than the operating bias (1.8 V vs Hg/HgO) at room temperature for 2 h showed a comparable performance to freshly activated samples. Another Ni-based OER catalyst electrodeposited from either a borate or phosphate buffer (NiCat) showed similar structures to the Co ones (CoCat) prepared using the same methods.406 Most importantly, the NiCat and CoCat prepared using this method shared some common features with the natural Mn4Ca OER catalyst center. The detailed structure study was recently elaborated by Bediako et al.407 Ni borate (Ni−Bi borateterminated nickel oxide) with a chemical formula of NiIIIO(OH)2/3(H2BO3)1/3·1.5H2O, synthesized using electrochemical 8690


Chemical Reviews

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photoanode directly decorated with NiO(OH). Recently, Sun et al. synthesized a high-quality nonstoichiometric NiOx thin film on an n-Si photoanode using a solution-based sol−gel method. This showed significantly higher catalytic activity than near stoichiometric, sputtered NiO.214 This film simultaneously served as a protection layer to increase the lifetime of the Si photoanode, an electrocatalyst to reduce the external bias, and a p-type coating for forming a rectifying junction to further provide a photovoltage. Compared to other results,242 an inferior water oxidation performance from this preliminary study (photocurrent density of 0.2 mA/cm2 at 1.23 V vs RHE, Tafel slope of 129 mV/dec, curve labled with a box with a circle inside in Figure 24) was presumably due to the high ohmic loss from the NiOx coating, interfacial losses between the NiOx and the Si, and a low density of surface active sites, as well as the poor catalytic activity of the dense NiOx under neutral pH conditions. The demonstrated NiOx-coated n-Si photoanode also suffered from low stability, primarily due to developing substantial porosity from conversion of Ni(OH)2 to NiOOH during the reaction and a subsequent passivation of the Si once in contact with the electrolyte. Combinatorial methods are also under investigation, adding alkaline metals or other transition metals into the uniform and pinhole-free NiOx film to further enhance the catalytic activity and conductivity397 without sacrificing photovoltage. To improve the poor catalytic activity of the sputtered NiOx, a Ru-incorporated NiOx composite was synthesized using a cosputtering technique.214 Gas-phase deposition allowed a conformal coating on nanotextured surfaces, which showed advantages over the sol−gel technique for protection purposes. The activity of NiRuOx was very sensitive to the preparation conditions, including RuO2 concentration, thickness, annealing environment and time, deposition environment and rate, measurement solution pH, and cyclic activation. All these factors provide additional tuning parameters for further optimization of activity and stability. The composite-coated nanotextured Si showed enhanced activity and stability compared with the sputtered NiOx as well as the Ru-coated n-Si in a phosphate-buffered Na2SO4 solution. Decomposition of a metal salt from a sol−gel is a very attractive method for controllable synthesis of nanophase catalysts with a high activity. However, this method typically generates a porous film of nanoparticles, which does not protect the underlying Si. To effectively utilize the high activity of NiOx made from the low-temperature sol−gel method, a method of protecting the Si is needed. Meanwhile, a pnjunction, made using a spin-on-dopant, can effectively separate the photogenerated carriers. Following this idea, Sun et al. reported an efficient water oxidation device based on a np+-Si| ITO:Au|NiOx424 (curve labeled with a circle in Figure 24). A layer of ITO with an intermediate Au layer (5 nm thick) was used to effectively conduct holes and protect the Si photoanode. Also, it was used as the anode for in situ solar cell measurements and as the contact for NiOx activation. This device demonstrated a maximum solar oxygen conversion efficiency of 0.07% under 0.52 sun illumination, which makes it among the best reported photoanodes for water oxidation without the use of surface textures. Very recently, Dai’s group used an ultrathin Ni metal film to protect a crystalline n-Si photoanode without removing the native oxides and demonstrated an operating lifetime under a water oxidation reaction425 of more than 80 h in aqueous solution after treating the Ni metal with Li. The Ni metal was directly coated on Si with a native oxide using electron-beam

Figure 24. OER electrocatalyst-decorated planar n-Si photoanodes for water oxidation. Data is extracted from original reports using software: (◊) n-Si|SiOx|Ni measured in 1 M KOH solution,425 (+) np+-Si|SiOx| CoOx measured in 1 M NaOH solution,431 (■) n-Si|SiOx|TiO2|Ir measured in 1 M NaOH solution,149 (triangle pointing right) n-Si| MnO measured in 1 M KOH solution,421 (▰) n-Si|α-Fe2O3 measured in 1 M KOH solution,242 (●) np+-Si|ITO:Au|NiOx measured in 1 M NaOH solution,424 (▼) npp+-Si|FTO|Ni−Bi measured in pH 9.2 0.5 M KBi and 1.5 M KNO3 solution,429 (⧫) npp+-Si|FTO|NiFeO measured in pH 9 0.5 M KBi and 1.5 M KNO3 solution,429 (triangle pointing left) npp+-Si|ITO|Co−Pi measured in 0.1 M KPi solution,390a (box with a circle inside) n-Si|SiOx|NiOx measured in pH 7 0.25 M PBS-buffered Na2SO4 solution,432 and (pentagon) np+-Si|IrOx measured in 1 M H2SO4 solution.433 Data derived from corresponding references.

Catalyzed photooxidation of water was successfully realized using this structure, while photodecomposition of n-Si was substantially suppressed within 30 min of operation at a current density of 25 mA/cm2 in 1 M KOH solution. Recently, Jun et al. demonstrated the application of a highquality hematite thin film coating using chemical vapor deposition (CVD) onto an n-Si photoanode for efficient water oxidation.242 A photocurrent of 1 mA/cm2 at 1.23 V vs RHE was achieved by coating a 10 nm thick Fe2O3 layer onto polished n-Si. Such a thin coating maintained the Si photoresponse without introducing strong electronic or optical modifications to the S/E interface. The system was optimized by converting the backside to highly doped n+-Si and utilizing a Si microwire array, eventually giving a record-breaking photocurrent of 20 mA/cm2 at 1.23 V vs RHE (curve designated with a diamond in Figure 24). Ni oxide has been widely used as a cocatalyst on transitionmetal oxide-based photocatalysts for oxygen evolution.422 The first demonstration of NiO coating on a Si photoelectrode was reported by Li et al.423 A layer of Ni metal with a thickness of 10−30 nm was first sputtered on a n+/p-Si substrate and further anodically treated to form NiO(OH). This catalyzed photoanode showed a much higher OER activity than Pt. Further improvement was realized by incorporation of W in the Ni film. This structure showed an extended lifetime compared to a Si 8691


Chemical Reviews

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(e-beam) evaporation. The uniform thin Ni metal served three functions: providing a Schottky junction with a high barrier height for charge separation; catalyzing the OER reaction after in situ conversion to NiOOH; and isolating the Si from oxidation. The high efficiency also relied on the quality of the insulating oxide and the MIS structure. This work so far represents the most stable Si photoanode demonstrated in the literature enabled by an earth-abundant OER catalyst with nearly 100 h lifetime under water oxidation conditions in moderate conditions and 24 h in 1 M KOH solution (Table 5).

Table 5. Comparative Performance of Heterogeneous Si Photoanodesa

catalysts RuO2

b (mV/ dec)

j0 (mA/cm2)

42

11.7 × 10−5 −5

203

IrO2

33

9.5 × 10

163

Mn−Cat

59

10−9

585

FeCoNiOx

31

Co−Bi

52

∼300 2 × 10−9 −8

499

Co−oxide

69

3.0 × 10

Co−Pi

60

6 × 10−8

Co3O4

49

2 × 10−7

Mn3O4/CoSe2

49

Ni−Bi

29

Ni3S2/Ni

159

NiFeO

33.7

Ni0.9Fe0.1Ox

30

NiAlFeOx a-FeOOH

37

Fe2(MoO4)3

35

NiFeOH/ CNT BaSrCoFeO

31

∼240

60

∼400

BaSrCoFeO/ LaSrMnO

50

>400

582

373 ∼400

5 × 10−15

441 310

520

J@EOER (mA/cm2)/ηSOCE (%)/condition

n-Si|SiOx|TiO2| Ir n-Si|SiOx|Ni

8.34/0.37/100 mW/cm2/1 M NaOH (aq) 10.47/0.14/200 mW/cm2/1 M KOH (aq) 5.3/0.23/100 mW/cm2/1 M KOH(aq) 17/0.75%/1 M NaOH (aq)

n-Si|MnO np+-Si|SiOx| CoOx n-Si|Fe2O3

Table 4. Kinetic Parameters for Some OER Catalysts ξ (mV@ 10 mA/cm 2 )

photoelectrode structure

n-Si|SiOx|NiOx condition

ref

5 M KOH (aq) 5 M KOH (aq) 0.1 M KH2PO4/ K2HPO4 (aq) 0.1 M KOH (aq) 1 M K−Bi (aq) 0.1 M NaH2PO4/ Na2HPO4 (aq) 0.1 M K−Pi (aq) 1 M KOH (aq) 0.1 M KOH (aq) 1 M K−Bi (aq) 0.1 M KOH (aq) 1 M KOH (aq) 1 M KOH (aq) 0.1 M NaOH 1 M Na2CO3 (aq) 1 M KOH (aq) 1 M KOH (aq) 0.1 M KOH (aq) 0.1 M KOH (aq)

412

npp+-Si|ITO| CoPi npp+-Si|FTO| NiBi npp+-Si|FTO| NiFeO np+-Si|ITO:Au| NiOx np+-Si|IrOx HIT-Si|ITO| NiOx np+-Si|TiO2|Ni islands

412 360

241 413

1.86/0.04/100 mW/cm2/1 M NaOH(aq) 0.13/0.006/100 mW/cm2/0.25 M PBS Na2SO4(aq) 0.86/0.04/100 mW/cm2/0.1 M K−Pi 3.81/0.09/100 mW/cm2/0.5 M K−Bi 1.5 M KNO3(aq) 1.9/0.05/100 mW/cm2/0.5 M K−Bi 1.5 M KNO3(aq) 1.97/0.07/52 mW cm−2/1 M NaOH(aq) 18/−/38.6 mW/cm2/1 M H2SO4(aq) 21/1.78/100 mW/cm2/1 M NaOH(aq) 11/0.32/125 mW cm−2/1 M KOH (aq)

stability

ref

8h

149

24 h

425

30 min

421

24 h

431

1h

242

30 min

432

12 h

390a 429 429

2.5 h

424

18 h

433 434

100 h

217

414

a

415

exponentially enhanced current density following the water oxidation reaction under the same applied bias and illumination level through the Si/Ni/electrolyte.426 In other words, an increased and then stabilized Faradaic efficiency (as the stabilization efficiency that Morrision et al. used and details of discussions can be found in refs 426,427) of water oxidation through Ni catalysts can be achieved at this point. One can think that if the water oxidation reaction is not kinetically favored, such as when using an inferior/ineffective alkaline catalyst with larger overpotential or under nonoptimal operation conditions (neutral or acidic solution), the Si oxidation reaction becomes more dominant through the defective oxide427c,428 and the lifetime is expected to be greatly degraded. Similarly, Nocera’s group presented buried junction Si photoanodes coated with Co−Pi (curve labeled with a left pointing triangle in Figure 24),390a NiFeO (curve labeled with a diamond in Figure 24), and Ni−Bi (curve labeled with a down pointing triangle in Figure 24)429 catalysts. In contrast to the approach used in ref 424, the pn-junction was constructed at the backside of the device and measured in 0.5 M potassium borate (KBi) at a pH of 9.2 with 1.5 M KNO3 as a supporting electrolyte, which effectively separated the chemical reaction interface with the catalysts from light absorption. This structure possesses advantages such as a minimized distortion of the incident light by the catalyst, the protective coating (FTO, ITO, and Ni films), and gas bubbles. However, in a practical solarfuel conversion device, the preferable side for incident light is on the OER side. Typically, this side can be amorphous Si in a multijunction configuration or a photoanode material in a photochemical diode configuration, with a wide band gap to absorb short wavelengths. Also, an oxide OER catalyst provides less distortion to the incident light despite its anodic coloration than a metallic or small band gap compound HER catalyst.

372 396 413 416 344 402 417 418 367 408 382 419

Given the fact that a 2 nm Ni film is hardly a uniform “isolation” coating for Si and Ni or, precisely, porous NiOOH in working condition under anodic bias is one of the best inorganic OER electrocatalyst in base, this work represents a strategy in which the Si is “kinetically” protected by introducing a more competitive reaction with fast reaction kinetics compared to reactions such as Si oxidation/etching and SiOx etching in KOH. In other words, a thin passivation oxide could be formed during the first scan under illumination when the catalyst is not activated (catalytic activity increases with scan cycles). This oxide does not continuously grow due to the 8692

Data derived from corresponding references.


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Table 6. Comparative Performance of Polymer-Modified Hybrid Si Photoanodes photoelectrode structure n-Si|Pt(5 Å)|polypyrrole(250 monolayer) n-Si|Pt(20 Å)|polypyrrole(400 monolayer) n-Si|polyacetylene n-Si|polyaniline(100 nm) n-Si|PEDOT:PSS(20 nm) n-Si|CuPc:I (80 nm) n-Si nanowire|PEDOT:I|CuPc (5−10 nm) n-Si|CuPc (100 nm)

η (%)/condition/electrolyte 2.8/60mW cm−2/0.1 M KI, 0.01 M I2, 0.1 M KCl (aq) 5.5/55mW cm−2/1 M KI, 0.1 M I2, 0.2 M KCl (aq)

stability/condition >160 h/9 mA/cm2 under 75 mW cm−2 >450 h/11 mA cm−2 55 mW cm−2

1.9/21mW cm−2 at λ = 632.8 nm/0.5 M LiI, 10 mM I2, 11 M 18 h/10 mA cm−2 3 mW/cm2 in 0.025 M KI, 0.002 M I2 (aq) VOC ≈ 0.2 V, JSC ≈ 6 mA cm−2/72.5 mW cm−2/1 M NaI, 1 mM >1.3 h/72.5 mW cm−2 in 11 M LiCl, 0.5 M LiI I2 (aq) (aq) pH = 0 −/−/0.2 M HI, 0.5 M H2SO4 (aq) >3 h/100 mW cm−2 at 0.1 V vs SCE VOC ≈ 0.4 V, JSC ≈ 0.4/250W mercury lamp/0.05 M Fe3+, 0.05 M Fe2+ (aq, pH = 4.2)

>30 min at 0.4 V vs SCE

ref 436 437 438 439 450 33 449 444

polypyrrole film on an n-Si surface using electrochemical deposition. The film showed a low resistance, resulting in an improved fill factor due to the rapid electron transfer kinetics and an improved photovoltage due to a lower degree of surface pinning.436 Further improvement to the conversion efficiency was realized by integrating a thin Pt film.437 Another hole conducting polymer is polyacetylene (CH)x, the simplest conjugated polymer. A n-Si|(CH)x photoanode was able to maintain the structural integrity and a current density after up to 23 h of operation at a low light intensity.438 The formed Schottky junction and the conductive nature of the polymer helped the photogenerated holes transfer to the electrolyte. Unlike polypyrrole or polyacetylene, a polyaniline film shows good conductivity for both holes and electrons, leading to its potential application as both photocathode and photoanode. Polyaniline-coated n-Si showed a significantly improved stability up to 72 h, while polyaniline-coated p-Si showed a cathodically shifted onset potential which improved the chargetransfer kinetics.439 Limited long-term stability from previous studies was believed due to the nonuniform coating and a weak adhesion of a thick film. Moreover, the optical properties of the polymer in the early development of the photoelectrode coating were not considered, such as the dielectric constant, the refractive index, and the absorption coefficient,440 which remain important design considerations for photoelectrodes. Application of a phthalocyanine (Pc, a covalently bonded molecule) as a dye to sensitize wide band gap materials, such as TiO2,441 WO3, and SnO2, has been demonstrated before, because of its visible light sensitivity.442 Their p-type semiconducting nature has enabled the metal phthalocyanines (M− Pc) to be broadly applied in organic optoelectronic devices.443 Pc films are potentially useful for photoelectrochemical cells because of their hydrophobicity and their catalytic properties.33,444 Applications of metal-free phthalocyanine (H2Pc),444 Cu−Pc,33 and Fe−Pc445 layers on n-Si have been conducted. These structures showed a rectifying current−voltage behavior, indicating that they formed a Schottky barrier for charge separation.446 The photocorrosion process was remarkably decreased on the Pc-coated n-Si, but the rate of formation of the insulating layer was greatly dependent upon the nature of the redox couple in the measurement electrolyte. Addition of iodine, which is a dopant for the Pc layer, led to larger photoeffects and a lower rate of photopassivation. However, the electrode was still unstable after irradiation for a long time. Bard et al. reported that the Pc-coated n-Si never showed longterm stability at a high light intensity (135 mW/cm2).33

A small band gap degenerate Tl2O3 film was also used on an n-Si photoanode using photoelectrochemical deposition. A band bending of 1 eV through the junction of n-Si and degenerate Tl2O3 with an energy band gap of 1.4 eV and a first direct transition of 2.2 eV was reported.430 This semitransparent film also showed antireflection properties but a significant absorption with increasing thickness. Interestingly, this coating showed catalytic activity similar to Pt and good stability in an alkaline solution under an anodic bias. Polarization curves and their measurement conditions are compared and summarized in Figure 24 and Table 5. 4.3. Organic Compounds

Similar to the inorganic approaches discussed above, an organic surface modification to the Si photoelectrode is capable of effectively unpin the surface Fermi level and regain control of the energy barrier at the Si interface while isolating the Si from contacting the aqueous solution and efficiently separating and conducting the photoinduced charge carriers to the solution for chemical reactions. Major concerns for an organic coating include its stability under illumination in a harsh environment (basic, acidic, or even neutral), its adhesion strength to the underlying substrate, and its charge transport efficiency. Despite these potential concerns for hybrid systems, the organic approach to enable Si does have some advantages. For example, a typical organic coating does not require a high-temperature or high-vacuum process and thus does not need expensive equipment for deposition, which could make the process relatively cheap and affordable. This also allows the deposition of polymers onto a variety of materials. Hybrid systems are typically realized in several different ways. In this section, we will review methods, including • use of conductive polymers, • surface functionalization through the modification of Si surface terminations, • introduction of redox mediators, • electroactive polymers containing redox couples, • homogeneous molecular catalysts. 4.3.1. Polymers. Besides the inorganic materials discussed in previous sections, an organic polymer can be used to protect the Si from passivation.435 A conductive polymer has advantages over a nonconductive one due to the lower ohmic loss from the coating. A hybrid photoanode using a solutionprocessed conductive polymer is a very interesting technique, which allows for large-scale processing of conformal coatings, leading to a cost-effective solar-fuel conversion system (Table 5). Skotheim et al. demonstrated addition of a conductive 8693


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Moreover, light absorption could be significant in the Pc coating, which would limit the Si response and also lead to a photoresponse and significant surface band bending at the Pc coating/electrolyte interface.447 Toshiyuki et al. demonstrated that the metal-free Pc outperformed the metal-containing Pcs, with higher surface kinetics and a higher activity as a photoanode coating.448 Recently, Yang et al. further coated CuPc on an n-Si nanowire array with an iodine-doped PEDOT polymer.449 Only H2 gas was generated due to introduced redox shuttles with a solar-fuel conversion efficiency of 3%. A conformal coating of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an efficient hole transport and protection layer was deposited on n-Si using a solution-based method.450 This layer provided a Schottky junction for efficient charge separation and was also electrocatalytically active, which eased the requirement for an additional electrocatalyst coating. Using a solution of HI and HBr, the cell had a solar hydrogen conversion efficiency of 2.5% without a significant loss of activity for nearly 20 h. However, a comparison to Pt reported in this work suggested an inferior activity of PEDOT for water oxidation reactions. Another independent study recently used PEDOT-coated n-Si nanowires and demonstrated for the first time the capability of hybrid devices for water oxidation in 1 M KOH solution (Table 6). 4.3.2. Surface Functionalization. Hydride-terminated Si (H−Si) shows the lowest density of states and thus the lowest recombination velocity,131 but it is not stable in aqueous solution, as discussed above. In contrast to polymer-coated Si, surface functionalization of Si can be realized through a molecular monolayer treatment. This involves forming different surface-terminated groups including alkyl groups,452 ethyl groups (C 2 H 5 ), methyl groups (CH 3 ), allyl groups (CH2CHCH2), thienyl groups (SC4H3),453 and even mixed functional groups. These terminated surfaces could maintain the electrical properties of Si, allow a facile charge transfer from the Si surface, and protect Si from oxidation. For example, H− Si, through a facile tunneling barrier, provided an improved stability in an aqueous solution.52,454 Meanwhile, this surface functionalization could also shift the band edge position due to the different dipole moment455 and manipulate the S/E energetics to allow the band edge position at the interface to be independent of changes in the solution pH. Moreover, surface functionalization could minimize the surface defects and thus the recombination velocities for an efficient Si photoelectrode. Moreover, functionalized Si surfaces can be derivatized by attaching redox couples, such as Fc,456 to further modify the charge-transfer kinetics. 4.3.3. Redox Couples. Organic or nonaqueous solvents were used to effectively limit the photopassivation of Si, in which dissolved redox couples were the electron donor instead of water.457 Redox couples can avoid the difficulty of fabricating a conformal metal contact, which is particularly problematic for nanostructured surfaces. Selection of efficient intermediate redox couples for different barrier heights is based on the different redox potentials of the redox couples.458 A redox couple with a redox potential close to the Si valence band is typically needed, for example, ferrocene/ferricenium (Fc+/0) couple, dimethylferrocene (Me2Fc+/0).117b,126 On the other hand, a redox couple with a potential close to the conduction band of Si is typically involved in the characterization of p-Si, for example, vanadium,459 and N,N-dimethyl-4,4-bipyridinium redox (MV2+/+).53b

The redox couple mediated-water splitting relies on the manipulation of charge transfer kinetics (A/A+ and B/B+),460 which can be described using the equations below p ‐ Si, hν

A+ + e− ⎯⎯⎯⎯⎯⎯→ A

(8)

catalyst

2A + 2H 2O ⎯⎯⎯⎯⎯⎯→ 2A+ + 2OH− + H 2(g) n ‐ Si, hν

B + h+ ⎯⎯⎯⎯⎯⎯→ B+

(9) (10)

catalyst

4B+ + 2H 2O ⎯⎯⎯⎯⎯⎯→ 4B + 4H+ + O2 (g)

(11)

A wide selection of electroactive redox couples, either covalently bonded to the surface or confined in supporting polymers close to the electrode surface, has also been studied and applied to Si photoelectrodes.31 For example, Wrighton et al. proposed a new structure containing a photoelectrochemically active ferrocene covalently attached to the surfacederivatized n-Si to stabilize the electrode from photocorrosion.35,461 This derivatized n-Si exhibited an acceptable durability in acidic conditions, while corrosion of the ferrocene coating happened in an alkaline environment.35 Results using this derivatized surface showed that the structure did protect the n-Si from oxidation for over 30 min, and the surfaceattached species were photoelectrochemically active. Mediation reactions of the Fc in this system were further investigated by Bocarsly et al. using a two-electron reductant, N,N,N′,N′tetramethyl-p-phenylenediamine (TMPD), which showed a separated one-electron transfer on a ferrocene-derived n-Si.462 In this study, the Si was first cleaned with HF and NaOH, followed by a ferrocene functionalization through the introduced surface groups.461a It was observed that n-Si without surface derivatization gave very irreproducible behavior at low TMPD concentrations, presumably because Si oxidation was competitive with the TMPD oxidation, while the derivatized nSi under illumination showed a mediated TMPD oxidation through regeneration of the surface ferrocene. Thus, this surface derivatization method successfully inhibited the Si photopassivation. Besides the covalent attachment of redox mediators onto the Si surface, a redox couple can be incorporated into a polymer. For example, a polymer incorporated with ferrocene redox ions was electroactive and applied to an n-Si photoanode surface.463 Because of the positive charges on the backbone, negatively charged ions were attracted to the polymer. Increased negatively charged ions on the polymer surface helped to increase the capability to capture holes generated from the n-Si under illumination. An improved photocurrent stability of a pSi surface decorated with a polymer containing N,N′-dialkyl4,4′-bipyridiniumredox (PQ2+/+) was reported by Bookbinder et al.,464 based on previous research on the liquid junction containing a PQ2+/+ redox couple.460 The coating significantly lowered the cathodic photocurrent onset potential by 0.3 V compared to the bare p-Si. However, this structure did not exhibit a significantly improved HER efficiency without applying a catalyst. Pt was then introduced into the polymer through ion exchange followed by photoreduction of PtCl62−, which resulted in a uniform catalyst distribution in the PQcontaining polymer.465 Another electroactive polymer containing a polybenzylviologen (PBV) with a thickness varying from 50 nm to ∼2 μm was deposited on a p-Si by Abruña and Bard.466 The PBV electroactive polymer dramatically catalyzed the water reduc8694


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complexes can be realized by effectively binding the enzyme to the substrate through active binding sites. Future research should focus on the intra- and intermolecular charge transfers required to reach a high activity by lowering the overpotential, minimizing back charge transfers or reactions, and increasing the exchange current. The light sensitivity of molecules and their photostability are also important. An interfacial protection layer between complex catalysts and the Si substrate is required to protect the Si from corrosion under idle condition. 4.3.5. Others. Early development on Si electrochemistry/ photoelectrochemistry involved the use of HF-containing electrolytes to etch away the insulating oxides in situ during Si operation in aqueous solution. Examples of electrolytes include HF/HNO3,471 HF buffered by HCl, and fluoride/ HF.472 It was also reported that stability improved when a concentrated electrolyte was used.473 This can be attributed to a decrease in water activity and a concomitant decrease in the rate of water penetration through the film.33 However, these methods, similar to redox couples, do not rely on water as an electron donor.

tion reaction by anodically shifting the onset potential and improving the photocurrent. Incorporating photoreduced Pt into the polymer could further enhance the H2 evolution, which would successfully regenerate the reduced PBV. For this system, the reduced photocurrent was mainly due to decomposition of the viologen even at a low light intensity. 4.3.4. Molecular Complexes. Inspired by nature’s hydrogenases for proton reduction and Mn-centered complexes in photosystem II for water oxidation,467 applying these highly active enzymes on organic light absorbers for a hybrid system is also promising. A dye-sensitized photoelectrochemical cell (DSPEC) can be realized by attaching molecular electrocatalysts to molecular dyes typically used in a dye-sensitized solar cell (DSSC).468 On the other hand, molecular catalysts on inorganic light absorbers have been also demonstrated. The first breakthrough of the hybrid system using a molecular complex on inorganic photoactive material was demonstrated by Nann et al. using [Fe−Fe] hydrogenase linked onto a layered nanocrystal InP photocatalyst469 to realize an electrocatalyzed proton reduction for H2 generation. The catalytic reaction was recognized through the Fe−H and/or S−H bond. The system demonstrated stable operation for 1 h at low overpotentials in a neutral electrolyte. Recently, a homogeneous [Fe−Fe] hydrogenase catalyst demonstrated robust H2 generation by reducing protons together with a p-Si photocathode (Figure 25).470 A Si surface functionalized with organic

4.4. Summary

Thermodynamic and kinetic issues of Si in aqueous solution are considered as the major problems preventing Si from becoming a practical photoelectrode for solar water splitting. In this section we reviewed research efforts aiming to solve these issues through a heterogeneous alteration at the semiconductor− electrolyte interface using metals, compound semiconductors, organic molecules, etc. An ideal coating should replace the troublesome photoelectrochemical interface between Si and the electrolyte using two interfacesan electrochemical interface and a photoelectrical interface (Figure 26). This coating must provide a strong field for efficient charge generation and separation without Fermi-level pinning, providing minimal recombination at the photoelectrical interface and maximal photovoltage. A strong, built-in asymmetry between the coating and the Si at the photoelectrical interface allows it to replace the direct junction between the Si and electrolyte and to simultaneously isolate the Si from photoanodic passivation through the thick oxide formation. Note that discussions on the Si oxidation on nonuniformly coated Si photoelectrodes and its effect have also been made in this section. This unique property of formation of a nondissolvable passivation oxide even in alkaline solution (1 M or 1000 h) and considering the corrosion rate in a harsh environment during electrochemical reaction (e.g., Cr-containing Ni metal as OER catalyst has a corrosion rate as low as 2.5 μm yearly483), a thin catalyst loading will not be satisfied unless routine regeneration of catalysts can be deployed. A thick and sufficient loading of mesoporous catalysts in a patterned format with a certain surface coverage could be a promising approach in which the substrate light absorption is independent of the catalyst loading (thickness) and only determined by the catalyst surface coverage. On the other hand, this approach does not provide an easy route for protection of the underlying Si, which could result in photopassivation of the Si during extended operation. Generally speaking, the above case is true not only for n-type semiconductors that have an anodic decomposition potential more negative than the water oxidation level when operating as photoanodes38a,484 but also for p-type semiconductors that have a cathodic reduction level more positive than the water reduction level.10,485 This decomposition, in terms of forming a passivation oxide (like Si), or losing stoichiometry, or dissolution, could cause a loss of catalysts/photoactive material, degradation of photoactivity, and eventually failure of operation. In these cases, an additional intermediate isolation layer between the porous electrocatalyst and the underlying photoactive material is recommended for protection purposes. When surveying the catalysts that have been developed thus far, the materials are typically coated on TCO substrates and subsequently undergo a high-temperature thermal treatment. Thus, one should be aware of the potential for interdiffusion of elements from the TCO to the catalysts and consequent loss of substrate conductivity when conducting temperature-depend8696


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2-ethylhexonate with Fe, Co, and Ni provides an alternative to nitrates for ink formulation.241 High-throughput screening of the compound library relies largely on a probe that sensitively detects the variations of local environments. The screening can work either in parallel or in series (automation needs to be combined for serial scheme). For example, a fluorescent dye, sensitive to the local pH change in water, was introduced by Mallouk et al. for screening the OER catalysts.490 A thin film (WO3:Pd) optical reflection sensor that was sensitive to H2 was put close to the cathode by McFarland et al. for screening of HER catalysts. More recently, a chromophore paint sensitive to O2 was placed close to the anode for screening of OER catalysts.491 A parallel screening scheme based on the analysis of gas bubble accumulation during OER/HER on the electrode surfaces was developed.492 With the parallel method, the position where the compound has high catalytic activity appears as a hotspot in the image, which is captured either under a camera or under a microscope.493 The composition of the catalyst at the hotspot can be identified for further characterization. Both oxides of noble metals (Ru−Ir) and non-noble metals (Ni−Fe−Co−O, with additives of metal ions that can serve as Lewis acid) are screened for the OER catalyst in this way. Raster positioning of a laser spot across the compound library provides a serial way to map the photocurrent for high-throughput screening.487 A galvo scanner is typically used to position the laser spots in a programmable manner. The positions where high photocurrent is measured are identified as hotspots, and the specific composition can be determined. For cross-library comparison, internal standards can be adopted as in ref 494. Through this spot rastering strategy the screening was not only applied for searching for ternary or quaternary oxide photoelectrodes (Ni− Fe−Co−Al oxides) but also applied for optimization of the concentration of codopant for existing compounds like Fe2O3 and BiVO4. A tiny droplet containing the electrolyte can also be used as a localized probe,495 and a serial scheme based on moving the liquid droplet across the compound library was recently applied for both the OER/HER catalysts and the photoelectrodes.496 The success of combinatorial screening of photoelectrode (or OER/HER catalysts, heterojunction, or contact497) from a compound library is based on the ability to separate the effects of composition from other factors (like morphology and crystallinity). While the composition is changing, either continuously or discretely, the other factors are assumed to be the same across the compound library. However, as both the morphology and the crystal structure might be intercorrelating with the composition change, false negative and false positive results might be obtained during screening. One obvious concern is that the oxides of certain compositions in the compound library might be prone toward chemical etching or photoetching, and the etching current will appear during photoelectrochemical characterization and thus cause false screening results.498 Even for the compounds that are stable during screening, the temporal response to photon excitation of the oxide photoelectrodes can also result in false screening. Screening of semiconductor photoelectrodes is especially prone to false results since interface states, which are typically involved in semiconducting devices, are very sensitive to the preparation methods. For a material to perform effectively as a photoelectrode, various factors have to be at the right place, like carrier mobility for the photoelectrodes. A poorly performing photoanode might be simply due to the poor catalytic center on

ent activity studies. There is an ongoing need for catalysts that can be processed at a low temperature that is compatible with sensitive substrates such as a multijunction amorphous Si solar cells.301 Finally, fundamental understanding of the degradation processes with a particular focus on developing nanoscale electrocatalysts is critical in order to design electrocatalysts with long-term stability. For example, Meier et al. recently reported an aging study using TEM for analysis.486 Multiple mechanisms of stability loss were revealed, including detachment, dissolution, Ostwald ripening, agglomeration, and support corrosion. Understanding these fundamental issues will assist in designing and optimizing the electrocatalyst coating on photoelectrodes. Despite the intensive research over the past decades, the field of artificial leaf for solar-fuel production is still frustrated by a lack of materials for both the semiconductor as the solar photon absorber and the electrocatalysts for HER and OER.20d To date, only a handful of semiconductor materials, like WO3, Fe2O3, and BiVO4, are identified for photoelectrochemical water splitting with acceptable efficiencies and reasonable stability in a narrow pH range. Catalysts for water electrolysis (either HER or OER) are usually noble metals, which are not economically viable for terawatt-scale manufacturing. In addition, currently there is lack of earth-abundant and stable OER electrocatalysts that can operate in strong acid. As a result, combinatorial research was recently used to search materials with suitable compositions for the purpose of solar-fuel production.487 Metal oxides, usually combinations of earthabundant metal elements, are the focus of screening for both the photoelectrodes and the OER electrocatalysts. The alloys of transition metals are the focus of screening for HER electrocatalysts. The two pillar methods to support combinatorial efforts, a high-throughput way for synthesis of the material library and a high-throughput way to screen the materials with desired properties, are successfully developed through various strategies. To achieve the high throughput for synthesis of compound libraries, both the analogue strategy and the digital strategy have been adopted to incorporate the composition gradient of individual elements during synthesis. The varied concentration of the elements inside the plume during gas-phase deposition is used to change the feed continuously (analogue). The ratio of elements in the compound can then be tuned by overlapping different regions of the plumes.488 On the other hand, the method of inkjet printing, introduced by the Parkinson group,489 can be viewed as a digital way to tune the ratio between individual elements in a discrete manner. The precursor inks in the cartridges can be delivered to the substrate in a programmable mannerthe chemical precursors get printed out like the combination of colors. Ink formulation for the precursors is very important to get a reliable compound library through printing. To avoid complications, the formulation recipe should be as simple as possible (no side effects might be introduced) and the inks for individual elements should be compatible with each other (to allow a uniform mixing of the elements). The precursor compound library is typically deposited on a glass substrate and will undergo further heat treatment to transform into metal oxides. For this reason, only precursors that allow solid-state reaction at 200 s−1) in the electrolyte for CO2 reduction to generate CO. The competition between the reduction of CO2 and water could be tuned by varying the concentration of the Re complex in the water. The Re complex catalyst exhibited selective reduction of CO2 over water. The origins of the selectivity and further deactivation were investigated and reported recently575 in order to understand the basic principles governing improved performance. Similarly, a Si surface, functionalized using a phenylethyl group, showed an improved catalytic current behavior over H−

5.3. O2 Reduction for H2O2 Generation

In addition to water splitting for H2 generation and CO2 reduction to hydrocarbons, H2O2 from O2 reduction is another promising route for solar energy storage.578 H2O2 has its advantages compared to H2 fuel, which requires a large H2 gas storage container. H2O2 can be also used as a fuel and oxidant in an H2O2 fuel cell for electricity generation. Electrochemical formation of H2O2 involves O2 reduction and water oxidation with a 2-electron process hν

2H+ + O2 + 2e− → H 2O2

(26)

Photosynthesis of H2O2 has been investigated on various semiconductors.579 The first demonstration of viable H2O2 production through a light-driven reduction of O2 was presented by Calabrese in 1981 using p-Si derivatized with a quinone system.580 This is similar to the process generally used for large-scale production of H2O2. Pt and Pd are still considered as the best oxygen reduction reaction (ORR) catalysts with a large exchange current density.581 Development of catalysts made of earth-abundant materials is also critical to enable large-scale production of hydrogen peroxide. A lot of work has been done to develop an efficient ORR catalyst for direct synthesis of H2O2 by electrochemically reducing O2 in aqueous media. Recent work using Hg to modify the surface of Pd and Ag nanoparticles showed improved activity and selectivity of the oxygen reduction reaction for H2O2 production.582 This is typically in an acid solution to prevent decomposition of the H2O2 and because reaction of atomic hydrogen or protons with oxygen thermodynamically favors production of water. New electrochemistry developed by Dow to produce alkaline hydrogen peroxide utilizes an electrolytic reduction of oxygen in a diluted sodium hydroxide solution. hv

H 2O + O2 + 2e− → HO−2 + OH−

(27)

On the other hand, transition metals and their compounds can also catalyze the decomposition reaction of H2O2. Therefore, discovering an electrocatalyst that can selectively produce H2O2 rather than H2O and meanwhile does not accelerate the decomposition process is necessary and still needs a significant research effort.

6. CONCLUSIONS AND PROSPECTS Over the past few decades, humans have dedicated tremendous efforts to solve the energy problem and to deal with the 8706


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system, can be an excellent platform to integrate with the advanced catalysts for these applications in the future.

consequences of the excessive use of fossil fuels. Solar energy is recognized as the ultimate solution to all of the energy and environmental crises due to its effectively unlimited available power. Mimicking natural photosynthesis in order to synthesize chemical fuels from light in a clean and cheap fashion is one of the promising approaches for converting and storing solar energy. Exploration of this approach using organic molecular photochemistry or inorganic semiconductor photoelectrochemistry has been successful. Si is widely used for photovoltaic applications, due to its superior advantages of low cost and narrow band gap matched to the solar spectrum. It is therefore a perfect candidate for the bottom cell of a tandem design. The thermodynamic and kinetic problems of Si in aqueous solution are considered as the major issues to solve in order to apply it to photoelectrochemical solar-fuel conversion. Surface alteration techniques are important for overcoming these intrinsic drawbacks. An ideal coating should satisfy the BORSA criteria, providing a strong built-in asymmetry, proper optical modulation, minimum resistance losses, chemical stability, and high activity toward the desired reactions. Strategies to enable Si for solar-fuel production are broadly reviewed in this article. These include the use of surface textures and coatings containing metallic, inorganic semiconductor, organic polymer, and small molecule materials. Significant successes obtained from these studies provide fundamental design guidance to develop a spontaneous water-splitting device for H2 production as a clean fuel. System prototypes based on two-photon or three-photon configurations such as a photoelectrical diode or a triple-junction amorphous Si solar cell are compared and design considerations are provided in this review. Moreover, hybrid configurations, by integrating a molecular sensitized solar cell, a solid-state/quantum dots sensitized solar cell, or an organic solar cell with a Si photocathode or Si photoanode, could be one of the alternative approaches for system integration, depending on the band gap of the water oxidation and water reduction components. In addition, cost-effective fabrication processes like inkjet printing and sol−gel processing as well as screening novel electrocatalysts through compositional and structural modification have been developed and are promising paths for development. Computational screening together with a combinatorial experimental verification provides an effective way to successfully identify novel, extremely durable, and selective photocatalysts and electrocatalysts with high activities. A highly integrated system for safe and stable production of fuels in a globally scalable fashion directly from sunlight still needs research efforts to address challenges that are typically common in electrolysis systems.583 These challenges are associated with the membrane properties of the solar watersplitting system, such as separation of products, neutralization of pH gradients, and ability to hold back pressure. Research should be focused on rational management of the efficiency of each step in this complex multiphysics and chemistry process as well as optimization of the overall performance. Such steps include light absorption, carrier collection and transport, gas evolution, system stability/capability of regeneration, catalyst activity/stability, ionic transportation, separation of products, heat and water flow management, etc. Finally, in addition to water splitting for H2/O2 generation, CO2 reduction and O2 reduction for liquid fuel generations are also promising yet challenging chemical reactions for solar energy storage in alternative fuels. Catalyst characterization and development are the main research focuses for these two pathways for solar-fuel conversion. Si, as an earth-abundant bottom cell in a tandem

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Ke Sun obtained his Ph.D. degree in Electrical and Computer Engineering from the University of California at San Diego in 2013 for his work on synthesis/characterization of nanomaterials and heterogeneous surface modification of Si for optoelectronics and solar water splitting. He is currently a postdoctoral researcher at the Joint Center of Artificial Photosynthesis (DOE Energy Innovation Hub) under the supervision of Professor Nathan Lewis at Caltech. He is continuously committed to solar-fuel research. Specifically, he is focusing on development of protective coatings for unstable photoelectrodes in aqueous environment, optimization of electrocatalyst loading, and optimization of Si microwire-based photoelectrodes.

Shaohua Shen obtained his Ph.D. degree in Thermal Engineering from Xi’an Jiaotong University in 2010. During 2008−2009 and 2011− 2012, he worked as a guest researcher at Lawrence Berkeley National Laboratory and a postdoctoral researcher at the University of California at Berkeley. He is currently an associate professor at Xi’an Jiaotong University, China. His research interests include synthesis of nanomaterials and development of devices for photocatalytic and photoelectrochemical solar energy conversion. 8707


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Yongqi Liang obtained his Ph.D. degree from Peking University, China (supervisor Prof. Dongsheng Xu) in 2005 for his work on controlled synthesis and assembly of nanowires for optoelectronic applications. With sustained interests on solar-fuel production during the past 9 years, he has been working with Dr. Roel van de Krol at TUDelft (from 2005 to 2009) and Prof. Bruce Parkinson at the University of Wyoming (from 2009 to 2012) on screening and optimizing transition-metal oxides for photoelectrochemical water splitting. He is now working with Prof. Johannes Messinger at Umea University as a postdoctoral researcher. His current research is on prototyping artificial leaf devices by borrowing insights from natural photosynthesis. He will soon join Dalian University of Technology as an associate professor and continue his research on energy conversion and storage.

Samuel S. Mao is Director of the Clean Energy Engineering Laboratory and an adjunct professor at the University of California at Berkeley. After receiving his Ph.D. degree from U.C. Berkeley in 2000, he has been leading a multidisciplinary research team studying functional materials and developing clean energy technologies. He has published 120 refereed articles that have received nearly 20 000 citations. He is also an inventor or coinventor of 20 patents, and has delivered invited, keynote, or plenary talks at more than 80 international conferences. He has served as a technical committee member, program review panelist, grant proposal reviewer, and national laboratory observer for the U.S. Department of Energy. He cochaired the 2011 Materials Research Society (MRS) annual spring meeting and the 2012 International Conference on Clean Energy. In 2013, he founded a private research institution, the Samuel Mao Institute of New Energy, which develops and commercializes sustainable energy and environmental technologies to meet the changing needs of society.

Paul E. Burrows obtained his Ph.D. degree from Queen Mary College, University of London, in 1989. From 1990 to 2000, he held research appointments at the Riken Institute in Japan, the University of Southern California, and Princeton University, where he was a Research Scholar until 2000. His work encompassed the first demonstration of organic molecular quantum well structures and advanced organic light-emitting devices, which led to the spinout of Universal Display Corporation. He subsequently joined Pacific Northwest National Laboratory as a Laboratory Fellow in the Energy Science and Technology Directorate. In 2008, he founded an independent technology consulting firm, Reata Research, and in 2013 he assumed the founding Senior Vice President for Research and Development at the Samuel Mao Institute of New Energy. He has more than 100 refereed publications and 94 issued U.S. patents.

Deli Wang obtained his Ph.D degree in Material Science from the University of California at Santa Barbara in 2001. After postdoctoral research at Harvard University, he joined the University of California at San Diego and currently is an associate professor in Electrical Engineering. His research interests include nanomaterials and nanofabrication, device structures and applications in optoelectronics, sensing, renewable energy, and wearable technology.

ACKNOWLEDGMENTS The authors would like to thank Clifford Kubiak (UCSD), Chengxiang Xiang (Caltech), and Johannes Messinger (Umeå University) for valuable discussions. This work has been supported by the University of California Senate Research Program (K.S. and D.W.), the National Natural Science Foundation of China (S.S.M.), the K&A Wallenberg Foundation and Umeå University (Y.L.), the U.S. Department 8708


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