Photocatalysis: From Fundamental Principles to Materials and

Review Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Photocatalysis: From Fundamental Principles to Materials and Applications Xiaogang Yang*,† and Dunwei Wang*,‡ †

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 193.93.195.151 on 11/15/18. For personal use only.

Key Laboratory of Micro-Nano Materials for Energy Storage and Conversion of Henan Province, Henan Joint International Research Laboratory of Nanomaterials for Energy and Catalysis, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, People’s Republic of China ‡ Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill, Massachusetts 02467, United States ABSTRACT: Photocatalysis represents a unique class of chemical transformations. It utilizes the energy delivered by light and drives reactions that are difficult, sometimes even impossible, to carry out in dark. When used for thermodynamically uphill reactions such as photosynthesis, photocatalysis promises a sustainable solution to large scale solar energy storage. Despite the longstanding interest in this process and research efforts, existing photocatalysis demonstrations are limited to academic laboratory settings. Chief among the reasons for the slow progress is the lack of suitable photocatalyst materials for large scale applications. For the purpose of effective light absorption, charge separation, and charge transfer, a large number of photocatalytic materials, including conventional semiconductors and emerging photoelectronic materials such as nanoscale plasmonic metal particles, quantum dots, and 2D materials, have been studied. This Review is written to summarize these recent efforts from a broad materials perspective and discuss possible strategies to move forward to practical implementations. We start with a discussion on the fundamental principles that govern photocatalysis in general and then move on to discuss the different classes of photocatalytic materials, covering various aspects of their properties, such as efficiency, stability, scalability, and cost. Afterward, we use model photocatalytic reactions including water splitting, CO2 reduction, and N2 fixation to demonstrate the applications of these photocatalysts. Our perspectives concerning where the field of photocatalysis is headed toward are provided at the end. KEYWORDS: photocatalysis, solar energy, materials, properties, applications

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Becquerel as early as in 1839,6 intent research on this topic did not gain mainstream attention until the late 1960s,7 thanks to efforts by pioneers in the field such as Boddy8 and Honda and Fujishima.9 Most recently, the research interest in photocatalysis benefits from the growing awareness of sustainability.10,11 One promising application of photocatalysis is to store solar energy directly in chemicals and, hence, the name of solar fuels and artificial photosynthesis.12,13 Before going further, we will first clarify the concept of photocatalysis as it has been used in the literature for two distinctly different processes (Figure 2).15 Strictly speaking, when a material uses light energy to drive thermodynamically uphill reactions (positive Gibbs free energy change, ΔG > 0, Figure 2a), the process should be regarded as photosynthesis. The material used in such a situation may be regarded as a “photocatalyst” only if the photon is considered as a reactant. Popular chemical transformations studied for this situation

INTRODUCTION Photocatalysis is inspired by natural photosynthesis.1,2 The fundamental processes driven by light are often referred to as the “Z-scheme” as shown in Figure 1, where the most important components are the two photosystems (PSI and PSII).3 Upon illumination, the chlorophylls in PSII absorb photons with a maximum wavelength of 680 nm (P680) and transfer the energy to PSII.4 The energy is then used to extract electrons from H2O, producing O2 by the water oxidation catalysts (WOCs). The electrons are separated and transferred to PSI, where the chlorophylls absorb photons with a maximum wavelength of 700 nm (P700).5 The energy of the newly absorbed photons further excites the electrons transferred from PSII, making them energetic enough to reduce nicotinamide adenine dinucleotide phosphate (NADP+ → NADPH). Together with the proton gradient built during the process, NADPH drives the downstream transformations such as CO2 to hydrocarbons by the Calvin cycle. The most important essence of natural photosynthesis is the ability to drive chemical reactions using optical energy. While the concept of photocatalysis may be traced back to Edmond © XXXX American Chemical Society

Received: August 13, 2018 Accepted: October 25, 2018

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DOI: 10.1021/acsaem.8b01345 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Z-scheme charge separation process of nature photosyntheses I and II with protein photocatalyst. P680* and P700* stand for the excited states of chlorophyll P680 and P700, respectively; NADP is the nicotinamide adenine dinucleotide phosphate reductase. Adapted by permission from Springer Nature, Nat. Mater. ref 3, Copyright 2012. In dashed box, P680 and P700 are dimers of chlorophyll a (green structures) in different environments, respectively. Reproduced with permission from ref 14. Copyright 2006 National Academy of Sciences.

Figure 3. Schematic artificial photosynthesis classification: (a) molecular dye photocatalyst; (b) traditional semiconductor-based photocatalyst; (c−e) new emerging (c) quantum-dots-based photocatalyst, (d) two-dimensional material-based photocatalyst, and (e) plasmonic metal-based photocatalyst; (f) traditional semiconductorbased photovoltaic power driven electrocatalysis as “photocatalysis”.

catalysts, and (e) plasmonic metal photocatalysts; as well as (f) traditional semiconductor-based photovoltaic-assisted catalysts. An obvious advantage of categorizing these implementations as shown in Figure 3 is that it allows us to discuss the different principles that govern light absorption and charge separation. For example, where molecular photocatalysts are concerned (Figure 3a),22 the molecular orbital theory will be most suitable to describe the system, where the alignment of the highest occupied molecular orbitals (HOMO) or the lowest unoccupied molecular orbital (LUMO) of the photocatalyst with the reactant molecular orbitals is most relevant. For semiconducting photocatalysts (Figure 3b), the band theory is most suitable.16 When the sizes of the photocatalysts are reduced to be comparable to the Bohr radii of excitons, the quantum effect becomes significant, where the optical and electrical behaviors are both highly sensitive to the sizes.23 This is the situation where quantum dots (QDs) photocatalysts attract significant attention (Figure 3c). Indeed, as will be detailed later in this Review, when the size is small enough, even conductive materials (e.g., carbon dots < 10 nm) could become semiconducting, opening up possibilities of using them for photocatalysis.24−26 To the end of using conductive materials for photocatalysis, metals have been applied as photocatalysts lately thanks to their plasmonic effects.27 Twodimensional (2D) materials (e.g., ultrathin films, monolayers of black phosphorus, transition metal oxides, and dichalcogenides, to name a few,28,29) are also explored as photocatalysts (Figure 3d). The interest in this class of materials originates from their unique structures, which gives rise to anisotropy desired for charge separation for photocatalytic applications.30,31 Lastly, we note that the inclusion of photovoltaic in conjunction with catalysts (Figure 3e) is mainly meant to be inclusive, as this class of applications represents efforts for the same purpose of using light to catalyze chemical reactions that are difficult (e.g., pollutant degradation) or thermodynamically unfavored (e.g., photosynthesis). The benefit is obvious: light absorption, as well as charge separation, and catalysis can be optimized separately on traditional semiconductor photovoltaic devices. Moreover, the photovoltaic component in such a system could be readily replaced by other renewable electricity such as wind. The challenges, however, are equally apparent. The costs of these implementations are often limited by the higher of the power generation or the catalytic unit. As far as the scope of this Review is concerned, we further note

Figure 2. Thermodynamics of uphill and downhill photocatalysis: (a) uphill process; (b) downhill process. Adapted with permission from ref 15. Copyright 2017, American Chemical Society.

include water splitting16,17 and CO2 reduction.18 Conversely, a material may use light to facilitate thermodynamically downhill reactions (ΔG < 0, Figure 2b). Since the material does not change the thermodynamics of the reaction but only changes the kinetics by establishing new reaction routes through absorption of optical energy, the material would fit the strict definition of photocatalyst. Examples in this category include the oxidation of phenol to hydroquinone by oxygen (ΔG° = −167.96 kJ/mol) or complete oxidation to CO2 and H2O (ΔG° = −3027.36 kJ/mol).19,20 Irrespective of the clear distinctions from a thermodynamic perspective, from a materials point of view, the considerations of photocatalyst properties such as light absorption, charge separation, and charge transfer are shared for the two categories of reactions as shown in Figure 2. As such, we adopt the broad definition by IUPAC on photocatalyst as “catalyst able to produce, upon absorption of light, chemical transformations of the reaction partners...”21 for the rest of this Review. With the broad term of photocatalyst defined, we next discuss the existing implementations of photocatalysis. Of the many ways to categorize the implementations, we chose to group them based on the types of materials involved, in line with the main focus of this Journal on materials research. In Figure 3, we present six general classes of implementations, including (a) molecular photocatalysts; (b) traditional semiconducting photocatalysts; and new emerging photocatalysts such as (c) quantum dot photocatalysts, (d) 2D photoB


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Figure 4. Particulate semiconductor photocatalysis: (a) Particles dispersion in single reactor. Republished with permission of Royal Society of Chemistry, from ref 61, Copyright 2009; permission conveyed through Copyright Clearance Center, Inc. (b) Two side reactions at single particle. Reprinted with permissions from ref 62. Copyright 2010 American Chemical Society. (c) Web chart of applicating criteria for particulate photocatalysis (nos. 1−5 stand for low to high).

reactions take place in close vicinity and photoelectrochemical (or photoelectrocatalytic) configurations where redox reactions are intentionally separated. We first use conventional semiconductor photocatalysts to illustrate the difference between the two different implementations. Next, we discuss new emerging materials (e.g., quantum dots, 2D materials and plasmonic materials), which display unique properties and features. As far as capturing solar light and converting the energy to chemical energy are concerned, it is probably most effective (albeit more expensive) to combine photovoltaics with electrolysis. Such an approach would fall in the second category as outlined above. Following this line of thoughts, we scrutinize these different implementations, focusing on how to best take advantage of the unique properties of photocatalysts with different structures and morphologies. For convenience in comparing these different approaches, a hexagon Web diagram is drawn for each implementation based on the fuzzy analytic hierarchy process (FAHP),52 in which the following factors are compared on a scale between 1 (lowest) and 5 (highest): cost, convenience, scalability, efficiency, stability, and reaction selectivity.53 Our idea is to provide an easy visual comparison of these different implementations in connection with the various practical considerations. 2.1. Particulate Semiconductor Photocatalysts. Since semiconductors have been studied largely in both particulate and film-like configurations. We separately reviewed them in sections 2.1 and 2.2. Among all implementations, the particulate dispersion approach may be the most popular in evaluating the photocatalytic performance, thanks to the simplicity of the instrumentation required. This particulate photocatalyst includes not only solid spheres but also hollow structure54 and other asymmetric morphologies. Research on this type of implementations also benefits tremendously from the facile synthesis of nanoparticles, such as the hot-injection method,55 hydrothermal precipitation,56 co-precipitation,57 thermal condensation reaction,58 ultrasonic exfoliation,59 and ball milling.60 Figure 4a shows a typical particle-based system for photocatalysis, which consists of the photocatalyst and reactive medium as two main components.17,61 Typically, both the reduction and the oxidation reaction take place on the surface of the same photocatalyst particle, in close vicinity (Figure 4b). To avoid precipitation of photocatalyst particles and to maximize light absorption, the system often requires stirring or flow agitation. Using a semiconductor nanoparticle as an example, we illustrate how photogenerated charges are separated and used for the redox reactions. Upon illumination, photogenerated electrons and holes migrate to spatially separated sites on the surfaces, where reduction and oxidation reactions take place (Figure 4b).62 Similar systems have been

that we leave out photocatalytic organic synthesis by homogeneous molecular catalysts32,33 because it is a unique class of chemical transformations that does not fit well with our discussions here. Similarly, molecular photocatalysts for water oxidation2 and dye-sensitized water splitting34 are not discussed, either. Instead, we focus on heterogeneous photocatalysts (Figure 3b−e). Even within this space, there are reviews focused on various aspects of photocatalysis, e.g., general and comprehensive reports on semiconductors, nanostructures, mechanisms, and optimization strategies,16,17,35−37 specific materials (ZnO, α-Fe2O 3, and TiO2),38−40 new emerging plasmonic materials,41−46 quantum dots,24−26 two-dimensional materials,29,47−49 and broad applications (bacterial disinfection,50 water splitting,16,17 and CO2 reduction18,51). Notwithstanding, a comprehensive review specifically focused on the broad scope of photocatalytic materials has been missing, which prompted us to write this Review. Our goal is to provide a platform for researchers to survey photocatalytic materials for side-by-side comparisons, so as to inspire new ideas on how to advance research on this exciting topic. Based on these considerations, we present the Review in four parts. First, we compare the general advantages and challenges of different implementations based on the different materials, including traditional semiconductors (e.g., Si, Ta3N5, TiO2, BiVO4, Fe2O3, WO3, Cu2O, Ag3PO4, CdS, and GaAs/ InGaP) and new emerging plasmonic metals (e.g., Ag, Au, Al, and Bi), QDs (e.g., carbon dots, graphene dots, CdSe QDs, and CdTe QDs), and 2D materials (e.g., black P, g-C3N4, BiOX, MoS2, and MXene). These materials were grouped mainly based on the different mechanisms by which light absorption and charge separation take place. Next, we discuss the principles that underpin charge separation, recombination, and transfer within these different materials. Third, we summarize the efficiency, stability and cost of existing photocatalysts in different categories. Toward the end, we contrast photocatalysts based on the chemical transformations they are used for, such as water splitting reactions for hydrogen generation, CO2 reduction, N2 reduction, and other organic reactions, as well as other promising future applications.

2. ADVANTAGES AND DISADVANTAGES OF DIFFERENT IMPLEMENTATIONS Photocatalysts feature a wide range of structures and morphologies, including nanoparticles, nanowires, 2D materials, planar films, 3D porous films, and complex hierarchical nanostructures. To best take advantage these photocatalysts, a variety of implementations have been developed. Broadly, these implementations may be grouped into two general categories, namely, particulate suspensions where redox C


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Figure 5. Photoelectrochemical configurations with semiconductors: (a) single photoelectrode with counter cathode under bias in a Honda− Fujishima cell. (Left image) Adapted by permission from Springer Nature, Nature ref 9, Copyright 1972. (Right image) Adapted with permission from ref 81. Copyright 2016 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/by-nc/4.0/. (b) Two photoelectrode without bias. (Right image) Adapted with permission from J.-W. Jang, C. Du, Y. Ye, Y. Lin, X. Yao, J. Thorne, E. Liu, G. McMahon, J. Zhu, A. Javey, J. Guo, and D. Wang.76. Copyright 2015, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). (c) Wireless unassisted tandem cells. Adapted with permission from ref 79. Copyright 2013 American Chemical Society. (d) Web chart of applicating criteria of photoelectrochemical photocatalysis (nos. 1−5 stand for low to high).

convenient for large scale implementations, and (d) short charge diffusion distance to reduce bulk charge recombination. Disadvantages of this system include (a) difficulty in product separation, which would be particularly problematic for reactions such as overall water splitting; (b) serious surface charge recombination due to the close vicinity of reduction and oxidation sites, (c) poor photocatalyst chemical stability due to the high surface areas and/or poor disperse stability,66 and (d) product crossover due to the close vicinity of redox sites. These considerations are summarized in Figure 4c as a hexagonal web chart. 2.2. Photoelectrochemical Semiconductor Photocatalysts. A simplistic photoelectrochemical (PEC) cell should consist of at least three components, no fewer than one photoelectrode, the electrolyte, and an external electric circuit. Often, thin film materials are used, which can be readily prepared on a conductive substrate through the sol−gel,67 solution deposition,68 chemical vapor deposition (CVD),69 atomic layer deposition (ALD),37 molecular beam epitaxy (MBE),70 sputtering deposition,71 thermal or e-beam evaporation,72 and electrodeposition methods,73 among others. In Figure 5, we show three types of PEC configurations. The first type features a single light absorber (either the photocathode or the photoanode) and a counter electrode. These two electrodes can be placed in separate cells which are connected by an ion-conductive membrane or in the same chamber. The second type features two light absorbers to utilize photons in a wider range of the solar spectrum. These two photoelectrodes may also be placed in either two separate cells or a single chamber. The third type features two electrodes (one or both are photoelectrodes) that are connected back-to-back, so that the external circuit is minimized. The last configuration is often

widely applied for photocatalytic dye degradation studies. During the process, dissolved O2 accept photogenerated electrons to yield •O2 radicals, and photogenerated holes can oxidize H2O to form •OH radicals. Both radicals show long lifetimes and are effective in oxidizing reactive substrates such as dye molecules (e.g., methyl blue, methyl orange, and rhodamine B). They are also good for degradation of organic pollutants, as well as for disinfection applications by oxidizing biological species. In addition to thermodynamically downhill reactions, particulate photocatalysis systems have been studied for thermodynamically uphill ones, including water splitting,61 CO2 reduction,63 and N2 reduction.64 The most obvious distinction of this system in comparison with photoelectrochemical ones is the absence of external circuitries. As such, no external bias can be applied to particulate photocatalysts. This feature brings up an important discussion point. Except for a few photocatalysts whose bandgaps are wide enough to enable overall redox reactions such as water splitting, the rest of the literature on particulate photocatalytic studies involve sacrificial electron donors (for hydrogen evolution and/or CO2 reduction reactions) or electron scavengers (for photooxidation of water and organic molecules).65 While these sacrificial reagents are indeed convenient for the study of the photocatalytic reactivity of specific half-reactions, and the knowledge generated by such studies can be highly valuable, presenting the system as “solar hydrogen production” or “solar CO2 reduction” can be misleading. This is because how these sacrificial reagents may interact with the intended reactions can be complex, and synergistic effects have been observed.65 We caution against such unnecessary hypes. Advantages for this photocatalysis system include (a) high surface area, (b) high density of active sites, (c) simple and D


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Figure 6. Quantum dots with tunable band structures: (a) Density of states of bulk, quantum well, quantum wire, and quantum dot. Reprinted with permission from K. E. Jasim.23. Copyright 2015 CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/). (b) Bandgap increases of a semiconductor material with size decrease. The lower panel shows Koole’s CdSe colloidal suspensions under UV excitation. Republished with permission of Royal Society of Chemistry, from ref 85, Copyright 2011; permission conveyed through Copyright Clearance Center, Inc. (c) Band edge position of PbS QDs shifting with thiophenol ligands. Adapted with permission from ref 87. Copyright 2015 American Chemical Society. (d) UV−vis absorption, PL excitation, and emission spectra of carbon quantum dots. Insets show photographs under visible and UV light. Adapted with permission from ref 84. Copyright 2013 John Wiley & Sons. (e) Web chart of the specifications of the QDs in photocatalysis (nos. 1−5 stand for low to high).

referred to as “wireless” in the literature. The first type (Figure 5a) was developed by Honda and Fujishima.9 The single photoelectrode is irradiated by photons whose energy is greater than the bandgap, resulting in electron excitation to the conduction band and leaving holes in the valence band. Take an n-type semiconductor photoanode as an example. A Schottky junction is formed at the interface between the semiconductor and the electrolyte, which is key to the working mechanisms of this system.74 More details of the mechanisms will be discussed in section 3. It suffices to summarize here that the photogenerated holes in the valence band emerge to the interface, where they are used to carry out the oxidation reactions.75 The excited electrons will be collected by the underlying current collector and transported through the external circuit to the counter electrode to drive the reduction reactions. If a p-type photocathode is used, electrons would be used on the photoelectrode for the reduction reactions, and holes are transported to the counter electrode for the oxidation reactions. The dual-absorber system as shown in Figure 5b is often referred to as the Z-scheme.76 The shorter wavelength portion of the illumination (often solar light) will be absorbed by the front electrode (usually a wide bandgap n-type photoanode); the longer wavelength light will transmit through the front photoelectrode and be absorbed by the rear narrower bandgap semiconductor (usually a p-type photocathode). In addition, the parallel dual-absorber system can also be used side by side. To take advantage of different parts of the spectrum, a spectrum separator can be employed as has been shown by Li et al.77 and Mi et al.,78 separately. Moreover, the two absorbers can be fabricated in a wireless tandem

configuration, saving the need for external circuits (Figure 5c).79 A critical issue in the last configuration is how to meet the thermodynamic needs without external inputs. Nocera et al. have demonstrated the utility of amorphous Si triple junctions for such an approach in a stand-alone device.80 Advantages of a PEC system include the following: (a) better spatial charge separation, (b) easy controls over the reaction pathways, (c) ease in supplementing photoenergy by external power supplies for thermodynamically uphill photosynthetic reactions, (d) convenient product separation, and (e) less stringent requirements for co-catalyst owing to the relatively low current densities (typically 10−20 mA/cm2 as opposed to 1 A/cm2 for electrolysis applications). For example, by carrying out the reduction and oxidation reactions at spatially well separated sites, we can minimize product crossover and reduce recombination. The application of an external bias can dramatically increase the lifetimes of charges.82 This is particularly useful to take advantage of earth abundant, low-cost materials that are not able to perform the overall reactions of H2O splitting and/or CO2 reduction (e.g., Fe2O3 or BiVO4) on their own. By controlling the potentials, we also gain control over the surface redox reactions with promoted forward reactions and suppressed backward and other parasitic chemical reactions. Lastly, this approach has proven a powerful characterization tool to understand the fundamental photoelectrochemical properties of various photocatalysts. For instance, it can be employed to discern the true functions of co-catalyst depositions.83 The knowledge will be extremely useful for powder-based photocatalysis, as well. Disadvantages of this system include the following: (a) the E


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Figure 7. Band structures of 2D materials: (a) 2D materials evolution (nanosheets; semiconductor/graphene composites; ultrathin layers). (b) Indirect band structures of bulk MoS2 and direct band of monolayer. Adapted with permission from ref 30. Copyright 2011 by the American Physical Society. (c) Band edge positions of MX2 as a function of the number of layers. Reprinted from ref 99, Copyright 2013, with the permission of AIP Publishing. (d) Web chart of the 2D materials for photocatalysis (nos. 1−5 stand for low to high).

synthesis of thin films could be cumbersome and, therefore, be expensive; (b) the interface between the photoelectrode and the current collector needs to be optimized; (c) membranes may be needed to separate products, which would significantly increase the cost of implementations; and (d) relatively poor scalability is due to complications of the engineering involved. A Web diagram summarizing these considerations is shown in Figure 5d. Sections 2.1 and 2.2 mainly concern “conventional” photocatalytic semiconductor materials as they are governed by the conical principles in terms of light−matter interactions. The charge separation mechanism is also well described by the Schottky-type diode semiconductor theory. A critical challenge of these photocatalysts is the low efficiencies. One reason for the low efficiencies is the mismatch of the light absorption by these materials and the solar spectrum. How to tune the light absorption for a better match and, hence, improved efficiencies will be discussed in section 3. 2.3. QDs Photocatalysts with Tunable Band Structures. Different from bulk semiconductors with fixed conduction and valence band positions, the optoelectronic properties of QDs are governed by unique principles. A critical feature of QDs is their small sizes, comparable to their Bohr radii (usually EF,SC). Light penetration, diffusion length, and space charge width can be determined by 1/α, LD, and WSC, respectively. (e) Charge separation in a semiconductor with a p−n junction. (f) Z-scheme charge separation on a p-type semiconductor and n-type semiconductor, where ohmic contact is formed between them.

light absorption and electrolysis. With the applications of power electronics, it becomes possible to concentrate solar energy collected by a large number of PVs, so as to minimize the cost of electrocatalysts and membranes. In this case, the cost of the overall system will be limited by the PV cells.127 Nevertheless, existing techno-economical analysis (TEA) shows that such an approach will be highly dependent on both the solar cells and the electrolyzers.127 Interested readers are guided to reviews on related issues.130,131

well. However, the governing principles of this approach are fundamentally different from other forms of implementations. We discuss this approach here simply because it involves a wide range of materials, for both high-efficiency solar cells and high-performance electrolysis. For instance, conventional solar cells124 consisting of single or multiple junctions involve Si of a variety of forms; single crystalline, polycrystalline, or amorphous could be and have been utilized for this approach. Thin film solar cells consisting of copper indium gallium selenide (CIGS)125 and cadmium telluride (CdTe)126 have also been tried for this approach. When used to drive chemical reactions, a critical consideration is to match the output voltage with the electrochemical potential required for the targeted chemical transformations. Consider water splitting as an example. Typical single-junction solar cells feature opencircuit voltages much lower than the nominal water splitting potential (often at or greater than 1.8 V to be kinetically meaningful). This problem can be solved by either connecting multiple PV cells in series or through a DC−DC power converter. In either case, a significant penalty has to be paid in terms of efficiency or cost or both.127 It is within this context that perovskite solar cells have gained great interest for solar water splitting applications. For instance, two perovskite solar cells were connected in series to power electrolysis of water at a 12.3% solar-to-hydrogen (STH) efficiency128 (Figure 9a,b). Alternatively, a perovskite solar cell was utilized to augment a BiVO4 photoanode-based PEC cell,81 enabling complete water splitting that would not be possible by either the PV cell or the PEC cell alone. From an engineering perspective, such an application can be further integrated. For instance, Figure 9c shows how a double-junction a-Si PV cell is added behind a Wdoped BiVO4 photoanode in a PEC configuration for water splitting.129 The considerations on how to balance the voltages are summarized in Figure 9d, where the solar cell consists of InGaP, GaAs, and GaInNAs(Sb), connected in series.124 The PV plus electrolysis approach offers distinct advantages, mostly in the considerations of technology readiness levels (TRLs). For example, it allows for separate optimizations of

3. FUNDAMENTAL PRINCIPLES UNDERPINNING VARIOUS PHOTOCATALYTIC MATERIALS With the different forms of implementations introduced, we next discuss the governing principles of the various approaches. When used for photocatalysis, conventional semiconductors, QDs, 2D materials, and plasmonic materials share commonalities in the considerations for efficient light absorption, effective charge separation, facile charge diffusion/migration, and fast charge transfer to drive chemical transformations. The detailed processes, however, vary greatly. For instance, the light absorption mechanisms are different. The photogenerated charges are separated by distinct mechanisms, as well. The lifetimes of the photogenerated charges vary by a few orders of magnitude, from picosecond to microsecond in semiconductors to ECBM). That is, these materials are expected to be thermodynamically stable under electrochemical conditions against self-oxidation and selfreduction. Unfortunately, most efficient photocatalysts do not meet such an expectation. For instance, the self-oxidation potential may be between the VBM and OER potentials (EOER < ϕox < EVBM), or the self-reduction potential may be between the CBM and HER potential (ECBM < ϕred < EHER).323 Or worse still, the self-oxidation potential could be more negative than OER potential, in which case the photocatalyst would be oxidized before it can be used to oxidize H2O. Similarly, when the self-reduction potential is more positive than HER potential, it would be impractical to use the photocatalyst for water reduction. To make the situation even more complex, the addition of applied bias and/or photoillumination could easily shift the quasiFermi levels of the photocatalyst under photocatalytic conditions, making it difficult to predict the relative stability for practical applications. In recognition of the importance of stability to photocatalytic applications, we present the next contents based on their stabilities. 4.1. Stable but Less-Efficient Photocatalysts. We first examine photocatalysts that are thermodynamically or kinetically stable. They include TiO2, Ta2O5, BiVO4, Fe2O3, WO3, and Au, among others.123 Figure 14 shows a few strategies aimed at improving the various processes within a photocatalytic system, including the light absorption/exciton, charge separation, and charge transfer. Among these strategies, nanostructuring has been extensively applied to improve the performance of photocatalysts.39 This is because most charge behaviors relevant to photocatalytic reactions feature characteristic length scales at the nanometer range. Furthermore, nanostructures are advantageous for photocatalytic applications because they offer high surface areas to promote chemical reactions. Examples shown in Figure 14 include Fe2O3 on FTO nanospikes (Figure 14a),324 nanoporous BiVO4 (Figure 14b),325 and helix BiVO4 decorated with WO3 nanorods arrays (Figure 14c).326 Additionally, forming heterojunctions at the nanoscale has been shown to enhance light absorption by, for instance, providing a second absorber that broadens the absorption spectrum of the system (Figure 14d).327 Also in this category, we include examples which include TiO2 and plasmonic photocatalysts, where TiO2 increases charge lifetimes with better charge separation (Figure 14e).123 To further elaborate on the strategies of improving photocatalyst performance, we present examples in Figure 14f−l that are not limited to nanostructures. The general ideas are similar to what was discussed in the previous paragraph. That is, light absorption, charge separation, and charge transfer should be improved for stable photocatalysts, so that higher efficiencies can be measured. For instance, TiO2 as a

(1)

where NH2 is the hydrogen production (mmol/s), ΔG is the free energy gain (237 kJ/mol for water splitting), Ptotal is the solar illumination influx (mW/cm2), and A is the device area (cm2). Besides the direct measurement of chemicals produced by this process, a more convenient method can be used in the case of photoelectrochemistry:193

STH =

|Jph |VtheoηF PtotalA

(2)

where the |Jph| is the photocurrent measured without externally applied bias (e.g., unassisted water splitting in a two-electrode configuration). Vtheo is the theoretical voltage (e.g., 1.23 V for water splitting) for the target reaction under equilibrium conditions. ηF is the Faradaic efficiency that converts electron to molecules (e.g., H2 and/or O2). The external quantum efficiency can be measured and expressed as194

IPCE =

Jph Jtheo

= ηabsηsepηtransf

(3)

where Jtheo is the theoretical photocurrent when the incident photons are converted in measured photocurrent Jph, with 100% efficiencies for light absorption (ηabs) to form excited electron−hole pairs, charge separation (ηsep), and charge transfer to surface redox pairs (ηtransf), in series. It is worth noting that the IPCE efficiencies only consider what percentage of incident photons can be used to generate electrons, namely, the photon efficiencies. They are distinctly different from the energy efficiency as described by STH. In other words, to achieve high energy conversion efficiencies, both Vph and photocurrent Jph should be maximized. For maximum photovoltages, it is desired to use large bandgap semiconductors, in which photogenerated charges could occupy the conduction band and valence band separated by a large energy gap. However, the highest theoretical photovoltage of a photoelectrode is determined by the quasi-Fermi levels of electrons and holes in the Schottky junction (Figure 10a), Vph < Eg − (Eredox − ECBM) for n-type and Vph < Eg − (EVBM − Eredox) for p-type. (Eredox − ECBM) and (EVBM − Eredox) correspond to the Schottky junction barrier height for n-type and p-type semiconductors, respectively.134 More discussions on the origin of the photovoltage in relation to the Fermi level of the semiconductor in dark and the redox potential of the electrolyte, as well as the quasi-Fermi level of the semiconductor in light, can be found elsewhere.195 As such, without considering the kinetic overpotentials, the theoretical photovoltages are defined by the band structures (band edge positions) and the specific redox reactions (Eredox). For maximum photocurrent densities, it is desired to use small bandgap semiconductors so as to maximize photons that can contribute to the photocurrents, because only photons with (1240 eV·nm) wavelengths shorter than λ ≤ E (eV) can be absorbed by the g

light absorber. Moreover, it is desired to use materials with high absorption coefficients because the optical depth is inversely 1 proportional to the absorption coefficient (δp = α ). For better charge separation, we must consider geometric and electric effects. The geometric effect depends on the sum of the depletion layer width (WSC) and the diffusion length (LD), the latter of which is proportional to the square root of the charge diffusion coefficient (or mobility) and lifetime (LD = τD ).135 The electric effect refers to the extent of band bending (e.g., Eredox − EFBfor n-type) in the depletion region.134 In the charge separation region (LD + WSC), bulk recombination of photogenerated charges is the main factor that limits charge separation efficiencies. In realistic systems, surface recombination and corrosion must be taken into account when considering charge behaviors. N


halide, n-type

halide, n-type

halide, n-type carbonate, p-type oxide, p-type

phosphate, n-type chalcogenide, n-type chalcogenide, n-type

oxide, n-type

oxyhalide, p- or n-type

oxyhalide, n-type oxyhalide, n-type carbonate, n-type

molybdate, n-type

vanadate, n-type

tungstate, n-type

element

nitride, n-type

chalcogenide, n-type

chalcogenide, n-type

AgCl

AgI Ag2CO3 Ag2O

Ag3PO4 Ag2S AgInS2

Bi2O3

BiOCl

BiOBr BiOI Bi2O2CO3

Bi2MoO6

BiVO4

Bi2WO6

C (dots)

C3N4

CdS

CdSe

categories

AgBr

material

O

−1.11 CB, 1.45 VB

−1.1458 −0.6237

−0.55242

−1.14 CB, 1.54 VB58 −0.52 CB, 1.88 VB214 −0.70 CB, 1.65 VB237 −0.3 CB, 1.4 VB214 −0.55 CB, 2.15 VB242

1.7

2.7

2.4

1.9−2.949

−0.53234

233

−0.53 CB, 2.05 VB234

∼2.4−3.1

0.169 0.25230

0.46 CB, 2.86 VB224 0.39 CB, 3.01 VB229

2.4 2.62 232

−0.33221 0.05222 0.08223

0.4217 0.15218 0.71220 −0.52217

disadvantage

fast hole mobility (50 cm2/V/s);238 long hole lifetime (35 ns);243 Bohr radius (4.9 nm);240 good visible light absorption; suitable CBM and VBM for water splitting

fast holes mobility (48238 or 15239 cm2/V/s); long hole lifetime (0.1−0.3 μs);239 Bohr radius (2.8 nm);240 strong visible light absorption; suitable CBM and VBM for water splitting; inexpensive

long carrier lifetime (5.456 ns;58 3.6−5.6 ns;235 37 μs236); low cost; high chemical and thermal stability; suitable CBM and VBM for water splitting; 2D layered structures; nontoxic

photocorrosion in alkaline solution at −0.117 to −0.277 V244

photocorrosion; or required sacrificial agent and pH control241

short charge lifetime; light absorption 2.6 >20 8.7

0 0 0 0 0 ∼0.6b 1.63 1.7 0.6 2.07 0

130 min ∼8 h 1538 h 100 h 4h 80 h 2400 h 203 h 1000 h 100 h 3 hd

346 347 348 349 350 351 352 353 342 354 355

a

hydrogen evolution reaction. bvs. SCE. c1.1 sun. dunassisted CO2 reduction;

Figure 16. Stable and efficient photocatalytic devices: (a) Energy schematic of the tandem layer structure under illumination. t1, t2, and O denote tunnel junctions and ohmic back-contact, respectively. (b) Stability assessment under AM 1.5G illumination and 0.6 V vs RHE. (Panels a and b) Adapted with permission from M. M. May, H.-J. Lewerenz, D. Lackner, F. Dimroth, and T. Hannappel.357. Copyright 2015, Licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). (c) Structure of the InGaP/GaAs tandem cell photoabsorber with TiO2 protective layer and Ni catalyst. (d) Short-circuit photocurrent stability and the solar-to-hydrogen conversion efficiency, in two-electrode configuration under 1 sun illumination in 1.0 M KOH(aq). Adapted with permission of Royal Society of Chemistry, from ref 358, Copyright 2015; permission conveyed through Copyright Clearance Center, Inc. in KOH.338 In another study, p-Cu2O (Eg = 2.2 eV) was protected for 2 h stable photoreduction of H2O with an amorphous Ga2O3 buffer layer and a TiO2 outer layer (Figure 15a).339 The poor stability of nTa3N5 (Eg = 2.1 eV) was found to be due to surface hydroxylation/ oxidation.309 The application of a thick crystalline GaN layer (50 nm) was shown to enable stable photooxidation of H2O at 1.2 V vs RHE for over 10 h (Figure 15b).340 The TiO2 protection layer was applied on Sb2Se3/CdS p−n junctions using ALD growth, and the resulting photoelectrode demonstrated a stable photocurrent density of 8.6 mA/cm2 (85% retention) at 0 V vs RHE for 10 h.341 In situ formed NiFe-OEC on Mo:BiVO4 photoanode in Figure 15c was reported to enable stable operations for 1100 h.342 Crystalline TiO2 (24−30 nm) was used as an electron-conductive layer to protect p-GaInP2 photocathode (Figure 15d), enabling photoreduction of H2O for

over 20 h.343 In a separate study, Yang et al. have analyzed the band structure offset between TiO2 and p-GaInP2. They reported that an energy barrier formed at the interface to benefit hole blocking and facilitate electron transfer, further illustrating the passivation effect of TiO2.344 In a different, and somewhat surprising study, Hu et al. reported that amorphous TiO2 (4−143 nm) could be used to protect Si, GaAs, and GaP for photooxidation reactions, as shown in Figure 15e.345 It is interesting to see that hole transfer through the thick layer of TiO2 was not impeded. This approach allowed for continuous oxygen evolution on a Si photoanode for over 100 h a photocurrent density > 30 mA/cm2. In Table 3 and Figure 15f, we summarize several representative photocatalyst materials and reported strategies to stabilize them. T


Review


Figure 17. Overall water splitting by two absorbers: (a) Schematic of overall water splitting on the Ru-modified SrTiO3:La,Rh/Au/BiVO4:Mo sheet. (b) HER and OER production rates on Cr2O3/Ru-modified SrTiO3:La,Rh/Au/BiVO4:Mo sheet. Adapted by permission from Springer Nature: Nat. Mater., ref 360, Copyright 2016. (c) Schematics of hematite photoanode and a-Si photocathode in a tandem configuration. (d) J−V behaviors of various hematite photoelectrodes. (Panels c and d) Adapted by permission from J.-W. Jang, C. Du, Y. Ye, Y. Lin, X. Yao, J. Thorne, E. Liu, G. McMahon, J. Zhu, A. Javey, J. Guo, and D. Wang.76, Copyright 2015, licensed under CC BY 4.0 (https://creativecommons.org/licenses/ by/4.0/). (e) O2 and (f) H2 detection for IrOx-decorated hematite photoanode and a-Si photocathode in acidic solution (pH 1.01). (Panels e and f) Adapted with permission from ref 361. Copyright 2015 John Wiley & Sons. 4.3. Stable, Efficient but High-Cost Photocatalysts. With all the various issues listed, we note that high-efficiency photocatalysis is not out of reach. Although some of the photocatalysts (e.g., III−V group) are not fundamentally stable (Table 2), they may be protected for long-term stability. Within the context of solar water splitting, which is one of the most difficult reactions to carry out due to its high thermodynamic demands, high-STH (up to 12.4% for 20 h) PEC demonstration was achieved on a triple-junction system (p-GaAs/nGaAs/p-GaInP2) by Khaselev and Turner in 1998.356 More recently, May and co-workers demonstrated a two-photoabsorber system consisting of GaInP (1.78 eV) and GaInAs (1.25 eV) for Z-scheme solar water splitting, in which the photoelectrode was protected by and decorated with AlInPOx, Rh, and RuO2 (Figure 16a).357 The device exhibited an STH efficiency of 14% (photocurrent density > 12 mA/cm2) for over 40 h (Figure 16b). In yet another work, Verlage et al. reported a tandem cell made of an InGaP top cell and a GaAs bottom cell, protected by TiO2 and decorated with Ni catalysts as a photoanode (Figure 16c).358 By using a Ni−Mo counter electrode, the authors observed stable performance of ηSTH > 10% for over 40 h (Figure 16d). Results like these are exciting because they show stable and highly efficient solar water splitting can indeed be achieved. Coincidentally, these systems all involve buried junctions; they also included more than one light absorber. This is because a single junction, or a single cell, does not meet the voltage requirement to split water. Precisely for this reason, these results also have significant implications. That is, when treated as photocatalysts, these materials are prohibitively expensive, making them impractical for large-scale utilizations.

significant attention. They include thermodynamically uphill reactions such as H2O splitting, CO2 reduction, and N2 reduction. For completeness, several emerging organic transformations are also mentioned toward the end of this section. 5.1. Solar Water Splitting. Water splitting holds a special place in the discussions on photocatalysis. For one, this is because modern efforts of photocatalysis are arguably motivated by the desire to directly split water using sunlight, which promises an energy storage solution to the issues connected to the intermittent nature of solar energy. The most intuitive argument would be that the process can produce H2 as a clean energy carrier, whose production and utilization do not involve CO2. From a perspective focused on fundamental sciences, solar water splitting is also at a pivotal position when it comes to discussions on solar energy conversion. This is because it is the first step in natural photosynthesis, the products of which (excited electrons) are of critical importance for downstream reactions such as CO2 reduction and N2 fixation. While electrons could be supplied by other sacrificial reagents in a laboratory setting, for extremely large scale utilizations, our only reliable source would be water. Indeed, solar water splitting has been frequently reviewed.16,17,83 We keep our discussions on this subject brief, with a focus on materials-related issues. Simplistically, the two half-reactions, such as hydrogen evolution reaction at 0 V vs RHE and oxygen evolution reaction at 1.23 V vs RHE of water splitting may be presented as follows:

5. VARIOUS APPLICATIONS OF PHOTOCATALYSTS Photocatalysts have been applied in a wide range of fields. In accordance with our main goal of introducing materials-related aspects of photocatalysts, we present in this section a few prototypical chemical transformations that have received

2H+ + 2e− → H 2 H 2O + 2h+ → 2H+ + U

(4) 1 O2 2

(5) DOI: 10.1021/acsaem.8b01345 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Review


Table 4. Electrochemical Potentials for Various CO2 Reduction Processes vs NHE in Aqueous Solution (pH 0 and pH 7), 25 °C, and 1 atm Gas Pressure18 cathodic reaction −

−

CO2 + e → CO2 (g) +

−

+

−

+

−

+

−

+

−

radical

−1.9

formic acid

−0.17

carbon monoxide

−0.10

(9)

formaldehyde

−0.08

(10)

methanol

(6)

CO2 (g) + 2H + 2e → HCO2 H(l)

E°(pH0)a

product

(7)

CO2 (g) + 2H + 2e → CO(g) + H 2O(l)

(8)

CO2 (g) + 4H + 4e → HCHO(l) + H 2O(l) CO2 (g) + 6H + 6e → CH3OH(l) + H 2O(l)

CO2 (g) + 8H + 8e → CH4(g) + H 2O(l)

(11)

methane

a

Values calculated based on thermodynamic Gibbs free energy from Lange’s handbook. hydration energy and Gibbs free energy of gas phase.

E′(pH7)b

b

19 b

−0.61 −0.53 c

−0.48

0.02

−0.38

0.17

−0.24

c

Values from ref 364. Calculated based on formaldehyde

Figure 18. Photocatalytic CO2 reduction: (a) Redox potentials of the relevant reactions vs the Cu2O band edges (at pH 0, not considering stability). (b) Spatial separation of redox sites on TiO2 crystals (top) and surface heterojunction (bottom). (c) CH4 production of the TiO2 samples prepared by varying HF amount. (Panels b and c) Adapted with permission from ref 158. Copyright 2014 American Chemical Society. (d) CO2 photoreduction to CO on the VZn-rich ZnIn2S4 monolayers. (e) CO production rate on the VZn-rich and VZn-poor ZnIn2S4 monolayers. (Panels d and e) Adapted with permission from ref 366. Copyright 2017 American Chemical Society. (f) CO2 reduction mechanism and CO production rate on CdS-Cu2S NRs. Adapted with permission from ref 367. Copyright 2015 American Chemical Society. (g) SEM and elemental mapping of Au3Cu NP on Si NW arrays. (h) Faradaic efficiency of CO on Au3Cu NP/Si NWs. (Panels g and h) Adapted with permission from ref 368. Copyright 2016 American Chemical Society. (i) Mechanism for the light-driven carboxylation reactions. Reprinted with permission from ref 369. Copyright 2012 John Wiley & Sons.

V


Review


were also applied for water splitting.362 One can envision that, with advanced synthesis, it might be possible to reduce the size of the photoelectrodes into particulate forms for true photocatalytic water splitting applications. There is a strong desire to carry out solar water splitting using earth abundant materials because they are the only ones amenable to largescale utilizations. 5.2. CO2 Reduction. Given the immense interest in CO2 reduction, it would be remiss not to include it for discussions on photocatalysts. From a chemistry perspective, the issues faced by efforts on photocatalytic CO2 reduction are not different from those on electrocatalytic CO2 reduction (eqs 6− 11 in Table 4).51 The ability to directly use light to power such thermodynamically uphill reactions, however, has significant implications. As such, this topic deserves special attention.363 In addition to the considerations for reaction selectivity on the reduction side, which are the focuses of most studies on electrocatalytic CO2 reduction, how to supply electrons using sources that are amenable for extremely large scale utilizations represents an important concern for photocatalytic CO2 reduction. That is, for practical applications, photocatalytic CO2 reduction should close the loop in terms of electron supplies. The most obvious choice would be to extract electrons from H2O through H2O oxidation, similar to how it is carried out in natural photosynthesis. For this subsection, we discuss two aspects of photocatalytic CO2 reduction, complete systems that can both reduce CO2 and oxidize H2O, as well as unique chemical systems that address the selectivity concerns. In the first example as shown in Figure 18a, Cu2O features CBM and VBM at −0.7 and 1.3 V vs NHE, respectively. The CBM is more negative than the equilibrium electrochemical potentials of CO2/HCOOH, CO2/CO, CO2/HCHO, CO2/ CH3OH, and CO2/CH4, meaning that photoelectrons in the conduction band of Cu2O are expected to reduce CO2 to a variety of organic molecules. However, since these reactions are in a narrow potential range (variations < 0.4 V) and all very close to the HER potential,365 it would be a great challenge to control product selectivity. Further development of photocatalysts will require the combination of co-catalysts to address the reaction selectivity issue. In another example, TiO2 was used to reduce CO2 for the production of CH4 (Figure 18b). The uniqueness of this study was that TiO2 with specific facets was prepared,158 on which photogenerated electrons and holes migrate to the {101)} and {001} facets, respectively. Product detection revealed that high selectivity of CH4 was obtained on TiO2 {101} facets (Figure 18c). Similar facet-dependent charge separation phenomena have been observed on BiVO4, albeit for water oxidation reactions.157 Such a facet-dependent effect is amplified on anisotropic materials such as 2D photocatalysts, especially with the presence of localized defects, which not only affect light absorption by introducing midgap states but also influence chemical reactivity by altering adsorption energies of various species. In one example, Jiao et al. prepared ZnIn2S4 monolayer with tunable Zn vacancy defect concentrations (Figure 18d). They observed higher charge density, more efficient charge transport, and ultrafast (∼15 ps) charge transfer from CB to trap states. 366 Consequently, the authors achieved a higher CO formation rate (33.2 μmol/g/h) on a VZn-rich ZnIn2S4 monolayer (Figure 18e). Similarly, the anisotropic effect was observed on Cu2S nanorods/Pt photocatalysts (top panel, Figure 18f), in which efficient formation of CO and methane from CO2 occurs by suppressing the HER reaction.367 Under illumination

For overall water splitting, the two half-reactions need to take place simultaneously and be balanced. Such a requirement poses significant challenges in material considerations. From a thermodynamic perspective, we need materials that can offer photoexcited electrons with minimum energies more negative than the HER reactions (eq 4); the materials should also offer photoexcited holes (h+) with maximum energies more positive than the OER reactions (eq 5). From a kinetic perspective, the two half-reactions need to feature similar reaction rates. Given that HER reactions essentially involve two electrons and two protons, and that OER reactions involve four electrons and four protons, we see that such a kinetic balance is extraordinarily difficult to achieve. In many photocatalytic systems, the water oxidation half-reaction is the limiting step of the overall performance. This background information helps us see why many studies involved sacrificial electron donors (e.g., ethanol) for the characterization of photocatalysts, especially photocatalysts in particulate forms.241 While such an approach is indeed justified, especially for the purpose of characterizing the fundamental properties of the photocatalysts, presenting such a system as “solar hydrogen generation” could be misleading, not mentioning that synergistic effects may exist between the sacrificial reagents and the photocatalysts. As such, cautions must be taken in comparing various photocatalytic systems when sacrificial reagents are involved. In the remainder of this subsection, we will only discuss a few photocatalyst systems that can enable overall water splitting. A relatively small number of photocatalysts have been studied for unassisted solar water splitting. One such example is graphitic carbon nitride (g-C3N4) nanocomposite photocatalysts for water splitting through a two-electron process that yields H2O2.359 To address the issue that H2O2 would poison the photocatalyst, the authors introduced carbon quantum dots to promote disproportionation of H2O2 that produces O2. More commonly, the single absorber usually suffers from low photovoltage or unsuitable band alignment. In another example, Au nanoparticles were placed at the interface between Mo-doped BiVO4 and La, Rh-co-doped SrTiO3. Photogenerated electrons from the conduction band of Mo:BiVO4 would recombine with photogenerated holes from the valence band of La, Rh:SrTiO3 (Figure 17a).360 With the help of Rubased HER co-catalysts, water reduction took place on SrTiO3, whereas water oxidation reaction took place on BiVO4 in the presence of RuOx OER co-catalysts. Gas product analysis carried out at low pressures (e.g., 10 kPa) showed that stoichiometric generation of H2 and O2 lasted for at least 10 h with an STH of 1.1% (Figure 17b). As far as Z-scheme photocatalysts are concerned, separated photoanodes and photocathodes are far easier to fabricate and manage. An example that involves earth abundant materials (Fe2O3 and Si) is shown in Figure 17c−f. In this example, nanostructured hematite was used as the photoanode for the oxidation of H2O. Amorphous Si was employed as the photocathode for the reduction of H2O.76 The system allowed for separate considerations of the photocurrent and photovoltage of the two electrodes (Figure 17c). Unassisted solar water splitting at ca. 0.6 mA/cm2 (corresponding to 0.91% ηSTH) was reported in 0.5 M phosphate (pH 11.8) when NiFeOx was used as a water oxidation catalyst (Figure 17d). The system was further shown to function in acidic solutions (e.g., pH 1.01) when IrOx was used as the water oxidation catalyst.361 H2 and O2 detection (Figure 17e,f) showed nearly 100% Faradaic efficiencies. Similar works on QDs-modified photoelectrodes W


Review


Figure 19. Photocatalytic N2 reduction: (a) Using plasmon enhanced (black) b-Si NWs. (b) Yield of ammonia over 24 h obtained on different photocatalysts. (c) Quantum efficiency of ammonia on a GNP/bSi/Cr photoelectrochemical cell. (Panels a−c) Adapted with permission from M. Ali, F. L. Zhou, K. Chen, C. Kotzur, C. L. Xiao, L. Bourgeois, X. Y. Zhang, and D. R. MacFarlane.376. Copyright 2016, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). (d) N2 fixation on the rutile TiO2 (110) surface. (e) ESR spectra for rutile JRC-TIO-6 (8) without N2 treatment. (f) pH effect on the NH3 formation on JRC-TIO-6 (8). (Panels d−f) Adapted with permission from ref 378. Copyright 2017 American Chemical Society. (g) Scheme for N2 adsorption on the O vacancy of the BiOBr (001) surface. (h) NH3 generation under visible light (λ > 420 nm). (i) Multicycle N2 reduction with BOB-001-OV. (Panels g−i) Adapted with permission from ref 379. Copyright 2015 American Chemical Society.

Haber−Bosch process (N2 + 3H2 → 2NH3), which requires high reaction temperatures (400−600 °C), high pressures (20−40 MPa), and a large amount of H2 that is derived from methane stream reforming.373 Consequently, the process suffers challenges such as low efficiencies and rapid catalyst deactivation.374 In principle, these issues may be addressed by photocatalytic reactions, in which N2 could be activated by photoexcitation through the application of photocatalysts. Indeed, pioneering work has been reported by Schrauzer and Guth as early as 1977.375 The fundamental mechanisms for N2 reduction are similar to water splitting and CO2 reduction. As shown in eqs 12 and 13, photogenerated holes oxidize H2O (1.23 V vs NHE) and photogenerated electrons reduce N2 to form NH3 (0.057 V vs NHE).20

by an Xe lamp, the CO formation rate was significantly higher on nanorods (NRs) for tip decoration (3.02 μmol/h/g) than for random decoration (0.039 μmol/h/g). In yet another example, Si nanowires were modified with Au3Cu nanoparticles for CO2 to CO conversion (Figure 18g).368 Scanning transmission electron microscopy and EDS mapping confirmed uniform distribution of Au3Cu NPs on Si nanowires. A Faradaic efficiency (FE) of CO production over 70% was achieved. It is noted, however, this demonstration was performed in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) ionic liquid (Figure 18h). As far as nonaqueous systems are concerned, the scope of the chemical reactions for CO2 reduction can be significantly broadened. For instance in Figure 18i, Liu et al. demonstrated the synthesis of antiinflammatory molecules (e.g., ibuprofen and naproxen) with a very high selectivity (97% and 64%, respectively) and yield (84% and 64%, respectively) on Si nanowires in acetonitrile.369 These studies involved sacrificial Al as an electron donor. Future efforts should be directed toward realizing similar selectivity and yield using H2O as the electron donor.370 5.3. N2 Reduction. At an annual rate of over 150 million tons, N2 reduction represents an artificial reaction of immense importance to modern civilization.371 By some estimates, it accounts for ca. 3−6% of total electric energy consumption globally, contributing significantly to CO2 emission.372 On a technical level, most of the reaction is carried out using the

N2(g) + 6H+ + 6e− → 2NH3(g)

(12)

2H 2O(l) + 4h+ → O2 (g) + 4H+

(13)

The overall reaction (E ∼ 1.17 V)

20

may be written as

3 O2 (g) (14) 2 A variety of photocatalysts have been successfully employed for this reduction. For example, a N2 photoreduction cell could be assembled as shown in Figure 19a, where N2 gas is bubbled into the solution and adsorbed on black Si nanowires/Au N2(g) + 3H2O(g) → 2NH3(g) +

X


Review

ACS Applied Energy Materials surfaces.376 The conversion of N2 to NH3 by photogenerated electrons was shown, where sacrificial SO32− was used to scavenge photogenerated holes. Maximum yield of NH3 was obtained when both Au nanoparticles and Cr co-catalysts were used (Figure 19b). Quantum yield measurements (Figure 19c) have revealed that the reaction is active in the entire visible range. Nevertheless, the solar-to-ammonia efficiency is still low due to a number of reasons. For instance, the NN bond is one of the most stable in nature (bond energy, 940.95 kJ/ mol); successful conversion from N2 to NH3 requires six electrons and six protons; few catalysts show obvious catalytic sites that are effective in activating adsorbed N2. One approach to address this challenge was to draw inspirations from the mechanisms of nitrogenase, where N2 is adsorbed at a reducing center.377 It was proposed that O vacancy could be exploited to enhance N2 absorption for subsequent reduction reactions. For example, Hirakawa et al. reported that commercial TiO2 could present a high density of surface O vacancies when irradiated by UV light.378 Figure 19d schematically illustrates rutile TiO2 (110) surfaces with alternative rows of 5-fold coordinated Ti4+ and bridging O (Ob) along the ⟨001⟩ direction. When vacancies form at the Ob sites, a relatively low donor level of the newly generated Ti3+ state appears as trapping sites below the CBM. From the ESR spectra in vacuum (Figure 19e), the top curve of the JRC-TiO-6 sample clearly shows a distinct signal (g = 2.004) from the bridging O vacancies. When the same sample was exposed to N2, the ESR signal diminished, confirming a unique N2 adsorption mode at the vacancy sites. Moreover, the pH of K3PO4 was shown to affect NH3 formation. More efficient transformation was observed at pH 7.0 than in acidic solutions (pH 4.0) or basic solutions (pH 10; Figure 19f). Furthermore, the effect of enhanced N2 reduction by O vacancies could be more effective when 2D layered photocatalysts were used. For instance, Li et al. prepared BiOBr nanosheets with O vacancies on the exposed {001} facets, where the authors showed activation of adsorbed N2 by interfacial electrons.379 The NN bond distance was shown to increase to 1.133 Å, which is between the triple-bond length of 1.078 Å and the double-bond length of 1.201 Å (e.g., diazene; Figure 19g). The results suggest that the weakened bond could be an important intermediate for further reduction by photoelectrons. Together, BiOBr 2D photocatalysts with high density of O vacancies displayed good activity toward ammonia formation under illumination with good recyclability (Figure 19h,i). O vacancies could also be generated on Bi5O7Br nanotubes64 or CuCr-LDH nanosheets380 for N2 reduction. For example, Hu et al. reported that vacancies in sulfide photocatalysts played an important role in N2 reduction.381 Similar to CO2 reduction, when it comes to N2 reduction, it is important to consider possible competing reactions. First, N2 reduction to NH3 features redox potentials close to HER. As such, water reduction may compete strongly with N2 reduction under photocatalytic conditions. Second, single-electron transfer processes for N2 reduction are negligible as they require extremely high overpotentials. Taken as a whole, finding effective photocatalysts that can enable N2 reduction with low overpotentials and high selectivity is of paramount importance. 5.4. Other Applications. In the interest of space, we limited our discussions as presented above to only a relatively small, but extremely important, range of photocatalytic chemical transformations. However, we note that as a class of catalysis, a wide range of chemical reactions can benefit

tremendously from photocatalysts. For example, one unique product of photoexcitation can be free radicals, which would be of great utility for reactions such as radical-based polymerization.382 Similarly, photocatalysts can be used to promote cationic polymerization.383 As a matter of fact, photochemical polymerization is a bedrock of modern society because it is a fundamentally important process for photolithography, although most of the processes do not involve heterogeneous catalysts. More recently, photocatalytic conversion of biomass (i.e., furfural alcohol, HMF) to aldehydes and acids has been achieved.384,385 In addition, the energy conversion system can be integrated into energy storage systems by approaches such as photoelectrochemical flow batteries.386,387 Cheng et al. used a Ta3N5 nanotube photoanode and a GaN nanowire/Si photocathode to charge an alkaline anthraquinone/ferrocyanide redox battery that demonstrated a 1.2 V and 3.0% solar-to-chemical conversion efficiency.388

6. PERSPECTIVES Research on new energy conversion and storage technologies has thrived during the past decade owing to our growing consciousness on the possible devastations by the existing schemes of energy utilization, which is primarily based on fossil fuels. The community has shared a sense of urgency in the need for sustainable development, one built on renewable energy sources.389 It is within this context that significant efforts have been attracted to photocatalysis. By itself, photocatalysis is by no means a new concept. But the latest research has benefited tremendously from advancements made in materials science and engineering.390 For instance, decades of research on nanomaterials has created a broad, solid knowledge base on nanoscale materials, within which charge behaviors are well understood.391 To this end, new phenomena have emerged. Photocatalysts with quantum confinement effects are a good example. Plasmonic photocatalysts are yet another example that is directly enabled by nanoscale synthesis.392 Similarly, 2D photocatalysts have become a popular subject thanks to research on their optical and electronic properties, which are highly relevant to photocatalytic reactions. Advancements in materials synthesis and characterization have also made it possible to form nanomaterials, photonic structures,393 and nanoscale heterojunctions, which contributes significantly to the conical semiconductorbased photocatalysts.394,395 The ability to control the crystallinity and defects on the surface and/or at the interface within length scales relevant to charge behaviors is a key reason for the succcesses.344 New materials characterization tools and techniques offered us insights that are critically important to future developments of this field.396−398 As such, we recognize that photocatalysis research is first and foremost materials research because the synthesis and understanding of photocatalyst as a class of materials are the pillars that underpin the field. From this perspective, we are at a best time in research on photocatalysis thanks to the continued advancement in functional materials research. Equally important to the materials aspect, photocatalysis is also chemistry research because it concerns important chemical transformations. We tried our best to cover a broad range of important such transformations, but we realize what is discussed here is only a small subset of possible utilizations of photocatalysts. On the most fundamental level, catalysts are important because they alter the reaction routes to either speed Y


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up (in the case of lowering activation barriers) or slow down (for passivation purposes) chemical reactions.399 The means by which the activation barrier can be surmounted are limited to thermal, electrochemical, and optical, corresponding the usual tools to power chemical reactions of thermal heating, electricity, and light. Within this context, we see that photocatalysis represents a pillar of chemical reactions. Indeed, great progress has been made to use light to enable unique transformations for the synthesis of high value organic molecules,22 which was not discussed by this Review due to the limitations of space. While these demonstrations are primarily carried out on homogeneous catalysts in combination with molecular sensitizers, there are no reasons why they cannot be extended to heterogeneous photocatalysts as discussed here. A new door could be opened up. Just for the purpose of solar energy harvesting and storage applications, there is still large room for improvement. Despite decades of intense research, the so-called solar fuel synthesis remains a scientific hobby in the sense that no practical applications have been demonstrated. This is because the problem is incredibly complex. Precisely for this reason, we are encouraged to work on the problem, since we see no other means of terawatts scale solar energy storage solutions when we finally move into an era where the primary power source is the sun. Along the same line, we envision that photocatalysis will play increasingly more important roles in the area of environmental remissions by providing a facile route toward pollutant removals. This is not only true for organic pollutants but also for inorganic ones, such as toxic ions like Cr or As in our fresh water systems.400 A fourth area for which photocatalysis is likely to make significant contributions is to revolutionize our chemical industry by enabling chemical transformations with significantly lower needs for energy inputs. Examples in this area include steam methane reforming, Fisher−Trosch process, and Harber− Bosch process.372 Each of these processes represents a branch of modern chemical industry with an enormous scale. They share similar challenges such as rapid catalyst deactivation due to the high temperatures needed. Photocatalysts could enable disruptive technologies to fundamentally change the situation. Yet another area of applications that is not covered by this Review is how to take advantage of the light-enabled reactions for medical purposes such as IR therapeutic treatments.401 On this front, QDs and plasmonic photocatalysts are particularly interesting, not only for imaging-based diagnosis but also for thermal therapy. Taken as a whole, we not only need new materials as photocatalysts but also expect novel engineering concepts to better exploit these new photocatalysts. It is critical to recognize that research in this area remains relatively underdeveloped. Fundamental understandings of how a particular system works well will likely play critical roles in promoting further developments. The interdisciplinary nature of photocatalysis calls for efforts from a variety of different backgrounds, including materials, physics, and chemistry. We hope this Review will serve as a catalyst to prompt discussions by experts in these different fields for accelerated advancements of this exciting area.

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Xiaogang Yang: 0000-0002-1142-3100 Dunwei Wang: 0000-0001-5581-8799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.Y. is supported by the National Natural Science Foundation of China (Grants U1604121 and 21673200) and the startup funding from XCU. D.W. is supported by the National Science Foundation (Grants CBET 1703663 and CBET 1703655), the U.S. Department of Energy (Grant DE-EE0008086), and the Massachusetts Clean Energy Center. We are deeply indebted to our co-workers, the names of whom would be too numerous to list here.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Y.). *E-mail: [email protected] (D.W.). Z


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Photocatalysis: From Fundamental Principles to Materials and

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