Challenges and prospects in solar water splitting and CO2 reduction

Review Cite This: ACS Catal. 2018, 8, 3602−3635

pubs.acs.org/acscatalysis

Challenges and Prospects in Solar Water Splitting and CO2 Reduction with Inorganic and Hybrid Nanostructures Jacek K. Stolarczyk,*,†,‡ Santanu Bhattacharyya,†,‡ Lakshminarayana Polavarapu,†,‡ and Jochen Feldmann†,‡ †

ACS Catal. 2018.8:3602-3635. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/25/18. For personal use only.

Photonics and Optoelectronics Group, Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität München, Amalienstraße 54, 80799 Munich, Germany ‡ Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Munich, Germany ABSTRACT: The inexorable rise of carbon dioxide level in the atmosphere, already exceeding 400 ppm, highlights the need for reduction of CO2 emissions. Harvesting solar energy to drive reverse chemical reactions to fuel combustion offers a possible solution. The produced chemical fuels (e.g. hydrogen, methane, or methanol) are also a convenient means of energy storage, not available in photovoltaic cells. This Review is focused on the heterogeneous photocatalytic water splitting and on CO2 reduction with nanostructured semiconductors, metals, and their hybrids. The stages of light absorption, charge separation and transfer, and surface reactions are discussed, together with possible energy-loss mechanisms and means of their elimination. Many novel materials have been developed in this active field of research, and this Review describes the concepts underpinning the continued progress in the field. The approaches which hold promise for substantial improvement in terms of efficiency, cost, and environmental sustainability are discussed in the second part. These include emerging materials (carbon dots and nitrides, bimetallic catalysts, perovskite oxides, 2D materials), more complex architectures of the photocatalyst (Z-scheme, self-assembly), and mechanisms (defect engineering, hot electron injection, redox mediators). The concluding part provides an outlook for the future directions in the field. KEYWORDS: solar energy, photocatalysis, semiconductor nanocrystals, CO2 reduction, water splitting, defect engineering

1. INTRODUCTION Year 2015 marked the first time in at least 800 000 years that the annual average carbon dioxide concentration in the atmosphere exceeded 400 ppm.1 CO2 content has been growing steadily since the preindustrial revolution level, around 275 ppm,2 but the increase has accelerated recently reaching more than 2 ppm per annum in the past decade (see Figure 1). Natural variations have occurred in the past, but the current increase is 2 orders of magnitude faster than in the postglaciation periods. This unprecedented rate is inextricably linked to human activities, in particular to the large-scale combustion of fossil fuels which annually leads to the release of 30Gt of CO2.3 Such amount, about a third of the total carbon cycle, disturbs the cycle and overwhelms the natural sequestration capacity.4 The resulting increase in atmospheric level of CO2a known greenhouse gasraises concern about its impact on global climate and highlights the need to reduce the emissions.5 In order to achieve this, renewable sourcesin particular solar and windhave been utilized to an increasing extent to meet present and future energy demand. In 2016, they already represented 20% of global supply, reaching 153 GW.6 Importantly, more than 50% of new power capacity around the world comes from renewables. In Europe, the contribution is even higher at 90%. This is a welcome development; however, the actual output is much smaller due to diurnal and annual cycles as well as weather variability. As there is no inherent © 2018 American Chemical Society

energy storage ability in the wind and solar photovoltaic supply system, the intermittent and partially unpredictable nature of these renewable sources means that the fossil fuel based power plants need to remain in operation to meet the demand at all times. Chemical fuels are attractive media for energy storage because of their very high specific energy (e.g., 55 MJ·kg−1 for methane), significantly higher than that of present-day batteries (less than 1 MJ·kg−1).7,8 Conveniently, the energy stored in their bonds can be then released at will by combustion. Because the total incident solar energy exceeds 5000-fold the global demand,9 harvesting this energy to produce fuels offers a potential solution to simultaneously address the energy supply and storage and to wean the mankind off fossil-fuel dependence.10 To this end, the incident solar photon energy can be used to drive the energetically uphill reactions and thereby to produce the high-energy compounds from water and carbon dioxide (see Figure 2a). The two most prominent reactions are splitting water into hydrogen and oxygen (eq 1) and reduction of CO2 to technologically relevant compounds, for instance methanol or methane, together with oxidation of water (eqs 2 and 3). Importantly, the subsequent combustion of the produced fuelsreferred to as solar fuelsdoes not lead to Received: February 26, 2018 Published: March 5, 2018 3602

DOI: 10.1021/acscatal.8b00791 ACS Catal. 2018, 8, 3602−3635

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Figure 1. (a) Anthropogenic carbon cycle; (b) Increase of annual CO2 levels in the atmosphere between 1959 and 2016, measured at Mauna Loa observatory.1

reaction, n equals 2, so that ΔE0 = 1.23 V. This means that the photon energy needs to be larger than 1.23 eV which corresponds (λ = hc/ΔE) to wavelengths shorter 1008 nm. In the case of CO2 reduction to methanol or methane, the ΔG0 are much larger, but the number of exchanged electrons is 6 and 8, respectively. In effect, the required photon energies are very similar to water splitting. Specifically, the photons with wavelengths shorter than 1041 and 1196 nm are suitable for CO2 reduction to methanol and methane, respectively. This demonstrates that UV, visible, and even near IR light has sufficient energy to drive the solar fuel generating reactions, even if this only sets the thermodynamic constraints, without any reference to achievable rates. The supply of energy to cover the required ΔG0 and the provision of catalytically active sites for the eqs 1−3 to take place are essentially two distinct functionalities which can be run separately. Several approaches, varying by the degree of their integration, have been proposed.9,11,12 In the first, a multijunction photovoltaic (PV) cell is used to absorb the photons and to provide the necessary potential for the desired reactions to proceed on an electrocatalyst immersed in aqueous medium or in contact with CO2-rich atmosphere.13,14 The two parts of the setup are only electrically coupled so that they can be spatially separated and individually optimized.15 This way, the advances in photovoltaics and electrocatalysis can be combined for improved performance. Indeed, solar-to-hydrogen conversion efficiency exceeding 15% has been obtained using this approach.16−18 The open circuit voltage of the cells is usually too low, so that two to three PV cells need to be put in series to provide the 1.6−1.7 V potential needed in practice to induce water splitting with more needed for CO2 reduction. Despite the efficiency advantages, the prospects of wider use of the combined PV-electrocatalytic devices are limited by their inherent complexity and the ensuing high fabrication costs per unit.19 A simpler, more integrated architecture is represented by a photoelectrochemical (PEC) cell wherein the light is absorbed by one or both of the electrodes.15,20−22 The (photo)anode and the (photo)cathode can be placed in separate compartments of the vessel in which the oxidation and reduction reactions proceed, respectively. They are electrically coupled through an external circuit which can include an additional PV cell in order to provide additional potential bias.23−26 In the latter configuration, the PEC cell has many features in common with the previously described PV-electrocatalytic devices.

Figure 2. (a) Closed cycle of solar energy storage by conversion of H2O and CO2 into chemical fuels. The energy can be released on demand by a reverse reaction. (b) and (c) Energy diagrams of photoactivated (b) exergonic and (c) endergonic reactions. Following the photoexcitation of the reactant R, both reactions become energetically favorable (ΔG0 < 0), represented by dotted red arrows.

additional CO2 emissions, because the carbon cycle is closed (cf. Figure 2a). H 2O → H 2 + 1/2O2

ΔG 0 = 237 kJ/mol

(1)

ΔG 0 = 689 kJ/mol

CO2 + 2H 2O → CH3OH + 3/2O2

(2)

CO2 + 2H 2O → CH4 + 2O2

0

ΔG = 800kJ/mol

(3)

The required free energy difference, ΔG , of eqs 1−3 sets the upper limit for the wavelength of the photons which can still induce the reactions. The potential difference can be calculated as ΔE0 = ΔG0/nF, where n is the number of exchanged electrons and F is the Faraday constant. For the water-splitting 0

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processes are referred to in vast majority of the papers as photocatalytic.39,43,44 This convention is also adhered to in this paper, because the distinction between the photocatalysis and photosynthesis is rather muddled in the reactions involving sacrificial agents. The caveat is that the appropriate consideration for the specific thermodynamic and kinetic environment of the solar fuel generation is given in the analysis. Accordingly, in this Review, we focus on the photocatalytic water splitting and carbon dioxide reduction using nanostructured photocatalysts. We first present the thermodynamic constraints and energy loss mechanisms of solar fuel generation with attention to both spatial and energetic aspects of carrier transport. We then follow on with a discussion of the subsequent steps of light absorption, charge separation/ transport, and finally the interfacial reactions. Since the early stages of the process are identical for water splitting and CO2 reduction, we discuss them together, only making the distinction in the later part where the specific reactions are involved. We believe it would not be instructive to attempt an exhaustive presentation of all possible approaches to induce these reactions. Instead, in the subsequent part, we focus on selected few which in our opinion hold promise for substantial improvement in terms of efficiency, cost and/or environmental sustainability. These relate to either new materials (carbon dots and carbon nitrides, bimetallic catalysts, perovskites), design of the photocatalyst (nanostructuring, Z-scheme, self-assembly), or mechanisms (hot electron injection, mediators, defect engineering). The concluding part provides an outlook for the future development of the field.

Charge balance is maintained by proton transport through a suitably permeable membrane (e.g., Nafion or glassy carbon) between the compartments which prevents transport of other reactants.27,28 The division of the reaction vessel space into compartments not only maintains different conditions in both parts, optimized for each half-reaction, but also facilitates separation of the products and prevents the back-reaction.29 In photocatalytic systems, described herein, the functions are fully integrated to absorb light and drive the desired reactions without any external circuitry or bias.27,30 Semiconductor nanocrystals (NCs), either dispersed in aqueous medium or grown/deposited on support, have been at the forefront of heterogeneous photocatalysis because many have favorable band gap and band edge positions, high extinction coefficient, efficient free charge generation, short carrier pathway to the surface as well as high surface area with a plentitude of catalytically active sites.31−33 Importantly, these properties are further tunable through modification of the size, shape, doping level, aggregation state, and surface functionalization of the NCs allowing for several degrees of freedom in optimizing the material.34,35 Although the photocatalysts usually comprise not just one semiconductor NC type but also other components to improve light absorption, charge separation or catalytic properties, the photocatalytic approach represents a paradigm shift in fabricating solar fuel devices. Instead of a complex device wired together, the process relies on multiple very simple reactors placed in a reaction medium.19,36 The economic analysis of the nanoparticle-based photocatalytic approach indicates that in comparison to other options, it has the potential for a substantial reduction in fabrication costs due to simplicity of the design, provided that the yield of the reactions improves.36,37 The challenge is that as both half-reactions proceed in the same reaction space, the prevention of various parasitic back reactions and product extraction become critical for achieving high efficiency. It should be noted that the term “photocatalytic” in the context of solar fuel generation has generated some controversy.29,38 As shown in panels b and c of Figure 2, absorption of light can be used to increase the rate of both exergonic and endergonic reactions. In the former case, the photoexcitation reduces the kinetic barrier, but from the thermodynamic standpoint, the reaction is spontaneous (ΔG0 < 0) irrespective of the light absorption. Dye degradation or oxidative pollutant removal are the typical examples of such reactions.39−41 This case conforms perfectly to the IUPAC definition of photocatalysis as a change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or inf rared radiation in the presence of a substancethe photocatalystthat absorbs light and is involved in the chemical transformation of the reaction partners.42 However, in the endergonic reaction the photon energy is required to invert the sign of the Gibbs free energy change to negative (cf. Figure 2c). The reaction is not just facilitated (i.e., made faster) by the light absorption; the photon is essentially a participant in the reaction and its energy is not recovered. The definition does not refer to the photon energy leaving some ambiguity, but it is often argued that such reaction should be called photosynthetic instead.29 In water splitting or CO2 reduction, such terminology is used to highlight the fact that the reverse reactions are thermodynamically feasible (cf. eqs 1−3), so that the rate of a spontaneous back reaction may be the limiting factor of the whole process.29 This is exacerbated by the presence of all reactants and products in the same medium. Nonetheless, the

2. PHOTOCATALYSIS WITH SEMICONDUCTOR NANOCRYSTALS 2.1. Thermodynamics. An incident photon, upon absorption by a semiconductor nanocrystal, generates an exciton, an electron−hole pair bound by Coulomb forces, which can dissociate into free charges. For the catalytic process to occur, the charges have to avoid recombination and migrate separately to the reaction sites on the surface, see Figure 3a. There, the electron and hole may reduce the adsorbed electron acceptor (e.g., a proton or a CO2 molecule) or oxidize an electron donor (e.g., OH− anion), respectively.33 This simplified spatial scheme of the process raises questions about the driving force for the charge separation and the interfacial transfer to the adsorbates which can only be answered by considering the energy diagram. As shown in Figure 3b, the absorption of an above-band gap photon leads to a promotion of an electron from the valence band to the conduction band and subsequent thermalization down to the conduction and valence band edges, ECB and EVB, respectively. In effect, the Fermi levels of the electrons and the holes are elevated to a so-called quasi-Fermi levels, corresponding to Fermi levels under illumination. Not every electronic excitation of a semiconductor can be used to drive photocatalytic reactions. In many semiconductors, the edges of the valence and conduction bands are composed of hybridized orbitals localized on different atoms. Hence, the excitation can be also seen as a partial electron transfer between the atoms and an initial step in the charge separation.45 This is the case for TiO2 in which the valence band edge is made of O 2p orbitals, while the conduction band edge is made of Ti 3d orbitals, leading to a O to Ti charge transfer p−d transition.46 However, in other transition metal oxides (e.g., NiO), these transitions require high energy, more than 3.5 eV. Lower3604


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Figure 3. (a) Photocatalytic processes on semiconductor nanocrystals involving photoexcitation and formation of electron−hole pair in the nanocrystal. The charges separately diffuse to the surface, where they can participate in reduction and oxidation reactions, respectively; (b) Energy diagram of the same process for a semiconductor with conduction band minimum located at ECB and valence band maximum at EVB, separated by a band gap Eg. The overpotentials, ΔE, shown in blue, provide the driving force for the transfer of the charges to the electron acceptor (reduction) and donor (oxidation) molecules.

Figure 4. (a) Energy loss mechanisms in solar fuel formation: (1) below band gap photons, (2) thermalization, (3) charge recombination, (4) overpotential, (5) back reaction, (6) separation and product extraction losses; (b) Semiconductor band alignment with possible levels of self-oxidation redox potential (left) and with two different hole scavengers (HS1 and HS2, shown on the right in blue and red), representing two potential energy balance scenarios. Item (7) represents a loss of stored energy due to the use of a sacrificial agent.

energy d−d transitions, in the visible range, occur within a d shell of the same Ni atom.47,48 There is little induced charge separation in such transitions. This leads to a fast recombination and, unfortunately, to only negligible contribution to the photocatalytic activity of the semiconductor.49 The quasi-Fermi electron and hole levels of the semiconductor are usually approximated by the conduction and valence band edge positions, respectively. Their difference not only has to be larger than the difference in reduction and oxidation potentials of the two half-reactions, but also the band edges should be appropriately aligned with respect to these redox potentials. Specifically, the conduction band minimum has to be above (i.e., more negative than) the reduction potential of the desired reaction, while the valence band minimum should be below the oxidation potential. Effectively, the band gap has to span the two values, resembling a Type I (straddling) band alignment in semiconductor junctions. In other words, it needs to provide overpotentials for both the transfer of an electron and hole to the electron acceptor and donor (see Figure 3b). If the alignment resembles Type II (staggered) case, that is the criterion is only met on one side, the other charge has to be extracted through a different reaction, usually involving a sacrificial agent. The difference between the redox potential of the reduction and oxidation half-reactions determines the amount of energy stored in the photocatalytic process.39 In general, the energy losses in the process stem either from a failure to utilize a photon in a desired redox process (i.e., quantum efficiency less than 1) or a partial loss of its energy. These mechanisms are illustrated in consecutive steps in Figure 4a. First, the photons with insufficient energy to excite the electron from the valence band to the conduction band are not absorbed (item 1). This represents a significant loss channel for wide band gap semiconductors with an absorption onset in the UV. On the other hand, for photons with energy larger than the band gap,

excitation results in hot charge carriers, but the excess photon energy is quickly lost through thermalization (shown only for an electron in the scheme, item 2). These two conditions correspond to the well-known Shockley−Queisser limit in single junction PV cells.50 However, they set only thermodynamic constraints. The actual absorption of the above bandgap photons depends on the electronic transition probability. Indirect or dipole forbidden transitions result in low absorption coefficients and lead to further photon losses. This implies that the semiconductors with a direct and allowed band gap transition are best suited, although if the charges are not transferred further quickly enough, the rate of Auger recombination will become significant. In general, charge recombination, either in the bulk of the semiconductor or at the surface, constitutes a third loss channel (item 3). Nonradiative recombination, where the energy is dissipated through heat, amounts to the loss of the photon energy, but a photon emitted through a radiative recombination may in fact be reabsorbed by the sample. Typically, however, the radiative recombination represents a competitive channel to photocatalytic processes, so that there is an anticorrelation between the quantum yields (QY) of photoluminescence (PL) and the catalytic process.51 When the nonradiative processesoften trap-mediateddominate, the increase in PL QY corresponds to elimination of nonradiative channels. As this effect is also beneficial for photocatalytic activity, in such cases a (positive) correlation between PL QY and catalytic QY may be observed.52 Therefore, the correlationor lack thereof provides an important insight into the dynamics of the charge transfer process and the role and density of defects as recombination sites. The overpotential between the band position and the relevant redox levels is beneficial for the electron transfer rate, but the corresponding energy is used up to drive the 3605


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at pH 7 is −0.197 V vs NHE. Compared with water reduction level at −0.41 V at pH 7, this leaves 0.207 eV stored per electron.61 Unfortunately, in a great many papers, stronger reducing agents are used (marked in red in Figure 4b), so that there is no stored energy at all. The system becomes energetically downhill (i.e., photocatalytic, but not photosynthetic) and even a direct reduction of water by the sacrificial agent is possible. Regrettably, this important consequence is not always mentioned in the reports. By analogy, silver ions,62 ferric ions,63 or persulfates64 are used as electron scavengers to focus just on the oxidation half-reaction. Such studies are of great value in development and optimization of the new catalyst for specific half-reaction, but a long-term sustainable solution has to involve either full water splitting to H2 and O2 or CO2 reduction together with O2 generation. The feasibility of the photocatalytic process relies on the stability of the photocatalyst. While the systems involving a scavenger are intrinsically limited by the depletion of the sacrificial agent, the stability of different materials differs from minutes to months. This is assessed by monitoring the formation rate of the product under long-time illumination. The durable systems in closed systems usually require repeated evacuation of the reaction chambers, as otherwise the buildup of the gases on the reaction headspace would counteract the further progress of the reaction. Nonetheless, the decrease in activity, unless caused by the depletion of sacrificial agent, usually manifests the photodegradation of the material by the photoexcited charges or byproducts (e.g., superoxides), to which many semiconductors are susceptible. In this context, efficient extraction of the charges from the photocatalyst to the adsorbed stable acceptors or to the medium, for example through redox mediators, can convey improved stability.19,61 These methods are discussed further in the manuscript. 2.2. Light Absorption. The thermodynamic constraints on the band alignment of the semiconductors, discussed previously, suggest that only 1.23 V is needed to split water, with a similar potential needed for CO2 reduction. Unfortunately, this is misleading, because the overpotentials needed to achieve overcome kinetic constraints increase this number significantly, especially on the oxidation side. In fact, most systems which have successfully achieved this aim have Eg larger than 3.5 eV65,66 and a deep, strongly oxidizing valence band. According to Marcus electron transfer theory, unless the process is in the inverted regime, the larger the overpotential, the faster the electron transfer is.67 This illustrates a common conflict of interest in choosing the semiconductor and necessitating a compromise. The wider band gap translates into larger overpotentials and thereby more efficient charge transfer, but it also shifts the absorption onset to shorter wavelengths. This would result in higher absorption losses, potentially canceling the benefits gained in charge transfer rate. The requirement for UV irradiation with wavelengths shorter than 350 nm significantly limits the absorption to only a small fraction of the solar spectrum. As most incident solar energy falls in the visible light range, several techniques have been developed to extend the absorption range without violating the thermodynamic constraints of photocatalysis. They either rely on using multiple below band gap photons to make up the necessary energy for the excitation (see Figure 5a) or on modification of the photocatalyst to absorb visible light. These correspond to decreasing the losses marked as item (1) in Figure 4a. There are two general methods to achieve the necessary modifications. They combine the semiconductor with

transfer (item 4 in the Figure 4a). Once the products are obtained, the thermodynamically feasible reverse reactions to eqs 1−3 between the products further decrease the efficiency of the process (item 5). Finally, the extraction and separation of the products (e.g., H2 and O2) for storage will require further energy input (item 6). This is of particular importance for nanocrystal-based photocatalysts suspended in water, because both products are generated in the same reaction space. Efficient separation, possibly through the use of selective permeation membranes, is critical to prevent back reactions in this case. The energy loss mechanisms described thus far correspond to semiconductors in which the photogenerated hole (or less commonly, the photoexcited electron) does not photooxidize (photoreduce) the semiconductor itself53 or its stabilizing ligands.54 However, this happens for example when the redox potential of the semiconductor (drawn in red in Figure 4b) lies above the water oxidation potential, as is typically observed in chalcogenide II−VI semiconductors such as CdS.55 In case of oxidation of lattice ions (e.g., S2− to SO42−),53 a partial dissolution of the nanocrystal ensues resulting in the degradation of the photocatalyst. At the same time, the oxidized ligands (e.g., alkanethiols) desorb and no longer coat the nanocrystal. This leads to uncontrolled aggregation of the crystals and their loss from the dispersion. Accordingly, the process ceases to be catalytic and instead becomes stoichiometric, where the nanocrystals and/or the ligands are the reactants themselves. In order to avoid the photooxidation, a sacrificial agent called a hole scavenger can be used to remove the hole before it does any damage.32 These are typically reducing agents such as sulfides, sulfites, thiosulfates, ethyl or isopropyl alcohols, or amines (trimethylamine TEA, triethanolamine TEOA).56 For the sacrificial agent to be effective, the redox potential of the scavenger has to be above the oxidation level of the semiconductor so that the hole is transferred to the scavenger (cf. Figure 4b, blue arrow). Analogous arguments on the reduction side require the use of an electron scavenger to prevent reductive photodegradation. The use of a sacrificial agent constitutes an additional loss mechanism, because the stored energy is accordingly decreased by the difference between redox level of the scavenger and the water oxidation (item 7 in Figure 4b). It also means that only one-half-reactions can deliver the desired product. The other half-reaction depletes the sacrificial agent. The use of scavengers in photocatalytic applications is more common than just necessitated by preventing photodegradation. They are also employed with semiconductors stable in water under illumination (e.g., TiO2). This is because the rates of oxidation and reduction half-reactions differ significantly, so that due to charge balancing the slower one becomes overall rate limiting.57,58 This creates difficulty in studying the faster process as there emerges a very complex relationship between the conditions and the reaction rate. Typically, the water oxidation is the slower process, because it requires a transfer of four holes to the acceptor and a formation of an O−O double bond.27 For this reason, the focus of many researchers is then solely on production of H2 as a fuel, with a surrogate reaction involving a hole scavenger on the oxidation side.32 Indeed, the quantum efficiency of such processes can reach nearly 100%.59,60 This appears at first as an ideal solution; however, it is accompanied by a reduction in the stored energy. This is still acceptable for instance for ethanol whose redox potential of 3606


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(Sr1−xNbO3, WO2−NaxWO3)82,83 were recently used for upconversion of infrared light to induce photocatalytic reactions requiring larger energy. Another approach relies on two photon absorption by two different materials with appropriately aligned energy bands. A most common manifestation of this technique is called Z-scheme and will be discussed further in the text in section 3.2. The second type of approaches relies on decreasing the effective band gap of the photocatalyst, in order to extend (redshift) the absorption onset. Sensitization of a wide band gap semiconductor with a visible light active molecule (a dye) with a suitably small difference between its LUMO and HOMO energy levels or with a nanocrystalline narrow band gap semiconductor (a quantum dot, QD) offers a flexible solution (see Figure 5b). It involves a charge carrier transport, typically an electron, as the semiconductor is used for the reduction reaction. To this end, a dye with ELUMO higher than the conduction band edge of the semiconductor can be chosen (cf. Figure 5b), such that upon excitation of the dye, the electron is injected to this conduction band, closely resembling the operation of a dye sensitized solar cell.84 It can then participate in the desired reduction reaction.85,86 The dye is afterward regenerated from its oxidized state by the hole scavenger. Band alignment precludes an electron transfer from the deep valence band of the semiconductor to the dye. Following this approach, TiO2, ZnO, Nb2O5, and other d0 oxides were sensitized with dyes (often Ru(bpy)32+ based) to reduce water87−89 or CO290,91 under visible light illumination. Similarly, cadmium chalcogenide QDs (e.g., CdS, CdSe) or PbS QDs, which have higher conduction band edge than these oxides, can be used for sensitization92,93 and subsequent reduction of water94,95 or CO296 on the wide band gap oxides.97 The wide band gap semiconductors can also be sensitized by plasmonic metal NPs, especially Au and Ag, which exhibit several orders of higher extinction (absorption and scattering) cross sections compared with typical dye molecules because of their strong localized surface plasmon resonance (LSPR).98−100 It can be defined as the collective oscillations of the free electron cloud of the metal NPs caused by interaction of electromagnetic wave.100 The LSPR of plasmonic NPs can be widely tuned across the visible-near-infrared region of the spectrum by varying their size, shape, and composition.98,101−103 The LSPR decays into hot electrons and holes on the metal NP surface at a time scale of 10−20 fs, whereupon these hot carriers may transfer to a semiconductor (Figure 5c), although the difficulty is that the process competes with the ultrafast thermalization of the carriers.101,103−105 Nonetheless, the process can in principle sensitize even the semiconductors with highly reducing conduction band, a feature very useful in further application in photocatalysis. The disadvantage of the sensitization is that the hole left on the sensitizer still needs to be taken care of by the scavenger, raising questions about the energy balance of the exercise. Therefore, an alternative approach is to decrease the band gap of the semiconductor itself while retaining the oxidative properties of the semiconductor. In oxide semiconductors, the valence band edge, made of O 2p orbitals is very low (approximately 2.9 V vs NHE at pH 0) providing a large overpotential for water oxidation (1.23 V vs NHE at pH 0). Meanwhile, on the reduction side, the margins are rather narrow focusing the efforts to decrease the band gap on lifting the valence band.68 To this end, introduction of nitrogen and hybridization of the O 2p orbitals with higher-energy N 2p

Figure 5. Enhancement of light absorption of a wide band gap semiconductor (SC) through (a) energy transfer (ET) from upconverting nanoparticle, (b) sensitization with a dye or a second semiconductor, (c) sensitization with a plasmonic material via transfer of a hot electron from the decay of a surface plasmon (SP), (d) hybridization of orbitals forming the valence band with higher-energy orbitals to lift the valence band edge, (e) introduction of interband states associated with lattice defects.

a visible light absorbing material (sensitizer) which then transfers the photoexcited charges to the semiconductor or require engineering the band gap of the semiconductor itself such that the superfluous overpotentials are reduced (see Figure 5b−e).68,69 Starting with employing below band gap photon, the appeal of the method derives from the possibility to use near-infrared (NIR) part of the solar spectrum, which does not have sufficient energy to drive the reactions 1−3 alone. Multiphoton absorption by quantum dots, where several photons are absorbed at once through virtual interband states, is not a practical solution because it occurs only under very high illumination intensity.70,71 However, in lanthanide-doped upconverting nanoparticles, where absorption proceeds sequentially via intermediate 4f states of the Ln3+ cations, the limitation is not as severe.72,73 In such nanoparticles, rare earth metal cations (Er3+, Yb3+, Tm3+, etc.) are embedded in host material, typically fluorides (YF3, NaYF4) or oxides (Y2O3, ZnO, CeO2). Through the choice of dopants and their concentration, the absorption can be tuned to infrared while the anti-Stokes emission to ultraviolet. This way, it is possible to enable excitation of a wide band gap semiconductor (e.g., TiO2) via energy transfer (see Figure 5a). In particular, a core− shell architecture, with the upconverting nanoparticle in the center and the photocatalyst in the shell, ensures efficient energy transfer and the exposure of the surface of the semiconductor for the photocatalytic applications.74 Direct doping of TiO2 with Er3+ cations has also been used to excite the oxide by infrared photons.75 So far, there have been reports on photocatalytic pollutant or dye degradation with Ln3+-based upconversion of infrared light,76−79 but very few on actual water splitting.80 The field, albeit promising, remains largely unexplored. In addition, graphene QD81 and metallic oxides 3607


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ACS Catalysis orbitals increases the valence band edge (see Figure 5d). This approach gives rise to a family of oxynitrides which absorb in the visible range.106 For instance, the band gap of tantalum oxide, Ta2O5, is larger than 3.7 eV,64 but for oxynitride, TaON, it is only 2.4 eV.107 The position of conduction band edge remains the same, meaning that TaON retains the reductive potential of Ta2O5. Thus, the band alignment suggests that TaON is an excellent candidate for photocatalytic applications under visible light irradiation, as has been shown in water splitting and CO2 reduction experiments.108−110 Further replacement of oxygen with nitrogen leads to tantalum nitride, Ta3N5, with an even smaller band gap of 2.1 eV. Although used in hydrogen generation,111 it tends to photooxidize under illumination which limits its applicability. On the other hand, low levels of N atoms create additional interband states which also allow for some absorption in the visible range. However, these dopant states act as recombination centers partially canceling the benefits of increased absorption.112 Hence, it appears that theperhaps nonstoichiometricoxynitrides are the most promising photocatalyst, better than N-doped oxides or nitrides. Zinc germanium and zinc gallium oxynitride serve as another examples of the family, successfully used in full water splitting and CO2 reduction.113−116 The valence band of wide band gap d0 oxides can be also lifted by incorporation of Ag+, Cu+, Pb2+, or Bi3+ cations. The hybridization of O 2p orbitals with the fully occupied metal s or d orbitals shifts the valence band edge upward to a more negative potential. For instance, replacement of sodium in a niobate with Ag+ decreases the band gap from 3.4 eV in NaNbO3 to 2.8 eV in AgNbO3. The conduction band, made of Nb 4d orbital remains unchanged, the new valence band is made of O 2p and Ag 4d orbitals.117 The perovskite crystalline structure is also preserved. AgTaO3 has a wider band gap by 0.6 eV due to higher energy of the Ta 5d orbitals. Despite the higher valence band, AgNbO3 is still suitable for photocatalytic water oxidation.118 At the same time, solid solutions between the silver niobate or tantalate and SrTiO3 allow tuning the position of the band gap and can be used for both water oxidation and reduction.119,120 An analogous effect is responsible for the visible light activity of copper niobates and tantalates,121 and provides a matching band alignment for photocatalytic water splitting.122,123 Meanwhile, in bismuth vanadate, BiVO4, and tungstate, Bi2WO6, it is the hybridization of O 2p orbitals with Bi 6s orbitals that lifts the valence band.124,125 The first principle calculations show that the vanadate is a direct semiconductor with a band gap of 2.4 eV, efficient charge separation due to equal effective masses of electrons and holes and very good hole mobility. It is considered to be one of the best photocatalysts for water oxidation.126−129 This has been attributed to distortion of the Bi−O polyhedrals by the Bi 6s2 lone pair130 and by preferences for different crystals facets for the electrons and holes.131 Both effects translate into good charge separation qualities. However, due to coupling between V 3d, Bi 6p, and O 2p orbitals the conduction band edge is also lowered (around 0 V vs NHE at pH 0), borderline for water reduction. Therefore, BiVO4 needs to operate with an electron scavenger or as part of a larger structure where the reduction proceeds elsewhere. It has been, nonetheless, shown to selectively produce small amounts of ethanol by the reduction of CO2.132 The wider band gap of Bi2WO6, around 2.75 eV, allows for both water reduction and oxidation under visible light133,134 as well as CO2 reduction to methane.135,136

Lattice point defects can also introduce individual interband states and increase visible light absorption (cf. Figure 5e). A notable example is a highly catalytically active black TiO2 with Eg reduced to 1.54 eV due to O vacancies, Ti4+ interstitials, and Ti3+ states.137,138 Similarly, nanostructures of oxygen deficient tungsten oxide W18O49 have a strong absorption in the red region due to oxygen vacancies.139,140 Ultrathin W18O49 nanowires have been shown to reduce CO2 to methane under visible light.141 The critical role of the vacancies was verified by exposure to H2O2 which turned (oxidized) the blue W18O49 to standard yellow WO3 and arrested the photocatalytic activity. 2.3. Charge Separation. In many inorganic semiconductors, the exciton binding energy is sufficiently low that thermal energy is able to dissociate the excitons into free charge carriers. Once created, these carriers need to transfer to the surface of the NC and maintain their separation as long as the desired redox reaction takes. Shorter lifetime would result in majority of the carriers recombining before they can participate in their respective reactions. Unfortunately, the carrier lifetimes are on the nanosecond scale in typical semiconductors such as TiO2142,143 or CdS.144 In contrast, the water reduction needs milliseconds, while the oxidation half-reaction requires even seconds to proceed.145 Clearly, the carrier lifetimes are not long enough for single semiconductors. In nanocrystalline materials the diffusion length is not too small; the problem usually lies in too short lifetime on the surface. To address this, the research is focused on either on improving the charge separation, and thereby lifetime, or on increasing the rate of the surface reactions. As shown in Figure 6, the carrier lifetime can be extended by using metal−semiconductor or semiconductor− semiconductor heterojunctions. Their band edges and/or Fermi levels (work functions) are aligned to facilitate charge transfer in such a direction that the carriers would eventually find themselves on the opposite sides of the junction.

Figure 6. Binary systems for improved charge separation: (a) n-type semiconductor-metal junction; (b) p-type semiconductor-metal junction; (c) two semiconductors with Type II (staggered) band alignment; (d) two semiconductors forming a Z-scheme bandalignment, involving a redox mediator molecule M. 3608


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processes were the same. While the primary product of water reduction is molecular hydrogen (see 4),27,32,147 there are numerous possible products for CO2 reduction.44,68,146 Protoncoupled and multielectron half-reactions of CO2 reductions to carbon monoxide, formic acid, formaldehyde, methanol, and methane are given by eqs 5−9. These are all C1 molecules, but other products containing more than one carbon atom bonded through C−C bonds are also observed. These include oxalic acid, ethanol, ethane, propene, acetaldehyde, glyoxal, and others. The variety of the products points to a complex reduction pathway which involves C−O bond breaking, C−H bond formation, and radical dimerization.68

A schematic representation of a metal−n-type semiconductor junction is shown in Figure 6a. If the Fermi level of the metal is lower than of the semiconductor, placing them in contact leads to electron transfer from the semiconductor to the metal. The ensuing band bending at the interface builds up a Schottky barrier for the back-transfer of an electron. Both the charge separation and subsequent transfer of the electron to the adsorbate molecule are improved.33 This is because the metal, especially noble metal, nanoparticles are better catalysts than the semiconductors as they discharge the electrons easier. The role of the metal nanoparticle as an electron sink in the metal− semiconductor composite results also in an upward shift of its Fermi level so that it becomes more reductive.92 In a p-type semiconductor-metal junction, the electrons can easily transfer to the metal, but the barrier impedes the movement of the holes there, again improving charge separation (see Figure 6b). Consequently, most photocatalysts are composed of semiconductor NCs decorated with metal nanoparticles (Pt, Pd, Au, Cu, etc.) acting as cocatalysts for the redox reactions.32,68,146 In some full water splitting solutions, two types of cocatalysts are used, one for water reduction and one for oxidation.147,148 It can be seen as an extension of Figure 3a, where the charge separation and transfer of the electron and hole to separate sites seemed arbitrary. The two different types of cocatalysts, acting as electron and hole sinks, respectively, provide the rationale for the directed transfer. Efficient charge separation can be also attained through contacting two semiconductors. In particular, in Type II band alignment, the electron and holes transfer in opposite direction leading to a long-lived charge separated state, as illustrated in Figure 6c.149 This is also achieved if just one semiconductor is photoexcited. Only one carrier type will then move across the interface leaving the other behind, in a process identical to sensitization, described previously (cf. Figure 5b). The Type II alignment is very common in photocatalytic systems, such as CdS/TiO2,150 Cu2S/CdS,151 or WO3/BiVO4.127 The latter two are focused on just one-half-reaction, H2 and O2 production, respectively. An inverted Type II alignment, presented in Figure 6d, can also be very helpful in separating the charges. At first, it seems contradictory to the preferential directions of charge transfer outlines above. For example, one could expect electron transfer from SC2 to SC1. To avoid this, a mediator redox pair (e.g., IO3−/I−) or material (e.g., reduced graphene oxide) is employed. The constraint is that it is selectively reduced at SC1 and oxidized SC2, effectively transferring an electron from SC1 to SC2, but not vice versa. Such arrangement, known as a Z-scheme, is at the same time a two photon absorption mechanism, where the lower energetic carriers recombine, while the more energetic carriers are used in photocatalysis. Effectively, the photon energies are partially added together, so that two semiconductors with a narrow band gap can be used instead of a wide band gap one. Despite the difficulties in finding the appropriate mediators, a Z-scheme, which mimics natural photosynthesis, is one of the more promising approaches to photocatalysis. It is described in more detail in section 3.2. 2.4. Surface Reactions. 2.4.1. CO2 Reduction. Charge carriers can participate in several reactions at the surface of the NC, involving electron transfer to the adsorbate molecule, a proton, hydroxyl anion, water, CO2 or other. It is only at this stage that the water reduction and CO2 reduction become competing reactions. At all earlier stages the pathways of both

2H+ + 2e− → H 2

0 Eredox = −0.41 V

CO2 + 2H+ + 2e− → HCOOH

(4)

0 Eredox = −0.61 V

CO2 + 2H+ + 2e− → CO + H 2O

0 Eredox = −0.53 V

CO2 + 4H+ + 4e− → HCHO + H 2O

(5) (6)

0 Eredox = −0.48 V

(7) +

−

+

−

CO2 + 6H + 6e → CH3OH + H 2O

0 Eredox

= −0.38 V (8)

CO2 + 8H + 8e → CH4 + 2H 2O

0 Eredox

= −0.24 V (9)

CO2 + e− → CO2 ·−

0 Eredox = −1.90 V

(10)

Interestingly, the redox potentials of CO2 reduction cluster around the water reduction potential that suggests they should be accessible with the same semiconductors. Unfortunately, this is deceptive, because the CO2 reduction actually proceeds through multiple intermediate steps where the kinetic barriers are much higher. It is considered that the highest barrier is associated with the first, single-electron step of reduction of CO2 to anion radical CO2•‑. The potentials of this reaction in the gas phase (10) is −1.9 V.152 The comparison with band edge positions of common semiconductors (Figure 7a) reveals that no semiconductor, save perhaps for a ferroelectric LiNbO3 (not shown), should be able to directly reduce CO2. The very high stability of the CO2 molecule derives from its linear structure with strong double C−O bonds. Donation of an electron to (C−O) π* orbital of the CO2 molecule requires breaking this symmetry to produce a bent anion radical with O−C−O angle equal 138°. The high associated energy means that the LUMO level of CO2 is higher than the conduction band edges of the semiconductors. There are several implications of such band alignment. First, CO2 reduction proceeds at a much lower rate than water reduction. Therefore, every CO2 reduction experiment run in the presence of water faces a very strong (and usually at least partially successful) competition from hydrogen generation. Second, because the CO2 reduction is difficult enough, the efforts of the researchers in the field remain only on the reduction side, with hole scavengers used for transferring the holes away. Indeed, very few reports combine CO2 reduction with water oxidation.146 Third, the activation of CO2 requires an additional input. As shown in Figure 7b, the primary method is the adsorption on the surface of a semiconductor, usually vacancy mediated.154 This is accompanied by a partial electron transfer to the CO2 molecule, inducing its bending and lowering its LUMO level. Similarly, CO2 may be also first reacted with electron donating molecules, for example tertiary 3609


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products of CO2 reduction indeed originate from carbon dioxide.167 Contaminants, adventitious adsorbates, or carbon residues from the synthesis were suggested as the source of carbon.168 In the isotope labeling experiments, where 13CO2 is used as an intentional C source, only up to one-sixth of the measured CH4 and CO were confirmed to originate from 13 CO2. These results demonstrate the critical importance of the isotope labeling to confirm the reduction pathway of CO2. This has become a de facto standard measurement; in fact, in some reports only the results with 13CO2 are described.169−171 Other control measurements, such as blanks in the presence of neutral atmosphere, are important on their own, but cannot be a replacement. Especially, if only a small production rate (several μmol/h·gcat) with a burst in the early stages is observed. The exact pathway for CO2 reduction to methane remains to be fully established.172,173 It appears to depend strongly on the semiconductor type, specific facet and defects type and density which partially stems from different possible binding modes of CO2 (cf. Figure 7e). Three main possibilities have been proposed: formaldehyde, carbene, and glyoxal pathways, named on the account of a unique intermediate.68 The first one is also referred to as fast hydrogenation, as the C−H bonds are formed before the C−O are broken. In contrast, the second starts, also known as fast deoxygenation, with breaking C−O bonds, followed by attachment of H atoms. The in the final one, formyl radicals are claimed to dimerize forming C−C bonds (in glyoxal), which then by subsequent steps of oxidation and reduction, including photo-Kolbe type decarboxylation, can be transformed into methane.173 This pathway provides a good explanation of formation of C2 and higher products during CO2 reduction. Recent DFT calculations on the TiO2 surface indicate that the fast hydrogenation pathway is feasible both on perfect surface and on oxygen vacancies, although the latter process is much faster.174 The fast deoxygenation pathway is not energetically allowed on neither perfect nor defective TiO2 surface. Other calculations support this conclusion,173 but the energy balance is strongly dependent on specific crystal facet (e.g., (001) vs (101) on anatase TiO2). The presence of water also affects the results because abundant water molecules saturate Ti sites, leaving only bridging O atoms for adsorbing CO2, which in turn determines the further steps of the reduction.175 Despite most of the computational work being focused on TiO2, copper(I) and copper(II) compounds are considered to be the best catalysts for CO2 reduction.176−178 Importantly, they provide selectivity for CO2 over water reduction. DFT calculations of CO2 adsorption on copper oxide surface suggest a strong affinity (−93 kJ/mol binding energy) for (001) surface.179 Adsorption is accompanied by partial electron transfer to the molecule, leaving it in the bent anion radical conformation. As discussed earlier, this is a perfect starting point for further reduction. In addition, the highly reducing conduction bands of copper oxides and chalcogenides are very suitable for CO2 reduction (Figure 7a), as is the ease with which copper ions accept or donate electron, forming Cu(0) or Cu+ species. Accordingly, Cu2O has been used standalone180 and as cocatalyst for niobate nanosheets, SrTiO3 nanorods and TiO2 particles to reduce CO2 into CO.181−184 Overgrowth of TiO2/Pt nanoparticles structures with Cu2O has been demonstrated to result in selective CO2 reduction through suppression of concurrent water reduction.185 The catalytic properties of copper(I) compounds are also exhibited by sulfides which have also been shown to reduce CO2 to CH4 and

Figure 7. (a) Band alignment of several common semiconductors with redox potentials of water reduction (blue solid line) and oxidation (dotted blue line), as well as single-electron CO2 reduction (red line) and four multielectron and multiproton CO2 reduction reactions to formic acid, formaldehyde, methanol, and methane. Adapted with permission from ref 68. Copyright 2013 Wiley-VCH. Possible CO2 activation mechanisms through: (b) adsorption on semiconductor surface, (c) electron donation from a free-electron pair or (d) a negatively charged atom. (e) Attachment modes of CO2 on TiO2 surface: linear, monodentate, and bidentate. Adapted with permission from ref 153. Copyright 2017 Royal Society of Chemistry.

amines (Figure 7c) or strongly electronegative atomic complexes (Figure 7d) to make it more reactive. Alternatively, the direct CO2 reduction to hydrocarbons may be replaced with a stepwise approach which uses hydrogen obtained separately in a water splitting process.155−157 CO2 may be then hydrogenated to methane using an industrial-scale exergonic Sabatier process:158 CO2 + 4H 2 → CH4 + 2H 2O

(11)

The reverse water gas shift reaction (rWGS) in which CO2 reacts with H2 to produce carbon monoxide and water offers another solution.159 The rWGS reaction is endothermic in nature (ΔH298 = 42.1 kJ/mol in standard atmospheric pressure), hence the conversion of CO2 increases with increasing temperature and excess H2 flowing, but a strong increase in the rate may by induced by supported metal and metal oxide catalysts.160,161 The produced CO, mixed with further amounts of H2 (syngas) can be then combined with Fischer−Tropsch synthesis to further hydrogenate CO to hydrocarbons over dedicated catalysts.3,162 This approach allows to replace a difficult photocatalytic CO2 reduction with water splitting and subsequent series of industrial-scale CO2 hydrogenation processes. In similar context, substantial efforts have been devoted to photocatalytically reduce CO2 to CO in the presence of hydrogen.163 Recent results on indium oxide In2O3−x(OH)y nanocrystals with multiple oxygen vacancies and hydroxyl radicals suggest this to be a viable alternative.164,165 As the efficiency of the direct conversion of CO2 to hydrocarbons remains to be very modest,68,166 there are also lingering doubts whether the hydrocarbons reported as 3610


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Figure 8. (a) Illustration of the Pt-decorated CdSe/CdS dot-in-rod architecture; (b) corresponding energy level alignment. Reproduced and adapted with permission from ref 190. Copyright 2010 American Chemical Society; (c) comparison of relative electron transfer and charge recombination rates in tip-decorated and randomly decorated CdS nanorods. (d) Relaxation of the bleach of the 1S exciton in transient absorption for bare and decorated nanorods. Reproduced with permission from ref 60. Copyright 2016 American Chemical Society.

CO.186,187 Several reports have shown that metallic copper cocatalysts improve CO2 reduction efficiency.188,189 2.4.2. Water Reduction. In contrast to CO2 reduction, several semiconductors have sufficiently reductive conduction bands to reduce water to hydrogen (cf. Figure 7a). These are usually combined with nanoparticulate cocatalyst to lower the necessary overpotential. Platinum NPs are exceptional cocatalysts in this context, requiring virtually no overpotential, so that the process can even proceed with Pt-decorated TiO2 NCs which have conduction band only about 150 mV above the reduction potential of water.32 On the other hand, CdS NCs provide a perfect model platform to study the process in greater detail. A combination of the metal−semiconductor and a Type II junction is very nicely illustrated in a work on efficient H2 generation by Pt tip-decorated CdSe/CdS dot-in-rod nanostructures.59,190 The CdSe dot is located asymmetrically with respect to the CdS nanorod center, while the Pt nanoparticle is placed at its far end, as shown in Figure 8a. Upon excitation of the nanorod, the dot acts as a hole sink, while the delocalized electron is transferred to the Pt nanoparticle. This provides very effective charge separation, both energetic across the two interfaces but also spatial along more than half the length of the nanorod (Figure 8b). This will significantly reduce the recombination rate and lead to high quantum efficiency of the H2 generation. Figure 8c shows the comparison between two CdS nanorod systems (without the CdSe dot). In one, the nanorods had a Pt nanoparticle grown at one tip, while in the other they were randomly decorated with multiple Pt nanoparticles. In the latter the average distance to the cocatalyst

particle is much shorter than in the former, resulting in much faster electron transfer rate (see Figure 8d). Despite that, the hydrogen generation rate is much higher on the former, because the longer-distance separation ensures much lower recombination rate. In further encouraging developments, recent experiments on Pt-decorated CdS nanorods have proven that obtaining 100% quantum efficiency of H2 generation under visible light irradiation (450−460 nm) is possible.59,60 Nonetheless, platinum costs and scarcity have driven the search for an earth-abundant alternative with Ni, Co, and Mo considered to be the most promising candidates.191 To this end, in a similar configuration to the Pt-CdS system above, nickel decorated CdS nanorods achieved 53% external quantum efficiency at very high pH.61 XPS measurements have shown that nickel was in the form of elemental Ni nanoparticles, possibly coated with a thin shell of hydroxide. This is consistent with the observations that nickel oxides and ferrites perform very well in the alkaline conditions in photoelectrochemical cells.13 Few reports describe photocatalytic activity of the ferrites under visible light; however, high crystallinity and large surface area evidently promotes higher activity for H2 generation.192,193 CdS NCs served also as a scaffold and sensitizer for transition-metal phosphide nanoparticles. For CdS coated with poly(vinylpyrrolidone), PVP, and decorated with either Ni2P or Co2P, highest formation rates of H2 generation were 34.8 mmol·h−1·g−1 and 19.4 mmol·h−1·g−1 under white LED illumination (λ > 420 nm) and with DL-mandelic acid as a hole scavenger.194 The latter value corresponded to a quantum efficiency of 6.8%. A modified synthesis which eliminated the 3611


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leading to very short Fe−O bonds. This confers very good adsorption affinity for the oxidation intermediates on the Fe sites. In contrast, the Ni sites appear to be inactive. An alternative explanation was given by Trotochaud et al., who suggested that the partial charge transfer with the Ni sites, induced by the Fe atoms, activates the former for the oxidation process.207 It is interesting to note that the nickel−iron oxides are active catalysts both for reduction and oxidation process.13 The abundance of both elements makes these compounds a potentially ideal catalysts for water splitting. This intriguing and welcome property of nickel−iron (Ni,Fe)OOH oxyhydroxides is shared by other oxides, in particular nickel oxyhydroxide, NiOx,208 and cobalt oxyhydroxide, CoOx,200 which are also efficient oxidation catalysts. Many of the water oxidation measurements are taken in photoelectrochemical setups, with the efficiency expressed by the photocurrent. In a purely photochemical setup, the detection of evolved O2 by gas chromatography is not a certain proof of O2 generation because of possible leakage of atmospheric oxygen into the reaction vessel. In order to provide evidence of such process, one solution is to monitor simultaneously nitrogen and calculate the oxygen to nitrogen ratio. The release of N2 is not expected, so an increase in the ratio indicates that oxygen is released in the experiment. A more definitive proof can be provided by isotope labeling where H218O is used as a putative oxygen source.

need for the PVP coating hampering the charge transfer enabled preparation of a CdS-Co2P photocatalyst generating H2 at a rate of 254 mmol·h−1·g−1, corresponding to 25.1% quantum efficiency.195 The values obtained with Ni2P and Cu3P were only slightly smaller. Interestingly, they were higher than those obtained for Pt cocatalyst under similar conditions. These results imply that the electron transfer reaction operates very efficiently. In earlier experiments, hydrogen generation by CdS nanorods decorated with photodeposited Pt clusters was measured using different hole scavengers.56 Interestingly, the rate of H2 generation correlated very well with the redox potential of the scavenger. This means that the faster the hole collection was, the faster the H2 generation. As the charge balance must be maintained, this in turn suggests that the hole transfer is the rate limiting step of the process, in agreement with further carrier dynamics studies.57,58 In metal chalcogenide nanocrystals, holes are trapped at the surface on the subpicosecond time scale.196,197 It appears, further efforts on improving H2 generation on semiconductor NC should be concentrated on more efficient extraction of the hole as this is an area where substantial progress can be made. 2.4.3. Water Oxidation. The water oxidation reaction, given by eq 12, involves transfer of four holes, a formation of O−O double bond and transient adsorption of reaction intermediates on the surface.198 The need to store the holes in the ongoing process results in slow kinetics of the reaction and the necessity of a large overpotential.199 In effect, despite the difference of only 1.23 V between the water reduction and oxidation reactions, the full water splitting requires the band gap of at least 2.3−2.4 eV. Few semiconductors absorb visible light, yet possess the appropriate band alignment to satisfy the thermodynamic constraints. Tantalum oxynitride is one of not many examples. Typical water oxidation catalysts such as WO3 or BiVO4 cannot simultaneously reduce water, therefore the they are either used in photoelectrochemical cells200 or as part of a Z-scheme (discussed in section 3.2) 2H 2O + 4h+ → O2 + 4H+

0 Eredox = 0.82 V

3. APPROACHES TO EFFICIENCY IMPROVEMENT In the previous section, we described in detail the stages of the photocatalytic water splitting and CO2 reduction. We have also discussed the energy loss mechanisms and ways to minimize them during the process. However, application of a single solution to a single problem rarely gives rise to the best photocatalyst. These usually combine several beneficial features (e.g., absorption and charge separation) to gain improvements in efficiency. In this section we highlight a selection of the concepts and aspects of photocatalysis which promise to bring substantial progress in the field. These include new photocatalyst design based on defect engineering, use of mediators, self-assembly, and plasmonic materials, which can act as both sensitizers and cocatalysts. It also includes emerging materials such as perovskite oxides, carbon dots, carbon nitrides, and bimetallic cocatalysts. 3.1. Photocatalyst Engineering. Defects in photocatalytic applications carry largely pejorative connotations which stem from their role is providing trap sites for Shockley−Read−Hall type charge recombination. However, recent studies suggest a more complex and positive contribution which in many cases balances the recombination losses.209,210 These benefits stem from three factors. First, the absorption by wide band gap semiconductors can be extended to visible range via oxygen vacancies providing interband states. In the case of TiO2, this can red-shift the absorption onset from 400 to 530 nm, because of the formation of additional donor energy states at 0.75 to 1.18 eV below the conduction band.210 Second, the surface defects provide adsorption sites for the reactants, such as CO2. Third, the free charges need to be trapped at or near the surface to participate in the redox reactions. In this way, defects function as reaction centers at the surface providing additional electron-accepting or -donating states in the bandgap. This allows for an altered reaction pathway, proceeding with significantly lowered kinetic barriers. In oxide semiconductors, the oxygen vacancies are associated with electron traps, while

(12)

Three main types of nanoparticulate materials are used as oxidation catalyst. The first group includes noble metals and their oxides (e.g., RuO2, IrO2), which have been the cocatalyst of choice in many oxidation experiments on oxide semiconductors.17,201,202 The hole transfer from CdS nanorods to deposited IrO2 nanocrystals has been shown to be rapid enough to generate oxygen and prevent the competing reaction of photooxidizing CdS.203 Interestingly, ultrasmall IrO2 nanocrystals, even without sensitizer, were able to oxidize water.204 The visible light activity was a little surprising, but possibly originated from d−d transitions in IrO2. Transition metal oxides offer an earth-abundant alternative to iridium or ruthenium-based cocatalysts. However, not all of them are suitable for water oxidation.205 Manganese(II) and nickel(II) oxide have too large band gaps, ∼4 eV, for reasonable solar light activity. Meanwhile in other cases (e.g., in FeO), the valence and conduction band edges are made of the orbitals on the same atom. This implies strong coupling between the orbitals. The resulting fast recombination of the charge carriers makes FeO an unsuitable candidate. On the other hand, DFT calculations indicate very favorable properties of nickel oxyhydroxide, NiOOH.205 Moreover, an impressive 500-fold increase in oxidation rate is obtained by doping NiOOH with Fe.206 The role of the Fe atoms is not entirely clear, but it is thought that their presence distorts the crystal structure, 3612


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Figure 9. (a) Band alignment of CdS nanorod with redox potentials of water reduction and oxidation vs pH of the medium. Violet arrows show either single step hole transfer to the scavenger at neutral pH and two-step process in high pH; (b) illustration of hole transfer mediated by the OH−/OH• redox pair. Reproduced with permission from ref 61. Copyright 2014 Nature Publishing Group; (c) illustration of a Z-scheme using RuO2/TaON and Pt/TaON components in the presence of IO3−/I− redox mediator. Reproduced with permission from ref 219. Copyright 2013 American Chemical Society.

hydroxyl groups are associated with hole traps.211 For instance, oxygen vacancies and/or surface hydroxyl groups have been shown to be indispensable in the hydrogenation of CO2 on indium oxide nanocrystals.164,165,169 The density of these defects correlates with photocatalytic performance. This is attributed to longer excited-state lifetimes deriving from improved charge separation, as the electrons and holes reside in different traps.170 The defects are also partially associated with the adsorption of water or carbon dioxide on the surface. Both processes can be greatly facilitated by surface oxygen vacancies. This is particularly important for CO2 reduction in which the adsorption and transfer of a single electron provide the highest barrier. DFT calculation show that, as would be expected, adsorption of CO2 on TiO2 is much easier on the oxygen vacancy site.174 This either leads to abstraction of oxygen from CO2 to form CO, or further reduction to CH4, as observed on nonstoichiometric WO3−x or TiO2−x nanocrystals.209 The Ti3+ defects and surface disorder are considered to be responsible for visible light activity of the black TiO2,137 but recent positron annihilation measurements on ultrasmall rutile TiO2 attribute the extended absorption to surface hydroxyl groups, without any Ti3+.212 The samples with highest concentration of surface defects exhibited very high H2 generation rate of 24.7 mmol·h−1·g−1. The reason behind this activity was thought to be the upward shift in the valence band edge and improved charge separation at the surface. Clearly the detrimental effects of (primarily) bulk defects are outweighed by the benefits. The engineering of the active surface of the photocatalyst can include also introduction of atoms of different type. In fact,

bimetallic catalysts have attracted attention recently as potential better cocatalysts for water splitting and CO2 reduction.213−216 Having two different atoms in the alloy catalyst enables precise tuning of the surface properties to match the geometry and electronegativity of the atoms of the adsorbate.217 Recent study on CO2 reduction by isolated Cu atoms in Pd surface serves as a good illustration of this effect.218 Copper is a known catalyst for CO2 reduction through its d-band reactivity, while palladium, due to strong binding to H atoms, suppresses the H2 generation. The optimal binding mode was found to be with Cu atom in the proximity of the O atom of the CO2, while the C atoms was close to Pd atoms with adsorption energy −0.463 eV. Two neighboring Cu atoms would not give such optimal binding affinity. Therefore, the arrangement with an isolated Cu atom confers excellent 96% selectivity of CO2 reduction to methane. This further proves the potential of directing the chemical reaction on the NC surface through precise surface engineering. 3.2. Mediators and Z-Scheme. The extraction of the charge carriers from the semiconductor competes with the recombination processes. As discussed before, the electron transfer is usually quicker than the hole transfer; therefore, the rate of the photocatalytic reaction is limited by the latter process. Its rate can be increased by using a more reducing hole scavenger, but this way the energetic benefit of the reaction decreases. At the same time, the transfer to the scavenger is limited by the access of the often large molecule to the surface. In particular, the NC stabilizing ligands present at the surface may pose an obstacle. This problem can be ameliorated by using a mediator molecule which transports the charge carrier (usually hole) away from the nanocrystal surface to the 3613


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Solid-state Z-schemes can circumvent the problems of the liquid phase redox pairs.223 As mentioned earlier, reduced graphene oxide can be used as an electron mediator to construct a Z-scheme of CoOx loaded BiVO4 and Pt-loaded metal sulfides (e.g., CuInS2) as the catalyst.224 Such scheme can be operated for water splitting or CO2 reduction. Owing to its reductive conduction band, CdS can be also used as a base for solid Z-scheme to photocatalytically reduce CO2 or split water as has been shown for CdS/WO 3 and CdS/CdWO4 systems.225,226 Moreover, the arrangement in a Z-scheme improves the photostability of the sulfide. The CO2 reduction has also been demonstrated in α-Fe2O3/Cu2O Z-scheme which takes advantage of the catalytic properties of Cu2O, described earlier. 3.3. Surface Plasmon Resonance Enhancement. As mentioned in the introduction, the plasmonic metal NPs, especially Au and Ag exhibit strong localized surface plasmon resonance (LSPR) in the visible part of the electromagnetic spectrum.98−100 The phenomenon can be tuned by varying their size, shape and composition, thus making them interesting candidates for visible light active photocatalysis.98,101−103 They exhibit high extinction (absorption and scattering) cross sections compared to typical dye molecules. The absorption cross section of plasmonic NPs increases with their size, which results in the generation of more hot electrons available for photocatalytic reaction.227 The higher extinction cross section of plasmonic nanoparticles leads to strong absorption of light, which results in the generation of heat on the vicinity of NP surface, and it can enhance the kinetics of photochemical transformations.228,229 In addition, they exhibit strong electromagnetic field enhancement near their surface by irradiation of light at their plasmon frequency, which can enhance the absorption of nearby photoactive medium.103,230 All these unique properties of plasmonic NPs make them promising candidates for visible light induced photocatalysis, as compared with conventional semiconductor catalysts.98,101−103,105,231−234 In general, there are three different ways to use plasmonic NPs for the benefit of photocatalytic reactions:98,102,103,105 (1) direct, hot electron-induced photocatalysis on plasmonic NP surface (Figure 10a),104 (2) hot-electron transfer from plasmonic NP to nearby semiconductor or metal (Figure 10b),235−238 (3) SPR-induced electromagnetic field enhancement (Figure 10c).239 These three approaches have been under extensive investigation in recent years for plasmon-enhanced enhanced photocatalytic as well as photoelectrochemical water splitting, CO2 reduction and other chemical reactions. In addition, plasmonic NPs are well-known for localized heat generation upon light illumination,240 which could also contribute to the enhancement of the surface reaction, even without additional transfer of hot electrons. All these processes can effectively contribute to plasmon enhanced water splitting process; however, it is difficult to distinguish the exact contributions from each individual process. In this section, we discuss recent advances of plasmon-enhanced photocatalytic water splitting reactions on nanocomposites composed of Au and Ag NPs with either semiconductors or other metals under visible light illumination and their underlying mechanisms behind the SPR-induced enhancement. 3.3.1. Hot-Electron-Induced Photocatalytic Water Splitting. Plasmonic NPs offer unique optical properties that are tunable across the visible-NIR region of the spectrum. The excitation of LSPR leads to an oscillation of electron density around the Fermi level and subsequently decays into hot

scavenger. Such molecule undergoes two redox reactions, one collecting the charge at the surface and the other when releasing the charge. For this reason, it is referred to as a “redox shuttle”. The mediated process is only beneficial if the two processes are faster than a direct transfer. A recent example of a redox pair OH−/OH• was reported for CdS nanorods, a material known for its susceptibility to self-oxidation by the photogenerated hole.61 However, at neutral conditions, the valence band hole is not able to oxidize a hydroxyl anion. The process utilizes a specific feature of the chalcogenide NCs that the potential determining ion is not the hydroxyl. Therefore, the band edges do not follow a Nernstian slope of −59 mV per pH unit, but the dependence is much less steep (see Figure 9a) at only −33 mV per pH unit. This leads to a crossover with the OH−/OH• pair at around pH 11. Beyond that, oxidation of OH− is possible. In effect, in alkaline conditions, the hole is scavenged by the OH− which becomes an OH• radical which in turn diffuses away and oxidizes the final hole scavenger (ethanol, see Figure 9b). The hydrogen generation quantum efficiency correspondingly grows from less than 0.5% to 53%, in agreement with the limiting role of the hole transfer. A very similar process was reported also for potassium niobate, K4Nb6O17, nanosheet with trifluoroacetic acid TFA•/TFA− redox pair.220 Increasing the ratio of TFA to niobate to 6, the H2 generation rate increased 32-fold. In these mediated systems, the charge is transported to a sacrificial agent. Z-scheme mechanism can be seen as an extension of this concept, wherein the charge - instead of being wasted in a sacrificial reaction−is transferred to a second semiconductor nanoparticle where the second half-reaction takes place.219 As the potential is built up through two absorbing semiconductors, the Z-scheme can utilize two narrow band gap materials but still perform a task of a single wide band gap semiconductor (cf. Figure 6d). Another major benefit is that semiconductors which are suitable only for one-halfreaction can still be used in a Z-scheme. For example, BiVO4 or WO3 are excellent materials for the oxidation side (photoanodes), but they lack the potential to reduce water. They are, nonetheless, among the most common materials in a Z-scheme oxidation side, together with rutile TiO2 and TaON. Usually, an oxidation cocatalyst used such as RuO2, IrO2, PtOx.221 Only rutile oxidizes water without any cocatalyst. On the reduction side, anatase TiO2, SrTiO3, or TaON are used, usually loaded with Pt as a cocatalyst. The major difficulty of the Z-scheme approach is finding a suitable mediator which is selectively reduced by the conduction band electrons of the oxidation side semiconductor (SC 1 in Figure 6d), but not by the higher energy conduction band of the other semiconductor. Analogously, it should not be oxidized by the more oxidizing holes of the SC 1, but only by the reduction side semiconductor. This severely limits the choice to almost only Fe3+/Fe2+ and IO3−/I− redox pairs.219 For example, Fe3+ absorbs preferentially to Fe2+ on rutile. Therefore, rutile can be used to oxidize water in the presence of Fe3+ as electron scavengers. A semiconductor with a band gap alignment appropriate for both water reduction and oxidation (e.g., TaON) can be used on both sides of the Z-scheme with RuO2 and Pt catalyst. (see Figure 9c).222 Importantly, the RuO2 is able to oxidize water even in the presence of I− ions, to provide the necessary selectivity for the mediator. 3614


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prolongs the hot carrier lifetime to enable plasmonic photocatalytic reactions such as water splitting and CO2 reduction. The generation of hot electrons is a quantum phenomenon, which occurs at the surface of NPs. Hence, the quantum effects and the generation of hot electrons depend on the size and the type of the metal.243−245 In addition, the optical excitation of plasmonic hot carriers in metal NPs depends on their shape, as shown by Govorov et al.244 They have theoretically predicted that plasmonic nanocubes exhibit more efficient hot carrier generation compared to nanospheres, while the latter act more effectively compared to nanoplates.244 In a recent study, it was found that spiky nanostars with strong electromagnetic field enhancement at the tips of the spikes can produce large number of hot carriers compared to nanorods and nanospheres.243 The shape-dependent hot electron generation is likely due to differences in plasmonic field enhancement factors and the inhomogeneity in electromagnetic fields inside plasmonic nanostructures.244 In addition, it has been calculated that silver NPs exhibit higher rate of hot electron generation compared to gold NPs due to higher field enhancement and narrow plasmon peaks.243,244 As discussed in previous sections, the key requirements for an efficient photocatalysts are high absorption cross section, efficient charge (electron−hole) separation and ultrafast injection of excited electrons to a reaction center. Although plasmonic NPs exhibit absorption cross sections, the fast hot-carrier relaxation (∼few ps) limits their catalytic efficiency. In spite of a significant progress recently in the field of hot-electron induced chemical transformations,98,246 the efficiency of photocatalytic water splitting directly on the surface of metal NPs is rather poor and insufficient for practical applications.101,104,105 Despite decent progress in theoretical understanding of generation and injection of hot-carrier into water molecules, to the best of our knowledge there are no experimental results on hot-electron induced water splitting directly on metal NPs. Nevertheless, the efficiency of hot-electron induced water splitting can be dramatically improved by coupling the plasmonic NPs to another metal or semiconductor. Recently, there is growing interest in using multimetallic anisotropic NPs as photocatalysts toward fundamental understanding of the role of surface plasmons in water splitting reactions.236,247−249 Metals with higher work functions can act as hot-electron acceptors for improving carrier separation (Figure 11e). For instance, Pt-tipped Au nanoprisms have been studied for single particle photocatalytic water splitting (Figures 11e,f).236,249 It was found that these structures are more efficient for water splitting compared with Pt-edged Au nanoprisms. As shown in Figure 11f, upon excitation with a 405 nm laser, interband transitions and various plasmon modes (in-plane dipole and multipole) generate charge carriers which then either recombine radiatively or transfer to Pt. At the end, the charge carriers can radiatively recombine or lead to H2 evolution by injection of electrons into water molecules. Single-particle PL measurements further revealed that the charge carrier generation, separation, and recombination depend on geometry and anisotropic nature of plasmonic NPs.249 Although these single-particle studies provide better understanding of the mechanism of plasmonic photocatalysis, hydrogen production under solar light irradiation of ensemble colloidal bimetallic NP photocatalysts in conventional bulk measurements is yet to be realized. However, it has been found that these bimetallic plasmonic NPs (for example, Au@Pt core−shell NPs) incorporated with semiconductors could be more effective in

Figure 10. (a) Schematic illustration of hot-electron-induced water splitting directly on spherical plasmonic nanoparticle by visible light excitation. (b) Schematic illustration of hot-electron transfer from plasmonic NP (e.g., a gold nanorod) to the nearby semiconductor (SC) or metal for efficient charge separation leading to water-splitting reaction. (c) Simulated optical-absorption map of Janus Au−TiO2 NP under visible light illumination showing electromagnetic field enhanced photocatalytic water splitting. Adapted with permission from ref 241. Copyright 2015 Wiley-VCH.

electrons and holes on the metal NP surface at a time scale of 10−20 fs, whereafter these hot electrons may transfer to nearby water or organic molecules driving the desired reactions (Figure 10a).101,103−105 However, such short lifetime of hot electrons limits the efficiency of plasmonic photocatalytic chemical reactions because the time scale of chemical transformations are on the order of microseconds to seconds.242 Therefore, the direct photocatalysis on pure plasmonic NPs is quite weak due to the large mismatch between hot carrier lifetime and reaction time scales, and this requires further research investigation. Therefore, it is important to find mechanistic paths to reduce the mismatch between hot carrier lifetime and reaction time scales to improve the efficiency of plasmon-mediated chemical reactions. One of the widely accepted approach for reducing the mismatch between fast hot carrier lifetime and slow reaction kinetics is based the delocalization of hot electrons with neighboring metals or semiconductors by plasmonmediated electron transfer process. The Schottky barrier at the metal−semiconductor interface enables the delocalization of hot electrons between metal and semiconductor, and thus it 3615


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Figure 11. (a) Schematic illustration of hot-electron-induced water splitting on Au nanorods decorated with TiO2 at the longitudinal edges in dumbbell-like morphology and (b) lack thereof on Au@TiO2 core−shell nanorods; (c) Comparison of visible-light-induced photocatalytic activity of Au@TiO2 dumbbell and core−shell NP catalysts for H2 evolution and (d) degradation of methylene blue (MB) dye. Adapted with permission from ref 235. Copyright 2016 American Chemical Society. (e) Schematic illustration of the mechanism of water splitting on platinum (Pt) coated Au nanotriangle under visible light excitation; (f) energy diagram for radiative decay of in-plane dipole surface plasmon resonance (DSPR), multipole surface plasmon resonance (MSPR), and interband excitation leading to charge separation (CS) and recombination (CR). Adapted with permission from ref 236. Copyright 2016 American Chemical Society.

and semiconductors, only plasmonically excited highly energetic electrons can only transfer to semiconductors to carry out the H2 evolution half-reaction. Furthermore, it has been shown that the hole left on the metal NPs can drive the oxygen evolution half-reaction on the metal surface.252 Consequently, it appears possible to carry out full water splitting under solar light illumination using plasmonic metal− semiconductor hybrid catalysts. A number of studies have shown that variety of plasmonicmetal/semiconductor hybrid systems exhibit higher photocatalytic water splitting efficiency under visible light illumination compared with pure semiconductors. Among all, Au (or Ag)-TiO2 nanohybrid systems have been extensively exploited for visible-light-induced photocatalytic full water splitting,101−103,105,232 such as in an autonomous plasmonic device using Pt capped Au nanorods@TiO2@Co arrays.256 In this device, the hot electrons generated by excitation of gold NRs inject into Pt leading to H2 evolution half reaction, while the holes transfer to Co-based molecular catalyst for O2 evolution. It has been measured that each nanorod produced 5 × 1013 H2 molecules per cm2/s under AM 1.5 illumination. Furthermore, it was found that the device efficiency is higher under full spectrum compared with a specific wavelength excitation. It is worthwhile to mention that AM 1.5 contains a small percentage of UV light, which can be absorbed by semiconductors and contributes as well to the water splitting process. Consequently, extra care should be taken in the analysis of the contribution of plasmon-enhanced water splitting under AM 1.5 illumination. Although the autonomous plasmonic solar approach exhibits higher operational stability compared with the semiconductor based devices, the external quantum efficiency is only 0.1%, which is very low for any practical use. Although solar radiation consists of a broad range of wavelengths, only a specific part of the spectrum at which plasmonic NPs absorbs can be more effective. The absorption efficiency of the device could be improved by using a panchromatic photosynthetic device,

H2 evolution compared to monometallic NPs, which can be attributed to the reduction of Schottky barrier at the metal and semiconductor interface.250 Hybrid photocatalysts made of various metals and semiconductors have been widely investigated for water splitting reaction as discussed earlier, where metal NPs (e.g., Au, Pt, and Pd) typically act as cocatalysts while semiconductor NCs are the photoabsorbers under UV light illumination.21,251 However, the situation reverses for visible light excitation, where plasmonic metal NPs strongly interact with light and generate hot electrons via plasmon excitation, which then transfer to semiconductors through which hydrogen evolution takes place (Figure 10b,c).98,103,252 Additionally, plasmonic NPs exhibit much higher absorption cross section compared with dye molecules, and their extinction wavelength is easily tunable across the visible-NIR region as discussed above. Femtosecond pump−probe spectroscopy with an IR probe revealed that plasmon-induced hot-electron transfer from Au NP to TiO2 occurs on a time scale of 240 fs and with 40% yield.253 Recently, Lian and co-workers have shown an efficient charge separation and hot-electron transfer in Au-CdSe nanorods on a time scale of 20 fs with a quantum efficiency of 24%.254 The hot-electron transfer rate varies depending on the type of plasmonic metal and semiconductor in hybrid system.255 For instance, the hot-electron transfer time in Au-CdS NRs is slower (90 ± 20 fs) compared with Au-CdSe NRs. The charge separation at the interface of plasmonic metal−semiconductor eventually facilitates the water splitting process on the semiconductor surface. One should remember the energy levels of semiconductors with respect to the standard reduction potential of hydrogen. Most of the conventional semiconductors used for water splitting reaction have their valence band energy at levels of 2−3 V, while their conduction levels lies between −1 and 0 V vs NHE.98 On the other hand, the SPR energy of plasmonic NPs is between 1 and 4 eV with respect to Fermi level. Considering the band alignment of metal 3616


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Figure 12. (a) Schematic representation for the influence of excitation wavelength (UV and visible light) on the selectivity of products distribution using bimetallic (Au, Cu)-TiO2 as photocatalysts. Reproduced with permission from ref 188. Copyright 2014 American Chemical Society. (b) Photocatalytic product yields obtained using three catalytic substrates (TiO2, Au/TiO2, and Au) under visible light (532 nm) upon irradiation for 15 h. (c) Energy level diagram of the band alignment of TiO2 (anatase), Au, and the redox potentials of CO2 (for different photocatalytic products: HCHO, CH3OH, and CH4) and H2O under visible light illumination. Adapted from ref 262. Copyright 2011 American Chemical Society.

enhanced by the electric field created upon excitation of LSPR.258 In particular, when semiconductors are in close contact with plasmonic NPs, the plasmonic near fields could strongly couple to the optical transitions of nearby semiconductor leading to enhanced absorption and electron−hole pair production. However, it is difficult to differentiate the fieldenhanced absorption of semiconductors from hot-electron transfer mechanism.98 In fact, these two processes can always exist together in the photocatalytic enhancement.259 For example, the enhanced photocatalytic efficiency of Janus AuTiO2 hybrids were attributed to both field enhancement241 and hot-electron transfer from Au NP to TiO2.235 Han and coworkers have shown that the visible-light-induced photocatalytic H2 production rate increases with an increase of the size of Au NPs in Janus Au-TiO2 nanohybrid composite.241 The enhancement was attributed to the increase of electric field enhancement with increase in the size of Au NP, which was confirmed by simulations (Figure 10c). On the other side, the increase in the size of Au NPs leading to efficient hot carrier separation as discussed in the previous section. Therefore, both processes could simultaneously contribute to the observed enhancement of H2 evolution through multiple electron transfer paths.259 In addition, the presence of plasmonic NPs increases the optical path length through resonant scattering leading to enhanced absorption and thus enhanced water splitting efficiency.260 Recently the enhanced photocatalytic H2 production was demonstrated for CdSe nanoplatelets sandwiched between Au NPs and a flat Au surface.261 The enhancement was attributed to the increase of charge carrier production and plasmon-mediated interfacial electron transfer in CdSe nanoplatelets by the high electric field intensity created between the Au NPs and a flat Au surface.

which can be fabricated by using a mixture of plasmonic NPs that together absorb over a wide range of wavelengths.257 The geometry and the interface of plasmonic metal− semiconductor hybrid systems play a crucial role in the absorption efficiency and thus photocatalytic efficiency. For instance, the absorption spectrum of Janus Au-TiO2 hybrid system could tuned across the visible range not only by varying the size and shape of Au NPs, but also by the size of TiO2 NPs.258 It has been shown that Janus Au-TiO2 generally exhibit higher photocatalytic activity over Au@TiO2 core−shell NPs (Figure 10c and Figure 11a−d).235,241 A recent study by Stucky and coworkers revealed the importance of geometry of AuTiO2 hybrid in photocatalytic water splitting. They have found that Janus type TiO2−Au-TiO2 dumbbells exhibit photocatalytic H2 evolution and dye degradation under visible light illumination, while Au@TiO2 core−shell NRs show neither.235 The enhancement of visible-light-induced water splitting was attributed to hot electron transfer from plasmonic metal to semiconductor. 3.3.2. Electromagnetic Field Enhancement. Plasmonic NPs are well-known for their ability to enhance the electromagnetic field through the excitation of localized surface plasmon resonance (LSPR). The plasmonic field enhancement phenomenon has been used in a wide range of applications such as metal-enhanced fluorescence, surface-enhanced Raman scattering, photodynamic therapy, photovoltaics, and solar-fuel generation.103 In the present context, the field enhancement generated by the plasmon can significantly contribute to the plasmon-enhanced photocatalytic water splitting efficiency of nearby semiconductors (Figure 10c).241 Although wide-bandgap semiconductors absorb mostly UV light, their absorption spectra show a small tail in the visible region. As a result, the weak visible light absorption of semiconductors can be greatly 3617


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Figure 13. (a) Schematic illustration of cubic perovskite crystal structure for ABO3 (A: yellow, BO6 units: green-red). Adapted with permission from ref 274. Copyright 2012 Elsevier. (b) Schematic representation of the synthesis of Ni@NiO core−shell structure on SrTiO3 catalyst. Reproduced with permission from ref 276. Copyright 2016 Royal Society of Chemistry.

presence of H2O because the conduction band of anatase TiO2 lies above the reduction potential of CO2/CH4 on the NHE scale. On the basis of band alignments, methane is the only favorable product because the reduction potential energies of CO2/HCHO and CO2/CH3OH are higher than the conduction band of TiO2 (Figure 12c). However, upon UV light excitation, additional products, including C2H6, CH3OH, and HCHO were detected in GC, suggesting a more complex mechanism, likely involving dimerization of intermediate radical species. UV light illumination can excite the d band electronic transitions of Au, which lies above the reduction potentials of CO2/C2H6, CO2/HCHO and CO2/CH3OH.262 In addition to hot-electron and electric-field-induced enhancements, the heat generated by the relaxation of excited plasmonic NPs could significantly contribute to the enhancement of CO2 hydrogenation as demonstrated in several studies.270 It is also worthy to note that the plasmonic sensitization offers possibilities not available in other approaches. This is because the high energy of a hot electron should be sufficient to overcome the barrier for single electron reduction of CO2 to CO2•‑. For this energy to be utilized, the hot electron would have to be transferred to a semiconductor with very highly reductive conduction band (e.g., LiNbO3) rather than commonly used TiO2. Therefore, while the reported efficiencies of CO2 conversion remain low, the unique advantages of hot electrons encourage further research in the field. 3.4. Perovskite Oxides. Perovskite oxides (ABO3) have been one of the most important class of semiconductor catalysts for highly efficient photocatalytic water splitting as well as CO2 reduction owing to their unique physical and optical properties that are tunable either by varying the constituent atoms or by doping with one or more cations.65,271−278 They can be directly used as powder catalysts to carry out photocatalytic reactions with easy recyclability. The ability to prepare perovskites with controllable structures and physicochemical properties have enabled the researchers to study the structure−photocatalytic activity relationship for better understanding of photocatalytic water splitting mechanism.272,275−277 The band gap of perovskites could be controlled so that they can be efficient catalysts under both UV and visible light illuminations.273,275,276 Importantly, the band alignment of perovskite oxides with respect to H2 and O2 evolution redox potentials enables full water splitting.30,272−276,279 Herein, we briefly discuss the well-studied classes of titanate, tantalate, and niobate perovskite oxides and

From the discussion in this section, it is clear that significant progress has been made over the years toward the engineering of plasmonic photocatalysts and fundamental understanding of plasmon-enhanced photocatalysis. Nonetheless, the overall efficiencies reported are quite low for any practical applications. 3.3.3. Plasmon-Enhanced Photocatalytic CO2 Reduction. Integration of plasmonic NPs with semiconductors has also been studied in the context of photocatalytic reduction of CO2.263−266 The adsorption of CO2 molecules on a surface is crucial for its catalytic activity, which underlines the need to properly engineer the surface of the metal−semiconductor nanocomposites,262,267−269 including those based on TiO2. The plausible mechanism is similar to plasmon-enhanced water splitting, but the chemical reactions are different. Additionally, plasmonic metal−semiconductor hybrids offer selectivity of products depending on the irradiation wavelength. As shown in Figure 12, visible light irradiation leads to the generation of hot electrons on the metal NP surface through plasmon excitation, which then can be transferred to either nearby semiconductor TiO2 or to the surface of nearby metal. In this context, the hot electrons were shown to preferentially transfer to oxidized Cu atoms rather than to the conduction band of TiO2.188 Eventually, they are injected into adsorbed CO2 molecules for the multistep reduction to methane (Figure 12).188 In addition, the presence of water leads to photocatalytic H2 evolution as side product through hot electron transfer from metal to semiconductor as discussed in previous section. However, it was found that CH4 production takes place only with visible light irradiation, while H2 evolution was observed for both UV and visible light illumination.188 Cronin and coworkers have found that the photocatalytic efficiency of AuTiO2 hybrid can be enhanced 24-fold when irradiated with a 532 nm wavelength light that coincides with the SPR of Au NPs.262 This enhanced photocatalytic efficiency was attributed to the enhanced absorption of TiO2 at 532 nm by the electromagnetic field enhancement of Au NPs. Interestingly, only methane was detected in gas chromatography of products obtained for bare TiO2, Au-TiO2, and bare Au (Figure 12b). In order to understand the selectivity of products obtained through CO2 reduction by controlling the illumination light wavelength, one has to look at the energy band alignment of semiconductor and metals with respect to the redox potentials of the corresponding reactions (Figure 12c). It is possible that the photoexcited electrons of TiO2 can favorably transfer to CO2 to initiate the reaction of CO2 to CH4 conversion in the 3618


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NiO particles loaded on the edge while the O2 evolution proceeds at the groove of the NaTaO3 nanostructure.65 In addition, sensitizing the perovskite oxides with other semiconductors285 or dye molecules286 can also significantly improve their photocatalytic activity. As mentioned above, the band gap of perovskite oxides (ABO3) can also be tuned by replacing some of the “B” atoms with other metals while still preserving their perovskite crystal structure, which are generally named as double-perovskite oxides with a general formula of “A2B′B″O6”. Very recently, Weng et al. have demonstrated the synthesis of barium bismuth niobate double perovskite (Ba2BiNbO6 or BBNO) with tunable band gap by varying the stoichiometry of Bi and Nb.277 In addition, the cation ‘A’ has an influence on the band gap of perovskite oxides.276 However, as discussed before, the band gap of perovskite oxides can be decreased by replacement of “A” with Ag+, which was attributed to the formation of a more negative valence band of Ag 4d and O 2p hybridized orbitals in comparison with O 2p orbitals.117 The replacement of “A” with alkali metal ions (A = Li, Na, and K) can lead to a variety of oxide perovskites with tunable band gap LiTaO3 > NaTaO3 > KTaO3 (Figure 14).287 It was found that these perovskites can

their hybrids with a special emphasis on the reported approaches for improvement of the yield of the photocatalytic water splitting. These compounds generally possess a chemical formula of ABO3, where “A” and “B” are metal cations. In the cubic crystal structure of perovskite oxide, atom “A” sits at the center of three-dimensional cubic unit cell made by eight corner-sharing BO6 octahedra (Figure 13a). The differences in the radii of cations in perovskites can lead to distortions in the crystal lattice, thus resulting in the transformation of cubic phase into various other phases such as orthorhombic, tetragonal, rhombohedral, and others. The changes in crystal structure can greatly influence their electronic band structures (especially band alignment), which controls their photocatalytic water splitting efficiency.274,280 For instance, Li et al. have shown that the photocatalytic efficiency of cubic phase NaNbO3 perovskite can be two times higher than orthorhombic NaNbO3 for H2 evolution and CO2 reduction.280 The difference in the photocatalytic activity of these two crystal phases of NaNbO3 was attributed to the differences in their band gaps and unique electronic structure of highly symmetric cubic phase NaNbO3, which is beneficial for the efficient charge transfer for enhanced photocatalysis. The photocatalytic water splitting efficiency of perovskites (ABO3) can be optimized via adjusting their constituent chemical components (A, B, and O) either by ion doping/ replacement or by doping with additional cocatalysts.65,274,281,282 The systematic investigation of electronic structure of d0 (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) transition metal oxides belonging to the perovskite family by Eng et al. revealed that the band gap depends on the electronegativity of the transition metal ion, the crystal structure and the bond (B− O−B) angles.283 It was also found that the band gap increases with decreasing the effective electronegativity of transition metal ions (Mo6+ > W6+ > Ti4+ ∼ Nb5+ > Ta5+). Moreover, doping of perovskite oxides with other metals can shift their absorption from UV to visible region for solar photocatalysis. For example, Mn-, Ru-, Rh-, and Ir-doped SrTiO3 perovskites exhibit intense absorption in the visible region, while the pristine SrTiO3 do not show any absorption in visible region. The evolution of new absorption bands in the visible region of the SrTiO3 absorption spectra upon doping with metal ions was attributed to the excitation from the discontinuous dopant energy levels to the conduction band of SrTiO3. As a result, it was found that the noble metal (Ru-, Rh-, and Ir)-doped SrTiO3 perovskites in the presence of Pt cocatalyst show enhanced H2 evolution from methanol/H2O solution under visible light illumination. The presence of cocatalysts can significantly enhance the photocatalytic activity of perovskite oxides similar to other semiconductors. Domen and co-workers have shown that the UV light-induced photocatalytic activity could be enhanced by 100 times in the presence of NiO cocatalyst (Figure 13b).284 Further characterization revealed that Ni@NiO core−shell structure on the surface of SrTiO3 through a reduction and reoxidation process. In addition, it was found that the NiO on the outer layer of Ni was partly transformed to Ni(OH)2 during the photocatalytic water splitting process.284 This study clearly demonstrate that the photocatalytic efficiency of perovskites could be improved by proper engineering of their hybrids with cocatalysts. Similarly, doping lanthanum into NaTaO3:NiO hybrid photocatalyst leads to 9-fold enhancement in the efficiency of stoichiometric generation of H2 and O2 under UV illumination.65 Mechanistic studies revealed that the H2 evolution takes place on the small

Figure 14. (a) Energy band structures of alkali (Li, Na, K) tantalate photocatalysts and NiO cocatalyst versus the normal hydrogen electrode (NHE); (b) Energy band diagram of Sr2M2O7 (M = Nb and Ta) photocatalysts, NiO cocatalyst and redox potentials of H2O. Adapted with permission from ref 30. Copyright 2009 Royal Society of Chemistry.

exhibit overall photocatalytic water splitting into H2 and O2 under UV light illumination and the efficiency follows the order of KTaO3 < NaTaO3 < LiTaO3, which agrees well with the conduction band energies relative to H2 and O2 evolution potentials on NHE scale. However, when these perovskites are coupled with NiO cocatalyst, NaTaO3 shows the highest activity because of the suitable conduction band level for efficient charge separation between NaTaO3 and NiO (Figure 14a).287 One of the problems with NiO cocatalysts is that the photocatalytic efficiency gradually deactivates over a few recycles of water splitting reaction due to changes in chemical 3619


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Figure 15. (a) Schematic diagram of g-C3N4 constructed from melem units; (b) Theoretical band gap of g-C3N4 compared to the reduction and oxidation potential of H2 and O2, respectively; Kohn−Sham orbitals for the valence band (c) and conduction band (d). Reproduced with permission from ref 305. Copyright 2009 Nature Publishing Group.

instability is a major challenge to be addressed for driving the field forward.292 3.5. Carbon-Based Nanomaterials. Although visible light absorbing inorganic QDs made of transition metal chalcogenides and metal oxide nanocrystals achieved high efficiency of solar fuel generation,32,61,293−295 their high toxicity, difficult synthetic procedures, poor sustainability, high rates of unwanted nonradiative recombination processes on trapped states and high costs limit their applicability.199,296,297 Carbon based luminescent nanomaterials and their hybrid systems (especially g-C3N4, luminescent carbon quantum dots and/or very specifically graphene quantum dots, soluble graphene oxide, carbon nanotubes etc.) are possible alternatives which could be very efficient, more robust and cost-effective as well as environmentally sustainable.293,298−300 In this context, great progress has been recently made in understanding the function of carbon nitride for heterogeneous photocatalysis.293,296,301−304 Although the compound is considered as one of the oldest known synthetic polymers after its first report by Berzelius and Liebig in 1834, it came to the forefront of heterogeneous photocatalysis after the pioneering work by Wang et al. in 2009.305 Carbon nitride possesses several allotropes, among them the graphitic species (g-C3N4) shows greatest stability in ambient medium and is considered the most applicable for visible-light-induced photocatalysis.306 Intrinsic modifications of carbon nitride (especially g-C3N4) to extend the visible light absorption and efficient charge separation by minimizing nonradiative recombination processes are one of the major goals of this particular field of material research. Normally the graphitic plane of g-C3N4 stems from tris-s-triazine/melem units and these oligomeric units are connected by in-plane amino group (Figure 15a). Planar organization of molecular units and compression of the

composition by reduction of NiO; however, the efficiency could be retained by treating the catalysts with NaOH solution.287 In addition, recently there has been a growing interest in the development of layered perovskite oxynitrides288 and oxychlorides289 for visible-light-induced photocatalytic water splitting, reviewed by Domen and co-workers.273 The interesting thing about layered perovskite photocatalysts is that the spacing between layers can act as reaction site for enhanced O2 evolution.290 These layered perovskites with proper conduction band alignment can split pure water into H2 and O2 without any additional cocatalysts. For instance, Sr2Ta2O7 perovskite splits water under UV illumination, while Sr2Nb2O7 with similar crystal structure did not show any activity without a cocatalyst.291 Moreover, the activity of Sr2Ta2O7 can be greatly enhanced in the presence of NiO as expected based on the conduction band alignments shown in Figure 14b.291 It is clear that a wide range of photocatalysts based on perovskite oxides have been reported for highly efficient water splitting both under UV and visible light illumination. Moreover, the fundamental understanding of the perovskite structure−photocatalytic activity has improved over the years. To the best of our knowledge, perovskite oxides are the most efficient photocatalysts reported to date with the solar-to-hydrogen conversion efficiency of over 1%. However, the current efficiency is still far from the target efficiency (10%) required for practical applications. Development of new and cost-effective perovskite photocatalysts together with optimized cocatalysts based on Z-scheme designs for proper band alignments for efficient overall water splitting into H2 and O2 would be one of the future directions of the field. Very recently, there has been few attempts in using organic−inorganic hybrid perovskite halides for improved water splitting; however, water 3620


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ACS Catalysis successive planes can be regulated by the condensation techniques, resulting in controlled electronic properties.305 In general, g-C3N4 has a band gap around ∼2.5−2.7 eV which is much higher to overcome the potential barrier for endothermic water splitting reactions (1.23 eV). Furthermore, density functional theory calculation showed that HOMO−LUMO band gap of melem unit (basic unit of g-C3N4) is much larger than its polymeric form and is highly nonisotropic depending on the polymerization direction (Figure 15b).305,307 Therefore, band gap can be decreased by simply increasing the degrees of polymerization i.e. tuning the condensation temperature. Moreover, nitrogen pZ orbital essentially contributes to the valence band of g-C3N4, while, conduction band is basically combinations of carbon pZ orbital (Figure 15c and d). As a result, after photoexcitation hole is localized on a nitrogen atom whereas electron are situated on a carbon atom. This arrangement increases the intrinsic charge transfer processes within the polymeric matrix of g-C3N4 and thereby promotes the overall photocatalytic water splitting. In addition, controlled morphology and specific defects are crucial for enhancing overall charge separation process through increasing stacking defects, grain boundaries, and termination sites on the edge of the ring.302,308−312 Moreover, the introduction of chemical defects (for instance by using urea) on the peripheral sites of g-C3N4 can also influence the interfacial charge separation process, which is beneficial for the photocatalytic reactions and can also be used for storage of solar energy for hydrogen generation in the dark.313−316 The modification of electronic property/band gap of g-C3N4 for enhanced charge separation can be also done by incorporating nonmetal ions (e.g, B, S, O, P, I) inside the graphitic domain affecting both the structure and electronic properties.318,319 Introducing foreign nonmetal ions decreases the band gap, increases the solar light absorption, promotes charge mobility, and also enhances active sites in g-C3N4 matrix. Among the possible dopants, phosphorus plays the most important role to increase the photocatalytic performance of g-C3N4.311,317,320 Detailed structural analysis suggests that P atoms replace carbon atoms in graphitic domain of C3N4 and form P−N bonds, which create additional trap sites in g-C3N4 matrix (Figure 16a).317 As a result, intrinsic charge separation efficiency increases throughout the g-C3N4 matrix. In addition, upon P doping, the band gap of g-C3N4 is narrowed from 2.98 to 2.66 eV, and an empty band (−0.16 eV vs SHE) arises below the conduction band.317 The allowed transition from valence band (VB) of g-C3N4 to the mid gap state extends the absorption tail into the visible region. Moreover, the mid gap states upon P doping do not function as recombination centers of photoexcited electron hole pair. Instead, they act as charge separation centers to capture the photogenerated electrons, leading to the enhancement of the photocatalytic activity (Figure 16b). Considering the major aspect of improved absorption cross section (especially in visible regions) followed by enhanced interfacial charge separation, one can also think about the fabrication of intimate heterojunctional hybrid systems by combing g-C3N4 with inorganic semiconductors (metal oxide/ hydroxides, metal chalcogenides, etc.).293,321,322 Easily tunable electronic features of g-C3N4 and well-known shape-/sizedependent excitonic features of inorganic counterpart help to manipulate band alignment (especially type-II) for controlled charge separation in this typical heterojunctional hybrid systems.293,322−324 In general, upon photoexcitation electron

Figure 16. (a) Top and side views of pure and P-doped g-C3N4 (Cgray, N-blue and P-orange); (b) Schematic representation of VB, CB, mid gap states, and overall photocatalytic mechanism of P-doped gC3N4. Reproduced with permission from ref 317. Copyright 2015 Royal Society of Chemistry.

can reach the inorganic counterpart and hole can reside on the aromatic domain of g-C3N4 which enhance the photocatalytic efficiency (Figure 17a and b). As one step forward, further improvement of photoinduced charge separation for g-C3N4 based heterojunctional hybrid systems can also be done by forming Z-scheme ternary hybrid (e.g., g-C3N4/nanocarbon/ ZnIn2S4, Ag3PO4/Ag/g-C3N4, etc.) The layer between the two semiconductor counterparts acts as an electronic bridge/charge mediator, which stores free charges as well as promotes faster charge separation,323.325 Furthermore, the enhanced charge separation process in type-II hybrid heterostructure of g-C3N4 with inorganic nanomaterials (especially metal oxides) have also been used for heterogeneous CO2 photoreduction to solar fuels, recently reviewed by Li et al.304 Considering the limits and challenges of g-C3N4 hybrid systems with inorganic nanocrystals regarding their sustainability issue, several carbon-based material having extended πconjugation have been utilized to make intimate hybrid hetero structure with g-C3N4 for efficient photocatalytic water splitting.293 Materials having effective Π-conjugation can essentially act as charge transporting as well as charge storing counterpart upon forming hybrid with g-C3N4. In addition, conjugated materials can also inhibit the charge carrier recombination on g-C3N4. This is one of the major issues for increasing overall photocatalytic efficiency.327,328 In this context, Liu et al. developed an all organic metal cocatalyst free composite systems for complete water splitting via stepwise two-electron/two-electron mechanism upon combining g-C3N4 with carbon particles.326 Modified valence and conduction bands were calculated to be 6.96 and 4.19 eV, respectively, for this typical hybrid system which increase the absorption 3621


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ACS Catalysis

Figure 17. (a) Schematic representation of visible light absorption and charge separation in g-C3N4−MoS2 2D hybrid heterostructure; (b) Charge density differences in the g-C3N4/MoS2 nanocomposite (red and blue regions represent charge accumulation and depletion, respectively). Reproduced with permission from ref 324. Copyright 2014 Royal Society of Chemistry; (c) UV−vis spectra of g-C3N4 with and without carbon dots; (d) band structure diagram of g-C3N4−CD hybrid. Reproduced with permission from ref 326. Copyright 2015 AAAS.

Figure 18. (a) Schematic representation of p-type and n-type domains in nitrogen doped GO-QD; (b) Relative production rate of H2 and O2 under visible light from nitrogen-doped GO-QD. Reproduced with permission from ref 329. Copyright 2014 Wiley-VCH; (c) Schematic representation of photocatalytic hydrogen generation in nitrogen doped graphitic carbon dot-NiP composite; (d) Radiative recombination (dotted lines) and charge transfer (solid lines) pathways under photocatalytic conditions for nitrogen doped graphitic carbon dot-NiP composite. Reproduced with permission from ref 338. Copyright 2017 Wiley-VCH.

potential for H2O to H2O2 or O2 is just above the valence band. This arrangement enables full water splitting (Figure 17d). In the initial two electron processes, water was reduced to H2 and

shoulder in visible region compare to pristine g-C3N4 (Figure 17c,d). In effect, the reduction level for protons remains below the conduction band of the composite, while the oxidation 3622


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ACS Catalysis

the aromatic domain.337 Result shows that graphitic nitrogen (covalently bonded with three carbon atoms in aromatic domain) increase the photoluminescence properties of N-CD; however, nitrogen atoms on the edge of the aromatic domain especially pyrrolic nitrogen (covalently bonded with two carbon atoms on the edge of the pentagonal ring) enhance the atomic scale defects which virtually enhance the photocatalytic behaviors. Likewise, introducing edge defects by heteroatom doping in carbon nanomaterials can also promote selective CO2 reduction to hydrocarbons As an additional approach, CDs/GO QDs can also act as efficient photosensitizer/light harvester while combining with other catalyst and indirectly enhance the overall photocatalysis process.339 Although, the applicability of CDs as efficient light harvester is very limited and one must need to form a composite devices for this particular purpose.340,341 For example, 3d transition-metal-based molecular proton reduction catalysts have become very promising while combining with CDs.339 A composite system of amorphous CDs with molecular nickel catalyst ([Ni(P2N2)2]2+) achieved the H2 production activity to 398 μmol·g−1·h−1 under full solar light irradiation at 1.5 AM intensity.298 In this particular system, CQD acts as an efficient photosensitizer which absorbs light in the UV and visible region and rapidly transfers electrons toward the molecular Ni catalyst (Figure 18c). The amount of solar light absorption virtually depends on the relative amount of aromatic domain compare to amorphous domain, and it increases with increasing CC bonds (enhanced π−π* transition) which indirectly affect the photocatalysis process in this typical composites.338 Furthermore, a nitrogen-containing aromatic domain can also increase the rate of the hole-quenching process which virtually increases the concentration of long-lived photogenerated electrons leading to the higher hydrogen production. Transient absorption spectroscopy can clearly shed light on the electron-transfer mechanism of this particular composite system and effect of CDs’ aromatic domain for the overall process (Figure 18d). Briefly, more than 90% of excited states for all CDs show a prominent transient absorption signal near 550 and 650 nm (characteristic transient absorption signals of CDs) with picosecond time scale. Whereas, almost 10% of the excited states possess long lifetime (up-to microsecond to second time scale) which correspond to the long-living photogenerated electrons. Furthermore, addition of hole scavengers significantly enhance the lifetime of the photogenerated electrons and even increases more with a gradual increase in the graphitic domain of CDs. As a result, efficient hydrogen generation has been observed with IQE of ∼5%. However, initial preassembly or compact hybrid formation between CDs and NiP can further improve the photocatalytic performance by avoiding a diffusion-assisted fast decay process of the long living species. In general, carbon dots and/or graphene QDs have been emerging as the rising star in renewable energy research. However, it is essential to unveil the uncertain structure of CDs in “bottom-up” synthetic approach and developing the fundamental knowledge on actual structure property correlation for their future applicability. 3.6. Two-Dimensional Photocatalysts. Over the past decade, two-dimensional (2D) materials such as carbon nitride, graphene, metal chalcogenides, and black phosphorus have received significant research attention for various potential applications owing to their unique optical, electronic, and mechanical properties.342−349 The former two (graphene and

oxidized to H2O2. In the second step, H2O2 decomposed to O2 and H2O. Therefore, this stepwise process made the overall water splitting to H2 and O2 kinetically favorable. Even though notable scientific progress on g-C3N4 and their hybrid materials have been made in the past few years, several disadvantages (e.g., the probability of charge recombination in interfacial region and partial solubility in water due to large polymer matrix) often require specific sacrificial agents and high-temperature combustion techniques for synthesis inhibit their practical applicability in renewable energy research.298,299 In this context, luminescent carbon dots and/or graphene QDs could be one of the best carbon-based alternatives because of their efficient photostability and high solubility in aqueous medium.298,300,329−331 Unlike other carbon-based nanomaterials (e.g., graphene, carbon nanotube, etc.), carbon dots/ graphene QDs possesses both sp2 and sp3 domains inside the tiny dots (

Challenges and prospects in solar water splitting and CO2 reduction

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