The Middle Road Less Taken: Electronic-Structure-Inspired Design of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation Junsang Cho,† Aaron Sheng,‡ Nuwanthi Suwandaratne,‡ Linda Wangoh,§ Justin L. Andrews,† Peihong Zhang,∥ Louis F. J. Piper,*,§ David F. Watson,*,‡ and Sarbajit Banerjee*,† †

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Departments of Chemistry and Materials Science and Engineering, Texas A&M University, College Station, Texas 77842-3012, United States ‡ Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States § Department of Physics, Applied Physics, and Astronomy, Binghamton University, Binghamton, New York 13902, United States ∥ Department of Physics, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States S Supporting Information *

CONSPECTUS: The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising MxV2O5 nanowires, where M is a p-block cation with a (n − 1)d10ns2np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II−VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The MxV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical “etch-a-sketch” intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of MxV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of β-PbxV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair MxV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

1. INTRODUCTION The catastrophic environmental impact of our reliance on geological deposits of hydrocarbons1,2 as our primary source of © XXXX American Chemical Society

Received: July 30, 2018

A

DOI: 10.1021/acs.accounts.8b00378 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Illustration of evolutionary strategy for designing MxV2O5/QD photocatalysts with programmable energetic offsets and kinetics of charge transfer. p-block cations are of interest for positioning states derived from filled s-subshells at the top of the valence band to facilitate hole extraction from photoexcited QDs. Integration of first-principles modeling, material synthesis, interfacial functionalization, and catalytic evaluation allows for effective exploitation of the opportunities for programmability available in the design space.

functions.6,7 This approach mimics photosynthesis, in which oxidation and reduction occur in separate photosystems. Many composite semiconductor-based photocatalysts have been investigated;6 however, real-world water-splitting applications have been precluded by inefficient light harvesting and redox catalysis, long-term instability, and high cost. The vexingly high overpotential of the water-oxidation half-reaction has often necessitated the use of wide-band-gap metal oxides with highly positive valence-band-edge potentials.6 Such materials are poor harvesters of visible light, and free energy is dissipated via the unnecessarily large water-oxidation driving force. Moreover, since simple binary and ternary semiconductors exhibit clearly defined valence- and conductionband edges and fixed band gaps, the ability to tune light harvesting, electronic structure, and charge transfer is immutably limited.

energy makes solar energy conversion arguably the greatest scientific and technological challenge of our generation.3,4 Solar-to-electrical energy conversion, using photovoltaics, is essential but insufficient. Variable insolation necessitates the storage of solar energy in fuels.3 Water splitting, or the disproportionation of H2O into H2 and O2, is the most promising strategy as it yields hydrogen, a high-energy-density fuel that can be combusted to release energy, with only water as byproduct.4,5 Water splitting is fundamentally challenging, however, as it requires the concerted transfer of four electrons and four protons.4,6 A single material or molecule that can mediate the entire sequence of solar light-harvesting, charge-transfer, and masstransport processes required to split water may never be identified. Instead, a promising strategy is to develop hybrid systems in which different components perform these various B


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Figure 2. Mid-gap states derived from p-block cations. (A) Hybridization of p-block ns2np0 cations with anionic p-states.17 (B) Energy-level diagram of cation-s and anion-p orbitals calculated from DFT (KohnSham eigenvalues are depicted). (C) Energies of relevant cationic-s and anionic-p states. (D) Crystal structures of d-block β-Ag0.33V2O5 and p-block β-Pb0.33V2O5 illustrating the lone-pair-induced distortion; (E) (I) SEM and (II) TEM images of β-PbxV2O5 nanowires; (F) HAXPES and XPS valence-band spectra of β-PbxV2O5. Panels (A)−(C) are reproduced with permission from ref 18. Copyright 2011 Royal Society of Chemistry. Panels (D) and (F) are reproduced with permission from ref 12. Copyright 2014 American Physical Society. Panel (E) is reproduced with permission from ref 11. Copyright 2013 Wiley-VCH.

mitigating the vexing challenge of the photocorrosion of QDs, which has severely constrained their use in photocatalysis.14 The strategy to engage intercalative mid-gap states in interfacial charge transfer while exploiting quantum confinement represents a new approach to solar energy conversion. The accessible design space comprises a vast matrix of potential NW/QD heterostructures (Figure 1), differing in composition and interfacial properties, enabling broad tunability of electronic properties as well as both the thermodynamics and kinetics of excited-state charge transfer. Variables include the type of V2O5 framework (e.g., singlelayered α- and γ′-, double-layered ε′, and tunnel-structured ζphases),15 the specific cation (M), the stoichiometry (x) of the cation, the composition and size of QDs, and the nature of the interface between NWs and QDs. These variables provide tremendous versatility and are central to the development of heterostructures with programmable properties. However, the complexity of this multidimensional parameter space represents a daunting challenge, which is difficult to resolve using a purely Edisonian approach. We are instead navigating the design space utilizing a mix of chemical intuition and the iterative integration of theory and experiment to converge on optimal catalyst architectures.

We have undertaken an alternative approach involving the theory-guided design, synthesis, characterization, and catalytic evaluation of hybrid photocatalysts comprising II−VI semiconductor quantum dots (QDs) interfaced with singlecrystalline MxV2O5 nanowires (NWs), where M is an intercalated p-block cation.8−14 MxV2O5 NWs possess midgap electronic states situated several hundred millivolts positive of the water-oxidation potential derived from the stereoactive lone-pairs of the intercalated p-block cations.11−13 These states, which are derived not from defects or dopants but from the intrinsic crystal structure of NWs, are appropriately situated to accept photogenerated holes from II−VI QDs for subsequent delivery to an oxidation catalyst.13 Indeed, we have measured ultrafast hole transfer across QD/NW interfaces.9,10 The energetic dispersion and occupancy of the mid-gap states are tunable through the stoichiometry and identity of the intercalating cation, whereas potentials of excitonic and surface states of QDs are tunable through size, composition, and surface functionalization. Thus, interfacial charge-transfer driving forces are broadly tunable. Notably, MxV2O5 NWs exhibit reversible mid-gap-state-mediated electrochemistry and are not susceptible to anodic corrosion.9 The observed ultrafast hole extraction from QDs further provides a powerful means of C


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Figure 3. Synthetic strategies and energetic offsets of MxV2O5/QDs heterostructures. (A) SILAR and LAA routes for constructing heterostructures. (B) SEM and TEM images for SILAR-derived (I, III) and LAA-derived (II) β-PbxV2O5/CdSe heterostructures. (C) Valence-band spectra of βPbxV2O5/CdSe heterostructures. (D) Energetic offsets of MxV2O5/QD heterostructures illustrating the thermodynamic driving forces. Panels (B) and (C) are reproduced with permission from ref 8. Copyright 2015 American Chemical Society.

2. INTERCALATIVE MID-GAP STATES DERIVED FROM p-BLOCK CATIONS At the heart of our programmable design of electronic structure is the ability to reconfigure periodic MxV2O5 solidstate compounds through facile topochemical methods.14−16 A variety of V2O5 frameworks can be stabilized with intercalated cations through hydrothermal synthesis; energies of valence and conduction bands of these compounds are determined by the V−O connectivity and the vanadium coordination environment, whereas hybridization of intercalated cations with V2O5 frameworks yields states with variable energy dispersion (and partial reduction of vanadium sites). Conduction band edges are typically V 3d-derived (with specific splitting dictated by the vanadium coordination geometry and hybridization with O 2p states), whereas valence band edges are primarily O 2p-derived.16 Topochemical leaching of cations from MxV2O5 at low temperatures stabilizes metastable V2O5 frameworks, many of which are only slightly

In this Account, we outline our evolutionary design scheme and summarize our computational and experimental research involving first-generation heterostructures comprising βPb0.33V2O5 NWs interfaced with cadmium chalcogenide QDs.8−10 The prediction and measurement of their electronic structure, highlighting favorable energetics for interfacial charge separation, is delineated, along with the measurement of their excited-state charge-transfer dynamics and photocatalytic performance. Additionally, we report on the theoryguided design, synthesis, and characterization of secondgeneration SnxV2O5/QD heterostructures with electronic structure programmed to engender excited-state charge transfer and photocatalysis.14 The programmability of interfacial coupling epitomizes the versatility of MxV2O5 frameworks and underscores the centrality of electronic structure design to facilitate the rational design of heterostructures and the systematic improvement of their photocatalytic performance. D


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Accounts of Chemical Research higher in energy than the thermodynamically stable α-V2O5 phase. Tunnel-structured ζ-V2O5 and puckered γ′-V2O5 are prominent examples of kinetically trapped compounds that are considerably modified in structure and covalency from the layered α-V2O5 phase;16 the intercalation of different cations within these frameworks provides a powerful means of reconfiguring electronic structure and thereby energetic offsets in our photocatalytic platform. Of particular interest are the filled s-subshells of post-transition-metal cations with the (n − 1)d10ns2np0 electronic configuration (Figure 1), which have the potential to yield mid-gap states derived from stereoactive lone pairs of electrons. Such states are typically positioned at the top of the valence band, where they can extract photogenerated holes from QDs if the appropriate energetic offsets can be established within heterostructures. The filled s-subshell of p-block cations is manifested prominently in structural distortions of solid-state compounds, seen for example, in the litharge structures of SnO and PbO, which have been ascribed to a second-order JahnTeller effect mediated by the hybridization of cationic ns and np states.17 A more recent view, the “revised lone pair model”, paints a more complex picture.18,19 These filled subshells are not, in fact, inert and tend to be strongly hybridized with anion p-states giving rise to filled bonding and antibonding states (Figures 2A,B, S1, and S2).17,18 The antibonding states can further hybridize with empty cation np0 states with a distortion of the local coordination geometry away from a centrosymmetric environment when the electronic stabilization thus derived offsets the destabilization resulting from coordinative undersaturation.18 The projected electron density of the stabilized antibonding state has classical “lone pair” character and is positioned at the top of the valence band (Figure S2).20 Effective orbital overlap with cation 5p or 6p states, which is necessary to facilitate a pronounced distortion, requires that the antibonding cation-s−anion-p-states have substantial 5s/ 6s-character. In turn, this necessitates effective mixing of cation 5s/6s and anion p-states, the extent of which strongly diverges as a function of the anion electronegativity and resultant energy of anion-p states (Figure 2C).18 Two important consequences are (a) oxides are considerably more likely than chalcogenides to exhibit pronounced lone-pair effects (due to the sharp change in electronegativity and energy positioning of p-states from oxygen to sulfur) and (b) 5s2 (Sn2+) states that show the least differential in energy exhibit stronger lone-pair distortions in oxides since 6s2 (Pb2+) states are relegated to even lower energies as a result of relativistic effects. Figure S1 contrasts calculated orbital-projected density of states plots of α-V2O5 and intercalated phases with Pb2+ and Sn2+-ions ensconced within the tunnels.14 The order of stability of lone-pair distortions arising from antibonding states is predicted as Sn > Pb > Sb > Bi > Te > Po.18 Figure 2D shows the pronounced lone-pair-induced structural distortion observed around the intercalating cation in β-PbxV2O5 that is not observed in its d-block counterpart β-AgxV2O5. Figure S2 shows an isosurface plot for the mid-gap states of β-PbxV2O5. Figure 2E and Figure 2F illustrates the pronounced antibonding and bonding states with considerable Pb 6s character observed by high-energy X-ray photoemission spectroscopy (HAXPES) measurements at the edge and deep within the valence band of β-PbxV2O5, respectively. The ability to modulate energies of lone-pair states at the top of the valence band provides an important design tool for

constructing heterostructures with states positioned to accept holes from interfaced photoexcited QDs. The ability to access this design space is greatly facilitated by our recent discovery of topochemical extraction and intercalation methods wherein cation extraction from MxV2O5 bronzes yields a metastable V2O5 framework that can be filled with other cations. Indeed, Ag-ions have been extracted from β-Ag0.33V2O5 by acid treatment, stabilizing the metastable ζ-V2O5 phase with open tunnels, which are subsequently filled with Mg-ions, by reaction with di-nbutylmagnesium or by electrochemical magnesiation, to stabilize a β-Mg0.33V2O5 phase inaccessible from direct synthesis.21 Such an “etch-a-sketch” approach to swapping cations provides unprecedented control over electronic structure of the hole acceptor and thereby facilitates tuning of energetic offsets within heterostructures (Figure 1).

3. MATERIALS SYNTHESIS AND THERMODYNAMICS OF CHARGE SEPARATION In designing the photocatalysts, we envisioned a mechanism wherein light harvesting by II−VI QDs is followed by ultrafast charge separation: photoexcited electrons are extracted by a hydrogen-evolution catalyst and used to reduce protons in aqueous media, whereas photogenerated holes are transferred to mid-gap states of MxV2O5 nanowires for transport to a water-oxidation catalyst. Figure 3 illustrates two distinctive methods to prepare NW/QD heterostructures: (1) linkerassisted assembly (LAA),22 wherein bifunctional ligands tether colloidal QDs to NWs with binding affinities explicable based on principles of hard−soft acidbase interactions and differences in surface potentials, and (2) successive ionic layer adsorption and reaction (SILAR),23 wherein NWs are immersed sequentially into solutions of ionic precursors of cadmium and chalcogenide. LAA allows for well-defined preformed QDs to be tethered to NWs. Controlling QD size and composition enables tunability of light harvesting, whereas the ability to modulate interfacial separation with properties of the molecular linker provides control over the dynamics of hole and electron transfer.22 In turn, SILAR is facile and yields QDs directly bonded to NWs with size and coverage tunable by the number of SILAR cycles. Figure 3B illustrates scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of β-Pb0.33V2O5/CdSe heterostructures (additional TEM images are shown in Figure S3). HAXPES measurements allow for delineation of energetic offsets (Figure 3C). Photogenerated holes in II−VI QDs relax rapidly to valenceband edges, which are comprised primarily of chalcogenide pstates. Concordant with Fajan’s rules, mixing of cationic and anionic states increases down a group, reflecting the greater covalency of heavier chalcogenide lattices.24 Thus, the energy positioning of p-states that constitute the valence band follows periodic trends in electronegativity: S > Se > Te.25 The deeper valence-band edges of CdS are better situated to facilitate hole transfer to mid-gap states of β-PbxV2O5 NWs and indeed represent our most successful first-generation heterostructures (Figure 3D). However, owing to its larger band gap, CdS is inefficient at harvesting the solar spectrum;10 furthermore, there is a 0.48 eV barrier to hole transfer from the valence band edge to mid-gap states of β-PbxV2O5 NWs. Based on the considerations outlined above and the expected energy positioning of 5s2 states relative to 6s2 states (Figure 2), βSnxV2O5/QD heterostructures were anticipated to resolve both E


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Figure 4. Interfacial charge-transfer kinetics and redox photocatalysis. (A) Competing desirable (blue, A−E) and undesired (red, 1−5) deactivation, charge-transfer, and charge-recombination processes. (B) Nanosecond TA spectra of bare NWs and heterostructures. (C,D) TA 3D color maps and (E,F) spectra for SILAR-derived (left) and LAA-derived (right) β-PbxV2O5/CdSe heterostructures. (G) TA kinetic traces and fits at 650 nm for LAA-derived β-PbxV2O5/cys-CdSe (red) and β-PbxV2O5/hcys-CdSe (blue). (H) Linear sweep voltammograms of β-PbxV2O5/CdSe and α-V2O5/CdSe-modified FTO electrodes under chopped illumination (insets: corresponding bare NWs). Panels (C)−(F) are adapted with permission from ref 9. Copyright 2016 American Chemical Society.

modulation of the QD lattice, and the specific p-block cation selected for the NW component.

issues, allowing for thermodynamically favorable hole transfer from the valence band edges of CdS and CdSe QDs (Figure 3D) and further enabling utilization of smaller-band gap CdTe QDs that harvest visible light more efficiently. Such heterostructures have been recently brought to fruition and show promising photocatalytic performance as discussed below.14 Quantum confinement, which primarily modifies conduction-band edges of QDs26 but has relatively little influence on valence-band edges, provides an additional lever to control driving forces for electron transfer to a reduction catalyst. In summary, thermodynamic driving forces for electron and hole transfer are amenable to tuning in our heterostructures through variation of QD size, compositional

4. CHARGE-TRANSFER KINETICS AND REDOX PHOTOCATALYSIS The photocatalytic performance of NW/QD heterostructures ultimately depends on the rates of desired excited-state charge separation and subsequent dark redox processes (A−E in Figure 4A) relative to those of competing pathways (1−5 in Figure 4A). Thus, predicting and measuring charge-transfer kinetics is essential and complementary to the aforementioned efforts to control interfacial thermodynamics. We have used transient absorption spectroscopy, on time scales from 10−13 to 10−5 s, to characterize excited-state charge-transfer within F


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Figure 5. Second-generation heterostructures. (A) Topochemical synthesis of metastable p-block MxV2O5 NWs and subsequent integration with QDs. (B) SEM and (C) false-colored TEM images of β-SnxV2O5/CdS heterostructures. (D) TA 3D color map of β-SnxV2O5/CdS heterostructures upon 360 nm excitation.

NW/QD heterostructures,9,10 as well as photoelectrochemical and photochemical measurements to evaluate the extent to which charge separation leads to productive redox chemistry. We focus here on our first-generation heterostructures. Transient absorbance (TA) spectra of β-Pb0.33V2O5 NWs exhibit two induced absorption bands with maxima at approximately 550 and 750 nm (Figure 4B), which arise from holes in mid-gap states and electrons in the conduction band, respectively.9 Both absorption bands are enhanced in TA spectra of Pb0.33V2O5/QD heterostructures relative to those of bare NWs.9,10 On time scales greater than 10−10 s, TA spectra of heterostructures exhibit no features attributable to excitonic excited states of QDs, even though excitonic lifetimes of dispersed QDs are on the order of 10−7 s (Figure 4B).9,10,27−29 These data reveal that photoexcitation of QDs is followed by the desired transfer of holes to mid-gap states of NWs, as well as the transfer of electrons to the conduction band of NWs (the latter being specific to binary heterostructures in the absence of electron acceptors). Electronhole recombination

is slowed dramatically within NW/QD heterostructures relative to bare β-Pb0.33V2O5−NWs, suggesting that the deposition of QDs may hinder trap-state-mediated recombination.9 Figure S4 plots the averaged TA spectra of α-V2O5 and ζ-V2O5, respectively, illustrating the difference in band gap. Polymorphs of V2O5 interfaced with QDs exhibit type-II band alignment; interfacial charge separation brings about hole accumulation on the QD surfaces in the absence of mid-gap states capable of mediating hole extraction. Ultrafast TA spectra of Pb0.33V2O5/QD heterostructures reveal differences in electron- and hole-transfer dynamics and subtle dependences on interfacial structure (SILAR versus LAA).9,10 Photoinduced hole transfer, from CdS and CdSe QDs to mid-gap states of β-Pb0.33V2O5 NWs, occurs on subpicosecond time scales. In contrast, a rapid but time-resolvable growth of the lower-energy tail of the hole-associated induced absorption, at wavelengths coincident with the first-excitonic bleaches of CdS and CdSe QDs, reveals that electron transfer from QDs to NWs (10−1210−11 s) occurs much more slowly G


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Accounts of Chemical Research than hole transfer (

The Middle Road Less Taken: Electronic-Structure-Inspired Design of

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