Solar Fuel Photocatalysis at Semiconductors and the Challenge of Improving Light Harvesting
Direct solar water splitting is in principle one of the most sustainable pathways to produce clean fuels such as hydrogen () from cheap and abundant resources; however, the paucity of photocatalysts that efficiently drive water splitting under solar irradiation presently limits its viability as a route to renewable fuels. Functional photocatalysts must be cheap, stable in water, and able to perform the three fundamental functions in the photocatalytic process: (1) absorb solar radiation to produce reactive charge carriers (i.e., electron–hole pairs); (2) efficiently transfer charge carriers to reactive sites at the semiconductor surface while minimizing electron–hole recombination; and (3) drive chemical reactions of interest with high selectivity and yield of the desired product. Semiconductor photocatalysts with band edges satisfying the thermodynamic requirements for both water oxidation and water reduction, such as titania (),1 are limited by poor sunlight harvesting as a consequence of their large optical bandgap. Narrower bandgap semiconductors that absorb in the visible region, such as and , do not photogenerate the required thermodynamic potentials for water splitting.23.–4
Multiple strategies have been investigated to improve light utilization by wide bandgap semiconducting photocatalysts, with the most common including doping to shift band potentials, and sensitization with plasmonic metal nanoparticles, quantum dots, or dyes.56.–7 Alternatively, the physical arrangement of the semiconductor can be controlled at the nanometer-to-micrometer scale to improve light harvesting. In the past decade, photonic crystals (PCs) of various morphologies have been investigated as a means of enhancing light utilization in semiconductor-based photocatalysts. We discuss below the recent trends in photocatalyst design that exploit the unique optical properties of PCs to improve sunlight utilization and highlight examples of how these photocatalyst structures are being used to enhance photochemical splitting of water.
Slow Photons and Photonic Crystals
To clarify how PCs can enhance photocatalytic processes, including generation, some background information on PCs and how they interact with light is necessary. Whether in pure or sensitized photocatalyst materials, the phenomenon characteristic of PCs that has been most commonly exploited to enhance light absorption in semiconductor photocatalysts is the “slow photon effect”—a slowing of light propagation in photonic materials at wavelengths near the photonic bandgap. PCs are ordered, periodic structures with alternating regions of high and low refractive index that satisfy Bragg diffraction conditions.89.10.–11 A “forbidden band” of radiation with wavelengths on the same order as the periodicity of the PC structure cannot propagate through the PC, leading to a strong reflectance. The center of this band of radiation is known as the stop band or photonic bandgap. At the high- (blue) and low-energy (red) regions of the stop band, photons are multiply scattered in the lower refractive index and higher refractive index portion of the PC, respectively (Fig. 1),12 thus effectively lowering the group velocity of light at the edges of the stop band.13 The propagation of slow photons increases their optical path length within the semiconductor or sensitizer and enhances the probability of generating charge carriers.
Although more exotic structures exist, inverse opals (IOs) are the most common PC structure utilized in photocatalysis. These structures are typically prepared by templating monodisperse polystyrene spheres in an opal arrangement and infiltrating the voids of the face-centered cubic (fcc) array with oxide precursors before removing the template by calcination12,1415.16.17.–18 (Fig. 2). The photonic bandgap () for the IO films obeys a modified version of Bragg’s law15 The exact frequency of the photonic bandgap is highly sensitive to the refractive indices of the solid and voids and will redshift as the refractive index of the pore-filling fluid increases (Fig. 3). The critical spatial dimension, , is controlled directly during IO synthesis; the stop band is synthetically tuned as a function of the spherical template diameter (Fig. 4).19
Coupling Photonic Crystals to Photocatalysis—Tuning the Red Edge of the Stop Band to the Photocatalyst Absorption Maximum
Tuning the Photonic Crystal Stop Band Around the Semiconductor Band Edge
Precise tuning of the stop band is necessary to exploit the slow photon effect for enhanced photocatalysis. Findings by Chen et al.2021.–22 established the ground rules for slow photon-enhanced photocatalysis. They applied anatase IO films with stop bands centered at wavelengths between 280 and 500 nm to the photocatalytic degradation of methylene blue under monochromatic UV radiation at .20 The most photocatalytically active anatase PCs featured stop bands at 345 nm–not 370 nm—because it is at the red edge of the stop band that light is most substantially slowed in the high index material. Monochromatic light of wavelengths matched to the stop band does not propagate in the PC. Anatase IO films with stop bands at 345 nm had a low-energy edge coincident with the 370-nm illumination (near the anatase band edge), enabling them to enhance degradation rates by a factor of 1.9. PC films with stop bands at 370 nm rejected the incident radiation and were less catalytically active than nonphotonic nanocrystalline controls. As predicted by Eq. (1), the positions of the PC film stop bands were sensitive to the incident angle of radiation; tilt angles of 10 deg, 20 deg, and 45 deg substantially shifted the stop bands of all films, changing the films that were most photocatalytically active.
When white-light was used to irradiate the films, the order of the reactivity for the series of stop bands changed: PCs with stop bands at 300 nm exhibited the most photocatalytic activity enhancement (with an enhancement factor of 2.3 relative nanocrystalline ). Under polychromatic radiation, the effects of reflection compete with the slow light effect at PCs. The wavelengths at which reflection occurs comprise a larger portion of the stop band than those wavelengths at the edges of the stop band, at which the slow photon effects dominate. Thus, positioning the stop band directly at the absorption edge can result in diminished photocatalytic activity under broad spectrum illumination.20 However, absorption by the dielectric material strongly diminishes photonic effects.23 Thus, positioning the stop band at energies higher (shorter wavelengths) than the band edge enhances band-edge excitation and at the same time minimizes reflective losses occurring at wavelengths between the stop band and its high-energy edge.
Tuning the Photonic Crystal Stop Band Around Sensitizer Excitation Maxima
The rules for matching the red edge of the stop band to excitation energy apply to sensitized PCs as well, whether they are sensitized with dyes, quantum dots, plasmonic metal nanoparticles, or atomic dopants; slow photon edges—rather than the stop band itself—should be matched to absorption of the sensitizer, with care taken to avoid reflective losses from the stop band. For example, slow photon effects when carefully matched to the 550-nm absorption of -doped PCs enhanced the broadband visible light photocatalytic activity for methylene blue degradation by about a factor of 2 compared to the equivalently -doped nonphotonic counterpart; doped PCs with more poorly matched stop bands exhibited much more modest enhancement.24
Curti et al.25 prescribe some standard experimental guidelines that will help clarify the extent to which the slow photon effect is responsible for changes in photocatalytic activity. They recommend that the photocatalytic performance of IOs be directly compared to disordered structures of the same material with comparable surface area and porosity to isolate the effects of surface area. Broadband excitation should be avoided to allow one to discriminate between photogenerated carriers originating from slow photons and those originating from normal bandgap excitation. Under these controlled experimental conditions, one can quantify the level of photocatalytic enhancement attributed to the slow photon effect and utilize the ground rules established in the seminal work by Chen et al.2021.–22 and others19,26 to engineer the stop band position for maximum photocatalytic activity.
Hydrogen Generation Photocatalysis at Photonic Crystals
Using the design principles described above, many researchers have exploited the slow photon effect to enhance generation by photochemical or photoelectrochemical water splitting in photocatalysts including ,2728.29.–30 ,19 ,31 ,32 and 18,33 (results and references are summarized in Table 1). At Pt-modified PCs,27 the photocatalytic efficiency of generation as a function of the stop band position obeyed the same trends as those observed for dye degradation reactions by Chen et al.20 The largest enhancement factor under broadband excitation of 2.5 occurred when the stop band was tuned to 342 nm—where the red edge overlapped the band edge.27
Diverse examples of photonic enhancement of photocatalytic and photoelectrochemical water splitting/H2 generation reactions. Note that conditions that may impact the enhancement factor, such as excitation conditions, use of hole scavengers in reaction slurry/electrolyte, and photoanode bias in photoelectrochemical measurements, are omitted for the sake of brevity.
|Material/morphology||Reaction||Enhancement factor||Comment||Reference number|
|IO||generation in methanol:water||2.5||Slow photon enhancement of generation photocatalysis||27|
|and IO||Photoelectrochemical water splitting||2.07 without Au and 2.86 with Au||Slow photon enhancement and plasmonic enhancement||28|
|IO||generation in formate:water||per surface area under UV excitation||Enhancement by IOs only observed at select excitation wavelengths; surface-area-normalized enhancement only marginally better than a disordered control||29|
|-doped NT array with top PC layer||Photoelectrochemical water splitting||10 under visible light||Combination of slow photon enhancement and visible light sensitization with dopant||30|
|nanoparticles deposited on graphene IO||Photoelectrochemical water splitting||1.4||Reduced electron–hole recombination and fast electron transfer||31|
|IO||Photoelectrochemical water splitting||near excitation wavelength of activity maximum; at low-activity wavelengths||Extensive angle-dependent study of slow photon enhancement; enhancement due to IO structure more pronounced at wavelengths where is initially less active||19|
|IO||Photoelectrochemical water splitting||3||Slow photon effects and reduced bulk recombination||32|
|and Au-modified IO||Photoelectrochemical water splitting||8 over entire potential range; additional 55% increase with Au nanoparticles||Combination of slow photon enhancement, increased surface area and improved charge transfer, and SPR enhancement||33|
|IO||Photoelectrochemical water splitting||3 to 7, depending on electrochemical potential||Combination of cation-substituted composite oxide chemistry and PC-enhanced photocatalysis; control size of mesopores in addition to size of macropores||18|
|C quantum IO||Photoelectrochemical water splitting||6||Combination of slow photon effects and visible light sensitization from C QDs||34|
|IO||generation in methanol:water||4.7||CdS activity enhanced at blue edge of stop band of passive support||35|
|Au on NTs array with PC top layer||Photoelectrochemical water splitting||2.5 compared to sample with no PC top layer||Maximum enhancement when Au SPR matched to PC photonic bandgap||36|
|IO||Photoelectrochemical water splitting||2||Coupling of plasmonic sensitization with slow-photon effects||37|
|templated from butterfly wing||Photoelectrochemical water splitting||2.3||Enhancement of photocatalysis by aperiodic structure (unclear if enhancement is surface area or photonic effects)||38|
|N-doped templated from leaf replica||Photocatalytic water generation||3.3 to 8||Demonstration of viability of structural approach (unclear if enhancement due to surface area or photonic effects)||39|
While narrower bandgap alternatives to for photocatalytic water splitting23.–4 may absorb more visible light, none features 100% photon-to-carrier conversion efficiency, and thus all can still benefit from improved light utilization. Alternatives to also suffer from drawbacks involving other parts of the photocatalytic process, such as valence/conduction band energies that are inefficient for water-splitting chemistry, and from fast electron–hole recombination kinetics. In particular, and suffer from high bulk recombination rates and thus benefit from the shortened carrier path lengths to the semiconductor surface featured in the IO architecture.18,3233.–34 Nan et al.34 show particularly strong enhancement factors of both surface-normalized photocurrents and photocatalytic oxygen and generation at the IO form of (Fig. 5). While the red edge of the stop band in their IO structures () overlaps well with the band edge of , their observed enhancement is probably due to more than the slow light effect alone. The simultaneous improvement of light management and electron–hole separation with IO forms of and enhances photoelectrochemical water splitting relative to non-PC forms of the semiconductors to a significantly greater extent than that achieved when expressing in PC form (about to for and compared to about to for , Table 1). Note that while the improvement is more substantial in the PC forms of and , the overall photoefficiencies are still typically lower than that achievable with .
Zhang et al.33 compared the photoelectrochemical water-splitting activity of IO and planar photoanodes. When the stop band was tuned to 513 nm, near the electronic bandgap of the oxide, photoelectrochemical activity was enhanced by about . Photoelectrochemical activity was still enhanced—but to a much lesser extent—when the stop band was tuned to a frequency at which slow photons did not overlap the electronic bandgap excitation of . The enhanced activity in the absence of the slow photon effect is attributed to improved charge separation imparted by the PC architectures. Zhou et al.18 used transient photocurrent decay to measure the extent to which IO architectures enhanced charge migration during photoelectrochemical water splitting in PCs. Carrier lifetimes were extended in photoanodes with a macroporous IO structure compared to those observed in disordered films. The longest lifetimes were measured in IO structures containing both macro and mesoporosity; photoelectrochemical water-splitting activity was enhanced by roughly to in such films.
Mitchell et al.35 applied a somewhat different approach to PC-enhanced photocatalysis, in which CdS nanocrystals (NCs) supported in a photocatalytically inactive zirconium oxide () PC performed photocatalysis directly, instead of acting as a sensitizer. They observed a maximum enhancement for generation under visible light illumination relative to nonphotonic at the “blue edge” of the PC stop band, suggesting that the supported CdS nanoparticles exploit slow photons in the lower-index voids in the PC—in contrast to the case in which the PC support itself is photocatalytic. Their results—combined with the myriad results that demonstrate the opposite case in which the PC itself acts as the photocatalyst material and enhancement of photocatalysis occurs via localization of light coincident with the red edge of the stop band in the PC—convincingly demonstrate that the localization of the different band edges in PC-derived photocatalysts can be effectively tailored to application via materials design.
Combining Photonic Crystal Architectures with Plasmonics
Plasmonic metal nanoparticles, in particular Au and Ag, have recently received substantial attention as sensitizers for solar water-splitting catalysts due to their chemical stability and surface plasmon–resonant frequencies that overlap the solar spectrum.4041.42.–43 Whereas PCs improve carrier generation at or near the electronic band edge, plasmonic sensitizers can extend the useful range of photocatalysts much farther into the red (i.e., to lower energy, longer-wavelength radiation). Properly applied, the combination of PCs and plasmonic sensitizers enhances photochemical/photoelectrochemical activity to a greater extent than either strategy alone.33,36,37,44,45 For example, Zhang et al.33 measured a roughly 26% enhancement in photoelectrochemical water splitting when they modified planar films with plasmonic Au nanoparticles, but measured nearly 55% enhancement when they modified PC with the same plasmonic Au nanoparticles. When plasmonic Au was present, the photocurrent response was extended beyond the electronic bandgap of at 530 to 560 nm.
Analogous to other cases of combining photonics with sensitizers, the optimum enhancements from PCs + plasmonic sensitizers are achieved when the slow photon regions are positioned to overlap the surface plasmon resonance (SPR) frequency maximum while simultaneously avoiding reflective losses in electronic bandgap excitation.36,37,44,45 A large increase in optical extinction occurs when the slow photon region of a PC is well-matched to the SPR.36,37 Zhang et al.36 coupled PC structures to nanotube (NT) arrays, creating NT/PC (NTPC) assemblies and subsequently deposited SPR-active Au NCs of different sizes onto the NTPCs. The intensity of the SPR response of Au NCs with SPR maxima well-coupled to the PC photonic bandgap was more strongly enhanced than those with more poorly matched SPR maxima (Fig. 6). The SPR-driven photoelectrochemical response (Fig. 7) and generation were similarly more strongly enhanced at the well-matched NTPCs. Zhang et al.37 achieved a maximum enhancement in photoelectrochemical water splitting when the red edge of the stop band of PC was positioned at 518 nm, close to the SPR frequency maximum of the Au particles, but at a position where the photonic stop band (at 450 nm) would not interfere with bandgap excitation of the . When the Bragg reflection of the PC overlapped the Au SPR, the plasmonic enhancement was significantly suppressed.
Butterflies and Leaves: Alternate, Bioinspired, Photonic Crystal Geometries for Photocatalysis
Nature provides examples of structures that have unique interactions with light—whether they are naturally occurring PCs or disordered structures that multiply scatter light—that have inspired structural photocatalyst mimics. Butterfly wings38,46,47 and artificial leaves39 have both recently been used as structural templates for photocatalysts. Butterfly wing-templated materials that feature regular, bicontinuous, gyroid structures with clear photonic bandgaps have been fabricated.46
Although not a PC, the Papilio helenus Linnaeus butterfly wing contains structural elements that improve light utilization, including honeycomb-like hole arrays that multiply scatter light, “ridges” that guide light into the hole array, and a bottom reflective layer that recaptures light escaping from the honeycomb arrays. Platinum-modified replicas of P. helenus Linnaeus butterfly wings can function as -generating photocatalysts.38 The wing-pattered photocatalyst strongly enhances ultrabandgap light absorption and shows a improvement in generation relative to nanoparticulate , although it was unclear how much of this enhancement could be attributed solely to surface area. Demonstrating the hierarchical possibilities of biostructure-derived photocatalytic architectures, Yan and coworkers47 deposited plasmonic gold nanorods onto bismuth vanadate (BVO) structures templated from wings of the butterfly Paplio nirius (Fig. 8) and achieved substantial photocatalytic activity at red and near-infrared light illumination. Zhou et al.39 replicated the hierarchical structure of Anemone vitifolia Buch leaves, which contain various components that serve to focus, scatter or guide light and enhance light–matter interactions. Using a two-step infiltration process, they produced an N-doped inorganic replica of the leaf structure that showed a roughly to enhancement in generation compared to nanoparticulate ; the activity was enhanced by another order of magnitude when modified with Pt nanoparticles.
Conclusions and Comments
Exploitation of the slow photon effect of photonic bandgap-containing materials requires rigorous matching of the red edge of the stop band with the desired excitation wavelength while avoiding absorption losses due to nonpropagation of light at the stop band. When PCs are correctly applied, photocatalytic activity enhancement factors of about 2 to 3 can be readily achieved, and in some cases enhancement of or more may be possible. It is also possible to achieve light-utilization enhancement via the more disorganized process of internal light scattering, which is less demanding in structural precision, but also more difficult to design in a rational way.
A couple of important caveats should be mentioned. First, application of the slow photon effect to photocatalysis is illumination angle dependent, and as such can only be effectively applied in film geometries in which the illumination angle can be well-controlled. Practical photocatalysts are often used as fluid-dispersed (suspended) powders to maximize both mass transport and light utilization. The reactor geometry limitation inherent to PCs (i.e., a thin-film form-factor) means that careful consideration must be given to mass transport and light harvesting when designing the interfaces. Additionally, solar fuels photocatalysis involves many challenging photophysical and photochemical steps. Light harvesting is critical, but even with substantial improvements in light harvesting, many photocatalyst materials are still 2 to 3 orders of magnitude (or more)—rather than a factor of 2 to 3—from practical viability. One important class of solar fuels photocatalytic reactions omitted from this section involve reduction of carbon dioxide () to hydrocarbons as its final step, rather than protons to . Photochemical and electrochemical reductions of comprise an enormous research challenge in its own right. Improvements on all photocatalytic fronts, however, continue apace; a number of composite solar fuels photocatalyst materials are approaching activity competitive with alternative routes to solar fuels and photonics could conceivably contribute enough additional efficiency to make them viable.
The authors would like to thank the Office of Naval Research for its support of this work. We also would like to thank Debra Rolison of the U.S. Naval Research Laboratory for helpful comments on this paper.
Jeremy J. Pietron received his PhD in analytical chemistry from the University of North Carolina, Chapel Hill, in 1998. After completing a National Research Council Postdoctoral Fellowship at the U.S. Naval Research Laboratory (NRL), Washington, DC, from 1999 to 2002, he joined the laboratory as a full-time staff scientist in 2003. His primary research interests are nanostructured materials, electrocatalysis, plasmonic nanomaterials, heterogeneous catalysis, and solar fuels photocatalysis.
Paul A. DeSario received his PhD in environmental engineering from Northwestern University in 2011. He joined the Surface Chemistry Branch of the U.S. NRL, Washington, DC, as a National Research Council Postdoctoral Fellow from 2011 to 2014 before joining the laboratory as a full-time staff scientist in 2015. His primary research interests are in the nanoscale design of catalytic materials for applications in energy conversion and remediation.