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Biomimicry designs for photoelectrochemical systems: Strategies to improve light delivery efficiency
Journal Pre-proof Biomimicry designs for photoelectrochemical systems: strategies to improve light delivery efficiency Enric Brillas, Albert Serrà, Sergi Garcia-Segura PII:
S2451-9103(20)30213-1
DOI:
https://doi.org/10.1016/j.coelec.2020.100660
Reference:
COELEC 100660
To appear in:
Current Opinion in Electrochemistry
Received Date: 2 October 2020 Revised Date:
15 November 2020
Accepted Date: 17 November 2020
Please cite this article as: Brillas E, Serrà A, Garcia-Segura S, Biomimicry designs for photoelectrochemical systems: strategies to improve light delivery efficiency, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2020.100660. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.
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Biomimicry designs for photoelectrochemical systems:
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strategies to improve light delivery efficiency
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Enric Brillasa,*, Albert Serràb,**, Sergi Garcia-Segurac,**
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a
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Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
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b
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Materials and Nanostructures, Feuerwerkerstrasse 39, CH-3602, Thun, Switzerland
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c
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Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona
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Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Mechanics of
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Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment. School of
85287, United States
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Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física,
Article submitted to be published in Current Opinion in Electrochemistry
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*Corresponding author: Enric Brillas ([email protected])
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**Albert Serrà ([email protected])
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***Sergi Garcia-Segura ([email protected])
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Abstract
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In the last years, photoelectrocatalysis has been developed to offer green and sustainable application
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to wastewater remediation, water disinfection, H2 production and CO2 reduction. The advances of
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these systems with new semiconductor photoelectrocatalysts that are photoexcitated with visible
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light giving photogenerated charge carriers efficiently separated in a photoelectrochemical cell, are
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explained. These studies do not consider the light transport as fundamental aspect of light-matter
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interaction, although the harvesting of photons at the semiconductor surface limits the quantum
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yield by the existing semiconductor architectures. This opinion review envisages biomimicry as an
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alternative natural guide for synthesizing more efficient photoelectrodes. Micro- and nanostructure
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shapes
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photoelectrochemical reactors with the light transport as indispensable element. The
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implementation of strategies of phototroph organisms to maximize light adsorption and the
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enhancement of photoelectrocatalytic surface area are analyzed as key factors for such bio-based
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photoelectrodes.
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Keywords
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Electrochemical water treatment; water splitting; CO2 reduction; bio-based structures; bio-inspired
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materials; photocatalysis; semiconductors
nature
are
identified
to
prepare
new
bio-inspired
photoelectrodes
for
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Photoelectrocatalytic technologies for environmental applications
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Photoelectrocatalytic (PEC) processes benefit from the synergistic hybridization of photocatalysis
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and electrochemistry. The photocatalytic effect was initially reported in the seminal work of
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Fujishima and Honda in 1972 [1] that quickly resulted in a highly productive line of research with
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thousands of scientific articles published annually. Photocatalytic processes exploit the intrinsic
37
properties of semiconductor materials and their interactive response with photons. The absorption
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of a photon of energy equal or higher to the semiconductor band gap energy (Eg) promotes the
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ejection of an electron from the full semiconductor’s valence band (VB) to its empty conductive
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band (CB) [2-4]. The photo-promotion of an electron to the CB (e−CB) results in the formation of a
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positively vacancy or hole in the VB (h+VB) following reaction (1). Photogenerated e−CB and h+VB
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are also known as charge carriers since are responsible of the electrical conductivity of
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semiconductor materials. Both charge carrier species are highly reactive and can enable different
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reactions of environmental interest. However, the charge separation on the semiconductor surface is
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representative of an excited and unstable state that tends to decrease the high energy state of the
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e−CB through the quick recombination of the photogenerated e−CB/h+VB pair by reaction (2) [5-7].
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Semiconductor + hν
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e−CB + h+VB
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The low quantum yield of photocatalytic processes is indeed explained by the recombination of
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charge carriers by reaction (2). Here, it is where electrochemical processes play a key role to ensure
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the stabilization of the highly reactive e−CB/h+VB pair on the semiconductor surface [8-10]. The use
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of semiconductor materials as photoelectrodes holds the promise of stabilizing charge carriers by
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inducing their charge separation through the application of an external electric field (i.e., defined
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bias potential or bias applied current) [11,12]. This positive effect is observed by the drastic
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increase in PEC performance when compared to electrocatalysis or photocatalysis as individual
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e−CB + h+VB
Semiconductor + heat
(1) (2)
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processes [13,14]. The high efficiency of PEC systems and the outstanding reactivity of
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photogenerated charge carriers on semiconductor surfaces as heterogeneous catalysts have
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delineated different research avenues in the context of sustainable development that are summarized
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in Figure 1.
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First, PEC has shown promise as water treatment technology for pollutant removal and disinfection.
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The charge carrier h+VB generates the strong oxidant hydroxyl radical (•OH) from water oxidation
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among other oxygen reactive species [15-18]. The oxidants produced degrade organic pollutants till
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their complete incineration to CO2 and enable bacteria inactivation mechanisms [19-24]. The low
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cost of these processes can set us a step closer to clean water access for all.
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Second, in the water-energy nexus PEC has demonstrated its relevance to sustain a green and
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sustainable hydrogen economy [25,26]. PEC offers a simpler procedure for H2 sysnthesis from
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water splitting as indirect solar energy harvesting tool [27-34]. Photogenerated charge carriers are
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responsible of cathodic reduction of H2 at the cathode of a divided electrochemical cell. The high
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efficiency and low energy cost of this process ensures its applicability as a more cost-effective
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alternative route for H2 production.
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Third, as resource recovery and climate change action. PEC processes can achieve CO2 reduction
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into added value products such as formic acid, formaldehyde and methanol [35,36]. This process is
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feasible in the cathode compartment of a divided cell where injected CO2 is reduced at a n-
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semiconductor photocathode or by coupling PEC with a biocatalyst [37-42]. Capture and
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transformation of CO2 can contribute to diminish the environmental impact of anthropogenic
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emissions of CO2 caused from the use of fossil fuels and deforestation. That also includes different
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existing opportunities for electrosynthesis of different products such as ammonia from nitrate
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reduction [43,44].
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The neglected relevance of light delivery in photoelectrochemical systems
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PEC processes are still at their infancy. Research it is being conducted at fundamental level and at
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low technology readiness levels. Despite of the undeniable inherent differences between the
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ultimate goals of PEC technology applications summarized in Figure 1, these different challenges
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have been approached with astonishingly similar research questions. Main efforts have been
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devoted to (i) develop understanding on fundamental PEC capabilities, (ii) elucidate reaction
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mechanisms, and (iii) achieve breakthroughs on the development of new photoelectrocatalytic
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materials.
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Titanium dioxide (TiO2) has been the gold-standard material in photocatalytic processes, with an
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outstanding record of over 26,000 articles published on photocatalytic applications till 2020. The
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major hindrances of TiO2 photocatalytic processes that has been discussed for decades in the
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literature is the wide band gap of 3.2 eV that requires UV irradiation for TiO2 photocatalytic
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activation [45-48]. This discussion was based on the reliance of energy intensive Hg-lamps as UV
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light source. These inefficient light sources were identified as the main cost driver for PC and PEC
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applications, but without considering life cycle assessments and the environmental impact of the use
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of mercury as hazardous chemical. Techno-economic discussions define that the inflection point for
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PEC competitiveness depends on the development of sunlight active photo(electro)catalysts and
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centered major research efforts in the material discovery. This critical thought made scientist
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worldwide set sails in a race for the searching of the holy grail of visible light-active photocatalysts.
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Thousands of papers exploring different approaches with a trial-error strategy have resulted from
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that effort, in which most of the times, every material reported is excellent but not benchmarked
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against any other catalyst in terms of performance or long-term stability (meaning more than 5
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consecutive cycles reusing the material). A large variety of semiconductor photocatalysts have been
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prepared and checked , including halide perovskite-based materials [49,50], CdS QD-sensitized
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BiOI/WO3 [51], hybrid cocatalysts [52], metal-organic frameworks [53,54], Cu2O [55,56], iron-
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based materials [57,58], inorganic graphitic, graphene and related compounds [59-64], MoS2 [65]
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and g-C3N4-based materials [66,67]. Being honest, despite of its relevance, this scientific race may
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have blinded our sight and distracted all of us from other relevant aspects that should be considered
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for technology translation.
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The recent addition of light-emitting diodes to the light-driven technologies toolbox has brought
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monochromatic, alternative non-energy intensive, and mercury-free light sources into play.
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Therefore, in an exercise of honesty we should agree upon the fact that photon source may not be a
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major hindrance after all for certain applications such as water treatment or CO2 reduction. Until
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reaching to that conclusion, we may have focused too much on the semiconductor material
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composition (e.g., doping, nanodecoration, composites, etc.) and completely neglected the other
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relevant aspect of PEC systems: the light delivery. Evaluation of PEC efficiency entails the
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effective use of photons delivered from the light source. Light transport is of major relevance for
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competitive operation of PEC technologies of any kind. In fact, at this point in the game we realize
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that we have not designed light transport frameworks that can be deployed or scaled-up in
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photoelectrochemical reactors.
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Biomimicry strategies to improve light delivery in photoelectrochemical systems
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Biomimicry identifies strategies in nature to solve problems in human design. Because organisms
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have developed highly evolved structures while surviving eons as the most competitive specimens
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following natural selection, already refined knowledge from observing and emulating nature’s
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optimal shapes and ecosystems can inspire the smart design of more effective materials [68-71]. As
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such, biomimicry will play a major role in developing optimized semiconductor architectures and at
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PEC systems level for improved light harvesting and use [72].
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In particular, the synthesis and design of photoelectrodes should seek smart sophisticated
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architectures and strategies among insects, plants, algae, and photosynthetic bacteria. These
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organisms’ anatomical features can reduce reflection or maximize cooperative light-harvesting
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mechanisms and exploit unique nanoscale effects to improve selectivity towards target species in
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low concentration in water matrices [72-76]. In the process, the biomimetic design of innovative
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materials and systems for improved light harvesting should be based on developing antireflective
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surfaces (Figure 2a) in front of hierarchical micro- and nanoscale architectures (Figure 2b) and/or
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macrostructures (Figure 2c) to facilitate light trapping [72].
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Many insects present optimized optical structures to improve their ability to attract mates and
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camouflage themselves. Such intricate surface architectures (e.g., moth eyes and butterfly wings)
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are based on a subwavelength diffraction grating mechanism, which results in minimum light
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reflection (Figure 2a). In the past two decades, significant efforts have been made to fabricate
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antireflective coatings based on the integration of tapered shapes such as cones, pillars and columns
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(Figure 3a), primarily inspired by moth eyes and butterfly wings [73,74]. In fact, those antireflective
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coatings are commonly called “moth-eye structures”. For example, the antireflective properties of
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month-eye TiO2 nanostructured films improved light-harvesting efficiency in perovskite solar cells
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– the power conversion efficiency was improved by approximately 11% (Figure 3a) [76].
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Plants and other photosynthetic organisms evolved to maximize their energy production via
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photosynthesis process. These organisms mastered the secrets of highly efficient photon harvesting.
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PEC is a heterogeneous catalytic process, which means that available surface is crucial for any final
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application of PEC technologies (Figure 1). However, available surface it is not only relevant in
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terms of reactive sites but also is required for photon absorption. The depth penetration of photons
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interacting with a semiconductor surface is defined in function of 1/α [75,76], where α is the
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absorption coefficient of the semiconductor at a given wavelength. The maximum depletion layer
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defines the light penetration, which have already justified the relevance of nanostructured materials
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for enhanced photoexcitation performance defined by the quantum yield. Thus, it is not a matter of
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enhancing JM available surface area but more of shaping architectures that enhance photons capture.
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This was observed in a recent work that explored different synthetic approaches of ZnO
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nanorods.[77]. Disorganized structures with crisscrossed nanorods decreased photoelectrode surface
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efficiently exposed to irradiation, thus diminishing PEC performance and quantum yield when
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compared to hierarchically organized structures. Flexible, hierarchical, fractal micro- and nanoscale
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architectures as well as efficient trapping mechanisms have been partly optimized, but biomimicry
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can translate these designs to a higher level of performance.
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Bio-inspired designs based on natural hierarchical architectures (e.g., fern leaves, green leaves, and
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cacti) exhibit excellent multilevel light scattering, long electron-diffusion, open and accessible
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pores, high surface-to-volume ratios, and high light absorption independent of the incident light
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angle. For example, bioinspired ZnO microferns (Figure 3b) [78] exhibit improved light-harvesting
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properties independent of that angle compared to ZnO nanorods or nanowires, while the porous
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morphology of N-doped ZnO photocatalysts (Figure 3c) [79], inspired on green leaves, increases
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light absorption up to 131%. From that perspective, complex bio-inspired architectures demonstrate
166
to be more efficient and smart designs for harvesting light.
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Another common means to improve light harvesting is mimicking the phototropic mechanism of
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sunflowers (i.e., self-orientation toward the sun throughout the day). Following that strategy,
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human-made omnidirectional trackers have exhibited improvement in solar energy-harvesting up to
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400% compared with non-phototropic materials (Figure 4) [80].
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nanostructured stimuli-responsive polymers that are able to align to the incident light direction in
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the three-dimensions. This strategy can be extended to many responsive materials and stimuli.
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Conclusions and key insights
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Photoelectrocatalytic technologies provide a wide range of sustainable applications that are key in
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the transition to a green society. During the last decade, researchers have focused on the
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development of new photoelectrocatalyst excitable under visible light. Fundamental knowledge and
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advance on advanced materials has have been gigantic but have neglected one of the two aspects
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involved in the material-light interaction: the light transport. Photoelectrocatalytic efficiency is
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tightly related to the efficient harvesting of photons at the semiconductor surfaces. Existing material
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This system is based on
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architectures may be limited on their photon capture capabilities which affects overall quantum
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yields. Biomimicry holds the potential to guide technology advancements to a next generation of
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more efficient and smart photoelectrodes. Emulating nature’s optimal shapes at the micro- and
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nanostructure is identified as the new challenge to ensure translation of to higher technology
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readiness level, in which electrochemical reactor designs and photoelectrodes consider the
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relevance of light transport as indispensable element of photoelectrocatalytic systems. New
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electrode architectures must consider and implement strategies of phototroph organisms to
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maximize light usage while increasing photoelectrocatalytic surface area.
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Acknowledgements
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This work was partially funded by the National Science Foundation (EEC-1449500) Nanosystems
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Engineering Research Center on Nanotechnology-Enabled Water Treatment
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of ro -p re lP na ur Jo Figure 1: Utilization of photoelectrocatalysis for sustainable and green economic development.
22
of ro -p re lP na ur Jo
Figure 2: Schematic representation of light interaction with (a) subwavelength diffraction grating, (b) hierarchical micro- and nanoscale architectures, and (c) macrostructures. Adapted with permission from [72].
23
of ro -p re lP na ur Jo Figure 3: (a) Optical and FE-SEM micrographs of corneal nipple arrays of peacock butterfly and moth‐eye patterned mp‐TiO2 layer. Adapted with permission from [73] and [74]. (b) Schematic representation of microfern leaf and FE-SEM micrographs of ZnO-based microferns. Scale bar: 5 mm. Adapted with permission from [78]. (c) Characterization of original Cinnamomum camphora leaf from macroscale to nanoscale and FE-SEM micrographs of artificial N-doped ZnO Cinnamomum camphora leaf. Adapted with permission from [79]. 24
of ro -p re lP na ur Jo Figure 4: (a) Schematic representation of phototropism mechanism of sunflowers and human-made omnidirectional trackers. (b) Schematic representation and (c) photos of human-made omnidirectional trackers immersed in water. (d, e) Optical micrographs of human-made omnidirectional tracker(s) in water aiming at a laser that illuminates from various zenith angles. Adapted with permission from [80].
25
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2451-9103(20)30213-1
DOI:
https://doi.org/10.1016/j.coelec.2020.100660
Reference:
COELEC 100660
To appear in:
Current Opinion in Electrochemistry
Received Date: 2 October 2020 Revised Date:
15 November 2020
Accepted Date: 17 November 2020
Please cite this article as: Brillas E, Serrà A, Garcia-Segura S, Biomimicry designs for photoelectrochemical systems: strategies to improve light delivery efficiency, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2020.100660. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.
1
Biomimicry designs for photoelectrochemical systems:
2
strategies to improve light delivery efficiency
3
Enric Brillasa,*, Albert Serràb,**, Sergi Garcia-Segurac,**
4
a
5
Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
6
b
7
Materials and Nanostructures, Feuerwerkerstrasse 39, CH-3602, Thun, Switzerland
8
c
9
Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona
of
-p
ro
Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Mechanics of
lP
re
Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment. School of
85287, United States
11
Jo
ur
na
10
Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física,
Article submitted to be published in Current Opinion in Electrochemistry
12
*Corresponding author: Enric Brillas ([email protected])
13
**Albert Serrà ([email protected])
14
***Sergi Garcia-Segura ([email protected])
1
Abstract
16
In the last years, photoelectrocatalysis has been developed to offer green and sustainable application
17
to wastewater remediation, water disinfection, H2 production and CO2 reduction. The advances of
18
these systems with new semiconductor photoelectrocatalysts that are photoexcitated with visible
19
light giving photogenerated charge carriers efficiently separated in a photoelectrochemical cell, are
20
explained. These studies do not consider the light transport as fundamental aspect of light-matter
21
interaction, although the harvesting of photons at the semiconductor surface limits the quantum
22
yield by the existing semiconductor architectures. This opinion review envisages biomimicry as an
23
alternative natural guide for synthesizing more efficient photoelectrodes. Micro- and nanostructure
24
shapes
25
photoelectrochemical reactors with the light transport as indispensable element. The
26
implementation of strategies of phototroph organisms to maximize light adsorption and the
27
enhancement of photoelectrocatalytic surface area are analyzed as key factors for such bio-based
28
photoelectrodes.
29
Keywords
30
Electrochemical water treatment; water splitting; CO2 reduction; bio-based structures; bio-inspired
31
materials; photocatalysis; semiconductors
nature
are
identified
to
prepare
new
bio-inspired
photoelectrodes
for
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2
Photoelectrocatalytic technologies for environmental applications
33
Photoelectrocatalytic (PEC) processes benefit from the synergistic hybridization of photocatalysis
34
and electrochemistry. The photocatalytic effect was initially reported in the seminal work of
35
Fujishima and Honda in 1972 [1] that quickly resulted in a highly productive line of research with
36
thousands of scientific articles published annually. Photocatalytic processes exploit the intrinsic
37
properties of semiconductor materials and their interactive response with photons. The absorption
38
of a photon of energy equal or higher to the semiconductor band gap energy (Eg) promotes the
39
ejection of an electron from the full semiconductor’s valence band (VB) to its empty conductive
40
band (CB) [2-4]. The photo-promotion of an electron to the CB (e−CB) results in the formation of a
41
positively vacancy or hole in the VB (h+VB) following reaction (1). Photogenerated e−CB and h+VB
42
are also known as charge carriers since are responsible of the electrical conductivity of
43
semiconductor materials. Both charge carrier species are highly reactive and can enable different
44
reactions of environmental interest. However, the charge separation on the semiconductor surface is
45
representative of an excited and unstable state that tends to decrease the high energy state of the
46
e−CB through the quick recombination of the photogenerated e−CB/h+VB pair by reaction (2) [5-7].
47
Semiconductor + hν
48
e−CB + h+VB
49
The low quantum yield of photocatalytic processes is indeed explained by the recombination of
50
charge carriers by reaction (2). Here, it is where electrochemical processes play a key role to ensure
51
the stabilization of the highly reactive e−CB/h+VB pair on the semiconductor surface [8-10]. The use
52
of semiconductor materials as photoelectrodes holds the promise of stabilizing charge carriers by
53
inducing their charge separation through the application of an external electric field (i.e., defined
54
bias potential or bias applied current) [11,12]. This positive effect is observed by the drastic
55
increase in PEC performance when compared to electrocatalysis or photocatalysis as individual
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of
32
e−CB + h+VB
Semiconductor + heat
(1) (2)
3
processes [13,14]. The high efficiency of PEC systems and the outstanding reactivity of
57
photogenerated charge carriers on semiconductor surfaces as heterogeneous catalysts have
58
delineated different research avenues in the context of sustainable development that are summarized
59
in Figure 1.
60
First, PEC has shown promise as water treatment technology for pollutant removal and disinfection.
61
The charge carrier h+VB generates the strong oxidant hydroxyl radical (•OH) from water oxidation
62
among other oxygen reactive species [15-18]. The oxidants produced degrade organic pollutants till
63
their complete incineration to CO2 and enable bacteria inactivation mechanisms [19-24]. The low
64
cost of these processes can set us a step closer to clean water access for all.
65
Second, in the water-energy nexus PEC has demonstrated its relevance to sustain a green and
66
sustainable hydrogen economy [25,26]. PEC offers a simpler procedure for H2 sysnthesis from
67
water splitting as indirect solar energy harvesting tool [27-34]. Photogenerated charge carriers are
68
responsible of cathodic reduction of H2 at the cathode of a divided electrochemical cell. The high
69
efficiency and low energy cost of this process ensures its applicability as a more cost-effective
70
alternative route for H2 production.
71
Third, as resource recovery and climate change action. PEC processes can achieve CO2 reduction
72
into added value products such as formic acid, formaldehyde and methanol [35,36]. This process is
73
feasible in the cathode compartment of a divided cell where injected CO2 is reduced at a n-
74
semiconductor photocathode or by coupling PEC with a biocatalyst [37-42]. Capture and
75
transformation of CO2 can contribute to diminish the environmental impact of anthropogenic
76
emissions of CO2 caused from the use of fossil fuels and deforestation. That also includes different
77
existing opportunities for electrosynthesis of different products such as ammonia from nitrate
78
reduction [43,44].
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The neglected relevance of light delivery in photoelectrochemical systems
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PEC processes are still at their infancy. Research it is being conducted at fundamental level and at
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low technology readiness levels. Despite of the undeniable inherent differences between the
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ultimate goals of PEC technology applications summarized in Figure 1, these different challenges
83
have been approached with astonishingly similar research questions. Main efforts have been
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devoted to (i) develop understanding on fundamental PEC capabilities, (ii) elucidate reaction
85
mechanisms, and (iii) achieve breakthroughs on the development of new photoelectrocatalytic
86
materials.
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Titanium dioxide (TiO2) has been the gold-standard material in photocatalytic processes, with an
88
outstanding record of over 26,000 articles published on photocatalytic applications till 2020. The
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major hindrances of TiO2 photocatalytic processes that has been discussed for decades in the
90
literature is the wide band gap of 3.2 eV that requires UV irradiation for TiO2 photocatalytic
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activation [45-48]. This discussion was based on the reliance of energy intensive Hg-lamps as UV
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light source. These inefficient light sources were identified as the main cost driver for PC and PEC
93
applications, but without considering life cycle assessments and the environmental impact of the use
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of mercury as hazardous chemical. Techno-economic discussions define that the inflection point for
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PEC competitiveness depends on the development of sunlight active photo(electro)catalysts and
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centered major research efforts in the material discovery. This critical thought made scientist
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worldwide set sails in a race for the searching of the holy grail of visible light-active photocatalysts.
98
Thousands of papers exploring different approaches with a trial-error strategy have resulted from
99
that effort, in which most of the times, every material reported is excellent but not benchmarked
100
against any other catalyst in terms of performance or long-term stability (meaning more than 5
101
consecutive cycles reusing the material). A large variety of semiconductor photocatalysts have been
102
prepared and checked , including halide perovskite-based materials [49,50], CdS QD-sensitized
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BiOI/WO3 [51], hybrid cocatalysts [52], metal-organic frameworks [53,54], Cu2O [55,56], iron-
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based materials [57,58], inorganic graphitic, graphene and related compounds [59-64], MoS2 [65]
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and g-C3N4-based materials [66,67]. Being honest, despite of its relevance, this scientific race may
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have blinded our sight and distracted all of us from other relevant aspects that should be considered
107
for technology translation.
108
The recent addition of light-emitting diodes to the light-driven technologies toolbox has brought
109
monochromatic, alternative non-energy intensive, and mercury-free light sources into play.
110
Therefore, in an exercise of honesty we should agree upon the fact that photon source may not be a
111
major hindrance after all for certain applications such as water treatment or CO2 reduction. Until
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reaching to that conclusion, we may have focused too much on the semiconductor material
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composition (e.g., doping, nanodecoration, composites, etc.) and completely neglected the other
114
relevant aspect of PEC systems: the light delivery. Evaluation of PEC efficiency entails the
115
effective use of photons delivered from the light source. Light transport is of major relevance for
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competitive operation of PEC technologies of any kind. In fact, at this point in the game we realize
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that we have not designed light transport frameworks that can be deployed or scaled-up in
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photoelectrochemical reactors.
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Biomimicry strategies to improve light delivery in photoelectrochemical systems
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Biomimicry identifies strategies in nature to solve problems in human design. Because organisms
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have developed highly evolved structures while surviving eons as the most competitive specimens
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following natural selection, already refined knowledge from observing and emulating nature’s
123
optimal shapes and ecosystems can inspire the smart design of more effective materials [68-71]. As
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such, biomimicry will play a major role in developing optimized semiconductor architectures and at
125
PEC systems level for improved light harvesting and use [72].
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In particular, the synthesis and design of photoelectrodes should seek smart sophisticated
127
architectures and strategies among insects, plants, algae, and photosynthetic bacteria. These
128
organisms’ anatomical features can reduce reflection or maximize cooperative light-harvesting
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mechanisms and exploit unique nanoscale effects to improve selectivity towards target species in
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low concentration in water matrices [72-76]. In the process, the biomimetic design of innovative
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materials and systems for improved light harvesting should be based on developing antireflective
132
surfaces (Figure 2a) in front of hierarchical micro- and nanoscale architectures (Figure 2b) and/or
133
macrostructures (Figure 2c) to facilitate light trapping [72].
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Many insects present optimized optical structures to improve their ability to attract mates and
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camouflage themselves. Such intricate surface architectures (e.g., moth eyes and butterfly wings)
136
are based on a subwavelength diffraction grating mechanism, which results in minimum light
137
reflection (Figure 2a). In the past two decades, significant efforts have been made to fabricate
138
antireflective coatings based on the integration of tapered shapes such as cones, pillars and columns
139
(Figure 3a), primarily inspired by moth eyes and butterfly wings [73,74]. In fact, those antireflective
140
coatings are commonly called “moth-eye structures”. For example, the antireflective properties of
141
month-eye TiO2 nanostructured films improved light-harvesting efficiency in perovskite solar cells
142
– the power conversion efficiency was improved by approximately 11% (Figure 3a) [76].
143
Plants and other photosynthetic organisms evolved to maximize their energy production via
144
photosynthesis process. These organisms mastered the secrets of highly efficient photon harvesting.
145
PEC is a heterogeneous catalytic process, which means that available surface is crucial for any final
146
application of PEC technologies (Figure 1). However, available surface it is not only relevant in
147
terms of reactive sites but also is required for photon absorption. The depth penetration of photons
148
interacting with a semiconductor surface is defined in function of 1/α [75,76], where α is the
149
absorption coefficient of the semiconductor at a given wavelength. The maximum depletion layer
150
defines the light penetration, which have already justified the relevance of nanostructured materials
151
for enhanced photoexcitation performance defined by the quantum yield. Thus, it is not a matter of
152
enhancing JM available surface area but more of shaping architectures that enhance photons capture.
153
This was observed in a recent work that explored different synthetic approaches of ZnO
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nanorods.[77]. Disorganized structures with crisscrossed nanorods decreased photoelectrode surface
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efficiently exposed to irradiation, thus diminishing PEC performance and quantum yield when
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compared to hierarchically organized structures. Flexible, hierarchical, fractal micro- and nanoscale
157
architectures as well as efficient trapping mechanisms have been partly optimized, but biomimicry
158
can translate these designs to a higher level of performance.
159
Bio-inspired designs based on natural hierarchical architectures (e.g., fern leaves, green leaves, and
160
cacti) exhibit excellent multilevel light scattering, long electron-diffusion, open and accessible
161
pores, high surface-to-volume ratios, and high light absorption independent of the incident light
162
angle. For example, bioinspired ZnO microferns (Figure 3b) [78] exhibit improved light-harvesting
163
properties independent of that angle compared to ZnO nanorods or nanowires, while the porous
164
morphology of N-doped ZnO photocatalysts (Figure 3c) [79], inspired on green leaves, increases
165
light absorption up to 131%. From that perspective, complex bio-inspired architectures demonstrate
166
to be more efficient and smart designs for harvesting light.
167
Another common means to improve light harvesting is mimicking the phototropic mechanism of
168
sunflowers (i.e., self-orientation toward the sun throughout the day). Following that strategy,
169
human-made omnidirectional trackers have exhibited improvement in solar energy-harvesting up to
170
400% compared with non-phototropic materials (Figure 4) [80].
171
nanostructured stimuli-responsive polymers that are able to align to the incident light direction in
172
the three-dimensions. This strategy can be extended to many responsive materials and stimuli.
173
Conclusions and key insights
174
Photoelectrocatalytic technologies provide a wide range of sustainable applications that are key in
175
the transition to a green society. During the last decade, researchers have focused on the
176
development of new photoelectrocatalyst excitable under visible light. Fundamental knowledge and
177
advance on advanced materials has have been gigantic but have neglected one of the two aspects
178
involved in the material-light interaction: the light transport. Photoelectrocatalytic efficiency is
179
tightly related to the efficient harvesting of photons at the semiconductor surfaces. Existing material
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This system is based on
8
architectures may be limited on their photon capture capabilities which affects overall quantum
181
yields. Biomimicry holds the potential to guide technology advancements to a next generation of
182
more efficient and smart photoelectrodes. Emulating nature’s optimal shapes at the micro- and
183
nanostructure is identified as the new challenge to ensure translation of to higher technology
184
readiness level, in which electrochemical reactor designs and photoelectrodes consider the
185
relevance of light transport as indispensable element of photoelectrocatalytic systems. New
186
electrode architectures must consider and implement strategies of phototroph organisms to
187
maximize light usage while increasing photoelectrocatalytic surface area.
188
Acknowledgements
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This work was partially funded by the National Science Foundation (EEC-1449500) Nanosystems
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Engineering Research Center on Nanotechnology-Enabled Water Treatment
191
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of ro -p re lP na ur Jo Figure 1: Utilization of photoelectrocatalysis for sustainable and green economic development.
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Figure 2: Schematic representation of light interaction with (a) subwavelength diffraction grating, (b) hierarchical micro- and nanoscale architectures, and (c) macrostructures. Adapted with permission from [72].
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of ro -p re lP na ur Jo Figure 3: (a) Optical and FE-SEM micrographs of corneal nipple arrays of peacock butterfly and moth‐eye patterned mp‐TiO2 layer. Adapted with permission from [73] and [74]. (b) Schematic representation of microfern leaf and FE-SEM micrographs of ZnO-based microferns. Scale bar: 5 mm. Adapted with permission from [78]. (c) Characterization of original Cinnamomum camphora leaf from macroscale to nanoscale and FE-SEM micrographs of artificial N-doped ZnO Cinnamomum camphora leaf. Adapted with permission from [79]. 24
of ro -p re lP na ur Jo Figure 4: (a) Schematic representation of phototropism mechanism of sunflowers and human-made omnidirectional trackers. (b) Schematic representation and (c) photos of human-made omnidirectional trackers immersed in water. (d, e) Optical micrographs of human-made omnidirectional tracker(s) in water aiming at a laser that illuminates from various zenith angles. Adapted with permission from [80].
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: