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A Coating Strategy to Achieve Effective Local Charge Separation for Photocatalytic Coevolution

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A Coating Strategy to Achieve Effective Local Charge Separation for Photocatalytic Coevolution A coating strategy to achieve effective local charge separation for photocatalytic coevolution Tianshuo Zhaoa,b, Rito Yanagia,b, Yijie Xua, Yulian Hea,b, Yuqi Songa, Meiqi Yanga,b, and Shu Hua,b,1 aDepartment of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520; and bEnergy Sciences Institute, Yale University, West Haven, CT 06516 Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved January 12, 2021 (received for review November 19, 2020) Semiconductors of narrow bandgaps and high quantum efficiency coevolution. Coevolution refers to the concurrent generation of have not been broadly utilized for photocatalytic coevolution of two products from the reduction and oxidation reactions in the H2 and O2 via water splitting. One prominent issue is to develop same photocatalyst sample space (21). These reactions can be effective protection strategies, which not only mitigate photocor- thermodynamically uphill. Only when their reverse reactions are rosion in an aqueous environment but also facilitate charge sepa- suppressed at the respective catalytic sites can the two products ration. Achieving local charge separation is especially challenging coexist rather than chemically recombine. For water-splitting re- when these reductive and oxidative sites are placed only nanome- actions, those reductive and oxidative sites at the photocatalyst– ters apart compared to two macroscopically separated electrodes liquid interfaces coevolve H2 and O2, or produce H2 and oxidize in a photoelectrochemical cell. Additionally, the driving force of redox mediators, or reduce redox mediators and evolve O ,con- charge separation, namely the energetic difference in the barrier 2 currently. Therefore, a coating for photocatalysts should simulta- heights across the two type of sites, is small. Herein, we used conformal coatings attached by nanoscale cocatalysts to transform neously allow reductive and oxidative charge transfer, from the two classes of tunable bandgap semiconductors, i.e., CdS and photocatalyst through the coating to the liquid; whereas the con- ventional coatings only enable single-direction charge transfer, GaInP2, into stable and efficient photocatalysts. We used hydro- gen evolution and redox-mediator oxidation for model study, and either reductive or oxidative. further constructed a two-compartment solar fuel generator that Recent publications imply that crystal facets, cocatalysts, or separated stoichiometric H2 and O2 products. Distinct from the local doping can cause charges to separate locally at the solid- single charge-transfer direction reported for conventional protec- state photoabsorbers (6, 22–24). Different chemical states of ENGINEERING tive coatings, the coating herein allows for concurrent injection of bifunctional cocatalysts were also postulated to facilitate charge photoexcited electrons and holes through the coating. The ener- separation (25, 26). However, the energetics under operational getic difference between reductive and oxidative catalytic sites conditions have not been quantified. Therefore, a mechanism was regulated by selectivity and local kinetics. Accordingly, the leading to effective charge separation needs to be generalized. charge separation behavior was validated using numerical simula- Our design principle is to separate charges at spatially disparate tions. Following this design principle, the CdS/TiO2/Rh@CrOx pho- sites of photocatalysts. It should be different from molecular tocatalysts evolved H2 while oxidizing reversible polysulfide redox photosynthesis, where the excited states are long-lived to favor μ · −1· −2 mediators at a maximum rate of 90.6 mol h cm by stacking charge accumulation and catalysis over charge recombination three panels. Powered by a solar cell, the redox-mediated solar (27). In contrast, irreversible hole scavengers cannot be regen- water-splitting reactor regenerated the polysulfide repeatedly erated to sustain continuous PC, despite that their fast kinetics and achieved solar-to-hydrogen efficiency of 1.7%. are often leveraged for competing against photooxidation (28). Without the chemical bias provided by the irreversible photocatalytic synthesis | charge separation | coatings | corrosion protection | reactor engineering Significance articulate photocatalysis (PC) is a promising platform for Particulate photocatalysis is a promising approach to solar fuels Psunlight-driven hydrogen production or CO2 reduction at scale (1–5). Although particulate photocatalysts have attained production at scale. Herein, we present a general design by nearly 100% quantum efficiency recently (6), the state-of-the-art using conformal coatings and attaching nanoscale cocatalysts solar-to-hydrogen (STH) conversion efficiency for oxides (5, 7), to achieve local charge separation and, at the same time, to stabilize photocatalysts that are easily photocorroded other- oxynitrides (8, 9), and oxysulfides (10), remains around 1% (11). wise. With spatial charge separation, the nanometer-spaced An alternative direction is to utilize narrow and tunable bandgap reductive and oxidative surface sites can coevolve to produce photoabsorbers, such as those II–VI and III–V semiconductors. H and O , or to produce H and oxidize redox mediators, or to They are known for strong optical absorption, high quantum 2 2 2 produce O and reduce redox mediators. This work investigates efficiency, and reasonable carrier lifetime (12–14). For example, 2 – – the charge separation strategy for the semiconductor/coating/ cadmium sulfide (CdS), and gallium indium phosphide (GaInP2) cocatalyst structure both by tuning barrier height energetics photoabsorbers have bandgaps of 2.42 and 1.87 eV, respectively. and by building numerical models. The use of GaInP2 as the top absorber in a photoelectrochemical water-splitting device showed 19% STH efficiency (15, 16). CdS- Author contributions: T.Z. and S.H. designed research; T.Z., R.Y., Y.X., Y.H., Y.S., and M.Y. and GaInP2-based photocatalysts, if utilized in a two-photosystem performed research; T.Z., R.Y., Y.X., Y.H., Y.S., and M.Y. analyzed data; and T.Z., R.Y., and scheme or redox-mediated water-splitting reactor, promise a leap S.H. wrote the paper. in STH efficiency (1, 17). However, these semiconductors suffer The authors declare no competing interest. from poor photostability in water, and their few-hour stability is This article is a PNAS Direct Submission. much shorter than the thousands of hours durability shown by Published under the PNAS license. oxides (5). 1To whom correspondence may be addressed. Email: [email protected]. Although coating-protected narrow bandgap semiconductors This article contains supporting information online at https://www.pnas.org/lookup/suppl/ have been broadly studied in photoelectrochemical devices doi:10.1073/pnas.2023552118/-/DCSupplemental. (18–20), they have not been investigated for photocatalytic Published February 8, 2021. PNAS 2021 Vol. 118 No. 7 e2023552118 https://doi.org/10.1073/pnas.2023552118 | 1of8 Downloaded by guest on September 27, 2021 scavengers, the charge separation in the presence of reversible can coexist due to their slow chemical recombination: They redox mediators is often not effective. transport away and eventually get separated. The charge sepa- Herein, we develop a coating approach that stabilizes photo- ration and recombination occur within semiconductor photo- catalysts and, in the meantime, facilitates spatial charge sepa- absorbers on the order of picoseconds to microseconds, while the ration during light-induced coevolution. As illustrated in Fig. 1A, timescale of catalysis, such as H2 evolution, is in microseconds to the semiconductor/coating/cocatalyst interface is fabricated by milliseconds. The rate of charge transfer between the photo- coating the semiconductor film with thin protective coatings, catalyst and liquid is far outcompeted by the rate of electron- followed by sparsely loading metal nanoparticles as cocatalysts hole recombination at a single catalytic site. Therefore, the on the coating surface. This simple two-step procedure trans- spatial separation of photoexcited electrons and holes should forms narrow-bandgap semiconductors, such as CdS powder suppress carrier recombination. The charges are accumulated or films and epitaxial GaInP2 layers, into stable and efficient pho- trapped at spatially disparate surface states of the photocatalysts, tocatalysts: Coevolution of hydrogen and reversible redox me- where they reside for much longer than the recombination life- − diators (A/A ) occurs at the cocatalyst sites and bare coating time of the photoabsorber. Therefore, efficient charge separa- surfaces, respectively. Those two types of sites are alternating tion schemes should be established for those reductive and along the liquid interface. Such a general scheme does not rely oxidative sites that are only nanometers apart. Otherwise, severe on specific morphology or crystal facets of the photocatalyst. We charge recombination will lead to low or even no photocatalytic therefore fabricated CdS/TiO2/Rh@CrOx particulate panels, activity. −1 −2 which continuously evolved H2 at 90.6 μmol·h ·cm , and Based on our previous study (29), efficient charge separation showed an internal quantum yield (IQY) of 44.3% at 438 nm in a can be realized in photocatalysts through the spatially varying 2− 2− solution of reversible polysulfide/sulfide (Sn /S ) redox cou- barrier heights along the same photocatalyst–liquid interface. It ples. TiO2 coating-stabilized
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