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Promotion of Atomic Hydrogen Recombination As an Alternative To Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H2 Ji Bong Joo, Robert Dillon, Ilkeun Lee, Yadong Yin, Christopher J. Bardeen, and Francisco Zaera1 Department of Chemistry, University of California, Riverside, CA 92521 Edited* by Gabor A. Somorjai, University of California, Berkeley, CA, and approved April 24, 2014 (received for review March 21, 2014) The production of hydrogen from water with semiconductor migrates to the metal and recombines to yield the final molecular photocatalysts can be promoted by adding small amounts of hydrogen product (Fig. 1B). metals to their surfaces. The resulting enhancement in photo- catalytic activity is commonly attributed to a fast transfer of the Results and Discussion excited electrons generated by photon absorption from the The dynamics of photon absorption, electronic excitation, and semiconductor to the metal, a step that prevents deexcitation electron migration was first assessed by using transient absorp- back to the ground electronic state. Here we provide experi- tion and time-resolved fluorescence spectroscopies. Time-resolved mental evidence that suggests an alternative pathway that fluorescence studies have proven to be particularly useful for the does not involve electron transfer to the metal but requires it characterization of photocatalysts, because fluorescence is one of to act as a catalyst for the recombination of the hydrogen the main channels by which the electron–hole pair recombines atoms made via the reduction of protons on the surface of the and the photon excitation is lost. A recent study from our lab- semiconductor instead. oratory using well-defined Au@Void@TiO2 yolk-shell nano- structures identified clear correlations among the lifetime of photocatalysis | hydrogen production | gold–titania | time-resolved the fluorescence decay, the rate at which hydrogen is produced fluorescence | core-shell nanostructures in photocatalysis, and the crystallinity of the titania shells: the CHEMISTRY more crystalline catalysts display both longer fluorescence life- hotocatalysis is a promising technology to address problems times and higher hydrogen production rates (Fig. S1) (19). Pin chemical synthesis (1), environmental remediation (2, 3), Surprisingly, however, similar fluorescence decay kinetics were and energy utilization (4). The potential for harvesting solar seen here with Au@Void@TiO2 versus Void@TiO2 samples energy and storing it as a chemical fuel using photocatalysts is [Fig. S1A; synthetic details of the latter catalyst are provided particularly appealing. Specifically, H2 may be produced via elsewhere (20, 21)], indicating that the photophysics of the ti- water splitting this way (5–7). In fact, many photocatalysts have tania shells is not affected by the presence of the metal. This been reported already capable of producing hydrogen out of behavior proved to be quite general: it was also observed with water, even if none of those are yet commercially viable. Such catalysts made by dispersing either gold or platinum nanoparticles photocatalysis rely on capturing photon energy via excitation of electrons from the valence band to the conduction band of ap- Significance propriate semiconductors such as titania, which is often cited as a prototypical example (8–10), creating an excited electron–hole The use of catalysis assisted by light is a promising route to pair that is then used to promote redox reactions (11, 12). convert solar energy into chemical fuels such as H2. Photon However, for the photocatalysts to be useful, the lifetime of the absorption promotes electrons in semiconductor catalysts from electron–hole pair needs to be long enough to be accessible for the valence to the conduction band of those solids, and the chemical conversions. The search for photocatalytic systems that excited electrons may be used to reduce protons in water to fulfill this and other key requirements continues, and would be produce hydrogen gas. Unfortunately, those electrons tend to greatly facilitated by a clearer understanding of the underlying rapidly return to their ground state instead. The addition of chemical process. metals has been shown to enhance photocatalytic activity, and It is well known that the addition of metals such as platinum or that has been explained by their ability to trap the excited gold to semiconductors enhances their activity as photocatalysts, electrons and quench the electron–hole recombination process in particular for water splitting to produce H2 (13–15). This ef- that neutralizes the photoexcitation. Here we challenge the fect is currently explained by a mechanism where the excited validity of this model and provide an alternative explanation electrons produced by absorption of light are transferred from for the enhancement. A new thinking about this mechanism the semiconductor to the metal before they have the opportunity may result in new searches for appropriate cocatalysts. to recombine with their corresponding holes and return to the ground electronic state (Fig. 1A) (16–18). In this scheme, the Author contributions: C.J.B. and F.Z. designed research; F.Z. conceived and designed all aspects of the project and coordinated the effort; Y.Y. designed the synthesis of core-shell protons from water are reduced on the surface of the metal, in catalysts; J.B.J. and R.D. performed research; J.B.J. carried out the synthesis of the cata- sites physically separated from those on the semiconductor, lysts, their characterization, and the evaluation of their photocatalytic activity; R.D. car- where oxygen production takes place. Here we present evidence ried out the photophysical measurements; I.L. carried out the X-ray photoelectron – that challenges this conventional explanation, and offer an al- spectroscopy and H D exchange measurements; C.J.B. helped design and supervised the photophysics experiments and helped conceive the overall project; I.L. and Y.Y. contrib- ternative mechanism for the promotion of photocatalysis by uted new reagents/analytic tools; J.B.J., R.D., and F.Z. analyzed data; and F.Z. wrote metals. Specifically, using the Au/TiO2 metal–semiconductor the paper. pair as a model system, we show that electron transfer to the The authors declare no conflict of interest. metal may not play a significant role in photocatalysis. Instead, *This Direct Submission article had a prearranged editor. the data support a model where the excited electron promotes 1To whom correspondence should be addressed. E-mail: [email protected]. the reduction of protons on the surface of the semiconductor, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. not the metal, and where the reduced atomic hydrogen then 1073/pnas.1405365111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1405365111 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 needed to detect the small number of electrons needed to pro- duce the quantity of H2 detected. However, it was determined that the fluorescence lifetime is sensitive to changes in the re- action environment (Fig. S3), in particular when methanol is added to the liquid phase (as required to promote H2 pro- duction). The interpretation of those data are complex and may involve changes in local electronic structure, but they do provide evidence that the states involved in photocatalysis and photo- luminescence are sensitive to the same environmental factors. On the whole, we believe that the time-resolved fluorescence data offer strong, if not irrefutable, support for the lack of par- ticipation of semiconductor-to-metal electron transfer steps in the photocatalytic production of H2. It should be said that it is well known that metals do accept charge from the semiconductor when photoexcited. In the case of gold, this can be detected by a shift in the frequency at which the plasmon resonance of the metal occurs (24), a fact that was Fig. 1. Schematic representation of the mechanisms (Upper) and electronic A transitions (Lower) proposed to explain the role of metals (Au) in the pho- also corroborated in our study (Fig. S4 ). However, it should be tocatalytic splitting of water with semiconductors (TiO2). (A) In the con- noted that no differences in this shift were observed in our ventional model, the metal acts as an electron trap that physically separates experiments with pure water versus with 10 vol% methanol sol- the excited electron used for proton reduction from the oxidation step that utions (Fig. S4A), even though the addition of the sacrificial occurs on the surface of the semiconductor. (B) Our alternative model pro- methanol was needed to promote hydrogen production (no H + 2 poses that H reduction occurs at semiconductor sites but that the resulting was seen to evolve from pure water). On the other hand, it has hydrogen atoms need to migrate to the metal to recombine and produce also been well established that most transition metals are effi- the final H2 product. cient catalysts for the recombination of adsorbed hydrogen atoms (25), the alternative mechanism proposed in this report. on the typical commercial titania P25 used in most photocatalysis That behavior was probed, and shown to be operative in the H2- studies, and was determined to also hold true in different envi- producing photocatalysts, by following the isotope scrambling of + B ronments and with various metal loadings, up to 10 wt% (Fig. 2 H2 D2 mixtures in the gas phase, as illustrated in Fig. S4 . The and Fig. S2). The observation that fluorescence from titania is not challenge, therefore, is to decouple the two effects in order to modified by the presence of a metal on its surface indicates that determine which one is the most pertinent to photocatalysis. In such metal does not help quench the radiative decay of the elec- tronic excitation due to photon absorption in any appreciable way, even if it does aid in the photochemical production of H2. Fur- thermore, no significant differences were seen in transient ab- sorption experiments with 200-fs resolution, which probe the initial events after photon absorption (Fig.
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