Letter pubs.acs.org/NanoLett
Perfect Photon-to-Hydrogen Conversion Efficiency Philip Kalisman, Yifat Nakibli, and Lilac Amirav* Schulich Faculty of Chemistry, Technion−Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *
ABSTRACT: We report a record 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water-splitting reduction half-reaction. This result was accomplished by utilization of nanoparticlebased photocatalysts, composed of Pt-tipped CdSe@CdS rods, with a hydroxyl anion−radical redox couple operating as a shuttle to relay the holes. The implications of such record efficiency for the prospects of realizing practical over all water splitting and solar-to-fuel energy conversion are discussed.
KEYWORDS: Photocatalysis, solar-to-fuel conversion, hydrogen, water splitting, nanoparticles
T
presented in Figure 1, has been widely studied optically13−15 and photocatalytically16−21 and is well-characterized. This architecture facilitates localization of holes,22,23 which are three-dimensionally confined to the CdSe, while delocalized electrons are transferred to the metal tip. Consequently, spatial separation over three different components was attained between the electrons and holes. This design enables efficient long lasting charge carriers’ separation, extending their availability for redox reactions. In addition, the formation of distinct and spatially segregated reaction sites contributes to minimizing the back-reaction of intermediates. Tailored variation of tip composition,24,25 quantum dot diameter,26 and rod length enable further control over light absorption and charge-separation events in this system. In pure water, recombination of the hole with the electron formerly residing on the Pt is still favorable over photochemical consumption (water oxidation), and the charge-separated state lifetime was estimated to be on the order of 10−7 s.27 The introduction of methanol as electron donor (hole-scavenger) affords the rods moderate activity toward hydrogen production. Utilization of isopropyl alcohol (IPA) as the hole-scavenger rather than methanol results in a 3-fold increase in activity toward the reduction half-reaction, and apparent quantum yields of up to 27% at 450 nm were reported.6,12 This indicates that hole scavenging could be the efficiency-limiting step of the process. This hypothesis was confirmed by Achraya et al. in their study of the effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals.28 Lian and coworkers19 later strengthened this theory and reported that the steady-state H2 generation efficiencies depend sensitively on the
o meet increasing global demand for energy, reduce global warming, help our environment, and prepare for the possible depletion of fossil fuel supplies in the coming years, it is necessary to develop alternative “clean” energy sources. One approach to addressing these problems is to use photocatalytic systems, which harvest sunlight and split water, producing molecular oxygen and hydrogen.1 The latter can be either stored and utilized as a transportable fuel or converted into energy-rich organic molecules to cope with the intermittent character of the solar radiation.2,3 After four decades of global research, this multistep reaction remains highly challenging. Systems that are sufficiently stable and efficient for practical use have not yet been realized. Semiconductor-mediated photocatalysis is initiated by the absorption of photons and the subsequent generation of electron−hole pairs. It relies on successful charge separation and migration of these carriers to the surface, where they can promote chemical reactions. In light of the numerous challenges and limited understanding of the photocatalytic chemical process, efforts are still devoted to studying the separate half-reactions. Through the use of sacrificial electron donors or acceptors, each half-reaction can be decoupled and studied separately.4,5 The world-record activity for visible light production of hydrogen was reported by Bao et al.,4 who obtained an apparent quantum yield of 60% at 420 nm from Pt-loaded CdS nanoporous structures. We report here a system and conditions that provide an exceptional, near-perfect efficiency for the water reduction half-reaction, with a turnover frequency of 360 000 mol of hydrogen per hour per mole of catalysts. We employ a well-controlled nanoparticle-based artificial system.6 It consists of a cadmium selenide (CdSe) quantum dot (seed) embedded asymmetrically within a cadmium sulfide (CdS) quantum rod7−10 with a Pt-reduction catalyst placed on the opposite end of the rod.11,12 The structure, which is © 2016 American Chemical Society
Received: November 25, 2015 Revised: January 14, 2016 Published: January 20, 2016 1776
DOI: 10.1021/acs.nanolett.5b04813 Nano Lett. 2016, 16, 1776−1781
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Figure 1. (A) Illustration of the nanoparticle photocatalyst depicting the photocatalytic cycle. (B) TEM and (C) high-resolution HAADF micrographs of a few representative metal-tipped CdSe@CdS rods.
holes. A typical sample contained about 6 × 1014 50 nm long rods with embedded seeds of 2.3 nm diameter. Other rod lengths and seed sizes were tested as well. The photocatalytic activity of the sample was measured for the water reduction half-reaction. Solutions of rods suspended in water with isopropyl alcohol (10% by volume) acting as an electron donor and KOH (0.15 g/mL at pH of >15) were placed in a custom-built gas-tight reaction cell purged with argon (at a controlled and variable flow rate). The samples were then illuminated with a LED at 455 nm, and the photon flux was measured with a power meter (typically working with 50 mW, which is equivalent to a photon flux of 1.15 × 1017 photons/ sec). The evolving hydrogen was analyzed in real-time using an appended gas chromatograph equipped with a thermal conductivity detector. Operation in continuous flow mode allowed for direct determination of the evolved gas production rate. Great care was devoted to the calibration and accuracy of the measurements, as detailed in the Supporting Information.
hole scavenger and are correlated with hole transfer rates to the scavenger (as measured by time-resolved fluorescence decay). Recently, Simon et al.29 proposed employing a hydroxyl anion−radical redox couple to efficiently relay the hole from the semiconductor to the scavenger. By operating the redox couple •OH−−OH as a shuttle, Simon et al. obtained hydrogen production with an apparent quantum yield of 50% from Nidecorated CdS nanorods. The holes in the CdS valence band are able to oxidize hydroxyl anions only at very high pH.30 The CdSe@CdS−Pt structure can be regarded as a closedcircuit photoelectrochemical cell with two electrodes that are only a few dozens of nm apart. Therefore, in addition to hydroxide anions acting as electron donors, the use of a supporting electrolyte such as KOH is beneficial for diffusion of charged species away from the electrode surface. Hence, we examined the activity of the Pt-tipped CdSe@CdS nanorods, toward hydrogen production, with the hydroxyl anion−radical redox couple operating as a shuttle to relay the 1777
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Figure 2. Photocatalytic quantum efficiencies for the hydrogen reduction half-reaction obtained with different samples of Pt-tipped CdSe@CdS nanorod photocatalysts using IPA as an electron donor at neutral pH conditions (dark blue) and with the addition of hydroxyl anion−radical redox couple operating at high pH (light blue). Middle-tone bars are for solutions with pH of 13. Samples are illustrated with CdSe seed size of 2.3 (green), 2.5 (yellow), and 2.7 nm (orange) and rod length ranging from 45 to 65 nm.
The apparent quantum efficiency of the sample, which is defined as QE = 2NH2/Nhυ, was determined by quantifying the amount of evolved hydrogen at a given photon flux. Results for a gallery of different samples are presented in Figure 2. At 50 mW of power we obtained a flow of (up to) 139 μL/min or 142 mmol H2 per hour per gram sample (see the Supporting Information for calculations), which corresponds to an apparent quantum yield of 100% at 455 nm. These numbers indicate that a hydrogen molecule is produced (on average) every 10 ms on each single rod. The result is a turnover frequency of 360 000 mol of hydrogen per hour per mole of catalysts. The results are highly reproducible and, as can be seen in Figure 2, we have obtained apparent quantum efficiencies of above 90% for numerous samples with varying seed size (ranging from 2.3 (illustrated in green) to 2.7 nm (illustrated in orange) and rod length (ranging from 45 to 65 nm). Interestingly, trends in the activity of different samples that were obtained when tested under neutral conditions (dark blue bars in Figure 2, or middle-tone blue bars for pH 13) are altered when operating under extreme basic environment (light blue, Figure 2). Similarly, samples with similar activity at neutral conditions might have very different activity at pH 15. This implies that the key parameter in determining the activity of a sample is related to its ability to mediate hole transfer on the rod surface, a process that might be highly sensitive to the ligand coverage. It is noteworthy that the system has a single Pt-reduction catalyst. The apparent quantum efficiency obtained from a sample with identical rods but with two Pt tips was only 58.5% (Figure 3). For a multi electron reaction such as hydrogen reduction, the photocatalyst design should only include a single co-catalytic site per each segment of the semiconductor capable of light excitation.12 This is to ensure that intermediates are formed at close proximity. The benefits of a single catalyst should become more pronounced in correlation with the number of electrons that are involved in the process and is expected to be critical for the proper design of photocatalysts aimed at water oxidation.
Figure 3. On the left, two TEM micrographs of CdSe@CdS nanorod photocatalysts with (A) single and (B) double Pt co-catalysts. Bar is equal to 25 nm. On the right, photocatalytic quantum efficiencies for the hydrogen reduction half-reaction obtained with the two samples using IPA as an electron donor and the hydroxyl anion−radical redox couple as hole shuttle. The advantage of utilizing a single cocatalytic site for this multi-electron reaction is evident.
The catalytic activity of the samples is highly sensitive to the pH conditions, as can be seen in Figure 4. The activity seems to increase exponentially with the solution pH, from pH 8 to about 15, where it saturates. Such saturation of the quantum efficiency for hydrogen production supports the hypothesis that the high pH is required for shifting the CdS valence band to allow for oxidation of hydroxyl anions.29 Reports in the literature state that the CdS bands shift with pH following a slope of only −33 mV/pH unit and do not follow the Nernstian 1778
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Our system provides light-driven H2 production with excellent longevity, sustaining its original high activity with no reduction for over 44 h of noncontinuous illumination (the longest period tested and at which point no noticeable degradation was observed). This can be seen in Figure 5, which presents a timeline for normalized relative hydrogen production. Over the course of the first 17 h, hydrogen is being produced at moderate rate with slow gradual increase in activity. At this point, the LED was turned off, the cell was opened, 1.5 g KOH + 500 μL IPA were added to the sample solution, and after purging, the illumination was turned on again (purple arrow, Figure 5). The addition of the base resulted in a sharp increase in the hydrogen production rate. Over the next 3 h, a decrease in production rate was observed. This is likely caused by the consumption and evaporation of IPA. IPA is oxidized by the hydroxyl radicals that are formed by photochemical oxidation. In the absence of IPA, hydroxyl radicals will be reduced to hydroxide anions in competition with hydrogen reduction. Indeed, each addition of 500 μL IPA to the sample solution (in the dark, followed by purging of the cell) resulted in recovery of the activity, which resumed its original high value (black arrow(s) in Figure 5). The fourth and last additions also included 500 μL of water to compensate for evaporation. After 11 such cycles, spread out over a week, the production rate, on average, maintained its original high activity. Given the impressive turnover frequency, and extended illumination time, a total of at least 16 × 106 moles of H2 can be produced from a single mole of rods without degrading the sample. This is highly promising for future incorporation of such samples into full artificial photosynthesis systems. Although the photocatalytic hydrogen generation presented here is not yet genuine solar-to-fuel energy conversion, and although these basic conditions are perhaps not always ideal, this record efficiency indicates that we have few if any problems
Figure 4. Relative performance of Pt-tipped CdSe@CdS rods for photocatalytic hydrogen production at different pH.
dependence on pH (−59 mV/pH unit), typical for many semiconductors.31,32 This enables a crossover at high pH between the valence band maximum and the redox potential for hydroxyl anion and radical. As a result, the hydroxyl anion− radical redox couple can be employed as a shuttle to efficiently relay the hole from the semiconductor to the scavenger. The holes in the CdS valence band are able to oxidize hydroxyl anions only at very high pH. However, there is only negligible difference in the oxidative power toward the hole scavenger (isopropyl alcohol). Hence, it appears that introducing hydroxyl anion as an alternative hole scavenger serves mainly a kinetic role. The observed saturation of the quantum efficiency for hydrogen production also provides reassurance for the measurement calibration.
Figure 5. H2 production permanence over time. The insert serves as a guide to the figure, showing when the illumination was turned on and off and indicating additions to the solution. The first 17 h of operation under neutral conditions were followed by introduction of KOH and additional isopropyl alcohol (IPA) as electron scavengers (purple arrow). The addition of 500 μL IPA is marked by black arrows. The sample’s high activity toward hydrogen production was maintained over a total of 44 h of noncontinuous illumination. 1779
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on the reduction side. Photons are being absorbed; charges are being generated and separated, successfully competing with recombination; charge transfer to the metal co-catalysts is efficient; charges are being consumed, and the hydrogen intermediate (absorbed protons) seems stable on a ms time scale. As a result of this record, near-unity, quantum efficiency, the discussion should no longer focus on the evolution of hydrogen and the optimization of conditions for its generation but rather on the prospects of realizing practical overall water splitting. On the basis of these results, we determined that the efficient removal of holes from the catalyst at less extreme conditions is perhaps the main hurdle to a sustainable and efficient visible-light-driven solar-to-fuel cycle. Another task that must be met before such system can be utilized is to treat what some consider the Achilles heel of CdS: its photochemical instability. It is particularly difficult to find a semiconductor system with suitable band gap and electron affinity for visible light absorption and for driving the subsequent redox chemistry, and CdS is among the few materials that met these strict requirements. It is unfortunately not suitable for overall water splitting because the prolonged irradiation of its suspensions leads to photocorrosion, and it therefore requires the use of sacrificial donors for photochemical H2 evolution from water. It was recently demonstrated that the addition of a second cocatalyst, such as IrO233 or Ru,34 which can scavenge the holes from the semiconductor and mediate their transfer to water, affords CdS-based structures the desired photochemical stability. Manipulation of charge carriers via simple design principles improved the activity. Identification of the bottleneck of the process supported setting up appropriate working conditions. The combination of the two enabled us to obtain perfect photon-to-hydrogen production efficiency. We believe that this approach of intelligent design will enable us to tackle the remaining challenges with an eye toward overall water splitting.
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was carried out in the framework of the Russell Berrie Nanotechnology Institute (RBNI) and the Nancy and Stephen Grand Technion Energy Program (GTEP). The authors acknowledge the generous support from the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (grant no. 152/11) as well as the Adelis Foundation support in research in renewable energy. We thank the Schulich Faculty of Chemistry and the Technion − Israel Institute of Technology for the renovated laboratory and startup package. Dr. Kalisman thanks the Schulich postdoctoral fellowship for their support.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04813. Details on the synthesis of cadmium selenide quantum dots and seeded rods, the photodeposition and colloidal growth of platinum metal tips, ligand exchange, particle characterization, hydrogen production rate determination, the calibration of hydrogen production measurements, assessment of solution pH, the photon flux, quantum efficiency calculation for hydrogen production, efficiency calculation, control experiments, QE for H2 production under different illumination wavelengths, and sample concentration. Figures showing representative TEM micrographs of CdSe@CdS nanoparticles, absorbance and fluorescence of example bare rods, photoluminescence spectra, a flow diagram of argon gas through the system, the gas-tight reaction cell, example chromatograms, hydrogen production for a sample under basic conditions, and quantum efficiency for hydrogen production. A table showing typical precursor amounts and conditions for CdSe@CdS growth. (PDF) 1780
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