Semiconductor Photocatalysis: “Tell Us the Complete Story!”
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contributions to the overall yield of the products in a photocatalytic reaction and present a photocatalytic mechanism that involves both reduction and oxidation steps and proper identification of products. Sacrificial donors employed in photocatalytic reduction are typically methanol, ethanol, i-propanol, triethanolamine, formate, ascorbic acid, EDTA, etc. These sacrificial donors are rich in carbon and hydrogen. Oxidation of these compounds (for example, methanol) produces C1 products as well as hydrogen (Figure 2), the details of which can be found elsewhere.2 The sequential oxidation of methanol and other alcohols, induced by photogenerated holes (SC(h)), produces aldehydes, carboxylic acid, CO2, and hydrogen and contributes to the measured yield of photocatalytic products (see for example reactions 1−3).
hotocatalytic production of solar fuels has become a major research area in solar energy conversion. The two most popular topics include reduction of CO2 into CO and C1 fuels and splitting of water into hydrogen and oxygen. New semiconductor photocatalysts are continually being designed to demonstrate their effectiveness for generating solar fuels. In many instances the net production of the products is in the range of micromoles to millimoles per hour. Yet, the authors often like to impress the readers with adjectives such as “highly eff icient” in the title and report up to 100% photon conversion efficiencies. The majority of these studies involve monitoring the yield of products, viz C1 products (CO, CH3OH, CH4, etc.) in the case of CO2 reduction and hydrogen in the case of reduction of H+ ions or water, thus focusing only on the reduction cycle. The oxidation cycle, the other half of the story, is seldom discussed in detail (Figure 1).
CH3OH + SC(h) → •CH3O + H+ → HCHO + H 2 (1) (2)
HCOOH + SC(h) → CO2 + H 2
(3)
In a similar fashion, other hole scavengers, such as ascorbic acid, trimethylamine, and EDTA, are also capable of generating hydrogen during their oxidation at the semiconductor interface. This undesired role of the sacrificial donor was discussed earlier by Bahnemann and co-worker in a commentary.2 To quote these authors, “Usually, most experimentalists are not even interested in these (oxidation) processes because all that they measure is the formation of hydrogen gas. As elaborated above, H2 will be formed through both the direct reductive as well as the indirect oxidative paths.”2 There is also another way sacrificial donors can enhance the yield of reduction processes. For example, alcohol oxidation is known to produce alkoxy radical intermediates, which are reductive in nature. They in turn can either inject electrons into the semiconductor particle or participate in the reduction process in an indirect way. The current “doubling effect” induced by such reductive species in a photoelectrochemical cell through the oxidation of alcohols and formate was established a long time ago.3−5 Scaiano and co-workers in their Viewpoint article published in this issue provide additional details on the influence of alcohols in the photocatalytic reduction.6 Another important aspect of the chemistry of the sacrificial donor such as S2− is its ability to undergo protonation when the salt is dissolved in water. For example, Na2S (a commonly used sacrificial donor for metal chalcogenide systems) dissolved in water forms SH− and makes the solution basic with pH in the range of 12−13 (eq 4):
Figure 1. Illustration of reduction and oxidation cycles as part of the overall photocatalysis. The hole oxidation of sacrificial donor is seldom discussed in detail.
It is a common knowledge that both reductive and oxidative processes occur in a photocatalytic reaction. In a true photocatalytic water-splitting reaction, hydrogen gas is generated through the reductive process and oxygen gas is generated through the oxidative process, and the yields of both products are monitored and reported.1 However, the oxygen production process is especially sluggish and often the bottleneck of the whole water-splitting process. In order to study the reduction process selectively in a photocatalytic system, it is a common practice to include sacrificial electron donors. The presence of a sacrificial donor such as alcohol in a photocatalytic reaction overcomes the kinetic limitation of the oxidation process and allows researchers to probe the reductive cycle selectively. Although such an approach helps to elucidate the electron-transfer mechanism, it does not accurately quantify the reduction yield. The oxidation of sacrificial donors often contributes to the products that are being considered as reduction products. Hence, it becomes imperative to evaluate the fate of these sacrificial donors and their © 2018 American Chemical Society
HCHO + H 2O + SC(h) → HCOOH + H 2
S2 − + H 2O ⇔ SH− + OH−
(4)
Published: March 9, 2018 622
DOI: 10.1021/acsenergylett.8b00196 ACS Energy Lett. 2018, 3, 622−623
Editorial
Cite This: ACS Energy Lett. 2018, 3, 622−623
ACS Energy Letters
Editorial
transition-metal oxides and chalcogenides as well as group II−VI semiconductor nanocrystals. The arrival of Prof. Sarma will help us to broaden the editorial expertise and allow us to maintain a rapid publication time.
Prashant V. Kamat, Editor-in-Chief
University of Notre Dame, Notre Dame, Indiana, United States
Song Jin, Senior Editor
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Figure 2. Schematic illustration of the photocatalytic reduction of water in the presence of methanol. The oxidation of methanol (sacrificial donor) in this example by photogenerated holes and hydroxyl radicals could result in the formation of H2 and other intermediates.
University of Wisconsin - Madison, Madison, Wisconsin, United States
AUTHOR INFORMATION
ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Song Jin: 0000-0001-8693-7010 Notes
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Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
Oxidation of SH in aqueous solution containing metal chalcogenide photocatalysts has also been shown to produce hydrogen.7,8 1 SH−+SC(h) → S + H 2 (5) 2 Thus, in a photocatalytic reaction involving metal chalcogenides as a photocatalyst and a S2−/SH− as sacrificial electron donor, both reductive and oxidative cycles can produce hydrogen. One should also take into consideration the pH of the solution (very low H+ ion concentration in alkaline solution) and should not indicate reduction of H+ ions as the possible reduction step in the overall reaction mechanism. Today, the majority of the photocatalytic reduction studies that employ alcohol or other sacrificial donors ignore their contributions in the overall photocatalytic reactions while estimating the photoconversion yield. The photocatalysis studies that employ alcohols or other sacrificial donors are mostly investigating their photocatalytic oxidation reactions instead of the water-splitting reactions where water is oxidized to generate oxygen gas. Surprisingly, often very little effort has been made to correlate the efficiencies of these two reactions using the same photocatalyst. We request our authors be more diligent in reporting photocatalytic efficiencies. We are committed to scrutinizing the validity of the claims made in the photocatalytic generation of solar fuels and rigorously checking the completeness of the mechanistic studies. We urge our authors to carefully study previous articles highlighting the best practices in photocatalysis9−13 and follow the best practices recommended in these articles. We at ACS Energy Letters regard the completeness of the mechanistic study backed by careful control experiments as an important criterion for considering papers for publication. A manuscript reporting a well-rounded study with new findings is likely to see success in ACS Energy Letters. Make sure you tell us the complete story! Welcoming New Senior Editor. We welcome Professor D. D. Sarma, of Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India, who will be joining our editorial team as a Senior Editor starting this month. Prof. Sarma holds the J.N. Tata Chair at Indian Institute of Science. He has published more than 460 scientific papers in peer-reviewed journals. Prof. Sarma brings us scientific expertise in the areas of semiconductor photophysics, strongly correlated electronic systems, and energy science. His group has worked extensively on the electronic structure of materials, primarily based on
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REFERENCES
(1) Chen, S.; Takata, T.; Domen, K. Particulate Photocatalysts for Overall Water Splitting. Nature Review Mater. 2017, 2, 17050. (2) Schneider, J.; Bahnemann, D. W. Undesired Role of Sacrificial Reagents in Photocatalysis. J. Phys. Chem. Lett. 2013, 4, 3479−3483. (3) Maeda, Y.; Fujishima, A.; Honda, K. The Investigation of Current Doubling Reactions on Semiconductor Photoelectrodes by Temperature Change Measurements. J. Electrochem. Soc. 1981, 128, 1731−1734. (4) Herrasti, P.; Peter, L. Photocurrent Doubling During the Oxidation of Formic Acid at n-CdS: An Investigation by Intensity Modulated Photocurrent Spectroscopy. J. Electroanal. Chem. Interfacial Electrochem. 1991, 305, 241−258. (5) Mandelbaum, P. A.; Regazzoni, A. E.; Blesa, M. A.; Bilmes, S. A. Photoelectro-Oxidation of Alcohols on Titanium Dioxide Thin Film Electrodes. J. Phys. Chem. B 1999, 103, 5505−5511. (6) Scaiano, J.; Hainer, A.; Hodgins, J.; Sandre, V.; Vallieres, M.; Lanterna, A. Photocatalytic Hydrogen Generation using Metaldecorated TiO2: Sacrificial Donors vs. True Water Splitting. ACS Energy Lett. 2018, 3, 542−545. DOI: 10.1021/acsenergylett.8b00152. (7) Serpone, N.; Borgarello, E.; Graetzel, M. Visible Light Induced Generation of Hydrogen from H2S in Mixed Semiconductor Dispersions: Improved Efficiency through Inter-Particle Transfer. J. Chem. Soc., Chem. Commun. 1984, 6, 342−343. (8) Muradov, N. Z.; Rustamov, M. I.; Guseinova, A. D.; Bazhutin, Y. V. Photocatalytic Production of Hydrogen from H2S Solutions over CdS/ Pt Colloids. React. Kinet. Catal. Lett. 1987, 33, 279−83. (9) Buriak, J. M.; Kamat, P. V.; Schanze, K. S. Best Practices for Reporting on Heterogeneous Photocatalysis. ACS Appl. Mater. Interfaces 2014, 6, 11815−11816. (10) Qureshi, M.; Takanabe, K. Insights on Measuring and Reporting Heterogeneous Photocatalysis: Efficiency Definitions and Setup Examples. Chem. Mater. 2017, 29, 158−167. (11) Kisch, H.; Bahnemann, D. Best Practice in Photocatalysis: Comparing Rates or Apparent Quantum Yields? J. Phys. Chem. Lett. 2015, 6, 1907−1910. (12) Rajeshwar, K.; Thomas, A.; Janáky, C. Photocatalytic Activity of Inorganic Semiconductor Surfaces: Myths, Hype, and Reality. J. Phys. Chem. Lett. 2015, 6, 139−147. (13) Chen, R.; Yang, C.; Cai, W.; Wang, H.-Y.; Miao, J.; Zhang, L.; Chen, S.; Liu, B. Use of Platinum as the Counter Electrode to Study the Activity of Nonprecious Metal Catalysts for the Hydrogen Evolution Reaction. ACS Energy Lett. 2017, 2, 1070−1075.
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DOI: 10.1021/acsenergylett.8b00196 ACS Energy Lett. 2018, 3, 622−623
