Unsung Heroes of Energy Research “Go and explore older literature if you are looking for a new research theme,” advice we often hear from senior scientists. It is not uncommon to see a new research trend emerge from the original concepts laid out 30 or more years ago. In several instances we recognize the derivative work without recognizing the original contributors. There exists a treasure of older literature that can provide inspiration to advance modern scientific research (Figure 1). This Editorial draws attention to
attention toward developing solar cells. The concept of QDSCs is essentially similar to the dye-sensitization effect with semiconductor QDs acting as sensitizers. One of the early works came from Luebke and Gerischer demonstrating the size effect of CdS in the sensitization of TiO2 films.6 The paper by Hodes and co-workers on nanocrystalline photoelectrochemical cells is one of the early papers discussing the basic principles of nanocrystalline semiconductor (or QD) solar cells.7 The hot injection method to prepare size-selective semiconductor quantum dots by Bawendi’s group helped to further advance this field.8 Photocatalysis. The use of the term “photocatalysis” or “photocatalyst” can be seen as early as 1911 in the work on light-induced processes in uranyl compounds containing oxalic acid.9 Although most of the early photocatalytic studies referred to natural photosynthesis, metal oxides such as ZnO emerged as a new photocatalyst. The role of electrons and holes in photocatalysis was elucidated in an early electrochemical study.10 Semiconductor-Assisted Dye Degradation. One of the pioneers in dye degradation in semiconductor-assisted photocatalysis was Sister Markham at St. Joseph College who demonstrated photoinduced dye degradation on ZnO as part of a teaching experiment (Figure 2).11 She also pointed out the
Figure 1. Older literature remains a great treasure of inspiration to advance modern scientific research.
a few examples of older literature on light energy conversion, many of which have failed to receive their well-deserved recognition. The concepts cited in these early works identify the early visionaries of energy conversion research. Photosensitization of Semiconductors. Dye-sensitized solar cells have been a prominent field with nearly 25000 published papers on dye-sensitization between 2000 and 2018 (Source: Web of Science). It is important to recognize that the original motivation for color photography came through the sensitization of silver halides with dyes. The energetic considerations of the dye-sensitization process was laid down by Gerisher and coworkers.1 The photosensitization of TiO2 single crystals by a ruthenium(II) tris bipyridyl complex was attempted by Clark and Sutin in 1977.2 The smaller surface area of single crystals limited the dye interaction and hence the efficiency of the solar cell. The collection of papers on photogenerated charges from semiconductor particles at an electrode by the Bard group,3 design of a mesoscopic transparent TiO2 film using the sol−gel method (referred to as a membrane)4 by the Anderson group, and effective utilization of mesoscopic TiO2 films in dyesensitization experiments by the Grätzel group5 has helped shape modern day dye-sensitized solar cells. Quantum Dot Solar Cells (QDSCs). The size-dependent optoelectronic properties of semiconductor nanocrystals (commonly referred to as quantum dots or QDs) have drawn © 2018 American Chemical Society
Figure 2. Early report on the photocatalytic degradation of dyes on ZnO. From ref 11 (Copyright American Chemical Society).
relevance of photoinduced processes at semiconductor interfaces to solar energy conversion. The early research of Honda’s group on the degradation of rhodamine and methylene blue dyes on CdS surfaces with complete mechanistic details12,13 remains a seminal piece of work. Similar dye degradation studies have now produced over 14000 publications, with more than 10000 of these papers using methylene blue (Source: Web of Science). Surprisingly, the majority of recent papers practice less rigor in scientific details than the original papers and reiterate the same basic concepts. Cesium Lead Halide Perovskites. The recent burst of activity in metal halide perovskites for designing high-efficiency solar cells has drawn significant interest in understanding the fundamental chemistry of the making of these materials. The chemistry of a lead halide complex or plumbate complex is well documented in the literature. For example, early work on Published: June 8, 2018 1394
DOI: 10.1021/acsenergylett.8b00792 ACS Energy Lett. 2018, 3, 1394−1395
Editorial
Cite This: ACS Energy Lett. 2018, 3, 1394−1395
ACS Energy Letters
Editorial
(2) Clark, W. D. K.; Sutin, N. Spectral Sensitization of n-type TiO2 Electrodes by Polypyridineruthenium(II) Complexes. J. Am. Chem. Soc. 1977, 99, 4676−4682. (3) Ward, M. D.; Bard, A. J. Photocurrent Enhancement Via Trapping of Photogenerated Electrons of TiO2 Particles. J. Phys. Chem. 1982, 86, 3599−3605. (4) Anderson, M. A.; Gieselmann, M. J.; Xu, Q. Titania and Alumina Ceramic Membranes. J. Membr. Sci. 1988, 39, 243−258. (5) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (6) Gerischer, H.; Luebke, M. A Particle Size Effect in the Sensitization of TiO2 Electrodes by a CdS Deposit. J. Electroanal. Chem. Interfacial Electrochem. 1986, 204, 225−227. (7) Hodes, G.; Howell, I. D. J.; Peter, L. M. Nanocrystalline Photoelectrochemical Cells. A New Concept in Photovoltaic Cells. J. Electrochem. Soc. 1992, 139, 3136−3140. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715. (9) Bruner, L.; Kozak, J. Information on the Photocatalysis I The Light Reaction in Uranium Salt Plus Oxalic Acid Mixtures. Z. Elektrochem. Angew. Phys. Chem. 1911, 17, 354−360. (10) Morrison, S. R.; Freund, T. Chemical Role of Holes and Electrons in ZnO Photocatalysis. J. Chem. Phys. 1967, 47, 1543−1551. (11) Markham, M. C. Photocatalytic Properties of Oxides. J. Chem. Educ. 1955, 32, 540−543. (12) Watanabe, T.; Takizawa, T.; Honda, K. Photocatalysis through Excitation of Adsorbates. 1. Highly Efficient N-deethylation of Rhodamine B Adsorbed to CdS. J. Phys. Chem. 1977, 81, 1845−1851. (13) Takizawa, T.; Watanabe, T.; Honda, K. Photocatalysis through Excitation of Adsorbates. 2. A Comparative Study of Rhodamine B and Methylene Blue on CdS. J. Phys. Chem. 1978, 82, 1391−1396. (14) Wells, H. L. Ü ber die Cäsium- und Kalium-Bleihalogenide. Z. Anorg. Chem. 1893, 3, 195−210. (15) Moller, C. K. Crystal Structure and Photoconductivity of Caesium Plumbohalides. Nature 1958, 182, 1436−1436. (16) Calvin, M. Solar-Energy by Photosynthesis. Science 1974, 184, 375−381. (17) Infelta, P. P.; Graetzel, M.; Fendler, J. H. Aspects of Artificial Photosynthesis. Photosensitized Electron Transfer and Charge Separation in Cationic Surfactant Vesicles. J. Am. Chem. Soc. 1980, 102, 1479−1483.
synthesizing cesium lead halides can be seen in the work published by Wells in 1893,14 whose crystal structure was later identified by Møller in 1958.15 Artificial Leaf or Artificial Photosynthesis. Following the discussion of light-induced processes in photosynthesis (Melvin Calvin received the Nobel Prize in Chemistry 1961 for his research on CO2 assimilation in plants), attention was quickly drawn to artificial photosynthesis or the artificial leaf in the 1970s.16 One of the early works of Joseph Katz was highlighted in the New York Times with the title “An Artif icial Leaf Helps in Photosynthesis Study” (https://www.nytimes.com/1975/12/19/ archives/an-artificial-leaf-helps-in-photosynthesis-study.html). Attempts were also made by Fendler and co-workers to establish the concepts of artificial photosynthesis by investigating electron transfer in vesicles.17 These and many other early visionary scientists have inspired numerous researchers to explore new molecular and semiconductor catalysts to split water or reduce carbon dioxide. It is important to explore the genealogy of a research field while undertaking a new research problem. We often read claims such as “breakthrough”, “for the first time”, or “novel concept” by authors to draw the attention of readers and news media. The examples cited here show why one needs to exercise caution before making such claims. Many early contributions from the last century have significantly influenced the popular topics of modern day energy research. The early works mentioned in this Editorial are not meant to be allinclusive. They are examples and not a complete list of all of the great contributions by many leading research groups. The older literature shows the brilliant minds of scientists who recognized the importance of energy research early on when accessibility to instrumentation was limited. The time span also shows the efforts and time it takes to evolve a major research theme. Hence, it is imperative to recognize such early contributions and give credit to the real inventors. Welcoming New Senior Editor. We welcome Professor Phillip Christopher, Chemical Engineering at University of California, Santa Barbara (UCSB), USA, who has joined our editorial team as a Senior Editor starting last month. Prof. Christopher holds Mellichamp Cluster Chair in Sustainable Manufacturing at UCSB. Prof. Christopher brings us scientific expertise in the areas of catalysis, metal- and semiconductor-assisted photocatalysis, and solar fuels. His group is currently engaged in research on single-atom catalysis and catalysis restructuring, CO2 conversion chemistry, and biomass conversion. His expertise in these areas will help us broaden our editorial expertise and provide our authors a rapid publication platform.
Prashant V. Kamat, Editor-in-Chief
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University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Gerischer, H.; Willig, F. Reaction of Excited Dye Molecules at Electrodes. Top. Curr. Chem. 1976, 61, 31−84. 1395
DOI: 10.1021/acsenergylett.8b00792 ACS Energy Lett. 2018, 3, 1394−1395
