Special Issue Preface pubs.acs.org/JPCC
Preface to the Festschrift in Honor of Professor Michael Grätzel, Pioneer of Mesoscopic Solar Cells During this period he gained interest in some select areas of chemistry such as the chemistry of colloids and polymers using spectroscopic methods, and these systems stayed in his focus for several years thereafter.
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PULSE RADIOLYSIS AND LASER FLASH PHOTOLYSIS The nuclear bombs dropped in Japan to bring a rapid conclusion to WWII caused profound damage not only to buildings but also to the human population on a large scale never witnessed before in human history. Scientific community was at a loss to understand fully the origin of huge lesions of human body exposed to the radiation. This set the scientific community to undertake a systematic study of the effects of high-energy radiation on matter. The discipline evolved quickly to be known as radiation chemistry. Investigation of primary photophysical processes following a short flash-light excitation was already an established technique in the 1960s called flash photolysis. With the introduction of Q-switched lasers, the time scale of investigation was extended to a few nanoseconds. Linear accelerators and van de Graff generators are excellent sources to generate short high-energy electron pulses (few nanoseconds duration with energies in excess of 1 MeV). Scientists started using such electron pulses in a methodology similar to laser flash photolysis. Pulse radiolysis thus evolved as a sister technique of flash photolysis to study the nature of the excited states and radical ion intermediates formed following the absorption of highenergy radiation by materials as solids and in the solution phase. Several laboratories across the globe developed as leading centers for pulse radiolysis studies. Notable ones are the Radiation Laboratory at the University of Notre Dame, Argonne National Laboratory in Chicago, Carnegie Mellon University in Pittsburgh, Hahn−Meitner Institute in Berlin, RIKEN in Tsukuba, Japan, and the University of Leeds in Leeds, U.K. Prof. Arnim Henglein of the Hahn−Meitner Institute, Berlin spent a sabbatical year at the Carnegie Mellon Institute working on pulse radiolysis. He decided to continue such studies upon his return to Berlin. Michael Grätzel started his doctoral thesis work under the guidance of Prof. Arnim Henglein at HMI Berlin. Following the optical absorption changes after short pulse excitation was the standard method used to characterize the photo- or radiation-induced primary products. Henglein and Grätzel added another powerful technique, viz., pulsed polarography. As part of his doctoral thesis work, Grätzel examined the properties of several short-lived radicals of organic molecules by monitoring their polarographic behavior on hanging mercury drops.
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e are immensely pleased to dedicate this special issue of The Journal of Physical Chemistry C to Prof. Michael Grätzel on his 70th Birthday. One topic that pervades all of the scientific research activities of Michael Grätzel spanning 45 years is “artificial photosynthesis”. It is the process of identification of photochemical methods by which the fundamental processes of natural photosynthesis are duplicated in a simpler but more effective manner. One template system to bring together key components that Grätzel identified and exploited elaborately is mesoscopic thin films composed of nanocrystalline semiconductor particles such as titanium dioxide. Systematic research by his group and many other laboratories across the globe over 25 years has led to the development of “mesoscopic solar cells” for efficient and direct conversion of sunlight to electricity. The “tunability” of each and every component of mesoscopic solar cells has given ample opportunities for chemists of every orientation (organic, inorganic, physical, and theoretical) to participate in the development of this new form of solar cells. More than 100 articles contributed by students, postdoctoral associates, collaborators, and colleagues to this special issue attest to the rich chemistry this field has brought up as well as the important role Michael Grätzel and his group have played in this area of science. Michael Grätzel was born in May 1944 in the small town of Dorfchemnitz, near Dresden in the former Eastern Germany. Dresden and its neighborhood were recovering from the vast destruction caused by World War II. The Grätzel family moved to Berlin so that young Michael Grätzel could pursue his first degree in chemistry at the Free University of Berlin following the completion of his primary and secondary level studies up to grammar school. Michael had already built an orientation to take up a research career in chemistry, and he undertook a project research in the area of polymer chemistry, receiving an M.S. degree also from the Free University of Berlin in 1968. © 2014 American Chemical Society
Special Issue: Michael Grätzel Festschrift Published: July 31, 2014 16303
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The Journal of Physical Chemistry C
Special Issue Preface
In the German academic tradition, researchers were keen to take up a teaching career, undertake semi-independent research work for a few years, and write up the key results in a dissertation known as “Habilitation”. Upon a successful defense of the Habilitation, German Universities confer the title of privatdocent (honorary lecturer) with associated rights to teach courses at the university level. As a member of the Argonne National Laboratory, Prof. J. Kerry Thomas developed in the 1960s parallel techniques that extended the time scale of pulse radiolysis studies to a few nanoseconds. In 1970, Prof. Thomas moved to the Radiation Laboratory of the University of Notre Dame in Indiana to continue mechanistic studies and to extend the time scales further to a few picoseconds. Michael Grätzel decided to go across the Atlantic to pursue prehabilitation work at the Radiation Laboratory of the University of Notre Dame in Indiana and joined the research group of Prof. J. Kerry Thomas.
organized molecular assemblies became a principal area of research of Michael Grätzel for several years thereafter. After two very productive years devoted to photophysical and radiolysis studies in the U.S., Michael Grätzel returned to Germany in 1974 to rejoin the radiation chemistry group (Bereich Strahlenchemie) of the Hahn−Meitner Institute, Berlin. Prof. Heinz Gerischer was a German chemist who spent nearly two decades at the Technical University of Munich studying the electrochemical behavior of semiconducting materials such as CdS, ZnO, GaAs, and silver halides in the dark and under illumination. The experimental techniques and the theoretical models he developed formed the foundations of photoelectrochemistry and extended them to the new area of photoelectrochemical energy conversion and storage. In 1970, Prof. Gerischer moved to Berlin to take up the directorship of one of the laboratories of the Max Planck Research Institute network, viz., Fritz-Haber Institute. Soon after his return to Germany, Dr. Grätzel wrote up his Habilitation thesis and was fortunate to defend it successfully before a jury headed by none other than Prof. Heinz Gerischer. For several years, Prof. Arnim Henglein and PD Dr. Michael Grätzel continued their studies of the spectral and redox properties of colloidal particles of noble metals and of semiconductors.
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PHOTOCHEMISTRY IN MICROHETEROGENEOUS SYSTEMS As young scientists, Michael Grätzel and Prof. J. Kerry Thomas decided to go beyond the conventional solute−solvent systems for time-resolved studies. They decide to venture onto a hitherto unexplored domain, viz., structural studies of colloidal and self-assembled systems using time-resolved techniques of laser photolysis and pulse radiolysis. Micelles formed by longchain surfactant molecules and lipid vesicles formed by lecithin were taken up as model systems of more complex biological systems. It was a novel, unorthodox initiative with several potential risks. The presence of impurities (oxygen and other chemicals present) considerably reduces the lifetimes of photoor radiation-induced excited states and intermediates. Hence time-resolved studies required the use of ultrapure solvents and solutes. Surfactants are the core components of detergents, and the detergency action is based on the ability of surfactant assemblies to sequester/extract dirt and accumulate them in their interiors. So there was a lot of skepticism on the reliability of data obtained in such systems. Light scattering by colloidal particles is another issue that has to be handled adequately. The presence of small islands or pockets of enhanced hydrophobicity effectively turn microheterogeneous systems of micellar and lipid vesicles into two-phase systems. Using surfactants, it is possible to solubilize a fairly high concentration of hydrophobic solutes (such as aromatic hydrocarbons) in aqueous solutions. Due to the existence of pockets of varying hydrophobicity, the solute distribution in such microheterogeneous systems is no longer isotropic, and classical descriptions of bimolecular reactions with conventional diffusion models are inadequate. Special theoretical models based on statistical distribution of solutes and molecular movements in restricted geometries have to be developed. Some of the kinetic models developed at Notre Dame using Poisson statistics have been successfully employed to describe quantitatively the photophysical properties and electron-transfer reactions of solutes immobilized in such microheterogeneous systems. One of the interesting observations made in the photophysical studies of aromatics was intense phosphorescence observed in aerated solutions of micellar aggregates at room temperature. The presence of longlived triplet states with intense radiative emission significantly extended the time scale for probing dynamical properties of these aggregates, such as the rate of entry and exit of solutes in and out of micelles. Studies of triplet-state processes in
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PHOTOPHYSICS AND PHOTOREDOX CHEMISTRY OF COLLOIDAL SEMICONDUCTORS In 1969, the engineering college of the University of Lausanne, Switzerland received a major upgrade as a national institute for the teaching of science and technology for the French-speaking region of the Switzerland. The Ecole Polytechnique of the University of Lausanne (EPUL) was upgraded to the Ecole Polytechnique of the Confederation (EPFL), on par with the well-known ETH of Zürich. The Swiss government provided considerable financial resources for a rapid stimulated growth. Two research groups dominated the physical chemistry research at EPFL, one in radiation chemistry by Prof. T. Gaumann and the other on the spectroscopy and photochemical reaction kinetics by Prof. Jürgen Troe. In 1976, Prof. Tröe took a call from the University of Göttingen and returned to Germany. High-impact research that Michael Grätzel carried out in Berlin and in the U.S. so impressed his mentors that they enthusiastically supported his nomination as “professor extraordinaire” (associate professor) of EPFL at the young age of 32 in 1976. The Physical Chemistry Institute of EPFL already had a 2.0 MeV Van de Graff generator for pulse radiolysis studies and a number of Q-switched lasers for laser photolysis studies on the nanosecond time scale, basic tools of Grätzel’s research. In the setting of his research group, Michael Grätzel was fortunate to attract several collaborators with whom he worked or interacted at either Berlin or Notre Dame (John Kiwi, Pierre Infelta, and K. Kalyanasundaram). By combining spectroscopic, theoretical, and time-resolved methods, Grätzel’s group set on to study the photophysical properties and interfacial electron transfer involving colloidal semiconductor particles. Oil embargo imposed by the countries of the Gulf states region during 1973 (and in 1979, albeit on a smaller scale) had a profound influence on the economics of several industrially developed countries and on the lifestyle of their population. People became aware and even conscious of the fact that the natural fossil fuel reserves are limited and that the world is consuming them at an alarmingly fast rate, accompanied by a gradual degradation of the atmosphere/environment we live in. 16304
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Special Issue Preface
It is the only photovoltaic cell that uses a molecular sensitizer to absorb light and generate charge carriers. In this fashion, it successfully mimics the process that green plants and algae use in natural photosynthesis. Through his highly innovative and painstaking research, Prof. Grätzel has maintained a leading position in the domain of mesoscopic solar cells since the inception of the new solar cells in 1991. His group conceived and molecularly engineered transition-metal sensitizers showing panchromatic light absorption and very high quantum yields of charge-carrier generation. Performing in-depth kinetic studies, he discovered that the electron injection from the excited sensitizer into the conduction band of titanium dioxide occurs on a femtosecond time scale, often involving vibrationally hot states, while the electron back-reaction takes place at a much slower rate. He also found that rapid cross-surface charge transport can occur within the self-assembled monolayers of surface-adsorbed dye molecules. These striking findings have stimulated extensive theoretical and experimental investigations of such ultrafast interfacial electron transfer reactions. Using time-resolved DFT theory, Grätzel and colleagues have performed theoretical studies to provide a rationale for the unique light absorption and ultrafast charge-separation features of the sensitizer molecules. Due to their excellent stability and unmatched performance, these ruthenium complexes remain the state-ofthe-art sensitizers in today’s mesoscopic injection solar cells. Meanwhile, Grätzel’s group continues to develop new organic dyes that show near-infrared response enhancing the efficiency of power generation from sunlight by these molecular photovoltaic systems. Grätzel also pioneered the use of organic hole conductors and ionic liquids to replace the redox electrolytes in the DSSC, which were initially based on volatile solvents. His publications on the solid-state hole conductor cell (Nature 1998) and a fundamental paper on new hydrophobic ionic liquids displaying low viscosity (Inorganic Chemistry 1996) have been cited 2002 and 3015 times, respectively, revealing the large impact of his work. The new hydrophobic low-viscosity ionic liquids that his team discovered have found widespread applications and are produced commercially. The breakthroughs made by his team in this area have dramatically increased the stability of the dye-sensitized solar cells under prolonged light soaking or heat stress, fostering their practical development for outside use. During his studies on the fundamental aspects of chargetransfer reactions in mesoscopic systems, Grätzel discovered that a rapid cross-surface charge transport can take place in selfassembled monomolecular layers of redox-active molecules, which are adsorbed at the surface of electrically insulating materials. This intriguing surface conduction process that occurs by charge percolation via hopping between adjacent electroactive molecules within the monolayer has meanwhile been exploited for molecular wiring of insulator nanocrystals and redox targeting of lithium ion battery cathode materials having very low electronic conductivity. One of the main advantages of the mesoporous layer composed of nanosized particles is the enormous surface area available for anchoring other molecules in a monolayer fashion without significant aggregation effects. This feature can be exploited in select materials for display applications. There are a number of molecules that undergo drastic color changes upon reduction (e.g., methyl viologen) or oxidation (e.g, tungsten bronzes). Anchoring of such molecules on high-surface-area mesoporous titanium dioxide layers leads to a display material
The possibility of using the abundant solar radiation reaching the earth’s surface has been previously invoked, even at the beginning of the 20th century, but only a few scattered studies examined the issues involved in depth. Attracted by finely divided photoactive inorganic materials that could be used in solar energy conversion, in a pioneering approach, Grätzel prepared nanoparticles of colloidal semiconductors and studied light-induced charge-carrier reactions in these systems by time-resolved laser photolysis. He also discovered that metal-oxide nanoparticles such as RuO2 act as very effective redox catalysts promoting the oxidation of water to oxygen by analogy to the water-splitting enzyme in photosystem II of green plants. One of his key findings during this period was that nanocrystalline wide band-gap oxide particles, such as colloidal TiO2, can be very effectively sensitized to visible light by covering them with a monolayer of a ruthenium dye endowed with appropriate anchoring groups. It was noted that the rate of photoinduced electron injection from the excited state of the sensitizer into the conduction band of the oxide nanoparticle was found to be many orders of magnitude faster than the dark back reaction, that is, the recapture of the electron by the oxidized dye. This later turned out to be of crucial importance for Grätzel’s success in developing mesoscopic solar cells. As with many other chemists, a small research note by Fujishima and Honda in 1972 on possible photoelectrochemical decomposition of water using TiO2 and wide band-gap titanates that are stable in aqueous solutions stimulated Michael Grätzel to devote specific attention to colloidal nanoparticles of titania. Fundamental studies on the properties of colloidal particles of metal dichalcogenides such as PbS and PbSe laid the groundwork for the development of semiconductor quantum dots showing size quantization effects in their absorption and emission properties. Research in this field was soon booming, and it continues to attract enormous attention by scientists in physics, chemistry, and material science alike. Grätzel’s research on nanocrystalline semiconductors in the 1980s laid the foundation for the invention of the mesoscopic solar cells, which he announced in a landmark paper in Nature in 1991. According to the ISI Web of Science index, this paper has been cited 12 100 times since then, with the number of citations growing every year. Over 1600 patents have been filed on the mesoscopic solar cells with maximum solar-to-electrical conversion efficiency in excess of 12% to date. The enormous attention that has been paid to this discovery is explained by the fact that the solar cell described in the Nature paper presented an entirely new paradigm in photovoltaic technology: mesoscopic solar cells. Mesoscopic solar cells, where the electron- and hole-conducting material form an interpenetrating network in contrast with conventional p-n solar cells that employ flat junctions, are presently being investigated worldwide by a very large number of scientists. The prototype of this new photovoltaic family is the dye-sensitized solar cell (DSSC), developed in Lausanne, which employs dye molecules or semiconductor quantum dots to sensitize a nanocrystalline wide band-gap semiconductor film. The key role that Grätzel has played in the development of this novel form of solar cell has been recognized by the scientific community, which refers to the dye-sensitized solar cell simply as “Grätzel Cell”. The revolutionary approach to use high-surface-area structure in a mesoporous morphology has allowed us to reach for the first time very high efficiencies in a photovoltaic conversion process that separates solar light harvesting and charge-carrier transport. 16305
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The Journal of Physical Chemistry C
Special Issue Preface
several EU network research programs under FP7 (ESCORT, SANS, MOLESOL, GLOBASOL, POWERWEAVE, NANOMATCELL) and COST. Prof. Grätzel has been a visiting professor at the Lawrence Berkeley Lab, University of California, National Renewable Energy Lab (NRE), Golden, CO, USA, Delft University of Science and Technology, National University of Singapore, and Nangyang Technolofical University of Singapore. In recognition of his scientific contributions, Prof. Grätzel has received honorary doctorates in 11 leading universities of Europe and South Asia. A unique feature of the dye -sensitized solar cell is total tunability of nearly every key component (oxide anode, cathode, dyes, electrolytes, redox mediator) and even the morphology of components and packaging (liquid electrolytes to quasi-solid-state and solid-state versions). Hence DSC has evolved as an active playground for any chemist to try out his expertise in synthesis, characterization, or theoretical modeling. During the last two decades, Prof. Grätzel has visited nearly every laboratory in various South and Southeast Asian countries actively working in the development of dye solar cells. He actively supported the setup of research laboratories in the area of solar energy conversion and storage by spending several weeks or months (planning, fund procurement, and installation) in Singapore, China, Korea, Saudi Arabia, and Taiwan. Wuhan University of Science and Technology at Wuhan, PR China has honored him by setting up a solar cell R&D lab bearing his name (Michael Grätzel Center for Mesoscopic Solar Cells). Michael Grätzel has published over 1100 papers in the area of time-resolved spectroscopy, heterogeneous and interfacial electron transfer, colloidal semiconductors, photochemical water splitting, and biosensors, to cite the main ones. His research publications have been cited extensively, over 122 000 times, with an h-index of 162, placing him among the top-ten most cited chemists in the world. A list of the 200 most-cited papers included in this issue highlights the key papers where there have been numerous follow-up studies. The curriculum vitae of Prof. Grätzel also included in this special issue lists the numerous awards and honors he has received in recognition of his contributions. It has been a great personal privilege and honor to be associated with Prof. Michael Grätzel for many years. We take this opportunity to thank all of the authors who contributed papers for this special issue.
where the color can be turned on electrochemically or photochemically. Using suitable structural modification, it is possible to vary the relevant redox potentials and spectrally tune the color changes. Grätzel’s group has investigated the spectral and photoelectrochemical behavior of a number of reagents for photochromic and electrochemical display applications. Systematic studies on nanocrystalline junctions prompted important new discoveries made in Lausanne in areas well beyond the solar cell field. Thus, Grätzel was the first to employ nanocrystalline materials in rechargeable batteries, which lead to the realization of high-power lithium ion batteries based on cathode materials such as LiMnPO4 that exhibit extreme lithium insertion and release. Thus, he was the first to employ nanocrystalline materials to realize high-power lithium ion batteries and to develop new electrochromic and electroluminescent displays as well as biosensors. Finally, Grätzel has made ground-breaking discoveries in the field of solar production of hydrogen. His recent work on water photolysis using mesocopic iron oxide films sets a new benchmark in this important field of fuel generation from sunlight.
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FAMILY EXPANSION
The 1980s also witnessed substantial changes in the personal family situation of Michael Grätzel. In 1978, a young American chemist, Dr. Carole Parker of Houston, Texas, joined the research group of Prof. Tino Gäumann at EPFL on a postdoctoral assignment upon completion of her thesis work at the University of Rice. At that time, Carole possibly did not imagine that her stint at EPFL would last much longer and that Lausanne itself would become her second home. Casual meetings of the young postdoc with the young and energetic professor Michael Grätzel in the chemistry building soon became regular, love blossomed, and the two finally got married in 1980. The Grätzels saw their son Chauncey and two lovely daughters Amy and Liliane born one after the other. Despite the heavy demand on her time of raising the three children, Carole keeps her interests in chemistry alive and remains a key member of Michael Grätzel’s research group. The Grätzels have managed to pass on their passion for science to all of their children, with them pursuing higher studies in science and engineering at the university level. Grätzel’s son Chauncey Grätzel did his undergraduate studies in microengineering at EPFL, followed by a year at the University of Texas at Austin. This short stint enabled another young American lady, Jenny, to the Grätzel family as the daughter-in-law of Michael Grätzel.
K. Kalyanasundaram Laboratory for Photonics and Interfaces (LPI), Institute of Chemical Sciences and Engineering (ISIC) Swiss Federal Institute of Technology at Lausanne (EPFL)
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INTERNATIONAL COLLABORATION Throughout four decades of teaching and research, Prof. Michael Grätzel actively collaborated with scientists from all four corners of the world. His ability to forge international collaboration and cooperation permitted him to receive largescale funding, and he has maintained a fairly large research group during the last 25 years (averaging about 50 doctoral students, postdocs and visiting scientists in any year). His research group consists of scientists coming from India, China, Japan, Korea, Germany, USA, Italy, U.K., Israel, Taiwan, Iran, and Thailand. Prof. Grätzel has been a popular and frequently sought speaker in international conferences. (He gives more than 50 keynote or invited lectures per year.) Nearly all leading laboratories of Western Europe have collaborated with the Lausanne group on the development of DSC supported by 16306
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