LIDA XING
NEWS & VIEWS RESEARCH a Epidendrosaurus ningchengensis
b Epidexipteryx ningchengensis
Figure 1 | The three known members of the Scansoriopterygidae. Fossils have been found from three members of this group of theropod dinosaurs: Epidendrosaurus ningchengensis, Epidexipteryx ningchengensis and the newly discovered form reported by Xu et al.4, named Yi qi. They are all unusual for theropods because the third finger is longer than the second, rather than the
have mitigated this problem. Still, we are left in a quandary: an animal with a strange structure that looks as if it could have been used in flight, borne by an animal that otherwise shows no such tendencies. And so far, there is no other plausible explanation for the function of this structure. Despite this aeronautic uncertainty, the paper is a milestone for another reason. The Yi qi fossil was found by a farmer, which is the case for many Chinese fossils. But Xu and colleagues provide more complete information about the geographical and geological provenance of their specimen than has accompanied other recent Chinese fossils collected by nonscientists. The Supplementary Information to
c Yi qi
other way round. It is considerably longer in Epidendrosaurus and even more so in Yi qi. The new dinosaur also sports a unique ‘styliform element’ on the wrist that seems to be made of bone and to have had a membranous structure attached. Reconstructions have been inferred from incomplete skeletons. Scale bars, 5 cm.
the paper documents how the authors verified the provenance of the specimen and even excavated fossils in associated local sediments. Moreover, they examined the specimen meti culously to be sure that none of its elements had been faked or restored. This is a key advance and sets the standard for future publications of specimens procured from third parties. The authors are due thanks for this diligence from the entire palaeontological community. ■ Kevin Padian is in the Department of Integrative Biology and the Museum of Paleontology, University of California, Berkeley, Berkeley, California 94720, USA. e-mail: [email protected]
MAT ERIALS CHEMISTRY
Organic polymers form fuel from water Porous polymers have joined the ranks of light-activated catalysts that split water into hydrogen, a carbon-free alternative to fossil fuels. Their properties are easily tuned — a big plus for the development of practically useful catalysts. V I J AY S . V YA S & B E T T I N A V. L O T S C H
T
he Sun could be harnessed as an unlimited source of energy by exploiting another naturally abundant resource: water. The light-induced splitting of water into oxygen and hydrogen generates storable chemical fuels that have no carbon footprint, potentially solving the world’s ever-increasing energy demands. The seemingly simple task of absorbing sunlight to split water requires a semiconductor catalyst, and inorganic catalysts are leading the field. Writing in the Journal of the American Chemical
Society, Sprick et al.1 demonstrate that organic photocatalysts (light-activated catalysts) may become just as useful as their inorganic counterparts, and offer intriguing opportunities for future research because the catalysts’ physical responses to light can be tailored. Designing photocatalytic materials for water splitting is far from easy: not only should they absorb light efficiently to form photoexcited states, but also the excitations should be long-lived and effectively lead to separation of charges at the catalyst’s surface, where the redox reactions needed for water splitting occur. Particulate photocatalysts often require
1. Chen, P., Dong, Z. & Zhen, S. Nature 391, 147–152 (1998). 2. Ji, Q. & Ji, S. Chin. Geol. 238, 38–41 (1997). 3. Long, J. & Schouten, P. Feathered Dinosaurs: The Origin of Birds (Oxford Univ. Press, 2008). 4. Xu, X. et al. Nature 521, 70–73 (2015). 5. Agnolín, F. L. & Novas, F. E. Avian Ancestors: A Review of the Phylogenetic Relationships of the Theropods Unenlagiidae, Microraptoria, Anchiornis and Scansoriopterygidae (Springer, 2013). 6. Padian, K. & Chiappe, L. in The Encyclopedia of Dinosaurs (eds Currie, P. J. & Padian, K.) 71–79 (Academic, 1997). 7. Padian, K. & Dial, K. P. Nature 438, E3; http:// dx.doi.org/10.1038/nature04354 (2005). 8. Rayner, J. M. V. J. Exp. Biol. 80, 17–54 (1979). 9. Pennycuick, C. J. Animal Flight (Arnold, 1972). This article was published online on 29 April 2015.
additional ‘sacrificial’ agents with a larger thermodynamic driving force than water to accept a light-generated charge carrier. This can dramatically facilitate charge separation, the largest bottleneck in the photocatalytic process. Moreover, the vast majority of photocatalysts are unlikely to work efficiently alone — a co-catalyst such as platinum or another noble metal is needed to lower the energy losses associated with hydrogen or oxygen evolution. Substantial research efforts are therefore focused on finding materials that offer better light harvesting, charge transport and conversion of water to hydrogen and oxygen. Most of these materials are inorganic semiconductors2, which are highly robust, but whose properties are often barely tunable. Although metal-free photocatalysts such as carbon nitrides have been reported3, soft organic polymers have not yet found their place in the race. Sprick and colleagues now highlight the potential of porous organic polymers for producing hydrogen. These soft materials offer ample opportunities for systematic engineering of their bandgap, a property that governs what part of the solar spectrum is absorbed, and that can be manipulated to improve the 7 M AY 2 0 1 5 | VO L 5 2 1 | NAT U R E | 4 1
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS
Total hydrogen evolved over 6 hours (µmol)
Bandgap (electronvolts)
effectiveness of photocatalysis. the rate of hydrogen evolution, a Organic materials formed from Sprick and colleagues devised a O O layers of two-dimensional atomic palladium-free synthesis for their B B Br Br HO OH O O B networks, which include crystalphotocatalysts, deliberately added Br Br line covalent organic frameworks4 platinum to some of their experiand amorphous conjugated micro ments, and performed other tests in Br Br O O B porous polymers (CMPs) 5, are which carbon monoxide was added B B Br Br HO OH O O chemically inert, thermally stable to ‘poison’ any traces of palladium. and have potentially useful optoAll of these studies suggested that electronic properties combined the rate of hydrogen evolution corwith high surface areas. They have relates more strongly with the optitherefore been widely used for gas cal bandgap than with noble-metal CMP1 to CMP15 adsorption, chemical sensing and content. b Increasing pyrene content 3.0 catalysis 6. These materials can Another notable feature of the be made from a broad range of CMP photocatalysts is their selec2.8 organic building blocks and bondtive visible-light activity, with 2.6 forming reactions, thus providing almost no activity observed in 2.4 an extensive toolbox for the sysultraviolet light. This unusual bias tematic fine-tuning of their strucrenders them ‘true’ visible-light 2.2 tural and physical properties. photocatalysts, boding well for the 2.0 Sprick et al. prepared a series of future design of photocatalysts that 15 different CMPs from phenylene absorb a large fraction of the visi1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 and pyrene building blocks, using ble-light spectrum with maximum palladium-catalysed reactions light-harvesting efficiency. 100 (Fig. 1). The polymers exhibit a Against the background of inorcontinuous variation in optical ganic semiconductor photocataly80 properties that depends on the sis, Sprick and co-workers’ findings 60 ratio of phenylene to pyrene units: highlight the power of organic increasing the pyrene content photocatalysis, with its armoury of 40 causes a gradual decrease in the molecular-engineering protocols 20 optical bandgap from 2.95 electronsuitable for producing complex volts to 1.94 eV, an effect that allows catalysts that have improved band0 the polymers to absorb increased gaps. But as with all breakthroughs, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 amounts of the solar spectrum. The there is more to be done: greater CMP researchers rationalize this trend by insight into charge-carrier dynamproposing that low-energy optical Figure 1 | Optical and photocatalytic properties of a series of porous ics and the nature of the catalyti1 excitations in the polymers become polymers. a, Sprick et al. have prepared 15 conjugated microporous polymers cally active sites will be required to dominated by contributions from (CMP1 to CMP15) from a mixture of phenylene-containing (red) and pyrene- further increase the efficiency of pyrene’s molecular orbitals as the containing (blue) building blocks, increasing the ratio of pyrene to phenylene polymer photocatalysts and make pyrene content rises. They also sug- across the series. b, The optical bandgap of the polymers decreases as the pyrene them practically viable. content increases, implying that the polymers’ ability to catalyse hydrogen gest that structural effects — such production from water when irradiated with visible light should increase across Tuning the catalytic activity as the formation of cyclic substruc- the series. In fact, the rate of hydrogen evolution peaked for CMP10, possibly will necessitate subtle control of tures (rings) and the strain within because of mechanisms that reduce the availability of electrons to take part in the polymers’ composition, and them — become more dominant the hydrogen-producing reaction in CMP11 to CMP15. especially of their crystallinity and as the pyrene component increases. microstructure. This poses grand The authors tested the porous polymers for the authors observed that hydrogen evolution synthetic challenges, and will probably require their ability to catalyse hydrogen evolution reaches a peak for the CMP that has a band- the development of alternative cheap, reversfrom water in visible light, using the organic gap of 2.33 eV (Fig. 1); it then declines for the ible polymerization protocols. Finally, a single compound diethylamine as a sacrificial elec- remaining polymers that have smaller band- porous polymer that catalyses ‘complete’ water tron donor. Remarkably, all of the polymers gaps. The authors suggest that recombination splitting — which does not require sacrificial promoted stable hydrogen evolution for at of separated charge carriers (which prevents electron donors — has yet to be realized. The least 6 hours, and the best polymer was shown electrons from being transferred from the race is on. ■ to work for more than 100 hours without polymer) becomes dominant in CMPs that much decline in activity. This behaviour was have smaller bandgaps, or that the kinetic Vijay S. Vyas and Bettina V. Lotsch are at the predicted by the researchers’ theoretical cal- barrier to electron transfer increases, both of Max Planck Institute for Solid State Research, culations, which showed that all the CMPs which would reduce hydrogen evolution. Stuttgart 70569, Germany, and the Chemistry have a strong thermodynamic driving force Remarkably, the polymers are active in the Department, University of Munich (LMU), to promote hydrogen evolution. Sprick et al. absence of any deliberately added noble met- Germany. observed no signs of light-induced degrada- als, providing a possible solution to the long- e-mail: [email protected]. tion of the CMPs in their experiments; such standing and much-researched question of Sprick, R. S. et al. J. Am. Chem. Soc. 137, 3265–3270 stability is a key prerequisite for any catalyst if how to minimize the amount of these expen- 1. (2015). it is to be more than just a laboratory curiosity. sive co-catalysts that is required. But as the 2. Simon, T. et al. Nature Mater. 13, 1013–1018 (2014). Wang, X. et al. Nature Mater. 8, 76–80 (2009). On the basis of the CMPs’ optical proper- authors point out, traces of palladium trapped 3. 4. Côté, A. P. et al. Science 310, 1166–1170 (2005). ties, the rate of hydrogen evolution would be in the CMPs during their synthesis might act as 5. Xu, Y., Jin, S., Xu, H., Nagai, A. & Jiang, D. Chem. Soc. 42, 8012–8031 (2013). expected to increase across the series of poly- masked co-catalysts. To rule out the possibility 6. Rev. Ding, S.-Y. & Wang, W. Chem. Soc. Rev. 42, 548–568 mers (that is, as the bandgap decreases). But that residual noble metals appreciably affect (2013). 4 2 | NAT U R E | VO L 5 2 1 | 7 M AY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
NEWS & VIEWS RESEARCH a Epidendrosaurus ningchengensis
b Epidexipteryx ningchengensis
Figure 1 | The three known members of the Scansoriopterygidae. Fossils have been found from three members of this group of theropod dinosaurs: Epidendrosaurus ningchengensis, Epidexipteryx ningchengensis and the newly discovered form reported by Xu et al.4, named Yi qi. They are all unusual for theropods because the third finger is longer than the second, rather than the
have mitigated this problem. Still, we are left in a quandary: an animal with a strange structure that looks as if it could have been used in flight, borne by an animal that otherwise shows no such tendencies. And so far, there is no other plausible explanation for the function of this structure. Despite this aeronautic uncertainty, the paper is a milestone for another reason. The Yi qi fossil was found by a farmer, which is the case for many Chinese fossils. But Xu and colleagues provide more complete information about the geographical and geological provenance of their specimen than has accompanied other recent Chinese fossils collected by nonscientists. The Supplementary Information to
c Yi qi
other way round. It is considerably longer in Epidendrosaurus and even more so in Yi qi. The new dinosaur also sports a unique ‘styliform element’ on the wrist that seems to be made of bone and to have had a membranous structure attached. Reconstructions have been inferred from incomplete skeletons. Scale bars, 5 cm.
the paper documents how the authors verified the provenance of the specimen and even excavated fossils in associated local sediments. Moreover, they examined the specimen meti culously to be sure that none of its elements had been faked or restored. This is a key advance and sets the standard for future publications of specimens procured from third parties. The authors are due thanks for this diligence from the entire palaeontological community. ■ Kevin Padian is in the Department of Integrative Biology and the Museum of Paleontology, University of California, Berkeley, Berkeley, California 94720, USA. e-mail: [email protected]
MAT ERIALS CHEMISTRY
Organic polymers form fuel from water Porous polymers have joined the ranks of light-activated catalysts that split water into hydrogen, a carbon-free alternative to fossil fuels. Their properties are easily tuned — a big plus for the development of practically useful catalysts. V I J AY S . V YA S & B E T T I N A V. L O T S C H
T
he Sun could be harnessed as an unlimited source of energy by exploiting another naturally abundant resource: water. The light-induced splitting of water into oxygen and hydrogen generates storable chemical fuels that have no carbon footprint, potentially solving the world’s ever-increasing energy demands. The seemingly simple task of absorbing sunlight to split water requires a semiconductor catalyst, and inorganic catalysts are leading the field. Writing in the Journal of the American Chemical
Society, Sprick et al.1 demonstrate that organic photocatalysts (light-activated catalysts) may become just as useful as their inorganic counterparts, and offer intriguing opportunities for future research because the catalysts’ physical responses to light can be tailored. Designing photocatalytic materials for water splitting is far from easy: not only should they absorb light efficiently to form photoexcited states, but also the excitations should be long-lived and effectively lead to separation of charges at the catalyst’s surface, where the redox reactions needed for water splitting occur. Particulate photocatalysts often require
1. Chen, P., Dong, Z. & Zhen, S. Nature 391, 147–152 (1998). 2. Ji, Q. & Ji, S. Chin. Geol. 238, 38–41 (1997). 3. Long, J. & Schouten, P. Feathered Dinosaurs: The Origin of Birds (Oxford Univ. Press, 2008). 4. Xu, X. et al. Nature 521, 70–73 (2015). 5. Agnolín, F. L. & Novas, F. E. Avian Ancestors: A Review of the Phylogenetic Relationships of the Theropods Unenlagiidae, Microraptoria, Anchiornis and Scansoriopterygidae (Springer, 2013). 6. Padian, K. & Chiappe, L. in The Encyclopedia of Dinosaurs (eds Currie, P. J. & Padian, K.) 71–79 (Academic, 1997). 7. Padian, K. & Dial, K. P. Nature 438, E3; http:// dx.doi.org/10.1038/nature04354 (2005). 8. Rayner, J. M. V. J. Exp. Biol. 80, 17–54 (1979). 9. Pennycuick, C. J. Animal Flight (Arnold, 1972). This article was published online on 29 April 2015.
additional ‘sacrificial’ agents with a larger thermodynamic driving force than water to accept a light-generated charge carrier. This can dramatically facilitate charge separation, the largest bottleneck in the photocatalytic process. Moreover, the vast majority of photocatalysts are unlikely to work efficiently alone — a co-catalyst such as platinum or another noble metal is needed to lower the energy losses associated with hydrogen or oxygen evolution. Substantial research efforts are therefore focused on finding materials that offer better light harvesting, charge transport and conversion of water to hydrogen and oxygen. Most of these materials are inorganic semiconductors2, which are highly robust, but whose properties are often barely tunable. Although metal-free photocatalysts such as carbon nitrides have been reported3, soft organic polymers have not yet found their place in the race. Sprick and colleagues now highlight the potential of porous organic polymers for producing hydrogen. These soft materials offer ample opportunities for systematic engineering of their bandgap, a property that governs what part of the solar spectrum is absorbed, and that can be manipulated to improve the 7 M AY 2 0 1 5 | VO L 5 2 1 | NAT U R E | 4 1
© 2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS
Total hydrogen evolved over 6 hours (µmol)
Bandgap (electronvolts)
effectiveness of photocatalysis. the rate of hydrogen evolution, a Organic materials formed from Sprick and colleagues devised a O O layers of two-dimensional atomic palladium-free synthesis for their B B Br Br HO OH O O B networks, which include crystalphotocatalysts, deliberately added Br Br line covalent organic frameworks4 platinum to some of their experiand amorphous conjugated micro ments, and performed other tests in Br Br O O B porous polymers (CMPs) 5, are which carbon monoxide was added B B Br Br HO OH O O chemically inert, thermally stable to ‘poison’ any traces of palladium. and have potentially useful optoAll of these studies suggested that electronic properties combined the rate of hydrogen evolution corwith high surface areas. They have relates more strongly with the optitherefore been widely used for gas cal bandgap than with noble-metal CMP1 to CMP15 adsorption, chemical sensing and content. b Increasing pyrene content 3.0 catalysis 6. These materials can Another notable feature of the be made from a broad range of CMP photocatalysts is their selec2.8 organic building blocks and bondtive visible-light activity, with 2.6 forming reactions, thus providing almost no activity observed in 2.4 an extensive toolbox for the sysultraviolet light. This unusual bias tematic fine-tuning of their strucrenders them ‘true’ visible-light 2.2 tural and physical properties. photocatalysts, boding well for the 2.0 Sprick et al. prepared a series of future design of photocatalysts that 15 different CMPs from phenylene absorb a large fraction of the visi1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 and pyrene building blocks, using ble-light spectrum with maximum palladium-catalysed reactions light-harvesting efficiency. 100 (Fig. 1). The polymers exhibit a Against the background of inorcontinuous variation in optical ganic semiconductor photocataly80 properties that depends on the sis, Sprick and co-workers’ findings 60 ratio of phenylene to pyrene units: highlight the power of organic increasing the pyrene content photocatalysis, with its armoury of 40 causes a gradual decrease in the molecular-engineering protocols 20 optical bandgap from 2.95 electronsuitable for producing complex volts to 1.94 eV, an effect that allows catalysts that have improved band0 the polymers to absorb increased gaps. But as with all breakthroughs, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 amounts of the solar spectrum. The there is more to be done: greater CMP researchers rationalize this trend by insight into charge-carrier dynamproposing that low-energy optical Figure 1 | Optical and photocatalytic properties of a series of porous ics and the nature of the catalyti1 excitations in the polymers become polymers. a, Sprick et al. have prepared 15 conjugated microporous polymers cally active sites will be required to dominated by contributions from (CMP1 to CMP15) from a mixture of phenylene-containing (red) and pyrene- further increase the efficiency of pyrene’s molecular orbitals as the containing (blue) building blocks, increasing the ratio of pyrene to phenylene polymer photocatalysts and make pyrene content rises. They also sug- across the series. b, The optical bandgap of the polymers decreases as the pyrene them practically viable. content increases, implying that the polymers’ ability to catalyse hydrogen gest that structural effects — such production from water when irradiated with visible light should increase across Tuning the catalytic activity as the formation of cyclic substruc- the series. In fact, the rate of hydrogen evolution peaked for CMP10, possibly will necessitate subtle control of tures (rings) and the strain within because of mechanisms that reduce the availability of electrons to take part in the polymers’ composition, and them — become more dominant the hydrogen-producing reaction in CMP11 to CMP15. especially of their crystallinity and as the pyrene component increases. microstructure. This poses grand The authors tested the porous polymers for the authors observed that hydrogen evolution synthetic challenges, and will probably require their ability to catalyse hydrogen evolution reaches a peak for the CMP that has a band- the development of alternative cheap, reversfrom water in visible light, using the organic gap of 2.33 eV (Fig. 1); it then declines for the ible polymerization protocols. Finally, a single compound diethylamine as a sacrificial elec- remaining polymers that have smaller band- porous polymer that catalyses ‘complete’ water tron donor. Remarkably, all of the polymers gaps. The authors suggest that recombination splitting — which does not require sacrificial promoted stable hydrogen evolution for at of separated charge carriers (which prevents electron donors — has yet to be realized. The least 6 hours, and the best polymer was shown electrons from being transferred from the race is on. ■ to work for more than 100 hours without polymer) becomes dominant in CMPs that much decline in activity. This behaviour was have smaller bandgaps, or that the kinetic Vijay S. Vyas and Bettina V. Lotsch are at the predicted by the researchers’ theoretical cal- barrier to electron transfer increases, both of Max Planck Institute for Solid State Research, culations, which showed that all the CMPs which would reduce hydrogen evolution. Stuttgart 70569, Germany, and the Chemistry have a strong thermodynamic driving force Remarkably, the polymers are active in the Department, University of Munich (LMU), to promote hydrogen evolution. Sprick et al. absence of any deliberately added noble met- Germany. observed no signs of light-induced degrada- als, providing a possible solution to the long- e-mail: [email protected]. tion of the CMPs in their experiments; such standing and much-researched question of Sprick, R. S. et al. J. Am. Chem. Soc. 137, 3265–3270 stability is a key prerequisite for any catalyst if how to minimize the amount of these expen- 1. (2015). it is to be more than just a laboratory curiosity. sive co-catalysts that is required. But as the 2. Simon, T. et al. Nature Mater. 13, 1013–1018 (2014). Wang, X. et al. Nature Mater. 8, 76–80 (2009). On the basis of the CMPs’ optical proper- authors point out, traces of palladium trapped 3. 4. Côté, A. P. et al. Science 310, 1166–1170 (2005). ties, the rate of hydrogen evolution would be in the CMPs during their synthesis might act as 5. Xu, Y., Jin, S., Xu, H., Nagai, A. & Jiang, D. Chem. Soc. 42, 8012–8031 (2013). expected to increase across the series of poly- masked co-catalysts. To rule out the possibility 6. Rev. Ding, S.-Y. & Wang, W. Chem. Soc. Rev. 42, 548–568 mers (that is, as the bandgap decreases). But that residual noble metals appreciably affect (2013). 4 2 | NAT U R E | VO L 5 2 1 | 7 M AY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
