Guest Commentary pubs.acs.org/JPCL
Nanocarbon Hybrids: Interactions with Luminophores to Applications in Energy Harvesting and Solar Fuel Production
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complexes and quantum dots, where bright emission of the luminophores stacked onto walls of CNTS is maintained without undergoing a significant quenching process. Although stability of the hybrids could be an issue, these hybrid materials could find applications in biochemical and sensor development. The second Perspective by Xiang and Yu (J. Phys. Chem. Lett. 2013, 4, 753−759) discusses the recent developments in one of the popular areas of graphene-based photocatalysts for hydrogen generation. Hydrogen, being one of the clean fuels, can be produced by photocatalytic water splitting into hydrogen and oxygen with the help of a catalyst and sunlight.7 Utilization of graphene as a substitute for noble metals (such as Pt) in the photocatalytic H2 production and enhancement of the photoactivity of graphene-based photocatalysts are the main focus of this Perspective. The authors have made a good effort summarizing recent significant advances in the design and efficient utilization of graphene-based photocatalysts in photocatalytic hydrogen generation. Mechanistically, the presence of graphene in these composites enhances the photocatalytic H2 production performance of photocatalysts in a number of ways, namely, by (i) reducing photogenerated electron and hole recombination, (ii) providing more adsorption and catalytic active sites, (iii) tuning the band gap size of the semiconductor, (iv) acting as a dye-like photosensitizer, and (iv) replacing platinum as a cocatalyst for water reduction. It is clear that further efforts are required to improve the overall performance by securing a firm understanding of the structure−reactivity relationships. For example, instead of commonly used binary nanocomposites, anisotropic multicomponent hybrid nanomaterials composed of graphene and different metals and/or semiconductors could be used, which would allow for fine tuning of the charge separation behavior through the interaction between multicomponent materials and graphene, leading to favorable band alignment. In this context, elegant studies on efficient separation of photoinduced charge carriers in graphene-based multicomponent composites to enhance photocatalytic activity using proper heterostructures by Imahori, Kamat, and co-workers,8 those on the graphene− CdSe composite by Kim and Park,9 and some of the elegant work of authors of the Perspective are noteworthy.10 The third Perspective from Anh and co-workers (J. Phys. Chem. Lett. 2013, 4, 831−841) is focused on the significance and application of utilizing graphene films for flexible organic and energy storage device applications. The authors bring out the importance of flexible graphene electrodes and discuss advantages and disadvantages of different synthesis methods, transfer of graphene onto desired substrates, chemical and electrostatic doping of the graphene film, and use of these flexible electrodes for a broad range of flexible devices such as photovoltaic, electronic, and electrochemical energy storage. Among the different synthesis procedures, the chemical vapor
arbon, one of the few elements known since ancient times, has become an indispensable material of modern civilization. It is abundant in Earth’s crust (∼0.2 wt %) and can be found in Nature in its elemental form as graphite, diamond, and coal.1 Carbon has been widely used in technology, with a record production of about 9 Gt/year.1 Nanostructured carbon allotropes have been extensively investigated in the past 2 decades, including single-walled carbon nanotubes (SWCNT),2 fullerenes,3 graphene,4,5 and their functionalized chemical derivatives. These materials exhibit desirable properties, including high carrier mobility, mechanical strength, optical absorption, and improved solubility of the chemically derivatized nanocarbons in organic solvents, relevant for energy harvesting and storage applications. In general, nanotubes, especially SWCNTs, due to their unique structural feature of high aspect ratio, have become novel materials in nanotechnology. Their helical structure determines the electronic and optical properties, the conductance, and the lattice structure, among others.2 Similarly, graphene exhibits exceptionally high electronic and thermal conductivity, optical transparency, and high specific surface area, combined with excellent mechanical flexibility and environmental stability. Consequently, both SWCNTs and graphene have stimulated interest in both academia and industry for various fundamental and applied purposes.4,5 In this issue of J. Phys. Chem. Lett., three inter-related Perspectives highlighting the recent progress in the development of nanocarbon hybrids for photochemical, photocatalytic, and energy storage applications are discussed. In the first Perspective (J. Phys. Chem. Lett. 2013, 4, 767−778), Mohanraj and Armaroli give an overview of the substantial work that has been done in the area of nanohybrids made out of chromophores/luminophores combined with CNTs through noncovalent interactions. By employing different self-assembly protocols involving intermolecular interactions (e.g., electrostatic, π−π stacking, etc.)6 and by utilizing the chemical nature of the photoactive component (organic or inorganic), a versatile combination of nanohybrids with interesting photophysical properties is highlighted. The authors have emphasized the improved chemical stability of the luminescent hybrids due to external decoration of the carbon scaffold with endohedral functionalized CNTs. These materials, owing to the wider vis− NIR absorbance and higher electric conductivity of CNTs, are found to be suitable for building organic photovoltaics (OPVs), eventually replacing those commonly used as electron acceptors in OPV. However, studies performed to date have demonstrated much weaker photoenergy conversion efficiencies, thus increasing the demand for better design and engineering of the photocells. Interestingly, as the authors rightly pointed out, confining organic guest molecules inside of CNTs offers the possibility of modifying the intrinsic electronic properties of the carbon scaffold, which is useful for building optoelectronic devices. Particular emphasis has been given to hybrid materials made of CNTs and inorganic luminophores, especially Ln(III) © 2013 American Chemical Society
Published: March 7, 2013 842
dx.doi.org/10.1021/jz400308p | J. Phys. Chem. Lett. 2013, 4, 842−843
The Journal of Physical Chemistry Letters
Guest Commentary
deposition (CVD)-grown graphene film on transition metals has the potential of scalable production of graphene electrodes for electronic and optoelectronic devices. Utilization of this material for solar cell application would still require a favorable work function and higher conductivity to facilitate rapid charge transfer. The mass production of graphene derivatives using bulk graphite will help in industrial-scale production for applications in large-energy-density lithium ion batteries. Nanocarbons, especially graphene as an electrode material for electrochemical capacitors, have also attracted much attention due to high power density, longer life cycles than batteries, and higher energy density than dielectric capacitors. In one example, high specific capacitances in the amount of 237 F/g have been reported in the case of flexible and uniform composites of graphene and polypyrrole, which are attributed to the favorable nucleation of new polymer chains at the defect site of the graphene surface.11 From the compiled information, it is clear that there is a great opportunity for monolayer and few-layer graphene-based conducting and transparent electrodes for applications in energy harvesting and storage. In summary, the Perspectives discussed here provide up-todate information on some of the key areas of nanocarbon research. Supramolecular nanohybrid formation via π−π stacking of organic and inorganic dye molecules seems to be a popular approach because such hybrids are not only useful for studying light-induced electron transfer but can also be useful for biochemical imaging and sensor applications. This is mainly due to retaining emission properties of the luminophore in the graphene−luminophore hybrids. Graphene decorated with multicomponent composites is a novel approach to build a new generation of water splitting catalysts because such heterostructures reveal enhanced photocatalytic activity. The same could be said for applications related to energy storage involving nanocarbons. Results on hybrids involving a blend of graphene, conducting polymers, or inorganic materials seem to be promising for the construction of high-capacity supercapacitors.12 The majority of the applications discussed here are on the lab-bench scale, with a great potential of commercialscale expansion. Much could be anticipated in the coming years from these fascinating nanocarbon materials.
(7) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655− 2661. (8) Hayashi, H.; Lightcap, I. V.; Tsujimoto, M.; Takano, M.; Umeyama, T.; Kamat, P. V.; Imahori, H. Electron Transfer Cascade by Organic/Inorganic Ternary Composites of Porphyrin, Zinc Oxide Nanoparticles, and Reduced Graphene Oxide on a Tin Oxide Electrode that Exhibits Efficient Photocurrent Generation. J. Am. Chem. Soc. 2011, 133, 7684−7687. (9) Kim, Y. K.; Park, H. How and to What Extent do Carbon Materials Catalyze Solar Hydrogen Production from Water? Appl. Catal., B 2012, 125, 530−537. (10) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Graphene-Based Semiconductor Photocatalysts. Chem. Soc. Rev. 2012, 41, 782−796 , and references cited therein. (11) Davies, A.; Audette, P.; Farrow, B.; Hassan, F.; Chen, Z.; Choi, J. Y.; Yu, A. Graphene-Based Flexible Supercapacitors: Pulse-Electropolymerization of Polypyrrole on Free-Standing Graphene Films. J. Phys. Chem. C 2011, 115, 17612−17620. (12) Pieta, P.; D’Souza, F.; Kutner, W. Preparation, Properties and Application of Polymer Composites of Carbon Nanotubes. In Handbook of Carbon Nano Materials; D’Souza, F., Kadish, K. M., Eds.; World Scientific Publishing: Singapore, 2011; Vol. 2, Chapter 21, pp 697−753.
Francis D’Souza
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University of North Texas, Denton, Texas, United States
AUTHOR INFORMATION
Notes
Views expressed in this Guest Commentary are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Cox, P. A. The Elements: Their Origin, Abundance, and Distribution; Oxford University Press: New York, 1989. (2) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56−58. (3) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (4) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (5) Geim, A. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (6) D'Souza, F.; Sandanayaka, A. S. D.; Ito, O. SWNT-Based Supramolecular Nanoarchitectures with Photosensitizing Donor and Acceptor Molecules. J. Phys. Chem. Lett. 2010, 1, 2586−2593. 843
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