View Online / Journal Homepage / Table of Contents for this issue
Downloaded by Sandia National Laboratories on 24 April 2012 Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30845B
This article is part of the
Porphyrins & Phthalocyanines web themed issue
Guest editors: Jonathan Sessler, Penny Brothers and Chang-Hee Lee All articles in this issue will be gathered together online at www.rsc.org/porphyrins
ChemComm
Dynamic Article Links
Cite this: Chem. Commun., 2012, 48, 4863–4865 www.rsc.org/chemcomm
COMMUNICATION
Downloaded by Sandia National Laboratories on 24 April 2012 Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30845B
Hierarchical cooperative binary ionic porphyrin nanocompositeswz Yongming Tian,ab Tito Busani,cd Gregory H. Uyeda,e Kathleen E. Martin,ad Frank van Swol,ad Craig J. Medforth,df Gabriel A. Montan˜oe and John A. Shelnutt*ag Received 6th February 2012, Accepted 2nd March 2012 DOI: 10.1039/c2cc30845b Cooperative binary ionic (CBI) solids comprise a versatile new class of opto-electronic and catalytic materials consisting of ionically self-assembled pairs of organic anions and cations. Herein, we report CBI nanocomposites formed by growing nanoparticles of one type of porphyrin CBI solid onto a second porphyrin CBI substructure with complementary functionality. Crystalline cooperative binary ionic (CBI) solids that are selfassembled from ionic porphyrin molecules provide a multifunctional class of micro- and nanoscale materials that strongly absorb visible and UV light.1 These CBI solids permit cooperative interactions between functionally complementary porphyrin subunits leading to interesting emergent collective properties that may be useful in a variety of opto-electronic and renewable energy-related applications. An important aspect of these CBI materials is the possibility of tuning their function in a predictable manner. While the basic properties of a porphyrin anion or cation can be controlled primarily by the choice of metals in the porphyrin rings, in the CBI crystal their spatial arrangement also determines function. For some CBI solids,1d,e it may already be possible to tailor functionality because, within limits, the nature of the metals in the porphyrin rings does not significantly alter the basic crystalline structure. Thus, metalloporphyrins may be chosen a
Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM 87106, USA Department of Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA c Universidade Nova de Lisboa at CENIMAT/I3N, Departamento de Cieˆncia dos Materials, Faculdade de Cieˆncias e Tecnologia and CEMOP-UNINOVA, 2829-516, Caparica, Portugal d Departments of Electrical and Computer Engineering, Chemical and Nuclear Engineering, and Biology, University of New Mexico, Albuquerque, NM 87106, USA e Center for Integrated Nanotechnologies, Los Alamos National Laboratories, Albuquerque, NM 87185, USA f REQUIMTE/Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆncias, Universidade do Porto, 4169-007 Porto, Portugal g Department of Chemistry, University of Georgia, Athens, GA 30602, USA. E-mail:
[email protected] w Electronic Supplementary Information (ESI) available: Large SEM images and XRD data and table of reflection angles for the Sn/Zn of the CBI clovers and nanocomposites. Included SEM images are the Sn/Sn clovers at 10%, 20%, and 40% loadings of Sn/Co material (Fig. S5), Zn/Zn clovers at 20% loading of Zn/Co material (Fig. S6), Zn/Sn clovers at 20% and 40% loadings of Sn/Sn material (Fig. S7), and Sn/Sn clovers at 20% and 40% loadings of Zn/Sn material (Fig. S8). See DOI: 10.1039/c2cc30845b z This article is part of the ChemComm ‘Porphyrins and phthalocyanines’ web themed issue. b
This journal is
c
The Royal Society of Chemistry 2012
to systematically alter the functionality of the solid without changing the spatial arrangement of the porphyrins. Conversely, we have recently shown that changing the peripheral substituents of just one of the porphyrin complexes while keeping the metals the same can significantly change the crystal structure, providing another dimension of variability to CBI materials.2 Initially, porphyrin-based CBI materials were prepared in the form of complex microscale morphologies, such as the four-leaf clover-like microstructures (with nanoscale features) shown in Fig. 1.1d These dendritic CBI structures form by diffusionlimited crystallization. The metals in the porphyrin anions and cations were chosen for their preferred metalloporphyin electronic functionalities, i.e., electron donor (Zn porphyrin) and acceptor (Sn porphyrin) abilities (Fig. 1a). Here, we show that different CBI solids that share the same crystal structure can be grafted together to form CBI nanocomposites that hierarchically organize the functional properties of the two types of CBI solids. The goal is to produce nanocomposites that can carry out multiple, spatially prescribed functions like those necessary for photosynthesis of carbon-based fuels using sunlight, including light harvesting, charge separation and transport, and catalysis. The Sn/Co CBI material (Fig. 1b) was chosen for the well known electrocatalytic activity of Co porphyrins in reducing CO23 and the generation of H2 and O2.4 The Sn/Co material is grafted onto the surface of Sn/Zn clovers that are efficient and stable photocatalytic/light-harvesting structures that reduce water to H2.1e The synthetic procedure developed produces the first hierarchical hybrid CBI structures shown in Fig. 2. The fabrication of these nanocomposites utilizes CBI materials with different metal combinations that are expected to have similar ionic crystal structures,1d,e enhancing the possibility of one CBI material nucleating and growing onto the other. The hybrid structures (Fig. 2) are obtained by first growing the Sn/Zn clover-like structures in Fig. 1a by mixing solutions of
Fig. 1 Sn/Zn (a) and Sn/Co (b) clover-like structures formed by diffusion-limited crystal growth upon mixing solutions of SnTPPS4 and ZnT(NEtOHPy)P4+or CoT(NEtOHPy)P4+.
Chem. Commun., 2012, 48, 4863–4865
4863
Downloaded by Sandia National Laboratories on 24 April 2012 Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30845B
Fig. 2 SEM images of CBI nanocomposite structures designed for possible visible light-driven CO2 reduction: Sn/Zn clovers with (a) 10% Sn/Co particles grown on surface, (b) 20% Sn/Co particles, (c) 40% Sn/Co particles, and (d) 60% Sn/Co particles.
Sn(OH)2 tetra(4-sulfonatophenyl)porphyrin4 (SnTPPS4) and Zn(II) tetra(N-hydroxyethyl-4-pyridiniumyl)porphyrin4+ (Zn(TNEtOHPyP4+)). After formation of the Sn/Zn clovers, solutions of a different pair of metalloporphyrins, SnTPPS4 and CoT(NEtOHPy)P4+, are added to the suspension of preformed Sn/Zn clovers. The Sn/Co solid, which in the absence of the Sn/Zn clovers forms Sn/Co clovers (Fig. 1b), nucleates and grows as nanoparticles onto the Sn/Zn clovers giving the nanocomposite structures shown in Fig. 2 with different loadings of the Sn/Co material. The SEM images of the Sn/Co–Sn/Zn nanocomposites, shown in Fig. 2, demonstrate that nanoparticles have grown onto the clover surface and that they become larger as the molar percent of Sn/Co material increases. At 10% Sn/Co, high magnification SEM images (Fig. S1, ESIw) show very small nanoparticles on the clover surface that are not readily visible in Fig. 2a. By 20% loading of the Sn/Co materials, particles as large as 50 nm are observed (bright spots in Fig. 2b), and by 40% the particles are large enough to significantly change the general appearance of the clovers (Fig. 2c). At 60%, the clover itself begins to take on the appearance of the pure Sn/Co clover (Fig. 1b). In order to verify that the particles on the clover surface indeed contain the Co porphyrin, the 20% Sn/Co–Sn/Zn clovers (Fig. 2b) were investigated by high-resolution SEM with element mapping by selected-area EDS (Fig. 3a). The numbered locations on the clover in Fig. 3 indicate the areas where EDS spot mapping experiments were performed. Areas 1 and 3 are locations where Sn/Co nanoparticles are grown onto the surface. The EDS spectra in Fig. 3b show that these areas contain Co. Areas 2, 4, and 5 that contain no nanoparticles show little or no Co peak. The commonality of the crystal structures of the CBI solids forming the morphologically similar clover-like structures with all four combinations of Zn and Sn in TPPS4 and T(NEtOHPy)P4+ [Sn/Zn, Zn/Sn, Zn/Zn, and Sn/Sn] was shown previously by XRD.1d Fig. S2 of the ESIw shows that the XRD patterns of the Sn/Zn clovers, the Sn/Co clovers, and the 20% and 40% hybrid structures are almost identical except for small angle displacements and intensity differences in the peaks. (See Table S1 and the caption of Fig. S2 of the ESI.w) Importantly, the XRD patterns for the hybrid structures show no additional reflections 4864
Chem. Commun., 2012, 48, 4863–4865
Fig. 3 (a) SEM image of a 20% Sn/Co–Sn/Zn clover coated with a 4 nm thick Au/Pd layer. The lines indicate regions where the EDS analysis was performed. (b) EDS spectra of spots on the 20% Sn/Co–Sn/Zn clover shown in (a). The Co peaks at 0.78 eV are observed for spots 1 and 3 where grafted Sn/Co nanoparticles are seen.
that might indicate a different crystal structure for the Sn/Co particles on the surface of the Sn/Zn clovers. Thus, the XRD data are consistent with the grafted Sn/Co CBI nanoparticles retaining the same basic crystal structure shared by the Sn/Zn and Sn/Co clovers. It is also possible that the nanoparticles might take on the crystallographic orientation of the underlying clover substrate. The fluorescence properties of the nanocomposites also reflect the different emission properties of the Sn/Co and Sn/Zn materials. The fluorescence properties of Sn/Zn, Sn/Co, and hybrid structures with 20% Sn/Co–Sn/Zn and 40% Sn/Co–Sn/ Zn were measured using a laser scanning confocal microscope and an excitation wavelength of 515 nm. At this excitation wavelength, the bulk fluorescence emission spectra for Sn/Zn and Sn/Co crystals are very similar except for the region between 560 and 620 nm (see Fig. S3 of the ESI.w) In this region, the Sn/Co clovers show a substantially increased fluorescence intensity compared to the Sn/Zn clovers that provides the ability to distinguish Sn/Zn material from Sn/Co material by their emission properties alone. To compensate for variations in clover thickness, fluorescence images of the four structure types (Sn/Zn and Sn/Co clovers and 20% and 40% hybrids) were measured simultaneously in two channels. Ch. 1 measured fluorescence in the region of 560–620 nm, where Sn/Co emission is greater than Sn/Zn emission, while Ch. 2 measured the region of 640–660 nm, where emission from the two crystal types is similar. Normalized images from the two channels and surface plots of the ratio between the two channels are shown for the four structures in Fig. 4 (see Fig. S4 of the ESIw for larger normalized images). For the Sn/Zn clover, the ratio between the two channels across the surface of the structure is relatively constant with a mean value of 0.38 and a relative standard deviation (RSD) of 8.0%. The ratio across the surface of the Sn/Co clover is similarly constant, but the magnitude of this ratio is much higher with a mean value of 0.84 and RSD of 7.5%. This difference reflects the different emission properties observed in the bulk emission spectra of the two clover types. For the 20% Sn/Co–Sn/Zn hybrid, the ratio across the surface is relatively constant but higher than that observed for the Sn/Zn clover, with a mean value of 0.43 and RSD of 7.7%. These results are consistent with the SEM image in Fig. 2b where small Sn/Co domains appear to be scattered across the underlying Sn/Zn This journal is
c
The Royal Society of Chemistry 2012
Downloaded by Sandia National Laboratories on 24 April 2012 Published on 06 March 2012 on http://pubs.rsc.org | doi:10.1039/C2CC30845B
Fig. 4 Surface plots of the ratio of Ch. 1 (560–620 nm) and Ch. 2 (640–660 nm) emission across single structures and the normalized fluorescence microscopy images for Ch. 1 (left) and Ch. 2 (right) for each plot. (a) Sn/Zn, (b) Sn/Co, (c) 20% Sn/Co–Sn/Zn, and (d) 40% Sn/Co–Sn/Zn. The scale bars in the normalized images are 2 mm.
clover surface. For the 40% Sn/Co–Sn/Zn hybrid, the ratio is not constant across the surface and in certain regions near the edges of the structure, the ratio becomes similar to that observed for the Sn/ Co clover. The mean value across the surface is 0.54 with an RSD of 27%. The variation observed across the surface is consistent with the extent of coverage shown in Fig. 2c and the morphology of the underlying Sn/Zn clover, which is thinner at the crystal edges. For the Sn/Zn, 20% Sn/Co–Sn/Zn, 40% Sn/Co–Sn/Zn, and Sn/Co structures, the mean ratio values from B40 crystals were found to be 0.31, 0.34, 0.44, and 0.74 with RSD values of 9.8, 11, 11, and 9.6%, respectively. To investigate the generality of synthetic strategies used in producing these hybrid CBI structures we attempted to prepare hybrid CBI clover-like structures with other metals in the porphyrins. SEM images are given in Fig. S5–S8 of the ESI.w Comparing the morphologies of these nanocomposites with the morphologies of the relevant clovers shown in Fig. S9, clear evidence of phase separation instead of particle grafting is seen. Examples of phase separation are the Zn/Co material added to the Zn/Zn clovers (Fig. S6) and Zn/Sn material added to the Sn/Sn clovers (Fig. S8a, Inset). In both cases, the morphologies of the clovers formed by the two CBI materials differ significantly, perhaps preventing grafting onto the preformed structures. Hierarchical nanocomposites (Fig. 2 and 3) demonstrate the grafting together of structurally similar but functionally dissimilar CBI solids. In this case, the underlying CBI clover consists of an electron donor (Zn porphyrin) and an electron
This journal is
c
The Royal Society of Chemistry 2012
acceptor (Sn porphyrin) giving an efficient light-harvesting substructure; the Sn/Co nanocrystals on the surface contain a Co porphyrin catalyst for CO2, O2, or H2O reduction. Such bio-inspired hierarchical nanoscale constructs have the potential for performing light-driven CO2 reduction to CO under visible light and mild conditions.3 The advent of CBI solids and now their hybrid nanocomposites has implications for producing a broad spectrum of advanced materials with applications in opto-electronics and renewable energy and provide guiding principles for producing CBI solids with myriad other non-porphyrin organic ions. Research supported by the United States Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. CJM is the recipient of a Marie Curie Fellowship from the Fundac¸a˜o para a Cieˆncia e a Tecnologia, Portugal and the Marie Curie Action Cofund. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DEAC04-94AL85000.
References 1 (a) Z. Wang, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc., 2004, 126, 15954–15995; (b) Z. Wang, K. J. Ho, C. J. Medforth and J. A. Shelnutt, Adv. Mater., 2006, 18, 2557–2560; (c) J. A. Shelnutt and C. J. Medforth, in Organic Nanomaterials, ed. G. Bottari and T. Torres, Wiley, 2012, p. in press; (d) K. E. Martin, Z. Wang, T. Busani, R. M. Garcia, Z. Chen, Y. Jiang, Y. Song, J. L. Jacobsen, T. T. Vu, N. E. Schore, B. S. Swartzentruber, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc., 2010, 132, 8194–8201; (e) Y. Tian, K. E. Martin, J. Y.-T. Shelnutt, L. Evans, T. Busani, J. E. Miller, C. J. Medforth and J. A. Shelnutt, Chem. Commun., 2011, 47, 6069–6071; (f) C. J. Medforth, Z. Wang, K. E. Martin, Y. Song, J. L. Jacobsen and J. A. Shelnutt, Chem. Commun., 2009, 7261–7277; (g) C. J. Medforth and J. A. Shelnutt, in Handbook of Porphyrin Science, ed. K. M. Kadish, K. M. Smith and R. Guilard, World Scientific Publishing, Hackensack, NJ, 2011, vol. 11, pp. 181–222; (h) C. M. Drain, A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109, 1630. 2 Y. Tian, C. Beavers, T. Busani, K. E. Martin, J. L. Jacobsen, B. S. Swartzentruber, F. van Swol, C. J. Medforth and J. A. Shelnutt, Nanoscale, 2012, 4, 1695–1700. 3 (a) N. Furuya and K. Matsui, J. Electroanal. Chem., 1989, 271, 181; (b) K. Leung, I. M. B. Nielsen, N. Sai, C. Medforth and J. A. Shelnutt, J. Phys. Chem. A, 2010, 114, 10174–10184. 4 (a) C. H. Lee, D. K. Dogutan and D. G. Nocera, J. Am. Chem. Soc., 2011, 133, 8775–8777; (b) A. J. Olaya, D. Schaming, P.-F. Brevet, H. Nagatani, T. Zimmermann, J. Vanicek, H.-J. Xu, C. P. Gros, J.-M. Barbe and H. H. Girault, J. Am. Chem. Soc., 2012, 134, 498–506.
Chem. Commun., 2012, 48, 4863–4865
4865
