Letter pubs.acs.org/NanoLett
Morphology-Controlled Self-Assembly and Synthesis of Photocatalytic Nanocrystals Yong Zhong,†,‡ Jiefei Wang,†,‡ Ruifang Zhang,†,‡ Wenbo Wei,†,‡ Haimiao Wang,†,‡ Xinpeng Lü,†,‡ Feng Bai,*,†,‡,§ Huimeng Wu,∥ Raid Haddad,§ and Hongyou Fan*,§,∥ †
Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Kaifeng 475004, People’s Repubic of China ‡ Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, China § Department of Chemical and Biological Engineering, The University of New Mexico/NSF Center for Micro-Engineered Materials, Albuquerque, New Mexico 87131, United State ∥ Sandia National Laboratories, Advanced Materials Laboratory, Albuquerque, New Mexico 87106, United States S Supporting Information *
ABSTRACT: Abilities to control the size and shape of nanocrystals in order to tune functional properties are an important grand challenge. Here we report a surfactant selfassembly induced micelle encapsulation method to fabricate porphyrin nanocrystals using the optically active precursor zinc porphyrin (ZnTPP). Through confined noncovalent interactions of ZnTPP within surfactant micelles, nanocrystals with a series of morphologies including nanodisk, tetragonal rod, and hexagonal rod, as well as amorphous spherical particle are synthesized with controlled size and dimension. A phase diagram that describes morphology control is achieved via kinetically controlled nucleation and growth. Because of the spatial ordering of ZnTPP, the hierarchical nanocrystals exhibit both collective optical properties resulted from coupling of molecular ZnTPP and shape dependent photocatalytic activities in photo degradation of methyl orange pollutants. This simple ability to exert rational control over dimension and morphology provides new opportunities for practical applications in photocatalysis, sensing, and nanoelectronics. KEYWORDS: Morphology control, self-assembly, photocatalytic nanocrystals, hierarchical, porphyrin
N
building block through micelle confined self-assembly.15−17 These alkanethiol-stabilized gold nanocrystals are hydrophobic and can be treated as a hydrophobic entity that can be individually encapsulated into the hydrophobic interior of a surfactant micelle in water. During the self-assembly, surfactants serve as active agents to reduce interfacial force between hydrophobic gold nanocrystals and water. The encapsulation of gold nanocrystals inside the surfactant micelles is thermodynamically favorable for further self-assembly (or nucleation and growth) into superlattices with well-defined shape. In this work, we extended this self-assembly induced micelle encapsulation synthetic method to fabricate porphyrin nanocrystals using the optically active precursor zinc-tetra(4-pyridyl) porphyrin (ZnTPP) (see Figure S1 in Supporting Information). Through confined noncovalent interactions of ZnTPP within surfactant micelles, nanocrystals with a series of morphologies including nanodisk, tetragonal rod, and hexagonal rod, as well as amorphous spherical particle are synthesized. Through combined π−π interaction and coordination of the peripheral
anocrystals synthesized from the noncovalent selfassembly of molecular precursor exhibit unique electronic and optical properties stemming from the molecular building blocks.1−8 More importantly, the ability to control size and shape provides enhanced properties due to size- and shapedependent effects and collective behavior from self-assembled vicinity building blocks.9,10 In a similar manner to biological systems, synthetic self-assembly relies on one or more noncovalent interactions, such as van der Waals forces, hydrogen bonding, aromatic π−π stacking, and axial coordination, to produce nanocrystals from a single molecule and provides an effective approach for morphology and sizecontrolled synthesis and self-assembly of molecular nanocrystals.11,12 For example, amphiphilic surfactants self-assembled into micelles that can be used as reactors for confined nucleation and growth of nanocrystals with controlled size and shape.13,14 A series of shapes such as spherical, rod, wire, cube, and so forth, can be accomplished by using the micelleconfined synthetic method. Depending on kinetic reaction condition, the size of the nanocrystals can also be routinely tuned from a few nanometers to a few microns. In previous work, we demonstrated formation of self-assembled nanocrystal superlattices using alkanethiol-stabilized gold nanocrystals as a © XXXX American Chemical Society
Received: October 6, 2014 Revised: October 30, 2014
A
dx.doi.org/10.1021/nl503761y | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Figure 1. Representative SEM images (first column), TEM images (second column), and size distribution (third column) of porphyrin nanocrystals with different morphologiesL nanoparticles (A−C), tetragonal nanorods (D−F), hexagonal porous nanodisks (G−I), and hexagonal nanorods (J− L).
aqueous solution with the basic surfactant solution under vigorous stirring with a range of volume ratios (see Experimental Details in Supporting Information). The concentration of the surfactants was kept greater than the critical micelle concentration (cmc) of the corresponding surfactants used to ensure formation of surface micelles. After mixture of the two solutions, acid−base neutralization reaction occurs, which deprotonates the tetrapyridinium cations and produces insoluble ZnTPPs. ZnTPPs are not soluble in water, thus they were automatically trapped into the hydrophobic micellar interiors. This is a similar process to what happens for hydrophobic entities to be trapped inside detergent mesophases. After that, self-assembly was driven by intermolecular axial coordination between the neutral pyridine groups and center Zn ions (Zn−N). Likely, other noncovalent interactions such as hydrophobic−hydrophobic interactions and aromatic π−π stacking between molecules or surfactants facilitate nucleation and growth of self-assembled ZnTPP nanostructures. The final products of the self-assembled porphyrin nanocrystals were collected either by filtering using micro and/ or nanopore filters or by centrifuging using a given rotation
pyridine groups to the core metal ions (Zn), ZnTPP selfassembled into highly ordered three-dimensional (3D) arrays. Because of the spatial ordering of ZnTPP, the hierarchical nanocrystals exhibit collective optical properties resulted from coupling of molecular ZnTPP and shape-dependent photocatalytic activities in photodegradation of methyl orange (MO) pollutants. ZnTPP is a macrocyclic molecular building block that exhibits photoactive. Its backbone structure has strong conjugated network and well-defined shape with dimension size of ∼1.6 nm × 1.6 nm × 0.5 nm. Self-assembled and ordered networks are formed through coordination with metal ions forming and/or π−π stacking. In addition, protonation of basic groups to form tetrapyridinium cation ZnTPP-H44+ ions provides greater solubility in water. Given that nonprotonated ZnTPP is not water soluble, an acidic aqueous solution with pH less than 2 was prepared to protonate its pyridyl groups and to dissolve ZnTPP in aqueous solutions, forming homogeneous solutions.18 In addition to the porphyrin acidic solutions another surfactant aqueous solution was prepared in basic condition with pH above 7. The self-assembly process was subsequently initiated by combining the ZnTPP-H44+ acidic B
dx.doi.org/10.1021/nl503761y | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
speed above 5000 rpm. The precipitates (porphyrin nanocrystals) were washed with DI water to remove extra surfactants. The porphyrin nanocrystals displayed high monodispersity and well-defined morphologies, including nanodisk, tetragonal rod, and hexagonal rod, as well amorphous spherical particle as illustrated in Figure 1A−L. Figure 1D shows a representative top-down scanning electron microscopy (SEM) image of the resulting ZnTPP nanocrystals that have tetragonal external morphology (shown as in Figure 1D inset) with narrow size distribution in both length and diameter. Substantially, all (nearly 100%) of the nanorods have a length of about 290 ± 20 nm, as represented in the population histogram of Figure 1F (left). In addition, the tetragonal nanorods also show similar monodispersity with regard to their diameters. As shown in Figure 1F (right), nearly all of the rods have a diameter of about 90 ± 15 nm. We found that the average size of the tetragonal nanorods (length and diameter) is tunable through adjusting surfactant concentrations and pH. In a general sense, the average length decreases with increasing surfactant concentrations while the average diameter decreases (see Figure S2 in Supporting Information); the average length increases with increasing of pH while the average diameter also increases along with pH (see Figure S3 in Supporting Information). Figure 2 shows the detailed microstructures of the ZnTPP tetragonal nanorods. High-resolution transmission electron microscopy (HRTEM) characterization (Figure 2A) reveals that these tetragonal nanorods show a uniform electron contrast without defects. HRTEM image (Figure 2A) demonstrates the ordered crystal structural arrays within a single crystalline wall structure. The periodicities along a- and b-axis (Figure 2A,B) are measured to be 1.1 and 1.3 nm respectively; the fast Fourier transformation (FFT) (Figure 2A inset) shows a single-crystalline pattern. The consistency in periodicity between the HRTEM image and reverse FFT further confirms the well-ordered nature of the nanocrystals. The crystal structures were further characterized by X-ray diffraction (XRD) shown in Figure 2C. All of the peaks can be indexed as monoclinic space group P21/c (14) with the unit cell dimensions a = 11.187 Å, b = 13.825 Å, c = 22.172 Å, α = β = 90°, and γ = 100.52°.19 Figure 2D,E shows the crystal structures simulated from the XRD data. In this structural model, combined π−π stacking and ligand-to-metal Zn coordination drives the formation of the 3D self-assembled ZnTPP nanocrystal network. It is advantageous for further nucleation and growth of the self-assembled ZnTPP crystalline network through such confined 3D cooperative self-assembly through Zn−N axial coordination and noncovalent π−π interactions, which favorably allows control over size (e.g., aspect ratio, etc.) and shape of the nanostructures that is not easily attainable with other methods.1,5 As we discussed above in the case of formation of tetragonal nanorods, solution pH and surfactant concentration are two key factors influence the nanocrystal nucleation and growth in reactions with a given concentration of porphyrin. Through detailed investigations by SEM and TEM, we have rationalized the morphology evolution of the porphyrin nanocrystals and their dependence on factors such as surfactant concentration and pH values as diagram in Figure 3. Within each individual phase region, along with increasing of surfactant concentration, the nanocrystal dimension increases. We observed that the nanocrystal dimension also increases when pH value increases. At given pH conditions, along with increasing of surfactant
Figure 2. Structure of the self-assembled rectangular ZnTPP nanorods. (A) HR-TEM of rectangular rods prepared using 0.5 mM ZnTPP (in 0.01 M HCl solution) and 0.01 M MTAB at pH 11.6. Inset shows the FFT of the TEM image in A. (B) Corresponding reverse fast Fourier transformation (RFFT) image built upon the FFT image in the inset of the HRTEM in A. (C) XRD spectra of the rectangular rods. The specimens were degassed at 50 °C for 48 h. (D) Simulated crystal structure of the rectangular rods, Zn (red), N (blue), and C (gray). (E,F) Simulated crystal structure of rectangular rods viewed along the crystallographic c axis.
Figure 3. Nanocrystal morphology evolution with the surfactant concentration and pH.
concentration, one can achieve morphology from irregular particles to those that have well-defined morphologies including nanodisk, tetragonal rod, and hexagonal rods. C
dx.doi.org/10.1021/nl503761y | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
measurement shows a maximum surface area of ∼460 m2/g for the nanodisk structure with ∼40% porosity. The porosity of these self-assembled nanocrystals is maintained after degassing at 50 °C for 48 h suggesting stability of the pore structure. Overall, there exist mesopores and macropores ranging from 3 to ∼20 nm. Our thermogravimetric analysis (TGA, Supporting Information Figure S4) results show these nanocrystals are thermally stable up to ∼550 °C before they are carbonized. Through optical characterization, we found that the selfassembled nanocrystals display collective optical properties resulting from coupling of their individual precursors within ordered assemblies. Absorption spectra of protonated ZnTPP precursors show red shift in comparison with the neutral ZnTPP building block (Supporting Information Figure S5). After the confined noncovalent self-assembly, the resulted crystalline networks not only preserve the optical characters of individual macrocyclic monomers but also exhibit collective optical behaviors (Supporting Information Figure S6). Absorption spectra of self-assembled nanocrystals display more complicated signals in comparison to those of individual ZnTPPs (Supporting Information Figure S5). This may be due to the coupling of conjugated π−π structure from individual ZnTPPs within the ordered assemblies. Further investigation of the optical behavior and correlation of the optical property and self-assembled structure is underway. ZnTPP exhibit similar molecular structure to photoactive molecules such as chlorophyll, heme, and so forth and provides analogous chemical properties such as photocatalysis that can be found in many biological energy transduction processes in plants, algae, and so forth.20 The catalytic activity for photodegradation of MO pollutant was investigated under visible light irradiation. Figure 5 shows the preliminary results of the photocatalytic activities for different ZnTPP nanocrystal networks. From Figure 5A, several interesting catalytic features are obvious. All ZnTPP nanocrystals display photocatalytic activity for photodegradation of MO; in addition, all ZnTPP nanocrystals show enhanced photocatalytic activity compared to commercial P25 catalysts that are currently used for photodegradation of MO; finally, the ZnTPP nanocrystals exhibit morphology-dependent photocatalytic activity. Tetragonal rods show the least photoactivity while the nanodisks display high activity. This might be because the tetragonal rods have the smallest pore surface area and nanodisks have the
Due to the formation of the periodic self-assembled crystalline nanostructure, some of the ZnTPP networks exhibit uniform microporous structures. Nitrogen sorption isotherms were performed to characterize the pore structures of the ZnTPP nanocrsytals. Nitrogen adsorption/desorption isotherms and corresponding micropore size distributions for different self-assembled ZnTPP nanocrsytals are shown in Figure 4, and the calculated porosity, surface area, and average
Figure 4. Nitrogen sorption isotherms obtained at 77 K for different ZnTPP nanocrystals.
Table 1. Pore Structure of Different ZnTPP Nanocrystals Measured through Nitrogen Sorption Isotherm porous disk hexagonal rod tetagonal rod
porosity (%)
BET surface area (m2/g)
pore size (nm)
40.4 23.6 18.2
457 301 32.8
3.58 3.14 22.0
micropore size are summarized in Table 1. The nitrogen isotherms are characteristic of type IV isotherms for the nanodisks and hexagonal rods with hysteresis. The tetragonal rods show type I isotherms without apparent hysteresis. BET
Figure 5. (A) Photocatalytic activities of Zn porphyrin nanocrystals. Tetragonal nanorods with 200 nm length (c), same concentration ZnTPP in DMF (d), same concentration ZnTPP in 0.01 M HCl (e), nanoparticles with 80 nm diameter (f), hexagonal nanowires with 2 μm length (g), hexagonal rods with 400 nm length (h), and hexagonal porous nanodisks (i) for photo degradation of MO molecules under visible light irradiation. The results from blank experiments, where no Zn Porphyrin nanocrystals were used (a) and commercial P25 (b) was used are also presented for comparison. (B) Cycling tests of photocatalytic activity of ZnTPP nanodisks under visible light irradiation. D
dx.doi.org/10.1021/nl503761y | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
■
largest pore surface area. The photodegradation efficiencies of MO can reach above 90%, when nanodisks were used. The cycling results (Figure 5B) essentially demonstrate the nanodisks nanocrystals can reused for efficient photodegradation of MO. More experiments are under way to study the sizeand shape-dependent photocatalytic property in different solvents and better-controlled conditions. In summary, the synthesis of Zn porphyrin nanocrystals was accomplished using the micelle-confined self-assembly process. Through surfactant-assisted noncovalent interactions, nanocrystals with controlled dimension and morphology were prepared. These nanostructures exhibit enhanced optical properties, versus individual monomers due to collective behaviors resulting from their assemblies. A phase diagram that describes morphology control is achieved via kinetically controlled nucleation and growth. The resulting nanocrystals exhibit shape-dependent photocatalytic degradation of the exemplary pollutant MO. This simple ability to exert rational control over dimension and morphology provides new opportunities for applications in photocatalysis, sensing,21 nanoelectronics,22 and optics.23,24
■
REFERENCES
(1) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 15954−15955. (2) Tian, Y.; Busani, T.; Uyeda, G. H.; Martin, K. E.; van Swol, F.; Medforth, C. J.; Montano, G. A.; Shelnutt, J. A. Chem. Commun. 2012, 48, 4863−4865. (3) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 109, 1630−1658. (4) Capito, R. M.; Azevedo, H. S.; Velichko, Y. S.; Mata, A.; Stupp, S. I. Science 2008, 319, 1812−1816. (5) Hu, J.-S.; Guo; Liang, H.-P.; Wan, L.-J.; Jiang, L. J. Am. Chem. Soc. 2005, 127, 17090−17095. (6) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407−2409. (7) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. Angew. Chem., Int. Ed. 2011, 50, 3376−3410. (8) Bai, F.; Sun, Z.; Wu, H.; Haddad, R. E.; Coker, E. N.; Huang, J. Y.; Rodriguez, M. A.; Fan, H. Nano Lett. 2011, 11, 5196−5200. (9) Tian, Y.; M. Beavers, C.; Busani, T.; Martin, K. E.; Jacobsen, J. L.; Mercado, B. Q.; Swartzentruber, B. S.; van Swol, F.; Medforth, C. J.; Shelnutt, J. A. Nanoscale 2012, 4, 1695−1700. (10) Zhong, Y.; Wang, Z.; Zhang, R.; Bai, F.; Wu, H.; Haddad, R.; Fan, H. ACS Nano 2013, 8, 827−833. (11) Zhao, Y.; Sakai, F.; Su, L.; Liu, Y.; Wei, K.; Chen, G.; Jiang, M. Adv. Mater. 2013, 25, 5215−5256. (12) Kokkoli, E.; Mardilovich, A.; Wedekind, A.; Rexeisen, E. L.; Garg, A.; Craig, J. A. Soft Matter 2006, 2, 1015−1024. (13) Wei, Z.; Matsui, H. Nat. Commun. 2014, 5, 1−8. (14) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60−103. (15) Bai, F.; Wang, D.; Huo, Z.; Chen, W.; Liu, L.; Liang, X.; Chen, C.; Wang, X.; Peng, Q.; Li, Y. Angew. Chem., Int. Ed. 2007, 46, 6650− 6653. (16) Fan, H. Y.; Leve, E.; Gabaldon, J.; Wright, A.; Haddad, R.; Brinker, C. Adv. Mater. 2005, 17, 2587−2590. (17) Fan, H. Y.; Chen, Z.; Brinker, C.; Clawson, J.; Alam, T. J. Am. Chem. Soc. 2005, 127, 13746−13747. (18) Bai, F.; Wu, H.; Haddad, R. E.; Sun, Z.; Schmitt, S. K.; Skocypec, V. R.; Fan, H. Chem. Commun. 2010, 46, 4941−4943. (19) Krupitsky, H.; Stein, Z.; Goldberg, I.; Strouse, C. E. J. Inclusion Phenom. 1994, 18, 177−192. (20) Blankenship, R. E. Photosynthetic Pigments: Structure and Spectroscopy. In Molecular Mechanisms of Photosynthesis; Blackwell Science Ltd: Cambridge, MA, 2008; pp 42−60. (21) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710−713. (22) Schwab, A. D.; Smith, D. E.; Bond-Watts, B.; Johnston, D. E.; Hone, J.; Johnson, A. T.; de Paula, J. C.; Smith, W. F. Nano Lett. 2004, 4, 1261−1265. (23) Winkelmann, C. B.; Ionica, I.; Chevalier, X.; Royal, G.; Bucher, C.; Bouchiat, V. Nano Lett. 2007, 7, 1454−1458. (24) Medforth, C. J.; Wang, Z.; Martin, K. E.; Song, Y.; Jacobsen, J. L.; Shelnutt, J. A. Chem. Commun. 2009, 7261−7277.
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, molecular formula, SEM/TEM images, and UV−vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
■
Letter
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (F.B.). *E-mail:
[email protected] (H.F.). Author Contributions
Y.Z., J.W., R.Z., W.W., H.W., X.L., F.B., H.W., and R.H. performed experiments. R.H. indexed XRD crystal structure. H.F. designed the idea and experiments. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. F.B. acknowledged the support from the National Natural Science Foundation of China (21422102, 21171049, 21403054, and 50828302), Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 13HASTIT009), and Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT1126). TEM studies were performed in the Department of Earth and Planetary Sciences at University of New Mexico. We acknowledge the use of the SEM facility supported by the NSF EPSCOR and NNIN grants. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. E
dx.doi.org/10.1021/nl503761y | Nano Lett. XXXX, XXX, XXX−XXX
