Twinning Driven Growth of Manganese Oxide Hollow Cones through Self-Assembly of Nanorods in Water David Portehault,†,§,⊥ Sophie Cassaignon,*,†,§ Emmanuel Baudrin,# and Jean-Pierre Jolivet§ UPMC UniV. Paris 6, UMR 7574 - Laboratoire Chimie de la Matiere Condensee de Paris, College de France, 11 place Marcelin Berthelot, 75231 Paris cedex 05, France, CNRS, UMR 7574 - Laboratoire Chimie de la Matiere Condensee de Paris, College de France, 11 place Marcelin Berthelot, 75231 Paris cedex 05, France, and UniV. de Picardie-Jules Verne, UMR 6007, Laboratoire de ReactiVite et Chimie des Solides, 33 rue Saint-Leu, 80039 Amiens cedex, France
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2562–2565
ReceiVed March 11, 2009; ReVised Manuscript ReceiVed April 21, 2009
ABSTRACT: Hollow γ-MnO2 nanocones are synthesized by precipitation in water at 95 °C through a “one-pot” procedure. Characterization of the particles is carried out using powder XRD, electron diffraction, FESEM, and TEM. The hexagonal-shaped base is shown to originate from specific {021} twinning. The growth mechanism is investigated and proposed to proceed through two steps. The first stage involves the growth of pyramidal seeds that direct the final shape of the particles. The second stage proceeds through oriented attachment of primary R-MnOOH nanorods on previously formed γ-MnO2 cones. Attachment is accompanied by phase transformation of the oxyhydroxide to the oxide by a topotactic reaction, because of the structural relations between both structures. This study reports on the first demonstration of specific twinning from oriented attachment. This also provides a deep insight into the role of structural defects for further design of complex and novel architectures. Additionally, the procedure developed herein occurs in water without any surfactant. It is therefore low cost and environmentally friendly. Defects in nanostructures not only modify the material properties, they also influence the nanoparticle morphology and the ordering between nanodomains.1 Thus, control of defect nature, orientation, and distribution is a challenging issue to tailor functional nanomaterials. In this context, oriented attachment, involving consistent aggregation between primary nanocrystals to form a singlecrystalline secondary particle, could provide novel ways of controlling such features,2 because it drives the formation of stacking faults and twinnings at the boundary between primary particles.2a,3 However, the ability to form specific defects has not yet been demonstrated. Furthermore, oriented attachment emerges as a mean to induce crystallization toward complex morphologies.2b,3,4 Procedures involving this mechanism usually require surface complexants in organic solvents with high environmental impact.2c,5 It is thus of great interest to develop nonpolluting aqueous syntheses through oriented attachment without any organic additive.3 Manganese oxides exhibit various properties in sorption, energy harnessing, catalysis, and magnetism.6 Among these compounds, ramsdellite is composed of double MnO6 chains (Figure SI-1a in the Supporting Information) linked together to form tunnels with a 1 × 2 octahedra cross-section.7a,b Ramsdellite is only observed in nature while the synthetic product γ-MnO2 contains pyrolusite (1 × 1 tunnels) intergrowths (de Wolff defects) and microtwinnings.7 γ-MnO2 has been obtained as nanorods, nanowires, urchinlike particles, and hollow spheres, mainly through precipitation in aqueous media.8 We present herein a “one-pot” synthesis in water, at 95 °C, of a γ-MnO2 compound with a morphology consisting of hollow nanocones and originating from specific twinning. The growth of each face proceeds by nanorod oriented attachment between two different phases (heterogeneous oriented attachment). This study provides insight into formation processes that could be involved in the synthesis of various novel nanoarchitectures, by combining “nonclassical” crystallization and “defect engineering”. The synthesis (see the Supporting Information) was carried out through the reaction between MnO4- and Mn2+ in water at 95 °C * To whom correspondence should be addressed. E-mail: sophie.cassaignon@ college-de-france.fr. † UPMC Univ. Paris 6. § CNRS. # Univ. de Picardie-Jules Verne. ⊥ Present affiliation: Max Planck Institute of Colloids and Interfaces, Golm (Germany).
Figure 1. XRD patterns obtained at different aging times. Triangles, γ-MnO2; stars, unindexed peaks attributed to ε-MnO2; pentagons, groutite R-MnOOH; circles, manganite γ-MnOOH.
for 1 day. Characteristic XRD (Figure 1) peaks of γ-MnO2 appear after aging for 1h50 min. Simultaneously, groutite R-MnOOH (tunnels with 1 × 2 cross-section, see Figure SI-1b in the Supporting Information) is observed. The (1j11) peak of manganite γ-MnOOH (tunnels with 1 × 1 cross-section) (see Figure SI-1c in the Supporting Information) is observed at 26.2 ° (2θ CuKR) after 2h40 min. The final powder (Figure 1, 1 day) is a mixture of Manganite γ-MnOOH, groutite R-MnOOH and γ-MnO2. Some peaks cannot be indexed to usual manganese (oxyhydr)oxides but have already been observed by Tu et al.9 and Ding et al. who pointed out the relation between these reflexions and ε-MnO2.6e This compound is a particular case of γ-MnO2 with a hexagonal structure originating from a perfect ordering along the a axis of ramsdellite (c axis of ε-MnO2) and a full disorder of the cationic sublattice in the (b, c) plane. Partial ordering can give rise to various superstructures.7e
10.1021/cg9002862 CCC: $40.75 2009 American Chemical Society Published on Web 05/08/2009
Communications
Crystal Growth & Design, Vol. 9, No. 6, 2009 2563
Figure 2. FESEM (a), TEM (b, c, e, i), SAED (d, f, h), HRTEM (j) images, and scheme of nanocones (g) obtained after aging at 95 °C for 1 day.
The thin XRD peak at 22.1° for γ-MnO2 indicates good coherence of the (1j10) planes. EDX analysis reveals only Mn and O elements. FESEM (Figure 2a), TEM, and selected area electron diffraction (SAED) (see Figure SI-2 in the Supporting Information) show that the sample obtained after 1 day is composed of two types of particles: nanowires of manganite and hollow cones with a hexagonal base (Figure 2b). Such a shape has been observed by Tu et al. after hydrothermal treatment of a layered manganese oxide.9 The cone height is ca. 400 nm. The base has a ca. 100 nm diameter and is not a perfect hexagon but has a corner angle of ca. 117 ° (Figure 2c, e, g). The SAED pattern of one edge of the pseudohexagon exhibits intense reflexions indexed to the ramsdellite structure with the a zone axis as the central cone axis (Figure 2d), thus confirming that the structure belongs to γ-MnO2. The c* axis is parallel to the edge. A superstructure along b* is highlighted by low-intensity spots (white arrows). SAED performed at the corner of the pseudohexagon exhibits two identical patterns belonging to the adjacent faces (Figure 2e-h) with an angle of 117° with respect to each other. {021} twinning planes separate the adjacent faces (Figure 2g, h). HRTEM performed on a cone laying on one lateral face
Figure 3. TEM (a-c, e-h), SAED (insert c), and FESEM (d) images of particles obtained upon aging. (c) Groutite nanorods. (g, h) Ultrathin sections. Dark arrows, between dashed lines: (a, b, e) central pillars or (f, h) boundaries between primary nanorods.
(Figure 2i, j) exhibits fringes running along the whole face with d-spacing (0.41 nm) consistent with the intense and thin (1j 10)ramsdellite XRD peak (Figure 1). Thus, preferential growth of faces is observed along [1j 10]*. Early elongated particles appear after aging for 1 h and 30 min (Figure 3a) with a central pillar (dark arrow, Figure 3a), which is shown by HRTEM (see Figure SI-3 in the Supporting Information) to be composed of disoriented crystallites of ca. 5 nm diameter. Some particles are stacked around the central pillar. Few faceted particles are also observed (Figure 3b). Meanwhile, groutite R-MnOOH nanorods are identified by SAED (Figure 3c). A bundle of aggregated groutite nanorods exhibits a highly coherent SAED pattern (Figure 3c), thus indicating that oriented attachement occurs between the R-MnOOH nanorods. Whatever
2564
Crystal Growth & Design, Vol. 9, No. 6, 2009
Scheme 1. Relation between Nanocone Morphology, Ramsdellite MnO2, and Groutite r-MnOOH Structures
the aging time, no γ-MnO2 single rod was observed. FESEM performed after aging 4.0 h (Figure 3d) shows that cone faces are composed of laterally aggregated nanorods, in agreement with Figure 3a. After aging for 1 day, cones exhibit a pyramidal shaped central part (dark arrow, Figure 3e), similar to the pillar observed after 1 h and 30 min. The assembly character is still observed in the final cones (dark arrows, Figure 3f) where primary nanorod boundaries are apparent. Ultrathin sections (Figure 3g, h) of an embedded final sample show some fragments of broken cones, which confirm that the particles originate from the assembly of nanorod building units. Cones are composed of a γ-MnO2-related compound. Resolution of the unidentified XRD and SAED will require further investigations, by taking into account cation ordering/disordering and microtwinning.7b-d Nevertheless, nondefectuous ramsdellite is used as a first simple model of γ-MnO2. Cone faces originate from γ-MnO2 nanorods stacked on their lateral (1j 10) faces (Scheme 1). The rod main axis is c, in agreement with the tunnel structure of ramsdellite. {021} twinning planes separate adjacent cone faces and have already been reported in defect investigations for γ-MnO2.7b Interestingly, {021} planes form an angle of 58.6° with the c axis (Scheme 1). Specific {021} twinning thus underlies the pseudohexagonal shape and a slight distortion from an ideal hexagon (117.2° according to the ramsdellite structure) ensures the continuity of the lattice within cones. Manganite γ-MnOOH is observed after the cone formation (Figure 2) and thus is not involved in the cone growth. Structural and morphological evolutions enable to propose a growth mechanism by a two step process. First, some poorly ordered particles are precipitated (Figures 1 (0-100 min) and 3a, and Figure SI-3 in the Supporting Information). This disorganized compound is metastable and undergo transformation accompanied by facetting of the particles (Figure 3b and step 1 of Scheme 2). Such an evolution is likely to occur through a solution pathway, in good agreement with results from Ding et al., who obtained pyramidal ε-MnO2 particles with a hexagonal symmetry through hydrothermal synthesis.6e The authors proposed that pyramids were formed through dissolution-recrystallization. No fulfilled cone was observed in our study, but final particles exhibit a central pillar similar to the ones in the initial samples (Figure 3e). Thus, it is proposed that ε-MnO2 faceted pyramids are preserved upon aging and act as seeds for further growth of γ-MnO2 hollow cones. The formation of faceted pyramidal seeds with a hexagonal base is a prerequisite for pseudohexagonal cone growth. The second step of formation consists in growth of external faces through stacking of nanorods on their lateral
Communications Scheme 2. Scheme for Face Formation of a Single Cone
(1j 10)ramsdellite faces (Figure 3a, f, g, h) with a very good ordering along [1j 10]* on each cone face (Figures 1 (1 day) and 2j). Therefore, oriented attachment of nanorods occurs on the previously formed cones (step 2 of Scheme 2).2a,b No γ-MnO2 nanorod was observed, contrary to groutite R-MnOOH nanorods, which appear after 1 h and 30 min (Figures 1 and 3c). Thus, it is proposed that attachment occurs between γ-MnO2 cones and R-MnOOH nanorods (step 2 of Scheme 2). The involved faces for such a process are (1j 10)ramsdellite and (101)groutite (Scheme 1). Such a process is favored by the isostructural relation between R-MnOOH and ramsdellite-related γ-MnO2. Moreover, in the aging conditions (pH ∼4), MnIVO2 compounds are more stable than MnIIIOOH.10 Evolution from groutite R-MnOOH to γ-MnO2 is thus thermodynamically favored. The oxidation of R-MnOOH to γ-MnO2 occurs by retention of the nanorod morphology and is thus likely to occur through a topotactic transformation (step 3 of Scheme 2). Early cones exhibit rough surfaces (Figure 3a), in agreement with reported aggregation driven growth mechanisms.2 Steps at primary nanorod boundaries are smoothed upon aging through Ostwald’s ripening (dissolution of steps and recrystallization in holes to decrease the interfacial energy) (Figures 2b, 3e, and 3f and step 4 of Scheme 2). Because cones are obtained in aging media with pH ∼4, where γ-MnO2 has low solubility,2b,3b,10 smoothing is believed to occur after aggregation. Surprisingly, a few early particles with smoothed surfaces are also observed (Figure 3b) and could originate from the metastability of the early compound, which increases its solubility. Moreover, Ostwald’s ripening can also account for the length of the attached nanorods surprisingly fitting with the width of the cone faces, depending on the distance between the rod and the cone top (Step 4 of Scheme 2). This dissolutioncrystallization process enables each cone faces to join together at the twinning planes. Finally, the imbrication of several hollow cones can be explained by a multisite nucleation of the cone edges on the pyramidal seed. In summary, the nanocone growth proceeds through two steps. First, the formation of pyramidal seeds drives the nucleation of the cone faces with a hexagonal symmetry. Second, oriented attachment of primary nanorods occurs on the external cone
Communications
Crystal Growth & Design, Vol. 9, No. 6, 2009 2565
faces. Specific twinning is preserved upon growth and underlies the final pseudohexagonal base. Heterogeneous oriented attachment and topotactic phase transformation are involved in the second step. This growth mechanism underlines original aspects of oxide crystallization pathways and brings out the crucial role of “nonclassical crystallization” mechanisms and defects in the design of elaborate nanoarchitectures.
Acknowledgment. The authors acknowledge Dr. Patricia Beaunier and Stephan Borensztajn (UPMC) for TEM and FESEM measurements, Dominique Jalabert (University of Orle´ans) for HRTEM measurements, and Anny Anglo (UPMC) for ultrathin sections. Supporting Information Available: Experimental details, structural representations, low-magnification TEM image of a compound obtained after aging 1 day, and HRTEM image of an initial seed (PDF). This information is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.-W.; Alivisatos, A. P. Science 2007, 317, 355–358. (b) Wu, X.; Jiang, P.; Ding, Y.; Cai, W.; Xie, S.; Wang, Z. L. AdV. Mater. 2007, 19, 2319–2323. (2) (a) Penn, L. R.; Banfield, J. F. Science 1998, 281, 969–971. (b) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549– 1557. (c) Niederberger, M.; Co¨lfen, H. Phys. Chem. Chem. Phys. 2006, 8, 3271–3287.
(3) (a) Huang, F.; Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 10470–10475. (b) Portehault, D.; Cassaignon, S.; Baudrin, E.; Jolivet, J.-P. Chem. Mater. 2007, 19, 5410–5417. (4) (a) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188–1191. (b) Polleux, J.; Pinna, N.; Antonietti, M.; Niederberger, M. AdV. Mater. 2004, 16, 436. (5) (a) Cho, K.-S.; Talapin, V.; Gaschler, W.; Murray, B. J. Am. Chem. Soc. 2005, 127, 7140–7147(b) Gehrke, N.; Co¨lfen, H.; Pinna, N.; Antonietti, M.; Nassif, N. Cryst. Growth Des. 2005, 5, 1317–1319. (6) (a) Sauvage, F.; Baudrin, E.; Tarascon, J.-M. Sens. Actuators, B 2007, 120, 638–644. (b) Thackeray, M. M. Prog. Solid State Chem. 1997, 25, 1. (c) Brousse, T.; Toupin, M.; Dugas, R.; Athouel, L.; Crosnier, O.; Belanger, D. J. Electrochem. Soc. 2006, 153, A2171. (d) Ding, Y.-S.; Shen, X.-F.; Sithambaram, S.; Gomez, S.; Kumar, R.; Crisostomo, V. M. B.; Suib, S. L.; Aindow, M. Chem. Mater. 2005, 17, 5382–5389. (e) Ding, Y.-S.; Shen, X.-F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. AdV. Funct. Mater. 2006, 16, 549–555. (7) (a) Fong, C.; Kennedy, B. J.; Elcombe, M. M. Zeit. Krist. 1994, 209, 941–945. (b) Hill, L. I.; Verbaere, A. J. Solid State Chem. 2004, 177, 4706–4723. (c) De Wolff, P. M. Acta Crystallogr. 1959, 12, 341. (d) Chabre, Y.; Pannetier, J. Prog. Solid State Chem. 1995, 23, 1–130. (e) Kim, C.-H.; Akase, Z.; Zhang, L.; Heuer, A. H.; Newman, A. E.; Hughes, P. J. J. Solid State Chem. 2006, 179, 753–774. (8) (a) Zhang, L.; Liu, Z.-H.; Lv, H.; Tang, X.; Ooi, K. J. Phys. Chem. C 2007, 111, 8418–8423. (b) Wang, X.; Li, Y. Chem.sEur. J. 2003, 9, 300–306. (c) Yuan, J.; Laubernds, K.; Zhang, Q.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966–4967. (9) Tu, S.; Racz, G. J.; Goh, T. B. Clays Clay Miner. 1994, 42, 321–330. (10) Pourbaix, M. Atlas d’Equilibres Electrochimiques; Gauthier-Villars: Paris, 1963.
CG9002862
