Ultrathin SnO2 nanosheets: Oriented attachment mechanism, nonstoichiometric

We successfully synthesized large-scale and highly pure ultrathin SnO2 nanosheets (NSs), with a minimum thickness in the regime of ca. 2.1 nm as determined by HRTEM and in good agreement with XRD refinements and AFM height profiles. Through TEM and HRTEM observations on time-dependent samples, we found that the asprepared SnO2 NSs were assembled by “oriented attachment” of preformed SnO2 nanoparticles (NPs). Systematic trials showed that well-defined ultrathin SnO2 NSs could only be obtained under appropriate reaction time, solvent, additive, precursor concentration, and cooling rate. A certain degree of nonstoichiometry appears inevitable in the well-defined SnO2 NSs sample. However, deviations from the optimal synthetic parameters give rise to severe nonstoichiometry in the products, resulting in the formation of Sn3O4 or SnO. This finding may open new accesses to the fundamental investigations of tin oxides as well as their intertransition processes. Finally, we investigated the lithium-ion storage of the SnO2 NSs as compared to SnO2 hollow spheres and NPs. The results showed superior performance of SnO2 NSs sample over its two counterparts. This greatly enhanced Li-ion storage capability of SnO2 NSs is probably resulting from the ultrathin thicknesses and the unique porous structures: the nanometer-sized networks provide negligible diffusion times of ions thus faster phase transitions, while the “breathable” interior porous structure can effectively buffer the drastic volume changes during lithiation and delithiation reactions. ■ INTRODUCTION Nowadays, the fast development and demand in industry intensively focus on various applications of nanomaterials. The main interest lies in more rational design and precise control over specialized morphologies of nanomaterials with tailored properties. Among various nanostructures, dimensionality is one of the most defining parameters which significantly control the ways in which materials behave. Until now, 0-D (quantum dots), 1-D (nanowires, nanotubes, nanorods), and 3-D crystals are abundantly documented, while 2-D structures are far less reported. Ever since the finding of graphene, the combination of unique molecular geometry with exciting properties triggered intensive efforts in synthesizing other 2D ultrathin nanosheet materials. Furthermore, 3-D hierarchical structures produced by self-assembly and higher order organization of nanosheet building blocks attract extensive attention due to their collective optical, electrical, and magnetic properties since the complicated spatial arrangement can provide both extraordinarily high activated surface area and robustness. However, it is still highly desirable to develop facile and reliable synthesis routes, which do not require catalysts, expensive or toxic templates, and tedious procedures for rational design of those hierarchical structures. Solution-based procedures (known as “bottom-up” approach) for the syntheses of nanomaterials are very promising to precisely control their sizes and morphologies. However, the crystallization from a solution is still not fully understood. Classical crystal growth mode has been used to interpret crystallization processes since about 100 years ago. It is represented by atom additions to preformed nuclei and dissolution of unstable phases with concomitantly reprecipitation of stable phases. Alternatively, a particle-based aggregation mode named “oriented attachment” mechanism was recently proposed by Penn and Banfield, in which secondary crystal could be achieved by irreversible and highly oriented attachments of primary particles. This mechanism has been confirmed in various systems and thought to be potentially active in anisotropic growth of novel nanocrystals. The driving force for this spontaneous procedure stems from the elimination of thermodynamically high energy surfaces. Crystallization in this mode is often characterized by sequential processes involving structural and compositional modifications of amorphous precursors and crystalline intermediates. Received: January 5, 2012 Published: January 18, 2012 Article