Reversible, Surface-Controlled Structure Transformation in Nanoparticles Induced by an Aggregation State

The structure of 3 nm ZnS nanoparticles differs from that of bulk ZnS and is shown to vary with the particle aggregation state. Dispersed or weakly aggregated nanoparticles in suspension have a more distorted internal structure than strongly aggregated nanoparticles. Reversible switching between distorted and crystalline structures can be induced by changing the aggregation state via slow drying and ultrasonic agitation. The transformation was analyzed using pair distribution function data from wide angle x-ray diffraction and the aggregation state monitored via small angle x-ray scattering. Molecular modeling provides insight into particle-particle interactions that induce the structural changes. The reversible nature also implies a low activation energy of nanoparticle transformation and indicates that distorted nanoparticles are not trapped in a metastable state.

Figure 1

XRD of ZnS nanoparticles illustrating structure switching due to changes in the aggregation rate. Panel (a) Samples x rayed in methanol showing the sequential steps: ($A$) ZnS nanoparticles synthesized in methanol; ($B$) after slowly drying in a 200 Torr atmosphere; ($C$) after ultrasonic treatment in methanol for 24 h; ($D$) after drying and resuspension in methanol; ($E$) after ultrasonic treatment for 24 h. Panel (b) XRD of ZnS nanoparticles treated similarly to the series in (a), but x rayed dry. ($A^{\prime }$) ZnS nanoparticles synthesized in methanol, rapidly dried in ${10}^{-2}\mathrm{Torr}$; ($B^{\prime }$) after soaking in methanol for 24 h and slowly drying in a 200 Torr atmosphere; ($C^{\prime }$) after ultrasonic treatment in methanol (see text for details) and rapid drying in ${10}^{-2}\mathrm{Torr}$; ($D^{\prime }$) after reemersion in methanol for 24 h and slow drying in a 200 Torr atmosphere; ($E^{\prime }$) after further ultrasonic dispersal in methanol and rapid drying in ${10}^{-2}\mathrm{Torr}$.

Figure 2

(a) Wide angle x-ray scattering (WAXS), (b) pair distribution function (PDF), and (c) small angle x-ray scattering (WAXS) pattern of ZnS nanoparticles in aggregation states $A^{\prime }$ to $E^{\prime }$ in Fig. 1b.

Figure 3

Molecular dynamics predictions of the structures of (a) an isolated and (b) an aggregated individual 3 nm ZnS nanoparticle, and the calculated WAXS patterns. The nanoparticles are shown in cross section to reveal internal structure. Isolated nanoparticles are distorted and give broad peaks in the calculated WAXS pattern (red line) in agreement with experimental curve $A^{\prime }$ (black line). Aggregated nanoparticles possess more ordered structures, which produce sharper peaks in the WAXS pattern (red line) in agreement with experimental curve $B^{\prime }$ (black line). The calculated WAXS pattern of a 3 nm undistorted sphalerite nanoparticle is also shown (green line). (c) A cross section through part of the aggregated MD cluster. The sphalerite 111 directions of each nanoparticle are shown beneath, indicating that the particles have aggregated without coalescence and coarsening.