Growth of Gold Nanorods: A SAXS Study

Abstract

Using simultaneous, in situ optical spectroscopy and time-resolved, small-angle X-ray scattering (SAXS), we have directly monitored the seeded growth of nearly monodisperse gold nanorods using hydroquinone as the reductant. Growth of the rods is much slower than with the ascorbate ion, allowing the rate of growth along both the longitudinal and transverse directions to be independently determined. The thickness of the stabilizing CTAB layer (3.2 ± 0.3 nm) has also been extracted. We find that increasing the hydrogen tetrachloroaurate­(III) concentration produces longer rods, while conversely, increasing the hydroquinone concentration reduces the final aspect ratio. The final number of gold rods is smaller than the initial number of seed particles and decreases in the presence of larger concentrations of HAuCl4. The SAXS data reveal an early transition from a spherical morphology to an ellipsoidal one and then to spherically capped cylinders. The growth curve exhibits at least three distinct regimes: an initial phase comprising spherical seed growth, followed by symmetry breaking and slow elongation. A third phase is marked by rapid rod growth and increases in the aspect ratio. This process is temporally well resolved from the initial symmetry breaking but typically occurs when the rods are around 6 nm in diameter using hydroquinone as the reductant. The results provide qualitative support for the “popcorn model” proposed by Edgar et al. [Formation of Gold Nanorods by a Stochastic “Popcorn” MechanismACS Nano2012611161125].

Investigating Growth of Gold Nanorods by Simultaneous Small‐Angle X‐Ray and Neutron Scattering

“…Due to the complementary information from simultaneous SAXS/SANS, extracting the CTAB layer density this way is inherently more reliable than previous studies on this topic. [23,38,39] Nevertheless, more detailed measurements should be performed using this method.…”

“…UV–vis spectra of AuNR synthesized during the simultaneous SAXS/SANS experiment show similar quality to particles synthesized in a recently presented in‐situ study as indicated by the transversal surface plasmon resonance at around 550 nm. [ 2,23 ]…”

“…UV-vis spectra of AuNR synthesized during the simultaneous SAXS/SANS experiment show similar quality to particles synthesized in a recently presented in-situ study as indicated by the transversal surface plasmon resonance at around 550 nm. [2,23] Transmission Electron Microscopy: TEM measurements were performed on a Philips CM30. The acceleration voltage was set to 300 kV with a LaB 6 cathode operated at a current of 24 mA.…”

“…Despite all these research efforts on CTAB aggregation and its influence on AuNR stabilization during synthesis, [19][20][21] the exact mechanism remains elusive. [10] This circumstance is mainly due to the inherent difficulty of commonly used experimental methods like SAXS, [16,22,23] UV-vis spectroscopy, [24] TEM, [8][9][10]12] and XANES [16] probing the organic parts in the solution and simultaneously correlating them to the evolving AuNR morphology. SANS has been a great tool to analyze a variety of organic nanostructures.…”

Tracking Structural Changes of Carbon Nanotube Electrodes for Lithium–Air Batteries by Time-Resolved Operando Wide- and Small-Angle X-ray Scattering

“…These multi-method time-resolved studies have been conducted in a variety of systems, although there are significant challenges in the interpretation. For instance, the growth of Au and Pt nanorods in an aqueous solution has been followed similarly, − and one of the authors has reported results from UV–visible absorption spectroscopy, SAXS, and NEXAFS …”