Understanding the initial nucleation mechanism of monodisperse nanocrystals (NCs) during synthesis process is an important prerequisite to control the desired sizes and to manipulate the properties of nanoscale materials. The acquisition of information for the small nanocluster nucleation process, however, still remains challenging. Here, using a continuous-flow in situ X-ray absorption fine structure (XAFS) spectroscopy for time-resolved studies, we have clarified the initial kinetic nucleation of Au clusters under the grain size of 1 nm for the classical Au NCs synthesis via the reduction of AuCl(4)(-) in aqueous solution. The in situ XAFS results present the experimental revelation of the formation of intermediate Cl(3)(-)Au-AuCl(3)(-) dimer and the subsequent higher complexes 'Au(n)Cl(n+x)' in the initial nucleation stage. We propose a kinetic three-step mechanism involving the initial nucleation, slow growth, and eventual coalescence for the Au NCs formation, which may be helpful for the synthesis of metallic nanomaterials.

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Environment-friendly proton-exchange-membrane fuel cells (PEMFC) are considered a possible answer to environmental and energy problems. [1][2][3][4][5][6][7][8] To make fuel-cell automobiles a reality, the activity and life of the Pt/C cathode catalyst must be improved. Towards this goal, we have developed a novel time-gating quick XAFS (QXAFS) technique with 1-s time resolution and an energy-dispersive XAFS (DXAFS) system with 4-ms time resolution. Using these techniques, we have observed the electrochemical reaction mechanism and found evidence for dynamic surface events involving Pt dissolution at the Pt/C cathode, the reaction kinetics of the electrontransfer processes, redox structural changes (eight elementary steps), and a significant time lag among those events for the first time under operando fuel-cell conditions. Measurement of the current in a PEMFC in real time shows that a power-on process from open circuit to an operating state brings about rapid electrochemical reactions on its electrode surfaces, which are completed within a few seconds. Such power-on and -off processes (voltage change from open-circuit voltage (OCV = e.g. 1.0 V) to operating voltage (e.g. 0.4 V)) with huge energy transfer are indispensable for commercial applications of fuel-cell systems. However, surface atoms of the active metal particles tend to dissolve slightly into the electrolyte that is in contact with the cathode catalyst layer, and an undesired Pt particle (or layer) deposits in the electrolyte. [9,10] This effect is a problem because automobiles, in particular, require continual repetition of the on/off processes with rapid changes in cell voltages to alter the cars speed.To overcome these serious problems, reaction mechanisms on the electrode surfaces must be investigated in situ during voltage-stepping processes in real time. However, to the best of our knowledge, there are no reports that have fully explored and determined the reaction kinetics of both the structural changes of the metal catalysts and the electrochemical reactions on the electrode surfaces in PEMFCs. We have investigated the mechanism of the electrochemical processes involved in rapid voltage-controlled processes on a Pt/C catalyst. From the viewpoints of electrification and structural changes in the Pt catalyst, in situ time-resolved quick X-ray absorption fine structure (QXAFS) spectroscopy was used to monitor directly the chemical bonding and electronic states in the Pt nanoparticles that act as the electrodes. [11][12][13][14][15][16][17] Acquisition of a QXAFS spectrum at a Pt L III edge requires at least 15 s because of the slow mechanical rotation of the monochromator; however, the time resolution (15 s) is too slow to observe the reaction mechanism of the target processes. Herein, we propose a novel time-gating quick XAFS (TG-QXAFS), with 1-s time resolution, for the first time. The in situ TG-QXAFS measurements of the potential stepping processes in real time revealed extraordinary time lags between electrification and the redox chemical p...