Troubleshooting the influence of trace chemical impurities on nanoparticle growth kinetics via electrochemical measurements.

Reproducibility issues resulting from particle growth solutions made with cetyltrimethylammonium bromide (CTAB) surfactant from different lots and product lines in a newly developed synthesis of monometallic palladium (Pd) tetrahexahedra (THH) nanoparticles are investigated via a multi-pronged approach. Time-resolved electrochemical measurements of solution potential, variation of chemical parameters in colloidal synthesis, and correlation to electrodeposition syntheses are used together to uncover the effects of the unknown contaminants on the chemical reducing environment during nanoparticle growth. Iodide-a known impurity in commercial CTAB-is identified as one of the required components for equalizing the reducing environment across multiple CTAB sources. However, an additional component-acetone-is critical to establishing the growth kinetics necessary to enable the reproducible synthesis of THH in each of the CTAB formulations. In one CTAB variety, the powdered surfactant contains too much acetone, and drying of the as-received surfactant and re-addition of solvent is necessary for successful Pd THH synthesis. The relevance of solvent impurities to the reducing environment in aqueous nanoparticle synthesis is confirmed via electrochemical measurement approaches and solvent addition experiments. This work highlights the utility of real-time electrochemical potential measurements as a tool for benchmarking of nanoparticle syntheses and troubleshooting of reproducibility issues. The results additionally emphasize the importance of considering organic solvent impurities in powdered commercial reagents as a possible shape-determining factor during shaped nanomaterials synthesis.

Nanomaterials Synthesis Discovery via Parallel Electrochemical Deposition.

Electrodeposition of nanoparticles is investigated with a multichannel potentiostat in electrochemical and chemical arrays. De novo deposition and shape control of palladium nanoparticles are explored in arrays with a two-stage strategy. Initial conditions for electrodeposition of materials are discovered in a first stage and then used in a second stage to logically expand chemical and electrochemical parameters. Shape control is analyzed primarily with scanning electron microscopy. Using this approach, optimized conditions for the electrodeposition of cubic palladium nanoparticles were identified from a set of previously untested electrodeposition conditions. The parameters discovered through the array format were then successfully extrapolated to a traditional bulk three-electrode electrochemical cell. Electrochemical arrays were also used to explore electrodeposition parameters reported in previous bulk studies, further demonstrating the correspondence between the array and bulk systems. These results broadly highlight opportunities for electrochemical arrays, both for discovery and for further investigations of electrodeposition in nanomaterials synthesis.

Bridging Colloidal and Electrochemical Nanoparticle Growth with In Situ Electrochemical Measurements.

ConspectusProspective applications involving the electrification of industrial chemical processes and electrical energy to chemical fuels interconversion as part of the energy transition to renewable energy sources have led to an increasing need for highly tailored nanostructures immobilized on electrode surfaces. Control of surface facet structure across material compositions is of particular importance for ensuring performance in such applications. Colloidal methods for producing shaped nanoparticles in solution are abundant, particularly for noble metals. However, significant technical challenges remain with respect to rationally designing syntheses for the novel compositions and morphologies required to sustainably enable the above technological advances as well as in developing methods for uniformly and reproducibly dispersing colloidally synthesized nanostructures on electrode surfaces. The direct synthesis of nanoparticles on electrodes using chemical reduction approaches remains challenging, though recent advances have been made for certain materials and structures. Electrochemical nanoparticle synthesis─where an applied current or potential instead of a chemical reducing agent drives the redox chemistry of nanoparticle growth─is poised to play an important role in advancing the fabrication of nanostructured electrodes. Specifically, this Account focuses on the colloidal-inspired design of electrochemical syntheses and the interplay between colloidal and electrochemical approaches in terms of understanding the fundamental chemical reaction mechanisms of nanoparticle growth. An initial discussion of the development of electrochemical particle syntheses that incorporate colloidal synthetic tools highlights the promising emergent capabilities that result from blending these two approaches. Furthermore, it demonstrates how existing colloidal syntheses can be directly translated to electrochemical growth on a conductive surface using real-time electrochemical measurements of the chemistry of the growth solution. Measuring the open circuit potential of a colloidal synthesis over time and then replicating that measured potential during electrochemical deposition leads to the formation of the same nanoparticle shape. These in situ open circuit and chronopotentiometric measurements also give fundamental insight about the changing chemical environment during particle growth. We highlight how these time-resolved electrochemical measurements, as well as correlated spectroelectrochemical monitoring of particle formation kinetics, enable the extraction of information regarding mechanisms of particle formation that is difficult to obtain using other approaches. This information can be translated back into colloidal synthesis design via a directed, intentional approach to synthetic development. We additionally explore the added flexibility of synthetic design for methods involving electrochemically driven reduction as compared to the use of chemical reducing agents. The Account concludes with a brief perspective on potential future directions in both fundamental studies and synthetic development enabled by this emerging integrated electrochemical approach.

Structural Dynamics of Metal Halide Perovskites during Photoinduced Halide Segregation.

Despite substantial research effort, photoinduced halide segregation in mixed halide perovskites continues to limit the available perovskite chemistries for use in optoelectronic applications. In this study, we present new insights into halide-segregation process through in situ X-ray diffraction measurements that reveal substantial structural changes in mixed-halide perovskites under excitation. We observe that photoinduced halide segregation leads to the formation of one iodide-rich and one bromide-rich perovskite composition whose Br:I ratios are the same (at 20 and 93% bromine, respectively), for a range of compositions of the pristine initial perovskite phase. This segregation reverses in the dark to re-form a mixed halide perovskite with the same lattice spacing as the pristine perovskite. From these results, we determine a kinetic rate for the formation and dissolution of these new crystalline phases and observe that the crystalline orientation is preserved through the light-segregation and dark-relaxation processes. Our results are consistent with a model of halide segregation where excitation causes changes in the free energy of mixing and ultimately the formation of a miscibility gap in the MAPb(IxBr1-x)3 phase diagram and should inform future works to model and manipulate the halide-segregation process in mixed-halide perovskites.