Lilac Amirav

Nanoheterostructure Cation Exchange: Anionic Framework Conservation

In ionic nanocrystals the cationic sublattice can be replaced with a different metal ion via a fast, simple, and reversible place exchange, allowing postsynthetic modification of the composition of the nanocrystal, while preserving its size and shape. Here, we demonstrate that, during such an exchange, the anionic framework of the crystal is preserved. When applied to nanoheterostructures, this phenomenon ensures that compositional interfaces within the heterostructure are conserved throughout the transformation. For instance, a morphology composed of a CdSe nanocrystal embedded in a CdS rod (CdSe/CdS) was exchanged to a PbSe/PbS nanorod via a Cu(2)Se/Cu(2)S structure. During every exchange cycle, the seed size and position within the nanorod were preserved, as evident by excitonic features, Z-contrast imaging, and elemental line scans. Anionic framework conservation extends the domain of cation exchange to the design of more complex and unique nanostructures.

Perfect Photon-to-Hydrogen Conversion Efficiency

We report a record 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water-splitting reduction half-reaction. This result was accomplished by utilization of nanoparticle-based photocatalysts, composed of Pt-tipped CdSe@CdS rods, with a hydroxyl anion-radical redox couple operating as a shuttle to relay the holes. The implications of such record efficiency for the prospects of realizing practical over all water splitting and solar-to-fuel energy conversion are discussed.

Luminescence Studies of Individual Quantum Dot Photocatalysts

Using far-field optical microscopy we report the first measurements of photoluminescence from single nanoparticle photocatalysts. Fluence-dependent luminescence is investigated from metal-semiconductor heterojunction quantum dot catalysts exposed to a variety of environments, ranging from gaseous argon to liquid water containing a selection of hole scavengers. The catalysts each exhibit characteristic nonlinear fluence dependence. From these structurally and environmentally sensitive trends, we disentangle the separate rate-determining steps in each particle across the very wide range of time scales, which follow the initial light absorption process. This information will significantly benefit the design of effective artificial photocatalytic systems for renewable direct solar-to-fuel energy conversion.

Less Is More: The Case of Metal Cocatalysts.

We provide evidence that for a multielectron reaction such as hydrogen reduction, the photocatalyst design should include only a single cocatalytic site per each segment of the semiconductor capable of light excitation. This is to ensure that intermediates are formed at close proximity. These findings are demonstrated by evaluating the efficiency for hydrogen production over a nanoparticle-based photocatalyst consisting of Pt-decorated CdSe@CdS rods. Rods decorated with a single Pt catalyst were found to be the most active for hydrogen production, with QE of 27%, while rods having two reduction sites reached QE of only 18% and rods with multiple sites showed very low activity. The advantage of using a single catalytic site became negligible when the rods were employed in catalyzing a single electron reaction. We believe the implications of this finding are of significance for the proper design of photocatalysts aimed at solar-to-fuel energy conversion.

Modular Synthesis of a Dual Metal–Dual Semiconductor Nano‐Heterostructure

Reported is the design and modular synthesis of a dual metal-dual semiconductor heterostructure with control over the dimensions and placement of its individual components. Analogous to molecular synthesis, colloidal synthesis is now evolving into a series of sequential synthetic procedures with separately optimized steps. We detail the challenges and parameters that must be considered when assembling such a multicomponent nanoparticle, and their solutions. This multicomponent nanosystem, Ru-CdSe@CdS-Pt, was designed to achieve charge carrier separation and directional transfer across different interfaces toward two separate redox catalysts. This heterostructure may potentially serve as a nanometric closed circuit photoelectrochemical cell.

The golden gate to photocatalytic hydrogen production

We demonstrate improved efficiency for the photocatalytic water splitting reduction half reaction by employing Au–Pt bimetallic cocatalysts. We employed nanoparticle-based photocatalysts consisting of CdSe@CdS rods tipped with Au, Pt, Au–Pt core–shell or Au decorated with Pt islands. By tailoring the composition and morphology of the Au–Pt bimetallic catalysts, we achieved more than a fourfold increase in activity for hydrogen production compared to pure Pt.

Designing Bimetallic Co-Catalysts: A Party of Two

The enhanced catalytic properties of bimetallic particles has made them the focus of extensive research. We compare the photocatalytic activity for hydrogen production of core-shell structures of Au@Pd and Au@(Au/Pd alloy) on seeded rods of CdSe@CdS and show that Au@alloy was superior toward hydrogen production. Our finding reveals that the promotion effects of Au in Pd originate both from the alteration of the electronic structure by the Au core as well as by the atomic rearrangement of the surface. Long-term monitoring of the activity of the photocatalysts offered insights into the dynamic processes during the illumination showing that the tip morphology influenced the stability of the hybrid structures. The Au core served as a physical barrier, protecting the CdS rod against cation exchange reactions with the Pd. The coupling of these factors to achieve synergistic effects is therefore a prime aspect in the rational design of efficient cocatalysts.

Photochemical oxidation on nanorod photocatalysts

The combination of photochemical oxidation with colloidal synthesis enables us to re-think the design of photocatalysts with an eye towards overall water splitting. Here, IrO2 nanoparticle oxidative catalysts that were photodeposited on colloidal CdSe@CdS nanorod photocatalysts revealed a mediated oxidative pathway, and afforded the rods remarkable photochemical stability under prolonged illumination in pure water.

IV-VI semiconductor nanocrystals for passive Q-switch in IR

The use of semiconductor nanocrystals as a passive Q-switch in an eye-safe laser system is demonstrated. These lasers recently became popular in laser radar, three-dimensional scanning, targeting, and communication applications. Such applications require the laser to operate under Q-switching, generating a laser pulse with duration on the order of tens of nanoseconds, and a peak power on the order of a megawatt. Semiconductor nanocrystals exhibit unique physical properties, associated with the quantum size effect. The PbS and PbSe nanocrystals show a size-tunable absorption resonance in the near IR spectral region (1000-3000 nm), saturable absorbing properties, suitable as a functional Q-switch in eye-safe lasers. The quantum confinement effect and the saturable absorption can be manifested only in high quality nanocrystals with a narrow size distribution and passivated surfaces. Thus, a special synthetic procedures have been used for the preparation of PbSe core, PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystals. Then, a passively Q-switched Er:glass laser has been assembled, while the laser output energy, Q-switch threshold energies, and pulse width have been measured.

Size Matters: Cocatalyst Size Effect on Charge Transfer and Photocatalytic Activity

Hybrid semiconductor-metallic nanostructures play an important role in a wide range of applications and are key components in photocatalysis. Here we reveal that the nature of a nanojunction formed between a semiconductor nanorod and metal nanoparticle is sensitive to the size of the metal component. This is reflected in the activity toward hydrogen production, emission quantum yields, and the efficiency of charge separation which is determined by transient absorption spectroscopy. A set of Ni decorated CdSe@CdS nanorods with different tip size were examined, and an optimal metal domain size of 5.2 nm was obtained. Remarkably, charge separation time constants were found to be nonvariant with metal tip size. It is proposed that electron transfer mechanism encompasses two consecutive but separate processes: slow charge migration along the rod toward the interface, followed by fast interface crossing of the electron from the semiconductor into the metal phase. The first migration step dominates the time constant for the charge separation process and is not affected by the metal size. The efficiency of charge separation on the other hand was found to be sensitive to metal size. It is suggested that Coulomb blockade charging energy and a size-dependent Schottky barrier contribute to the metal size effect on charge transfer probability across the semiconductor-metal nanojunction. These two opposing trends result in an optimal metal size domain for the cocatalyst. This work is expected to benefit a broad range of applications utilizing semiconductor-metal nanocomposites.