Improving the catalytic activity of semiconductor nanocrystals through selective domain etching.

Colloidal chemistry offers an assortment of synthetic tools for tuning the shape of semiconductor nanocrystals. While many nanocrystal architectures can be obtained directly via colloidal growth, other nanoparticle morphologies require alternative processing strategies. Here, we show that chemical etching of colloidal nanoparticles can facilitate the realization of nanocrystal shapes that are topologically inaccessible by hot-injection techniques alone. The present methodology is demonstrated by synthesizing a two-component CdSe/CdS nanoparticle dimer, constructed in a way that both CdSe and CdS semiconductor domains are exposed to the external environment. This structural morphology is highly desirable for catalytic applications as it enables both reductive and oxidative reactions to occur simultaneously on dissimilar nanoparticle surfaces. Hydrogen production tests confirmed the improved catalytic activity of CdSe/CdS dimers, which was enhanced 3-4 times upon etching treatment. We expect that the demonstrated application of etching to shaping of colloidal heteronanocrystals can become a common methodology in the synthesis of charge-separating nanocrystals, leading to advanced nanoparticles architectures for applications in areas of photocatalysis, photovoltaics, and light detection.

Photocatalytic Applications of Colloidal Heterostructured Nanocrystals: What's Next?

Recent progress in the colloidal synthesis of inorganic nanocrystals has led to the realization of complex, multidomain nanoparticle morphologies that give rise to advanced optoelectronic properties. Such nanocomposites are particularly appealing for photocatalytic applications where tunable absorption, extensive charge separation, and large surface-to-volume ratios are important. To date, heterostructured nanocrystals featuring a metal catalyst and a semiconductor "chromophore" component have shown compelling efficiencies in photoreduction reactions, including sacrificial hydrogen production. Time-resolved optical studies have attributed their success to a near-complete separation of photoinduced charges across dissimilar nanoparticle domains. The spectroscopy approach has also identified the key performance-limiting factors of nanocrystal catalysts that arise from inefficient extraction of photoinduced charges to catalytic sites. Along these lines, the main scope of present-day efforts targets the improvement of interstitial charge transfer pathways across the chromophore-catalyst assembly through the design of high-quality stoichiometric interfaces.

Enhanced lifetime of excitons in nonepitaxial Au/CdS core/shell nanocrystals.

The ability of metal nanoparticles to capture light through plasmon excitations offers an opportunity for enhancing the optical absorption of plasmon-coupled semiconductor materials via energy transfer. This process, however, requires that the semiconductor component is electrically insulated to prevent a "backward" charge flow into metal and interfacial states, which causes a premature dissociation of excitons. Here we demonstrate that such an energy exchange can be achieved on the nanoscale by using nonepitaxial Au/CdS core/shell nanocomposites. These materials are fabricated via a multistep cation exchange reaction, which decouples metal and semiconductor phases leading to fewer interfacial defects. Ultrafast transient absorption measurements confirm that the lifetime of excitons in the CdS shell (τ ≈ 300 ps) is much longer than lifetimes of excitons in conventional, reduction-grown Au/CdS heteronanostructures. As a result, the energy of metal nanoparticles can be efficiently utilized by the semiconductor component without undergoing significant nonradiative energy losses, an important property for catalytic or photovoltaic applications. The reduced rate of exciton dissociation in the CdS domain of Au/CdS nanocomposites was attributed to the nonepitaxial nature of Au/CdS interfaces associated with low defect density and a high potential barrier of the interstitial phase.

Harvesting solar energy by means of charge-separating nanocrystals and their solids.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials. Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system. To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells.

The effect of the charge-separating interface on exciton dynamics in photocatalytic colloidal heteronanocrystals.

Ultrafast transient absorption spectroscopy was used to investigate the nature of photoinduced charge transfer processes taking place in ZnSe/CdS/Pt colloidal heteronanocrystals. These nanoparticles consist of a dot-in-a-rod semiconductor domain (ZnSe/CdS) coupled to a Pt tip. Together the three components are designed to dissociate an electron-hole pair by pinning the hole in the ZnSe domain while allowing the electron to transfer into the Pt tip. Separated charges can then induce a catalytic reaction, such as the light-driven hydrogen production. Present measurements demonstrate that the internal electron-hole separation is fast and results in the localization of both charges in nonadjacent parts of the nanoparticle. In particular, we show that photoinduced holes become confined within the ZnSe domain in less than 2 ps, while electrons take approximately 15 ps to transition into a Pt tip. More importantly, we demonstrate that the presence of the ZnSe dot within the CdS nanorods plays a key role both in enabling photoinduced separation of charges and in suppressing their backward recombination. The implications of the observed exciton dynamics to photocatalytic function of ZnSe/CdS/Pt heteronanocrystals are discussed.