Nano Research最新文献

CdS quantum dots (QDs) have been extensively studied as photocatalysts and sensitizers for visible-light-driven water reduction. However, their efficiencies are limited by the need to accumulate sufficient redox equivalents to produce H₂ and consequent photocorrosion associated with slow hole-transfer rates. To address these limitations, we report the formation of CdS QD assemblies (aerogels, AGs) capable of facilitating energy/charge transport between individual QDs, and evaluate their performance as photocatalysts for hydrogen evolution as a function of structure, wurtzite (w-) vs. zincblende (zb-), and different annealing temperatures. The formation of AGs from QDs resulted in increased rates of H₂ production under visible light illumination: from 1458 (QD) to 6650 (AG) µmol_H2·h⁻¹·g⁻¹ on zbCdS and from 1221 (QD) to 3325 (AG) µmol_H2·h⁻¹·g⁻¹ on wCdS. This is attributed to exciton delocalization between adjacent QDs facilitating charge/energy transport. Thermal processing of CdS AGs up to 250 °C improved their activity, increasing the degree of exciton delocalization, while annealing them to 300 °C caused sintering of the primary QD particles within the AGs and a decrease in activity associated with loss in surface area. The best photocatalyst, zbCdS AG annealed at 250°C, had an average H₂ production rate of 13,604 ± 2017 µmol_H2·h⁻¹·g⁻¹, an apparent quantum yield of 2.8% at 425 ± 12.5 nm, and was stable for 2 h before beginning to deactivate due to photocorrosion. This study confirms the potential of CdS AGs as matrixes for the design of more active and stable composite photocatalysts for water splitting.

Precise modulation of photoluminescence (PL) of nanomaterials by external control is of great interest in such diverse areas as photocatalysis, memory and sensing. Recent studies have combined colloidal quantum dots (QDs) with photochromic molecules to construct optically switchable PL systems. However, it still remains challenging to switch the PL on and off in the near-infrared (NIR) region with multi-stimuli such as light and heat. Here, we present light and heat triggered modulation of the NIR PL of PbS QDs using adjacent spiropyran derivatives. The NIR PL of PbS was reversibly switched on and off through the isomerization process of spiropyran molecules that can be triggered by either light irradiation or heating. The PL intensity of the off state is low enough to yield an on/off ratio as high as 54. Transient absorption measurements revealed ultrafast photoinduced hole transfer from PbS to spiropyran, the rate and efficiency of which depend critically on the driving force that can be deeply modulated through spiropyran isomerization. This study not only establishes a novel multi-stimuli switchable PL system in the NIR, but also provides fundamental guidelines for the design for such systems for a variety of emerging applications.

Magic-sized (CdSe)₁₃ clusters (MSCs) represent a material class at the boundary between molecules and quantum dots that exhibit a pronounced and well separated excitonic fine structure. The characteristic photoluminescence is composed of exciton bandgap emission and a spectrally broad mid-gap emission related to surface defects. Here, we report on a thermally activated energy transfer from fine-structure split exciton states to surface states by using temperature dependent photoluminescence excitation spectroscopy. We demonstrate that the broad mid-gap emission can be suppressed by a targeted Mn-doping of the MSC leading to the characteristic orange luminescence of the ⁴T₁ → ⁶A₁ Mn²⁺ transition. The energy transfer to the Mn²⁺ states is found to be significantly different than the transfer to the surface defect states, as the activation of the dopant emission requires a spin-conserving charge carrier transfer that only dark excitons can provide.

The field of colloidal nanocrystals has witnessed enormous progress in the last three decades. For many families of nanocrystals, wet-chemical syntheses have been developed that allow control over the crystal shape and dimensions, from the three-dimensional down to the zero-dimensional case. Additionally, careful control of surface chemistry has enabled the prevention of non-radiative recombination, thus allowing the detailed study of confined charge carriers and excitons. This has led to a vast amount of applications of nanocrystals in displays, labels, and lighting. Here, we discuss how this expertise could benefit the rapidly advancing field of quantum materials, where the coherence of electronic wave functions is key. We demonstrate that colloidal two-dimensional nanocrystals can serve as excellent model systems for studying topological phase transitions, particularly in the case of quantum spin Hall and topological crystalline insulators. We aim to inspire researchers with strong chemical expertise to explore the exciting field of quantum materials.

To enhance the reproducibility and scale up the synthesis of colloidal quantum dots (QDs), continuous flow synthesis is an appealing alternative to the widely used batch synthesis. Amongst other advantages, the strongly enhanced heat and mass transfer in small tubular reactors combined with controlled pressure can be cited. Nonetheless, the widespread use of this technique is hampered by special requirements such as the absence of solid or gaseous products and the room-temperature solubility of precursors. Therefore, the transfer of established reaction conditions from batch to flow is not straightforward and in most reported works the optical properties of the obtained QDs lag behind those prepared in batch reactions. This is also the case for PbS-based QDs, which are established near infrared (NIR) absorbers/emitters. Here we identified experimental conditions giving access to high-quality PbS core and PbS/CdS core/shell QDs obtained in an automated, easily scalable continuous flow synthesis. In particular, substituted thioureas have been selected as the sulfur source and ex-situ synthesized lead and cadmium oleate as the metal precursors, and appropriate solvent mixtures have been identified for each precursor. Highly luminescent PbS/CdS QDs emitting at the target wavelengths 940 and 1130 nm of special interest for NIR light-emitting diodes have been prepared, exhibiting a photoluminescence quantum yield up to 91%.

We synthesized heterostructures by tethering Ni(II)-doped CdS (Ni:CdS) quantum dots (QDs) to β-Pb_0.33V₂O₅ nanowires (NWs) using L-cysteine as a molecular linker, and we evaluated the influence of doping on their redox photocatalytic reactivity. We initially hypothesized that incorporating Ni:CdS QDs into heterostructures could alter excited-state dynamics and mechanisms, and that the localization of excited electrons on Ni 3d states could promote redox photocatalytic mechanisms including reduction of CO₂. Isolated Ni:CdS QDs were ferromagnetic, and they exhibited enhanced photocatalytic hydrogen evolution and photostability relative to undoped CdS QDs. Both Pb_0.33V₂O₅/CdS heterostructures (with undoped QDs) and Pb_0.33V₂O₅/Ni:CdS heterostructures (with Ni(II)-doped QDs) exhibited substantial energetic overlap between valence-band states of QDs and intercalative mid-gap states of β-Pb_0.33V₂O₅ NWs. Within Pb_0.33V₂O₅/CdS heterostructures, photoexcitation of CdS QDs was followed by rapid (50–100 ps) transfer of both holes and electrons to β-Pb_0.33V₂O₅ NWs. In contrast, within Pb_0.33V₂O₅/Ni:CdS heterostructures, holes were transferred from Ni:CdS QDs to β-Pb_0.33V₂O₅ NWs within 100 ps, but electrons were transferred approximately 20-fold more slowly. This difference in electron- and hole-transfer kinetics promoted charge separation across the Pb_0.33V₂O₅/Ni:CdS interface and enabled the photocatalytic reduction of CO₂ to CO, CH₄, and HCO₂H with > 99.9% selectivity relative to the reduction of H⁺ to H₂. These results highlight the opportunity to fine-tune dynamics and mechanisms of excited-state charge-transfer, and mechanisms of subsequent redox half-reactions, by doping QDs within heterostructures. Moreover, they reveal the promise of heterostructures comprising QDs and M_xV_yO₅ materials as CO₂-reduction photocatalysts.