[en] The increase of carbon dioxide (CO₂) emission is responsible for global climate change. To mitigate global warming, a great deal of research effort has shifted the focus to effectively converting CO₂ molecules into hydrocarbons. In this review, we highlight the progress in photocatalytic CO₂-to-hydrocarbon conversion, with an emphasis on three strategies: size control, composition control, and facet control of cocatalysts for the enhancement of CO₂ reduction activity and selectivity. Recent progress in photocatalyst design is reviewed in the context of the binding energy and activation energy of CO₂ and the intermediate species. (topical review)

[en] We have investigated the fluorescence properties of colloidal suspensions conntaining quantum dot (QD)/silica hybrid particles. First, we synthesized QD/silica hybrid particles with silica–QD–silica (SQS) core–shell–shell geometry, and monitored the quantum efficiencies of their suspensions at various particle concentrations. We found that the quantum efficiency (QE) of SQS particles in deionized (DI) water was much lower than that of the QDs even at low particle concentration, mainly due to the light scattering of emitted photons at the silica/water interface, followed by reabsorption by QDs. As the concentration of SQS particles was increased, both light scattering and reabsorption by QDs became more important, which further reduced the QE. Refractive index-matched solvent, however, reduced light scattering, yielding greater QE than DI water. Next, we induced aggregation of SQS particles, and found that QE increased as particles aggregated in DI water because of reduced light scattering and reabsorption, whereas it remained almost constant in the refractive index-matched solvent. Finally, we studied aggregation of highly concentrated silica particle suspensions containing a low concentration of SQS particles, and found that QE increased with aggregation because light scattering and reabsorption were reduced. (paper)

[en] Chemically-synthesized FePt nanocrystals must be annealed at a high temperature (>550 deg. C) to induce the hard ferromagnetic L 1₀ phase. Unfortunately, the organic stabilizer covering these nanocrystals degrades at these temperatures and the nanocrystals sinter, resulting in the loss of control over nanocrystal size and separation in the film. We have developed a silica overcoating strategy to prevent nanocrystal sintering. In this study, 6 nm diameter FePt nanocrystals were coated with 17 nm thick shells of silica using an inverse micelle process. Magnetization measurements of the annealed FePt-SiO₂ nanocrystals indicate ferromagnetism with a high coercivity at room temperature. Magnetic force microscopy (MFM) results show that the film composed of nanocrystals behaves as a dipole after magnetization by an 8 T external field. The individual nanocrystals are modelled as single-domain particles with random crystallographic orientations. We propose that the interparticle magnetic dipole interaction is weaker than the magnetocrystalline energy in the remanent state, leading to an unusual material with no magnetic anisotropy and no domains. Films of these nanoparticles are promising candidates for magnetic media with a data storage density of ∼Tb/in²

[en] PbS crystals of cubic, octahedral, and dendritic shapes are synthesized in an aqueous solution that contains supramolecular complexes of β-cyclodextrin (CD) and hexadecyltrimethylammonium bromide (CTAB). The morphology of the PbS crystals depends on the concentration of CD or CTAB in the reaction solution; for example, the branched dendritic structures evolve with an appropriate molar ratio of CD/CTAB supramolecular complexes and reaction time. Regardless of the CD/CTAB molar ratios, octahedral PbS crystals are observed at all compositions of CD/CTAB for the reaction times of 1–5 h, while self-assembled branched/dendritic structures are obtained only for CD/CTAB molar ratios of 0.5, 1, and 2 after a prolonged reaction, e.g., for 24–48 h. Systematic investigation reveals that both reaction time and CD/CTAB molar ratio are responsible for self-assembled branched/dendritic structures of octahedral crystals

[en] Commercially viable synthesis of InP nanocrystals (NCs) involves highly pyrophoric phosphorus (P) precursor, tris(trimethylsilyl) phosphine (TMS₃P). Finding a cheap and safe alternative would be the holy grail. We report the synthesis of InP NCs using triphenyl phosphite, an inexpensive and relatively safe phosphorous source. By reacting indium chloride and triphenyl phosphite, we obtained large-sized and black-colored InP NCs, yet without any distinct feature that shows quantum confinement effect. Addition of ZnCl₂ resulted in InP NCs with controlled size, which was manifested in the shift of 1S peak in absorption spectra. By coating ZnS shell on InP NCs, we achieved photoluminescence with some extent of trap emission, showing maximum total quantum yield (QY) of 23% (8% of band-edge emission QY). We used ³¹P nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY), and mass spectrometry (MS) to assign intermediates and following mechanisms of the InP synthesis using triphenyl phosphite. The development of this safe and cost-effective P precursor opens broader opportunity space for large-scale production of InP NC.

[en] Highlights: • Size-tunable energy states of conduction band in CdSe quantum dots enable systematic control of product selectivity for CO₂ reduction. • The energy gap between conduction band edge and redox potential of each reduction product correlates with their production rate. • The electron transfer kinetics proves to alter the selectivity of CO₂ reduction. We investigate the product selectivity of CO₂ reduction using NiO photocathodes decorated with CdSe quantum dots (QDs) of varying size in a photoelectrochemical (PEC) cell. Size-tunable and quantized energy states of conduction band in CdSe QDs enable systematic control of electron transfer kinetics from CdSe QDs to NiO. It turns out that different size of CdSe QDs results in variation in product selectivity for CO₂ reduction. The energy gap between conduction band edge and redox potential of each reduction product (e.g., CO and CH₄) correlates with their production rate. The size dependence of the electron transfer rate estimated from the energy gap is in agreement with the selectivity of CO₂ reduction products for all reduction products but CO. The deviation in the case of CO is attributed to sequential conversion of CO into CH₄ with CO adsorbed on electrode surface. Based on a premise that the CdSe QDs would exhibit similar surface configuration regardless of QD size, it is concluded that the electron transfer kinetics proves to alter the selectivity of CO₂ reduction.

[en] We present facile synthesis of bright CdS/CdSe/CdS@SiO₂ nanoparticles with 72% of quantum yields (QYs) retaining ca 80% of the original QYs. The main innovative point is the utilization of the highly luminescent CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) as silica coating seeds. The significance of inorganic semiconductor shell passivation and structure design of quantum dots (QDs) for obtaining bright QD@SiO₂ is demonstrated by applying silica encapsulation via reverse microemulsion method to three kinds of QDs with different structure: CdSe core and 2 nm CdS shell (CdSe/CdS-thin); CdSe core and 6 nm CdS shell (CdSe/CdS-thick); and CdS core, CdSe intermediate shell and 5 nm CdS outer shell (CdS/CdSe/CdS-SQW). Silica encapsulation inevitably results in lower photoluminescence quantum yield (PL QY) than pristine QDs due to formation of surface defects. However, the retaining ratio of pristine QY is different in the three silica coated samples; for example, CdSe/CdS-thin/SiO₂ shows the lowest retaining ratio (36%) while the retaining ratio of pristine PL QY in CdSe/CdS-thick/SiO₂ and SQW/SiO₂ is over 80% and SQW/SiO₂ shows the highest resulting PL QY. Thick outermost CdS shell isolates the excitons from the defects at surface, making PL QY relatively insensitive to silica encapsulation. The bright SiO₂-coated SQW sample shows robustness against harsh conditions, such as acid etching and thermal annealing. The high luminescence and long-term stability highlights the potential of using the SQW/SiO₂ nanoparticles in bio-labeling or display applications. (paper)

[en] Highlights: • CdSe/PbSe type II nanorod heterostructures are prepared by cation exchange. • Transient absorption study reveals significant carrier trapping at defect sites formed in the cation exchange process. • Transient absorption study reveals the reduction of defect density and increase of carrier lifetime by annealing the nanorod. Cation exchange occurs via defect initiated solid-state diffusion, a process that can lead to defect formations. The effect of such inherent defect formation on carrier dynamics of cation-exchanged heterostructures remains poorly understood. Herein, we report exciton dynamics in type II CdSe/PbSe heterostructure nanorods formed via cation exchange. The majority of electrons in CdSe domains decays in 5 ps due to ultrafast carrier trapping. The defect generated by cation exchange can be healed by annealing the as-synthesized CdSe/PbSe heterostructure nanorods. This study suggests a strategy for improving properties of heteronanostructures prepared by cation exchange for applications in photovoltaics and photocatalysis.

[en] The past decade has seen a dramatic surge in the power conversion efficiency (PCE) of next-generation solution-processed thin-film solar cells rapidly closing the gap in PCE of commercially-available photovoltaic (PV) cells. Yet the operational stability of such new PVs leaves a lot to be desired. Specifically, chemical reaction with absorbers via high-energy photons transmitted through the typically-adapted metal oxide electron transporting layers (ETLs), and photocatalytic degradation at interfaces are considered detrimental to the device performance. Herein, the authors introduce a device architecture using the narrow-gap, Indium Arsenide colloidal quantum dots (CQDs) with discrete electronic states as an ETL in high-efficiency solution-processed PVs. High-performing PM6:Y6 organic PVs (OPVs) achieve a PCE of 15.1%. More importantly, as the operating stability of the device is significantly improved, retaining above 80% of the original PCE over 1000 min under continuous illumination, a Newport-certified PCE of 13.1% is reported for nonencapsulated OPVs measured under ambient air. Based on operando studies as well as optical simulations, it suggested that the InAs CQD ETLs with discrete energy states effectively cut-off high-energy photons while selectively collecting electrons from the absorber. The findings of this works enable high-efficiency solution-processed PVs with enhanced durability under operating conditions. (© 2022 Wiley‐VCH GmbH)

[en] Films of octadecyl-capped Si nanoparticles (NPs) (diameter, 3.4 ± 0.7 nm) prepared by drop-coating on indium tin oxide (ITO) showed electrogenerated chemiluminescence (ECL) for both cathodic and anodic potential sweeps in KOH solutions containing peroxydisulfate. The redox potentials of the Si NPs can be estimated as approximately -0.9 and +0.95 V (versus Ag|AgCl) based on the anodic potential for the onset of ECL minus the ECL peak energy. The ECL exhibits a relatively broad spectrum (FWHM = 160 nm) with a peak wavelength of ∼670 nm (1.85 eV), similar to the photoluminescence spectra. In electrochemical studies in KOH solution in the absence of peroxydisulfate, an anodic current peak appears at about -1 V (versus Ag|AgCl) following a scan to negative potentials. A similar peak has been observed during the etching of a bulk single crystal Si electrode in alkaline aqueous solution. Unpassivated surface sites of Si NPs seem to be etched at potentials negative of the anodic oxidation peak