Facet-Selective Deposition of Ultrathin Al2 O3 on Copper Nanocrystals for Highly Stable CO2 Electroreduction to Ethylene.

Catalysts based on Cu nanocrystals (NCs) for electrochemical CO2 -to-C2+ conversion with high activity have been a subject of considerable interest, but poor stability and low selectivity for a single C2+ product remain obstacles for realizing sustainable carbon-neutral cycles. Here, we used the facet-selective atomic layer deposition (FS-ALD) technique to selectively cover the (111) surface of Cu NCs with ultrathin Al2 O3 to increase the exposed facet ratio of (100)/(111), resulting in a faradaic efficiency ratio of C2 H4 /CH4 for overcoated Cu NCs 22 times higher than that for pure Cu NCs. Peak performance of the overcoated catalyst (Cu NCs/Al2 O3 -10C) reaches a C2 H4 faradaic efficiency of 60.4 % at a current density of 300 mA cm-2 in 5 M KOH electrolyte, when using a gas diffusion electrode flow cell. Moreover, the Al2 O3 overcoating effectively suppresses the dynamic mobility and the aggregation of Cu NCs, which explains the negligible activity loss and selectivity degradations of Cu NCs/Al2 O3 -10C shown in stability tests.

Retraction: Phosphorus-Rich Colloidal Cobalt Diphosphide (CoP2 ) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution.

Adv. Mater. 2019, 31, 1900813. https://doi.org/10.1002/adma.201900813 The above article, published online on May 6, 2019, in Wiley Online Library (https://doi.org/10.1002/adma.201900813), has been retracted by agreement between the authors, the journal Editor in Chief Jos Lenders, and Wiley-VCH GmbH. The retraction has been agreed on following concerns raised by a third party and a subsequent investigation at Wake Forest University. Data integrity issues were found in Figures 2c, 4d, S13a, S13b, S15, and S37. As a result, the authors consider the conclusions of this article invalid.

Retraction: Colloidal Cobalt Phosphide Nanocrystals as Trifunctional Electrocatalysts for Overall Water Splitting Powered by a Zinc-Air Battery.

Adv. Mater. 2018, 30, 1705796 https://doi.org/10.1002/adma.201705796 The above article, published online on January 15, 2018, in Wiley Online Library (https://doi.org/10.1002/adma.201705796), has been retracted by agreement between the authors, the journal Editor in Chief Jos Lenders, and Wiley-VCH GmbH. The retraction has been agreed on following concerns raised by a third party and a subsequent investigation at Wake Forest University. Data integrity issues were found in Figures 1a, S2b, and S17. As a result, the authors consider the conclusions of this article invalid.

Perspective on the prospects of a carrier multiplication nanocrystal solar cell.

This article presents a perspective on the experimental and theoretical work to date on the efficiency of carrier multiplication (CM) in colloidal semiconductor nanocrystals (NCs). Early reports on CM in NCs suggested large CM efficiency enhancements. However, recent experiments have shown that CM in nanocrystalline samples is not significantly stronger, and often is weaker, than in the parent bulk when compared on an absolute photon energy basis. This finding is supported by theoretical consideration of the CM process and the competing intraband relaxation. We discuss the experimental artifacts that may have led to the apparently strong CM estimated in early reports. The finding of bulklike CM in NCs suggests that the main promise of quantum confinement is to boost the photovoltage at which carriers can be extracted. With this in mind, we discuss research directions that may result in effective use of CM in a solar cell.

Correction: Synthesis of lead-free Cs3Sb2Br9 perovskite alternative nanocrystals with enhanced photocatalytic CO2 reduction activity.

Correction for 'Synthesis of lead-free Cs3Sb2Br9 perovskite alternative nanocrystals with enhanced photocatalytic CO2 reduction activity' by Chang Lu et al., Nanoscale, 2020, 12, 2987-2991, https://doi.org/10.1039/C9NR07722G.

Scalable neutral H2O2 electrosynthesis by platinum diphosphide nanocrystals by regulating oxygen reduction reaction pathways.

Despite progress in small scale electrocatalytic production of hydrogen peroxide (H2O2) using a rotating ring-disk electrode, further work is needed to develop a non-toxic, selective, and stable O2-to-H2O2 electrocatalyst for realizing continuous on-site production of neutral hydrogen peroxide. We report ultrasmall and monodisperse colloidal PtP2 nanocrystals that achieve H2O2 production at near zero-overpotential with near unity H2O2 selectivity at 0.27 V vs. RHE. Density functional theory calculations indicate that P promotes hydrogenation of OOH* to H2O2 by weakening the Pt-OOH* bond and suppressing the dissociative OOH* to O* pathway. Atomic layer deposition of Al2O3 prevents NC aggregation and enables application in a polymer electrolyte membrane fuel cell (PEMFC) with a maximum r(H2O2) of 2.26 mmol h-1 cm-2 and a current efficiency of 78.8% even at a high current density of 150 mA cm-2. Catalyst stability enables an accumulated neutral H2O2 concentration in 600 mL of 3.0 wt% (pH = 6.6).

Synthesis of lead-free Cs3Sb2Br9 perovskite alternative nanocrystals with enhanced photocatalytic CO2 reduction activity.

A synthetic method for uniform and pure Cs3Sb2Br9 NCs has been developed. Cs3Sb2Br9 NCs exhibit a 10-fold increase in activity for the photocatalytic CO2 reduction reaction compared to CsPbBr3 NCs, achieving 510 µmol CO g-1 cat. after 4 h. Density functional theory shows that Cs3Sb2Br9 surfaces sufficiently expose Sb to allow reactivity, as opposed to the unreactive CsPbBr3 surface.

Colloidal silver diphosphide (AgP2) nanocrystals as low overpotential catalysts for CO2 reduction to tunable syngas.

Production of syngas with tunable CO/H2 ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liquid fuel production. Ag is a benchmark electrocatalysts for CO2-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP2 nanocrystals (NCs) with a greater than 3-fold reduction in overpotential for electrochemical CO2-to-CO reduction compared to Ag and greatly enhanced stability. Density functional theory calculations reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a maximum Faradaic efficiency is achieved at an average silver valence state of +1.08 in AgP2 NCs. A photocathode consisting of a n+p-Si wafer coated with ultrathin Al2O3 and AgP2 NCs achieves an onset potential of 0.2 V vs. RHE for CO production and a partial photocurrent density for CO at -0.11 V vs. RHE (j-0.11, CO) of -3.2 mA cm-2.

Molecular Rectifiers on Silicon: High Performance by Enhancing Top-Electrode/Molecule Coupling.

One of the simplest molecular-scale electronic devices is the molecular rectifier. In spite of considerable efforts aimed at understanding structure-property relationships in these systems, devices with predictable and stable electronic properties are yet to be developed. Here, we demonstrate highly efficient current rectification in a new class of compounds that form self-assembled monolayers on silicon. We achieve this by exploiting the coupling of the molecules with the top electrode which, in turn, controls the position of the relevant molecular orbitals. The molecules consist of a silane anchoring group and a nitrogen-substituted benzene ring, separated by a propyl group and imine linkage, and result from a simple, robust, and high-yield synthetic procedure. We find that when incorporated in molecular diodes, these compounds can rectify current by as much as 3 orders of magnitude, depending on their structure, with a maximum rectification ratio of 2635 being obtained in ( E)-1-(4-cyanophenyl)- N-(3-(triethoxysilyl) propyl)methanimine (average Ravg = 1683 ± 458, at an applied voltage of 2 V). This performance is on par with that of the best molecular rectifiers obtained on metallic electrodes, but it has the advantage of lower cost and more efficient integration with current silicon technologies. The development of molecular rectifiers on silicon may yield hybrid systems that can expand the use of silicon toward novel functionalities governed by the molecular species grafted onto its surface.

Phosphorus-Rich Colloidal Cobalt Diphosphide (CoP2 ) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution.

Developing earth-abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high-performance solar-to-hydrogen energy conversion process. Herein, phosphorus-rich colloidal cobalt diphosphide nanocrystals (CoP2 NCs) are synthesized via hot injection. The CoP2 NCs show a Pt-like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve -10 mA cm-2 and a very low Tafel slope of 32 mV dec-1 . Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP2 NCs is further demonstrated in a metal-insulator-semiconductor photoelectrochemical device consisting of bottom p-Si light absorber, atomic layer deposition Al-ZnO passivation layers, and the CoP2 cocatalyst. The p-Si/AZO/TiO2 /CoP2 photocathode shows a photocurrent density of -16.7 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p-Si and AZO, the corrosion-resistance of the pinhole-free TiO2 protective layer, and the fast HER kinetics of the CoP2 NCs.

Size-dependent charge collection in junctions containing single-size and multi-size arrays of colloidal CdSe quantum dots.

This paper describes the electrical characteristics of junctions composed of three-dimensional arrays of colloidal CdSe quantum dots (QDs) with tin-doped indium oxide (ITO)/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) and eutectic Ga-In (EGaIn) electrodes. It focuses on a comparison of junctions containing QDs of one size to those of arrays containing QDs of multiple sizes. This comparison makes it possible to estimate the relative contributions of transport across various interfaces (e.g., between the QDs and between the QDs and the electrodes) to the observed electrical characteristics of the junction and to evaluate the dependence of these contributions on the locations of various sizes of QDs within the junction. The junctions were diodes, and their turn-on voltage depended on the size of the QDs next to the PEDOT:PSS. We describe this dependence using a Marcus model to estimate the barrier for charge transfer induced by the difference in energies between the orbitals of the QDs and the valence band of PEDOT:PSS.

The use of size-selective excitation to study photocurrent through junctions containing single-size and multi-size arrays of colloidal CdSe quantum dots.

This paper describes a study of the generation and flow of photocurrent through junctions containing three-dimensional arrays of colloidal CdSe quantum dots (QDs) of either a single size or multiple sizes. The electrodes were indium tin oxide (ITO) covered with a thin layer of poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) and a eutectic alloy of Ga and In (EGaIn). We measured the current-voltage characteristics of the junctions in the dark and under illumination, with various sources and wavelengths of excitation, and their photocurrent action spectra. Size-selective photoexcitation of the arrays of multiple sizes of QDs helped to determine (i) the location of the interface at which photoinduced separation of charge occurred, (ii) whether the energy absorbed by the QDs was redistributed before separation of charge, and (iii) the dependence of the photovoltage on the locations of various sizes of QDs within the junction. This research is a step toward the use of QDs for harvesting light and for transporting energy and charge in devices-for example, solar cells and photodetectors-that operate at zero bias.

A broadband Fourier transform microwave spectrometer based on chirped pulse excitation.

Designs for a broadband chirped pulse Fourier transform microwave (CP-FTMW) spectrometer are presented. The spectrometer is capable of measuring the 7-18 GHz region of a rotational spectrum in a single data acquisition. One design uses a 4.2 Gsampless arbitrary waveform generator (AWG) to produce a 1 mus duration chirped pulse with a linear frequency sweep of 1.375 GHz. This pulse is sent through a microwave circuit to multiply the bandwidth of the pulse by a factor of 8 and upconvert it to the 7.5-18.5 GHz range. The chirped pulse is amplified by a traveling wave tube amplifier and broadcast inside the spectrometer by using a double ridge standard gain horn antenna. The broadband molecular free induction decay (FID) is received by a second horn antenna, downconverted, and digitized by a 40 Gsampless (12 GHz hardware bandwidth) digital oscilloscope. The second design uses a simplified pulse generation and FID detection scheme, employing current state-of-the-art high-speed digital electronics. In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 Gsampless AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 Gsampless digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7-18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle-Flygare-type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.

Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution.

Colloidal semiconductor quantum dot (CQD)-based photocathodes for solar-driven hydrogen evolution have attracted significant attention because of their tunable size, nanostructured morphology, crystalline orientation, and band gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm2 (AM-1.5G, 100 mW/cm2) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j0) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V versus RHE and long-term stability with negligible degradation. In the acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited because of photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j0 of -2.14 mA/cm2 is obtained with only 8.3% activity degradation after 6 h, compared with 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge-transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production.

Colloidal Cobalt Phosphide Nanocrystals as Trifunctional Electrocatalysts for Overall Water Splitting Powered by a Zinc-Air Battery.

Highly efficient and stable electrocatalysts, particularly those that are capable of multifunctionality in the same electrolyte, are in high demand for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). In this work, highly monodisperse CoP and Co2 P nanocrystals (NCs) are synthesized using a robust solution-phase method. The highly exposed (211) crystal plane and abundant surface phosphide atoms make the CoP NCs efficient catalysts toward ORR and HER, while metal-rich Co2 P NCs show higher OER performance owing to easier formation of plentiful Co2 P@COOH heterojunctions. Density functional theory calculation results indicate that the desorption of OH* from cobalt sites is the rate-limiting step for both CoP and Co2 P in ORR and that the high content of phosphide can lower the reaction barrier. A water electrolyzer constructed with a CoP NC cathode and a Co2 P NC anode can achieve a current density of 10 mA cm-2 at 1.56 V, comparable even to the noble metal-based Pt/C and RuO2 /C pair. Furthermore, the CoP NCs are employed as an air cathode in a primary zinc-air battery, exhibiting a high power density of 62 mW cm-2 and good stability.

Imbedded Nanocrystals of CsPbBr3 in Cs4 PbBr6 : Kinetics, Enhanced Oscillator Strength, and Application in Light-Emitting Diodes.

Solution-grown films of CsPbBr3 nanocrystals imbedded in Cs4 PbBr6 are incorporated as the recombination layer in light-emitting diode (LED) structures. The kinetics at high carrier density of pure (extended) CsPbBr3 and the nanoinclusion composite are measured and analyzed, indicating second-order kinetics in extended and mainly first-order kinetics in the confined CsPbBr3 , respectively. Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr3 indicates enhanced oscillator strength consistent with earlier published attribution of the sub-nanosecond exciton radiative lifetime in nanoprecipitates of CsPbBr3 in melt-grown CsBr host crystals and CsPbBr3 evaporated films.

An atomic layer deposition chamber for in situ x-ray diffraction and scattering analysis.

The crystal structure of thin films grown by atomic layer deposition (ALD) will determine important performance properties such as conductivity, breakdown voltage, and catalytic activity. We report the design of an atomic layer deposition chamber for in situ x-ray analysis that can be used to monitor changes to the crystal structural during ALD. The application of the chamber is demonstrated for Pt ALD on amorphous SiO2 and SrTiO3 (001) using synchrotron-based high resolution x-ray diffraction, grazing incidence x-ray diffraction, and grazing incidence small angle scattering.

In Vacuo Photoemission Studies of Platinum Atomic Layer Deposition Using Synchrotron Radiation.

The mechanism of platinum atomic layer deposition using (methylcyclopentadienyl)trimethylplatinum and oxygen is investigated with in vacuo photoemission spectroscopy at the Stanford Synchrotron Radiation Lightsource. With this surface-sensitive technique, the surface species following the Pt precursor half cycle and the oxygen counter-reactant half cycle can be directly measured. We observed significant amounts of carbonaceous species following the Pt precursor pulse, consistent with dehydrogenation of the precursor ligands. Significantly more carbon is observed when deposition is carried out in the thermal decomposition temperature region. The carbonaceous layer is removed during the oxygen counter reactant pulse, and the photoemission spectrum shows that a layer of adsorbed oxygen remains on the surface as previously predicted.

Efficient luminescent down-shifting detectors based on colloidal quantum dots for dual-band detection applications.

A colloidal quantum dot (QD) luminescent down-shifting (LDS) layer is used to sensitize an InGaAs short wavelength infrared photodetector to the near UV spectral band. An average improvement in the external quantum efficiency (EQE) from 1.8% to 21% across the near UV is realized using an LDS layer consisting of PbS/CdS core/shell QDs embedded in PMMA. A simple model is used to fit the experimental EQE data. A UV sensitive InGaAs imaging array is demonstrated and the effect of the LDS layer on the optical resolution is calculated. The bandwidth of the LDS detector under UV illumination is characterized and shown to be determined by the photoluminescence lifetime of the QDs.