With porous shells and mobile cores, yolk-shell nanostructures provide great structural advantage for mass transport-related applications such as photocatalysis. In this work, Au–Cu7S4 yolk-shell ...nanostructures are synthesized from Au–Cu2O core-shell templates. The Cu7S4 shell is then converted to CdS through a cation exchange process to produce Au–CdS yolk-shell photocatalysts for hydrogen generation. Ultrafast transient absorption and finite-difference time-domain simulation are used to investigate electronic interaction between Au nanoparticle core and the surrounding CdS shell. Additionally, a new method is presented to simulate chemical transport and quantitatively compare diffusion kinetics by monitoring mass transport through the porous CdS shell with dye molecules as optical probes. The highest hydrogen generation rate of 3390 μmol g−1 h−1, corresponding to an adequate apparent quantum yield of 4.22% at 420 nm, is achieved for Au–CdS with the largest void size. The enhancement in photocatalytic performance with increase in void size is mostly attributed to improved mass transport kinetics, with additional gains from more efficient charge transfer and stronger surface plasmon resonance-mediated near-field effects. This comprehensive study demonstrates that void size is a critical structural parameter in optimizing the performance of yolk-shell nanostructures for photocatalysis or other mass-transport related applications.
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•Yolk-shell nanostructures are demonstrated as an emerging photocatalyst paradigm.•The activity of hydrogen generation is enhanced with increase in void size.•Efficient charge transfer and strong SPR-mediated near-field effects are observed.•Control over the void size can improve mass transport kinetics across the shell.
CH3NH3PbBr3 perovskite nanocrystals (PNCs) of different sizes (ca. 2.5–100 nm) with high photoluminescence (PL) quantum yield (QY; ca. 15–55 %) and product yield have been synthesized using the ...branched molecules, APTES and NH2‐POSS, as capping ligands. These ligands are sterically hindered, resulting in a uniform size of PNCs. The different capping effects resulting from branched versus straight‐chain capping ligands were compared and a possible mechanism proposed to explain the dissolution–precipitation process, which affects the growth and aggregation of PNCs, and thereby their overall stability. Unlike conventional PNCs capped with straight‐chain ligands, APTES‐capped PNCs show high stability in protic solvents as a result of the strong steric hindrance and propensity for hydrolysis of APTES, which prevent such molecules from reaching and reacting with the core of PNCs.
Perowskit‐Nanokristalle (PNCs) unterschiedlicher Größe mit hohen Photolumineszenz‐Quantenausbeuten wurden unter Verwendung verzweigter APTES‐Liganden synthetisiert. Die APTES‐beschichteten PNCs sind hoch beständig in protischen Lösungsmitteln, da ihr Kern durch die sterischen Eigenschaften und das Hydrolyseverhalten von APTES geschützt wird.
Abstract
Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced ...by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi
2
O
4
. The optimized overlayer (Cu
1.5
TiO
z
) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.
In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell
via
a microwave-assisted cation exchange ...approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns-μs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed.
We report anomalous size-dependent photoluminescence intensity variation of PbS quantum dots with the formation of a thin CdS shell.
On the nanoscale, size and structure are powerful dictators of optical and electronic response. As such, they may be rationally manipulated in order to obtain desired properties for specific ...applications. The dissertation projects herein are aimed toward understanding how structure affects the photophysical properties of three nanoparticle systems: hollow gold nanospheres (HGNs), doped α-Fe2O3 nanostructures, and PbS/CdS core/shell quantum dots (QDs). The dissertation is divided into two parts: Part I: The Highly Tunable Hollow Gold Nanosphere: Synthesis, Size, and Surface Morphology and Part II: Ultrafast Charge Carrier Dynamics of Hematite Nanostructures and PbS/CdS Quantum Dots. Part I focuses on synthetic control and characterization of HGNs, solvent-filled plasmonic shells of gold ranging from 20-200 nm in diameter. HGNs have shown promising performance in drug loading, targeted delivery, surface-enhanced Raman scattering, and photothermal therapy. Their optical properties are very sensitive to their aspect ratio, the ratio of diameter to shell thickness. As such, a well-controlled synthesis is highly desired. In Chapter 1, the HGN synthesis was updated to enable simultaneous control of both diameter and SPR while maintaining monodispersity and uniformity of the resultant shells. This was possible through a detailed and systematic investigation of the synthesis of the sacrificial cobalt-based scaffolds onto which HGNs are formed through galvanic exchange. In Chapter 2, additional synthetic adjustments were introduced to systematically control the HGN surface morphology from smooth to very bumpy. Rugose structures are perhaps the most versatile nanoparticles, with an increased density of active sites for catalysis as well as local electric field enhancement around the surface features for sensing and detection. As hollow particles have displayed enhanced plasmonic performance in comparison with their solid counterparts for a number of applications, the combination of hollow cores and rugose surfaces is highly attractive. One of the most attractive applications of HGNs is plasmonic photothermal therapy (PTT). In this application, plasmonic nanoparticles are targeted to cancer cells where they convert incident light to heat, raising the temperature of their environment above the point of cell viability. A systematic comparison of HGNs with different surface morphologies revealed that bumpy HGNs retain the excellent photothermal conversion efficiency (PCE) of their smooth counterparts. Next, in Chapter 3, PCE was investigated for HGNs of different diameters, theoretically and experimentally. The findings revealed that 50 nm HGNs generate ~2 times the heat per µg gold as their 70 nm counterparts and ~1.5 times the heat per µg gold as their 30 nm counterparts. In vitro HGN-mediated PTT of oral squamous cell carcinoma was also carried out. Ongoing efforts are needed to assess the PCE and potential size dependence of HGNs in vitro and in vivo. In Part II, transient absorption spectroscopy (TAS) is used to probe the charge carrier dynamics in ?-Fe2O3 (hematite) and PbS/CdS nanostructures. In both systems, structural modification has been crucial for obtaining enhanced photophysical performance. Hematite is considered to be an especially promising material for solar water splitting due to its chemical stability, abundance, and non-toxicity. Additionally, its band gap of approximately 2.0–2.2 eV facilitates the absorption of about 40% of incident solar light. However, it suffers from a number of limitations which hinder its practical implementation as a photoanode material. Chapter 4 discusses these limitations and structural approaches to overcome them, including nanostructuring and doping to facilitate charge transfer and improve PEC performance. Charge carrier recombination dynamics are investigated before and after doping to aid understanding of the mechanism of performance enhancement in Zr and Ti-doped hematite films. Although nanostructuring and doping are beneficial, performance gains have been modest and a review of the current approaches for the rational design of hematite heterostructures is provided in Chapter 5. In these reports, TAS has been employed as a useful tool to gain deeper insight into the mechanisms of photogenerated electron-hole recombination and their relation to PEC performance. Finally, in Chapter 6, the passivation of PbS QDs with a CdS shell, and the dependence of the passivation on particle size, is investigated. Semiconductor QDs like PbS are highly attractive components in solar cells, sensing, and detection due to their size-dependent optical and electronic properties. Broad absorption, narrow photoluminescence (PL), and high PL quantum yield arise from quantum confinement. The formation of a thin shell atop the QD core has become a common method for passivating the surface bonds and thereby enhancing and stabilizing resultant PL. In this work, it was found that surface passivation of PbS with a CdS shell is core-size-dependent; ultrasmall PbS QDs did not benefit from CdS passivation like their larger counterparts, but instead experienced a decrease in PL. Coupling TAS with steady state PL and QY measurements enabled a comprehensive understanding of the radiative and nonradiative relaxation pathways of the PbS/CdS nanosystem. Insights into the size-dependent variation in trap state density and relaxation pathways may serve as a guide for future structural modification.
CH3 NH3 PbBr3 perovskite quantum dots (PQDs) are synthesized by using four different linear alkyl phosphonic acids (PAs) in conjunction with (3-aminopropyl)triethoxysilane (APTES) as capping ligands. ...The resultant PQDs are characterized by means of XRD, TEM, Raman spectroscopy, FTIR spectroscopy, UV/Vis, photoluminescence (PL), time-resolved PL, and X-ray photoelectron spectroscopy (XPS). PA chain length is shown to control the PQD size (ca. 2.9-4.2 nm) and excitonic absorption band positions (λ=488-525 nm), with shorter chain lengths corresponding to smaller sizes and bluer absorptions. All samples show a high PL quantum yield (ca. 46-83 %) and high PL stability; this is indicative of a low density of band gap trap states and effective surface passivation. Stability is higher for smaller PQDs; this is attributed to better passivation due to better solubility and less steric hindrance of the shorter PA ligands. Based on the FTIR, Raman, and XPS results, it is proposed that Pb2+ and CH3 NH3 + surface defects are passivated by R-PO3 2- or R-PO2 (OH)- , whereas Br- surface defects are passivated by R-NH3 + moieties. This study establishes the combination of PA and APTES ligands as a highly effective dual passivation system for the synergistic passivation of multiple surface defects of PQDs through primarily ionic bonding.
Functional nanoscale interfaces that promote the transport of photoexcited charge carriers are fundamental to efficient hydrogen production during photoelectrochemical (PEC) splitting of water. Here, ...the realization of a functional one‐dimensional nanostructure achieved through surface engineering of hematite (α‐Fe2O3) nanorods with a TiO2 overlayer is reported. The surface‐engineered hematite nanostructure exhibits significantly improved PEC performance as compared to untreated α‐Fe2O3, with an increase in the maximum incident photon‐to‐current efficiency (IPCE) of nearly 400% at 350 nm. While addition of the TiO2 overlayer did not alter the lifetime of photoexcited charge carriers, as evidenced from transient absorption spectroscopy, it is found that the presence of TiO2 could enhance oxygen electrocatalysis by interfacial electron enrichment, largely attributed to enhanced O(2p)−Fe(3d) hybridization. Moreover, the interfacial electronic structure revealed from XANES measurements of the α‐Fe2O3/TiO2 nanorods suggests that photoexcited holes in α‐Fe2O3 may efficiently transfer through the TiO2 overlayer to the electrolyte while electrons migrate to the external circuit along the one‐dimensional nanorods, thereby promoting charge separation and enhancing PEC splitting of water.
An interfacial α‐Fe2O3/TiO2 structure is rationally designed and fundamentally characterized to explore its electronic structure evolution for the synergy of the promoted interface charge transfer processes and the accelerated water oxidation electrocatalysis, contributing to the greatly enhanced photoelectrochemical water splitting activities.
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