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1.
J Phys Chem B ; 127(41): 8961-8973, 2023 Oct 19.
Artículo en Inglés | MEDLINE | ID: mdl-37802098

RESUMEN

Poly(ethylene glycol) (PEG) ligands can inhibit proteins and other biomolecules from adhering to underlying surfaces, making them excellent surface ligands for nanocrystal (NC)-based drug carriers. Quantifying the PEG ligand shell morphology is important because its structure determines the permeability of biomolecules through the shell to the NC surface. However, few in situ analytical tools can reveal whether the PEG ligands form either an impenetrable barrier or a porous coating surrounding the NC. Here, we present a Förster resonance energy transfer (FRET) spectroscopy-based approach that can assess the permeability of molecules through PEG-coated ZnO NCs. In this approach, ZnO NCs serve as FRET donors, and freely diffusing molecules in the bulk solution are FRET acceptors. We synthesized a series of variable chain length PEG-silane-coated ZnO NCs such that the longest chain length ligands far exceed the Förster radius (R0), where the energy transfer (EnT) efficiency is 50%. We quantified the EnT efficiency as a function of the ligand chain length using time-resolved photoluminescence lifetime (TRPL) spectroscopy within the framework of FRET theory. Unexpectedly, the longest PEG-silane ligand showed equivalent EnT efficiency as that of bare, hydroxyl-passivated ZnO NCs. These results indicate that the "rigid shell" model fails and the PEG ligand shell morphology is more likely porous or in a patchy "mushroom state", consistent with transmission electron microscopy data. While the spectroscopic measurements and data analysis procedures discussed herein cannot directly visualize the ligand shell morphology in real space, the in situ spectroscopy approach can provide researchers with valuable information regarding the permeability of species through the ligand shell under practical biological conditions.

2.
Proc Natl Acad Sci U S A ; 120(15): e2220333120, 2023 Apr 11.
Artículo en Inglés | MEDLINE | ID: mdl-37011201

RESUMEN

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, "cool" carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS2. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm2 areas by intimately coupling ML-MoS2 to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future two-dimensional (2D) semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuel applications.

3.
Artículo en Inglés | MEDLINE | ID: mdl-34723470

RESUMEN

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance. Photocurrent mapping and local photocurrent-potential (i-E) measurements of MoS2 nanoflakes exfoliated from naturally occurring bulk crystals revealed the presence of n- and p-type domains within the same nanoflake. Interestingly, the n- and p-type domains in the natural MoS2 nanoflakes were equally efficient for iodide oxidation and tri-iodide reduction (IQE values exceed 80%). At the single domain-level, the natural MoS2 nanoflakes were nearly as efficient as nanoflakes exfoliated from synthetic n-type MoS2 crystals. Single domain-level i-E measurements explain why natural MoS2 nanoflakes exhibit an n-type to p-type photocurrent switching effect in ensemble-level measurements: the n- and p-type diode currents from individual domains oppose each other upon illuminating the entire nanoflake, resulting in zero photocurrent at the switching potential. The doping heterogeneity effect is likely due to nonideal stoichiometry, where p-type domains are S-rich according to XPS measurements. Although this doping heterogeneity effect limits photoanode or photocathode performance, these findings open the possibility to synthesize efficient TMD nanoflake photocatalysts with well-defined lateral p- and n-type domains for enhanced charge separation.

4.
Front Chem ; 9: 651248, 2021.
Artículo en Inglés | MEDLINE | ID: mdl-33816440

RESUMEN

Understanding how particle size and morphology influence ion insertion dynamics is critical for a wide range of electrochemical applications including energy storage and electrochromic smart windows. One strategy to reveal such structure-property relationships is to perform ex situ transmission electron microscopy (TEM) of nanoparticles that have been cycled on TEM grid electrodes. One drawback of this approach is that images of some particles are correlated with the electrochemical response of the entire TEM grid electrode. The lack of one-to-one electrochemical-to-structural information complicates interpretation of genuine structure/property relationships. Developing high-throughput ex situ single particle-level analytical techniques that effectively link electrochemical behavior with structural properties could accelerate the discovery of critical structure-property relationships. Here, using Li-ion insertion in WO3 nanorods as a model system, we demonstrate a correlated optically-detected electrochemistry and TEM technique that measures electrochemical behavior of via many particles simultaneously without having to make electrical contacts to single particles on the TEM grid. This correlated optical-TEM approach can link particle structure with electrochemical behavior at the single particle-level. Our measurements revealed significant electrochemical activity heterogeneity among particles. Single particle activity correlated with distinct local mechanical or electrical properties of the amorphous carbon film of the TEM grid, leading to active and inactive particles. The results are significant for correlated electrochemical/TEM imaging studies that aim to reveal structure-property relationships using single particle-level imaging and ensemble-level electrochemistry.

5.
J Chem Phys ; 154(5): 054704, 2021 Feb 07.
Artículo en Inglés | MEDLINE | ID: mdl-33557543

RESUMEN

Energy transfer measurements are widely used to measure the distance between donors and acceptors in heterogeneous environments. In nanocrystal (NC)-molecule donor-acceptor systems, NC defects can participate in electronic energy transfer (EnT) in a defect-mediated EnT process. Here, we explore whether ensemble-level spectroscopy measurements can quantify the distance between the donor defect sites in the NC and acceptor molecules. We studied defect-mediated EnT between ZnO NCs and Alexa Fluor 555 (A555) because EnT occurs via emissive NC defect sites, such as oxygen vacancies. We synthesized a size series of ZnO NCs and characterized their radii, concentration, photoluminescence (PL) lifetime, and defect PL quantum yield using a combination of transmission electron microscopy, elemental analysis, and time-resolved PL spectroscopy. The ZnO defect PL decay kinetics were analyzed using the stochastic binding (SB) and restricted geometry (RG) models. Both models assume the Förster point dipole approximation, but the RG model considers the geometry of the NC donor in the presence of multiple acceptors. The RG model revealed that the emissive defect sites are separated, on average, 0.5 nm from the A555 acceptor molecules. That is, the emissive defect sites are predominantly located at or near the surface of large NCs. The SB model revealed the average number of A555 molecules per NC and the equilibrium binding constant but did not provide meaningful information regarding the defect-acceptor distance. We conclude that ensemble-level EnT measurements can reveal the spatial distribution of defect sites in NCs without the need for interrogating the sample with a microscope.

6.
ACS Appl Mater Interfaces ; 12(13): 15034-15042, 2020 Apr 01.
Artículo en Inglés | MEDLINE | ID: mdl-32141285

RESUMEN

Substrates influence the electrical and optical properties of monolayer (ML) MoS2 in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS2. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS2. Here, we report spatially resolved PL, Raman, and photo-electrochemical current measurements of 5-10 µm-wide ML-MoS2 triangles deposited on pairs of indium-doped tin oxide (ITO) electrodes that are separated by a narrow insulating quartz channel [i.e., an ITO interdigitated array (IDA) electrode]. Optical microscopy images and atomic force microscopy measurements revealed that the ML-MoS2 triangles lie conformally on the quartz and ITO substrates. The PL spectrum of MoS2 shifts and decreases in intensity in the ITO region, which can be attributed to differences in nonradiative and radiative exciton decay channels. Raman spectra showed no significant peak shifts on the two substrates that would have indicated a substrate-induced doping effect. We spatially resolved the photo-electrochemical current because of hole-induced iodide oxidation and observed that ML-MoS2 produces lower photocurrents in the quartz region than in the ITO region. The correlated PL, Raman, and photocurrent mapping data show that the MoS2/quartz interface diminishes fast nonradiative exciton decay pathways but the photocurrent response in the quartz region is likely limited by inefficient in-plane carrier transport to the ITO electrode because of carrier recombination with S vacancies in MoS2 or charged impurities in the quartz substrate. Nonetheless, the experimental methodology presented herein provides a framework to evaluate substrate engineering strategies to tune the (photo)electrocatalytic properties of two-dimensional materials.

7.
Anal Chem ; 91(23): 14983-14991, 2019 Dec 03.
Artículo en Inglés | MEDLINE | ID: mdl-31682115

RESUMEN

Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which warrants their investigation at the single-nanoparticle level. Here we demonstrate a nondestructive high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level. Electro-optical measurements relate the optical density change of a nanoparticle to redox changes of elements in the particle under working electrochemical conditions. We benchmarked this technique by studying Li-ion insertion in hexagonal tungsten oxide (h-WO3) nanorods during chronoamperometry and cyclic voltammetry. Interestingly, the optically detected current response revealed underlying processes that are hidden in the conventional electrochemical current measurements. This imaging technique may be applied to 13 nm particles and a wide range of electrochemical systems such as electrochromic smart windows, batteries, solid oxide fuel cells, and sensors.

8.
Nano Lett ; 19(12): 9084-9094, 2019 12 11.
Artículo en Inglés | MEDLINE | ID: mdl-31738855

RESUMEN

Monolayer heterojunctions such as MoS2/WS2 are attractive for solar energy conversion applications because the interfacial electric field spatially separates charge carriers in less than 100 fs. Photoelectrochemical cells represent an intriguing platform to collect the spatially separated carriers. However, the recombination, transport, and interfacial charge transfer processes that take place following the ultrafast charge separation step have not been investigated. Here we demonstrate novel charge recombination and transport pathways in monolayer MoS2/WS2 photoelectrochemical cells by spatially resolving the net collection of carriers (i.e., the photocurrent) at the single nanosheet level. We discovered an excitation-wavelength-dependent recombination pathway that depends on the heterojunction stacking configuration and the carrier generation profile in the heterostructure. Photocurrent mapping measurements revealed that charge transport occurs parallel to the layers over micrometer-scale distances even though the indium tin oxide electrode and liquid electrolyte provide efficient charge extraction pathways via intimate electron- and hole-selective contacts. Our results reveal how composition heterogeneity influences the performance of bulk heterojunction electrodes made from randomly oriented nanosheets and provide critical insight into the design of efficient heterojunction photoelectrodes for solar energy conversion applications.

9.
J Chem Phys ; 151(18): 180901, 2019 Nov 14.
Artículo en Inglés | MEDLINE | ID: mdl-31731844

RESUMEN

Semiconductor photoelectrochemistry is a fascinating field that deals with the chemistry and physics of photodriven reactions at solid/liquid interfaces. The interdisciplinary field attracts (electro)chemists, materials scientists, spectroscopists, and theorists to study fundamental and applied problems such as carrier dynamics at illuminated electrode/electrolyte interfaces and solar energy conversion to electricity or chemical fuels. In the pursuit of practical photoelectrochemical energy conversion systems, researchers are exploring inexpensive, solution-processed semiconductor nanomaterials as light absorbers. Harnessing the enormous potential of nanomaterials for energy conversion applications requires a fundamental understanding of charge carrier generation, separation, transport, and interfacial charge transfer at heterogeneous nanoscale interfaces. Our current understanding of these processes is derived mainly from ensemble-average measurements of nanoparticle electrodes that report on the average behavior of trillions of nanoparticles. Ensemble-average measurements conceal how nanoparticle heterogeneity (e.g., differences in particle size, shape, and surface structure) contributes to the overall photoelectrochemical response. This perspective article focuses on the emerging area of single particle photoelectrochemistry, which has opened up an exciting new frontier: direct investigations of photodriven reactions on individual nanomaterials, with the ability to elucidate the role of particle-dependent properties on the photoelectrochemical behavior. Here, we (1) review the basic principles of photoelectrochemical cells, (2) point out the potential advantages and differences between bulk and nanoelectrodes, (3) introduce approaches to single nanoparticle photoelectrochemistry and highlight key findings, and (4) provide our perspective on future research directions.

10.
Proc Natl Acad Sci U S A ; 116(26): 12666-12671, 2019 06 25.
Artículo en Inglés | MEDLINE | ID: mdl-31160443

RESUMEN

Nanomaterials have tremendous potential to increase electrochromic smart window efficiency, speed, and durability. However, nanoparticles vary in size, shape, and surface defects, and it is unknown how nanoparticle heterogeneity contributes to particle-dependent electrochromic properties. Here, we use single-nanoparticle-level electro-optical imaging to measure structure-function relationships in electrochromic tungsten oxide nanorods. Single nanorods exhibit a particle-dependent waiting time for tinting (from 100 ms to 10 s) due to Li-ion insertion at optically inactive surface sites. Longer nanorods tint darker than shorter nanorods and exhibit a Li-ion gradient that increases from the nanorod ends to the middle. The particle-dependent ion-insertion kinetics contribute to variable tinting rates and magnitudes across large-area smart windows. Next, we quantified how particle-particle interactions impact tinting dynamics and reversibility as the nanorod building blocks are assembled into a thin film. Interestingly, single particles tint 4 times faster and cycle 20 times more reversibly than thin films made of the same particles. These findings allow us to propose a nanostructured electrode architecture that optimizes optical modulation rates and reversibility across large-area smart windows.

11.
ACS Appl Mater Interfaces ; 11(21): 19207-19217, 2019 May 29.
Artículo en Inglés | MEDLINE | ID: mdl-31070890

RESUMEN

Understanding light-matter interactions in transition-metal dichalcogenides (TMDs) is critical for optoelectronic device applications. Several studies have shown that high intensity light irradiation can tune the optical and physical properties of pristine TMDs. The enhancement in optoelectronic properties has been attributed to a so-called laser annealing effect that heals chalcogen vacancies. However, it is unknown whether laser annealing improves functional properties such as photocatalytic activity. Here, we show that high intensity supra band gap illumination improves the photoelectrochemical activity of MoSe2 nanosheets for iodide oxidation in indium doped tin oxide/MoSe2/I-, I3-/Pt liquid junction solar cells. Ensemble-level photoelectrochemical measurements show that, on average, illuminating MoSe2 thin films with 1 W/cm2 532 nm excitation increases the photoelectrochemical current by 142% and shifts the photocurrent response to more favorable (negative) potentials. Scanning photoelectrochemical microscopy measurements reveal that pristine bilayer (2L)-MoSe2, trilayer (3L)-MoSe2, and multilayer-thick nanosheets are initially inactive for iodide oxidation. The light treatment activates 2L-MoSe2 and 3L-MoSe2 materials, and the activation process initiates at the edge sites. The photocurrent enhancement is more significant for 2L-MoSe2 than for 1L-MoSe2. Multilayer-thick MoSe2 remains inactive for iodide oxidation even after the laser treatment. Our microscopy measurements reveal that the laser-induced enhancement effect depends critically on MoSe2 layer thickness. X-ray photoelectron spectroscopy measurements further show that the laser treatment oxidizes Mo(IV) species that are initially associated with Se vacancies. Ambient oxygen fills the Se vacancies and removes trap states, thereby increasing the overall photogenerated carrier collection efficiency. To the best of our knowledge, this work represents the first report on using laser to enhance the photoelectrocatalytic properties of few-layer-thick TMDs. The simple and rapid laser annealing procedure is a promising strategy to tune the reactivity of TMD-based photoelectrochemical cells for electricity and chemical fuel production.

12.
Nano Lett ; 19(5): 2960-2967, 2019 05 08.
Artículo en Inglés | MEDLINE | ID: mdl-30913393

RESUMEN

Ultrathin photovoltaics made of MoS2 or WSe2 have the potential to convert solar energy to electricity with high efficiency because all photogenerated carriers are produced at a charge-collecting interface. However, solid-state monolayer photovoltaic devices typically require that charge carriers travel parallel to, instead of perpendicular to, the three atom-thick material toward charge-collecting contacts. Parallel charge transport across long distances decreases energy conversion efficiency. Here we demonstrate proof-of-concept monolayer and bilayer TMD|I-,I3-|Pt photoelectrochemical solar cells that use a conformal liquid electrolyte junction for efficient perpendicular charge transport over significantly larger active areas than solid state systems. Efficient perpendicular charge transport is evidenced by high peak internal quantum efficiencies (IQE) of 44.2, 9.1, and 10.5% for 0.4 mm2 MoS2, WSe2, and MoS2/WSe2 domains in a predominantly monolayer MoS2/WSe2 film. The monochromatic energy conversion efficiencies are competitive with state-of-the-art solid-state monolayer heterojunction photovoltaics. However, inefficient light absorption limits the overall power conversion efficiency to 0.19% for this planar geometry monolayer photovoltaic system. Interestingly, correlated Raman and scanning photoelectrochemical microscopy measurements revealed a nonlinear scaling relation between IQE and layer thickness for MoS2, WSe2, and MoS2/MoS2/WSe2 domains within the heterojunction film. Specifically, the monochromatic energy conversion efficiency of bilayer MoS2 is an order of magnitude greater than monolayer MoS2 and MoS2/MoS2/WSe2 and it exceeds MoS2/WSe2 by a factor of 4. The structure/function relationships are hidden in ensemble-level photoelectrochemical measurements. Although nanostructured or plasmonic electrode architectures are still needed to enhance overall light absorption, our study shows that the liquid junction approach represents a simple and rapid strategy to screen ultrathin TMD materials combinations, tune interfacial energetics, and make conformal electrical contacts in photoelectrochemical energy conversion systems for electricity or solar fuels production.

13.
Nano Lett ; 19(2): 958-962, 2019 02 13.
Artículo en Inglés | MEDLINE | ID: mdl-30615831

RESUMEN

Particle-particle interfaces are ubiquitous in nanostructured photoelectrodes and photovoltaics, which are important devices for solar energy conversion. These interfaces are expected to cause performance losses in these devices, but how much loss they would incur is poorly defined. Here we use a subparticle photoelectrochemical current measurement, in combination with specific photoelectrode configurations, to quantify the current losses from single particle-particle interfaces formed between individual TiO2 nanorods operating as photoanodes in aqueous electrolytes. We find that a single interface leads to ∼20% photocurrent loss (i.e., ∼80% retention of the original current). Such quantitative, first-of-its-kind, information provides a metric for guiding the optimization and design of nanostructured photoelectrodes and photovoltaics.

14.
ACS Appl Mater Interfaces ; 10(33): 27780-27786, 2018 Aug 22.
Artículo en Inglés | MEDLINE | ID: mdl-30019887

RESUMEN

Transition metal dichalcogenides (TMDs) such as MoSe2 and WSe2 are efficient materials for converting solar energy to electrical energy in photoelectrochemical photovoltaic cells. One limiting factor of these liquid junction solar cells is that photogenerated oxidation products accumulate on the electrode surface and decrease the photocurrent efficiency. However, it is unclear where the reaction products accumulate on the electrode surface and how they impact the local photoelectrochemical response. This open question is especially important for the structurally heterogeneous TMD nanoflake thin-film electrodes that are promising for large-area solar energy conversion applications. Here, we use a single-nanoflake photoelectrochemical and Raman microscopy approach to probe how the photogenerated I2/I3- products impact the photocurrent collection efficiency and the onset potential in MoSe2-nanoflake|I-/I2|Pt photoelectrochemical solar cells. We observed localized I2/I3- deposition on all types of MoSe2 nanoflake surface motifs, including basal planes, perimeter edges, and interior step edges. Illuminated nanoflake spots with the highest photocurrent collection efficiency are the first to be limited by I2/I3- formation under high-intensity illumination. Interestingly, I2/I3- formation occurs on illuminated surface spots that have the lowest photocurrent onset potential for iodide oxidation, corresponding to the highest open circuit voltage ( VOC). The VOC shifts could be attributed to variations in the surface reaction kinetics or doping density across the nanoflake. Our results highlight important limiting factors of nanoflake thin-film TMD liquid junction photovoltaics under concentrated solar illumination intensities.

15.
ACS Cent Sci ; 3(11): 1189-1197, 2017 Nov 22.
Artículo en Inglés | MEDLINE | ID: mdl-29202021

RESUMEN

Compared with their monometallic counterparts, bimetallic nanoparticles often show enhanced catalytic activity associated with the bimetallic interface. Direct quantitation of catalytic activity at the bimetallic interface is important for understanding the enhancement mechanism, but challenging experimentally. Here using single-molecule super-resolution catalysis imaging in correlation with electron microscopy, we report the first quantitative visualization of enhanced bimetallic activity within single bimetallic nanoparticles. We focus on heteronuclear bimetallic PdAu nanoparticles that present a well-defined Pd-Au bimetallic interface in catalyzing a photodriven fluorogenic disproportionation reaction. Our approach also enables a direct comparison between the bimetallic and monometallic regions within the same nanoparticle. Theoretical calculations further provide insights into the electronic nature of N-O bond activation of the reactant (resazurin) adsorbed on bimetallic sites. Subparticle activity correlation between bimetallic enhancement and monometallic activity suggests that the favorable locations to construct bimetallic sites are those monometallic sites with higher activity, leading to a strategy for making effective bimetallic nanocatalysts. The results highlight the power of super-resolution catalysis imaging in gaining insights that could help improve nanocatalysts.

16.
J Am Chem Soc ; 139(36): 12623-12631, 2017 09 13.
Artículo en Inglés | MEDLINE | ID: mdl-28797166

RESUMEN

Since their invention in the 1950s, composite carbon electrodes have been employed in a wide variety of applications, ranging from batteries and fuel cells to chemical sensors, because they are easy to make and pattern at millimeter scales. Despite their widespread use, traditional carbon composite electrodes have substandard electrochemistry relative to metallic and glassy carbon electrodes. As a result, there is a critical need for new composite carbon electrodes that are highly electrochemically active, have universal and easy fabrication into complex geometries, are highly conductive, and are low cost. Herein, a new solvent-based method is presented for making low-cost composite graphite electrodes containing a thermoplastic binder. The electrodes, which are termed thermoplastic electrodes (TPEs), are easy to fabricate and pattern, give excellent electrochemical performance, and have high conductivity (700 S m-1). The thermoplastic binder enables the electrodes to be hot embossed, molded, templated, and/or cut with a CO2 laser into a variety of intricate patterns. Crucially, these electrodes show a marked improvement in peak current, peak separation, and resistance to charge transfer over traditional carbon electrodes. The impact of electrode composition, surface treatment (sanding, polishing, plasma treatment), and graphite source were found to significantly impact fabrication, patterning, conductivity, and electrochemical performance. Under optimized conditions, electrodes generated responses similar to more expensive and difficult to fabricate graphene and highly oriented pyrolytic graphite electrodes. The TPE electrode system reported here provides a new approach for fabricating high performance carbon electrodes with utility in applications ranging from sensing to batteries.


Asunto(s)
Electrodos , Grafito , Plásticos , Solventes/química , Conductividad Eléctrica , Microscopía Electrónica de Rastreo , Espectrometría Raman , Propiedades de Superficie
17.
Nature ; 530(7588): 77-80, 2016 Feb 04.
Artículo en Inglés | MEDLINE | ID: mdl-26842056

RESUMEN

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled--yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resolution imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod--even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

18.
ACS Appl Mater Interfaces ; 6(24): 21916-20, 2014 Dec 24.
Artículo en Inglés | MEDLINE | ID: mdl-25478932

RESUMEN

Cadmium selenide quantum dots covalently attached to and photosensitizing single-crystal TiO2 surfaces are observed to corrode under illumination in aqueous electrolyte containing iodide as a regenerator. Comparison of photocurrent spectra before and after long-term monochromatic illumination indicated that the CdSe QD sensitizers photocorroded and decreased in size until their band gap energy exceeded the excitation energy. This wavelength-dependent photoelectrochemical etching mechanism can be used to tune the size distribution of surface adsorbed QDs and may account for the instability of QD sensitized solar cells that do not employ sulfide-based electrolytes.

19.
Annu Rev Phys Chem ; 65: 395-422, 2014.
Artículo en Inglés | MEDLINE | ID: mdl-24423372

RESUMEN

Nanoparticles are among the most important industrial catalysts, with applications ranging from chemical manufacturing to energy conversion and storage. Heterogeneity is a general feature among these nanoparticles, with their individual differences in size, shape, and surface sites leading to variable, particle-specific catalytic activity. Assessing the activity of individual nanoparticles, preferably with subparticle resolution, is thus desired and vital to the development of efficient catalysts. It is challenging to measure the activity of single-nanoparticle catalysts, however. Several experimental approaches have been developed to monitor catalysis on single nanoparticles, including electrochemical methods, single-molecule fluorescence microscopy, surface plasmon resonance spectroscopy, X-ray microscopy, and surface-enhanced Raman spectroscopy. This review focuses on these experimental approaches, the associated methods and strategies, and selected applications in studying single-nanoparticle catalysis with chemical selectivity, sensitivity, or subparticle spatial resolution.


Asunto(s)
Catálisis , Nanopartículas/química , Nanotecnología/métodos , Técnicas Electroquímicas/instrumentación , Técnicas Electroquímicas/métodos , Microanálisis por Sonda Electrónica/instrumentación , Microanálisis por Sonda Electrónica/métodos , Diseño de Equipo , Microscopía Fluorescente/instrumentación , Microscopía Fluorescente/métodos , Microscopía de Sonda de Barrido/instrumentación , Microscopía de Sonda de Barrido/métodos , Nanopartículas/ultraestructura , Nanotecnología/instrumentación , Imagen Óptica/instrumentación , Imagen Óptica/métodos , Espectrometría Raman/instrumentación , Espectrometría Raman/métodos , Resonancia por Plasmón de Superficie
20.
J Am Chem Soc ; 133(44): 17521-3, 2011 Nov 09.
Artículo en Inglés | MEDLINE | ID: mdl-21961793

RESUMEN

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars.


Asunto(s)
Cloruros/química , Cloruros/efectos de la radiación , Marte , Minerales/química , Óxidos/química , Percloratos/química , Procesos Fotoquímicos/efectos de la radiación , Minerales/efectos de la radiación , Oxidación-Reducción/efectos de la radiación , Óxidos/efectos de la radiación , Percloratos/efectos de la radiación , Rayos Ultravioleta
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