Engineering topics
El-Sayed, Mostafa A.
Publications and source records attributed to El-Sayed, Mostafa A..
Enhancing the rate of electrochemical nitrogen reduction reaction for ammonia synthesis under ambient conditions using hollow gold nanocages
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Shape tunable plasmonic nanoparticles
Noble metal nanoparticles and methods of their use are provided. Certain aspects provided solid noble metal nanoparticles tuned to the near infrared. The disclosed nanoparticles can be used in molecular imaging, diagnosis, and treatment. Methods for imaging cells are also provided.
Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction
In 2 O 3-x (OH) y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water–gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In 2 O 3-x (OH) y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO 2 reduction.
Time Resolved Optical Studies on The Plasmonic Field Enhancement of Bacteriorhodopsin Proton Photo-current: Final Technical Report Covering Aug 31, 2015–Aug 31, 2016
Our research continues to focus on the effects of plasmonic nanoparticles on organic and inorganic light-harvesting materials. Recent work has focused on the synthesis of stabilized gold nanoparticles to enhance the efficiency of dye-sensitized solar cells (DSSCs). Two major concerns in dye sensitized solar cells (DSSCs) are efficient light absorption and charge collection. Charge collection typically suffers because transport of electrons through the mesoporous TiO 2 substrate is slow. Thus, one obvious way to improve charge collection is to reduce the thickness of the TiO 2 . Alternatively, a form of TiO 2 with fewer grain boundaries, such as nanotubes, could be used in place of sintered nanospheres. Unfortunately, both of these solutions end up reducing the amount of surface area available to adsorb dye molecules. This directly reduces the percentage of photons absorbed. This problem could be avoided if dye molecules with larger absorption were designed; although synthetic chemists seem to be pushing the limits of what is achievable. Plasmonic nanoparticles offer an alternative way to boost light absorption. It is well known that plasmonic nanoparticles can enhance the local electric field of resonant frequencies of light. If this were in the same spectral region as the dye’s absorption band it would increase the percentage of absorbed photons. One concern is that if the nanoparticles are too close to the dye molecules they can quench the excited state. To avoid this problem, we prepared gold nanoparticles with a silica shell. This limited the amount of quenching while still permitting some enhancement of absorption. Unfortunately, we ran into some serious issues. The iodide based electrolyte etched the gold nanoparticles, completely dissolving them within a few hours. The silica shell should have provided protection but there were pinholes through which iodide could diffuse. Increasing the thickness of the silica to over 10 nm prevented etching but also limited any photon absorption enhancement.
Probing Structural Evolution and Charge Storage Mechanism of NiO 2 H x Electrode Materials using In Operando Resonance Raman Spectroscopy
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Probing the Charge Storage Mechanism of a Pseudocapacitive MnO 2 Electrode Using in Operando Raman Spectroscopy
Although manganese oxide (MnO 2 ) has been extensively studied as an electrode material for pseudocapacitors, a clear understanding of its charge storage mechanism is still lacking. Here we report our findings in probing the structural changes of a thin-film model MnO 2 electrode during cycling using in operando Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quantitatively with the size (Li + , Na + , and K + ) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudocapacitive MnO 2 . Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO 2 agree with the experimental observations of cation-size effect on the positions of Raman bands. Additionally, the cation-size effects on spectral features at different potentials of MnO 2 electrode are correlated quantitatively with the amount of charge stored in the MnO 2 electrode. The understanding of the structural changes associated with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochemical performances of the MnO 2 electrode.
A Step Toward Efficient Panchromatic Multi-Chromophoric Sensitizers for Dye Sensitized Solar Cells
Panchromatic dyes with absorption profiles extending into the near infra-red are of interest to researchers in the field of dye sensitized solar cells (DSSCs), as they offer potential access to a wide energy range of photons necessary to enhance solar to electric power conversion efficiencies (PCEs). In this report, a porphyrin with a Soret band absorbing at high energy is combined with a squaraine absorbing at low energy $via$ an acetylene linker to form a bichromophoric sensitizer with molar extinctions on the order of 10 5 M -1 cm -1 and an incident photon-to-current efficiency (IPCE) onset of ~850 nm. Various bulky substituents were installed on both the porphyrin and squaraine moieties, and conjugation was increased with π-bridge spacers to achieve a PCE of 7.6%, which is up to 15% higher than a comparable squaraine-only dye. For the most part, charge injection dynamics indicate slower charge injection rates and lower injection quantum yields for these bichromophoric sensitizers compared to non-porphyrin squaraine-based DSSC sensitizers. Nevertheless, higher PCE was observed for most porphyrincontaining dyes due largely to increased panchromaticity.
Near-infrared asymmetrical squaraine sensitizers for highly efficient dye sensitized solar cells: the effect of bridges and anchoring groups on solar cell performance
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Tribute to A. W. Castleman, Jr.
It is with great pleasure that we join our many colleagues who contributed to this special issue of The Journal of Physical Chemistry A in dedicating it in honor of Professor A. W. “Will” Castleman, Jr. Will is a pioneer in the field of cluster science, and the far-reaching impact of his career is demonstrated by the broad range of topics covered in this issue. This diversity validates Will’s teaching that “fundamental research in cluster science allows you to explore any problem you want”. This ability to extend to other research areas comes because, as Will has shown both rigorously and elegantly, clusters are an intermediate state of matter that bridge the gap between molecular and bulk levels.
High-temperature surface enhanced Raman spectroscopy for in situ study of solid oxide fuel cell materials
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