Ying-Chih Pu

Au Nanostructure-Decorated TiO2 Nanowires Exhibiting Photoactivity Across Entire UV-visible Region for Photoelectrochemical Water Splitting

Here we demonstrate that the photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 to 800 nm by manipulating the shape of the decorated Au nanostructures. The samples were prepared by carefully depositing Au nanoparticles (NPs), Au nanorods (NRs), and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays. As compared with bare TiO2, Au NP-decorated TiO2 nanowire electrodes exhibited significantly enhanced photoactivity in both the UV and visible regions. For Au NR-decorated TiO2 electrodes, the photoactivity enhancement was, however, observed in the visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO2 nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV-visible region. Monochromatic incident photon-to-electron conversion efficiency measurements indicated that excitation of surface plasmon resonance of Au is responsible for the enhanced photoactivity of Au nanostructure-decorated TiO2 nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was attributable to the effective surface passivation of Au NPs. Furthermore, 3D finite-difference time domain simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO2 upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was partially due to the increased optical absorption of TiO2 associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems to effectively harvest the entire UV-visible light for solar fuel production.

Organolead Halide Perovskite Nanocrystals: Branched Capping Ligands Control Crystal Size and Stability

CH3 NH3 PbBr3 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.

Optical Properties and Exciton Dynamics of Alloyed Core/Shell/Shell Cd1–xZnxSe/ZnSe/ZnS Quantum Dots

In this study we introduce a new method for the one-pot synthesis of core/shell/shell alloyed Cd1-xZnxSe/ZnSe/ZnS QDs and examine the effect of the shell coating on the optical properties and exciton dynamics of the alloy core. The photoluminescence (PL) quantum yield is greatly enhanced after shell growth, from 9.6% to 63%. The exciton dynamics were studied by time correlated single photon counting (TCSPC) and fit using integrated singular value decomposition global fitting (i-SVD-GF), which showed the biexponential observed lifetimes on the nanosecond time scale remain the same after shell growth. Using ultrafast transient absorption (TA) spectroscopy and SVD-GF, we have determined that surface passivation by ZnSe and ZnSe/ZnS shells reduces nonradiative recombination primarily on the picosecond time scale. These findings are helpful in directing the development of the next generation of QDs for biological labeling and other applications.

Multicolored Cd1−xZnxSe quantum dots with type-I core/shell structure: single-step synthesis and their use as light emitting diodes

We developed a single-step hot-injection process to synthesize Cd1-xZnxSe quantum dots (QDs) with tunable emission wavelengths. The multiple emission colors of the Cd1-xZnxSe QDs resulted from the variation in their compositions (x value) with the reaction time. Because of the higher reactivity of the Cd precursor, QDs whose composition was rich in CdSe were generated at the beginning of the reaction. As the reaction proceeded, the later-formed ZnSe shell was simultaneously alloyed with the core, giving rise to a progressive alloying treatment for the grown QDs. During the reaction period, the emission color of the Cd1-xZnxSe QDs shifted from red to orange, to yellow, to green and finally to blue. A light emitting diode (LED) composed of multilayers of ITO/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/poly(3-hexylthiophene) blended with Cd1-xZnxSe QDs/Al was fabricated to test the electroluminescence (EL) properties of the QDs. The EL results show high color purity for the emission from LED devices containing Cd1-xZnxSe QDs, revealing that the as-synthesized QDs can be easily processed and integrated into a light-emitting device without using a complicated procedure. The findings from the present work also demonstrate the advantage of using the current single-step synthetic approach to obtain a batch of Cd1-xZnxSe QDs that may emit different colors in prototype LEDs.

Au/ZnS core/shell nanocrystals as an efficient anode photocatalyst in direct methanol fuel cells.

Au/ZnS core/shell nanocrystals with controllable shell thicknesses were synthesized using a cysteine-assisted hydrothermal method. Incorporating Au/ZnS nanocrystals into the traditional Pt-catalyzed half-cell reaction led to a 43.3% increase in methanol oxidation current under light illumination, demonstrating their promising potential for metal/semiconductor hybrid nanocrystals as the anode photocatalyst in direct methanol fuel cells.

An electrochemical method to enhance the performance of metal oxides for photoelectrochemical water oxidation

It has recently been demonstrated that the photoelectrochemical performance of a number of metal oxides can be substantially improved by controllably increasing their carrier densities through controlled introduction of defects such as oxygen vacancies. The creation of defects can be achieved via different synthetic methods, including hydrogenation, thermal treatment in oxygen deficient environment, chemical reductions, ion bombardment and electrochemical reductions. Here we report a general strategy to prepare oxygen-deficient metal oxides, including WO3, TiO2 (rutile and anatase), BiVO4, and ZnO, by electrochemically induced formation of low valent metal species. These electrochemically treated metal oxides show significantly enhanced photoactivity, as a result of improved charge injection and charge separation efficiency. The reported electrochemical method in this work represents a simple, rapid, highly scalable and safe approach to prepare high performance metal oxide photoanodes.

Shell-thickness dependent electron transfer and relaxation in type-II core–shell CdS/TiO2 structures with optimized photoelectrochemical performance

Core–shell CdS/TiO2 structures are promising for solar-to-fuel conversion applications because their ideal type-II band alignment helps effective charge transfer to form the CdS+/TiO2− system. A better understanding of the charge carrier dynamics is critical to provide guiding principles for designing photoelectrochemical (PEC) devices. Hence, TiO2 shell-thickness dependent charge carrier dynamics and competition between electron relaxation in CdS (e.g. recombination and trapping) and electron transfer from CdS to TiO2 were investigated using ultrafast transient absorption (TA) spectroscopy. The results indicate that the CdS/TiO2 nanocomposite with a molar ratio of 2 : 1 exhibits the highest electron transfer rate constant of ET = 2.71 × 1010 s−1, along with an electron relaxation rate of CdS/TiO2 = 3.43 × 1010 s−1, resulting in an electron transfer quantum efficiency of QET = 79%, which also corresponds to the best PEC hydrogen generation in the CdS/TiO2 core–shell composites. However, the electron transfer rate decreases with increasing thickness of the TiO2 shell consisting of aggregated nanoparticles. One possible explanation is that the CdS and TiO2 form relatively larger, separate particles, or less conforming small particles, with poor interfaces with increasing TiO2, thereby reducing electron transfer from CdS to TiO2, which is supported by SEM, and TEM data and consistent with PEC results. The thickness and morphology dependence of electron transfer and relaxation provides new insight into the charge carrier dynamics in such composite structures, which is important for optimizing the efficiency of PEC for solar fuel generation applications.

Uniform carbon-coated CdS core–shell nanostructures: synthesis, ultrafast charge carrier dynamics, and photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using solar energy has received widespread attention, and strong performance photocatalysts are highly desired. In this work, uniform carbon-coated CdS nanostructures have been fabricated using ascorbic acid as the carbon source by a facile hydrothermal method and characterized using transmission electron microscopy (TEM). The thickness of the carbon layer can be well controlled by the amount of ascorbic acid added during the reaction. Compared to pristine CdS, carbon-coated CdS nanostructures exhibit stronger light absorption and more efficient electron transfer as determined by absorption and photoluminescence (PL) spectroscopy. Ultrafast charge carrier dynamics in the composite CdS/C structures were studied using femtosecond transient absorption (TA) spectroscopy, which revealed direct evidence of effective charge transfer from CdS to the carbon layer. In addition, the CdS/C composites were employed as photoanodes for PEC hydrogen generation, which showed significant improvement in photoactivity over pristine CdS nanospheres. The photocurrent density (−1.0 V vs. Ag/AgCl) of one of the composite structures, CdS/7-C, exhibited ∼20 times enhancement compared with that of pristine CdS. The enhanced PEC property can be attributed to increased light scattering and consequently the light harvesting throughout the whole spectral wavelength, and the effective electron transfer from CdS to the carbon layer. Such carbon-coated semiconductor composites based on a simple and low-cost synthesis method should be useful in PEC as well as other applications such as photovoltaics, detectors and sensors.

Studies on the annealing and antibacterial properties of the silver-embedded aluminum/silica nanospheres

Substantial silver-embedded aluminum/silica nanospheres with uniform diameter and morphology were successfully synthesized by sol-gel technique. After various annealing temperatures, the surface mechanisms of each sample were analyzed using scanning electron microscope, transmission electron microscope, and X-ray photoelectron spectroscopy. The chemical durability examinations and antibacterial tests of each sample were also carried out for the confirmation of its practical usage. Based on the result of the above analyses, the silver-embedded aluminum/silica nanospheres are eligible for fabricating antibacterial utensils.

Design of Novel Metal Nanostructures for Broadband Solar Energy Conversion

Solar power holds great potential as an alternative energy source, but current photovoltaic cells have much room for improvement in cost and efficiency. Our objective was to develop metal nanostructures whose surface plasmon resonance (SPR) spectra closely match the solar spectrum to enhance light absorption and scattering. We employed the finite-difference time-domain simulation method to evaluate the effect of varying key parameters. A novel nanostructure with SPR absorption matching a region of the solar spectrum (300 to 1500 nm) that contains 90% of solar energy was successfully designed. This structure consists of a large gold-silica core-shell structure with smaller gold nanoparticles and nanorods on its surface. Such complex nanostructures are promising for broad and tunable absorption spectra. In addition, we investigated the SPR of silver nanoparticle arrays, which can achieve scattering close to the solar spectrum. We demonstrated an improvement in efficiency of over 30% with optimal nanoparticle radius and periods of 75 nm and 325 nm, respectively. In combination, our studies enable high-efficiency, tunable, and cost-effective enhancement of both light absorption and scattering, which has potential applications in solar energy conversion as well as biomedical imaging.