Abraham Wolcott

Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting

We report the rational synthesis of nitrogen-doped zinc oxide (ZnO:N) nanowire arrays, and their implementation as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Nanowires with a controlled N concentration (atomic ratio of N to Zn) up to approximately 4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to approximately 4%. Incident-photon-to-current-efficiency measurements carried out on PEC cell with ZnO:N nanowire arrays as photoanodes demonstrate a significant increase of photoresponse in the visible region compared to undoped ZnO nanowires prepared at similar conditions. Mott-Schottky measurements on a representative 3.7% ZnO:N sample give a flat-band potential of -0.58 V, a carrier density of approximately 4.6 x 10(18) cm(-3), and a space-charge layer of approximately 22 nm. Upon illumination at a power density of 100 mW/cm(2) (AM 1.5), water splitting is observed in both ZnO and ZnO:N nanowires. In comparison to ZnO nanowires without N-doping, ZnO:N nanowires show an order of magnitude increase in photocurrent density with photo-to-hydrogen conversion efficiency of 0.15% at an applied potential of +0.5 V (versus Ag/AgCl). These results suggest substantial potential of metal oxide nanowire arrays with controlled doping in PEC water splitting applications.

Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays

Dense and aligned TiO2 nanorod arrays are fabricated using oblique-angle deposition on indium tin oxide (ITO) conducting substrates. The TiO2 nanorods are measured to be 800-1100 nm in length and 45-400 nm in width with an anatase crystal phase. Coverage of the ITO is extremely high with 25 x 10(6) mm(-2) of the TiO2 nanorods. The first use of these dense TiO2 nanorod arrays as working electrodes in photoelectrochemical (PEC) cells used for the generation of hydrogen by water splitting is demonstrated. A number of experimental techniques including UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoelectrochemistry are used to characterize their structural, optical, and electronic properties. Both UV/Vis and incident-photon-to-current-efficiency measurements show their photoresponse in the visible is limited but with a marked increase around approximately 400 nm. Mott-Schottky measurements give a flat-band potential (V(FB)) of +0.20 V, a carrier density of 4.5 x 10(17) cm(-3), and a space-charge layer of 99 nm. Overall water splitting is observed with an applied overpotential at 1.0 V (versus Ag/AgCl) with a photo-to-hydrogen efficiency of 0.1%. The results suggest that these dense and aligned one-dimensional TiO2 nanostructures are promising for hydrogen generation from water splitting based on PEC cells.

Quasi-core-shell TiO2/WO3 and WO3/TiO2 nanorod arrays fabricated by glancing angle deposition for solar water splitting

Unique quasi-core-shell nanorod arrays of TiO2/WO3 and WO3/TiO2 are fabricated on indium tin oxide coated glass substrates by dynamic shadowing growth using glancing angle deposition. The resulting heterostructures are characterized by X-Ray diffraction, UV-vis absorption spectroscopy, scanning electron microscopy, and photoelectrochemical measurements. The incident-photon-to-current-efficiency and absorbance measurements show that the TiO2-core/WO3-shell structures have a distinct photoresponse in the UV range, with wavelength λ ≤ 400 nm, while the WO3-core/TiO2-shell structures show stronger visible light absorption and photocurrent out to λ ∼ 600 nm. Mott-Schottky measurements give a flat-band potential of −0.28 V, a carrier density of 1.47 × 1020 cm−3, and a space charge layer of 100 nm for the WO3-core/TiO2-shell samples. These results suggest that the quasi-core-shell nanorods preserves the optical properties and water splitting performance of the core while the surface properties such as the flat band potential of the nanorods are modified by the shell. This approach affords a simple and powerful method for designing nanostructures with improved photoelectrochemical properties.

Determination of the Exciton Binding Energy in CdSe Quantum Dots

The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

Photoluminescence spectroscopy of bioconjugated CdSe∕ZnS quantum dots

The authors performed scanning photoluminescence (PL) spectroscopy on CdSe∕ZnS core/shell quantum dots (QDs) bioconjugated to Interleukin 10 (IL10) antibody. The influence of IL10 on the QD photoluminescence spectra was explored on samples dried on solid substrates at various temperatures. A “blue” up to 15nm spectral shift of the PL maximum was observed on the bioconjugated QDs. The spectral shift is strongly increased after samples annealing above room temperature. A mechanism of the observed effect is attributed to changes in the QD electronic energy levels caused by local electric or stress field or chemical reactions which occurred on the QD surface.

Direct mapping of hot-electron relaxation and multiplication dynamics in PbSe quantum dots.

How hot electrons relax in semiconductor quantum dots is of critical importance to many potential applications, such as solar energy conversion, light emission, and photon detection. A quantitative answer to this question has not been possible due in part to limitations of current experimental techniques in probing hot electron populations. Here we use femtosecond time-resolved two-photon photoemission spectroscopy to carry out a complete mapping in time- and energy-domains of hot electron relaxation and multiexciton generation (MEG) dynamics in lead selenide quantum dots functionalized with 1,2-ethanedithiols. We find a linear scaling law between the hot electron relaxation rate and its energy above the conduction band minimum. There is no evidence of MEG from intraband hot electron relaxation for excitation photon energy as high as three times the bandgap (3E(g)). Rather, MEG occurs in this system only from interband hot electron transitions at sufficiently high photon energies (~4E(g)).

Ligand-mediated modification of the electronic structure of CdSe quantum dots.

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions.

Surface Structure of Aerobically Oxidized Diamond Nanocrystals.

We investigate the aerobic oxidation of high-pressure, high-temperature nanodiamonds (5–50 nm dimensions) using a combination of carbon and oxygen K-edge X-ray absorption, wavelength-dependent X-ray photoelectron, and vibrational spectroscopies. Oxidation at 575 °C for 2 h eliminates graphitic carbon contamination (>98%) and produces nanocrystals with hydroxyl functionalized surfaces as well as a minor component (<5%) of carboxylic anhydrides. The low graphitic carbon content and the high crystallinity of HPHT are evident from Raman spectra acquired using visible wavelength excitation (λexcit = 633 nm) as well as carbon K-edge X-ray absorption spectra where the signature of a core–hole exciton is observed. Both spectroscopic features are similar to those of chemical vapor deposited (CVD) diamond but differ significantly from the spectra of detonation nanodiamond. The importance of these findings to the functionalization of nanodiamond surfaces for biological labeling applications is discussed.

Reactive ion etching: Optimized diamond membrane fabrication for transmission electron microscopy

Commonly used preparation method for thin diamond membranes by focused ion beam (FIB) techniques results in surface damage. Here, the authors introduce an alternative method based on reactive ion etching (RIE). To compare these methods, cross-sectional samples are produced in single crystal diamond, a material that has generated growing interest for a variety of applications. The samples are examined by Raman spectroscopy and high-resolution transmission electron microscopy (TEM). Raman spectra indicate that the crystalline structure of the RIE-processed diamond is preserved, while the FIB-processed diamond membrane has a broad-background sp2 feature. Atomic-resolution TEM imaging demonstrates that the RIE-based process produces no detectable damage, while the FIB-processed sample has an amorphous carbon layer of about 11 nm thick. These findings show that the RIE-based process allows the production of diamond TEM samples with reduced near-surface damage and can thus enable direct examination of growth de...

Synthesis and characterization of gold nanoparticle aggregates as novel substrates for surface-enhanced Raman scattering

Novel gold nanoparticle aggregates have been synthesized using simple colloidal chemistry techniques. The electronic absorption spectra of the aggregates can be manipulated by controlling the synthetic conditions. The aggregates have been demonstrated for the first time to exhibit strong activity for surface-enhanced Raman scattering (SERS). SERS studies were performed using rhodamine 6G (R6G), a molecule which normally does not show SERS enhancement on gold surfaces, showed an enhancement factor on the order of 109, which is similar to or better than most ensemble averaged SERS enhancement factors reported to date. The results demonstrate that these gold nanoparticle aggregates are promising for SERS applications in detection and analysis of molecules.