Double-Sided CdS and CdSe Quantum Dot Co-Sensitized ZnO Nanowire Arrays for Photoelectrochemical Hydrogen Generation

We report the design and characterization of a novel double-sided CdS and CdSe quantum dot cosensitized ZnO nanowire arrayed photoanode for photoelectrochemical (PEC) hydrogen generation. The double-sided design represents a simple analogue of tandem cell structure, in which the dense ZnO nanowire arrays were grown on an indium−tin oxide substrate followed by respective sensitization of CdS and CdSe quantum dots on each side. As-fabricated photoanode exhibited strong absorption in nearly the entire visible spectrum up to 650 nm, with a high incident-photon-to-current-conversion efficiency (IPCE) of ∼45% at 0 V vs Ag/AgCl. On the basis on a single white light illumination of 100 mW/cm2, the photoanode yielded a significant photocurrent density of ∼12 mA/cm2 at 0.4 V vs Ag/AgCl. The photocurrent and IPCE were enhanced compared to single quantum dot sensitized structures as a result of the band alignment of CdS and CdSe in electrolyte. Moreover, in comparison to single-sided cosensitized layered structures, this double-sided architecture that enables direct interaction between quantum dot and nanowire showed improved charge collection efficiency. Our result represents the first double-sided nanowire photoanode that integrates uniquely two semiconductor quantum dots of distinct band gaps for PEC hydrogen generation and can be possibly applied to other applications such as nanostructured tandem photovoltaic cells

Ultrasmall Single-Crystal Indium Antimonide Nanowires

We report the rational synthesis of ultrasmall indium antimonide (InSb) nanowires down to 4.5 nm diameter. To achieve uniform InSb nanowires, we designed and performed the synthesis via a vapor−liquid−solid growth mechanism, where monodispersed gold colloids were used as a catalyst. The growth was carried out in a home-built three-zone chemical vapor deposition system, which allows continuous tunability of respective indium and antimony vapor pressure via separate temperature control. Several parameters are important for achieving successful nanowire growth, including the use of catalysts and tuning of the V/III ratio. Scanning electron microscopy revealed that InSb nanowires had uniform diameters with lengths up to several micrometers, and their sizes were defined by gold nanoparticles. High-resolution transmission electron microscopy structural characterization showed that as-prepared InSb nanowires were single crystals elongating along the ⟨111⟩ direction, regardless of wire diameter. Their chemical compositions were characterized by Raman spectroscopy and electron energy loss spectroscopy. The ability to rationally prepare ultrasmall, single-crystal InSb nanowires opens up new opportunities for studying the size-associated fundamental properties and provides insights for potential nanodevice applications

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 ∼4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to ∼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 ∼4.6 × 1018 cm−3, and a space-charge layer of ∼22 nm. Upon illumination at a power density of 100 mW/cm2 (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

Hydrogen-Treated TiO₂ Nanowire Arrays for Photoelectrochemical Water Splitting

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO2 (H:TiO2) nanowires were prepared by annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO2 nanowires, H:TiO2 samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO2 samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO2 nanowire sample yields a photocurrent density of ∼1.97 mA/cm2 at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm2 from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1.63%. After eliminating the discrepancy between the irradiance of the xenon lamp and solar light, by integrating the incident-photon-to-current-conversion efficiency (IPCE) spectrum of the H:TiO2 nanowire sample with a standard AM 1.5G solar spectrum, the STH efficiency is calculated to be ∼1.1%, which is the best value for a TiO2 photoanode. IPCE analyses confirm the photocurrent enhancement is mainly due to the improved photoactivity of TiO2 in the UV region. Hydrogen treatment increases the donor density of TiO2 nanowires by 3 orders of magnitudes, via creating a high density of oxygen vacancies that serve as electron donors. Similar enhancements in photocurrent were also observed in anatase H:TiO2 nanotubes. The capability of making highly photoactive H:TiO2 nanowires and nanotubes opens up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells, and photocatalysis