Navid Soheilnia

Solution phase synthesis of carbon quantum dots as sensitizers for nanocrystalline TiO2 solar cells

Carbon n quantum dots (CQDs) have recently emerged as viable alternatives to traditional semiconductor quantum dots because of their facile and low cost synthesis, long term colloidal stability, and low environmental and biological toxicity. The compatible surface chemistry, good solubility in polar solvents and extensive optical absorption throughout the visible and near-infrared wavelength regions render CQDs as potentially useful sensitizers for photovoltaic applications. Presented herein is a new strategy for the solution phase synthesis of water-soluble, colloidally stable CQDs and a preliminary exploration of their utilization as sensitizers in nanocrystalline TiO2 based solar cells. Under AM 1.5 illumination, the Voc and FF values reach 380 mV and 64% respectively, achieving a power conversion efficiency of 0.13%.

High-efficiency dye-sensitized solar cell with three-dimensional photoanode.

Herein, we present a straightforward bottom-up synthesis of a high electron mobility and highly light scattering macroporous photoanode for dye-sensitized solar cells. The dense three-dimensional Al/ZnO, SnO(2), or TiO(2) host integrates a conformal passivation thin film to reduce recombination and a large surface-area mesoporous anatase guest for high dye loading. This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs. An increase in photovoltage of up to 110 mV over state-of-the-art DSC is demonstrated.

Graphene Oxide−Periodic Mesoporous Silica Sandwich Nanocomposites with Vertically Oriented Channels

This paper describes the synthesis and characterization of single-layer graphene oxide-periodic mesoporous silica sandwich nanocomposites. Through a comprehensive exploration of the synthesis conditions, it has proven possible to create the first example of a graphene oxide-periodic mesoporous silica nanocomposite in which hexagonal symmetry PMS film grows on both sides of the graphene oxide sheets with the mesoporous channels vertically aligned with respect to the graphene oxide surface. The formation of this novel architecture is found to be very sensitive to pH, the ratio of surfactant template to graphene oxide, the amount of silica precursor, and the temperature of the synthesis. On the basis of the collected data of a multi-technique analysis, it is proposed that the mode of formation of the nanocomposite involves the co-assembly of silicate-surfactant admicelles on opposite sides of graphene oxide platelets acting thereby as a template for growth of vertical mesopores off the platelet surface. These composites showed semiconductive behavior with electrical conductivity sensitively responding to analyte vapor exposure. The discovery of graphene oxide-periodic mesoporous silica sandwich nanocomposites will provide new opportunities for research that exploits the synergism of the graphene oxide and periodic mesoporous silica parts.

Enhanced Hematite Water Electrolysis Using a 3D Antimony-Doped Tin Oxide Electrode

We present herein an example of nanocrystalline antimony-doped tin oxide (nc-ATO) disordered macroporous inverse opal 3D electrodes as efficient charge-collecting support structures for the electrolysis of water using a hematite surface catalyst. The 3D macroporous structures were created via templating of polystyrene spheres, followed by infiltration of the desired precursor solution and annealing at high temperature. Using cyclic voltammetry and electrochemical impedance spectroscopy, it was determined that the use of this 3D transparent conducting oxide with a hematite surface catalyst allowed for a 7-fold increase in active surface area for water splitting with respect to its 2D planar counterpart. This ratio of surface areas was evaluated based on the presence of oxidized trap states on the hematite surface, as determined from the equivalent circuit analysis of the Nyquist plots. Furthermore, the presence of nc-ATO 2D and 3D underlayer structures with hematite deposited on top resulted in decreased charge transfer resistances and an increase in the number of available active surface sites at the semiconductor-liquid junction when compared to hematite films lacking any nc-ATO substructures. Finally, absorption, transmission, and reflectance spectra of all of the tested films were measured, suggesting the feasibility of using 3D disordered structures in photoelectrochemical reactions, due to the high absorption of photons by the surface catalyst material and trapping of light within the structure.

Organic light-emitting diode microcavities from transparent conducting metal oxide photonic crystals.

We report herein on the integration of novel transparent and conducting one-dimensional photonic crystals that consist of periodically alternating layers of spin-coated antimony-doped tin oxide nanoparticles and sputtered tin-doped indium oxide into organic light emitting diode (OLED) microcavities. The large refractive index contrast between the layers due the porosity of the nanoparticle layer led to facile fabrication of dielectric mirrors with intense and broadband reflectivity from structures consisting of only five bilayers. Because our photonic crystals are easily amenable to large scale OLED fabrication and simultaneously selectively reflective as well as electronically conductive, such materials are ideally suited for integration into OLED microcavities. In such a device, the photonic crystal, which represents a direct drop-in replacement for typical ITO anodes, is capable of serving two necessary functions: (i) as one partially reflecting mirror of the optical microcavity; and (ii) as the anode of the diode.

Periodic Macroporous Nanocrystalline Antimony-Doped Tin Oxide Electrode

Optically transparent and electrically conductive electrodes are ubiquitous in the myriad world of devices. They are an indispensable component of solar and photoelectrochemical cells, organic and polymer light emitting diodes, lasers, displays, electrochromic windows, photodetectors, and chemical sensors. The majority of the electrodes in such devices are made of large electronic band-gap doped metal oxides fashioned as a dense low-surface-area film deposited on a glass substrate. Typical transparent conducting oxide materials include indium-, fluorine-, or antimony-doped tin oxides. Herein we introduce for the first time a transparent conductive periodic macroporous electrode that has been self-assembled from 6 nm nanocrystalline antimony-doped tin oxide with high thermal stability, optimized electrical conductivity, and high quality photonic crystal properties, and present an electrochemically actuated optical light switch built from this electrode, whose operation is predicated on its unique combination of electrical, optical, and photonic properties. The ability of this macroporous electrode to host active functional materials like dyes, polymers, nanocrystals, and nanowires provides new opportunities to create devices with improved performance enabled by the large area, spatially accessible and electroactive internal surface.

See-Through Dye-Sensitized Solar Cells: Photonic Reflectors for Tandem and Building Integrated Photovoltaics

See-through dye-sensitized solar cells with 1D photonic crystal Bragg reflector photoanodes show an increase in peak external quantum efficiency of 47% while still maintaining high fill factors, resulting in an almost 40% increase in power conversion efficiency. These photoanodes are ideally suited for tandem and building integrated photovoltaics.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

Thermal treatment of ultrathin films of hematite (α-Fe2 O3 ) under an atmosphere of 5u2009% H2 in Ar is presented as a means of activating α-Fe2 O3 towards the photoelectrochemical splitting of water. Spin-coated films annealed in air exhibited no photoactivity, whereas films treated in hydrogen exhibited a photocurrent response. X-ray photoelectron spectroscopy and UV/Vis absorption spectroscopy results showed that the H2 -treated films contain oxygen vacancies, which suggests improved charge transport. However, Tafel slopes, scan-rate dependent measurements, and kinetic analyses performed by using H2 O2 as a hole scavenger suggested that surface modification may also contribute to their induced photoactivity. Electrochemical impedance spectroscopy results revealed the buildup of a surface trap capacitance at the point of photocurrent onset for the hydrogen-treated films under illumination. A decrease in charge trapping resistance was also observed, which suggests improved transport of charges away from the surface.

Pd@HyWO3–x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect

The design of photocatalysts able to reduce CO2 to value-added chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO2 reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO3. Specifically, we focus attention on the use of Pd nanocrystal decorated WO3 nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO2. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet-visible-near infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, H yWO3- x, as the catalytically active species formed via the H2 spillover effect by Pd. The existence in H yWO3- x of Brønsted acid hydroxyls OH, W(V) sites, and oxygen vacancies (VO) facilitate CO2 capture and reduction reactions. Under solar irradiation, CO2 reduction attains CO production rates as high as 3.0 mmol gcat-1 hr-1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO2 by Pd@H yWO3- x.