George P. Demopoulos

Near-Infrared Sunlight Harvesting in Dye-Sensitized Solar Cells Via the Insertion of an Upconverter-TiO2 Nanocomposite Layer

Er(3+),Yb(3+) co-doped LaF₃-TiO₂ nanocomposites (UC-TiO₂) are inserted as a middle layer in a novel tri-layer photoanode design (see Figure) of a dye-sensitized solar cell (DSSC). The Er(3+),Yb(3+) co-doped LaF₃ part of the nanocomposite helps capture near-infrared (NIR) light by converting it into visible light absorbable by the dye hence opening the road for the development of DSSCs with higher conversion efficiency and photocurrent output.

Further Understanding of the Adsorption Mechanism of N719 Sensitizer on Anatase TiO2 Films for DSSC Applications Using Vibrational Spectroscopy and Confocal Raman Imaging

Vibrational spectroscopic studies of N719 dye-adsorbed TiO(2) films have been carried out by using SERRS, ATR-FTIR, and confocal Raman imaging. The high wavenumber region (3000-4000 cm(-1)) of dye adsorbed TiO(2) is analyzed via Raman and IR spectroscopy to investigate the role of surface hydroxyl groups in the anchoring mode. As a complementary technique, confocal Raman imaging is employed to study the distribution features of key dye groups (COO-, bipyridine, and C=O) on the anatase surface. Sensitized TiO(2) films made from two different nanocrystalline anatase powders are investigated: a commercial one (Dyesol) and our synthetic variety produced through aqueous synthesis. It is proposed the binding of the N719 dye to TiO(2) to occur through two neighboring carboxylic acid/carboxylate groups via a combination of bidentate-bridging and H-bonding involving a donating group from the N719 (and/or Ti-OH) units and acceptor from the Ti-OH (and/or N719) groups. The Raman imaging distribution of COO(-)(sym) on TiO(2) was used to show the covalent bonding, while the distribution of C=O mode was applied to observe the electrostatically bonded groups.

Speciation and separation of rhodium (III) from chloride solutions: a critical review

Abstract The recovery and purification of rhodium has always been difficult because of its complex aqueous chemistry in chloride solutions. A review of this chemistry, with special emphasis on speciation, is presented. Using available kinetic data, a new speciation diagram is proposed and it is shown that, even in relatively strong chloride solutions, there are at least two different rhodium aquo/chloro anionic species. In fact, it is shown that aquation may be even more prevalent than previously thought. This has serious implications on the behaviour of such solutions towards modern recovery processes such as solvent extraction. In addition, published solvent extraction and ion exchange separation schemes are reviewed, demonstrating the difficulties that have been encountered in devising a suitable process, due to the aqueous chemistry of rhodium.

Optimizing Cu removal/recovery in a biosorption column

Biosorption of Cu2+ by Sargassum fluitans seaweed biomass protonated by an acidic wash or loaded with Ca2+ is based on ion exchange. The uptake of Cu2+ is respectively accompanied by a release of either H+ or Ca2+ into the solution phase. The effects of Ca-, H- and HCa-cycles on the performance of a continuous-flow biosorption fixed-bed were established. The Ca-cycle applied to Sargassum biomass in a packed bed led to a high degree of a column utilization but did not allow an effective Cu recovery. The H-cycle permitted 100% Cu recovery but also shortened the sorption column service time. The combined CaH-cycle was shown to be inefficient due to the time consuming regeneration of biomass from the H-form to the Ca-form. Biomass pretreatment with 1% (w) solution of CaCl2 and with 0.1 m HCl resulted in the same Cu uptake of 75 mg/g. The Ca-pretreated biomass lost approximately 30% of its Cu capacity with subsequent acidic wash. The equilibrium aspects of Cu removal and recovery in a biosorption column were analyzed through the concept of ion-exchange isotherms. The dynamics of Cu sorption and of biomass regeneration in a fixed-bed column was predicted by numerically solving the equations of a proposed ion-exchange model.

Enhanced Performance of Dye-Sensitized Solar Cells by Utilization of an External, Bifunctional Layer Consisting of Uniform β-NaYF4:Er3+/Yb3+ Nanoplatelets

Uniform β-NaYF(4):Er(3+)/Yb(3+) hexagonal nanoplatelets were synthesized via a modified hydrothermal route, and the nanoplatelets were applied as an external, bifunctional layer in a novel DSC configuration consisting of only one internal TiO(2) transparent layer. Approximately 10% enhancements of photocurrent and overall DSC efficiency are demonstrated by the addition of the external layer, which exhibits two functions of light reflecting and near-infrared (NIR) light harvesting. The novel DSC configuration not only simplifies the DSC fabrication process but also eliminates charge recombination induced by the conducting up-converting nanocrystals when used internally thus opening the path for other more efficient up-converting nanocrystals to be designed and applied.

Precipitation of crystalline scorodite (FeAsO4 · 2H2O) from chloride solutions

Abstract Crystalline scorodite was produced under ambient pressure at temperatures in the range 80–95°C, employing a supersaturation-controlled precipitation procedure. Failure to control supersaturation led to the production of amorphous precipitate. The procedure involved step-wise neutralization under a low supersaturation environment in the presence of seed. Acidic chloride solutions (1.0–6.0 M Cl− total), containing 2 g/1 As(V) and a variable Fe(III) / As(V) molar ratio ( 1 1 , 2 1 and 4 1 were used. In the absence of seed, crystalline scorodite was produced at 95°C only on the walls of the precipitation reactor. Use of seed made the precipitation of crystalline scorodite possible at even lower temperatures (i.e. 80°C), although a lower supersaturation level had also to be maintained. The presence of SO42− in the chloride solutions was found to have an inhibitory effect on the crystallization of scorodite, resulting in a slowing down of the precipitation process. Neutralization of high Fe/As ratio solutions was found to go through sequential precipitation of crystalline scorodite (up to pH ≈1.1), followed by precipitation of β-goethite.

Acidity, valency and third-ion effects on the precipitation of scorodite from mixed sulfate solutions under atmospheric-pressure conditions

The present study is focused on the precipitation of scorodite from mixed sulfate media at 95 °C under atmospheric pressure. In particular, this study explores the effects of acidity (pH), valency [Fe(II)/Fe(III), As(III)/As(V)], and solution composition (third cation/anion) on the yield, crystallinity, and stability (leachability) of scorodite precipitates. Thus, it was found that the precipitation of crystalline scorodite can be achieved without stringent pH control once the precipitation has started. Nonetheless, the selection of the initial pH is critical to avoid the formation of an amorphous precipitate. A leachability as low as 0.5 mg/L As at pH 5 and 22 °C (TCLP-like test) is obtained when the initial molar ratio Fe(III):As(V) is increased to 3:1, but the precipitation yield is very low. When Fe(II) is used as excess iron, the precipitate solubility drops to 0.2 mg/L As with a yield exceeding 80 pct in 2.5 hours. The stability of the product is not measurably affected by the presence of Cu2+, Zn2+, Ni2+, Co2+, Mn2+, SO42−, and NO3−. The presence of PO43−, however, leads to the formation of crystalline phosphate-containing scorodite precipitates of somewhat reduced stability. In most cases, the TCLP leachability of the precipitate was found to be between 1 and 3 mg/L As, and never exceeded the regulatory limit of 5 mg/L As.

New insight into the atomic-scale bulk and surface structure evolution of Li4Ti5O12 anode.

Identifying the structure of electrodes at atomic-scale remains a key challenge but is a fertile realm for groundbreaking fundamental research in the advanced Li-ion battery material field. In this context, the subtle structure evolution taking place during lithiation/delithiation in the bulk/surface of Li(4)Ti(5)O(12) spinel (LTO) was probed using scanning transmission electron microscopy and found to undergo significant structure torque, namely Ti-O bond stretching/shrinking at different state-of-charge (SOC), which is not identified previously. This kind of nanostructure change plays an important role in facilitating the formation of capturing centers for the electron/hole pairs in a 3.80 eV insulating material as is LTO. Furthermore, with the aid of electron energy loss spectroscopy, the spontaneous charge transfer process, Ti(3+) ↔ e(-) + Ti(4+), was confirmed in the fully lithiated Li(7)Ti(5)O(12) surface as an essential step of the gas-releasing phenomenon. This new insight paves the way toward deeper comprehension and ultimately control of the electrochemical process for this and other important Li-ion battery materials.

Size-Dependent Maximization of Upconversion Efficiency of Citrate-Stabilized β-phase NaYF4:Yb3+,Er3+ Crystals via Annealing

Upconversion materials show great potential in converting infrared light to visible for many optoelectronic and photovoltaic devices. One of the most promising upconverting materials is Yb(3+),Er(3+)- doped β-NaYF4. In this study, annealing is shown to have a significant impact on the phase, morphology, and upconversion luminescence of β-NaYF4:Yb(3+),Er(3+) crystals of varying sizes (300 nm, 700 nm, and 2.3 μm, respectively) prepared by hydrothermal synthesis stabilized with sodium citrate. Upconversion luminescence is maximized via annealing while maintaining crystal shape and size dispersity up to a temperature dependent on initial size, with NIR-to-visible quantum yields of 2-5%. Further temperature increases result in growth and agglomeration, increasing luminescence, followed by transformation to the α-cubic phase resulting in decreases in overall upconversion performance and shifts to dominant red emission. This study establishes the critical link between annealing temperature and maximal upconversion luminescence in β-NaYF4:Yb(3+),Er(3+) crystals, while maintaining particle morphology, which can be very important for technological application.

Acid pressure oxidation of pyrite: reaction kinetics

Abstract The pressure oxidation (H2SO4-O2) kinetics of narrow-sized pyrite particulates were experimentally determined in the temperature range 140–180°C and pressure range 5–20 atm. The oxidation kinetics were found to follow a shrinking core model, with the surface chemical reaction as the rate-controlling step. The reaction proceeds to completion only at temperatures exceeding 160°C, but at temperatures below 160°C the formation of liquid elemental sulphur blocks the grain surface and eventually terminates the reaction before completion. The activation energy was found to be 46.2 kJ mol−1 between 140 and 160°C, and 110.5 kJ mol−1 between 160 and 180°C. The reaction order with respect to oxygen pressure was found to depend on temperature and pressure. First order dependency was found at 140–160°C/5–20 atm and at 160–180°C/5–10 atm. The order becomes 0.5 at elevated temperatures and pressures (160–180°C/10–20 atm). Finally, appropriate rate equations were developed for the two temperature ranges.