Increasing demands for energy and environmental concern have caused many researchers to look to alternative sources of energy. Solar energy has been a main area of focus as this energy is highly abundant and has the potential to solve the problem. Many solar farms today use silicon based solar panels, but these types of solar panels are costly both energetically and financially. Inorganic thin film light absorbing layers, such as CuInGaSe2 (CIGS), CuInS2 (CIS) and Cu2 ZnSnS4 (CZTS) are showing great promise as a replacement. These are CIGS based layers with the Ga removed (CIS) or the In from CIS replaced with Zn and Sn. They have reported lab efficiencies of around 10%. The drawback of these two materials is the high temperature annealation in sulfur atmosphere needed to achieve the high efficiency cells. We are trying to overcome this barrier with a one-pot, annealation free synthesis of the CIS and CZTS layers. A photoelectrochemical (PEC) measurement is performed on the material so the starting stoichiometry can be optimized to give the material with the largest photocurrent density. Further characterization is utilized to investigate their morphology (SEM and TEM), composition (XRF,XPS, EDX, HRTEM), structure (XRD, HRTEM), optical properties (UV- Vis, action spectra) and the kinetics of the photoreaction (PEC IMPS). After optimization, different methods of depositing not only the light-absorbing layer but also the other layers that compose a full solar device will be tested layer-by-layer. These methods include printing techniques, electrophoretic deposition and chemical bath deposition.
CZTS shows the most promise as a new absorbing layer. It is composed of earth abundant low cost element and if it can be synthesized in a way that is scalable, environmentally friendly, relatively cheap with comparable efficiencies to the current solar options it may be a solution to the energy crisis.
The overarching goal of our research to investigate the suitability of room temperature ionic liquids (RTILs) in biphasic metal ion extraction for use in spent nuclear fuel (SNF) reclamation. RTILs have demonstrated high metal extraction efficiencies, however loss of the anionic or cationic components through ion exchange present a serious challenge. Therefore, the first step was to find a RTIL that was sufficiently hydrophobic and we used our facile biphasic electrochemistry at water|RTIL (w|RTIL) and water|organic (w|o) solvent interfaces in order to discriminate between different candidates. The technique we employed used a micro-interface housed at the tip of a pulled glass capillary; the aqueous phase inside the pipette and immersed in organic or RTIL phase. By using a micro- interface we greatly reduced the volume of material required. Since RTILs can be expensive, this made direct w|RTIL assessments more feasible. Many commercially available RTILs were tested, however none were adequate. So we prepared a novel RTIL in-house at a greatly reduced cost (more than 10 times cheaper). Throughout the RTIL synthesis we were able to introduce measures that both simplified the preparation procedure and allowed for facile, quick purification of the product at quantitative yields. It is these kinds of innovations which will help lower the cost of RTILs and make them more attractive to large scale industrial processes.
By adding ligands to the organic or RTIL phase the transfer of metal ions from water to these phases can be greatly increased. Using electrochemistry at the biphasic interface, we were able to quantify several metal-ligand complexation reactions, individually, so that the selectivity and efficiency could be compared. This also allowed me to show that RTILs were 100 000 times better than organic solvents in metal ion extraction. we also observed a greater selectivity for strontium versus uranium or cesium using two ligands common to contemporary SNF industrial reprocessing. 95% of SNF is useable uranium that has been contaminated with fission byproducts that, in turn, include high amounts of 90Sr and 137Cs isotopes; major contributors to its radioactivity. This metal ion differentiation result demonstrates the importance of this work.
These small scale experiments also served as a proof-of-concept that electrochemical, rather than the typical physical means of separation, is possible. Physical separation requires a great deal of engineering and mechanical moving parts; owing to the inherent radioactivity of the SNF being separated, maintenance of this equipment is prohibitively expensive. If an electrochemical method were to be used, the mechanical engineering could be reduced and thus the cost would go down. This would make reprocessing SNF more attractive and we could avoid long-term geological disposal which has all kinds of potentially nasty repercussions.
In conclusion, we were able to test and synthesize a host of RTILs and investigate their suitability towards metal ion extraction. Critically, the selectivity and efficiency of biphasic separations was quantified and it is this that should have a lasting impact on how we view SNF reprocessing.
Electrogenerated chemiluminescence (ECL) is the process in which electrogenerated radical anions and cations combine in solution, produce an excited species via electron transfer and emit light. ECL has become a powerful analytical technique in immunoassays, food and water testing, trace metal determination, and biomolecule detection. In our laboratory, we study a variety of compounds, ranging from metal complexes, organic molecules, modified nucleobases, dyes, polymers, atomic clusters, nanocrystals in the visible and near-infrared regions. We perform cyclic voltammetry (CV), differential pulse voltammetry (DPV), and ECL to study the mechanisms in which light is generated from the compounds in solutions
ECL via annihilation is the process by which a luminophore species in solution is scanned or pulsed to its first oxidation and reduction potentials at a working electrode. The generation of its radical anion and radical cation in the vicinity of the electrode will produce the excited state species. ECL results from the excited state species relaxing back down to its ground state and emitting light.
An alternative to annihilation ECL is using a coreactant system. Here, one directional potential scanning at the working electrode in a solution containing the luminophore species with added coreactant (such as benzoyl peroxide, BPO) is performed. Radicals are generated from the luminophore, and coreactant that reacts to give off an excited species that emits light.
In order to see the evolution of light emission, we have developed a new technique called spooling that captures the spectrum every second for a given amount of time period while scanning the compound using CV. With the spooling technique we can track the potential at which ECL emission occurs. Using spooling we can identify multiple ECL emissions individually ECL is a valuable analytical technique that is cost effective, fast, highly sensitive and selective and tunable, requiring minimal amount of compound and solvent.
Scanning electrochemical microscopy (SECM) involves the measurement of electrochemically active species through an ultramicroelectrode (UME) (an electrode with a diameter of the order a few nm to 25 micrometers) when it is held or moved in a solution in the vicinity of a substrate. Since the electrochemical current is proportional to the concentration of a redox species at the substrate, which can be live cells or metal alloys, any change in their chemical activities will perturb the electrochemical responses of the probe. This perturbation provides information about the nature and activities at the substrate. For instance, reactive oxygen species (ROS) released from a single live cell have been found by us to be a good molecular probe for its physiology and pathophysiology. SECM has been also applied to various metallic materials very important to our daily life. We have constructed grain microstructure maps of Ti metal alloys, UO2+x (x=0.001 to 0.33) and Zr alloys based on reactivity images from SECM, providing insights into relationship between their structures and reactivities.
While the SECM has become a very powerful tool to determine fast heterogeneous kinetics, image chemical and biological activity, fabricate interdigitated arrays, its power would be increased if it was combined with other techniques. Only a few attempts have been reported on nanoelectrode fabrication and combinations of SECM with other techniques: SECM/photoelectrochemical microscopy, SECM/ atomic force microscopy (AFM), and SECM /AFM /near-field scanning optical microscopy (NSOM).
We are working hard in the aspects of probe fabrication, probe displacement, scanning speeds, and probe-to-substrate distance adjustment for further development of SECM. We have found a novel strategy to perform constant-distance AC SECM imaging: negative AC feedback is independent on the nature of the substrate. This is significant since the probe-to-substrate distance can be kept constant by simply setting up the constant AC current. Simultaneous AC and DC SECM images will discriminate topographical from chemical information.