Science, Engineering, Innovation, Communication
Our world was built on the human spirit of collaboration. The 21st century presents unique challenges as a result of the science illiteracy that threatens to destabilize the world as we know it. Here I highlight and summarize some of my previous publications. Clear and effective science communication is the best disinfectant for disinformation. If we lose our ability to collaborate socially, which some do appear to seek, we lose the future for generations to come.
If you're interested in seeing the full catalog of my scientific publications, my Google Scholar profile can be found here.
If you're interested in seeing the full catalog of my scientific publications, my Google Scholar profile can be found here.
Microstructure and electrochemical properties of high performance graphene/manganese oxide hybrid electrodes†
Fatima Hamade, Emmy Radich‡ and Virginia A. Davis*
DOI: 10.1039/D1RA05323J (Paper) RSC Adv., 2021, 11, 31608-31620
Abstract
Hybrids consisting of 2D ultra-large reduced graphene oxide (RGO) sheets (∼30 μm long) and 1D α-phase manganese oxide (MnO2) nanowires were fabricated through a versatile synthesis technique that results in electrostatic binding of the nanowires and sheets. Two different hybrid (RGO/MnO2) compositions had remarkable features and performance: 3:1 MnO2/RGO (75/25 wt%) denoted as 3H and 10:1 MnO2/RGO (90/10 wt%) denoted as 10H. Characterization using spectroscopy, microscopy, and thermal analysis provided insights into the microstructure and behavior of the individual components and hybrids. Both hybrids exhibited higher specific capacitance than their individual components. 3H demonstrated excellent overall electrochemical performance with specific capacitance of 225 F g−1, pseudocapacitive and electrochemical double-layer capacitance (EDLC) contributions, charge-transfer resistance <1 Ω, and 97.8% capacitive retention after 1000 cycles. These properties were better than those of 10H; this was attributed 3H's more uniform distribution of nanowires enabling more effective electronic transport. Thermal annealing was used to produce reduced graphene oxide (RGO) that exhibited significant removal of oxygen functionality with a resulting interlayer spacing of 0.391 nm, higher D/G ratio, higher specific capacitance, and electrochemical properties representing more ideal capacitive behavior than GO. Integrating ultra-large RGO with very high surface area and MnO2 nanowires enables chemical interactions that may improve processability into complex architectures and electrochemical performance of electrodes for applications in electronics, sensors, catalysis, and deionization.
Carrier Lifetime and Charge Separation in K+-Doped CZTS Nanocrystals
Animesh Mondal, Christopher Scheinert, and Emmy J. Radich*
ACS Appl. Energy Mater. 2019, 2, 1, 250–259
Publication Date: November 9, 2018
https://doi.org/10.1021/acsaem.8b01168
Abstract
Cu2ZnSnS4 (CZTS) nanocrystals are important materials for next generation solar energy capture and conversion strategies. CZTS contains earth-abundant materials and has optimum band gap energy and high absorption coefficient. However, progress in utilizing CZTS nanocrystals is impeded by the numerous crystal and morphological defects present without high-temperature annealing. These defects commonly result in reduced carrier diffusion length and carrier lifetime. One recent and promising direction for employing CZTS nanocrystals in solar energy capture and conversion strategies is through remediation of defects by doping with alkali earth metal ions such as Li+, Na+, and/or K+. Here we focus on the K+ cationic doping since it has demonstrated the highest efficiencies but without a fundamental investigation into the underlying drivers. We first developed a flexible and reproducible synthesis route to prepare K+-doped CZTS nanocrystals with low polydispersity. We employed undoped and K+-doped nanocrystals to fabricate CZTS photocathodes and evaluated their photoresponsiveness in a photoelectrochemical cell. K+-doped CZTS nanocrystals show the previously reported trend of improved photocurrent density. We obtained further insight into the role of K+ dopant using Raman spectroscopy, X-ray photoelectron spectroscopy, and transient absorption spectroscopy. K+-doping of CZTS nanocrystals boosts charge carrier lifetime and enables better charge extraction efficiency to boost photocurrent. Improved carrier lifetime is attributed to remediation of binary/tertiary impurity phases and surface anion vacancies to yield a higher degree of phase purity and a lower degree of surface electron traps in CZTS.
Ni(OH)2 as Hole Mediator for Visible Light-Induced Urea Splitting
Rong Zhao, Grant Schumacher, Stephen Leahy, and Emmy J. Radich*
Cite this: J. Phys. Chem. C 2018, 122, 25, 13995–14003
Publication Date: April 15, 2018
https://doi.org/10.1021/acs.jpcc.8b01116
Abstract
Urea is a small molecule produced in millions of tons per day and is ubiquitous in nature. Biological treatment is commonly used to oxidize the urea wastewater produced each day across the world, which produces additional solid waste and eliminates any potential for utilizing the stored chemical energy within. A solar waste-to-fuels concept is presented to synergistically produce hydrogen fuel from visible sunlight while remediating urea wastewaters. A cascade semiconductor-catalyst electrode assembly was designed to drive the photoconversion of urea to hydrogen. Proper band energy alignment facilitates catalyst activation via hole transfer across the semiconductor–catalyst interface. Specifically CdS-sensitized TiO2 with Ni(OH)2 urea electrocatalyst on fluorine-doped tin oxide coated glass was employed as photoanode. The steady-state response of the semiconductor–catalyst electrode is investigated in a photoelectrochemical cell, and charge transfer and recombination kinetics are elucidated to identify limiting charge-transfer reactions within the electrode architecture. Back electron transfer from semiconductor to catalyst is found to be competitive with urea oxidation reaction, which hinders steady-state photoconversion efficiency. Furthermore, the photoanode rapidly decomposes in urea electrolyte solutions as a result of the water-mediated photocorrosion of chalcogenide electrodes. Passivation of CdS with ZnS prior to catalyst deposition significantly improves open-circuit potential and photostability.
Is Graphene a Stable Platform for Photocatalysis? Mineralization of Reduced Graphene Oxide With UV-Irradiated TiO2 Nanoparticles
Emmy J. Radich , Anthony L. Krenselewski , Jiadong Zhu , and Prashant V. Kamat* Chem. Mater. 2014, 26, 15, 4662–4668
Publication Date: July 21, 2014
https://doi.org/10.1021/cm5026552
Abstract
The recent thrust in utilizing reduced graphene oxide (RGO) as a support for nanostructured catalyst particles has led to the claims of improved efficiency in solar cells, fuel cells, and photocatalytic degradation of pollutants. Specifically, the robust TiO2 system is often coupled with RGO to improve charge separation and facilitate redox reactions. Here, we probe the stability of RGO in the presence of UV-excited TiO2 in aqueous media and establish its reactivity toward OH• radicals, a primary oxidant generated at the TiO2 surface. By probing changes in absorption, morphology, and total organic carbon content (TOC), we conclusively demonstrate the vulnerability of RGO toward OH• attack and raise the concern of its use in many applications where OH• are likely to be formed. On the other hand, the OH• radical-mediated mineralization could also enable new approaches in tackling environmental remediation of nanocarbons such as RGO, carbon nanotubes, and fullerenes.
How Does a SILAR CdSe Film Grow? Tuning the Deposition Steps to Suppress Interfacial Charge Recombination in Solar Cells
Matthew A. Becker, Emmy J. Radich, Bruce A. Bunker, and Prashant V. Kamat*
Cite this: J. Phys. Chem. Lett. 2014, 5, 9, 1575–1582
Publication Date: April 11, 2014
https://doi.org/10.1021/jz500481v
Abstract Successive ionic layer adsorption and reaction (SILAR) is a popular method of depositing the metal chalcogenide semiconductor layer on the mesoscopic metal oxide films for designing quantum-dot-sensitized solar cells (QDSSCs) or extremely thin absorber (ETA) solar cells. While this deposition method exhibits higher loading of the light-absorbing semiconductor layer than direct adsorption of presynthesized colloidal quantum dots, the chemical identity of these nanostructures and the evolution of interfacial structure are poorly understood. We have now analyzed step-by-step SILAR deposition of CdSe films on mesoscopic TiO2 nanoparticle films using X-ray absorption near-edge structure analysis and probed the interfacial structure of these films. The film characteristics interestingly show dependence on the order in which the Cd and Se are deposited, and the CdSe–TiO2 interface is affected only during the first few cycles of deposition. Development of a SeO2 passivation layer in the SILAR-prepared films to form a TiO2/SeO2/CdSe junction facilitates an increase in photocurrents and power conversion efficiencies of quantum dot solar cells when these films are integrated as photoanodes in a photoelectrochemical solar cell.
Charge Transfer Mediation Through CuxS. The Hole Story of CdSe in Polysulfide
Emmy J. Radich, Nevin R. Peeples, Pralay K. Santra, and Prashant V. Kamat*
J. Phys. Chem. C 2014, 118, 30, 16463–16471
Publication Date: February 7, 2014
https://doi.org/10.1021/jp4113365
Abstract Hole transfer to dissolved sulfide species in liquid junction CdSe quantum dot sensitized solar cells is relatively slow when compared to electron transfer from CdSe to TiO2. Controlled exposure of cadmium chalcogenide surfaces to copper ions followed by immersion in sulfide solution promotes development of the interfacial CuxS layer, which mediates hole transfer to polysulfide electrolyte by collection of photogenerated holes from CdSe. In addition, CuxS was also found to interact directly with defect states on the CdSe surface and quench emission characteristic of electron traps resulting from selenide vacancies. Together these effects were found to work in tandem to deliver 6.6% power conversion efficiency using Mn-doped CdS and CdSe cosensitized quantum dot solar cells. Development of an n–p interfacial junction at the photoanode–electrolyte interface in quantum dot solar cells unveils new means for designing high efficiency liquid junction solar cells.
Nickel-Doped MnO2 Nanowires Anchored onto Reduced Graphene Oxide for Rapid Cycling Cathode in Lithium Ion Batteries
Emmy J. Radich, Yong-Siou Chen, and Prashant V. Kamat
Published 3 October 2013
ECS Journal of Solid State Science and Technology, Volume 2, Number 10
https://doi.org/10.1149/2.023310jss
Abstract
Nickel-doped MnO2 nanowires were synthesized directly onto reduced graphene oxide (RGO) to generate a composite cathode material with improved high-rate cycling characteristics. The presence of RGO improves the electrochemical characteristics of the cathode in Li-ion half-cell architecture. Cyclic voltammetry, electrochemical impedance spectroscopy, and electrode cycling are confirm that RGO plays a major role in enhancing the ability of the NixMn(1-x)O2 to reversibly intercalate lithium ions at 1C rate. The chronocoulometric response of the RGO-based electrode shows the improvements originate from faster reaction kinetics and transport of Li+ coupled with increased specific capacitance and Li+ adsorption.
Making Graphene Holey. Gold-Nanoparticle-Mediated Hydroxyl Radical Attack on Reduced Graphene Oxide
Emmy J. Radich and Prashant V. Kamat*
Cite this: ACS Nano 2013, 7, 6, 5546–5557
Publication Date: May 5, 2013
https://doi.org/10.1021/nn401794k
Abstract
Graphene oxide (GO) and reduced graphene oxide (RGO) have important applications in the development of new electrode and photocatalyst architectures. Gold nanoparticles (AuNPs) have now been employed as catalyst to generate OH• and oxidize RGO via hydroxyl radical attack. The oxidation of RGO is marked by pores and wrinkles within the 2-D network. Nanosecond laser flash photolysis was used in conjunction with competition kinetics to elucidate the oxidative mechanism and calculate rate constants for the AuNP-catalyzed and direct reaction between RGO and OH•. The results highlight the use of the AuNP-mediated oxidation reaction to tune the properties of RGO through the degree of oxidation and/or functional group selectivity in addition to the nanoporous and wrinkle facets. The ability of AuNPs to catalyze the photolytic decomposition of H2O2 as well as the hydroxyl radical-induced oxidation of RGO raises new issues concerning graphene stability in energy conversion and storage (photocatalysis, fuel cells, Li-ion batteries, etc.). Understanding RGO oxidation by free radicals will aid in maintaining the long-term stability of RGO-based functional composites where intimate contact with radical species is inevitable.
Origin of Reduced Graphene Oxide Enhancements in Electrochemical Energy Storage
Emmy J. Radich and Prashant V. Kamat*
Cite this: ACS Catal. 2012, 2, 5, 807–816
Publication Date: March 27, 2012
https://doi.org/10.1021/cs3001286
Abstract
Reduced graphene oxide (RGO) has become a common substrate upon which active intercalation materials are anchored for electrochemical applications such as supercapacitors and lithium ion batteries. The unique attributes of RGO, including high conductivity and porous macrostructure, are often credited for enhanced cycling and capacity performance. Here we focus on probing the electrochemical response of α-MnO2/RGO composite used as an electrode in a lithium ion battery cell and elucidating the mechanistic aspects of the RGO on the commonly observed improvements in cycling and capacity. We find that electron storage properties of RGO enables better electrode kinetics, more rapid diffusion of Li+ to intercalation sites, and a greater capacitance effect during discharge. Further investigation of the length of the one-dimensional nanowire morphology of the α-MnO2 has allowed us to differentiate between the innate characteristics of the MnO2 and those of the RGO. RGO coupled with long nanowires (>5 μm) exhibited the best performance in all tests and retained ∼150 mAh/g capacity after 20 cycles at 0.4C rate.
Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2–/Sn2– at the Counter Electrode
Emmy J. Radich, Ryan Dwyer, and Prashant V. Kamat*
Cite this: J. Phys. Chem. Lett. 2011, 2, 19, 2453–2460
Publication Date: September 13, 2011
https://doi.org/10.1021/jz201064k
Abstract
Polysulfide electrolyte that is employed as a redox electrolyte in quantum dot sensitized solar cells provides stability to the cadmium chalcogenide photoanode but introduces significant redox limitations at the counter electrode through undesirable surface reactions. By designing reduced graphene oxide (RGO)-Cu2S composite, we have now succeeded in shuttling electrons through the RGO sheets and polysulfide-active Cu2S more efficiently than Pt electrode, improving the fill factor by ∼75%. The composite material characterized and optimized at different compositions indicates a Cu/RGO mass ratio of 4 provides the best electrochemical performance. A sandwich CdSe quantum dot sensitized solar cell constructed using the optimized RGO-Cu2S composite counter electrode exhibited an unsurpassed power conversion efficiency of 4.4%.
Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked versus Direct Attachment
Douglas R. Pernik, Kevin Tvrdy, Emmy J. Radich, and Prashant V. Kamat*
Cite this: J. Phys. Chem. C 2011, 115, 27, 13511–13519
Publication Date: May 27, 2011
https://doi.org/10.1021/jp203055d
Abstract
Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO2 films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO2 films. The adsorption constant Kad for submonolayer coverage was (6.7 ± 2.7) × 103 M–1 for direct adsorption and (4.2 ± 2.0) × 104 M–1 for MPA-linked assemblies. Prolonged exposure of a TiO2 film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO2 was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO2 from directly adsorbed QDs (kET = 7.2 × 109 s–1) compared with MPA-linked QDs (kET = 2.3 × 109 s–1). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO2 and designing efficient photoanodes for QDSSCs.
