Dalaver H. Anjum

A Highly Selective Copper–Indium Bimetallic Electrocatalyst for the Electrochemical Reduction of Aqueous CO2 to CO

The challenge in the electrochemical reduction of aqueous carbon dioxide is in designing a highly selective, energy-efficient, and non-precious-metal electrocatalyst that minimizes the competitive reduction of proton to form hydrogen during aqueous CO2 conversion. A non-noble metal electrocatalyst based on a copper-indium (Cu-In) alloy that selectively converts CO2 to CO with a low overpotential is reported. The electrochemical deposition of In on rough Cu surfaces led to Cu-In alloy surfaces. DFT calculations showed that the In preferentially located on the edge sites rather than on the corner or flat sites and that the d-electron nature of Cu remained almost intact, but adsorption properties of neighboring Cu was perturbed by the presence of In. This preparation of non-noble metal alloy electrodes for the reduction of CO2 provides guidelines for further improving electrocatalysis.

Spin-cast bulk heterojunction solar cells: a dynamical investigation.

Spin-coating is extensively used in the lab-based manufacture of organic solar cells, including most of the record-setting solution-processed cells. We report the first direct observation of photoactive layer formation as it occurs during spin-coating. The study provides new insight into mechanisms and kinetics of bulk heterojunction formation, which may be crucial for its successful transfer to scalable printing processes.

Hollow Au@Pd and Au@Pt core–shell nanoparticles as electrocatalysts for ethanol oxidation reactions

Hybrid alloys among gold, palladium and platinum become a new category of catalysts primarily due to their enhanced catalytic effects. Enhancement means not only their effectiveness, but also their uniqueness as catalysts for the reactions that individual metals may not catalyze. Here, preparation of hollow Au@Pd and Au@Pt core–shell nanoparticles (NPs) and their use as electrocatalysts are reported. Galvanic displacement with Ag NPs is used to obtain hollow NPs, and higher reduction potential of Au compared to Ag, Pd, and Pt helps to produce hollow Au cores first, followed by Pd or Pt shell growth. Continuous and highly crystalline shell growth was observed in Au@Pd core–shell NPs, but the sporadic and porous-like structure was observed in Au@Pt core–shell NPs. Along with hollow core–shell NPs, hollow porous Pt and hollow Au NPs are also prepared from Ag seed NPs. Twin boundaries which are typically observed in large size (>20 nm) Au NPs were not observed in hollow Au NPs. This absence is believed to be due to the role of the hollows, which significantly reduce the strain energy of edges where the two lattice planes meet. In ethanol oxidation reactions in alkaline medium, hollow Au@Pd core–shell NPs show highest current density in forward scan. Hollow Au@Pt core–shell NPs maintain better catalytic activities than metallic Pt, which is thought to be due to the better crystallinity of Pt shells as well as the alloy effect of Au cores.

Nanostructured cobalt sulfide-on-fiber with tunable morphology as electrodes for asymmetric hybrid supercapacitors

Porous cobalt sulfide (Co9S8) nanostructures with tunable morphology, but identical crystal phase and composition, have been directly nucleated over carbon fiber and evaluated as electrodes for asymmetric hybrid supercapacitors. As the morphology is changed from two-dimensional (2D) nanoflakes to 3D octahedra, dramatic changes in supercapacitor performance are observed. In three-electrode configuration, the binder-free Co9S8 2D nanoflake electrodes show a high specific capacitance of 1056 F g−1 at 5 mV s−1vs. 88 F g−1 for the 3D electrodes. As sulfides are known to have low operating potential, for the first time, asymmetric hybrid supercapacitors are constructed from Co9S8 nanostructures and activated carbon (AC), providing an operation potential from 0 to 1.6 V. At a constant current density of 1 A g−1, the 2D Co9S8, nanoflake//AC asymmetric hybrid supercapacitor exhibits a gravimetric cell capacitance of 82.9 F g−1, which is much higher than that of an AC//AC symmetric capacitor (44.8 F g−1). Moreover, the asymmetric hybrid supercapacitor shows an excellent energy density of 31.4 W h kg−1 at a power density of 200 W Kg−1 and an excellent cycling stability with a capacitance retention of ∼90% after 5000 cycles.

Molybdenum carbide–carbon nanocomposites synthesized from a reactive template for electrochemical hydrogen evolution

Molybdenum carbide nanocrystals (Mo2C) with sizes ranging from 3 to 20 nm were synthesized within a carbon matrix starting from a mesoporous graphitic carbon nitride (mpg-C3N4) template with confined pores. A molybdenum carbide phase (Mo2C) with a hexagonal structure was formed using a novel synthetic method involving the reaction of a molybdenum precursor with the carbon residue originating from C3N4 under nitrogen at various temperatures. The synthesized nanocomposites were characterized using powder X-ray diffraction (XRD), temperature-programmed reaction with mass spectroscopy (MS), CHN elemental analyses, thermogravimetric analyses (TGA), nitrogen sorption, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results indicated that the synthesized samples have different surface structures and compositions, which are accordingly expected to exhibit different electrocatalytic activities toward the hydrogen evolution reaction (HER). Electrochemical measurements demonstrated that the sample synthesized at 1323 K exhibited the highest and most stable HER current in acidic media, with an onset potential of −100 mV vs. RHE, among the samples prepared in this study. This result is attributed to the sufficiently small particle size (∼8 nm on average) and accordingly high surface area (308 m2 g−1), with less oxidized surface entrapped within the graphitized carbon matrix.

Hydrogen production by tuning the photonic band gap with the electronic band gap of TiO2

Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

Efficient inverted bulk-heterojunction solar cells from low-temperature processing of amorphous ZnO buffer layers

In this report, we demonstrate that solution-processed amorphous zinc oxide (a-ZnO) interlayers prepared at low temperatures (∼100 °C) can yield inverted bulk-heterojunction (BHJ) solar cells that are as efficient as nanoparticle-based ZnO requiring comparably more complex synthesis or polycrystalline ZnO films prepared at substantially higher temperatures (150–400 °C). Low-temperature, facile solution-processing approaches are required in the fabrication of BHJ solar cells on flexible plastic substrates, such as PET. Here, we achieve efficient inverted solar cells with a-ZnO buffer layers by carefully examining the correlations between the thin film morphology and the figures of merit of optimized BHJ devices with various polymer donors and PCBM as the fullerene acceptor. We find that the most effective a-ZnO morphology consists of a compact, thin layer with continuous substrate coverage. In parallel, we emphasize the detrimental effect of forming rippled surface morphologies of a-ZnO, an observation which contrasts with results obtained in polycrystalline ZnO thin films, where rippled morphologies have been reported to improve efficiency. After optimizing the a-ZnO morphology at low processing temperature for inverted P3HT:PCBM devices, achieving a power conversion efficiency (PCE) of ca. 4.1%, we demonstrate inverted solar cells with low bandgap polymer donors on glass/flexible PET substrates: PTB7:PC71BM (PCE: 6.5% (glass)/5.6% (PET)) and PBDTTPD:PC71BM (PCE: 6.7% (glass)/5.9% (PET)). Finally, we show that a-ZnO based inverted P3HT:PCBM BHJ solar cells maintain ca. 90–95% of their initial PCE even after a full year without encapsulation in a nitrogen dry box, thus demonstrating excellent shelf stability. The insight we have gained into the importance of surface morphology in amorphous zinc oxide buffer layers should help in the development of other low-temperature solution-processed metal oxide interlayers for efficient flexible solar cells.

Direct Chemical Synthesis of MnO2 Nanowhiskers on Transition-Metal Carbide Surfaces for Supercapacitor Applications

Transition-metal carbides (MXenes) are an emerging class of two-dimensional materials with promising electrochemical energy storage performance. Herein, for the first time, by direct chemical synthesis, nanocrystalline ε-MnO2 whiskers were formed on MXene nanosheet surfaces (ε-MnO2/Ti2CTx and ε-MnO2/Ti3C2Tx) to make nanocomposite electrodes for aqueous pseudocapacitors. The ε-MnO2 nanowhiskers increase the surface area of the composite electrode and enhance the specific capacitance by nearly 3 orders of magnitude compared to that of pure MXene-based symmetric supercapacitors. Combined with enhanced pseudocapacitance, the fabricated ε-MnO2/MXene supercapacitors exhibited excellent cycling stability with ∼88% of the initial specific capacitance retained after 10000 cycles which is much higher than pure ε-MnO2-based supercapacitors (∼74%). The proposed electrode structure capitalizes on the high specific capacitance of MnO2 and the ability of MXenes to improve conductivity and cycling stability.

The temperature-dependent microstructure of PEDOT/PSS films: insights from morphological, mechanical and electrical analyses

Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) is a widely used conductive polymer in the field of flexible electronics. The ways its microstructure changes over a broad range of temperatures remain unclear. This paper describes microstructure changes at different temperatures and correlates the microstructure with its physical properties (mechanical and electrical). We used High-Angle Annular Dark-Field Scanning Electron Microscopy (HAADF-STEM) combined with electron energy loss spectroscopy (EELS) to determine the morphology and elemental atomic ratio of the film at different temperatures. These results together with the Atomic Force Microscopy (AFM) analysis provide the foundation for a model of how the temperature affects the microstructure of PEDOT/PSS. Moreover, dynamic mechanical analysis (DMA) and electrical characterization were performed to analyze the microstructure and physical property correlations.

Effect of rapid thermal annealing on strain in ultrathin strained silicon on insulator layers

The fabrication of ultrathin strained silicon directly on insulator is demonstrated and the thermal stability of these films is investigated. Ultrathin (∼13 nm) strained silicon on insulator layers were fabricated by epitaxial growth of strained silicon on relaxed SiGe, wafer bonding, and an etch-back technique employing two etch-stop layers for improved across wafer thickness uniformity. Using 325 nm Raman spectroscopy, no strain relaxation is observed following rapid thermal annealing of these layers to temperatures as high as 950 °C. The thermal stability of these films is promising for the future fabrication of enhanced performance strained Si ultrathin body and double-gate metal-oxide-semiconductor field-effect transistors.