Venkat Kalyan Vendra

Tungsten oxide-coated copper oxide nanowire arrays for enhanced activity and durability with photoelectrochemical water splitting

The deposition of crystalline CuWO4 and WO3 layers on copper oxide nanowire arrays resulted in a five-fold improvement of the photocurrent density (∼1.7 mA cm−2 at −0.1 V vs. RHE) over titania-coated copper oxide NW arrays. The deposition of WO3 or CuWO4 reduced the CuO phase impurities in Cu2O. The improvement in the phase purity of the nanowire arrays led to a considerable enhancement in the resulting photoactivity. These protective coatings have been shown to play a dual role of marginally extending the durability of the electrode and promoting charge separation.

New Visible Light Absorbing Materials for Solar Fuels, Ga(Sbx)N1−x

A novel visible-light-absorbing dilute alloy, Ga(Sbx)N1-x is synthesized by metal organic chemical vapor deposition (MOCVD) for solar hydrogen production. Significant bandgap reduction of GaN, from 3.4 eV to 1.8 eV, is observed, with a low (2%) incorporation of antimonide, and the lattice expansion is in agreement with our first-principles calculations. The band edges of Ga(Sbx)N1-x are found to straddle the water redox potentials showing excellent suitability for solar water splitting.

High rate capacity retention of binder-free, tin oxide nanowire arrays using thin titania and alumina coatings

This paper reports the use of thin titania or alumina coatings on tin oxide nanowire arrays for high cyclability electrodes for lithium-ion batteries. We demonstrate that such coatings can significantly reduce irreversible capacity loss associated with the formation of a solid electrolyte interface and improve the capacity retention at high rates. Specifically, tin oxide nanowires grown on stainless steel substrates were conformally coated with thin films of either titania or alumina using atomic layer deposition and were tested as anodes in coin cells. Both titania and alumina coatings resulted in no initial capacity loss due to solid electrolyte interface formation in the first cycle. Tin oxide nanowire array electrodes coated with 5 nm thick titania layer and 1 nm thick alumina layer retained capacities of 767 and 725 mA h g−1 after 30 cycles using current density of 700 mA g−1. Both electrodes retained capacity around 664 mA h g−1 after 30 cycles using a current density of 1500 mA g−1, respectively. The results indicate that thin coatings acted as mechanical shells preserving the electrode nanostructure morphology necessary for high capacity retention. The study also showed that within the first two cycles, tin migrates out forming nanoclusters on the surface of nanowires due to both stress enhanced diffusion and the Kirkendall effect. The presence of tin nanoclusters on the surface of protective layers further enhances high rate capability.

Nanowires as semi-rigid substrates for growth of thick, InxGa1−xN (x > 0.4) epi-layers without phase segregation for photoelectrochemical water splitting

Here, we show that GaN nanowires (diameter <30 nm) can be used as strain relaxing substrates for the heteroepitaxial growth of stable In(x)Ga(1-x)N alloys of controlled composition and thickness. Thinner nanowires with their smaller interfacial area reduce the heteroepitaxial stress. Also, the limited adatom diffusion length scales on the thinner nanowires aid in reducing the kinetic segregation effects. In addition to being single crystal templates for heteroepitaxial growth, these thick single crystal overlayers on nanowire substrates can provide suitable architectures for photoelectrochemical applications. The stability and crystallinity of the In(x)Ga(1-x)N layers are preserved by the nanowires acting as compliant substrates. Photoelectrochemical water splitting requires In(x)Ga(1-x)N alloys with a 2.2-1.6 eV band gap (i.e. 0.45 < x < 0.65) and 150-200 nm film thickness for efficient light absorption and carrier generation. At such compositions, the In(x)Ga(1-x)N alloys are inherently unstable, the thickness-dependent stress builds up during the commonly employed heteroepitaxial growth methods, and adds to the instability causing phase segregation and property degradation. A dependence of the growth morphology on the GaN nanowire growth orientation was observed and a growth mechanism is presented for the observed orientation dependent growth on a-plane and c-plane GaN nanowires. Photoactivity of GaN and In(x)Ga(1-x)N films on GaN nanowires is also investigated which shows a distinct difference attributable to GaN and In(x)Ga(1-x)N, demonstrating the advantages of using nanowires as strain relaxing substrates.

Scalable synthesis and surface stabilization of Li2MnO3 NWs as high rate cathode materials for Li-ion batteries

Li2MnO3 nanowires (NWs) are synthesized using a scalable two-step process involving a solvo-plasma technique, utilizing inexpensive precursors such as commercially available MnO2 microparticle powders and KCl, followed by a solid state lithiation process. Lithium manganese oxide (Li2MnO3) nanowires exhibited high capacity retention of 120 mA h g−1 in the 2–4.5 V voltage window even at high C-rates such as 20 C. The specific capacity of the Li2MnO3 NWs gradually increased with cycling and subsequently stabilized. Further, the Li2MnO3 NW cathodes exhibited no loss in the capacity for 100 cycles with close to 100% coulombic efficiency. Most importantly, single crystalline Li2MnO3 nanowires with short transport length scales for Li, O and Mn atoms along the radial direction allow for the formation of a thick and conformal LiMn2O4 shell resulting in increased capacity, excellent capacity retention and high coulombic efficiencies.

Nanowire architectures for iodide free dye-sensitized solar cells

In this study, we show that the performance of iodide free redox couples in dye-sensitized solar cells could be significantly improved by engineering the electron transport and surface properties of the electrode materials. Specifically, tin oxide nanowires electrophoretically coated with titania nanoparticles and subsequently passivated with a submonolayer of alumina by atomic layer deposition show a remarkable ten-fold increase in short-circuit current densities over those obtained with titania nanoparticles, even when a typical N-719 dye is used for sensitization. Comparison of the performance of different electrode materials such as nanowires, nanoparticles and nanowire–nanoparticle hybrid architectures of tin oxide and titania suggests that fast electron transport helps in improving the short-circuit current density with ferrocene/ferrocenium and TEMPO redox couples. The nature of the surface trap states and their passivation have a significant effect on the electron lifetimes in the semiconductor and the resulting open-circuit voltage with these redox couples. The higher electron diffusion lengths with the tin oxide nanowire based architectures allow for thicker electrodes with enhanced dye loading. The analysis of literature data on DSCs made using different dyes and alternate redox couples suggests smaller delta G or reorganization energy for non-ruthenium based dyes.

Implantable thin-film porous microelectrode array (P-MEA) for electrical stimulation of engineered cardiac tissues

We have designed, fabricated, and validated a novel porous, multielectrode array (P-MEA) device capable of low-voltage electrical stimulation of engineered cardiac tissues (ECTs). The primary advantage of this device is the ability to successfully function at a very low voltage thus minimizing any undesirable oxidative by-products in the culture environment or cell injury. Major features of our P-MEA include dimensions of 10 mm width and 82 mm length, four arms to allow movement of the individual pads within ECTs, each embedded electrode arm incorporates eight 100 µm×200 µm rectangular pores surrounding a 950 µm×340 µm exposed electrode, large pads on either side of the porous embedded device to function as current return electrodes, suture holes to aid in vivo suturing and stabilization, and an eight electrode connector pads. Average thickness of the Ni/Au electrodes was 20 nm of nickel and 400 nm of gold, an average electrode film thickness of 0.4 µm, and a double polyimide layer thickness of 16 µm. Electrode resistance ranged from 69.45 O to 78.52 O. Electrochemical impedance spectroscopy confirmed that the P-MEA operates in the 0.01 V to 1.0 V range with favorable charge transfer characteristics. Proof of principle experiments confirmed the ability of the P-MEA to effectively embed within ECT and electrically stimulate ECT during chronic, in vitro culture. Histology imaging shows that the embedding of the device has no adverse effects on the ECT and the cardiomyocytes are aligned within the tissue. Experiments are ongoing to evaluate the role of electrical stimulation on the maturation and function of ECTs.

Engineering the Photoanode Using Scalable Hybrid Nanostructures

Dye sensitized solar cells (DSSCs) have garnered a great deal of interest as a cost-effective technology for large-scale manufacturing. Engineered inorganic hybrid nanostructures can improve the performance of DSSC’s without affecting the cost effectiveness of the devices. Here, we present a concept of engineered hybrid nanostructures, incorporating appropriate selection of nanowire and nanoparticle materials, to enhance the charge transport and reduce the recombination within the photoanode. Low recombination properties of this photoanode allow flexibility in choosing the redox couple for increasing open circuit voltage.Copyright