fa Yan

Nanostructured Fe3O4/SWNT Electrode: Binder‐Free and High‐Rate Li‐Ion Anode

Rechargeable Li-ion batteries are currently being explored for high-power applications such as electric vehicles. However, in order to deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes made from durable, nontoxic, and inexpensive materials with a high charge/discharge rate and a high reversible capacity. Transition metal oxides such as Fe3O4, Fe2O3, MoO3, and Co3O4 [1–5] are capable of Liþ insertion/ extraction in excess of 6 Liþ per formula unit, resulting in a significantly larger reversible capacity than commercially employed graphite. In contrast to the intercalation mechanism that occurs for graphite, the transitionmetal oxides are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Liþ to form Li2O. [1,2,6] This usually leads to large volume expansion and destruction of the structure upon electrochemical cycling, especially at high rate. Hence, optimizing particle size and mixing the particles with various carbon additives have been employed to improve the reversible capacity and rate capability of metal oxide electrodes. Among the transition metal oxides, Fe3O4 is both nontoxic and abundant (inexpensive) and is thus considered one of the most promising electrode materials. However, a truly durable high-rate capability and a high capacity for metal oxide based electrodes including Fe3O4 have not yet been achieved. To achieve high-rate capability and high capacity using metal oxide nanoparticles mixed with carbon materials, there are three key issues that must be considered: i) the size of the nanoparticles must be optimized such that rapid Li-ion diffusion and reaction with metal oxide nanoparticles are achieved, ii) an optimized carbon matrix must be developed that ensures both electrical conductivity and good thermal conductivity (to improve heatdissipation), and iii) the conductive additive must maintain a flexible and strong matrix that accommodates large volume changes. In most conventional electrodes, metal oxide nanoparticles are directly mixed with a carbon additive and a binder to help maintain electrical conductivity, and the large volume expansion then results inmechanical degradation of the electrode when cycled at high rate. Here we employ the unique properties of highly crystalline and long single-walled carbon nanotubes (SWNTs) to simultaneously address all of the three key issues with a simple two-step process to synthesize Fe3O4 nanoparticles embedded uniformly in an interconnected ‘‘SWNT net.’’ Furthermore, no polymer binder is required to maintain electrical conductivity. The electrodes contain 95wt% active material with only 5wt% SWNTs as the conductive additive (typical electrodes contain 80wt% active material and 20wt% conductive and binder additives). Most importantly, by using these binder-free electrodes, we have demonstrated a high reversible capacity of 1000mAhg 1 ( 2000mAh cm ) at C rate as well as high-rate capability and stable capacities of 800mAhg 1 at 5C (both for over 100 deep charge/discharge cycles) and 600mAhg 1 at 10C. Raman spectroscopy suggests that this remarkable rate capability is achieved because the Fe3O4 nanoparticles are actually bound to the flexible nanotube net. We also believe that this fabrication method may be employed for other active materials to achieve a binder-free, high-rate, and durable electrode. The FeOOH nanorods, employed as a precursor in the electrode fabrication process, have a width of 50 nm, length of 250 nm, and thickness of 20 nm and are formed with a simple hydrothermal process. X-ray diffraction (XRD) spectra of the as-prepared nanorods and reference a-FeOOH phase (goethite, JCPDS 81-0463) are shown in Figure 1a. All of the reflection peaks can be indexed to the tetragonal a-FeOOH phase. Next we created Fe3O4 nanoparticles embedded in an interconnected SWNTnetwork using FeOOHnanostructures and SWNTs as precursors for a vacuum-filtration and subsequent annealing process. We found that annealing the FeOOH nanorods without SWNTs to 450 8C in an argon atmosphere leads to a mixture of a-Fe2O3 (hematite) and Fe3O4 (magnetite) as indicated by the XRD patterns in Figure 1b. The peaks marked with * are indexed to the Fe3O4 phase (JCPDS 88-0315) and the remainder of the diffraction peaks are indexed to a-Fe2O3, (JCPDS 33-0664). In contrast, annealing FeOOHnanorods mixed with 5wt% SWNTs at 450 8C in an argon atmosphere leads to the complete reduction of FeOOH to Fe3O4, as indicated in Figure 1c. It is therefore evident that the SWNTs actually facilitate the formation of Fe3O4 nanoparticles, enabling excellent Fe3O4 nanoparticle/SWNT electronic and mechanical contact, which is further confirmed by the Raman spectroscopy analysis discussed later. The elegant morphology of the Fe3O4 nanorods embedded uniformly in the SWNT net is clearly depicted in the scanning electronmicroscope (SEM) image of Figure 2a. Figure 2b displays

Microstructure and Pseudocapacitive Properties of Electrodes Constructed of Oriented NiO-TiO2 Nanotube Arrays

We report on the synthesis and electrochemical properties of oriented NiO-TiO(2) nanotube (NT) arrays as electrodes for supercapacitors. The morphology of the films prepared by electrochemically anodizing Ni-Ti alloy foils was characterized by scanning and transmission electron microscopies, X-ray diffraction, and photoelectron spectroscopies. The morphology, crystal structure, and composition of the NT films were found to depend on the preparation conditions (anodization voltage and postgrowth annealing temperature). Annealing the as-grown NT arrays to a temperature of 600 °C transformed them from an amorphous phase to a mixture of crystalline rock salt NiO and rutile TiO(2). Changes in the morphology and crystal structure strongly influenced the electrochemical properties of the NT electrodes. Electrodes composed of NT films annealed at 600 °C displayed pseudocapacitor (redox-capacitor) behavior, including rapid charge/discharge kinetics and stable long-term cycling performance. At similar film thicknesses and surface areas, the NT-based electrodes showed a higher rate capability than the randomly packed nanoparticle-based electrodes. Even at the highest scan rate (500 mV/s), the capacitance of the NT electrodes was not much smaller (within 12%) than the capacitance measured at the slowest scan rate (5 mV/s). The faster charge/discharge kinetics of NT electrodes at high scan rates is attributed to the more ordered NT film architecture, which is expected to facilitate electron and ion transport during the charge-discharge reactions.

Doping of ZnO by group-IB elements

The authors present their first-principles calculations of doping effects in ZnO with group-IB elements such as Cu, Ag, and Au. The calculated transition energies e(0∕−) for substitutional Cu, Ag, and Au are 0.7, 0.4, and 0.5eV, respectively. The calculated formation energies are very low for these group-IB elements on the substitutional sites, but rather high at the interstitial sites under oxygen-rich growth conditions. Under the conditions, the formation of major hole-killer defects, such as oxygen vacancies and Zn interstitial, are suppressed. Thus, Ag may be a good candidate for producing p-type ZnO.

Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn2SnO4, thin films

Single-phase, spinel zinc stannate (Zn2SnO4) thin films were grown by rf magnetron sputtering onto glass substrates. Uniaxially oriented films with resistivities of 10−2–10−3 Ω cm, mobilities of 16–26 cm2/V s, and n-type carrier concentrations in the low 1019 cm−3 range were achieved. X-ray diffraction peak intensity studies established the films to be in the inverse spinel configuration. 119Sn Mossbauer studies identified two octahedral Sn sites, each with a unique quadrupole splitting, but with a common isomer shift consistent with Sn+4. A pronounced Burstein–Moss shift moved the optical band gap from 3.35 to as high as 3.89 eV. Density-of-states effective mass, relaxation time, mobility, Fermi energy level, and a scattering parameter were calculated from resistivity, Hall, Seebeck, and Nernst coefficient transport data. Effective-mass values increased with carrier concentration from 0.16 to 0.26 me as the Fermi energy increased from 0.2 to 0.9 eV above the conduction-band minimum. A bottom-of-the-band ...

Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction

Due to its large band gap (3.2 eV), TiO2 cannot absorb sun light effectively. To reduce its band gap, various approaches have been attempted; most of them are using doping to modify its band structure. Using first-principles band structure calculations, we show that unlike the rutile phases, the band gap of TiO2 in the anatase phase can be effectively reduced by applying stress along a soft direction. We propose that this approach of tuning the band gap by applying stress along soft direction of a layered semiconductor is general and should be applicable to other anisotropic materials.

Enhanced Photoelectrochemical Responses of ZnO Films Through Ga and N Codoping

We report on the crystallinity and photoelectrochemical (PEC) response of ZnO thin films codoped by Ga and N. The ZnO:(Ga,N) thin films were deposited by cosputtering at room temperature and followed by postannealing at 500°C in air for 2h. We found that ZnO:(Ga,N) thin films exhibited significantly enhanced crystallinity compared to ZnO doped solely with N at the same growth conditions. Furthermore, ZnO:(Ga,N) thin films exhibited enhanced N incorporation over ZnO doped solely with N at high temperatures. As a result, ZnO:(Ga,N) thin films achieved dramatically improved PEC response, compared to ZnO thin films doped solely with N at any conditions. Our results suggest a general way to improve PEC response for wide-band-gap oxides.

Chemical vapor deposition-formed p-type ZnO thin films

We have fabricated nitrogen-doped zinc oxide (ZnO) films that demonstrate p-type behavior by using metalorganic chemical vapor deposition. In our experiment, diethylzinc is used as a Zn precursor, and NO gas is used to supply both O and N to form a N-doped ZnO (ZnO:N) film. With these precursors, we have routinely reached an N concentration in the ZnO films of about 1–3 at. %. When the N concentration level is higher than 2 at. %, the films demonstrate p-type characteristics. The carrier concentration of the films varies from 1.0×1015 to 1.0×1018 cm−3, and mobilities are mainly in the 10−1 cm2 V−1 s−1 range. The lowest film resistivity achieved is ∼20 Ω cm.

Conformal Surface Coatings to Enable High Volume Expansion Li-Ion Anode Materials

An alumina surface coating is demonstrated to improve electrochemical performance of MoO(3) nanoparticles as high capacity/high-volume expansion anodes for Li-ion batteries. Thin, conformal surface coatings were grown using atomic layer deposition (ALD) that relies on self-limiting surface reactions. ALD coatings were tested on both individual nanoparticles and prefabricated electrodes containing conductive additive and binder. The coated and non-coated materials were characterized using transmission electron microscopy, energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, and galvanostatic charge/discharge cycling. Importantly, increased stability and capacity retention was only observed when the fully fabricated electrode was coated. The alumina layer both improves the adhesion of the entire electrode, during volume expansion/contraction and protects the nanoparticle surfaces. Coating the entire electrode also allows for an important carbothermal reduction process that occurs during electrode pre-heat treatment. ALD is thus demonstrated as a novel and necessary method that may be employed to coat the tortuous network of a battery electrode.

Electronic, structural, and magnetic effects of 3d transition metals in hematite

We present a density-functional theory study on the electronic structure of pure and 3d transition metal (TM) (Sc, Ti, Cr, Mn, and Ni) incorporated α-Fe2O3. We find that the incorporation of 3d TMs in α-Fe2O3 has two main effects such as: (1) the valence and conduction band edges are modified. In particular, the incorporation of Ti provides electron carriers and reduces the electron effective mass, which will improve the electrical conductivity of α-Fe2O3. (2) The unit cell volume changes systematically such as: the incorporation of Sc increases the volume, whereas the incorporation of Ti, Cr, Mn, and Ni reduces the volume monotonically, which can affect the hopping probability of localized charge carriers (polarons). We discuss the importance of these results in terms of the utilization of hematite as a visible-light photocatalyst.

Synthesis of band-gap-reduced p-type ZnO films by Cu incorporation

p-type ZnO thin films with significantly reduced band gaps were synthesized by heavy Cu incorporation at room temperature and followed by postdeposition annealing at 500°C in air for 2h. All the films were synthesized by rf magnetron sputtering on F-doped tin oxide-coated glass. The p-type conductivity was confirmed by Mott-Schottky plots and illuminated I-V analysis. The Cu+1 acceptor states (at substitutional sites) and their band-gap reduction were demonstrated by UV-visible absorption and x-ray excited valence band measurements.