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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Electrochemical Synthesis of Highly Oriented, Transparent, and Pinhole-Free ZnO and Al-Doped ZnO Films and Their Use in Heterojunction Solar Cells

Electrochemical synthesis conditions using nonaqueous solutions were developed to prepare highly transparent (T > 90%) and crystalline ZnO and Al-doped ZnO (AZO) films for use in solar energy conversion devices. A focused effort was made to produce pinhole-free films in a reproducible manner by identifying a key condition to prevent the formation of cracks during deposition. The polycrystalline domains in the resulting films had a uniform orientation (i.e. the c-axis perpendicular to the substrate), which enhanced the electron transport properties of the films. Furthermore, electrochemical Al doping of ZnO using nonaqueous media, which was demonstrated for the first time in this study, effectively increased the carrier density and raised the Fermi level of ZnO. These films were coupled with an electrodeposited p-type Cu 2 O to construct p-n heterojunction solar cells to demonstrate the utilization of these films for solar energy conversion. The resulting n-ZnO/p-Cu 2 O and n-AZO/p-Cu 2 O cells showed excellent performance compared with previously reported n-ZnO/p-Cu 2 O cells prepared by electrodeposition. In particular, replacing ZnO with AZO resulted in simultaneous enhancements in short circuit current and open circuit potential, and the n-AZO/p-Cu 2 O cell achieved an average power conversion efficiency (η) of 0.92 ± 0.09 %. Furthermore, the electrodeposition condition reported here will offer a practical and versatile way to produce ZnO or AZO films, which play key roles in various solar energy conversion devices, with qualities comparable to those prepared by vacuum-based techniques.

14 SOLAR ENERGY↗
DOE OSTI2015

Electronic band structure of ZnO-rich highly mismatched ZnO{sub 1−x}Te{sub x} alloys

We synthesized ZnO{sub 1−x}Te{sub x} alloys with Te composition x < 0.23 by using pulsed laser deposition. Alloys with x < 0.06 are crystalline with a columnar growth structure while samples with higher Te content are polycrystalline with random grain orientation. Electron microscopy images show a random distribution of Te atoms with no observable clustering. We found that the incorporation of a small concentration of Te (x ∼ 0.003) redshifts the ZnO optical absorption edge by more than 1 eV. The minimum band gap obtained in this work is 1.8 eV for x = 0.23. The optical properties of the alloys are explained by the modification of the valence band of ZnO, due to the anticrossing interactions of the localized Te states with the ZnO valence band extended states. Hence, the observed large band gap reduction is primarily originating from the upward shift of the valence band edge. We show that the optical data can be explained by the band anticrossing model with the localized level of Te located at 0.95 eV above the ZnO valence band and the band anticrossing coupling constant of 1.35 eV. These parameters allow the prediction of the compositional dependence of the band gap as well as the conduction and the valence band offsets in the full composition range of ZnO{sub 1−x}Te{sub x} alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Formation of Luminescence Spectra and Emission Intensity in the UV and Visible Spectral Regions for n-ZnO/p-GaN and n-ZnO/p-ZnO Structures when Depositing ZnO Films by High-Frequency Magnetron Sputtering

The emission spectra of structures based on ZnO films deposited by high-frequency magnetron sputtering are studied. Clearly pronounced emission lines associated with the recombination of free (λ = 363 nm) and bound excitons (λ = 377, 390, 410 nm) are observed in the PL (photoluminescence) spectra (T = 300 K) of n-ZnO/p-GaN:Mg structures, and no substantial emission is observed in the impurity PL region λ = 450–600 nm. Only emission lines characteristic of n-ZnO (λ = 374 nm) are observed in the EL (electroluminescence) spectra of n-ZnO/p-ZnO structures (T = 300 K).

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural stability and defect energetics of ZnO from diffusion quantum Monte Carlo

We have applied the many-body ab initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure and the energetics of the oxygen vacancy, zinc interstitial, and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are challenging for density functional theory (DFT) approximations. DMC agrees with experimental measurements to within 0.3 eV, including the band-gap of ZnO, the ionization potential of O and Zn, and the atomization energy of O{sub 2}, ZnO dimer, and wurtzite ZnO. DMC predicts the oxygen vacancy as a deep donor with a formation energy of 5.0(2) eV under O-rich conditions and thermodynamic transition levels located between 1.8 and 2.5 eV from the valence band maximum. Our DMC results indicate that the concentration of zinc interstitial and hydrogen impurities in ZnO should be low under n-type and Zn- and H-rich conditions because these defects have formation energies above 1.4 eV under these conditions. Comparison of DMC and hybrid functionals shows that these DFT approximations can be parameterized to yield a general correct qualitative description of ZnO. However, the formation energy of defects in ZnO evaluated with DMC and hybrid functionals can differ by more than 0.5 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Thermoelectric properties of rocksalt ZnO from first-principles calculations

Zinc oxide (ZnO) undergoes a pressure-induced structural transition from its normal ambient-pressure wurtzite (WZ) phase to a rocksalt (RS) phase around 10 GPa. A recent experiment shows that the high-pressure RS ZnO phase can be recovered and stabilized at ambient conditions, which raises exciting prospects of expanding the range of properties of ZnO. For a fundamental understanding of the RS ZnO phase, we have performed first-principles calculations to determine its electronic, phonon, and thermodynamic properties at high (20 GPa) and ambient (0 GPa) pressure. Furthermore, we have calculated its electrical and thermal transport properties, which allow an evaluation of its thermoelectric figure of merit ZT at different temperature and doping levels. Our calculations show that the ambient-pressure RS ZnO phase can reach ZT values of 0.25 to 0.3 under both n-type and p-type doping in a large temperature range of 400 K to 800 K, which is considerably lower than the temperature range of 1400 K to 1600 K where WZ ZnO reaches similar ZT values. These results establish RS ZnO as a promising material for thermoelectric devices designed to operate at temperatures desirable for many heat recovery applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Al-doped ZnO seed layer-dependent crystallographic control of ZnO nanorods by using electrochemical deposition

Highlights: • Polar and semipolar ZnO NRs were successfully achieved by hydrothermal synthesis. • Semipolar and polar ZnO NRs were grown on ZnO and AZO/m-sapphire, respectively. • Al % of AZO/m-sapphire enhanced the lateral growth rate of polar ZnO NRs. - Abstract: We investigated the effect of an Al-doped ZnO film on the crystallographic direction of ZnO nanorods (NRs) using electrochemical deposition. From high-solution X-ray diffraction measurements, the crystallographic plane of ZnO NRs grown on (1 0 0) ZnO/m-plane sapphire was (1 0 1). The surface grain size of the (100) Al-doped ZnO (AZO) film decreased with increasing Al content in the ZnO seed layer, implying that the Al dopant accelerated the three-dimensional (3D) growth of the AZO film. In addition, it was found that with increasing Al doping concentration of the AZO seed layer, the crystal orientation of the ZnO NRs grown on the AZO seed layer changed from [1 0 1] to [0 0 1]. With increasing Al content of the nonpolar (1 0 0) AZO seed layer, the small surface grains with a few crystallographic planes of the AZO film changed from semipolar (1 0 1) ZnO NRs to polar (0 0 1) ZnO NRs due to the increase of the vertical [0 0 1] growth rate of the ZnO NRs owing to excellent electrical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

In Situ TEM Investigation of ZnO Nanowires during Sodiation and Lithiation Cycling

The development of sodium-ion batteries has drawn lots of attention recently due to the low-cost and eco-friendly sodium source. However, a fundamental understanding of the sodiation behavior for commonly used electrode materials is still limited. Here, combining in situ transmission electron microscopy, aberration-corrected scanning transmission electron microscopy, and ex situ battery cycling tests, the lithiation and sodiation behavior of ZnO nanowires is investigated. The findings show a direct correlation between the mechanical behavior of the lithiated/sodiated ZnO nanowires and their electrochemical cyclability. The mechanical brittleness of LiZn and the formation of nanocracks lead to the poor cyclability of Li-ion batteries with a ZnO anode. However, the sodiated ZnO nanowires show profuse dislocation plasticity. The observed high-density dislocations offer the sodiated ZnO anode more ductility and subsequently better cyclability than its Li-ion counterpart. The results reveal the importance of understanding the correlation between mechanical properties of battery electrodes and their cycling abilities.

Asayesh-Ardakani, Hasti↗
DOE OSTI2017

Growth and characterization of ZnO nanostructure on TiO{sub 2}-ZnO films as a light scattering layer for dye sensitized solar cells

Graphical abstract: Pictorial representation of (a,b) electron transport path in nanoparticles and nanorods (red arrow marks), and (c,d) light scattering effect on TiO2-ZnO film without and with light scattering layer (blue, red and green arrow marks are incident, transmit and scattering light, respectively). - Abstract: In the present work, we studied the effect of light scattering layer on the performance of DSSCs. TiO{sub 2}-ZnO thin films were coated onto FTO glass substrate by spin coating technique as a photoanode for DSSCs. ZnO nanorods with flower structures were potentiostatically grown on TiO{sub 2}-ZnO films at various potentials from −1.1 to 1–1.3 V as a light scattering layer. XRD patterns reveal that as the deposition potentials increase the growth of ZnO nanorods increases along with (101) plane orientation. FE-SEM images display agglomerated grains and nanorods with flower structures for TiO{sub 2}-ZnO film, and ZnO has grown on TiO{sub 2}-ZnO film at a deposition potential of −1.3 V. UV–vis spectra confirm the TiO{sub 2}-ZnO with ZnO nanorods having a higher light scattering effect, absorbance and dye loading than that of another film. The efficiency of the DSSCs consisting of without and with scattering layer is estimated to be 0.79 and 1.81%, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Single-Doped and Multidoped Transition-Metal (Mn, Fe, Co, and Ni) ZnO and Their Electrocatalytic Activities for Oxygen Reduction Reaction

The electrochemical oxygen reduction reaction (ORR) is the limiting half-reaction of fuel cells, which is mediated by using platinum-based catalysts. Hence, the development of low-cost, active ORR catalysts is highly required to make fuel cell technology commercially available. In this report, transition-metal (TM; Mn, Fe, Co, and Ni) single-doped and multidoped (MD) ZnO nanocrystals (ZNs) were prepared for use as ORR catalysts using a simple precipitation method. The effects of single doping and multidoping on the structure, morphology, and properties of the TM-doped ZNs were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence, X-ray photoelectron microscopy, electron paramagnetic resonance, and Raman and photoluminescence (PL) spectroscopies. The XRD results reveal that synthesized ZnO samples retained a pure hexagonal wurtzite crystal structure, even at high levels of multidoping (nominal 20%). SEM analyses show that the morphology of the prepared ZNs varies with the doping elements, doping mode, and amounts of doping. The observation of peak shifting and peak intensity changes in Raman studies confirms the presence of dopants in ZnO. The PL investigation reveals that the incorporation of dopants into the ZnO structure increases the oxygen vacancies within the materials. The highest oxygen vacancies were present in Mn-doped ZnO and 15% MD ZnO among the single-doped and MD samples, respectively. Linear-sweep voltammetry studies conclude that doped ZnO shows enhanced ORR activity compared to the undoped samples. The Mn-doped ZnO and 15% MD ZnO exhibited the highest ORR activity among the prepared single-doped and MD ZN samples, respectively. In comparison, single doping showed better ORR activity than the multidoping system. The enhanced ORR activity of the synthesized ZN materials correlates with the amount of oxygen vacancies present in the samples. Finally, the enhanced activity of TM-doped ZnO suggests that these materials can be used as potential, low-cost electrocatalysts for ORR in fuel cell technology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Acceptors in ZnO

Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence indicates these point defects have acceptor levels 3.2, 1.4, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO{sub 2} contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals is attributed to an acceptor, which may involve a Zn vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g{sub ⊥} = 2.0015 and g{sub //} = 2.0056, along with an isotropic center at g = 2.0035.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Optimization and understanding of ZnO nanoarray supported Cu-ZnO-Al 2 O 3 catalyst for enhanced CO 2 -methanol conversion at low temperature and pressure

Cu-ZnO-Al 2 O 3 is the most widely applied catalyst for CO 2 hydrogenation to methanol. However, it is still a challenge to produce methanol using this catalyst under low-temperature (<250 °C) and low-pressure (<10 bar) conditions with desirable yield and selectivity. In this work, by tuning the experimental processing parameters such as solvent, loading amount, and annealing temperature, highly improved ZnO nanoarray supported Cu-ZnO-Al 2 O 3 catalysts have been successfully demonstrated. Here, by using organic solvent (N,N-dimethylformamide (DMF), acetone, or isopropanol) for dip-coating loading process instead of deionized (DI) water, Cu-ZnO-Al 2 O 3 nanocatalysts was comparatively better dispersed on the nanorod array support with populated and abundant active sites, thus enhancing the methanol yield. With the control of the loading amount and annealing temperature, finely distributed Cu nanoparticles were obtained on the ZnO nanorod surfaces to enhance the interactions between Cu and ZnO nanorod surfaces. Further improvement of the catalyst performance is demonstrated by tuning the reaction space velocity. At 200 °C and 10 bar conditions, the optimized catalyst achieved a methanol yield of 6.46 mol h –1 kg –1 with 100 % selectivity. The good stability after prolonged testing of the catalysts demonstrates the potential practical implementation. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements under the 1 bar reveal that the CO 2 hydrogenation to methanol on the ZnO nanoarray supported Cu-ZnO-Al 2 O 3 catalyst follows the CO reaction pathway, due to the surface oxygen vacancies on ZnO nanorods which facilitate CO 2 dissociation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Hierarchical assembly of ZnO nanowire trunks decorated with ZnO nanosheets for lithium ion battery anodes

Hierarchical architectures composed of nanomaterials in different forms are essential to improve the performance of lithium-ion battery (LIB) anodes. Here, we systematically studied the effects of hierarchical ZnO nanostructures on the electrochemical performance of LIBs. ZnO nanowire (NW) trunks were decorated with ZnO NWs or ZnO nanosheets (NSs) by successive hydrothermal synthesis to create hierarchical three-dimensional nanostructures. The branched ZnO NSs on the ZnO NW trunk exhibited a two-fold higher specific gravimetric capacity compared to ZnO NWs and branched ZnO NWs on ZnO NW trunks after 100 cycles of charging–discharging at 0.2C (197.4 mA g –1 ). The improvement in battery anode performance is attributable to the increased interfacial area between the electrodes and electrolyte, and the void space of the branched NSs that facilitates lithium ion transport and volume changes during cycling.

25 ENERGY STORAGE↗
DOE OSTI2014

In vitro cytotoxicity tests of ZnO‐Bi{sub 2}O{sub 3}‐Mn{sub 2}O{sub 3}-based varistor fabricated from ZnO micro and nanoparticle powders on L929 mouse cells

The present study investigated the cytotoxicity of ZnO‐Bi{sub 2}O{sub 3}‐Mn{sub 2}O{sub 3}-varistors. To this effect, ZnO‐Bi{sub 2}O{sub 3}‐Mn{sub 2}O{sub 3} varistors fabricated from ZnO micro-and nanoparticle powders are prepared via conventional ceramic processing method. The effects of ZnO particle size on the properties of ZnO varistors are also investigated. The strong solid-state reaction during sintering may be attributed to the high surface area of the 20 nm ZnO nanoparticles that promote strong surface reaction. The intensity of XRD peaks reflected the high degree of crystallinity of the ZnO nanoparticles. However, the width of the peaks in case of ZnO nanoparticles has increased due to the quantum size effect. The cytotoxicity evaluation of ZnO varistor was conducted on mouse connective tissue fibroblast cells (L929) using Trypan Blue Exclusion Assay analysis. The results show that the four types of varistor samples lead to cellular mitochondrial dysfunction, morphological modifications and apoptosis at the various concentration range and the toxic effects are obviously displayed in high concentration samples. 20nm-VDR is the most toxic materials followed by 40nm-VDR, P8-VDR, and W4-VDR in a descending order.

36 MATERIALS SCIENCE↗
DOE OSTI2018

First principles study of the structural, electronic, and optical properties of Sn 2+ -doped ZnO–P 2 O 5 glasses [First principles study of the structural, electronic and optical properties of Sn-doped ZnO-P2O5 glasses]

The atomic-structural, electronic, and optical properties of Sn-doped ZnO–P 2 O 5 glasses are examined using combined semi-empirical and density functional theory calculations. From model structures for two different ZnO–P 2 O 5 glass compositions, it is found that these glasses consist of nearly ideal tetrahedral PO 4 unit and largely distorted ZnO 4 and ZnO 5 units. The concentration of ZnO 5 units is calculated to increase and exceed that of ZnO 4 as the amount of ZnO in the glass is increased. Also, the concentration of terminal oxygen atoms shared by two Zn neighbors is largely increased for 70ZnO:30P 2 O 5 (70:30) glass compared to 60:40 glass, which can be rationalized by electrostatic bond strength calculations. Regarding medium-range order, calculations show that Q 1 and Q 2 structures (where Q n refers to structures containing n bridging oxygen atoms on PO 4 tetrahedra) are dominant for the 60:40 glass, while Q 0 and Q 1 prevail for the 70:30 glass, in good agreement with experimental observations. For the Sn-doped ZnO–P 2 O 5 glass, various Sn coordination geometries are found, with trigonal pyramidal SnO 3 and largely distorted pyramidal SnO 4 configurations being dominant and a small fraction of two-fold and five-fold coordinated Sn configurations also present. From combined cluster and glass model calculations, two-fold coordinated Sn are found to exhibit slightly smaller optical transition energy than three-fold and four-fold coordinated Sn. However, local variations in coordination structures throughout the glass likely result in a broad singlet transition (S 0 →S 1 ) around 5.4 eV for all the different Sn configurations. Finally, a triplet transition (S 0 →T 1 ) driven by spin-orbit coupling can occur around 4.5 – 4.8 eV, but the magnitude of this transition is predicted to be much smaller than the spin-allowed singlet transition.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films

We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses Ge-oxide but subsequent RTA leads to crystalline Ge structure without the oxide layer. In order to study the occupied electronic structure, HAXPES has been utilized. The peak separation between the Zn 2p and Ge 3d shows no significant change due to RTA. This implies little change in the valence band offset due to RTA.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Electrical and structural properties of ZnO synthesized via infiltration of lithographically defined polymer templates

We investigate the electrical and structural properties of infiltration-synthesized ZnO. In-plane ZnO nanowire arrays with prescribed positional registrations are generated by infiltrating diethlyzinc and water vapor into lithographically defined SU-8 polymer templates and removing organic matrix by oxygen plasma ashing. Transmission electron microscopy reveals that homogeneously amorphous as-infiltrated polymer templates transform into highly nanocrystalline ZnO upon removal of organic matrix. Field-effect transistor device measurements show that the synthesized ZnO after thermal annealing displays a typical n-type behavior, ~1019 cm -3 carrier density, and ~0.1 cm 2 V -1 s -1 electron mobility, reflecting highly nanocrystalline internal structure. The results demonstrate the potential application of infiltration synthesis in fabricating metal oxide electronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Hydrogen-related complexes in Li-diffused ZnO single crystals

Zinc oxide (ZnO) is a wide band gap semiconductor and a potential candidate for next generation white solid state lighting applications. In this work, hydrogen-related complexes in lithium diffused ZnO single crystals were studied. In addition to the well-known Li-OH complex, several other hydrogen defects were observed. When a mixture of Li{sub 2}O and ZnO is used as the dopant source, zinc vacancies are suppressed and the bulk Li concentration is very high (>10{sup 19 }cm{sup −3}). In that case, the predominant hydrogen complex has a vibrational frequency of 3677 cm{sup −1}, attributed to surface O-H species. When Li{sub 2}CO{sub 3} is used, a structured blue luminescence band and O-H mode at 3327 cm{sup −1} are observed at 10 K. These observations, along with positron annihilation measurements, suggest a zinc vacancy–hydrogen complex, with an acceptor level ∼0.3 eV above the valence-band maximum. This relatively shallow acceptor could be beneficial for p-type ZnO.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗