Ionic conductivity in oxide heterostructures: the role of interfaces

Rapidly growing attention is being directed to the investigation of ionic conductivity in oxide film heterostructures. The main reason for this interest arises from interfacial phenomena in these heterostructures and their applications. Recent results revealed that heterophase interfaces have faster ionic conduction pathways than the bulk or homophase interfaces. This finding can open attractive opportunities in the field of micro-ionic devices. The influence of the interfaces on the conduction properties of heterostructures is becoming increasingly important with the miniaturization of solid-state devices, which leads to an enhanced interface density at the expense of the bulk. This review aims to describe the main evidence of interfacial phenomena in ion-conducting film heterostructures, highlighting the fundamental and technological relevance and offering guidelines to understanding the interface conduction mechanisms in these structures.     

Ionically conducting oxide composite thin films and heterostructures

Ionic conductivity in oxide composite thin films has recently drawn growing attention due to the unique interfacial phenomena and huge application potential. In this review article the author presents the recent progress in three materials classes including (1) the single-layer heteroepitaxial structures, (2) multilayer structures, and (3) vertically aligned heteroepitaxial structures. This review aims to give an overview of this field and offer guidelines to understand the interfacial phenomena. Two promising research directions in this field are proposed at the end, including understanding the interfacial phenomena in heterostructures, and exploring new methods which can separate thin films from their substrates.

This review was submitted as part of the 2017 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged     

Longitudinal conductivity of LaF3/SrF2 multilayer heterostructures

LaF₃/SrF₂ multilayer heterostructures with thicknesses of individual layers in the range 5–100 nm have been grown on MgO(100) substrates using molecular beam epitaxy. The longitudinal conductivity of the films has been measured using impedance spectroscopy in the frequency range 10^?1–10⁶ Hz and a temperature range 300–570 K. The ionic DC conductivities have been determined from Nyquist impedance diagrams and activation energies from the Arrhenius–Frenkel equation. An increase of the DC conductivity has been observed to accompany decreased layer thickness for various thicknesses as small as 25 nm. The greatest conductivity has been shown for a multilayer heterostructure having thicknesses of 25 nm per layer. The structure has a conductivity two orders of magnitude greater than pure LaF₃ bulk material. The increasing conductivity can be understood as a redistribution of charge carriers through the interface due to differing chemical potentials of the materials, by strong lattice-constant mismatch, and/or by formation of a solid La_1-xSr_xF_3-x solution at the interface during the growth process.     

Computer Modeling of Ionic Conductivity in Low Temperature Doped Ceria Solid Electrolytes

Solid oxides, such as ceria (CeO₂) doped with cations of lower valance, are potential electrolytes for future solid oxide fuel cells. This is due to the theoretically high ionic conductivity at low operation temperature. This paper investigates the feasibility of two potential electrolytes which are samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) to replace the traditional yttria-stablized zirconia (YSZ). Molecular simulation techniques were employed to study the influence of different dopant concentrations at different operation temperatures on the ionic conductivity from the atomistic perspective. Simulation results show that the optimized ionic conductivity occurs at 11.11mol% concentration using both dopants of Gd₂O₃ and Sm₂O₃. The temperature effect was also examined under a fixed concentration simulation to check how low temperature they still function. The predicted ionic conductivities have been verified with published experimental results and show reasonable agreements. This simulation technique reveals a clear picture with qualitative and quantitative connection between the choice of the dopant and the improvement of the ionic conductivity of fuel cell electrolytes.     

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

Abstract

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.     

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.     

Gas-chromism in ultrasonic spray pyrolyzed tungsten oxide thin films

A simple and inexpensive ultrasonic spray pyrolysis (USP) technique has been employed to deposit tungsten oxide (WO₃) thin films by spraying 2.0 mM aqueous ammonium metatungstate solution onto the amorphous glass substrates kept at 250°C. The films were further annealed at 400°C for 4 h in air. X-ray diffraction (XRD) technique was used to determine the crystallinity and to identify the WO₃ phases. It was found that the films were sub-stoichiometric, WO_3-z. To study gas-induced properties, a catalyzing layer of platinum (Pt) was sputtered onto it. The gas-induced electrical and optical properties of Pt/WO₃/glass samples were studied and results reported. It was found that electrical resistivity decreased by a factor of 10 within 2 min and stabilized after 15 min, after H₂ gas exposure. Similarly the optical transmittance of the samples attenuated from 55% to 10% within 15–20 min. The reversible changes in electrical resistivity and optical transmittance were observed when the samples were exposed to oxygen. The response times and sensitivity of the samples were estimated.     

A method for the accurate measurement of the complex conductivity of high-Tc superconductive thin films

In this paper, we introduce a method which makes it possible to rapidly and accurately determine the penetration depth as a function of temperature for superconductive thin film samples. A key feature of the approach described here is that it is derived only from electrodynamic definitions and makes no use of a selected model of superconductivity. Another advantage relative to some of the alternative methods presently in use is that it can be expected to give useful results for films with thicknesses that are as much as three times the zerotemperature effective penetration depth. When combined with an accurate evaluation ofR _S as a function of temperature for the same samples, these penetration depth data enable the computation of the complex conductivity for a wide variety of samples. One shortcoming of the method is the fact that it performs well only at temperatures below about 0.95T _c.     

Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites

We have demonstrated a facile and efficient strategy for the fabrication of soluble reduced graphene oxide sheets (RGO) and the preparation of titanium oxide (TiO₂) nanoparticle-RGO composites using a modified one-step hydrothermal method. It was found that graphene oxide could be easily reduced under solvothermal conditions with ascorbic acid as reductant, with concomitant growth of TiO₂ particles on the RGO surface. The TiO₂-RGO composite has been thoroughly characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy, atomic force microscopy, and transmission electron microscopy) have been employed to probe the morphological characteristics as well as to investigate the exfoliation of RGO sheets. The TiO₂-RGO composite exhibited excellent photocatalysis of hydrogen evolution.      

Low-temperature deposited Titanium-doped zinc oxide thin films on the flexible PET substrate by DC magnetron sputtering

Transparent conducting Titanium-doped zinc oxide thin films (TZO) with high transparency and relatively low resistivity were firstly deposited on water-cooled polyethylene terephthalate (PET) substrates at room temperature by DC magnetron sputtering. The microstructure, optical and electrical properties of the deposited films were investigated and discussed. The XRD patterns show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity decreases when the sputtering power increases from 45 W to 60 W. However, as the puttering power continue increases from 60 W to 90 W, the electrical resistivity increases rapidly. When the puttering power is 60 W, the films deposited on PET substrate have the lowest resistivity of 4.72 × 10⁻⁴ Ω cm and a relatively high transmittance of above 92% in the visible range.     

基于离子阻挡法测量电子-离子混合导体中离子电导率的研究

近年来, 具有诸多新的物理性质和优良热电性能的类液态材料获得了研究人员的广泛关注。离子电导率对理解这类热电材料中的离子迁移行为非常关键。但是, 由于类液态热电材料中的离子电导率在总电导率中的贡献很小, 因此很难利用传统方法精确测量其离子电导率。本研究基于Yokota提出的离子阻挡法, 尝试利用自主搭建的设备测量类液态热电材料的离子电导率。本文详细介绍了该设备的基本构造、测试原理以及测试步骤, 分析了帕尔贴效应/塞贝克效应、离子析出、电压测试点的位置和氧化挥发等因素对测试准确性的影响, 并提出了相应的解决方案, 成功测量了几种具有代表性的类液态热电材料的离子电导率。     

锶镁共掺对Na0.5Bi0.5TiO3氧离子导体电学性能的影响分析

通过固相反应法制备单一结构的锶镁共掺的Na0.5Bi0.48Sr0.02Ti0.98Mg0.02O2.97氧离子导体,利用交流阻抗谱和内耗温度谱分别研究锶镁共掺对Na0.5Bi0.5TiO3材料晶粒电导率及氧离子扩散的影响。在593K时,Na0.5Bi0.48Sr0.02Ti0.98Mg0.02O2.97材料的晶粒电导率可以达到5.31×10^-4S/cm,比母体Na0.5Bi0.5TiO3材料在同温度下的晶粒电导率高一个数量级,甚至超过了Na0.5Bi0.5Ti0.98Mg0.02O2.98样品在673K温度下的晶粒电导率。在锶镁共掺的Na0.5Bi0.48Sr0.02Ti0.98Mg0.02O2.97材料中观察到一个氧弛豫内耗峰,其弛豫参数为:E=0.85eV,τ0=7.4×10^-14s。结合弛豫参数和结构分析,Sr^2+的掺杂在一定程度上可以增大氧离子扩散的自由体积,较大的自由体积、较高的可动氧空位浓度和较好的氧空位可动性是NBT-SrMg2样品晶粒电导率相较于NBT-Mg2样品大幅提高的主要原因。     

连续离子层吸附与反应法(SILAR)生长ZnO多晶薄膜的研究

采用连续离子层吸附与反应法(SILAR),以锌氨络离子([Zn(NH₃)₄]²⁺)为前驱体溶液,在玻璃衬底上沉积了ZnO薄膜,以XRD和SEM等手段分析了薄膜的晶体结构和表面、断面形貌,考察了空气气氛下的退火过程对ZnO薄膜晶体结构与微观形貌的影响,并初步探讨了以SILAR方法沉积ZnO薄膜的机理.结果表明,经200次SILAR沉积循环,所得ZnO薄膜为红锌矿结构的多晶薄膜,沿<002>方向择优生长;薄膜表面致密、光滑均匀,厚度约800nm.退火处理使ZnO薄膜氧缺位减少,晶粒沿c轴取向增强;随退火温度升高,锌间隙原子增加;500℃退火时,ZnO薄膜发生再结晶.减小前驱体溶液的[NH₃·H₂O]/[Zn²⁺]比率可提高ZnO薄膜生长速率.     

Modulating conductivity type of cuprous oxide (Cu2O) films on copper foil in aqueous solution by comproportionation

Cuprous oxide (Cu₂O) is an attractive material for photoelectrochemical (PEC) hydrogen production or photovoltaic application, because of its appropriate band gap, low material cost and non-toxic. In this paper, Cu₂O films were obtained by comproportionation in acid cupric sulfate solutions with varying concentrations of potassium nitrate. Photoelectrochemical and electrochemical experiments, such as zero-bias photocurrent responses, voltammograms, and Mott-Schottky measurements, show that the Cu₂O films grown in low (≤0.75 mol dm^–3) and high (≥1.00 mol dm^–3) nitrate ion concentrations presented n-type and p-type conductivity, respectively. Open circuit potential and polarization behavior were monitored to investigate the mechanism of modulating conductivity type. Nitrate ions consume protons in the plating solution during comproportionation with different concentrations of nitrate ions creating different pH at the Cu₂O/solution interface. This gradient leads to the transformation of Cu₂O films conductivity changing from n-type to p-type with increasing the concentration of nitrate ions in the plating solution. This method could be used to fabricate homojunction electrode on metal substrate for PEC hydrogen production or photoelectric application.     

Enhanced electrical and thermal conductivity of modified poly(acrylonitrile-co-butadiene)-based nanofluid containing functional carbon black-graphene oxide

Solution refluxing and high-pressure homogenization technique were reported for synthesizing nanofluids based on modified poly(acrylonitrile-co-butadiene) (M-PANB) as base fluid and carbon black (CB)/carbon black-graphene oxide (CB-GO) as filler. The physiochemical properties were studied to analyze the structure, morphology, thermal and electrical conductivity. FTIR analysis corroborated the structure of CB-GO nanobifiller and nanocomposite. Microstructure analysis of M-PANB/CB-GO revealed good dispersion of CB-GO nanosheets, while CB series showed granular distribution. XRD studies confirmed amorphous structure of M-PANB/CB-GO nanocomposite. Thermal conductivity of nanofluid was found to increase upto 1.41 W/mK for 10 wt.% CB-GO loading and electrical conductivity was increased to 2.5 × 10^?3 Scm^?1.     

Thickness-dependent properties of sprayed iridium oxide thin films

Iridium oxide thin films with variable thickness were deposited by spray pyrolysis technique (SPT), onto the amorphous glass substrates kept at 350 °C. The volume of iridium chloride solution was varied to obtain iridium oxide thin films with thickness ranging from 700 to 2250 Å. The effect of film thickness on structural and electrical properties was studied. The X-ray diffraction (XRD) studies revealed that the as-deposited samples were amorphous and those annealed at 600 °C for 3 h in milieu of air were polycrystalline IrO₂. The crystallinity of Ir-oxide films ameliorate with film thickness thereby preferred orientation along (1 1 0) remains unchanged. The infrared spectroscopic results show Ir–O and Ir–O₂ bands. The room temperature electrical resistivity (ρ_RT) of these films decreases with increase in film thickness. The p-type semiconductor to metallic transition was observed at 600 °C.     

An analysis of the electrical conductivity in BaSO4-added Ag2SO4 solid electrolyte system

The compositions (1 −x)Ag₂SO₄−(x)BaSO₄, wherex=0·01 to 0·6, were prepared by slow cooling of the melt. The extent of the solid solubility of Ba²⁺ in Ag₂SO₄ was determined by X-ray powder diffraction and scanning electron microscopy. The bulk conductivity of each sample was obtained using a detailed impedance analysis. The partial substitution of Ba²⁺ results in the enhancement of conductivity in compliance with the classical aliovalent doping theory. A simplistic model based on lattice distortion (expansion) due to partial substitution of Ag⁺ by the bigger Ba²⁺ has been considered to explain enhanced conductivity. Beyond solid-solubility limit (5·27 mole%) the BaSO₄-dispersed Ag₂SO₄ conductivity follows the usual trend seen in binary systems. An increase in conductivity in this case is discussed in the light of interfacial reactions and surface defect chemistry. The maximum conductivity in 20 mole% BaSO₄ dispersed Ag₂SO₄ is due to percolation threshold.     

Thickness dependence of structural and optical properties of indium tin oxide nanofiber thin films prepared by electron beam evaporation onto quartz substrates

Indium tin oxide (ITO) thin films, produced by electron beam evaporation technique onto quartz substrates maintained at room temperature, are grown as nanofibers. The dependence of structural and optical properties of ITO thin films on the film thickness (99-662 nm) has been reported. The crystal structure and morphology of the films are investigated by X-ray diffraction and scanning electron microscope techniques, respectively. The particle size is found to increase with increasing film thickness without changing the preferred orientation along (2 2 2) direction. The optical properties of the films are investigated in terms of the measurements of the transmittance and reflectance determined at the normal incidence of the light in the wavelength range (250-2500 nm). The absorption coefficient and refractive index are calculated and the related optical parameters are evaluated. The optical band gap is found to decrease with the increase of the film thickness, whereas the refractive index is found to increase. The optical dielectric constant and the ratio of the free carrier concentration to its effective mass are estimated for the films.     

Structure and AC conductivity of nanocrystalline Yttrium oxide thin films

Yttrium oxide (Y₂O₃) thin films were grown at substrate temperatures (T_s) ranging from room temperature (RT) to 500 °C and their structural and electrical properties were evaluated. The results indicate that Y₂O₃ films grown at RT-100 °C were amorphous (a-Y₂O₃). Y₂O₃ films began to show cubic phase (c-Y₂O₃) at T_s = 200 °C. The average grain size varies from 5 to 40 nm as a function of T_s. Room temperature ac electrical conductivity increases from 0.4 (Ω-m)^− 1 to 1.2 (Ω-m)^− 1 with increasing T_s from RT to 500 °C. The frequency dispersion of the electrical resistivity reveals the hopping conduction mechanism. Frequency dispersion of the electrical resistivity fits to the modified Debye's function, which considers more than one ion contributing to the relaxation process. The mean relaxation time decreases from 2.8 to 1.4 μs with increasing T_s indicating that the effect of microstructure of the Y₂O₃ films is significant on the electrical properties.