Nanoporous NiO Films Research Articles

Influence of electrodeposition-dealloying and silver nanoparticles on the electrochemical properties of nanoporous NiO films for supercapacitor

Nanoporous NiO films are prepared by electrochemical dealloying the Zn-Ni alloys electrodeposited on Ni substrates followed by self-oxidizing method. The electrodeposition-dealloying parameters are investigated. The optimal nanoporous NiO film exhibits a specific capacitance of 1212.0 F/g at 1 A/g. The capacitance retention rate can reach 71.9 % at 20 A/g. The improved electrochemical properties are attributed to the hierarchical porous structure, which can provide large specific surface areas with enough exposed active sites and enhance the mass transport. On the basis of this, nanoporous NiO films are decorated with Ag nanoparticles with an average size of 50.6 nm using photoreduction method. Owing to the good ohmic contact between Ag and NiO, the nanoporous NiO-Ag films possess an increased specific capacitance up to 3592.9 F/g three times higher than the nanoporous NiO film. The capacitance retention rate reaches 84.5 % at 20 A/g. An asymmetric supercapacitor NiO-Ag//AC exhibits an energy density of 116.88 Wh/kg at 1720 W/kg. An energy density of 85.41 Wh/kg is maintained at 36600 W/kg. These boosted electrochemical performances are mainly owing to the synergistic effects of unique hierarchical porous structure and high electronic conductivity. This study provides a new approach to high performance electrodes for supercapacitors.

Combined chemoresistive and in situ FTIR spectroscopy study of nanoporous NiO films for light-activated nitrogen dioxide and acetone gas sensing

The chemoresistive sensor response of nanoporous NiO films prepared by advanced gas deposition was investigated by combined resistivity and in situ FTIR spectroscopy, with and without simultaneous light illumination, to detect NO2 and acetone gases. The sensitivity towards NO2 increased dramatically under UV irradiation employing 275 nm light. Improved sensitivity was observed at an elevated temperature of 150 °C. In situ FTIR measurements were performed to record the transient gas adsorption/desorption processes. The sustained sensitivity and repeatability for NO2 sensing could be attributed to reversible surface-nitro and nitrate species formation, which are stable on the surface at relative humidity up to 40%. In contrast, acetone sensing results in irreversible decomposition and accumulation of reaction products on the NiO sensor surface, covering the surface and limiting gas sensing. Implications of the study for improved and sustained NiO gas sensor properties in gas mixtures are discussed.

Preparation and gas-sensing properties of very thin sputtered NiO films

Abstract We present results on very thin NiO films which are able to detect 3 ppm of acetone, toluene and n-butyl acetate in synthetic air and to operate at 300°C. NiO films with 25 and 50 nm thicknesses were prepared by dc reactive magnetron sputtering on alumina substrates previously coated by Pt layers as heater and as interdigitated electrodes. Annealed NiO films are indexed to the (fcc) crystalline structure of NiO and their calculated grain sizes are in the range from 22 to 27 nm. Surface morphology of the examined samples was influenced by a rough and compact granular structure of alumina substrate. Nanoporous NiO film is formed by an agglomeration of small grains with different shapes while they are created on every alumina grain.

Room temperature detection and modelling of sub-ppm NO2 by low-cost nanoporous NiO film

NiO-based NO2 sensors operating at room temperature are attracting great attentions due to their promising energy and cost saving performances. However, only a few reports showed high sensitivity and selectivity in the sub-ppm concentration range and low limit of detection (LoD). In this work, we designed and fabricated by a low-cost chemical bath deposition (CBD) and thermal annealing a nanostructured and porous NiO thin film (nanoporous NiO film) composed of NiO nanoparticles (30−50 nm). The nanoporous NiO film was then applied as sensing element for the NO2 detection at room temperature, demonstrating a high response to sub-ppm level NO2 (140 ppb), excellent selectivity and stability, and a very low LoD of 20 ppb. The NO2 sensing mechanism was investigated and satisfactorily modelled by two energetically different and independent adsorption sites at room temperature. Both sites contribute to the NO2 detection at room temperature while only one site contributes at higher temperatures. The described low-cost fabrication method and the discussed superior NO2 sensing performances at room temperature make the nanoporous NiO film a promising NO2 sensor for environmental monitoring.

Transparent nanoporous P-type NiO films grown directly on non-native substrates by anodization

While electrochemical anodization has been used to form a number of nanostructured n-type semiconducting metal oxides for optoelectronic device applications, there exists a dearth of p-type metal oxide films that are solution processable. Herein, we formed p-type semiconducting NiO films by vacuum depositing Ni thin films on non-native substrates (transparent conductive oxide (TCO)-coated glass substrates and silicon wafers) using magnetron sputtering, and subsequently anodizing and annealing the Ni films. The Ni films were subjected to electrochemical anodization in diethylene glycol based organic electrolytes and subsequently annealed at 600 °C to form nanoporous NiO films with a pore size of ~ 20 nm. Runaway etching is a key issue in Ni anodization which was mitigated through the use of ice bath cooling and galvanostatic anodization. The choice of substrate is found to be critical to the resulting morphology owing to the differing surface roughness. Crystalline NiO is found to have formed from Ni(OH)2 and NiOOH during annealing, and an additional NiSi layer is noted for NiO films on Si wafers. The bandgap of the NiO was estimated to be 3.5 eV. Electrochemical impedance spectroscopy and Mott–Schottky analysis confirmed p-type semiconducting behaviour, and enabled measurement of an acceptor density (NA) of 2.85 × 1018 cm−3 and a flatband potential (VFB) of 0.687 V versus Ag/AgCl.

Improved electrochromic properties of nanoporous NiO film by NiO flake with thickness controlled by aluminum

Nanoporous NiO thin films were fabricated on indium tin oxide glass by simple chemical bath deposition (CBD). Dense porous structure was formed by NiO flakes with thickness controlled by aluminum through CBD. Nanoporous NiO thin films with high porosity showed excellent electrochromic properties, with transmittance change of 77% at 550 nm and coloration efficiency of 99.5 cm2/C. Electrochemical properties of these thin films were studied in a 0.1 M KOH electrolyte by cyclic voltammetry and chronoamperometry measurements. These nanoporous NiO thin films showed high porosity structure with excellent electro-reversibility. They are also very stable with long cyclic durability.

Effect of Polyethylene Glycol on the NiO Photocathode

In this study, a uniform nanoporous NiO film, with a thickness of up to 2.6 μm, was prepared using polyethylene glycol (PEG). The addition of PEG significantly decreased the cracks in the NiO film and prevented the peeling of the NiO film from a fluorine-doped tin oxide substrate. The NiO cathode was prepared using CdSeS quantum dots (QDs) as the sensitizer, with an optimized photoelectric conversion of 0.80%. The optimized QD-sensitized NiO films were first assembled with the TiO2 anode to prepared QD-sensitized p–n-type tandem solar cells. The open circuit voltage was greater than that obtained using the separated NiO cathode or TiO2 anode.

Hybrid PbS Quantum Dot/Nanoporous NiO Film Nanostructure: Preparation, Characterization, and Application for a Self-Powered Cathodic Photoelectrochemical Biosensor.

This work reports the first preparation and characterization of a hybrid PbS quantum dot (QD)/nanoporous NiO film nanostructure as well as its application for novel self-powered cathodic photoelectrochemical (PEC) sensing. Specifically, we synthesized the thioglycolic acid-capped PbS QDs and then assembled them onto the hydrothermally fabricated three-dimensional (3D) NiO nanostructured films on the transparent indium tin oxide-coated glass substrates, followed by the subsequent conjugation with the glucose oxidase as a model biocatalyst. Favorable alignment existed between the NiO and PbS QDs, and the as-obtained p-type heterostructure was characterized by various techniques and found to have good PEC activities. In the self-powered PEC biosensing of glucose, the system exhibited high sensitivity toward the presence of dissolved oxygen in the electrolyte, and thereby, a novel PEC enzymatic sensor was developed. With a PbS QD/3D NiO nanofilm, this work manifested the great promise of a heterostructure photocathode for a self-powered PEC biosensor that to the best of our knowledge has not been reported. We believe that it could inspire more interest in the design and development of numerous other p-type heterostructures for advanced self-powered PEC biosensors.

Photoelectrochemistry by Design: Tailoring the Nanoscale Structure of Pt/NiO Composites Leads to Enhanced Photoelectrochemical Hydrogen Evolution Performance.

Photoelectrochemical hydrogen evolution is a promising avenue to store the energy of sunlight in the form of chemical bonds. The recent rapid development of new synthetic approaches enables the nanoscale engineering of semiconductor photoelectrodes, thus tailoring their physicochemical properties toward efficient H2 formation. In this work, we carried out the parallel optimization of the morphological features of the semiconductor light absorber (NiO) and the cocatalyst (Pt). While nanoporous NiO films were obtained by electrochemical anodization, the monodisperse Pt nanoparticles were synthesized using wet chemical methods. The Pt/NiO nanocomposites were characterized by XRD, XPS, SEM, ED, TEM, cyclic voltammetry, photovoltammetry, EIS, etc. The relative enhancement of the photocurrent was demonstrated as a function of the nanoparticle size and loading. For mass-specific surface activity the smallest nanoparticles (2.0 and 4.8 nm) showed the best performance. After deconvoluting the trivial geometrical effects (stemming from the variation of Pt particle size and thus the electroactive surface area), however, the intermediate particle sizes (4.8 and 7.2 nm) were found to be optimal. Under optimized conditions, a 20-fold increase in the photocurrent (and thus the H2 evolution rates) was observed for the nanostructured Pt/NiO composite, compared to the benchmark nanoparticulate NiO film.

Adsorption Behavior of I3- and I- Ions at a Nanoporous NiO/Acetonitrile Interface Studied by X-ray Photoelectron Spectroscopy.

The adsorption of I- and I3- anions, i.e., the two species constituting the most common redox couple of dye-sensitized solar cells (DSCs), onto the surface of screen-printed nanoporous NiO was studied by means of X-ray photoelectron spectroscopy (XPS). Nanoporous NiO films were deposited on transparent metallic fluorine-doped tin oxide (FTO) and polarized as working electrodes in a three-electrode cell with differently concentrated I-/I3- electrolytes to simulate the different conditions experienced by the NiO cathodes during the lifecycle of a p-type DSC (p-DSC) at those atomic sites not passivated by the dye. Bare NiO films were tested also as photocathodes of nonsensitized p-DSCs. The ex situ XPS analysis of I 4d ionization region of both reference and electrochemically treated NiO films showed that the presence of native and electrochemically generated Ni3+ and Ni4+ centers induces fast adsorption/desorption of I- ions and catalyzes their oxidation to I3- ions. The adsorption phenomena generated by I- and I3- species on nanoporous NiO electrodes can also induce an effect of electrochemical passivation toward a fraction of charged Ni sites. Such an effect would render these sites inactive for the further realization of those photoelectrochemical processes at the basis of the operation of a p-DSC.

The construction of tandem dye-sensitized solar cells from chemically-derived nanoporous photoelectrodes

A tandem dye-sensitized solar cell (tandem-DSSC) was synthesized on the basis of thin-film semiconductor electrodes. The nanoporous p-type NiO films were successfully obtained by simultaneous deposition of Al and Ni, followed by selective etching of Al and oxidation. Likewise, the n-type photoanode was made where Ag was etched in nitric acid after the initial formation of Ag/TiO2 nanocomposites. Such dye-sensitized photoelectrodes were combined to construct a tandem solar cell which exhibited an enhanced open-circuit voltage. Also, the tandem devices were subjected to various light fluxes to correlate the experimental cell parameters (open-circuit voltage, short-circuit current, fill factor, recombination shunt resistance, etc.) with the ideal one-diode model. Interestingly, impedance spectra of the tandem cell was well matched with the parameters from each of the n-type or p-type DSSC, indicative of successfully-designed tandem structure.

Fabrication and characterization of a nanoporous NiO film with high specific energy and power via an electrochemical dealloying approach

A three-dimensional (3D) nanoporous NiO film was fabricated via a two-step process using an electrochemical route. This process included electrodeposition of the Ni/Zn alloy film and electrochemical dealloying using a direct-current power source. The scanning electron microscopy images suggest that the film has an irregular 3D interconnected nanosheet structure with open channels. The adsorption–desorption isotherms indicate that the as-prepared NiO film had a high specific surface area of 198mg−1 and a narrow pore size distribution, with two peaks at 2.7 and 5.1nm. The specific capacitance of the sample reached 1670Fg−1 at a discharge current density of 1Ag−1. In addition, the as-prepared nanoporous film exhibited high performance during a long-term cycling test. The maximum values for the specific energy and specific power at the 1.1V potential window were 170 and 27.5kWkg−1, respectively.

Facile ammonia-induced fabrication of nanoporous NiO films with enhanced lithium-storage properties

Nanoporous NiO films directly grown on the foam Ni are fabricated via a facile ammonia-induced route. As anode materials for lithium-ion batteries, nanoporous NiO films exhibit an outstanding rate capacity of 280mAh·g−1 at 10C rate and high reversible capacity of 543mAh·g−1 after 100cycles at 0.2C rate. The fabrication strategy offers a novel approach to fabricate other thin film materials.

Intense Ultraviolet Photoluminescence Observed at Room Temperature from NiO Nano-porous Thin Films Grown by the Hydrothermal Technique

ABSTRACTWe have successfully formed high-quality nanoporous NiO films by the hydrothermal technique and observed intense ultraviolet (UV) luminescence at room temperature. The SEM image reveals nanoporous NiO films with pore diameters from 70 to 500 nm. The results of XRD, Micro Raman and FTIR characterizations confirm the cubic structure of NiO. The optical band gaps estimated from the absorption spectrum are found to be 3.86 and 4.51 eV. The former is similar to that of bulk NiO, while the latter is much higher than that of bulk NiO. The increased band gap was attributed to the quantum confinement in the NiO nanocrystals, which may be present in the nanoporous NiO film. The room-temperature photoluminescence (PL) spectrum shows a peak of intense luminescence at 3.70 eV and several other peaks in the UV and near-UVwavelength regions. The intense UV luminescence at 3.70 eV was associated with the near band-edge emission and the others with defect-related emission. The high-quality wall of nanoporous NiO with a large surface-to-volume ratio provided the intense UV emission.

High-performance three-dimensional nanoporous NiO film as a supercapacitor electrode

A three-dimensional nanoporous NiO film was fabricated using a two-step process through an electrochemical route. The as-prepared NiO film exhibited a highly nanoporous structure and high surface area (264 m2 g−1). The textural characterization of the film and its electrochemical performance as an electrochemical electrode were investigated. The electrode showed a high specific capacitance (1776 F g−1), power density (89 Wh kg−1), and energy density (16.5 kW kg−1). In addition, the electrode exhibited high and stable specific capacitance retention after a long cycle test in KOH solution. More importantly, the power density met the power requirements of the Partnership for a New Generation of Vehicles (PNGV).

Enhanced electrochromic performance of nanoporous NiO films

NiO films have been prepared via sol–gel dip coating method at 300 °C followed by annealing them at 300 °C for different durations (15–60 min). XRD spectra of the films reveal the amorphous nature of the films annealed for 15–45 min. The films annealed for 60 min exhibit crystalline nature with particle size of 7 nm. Good amount of porosity with an average pore size of 46 nm could be observed through the field emission SEM images for the films annealed for 30 min. The electrochromic properties have been studied using cyclic voltammetry (CV) and in-situ spectro-electrochemical techniques. The films annealed for 30 min exhibit maximum anodic/cathodic diffusion coefficient values. The same films exhibit the maximum color change (82.5%) with photopic constrast ratio of 12.16. The colouration and bleaching time of these films was 3.5 and 1.3 s, respectively. The films annealed for 30 min at 300 °C appear to show optimal electrochromic properties.

Facile fabrication of porous NiO films for lithium-ion batteries with high reversibility and rate capability

We report the high-rate capability and good cyclability of three-dimension nanoporous NiO films as the anodes of lithium-ion batteries. The NiO films are fabricated by immersing foam nickel substrates in an 80 °C aqueous solution containing ammonia and potassium peroxydisulfate, and subsequent heat treatment at 500 °C. At a rate of 1.0 C, the film electrodes maintain a capacity of 560 mAh g−1 as well as capacity retention of 97% after 100 discharge/charge cycles. When the current density is increased to 14C, 42% of the capacity can be retained. Owing to the ease of large-scale fabrication and superior electrochemical performance, these NiO films will be promising anodes for high-energy-density lithium-ion batteries.

Syntheses of NiO nanoporous films using nonionic triblock co-polymer templates and their application to photo-cathodes of p-type dye-sensitized solar cells

We prepared nanoporous NiO films from NiCl 2 in water/ethanol mixed solution, using a series of polyethyleneoxide–polypropyleneoxide–polyethyleneoxide (PEO–PPO–PEO) triblock co-polymers as template, and examined them as p-type NiO electrodes sensitized with a merocyanine dye. Triblock co-polymers with high PEO/PPO ratio leaded to three-fold higher photocurrent in comparison with low PEO/PPO polymers. Use of high PEO/PPO templates resulted in NiO membrane structure formed by uniformly distributed NiO nano particles with small interparticle voids, giving large surface area for dye adsorption, while low PEO/PPO templates resulted in larger void volume and less surface area. Photocurrent under simulated solar light was sum of the currents generated by NiO direct excitation and by dye-sensitization at spectra below 450 nm and from 450 to 590 nm, respectively.