Next Article in Journal
Rotational Shearing Interferometer in Detection of the Super-Earth Exoplanets
Next Article in Special Issue
Uncovering the Use of Fucoxanthin and Phycobiliproteins into Solid Matrices to Increase Their Emission Quantum Yield and Photostability
Previous Article in Journal
Automation System for the Flexible Sample Preparation for Quantification of Δ9-THC-D3, THC-OH and THC-COOH from Serum, Saliva and Urine
Previous Article in Special Issue
Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 12
Issue 6
10.3390/app12062839
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

NiO-Based Electronic Flexible Devices

by
Marilena Carbone
Department of Chemical Science and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
Appl. Sci. 2022, 12(6), 2839; https://doi.org/10.3390/app12062839
Submission received: 2 January 2022 / Revised: 6 March 2022 / Accepted: 7 March 2022 / Published: 10 March 2022
(This article belongs to the Special Issue Women in Materials Science)
Browse Figures
Versions Notes

Abstract

:
Personal, portable, and wearable electronics have become items of extensive use in daily life. Their fabrication requires flexible electronic components with high storage capability or with continuous power supplies (such as solar cells). In addition, formerly rigid tools such as electrochromic windows find new utilizations if they are fabricated with flexible characteristics. Flexibility and performances are determined by the material composition and fabrication procedures. In this regard, low-cost, easy-to-handle materials and processes are an asset in the overall production processes and items fruition. In the present mini-review, the most recent approaches are described in the production of flexible electronic devices based on NiO as low-cost material enhancing the overall performances. In particular, flexible NiO-based all-solid-state supercapacitors, electrodes electrochromic devices, temperature devices, and ReRAM are discussed, thus showing the potential of NiO as material for future developments in opto-electronic devices.

Graphical Abstract

1. Introduction

Personal, portable and wearable electronics have become widely used in everyday life. Smart watches, smart glasses, fitness trackers, Bluetooth headsets are daily-life objects, which require the application of flexible electronic components. Flexible electronic technology offers a wide-variety of applications in cutting-edge areas such as biosensors [1,2,3,4,5,6], E-skin in robotics and prostheses [7], epidermal electronics [8], and biomedical instruments [9,10,11,12]. Furthermore, tactile sensors, can be built with signal processing technologies used to interpret the measured information from tactile sensors and/or sensors for other sensory modalities [13]. In addition, tools which are usually rigid, such as electrochromic devices, may also have wider applications if they are fabricated with flexible characteristics [14]. The assembling of these devices largely depends on the availability of flexible powering units such as batteries [15,16,17,18,19] solar cells [20], electrodes [21,22], transistors [23,24,25,26,27], display devices [28] conformable RFID (radio frequency identification) tags [29], and resistive random-access memories [30]. Traditional electronics are largely based on the integrated circuits mostly fabricated in rigid and planar semiconductor wafers, which fueled extensive research in associated phenomena on silicon surfaces [31,32,33,34,35,36,37,38,39,40,41]. However, they have limited applications in manufacturing flexible electronic devices due to the intrinsic rigidity of silicon wafers and the complex fabricating processes of its microstructure. Flexible materials, on the other hand, allow for large mechanical deformation, such as bending, twisting, stretching, and folding, which may fit irregular, soft, or moving objects such as human skin and wrinkled clothes [42,43]. Therefore, new materials have started being fabricated which are based on silicon nanoparticles integrated in elastomers [44], along with advanced materials and new architecture designs [45]. Simultaneously, supercapacitors with higher power density, faster recharge capability, safer operation, and longer cycle performance have become in demand to guarantee the devices operations with additional flexibility requirements. Materials such as graphene have emerged as a promising solution for building efficient, flexible electrochemical devices due to its unique electronic properties and high optical transmittance [46,47,48,49]. However, high sheet resistance, low work function of pristine material [49], poor mechanical properties in presence of defects [50], and high strain limits curtail the actual graphene applications [51].
Combining flexibility requirements and enhanced performances may be achieved by integrating nanomaterials with the desired properties with suited flexible substrates resistant to deformations, mechanical stress, and of long duration.
Transition metal oxides and particularly NiO have the advantage of high storage capacitance [52], optical and electrical properties [53], transport properties on surfaces [54] as well as in perovskite solar cells [55]. Their nanoparticles synthesis may be achieved in several, environmentally friendly ways [56,57,58,59,60,61,62,63], with several shapes, arrangements, and super-structures [64]. Depending on the specific size and morphological characteristics, they can be employed in the construction of several types of devices for different purposes, which include chemosensing [65,66,67,68], supercapacitive devices [69,70,71], solar cells [72,73], and resistive memories [74,75,76].
Integrated NiO-based flexible materials can be achieved in several ways. NiO nanoparticles or thin films can be deposited on flexible materials such as PET (polyethylene terephthalate) or PEN (polyethylene naphthalate) substrates; NiO nanoparticles can be embedded in the flexible materials in the preparation phase, and NiO leagues can be de-alloyed [77] with a specific procedure that ensures ultimate pliability.
In the present mini-review, an overview of NiO synthetic methods and properties are reported as well as recent progresses in the fabrication and performances of NiO-based flexible devices. In particular, the latest constructions will be presented of NiO-flexible supercapacitors, electrodes, electrochromic devices, solar cells, temperature sensor devices, and resistive random-access memory.

2. Synthesis and Properties of NiO Nanoparticles and Thin Films

The NiO synthetic procedures largely condition its properties and make it suitable for different applications. Solvothermal (including sol-gel) and hydrothermal syntheses are among the most largely used methods, since they usually provide a facile way to tune size and shape of the NiO particles. Several parameters can be varied in both cases, such as type of Ni salt, type of base, temperature, solvent, and possible employment of a template. The employment of an autoclave also influences the course of the reaction due to the vapor pressure in a sealed vessel. Finally, the calcination phase, which typically follows, also plays a role, since it can shape the material depending on the temperature ramp and final temperature.
In addition, the phytogenic synthesis has become an interesting variant of the sol-gel method, where leaf extracts of plants such as G. Sylvestre [78] or Ageratum conyzoides L. [79] are used to form a gel which is to be removed during the calcination phase. NiO thin films can be achieved through several techniques: RF sputtering, electron beam evaporation, DC magnetron sputtering, anodic and cathodic electrodeposition, chemical vapor deposition, and nanoparticles electrospray [80]. A sketch of common synthetic pathways of NiO is reported in Figure 1, along with two examples of NiO thin film synthesis.
Finally, the dealloying technique has been successfully employed for achieving all-solid flexible materials, starting from various metals and metal oxides as an implementation of the achievement of flexible porous gold by the removal of silver from a gold-silver alloy with nitric acid [81].
The type of conditions employed in the synthesis reflect onto the NiO properties. There are several examples of tuning the NiO features via the employment, for instance, of different types of bases which may influence the precursors structural parameters, such as the planar distances. The NiO morphology and the ensuing capacitance can be influenced by the calcination procedure [52,64,82]. The size of NiO nanoparticles is directly correlated to the band gap and can be estimated through the Brus relationship for spherical shapes [83]. Similarly, size and shape of the nanoparticles impact the NiO capacitance which is among the mostly exploited properties. Other relevant properties useful for building devices are the resistance variation with the temperature (Seebeck effect), the refractive index variation as a function of light wavelength, the hole mobility, and the resistance variation upon voltage application, which can be used for temperature devices, electrochromic devices, solar cells, and ReRAM, respectively.
The employment of NiO, either as nanoparticles or as thin layers in the actual construction of the various devices requires the embedding of NiO into matrices or the deposition on supports and ensuing incorporation in the device, with associated issues of integration feasibility and configuration design. The interaction NiO-matrix or NiO-support may influence the NiO properties through synergistic effects. Furthermore, the flexibility requirement may introduce the deterioration upon deformation, thus affecting the overall properties of the devices.
In Table 1, a few examples of capacitance dependency of NiO from morphologies as well as matrix and coating interactions are reported.
The employment of NiO on a large scale needs to be offset against major drawbacks such as allergenicity and toxicity. In contact with the skin, NiO can cause allergic reactions [94] which is a problem for wearable devices exposing NiO on the external layer. Furthermore, it is especially harmful when inhaled [95], since it can induce an increased risk of lung cancer, as observed in epidemiological studies on rats [96]. In addition, the pouring out of NiO in waters and its bio-accumulation can represent an environmental hazard [97].

3. NiO-Based Flexible Devices

The usage of flexible, personal, portable, and wearable electronics and devices is subjected to the availability of similarly flexible storage apparatuses for power supply. Therefore, the first part of the mini-review is dedicated to the state-of-the-art NiO-based supercapacitors and electrodes (Section 3.1). Recently developed NiO-based flexible devices will be described in the following Section 3.2, Section 3.3, Section 3.4 and Section 3.5.

3.1. Supercapacitors and Electrodes

Good flexibility as well as good electrochemical performances are achieved with NiO based cable-like all-solid-state flexible supercapacitor (CAF) [98]. In a recent report, a highly flexible electrode was constructed by achieving a sandwich of an interlayer of metallic glass (MG) and two outer layers of nanoporous nickel coated with nickel oxide/hydroxide (np-NiOxHy@Ni). The assembling of the sandwich was achieved in several steps, schematically reported in Figure 2. Metallic glass ribbons were obtained by arc-melting of pure metals (99.99 wt.%) under argon to yield the master alloy of Ni40Zr20Ti40, followed by rapid quenching of the re-melted alloys onto a spinning copper roller [99,100] (Figure 2a). The outer layers were generated by immersing the as-spun ribbons in 0.05 M HF solutions for 4 h at 298 K to synthesize sandwich-like flexible np-NiOxHy@Ni/MG/np-NiOxHy@Ni composite electrodes (Figure 2b). Cable-like all-solid-state flexible supercapacitors (CAFSs) were prepared by soaking the nanoribbons in a gel obtained by heating an aqueous solution of PVA/KOH (Figure 2c). Then, two ribbons were twined on a rubber rod in the opposite direction and coated with gel electrolyte (Figure 2d). Finally, the CAFSs were prepared by wrapping with a thin cling film. Electrochemical measurements indicate that the electrode is essentially a battery-type material, also exhibiting pseudocapacitive behavior with a capacitance of 778 F cm−3 at 1 A cm−3 in KOH solutions and a retention of 80.3% when the current density increases by 128-fold, as well as a cycle stability of 100% capacitance retention after 8000 cycles.
The flexibility of the CAFSs was tested by performing cyclic voltammetry (CV) measurements with electrodes subjected to bending or twisting. The capacitance slightly increases by bending a 4 cm long electrode, until the distance between the extremes reaches 2 cm (nearly a U shape). With a further decrease of the distance, the capacitance slightly reduced. A capacitance retention of 96.4% could be obtained when the electrode was twisted into a shape of an “8”. In addition, a capacitance retention of 92.4% was obtained after 1500 bending cycles.
Dealloying was also used for preparing a flexible Co(OH)2/NiOxHy@Ni hybrid [101] with interwoven Co(OH)2 nanopetals electrochemically deposited on the skeleton surface of np-NiOxHy electrodes. This was achieved by electrodeposition, using a three electrode cell immersed in a Co(NO3)2⋅6H2O aqueous solution, where np-NiOxHy@Ni substrate acts as working electrode, a platinum plate is used as counter electrode and Ag/AgCl as reference electrode. Owing to the hierarchical porous structure and synergetic effect of Co(OH)2 and NiOxHy electroactive materials, the Co(OH)2/np-NiOxHy@Ni hybrid electrode delivered a voltage window of −0.1~0.6 V and a specific capacitance of 1421.1 F cm−3 at 0.5 A cm−3, higher than the single transition-metal electrodes. Mounted on a watch, the supercapacitor could guarantee 45 min autonomy while being subjected to different deformations.

NiO-Plastisol Electrodes

Cheap, practical, flexible free standing electrodes were achieved by mixing synthetic graphite, with plasticizer agents (i.e., high molecular weight polyvinylchloride powder (PVC), bis(2-ethylhexyl)adipate (BEA) and THF [102]), followed by stirring, sonication, and solvent evaporation at room temperature overnight. NiO nanoparticles could be incorporated into the free-standing electrode by mixing them to the slurry, prior to the evaporation phase [103]. The resulting electrodes displayed a quite homogenous distribution of nanoparticles. Furthermore, NiO of different shapes and morphologies and at variable concentrations could be included.
In Figure 3A through c, the flexible NiO-decorated free standing electrode is shown. It can be cut with scissors (Figure 3A) in any kind of shape and size (Figure 3B), the original dimension being solely dependent on the size and shape of the glass tray hosting the slurry during the evaporation phase. Different morphologies of NiO were achieved by varying the synthesis condition, in particular the precipitating agent and the calcination temperature. More in detail, triethylamine, NaOH and urea reacted with Ni(NO3)2 to yield Ni(OH)2, converted into NiO by calcination at either 400 °C or 600 °C. The corresponding samples were labelled A (triethylamine), S (NaOH) or U (urea), followed by a 4 (400 °C) or 6 (600 °C). Overall, A4, A6, S6 and U6 NiO-decorated plastic electrodes were implemented for non-enzymatic amperometric detection of H2O2 in milk. The A4-NiO/plastisol displays the highest current density of all probed electrodes in response to H2O2 detection. This also corresponded to the best NiO dispersion within the plastisol matrix, though still retaining its original flower shape, as it can be appreciated at the backside of the electrode (Figure 3C—A4). A selection of SEM images of the flexible NiO-doped electrodes (front- and backside) is reported in Figure 3C. When probed on commercial milk, the A4-NiO/plastisol electrode reached a LOD of 5 μM and a linearity range up to 4 mM.

3.2. Resistive Random Access Memory

Resistive random access memory (ReRAM) is considered the most promising among the flexible non-volatile memories (NVMs) due to its simple metal-insulator-metal (MIM) structure, high switching speed, low power consumption, low operating temperature, and high packaging density. Flexible ReRAM is essential in smart wearable electronics and all-in-one fully flexible electronic system, since flexible memory is a fundamental component for data processing, storage, and communication with external devices.
NiO based ReRAM has the advantage of stable switching properties at specific conditions, lower operation set/reset current and forming voltage in comparison to other metal oxides. A flexible NiO ReRAM device with good operation characteristics was achieved, based on a solution process using NiO nanocrystals (NC), on flexible PET substrate [104]. The construction of the ReRAM devices consisted in the deposition of a 100-nm-thick platinum (Pt) and a 20 nm-thick titanium (Ti) adhesion layer on a polymer substrate using an e-beam evaporator. A dispersion of NiO-NCs and isopropyl-alcohol (IPA) was dropped on the flexible PET substrate, and the NiO-NC solution pressed by a poly—dimethylsiloxane (PDMS) mold while heating at 80 °C. The excess of coated NiO-NC solution leaking out, as well as the residual layer, were removed and the PDMS mold was demolded from the substrate, leaving a NiO nanopillar array fabricated on the substrate without NC residue. The electric signal and bias voltage were probed by a Pt/Ti-coated Si AFM tip contacted on nanoscale ReRAM devices, by applying a voltage on the top electrode, while the bottom electrode was grounded, and the resistance values of the NiO ReRAM were read at 0.01 V. The flexible NiO ReRAM device exhibited unipolar resistive switching behaviors corresponding to those of an evaporated NiO thin film. The switching behavior was initiated at 11 V with 0.04 V and 1 μA current compliance by a conductive filament forming operation. For the reset switching of the NiO flexible ReRAM, the voltage was swept from 0 V to 5 V with 0.02 V. The resistance of the NiO ReRAM device changed from 102 Ω to 107 Ω at 2.4 V for reset switching, and the set switching was operated in reverse at 3.4 V. The flexibility of the device is demonstrated by a nearly 180° bending as shown in Figure 4, along with the SEM imaging indicating the NiO-NC patterning of the PET substrate.

3.3. Electrochromic Devices

Electrochromism is the reversible color change of an electrochromic device (ECD) under an applied voltage. NiO is among the transition metals reportedly displaying the best electrochromic properties as anodic material, though associated drawbacks may be a low optical modulation, short-term stability, and slow switching time. These pitfalls can be overcome by modulating the NiO properties through synthetic strategies. A facile solid-state strategy for the preparation of NiO particles [105] foresaw a solid-state calcination of nickel acetate dihydrate at 600–900 °C, followed by NiO ink preparation by mixture with solvents (chlorobenzene, a 2:1 isopropanol-water mixture, or chloroform) sonication and sedimentation. According to this procedure, the ink was subsequently spin-coated on cleaned ITO glass and flexible Ag/PET substrates. The latter, in particular, was achieved by subsequently coating AgNanoWire (AgNW), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS, 5 wt% in ethanol) and NiO ink onto the PET substrate. Analogous films were synthesized using WO3 particles. Devices were built using a NiO film as working electrode and PEDOT:PSS/AgNW/PET or WO3—coated PEDOT: PSS/AgNW/PET as the counter electrodes. The NiO and NiO/WO3 ECDs on ITO substrate exhibited a transmittance variation of ΔT = ~84% at 700 nm between −3.0 and 0 V. The flexible NiO/WO3 device achieved ΔT = ~38% at 700 nm between 2.0 V–0 V for, indicating a potentially use in smart windows and optical devices.
The properties of NiO can be modified by doping, for instance with tungsten to achieve Ni1−xWxO, which is suitable as anode in electrochromic devices. In this framework, the employment of doped oxides required a preliminary optimization procedure to determine the most suited Ni1−xWxO composition. To this purpose, the electrochromic properties of Ni1−xWxO were evaluated by varying the W atomic percentage in a range between 0 and 20% and performing measurements in lithium perchlorate and propylene carbonate. Enhanced cycle stability with moderate optical modulation was obtained for a Ni1−xWx oxide film with at x = 0.024 [106]. Flexible electrochromic devices employing Ni1−xWxO were fabricated on a PET substrate with monolayered graphene as a platform, thus creating stacked layers of PET, ITO, graphene, WO3 and Ni1−xWxO thin films. In particular, the flexible electrochromic device was achieved by growing a single layered graphene on Cu foil (99.8%) by rapid thermal chemical vapor deposition (RTCVD). A procedure of DC magnetron sputtering at RT and subsequent post-annealing process at 250 °C was used to fabricate highly crystalline ITO (c-ITO) on graphene/Cu foil with a thickness of 60 nm, which was afterwards continuously transferred onto a PET substrate. The c-ITO/graphene/PET electrode showed a sheet resistance of ~45 Ω/sq and a transmittance of ~92% at a wavelength of 550 nm. The flexible EC devices were fabricated by laminating the WO3 film and optimizing the Ni0.976W0.024O film deposited on the c-ITO/graphene/PET substrate, using Li-based polymeric solid-state electrolyte. The flexible EC device showed stable cycling performances, maintaining an optical modulation of ΔT ≈ 40%. In addition, the flexible EC device showed stable optical modulation, with a ~2% reduction after 1000 cycles.

3.4. Temperature Devices

Flexible, resistive temperature detectors (RTDs) allow temperature measurements in wearable devices. In addition, the coupling to RFID converts them into wireless sensors which can be used for checking the body temperature, short-life span food and medications. The main requirements of such devices are a high and linear temperature coefficient of resistance (TCR), high resistance to bending and deformation and simplicity of design and measurements. A Ni/NiO based resistive temperature detector (RTDs) was proposed with such characteristics, based on the layered deposition on a PET fiber [107]. The device was fabricated by depositing a Ni/NiO bilayer on a bare cylindrical PET fiber pre-treated by sonication in acetone for 10 min, followed by sonication in methanol for 10 more min, washing and drying. NiO and Ni were deposited on the rotating PET fiber substrate by Radio Frequency magnetron sputtering, ensuring simultaneous argon and oxygen flow for NiO deposition and argon flow for the Ni thin film second coat. Resistance variation measurements with the temperature indicated that the device had a TCR of 3.8 × 10−3 °C−1 and a linearity of 0.9852. The bending process simulated movements through daily activities. After 16,000 bends the TCR of the device was 1.0 × 10−4 °C−1 with a linearity of 0.9065. Additional tests were performed washing with tap water and laundry detergent and exposing to acidic and basic environment. The electrical properties including resistance, TCR and linearity of slope remained unaffected when subjected to the different conditions. Moreover, the device maintained about 75% of the original TCR after being exposed to an acidic medium.

3.5. Solar Cells

Flexible inverted perovskite solar cells (PSCs) are achieved using a glass/ITO substrate as conductive transparent layer, and NiOx as an effective hole-transport material. Several variants were probed, with the purpose of achieving a high-power conversion efficiency (PCE) while keeping flexibility. Compared to normal PSCs, inverted flexible PSCs are light weighted, compatible with different surfaces, and portable, thus meeting the demands of the electronics industry [108,109]. Their further development, however, depends on the quality and characteristics of the hole transport layer (HTL). In this regard, NiOx is a promising candidate, owing to its low-temperature processing and good electron-blocking ability. In a relatively simple construction [110] NiOx nanofilms were deposited on ITO-PEN by spin coating from a high quality NiOx nanoparticle solution pre-synthesized by precipitation. Perovskite was deposited on top of the NiOx layer by spin coating, after achieving a precursor by reacting PbI2 and CH3NH3I in a mixture of DMSO and DMF, dripping chlorobenzene during spinning and baking at 100 °C. A thin layer of phenyl-C61-butyric acid methylester and a layer of Ag 150 nm thick completed the inverted planar heterojunction solar cells. The as-prepared NiOx films had excellent hole extraction and electron blocking property, thus providing a good hole contact for perovskite solar cells. A maximum PCE of 16.47% (steady state PCE of 16.21%) was achieved for the NiOx-based perovskite solar cell on the ITO-PEN glass substrate.
In order to improve the band structure alignment between low-temperature-processed NiOx and the perovskite layer, amino-functionalized graphene quantum dots (AGQDs) were included in the NiOx film as a dual-role additive. In addition, the AGQDs can provide abundant N atoms at the modified NiOx layer surface to enhance the crystallization of the perovskite film by a Lewis base-acid interaction. On the other hand, the AGQDs can optimize the band structure alignment between the NiOx and perovskite layers, facilitating hole extraction at the NiOx/perovskite interface [111]. The overall building up of AGQDs-aided NiO/peroskite flexible solar cells requires the separate synthesis of AGQDs, NiOx ink and subsequent assembling on PEN/ITO substrate after etching. More in detail, AGQDs were achieved by RT reaction of graphene sheets with ammonia, followed by autoclave treatment at 150 °C for 5 h and dialysis of the cooled solution. NiOx was prepared by reaction of Ni(NO3)2 with NaOH at pH 10, drying of the precipitate at 80 °C and calcination at 275 °C for 2 h. The pristine NiOx ink was prepared by dispersing 20 mg of NiOx powder in 1 mL of deionized water. AGQD-doped NiOx inks were obtained by directly mixing AGQD aqueous solutions with the pristine NiOx ink. The pristine and AGQD-doped NiOx inks were spin-coated on the flexible substrates, heated at 120 °C for 15 min and transferred to a N2-filled glovebox to prepare the perovskite films. The resulting inverted flexible PSC was subjected to automatic bending and exhibited an efficiency of 18.10%, good air stability, retention of 88% of its initial efficiency after continuously bending 1000 times and a retention of 64% of the initial efficiency after 30 days.

4. Electronic Flexible Devices Based on Other Oxides

Flexible supercapacitors are typically achieved by embedding an active material in a flexible support, usually a carbon-based material, including active carbon, graphene, carbon nanotubes, aerogel and hydrogels [112,113], unless techniques such as the dealloying can be applied. More recently, the use of paper supports was implemented for one-time only devices [114]. Several transition metal oxides (TMO), alone and in association with conducting polymers, were probed on flexible supports to achieve supercapacitors. ZnO/carbon nanofibers attained by electrospinning showed a specific capacitance > 100 Fg−1 and retention capability of 93.90% after 2000 continuous charge/discharge cycles [115]. A porous carbonized cotton/ZnO/CuS composite electrode showed a specific capacitance of 1830 mF cm−2 at 2 mA cm−2 and 74% retention with the current density increasing from 2 to 10 mA cm−2 and a loss of 14.8% of the initial capacitance after 5000 cycles [116]. An Fe2O3/Graphene aerogel hybrid electrode was used to build a highly flexible all-solid-state symmetric supercapacitor device, which could be bent by several angles providing a high specific capacitance, i.e., 440 Fg−1 and 90% capacitance retainment over 2200 cycles [117]. Confined anatase TiO2 nanoparticles were achieved in conductive activated carbon interconnected nanopores. The assembled cells with porous carbon cathodes displayed a specific capacity of ≈140 mAh g−1 at slow charge and ≈60 mAh g−1 at a 3.5 s fast charge [118]. Compared to the most recent NiO-based supercapacitors or electrodes, these systems show either a slightly lower performance or lower retention of properties upon deformation.
Flexible temperature devices have been fabricated using many different types of active materials on flexible supports deposited with different techniques. The most common active materials in temperature sensors are Pt and Au. They were deposited on a polyimide film after a peeling process, for monitoring the temperature in the range 20–120 °C and displayed a TCR value of 0.0032 °C−1 [119]. Cr and Au thin layers were sputtered on pre-stretched PDMS to minimize the tensile strain [120] and create a reversible bendable and stretchable flexible temperature device.
Ag lines on polyimide were evenly distributed by inkjet printing and constituted a bendable and flexible sensor for measuring body surface temperatures in the 20–60 °C range, with an average sensitivity of 2.23 × 103 °C−1 and less than 5% hysteresis [121].
ReRAMs are typically constituted of a top and a bottom electrode interlayered either by a semiconductor or by an insulating layer. The ReRAM performances are usually related to the interlayer, though the choice of the electrodes also plays a role. As for the interlayers, they are usually categorized into two groups, namely organic and inorganic. The organic interlayers include, among the others, silk protein/fibroin, nanocellulose and albumen [122,123,124]. These devices, however, tend to be rather humidity sensitive and not very stable in long terms operations. Metal oxides are typical inorganic interlayers, and a vast amount of electrode interlayer combinations was probed. Bipolar ReRAM devices were achieved, for instance, based on Pd/HfOx-Ag NPs/TiN [125], or Ag/MnO-Ta2O5/Pt [126]. The former operates at a set voltage of 2.2 V and reset voltage at −2.2 V, the latter at a set voltage of 0.8 V and reset voltage at −1.1 V. Unipolar ReRAM are more difficult to achieve. Some examples are provided by Al/HfOx/Al (set voltage 1.8 V, reset voltage 0.8 V) [127] and Pt/ZnO/Pt (set voltage 1.1–2.3 V, reset voltage 0.4–1 V) [128]. The issue of flexibility of ReRAM, however, is difficult to tackle, also due to the necessity of incorporating the electrodes in the device. This was achieved, so far in a Ni/Sm2O3/ITO device, which can operate at very low power (set: 0.25 V, 100 μA and vice versa, reset: 0.20 V, 49 μA) [129] or more recently with Ag/Au-chitosan/Au (bipolar, set voltage of 2.5 V) [130], which is biocompatible, but does not overcome the costs and fabrication related issues.
The development of flexible electrochromic devices is connected to the possibility of achieving a uniform layer of color changing material on a flexible substrate, capable of retaining the properties under stress and over time. Besides NiO based materials, recent developments were achieved using 2D TiO2/MXene heterostructures on PET [131]. More in detail, TiO2/Ti3C2Tx heterostructures were employed where the Ti3C2Tx film serves as the transparent conductive electrode, and the TiO2 film serves as the EC layer in EC devices. The heterostructure was prepared through an aerogel formation, dispersion in IPA and ensuing deposition on the substrate. At 550 nm, the transmittance variation reaches its maximum at around 60%, and it is largely retained upon 1000 EC cycles (92%), thus offering a practical alternative to NiO based EC devices.
A different issue is the employment NiO in perovskite flexible solar cells since it is a rather standard layer for the hole transport and difficult to replace.
On average, NiO-based devices have the advantage of a low cost, especially when compared to Au/Pt based thermometers, or to HfOx ReRAM, as well as a better retention of properties upon deformation. In addition, NiO-based devices have more stable switching properties.

5. Future Perspectives and Conclusions

Flexible devices are largely in demand due to the rising tendency for good fitting, comfortable, easy-to-use tools and materials, which can feed the creativity of fashion designs. In addition, practicality, improved operativity and portability of smart devices for well-being aid the monitoring of vital parameters for personalized drug-delivery based on actual needs (all-in-all helping to maintain a healthy lifestyle).
To a larger scale, modern, eco-sustainable architecture requires the use of materials which are adaptable, versatile, and energetically self-standing. The future development of flexible devices is very much connected to the development of integrated flexible components with homogenous characteristics of durability which ensure the proper functioning of the devices, a low-cost production and can be easily recharged if necessary. In this framework, some recently developed flexible tools were presented based on NiO as low-cost active material, which can give rise to improved flexible technology for the pursue of personalized items and tailored devices.
NiO-based flexible supercapacitors may b achieved by the de-alloying, a technique that allows the fabrication of all-solid-state devices with associated advantages over the employment of liquid electrolytes (which may be affected by leakage and transport issues). These devices are characterized by a capacitance comparable to analoguesbased on 2D materials and higher resilience to repeated bending and deformation cycles [132]. The embedding of NiO is fundamental to enhance the electrochemical response of the plastisol towards specific targets such as H2O2. Overall, the hybrid NiO-plastisol material has performances, to some extent, comparable to other non-flexible electrodes (see Table 3 in Ref. [103]), with the additional advantage of its flexibility.
The examined NiO flexible temperature device showed a large resistance to harsh basic and acidic conditions (as compared, for instance, to graphene ribbons-based devices) and less sensitivity to the oxidative conditions in the synthesis procedure (which affects the reproducibility of the production of graphene derivatives-based devices) [133].
Flexible resistive random-access memory devices are usually achieved by employing a supple support topped by an active layer. Several options were suggested as active material, such as black phosphorous [134] or graphene oxide [135]. However, in both cases, limitations arise from the difficulty of handling the material in safe conditions or to achieve stability and/or stable performances (in addition to the employment of expensive connection Au layers) [135].
NiO-based electrochromic devices have better optical contrast, shorter coloring and bleaching times as compared to other inorganic analogues such as Ag-nanowire based ones [136], and cheaper manufacturing, due to lower cost of production materials.
For solar cells the use of NiO is, in a way, essential, since it is routinely used as HTL, the real challenge being the insertion of the NiO component in flexible structures while keeping or improving the conversion efficiency.
However, the manufacturing techniques may strongly impact the performances of the electronic devices. The retention of properties and consequent scalability of the processes is usually more favorable for flexible materials prepared by physical deposition methods. However, these are lengthy and costly procedures. Therefore, the development of solution-processable materials is preferred for device assembling. In this regard, the molding as well as the ink deposition and printing techniques are the most promising for the fabrication of devices. Furthermore, a correct assessment of the performances upon stress would require several deformation tests, including stretching and compression (besides bending), especially for the electronic materials destined to wearable devices. Therefore, a complete evaluation of performances under stress is advisable for the planning of a device.
In general, NiO provides the benefits of a low-cost material, which can be easily adapted to the specific needs through tailored synthetic pathways. The use of NiO may be of source of concern when included into devices which come in contact with the skin since it can induce allergic reactions. Furthermore, it can be considered a bio-hazard since it enhances the risks of lungs cancer. Therefore, sealing must be ensured of the NiO containing component to avoid leakage and ensuing adverse responses.
In summary, NiO-based flexible devices provide quite often a practical, efficient, resilient, and low-cost option to fulfill the increasing demand of flexible technology.

Funding

This research received no external funding.

Institutional Review Board Statement

Not Applicable.

Informed Consent Statement

Not Applicable.

Conflicts of Interest

The author declares no conflict of interest.

References

  1. Mannsfeld, S.C.; Tee, B.C.; Stoltenberg, R.M.; Chen, C.V.; Barman, S.; Muir, B.V.O.; Sokolov, A.N.; Reese, C.; Bao, Z. Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nat. Mater. 2010, 9, 859. [Google Scholar] [CrossRef] [PubMed]
  2. Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D.N.; Hata, K. A stretchable carbon nanotube strain sensor for human-motion detection. Nat. Nanotechnol. 2011, 6, 296. [Google Scholar] [CrossRef] [PubMed]
  3. Yang, Y.; Gao, W. Wearable and flexible electronics for continuous molecular monitoring. Chem. Soc. Rev. 2019, 48, 1465–1491. [Google Scholar] [CrossRef] [PubMed]
  4. Schwartz, G.; Tee, B.C.K.; Mei, J.; Appleton, A.L.; Kim, D.H.; Wang, H.; Bao, Z. Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nat. Commun. 2013, 4, 1859. [Google Scholar] [CrossRef] [PubMed]
  5. Choong, C.L.; Shim, M.B.; Lee, B.S.; Jeon, S.; Ko, D.-S.; Kang, T.-H.; Bae, J.; Lee, S.-H.; Byun, K.-E.; Im, J.; et al. Highly stretchable resistive pressure sensors using a conductive elastomeric composite on a micropyramid array. Adv. Mater. 2014, 26, 3451. [Google Scholar] [CrossRef] [PubMed]
  6. Trung, T.Q.; Lee, N.E. Flexible and stretchable physical sensor integrated platforms for wearable human-activity monitoring and personal healthcare. Adv. Mater. 2016, 28, 4338–4372. [Google Scholar] [CrossRef] [PubMed]
  7. Zhu, H.; Wang, X.H.; Liang, J.; Lv, H.; Tong, H.; Ma, L.; Hu, Y.; Zhu, G.; Zhang, T.; Tie, Z.; et al. Versatile electronic skins for motion detection of joints enabled by aligned few-walled carbon nanotubes in flexible polymer composites. Adv. Funct. Mater. 2017, 27, 1606604. [Google Scholar] [CrossRef]
  8. Kim, D.H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S.M.; Tao, H.; Islam, A.; et al. Epidermal electronics. Science 2011, 333, 838. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  9. Kenry, J.C.Y.; Lim, C.T. Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications. Microsyst. Nanoeng. 2016, 2, 16043. [Google Scholar] [CrossRef] [PubMed]
  10. Lacour, S.P.; Benmerah, S.; Tarte, E.; FitzGerald, J.; Serra, J.; McMahon, S.; Fawcett, J.; Graudejus, O.; Yu, Z.; Morrison, B. Flexible and stretchable micro-electrodes for in vitro and in vivo neural interfaces. Med. Biolog. Eng. Comput. 2010, 48, 945–954. [Google Scholar] [CrossRef] [PubMed]
  11. Jeong, J.W.; Kim, M.K.; Cheng, H.; Yeo, W.-H.; Huang, X.; Liu, Y.; Zhang, Y.; Huang, Y.; Rogers, J.A. Capacitive epidermal electronics for electrically safe, long-term electrophysiological measurements. Adv. Healthc. Mater. 2014, 3, 642–648. [Google Scholar] [CrossRef] [PubMed]
  12. Yang, Y.; Yang, X.; Zou, X.; Wu, S.; Wan, D.; Cao, A.; Liao, L.; Yuan, Q.; Duan, X. Ultrafine graphene nanomesh with large on/off ratio for high-performance flexible biosensors. Adv. Funct. Mater. 2017, 27, 1604096. [Google Scholar] [CrossRef]
  13. Zou, L.; Ge, Z.C.; Wang, J.; Cretu, E.; Li, X. Novel Tactile Sensor Technology and Smart Tactile Sensing Systems: A Review. Sensors 2017, 17, 2653. [Google Scholar] [CrossRef] [PubMed]
  14. Eh, A.L.-S.; Tan, A.W.M.; Cheng, X.; Magdassi, S.; Lee, P.S. Recent Advances in Flexible Electrochromic Devices: Prerequisites, Challenges, and Prospects. Energy Technol. 2018, 6, 33–45. [Google Scholar] [CrossRef]
  15. Chen, Z.; To, J.W.F.; Wang, C.; Lu, Z.; Liu, N.; Chortos, A.; Pan, L.; Wei, F.; Cui, Y.; Bao, Z. A three-dimensionally interconnected carbon nanotube-conducting polymer hydrogel network for high-performance flexible battery electrodes. Adv. Energy Mater. 2014, 4, 1400207. [Google Scholar] [CrossRef]
  16. Lin, H.; Weng, W.; Ren, J.; Qiu, L.; Zhang, Z.; Chen, P.; Chen, X.; Deng, J.; Wang, Y.; Peng, H. Twisted aligned carbon nanotube/silicon composite fiber anode for flexible wire-shaped lithium-ion battery. Adv. Mater. 2014, 26, 1217–1222. [Google Scholar] [CrossRef] [PubMed]
  17. Gwon, H.; Hong, J.; Kim, H.; Seo, D.-H.; Jeon, S.; Kang, K. Recent progress on flexible lithium rechargeable batteries. Energy Environm. Sci. 2014, 7, 538–551. [Google Scholar] [CrossRef]
  18. Chen, L.; Zhou, G.; Liu, Z.; Ma, X.; Chen, J.; Zhang, Z.; Ma, X.; Li, F.; Cheng, H.-M.; Ren, W. Scalable clean exfoliation of high-quality few-layer black phosphorus for a flexible lithium ion battery. Adv. Mater. 2016, 28, 510–517. [Google Scholar] [CrossRef] [PubMed]
  19. Mo, R.; Rooney, D.; Sun, K.; Yang, H.Y. 3D nitrogen-doped graphene foam with encapsulated germanium/nitrogen-doped graphene yolk-she, ll nanoarchitecture for high-performance flexible Li ion battery. Nat. Commun. 2017, 8, 13949. [Google Scholar] [CrossRef] [PubMed]
  20. Li, X.; Li, P.; Wu, Z.; Luo, D.; Yu, H.-Y.; Lu, Z.-H. Review and perspective of materials for flexible solar cells. Mater. Rep. Energy 2021, 1, 100001. [Google Scholar] [CrossRef]
  21. Dong, S.; Xi, J.; Wu, Y.; Liu, H.; Fu, C.; Liu, H.; Xiao, F. High loading MnO2 nanowires on graphene paper: Facile electrochemical synthesis and use as flexible electrode for tracking hydrogen peroxide secretion in live cells. Anal. Chim. Acta 2015, 853, 200–206. [Google Scholar] [CrossRef]
  22. Nagaraju, G.; Raju, G.S.R.; Ko, Y.H.; Yu, J.S. Hierarchical Ni–Co layered double hydroxide nanosheets entrapped on conductive textile fibers: A cost-effective and flexible electrode for high-performance pseudocapacitors. Nanoscale 2016, 8, 812–825. [Google Scholar] [CrossRef]
  23. Pu, J.; Yomogida, Y.; Liu, K.-K.; Li, L.-J.; Iwasa, Y.; Takenobu, T. Highly flexible MoS2 thin film transistors with ion gel dielectrics. Nano Lett. 2012, 12, 4013–4017. [Google Scholar] [CrossRef]
  24. Lee, G.H.; Yu, Y.J.; Cui, X.; Petrone, N.; Lee, C.-H.; Choi, M.S.; Lee, D.-Y.; Lee, C.; Yoo, W.J.; Watanabe, K.; et al. Flexible and transparent MoS2 field-effect transistors on hexagonal boron nitride-graphene heterostructures. ACS Nano 2013, 7, 7931–7936. [Google Scholar] [CrossRef]
  25. Chortos, A.; Lim, J.; To, J.W.F.; Vosgueritchian, M.; Dusseault, T.J.; Kim, T.H.; Hwang, S.; Bao, Z. Highly stretchable transistors using a microcracked organic semiconductor. Adv. Mater. 2014, 26, 4253–4259. [Google Scholar] [CrossRef]
  26. Park, S.; Shin, S.H.; Yogeesh, M.N.; Lee, A.L.; Rahimi, S.; Akinwande, D. Extremely high-frequency flexible graphene thin-film transistors. IEEE Electron Device Lett. 2016, 37, 512–515. [Google Scholar] [CrossRef]
  27. Kim, Y.H.; Lee, E.Y.; Lee, H.H.; Seo, S. Characteristics of reduced graphene oxide quantum dots for a flexible memory thin film transistor. ACS Appl. Mater. Interfaces 2017, 9, 16375–16380. [Google Scholar] [CrossRef]
  28. Liang, J.; Li, L.; Niu, X.; Pei, Q. Elastomeric polymer light-emitting devices and displays. Nat. Photonics 2013, 7, 817–824. [Google Scholar] [CrossRef]
  29. Jung, M.; Kim, J.; Noh, J.; Lim, N.; Lim, C.; Lee, G.; Kim, J.; Kang, H.; Jung, K.; Leonard, A.D.; et al. All-printed and roll-to-roll-printable 13.56-MHz-operated 1-bit RF tag on plastic foils. IEEE Trans. Electron Devices 2010, 57, 571. [Google Scholar] [CrossRef]
  30. Gupta, V.; Kapur, S.; Saurabh, S.; Grover, A. Resistive Random Access Memory: A Review of Device Challenges. IETE Tech. Rev. 2020, 37, 377–390. [Google Scholar] [CrossRef]
  31. Casaletto, M.P.; Zanoni, R.; Carbone, M.; Piancastelli, M.N.; Aballe, L.; Weiss, K.; Horn, K. Ethylene adsorption on Si(100)2 × 1: A high-resolution photoemission study. Phys. Rev. B 2000, 62, 17128–17133. [Google Scholar] [CrossRef]
  32. Kim, J.W.; Carbone, M.; Tallarida, J.H.; Dil, M.; Horn, K.; Casaletto, M.P.; Flammini, R.; Piancastelli, M.N. Adsorption of 2,3-butanediol on Si(100). Surf. Sci. 2004, 559, 179–185. [Google Scholar] [CrossRef]
  33. Carbone, M.; Caminiti, R. Adsorption states and site conversions of phenylacetylene on Si(100)2 × 1 calculated by DFT. J. Theor. Comput. Chem. 2012, 11, 1089–1099. [Google Scholar] [CrossRef]
  34. Carbone, M.; Piancastelli, M.N.; Zanoni, R.; Comtet, G.; Dujardin, G.; Hellner, L. Methanol adsorption on Si(111)-(7 × 7), investigated by core-line photoemission and mass spectrometry of photodesorbed ions. Surf. Sci. 1997, 370, L179–L184. [Google Scholar] [CrossRef]
  35. Carbone, M.; Zanoni, R.; Piancastelli, M.N.; Comtet, G.; Hellner, L.; Mayne, A. Adsorption of ethylene on Si(111)7 × 7 by synchrotron radiation photoemission. J. Electron Spectrosc. 1995, 76, 271–276. [Google Scholar] [CrossRef]
  36. Carbone, M.; Piancastelli, M.N.; Casaletto, M.P.; Zanoni, R.; Besnard-Ramage, M.J.; Comtet, G.; Dujardin, G.; Hellner, L. Phenol adsorption on Si (111)7 × 7 studied by synchrotron radiation photoemission and photodesorption. Surf. Sci. 1999, 419, 114–119. [Google Scholar] [CrossRef]
  37. Flammini, R.; Cecchetti, D.; Tagliatesta, P.; Carbone, M. Multiple options for phenol on Si(111)7 × 7 revealed by high resolution photoemission. Surf. Sci. 2020, 692, 121510. [Google Scholar] [CrossRef]
  38. Carbone, M.; Zanoni, R.; Piancastelli, M.N.; Comtet, G.; Dujardin, G.; Hellner, L. Synchrotron radiation photoemission and photostimulated desorption of deuterated methanol on Si(111)7 × 7 and Si(100)2 × 1. Surf. Sci. 1996, 352–354, 391–395. [Google Scholar] [CrossRef]
  39. Carbone, M.; Piancastelli, M.N.; Paggel, J.J.; Weindel, C.; Horn, K. A high-resolution photoemission study of ethanol adsorption on Si(111)-(7 × 7). Surf. Sci. 1998, 412–413, 441–446. [Google Scholar] [CrossRef]
  40. Carbone, M.; Caminiti, R. Fragmentation pathways of acetic acid upon adsorption on Si(100)2 × 1. Surf. Sci. 2008, 602, 852–858. [Google Scholar] [CrossRef]
  41. Carbone, M.; Meloni, S.; Caminiti, R. Dissociative versus molecular adsorption of phenol on Si (100) 2 × 1: A first-principles calculation. Phys. Rev. B 2007, 76, 085332. [Google Scholar] [CrossRef]
  42. Hammock, M.L.; Chortos, A.; Tee, B.C.K.; Tok, J.B.-H.; Bao, Z. 25th anniversary article: The evolution of electronic skin (e-skin): A brief history, design considerations, and recent progress. Adv. Mater. 2013, 25, 5997. [Google Scholar] [CrossRef] [PubMed]
  43. Rogers, J.A.; Someya, T.; Huang, Y. Materials and mechanics for stretchable electronics. Science 2010, 327, 1603. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  44. Ying, M.; Bonifas, A.P.; Lu, N.; Su, Y.; Li, R.; Cheng, H.; Ameen, A.; Huang, Y.; Rogers, J.A. Silicon nanomembranes for fingertip electronics. Nanotechnology 2012, 23, 344004. [Google Scholar] [CrossRef] [PubMed]
  45. Zou, M.; Ma, Y.; Yuan, X.; Hu, Y.; Liu, J.; Jin., Z. Flexible devices: From materials, architectures to applications. J. Semicond. 2018, 39, 011010. [Google Scholar] [CrossRef]
  46. Valentini, F.; Roscioli, D.; Carbone, M.; Conte, V.; Floris, B.; Palleschi, G.; Flammini, R.; Bauer, E.M.; Nasillo, G.; Caponetti, E. Oxidized graphene in ionic liquids for assembling chemically modified electrodes: A structural and electrochemical characterization study. Anal. Chem. 2012, 84, 5823–5831. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  47. Valentini, F.; Roscioli, D.; Carbone, M.; Conte, V.; Floris, B.; Bauer, E.M.; Ditaranto, N.; Sabbatini, L.; Caponetti, E.; Chillura-Martino, D. Graphene and ionic liquids new gel paste electrodes for caffeic acid quantification. Sens. Actuat. B Chem. 2015, 212, 248–255. [Google Scholar] [CrossRef]
  48. Valentini, F.; Carbone, M.; Palleschi, G. Graphene oxide nanoribbons (GNO), reduced graphene nanoribbons (GNR), and multi-layers of oxidized graphene functionalized with ionic liquids (GO-IL) for assembly of miniaturized electrochemical devices. Anal. Bioanal. Chem. 2013, 405, 3449–3474. [Google Scholar] [CrossRef]
  49. Han, T.-H.; Kim, H.; Kwon, S.-J.; Lee, T.-W. Graphene-based flexible electronic devices. Mater. Sci. Eng. R Rep. 2017, 118, 1–43. [Google Scholar] [CrossRef]
  50. Lee, S.-M.; Kim, J.-H.; Ahn, J.-H. Graphene as a flexible electronicmaterial: Mechanical limitations by defectformation and efforts to overcome. Mater. Today 2015, 18, 336–344. [Google Scholar] [CrossRef]
  51. Petrone, N.; Meric, I.; Chari, T.; Shepard, K.L.; Hone, J. Graphene Field-Effect Transistors for Radio-Frequency Flexible Electronics. IEEE J. Electron Devices Soc. 2015, 3, 44–48. [Google Scholar] [CrossRef]
  52. Carbone, M.; Missori, M.; Micheli, L.; Tagliatesta, P.; Bauer, E.M. NiO pseudocapacitance and optical properties: Does the shape win? Materials 2020, 13, 1417. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  53. Guillén, C.; Herrero, J. Influence of Acceptor Defects on the Structural, Optical and Electrical Properties of Sputtered NiO Thin Films. Phys. Status Solidi A 2021, 218, 2100237. [Google Scholar] [CrossRef]
  54. Mallick, G.; Labh, J.; Giri, L.; Pandey, A.C.; Karna, S.P. Facile synthesis and electron transport properties of NiO nanostructures investigated by scanning tunneling microscopy. AIP Adv. 2017, 7, 085007. [Google Scholar] [CrossRef] [Green Version]
  55. Kwon, U.; Kim, B.-G.; Nguyen, D.C.; Park, J.-H.; Ha, N.Y.; Kim, S.-J.; Ko, S.H.; Lee, S.; Lee, D.; Park, H.J. Solution-Processible Crystalline NiO Nanoparticles for HighPerformance Planar Perovskite Photovoltaic Cells. Sci. Rep. 2016, 6, 30759. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  56. Donia, D.; Bauer, E.M.; Missori, M.; Roselli, L.; Cecchetti, D.; Tagliatesta, P.; Gontrani, L.; Carbone, M. Room Temperature Syntheses of ZnO and Its Structure. Symmetry 2021, 13, 733. [Google Scholar] [CrossRef]
  57. Donia, D.T.; Carbone, M. Fate of the nanoparticles in environmental cycles. Int. J. Environ. Sci. Technol. 2019, 16, 583–600. [Google Scholar] [CrossRef]
  58. Carbone, M. Cu-Zn-Co nanosized mixed oxides prepared from hydroxycarbonate precursors. J. Alloys Compd. 2016, 688, 202–209. [Google Scholar] [CrossRef]
  59. Carbone, M.; Nesticò, A.; Bellucci, N.; Micheli, L.; Palleschi, G. Enhanced performances of sensors based on screen printed electrodes modified with nanosized NiO particles. Electrochim. Acta 2017, 246, 580–587. [Google Scholar] [CrossRef]
  60. Carbone, M.; Briancesco, R.; Bonadonna, L. Antimicrobial power of Cu/Zn mixed oxide nanoparticles to Escherichia coli. Environ. Nanotech. Monitor. Manag. 2017, 7, 97–102. [Google Scholar] [CrossRef]
  61. Carbone, M. Zn defective ZnCo2O4 nanorods as high capacity anode for lithium ion batteries. J. Electroanal. Chem. 2018, 815, 151–157. [Google Scholar] [CrossRef]
  62. Carbone, M.; Sabbatella, G.; Antonaroli, S.; Remita, H.; Orlando, V.; Biagioni, S.; Nucara, A. Exogenous control over intracellular acidification: Enhancement via proton caged compounds coupled to gold nanoparticles. Biochim. Biophys. Acta Gen.-Subj. 2015, 1850, 2304–2307. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  63. Sabbatella, G.; Antonaroli, S.; Diociauti, M.; Nucara, A.; Carbone, M. Synthesis of proton caged disulphide compounds for gold nanoparticle functionalization. New J. Chem. 2015, 39, 2489–2496. [Google Scholar] [CrossRef]
  64. Carbone, M.; Bauer, E.M.; Micheli, L.; Missori, M. NiO morphology dependent optical and electrochemical properties. Colloids Surf. A 2017, 532, 178–182. [Google Scholar] [CrossRef]
  65. Nakate, U.T.; Ahmad, R.; Patil, P.; Yud, Y.T.; Hahn, Y.-B. Ultra thin NiO nanosheets for high performance hydrogen gas sensor device. Appl. Surf. Sci. 2020, 506, 144971. [Google Scholar] [CrossRef]
  66. Carbone, M.; Tagliatesta, P. NiO grained-flowers and nanoparticles for ethanol sensing. Materials 2020, 13, 1880. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  67. Mokoena, T.P.; Swart, H.C.; Motaung, D.E. A review on recent progress of p-type nickel oxide based gas sensors: Future perspectives. J. Alloys Compd. 2019, 805, 267–294. [Google Scholar] [CrossRef]
  68. Carbone, M. CQDs@NiO: An efficient tool for CH4 sensing. Appl. Sci. 2020, 10, 6251. [Google Scholar] [CrossRef]
  69. Dhas, S.D.; Maldar, P.S.; Patil, M.D.; Nagare, A.B.; Waikar, M.R.; Sonkawade, R.G.; Moholkar, A.V. Synthesis of NiO nanoparticles for supercapacitor application as an efficient electrode material. Vacuum 2020, 181, 109646. [Google Scholar] [CrossRef]
  70. Xiong, S.; Jiang, S.; Wang, J.; Lin, H.; Lin, M.; Weng, S.; Liu, S.; Jiao, Y.; Xu, Y.; Chen, J. A high-performance hybrid supercapacitor with NiO derived NiO@Ni-MOF composite electrodes. Electrochim. Acta 2020, 340, 135956. [Google Scholar] [CrossRef]
  71. Xiao, H.; Yao, S.; Liu, H.; Qu, F.; Zhang, X.; Wun, X. NiO nanosheet assembles for supercapacitor electrode materials. Progr. Nat. Sci. 2016, 26, 271–275. [Google Scholar] [CrossRef] [Green Version]
  72. Di Girolamo, D.; Di Giacomo, F.; Matteocci, F.; Marrani, A.G.; Dini, D.; Abate, A. Progress, highlights and perspectives on NiO in perovskite photovoltaics. Chem. Sci. 2020, 11, 7746–7759. [Google Scholar] [CrossRef] [PubMed]
  73. Guo, T.; Zhang, Z.; Yu, L.; Yuan, H.; Zhang, J.; Liu, X.; Hu, Z.; Zhu, Y. Synthesis of well dispersed NiO ink for efficient perovskite solar cells. J. Alloys Compd. 2021, 860, 157889. [Google Scholar] [CrossRef]
  74. Ielmini, D.; Nardi, F.; Cagli, C. Physical models of size-dependent nanofilament formation and rupture in NiO resistive switching memories. Nanotechnology 2011, 22, 254022. [Google Scholar] [CrossRef] [PubMed]
  75. Ielmini, D.; Nardi, F.; Cagli, C.; Lacaita, A.L. Size-Dependent Retention Time in NiO-Based Resistive-Switching Memories. IEEE Electr. Device Lett. 2010, 31, 353–355. [Google Scholar] [CrossRef]
  76. Lee, E.M.; Ahn, Y.; Son, J.Y. Electric field control of magnetization reversal in conducting filament nanostructures in NiO resistive random access memory. J. Alloys Compd. 2020, 840, 155748. [Google Scholar] [CrossRef]
  77. Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nat. Nanotechnol. 2011, 6, 232–236. [Google Scholar] [CrossRef] [PubMed]
  78. Kumar, P.V.; Ahamed, A.J.; Karthikeyan, M. Synthesis and characterization of NiO nanoparticles by chemical as well as green routes and their comparisons with respect to cytotoxic effect and toxicity studies in microbial and MCF-7 cancer cell models. SN Appl. Sci. 2019, 1, 1083. [Google Scholar] [CrossRef] [Green Version]
  79. Ariyanta, H.A.; Ivandini, T.A.; Yulizar, Y. Novel NiO nanoparticles via phytosynthesis method: Structural, morphological and optical properties. J. Mol. Struct. 2021, 1227, 129543. [Google Scholar] [CrossRef]
  80. Ukoba, K.O.; Eloka-Eboka, A.C.; Inambao, F.L. Review of nanostructured NiO thin film deposition using the spray pyrolysis Technique. Renew. Sust. Energ. Rev. 2018, 82, 2900–2915. [Google Scholar] [CrossRef]
  81. Erlebacher, J.; Aziz, M.J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Evolution of nanoporosity in dealloying. Nature 2001, 45, 451–453. [Google Scholar] [CrossRef] [PubMed]
  82. Meher, S.K.; Justin, P.; Rao, G.R. Nanoscale morphology dependent pseudocapacitance of NiO: Influence of intercalating anions during synthesis. Nanoscale 2011, 3, 683. [Google Scholar] [CrossRef] [PubMed]
  83. Brus, L.E. Electronic wave functions in semiconductor clusters: Experiment and theory. J. Phys. Chem. 1986, 90, 2555–2560. [Google Scholar] [CrossRef]
  84. Raj, B.G.S.; Natesan, B.; Asiri, A.M.; Wu, J.J.; Anandan, S. Pseudocapacitive properties of nickel oxide nanoparticles synthesized via ultrasonication approach. Ionics 2019, 26, 953–960. [Google Scholar]
  85. Sun, W.; Xiao, L.; Wu, X. Facile synthesis of NiO nanocubes for photocatalysts and supercapacitor electrodes. J. Alloys Compd. 2019, 772, 465–471. [Google Scholar] [CrossRef]
  86. Vidhyadharan, B.; Zain, N.K.M.; Misnon, I.I.; Aziz, R.A.; Ismail, J.; Yusoff, M.M.; Jose, R. High performance supercapacitor electrodes from electrospun nickel oxide nanowires. J. Alloys Compd. 2014, 610, 143–150. [Google Scholar] [CrossRef]
  87. Yuan, C.; Li, J.; Hou, L.; Yang, L.; Shen, L.; Zhang, X. Facile growth of hexagonal NiO nanoplatelet arrays assembled by mesoporous nanosheets on Ni foam towards high-performance electrochemical capacitors. Electrochim. Acta 2012, 78, 532–538. [Google Scholar] [CrossRef]
  88. Yang, H.; Zou, J. Controllable preparation of hierarchical NiO hollow microspheres with high pseudo-capacitance. Trans. Nonferrous Met. Soc. China 2018, 28, 1808–1818. [Google Scholar] [CrossRef]
  89. Xu, K.; Zou, R.; Li, W.; Xue, Y.; Song, G.; Liu, Q.; Liu, X.; Hu, J. Self-assembling hybrid NiO/Co3O4 ultrathin and mesoporous nanosheets into flower-like architectures for pseudocapacitance. J. Mater. Chem. A 2013, 1, 9107–9113. [Google Scholar] [CrossRef]
  90. Hakamada, M.; Abe, T.; Mabuchi, M. Electrodes from carbon nanotubes/NiO nanocomposites synthesized in modified Watts bath for supercapacitors. J. Power Sources 2016, 325, 670–674. [Google Scholar] [CrossRef] [Green Version]
  91. Wu, X.; Wang, Q.; Zhang, W.; Wang, Y.; Chen, W. Nano nickel oxide coated graphene/polyaniline composite film with high electrochemical performance for flexible supercapacitor. Electrochim. Acta 2016, 211, 1066–1075. [Google Scholar] [CrossRef]
  92. Qiu, D.; Ma, X.; Zhang, J.; Lin, Z.; Zhao, B. In situ synthesis of mesoporous NiO nanoplates embedded in a flexible graphene matrix for supercapacitor electrodes. Mater. Lett. 2018, 232, 163–166. [Google Scholar] [CrossRef]
  93. Ran, F.; Yang, H.; Wu, Y.; Zhao, X.; Tan, Y.; Liu, Y.; Niu, X.; Chen, Y.; Kong, L.; Kang, L. Facile preparation of porous nickel oxide membrane for flexible supercapacitors electrode via phase-separation method of polymer. Mater. Res. Bull. 2018, 103, 25–31. [Google Scholar] [CrossRef]
  94. Buxton, S.; Garman, E.; Heim, K.E.; Lyons-Darden, T.; Schlekat, C.E.; Taylor, M.D.; Oller, A.R. Concise Review of Nickel Human Health Toxicology and Ecotoxicology. Inorganics 2019, 7, 89. [Google Scholar] [CrossRef] [Green Version]
  95. Horie, M.; Stowe, M.; Tabei, M.; Kuroda, E. Metal ion release of manufactured metal oxide nanoparticles is involved in the allergic response to inhaled ovalbumin in mice. Occup. Dis. Environ. Med. 2016, 4, 17. [Google Scholar] [CrossRef] [Green Version]
  96. Oller, A.R. Respiratory carcinogenicity assessment of soluble nickel compounds. Environ. Health Perspect. 2002, 110, 841–844. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  97. Sousa, C.A.; Soares, H.M.V.M.; Soares, E.V. Toxic effects of nickel oxide (NiO) nanoparticles on the freshwater alga Pseudokirchneriella subcapitata. Aquat. Toxicol. 2018, 204, 80–90. [Google Scholar] [CrossRef] [PubMed]
  98. Qin, C.; Zheng, D.; Hu, Q.; Zhang, X.; Wang, Z.; Li, Y.; Zhu, J.; Ou, J.Z.; Yang, C.; Wang, Y. Flexible integrated metallic glass-based sandwich electrodes for high-performance wearable all-solid-state supercapacitors. App. Mater. Today 2020, 19, 100539. [Google Scholar] [CrossRef]
  99. Wang, Z.F.; Zhang, X.M.; Liu, X.L.; Wang, Y.C.; Zhang, Y.G.; Li, Y.Y.; Zhao, W.M.; Qin, C.L.; Mukanova, A.; Bakenov, Z. Bimodal nanoporous NiO@Ni–Si network prepared by dealloying method for stable Li-ion storage. J. Power Sources 2020, 449, 227550. [Google Scholar] [CrossRef]
  100. Wang, Z.F.; Fei, P.Y.; Xiong, H.Q.; Qin, C.L.; Zhao, W.M.; Liu, X.Z. CoFe2O4 nanoplates synthesized by dealloying method as high performance Li-ion battery anodes. Electrochim. Acta 2017, 252, 295–305. [Google Scholar] [CrossRef]
  101. Zhao, F.; Zheng, D.; Liu, Y.; Pan, F.; Deng, Q.; Qin, C.; Li, Y.; Wang, Z. Flexible Co(OH)2/NiOxHy@Ni hybrid electrodes for high energy density supercapacitors. Chem. Eng. J. 2021, 415, 18871. [Google Scholar] [CrossRef]
  102. Marsilia, M.; Susmel, S. Free-standing Plastic electrodes: Formulation, electrochemical characterization and application to dopamine detection. Sensor. Actuat. B Chem. 2018, 255, 1087–1096. [Google Scholar] [CrossRef]
  103. Carbone, M.; Aneggi, E.; Figueredo, F.; Susmel, S. NiO-nanoflowers decorating a plastic electrode for the non-enzymatic amperometric detection of H2O2 in milk: Old issue, new challenge. Food Control 2022, 132, 108549. [Google Scholar] [CrossRef]
  104. Kima, S.-J.; Lee, H.; Hong, S.-H. Solution-processed flexible NiO resistive random access memory device. Solid State Electron. 2018, 142, 56–61. [Google Scholar] [CrossRef]
  105. Rakibuddin, M.; Shinde, M.A.; Kim, H. Sol-gel fabrication of NiO and NiO/WO3 based electrochromic device on ITO and flexible substrate. Ceram. Int. 2020, 46, 8631–8639. [Google Scholar] [CrossRef]
  106. Lee, S.J.; Lee, T.-G.; Nahm, S.; Kim, D.H.; Yang, D.J.; Han, S.H. Investigation of all-solid-state electrochromic devices with durability enhanced tungsten-doped nickel oxide as a counter electrode. J. Alloys Compd. 2020, 815, 152399. [Google Scholar] [CrossRef]
  107. Appiagyei, A.B.; Banua, J.; Han, J.I. Flexible and patterned-free Ni/NiO-based temperature device on cylindrical PET fabricated by RF magnetron sputtering: Bending and washing endurance tests. J. Ind. Eng. Chem. 2021, 100, 372–382. [Google Scholar] [CrossRef]
  108. Cao, B.; Yang, L.; Jiang, S.; Lin, H.; Wang, N.; Li, X. Flexible Quintuple Cation Perovskite Solar Cells with High Efficiency. J. Mater. Chem. A 2019, 7, 4960–4970. [Google Scholar] [CrossRef]
  109. Jo, J.W.; Seo, M.-S.; Park, M.; Kim, J.-Y.; Park, J.S.; Han, I.K.; Ahn, H.; Jung, J.W.; Sohn, B.-H.; Ko, M.J.; et al. Improving Performance and Stability of Flexible Planar-Heterojunction Perovskite Solar Cells Using Polymeric Hole-Transport Material. Adv. Funct. Mater. 2016, 26, 4464–4471. [Google Scholar] [CrossRef]
  110. Yin, X.; Chen, P.; Que, M.; Xing, Y.; Que, W.; Niu, C.; Shao, J. Highly Efficient Flexible Perovskite Solar Cells Using Solution-Derived NiOx Hole Contacts. ACS Nano 2016, 10, 3630–3636. [Google Scholar] [CrossRef]
  111. Wang, Z.; Rong, X.; Wang, L.; Wang, W.; Lin, H.; Li, X. Dual Role of Amino-Functionalized Graphene Quantum Dots in NiOx Films for Efficient Inverted Flexible Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 8342–8350. [Google Scholar] [CrossRef] [PubMed]
  112. Rehman, S.; Bi, H. Electrodes for Flexible Integrated Supercapacitors, from Flexible Supercapacitor Nanoarchitectonics. Scrivener Publ. LLC 2021, Chapter 1, 1–26. [Google Scholar]
  113. Shi, S.; Xu, C.; Yang, C.; Li, J.; Du, H.; Li, B.; Kang, F. Flexible supercapacitors. Particuology 2013, 11, 371–377. [Google Scholar] [CrossRef]
  114. Zhang, Y.-Z.; Wang, Y.; Cheng, T.; Lai, W.-Y.; Pang, H.; Huang, W. Flexible supercapacitors based on paper substrates: A new paradigm for low-cost energy storage. Chem. Soc. Rev. 2015, 44, 5181–5199. [Google Scholar] [CrossRef] [PubMed]
  115. Chee, W.; Lim, H.; Zainal, Z.; Harrison, I.; Huang, N.; Andou, Y.; Chong, K.; Pandikumar, A. Electrospun nanofiber membrane as ultrathin flexible supercapacitors. RSC Adv. 2017, 7, 12033–12040. [Google Scholar] [CrossRef] [Green Version]
  116. Zhai, S.; Jin, K.; Zhou, M.; Fan, Z.; Zhao, H.; Li, X.; Zhao, Y.; Ge, F.; Cai, Z. A novel high performance flexible supercapacitor based on porous carbonized cotton/ZnO nanoparticle/CuS micro-sphere. Colloids Surf. A Physicochem. Eng. Asp. 2020, 584, 124025. [Google Scholar] [CrossRef]
  117. Khattak, A.M.; Yin, H.; Ghazi, Z.A.; Liang, B.; Iqbal, A.; Khan, N.A.; Gao, Y.; Li, L.; Tang, Z. Three dimensional iron oxide/graphene aerogel hybrids as all-solid- state flexible supercapacitor electrodes. RSC Adv. 2016, 6, 58994–59000. [Google Scholar] [CrossRef]
  118. Fu, W.; Zhao, E.; Ma, R.; Sun, Z.; Yang, Y.; Sevilla, M.; Fuertes, A.B.; Magasinski, A.; Yushin, G. Anatase TiO2 confined in carbon nanopores for high-energy Li-Ion hybrid supercapacitors operating at high rates and subzero temperatures. Adv. Energy Mater. 2020, 10, 1902993. [Google Scholar] [CrossRef]
  119. Gwo-Bin, L.; Fu-Chun, H.; Chia-Yen, L.; Jiun-Jih, M. A new fabrication process for a flexible skin with temperature sensor array and its applications. Acta Mech. Sin. 2003, 20, 27–32. [Google Scholar] [CrossRef]
  120. Yu, C.; Wang, Z.; Yu, H.; Jiang, H. A stretchable temperature sensor based on elastically buckled thin film devices on elastomeric substrates. Appl. Phys. Lett. 2009, 95, 141912. [Google Scholar] [CrossRef] [Green Version]
  121. Dankoco, M.D.; Tesfay, G.Y.; Benevent, E.; Bendahan, M. Temperature sensor realized by inkjet printing process on flexible substrate. Mater. Sci. Eng. B 2015, 205, 1–5. [Google Scholar] [CrossRef]
  122. Wang, H.; Zhu, B.; Ma, X.; Hao, Y.; Chen, X. Physically Transient Resistive Switching Memory Based on Silk Protein. Small 2016, 12, 2715–2719. [Google Scholar] [CrossRef] [PubMed]
  123. Wang, H.; Zhu, B.; Wang, H.; Ma, X.; Hao, Y.; Chen, X. Ultra-Lightweight Resistive Switching Memory Devices Based on Silk Fibroin. Small 2016, 12, 3360–3365. [Google Scholar] [CrossRef] [PubMed]
  124. Chen, Y.C.; Yu, H.C.; Huang, C.Y.; Chung, W.L.; Wu, S.L.; Su, Y.K. Nonvolatile Bio-Memristor Fabricated with Egg Albumen Film. Sci. Rep. 2014, 5, 1002201–1002212. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  125. Wu, Q.; Banerjee, W.; Cao, J.; Ji, Z.; Li, L.; Liu, M. Improvement of durability and switching speed by incorporating nanocrystals in the HfOx based resistive random access memory devices. Appl. Phys. Lett. 2018, 113, 023105. [Google Scholar] [CrossRef]
  126. Hu, Q.; Park, M.R.; Abbas, H.; Kang, T.S.; Yoon, T.S.; Kang, C.J. Forming-free resistive switching characteristics in tantalum oxide and manganese oxide based crossbar array structure. Microelectron. Eng. 2018, 190, 7–10. [Google Scholar] [CrossRef]
  127. Chen, Y.C.; Chang, Y.F.; Wu, X.; Zhou, F.; Guo, M.; Lin, C.Y.; Hsieh, C.-C.; Fowler, B.; Chang, T.-C.; Lee, J.C. Dynamic conductance characteristics in HfO2 based resistive random access memory. RSC Adv. 2017, 7, 12984–12989. [Google Scholar] [CrossRef] [Green Version]
  128. Seo, J.W.; Baik, S.J.; Kang, S.J.; Lim, K.S. Characteristics of ZnO thin film for the resistive random access memory. MRS Online Proc. Libr. Arch. 2010, 1250. [Google Scholar] [CrossRef]
  129. Mondal, S.; Chueh, C.-H.; Pan, T.-M. High-Performance Flexible Ni/Sm2O3/ITO ReRAM Device for Low-Power Nonvolatile Memory Applications. IEEE Electron Dev. Lett. 2013, 9, 1145–1147. [Google Scholar] [CrossRef]
  130. Raeis-Hosseini, N.; Rho, J. Solution-Processed Flexible Biomemristor Based on Gold-Decorated Chitosan. ACS Appl. Mater. Interfaces 2021, 13, 5445–5450. [Google Scholar] [CrossRef]
  131. Li, R.; Ma, X.; Li, J.; Cao, J.; Gao, H.; Li, T.; Zhang, X.; Wang, L.; Zhang, Q.; Wang, G.; et al. Flexible and high-performance electrochromic devices enabled by self-assembled 2D TiO2/MXene heterostructures. Nat. Commun. 2021, 12, 1587. [Google Scholar] [CrossRef] [PubMed]
  132. Chi, K.; Zhang, Z.; Xi, J.; Huang, Y.; Xiao, F.; Wang, S.; Liu, Y. Freestanding Graphene Paper Supported Three-Dimensional Porous Graphene−Polyaniline Nanocomposite Synthesized by Inkjet Printing and in Flexible All-Solid-State Supercapacitor. ACS Appl. Mater. Interfaces 2014, 6, 16312–16319. [Google Scholar] [CrossRef] [PubMed]
  133. Gong, X.; Zhang, L.; Huang, Y.; Wang, S.; Pan, G.; Li, L. Directly writing flexible temperature sensor with graphene nanoribbons for disposable healthcare devices. RSC Adv. 2020, 10, 22222–22229. [Google Scholar] [CrossRef]
  134. Hao, C.; Wen, F.; Xiang, J.; Yuan, S.; Yang, B.; Li, L.; Wang, W.; Zeng, Z.; Wang, L.; Liu, Z.; et al. Liquid-Exfoliated Black Phosphorous Nanosheet Thin Films for Flexible Resistive Random Access Memory Applications. Adv. Funct. Mater. 2016, 13, 2448–2455. [Google Scholar] [CrossRef]
  135. Aziz, T.; Wei, S.; Sun, Y.; Ma, L.-P.; Pei, S.; Dong, S.; Ren, W.; Liu, Q.; Cheng, H.-M.; Sun, D.-M. High-performance flexible resistive random access memory devices based on graphene oxidized with a perpendicular oxidation gradient. Nanoscale 2021, 254, 97–105. [Google Scholar] [CrossRef] [PubMed]
  136. Shinde, M.A.; Kim, H. Flexible electrochromic device with simple solution processed stable silver nanowire based transparent conductive electrodes. Synth. Met. 2019, 254, 97–105. [Google Scholar] [CrossRef]
Applsci 12 02839 g001 550
Figure 1. Sketch of the most common preparation methods of NiO nanoparticles and thin films: (a) phytosynthesis, (b) precipitation; (c) hydrothermal synthesis, (d) electrospray of nanoparticles to achieve thin film, (e) an example of RF sputtering technique for thin film synthesis. The blue and orange balls represent Ar and O2 gases.
Figure 1. Sketch of the most common preparation methods of NiO nanoparticles and thin films: (a) phytosynthesis, (b) precipitation; (c) hydrothermal synthesis, (d) electrospray of nanoparticles to achieve thin film, (e) an example of RF sputtering technique for thin film synthesis. The blue and orange balls represent Ar and O2 gases.
Applsci 12 02839 g001
Applsci 12 02839 g002 550
Figure 2. Scheme of the synthesis of sandwich-structured NiO-based flexible electrode and subsequently assembling of CAFS device (Reproduced from [98]. Copyright 2020, Elsevier: (a) Ni-based MG ribbon, (b) sandwich-like flexible electrode, (c) flexible electrodes after coating with PVA-KOH electrolytes, (d) flexible cable-like structure. (e) flexible all-solid-state symmetric supercapacitor.
Figure 2. Scheme of the synthesis of sandwich-structured NiO-based flexible electrode and subsequently assembling of CAFS device (Reproduced from [98]. Copyright 2020, Elsevier: (a) Ni-based MG ribbon, (b) sandwich-like flexible electrode, (c) flexible electrodes after coating with PVA-KOH electrolytes, (d) flexible cable-like structure. (e) flexible all-solid-state symmetric supercapacitor.
Applsci 12 02839 g002
Applsci 12 02839 g003 550
Figure 3. Flexible NiO-based plastisol free-standing electrode. (A) Photograph of the flexible, croppable electrode. (B) An example of a customized shape of NiO-plastisol sheet and an indication of the dimensions. (C) SEM images of front and back sides the NiO-plastisol electrodes obtained with NiO of different morphologies (Reproduced from [103], with modifications).
Figure 3. Flexible NiO-based plastisol free-standing electrode. (A) Photograph of the flexible, croppable electrode. (B) An example of a customized shape of NiO-plastisol sheet and an indication of the dimensions. (C) SEM images of front and back sides the NiO-plastisol electrodes obtained with NiO of different morphologies (Reproduced from [103], with modifications).
Applsci 12 02839 g003
Applsci 12 02839 g004 550
Figure 4. SEM image of a flexible NiO ReRAM device on flexible PET substrate. (Reproduced from [104], Copyright 2021, Elsevier).
Figure 4. SEM image of a flexible NiO ReRAM device on flexible PET substrate. (Reproduced from [104], Copyright 2021, Elsevier).
Applsci 12 02839 g004
Table
Table 1. Capacitance of NiO with various morphologies and composites.
Table 1. Capacitance of NiO with various morphologies and composites.
NiO MorphologyCapacitanceReference
Nanoparticles423 F/g at 0.5 mA/cm2[84]
Nanoparticles1012 mF/cm2 at 1 mA/cm2[85]
Nanowires670 F/g at 1 A/g[86]
Nanoplates array1124 F/g at 2 A/g[87]
Microspheres1482 F/g at 0.5 A/g[88]
NiO Composites
NiO@CeO2 (5 wt%) Flower-like microspheres2155.6 F/g at 1 A/g[89]
NiO@CNT Particles2480 F/g at 0.5 A/g[90]
NiO-GNS/PANI1409 F/g[91]
NiO/GNS1050 F/g[92]
M-NiO-G//AC154.0 F/g[93]
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Carbone, M. NiO-Based Electronic Flexible Devices. Appl. Sci. 2022, 12, 2839. https://doi.org/10.3390/app12062839

AMA Style

Carbone M. NiO-Based Electronic Flexible Devices. Applied Sciences. 2022; 12(6):2839. https://doi.org/10.3390/app12062839

Chicago/Turabian Style

Carbone, Marilena. 2022. "NiO-Based Electronic Flexible Devices" Applied Sciences 12, no. 6: 2839. https://doi.org/10.3390/app12062839

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop