Nanocrystalline NiO Thin Films Research Articles

Room temperature RF magnetron sputtered nanocrystalline NiO thin films for highly responsive and selective H2S gas sensing at low ppm concentrations

We report on the H2S gas sensing performance of nanocrystalline NiO thin films sputtered on alumina substrate via RF magnetron sputtering using NiO target at room temperature. Nanocrystalline nature with granular morphology of the films was observed along with the hydrophobic nature (θw ∼ 123.5°), as investigated by GIXRD, FESEM, and wettability measurements. PL spectrum indicates the presence of oxygen or nickel vacancies/defects, which was further confirmed by the XPS measurements. The H2S gas sensing characteristics of NiO thin films have been studied at different operating temperatures (200–425 °C) and gas concentrations (1–200 ppm). It shows the highest sensor response (Rg/Ra) of ∼ 28.8 for 200 ppm H2S gas at 400 °C (with a fast response/recovery time of ∼ 108 s/47 s). Further, NiO thin films confirm the high sensitivity, selectivity, and stability towards H2S gas with detection limit of 135 ppb. The H2S gas sensing mechanism with NiO is also explained.

Influence of annealing temperature on the structural, morphological, and optical properties of nanocrystalline NiO and Cu-doped NiO thin films and its application in acetone gas sensing

Influence of annealing temperature on the structural, morphological, and optical properties of nanocrystalline NiO and Cu-doped NiO thin films and its application in acetone gas sensing

Effect of Substrate Temperature on the Growth and Properties of Nanocrystalline NiO Thin Films

In the present research, the NiO thin films have been deposited by pneumatic spray method at various substrate temperatures. The main object is to find a good structural, optical and electrical properties of NiO tin films with substrate temperature effected. A set of deposition temperature (380, 400, 420, 440 and 460 °C), the NiO thin film was deposited on glass substrate with 0.1 M. Nanocrystalline films with a polycrystalline cubic crystal with (111) and (200) peaks, it is a strong (111) preferred orientation. The minimum value of grain size G = 16.88 nm was obtained for deposited film at 420 °C. The optical spectra show that the NiO thin films become transparency in visible region, but the film deposited at 460 °C have a high transmission. The band gap energy was located between 3.43 and 3.63 eV, the minimum was obtained for sprayed film at 440 °C. The minima value of Urbach energy was 0.25 eV, it also was found at 440 °C. The electrical property of deposited NiO films at 440 °C has a minimum value of electrical resistivity. The best structure, optical and electrical characterizations of NiO thin films are achieved and begin from deposited temperature of 420 °C.

Effect of substrate bias on surface morphology, electrical and optical properties of sputter deposited nanocrystalline nickel oxide films

Nanocrystalline NiO thin films were deposited by reactive magnetron sputtering under argon and oxygen gas environment on glass substrate and p-type silicon (1 0 0) substrate at different negative substrate biasing varying from 0 V to −90 V. The phases and grain sizes were evaluated using X-ray diffraction technique. Microstructural analysis was done under field emission scanning electron microscopy (FESEM). Resistivity and direct band gap were measured by four probe methods and UV–visible photospectrometer, respectively. Direct band gap values were found to increase from ≈2.6 eV to ≈4.5 eV with increase in negative substrate bias due to decrease in grain size.

Fluctuation-enhanced and conductometric gas sensing with nanocrystalline NiO thin films: A comparison

Fluctuation-enhanced and conductometric gas sensing with nanocrystalline NiO thin films: A comparison

Band Gap Engineering in Spray Pyrolysis Grown Nanocrystalline NiO Thin Films by Fe Doping

Band Gap Engineering in Spray Pyrolysis Grown Nanocrystalline NiO Thin Films by Fe Doping

Interfacial electron transfer yields in dye-sensitized NiO photocathodes correlated to excited-state dipole orientation of ruthenium chromophores

Interface dynamics of nanocrystalline NiO thin films sensitized with two ruthenium polypyridyl chromophores have been investigated to examine the influence that excited-state dipole orientation and the position of the bipyridine radical formed in the charge-separated state have on interfacial electron transfer yields. In ultrafast transient absorption experiments, the charge separated state is observed on the nanosecond timescale for the trifluoromethyl-substituted chromophore, [Ru(flpy)2(dcb)]2+ (flpy = 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, dcb = 4,4′-dicarboxy-2,2′-bipyridine), but not for [Ru(bpy)2(dcb)]2+ (bpy = 2,2′-bipyridine). Differences are attributed to the positioning of the bipyridine radical formed in the charge separated state; for [Ru(flpy)2(dcb)]2+, the electron is localized on the flpy ligand distal to the surface, whereas for [Ru(bpy)2(dcb)]2+, the electron is localized on the dcb ligand, proximal to the NiO surface. Enhanced photovoltaic performance is observed for dye-sensitized solar cell devices prepared with [Ru(flpy)2(dcb)]2+, demonstrating that enhanced charge separation can be correlated with device efficiency.

Highly selective NO2 sensor based on p-type nanocrystalline NiO thin films prepared by sol–gel dip coating

Nanocrystalline NiO thin films were synthesized on ITO substrates by a sol–gel dip coating method using the mixture of NiC4H6O4·4H2O, NH3·H2O and C3H8O as the precursor. Structure of NiO thin films was characterized by means of X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy, and the results showed that the films were composed of cubic NiO nanocrystalline particles having diameters in the range of 20–30nm, which provides a large specific surface area. NiO thin films showed p-type semiconductor behavior due to the non-stoichiometry. A band gap of 3.75eV was obtained by UV–vis measurement. The sensor based on NiO thin films demonstrated a good reversibility and repeatability to NO2 at evaluated operating temperatures. The films also showed an excellent NO2 selectivity compared to formaldehyde, ethanol, methanol, xylene, hydrogen, ammonia and methane at 150°C, demonstrating a potential application in low-energy consumption gas sensor devices.

Fluctuation-enhanced and conductometric gas sensing with nanocrystalline NiO thin films: A comparison

Nanocrystalline thin films of NiO were prepared by advanced reactive gas deposition, and their responses to formaldehyde, ethanol and methane gases were studied via fluctuation-enhanced and conductometric methods Thin films with thicknesses in the 200–1700-nm range were investigated in as-deposited form and after annealing at 400 and 500°C. Morphological and structural analyses showed porous deposits with NiO nanocrystals having face-centered cubic structure. Quantitative changes in frequency-dependent resistance fluctuations as well as in DC resistance were recorded upon exposure to formaldehyde, ethanol and methane at 200°C. The response to formaldehyde was higher than that to ethanol while the response to methane was low, which indicates that the NiO films exhibit significant selectivity towards different gaseous species. These results can be reconciled with the fact that formaldehyde has a nucleophilic group, ethanol is an electron scavenger, and methane is hard to either reduce or oxidize. The gas-induced variations in DC resistance and resistance fluctuations were in most cases similar and consistent.

Sputtered nanocrystalline NiO thin films for very low ethanol detection

We present results on nanocrystalline NiO chemoresistive films which are able to detect 5 ppm of ethanol vapor in air and to operate at 250 °C. NiO films with 50 and 100 nm thicknesses were prepared by DC reactive magnetron sputtering on alumina substrates previously coated by Pt layers as microheaters and interdigitated electrodes. NiO-based sensors with 50 nm thickness exhibited three times higher relative sensitivity and were approximately 2 times faster than the sensor element with 100 nm thickness in the whole ethanol concentration range. These 50 nm thick NiO films are created by nanocrystals with lateral size of about 30 nm homogenously covering the Pt/alumina surface. On the other hand, NiO films with 100 nm thickness are porous, divided in parts and lapped by smaller grains (∼15 nm) in several overlayers.

Structural and optical evaluations of deposited nanocrystalline NiO thin films via a Ni(II)–8-hydroxyquinolate complex by the static step-by-step soft surface reaction technique for optoelectronic applications

The static step-by-step soft surface reaction (SS-b-SSR) technique was implemented as an efficient soft chemical methodology for the synthesis of nanocrystalline NiO thin films. The target nanocrystalline NiO was produced upon thermal annealing of deposited thin films of the Ni(II)–8-hydroxyquinolate complex, Ni(II)–(8HQ)2. The (SS-b-SSR) technique was also used to initially produce the Ni(II)–(8HQ)2 complex with a first step deposition of 8HQ on a glass substrate, followed by a second soft reaction step with the Ni(II) ion. The assembled thin films of nanocrystalline Ni(II)–(8HQ)2 and NiO were characterized by elemental analysis, FT-IR, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The results of XRD revealed that the diffraction peaks of NiO are well crystalline and corresponding to a face centered cubic crystal structure. The mean crystallite size (D) and dislocation density (δ) were found to be 25nm and 1.60×10−3nm−2, respectively. The photoluminescence (PL) spectrum indicated a single broad peak, a near-band-edge emission in the ultraviolet region, and a relatively weak blue-green light (visible) emission at ∼458.5nm. The refractive index (n) and the absorption index (k) were calculated using spectrophotometric measurements of T(λ) and R(λ). The analysis of the spectral behavior of the absorption coefficient (α), in the absorption region revealed direct transitions and the energy gap was calculated as 3.47eV. The temperature dependence of the resistivity ρ(T) and thermoelectric power S(T) were investigated. The photocurrent properties of the device under various illuminations were also explored and it was found that the reverse bias voltage was increased by increasing the photo-illumination intensity. The transient photocurrent results indicated that the photocurrent under illumination is higher than the dark current.

Effects of grain refinement and disorder on the electronic properties of nanocrystalline NiO

Nanocrystalline NiO thin films have been grown on different substrates by RF magnetron sputtering with mixed O2-Ar plasma composition. The oxygen content of the plasma was varied between 0 and 100 %. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray pho- toelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). SEM results reveal a grain refinement when oxygen is added to the plasma. This effect can also be observed by XRD, where an analysis of the peak width confirms this decrease in the grain size. The analysis of EXAFS data shows that the presence of O2 in the plasma induces lattice disorder, as evidenced by the observed increase of the Debye-Waller factor. These microstructural changes modify the electronic structure of the NiO thin films. The spectral line shape in Ni 2p XPS spectra shows clear differences between samples grown with and without O2 in the plasma. These differences can be explained in terms of the observed structural changes.

Effect of Cu Doping on the Gas Sensing Properties of Nano-Crystalline NiO Thin Films

Effect of Cu Doping on the Gas Sensing Properties of Nano-Crystalline NiO Thin Films

Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

Evolution of hydrogen gas sensing properties of sol–gel derived nickel oxide thin film

Hydrogen sensing properties of nanocrystalline NiO thin films are investigated. NiO thin films were fabricated by a sol–gel method with special efforts to change the porosity of coated films by a multi-step coating and annealing processes. The experimental results indicated that the multi-step annealed NiO thin films had a higher porosity than films fabricated by a typical sol–gel process. When the thicknesses of NiO films were increased, the grain size increased and the sheet resistance of the film decreased. The highest gas response was detected at 175°C for two layered samples. NiO films fabricated by multi-step annealing showed a better response to hydrogen than NiO fabricated by a typical sol–gel process. The H2 gas response was decreased with increasing the thickness and decreasing the porosity of NiO films. A high response value of 68% was obtained for 3000ppm of H2 at 175°C for optimized sample by a multiple annealing process. The effect of humidity on the gas sensing performance of the sensor was studied and the devices were also tested for several cycles to study the repeatability of the sensors. The cross sensitivity measurements showed that the sensor could be used to monitor hydrogen in a likelihood mixture of reducing gases.

Preparation of nanocrystalline nickel oxide thin films by sol–gel process for hydrogen sensor applications

Preparation of nanocrystalline NiO thin films by sol–gel method and their hydrogen (H2) sensing properties were investigated. The thin films of NiO were successfully deposited on the glass and SiO2/Si substrate by a sol–gel coating method. The films were characterized for crystallinity, electrical properties, surface topography and optical properties as a function of calcination temperature and substrate material. It was found that the films produced by this method were polycrystalline and phase pure NiO. The H2 gas sensitivity of these films was studied as a function of H2 concentration and calcination temperature. The results indicated that the sol–gel derived NiO films could be used for the fabrication of H2 gas sensors to monitor low concentration of H2 in air quantitatively at low temperature range (<200°C).

Ultra smooth NiO thin films on flexible plastic (PET) substrate at room temperature by RF magnetron sputtering and effect of oxygen partial pressure on their properties

Transparent p-type nickel oxide thin films were grown on polyethylene terephthalate (PET) and glass substrates by RF magnetron sputtering technique in argon + oxygen atmosphere with different oxygen partial pressures at room temperature. The morphology of the NiO thin films grown on PET and glass substrates was studied by atomic force microscope. The rms surface roughnesses of the films were in the range 0.63–0.65 nm. These ultra smooth nanocrystalline NiO thin films are useful for many applications. High resolution transmission electron microscopic studies revealed that the grains of NiO films on the highly flexible PET substrate were purely crystalline and spherical in shape with diameters 8–10 nm. XRD analysis also supported these results. NiO films grown on the PET substrates were found to have better crystalline quality with fewer defects than those on the glass substrates. The sheet resistances of the NiO films deposited on PET and glass substrates were not much different; having values 5.1 and 5.3 kΩ/□ and decreased to 3.05, 3.1 kΩ/□ respectively with increasing oxygen partial pressure. The thicknesses of the films on both substrates were ∼700 nm. It was also noted that further increase in oxygen partial pressure caused increase in resistivity due to formation of defects in NiO.

Band gap widening of nanocrystalline nickel oxide thin films via phosphorus doping

Phosphorus doped nanocrystalline NiO thin films were synthesized using radio frequency magnetron sputtering of a prefabricated target on glass and silicon substrates in argon atmosphere. X-ray diffraction studies confirmed the good crystallinity and proper phase formation. Phosphorus doping in NiO films was confirmed from the binding energy determination by X-ray photoelectron spectroscopic studies. Morphological information was obtained from the atomic force microscopic measurement. Determination of band gaps from the UV–vis–NIR spectrophotometric measurement showed that it increased from 3.66 to 3.81 eV corresponding to undoped and 10% P doped NiO thin films.

Chemically deposited nanocrystalline NiO thin films for supercapacitor application

A chemical synthesis process for the fabrication of NiO is described. In the present work, we report a chemical route for preparation of nickel hydroxide thin films on the glass substrate from solution containing Ni (2+) ions and ammonia. The deposition process based on the thermal decomposition of ammonia complex with nickel ions at temperature 333 K. As-prepared films were annealed at 623 K in air for 2 h to remove hydroxide phase and used for structural, surface morphological and optical characterizations. It was found that after annealing β-phase of Ni(OH) 2 was transformed to a cubic phase structured NiO. Honeycomb like surface morphology of “β-Ni(OH) 2” films was merged and resulted in decrease in porosity after annealing. A red shift in band gap energy was observed after annealing. The capacitive characteristics of the films deposited on steel substrate were investigated in KOH electrolyte using cyclic voltammetry. The supercapacitive properties of NiO were strongly affected by scan rate, concentration of electrolyte and deposited mass of the material. The maximum specific capacitance of 167 F g −1 was obtained in 2 M KOH electrolyte.

Au-NiO nanocrystalline thin films for sensor application

Nanocrystalline NiO thin films were deposited by dc reactive magnetron sputtering in a mixture of oxygen and argon and subsequently coated by Au on a NiO film surface. Very thin Au overlayers with a thickness of about 1 and 7 nm have been prepared by magnetron sputtering. Then, the surface modified NiO films have been analysed by TEM, EDX and SEM. NiO thin films showed a polycrystalline structure with the size of nanocrystals ranging from a few nanometers to 10 nm. Electrical responses of NiO-based structure towards hydrogen have been measured.