Study of sensitivity and selectivity of α-Fe2O3 thin films for different toxic gases and alcohols

This paper presents a detailed study on the sensitivity and selectivity of α-Fe₂O₃ thin films produced by deposition of Fe and post-deposition annealed at two temperatures of 600 °C and 800 °C with flow of oxygen for application as a sensor for toxic gases including CO, H₂S, NH₃ and NO₂ and alcohols such as C₃H₇OH, CH₃OH, and C₂H₅OH. The crystallographic structure of the samples was studied by X-ray diffraction (XRD) method while an atomic force microscope (AFM) was employed for surface morphology investigation. The electrical response of the films was measured while they were exposed to various toxic gases and alcohols in the temperature range of 50–300 °C. The sample annealed at higher temperature showed higher response for different gases and alcohols tested in this work which can be due to the higher resistance of this sample. Results also indicated that the α-Fe₂O₃ thin films show higher selectivity to NO₂ gas relative to the other gases and alcohols while the best sensitivity is obtained at 200 °C. The α-Fe₂O₃ thin film post-deposition annealed at 800 °C also showed a good stability and reproducibility and a detection limit of 10 ppm for NO₂ gas at the operating temperature of 200 °C.     

Novel hybrid self-assembly of an ultralarge ZnO macroflower and defect intensity-induced photocurrent and photocatalytic properties by facile hydrothermal synthesis using CO(NH2)2–N2H4 as alkali sources

Different flower-like ZnO nanoarchitectures were synthesized by a facile hydrothermal method using CO(NH₂)₂ and N₂H₄ as alkali sources simultaneously. A novel ultralarge ZnO macroflower was constructed by the ultrathin leaf-like nanobelts, hollow semisphere-like, sphere-like and apple-shaped nanoparticles simultaneously. The diameter of an individual flower can reach 90 µm. Meanwhile, three or five flower-like ZnO nanostructures with different diameters, lengths and tips (Planar, semi-pyramid, and/ or pyramid tips) were formed simultaneously under the same reaction condition. XRD shows that all the ZnO crystals possess the hexagonal wurtzite structure. When the samples range from S1 to S5, the crystallinity is improved. EDX shows that the Zn/ O atom ratio of S1–S5 is close to the 1:1 stoichiometric ratio, and that of S3 is almost equal to 1:1. FTIR indicates that S4 and S5 are pure. However, the surface of S1, S2 and S3 adsorbs the CO₃²⁻ group. The reflectance of S1–S4 in the range of 300–370 nm is inversely proportional to that in visible region. Meanwhile, when the grain size of S1–S4 decreases, their band gap increases. The Raman results of S1 and S5 are different from those of S4 and exhibit the higher crystal quality, which are favorable for the improvement of photocatalytic performance. S1 and S5 exhibit the highest photocatalytic performance and decompose 65% and 70% of MB within 50 min respectively. The photocatalytic activity and photocurrent strongly depend on the defect intensity of the ZnO crystals. The ZnO photocatalyst of S5 is still stable and efficient after three cycles. However, the photocatalytic activity of S1 decreases continuously, due to its unique morphology and the adsorption of intermediates. In addition, A hybrid self-assembly process of the ultralarge ZnO macroflower was proposed.     

Surfactant free hydrothermally derived ZnO nanowires,nanorods, microrods and their characterization

ZnO nanowires, nanorods and microrods have been prepared by an organic-free hydrothermal process using ZnSO₄ and NaOH/NH₄OH solutions. The powder X-ray diffraction (PXRD) patterns reveal that the ZnO nano/microrods are of hexagonal wurtzite structure. The Fourier transform infrared (FT-IR) spectrum of ZnO powder shows only one significant spectroscopic band at around 417 cm^?1 associated with the characteristic vibrational mode of Zn–O bonding. The thickness 75–300 nm for ZnO nanorods and 0.2–1.8 μm for microrods are identified from SEM/TEM images. UV–visible absorption spectra of ZnO nano/microrods show the blue shift. The UV band and green emission observed in photoluminescence (PL) spectra are due to free exciton emission and singly ionized oxygen vacancy in ZnO. Finally, the mechanism for organic-free hydrothermal synthesis of the ZnO nano/microrods is discussed.     

Synthesis of highly selective and sensitive Cu-doped ZnO thin film sensor for detection of H2S gas

Copper (Cu) doped zinc oxide (ZnO) thin films were successfully prepared by a simple sol-gel spin coating technique. The effect of Cu doping on the structural, morphology, compositional, microstructural, optical, electrical and H₂S gas sensing properties of the films were investigated by using XRD, FESEM, EDS, FTIR, XPS, Raman, HRTEM, and UV–vis techniques. XRD analysis shows that the films are nanocrystalline zinc oxide with the hexagonal wurtzite structure and FESEM result shows a porous structured morphology. The gas response of Cu-doped ZnO thin films was measured by the variation in the electrical resistance of the film, in the absence and presence of H₂S gas. The gas response in relation to operating temperature, Cu doping concentration, and the H₂S gas concentration has been systematically investigated. The maximum H₂S gas response was achieved for 3 at% Cu-doped ZnO thin film for 50 ppm gas concentration, at 250 °C operating temperature.     

Structural,optical and photocatalytic properties of MgO/ZnO nanocomposites prepared by a hydrothermal method

MgO/ZnO nanocomposites were synthesized by a hydrothermal method at 180 °C for 15 h. The XRD results showed that MgO phase occurred over the whole range of Mg concentration used. The MgO induced a growth of crystallite size of ZnO. The particle shape of ZnO altered from an agglomerated nanosheet to a hexagonal platelet when loading with MgO and MgO was shaped in small rod structures. The optical band gap was increased from 3.190 to 3.225 eV. The photocatalytic degradation depended upon the irradiation time and MgO loading contents. In this study, the 5 mol% MgO/ZnO nanocomposite exhibited the best photocatalytic degradation of 98.3% during irradiation by blacklight for 90 min.     

Photocatalytic degradation of methyl orange and gas-sensing performance of nanosized ZnO

ZnO nanoparticles were synthesized by calcining composites of zinc nitrate and poly(vinyl pyrrolidone) (PVP, molecular weight 30 000) at a mass ratio of 1:2 at 500 °C for 2 h. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to characterize the as-synthesized ZnO nanoparticles. The particles ranged in size from 30 to 50 nm. Infrared spectra of PVP and the PVP+Zn(NO₃)₂·6H₂O composite revealed coordination between the carbonyl (C=O) of PVP and Zn²⁺ of zinc nitrate, which led to a uniform nanoparticle morphology. The gas-sensing properties and photocatalytic performance of the final product were systematically investigated. The results show that the ZnO nanoparticles exhibit both a high response for ethanol detection and excellent photocatalytic activity for degradation of methyl orange under UV irradiation for 30 min.     

Room temperature NO2 gas sensing properties of DBSA doped PPy–WO3 hybrid nanocomposite sensor

We demonstrate the chemiresistive NO₂ gas sensor based on DBSA doped PPy–WO₃ hybrid nanocomposites operating at room temperature. The sensor was fabricated on glass substrate using simple and cost effective drop casting method. The gas sensing performance of sensor was studied for various toxic/flammable analytes like NO₂, C₂H₅OH, CH₃OH, H₂S and NH₃. The sensor shows higher selectivity towards NO₂ gas with 72% response at 100 ppm. Also the sensor can successfully detect low concentration of NO₂ gas upto 5 ppm with reasonable response of 12%. Structural, morphological and compositional analyses evidenced the successful formation of DBSA doped PPy–WO₃ hybrid nanocomposite with uniform dispersion of DBSA into PPy–WO₃ hybrid nanocomposite and enhance the gas sensing behavior. We demonstrated that DBSA doped PPy–WO₃ hybrid nanocomposite sensor films shows excellent reproducibility, high stability, moderate response and recovery time for NO₂ gas in the concentration range of 5–100 ppm. A gas sensing mechanism based on the formation of random nano p–n junctions distributed over the surface of the sensor film has been proposed. In addition modulation of depletion width takes place in sensor on interaction with the target NO₂ gas has been depicted on the basis of schematic energy band diagram. Impedance spectroscopy was employed to study bulk, grain boundary resistance and capacitance before and after exposure of NO₂ gas. The structural and intermolecular interaction within the hybrid nanocomposites were explored by Raman and X-ray photoelectron spectroscopy (XPS), while field emission scanning electron microscopy (FESEM) was used to characterize surface morphology. The present method can be extended to fabricate other organic dopent-conducting polymer–metal oxide hybrid nanocomposite materials and could find better application in the gas sensing.     

Investigation of the properties of ferromagnetic ZnO:Cr2O3 nanocomposites

Present work focuses on the structural, optical and magnetic properties of ZnO:Cr₂O₃ nanocomposites. ZnO nanoparticles were synthesized and the structure was confirmed using powder x-ray diffraction. ZnO nanoparticles was grown in the hexagonal wurtzite structure with the preferential orientation along (101) plane. ZnO:Cr₂O₃ composites have been synthesized by doping different concentration of Cr₂O₃ (1, 3 and 5 wt%) into ZnO. The incorporation of Cr₂O₃ was confirmed using Fourier transformed infrared spectroscopy. UV–visible absorption spectra have been observed and interpreted for the determination of optical constants of ZnO:Cr₂O₃ composites. The optical constants like optical band gap, refractive index were determined and the effect of Cr₂O₃ on these constants was investigated. Relation between optical band gap and the refractive index were obtained. Magnetic studies using vibrating sample magnetometer reveal the ferromagnetism at 150 K in the composites with 3 and 5 wt% of Cr₂O₃.     

Heterostructure of CdO-ZnO nanoparticles intercalated on PANI matrix for better thermal and electrochemical performance

New heterostructure of CdO-ZnO nanoparticles intercalated on PANI matrix (CZP) have been synthesized by two step solution route: firstly, CdO-ZnO heterostructure was prepared by the simple chemical precipitation method and the resulting materials are effectively coupled on PANI by the chemical oxidative polymerization method in acidic medium using APS as oxidant. The resulting respective hybrid materials of CdO, ZnO and PANI matrix were characterized and confirmed (functional groups and crystalline nature) by FTIR, RAMAN and XRD analysis. From the HR-SEM analysis, hybrid materials of CdO-ZnO/PANI matrix has agglomerated hexagonal structure was derived from the granular structure of PANI. Thermo gravimetric analysis showed that CZP hybrid composite has higher thermal stability than ZnO-PANI and PANI matrix. Surface area and pore volume of the hybrid heterostructure obtained to be 54.52 m²/g and 0.14 cm³/g respectively. The electrical properties of CdO-ZnO particles intercalated on PANI matrix are evaluated in three different electrolyte solutions. In the sense, 1 M H₂SO₄ electrolyte solution exhibits better current response than the other electrolyte solution. From the results shows that the hybrid hetero structure of (CZP) exhibited better thermal and electrochemical performances than ZnO-PANI hybrid and the capacitance retention showing 72.6% after 500 cycles at a current density of 0.5 mA g⁻¹, thus the electrode materials possess good specific capacitance and cycle stability. Hence the CZP is a promising material in the field of super capacitor applications.     

High thermal annealing effect on structural and optical properties of ZnO–SiO2 nanocomposite

The effect of annealing temperature on photoluminescence (PL) of ZnO–SiO₂ nanocomposite was investigated. The ZnO–SiO₂ nanocomposite was annealed at different temperatures from 600 °C to 1000 °C with a step of 100 °C. High Resolution Transmission Electron Microscope (HR-TEM) pictures showed ZnO nanoparticles of 5 nm are capped with amorphous SiO₂ matrix. Field Emission Scanning Electron Microscope (FE-SEM) pictures showed that samples exhibit spherical morphology up to 800 °C and dumbbell morphology above 800 °C. The absorption spectrum of ZnO–SiO₂ nanocomposite suffers a blue-shift from 369 nm to 365 nm with increase of temperature from 800 °C to 1000 °C. The PL spectrum of ZnO–SiO₂ nanocomposite exhibited an UV emission positioned at 396 nm. The UV emission intensity increased as the temperature increased from 600 °C to 700 °C and then decreased for samples annealed at and above 800^°C. The XRD results showed that formation of willemite phase starts at 800 °C and pure willemite phase formed at 1000 °C. The decrease of the intensity of 396 nm emission peak at 900 °C and 1000 °C is due to the collapse of the ZnO hexagonal structure. This is due to the dominant diffusion of Zn into SiO₂ at these temperatures. At 1000 °C, an emission peak at 388 nm is observed in addition to UV emission of ZnO at 396 nm and is believed to be originated from the willemite.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

Effect of H2O2 concentration on electrochemical growth and properties of vertically oriented ZnO nanorods electrodeposited from chloride solutions

In this work, ZnO nanostructures are electrodeposited on a transparent conducting glass from chloride baths. The influence of H₂O₂ concentration on the electrochemical characteristics has been studied using cyclic voltammetry (CV) and chronoamperometry (CA) techniques. From the analysis of the current transients on the basis of the Scharifker–Hills model, it is found that nucleation mechanism is progressive with a typical three-dimensional (3D) nucleation and growth process; independently with the concentration of H₂O₂. However, the nucleation rate of the ZnO changes with the increase of H₂O₂ concentration. The Mott–Schottky measurements demonstrate an n-type semiconductor character for all samples with a carrier density varying between 5.14×10¹⁸ cm⁻³ and 1.47×10¹⁸ cm⁻³. Scanning electron microscopy (SEM) observations show arrays of vertically aligned ZnO nanorods (NRs) with good homogeneity. The X-ray diffraction (XRD) patterns show that the ZnO deposited crystallises according to a hexagonal Würtzite-type structure and with the c-axis perpendicular to the electrode surface. The directional growth along (002) crystallographic plane is very important for deposits obtained at 5 and 7 mM of H₂O₂. The high optical properties of the ZnO NRs with a low density of deep defects was checked by UV–vis transmittance analyses, the band gap energy of films varies between 3.23 and 3.31 eV with transparency around 80–90%.     

Structural,optical and transport properties of SxZnO

In this paper, S-doped ZnO (S_xZnO) was prepared using sol-gel method at different S amounts. The structural, optical and transport properties were investigated. The introduction of S atoms into the ZnO network was found to lower the crystallization level which results in reducing the crystallite size up to x=0.3. The doping process is confirmed by the observed peak at ~610 cm⁻¹ in the ATR spectrum related to the Zn-S linking. EDX mapping shows a homogeneous distribution of S atoms on the particles surface. The best compromise between the band gap (Eg=2.96 eV), the charge carriers (N_A=2.139×10²² cm⁻³), the conductivity (σ=5.56×10⁻⁴ Ω⁻¹ m⁻¹) and the mobility (µ=16.26×10⁻¹⁴ m² V⁻¹ s⁻¹) is obtained for x=0.1. The conduction mechanism is assumed by small hopping polaron. The S-doping has impacted positively the photocatalytic activity of ZnO, with particularly high performance for S_0.2ZnO.     

Influence of In doping on the structural,optical and acetone sensing properties of ZnO nanoparticulate thin films

Indium-doped zinc oxide (ZnO) nanoparticle thin films were deposited on cleaned glass substrates by spray pyrolysis technique using zinc acetate dihydrate [Zn(CH₃COO)₂ 2H₂O] as a host precursor and indium chloride (InCl₃) as a dopant precursor. X-ray diffraction results show that all films are polycrystalline zinc oxide having hexagonal wurtzite structure. Upon In doping, the films exhibit reduced crystallinity as compared with the undoped film. The optical studies reveal that the samples have an optical band gap in the range 3.23–3.27 eV. Unlike the undoped film, the In-doped films have been found to have the normal dispersion for the wavelength range 450–550 nm. Among all the films investigated, the 1 at% In-doped film shows the maximum response 96.8% to 100 ppm of acetone in air at the operating temperature of 300 °C. Even at a lower concentration of 25 ppm, the response to acetone in this film has been found to be more than 90% at 300 °C, which is attributed to the smaller crystallite size of the film, leading to sufficient adsorption of the atmospheric oxygen on the film surface at the operating temperature of 300 °C. Furthermore, In-doped films show the faster response and recovery at higher operating temperatures. A possible reaction mechanism of acetone sensing has been explained.     

Multifunctional device based on ZnO:Fe nanostructured films with enhanced UV and ultra-fast ethanol vapour sensing

Extensive application requests on high-performance gas sensors and photodetectors reveal the importance of controlling semiconducting oxide properties. Sensing properties of ZnO nano- and micro-structures can be tuned and their functional performances can be enhanced more efficiently by metal-doping. Here, we report the synthesis of crystalline Fe-doped ZnO (ZnO:Fe) nanostructured films via a cost-effective and simple synthesis from chemical solutions (SCS) approach followed by rapid thermal annealing (RTA) with excellent potential for the development of multifunctional devices for UV and ethanol (C₂H₅OH) vapour sensing. The effects of two types of thermal annealing on the ZnO:Fe morphology, the crystallinity, the electronic and the vibrational properties, the UV radiation and the gas sensing properties are investigated. The experimental results indicate an increase in UV response (I_UV/I_DARK~10⁷) of as-grown ZnO nanostructured films by Fe-doping, as well as an essential improvement in rise and decay times due to RTA effects at 725 °C for 60 s. In comparison with un-doped samples, ZnO:Fe (0.24 at%) specimens showed a response to ethanol which is enhanced by a factor of two, R_air/R_gas~61. It was demonstrated that by using Fe-doping of ZnO it is possible to reduce essentially the response τ_r and recovery times τ_d of the multifunctional device. The involved gas sensing mechanism is discussed in detail in this paper. The presented results could be of great importance for the application of RTA and doping effects for further enhancement of UV detection and gas sensing performances of the ZnO:Fe nanomaterial-based multifunctional device.     

Chemical bonding states and local structures of the oriented hexagonal BCN films synthesized by microwave plasma CVD

In order to synthesize oriented hexagonal boron carbonitride (h-BCN) films, borane-triethylamine complex (C₆H₁₈BN) was used as a single-source precursor. The films were deposited on Si (1 0 0) substrate by microwave plasma-enhanced chemical-vapor deposition using CH₄+H₂ as the carrier gas. The deposition was performed at different microwave powers of 200–500 W at working pressure of 5.0 Torr. The microhardness, estimated by nano-indentation test, of the films was found to be around 4 GPa. Fourier transform infrared spectroscopy (FT-IR) confirmed the formation of hexagonal BCN phase in a short-range order. The chemical composition and the local structures of films were studied by X-ray photoelectron spectroscopy (XPS) and the near-edge X-ray absorption fine structure (NEXAFS) spectroscopic measurements. XPS revealed that B, C and N atoms in the deposited films are in various chemical environments such as B–N, B–C, C–N and B–C–N atomic hybrid configuration. The NEXAFS measurement suggested that the B atoms are bonded not only to the N atoms but also to the C atoms to form various local structures of sp² B–C–N hybrid configurations. The polarization dependence of NEXAFS suggested that the local structures of the sp² BCN layers have different atomic orientations to the substrate.     

Nanotribological properties of ALD-processed bilayer TiO2/ZnO films

TiO₂/ZnO films grown by atomic layer deposition (ALD) demonstrated nanotribological behaviors using scratch testing. TEM profiles obtained an amorphous structure TiO₂ and nanocrystalline structure ZnO, whereas the sample has significant interface between the TiO₂/ZnO films. The experimental results show the relative XRD peak intensities are mainly contributed by a wurtzite oxide ZnO structure and no signal from the amorphous TiO₂.With respect to tribology, increased friction causes plastic deformation between the TiO₂ and ZnO films, in addition to delamination and particle loosening. The plastic deformation caused by adhesion and/or cohesion failure is reflected in the nanoscratch traces. The pile-up events at a loading penetration of 30 nm were measured at 21.8 μN for RT, 22.4 μN for 300 °C, and 36 μN for 400 °C. In comparison to the other conditions, the TiO₂/ZnO films annealed at 400 °C exhibited higher scratch resistance and friction with large debris, indicating the wear volume is reduced with increased annealing temperature and loading.     

Structural,magnetic and dielectric properties of nanocrystalline cobalt ferrite by wet hydroxyl chemical route

Nanopowders of CoFe₂O₄ are synthesized via wet chemical co-precipitation processing at pH 8. The synthesized nanoferrite powders are annealed at various temperatures (350 °C, 700 °C and 1050 °C) and are characterized. X-ray diffraction (XRD) patterns indicate the crystalline nature of CoFe₂O₄ nanopowders. Transmission electron microscope (TEM) investigations show, anisotropic shapes like cubic, hexagonal and spherical morphology of nanoparticles with average particle size 38–85 nm. Dielectric constant decreases as the frequency increases. Low value of dielectric loss at higher frequencies suggests the material is suitable for high frequency applications. AC conductivity increases with frequency. The saturation magnetization (M_s), remanant magnetism (M_R) and coercivity (H_C) increases with applied field.     

Post-annealing modification in structural properties of ZnO thin films on p-type Si substrate deposited by evaporation

Zinc oxide (ZnO) films of thickness ∼380 nm were deposited on p-type Si (1 1 1) substrate maintained at 300 °C under 3×10⁻⁶ Torr by a radio frequency (RF) heating source. Transmission Fourier transform infrared (FTIR) spectrum exhibited a clear Zn–O bond excitation frequency of ∼408 cm⁻¹. X-ray diffraction spectrum demonstrated four peaks (P₁–P₄) at 2θ (deg) ∼36±0.06, 40±0.09, 82±0.17 and 86±0.2, which originated from (1 0 0), (0 0 2), (2 0 1) and (0 0 4) hexagonal planes, respectively. P₂ being the highest intensity peak indicated that the growth of ZnO predominantly occurred along the c-axis i.e. (0 0 2) plane. Micrographs of the samples obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) identically displayed scattered nanocrystallites, which grew bigger with the increase of sample annealing temperature (°C) in the range of 400–1000. AFM pictures, in particular, exposed the hexagonal structure of the deposited films along with voids. However, ZnO composition ∼6:1 (Zn:O) as calculated from the energy dispersive spectrum (EDS) revealed that the formation of ZnO was not stoichiometric, rather of Zincsuboxide structure ZnO_x (x<1). Arrhenius plot of the resistivity data yielded a donor level (zinc interstitial and/or Zn–on–O site) with ionization energy E_c–1.26 eV, thereby it supports our measured results, in general.     

Sensing characteristics of zinc oxide thin films deposited by spray pyrolysis onto tubular Pyrex substrate

This paper presents NO₂ sensing properties of ZnO thin films grown onto tubular Pyrex substrate using the spray pyrolysis method. The sensor response was found to depend essentially on four parameters: chemical composition, structure, morphology and operating temperature. The crystallinity and morphology of the as-preapred films were analyzed using X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM). The sensing properties of ZnO toward NO₂ were investigated at different operating temperatures and NO₂ concentrations. Optimization of the preparation conditions show that ZnO thin film deposited during 15 min exhibit the highest sensitivity with fast response and recovery time.