Doped ZnO nanostructures with selected elements - Structural,morphology and optical properties: A review

ZnO is a unique semiconductor that used for various applications. The preparation method, crystal morphology and particle size have important role in its physical properties. This article reviews the structural, morphology and optical properties of zinc oxide nanostructures grown up by different synthesis methods. The effect of adding various impurities which categorized by periodic table groups, on its structural and optical properties have been studied.     

Green synthesized zinc oxide nanoparticles: Effect of polyethylene glycol and chitosan on structural,optical and morphological analysis

The use of plant extracts in the synthesis of nanoparticles has several advantages over traditional chemical-based synthesis methods. The synthesis of ZnO, Polyethylene Glycol ZnO (PEG), and Chitosen (Chit) ZnO nanoparticles using moringa oleifera leaf extracts as an effective reducing agent is described in this report. The colour of the reaction mixture changes from yellowish brown to white as nano metal oxide forms. Surface modifying agents for ZnO nano oxide included polyethylene glycol and chitosan. UV–Vis and FTIR spectroscopy were used to investigate the optical characteristics of produced nano oxides. The structural, elemental analysis, and morphology of nano oxides were studied using XRD, FESEM, EDAX, and HRTEM techniques. XRD validated the hexagonal wurtzite structure, and the average crystallite size of the ZnO, PEG-ZnO, and Chit-ZnO nanoparticles were 35 nm, 32 nm, and 28 nm, respectively. The band gaps of ZnO, PEG-ZnO, and Chit-ZnO nanoparticles were found to be 3.12 eV, 3.18 eV, and 3.14 eV, respectively, using UV–Visible analysis. ZnO, PEG-ZnO, and Chit-ZnO nanoparticles were found to be generally spherical and nano in size, according to SEM and TEM examination. According to the findings, moringa oleifera leaf extract-assisted ZnO-based nanoparticles are promising materials for various applications.     

Structural,magnetic and optical properties of BiFeO3 synthesized by the solvent-deficient method

Bismuth ferrite (BiFeO₃, BFO) powders were synthesized by a simple and cost-effective solvent-deficient method using bismuth nitrate, iron nitrate, and ammonium bicarbonate as the only precursors. Single phase BiFeO₃ powder was fabricated after the Bi:Fe ratio was adjusted and after the precursor mixture was calcined for one hour at 600 °C. We investigated the formation reactions, crystal structure, particle size distribution, magnetic and optical properties of synthesized BiFeO₃. X-ray diffraction revealed the formation of well-crystallized BFO nanocrystallites starting at a temperature of 450 °C. BiFeO₃ powder calcined at 600 °C showed very weak ferromagnetism at room temperature which is different from the linear M–H relationship in bulk BiFeO₃ ceramics. The remnant magnetization value (Mr) was found to be 5 × 10⁻³ emu g⁻¹ and a coercive field value (Hc) nearly 500 Oe. The UV–visible spectra showed maximum adsorption at ∼464 nm with a derived bandgap value of 1.85 (1.8449 ± 0.0013) eV for BFO nanocrystallites supporting their potential application as visible light-response photocatalysts. Direct bandgap value obtained from reflectance measurement is determined to be 2.25 (2.2464 ± 0.0065) eV.     

Effect of cations and anions on properties of zinc oxide particles synthesized in supercritical water

Hydrothermal synthesis of zinc oxide fine particles from zinc salt (Zn(CH₃COO)₂, ZnSO₄, Zn(NO₃)₂) and alkali metal hydroxide (LiOH, KOH) aqueous solution was carried out with a Ti alloy batch reactor in supercritical water. Particle size synthesized in LiOH solution was relatively smaller than that in KOH. Emission spectra of the particle produced from ZnSO₄ and LiOH aqueous solution shows the highest intensity among these systems. Hydrothermal synthesis of zinc oxide fine particles from Zn(NO₃)₂ and LiOH solution was also carried out with a flow-through apparatus for continuous production and rapid heating of the starting solution to supercritical states. Nanoparticles having an average particle diameter of 16 nm was produced at 659 K and 30 MPa.     

Continuous synthesis of surface-modified zinc oxide nanoparticles in supercritical methanol

Continuous synthesis of surface-modified zinc oxide (ZnO) nanoparticles was examined using surface modifiers (oleic acid and decanoic acid) in supercritical methanol at 400 °C, 30 MPa and a residence time of ∼40 s. Wide angle X-ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles retained ZnO crystalline structure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the surface modifiers changed drastically the size and morphology of the ZnO nanoparticles. When the molar ratio of oleic acid to Zn precursor ratio was 30, 10 nm size particles with low degree of aggregation were produced. The surface-modified ZnO nanoparticles had higher BET surface areas (29-36 m²/g) compared to unmodified ZnO particles synthesized in supercritical water (0.7 m²/g). Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) indicated that aliphatic, carboxylate and hydroxyl groups were chemically attached on the surface of ZnO nanoparticles. Long-term (80 days) dispersion test using ultraviolet transmittance showed that the surface-modified ZnO particles had enhanced dispersion stability in ethylene glycol.     

Cu-doped ZnO nanorod arrays: the effects of copper precursor and concentration

Cu-doped ZnO nanorods have been grown at 90°C for 90 min onto a quartz substrate pre-coated with a ZnO seed layer using a hydrothermal method. The influence of copper (Cu) precursor and concentration on the structural, morphological, and optical properties of ZnO nanorods was investigated. X-ray diffraction analysis revealed that the nanorods grown are highly crystalline with a hexagonal wurtzite crystal structure grown along the c-axis. The lattice strain is found to be compressive for all samples, where a minimum compressive strain of −0.114% was obtained when 1 at.% Cu was added from Cu(NO₃)₂. Scanning electron microscopy was used to investigate morphologies and the diameters of the grown nanorods. The morphological properties of the Cu-doped ZnO nanorods were influenced significantly by the presence of Cu impurities. Near-band edge (NBE) and a broad blue-green emission bands at around 378 and 545 nm, respectively, were observed in the photoluminescence spectra for all samples. The transmittance characteristics showed a slight increase in the visible range, where the total transmittance increased from approximately 80% for the nanorods doped with Cu(CH₃COO)₂ to approximately 90% for the nanorods that were doped with Cu(NO₃)₂.     

Photoluminescence of zinc oxide nanopowder synthesized by a combustion method

Zinc oxide (ZnO) nanopowder was synthesized by a simple and quick combustion method using zinc nitrate as a precursor and glycine as a fuel material. The starting materials were mixed at room temperature and spontaneous ignition of which resulted into the ZnO nanopowder. The synthesized nanopowder was characterized by X-ray diffraction (XRD), scanning electronic microscope (SEM), Infrared (IR) spectrophotometer and spectroflurometer in order to study the structural, morphological, compositional and photoluminescence (PL) properties. The ZnO powder shows polycrystalline nature with preferential peak (101) having crystallite size 25 nm. A significant band at 532 cm⁻¹ in the IR spectrum corroborates the presence of characteristic band of ZnO. Room temperature photoluminescence spectrum of the synthesized nanopowder exhibits a dominant, sharp and strong ultraviolet (UV) emission with a suppressed deep-level emission indicating good crystal quality and optical properties.     

A comparative study of magnetic,photocatalytic and dielectric properties of NiO nanoparticles synthesized by sol-gel and composite hydroxide mediated method

In this study, Nickel oxide (NiO) nanoparticles were synthesized by conventional sol-gel (SG) route using citric acid as an end-capping agent and composite-hydroxide-mediated (CHM) approach. XRD and FTIR confirmed the formation of NiO nanoparticles and the average crystallite size was found to be 68 and 34 nm for SG and CHM method, respectively. Raman spectroscopy revealed higher intensity of one-phonon longitudinal optical (1LO) mode in CHM nanoparticles which is attributed to the presence of defects such as Ni or oxygen vacancies. The two-magnon (2 M) band at 1472 cm⁻¹ confirmed antiferromagnetic (AFM) nature of SG nanoparticles whereas its suppression in CHM nanoparticles indicate superparamagnetic (SPM) nature of the nanoparticles due to decrease in average crystallite size. The magnetic measurements also confirmed the AFM and SPM state of SG and CHM nanoparticles, respectively in accordance with Raman spectroscopy. A sharp and low blocking temperature peak in ZFC/FC also indicates smaller and uniform sized NiO nanoparticles synthesized by CHM method. Optical studies revealed a large value of energy band gap for CHM nanoparticles due to their small average crystallite size and is attributed to quantum confinement effect. A higher photocatalytic performance was observed for CHM nanoparticles due to their large surface area and higher concentration of oxygen vacancies in accordance with Raman spectra. A higher dielectric constant was also observed for CHM nanoparticles due to increased number of grains per unit volume in smaller crystallites. In summary, CHM is one of the most simple synthesis approach that provides fine single-phase NiO nanoparticles at low calcined temperature with improved magnetic, photocatalytic and dielectric properties as compared to SG method.     

Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature

Vertically aligned ZnO nanorods (NRs) on aluminum-doped zinc oxide (AZO) substrates were fabricated by a single-step aqueous solution method at low temperature. In order to optimize optical quality, the effects of annealing on optical and structural properties were investigated by scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. We found that the annealing temperature strongly affects both the near-band-edge (NBE) and visible (defect-related) emissions. The best characteristics have been obtained by employing annealing at 400°C in air for 2 h, bringing about a sharp and intense NBE emission. The defect-related recombinations were also suppressed effectively. However, the enhancement decreases with higher annealing temperature and prolonged annealing. PL study indicates that the NBE emission is dominated by radiative recombination associated with hydrogen donors. Thus, the enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors. On the other hand, the reduction of visible emission is mainly attributed to the annihilation of OH groups. Our results provide insight to comprehend annealing effects and an effective way to improve optical properties of low-temperature-grown ZnO NRs for future facile device applications.     

Multifunctional ZnO/Fe-O and graphene oxide nanocomposites: Enhancement of optical and magnetic properties

ZnO is a semiconductor with a great interest, but it has several deficiencies which limit its use in technologic applications. One important limitation is having the band gap in the UV which reduces its use in optical devices. To solve this problem, in this work, composites based in ZnO with goethite and graphene oxide (GO) by sol-gel are prepared. The obtained samples (powders and thin films) were characterized microstructurally (DTA, XRD, micro-Raman, FE-SEM), optically (transmittance and photoluminescence) and magnetically (SQUID). The ZnO band gap of multifunctional composites shows a red-shift towards visible range (E_g ∼3.01 eV) with high transmittance ∼85% (thickness of 362 nm) over the visible wavelength range. A long-range magnetic order at room temperature appears in these nanocomposites (Ms = 1.60·10⁻² emu/g). The combination of both dopants allows modifying the functional properties of ZnO, opening a great field of applications in ZnO composites, such as spintronic and optoelectronic devices.     

Effect of pH on the morphology and optical properties of modified ZnO particles by SDS via a precipitation method

ZnO particles were synthesized directly from an aqueous solution of zinc acetate dihydrate in the presence of sodium dodecyl sulfate (SDS) and sodium hydroxide at 70 °C. The morphological changes were investigated in the range of pH 8-12. The hexagonal prism-like shape was formed at pH 8 and 10 by inhibition of growth along the c direction whereas the small rod-like shape was observed at pH 12. The estimated band gap and the room temperature photoluminescence intensity in a visible region are dependent upon the geometrical shape and size of the ZnO particles.     

Microwave sintering of nano-sized ZnO synthesized by a liquid route

Zinc oxide is a widely used material in various applications in electronic, optic, and spintronic fields, in particular. The control of the final properties of ZnO requires the mastering of the final microstructure. To achieve this goal, the grain growth of ZnO has been examined as a function of the sintering conditions, in particular in using a specific microwave sintering method. In order to get nano-sized ZnO powder as a starting material, a liquid route was implemented. The latter is based on the direct precipitation of a zinc oxalate solution. After thermal treatment, pure ZnO powder was obtained with a very narrow grain size distribution, centered at around 20 nm. The sintering of this powder was then carried out in conventional and microwave furnaces. While an important grain growth occurs during the conventional sintering, it is shown that microwave sintering allows us to maintain the grain size at the nano-metric scale.     

Role of defects in tailoring optical,electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase würtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV–Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu²⁺ and Cu¹⁺ and showed a clear increase in the Cu²⁺ intensity relative to Cu¹⁺, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films’ optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (E_g). This is correlated with a clear reduction in the blue emission luminescence band associated with the V_Zn and in the yellow emission band associated with the O_i. On the other hand, we observed a clear enhancement in the green emission band originating from the V_O, and in the emission band related to possible transitions from Zn_i levels to O_i levels. The slight reduction in the E_g signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu²⁺ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the E_g values and a negligible effect on the distribution of the native defects.     

In-situ study of mass and current density for electrophoretic deposition of zinc oxide nanoparticles

In this study, zinc oxide nanoparticles were synthesized by the hydrothermal microwave-assisted method, followed by its deposition using electrophoretic deposition (EPD) method. An investigation of the characteristics of the synthesized nanoparticles was carried out using X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The morphology and size distribution of the nanoparticles were examined by the images obtained from Transmission Electron Microscope (TEM). The in-situ variations of mass and current density were investigated during the EPD. The effect of different parameters such as the solvent type at various voltages (20 and 40 V) was investigated on EPD kinetics. By increasing the voltage from 20 to 40 V in the methanol, the mass of the deposited film was increased up to about 38%. Similarly, in the ethanol, an increase equal to 39% was observed. The morphology and porosity of deposited nanoparticles were studied by analyzing the images of the Scanning Electron Microscope (SEM). It was observed that the porosity of the film in the ethanol was more than the methanol, at similar potentials. The increase in porosity at the voltage of 20 V was almost 3.1% and at 40 V, it was approximately 4.4% with respect to methanol. Initial current densities in methanol at 20 and 40 V were about 18 and 29% more than ethanol, respectively.     

Tuning of photoluminescence colour in zinc oxide nanoparticles via lithium doping

Zinc acetate dihydrate, lithium hydroxide monohydrate, and anhydrous ethanol as starting compounds are used for the synthesis of Li containing zinc oxide nanoparticles with different emission colours. The emission colour depends on addition of lithium hydroxide and changes from cyan to green-yellow. Photoluminescence spectra of synthesized nanoparticles are recalculated into CIExyY colorimetric model which characterizes the emission colour considering the whole distribution of intensities of the obtained spectra in visible region. This study gives an overlook of the changes which occur when changing the amount of dopant in the process of synthesis. In addition to colour characterization, nanoparticle size distributions and aggregation are discussed. The results demonstrate that the applied method is able to produce lithium-doped zinc oxide nanoparticles with tuneable emission within the range of about 60 nm.     

Preparation and antibacterial properties of titanium-doped ZnO from different zinc salts

To research the relationship of micro-structures and antibacterial properties of the titanium-doped ZnO powders and probe their antibacterial mechanism, titanium-doped ZnO powders with different shapes and sizes were prepared from different zinc salts by alcohothermal method. The ZnO powders were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), and the antibacterial activities of titanium-doped ZnO powders on Escherichia coli and Staphylococcus aureus were evaluated. Furthermore, the tested strains were characterized by SEM, and the electrical conductance variation trend of the bacterial suspension was characterized. The results indicate that the morphologies of the powders are different due to preparation from different zinc salts. The XRD results manifest that the samples synthesized from zinc acetate, zinc nitrate, and zinc chloride are zincite ZnO, and the sample synthesized from zinc sulfate is the mixture of ZnO, ZnTiO₃, and ZnSO₄ · 3Zn (OH)₂ crystal. UV-vis spectra show that the absorption edges of the titanium-doped ZnO powders are red shifted to more than 400 nm which are prepared from zinc acetate, zinc nitrate, and zinc chloride. The antibacterial activity of titanium-doped ZnO powders synthesized from zinc chloride is optimal, and its minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are lower than 0.25 g L⁻¹. Likewise, when the bacteria are treated by ZnO powders synthesized from zinc chloride, the bacterial cells are damaged most seriously, and the electrical conductance increment of bacterial suspension is slightly high. It can be inferred that the antibacterial properties of the titanium-doped ZnO powders are relevant to the microstructure, particle size, and the crystal. The powders can damage the cell walls; thus, the electrolyte is leaked from cells.     

A novel thermal decomposition method for the synthesis of ZnO nanoparticles from low concentration ZnSO4 solutions

In this research work, ZnO nanoparticles were prepared by direct thermal decomposition method with Zn₄(SO₄)(OH)₆·0.5 H₂O as a precursor. The precursor was calcinated in air for 1 h at 825 °C. Samples were characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), infrared spectrum (IR), and scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD, EDS, and IR results indicated that the ZnO nanoparticles were pure. The average crystallite and particle size of the ZnO nanoparticles were estimated to be 87 nm and 92 nm by XRD and TEM, respectively. The SEM and TEM images showed that the ZnO nanoparticles were of spherical shape. The simplicity of the present method suggests its potential application at industrial scale as a cheap and convenient way to produce pure ZnO nanoparticles from low concentration ZnSO₄ solutions.     

Effect of ignition temperature and fuel amount on photocatalytic activity of solution combustion synthesized ZnO

Nanocrystalline zinc oxide (ZnO) photocatalyst has been synthesized by a simple solution combustion method using zinc nitrate as the oxidizer and urea as the fuel. Effect of fuel to oxidizer ratio and ignition temperature on the mechanism of combustion synthesis, crystallinity, morphology, surface area, and optical properties were investigated by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), Diffuse reflectance UV–Visible spectra and Photoluminescence analyses. Photocatalytic activity of the synthesized materials was evaluated by degrading an azo dye at ambient temperature and solution pH. The prepared photocatalysts at the fuel-rich condition possess small crystallite size and more surface area; consequently, a higher photocatalytic dye degradation capability.The powder samples synthesized at the fuel-oxidant ratio of 1.8 and the ignition temperature at 400 °C have shown the maximum percentage (99%) of dye degradation in 180 min. The pseudo-first order photocatalytic dye degradation rate constant of a catalyst sample synthesized at the fuel-oxidant ratio of 1.8 was 0.0253 min⁻¹and it is 3.14 and 2.88 times higher than that of samples synthesized at fuel-oxidant ratios of 0.6 and 1.The outcomes of the present article help to design more pronounced experiments for the synthesis of photocatalysts by varying ignition temperatures and fuel amounts.     

A green chemical method for synthesis of ZnO nanoparticles from solid-state decomposition of Schiff-bases derived from amino acid alanine complexes

As a part of the desire to save the environment via green chemistry practices, we report a novel method to synthesize ZnO nanoparticles from nontoxic and biocompatible chemicals where no pollutant or combustible side product is produced. In this recipe, a binary Zn(II) Schiff-base complex is obtained from alanine where water is used as solvent and a biologically compatible amino acid instead of toxic amines is used as a nitrogen source. The Schiff-base complex is subsequently heat treated to synthesize ZnO particles via a solid-state decomposition process. The effect of post heat treatment temperature (400, 500, and 600 °C) on microstructure and defect content of ZnO nanoparticles is investigated. The formation of single phase ZnO particles is confirmed by XRD θ–2θ patterns and FTIR spectra. TEM and SEM micrographs indicate the formation of nanoparticles with a particle size of 50–110 nm for different heat treatment temperatures. Combing XRD, FTIR, and PL results, it is revealed that the samples heat treated at intermediate temperatures (500 °C) possess the lowest defect concentration and a favorable crystallinity. This study emphasizes on green chemistry and synthesis of nanomaterials through ecofriendly methods to save our planet and its reservoirs for future and next generations.