Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition

Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.40°. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm⁻¹, whereas the E₂ mode of ZnO at 436 cm⁻¹ can barely be identified. Annealing at 500-800 °C causes decrease of 275, 582, 640 and 720 cm⁻¹ and increase of 436 cm⁻¹ intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of ∼10²¹/cm³ was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of Zn_I-N_O or (N₂)_O that act as donor/double donors.     

Structural, optical and electronic properties of P doped p-type ZnO thin film

P doped ZnO films were grown on quartz by radio frequency-magnetron sputtering method using a ZnO target mixed with 1.5 at% P₂O₅ in the atmosphere of Ar and O₂ mixing gas. The as-grown P doped ZnO film showed n-type conductivity, which was converted to p-type after 800 °C annealing in Ar gas. The P doped ZnO has a resistivity of 20.5 Ω cm (p∼2.0×10¹⁷ cm⁻³) and a Hall mobility of 2.1 cm² V⁻¹ s⁻¹. XRD measurement indicated that both the as-grown and the annealed P doped ZnO films had a preferred (0 0 2) orientation. XPS study agreed with the model that the P_Zn-2V_Zn acceptor complex was responsible for the p-type conductivity as found in the annealed P-doped ZnO. Temperature-dependent photoluminescence (PL) spectrum showed that the dominant band is located at 3.312 eV, which was attributed to the free electronic radiative transition to neutral acceptor level (FA) in ZnO. The P_Zn-2V_Zn acceptor complex level was estimated to be at E_V=122 meV.     

Annealing and surface conduction on Hydrogen peroxide treated bulk melt-grown,single crystal ZnO

We report on the studies carried out on hydrogen peroxide treated melt-grown, bulk single crystal ZnO samples. Results show the existence of two shallow donors in the as-received ZnO samples with energy levels (37.8±0.3) meV that has been suggested as Zn_i related and possibly H-complex related and (54.5±0.9) meV, which has been assigned to an Al-related donor. Annealing studies performed on the hydrogen peroxide treated samples reveal the existence of a conductive channel in the samples in which new energy levels have been observed, Zn vacancies, related to the Group I elements, X_Zn. The surface donor volume concentration of the conductive channel was calculated from a theory developed by Look (2007) [1]. Results indicate an increase in the surface volume concentration with increasing annealing temperature from 60×10¹⁷ cm⁻³ at 200 °C to 4.37×10¹⁸ cm^-3 at 800 °C.     

Novel porous CuO microrods: synthesis,characterization, and their photocatalysis property

Porous copper oxide microrods have been synthesized via calcining copper glycinate monohydrate microrod precursor which was prepared in mild conditions without any template or additive. Several techniques, such as X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller (BET) N₂ adsorption–desorption analyses, were used to characterize the structure and morphology of the products. Scanning electron microscopy (SEM) analyses show that the precursor consists of a large quantity of uniform rod-like micro/nanostructures with typical lengths in the range of 25–40 µm and diameters in the range of 0.1–0.35 µm. The microrod-like precursors transformed into porous microrod products after calcination at 450 °C in flow air for 2 h. The BET surface area of the porous CuO microrods was calculated to be 8.5 m² g⁻¹. In addition, the obtained porous CuO microrods were used as catalysts to photodegrade rhodamine B (RhB), methyl orange, methylene blue, eosin B, and p-nitrophenol. Compared with commercial CuO powders, the as-prepared porous CuO microrods exhibit superior properties on photocatalytic decomposition of RhB due to their porous hierarchical structures.     

Comparative study of ethanol sensor based on gold nanoparticles: ZnO nanostructure and gold: ZnO nanostructure

Gold colloid:ZnO nanostructures were prepared from Zn powder by using thermal oxidation technique on alumina substrates, then it was impregnated by gold colloid for comparative study. The gold colloid is the solution prepared by chemical reduction technique; it appeared red color for gold nanoparticle solution and yellow color for gold solution. The heating temperature and sintering time of thermal oxidation were 700 °C and 24 h, respectively under oxygen atmosphere. The structural characteristics of gold colloid:ZnO nanostructures and pure ZnO nanostructures were studied using filed emission scanning electron microscope (FE-SEM). From FE-SEM images, the diameter and length of gold colloid:ZnO nanostructures and ZnO nanostructures were in the ranges of 100-500 nm and 2.0-7.0 μm, respectively. The ethanol sensing characteristics of gold colloid:ZnO nanostructures and ZnO nanostructures were observed from the resistance alteration under ethanol vapor atmosphere at concentrations of 50, 100, 200, 500, and 1000 ppm with the operating temperature of 260-360 °C. It was found that the sensitivity of sensor depends on the operating temperature and ethanol vapor concentrations. The sensitivity of gold colloid:ZnO nanostructures were improved with comparative pure ZnO nanostructures, while the optimum operating temperature was 300 °C. The mechanism analysis of sensor revealed that the oxygen species on the surface was O²⁻.     

Ripple topography on thin ZnO films by grazing and oblique incidence ion sputtering

We have investigated the formation and growth of nano sized ripple topography on ZnO thin films by 10 keV O¹⁺ bombardment at impact angles of 80° and 60°, varying the ion fluence from 5 × 10¹⁶ to 1 × 10¹⁸ ions/cm². At 80° the ripples are oriented along the ion beam direction whereas at 60° it is perpendicular to the ion beam direction. The developed ion induced structures are characterized by atomic force microscopy (AFM) and the alignment, variation of rms roughness, wavelength and correlation length of the structures are discussed with the existing model and basic concept of ion surface interaction.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Fabrication of p-type Li-doped ZnO films by pulsed laser deposition

p-Type ZnO thin films have been realized via doping Li as acceptor by using pulsed laser deposition. In our experiment, Li₂CO₃ was used as Li precursor, and the growth temperature was varied from 400 to 600 °C in pure O₂ ambient. The Li-doped ZnO film prepared at 450 °C possessed the lowest resistivity of 34 Ω cm with a Hall mobility of 0.134 cm² V⁻¹ s⁻¹ and hole concentration of 1.37 × 10¹⁸ cm⁻³. X-ray diffraction (XRD) measurements showed that the Li-doped ZnO films grown at different substrate temperatures were of completely (0 0 2)-preferred orientation.     

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al₂O₃ substrate to form an intermediate ZnAl₂O₄ spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm⁻¹ is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.     

Effect of ZnO on phase emergence, microstructure and surface modifications of calcium phosphosilicate glass/glass-ceramics having iron oxide

The effect of ZnO on phase emergence and microstructure properties of glass and glass-ceramics with composition 25SiO₂-50CaO-15P₂O₅-(10 − x)Fe₂O₃-xZnO (where x = 0, 2, 5, 7 mol%) has been studied. They have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Surface modifications of these glass-ceramics in simulated body fluid have been studied using Fourier transform infrared reflection spectroscopy (FTIR), XPS and SEM. Results have shown a decrease in the fraction of non-bridging oxygen with increase in zinc oxide content. Emergence of crystalline phases in glass-ceramics at different heat treatment temperatures was studied using XRD. When glass is heat treated at 800 °C calcium phosphate, hematite and magnetite are developed as major phases in the glass-ceramics samples with ZnO up to 5 mol%. In addition to these, calcium silicate (Ca₃Si₂O₇) phase is also observed when glass is heat treated at 1000 °C. The microstructure of the glass-ceramics heat treated at 800 °C exhibits the formation of nano-size (40-50 nm) grains. On heat treatment at 1000 °C crystallites grow to above 50 nm size and more than one phase are observed in the microstructure. The formation of thin flake-like structure with coarse particles is observed at high zinc oxide concentration (x = 7 mol%). In vitro studies have shown the surface modifications and formation of Ca-P-rich layer on the glass-ceramics when immersed in simulated body fluids (SBF) for different durations. The bioactive response was found to depend on ZnO content.     

Excimer laser processing of ZnO thin films prepared by the sol-gel process

ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH] and isopropanol. The deposited films were dried at 50 and 300 °C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm⁻² on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol-gel processing route differs significantly from ZnO sol-gel films prepared by conventional furnace annealing which requires temperatures above 450 °C for the formation of crystalline ZnO.     

Preparation and characterization of bifunctional, Fe3O4/ZnO nanocomposites and their use as photocatalysts

Bifunctional magnetic-optical Fe₃O₄/ZnO nanocomposites with different molar ratio were successfully synthesized by a facile two-step strategy. Compared with the other methods, it was found to be mild, inexpensive, green, convenient and efficient. Fe₃O₄ nanocrystal was used as seed for the deposit and growth of ZnO nanoparticle. A series of the characterizations manifested that the combination of Fe₃O₄ with ZnO nanoparticles was successful. Photocatalytic activity studies confirmed that as-prepared nanocomposites had excellent photodegradating behavior to Methyl Orange (MO) compared to the pure ZnO nanoparticles. The results showed that the degradation percentage of MO was about 93.6% for 1 h when the amount of catalyst was 0.51 g L⁻¹ and initial concentration of MO was 6 × 10⁻⁵ mol L⁻¹ in the pH 7 solution. Moreover, the kinetics of photocatalytic degradation reaction could be expressed by the first-order reaction kinetic model. Furthermore, the Fe₃O₄/ZnO nanocomposites could be also served as convenient recyclable photocatalysts because of their magnetic properties.     

Cu-Zn-Al mixed metal oxides derived from hydroxycarbonate precursors for H2S removal at low temperature

One series of Cu-Zn and two series of Cu-Zn-Al hydroxycarbonate precursors with varying metal molar ratios were prepared via co-precipitation or multi-precipitation method, and the mixed metal oxides obtained by calcination of the precursor materials were used as adsorbents for H₂S removal in the range of 25-100 °C. The results of H₂S adsorption tests showed that these mixed oxides, especially two series of Cu-Zn-Al mixed metal oxides exhibited markedly high breakthrough sulfur capacities (ranging from 4.4 to 25.7 g S/100 g-sorbent with increase of Cu/Zn molar ratio) at 40 °C. Incorporation Cu and/or Al decreased the mean crystalline sizes of ZnO and CuO species in the Cu-Zn and Cu-Zn-Al mixed metal oxide adsorbents by decreasing of mean crystalline sizes of hydroxycarbanate phases mainly including hydrozincite, aurichalcite and malachite, segregation of Al phase, etc. Higher breakthrough sulfur capacity of each adsorbent in two ternary series than that of the corresponding adsorbent in binary series should be ascribed to the enhancement of the dispersion of ZnO and/or CuO species with incorporation of aluminum, thereby increasing the overall rate of reaction between the adsorbent and H₂S by reducing the thickness of potential sulfide shell on the outer layer of the oxide crystalline grains and increasing the area of the interface for the exchange of HS⁻/S²⁻ and O²⁻. For each series of adsorbents, the breakthrough sulfur capacity increased with the increase of Cu/Zn molar ratio regardless of changes of the dispersion of CuO and/or ZnO. This phenomenon might be mainly attributed to faster rate of the lattice diffusion of HS⁻, S²⁻ and O²⁻ or exchange of HS⁻/S²⁻ and O²⁻ during the sulfidation of CuO than that during the sulfidation of ZnO due to less rearrangement of the anion lattice.     

Luminescence anisotropy of ZnO microrods

The local features of light emission from ZnO microrods were studied: it is revealed that ZnO luminescence spectra are significantly influenced by the crystal morphology. It is shown that the near and edge ultraviolet emission occurs primarily from the top (0001) planes of ZnO microrods; while the defect related visible emission was found to occur dominantly from the side facets. The room temperature cathodoluminescence analysis revealed that visible emission consists of a few overlapping peaks, arising due to recombination on common points and surface defects (Zn_i, V_o, V_o⁰/V_o^?*_, V_o^** and surface defects.). While at low temperature, only the luminescence due to neutral donor bound exciton (D⁰X) emission is observed. The data obtained suggest that the light emission spectra of ZnO material of diverse morphology cannot be directly compared, although some common spectral features are present.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

Magnetic and bioactivity evaluation of ferrimagnetic ZnFe2O4 containing glass ceramics for the hyperthermia treatment of cancer

Glass ceramics of the composition xZnO·25Fe₂O₃·(40−x)SiO₂·25CaO·7P₂O₅·3Na₂O were prepared by the melt-quench method using oxy-acetylene flame. Glass-powder compacts were sintered at 1100 °C for 3 h and then rapidly cooled at −10 °C. X-ray diffraction (XRD) revealed 3 prominent crystalline phases: ZnFe₂O₄, CaSiO₃ and Ca₁₀(PO₄)₆(OH)₂. Vibrating sample magnetometer (VSM) data at 10 KOe and 500 Oe showed that saturation magnetization, coercivity and hence hysteresis area increased with the increase in ZnO content. Nano-sized ZnFe₂O₄ crystallites were of pseudo-single domain structure and thus coercivity increased with the increase in crystallite size. ZnFe₂O₄ exhibited ferrimagnetism due to the random distribution of Zn²⁺ and Fe³⁺ cations at tetrahedral A sites and octahedral B sites. This inversion/random distribution of cations was probably due to the surface effects of nano-ZnFe₂O₄ and rapid cooling of the material from 1100 °C (thus preserving the high temperature state of the random distribution of cations). Calorimetric measurements were carried out using magnetic induction furnace at 500 Oe magnetic field and 400 KHz frequency. The data showed that maximum specific power loss and temperature increase after 2 min were 26 W/g and 37 °C, respectively for the sample containing 10% ZnO. The samples were immersed in simulated body fluid (SBF) for 3 weeks. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDX) and XRD results confirmed the growth of precipitated hydroxyapatite phase after immersion in SBF, suggesting that the ferrimagnetic glass ceramics were bioactive and could bond to the living tissues in physiological environment.     

Simultaneous observation of oxygen uptake curves and electronic states during room-temperature oxidation on Si(0 0 1) surfaces by real-time ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy was used to measure the oxygen uptake, changes in work function due to the surface dipole layer of adsorbed-oxygen atoms, Δ?_SDL, and changes in band bending due to the defect-related midgap state, ΔBB, simultaneously during oxidation on Si(0 0 1) surface at room-temperature, RT, under an O₂ pressure of 1.3 × 10⁻⁵ Pa. The oxygen dosage dependence of Δ?_SDL revealed that dissociatively adsorbed-oxygen atoms occupy preferentially dimer backbond sites at the initial stage of Langmuir-type adsorption, which is associated with a rapid increase of ΔBB. When raising temperature to ∼600 °C, such preferential occupation of the dimer backbond sites by oxygen atoms is less significant and ΔBB becomes smaller in magnitude. The observed relation between Δ?_SDL and ΔBB indicates that point defects (emitted Si atoms + vacancies) are more frequently generated by oxygen atoms diffusing to the dimer backbond sites at lower temperature in RT −600 °C.     

A novel chemical synthesis and characterization of Mn3O4 thin films for supercapacitor application

Mn₃O₄ thin films have been prepared by novel chemical successive ionic layer adsorption and reaction (SILAR) method. Further these films were characterized for their structural, morphological and optical properties by means of X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), field emission scanning electron microscopy (FESEM), wettability test and optical absorption studies. The XRD pattern showed that the Mn₃O₄ films exhibit tetragonal hausmannite structure. Formation of manganese oxide compound was confirmed from FTIR studies. The optical absorption showed existence of direct optical band gap of energy 2.30 eV. Mn₃O₄ film surface showed hydrophilic nature with water contact angle of 55°. The supercapacitive properties of Mn₃O₄ thin film investigated in 1 M Na₂SO₄ electrolyte showed maximum supercapacitance of 314 F g⁻¹ at scan rate 5 mV s⁻¹.