Porous Zinc Oxide Research Articles

Hierarchically Assembled Porous ZnO Nanoparticles: Synthesis, Surface Energy, and Photocatalytic Activity

We show an effective experimental method to prepare hierarchically porous zinc oxide (ZnO) spherical nanoparticles through a self-assembly pathway using surface-modified colloidal ZnO nanocrystallites as the building blocks and P-123 copolymers as the template in aqueous solution. Copolymers are thoroughly removed by Soxhlet extraction with ethanol and calcination at 400 °C. The final products have been characterized by X-ray powder diffraction (XRD) pattern, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and photoluminescence (PL) spectroscopy. On the basis of calorimetric measurements reported separately, the surface enthalpy (γ) of the hydrated porous ZnO is 1.42 ± 0.21 J/m2, in good agreement with that of ZnO nanoparticles. The calorimetric results support the presence of self-assembled ZnO nanocrystallites in the nanoporous ZnO. Photocatalytic activitiy of porous ZnO nanoparticles has been tested on the photodegradation of phenol under ambient condition, indicating that porous ZnO nanoparticles show superior activity to TiO2 nanoparticles (PC-500).

Thermoelectric properties of porous zinc oxide ceramics doped with praseodymium

We evaluated praseodymium (Pr) doping effects on thermoelectric properties of porous zinc oxide (ZnO) ceramics. The low density ceramics composed of ZnO and an additive of Pr (0.5 and 0.1 mol%) oxide were prepared by sintering processes in different atmospheres (air and oxygen), where the additives were Pr6O11 known for phase transformations and the trioxide Pr2O3 with low valence state different from Pr6O11. Thermoelectric properties of the samples were measured between 313 K and 903 K. At high-temperature around 590 K, some samples showed a maximum of electrical resistivity. We discussed the origin of the maximum on the basis of carrier transport model on gas sensor and varistor. From the results, The maximum appearance was attributable to an interchange of carriers through defects complex closely related with enhanced zinc vacancies acceptor-like in the vicinity of grain boundaries (GB), where the defect complex seemed to be caused by Pr with valence state between 3+ and 3.78+ around GB. The doping Pr into ZnO matrix is expected to be advantageous to improve thermoelectric power.

Morphology and optoelectronic properties of ZnO rod array/conjugated polymer hybrid films

Vertically aligned zinc oxide (ZnO) nanorods were grown on the ITO glass and then coated with the conjugated polymer poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) to make the hybrid films. Nanorods with different diameters were synthesized to study the influences of ZnO nanorod morphology and polymer infiltration on the photocurrent and optical properties of the hybrid films. Increasing the growth time leads to the formation of ZnO rod array with large rod diameter, large surface area and small inter-rod distance. Small inter-rod distance hinders the filling of DB-PPV into the porous ZnO rod microstructure and lowers the PN junction area. It leads to lower photocurrent of the hybrid film. The red shift of the photoluminescence spectra suggests that filling the polymer into the ZnO rod microstructure favors more planar molecular orientations of the conjugated polymers and leads to an increase in the effective conjugation length.

Kinetics of Zinc Oxide Sulfidation for Packed-Bed Desulfurizer Modeling

The sulfidation process of porous zinc oxide sorbent with hydrogen sulfide can be described in five steps after external mass transfer and pore diffusion. They are surface adsorption of hydrogen sulfide gas on zinc oxide sorbent and dissociation of the gas molecule on the sorbent surface, followed by reversible surface reactions, sulfide ion migration under the surface, and sulfidation penetration into the solid crystallite. On the basis of the understanding of this chemistry, an empirical rate law for the intrinsic kinetics of the sulfidation process was derived in this study. Kinetics modeling results using this reversible, adsorption, and ion-migration (RAIM) model were found, consistent with selected experiments of single-particle sulfidation. Modeling results were also comparable with several well-defined sulfidation models in the literature. The intrinsic kinetics of a porous ZnO sorbent G-72E were measured in a microflow packed column and calculated using the RAIM model, using a finite difference approach. The effective pore diffusivity of gaseous hydrogen sulfide in the porous zinc oxide pellet was calculated using the general process modeling system (gPROMS). Finally, design calculation for a full-scale packed desulfurizer was performed using the gPROMS distributed reactor model. Case studies were presented for hydrogen sulfide removal from natural gas in a simulated fuel-processing train for syngas production.

One-step Preparation and Photosensitivity of Size-quantized Cadmium Chalcogenide Nanoparticles Deposited on Porous Zinc Oxide Film Electrodes

Abstract Size-quantized cadmium chalcogenide nanoparticles were effectively deposited on a porous zinc oxide particulate film electrode by a one-step hydrothermal method in which an increase in the concentration of 3-mercaptopropionic acid reduced the size of deposited nanoparticles. The photoelectrochemical properties of the resulting electrodes varied depending on the kind of metal chalcogenide nanoparticles deposited and their size.

Tailored synthesis of ZnO:Er(III) nanosystems by a hybrid rf-sputtering/sol-gel route

The rational design of rare-earth doped materials represents one of the major goals of current research in the field of optoelectronics. In this study, ZnO:Er(III) nanosystems were synthesized by means of a hybrid rf-sputtering/sol-gel (SG) route, consisting of the erbium (guest) rf-sputtering on porous zinc oxide xerogels (host) obtained by the SG method, followed by ex situ thermal treatments in air (300–600°C, 1–5h). Particular attention was focused on tailoring the erbium content and distribution in the resulting systems by optimization of the processing conditions. The structural, morphological, and compositional characteristics and their interplay with the synthesis procedure were investigated by glancing incidence x-ray diffraction, x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, secondary ion mass spectrometry, UV-visible-near IR absorption spectroscopy, and atomic force microscopy. The obtained results highlight the advantages and versatility of the adopted preparation strategy in tailoring the amount and distribution of Er(III) species in high purity ZnO matrices, resulting in nanosystems endowed with compositional and morphological homogeneity at nanoscale level.

Production of porous nanostructured zinc oxide thin films by pulsed laser deposition

Pulsed laser deposition has been used to produce porous nanostructured zinc oxide (ZnO) thin films. A laser produced ZnO plasma plume was deposited at a highly oblique incident angle onto rotating silicon (Si) substrates resulting in porous thin films consist of isolated ZnO posts or helices with diameters around 100nm. These posts and helices were formed due to a self-shadowing mechanism, at fast and slow substrate rotation speeds, respectively. These films were deposited on Si substrates at room temperature at the presence of 0.07Torr oxygen and the films were annealed at 600°C for 2h after deposition. Strong UV and negligible visible photoluminescence emissions were observed from these ZnO films.

Electrodeposition of porous zinc oxide electrodes in the presence of sodium laurylsulfate

Thin porous films of crystalline ZnO have been prepared by cathodic electrodeposition from aqueous oxygen saturated zinc chloride solution in the presence of additives, such as different small organic acids (benzoic acid, p-toluenesulfonic acid, benzenetetracarboxylic acid) and a surfactant (sodium laurylsulfate). The small negatively charged additives influence the growth rate of ZnO due to the increase in the current density during electrodeposition with increasing number of functional groups. However, they do not change the morphology of the resulting ZnO films.The influence of sodium laurylsulfate on the growth rate of ZnO depends on its concentration. As long as the concentration is low there is only slight effect. However, if the provided concentration is high enough to achieve formation of micelles and to get assembly of these micelles on the charged electrode surface, the current density found during electrochemical deposition of ZnO was extremely high, indicating a strongly increased growth rate due to the catalysis by sodium laurylsulfate. The resulting films show remarkably different morphology, large surface area and good porosity. Sodium laurylsulfate incorporated in the ZnO films could be removed almost completely from the pores by extraction with ethanol without affecting the crystallinity of the material.

Fabrication of 2D and 3D ordered porous ZnO films using 3D opal templates by electrodeposition

Both two- and three-dimensional ordered porous zinc oxide films were fabricated by electrodeposition using three-dimensional polystyrene opal templates. Scanning electron microscope measurements showed that the dimensions (monolayer or multilayer) of porous zinc oxide films can be controlled by selecting zinc nitrate in different solvents (water or mixed ethanol–water solvents). Optical measurements showed that the porous structures generated in mixed ethanol–water solutions exhibited the photonic stopband, indicating large-scale crystalline order in the samples. It was found that the position of stopband center and the surface morphologies in the three-dimensional zinc oxide porous films were dependent on the ethanol content in the electrolyte. The influence of ethanol content on the porous films was discussed.

Fabrication of 2D Ordered Porous ZnO Films Using 3D Opal Templates by Electrodeposition

Abstract The fabrication of two-dimensionally ordered porous zinc oxide films using three-dimensional colloidal crystal templates by electrodeposition is reported. The hydrophilicity of the substrate and the hydrophobicity of the template are utilized to control the generated structures.

Dense and porous ZnO thin films produced by pulsed laser deposition

Dense and porous zinc oxide (ZnO) thin films were deposited onto silicon substrates in vacuum and in 100 mTorr O 2 at room temperature by pulsed laser deposition using 15 ns krypton fluoride (KrF), λ = 248 nm, laser pulses with laser fluence of 3 J cm −2. The structural, morphological, optical and photoluminescence properties of the as-grown and annealed ZnO thin films were studied. O 2 background gas during deposition and post-annealing treatment were essential to obtain a crystalline structure and strong ultraviolet (UV) luminescence emissions.

Zinc oxide as an ozone sensor

This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet (UV) photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4mW∕cm2. On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose, with a response frequency above 15kHz, thus showing improved ozone ac sensing discrimination.

Growth of layered basic zinc acetate in methanolic solutions and its pyrolytic transformation into porous zinc oxide films

Layered basic zinc acetate (LBZA), Zn 5(OH) 8(CH 3COO) 2·2H 2O, was deposited on glass substrates by a chemical bath deposition (CBD) method using methanolic solutions of zinc acetate dihydrate. The substrates were put into bottles filled with the solutions and sealed up and were kept at 60 °C in a drying oven. Immersion time necessary for the deposition of LBZA films was typically more than 28 h. This was a key to inducing heterogeneous nucleation of LBZA through control over a degree of supersaturation in the evolution of a unique, nest-like morphology. Hydration water contained by zinc acetate dihydrate was quantitatively enough to promote hydrolysis of zinc acetate. The LBZA films were transformed into nanocrystalline, porous ZnO films without morphological deformation by heating at 150 °C in air. A mechanism of the formation of the nest-like morphology was discussed based on nonaqueous solution reactions, nucleation, and crystal growth during the CBD process.

The mechanisms and kinetics of reduction of zinc oxide solid solutions

The reduction of dense zinc oxide (ZnO) samples with and without impurity oxide additions in CO/CO 2 and CO/N 2 gas mixtures has been investigated at temperatures between 1173 and 1373 K. The microstructural characterization of the reacted and partially reduced samples was complemented with measurements of the rates of the reactions. The kinetics of reduction of ZnO were found to be dependent upon the CO partial pressure, temperature, degree of reduction, and the type of impurity oxides (i.e., FeO, MgO, CaO, SiO 2 , MnO, Al 2 O 3 ) in the samples. The reduction of ZnO solid solutions resulted in the formation of a planar ZnO reaction interface and a porous product layer composed of impurity oxides, whereas the reduction of pure ZnO resulted in selective attack of particular crystal planes and an irregular growth interface. An interface reaction model has been proposed for the observed reaction kinetics and product microstructures for the reduction of ZnO solid solutions. From the analysis of the relative rates of the various chemical reactions and mass transport processes, the rates of reduction of ZnO with and without impurity oxide addition were found to be controlled jointly by chemical reaction at the ZnO reaction interface and pore diffusion of reducing gas through the porous ZnO layer or reaction product layer.

Observation of an Electromagnetically Driven Temperature Wave in Porous Zinc Oxide During Microwave Heating

AbstractPropagation of a sharp temperature wave was observed during microwave heating of porous zinc oxide in nitrogen and argon atmospheres. This wave initiated from the center of the sample and traveled at an average velocity of 0.2 cm/min towards its surface. This temperature wave was attributed to an anomalous peak in the imaginary part of the complex permittivity possibly caused by desorption of chemisorbed oxygen from the surfaces of ZnO crystallites.

Reactivity evolution during sulfidation of porous zinc oxide

The sulfidation of porous zinc oxide in hydrogen sulfide—nitrogen mixtures was studied in a thermogravimetric analysis system. Experiments were carried out with particles in the range 52–350 μm, at temperatures from 300 to 600°C, and with different hydrogen sulfide concentrations. The analysis of the experimental results showed that the sulfidation reaction was of first-order with respect to the H 2S concentration, and that the conversion vs time trajectories depended strongly on the particle size. The generalized pore model of Yu and Sotirchos, a detailed model for diffusion, reaction, and structure evolution in chemically reacting porous media, was used to analyze the experimental data, and good agreement between model and experiment was observed. The intraparticle diffusional limitations and their evolution with the progress of the reaction were found to be the factor influencing most the sorptive capacity of the ZnO sorbet for H 2S removal.

Ellipsometric characterization of surface oxidation in polycrystalline Zn 3P 2 thin films

Zn 3P 2 is II–V semiconductor that has been studied as a light-absorbing material in highly efficient photovoltaic heterojunctions and in photodetectors. The optical properties of the films, as measured by spectroscopic ellipsometry, indicate the existence of two absorption peaks at 2.7 eV and 4.0 eV, as in the case of monocrystalline material. These peaks have a soft shape because of the polycrystalline character, roughness and surface oxide of the film. The spectral ellipsometric analysis, after surface etching, revealed notable evolutions of the optical spectra. Real-time ellipsometric analysis of the surface evolution, at a fixed wavelength, was performed in air at room temperature after surface etching. The results indicate the formation of a very thin and porous native zinc oxide layer with an exponentially decreasing growth rate. The results are discussed in terms of surface changes, such as oxidation, loss of stoichiometry and porosity which are of great interest for discerning the kinetics of surface degradation, with a view to the applications of these films in optoelectronic devices.

Experimental evaluation of a class of distributed pore size models for gas-solid reactions with solid product

The performance of a mathematical model for transport, reaction, and structure evolution during gas-solid reactions involving porous solids and solid product was evaluated using experimental reactivity evolution data for the sulfidation of porous zinc oxide and the sulfation of limestone calcines. It was found that the mathematical model can satisfactorily approximate the experimental data provided that allowance is made for the formation of inaccessible pore space in the course of the reaction. Comparison of model predictions and experimental data showed that the intraparticle diffusional limitations are among the factors having the most influence on the behavior of these systems. As a result, the variation of the reactivity of the solids is strongly influenced not only by the pore size distribution of the solids but by the connectivity of their pore space as well.

Gas Sensing Characteristics of Porous ZnO and Pt/ZnO Ceramics

Changes in resistivity and chemical changes in reducing gases were measured for porous zinc oxide ceramics with and without a platinum catalyst at 300° and 400°C to examine the gas sensing mechanism and the effect of platinum additions. Reducing gases were oxidized to CO2 and Hz2 on the sensor surfaces. Platinum addition promoted the oxidation of reducing gases but did not lead to an increase in the resistivity change at 400°C. The reaction sequences for the gas sensing process are proposed, taking into account partially oxidized intermediates of hydrocarbons and oxidation on platinum without an electron transfer process.

Photodesorption in Zinc Oxide Semiconductor

Dark and photoconductivity in sintered zinc oxide have been studied by Morrison and Melnick as an oxygen chemisorption process. An order of magnitude calculation shows that a consequence of Melnick's theory of photoconductivity should be an observable new effect which we call photodesorption (the release of surface adsorbed gases by light). Measurement of the photodesorption from sintered porous zinc oxide simultaneously with the photoconductivity is described.