Nanoporous ZnO Research Articles

An environment-benign solvothermal method for the synthesis of flower-like hierarchical nickel and zinc compounds and their transformation to nanoporous NiO and ZnO

Flower-like hierarchical Ni3(NO3)2(OH)4 and Zn5(OH)8(NO3)2(H2O)2 compounds were synthesized by a simple solvothermal method. Absolute ethanol was adopted as solvent, and the precursor is the corresponding nitrate. The as-prepared products were studied by XRD, SEM, TEM, and SAED. It is noted that compared to the conventional oil–water surfactant system, this biosafe alcohol system is simpler and environmentally more benign. Through calcination of the as-synthesized nickel and zinc compounds, nanoporous NiO and ZnO were generated. The optical properties of the nanoporous NiO and ZnO microcrystals have been discussed in terms of photoluminescence (PL) at room temperature. An emission band centered at ca. 2.3 eV that could only be found by cathodoluminescence was observed for the NiO microcrystals. We tentatively assign the peak to the magnetic exciton state 3d82p and relate it to the unique nanostructure of the nanoporous NiO microcrystals.

From nanowires to hierarchical structures of template-free electrodeposited ZnO for efficient dye-sensitized solar cells

A new and original method for the electrochemical growth of ZnO nanocrystalline porous layers and multiscale hierarchical structures is described. The structures are designed by simply playing with the growth conditions and without any use of template or additive in the aqueous deposition bath. Two types of hierarchical structures are described combining electrodeposited ZnO nanowire arrays and a nanoporous layer: nanowire arrays covered by a conformal nanoporous layer and nanowire arrays embedded in a nanoporous layer. The global performances of dye sensitized solar cells (DSSCs) fabricated using the hierarchical structures are higher than those found for nanoparticulate sol–gel ZnO films and for the two basic electrodeposited structures. Films made of nanowires embedded in a nanocrystalline matrix show a maximum energy conversion efficiency of ∼4.1%. The wires play several important beneficial roles in the presented structures since they permit the electrodeposition of thick nanoporous ZnO films which immobilize a large amount of dye, they act as preferential electron pathways for efficient charge collection and, due to their size, they enhance the light trapping in the photoanode and hence increase the light diffusion length before its harvesting by the dye. Another interest of the proposed ZnO hierarchical structures is a synthesis as well as an applied post-growth thermal treatment performed below 150 °C in soft environments which are then perfectly compatible with lightweight plastic flexible and other fragile substrates.

Tunable Nanostructure and Photoluminescence of Columnar ZnO Films Grown by Plasma Deposition

Nanoporous ZnO thin films presenting a tunable nanostructure and photoluminescence (PL) were grown by plasma enhanced vapor deposition on surface oxidized Si substrates. These films consist of c-ax...

Electrodeposited Nanoporous versus Nanoparticulate ZnO Films of Similar Roughness for Dye-Sensitized Solar Cell Applications

We present a comparative study of two different ZnO porous film morphologies for dye-sensitized solar cell (DSSC) fabrications. Nanoparticulate ZnO was prepared by the doctor-blade technique starting from a paste containing ZnO nanoparticles. Nanoporous ZnO films were grown by a soft template-assisted electrochemical growth technique. The film thicknesses were adjusted at similar roughness of about 300 in order to permit a worthy comparison. The effects on the cell performances of sensitization by dyes belonging to three different families, namely, xanthene (eosin Y) and indoline (D102, D131, D149 and D205) organic dyes as well as a ruthenium polypyridine complex (N719), have been investigated. The mesoporous electrodeposited matrix exhibits significant morphological changes upon the photoanode preparation, especially upon the dye sensitization, that yield to a dramatic change of the inner layer morphology and increase in the layer internal specific surface area. In the case of indoline dyes, better efficiencies were found with the electrodeposited ZnO porous matrixes compared to the nanoparticulate ones, in spite of significantly shorter electron lifetimes measured by impedance spectroscopy. The observation is interpreted in terms of much shorter transfer time in the oxide in the case of the electrodeposited ZnO films. Among the tested dyes, the D149 and D205 indoline organic dyes with a strong acceptor group were found the most efficient with the best cell over 4.6% of overall conversion efficiency.

Symmetrically and unsymmetrically substituted carboxy phthalocyanines as sensitizers for nanoporousZnOfilms

The photoelectroectrochemical studies of water soluble octacarboxylated oxotitanium (OTiOCPc) , zinc (ZnOCPC) , hydroxyaluminium ((OH)AlOCPc) , dihydroxysilicon ((OH)2SiOCPc) , hydroxygallium (OHGaOCPc) and low symmetry zinc monocarboxy (ZnMCPc) phthalocyanines were performed. The dyes were adsorbed to nanoporous ZnO electrodeposited in the presence of eosin Y as structure directing agent (SDA) on FTO substrates by refluxing or soaking the films in a solution containing the dye of interest such that a full surface coverage was achieved. High external (IPCE) and internal (APCE) quantum efficiencies of up to 50.6% and 96.7% were achieved for the OTiOCPc complex. There was a lower overall cell efficiency for cells sensitized with phthalocyanines containing hydroxyl as axial ligand ZnO/(OH)2SiOCPc , ZnO/(OH)GaOCPc and (OH)AlOCPc because of strong aggregation on the surface of the electrodes. To further suppress dye aggregation, the zinc complex of a new monocarboxylated phthalocyanine sensitizer with bulky naphtho side groups (ZnMCPc) was employed. Among the studied sensitizers, ZnMCPc gave the highest overall cell efficiency of phthalocyanine electrodeposited on ZnO of η = 0.48%.

Electrodeposition of nanoporous ZnO on Al-doped ZnO leading to a highly organized structure for integration in Dye Sensitized Solar Cells

In the present study, we propose an improvement of the anode configuration in Zinc Oxide based Dye Sensitized Solar Cells (DSSC). Instead of the classical configuration, which is composed by two different metal oxides: one transparent conducting oxide (TCO) for the substrate and one nanostructured metal oxide for supporting the dye, the new approach is to use ZnO as unique material. Thus, nanoporous zinc oxide films have been electrodeposited on a sputtered Al doped ZnO layers with varying thicknesses up to 6 μ m. The evolution of the porosity of the structure has been studied by scanning electron microscope (SEM) and electrochemical impedance spectroscopy and compared with standard nanoporous ZnO grown on fluorine doped tin oxide (SnO2 :F noted FTO).This results firstly in the modification of the nanoporous structure morphology and secondly a better adhesion between the nanoporous layer and the substrate. Organization in the nanoporous material is enhanced with regular pores arrays and perpendicular to the substrate. Dye sensitized solar cells based on this simplified architecture present efficiencies up to 4.2% and 4.5% with N719 and D149 respectively as sensitizers. Higher fill factor and Voc are found in comparison with the one obtained for deposition on the classical transparent conducting oxide (FTO), which denote improved electrical transfer properties.

Sensitization of nano-porous ZnO photo-anode by a conjugated conducting polymer

Hybrid ZnO films are prepared via one-step electrochemical process. Extraction of organic component from hybrid films results tiny wires like ZnO columns perpendicular to the substrate. Visible light sensitive-conjugated polymer poly(2-methoxy-5-[2 ethylhexyloxy]-1-4-phenylenevinylene, MEH–PPV) was embedded in highly porous ZnO ceramic by a solvent vaporization technique. An attempt was made to fabricate polymer sensitized photovoltaic cell by coupling polymer embedded ZnO electrodes with an electrolyte. Maximum photovoltage of 490 mV is observed for the cell with the configuration of ZnO|MEH–PPV| I −/ I 3 − cell.

Low-Cost UV-IR Dual Band Detector Using Nonporous ZnO Film Sensitized by PbS Quantum Dots

A low-cost photoconductive dual-band detector was prepared using a nanoporous ZnO film sensitized by PbS quantum dots (QDs). At room temperature the device shows a UV response in the wavelength range of 200–400 nm with a 370 nm peak responsivity of 4.0 × 105 V/W and a vis-NIR response from 500 to 1400 nm with a 700 nm peak responsivity of 5.4 × 105 V/W. By increasing the size of the PbS QD, the response can be extended up to 2.9 μm. It is suggested that the UV response is a result of interband absorption of UV radiation by ZnO and the IR response comes from the absorption of PbS QDs.

Synthesis of Novel Flower-Like Zn(OH)F via a Microwave-Assisted Ionic Liquid Route and Transformation into Nanoporous ZnO by Heat Treatment

Zinc hydroxide fluoride (Zn(OH)F) with novel flower-like morphology has been prepared via a microwave-assisted ionic liquid route. The flower-like Zn(OH)F particle has six petals and every petal is composed of lots of acicular nano-structure. Nanoporous ZnO is obtained by thermal decomposition of as-prepared Zn(OH)F in air, and the flower-like morphology is well retained. In the process of synthesis, ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate is used as both the reactant and the template.

Nanoporous structures on ZnO thin films

Porous structures were formed on ZnO thin films which were grown by an electrochemical deposition (ECD) method. The growth processes were carried out in a solution of dimethylsulfoxide (DMSO) zinc perchlorate, Zn(ClO4)2, at 120 ∘C on indium tin oxide (ITO) substrates. Optical and structural characterizations of electrochemically grown ZnO thin films have shown that the films possess high (0002) c-axis orientation, high nucleation, high intensity and low FWHM of UV emission at the band edge region and a sharp UV absorption edge. Nanoporous structures were formed via self-assembled monolayers (SAMs) of hexanethiol (C6SH) and dodecanethiol (C12SH). Scanning electron microscope (SEM) measurements showed that while a nanoporous structure (pore radius 20 nm) is formed on the ZnO thin films by hexanathiol solution, a macroporous structure (pore radius 360 nm) is formed by dodecanethiol solution. No significant variation is observed in X-ray diffraction (XRD) measurements on the ZnO thin films after pore formation. However, photoluminescence (PL) measurements showed that green emission is observed as the dominant emission for the macroporous structures, while no variation is observed for the thin film nanoporous ZnO sample.

Hydrogen Gas Sensors Using Anodically Prepared and Surface Modified Nanoporous ZnO Thin Films

Hydrogen Gas Sensors Using Anodically Prepared and Surface Modified Nanoporous ZnO Thin Films

Low temperature methane sensing by electrochemically grown and surface modified ZnO thin films

Abstract The functional characteristics of the planar resistive and MIM (metal-insulator-metal) sensors using electrochemically grown nanocrystalline–nanoporous ZnO thin films and surface modified by dipping in an aqueous solution of PdCl 2 were investigated for methane sensing. It was found that the operating temperature was substantially reduced to 70 °C and 100 °C for the two different configurations, respectively, after this interesting and somewhat novel surface modification step. A high purity Zn was anodized to produce ZnO thin films using a Pt cathode, a calomel reference electrode and a 0.3 M oxalic acid electrolyte. Pd–Ag (26%) was used as the catalytic metal contact to ZnO to fabricate a resistive and an MIM configuration. The response of the order of ∼48, a response time of ∼4.5 s and a recovery time of ∼22.7 s were obtained for the planar resistive structures, while the MIM structures showed a response of the order of ∼32, a response time ∼2.7 s and a recovery time of ∼16 s. The sensors were studied in the presence of 1% methane in nitrogen and in synthetic air in separate experiments. The performance was somewhat reduced in synthetic air for both the sensor structures while maintaining the optimum operating temperature the same. Both the sensors were stable in 1% methane in nitrogen as well as in 1% methane in synthetic air.

Self-Organized and High Aspect Ratio Nanoporous ZnO prepared by Anodization

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Growth Mechanism and Photonic Behaviours of Nanoporous ZnO Microcheerios

Nanoporous ZnO microcheerios were fabricated in a controlled fashion by oxidizing the DC sputtered metallic Zn crystals. The growth mechanism and photonic behavior of the materials with such a nano/microstructure were investigated and characterized in detail. It was demonstrated that the large surface energy caused by the growth steps around the equator of the metallic Zn crystals with a bi-hexagonal pyramid structure is essential for the growth of the cheerios. In particular, the nanoporous structure acts as a scaffold to provide an enormous surface area for further oxidation, resulting in composition gradients that drive Zn atoms from the core of crystals to the surface of the scaffold via the ZnO/Zn interface by self-diffusing. Such oriented diffusion behavior gives rise to the formation of ZnO microcheerios, which consist of nanowhiskers with radial orientations, having an interesting photonic behavior. As a result a high efficiency exciton emission and a visible emission that covers a wide span of wavelengths from 450 to 900 nm are observed. It is demonstrated that the visible spectra consist of five individual emissions dominated by various mechanisms. Such emissions can also be tuned precisely by manipulating the defect chemistry and the size of nanopores with the controlled synthesis process.

Postpressing dependence of the effective electron diffusion coefficient in electrophoretically prepared nanoporous ZnO and TiO2films

The porosity of electrophoretically prepared nanoporous ZnO and TiO2films was systematically decreased by postpressing at different pressures. The nanoporous structure of the films was fixed by sintering after the postpressing procedure. The postpressing-induced change of the internal surface area of the nanoporous films was monitored using the dye-removal technique. The effective electron diffusion coefficient (Deff) of the unpressed nanoporous films depended on the thickness according to Fick’s second law. When pressed, the diffusion coefficient of the films increases significantly. In nanoporous TiO2, the increase ofDefffollows the percolation theory where transport rate depends on the particle-coordination number. In contrast to the TiO2films, the value ofDeffof pressed nanoporous ZnO films changed with the porosity much stronger than one would expect from the percolation theory with hard spheres. This property has been attributed to the strong increase of necking between ZnO nanoparticles with increasing pressure as indicated by a strong decrease of the internal surface area.

The superior performance of the electrochemically grown ZnO thin films as methane sensor

Pd–Ag/ZnO/Zn and Rh/ZnO/Zn MIM (metal–insulator–metal) gas sensors were fabricated using nanoporous ZnO thin films, obtained by an electrochemical deposition method in the absence and presence of UV light. A high-purity Zn anode, a Pt cathode, a calomel reference electrode and a 0.3-M oxalic acid electrolyte were used for deposition. Pd–Ag (26%) and Rh were used separately as the catalytic metal electrodes to fabricate the two different types of MIM configurations. A gas response of the order of 3.85 ± 2, a response time of 5 ± 0.5 s and a recovery time of 16 ± 0.5 s were obtained with the Pd–Ag contact, while the Rh contact showed a response of the order of 4.82 ± 2, a response time of 24 ± 0.5 s and a recovery time of 72 ± 0.5 s, at the optimum temperature of 220 °C, which is the lowest temperature so far reported for metal oxide sensors to sense 1% methane in a N2 carrier gas. The undoped zinc oxide thin films grown by UV-assisted electrochemical anodization of high-purity Zn demonstrated a better performance for methane sensing. The experiments were repeated in synthetic air and a somewhat reduced performance was observed. The selectivity in the presence of hydrogen and the stability of the sensors were studied.

Nanoporous ZnO thin films deposited by electrochemical anodization: effect of UV light

Nanoporous ZnO thin films were deposited by electrochemical anodization of high purity Zn at room temperature using Pt counter electrode, calomel reference electrode and oxalic acid as the electrolyte. The crystallinity and the surface morphology were studied by X-ray diffraction (XRD) and Field emission scanning electron microscope (FESEM). The variation in molar concentration of oxalic acid during anodization had significant effect on the crystal size and the pore size particularly in the presence of UV light. An increase in room temperature band gap from 3.25 to 3.87 eV of ZnO film grown in 0.3 M oxalic acid indicates a quantum confinement effect and it was further confirmed by a blue shift of the photoluminescence (PL) spectra. A possible mechanism of the anodization and the photoetching in the presence of UV light of the ZnO film have been suggested.

Crystalline ZnO with an enhanced surface area obtained by nanocasting

The authors report the synthesis of nanoporous ZnO, which exhibits a periodically ordered, uniform pore system with crystalline pore walls. The crystalline structure is investigated by x-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The large specific surface area and the uniformity of the pore system are confirmed by nitrogen physisorption. Raman spectroscopy along with low-temperature photoluminescence measurements confirms the high degree of crystallinity and gives insight into defects participating in the radiative recombination processes.

Cathodic Electrodeposition of Nanocrystalline ZnO Films for Dye-Sensitized Solar Cell Applications

Solar cells based on dye-sensitized nanostructured metal oxides are promising for low-cost solar energy conversion and are intensively investigated nowadays. In this paper, the possibilities to use nanocrystalline ZnO electrodes in dye-sensitized solar cells are investigated. ZnO films have been grown on fluorine-doped SnO 2 coated glass by cathodic electrodeposition from a simple aqueous zinc nitrate solution containing organic additives. Under optimal deposition conditions, nanoporous ZnO films with grain sizes between 20 and 40 nm are obtained. X-ray diffraction and scanning electron microscope are used to characterize the films. Nanoporous ZnO electrodes, dye-sensitized with a ruthenium bipyridyl complex, are used as photoanodes in solar cells. The solar cell performance is studied and the cell exhibits a short circuit photocurrent, an open-circuit photovoltage and the photoelectron conversion efficiency of 1.379 mA/cm 2 , 0.612 V and 1.47%, respectively.

Self-Assembly of Methylzinc–Polyethylene Glycol Amphiphiles and Their Application to Materials Synthesis

A new kind of amphiphiles containing an organometallic headgroup, an alkyl zinc alkoxide, self-assembles to form [MeZnOPEGn] gel (PEG: polyethylene glycol; n=400; see picture, left), which in turn serves as a precursor to afford nanoporous ZnO (right). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2007/z602262_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.