The study of growth mechanism as a key for large-scale vapor-phase synthesis of ZnO nanostructures

The growth of zinc oxide (ZnO) nanostructures is one of the main topics in today\u27s Material Science. These nanostructures have several proved or potential application in many fields, such as optoelectronics, photovoltaics, transparent electronics, gas-sensing, "piezo-tronics", etc. For real-world technological application of these nanostructures, large-scale and low-cost synthesis processes are strongly required. Many different growth processes have been presented in literature for ZnO nanostructures, "wet" chemical syntheses probably represent the easiest way to obtain them with high yields and low cost. However, ZnO nanostructures with a large variety of morphologies can be obtained also by vapor-phase growth processes, with high crystal perfection grade. If no catalyst is used in the growth process, a very low dopant/impurity content is also obtained (a fundamental request for some applications), but reproducibility and yield are generally much lower than those of chemical syntheses. In this presentation a deep-study of the vapor-phase nucleation and growth mechanisms of selected ZnO nanostructures (nanorods, nanotetrapods, nanowires and nanobelts) is presented. By mean of the obtained results, it has been possible to enhance the growth procedures and strongly improve the synthesis yields. Since the higher purity is generally one of the main advantages of vapor-phase growth process, only zinc and oxygen have been used as reagents, avoiding any catalyst or precursor. Metallic Zn has been preferred to ZnO as starting material to keep growth temperature as low as possible (generally <500?C). Large amounts of ZnO tetrapods have been obtained by a continuous streaming reaction, directly in the vapor phase. On the other side, thin aligned ZnO nanorods rods have been grown over some square centimeters areas, by mean of a ZnO layer. Then, also randomly oriented long nanowires have been obtained on several substrates, with homogeneous distribution. The role of Zn/O ratio, supersaturation and seeding-substrates is discussed in the different cases and results are compared, together with structural and optical characterizations. Moreover, some examples of functionalization of the obtained nanostructures and feasible applications are described

Growth mechanism of aligned ZnO nanorods by vapour phase process

ZnO is an important and versatile functional semiconducting material. In recent years one-dimensional nanostructures (wires, tubes, tetrapods, etc.) have received increased attention not only for their specific properties but also for the fabrication of nanoscale devices. Among these structures the parallelly aligned, column-shaped nanorods, orthogonal to the growth substrate plane, are particularly interesting for many applications such as dye sensitized solar cells, transistors, nanogenerators, short-wavelength nanolasers, etc. Many different techniques have been reported for the growth of ZnO nanostructures but thermal evaporation turns out to be one of the most convenient when considering the high quality and purity of the grown crystals, the simplicity of the growth apparatus and the easiness in the scaling-up of the process. In this communication the authors report on a selective growth process as regards well-aligned ZnO nanorods arrays extended up to a few cm2. The method, which is based on thermal evaporation and controlled oxidation, includes nucleation and growth kinetics control, adjustment of local growth temperature, selection of appropriate source materials and chemical composition of the substrate. The optimized growth parameters allowed to obtain arrays of (0001)-oriented, vertically aligned single crystals with length and diameters within 1-3 microns and 20-10 nm respectively. It is in particular pointed out: (a) the formation of sub-micrometric metal Zn clusters during the metal source evaporation; (b) their adhesion to a ZnO buffer layer (about 300 nm thick) previously deposited on the substrate (glass, Si, ...), which enables to keep an appropriate cluster distribution while avoiding larger clusters/drops formation thanks to suitable "surface wettability" conditions; (c) the subsequent growth of ZnO nanorods owing to the contemporary presence of Zn vapours, oxygen and Zn clusters on the substrate, these last ones acting as selective nucleation points. On the ground of what above and consequent process re-adjustments, the here proposed method, with its elevated yields and reproducibility as well as low production costs, turns out to be especially well-suited for large-scale application requirements

Study of nucleation and growth mechanisms for optimized and large-scale synthesis of aligned ZnO nanorods for photovoltaic applications

The growth of ordered ZnO nanostructures is very important for many application fields, as optoelectronics, photovoltaics, piezo-electric power generation, etc. In particular, vertically aligned ZnO nanorods can be employed as 3D electron-harvesting TCO, capable of inducing multiple absorptions by light scattering phenomena in hybrid third generation solar cells. However, for photovoltaic application it is necessary to produce large area substrates uniformly covered by nanorods with controlled dimensions. Aligned ZnO nanorods can be obtained in highly ordered arrays over different substrates both by wet chemical methods and by vapour-phase processes. Not-catalyzed vapour growth processes are those that can supply nanostructures with the lowest concentration of undesired impurities. This aspect is fundamental for photovoltaic application in order to prevent negative effects on electron transport properties. In this work, we report the preparation of a large area (few square centimetres) nanostructured TCO on commercial glass substrate, constituted by a highly conducting ZnO:Al layer (by Pulsed Electron Deposition) and vertically aligned ZnO nanorods homogeneous in length and diameter over the whole substrate. The latter have been obtained by a vapour-phase process, starting from metallic Zn where only argon and oxygen flows are used, with a maximum temperature lower than 500?C (which is compatible with low-cost and application-oriented glass substrates). This goal has been achieved after an in-depth study of the nucleation and growth mechanisms of ZnO nanorods. The influence of substrate material and its crystallographic properties, as well as the results obtained with different growth parameters (temperature, flows, time) are discussed. Above all, it has been observed that the condensation of small Zn clusters on the polar surface of a ZnO film can reproducibly nucleate the growth of ZnO well-aligned rods, perpendicularly to the substrate face. These clusters can be formed by a proper control of temperature gradients in order to obtain locally a zinc vapour pressure larger than the equilibrium one of liquid phase. Once nucleation has occurred, the growth of the nanorods follows until zinc and oxygen are supplied. Diameter of nanorods can thus be intentionally modified in the 20-200 nm range, and length up to 3-4 microns. Moreover, it has been evidenced that during the growth process a ZnO wetting layer of controllable thickness can be deposited between the ZnO:Al layer and nanorods

A new approach for the synthesis of ZnO nanoparticles sensitized with metal chalcogenides

The present communication is a response to renewed interest in nanostructure based "coupled compounds", like ZnO-MeX (where Me = Cd, Pb,... and X = S, Se) which can find extensive use in the fabrication of a number of solid state devices, such as photoconductive, solar cells, electroluminescent cells, photocatalysts. Various oxide semiconductors, like TiO2 and ZnO, are known to have appropriate properties for these applications, although there are some drawbacks associated with their use: (i) charge carrier recombination occurs within a few nanoseconds, (ii) band edge absorption threshold does not allow the utilization of visible light. One of the main approaches to overcome these particular limitations involves the contact of the semiconductor particle with another semiconductor, called "sensitizer". For example, this is the case of nanostructured ZnO particles combined with metal chalcogenides. Infact it is known that in these coupled systems the absorption threshold is extended to the visible region and the photogenerated electrons are quickly transferred from sulphide/selenide layer into to the lower lying conduction band of ZnO, thus limiting recombination effects. In order to produce this type of material we have combined ZnO nanoparticles, in the specific "tetrapod" morphology, with nanoparticles of metal chalcogenides. The main innovative aspects of the preparation procedure are the following: &#61485; the use of appropriate organic solvents to keep both ZnO and the formed metal chalcogenides completely suspended and dispersed in the liquid; &#61485; an in situ direct formation of metal chalcogenides keeping pH value in the range 6-8 (no use of ammonia salt or complexing agents); &#61485; the limited use of chemical reagents, i.e. only metal and sulphur/selenium precursors are involved; &#61485; the possibility to deposit the "coupled compounds" in form of thin films directly from the liquid suspension onto the substrates (silicon, alumina, glass, TCO layers, etc.). This paper reports details on the preparation procedure, results of morphological and structural investigations (XRD, SEM), compositional analysis (EDS microanalysis) and optical-electrical measurements (I-V, impedance spectroscopy, etc.), which point out the great potentiality of the proposed method for the synthesis of different "sensitized nano-compounds"

Enhanced aldehydes detection by ZnO nano-tetrapod based gas sensors

Metal oxides are very important materials in gas-sensing and the possibility to obtain them as crystalline nanostructures represents an essential chance to improve sensors sensitivity an lifetime. Zinc oxide (ZnO) is a versatile material that is today widely studied because of the large number of possible application fields. The availability of this material in a large number of nanostructures makes it very interesting for the realization of gas sensors. In this field ZnO nano-tetrapods can find a suitable and reliable application, since they can be obtained by vapour phase growth, starting from metallic Zn, with very large yield and low production costs. In the present work authors report the excellent results obtained in: (i) developing an optimized growth process for the production of ZnO tetrapods, (ii) realizing a gas sensing device based on these nanostructures and (iii) the very promising results obtained in the detection of some volatile organic compounds (VOC). In particular a very high response and a remarkable sub-ppm detection limit is demonstrated for aldehydes. Furthermore, the reaction mechanisms, which take place on the surface of ZnO tetrapods, are discussed as a function of temperature and it is shown that the response curves measured at different temperatures can provide a powerful tool for adding selectivity to aldehydes detection towards particular interfering compounds (e.g. alcohols)

The challenge for large-scale vapor-phase growths of not-catalyzed ZnO nanostructures: purity vs. yield

ZnO nanostructures are today a very important research topic because their proved (or even just "potential") properties promoted huge studies in many different application fields, such as optoelectronics, photovoltaics, spintronics, gas sensing, photocatalysis, piezo-electric applications, etc. Since a reproducible large-scale production is essential for a likely use of these nanostructures in any industrial application, large efforts have been done to control and stabilize their synthesis processes. Good results have been obtained in vapor phase growths of nanorods and nanowires, by mean of metal catalysts (such as Au, Pt or Ni particles). On the other side, large and controlled production of some ZnO nanostructures have been realized by wet chemical processes. Unfortunately both these approaches are intrinsically affected by the introduction of impurities in the nanocrystals\u27 structure. Indeed, even very low impurity levels may have a strong effect on the physical properties of these semiconducting nanostructures. Catalyst-free vapor-phase growth techniques should not be affected by the same impurity levels if high purity sources and gases are employed. Unfortunately, the synthesis control is generally more difficult in this kind of processes. In the present work authors show the results obtained in the optimization of three different growth processes, for a large-scale oriented production of (i) ZnO tetrapods, (ii) ZnO nanorods and (iii) ZnO long nanowires. All the described processes share a catalyst-free growth and the use of high purity metallic Zn, O2 and inert carrier gas (Ar) only. The properties of the obtained ZnO nanostructures have been characterized and, hence, pros and cons of the used approach have been discussed

Method for large area deposition of ZnO tetrapod nanostructures

Among several morphologies ZnO presents one quite characteristic nano-crystalline structure, usually reported as "tetrapod" (TP), which consists of four needle-shaped "legs" connected at one common end and respectively arranged as axes of a tetrahedron (Fig. 1). Using an appropriate vapor-solid process, ZnO TPs are produced in large quantity (tens of mg per run) and, when removed from the growth reactor, they appear well assembled like a light white-grey "sponge". These aggregate structures mainly consist of TP nanocrystals, though usually they might also include unreacted Zn metal particles, nanosized ZnO powders and/or partially oxidized ZnO1-x nanostructures/powders. In order to "purify" as-grown TPs from the other undesired structures, the authors propose a multi-step process summarized as following: (1) post growth thermal annealing in vacuum (evaporation of metal Zn) and, subsequently, in oxygen atmosphere (oxidation of not completely reacted particles of ZnO1-x); (2) suspension of all the reaction products in appropriate liquids, in which ZnO is insoluble, in order to decant and to separate TPs from spurious structures; (3) room-temperature deposition of purified ZnO TPs on proper substrates (glass, silicon, alumina, etc., depending on the final application), whose sizes can vary from a few square mm up to many square cm; (4) heating at moderate temperature under low vacuum to remove traces of organic solvent and to favour the sticking of ZnO TPs on the substrates. The described procedure is highly valuable as it allows the achievement of homogeneous distribution of purified ZnO nanostructures on large substrates and a room-temperature deposition process which avoids detrimental interaction of ZnO TPs with substrate material, or with metal contacts previously deposited on the substrates. In practice, the proposed method is a new way to prepare large area films of metal oxides nanostructures, ready for device production. Application of the process to gas sensor fabrication and hybrid compounds (ZnO-MeS) preparation is also reported

Growth And Characterization Of ZnO Nanostructures By A Self-Catalythic CVD Process

In the last years, many papers have dealt with the topic of metal oxide nanowire growth and characterization. In particular, it is widely reported that Zinc Oxide (ZnO) can be obtained in a large variety of nanostructures with different morphologies, namely nanowires, nanorods, nanotetrapods (or simply "tetrapods"), nanotapes or nanoribbons, etc. The possibility to obtain this material in different nanosized crystalline structures is particularly interesting in view of several application fields, e.g. chemical sensors, solar-cells, optoelectronics. However, different nanostructures often appear mixed on the same substrate, thus limiting the possibility of exploitation for applications

Growth of ZnO tetrapods for gas sensor application

Zinc Oxide (ZnO) nanostructures have been obtained by vapour phase growth. Tetrapods have been grown in a reproducible way and separated from the other possible ZnO nano-morphologies, by the optimization of growth parameters. The further deposition of these nanostructures on an alumina substrate with contacts and heater, allowed us to test the gas sensing properties of the obtained ZnO tetrapods with different gases

Photocatalytic activity of ZnO nanostructures grown by vapour and solution

Zinc oxide (ZnO) is one of the most studied functional materials in the last years because it matches the opportunity to be easily synthesized in nanocrystalline form (with different morphologies and by different growth techniques) with a very large number of possible applications in different fields (optoelectronics, photovoltaics, piezoelectric devices, gas-sensing and bio-sensing, photocatalysis, spintronics, nano power generators, cantilever production, etc.). In the present contribution we report on ZnO nanocrystalline structures, with the same wurtzite structure but different morphologies, synthesized by vapour-phase and by solution growth techniques. More in detail, ZnO nanotetrapods and nanopowders with different size have been obtained by a combination of metallic Zn thermal evaporation and controlled oxidation in a non-standard CVD (chemical vapour deposition) reactor where temperature have been set in the 450?C-650?C range. Other ZnO multi-branched nanostructures, resulting from aggregation or twinning of nanosized crystals, have been grown in aqueous solution of zinc salts and hexamine, in slightly alkaline medium below 100?C. Moreover, ZnO nanopowders have been obtained by thermal decomposition of a metallorganic gel precursor, resulting from dehydration of a zinc citrate solution. Different nanopowders samples have been prepared with different decomposition processes (time, temperature). ZnO nanostructures have been later on deposited on a photocatalysis-inert substrate (glass) from an alcoholic suspension at room temperature by forcing solvent evaporation and surface cleaning for a few minutes at 200?C in low vacuum. The obtained specimens are about 50 cm2 in size. All the ZnO nanostructure films are characterized by high porosity and high surface-to-volume ratios, which are generally basic requirements for the application in photocatalysis of gaseous species. The samples have been characterised by scanning electron microscopy and tested for photocatalytic degradation activity of airborne pollutant using a stirred flow photoreactor irradiated with UV-A. The measurements were carried out using ethylbenzene as organic target pollutant at concentration level typically found in ambient conditions. The samples demonstrated a good photocatalytic activity in the degradation of ethylbenzene in air