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WO2006008175A1 - Synthese par flamme rapide de nanotiges de zno dopees a facteur de forme controle - Google Patents

Synthese par flamme rapide de nanotiges de zno dopees a facteur de forme controle Download PDF

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WO2006008175A1
WO2006008175A1 PCT/EP2005/007995 EP2005007995W WO2006008175A1 WO 2006008175 A1 WO2006008175 A1 WO 2006008175A1 EP 2005007995 W EP2005007995 W EP 2005007995W WO 2006008175 A1 WO2006008175 A1 WO 2006008175A1
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dopant
metal
zno
fsp
nanorods
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Murray J. Height
Lutz Maedler
Sotiris Pratsinis
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Eth Zurich
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
    • C01B13/14—Methods for preparing oxides or hydroxides in general
    • C01B13/34—Methods for preparing oxides or hydroxides in general by oxidation or hydrolysis of sprayed or atomised solutions
    • C—CHEMISTRY; METALLURGY
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    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G9/00—Compounds of zinc
    • C01G9/02—Oxides; Hydroxides
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    • C01G9/00—Compounds of zinc
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    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/77—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by unit-cell parameters, atom positions or structure diagrams
    • C—CHEMISTRY; METALLURGY
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    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
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    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
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    • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
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    • C01P2004/16—Nanowires or nanorods, i.e. solid nanofibres with two nearly equal dimensions between 1-100 nanometer
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    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
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    • C01P2006/12—Surface area

Definitions

  • the present invention concerns a flame spray pyrolysis method (FSP) for the production of metal oxide nanorods, in particular zinc oxide nanorods with controlled aspect ratio.
  • FSP flame spray pyrolysis method
  • the invention especially concerns a single-step, continuous, and readily scaleable method.
  • Nanorods are nanoscale solid structures with one characteristic dimension larger than the other. This disparity gives an aspect-ratio (ratio of length to width) greater than unity and a dominant linear, 1-dimensional nature to the structure. Nanorods are similar in geometry to nanowires, nanofilaments and nanofibers, the term rod, however, implies a shorter length.
  • nanorods may exhibit many unique electronic, optical and mechanical properties that appeal to a range of applications.
  • the state of the art suggests the use of nanorods in the area of electronics, e.g. in transistors, diodes, interconnects and other nanoscale circuit features (Wang, 2003; Xia et al . , 2003), and due to their high surface-to-volume ratio they are suggested as constituents for electro-chemical sensors (Xia et al . , 2003) .
  • Proposed optical applications are e.g. field- emission displays, photoluminescent materials, optical switches and non-linear optical filters and converters (Xia et al . , 2003) .
  • Nanoscale linear structures such as nanorods are also disclosed to exhibit enhanced strength compared to the analogous bulk materials (Xia et al . , 2003) .
  • Proposed mechanical and electro-mechanical applications include high-strength polymer-composites (Manhart et al . , 2000), actuator devices (Lin and Chang, 2002) and strain sensors. Due to the linearity of the nanorods, Tani et al . suggest their use in novel conducting polymers or thermo-electric applications (Tani et al . , 2001) .
  • Nanorod synthesis techniques typically include vapor or wet-chemistry routes.
  • Vapor phase methods include evaporation/condensation (Sun et al . , 2004; Wen et al . , 2003) , pulsed-laser deposition (Kawakami et al., 2003), chemical vapor deposition (Liu et al . , 2004), or gas-phase reaction (Yan et al. , 2003) .
  • the evaporation/condensation technique has been widely used to synthesize anisotropic materials including nanorods, nanowires and nanobelts (Kong et al., 2004; Wen et al., 2003; Yan et al . , 2003) .
  • Wet chemistry methods include solvothermal (Patzke et al . , 2002) , hydrothermal (Cheng and Samulski, 2004; Liu and Zeng, 2003) , and sol-gel techniques (Guo et al., 2002; Vayssieres, 2003) . While these techniques are capable of well-controlled material uniformity and properties, they are limited in their scale-up potential.
  • nanorods A variety of mechanisms have been proposed for formation of nanorods.
  • ZnO zinc oxide
  • the introduction of certain impurities such as indium (Wen et al . , 2003), tin (Gao et al . , 2003) , selenium (Sun et al . , 2004), antimony (Zeng et al . , 2004) , cerium (Cheng et al . , 2004) and other elements was found to induce preferential growth within specific crystal planes (Kong et al., 2004) resulting in asymmetric crystals.
  • Another nanorod growth possibility is through control of the reactant composition ratio.
  • Yan and coworkers made ZnO nanorods (among other interesting morphologies) by carefully tuning the ratio of oxygen to zinc precursor (Yan et al., 2003) .
  • the oxygen stoichiometry has been shown also to influence nanorod growth during evaporation/condensation (Leung et al . , 2004; Tseng et al . , 2003) and chemical vapor deposition (Liu et al . , 2004) syntheses.
  • Flame synthesis is a technique that can be readily scaled to produce nanoscale materials in high- volume at low-cost (M ⁇ ller et al . , 2003) .
  • Flame-generated materials are generally dominated by spherical primary particles and chain-like agglomerates (Pratsinis, 1998) .
  • Carbon nanotubes have been made using flames (Height et al., 2003; Height et al . , 2004) however such pronounced linear features as characteristic for nanorods have not been observed for flame-synthesized inorganic materials.
  • Short rod-like particles have been observed in flames before (Akhtar et al . , 1992), however their formation was neither controlled nor a dominant feature of the synthesized materials (Tani et al . , 2002a) .
  • the flame spray pyrolysis (FSP) method for the production of metal- oxide nanorods is manifested by the features that at least one dopant metal precursor is added to a precursor solution of the predominant metal in amounts of 1 to 15 atom-% referred to the amount of predominant metal, said dopant having an higher valency than the predominant metal, and wherein the crystal lattice of the pure metal oxide formed by FSP is in a hexagonal closed crystal structure.
  • FSP flame spray pyrolysis
  • the coordination number of the dopant atoms within the product crystal lattice is higher than the coordination number of the predominant metal atoms. Also preferred is that the dopant atoms and/or ions and the predominant metal atoms/ions have sizes varying by less than 20 % referred to the atomic radius of the predominant metal as 100 %.
  • FSP flame spray pyrolysis
  • FSP is a combustion-based process whereby a liquid solvent containing various dissolved precursor species is injected and atomized into a flame (Madler et al., 2002) .
  • the carrier solvent provides energy for combustion and the flame temperature causes evaporative release of the precursors from the spray droplets into the gaseous environment where reactions proceed to form nanoparticles of the desired composition.
  • the use of liquid precursors gives wide flexibility and accurate control of precursor composition while the high temperature and reactive conditions of the flame provide a favourable environment for the rapid formation of nanosized particles.
  • FSP has been demonstrated in the synthesis of a wide variety of metal and mixed-metal oxides, including ZnO (Tani et al . , 2002a) and mixed ZnO/SiO 2 (Tani et al . , 2002b) .
  • the above described FSP method can be further developed such that it allows the rapid and direct synthesis of nanorod particulate metal oxides with close control of the nanorod geometry, in particular the aspect ratio, through addition of suitable dopant precursors (in the case of ZnO e.g. indium and tin dopant precursors) into the precursor solution that is injected and atomized into the flame .
  • suitable dopant precursors in the case of ZnO e.g. indium and tin dopant precursors
  • dopants in the FSP is especially noticeable in the case of metal oxides forming hexagonal close packed (HCP) crystal structures such as ZnO crystallites having wurtzite lattice structure.
  • HCP hexagonal close packed
  • Addition of indium or tin dopant species to such ZnO dramatically reduced the size of the (002) plane, leading to nanorod features with controlled aspect ratios as confirmed by TEM and BET analysis.
  • Doping with lithium gave no change in the ZnO crystal texture.
  • Nanorod formation in general can primarily be attributed to dopants with higher valency and/or coordination relative to the predominant metal of the metal oxide.
  • doped ZnO both, indium and tin, have higher valency and coordination compared to zinc.
  • the incorporation of dopants having the above outlined characteristics leads to structural disruption and reduced growth within the suitable lattice planes.
  • the atomic or ionic size might have an influence.
  • the size is only relevant in so far as similarly sized dopants are readily incorporated into the metal plane of the metal oxide crystal where they have a disruptive coordination due to the different valency from the predominant metal. This limits the growth in the crystal plane parallel to the metal atoms thereby altering the aspect ratio of the crystal.
  • the atomic radius of indium, tin, and lithium In 1.55 A; Sn 1.45 A; Li 1.45 A) (Slater, 1964) are similar to zinc (1.35 A) (Slater, 1964) and the atomic radii of Sn and Li are the same.
  • the size of the dopant atoms is unlikely to be a relevant parameter for alteration of the shape of the crystal to induce the observed rod-like geometry. It has, however, a relevance in so far as similarly sized dopants are more readily incorporated into the crystal lattice and thus bear less risk of influencing the crystal lattice geometry.
  • a higher valency of the dopant than the valency of the predominant metal is assumed to be a relevant driving force for nanorod formation as well as coordination effects.
  • the higher valency of dopants relative to the predominant metal lead to greater coordination with surrounding 0 atoms than for the predominant metal's atoms within the same crystal plane. It is assumed - without wanting to be bound by any theory - that the higher valency dopant atoms have a disrupting influence on the metal plane of the metal oxide lattice, hindering crystal growth within the metal layer (Kong et al . , 2004) , i.e.
  • a dopant with a higher valency than the predominant metal has a greater tendency to strongly bond with oxygen atoms in the crystal and so selectively disrupt the bonding within specific crystal lattice planes leading to the inhibited growth in that plane and an elongated crystal shape for the overall particle.
  • This hindering effect is observed in the decreasing (002) crystal plane size as the dopant concentration is increased.
  • the more pronounced nanorod forming influence of some of the dopants may be attributed to higher valency, and larger disruptive influence, of e.g. tin with regard to indium.
  • lower valency means that such dopant atoms have minimal disruptive influence within the metal atom layer and so little effect on the size of the (002) plane.
  • the elemental composition of the obtained nanorods will be identical to the metal composition in the FSP liquid precursor solution as confirmed by energy dispersive x-ray spectroscopy (EDXS) .
  • dopant metal While in general only one dopant, metal will be incorporated, due to the similar sizes preferred, also different dopant metals may simultaneously be incorporated by using a precursor solution comprising more than one dopant precursor.
  • the FSP synthesis method delivers precursor compounds in liquid, or rather dissolved form to a high temperature flame environment where the precursors vaporize, oxidize, and subsequently form metal-oxide clusters that coagulate into nanometer-scale particles. Particle growth can continue via surface reactions and collisions with other particles (Madler et al., 2002) . Precursor reaction and particle formation occur in the vapor-phase, therefore the initial stage of the nanorod formation process is the formation of primary particles from the vapor. In the early stages of the flame, particles are at a high temperature, for many metal oxides at a temperature higher than their melting point. This is e.g. true for ZnO having a melting point of 1970 0 C. Particles formed in the early stages of the flame are spherical in shape and liquid-like. However, in the latter region of the flame, particles cool rapidly, solidify and crystallize to be collected on a filter.
  • the dopants influence the particle formation and morphology in two regions of the flame. Firstly, during the initial stages of the flame, the presence of the dopants may affect the size of the primary particles by influencing the initial cluster formation and the sintering and growth rate of the particles (Akhtar et al . , 1994) . Secondly, during the late stages of the flame, the gas temperature decreases significantly and the particles crystallize. During this cooling process, the dopant species within the particles are assumed to begin to influence the formation of the crystal structure as described previously, disrupting growth in the (002) plane. The final nanorod shapes therefore could be attributed to the annealing and rearrangement of the particle during the later stages of the flame.
  • the FSP is performed as described by Muller et al . 2003, but in particular within the following parameter ranges:
  • the metal oxide of the present invention will in general be predominantly in the form of nanorods .
  • Predominantly here means that analysis of the XRD pattern for the bulk powder according to the fundamental parameter method (described elsewhere in this document) will yield an aspect ratio (length to diameter) greater than 1, preferably greater than 1.5 and most preferably greater than 2.
  • Suitable precursors for the metal oxide are e.g. organometallic compounds, in particular zinc naphthenate, and suitable dopant precursors are e.g. also organometallic compounds, in particular indium , acetylacetonate and stanneous-2-ethyl hexanoate .
  • the metal oxides in nanorod form of the present invention exhibits at least one of the following properties:
  • FIG. 1 is a schematic diagram of the flame spray pyrolysis (FSP) nozzle (shown in cross-section) and associated flow control and sample collection systems (Madler et al . , 2003) .
  • FSP flame spray pyrolysis
  • Figure 2 shows the powder X-ray diffraction (XRD) patterns for pure ZnO and 1 to 10 at% In-doped ZnO. Crystal plane indices for each peak are shown in the upper frame.
  • XRD powder X-ray diffraction
  • Figure 3 shows crystallite size analysis.
  • the top panel ( Figures 3a,b,c)gives detail of XRD patterns for ZnO doped with a) In, b) Sn and c) Li. The peaks correspond to the (100) , (002), and (101) planes of the wurtzite structure at 31.8, 34.5, and 36.3 2 ⁇ degrees respectively.
  • the lower panel ( Figures 3d,e,f) indicates control of ZnO crystal plane sizes ( ⁇ -(100) ; O- (002) ) as a function of dopant concentration for d) In, e) Sn, and f) Li, together with BET equivalent diameter (D) for each of the doped ZnO samples.
  • the Sn-doped ZnO shows a steeper variation than the In-doped ZnO, in agreement with XRD. Annealed samples (filled symbols) and associated temperatures are shown in d) and e) .
  • Figure 4 shows the aspect ratio control, wherein the left part, designated a, shows the ZnO lattice along with the calculation of XRD lattice aspect ratio from the ratio of (100) to (002) XRD crystal plane size measurements.
  • the inset graphics illustrate the idealized geometry associated with the aspect ratio for the as-prepared In-doped ZnO samples .
  • Figure 5 shows transmission electron microscope (TEM) images of a) pure ZnO also showing the scale marker that is also valid for all images except for image i) . b-f) 2-10 at . % In-doped ZnO and 4 at . % g) Sn- and h) Li-doped ZnO.
  • the crystal morphology of the In- doped ZnO clearly changes from spheroidal (pure ZnO) , to cuboid and to increasingly rod-like as In concentration increases, consistent with XRD ( Figure 3a,d) .
  • Image (i) shows a high-resolution TEM of a single 10 at. % In-doped ZnO nanorod crystal .
  • Figure 6 shows the interplanar d-spacing of (100) , (002), and (101) peaks for ZnO doped with various concentrations of indium.
  • the (101) spacing is normalized to the undoped value and serves here as an internal reference. Relative to the (101) plane, the (100) spacing remains constant while the (002) spacing shifts towards higher values as dopant concentration increases .
  • the selective expansion of the (002) plane separation indicates dopants associate with the (002) plane consistent with the incorporation of the dopant atoms within the Zn wurtzite lattice planes.
  • Figure 7 shows HRTEM images of 10 at . % In- doped ZnO showing lattice planes for cross-sectional (a) and axial (b) directions of nanorod crystals. The measured spacings are consistent with the (002) (ca. 2.6A; a) and (001) (ca. 2.8A; b) planes respectively, confirming the nanorod length along the c-axis .
  • Figure 8 shows the dopant coordination, wherein a) shows a Raman spectroscopy analysis (632 nm excitation) of Sn-doped ZnO powders. The peaks at 570 cm ⁇ l and 670 cm ⁇ l are associated with different coordination states of the Sn dopant with the latter peak reflecting higher coordination, b) shows the ratio of the areas (circles) below the peak at 670 cm ⁇ l to that below 570 cm "1 . This ratio is consistent with the XRD lattice aspect ratio (diamonds) ( Figure 4b) .
  • FIG. 1 A diagram of an experimental apparatus suitable for the FSP method for the production of metal oxide nanorods is shown in Figure 1.
  • Figure 1 A diagram of an experimental apparatus suitable for the FSP method for the production of metal oxide nanorods is shown in Figure 1.
  • a dispersion gas flow rate of 5 L/min and a pressure drop of 1.5 bar was maintained across the nozzle during FSP operation.
  • a sheath gas flow of 5 L/min of oxygen was issued concentrically around the nozzle to stabilize and contain the spray flame .
  • the precursor liquid feed was supplied at 5 ml/min using a rate-controlled syringe pump (Inotech R232) and all gas flows (Pan Gas, >99.95%) were metered using mass flow controllers (Bronkhorst) .
  • a water-cooled, stainless-steel filter housing supported a glassfiber sheet (Whatman GF/D; 25.7 cm diameter) for collection of the flame-produced powder with the aid of a vacuum pump (Busch) .
  • the basis liquid precursor solution was composed of toluene (Fluka, 99.5%) and zinc naphthenate (STREM Chemicals, 65% in mineral spirits) .
  • Dopant species were indium (indium acetyl acetonate; Aldrich, 99.99%) , tin (stannous 2-ethyl hexanoate; Aldrich 95%), and lithium (lithium tert-butoxide; Aldrich, 1. OM in tetrahydrofuran) .
  • Li-doped ZnO samples were also prepared using lithium acetylacetonate (Aldrich, 97%) resulting in identical powders to the tert-butoxide precursor.
  • Dopant concentrations ranged between 1 and 10 atom percent with respect to the Zn metal.
  • the total metal concentration for each precursor solution was 0.5 mol/L.
  • Thermal stability of the powders was investigated by annealing powders in a Carbolite CWF1300 temperature programmed oven. Samples were heated at 5 °C/minute to either 700 0 C or 900 °C, maintained at that temperature for 5 hours, followed by cooling at 10 °C/minute.
  • the flame-generated powders were characterized using powder X-ray diffraction (XRD) with a Bruker AXS D8 Advance spectrometer at 2 ⁇ (Cu-Ka) 20 to 70°, step size of 0.03°, and scan speed of 0.6 °/min (source 40 kV, 40 mA) .
  • XRD patterns were analyzed using the Fundamental Parameter (FP) method to match the profile of individual peaks within each XRD pattern, allowing extraction of crystal size information (Cheary and Coelho, 1992) .
  • FP Fundamental Parameter
  • BET adsorption isotherms and specific surface area analysis was performed using a MicroMeritics TriStar 3000 system after degassing in nitrogen for 1.5 hours at 150 °C.
  • the specific surface area (SSA) was measured using 5-point nitrogen adsorption at 77 K.
  • the BET equivalent diameter was evaluated from the measured SSA for each sample, assuming a spherical primary particle geometry and a composition-corrected density.
  • Transmission electron microscopy analysis was carried out with a Phillips CM30ST microscope (LaB6 cathode, 300 kV) .
  • a confocal Raman microscope (Labram, Jobin Yvon, ex DILOR Instruments SA) was used to acquire Raman spectra.
  • An internal HeNe laser at 632.8 nm with approximately 2.5 mW power was used for ex situ measurements.
  • Raman spectra were recorded in the spectral range from (200 to 1050 cm “ ⁇ " ⁇ ) with a line resolution of 4 cm '1 .
  • Raman band positions were calibrated against the spectrum of a neon lamp (Penray, Oriel) . With the confocal hole at 500 micrometers the spatial resolution was between 3-5 ⁇ m 3 .
  • Figure 2 shows X-ray diffraction (XRD) patterns for pure and indium doped ZnO (1 to 10 atom %) .
  • the XRD-pattern of pure ZnO shows the typical reflections of ZnO with the crystal plane assignments indicated in the upper portion of the frame. Patterns for indium doped ZnO were arranged upwards from the ZnO pattern with increasing dopant concentration. All patterns were normalized relative to the intensity of the
  • the XRD patterns for each of the In-, Sn-, and Li-doped ZnO samples exhibited the same characteristic pattern of the pure ZnO with wurtzite crystal structure ( Figure 2) .
  • the structural agreement between the doped and pure ZnO patterns indicates that each of the dopant atoms are fully incorporated into the ZnO crystal lattice without significantly altering the packing structure of the parent lattice.
  • the preservation of the wurtzite lattice structure could also be confirmed by electron diffraction and Raman spectroscopy measurements (not shown) .
  • the (002) peak exhibits a dramatic reduction in intensity and a peak broadening with increasing dopant concentration.
  • the Li- doped ZnO pattern showed to be relatively insensitive to dopant concentration.
  • the decreasing intensity and increased broadening of the (002) peak found is consistent with the (002) crystal size becoming smaller, while the (101) crystal size appeared to remain relatively constant.
  • the XRD patterns were analyzed using the TOPAS software (Bruker, 2000) .
  • the Fundamental Parameter (FP) method was used to fit the profile of individual peaks within each XRD spectrum, allowing extraction of crystal size information (Cheary and Coelho, 1992) .
  • the FP method was used to determine the average crystal size associated with the (100) and (002) peaks in each pattern.
  • FIG. 3 lower panel, i.e. d, e and f, shows the average crystal plane size associated with the (100) and (002) peaks in each pattern as a function of dopant concentration.
  • the (002) crystal size decreased by a factor of five from 27 nm to 5 nm while indium concentration was increased from 0 to 8 at .% ( Figure 3d) .
  • the (001) size increased slightly from 18 to 20 nm before gradually declining to 16 nm.
  • the Sn-doped ZnO showed a similar, yet more steep trend than the In- doped ZnO with the (002) crystal size decreasing from 27 nm to 6 nm with the addition of only 4 at . % .
  • the corresponding change in size ' for the (100) plane was 18 to 14 nm with 4 at . % tin dopant (similar effect for 8% In-doped ZnO) .
  • Both In and Sn selectively reduced the size of the (002) plane while only slightly decreasing the size of the (100) plane.
  • the crystal sizes for the Li-doped ZnO remained unaffected even for dopant concentrations up to 10 at . % .
  • the specific surface area (SSA) increased steadily from 53 to 77 m 2 /g as indium concentration increased from 0 to 10 at . % and from 53 to 85 m 2 /g as tin concentration increased from 0 to 6 at . % followed by a decrease to 75 m 2 /g at doping up to 10 at.%.
  • the Li-doped ZnO exhibited the same SSA as the pure ZnO (53 m 2 /g) for dopant concentrations up to 6 at.% and then decreased to 43 m 2 /g at doping up to 10 at.%.
  • the BET equivalent diameter (dBET) for the In-, Sn-, and Li-doped ZnO samples are shown also as a function of dopant concentration in Figure 3d,e,f (squares) .
  • the thermal stability of the In-doped nanorods was investigated by annealing for 5 hours at either 900 or 700 0 C ( Figure 3d) .
  • the XRD size of the (100) and (002) planes of the pure ZnO powders increased dramatically from 23 ⁇ 5 nm (as prepared) to 65 ⁇ 2 nm for 700 0 C and 133 ⁇ 2 nm for 900 °C.
  • the size of the (100) plane increased from the as-prepared value of 16.7 nm to 24.7 nm (700 0 C) and 33.0 nm (900 0 C) ( Figure 3d) .
  • the size of the (002) plane remained relatively close to the as-prepared value of 6.0 nm, increasing to 6.9 nm (700 0 C) and 7.8 nm (900 0 C) ( Figure 3e) .
  • the 6 at . % Sn-doped ZnO samples showed a similar increase in the (100) plane size from 12.1 nm (as prepared) to 16.0 nm (700 0 C) and the (002) plane size changed from 4.4 nm (as-prepared) to 5.9 nm (700 0 C) .
  • the size of both the (100) and (002) planes increased dramatically to 98.1 nm and 35.3 nm respectively (not shown in Figure 3e) .
  • Zinc oxide has a hexagonal-close-packed wurtzite structure in which the crystal is composed of alternating planes of Zn atoms and 0 atoms ( Figure 4a) .
  • the (002) plane lies in parallel to the 0 and Zn planes while the (100) plane lies perpendicular to the (002) plane, intersecting alternating layers of Zn and O.
  • the crystallite size of the (002) and (100) planes can essentially be considered as metrics of 'Diameter' (D) and 'Length' (L) respectively ( Figure 4a) .
  • the crystal aspect ratio (L/D) can therefore be estimated by the ratio of (100) and (002) crystal plane sizes, designated here as the XRD lattice aspect ratio.
  • Figure 4b shows the variation of XRD lattice aspect ratio with dopant concentration for the In-, Sn-, and Li-doped ZnO as calculated from the crystal plane size measurements.
  • the XRD lattice aspect ratio was found to increase steadily by a factor of 5 from 0.6 to 3.1 as dopant concentration was increased from 0 to 10 at.%.
  • the Sn-doped ZnO was also found to increase in XRD lattice aspect ratio as more dopant is added, however tin appeared to have a stronger influence than indium at lower dopant concentrations.
  • the XRD lattice aspect ratio of the Li-doped ZnO remained essentially the same as the one of pure ZnO as the Li-dopant concentration was increased up to 10 at . % .
  • the annealed pure ZnO samples showed a dramatic jump in size of both the (100) and (002) planes upon annealing, giving an XRD lattice aspect ratio of unity for both temperatures.
  • the In-doped ZnO samples showed a selective increase in size of the (100) plane yielding an increase in XRD lattice aspect ratio from 2.8 (as-prepared) to 3.6 (700 0 C) and 4.2 (900 0 C) ( Figure 4b) .
  • the XRD lattice aspect ratio remained constant at 2.7 for (700 °C) and slightly increased to 2.8 for (900 0 C) .
  • Figure 5 shows a series of TEM images for (a) pure ZnO along with (b to f) showing 2 to 10 at . % In- doped ZnO.
  • Pure ZnO was found to contain mainly spheroidal particles typically with diameters of 20 nm with occasional rod-like structures, consistent with Tani and coworkers (Tani et al . , 2002a) .
  • These short rod-like features seen among the mainly spheroidal particles are assumed to arise from a slight oxygen deficiency for some particles as they form in the flame, in-line with size effects caused by oxygen stoichiometry variation (Tseng et al., 2003; Yan et al . , 2003) .
  • % In- doped ZnO shows particles with cuboid morphology, while for 4 at .% doping (c) the particles begin to exhibit an elongated morphology. Between 6, 8, and 10 at.% (d,e,f) the particles were found to exhibit increasing rod-like appearance. The 10 at.% In-doped ZnO sample (f) in particular was found to be dominated by nanorod features with diameters between 10 and 20 nm and lengths up to about 100 nm. The XRD measurements ( Figure 4), showing an increased XRD lattice aspect ratio with increased dopant concentration, are consistent with these TEM images.
  • Figure 6 shows a plot of the displacement of the (100) and (002) peaks from the (101) peak as a function of indium dopant concentration.
  • the (100) and (002) peaks remained at identical positions to the undoped sample.
  • the maxima of the (002) peak shifted towards higher interplanar d-spacing (smaller 2 ⁇ ) values while the (100) peak remained at the same d-spacing.
  • the dopants are incorporated within the wurtzite lattice, it could be shown that the lattice geometry is altered for higher indium and tin concentrations with respect to the 2 ⁇ position of the (002) peak (see Figures 3a, 3b) . Comparing the relative displacement of the maxima of the (100) and (002) peaks relative to the position of the (101) peak, treated here as an internal reference, it was possible to gain an insight of the distortion in crystal geometry. For dopant concentrations up to 6 at.%, the (100) and (002) peaks remained at identical positions with those of pure ZnO.
  • the incorporated dopants may influence the surrounding lattice either by their atom size, their valency or their degree of coordination.
  • the atomic radii of In (1.55 A) , Sn (1.45 A) and Li (1.45 A) are very similar to the atomic radius of Zn (1.35 A) (Slater, 1964) so the size of the dopant atoms is unlikely to induce the observed rod-like geometry. Similar dopant sizes can create a substitutional defect as with Sn and Al doping on Ti ⁇ 2 (Akthar et al . , 1994) .
  • Figure 8a shows Raman spectra for 0 to 10 at.% Sn-doped ZnO. Each spectrum was normalized to the amplitude of the E2 mode (437 cm “1 ) of the wurtzite. All Sn-doped powders were found to exhibit two additional peaks at 570 cm “1 and 670 cm “1 . These peaks are indicative of vibrations associated with the tin dopant atoms within the crystal while the peak at 670 cm” 1 was associated with a more highly coordinated structure than that of the 570 cm "1 peak (Porotnikov et al . 1983) .
  • Figure 8b shows the ratio (diamonds) of areas below the 670 cm “1 peak to that below the 570 cm “1 peak as a function of dopant concentration.
  • Oxidic nanotubes and nanorods - anisotropic modules for a future nanotechnology Angewandte Chemie International Edition, 41: 2446-2461.

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Abstract

La présente invention a trait à un procédé de pyrolyse au pistolet à flamme pour la production de nanotiges dans lequel au moins un précurseur de métal dopant est additionné à une solution précurseur du métal majoritaire en des quantités égales ou inférieures à 15 % en atomes par rapport au métal majoritaire, ledit dopant présentant une valence supérieure à celle du métal majoritaire, et dans lequel le réseau cristallin de l'oxyde métallique pur formé par la pyrolyse au pistolet à flamme se trouve dans un réseau hexagonal compact. Dans des modes de réalisation préférés, le nombre de coordination des atomes de dopant est supérieur au nombre de coordination des atomes du métal majoritaire et les atomes et/ou ions du dopant et les atomes/ions du métal majoritaire ont des tailles à variation de moins de 20 % en atomes
PCT/EP2005/007995 2004-07-23 2005-07-22 Synthese par flamme rapide de nanotiges de zno dopees a facteur de forme controle WO2006008175A1 (fr)

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CN100360421C (zh) * 2006-07-31 2008-01-09 浙江理工大学 一种氧化锌纳米棒的制备方法
WO2008128821A1 (fr) * 2007-04-19 2008-10-30 Evonik Degussa Gmbh Composite de couches comprenant une couche d'oxyde de zinc pyrogène et transistor à effet de champ comprenant ce composite
WO2009080896A1 (fr) * 2007-12-20 2009-07-02 Beneq Oy Dispositif et procédé pour produire des particules

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100360421C (zh) * 2006-07-31 2008-01-09 浙江理工大学 一种氧化锌纳米棒的制备方法
WO2008128821A1 (fr) * 2007-04-19 2008-10-30 Evonik Degussa Gmbh Composite de couches comprenant une couche d'oxyde de zinc pyrogène et transistor à effet de champ comprenant ce composite
JP2010525560A (ja) * 2007-04-19 2010-07-22 エボニック デグサ ゲーエムベーハー 熱分解法酸化亜鉛層を含む層複合材料及び該複合材料を含む電界効果トランジスタ
KR101156280B1 (ko) 2007-04-19 2012-06-13 포르슝스젠트룸 카를스루에 게엠베하 발열성 산화 아연 층을 포함하는 층 복합재 및 그 복합재를 포함하는 전계-효과 트랜지스터
CN101675506B (zh) * 2007-04-19 2012-08-08 赢创德固赛有限责任公司 包含热解法氧化锌层的层状复合体和包含该复合体的场效应晶体管
US8907333B2 (en) 2007-04-19 2014-12-09 Evonik Degussa Gmbh Pyrogenic zinc oxide-comprising composite of layers and field-effect transistor comprising this composite
WO2009080896A1 (fr) * 2007-12-20 2009-07-02 Beneq Oy Dispositif et procédé pour produire des particules
EA017446B1 (ru) * 2007-12-20 2012-12-28 Бенек Ой Устройство и способ для получения частиц

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