Flame synthesis of zinc oxide nanocrystals - Semantic Scholar
Materials Science and Engineering B 178 (2013) 127–134
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Flame synthesis of zinc oxide nanocrystals Wilson Merchan-Merchan ∗ , Moien Farmahini Farahani School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019, USA
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 Available online 9 November 2012 Keywords: Nanocrystals Flame synthesis Zinc oxide nanostructures
a b s t r a c t Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counterflow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ∼99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 m. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ∼0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and applications of zinc oxide (ZnO) nanostructures have significantly increased during the last years due to their unique mechanical and electronic properties. High heat capacity and heat conductivity along with low thermal expansion and high melting temperature of zinc oxides make this compound very unique. Additionally, the high piezoelectric tensor property of ZnO makes it a technologically important material for many piezoelectrical applications which require significant electromechanical coupling. Due to their unique electronic properties ZnO nanostructures can be employed as important components in the semiconductor industry such as wide band gap semiconductors. The band gap of ZnO is approximately 3.37 eV at room temperature which allows higher breakdown voltages and an ability to work as high electric field emitters. Structures made of ZnO possess gas sensitive properties, high excitonic binding energy with high thermal and chemical stabilities [1–3]. For those reasons, structures of ZnO have been considered as important components for applications of photocatalysts [4], gas sensors [5], ultraviolet (UV)
∗ Corresponding author. Tel.: +1 405 325 1754; fax: +1 405 325 1088. E-mail address: [email protected] (W. Merchan-Merchan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.10.031
light-emitting diodes (LED) and UV lasing [6,7], field emission devices [8], biological probes [9] and solar cells [10]. At the present time, a variety of 1D and 3D nanostructured ZnO crystals have been formed such as nanorods [11,12], nanowires [13], nanowhiskers [14], nanobelts (nanoribbons) [15], nanospings [16], nanocombs [17], nanowalls [18], nanobridges [19], nanonails [20], nanohelices [21], and hierarchical nanostructures [22]. Various techniques have been developed for the preparation of ZnO nanostructures, including vapor-phase transport [23], chemical vapor deposition (CVD) [24], plasma enhanced CVD [25], reactive magnetic sputtering [26], spray pyrolysis [27], sol–gel techniques [28], electrochemical deposition [29], and hydrothermal deposition [30,31] among others. Although these methods are capable of producing ZnO structures, they are generally limited by the complexity of the process, scalability and purity of the products. Some of these methods are composed of a multi-step process which is time consuming and expensive. A flame is an exothermic system with a self-sustaining isothermal oxidation reaction medium. In this highly energetic medium, synthesis of nanomaterials represents a single step process resulting in a rapid, continuous, and inexpensive method to synthesize nanomaterials compared to other existing methods. For a thermalbased system to be suitable for the synthesis of transition metal oxide (TMO) nanomaterials it must contain several essential components: (i) a source of oxygen or oxygen free radicals; (ii) allow for
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Fig. 1. (a) A cut view of a flame section formed in a counter-flow reactor with a schematic of the sleeve containing a Zn probe both introduced in the oxygen-rich zone of the flame; (b) schematic illustrating the flame synthesis method inside the sleeve with corresponding flow direction and probe inserted through the sleeve; (c) chemical species and flame temperature profile of the flame predicted by a numerical simulation based on a multi-step reaction mechanism [42]; Z represents the distance from the edge of the fuel nozzle to the center diameter of the sleeve opening. The tip of the Zn probe is placed at a distance of approximately 5 mm from the edge of the adjustable window.
an efficient introduction of the raw material “elemental source”; and (iii) a source of heat to create a chemical intercalation to “decompose the raw material and allow intercalation with oxygen free radicals” to form the metal oxide structures. Compared to current methods employed for the synthesis of ZnO structures, the flame method represents an auto-thermal process that is capable of providing an optimal source of heat for achieving desired synthesis conditions. The flame medium can also be tailored to be rich in intermediate oxygen and oxygen radicals that can be formed at very high concentrations during intense homogeneous reactions. Recently, a number of works have shown that flames are successful in preparing various types of elongated TMO nanostructures [32–34]. Furthermore, several studies have shown that various flame geometries can be employed to synthesize a variety of carbon nanoforms such as carbon nanotubes [35–40] and fullerenes [41]. It is the ability to easily introduce different flame parameters to control the temperature (heat source) and chemical species within the system that makes flames very attractive for nanomaterial synthesis. Different flame structures can be obtained by varying the fuel type and additive, equivalence ratio, gas velocity, oxygen content in the oxidizer stream, and even by varying the pressure. In this paper we show that a flame formed using a counterflow reactor can be successfully employed to synthesize unique ZnO nanocrystals in a rapid process requiring no more than a few minutes. In this method a Zn probe is inserted inside the flame medium through a sleeve where it interacts with O, OH, O2 , and other oxidative species in the flame to form ZnO nanocrystals on the surface of the probe.
2. Experimental setup A stable flame formed by a counter-flow reactor is employed for the synthesis of ZnO nanostructures. The reactor contains two opposite gas flows (fuel and oxidizer) that are separated by a one inch gap where the flame forms, Fig. 1(a). The fuel (96%CH4 + 4%C2 H2 ) is introduced from the top nozzle and the oxidizer (50%O2 + 50%N2 ) is supplied from the bottom nozzle.
The gas flows are controlled with electronic mass flow meters providing accuracy within 1.5%. The fuel and oxidizer flows impinge against each other to form the stagnation plane. The experiments were conducted with constant fuel and oxidizer strain rates equal to 20 s−1 . Zinc wires, with a diameter of 1 mm, were used as the material source. Due to the low melting point of Zn (∼419 ◦ C) the probe was sequestered inside a sleeve in order to protect it from the high temperature zone of the flame. An insert containing an opening at the center of one of its sides is introduced at the end of the sleeve exposed to the flame medium. The insert and sleeve were placed in the flame so that the gas flows perpendicular relative to the position of the Zn probe, Fig. 1(b). The probe and sleeve were introduced radially at various flame heights (Z). Z represents the distance from the edge of the fuel nozzle to the center diameter of the sleeve opening, Fig. 1(a). The flame height is used in our experiments as a means to control the growth of the nanostructures. It is evident from Fig. 1(c) that at various Z positions the flame temperature and chemical species present within the flame varies significantly [42]. 3. Results and discussion Fig. 2 represents SEM images of the as-grown nanomaterials deposited on the surface of a Zn probe inserted in a flame volume at the flame height of 12 mm for 2 min residence time. It is evident that a high density of nanostructures can be grown on the surface of the probe. It is also observed that the bases of the structures have a larger diameter size than the tips. The diameter of the rods is less than 100 nm and of several microns in length. Grown rods have an approximate length-to-diameter aspect ratio of 10–30 with a growth rate of 8–17 nm/s. Scanning of the probe with SEM showed that the entire surface of the probe is covered with 1-D nanocrystals. Fig. 2(b) and (c) represents SEM images showing a close up view of the selected areas in Fig. 2(a). Although, the structures grow in different directions, they are perpendicular to the surface of the probe, Fig. 2(b) and (c). The structures were synthesized using a sleeve opening with a diameter size of 2 mm. It was found that by altering the position of the probe, different types of structures can be grown due to the significant changes in
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Fig. 2. SEM images of zinc oxide nanorods formed on the surface of the zinc probe; (a) low resolution SEM showing the high density growth of nanorods; ((b) and (c)) high resolution SEM images of selected areas in (1) and (2) in (a). The Zn probe is inserted in the flame medium at Z = 12 mm with a 2 mm opening size sleeve.
the temperature and the content of oxygen species near the surface of the probe. The repositioning of the probe to Z = 11 mm (with the same size opening on the one side of the sleeve, 2 mm) resulted in larger polyhedral crystals synthesized on the surface of the probe (Fig. 3). Large ZnO polyhedral crystals similar to the ones described here are referred to in the literature as ZnO microprism (ZMP) [43]. The ZMPs can have a large number of facets with rectangular, pentagonal, and hexagonal cross-sectional areas (Fig. 3). High resolution SEM reveals that the ZMPs have a protruding nanorod at their tips (white arrows Fig. 3(a) and (b)) creating a needle-like structure. The protruding nanorods at the top of the ZMP crystals have diameters of less than 100 nm and are several hundreds of nanometers in length. It is interesting to note that the protruding nanorods have similar morphological
characteristics to the structures grown in the lower part of the flame (Z = 12 mm), Fig. 2. That is, they have a nanoscale diameter size and the tip gradually becomes pointy suggesting that these structures grow in a similar manner. Similar to the previous testing position, the protective sleeve assures that the probe is not instantly oxidized and consumed in the flame medium. The counter-flow flame has strong axial temperature (up to 1750 ◦ C/cm) and chemical composition gradients. At the flame position of Z = 11 mm the flame temperature is predicted to be approximately 2100 ◦ C compared to ∼2500 ◦ C at Z = 12 mm. The flame environment at the flame height of Z = 12 mm contains 2 mol% of O2 and 0.5 mol% of O compared to no oxygen–oxygen/radicals at the flame position of 10.5 mm (Fig. 1(c)). Thus, the position of the probe/sleeve can significantly affect the results of the nanomaterial synthesis. The effect
Fig. 3. SEM images of structures formed on the surface of the Zn probe inserted in the flame medium at Z = 11 mm with a 2 mm sleeve opening size; (a) shows that a high density of polyhedral crystal structures can be synthesized; (b) high resolution SEM image of a selected area in (a) shows that the larger polyhedral crystals have a protruding nanorod at their tips; (c) a close-up view shows that the larger crystals have a high number of facets with their edges well defined.
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Fig. 4. Structural and chemical composition analysis of the Zn oxide crystals formed in the flame medium; (a) low resolution TEM image of a nanorod with its tip gradually decreasing into the shape of pipet-like structure; (b) low resolution TEM image of a Zn oxide nanorod; (c) HR-TEM image of the same nano-rod (circled area in (b)) showing well ordered crystal structure with characteristic d-spacing corresponding to {0 0 2} planes; (d) EDS spectrum collected on the structure shows the presence of Zn and oxygen and SAED with diffraction spots confirming the crystallinity of the structure. The growth of the nanorod is in the [0 0 1] direction.
of temperature and oxygen on the flame synthesis of ZnO nanostructures correlates well with other complex thermal methods where it is shown that both temperature and oxygen content can greatly affect morphology and size of ZnO structures [2,3,24,44,45]. Grown nanorods on the tip of the ZMP crystals have length-todiameter aspect ratios in the range of 2–8, and have a growth rate of 5–15 nm/s. Zinc oxide structures possessing higher [30,46] and lower [47,48] aspect ratio can be found in literature. For TEM studies, samples were prepared by suspending the removed synthesized structures in methanol. The solutions were sonicated until the material was well dispersed in the solvent. A drop of the suspension was then placed on a copper substrate/carbon-film of the microscope specimen grid and dried. HR-TEM and SAED imaging of the grown structures reveals the structural uniformity and highly ordered crystalline structure as shown in TEM images in Fig. 4. Fig. 4(a) represents a low resolution TEM image of a nanarod whose diameter appears to gradually taper toward its tip to the shape of a pipet-like structure. TEM and HR-TEM reveals that the structures possess a high degree of perfection. Fig. 4(c) represents atomic resolution of a typical nanorod (from the circled area in Fig. 4(b)). The lattice fringes from TEM studies correspond to a d-spacing of approximately 0.26 nm that closely matches the (0 0 2) plane of hexagonal ZnO (Wurtzite) cell. It is well established in the literature that ZnO has three fast growth directions [0 0 0 1], [1 0 1 0] and [2 1 1 0] [49–52]. The work of Li et al. showed that the growth rate of the basal plane [0 0 0 1] of Zn oxide structures is preferential and it grows at much faster rates than the facet sides [53]. Once the facets appear, they do not disappear, crystal faces with this behavior cause the development of the gradual reduction in cross-sectional area along the growth direction and hence yielding the pointy tip morphology. Since no catalyst was employed in the experiments and also no condensate structures such as droplet can be observed in the SEM and/or TEM images, the growth mechanism of ZnO nanostructures can be explained through the vapor–solid (VS) mechanism. As for the VS mechanism, it is very typical that the nanorods or nanowires reduce gradually in diameter during growth and possess a sharp tip [48,1]. HR-TEM imaging of a typical nanorod at different points shows that they are free of dislocations and structural defects indicating that they consist of a single crystal. The HR-TEM analysis and SAED diffraction pattern with the diffraction spots confirms the crystallinity of the structure and growth of the Zn nanorod in the [0 0 1] direction, Fig. 4(b). EDS conducted on the synthesized products revealed that the structures are indeed composed of zinc and oxygen (Fig. 4(d)).
A gas adsorption analyzer (Micromeritics ASAP 2000) was employed to measure the surface area of the formed crystals based on the Brunauer–Emmett–Teller (BET) theory. The BET results show that the ZnO crystals synthesized in the flame have a surface area less than 1 m2 /g. In this work we show that ZnO nanorods and ZMP crystals with very high crystallinity are formed in a combustion method. Fig. 5 represents TEM and SEM images of grown crystals at the earlier, intermediate, and final stages. We propose that the growth mechanism is composed of two different processes: (1) the global growth mechanism which is responsible for the formation of vapors by the evaporation of the base metal as it is partially exposed to the flame medium. The Zn atoms are intercalated with oxygen atoms from the flame to form ZnO nuclei. (2) Once the nuclei are formed, elongated structures start to form through the vapor–solid (VS) growth mechanism. For the first step, the mechanism involves the partial interaction of a high purity Zn probe (in the form of a 1 mm diameter wire) with a high temperature flame that is rich in oxygen and oxygen radicals. It is important to note that the temperature of the 1 mm diameter probe is much higher closer to the tip of the sleeve where the flame–probe interaction takes place. That is, a high rate of zinc vapor is produced from the part of the probe that is exposed to the flame due to the low melting point of Zn which is ∼419 ◦ C. As the process continues Zn atoms join oxygen atoms from the flame to form ZnO. The Zn oxide is deposited on the surface of the probe and can act as nuclei to form elongated structures. A continuous influx of ZnO vapors can be further deposited on previously formed rods to further crystallize complex 1-D or 3-D structures. A recent work by Li et al. [53] reports the formation of ZnO micro- and nano-structures prepared by thermal evaporation that very closely resemble our flame synthesized ZnO structures reported here. In that study, the structures are grown in a tube chamber operating at ∼1000 ◦ C with various oxygen percentages. It is proposed that at the applied temperature condition Zn vapors are mass-produced from the Zn source material and later combine with the oxygen atoms in the gas-phase to form ZnO nuclei. In our previous works on the flame synthesis of TMO nanostructures we showed that metal oxide/hydroxides are first formed on one side of the base probe located in the higher temperature of the flame [54,55]. These oxides/hydroxides are further evaporated and transported by the gas flow to the lower temperature side of the probe where they are deposited in the form of transition metal oxide nanostructures. In the Zn oxide case, the melting and boiling point temperature of ZnO is approximately 1975 ◦ C and 2360 ◦ C, respectively. Therefore, our experimental results suggest that the
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Fig. 5. Proposed growth mechanism of ZnO nanostructures.
Zn in the vapor phase must combine with oxygen atoms and deposit along the surface of the probe to serve as nuclei and material source for the continuous growth of the ZnO structures. The density of metal and metal/oxide vapor and consequently the nuclei can be controlled by the simple relocation of the probe within the flame. Higher environment temperature results in higher Zn vapor formation thus higher density of nuclei. Experiments conducted in vapor phase transport using Au-catalyzed Si substrates for the synthesis of ZnO nanostructures have shown that a wide variation in morphology of the synthesized structures can be obtained by a small variation of temperature [48]. In that study it was shown that at the growth temperature of 800 ◦ C mainly 1D rod-like ZnO structures are synthesized compared to larger hexagonally faceted 3D structures obtained by the increase of the growth temperature by only ∼100 ◦ C. The concentration of oxygen in the environment can be also used to yield ticker or slender ZnO structures [1]. The effect of variation of temperature for the growth of the Zn oxide structures is also very evident in our flame synthesis method. At the flame position of Z = 12 mm the local flame temperature generated by the counter-flow reactor is higher than that at Z = 11 mm by approximately 280 ◦ C [42]. In our experiments we can observe that a higher growth temperature (Z = 12 mm) results in the surface of the probe being covered with mostly 1D ZnO nanorods. However, in the lower part of the flame (Z = 11 mm) where the temperature is lower the probe is covered with ZMPs containing a high number of facets and a nanorod extruding from the tip. The effect of residence time has also been studied for the synthesis and morphology control of metal oxide channels in flames [54].
The flame temperatures and concentration of chemical species used to correlate with nanomaterial synthesis were predicted by employing a numerical simulation reported by Beltrame et al. using the same flame reactor and similar flame parameters used in that study [42]. The chemical kinetic model was developed by employing GRI-MECH 2.11, C6 chemistry and a soot formation mechanism. Also it is important to note that the temperature of the 1 mm probe used for Zn-oxide nanostructure synthesis is always lower than the local flame temperature due to the radiant and conductive heat losses. A probe inserted in the flame volume at Z = 11 mm results in lower Zn vapor supersaturation which leads to the formation of ZnO microprism (ZMP) crystals with a high number of facets and pointy tip morphology. Our results show that ZMP crystals of a few micrometers in length are first formed on the surface of the Zn probes at this flame position. SEM images also show that the tips of the ZMP crystals have a structural defect in the form of a nanometer sized hole (Fig. 5(a1)). The hole constitutes an instable site that is ideal for a second nucleation process [56]. As the growth proceeds and the probe is consumed, the diameter of the probe reduces. Consequently, the temperature of the probe increases, leading to a higher vapor supersaturation and to a level that favors the synthesis of 1D nanorods of high crystallinity extruding from the tip of the ZMP crystals. The arrow in the SEM image in Fig. 5(a1) points to the nanosized hole or indentation at the tip of a ZMP crystal revealing the early stage growth of the strutting nanorod (second nucleation). TEM analysis confirms that the nanorod emerging from the indentation (as observed by SEM) is indeed a part of the larger crystal,
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Fig. 6. (a) SEM image collected on the surface of a Zn probe reveals the presence of a high density of complex crystal formation. Arrows 1 and 2 point to several slender structures that have diamond and rectangular shape growing on the surface of a rod, respectively. Arrow 3 points to several cubic structures grown on the surface of a nanorod. Arrows 4–6 point to crystal bases that have pentagonal, circular, and rectangular cross sections; ((b)–(e)) higher magnification of selected areas in (a); ((f)–(j)) HR-SEM images showing that the ZMPs have a protruding nanorod at their tips.
Fig. 5(a2). That is, after sonication of the crystals in methanol, the nanorod in its earlier growth stage remained at the tip of the ZMP. It is not clear what activates the growth of the nanorods on the tip of the ZMPs, but it appears that the nanosized hole on the tip of the ZMP crystal serves as an auto-catalyst similar to the VLS growth model. Once the nucleation of the nanorod is initiated it continues to grow in a preferential direction away from the tip of the ZMP crystal. Close inspection of the SEM image (Fig. 5(b1)) reveals that the size of the ZMP crystals remained the same (black arrow) but the length of the protruding rods at the tips of the crystals significantly increased as pointed out by a white arrow. The protruding rods are of aspect ratio resembling solid pipet-like structures. The TEM image, Fig. 5(b2) clearly shows that the base and the tip both form a single crystal. The pipet-like structures are modified to needlelike nanorods due to the continuous deposition on their surfaces by ZnO species, Fig. 5(c). Close inspection shows that the crosssectional area at the base of the pipet-like structures (Fig. 5(b1) and (b2)) coincides with the base of the needle-like structures (Fig. 5(c1) and (c2)). Further deposition of the ZnO vapors on the surface of existing structures results in the transformation of structures, Fig. 5(d). The SEM image, Fig. 5(d1), shows structures with larger bases and pointy tips as highlighted by black arrows. The nanorods have lengths of several microns and a gradual reduction in diameter. They also appear to be flexible, for instance, the
structure in the inset in Fig. 5(d1) has a “U” shape curve/bend at its tip which shows that some of the ZnO structures can have very small bending radius. The black arrows in Fig. 5(d2) highlight needle-like structures and the white arrows highlight the nanorods protruding from the tips of the needle-like structures. Fig. 5(d3) is a close up view of a selected area in Fig. 5(d2) highlighting the transformation from needle-like to nanorod structure. The flame formed ZMPs and nanorods have close aspect ratios in comparison to similar structures in the literature [43] suggesting that the growth is mandated by a certain growth mechanism rather than the method used. The hypothetical growth mechanism of the ZnO structures discussed above shows that a continuous presence of ZnO vapors (source) are necessary to form and transform the shapes of the crystals. The precipitation of ZnO on the surface of the activated rods is necessary for continuing growth of the structures. That is, the initial structures themselves (rods) play the role of support for the deposition of ZnO vapors to transform into crystals of various shapes. The rate and availability of ZnO vapors can significantly change the morphology of the structures. To support the above mechanism additional experiments were conducted by increasing the residence time of the probe/sleeve inside the flame volume. By increasing the residence time of the probe (∼3 min), the amount of ZnO vapors inside the sleeve increases and this should lead to crystal formation containing
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Fig. 7. TEM image of a nanobead-like structure with corresponding SAED and EDS spectra in selected crystal locations “rod” and “cubic”.
different characteristics on the surface of the Zn probe. Indeed, the formation of only well faceted large crystals or ZMPs with a short nanorod protruding from the tip was not observed in the experiment results performed at Z = 11 mm with a sleeve diameter of 2 mm. Instead, other unique complex crystal structures are formed (Fig. 6). Further crystallization is formed by vapor deposition directly on the nanocrystal surfaces to form more complex structures. It can be observed (Fig. 6) that some of the elongated rods have several slender polyhedral structures attached to their surfaces as if decorating the protruding nanorod. The arrows 1 and 2 in Fig. 6(a1) point to diamond and rectangular shaped structures attached to the surface of two different nanorods, respectively. Arrow 3 points to several cubic structures attached to the surface of a single nanorod. Arrows 4–6 point to ZMPs that have pentagonal, circular, and rectangular cross-sections, respectively. The hypothesis is further supported by noting that the rods are independent from the “decorative” polyhedral structures. The TEM images in Fig. 6(b) and (c) show a rod outside the polyhedral and partially embedded at its base as pointed out by the arrows. This characteristic is much more clearly observed in Fig. 6(f) and (g) where slender polyhedral structures are fused diagonally to the surface of a nanorod. The rods have a darker contrast compared to the slender polyhedral structures. It appears that the vapor–solid mechanism and finite size of the formed eutectic droplets along with a low driving force of crystal growth results in small cross-sectional size polyhedral structures. The low driving force of the crystal formation can be defined as a ratio of the chemical potential difference between two phases over temperature. The direct deposition of smaller crystals on the surface of the nanorod is evident as pointed out by the arrow in Fig. 6(d). The nucleation of the polyhedral structures appears to occur simultaneously at several locations on the surface of the formed nanorods due to the continuous precipitation of ZnO vapors. Fig. 6(e) shows a nanorod with several decorative cubical structures with a sudden direction change of ∼90◦ . The similar bending of a rod with earlier deposition of the “decorative” structures is evident in Fig. 6(a) (set of white arrows) suggesting that a further influx of ZnO vapor will lead to a mature structure similar to the one shown in Fig. 6(e). Fig. 6(f)–(j) shows that the ZMPs all have a nanorod protruding from the indentation (darker contrast) present at the center on their tips as pointed out by the arrows. Fig. 7(a)–(e) shows TEM images, SAED, and EDS spectra of a nanorod with several cubic crystals attached to its surface forming a “nanobead-like” structure. It is interesting to note that even after several minutes of sonication, which is required for TEM analysis, the small cubical structures and the rod forming the “nanobead-like structure” remained together resembling a single crystal, Fig. 7(a). The EDS spectra in Fig. 7(d) and (e) are of the nanorod and of a
cubical crystal, respectively. The EDS analysis using a focused electron beam with approximately the same diameter as that of the nanobead-like structure shows the presence of Zn, O, and C. The signal of the carbon peak in the EDS spectrum taken across the entire structure is very high and it is coming from the film supporting the nanobead-like structure, Fig. 7(a) and (d). EDS spectrum taken with a focused electron beam with approximately the same diameter as the nanorod shows that the carbon peak intensity is nonexistent, Fig. 7(e). However, the Zn peak is quite high. The SAED collected from both, the nanorod (arrow in the inset Fig. 7d) and the polyhedral structure (arrow in the inset Fig. 7e), show that they are nearly identical (Fig. 7(b) and (c)). This association is quite analogous to our proposed mechanism that the initial structures themselves play the role of supports for the deposition of Zn oxide vapors to transform into various solid shapes.
4. Conclusions The synthesis of ZnO nanocrystals is performed using Zn probes inserted in a counter-flow flame medium formed using methane/acetylene and oxygen-enriched air streams. The source material is introduced in the form of a solid wire through a sleeve. For the synthesis of the ZnO nanocrystals the sleeve containing the zinc probe is introduced in a region of the flame that is rich in O, OH, O2 , and other oxidative species. The shape and structural parameters of the grown crystals strongly depend on the flame position. The variation of flame position from Z = 12 mm (were the predicted flame temperature is ∼2500 ◦ C) to Z = 11 mm (predicted flame temperature 2100 ◦ C) lead to modification of crystal morphology from short thick based ZMPs with a short protruding nanorod at their tips to very elongated nanorods. The diameters of the protruding nanorods in both cases are less than 100 nm. However, the physical characteristics of the crystal bases remained approximately the same. The proposed growth mechanism of the nanocrystals is composed of two different processes: (i) the global growth mechanism which is responsible for the formation of vapors by the evaporation of the base metal as it is partially exposed to the flame medium. The Zn atoms are intercalated with oxygen atoms from the flame to form ZnO nuclei, and (ii) once the nuclei are formed elongated structures start to form through the vapor–solid (VS) growth mechanism. This mechanism is indirectly confirmed by increasing the residence time of the wire/sleeve inside the flame volume. This strongly suggests that the initial structures themselves (rods) play the role of support for the deposition of Zn oxide vapors to transform into various solid shapes. The protruding nanorods have slender polyhedral structures that have diamond, rectangular, and cubical shape as if decorating the nanorods.
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Acknowledgments The support of this work by the National Science Foundation through the Grants CTS-0854433 and CTS-0854006 is gratefully acknowledged. The authors would like to extend special thanks to Dr. Alan Nicholls, Dr. Ke-Bin Low, and Ms. Kristina Jarosius from the University of Illinois at Chicago Research Resource Center for assistance in TEM and SEM studies, encouragement and helpful discussions. We would also like to thank Dr. Rolf Jentoft from the School of Chemical, Biological and Materials Engineering at the University of Oklahoma for help with surface area and porosity studies on the flame formed structures. References [1] F. Li, Z. Li, F. Jin, Physica B 403 (2008) 664–669. [2] N.K. Park, G.B. Han, J.D. Lee, S.O. Ryu, T.J. Lee, W.C. Chang, C.H. Chang, Current Applied Physics 6S1 (2006) 176–181. [3] X.N. Zhang, C.R. Li, Z. Zhang, Applied Physics A 82 (2006) 33–37. [4] T.I.H. Yumoto, S.J. Li, T. Sako, K. Nishiyama, Thin Solid Films 345 (1999) 38–41. [5] A. Uedono, T. Koida, A. Tsukazaki, M. Kawasaki, Z.Q. Chen, S.F. Chichibu, H. Koinuma, Journal of Applied Physics 93 (2003) 2481. [6] J.H. Lim, C.K. Kang, K.K. Kim, I.K. Park, D.K. Hwang, S.J. Park, Advanced Materials 18 (2006) 2720–2724. [7] Z.P. Wei, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, X.W. Fan, Z.K. Tang, Applied Physics Letters 90 (2007) 042113. [8] S. Xiao-Ping, Y. Ai-Huan, H. Ye-Min, J. Yuan, X. Zheng, H. Zheng, Nanotechnology 16 (2005) 2039–2043. [9] G.I. Dovbeshko, O.P. Pennytska, E.D. Obraztsova, Y.V. Shtogun, Chemical Physics Letters 372 (2003) 432–437. [10] K. Keis, E. Magnusson, H. Lindström, S.E. Lindquist, A. Hagfeldt, Solar Energy Materials and Solar Cells 73 (2002) 51–58. [11] C. Li, G. Fang, N. Liu, J. Li, L. Liao, F. Su, G. Li, X. Wu, X. Zhao, Journal of Physical Chemistry C 111 (2007) 12566–12571. [12] Z. Guo, D. Zhao, D. Shen, F. Fang, J. Zhang, B. Li, Crystal Growth and Design 7 (2007) 2294–22996. [13] L. Vayssieres, Advanced Materials 15 (2003) 464–466. [14] C. Xu, Z. Liu, S. Liu, G. Wang, Scripta Materialia 48 (2003) 1367–1371. [15] Y.X. Chen, X.Q. Zhao, J.H. Chen, Materials Letters 62 (2008) 2369–2371. [16] P.X. Gao, Z.L. Wang, Small 1 (2005) 945–949. [17] Z.L. Wang, X.Y. Kong, J.M. Zuo, Physical Review Letters 91 (2003) 185502. [18] H.T. Ng, J. Li, M.K. Smith, P. Nguyen, A. Cassell, J. Han, M. Meyyappan, Science 300 (2003) 1249. [19] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Letters 3 (2003) 235–238. [20] X. Zhang, Y. Zhang, J. Xu, Z. Wang, X. Chen, D. Yu, P. Zhang, H. Qi, Y. Tian, Applied Physics Letters 87 (2005) 123111. [21] P.X. Gao, Y. Ding, W. Mai, W.L. Hughes, C. Lao, Z.L. Wang, Science 309 (2005) 1700–1704. [22] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Journal of Materials Chemistry 14 (2004) 770–773. [23] R.T.R. Kumar, E. McGlynn, C. McLoughlin, S. Chakrabarti, R.C. Smith, J.D. Carey, J.P. Mosnier, M.O. Henry, Nanotechnology 18 (2007) 1215704. [24] P.C. Chang, Z. Fan, D. Wang, W.Y. Tseng, W.A. Chiou, J. Hong, J.G. Lu, Chemistry of Materials 16 (2004) 5133–5137. [25] R. Yang, J. Zheng, W. Li, J. Qu, X. Zhang, X. Li, Materials Chemistry and Physics 129 (2011) 693–695. [26] S. Singh, R. Kumar, T. Ganguli, R.S. Srinivasa, S.S. Major, Journal of Crystal Growth 310 (2008) 4640–4646. [27] A. Bashir, P.H. Wöbkenberg, J. Smith, J.M. Ball, G. Adamopoulos, D.D.C. Bradley, T.D. Anthopoulos, Advanced Materials 21 (2009) 2226–2231. [28] Y. Masuda, K. Kato, Crystal Growth and Design 9 (2009) 3083–3088. [29] A. Ashida, A. Fujita, Y. Shim, K. Wakita, A. Nakahira, Thin Solid Films 517 (2006) 1461–1464. [30] D.S. Kang, S.K. Han, J.H. Kim, S.M. Yang, J.G. Kim, S.K. Hong, D. Kim, H. Kim, J.H. Song, Journal of Vacuum Science and Technology B 27 (2009) 1667–1672. [31] M. Guo, P. Diao, S. Cai, Journal of Solid State Chemistry 178 (2005) 1864–1873. [32] W. Merchan-Merchan, A.V. Saveliev, M. Desai, Nanotechnology 20 (2009) 47601.
[33] W. Merchan-Merchan, A.V. Saveliev, W.C. Jimenez, Proceedings of the Combustion Institute 33 (2011) 1899–1908. [34] F. Xu, X. Liu, S.D. Tse, F. Cosandey, B.H. Kear, Chemical Physics Letters 449 (2007) 175–181. [35] L. Yuan, K. Satio, C. Pan, F.A. Williams, A.S. Gordon, Chemical Physics Letters 340 (2001) 237. [36] R.L. Vander Wal, Chemical Physics Letters 324 (2000) 217–223. [37] M.J. Height, J.B. Howard, J.W. Tester, J.B. Vander Sande, Carbon 42 (2004) 2295–2307. [38] F. Xu, X. Liu, S.D. Tse, Carbon 44 (2006) 570–577. [39] T.X. Li, H.G. Zhang, F.J. Wang, Z. Chen, K. Saito, Proceedings of the Combustion Institute 31 (2007) 1849–1856. [40] W. Merchan-Merchan, A.V. Saveliev, L. Kennedy, W. Cuello Jimenez, Progress in Energy and Combustion Science 36 (2010) 696–727. [41] W.J. Grieco, A.L. Lafleur, K.C. Swallow, H. Richter, K. Taghizadeh, J.B. Howard, Proceedings of the Combustion Institute 27 (1998) 1669–1675. [42] A. Beltrame, P. Porshnev, W. Merchan-Merchan, A. Saveliev, A. Fridman, L.A. Kennedy, O. Petrova, S. Zhdanok, F. Amoury, O. Charon, Combustion and Flame 124 (2001) 295–310. [43] H.B. Lu, L. Liao, H. Li, D.F. Wang, J.C. Li, Q. Fu, B.P. Zhu, Y. Wu, Physica E 40 (2008) 2931–2936. [44] J. Zhang, Y. Yang, B. Xu, F. Jiang, J. Li, Journal of Crystal Growth 280 (2005) 509–515. [45] D.H. Liu, L. Liao, J.C. Li, H.X. Guo, Q. Fu, Materials Science and Engineering B 121 (2005) 77–80. [46] J. Qiu, X. Li, W. He, S.J. Park, H.K. Kim, Y.H. Hwang, J.H. Lee, Y.D. Kim, Nanotechnology 20 (2009) 155603. [47] H. Saitoh, M. Satoh, N. Tanaka, Y. Ueda, S. Ohshio, Japanese Journal of Applied Physics 38 (1999) 6873–6877. [48] R.T.R. Kumar, E. McGlynn, M. Biswas, R. Saunders, G. Trolliard, B. Soulestin, J.R. Duclere, J.P. Mosnier, M.O. Henry, Journal of Applied Physics 104 (2008) 084309-1–084309-11. [49] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [50] Y.F. Zhu, G.H. Zhou, H.Y. Ding, A.H. Liu, Y.B. Lin, N.L. Li, Physica E 42 (2010) 2460–2465. [51] H.J. Fan, A.S. Barnard, M. Zacharias, Applied Physics Letters 90 (2007) 1431161–143116-3. [52] G.H. Lee, M.S. Kim, Journal of the Ceramic Society of Japan 118 (2010) 269–271. [53] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, Journal of Crystal Growth 203 (1999) 186–196. [54] W. Merchan-Merchan, A.V. Saveliev, L.A. Kennedy, Chemical Physics Letters 422 (2006) 72–77. [55] W. Merchan-Merchan, A.V. Saveliev, A.M. Taylor, Micron 40 (2009) 821–826. [56] W.J. Li, E.W. Shi, W.-Z. Zhong, Z.W. Yin, Journal of Crystal Growth 203 (1999) 186–196. Wilson Merchan-Merchan received his Ph.D. from the University of Illinois at Chicago in the areas of Combustion & Nanotechnology in 2005. Currently he holds a faculty position at the University of Oklahoma. One of his areas of research areas focuses on the application of flames for the synthesis of 1-D and 3-D transition metal oxide nanoand micron-sized structures.
Moien Farmahini-Farahani is Ph.D. Candidate at the University of Oklahoma, major in mechanical engineering. He is currently doing research on flame synthesis of transition metal oxides nano-structures.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Flame synthesis of zinc oxide nanocrystals Wilson Merchan-Merchan ∗ , Moien Farmahini Farahani School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019, USA
a r t i c l e
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Article history: Received 25 April 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 Available online 9 November 2012 Keywords: Nanocrystals Flame synthesis Zinc oxide nanostructures
a b s t r a c t Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counterflow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ∼99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 m. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ∼0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and applications of zinc oxide (ZnO) nanostructures have significantly increased during the last years due to their unique mechanical and electronic properties. High heat capacity and heat conductivity along with low thermal expansion and high melting temperature of zinc oxides make this compound very unique. Additionally, the high piezoelectric tensor property of ZnO makes it a technologically important material for many piezoelectrical applications which require significant electromechanical coupling. Due to their unique electronic properties ZnO nanostructures can be employed as important components in the semiconductor industry such as wide band gap semiconductors. The band gap of ZnO is approximately 3.37 eV at room temperature which allows higher breakdown voltages and an ability to work as high electric field emitters. Structures made of ZnO possess gas sensitive properties, high excitonic binding energy with high thermal and chemical stabilities [1–3]. For those reasons, structures of ZnO have been considered as important components for applications of photocatalysts [4], gas sensors [5], ultraviolet (UV)
∗ Corresponding author. Tel.: +1 405 325 1754; fax: +1 405 325 1088. E-mail address: [email protected] (W. Merchan-Merchan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.10.031
light-emitting diodes (LED) and UV lasing [6,7], field emission devices [8], biological probes [9] and solar cells [10]. At the present time, a variety of 1D and 3D nanostructured ZnO crystals have been formed such as nanorods [11,12], nanowires [13], nanowhiskers [14], nanobelts (nanoribbons) [15], nanospings [16], nanocombs [17], nanowalls [18], nanobridges [19], nanonails [20], nanohelices [21], and hierarchical nanostructures [22]. Various techniques have been developed for the preparation of ZnO nanostructures, including vapor-phase transport [23], chemical vapor deposition (CVD) [24], plasma enhanced CVD [25], reactive magnetic sputtering [26], spray pyrolysis [27], sol–gel techniques [28], electrochemical deposition [29], and hydrothermal deposition [30,31] among others. Although these methods are capable of producing ZnO structures, they are generally limited by the complexity of the process, scalability and purity of the products. Some of these methods are composed of a multi-step process which is time consuming and expensive. A flame is an exothermic system with a self-sustaining isothermal oxidation reaction medium. In this highly energetic medium, synthesis of nanomaterials represents a single step process resulting in a rapid, continuous, and inexpensive method to synthesize nanomaterials compared to other existing methods. For a thermalbased system to be suitable for the synthesis of transition metal oxide (TMO) nanomaterials it must contain several essential components: (i) a source of oxygen or oxygen free radicals; (ii) allow for
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Fig. 1. (a) A cut view of a flame section formed in a counter-flow reactor with a schematic of the sleeve containing a Zn probe both introduced in the oxygen-rich zone of the flame; (b) schematic illustrating the flame synthesis method inside the sleeve with corresponding flow direction and probe inserted through the sleeve; (c) chemical species and flame temperature profile of the flame predicted by a numerical simulation based on a multi-step reaction mechanism [42]; Z represents the distance from the edge of the fuel nozzle to the center diameter of the sleeve opening. The tip of the Zn probe is placed at a distance of approximately 5 mm from the edge of the adjustable window.
an efficient introduction of the raw material “elemental source”; and (iii) a source of heat to create a chemical intercalation to “decompose the raw material and allow intercalation with oxygen free radicals” to form the metal oxide structures. Compared to current methods employed for the synthesis of ZnO structures, the flame method represents an auto-thermal process that is capable of providing an optimal source of heat for achieving desired synthesis conditions. The flame medium can also be tailored to be rich in intermediate oxygen and oxygen radicals that can be formed at very high concentrations during intense homogeneous reactions. Recently, a number of works have shown that flames are successful in preparing various types of elongated TMO nanostructures [32–34]. Furthermore, several studies have shown that various flame geometries can be employed to synthesize a variety of carbon nanoforms such as carbon nanotubes [35–40] and fullerenes [41]. It is the ability to easily introduce different flame parameters to control the temperature (heat source) and chemical species within the system that makes flames very attractive for nanomaterial synthesis. Different flame structures can be obtained by varying the fuel type and additive, equivalence ratio, gas velocity, oxygen content in the oxidizer stream, and even by varying the pressure. In this paper we show that a flame formed using a counterflow reactor can be successfully employed to synthesize unique ZnO nanocrystals in a rapid process requiring no more than a few minutes. In this method a Zn probe is inserted inside the flame medium through a sleeve where it interacts with O, OH, O2 , and other oxidative species in the flame to form ZnO nanocrystals on the surface of the probe.
2. Experimental setup A stable flame formed by a counter-flow reactor is employed for the synthesis of ZnO nanostructures. The reactor contains two opposite gas flows (fuel and oxidizer) that are separated by a one inch gap where the flame forms, Fig. 1(a). The fuel (96%CH4 + 4%C2 H2 ) is introduced from the top nozzle and the oxidizer (50%O2 + 50%N2 ) is supplied from the bottom nozzle.
The gas flows are controlled with electronic mass flow meters providing accuracy within 1.5%. The fuel and oxidizer flows impinge against each other to form the stagnation plane. The experiments were conducted with constant fuel and oxidizer strain rates equal to 20 s−1 . Zinc wires, with a diameter of 1 mm, were used as the material source. Due to the low melting point of Zn (∼419 ◦ C) the probe was sequestered inside a sleeve in order to protect it from the high temperature zone of the flame. An insert containing an opening at the center of one of its sides is introduced at the end of the sleeve exposed to the flame medium. The insert and sleeve were placed in the flame so that the gas flows perpendicular relative to the position of the Zn probe, Fig. 1(b). The probe and sleeve were introduced radially at various flame heights (Z). Z represents the distance from the edge of the fuel nozzle to the center diameter of the sleeve opening, Fig. 1(a). The flame height is used in our experiments as a means to control the growth of the nanostructures. It is evident from Fig. 1(c) that at various Z positions the flame temperature and chemical species present within the flame varies significantly [42]. 3. Results and discussion Fig. 2 represents SEM images of the as-grown nanomaterials deposited on the surface of a Zn probe inserted in a flame volume at the flame height of 12 mm for 2 min residence time. It is evident that a high density of nanostructures can be grown on the surface of the probe. It is also observed that the bases of the structures have a larger diameter size than the tips. The diameter of the rods is less than 100 nm and of several microns in length. Grown rods have an approximate length-to-diameter aspect ratio of 10–30 with a growth rate of 8–17 nm/s. Scanning of the probe with SEM showed that the entire surface of the probe is covered with 1-D nanocrystals. Fig. 2(b) and (c) represents SEM images showing a close up view of the selected areas in Fig. 2(a). Although, the structures grow in different directions, they are perpendicular to the surface of the probe, Fig. 2(b) and (c). The structures were synthesized using a sleeve opening with a diameter size of 2 mm. It was found that by altering the position of the probe, different types of structures can be grown due to the significant changes in
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Fig. 2. SEM images of zinc oxide nanorods formed on the surface of the zinc probe; (a) low resolution SEM showing the high density growth of nanorods; ((b) and (c)) high resolution SEM images of selected areas in (1) and (2) in (a). The Zn probe is inserted in the flame medium at Z = 12 mm with a 2 mm opening size sleeve.
the temperature and the content of oxygen species near the surface of the probe. The repositioning of the probe to Z = 11 mm (with the same size opening on the one side of the sleeve, 2 mm) resulted in larger polyhedral crystals synthesized on the surface of the probe (Fig. 3). Large ZnO polyhedral crystals similar to the ones described here are referred to in the literature as ZnO microprism (ZMP) [43]. The ZMPs can have a large number of facets with rectangular, pentagonal, and hexagonal cross-sectional areas (Fig. 3). High resolution SEM reveals that the ZMPs have a protruding nanorod at their tips (white arrows Fig. 3(a) and (b)) creating a needle-like structure. The protruding nanorods at the top of the ZMP crystals have diameters of less than 100 nm and are several hundreds of nanometers in length. It is interesting to note that the protruding nanorods have similar morphological
characteristics to the structures grown in the lower part of the flame (Z = 12 mm), Fig. 2. That is, they have a nanoscale diameter size and the tip gradually becomes pointy suggesting that these structures grow in a similar manner. Similar to the previous testing position, the protective sleeve assures that the probe is not instantly oxidized and consumed in the flame medium. The counter-flow flame has strong axial temperature (up to 1750 ◦ C/cm) and chemical composition gradients. At the flame position of Z = 11 mm the flame temperature is predicted to be approximately 2100 ◦ C compared to ∼2500 ◦ C at Z = 12 mm. The flame environment at the flame height of Z = 12 mm contains 2 mol% of O2 and 0.5 mol% of O compared to no oxygen–oxygen/radicals at the flame position of 10.5 mm (Fig. 1(c)). Thus, the position of the probe/sleeve can significantly affect the results of the nanomaterial synthesis. The effect
Fig. 3. SEM images of structures formed on the surface of the Zn probe inserted in the flame medium at Z = 11 mm with a 2 mm sleeve opening size; (a) shows that a high density of polyhedral crystal structures can be synthesized; (b) high resolution SEM image of a selected area in (a) shows that the larger polyhedral crystals have a protruding nanorod at their tips; (c) a close-up view shows that the larger crystals have a high number of facets with their edges well defined.
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Fig. 4. Structural and chemical composition analysis of the Zn oxide crystals formed in the flame medium; (a) low resolution TEM image of a nanorod with its tip gradually decreasing into the shape of pipet-like structure; (b) low resolution TEM image of a Zn oxide nanorod; (c) HR-TEM image of the same nano-rod (circled area in (b)) showing well ordered crystal structure with characteristic d-spacing corresponding to {0 0 2} planes; (d) EDS spectrum collected on the structure shows the presence of Zn and oxygen and SAED with diffraction spots confirming the crystallinity of the structure. The growth of the nanorod is in the [0 0 1] direction.
of temperature and oxygen on the flame synthesis of ZnO nanostructures correlates well with other complex thermal methods where it is shown that both temperature and oxygen content can greatly affect morphology and size of ZnO structures [2,3,24,44,45]. Grown nanorods on the tip of the ZMP crystals have length-todiameter aspect ratios in the range of 2–8, and have a growth rate of 5–15 nm/s. Zinc oxide structures possessing higher [30,46] and lower [47,48] aspect ratio can be found in literature. For TEM studies, samples were prepared by suspending the removed synthesized structures in methanol. The solutions were sonicated until the material was well dispersed in the solvent. A drop of the suspension was then placed on a copper substrate/carbon-film of the microscope specimen grid and dried. HR-TEM and SAED imaging of the grown structures reveals the structural uniformity and highly ordered crystalline structure as shown in TEM images in Fig. 4. Fig. 4(a) represents a low resolution TEM image of a nanarod whose diameter appears to gradually taper toward its tip to the shape of a pipet-like structure. TEM and HR-TEM reveals that the structures possess a high degree of perfection. Fig. 4(c) represents atomic resolution of a typical nanorod (from the circled area in Fig. 4(b)). The lattice fringes from TEM studies correspond to a d-spacing of approximately 0.26 nm that closely matches the (0 0 2) plane of hexagonal ZnO (Wurtzite) cell. It is well established in the literature that ZnO has three fast growth directions [0 0 0 1], [1 0 1 0] and [2 1 1 0] [49–52]. The work of Li et al. showed that the growth rate of the basal plane [0 0 0 1] of Zn oxide structures is preferential and it grows at much faster rates than the facet sides [53]. Once the facets appear, they do not disappear, crystal faces with this behavior cause the development of the gradual reduction in cross-sectional area along the growth direction and hence yielding the pointy tip morphology. Since no catalyst was employed in the experiments and also no condensate structures such as droplet can be observed in the SEM and/or TEM images, the growth mechanism of ZnO nanostructures can be explained through the vapor–solid (VS) mechanism. As for the VS mechanism, it is very typical that the nanorods or nanowires reduce gradually in diameter during growth and possess a sharp tip [48,1]. HR-TEM imaging of a typical nanorod at different points shows that they are free of dislocations and structural defects indicating that they consist of a single crystal. The HR-TEM analysis and SAED diffraction pattern with the diffraction spots confirms the crystallinity of the structure and growth of the Zn nanorod in the [0 0 1] direction, Fig. 4(b). EDS conducted on the synthesized products revealed that the structures are indeed composed of zinc and oxygen (Fig. 4(d)).
A gas adsorption analyzer (Micromeritics ASAP 2000) was employed to measure the surface area of the formed crystals based on the Brunauer–Emmett–Teller (BET) theory. The BET results show that the ZnO crystals synthesized in the flame have a surface area less than 1 m2 /g. In this work we show that ZnO nanorods and ZMP crystals with very high crystallinity are formed in a combustion method. Fig. 5 represents TEM and SEM images of grown crystals at the earlier, intermediate, and final stages. We propose that the growth mechanism is composed of two different processes: (1) the global growth mechanism which is responsible for the formation of vapors by the evaporation of the base metal as it is partially exposed to the flame medium. The Zn atoms are intercalated with oxygen atoms from the flame to form ZnO nuclei. (2) Once the nuclei are formed, elongated structures start to form through the vapor–solid (VS) growth mechanism. For the first step, the mechanism involves the partial interaction of a high purity Zn probe (in the form of a 1 mm diameter wire) with a high temperature flame that is rich in oxygen and oxygen radicals. It is important to note that the temperature of the 1 mm diameter probe is much higher closer to the tip of the sleeve where the flame–probe interaction takes place. That is, a high rate of zinc vapor is produced from the part of the probe that is exposed to the flame due to the low melting point of Zn which is ∼419 ◦ C. As the process continues Zn atoms join oxygen atoms from the flame to form ZnO. The Zn oxide is deposited on the surface of the probe and can act as nuclei to form elongated structures. A continuous influx of ZnO vapors can be further deposited on previously formed rods to further crystallize complex 1-D or 3-D structures. A recent work by Li et al. [53] reports the formation of ZnO micro- and nano-structures prepared by thermal evaporation that very closely resemble our flame synthesized ZnO structures reported here. In that study, the structures are grown in a tube chamber operating at ∼1000 ◦ C with various oxygen percentages. It is proposed that at the applied temperature condition Zn vapors are mass-produced from the Zn source material and later combine with the oxygen atoms in the gas-phase to form ZnO nuclei. In our previous works on the flame synthesis of TMO nanostructures we showed that metal oxide/hydroxides are first formed on one side of the base probe located in the higher temperature of the flame [54,55]. These oxides/hydroxides are further evaporated and transported by the gas flow to the lower temperature side of the probe where they are deposited in the form of transition metal oxide nanostructures. In the Zn oxide case, the melting and boiling point temperature of ZnO is approximately 1975 ◦ C and 2360 ◦ C, respectively. Therefore, our experimental results suggest that the
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Fig. 5. Proposed growth mechanism of ZnO nanostructures.
Zn in the vapor phase must combine with oxygen atoms and deposit along the surface of the probe to serve as nuclei and material source for the continuous growth of the ZnO structures. The density of metal and metal/oxide vapor and consequently the nuclei can be controlled by the simple relocation of the probe within the flame. Higher environment temperature results in higher Zn vapor formation thus higher density of nuclei. Experiments conducted in vapor phase transport using Au-catalyzed Si substrates for the synthesis of ZnO nanostructures have shown that a wide variation in morphology of the synthesized structures can be obtained by a small variation of temperature [48]. In that study it was shown that at the growth temperature of 800 ◦ C mainly 1D rod-like ZnO structures are synthesized compared to larger hexagonally faceted 3D structures obtained by the increase of the growth temperature by only ∼100 ◦ C. The concentration of oxygen in the environment can be also used to yield ticker or slender ZnO structures [1]. The effect of variation of temperature for the growth of the Zn oxide structures is also very evident in our flame synthesis method. At the flame position of Z = 12 mm the local flame temperature generated by the counter-flow reactor is higher than that at Z = 11 mm by approximately 280 ◦ C [42]. In our experiments we can observe that a higher growth temperature (Z = 12 mm) results in the surface of the probe being covered with mostly 1D ZnO nanorods. However, in the lower part of the flame (Z = 11 mm) where the temperature is lower the probe is covered with ZMPs containing a high number of facets and a nanorod extruding from the tip. The effect of residence time has also been studied for the synthesis and morphology control of metal oxide channels in flames [54].
The flame temperatures and concentration of chemical species used to correlate with nanomaterial synthesis were predicted by employing a numerical simulation reported by Beltrame et al. using the same flame reactor and similar flame parameters used in that study [42]. The chemical kinetic model was developed by employing GRI-MECH 2.11, C6 chemistry and a soot formation mechanism. Also it is important to note that the temperature of the 1 mm probe used for Zn-oxide nanostructure synthesis is always lower than the local flame temperature due to the radiant and conductive heat losses. A probe inserted in the flame volume at Z = 11 mm results in lower Zn vapor supersaturation which leads to the formation of ZnO microprism (ZMP) crystals with a high number of facets and pointy tip morphology. Our results show that ZMP crystals of a few micrometers in length are first formed on the surface of the Zn probes at this flame position. SEM images also show that the tips of the ZMP crystals have a structural defect in the form of a nanometer sized hole (Fig. 5(a1)). The hole constitutes an instable site that is ideal for a second nucleation process [56]. As the growth proceeds and the probe is consumed, the diameter of the probe reduces. Consequently, the temperature of the probe increases, leading to a higher vapor supersaturation and to a level that favors the synthesis of 1D nanorods of high crystallinity extruding from the tip of the ZMP crystals. The arrow in the SEM image in Fig. 5(a1) points to the nanosized hole or indentation at the tip of a ZMP crystal revealing the early stage growth of the strutting nanorod (second nucleation). TEM analysis confirms that the nanorod emerging from the indentation (as observed by SEM) is indeed a part of the larger crystal,
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Fig. 6. (a) SEM image collected on the surface of a Zn probe reveals the presence of a high density of complex crystal formation. Arrows 1 and 2 point to several slender structures that have diamond and rectangular shape growing on the surface of a rod, respectively. Arrow 3 points to several cubic structures grown on the surface of a nanorod. Arrows 4–6 point to crystal bases that have pentagonal, circular, and rectangular cross sections; ((b)–(e)) higher magnification of selected areas in (a); ((f)–(j)) HR-SEM images showing that the ZMPs have a protruding nanorod at their tips.
Fig. 5(a2). That is, after sonication of the crystals in methanol, the nanorod in its earlier growth stage remained at the tip of the ZMP. It is not clear what activates the growth of the nanorods on the tip of the ZMPs, but it appears that the nanosized hole on the tip of the ZMP crystal serves as an auto-catalyst similar to the VLS growth model. Once the nucleation of the nanorod is initiated it continues to grow in a preferential direction away from the tip of the ZMP crystal. Close inspection of the SEM image (Fig. 5(b1)) reveals that the size of the ZMP crystals remained the same (black arrow) but the length of the protruding rods at the tips of the crystals significantly increased as pointed out by a white arrow. The protruding rods are of aspect ratio resembling solid pipet-like structures. The TEM image, Fig. 5(b2) clearly shows that the base and the tip both form a single crystal. The pipet-like structures are modified to needlelike nanorods due to the continuous deposition on their surfaces by ZnO species, Fig. 5(c). Close inspection shows that the crosssectional area at the base of the pipet-like structures (Fig. 5(b1) and (b2)) coincides with the base of the needle-like structures (Fig. 5(c1) and (c2)). Further deposition of the ZnO vapors on the surface of existing structures results in the transformation of structures, Fig. 5(d). The SEM image, Fig. 5(d1), shows structures with larger bases and pointy tips as highlighted by black arrows. The nanorods have lengths of several microns and a gradual reduction in diameter. They also appear to be flexible, for instance, the
structure in the inset in Fig. 5(d1) has a “U” shape curve/bend at its tip which shows that some of the ZnO structures can have very small bending radius. The black arrows in Fig. 5(d2) highlight needle-like structures and the white arrows highlight the nanorods protruding from the tips of the needle-like structures. Fig. 5(d3) is a close up view of a selected area in Fig. 5(d2) highlighting the transformation from needle-like to nanorod structure. The flame formed ZMPs and nanorods have close aspect ratios in comparison to similar structures in the literature [43] suggesting that the growth is mandated by a certain growth mechanism rather than the method used. The hypothetical growth mechanism of the ZnO structures discussed above shows that a continuous presence of ZnO vapors (source) are necessary to form and transform the shapes of the crystals. The precipitation of ZnO on the surface of the activated rods is necessary for continuing growth of the structures. That is, the initial structures themselves (rods) play the role of support for the deposition of ZnO vapors to transform into crystals of various shapes. The rate and availability of ZnO vapors can significantly change the morphology of the structures. To support the above mechanism additional experiments were conducted by increasing the residence time of the probe/sleeve inside the flame volume. By increasing the residence time of the probe (∼3 min), the amount of ZnO vapors inside the sleeve increases and this should lead to crystal formation containing
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Fig. 7. TEM image of a nanobead-like structure with corresponding SAED and EDS spectra in selected crystal locations “rod” and “cubic”.
different characteristics on the surface of the Zn probe. Indeed, the formation of only well faceted large crystals or ZMPs with a short nanorod protruding from the tip was not observed in the experiment results performed at Z = 11 mm with a sleeve diameter of 2 mm. Instead, other unique complex crystal structures are formed (Fig. 6). Further crystallization is formed by vapor deposition directly on the nanocrystal surfaces to form more complex structures. It can be observed (Fig. 6) that some of the elongated rods have several slender polyhedral structures attached to their surfaces as if decorating the protruding nanorod. The arrows 1 and 2 in Fig. 6(a1) point to diamond and rectangular shaped structures attached to the surface of two different nanorods, respectively. Arrow 3 points to several cubic structures attached to the surface of a single nanorod. Arrows 4–6 point to ZMPs that have pentagonal, circular, and rectangular cross-sections, respectively. The hypothesis is further supported by noting that the rods are independent from the “decorative” polyhedral structures. The TEM images in Fig. 6(b) and (c) show a rod outside the polyhedral and partially embedded at its base as pointed out by the arrows. This characteristic is much more clearly observed in Fig. 6(f) and (g) where slender polyhedral structures are fused diagonally to the surface of a nanorod. The rods have a darker contrast compared to the slender polyhedral structures. It appears that the vapor–solid mechanism and finite size of the formed eutectic droplets along with a low driving force of crystal growth results in small cross-sectional size polyhedral structures. The low driving force of the crystal formation can be defined as a ratio of the chemical potential difference between two phases over temperature. The direct deposition of smaller crystals on the surface of the nanorod is evident as pointed out by the arrow in Fig. 6(d). The nucleation of the polyhedral structures appears to occur simultaneously at several locations on the surface of the formed nanorods due to the continuous precipitation of ZnO vapors. Fig. 6(e) shows a nanorod with several decorative cubical structures with a sudden direction change of ∼90◦ . The similar bending of a rod with earlier deposition of the “decorative” structures is evident in Fig. 6(a) (set of white arrows) suggesting that a further influx of ZnO vapor will lead to a mature structure similar to the one shown in Fig. 6(e). Fig. 6(f)–(j) shows that the ZMPs all have a nanorod protruding from the indentation (darker contrast) present at the center on their tips as pointed out by the arrows. Fig. 7(a)–(e) shows TEM images, SAED, and EDS spectra of a nanorod with several cubic crystals attached to its surface forming a “nanobead-like” structure. It is interesting to note that even after several minutes of sonication, which is required for TEM analysis, the small cubical structures and the rod forming the “nanobead-like structure” remained together resembling a single crystal, Fig. 7(a). The EDS spectra in Fig. 7(d) and (e) are of the nanorod and of a
cubical crystal, respectively. The EDS analysis using a focused electron beam with approximately the same diameter as that of the nanobead-like structure shows the presence of Zn, O, and C. The signal of the carbon peak in the EDS spectrum taken across the entire structure is very high and it is coming from the film supporting the nanobead-like structure, Fig. 7(a) and (d). EDS spectrum taken with a focused electron beam with approximately the same diameter as the nanorod shows that the carbon peak intensity is nonexistent, Fig. 7(e). However, the Zn peak is quite high. The SAED collected from both, the nanorod (arrow in the inset Fig. 7d) and the polyhedral structure (arrow in the inset Fig. 7e), show that they are nearly identical (Fig. 7(b) and (c)). This association is quite analogous to our proposed mechanism that the initial structures themselves play the role of supports for the deposition of Zn oxide vapors to transform into various solid shapes.
4. Conclusions The synthesis of ZnO nanocrystals is performed using Zn probes inserted in a counter-flow flame medium formed using methane/acetylene and oxygen-enriched air streams. The source material is introduced in the form of a solid wire through a sleeve. For the synthesis of the ZnO nanocrystals the sleeve containing the zinc probe is introduced in a region of the flame that is rich in O, OH, O2 , and other oxidative species. The shape and structural parameters of the grown crystals strongly depend on the flame position. The variation of flame position from Z = 12 mm (were the predicted flame temperature is ∼2500 ◦ C) to Z = 11 mm (predicted flame temperature 2100 ◦ C) lead to modification of crystal morphology from short thick based ZMPs with a short protruding nanorod at their tips to very elongated nanorods. The diameters of the protruding nanorods in both cases are less than 100 nm. However, the physical characteristics of the crystal bases remained approximately the same. The proposed growth mechanism of the nanocrystals is composed of two different processes: (i) the global growth mechanism which is responsible for the formation of vapors by the evaporation of the base metal as it is partially exposed to the flame medium. The Zn atoms are intercalated with oxygen atoms from the flame to form ZnO nuclei, and (ii) once the nuclei are formed elongated structures start to form through the vapor–solid (VS) growth mechanism. This mechanism is indirectly confirmed by increasing the residence time of the wire/sleeve inside the flame volume. This strongly suggests that the initial structures themselves (rods) play the role of support for the deposition of Zn oxide vapors to transform into various solid shapes. The protruding nanorods have slender polyhedral structures that have diamond, rectangular, and cubical shape as if decorating the nanorods.
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Moien Farmahini-Farahani is Ph.D. Candidate at the University of Oklahoma, major in mechanical engineering. He is currently doing research on flame synthesis of transition metal oxides nano-structures.
