- Email: [email protected]
Flame synthesis of molybdenum oxide whiskers
Chemical Physics Letters 422 (2006) 72–77 www.elsevier.com/locate/cplett
Flame synthesis of molybdenum oxide whiskers Wilson Merchan-Merchan b
a,b
, Alexei V. Saveliev
b,*
, Lawrence A. Kennedy
b
a Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 2039 ERF, m/c 251, 842 West Taylor Street, Chicago, IL 60607, USA
Received 17 January 2006; in final form 16 February 2006 Available online 10 March 2006
Abstract The spectacular growth of molybdenum oxide hollow and solid whiskers with rectangular and circular cross-section is reported. The synthesis is performed using molybdenum probes inserted in an opposed-flow methane oxy-flame. The solid rectangular and circular whiskers with characteristic cross-sectional dimensions from 100 nm to 4 lm and hollow rectangular channels with wall thickness from 50 to 100 nm are grown on 1-mm diameter probes inserted at the high temperature zone on the oxidizer side of the flame front. The shape and structural parameters of grown whisker materials strongly depend on the flame position (temperature) and probe diameter. Well-defined elongated crystal structures with a large number of facets and complex symmetry were grown on the probes with a diameter of 0.25 mm. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Flames have been successfully employed for growth of a variety of nano- and micro-materials such as fullerenes [1– 3], carbon nanotubes [4–8], carbon whiskers, diamond crystals [9–11] and other nanomaterials such as carbides and oxides of various metals [12,13]. In recent years, much effort has been devoted to the study of molybdenum oxides and related materials. It has been shown that molybdenum oxides possess unique catalytic and electronic properties and have potential applications in chemical synthesis, petroleum refining, recording media and sensors [14–17]. A variety of uses facilitates the development of novel synthesis methods for the generation of molybdenum oxides and related materials with desirable structure, chemical activity, and physical properties. For catalytic applications several techniques have been developed to synthesize nanodispersed molybdenum oxides directly on alumina and silica support surfaces [14]. A number of studies have been devoted to the growth of MoO2 nano-particles and wires on various substrates. Zhou and co-workers [17] have reported the formation of MoO2 nanowire arrays formed *
Corresponding author. Fax: +1 312 413 0447. E-mail address: [email protected] (A.V. Saveliev).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.02.040
by employing a high temperature heating process in a restricted vacuum chamber. Alignment, vertical orientation, and high degree of order are some of the physical characteristics of the formed nanowires. In this Letter, the flame synthesis of molybdenum oxide whiskers of various structures is reported. 2. Experimental The flame generated by the counter-flow burner (Fig. 1) is formed by two opposed gas streams; the fuel (96% CH4 + 4% C2H2) is supplied from the top nozzle and the oxidizer (50% O2 + 50% N2) is introduced from the bottom nozzle. The nozzles are separated by a distance of 2.54 cm. The fuel and oxidizer flows impinge against each other with a strain rate equal to 20 s1. A diffusion flame is established on the oxidizer side of the stagnation plane. Molybdenum probes were inserted into the flame zone parallel to the flame front at various distances Z from the center of the probe to the edge of the fuel nozzle. The molybdenum wires with a purity of 99.95% and diameters of 0.25 and 1 mm were used as the probes. The probes were exposed to the flame for time intervals from 30 s to 20 min. The formed materials were further analyzed using scanning and transmission electron
W. Merchan-Merchan et al. / Chemical Physics Letters 422 (2006) 72–77
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73
(b)
96%CH4 + 4%C2H2
Stagnation plane
0
10
2500
O
2
CH
2000
Protective Shield
Molybdenum Probe
Flame Front
-1
Mole fraction
Z
CO
4
CH
10
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T
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OH -2
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1000
Temperature, ºC
Exhaust
O 500
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0 5
N2
50%O2/ 50%N2
N2
10
15
20
Z, mm
Fig. 1. Opposed-flow flame used in synthesis of molybdenum whiskers: (a) schematic of the experimental apparatus, (b) numerical predictions of temperature profile and distribution of major chemical species in (96% CH4 + 4% C2H2)/(50% O2 + 50% N2) flame, the simulations are performed using multi-step reaction mechanism [3].
microscopy (SEM and TEM) methods. The details of the experimental setup and electron microscopy methods are described in the related studies on carbon nanomaterial synthesis [18–20]. Temperature of a small diameter probe inserted in a cross flow is defined by a balance of convection and radiation. Due to the intensive radiant heat losses the temperature of the probe can be lower than the flame temperature by a few hundred degrees. To account for this effect the temperature of the probe was directly measured by an optical pyrometer in each performed experiment. 3. Results and discussion The opposed-flow flame is characterized by strong axial gradients of temperature and chemical species (Fig. 1b). The temperature gradients reach 2000 K/cm and the chemical environment rapidly changes from a hydrocarbon-rich zone on the fuel side of the flame to the oxygenrich zone on the oxidizer side. As a result, the probe position in the flame strongly affects the synthesis processes. The results can also be affected by the diameter of the probe when it is comparable to the thickness of the flame front. Fig. 2 represents SEM images collected at different resolutions on the surface of a 1-mm diameter molybdenum probe exposed to the flame at a height of Z = 12.0 mm (wire temperature 1200 °C) for 2 min. At this particular flame position, the whiskers of rectangular and square cross-section are observed along with the circular ones (Fig. 2). The tips of rectangular structures exhibit small depressions of regular shape suggesting their crystalline structure. The tips of the circular whiskers have a domecapped shape suggesting the presence of liquid matter during the synthesis (Fig. 2b). In several areas, SEM analysis also revealed that some rectangular and circular whiskers have open tips (Fig. 2c) exhibiting their hollow structure.
Characteristic sizes of rectangular and circular objects were determined from several SEM images. The rectangular structures have side dimensions ranging from 0.5 to 4 lm. The circular structures appear to be much smaller; their diameters are less than 1 lm with an average diameter close to 0.33 lm. Some of the circular whiskers have characteristic nanoscale dimensions. The dominance of circular geometry at submicron and nanoscale range could be related to the small relative surface area and, hence, reduced surface energy. The tip structure also supports the vapor–liquid–solid growth mechanism. Under the influence of the surface tension, a liquid particle formed on the tip of the growing whisker acquires a semi-spherical shape that influences the formation of cylindrical whisker body. The repositioning of the 1-mm probe to the flame height of Z = 11.0 mm (wire temperature 1150 °C) led to the essential variation of the synthesized material structures. As in the previous case, the probe was exposed to the flame for a period of 2 min. Performed SEM analysis revealed that the surface of the probe is covered with densely packed micron-size structures that have shape of hollow rectangular channels (Fig. 3a). It is observed that the slender, prismatic, four-faced structures are completely hollow, the inside cavities are very large and devoid of other materials. The unique morphology with large cavities, nano-sized walls, sharp edges and high specific surface gives the structure a significant importance, for example, in medical applications. The large cavities can also be useful for a variety of applications including storage of liquids or nanoparticles, material reinforcements, or as a significant component in the fabrication of MEMS devices, etc. Aluminum-rich mullite structures with similar shapes have been grown by using a multi-step solid–vapor process by Xiang and co-workers [21]. The reported structures have perfectly rectangular cross-sectional areas and are completely hollow, very much resembling the structures studied
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Fig. 2. Representative SEM images collected on the surface of a Mo probe inserted at the flame height of 12 mm for 2 min: (a) a low resolution SEM image collected on the Mo surface, (b) higher resolution imaging analysis shows the presence of rectangular (1), square (2), and circular fiber-like structures (3,4) (c) some tips of the circular and rectangular structures are open showing the inner hollow cavities.
Fig. 3. Representative SEM images collected on a Mo probe inserted at the flame height of 11.0 mm for sampling time of 2 min: (a) SEM image shows the presence of channel-like structures with rectangular and square morphologies, the structures appear to be completely hollow, their corners are welldefined, with lengths slightly longer at one of the corners, as shown by arrows; (b) High resolution TEM image exhibiting the lattice structure on the edge of the channel with measured lattice distance of 0.36 nm corresponding to ð111Þ plane of a monoclinic MoO2; (c) EDX of elemental spectrum acquired using SEM.
here. It is suggested that the sharp-tip hollow structures can be useful for penetrating biological tissues for drug delivery at the microscale regime. SEM analysis of the probe surface reveals that the structures grown at the flame height of Z = 11.0 mm are substantially larger than the ones synthesized at Z = 12 mm. The rectangular side dimensions range from 1 to 7 lm with average of 3.2 lm. Another interesting physical characteristic present in some of the structures is that their length varies slightly at the tips creating sharp edges, as shown by arrows in Fig. 3a. High resolutions TEM, X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED) microscopy were employed to characterize the elemental composition and structure of the micro-channels.
For this analysis, the formed microstructures were transferred to a TEM copper-substrate/carbon film grid. HRTEM and SAED employed on the grown structures reveals the structural uniformity and highly ordered crystalline structure as shown in Fig. 3b. It can also be observed that the wall of the rectangular whisker is free of dislocations and structural defects. A lattice spacing of 0.36 nm was measured; it corresponds to ð111Þ plane of a monoclinic MoO2 cell. The SAED collected from the walls of the structure confirmed the high degree of crystallinity present in the material by reflecting the regular pattern of diffraction spots (Fig. 3b, insert). EDS analysis conducted on the asgrown structures revealed the presence of carbon, oxygen, and molybdenum (Fig. 3c). The carbon readings are attributed to the carbon tape holding the probe. Generally, EDS
W. Merchan-Merchan et al. / Chemical Physics Letters 422 (2006) 72–77
analysis can be employed as an excellent tool for obtaining the elemental information of materials; however, it provides largely inconclusive information on the chemical composition due to the large difference in the sensitivities for molybdenum and oxygen. A very recent work by Suemitsu and co-workers [13] reports the formation of molybdenum oxide hollow fiber crystals possessing rectangular cross-sections. The physical and chemical characteristics of those crystals closely resemble the rectangular structures reported here. In the Suemitsu study, it was reported that the rectangular structures were formed on the surface of a Mo substrate exposed to an oxygen–acetylene combustion flame for a period of 90 min. SEM images were collected at various probe locations in order to gain insights into the mechanism of molybdenum whisker growth. Fig. 4 shows characteristic distribution of the whisker deposits on the surface of the molybdenum probe. The probe with a diameter of 1 mm inserted in the flame near the flame front is exposed to the strong temperature gradients and variations of the chemical composition. The low velocity gas flow in this location is directed from the oxidizer nozzle to the stagnation plane. The recorded distribution of deposited materials suggests the transfer of molybdenum oxides by the gas flow from the side of the wire exposed to the high temperature oxidizing environment (T1) and their further crystallization on the wire surfaces downstream at a region of lower temperature (T2), see Fig. 4. The molybdenum wire inserted in the flame medium is oxidized from the oxidizer side forming several characteristic oxide layers [22]. The oxidizer side of the probe posi-
Fuel
Probe Cross-sectional Area
T2
Deposits
MoO2
MoO2
MoO3
...
..
. .. .
.
MoO3
T1
O, O2 Fig. 4. Mechanism of MoO2 whisker growth on 1-mm molybdenum wire inserted into the opposed-flow flame, T1 > T2.
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tioned at Z = 12 mm is exposed to the flame environment containing 5 mol% of O2 and 1 mol% of O at flame temperature of 2500 °C. The corresponding numbers for the probe position of 11 mm are 0.2%, 0.2%, and 2400 °C (Fig. 1b). A layer of MoO2 is first formed coating the Mo substrate, followed by a layer containing different molybdenum-oxides MoxO3x1 and finally by an outmost layer of MoO3. The MoO3 is relatively volatile with a melting point and sublimation temperature of 795 and 1155 °C, respectively. The evaporated MoO3 is transported by the gas flow along the wire surface. Its deposition downstream is affected by two main factors: the lower temperature of the environment and change of the chemical flame composition. It should be noted, that due to intense radiative losses the wire temperature is always lower than the local flame temperature. The high thermal conductivity of the wire also contributes to the reduction of temperature gradients. However, the temperature drop along the wire surface can be sufficient to induce condensation of the evaporated material. This factor is strongly augmented by the chemical transformation of MoO3 to MoO2 as the flame environment changes from oxidizing to reducing across the flame front. Further whiskers growth can proceed following vapor– liquid–solid or vapor–solid mechanism depending on temperature and other process parameters. At high temperatures, liquid drops of eutectic composition are formed on the tips of the growing whiskers; they serve as intermediates that accept new material from the gas phase and convert it to crystals. The characteristic structures of the tips observed on SEM images partially confirm this hypothesis (Fig. 2). The vapor–liquid–solid mechanism and finite size of formed liquid droplets along with a low driving force of crystal growth (usually defined as a ratio of the chemical potential difference between two phases over temperature [23]) result in small cross-sectional sizes of the grown whiskers. At lower temperature (Z = 11 mm) direct deposition from the gas phase takes place. The driving force increases and the growth instability leads to preferential deposit of new material along the edges of the growing whisker (Berg effect) producing characteristic narrow wall structures with large cross-sectional sizes [23]. Only a limited amount of material can diffuse to the center of the whisker; as a result the center remains hollow. To support the above mechanism additional experiments were conducted with 0.25-mm diameter probes. The small diameter of the probes results in temperature and chemical composition that is practically uniform along their surfaces. Indeed, the formation of rectangular and circular whiskers was not observed in the experiments performed at flame positions of 11 and 12 mm. Instead, at flame position of 10 mm (wire temperature 1100 °C), well developed elongated crystal structures with a high number of facets and complex symmetry were generated. Fig. 5 represents low resolution SEM images collected on the surface of the 0.25-mm probe; it shows the structures of various shapes and symmetries densely covering the probe.
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Fig. 5. Representative SEM images collected on 0.25-mm diameter Mo probe inserted at the flame height of 10.0 mm for 2 min sampling time: (a) a group of very symmetrical structures containing very unusual tips, (b) a group of structures with different facets with very sharp tips, see arrows, (c) the synthesis of micron-size 3-dimensional structures, (d) EDS of elemental spectrum acquired using SEM; this particular spectrum was collected from the material depicted on the SEM insert.
The structures are well aligned and appear to grow perpendicular to the surface of the probe. Their lengths are from 10 to 30 lm with cross-sections from 1 to 5 lm. The structures have very uniform shape and are highly symmetrical. All of the present structures have an intriguing similarity in the shape of their tips. The tips of the structures have a triangular shape (Fig. 5, insets). It is observed that on certain types of the grown structures the cross-sectional area at the middle of the objects tends to be largest and decreases at the bases and tips. Small plates are present near the roots of the crystals suggesting development of dendritic structures. Fig. 5a represents a SEM image showing a group of structures with very unusual shape and symmetry. Another structural type is shown in Fig. 5b. These structures have a high number of facets with pointy tip morphology (Fig. 5b, insets). Fig. 5c shows highly elongated triangular structures. These structures tend to form multiple crystalline plates extending from the triangular edges at the root of their bodies. The plates can further develop into independent crystals forming highly developed dendritic networks (Fig. 5d, insets). Analysis of elemental composition using EDS shows the presence molybdenum, oxygen, and car-
bon. The recorded molybdenum to oxygen ratio is higher in comparison to rectangular and circular whiskers. The intensity of the carbon peak in the spectrum is also increased and can be produced both by the carbon film holder and by the carbon present in the structure itself. The mechanism of growth for the various types of structures reported here is not completely understood and more extensive studies are under way in order to comprehend their structure, properties, and synthesis process. 4. Conclusions The synthesis of molybdenum oxide hollow and solid whiskers with rectangular and circular cross-section is performed using molybdenum probes inserted in opposed-flow flame formed by methane/acetylene and oxygen enriched air streams. The solid rectangular and circular whiskers with characteristic cross-sectional dimensions from 100 nm to 4 lm and hollow rectangular channels with wall thickness from 50 to 100 nm are grown on 1-mm diameter probes inserted at the high temperature zone on the oxidizer side of the flame front. The shape and structural parameters of grown whisker materials strongly depend
W. Merchan-Merchan et al. / Chemical Physics Letters 422 (2006) 72–77
on the flame position and probe diameter. The variation of flame position of 1-mm probe from Z = 12 mm (wire temperature 1200 °C) to Z = 11 mm (wire temperature 1150 °C) lead to modification of whisker morphology from solid to hollow and was accompanied by a large size increase. The proposed mechanism of whisker growth involves transfer of molybdenum in the form of MoO3 from the side of the probe exposed to the high temperature oxidizer stream to the low temperature reducing atmosphere of the fuel side where molybdenum species are crystallized in the form of MoO2 whiskers. This mechanism is indirectly confirmed by the use of small diameter probes; the rectangular and circular whisker structures are not observed in this case at the same flame positions. Instead, well-defined elongated crystal structures with a large number of facets and complex symmetry were grown on the probes with a diameter of 0.25 mm at flame position Z = 10 mm (wire temperature 1100 °C). Acknowledgements The support by the National Science Foundation (grant CTS-0304528) is gratefully acknowledged. The authors wish to extend special thanks to Dr. Alan Nicholls, Mr. John Roth, Ms. Linda Juarez, Ms. Kristina Jarosius, and Mr. Kyle Tabor for their day-to-day assistance, help in SEM and TEM studies, and helpful discussions. References [1] J.B. Howard, J.T. McKinnon, Y. Makarovsky, A.L. Lafleur, M.E. Johnson, Nature 352 (1991) 139.
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[2] W.J. Grieco, J.B. Howard, C.L. Rainey, J.B. Vander Sande, Carbon 38 (2000) 597. [3] M. Silvestrini, W. Merchan-Merchan, H. Richter, A. Saveliev, L.A. Kennedy, Proc. Combust. Inst. 30 (2005) 2545. [4] J.B. Howard, D.K. Chowdhury, J.B. Vander Sande, Nature 370 (1994) 603. [5] L. Yuan, L. Tianxiang, S. Kozo, Carbon 41 (2003) 1889. [6] L. Yuan, K. Saito, W.Z. Hu, Chem. Phys. Lett. 346 (2001) 23. [7] R.L. Vander Wal, Chem. Phys. Lett. 324 (2000) 217. [8] R.L. Vander Wal, Carbon 40 (2002) 2101. [9] T. Abe, M. Suemitsu, J. Crystal Growth 183 (1998) 183. [10] K. Okada, S. Komatsu, T. Ishigaki, S. Matsumoto, Y. Moriyoshi, J. Crystal Growth 116 (1992) 307. [11] T. Abe, M. Suemitsu, N. Miyamoto, N. Sato, J. Appl. Phys. 73 (1993) 971. [12] M. Suemitsu, T. Abe, H.-J. Na, H. Yamane, Jpn. J. Appl. Phys. 44 (2005) L449. [13] T. Abe, M. Suemitsu, N. Miyamoto, J. Appl. Phys. 74 (5) (1993) 3531. [14] N.A. Dhas, A. Gedanken, Chem. Mater. 9 (1997) 3144. [15] M. Anpo, M. Kondo, Y. Kubokawa, C. Louis, M. Che, J. Chem. Soc., Faraday Trans. 1 (84) (1988) 771. [16] Y. Liu, Y. Qian, M. Zhang, Z. Chen, C. Wang, Mater. Res. Bull. 31 (1996) 1029. [17] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Chem. Phys. Lett. 382 (2003) 443. [18] A. Saveliev, W. Merchan-Merchan, L.A. Kennedy, Combust. Flame 135 (2003) 27. [19] W. Merchan-Merchan, A.V. Saveliev, L.A. Kennedy, Carbon 42 (2004) 599. [20] W. Merchan-Merchan, A.V. Saveliev, L.A. Kennedy, A.A. Fridman, Chem. Phys. Lett. 354 (2002) 20. [21] X.Y. Kong, Z.L. Wang, J. Wu, Adv. Mater. 15 (2003) 1445. [22] T. Saburi, H. Murata, T. Suzuki, Y. Fujii, K. Kiuchi, J. Plasma Fusion Res. 78 (2002) 3. [23] I. Sunagawa, Crystals. Growth, Morphology and Perfection, Cambridge University Press, Cambridge, 2005, p. 74.
Flame synthesis of molybdenum oxide whiskers Wilson Merchan-Merchan b
a,b
, Alexei V. Saveliev
b,*
, Lawrence A. Kennedy
b
a Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 2039 ERF, m/c 251, 842 West Taylor Street, Chicago, IL 60607, USA
Received 17 January 2006; in final form 16 February 2006 Available online 10 March 2006
Abstract The spectacular growth of molybdenum oxide hollow and solid whiskers with rectangular and circular cross-section is reported. The synthesis is performed using molybdenum probes inserted in an opposed-flow methane oxy-flame. The solid rectangular and circular whiskers with characteristic cross-sectional dimensions from 100 nm to 4 lm and hollow rectangular channels with wall thickness from 50 to 100 nm are grown on 1-mm diameter probes inserted at the high temperature zone on the oxidizer side of the flame front. The shape and structural parameters of grown whisker materials strongly depend on the flame position (temperature) and probe diameter. Well-defined elongated crystal structures with a large number of facets and complex symmetry were grown on the probes with a diameter of 0.25 mm. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Flames have been successfully employed for growth of a variety of nano- and micro-materials such as fullerenes [1– 3], carbon nanotubes [4–8], carbon whiskers, diamond crystals [9–11] and other nanomaterials such as carbides and oxides of various metals [12,13]. In recent years, much effort has been devoted to the study of molybdenum oxides and related materials. It has been shown that molybdenum oxides possess unique catalytic and electronic properties and have potential applications in chemical synthesis, petroleum refining, recording media and sensors [14–17]. A variety of uses facilitates the development of novel synthesis methods for the generation of molybdenum oxides and related materials with desirable structure, chemical activity, and physical properties. For catalytic applications several techniques have been developed to synthesize nanodispersed molybdenum oxides directly on alumina and silica support surfaces [14]. A number of studies have been devoted to the growth of MoO2 nano-particles and wires on various substrates. Zhou and co-workers [17] have reported the formation of MoO2 nanowire arrays formed *
Corresponding author. Fax: +1 312 413 0447. E-mail address: [email protected] (A.V. Saveliev).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.02.040
by employing a high temperature heating process in a restricted vacuum chamber. Alignment, vertical orientation, and high degree of order are some of the physical characteristics of the formed nanowires. In this Letter, the flame synthesis of molybdenum oxide whiskers of various structures is reported. 2. Experimental The flame generated by the counter-flow burner (Fig. 1) is formed by two opposed gas streams; the fuel (96% CH4 + 4% C2H2) is supplied from the top nozzle and the oxidizer (50% O2 + 50% N2) is introduced from the bottom nozzle. The nozzles are separated by a distance of 2.54 cm. The fuel and oxidizer flows impinge against each other with a strain rate equal to 20 s1. A diffusion flame is established on the oxidizer side of the stagnation plane. Molybdenum probes were inserted into the flame zone parallel to the flame front at various distances Z from the center of the probe to the edge of the fuel nozzle. The molybdenum wires with a purity of 99.95% and diameters of 0.25 and 1 mm were used as the probes. The probes were exposed to the flame for time intervals from 30 s to 20 min. The formed materials were further analyzed using scanning and transmission electron
W. Merchan-Merchan et al. / Chemical Physics Letters 422 (2006) 72–77
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(b)
96%CH4 + 4%C2H2
Stagnation plane
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O
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CO
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CH
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OH -2
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1000
Temperature, ºC
Exhaust
O 500
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10
0 5
N2
50%O2/ 50%N2
N2
10
15
20
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Fig. 1. Opposed-flow flame used in synthesis of molybdenum whiskers: (a) schematic of the experimental apparatus, (b) numerical predictions of temperature profile and distribution of major chemical species in (96% CH4 + 4% C2H2)/(50% O2 + 50% N2) flame, the simulations are performed using multi-step reaction mechanism [3].
microscopy (SEM and TEM) methods. The details of the experimental setup and electron microscopy methods are described in the related studies on carbon nanomaterial synthesis [18–20]. Temperature of a small diameter probe inserted in a cross flow is defined by a balance of convection and radiation. Due to the intensive radiant heat losses the temperature of the probe can be lower than the flame temperature by a few hundred degrees. To account for this effect the temperature of the probe was directly measured by an optical pyrometer in each performed experiment. 3. Results and discussion The opposed-flow flame is characterized by strong axial gradients of temperature and chemical species (Fig. 1b). The temperature gradients reach 2000 K/cm and the chemical environment rapidly changes from a hydrocarbon-rich zone on the fuel side of the flame to the oxygenrich zone on the oxidizer side. As a result, the probe position in the flame strongly affects the synthesis processes. The results can also be affected by the diameter of the probe when it is comparable to the thickness of the flame front. Fig. 2 represents SEM images collected at different resolutions on the surface of a 1-mm diameter molybdenum probe exposed to the flame at a height of Z = 12.0 mm (wire temperature 1200 °C) for 2 min. At this particular flame position, the whiskers of rectangular and square cross-section are observed along with the circular ones (Fig. 2). The tips of rectangular structures exhibit small depressions of regular shape suggesting their crystalline structure. The tips of the circular whiskers have a domecapped shape suggesting the presence of liquid matter during the synthesis (Fig. 2b). In several areas, SEM analysis also revealed that some rectangular and circular whiskers have open tips (Fig. 2c) exhibiting their hollow structure.
Characteristic sizes of rectangular and circular objects were determined from several SEM images. The rectangular structures have side dimensions ranging from 0.5 to 4 lm. The circular structures appear to be much smaller; their diameters are less than 1 lm with an average diameter close to 0.33 lm. Some of the circular whiskers have characteristic nanoscale dimensions. The dominance of circular geometry at submicron and nanoscale range could be related to the small relative surface area and, hence, reduced surface energy. The tip structure also supports the vapor–liquid–solid growth mechanism. Under the influence of the surface tension, a liquid particle formed on the tip of the growing whisker acquires a semi-spherical shape that influences the formation of cylindrical whisker body. The repositioning of the 1-mm probe to the flame height of Z = 11.0 mm (wire temperature 1150 °C) led to the essential variation of the synthesized material structures. As in the previous case, the probe was exposed to the flame for a period of 2 min. Performed SEM analysis revealed that the surface of the probe is covered with densely packed micron-size structures that have shape of hollow rectangular channels (Fig. 3a). It is observed that the slender, prismatic, four-faced structures are completely hollow, the inside cavities are very large and devoid of other materials. The unique morphology with large cavities, nano-sized walls, sharp edges and high specific surface gives the structure a significant importance, for example, in medical applications. The large cavities can also be useful for a variety of applications including storage of liquids or nanoparticles, material reinforcements, or as a significant component in the fabrication of MEMS devices, etc. Aluminum-rich mullite structures with similar shapes have been grown by using a multi-step solid–vapor process by Xiang and co-workers [21]. The reported structures have perfectly rectangular cross-sectional areas and are completely hollow, very much resembling the structures studied
74
W. Merchan-Merchan et al. / Chemical Physics Letters 422 (2006) 72–77
Fig. 2. Representative SEM images collected on the surface of a Mo probe inserted at the flame height of 12 mm for 2 min: (a) a low resolution SEM image collected on the Mo surface, (b) higher resolution imaging analysis shows the presence of rectangular (1), square (2), and circular fiber-like structures (3,4) (c) some tips of the circular and rectangular structures are open showing the inner hollow cavities.
Fig. 3. Representative SEM images collected on a Mo probe inserted at the flame height of 11.0 mm for sampling time of 2 min: (a) SEM image shows the presence of channel-like structures with rectangular and square morphologies, the structures appear to be completely hollow, their corners are welldefined, with lengths slightly longer at one of the corners, as shown by arrows; (b) High resolution TEM image exhibiting the lattice structure on the edge of the channel with measured lattice distance of 0.36 nm corresponding to ð111Þ plane of a monoclinic MoO2; (c) EDX of elemental spectrum acquired using SEM.
here. It is suggested that the sharp-tip hollow structures can be useful for penetrating biological tissues for drug delivery at the microscale regime. SEM analysis of the probe surface reveals that the structures grown at the flame height of Z = 11.0 mm are substantially larger than the ones synthesized at Z = 12 mm. The rectangular side dimensions range from 1 to 7 lm with average of 3.2 lm. Another interesting physical characteristic present in some of the structures is that their length varies slightly at the tips creating sharp edges, as shown by arrows in Fig. 3a. High resolutions TEM, X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED) microscopy were employed to characterize the elemental composition and structure of the micro-channels.
For this analysis, the formed microstructures were transferred to a TEM copper-substrate/carbon film grid. HRTEM and SAED employed on the grown structures reveals the structural uniformity and highly ordered crystalline structure as shown in Fig. 3b. It can also be observed that the wall of the rectangular whisker is free of dislocations and structural defects. A lattice spacing of 0.36 nm was measured; it corresponds to ð111Þ plane of a monoclinic MoO2 cell. The SAED collected from the walls of the structure confirmed the high degree of crystallinity present in the material by reflecting the regular pattern of diffraction spots (Fig. 3b, insert). EDS analysis conducted on the asgrown structures revealed the presence of carbon, oxygen, and molybdenum (Fig. 3c). The carbon readings are attributed to the carbon tape holding the probe. Generally, EDS
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analysis can be employed as an excellent tool for obtaining the elemental information of materials; however, it provides largely inconclusive information on the chemical composition due to the large difference in the sensitivities for molybdenum and oxygen. A very recent work by Suemitsu and co-workers [13] reports the formation of molybdenum oxide hollow fiber crystals possessing rectangular cross-sections. The physical and chemical characteristics of those crystals closely resemble the rectangular structures reported here. In the Suemitsu study, it was reported that the rectangular structures were formed on the surface of a Mo substrate exposed to an oxygen–acetylene combustion flame for a period of 90 min. SEM images were collected at various probe locations in order to gain insights into the mechanism of molybdenum whisker growth. Fig. 4 shows characteristic distribution of the whisker deposits on the surface of the molybdenum probe. The probe with a diameter of 1 mm inserted in the flame near the flame front is exposed to the strong temperature gradients and variations of the chemical composition. The low velocity gas flow in this location is directed from the oxidizer nozzle to the stagnation plane. The recorded distribution of deposited materials suggests the transfer of molybdenum oxides by the gas flow from the side of the wire exposed to the high temperature oxidizing environment (T1) and their further crystallization on the wire surfaces downstream at a region of lower temperature (T2), see Fig. 4. The molybdenum wire inserted in the flame medium is oxidized from the oxidizer side forming several characteristic oxide layers [22]. The oxidizer side of the probe posi-
Fuel
Probe Cross-sectional Area
T2
Deposits
MoO2
MoO2
MoO3
...
..
. .. .
.
MoO3
T1
O, O2 Fig. 4. Mechanism of MoO2 whisker growth on 1-mm molybdenum wire inserted into the opposed-flow flame, T1 > T2.
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tioned at Z = 12 mm is exposed to the flame environment containing 5 mol% of O2 and 1 mol% of O at flame temperature of 2500 °C. The corresponding numbers for the probe position of 11 mm are 0.2%, 0.2%, and 2400 °C (Fig. 1b). A layer of MoO2 is first formed coating the Mo substrate, followed by a layer containing different molybdenum-oxides MoxO3x1 and finally by an outmost layer of MoO3. The MoO3 is relatively volatile with a melting point and sublimation temperature of 795 and 1155 °C, respectively. The evaporated MoO3 is transported by the gas flow along the wire surface. Its deposition downstream is affected by two main factors: the lower temperature of the environment and change of the chemical flame composition. It should be noted, that due to intense radiative losses the wire temperature is always lower than the local flame temperature. The high thermal conductivity of the wire also contributes to the reduction of temperature gradients. However, the temperature drop along the wire surface can be sufficient to induce condensation of the evaporated material. This factor is strongly augmented by the chemical transformation of MoO3 to MoO2 as the flame environment changes from oxidizing to reducing across the flame front. Further whiskers growth can proceed following vapor– liquid–solid or vapor–solid mechanism depending on temperature and other process parameters. At high temperatures, liquid drops of eutectic composition are formed on the tips of the growing whiskers; they serve as intermediates that accept new material from the gas phase and convert it to crystals. The characteristic structures of the tips observed on SEM images partially confirm this hypothesis (Fig. 2). The vapor–liquid–solid mechanism and finite size of formed liquid droplets along with a low driving force of crystal growth (usually defined as a ratio of the chemical potential difference between two phases over temperature [23]) result in small cross-sectional sizes of the grown whiskers. At lower temperature (Z = 11 mm) direct deposition from the gas phase takes place. The driving force increases and the growth instability leads to preferential deposit of new material along the edges of the growing whisker (Berg effect) producing characteristic narrow wall structures with large cross-sectional sizes [23]. Only a limited amount of material can diffuse to the center of the whisker; as a result the center remains hollow. To support the above mechanism additional experiments were conducted with 0.25-mm diameter probes. The small diameter of the probes results in temperature and chemical composition that is practically uniform along their surfaces. Indeed, the formation of rectangular and circular whiskers was not observed in the experiments performed at flame positions of 11 and 12 mm. Instead, at flame position of 10 mm (wire temperature 1100 °C), well developed elongated crystal structures with a high number of facets and complex symmetry were generated. Fig. 5 represents low resolution SEM images collected on the surface of the 0.25-mm probe; it shows the structures of various shapes and symmetries densely covering the probe.
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Fig. 5. Representative SEM images collected on 0.25-mm diameter Mo probe inserted at the flame height of 10.0 mm for 2 min sampling time: (a) a group of very symmetrical structures containing very unusual tips, (b) a group of structures with different facets with very sharp tips, see arrows, (c) the synthesis of micron-size 3-dimensional structures, (d) EDS of elemental spectrum acquired using SEM; this particular spectrum was collected from the material depicted on the SEM insert.
The structures are well aligned and appear to grow perpendicular to the surface of the probe. Their lengths are from 10 to 30 lm with cross-sections from 1 to 5 lm. The structures have very uniform shape and are highly symmetrical. All of the present structures have an intriguing similarity in the shape of their tips. The tips of the structures have a triangular shape (Fig. 5, insets). It is observed that on certain types of the grown structures the cross-sectional area at the middle of the objects tends to be largest and decreases at the bases and tips. Small plates are present near the roots of the crystals suggesting development of dendritic structures. Fig. 5a represents a SEM image showing a group of structures with very unusual shape and symmetry. Another structural type is shown in Fig. 5b. These structures have a high number of facets with pointy tip morphology (Fig. 5b, insets). Fig. 5c shows highly elongated triangular structures. These structures tend to form multiple crystalline plates extending from the triangular edges at the root of their bodies. The plates can further develop into independent crystals forming highly developed dendritic networks (Fig. 5d, insets). Analysis of elemental composition using EDS shows the presence molybdenum, oxygen, and car-
bon. The recorded molybdenum to oxygen ratio is higher in comparison to rectangular and circular whiskers. The intensity of the carbon peak in the spectrum is also increased and can be produced both by the carbon film holder and by the carbon present in the structure itself. The mechanism of growth for the various types of structures reported here is not completely understood and more extensive studies are under way in order to comprehend their structure, properties, and synthesis process. 4. Conclusions The synthesis of molybdenum oxide hollow and solid whiskers with rectangular and circular cross-section is performed using molybdenum probes inserted in opposed-flow flame formed by methane/acetylene and oxygen enriched air streams. The solid rectangular and circular whiskers with characteristic cross-sectional dimensions from 100 nm to 4 lm and hollow rectangular channels with wall thickness from 50 to 100 nm are grown on 1-mm diameter probes inserted at the high temperature zone on the oxidizer side of the flame front. The shape and structural parameters of grown whisker materials strongly depend
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on the flame position and probe diameter. The variation of flame position of 1-mm probe from Z = 12 mm (wire temperature 1200 °C) to Z = 11 mm (wire temperature 1150 °C) lead to modification of whisker morphology from solid to hollow and was accompanied by a large size increase. The proposed mechanism of whisker growth involves transfer of molybdenum in the form of MoO3 from the side of the probe exposed to the high temperature oxidizer stream to the low temperature reducing atmosphere of the fuel side where molybdenum species are crystallized in the form of MoO2 whiskers. This mechanism is indirectly confirmed by the use of small diameter probes; the rectangular and circular whisker structures are not observed in this case at the same flame positions. Instead, well-defined elongated crystal structures with a large number of facets and complex symmetry were grown on the probes with a diameter of 0.25 mm at flame position Z = 10 mm (wire temperature 1100 °C). Acknowledgements The support by the National Science Foundation (grant CTS-0304528) is gratefully acknowledged. The authors wish to extend special thanks to Dr. Alan Nicholls, Mr. John Roth, Ms. Linda Juarez, Ms. Kristina Jarosius, and Mr. Kyle Tabor for their day-to-day assistance, help in SEM and TEM studies, and helpful discussions. References [1] J.B. Howard, J.T. McKinnon, Y. Makarovsky, A.L. Lafleur, M.E. Johnson, Nature 352 (1991) 139.
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