WO2007087708A1

WO2007087708A1 - Organic nanofluids, method and reactor for synthesis thereof

Info

Publication number: WO2007087708A1
Application number: PCT/CA2007/000122
Authority: WO
Inventors: Sylvain Coulombe; Jason Tavares
Original assignee: Mcgill University
Priority date: 2006-01-31
Filing date: 2007-01-30
Publication date: 2007-08-09

Abstract

A method and reactor for in-situ synthesis, stabilization and dispersion of nanoparticles in a organic host fluid, and nanofluids containing nanoparticles that are coated in-situ with a surface layer compatible with the organic host fluid.

Description

TITLE OF THE INVENTION

Organic nanofluids, method and reactor for synthesis thereof

FIELD OF THE INVENTION

[0001] The present invention relates to organic nanofluids. More specifically, the present invention is concerned with organic nanofluids, and a method and a reactor for synthesis thereof.

BACKGROUND OF THE INVENTION

[0002] Nanofluids are two-phase mixtures comprising a continuous phase, consisting of a liquid host, and a dispersed phase of nanoparticles.

[0003] Nanofluids are made by suspending nanoscale particles of materials such as carbon-based structures like fullerenes and carbon nanotubes (CNT), bare metal or metal oxides such as copper and copper oxide respectively in liquids such as oil, water and radiator fluid (a mixture mostly of water and ethylene glycol). Such adding nanoscale particles to the fluids leads to enhanced or new heat transfer, electric, magnetic and/or optical properties.

[0004] In US patent 6,221 ,275, Choi et al. disclose a method for introducing to a fluid particles having thermal conductivities higher than the thermal conductivity of the fluid. In this method, a stabilization step is required, whereby a stabilizing agent is added to the nanofluid obtained.

SUMMARY OF THE INVENTION

[0005] More specifically, in accordance with the present invention, there is provided an integrated vacuum reactor for fabricating organic nanofluids containing an organic host fluid and nanoparticles that are coated in-situ with a surface layer compatible with the organic host fluid, comprising: a nanoparticle synthesis region including a solid source and an energy source to produce a high-density cloud of vapors from the solid source; a nanoparticle nucleation and growth region, in which a cold inert gas supersaturates the cloud of vapors and transports as-formed nanoparticles away from the nanoparticle synthesis region; a coating region, where a uniform and electrodeless glow discharge plasma is generated and plasma polymerization occurs onto a surface of incoming nanoparticles; and a contact region, comprising a film of the host fluid, receiving a stream of coated nanoparticles and gas, where the coated nanoparticles come into contact with the host fluid, yielding an organic nanofluid containing the organic host fluid and coated nanoparticles.

[0006] There is further provided a method for in-situ synthesis, stabilization and dispersion of nanoparticles in a host fluid comprising the steps of producing a high density cloud of vapors; quenching the vapors with an inert cooling gas, thereby forming nano-sized particles; in-flight coating the nano-sized particles by plasma polymerization with a surface layer compatible with the host fluid; and dispersing the coated nanoparticles in the host fluid into an organic nanofluid.

[0007] There is further provided an organic nanofluid comprising an organic fluid and a suspension of in-situ surface-stabilized nanoparticles, the nanoparticles being coated in-situ with a stabilization layer compatible with the organic host fluid.

[0008] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the appended drawings:

[0010] Figure 1 is a schematic view of a first embodiment of a reactor according to a first aspect of the present invention;

[0011] Figure 2 is a schematic view of a second embodiment of a reactor according to the first aspect of the present invention;

[0012] Figure 3 is a flowchart of a method according to an embodiment of an aspect of the present invention;

[0013] Figure 4 is a FE-SEM image of uncoated copper nanoparticles, collected over a fine polymeric filter, showing significant agglomeration;

[0014] Figure 5 (A) is a FE-SEM image of copper nanoparticles coated with an organic layer, produced by a reactor similar to that of Figure 1 , and collected over a fine polymeric filter, showing a very limited agglomeration; (B) and (C) are FE-SEM images of individual copper nanoparticles coated with an organic layer, the dark core corresponding to copper, the organic layer appearing translucent; and

[0015] Figure 6 is a FTIR spectrum of a collection of copper nanoparticles coated with an organic layer.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0016] According to an aspect of the present invention, there is provided an integrated vacuum reactor for fabricating organic nanofluids.

[0017] The reactor is capable of operating at reduced pressures, between 1 and 200 torr for example.

[0018] As illustrated in the embodiments of Figures 1 and 2 of the appended drawings, the reactor 20 comprises a nanoparticle synthesis region 22; a nanoparticle nucleation and growth region 24; a nanoparticle surface functionalization and coating region 26; a nanoparticles/ host fluid contact region 28; and, optionally, a nanofluid recirculation circuit 30.

[0019] In the nanoparticle synthesis region 22, a high-density metal vapor cloud is generated by erosion of a cathode 21 under a low-pressure, pulsed or nonpulsed, electric arc.

[0020] The cathode may be a metal cathode or a carbon cathode.

[0021] Pulsed electric arcs can be self-triggered using a capacitor charging power supply unit, whereby an arc is initiated every time the voltage across an inter-electrode gap exceeds the breakdown voltage of the gap. Alternatively, an ablation laser may trigger the arcs, the pulses of the laser producing micro- plasmas upon impact with the cathode, when using metal or carbon/metal cathodes. Still alternatively, a powerful laser (pulsed CO₂ laser) may be used to ablate a solid target and to form a dense cloud of metallic or carbon/metal vapors, or a high-temperature furnace (temperature range between 500 and 650 ⁰C) may be used to sublimate fullerenes C₆o from a fullerenes-rich extract. [0022] Synthesis of carbon nanotubes can be done by a cathodic arc evaporation process using a carbon cathode containing the metal catalyst particles, such as Ni, Ni/Cu alloys, etc.

[0023] Synthesis of carbon nanotubes can also be done by a combined cathodic arc evaporation/condensation process, whereby a cloud of vapors is generated from a metal catalyst cathode, and carbon nanotubes are synthesized onto the metal nanoparticles with a gaseous hydrocarbon, in an in-flight thermal chemical vapor deposition (CVD) process. In this case, the synthesis region 22 comprises a thermal CVD part where the metal nanoparticles are used as host catalysis sites for the synthesis of the CNT from the hydrocarbon gas. The thermal CVD part comprises heated walls to maintain the desired reaction temperature.

[0024] The cathodic arc evaporation/condensation process for the synthesis of catalyst nanoparticles can be replaced by using a fine resistively heated wire which is also known to lead to the formation of fine nanoparticles by nucleation/condensation of the metal atoms released from its surface.

[0025] In the nanoparticle nucleation and growth region 24, a cold inert gas is injected for supersaturating the vapors, thereby forcing the nucleation process, and transporting the as-formed nanoparticles away from their source.

[0026] In the in-flight coating formation region 26, an uniform and electrodeless glow discharge plasma is generated. This plasma operates with various gases, injected into the in-flight coating formation region 26, including organic vapors, in order to favor a plasma polymerization process onto the surface of the nanoparticles. The plasma can be sustained by capacitive or inductive coupling. Radio-frequency excitation of 13.56 MHz for example has been successfully tested at reactor pressures of about 20 torr or less. Audio frequencies of a few kHz or less may be used as well, especially at higher reactor pressures.

[0027] The stream of nanoparticles and gas flows through the in-flight coating formation region 26 downstream of the point of injection of the various gases including organic vapors.

[0028] The host organic liquid, injected in the in-flight coating formation region 26, or downstream from this region, forms a stable film flowing on the reactor inside walls, in the case of a vertical reactor as illustrated in Figure 1 , or forms a flowing film on the bottom inner wall of an horizontal reactor as illustrated in Figure 2.

[0029] These inside walls of the vertical reactor or this bottom inner wall of the horizontal reactor provide a contact region 28 between the nanoparticles and the host liquid.

[0030] As it is transported by convection, along the axis of a tubular vertical reactor for example, the stream of coated nanoparticles and gas diffuses by thermophoresis and normal diffusion towards the inner surface of the reactor, where the coated nanoparticles come into contact with the organic host fluid.

[0031] A nanofluid recirculation circuit 30, consisting of a circulation pump, a reservoir, and corresponding tubing for example, may be provided to enrich the nanofluid to a target level, in an iterative process.

[0032] The geometry and dimensions of the reactor can be adapted based on the specific application of interest, and the scale of the production. It is found that a vertically mounted tubular reactor as illustrated in Figure 1 is simple and efficient by providing an increased contacting surface, the entire reactor assembly may be mounted vertically.

[0033] The reactor may also be mounted so that the contacting surface is horizontal, as illustrated in Figure 2.

[0034] A method according to an embodiment of another aspect of the present invention, as shown in Figure 3, comprises the generation of nanoparticle precursors (step 110); the homogeneous nucleation and growth of nanoparticles (step 120); the functionalization and coating of the surface of the nanoparticles (step 130); and dispersion of the nanoparticles in an organic host fluid (step 140).

[0035] Optionally, the method further comprises recirculating the obtained organic nanofluid (step 150), to enrich the organic nanofluid to a target level.

[0036] In step 110, a range of solid precursors may be used, including metals, in the case when metal-organic liquid nanofluids are desired, and carbon, in the case when fullerenes and CNT-organic liquid nanofluids are desired, for example.

[0037] For the synthesis of nanofluids containing coated metal nanoparticles or CNT, in step 110 a cloud of metal vapors is generated from the erosion of a cathode 21 by an electric arc, in the nanoparticle synthesis region 22 of a reactor. Operating the arc at chamber pressures in a range between about 1 and about 10 torr (range between about 133 and about 1333 Pa) ensures compatibility with the subsequent step of surface functionalization of the nanoparticles (step 130), and favors high cathode erosion rates. The erosion rates and consequently, the flux of metal vapors leaving the cathode surface, are controlled through the arc current. [0038] A pulsed-arc configuration 23 can be used in order to minimize the thermal load to the cathode and to maximize the instantaneous current, i.e. erosion rate. The electric arc can be initiated by gaseous breakdown with a self-triggering capacitor circuit or by laser triggering.

[0039] For CNT production according to cathodic arc evaporation process described hereinabove, a graphite cathode hosting metal inclusions may be used for example, formed metal nanoparticles providing catalytic surfaces for the growth of CNT.

[0040] Alternatively, an ablation laser such as a pulsed CO2 laser may be used to produce the cloud of vapors.

[0041] Alternatively, a resistively heated metal filament may be used to produce the cloud of vapors.

[0042] The high-density cloud of vapors thus generated is transported away from the solid source thereof and cooled by a stream of inert gas flowing in the inter-electrode gap defined by the cathode 21 and anode 25. The erosion/evaporation products are transported downstream of the source, nano- sized particles being formed by the rapid quenching of the vapors, as a result of the supersaturation and homogeneous nucleation phenomena.

[0043] In the case of CNT synthesis by the cathodic arc process, the inert cooling gas is preferably helium and the formed metal nanoparticles act as host catalyst sites for the growth of the CNT.

[0044] In the case of CNT synthesis by the combined cathodic arc (or hot filament) and thermal CVD process discussed hereinabove, an inert gas such as argon can be used for the supersaturation of the metal vapor clouds and a gaseous hydrocarbon can be used for precursor for the growth by thermal CVD of CNT onto the metal nanoparticles.

[0045] The formed nanoparticles or CNT are then transported into a coating region of the reactor for in-flight coating, where a radio frequency (RF) capacitively coupled glow discharge plasma is maintained under reduced pressure conditions, between about 1 and about 50 torr for example. This low pressure sustains a stable, large volume uniform glow discharge plasma medium allowing efficient plasma polymerization.

[0046] Other types of electrical energy transfer to the polymerization plasma may be used, such as inductive coupling, as well as higher reactor pressures, providing that the plasma remains diffuse and fills the available volume.

[0047] The monomer gas used in the plasma polymerization process may originate from two sources: either from the vapor of the organic host liquid present in the reactor, or from the injection of a foreign gas that mixes with the already present organic gas.

[0048] The method allows an in-situ stabilization of the nanoparticles, whereby the nanoparticles are coated, in-situ, by plasma polymerization with a surface layer compatible with the organic host fluid. Such coating forms a solid- liquid interlayer acting as a stabilizing agent for the suspension of nanoparticles in the host fluid, thus eliminating the need of a further step of stabilization by using a stabilizing foreign chemical which properties are likely to degrade under real operating conditions and/or to affect the performance of the resulting nanofluid.

[0049] The method allows a control of the size of the nanoparticles, of the chemical composition and thickness of the coating, and the weight content of nanoparticles in the nanofluid.

[0050] The reactor pressure, the inert gas flow rate and the length of quenching zone may be adjusted to control the size of the metal nanoparticles.

[0051] The composition of the host fluid or foreign gas may be selected to control the composition of the coating.

[0052] The RF plasma power, length of plasma polymerization zone, and content in plasma polymerization gas are used to control the thickness of the coating.

[0053] The weight content of thenanoparticles into the host fluid is impacted by the flow rate of the recirculation loop and processing time.

[0054] The method may be performed in continuous, and allows scalability while yielding high throughput.

[0055] The method may be used to synthesize organic nanofluids containing surface-coated metal nanoparticles and also fullerenes- and CNT- based nanofluids.

[0056] When using a vertical reactor as illustrated in Figure 1 of the appended drawings, the organic host fluid is injected through a flange separating the nanoparticle synthesis region 22 from the in-flight coating region 26, and forms a stable film falling on the reactor inside wall. The organic vapor liberated from the organic host fluid, and the foreign gas in the case of the injection of a foreign gas as discussed hereinabove, reach the glow discharge plasma area of the reactor, where they are excited and dissociated by electron-impact collisions. Such process leads to the formation of active chemical fragments that deposit onto the surface of the nanoparticles. This plasma polymerization process on the surface of the nanoparticles leads to the formation of a dense organic coating. Such coating stabilizes the nanoparticles by reducing their surface free energy and enhances the compatibility of the nanoparticles with the organic host fluid.

[0057] As shown in Figure 1 , the host fluid may be injected downstream of the plasma polymerization region of the reactor (see arrow A), instead of upstream thereof (see arrow B), in which case direct contact of the polymerization plasma with the host fluid is avoided thus limiting the extent of plasma-induced chemical reactions in the organic host fluid.

[0058] Further downstream in a vertical reactor, a falling film of the organic liquid is exposed to the stream of coated nanoparticle and gas. In the case of a horizontal contact zone, a film of the organic liquid flowing on the inner bottom wall of the reactor is exposed to the stream of coated nanoparticles and gas. In both cases, this leads to the dispersion of the nanoparticles into the organic fluid.

[0059] The nanofluid thus produced may be re-circulated in a recirculation circuit until a target loading of nanoparticles is reached.

[0060] As mentioned hereinbefore, the vapor of the organic host fluid alone may be used as the precursor gas for the organic film formation. The expected range of temperatures for the falling liquid film is comprised in a range between about 25 and about 8O⁰C. As an indicator of the vapor pressures involved, ethylene glycol (C₂H₆O₂), a well-known organic heat transfer fluid, has a vapor pressure of about 0.06 torr at 25⁰C while it reaches about 0.7 torr at 5O⁰C. Such vapor pressure range gives rise to dilution ratios with the inert gas of the order of a few %, which is adequate for the plasma polymerization process. Using organic fluids such as methanol and ethanol for example may be contemplated, and involves operating the reactor at significantly higher pressures due to the higher vapor pressures of these organic fluids.

[0061] In the case where the host fluid is a polar organic solvent, additional gaseous chemical compounds known to produce polar functionalities by plasma polymerization might be contemplated as precursors.

[0062] According to an embodiment of still a further aspect of the present invention, there is provided an organic nanofluid comprising a low-vapor pressure organic fluid, such as ethylene glycol for example, and a suspension of metal nanoparticles, carbon nanotubes or fullerenes, coated in-situ with a thin stabilization layer compatible with the organic host fluid. Such a coating is likely to maintain its stabilizing properties under real use conditions, thereby avoiding the need of a surfactant.

[0063] Fine metal nanoparticles show strong optical activity, in particular in terms of absorptivity, in the UV and visible ranges. Such property can be used for the development of liquid-based optical filters for solar-panel applications and windows, increasingly powerful dye lasers, and random lasers (lasers which do not require an optical cavity for the light amplification) for example. Furthermore, nanofluids containing magnetic nanoparticles, such as iron for example, could be pumped with magnetic fields (no need for mechanical pumps) and used as tracer fluids.

[0064] As shown with nanoparticles coated with a dense organic layer, in

Figures 4 and 5, the coating on the nanoparticles limits or eliminates the agglomeration of the nanoparticles. In Figure 6, peaks at 2953 cm^"1, 2924 cm^"1 and 2855 crrf¹ of the FT-IR spectrum are associated to saturated C-H stretch bonds, a peak at 1454 cm^"1 to a C-H bending bond and a peak at 1361 cm^"1 to the C-H bending bond of the CH₃ group. No unsaturated C-H and C-C bonds are observed in the spectrum. The coating therefore presents a macromolecular organic structure, in contrast to a graphite-like structure.

[0065] The present nanofluids containing surface-stabilized nanoparticles,

C₆₀ fullerenes or CNT having superior properties under real operating conditions may be used in the field of heat transfer for example, and allows reductions in heat exchanger size, storage needs, pumping capacity, devices' volume and mass. Due to the increasing demand on engines and microelectronic devices, heat transfer fluids with considerably enhanced heat conductivity are required.

[0066] Such nanofluids may also find uses in optics, as filters (UV-VIS) and lasing media (random lasers) for example. Magnetic nanofluids (ex. with Fe nanoparticles) may be used with electromagnetic pumps and as biofluids.

[0067] As people in the art will appreciate, the present nanofluids may therefore find a range of applications as heat transfer fluids (ex. automotive and chemical processing industries, microelectronic and microfluidic devices, magnetically-controllable fluids (ferrofluids), optics (ex. filters and absorbers, random lasers), biomedical engineering (ex. tracer fluids and fluid vectors).

[0068] Although the present invention has been described hereinabove by way of specific embodiments thereof, it can be modified, without departing from the nature and teachings of the subject invention as described herein.

Claims

WHAT IS CLAIMED IS:

1. A integrated vacuum reactor for fabricating organic nanofluids containing an organic host fluid and nanoparticles that are coated in-situ with a surface layer compatible with the organic host fluid, comprising: a nanoparticle synthesis region including a solid source and an energy source to produce a high-density cloud of vapors from said solid source; a nanoparticle nucleation and growth region, in which a cold inert gas supersaturates the cloud of vapors and transports as-formed nanoparticles away from the nanoparticle synthesis region; a coating region, where a uniform and electrodeless glow discharge plasma is generated and plasma polymerization occurs onto a surface of incoming nanoparticles; and a contact region, comprising a film of the host fluid, receiving a stream of coated nanoparticles and gas, where the coated nanoparticles come into contact with the host fluid, yielding an organic nanofluid containing the organic host fluid and coated nanoparticles.

2. The reactor of claim 1 , wherein said energy source if one of: i) an erosion source and ii) an evaporation source.

3. The reactor of claim 1 , further comprising a nanofluid recirculation circuit.

4. The reactor of claim 1 , said regions being located successively along a vertical axis of said reactor, the host organic liquid, injected at one of: i) upstream and ii) downstream of the coating region, forming a stable film flowing on inside walls of the reactor in a downstream section of the reactor, said inside walls forming said contact region.

5. The reactor of claim 1 , having a tubular geometry.

6. The reactor of claim 1 , wherein said solid source is one of: i) a metal cathode, ii) a carbon cathode, and iii) a carbon/metal cathode; said energy source being selected in the group consisting of: low-pressure electric arcs; arcs triggered by an ablation laser; an ablation laser; a high-temperature furnace; and a resistively heated metal filament.

7. The reactor of claim 1 , wherein said nanoparticle synthesis region further comprises a thermal chemical vapor deposition (CVD) part, the solid source being a metal catalyst cathode, the cloud of vapors obtained containing metal nanoparticles, carbon nanotubes (CNT) being synthesized onto the metal nanoparticles with an hydrocarbon gas by in-flight thermal chemical vapor deposition (CVD).

8. The reactor of claim 1 , wherein gases, including organic vapors, are injected into said coating region.

9. The reactor of claim 7, wherein the thermal CVD part is provided with heated walls, the metal nanoparticles being used as host catalysis sites for the synthesis of CNT from the hydrocarbon gas.

10. The reactor of claim 1 , operating at pressures comprised in the range between 1 and 200 torr.

1 1 . A method for in-situ synthesis, stabilization and dispersion of nanoparticles in a host fluid, comprising the steps of producing a high density cloud of vapors; quenching the vapors with an inert cooling gas in a quenching zone thereby forming nano-sized particles; in-flight coating the nano-sized particles by plasma polymerization with a surface layer compatible with the host fluid; and dispersing the coated nanoparticles in the host fluid into an organic nanofluid.

12. The method of claim 11 , wherein said step of producing a high- density cloud of vapors comprises one of: i) eroding a solid source using an electric arc; ii) ablating a solid source using a laser; iii) sublimation a solid source in a high-temperature furnace; and iv) thermal evaporation of a filament.

13. The method of claim 1 1 , wherein said step of producing a high density cloud of vapors comprises using solid precursors selected in the group consisting of metals and carbon.

14. The method of claim 11 , wherein said step of producing a high density cloud of vapors comprises using solid metal precursors, the nano-sized particles formed in the quenching zone being metallic, and the host fluid being an organic liquid.

15. The method of claim 1 1 , wherein said step of producing a high density cloud of vapors comprises using carbon as precursors, the nano-sized particles formed in the quenching zone being ones of: fullerenes and CNT, the host fluid being an organic liquid.

16. The method of claim 11 , wherein said step of producing a high density cloud of vapors comprises using a graphite cathode hosting metal inclusions, the nano-sized particles formed in the quenching zone being metal nanoparticles acting as a catalytic surface for the growth of CNT.

17. The method of claim 16, wherein the inert cooling gas is helium.

18. The method of claim 1 1 , wherein said step of producing a high density cloud of vapors comprises using a metal catalyst cathode to generate a metal vapor cloud, the nano-sized particles formed in the quenching zone being metal nanoparticles; carbon nanotubes (CNT) being synthesized onto the metal nanoparticles with an hydrocarbon gas by in-flight thermal chemical vapor deposition (CVD).

19. The method of claim 18, wherein the inert cooling gas is argon.

20. The method of claim 1 1 , wherein a glow discharge plasma is maintained under reduced pressure conditions.

21. The method of claim 20, wherein the glow discharge plasma is maintained under a pressure comprised in the range between 1 and 50 torr.

22. The method of claim 11 , wherein a stable, large volume uniform glow discharge plasma is maintained under a pressure comprised in the range between 1 and 50 torr.

23. The method of claim 1 1 , wherein said plasma polymerization process uses monomer gas originating from at least one of: i) vapors of the organic host liquid, and ii) vapors of the organic host liquid together with addition of an injected foreign gas.

24. The method of claim 1 1 , wherein said plasma polymerization process forms a coating on the surface of the nano-sized particles.

25. The method of claim 1 1 , comprising controlling the size of the nano-sized particles by adjusting at least one of: i) the reactor pressure, ii) the inert gas flow rate, and iii) a length of the quenching zone.

26. The method of claim 23, comprising controlling a chemical composition of the surface layer compatible with the host fluid by selecting at least one of: i) the composition of the host fluid and ii) the composition of the foreign gas.

27. The method of claim 11 , comprising controlling a chemical composition of the surface layer compatible with the host fluid by selecting the composition of the host fluid.

28. The method of claim 11 , comprising controlling the thickness of the surface layer compatible with the host fluid by adjusting at least one of: i) the plasma power; ii) the length of the plasma polymerization zone; and ii) the content in plasma polymerization gas.

29. The method of claim 11 , further comprising the step of enriching the obtained organic nanofluid to a target level.

30. The method of claim 29, comprising controlling a weight content of the nanoparticles in the nanofluid by adjusting the flow rate of a recirculation loop and processing time.

31 . The method of claim 11 , for producing nanofluids containing surface-coated metal nanoparticles and fullerenes- and CNT-based nanofluids.

32. An organic nanofluid comprising an organic fluid and a suspension of surface-stabilized nanoparticles, the nanoparticles being coated in- situ with a stabilization layer compatible with the organic host fluid.

33. The organic nanofluid of claim 32, wherein said organic fluid is an organic solvent.

34. The organic nanofluid of claim 32, wherein said organic fluid is one of: i) ethylene glycol; ii) methanol and iii) ethanol.

35. The organic nanofluid of claim 32, wherein said suspension of nanoparticles comprises one of: i) metal nanoparticles, ii) metal oxides, iii) carbon nanotubes and iv) fullerenes.