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Noble metals supported on carbon nanotubes using supercritic

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Athirah Syamila

on 16 November 2015

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Transcript of Noble metals supported on carbon nanotubes using supercritic

Carbon Nanotube (CNT)
A Journal Review
Noble metals supported on carbon nanotubes using supercritical fluids for the preparation of composite materials: A look at the interface
This paper presents a look at the CNTs-metal interface as a function of the functionalization technique.
1) Raw CNTs

2) CNTs functionalized with the reference HNO3/H2SO4 method.

3) CNTs functionalized with our approach using CO2/EtOH/H2O/H2O2 under temperature and pressure.
Three types of CNTs are considered for deposition:
Small size, high chemical stability and large surface area.
CNTs have also been considered as new supports for metal catalysts.
Nur Syamila Athirah bt Mohd Noor
1121149
Three main techniques are used to synthesize carbon nanotubes:
- Arc discharge
- Laser ablation
- Chemical vapour deposition (CVD)
Arc discharge technique has been used to synthesize fullerenes for the first time and has been historically used by Ijima.
The drawbacks of arc discharge and laser ablation techniques are the low quantities of CNTs obtained because of the size limitations of the carbon source and the formation of byproducts requiring a purification step.
However, the method commonly used leads to bundles made of hundreds of tangled single nanotubes with rather inert surface.
This implies:
- a lack of dispersibility of CNTs in the inorganic matrix and
- weak interactions between metal and CNTs.
To solve this problem, functionalization techniques have attracted a huge interest.
The interface between CNTs and metallic or inorganic materials is important.
Defects may decrease the composite properties.
Organic and inorganic CNTs functionalization appears to be a good mean for overcoming the lack of dispersibility and optimizing interfaces.
The functionalization techniques are to remove the metallic particles used as catalysts for the synthesis of CNTs.
A treatment with a mixture of HNO3 and H2SO4 at 80 °C seems to be the most efficient method, but it requires further purification and drying steps of CNTs because of the treatment occurs in solution.
Recently, a mixture of CO2/EtOH/H2O/H2O2 is used at high pressure and high temperature.
Precious metals as Ru, Rh, Pd, Ag, Au, Pt : The metallic precursors can be first added to the carbon source used for the carbon growth, but this route implies harsh conditions.
Another way is the deposition of metallic nanoparticles on the walls of the CNTs.
This can be carried out in solution but the yield of the reaction is low and an important amount of waste is generated.
Alternative approaches with supercritical carbon dioxide and water have been used as green media for the synthesis of new materials and especially CNTs-metal composite materials.
According to the nature of the deposited metal, the homogeneity and the size of the coating can vary.
Thus Ti, Ni and Pd form continuous and quasi-continuous coating while Au, Al and Fe form only discrete particles on CNTs surface.
To understand these behaviors, studies at the interface level are necessary.
Only few characterizations of this CNTs-metal interface have been reported from chemical and crystallographic point of views.
Some calculations have been realized and allow explaining the experimental data.
One can however wonder if the CNTs-metal interface depends on the functionalization method and/or deposition process.
The aim of this paper is to report a simple, rapid and green procedure to homogeneously decorate CNTs with metal nanoparticles (Ag, Pd) and to fully characterize the deposition and the CNTs-metal interface as a function of the functionalization technique.
Procedure
1) The experiments were performed using a 50 cm3 stirred stainless steel vessel reactor operating up to 400 °C and 40 MPa. The used mixture is 95/5 M CO2/EtOH mixture (Tc = 55 °C, pc = 9.5 MPa).

3) CNTs were dispersed in ethanol containing the palladium precursor (hexafluoroacetylacetonate of palladium) or the silver precursor ((1,5-cyclooctadiene) hexafluoroacetylacetonate of silver); this solution was added into the high pressure and high temperature reactor.

4) The reactor was closed, filled with hydrogen and then carbon dioxide and reaches the operating conditions.

5) After 1 h reaction time, pure supercritical CO2 flows through the reactor to remove ethanol and organic residues from metallic precursor.

6)The dried CNTs were then recovered without any additional filtering step.
7) Samples for transmission electron microscopy (TEM) were prepared by suspending the CNTs powder in alcohol by ultrasonication and depositing a drop of the suspension on a copper grid covered with a carbon film.

8) The grid was finally air-dried for 15 min. TEM and high-resolution TEM (HRTEM) observations as well as Energy Dispersive Spectroscopy (EDX) analyses were performed using a Jeol 2200FS equipped with a field emission gun operating at 200 kV and with a point resolution of 0.23 nm.

9) High-resolution TEM micrographs were acquired with a Gatan Ultrascan CCD 2k*2k and digital diffractograms were calculated using the Gatan Digital Micrograph software. Moreover, in order to be representative and statistically meaningful, several images from several regions of a sample were recorded and the most characteristic results are presented here.
10) The tilt series for electron tomography were acquired on a JEOL 2100F transmission electron microscope equipped with a field emission gun operating at 200 kV, a spherical aberration probe (Cs) corrector and a GATAN tridiem energy filter.

11) The acquisition software used to record the tilt series was Digital Micrograph. Several bright field TEM tilt series were acquired for each specimen between 70° and −70°, with a tilt increment given by a 2° Saxton scheme. A total number of about 101 TEM 2048*2048 pixel images were recorded during 45 min and no beam damage was observed during the acquisition.

12) The image alignment and volume reconstruction were performed by using the IMOD software [51] whereas the quantitative analysis and the 3D visualization of the reconstructed volumes have been done by using ImageJ and 3D Slicer software.
On these XRD patterns, the wide peak around 26° is attributed to CNTs whereas peaks at 40, 47 and 68° to palladium. One can see that these peaks are growing with the targeted quantity of palladium which means that more and more palladium is deposited. However, as far as deposition on raw CNTs is concerned, an additional peak can be observed at 34° which corresponds to the formation of PdO. These results show that functionalization of CNTs seems to have a strong effect on the deposition nature.
RESULT
The size of metallic particles has been determined from the XRD patterns using the Scherrer formula. The average size is around 10 nm for all samples but for raw CNTs this value increases to 25 nm when more palladium is deposited. This effect is not visible on functionalized CNTs. The functionalization seems to be useful to reduce the size of deposited particles by favoring nucleation of new particles instead of growth of existing particles.
The size of metallic particles has been determined from the XRD patterns using the Scherrer formula. The average size is around 10 nm for all samples but for raw CNTs this value increases to 25 nm when more palladium is deposited. This effect is not visible on functionalized CNTs. The functionalization seems to be useful to reduce the size of deposited particles by favoring nucleation of new particles instead of growth of existing particles.
CONCLUSION
This study has demonstrated that Pd and Ag can be successfully deposited on CNTs using supercritical CO2/EtOH mixture. The quality and nature of the deposit strongly depend on the functionalization technique of CNTs. The depositions on raw CNTs lead to palladium oxide whereas the one on HNO3/H2SO4-CNTs leads to the formation of Ag2SO4 and PdSO4. The chemistry of the interface has been highlighted by XPS: oxygen bond formed between palladium and CNTs. From a crystallographic point of view HRTEM observations have shown that {1 1 1} planes of Pd are parallel to graphite sheets and the relative arrangement of palladium and carbon atoms can be deduced from the distances between them in these planes. Finally, the deposition in supercritical CO2/EtOH offers a uniform deposition of metallic nanoparticles of about 10 nm on the CNTs inside the bundles as demonstrated by electron tomography. They are generally made by the agglomeration of 4 or 5 smaller nanoparticles. The increase of precursor quantity should allow to fill the bundles for which a high volume fraction of vacuum remains.
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