Synthesis and characterization of catalytic shells self assembled onto nano-sized SnO2 cores
C.A. Moina 1, L.B. Fraigi2, A. Weinstock2 1Instituto Nacional de Tecnología Industrial, INTI- Electrodeposición y Procesos Superficiales, Argentina.
2 Instituto Nacional de Tecnología Industrial, INTI- Electrónica e Informatica, Argentina.
*C. Moina, Phone : +5411 4754 , email: moina@inti.gov.ar
Abstract
SnO2 nanopowders were surface modified with derivatives of the propionic acid. 3-mercaptopropionic, 2mercaptopropionic, 3-aminopropionic and 2-amino-3-mercaptopropionic acids were used as modifiers. Pt nanoparticles were self-assembled on the functionalized SnO2. The self-assembling of the metallic nanoparticles was confirmed by FTIR and TEM. The attachment of the Pt clusters to the functionalized oxide was strongly dependent of the molecular linker. The differences in reactivity can be explained in terms of the structural characteristics of the surface complex formed.
Keywords: Metal nanoparticles additivated SnO2, functionalized SnO2, gas sensors
Introduction
Catalytic additives are widely employed to improve the sensing characteristics of oxide-based solid-state gas sensors. The most frequently used additives are noble metals such as Pt, Pd and Au. In thick film techniques, several wet methods to introduce the additives in the sensing oxide matrix have been proposed, most of which are based in two basic procedures: a) impregnation of the oxide with a salt solution of the additive followed by a heat treatment [1]; and b) introduction of a salt of the additive during the synthesis of the oxide [2,3]. In both cases the additive is introduced as a molecular species (PtCl6-, PdCl2, etc.), which undergoes a chemical change to its final form, presumably metallic clusters, during a pyrolitic process.
Recently we have proposed a new method to attach catalytic nanoparticles to the surface of SnO2 nanocrystals [4]. Briefly, in a two steps procedure a catalytic shell is formed on the surface of an oxide core. The shell consists of a self-assembled monolayer of a molecular linker, which provides anchorage to the catalytic metallic clusters. The core-shell assembly can be used as the starting
material for manufacturing sensors with improved characteristics.
This approach presents several advantages: i) the chemically synthesized metal particles are small (typically 1.5-4 nm) and highly catalytic; ii) the catalyst load can be easily controlled by varying the self-assembling conditions; and iii) different blends of metal clusters can be formulated, opening the possibility of fine-tuning the sensibility of the sensors towards different gases. In the present communication both, the synthesis procedures to obtain the dual systems and its characterization by infrared spectroscopy are described in detail.
Experimental
SnO2 (Merck φ ≅ 50 nm), colloidal Pt nanoparticles (glycol method, φ ≅ 4 nm) and mercapto and amino derivatives of the propionic acid (3mercaptopropionic, 2-mercaptopropionic, 3aminopropionic and 2-amino-3-mercaptopropionic acids, p.a. grade) were used in the different experiments.
Synthesis of Pt nanoparticles
Pt metallic nanoparticles (NP Pt) were obtained by reduction of the chloroplatinic acid in ethylenglycol at 150° C in N2 atmosphere. The experimental details were published elsewere[4].
Functionalization of SnO2
a) Cysteine: SnO2(Merck) was placed in contact with a methanolic solution of cysteine (cys, 2amino-3-mercaptopropionic acid.). The molar ratio was fixed at 1:3. The pH was adjusted at 8 with a methanolic solution of NaOH 0.5M. The mix was stirred for 3 h. The resulting precipitate was decanted, washed several times with methanol and dried at 70º.
b) Alanine: SnO2 and an aqueous solution of alanine (3-aminopropionic acid) in a molar ratio 1:3 were stirred for 3 h. The pH was kept at 4.5. The precipitate was decanted, washed with water and dried at 70º.
c)3-MPA: SnO2 and an aqueous solution of 3mercaptopropionic acid, molar ratio 1:2, were stirred for 3 h. The pH was kept at 5. The precipitate was decanted, washed with water and dried at 70º.
d)2-MPA: SnO2 and an aqueous solution of 2mercaptopropionic acid, molar ratio 1:2, were stirred for 3 h. The pH was kept at 5. The precipitate was decanted, washed with water and dried at 50º.
Self assembling of Pt NP
The chemically modified SnO2 powders were suspended in methanol and sonicated by 10`. A solution of Pt NP was added in order to obtain a ratio SnO2: Pt 100:1 p/p. The mixes were stirred for 3 h, decanted by centrifugation, washed three times with methanol and dried at 50º. The final colour of the powders varied from dark to pale grey, depending on the molecular linker.
The powders thus obtained were characterized by transmission electron microscopy (TEM) and diffuse reflectance-absorption infrared Fourier transform spectroscopy (DRIFT).
Results
TEM
The presence of metallic particles on the surface of the SnO2 has been confirmed by TEM. In Figure 1 a high magnification TEM image of one of the samples is shown. The micrograph shows an agglomeration of tin oxide nanocrystals with an average size of 50 nm. The grains are covered by Pt nanoparticles with an apparent diameter of 4 nm.
50 nm
Figure1-TEM image of Pt noparticles onto of SnO2
It is interesting to note that the NP are somehow aligned onto the surface forming “nanoleads”. This is probably a decoration effect due to the preferential deposition of the NP along imperfections in the SnO2 nanocrystals, where the electrostatic interactions oxide-Pt NP are stronger.
FTIR
Is in general accepted that the carboxylic acids anchor oxides through the carboxylate moiety [5,6]. The infrared spectra of the adsorbed carboxylate ion has a strong antisymmetric (νas) and medium intensity symmetric (νa) stretch absorption at 16501510 and 1450-1250 cm-1, respectively. There are three common coordination modes: unidentade, chelating bidentade and bridging bidentade. The presence of functional groups with complexing properties (such as –NH2 and –SH, in the present case) can led to the formation of complex surface structures involving more than one group [7].
Absorbance
1,0
0,8
0,6
cys
0,4
0,2
cys+Pt
0,0
4000
3500
3000
2500
2000
1500
1000
500
wavenumber(cm-1)
Fig.2:FTIR spectra of SnO2/Cys and SnO2/Cys/Pt
Absorbance Absorbance
In Figure 2 the infrared spectra of SnO2 functionalized with cysteine is shown. Several distinctive bands can be observed. The broad band centered at 3000 cm-1 can be assigned to the stretching modes of the –N-H bonds, while the band at around 2550 cm-1 corresponds to the stretching of the S-H bond [8]. The band at 2100 cm-1 indicates the presence of extensive hydrogen bonding due to the amino groups. The fingerprint region shows a complex vibrational structure. The bands at 2585 and 1420 cm-1 can be assigned to the asymmetric and symmetric stretching modes of the adsorbed carboxylate moiety; while the bands around 1500 cm-1 are deformation modes of the amino group [8].From the spectra can be concluded that the cysteine is anchored through the carboxylate, with the amino and thiol groups relatively free for further reaction. When Pt NP are self- assembled onto the modified SnO2 the DRIFT spectra depicts changes in the bands. Noteworthy, the band at 2550 cm-1 is absent indicating that the cysteine molecules are coordinated to the Pt NP through the sulfur of the thiol moiety. On the other hand, there is a noticeable quenching of the bands due to the amine (at 3000, 2100 and 1500 cm-1). This suggest that the amino group is also involved in the bonding to the Pt NP. Interestingly, a new band appears at around 1740 cm-1 that is indicative of the stretching of the C=O. Thus in the new configuration cysteine forms an unidentade complex with the SnO2 while the Pt NP are bonded through the –N and the –S forming a five-membered ring. Has been noted that planar five-rings are stable structures [7].
1,1
1,0
0,9
0,8
0,7
3MP
0,6
0,5
0,4
3MP+Pt
0,3
0,2
0,1
4000
3500
3000
2500
2000
1500
1000
500
wavenumber(cm-1)
Fig.3:FTIR spectra of SnO2/3MP and SnO2/3MP/Pt
In Figure 3 the infrared spectra of 3-MPA/SnO2 and 3-MPA/SnO2/Pt NP are shown. In the first case, there is a band centered at 3400 cm-1 which
indicates that the surface is heavily hydroxylated. There is a band at 2550 cm-1 due to the –S-H bond
and a group of bands and shoulders in the region 1750-1350 cm-1 probably due to the coexistence of more than configuration of the carboxylate. When
the Pt NP are assembled, the band at 2550 vanishes indicating the formation of –S-Pt bond. The two broad bands centered at 1620 and 1370 cm-1
correspond to the νas and νa of the carboxylate, respectively.
Absorbance
0,55
0,50
0,45
0,40
0,35
0,30
0,25
0,20
2MP
0,15
0,10
0,05
3MP+Pt
0,00
-0,05
4000
3500
3000
2500
2000
1500
1000
500
wavenumber(cm-1)
Fig. 4:FTIR spectra of SnO2/2MP and SnO2/2MP/Pt
In Figure 4 the spectra of the 2-MPA systems are shown. The most prominent feature is the disappearance of the band due to the S-H stretching at 2550 cm-1 , even in the absence of Pt NP. Then, it is apparent that the –SH in α position contributes to the anchoring of the acid to the oxide. This can explain the low affinity toward Pt binding of the SnO2 functionalized with 2-MPA.
0,8
0,7
0,6
0,5
ala
0,4
0,3
ala+Pt
0,2
0,1
4000
3500
3000
2500
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1500
1000
500
wavenumber(cm-1)
Fig.5:FTIR spectra of SnO2/Ala and SnO2/Ala/Pt
Figure 5 shows the infrared spectra of SnO2/alanine and SnO2/alanine/Pt NP. In the first case, the spectra
depict a broad band centered at 3100 cm-1 and a band at 1570 cm-1 that can be assigned respectively to the stretching and deformation modes of the amine [9]. The shoulder at around 1640 cm-1 and the band at 1400 cm-1 corresponds to the stretching modes of the carboxylate. When Pt NP are assembled, there is a noticeable quenching of the bands due to the amine modes, suggesting that Pt is bonded through the –NH2.
Discussion
The derivatives of the propionic acid studied presents different affinities toward the Pt NP. In order of decreasing affinity: Cys > 3-MPA > Alanine > 2-MPA.
Cysteine shows an strong binding to Pt, perhaps due to a five membered ring where both, -NH2 in position α and the –SH in position β are involved. This bi-functional complex seems to be highly stable, leading to a cuantitative transfer of the NP toward the modified oxide. 3-MPA is also an effective monodentate ligand that form stable bonds with the thiol group. Alanine on the other hand is not as effective as ligand. This is due to the limited affinity of the amines toward Pt, as compared to thiols. The 2-MPA presented the poorest affinity for Pt NP, despite the presence of a thiol group. The lack of reactivity seems to be related to the tendency of the –SH to bind to the oxide. To effectively attach to the Pt NP, the surface complex must rearrange which is probably energetically too costly to be effective.
dependent on the structural characteristics of the complexes formed on the surface of the oxide. The thiol group in β position presents a higher affinity toward Pt NP. This effect is reinforced by the presence of ligands like –NH2 in α position.
References
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[7] T. Rajh, D.M. Tiede, M.C. Thurnauer; J. Non-Cryst. Solids 205-207 (1996) 815.
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Conclusions
The effectiveness of different bi- and threedentade ligands to bind SnO2 and Pt NP is
Ver+/-
