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Self-assembled hybrid films of phosphotungstic acid and aminoalkoxysilanes onSiO2/Si surfaces
Adriano L. Souza a, Lygia A. Marques b, Marcos N. Eberlin b, Pedro A.P. Nascente c,Paulo S.P. Herrmann Junior d, Fabio L. Leite e, Ubirajara P. Rodrigues-Filho a,⁎a Universidade de São Paulo (USP), Instituto de Química de São Carlos, P.O. Box 780, 13560–970, São Carlos, São Paulo, Brazilb Universidade Estadual de Campinas (UNICAMP), Instituto de Química, Laboratório Thomson de Espectrometria de Massas, 13083-970, Campinas, São Paulo, Brazilc Universidade Federal de São Carlos (UFSCar), Departamento de Engenharia de Materiais, 13565-905, São Carlos, São Paulo, Brazild Empresa Brasileira de Pesquisa Agropecuária, Embrapa Instrumentação Agropecuária, Laboratório Nacional de Nanotecnologia para o Agronegócio, 13560-970, P.O.Box 741, São Carlos,São Paulo, Brazile Universidade Federal de São Carlos (UFSCar), Campus de Sorocaba, P. O. Box 3031, 18052-780, Sorocaba, São Paulo, Brazil
⁎ Corresponding author at: Universidade de São PaulCarlos, P.O. Box 780, 13560-970, São Carlos, Brazil. Tel./
Article history:Received 20 October 2010Received in revised form 21 December 2011Accepted 23 December 2011Available online 30 December 2011
Keywords:3-aminopropyltriethoxysilaneN-(3-(trimethoxysilyl)-propyl)-ethylenediamineX-ray photoelectron spectroscopyAtomic force microscopyalkoxysilanesSelf-assembled filmsPhosphotungstic acid
The present paper describes the influence of the chemical structure of two aminoalkoxysilanes: 3-aminopropyltriethoxysilane (APTS) and N-(3-(trimethoxysilyl)-propyl)-ethylenediamine (TSPEN) on themorphology of thin layer hybrid films with phosphotungstic acid (HPW), a Keggin heteropolyanion. X-rayphotoelectron spectroscopy analyses indicated that both silane films showed protonated amine species inter-acting with the heteropolyanion by electrostatic forces as well as the presence of secondary carbamate an-ions. The hybrid films have different surface morphology according to atomic force microscopy analyses.The hybrid film with TSPEN forms flatter surfaces than the hybrid film with APTS. This effect is ascribed tohigher flexibility and chelating ability of the TSPEN on adsorbed molecules. Ultrasonication effect on surfacemorphology of the hybrid film with APTS plays a fundamental role on surface roughness delivering enoughenergy to promote surface diffusion of the HPW heteropolyanions. This diffusion results in agglomerate for-mation, which corroborates with the assumption of electrostatic bonding between the HPW heteropolya-nions and the protonated amine surface. These hybrid films could be used for electrochemical sensordesign or to build photochromic and electrochromic multilayers.
The modification of surfaces by silylation producing self-assembled monolayers (SAMs) [1] has contributed to applications inseveral areas such as sensors and biosensors [2–5] as well as the im-mobilization of metallic catalysts [6–8]. For these purposes, the si-lanes usually consist of three surface reactive groups, with labilealkoxide or chlorine, at one extremity and an organofunctionalgroup (for example, a nitrogenated Lewis base) at the other.
The hydrolysis reaction of the labile alkoxy or chloro groups re-sults in silanol groups which can react with themselves throughcondensation reaction leading to oligomers linked by bridges of si-loxanes or react with the hydroxyl groups on the surface [9]. The ki-netics of these hydrolysis reactions [10,11], control the silane filmmorphology and adhesion. Nanostrands of oligo-aminosiloxanescan be obtained by deposition of aged hydrolyzed solutions of 3-aminopropyltriethoxysilane (APTS) on glass surface [12]. Also,
o, Instituto de Química de Sãofax: +55 16 3373 9976.ho).
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non-ordered aggregates or agglomerates, deriving from these reac-tions, previously deposit on the surfaces resulting in random andrough film morphologies [13,14]. Kornherr et al. [15], studying theearly steps of silane adsorption on polar surfaces, simulated the ad-sorption of octyltrihydroxysilane, 3-aminopropyl-trihydroxysilane,and 3-mercaptopropyltrihydroxysilane single molecules on ZnOsurface from isopropanol solution by molecular dynamics calcula-tions. The results indicated that the more polar silane adsorbs onthe surface with the backbone parallel to the oxide surface, there-fore maximizing the interaction between both polar groups (OHand NH2), and the surface silanol sites (SiOH). For instance, theother two silane molecules showed parallel and orthogonal adsorp-tion, with higher stability for the orthogonal adsorption mode.Fiorilli et al. [16] presented the same conclusion for the adsorptionof APTS on silicon substrates via vapor process. Wu et al. [17], inves-tigating the adsorption of 4-amino-butyldimethylmethoxysilane(ABS) and octadecyltrichlorosilane (OTS) from non polar solventson wet chemically oxidized silicon wafer surfaces, confirmedthe formation of a Langmuir monolayer for OTS and a three-dimensional multilayer film up to 7.0 nm in thickness for ABS ad-sorption. The adsorption of ABS is therefore very sluggish. Cao et
Fig. 1. Structure of the HPW in a ball and stick model.
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al. [18] investigated the use of supercritical CO2 as solvent for silyla-tion reaction with alkylchlorosilanes on nanosized silica gel and sil-icon wafer. They emphasized the role of surface water film onsilylation process in dried solvent and the formation of oligomeric spe-cies on the surface of di- and tri- chlorosilanes resulting in multilayerfilms in contrast to monolayer formation of mono-chlorosilanes.Watson et al. [19] studied the influence of solvent polarity on the ad-sorption of an ureido silane on E-glass fibers and the influence of thepolarity of the washing solvent on the physisorbed silane removaland on the reorientation of chemisorbed silane on the surface. Allenet al. [20] studied the adsorption of 3-aminopropyltrimethoxysilanefrom dry and diluted bicyclohexyl solution by time-of-flight second-ary ion mass spectrometry (ToF-SIMS) obtaining homogeneous thinfilms with thickness of up to 2 monolayers. Their depth-profile ToF-SIMS results in the H- and O− species that lead them to characterizethe presence of a water ad-layer on top of the aminosilane film.These results are in agreement with our previous results obtainedby laser desorption ionization mass spectrometry for the 3-imidazolylpropyltrimethoxysilane from diluted acetonitrile solu-tion [21]. Recently, Ofir et al. [22] proposed a synthesis strategyfor fabricating a dense amine functionalized SAM on hydroxylatedsurfaces using (11-Bromoundecyl)-trichlorosilane (BUTCS) and (3-bromopropyl)-trichlorosilane (BPTCS) precursor layers. The tech-nique has proven useful for obtaining an amine-terminated SAMbased on the BUTCS but fails for the BPTCS-derived layer. This differenceis ascribed to the less ordered and less dense layer formed by BPTCS,thus making it less durable to the basic environment at the last step ofthe reaction, resulting in its partial or complete removal. The results ofOfir et al. [22] and those fromWu et al. [17] indicate therefore the betterordering of the less polar silanes, alkyl- and bromo- silanes, and longerchain silanes, octadecyl- and undecyl-silanes.
The formation of the self-assembled hybrid films is currently usedin applications such as electrochemical sensor [23], catalysis [24] andelectrocatalysis [25]. Some examples can be found in the literaturewhere coordination compounds [26,27] and polyoxoanions [28,29]have been immobilized on organic layers for different purposes.
Keggin-type heteropolyanions are highly symmetric anionicoxocluster units of c.a. 10 Å in diameter. They are basically composedof molybdenum, tungsten, or vanadium atoms — called addendaatoms, which are surrounded by six oxygen atoms in a C4v symmetry.They are also assembled in trimers and tetramers sharing edges andcorners [30]. In the centre of the structure there are other elementscalled heteroatoms, linked to four oxygens, in a Td symmetry. Theheteroatoms can be almost any transition metal and non-metal
Fig. 2. Chemical structures of the aminoal
atoms. These heteropolyoxometalates can be envisaged as molecularmixed oxides or quantum dots of the parent mixed-oxide structures,as the [PW12O40]3− cluster (phosphotungstic acid, whose abbrevia-tion is HPW), which has a diameter of 10.7 Å and is shown in Fig. 1.The unique size and architecture of these compounds have madethem useful in several areas such as in materials science, catalysis,and medicine [31].
In this study, self-assembled hybrid films of the HPW with ami-noalkoxysilanes bearing one and two amine functions were studied.The two aminoalkoxysilanes chosen were APTS and N-(3-(trimethox-ysilyl)-propyl)-ethylenediamine (TSPEN) (Fig. 2), which wereadsorbed on the wet-chemically grown SiO2 layers over n-type Si(100), named SiO2/Si. The hybrid film was obtained by further ad-sorption of the HPW on the silane SAMs. The influence of the chainlengths and the number of Brønsted basic amine functions of the si-lanes on the morphology of the hybrid films is also discussed. Thesesystems formed herein could be used to develop electrochromic de-vices and/or photochromic sensors since the polyoxotungstate under-go multiple reversible reactions of electron transfer, preserving themolecular structure [32].
The adsorption times of the silanes in the SiO2/Si surfaces wereoptimized by the static contact angles (CAs) measurements. To verifythe possible hydrolysis or agglomeration of these silanes, the silanesolutions in chloroform were studied by electrospray ionizationmass spectrometry (ESI-MS). The self-assembled hybrid films werecharacterized by X-ray photoelectron spectroscopy (XPS) and byatomic force microscopy (AFM).
2. Materials and methods
2.1. Materials
Plates of 1 cm2 were cut from n-Si (100) wafers (Goodfellow Cam-bridge Ltd.). The chemicals used in this study were APTS, TSPEN (GESilicones do Brasil and Acros), acetone (HPLC grade, Sigma-Aldrich),chloroform (anhydrous, Sigma-Aldrich), HPW (Sigma-Aldrich), HF(40%), H2SO4 (95–98%) and H2O2 (29%) from Synth. All reagentswere used as received. The resistivity of the deionized water was18.2 MΩ cm−1 and the nitrogen gas was purchased from AGA.
2.2. Silicon wafer preparation
The silicon surface was previously treated in order to obtainsmooth and clean surfaces of SiO2/Si. The wafers were immersed in10% (v/v) HF solution for 10s at room temperature, rinsed with plentyof deionized water and then they were immersed in “piranha solu-tion”, which consists of a mixture of H2SO4 and H2O2 (2:1-v/v) for5 min. After that, the substrates were rinsed with copious amountsof deionized water and dried under a nitrogen stream.
2.3. Preparation of hybrid films
The adsorption method of the layers was the same as thatreported in [21] referring to liquid-phase adsorption. In this method,the SiO2/Si plates were immersed directly in 0.1% (v/v) APTS or TSPENchloroform solutions at 298 K. The samples were then washed ultra-sonically with chloroform for 15 min in order to remove the physical-ly adsorbed silane overlayer and then dried under a nitrogen streamat room temperature. The immersion time was optimized by static
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contact angles measurements. The optimal adsorption time corre-sponded to the maximum value of the water static contact anglesince the silylation process replace highly polar and hydrogen bond-ing silanol groups by the less polar and weaker hydrogen bondinggroup alkylamine.
The hybrid films were produced by dipping the silylated plates inthe 2.10−3 mol L−1 solution of the HPW in acetone; for each silanethe immersion time used was the same previously optimized bywater static contact angles measurements. Afterwards, the plateswere washed ultrasonically with acetone for 15 min in order to re-move the physically adsorbed silanes and dried under a nitrogenstream at room temperature.
2.4. Electrospray Ionization Mass Spectrometry (ESI-MS)
ESI-MS spectra were obtained using a Waters Micromass QTof hy-brid quadrupole time-of-flight mass spectrometer operating at 7000mass resolution and 5 ppm mass accuracy using typical analyticalconditions as described elsewhere [33].
2.5. Water Static Contact Angles (CAs) measurements
The water static contact angle were measured in a home-built go-niometer composed of a digital CCD camera connected to a horizontaloptical microscope (MM Optics, São Carlos, Brazil) and interfaced to amicrocomputer. The water drop volumes were set at 10 μL.
2.6. X-Ray Photoelectron Spectroscopy (XPS)
The XPS measurements were performed using a spectrometerVSW HA-100 with spherical analyzer and non-monochromatic AlKα radiation (1486.6 eV). The pressure in the chamber was in therange of 10−6 Pa. The high-resolution spectra were measured withconstant analyzer pass energies of 44 eV, which produce a fullwidth at half-maximum line width of 1.6 eV for the Au(4f7/2) line.The take-off angle used for spectra acquisition was 90°. The spectrawere charge-corrected and the chemical compositions of the filmswere verified as previously described [21]. No evidence for photode-composition of the samples due to X-ray irradiation was observedin the time scale of the measurements.
All XPS peaks in insulating samples spectra were fitted accordingto Leclercq and Pireaux's method, using Gaussian–Lorentzian peaksimplemented in the Winspec package kindly supplied by ProfessorPireaux [34].
Fig. 3. ESI-MS spectra for the solutions 0.1% (v/v) in chloroform of the aminoalkoxysi-lanes. APTS (a) and TSPEN (b).
2.7. Atomic Force Microscopy (AFM)
AFM images were taken in a Topometrix microscope, model Dis-coverer TMX 2010, using silicon nitride tips with radius of 50 nmand a cantilever (V shape) with spring constant of 0.09±0.02 N/m(nominal value). The measurements were acquired at room tempera-ture and relative humidity of 40%. All images were obtained in thecontact mode at a scan rate of 2 Hz. The root-mean-square roughness(RRMS) and the agglomerates analysis were calculated using the Scan-ning Probe Image Processor (SPIP TM) software from Image Metrolo-gy, Denmark.
Two usual roughness parameters can be extracted from the AFManalysis. They are the Average Roughness (Ra) and the RRMS and aredefined by equations below:
Ra ¼1L
� �∫L0 F xð Þj jdx ð1Þ
RRMS ¼1L
� �∫L
0
F xð Þ½ �2dx" #1
2
ð2Þ
where F(x) is the profile height above datum and L is the samplinglength [35]. Ra is the arithmetic average of the absolute values ofthe profile height deviations from the mean line recorded withinthe evaluation length. Also known as Arithmetic Average or Center
Fig. 4. Long scan XP-spectra of the self-assembled films on SiO2/Si surfaces. APTS (a)and HPW/APTS (b).
Fig. 6. High resolution W4f XP-spectrum of the self-assembled HPW/APTS/SiO2/Si hy-brid film.
Fig. 5. High resolution N1s XP-spectra of the self-assembled films on SiO2/Si surfaces.APTS (a) and HPW/APTS (b).
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Line Average, Ra is therefore the area between the roughness profileand its mean line or the integral of the absolute value of the rough-ness profile height over the evaluation length. RRMS is the root meansquare of the profile height deviations from the mean line, recordedwithin the evaluation length. Ra and RRMS are both representationsof surface roughness, but each is calculated differently. Ra is calculat-ed as the average roughness of microscopic peaks and valleys of a sur-face and RRMS is calculated as the root mean square of microscopicpeaks and valleys of a surface.
The main difference in the two scales is that RRMS amplifies occa-sional high or low readings, while Ra simply averages them. For agiven surface, therefore, the RRMS value will be higher than the Ra
value (by approximately 11%). Thus, RRMS is used to control veryfine surfaces in scientific measurements and statistical evaluations.Ra is not a good discriminator for different types of surfaces (no dis-tinction is made between peaks and valleys) and is not a good mea-sure of sealed surfaces. In addition to this, Ra can takes less accountof the variations of the low frequencies [36–38]. For these reasons,RRMS was used in this study.
Table 2Binding energy values for the components in the N 1 s XP-spectra of the self-assembledfilms on SiO2/Si surfaces.
The surface morphology of thin films can be driven by solutionand surface chemistry. By solution chemistry effects we mean any ag-glomeration phenomena or reaction that result in oligomer or macro-molecule adsorption on surfaces. There is a hydrolytic tendency ofsilanes in solution and formation of oligomeric and polymeric species,therefore in this article we have studied the solution chemistry ofAPTS and TSPEN to confirm that the morphology of our films could
Fig. 7. High resolution N1s XP-spectra of the self-assembled films on SiO2/Si surfaces.TSPEN (a) and HPW/TSPEN (b).
Fig. 8. High resolution W4f XP-spectrum of the self-assembled HPW/TSPEN/SiO2/Si hy-brid film.
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only be assigned to surface chemistry caused by reaction on surfacegroups. Therefore in the next section we describe the solution behav-ior of these silanes in our experimental conditions.
3.1. Solution chemistry
ESI-MS spectra of the silane solutions were obtained to verify ag-glomeration, hydrolysis, and/or oligomer formation from silanes and
(a)
(c)
Fig. 9. AFM images of the self-assembled films on SiO2/Si surfaces in scale of 20×20 μm
are displayed in Fig. 3(a) and (b) for the solutions of APTS andTSPEN, respectively.
The ESI-MS spectrum of the APTS solution 0.1% (v/v) in chloro-form is characterized mainly by the detection of the protonated mol-ecule [M+H]+ ofm/z 222, as shown in Fig. 3(a). The ion ofm/z 176 isascribed to [M+H−EtOH]+. The other ions of higher m/z, that is, ofm/z 277 and 323, are ascribed to [M+K+NH3]+ and [M+K+NH3+EtOH]+, respectively. The ESI-MS spectrum of the TSPEN solution0.1% (v/v) in chloroform, Fig. 3(b), is characterized mainly by thedetection of the two ions of m/z 223 and 237. The first is ascribed to[M+H]+, and the second to an homologue, that is, [M+CH2+H]+.This is an important statement since we can now be sure that the ad-sorption is only of single alkoxy silane molecules and not silanol mol-ecules or oligomer siloxane molecules. Therefore, any agglomerateformation on the surface is a surface driven process and is not relatedto adsorption of agglomerates formed in solution.
3.2. Surface chemistry
3.2.1. CAs measurementsTable 1 shows the temporal water contact angle behavior of the
self-assembled films of aminosilanes on the SiO2/Si surfaces. It is as-sumed that after 40 min the constant values denote the steady stateconditions. This assumption is likely to be correct since the achieve-ment of the steady state would result in no more changes in thechemical or morphological surface features and a constant surface en-ergy. This result is similar to values of contact angles for surfacesmodified with amine groups [17]. The increase in contact angles
(b)
(d)
. APTS (a), TSPEN (b), HPW/APTS hybrid film (c) and HPW/TSPEN hybrid film (d).
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with the adsorption time indicates that these silanes adsorb with theamine group interacting with silanol groups on the surface as de-scribed by Kornherr et al. [15] for silane adsorption on ZnO surface.It turns out that the silanes would be adsorbed with non-preferential absorption in the experimental conditions reported here-in with a random orientation of the molecules in relation to thesurface.
3.2.2. Hybrid HPW–aminosilanes filmsThe most important features of the long-scan XP-spectra for the
immobilization of the HPW and aminosilanes on the SiO2/Si surfacesare the presence of the N1s and W4f peaks, confirming the modifica-tion of the surfaces with each aminosilane and the HPW. Fig. 4(a)shows the long scan spectrum of the APTS/SiO2/Si film as a represen-tative spectrum for both silane films. This spectrum is representativesince these silanes have the same features, confirming the adsorptionof the silanes. Fig. 4(b) shows the long scan spectrum for the HPW/APTS/SiO2/Si hybrid film with the W4f peaks. These peaks confirmthe presence of the HPW. The spectrum for the film HPW/TSPEN/SiO2/Si hybrid film showed the same features (not shown).
The high resolution N1s XP-spectra for the films APTS/SiO2/Siand HPW/APTS/SiO2/Si are exhibited in Fig. 5. In each case therewas the appearance of an asymmetric peak, which was fitted withthree components assigned to amine groups (–NH2), carbamategroups (–NHCOO−) and protonated amine groups (–NH3
+) [39,40].Table 2 shows the binding energy values for these species. The pro-tonation could result from the interaction between the APTS and thesurface silanol groups with a consecutive proton transfer [39], pro-ton transfer from water ad-layer or vapor, or by proton transferfrom HPW to the amine groups. For the hybrid film, the last hypoth-esis is more likely correct considering the superacidity of the phos-photungstate. The carbamate groups result from the reactionbetween the APTS and atmospheric CO2 [12]. More evidence of theformation of carbamate groups can be obtained by the C1s XP-spectra (not shown). They exhibited an asymmetric peak, whichwas fitted with three components at 285.0, 286.7, and 289.2 eV
(a)
Fig. 10. AFM images of the self-assembled films on SiO2/Si surfaces in scale of 20×20 μm(RRMS=1.2 nm) (b).
where the last component was assigned to the carbamate groups[41,42]. Similar C1s XP-spectra were obtained for the TSPEN/SiO2/Si film before and after the adsorption of the HPW (not shown).
The W4f XP-spectrum for the HPW/APTS/SiO2/Si hybrid film, Fig. 6,shows an asymmetric doublet peak, which was fitted with two spin-orbit doublets. The first species appears at 34.7 eV and the second at36.4 eV. They were ascribed to oxidation states W5+ and W6+, respec-tively [40]. The appearance of the W5+ oxidation state is assigned to aminor photo-reduction of the sample after X-ray exposure.
The N1s XP-spectra for the films TSPEN/SiO2/Si and HPW/TSPEN/SiO2/Si are shown in Fig. 7. In each case, there was also the appear-ance of an asymmetric peak, which was fitted with three componentsassigned to the amine groups (–NH2), carbamate groups (NHCOO−)and protonated amine groups (NH3
+). Table 2 shows the binding en-ergy values for these species. The reasons for the appearance ofthese species are analogous to the APTS/SiO2/Si film and HPW/APTS/SiO2/Si hybrid film cases. Interestingly, the relative concentrations ofthe ammonium and carbamate species in the HPW/TSPEN/SiO2/Si hy-brid film are not the same as observed in the HPW/APTS/SiO2/Si hy-brid film. This difference could be explained by the highernucleophilicity of TSPEN when compared to APTS, resulting in ahigher stability of carbamate groups formed with TSPEN.
The W4f XP-spectrum for the HPW/TSPEN/SiO2/Si hybrid film,Fig. 8, shows an asymmetric spin orbit doublet peak, which was fittedwith two spin-orbit components. The first species presents a W 4f7/2peak at 34.8 eV and the second species, at 36.5 eV. They were ascribedto oxidation states W5+ and W6+, respectively, as described before.
The AFM images of the self-assembled films are shown in Fig. 9.When HPW is adsorbed onto each silane film, the films become rough-er with the appearance of globular deposits (Fig. 9(c) and (d)), con-trasting with the flatter silane films (Fig. 9(a) and (b)). The RRMS
values and the size of globules (D) are shown in Table 3. The formationof large globules in the hybrid film with APTS (100 nm≤D≤400 nm)were assigned to supramacromolecular agglomerates on the surfaceformed by clustering or crystallization of several HPW on the silanelayer, as also evidenced in [43], increasing the RRMS. The flatter surfaceof the hybrid film (RRMS=0.8 nm)with TSPEN reflects the possibility ofthis silane to chelate the phosphotungstate preventing the formation ofagglomerates during the ultrasonication stage. It turns out that thischelating mode results in better accommodation of the phosphotung-state anion, [PW12O40]3−, inside the silane film. Therefore, this betterincorporation achieved decreases the possibility of interaction amongthe ion pairs.
In order to verify the role of the ultrasonication stage on the HPWagglomeration on the silane film, an additional AFM study was con-ducted with the HPW/APTS/SiO2/Si hybrid film, which was a rougher
(b)
without the ultrasound stage. APTS (RRMS=0.06 nm) (a) and HPW/APTS hybrid film
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film. Fig. 10 shows AFM images of the APTS/SiO2/Si (Fig. 10(a)) andHPW/APTS/SiO2/Si (Fig. 10(b)) films without the ultrasonicationstage. In this situation, the RRMS values were 0.06 nm and 1.2 nm forthe APTS/SiO2/Si and HPW/APTS/SiO2/Si films respectively, in imagesof 20×20 μm.
Semi-quantitative information can be extracted from N/Si and W/Si atomic ratios from XPS results for the APTS/SiO2/Si and HPW/APTS/SiO2/Si films. For the films containing only APTS, the N/Si atomic ra-tios values were 0.077 and 0.12, respectively with and without theuse of the ultrasound. The smaller value found for the film subjectedto the ultrasound can be attributed to the removal of physicallyadsorbed molecules of APTS of the substrate.
A similar conclusion can be obtained when the W/Si atomic ratiosfor the HPW/APTS/SiO2/Si hybrid films are analyzed. The W/Si ratiosfound were 0.13 and 2.3, respectively with and without the use ofthe ultrasound. Again, the lowest value of atomic ratio obtained forthe film subjected to the ultrasound can be related to removal ofHPW molecules from the film.
So, the clustering effect of the ultrasonication becomes evidentdue to the great increase in the RRMS values for the films, as depictedin Table 3. Indeed, this clustering effect on the ultrasound is quite ev-ident due to the electrostatic nature of the [PW12O40]3−/NH3
+ interac-tion in the hybrid films.
4. Summary and conclusion
In this paper, we have evaluated the influence of a number ofamine groups of two aminoalkoxysilanes on the morphology of self-assembled hybrid films with one Keggin-type heteropolyanion,HPW on silicon wafers. For this evaluation, the adsorption times ofthe aminosilanes were optimized by using the static contact anglesmeasurements. The hypothesis of formation of aggregates or agglom-erates in solution due to hydrolysis and condensation reactions ofthese aminosilanes was discarded by ESI-MS studies. The immobiliza-tions of the molecules and the identification of the present specieswere evidenced by XPS analyses. The AFM measurements showedthat the hybrid film with TSPEN is flatter than the hybrid film withAPTS due to the two amine groups in the TSPEN, which “chelate” toHPW to better accommodate the [PW12O40]3− inside the silane film.Silane films are strongly attached to the surface by covalent siloxanebonds, and HPW adsorbs through electrostatic forces on the surfaces,enabling us to design the surface morphology by sonication. The re-sults herein reported expand the understanding of both the solutionchemistry of aminosilanes and the surface chemistry of hybrid films.They can also help for the development of sensors and the applicationof these SAMs in catalysis and electrocatalysis or electrochromicdevices.
Acknowledgments
The authors acknowledge CNPq (Conselho Nacional de Desenvolvi-mento Científico e Tecnológico), FAPESP (Fundação de Amparo à Pes-quisa do Estado de São Paulo) for financial assistance and ALS thanksthe CNPq and CAPES (Coordenação de Aperfeiçoamento de Pessoal deNível Superior) for the MSc and PhD fellowships respectively. The au-thors are grateful to Prof. Richard Landers (IFGW/UNICAMP) and RitaC. G. Vinhas for assistance with the XPS measurements.
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