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SIMS analysis of conducting polypyrrole–silica gel composites
Synthetic Metals 113 Ž2000. 53–63 www.elsevier.comrlocatersynmet
SIMS analysis of conducting polypyrrole–silica gel composites Christian Perruchot a , Mohamed M. Chehimi a,) , Michel Delamar a , John A. Eccles b, Timothy A. Steele b, Craig D. Mair b a
(ITODYS), UniÕersite´ Paris 7, Denis Diderot, CNRS (UPRESA 7086), 1 rue Guy de la Brosse, Institut de Topologie et de Dynamique des Systemes ` 75005 Paris, France b Millbrook Instrument, Blackburn Technology Centre, Challenge Way, Blackburn, Lancashire BBI 5QB, England, UK Received 1 June 1999; received in revised form 20 October 1999; accepted 26 November 1999
Abstract Secondary ion mass spectrometry ŽSIMS. has been used to characterize novel hybrid conducting polypyrrole–silica gel composites. The silica gel particles act as a high surface inorganic substrate for the in situ chemical synthesis of polypyrrole ŽPPy. in aqueous media. However, the conventional method using untreated silica gel as a host material for pyrrole polymerisation led to insulating PPy–silica composites, which exhibit a silica-rich surface. By contrast, pre-treatment of the silica gel particles by aminopropyltriethoxysilane ŽAPS. prior to polymerisation results in conducting PPy–APS–silica composites, the surface of which was found to be PPy-rich. The present study uses the unique capabilities of SIMS to distinguish unambiguously between fragments from the silane coupling agent and the conducting PPy overlayers. Specific negative ions from PPy and the underlying substrate particles were effective in monitoring the change in the PPy content at the surface of the composites vs. the initial concentration of APS used to pre-treat silica gel particles. This SIMS analysis clearly demonstrated that APS is effective in increasing the surface PPy content and thus confirms previously published XPS data based on some necessary assumptions. q 2000 Elsevier Science S.A. All rights reserved. Keywords: SIMS; Conducting polypyrrole; Silica gel; Silane coupling agent; Composite materials
1. Introduction Inherent conducting polymers ŽICPs. constitute a new class of polymers with particular interest owing to their physical and chemical properties w1x. Polypyrrole ŽPPy. is one of the most studied ICP due to its relative stability but suffers poor processability. To overcome these limitations, many research groups focused on the preparation of conducting PPy-based composite materials w2x. Extensive research on such materials has the main objective to synthesize novel composites, which combine both properties of the conducting polymer Žconductivity w3x, redox properties w4x, ion and proton exchange w5x, Lewis acid–base interactions w6x, optical properties w7x, etc.. and those of the mineral host material on the one hand, and to improve the processability of insoluble and infusible conducting PPy polymer on the other hand w8x. Maeda and Armes w9x
)
Corresponding author. Fax: q33-1-4427-6814. E-mail address: [email protected] ŽM.M. Chehimi..
reported the synthesis of nanoscale PPy–silica composites. However, such materials exhibit a silica-rich surface as found by XPS analysis w10x. Furthermore, conductivity measurements showed that PPy–silica composites are poorly conducting or even insulating thus confirming that they do have a silica-rich surface. Of relevance to the present study, Chriswanto et al. w11x synthesized PPy–silica gel composite and used it as novel stationary phase in high performance liquid chromatography ŽHPLC.. Such stationary phase was further found to have reversed phase and ion-exchange properties w5x. However, we found by X-ray photoelectron spectroscopy ŽXPS. w12x that composites synthesized using a similar protocol are silica-rich surface materials and thus, the chromatographic performances of PPy may be obscured by those of silica w13x. In order to obtain similar composite materials with rather a PPy-rich surface, we have suggested to modify the protocol of Chriswanto et al. w11x by pre-treating the silica gel particles by a common silane coupling agent, aminopropyltriethoxy silane ŽAPS.. In this case, the silica–APS particles acted as a porous host for the in situ polymerisa-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 0 3 - 3
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C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
tion of pyrrole in water. A free-flowing powder of PPy– APS–silica composites was obtained w12x. This material exhibits a high relative surface content of PPy at the silica surface and an increase of conductivity by at least four orders of magnitude is observed Žup to 0.1 Srcm.. Although APS significantly affected the specific surface area Ždecrease from 430 to 225 m2rg. of silica gel particles, we still obtained a fairly high specific surface area for the PPy–APS–silica end-product Ž180 m2rg. compared to bulk PPy powder Ž15–25 m2rg. w14x. Prior to synthesis of the conducting composites, we have determined an adsorption isotherm of APS by means of XPS w14x and found that it fitted the Langmuir type as for planar quartz surfaces w15x. The plateau value was reached for an initial concentration of APS of 1%. It is for this value that conductivity of the PPy–silica–APS became measurable and reached 2.7 10y2 Srcm, whereas for an initial concentration of 0.5% only, the composites were much less conductive Ž10y5 Srcm.. These improvements resulted from only a slight increase from 6 to 9 wrw % of PPy content as determined by thermogravimetric analysis ŽTGA.. Although the XPS was extremely useful in monitoring the changes in the surface composition of silica due to APS and PPy, it was not possible to distinguish between the nitrogen and carbon species arising from both APS and PPy. Thus, the determination of the relative proportion of PPy at the surface of the conducting PPy–APS–silica composites was difficult to determine, and some assumptions were made in this regard w14x. In this study, we would like to explore the capabilities of secondary ion mass spectrometry ŽSIMS. to complement our recent studies of the surface composition of the conducting composites mentioned above. SIMS has a superior surface specificity Žca. 1 nm. compared to XPS Žca. 5–10 nm.. In addition, it detects specific fragments from the materials under test in either the positive or the negative mode of detection w16x. SIMS has been widely used to characterize conventional polymers and polymer blends w17x on the one hand, and silane coupling agent-modified substrates on the other hand w18x. However, few SIMS studies onto conducting polymers have been reported until now w19–24x. Concerning conducting PPy, Hearn et al. w21x examined by ToF-SIMS the nature of aromatic sulphonate anion dopant in various PPy-textile fibers. Abel et al. w22x reported ToF-SIMS characterization of electrochemically conducting PPy films with various anion dopants. It was clearly shown that specific fragments could be assigned to both the PPy backbone and anion dopant in the negative ion mode. In a subsequent paper, the capabilities of ToF-SIMS were explored to determine adsorption isotherms of PMMA onto sulfate-doped PPy powder, which were found to be of the Langmuir type w23x. Chehimi et al. w24x have also used ToF-SIMS to study the adsorption of polymer blends and a block copolymer onto PPy powders. In this paper, we describe the use of SIMS as a complementary surface analytical technique to XPS to our on
going research programme on conducting polymer composites. We investigated the effect of aminopropyl-silane ŽAPS. pre-treatment of silica gel particles before PPy coating and determine surface properties of the resulting novel hybrid conducting PPy–silica gel composites.
2. Experimental 2.1. Synthesis of aminopropyltriethoxysilane-grafted silica gel particles APS ŽAcros, 0.5% to 3% vrv. was first hydrolysed 6 h in a 200 ml de-ionized waterrethanol Ž1r9 vrv. solution and then 2 g of bare silica gel ŽMerck, diameter in the 60 to 125 mm range, Vporous s 0.75 cm3rg,. was added to the solution and stirred overnight. This mixture was Buchner¨ filtered, rinsed with 50 ml of de-ionized waterrethanol Ž1r9 vrv. solution to remove the excess of physically adsorbed silane coupling agent, and dried overnight in a dessicator. 2.2. Synthesis of PPy–silica particles 2.2.1. PPyTS–silica Pyrrole ŽAcros, 1 ml. was added to a suspension of 2 g of silica particles in 50 ml of pentane ŽProlabo, normapur grade. and the mixture was stirred in a fumehood until free-flowing pyrrole-coated silica particles were obtained. These particles were then added to 100 ml of aqueous solution of iron chloride ŽAldrich, 36.0 mmol. and paratoluenesulfonate sodium salt ŽAldrich, 36.0 mmol. at room temperature. This solution was stirred for 24 h and the resulting black PPy–silica particles were vacuum-filtered, washed with copious amounts of de-ionized water, and then dried in a dessicator overnight. 2.3. Synthesis of PPy-coated, APS-grafted silica particles 2.3.1. PPyTS–APS–silica The APS coupling agent was first grafted onto the silica particles before pyrrole coating and polymerization mentioned above. 2.4. Synthesis of PPy powders 2.4.1. PPyTS Pyrrole Ž1.00 ml, 14.4 mmol. was added via syringe to 100 ml of a stirred aqueous solution containing iron chloride Ž36.0 mmol. and para-toluenesulfonate sodium salt Ž36.0 mmol. at room temperature. The reaction solution was stirred for 24 h and the resulting black precipitate was
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
vacuum-filtered and washed with copious amounts of deionized water until the washings were clear. The powder was then dried in a dessicator overnight and sieved to 180 mm diameter before analysis.
2.5. SIMS analyses SIMS analyses were performed using a Chemical Microscope, an automated benchtop SIMS system from Millbrook Instruments w25a,bx. In this instrument, the primary beam comprises 6 keV Gaq ions, focused to a 50-mm spot size and rastered over the sample surface to irradiate a larger area. The mass spectrometer is a 300 mrz quadrupole with low energy secondary ion collection optics; dynamic emittance matching is employed to maximize collection efficiency. Charge neutralisation for insulating samples is provided by an auxiliary electron gun using indirect irradiation and an oscillating sample bias, a design based on the system devised by Gilmore and Seah w26x. Samples for analysis were prepared by pressing each of the powdered materials into indium foil, taking care to ensure complete coverage. Fresh samples were used to acquire both the positive and negative spectra. Charge neutralisation was required only for the untreated silica gel
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sample. The averaged primary ion dose used to acquire each survey spectrum Ž2–200 Da. was below 1 = 10 12 ions cmy2 , well within the accepted static SIMS limit. Significant attenuation of higher mass peaks was in fact noted for primary ion doses above 5 = 10 12 ions cmy2 . For the quantitative study of the effect of varying APS pre-treatment, the averaged primary ion dose therefore had to be kept below 1 = 10 12 ions cmy2 per sample. This was achieved by acquiring negative mass spectra only over the range 55–85 Da; this narrow mass range contained peaks characteristic of both the substrate and the PPy, but the restricted mass range allowed a longer dwell time at each point, and hence, much improved counting statistics. Five samples were analysed for each APS pre-treated material. Since the Chemical Microscope features fast sample loading and the facility to pre-set standard experimental parameters, analysis of each sample was completed in under 10 min.
3. Results and discussion All materials were characterized in both positive Žexcept for bulk PPy powder. and negative ion mode in order to determine specific features due to silica gel particles,
Table 1 Peak identification for silica, silica–APS, silica–PPy and silica–APS–PPy materials in positive ion mode mrz 12 15 17 18 26 27 28 29 30 31 39 41 42 43 44 45 55 56 57 69 71 91 93 131
Formula
Silica
q
C CHq 3 NHq 3 q NH 4 CNqrC 2 Hq 2 CNHq SiqrC 2 Hq 4 SiHqrC 2 Hq 5 CH 4 Nq q CH 5 N CH 3 Hq 3 C 3 Hq 5 q C 3 Hq 6 rC 2 H 4 N C 3 Hq 7 C 2 H 6 Nq SiOHq C 4 Hq 7 C 3 H 6 NqrSiC 2 Hq 4 C 4 Hq 9 69 q C 5 Hq 9 r Ga 71 q C 5 Hq r Ga 11 C 7 Hq 7 C 7 Hq 9
U
U U
U U U U U U U U U U
Silica–APS
Silica–PPy
Silica–APS–PPy
U U U U
U U
U U U U U U U U U U U U U U U U U U U
U U U
U U U U U U U U U U U U U U U U U U U U U U U
U U U U U U U U U U U
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
56
silane coupling agent and PPy overlayer, respectively. Since SIMS is by nature a destructive method, we used different samples from the same batch in order to determine average relative peak mass intensity. PPy–APS–silica and PPy–silica were the composite materials of interest, while fresh silica, silica–APS, and PPy bulk powder were used as reference materials. Table 1 and Table 2 report peak assignment of bare silica gel, APS-treated silica gel particles, hybrid PPy–silica gel composite and bulk PPy powder detected in positive and negative ion mode, respectively. Fig. 1 shows the positive ion mode spectra of untreated silica gel particles Ža., APS-treated silica particles Žb., and hybrid PPy–silica composite Žc. in the 2–200 amu mass range, respectively. For silica gel particles, Fig. 1a shows typical siliconcontaining fragments such as Siq and SiOHq. Additional weak peaks due to low carbon contamination are also detected as previously observed by XPS analysis w12x. Fig. 1b shows the positive ion mode spectra of APStreated silica gel particles. In addition to features observed for the untreated silica gel particles, unambiguous characteristic peaks due to APS are also detected. Characteristic fragments are assigned to amino end functionality Ži.e., q q NHq and CH 5 Nq or CH 3 Oq ., which 3 , NH 4 , CH 4 N were previously reported for APS-treated substrates w27x. An additional peak at 56 amu is clearly detected and can be assigned to C 3 H 6 Nq fragment w28x. Moreover, the clusters of peaks at 39, 41, 43 amu, 53, 55, 57 amu and 69, 71 amu are typical aliphatic hydrocarbon fragments of the form C n H 2 ny3 , C n H 2 ny1 and C n H 2 nq1 , arising from the propyl chain. It is to note that they are more intense than
those arising from the untreated silica gel. A low intensity peak is also detected at 131 amu. This fragment is assigned to the following structure w28x:
Nevertheless, indium foil may be irradiated by the primary beam and emits InOq fragment at mrz s 131 amu. This is in line with a peak observed at mrz s 115 amu, corresponding to Inq. However, these peaks are not observed simultaneously for silica gel particles and silica–APS–PPy composite material. Moreover, in case of silica–APS–PPy composite material, the absence of these peaks at 115 and 131 amu can be interpreted in term of homogeneous and compact PPy coating onto APS-treated silica particles. It is worth noting that in the case where both peaks at mrz s 115 and 131 amu are observed, the intensity ratio 131r115 is much larger for silica–APS than for silica–PPy. Therefore, one can conclusively attribute the peak at mrz s 131 amu to the previously reported structure than to InOq fragment. It is also to note that APS overlayer leads to a slight decrease of the 45qr28q ŽSiOHqrSiq . peak intensity ratio ŽPIR. from 0.4 to 0.36 that is a slight depletion of silica. However, APS-treated silica particles still exhibit a silica-rich surface.
Table 2 Peak identification for silica, silica–APS, silica–PPy, bulk PPy powder and silica–APS–PPy materials in negative ion mode mrz 12 13 16 17 24 25 26 32 35 37 48 60 61 64 76 77 80 171
Formula y
C CHy Oy OHy Cy 2 C 2 Hy CNyrC 2 Hy 2 Oy 2 Cly Cly SOy SiOy 2 SiO 2 Hy y SO 2 SiOy 3 SiO 3 Hy SOy 3
Silica
Silica–APS
Silica–PPy
PPy
Silica–APS–PPy
U U U U U U U U U
U U U U U U U U U U
U U U U U U U U U U
U U U U U U U U U U U
U U
U U
U U
U U
U U U U U U
U U U U U U U U U U U U U U U
U
U
U U
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
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Fig. 1. SIMS positive ion mode spectra Ž0–200 amu. of untreated silica gel particles Ža., APS-treated silica gel particles Žb., and hybrid PPy–silica gel composite materials Žc.. For each spectrum, enlargements are shown for 100–200 amu region.
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Fig. 2. SIMS negative ion mode spectra Ž0–200 amu. of untreated silica gel particles Ža., APS-treated silica gel particles Žb., hybrid PPy–silica gel composite material Žc., and bulk PPy powder Žd.. For each spectrum, enlargements are shown for the 100–200-amu region.
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
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Fig. 2 Ž continued ..
Fig. 1c shows the positive ion spectrum of PPy–silica gel composite. In addition to the silica features, some characteristic peaks from the polymer chain are also detected. The PPy overlayer led to a relative increase of features at 15 and 27 amu, which are typical of CHq 3 and CNHq characteristic fragments of the polymer chain w22x. Additional clusters at 39, 41, 43 amu and 53, 55, 57 amu are also detected and can be assigned to aliphatic carbon fragments of PPy chain. Para-toluenesulfonate anion dopant also gives a specific fragment in the positive ion mode w22x. A typical feature at 91 amu can be assigned to the following structures:
However, the most stable structure known of C 7 Hq 7 fragment is that of tropyllium. Although both silica and PPy chain contributed to the SIMS signal, this study supports the view that PPy–silica material still exhibits a silica-rich surface as shown by the relative intense signals at 28 and 45 amu arising from the host silica gel substrate. This result is in good agreement with previously reported XPS study of similar hybrid PPy–silica gel particles w14x and PPy–silica nanocomposites w10x. Fig. 2 shows the negative ion mode spectra of untreated silica gel particles Ža., APS-treated silica gel particles Žb., hybrid PPy–silica composite Žc., and bulk PPy powder Žd. in the 2–200 amu mass range, respectively. For the untreated silica gel particles, Fig. 2a displays two intense peaks at 16 and 17 amu, corresponding to Oy
and OHy, respectively. In addition, typical ions of silica can be detected at 60, 61, 76, 77 amu and assigned to y y y SiOy species, respectively 2 , SiO 2 H , SiO 3 , and SiO 3 H w16x. Weak signals are recorded at 24 and 25 amu, probably due to low carbon contamination species. This is in good agreement with the positive ion mode as mentioned above and XPS analysis of bare silica gel particles w12x. For APS-treated silica gel particles, Fig. 2b shows few new specific peaks compared to the untreated silica gel particles. The 17r16 PIR slightly increases due to the increase of oxygen content at the outermost silica surface and silanol group due to APS treatment. Another possible reason for such increase in the OHyrOy peak ratio is that APS may adsorb onto silica via the amino group w29,30x, an interaction, which permits the hydrolysed silane OH group to be at the outermost layer. Additional peaks at 24, 25, and 26 amu can be detected and assigned to carbon species and CNy fragments, respectively. These results are in good agreement with those previously reported by Wang and Jones w29,30x. This study confirms the presence of a thin overlayer of the aminosilane coupling agent at the silica gel surface. However, no structural information can be obtained concerning the interaction mechanism of the coupling agent with the silica gel particles. Moreover, since we used a low APS concentration to treat the silica particles, no characteristic features above 150 amu were detected. Negative ion mode spectrum of hybrid PPy–silica gel composite is shown in Fig. 2c. The main information arising from the conducting polymer is a typical fragment at 26 amu, which is due to CNy anion generated by the PPy chain w22x. A significant increase of the intensity of features at 24 and 25 amu coming from hydrocarbon fragments is also observed. However, little structural information can be gained from the negative ion mode of PPy
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Fig. 3. SIMS positive ion mode spectrum Ž0–200 amu. of PPy–APS–silica gel composite material. Enlargement is shown for the 100–200-amu region.
probably due to the relative stability of its backbone under the ion beam. Such stability is perhaps driven by the partially crosslinked structure of PPy chain w31x. Additional features arising from para-toluenesulfonate anion dopant are also clearly detected. Indeed, the fragmentation of para-toluenesulfonate anion gives characteristic peaks at 64 and 80 amu. These peaks are attributed to SOy 2 and SOy 3 species. However, there is no evidence of paratoluenesulfonate anion dopant at 171 amu. This is likely to be due to the low PPy content at the surface of untreated silica gel particles as determined by TGA and XPS study w14x. Since iron chloride is used as oxidant for the chemical synthesis of PPy, halide Cly is inserted as a co-dopant,
thus, leading to the detection of mass peaks at 35 and 37 amu and to an the intensity ratio 3:1 Žnatural isotopic abundance.. It is to note that very low concentration of chloride species were detected by XPS whereas, in this present work, these ions are readily detected by SIMS due to its superior detection limit. Moreover, halides species are very stable ions. Since positive and negative ion mode spectra of PPy– silica composite are very similar to that of untreated silica gel particles, SIMS analysis confirms that the PPy–silica composite exhibits a silica-rich surface. This result is in good agreement with previously reported XPS analysis of such material w14x.
Fig. 4. SIMS negative ion mode spectrum Ž0–200 amu. of PPy–APS–silica gel composite material. Enlargement is shown for the 100–200-amu region.
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For the bulk PPy powder ŽFig. 2d., the main peaks arising from the PPy chain is the cluster at 24–26 amu characteristic of carbon and CNy species as previously detected for PPy–silica composite. Oxygen species are also detected as shown by the two peaks at 16 ŽOy. and 17 ŽOHy. amu. This must be due to oxidized carboxylic group as already reported by XPS analysis of bulk PPy powder or film w32x or due to the fragmentation of paratoluenesulfonate anion dopant w16x. However, it is to note that no characteristic peak above 50 amu is detected, confirming the relative stability of PPy chain under the primary ion beam. Additional characteristic features arising from the fragmentation of para-toluenesulfonate anion dopant are clearly detected at 64 and 80 amu, due to SOy 2 and SOy 3 species, respectively. The co-dopant chloride cluster is also detected at 35 and 37 amu with a 3:1 intensity ratio as described above.
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Fig. 3 and Fig. 4 show the spectra of PPy–APS–silica gel composite in the 2–200 amu mass range in the positive and negative ion mode, respectively. Additional peaks arising from both PPy chain and APS contribute to the SIMS signal. Peak assignment is similar to that reported above for the reference materials silica, APS-treated silica gel, and PPy–silica gel particles in Table 1 and Table 2. However, Fig. 3 and Fig. 4 drastically contrast with Fig. 1a–c and Fig. 2a–c, respectively. Indeed, there is a sharp increase of all features arising from both the conducting PPy chain and para-toluenesulfonate anion dopant compared to the reference materials silica, APS-treated silica gel particles, and PPy–silica composite in both positive and negative ion mode. All peaks mentioned above for PPy–silica composite become clearly distinguished from the background noise and unambiguous assignment can be made for both PPy chain and para-toluenesulfonate anion
Fig. 5. SIMS narrow range spectra Ž55–85 amu. of PPy–silica gel Ža. and PPy–1% APS–silica composite Žb. materials in negative ion mode.
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mined the relative peaks intensities ratio from paratoluenesulfonate to APS-treated silica as a function of the initial APS concentration used to pre-treat the silica particles prior to pyrrole polymerization. The PIR is defined as follows: PIR s
Ž ISO . q Ž ISO . Ž ISiO . q Ž ISiO . y 2 y 2
Fig. 6. Relative PIR vs. the initial APS concentration used to pre-treat silica gel particles prior to pyrrole polymerization.
dopant. Moreover, in the negative ion spectrum, a characteristic feature at 171 amu is observed and is attributed to para-toluenesulfonate w22x anion dopant having the following structure:
Since the PPy chain only gives weak specific fragments under the ion beam, we chose to monitor specific features of both APSrsilica and para-toluenesulfonate anion dopant in order to determine the effect of initial APS concentration used to pre-treat silica gel prior to pyrrole polymerization. Negative ion mode yields fewer peaks, but they are more specific to each component. The specific features used to characterize APSrsilica are at 60 and 76 y amu, corresponding to SiOy 2 and SiO 3 , respectively. The fragmentation of para-toluenesulfonate anion dopant gives two specific features at 64 and 80 amu, assigned to SOy 2 and SOy 3 species as reported above. The latter peaks are more intense than that of para-toluenesulfonate at 171 y y amu. It is important to note that SiOy 2 –SiO 3 and SO 2 – y SO 3 peaks can be detected in a narrow range SIMS spectrum. This allows a fast acquisition spectrum with a minimum ion beam damage of the material under test. Moreover, mass resolution and transmission effects are minimized, allowing a more accurate SIMS quantification w16x. Fig. 5 displays narrow range spectra of PPy–silica gel ŽFig. 5a. and PPy–1% APS–silica ŽFig. 5b. composites in the negative ion mode. Fig. 5 brings a strong supporting evidence for the massive deposition of PPy at the surface of silica gel particles resulting from APS pre-treatment, as judged by the relative increase of the specific fragments of para-toluenesulfonate to APS-treated silica. In order to quantify the increase of the conducting PPy content at the silica gel surface induced by APS pre-treatment, we deter-
y 3
y 3
where I x is the peak intensity. Fig. 6 shows a plot of the relative PIR vs. initial APS concentration used to pre-treat silica gel particles prior to pyrrole polymerization. Fig. 6 shows a sharp increase of the PIR with APS pre-treatment. For PPy–1% APS–silica composite, the PIR is eight times higher than that observed for PPy–silica gel composite and for PPy–3% APS–silica gel composite material, PIR almost reaches a plateau value ca. 10 times higher than for the reference material PPy– silica gel composite. Note that this signal is recorded for only 6 to 12 wt.% of the conducting PPy in the composite materials as determined by TGA w14x. We have also monitored the doping level of PPy overlayer by comparing the y. y ŽSOy Ž y 2 q SO 3 peak intensities to that of CN . The SO 2 q y. y. Ž SO 3 r CN PIR remains constant for PPy deposited onto untreated and APS-treated silica gel particles. Moreover, the doping level ŽSrN atomic ratio. as determined by XPS analysis as a function of the initial APS concentration was matching that of bulk para-toluenesulfonate doped PPy powder w14x. Thus, APS pre-treatment of silica gel before pyrrole polymerization does not affect the doping level in PPy. This SIMS study based on specific fragments of silica, APS, and PPy confirms the hypothesis that conducting PPy–APS–silica materials have a PPy-rich surface as it was deduced from the multitechnique approach in our recently published work w14x. This increase in PPy surface content is probably due to favourable specific interactions of the Lewis acid–base type between the basic amino group Ž n donor. and the acidic N – H bonds Ž s U acceptor. andror the positively charged PPy backbone Ž n acceptor. w33–35x as shown in Scheme 1. Wettability measurements and simple PPy–substrate adhesion peel test are in progress to confirm this hypothesis using planar glass plate.
Scheme 1.
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4. Conclusion SIMS analyses were effective in monitoring the change in surface composition of silica due to polypyrrole coating vs. the initial concentration of aminopropyl–silane coupling agent used to pre-treat silica gel particles. Unambiguous features of each component Žsilica, aminopropyltriethoxy–silane, and PPy. can be detected in both positive and negative ion modes. It is clearly demonstrated that hybrid PPy–silica particles exhibit silica-rich surface. In contrast, APS-treatment of silica gel before pyrrole polymerization led to a sharp increase of the PPy content at the silica gel surface. This study shows the importance of pre-treatment of silica gel substrate by amino–silane coupling agent in order to produce a PPy-rich surface material and thus, confirms our previous multitechnique study of the role of silane coupling agent in the preparation of conducting PPy–silica composites.
Acknowledgements CP would like to thank the French Ministry of Education and Research for the provision of a scholarship and Millbrook Instrument for travel funds to UK. The authors wish to thank Dr. M.-L. Abel ŽUniversity of Surrey. for helpful discussion.
References w1x T.A. Skotheim ŽEd.., Handbook of Conducting Polymers 1 and 2 Marcel Dekker, New York, 1986. w2x M. Aldissi ŽEd.., Intrinsically Conducting Polymers: An Emerging Technology, Kluwer Academic Publishing, Dordrecht, 1993. w3x W.R. Salaneck, I. Lundstrom, ¨ B. Ramby ŽEds.., Conjugated Polymers and Related Materials, Oxford Univ. Press, London, 1993. w4x E.T. Kang, K.G. Neoh, K.L. Tan, Adv. Polym. Sci. 106 Ž1993. 135. w5x H. Ge, G.G. Wallace, React. Polym. 18 Ž1992. 133. w6x M.M. Chehimi, M.L. Abel, E. Pigois-Landureau, M. Delamar, Synth. Met. 60 Ž1993. 183. w7x M.A. De Paoli, S. Paserini, B. Scrosati, Adv. Mater. 2 Ž1990. 480. w8x J. Rodriguez, H.J. Grande, T.F. Otero, in: H.S. Nalwa ŽEd.., Handbook of Organic Conducting Molecules and Polymers 2 Wiley, 1997, p. 415.
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w9x S. Maeda, S.P. Armes, J. Mater. Chem. 4 Ž1994. 935. w10x S. Maeda, M. Gill, S.P. Armes, I.W. Fletcher, Langmuir 11 Ž1995. 1899. w11x H. Chriswanto, H. Ge, G.G. Wallace, Chromatographia 37 Ž1993. 423. w12x C. Perruchot, M.M. Chehimi, D. Mordenti, M. Briand, M. Delamar, J. Mater. Chem. 8 Ž1998. 2185. w13x J.R. Conder, J. High Resolut. Chromatogr. Chromatogr. Commun. 5 Ž1982. 341. w14x C. Perruchot, M.M. Chehimi, M. Delamar, F. Fievet, Surf. Interface Anal. 26 Ž1998. 689. w15x D. Kowalczyk, S. Slomkowski, M.M. Chehimi, M. Delamar, Int. J. Adhes. Adhes. 16 Ž1996. 227. w16x D. Briggs, A. Brown, J.C. Vickerman ŽEds.., Handbook of Static Secondary Ion Mass Spectroscopy ŽSIMS., Wiley, Chichester, 1989. w17x N.M. Reed, J.C. Vickerman, in: L. Sabbatini, P.G. Zambonin ŽEds.., Surface Characterization of Advanced Polymers, VCH, Weinheim, 1993, p. 83. w18x E.P. Plueddmann, in: K.L. Mittal ŽEd.., Silanes and Other Coupling Agents, VSP, Ultrecht, The Netherlands, 1992. w19x G. Morea, L. Sabbatini, R.H. West, J.C. Vickerman, Surf. Interface Anal. 18 Ž1992. 421. w20x G. Morea, L. Sabbatini, P.G. Zambonin, A.J. Swift, R.H. West, J.C. Vickerman, Macromolecules 24 Ž1991. 3630. w21x M.J. Hearn, I.W. Fletcher, S.P. Church, S.P. Armes, Polymer 34 Ž1993. 262. w22x M.L. Abel, S.R. Leadley, M.A.M. Brown, J. Petitjean, M. Chehimi, J.F. Watts, Synth. Met. 66 Ž1994. 85. w23x M.L. Abel, M.M. Chehimi, A.M. Brown, S.R. Leadley, J.F. Watts, J. Mater. Chem. 5 Ž1995. 845. w24x M.M. Chehimi, M.L. Abel, F. Fricker, M. Delamar, A. Jada, A.M. Brown, J.F. Watts, J. Chim. Phys. 95 Ž1998. 1282. w25ax A.J. Eccles, T.A. Steele, in: G. Gillen, R. Lareau, J. Bennett, F. Stevie ŽEds.., Secondary Ion Mass Spectrometry SIMS XI, John Wiley, Chichester, 1998, p. 775. w25bx A.J. Eccles, T.A., Steele, A.W. Robinson, Broadening the horizons of SIMS: the low cost chemical microscope, Appl. Surf. Sci. 144 Ž1999. 100. w26x I.S. Gilmore, M.P. Seah, Surf. Interface Anal. 23 Ž1995. 191. w27x B.N. Eldridge, L.P. Buchwater, C.A. Chess, M.J. Golldberg, R.D. Goldblattcand, F.P. Novak in Ref. 18, p. 305. w28x W.J. Van Ooij, A. Sabata in Ref. 18, p. 323. w29x D. Wang, F.R. Jones, J. Mater. Sci. 28 Ž1993. 1396. w30x D. Wang, F.R. Jones, J. Mater. Sci. 27 Ž1992. 36. w31x H. Naarman, Synth. Met. 41–43 Ž1991. 1. w32x E.T. Kang, K.G. Neoh, Y.K. Ong, K.L. Tan, B.T.G. Tan, Macromolecules 24 Ž1991. 2822. w33x F.M. Fowkes, D.W. Dwight, D.A. Cole, T.C. Huang, J. Non-Cryst. Solids 120 Ž1990. 47. w34x F.M. Fowkes, J. Adhes. Sci. Technol. 1 Ž1987. 7. w35x D.E. Leyden ŽEd.., Silanes, Surfaces and Interfaces, Gordon and Breach, New York, 1985.
SIMS analysis of conducting polypyrrole–silica gel composites Christian Perruchot a , Mohamed M. Chehimi a,) , Michel Delamar a , John A. Eccles b, Timothy A. Steele b, Craig D. Mair b a
(ITODYS), UniÕersite´ Paris 7, Denis Diderot, CNRS (UPRESA 7086), 1 rue Guy de la Brosse, Institut de Topologie et de Dynamique des Systemes ` 75005 Paris, France b Millbrook Instrument, Blackburn Technology Centre, Challenge Way, Blackburn, Lancashire BBI 5QB, England, UK Received 1 June 1999; received in revised form 20 October 1999; accepted 26 November 1999
Abstract Secondary ion mass spectrometry ŽSIMS. has been used to characterize novel hybrid conducting polypyrrole–silica gel composites. The silica gel particles act as a high surface inorganic substrate for the in situ chemical synthesis of polypyrrole ŽPPy. in aqueous media. However, the conventional method using untreated silica gel as a host material for pyrrole polymerisation led to insulating PPy–silica composites, which exhibit a silica-rich surface. By contrast, pre-treatment of the silica gel particles by aminopropyltriethoxysilane ŽAPS. prior to polymerisation results in conducting PPy–APS–silica composites, the surface of which was found to be PPy-rich. The present study uses the unique capabilities of SIMS to distinguish unambiguously between fragments from the silane coupling agent and the conducting PPy overlayers. Specific negative ions from PPy and the underlying substrate particles were effective in monitoring the change in the PPy content at the surface of the composites vs. the initial concentration of APS used to pre-treat silica gel particles. This SIMS analysis clearly demonstrated that APS is effective in increasing the surface PPy content and thus confirms previously published XPS data based on some necessary assumptions. q 2000 Elsevier Science S.A. All rights reserved. Keywords: SIMS; Conducting polypyrrole; Silica gel; Silane coupling agent; Composite materials
1. Introduction Inherent conducting polymers ŽICPs. constitute a new class of polymers with particular interest owing to their physical and chemical properties w1x. Polypyrrole ŽPPy. is one of the most studied ICP due to its relative stability but suffers poor processability. To overcome these limitations, many research groups focused on the preparation of conducting PPy-based composite materials w2x. Extensive research on such materials has the main objective to synthesize novel composites, which combine both properties of the conducting polymer Žconductivity w3x, redox properties w4x, ion and proton exchange w5x, Lewis acid–base interactions w6x, optical properties w7x, etc.. and those of the mineral host material on the one hand, and to improve the processability of insoluble and infusible conducting PPy polymer on the other hand w8x. Maeda and Armes w9x
)
Corresponding author. Fax: q33-1-4427-6814. E-mail address: [email protected] ŽM.M. Chehimi..
reported the synthesis of nanoscale PPy–silica composites. However, such materials exhibit a silica-rich surface as found by XPS analysis w10x. Furthermore, conductivity measurements showed that PPy–silica composites are poorly conducting or even insulating thus confirming that they do have a silica-rich surface. Of relevance to the present study, Chriswanto et al. w11x synthesized PPy–silica gel composite and used it as novel stationary phase in high performance liquid chromatography ŽHPLC.. Such stationary phase was further found to have reversed phase and ion-exchange properties w5x. However, we found by X-ray photoelectron spectroscopy ŽXPS. w12x that composites synthesized using a similar protocol are silica-rich surface materials and thus, the chromatographic performances of PPy may be obscured by those of silica w13x. In order to obtain similar composite materials with rather a PPy-rich surface, we have suggested to modify the protocol of Chriswanto et al. w11x by pre-treating the silica gel particles by a common silane coupling agent, aminopropyltriethoxy silane ŽAPS.. In this case, the silica–APS particles acted as a porous host for the in situ polymerisa-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 0 3 - 3
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C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
tion of pyrrole in water. A free-flowing powder of PPy– APS–silica composites was obtained w12x. This material exhibits a high relative surface content of PPy at the silica surface and an increase of conductivity by at least four orders of magnitude is observed Žup to 0.1 Srcm.. Although APS significantly affected the specific surface area Ždecrease from 430 to 225 m2rg. of silica gel particles, we still obtained a fairly high specific surface area for the PPy–APS–silica end-product Ž180 m2rg. compared to bulk PPy powder Ž15–25 m2rg. w14x. Prior to synthesis of the conducting composites, we have determined an adsorption isotherm of APS by means of XPS w14x and found that it fitted the Langmuir type as for planar quartz surfaces w15x. The plateau value was reached for an initial concentration of APS of 1%. It is for this value that conductivity of the PPy–silica–APS became measurable and reached 2.7 10y2 Srcm, whereas for an initial concentration of 0.5% only, the composites were much less conductive Ž10y5 Srcm.. These improvements resulted from only a slight increase from 6 to 9 wrw % of PPy content as determined by thermogravimetric analysis ŽTGA.. Although the XPS was extremely useful in monitoring the changes in the surface composition of silica due to APS and PPy, it was not possible to distinguish between the nitrogen and carbon species arising from both APS and PPy. Thus, the determination of the relative proportion of PPy at the surface of the conducting PPy–APS–silica composites was difficult to determine, and some assumptions were made in this regard w14x. In this study, we would like to explore the capabilities of secondary ion mass spectrometry ŽSIMS. to complement our recent studies of the surface composition of the conducting composites mentioned above. SIMS has a superior surface specificity Žca. 1 nm. compared to XPS Žca. 5–10 nm.. In addition, it detects specific fragments from the materials under test in either the positive or the negative mode of detection w16x. SIMS has been widely used to characterize conventional polymers and polymer blends w17x on the one hand, and silane coupling agent-modified substrates on the other hand w18x. However, few SIMS studies onto conducting polymers have been reported until now w19–24x. Concerning conducting PPy, Hearn et al. w21x examined by ToF-SIMS the nature of aromatic sulphonate anion dopant in various PPy-textile fibers. Abel et al. w22x reported ToF-SIMS characterization of electrochemically conducting PPy films with various anion dopants. It was clearly shown that specific fragments could be assigned to both the PPy backbone and anion dopant in the negative ion mode. In a subsequent paper, the capabilities of ToF-SIMS were explored to determine adsorption isotherms of PMMA onto sulfate-doped PPy powder, which were found to be of the Langmuir type w23x. Chehimi et al. w24x have also used ToF-SIMS to study the adsorption of polymer blends and a block copolymer onto PPy powders. In this paper, we describe the use of SIMS as a complementary surface analytical technique to XPS to our on
going research programme on conducting polymer composites. We investigated the effect of aminopropyl-silane ŽAPS. pre-treatment of silica gel particles before PPy coating and determine surface properties of the resulting novel hybrid conducting PPy–silica gel composites.
2. Experimental 2.1. Synthesis of aminopropyltriethoxysilane-grafted silica gel particles APS ŽAcros, 0.5% to 3% vrv. was first hydrolysed 6 h in a 200 ml de-ionized waterrethanol Ž1r9 vrv. solution and then 2 g of bare silica gel ŽMerck, diameter in the 60 to 125 mm range, Vporous s 0.75 cm3rg,. was added to the solution and stirred overnight. This mixture was Buchner¨ filtered, rinsed with 50 ml of de-ionized waterrethanol Ž1r9 vrv. solution to remove the excess of physically adsorbed silane coupling agent, and dried overnight in a dessicator. 2.2. Synthesis of PPy–silica particles 2.2.1. PPyTS–silica Pyrrole ŽAcros, 1 ml. was added to a suspension of 2 g of silica particles in 50 ml of pentane ŽProlabo, normapur grade. and the mixture was stirred in a fumehood until free-flowing pyrrole-coated silica particles were obtained. These particles were then added to 100 ml of aqueous solution of iron chloride ŽAldrich, 36.0 mmol. and paratoluenesulfonate sodium salt ŽAldrich, 36.0 mmol. at room temperature. This solution was stirred for 24 h and the resulting black PPy–silica particles were vacuum-filtered, washed with copious amounts of de-ionized water, and then dried in a dessicator overnight. 2.3. Synthesis of PPy-coated, APS-grafted silica particles 2.3.1. PPyTS–APS–silica The APS coupling agent was first grafted onto the silica particles before pyrrole coating and polymerization mentioned above. 2.4. Synthesis of PPy powders 2.4.1. PPyTS Pyrrole Ž1.00 ml, 14.4 mmol. was added via syringe to 100 ml of a stirred aqueous solution containing iron chloride Ž36.0 mmol. and para-toluenesulfonate sodium salt Ž36.0 mmol. at room temperature. The reaction solution was stirred for 24 h and the resulting black precipitate was
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
vacuum-filtered and washed with copious amounts of deionized water until the washings were clear. The powder was then dried in a dessicator overnight and sieved to 180 mm diameter before analysis.
2.5. SIMS analyses SIMS analyses were performed using a Chemical Microscope, an automated benchtop SIMS system from Millbrook Instruments w25a,bx. In this instrument, the primary beam comprises 6 keV Gaq ions, focused to a 50-mm spot size and rastered over the sample surface to irradiate a larger area. The mass spectrometer is a 300 mrz quadrupole with low energy secondary ion collection optics; dynamic emittance matching is employed to maximize collection efficiency. Charge neutralisation for insulating samples is provided by an auxiliary electron gun using indirect irradiation and an oscillating sample bias, a design based on the system devised by Gilmore and Seah w26x. Samples for analysis were prepared by pressing each of the powdered materials into indium foil, taking care to ensure complete coverage. Fresh samples were used to acquire both the positive and negative spectra. Charge neutralisation was required only for the untreated silica gel
55
sample. The averaged primary ion dose used to acquire each survey spectrum Ž2–200 Da. was below 1 = 10 12 ions cmy2 , well within the accepted static SIMS limit. Significant attenuation of higher mass peaks was in fact noted for primary ion doses above 5 = 10 12 ions cmy2 . For the quantitative study of the effect of varying APS pre-treatment, the averaged primary ion dose therefore had to be kept below 1 = 10 12 ions cmy2 per sample. This was achieved by acquiring negative mass spectra only over the range 55–85 Da; this narrow mass range contained peaks characteristic of both the substrate and the PPy, but the restricted mass range allowed a longer dwell time at each point, and hence, much improved counting statistics. Five samples were analysed for each APS pre-treated material. Since the Chemical Microscope features fast sample loading and the facility to pre-set standard experimental parameters, analysis of each sample was completed in under 10 min.
3. Results and discussion All materials were characterized in both positive Žexcept for bulk PPy powder. and negative ion mode in order to determine specific features due to silica gel particles,
Table 1 Peak identification for silica, silica–APS, silica–PPy and silica–APS–PPy materials in positive ion mode mrz 12 15 17 18 26 27 28 29 30 31 39 41 42 43 44 45 55 56 57 69 71 91 93 131
Formula
Silica
q
C CHq 3 NHq 3 q NH 4 CNqrC 2 Hq 2 CNHq SiqrC 2 Hq 4 SiHqrC 2 Hq 5 CH 4 Nq q CH 5 N CH 3 Hq 3 C 3 Hq 5 q C 3 Hq 6 rC 2 H 4 N C 3 Hq 7 C 2 H 6 Nq SiOHq C 4 Hq 7 C 3 H 6 NqrSiC 2 Hq 4 C 4 Hq 9 69 q C 5 Hq 9 r Ga 71 q C 5 Hq r Ga 11 C 7 Hq 7 C 7 Hq 9
U
U U
U U U U U U U U U U
Silica–APS
Silica–PPy
Silica–APS–PPy
U U U U
U U
U U U U U U U U U U U U U U U U U U U
U U U
U U U U U U U U U U U U U U U U U U U U U U U
U U U U U U U U U U U
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
56
silane coupling agent and PPy overlayer, respectively. Since SIMS is by nature a destructive method, we used different samples from the same batch in order to determine average relative peak mass intensity. PPy–APS–silica and PPy–silica were the composite materials of interest, while fresh silica, silica–APS, and PPy bulk powder were used as reference materials. Table 1 and Table 2 report peak assignment of bare silica gel, APS-treated silica gel particles, hybrid PPy–silica gel composite and bulk PPy powder detected in positive and negative ion mode, respectively. Fig. 1 shows the positive ion mode spectra of untreated silica gel particles Ža., APS-treated silica particles Žb., and hybrid PPy–silica composite Žc. in the 2–200 amu mass range, respectively. For silica gel particles, Fig. 1a shows typical siliconcontaining fragments such as Siq and SiOHq. Additional weak peaks due to low carbon contamination are also detected as previously observed by XPS analysis w12x. Fig. 1b shows the positive ion mode spectra of APStreated silica gel particles. In addition to features observed for the untreated silica gel particles, unambiguous characteristic peaks due to APS are also detected. Characteristic fragments are assigned to amino end functionality Ži.e., q q NHq and CH 5 Nq or CH 3 Oq ., which 3 , NH 4 , CH 4 N were previously reported for APS-treated substrates w27x. An additional peak at 56 amu is clearly detected and can be assigned to C 3 H 6 Nq fragment w28x. Moreover, the clusters of peaks at 39, 41, 43 amu, 53, 55, 57 amu and 69, 71 amu are typical aliphatic hydrocarbon fragments of the form C n H 2 ny3 , C n H 2 ny1 and C n H 2 nq1 , arising from the propyl chain. It is to note that they are more intense than
those arising from the untreated silica gel. A low intensity peak is also detected at 131 amu. This fragment is assigned to the following structure w28x:
Nevertheless, indium foil may be irradiated by the primary beam and emits InOq fragment at mrz s 131 amu. This is in line with a peak observed at mrz s 115 amu, corresponding to Inq. However, these peaks are not observed simultaneously for silica gel particles and silica–APS–PPy composite material. Moreover, in case of silica–APS–PPy composite material, the absence of these peaks at 115 and 131 amu can be interpreted in term of homogeneous and compact PPy coating onto APS-treated silica particles. It is worth noting that in the case where both peaks at mrz s 115 and 131 amu are observed, the intensity ratio 131r115 is much larger for silica–APS than for silica–PPy. Therefore, one can conclusively attribute the peak at mrz s 131 amu to the previously reported structure than to InOq fragment. It is also to note that APS overlayer leads to a slight decrease of the 45qr28q ŽSiOHqrSiq . peak intensity ratio ŽPIR. from 0.4 to 0.36 that is a slight depletion of silica. However, APS-treated silica particles still exhibit a silica-rich surface.
Table 2 Peak identification for silica, silica–APS, silica–PPy, bulk PPy powder and silica–APS–PPy materials in negative ion mode mrz 12 13 16 17 24 25 26 32 35 37 48 60 61 64 76 77 80 171
Formula y
C CHy Oy OHy Cy 2 C 2 Hy CNyrC 2 Hy 2 Oy 2 Cly Cly SOy SiOy 2 SiO 2 Hy y SO 2 SiOy 3 SiO 3 Hy SOy 3
Silica
Silica–APS
Silica–PPy
PPy
Silica–APS–PPy
U U U U U U U U U
U U U U U U U U U U
U U U U U U U U U U
U U U U U U U U U U U
U U
U U
U U
U U
U U U U U U
U U U U U U U U U U U U U U U
U
U
U U
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
57
Fig. 1. SIMS positive ion mode spectra Ž0–200 amu. of untreated silica gel particles Ža., APS-treated silica gel particles Žb., and hybrid PPy–silica gel composite materials Žc.. For each spectrum, enlargements are shown for 100–200 amu region.
58
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
Fig. 2. SIMS negative ion mode spectra Ž0–200 amu. of untreated silica gel particles Ža., APS-treated silica gel particles Žb., hybrid PPy–silica gel composite material Žc., and bulk PPy powder Žd.. For each spectrum, enlargements are shown for the 100–200-amu region.
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
59
Fig. 2 Ž continued ..
Fig. 1c shows the positive ion spectrum of PPy–silica gel composite. In addition to the silica features, some characteristic peaks from the polymer chain are also detected. The PPy overlayer led to a relative increase of features at 15 and 27 amu, which are typical of CHq 3 and CNHq characteristic fragments of the polymer chain w22x. Additional clusters at 39, 41, 43 amu and 53, 55, 57 amu are also detected and can be assigned to aliphatic carbon fragments of PPy chain. Para-toluenesulfonate anion dopant also gives a specific fragment in the positive ion mode w22x. A typical feature at 91 amu can be assigned to the following structures:
However, the most stable structure known of C 7 Hq 7 fragment is that of tropyllium. Although both silica and PPy chain contributed to the SIMS signal, this study supports the view that PPy–silica material still exhibits a silica-rich surface as shown by the relative intense signals at 28 and 45 amu arising from the host silica gel substrate. This result is in good agreement with previously reported XPS study of similar hybrid PPy–silica gel particles w14x and PPy–silica nanocomposites w10x. Fig. 2 shows the negative ion mode spectra of untreated silica gel particles Ža., APS-treated silica gel particles Žb., hybrid PPy–silica composite Žc., and bulk PPy powder Žd. in the 2–200 amu mass range, respectively. For the untreated silica gel particles, Fig. 2a displays two intense peaks at 16 and 17 amu, corresponding to Oy
and OHy, respectively. In addition, typical ions of silica can be detected at 60, 61, 76, 77 amu and assigned to y y y SiOy species, respectively 2 , SiO 2 H , SiO 3 , and SiO 3 H w16x. Weak signals are recorded at 24 and 25 amu, probably due to low carbon contamination species. This is in good agreement with the positive ion mode as mentioned above and XPS analysis of bare silica gel particles w12x. For APS-treated silica gel particles, Fig. 2b shows few new specific peaks compared to the untreated silica gel particles. The 17r16 PIR slightly increases due to the increase of oxygen content at the outermost silica surface and silanol group due to APS treatment. Another possible reason for such increase in the OHyrOy peak ratio is that APS may adsorb onto silica via the amino group w29,30x, an interaction, which permits the hydrolysed silane OH group to be at the outermost layer. Additional peaks at 24, 25, and 26 amu can be detected and assigned to carbon species and CNy fragments, respectively. These results are in good agreement with those previously reported by Wang and Jones w29,30x. This study confirms the presence of a thin overlayer of the aminosilane coupling agent at the silica gel surface. However, no structural information can be obtained concerning the interaction mechanism of the coupling agent with the silica gel particles. Moreover, since we used a low APS concentration to treat the silica particles, no characteristic features above 150 amu were detected. Negative ion mode spectrum of hybrid PPy–silica gel composite is shown in Fig. 2c. The main information arising from the conducting polymer is a typical fragment at 26 amu, which is due to CNy anion generated by the PPy chain w22x. A significant increase of the intensity of features at 24 and 25 amu coming from hydrocarbon fragments is also observed. However, little structural information can be gained from the negative ion mode of PPy
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C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
Fig. 3. SIMS positive ion mode spectrum Ž0–200 amu. of PPy–APS–silica gel composite material. Enlargement is shown for the 100–200-amu region.
probably due to the relative stability of its backbone under the ion beam. Such stability is perhaps driven by the partially crosslinked structure of PPy chain w31x. Additional features arising from para-toluenesulfonate anion dopant are also clearly detected. Indeed, the fragmentation of para-toluenesulfonate anion gives characteristic peaks at 64 and 80 amu. These peaks are attributed to SOy 2 and SOy 3 species. However, there is no evidence of paratoluenesulfonate anion dopant at 171 amu. This is likely to be due to the low PPy content at the surface of untreated silica gel particles as determined by TGA and XPS study w14x. Since iron chloride is used as oxidant for the chemical synthesis of PPy, halide Cly is inserted as a co-dopant,
thus, leading to the detection of mass peaks at 35 and 37 amu and to an the intensity ratio 3:1 Žnatural isotopic abundance.. It is to note that very low concentration of chloride species were detected by XPS whereas, in this present work, these ions are readily detected by SIMS due to its superior detection limit. Moreover, halides species are very stable ions. Since positive and negative ion mode spectra of PPy– silica composite are very similar to that of untreated silica gel particles, SIMS analysis confirms that the PPy–silica composite exhibits a silica-rich surface. This result is in good agreement with previously reported XPS analysis of such material w14x.
Fig. 4. SIMS negative ion mode spectrum Ž0–200 amu. of PPy–APS–silica gel composite material. Enlargement is shown for the 100–200-amu region.
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For the bulk PPy powder ŽFig. 2d., the main peaks arising from the PPy chain is the cluster at 24–26 amu characteristic of carbon and CNy species as previously detected for PPy–silica composite. Oxygen species are also detected as shown by the two peaks at 16 ŽOy. and 17 ŽOHy. amu. This must be due to oxidized carboxylic group as already reported by XPS analysis of bulk PPy powder or film w32x or due to the fragmentation of paratoluenesulfonate anion dopant w16x. However, it is to note that no characteristic peak above 50 amu is detected, confirming the relative stability of PPy chain under the primary ion beam. Additional characteristic features arising from the fragmentation of para-toluenesulfonate anion dopant are clearly detected at 64 and 80 amu, due to SOy 2 and SOy 3 species, respectively. The co-dopant chloride cluster is also detected at 35 and 37 amu with a 3:1 intensity ratio as described above.
61
Fig. 3 and Fig. 4 show the spectra of PPy–APS–silica gel composite in the 2–200 amu mass range in the positive and negative ion mode, respectively. Additional peaks arising from both PPy chain and APS contribute to the SIMS signal. Peak assignment is similar to that reported above for the reference materials silica, APS-treated silica gel, and PPy–silica gel particles in Table 1 and Table 2. However, Fig. 3 and Fig. 4 drastically contrast with Fig. 1a–c and Fig. 2a–c, respectively. Indeed, there is a sharp increase of all features arising from both the conducting PPy chain and para-toluenesulfonate anion dopant compared to the reference materials silica, APS-treated silica gel particles, and PPy–silica composite in both positive and negative ion mode. All peaks mentioned above for PPy–silica composite become clearly distinguished from the background noise and unambiguous assignment can be made for both PPy chain and para-toluenesulfonate anion
Fig. 5. SIMS narrow range spectra Ž55–85 amu. of PPy–silica gel Ža. and PPy–1% APS–silica composite Žb. materials in negative ion mode.
62
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
mined the relative peaks intensities ratio from paratoluenesulfonate to APS-treated silica as a function of the initial APS concentration used to pre-treat the silica particles prior to pyrrole polymerization. The PIR is defined as follows: PIR s
Ž ISO . q Ž ISO . Ž ISiO . q Ž ISiO . y 2 y 2
Fig. 6. Relative PIR vs. the initial APS concentration used to pre-treat silica gel particles prior to pyrrole polymerization.
dopant. Moreover, in the negative ion spectrum, a characteristic feature at 171 amu is observed and is attributed to para-toluenesulfonate w22x anion dopant having the following structure:
Since the PPy chain only gives weak specific fragments under the ion beam, we chose to monitor specific features of both APSrsilica and para-toluenesulfonate anion dopant in order to determine the effect of initial APS concentration used to pre-treat silica gel prior to pyrrole polymerization. Negative ion mode yields fewer peaks, but they are more specific to each component. The specific features used to characterize APSrsilica are at 60 and 76 y amu, corresponding to SiOy 2 and SiO 3 , respectively. The fragmentation of para-toluenesulfonate anion dopant gives two specific features at 64 and 80 amu, assigned to SOy 2 and SOy 3 species as reported above. The latter peaks are more intense than that of para-toluenesulfonate at 171 y y amu. It is important to note that SiOy 2 –SiO 3 and SO 2 – y SO 3 peaks can be detected in a narrow range SIMS spectrum. This allows a fast acquisition spectrum with a minimum ion beam damage of the material under test. Moreover, mass resolution and transmission effects are minimized, allowing a more accurate SIMS quantification w16x. Fig. 5 displays narrow range spectra of PPy–silica gel ŽFig. 5a. and PPy–1% APS–silica ŽFig. 5b. composites in the negative ion mode. Fig. 5 brings a strong supporting evidence for the massive deposition of PPy at the surface of silica gel particles resulting from APS pre-treatment, as judged by the relative increase of the specific fragments of para-toluenesulfonate to APS-treated silica. In order to quantify the increase of the conducting PPy content at the silica gel surface induced by APS pre-treatment, we deter-
y 3
y 3
where I x is the peak intensity. Fig. 6 shows a plot of the relative PIR vs. initial APS concentration used to pre-treat silica gel particles prior to pyrrole polymerization. Fig. 6 shows a sharp increase of the PIR with APS pre-treatment. For PPy–1% APS–silica composite, the PIR is eight times higher than that observed for PPy–silica gel composite and for PPy–3% APS–silica gel composite material, PIR almost reaches a plateau value ca. 10 times higher than for the reference material PPy– silica gel composite. Note that this signal is recorded for only 6 to 12 wt.% of the conducting PPy in the composite materials as determined by TGA w14x. We have also monitored the doping level of PPy overlayer by comparing the y. y ŽSOy Ž y 2 q SO 3 peak intensities to that of CN . The SO 2 q y. y. Ž SO 3 r CN PIR remains constant for PPy deposited onto untreated and APS-treated silica gel particles. Moreover, the doping level ŽSrN atomic ratio. as determined by XPS analysis as a function of the initial APS concentration was matching that of bulk para-toluenesulfonate doped PPy powder w14x. Thus, APS pre-treatment of silica gel before pyrrole polymerization does not affect the doping level in PPy. This SIMS study based on specific fragments of silica, APS, and PPy confirms the hypothesis that conducting PPy–APS–silica materials have a PPy-rich surface as it was deduced from the multitechnique approach in our recently published work w14x. This increase in PPy surface content is probably due to favourable specific interactions of the Lewis acid–base type between the basic amino group Ž n donor. and the acidic N – H bonds Ž s U acceptor. andror the positively charged PPy backbone Ž n acceptor. w33–35x as shown in Scheme 1. Wettability measurements and simple PPy–substrate adhesion peel test are in progress to confirm this hypothesis using planar glass plate.
Scheme 1.
C. Perruchot et al.r Synthetic Metals 113 (2000) 53–63
4. Conclusion SIMS analyses were effective in monitoring the change in surface composition of silica due to polypyrrole coating vs. the initial concentration of aminopropyl–silane coupling agent used to pre-treat silica gel particles. Unambiguous features of each component Žsilica, aminopropyltriethoxy–silane, and PPy. can be detected in both positive and negative ion modes. It is clearly demonstrated that hybrid PPy–silica particles exhibit silica-rich surface. In contrast, APS-treatment of silica gel before pyrrole polymerization led to a sharp increase of the PPy content at the silica gel surface. This study shows the importance of pre-treatment of silica gel substrate by amino–silane coupling agent in order to produce a PPy-rich surface material and thus, confirms our previous multitechnique study of the role of silane coupling agent in the preparation of conducting PPy–silica composites.
Acknowledgements CP would like to thank the French Ministry of Education and Research for the provision of a scholarship and Millbrook Instrument for travel funds to UK. The authors wish to thank Dr. M.-L. Abel ŽUniversity of Surrey. for helpful discussion.
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