C. Malitesta

NEW FINDINGS ON POLYPYRROLE CHEMICAL STRUCTURE BY XPS COUPLED TO CHEMICAL DERIVATIZATION LABELLING

Abstract Polypyrrole electrosynthesised from aqueous solutions has been investigated by XPS, both in the conducting (PPY) and in the so-called overoxidised (PPYox) state. An accurate analysis of high resolution spectra, including cross-checking between the independent fittings of different though related signals (e.g. C1s and O1s), is presented, enlightening novel findings on polypyrrole structure. In particular, it suggested, for the first time, that overoxidation breaks polymer chains, producing new oxygen function on α-C (mainly Cue5fbO). The application of a new chemical derivatization (CD) method to the labelling of this key functional group (Cue5fbO) would confirm this feature.

Interference-free glucose sensor based on glucose-oxidase immobilized in an overoxidized non-conducting polypyrrole film

SummaryAn amperometric glucose sensor is described; it is based on glucose oxidase immobilized in an overoxidized non-conducting polypyrrole membrane. The overoxidation process of polypyrrole produces a permselective, antifouling membrane capable of rejecting ascorbate, urate, acetaminophen and cysteine, as well as proteins and other surface active components typically present in serum. The amperometric assay of glucose in serum correlates well with an established routine procedure based on an enzymatic-colorimetric method.

Electrosynthesised thin polymer films: the role of XPS in the design of application oriented innovative materials

The paper reviews some significant pieces of work carried out in the authors’ laboratory in the course of about three decades and concerned with the development of market oriented devices, which exploit the singular characteristics of electrosynthesised polymers. The strategic role of X-Ray Photoelectron Spectroscopy is underlined not only as a powerful technique for the characterization of these thin films, but mainly as an unvaluable tool to feedback optimization procedures both in film synthesis and modification. Case studies relevant to the development of permselective membranes for biosensors, of biocompatible coatings and active layers for gas sensors have been selected and reviewed. Future trends and prospects of the work in progress are also described.rnrnPrefacernrnDuring electrochemical experiments electrode materials often undergo more or less pronounced modifications of their surface composition, for example by chemical interaction with the media and/or as a consequence of the applied potential. These phenomena, should they be promoted deliberately or completely undesired, can influence the behaviour of the electrode systems particularly when surface species are directly involved in the electrochemical processes.rnrnThis can lead to a lack of molecular specificity in electrochemical measurements so that the knowledge of the chemical composition of the electrode surface becomes of paramount importance. In this context, surface analysis techniques have played, and still play, a key-role in the characterization of surface chemically-modified electrodes.rnrnIn the course of a systematic investigation performed in our laboratory in the period 1960–1975, aimed at the development of oxygen and hydrogen electrodes to be employed in medium temperature fused salt fuel cells, irreversible potentiometric behaviours, largely dependent on the working temperature, and Nernstian behaviours different from those expected on the basis of the overall electrode processes, were observed. These results were rationalised by hypothesizing potential-determining steps involving solid species (metal oxides) present on the electrode surface [1–4]. In the following years, combined potentiometric and XPS studies were performed on those (and others) electrode systems [5–9] and the results obtained well supported the proposed mechanistic models through the detection and identification of solid surface species.rnrnAs an example of this research activity, results obtained on the system (Ni)CO2,O2/CO3= in a (Na,K)NO3 equimolar mixture will be briefly reviewed. Potentiometric measurements performed in the temperature range 507<T<623 K in molten nitrates containing a carbonate ion concentration in the range 10−5<[CO3=]<10−2 mol kg−1 and fluxed with a mixture of CO2 and O2 at variable partial pressures indicated that at the highest tested temperature the system was irreversible and the potential was independent of oxygen concentration. This was interpreted on the basis of the following mechanistic modelrnrnMechanism I:rn(1)2NiO+CO3= ⇔ NiO-O-NiO+CO2 +2e(fast)rnrn(2)Ni2O3 ⇒ 2NiO+1/2 O2(slow)rnrn-----------------------------------------------------rnrn(3)CO3= ⇔ CO2+1/2 O2+2ernWhere reaction 1 represents the potential-determining step and reaction 3 the overall [10,3] electrode process.rnrnOn the other hand a dependence of the potential on O2 (other than CO2) was seen at lower temperatures. At the same time a large irreversibility of the system was still present. To explain this behavior, the presence of a second parallel mechanism, increasingly competitive with the first one as the temperature decreases, was proposed.rnrnMechanism II:rn(4)2NiO+CO3=⇔ NiO–CO2–NiO +1/2 O2 +2e(fast)rnrn(5)NiO–CO2–NiO ⇒ 2NiO +CO2(slow)rnrn------------------------------------------------------------rnrnCO3= ⇔ CO2+1/2 O2+2e(3)rnrnrnAs is apparent, both mechanisms involve, in the potential determining step, solid species present on the electrode surface. Due to the strong oxidising power of nitrate melts, the following reactions can occur when nickel electrodes are immersed in such a solventrnNi +NO3−⇔ NiO +NO2−rnrn2NiO +NO3− ⇔ Ni2O3+NO2−rnrnNi2O3+Ni⇔ 3NiOrnrnrnAn XPS study was carried out on this system in order to verify the reliability of the proposed mechanistic models. Spectra recorded on nickel foils maintained in contact with the melt at two different temperatures and for different lapses of time are reported in Figs. 1 and 2rnDownload high-res image (149KB)rnDownload full-size imagernrnrnrnFig. 1. Typical photoelectron spectra relevant to the nickel 2p3/2 level recorded at the following metal-melt times of contact (in min): (a)0.0, (b)5.0, (c)30.0, (d)90.0, (e)120.0. T=511 K. The arrows define the range in which binding energies of some nickel compounds (NiO, Ni2O3, Ni(OH)2, NiCO3) fall.rnrnrnDownload high-res image (87KB)rnDownload full-size imagernrnrnrnFig. 2. Typical photoelectron spectra relevant to the nickel 2p3/2 level recorded at the following metal-melt times of contact: (a’) 0.0, (b’) 5.0 s, (c’) 5.0 min. T=609 K. The arrows define the range in which binding energies of some nickel compounds (NiO, Ni2O3, Ni(OH)2, NiCO3) fall.rnrn. The presence of oxidised species is clear; Ni2O3 and Ni(OH)2 species seem to be present in the first tract of the experiment, while, for longer periods of contact, NiO becomes the predominant one. This is consistent with the fact that Ni2O3 is the primary species formed on nickel-oxygen surfaces [11,12], since kinetically favoured, and with the observation that NiO is the only known stable “bulk” oxide.rnrnIn order to test for the presence of carbonate species [involved in the proposed mechanism II] XPS spectra were recorded on nickel foils maintained in contact with the melt for different lapses of time, under two different gas atmospheres, both at low (511 K) and high (609 K) temperature. The C1s signals reported in Figs. 3 and 4rnDownload high-res image (82KB)rnDownload full-size imagernrnrnrnFig. 3. Typical photoelectron spectra relevant to C1s region recorded on nickel samples under the following working conditions. Metal-melt times of contact (in min): (a’) 10.0, (b’) 30.0, (c’) 120.0. T=511 K; gas atmosphere: CO2=0.9 atm, O2=0.1atm.rnrnrnDownload high-res image (62KB)rnDownload full-size imagernrnrnrnFig. 4. Typical photoelectron spectra relevant to C1s region recorded on nickel samples under the following working conditions. Metal-melt times of contact (in min): (a’) 10.0, (b’) 30.0, (c’) 120.0. T=609 K; gas atmosphere: CO2=0.9 atm, O2=0.1atm.rnrnclearly show that at low temperature a carbonate peak (ca. 290 eV) is present, whose intensity depends on carbon dioxide partial pressure and on the time of contact between nickel substrate and molten nitrates. At high temperatures, whatever the composition of the gas mixture and the time of immersion of the nickel foils, no carbonate peak could be recorded. As expected, at this temperature the formation of a nickel carbonate species is strongly prevented because of its thermal decomposition.rnrnIn conclusion, XPS investigation unequivocally showed that solid species (carbonate and/or oxides) are formed on the surface of nickel electrode, thus confirming the hypothesis that the large potentiometric irreversibility as well as the marked dependence of the potentiometric behaviour on the temperature were consequences of the electrode chemical corrosion and could be explained by the establishment of two competitive mechanistic models (mechanisms I and II).rnrnAs a more general conclusion, the research activity carried out by our group in the field of fuel cells contributed further evidence of the great potential of XPS in providing definitely the “chemical situations” existing at modified electrode surfaces.

Analytical characterization of electrode surface by X-ray photoelectron spectroscopy. β-PbO2-based cathode in voltage-compatible lithium cells

Lead dioxide cathodes, discharged in a lithium cell at significant points of the discharge curve, have been analysed by XPS and information obtained on the surface chemical composition has been used to elucidate the cell discharge mechanism.General and particular problems and complications arising in XPS investigations when ‘real’ samples are involved are critically outlined with obvious reference to the intrinsic difficulty in the surface analytical characterization of a mixture of lead compounds. An approach is suggested which stresses how even semiquantitative determinations could play a key role in the speciation analysis.

Electrochemical lithiation of Pb3O4

Abstract Pb 3 O 4 has been investigated as a cathode material for voltage compatible, nonaqueous lithium cells and its good performance has been demonstrated. The mechanism of the process has been elucidated by undertaking X-ray and XPS analyses of the discharge products at significant points of the discharge curve. Pb(II) was unambiguously recognized by the latter technique, thus evidencing a two-step reduction mechanism en route to Pb(O) during lithiation.

Schottky diodes and field-effect transistors based on conjugated thiophenes

Abstract In this paper we present a study of organic devices such as Schottky diodes and field-effect transistors based on thiophene oligomers/polymers. These organic semiconductors can be prepared in the form of a thin layer on a number of substrates, such as glass and flexible plastic. This makes these devices particularly interesting because of the potential advantages they possess in terms of flexibility, low weight, and low cost. The effects produced by rapid thermal annealing on α-sexithiophene (α-6T) thin film transistors (TFTs) are shown to be very important both for the improvement of the on/off ratio of the devices and as a means to shed some light on the understanding of the transport mechanisms in these devices. The electrochemical investigation of the polybithiophene/TiO2 heterojunction clearly shows the rectifying character of the device.

Analytical X-ray photoelectron spectroscopic investigation of the modification of polybithiophene (pbT) under electrochemical cycling

Electrochemically synthesised polybithiophene (pbT) films have been analysed by XPS, as grown and after galvanostatic cycling, to test their chemical stability under use and to obtain more information about the doping–undoping process.It has been shown that working conditions can heavily influence the behaviour of the conducting polymer under cycling. In particular, a water content higher than 0.03% v/v in the acetonitrile solvent can promote irreversible modification of the surface chemistry of pbT. Surprisingly, the dopant ClO–4 anions can be reduced on the polymer-modified platinum electrode and the reduction products react with the pbT backbone.

Applicability of chemical derivatization – X-ray photoelectron spectroscopy (CD–XPS) to the characterization of complex matrices: case of electrosynthesized polypyrroles

Abstract Problems arising in the application of XPS coupled with derivatization reactions (CD-XPS) to the characterization of complex matrices are stressed by considering the case of polypyrroles electrosynthesized from aqueous solution. In these materials oxygen– and nitrogen–carbon functionalities are contemporaneously present, as well as acidic, basic, charged and reducible groups. The derivatizing agents titanium di-isopropoxide-bis(2,4-pentanedionate) (TAA), trifluoroethanol (TFE) and pentafluorophenylhydrazine (PFPH) were used to label the C–OH, COOH and Cue605O groups, respectively, in the pristine (PPY) and in the overoxidized (PPYox) polymer. The case shows that, when real samples are analysed, only the correlation among the information present in all the available spectra can supply the key to the interpretation of CD-XPS data, such as the actual stoichiometry and secondary effects. In particular, it is showed that PFPH cannot be used as a reliable derivatizing agent for Cue605O groups in PPYox, due to the competition of a side reaction (already reported for hydrazines) occurring on this particular substrate. This effect may be disregarded if a thorough control of the stoichiometry is not performed.

Photoelectrochemical behaviour and XPS characterization of a (Ti,Al,V)O2 film obtained by non-conventional anodic oxidation of a commercial TiAlV alloy

Abstract With the aim to improve the photoelectrochemical behaviour and to reduce the optical band gap of TiO 2 without sacrificing its stability, special n -type (Ti,Al,V)O 2 films have been prepared by anodically oxidizing a commercial titanium alloy following a non-conventional technique. Such alloy contained 6 w/o aluminium and 4 w/o vanadium. The spectral response of (Ti,Al,V)O 2 was dramatically shifted toward the visible region with respect to the response of the parent TiO 2 , the band-gap energy being about 2 V. The photocurrent-voltage characteristics have also been examined, and the flat-band potential determined both from the photocurrent onset potential and from the material bulk electronegativity. X-ray Photoelectron Spectroscopy (XPS or ESCA) coupled with argon ion sputtering has been used to obtain information about the in-depth concentration profile of dopants (Al, V) in the oxidized films. The lack of vanadium ions and the lower content of aluminium ions than that expected on the basis of bulk composition have been clearly evidentiated. The photoelectrochemical behaviour is then discussed both in the light of XPS results and on considering previous works in the field.

Surface characterization of anodic titanium dioxide films for photoelectrochemical solar cells

Abstract Three different titanium dioxide films have been prepared by anodic oxidation of titanium, and their morphology, structure and surface composition are examined, the latter by ESCA analysis. A mutual relationship has been found between the presence of metal atoms, such as sodium, in the bulk of the material and its response in photoelectrochemical cells.