Chain-Length-Identification Strategy in Zinc Polyphosphate Glasses by Means of XPS and Tof-SIMS
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Anal Bioanal Chem (2012) 403:1415–1432 DOI 10.1007/s00216-012-5836-7 ORIGINAL PAPER Chain-length-identification strategy in zinc polyphosphate glasses by means of XPS and ToF-SIMS Maura Crobu & Antonella Rossi & Filippo Mangolini & Nicholas D. Spencer Received: 10 September 2011 /Revised: 29 January 2012 /Accepted: 3 February 2012 /Published online: 27 March 2012 # Springer-Verlag 2012 Abstract The surface chemistry of amorphous zinc poly- linked through P–O–P bonds predominate in the metaphos- phosphates of different compositions (ranging from zinc meta- phate composition, while when the zinc content is increased, phosphate to zinc orthophosphate) has been investigated by the chains become shorter, ultimately being replaced by PO4 means of X-ray photoelectron spectroscopy (XPS) and time- monomers in the orthophosphate composition. of-flight secondary-ion mass spectroscopy (ToF-SIMS). The identification of the chain length of zinc polyphosphates by Keywords Polyphosphate glasses . X-ray photoelectron XPS was on the basis of the integrated intensity ratio of the spectroscopy. Time-of-flight secondary-ion mass bridging (P–O–P) and nonbridging (P0OandP–O–M) oxy- spectroscopy. Fourier transform IR spectroscopy gen peaks used for fitting the oxygen 1s signal, the shift of the P2p3/2 signal towards lower binding energies and the modi- fied Auger parameter towards higher values as the zinc con- tent increases. The discrimination of the polyphosphate chain Introduction lengths was also achieved by ToF-SIMS, by comparing the intensities of selected characteristic phosphate fragments. Polyphosphate glasses consist of cross-linked networks, Both techniques appear to be suitable for the investigation of whose building blocks are phosphate tetrahedra. By linking polyphosphate glasses in applications such as tribology, where through covalent bridging oxygen (BO) atoms, these tetrahe- there is a need to identify the chain length present in the dra can form various polyphosphate species [1, 2]. In the case outermost monolayer of the film. Fourier-transform infrared of phosphorus pentoxide (P2O5), there are three tetrahedral (FT-IR) spectroscopy was used to characterize the bulk com- linkages (i.e., three oxygen atoms covalently bind the phos- pounds. The FT-IR studies showed that long-chain structures phate tetrahedra), leaving, as a consequence, one terminal oxygen (P0O) present in the network. On the basis of the number of BO atoms, the tetrahedra present in P2O5 are Electronic supplementary material The online version of this article therefore classified as Q (see Fig. 1). When a modifying (doi:10.1007/s00216-012-5836-7) contains supplementary material, 3 which is available to authorized users. oxide is added to P2O5, the network is depolymerized as a consequence of the substitution of P–O–P bonds by P–O–M M. Crobu : A. Rossi : F. Mangolini : N. D. Spencer (*) Laboratory for Surface Science and Technology, bonds (with M being a metal). Thus, a wide range of compo- Department of Materials, ETH Zurich, sitions and structures can be obtained depending on the metal Wolfgang-Pauli-Strasse 10, oxide to P2O5 ratio , i.e., the oxygen-to-phosphorus ratio, [O]/ 8093 Zurich, Switzerland [P] [1, 3, 4]. When [O]/[P] is 3, the structure is characterized e-mail: [email protected] by long chains consisting of Q2 tetrahedra (i.e., phosphate A. Rossi (*) tetrahedra sharing two oxygen atoms): this is known as the Dipartimento di Chimica Inorganica ed Analitica, metaphosphate region. Changing to higher oxide content, the Università degli Studi di Cagliari, INSTM Unit, average chain length decreases, resulting, eventually, in Q Cittadella Universitaria di Monserrato, 1 09100 Cagliari, Italy dimers (pyrophosphate, [O]/[P] of 3.5) and Q0 monomers e-mail: [email protected] (orthophosphate, [O]/[P] of 4) [5–7]. 1416 M. Crobu et al. in poor water durability, limiting, therefore, the possible practical uses of those materials. On the other hand, this poor chemical durability to hydrolysis makes these com- pounds useful as bioactive functional materials in applica- tions such as hard and soft tissue engineering and controlled- release glasses [21]. The introduction of dopants can also be used for tuning their biocompatibility [22], their biological functionalities [21], and for improving their chemical durability [23]. Polyphosphate glasses have also been found to be re- sponsible for the good antiwear properties of phosphorus- based engine-oil additives [24–26]. These additives are able to react at the interface between two sliding surfaces to form a reaction film, the so-called tribofilm. In the field of tribol- ogy, the hope of understanding the antiwear mechanism of phosphorus-based additives has led to renewed interest in investigating the mechanical and tribochemical properties of zinc and iron polyphosphates [27, 28]. The films formed on tribostressed surfaces generally have a thickness ranging Fig. 1 Terminology for different composition and structures in poly- from a few nanometers to 200 nm, and for this reason the phosphate glasses application of surface analytical techniques to the character- ization of polyphosphate glasses is very important. The possibility of synthesizing zinc polyphosphate glasses A relevant contribution to the understanding of the forma- over a wide compositional range is the main reason why they tion of glassy tribofilm polyphosphates has been provided by have often been chosen, among many other different binary the application of many surface-analytical techniques, such as phosphates glasses, for detailed structural studies. Zinc phos- X-ray photoelectron spectroscopy (XPS) [24, 29, 30], Auger phate glasses show a discontinuity in the composition-property electron spectroscopy [31], and X-ray absorption near-edge dependence of the refractive index, mass density, and glass- spectroscopy (XANES) [32]. The analytical questions to be transition temperature in the transition from ultraphosphate to addressed when chemical reactions are occurring at a surface polyphosphate composition [8, 9]. It has been observed, for under mechanical stress are many and various, but all are crucial. example, that the glass-transition temperatures values go Not only it is required to identify the elements, the nature of the through a minimum between metaphosphate and pyrophos- chemical bonds, and the thickness and composition of layers— phate composition. A change in coordination of the Zn2+ cation very often only a few nanometers thick—but also knowledge from 4 to 6 at the metaphosphate composition has been hy- of the composition in both in the lateral and the vertical pothesized as an explanation for this anomalous behavior, direction is needed, if possible obtained in a nondestructive which is in common only with that of magnesium phosphate way. To date there is no single technique that provides all this glasses. However, this theory has not been supported by exper- information, and only the combined application of different iment. In contrast, the Zn coordination number was found to be spectroscopies can help in understanding the surface chemistry nearly constant with the composition, with an average value of of challenging systems such as those characteristic of tribolo- 4[8–13]. Raman spectroscopy [3, 14], infrared (IR) spectros- gy. XPS is one of the most versatile techniques, since it allows copy [14–18], and nuclear magnetic resonance (NMR) spec- identification of the elements (except H and He), it provides troscopy [5, 7, 19] have been commonly employed for information on the chemical bonds between the elements, and, studying the structure of polyphosphates. under certain circumstances, it can provide the thickness and Research on polyphosphate glasses has recently been composition even of multilayer systems, resolving the surface stimulated by a wide range of technical uses of these com- composition in the nanometer range [33, 34]. Auger electron pounds. The materials are good candidates for various opti- spectroscopy is unique for its high lateral resolution, which cal applications, such as optical waveguides and high-power may reach a few tens of nanometers, but it fails in providing lasers. They have been considered as materials for nuclear- information at the molecular level, and problems may arise if waste immobilization and, thanks to their very high thermal the films are not conductive. XANES has also been applied for expansion coefficient, as materials for hermetic seals. the determination of polyphosphate chain lengths [32]. This Phosphate-based glasses have unique dissolution properties spectroscopy, in total electron yield and fluorescence yield in aqueous-based fluids: The P–O–P linkages present in the detection modes at the P K-edge, provides information that is glass networks can be easily hydrolyzed [20], which results averaged over sampling depths that have been estimated as Chain-length-identification strategy in zinc polyphosphate glasses 1417 5 nm and about 60 nm, respectively, with a lateral resolution of adsorption and reaction of phosphate-type lubricant addi- 500 nm [32]. A practical limitation of its applicability is that it tives (zinc- and sulfur-free) on ferrous materials by ToF- relies on synchrotron radiation. An extensive XANES investi- SIMS. The capabilities of such a technique have not been gation of chain lengths has been published for sodium poly- pushed to the limits in tribology so far. The recent phosphate glasses [35]. introduction of polyatomic primary ion sources has XPS analysis of phosphate glasses was first performed in been found to improve the yield of secondary ions, the 1970s. It was observed that the O 1s signal could be curve- reducing the surface damage compared with that for fitted with two components: the first component was assigned monoatomic sources. In this way it is possible to eject to the bridging P–O–P oxygen atoms (BO atoms), whereas the and detect high-mass molecular fragments with good second component included two kinds of terminal oxygen signal-to-noise ratio. This approach has been applied atoms, P0OandP–O–M.