ToF-SIMS of polyphosphate glasses Maura Crobu, a Antonella Rossi a,b * and Nicholas D. Spencer a * The increasing interest in phosphate glasses is due to the fact that they are considered to be bioactive functional materials in applications such as hard and soft tissue engineering and also that they possess interesting optical properties. These include low dispersion, high refractive index and high transparency for ultraviolet light. They have also been found to be responsible for the good anti-wear properties of phosphorus-based engine-oil additives. More recently, the low processing temperature of zinc and iron phosphates, as well as their chemical durability, has opened the possibility for new applications, such as the immobilization of nuclear wastes. Understanding the surface chemistry of such glasses turns out to be a key step for inferring their underlying mechanisms of action. In this work, zinc phosphate and iron phosphate glasses of different chain lengths were synthesized and characterized by means of time-of-flight secondary ion mass spectroscopy (ToF-SIMS). ToF-SIMS was able to successfully differentiate between the samples of different composition. A characteristic pattern of phosphate fragments was observed in the negative-mode ToF-SIMS spectra. The most intense peaks could be assigned to the phosphate fragments PO À , PO 2 À , PO 3 À and PO 4 À . Toward higher masses, fragments containing up to four phosphorus atoms with a periodicity of PO 2 were observed: FePO 3 À , ZnPO 3 À , FePO 4 À , ZnPO 4 À , FeP 2 O 6 À , ZnP 2 O 6 À , FeP 2 O 7 À , ZnP 2 O 7 À ; FeP 3 O 8 À , ZnP 3 O 8 À , FeP 3 O 9 À and ZnP 3 O 9 À . This pattern characterizes the spectra of all the glasses under investigation. For the first time, a method is proposed that allows the discrimination between polyphosphates of different chain lengths, by comparing the intensities of the relevant ToF-SIMS peaks. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: polyphosphate glasses; time-of-flight secondary ion mass spectroscopy; ToF SIMS Introduction Polyatomic or ‘cluster’ primary-ion bombardment has been described as the most exciting recent development in ToF-SIMS. [1] The use of cluster projectiles improves the ion-formation efficiency, allowing the detection of high-mass molecular fragments with good signal-to-noise ratio. Moreover it has been observed that the related surface damage is significantly lower than that of monoatomic projectiles, because the deposition of collision energy is closer to the surface. [1,2] These improvements allow the static limit to be overcome, broadening the possible applications of ToF-SIMS to many analytical systems, especially organic and biological samples. Polyphosphate glasses have been found to be responsible for the good anti-wear properties of phosphorus-based engine-oil additives, such as the zinc dialkyl dithiophosphates. [3] Depending on the temperature, these additives may rearrange in the lubricant solution to form reactive species that, thanks to a series of chemical reactions at the interface between two steel sliding surfaces, form a reaction film—the so-called tribofilm. In the field of tribology, the goal of understanding the anti-wear mechanism of phosphorus-based additives has led to renewed interest in investigating the mechanical and tribochemical properties of zinc and iron polyphosphates. [4,5] X-ray photoelectron spectroscopy (XPS) has been already extensively applied to tribology to investi- gate the surface chemistry of the samples after tribological stress. [6–11] ToF-SIMS can provide complementary information to XPS, thanks to its monolayer surface sensitivity combined with excellent lateral resolution (in the order of 100 nm). ToF-SIMS imaging of elements (Zn + , Fe + , Ca + ,P + and S + ) and low-mass ions (PO À , PO 2 À and PO 3 À ) has been already successfully applied to tribological samples. [12–15] Murase investigated adsorption and reaction of phosphate-type lubricant additives (zinc and sulfur-free) on ferrous materials by ToF-SIMS. [16] In a recent study in our research group, it has been shown that the spectra obtained upon bombarding with Bi 3 ++ cluster projectiles contained molecular fragments of masses >700 amu and the intensities of the molecular fragments at high mass could be used to discriminate between zinc polyphosphates of different chain length. [17] In this work, bulk glassy iron polyphosphates of different chain lengths have been investigated by ToF-SIMS. These results will be discussed in comparison with those from zinc polyphosphates. Experimental Glass synthesis and preparation for surface analysis The sample preparation and characterization have been described in Crobu et al. [4,17] Four different chain lengths were synthesized with different compositions: iron orthophosphate ([O]/[P] = 4), iron pyrophosphate ([O]/[P] = 3.5), iron metaphosphate ([O]/[P] = 3) and ‘iron polyphosphate 1.5 ’ with [O]/[P] = 3.167. In this paper, * Correspondence to: Nicholas D. Spencer, Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland E-mail: nspencer@ethz.ch Antonella Rossi, Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, INSTM unit Cittadella Universitaria di Monserrato, I-09100 Cagliari, Italy E-mail: rossi@unica.it a Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland b Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, INSTM unit Cittadella Universitaria di Monserrato, I-09100 Cagliari, Italy Surf. Interface Anal. (2012) Copyright © 2012 John Wiley & Sons, Ltd. SIMS proceedings paper Received: 9 October 2011 Revised: 29 May 2012 Accepted: 15 June 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/sia.5111