Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 375–378 Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena j our na l ho me page: www.elsevier.com/locate/elspec Functionalized nanoparticles in aqueous surroundings probed by X-ray photoelectron spectroscopy Johan Söderström a,∗ , Niklas Ottosson a , Wandared Pokapanich a , Gunnar Öhrwall b , Olle Björneholm a a Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden b Max-lab, Lund University, Box 118, 221 00 Lund, Sweden a r t i c l e i n f o Article history: Received 7 December 2010 Received in revised form 15 February 2011 Accepted 21 February 2011 Available online 30 March 2011 Keywords: Nanoparticle Soft X-ray Photoelectron spectroscopy Liquid colloid Liquid micro-jet a b s t r a c t In this paper we present the first core-level photoelectron spectroscopic study of solid nanoparticles in liquid water. The particles are SiO 2 -based with an average diameter of 70 nm and functionalized with carboxylic groups. Despite that the sample is very dilute, we show that it is possible to obtain reasonable photoemission signal containing chemical information about both the ligands and the outermost parts of the SiO 2 -based core of the nanoparticles. We argue that this is due to a significant enrichment of the dispersed particles at the liquid/vapor interface. This proof-of-principle study expands the field of X-ray photoelectron spectroscopy by adding a new, wide and important class of systems that can be studied. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanoparticles (NPs) exhibit size-dependent properties that dif- fer significantly from those observed for larger particles or the infinite solid of the same constituent material, which often is related to the increased surface-to-volume ratio with decreasing size. If a particle is sufficiently small, the curvature can strain the bonding of surface atoms/molecules and thereby dramatically alter the particle’s surface reactivity – a property that can be tailored and exploited for efficient catalytic reactions [1]. Many current, and pos- sibly also future, chemical applications employ NPs in contact with a liquid medium, see e.g. Refs. [2,3]. Due to their useful proper- ties, NPs are today used in large-scale industrial applications and are thus gradually becoming more frequent in our surroundings, even though their potential toxicity and impact on natural ecosys- tems are not well understood at present [4,5]. It is therefore of both practical and fundamental interest to increase our molecular-scale understanding of the complex and important NP containing liquid systems. X-ray photoelectron spectroscopy (XPS) is a powerful tool for investigations of free atoms, molecules and solid samples. The method provides a local probe of the sample’s electronic structure with elemental and site-specificity and is thus, in principle, ide- ally suited for fundamental characterization of NPs. In recent years ∗ Corresponding author. Tel.: +46 18 471 36 05. E-mail address: Johan.Soderstrom@fysik.uu.se (J. Söderström). XPS has proven to be useful in the study of free clusters [6] (and references therein) and free NPs [7,8], it should be noted that the technique to generate free NPs employed in Ref. [7] might leave the NP surrounded by a large number of water molecules – this is not specified in the paper but our understanding is that this might be the case. However, no one has yet demonstrated the applicability of XPS to NPs dispersed in liquid media. In addition to overcoming the experimental challenges associated with performing XPS on volatile liquids – something which was only recently overcome with the development of the liquid micro-jet technique [9,10] – dispersed NPs present new challenges given the low fraction of NPs dispersed in a liquid system. Firstly, this means that the samples are typically very dilute, presenting a sensitivity challenge. Secondly, the NPs may easily precipitate or agglomerate in the nozzle of the liquid micro-jet, thus presenting sample handling challenges. As the NPs are orders of magnitude larger than atomic or small molec- ular ions fewer agglomeration steps are needed in order to block the nozzle of the liquid jet. Initial tries indicates that the blocking of the nozzle strongly depends on the ligands, most likely the risk of blocking the nozzle also depends on the concentration and size of the NPs. In this paper we present the first results from NPs dispersed in liquid studied with soft X-ray photoelectron spectroscopy. The aim of the paper is not to perform a complete characterization of the quite complex ligand-covered and dye-doped NPs dispersed in liq- uid water. It is instead to demonstrate that XPS indeed can be used to probe the electronic structure of NPs in an aqueous medium, 0368-2048/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2011.02.006