1 Single Nanoparticle SERS Probes of Ion Intercalation in Metal-Oxide 2 Electrodes 3 Li Li, †,‡ Ullrich Steiner, † and Sumeet Mahajan* ,†,‡ 4 † Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom 5 ‡ Institute of Life Sciences and Department of Chemistry, Highfield Campus, University of Southampton, Southampton, SO17 1BJ, 6 United Kingdom 7 * S Supporting Information 8 ABSTRACT: Probing ion-intercalating processes in electro- 9 des is hugely important for batteries, supercapacitors, and 10 photovoltaic devices. In this work we use single-nanoparticle 11 (NP) probes to see real-time molecular changes correlated to 12 electrochemically modulated ion-intercalation in metal-oxide 13 electrodes. Using surface-enhanced Raman spectroscopy 14 (SERS) transduced by single NP probes, we observe that 15 the Raman frequencies and spectral intensities of the adsorbed 16 molecules vary on cycling the electrochemical potential on a 17 vanadium-oxide electrode. The potential-dependent frequency 18 shifts in SERS from an electrochemically inert molecule are attributed to a Stark effect induced by chemical and structural 19 changes as a result of ion-intercalation processes in vanadium oxide. Our study opens up a unique strategy to explore adsorbates 20 and molecular reaction pathways on ion-intercalating materials and semiconducting interfaces. 21 KEYWORDS: Transition metal oxides, surface-enhanced Raman scattering (SERS), nanoparticles, plasmons, interfacial reactions 22 T he in situ investigation of interfacial reactions has long 23 attracted great scientific and technological interest for a 24 better understanding of their fundamental mechanisms and the 25 improvement of device performance. The combination of 26 surface-enhanced Raman spectroscopy (SERS) with electro- 27 chemical processes is a powerful way to probe interfacial 28 reactions. 1-4 The fingerprint nature of the Raman spectrum 29 and the significantly enhanced signal intensities of SERS make 30 this an ideal surface analysis technique to probe interfacial 31 chemistry at the molecular level. Conventional electrochemical- 32 SERS works only on a limited type of metal electrodes, such as 33 coinage metals (Au, Ag, and Cu) 5 and transition metals (Rh, 34 Pd, and Ni), 6 or their combination such as in the borrowed 35 SERS approach utilizing ultrathin films on SERS-active Au 36 substrate. 7 Moreover, nanostructured substrates are typically 37 required to achieve sufficient SERS enhancement. 1,6 This limits 38 its potential usage especially for nonmetallic, for example, 39 metal-oxide semiconductor surfaces which are widely used for 40 example in lithium-ion batteries, 8 supercapacitors, 9 and photo- 41 voltaic devices. 10 The study of adsorbates at metal-oxide 42 interfaces is therefore important not only to further our 43 understanding of interfacial reactions but also for device design 44 and performance improvement of several modern energy 45 materials. 46 Metal nanoparticles (NPs) that exhibit surface plasmon 47 resonances excited by visible light (i.e., Au and Ag NPs) can 48 greatly concentrate the local electromagnetic field. This has 49 been used for enhancing the performance in applications of 50 photovoltaics, 11 water splitting, and photocatalysis, 12 in 51 addition to their well-known function of enhanced Raman 52 scattering. 5 However, they have never been applied to monitor 53 and probe ion-intercalation processes although they have been 54 employed as Raman enhancers on non-SERS active substrates, 55 including single crystal metals, silicon, indium tin oxide, and 56 microscope slide glass. 2,13-15 However, in these studies 57 primarily aggregates of NPs or large sized particles were 58 utilized for generating Raman signals. Electrochemical SERS on 59 transition metal oxides has also not been reported. Here we use 60 nanojunctions formed by single NPs at a transition metal-oxide 61 interface. This allows changes due to ion intercalation, which is 62 unique to such transition metal oxides, on electrochemical 63 modulation to be monitored by SERS. 64 Vanadium oxide is an abundant, low-cost metal-oxide 65 material with multiple valence states and a layered structure 66 which can accommodate large number of ions while 67 maintaining structural integrity, making it well-suited as ion 68 intercalation material. 16-18 Upon applying an electric potential, 69 vanadium oxide undergoes a redox reaction accompanied by 70 the intercalation/extraction of ions into/out of its structure as a 71 result of which the interface remains largely unpolarized. In our 72 experiments, we employ a sandwich junction geometry that is 73 assembled by placing AuNPs onto the vanadium oxide film. 74 SERS with these single NP probes reports the effect of local ion 75 intercalation/extraction caused by electrochemical processes. Received: September 18, 2013 Revised: December 18, 2013 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl403485e | Nano Lett. XXXX, XXX, XXX-XXX amc00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i4 HF01:4180 | 2.0 alpha 39) 2013/10/21 02:46:00 | PROD-JCAVA | rq_3100394 | 1/07/2014 14:50:00 | 4 | JCA-DEFAULT