Catalytic processes monitored at the nanoscale with tip-enhanced Raman spectroscopy Evelien M. van Schrojenstein Lantman 1 , Tanja Deckert-Gaudig 2 , Arjan J. G. Mank 1,3 , Volker Deckert 2,4 * and Bert M. Weckhuysen 1 * Heterogeneous catalysts play a pivotal role in the chemical industry, but acquiring molecular insights into functioning cat- alysts remains a significant challenge 1–4 . Recent advances in micro-spectroscopic approaches have allowed spatiotemporal information to be obtained on the dynamics of single active sites and the diffusion of single molecules 5,6 . However, these methods lack nanometre-scale spatial resolution and/or require the use of fluorescent labels. Here, we show that time-resolved tip-enhanced Raman spectroscopy can monitor photocatalytic reactions at the nanoscale. We use a silver- coated atomic force microscope tip to both enhance the Raman signal and to act as the catalyst. The tip is placed in contact with a self-assembled monolayer of p-nitrothiophenol molecules adsorbed on gold nanoplates. A photocatalytic reduction process is induced at the apex of the tip with green laser light, while red laser light is used to monitor the trans- formation process during the reaction. This dual-wavelength approach can also be used to observe other molecular effects such as monolayer diffusion. Classical spectroscopic techniques for the study of heterogeneous catalysts at work, including infrared, ultraviolet–visible, Raman or fluorescence spectroscopy, are limited in terms of their spatial resol- ution to the micrometre scale because of the diffraction limit of light. Molecular techniques such as Raman spectroscopy commonly have the added disadvantage of relatively low signal intensity. Techniques that do have the spatial resolution required to look at individual catalytic particles often only image dense elements and therefore only the catalytic particle itself 3,7 . Fortunately, recent advances have opened up possibilities to overcome both limit- ations 8 . More specifically, tip-enhanced Raman spectroscopy (TERS) is a promising nano-spectroscopic technique that combines atomic force microscopy (AFM) with the phenomenon of surface- enhanced Raman scattering (SERS). Through a thin coating of, for example, silver or gold on an AFM tip, a dramatic intensity boost of the Raman signal can be obtained at the end of the tip, while maintaining the spatial resolution of the AFM 9–13 (Fig. 1). Following excitation with an appropriate wavelength, a strong field enhancement is induced at the apex of the coated tip. This results in highly enhanced Raman scattering signals in molecules in the direct vicinity of the tip. The observation area has been esti- mated to have a diameter of ≤10 nm (ref. 14), and is independent of the wavelength used and well below the diffraction limit. Although the combination of TERS and (heterogeneous) cataly- sis is very promising, only a limited number of preliminary studies can be found in the literature to date 15,16 . None of these studies goes as far as studying a catalytic reaction in real time. In the present work, a photocatalytic process was selected where the silver- coated TERS tip itself acts as the catalyst. Self-assembly of the mol- ecules on gold nanoplates ensures that no more than a monolayer of reactant is studied. As only the utmost point of the tip is in contact with the reactant, this is a first-of-a-kind study of a single catalytic ‘particle’ in action. The combination of two laser sources is crucial for controlled observation of the reaction at the catalytic site. On excitation with a short wavelength, a reaction can be temporarily activated. A longer-wavelength excitation is dedicated only to the monitoring of the sample, and does not trigger the reaction. With this approach, it is possible to differentiate between the processes that occur natu- rally on the sample (such as diffusion and orientation effects) and the actual photocatalysed reaction. It is known that the reduction of p-nitrothiophenol (pNTP) can be activated with green light excitation, by means of a charge-trans- fer mechanism, when a SERS-active silver substrate is present 17–19 ; this would otherwise require ultraviolet light for excitation. The origins of the 1,390 and 1,440 cm 21 bands in the spectrum of the product of this reduction are currently subject to debate, with the literature suggesting either p-aminothiophenol (pATP) or p,p ′ -dimercaptoazobisbenzene (DMAB) 20–23 . The exact interpret- ation is beyond the scope of this Letter and is not important for the main message of the work. From the Raman spectra it is possible to assign the spectra to either reagent (pNTP) or product and thereby study the reaction. For ease of discussion we chose to call the product DMAB and have named the related vibrations accordingly. SERS measurements using 532 nm laser excitation on colloidal silver SERS substrates show a clear spectral change over time (Fig. 2). After ≏120 s of laser exposure, the most prominent peak of the pNTP at 1,335 cm 21 (n NO2,sym ) decreases 24 , while DMAB- related peaks at 1,140 (b C–H ), 1,380 (n NN þ n CC þ n C–H ) and 1,440 (n NN þ n CC þ b C–H ) cm 21 increase in intensity 21 , indicating the (partial 23 ) conversion of pNTP to DMAB. More extensive SERS studies (Supplementary Fig. S1) confirm that the reaction requires both green laser excitation and silver. As these studies also confirm that no reaction occurs under excitation at 633 nm, the use of this excitation wavelength provides an unobtrusive way of monitoring the extent of the reaction. SERS measurements can yield valuable information on a ‘bulk’ scale, but are limited in studies near single-molecule levels. With TERS, the observation area is well below the diffraction limit of light. As the SERS activity is localized at the tip apex, which is also the catalytic particle in this arrangement, the system is flexible with respect to the material under study and position on the sample. pNTP is allowed to self-assemble into a monolayer on a substrate of 1 Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterial Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands, 2 Institute of Photonic Technology, Albert-Einstein-Strasse 9, D-07745 Jena, Germany, 3 Molecular and Surface Analysis, Philips Innovation Services, High Tech Campus 11, 5656 AE Eindhoven, The Netherlands, 4 Physical Chemistry, Friedrich-Schiller University Jena, Helmholtzweg, 07743 Jena, Germany. *e-mail: volker.deckert@ipht-jena.de; b.m.weckhuysen@uu.nl LETTERS PUBLISHED ONLINE: 19 AUGUST 2012 | DOI: 10.1038/NNANO.2012.131 NATURE NANOTECHNOLOGY | VOL 7 | SEPTEMBER 2012 | www.nature.com/naturenanotechnology 583 © 201 2 M acmillan Publishers Limited. All rights reserved.