Approaches to Single-Nanoparticle Catalysis - Chen Groupchen.chem.cornell.edu/publications/annurev-physchem-040513-103729.pdf · Methods for electrochemical measurements of single-nanoparticle

PC65CH18-Chen ARI 26 December 2013 14:37

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Approaches toSingle-Nanoparticle CatalysisJustin B. Sambur and Peng ChenDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14850;email: [email protected]

Annu. Rev. Phys. Chem. 2014. 65:395–422

The Annual Review of Physical Chemistry is online atphyschem.annualreviews.org

This article’s doi:10.1146/annurev-physchem-040513-103729

Copyright c© 2014 by Annual Reviews.All rights reserved

Keywords

electrochemistry, single-molecule fluorescence, surface plasmonresonance, X-ray microscopy, surface-enhanced Raman

Abstract

Nanoparticles are among the most important industrial catalysts, with ap-plications ranging from chemical manufacturing to energy conversion andstorage. Heterogeneity is a general feature among these nanoparticles, withtheir individual differences in size, shape, and surface sites leading to vari-able, particle-specific catalytic activity. Assessing the activity of individualnanoparticles, preferably with subparticle resolution, is thus desired and vi-tal to the development of efficient catalysts. It is challenging to measurethe activity of single-nanoparticle catalysts, however. Several experimentalapproaches have been developed to monitor catalysis on single nanoparti-cles, including electrochemical methods, single-molecule fluorescence mi-croscopy, surface plasmon resonance spectroscopy, X-ray microscopy, andsurface-enhanced Raman spectroscopy. This review focuses on these exper-imental approaches, the associated methods and strategies, and selected ap-plications in studying single-nanoparticle catalysis with chemical selectivity,sensitivity, or subparticle spatial resolution.

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Review in Advance first posted online on January 9, 2014. (Changes may still occur before final publication online and in print.)

Changes may still occur before final publication online and in print

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1. INTRODUCTION

As catalysts, nanoscale particles have a significant impact on our society, given that “one-third ofmaterial gross national product in the United States involved a catalytic process somewhere in theproduction chain” (1). Modern synthetic procedures can allow for the production of nanoparti-cles with impressive control over their size, shape, and composition (2, 3). Yet at the individualparticle level, structural heterogeneity is still prevalent, especially for industrial nanoscale cata-lysts. This structural heterogeneity inevitably leads to particle-specific activity in catalysis, whichis masked in conventional ensemble-averaged activity measurements. Revealing such behavior,as well as identifying champion or spectator nanoparticles, requires single-nanoparticle-levelmeasurements.

An ideal experiment would acquire real-time, simultaneous information on the structure, chem-ical bonding, and activity of a single nanoparticle (or even a single active site). It is certainly anenormous challenge and has not yet been achieved to our knowledge. But impressive advances havebeen made in the past decade, and a number of approaches now exist that can monitor catalytic re-actions on single nanoparticles, including electrochemical methods, single-molecule fluorescencemicroscopy, surface plasmon resonance spectroscopy, X-ray microscopy, and surface-enhancedRaman spectroscopy. Some of these approaches have subparticle spatial resolution in interrogat-ing the activity, structure, chemical state, or composition of a working catalyst nanoparticle underreaction conditions.

This review focuses on these experimental approaches for single-nanoparticle catalysis. Foreach approach, we discuss the general principle, the experimental methods and strategies to realizethe approach, and a few selected applications, along with discussions of features, generalities,and challenges. We mainly focus on studies of nanoscale catalyst particles because macroscopicparticles or bulk materials can usually be studied with conventional analytical techniques. Extensiveadvances have also been made in atomic-scale structural studies of nanoscale particles, includingcatalysts, but they are out of the scope of this review, and we direct readers to other reviews onstructural characterizations of catalytic materials (4–6).

2. ELECTROCHEMICAL DETECTION OFSINGLE-NANOPARTICLE CATALYSIS

2.1. Principle

Electrochemical reactions are associated with an electrical current across an electrode/solutioninterface. This current is quantitatively proportional to the rate of reactants consumed or productsgenerated, and thus to the underlying reaction kinetics.

Recent advances in the fabrication of nanometer-sized electrodes have led to tremendousgrowth in nanoelectrochemistry (recently reviewed in 7, 8). Owing to their small electroactivearea, nanoelectrodes exhibit low background currents and are capable of measuring small currents(∼pA) associated with single molecules (9), single-electron transfer events (10), electrochemicalproperties of nanoparticle solutions (11), and high-resolution electrochemical imaging of bulksurfaces (12). Here we focus on electrochemical approaches for the study of reactions or catalysison isolated nanostructures.

The challenges with the electrochemical approach are to isolate a single nanoparticle andmeasure the small current associated with the redox reactions on its surface. Below we reviewthree electrochemical methods that are capable of measuring catalysis on single nanoparticles(Figure 1).

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Figure 1Methods for electrochemical measurements of single-nanoparticle (NP) catalysis. (a) The current responseof an ultramicroelectrode (UME) scanned over an immobilized nanocatalyst can be used to simultaneouslyimage the catalyst (bottom) and monitor catalytic activity. Panel a, bottom, adapted from Reference 13 withpermission from The Royal Society of Chemistry. (b) A single-NP catalyst attached to a noncatalytic UME canbe used to study single-NP electrocatalysis. (Bottom) Linear sweep voltammograms show the size-dependentbehavior of Au NPs. Panel b, bottom, adapted with permission from Reference 14. Copyright 2010 AmericanChemical Society. (c) A freely diffusing NP (i ) collides and (ii ) temporarily adsorbs onto a noncatalyticUME, resulting in a catalytic current spike during its residence time on the electrode (bottom). Panelc, bottom, reproduced with permission from Reference 15. Copyright 2010 American Chemical Society.

2.2. Electrochemical Scanning Probe Microscopy

Scanning electrochemical microscopy (SECM) was developed to image the electrochemical prop-erties of surfaces (recently reviewed in 16, 17). In general, SECM for single-nanoparticle catalysisinvolves scanning a nanoelectrode over spatially separated nanoparticles supported on an elec-trode surface that is not catalytically active for the reaction of interest (Figure 1a). Stimmingand coworkers (13) first used this method to study the catalysis of single Pd nanoparticles on abulk Au (111) electrode. After locating and imaging a nanoparticle (Figure 1a), they polarizedthe potential of the scanning nanoelectrode anodically (independent of the Au support electrode)so that hydrogen evolved from the single Pd nanoparticle could subsequently be oxidized at thenanoelectrode, generating a detectable electrochemical current. They showed that under condi-tions of high mass transport, the hydrogen evolution kinetics of Pd nanoparticles differed frombulk Pd (13), and the size effect can be explained by the thickness variation of the support-inducedstrain at the surface of the Pd nanoparticles (18).

Using a similar method, Chen & Kucernak (19, 20) studied single Pt nanoparticles depositedon carbon electrodes and discovered alternative mechanisms for the oxygen reduction reactionand the hydrogen oxidation reaction under conditions of extremely high mass transport. Bard andcoworkers further used SECM to synthesize and discover catalysts by screening the activity ofelectrocatalysts (21) and photoelectrocatalyst arrays (22).

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Unwin and coworkers recently introduced a new scanning-probe-based method, scanning elec-trochemical cell microscopy (reviewed in 23), in which the scanning probe is a pulled theta pipetthat acts as a mobile electrochemical cell containing the electrolyte and the working, counter, andreference electrodes (24). They acquired reactivity maps of single Pt nanoparticles deposited onsingle-walled carbon nanotubes (25). They found large particle-to-particle activity differences, aswell as particle preferences for reduction versus oxidation reactions, which could be attributedto differences in the nanoparticles’ nanofaceting (25). The Unwin group also used scanning elec-trochemical cell microscopy to study the redox activity of complex nanoscale environments, suchas the electrocatalysis of carbon nanotubes (26), metal nanowires (27), the landing and catalyticcharacterization of Au nanoparticles (28), and single or multilayer graphene (29).

2.3. Direct Attachment of a Single Nanoparticle to an Ultramicroelectrode

Instead of using the probe electrode as a detector, Bard and coworkers (30) directly attached asingle 30-nm Pt nanoparticle to the end of a carbon fiber ultramicroelectrode (UME) (Figure 1b).Electrochemical reactions on this single nanoparticle directly lead to a current-potential responsemeasured via the UME. The attachment involved functionalizing the carbon fiber with bifunc-tional linker molecules that covalently bonded the Pt nanoparticle to the electrode surface. Theyused electron microscopy to characterize the single nanoparticle attached to the UME. Zhang andcoworkers (14) employed this methodology to attach various-sized Au nanoparticles to amine-functionalized Pt nanoelectrodes with 2–10-nm tip sizes. They observed a size-dependent over-potential of Au nanoparticles in catalyzing the oxygen reduction reaction, with smaller particleshaving larger overpotentials (Figure 1b).

In a related study, Wang and coworkers (31) etched a Pt disk electrode to fabricate a nanoelec-trode with a single Pt nanowire protruding from the tip. The protruding nanosized tip effectivelyserves as a single-nanowire electrode. The electrocatalytic activity of the nanowire was dependenton the tip radius, and the tip exhibited higher activity than the nanowire side walls for the oxygenreduction reaction.

2.4. Electrocatalytic Current Amplification via Single-Nanoparticle Collisions

The method of electrocatalytic current amplification via single-nanoparticle collisions, first de-veloped by Xiao & Bard (32), enables the detection of single nanoparticles in solution. Here,a UME, which itself is inactive for the desired electrocatalytic reaction in the applied potentialrange, is immersed in a solution containing nanoparticles and reactant species (Figure 1c). Whilediffusing in solution, the nanoparticles cannot electrocatalyze the reaction because of a lack ofelectrical contact with the electrode (no current response) (part i in Figure 1c). Upon the collisionof a nanoparticle with the UME, an electrical contact is established, and electrocatalytic reactionscommence (part ii in Figure 1c), generating an abrupt current response while the nanoparticletemporarily resides on the electrode. The low concentration of the nanoparticles ensures that nomore than one nanoparticle collides with the UME at any instant, leading to single-nanoparticleelectrocatalysis measurements.

Xiao & Bard (32) initially used this method to detect single Pt nanoparticle-catalyzed protonreduction and H2O2 reduction reactions on carbon fiber electrodes. The method was furtherapplied to study monolayer-passivated nanoparticles colliding with UMEs (33), water oxidationon IrOx nanoparticles (15), NaBH4 oxidation on Au nanoparticles (34), and hydrazine oxidation onPt nanoparticles (35). Bard and coworkers (36, 37) further showed that the current transients can beused to estimate the size distributions, concentrations, and diffusion coefficients of nanoparticles.

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Recently, Fernando et al. (38) extended this method to study the photocatalytic oxidation ofmethanol by semiconducting anatase TiO2 nanoparticles, demonstrating particle size-dependentstochastic photocurrent steps.

2.5. Features, Generality, and Challenges

The above three electrochemical methods share the use of UMEs to suppress background currentsand measure picoampere catalytic currents using commercial potentiostats. In SECM, dependingon the probe-sample distance, the probe may not detect all molecules generated from the nanopar-ticle, so quantification of the reaction can be underestimated [although collection efficiencies ofthe order of 95% have been reported (39)]. Electrodes with tip radii less than 100 nm can befabricated via established procedures in the literature (40); the commercial electrodes are largerin tip radius (<10 μm). In principle, all redox reactions can be investigated using electrochemicaldetection, such as those in photoelectrochemical cells and fuel cells. SECM and direct nanoparti-cle attachment methods can probe real-time temporal behavior, such as catalyst deactivation, andstructural characterization via electron microscopy is possible.

Challenges are also present for these electrochemical methods. For SECM, high-resolutionelectrochemical imaging generally requires isolated nanoparticles dispersed on flat substrates. Forthe direct attachment method, every experiment requires the fabrication of a nanoelectrode, whichcould be arduous. Both the scanning probe and direct attachment methods are serial measurements,so the overall data throughput is low. The nanoparticle collision current amplification methodcan detect many nanoparticles very quickly, but it lacks the ability to structurally characterize thenanoparticle that caused the current transient. It is difficult to achieve single-reaction resolutionusing the current detection approach because each reaction only results in the passage of one ora few electrons.

3. SINGLE-MOLECULE FLUORESCENCE MICROSCOPY FORSINGLE-NANOPARTICLE CATALYSIS

3.1. Principle

A direct way to measure catalysis by a single nanoparticle is to detect its reaction products. Thisrequires ultrasensitive detection of the product molecules, as only a few product molecules maybe generated by a single nanoparticle during a limited time. Single-molecule fluorescence mi-croscopy, with its single-molecule sensitivity, has made this ultrasensitive detection possible. In thisapproach, a fluorogenic catalytic reaction is studied, in which a nonfluorescent reactant molecule isconverted to a highly fluorescent product molecule on a nanoparticle catalyst. Under illumination,each product molecule can emit hundreds to thousands of photons, enabling its single-moleculedetection by, for example, an electron-multiplying CCD (charge-coupled device) camera.

Single-molecule microscopy of fluorogenic reactions was initially developed for studyingsingle-enzyme catalysis (41–45). Hofkens and coworkers (46) were the first to apply this approachin 2006 to study heterogeneous catalysis on micrometer-sized layered double hydroxide crystals. Ithas been used by Majima and coworkers (47) to study photocatalysis on TiO2 nanostructures andby our group to study catalysis on single metal nanoparticles (48). In 2005, Majima and coworkers(49) used indirect fluorescence detection to image reactive oxygen species generated by photo-catalysis on TiO2 films at the single-molecule level. In the following, we review the experimentalmethods for single-molecule fluorescence microscopy of nanocatalytic systems, as well as recentstudies demonstrating selected topics. Several recent reviews also cover related topics (50–53).



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Figure 2Single-molecule microscopy of fluorogenic reactions on nanocatalysts. (a) Schematic illustration of a fluorogenic reaction catalyzed onthe surface of a pseudo-1D catalyst. (b) Experimental scheme (not to scale) using a prism-based total internal reflection fluorescencemicroscope and a microfluidic reactor made between a transparent support and a coverslip. The fluorescence emission from a singlemolecule is detected by an electron-multiplying CCD camera. (c) Segment of the fluorescence intensity versus time trajectory from asingle 6-nm Au nanoparticle catalyzing the fluorogenic reductive N-deoxygenation reaction of resazurin to resorufin. Panel c adaptedfrom Reference 48.

Relatedly, conventional fluorescence microscopy has also been used to study catalysis by singlemacroscopic particles, such as zeolite crystals (54–56), photocatalysis on anatase TiO2 crystals(57), and even industrial fluidic cracking catalyst particles (58).

3.2. Experimental Methods for Single-Molecule Fluorescence Microscopy

Figure 2a,b illustrates the general scheme of single-molecule fluorescence microscopy of fluo-rogenic reactions. Immobilized catalysts convert nonfluorescent reactants to highly fluorescentproduct molecules, which are temporarily adsorbed on the catalyst surface (Figure 2a), facil-itating their detection at the single-molecule level. Low densities of catalyst particles on thesupport ensure that there is only one particle within a diffraction-limited lateral resolution ofthe optical microscope (approximately half of the wavelength of the fluorescence light). A mi-crofluidic reactor (Figure 2b) can be used to provide a continuous supply of reactants to studysteady-state reaction kinetics, alter conditions that affect the catalytic rate, and prevent productaccumulation.

Several configurations may be used to excite and detect the fluorescence of single molecules.In wide-field imaging, fluorescence signals from single molecules are imaged by a camera andcontinuously recorded with millisecond time resolution. Many particles can be imaged in parallel.Total internal reflection excitation is often used to suppress background fluorescence by limitingthe excitation within a thin layer (∼150 nm) of the transparent support surface (Figure 2b), insteadof epi-illumination in which the excitation light passes through the sample.

In confocal imaging, the laser excitation is tightly focused into a diffraction-limited spot, and apoint detector, such as an avalanche photodiode, typically detects the emission. Raster scanning thesample or the laser subsequently leads to an image. Although it suffers from low data throughput,confocal imaging offers faster time resolution compared to wide-field imaging owing to fast de-tector responses. For example, Scaiano and coworkers (59) recently used nanosecond fluorescencelifetime imaging to examine the single-particle photocatalytic oxidation of 9-anthraldehyde on col-loidal and supported Au nanoparticles, revealing the intermittency of surface plasmon–mediatedactivity during catalysis.

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3.3. Real-Time Single-Turnover Kinetics of Single Nanoparticles

Using wide-field imaging with total internal reflection fluorescence excitation, our group stud-ied the real-time catalytic kinetics of pseudospherical 6-nm Au nanoparticles (48, 60). Figure 2cshows an exemplary fluorescence intensity trajectory from a single 6-nm Au nanoparticle cat-alyzing the fluorogenic N-deoxygenation reaction of resazurin to resorufin (48). The trajectoryshows digital, stochastic on-off signals that contain rich information regarding the reaction kinet-ics and mechanism. The probabilistic τ on and τ off values correspond to the residence time of aproduct molecule on the nanoparticle and the waiting time from the previous product desorptionto the subsequent catalytic product formation reaction, respectively. Their statistical propertiesare defined by, and report on, the underlying reaction kinetics. The catalytic formation reac-tion followed a Langmuir-Hinshelwood mechanism, with large reactivity heterogeneity amongindividual nanoparticles. Two product desorption pathways were identified—a direct desorptionpathway and a reactant-assisted desorption pathway—and individual nanoparticles showed differ-ent selectivity in these two pathways, with the selectivity further tunable by the particle size (61).The real-time kinetics also uncovered temporal catalytic dynamics of individual nanoparticles,which could be attributed to catalysis-induced and spontaneous dynamic surface reconstructing,which is further dependent on the nanoparticle size (61) and catalyst materials (62).

This real-time, single-turnover kinetics approach was also applied to other single-nanocatalystsystems. Majima’s group (47) used fluorogenic probes to study reactive oxygen species such assinglet oxygen, 1O2, and hydroxyl radicals, ·OH, that were generated from irradiated TiO2

photocatalysts. The authors were able to identify 1O2 formation exclusively from N-doped TiO2

and quantified the bimolecular rate constant between hydroxyl radicals and the fluorescent probeand also the effective diffusion rate of ·OH. Our group further studied the catalytic behaviorsof single Pt nanoparticles in two different fluorogenic reactions, which uncovered the structuralsensitivity or insensitivity of Pt nanoparticles in the two reactions (62). Using the same approach,we also studied catalysis by single Au nanorods and nanoplates (63, 64), as well as electrocatalysisby carbon nanotubes (65). Alivisatos and coworkers (66) studied the fluorogenic photocatalyticoxidation of amplex red to resorufin using Sb-doped TiO2 nanorods. Their study revealed thatactive sites around trapped holes showed higher activity, stronger binding ability, and a differentdissociation mechanism than active sites around trapped electrons.

3.4. Single-Turnover Activity Mapping with Diffraction-Limited Resolution

For catalysts with micrometer or larger lateral dimensions, one can resolve the spatial distributionof single turnovers directly with diffraction-limited wide-field fluorescence imaging. Hofkensand coworkers (46) observed acid-catalyzed transformations of fluorogenic reactants on singlemicrometer-sized layered double hydroxide crystals, demonstrating that ester hydrolysis pro-ceeded on the lateral crystal faces, whereas transesterification occurred on the outer crystal surface.The number of fluorescent bursts accumulated over time indicates hot spots of transesterificationactivity at the edges of the crystal (Figure 3a). They further used diffraction-limited imaging ofsingle turnovers to demonstrate the spatially inhomogeneous product formation and local orien-tation of molecules within single zeolite crystals (54, 67, 68).

Majima and coworkers used diffraction-limited imaging to reveal the temporal and spatialdistributions of reactive oxygen species generated from photoexcited TiO2 nanoparticles (47) andmesoporous nanotubes (69). Figure 3 shows that under band gap illumination, the location ofintensity bursts from fluorogenic reactions within a one-dimensional (1D) mesoporous tube wasidentified and then correlated with the nanotube’s transmission image. They determined that



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Fluorescence image Accumulated intensity

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Figure 3Single-molecule diffraction-limited imaging of catalysis. (a) Fluorescence image (left) of catalytic reactionsoccurring on a single layered double hydroxide crystal. The accumulated spot intensity (right) on the samecrystal over 256 consecutive images reveals reactivity along crystal edges. Panel a adapted from Reference 46by permission from Macmillan Publishers Ltd., copyright 2006. (b) Transmission image (left) of a single TiO2nanotube and the corresponding fluorescence image (right) revealing subparticle reactivity heterogeneity.Panel b reprinted with permission from Reference 69. Copyright 2009 American Chemical Society.

the diffusion of reactants within the mesoporous nanotube had a significant impact on catalyticactivity. Majima’s group (56) extended this approach to image photocatalytic active sites on asingle titanosilicate zeolite.

3.5. Superresolution Imaging of Single-(Nano)Particle Catalysis

Wide-field imaging of single-molecule fluorescence also enables nanometer localizations of in-dividual molecules (70, 71). This is typically achieved by fitting the fluorescence point spreadfunction with a 2D Gaussian function (Figure 4a,b); the larger the photon counts are, the higheris the localization precision. By localizing the reaction products one at a time, one can image prod-uct formation with a resolution of tens of nanometers (i.e., superoptical resolution). This strategyis the basis for (f )PALM/STORM (72–74), two related superresolution imaging techniques widelyused in biological sciences. One of the early implementations of superoptical resolution imagingin catalysis was to resolve nanometer-sized reactive sites on carbon nanotube electrocatalysts (65).

Building upon their previous diffraction-limited imaging work with the fluorogenic reactioninvolving furfuryl alcohol (46), Hofkens and coworkers (75) used superresolution imaging to mapcatalytic activity on zeolite needles (Figure 4c). They were able to resolve intraparticle activitydifferences along the axis of the needle (Figure 4e). Kinetic analysis further revealed three distinctzones along the rod axis that could be attributed to the mechanism of zeolite crystal growth. Usingsuperresolution imaging, Hofkens and coworkers (76) further observed mass transfer limitationsfor individual reactions inside Ti-MCM-41 particles, revealing direct information on the Thielemodulus and effectiveness factor of the specific intraparticle diffusion-limited catalytic process.

Majima and coworkers (77) used superresolution imaging of reductive photocatalysis to identifyactive sites on TiO2 crystals. They were able to perform one-to-one correlation between super-resolution and electron microscopy imaging. Figure 4d shows a scanning electron microscopy(SEM) image of a single TiO2 crystal with large surface cracks identifiable on the surface. Inter-estingly, these large defects served as highly active sites. Owing to the large lateral dimensions ofthe crystals, the authors were able to study facet-specific charge carrier generation and transportby focusing the laser and localizing excitation to specific crystal faces. When exciting only the top

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Figure 4Superresolution imaging of nanocatalytic processes. (a) Fluorescence image of a single molecule. (b) 3Drepresentation of the image in panel a, and fitting it with a 2D Gaussian function, yielding the centerposition of the molecule (red cross in panel a). (c, top) Representative scanning electron microscopy (SEM)image of a zeolite needle sample and (bottom) a scatter plot of locations of product molecules generated froma single needle during furfuryl alcohol oligomerization. (d, top) SEM image of a TiO2 crystal withmacroscopic defects (circles) and (bottom) localized positions of product molecules generated on {001} (blue)and {101} (red ) facets in catalyzing boron-dipyrromethene dye reduction. (e, top) SEM image of a singlemesoporous silica-coated Au nanorod and (bottom) localized positions of product molecules from thenanorod in catalyzing an oxidative N-deacetylation reaction. Each dot represents one product molecule. Theinset shows a transmission electron microscopy image of the sample. Panels a, b, and e reproduced fromReference 63. Panel c reproduced by permission from Reference 75. Panel d adapted with permission fromReference 77. Copyright 2011 American Chemical Society.

{001} facet, they observed that product molecules were dominantly detected on the edges of thecrystal (Figure 4d ), attributable to facet-specific electron trapping on the {101} facets. Majimaand coworkers extended their superresolution imaging studies to metal nanoparticle-modifiedTiO2 mesocrystals (78), micrometer-sized crystals (79), and recently 2D nanocrystals (80), whichprovided insight into relationships between charge carrier generation and transport, as well asinto plasmonic nanoparticle-catalyzed reactions.

We have also used superresolution imaging to map catalysis on pseudo-0D, -1D, and -2Dnanoscale catalysts (63, 64, 81). For a pseudo-1D nanocatalyst (i.e., a single Au nanorod encapsu-lated in mesoporous silica), we mapped its individual products from an oxidative N-deacetylationreaction in direct correlation with its SEM image (Figure 4e) (63). We showed that the nanorodends are generally more active than their side facets. Strikingly, we observed that along the sidefacets of a single Au nanorod, the activity exhibited a linear gradient from the middle toward thetwo ends, attributable to an underlying defect density gradient that presumably formed during theseeded solution-phase growth of the nanorods (63). A related radial activity gradient was observedon Au nanoplates via superresolution imaging of catalysis in a different catalytic reaction (64). Wefurther showed that this imaging strategy could be used as a scalable parallel method to screen theactivity of a large number of catalyst particles, and the results could be extrapolated to catalyticreactions that do not involve fluorescent molecules (81).



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Single-molecule fluorescence microcopy allows for real-time, high-throughput, single-turnoverobservations of catalytic processes on a wide range of catalyst materials and sizes under ambientconditions. The temporal resolution of wide-field imaging experiments is typically in the mil-lisecond time region, although it may be possible to achieve nanosecond time resolution withconfocal microscopy. Using superresolution imaging, one can localize the position of a fluores-cent molecule down to subnanometer precision (82). Nanometer resolution along the z axis canalso be achieved based on single-molecule localizations (83). The optical microscopy setup canbe integrated readily into other optical techniques, such as micro-infrared or Raman (to studyrelatively large >1-μm catalysts) (84, 85), electrochemistry (86), scanning probe microscopy (87),and electron microscopy (88).

A requirement for applying single-molecule fluorescence microscopy is that products (or re-actants) need to be fluorescent. Catalytic reactions that do not involve fluorescent molecules,such as those in fuel cells, batteries, and photoelectrochemical cells, cannot be studied directly.To overcome this challenge, one can design fluorogenic reactions to study particular chemicaltransformations, for example, as in Majima and coworkers’ (89) study to probe interfacial electrontransfer reactions. One can also use indirect detection, which employs a subsequent fluorogenicreaction to detect nonfluorescent products in an initial reaction of interest, as demonstrated byMajima and coworkers (49) to detect reactive oxygen species. Alternatively, Amirav & Alivisatos(90) used fluence-dependent measurements in different media to show that single-particle photo-luminescence could be correlated with photocatalytic activity. Moreover, we recently showed thatactivity correlations between different reactions occurring on the same catalysts can be made so thatresults from fluorogenic reactions can be extrapolated to general, nonfluorogenic reactions (81).

4. SURFACE PLASMON RESONANCE SPECTROSCOPY FORSINGLE-NANOPARTICLE CATALYSIS

4.1. Principle

Metal nanoparticles support localized surface plasmon resonances (LSPRs), which are coherentoscillations of conduction electrons. The LSPR wavelength of a metal nanoparticle depends on itssize, shape, composition, local environment, and electron density. For Au and Ag nanoparticles,their LSPR wavelengths are in the ultraviolet/visible/near-infrared regime, in which the scatteringspectrum of individual nanoparticles can be measured with dark-field microscopy. Research onnanoparticle LSPR has led to applications in sensing, catalysis, and solar energy conversion. Wedirect the readers to comprehensive reviews regarding the theory, experiments, and applicationsof LSPR spectroscopy (91, 92).

Catalytic reactions can lead to changes in the chemical or physical properties of a catalystparticle, as well as in the local environment, as reactant molecules are consumed and productmolecules are generated on the catalyst surfaces. The LSPR approach to single-nanoparticlecatalysis is to detect shifts in the LSPR wavelength caused by these changes (93). Here we reviewthe method to measure LSPR spectra of single nanoparticles and several strategies that use LSPRto measure catalytic reactions on single nanoparticles.

4.2. Experimental Method for Single-Nanoparticle Localized SurfacePlasmon Resonance

Dark-field microscopy (Figure 5a) is often used to measure the LSPR spectrum of a singleplasmonic nanoparticle. A high–numerical aperture condenser is used to illuminate the sample

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Figure 5Localized surface plasmon resonance (LSPR) approach to single-nanoparticle catalysis. (a) Schematic ofdark-field microscopy for single-nanoparticle LSPR measurements. A high–numerical aperture dark-fieldcondenser illuminates the sample with white light ( green rays), and the scattered light is collected with alow–numerical aperture objective and imaged with a CCD (charge-coupled device) camera. (b) Schematic ofthe use of a slit (two red lines) to select the scattering signal from a single nanoparticle (white spot) from thedark-field image. The scattered light is then passed through a spectrometer to separate the differentwavelengths, which are imaged on the CCD camera at different locations perpendicular to the slit direction.(c) Integration of the signal from the slit region in panel b, yielding a single-nanoparticle scattering spectrum.

with a hollow cone of light. Scattered light from the nanoparticles is collected by a low–numericalaperture objective, in which the transmitted incident light is rejected and individual nanoparticlesappear as bright spots on a dark background. To acquire the scattering spectrum of a singlenanoparticle, a slit is introduced into the detection path (Figure 5b) to spatially select the scatteredlight from a single nanoparticle. The scattering signal is passed through a spectrometer to dispersephotons of different wavelengths, which are imaged by a camera.

4.3. Direct Localized Surface Plasmon Resonance Detection of SinglePlasmonic Nanoparticle Catalysis

For plasmonic nanocatalysts, their LSPR spectra can be measured to directly report on the catalyticreactions occurring on their surfaces. Using this strategy, Mulvaney and coworkers (94) observedascorbic acid oxidation reactions on single Au decahedron nanoparticles (Figure 6a). The oxi-dation of ascorbic acid molecules causes electron injection into the Au nanoparticle, resulting inelectron accumulation and a concurrent blue shift of the nanoparticle’s LSPR wavelength; the sub-sequent reaction of the Au nanoparticle with O2 depopulates the accumulated electrons, and theLSPR wavelength shifts back (Figure 6b). The shift in LSPR wavelength caused by the electrondensity change can be quantified using the Rayleigh equation. From the size of the nanoparticle(∼107 Au atoms) and the LSPR shift (∼20 nm), the authors calculated that 4,600 electrons/swere transferred to the nanoparticle during ascorbic acid oxidation, and correspondingly 65 O2

molecules would be reduced to return the nanoparticle to the initial state.Eo et al. (97) used a similar strategy to study the electron transfer rates of high-index

facets of elongated tetrahexahedryl Au nanoparticles and low-index facets of Au nanorods in the4-nitrophenol reduction reaction. This reaction also causes electron density changes on the Aucatalysts. They observed that the high-index facets were capable of accepting electrons seven timesfaster from reducing agents and transferring electrons to the reactant 2.5 times faster than thelow-index facets.

The direct LSPR strategy can also monitor chemical reactions on nanoparticles by utilizingthe particle-shape sensitivity of the LSPR wavelength. Mulvaney and coworkers (94) used this



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strategy to monitor the reductive Au deposition on a single Au nanoparticle. Starting with alow–aspect ratio Au seed, reductive Au+ deposition on it leads to growth into a high–aspect ratioAu nanorod, which has a distinct scattering spectrum from the initial seed. A few other studieshave since appeared that monitor the in situ growth of nanocrystals with LSPR spectroscopy(98, 99).

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4.4. Indirect Localized Surface Plasmon Resonance Detection of SingleNonplasmonic Nanoparticle Catalysis

Many catalytic materials do not support a measurable LSPR, at least in the ultraviolet/visible/near-infrared region where dark-field microscopy can be readily performed. Several indirect LSPRdetection strategies have been developed to circumvent this problem, as described below.

4.4.1. Core@shell strategy. Kasemo and coworkers (100, 101) used arrays of Au nanodisksas an indirect plasmonic sensing platform to monitor CO and H2 oxidation on overlaying Ptnanoparticles at the ensemble level. Alivisatos and coworkers (95) extended this indirect detectionstrategy to study catalysis on single Au@Pd core@shell nanoparticles of various shapes (Figure 6c).The Au core is LSPR active and serves as the reporter while the Pd shell is the catalyst and also thesubstrate for the hydrogen uptake reaction. Upon chemisorption onto the Pd shell, H2 dissociatesand the atomic hydrogen diffuses into the Pd to form PdH, which causes a change in the dielectricenvironment of the Au core and a concurrent change in its LSPR wavelength (Figure 6d,e).By examining the hydrogen uptake trajectories of many nanoparticles under cyclic hydrogendosage, they observed shape-dependent hydrogen uptake among triangular plates, icosahedrons,and decahedrons of Au@Pd nanoparticles.

Using a similar strategy, Song and coworkers (102) synthesized core@shell nanoparticle cat-alysts to study hydrogen evolution by platinized CdS photocatalysts. Plasmon band shifts of Auportions within their core@shell particles showed significant particle-to-particle heterogeneity,and the kinetic analysis of the LSPR wavelength shift allowed for calculation of the rate constant,diffusion coefficient, and average distance between active sites of Pt/CdS and the Au domains.

4.4.2. Plasmonic antenna strategy. Physically connecting the plasmonic reporter and ananocatalyst could alter the catalytic activity of the native catalyst. To circumvent this require-ment, Alivisatos and coworkers (96) used a plasmonic nanoparticle antenna strategy to monitorcatalysis on a nearby catalyst particle at the single-nanoparticle level. They used electron beamlithography to position a triangular Au nanoprism at nanometer distances away from a pseudo-spherical Pd nanoparticle (Figure 6f ). The triangular Au nanoprism acts as a sensor through itsLSPR while the Pd nanoparticle undergoes hydrogen gas uptake. Upon hydrogen adsorption ontothe Pd nanoparticle and subsequent reaction, the dielectric environment of the Au nanoprismis altered, causing a shift in its LSPR wavelength (Figure 6g,h). The Pd nanoparticle is alsopositioned next to a corner of the Au nanoprism, where the electric field from the LSPR is←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 6(a,b) Direct localized surface plasmon resonance (LSPR) detection. (a) Scanning electron micrograph (SEM)image of an Au decahedron nanoparticle. (b) Scattering spectra of the nanoparticle shown in panel a versusthe reaction time. The oxidation of ascorbic acid by O2 on single Au nanoparticles causes the accumulationof electrons in the nanoparticle. At longer times, the particle returns to its initial electronic state by reducingoxygen to water. Panels a and b adapted from Reference 94 by permission from Macmillan Publishers Ltd.,copyright 2008. (c–e) Indirect LSPR detection via the core@shell strategy. (c) High-resolution transmissionelectron microscopy images of Au@Pd core@shell nanocrystals of triangular nanoplates (top) andicosahedrons (bottom). (d ) Scattering spectra of an Au@Pd triangular plate as a function of two hydrogenpressure cycles. (e) LSPR maximum wavelength during absorption (solid triangles) and desorption (opentriangles). Panels c–e adapted with permission from Reference 95. Copyright 2011 American ChemicalSociety. ( f–h) Indirect LSPR detection via the antenna strategy. ( f ) SEM image of a single Pd-Au antenna(bottom), which appears as a diffraction-limited spot in the dark-field image (top). ( g) Evolution of scatteringspectra from the Au antenna as the hydrogen partial pressure is cycled from 0 torr to 33 torr. (h) Behavior ofthe resonance peak versus partial pressure. Panels f–h reprinted from Reference 96 by permission fromMacmillan Publishers Ltd., copyright 2011.



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Figure 7Plasmonic film-supported nanoparticle strategy. (a) Surface plasmon resonance electrochemical currentimage of a single nanoparticle as a function of the applied potential. (b) Cyclic voltammogram of the samesingle nanoparticle obtained by integrating the current density over the images in panel a. Figure adaptedfrom Reference 105 by permission from Macmillan Publishers Ltd., copyright 2012.

strongest and thus maximizes the LSPR wavelength shift upon hydrogen reaction with the Pdnanoparticle. This antenna strategy extends the range of materials and reactions that can be studiedat the single-particle level with indirect, optical methods. Using a related strategy, Tittl et al. (103)investigated hydrogen dissociation and subsequent uptake on 15-nm Pd films using individualsilica-coated Au nanoparticles as plasmonic reporters. Their strategy enabled the simultaneousprobing of chemical reactions at various surface morphologies and local reaction environments.

4.4.3. Plasmonic film-supported nanoparticle strategy for electrocatalysis. Tao and cowork-ers introduced a strategy related to the core@shell strategy that uses a plasmonic metal film asthe reporter rather than a plasmonic nanoparticle. They initially used this strategy to image localelectrochemical processes (104) and later extended it to study the electrocatalytic activity of sin-gle Pt nanoparticles and low-density arrays of nanoparticles supported on an Au film (105). Theoptically thin (∼50 nm) Au film serves as the catalytically inactive plasmonic support and workingelectrode for the proton reduction reaction catalyzed by Pt nanoparticles. At negative appliedpotentials, Pt-nanoparticle-catalyzed proton reduction decreases the refractive index near the Auelectrode surface, resulting in a detectable SPR peak shift that is quantitatively proportional to theelectrochemical current. They demonstrated that the technique could be used to simultaneouslyimage and quantitatively screen the activity of variable density arrays of ∼105 Pt nanoparticles.Figure 7a shows electrochemical current images of a single Pt nanoparticle versus the appliedvoltage. The tail at negative applied bias results from scattering of the surface plasmonic waveby the single nanoparticle. Simulation of the images as a function of applied bias allowed for themeasurement of current-voltage behavior for single nanoparticles (Figure 7b). Their strategy isnot limited to electrochemical applications and can be applied other catalytic reactions.


The SPR-based approach to single-nanoparticle catalysis is sensitive to the chemical, physical,and local environment of the nanoparticle and can be used to directly or indirectly report on

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catalytic processes. Regardless of the specific strategy, the approach requires a plasmonic material.Plasmonic materials such as Au and Ag are often used because commercial CCD cameras canreadily detect their SPR spectra in the visible regime. Theoretical simulations can be used topredict changes in the LSPR wavelength upon changes in the physical or chemical properties of aparticle during catalysis, facilitating data interpretation (106). The technique has also been shownto be high throughput (98, 105).

The plasmonic approach to single-nanoparticle catalysis also faces technical challenges. TheLSPR of a metal nanoparticle is typically measured by its Rayleigh scattering spectrum. TheRayleigh scattering intensity from a nanoparticle with diameter d is proportional to d 6. With de-creasing particle size, measuring the scattering spectrum of a single nanoparticle becomes increas-ingly difficult. With a conventional dark-field microscope, one can easily measure the scatteringspectrum of nanoparticles down to ∼50 nm, beyond which more specialized instrument config-urations are needed. Shifts in the LSPR require substantial perturbations to the particle shape,size, or electron density (e.g., the injection of hundreds to thousands of electrons). The techniqueis not currently amenable to study nanocatalysis with single-reaction resolution. In addition, theLSPR peak wavelength is a property of the entire nanoparticle, and thus subparticle informationis currently inaccessible.

5. X-RAY MICROSCOPY OF SINGLE-(NANO)PARTICLE CATALYSIS

5.1. Principle

A heterogeneous catalysis reaction that involves redox causes changes in the oxidation states orcoordination environments of the atoms in the catalyst during the catalytic cycle. Therefore, oneway to measure catalysis on single nanoparticles is to monitor these changes by X-ray absorptionspectroscopy (XAS), including X-ray absorption near-edge structure and extended X-ray absorp-tion fine structure. XAS involves the excitation of core-level electrons of atoms, in which theabsorption energy threshold (i.e., the absorption edge) is characteristic of the element, and thenear-edge features and extended fine structures inform its oxidation state and local coordinationenvironment (107).

XAS has been widely used as an ex situ technique to characterize bulk solids or an ensemble ofnanoparticle catalysts (reviewed in 108). Technological advancements in X-ray optics and detectorshave now made it possible to perform XAS with ∼10–40-nm spatial resolution using synchrotronsources. Advancements in the design of nanoreactors have also led to in situ XAS measurementsin both liquid and high-pressure gaseous environments with nanometer spatial resolution in threedimensions (109, 110).

Here we focus on in situ transmission X-ray microscopy studies that provide nanoscale chemicaland structural information of single working catalyst particles. We refer the readers to recentreviews for in-depth discussions of instrumentation, experimental methods, and data analysis ofin situ XAS measurements on heterogeneous catalysts (111–114). In situ X-ray imaging studies ofbattery electrodes (115) and fuel cell electrodes (116, 117) are not included here.

5.2. Experimental Methods for Single-(Nano)Particle In SituTransmission X-Ray Microscopy

X-ray imaging studies are performed at synchrotron sources that can provide a high flux ofmonochromatic X-rays tunable over a broad energy range (118). Depending on the experimentobjectives, one can use soft (200–2,000 eV) or hard (>2,000 eV) X-rays. Soft X-rays can be usedto excite the core levels of the lighter elements; they have a short penetration depth and can thus



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provide more surface-sensitive information, albeit with restricted sample thickness (2–3 μm thick)(119). Hard X-rays can probe the core levels of heavier elements; they interact less with the envi-ronment and penetrate deeper into samples, allowing for thicker reactor dimensions and samples(>20 μm) (120). The energy resolution of X-rays is of the order of 0.1–0.5 eV (111, 121).

For in situ nanoscale imaging of catalysts, two X-ray microscopy techniques have been mainlyused: scanning transmission X-ray microscopy (STXM) and full-field TXM. For each technique,several experimental setups are available at various synchrotron facilities (122, 123); detailed con-figurations have been described (118).

STXM uses point excitation of the sample with a focused beam (down to 10–40 nm in size;Figure 8a). An image is acquired by raster scanning the sample. This process is repeated for a rangeof excitation energies so that full spectral information can be obtained with a spatial resolutiondown to ∼15 nm. The experimental setup of STXM is conceptually related to confocal imagingin fluorescence microscopy. Full-field TXM uses a larger, micrometer-sized beam to irradiate asample, and the transmitted X-rays are detected with an X-ray CCD camera. An image is acquiredwithout raster scanning, analogous to wide-field optical microscopy. The spatial resolution offull-field TXM is approximately 30 nm with a single, flat field of view of up to 30 × 30 μm2.For both STXM and full-field TXM, the images can be obtained at many different orientationsof the sample relative to the incident beam. One can then use these images for tomographicalreconstruction to obtain 3D information (124, 125).

5.3. Nanoscale Scanning Transmission X-Ray Microscopyof a Working Catalyst

De Smit et al. (126) used STXM to study a working Fe2O3/CuO/K2O/SiO2 Fischer-Tropschsynthesis (FTS) catalyst. The exact nature of the active phase has been debated. The authors useda special ∼50-μm-thick nanoreactor, capable of reaching 500◦C, to alleviate the attenuation ofsoft X-rays.

Figure 8b,d shows chemical contour maps of the catalyst, constructed by analyzing the spatiallyresolved XAS spectra (Figure 8c,e), before and after exposure to a reductive (H2) and catalytic (H2

and CO) environment. De Smit et al. found that only two species were present in the native catalyst:∼25% α-Fe2O3 and 75% SiO2. Upon exposure to H2 gas and synthesis gas (a mixture of H2 andCO), a mixture of iron oxides and metallic iron species was observed in varying concentrationacross the catalyst. As seen in Figure 8b,d, the size and shape of the particle change significantly,indicating morphological changes in the sample as a result of the transition from Fe2O3 to Fe3O4,Fe2SiO4, and Fe0 (126). These complex in situ structural and chemical changes of single particleshighlight the possible complication of interpreting ex situ bulk XAS results. By analyzing thecarbon K-edge spectra, de Smit et al. further found that sp2 carbon was preferentially adsorbed oniron-rich regions, whereas sp3 carbon (attributed to reaction products) was found in iron-deficientregions. This led to their proposal that iron carbide formed in the iron-rich region, and thepresence of hydrocarbon species in iron-deficient regions suggested a spillover of products fromthe metal to the support, preventing the blocking of the active sites of the catalyst.

Many in situ STXM studies of working catalysts have since appeared, mainly from the Weck-huysen group, focusing on the support effects of FTS catalysts (127), the density and nature ofactive sites for fluid cracking catalysts (128), and the methanol-to-hydrocarbon process withinmicroporous silicoaluminophosphates and aluminosilicates (129, 130). Recently, Aramburo et al.(131) published a 3D STXM study with soft X-rays to determine the distribution of aluminum inzeolites. Tada et al. (132) used scanning hard X-ray imaging to identify the catalytically active andinactive phases of a nickel oxide–supported nanoparticle catalyst for methane steam reforming

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Figure 8(a) Schematic of the scanning transmission X-ray microscopy (STXM) setup. X-rays are focused on thesample using a Fresnel-type zone plate lens. An order-sorting aperture filters out higher-order diffractionorders. The sample is supported on the ∼10-nm-thick SiNx windows within the nanoreactor(∼500 × 500 × 50 μm3 in dimension). An image is acquired by moving the sample position with ∼35-nmprecision in x, y, and z and collecting transmitted X-rays with an X-ray detector. (b–e) Nanoscale STXM of aworking catalyst. (b,d ) STXM images of a 400 nm × 750 nm region of a Fischer-Tropsch synthesis catalystparticle (b) before and (d ) after 4 h in synthesis gas (a mixture of H2 and CO) at 250◦C. The black boxes inpanels b and d indicate specific sampling regions for XAS spectra shown in panels c and e of the Fe L2 and L3edges, all normalized to a maximum absorption of 1. ( f,g) 3D full-field imaging. Snapshots of an X-raynanotomography video of an individual fresh catalyst particle. Red represents Fe (the species Fe2O3, Fe3O4,and Fe2TiO5); green represents Zn (ZnO), and white/yellow/orange represent Ti + K (TiO2 and K2O),with white used for the highest concentration. Panels a–e reprinted by permission from Reference 126,Macmillan Publishers Ltd., copyright 2008. Panels f and g reproduced with permission from Reference 120.

to produce syngas. O’Brien et al. (133) used a similar method to probe solid-state changes of amillimeter-sized Ni/alumina catalyst during calcination and CO methanation.

5.4. Three-Dimensional Full-Field Imaging of a Working Catalyst

Utilizing the longer penetration depth of hard X-rays, Gonzalez-Jimenez et al. (120) performedfull-field TXM and 3D tomography on a 20-μm iron oxide Ruhrchemie catalyst particle promoted



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with titanium, zinc, and potassium oxides with 30-nm spatial resolution. Figure 8f,g shows snap-shots from a nanotomography video that illustrate the distribution of chemical species throughoutthe fresh catalyst. The authors monitored the evolution of the active Fe phases of the catalyst dur-ing the Fischer-Tropsch-to-olefins reaction at 350◦C and 10 bar in (H2/CO = 1). They concludedthat the most selective catalyst material corresponded to a catalyst material comprising a growingamount of Fe3O4, followed by Fe2O3 and Fe2TiO5.

Weckhuysen and coworkers (119) also used hard X-ray full-field TXM to investigate Co speciesgenerated during catalysis on a working TiO2-supported Co FTS catalyst in two and three di-mensions with 30-nm spatial resolution. Nanoscale depth profiling of elements revealed that Co isheterogeneously concentrated in the center of the catalyst, and no oxidation of Co was observed.


Advances in X-ray technology have pushed the spatial resolution to 10 nm, even at high tempera-ture and pressure (134). The use of hard or soft X-rays allows for the specific probing of elements,separating the catalyst material from catalytic products. The penetration depth of hard X-raysallows for the internal interrogation of mesoporous samples. Nanoreactor designs can furtherallow for high-throughput, 3D chemical/structural imaging in high-pressure gaseous and liquidsystems (111). Our review focuses primarily on the use of X-ray transmission detection modes; itis noteworthy that TXM can also be conducted with other detection modes, such as photoelectronemission and X-ray fluorescence (111, 114). The photoelectrons only have a mean free path of<10 nm in the solid, making this detection mode extremely surface sensitive.

Currently, the working distance between X-ray focusing optics and the sample is short, whichputs a strain on the nanoreactor design. The high flux of X-rays can incur significant damageto the sample, albeit reduced damage compared with scanning transmission electron microscopyand electron energy-loss spectroscopy (111, 121). To achieve sufficient contrast in a scanningtransmission X-ray detection experiment, one must ensure that the sample is not too dilute,making it difficult to examine a single, small, <10-nm catalyst particle. STXM images can requiretens of minutes to acquire; thus it is difficult to track real-time catalytic processes.

6. SURFACE-ENHANCED RAMAN SPECTROSCOPY FORSINGLE-NANOPARTICLE CATALYSIS

6.1. Principle

Reactants and products in catalytic reactions often have distinct vibrational features that can bemeasured by Raman spectroscopy. The Raman signals of molecules are greatly enhanced (∼106) inthe vicinity of an excited plasmonic material (typically Au or Ag), an effect called surface-enhancedRaman scattering (SERS) (135, 136). Single-molecule SERS (137–140) and real-time monitoringof chemical reactions via SERS (141–143) have been demonstrated. We refer the readers to otherreviews on the history, theory, and applications of SERS (144–147).

Owing to its high sensitivity and selectivity, SERS has the potential to observe single-nanoparticle catalysis in real time, for which a plasmonic catalyst is needed. Alternatively, a metallicprobe tip [e.g., in atomic force microscopy (AFM) and scanning transmission microscopy (STM)]in close proximity can be used to provide the electromagnetic field enhancement necessary forSERS detection of surface adsorbed molecules (148–150). The advantage of tip-enhanced Ramanspectroscopy (TERS) is its ability to spatially resolve spectroscopic information (recently reviewedin 151–154), even with single-molecule resolution (155, 156). TERS has been applied to study

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Figure 9(a,b) Experimental setups for tip-enhanced Raman spectroscopy (TERS) and surface-enhanced Raman scattering (SERS).(a) Combined confocal Raman/atomic force microscope setup for TERS. The plasmonic enhancement occurs at the plasmonic tip.(b) Confocal Raman microscope setup to measure Raman spectra of reactants or products adsorbed on single plasmonic nanoparticlesthat are also catalysts. (c) Single-particle catalysis via TERS. Shown are the time-dependent SERS spectra acquired with the setup inpanel a of the photocatalytic reduction of pNTP (top spectrum) to DMAB. Spectra were acquired every 12 s. (d,e) Single-particlecatalysis via SERS. (d ) Scanning electron microscopy image of a single roughened Ag nanocatalyst. (e) Raman image of the catalystcollected at the 1,335-cm−1 Raman peak corresponding to the v(NO2) peak of pNTP. Panels a and c reproduced from Reference 158by permission from Macmillan Publishers Ltd., copyright 2012. Panels d and e reproduced with permission from Reference 159.

heterogeneous catalysis on bulk substrates (142, 157). Here we review how TERS and SERS havebeen applied to study catalysis on single nanocatalysts.

6.2. Experimental Methods for Single-Nanoparticle Tip-EnhancedRaman and Surface-Enhanced Raman Spectroscopies

Figure 9 illustrates two experimental methods to study single-nanoparticle catalysis via SERS.Figure 9a shows a TERS setup, combining a confocal Raman microscope and an AFM adoptedby Weckhuysen and coworkers (158). The Ag-coated AFM tip acts as both a TERS tip and acatalyst; the tip apex interacts with reactant molecules adsorbed on the Au nanoplate and thusrepresents a single catalytic particle. The Au nanoplate support also enhances the Raman signal.Various TERS setups have been reviewed, and they mainly differ in the type of scanning probeconfiguration, illumination geometry, and excitation/detection configuration (153).

Figure 9b illustrates a general confocal Raman microscope setup that uses focused laser illumi-nation to excite a small region that contains a single plasmonic nanoparticle. The Raman signalsof the reactants and products on the nanoparticle are collected via the microscope objective. Inthis method, the nanoparticle both acts as a catalyst and enhances the Raman signals.



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6.3. Nanoscale Catalytic Processes via Tip-Enhanced Raman Spectroscopy

Using the setup in Figure 9a, Weckhuysen and coworkers (158) monitored the photocatalyticreduction of p-nitrophenol (pNTP) at the illuminated Ag tip. The Au nanoplate support ensuresthat no more than a monolayer of reactant molecules is present, and it also further enhances theTERS signal. The photocatalytic reaction was initiated with a 532-nm laser, whereas a 633-nmlaser was used for Raman excitation. By acquiring the Raman spectra of the molecules on the Aunanoplates as a function of 532-nm illumination time (Figure 9c), the authors found that pNTPwas catalytically converted to p,p′-dimercaptoazobisbenzene (DMAB). Moreover, the completemonolayer coverage of the pNTP reactant was disturbed during catalysis, through either thechange in functionality from NO2 to NH2 or dimerization.

Around the same time, Sun et al. (160) used a high-vacuum STM-TERS setup to investigateplasmon-driven 4-nitrobenzenethiol dimerization to dimercaptoazobenzene. They were able tocontrol the plasmon-driven reaction by laser power or by the tip-sample distance (and thereforeSERS enhancement factor). Their study holds promise for future investigations that will combineatomic-resolution STM imaging with localized spectroscopic information of surface-catalyzedreactions.

6.4. Catalysis via Single-Particle Surface-Enhanced Raman Spectroscopy

Kang et al. (159, 161) studied the same photocatalytic pNTP-to-DMAB conversion reactionon single Ag particles (Figure 9d ) using a setup similar to that in Figure 9b. The large(∼2-μm diameter), highly roughened Ag particle is nanostructured on its surface, which acts asan efficient SERS substrate. Figure 9e shows a Raman image acquired at the 1,335-cm−1 Ramanpeak, corresponding to the v(NO2) peak of pNTP. The authors studied the effects of excitationenergy and power on the rate of the plasmon-driven reduction of pNTP on individual Ag particlesand showed that 532-nm excitation increased the reaction rate more effectively than 633-nm exci-tation, confirming that the reaction is induced by excitation of surface plasmons on the Ag particle.


SERS potentially offers the general, real-time, direct, quantitative detection of reactant consump-tion and product generation on a single catalyst particle. In addition, Raman spectroscopy canpotentially yield complementary chemical and structural information of the catalyst particles be-sides the reactants and products (162). Intermediates in the catalytic process may also be identified(163). Single-molecule sensitivity via SERS is also possible in both liquid and gaseous environ-ments (137, 138, 164, 165). Combining the confocal Raman microscope with high-resolutionscanning probe microscopy allows for ultrahigh resolution imaging of catalysts to be correlatedwith single-molecule SERS spectra. Zhang et al. (156) recently demonstrated that an STM-basedTERS approach could resolve the inner structure and surface configuration of a single molecule ona plasmonic Ag (111) substrate. TERS has the distinct advantage that the tip can spatially localizethe electromagnetic field enhancement to a nanoscale region of interest on any type of catalyst.Thus, the TERS approach holds tremendous promise toward the identification of atomic-scaleactive sites in heterogeneous catalysis.

Challenges are also present. At least one component of the experimental setup must include amaterial that serves to boost the Raman signal of molecules via SERS, typically metallic particlessuch as Au and Ag. Real-time single-molecule kinetics studies are limited by the integration timeneeded to acquire a spectrum (1 to 10 s). For TERS, the fabrication of tips/samples that provide a

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known TERS enhancement factor is critical for the quantitative evaluation of catalysis data (152).Solution-phase catalysis experiments can be difficult too, because reactant/product molecules inthe near surface region are difficult to exclude from those specifically adsorbed to the catalyst.

7. CONCLUDING REMARKS

Above we review experimental approaches that allow for the study of catalysis on single nanopar-ticles, with selected examples illustrating their applications. Each approach relies on the sensitivedetection of photon or electron signals that report on the reactants, products, catalyst of the re-action, or changes these reactions evoke on the surroundings. Each approach is distinguished bystrengths and limitations with regard to chemical selectivity, sensitivity, and spatial resolution.The types of information available from these approaches are both complementary and corrobo-rating. Thus, when used in combination, they have the potential to provide unprecedented insightsinto the structure-activity-dynamics correlation of nanoscale catalysis. New approaches will likelycontinue to emerge that will achieve even greater spatial and temporal resolution in an attempt toidentify and study the atomic-scale active sites in nanoscale heterogeneous catalysis.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

J.B.S acknowledges the National Science Foundation for an ACC-F Postdoctoral Fellow-ship (NSF-1137217). P.C. acknowledges support from the Department of Energy (DE-FG02-10ER16199), Army Research Office (W911NF0910232), and National Science Foundation(CBET-1263736).

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Approaches to Single-Nanoparticle Catalysis - Chen Groupchen.chem.cornell.edu/publications/annurev-physchem-040513-103729.pdf · Methods for electrochemical measurements of single-nanoparticle