Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

Laboratnn Cor

E-m

journal homepage: www.elsevier.com/locate/nima

Study of electro-chemical properties of metal–oxide interfaces using a newlyconstructed ambient pressure X-ray photoelectron spectroscopy endstation

Funda Aksoy a,b,n, Michael E. Grass a,c, Sang Hoon Joo d, Naila Jabeen a,e, Young Pyo Hong a,c, Zahid Hussain a,Bongjin S. Mun c, Zhi Liu a,nn

a Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94709, USAb Department of Physics, C- ukurova University, Adana 01330, Turkeyc Department of Applied Physics, Hanyang University-ERICA, Gyeonggi-do 426-791, Republic of Koread School of Nano-Biotechnology and Chemical Engineering, UNIST, Banyeon-ri 100, Ulsan 689-798, Republic of Koreae Nanoscience & Catalysis Div., National Centre For Physics, Islamabad 44000, Pakistan

a r t i c l e i n f o

Available online 31 December 2010

Keywords:

Electron energy analyzer

Ambient pressure X-ray photoelectron

spectroscopy

Catalysis

Electrochemistry

Rh nanoparticles

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.12.171

esponding author at: Advanced Light Source,

ory, Berkeley, CA 94709, USA.

responding author.

ail addresses: faksoy@lbl.gov (F. Aksoy), zliu2

a b s t r a c t

In this report, we briefly describe the general design principles and construction of a newly developed

ambient pressure X-ray photoelectron spectroscopy system. This system provides an imaging mode with

o20 mm spatial resolution in one dimension as well as an angle-resolved mode. The new imaging mode

enables us to study structured surfaces under catalytically and environmentally relevant conditions. To

illustrate this capability, in situ studies on a Au–SiO2 heterojunction and Rh–TiO2 metal–support system

are presented. This new system can probe structured surfaces near ambient pressure as a function of

temperature, pressure, electrical potential, local position, and time. It is a valuable in situ tool to detect

material transformations at the micrometer scale.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Creating a carbon-neutral energy future is one of the greatestchallenges that human society has ever faced. Given the urgency ofthis task and the speed of industrial development, the research anddevelopment cycle for new materials and technology is becomingshorter and shorter. We cannot afford to evaluate the long-term andlarge-scale environmental impacts of new materials, infrastructures,and new technologies through a traditional multi-decade researcheffort. Given the scale of industrial productivity nowadays and thesheer volume of new materials that will be produced and used, wecannot afford not to do it either—the implications of environmentaland economical impact are too important to ignore. We must be ableto accurately predict long-term impacts in a time frame commen-surate with the speed of research and development to provideuseful, timely feedback.

One solution is to detect material transformations at themicrometer to nanometer scale in situ. By shrinking the lengthscale, we can evaluate long-term, large-dimension changes withina reasonable time span. Many in situ surface-sensitive techniques,such as synchrotron radiation based ambient pressure X-rayphotoelectron spectroscopy (APXPS) [1,2], are ideal in detecting

ll rights reserved.

Lawrence Berkeley National

@lbl.gov (Z. Liu).

small changes at the gas–solid and liquid–solid interface. Mostlong-term change, such as corrosion and the degradation ofcatalysts and batteries or steel-reinforced concrete, results fromslow chemical reactions at surfaces. If we can understand how suchchemical reactions start and develop, we can predict the afore-mentioned long-term impacts to provide timely feedback.

To achieve such a goal, we have constructed and commissioneda high performance APXPS endstation at beamline (BL) 9.3.2 at theAdvanced Light Source [2]. The versatility and high transmission ofthe newly designed electron lens system enable us to use fewerphotons (less beam damage), provide in situ spectroscopic andmicroscopic information of surfaces, collect high quality data in ashorter time, and extend experiments to higher pressure.

In this report, we will briefly describe the general design andconstruction of this new APXPS system with high electron trans-mission and detection efficiency to reduce radiation exposure anddata acquisition time. More detailed information can be found inthe previous report [3]. We will also present two examplesdemonstrating how the newly developed imaging mode of thissystem can help us to study more complex materials undercatalytically and environmentally relevant conditions.

2. General design and performance of new APXPS system

This new APXPS endstation is based on the Scienta R4000HiPP system with a two-dimensional detector consisting of two

F. Aksoy et al. / Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265 261

multichannel plates coupled to a phosphor screen and charge-coupled device camera. As shown in Fig. 1, the electrostatic lenssystem is integrated to incorporate four separated pumpingsections within the lens housing. To enhance the energy andspatial/angular resolution, the pre-lens section and analyzer havedouble m-metal shielding layers throughout.

The main chamber is equipped with a four-axes motor systemfor computer control of sample motion using step motors withsubmicron precision. The sample holder is a Thermionics STLCplaten with a HeatWave ceramic coated button heater for oxidizingenvironments. We are currently planning to replace the buttonheater with an infrared laser to reduce background reactionsduring catalytic studies. A custom built manipulator is used, whichcan be cooled using water, chiller fluid, or liquid nitrogen.

There are many design criteria for an APXPS system. Mostimportantly, the analysis chamber must be separated from theelectron detector (analyzer), which operates in high vacuum. This isaccomplished by differential pumping through a series of aper-tures. The radius of the aperture between the analysis chamber andthe first pumping section, R, is the key parameter. R, typically0.1–0.5 mm, not only determines the gas conductance from theanalysis chamber into the analyzer lens column but also sets theminimum distance between the sample and aperture. This mini-mum distance determines the upper limit of workable pressure.The radius of our first aperture is 0.425 mm.

Due to the pressure drop across the first aperture, the pressuredecreases as the sample moves toward the first aperture.The pressure along the perpendicular axis of the first aperture isgiven by [1]

PðzÞ ¼P0

21�

zffiffiffiffiffiffiffiffiffiffiffiffi1þz2p

� �ð1Þ

where z is the distance between the sample and the center of theaperture (z¼0). Using this equation, the sample must be placed at adistance greater than 2R (zr�2R) to ensure that the gas pressureat the sample is at least 95% of analysis chamber pressure. The

Fig. 1. Schematic cross-section drawing of ALS-Scienta HiPP4000 APPES system. The syst

The electrostatic lens system is integrated with four separated pumping stages in pre-lens

analysis chamber. Left inset—zoom-in view of region near sample and first aperture. B

detector when analyzer is working in angular mode and imaging mode.

attenuation of the photoelectron signal in a gas environmentdepends on the gas species, gas pressure, and the kinetic energyof electrons. For given pressure P, this attenuation is given as

Ip=Ivac ¼ expð�zPseðKEÞ=kBTÞ ð2Þ

where Ip is the signal at pressure P, Ivac is the signal in vacuum, kB isthe Boltzmann constant, andse is the electron-gas scattering cross-section.

If we define the maximum operational pressure Pmax to be thepressure at which Ip=Ivac ¼ e�2 (13%) [1] and assume that thesample is placed at a distance 2R from the first aperture, we cancalculate the maximum pressure from Eqs. (1) and (2). We findPmax ¼ kBT=Rse. For our system with a 0.425 mm radius, themaximum operational pressure is about 1.8 Torr for oxygen.

This result highlights why a synchrotron based light source isideal for APXPS. Due to the high brilliance of 3rd generationsynchrotrons, the X-ray can be focused to a small spot. Therefore,smaller R can be used and higher maximum pressure can beachieved. However, the minimum spot size at BL 9.3.2 is relativelylarge, 0.5�1.5 mm2. The photon footprint on the sample isfurther enlarged because of the 151 incident angle. The size ofthe first aperture is smaller than the photon spot on the sample.Therefore, we need to determine the aperture radius that providesmaximum photoelectron counts under typical operating condi-tions. The total electron count is proportional to the product of theuseful aperture size (usable photon flux) and attenuation ratiogiven by Eq. (2), so the optimum aperture radius for a givenpressure can be derived through the optimization of this product.Our aperture radius, 0.425 mm, was designed to have maximumcounts at �0.8 Torr.

Using a dispersive hemispherical analyzer with a two-dimen-sional detector system, one can record data in two dimensions. Asshown in Fig. 1 (bottom left inset), one dimension (verticaldirection here) represents the energy dispersion direction andthe other (horizontal direction) typically represents the take-offangle in most analyzers to perform angle-resolved photoelectron

em consists of a pre-lens section and a Scienta R4000 hemispherical energy analyzer.

section labeled as S1–S4. X-ray from ALS was delivered through a silicon nitride into

ottom inset—from left to right, false color images from two-dimensional electron

F. Aksoy et al. / Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265262

spectroscopy. We developed a different lens voltage table (voltageson electron lens elements at any given electron kinetic energy) touse the horizontal direction for spatial imaging. Photoelectronsoriginating from the sample at different positions in the horizontaldirection (y) will be refocused on the detector to reproduce a ‘‘one-dimensional image’’ (Fig. 1 bottom right inset). Using this imagingmode, we can perform spectromicroscopy of a structured sample inone dimension with 20 mm resolution. Two examples are givenhere to illustrate the capability of this mode of operation.

The first priority of an APXPS system design is to optimize PEtransmission. The pre-lens should be able not only to reduce thepressure over a very short distance but also to collect a largefraction of the PEs. At the same time, the energy solution required,the size of the excitation spot, and flexibility in lens mode operationall affect the electron lens table design. We would like to design asystem that is very flexible, gives a high experimental throughput,and maximum uptime.

We will briefly describe the design of our pre-lens system. Onecan find a more detailed description of this pre-lens design inRef. [3]. A key feature of the Scienta R4000 HiPP pre-lens design isthe 1201 cone shape first aperture. Such an open first cone allowsfor the possibility to put a second conical lens element very close tothe first aperture. It allows the electron optics to control thetrajectories very early in the path, making it possible to keep thetrajectories close to the optical axis. Second, this differentialpumping stage has a high PE transmission without requiring anode of the electron trajectories. Due to the spherical aberrationsinherent in electrostatic lens systems, good point-to-point imagingis limited to relatively small angular intervals. Electrostatic lenssystems also have strong chromatic aberrations inherently. Theposition of a node will be a function of the kinetic energy, which canaffect the PE transmission. This is especially important to considerwhen optimizing for high transmission with large pass energy,when a two-dimensional detector simultaneously collects over alarge energy interval. The second differential pumping stagecomprises additional lens elements and ends with an exit slit.The size of this slit is matched to the hemispherical analyzerentrance slit and the position is in the standard sample position ofthe R4000 lens. The use of a matched exit slit gives sufficientdifferential pumping properties at this stage without reducing theelectron throughput. Most importantly, such a slit provides thedynamical freedom necessary to operate complex lens modesstably and continuously over large energy ranges. The pre-lensmoves the effective source point of the R4000 lens 100 mm furtherfrom the analyzer and matches the size and angular divergence ofthe beam to the analyzer acceptance properties. In a typicaloperation, a magnified image of the sample spot is created onthe exit slit. The imaging properties of the pre-lens are best for amagnification of around five times. In the current design setup, thesample, first aperture, and exit slit of the pre-lens are all at groundpotential, so that the pre-lens does not change the energy of the PEs.According to Liouville’s theorem, the product of the spatial andangular lens magnifications is then unity. The R4000 lens willtherefore receive PEs where the angular distribution has beencompressed into a range that is well adapted to its normaloperation parameters. Since the pre-lens can control the electrontrajectories at such an early stage, a very large angular range can betransformed into a well collimated beam through the exit slit of thepre-lens. Dependent on the chosen slit size and energy resolution,virtually all electrons that pass through the exit slit of the pre-lenswill reach the detector. For kinetic energies below 500 eV and spotsizes similar to the radius of the first aperture, the total transmis-sion under UHV conditions of the system is not much different fromthat of a normal R4000 lens. To calculate lens tables for morecomplex situations, the complete lens system must be treated bythe optimization routines.

3. Results and discussion

3.1. Spatiotemporal spectroscopy of Au/SiO2 heterojunction at near

ambient condition

Junctions between two types of materials are common in manyapplications, including silicon-based electronics, fuel cells, bat-teries, welded joints, etc. By adding a 2D detector and a spatialresolution lens voltage table to the APXPS system, the surfaces ofsuch junctions can be studied in situ with elemental and chemicalspecificity. To illustrate this capability, we report a simple systemconsisting of a grounded Au stripe 100 mm wide patterned on a Siwafer with a thermally grown SiO2 layer. The image from the two-dimensional electron detector is shown in the right inset at thebottom of Fig. 1. x-Axis gives the sample position of photoelectronemission and y-axis gives the binding energy of the photoelectrons.One can clearly see the Au 4f and Si 2p peaks from the 100 mm Aubar and SiO2 layer, respectively. A vertical cut of the image alongy-axis at a given x position will give an integrated XPS spectrum ofthe sample. This spectrum provides elemental and chemicalcomposition information of the sample at that particular x position.A horizontal cut of the image along x-axis across a given core levelpeak will give the spatial intensity distribution of that element. Thisprovides information of the element concentration across thesample surface. In addition, the local electrical surface potentialchange can be determined from the changes in kinetic energy of thephotoemission peak as shown below.

For this test, the sample was heated in 200 mTorr O2 to burn offcarbon from the surface while simultaneously scanning the Au 4f, Si2p, and C 1s regions (�1 min per set of scans). In Fig. 2, we show asequence of spectral images of the Si 2p region taken while heating.Initially, the Si 2p binding energy is 104.5 eV across the image,corresponding to Si4+ from the SiO2 sample. The Si 2p peak starts toshift to a higher binding energy after frame 30 (T�500 K). The Si 2pshift is smaller for the region near the Au bar and becomes largerfurther away. This shift to higher binding energy (up to 1.0 eV) is nota chemical shift, but indicates charging of the SiO2 surface. The peakreturns to its original binding energy over the course of 10 min.

To understand the cause of this Si 2p peak shift, we plot thesurface color map of the binding energy shift vs. position and timealong with the C 1s peak intensity during the same time period inFig. 3. In the map, x-axis represents time and y-axis represents theposition of the sample. The Si 2p peak shift coincides with thedisappearance of surface carbon from SiO2. It is quite plausible thatthe surface carbon layer provides a conductive pathway forelectrons from the sample holder via the Au stripes to compensatethe photoelectron loss from the SiO2 layer. As the temperatureincreases to �500 K, the carbon is removed from the SiO2 surface asCO2 under 200 mTorr of O2. There is no conductive path for theelectron after the surface carbon layer is removed. The insulatingSiO2 layer starts to charge positively due to the loss of photoelec-trons. The surface potential shifts toward more positive values,thus the apparent binding energy of the Si 2p peak shifts to highervalues. It is also possible that extra charge transfer happens fromthe SiO2 to C during the C oxidation process. This can cause such asurface potential shift as well.

It is less clear why the Si 2p binding energy returns to its originalvalue after 10 min. It could result from photon beam inducedconductivity. However, photon induced conductivity, like electronbeam induced conductivity (EBIC) [4,5], is sensitive to the material,the thickness of the insulating layer, and the photon energy [6–8]. Itis beyond the scope of this paper to address all these issues.Nevertheless, we have shown that we can use APXPS to quantita-tively characterize transient charging of thin insulating materials atmetal–insulator junctions. Surface charging and discharging occurduring electron beam lithography (printing) and directly impact

Fig. 2. Time sequence of Si 2p spectral image taken at imaging mode. x-Axis is binding energy of photoelectrons. y-Axis is relative position on sample; each detector camera

pixel corresponds to 1 mm on sample. Each frame is taken 1 min after previous frame and frame number is labeled at bottom left of each frame. Position of Au bar can be

determined by secondary electron tail of Au 4f peak, which can be clearly seen in middle of each frame.

Pos

ition

(µm

)C

1s

Inte

nsity

(CP

S)

Time (min)

Time (min)

10 20 300

10 20 300

Cha

nge

in B

E (e

V)

Fig. 3. Top—integrated C 1s intensity vs. time plot over whole region during heating. Bottom—surface plot of changes of Si 2p binding energy (BE) vs. position and time. It

shows that there is simultaneously positive BE shift, i.e. positive surface potential shift, across SiO2 surface when surface carbon disappears. BE returns to original value in

about 10 min (70.2 eV).

F. Aksoy et al. / Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265 263

F. Aksoy et al. / Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265264

beam placement accuracy [9–11]. APXPS can be used as an in situcharacterization tool for EBIC research on photo resist layers andother insulating materials. Moreover, the new imaging mode of thisendstation can provide valuable local chemical information as wellas local surface potential information during chemical reactions.The correlations between local chemical processes and local sur-face potentials associated with such changes under operatingconditions are crucial to understand important electro-chemicaldevices such as batteries and fuel cells. In fact, several studies inthis area have been carried out using this endstation [12–15].

3.2. Catalyst support interaction at Rh–TiO2 interface

Supported metal catalysts have been studied extensively due totheir technological importance. The interactions between smallmetal nanoparticles and the underlying transition metal oxidesupport are of great interest since the discovery of ‘‘synergeticpromotion’’ from oxides support 80 years ago [16]. Considerableefforts have been directed at understanding this so called strong-metal–support-interaction (SMSI) over the last three decades.Different models [17–21] were proposed, such as alloy formation,electronic effects (charge transfer), and geometric effects (encap-sulation or decoration of the metal by the oxide). SMSI is aphenomenon that exists only at an interface. Understanding theseSMSI systems thus critically depends on advanced surface techni-ques. A meaningful study of metal–support interaction must beable to detect changes in the valence and oxidation state of metaland support, and also differentiate between the metal and supportsurface and the metal–support boundary. XPS is an important toolin determining changes in valence and oxidation states. It is notsurprising that APXPS has been used to study these systems [22,23]

We studied a Rh–TiO2 model system under hydrogen oxidationconditions. Herein, we report the results under reaction conditionsof 573 K, 100 mTorr H2+100 mTorr O2. A more detailed study ofthis system will be reported elsewhere [24]. Using the newly

C 1s Rh 3d

Ti 3pVB

X (m

m)

295 290 285 280 275

X (m

m)

320 316 312 308 304

42 40 38 36 34 32 306 4 2 0 -2Binding Energy (eV) Binding Energy (eV)

0.1

0.0

-0.1

-0.2

0.1

0.0

-0.1

-0.2

0.1

0.0

-0.1

-0.2

0.1

0.0

-0.1

-0.2

x

TiO2

5nm Rhnanoparticles

Binding Energy (eV) Binding Energy (eV)

X (m

m)

X (m

m)

Fig. 4. (a) Spectral images of valence band (VB), Ti 3p, C 1s, and Rh 3d region at TiO2/Rh–

regions away from interface. Red dotted line represents spectrum from TiO2 region an

schematic drawing of side view of half covered Rh–TiO2 sample, the dashed line indicating

figure, the reader is referred to the web version of this article.)

developed imaging mode, we can scan the support only surface andmetal–support surface simultaneously. We can easily differentiatethe changes on the support surface and the metal–support surface.

Rh nanoparticles (NPs) of 5 nm were synthesized following theone-step polyol synthesis method described previously [25,26].Monolayer films of 5 nm particles were then prepared in aLangmuir–Blodgett (LB) trough. To image both the TiO2 only regionand the Rh–TiO2 region simultaneously, we coated only half of aTiO2 wafer with NPs as shown in Fig. 4 (inset on the left).

After moving the TiO2/Rh–TiO2 interface into the detectorregion, we used the imaging mode of the spectrometer to monitorthe core level peaks of interest at various hydrogen oxidationreaction conditions. In Fig. 4(a), we show spectral images of thevalence band (VB), Ti 3p, C 1s, and Rh 3d at the near-interface regiontaken at 573 K with a photon energy of 500 eV under 100 mTorr H2

and 100 mTorr O2. The interface between the TiO2 only region andRh NP coated TiO2 region is clearly visible. Furthermore, the VBspectral image showed the clear band offset between TiO2 only andthe Rh NP coated TiO2 region. The VB maximum of the TiO2 onlyregion lies �3.0 eV below the Fermi level while that of the Rh NPcoated TiO2 region coincides with the Fermi level due to themetallic nature of the Rh NPs. Such an image provides directvisualization of the band offset across the metal–semiconductorjunction. Because we can map both sides of the junction at the sametime, real time changes of such an offset can be obtained underdifferent conditions.

As previously mentioned, we can obtain the photoemissionspectra by taking a cut along the energy direction. We presentspectra of VB, Ti 3p, C 1s, and Rh 3d in Fig. 4(b). Two spectra for eachVB and core level are shown. The red dotted line represent spectrafrom the TiO2 region and the black dotted line are spectra from theRh–TiO2 region. They are obtained by integrating the pixel countsof the shaded area in each respective region. From peak fitting, wecannot only identify the different chemical species but alsocompare the relative surface potential difference between these

Binding Energy (eV)

VB

7x104C 1s

Ti 3p

7x105

Inte

nsity

(a. u

.)

316 314 312 310 308 306 304Binding Energy (eV)

X: (0.1mm, 0.17mm)

X: (-0.2mm, -0.13mm)

Rh 3d

2.5x104

2.0

1.5

1.0

0.5

Inte

nsity

(a. u

.)

6 4 2 0 -2

X: (-0.2mm, -0.13mm)

X: (0.1mm, 0.17mm)

6

5

4

3

Inte

nsity

(a. u

.)

292 290 288 286 284 282 280Binding Energy (eV)

X: (0.1mm, 0.17mm)

X: (-0.2mm, -0.13mm)

1.0x105

0.8

0.6

0.4

0.2

Inte

nsity

(a. u

.)

44 42 40 38 36 34 32Binding Energy (eV)

X: (-0.2mm, -0.13mm)

X: (0.1mm, 0.17mm)

6

5

4

3

2

1

316 314 312 310 308 306 304Binding Energy (eV)

X: (0.1mm, 0.17mm)

X: (-0.2mm, -0.13mm)

TiO2 interface. (b) Integrated XPS spectra of VB, Ti 3p, C 1s, and Rh 3d from shaded

d black dotted line represents spectrum from Rh–TiO2 region. Left inset shows a

where TiO2/Rh–TiO2 interface is. (For interpretation of the references to color in this

x

TiO2

5nm Rh NPs

3.2eV 3.0eV

0.2eVFermi Level (EB = 0)

3.2eV

Band diagram of TiO2 only surface

Band diagram of Rh-TiO2 surface

316

Inte

nsity

(A. U

.)

Binding Energy (eV)

Rh3d500eV

0.6/0.7eV(0.4/0.5+0.2)Fermi Level (EB = 0)

314 312 310 308 306 304

Fig. 5. (a) Schematic drawings of surface band bending of TiO2 only region (top) and Rh–TiO2 region (bottom). There is 0.4–0.5 eV more upward band bending at Rh–TiO2

region due to charge transfer between TiO2 support and Rh NPs. (b) Fitting of Rh 3d spectrum taken from Rh–TiO2 region. It can be fitted with two doublets. The red one belongs

to metallic Rh atoms and the blue one belongs to Rh+, which has a Rh 3d5/2 BE of 307.6 and 308.5 eV, respectively. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

F. Aksoy et al. / Nuclear Instruments and Methods in Physics Research A 645 (2011) 260–265 265

two surfaces. We found that the binding energy (BE) of the C 1speak on the TiO2 only region is �0.4 eV higher than that of theRh–TiO2 region. A similar 0.5 eV was observed for Ti 3p. There wereno line shape changes for either the C 1s or Ti 3p spectra. Thus, thisBE shift was not due to a chemical shift, but a surface potentialoffset due to band bending at the interface. The band bends upward�0.4–0.5 eV more from the TiO2 to the Rh–TiO2 surface at thisreaction condition. This results from charge transfer between theTiO2 support and the Rh NPs, a direct indication of metal–supportinteraction. Combining this additional shift with the 3.0 eV VBoffset found in the VB spectra, we can plot the surface banddiagrams of both regions in Fig. 5(a) assuming a 3.2 eV bandgap forTiO2. This upward band bending at the metal–oxide surface hasbeen observed on other metal–TiO2 systems [27–29]. It is animportant factor leading to the SMSI effect. The detailed fittingof Rh 3d spectra is shown in Fig. 5(b). There are two Rh species—oneis metallic Rh with a BE of 307.6 eV and the other is assigned to Rh1 +

at 308.5 eV. We also found that the band bending of Rh–TiO2 andoxidation state of Rh NPs change under different reaction condi-tions. The complete results will be published elsewhere [24].

In this section, we demonstrated that using the imaging mode ofAPXPS, we can easily differentiate chemical changes and electronicchanges on a support surface and the metal–support surface andmonitor them in situ simultaneously. This newly developed techni-que is a very powerful tool for studying heterogeneous catalysis.

4. Conclusions

We described the general design principles and construction of anewly developed APXPS system with high electron transmission anddetection efficiency. This system provides an imaging mode witho20 mm spatial resolution in one dimension as well as an angle-resolved mode. The new imaging mode enables us to study structuredsurfaces under catalytically and environmentally relevant conditions.We also reported in situ studies on a Au–SiO2 heterojunction andRh–TiO2 metal–support system. Information of both local chemicalchanges and local surface potentials associated with such changes canbe obtained. This new system can probe structured surfaces nearambient pressure as a function of temperature, pressure, potential,local position, and time. It is a valuable tool to detect materialtransformations at the micrometer scale.

Acknowledgements

The ALS and the Molecular Foundry are supported by theDirector, Office of Science, Office of Basic Energy Sciences, of theU.S. Department of Energy under Contract no. DE-AC02-05CH11231. M.E.G. would like to thank the support of the ALS PostdoctoralFellowship program. M.E.G, Y.P.H, and B.S.M. would like to thankthe support from the Research Fund of HYU (HYU-2010-T).

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