investigation of the gas sensing properties of au/mnox: response to co exposure and comparison to...

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 26, 1035È1049 (1998)

Investigation of the Gas Sensing Properties ofResponse to Exposure andAu/MnO

x: CO

Comparison to Pt/SnO2

Bharat Srinivasan and Steven D. Gardner*Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA

The surface conductivity changes of 2 at.% and 2 at.% have been measured duringAu/MnOx, MnO

xPt/SnO

2cyclical exposures to CO and dry air at 130 ÄC. At each stage of the gas exposures, the corresponding surfacecompositions were determined using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS).The results have been used to gain insight into the surface processes that are responsible for the observed conduc-tivity behavior. Upon CO exposure, the extent of conductivity increase is di†erent for each specimen and a syn-ergistic interaction between Au and is demonstrated. Whereas exposure to dry air (or oxygen) causes theMnO

xconductivity of to decrease, the and specimens continue to exhibit positive changes inPt/SnO2

Au/MnOx

MnOxsurface conductivity. This is believed to be due to sufficiently high partial pressures of oxygen that transform the

materials from n-type semiconductors to p-type. The surface analysis data implicate the importanceMnOx-based

of hydroxyl groups and/or adsorbed water to the CO gas sensing mechanism on all three materials, but there isevidence suggesting that their role is di†erent with respect to and The potential advantages ofAu/MnO

xPt/SnO

2.

combining and into a single gas sensor are also considered. 1998 John Wiley & Sons, Ltd.Au/MnOx

Pt/SnO2

(

KEYWORDS: sensors ; gold ; manganese oxide ; platinum; tin oxide ; XPS; ISS ; x-ray photoelectron spectroscopy

INTRODUCTION

The need to detect trace amounts of potentially hazard-ous gases has become increasingly important due toconcerns over environmental pollution, occupationalsafety and combustion efficiency. In order to addressthese concerns, numerous commercial instruments havebeen made available that detect a variety of gases,including CO, alcohols and hydro-H2 , O2 , CO2 , NO

x,

carbons.1h3 Many of these instruments are based uponsolid-state gas sensors composed of semiconductingmetal oxides. In such cases, the detection schemes relyon surface phenomena that occur upon exposure of thesensor to the targeted gas. For example, the electricalconductivity of a semiconducting metal oxide such as

can be altered by adsorption of gases from theSnO2ambient. It follows that a calibrated response may beestablished between the surface conductivity andSnO2the concentration of the target gas in the ambient. Thenet result is a rather simple and inexpensive gas sensingdevice.

Despite the recent advances in semiconductor gassensing instrumentation, the underlying technologyitself must evolve signiÐcantly in order to addresscertain performance deÐciencies. Perhaps the most criti-cal problem associated with current semiconducting gassensing devices is a lack of selectivity toward a speciÐcgas. In addition, there is often the problem of gradual

* Correspondence to : S. D. Gardner, Department of ChemicalEngineering, Mississippi State University, Mississippi State, MS39762, USA

sensor deactivation. Considerable response times canalso hinder the performance and overall applicability ofgas sensors. In short, there is a critical need to enhancethe selectivity, lifetime stability and response time ofsemiconductor gas sensors while maintaining acceptablesensitivity toward the gas(es) to be monitored.

In previous research, has exhibited unpar-An/MnOxalleled performance as a low-temperature CO oxidation

catalyst.4h6 In particular, has consistentlyAu/MnOxproven to be superior to CO oxidation catalysts based

upon the latter of which is one of the mostSnO2 ,widely studied and successful semiconducting gas sensormaterials for CO detection. Given the excellent catalyticproperties of and the fact that catalyticAu/MnO

xproperties are ultimately related to gas sensing charac-teristics, it is highly probable that this material will alsoÐnd applications in high performance gas sensinginstrumentation designed to detect reducing gases suchas CO.

Much of the current development in gas sensing tech-nology has been the result of empirical testing andevaluation. However, in order to address the per-formance issues cited above, basic research is needed tounderstand better the fundamental surface processesthat are ultimately responsible for the gas sensingmechanism(s) in place. In the present research, experi-ments have been performed to investigate the gassensing properties (based on electrical conductivitymeasurements) of a novel manganese-oxide-supportedgold catalyst and furthermore to explore(Au/MnO

x)

the surface phenomena connected with the conductivitychanges. The surface conductivity of hasAu/MnO

xbeen evaluated during stepwise, sequential exposures toCO and dry air and the resulting surface composition

CCC 0142È2421/98/131035È15 $17.50 Received 11 July 1997( 1998 John Wiley & Sons, Ltd. Accepted 11 August 1998

1036 B. SRINIVASAN AND S. D. GARDNER

has been determined using x-ray photoelectron spec-troscopy (XPS) and ion scattering spectroscopy (ISS).Similar data have also been acquired for andMnO

xand the information is used to gain morePt/SnO2 ,insight into the gas sensing behavior observed.

EXPERIMENTAL

Materials and synthesis

The specimens were prepared via coprecipi-Au/MnOxtation, whereby aqueous solutions of andHAuCl4were added dropwise to stirring atMn(NO3)2 Na2CO3room temperature.7,8 The concentration of the precur-

sor solutions was adjusted to yield specimens whose Aucontent was 2 at.% based on Au and Mn only. Theresulting precipitate was washed twice with hot (80 ¡C)deionized water and then dried in air at 110 ¡C for 24 h.The material was crushed and subsequently calcined at400 ¡C in air for 4 h. Specimens of were preparedMnO

xin an identical manner, except that was notHAuCl4added during the coprecipitation step.Platinized tin oxide was prepared using an impregna-

tion method.9 Tin(IV) oxide powder (Aldrich, 99.9%,325 mesh) was impregnated with an aqueous solution of

followed by drying in air at 100 ¡C and calcin-H2PtCl6 ,ation at 600 ¡C for 5 h. The amount of platinum saltwas adjusted to yield a 2 at.% specimen.Pt/SnO2The powdered materials were prepared for conduc-tivity measurements by pressing them into small alu-minium cups. The compressed powders formed disksthat were 1.2 cm in diameter and D0.07 cm thick.

Surface conductivity measurements

The conductivity of the model sensors was evaluatedusing a standard four-point probe method.10 In orderto allow for in situ XPS and ISS surface analysis(described in the following section), the conductivityexperiments had to be carried out in a vacuum chamberadjacent to the main XPS/ISS analytical chamber. Acustom, four-point probe compatible with ultrahighvacuum (UHV) was constructed for this purpose, basedon designs utilized in similar research.11,12 The conduc-tivity probe contained four spring-loaded, gold-platedpins (IDI, Inc., Model S-I-J-3.8-D) arranged in a squarepattern. Surface conductivity measurements were madeby passing an electric current (Keithley Model 220 pro-grammable current source) through two adjacent pinsand measuring the resulting voltage (Keithley Model2000 multimeter) across the remaining two pins in thesquare array. Computer data acquisition (Keithley Test-Point software) was used to provide accurate records ofconductivity vs. time. The probeÏs performance wasevaluated using an n type silicon wafer (volume resis-tivity of 178 ) É cm) obtained from the National Insti-tute of Standards and Technology (NIST). Using anappropriate correction factor,13,14 the probe measureda volume resistivity within 3% of the NIST standard. Inthe present work, the specimen surface conductivities(1/)) are expressed as the reciprocal of the surface

resistivity, which, in turn, is deÐned as the volume resis-tivity divided by the thickness of the specimen.15

In preparation for the conductivity measurements, anair-exposed specimen was situated on a heater platforminside the vacuum chamber, which rendered the speci-men isolated from ground. The sample was subse-quently heated to 130 ¡C, which is a typical operationtemperature for many gas sensors composed of tinoxide. The four point probe was then lowered onto thespecimenÏs surface. Initially, a baseline conductivity wasrecorded corresponding to the UHV environment. Afterthe baseline conductivity had stabilized, ultrahighpurity (UHP) carbon monoxide or dry air was admittedinto the vacuum chamber through a precision leakvalve. Typical gas pressures were maintained near 0.5Torr during each exposure period, which lasted untilthe conductivity signal did not vary signiÐcantly withtime. The chamber was evacuated between successivegas exposures, enabling the CO and dry air to be cycledand the reversibility of the sensor response to beobserved in each case.

Surface characterization

The model gas sensors were analyzed using a PhysicalElectronics (PHI) Model 1600 surface analysis system(base pressure 10~9 Torr). The instrumentation is con-Ðgured to enable XPS and ISS from the specimenswithout the need for intermittent ambient air exposure.The surface analysis experiments were initially con-ducted on the air-exposed specimens and subsequentlyafter each of the CO and dry air exposures describedabove. In this manner, the surface composition of eachmodel gas sensor was referenced to the surface conduc-tivity measured during the previous gas exposure.

The XPS/ISS instrumentation is based on the Physi-cal Electronics Model 10-360 spherical capacitor energyanalyzer (SCA) equipped with an Omni Focus III small-area lens (800 lm diameter analysis area) and multi-channel detection (MCD) technology. Both the XPSand ISS data were acquired with the SCA operating inthe Ðxed analyzer transmission (FAT) mode. The XPSspectra were acquired using an achromatic Al Ka(1486.6 eV) x-ray source operated at 300 W. Surveyspectra were collected over the range 0È1100 eV with apass energy of 46.95 eV, and high-resolution XPSspectra were acquired from signiÐcant peaks in thesurvey spectra using a pass energy equal to 23.50 eV. Inall cases, the photoelectron take-o† angle was 30¡.Depending upon the specimen being analyzed, the XPSbinding energies were referenced to the Mn 3d states inthe valence band (D4 eV) or the Sn peak posi-3d5@2tion corresponding to (486.4 eV). The Mn 3dSnO2peak position serves as a good reference because it hasbeen shown to be essentially stationary between thecompositions MnO and Whereas the loca-Mn2O3 .16tion of the Sn peak may vary up to 0.5 eV3d5@2between the SnO and chemical states,17,18 theSnO2assumed location of 486.4 eV is most consistent with thecorresponding XPS quantitative results from thepresent study. The composition of each specimen wasestimated from the relative XPS peak areas after Shirleybackground subtraction (PHI Matlab software, Version4.0) and application of atomic sensitivity factors appro-

Surf. Interface Anal. 26, 1035È1049 (1998) ( 1998 John Wiley & Sons, Ltd.

GAS SENSING PROPERTIES OF Au/MnOx

1037

priate for the transmission characteristics of the Physi-cal Electronics SCA.19 The compositions are based onthe assumption of a homogenous surface and thereforethey should be regarded as approximations.

The ISS spectra were acquired using 3He` ions and ascattering angle of 135¡. The ion beam current densitywas D250 nA cm~2 with a beam diameter of D5 mm.The SCA pass energy used for ISS was 187.85 eV.Typical data acquisition times were 1È2 min corre-sponding to a scattered ion energy ratio ranging(E/E0)from 0.3 to 1.0. During the ISS experiments, the systempressure was maintained near 10~7 Torr.

RESULTS AND DISCUSSION

Surface conductivity measurements from Au/MnOx

The surface conductivity of 2 at.% appears inAu/MnOxFig. 1 as a function of two complete cyclical exposures

to CO and dry air. Initially, the surface conductivity isD0.55] 10~3 )~1, which corresponds to the vacuumenvironment with the sample maintained at 130 ¡C.Upon exposure to D0.78 Torr of CO, D15 s elapsesbefore the conductivity increases sharply, ultimatelyreaching a value near 1.8] 10~3 )~1. This equates tomore than a threefold increase in the surface conductivi-ty, most of which occurs during a period spanning only40 s. The data have not been smoothed and it isbelieved that the irregularities in the peak shapes aredue to pressure Ñuctuations during gas admission andevacuation, both of which were performed manually.Once the signal began to stabilize in the CO environ-ment, the chamber was evacuated, after which thesurface conductivity decreased rapidly to a point nearthe initial baseline value. This reversible conductivitychange is consistent with CO oxidation experiments,where it has been observed that outgassing An/MnO

x

during reaction rate measurements (accomplished bytemporarily replacing the CO and reaction gas withO2He) has a relatively minor inÑuence on the ensuingactivity when the reaction gas is diverted back to thereactor.5

The change (increase) in surface conductivity ofupon exposure to CO (Fig. 1) is consistentAu/MnO

xwith the behavior of an n-type semiconducting metaloxide such as In such cases it is believed that theSnO2 .increase in surface conductivity may be attributed to :electron donation to the substrate that occurs upon COchemisorption ; reaction between CO and chemisorbedoxygen species, and/or partial reduction of the substratevia reaction between CO and lattice oxygen.20h25 Con-versely, when an n-type semiconducting metal oxide isexposed to oxygen, there is a corresponding decrease inthe surface conductivity because oxygen behaves as anelectron acceptor. Note in Fig. 1, however, that whenthe specimen is exposed to dry air (D0.70Au/MnO

xTorr) there is an increase in surface conductivity of theorder of that observed for CO exposure. When the dryair is evacuated from the chamber, the surface conduc-tivity once again returns to the baseline value, suggest-ing that the surface interaction is essentially reversible.A possible explanation for this behavior is discussed inthe following sections.

Further inspection of Fig. 1 reveals that the overalltrends in the surface conductivity continueAu/MnO

xfor the second cycle of gas exposures. Relative to theÐrst CO exposure, the CO pressure was stabilized morequickly during the second exposure, and hence the peakin narrower, and the increase in surface conductivity isslightly less. In fact, if the specimen isAu/MnO

xexposed to four cyclical CO exposures (no dry airinvolved), the change in surface conductivity (notshown) is progressively diminished by D50%. Thiswould be consistent with gradual consumption of activesurface oxygen species as CO oxidation proceeded inthe absence of gas-phase oxygen. The dry air exposureonce again increases the surface conductivity, but to a

Figure 1. The surface conductivity of 2 at.% vs. time as a function of sequential exposures to CO and dry air at 130 ¡C. TheAu/MnOx

arrows indicate the points at which the indicated changes were made to the gas-phase composition.

( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 1035È1049 (1998)


lesser extent compared to the Ðrst exposure. It is pos-sible that the decreased response is related to surfacereactions that attempt to replenish the active oxygensites that are consumed by interaction with CO. The

conductivity proÐles resulting from fourAu/MnOxcyclical dry air exposures (not shown) remain essentially

identical. Therefore, the surface is altered byAu/MnOxprior CO exposure, which, in turn, alters the extent of

interaction with the dry air.

Surface conductivity measurements from MnOx

Figure 2 illustrates the surface conductivity proÐle ofthe specimen as a function of cyclical exposuresMnO

xto CO and dry air at 130 ¡C. Note that although themeasured surface conductivity of remains aboutMnO

xone order of magnitude below that measured from(Fig. 1), the changes in the surface conduc-Au/MnO

xtivity upon gas exposure are still signiÐcant(approximately twofold) for both CO and dry air.Nevertheless, the conductivity changes are far belowthose observed for It is apparent, therefore,Au/MnO

x.

that the addition of Au to inÑuences the surfaceMnOxconductivity responses to CO and dry air. This is some-

what expected because it has been shown that 2%is D10 times more active toward low-Au/MnO

xtemperature (D50 ¡C) CO oxidation than MnOxalone.26

The overall characteristics of the conductivityMnOxproÐle are similar to those of the specimen.Au/MnO

xFor example, the baseline conductivity is largelyregained upon exposing to the vacuum environ-MnO

xment after each gas is cycled through. The responsetimes are also small, of the order of 10È20 s. Perhapsthe most intriguing similarity is the increase in surfaceconductivity that results upon exposure to dry air. Asmentioned previously, that is not the usual responseobserved for an n-type semiconducting metal oxide.

Previous investigations of surface conductivityMnOxare not extensive enough to corroborate this enhanced

conductivity response to air or oxygen.27h29 In fact, ithas been observed previously that does notMnO

xexhibit measurable conductivity changes upon exposureto air or oxygen.27,28 Where details of the oxygen expo-sure were given,27 the partial pressure was 0.065 Torr,presumably at room temperature. In agreement withdata acquired in the present investigation, these condi-tions are too mild to initiate a signiÐcant change in thesurface conductivity of MnO

x.

Surface conductivity measurements from Pt/SnO2

The surface conductivity proÐle acquired from the 2at.% specimen is shown in Fig. 3. These dataPt/SnO2may be compared to the surface conductivities mea-sured from (Fig. 1) and (Fig. 2) underAu/MnO

xMnO

xsimilar conditions. All three materials respond rapidlyto changes in the gas-phase composition. Upon COexposure, the surface conductivity increasesPt/SnO2from 0.8 ] 10~4 )~1 to D3.8] 10~4 )~1, which isgreater than the net change in conductivity exhibited by

in Fig. 1. Beyond the initial CO gas expo-Au/MnOxsure, however, the behavior observed from isPt/SnO2quite di†erent from both samples based on MnO

x.

After chamber evacuation the surface conduc-Pt/SnO2tivity does not return to the initial level prior to the COdosing. Instead, the conductivity slowly drifts down-ward to D3.5] 10~4 )~1. Subsequent exposure todry air quickly decreases the conductivity, but not tothe original baseline value. Upon oxygen evacuation,there is a brief increase in the surface conductivity fol-lowed by a decay that returns the conductivity to apoint near the starting value. During the second gascycle, the behavior noted above is essentially the same.Noteworthy exceptions are the increased response tothe second CO exposure, and the increased reversibility

Figure 2. The surface conductivity of vs . time as a function of sequential exposures to CO and dry air at 130 ¡C. The arrows indicateMnOx

the points at which the indicated changes were made to the gas-phase composition.



1039

Figure 3. The surface conductivity of 2 at.% vs. time as a function of sequential exposures to CO and dry air at 130 ¡C. The arrowsPt/SnO2

indicate the points at which the indicated changes were made to the gas-phase composition.

of the conductivity response upon CO evacuation. Con-ductivity data were also acquired from the as-received

support. Its conductivity proÐle is very similar toSnO2that described above for but the overall con-Pt/SnO2 ,ductivities are one order of magnitude less and theincrease in conductivity upon CO exposure is aboutthreefold.

The behavior above for (and may bePt/SnO2 SnO2)compared to results from similar investigations. Theconductivity response of diode-type sensorsPt/SnO2has been investigated using 1000 ppm of CO in air at90 ¡C.30 The times required to reach 50% and 90% ofthe total conductivity change were 1 min and 20 min,respectively. Unlike the situation in Fig. 3, however,after removal of CO from the air stream the conductivi-ty returned to its original value (in other words, theresponse was essentially reversible). The conductivity ofporous pellets has been evaluated at 300 ¡C inSnO2response to pulses of various di†erent gases in air.2Upon exposure to 1 vol.% of CO in air, the con-SnO2ductivity increased approximately twofold. When pureair Ñow was restored, the conductivity was nearlyreturned to the original baseline value.

Previous investigations have also been performedthat are more similar in nature to the present study. Theconductivity of thin Ðlms has been evaluatedPd/SnO2in a vacuum chamber in which the sample temperaturewas maintained at 130 ¡C.31 The specimen was exposedto cyclical doses of and (10~3 Pa) with inter-H2 O2mittent vacuum. Hydrogen, like CO, is a reducing gasthat caused the conductivity to increase. AsPd/SnO2observed in the case of Fig. 3, subsequent vacuum expo-sure did not return the conductivity to itsPd/SnO2original value. In fact, the ensuing oxygen exposuredecreased the surface conductivity below the baselinevalue. When the gas cycles were repeated, the responseto was enhanced just as observed in Fig. 3 for CO. ItH2was concluded from those experiments that the oxygenreacted with residual hydrogen that remained adsorbedafter vacuum exposure, and the further decrease in con-

ductivity was caused by additional oxygen chemisorp-tion.

Surface characterization

The increased conductivity of upon COAu/MnOxexposure might have resulted from chemisorption of

CO with subsequent electron transfer to the bulk solid.Given the CO oxidation activity of this material,however, it is more reasonable to speculate that themajority of the CO reacted with the surface to form

This reaction is known to occur on at tem-CO2 . MnO2peratures as low as [15 ¡C and in the absence of gas-phase oxygen.29 It has been proposed that the activesurface oxygen is present as and/or O~.29 Reac-O2~tion between CO and these surface oxygen species, withsubsequent desorption of neutral would thereforeCO2 ,increase the surface conductivity.

The situation appears to be more complex when thespecimen is exposed to dry air. As depictedAu/MnO

xin Fig. 1, the surface conductivity increases whenis dosed with dry air. Measurement of theAu/MnO

xSeebeck voltage on both the and theAu/MnOx

MnOxspecimens (in ambient air) indicates that these materials

are n-type semiconductors. Therefore, it is expected thatair exposure would cause the surface conductivity todecrease in the manner observed for other n-type semi-conductors such as Whereas air (or oxygen)SnO2 .exposure usually decreases the availability of conduct-ing electrons on n-type semiconductors, in the presentcase there is a mechanism in place that increases thecharge carrier concentration on the surface of

(and during air exposure at theseAu/MnOx

MnOx)

conditions. The same behavior depicted in Figs 1 and 2is observed when the dry air is replaced with oxygen.

In order to help understand the behavior observed inFigs 1È3, the and specimensAu/MnO

x, MnO

xPt/SnO2have been characterized using XPS and ISS. These

surface analysis experiments were performed at each



stage of evacuation, as illustrated in Figs 1È3. In addi-tion, the air-exposed specimens were analyzed beforeand after heating to 130 ¡C in a vacuum, which corre-sponds to the initial state prior to the conductivity mea-surements. For the latter two cases, two consecutive setsof XPS/ISS spectra were acquired to assess the extent ofsurface degradation resulting from the analysis pro-cedures themselves. In this manner, a complete historyof each specimen has been recorded in terms of thesurface composition.

The XPS/ISS results for Au/MnOx

The XPS survey spectra acquired from the Au/MnOxspecimen (not shown) reveal that, in addition to man-

ganese, oxygen and gold, peaks corresponding tocarbon and sodium are present as well. The latter twoelements most likely originate from the sodium carbon-ate utilized in the synthesis procedure. Only traceamounts of chlorine are detected by XPS, probably dueto the gold precursor High-resolution XPS(HAuCl4).spectra were recorded for the elements above and thedata have been used to estimate the surface composi-tions appearing in Table 1. The time required to recordeach XPS spectrum was approximately 3 h. The data inTable 1 correspond to two spectra obtained consecu-tively from the air-exposed specimen at room tem-perature (a and b), two spectra each obtained directlyafter heating the air-exposed specimen for 1 h at 130 ¡Cin a vacuum (c and d), and spectra (eÈh) acquired aftereach of the four evacuation stages depicted in Fig. 1.Inspection of Table 1 reveals that the stoichiom-MnO

xetry is approximately on the air-exposed sur-MnO1.4faces (a and b), which would be consistent with anaverage composition between and if allMn3O4 Mn2O3of the oxygen is assumed to be associated with the man-ganese. It is believed that this is a reasonable Ðrstapproximation for both and evenAu/MnO

xMnO

xthough a small amount of oxidized carbon is detected inthe corresponding C 1s spectra (not shown) and a frac-tion of the oxygen present as water or hydroxyls maynot be associated with the manganese. There is agradual increase in the Mn/O atomic ratio upon com-pletion of the second heating at 130 ¡C (d). Therefore,heating in a vacuum appears to slightly reduce the

oxygen content of the support. During the gasMnOxexposure sequence (eÈh), XPS indicates that the Mn/O

atomic ratio is D0.9 and does not vary appreciably.Note also that the carbon content on the air-exposedspecimen is signiÐcantly reduced at the onset of the gasexposures.

Inspection of the high resolution XPS spectra pro-vides additional information about the Au/MnO

xsurface. Figure 4 illustrates the Mn 2p XPS spectraacquired from the specimen as a function ofAu/MnO

xeach treatment stage identiÐed in Table 1 above. TheMn peak position remains nearly stationary [email protected] eV. As discussed later, slight deviations in thepeak positions are believed to be due to di†erentialsurface charging. This binding energy assignment corre-sponds closely to those tabulated for Mn3O4 , Mn2O3and MnOOH, but it is slightly below that of

Figure 4. The high-resolution Mn 2p and O 1s XPS spectraacquired from 2 at.% as a function of the surface treat-Au/MnO

xments listed in Table 1. The arrows mark the location of satellitefeatures that aid in chemical state identification. The vertical linesserve as an arbitrary reference to illustrate the peak shifts.

Table 1. Relative XPS atomic concentrations for Au/MnOx

Atomic concentrations (%)

Sample treatment Mn O C Au Na Mn/c Mn/O

(a) Air-exposed (first spectrum) 34.1 48.5 15.7 1.0 0.7 2.2 0.7

(b) Air-exposed (second spectrum) 35.0 47.3 16.1 1.1 0.6 2.2 0.7

(c) Heated to 130 ¡C in vacuum 34.7 45.5 18.1 1.0 0.7 1.9 0.8

for 1 h (first spectrum)

(d) Heated to 130 ¡C in vacuum 38.0 46.7 13.3 1.0 1.0 2.9 0.8

for 1 h (second spectrum)

(e) Exposure to Á0.5 Torr of CO 41.5 48.7 8.2 1.2 0.4 5.1 0.9

at 130 ¡C(f) Exposure to Á0.5 Torr of dry air 41.9 48.9 7.2 1.2 0.9 5.8 0.9

at 130 ¡C(g) Exposure to Á0.5 Torr of CO 42.3 49.4 6.0 1.1 1.1 7.1 0.9

at 130 ¡C(h) Exposure to Á0.5 Torr of dry air 44.2 50.1 3.4 1.1 1.2 13.0 0.9

at 130 ¡C



1041

The discrepancies among these tabu-MnO2 .18,19,32,33lated data and the narrow range over which they occure†ectively prevent an accurate determination of themanganese chemical oxidation state from the Mn 2p3@2binding energy alone. More information is availablefrom the Mn 2p satellite structure. It is known thatmanganese exhibits satellite features that are shifted tohigher binding energy from the main 2p peaks by D5eV for MnO and D10È11 eV for andMn3O4 , Mn2O3Metallic manganese does not exhibit aMnO2 .16,32,342p satellite structure. Satellite features are indeedpresent in Fig. 4, as indicated by the two arrows. Theshoulder on the Mn peaks near 647 eV is consis-2p3@2tent with the presence of MnO, whereas the feature near663 eV would be consistent with the presence of

and/or Upon progressingMn3O4 , Mn2O3 MnO2 .from spectrum (a) to (d), digital superposition of thespectra reveals that the size of the shoulder near 647 eVslightly increases, indicating an enhanced presence ofMnO. Therefore, some reduction of the supportMnO

xprobably occurs during the vacuum heating as dis-cussed above. Otherwise, the relative size and shape ofthese satellite features do not change signiÐcantly withrespect to the surface treatments. The data in Fig. 4suggest that the surface composition of the Au/MnO

xspecimen is quite complex, with several stoichio-MnOxmetries potentially present. A similar conclusion has

been reached in a previous XPS investigation ofThe extent of Mn 3s multiplet splittingAu/MnO

x.3

might ordinarily be used to help identify the chemicalstates of manganese in the specimen, butAu/MnO

xunfortunately the Au 4f peaks overlap the Mn 3s fea-tures.

Additional information about the surfaceAu/MnOxis available from the O 1s XPS spectra, which also

appear in Fig. 4. Again, the data are arranged accordingto the notation in Table 1. It is evident that the main O1s peak exhibits an itinerant nature as a function of thesurface treatments ranging from D529.8 to 530.4 eV.Some of this may be due to di†erential surface charging,but when the peaks are superimposed their shapesdi†er, which would suggest that some of the peak shiftis due to a changing chemical environment. The bindingenergies of the main O 1s peak (D530 eV) correspondto those typical of metal oxides.19,33 Previous investiga-tions of manganese oxides have recorded O 1s bindingenergies for several stoichiometries at room tem-MnO

xperature, including MnO (529.6 eV32 and 530.8 eV)34,(529.6 eV)32, (530.1 eV)32 andMn3O4 Mn2O4 MnO2(529.7 eV).32 Perhaps the most interesting features in

Fig. 4 are the shoulders that appear on the high-binding-energy side of the main O 1s peaks. These fea-tures represent hydroxyl groups (D532 eV) andadsorbed water (D533 eV). Considering the C 1sspectra (not shown), a portion of the O 1s shoulder mayalso be due to oxygen bound in carbonate and/or bicar-bonate groups (D531.5 eV). The surface concentrationof carbonate/bicarbonate species progressivelydecreases during the series of surface treatments, sug-gesting that they probably originate from the sodiumcarbonate precursor. It is well known that sur-MnO

xfaces contain an abundance of adsorbed water andhydroxyl groups.35h38 Temperature-programmeddesorption (TPD) studies of have indicated thatMnO2the majority of surface water and hydroxyls do not

desorb at temperatures below D200 ¡C.35,36 In thepresent case, the specimen was heated toAu/MnO

xonly 130 ¡C and this further supports the chemicalassignments for the O 1s peak shoulders in Fig. 4. TheO 1s shoulder in the specimen acquired from the air-exposed surface [Fig. 4(a)] is quite broad,Au/MnO

xand after a short time in the vacuum system XPS [Fig.4(b)] detects an increase in the size of the shoulder. Thismight be due to hydrogen or water di†usion from thebulk. If the main O 1s peaks are aligned and superim-posed, the relative size of the peak shoulder steadilydecreases upon progressing through the remainingsurface treatments [Fig. 4(b)È(h)]. Nevertheless, a sig-niÐcant amount of water, hydroxyl groups and perhapscarbonate/bicarbonate species remain at the end of thegas exposure cycle. It has been proposed that surfacehydroxyls and bicarbonate intermediates play animportant role in low-temperature CO oxidation overboth and The detection ofAu/MnO

xPt/SnO2 .8,39h42

these species on the present specimen mayAu/MnOxalso indicate their importance in the corresponding CO

gas sensing mechanism.The addition of gold to manganese oxide has already

been shown to alter the CO oxidation activity of MnOxand Figs 1 and 2 reveal that there is a change in the gas

sensing properties as well. In order to understand howthe gold alters the gas sensing characteristics of MnO

x,

it is necessary to determine the chemical state of the Auon the surface. The Au 4f XPS spectra acquired fromthe specimen appear in Fig. 5 for each of theAu/MnO

xsurface treatments shown in Table 1. The signal inten-sity is rather low due to the small amount of gold onthese surfaces and some surface charging also appearsto be evident. The average Au peak position is4f7@2near 84.2 eV, which is slightly greater than the binding

Figure 5. The high-resolution Au 4f XPS spectra acquired from 2at.% as a function of the surface treatments listed inAu/MnO

xTable 1. The arrows mark the approximate location of the under-lying Mn 3s peaks. The vertical lines denote binding energyassignments with respect to the peak.4f

7@2( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 1035È1049 (1998)


energy reference for bulk metallic gold (84.0 eV) andconsiderably below that of (D86 eV). TheAu2O3binding energy of small gold particles (less than D30 Óin diameter) supported on and has beenAl2O3 SiO2shown to be greater than that of metallic gold by asmuch as 1 eV.43,44 When the gold particle size was pro-gressively increased, the binding energy shifted towardthat of bulk metallic gold. This behavior might be dueto real di†erences in the electronic structure of smallparticles.

Haruta and co-workers have investigated several dif-ferent catalysts consisting of 5 at.% and theyAu/Fe2O3measured Au binding energies ranging from 83.94f7@2eV45 to near 84.5 eV.46 In the latter case, transmissionelectron microscopy (TEM) detected hemispherical goldparticles D30È40 in diameter in addition to smallÓraft-like gold clusters comprised of about three Auatoms each. The relatively high dispersion of small goldparticles (40%) and their tight contact with the ironoxide phase may have promoted enhanced chemicalinteraction with the support. This, in turn, might alterthe electronic properties of the gold particles relative tothose of bulk metallic gold. This scenario would bequalitatively consistent with XPS/ISS investigations of

and The goldAu/Fe2O3 ,47 Au/Co3O4 47 Au/MnOx.8

particle size distribution on the present cata-Au/MnOxlyst has not been determined and the existence of a

chemical interaction between the Au and MnOxsupport can only be speculated given the current data.

However, small, highly dispersed gold particles appearto be a prerequisite for the high CO oxidation activityof supported gold catalysts.46 Note also the shouldersthat appear on the Au 4f peaks in Fig. 5 (indicated byarrows). These features are due to the Mn 3s peak (D83eV) and the accompanying multiplet splitting(D89 eV). If the Mn 3s features were not present, theapparent binding energy of the Au peaks would be4f7@2slightly increased, providing additional support for thearguments above.

Thus far, the XPS data have provided an indicationof the composition and those data pertain toAu/MnO

xa region encompassing the outer 50È100 of theÓsurface. Further information is available by supplement-ing XPS with ISS. The advantage of ISS is its extremesurface sensitivity (outermost atomic layer only). Figure6 contains the ISS spectra acquired from Au/MnO

xwhere the data have been conÐgured like those in theprevious XPS spectra. The spectra have all been scaledto a common (and arbitrary) Mn peak intensity tofacilitate comparisons. The surface of the air-exposed

specimen [Fig. 6(a)] is oxygen rich and theAu/MnOxgold peak is barely discernible. Carbon is also present,

as well as sodium and chlorine. Upon reaching the Ðnalheating cycle (130 ¡C for 1 h in a vacuum), which corre-sponds to the surface condition prior to the initial COgas exposure in Fig. 1, Fig. 6(d) indicates that the rela-tive oxygen presence is reduced whereas the gold andchlorine signals are more prominent. When the

is Ðrst exposed to CO [Fig. 6(e)], the relativeAu/MnOxconcentrations of carbon, sodium, oxygen and chlorine

are all diminished. After subsequent exposure to dry air,Fig. 6(f ) reveals an increase in the oxygen signal, whichwould be consistent with replenishment of oxygen lostduring prior CO reduction of the surface. The oxygenconcentration follows a similar trend during the second

Figure 6. The ISS spectra acquired from 2 at.% as aAu/MnOx

function of the surface treatments listed in Table 1. The elementssuperimposed at the top of the figure indicate their expected peakpositions based on the binary elastic scattering equation thatgoverns ISS.

CO/dry air cycle [Figs 6(g) and 6(h)], although to alesser extent. Note that there is practically no carbondetected by ISS during the gas exposure sequence[excepting perhaps Fig. 6(e)]. Several factors may beinvolved. Carbon has a high neutralization coefficientfor 3He` and the ISS sensitivity decreases with decreas-ing atomic number of the target atom. It is possible thatCO adsorbs on the surface such that theAu/MnO

xcarbon atom is physically shielded from the probe ionsby oxygen or hydrogen. Shielding of the carbon atomwould also be likely if a unidentate or bidentate carbon-ate species is present. However, considering the conduc-tivity data in Fig. 1, the ISS spectra are probablyindicative of CO desorption from the surfaceAu/MnO

xas a result of vacuum exposure, i.e. the reversibleadsorption of CO would be consistent with the overallreversibility of the conductivity upon COAu/MnO

xexposure and subsequent evacuation (Fig. 1). The lackof strongly bound CO is also consistent with the XPS C1s spectra (not shown) and the progressive reduction inthe concentration of surface carbon. Note also that thesurface concentration of gold appears to oscillate duringthe gas exposure sequence, being greatest after the dryair exposures [Figs 6(f ) and 6(h)].

The fact that sodium and chlorine are detected byXPS and ISS raises interesting questions about theirpotential inÑuence on the surface conductivity. It is dif-Ðcult to prepare specimens that are chlorine-Au/MnO

xfree using the preparation method previously described.However, washing the precipitates thoroughly with hotwater signiÐcantly reduces the presence of surface chlo-rine. Chlorine is known to be detrimental to the per-formance of many CO oxidation catalysts and this maybe true with respect to CO gas sensing as well. TheinÑuence of sodium on surface conductivity could be



1043

examined by using a di†erent precipitating agent otherthan such as or ThisNa2CO3 , K2CO3 Li2CO3 .approach has been utilized previously to investigatehow the presence of Na, K and Li alters the low-temperature CO oxidation activity of Au/MnO

x.5

The XPS/ISS results for MnOx

A set of XPS and ISS spectra was also acquired fromthe specimen in order to determine how the pres-MnO

xence of gold inÑuences the surface conductivity. Table 2contains the relative atomic concentrations as deter-mined from the high resolution XPS spectra. TheMnO

xletter designations for the surface treatments are identi-cal to those presented above in Table 1. When the datain Table 2 are compared to those in Table 1(MnO

x)

it is apparent that the Mn/O atomic ratio(Au/MnOx),

is slightly lower overall on the surface. As dis-MnOxcussed above, if all of the oxygen is assumed to be

associated with the manganese, Table 2 indicates thatthe stoichiometry on the air-exposed specimenMnO

x(a) is (compared to forDMnO1.7 MnO1.4 Au/MnOx).

Upon progressing through the series of surface treat-ments, the Mn/O atomic ratio on ultimatelyMnO

xincreases to 0.9, which is the value determined forfor the same stage of surface treatment. TheAu/MnO

xdata in Tables 1 and 2 also indicate that the sodiumconcentrations are similar for both andMnO

xand the relative concentration of carbon onAu/MnOxboth specimens decreases sharply after the initial CO

exposure.The di†erences in the Mn/O atomic ratios may be an

indication that the average Mn oxidation states di†erbetween and The Mn 2p XPSMnO

xAu/MnO

x.

spectra appear in Fig. 7 for and they may beMnOxcompared directly to Fig. 4 Surface charg-(Au/MnO

x).

ing is slightly more extensive on which may beMnOx,

due to the lack of gold. If the Mn 2p spectra in Fig. 7are digitally superimposed, it is seen that the satellitefeature near 647 eV increases from spectrum (a) to (c)and remains nearly stationary thereafter. This trend isindicative of increased MnO formation, which wouldalso explain changes in the shape of the main 2p3@2peaks. This is also consistent with the assertion thatsome of the is reduced during the surface treat-MnO

x

Figure 7. The high-resolution Mn 2p and O 1s XPS spectraacquired from as a function of the surface treatments listedMnO

xin Table 2. The arrows mark the location of satellite features thataid in chemical state identification. The vertical lines serve as anarbitrary reference to illustrate the peak shifts.

ments. Nevertheless, these Mn 2p spectra appear similarto those in Fig. 4 corresponding to WhenAu/MnO

x.

the Mn 2p spectra in Figs 4 and 7 are scaled and super-imposed, the satellite feature near 647 eV is slightlygreater for from spectrum (a) to (d). This isAu/MnO

xlargely consistent with the Mn/O stoichiometry shownin Tables 1 and 2. Beyond spectrum (d) there are nosigniÐcant variations in the Mn 2p line shapes thatwould otherwise indicate a measurable di†erence in thedistribution of manganese oxidation states.

As mentioned previously, the extent of Mn 3s multi-plet splitting may be used to help determine the chemi-cal state(s) of manganese and, unlike the case for

the Mn 3s features on are notAu/MnOx, MnO

xmasked by the Au 4f peaks. The Mn 3s XPS spectra for

Table 2. Relative XPS atomic concentrations for MnOx

Atomic concentrations (%)

Sample treatment Mn O C Na Mn/C Mn/O

(a) Air-exposed (first spectrum) 27.1 48.4 23.7 0.8 1.1 0.6

(b) Air-exposed (second spectrum) 29.5 49.0 21.3 0.3 1.4 0.6

(c) Heated to 130 ¡C in vacuum 32.9 48.6 17.4 1.0 1.9 0.7

for 1 h (first spectrum)

(d) Heated to 130 ¡C in vacuum 39.3 48.4 10.9 1.4 3.6 0.8

for 1 h (second spectrum)

(e) Exposure to Á0.5 Torr of CO 40.8 51.8 6.3 1.2 6.5 0.8

at 130 ¡C(f) Exposure to Á0.5 Torr of dry air 41.0 52.5 5.0 1.5 8.2 0.8

at 130 ¡C(g) Exposure to Á0.5 Torr of CO 42.3 51.3 4.4 2.0 9.6 0.8

at 130 ¡C(h) Exposure to Á0.5 Torr of dry air 44.7 52.0 2.4 0.8 18.6 0.9

at 130 ¡C



Figure 8. The high-resolution Mn 3s XPS spectra acquired fromas a function of the surface treatments listed in Table 2. TheMnO

xdistance between the vertical lines approximates the extent ofmultiplet splitting.

are illustrated in Fig. 8 and they correspond toMnOxeach of the surface treatments listed in Table 2. The two

vertical lines drawn on the Ðgure indicate that thedegree of multiplet splitting is essentially constant atD5.4 eV, which is consistent with the presence of

and Similar results have beenMn2O3 Mn3O4 .16,32observed in a previous investigation of TheAu/MnO

x.8

predominance of and is largely consis-Mn2O3 Mn3O4tent with the Mn 2p XPS spectra in Figs 4 and 7 andthe Mn/O atomic ratios appearing in Tables 1 and 2.

Thus far the Mn 2p and Mn 3s spectra are unable toidentify signiÐcant di†erences between andMnO

xwith respect to the manganese chemicalAu/MnOxenvironment. However, information contained in the

XPS O 1s spectra provide a more useful comparisonbetween the two specimens. The XPS O 1s spectraacquired from appear in Fig. 7. Similar to theMnO

xcase for (Fig. 4), these O 1s spectra exhibitAu/MnOxsigns of di†erential surface charging, but useful informa-

tion is still available. In particular, the relative size ofthe high-binding-energy shoulder corresponding tohydroxyl groups and adsorbed water varies consider-ably when compared to the corresponding spectra inFig. 4 for Although the overall trend is stillAu/MnO

x.

a decrease in the presence of water/hydroxyl groups, thespecimen maintains a higher relative concen-Au/MnO

xtration of these groups for all but the initial O 1s spec-trum [Figs 4(a) and 7(a)] representing the air-exposedsurface. These data, therefore, further support the con-tention that hydroxyl groups and/or adsorbed waterplay an important role in the low-temperature CO oxi-dation activity of It follows that these dataAu/MnO

x.

also provide evidence that surface hydroxyls andadsorbed water are important to the CO gas sensingmechanism as well, the superior performance of

having been established in Figs 1 and 2.Au/MnOx

As a Ðnal means of comparison between andMnOxISS spectra are presented in Fig. 9 corre-Au/MnO

x,

sponding to The main features in these spectraMnOx.

are due to oxygen, sodium and manganese, but peaksdue to carbon and chlorine are also evident in somecases. As observed in the ISS spectra acquired from

(Fig. 6), the carbon that is detected by ISS onAu/MnOxessentially disappears at the point where theMnO

xinitial CO exposure takes place [Fig. 9(e)]. This may berelated to the fact that the surface conductivity,MnO

xlike that of is reversible with respect to COAu/MnOx,

exposure. It is interesting that ISS also detects Cl on thespecimen. In this case, none of the precursorMnO

xchemicals contain Cl explicitly, and separate glasswarewas used during the synthesis of andMnO

xAu/MnO

xto avoid cross-contamination from residual HAuCl4 .Therefore, other than the new glassware itself orperhaps the drying oven environment, the source of theCl on is unknown.MnO

xExamining the major features in Fig. 9, the relativeintensity of the Na peak gradually increases upon pro-gressing through the series of surface treatments, whichagain raises questions about the potential role of Na indetermining the surface conductivity on as wellMnO

xas Ion scattering spectroscopy indicates thatAu/MnOx.

the O/Mn atomic ratio on is greatest after theMnOxinitial spectrum is acquired from the air-exposed speci-

men [Fig. 9(a)]. Continued vacuum exposure andheating [Fig. 9(b)È(d)] decreases the relative oxygencontent to a level that remains essentially constantthroughout the gas exposure sequence [Fig. 9(e)È(h)].This is unlike the case for (Fig. 6), where ISSAu/MnO

xindicates that the oxygen content decreases upon COexposure and increases upon exposure to dry air.

Figure 9. The ISS spectra acquired from as a function ofMnOx

the surface treatments listed in Table 2. The elements superim-posed at the top of the figure indicate their expected peak posi-tions based on the binary elastic scattering equation that governsISS.



1045

The XPS/ISS results for Pt/SnO2

Materials based on particularly andSnO2 , Pt/SnO2comprise a large number of gas sensorsPd/SnO2 ,designed to detect reducing gases such as CO and H2 .Given their success, these materials provide a usefulbenchmark to which and may beAu/MnO

xMnO

xcompared. In the present case, a model gas sensor con-sisting of 2 at.% was synthesized. The XPSPt/SnO2qualitative results (not shown) indicate that the Sn/Ostoichiometry is approximately that of and theSnO2actual Pt content is close to 3 at.% based on Pt, Sn andO. The conductivity response of this specimenPt/SnO2has already been presented in Fig. 3, where signiÐcantdi†erences have been noted when compared to the con-ductivity curves of (Fig. 1) and (Fig. 2)Au/MnO

xMnO

xfor the same conditions. It is helpful therefore to investi-gate the surface composition of to determinePt/SnO2the factors that might be responsible and which mayprovide additional insight into the behavior observedfor andAu/MnO

xMnO

x.

The XPS Sn 3d spectra acquired from arePt/SnO2shown in Fig. 10 and they are conÐgured according tosurface treatment exactly as the spectra presented abovefor and The main Sn peak atAu/MnO

xMnO

x. [email protected] eV is typical of but some SnO (D486 eV)SnO2may also be present.17,18,48 Formation of SnO might

also be promoted by exposures to a vacuum and CO,both of which are reducing atmospheres.49 When the

specimen is heated to 130 ¡C in a vacuum for 1Pt/SnO2h [Fig. 10(c)], a shoulder near 484.6 eV appears on theSn peak. This is consistent with the formation of3d5@2metallic Sn, which indicates that a portion of the SnO2support has been reduced. The bare support wasSnO2also subjected to the same surface treatments, and XPS

Figure 10. The high-resolution Sn and O 1s XPS spectra3d5@2

acquired from 2 at.% The spectra are arranged accordingPt/SnO2.

to surface treatment exactly as presented for andAu/MnOx

MnOx

(see Tables 1 and 2).

did not detect any metallic Sn. It has been proposedthat the presence of Pt catalyzes the reduction of Sn andthat portions of the metallic Sn may become alloyedwith Pt.39,50 The data in Fig. 10 are qualitatively con-sistent with this argument. The presence of metallic Snreaches a maximum after the second vacuum heat treat-ment at 130 ¡C [Fig. 10(d)] and its relative surface con-centration continues to diminish progressing throughthe gas exposure sequence [Fig. 10(e)È(h)]. The spec-trum in Fig. 10(h) acquired after the second (and Ðnal)dry air exposure still exhibits broadening to lowerbinding energy, indicating that some of the reduced tinremains on the surface. It is interesting to recall fromFig. 3 that the conductivity response to CO isPt/SnO2enhanced during the second cycle of dosing. Thegradual disappearance of metallic Sn as the gas expo-sure cycle progresses may be germane to this obser-vation.

The addition of platinum or palladium to isSnO2well known to enhance the sensor response and/orselectivity to certain gases, including CO and ThisH2 .is also the case in the present investigation based onconductivity measurements of the bare supportSnO2exposed to CO and dry air at 130 ¡C (not shown). Thechemical state of the Pt may provide important infor-mation regarding its inÑuence on the enhanced conduc-tivity response to CO. Figure 11 illustrates the Pt 4fXPS spectra acquired from the specimen at thePt/SnO2various stages of treatment. The spectrum in Fig. 11(a)corresponds to the air-exposed specimen prior toheating and it is indicative of primarily platinum oxides(D74È75 eV), hydroxides (D72.8 eV) and/orPtwOwSn species (D72.3 eV) with relatively littlemetallic platinum present (D71 eV).41,51,52 A sub-sequent spectrum taken immediately thereafter [Fig.

Figure 11. The high-resolution Pt 4f XPS spectra acquired from 2at.% The spectra are arranged according to surface treat-Pt/SnO

2.

ment exactly as presented for and (see Tables 1Au/MnOx

MnOx

and 2). The vertical lines denote binding energy assignments withrespect to the peak.4f

7@2( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 1035È1049 (1998)


11(b)] indicates that the fraction of Pt that is metallicincreases with a corresponding reduction in the relativeamount of Pt present as oxides and hydroxides. Heatingthe specimen in a vacuum [Figs 11(c) and 11(d)] furtherincreases the relative concentration of metallic Pt withthe growth of Pt¡ crystallites. This trend gradually con-tinues through the CO/dry air gas exposures but all ofthe Pt is never completely reduced to metallic form.This is evidenced by the shoulder on the Pt peaks4f7@2near 72.4 eV [particularly noticeable in Figs 11(g) and11(h)], which is indicative of PtwOwSn and/or

The predominance of metallic Pt on thePt(OH)2 .present surface is analogous to results fromPt/SnO2similar investigations with respect to the (metallic)chemical state of Pd detected in gas sensors.53Pd/SnO2Based on investigations of CO oxidation on tin oxidesupported metals,54 Pt may serve to increase thenumber of CO chemisorption sites on and alsoPt/SnO2promote subsequent reactions with lattice oxygen toform Both CO chemisorption and loss of latticeCO2 .oxygen would act to increase the conductancePt/SnO2upon CO exposure. The presence of would bePt(OH)2consistent with the observed enhancement of CO oxida-tion over in the presence of water vapor,Pt/SnO2although the mechanism involved is still not completelyunderstood.40,42

The potential importance of hydroxides in the COgas sensing mechanism on may be assessedPt/SnO2further by inspection of the XPS O 1s spectra depictedin Fig. 10. The major peak in each spectrum near 530.4eV is due primarily to tin oxide,19 although the Pt 4fspectra above would indicate some contribution fromplatinum oxides. Hydroxide species are indicated by thehigh-binding-energy shoulder on the main peaks locatednear 531.5 eV.19 Heating the specimen in aPt/SnO2vacuum slightly reduces the presence of hydroxyls [Figs10(c) and 10(d)]. Upon CO exposure [Fig. 10(e)], thesize of the hydroxyl feature remains essentiallyunchanged. If the O 1s spectra in Figs 10(e) (COexposure) and 10(f ) (dry air exposure) are superimposed,it becomes apparent that the dry air exposure slightlyincreases the surface concentration of hydroxyls. Sub-sequent exposures to CO and dry air continue todecrease and increase, respectively, the presence ofsurface hydroxyls. Such behavior would be consistentwith the participation of hydroxyl groups in the CO gassensing mechanism. In addition, recall from Fig. 3 thatthe conductivity increase becomes greaterPt/SnO2during the second CO exposure and this coincides withan increased relative concentration of hydroxyl groupsafter the previous dry air exposure. Note, however, thatthe relative concentration of surface hydroxyls (and/oradsorbed water) detected on (Fig. 10) is muchPt/SnO2less than that detected on (Fig. 4) for theAu/MnO

xsame conditions. This may ultimately prove to be animportant factor that determines the gas sensing char-acteristics of each material.

Additional details about the surface com-Pt/SnO2position may be obtained from ISS. Figure 12 containsthe ISS spectra acquired from the specimen asPt/SnO2a function of the eight di†erent surface treatments.These spectra have all been scaled to a common Snpeak intensity to aid comparisons. The outermostatomic layer of the air-exposed surface [Figs 12(a) and12(b)] is comprised of carbon and chlorine in addition

Figure 12. The ISS spectra acquired from 2 at.% ThePt/SnO2.

spectra are arranged according to surface treatment exactly as pre-sented for and (see Tables 1 and 2). The elementsAu/MnO

xMnO

xsuperimposed at the top of the figure indicate their expected peakpositions based on the binary elastic scattering equation thatgoverns ISS.

to oxygen, tin and platinum. Hydrogen will also bepresent, but ISS is not sensitive to hydrogen. Afterheating the specimen in a vacuum at 130 ¡CPt/SnO2[Figs 12(c) and 12(d)], only oxygen, tin and platinumare detected by ISS, and the oxygen signal decreasessigniÐcantly. In addition, the Pt/Sn peak intensity ratiois reduced. These observations have all been shown tobe indicative of PtSn alloy formation.39,50 In thepresent case, this would be consistent with Figs 10(c)and 10(d) and the detection of metallic Sn by XPS.During the CO/dry air gas cycling [Fig. 12(e)È(h)] thePt/Sn peak intensity ratio increases slightly, which coin-cides with the disappearance of much of the metallic Snaccording to Fig. 10. Note also that the oxygen signalincreases after the dry air exposures [Figs 12(f ) and12(h)], which is consistent with the rise and fall inhydroxyl group concentration observed in the corre-sponding XPS O 1s spectra above. In addition, there isa gradual resurgence in the concentration of chlorine atthe surface as the gas exposure sequence progresses.

An interesting feature in Fig. 12 is the lack of a sig-niÐcant carbon ISS peak, particularly in the spectra cor-responding to the CO gas exposures [Figs 12(e) and12(g)]. Recall from Fig. 3 that the CO conductivityresponse of is not reversible when the speci-Pt/SnO2men is subsequently exposed to a vacuum. If this is dueto residual chemisorbed CO on the surface, then it isreasonable to expect an increase in the carbon and/oroxygen ISS peak intensity. After the Ðrst CO dosing[Fig. 12(e)] there is a small increase in the relativeoxygen presence, but no carbon is detected. This may bedue to vertical orientation of the chemisorbed CO mol-ecules and shadowing of the underlying carbon atoms,such as that observed on polycrystalline Pt surfaces.55Based on infrared spectra of CO/air interaction with



1047

the CO may react to form surface carbonateSnO2 ,groups whose oxygen atoms might shield carbon fromISS detection.56 Hydrogen may also be responsible forshadowing e†ects. Whereas some of the CO may desorbupon vacuum exposure, total CO desorption is not con-sistent with the irreversible conductivity responseshown in Fig. 3. Subsequent dry air exposure increasesthe oxygen signal, as shown in Fig. 12(f ). The oxygensignal slightly decreases after the second CO exposure[Fig. 12(g)], which may result from surface reactions toyield and a corresponding conductivity increase. ItCO2is uncertain whether a portion of this remaining oxygenmay be attributed to chemisorbed CO. Little additionalinformation is available from XPS due to the low inten-sity of the C 1s peaks detected in these cases.

Surface conductivity characteristics of andAu/MnOxMnO

x

The determination of surface compositions at variousstages of the surface conductivity proÐles has provideduseful information about the gas sensing characteristicsof and yet the data areAu/MnO

x, MnO

xPt/SnO2 ,

unable to provide a clear understanding with respect tothe conductivity increase exhibited by andAu/MnO

xupon exposure to dry air. As explained pre-MnOxviously, these materials have been determined to be

n-type semiconductors and therefore the typicalresponse to dry air or oxygen would be a decrease insurface conductivity. Instead, these materials behave asif they are p-type semiconductors in the presence of anoxidizing atmosphere.

Semiconductor materials may exhibit a wide range ofresponses to both reducing and oxidizing gases. Impor-tant factors that a†ect the conductivity changes includetemperature and the composition of the sensor itself.For example, it has been demonstrated that uponheating a tin oxide pellet from 300 ¡C to 510 ¡C, theselectivity toward CO is diminished in favor ofenhanced selectivity for The surface composi-CH4 .57tion of a sensor may be altered by the ambient gas-phase conditions and surface reactions. In certain cases,this may not only cause changes in the sensor conduc-tivity, but it may also a†ect the directionality of thesensor response. This has been demonstrated by mea-suring the surface conductivity of a barium titanateceramic sensor as a function of oxygen partial pres-sure.58 As the oxygen partial pressure was increased, theconductivity of the sensor initially decreased, traverseda minimum and subsequently began to increase. Inother words, the sensorÏs response characteristicsswitched from n-type to p-type. A similar response tooxygen has been observed for a gas sensor with thecomposition Additional exam-BaFe0.67Ta0.33O3~x

.57ples of this behavior are represented by BaSn0.9Zr0.1O3(exposure to increasing concentrations of in air)57H2Sand Al-doped ZnO (exposure to increasing concentra-tions of in air).59 It has also been demonstratedNH3that the conductivity increase usually observed uponexposing to CO is changed to a conductivitySnO2decrease when the sensor is equilibrated with highoxygen partial pressures prior to the CO exposure.60

It has been stated that, in principal, any semicon-ductor may transform from n-type to p-type given the

right circumstances,57 and the examples above appearto support this statement. An n-type semiconductormay exhibit n-type behavior toward oxygen at low con-centrations, but a p-type response may be observed athigher oxygen partial pressures. The two extremeswould be separated by a minimum in the conductivityproÐle. In the present case, the p-type response of

and to dry air may be the result ofAu/MnOx

MnOxoxygen partial pressures that are high enough at the

given temperature to place the sensor response in ap-type region. This condition might arise from adecreased concentration of electron charge carriers dueto the formation of oxygen ions on the sensor surface.At a sufficiently high oxygen ion surface concentration,the decreased contribution of electrons in the conduc-tion band may become more than o†set by the numberof holes created in the valence band.57 As a result, thesensor would begin to exhibit p-type behavior.

Experiments were performed to determined if theand surface conductivities eventuallyAu/MnO

xMnO

xshow negative deviation from the baseline responseupon exposure to lower partial pressures of oxygen (seeFigs 1 and 2). As the oxygen partial pressure is system-atically reduced, the e†ect is only to attenuate the con-ductivity increases, ultimately to a point where noresponse can be discerned. Therefore, it appears that theregion of n-type response to oxygen occurs at anoxygen partial pressure that is too low to e†ectivelymeasure a conductivity change for the conditions of thisinvestigation.

SUMMARY

The gas sensing properties of 2 at.% haveAu/MnOxbeen investigated as a function of successive exposures

to CO and dry air at 130 ¡C. The resulting surface con-ductivity proÐles were then correlated to the surfacecomposition as determined by XPS and ISS at interme-diate stages of the gas exposures. Identical experimentswere performed on and 2 at.% and theMnO

xPt/SnO2results have been compared to help understand the

surface phenomena responsible for the measured con-ductivity changes.

Relative to alone, the specimenMnOx

Au/MnOxexhibits an enhanced conductivity response to both CO

and dry air, suggesting a synergistic interaction betweenAu and the support. In both cases, exposure toMnO

xCO and dry air (or oxygen) results in a conductivityincrease that is atypical for these and other n-type semi-conductors. The conductivity changes are essentiallyreversible in each case. The surface analysis data suggestthat the chemical state of Mn on andMnO

xAu/MnO

xis similar, indicating a mixture primarily of andMn3O4although MnO is also detected. On the con-Mn2O3 ,trary, signiÐcant di†erences are noted with respect tothe presence of adsorbed water and hydroxyl groups,the concentration of which is greater on Au/MnO

x.

Because the presence of hydroxyl groups has often beenshown to promote CO oxidation performance onsimilar materials, this correlates well with the superiorCO oxidation activity of (relative toAu/MnO

xMnO

x)

and it is also consistent with the enhanced conductivityresponse of From XPS it was concludedAu/MnO

x.



that gold is probably present in the metallic state onAu/MnO

x.

Under the conditions of the investigation, thespecimen exhibits the largest conductivityPt/SnO2change (increase) upon exposure to CO. Unlike MnO

xand however, the response to CO is notAu/MnOx,

reversible. This may be due to residual CO chemisorbedon the surface, but XPS and ISS are not able to yieldconclusive evidence. Subsequent exposure of Pt/SnO2to dry air decreases the measured conductivity. Smallamounts of reduced Sn are detected on the Pt/SnO2specimen (possibly alloyed with Pt), the presence ofwhich decreases upon progressing through two com-plete cycles of CO and dry air exposure. However,because the conductivity change of is evenPt/SnO2greater during the second CO exposure, the presence ofreduced Sn may adversely a†ect the sensor response.Hydroxyl groups are also detected on andPt/SnO2their surface concentration decreases and increases,respectively, upon CO and dry air exposure. This maybe an indication of their importance to the gasPt/SnO2sensing mechanism. In all cases the relative fraction ofoxygen on present as hydroxyl groups/Pt/SnO2adsorbed water is less than that measured on Au/MnO

xand this is in agreement with the previous statementand the superior CO oxidation activity of Au/MnO

x.

The fact that the CO conductivity change is not as greatfor is consistent with di†erent CO gasAu/MnO

xsensing mechanisms with respect to utilization of

surface hydroxyl functions. This assertion is further sup-ported by XPS, which indicates that a signiÐcant frac-tion of Pt is present as The performance ofPt(OH)2 .the sensor has not been optimized in theAu/MnO

xpresent investigation, and research is currently underway to determine the selectivity characteristics ofseveral materials and how they ultimatelyAu/MnO

xcorrelate to the surface composition.Finally, the positive change in conductance exhibited

by (and upon dry air exposure isAu/MnOx

MnOx)

attributed to a transformation from n-type conductivityto p-type conductivity, brought about by sufficientlyhigh partial pressures of oxygen. This result is particu-larly interesting because it presents motivation to inves-tigate the combination of and in aAu/MnO

xPt/SnO2single CO gas sensor. The opposite direction of their

conductivity changes upon air exposure may have acancellation e†ect, which might further enhance thepositive conductivity change exhibited during CO expo-sure.

Acknowledgement

This research was supported by the US Environmental ProtectionAgency under grant no. R82-3130-010. The authors extend thanks toDr John Yeager (Keithley Instruments) for his useful advice regardingthe conductivity measurements. Thanks also go to Dr Jim Ehrstein(National Institute of Standards and Technology) for providing thesilicon wafer surface conductivity standard.

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