measurement of the chemical and morphological changes that occur on gold surfaces following thermal...

Analytica Chimica Acta 496 (2003) 279–287

Measurement of the chemical and morphological changes thatoccur on gold surfaces following thermal desorption and

acid dissolution of adsorbed mercury

Todd Morris, Jia Sun, Greg Szulczewski∗Department of Chemistry, The University of Alabama, Box 870336, 6th Avenue, Lloyd Hall, Tuscaloosa, AL 35487, USA

Received 14 October 2002; accepted 17 October 2002

Abstract

We have studied the chemical and morphological changes that occur in mercury-covered polycrystalline gold films afterthermal annealing and acid dissolution. Atomic force microscopy images show that acid etching causes significant morpho-logical changes in the gold films. The morphological changes following acid dissolution can be explained by the preferentialdissolution of Hg at grain boundaries in polycrystalline films, which causes the nucleation of larger gold islands and creationof voids. X-ray photoelectron spectroscopy indicates that not all of the Hg can be dissolved from the polycrystalline goldfilms with concentrated nitric acid, but heating above 500 K is sufficient to remove the Hg. Thermal desorption of Hg from aAu(1 1 1) single crystal was also studied under ultrahigh vacuum conditions. Desorption peaks were observed at∼240 and∼380 K and were assigned to the multilayer and monolayer, respectively. These findings are important since they suggest mer-cury cannot be completely removed from polycrystalline gold surfaces (i.e. electrodes) by acid dissolution. The implicationsof cleaning gold-based mercury sensors are discussed.© 2003 Elsevier B.V. All rights reserved.

Keywords: Atomic force microscopy; Temperature programmed desorption; Mercury; Gold

1. Introduction

The adsorption of mercury atoms and ions on goldsurfaces is important to several analytical techniques.For example, under-potential deposition of mercuryions on a gold electrode followed by anodic strip-ping is a sensitive method of detection for ionic mer-cury [1,2]. Adsorption of gas phase Hg atoms ontogold “traps” is used to pre-concentrate the element foratomic absorption[3] and fluorescence[4] methods.In these optical techniques mercury isdesorbed from

∗ Corresponding author. Tel.:+1-205-348-0610;fax: +1-205-348-9104.E-mail address: [email protected] (G. Szulczewski).

the gold surface by heating and the concentration ofgas phase Hg atoms is quantified by either the absorp-tion or emission of a photon at 253.7 nm. In addition,gravimetric[5,6], optical [7], and electrical[8] trans-ducers can detect Hgadsorption on gold surfaces. Dueto the importance of the Hg/Au interface to these an-alytical techniques there have been a number of fun-damental studies of Hg adsorption on gold surfaces.By way of introduction, we briefly summarize the im-portant findings that are relevant to the work in thismanuscript.

In 1985, Schroeder et al.[9], reviewed the useof gold and silver as collection media for elemen-tal Hg and mercury compounds such as HgCl2 andHg(CH3)2. The majority of the work summarized by

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0003-2670(03)01007-9

280 T. Morris et al. / Analytica Chimica Acta 496 (2003) 279–287

Schroeder et al. was conducted on poorly character-ized gold or silver (i.e., films, wires, sponges and/orwool). Since 1985 several surface science studieshave measured Hg adsorption kinetics, structure, andcoverage on well-defined gold surfaces. In particu-lar, Glaunsinger and co-workers[10–12]conducted aseries of systematic experiments to understand howHg adsorption changes the resistivity of thin goldfilms. Specifically they examined the role of filmmorphology (i.e., polycrystalline versus single crys-tal films) with scanning tunneling microscopy. Themain finding from their work is that Hg preferen-tially adsorbs at grain boundaries in the polycrys-talline films. In contrast, on a single crystal surfacewith very flat terraces mercury forms a network-likestructure. Recently, Levlin et al.[13] confirmed theobservation that Hg forms two-dimensional islandson Au(1 1 1). However, they proposed that some ofthe Hg and Au atoms exchange place in a concertedprocess.

We previously reported that the transition from ad-sorption to absorption on polycrystalline gold filmscan be monitored by spectroscopic ellipsometry, sur-face plasmon resonance spectroscopy, and X-ray pho-toelectron spectroscopy (XPS)[14]. We exposed cleangold films to 15 ppm Hg vapor in air. Between 1 and10 min adsorption is rapid, but slows between 15 and20 min as the number of unoccupied sites decreases.Beyond 30 min Hg diffused into the bulk to createavailable adsorption sites for Hg. Our conclusion wassupported by the fact that no significant Hg uptake wasobserved beyond 30 min on a Au(1 1 1) single crys-tal. Although the adsorption of Hg onto Au has beenwell studied there have been very few studies on thedesorption process.

In order to successfully use gold films or elec-trodes in a sensor it is critical that all the mercury isreleased from the gold surface before a subsequentanalysis. Watson et al.[15] has recently noted thattrace amounts of mercury are retained on polycrys-talline gold electrodes (used for stripping methods)even after extensive cleaning protocols. However,Watson et al. did not investigate the morphologyof the electrodes. As a result, the objective of ourstudy was to monitor the chemical and morpholog-ical changes that occurs in Hg covered polycrys-talline Au films following thermal desorption andacid dissolution. To better understand the thermal

desorption process we performed experiments ona Au(1 1 1) single crystal under ultrahigh vacuumconditions.

2. Experimental

XPS and temperature programmed desorption(TPD) were conducted in two stainless steel ultra-high vacuum chambers (base pressure∼2×10−8 Pa).X-ray photoelectron spectra were measured with aKratos Axis 160 using Al K� monochromatic ra-diation at 1486.7 eV. A hemispherical analyzer wasused to measure the kinetic energy of the photoelec-trons at a pass energy of 80 eV. This chamber didnot allow for temperature control of the sample. TPDexperiments were carried out in a chamber designedfor sample heating and cooling. A 6.4 mm × 2 mmAu(1 1 1) crystal (monocrystals) had two 0.5 mm slotscut into opposite sides. Tungsten wires were firmlypressed into the grooves and spotwelded to two Wrods, which were in contact with a liquid nitrogencooled reservoir and electrically isolated from ground.As a result, direct current was passed through the Wwires to resistively heat the Au crystal. A hole wasdrilled into the edge to hold a type K thermocouple.A Eurotherm (model 2040) temperature controllerthat utilized adaptive feedback loop technology wasused to regulate a DC power supply and generate alinear heating rate. The single crystal was cleanedby repeated cycles of sputtering with 1000 eV Arions and annealing to 900 K. Mercury vapor wasadmitted into the chamber through a variable leakvalve. In a nitrogen filled glove box, elemental mer-cury (99.999%) was transferred into a glass tube tometal joint. The tube was connected directly to theleak valve in the dry box. Several freeze pump thawcycles were used to remove the nitrogen. When theleak value was fully open the pressure in the chamberincreased to∼2 × 10−8 Pa (uncorrected ionizationgauge reading). We use the common definition that1 Langmuir (L) equals 1.33 × 10−4 Pa s. This is thepressure–time required for every site on the surfaceto experience one collision. We used a Stanford Re-search Systems (model RGA 300) quadrupole massspectrometer (QMS) as the detector in the TPD ex-periments, which can measure partial pressures downto ∼1.3 × 10−12 Pa. We wrote a computer program

T. Morris et al. / Analytica Chimica Acta 496 (2003) 279–287 281

to record the intensity of the ions from the QMS. Theheating rate was 2 K/s and we recorded the ion signalfor every 0.5 s.

Polycrystalline gold films (∼200 nm) were preparedby vapor deposition of high purity gold (99.995%from Alfa Aesar) onto Si(1 0 0) wafers covered with a2 nm Cr adhesion layer under high vacuum conditions[16]. X-ray diffraction of the as-deposited gold filmsexhibit intense scattering from the (1 1 1) plane. Thefilms were exposed to air saturated with Hg vapor inthe following manner. A test tube containing∼1 g ofelectronic grade Hg (99.999%) was placed in a glassjar. After the lid to the jar was sealed the Hg con-centration equilibrates to∼15 mg/l or about 15 ppmat room temperature[17]. AFM of the gold films wasperformed with a Digital Instruments Dimension 3000

Fig. 1. X-ray photoelectron spectra in the Hg(4f) region after exposing polycrystalline gold films to∼15 ppm Hg for 30 min and subsequentlyplaced in nitric acid for: (a) 0 min; (b) 1 min; (c) 10 min.

using Si tips in tapping mode. The manufacturer ofthe tips (MikroMasch) reports the average radius ofcurvature of the tips to be less than 10 nm.

3. Results and discussion

In Fig. 1 we show X-ray photoelectron spectra ofpolycrystalline gold films in the Hg(4f) region afterexposure to Hg vapor for 30 min and immersion in ni-tric acid for various periods of time. The mean freepath of the Hg(4f) photoelectrons has been measuredto be ∼1 nm [18]. Consequently 99% of the signalmust come from the topmost∼3 nm of the film. InFig. 2we plot the amount of Hg remaining on the goldsurface after immersion in nitric acid. It is evident that


Fig. 2. Amount of Hg remaining on gold films following acid dissolution.

the mercury is rapidly removed from the film. How-ever, we could not completely remove all the mer-cury. Our results supports the findings of Watson et al.[15].

In Fig. 3 we show a series of representative AFMimages of polycrystalline Au films that were exposedto ∼15 ppm Hg in air for 30 min followed by immer-sion into concentrated nitric acid for 1, 10, and 30 min.The samples were rinsed with copious amounts ofde-ionized water and dried with a stream of high pu-rity nitrogen gas (99.99%) before imaging. Severalobservations are noteworthy. First, the morphologyof clean gold films placed in nitric acid does notchange (image not shown). Second, it is evident thatwith increasing time in the nitric acid the gold filmsroughen. The as-deposited gold films contains islandsthat are∼50 nm in diameter and 1–2 nm in height. Af-ter 30 min of Hg exposure and 10 min of nitric acid,some Au islands have grown to 5–10 nm in height(as indicated by the bright regions in the images). Fi-nally after 30 min in nitric acid several Au islandshave reached 10–15 nm in height. Also, it is apparentthat many voids remain next to the Au islands thathave grown, but most of the grains in the film are un-changed. The morphological changes were observedon many samples and different regions of a given sam-

ple. We must emphasize that the images are a con-volution of the surface topology and tip geometry,not the actual surface morphology. In related experi-ments (AFM images not shown) we varied the mer-cury exposure time and kept the dissolution time fixedat 30 min. There are only small changes in the mor-phology for short exposures (i.e., less than 15 min).Longer Hg exposure (15–30 min) are required to ob-serve the changes in morphology, presumably becausemore Hg diffuses into the near-surface region at thegrain boundaries (see discussion below).

The changes in morphology that we observe areconsistent with a dealloying process[19,20]. Dealloy-ing is the dissolution of the least noble metal from analloy. There have been studies of dealloying in CuAu[21,22]and AgAu alloys[19,20], but to our knowledgethere are no reports of Hg dissolution from Au. In bi-nary alloys of Ag and Au it has been documented thatselective dissolution of Ag in acidic solution producesa “spoongy” or “nanoporous” gold film[19,20]. Fortyproposed a physical model that is consistent with theobserved morphology[19]. The element that is oxi-dized (in our case Hg) creates vacancies in the alloy.The more noble metal atoms (in our case Au) re-orderthe dis-ordered surface. As a result, island growth be-gins in concert with void formation.


Fig. 3. AFM images of polycrystalline gold films exposure to∼15 ppm Hg vapor for 30 min and placed in nitric acid for: (a) 1 min; (b)10 min; (c) 30 min.

We also used AFM to monitor changes imagesof polycrystalline Au films that were exposed to∼15 ppm Hg in air for 30 min and heated to∼50,100, and 150◦C under vacuum for 30 min. We foundno discernable difference in the morphology of an-nealed Hg covered and clean polycrystalline Au films(AFM images not shown). InFig. 4 we show the

X-ray photoelectron spectra after thermal annealingthe polycrystalline gold films for 30 min and heatedunder vacuum.Fig. 5 shows the amount of mercuryremaining on the gold surface after the annealingtemperature. The amount of Hg remaining on thefilm decreases exponentially with increasing temper-ature. We have taken Au films that retained Hg after


Fig. 4. X-ray photoelectron spectra in the Hg(4f) region after exposing gold films to∼15 ppm Hg for 30 min (a) and heated to (b) 50◦C,(c) 100◦C, and (d) 150◦C.

nitric acid treatment and heated them to 150◦C un-der vacuum for 30 min. XPS indicates that∼0.5%Hg remains on these Au films. This suggests that theHg is not elemental but most likely a stable alloy orcompound under these conditions.

The results from a series of TPD experiments ona Au(1 1 1) single crystal under ultrahigh vacuum areshown inFig. 6. We monitored the202Hg signal withthe QMS since it is the most abundant isotope. Itcan be seen that as the exposure increases from 2 to32 L several desorption features appear. The peak at∼240 K does not saturate with increasing exposureand is characteristic of zero order desorption kinetics[23]. There are two peaks at∼310 and∼250 K thatsaturate at high exposures. We note that doses above4 L at 300 K result in a broad desorption peak near400 K (data not shown). The desorption yield of a 4 Ldose at 300 K is about 10 times less than the samedose at 140 K, which indicates the sticking coefficientof Hg on Au(1 1 1) depends on temperature. To ourknowledge these are the first published TPD spectra of

Hg from a Au single crystal surface. Very few studiesof Hg adsorption/desorption on single crystal surfaceshave been published, the exceptions being Fe(1 0 0)[24], W(1 0 0) [25], Ni(1 1 1) [26–28], Ni(1 0 0) [29],Ag(1 0 0)[30,31], and Cu(1 0 0)[31,32], surfaces. Webegin our discussion by comparing our TPD spectrato the other coinage metals.

Kime et al. [31] have dosed Hg onto Cu(1 0 0)surface held at 140 K and observed four desorptionpeaks. Following a 1.9 L dose a single desorptionpeak was observed at∼235 K. After a 3.8 L dose thepeak at∼240 K shifts to∼245 K and new peak de-veloped at∼285 K. A dose of 7.6 L produced a newpeak near 265 K. Finally after a 11.4 L dose a fourthpeak seems to appear near 260 K. They had previ-ously used helium atom diffraction and low-energydiffraction measurements[32] to establish a relation-ship between overlayer structure and Hg coverage.They reported that a 6 L dose was sufficient to formthe first monolayer with saturation coverage of 0.2 Hgatoms per Cu atom. A second layer forms after∼a


Fig. 5. Amount of mercury remaining on gold films before (25◦C) and after annealing.

15 L dose and yields a coverage of∼0.6 Hg atomsper Cu atom. Above 18 L the Hg layer is dis-ordered.On Ag(1 0 0) the thermal desorption spectra of Hg aresimpler, but more difficult to interpret. There is onlyone desorption peak near 240 K when Hg is dosed at120 K. The peak temperature of this peak graduallyincreased with larger doses. Dowben et al.[30] usedsoft X-ray photoemission spectroscopy to determinethe relationship between exposure and surface cover-age. They report that after a 3, 6 and 20 L dose thefirst, second and seventh layers form, respectively. Wecan now use the thermal desorption data on Cu(1 0 0)and Ag(1 0 0) to help assign the desorption featureswe observed on Au(1 1 1). The first obvious differ-ence is that Hg adsorbs more strongly to Au thanCu or Ag since the highest desorption peaks are wellabove room temperature. The two peaks at 380 and417 K are difficult to assign. Perhaps the highest peakis desorption from defect sites or sub-surface Hg (ifthe place exchange adsorption model of Levlin et al.[13] is correct). However, we can conclude that lateralinteractions are important because the peak temper-ature increases about 50 K as the coverage increases.

We can certainly assign the lowest temperature peakat 235 K to multilayer desorption because this peaknever saturates with increasing exposure. That al-lows us to assign the peaks at∼303 and∼246 K toa second and third layer, respectively. Unfortunatelywe do not have TPD and XPS in the same cham-ber and cannot report an absolute surface coverage.Furthermore, we cannot exclude the possibility thatsome of the Hg diffuses into the bulk during heat-ing. One more observation deserves comment. Boththe monolayer and multilayer desorption peaks existat the lowest doses we studied (i.e., 0.1 L) at 140 K,which suggests mercury prefers to form two- andthree-dimension islands before fully covering the Ausurface. We can estimate the absolute surface cover-age of our smallest dose (i.e., 0.1 L) as follows. Wecalculate that a pressure of∼9.3 × 10−4 Pa is re-quired for a monolayer of Hg collisions with a 1 cm2

surface area[33]. If the sticking coefficient is unitythen a 0.1 L dose is about 1/70 of a monolayer or∼2 × 1013 Hg atoms cm2. This represents an upperlimit of coverage since the sticking coefficient maybe less than unity.


Fig. 6. TPD spectra of Hg following doses of (a) 2 l, (b) 4 l, (c) 8 l, (d) 16 l, and (e) 32 l. The Au(1 1 1) crystal was held at 140 K duringadsorption. The spectra are offset for clarity.

4. Conclusions

Mercury desorption from covered polycrystallinegold films is complete by heating to 500 K. In con-trast, acid dissolution does not completely removeadsorbed Hg and induces a gold island growth dueto Hg vacancies created at grain boundaries in poly-crystalline films. On a Au(1 1 1) single crystal underultrahigh vacuum conditions we have studied thethermal desorption mechanism of Hg atoms. Wefind that multilayers desorb near 240 K; a secondlayer desorbs near 300 K; and the first layer desorbsnear 380 K. We suggest that care must be exer-cised when using mechanically polished/acid treatedgold electrodes in electrochemical stripping experi-ments since it is likely that this regeneration proce-

dure will not remove all the mercury from the goldsurface.

Acknowledgements

GS thanks The School of Mines and Energy De-velopment at The University of Alabama for partialsupport of this work. We acknowledge The NationalScience Foundation for use of shared instrumen-tation (XPS and AFM) through the Materials andResearch Science and Engineering Center (Grant#DMR-98-09423). We thank Franklin Leach forhelp in the construction of the UHV chamber andMark Tomich for writing the TPD data acquisitionsoftware.


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Greg Szulczewski was born in Detroit,MI in 1966. In 1989 he graduated fromthe University of Michigan with a B.S.in Chemistry and in 1995 he receivedPh.D. in Physical Chemistry from WayneState University. In 1995 he and his wife,Shawna, moved to Austin, TX. After a2 year postdoctoral experience with Prof.Mike White at The University of Texas hemoved to Tuscaloosa, AL and began his

academic carrier at The University of Alabama. In 2002 Greg wasappointed as Director of Undergraduate Studies in Chemistry. Greghas given over 20 invited scientific presentations and publishedover 25 papers in the area of surface science. His research isfocused on the preparation and characterization of surfaces andthin films for applications in sensors, separations, optoelectronics,data storage, molecular electronics and catalysis.

measurement of the chemical and morphological changes that occur on gold surfaces following thermal...

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