Bowker M., Davies P.R. (Eds.) Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

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Figure 7.3 High-pressure reactor and

STM body. (a) A view of the whole reactor:

(1) cell lid, (2) cell neck, (3) middle stage,

(4) bottom stage, (5) pin-socket contacts

for wiring of STM body at the interface of

high-vacuum and high-pressure environment,

(6) a pin on the lever of the cell lid for

mounting, (7) Swagelok fitting welded on the

tube of the cell lid for gas introduction,

(8) Swagelok fitting welded on the button

stage of reactor for gas exit, (9) port of

reactor for sample and tip transfer. (b) Side

view of the STM body: (1) Wall of STM body

coated with a layer of gold, (2) hexagonal

sapphire, (3) CuBe spring plates, (4) receiver

of tip holder, (5) screw to adjust the pressure

applied to hexagonal sapphire, (6) hole to

assemble sample stage to STM body. (c)

Scheme of pin-socket alignment for wiring. Set

III is the contacts glued to the holes on top of

STM body; Set II is the contacts glued to the

holes on top of cell lid of the reactor; Set I is the

contacts glued to wires from docking scaffold.

(d) Scheme showing the assembly of three

packs of shear piezoelectric plates between the

hexagonal sapphire and the wall of the STM

body and the assembly of the scanning tube: (1)

a set of piezoelectric plates, (2) wall of STM

body, (3) hexagonalsapphire,(4) scanningtube.

(e) Scheme showing how the four shear piezo-

electric plates of one set work for coarse

approach.

194

7 Surface Mobility of Atoms and Molecules Studied with High-Pressure STM

a sample stage assembled at the end of the STM body, and wire connections to

these parts. The coarse approach is carried out by six sets of shear piezoelectric plates

(three of them are schematically shown in a bottom view of STM body in Figure 7.3d)

located between a hexagonal sapphire (Figure 7.3b2) and the wall of the STM body.

One side of each shear piezoelectric set is glued to the internal wall of the STM body

through an alumina plate for thermal isolation while the other side is glued to another

alumina plate that contacts the surface of the hexagonal sapphire (Figure 7.3d and e).

By applying negative or positive voltages to the ﬁrst/third and the second/fourth

piezoelectric plates, the lateral force moves the hexagonal sapphire forward and

backward (Figure 7.3e). A single piezoelectric scanning tube is glued to an alumina

disk that is in turn glued to one end of the hexagonal sapphire. Five Kapton wires are

glued to the ﬁve components ( þx, x, þy, y, z) of the scanning tube through holes

on the alumina disk. Another alumina disk is glued to the other end of the scanning

tube onto which a bowl-shaped tip receiver is glued (Figure 7.3b4). The central part

of this tip receiver is a SmCo magnet. The tip change mechanism is described below.

A ﬂexible coaxial wire is glued to this tip receiver for transmitting the tunneling

current.

A sample assembly stage is screwed to the end of the STM body (Figure 7.3b6),

which is thermally and electrically insulated from it by three precisely aligned

sapphire balls and insulating washers. For sample transfer and tip change, a port

is opened on the wall of the bottom stage of the high-pressure reactor (Figure 7.3a9).

A bayonet seal (inset of Figure 7.3a) was fabricated to seal the port on the reactor. This

sealing keeps the pressure in vacuum chamber lower than 5 10

7

Torr when the

high-pressure reactor is ﬁlled with 1000 Torr N

. The naturally leaked gases from the

sealing interface between different sections of the reactor can be analyzed in situ

with a mass spectrometer during reaction.

The in situ sample heating during reaction is carried out by a light source outside

the high-pressure reactor to avoid heating elements in the high-pressure environ-

ment. It consists of a halogen lamp with an elliptical reﬂector that focuses the

radiation onto the sample through a sapphire window welded at the bottom of the

reactor (Figure 7.4). The distance between the lamp and the reactor can be adjusted to

focus the light on the back of the sample for efﬁcient heating. The heating rate can be

controlled by adjusting the power supplied to the lamp. A K-type thermocouple was

spot-welded to the sample stage for both sample bias and temperature measure-

ments. Another thermocouple was attached to the STM body to monitor the

temperature of the shear piezoelectric plates during sample heating. Thus, thermal

diffusion and possible increases in the temperature of the STM body can be

simultaneously monitored when the sample in the high-pressure reactor is heated.

Replacement of the STM tip is accomplished using a magnetic tip exchanger with

the same geometry as the sample holder (Figure 7.5a) and a tip holder (Figure 7.5b).

The tip exchanger can be easily transferred to and from the high-pressure reactor

(Figure 7.5c), the storage disk, and the load-lock system.

Figure 7.6 schematically shows the setup of gas introduction for the high-pressure

reactor. For gas introduction, the male part of a Swagelok ﬁtting (4 in Figure 7.6)

is welded on a 1/8 in. tube of the cell lid (3 in Figure 7.6). A 1/32 in. PEEK tube

7.3 High-Pressure STM Technique and Instrumentation

195

(7 in Figure 7.6) capable of supporting high pressure was glued to a 1/16 in. stainless

steel tube (6 in Figure 7.6) assembled into a female part of the Swagelok ﬁtting (5 in

Figure 7.6). Another end of the PEEK tube is also glued and assembled to another

Swagelok ﬁtting welded to a hole in a double-sided CF ﬂange. This design isolates the

reactant gases in the high-pressure reactor from the vacuum environment of the STM

chamber. The high-pressure gases in the reactor can be pumped down by a turbo

molecular pump to obtain a UHV environment after completion of a high-pressure

experiment,thereforequicklyrestoringa UHVenvironment.Thus,thishigh-pressure

reaction system can work under both UHV and high pressure, offering the capability

of studying catalysts over the entire pressure range from 1 10

10

to 5000 Torr.

In addition, the reactions can be carried out using batch or ﬂow reactor mode.

Atomically resolved images of inert HOPG under 700 Torr N

and of hex-Pt(1 0 0)

prepared in UHV can be routinely obtained with this homebuilt high-pressure high-

temperature STM reactor/UHV system.

Figure 7.4 STM chamber in situ sample heating system.

(1) Halogen lamp, (2) elliptical reflector, (3) sapphire window

welded at the center of the bottom of the high-pressure reactor,

(4) the assembled sample, (5) high-pressure reactor. Turquoise

dashed line shows the alignment of light beam and sample center.

196

7 Surface Mobility of Atoms and Molecules Studied with High-Pressure STM

7.4

Mobility and Flexibility of Catalyst Surfaces at High-Pressure High-Temperature

Reaction Conditions

The exploration of the surface of single-crystal model catalysts has a long history.

Electron microscopy and ﬁeld-emission microscopy developed in the 1950s revealed

the existence of surface steps and kinks, which led to the development of the

heterogeneous rigid surface model (Figure 7.7) proposed in 1960s [21]. In this

model, a surface is heterogeneous and the atoms of the surface layer are arranged at

the exactly same sites as those in the bulk. That is, the location of surface atoms can be

exactly provided by the structural parameters of a bulk layer. This model ignored

Figure 7.5 Parts used for a convenient tip

exchange. (a) Tip exchanger: (1) the central

slot that has a size between the stopping

disk and outer diameter of the tube of the

tip holder, (2) magnets. (b) Tip holder with

an empty tube for placing a tip: (1) empty

tube for placing a STM tip, (2) stopping disk.

scanning tube. The arrow shows the hidden

bowl at the end of the scanning tube.

7.4 Mobility and Flexibility of Catalyst Surfaces

197

the surface effects of relaxation and reconstruction. But it was reasonable to some

extent at that time because it could rationalize the kinetics of crystal growth and the

site-dependent performance of chemisorption and charge distribution. The new

results obtained with the surface analytic techniques developed in 1980s showed that

this heterogeneous rigid model was not correct. We put forward the concept of the

ﬂexible surface in 1991 [13, 22]. One structural evidence of the ﬂexibility of surfaces is

the inward relaxation of the ﬁrst layer of a clean metal surface to produce a short

Figure 7.6 Scheme showing the introduction of

high-pressure reactant gases to reactor when

the reactor is under UHV environment of the

STM chamber. (1) STM chamber, (2) high-

pressure reactor, (3) stainless steel tube

(ID ¼1/8 in.), (4) and (10) male part of a

Swagelok fitting, (5) and (9) female part of a

Swagelok fitting, (6) and (8) silica-coated

stainless steel tube (ID ¼1/16 in.), (7) PEEK

tubing (OD ¼1/16 in., ID ¼1/32 in.), (11)

double-sided CF flange, (12) and (13) angle

valves, (14) angle valve for pumping reactant

gases after completion of a batch mode high-

pressure experiments, (15) vessel for mixing

different reactant gases, (16) and (17) variable

leak valves, (18) Baratron capacitance mano-

meter for pressure measurements, (19) and

(20) gas filters, (21) and (22) gas cylinders,

and (23) angle valve.

198

7 Surface Mobility of Atoms and Molecules Studied with High-Pressure STM

interlayer distance in contrast to the distance between two adjacent bulk layers

(Figure 7.8). In fact, extensive studies of surface structure conﬁrm the general

phenomena of surface relaxation as shown in Figure 7.9. Moreover, we discovered

two rules that the ﬂexibility of metal surfaces generally follow [22]: (a) the closer

packed and lower Miller index surfaces on which atoms have large coordination are

generally less ﬂexible and (b) , more importantly, the ﬂexibility of chemisorbed

surfaces results in possible surface reactions induced by several factors, including

adsorption and thermal-induced restructuring. This model gave a clearer picture of

the catalyst surface [13, 22].

The proposed ﬂexibility of the surface on the basis of vacuum surface science

approach is deﬁnitely applicable to the catalyst surface under an environment of

high pressure of reactive gases. As mentioned above, entropy could signiﬁcantly

contribute to the surface structure and ﬂexibility at high-pressure conditions. Our

recent studies showed that the surface mobility of catalyst surfaces is signiﬁcantly

increased at high pressures. Thus, catalyst surfaces under high pressures of reactant

gases could be considered as mobile surfaces in some cases. Here, we use the

Figure 7.7 Heterogeneous rigid surface model.

Figure 7.8 Restructuring of surface atoms at step sites of a clean surface.

7.4 Mobility and Flexibility of Catalyst Surfaces

199

pressure-dependent adsorption and surface structure of hex-Pt(1 0 0) under an

environment of CO as an example to illustrate the mobility of the metal atoms of

catalyst surfaces. Figure 7.10 is one image of the clean hex-Pt(1 0 0) surface prepared

in UHV. The surface structures and the nanoclusters formed at different pressures

of CO are shown in Figures 7.11 to 7.13. Overall, the surface of hex-Pt(1 0 0)

Figure 7.10 STM image of a clean hex-Pt(1 0 0) surface.

211

111

310

bcc

fcc fcc

expt. theory

1 2 3 4 5 6 7

ROUGHNESS

-40

-35

-30

-25

-20

-15

-10

-5

Δ(d

)

/Δ(d

)

bulk

(%)

}}

110

511

221

210

332

321

110

221

322

320

510

311

332

320

322

331

410

411

210

001

331

311

Figure 7.9 Experimental and theoretical first-layer relaxation

(in percentage) as a function of roughness (¼ l/packing density)

for several bcc and fcc surfaces.

200

7 Surface Mobility of Atoms and Molecules Studied with High-Pressure STM

undergoes a phase change and forms nanoclusters under certain pressures of CO at

room temperature. More importantly, the size and shape of the nanoclusters

formed are pressure-dependent. At a low pressure of 5 10

9

Torr (Figure 7.11),

a portion of the surface is preferentially lifted along the [0 1 1] direction. However,

when a clean hex-Pt(1 0 0) surface is exposed to approximately 10

5

Torr CO, islands

with a rectangular shape and a size of 1.5–4.0 nm are formed (Figure 7.12). There is

no preferential growth along any direction. At a high pressure of 1 Torr CO,

Figure 7.12 STM image of a lifted Pt(1 0 0) surface at a condition of 5 10

7

Torr CO.

Figure 7.11 STM image of a lifted Pt(1 0 0) surface at a condition of 5 10

9

Torr CO.

7.4 Mobility and Flexibility of Catalyst Surfaces

201

nanoclusters with a round or ellipse shape are formed on the whole surface.

Figure 7.14 shows the evolution of the surface structure formed at approximately

5 10

7

Torr CO in a time interval of 28 min. The landmarks in Figure 7.14 are

used to identify the same location between two adjacent images since thermal drift

is present at 300 K. This ﬁgure clearly shows the shape and size of the formed

nanoclusters. During this time interval, the shape of islands changes quickly and a

portion of the atoms in adjacent nanoclusters move together and then form new

nanoclusters, indicating signiﬁcant surface mobility for the lifted Pt atoms at this

pressure. In fact, under 1 Torr CO, clean hex-Pt(1 0 0) is dramatically roughened and

forms highly dense nanoclusters with a size of 1–3 nm on the whole surface

(Figure 7.13). It suggests that a dramatic restructuring of the catalyst surface is

induced by the presence of a high pressure of reactant gas. Notably, no atomically

resolved image can be obtained from the catalyst surface in an environment with

CO pressure higher than 10

4

Torr, whereas atomically resolved images can be

obtained at a pressure of 10

10

–10

5

Torr CO. This suggests higher mobility of Pt

atoms on the catalyst surface under higher pressure of CO.

Temperature is another factor in surface mobility as evidenced by the aggregation

of surface islands when the lifted islands are heated from the original 25–150



(Figure 7.15). Figure 7.15a is the STM of hex-Pt(1 0 0) exposed to 1 Torr CO at room

temperature. Clearly, the surface formed at this pressure of CO consists of Pt

nanoclusters with a size of 2–3 nm. When this surface is heated in situ to 150



the lifted surface structure signiﬁcantly changes (Figure 7.15b). The clusters formed

at room temperature largely aggregate together at 150



C, forming clusters with a size

of 3–7 nm. This aggregation illustrates again the high ﬂexibility and mobility of a

catalyst surface.

Surface mobility and ﬂexibility were evidenced by dramatic reconstruction of the

Pt(1 0 0) surface upon annealing this surface at 150



C for 4–5 h in 1–1.6 atm of

Figure 7.13 STM image of Pt(1 0 0) surface at a condition of 1 Torr CO.

202

7 Surface Mobility of Atoms and Molecules Studied with High-Pressure STM

reactant gases (H

, or CO) [13, 23]. After exposing a clean Pt(1 1 0) surface to

1.6 atm H

and annealing at 150



C for 5 h in this hydrogen environment, the

surface is dominated by parallel rows that are the formed (1 n) missing rows

randomly nested (Figure 7.16a). This clearly demonstrates reconstruction of a

catalyst surface induced by a reactant gas at high temperature and high pressure.

Compared to the restructuring driven by hydrogen, oxygen induces the faceting of

the Pt(1 1 0) surface. After heating in 1 atm O

for 5 h, this catalyst surface consists

of microfaceted (1 1 1) with a width of 30–60 Å separated by steps (Figure 7.16b).

The reconstruction of Pt(1 1 0) into numerous microfaceted Pt(1 1 1) surfaces

could be driven by selective adsorption of oxygen atom on face-centered cubic

threefold hollow sites [24]. Thus, the transformation from Pt(1 1 0) to small Pt(1 1 1)

surfaces is deﬁnitely energetically favorable. When a clean Pt(1 1 0) surface is

Figure 7.14 STM images of a lifted Pt(1 0 0)

surface at condition of 5 10

7

Torr CO:

(a) t ¼t

min, (b) t ¼t

þ 10 min, (c) t ¼t

19 min, and (d) t ¼t

þ 28 min. All the images

have the same size of 14.1 nm 14.1 nm. They

were obtained with the same tunneling

condition and scanning speed. Landmarks 1, 2,

and 3 represent the same cluster between

(a) and (b), between (b) and (c), and between

7.4 Mobility and Flexibility of Catalyst Surfaces

203