Reduction of Pt(II) by H2: effects of citrate and NaOH and reaction mechanism

Article abstract of DOI:10.1021/jp000801o

Aqueous solutions of Pt(II) (aged PtCl₄^2- solutions) are reduced by hydrogen in the absence of a stabilizer and in the presence of stabilizing sodium citrate or/and sodium hydroxide. The rate of reduction is measured optically and conductometrically, and the colloidal nanoparticles, which are finally formed, are studied by high-resolution electron microscopy. In dilute solutions, the rate of reduction is proportional to the square of the Pt(II) concentration. In the presence of citrate and NaOH, the reduction occurs more slowly. A mechanism, in which intermediate free Pt atoms do not appear, is discussed. Monovalent platinum is postulated as the first intermediate, and an autocatalytic contribution to the reduction is observed. Particle growth in a chain reaction is also discussed. In the presence of NaOH, the reaction occurs in two steps: formation of a hydroxilated precursor and reduction of the latter. Depending on the concentration of the stabilizing additives, colloids consisting of polyhedra of different shape (cubic, tetrahedral, and triculent octahedral) are formed. Stabilization by citrate plus NaOH yields particularly long lifetimes of the colloidal solutions.

Full text of DOI:10.1021/jp000801o

J. Phys. Chem. B 2000, 104, 6767-6772
6767
Reduction of Pt(II) by H2: Effects of Citrate and NaOH and Reaction Mechanism
Arnim Henglein*
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556
Michael Giersig
Hahn-Meitner Institut, 14109 Berlin, Germany
ReceiVed: March 1, 2000; In Final Form: May 9, 2000
2-
Aqueous solutions of Pt(II) (aged PtCl4 solutions) are reduced by hydrogen in the absence of a stabilizer
and in the presence of stabilizing sodium citrate or/and sodium hydroxide. The rate of reduction is measured
optically and conductometrically, and the colloidal nanoparticles, which are finally formed, are studied by
high-resolution electron microscopy. In dilute solutions, the rate of reduction is proportional to the square of
the Pt(II) concentration. In the presence of citrate and NaOH, the reduction occurs more slowly. A mechanism,
in which intermediate free Pt atoms do not appear, is discussed. Monovalent platinum is postulated as the
first intermediate, and an autocatalytic contribution to the reduction is observed. Particle growth in a chain
reaction is also discussed. In the presence of NaOH, the reaction occurs in two steps: formation of a
hydroxilated precursor and reduction of the latter. Depending on the concentration of the stabilizing additives,
colloids consisting of polyhedra of different shape (cubic, tetrahedral, and triculent octahedral) are formed.
Stabilization by citrate plus NaOH yields particularly long lifetimes of the colloidal solutions.
Introduction
Experimental Section
Colloidal platinum in aqueous solution is readily prepared
by chemical reduction of Pt(II) and Pt(IV) compounds. Rampino
and Nord reported in 1941 that platinum hydroxide, which is
As has previously been described, the reduction of a freshly
prepared PtCl4 solution occurs much more slowly than that
2-
of an aged solution.2 A 10-2 M K2PtCl4 (99.99%, Aldrich) stock
solution was prepared and aged for at least 2 days. This solution
was then diluted to obtain the desired concentration. The
reduction occurred in a glass vessel, which had sidearms
carrying an optical cuvette (optical path given on the ordinate
axis of the spectra) and a homemade conductivity cell (glassy
carbon electrodes; 3-mm diameter; 5-mm distance; cell constant
of 1.23 cm-1). Absorption and conductivity measurements could
therefore be made without bringing the solution into contact
with air. The vessel was also equipped with a septum; solution
could therefore be removed or substances added via a syringe.
The solution was first bubbled with argon and then very
vigorously flushed with hydrogen for 90 s. The vessel then was
closed and, immediately afterward, the first spectrum or
conductivity measurement was recorded. Depending on the
composition of the solution, the formation of the colloid lasted
an hour or up to several days. During this time, the vessel was
slightly shaken to secure saturation of the solution with
hydrogen. The volume of the vessel was 150 mL, and the
volume of the solution 20-50 mL. Considering the much higher
H2 concentration in the gas than in the liquid phase, there was
always a vast excess of hydrogen in the reacting system.
The samples were observed in a high-resolution transmision
electron microscope, Phillips 12, operating at 120 kV, as
described previously.7
2-
made by boiling a PtCl4 solution in the presence of the
required amount of NaOH, is reduced by hydrogen gas to yield
an efficient hydrogenation catalyst.1 The reduction by hydrogen
was more recently applied again, using an aged PtCl42- solution
that contained mainly PtCl2(H2O)2: in the presence of poly-
acrylate or polyphosphate, nicely facetted 5-10 nm nanopar-
ticles were obtained.2,3 The reduction of PtCl6 by citrate in
2-
boiling water was introduced by Turkevich4 and has, ever since,
been applied in many laboratories. This method yields small
particles of about 3 nm in size, which exhibit many catalytic
effects. However, it has more recently been recognized that these
particles contain a certain contribution of a higher valence state
of platinum. This manifests itself in the destabilization they
undergo upon exposure to hydrogen gas and the formation of
colored Chini complexes upon the exposure to CO.5 Studies
on the interaction of these particles with reducing free radicals
have also shown that some kind of higher oxidation state is
present.6
In the present work, some features of the H2 reduction of
Pt(II) are investigated, and the nature of the nanoparticles formed
is studied by electron microscopy. It was found that the reduction
by H2 at ambient temperature in the presence of sodium citrate
yields Pt sols of particularly long lifetime. Citrate is not the
reducing agent in these experiments; it acts solely as a stabilizer.
A similar study on the capping action of citrate has recently
been performed in the case of silver reduction.7 In addition,
the effect of NaOH is studied. NaOH causes changes in the
structure of the starting Pt(II) material and also decreases the
rate of reduction.
Results
Kinetic Measurements. An aged solution containing Pt(II)
mainly as PtCl2(H2O)2 has a UV absorption that increases
toward shorter wavelengths,2 with an absorption coefficient at
200 nm of 1.24 × 104 M-1cm-1. Colloidal platinum exhibits
* Author to whom correspondence should be addressed.
10.1021/jp000801o CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/29/2000

6768 J. Phys. Chem. B, Vol. 104, No. 29, 2000
Henglein and Giersig
Figure 2. Semilog plot of the relative concentration of Pt(II) vs time
as derived from conductivity measurements (O). The initial concentra-
tion was 2.0 × 10-4 M. The dotted second-order curve (b) was
calculated from the early part (40% conversion) of the experimental
curve. No stabilizer was added.
Figure 1. 200-nm absorption of Pt(II) (aged PtCl42-) as a function of
time at various initial Pt(II) concentrations. The quantity “Pt(II)
concentration times optical path length” was kept constant. The
absorbance at the completion of the reduction was 0.45. No stabilizer
was added.
PtCl2(H2O)2 + H2 f Pt + 2 Cl- + 2 H+ + 2 H2O (1)
The relative Pt(II) concentration is equal to (C∞ - Ct)/C∞
-
less absorption at this wavelength. Thus, the course of the
reaction can be monitored by recording the decrease in the 200-
nm absorption as a function of time. In Figure 1, several initial
concentrations of Pt(II) were used; the solutions did not contain
a stabilizer. All solutions had the same initial absorbance of
1.24, as optical cells of different path length were used, so that
the quantity “initial concentration times path length” was kept
constant. Thus, the curves represent the relative Pt(II) contration
as a function of time. If the rate of reduction were proportional
to the Pt(II) concentration, then all points would lie on a single
curve. However, it can be seen that the negative slope of the
curves becomes greater with increasing Pt(II) concentration. At
the lower concentrations shown in the figure, the rate is
proportional to the square of the concentration; this indicates a
complex mechanism of reduction. At the higher concentrations,
the order with respect to the Pt(II) concentration is even higher.
Co), where Co and C∞ are the respective conductivities at time
t ) 0 and at the end of reaction. The conductivity measurements
corroborated the above conclusions about the order of the
reaction rate with respect to the initial Pt(II) concentration. The
conductivity curves yielded more detailed information, as shown
in Figure 2. At about 40% conversion, the negative slope of
the curve becomes noticeably larger, which indicates an increase
in the specific rate. Toward the end of reaction, the slope finally
decreases continuously. Assuming that the reaction rate is
proportional to the square of the Pt(II) concentration (on the
basis of the optical measurements in Figure 1), the pointed curve
in Figure 2 was calculated by extending the experimental curve
at 40% conversion. The faster decrease of the experimental curve
is attributed to an autocatalytic contribution to the reaction rate,
similar to the autocatalytic effect that has been reported for the
reduction of an iridium(I) complex by H2.8 It has to be pointed
out that the presentation in Figure 2 does not quantitatiVely
describe the autocatalytic contribution, as the choice of the point
of extension of the experimental curve at 40% is rather
ambiguous.
Moreover, the various curves are almost straight lines over a
wide range of concentrations without a significant induction
period. With increasing time, the Pt(II) concentration decreases,
and the curves should therefore have a smaller slope (because
of the above-mentioned higher order), which is not the case.
This indicates an increase in specific rate with time along a
curve, i.e., an autocatalytic contribution to the reduction. No
precipitation of the colloid formed took place during the
measurements. During aging of the colloids for several days, a
black precipitate appeared, which could initially be repeptised,
but which became insoluble at longer aging times.
In Figure 3, the 200-nm absorption of Pt(II) in a 2.0 × 10-4
M solution was measured as a function of time in the absence
and presence of 4.0 × 10-4 M sodium citrate. A strong retarding
effect of citrate can be recognized. The absorption spectrum of
PtCl2(H2O)2 is not changed upon the addition of sodium citrate
to the solution. It is therefore concluded that PtCl2(H2O)2 is
still the reaction partner of H2. The colloidal particles formed
in the presence of citrate are stable for weeks and months. In
our former study, polyacrylate was found to be a good
stabilizer;2 however, a comparison revealed that citrate exerts
a stronger protecting effect.
In the experiments of Figures 4 and 5, the effect of NaOH
was investigated. When NaOH is added to a Pt(II) solution,
the spectrum of PtCl2(H2O)2 is changed: an absorption band at
214 nm grows in with increasing NaOH concentration until the
molar Pt(II)/NaOH ratio is 1:1 (ꢀ214 ) 9.4 × 103 M-1cm-1).
An additional increase in NaOH concentration does not lead to
a further increase in the intensity of this band. It is concluded
that a well-defined species, such as PtCl(OH)(H2O)2 or
The change in the 200-nm absorbance is equal to the decrease
in the Pt(II) absorption plus an increase due to the absorption
of the final product. If the absorption spectrum of the product
is the same at all times, then the change in the 200-nm
absorbance is truly a measure of the rate of Pt(II) consumption.
In fact, the absorption of colloidal platinum in the visible region
developed at about the same time as the Pt(II) absorption
disappeared. A more reliable method for measuring the reaction
rate, which does not depend on the nature of the product,
consists of recording the conductivity of the solution as a
function of time. Hydrogen ions and chloride anions are formed
during the reduction

Reduction of Pt(II) by H2
J. Phys. Chem. B, Vol. 104, No. 29, 2000 6769
Figure 5. Absorption spectrum of a solution containing 5.0 × 10-4
M aged PtCl4 and 1.0 × 10-3 NaOH at different times of exposure
2-
to hydrogen. The full UV-vis spectrum of the final colloid is shown in
the inset.
Figure 3. Concentration vs time curves for the reduction of 2.0 ×
10-4 M PtCl2(H2O)2 in the absence and presence of 4.0 × 10-4
M
citrate. The final absorption spectra were identical with that in the inset
of Figure 5.
Figure 4. Absorption spectra. (a) Aged 5.0 × 10-4 M PtCl42- solution
Figure 6. Absorbancies at 214 nm [initially present Pt(II) complex]
and at 280 nm (final platinum colloid) as functions of time. The solution
initially contained 5.0 × 10-4 M Pt(II) and 5.0 × 10-4 M NaOH.
[containing mainly PtCl2(H2O)2]. (b) After addition of 1.0 × 10-3
M
NaOH to solution in a. (c) After aging of solution in b for 30 h.
PtCl2(OH)(H2O)-, is formed upon the addition of NaOH, which
we call the first product of hydrolysis. This product, however,
is not stable upon further addition of NaOH: the 214-nm band
decreases within 1-2 days, the final product of hydrolysis
showing a nonstructured increase in absorption toward the
shorter UV wavelengths. This can be seen from Figure 4. The
final hydrolysis product probably is platinum hydroxide.
In Figure 5, the absorption spectrum of a 5.0 × 10-4 M Pt-
(II) solution containing 1.0 × 0-3 M NaOH is shown at various
times of exposure to H2. A comparison with the reduction of
5.0 × 10-4 M Pt(II) in the absence of NaOH in Figure 1 shows
that NaOH drastically decreases the rate of reaction. In Figure
6, the absorbancies at 214 nm [Pt(II) species] and at 280 nm
(Pt nanoparticles) are plotted as functions of the reaction time.
Although there is a decrease in the 214-nm absorption of the
Pt(II) species right at the beginning, the 280-nm absorption of
nanoparticles does not start before ca. 40 h. Under the conditions
of Figure 5, the reaction obviously occurs in two steps: (1)
transformation of the first hydrolysis product (which is not
reduced by H2) into a second hydrolysis product, as described
in Figure 4, and (2) reduction of the latter to yield colloidal
platinum. The final colloid has an aborption spectrum with a
UV maximum. This spectrum agrees with the calculated one,9
as has previously been discussed in detail2 (inset of Figure 5).
Upon aging of the colloidal solution formed, the spectrum
becomes flatter as the particles aggregate. This effect can be
largely avoided by subsequent addition of 2.0 × 10-4
M
stabilizer (either citrate or polyacrylate) to the solution.
In the experiment of Figure 7, the processes of precursor
formation and reduction of the precursor are separated. A 5.0
× 10-4 M Pt(II) plus 1.0 × 10-3 M NaOH solution was aged
under argon for 2 days. Hydrogen was then added, and the
absorption spectrum determined at various times afterward. One
can see that the formation of the colloid in the aged solution
requires about 1 h. The rate of reduction still is much slower
than that of reduction of 5.0 × 10-4 M Pt(II) in the absence of
NaOH (see Figure 1).
The effects of citrate and NaOH were also combined by
reducing aged Pt(II) solutions that contained both additives. It
turned out that platinum sols of especially long lifetime, which
contain nicely shaped nanoparticles with a rather narrow size
distribution, are formed under these conditions.
Electron Microscopy. The particles are polyhedra, similar
to those that are formed in the presence of polyacrylate as a
stabilizer.2,3 Typical examples are shown in Figure 8. In the
absence of citrate (Figure 8, left), relatively large particles with

6770 J. Phys. Chem. B, Vol. 104, No. 29, 2000
Henglein and Giersig
Discussion
Considerations of the Mechanism of Reduction. If the first
step of the reduction consisted of the interaction of H2 with
one molecule of the Pt(II) compound, as indicated by eq 1, then
a free platinum atom would be formed. However, one calculates
that the reaction is highly endoergic, with ∆G ) 3.4 eV, using
the known free enthalpies of formation of the species involved
in eq 1.10 The reason for this endoergicity is the high free
enthalpy of formation of 5.4 eV of the free Pt atom. It is
therefore concluded that free Pt atoms do not appear as
intermediates in colloid formation. Another reaction route could
lead to PtH as the first reduction product.
PtCl2(H2O)2 + 1.5H2 f PtH + 2Cl- + 2H+ + 2H2O (2)
Figure 7. Absorption spectrum at various times after exposure to H2.
The solution contained 5.0 × 10-4 M Pt(II) and 1.0 × 10-3 M NaOH;
it was aged under argon for 2 days before the exposure to hydrogen.
Using a Pt-H bond energy of 3.47 eV,11 one calculates
∆G)2.03 eV; we again exclude this reaction path because of
the high endoergicity involved.
a narrow size distribution are present. Closer inspection shows
that they consist of two populations, i.e., cubes and truncated
octahedra, the latter being a bit larger. Tetrahedra are practically
absent. In the presence of citrate, the particles are smaller, and
tetrahedra appear more frequently with increasing citrate
concentration. The explanation for this phenomenon has already
been given for polyacrylate stabilized particles:3 tetrahydra are
formed first, which then grow into cubes or truncated octahedra,
unless their growth is hampered by adsorbed stabilizer. Note
that a very small citrate concentration (6 × 10-5 M in Figure
8, middle) exerts a significant effect on the particle properties,
a further increase in citrate concentration by almost one order
of magnitude (5 × 10-4 M in Figure 8, right) having little
additional effect. It thus seems that the coverage of the particles
by citrate is nearly complete already at the lower concentration.
No significant differences in the size or shape of the particles
were observed, when they were formed in the presence or
absence of NaOH. The main effect of NaOH consists in a better
stabilization of the particles. This might be due to a lesser extent
of protonation of citrate in the presence of NaOH, the citrate
thus acting as a better stabilizer.
The second-order behavior of the reaction (Figure 1) entices
one to postulate the involvement of two molecules of PtCl2-
(H2O)2 in the first reduction event.
H2 + PtCl2(H2O)2 a [H2-PtCl2(H2O)2]
(3a)
PtCl2(H2O)2 + [H2-PtCl2(H2O)2] f
2Pt+ + 4Cl- + 4H2O + 2H+ (3b)
H2 + 2PtCl2(H2O)2 f 2Pt+ + 4Cl- +
4H2O + 2H+ (4)
Colloidal particles are then formed via dismutation and coa-
lescence reactions of Pt+. Nothing seems to be known about
the thermodynamic and electrochemical properties of the
monovalent state of platinum. The free enthalpy of formation
of Pt+ can be calculated as follows, by making a resonable
aq
guess of the free enthalpy of its hydration: Platinum appears
before gold in the periodic table, and its molar volume is 11%
smaller than that of gold. We thus conclude that Pt+ is smaller
It was observed that the particles were larger and had a more
uniform distribution in size and shape when they were formed
at a higher initial Pt(II) concentration. This can be seen from
Figure 9, in which Pt(II) was reduced at two concentrations,
the citrate concentration being 2.0 × 10-4 M in both cases.
than Au+, and that its free enthalpy of hydration, ∆Ghydr
,
therefore is more negative than that of Au+. The free hydration
enthalpy of the latter is calculated as -6.3 eV from known
thermodynamic data;12 thus, it does not seem unreasonable to
Figure 8. Electron micrographs of Pt particles formed at different concentrations of citrate, 5.0 × 10-4 M Pt(II), and 7.0 × 10-4 M NaOH.

Reduction of Pt(II) by H2
J. Phys. Chem. B, Vol. 104, No. 29, 2000 6771
Figure 9. Electron micrographs of particles formed at different initial Pt(II) concentrations and 2.0 × 10-4 M citrate.
assume a value of -7.0 eV for the free enthalpy of hydration
of Pt+. The combination of this value with known thermody-
namic data allows one to calculate the free enthalpy of the redox
system
x - 2) chemisorbed H atoms. On the other hand, monovalent
platinum may be formed
Ptn-H + PtCl2(H2O)2 f Ptn+1+ + H+ + 2Cl- + 2H2O (9)
Ptm a Pt+aq + e-
(5)
or
Ptn-H + PtCl2(H2O)2 f Ptn + H+ + Pt+ + 2Cl- + 2H2O
on the standard electrochemical scale, eEo ) ∆Gfo(Pt+g) - Wh
+ ∆Ghydr(Pt+) (g, gas phase; aq, solution; m, bulk metal; Wh )
4.5 eV, work function of the standard hydrogen electrode). With
∆Gfo(Pt+g) ) 14.3 eV10 and ∆Ghydr(Pt+) ) -7.0 eV, the free
enthalpy of the equilibrium in eq 5 is +2.8 eV. With this value,
one calculates that the overall reaction 4 has ∆G ) 1.0 eV under
standard conditions and is practically thermoneutral at pH ) 7
and dilute solution (10-3 M). It thus seems plausible, although
not proven by direct experiment, that the colloid is formed
through the monovalent state of platinum. Moreover, it is
(10)
Reactions 8 and 9 would lead to an increase in particle size,
whereas reaction 10 would lead to the formation of new nuclei.
It is also conceivable (on the basis of the high positive potential
of the redox system in eq 5) that monovalent platinum is reduced
by hydrogen.
Ptn+1+ + H2 f H-Ptn+1 + H+
(11)
2+
conceivable that Pt2 is directly formed in reaction 4, i.e., a
Note that reactions 9 and 11 constitute the growth steps of a
chain mechanism for the autocatalytic reduction of Pt(II). This
case is possibly fulfilled at high PtCl2(H2O)2 concentrations,
for which the order of reaction becomes higher (Figure 1); the
growth of the nuclei would become faster, which would explain
why larger particles are formed at the higher Pt(II) concentration
in Figure 9.
species analogous to Pd22+, which was observed in the radiolytic
reduction of the homologous ion Pd(II).13 Dismutation and
coalescence reactions of Pt+ (or Pt22+) may finally lead to
metallic nanoparticles.
We can finally examine a reaction path, in which Pt2 appears
as the first product of reduction.
Effect of NaOH. The most startling effect of NaOH consists
2PtCl2(H2O)2 + 2H2 f Pt2 + 4Cl- + 4H+ + 4H2O (6)
of the decrease in the rate of reaction by at least an order of
1
magnitude. For example, whereas it takes about
/ h for
4
Using a bond strength of 3.70 eV for Pt2,11 one obtains ∆G )
3.1 eV, i.e., a high endoergicity. Thus, Pt+ formation, as
discussed above, seems to be the reaction route with the lowest
energy barrier.
The atoms of some other precious metals, such as iridium,
have large free enthalpies of formation, too. It seems probable
that reduction by H2 also does not proceed through intermediate
free atoms, as has been postulated.8
The autocatalytic behavior of the reduction of Pt(II) can be
understood in the facilitation of reaction 2 on the surface of the
colloidal particles, Ptn, which are formed in the early stages of
reaction
complete reduction of a 5.0 × 10-4 M Pt(II) solution, about 60
h are required when 1.0 × 10-3 M NaOH is present. In both
cases, the final particles have about the same size (∼12 nm).
However, they have a more narrow size distribution in the
presence of NaOH, and the high content of cubic structures is
of interest from a preparative point of view. The stability of
the particles is also different in the two cases. The particles are
more strongly agglomerated when formed in the absence of
NaOH, the solution having a dark color; in the presence of
NaOH, the solution color is yellow-brown, and the spectrum
agrees with the theoretical spectrum of well-separated particles
(inset of Figure 5).
As has already mentioned by Rapino and Nord,1 platinum
hydroxide is formed in the reaction of NaOH with PtCl42-. Our
findings indicate that this reaction occurs at ambient temperature
in two steps. The first product is formed practically instanta-
neously. It is, however, nonreactive toward hydrogen. As
mentioned above, we tentatively attribute it to the structure
PtCl(OH)(H2O)2-; its charge possibly prevents it from forming
Ptn + x/2 H2 a Ptn-xH
Ptn-xH + PtCl2(H2O)2 f
(7)
Ptn+1-(x - 2)H + 2H+ + 2Cl- + 2H2O (8)
where Ptn-xH [or Ptn-(x - 2)H] are particles carrying x (or

6772 J. Phys. Chem. B, Vol. 104, No. 29, 2000
Henglein and Giersig
the dimeric complex of eq 3a. The conversion to the final
hydroxide, possibly being Pt(OH)2(H2O)2, requires almost 2
days. This hydroxide is believed to be reduced by H2, a process
still much slower than the reduction non-hydroxilated Pt(II).
Effect of Citrate. Citrate is a good stabilizer of the colloidal
particles produced in the H2 reduction of Pt(II). The reduction
occurs significantly more slowly in the presence of this
stabilizer. Because the structure of the Pt(II) compound present
is not changed by citrate, the decrease in the reaction rate cannot
be explained by a change in the structure of the Pt(II) compound.
With increasing citrate concentration, the size of the particles
decreases, i.e., more particles are formed. This is understood in
terms of adsorbed citrate retarding the growth of the particles.
A similar effect of polyacrylate has previously been found.3
From a practical point of view, citrate has the advantage of
stabilizing the particles more efficiently than polyacrylate,
especially at higher Pt(II) concentrations.
electron microscope measurements. The work described herein
was supported by the Office of Basic Energy Sciences of the
U.S. Department of Energy. This is Contribution No. 4205
NDRL from the Notre Dame Radiation Laboratory.
References and Notes
(1) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 2745.
(2) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99,
14129.
(3) (a) Ahmadi, T. S.; Wang, L.; Henglein, A.; El-Sayed, M. A. Chem.
Mater. 1996, 8, 1161. (b) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.;
Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (c) Petroski, J.
M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998,
102, 3316.
(4) Aika, K.; Ban, L. L.; Okura, I.; Namba, S.; Turkevich, J. J. Res.
Inst. Catal., Hokkaido UniV. 1976, 24, 45.
(5) Henglein, F. A. J. Phys. Chem. B 1997, 101, 5889.
(6) Henglein, A.; Lindig, B.; Westerhausen, J. Radiat. Phys. Chem.
1984, 23, 199.
Final Remarks
(7) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999, 103, 9533.
(8) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382.
2-
The reduction of aged PtCl4 by hydrogen in the presence
of citrate is a useful method for the preparation of colloidal
platinum. The particles formed do not have the shortcomings
mentioned above when they are formed via the reduction by
citrate. The reduction by H2 is a complex phenomenon. In the
present work, some ideas about this complexity are expressed.
(9) Creighton; J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991,
87, 881.
(10) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous
Solution; Marcel Dekker: New York, 1985.
(11) Handbook of Chemistry and Physics, 75th ed.; CRC Press: London,
1995.
2-
It may be mentioned that PdCl4 can also be reduced by
(12) The solvation free enthalpy of Au+ is calculated as ∆Ghydr(Au+)
hydrogen to yield colloidal particles, and the mechanism seems
to be quite similar. The particles prepared in the present work
are also used to synthesize bi- and trimetallic structures in
combination with other metals and to investigate fast electronic
processes in femtosecond laser experiments, as we have recently
reported for other systems.14
) eE°(Au+/Aum) - ∆Gfo(Auo ) - IE(Auo) + Wh. With eE° ) 1.83 eV,
∆Gfo(Auo ) ) 3.38 eV, Wh ) g4.5 eV, and IE(Auo) ) 9.23 eV,10 ∆Ghydr
-
g
(Au+) ) 6.3 eV. (E°, standard redox potential; IE, energy of ionization).
The hydration free energies of Pto and Auo, which are expected to be very
small, are disregarded in our calculations.
(13) Michaelis, M.; Henglein, A. J. Phys. Chem. 1992, 96, 4719.
(14) (a) Hodak, J. H.; Henglein, A.; Hartland, G. V. J. Chem. Phys.
1999, 111, 8613. (b) Hodak, J.; Henglein, A.; Hartland, G. V. J. Chem.
Phys. 2000, 112, 5942. (c) Henglein, A. J. Phys. Chem. B 2000, 104, 2201.
(d) Hodak, J. H.; Henglein, A.; Hartland, G. V. J. Phys. Chem. B, in press.
Acknowledgment. The authors thank Prof. D. Meisel for
valuable discussions and Mrs. U. Bloek for assistance in the