Study of reactions of silver and sulfur clusters

Article abstract of DOI:10.1021/jp984560k

The reactions between silver and sulfur clusters are studied with a laser double ablation reactor. The main reaction products are [AgS₄]⁺, [AgS₈]⁺, [AgS₁₂]⁺, and [AgS₁₆]⁺, which have different compositions from those of Ag/S clusters produced from Ag/S mixed sample by laser ablation. The photolysis of the products with a 193 nm excimer laser indicates that [AgS₁₂]⁺ and [AgS₁₆]⁺ have a photodissociation efficiency much higher than that of [AgS₄]⁺ and [AgS₈]⁺. The structure of the products and the mechanism of the reactions to form products [AgS₁₂]⁺ and [AgS₁₆]⁺ are discussed.

Full text of DOI:10.1021/jp984560k

J. Phys. Chem. B 1999, 103, 3337-3339
3337
Study of Reactions of Silver and Sulfur Clusters
Peng Liu, Chunying Han, Zhen Gao,* Fanao Kong, and Qihe Zhu
State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100080, People’s Republic of China
ReceiVed: NoVember 30, 1998; In Final Form: February 15, 1999
The reactions between silver and sulfur clusters are studied with a laser double ablation reactor. The main
reaction products are [AgS4]+, [AgS8]+, [AgS12]+, and [AgS16]+, which have different compositions from
those of Ag/S clusters produced from Ag/S mixed sample by laser ablation. The photolysis of the products
with a 193 nm excimer laser indicates that [AgS12]+ and [AgS16]+ have a photodissociation efficiency much
higher than that of [AgS4]+ and [AgS8]+. The structure of the products and the mechanism of the reactions
to form products [AgS12]+ and [AgS16]+ are discussed.
Introduction
transition of the composition from the gas phase to the
condensed phase.
The compounds composed of silver and sulfur elements exist
in nature with the composition of Ag2S and three polymorphs:
monoclinic at room temperature, body-centered cubic (bcc) at
176 °C, and face-centered cubic (fcc) at 576 °C.1 These kinds
of compounds have important applications in optics and
electronics.2-6 It has been known that the compounds can be
used to prepare superionic conductive materials because of their
attractive ionic conduction properties.2 It was also reported that
by the doping of Ag2S in CdS on Pt electrodes the efficiency
of splitting H2S into H2 and S could be much improved.3 Sulfur
sensitization is one of the most important stages for the
formation of latent images composed of Ag in the photographic
emulsion-making process,4 and it is attributed to the Ag/S cluster
sensitization center. Applications of Ag2S compound as pho-
tovoltaic cells and infrared detectors have also been reported.5,6
Another kind of reaction is a chemical reaction between
clusters or between clusters and atoms or molecules,8,9,17,18
which have been performed in the gas phase by Fourier-
transform ion-cyclotron resonance (FTICR) spectrometer19 and
a flow tube reactor (FTR).20 The reactions of all the transition
metal ions, [M+], with sulfur have been reported,8 and the study
with Ag+ was highlighted.9 The reaction between silver ion Ag+
and S8 studied in a FTICR gave the products [AgS4]+, [AgS8]+,
and [AgS16]+. The compositions of these products are totally
different from that of condensed phase compounds or that of
Ag/S clusters produced from a Ag/S mixed sample,11 without
composition of a Ag2S unit. Herein, we report on a study of
the reactions between silver and sulfur clusters by a new
apparatus called a laser double ablation reactor (LDAR). LDAR
has a simple construction with only two solid samples. Thus
the clusters are produced by laser ablation of the two samples,
forming ions, and the ions collide to react with each other. The
experiments show that the reaction products obtained by LDAR
are similar to that obtained by FTICR,8,9 indicating that this
new technique is very effective for study of the reactions
between clusters. In this paper, we also present the study of the
photodissociation of the reaction products.
-
Molecular sulfur can be reduced to S2 by most metals in
condensed phase chemistry,7 although sulfur does not react with
metal ions in solution.8,9 However, the reactions in the con-
densed phase are influenced strongly by the environment;
therefore, the reactions in the gas phase are preferred to better
understand the chemical reaction mechanisms.
The reactions of clusters were performed in the gas phase
using various experimental conditions.8-18 With a high-intensity
focused laser beam, a solid sample can be vaporized. The
vaporized species collide with each other, emit with speed of
sound, and cool in the gas phase to form cluster ions by ion-
molecule reactions or neutral clusters by atom-molecule
reactions.10-15 This kind of reaction is a clustering reaction. We
have studied clustering reactions of silver and sulfur elements
by laser ablation of a sample made of silver and sulfur.10 The
mass spectrum indicated that the binary Ag/S clusters were
related by an incremental unit of Ag2S. The most abundant
cluster series in the mass spectrum had the composition of [Ag-
(Ag2S)n]+ (n ) 1, 2, 3, ...). It should be noted that the
composition of Ag/S clusters with the unit, Ag2S, was coincident
with that of silver sulfide compounds in nature. The mass
spectrum also indicated that the regularity of composition for
small clusters was not marked, and when the clusters became
larger, it appeared prominently. This feature might display the
Experiment
Figure 1 shows the construction of an LDAR, which is
attached to the homemade tandem time-of-flight mass spec-
trometer (TOF MS)21 for detection. The details of the LDAR
were reported in our previous work.22 Herein, only a brief
description is presented.
As shown in Figure 1, the LDAR contains two solid samples,
sample 1 and sample 2, separated from each other. In the present
experiments, sample 1 is made of silver and sample 2 is made
of sulfur. The two samples are shaped as disks (thickness is 5
mm, radius is 5 mm), and a hole is drilled in sample 2. The
double frequency beam of a Nd:YAG laser (532 nm) is directed
through the hole of sample 2 so as to be normal to the surface
of sample 1. The laser beam diameter is larger than the hole in
sample 2 so that the central part of laser beam ablates the surface
of sample 1 through the hole of sample 2 and the outer part of
laser beam ablates the surface of sample 2. Because the energy
of the central part of the laser beam is stronger than that of the
* To whom correspondence should be addressed.
10.1021/jp984560k CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/08/1999

3338 J. Phys. Chem. B, Vol. 103, No. 17, 1999
Liu et al.
Figure 1. Laser double ablation reactor.
Figure 2. Mass spectrum of positive ions produced from a silver
sample and a sulfur sample separated by 6 mm.
outer part, we used a sulfur sample with lower melting and
boiling points than silver. Neutral atoms, molecules, and ions
were simultaneously emitted with the speed of sound from each
sample using ablation with a laser beam, and clustering had
occurred. The clusters produced from sample 1 were passed
through the hole of sample 2 and reacted with the clusters
produced from sample 2.
The independent clustering of the emitted species from each
sample is very important to study the reactions between clusters
produced from samples 1 and 2, and it should be completed
prior to the reactions between clusters. This condition is called
clustering first and reacting second (CFRS).22 To satisfy the
condition of CFRS, the value of the distance between samples
1 and 2 plays an important role. If the distance is too short,
then the emitted species from the two samples meet first, and
clustering occurs. This process is similar to that produced with
one sample that contains all elements in samples 1 and 2 without
reaction between clusters. On the other hand, the distance
between samples 1 and 2 cannot be too long. If it is, then the
amount of the reaction products greatly decreases and detection
might be difficult. The experiments indicate that with a laser
fluence of 100 mJ/cm2 and a 1 mm diameter hole in sample 2,
the distances in the range of 5-10 mm between samples 1 and
2 are effective. In the present experiments, the distance between
sample 1 (silver) and sample 2 (sulfur) was set to 6 mm. The
laser beam was focused with a lens (f ) 545 mm), and the
energy of laser beam on the surface of sample 1 was about 108
W/cm2. The reaction products were detected by the first stage
of the tandem TOF MS.22 In the second stage of the tandem
TOF MS,22 photodissociation of the reaction products was
performed with a 193 nm laser beam. The ion detections were
all performed with dual microchannel plate detectors, and the
signals (mass spectrum) were recorded with a transient recorder
(10 MHz bandwidth). Typically, the signals with 1000-10 000
laser shots were accumulated and stored in an IBM-PC
computer.
Figure 3. Mass spectrum of positive ions produced from a mixed
sample of silver/sulfur (AgS ) 1:1).
having ions of greater intensity than the latter. This result shows
that the Ag/S cluster positive ions grow by the incremental unit
of Ag2S.
Willett et al. studied the reactions between Ag+ and S8 by
FTICR8,9 and obtained the following results:
Ag+(g) + S8(g) f [AgS4]+(g) + S4(g)
[AgS4]+(g) + S8(g) f [AgS8]+(g) + S4(g)
[AgS8]+(g) + S8(g) f [AgS16]+(g)
It is clear that the reaction products shown above are similar to
those obtained with LDAR except for the reaction product
[AgS12]+.
These results indicate that the products from LDAR are
different from those produced from a Ag/S mixed sample. The
former is produced by reactions between silver and sulfur
clusters, while the latter is produced by a clustering process of
silver and sulfur elements vaporized by laser ablation.
The reaction products [AgSn]+ (n ) 4, 8, 12, 16) were
photodissociated with a 193 nm laser. The photofragments are
listed in Table 1. It is known from Table 1 that for [AgS4]+ no
conspicuous fragment is detected except for a small Ag+, while
the [AgS8]+ is photodissociated to Ag+, [AgS2]+, and [AgS4]+.
[AgS12]+ and [AgS16]+ have photofragments mainly as [AgS4]+
and [AgS8]+, respectively. Table 1 also shows that the photo-
dissociation efficiency of [AgS12]+ and [AgS16]+ is much higher
than that of [AgS4]+ and [AgS8]+.
Results and Discussion
The mass spectrum of the reaction products is shown in Figure
2. The mass peaks presented in the mass spectrum mainly have
the composition of [AgSn]+ (n ) 0-16). It is evident that the
mass peaks [AgS4]+, [AgS8]+, [AgS12]+, and [AgS16]+ are very
distinct as magic numbers, indicating that these products have
more stable structures than the others. For comparison, the mass
spectrum of positive cluster ions produced from one Ag/S mixed
sample by laser ablation is presented in Figure 3. The mass
spectrum mainly consists of two series, [Ag(Ag2S)n]+ (n ) 1,
2, 3, ...) and [Ag3(Ag2S)n]+ (n ) 1, 2, 3, ...), and the former

Study of Reactions of Silver and Sulfur Clusters
J. Phys. Chem. B, Vol. 103, No. 17, 1999 3339
TABLE 1: Photodissociation Percentagea of Reaction
IP/Ag ) 7.57 eV),26 so the positive sulfur cluster ions are
relatively less easy to generate.
Products [AgSn]+ (n ) 4, 8, 12, 16)
The study7 of sulfur in the condensed phase shows that sulfur
atoms can easily bond to each other to form molecules with
undetermined atom numbers. The structure of orthogonal sulfur
with 16 S8 in a unit cell is the most stable at room temperature
and atmosphere. The ring S8 molecule has a structure similar
to the skull structure with D4d symmetry. Gas phase sulfur exists
mainly in the forms of S6, S7, and S8, while S8 is the most
abundant at the temperature below 673 K. This fact supports
our assumption that [AgS12]+ and [AgS16]+ are produced
through the addition of S8 on [AgS4]+ and [AgS8]+, respectively.
In comparison, we can see that the reaction products and the
reaction mechanism by LDAR are different from those by
FTICR. The former is of the reactions between the emitted
species (atoms, molecules, and clusters) generated by laser
ablation. In this case, the product species is more various than
that generated by FTICR, and there also exist many possible
reaction channels for the product formation.
photofragments
products
Ag+
[AgS2]+
1.05
[AgS3]+
0.50
[AgS4]+
[AgS8]+
[AgS4]+
[AgS8]+
[AgS12]+
[AgS16]+
0.56
0.82
0.68
16.0
10.5
a Photodissociation percentage ) (intensity of fragments/sum of
intensity of fragments and product) × 100.
From the discussion as above, we can describe the process
of cluster reactions by LDAR as follows: By laser ablation,
silver and sulfur samples are vaporized, and then the clustering
occurs. Silver ions from the silver sample react with sulfur atoms
or clusters from sulfur sample to produce [AgS4]+ and [AgS8]+,
and then [AgS4]+ and [AgS8]+ react with S8 to form [AgS12]+
and [AgS16]+, respectively.
Figure 4. Mass spectrum of positive ions produced from a sample of
silver.
Acknowledgment. The authors thank Professor G. D. Willett
and Dr. K. J. Fisher of the University of New South Wales for
the discussion of the experimental results. This research is
funded by National Natural Science Foundation of China.
From the distribution of photofragments, we can observe that
[AgS4]+ and [AgS8]+ are the main photofragments of [AgS12]+
and [AgS16]+; i.e., they arise from the same dissociation channel
in which S8 is stripped from [AgS12]+ and [AgS16]+, respec-
tively. So, it might be suggested that in the structure of [AgS12]+
and [AgS16]+ there are weak combinations between S8 and
[AgS4]+ and [AgS8]+, respectively. For the photodissociation
of [AgS4]+ and [AgS8]+, there are few fragments (see Table
1). This means that there may not be such weak interactions in
their structure. In the condensed phase, the S-S bond strength
(4.40 eV) is stronger than a S-Ag bond (2.25 eV),23,24 but all
bond strength are lower than the energy of a 193 nm photon
(6.42 eV). We can suggest that in [AgS4]+ and [AgS8]+ the
silver atom is bonded to the above two sulfur atoms. Fisher et
al. studied the reaction between Ag+ and HSC6H5 in the gas
phase and calculated the geometry energy surface of the product
[AgHSC6H5]+.8,9 For the geometry-optimized structure of
[AgHSC6H5]+ with the lowest energy, the silver atom is bonded
to the sulfur atom. This kind of bond may appear in the structure
of [AgS12]+ and [AgS16]+. So, [AgS12]+ and [AgS16]+ have
different bonding characters from [AgS4]+ and [AgS8]+.
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+
Eyler et al.25 studied the reactions between Ag2 and
diethylaniline (DEAN), and between Ag2+ and dimethylaniline
+
(DMAN). The reaction products of AgLn (L ) DEAN,
+
DMAN) were obtained. This suggests that Ag+ and Ag2 can
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form of AgLn+. Figure 4 is the mass spectrum of the positive
ions produced from the silver sample by 532 nm laser ablation.
+
+
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.
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