Hydrogen-bonded CdS nanoparticle assemblies on electrodes for photoelectrochemical applications

Article abstract of DOI:10.1002/anie.200463055

(Chemical Equation Presented) Trapping the light fantastic-ally! Enhanced photocurrents are observed for hydrogen-bonded gold nanoparticle/CdS nanoparticle arrays associated with a gold electrode surface (see scheme) as a result of charge separation and c

Full text of DOI:10.1002/anie.200463055

Communications
Nanotechnology
structures.[10] However, the use of hydrogen bonds to assem-
ble nanoparticle structures has been less explored. The
aggregation of silver nanoparticles[11] or gold nanoparticles[12]
by interparticle hydrogen bonds of the capping monolayers
was reported. Also, modified gold nanoparticles that form
hydrogen-bonded host–guest complexes have been synthe-
sized[13] and applied to the development of an electrochemical
sensor for phosphate.[13b] To date, the use of hydrogen bonds
to fabricate layered nanoparticle structures on surfaces is
little explored, but the formation of a multilayer of gold
nanoparticles, modified by carboxylic acids, using polyvinyl-
pyridine as the cross-linker in a layer-by-layer deposition
process was examined.[14] However, the assembly of func-
tional hydrogen-bonded layered nanoparticle structures on
surfaces is unknown. Herein, we report the assembly of
cadmium sulfide nanoparticles and gold nanoparticle/cad-
mium sulfide nanoparticle hybrids on gold surfaces through
Hydrogen-Bonded CdS Nanoparticle Assemblies
on Electrodes for Photoelectrochemical
Applications**
Ronan Baron, Chih-Hao Huang, Dario M. Bassani,*
Avital Onopriyenko, Maya Zayats, and Itamar Willner*
The organization of functional semiconductor nanoparticles
on surfaces has been extensively examined in recent years,[1]
and the use of organized semiconductor nanoparticle systems
for photoelectrochemical applications has attracted specific
research efforts.[2] For example, core–shell nanoparticle
systems were reported to behave as organized matrices that
assist charge separation and enhance photocurrent generation
when deposited on electrodes.[3] Similarly, the electropolyme-
rization of CdS semiconductor nanoparticles in conductive
bis-aniline films proved to result in an organized system that
facilitated the charge separation of electron–hole pairs,
generated upon photoexcitation of the semiconductor nano-
particles, and the transport of conduction-band electrons to
the electrode and led to improved photocurrents.[4] The
deposition of layered semiconductor nanoparticles on surfa-
ces by using electrostatic interactions[5] or covalent bonds[6]
has been described. Likewise, the assembly of semiconductor
nanoparticles on electrode supports by using molecular[7] or
supramolecular[8] cross-linkers and hybrid layered metal
nanoparticle/semiconductor nanoparticle assemblies[9a] or
carbon nanotube/semiconductor nanoparticle systems[9b]
were studied. The relationship between structure and photo-
electrochemical function in these nanostructures was charac-
terized, and the assembly of these hybrid systems proved to be
an effective means to improve the generation of photo-
currents by assisting charge separation.
complementary
barbiturate–triaminodiazine
hydrogen-
bonded interactions and the application of the systems for
the generation of photocurrents.
The thioacetyl-protected barbiturate 1 and its comple-
mentary hydrogen-bonding aminopyrimidine partner
3
(Scheme 1) were used to modify the CdS or Au nanoparticles.
In other organized systems, hydrogen-bonded interactions
are an important motif in the generation of supramolecular
[*] C.-H. Huang, Dr. D. M. Bassani
Centre de Recherche en Chimie Molꢀculaire
LCOO CNRS UMR 5802
Universitꢀ Bordeaux 1
351, Cours de la Libꢀration, 33405 Talence cedex (France)
Fax: (+33)5-4000-6158
E-mail: d.bassani@lcoo.u-bordeaux1.fr
Scheme 1. Synthesis and structures of barbiturate compounds 1 and 2
as well as their triaminodiazine complements 3 and 4. n=5 for 1 and
2; n=6 for 3 and 4.
Dr. R. Baron, A. Onopriyenko, M. Zayats, Prof. I. Willner
Institute of Chemistry and
the Farkas Center for Light-Induced Processes
The Hebrew University of Jerusalem
Jerusalem 91904 (Israel)
Fax: (+972)-2-652-7715
E-mail: willnea@vms.huji.ac.il
The capped nanoparticles were then organized on surfaces by
using complementary hydrogen bonds as the driving force.
The CdS nanoparticles (4–5 nm diameter) were prepared
according to reported methods,[15] and the resulting nano-
particles were treated in a solution of heptane/THF with the
thiol derivatives 2 or 4. The average loading of the CdS
nanoparticles with 2 or 4 was determined spectroscopically by
monitoring the absorption spectra of the CdS nanoparticles in
solution before and after thiol modification. The subtraction
of the respective absorbance spectra led to the net absorbance
of the capping reagents associated with the CdS nanoparticles.
[**] This work was supported by the German–Israeli Program (DIP) as
well as the French MRT and CNRS (AC Nanosciences–Nano-
technologies). M.Z. acknowledges the Israeli Ministry of Science for
a Levi Eshkol fellowship. The authors are grateful to Prof. D.
Mandler and A. Becker (The Hebrew University of Jerusalem) for
assistance with the FTIR measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200463055
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From the absorbance value at l = 270 nm of a solution of
known concentration of the CdS nanoparticles (the extinction
coefficient (e) of 4–5-nm CdS nanoparticles is 7 ꢀ
105 mÀ1 cmÀ1),[16] we estimate the average loading of 2 and 4
to be 172 and 190 units per particle, respectively.
Figure 1 depicts the methods to assemble the hydrogen-
bonded CdS nanoparticles on gold electrodes. The systems
were assembled on Au/quartz piezoelectric crystals to enable
thiolated barbiturate 2 on the gold-coated glass, a broad band
at n¯ = 3250 cmÀ1 for the N-H group and three sharp bands at
n¯ = 1747, 1716, and 1687 cmÀ1 for the three different carbonyl
functionalities were observed. These bands are consistent
with the reported values of IR peaks for barbiturate
derivatives.[17] A drop of a solution that contained the CdS
nanoparticles capped with the thiolated triaminodiazine 4 was
dried on the gold surface, and the residue revealed a broad
band at n¯ = 3255 cmÀ1 (N-H) and an intense band at
1574 cmÀ1 (C N). For the hydrogen-bonded assembly in
=
which the 4-capped CdS nanoparticles are linked to the 2-
functionalized gold surface, two bands at n¯ = 3262 and
3031 cmÀ1 were observed. The appearance of a wide band at
n¯ = 3031 cmÀ1 was previously attributed to the hydrogen-
À
bonded N H bond between the aminoazine and barbiturate
carbonyl functionalities.[18] Similarly, the three sharp bands of
the carbonyl group of the barbiturate monolayer are reduced
to two bands and shifted to n¯ = 1702 and 1698 cmÀ1 upon
formation of the hydrogen-bonded assembly with the 4-
capped CdS nanoparticles. These shifts are in good agreement
with the reported IR bands of hydrogen-bonded assemblies
between barbiturate and triaminodiazine derivatives.[17]
In the second configuration of the hydrogen-bonded CdS
nanoparticles, as depicted in Figure 1b, 4 was assembled on
the Au/quartz crystal with a surface coverage of 6.4 ꢀ
10À10 molecmÀ2. The 2-functionalized CdS nanoparticles
were then bound through hydrogen bonds to the surface
with a surface coverage of 3.3 ꢀ 1012 particlescmÀ2. The
formation of this hydrogen-bonded CdS nanoparticle assem-
bly was also supported by FTIR measurements. Similar to the
spectral changes discussed in detail above for system a,
analogous shifts for system b were observed. New stretching
frequencies for the hydrogen-bonded carbonyl group at n¯ =
1700 and 1705 cmÀ1 as well as the non-bonded carbonyl
functionalities associated with the particles at n¯ = 1730, 1716,
and 1683 cmÀ1 are the most indicative bands for the formation
of the hydrogen-bonded system. A new, less intense band at
n¯ = 3035 cmÀ1, which corresponds to the hydrogen-bonded
À
N H bond to the barbiturate carbonyl functions that cap the
CdS nanoparticles, was also observed.
Figure 1. Schematic presentation of hydrogen-bonded CdS nanoparti-
cle assemblies in which a) 2 is bound to the gold electrode and 4 is
bound to the CdS nanoparticles, and b) 4 is bound to the electrode
and 2 is associated with the CdS nanoparticles.
The resulting surfaces were also characterized by absorp-
tion spectroscopy. Figure 2a shows the absorption spectrum
of the gold surface functionalized with CdS nanoparticles,
generated by the association of the 4-modified CdS nano-
particles with the 2-functionalized surface. Figure 2b shows
the photocurrent action spectrum of the electrodes modified
with CdS nanoparticles, prepared according to system a
(Figure 1a). The photocurrents were recorded in anhydrous
THF in the presence of triethylamine (Et3N) as the electron
donor and tetra-n-butylammonium hexafluorophosphate as
the electrolyte. The photocurrent action spectrum follows the
absorbance pattern of the CdS nanoparticles. Furthermore,
control experiments indicated that in the absence of either the
electron donor or the CdS nanoparticles, no photocurrent was
generated in the system. Figure 2c shows the effect of the
applied potential on the magnitude of the resulting photo-
current. As the applied potential is negatively shifted, the
photocurrent decreases and is completely blocked at about
À0.5 V (vs SCE). These results allow us to define the
the characterization of the loading of the different compo-
nents on the surfaces by microgravimetric quartz-crystal-
microbalance (QCM) analyses. In one configuration, Fig-
ure 1a, the thiolated barbiturate 2 was linked to the Au/quartz
surface. From the frequency change (Df), its surface coverage
was calculated to be 6.6 ꢀ 10À10 molecmÀ2. The monolayer-
modified surface was then treated with the 4-functionalized
CdS nanoparticles in dry THF. From the frequency change
(Df = À420 Hz in air), we estimate the surface coverage of the
CdS nanoparticles to be 3.1 ꢀ 1012 particlescmÀ2 (note, the
calculation took into account the loading of the CdS nano-
particles with 4). Thus, presumably, about 32 monolayer units
participate in the hydrogen-bonded association of a CdS
nanoparticle to the surface. Further support that the CdS
nanoparticles are linked to the electrode by hydrogen bonds
was obtained by FT-IR experiments. For the monolayer of
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Figure 2. a) Absorbance spectrum of the CdS nanoparticle layer on a gold-coated glass support prepared according to Figure 1a; b) photocurrent
action spectrum corresponding to the CdS nanoparticle assembly shown in Figure 1a; c) photocurrent recorded as a function of the applied
potential at l=410 nm (SCE=standard calomel electrode); d) light-induced “ON” and “OFF” states of switchable photocurrents generated upon
irradiation of the functionalized electrode at l=410 nm. All data were recorded in THF solutions containing triethylamine (2ꢁ10À2 m) and tetra-n-
butylammonium hexafluorophosphate (0.1m).
mechanism for the generation of photocurrents in the system.
Upon photoexcitation of the CdS nanoparticles, an electron–
hole pair is generated in the nanoparticles. The transfer of
conduction-band electrons to the electrode followed by the
oxidation of the electron donor by valence-band holes leads
to the formation of the steady-state photocurrent. Biasing the
electrode potential at À0.5 V (vs SCE) blocks the transfer of
conduction-band electrons to the electrode which prohibits
the generation of photocurrents. By knowing the absorbance
features of the CdS nanoparticle interface, the efficiency of
light to electrical-energy conversion was calculated to be
0.5%. The functionalized electrode revealed reasonable
stability. Figure 2d shows the cyclic “ON” and “OFF”
formation of light-induced photocurrents in the system. The
electrode was stable for at least 3 days upon storage in the
dark under dry conditions, whilst less than 10% decrease in
the photocurrents were observed after immersion for 5 hours
in the THF/electrolyte solution.
Further support that the CdS nanoparticle interface is
stabilized by hydrogen-bonding interactions was obtained by
examining the effect on the resulting photocurrent of the
addition of 10% water to the THF/electrolyte solution
Figure 3. Effect of added water on the photocurrent spectra generated
by the CdS nanoparticle assembly shown in Figure 1a. Data recorded
in THF solutions containing triethylamine (2ꢁ10À2 m) and tetra-n-butyl-
ammonium hexafluorophosphate (0.1m). Photocurrent spectra were
recorded a) before and (b–f) after the addition of 10% water to the so-
(Figure 3). As the time interval after the addition of H2O
becomes longer, the resulting photocurrent decreases. Simul-
taneous QCM analyses indicated that the addition of H2O to
the CdS nanoparticle modified Au/quartz surface resulted in
the dissociation of CdS nanoparticles from the electrode
support. Interestingly, however, we did not notice desorbtion
of the photoactive interface within 10 minutes after the
addition of 2% H2O to the THF/electrolyte solution.
Increasing the concentration of H2O enhanced the dissocia-
tion rate of the CdS nanoparticles. The relative stability of the
CdS nanoparticle interface is therefore attributed to the
multipoint hydrogen-bonded association of the nanoparticles
to the monolayer which results in the synergetic binding of the
nanoparticles to the surface.
The photocurrent action spectrum observed for the
system described in Figure 1b is depicted in the Supporting
Information. The efficiency of conversion of light to electrical
energy in this system was estimated to be about 0.4%. This
electrode reveals similar properties and functions to the
electrode configuration outlined in Figure 1a. The addition of
lution in THF: b) 4 min, c) 8 min, d) 12 min, e) 16 min, and f) 20 min.
10% of H2O to this system similarly resulted in the time-
dependent degradation of the hydrogen-bonded interface and
decrease of the photocurrent.
In light of previous observations that employed layered
gold nanoparticle/CdS nanoparticle hybrid systems for the
enhanced generation of photocurrents,[9a] we decided to use
hydrogen bonds as a means to construct gold nanoparticle/
CdS nanoparticle composites (Figure 4). Citrate-stabilized
gold nanoparticles[19] (12 Æ 1 nm) were linked by a thiolated
monolayer to a Au/quartz surface and a semitransparent (Au
film thickness ꢀ 50 nm) gold-coated glass slide. Then a
monolayer of 4 was assembled on the gold nanoparticles,
with an average loading of 5.8 ꢀ 103 molecules of 4 per particle
determined by QCM measurements. CdS nanoparticles
functionalized with 2 were then linked through hydrogen
bonds to the gold nanoparticles. QCM measurements of the
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the system. The results are compared to the related hydrogen-
bonded CdS nanoparticle assembly that lacks the gold
nanoparticles (Figure 1b). The depicted photocurrent action
spectra were normalized to exhibit identical surface coverage
of the CdS nanoparticles. The photocurrent observed in the
gold nanoparticle/CdS nanoparticle composite system is
approximately 2.6-fold enhanced relative to the system that
lacks the gold nanoparticles. The enhanced photocurrent in
the gold nanoparticle/CdS nanoparticle composite system is
attributed to the charge separation of the electron–hole pair
that is generated upon the photochemical excitation of the
CdS nanoparticles; that is, the transfer of the conduction-
band electrons to the gold nanoparticles spatially separates
the electron–hole pair, thus leading to the retardation of the
electron–hole recombination. This delay facilitates the effec-
tive scavenging of the valence-band holes by the electron
donor as well as the transfer of the electrons trapped by the
gold nanoparticles to the electrode, processes that lead to the
enhanced generation of photocurrents. There is a substantial
difference between the hydrogen-bonded bulk gold surface/
CdS nanoparticle configurations (Figure 1a and b) and the
hydrogen-bonded gold nanoparticle/CdS nanoparticle array
(Figure 4) in facilitating charge separation. A comparison of
the surface coverage of molecular components on the
functionalized gold nanoparticles or the CdS nanoparticles
with the surface coverage by 2, 4, or propanedithiol on the
bulk gold surfaces indicates that the surface coverage of the
bulk gold support is 50- to 100-fold higher than the loadings of
the particles. This result implies that the CdS nanoparticles
which are linked to the electrode exist in a rigid configuration,
whereas structural flexibility exists within the gold nano-
particle/CdS nanoparticle array. This flexibility facilitates the
quenching of the conduction-band electrons and enhances
charge separation. Furthermore, note that the schematic
configuration depicted in Figure 4 is a simplified configura-
tion. The CdS nanoparticles as well as the gold nanoparticles
exhibit irregular surface roughness. As a result, the nano-
particles may exist at a substantially closer distance than that
dictated by the hydrogen-bonded complex which would
facilitate the interparticle charge transport. The special
separation of the electron and hole in two different particles
then retards the recombination reactions and enhances the
photocurrent.
Figure 4. Schematic representation of the layered hydrogen-bonded
gold nanoparticle/CdS nanoparticle assembly associated with a gold
electrode in which the gold nanoparticles are functionalized with 4 and
the CdS nanoparticles are modified with complementary 2.
parallel assembly of the layered nanoparticle system on a Au/
quartz piezoelectric crystal indicated that the surface cover-
age of the base dithiol monolayer was 8 ꢀ 10À10 molecmÀ2 and
the surface coverage of the gold nanoparticles and the CdS
nanoparticles corresponded to 7.0 ꢀ 1010 and 2.6 ꢀ 1012 parti-
clescmÀ2, respectively.
Figure 5a, curve 1, shows the absorption spectrum of the
primary layer of gold nanoparticles on the gold surface. The
characteristic plasmon absorbance is clearly visible at l =
520 nm. From the values of the absorbance and the extinction
Figure 5. a) Absorbance spectra on a gold-coated glass support of
1) the gold nanoparticle layer and 2) the gold nanoparticle/CdS nano-
particle assembly shown in Figure 4. b) Photocurrent action spectrum
corresponding to the gold nanoparticle/CdS nanoparticle assembly.
Data recorded in THF solutions containing triethylamine (2ꢁ10À2 m)
and tetra-n-butylammonium hexafluorophosphate (0.1m).
The addition of water to the composite gold nanoparticle/
CdS nanoparticle system resulted in the dissociation of the
CdS nanoparticles, as observed from microgravimetric QCM
measurements, absorption spectra, and photocurrent meas-
urements (see Figure 6). These results imply that the CdS
nanoparticles are linked to the gold nanoparticles by com-
plementary hydrogen bonds that are dissociated by water,
whereas the gold nanoparticles themselves are tightly bound
to the gold support by the dithiol bridging monolayer.
In conclusion, the present study has employed comple-
mentary hydrogen bonds to fabricate metal and semiconduc-
tor nanoparticle arrays on surfaces and has demonstrated the
functional use of the resulting nanostructures for the gen-
eration of photocurrents. Although the results may have a
limited impact for energy conversion owing to the low
photocurrent values, the study highlights fundamental effects
coefficient of 12-nm gold nanoparticles (e = 3 ꢀ
108 mÀ1 cmÀ1),[20] the surface coverage of the gold nanoparti-
cles was estimated to be 6.9 ꢀ 1010 particlescmÀ2, in excellent
agreement with the QCM measurements. Figure 5a, curve 2,
shows the corrected absorption spectrum of the system after
linking the 2-functionalized CdS nanoparticles to the surface,
with the background absorbance of the primary gold nano-
particles subtracted from the spectrum of the composite. The
absorbance spectrum of the CdS nanoparticles is clearly
identified and exhibits a characteristic shoulder at l = 420 nm.
Photocurrents from the electrode functionalized with the
gold nanoparticle/CdS nanoparticle array were then mea-
sured. Figure 5b shows the photocurrent action spectrum of
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Preparation of the CdS nanoparticle assemblies: (The character-
ization of compounds and is detailed in the Supporting
1
3
Information.) The acetyl protecting groups in compounds 1 and 3
were removed by hydrolysis under basic conditions in THF by the
addition of approximately 10 mg of NH4OH. CdS nanoparticles,
which were prepared according to the reported methods,[15] were
capped by mixing solutions of CdS nanoparticles (4 mL) with a
solution of 1 or 3 (5 ꢀ 10À3 m) in THF (2 mL). The mixture was stirred
overnight, then pyridine (0.5 mL) was added, and the resulting
particles were collected by centrifugation, washed with THF (ꢀ 3),
and dissolved in THF (2 mL). This solution of nanoparticles was then
used to assemble the functionalized electrodes.
The electrode configurations shown in Figure 1a and b were
prepared by the following procedure: Au/quartz-crystal electrodes
and gold-plated glass electrodes were cleaned in boiling ethanol prior
to modification. The electrodes were then modified with either 2 or 4
(5 ꢀ 10À3 m overnight in THF) and then immersed for 30 min in the
solution of the CdS nanoparticles in THF that were capped with the
complementary compound. After each modification step, the electro-
des were washed with THF (ꢀ 3).
For the configuration presented in Figure 4, the electrodes were
modified with a propanedithiol monolayer (10À2 m, overnight in
methanol) and then immersed overnight in an aqueous solution of
citrate-stabilized gold nanoparticles. The electrodes were then
modified by the procedure described above for the configuration of
Figure 1b.
FT-IR spectra were obtained using a Bruker Equinox 55 instru-
ment. The measurements were made directly on gold-coated glass
slides in the dry phase under argon, and 1000 scans were averaged for
each curve.
Figure 6. Effect of added water on the photocurrent spectra generated
by the gold nanoparticle/CdS nanoparticle assembly shown in
Figure 4. Data were recorded in THF solutions containing triethylamine
(2ꢁ10À2 m) and tetra-n-butylammonium hexafluorophosphate (0.1m).
Photocurrent spectra were recorded a) before and (b–f) after the
addition of 10% water to the solution in THF: b) 4 min, c) 8 min,
d) 12 min, e) 16 min, f) 20 min, g) 24 min, and h) 28 min.
of hydrogen-bonded complexes on charge transport at semi-
conductor nanoparticle/electrode interfaces. The fact that the
presented systems are in a monolayer/bilayer nanoparticle
configuration suggests, however, that by tailoring hydrogen-
bonded composites of higher complexity the resulting photo-
currents could be further enhanced.
The estimation of the number of hydrogen bonds between the
CdS nanoparticles and the complementary base monolayer associated
with the gold support was based on some rough geometrical
approximations. By knowing the average particle diameter, the
footprint area of the particles with the monolayer was calculated. By
knowing the total loading of the CdS nanoparticles with 2 or 4 (the
surface coverage of the gold surface is substantially higher than the
coverage of the particles), we were able to estimate the number of
hydrogen bonds between the nanoparticles and the surface by also
assuming that all the units associated with the footprint area form the
hydrogen bonds with the surface.
Experimental Section
Photocurrent experiments were performed using a homebuilt instru-
ment described previously.[7] All spectra were recorded under argon
on solutions in THF (0.1m) that contained triethylamine (Et3N; 2 ꢀ
10À2 m) as a sacrificial electron donor and tetra-n-butylammonium
hexafluorophosphate (0.1m) as electrolyte. The electrodes used were
piezoelectric Au/quartz crystals, which allowed us to determine the
surface coverage as well as record the photocurrent. The photo-
current spectra were normalized by using the surface coverage of CdS
nanoparticles on the piezoelectric crystals.
Received: December 24, 2004
Revised: March 6, 2005
Published online: May 24, 2005
Keywords: gold · hydrogen bonds · nanostructures ·
.
photoelectrochemistry · semiconductors
Absorbance spectra and quantum efficiencies were measured
using glass slides coated with a 3-nm layer of chromium and a 50-nm
layer of gold (Analytical-m Systems, Germany). The absorbance
spectra were obtained after subtraction of the background absorb-
ance of the gold coating. The quantum efficiency was calculated by
determining the light intensity absorbed by the CdS nanoparticle
systems and the photocurrent intensities generated by the respective
electrodes. The intensity of the light absorbed by the CdS nano-
particle layered system was determined by monitoring the intensity of
the light transmitted through the gold-coated glass and the light
reflected from the coated surface, and then subtracting the respective
values for the transmitted/reflected light for the gold surface modified
with CdS nanoparticles. To determine the intensity of absorbed light
in the gold nanoparticle/CdS nanoparticle system, the intensity of the
transmitted/reflected light in the gold nanoparticle/CdS nanoparticle
system was substracted from the intensities of the transmitted/
reflected light from the gold surface modified with gold nanoparticles.
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