Electrostatically self-assembled polyoxometalates on molecular-dye- functionalized diamond

Article abstract of DOI:10.1021/ja908131t

We have successfully immobilized phosphotungstic acid (PTA), a polyoxometalate, on the surface of boron-doped diamond (BDD) surface through electrostatic self-assembly of PTA on pyridinium dye-functionalized-BDD. The inorganic/organic bilayer structure on BDD is found to exhibit fast surface-confined reversible electron transfer. The molecular dye-grafted BDD can undergo controllable electrical stripping and regeneration of PTA which can be useful for electronics or sensing applications. Furthermore, we have demonstrated the use of PTA as a molecular switch in which the direction of photocurrent from diamond to methyl viologen is reversed by the surface bound PTA. Robust photocurrent converter based on such molecular system-diamond platform can operate in corrosive medium which is not tolerated by indium tin oxide electrodes.

Full text of DOI:10.1021/ja908131t

Published on Web 12/02/2009
Electrostatically Self-Assembled Polyoxometalates on
Molecular-Dye-Functionalized Diamond
Yu Lin Zhong, Wibowo Ng, Jia-Xiang Yang, and Kian Ping Loh*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543
Received May 22, 2009; E-mail: chmlohkp@nus.edu.sg
Abstract: We have successfully immobilized phosphotungstic acid (PTA), a polyoxometalate, on the surface
of boron-doped diamond (BDD) surface through electrostatic self-assembly of PTA on pyridinium dye-
functionalized-BDD. The inorganic/organic bilayer structure on BDD is found to exhibit fast surface-confined
reversible electron transfer. The molecular dye-grafted BDD can undergo controllable electrical stripping
and regeneration of PTA which can be useful for electronics or sensing applications. Furthermore, we
have demonstrated the use of PTA as a molecular switch in which the direction of photocurrent from diamond
to methyl viologen is reversed by the surface bound PTA. Robust photocurrent converter based on such
molecular system-diamond platform can operate in corrosive medium which is not tolerated by indium tin
oxide electrodes.
chemistry include spontaneous grafting via charge exchange6
or the electrochemical grafting of in situ generated aryldiazo-
nium salts.7
I. Introduction
Diamond is an extraordinary material with outstanding
properties such as optical transparency, chemical robustness,
wide electrochemical potential window, and biocompatibility.
The chemical vapor deposition (CVD) of boron-doped nanoc-
rystalline diamond (NCD) on glass and quartz1 opens up
possibilities of using diamond as an alternative electrode for
photovoltaics compared to optically transparent electrodes
(OTEs).2 The surface of diamond can be considered as a solid
organic template, and chemically robust C-C bonds can be
formed via covalent coupling. To derivatize diamond with
functional molecules, photochemical coupling with alkenes3 as
well as electrochemical modification via reduction of aryldia-
zonium salts had been applied successfully. Aryldiazonium salts
have been applied to functionalize various metals and semi-
conductors4 in many fields ranging from combinatorial chemistry
to molecular electronics.5 Variations of the diazonium coupling
The functionalization of diamond by electrochemical reduc-
tion of aryldiazonium salts was first reported by Kuo et al.8
and much interest was generated subsequently,9 especially in
connection with the biofunctionalization of diamond.10 Precise
control of grafted organics density via electrochemical reduction
of aryldiazonium salts is not a straightforward task. A compre-
hensive review by Pinson and Podvorica11 discussed monolayer
grafting achieved by the careful control of grafting parameters
such as the amount of charge supplied during electrochemical
grafting and the concentration of aryldiazonium salts. A Monte
Carlo simulation of attaching 4-bromobenzene on Si(111)
performed by Allongue et al.12 indicated that the formation of
organized domains is related to the higher probability of grafting
a molecule in the neighborhood of a first attached molecule,
arising from π-π stacking interactions between neighboring
adsorbates. The importance of stacking interactions was also
(1) Williams, O.; Nesladek, M. In Physics and Applications of CVD
Diamond; Koisumi, S., Nebel, C. E., Nesladek, M., Eds.; Wiley-VCH:
New York, 2008; p 13.
(2) Zhong, Y. L.; Midya, A.; Ng, Z.; Chen, Z.-K.; Daenen, M.; Nesladek,
M.; Loh, K. P. J. Am. Chem. Soc. 2008, 130, 17218.
(5) (a) Coulon, E.; Pinson, J.; Bourzat, J.-D.; Commerc¸on, A.; Pulicani,
J.-P. J. Org. Chem. 2002, 67, 8513. (b) Kosynkin, D. M.; Tour, J. M.
Org. Lett. 2001, 3, 993.
(6) Adenier, A.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.;
Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491–501.
(7) Baranton, S.; Be´langer, D. J. Phys. Chem. B 2005, 109, 24401.
(8) Kuo, T. C.; McCreery, R. L.; Swain, G. M. Electrochem. Solid-State
Lett. 1999, 2, 288.
(3) (a) Yang, W.; Auciello, O.; Bulter, J. E.; Cai, W.; Carlisle, J. A.; Gerbi,
J. E.; Gruden, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N.,
Jr.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253. (b) Hartl,
A.; Schmich, E.; Garrido, J. A.; Hernado, J.; Catharion, S. C. R.;
Walter, S.; Feulner, P.; Kromka, A.; Steinmuller, D.; Stutzmann, M.
Nat. Mater. 2004, 3, 736. (c) Zhong, Y. L.; Chong, K. F.; May, P. W.;
Chen, Z.-K.; Loh, K. P. Langmuir 2007, 23, 5824. (d) Chong, K. F.;
Loh, K. P.; Vedula, S. R. K.; Lim, C. T.; Sternschulte, H.; Steinmu¨ller,
D.; Sheu, F.-S.; Zhong, Y. L. Langmuir 2007, 23, 5615.
(9) (a) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir
2004, 20, 11450. (b) Shin, D.; Rezek, B.; Tokuda, N.; Takeuchi, D.;
Watanabe, H.; Nakamura, T.; Yamamoto, T.; Nebel, C. E. Phys. Status
Solidi A 2006, 203, 3245. (c) Uetsuka, H.; Shin, D.; Tokuda, N.; Saeki,
K.; Nebel, C. E. Langmuir 2007, 23, 3466.
(4) (a) Jiang, D.; Sumpter, B. G.; Dai, S. J. Am. Chem. Soc. 2006, 128,
6030. (b) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.;
Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am.
Chem. Soc. 2004, 126, 370. (c) Corgier, B. P.; Marquette, C. A; Blum,
L. J J. Am. Chem. Soc. 2005, 127, 18328. (d) Marwan, J.; Addou, T.;
Be´langer, D. Chem. Mater. 2005, 17, 2395. (e) Price, B. K.; Hudson,
J. L.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 14867.
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Zhou, Y. L.; Zhi, J. F. Electrochem. Commun. 2006, 8, 1811. (c) Shin,
D.; Tokuda, N.; Rezek, B.; Nebel, C. E. Electrochem. Commun. 2006,
8, 844.
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Bernard, M. -C. J. Electroanal. Chem. 2003, 550-551, 161.
9
10.1021/ja908131t  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 18293–18298 18293

A R T I C L E S
Zhong et al.
Scheme 1. Synthesis of 1-butyl-4-methylpyridinium Bromide
demonstrated by Lud et al.13 in the formation of biphenyl self-
assembled monolayers on doped ultrananocrystalline diamond
via spontaneous grafting without electrochemical induction. In
addition, the chemical reduction potential of the different
aryldiazonium salts with respect to the Fermi level of the solid
surface controls the extent of reaction.14 Thus far, most of the
diazonium salts grafted are restricted to nitrophenyl or bro-
mophenyl moieties due to the ease of acquiring these com-
mercially. In principle the concept of diazonium grafting can
be extended to a wide range of molecules, including organic
dyes or charged molecules, although this has not been explored
on the diamond platform.
Due to the rich and versatile electrochemical and photo-
chemical properties of polyoxometalates (POM), they have
attracted great attention in field of analytical chemistry, catalysis,
biology, medicine, and materials science.15 Of particular interest
is the bottom up fabrication of POM-based materials with atomic
precision through growth processes determined by self-assembly
for constructing modular molecular nanosystems. Some of the
popular methods include the use of Langmuir-Blodgett (LB)
technique16 and electrostatic layer-by-layer self-assembly (ELSA)
method17 to provide control of film thickness, structure, and
function. To create a more robust device, schemes based on
the covalent attachments of POMs through organic linkers had
been developed.18 Herein we report a strategy to graft a dye
molecule onto diamond via the facile electrochemical reduction
of in situ generated aryldiazonium salt, which allows subsequent
electrostatic assembly of phosphotungstic acid (PTA), a type
of POM, onto the positively charged terminal pyridinium end
of the coupled dye. The electrostatic interactions among the
charged groups can help to stabilize film structure and prevent
molecular aggregation, leading to the formation of well-ordered
molecular clusters.
washed with ethyl acetate and subsequently dried under vacuum
with a yield of 50%.19
4-Aminobenzaldehyde (2). Grounded sulfur flakes (5 g), sodium
sulfide nanohydrate (10 g, 41.7 mmol), and sodium hydroxide (9
g, 0.223 mol) were added to 200 mL of deionized water and then
heated for 20 min on a steam bath. Next, the mixture was added to
a hot solution of 4-nitrotoluene (16.7 g, 0.103 mol) in 95% ethanol
(100 mL), and the resulting mixture was refluxed at 80 °C for 3 h.
Deionized water (300 mL) was added to this mixture which was
then subjected to rapid vacuum distillation. The distillation was
terminated when the volume of residue decreased to about 200 mL.
Following that, the residual flask was immersed in an ice bath for
2 h for crystallization. Yellow-orange crystals of the product were
obtained via vacuum filtration and then purified by washing with
(200 mL) of cold, deionized water. The product was dried over
potassium hydroxide pellets in a vacuum desiccator for 24 h, giving
a yield of approximately 45%.20
4-(4-Aminostyryl)-1-butylpyridinium Bromide (3). 1-Butyl-
4-methylpyridinium bromide (4 g, 29.2 mmol) and 4-aminoben-
zaldehyde (2.36 g, 19.5 mmol) were added to 95% ethanol (60
mL) and refluxed at 80 °C for 24 h (see Scheme 2). The mixture
was then allowed to cool gradually before placing it in an ice bath
for crystallization to occur. Reddish-purple crystals were obtained
1
by vacuum filtration with a yield of 75%. H NMR (DMSO, 300
MHz) δ: 0.90 (3H, t, J ) 7.50 Hz), 1.26 (2H, m), 1.86 (2H, m),
4.40 (2H, t, J ) 7.20 Hz), 6.01 (2H, s), 6.62 (2H, d, J ) 8.70 Hz),
7.09 (1H, d, J ) 15.90 Hz), 7.46 (2H, d, J ) 8.70 Hz), 7.86 (1H,
d, J ) 16.2 Hz), 8.03 (2H, d, J ) 6.90 Hz), 8.75 (2H, d, J ) 6.90
Hz). 13C NMR (DMSO, 300 MHz): 13.30, 18.75, 32.41, 58.81,
113.74, 116.33, 122.23, 122.41, 130.53, 142.57, 143.39, 152.10,
153.84. ESI-MS: m/z 254.2, 197.3. FT-IR (KBr, cm-1): 3349, 3285,
3181, 3032, 2962, 2917, 1619, 1585, 1470, 1375, 1313, and 1165.
Diamond Thin Film. Polycrystalline boron-doped diamond
(BDD) films (50 µm thick) were grown on p-type Si substrates in
a commercial 2.45 GHz microwave plasma reactor (Astex) using
methanol and boron oxide mixtures. The BDD samples had a
surface resistance of 10 Ω cm and the boron doping level was
approximately 1020 cm-3. These BDD samples were used for
electrochemical experiments due to their high conductivity. Opti-
cally transparent diamond electrode was fabricated by the CVD of
a thin layer of boron-doped diamond film on quartz substrate (160
nm thick, boron concentration 7 × 1020 cm-3) according to
published procedures.1
Acid cleaning and hydrogen plasma cleaning of diamond were
used for all diamond samples. Metallic impurities were first
dissolved in hot aqua regia (HNO3/HCl ) 1:3), followed by the
removal of organic impurities from the diamond samples by hot
“piranha” solution (H2O2/H2SO4 ) 1:3) at 90 °C for 1 h. Hydrogen
termination of diamond samples was attained by microwave
hydrogen plasma treatment using 800 W microwave power and
300 sccm of hydrogen gas flow for 15 min.
II. Experimental Section
Chemical Reagents. All chemicals purchased were of the purest
grade and used as received from Sigma-Aldrich unless otherwise
stated. Phosphotungstic acid hydrate (H3[P(W3O10)4]·xH2O), also
known as tungstophosphoric acid, was used as received. All solvents
used for reaction and rinsing were of HPLC grade unless otherwise
stated. All dilution and preparation of electrolytes for electrochemi-
cal work were made with Nanopure water (18.0 MΩ·cm).
Organic Synthesis. 1-Butyl-4-methylpyridinium Bromide
(1). 1-Bromobutane (10 mL, 0.102 mol) was added to freshly
distilled 4-picoline (12.1 mL, 0.112 mol) under nitrogen; 95%
ethanol (125 mL) was added, and the mixture was refluxed for 24 h
(see Scheme 1). Light brownish-yellow crystals were precipitated
upon cooling in the refrigerator. The product was filtered and
(13) Lud, S. Q.; Steenackers, M.; Jordan, R.; Bruno, P.; Gruen, D. M.;
Feulner, P.; Garrido, J. A.; Stutzmann, M. J. Am. Chem. Soc. 2006,
128, 16884.
Surface Reaction Conditions. The grafting of the molecular
dye on H-terminated BDD via electrochemical reduction of in situ
generated aryldiazonium salt was carried out by applying five cyclic
voltammetric scans between +0.5 and -0.8 V (vs Ag/AgCl) with
a scan rate of 100 mV/s, in a solution of 1 mM of the tailor-made
molecular dye (3) and 1 mM NaNO2 in 0.5 M HBF4. The grafted
substrate was then ultrasonicated in acetone to remove any
physisorbed molecules, followed by rinsing in ethanol and water.
(14) Zhong, Y. L.; Loh, K. P.; Midya, A.; Chen, Z.-K. Chem. Mater. 2008,
20, 3137.
(15) (a) Hill, C. L. Chem. ReV. 1998, 1, 98 (Thematic Issue). (b) Pope,
M. T.; Mu¨ller, A. Polyoxometalate Chemistry: From Topology Via
Self-Assembly to Applications; Kluwer: Dordrecht, The Netherlands,
2001.
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Du, Z.-L.; Wong, W.-Y. Chem. Mater. 2007, 19, 1704.
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Soc. 2002, 124, 12279.
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41, 501. (b) Errington, R. J.; Petkar, S. S.; Horrocks, B. R.; Houlton,
A.; Lie, L. H.; Patole, S. N. Angew. Chem., Int. Ed. 2005, 44, 1254.
(c) Lu, M.; Nolte, W. M.; He, T.; Corley, D. A.; Tour, J. M. Chem.
Mater. 2009, 21, 442.
(19) Behar, D.; Neta, P.; Schultheisz, C. J. Phys. Chem. A 2002, 106, 3139.
(20) (a) Campaigne, E.; Budde, W. M.; Schaefer, G. F. Org. Synth. 1963,
4, 31. (b) Ogata, Y.; Kawasaki, A.; Sawaki, Y.; Nakagawa, Y. Bull.
Chem. Soc. Jpn. 1979, 52, 2399.
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Electrostatically Self-Assembled Polyoxometalates
A R T I C L E S
Scheme 2. Synthesis of 4-(4-Aminostyryl)-1-butylpyridinium
Bromide
Electrostatic self-assembly of phosphotungstic acid (PTA) was
performed by immersing the molecular-dye-functionalized BDD
(BDD + dye) in 10 mM PTA for 1 min. After all coupling steps,
the substrates were rinsed copiously with acetone, ethanol, and
ultrapure water to remove any physisorbed molecules.
Instrumentations. X-ray photoelectron spectroscopy (XPS) was
performed with a Phobios 100 electron analyzer (SPECS GmbH)
equipped with 5 channeltrons, using an unmonochromated Al KR
X-ray source (1486.6 eV). The pass energy of the hemisphere
analyzer was set at 50 eV for wide scan and 20 eV for narrow
scan, while the takeoff angle was fixed at normal to the sample.
Ultraviolet photoelectron spectroscopy (UPS) was performed using
He I UV lamp (VG Microtech, UK) with the same electron analyzer
at pass energy of 1 eV. The sample was biased at -5 V to overcome
the analyzer function of 4.44 eV in order to observe a distinct low
energy cutoff.
Scheme 3. Electrochemical Grafting of Molecular Pyridinium Dye
Followed by Electrostatic Self-Assembly of PTA
The thickness of grafted molecular film was characterized by
atomic force microscopy (AFM) scratching experiments performed
using the NanoMan AFM system (Veeco Instruments and Process
Metrology, USA) operating under contact mode for scratching and
tapping mode for imaging. A large force of >120 nN was employed
to scratch an area of 1 µm2 with complete removal of organic, and
thereafter an area of 5 µm2 was directly imaged under the tapping
mode. BudgetSensor Multi75 cantilever (Innovative Solutions
Bulgaria Ltd., Bulgaria) with a typical resonant frequency of 75
kHz and a force constant of 3 N/m was employed throughout this
study. Loading forces during the scratching experiments were
calculated from force curve measurements for each cantilever.
All electrochemical measurements were carried out with an
Autolab PGSTAT30 potentiostat (Eco Chemie, The Netherlands).
Photoelectrochemical measurements were performed in 0.1 M
Na2SO4 solution containing 5 mM methyl viologen (MV2+) as an
electron carrier, in a custom-made photoelectrochemical Teflon cell
with three-electrode configuration system: a diamond working
electrode, Ag/AgCl reference electrode (3.0 M KCl), and a Pt mesh
counter-electrode.2 Top contact on the sample surface (spotted with
Ag paste for improved contacts) was made through Au-plated strips.
The area of the sample exposed to light was 0.196 cm2.
Action spectrum was obtained by a monochromatic irradiation
produced by a Newport 300 W xenon light source, through a
Newport Cornerstone 260 monochromator. The light intensity was
measured by a Newport calibrated Si detector to be 0.35 mW/cm2
at 440 nm. The currents were measured in light intensity under
steady state.
Solid-state UV-visible spectra were acquired using UV-visible
spectrophotometry (Shimadzu UV-2450). A 1 cm × 1 cm H-
terminated boron-doped nanocrystalline diamond grown on quartz
plate was used to calibrate the instrument before characterization
and employed as a reference throughout the experiment. Another
identical substrate was grafted with dye and later immersed in the
PTA solution for the electrostatic self-assembly.
(AM1).24 The AM1 was chosen because it contains parameters for
elements that this molecular dye contains (H, C, N, F, and B).25
The semiempirical calculation gives similar results as DFT calcula-
tions; the degree of error is less than 4%. After geometry
optimization, the UV-visible spectrum of the dye was simulated
by calculating the electronic transition using VAMP.
III. Results and Discussion
Diazonium Functionalization and Electrostatic Self-As-
sembly of PTA. The hierarchical assembly begins with the use
of a tailor-made highly π-conjugated chromophore (3) which
consists of a terminal pyridinium moiety, electron-rich phenyl-
ethyl core, and a reactive aniline group for diazotization, as
shown in Scheme 3. The presence of pyridinium ion helps to
solubilize the otherwise insoluble molecular wire in aqueous
acidic solution, where NaNO2 was added to diazotize the aniline
group. The grafting of the molecular dye on hydrogen-
terminated boron-doped diamond initiates with the electro-
chemical reduction of in situ generated aryldiazonium salt on
the surface. Owing to the nature of free radical grafting and
excessive reduction voltage (5 cycles to -0.8 V, vs Ag/AgCl),
the possibility of multilayer formation cannot be excluded. The
sample was subsequently rinsed and immersed in phosphotung-
stic acid (PTA) solution for electrostatic self-assembly. The
advantages of electrochemical grafting via in situ generated
aryldiazonium salts include the simplicity of storage, ease of
working with stable aniline precursors, and compatibility with
lithographical patterning. In Figure 1, the electrochemical
grafting of in situ generated aryldiazonium salt can be judged
from the board reduction peak (at 0 V vs Ag/AgCl) observed
only in the first CV scan. The absence of reduction peak in
subsequent scans indicates that the diamond electrode was fully
passivated by bulky molecular dye after one scan.
The absorption spectrum and structural configuration of the dye
was simulated using both density functional theory (DFT) and
semiempirical methods. A comparison between DFT and the
semiempirical method regarding the geometry of the molecular dye
was made. DFT calculation was performed using the DMol3 code.21
The exchange and correlation energies were calculated with the
PWC functional of the local density approximation (LDA).22
Semiempirical quantum mechanical calculation was performed
using the VAMP (Vienna ab initio Molecular-Dynamics Package)23
modules with the Austin model Hamiltonian of first degree
Surface Science Characterization. The successful grafting of
molecular dye was confirmed by X-ray photoelectron spectros-
copy (XPS) as evident by the XPS N 1s peak at 402.1 eV
(21) (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) Delley, B. J. Chem.
Phys. 2000, 113, 7756.
(22) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244.
(23) Clark, T.; Alex, A.; Beck, B.; Burkhardt, F.; Chandrasekhar, J.; Gedeck,
P.; Horn, A. H. C.; Hutter, M.; Martin, B.; Rauhut, G.; Sauer, W.;
Schindler, T.; Steinke, T. Vamp 8.1; University of Erlangen: Erlangen,
Germany, 2002.
(24) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. J. Am.
Chem. Soc. 1985, 107, 3902.
(25) (a) Dewar, M. J. S.; Jie, C.; Zoebisch, E. G. Organometallics 1988,
7, 513. (b) Dewar, M. J. S.; Zoebisch, E. G. J. Mol. Struct.: Theochem
1988, 180, 1.
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A R T I C L E S
Zhong et al.
Figure 1. Cyclic voltammetry (CV) scans of H-terminated BDD in 1 mM
molecular dye with equivalent NaNO2 in 0.5 M HBF4, from +0.5 to -0.8
V (vs Ag/AgCl) at 0.1 V/s.
Figure 4. Tapping mode AFM image of scratched area on (a) molecular-
dye-functionalized BDD (BDD + dye) and (c) electrostatically self-
assembled PTA on dye-functionalized BDD (BDD + dye + PTA). (Bottom)
Average line profiles through the scratched area showing an average height
difference of 2.3 ((0.3) nm for (a) BDD + dye and 5.4 ((0.5) nm for (c)
BDD + dye + PTA. (Center) (b) Computed heights of molecular dye and
bilayer are 1.6 and 2.0 nm, respectively. The optimized structure shown
has an additional methyl group (faded) to simulate the C-C bonding of
the dye to the diamond surface.
Table 1. Elemental Composition of BDD + Dye
integral (RSF corrected)
ratio
N 1s (402.1 eV)
B 1s
F 1s
4280
4270
17500
1
1
4.1
Table 2. Elemental Composition of BDD + Dye + PTA
Figure 2. X-ray photoelectron spectroscopy (XPS) (a) N 1s spectrum of
molecular-dye-functionalized BDD (BDD + dye). (b) B 1s and (c) F 1s
spectrum of (i) molecular-dye-functionalized-BDD (BDD + dye) and (ii)
electrostatically self-assembled PTA on dye-functionalized BDD
(BDD + dye + PTA).
integral (RSF corrected)
ratio
expected ratio
N 1s (402.1 eV)
W 4d5/2 (260.7 eV)
2650
9220
1
3.5
1
4
Following ion exchange with PTAs, the BF4 anions were
replaced by the PTA. This is evidenced by the emergence of
the XPS W 4d peaks as shown in Figure 3b and the diminished
XPS B 1s and F 1s signals (Figure 2 and Table 1). Such an ion
exchange method is a very versatile method to build up
organic-inorganic hybrid films. From Table 2, the elemental
ratio of N:W was calculated to be 1:3.5, which is close to the
expected ratio of 1:4 to maintain electrical neutrality assuming
3-
one PW12O40 is electrostatically bonded to three pyridinium
groups. This suggests that most of the surface pyridinium groups
were accessible to electrostatic binding with PTAs.
The thickness of the grafted molecular dye as well as the
subsequently assembled PTA on diamond was verified by atomic
force microscopy (AFM) scratching experiment. AFM was
performed on a boron-doped single crystal diamond, C(100),
with root-mean-square roughness of 0.165 nm. The corrugation
of the clean surface is smaller than 10% of the step height
fluctuations after the electrochemical grafting of the molecular
dye and PTA adlayers on the surface. In AFM contact mode, a
large force of more than 120 nN was employed to scratch an
area of 1 µm2 with complete removal of organic dye, and
thereafter an area of 5 µm2 was directly imaged under the
tapping mode. Multiple domains were randomly chosen for
imaging to get statistical sampling. From Figure 4, an average
height profile was drawn through the scratched trench perpen-
dicular to the scanning direction of the tip to minimize error
arising from artifacts along the scanning direction. Multiple
height measurements were taken within the sampling box (∼4.4
µm × 0.8 µm) drawn on the AFM image. The average molecular
Figure 3. X-ray photoelectron spectroscopy (XPS) (a) N 1s of electrostati-
cally self-assembles PTA on dye-functionalized BDD (BDD + dye + PTA)
and (b) W4d spectra of (i) molecular-dye-functionalized-BDD (BDD + dye)
and (ii) BDD + dye + PTA.
attributed to the pyridinium nitrogen (Figure 2a). The small peak
at 399.8 eV is attributed to the formation of azo bridge in the
molecular film which is characteristic of such an electrochemical
grafting method.26 From Table 1, the XPS elemental composi-
tion, after correction for relative atomic sensitivity factors, shows
that original bromide counterions of the molecular dye were
replaced by BF4- anion from HBF4 and every surface pyridinium
-
cation (at 402.1 eV) was matched with a BF4 counterion.
(26) Lyskawa, J.; Be´langer, D. Chem. Mater. 2006, 18, 4755.
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Electrostatically Self-Assembled Polyoxometalates
A R T I C L E S
Figure 6. CV scans of electrostatically self-assembled PTA on molecular-
dye-functionalized BDD (BDD + dye + PTA) in 0.5 M HCl at scan rate
of 1 V/s after two CV cycles to -1 V (black), to -1.5 V (red), to -2 V
(blue) and after regeneration of PTA by reimmersion in PTA solution
(green).
Figure 5. Cyclic voltammetry (CV) scans of electrostatically self-assembled
PTA on dye-functionalized BDD (BDD + dye + PTA) in 0.5 M HCl at
varying scan rate from 0.5 to 4 V/s. Inset shows plot of cathodic peak current
at -0.48 V(9) and -0.65 V(b) and anodic peak current at -0.58 V(2) vs
scan rate.
dye thickness was found to be 2.3 ( 0.3 nm. This indicates
that the organic film consisted of predominantly two monolayers.
The definition of two monolayers is based on the theoretically
optimized length of a first layer molecular dye covalently
coupled to a second branched layer as shown in Figure 4b, where
the energetically optimized structure is determined to have a
projected vertical height of 2 nm. After electrostatic self-
assembly of PTA, the increased average film thickness of 5.4
( 0.5 nm confirmed the presence of PTA as shown in Figure
4c.
Electrochemical Characterization. Figure 5 shows the CV
scans of electrostatically self-assembled PTA on dye-function-
alized BDD (BDD + dye + PTA) in 0.5 M HCl, collected at
different scan rates. It should be pointed out that experiments
in harsh acidic conditions are impossible to perform with indium
tin oxide (ITO) because of its chemical instability in acid. The
advantage of using diamond is its chemical robustness.27 The
cathodic peak currents (at -0.48 and -0.65 V) and anodic peak
current (at -0.58 V) show linear dependence on scan rate, which
is indicative of a surface confined redox process (Figure 5, inset).
The peak potentials remain constant up to the fastest scan rate
of 400 mV/s, which indicate fast electron transfer between the
BDD and PTA.
Figure 7. (a) Action spectrum of H-terminated BDD (black), molecular-
dye-functionalized BDD (BDD + dye) at -0.1 V (vs Ag/AgCl) bias (red)
and BDD + dye at +0.1 V bias (blue). (b) Action spectrum of electrostati-
callyself-assembledPTAonmolecular-dye-functionalizedBDD(BDD +dye +PTA)
at -0.1 V bias (black) and at +0.1 V bias (red). (c) Schematic drawing of
energy level alignments (with respect to vacuum level) between BDD/dye/
MV2+ and BDD/dye/PTA/MV2+ and their preferred electron flow direction.
Using the Randles-Sevcik equation for a Nernstian reaction
at the adsorbate monolayer (eq 1),28
can be cathodically stripped off (> -1 V) and regenerated by
reimmersing in PTA solution (Figure 6). Such controllable
release of electrostatically self-assembled PTA may be useful
for electronics or sensing applications.
ip ) (9.39 × 105)n2νAΓ
(1)
where ip is the peak current in A, n is number of electrons, ν is
the scan rate in V/s, A is the electrode surface area in cm2, and
Γ is the coverage in mol/cm2. The plot of peak current at -0.48
V versus scan rate yields a straight line of equation: y ) (9.54
× 10-5)x, which fits the data with an R2 value of 0.998. With
n ) 1 and A ) 0.283 cm2, Γ was calculated to be 2.2 × 1014
molecules/cm2, which corresponds to about two monolayers of
PTA.29 This result correlates well with our AFM studies and
proves the formation of bilayered molecular film in which all
the pyridinium ions are accessible to PTA. It is known that the
surface coverage is directly related to the total charge of the
PTA anion and can be controlled from submonolayer to
multilayer coverage by adjusting the ionic strength of the dipping
solutions. Interestingly, the electrostatically self-assembled PTA
Photoelectrochemical Characterization. Photoelectrochemical
measurements were performed in 0.1 M Na2SO4 solution
containing 5 mM methyl viologen (MV2+) as electron carrier
under variable wavelength monochromatic irradiation. From
Figure 7a, the action spectrum of H-terminated BDD shows a
photocurrent peak at around 250 nm which is attributed to the
absorbance characteristic of methyl viologen.30 A small board
peak at 520 nm is possibly due to defect-related absorption
inherent with CVD diamond.31 After grafting of the molecular
dye, it led to the appearance of a photocurrent peak at 350 nm
attributedtothemoleculardyeabsorptionatthediamond-molecular
dye-solution interfaces. The VAMP-simulated absorption
maximum of the molecular dye is calculated to be 340 nm
(Figure 8), which agrees very well with the absorption peak at
(27) Stotter, J; Show, Y.; Wang, S.; Swain, Greg Chem. Mater. 2005, 17,
4880.
(28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 2000.
(29) Jiang, K.; Zhang, H.; Shannon, C.; Zhan, W. Langmuir 2008, 24, 3584.
(30) Zak, J. K.; Bulter, J. E.; Swain, G. M. Anal. Chem. 2001, 73, 908.
(31) Nesla´dek, M.; Meykens, K.; Stals, L. Phys. ReV. B 1996, 54, 5552.
9
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A R T I C L E S
Zhong et al.
V (vs Ag/AgCl) is used to determine its chemical potential
below vacuum level (4.3 eV).
The electrostatic assembly of PTA on the dye-diamond
surface broadens the photocurrent conversion spectra window
to 500 nm and beyond. It is well-known that the UV irradiation
of PTA film can result in the appearance of absorption bands
in the visible and near-IR regions arising from photoinduced
reduction of W6+ into W5+, giving rise to intervalence charge
transfer bands between adjacent W5+ and W6+ sites.33 PTA,
being a photochromic molecule,34 can trap photogenerated
electrons accompanied by the conversion of W6+ into W5+ ions,
the photochromic response being drastically enhanced in the
presence of photohole scavengers like methyl viologen.35 The
presence of these convertible W6+ and W5+ redox species can
give rise to both cathodic and anodic currents.32 The simulta-
neous injection of electrons and holes into the interstitial sites
in PTA can be driven photoelectrochemcially, thus in this case
at both negative and positive bias, we can observe current flow
with quite similar magnitudes in Figure 7b. The ability to have
current reversal at the same applied voltage by a reversible
tethering of molecular species on the electrode surface may be
useful for controlling the physical properties of electrochemical
deposited metallic nanostructures. Building photochromic ma-
terials like PTA on an optically transparent and conducting
diamond platform opens up the possibility of combining
photochromic coloration with electrochemical erasing.
Figure 8. (a) UV-visible spectrum of molecular dye simulated using
VAMP. (b) Solid-state UV-visible spectrum of (i) molecular-dye-func-
tionalized BDD (BDD + dye) and (ii) after electrostatic self-assembled of
PTA (BDD + dye + PTA).
353 nm in the UV-visible spectrum of molecular dye grafted
on diamond (BDD + dye). It can be seen that the photocurrent
peak at 350 nm for BDD + dye (Figure 7a) mirrors the
absorption peak in the UV-visible spectrum at 353 nm (Figure
8b). After electrostatic binding to PTA, the absorption peak of
the composite film is slightly broadened and red-shifted to 366
nm with similar absorption intensity. However, the action
spectrum of BDD + dye + PTA at the same applied bias (-0.1
V vs Ag/AgCl) shows a significantly reduced photocurrent
density compared to BDD + dye despite their similar absorption
intensity. This implies that the addition of PTA impedes the
preferred electron flow.
IV. Conclusion
We have demonstrated a method to build a hierachical
assembly of organic-inorganic hybrid films on diamond. The
versatility of this method is that different inorganic systems can
be assembled on the organic wire platform by electrostatic
assembly. In particular, an inorganic system based on polyoxo-
metalate provides functional components for constructing
modular molecular nanosystems which can mediate charge
transfer between diamond and electrolyte. The chemical stability
and optical transparency of the diamond platform allows optical
and electrical actuation of such nanosystems in corrosive media,
as demonstrated by the reversible tethering of PTA Via cathodic
stripping and electrostatic assembly.
Before the addition of PTA, the preferred electron flow
direction is from LUMO of the dye to the electron carrier, MV2+,
and this is favored by the application of a negative bias. The
application of a small positive bias (+0.1 V) caused a reduction
in the photocurrent obtained throughout the spectrum, as the
positive bias retarded the preferred electron flow direction from
LUMO of dye to the electron carrier, MV2+. We found that the
electrostatically bound PTA molecules could modulate the
direction of current flow from the diamond to MV2+. In the
absence of the PTA molecules, at +0.1 V electrode bias (Figure
7a), the current is negative. In the presence of surface-bound
PTA molecules, at the same applied voltage of +0.1 V (Figure
7b), the current becomes positive. The reversal of current can
be explained by the chemical potential of PTA which lies below
that of MV2+, thus favoring electron flow from the latter to the
former, as shown in the energy level alignment diagram in
Figure 7c. Following the photoexcitation of the dye, a hole is
created in the HOMO. In this case, electron is transferred from
the MV+ to PTA, and from PTA to the HOMO of the dye. The
energy levels of the BDD and grafted molecular dye, with
respect to vacuum level, were determined using ultraviolet
photoelectron spectroscopy (see Supporting Information). The
chemical potential of MV2+/MV+ is taken to be 4.0 eV below
vacuum level.32 The first redox potential of PTA at around -0.1
Acknowledgment. K. P. Loh acknowledges ARF-MOE Grant
“Structure and Dynamics of Molecular Self-Assembled Layers.
R-143-000-344-112.”
Supporting Information Available: UPS data for determina-
tion of energy levels. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA908131T
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