Photoelectrochemical Responses from Polymer-coated Plasmonic Copper Nanoparticles on TiO2
Taishi Yamaguchi, Emiko Kazuma, Nobuyuki Sakai, and Tetsu Tatsuma*
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(Received May 14, 2012; CL-120616; E-mail: tatsuma@iis.u-tokyo.ac.jp)

A TiO2 electrode with Cu nanoparticles protected with poly(vinyl alcohol) exhibits
photopotential and photocurrent responses due to localized surface plasmon resonance,suggesting that it could be applied to cost-effective plasmonic devices.

REPRINTED FROM

Vol.41 No.10

2012 p.1340–1342
CMLTAG
October 5, 2012

The Chemical Society of Japan
Published on the web October 20, 2012; doi:10.1246/cl.2012.1340

doi:10.1246/cl.2012.1340
Published on the web October 20, 20121340

Photoelectrochemical Responses from Polymer-coated Plasmonic Copper Nanoparticles on TiO2
Taishi Yamaguchi, Emiko Kazuma, Nobuyuki Sakai, and Tetsu Tatsuma*
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(Received May 14, 2012; CL-120616; E-mail: tatsuma@iis.u-tokyo.ac.jp)
Plasmonic Cu nanoparticles photocatalytically deposited on
a TiO2 electrode show photocorrosion under visible light. The
electrode protected with poly(vinyl alcohol) exhibits photopotential and photocurrent responses due to localized surface
plasmon resonance, suggesting that it could be applied to costeﬀective plasmonic devices.

Noble metal nanoparticles (NPs) such as Au, Ag, and Cu
absorb and scatter visible light due to localized surface plasmon
resonance (LSPR).1 Since intensity and wavelength of LSPR
depend on surrounding dielectric environment, as well as
particle size and shape, the NPs are applied to chemical sensing
and biosensing.2 Additionally, since electric ﬁelds localize
around NPs due to LSPR,1 NPs are used as light-harvesting
antennae for surface-enhanced Raman scattering,3 ﬂuorescenceenhancement,4 and photocurrent enhancement.5 We foundplasmon-induced charge separation at interfaces between Au
or Ag NPs and metal oxides such as TiO2.6 The plasmoninduced charge separation has also been studied7 and used8­10 bymany other research groups. In the case of Au NPs, which arestable during the charge separation process, the system has been
exploited for photovoltaic devices6d­6f,8 and photocatalysis.6d,9On the other hand, Ag NPs, which are less stable than Au NPs,
are oxidized to Ag+ ions as a result of the plasmon-induced
charge separation and are re-reduced to Ag NPs by TiO2
photocatalysis under UV light.6a,6b,11 These apparently reversible
photoelectrochemical reactions have been applied to multicolor6a,6b and infrared6g photochromism and other photochromic
processes10 and photomorphing hydrogels.11
Cu NPs also exhibit LSPR in the visible light region and thework function of Cu (4.65 eV) lies between Ag (4.0 eV) and Au(5.1 eV).12 For these reasons, plasmon-induced charge separation
might also be possible at the Cu NP­TiO2 interface. Since Cu
is much less expensive than Au and Ag, more cost-eﬀective
plasmonic devices could be developed by use of Cu NPs. New
functionalities could also be expected for Cu NP­TiO2 systems.
However, most of the previous work on Cu­TiO2 systems has
been for antibacterial eﬀects.13 In addition, it is known that Cu
NPs are easily oxidized and lose their plasmon resonancegradually under ambient conditions.14 For stabilization of Cu
NPs, Kominami and co-workers9b alloyed them with Au. To
stabilize pure Cu NPs, it is eﬀective to protect them with acapping agent.14,15 However, perfect protection would insulate
Cu NPs from the surrounding system and inhibit observation ofthe charge separation through photoelectrochemical reactions. In
the present work, we deposit Cu NPs on TiO2 by photocatalytic
reactions16 and coat the deposited NPs with poly(vinyl alcohol)(PVA) to mildly protect the NPs from extensive oxidation during
photoelectrochemical processes. As a result, photoelectrochemical responses based on LSPR of Cu NPs are observed undervisible light.
Chem. Lett. 2012, 41, 1340­1342

TiO2 suspension (75%, STS-21, Ishihara Sangyo) was spincoated at 1500 rpm for 10 s on a cleaned ITO-coated glass
substrate (40 © 12.5 © 1 mm3) and sintered (temperature wasraised up to 723 K at 1.5 K min¹1 and kept at the temperature for1 h) under atmospheric conditions, followed by irradiation withblack light (ca. 1 mW cm¹2) for >1 day to clean the surface
photocatalytically. A 0.1 mol L¹1 aqueous solution of Cu(CH3COO)2 (pH 5.4, 0.15 mL) was mixed with ethanol (3 mL),deaerated by N2 gas and was applied to the prepared anatase
TiO2 ﬁlm surface (0.15 mL). The ﬁlm was irradiated with UVlight (310 nm, 5 mW cm¹2) by using a Hg­Xe lamp under humid
N2 gas ﬂow for 5 min. As a result, the initially colorless TiO2
ﬁlm turned gray and a broad extinction (= absorption + scattering) peak characteristic of LSPR16 was observed at around
600 nm in the extinction spectrum (Figure 1a). Actually,
particles 20­40 nm in diameter were observed on the TiO2 ﬁlm
by SEM. Under UV light, electrons in the valence band of TiO2
are excited to the conduction band (¹0.48 V vs. NHE in a
pH 5.4 aqueous medium). Cu2+ ions are reduced to metallic CuNPs (E(Cu0/2+) = +0.34 V vs. NHE) by the excited electronsand positive holes generated in the valence band are consumedby oxidation of ethanol. The extinction spectrum of a 30 nm
Cu NP on TiO2 (Figure 1c, inset) was simulated on the basisof ﬁnite diﬀerence time domain (FDTD).16 The calculated
extinction peaks at 620 nm (Figure 1c), supporting that theexperimentally observed extinction peak is due to LSPR of Cu
NPs. The broader peak of the experimentally observed spectrum
than that of the calculated one is explained in terms of dispersion
in the particle size.
When the Cu NP­TiO2 sample was left in air under dark
conditions, the extinction peak gradually deteriorated as shownin Figure 1a. The sample left for 60 min was subjected to X-ray
photoelectron spectroscopy (XPS) and satellite peaks speciﬁc toCuO and a Cu LMM Auger signal ascribed to Cu+ were observed
as well as peaks ascribed to Cu0, whereas only the peaks for Cu0
were observed for as-deposited NPs. We therefore conclude that
the Cu NPs were partially oxidized by ambient oxygen to Cu
oxides such as CuO and Cu2O and the LSPR signal deteriorated.
Thus we spin-coated as-deposited Cu NPs with 3.4 wt %aqueous poly(vinyl alcohol) (PVA, Mw: 2000) followed bydrying in air at 393 K for 5 min to protect the NPs fromoxidation. The coating red-shifted the extinction peak of Cu NPs
by 25 nm due to an increased refractive index (ca. 1.5) aroundthe NPs (Figure 1b). The PVA-coated sample exhibited nosigniﬁcant change in the spectrum for at least 60 min in air underdark conditions, indicating that the Cu NPs were protected byPVA from oxidation in this period of time. Coating withpoly(vinylpyrrolidone) (PVP) also protected Cu NPs fromoxidation. PVA and PVP adsorb on the Cu surface by van derWaals interaction.14d
The PVA-coated Cu NPs were repeatedly irradiated withvisible light (480­700 nm, 15.5 mW cm¹2) in air. As a result,© 2012 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/1341

Figure 2. (a) Photopotential and (b) photocurrent action
spectra (latter: in the presence of 0.1 mol L¹1 triethanolamine)of the PVA-coated Cu NP­TiO2 electrode. Extinction spectra of
the electrode are also shown.

Figure 1. (a, b) Changes in the extinction spectra in air underdark conditions of (a) bare and (b) the PVA-coated Cu NPs
deposited on TiO2, respectively. (b inset) Changes in the peakextinction under repeated irradiation with visible light (480­700 nm, 15.5 mW cm¹2). (c) Simulated spectrum of a single Cu
NP on TiO2 (30 nm diameter).
the extinction peak height decreased during the irradiation(Figure 1b, inset) likely due to the plasmon-induced charge
separation. In the case of Ag NPs on TiO2, the charge separation
results in oxidation of Ag to Ag+ ions and reduction of ambient
O2.6a,11,17 Similar redox reactions possibly take place for thepresent system with Cu NPs.
Therefore the possible plasmon-induced charge separation
for the present system was examined by photoelectrochemicalmeans. In the case of TiO2-coated ITO electrodes with Au NPs
or Ag NPs, negative shifts of open-circuit potential and shortcircuit anodic currents are observed under visible light.6c,6d,8,17 If
the charge separation occurs also at the Cu NP­TiO2 interface,similar photoelectrochemical responses should be obtained.
Cu NPs were deposited on a TiO2-coated ITO electrode, andcoated with PVA and dried to obtain a PVA-coated Cu NP­TiO2electrode. This electrode as the working electrode and a Pt
counter electrode were immersed in acetonitrile containing
0.01 mol L¹1 tetrabutylammonium perchlorate (12 mL) as anelectrolyte. No signiﬁcant change in the spectrum was observed,Chem. Lett. 2012, 41, 1340­1342

indicating that the PVA coating was thicker than the penetration
length of the localized electric ﬁeld. Although the working
electrode was illuminated repeatedly (480­700 nm, 15.5mW cm¹2), no signiﬁcant changes in the open-circuit potential(vs. counter electrode) were observed. This may be explained interms of poor permeability of the dried PVA ﬁlm to electrolyte
ions. Similar behavior is observed for a hydrophilic conducting
polymer-coated electrode, which exhibits electrochemical responses in an organic medium only after addition of a certain
amount of water, which swells the polymer ﬁlm.18 We therefore
added 0.8 mL of water to the electrolyte, and observed shifts inthe potential (ca. ¹35 mV) during the light irradiation.
A TiO2-coated ITO electrode without Cu NPs was coatedfurther with PVA and was subjected to control experiments.
However, no changes in the potential were observed underillumination. Next we examined photopotential action spectrumof the PVA-coated Cu NP­TiO2 electrode under monochromatic
light irradiation (full width at half-maximum: 10 nm, 5 © 1015
photons cm¹2 s¹1) (Figure 2a). The action spectrum is roughly inagreement with the extinction spectrum of the electrode. Theseresults indicate that Cu NPs and their plasmon resonance are
essential for the photopotential responses.
We also examined photocurrents from the PVA-coated CuNP­TiO2 electrode in the electrolyte containing 0.1 mol L¹1
triethanolamine as an electron donor under monochromaticlight. As a result, anodic photocurrents were observed(Figure 2b). Since no signiﬁcant photocurrents were observed
for the electrode without Cu NPs, we ascribe the photocurrents
to the plasmon resonance of the Cu NPs. The peak wavelengthof the action spectrum is shorter than that of the extinction
spectrum by ²20 nm. These results suggest that Cu NPs ofwhich peak wavelengths are short contribute largely to thephotocurrents. Since the resonance peak of Cu NPs red shifts by
PVA coating, the resonance at the shorter wavelength may be
attributed to Cu NPs with a thin PVA coating or a much swollen© 2012 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/

1342
PVA coating, of which dielectric constant is close to that ofwater, rather than that of dried PVA. The thin or swollen coating
may facilitate supply of the electron donor (i.e., triethanolamine)from the bulk electrolyte to Cu NPs, and removal of the oxidized
donor to the bulk electrolyte.
Although the peak of the photopotential action spectrum
also blue-shifted from that of the extinction spectrum, the shift(¯20 nm) was smaller than that for the photocurrent action
spectrum. This is reasonable because the photopotential response
does not need electron donors, unlike the photocurrent response.
Therefore, the PVA coating does not interfere with the photopotential responses, unless the NPs are completely insulated.
In the case of Au and Ag NPs, ITO/TiO2/NP electrodes
exhibit negative photopotential shifts and anodic photocurrents
as described above,6c,6d,17a while ITO/NP/TiO2 electrodes
exhibit positive photopotential shifts and cathodic photocurrents.6e,17b The polarity inversion of photoresponses undervisible light, which is not observed when TiO2 is excited by
UV light, strongly supports electron injection from resonant NPs
to TiO2, whereas the inversion cannot be explained in terms ofplasmonic enhancement of excitation of electrons from TiO2
surface states to conduction band. The plasmon-based photoresponses of the present system may also be due to similarelectron injection from Cu NPs to TiO2. If an n-type semiconductor is in contact with a metal of which work function isgreater than electron aﬃnity of the semiconductor, a Schottky
barrier forms at the junction. This holds for junctions of anatase
TiO2 (ca. 3.9 eV19) with Au, Ag, and Cu (5.1, 4.0, and 4.65 eV,respectively12). The barrier may contribute to the charge
separation by preventing the separated charges from recombination.
Thus we found for the ﬁrst time that pure Cu NPs depositedon TiO2 exhibit photopotential and photocurrent responses. The
low response (incident photon to current conversion eﬃciency:
ca. 0.01%) is likely due to PVA, which improves stability atthe expense of eﬃciency. Since the plasmon-induced charge
separation is enhanced at the sites at which oscillating electricﬁeld is strongly localized,20 employment of NPs with such ﬁelds(e.g., NPs with sharp tips) would improve the eﬃciency. On theother hand, we have demonstrated that Ag NPs can be protectedby coating the NPs with TiO2.17b A similar strategy would give
more stable and practical photoelectrodes with Cu NPs without
undue deterioration in the eﬃciency. The photoelectrochemicalcorrosion in the absence of the polymer coating could also beexploited for photofunctionalities. Ag NPs, of which resonancewavelength largely varies as the particle size changes on TiO2,can be applied to multicolor photochromism.6a,6b Since resonance wavelength of Cu NPs is less sensitive than that of Ag
NPs to changes in the particle size,16 monochromatic photochromism may be a potential application of Cu NPs.
This work was conducted as a part of “R&D on Innovative
PV Power Generation Technology” which The University ofTokyo contracted with NEDO. EK thanks a JSPS Research
Fellowship for Young Scientists.
Paper based on an invited lecture at the International
Association of Colloid and Interface Scientists, Conference(IACIS2012), Sendai, Japan, May 13­18, 2012.

Chem. Lett. 2012, 41, 1340­1342

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