Nanocrystalline TiO2-catalyzed photoreversible color switching

Article abstract of DOI:10.1021/nl500378k

We report a novel photoreversible color switching system based on the photocatalytic activity of TiO₂ nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO₂ nanocrystals under UV irradiation, and the recoloration process operates by the TiO ₂-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications.

Full text of DOI:10.1021/nl500378k

Letter
pubs.acs.org/NanoLett
Nanocrystalline TiO2‑Catalyzed Photoreversible Color Switching
Wenshou Wang,† Miaomiao Ye,‡ Le He,† and Yadong Yin*,†
†Department of Chemistry, University of California, Riverside, California 92521, United States
‡Institute of Municipal Engineering, Zhejiang University, Hangzhou, 310058, P. R. China
S
* Supporting Information
ABSTRACT: We report a novel photoreversible color switching system based on the
photocatalytic activity of TiO2 nanocrystals and the redox-driven color switching property
of methylene blue (MB). This system rapidly changes from blue to colorless under UV
irradiation and recovers its original blue color under visible light irradiation. We have
identified four major competing reactions that contribute to the photoreversible switching,
among which two are dominant: the decoloration process is mainly driven by the reduction
of MB to leuco MB by photogenerated electrons from TiO2 nanocrystals under UV
irradiation, and the recoloration process operates by the TiO2-induced self-catalyzed
oxidation of LMB under visible irradiation. Compared with the conventional color
switching systems based on photoisomerization of chromophores, our system has not only
low toxicity but also significantly improved switching rate and cycling performance. It is
envisioned that this photoreversible system may promise unique opportunities for many
light-driven actuating or color switching applications.
KEYWORDS: TiO2 nanocrystals, photoreversible, color switching, reduction−oxidation reaction, methylene blue
he development of new materials for actuators that
reversibly change color in response to external stimuli,
T
such as electric or magnetic field, mechanical stress, temper-
ature change, or chemical reaction, has attracted a great deal of
attention for their important applications in sensing devices,
display and signage technologies, and security features.1−5
A
particularly intriguing possibility is offered by light-responsive
materials, which can be remotely controlled and rapidly
changed in a clean and noninvasive manner without the need
of direct contact with the actuators.6−10 The common
photoresponsive materials, however, mainly operate based on
the photoisomerization of constitute chromophores, such as
azobenzene, which undergo cis−trans isomerization upon
alternating irradiation with ultraviolet (UV) and visible
light.6,11 In addition to photoisomerization, the chromophores
also experience competing thermal back relaxation and
sometimes other side reactions which, although slow, are
unavoidable and give rise to serious problems such as lack of
stability and controllability,10,12,13 leading to low efficiency both
in color switching rate and cycling performance.
Methylene blue (MB, tetramethylthionine chloride,
C16H18ClN3S) is a heterocyclic aromatic dye that is brightly
blue colored in an oxidizing environment. Upon reduction,
however, it changes to leuco methylene blue (LMB), which is
colorless.14 The relationship between the oxidized form (MB)
and its stable reduced leuco form (LMB) is summarized in
Figure 1. Photochemical reduction of MB to LMB using
semiconductors in photoelectrochromisman area once very
attractive in the field of solar to electrical energy conversion
had been widely studied in the 1970s.15 Composite systems
containing a semiconductor material such as TiO2 (Degussa
P25), MB, and a sacrificial electron donor (SED) were also
Figure 1. Schematic illustration of the photoreversible color switching
between MB (blue) and LMB (colorless) catalyzed by TiO2
nanocrystals. The chloride ion is omitted in the molecular structure
of MB. Reactions 1−4 are shown here.
developed previously as oxygen indicators in food and
pharmaceutical industries to ensure freshness and quality
control of the products.16−18 In this case, blue MB is
transformed into colorless LMB upon UV irradiation, where
the holes photogenerated from TiO2 oxidize SED, and the
remaining photogenerated electrons reduce MB to LMB
(Reaction 1 in Figure 1). Recoloration occurs when the system
is exposed to oxygen, inducing back electron transfer from
Received: January 29, 2014
Revised: February 16, 2014
Published: February 20, 2014
© 2014 American Chemical Society
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Figure 2. (a) TEM image, (b) XRD, and (c) UV−vis spectrum of TiO2 nanocrystals prepared by a high temperature hydrolysis reaction using P123
as a capping ligand. Inset in (c): a digital photograph of a concentrated aqueous dispersion of the TiO2 nanocrystals in a glass vial.
Figure 3. Photoreversible color switching of the TiO2 nanocrystals/MB/water mixture in response to UV and visible light irradiation: (a) UV−vis
spectra showing the decoloration process under UV irradiation; (b) UV−vis spectra showing the recoloration process under visible light irradiation.
Inset: digital photographs showing the reappearance of the color after visible light irradiation for various lengths of time. (c) UV−vis spectra of the
sample in 11 cycles of repeated UV and visible irradiation. (d) The absorption intensity at ∼664 nm recorded after each UV and visible irradiation
for continuous 11 cycles.
LMB to oxygen (electron acceptors) and producing a blue
colored oxidized form of MB. Interestingly, very similar
reactions may also occur when MB is used as a test model
pollutant in semiconductor photocatalysis, where photobleach-
ing of MB can be due to reduction to its leuco form rather than
total decomposition, especially at low concentrations of
dissolved oxygen.18 In this case, the MB to LMB transition
may lead to inaccurate estimation of the photocatalytic activity
of the semiconductor catalysts.
Herein we report the design of colloidal TiO2 nanocrystals
for enabling reversible and considerably fast color switching
based on the photoelectrochromic reactions of MB in response
to UV and visible light irradiation. With the assistance of TiO2
nanocrystals, UV irradiation can rapidly bleach the original blue
color of MB, while the resulting colorless system can be
switched back to blue through a relatively short exposure to
visible light. The key to the reversible photoelectrochromic
switching is the prevention of photodecomposition of the dye.
This goal is achieved first by synthesizing TiO2 nanocrystals
with small sizes and low crystallinity, which correspond to
considerably low photocatalytic activity toward dye decom-
position. We also use a nonionic polymer poly(ethylene
glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol)
(P123) as a capping ligand to bind on the nanocrystal surface
for maintaining colloidal stability of TiO2 nanocrystals and
more importantly, as an effective SED to scavenge photo-
generated holes, preclude photodegradation of the dye, and
ensure sufficient electrons for rapid reduction of MB to LMB.19
An added benefit of using P123 as SED is that alkoxy radicals
generated initially by hole oxidation may further reduce MB at
the TiO2/MB interface.15 To further minimize the irreversible
photocatalytic decomposition of MB, the solution is kept at
slightly acidic condition (pH = ∼4) to limit the formation of
oxidative hydroxyl radicals (E = 2.80 V vs normal hydrogen
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electrode), which are generated by the reaction of the holes
either with the hydroxyl groups on the oxide surface or with
water favorably at higher pH.20−22 Our unique synthesis
involving a high-temperature hydrolysis reaction also enables
carbon doping in TiO2 nanocrystals, which leads to
considerable broad absorption in the visible range. The
backward switching from colorless LMB to blue MB can thus
be initiated by visible light irradiation of the TiO2 nanocrystals,
and further completed by rapid self-catalytic oxidation of LMB
by dissolved oxygen. The light-responsive color switching of the
TiO2 nanocrystals/MB/water system can be repeated for more
than 10 cycles with fully recovery, making them promising
materials for possible light-driven actuating or color switching
applications.
A typical synthesis of TiO2 nanocrystals involves high
temperature hydrolysis of TiCl4 with NH4OH as a catalyst,
poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene
glycol) (P123) as a surfactant, and diethylene glycol (DEG) as
a polar solvent. Introduction of NH4OH provides water and
also increases the alkalinity of the reaction system, both
favoring the controlled hydrolysis of TiCl4. Heating the mixture
at 220 °C for 3 h produces colloidal TiO2 nanocrystals. As
shown in a representative TEM image in Figure 2a, these
nanocrystals are small particles with elongated shape and an
average size below ∼5 nm. A low-magnification TEM image is
also included in the Supporting Information as Figure S2. No
obvious precipitation can be found even after keeping the
nanocrystals in the aqueous solution for six months,
demonstrating an excellent dispersity in water. The crystalline
structure of the TiO2 nanocrystals was confirmed by X-ray
powder diffraction (XRD) analysis (Figure 2b), with all
diffraction peaks being indexed to anatase phase with cell
constants of a = 3.7852 Å and c = 9.5139 Å (JCPDS card no.
21-1272). The diffraction peaks are significantly broadened,
indicating low crystallinity and small sizes of the products. The
absorption spectrum of TiO2 nanocrystals shown in Figure 2c
exhibits a strong absorption from 250 to 375 nm with a broad
shoulder extending from 375 to 700 nm. The absorption
shoulder in the visible range is consistent with the light-brown
color of a concentrated solution of the TiO2 nanocrystals (inset
in Figure 2c), which is most possibly due to carbon doping
from P123 and/or DEG during the high temperature hydrolysis
reaction.23
mixture with UV light for 10 s and then visible light for 15
min. As shown in Figure 3c, the recorded spectra nearly
overlapped. Figure 3d plots the intensity of the main absorption
peak at ∼664 nm against 10 cycles of UV and visible light
irradiation, further demonstrating the complete reversibility of
color switching. Compared with previous reports,4,24 our
photoreversible color switching system was able to minimize
the photodecomposition and demonstrated only a slightly
reduced reversibility in 10 decoloration-recovering cycles. On
the other hand, photodecomposition, although minimal, still
occurred, which led to reduced reversibility over many cycles of
reactions. Full recoloration can also occur if the UV-irradiated
system was kept under laboratory ambient light, however with a
substantially longer recovering time of 150 min (Figure S3).
Again, decoloration and recoloration can be cycled more than
10 times. In addition, recoloration occurring under dark
conditions needed much longer time (240 min, Figure S4). It
is therefore clear that visible light irradiation can significantly
promote the recoloration rate of the system (Figure S5).
Compared with the color switching system based on
photoisomerization of chromophores, our TiO2 nanocrystals/
MB/water system exhibits superior performance in terms of
switching rates, reversibility, and repeatability. Additional
advantages of our system for practical application may also
include low toxicity, low cost, and low environmental impact, as
both titania particles and MB have already been widely used in
cosmetic, medical, and other industries.25,26 Compared with
organic chromophores, semiconductor titania nanocrystals also
have superior photostability, orders of magnitude longer excited
state lifetimes, electronic states, and associated optical proper-
ties that can be engineered by controlling the size, shape, and
other characteristics during synthesis.27
To confirm the MB/LMB transition, liquid chromatog-
raphy−mass spectroscopy (LC-MS) was used to analyze the
mixture after UV irradiation (colorless) and the completely
recovered sample after visible light irradiation (blue), with
aqueous solutions of pure TiO2 nanocrystals and MB in water
as control samples. The results are shown in Figure 4. A series
of peaks were observed in the LC-MS chromatogram for
aqueous TiO2 nanocrystals dispersion (Figure 4a), which were
analyzed by mass spectroscopy and indexed to fragments
We examined the response of the TiO2 nanocrystals/MB/
water system to irradiation of UV and visible light. Upon
irradiation with UV light under a 300 W Hg lamp equipped
with a 365 nm filter (∼8.2 × 102 μw/cm2), the blue color of the
nanocrystal dispersion disappeared completely in a short period
of 10 s. To capture the change of absorption with irradiation
time, we recorded the spectra under UV light with a decreased
intensity (∼0.5 × 102 μw/cm2). As shown in the time-
dependent UV/vis spectra in Figure 3a,b, the absorption peak
of the mixture at ∼664 nm decreased in intensity with longer
irradiation and eventually disappeared completely after 2.5 min
of UV exposure. The absorption spectra of the colorless system
fully recovered after 15 min of irradiation under visible light, in
consistent with the recoloration of the solution from colorless
to blue (inset in Figure 3b). Throughout the process, the
system remains a transparent dispersion without any
aggregation. To evaluate the potential of the color switching
system for practical applications, the decoloration and
recoloration processes were repeated for 10 times by
sequentially irradiating the TiO2 nanocrystals/MB/water
Figure 4. LC-MS chromatograms of aqueous solutions of (a) TiO2
nanocrystals, (b) MB, (c) TiO2 nanocrystals, and MB mixture after
UV irradiation of 10 s (colorless state), and (d) mixture of TiO2
nanocrystals and MB after recoloration by visible irradiation
(recovered blue state).
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derived from surface bound P123 (Figure S6). Typical LC-MS
chromatogram of MB dissolved in water displays three peaks
(Figure 4b), which can be indexed to m/z of 256, 270, and 284
by mass spectroscopy (Figure S7), corresponding to azure A,
azure B, and MB+ (losing one Cl−), respectively.28,29
Interestingly, the colorless and recolored samples displayed
superimposable LC-MS chromatograms (Figure 4c, d), making
it reasonable to conclude that negligible photodecomposition of
MB occurred during the color switching process.
To further understand the recoloration process, the colorless
TiO2 nanocrystals/LMB/water system were irradiated with
visible light by passing a series of bandpass filters (fwhm 25
nm) for a defined period of time (30 min), and the
corresponding UV−vis spectra are shown in Figure 5a.
been reported previously by following a photooxidative
quenching mechanism:30 LMB → LMB*; and then 2LMB* +
O2 → 2MB + 2OH−, where * denotes an electronically excited
state. The important role of oxygen in this reaction has been
confirmed by the fact that irradiation of LMB by using a
bandpass filter of 370 nm, in the absence of oxygen resulted in
no significant change in its color and absorption spectrum
(Figure S8). Note that, when the system was irradiated with
broad UV light produced by a powerful Hg lamp, the TiO2
nanocrystals were excited, leading to predominant reduction of
MB to LMB and subsequently no observance of backward
photooxidation of LMB.
The enhanced recoloration rate by visible light can be
attributed to self-catalyzed oxidation of LMB that is initiated by
TiO2 nanocrystals, which have an absorption shoulder in visible
region. Although further exploration is required to understand
their exact role in this recoloration process, it is reasonable to
believe that the TiO2 nanocrystals absorb visible light energy
and then excite the LMB to LMB*, which then reacts with
dissolved oxygen to produce MB, in a way similar to the above-
mentioned photooxidative quenching mechanism under UV
irradiation. When a small amount of MB molecules are
produced, they strongly absorb visible light, transfer the energy
to LMB molecules, and further promote their oxidation to MB,
leading to a self-catalyzed oxidation reaction (Reaction 3) and
subsequently rapid conversion of LMB to MB. Note that the
self-catalyzed oxidation of LMB by MB under visible light is
proposed for the first time by this work. In order to confirm
this reaction mechanism, we prepared a LMB solution by
reducing MB with ascorbic acid under visible light by following
a procedure reported by Lee and Mills.30 While the solution
remained colorless under extended visible light irradiation, a
small addition of MB solution to the system instantly initiated
the recoloration process which proceeds to completion within
seconds (Figure S9).
Clearly, the oxygen dissolved in the system again plays a key
role in the conversion of LMB to MB under visible light. In the
absence of oxygen, photooxidation of LMB cannot occur
(Figure S8).19 Instead, a reverse reaction of reduction of MB
(Reaction 4) may take place. When an aqueous mixture
containing TiO2 nanocrystals and MB was purged with
nitrogen to remove the dissolved oxygen, visible irradiation
led to gradual disappearance of blue color, as shown in the
time-dependent UV−vis spectra in Figure 5b, suggesting
reduction of MB by photoelectrons generated from TiO2
nanocrystals by visible irradiation due to their considerable
absorption in visible region. We believe this process may also
operate when oxygen is present, but it is much slower than the
self-catalyzed oxidation of LMB to MB, leading to overall fast
recoloration. Conversely, purging oxygen through the LMB
solution by bubbling air can significantly enhance the
recovering rate from LMB to MB, further demonstrating that
oxygen dissolved in the system plays a key role in the
recovering process (Figure S4b and 5d).
Figure 5. (a) UV−vis spectra of recoloration process of a colorless
TiO2 nanocrystals/LMB/water mixture upon irradiation using visible
lights passing through different bandpass filters (fwhm 25 nm). Each
irradiation lasts 30 min. (b) UV−vis spectra of a blue colored TiO2
nanocrystals/MB/water mixture initially purged with nitrogen for 20
min then followed by visible light irradiation for different amounts of
time.
The four major reactions involved in the color switching
process are summarized in Figure 1. Upon UV irradiation, the
TiO2 nanocrystals are excited to produce oxidative holes
(Eo(h+) = +2.91 V) and reductive electrons (Eo(e−) = −0.32 V)
which are diffused to the surface of the nanocrystals. As the
holes are captured by surface bound SED molecules including
both P123 and its possible derivatives, the surviving photo-
generated electrons reduce blue MB to colorless LMB
(Eo(MB/LMB) = +0.53 V) by following Reaction 1. The
Interestingly, all of the light with bandpass filters at different
wavelengths, such as 370, 400, 450, 500, 550, 600, 650, and 700
nm, leads to enhanced recoloration rate than the case of
laboratory ambient conditions. A particular surprise is the
enhanced recoloration rate from LMB to MB by using a
bandpass filter of 370 nm, which seems to be contradictory to
the observation of decoloration of MB by UV exposure. In fact,
this is due to the UV driven photooxidation of LMB in the
presence of abundant dissolved oxygen (Reaction 2), which has
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Figure 6. Photoreversible color switching of a TiO2 nanocrystals/RhB/water mixture upon UV and visible light irradiation: (a) UV−vis spectra of
the decoloration process under UV irradiation. (b) UV−vis spectra of the recoloration process under laboratory ambient conditions. (c) UV−vis
spectra of recoloration process under visible light irradiation. (d) The UV−vis spectra of a typical TiO2 nanocrystals/RhB/water system during three
cycles of repeated UV and visible light irradiation.
tight binding of SED molecules to the TiO2 nanocrystal surface
benefits not only the rapid electron-transfer from TiO2 to MB
but also the high stability of the colloidal system, both favoring
a high color switching rate. While photobleaching of MB is
dominant during UV irradiation, the resulting LMB can be
partially oxidized simultaneously into MB by dissolved oxygen
through Reaction 2.30 Under strong UV irradiation, Reaction 1
is however dominant as it is driven photocatalytically by excited
TiO2 nanocrystals. On the other hand, the recoloration process
essentially involves the oxidation of LMB by dissolved oxygen,
which however is very slow (150 min) under laboratory
ambient condition. Visible light irradiation can dramatically
enhance the recoloration rate by photocatalytic initiation of
LMB to MB transition followed by a visible light-driven self-
catalyzed LMB oxidation (Reaction 3). A competing reverse
reaction (Reaction 4) may exist during recoloration, thanks to
the photocatalytic activity of carbon doped TiO2 nanocrystals
in the visible region. However, this reaction is substantially
suppressed when the solution contains abundant O2 so that the
overall reaction is predominantly the oxidation of LMB to MB
(Reaction 3).
As pointed out previously, the cyclability of the photo-
electrochromic system is mainly attributed to the relatively low
photocatalytic activity of TiO2 nanocrystals toward photo-
decomposition of the dye and the efficient hole scavenging by
surface bound P123 as SED. For comparison, a simple aqueous
mixture containing Degussa P25 TiO2, MB, and P123 was
irradiated by UV light under laboratory ambient conditions
(Figure S10). The blue color of the system disappeared totally
after UV irradiation of 15 min. But after centrifuging for 5 min
to remove TiO2 particles, only a very weak absorption peak of
MB at ∼660 nm was observed, which did not change in
intensity even after aging for another 7.5 h, indicating
irreversible decomposition of the MB molecules.
The color switching scheme can be extended to other dyes
which change colors based on redox reactions. For example, as
shown in Figure 6, Rhodamine B (RhB), an important
representative of xanthene dyes well-studied for its good
stability as dye laser materials,31 also exhibited reversible color
switching by UV/visible light irradiation when it was mixed
with TiO2 nanocrystals. Upon UV irradiation, its absorption
peak at ∼556 nm decreased gradually in intensity and
disappeared completely after 6 min (Figure 6a). Under ambient
conditions, the absorption peak gradually recovered to ∼75% of
the original value after 15 h (Figure 6b). Further elongating the
maintaining time to 24 h did not lead to apparent change in the
absorption intensity. Similar to the case of MB, the recovering
rate of RhB from leuco RhB (LRhB) can be significantly
enhanced by visible light irradiation, with a full recovery in 55
min (Figure 6c). The longer light-responsive time of RhB in
comparison to MB is consistent with its higher chemical
stability. Also, unlike the MB system which allows more than 10
times of reversible color switching, full recovery becomes
difficult for the RhB system after 3 cycles of reactions (Figure
6d).
In summary, we have demonstrated a photoreversible color
switching system by integrating photocatalytic property of TiO2
nanocrystals with the redox-driven color change property of
MB. This system can be rapidly switched from blue to colorless
by UV irradiation and then back to original blue by visible light
irradiation, and such color switching cycles can be repeated for
more than 10 times. The decoloration process is dictated by
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reduction of MB by electrons generated in TiO2 nanocrystals
under UV irradiation, while the recoloration process is driven
by visible light irradiation, which excites TiO2 nanocrystals and
initiates self-catalyzed oxidation of LMB. In comparison to the
previous color switching systems based on photoisomerization
of chromophores, the current system has key advantages in
lower production cost and environmental impact and
significantly higher switching rate and cycling performance.
We believe this photoreversible system can find potential use in
many practical applications that require photoactuation and
color switching, such as photorewritable papers, security
features, and other information recording and displaying
devices.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental details, emission spectrum of the
visible lamp, TEM image of TiO2 nanocrystals, plots of the
percent of MB recovered from LMB, LC-MS chromatogram
analyses, and UV−vis spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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*E-mail address: yadong.yin@ucr.edu. Tel.: +1-951-827-4965.
Fax: +1-951-827-4713.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We are grateful for the financial support from the U.S.
Department of Energy (DE-FG02-09ER16096). We also thank
Prof. Jingsong Zhang for helpful discussion and providing
optical filters.
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dx.doi.org/10.1021/nl500378k | Nano Lett. 2014, 14, 1681−1686