Visible-light sensitive Cu(ii)-TiO2 with sustained anti-viral activity for efficient indoor environmental remediation

Article abstract of DOI:10.1039/c5ta03756e

Visible-light sensitive photocatalysts are desirable for indoor environmental remediation applications. Photocatalysts used for indoor environmental applications must have efficient visible-light activity and sustainable function under dark conditions, as indoor light instruments are typically switched off during the night. Herein, we report the synthesis and optimization of a highly visible-light sensitive Cu(ii)-TiO₂ nanocomposite with sustained anti-viral activity under dark conditions. The synthesized Cu(ii)-TiO₂ exhibited superior volatile organic compound decomposition and anti-viral activity under visible-light irradiation. Its quantum efficiency for the decomposition of gaseous 2-propanol reached 68.7%. In addition, Cu(ii)-TiO₂ completely inactivated the bacteriophage within 30 min of visible-light irradiation. Notably, the Cu(ii)-TiO₂ photocatalyst also exhibited sustained anti-viral activity under dark conditions after visible-light irradiation treatment. Taken together, these findings indicate that the prepared Cu(ii)-TiO₂ is potentially an effective risk-reduction material for indoor applications.

Full text of DOI:10.1039/c5ta03756e

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Materials Chemistry A
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Visible-light sensitive Cu(II)–TiO2 with sustained
anti-viral activity for efficient indoor environmental
remediation†
Cite this: DOI: 10.1039/c5ta03756e
b
b
bc
ad
*
*
and Masahiro Miyauchi
Min Liu, Kayano Sunada, Kazuhito Hashimoto
Visible-light sensitive photocatalysts are desirable for indoor environmental remediation applications.
Photocatalysts used for indoor environmental applications must have efficient visible-light activity and
sustainable function under dark conditions, as indoor light instruments are typically switched off during
the night. Herein, we report the synthesis and optimization of a highly visible-light sensitive Cu(II)–TiO2
nanocomposite with sustained anti-viral activity under dark conditions. The synthesized Cu(II)–TiO2
exhibited superior volatile organic compound decomposition and anti-viral activity under visible-light
irradiation. Its quantum efficiency for the decomposition of gaseous 2-propanol reached 68.7%. In
addition, Cu(II)–TiO2 completely inactivated the bacteriophage within 30 min of visible-light irradiation.
Notably, the Cu(II)–TiO2 photocatalyst also exhibited sustained anti-viral activity under dark conditions
after visible-light irradiation treatment. Taken together, these findings indicate that the prepared
Cu(II)–TiO2 is potentially an effective risk-reduction material for indoor applications.
Received 24th May 2015
Accepted 17th July 2015
DOI: 10.1039/c5ta03756e
www.rsc.org/MaterialsA
develop visible-light-sensitive photocatalysts for indoor envi-
ronmental purication applications.
1. Introduction
Over the past few decades, attempts to develop visible-light-
sensitive photocatalysts have focused on the doping of TiO2
with various transition metal ions or anions. Despite these
efforts, most systems are not suitable for practical indoor
applications because of their low quantum efficiencies (QEs)
caused by the carrier recombination centers in metal-ion-doped
TiO2 or the low oxidation power and mobility of photogenerated
holes in non-metal-doped TiO2.11–13 As an alternative strategy to
doping, our group demonstrated that the surface modication
of TiO2 with Cu(II) or Fe(III) nanoclusters increases the visible-
light-sensitivity of the resulting material without inducing
impurity levels in the band gap.14–19 In the case of Cu(II) nano-
cluster-graed TiO2 (Cu(II)–TiO2), electrons in the valence band
(VB) of TiO2 are excited to the Cu(II) nanoclusters under visible-
light irradiation through an interfacial charge transfer (IFCT)
process. The introduction of excited electrons into the Cu(II)
nanoclusters leads to the formation of Cu(I) species, which
efficiently reduce oxygen molecules. During the IFCT process,
holes with strong oxidative power for the decomposition of
organic compounds are generated in the deep VB of TiO2.19
Thus, nanocluster-modied TiO2 is a promising material for
visible-light-sensitive photocatalytic applications.
Indoor air quality is an important health and safety factor due to
its inuence on population health and wellbeing.1,2 The air
levels of volatile organic compounds (VOCs) are of particular
concern because these harmful chemicals cause a wide range of
human health problems, including sick house syndrome.3
Infectious pathogens are also frequently encountered in our
daily environment and can adversely impact human health.4,5
As a promising solution for environmental remediation, pho-
tocatalytic oxidation using semiconductors has attracted
considerable attention. However, because most efficient pho-
tocatalysts are wide band-gap semiconductors, such as TiO2,
which are only activated by ultraviolet (UV) light irradiation,
they have been limited to outdoor applications.6–9 As indoor
light sources contain low levels (below several mW cmꢀ2 in white
light uorescent light)10 or no UV light (white incandescent light
and white light emitting diodes [LEDs]), it is highly desirable to
aDepartment of Metallurgy and Ceramics Science, Graduate School of Science and
Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo
152-8552, Japan. E-mail: mmiyauchi@ceram.titech.ac.jp
bResearch Center for Advanced Science and Technology, The University of Tokyo, 4-6-1
Komaba, Meguro-ku, Tokyo 153-8904, Japan. E-mail: hashimoto@light.t.u-tokyo.ac.jp
cDepartment of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-
1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
Practical photocatalytic systems for indoor environmental
remediation require the following characteristics: (i) composed
of robust inorganic materials that are nontoxic and derived
from naturally abundant elements; (ii) visible-light sensitivity
with strong oxidation power; and (iii) anti-pathogenic (anti-
bacterial and antiviral) effects, even under dark conditions.
dJapan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-
0012, Japan
† Electronic supplementary information (ESI) available: ICP, XPS, XRD, SEM,
UV-Vis,
photocatalytic
performance
measurements
and
calculated
photocatalytic performances of Cu(II)–TiO2 samples. See DOI: 10.1039/c5ta03756e
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Cu(II) or Fe(III) nanocluster-graed TiO2 satisfy the rst Cu(II)–TiO2 and was determined by evaluating the following
requirement, as they consist of non-toxic and earth-abundant parameters: temperature of annealing on TiO2 powder, aqueous
elements.20–22 Regarding visible-light sensitivity, the reaction HCl treatment to clean the TiO2 surface, pH value for Cu(II)
rate and QE of Cu(II)-graed TiO2 are reportedly lower than graing, and amount of Cu(II) ions in aqueous solution.
those of Fe(III)-graed TiO2.13–16 For the third requirement,
copper oxides, particularly copper monoxide, are reported to
have strong anti-viral activity, even under dark conditions,20–23
as compared to iron-based oxides. Therefore, the optimization
and improvement of Cu(II)–TiO2, particularly with respect to
increasing the visible-light activity and the sustained anti-viral
function of Cu(II) nanocluster-modied TiO2 are key challenges
for developing efficient indoor applications for environmental
remediation.
In this work, we comprehensively optimized the synthesis
conditions for graing Cu(II) nanoclusters onto TiO2 to
increase the reaction rate and QE of this photocatalyst. In
addition to achieving the highest visible-light activity among
the reported Cu(II)–TiO2 photocatalysts, we demonstrated that
Cu(II)–TiO2 has sustained anti-viral activity, even under dark
conditions aer visible-light irradiation. Irradiation of
Cu(II)–TiO2 with visible light causes the Cu(I) species to reach
an excited state, which is critical for the anti-viral function
under both light and dark conditions. The Cu(II)–TiO2 material
is photocatalytically activated by indoor light irradiation and
also maintains the anti-viral activity under dark conditions.
Due to these properties, the presently synthesized Cu(II)–TiO2
photocatalyst has great potential as an effective material for
indoor purication applications aimed at minimizing risks to
individual and public health.
2.2 Sample characterization
The crystal structures of the prepared Cu(II)–TiO2 nano-
composites were measured by powder X-ray diffraction (XRD) at
room temperature on a Rigaku D/MAX25000 diffractometer
with a copper target (l ¼ 1.54056 A). Elemental analyses of the
˚
samples were performed using an inductively coupled plasma-
atomic emission spectrometer (ICPAES; P-4010, Hitachi).
UV-visible absorption spectra were recorded by a diffuse
reection method using a UV-2550 spectrometer (Shimadzu).
The morphologies of the prepared TiO2 nanocomposites were
investigated by scanning electron microscopy (SEM) using a
Hitachi SU-8000 apparatus and transmission electron micros-
copy (TEM) using a Hitachi HF-2000 instrument with an accel-
eration voltage of 200 kV. The specic surface areas of the
samples were determined from nitrogen absorption data at
liquid nitrogen temperature using the Barrett–Emmett–Teller
(BET) technique.24 Briey, the samples were degassed at 200 ꢁC
and the pressure was kept below 100 mTorr using a Micro-
meritics VacPrep 061 instrument for a minimum of 2 h prior to
the analysis. Surface compositions were studied by X-ray
photoelectron spectroscopy (XPS; model 5600, Perkin-Elmer).
The binding energy data were calibrated with reference to the C
1s signal at 284.5 eV. The thermogravimetric analysis of the
samples was performed using a Rigaku Thermo plus TG 8120
apparatus within a temperature range between 298 and 1473 K
ꢀ1
ꢁ
2. Experimental section
in air atmosphere and at a heating rate of 10 C min
.
2.1 Synthesis of Cu(II)–TiO2 samples
2.3 Evaluation of photocatalytic properties
Cu(II)–TiO2 samples were prepared using a modied impreg-
nation method, as described previously.19 Briey, commercial The decomposition of gaseous (2-propanol) IPA in air atmo-
TiO2 (MT-150A, Tayca Co.; rutile phase, 15 nm grain size) was sphere was chosen as a probe to evaluate the photocatalytic
annealed at 950 ꢁC for 3 h. The calꢁcined TiO2 was treated with a activities of the samples. For the analysis, 0.3 g powder samples
6 M HCl aqueous solution at 90 C for 3 h under continuous were evenly dispersed on the bottom of a circular glass dish
stirring. Aer passing the reaction mixture through
a
(5.5 cm2), which was set in the center of a 500 mL cylindrical
membrane lter (0.025 mm, Millipore), the products were glass vessel reactor. Aer the vessel was sealed with a rubber
collected and then washed thoroughly with distilled water. The O-ring and a quartz cover, the reactor was evacuated and lled
obtained TiO2 powder was dried at 110 ꢁC for 24 h and was then with fresh synthetic air. To avoid organic contamination of the
ground into a ne powder using an agate mortar and pestle. For sample surface, the vessel was pre-illuminated with an Xe lamp
the graing of Cu(II) nanoclusters, 1 g TiO2 powder was (Luminar Ace 210, Hayashi Tokei Works) until the CO2 gener-
dispersed in 10 mL distilled water, to which copper chloride ation rate was less than 0.02 mmol per day. The vessel was
dihydrate (CuCl2$2H2O, Wako, 99.9%) was then added. The re-evacuated and re-lled with fresh synthetic air, and the
weight fraction of Cu relative to TiO2 was set to 0.1% and the pH internal pressure was maintained at approximately 1 atm. To
value of the solution was adjusted to 12 using sodium hydroxide begin the decomposition analysis, 300 ppm of gaseous IPA was
(NaOH). The resulting suspension was heated at 90 ꢁC under injected into the vessel. Prior to light irradiation, the vessel was
continuous stirring for 1 h in a vial reactor. The obtained kept in the dark for a sufficient time (approximately 12 h) to
product was ltered through a 0.025 mm membrane lter achieve absorption/desorption equilibrium of IPA on the pho-
(Millipore) and then washed with sufficient amounts of distilled tocatalyst surface. The vessel was then irradiated by visible light
water. Finally, the obtained product (denoted as Cu(II)–TiO2 with wavelengths from 420 to 530 nm emitted from an Xe lamp
(950-HCl-12)) was dried at 110 ꢁC for 24 h and was subsequently equipped with a combination of glass lters (B-47, L-42, and
ground into a ne powder using an agate mortar and pestle. The C-40C; AGC Techno Glass). The light intensity was measured
above-mentioned procedure is the typical synthesis condition using a spectroradiometer (USR-40D, Ushio) and was set to
used to achieve the optimum photocatalytic activity of 1 mW cmꢀ2. During the light irradiation, 1 mL gaseous samples
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were periodically extracted from the reaction vessel to detect the which resulted in a decrease of the powder weight between 400
ꢁ
concentrations of IPA, acetone and CO2 using a gas chromato- and 500 C. The exothermic peak was attributed to the crystal-
graph (model GC-8A, Shimadzu Co., Ltd).
lization of the powder,25 indicating that the TiO2 powder
obtained at approximately 935 C possessed good crystallinity.
ꢁ
The XRD patterns of the annealed powders showed that all of
the samples had a rutile structure (JCPDS card no. 21-1276). In
addition, the diffraction peaks became sharper with increasing
annealing temperature, indicating that the crystallinity of the
samples was improved by the annealing treatment. Fig. 1b
shows the inuence of the pre-annealing temperature on the
specic surface area and crystallite size of TiO2, as determined
by Scherrer's equation.26 The annealing treatment resulted in an
increase in the crystallite size of the TiO2 powder (from ꢃ15 to
ꢃ125 nm), but markedly decreased the total surface area. SEM
image analysis clearly showed that the particle size increased
with increasing annealing temperature (Fig. S2†). Notably,
performing the annealing step at over 950 ꢁC resulted in
signicant crystal growth and caused the color of the powder to
change from white to brown (Fig. S3†), indicating the intro-
duction of oxygen defects or generation of Ti3+ species into the
sample.17 Oxygen defects or Ti3+ species have been demon-
strated to reduce the photocatalytic performance of TiO2.17
Furthermore, the surface areas of the samples obtained at
950 and 1150 ꢁC were ꢃ5.8 and ꢃ1.2 m2 gꢀ1, respectively, which
are markedly smaller than the ꢃ110 m2 gꢀ1 area of the starting
TiO2 powders.
Fig. 2 shows the TEM and energy-dispersive X-ray spectros-
copy (EDS) analyses of the Cu(II)–TiO2 (950-HCl-12) sample,
which was prepared by pre-annealing TiO2 at 950 ꢁC, acid
treating before Cu(II) graing, and graing the Cu(II) clusters
under pH 12 conditions. These conditions were determined to
result in optimal photocatalytic activity of the prepared mate-
rial, as described in detail below. The TEM analysis revealed
that Cu(II) nanoclusters were well dispersed on the surface of
TiO2 (Fig. 2a). High-resolution TEM (HRTEM) image analysis
showed that the Cu(II) nanoclusters were ꢃ3 nm in size and well
attached to the TiO2 surface (Fig. 2b). EDS point analysis
(Fig. 2c) revealed that the nanoclusters were composed of Cu.
We also measured the XPS spectra of the Cu(II)–TiO2
(950-HCl-12) sample (Fig. S4†) and conrmed that Cu(II) species
were present on the TiO2 surface. The optical absorption
properties of Cu(II)–TiO2 and bare TiO2 were recorded and are
shown in the ESI (Fig. S5†). For bare TiO2, no absorption was
observed in the UV-vis spectra at wavelengths shorter than
2.4 Evaluation of anti-viral activity
Cu(II)–TiO2 was coated on a glass substrate (2.5 cm ꢂ 2.5 cm) by
simply drop casting 150 mL of an ethanol suspension of
Cu(II)–TiO2 (1 mg mLꢀ1). The total amount of Cu(II)–TiO2 coated
on the substrate was 0.3 mg/6.25 cm2, which is equal to 0.48 g
mꢀ2. The glass surface was dried and then sterilized at 120 C
ꢁ
for 3 h. Qb bacteriophage (NBRC 20012) and Escherichia coli
(NBRC 13965) as the host bacterium were used in the evaluation
experiment. A stock suspension of Qb bacteriophage (ꢃ1.2 ꢂ
1011 plaque forming units per mL [PFUs mLꢀ1]) was used to
infect E. coli cells at 35 ꢁC for 10 min, and then was plated onto a
double-layered medium, which was prepared with Nutrient
Broth (Difco) and agar (Difco) by adjusting the agar concen-
tration of the bottom layer to 1.5% and that of the top layer to
0.5%. The plates were then further incubated overnight at 35 ꢁC,
and the top agar layer containing the cells and bacteriophage
was then collected and added to 2 mL per plate SM buffer
(0.1 M NaCl, 8 mM MgSO4, 50 mM Tris–HCl [pH 7.5] and 0.1%
gelatin) at 4 ꢁC overnight. The solution was centrifuged
ꢁ
(8000ꢂg, 4 C, 20 min) and the resulting supernatant contain-
ing the bacteriophage was collected and ltered (0.22 mm,
Millipore, MA). The bacteriophage solution was diluted with
PBS to give approximately 2.5 ꢂ 109 PFUs mLꢀ1 and 50 mL of the
diluted suspension was then pipetted onto the Cu(II)–TiO2
coated glass. The glass samples were irradiated using 10 W
white light bulbs at an illuminance of 1000 lux. The light illu-
minance was measured using an illuminance meter (IM-5,
Topcon), and irradiation with visible light with wavelengths
above 400 nm was performed using a white uorescence light
bulb through a UV cutoff lm. Aer irradiation, the bacterio-
phage suspension was collected into SM buffer (10 mL). An
appropriate dilution of the collected suspension was used to
infect E. coli cells, which were then plated onto nutrient agar
medium using the above-described double-layer method to
determine the number of plaques.
3. Results and discussion
3.1 Improvement of visible-light sensitivity of Cu(II)–TiO2
To improve the visible-light activity of Cu(II)–TiO2, we investi-
gated the effect of the synthesis conditions, including the pre-
annealing treatment of bare TiO2 powder, washing treatment of
TiO2 under aqueous acid conditions, pH during the graing of
Cu(II) nanoclusters, and the amount of Cu(II) nanoclusters, on
the photocatalytic activity. To investigate the effect of the pre-
annealing conditions for TiO2 powder, XRD patterns of TiO2
powdꢁer annealed for 3 h at temperatures ranging from 300 to
1150 C were recorded (Fig. 1a). TG-DTA analysis was also per-
formed and revealed that endothermic and exothermic peaks of
the TiO2 powder appeared at approximately 440 and 935 ꢁC,
respectively (ESI, Fig. S1†). The endothermic peak mainly orig-
Fig. 1 (a) XRD patterns of TiO2 samples obtained at various annealing
temperatures and (b) crystallite sizes and surface areas of TiO2 versus
inated from the removal of surface-absorbed compounds,25 annealing temperature.
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420 nm other than the band-gap excitation of rutile TiO2. nding indicates that TiO2 with good crystallinity and few
However, aer the graing of Cu(II) nanoclusters on the defects, meaning less recombination centers, is important for
TiO2 surface, new absorptions in the regions of 420–550 and the photocatalytic activity of the Cu(II)–TiO2 system, even
700–800 nm were observed (Fig. S5†). The slight increase in though the TiO2 surface area is signicantly decreased. In
absorption in the former region was attributed to the IFCT addition to crystallinity, aqueous acid treatment of the TiO2
process of VB electrons to surface Cu(II) nanoclusters, whereas powder prior to the graing of Cu(II) clusters is critical for the
the increase in the latter region was assigned to a simple d–d removal of surface contaminants and achieving high photo-
transition of Cu(II).14
catalytic performance. Previous studies have reported that the
The photocatalytic activity of the Cu(II)–TiO2 samples was presence of contaminants, such as alkali and alkaline earth
evaluated by the visible-light-induced oxidation of IPA, which is elements (Na+ or Ca2+), markedly deteriorates the photocatalytic
frequently used as a representative gaseous VOC and is a activity of TiO2.29,30 The present XPS spectra analysis revealed
harmful indoor air pollutant.27 The photocatalytic oxidative that surface contaminants, including Na+ and Ca2+, were
decomposition of IPA proceeds via the formation of acetone as removed by acid treatment in hydrochloric acid (HCl) solution
an intermediate, followed by the further decomposition of (Fig. S7†). Owing to the high acid stability of TiO2, no obvious
acetone to the nal products, CO2 and H2O (Fig. 3a).28 During difference could be observed in the morphologies of the TiO2
the photocatalytic tests, the light intensity was adjusted to samples with or without acid treatment (Fig. S8†). Thus, the QE
ꢃ1 mW cmꢀ2, which corresponds to an illuminance of ꢃ300 lux value of Cu(II)–TiO2 was improved from 13.2% to 30.3% by acid
and is comparable to the intensity of typical indoor uorescent treatment (950-HCl-7 in Table 1), since the surface contaminant
and LED light. The wavelength of the irradiation light ranged was removed by acid treatment, resulting in efficient charge
from 420 to 530 nm, and the initial IPA concentration was set to transfer between TiO2 and Cu(II) clusters.
300 ppmv (ꢃ6 mmol). For all samples shown in Fig. 3, Cu(II)
In the above experiment, Cu(II) graing was performed
nanoclusters were graed at pH 7. The photocatalytic perfor- under neutral pH conditions. To examine the effect of pH
mance of the TiO2 samples increased with increasing annealing during graing on the photocatalytic activity of Cu(II)–TiO2, we
temperature up to 950 ꢁC (Fig. 3b). The performance of the TiO2 next attempted to optimize the pH of the solution used for the
sample obtained at an annealing temperature of 950 ꢁC was graing of Cu(II) nanoclusters. We used the TiO2 powder
markedly higher than that of the un-annealed sample. The QE prepared using a pre-annealing temperature of 950 ꢁC and HCl
was determined by the reaction rate (R) and number of absor- treatment as a starting material. From the UV-Vis spectra of the
bed photons (Fig. S6†). For the sample prepared by Cu(II)–TiO2 samples prepared at different pH values, it can be
pre-annealing treatment at 950 ꢁC, the QE was 13.2%. However, seen that visible-light absorption caused by either the IFCT
when the annealing temperature exceeded 950 ꢁC, the photo- process or the d–d transition of Cu(II) increased with increasing
catalytic activity of the samples markedly decreased. These alkalinity (Fig. 4a). The increase of the d–d transition of Cu(II)
results are attributable to the formation of oxygen or Ti3+ indicates that the amount of Cu(II) had increased in the samples
defects at high temperature. Thus, the sample obtained at prepared at higher pH, a nding that was conrmed by XPS and
ꢁ
1150 C exhibited the lowest photocatalytic performance. This ICP measurements (Fig. S9 and Table S1†). At pH values below
12, only a limited amount of Cu(II) nanoclusters (less than 20%
of added CuCl2 salt in aqueous solution) was graed on the
TiO2 surface. In contrast, when the pH value was above 12,
nearly all Cu(II) ions in solution were graed onto the TiO2
surface as nanoclusters. Thus, the IFCT visible-light absorption
sharply increased at pH values over 12 (Fig. 4a, inset). The
observed inuence of the pH on the amount of graed Cu(II)
nanoclusters may be due to the dependence of the surface
charge of TiO2 on pH. Under strong acidic (HCl) conditions, the
Fig. 2 (a) TEM and (b) HRTEM images of Cu(II)–TiO2 (950-HCl-12).
Nanoclusters (indicated by red arrows) were highly dispersed on the
TiO2 surface. In (b), the short dashed curve outlines a single Cu(II) Fig. 3 (a) Representative time-dependent gas concentrations during
nanocluster. The good attachment of nanoclusters and TiO2 can be IPA decomposition by the Cu(II)–TiO2 (950-7) sample under visible-
clearly observed. (c) EDS point analysis of the Cu(II) nanocluster on the light irradiation. (b) Comparative analysis of CO2 generation by Cu(II)–
point (i) in panel (b).
TiO2 synthesized at different temperatures under the same conditions.
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Table 1 Summary of the photocatalytic performances of the Cu(II)–TiO2 samples under different experimental conditions
Initial photons
Absorbed photons
(quanta secꢀ1 cmꢀ2
Reaction rate
Experimental conditions
pH dependence
pH
Cu(II) amount
(quanta sꢀ1 cmꢀ2
)
)
(mmol hꢀ1
)
QE (%)
2
4
7
10
12
14
12
12
12
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.05%
0.25%
0.5%
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
1.3 ꢂ 1016
4.43 ꢂ 1014
4.53 ꢂ 1014
4.62 ꢂ 1014
4.84 ꢂ 1014
5.82 ꢂ 1014
6.48 ꢂ 1014
4.98 ꢂ 1014
7.94 ꢂ 1014
9.55 ꢂ 1014
0.07
0.10
0.14
0.24
0.40
0.05
0.32
0.33
0.13
15.8
22.1
30.3
49.6
68.7
7.7
64.2
41.6
13.6
Cu(II) amount dependence
TiO2 surface is positively charged, as the pH value of the activity of Cu(II)–TiO2. Notably, the sample exhibited the
isoelectric point of bare TiO2 is neutral at a pH of approximately highest performance at a pH of 12. The calculated QE and R
6.31 Therefore, high pH conditions cause a negative shi in the for this sample were 68.7% and 0.40 mmol hꢀ1, respectively
surface TiO2 charge, allowing for the efficient graing of Cu(II) (Table 1), which have been the highest values reported to date
clusters. It is noted that we could not see the obvious loading for a Cu(II)–TiO2 photocatalytic system.14,17,19,21,22 A further
amount dependence on photocatalytic activities, when the pH increase in the pH value resulted in a drastic decrease of the
condition was neutral (Fig. S10†). These results also imply that photocatalytic performance, because the Cu(II) nanoclusters
the TiO2 surface is positively charged aer the acid treatment, were changed into CuO crystals, leading to a marked decrease
and that the amount of loaded Cu(II) nanoclusters strongly in the multi-electron reduction reaction rate and the reduced
depends on the pH of the solution. It is also possible that the consumption of photogenerated electrons.19
dependence of the Cu(II) nanocluster graing efficiency on pH
We also attempted to optimize the amount of Cu(II) graed
is due to the hydrolysis of Cu(II) ions. Cu(II) ions are poorly on the TiO2 surface at pH 12 (Fig. 5). ICP measurements showed
soluble in highly alkaline solutions and precipitate as black- that nearly all of the Cu(II) was graed on the surface of TiO2 at
brownish CuO particles, as shown in Fig. S11.† In the present pH 12 and that the total amount of graed Cu(II) increased with
study, when the pH value of Cu(II) solution was increased to 14, increasing levels of the initial Cu(II) concentration (Table S2†).
we conrmed that CuO crystals were formed (Fig. S12†). Our Consistent with this nding, the visible-light absorption of the
group previously demonstrated that bulk crystal CuO particles Cu(II)–TiO2 samples increased with increasing amounts of
poorly consume photogenerated electrons,19 which is crucial for graed Cu(II) (Fig. 5a). Measurements of the photocatalytic
the IFCT process and the multi-electron reduction reaction. At performances of the Cu(II)–TiO2 samples showed that their
pH values below 12, the heterogeneous hydrolysis of Cu(II) ions visible-light activities were rst increased with increasing
occurs on the surface of TiO2 powder, resulting in a good amounts of Cu(II). When the amount of Cu(II) exceeded 0.1%,
junction between the Cu(II) nanoclusters and TiO2 surface. the photocatalytic activity of Cu(II)–TiO2 decreased. The results
Under the present experimental conditions, a pH value of demonstrated that 0.1% is the optimal amount of graed Cu(II)
12 was optimal for the graing of Cu(II) nanoclusters. Further, nanoclusters for Cu(II)–TiO2. Notably, the photocatalytic activity
XPS spectra showed that the chemical states of elemental Ti and of Cu(II)–TiO2 prepared under these conditions was markedly
O were similar to those of Cu(II)–TiO2 prepared at pH 12 and superior to that of TiO2ꢀxNx, which is considered to be one of
bare TiO2 (Fig. S13†). Taken together, the results indicate that the most efficient visible-light photocatalyts.32 The calculated
the chemical state and environment surrounding the Cu(II) QE of the TiO2ꢀxNx samples was only 3.9%, which is dramati-
nanoclusters in the Cu(II)–TiO2 sample prepared at pH 12 were cally lower than that of our Cu(II)–TiO2 (950-HCl-12) sample of
highly similar to those of a previously characterized Cu(II)–TiO2 68.7% (Table 1).
system.14,19
Fig. 4b shows the photocatalytic performances of the
Cu(II)–TiO2 (950-HCl) samples prepared at different pH
values. It can be seen that the photocatalytic activity of
Cu(II)–TiO2 increased with increasing pH during the graing
of Cu(II) nanoclusters. The limited photocatalytic perfor-
mance of the samples prepared at pH values less than 12 is
likely attributable to the limited amount of Cu(II) nano-
clusters on the positively charged TiO2 surface. As the pH
increased, the TiO2 surface became more negative, and the
amount of surface-graed Cu(II) nanoclusters increased,
Fig. 4 (a) UV-Vis spectra for Cu(II)–TiO2 (950-HCl) samples prepared
leading to a corresponding increase in the photocatalytic
at different pH values. (b) CO2 generation curves for the prepared
samples under visible-light irradiation.
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viral activity of Cu(II)–TiO2 (950-HCl-12) was enhanced when
strong visible-light sources, such as a uorescent lamp, or UV
light sources, such as a black light bulb, were used for the
irradiation treatment (Fig. S14†).
The anti-bacterial and anti-viral properties of TiO2 under
light irradiation have been widely investigated33,34 and are
attributed to the direct oxidation of virus by photogenerated
holes or reactive radical species, such as cOH radical, O2ꢀ, and
H2O2.35 However, the inactivation speed of bacteria and virus on
TiO2 was limited by the step of disordering the outer
membranes of bacteria and virus.33 The anti-viral effects of Cu
metal, Cu oxides, and Cu(II) ions have also been determined.23,36
The anti-viral effects on Cu species are attributed to the block-
ing of functional groups of proteins and inactivation of
enzymes.23 Our recent study demonstrated that Cu(I) species in
Cu nanoclusters are much more effective than Cu metals or
Cu(II) species for enhancing the anti-viral and anti-bacterial
effects of TiO2 photocatalysts.20 Irie et al.19 investigated the role
of Cu(II) nanoclusters in electron-trapping by in situ X-ray
absorption ne structure (XAFS) analysis under visible-light
irradiation in the presence of IPA and absence of oxygen and
found that Cu(I) was generated under these conditions. There-
fore, the presently observed anti-viral activity of Cu(II)–TiO2
(950-HCl-12) under dark conditions aer visible-light irradia-
tion was induced by the Cu(I) species in the solid-state Cu(II)
nanoclusters. Previous studies have demonstrated that the
combination of TiO2 with CuO or Cu2O resulted in visible-light
absorption and electron trapping among TiO2 and these copper
oxides under visible-light irradiation.37,38 The trapped electrons
lead to the formation of highly reduced states of TiO2/Cu(I)
species, which are highly stable, even under oxygen-saturated
conditions, at the interface between TiO2 and Cu(II) species.39–41
Fig.
5 (a) UV-Vis spectra for Cu(II)–TiO2 samples with different
amounts of grafted Cu(II) at pH 12 and (b) photocatalytic activities of
the Cu(II)–TiO2 samples in (a) and N-doped TiO2 under visible-light
irradiation.
3.2 Anti-viral activity of Cu(II)–TiO2
In addition to the photocatalytic properties of TiO2, the anti-
bacterial and anti-viral properties of TiO2 under light irradia-
tion are also important for its practical applications.33–35 The
photocatalytic inactivation of pathogenic microorganisms by
Cu(II)–TiO2 was evaluated using thin lm samples that were
prepared by coating suspensions of Cu(II)–TiO2 (950-HCl-12) on
glass substrates. Bacteriophage Qb was used as a model virus to
measure the anti-viral activity of Cu(II)–TiO2.23 The anti-viral
activity of the Cu(II)–TiO2 nanocomposite was tested by the
plaque assay according to the standard evaluation procedure for
the photocatalytic antiviral effect (Japanese Industrial Stan-
dards, JIS R1756). In the assay, test solutions containing
bacteriophage Qb were used to infect E. coli (NBRC 13965), and
the bacteriophage Qb concentration was determined from the
number of PFUs mLꢀ1
.
As can be seen in Fig. 6a, the Cu(II)–TiO2 (950-HCl-7) sample
exhibited only a negligible degree of bacteriophage inactivation
in the dark, whereas obvious inactivation was observed under
visible-light irradiation. Specically, viral inactivation reached
99% and 99.99% under visible-light irradiation for 1 and 2 h,
respectively. Notably, the Cu(II)–TiO2 (950-HCl-12) displayed a
log10(6.5) reduction of bacteriophage aer only 30 min of
visible-light irradiation (Fig. 6b). This bacteriophage inactiva-
tion performance was identical to that of our previously repor-
ted CuxO/TiO2 samples.20 Fig. 6c shows the effect of pre-
irradiation treatment on the anti-viral activity of the Cu(II)–TiO2
(950-HCl-12) sample under dark conditions. Aer 1 h of visible
light pre-irradiation treatment, followed by 1 h incubation
under dark conditions, 99.9% of viral particles were inactivated.
The anti-viral performance of Cu(II)–TiO2 (950-HCl-12) under
dark conditions was superior to that of Cu(II)–TiO2 (950-HCl-7,
Fig. 6a) and conventional Cu(II)–TiO2 (950-7) samples under
visible-light irradiation,20 as only two orders (99%) of virus were
inactivated by the Cu(II)–TiO2 samples during visible-light irra-
diation for 1 h. As shown in Fig. 6b, the Cu(II)–TiO2 (950-HCl-12)
sample only exhibited low anti-viral activity under dark condi-
tions without irradiation treatment, conrming that the anti-
viral activity of this photocatalytic material in the dark was
attributable to the pre-irradiation treatment. Furthermore, the
anti-viral activity was directly inuenced by the intensity of the
light used for the pre-irradiation treatment. The sustained anti-
Fig. 6 Inactivation of Qb bacteriophage by (a) Cu(II)–TiO2 (950-HCl-7)
and (b) Cu(II)–TiO2 (950-HCl-12) under dark conditions and visible-
light irradiation, and by (c) Cu(II)–TiO2 (950-HCl-12) under dark
conditions after pre-irradiation treatment with visible-light.
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Fig. 7 Proposed processes of photocatalysis and inactivation of viruses and bacteria under visible-light irradiation and dark conditions.
The photogenerated electrons are trapped and stored in metal activity of the TiO2 (950-HCl-12) sample was stable in air with a
oxide semiconductors as donors and are gradually oxidized turnover number of greater than 60. These results indicate that
under air under dark conditions.37–41
both the photocatalytic VOC decomposition and anti-patho-
Fig. 7 shows the possible mechanism underlying the genic effects of our synthesized Cu(II)–TiO2 (950-HCl-12) nano-
sustained anti-viral activity of our photocatalyst in the absence composites can be sustained for long-term operation in indoor
of light irradiation. Under visible-light irradiation, electrons in environments. These properties suggest that Cu(II)–TiO2
the VB of TiO2 are excited to the surface Cu(II) nanoclusters in a (950-HCl-12) is a promising material for indoor environmental
process that represents a form of IFCT and results in the risk-reduction applications.
transformation of Cu(II) into Cu(I), with holes remaining in the
VB. The generated Cu(I) can efficiently reduce oxygen molecules
4. Conclusion
via
a multi-electron reduction process that regenerates
Cu(II).14–16 The holes generated in the VB of TiO2 possess strong In summary, we succeeded in preparing highly visible-light-
oxidation power to decompose VOCs, a property that explains active Cu(II)–TiO2 nanocomposites with sustained anti-viral
the high photooxidation activity of the TiO2 (950-HCl-12) properties through comprehensive optimization of the
nanocomposites for IPA decomposition under visible-light synthesis conditions, such as pre-annealing treatment of bare
irradiation. In addition, it has been demonstrated that Cu(I) TiO2 powder, washing treatment of TiO2 under aqueous acid
species are very effective for enhancing the anti-viral and anti- conditions, and the pH value of the solution and the amount of
bacterial effects of TiO2 photocatalysts even under dark condi- graed Cu(II) nanoclusters. By optimizing the crystallinity, the
tions.20 Following irradiation with visible light, holes generated interfacial junction between TiO2 and Cu(II) nanoclusters, and
in the VB of TiO2, in combination with Cu(I) species, can also the amount of Cu(II) nanoclusters, the synthesized Cu(II)–TiO2
attack the outer membrane, proteins, and nucleic acid (DNA nanocomposites exhibited efficient IFCT and reductive energy
and RNA) of viruses and bacteria, resulting in their death and storage properties. Thus, the photocatalytic nanocomposites
inactivation.20 Therefore, the obtained TiO2 (950-HCl-12) efficiently decomposed VOCs and had strong anti-pathogenic
nanocomposites exhibited a marked activity for anti-bacteria effects under indoor conditions. Notably, the nanocomposites
and antivirus under visible light irradiation. For the sustain- displayed sustained anti-viral activity under dark conditions
ability of antivirus activity under dark conditions, the remain- aer light irradiation. Based on these properties, our
ing Cu(I) species play an important role. It has been reported Cu(II)–TiO2 nanocomposites are promising materials for risk-
that the combination of TiO2 with copper oxides, such as CuO reduction applications in indoor environments. In addition, the
or Cu2O, not only resulted in visible-light absorption but also synthesis approach described here represents a fundamental
electron trapping at the interfaces among TiO2 and these copper route to develop efficient nanocomposites for numerous target
oxides under visible-light irradiation.37,38 Trapped electrons in applications.
Cu2+ species as Cu1+ states are thermodynamically more stable
than those in the conduction band (CB) of TiO2,37,38 since the
Acknowledgements
redox potential of Cu2+/Cu+ in the Cu(II) nanoclusters is more
positive than that in the CB of TiO2. Thus, a part of electrons This work was performed under the management of the Project
could be trapped in Cu(II) nanoclusters near the interfaces, to Create Photocatalysts Industry for Recycling-Oriented Society
resulting in the prevention of recombination with holes in the supported by the New Energy and Industrial Technology
VB.42 These trapped electrons in Cu(II) nanoclusters as Cu1+ Development Organization (NEDO) in Japan. This research was
species contribute to the sustained anti-bacterial and anti-viral also supported by the ACT-C program of the Japan Science and
properties under dark conditions. Moreover, the photocatalytic Technology (JST) Agency.
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21 M. Liu, X. Q. Qiu, M. Miyauchi and K. Hashimoto, J. Mater.
Chem. A, 2014, 2, 13571–13579.
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