Photocatalytic, photoelectrochemical, and antibacterial activity of benign-by-design mechanochemically synthesized metal oxide nanomaterials

Article abstract of DOI:10.1016/j.cattod.2016.07.010

In the search for highly active and stable photocatalysts, significant efforts are devoted to find both new materials and innovative synthetic methods. In this study, an environmentally friendly and sustainable approach, dry reactive milling, was employed to synthesize two different semiconducting oxide nanomaterials, namely TiO₂and ZnO using polysaccharides as sacrificial templates. The as synthesized nanomaterials were characterized by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, diffuse reflection UV-vis and Raman spectroscopy, and N₂adsorption tests. Their photocatalytic activity was tested in ethanol degradation, followed by gas chromatographic analysis. Photoelectrochemical measurements were performed to assess the optoelectronic properties and the antimicrobial activity of these photocatalysts was also tested under visible light irradiation. Overall, we found that the performance of the synthesized nanomaterials was comparable to the benchmark P25 EVONIK titania, with ZnO exhibiting a remarkably superior antibacterial activity against E. coli.

Full text of DOI:10.1016/j.cattod.2016.07.010

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Photocatalytic, photoelectrochemical, and antibacterial activity of
benign-by-design mechanochemically synthesized metal oxide
nanomaterials
a,b
a,b,c
a
a
Gergely F. Samu , Ágnes Veres
, Szabolcs P. Tallósy , László Janovák ,
a,c
d
d,∗∗
a,b,∗
Imre Dékány , Alfonso Yepez , Rafael Luque
Department of Physical Chemistry and Materials Science University of Szeged, Aradi sq. 1, 6720 Szeged Hungary
MTA-SZTE “Lendület” Photoelectrochemistry Research Group Szeged, Rerrich sq. 1, 6720 Hungary
MTA-SZTE Supramolecular and Nanostructured Materials Research Group, Universiy of Szeged, Dóm Sq. 8., 6720, Szeged Hungary
Departamento de Quimica Organica, Universidad de Cordoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014, Cordoba
, Csaba Janáky
a
b
c
d
Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
In the search for highly active and stable photocatalysts, significant efforts are devoted to find both
new materials and innovative synthetic methods. In this study, an environmentally friendly and sus-
tainable approach, dry reactive milling, was employed to synthesize two different semiconducting oxide
nanomaterials, namely TiO2 and ZnO using polysaccharides as sacrificial templates. The as synthesized
nanomaterials were characterized by powder X-ray diffraction, transmission electron microscopy, scan-
ning electron microscopy, diffuse reflection UV-vis and Raman spectroscopy, and N2 adsorption tests.
Their photocatalytic activity was tested in ethanol degradation, followed by gas chromatographic anal-
ysis. Photoelectrochemical measurements were performed to assess the optoelectronic properties and
the antimicrobial activity of these photocatalysts was also tested under visible light irradiation. Overall,
we found that the performance of the synthesized nanomaterials was comparable to the benchmark P25
EVONIK titania, with ZnO exhibiting a remarkably superior antibacterial activity against E. coli.
Received 21 May 2016
Received in revised form 6 July 2016
Accepted 14 July 2016
Available online xxx
Keywords:
Green synthesis
Photocatalysis
Environmental remediation
Mechanochemistry
Nanostructures
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
While there is an extensive and exponentially growing lit-
erature covering various fundamental and application oriented
aspects of semiconductor photocatalysis (ranging from enhanced
light absorption, through size effects, to crystallinity related phe-
nomena) [2], much less attention has been devoted to synthetic
procedures targeting a benign-by-design approach for nanoma-
terials preparation. TiO2 nanomaterials have been extensively
synthesized by means of different strategies [11], mostly related
to sol–gel or hydrothermal methods. Despite these efforts, there
is a continuously growing need for new methods, which result in
crystalline samples with high specific surface areas and excelling
physicochemical properties.
In addition, time and energy-efficient methods came to the fore-
front of interest recently, because they offer shorter energy payback
time, which is of prime importance in all solar energy application
schemes. For example, solution combustion synthesis [12,13] can
be an attractive approach where the exothermicity of the reac-
tion together with the release of different gases results in the
formation of crystalline nanoparticles. Mechanochemical protocols
ensure a rapid, mild, simple, and highly reproducible alternative to
Sunlight is undoubtedly the most valuable resource in the
quest for a sustainable chemical and energy industry [1]. Photo-
catalytic (for thermodynamically downhill) and photo-driven (for
thermodynamically uphill) processes [2] can both contribute to the
efficient and selective transformation of raw materials to either
useful fuels or chemicals. In this regard, nanoparticles of oxide
semiconductors are attractive candidates to be employed in envi-
ronmental remediation [3,4] (e.g., water purification), solar energy
conversion (i.e., water splitting [5,6] or CO2 reduction [7,8]), and
biomass valorization [9,10] to obtain value-added products from
earth abundant resources.
∗
Corresponding author at: Rerrich Sq. 1, Szeged, H6720, Hungary.
Corresponding author at: Ctra Nnal IV-A, Km 396, E14014, Cordoba, Spain.
E-mail addresses: q62alsor@uco.es (R. Luque), janaky@chem.u-szeged.hu
∗
∗
(
C. Janáky).
http://dx.doi.org/10.1016/j.cattod.2016.07.010
920-5861/© 2016 Elsevier B.V. All rights reserved.
0
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(i.e., 2 g zinc nitrate milled with 8 g starch) in a 125 cm3 stainless
steel recipient of a Retsch PM100 planetary ball mill at 350 rpm for
30 min (optimized conditions) [24]. 18 stainless steel balls of 1 cm
diameter were employed. Upon milling, the slightly colored solids
were directly transferred to a ceramic vessel and subsequently cal-
conventional methods [14]. In addition, solvent-free (dry milling)
mechanochemistry can offer additional remarkable possibilities in
the development of advanced catalytically active materials [14].
For example, a composite of two different metal oxides (ZnO and
SnO ) has been reported to be obtained in a one pot synthesis [15].
2
◦
Mechanochemically synthesized ZnO could also be directly embed-
ded into synthetic polymers during the synthesis with promising
uses as antibacterial coating [16].
cined in air at 600 C for 4 h. Calcination temperature was selected
based on previous thermal decomposition studies which indicated
◦
that most organics were removed from the material after 500 C
One important drawback of these methods is the limited control
over particle size and morphology (thus specific surface area) of the
resultant material. The application of soft and hard templates is a
feasible avenue to circumvent this issue. There are nice examples in
the literature for template synthesis [17–19], where the shape and
size of the synthesized nanostructures were precisely controlled.
Most of these studies, however, employed synthetic polymer or
anodized alumina templates, which are prepared in procedures
with significant environmental footprint [20]. As an alternative,
biopolymers can also be used as sacrificial templates, thus achiev-
ing biomass valorization while synthesizing oxide semiconductor
nanostructures [21]. Different metal oxides and metal/metal oxide
hybrids were obtained in this manner, such as CeO2 [22] and
[24].
2.3. Characterization methods
Diffuse reflectance UV-vis spectra were recorded by an Avantes
AvaSpec2048 equipped with an Avasphere-50 type integrating
TM
sphere. Raman spectra were obtained with a Thermo Scientific
TM
DXR Raman microscope at an excitation wavelength of 532 nm,
applying 10 mW laser power, and averaging 20 spectra with an
exposition time of 6 s. The X-ray diffractograms of the powdered
photocatalyst samples were recorded on a Philips X-ray diffrac-
tometer (XRD) (PW 1930 generator, PW 1820 goniometer) with Cu
K␣ (␭ = 0.1542 nm) as the radiation source at ambient temperature,
in the 10–70 (2ꢁ) range applying 0.02 (2ꢁ) step size. For Rietveld
refinement the software GSAS [25] was used with an EXPGUI [26]
graphical user interface. Transmission electron microscopic (TEM)
investigation was performed using a FEI Tecnai G2 20 X-Twin type
instrument, operating at an acceleration voltage of 200 kV. Scan-
ning Electron Microscopic (SEM) images were captured on a Hitachi
S-4700 FE-SEM instrument.
TiO /Au [23].
2
◦
◦
We have previously reported the benign-by-design preparation
of ZnO nanocrystals via an efficient dry reactive milling methodol-
ogy using Zn(NO ) as metal precursor and various polysaccharides
3
2
(
including a biomass-derived agar extracted from macroalgae) as
sacrificial templates [24]. This approach united the benefits of
mechanochemistry and template synthesis, while employing a
template from environmentally sustainable sources.
In continuation with such studies, the proposed work was
aimed to: (i) study the feasibility of the mechanochemical tem-
plating approach for a range of photoactive nanomaterials as
well as (ii) to investigate the photoelectrochemical, photocatalytic,
and antimicrobial properties of mechanochemically synthesized
nanostructures. In this regard, two different oxide semiconductors
Specific surface area of the powdered photocatalyst samples was
determined by a Micromeritics gas sorption analyzer (Gemini Type
2375) at 77 K in liquid nitrogen. The adsorption and desorption
branches of the isotherms were determined. Prior to measurements
the samples were pre-treated in vacuum (ca. 0.01 Torr) at 393 K for
2 h. The sample holder was loaded with ca. 0.1–0.3 g sample. The
adsorption isotherms were analyzed by means of the BET equation.
Photoelectrochemical measurements were performed with an
Autolab PGSTAT302 instrument, in a classical one-compartment,
three-electrode electrochemical cell. The various metal oxide
nanoparticles were spray coated from a 2-propanol solution
(
ZnO, TiO ) were prepared via dry reactive milling, using polysac-
2
charides such as starch as biotemplate. The most important finding
of this study was that the performance of mechanochemically
synthesized nanomaterials was similar to those of a commercial
benchmark material (EVONIK P25 TiO ) in terms of photocatalytic
2
−
3
−2
)
and photoelectrochemical activities, while ZnO exhibited an out-
standing antimicrobial activity.
(1 mg cm concentration) on ITO glass electrodes (∼0.1 mg cm
and were used as working electrodes. A large Pt foil counterelec-
trode and an Ag/AgCl/3 M KCl reference electrode completed the
cell setup. The light source was a 300 W Hg-Xe arc lamp (Hama-
matsu L8251). The radiation source was placed 3 cm away from
the working electrode surface. Photovoltammetry profiles were
recorded in both 0.1 M Na SO and 0.1 M Na SO electrolyte, using
2
. Experimental section
2.1. Materials
2
3
2
4
−
1
a slow potential sweep (2 mV s ) in conjunction with interrupted
irradiation (0.1 Hz) on the semiconductor coated electrodes. All
The metal oxide precursors, namely Zn(NO ) ·6H O (>99%), tita-
3
2
2
nium isopropoxide (>97%) were all purchased from Sigma Aldrich.
Commercially available P25 TiO2 (EVONIK) was used for bench-
marking purposes. Na SO (Alfa Aesar, anhydrous 99%) and Na SO
◦
procedures were performed at ambient temperature (20 ± 2 C).
The photocatalytic activity of the photocatalyst films was
probed by ethanol degradation tests under LED-light illumination
2
4
2
3
(
(
Sigma Aldrich, >98%) were used in all the photoelectrochemical
PEC) experiments along with N2 (Messer, 99.995%) gas. All chem-
(General Electric, Hungary, 7 W, ␭ = 405 nm) [27]. Photooxidation of
ethanol vapor on catalyst films was performed in a circulation reac-
icals were of the highest purity commercially available, and were
studied without further purification. Deionized water (MilliPore,
3
◦
tor (volume ca. 165 cm ) at 25.0 ± 0.1 C. The light source was fixed
2
at 50 mm distance from the 25 cm films. The irradiance reach-
1
8 Mꢀ) was used to prepare all solutions.
−
2
ing the sample was 14.8 mW cm (determined by actinometry).
After injection of ethanol and water vapor, the system was left to
stand for 30 min for the establishment of adsorption equilibrium,
and C0 was always determined after the adsorption completed.
The composition of vapor phase was analyzed by a gas chromato-
graph (Shimadzu GC-14B) equipped with a thermal conductivity
(TCD) and a flame ionization detector (FID). The flow rate of the gas
mixture in the photoreactor system was 375 cm min . The initial
concentration of ethanol was 0.36 ± 0.018 mmol dm at a relative
humidity of ∼70%.
2.2. Synthetic procedure
The preparation of bio-templated nanomaterials was carried
out employing a ball milling protocol similar to that previously
reported by the Luque group [24]. In a typical experiment, the
desired quantity of metal precursors, namely Zn(NO ) ·6H O and
3
−1
3
2
2
−
3
titanium isopropoxide were milled separately with a certain quan-
tity of starch to reach a 1:4 metal precursor/starch weight ratios
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Fig. 1. XRD patterns for TiO2 (A) and ZnO (B) samples. In (A) the crystal facets were assigned for all diffractions (A: anatase, R: rutile), and Rietveld refinement is also shown.
The antibacterial tests were carried out according to the modi-
fied EN ISO 27447:2009 standard. Prior to the antibacterial activity
measurements, the photocatalysts were immobilized in an inert
polymer film (Plextol©). The nanohybrid films were then steril-
ized and activated by UV light irradiation for an hour (light Source:
LightTech GCL307T5L/Cell lamp ␭ = 250 nm). The bacterial sus-
pensions (Escherichia coli ATCC 29522) were spread uniformly
multiphasic composition. Diffractions from both anatase ((JCPDS
No. 78-2486), space group I41/amd)) and rutile ((JCPDS No. 76-
1940), space group P42/mm)) phases were identified, as majority
and minority phases, respectively. Rietveld refinement was carried
out for the TiO2 samples (Fig. 1A), and a 75% anatase, 25% rutile
composition was determined. We note that this phase composi-
tion is similar to that of EVONIK P25 TiO , the most frequently
2
(
100 ␮L) on the surface of the nanohybrid films. The films were
studied benchmark photocatalyst [29]. A minor difference lies in
the presence of an amorphous titania phase in EVONIK P25 (∼10%)
[30,31]. Full profile fitting (as a part of Rietveld refinement process)
was employed to estimate the average size of crystalline domains
in the samples. Using this method all crystallographic directions
were taken into account (thus average crystallite diameters are
given) and also the error caused by the instrumental broadening
was subtracted. The analysis resulted in an average crystallite size
of 22 nm for the majority anatase phase and 26 nm for the minor-
ity rutile phase in the TiO2 sample. The ZnO sample featured a
well-defined wurtzite phase ((JCPDS No. 36-1451), space group
P6(3)mc), in good agreement with previous findings from the Luque
group [24]. This oxide exhibited remarkable crystallinity with an
estimated particle size of 44 nm (from full profile fit).
illuminated with an LED-light source (General Electric, Hungary,
7
−
2
W, ␭max = 405 nm, irradiance = 0.4 mW cm ) [27]. The exposure
times were 0, 30, 60, 90, and 120 min, and the distance of the light
source from the samples was 35 cm [27,28]. After different times
of illumination, the inoculated films were placed into a new ster-
ile Petri dish by sterile tweezers. After illumination, the inocula
were washed out from the activated nanohybrid films with 5 cm3
physiological saline (0.9%) to regain all surviving bacteria from the
uneven surface of the samples. For the counting of surviving bacte-
ria, 100 ␮L from the recovered bacterial suspensions was streaked
◦
on Mueller–Hinton plates, which were incubated at 37 C overnight
−
3
(
24 h). After the incubation period, colony-forming units (cfu cm
)
were counted and converted into cell number of surviving bacteria
per mL of the original inocula.
UV-vis diffuse reflectance spectroscopy (DRS) was employed
to characterize the optical properties of the synthetized materials
(
Fig. 2A). The bandgap of the prepared semiconductors was esti-
3
. Results and discussion
mated by deriving the appropriate Tauc-plots using the following
equation:
3.1. Physical characterization
ꢀ
ꢁ
hꢂ˛)1 = A hꢂ − Eg
/n
(1)
(
XRD patterns were recorded to identify the crystal phases of the
formed oxides, as well as to probe the overall crystallinity of the
samples (Fig. 1). In the case of TiO , the measurements indicated a
where h is the Planck-constant, ␯ is the photon frequency, ␣ is
the absorption coefficient, A is a proportionality constant, Eg is
2
Fig. 2. DRS-UV-vis spectra (A), tauc plots (B), and Raman spectra (C) of the TiO2 and ZnO samples.
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Fig. 3. TEM images of TiO2 (A, B) and ZnO (D, E) together with the particle size distributions (C and F, respectively).
the bandgap and n is a constant related to the nature of the elec-
ride template during the ball-milling: after the nucleation step the
formed oxide nanoparticles were grown until they have reached
another oxide particle. The particle size distribution was deter-
mined by measuring the size of 300 particles for both oxides. We
note that the TEM images only show a 2D projection of the 3D
particles, thus the observed particle size distribution is actually a
distribution of the projected dimension of the particles. As for TiO2,
an average particle size of 19 ± 6 nm was obtained, in good agree-
ment with the size of the crystalline domains calculated from XRD
(also see Fig. 1). The different kinetics of the nucleation and growth
for the two oxides is also reflected in the larger particle size of ZnO
(52 ± 30 nm).
tronic transition. ZnO is a direct bandgap semiconductor and for
direct allowed transition n = 0.5. The picture is murkier for TiO2,
because different polymorphs exhibit different properties [32,33].
Anatase (contained as major phase in our samples) has an indi-
rect bandgap, therefore in this case n = 2. The absorption coefficient
can be estimated by calculating the Kubelka–Munk function from
the reflectance data. The Tauc plot was then derived by plotting
/n
(
␣h␯)1 vs. h␯ where the point of intersection of the tangent line
and the horizontal axis will yield the bandgap value. Using this
method, bandgaps of 3.04 eV (TiO ) and 3.22 eV (ZnO) were esti-
2
mated (Fig. 2B). Both of these values agree well with previously
reported literature data [34,35].
A closer inspection of the particles via HR-TEM revealed vis-
ible lattice fringes for both materials. An interplanar spacing of
Raman spectroscopic measurements were carried out to further
probe the synthesized materials. In the case of the TiO samples, all
0.353 nm for the TiO sample (Fig. 4A), corresponding to the anatase
2
2
Raman modes were ascribed to the anatase phase of TiO (in accor-
(101) lattice plane may be discerned. For the ZnO sample (Fig. 4B)
this value was 0.250 corresponding to the (101) lattice plane.
These HR-TEM data are also consistent with the positions of the
most intensive diffractions in the XRD patterns presented earlier
(0.352 nm and 0.248 nm).
SEM images were also taken to probe the overall morphology
of the oxide samples. As shown in Fig. 4C and D, both materials
have a nanoporous structure, with larger primary particle size in
the case of ZnO. These images are additional proofs for the synthetic
mechanism, resulting in a coherent interconnected nanostructure,
as outlined above.
2
dance with previous XRD data) [36,37]. Interestingly, the Raman
modes of the minority rutile phase were completely absent. We
note that this behavior was also perceived in the case of EVONIK
P25 (not shown here). This observation, together with other liter-
ature data [38] suggests that this small amount of rutile phase is
invisible for Raman spectroscopy. The Raman spectrum of the ZnO
sample was also in close agreement with reported literature data
[
39].
TEM experiments were conducted to gain direct information
on particle size and morphology of the mechanochemically syn-
thesized nanomaterials (Fig. 3). The most prominent observation
of these studies was that the synthesis did not result in individual
nanoparticles, but instead in a porous nanostructured network. This
tendency was witnessed for both TiO2 and ZnO (Fig. 3A–D, respec-
tively). Notably, such fractal-type nanostructured morphology is
indeed beneficial in various applications where rapid charge car-
Specific surface area of the samples were measured by N2
adsorption (Fig. 5), analyzed by using the BET-isotherm. The
2
−1
2
−1
obtained specific surface areas (30 m g and 15 m g for TiO2
and ZnO) were in accordance with the measured relative particle
sizes. At the same time, these numbers are smaller than expected
simply from the particle size. This observation can be attributed
to the fusion of individual nanoparticles (i.e., formation of the net-
−
+
rier extraction (i.e., e /h transport) is required [40]. Formation of
such networks was rationalized by the presence of the polysaccha-
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Fig. 4. HR-TEM images of TiO2 (A) and ZnO (B) samples, displaying the presence of lattice fringes. SEM images of TiO2 (C) and ZnO (D) samples.
Fig. 5. N2 adsorption isotherms of TiO2 (A) and ZnO (B) at 77 K.
work structure) during the synthesis (see also TEM and SEM images
in Fig. 4).
[41]. Electron-hole pairs are generated during illumination of a
semiconductor electrode surface upon absorption of light. How-
ever, not all of the photogenerated charge carriers can be extracted,
because these are prone to recombination, which can proceed in
several different pathways.
3.2. Photoelectrochemical characterization
Fig. 6A shows the photoresponse of the spray coated semicon-
Linear sweep photovoltammetric experiments are often used
ductor electrodes in Na SO4 electrolyte. In this electrolyte, the
2
as the first step in assessing electrochemical processes under
illumination. During these measurements, the irradiation of the
semiconductor electrode is periodically interrupted while a slow
scan of the potential is applied. This allows the simultaneous mea-
surement of dark and light-induced response of the studied systems
photoresponse can be mainly ascribed to the photooxidation of
water. During illumination, the measured anodic currents indi-
cated an n-type behavior for both systems. The electrolyte was then
changed to Na SO3 (a more effective hole-scavenger) to see how
2
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Fig. 6. Comparison of the photoelectrochemical activity of the TiO2 and ZnO samples in 0.1 M Na2SO4 (A) and in 0.1 M Na2SO3 (B) applying a sweep rate of 2 mV s−1, using a
00 W Hg-Xe arc lamp.
3
much role does recombination and surface reaction kinetics play
in the overall performance of the studied materials. Such recorded
photovoltammograms are shown in Fig. 6B where the photocurrent
mainly arises from the photooxidation of sulfite ions. The magni-
tude of the photocurrent increased for both semiconductors in a
similar manner.
synthesized materials under practically relevant circumstances,
the photooxidation of ethanol vapor was performed in a circula-
tion reactor, where a constant flow rate of the gas mixture was
maintained while the composition of vapor phase was analyzed by
gas chromatography. Fig. 7A depicts the converted ethanol amount
as a function of time, normalized by the surface area of the catalyst
(as deduced from BET measurements, see also Fig. 5). As seen in
Fig. 7, the two TiO2 samples exhibited a comparable activity, while
ZnO outperformed both mechanochemically synthesized TiO2 as
well as its commercial P25 counterpart. To further quantify the
efficiency of the photocatalysts, the apparent reaction rates (k0)
were determined as the slope of
Finally, the onset potential of the voltammograms in Fig. 6B
(
when there are no kinetic constraints) can be related to the Fermi
level of the oxide samples [41]. As both samples are n-type semi-
conductors, the position of their conduction band (CB) edge was
estimated (−0.95 V and −0.70 V vs. Ag/AgCl at pH 10, for TiO2 and
ZnO, respectively) [42,43].
ꢀ
ꢁ
−
ln c/c0 = kꢀt
(2)
3
.3. Photocatalytic and antimicrobial studies
where c is the concentration of ethanol, c is the initial concentra-
tion of ethanol and t is irradiation time (this approach was feasible
because the reaction followed a pseudo-first order kinetics). The
0
Since photoelectrochemical studies have proved the photoac-
tivity of our samples (Fig. 6), we further studied their applicability
in two different solar energy utilization schemes. The first one
was photocatalytic environmental remediation, using ethanol as
a test molecule to model the photodegradation of volatile organic
compounds (VOCs) [44]. In this reaction a series of chemical trans-
formations and the generation of intermediate products occur
on the TiO2 surface, until the complete mineralization of ethanol
ꢀ
apparent reaction rate (k ) values were also normalized by the BET
surface area values to compare the specific photocatalytic activity
−
4
−4
of the catalysts. The obtained values were 1.56·10 , 2.05·10 , and
−
4
−1
−2
1.90·10 g min
m ; for TiO , ZnO, and P25 (TiO ) respectively.
2
2
These values are in good agreement with the trends presented in
Fig. 7A. The comparable values obtained for the two different TiO2
samples confirm that after peeling-off the obvious surface area
to CO2 and H O is achieved. [44]. To probe the photoactivity of
2
Fig. 7. Surface area normalized photocatalytic activity of the oxide samples towards ethanol degradation. (A) Comparison of the antibacterial activity of the synthesized TiO2
and ZnO samples, as assessed by the ISO 27447:2009 standard. The activity of the polymer binder and the benchmark P25 TiO2 is also shown. (B).
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Fig. 8. The scheme of the experimental setup for the antibacterial tests according to standard ISO 27447:2009 (1 LED lamp, 2 cover glass, 3 closed photoreactor, 4 Petri dish,
inoculum, 6 photocatalyst/polymer nanohybrid film, 7 wet wipes).
5
effect [2] the other factors governing the photocatalytic activity
e.g., crystallinity, phase composition) are essentially the same for
these two titania samples.
Acknowledgements
(
The authors wish to express their deep recognition of Prof.
András Dombi for his important contribution in bringing the engi-
neers perspective into the topic of photocatalysis for environmental
remediation. CJ gratefully acknowledges the support from the Hun-
garian Academy of Science, through its “Momentum” Excellence
Program. RL gratefully acknowledges Consejeria de Ciencia e Inno-
vación, Junta de Andalucía for funding project P10FQM-6711. The
authors thank the four anonymous Reviewers for their constructive
criticism on an earlier version of this manuscript.
The antibacterial activity of the photocatalysts was subse-
quently investigated using the modified ISO 27447:2009 standard
test. After selected exposure times (0, 30, 60, 90, and 120 min), the
3
cell number of the surviving bacteria per cm of the original inoc-
ula was calculated (see the experimental setup in Fig. 8). As seen
in Fig. 7B, mechanochemically synthesized TiO2 showed very lit-
tle activity (similar to that of the pure polymer binder) and similar
to that of P25, which can be ascribed to the direct effect of irra-
diation. Contrastingly, biotemplated ZnO exhibited a remarkable
antimicrobial activity, where all bacteria were killed after 90 min.
Clearly, this performance significantly predated that of the bench-
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