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

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Photocatalytic, photoelectrochemical, and antibacterial activity of benign-by-design mechanochemically synthesized metal oxide nanomaterials Gergely F. Samu a,b , Ágnes Veres a,b,c , Szabolcs P. Tallósy a , László Janovák a , Imre Dékány a,c , Alfonso Yepez d , Rafael Luque d,∗∗ , Csaba Janáky a,b,∗ a

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 c MTA-SZTE Supramolecular and Nanostructured Materials Research Group, Universiy of Szeged, Dóm Sq. 8., 6720, Szeged Hungary d Departamento de Quimica Organica, Universidad de Cordoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014, Cordoba Spain b

a r t i c l e

i n f o

Article history: 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

a b s t r a c t 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 TiO2 and 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 N2 adsorption 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. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Sunlight is undoubtedly the most valuable resource in the quest for a sustainable chemical and energy industry [1]. Photocatalytic (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 environmental 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: [email protected] (R. Luque), [email protected] (C. Janáky).

While there is an extensive and exponentially growing literature covering various fundamental and application oriented aspects of semiconductor photocatalysis (ranging from enhanced light absorption, through size effects, to crystallinity related phenomena) [2], much less attention has been devoted to synthetic procedures targeting a benign-by-design approach for nanomaterials 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 forefront 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 reaction 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

http://dx.doi.org/10.1016/j.cattod.2016.07.010 0920-5861/© 2016 Elsevier B.V. All rights reserved.

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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 SnO2 ) has been reported to be obtained in a one pot synthesis [15]. Mechanochemically synthesized ZnO could also be directly embedded into synthetic polymers during the synthesis with promising uses as antibacterial coating [16]. 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 achieving 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 TiO2 /Au [23]. We have previously reported the benign-by-design preparation of ZnO nanocrystals via an efficient dry reactive milling methodology using Zn(NO3 )2 as metal precursor and various polysaccharides (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 templating 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 (ZnO, TiO2 ) were prepared via dry reactive milling, using polysaccharides 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 TiO2 ) in terms of photocatalytic and photoelectrochemical activities, while ZnO exhibited an outstanding antimicrobial activity. 2. Experimental section 2.1. Materials The metal oxide precursors, namely Zn(NO3 )2 ·6H2 O (>99%), titanium isopropoxide (>97%) were all purchased from Sigma Aldrich. Commercially available P25 TiO2 (EVONIK) was used for benchmarking purposes. Na2 SO4 (Alfa Aesar, anhydrous 99%) and Na2 SO3 (Sigma Aldrich, >98%) were used in all the photoelectrochemical (PEC) experiments along with N2 (Messer, 99.995%) gas. All chemicals were of the highest purity commercially available, and were studied without further purification. Deionized water (MilliPore, 18 M) was used to prepare all solutions. 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(NO3 )2 ·6H2 O and titanium isopropoxide were milled separately with a certain quantity of starch to reach a 1:4 metal precursor/starch weight ratios

(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 calcined 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 [24].

2.3. Characterization methods Diffuse reflectance UV-vis spectra were recorded by an Avantes AvaSpec2048 equipped with an Avasphere-50 type integrating sphere. Raman spectra were obtained with a Thermo ScientificTM DXRTM 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 diffractometer (XRD) (PW 1930 generator, PW 1820 goniometer) with Cu K␣ (␭ = 0.1542 nm) as the radiation source at ambient temperature, in the 10–70◦ (2) range applying 0.02◦ (2) 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. Scanning Electron Microscopic (SEM) images were captured on a Hitachi S-4700 FE-SEM instrument. 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 (1 mg cm−3 concentration) on ITO glass electrodes (∼0.1 mg cm−2 ) and were used as working electrodes. A large Pt foil counterelectrode and an Ag/AgCl/3 M KCl reference electrode completed the cell setup. The light source was a 300 W Hg-Xe arc lamp (Hamamatsu L8251). The radiation source was placed 3 cm away from the working electrode surface. Photovoltammetry profiles were recorded in both 0.1 M Na2 SO3 and 0.1 M Na2 SO4 electrolyte, using a slow potential sweep (2 mV s−1 ) in conjunction with interrupted irradiation (0.1 Hz) on the semiconductor coated electrodes. All 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 (General Electric, Hungary, 7 W, ␭ = 405 nm) [27]. Photooxidation of ethanol vapor on catalyst films was performed in a circulation reactor (volume ca. 165 cm3 ) at 25.0 ± 0.1 ◦ C. The light source was fixed at 50 mm distance from the 25 cm2 films. The irradiance reaching the sample was 14.8 mW cm−2 (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 chromatograph (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 cm3 min−1 . The initial concentration of ethanol was 0.36 ± 0.018 mmol dm−3 at a relative humidity of ∼70%.

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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 modified 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 sterilized and activated by UV light irradiation for an hour (light Source: LightTech GCL307T5L/Cell lamp ␭ = 250 nm). The bacterial suspensions (Escherichia coli ATCC 29522) were spread uniformly (100 ␮L) on the surface of the nanohybrid films. The films were illuminated with an LED-light source (General Electric, Hungary, 7 W, ␭max = 405 nm, irradiance = 0.4 mW cm−2 ) [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 sterile 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 bacteria, 100 ␮L from the recovered bacterial suspensions was streaked on Mueller–Hinton plates, which were incubated at 37 ◦ C overnight (24 h). After the incubation period, colony-forming units (cfu cm−3 ) were counted and converted into cell number of surviving bacteria per mL of the original inocula. 3. Results and discussion 3.1. Physical characterization 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 TiO2 , the measurements indicated a

multiphasic composition. Diffractions from both anatase ((JCPDS No. 78-2486), space group I41/amd)) and rutile ((JCPDS No. 761940), 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 composition is similar to that of EVONIK P25 TiO2 , the most frequently 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 minority 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). 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 estimated by deriving the appropriate Tauc-plots using the following equation: (h˛)

1/n



= A h − Eg



(1)

where h is the Planck-constant, ␯ is the photon frequency, ␣ is the absorption coefficient, A is a proportionality constant, Eg is

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 electronic 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 indirect 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 (␣h␯)1/n 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 (TiO2 ) and 3.22 eV (ZnO) were estimated (Fig. 2B). Both of these values agree well with previously reported literature data [34,35]. Raman spectroscopic measurements were carried out to further probe the synthesized materials. In the case of the TiO2 samples, all Raman modes were ascribed to the anatase phase of TiO2 (in accordance 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 literature 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 synthesized 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, respectively). Notably, such fractal-type nanostructured morphology is indeed beneficial in various applications where rapid charge carrier extraction (i.e., e− /h+ transport) is required [40]. Formation of such networks was rationalized by the presence of the polysaccha-

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 determined 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 agreement 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). A closer inspection of the particles via HR-TEM revealed visible lattice fringes for both materials. An interplanar spacing of 0.353 nm for the TiO2 sample (Fig. 4A), corresponding to the anatase (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. Specific surface area of the samples were measured by N2 adsorption (Fig. 5), analyzed by using the BET-isotherm. The obtained specific surface areas (30 m2 g−1 and 15 m2 g−1 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-

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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). 3.2. Photoelectrochemical characterization Linear sweep photovoltammetric experiments are often used 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 measurement of dark and light-induced response of the studied systems

[41]. Electron-hole pairs are generated during illumination of a semiconductor electrode surface upon absorption of light. However, not all of the photogenerated charge carriers can be extracted, because these are prone to recombination, which can proceed in several different pathways. Fig. 6A shows the photoresponse of the spray coated semiconductor electrodes in Na2 SO4 electrolyte. In this electrolyte, the photoresponse can be mainly ascribed to the photooxidation of water. During illumination, the measured anodic currents indicated an n-type behavior for both systems. The electrolyte was then changed to Na2 SO3 (a more effective hole-scavenger) to see how

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Fig. 6. Comparison of the photoelectrochemical activity of the TiO2 and ZnO samples in 0.1 M Na2 SO4 (A) and in 0.1 M Na2 SO3 (B) applying a sweep rate of 2 mV s−1 , using a 300 W Hg-Xe arc lamp.

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 magnitude of the photocurrent increased for both semiconductors in a similar manner. 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 semiconductors, 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]. 3.3. Photocatalytic and antimicrobial studies Since photoelectrochemical studies have proved the photoactivity 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 transformations and the generation of intermediate products occur on the TiO2 surface, until the complete mineralization of ethanol to CO2 and H2 O is achieved. [44]. To probe the photoactivity of

synthesized materials under practically relevant circumstances, the photooxidation of ethanol vapor was performed in a circulation 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





−ln c/c0 = k t

(2)

where c is the concentration of ethanol, c0 is the initial concentration of ethanol and t is irradiation time (this approach was feasible because the reaction followed a pseudo-first order kinetics). The apparent reaction rate (k ) values were also normalized by the BET surface area values to compare the specific photocatalytic activity of the catalysts. The obtained values were 1.56·10−4 , 2.05·10−4 , and 1.90·10−4 g min−1 m−2 ; for TiO2 , ZnO, and P25 (TiO2 ) respectively. 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

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, 5 inoculum, 6 photocatalyst/polymer nanohybrid film, 7 wet wipes).

effect [2] the other factors governing the photocatalytic activity (e.g., crystallinity, phase composition) are essentially the same for these two titania samples. The antibacterial activity of the photocatalysts was subsequently investigated using the modified ISO 27447:2009 standard test. After selected exposure times (0, 30, 60, 90, and 120 min), the cell number of the surviving bacteria per cm3 of the original inocula was calculated (see the experimental setup in Fig. 8). As seen in Fig. 7B, mechanochemically synthesized TiO2 showed very little activity (similar to that of the pure polymer binder) and similar to that of P25, which can be ascribed to the direct effect of irradiation. Contrastingly, biotemplated ZnO exhibited a remarkable antimicrobial activity, where all bacteria were killed after 90 min. Clearly, this performance significantly predated that of the benchmark P25 TiO2 . We note that dark control experiments were also carried out and at 50% of the activity can be attributed to the irradiation in all cases. These observations prove that ZnO nanoparticles, obtained via biomass-templated solventless ball milling, possess the well-known excellent antimicrobial activity of ZnO nanomaterials [16,27].

4. Conclusions In this study, mechanochemical synthesis of two different transition metal oxides (TiO2 , ZnO) was carried out via a simple dry milling biotemplating process using starch as sacrificial template. Both oxides exhibited excellent crystallinity, with mixed anatase/rutile phases in TiO2 (with a composition similar to EVONIK P25 TiO2 ) [29–31]. and a pure wurtzite phase for ZnO. As a result of the presence of the bioorganic template during the synthesis, both oxides had an interconnected porous structure, as confirmed by both SEM and TEM images. The key message of this study is that oxide semiconductor photocatalysts obtained via an environmentally friendly and sustainable approach are in no means inferior to their counterparts obtained by synthetic methods with significantly larger environmental footprint. In fact, ZnO massively outperformed the benchmark EVONIK P25 TiO2 in the antibacterial tests. We note here that comparative studies were also performed with other ZnO materials (sol-gel colloids and electrodeposited films) and very similar data were obtained in the different studies. This is a significant outcome underlining the utility of biotemplateassisted mechanochemistry for generating nanoparticles [10] for any catalytically-relevant application where crystallinity and surface area play a key role. We believe that these findings will encourage other researchers to use this synthetic methodology to obtain other photocatalysts to be employed in various solar energy application schemes.

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