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ChemEngineering 2019, 3(1), 3; https://doi.org/10.3390/chemengineering3010003

Article
Photocatalytic and Antimicrobial Properties of Ag2O/TiO2 Heterojunction
1
Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
2
Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, 60-965 Poznan, Poland
3
Department of Process Engineering and Chemical Technology, Gdansk University of Technology, 80-233 Gdansk, Poland
4
Institute of Chemical and Environmental Engineering, West Pomeranian University of Technology, Szczecin, Pulaskiego 10, 70-322 Szczecin, Poland
*
Authors to whom correspondence should be addressed.
Received: 5 December 2018 / Accepted: 2 January 2019 / Published: 8 January 2019

Abstract

:
Ag2O/TiO2 heterojunctions were prepared by a simple method, i.e., the grinding of argentous oxide with six different titania photocatalysts. The physicochemical properties of the obtained photocatalysts were characterized by diffuse-reflectance spectroscopy (DRS), X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) with an energy dispersive X-ray spectroscopy (EDS). The photocatalytic activity was investigated for the oxidative decomposition of acetic acid and methanol dehydrogenation under UV/vis irradiation and for the oxidative decomposition of phenol and 2-propanol under vis irradiation. Antimicrobial properties were tested for bacteria (Escherichia coli) and fungi (Candida albicans and Penicillium chrysogenum) under UV and vis irradiation and in the dark. Enhanced activity was observed under UV/vis (with synergism for fine anatase-containing samples) and vis irradiation for almost all samples. This suggests a hindered recombination of charge carriers by p-n heterojunction or Z-scheme mechanisms under UV irradiation and photo-excited electron transfer from Ag2O to TiO2 under vis irradiation. Improved antimicrobial properties were achieved, especially under vis irradiation, probably due to electrostatic attractions between the negative surface of microorganisms and the positively charged Ag2O.
Keywords:
heterogeneous photocatalysis; nanocomposites; heterojunction; Ag2O; TiO2; antimicrobial properties

1. Introduction

Recently, the application potential of semiconductor photocatalysis, related to environmental remediation and renewable energy processes [1], has been gaining importance in environmentally clean and future technologies [2,3,4]. Titanium(IV) oxide (TiO2, titania) is an n-type semiconductor of great importance for applied photocatalysis [5,6,7]. The advantages of this material are an exceptional high photocatalytic efficiency, chemical inertness, long-term stability and photo-corrosion resistance. The current challenge for the development of technologies based on photocatalysis is still the design of TiO2-originated material with high efficiency in both UV and visible light (vis) irradiation. It should be pointed out that titania is inactive in vis range due to its wide bandgap, and quantum yield under UV irradiation is much lower than 100%, due to the recombination of charge carriers (typical for all semiconductors). Therefore, the introduction of vis absorption into titania and the hindering of charge carriers’ recombination would enable its broad application also under natural sunlight. Therefore, the design of new photocatalysts based on TiO2 should consider the above mentioned issues [8,9,10,11].
One of the main strategies to improve the performance of titania is its coupling with other semiconductors [12]. It is considered that the heterojunction of titania (n-type semiconductor) and p-type semiconducting material should result in an efficient charge separation, and thus, an increase in the lifetime of charge carriers [13]. To introduce the vis response, a p-type semiconductor with a lower bandgap energy than TiO2 is necessary. A promising candidate for a narrow bandgap semiconductor to couple with TiO2 is low-cost silver(I) oxide (Ag2O, argentous oxide). The band gap energy of Ag2O has been experimentally determined to be 1.3 ± 0.3 eV [14]. However, Ag2O is not often used as a single photocatalyst, because it is unstable and photo-sensitive under light irradiation [15,16,17,18]. Furthermore, Ag2O, with a body-centered cubic crystal structure (the same as in the case of Cu2O), allows a large variety of particle morphologies to be synthesized [16,17,19,20]. Silver(I) oxide has also strong antibacterial properties, which might depend on the shape of Ag2O particles [19].
The modification of titania with silver nanoparticles (NPs) or mixed silver forms (Ag(0), Ag+, Ag2+) to prepare photocatalysts has already been performed by many research groups [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. However, in the case of Ag2O/TiO2 system, there are only a few research papers and the mechanism of the coupling is still not fully understood [38,39,40,41,42,43,44,45,46,47,48,49]. The consequence of the p-n heterojunction is the movement of the photogenerated electrons to the conduction band of n-type TiO2 and the photogenerated holes to the valence band of p-type Ag2O, which promotes an interfacial electron transfer causing the reduction of the charge recombination effect [42]. Different morphological forms of anatase were used to prepare Ag2O/TiO2, such as nanofibers [39,49], nanobelts [40,46], nanosheets [41] and microspheres [47]. Rutile [42,50] and P25 (highly active, commercial titania sample containing anatase (76–80%), rutile (13–15%) and amorphous phase (6–11%) [51]) [44,45] were also considered in these preparations. It is important to mention that in the majority of studies, organic dyes were used as test compounds to check photocatalytic activity, even in the vis range of irradiation, which is not an appropriate selection, due to the possibility of titania sensitization [52]. Moreover, there is a lack of complex discussion about the effect of the titania matrix on the efficiency of the Ag2O/TiO2 system. Therefore, in this study, the p-n heterojunction of Ag2O and TiO2 has been investigated for photocatalytic decomposition of organic compounds and the inactivation of microorganisms (bacteria and fungi) in dependence on the titania matrix (anatase, rutile and mixed-phase titania).

2. Materials and Methods

2.1. Preparation of Ag2O/TiO2 Photocatalysts

TiO2 samples were supplied by several sources: P25 (AEROXIDE® TiO2 P25, Nippon Aerosil, Tokyo, Japan), ST01, (ST-01, Ishihara Sangyo, Osaka, Japan) ST41 (ST-41, Ishihara Sangyo), TIO6 (TIO-6, Catalysis Society of Japan, Tokyo, Japan), RUT and RUT’ (rutile nanopowder and predominantly rutile, Merck, Darmstadt, Germany). Ag2O was supplied by Wako Pure Chemicals. All materials were used as received, without further processing. Ag2O/TiO2 composites were prepared by the physical mixing of Ag2O and TiO2 powders in an agate mortar, i.e., the titania sample was mixed with Ag2O to prepare composites containing 1 wt% of Ag2O (5, 10, and 50 wt% were also tested for some reactions). The time of grinding (5 min) was the same for all samples to ensure the appropriate uniformity of the prepared composite powders.

2.2. Characterization

The UV/vis diffuse reflectance spectra (DRS) were recorded on JASCO V-670 (JASCO, Tokyo, Japan), equipped with a PIN-757 integrating sphere, using BaSO4 as a reference. Diffraction patterns (XRD) were collected on an X-ray diffractometer (Rigaku intelligent XRD SmartLab with a Cu target). The crystallite size was estimated by the diffraction peak using the Scherrer equation. The morphology of samples and distribution of Ag2O were observed by scanning transmission electron microscopy, equipped with an energy-dispersive X-ray spectroscopy (STEM-EDS, HD-2000, HITACHI, Tokyo, Japan). The content of silver was estimated by atomic absorption spectroscopy (AAS) after Ag dissolution from Ag2O/TiO2 in aqua regia.

2.3. Photocatalytic Activity Tests

The photocatalytic activities of the prepared photocatalysts were tested in four reaction systems: (1) Decomposition of acetic acid under UV/vis irradiation, (2) dehydrogenation of methanol under UV/vis irradiation, (3) decomposition of phenol under vis irradiation (λ > 400 nm: Xe lamp, water IR filter, cold mirror and cut-off filter L42), and (4) oxidation of 2-propanol (λ > 420 nm: Xe lamp, water IR filter, cold mirror and cut-off filter Y45). For the activity testing, 50 mg of photocatalyst was suspended in 5 mL of aqueous solution of (1) acetic acid (5 vol%), (2) methanol (50 vol%), (3) phenol (20 mg/L), and 2-propanol (5 vol%). The suspension for reaction (2) was bubbled with argon before irradiation. The 35-mL testing tubes were sealed with rubber septa, continuously stirred and irradiated in a thermostated water bath. Amounts of liberated (1) carbon dioxide in a gas phase, (2) hydrogen in a gas phase, (3) phenol, benzoquinone, and (4) acetone in a liquid phase (after powder separation; 0.3-mL sample was withdrawn for analysis) were determined by gas (GC-TCD (1-2), GC-FID (4)), and liquid (HPLC (3)) chromatography. The irradiation set-ups were described elsewhere [53]. In brief, the photoreactor for vis activity testing consisted of two parts (Figure 1). In the first part, light emitted from an Xe lamp (equipped with a cold mirror) passed through a water bath, and then through a removable UV cut-off filter (to eliminate IR and UV radiation, respectively). In the second part, two test-tubes were placed behind the filter at the same distance from the light source (4.5 cm × 4.5 cm—the irradiation window). Their contents were continuously stirred by magnetic stirrers and kept at a constant temperature by the thermostated water. To achieve the water circulation through the two parts (to avoid filter damage and to achieve constant reaction temperature) and to avoid UV light penetration into the second part, a separating wall was mounted in the first part parallel to the light beam. The cooling water was circulated over the separating wall and then through the hole at the bottom of the partition wall between the two parts. All inner parts of the reactor were covered with black tape and both parts were closed with metal covers. The light intensity measured in the front of the testing tubes (in the second part) was ca. 9.2 mW cm−2 and 11.3 mW cm−2 for reaction system (3) and (4), respectively.

2.4. Bactericidal Activity Test

The bactericidal activity of the prepared samples was examined by the suspension method: 50 mg of samples (1 and 5 wt% Ag2O/ST01, Ag2O (2.5 mg; the same content as that in 5 wt% Ag2O/ST01 sample) and bare ST-01) were dispersed in 7.0 mL of Escherichia coli K12 (E. coli K12, ATCC29425) suspension at a concentration of ca. 0.180 Abs at 630 nm (ca. 1–5 × 108 cells/mL) in a test tube and irradiated with an Xe lamp and a (i) CM1 and Y-45 filter for vis experiments (λ > 420 nm with ca. 20.0 mW·cm−2), or (ii) CM2 and UV-D36B filter for UV experiments (300 < λ < 420 nm with ca. 1.1 mW·cm−2) under continuous stirring in a thermostated water bath (photoreactor described in previous reports [53] and shown in Figure 1), or kept in the dark under stirring (500 rpm). The suspensions were diluted and aliquots of the suspensions were inoculated on a plate count agar (Becton, Dickinson and Company, Franklin Lakes, NJ, USA) at 0, 0.5, 1, 2, and 3 h. Media were incubated at 37 °C overnight and then the colonies were counted.

2.5. Fungicidal Activity Test

2.5.1. Suspension Method

50 mg of samples (1 and 5 wt% Ag2O/ST01, Ag2O and bare ST01) was dispersed in a test tube in 7.0 mL of Candida albicans (C. albicans, isolated from patients (throat smear) with immunodeficiency disorders that cause candidiasis (IIT&EE ZUT collection)) suspension at a concentration of ca. 0.400 Abs at 630 nm (ca. 1 × 104 cells/mL) and irradiated with UV and vis (same reactor and conditions as for bactericidal tests (2.4)) or kept in the dark under stirring (500 rpm). Serial dilutions were prepared, and aliquots of the suspensions were inoculated on sabouraud dextrose agar (Becton, Dickinson and Company, Franklin Lakes, NJ, USA) media at 0, 0.5, 1, 2 and 3 h. Media were incubated at 37 °C overnight and then the colonies were counted.

2.5.2. Spore Counting Method

20 g/L of the samples (1 and 5 wt% Ag2O/ST01 and bare ST01) and 0.2 g/L and 1.0 g/L of Ag2O (corresponding to Ag2O amount in 1 and 5 wt% Ag2O/ST-01, respectively) in Malt Extract Agar (Merck, Darmstadt, Germany) were autoclaved at 121 °C for 10 min, and the agar slants were prepared. Spores of Penicillium chrysogenum (P. chrysogenum) were suspended in 8.5 g/L NaCl aqueous solution, 20 µL of 10,000 spores/µL suspension was inoculated on the agar slants and irradiated with fluorescence light (FL) or kept in the dark for 5 days. After cultivation, spores were collected with 0.05% triton X-100 in 8.5 g/L NaCl aqueous solution by vortex mixing. Spore suspensions were diluted 10 or 50 times with 8.5 g/L NaCl aqueous solution and counted.

3. Results and Discussion

3.1. Characterization of Ag2O/TiO2 Samples

Six titania samples, which are commercially available, i.e., anatase-predominant samples (ST01 and ST41), rutile-predominant samples (TIO6, RUT and RUT’) and mixed-phase titania (P25), were used for the preparation of silver(I) oxide/titania nanocomposites. Crystalline properties, i.e., content and crystallite size of obtained photocatalysts are shown in Table 1.
Ag2O was detected in all coupled samples (Table 1 and Figure 2), and its content varied depending on the titania used (from 0.1 to 0.7 wt%). Such varieties in the Ag2O content are quite reasonable considering that only 1 wt% of Ag2O was used for titania modification (low-content modifiers are quite often undetectable). The crystallite sizes of titania (anatase and rutile) practically did not change after grinding (original sizes reported previously [6]). In contrast, the crystallite size of Ag2O decreased from about 60 nm to even 3 nm, resulting from the lower hardness of Ag2O [54] than that of titania. Interestingly, it was found that the change in Ag2O size corelated with TiO2 crystallite sizes, i.e., the smaller the crystallites of TiO2 were, the larger was a decrease in the Ag2O size after grinding. Moreover, the exact content of silver was confirmed in the Ag2O/ST01 sample (for 5 wt% of Ag2O, i.e., 4.65 wt% of Ag) by AAS reaching 4.66 wt% and 4.60 wt% of Ag for solid and suspended (for precipitate; same suspension as that used for photocatalytic activity tests) samples, respectively.
Diffuse reflectance spectra (DRS) of the obtained composites and exemplary DRS of bare titania (ST41) and Ag2O are shown in Figure 3. Titania can only absorb UV light, and depending on its polymorphic form (anatase vs. rutile) a different absorption edge is observed, i.e., bathochromic shift for rutile samples is caused by its narrower bandgap. Silver oxide is black, and thus can absorb all photons in entire range of 200–800 nm with maximum at ca. 500–550 nm, which corresponds to 2.25-eV bandgap [55]. After the titania grinding with Ag2O, the absorption at vis range appeared with different intensities and positions of its maximum (500–700 nm) depending on the titania.
Exemplary STEM (with EDS mapping) images of Ag2O/ST01 sample are shown in Figure 4. It was found that indeed NPs of Ag2O were much smaller (≤15 nm) than that used for the sample preparation, confirming the XRD results (Table 1). Ag2O deposits (nano-sized and NPs) were uniformly distributed on the titania matrix. Additionally, atomic percentages of oxygen, titanium and silver (64.1%, 34.2% and 1.8%) confirmed that stable p-n heterojunction could be prepared by simple grinding.

3.2. Photocatalytic Activity

3.2.1. UV/Vis-Induced Methanol Dehydrogenation

Photocatalytic activity was investigated under UV/vis irradiation for two reactions: (i) Methanol dehydrogenation under anaerobic conditions (H2 system), and (ii) decomposition of acetic acid under aerobic conditions (CO2 system; discussed in the next section (Section 3.2.2)). Obtained data of methanol dehydrogenation are shown in Figure 5.
At first, the influence of the Ag2O content was tested for the Ag2O/ST01 sample in the H2 system, and it was found that 1 wt% resulted in the highest level of photocatalytic activity, e.g., an increase in the Ag2O content from 1 to 10 wt% resulted in a decrease in photocatalytic activity by ca. half, probably due to the inner-filter effect (significant darkening of irradiated suspension). Therefore, for further studies, the composites containing 1 wt% of Ag2O were used.
For almost all the samples, the addition of inactive Ag2O to titania caused a significant increase in H2 liberation. The most active system with an obvious synergistic effect was Ag2O/ST01, where the addition of 1 wt% of Ag2O enhanced the reaction rate over 30 times. The only exception was the P25 matrix, where activity suddenly decreased after grinding with Ag2O. P25 was also used as a titania base by Ren et al. [56], where it was shown that a too high content of Ag2O (above 5 wt%) caused a drop in photocatalytic activity during the methyl orange degradation. Possible reactions for the heterojunction Ag2O/TiO2 system in methanol dehydrogenation under deaerated conditions are presented below.
TiO2 + hν → e + h+
Ag2O + hν → e + h+
CH3OH + 2h+ → HCHO + 2H+
H+ + e → 0.5 H2
Ag+ + e → Ag
Under UV/vis irradiation both semiconductors (TiO2 and Ag2O) are excited, which causes the generation of charge carriers, i.e., electrons and holes. Methanol works as a hole scavenger to hinder the hole-electron recombination. Therefore, photogenerated electrons can easily reduce protons, resulting in hydrogen formation. It is proposed that the enhanced photocatalytic activity by heterojunction between two oxides might be caused by a Z-scheme mechanism resulting in more reductive properties of Ag2O/TiO2 than that of bare TiO2 (more negative CB of Ag2O than that of TiO2). Figure 6 presents the possible mechanism of methanol dehydrogenation. Considering the darkening of suspension during irradiation (right parts of Figure 5 vs. Figure 7) in highly reductive conditions (anaerobic, methanol, UV), it is proposed that Ag2O was reduced forming zero-valent silver deposits (+1.17 V vs. SHE) being the co-catalyst for hydrogen evolution [22,23,24,57].

3.2.2. UV/Vis-Induced Acetic Acid Oxidation

The photocatalytic activity of the obtained Ag2O/TiO2 photocatalysts was also measured in an acetic acid oxidative decomposition reaction. The obtained rates of the CO2 generation are presented in Figure 7.
As a reference material, bare Ag2O was also tested, and it was found that Ag2O was almost inactive also for CO2 liberation. For almost all Ag2O/TiO2 composites, the photocatalytic activity was much higher than that by the respective bare TiO2 samples. Two-times improvement was reached for anatase-predominant samples (ST01 and ST41). It is proposed that through similarity to another heterojunction system Cu2O/TiO2 [58], two kinds of mechanisms could be proposed, i.e., Z-scheme and p-n heterojunction (II type), as shown in Figure 8. The first mechanism (recombination between e (TiO2) and h+ (Ag2O)) is preferential for high photocatalytic activity, due to the high oxidation (h+ in titania) and reduction (e in argentous oxide) potential of the obtained composites. It should be pointed out that the photocatalytic activity of the oxidation reactions depends on the oxidation potential of holes, and anatase has been reported as a stronger oxidant than rutile [59]. Therefore, the higher activities of Ag2O/anatase than Ag2O/rutile could be explained by the more positive position of its valence band (VB) [60]. Accordingly, the more negative potential of the conduction band (CB) in rutile than that in anatase might result in the formation of a II type heterojunction (e migration from CB of Ag2O to CB of TiO2 and an opposite migration of VB h+) with less redox potential (CB of titania and VB of argentous oxide).

3.2.3. Visible Light Photocatalytic Activity

The vis photocatalytic activity of the obtained Ag2O/TiO2 composites was investigated for two reactions: (1) Oxidative decomposition of phenol and (2) oxidation of 2-propanol. Obtained data are shown in Figure 9 and Figure 10.
Under vis irradiation, titania was inactive due to its wide bandgap, and thus only argentous oxide could absorb photons, and indeed exhibited high vis activity. It was found that the modification of vis-inactive titania with argentous oxide resulted in the appearance of a vis response for all titania samples. The highest vis activity was found for rutile samples. It is proposed that the higher activity of rutile-containing samples could be caused by the lower redox potential of the excited rutile (more negative CB) than that of anatase [59,60] facilitating electron migration from excited Ag2O to CB of titania, as shown in Figure 11. Buchalska et al. observed a more efficient O2•− generation on rutile than that on anatase, and thus a higher activity for photo-reactions involving photo-generated electrons as the predominant mechanism pathway. Consequently, higher activities of rutile than anatase have already been found for different vis systems, such as (1) plasmonic photocatalysts of gold-modified titania with “hot” (plasmonic) electron transfer from gold to CB of titania [53], and (2) Cu2O/TiO2 heterojunctions with an electron transfer from CB of cuprous oxide to CB of titania [61]. Nonetheless, the activity of bare argentous oxide was higher than that by Ag2O/TiO2 photocatalysts (1 h of irradiation was sufficient for the complete decomposition of phenol) confirming that Ag2O with a narrow bandgap [49,61] was more vis active than TiO2 and Ag2O/TiO2. Bare argentous oxide was not stable and could not be recycled after reaction, and thus, could not be treated as a “(photo)catalyst” (Similar results showing higher activity of Ag2O than its composites with TiO2 have already been published by Ren et al. [56] and Zhou et al. [40].). Therefore, it is concluded that the formation of Ag2O/TiO2 heterojunctions resulted in the preparation of more stable photocatalysts with high activity at a broad range of irradiation (UV and vis).

3.3. Antimicrobial Properties of Ag2O/TiO2

It is widely known that silver cations possess high antimicrobial activity against various microorganisms, such as gram-negative/positive bacteria, fungi and protozoa. There are two main mechanisms of bactericidal activity of Ag+, i.e., (i) the damage of the bacterial membrane [62], and (ii) deactivation of cellular enzymes [63], due to the high affinity of Ag+ for bacterial membrane (negatively charged) and thiol groups of enzymes. Additionally, bactericidal activity of Ag2O has recently been reported [64,65,66]. For example, it was proposed that chitosan-poly vinyl pyrrolidone-silver oxide nanoparticles possess higher bactericidal effect against gram-positive bacteria than gram-negative ones [14].
Figure 12 shows the bactericidal activity of Ag2O/ST01, bare Ag2O and bare TiO2 (ST01) under UV, vis, and in the dark. It was found that bare titania was active only under UV irradiation, confirming that generated oxygen species could decompose bacterial cells. The highest antibacterial activity was observed for bare argentous oxide, similar to all tested conditions. In the cases of 1 wt% and 5 wt% Ag2O/ST01, the order of activities was vis > UV > dark. Moreover, it was found that the activities of 5 wt% Ag2O/ST01 were higher than those of 1 wt% Ag2O/ST01 in all conditions. Thus, it is suggested that the high activity is directly correlated with the amount of Ag2O. Moreover, considering that the large amount of Ag was released from the photocatalyst surface (66, 68, and 78% under vis, dark and UV, respectively), it is proposed that the dissolved Ag could attribute to the overall activity. The worse dark activity of Ag2O/ST01 than that by bare Ag2O could be caused by the hindered contact between bacteria and Ag2O when TiO2 partly covered its surface (interface decrease). However, it is clear that both UV and vis irradiation increased antibacterial activity, confirming the positive effect of photocatalysis. It is proposed that under vis irradiation, Ag2O is excited with the simultaneous electron transfer to CB of titania resulting in either: (i) Positively charged Ag2O (could be considered as local state) and its easier adsorption on bacterial cells, (ii) generation of ROS on TiO2, (iii) direct oxidation by positive holes from VB of Ag2O. Under UV irradiation, two mechanisms of photocatalysis could be considered, i.e., Z-scheme and p-n heterojunction (already discussed for photocatalytic oxidation of acetic acid). Considering the worse activity under UV than that under vis irradiation, the Z-scheme mechanism (for anatase/argentous oxide) seems more probable as the p-n junction should result in more positively charged Ag2O (electron transfer from CB of Ag2O to CB of TiO2 and hole transfer from VB of TiO2 to VB of Ag2O), and thus higher activity than that in vis would be expected.
Antifungal activities were tested by two methods, i.e., suspension method (yeast C. albicans) and spore counting method (filamentous P. chrysogenum). Figure 13 shows the survival ratio of C. albicans on Ag2O/ST01, Ag2O and TiO2 under UV, vis and in the dark in suspension method. Similarly, to bactericidal effect, bare Ag2O was the most active independently on applied conditions (little higher activity: UV > dark > vis). Bare titania was only slightly active under UV irradiation (confirming Yang et al. study that Candida sp. were sensitive to photocatalysis [67]), and Ag2O/ST01 was active only at higher content of argentous oxide (5 wt%) suggesting that Ag2O was responsible for antifungal effect. Dark activity of Ag2O/ST01 (5 wt%) was much lower than that of bare Ag2O, probably due to decreased interface Ag2O-fungus as titania partly covered argentous oxide. However, significant increase in activity was observed under vis irradiation confirming that direct contact between fungal cells and argentous oxide is a key-factor for high antifungal effect (under vis irradiation Ag2O would be positively charged because of an electron transfer to CB of titania). In contrast, a decrease in activity under UV irradiation could confirm Z-scheme mechanism resulting in electron-rich Ag2O surface (under UV irradiation both semiconductors would be excited with CB(TiO2) electron and VB(Ag2O) hole recombination leaving CB(Ag2O) electrons and VB(TiO2) holes). It has been reported that cell walls of C. albicans are negatively charged [68], and thus the repulsion between fungus and photocatalyst could be detrimental for antifungal activity.
Fungicidal activity against P. chrysogenum was investigated by the spore-counting method (Figure 14). Both Ag2O/ST01 samples (1 and 5 wt%) remarkably inhibited the generation of spores, and the formation of mycelium under fluorescence light (FL). Whereas, in contrast to yeast fungi and bacteria, the activity of bare Ag2O was negligible (difficulty in Ag2O adsorption on fungi in agar). In addition, 1 wt% Ag2O/ST01 suppressed the growth of mycelium more efficiently than 5 wt% Ag2O/ST01 under FL, corresponding to the photocatalytic activity of methanol dehydrogenation where the highest activity was obtained for the 1 wt% Ag2O/ST01 sample. Therefore, it is proposed that the enhanced photocatalytic activity inhibited the fungal growth. However, samples in the dark showed higher activity than irradiated ones, especially bare ST01, in spite of the growth of mycelium. It should be pointed out that the number of spores formed in the control sample under FL was larger than that in the dark, since the light stimulated (was required for) fungal growth and sporulation [69,70,71,72]. It was reported that filamentous (mold) fungi showed a high variability (depending on the strain and place of isolation) in the reaction with titania, light, and combined titania/light [72,73]. It should be also taken into consideration that titania might enhance the growth of plants. Both the plant and fungus kingdom share many characteristic, such as the nucleus and cell organelles enclosed within membranes, cell walls made of cellulose, lack of ability to move and thread-like structures (fungal and plant roots) [74]. Therefore, in the case of ST01, it is proposed that the coexistence of titania and light accelerates the fungal growth (growth stimulation>photocatalytic activity).

4. Summary and Conclusions

In summary, a hybrid photocatalyst was prepared by a simple and cheap method, i.e., the grinding of silver(I) oxide and titania. The application of different kinds of titania photocatalysts allowed the preparation of samples significantly differed by resultant properties, and thus photocatalytic activities. It was found that although grinding did not change the crystal form of composites, titania crushed large particles/crystallites of Ag2O, resulting in the formation of uniformly mixed composites.
A large improvement of photocatalytic activities was observed under UV/vis irradiation for methanol dehydrogenation and acetic acid oxidation. Two kinds of mechanisms could be responsible for this activity enhancement, i.e., Z-scheme and p-n junction. It is proposed that the mechanism might differ depending on the crystalline composition. For example, p-n junction could be more probable for the rutile-containing composite with a more negative CB level.
Additionally, it was found that titania modified with only 1 wt% of Ag2O achieved a vis response by being active for the oxidative decomposition of phenol and 2-propanol. The composites containing rutile showed higher vis activity (similarly to previously reported plasmonic photocatalysts (Au/TiO2) and Cu2O/TiO2 photocatalysts) suggesting that the more negative position of rutile CB could facilitate an electron transfer from vis-excited Ag2O.
Ag2O/TiO2 photocatalysts exhibited high antimicrobial activities. Although antibacterial and anti-yeast activity was mainly caused by Ag2O presence, independently on applied conditions (UV, vis and dark), antifungal activity for mold fungi was mainly caused by Ag2O/TiO2 composites. It was found that vis irradiation could significantly improve antibacterial and anti-yeast activities, due to the electrostatic attraction between the negative surface of microorganisms and the positively charged Ag2O (after electron transfer to titania).

Author Contributions

M.J. and E.K. conceived and designed the experiments; M.E.-K., M.J., Z.B., A.M.-S. and E.K. interpreted the data and wrote the paper; M.E.-K. performed the antimicrobial tests and STEM/EDS; K.W. and D.Z. performed XRD, DRS and photocatalytic experiments; D.Z. prepared samples; K.W. drew the schematic mechanisms. All authors read, corrected, and approved the final version of manuscript.

Funding

The research was founded in part by the (i) Fusion Emergent Research project by the MEXT program of Integrated Research Consortium on Chemical Sciences (IRCCS), (ii) Institute for Catalysis, Hokkaido University (HU) within the framework of the research cluster on Plasmonic Photocatalysis, and (iii) Innovation Program for Foreign Professors (HU).

Acknowledgments

The authors thank Bunsho Ohtani for sound advice and unlimited access to laboratory equipment and Maxime Berry for his assistance in the preliminary study. M. J. acknowledges Hokkaido University for the guest lecturer position (2016–2017).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photograph of photoreactor (top view): (1) Irradiation source (Xe lamp with cold mirror), (2) first part, (3) second part, (4) separating wall, (5) UV cut-off filter, (6) testing tube, (7) water inlet, (8) water outlet.
Figure 1. Photograph of photoreactor (top view): (1) Irradiation source (Xe lamp with cold mirror), (2) first part, (3) second part, (4) separating wall, (5) UV cut-off filter, (6) testing tube, (7) water inlet, (8) water outlet.
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Figure 2. XRD diffractograms of Ag2O/TiO2 samples: (a) Ag2O/P25, (b) Ag2O/ST01, (c) Ag2O/ST41, (d) Ag2O/TIO6, (e) Ag2O/RUT, (f) Ag2O/RUT’. (Insets) XRD patterns of Ag2O (after subtraction of titania peaks).
Figure 2. XRD diffractograms of Ag2O/TiO2 samples: (a) Ag2O/P25, (b) Ag2O/ST01, (c) Ag2O/ST41, (d) Ag2O/TIO6, (e) Ag2O/RUT, (f) Ag2O/RUT’. (Insets) XRD patterns of Ag2O (after subtraction of titania peaks).
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Figure 3. Diffuse reflectance spectra of Ag2O/TiO2 samples.
Figure 3. Diffuse reflectance spectra of Ag2O/TiO2 samples.
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Figure 4. STEM (a) and EDS (Ti- (b), Ag (c) and merge of Ti and Ag (d)) images of Ag2O/ST01.
Figure 4. STEM (a) and EDS (Ti- (b), Ag (c) and merge of Ti and Ag (d)) images of Ag2O/ST01.
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Figure 5. UV/vis photocatalytic efficiency of Ag2O/TiO2 samples with a different titania matrix in methanol dehydrogenation reaction (a) with respective photographs of samples after 45-min irradiation (b).
Figure 5. UV/vis photocatalytic efficiency of Ag2O/TiO2 samples with a different titania matrix in methanol dehydrogenation reaction (a) with respective photographs of samples after 45-min irradiation (b).
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Figure 6. The schematic mechanism of Z-scheme for UV/vis-induced methanol dehydrogenation with Ag(0)/Ag2O/TiO2 photocatalysts.
Figure 6. The schematic mechanism of Z-scheme for UV/vis-induced methanol dehydrogenation with Ag(0)/Ag2O/TiO2 photocatalysts.
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Figure 7. UV/vis photocatalytic efficiency of Ag2O/TiO2 samples with a different titania matrix in acetic acid oxidation (a) with respective photographs of samples after 45-min irradiation (b).
Figure 7. UV/vis photocatalytic efficiency of Ag2O/TiO2 samples with a different titania matrix in acetic acid oxidation (a) with respective photographs of samples after 45-min irradiation (b).
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Figure 8. The schematic mechanisms of oxidative decomposition of acetic acid under UV/vis irradiation with Ag2O/TiO2 photocatalysts: Z-scheme for anatase (a) and heterojunction (type II) for rutile (b).
Figure 8. The schematic mechanisms of oxidative decomposition of acetic acid under UV/vis irradiation with Ag2O/TiO2 photocatalysts: Z-scheme for anatase (a) and heterojunction (type II) for rutile (b).
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Figure 9. Change in phenol (a) and benzoquinone (b) concentrations during vis irradiation (λ > 400 nm) of Ag2O/TiO2 photocatalysts.
Figure 9. Change in phenol (a) and benzoquinone (b) concentrations during vis irradiation (λ > 400 nm) of Ag2O/TiO2 photocatalysts.
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Figure 10. Vis (λ > 420 nm) photocatalytic activity of Ag2O/TiO2 photocatalysts for 2-propanol oxidation.
Figure 10. Vis (λ > 420 nm) photocatalytic activity of Ag2O/TiO2 photocatalysts for 2-propanol oxidation.
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Figure 11. The schematic mechanisms of oxidative decomposition of 2-propanol and phenol under vis irradiation with Ag2O/TiO2 photocatalysts for anatase (a) and rutile (b).
Figure 11. The schematic mechanisms of oxidative decomposition of 2-propanol and phenol under vis irradiation with Ag2O/TiO2 photocatalysts for anatase (a) and rutile (b).
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Figure 12. Survival ratio of E. coli K12 bacteria in the presence of 5 wt% Ag2O/ST01 (a), 1 wt% Ag2O/ST01 (b), Ag2O (c) and bare ST01 (d) under UV irradiation (purple, 300 < λ < 420 nm), vis irradiation (green, λ > 420 nm), and in the dark (grey).
Figure 12. Survival ratio of E. coli K12 bacteria in the presence of 5 wt% Ag2O/ST01 (a), 1 wt% Ag2O/ST01 (b), Ag2O (c) and bare ST01 (d) under UV irradiation (purple, 300 < λ < 420 nm), vis irradiation (green, λ > 420 nm), and in the dark (grey).
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Figure 13. Survival ratio of C. albicans in the presence of 5 wt% Ag2O/ST01 (a), 1 wt% Ag2O/ST01 (b), Ag2O (c) and bare ST01 (d) under UV irradiation (purple, 300 < λ < 420 nm), vis irradiation (green, λ > 420 nm), and in the dark (grey).
Figure 13. Survival ratio of C. albicans in the presence of 5 wt% Ag2O/ST01 (a), 1 wt% Ag2O/ST01 (b), Ag2O (c) and bare ST01 (d) under UV irradiation (purple, 300 < λ < 420 nm), vis irradiation (green, λ > 420 nm), and in the dark (grey).
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Figure 14. Sporulation after five days growth of P. chrysogenum under florescence light and in the dark (a) and representative photographs of slants under florescence light (b) and in the dark (c).
Figure 14. Sporulation after five days growth of P. chrysogenum under florescence light and in the dark (a) and representative photographs of slants under florescence light (b) and in the dark (c).
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Table 1. Properties of Ag2O/TiO2 photocatalysts.
Table 1. Properties of Ag2O/TiO2 photocatalysts.
Sample NameCrystalline Content * (%)Crystallite Size (nm)
AnataseRutileAg2OAnataseRutileAg2O
Ag2O/P2582.516.90.621.838.518.3
Ag2O/ST0199.9-0.17.8-3.2
Ag2O/ST4199.6-0.475.7-7.8
Ag2O/TiO67.392.20.56.315.913.0
Ag2O/RUT2.097.40.638.986.129.8
Ag2O/RUT’11.587.40.7109.2170.141.3
Ag2O--100--58.7
* Crystalline composition without consideration of amorphous phase.

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