Photocatalytic activity of Ag3PO4 and some of its composites under non-filtered and UV-filtered solar-like radiation

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Photocatalytic activity of Ag3PO4 and some of its composites under nonfiltered and UV-filtered solar-like radiation Katarína Baďurováa, Olivier Monforta, Leonid Satrapinskyyb, Ewa Dworniczekc, ⁎ Grażyna Gościniakc, Gustav Plescha, a Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovicova 6, Mlynska Dolina, 842 15 Bratislava IV, Slovakia b Department of Experimental Physics, Faculty of Mathematics Physics and Informatics, Comenius University in Bratislava, Mlynska Dolina, 842 48 Bratislava IV, Slovakia c Department of Microbiology, Wroclaw Medical University, 50 368 Wroclaw, Poland

A R T I C L E I N F O

A BS T RAC T

Keywords: Ag3PO4 Composite Photocatalysis UV-filter Antimicrobial

Silver phosphate is a promising photocatalyst since its energy band gap is situated in the visible range (Eg≈2.4 eV), thus this material is a potential candidate for replacing titania which is photoactive only under UV. However, Ag3PO4 suffers of photocorrosion and therefore composites should be prepared to limit this detrimental effect. In this work, pure Ag3PO4 and its composites with AgI, TiO2, and hydroxyapatite were prepared by using various methods. The photoactivity of the materials was evaluated by their ability to decolorize methylene blue and to mineralize phenol under non-filtered and UV-filtered artificial solar-like radiation. The use of UV cut-off filter enhanced the photocatalytic activity of pure silver phosphate by limiting the photocorrosion of silver(I) into Ag°. For composites with AgI and TiO2, despite their lower photoactivity compared to pure Ag3PO4, the efficiency in mineralization of phenol after repeated run is stabilized by using UV cut-off filter. On the other hand, the photocatalytic efficiency of Ag3PO4 composites containing hydroxyapatite remained low mainly due to high absorption properties of hydroxyapatite. The photoactive samples showed excellent photoinduced antimicrobial properties where Gram-negative E. coli was more susceptible to photocatalytic deactivation than Gram-positive S. aureus (MRSA).

1. Introduction Nowadays the purification of water from toxic chemical pollutants and microbial contamination is a critical environmental issue [1]. Heterogeneous photocatalysis that involves semiconducting materials is considered today as a promising and efficient solution [1,2]. Silver phosphate (Ag3PO4) is a semiconductor that exhibits a direct and an indirect energy band gap (Eg) at 2.45 and 2.36 eV, respectively, i.e. at λ < 530 nm [3–5]. It is a yellow material almost insoluble in water (0.02 g/L at 25 °C) and it has a body-centered cubic structure composed of PO4 tetrahedra with 6 pairs of deformed AgO4 tetrahedra at each cubic face [3,6–8]. The valence band maximum (VBM) and conduction band minimum (CBM) is situated at 2.67 V and 0.22 V, respectively. Therefore the position of Eg is suitable for efficient photo-oxidative process e. g. photodegradation of organic pollutants mainly due to the strong oxidative ability of photogenerated holes (h+) [3,9]. For instance, Ag3PO4 powder exhibited total mineralization of phenol after 5 h under ⁎

simulated sunlight while organic dyes such as methylene blue and rhodamine B were degraded within 15 min under visible light [10–12]. Moreover, Ag3PO4 showed higher photocatalytic activity compared to other visible light-responsive photocatalysts like BiVO4 and N-doped TiO2 [3,9–11,13]. However, Ag3PO4 is photosensitive, thus Ag(I) is easily reduced to metallic silver under irradiation by reaction with photogenerated electrons (e−) [5,15]. This is due to the position of the redox potential E(Ag+/Ag) that is situated within Eg [5]. Therefore, the photogenerated e− cannot react with O2 to form superoxide radical due to the energetic potential value of E(O2/O2•−) that is more negative than the CBM [13]. To limit the photocorrosion of silver phosphate and to increase its stability and performance, composites of Ag3PO4 have been developed resulting in e− and h+ transfer between the conduction band (CB) and the valence band (VB) of the components [5,16–18]. Such materials have improved photocatalytic activity partly due to an increase in charge carriers lifetime and better e−/h+ pair separation [14,17,18,42]. Composites of Ag3PO4 with TiO2, AgX (X=Cl, Br, I), and

Corresponding author. E-mail address: [email protected] (G. Plesch).

http://dx.doi.org/10.1016/j.ceramint.2016.11.217 Received 24 October 2016; Received in revised form 28 November 2016; Accepted 30 November 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Badurová, K., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.11.217

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washed with distilled water. Finally both AGP-p and AGP-m samples were dried at 70 °C for 12 h. Composites of Ag3PO4 with AgI (AGP/AgI), TiO2 (AGP/P25), and hydroxyapatite were prepared by ion exchange method, impregnation, and co-precipitation, respectively, similarly to the procedures reported in [19,21,24]. In the case of Ag3PO4/hydroxyapatite, composites were prepared using crystalline calcined hydroxyapatite (AGP/HA) and amorphous uncalcined tricalcium phosphate (AGP/HAN), respectively. First, HA and HAN powders were mixed in 0.1 M AgNO3 and NH3 solution, respectively, and then 0.1 M Na2HPO4·12H2O was added. The mixture was vigorously stirred for 2 h and subsequently, the asprepared powders were washed several times with distilled water and dried at 80 °C for 24 h. The phase composition of samples was characterized by X-Ray powder diffraction (XRD) using Philips PW 1050. The morphology and particle size of the as-prepared powders were characterized by scanning electron microscopy (SEM) equipped of Energy Dispersive X-Ray Spectroscopy (EDX) on Tesca Lyra III microscope.

hydroxyapatite (HAp) have been studied and they showed enhanced photooxidative properties under visible light in the degradation of organic dyes [3,4,15,16,19–24]. Hydroxyapatite is an interesting material due to its biocompatibility. Calcined and uncalcined hydroxyapatite exhibit differences in structure, morphology and charge surface [35,36]. Moreover, amorphous calcium phosphate has demonstrated better osteoconductivity and crack resistance than hydroxyapatite, and its intrinsic porosity is useful for loading proteins, genes, RNA and drugs [37]. Silver phosphate is also a promising material for antimicrobial applications. So far, however, spectrum of its bactericidal activity is poorly understood [25]. The bacteria of Escherichia coli and Staphylococcus aureus constitute significant pathogens inhabiting at both hospital and natural environment of humans. Methicillin resistant S. aureus (MRSA) which is resistant to all beta-lactam antibiotics is frequently involved in healthcare-and community- associated infections that are difficult to treat [26]. MRSA survives in aquatic environment including rivers, seas and swimming pool water [27]. The pathogenic strains of E. coli, responsible for diarrheal illness, are typically transmitted with contaminated food and water [28,29]. In this work, Ag3PO4 powder was prepared using two simple and low cost methods i.e. precipitation and mechanical homogenization of powder precursors. Several composites of Ag3PO4 with AgI, HAp and TiO2 were also synthesized. The prepared photocatalysts were characterized by XRD and SEM, and their photocatalytic properties were studied under non-filtered and UV-filtered solar-like radiation. In addition, photoinduced antimicrobial properties were investigated under solar-like radiation and in the dark on the deactivation of microorganism, particularly E. Coli and MRSA. The photocatalytic properties of Ag3PO4-based materials were evaluated in the photo-oxidative degradation of two different organic substrates e. g. decolorization of methylene blue (MB) and mineralization of phenol. The full degradation of phenol is an important feature for potential applications in waste water cleaning process where micropollutants or residual drugs that are highly toxic and harmful should be removed [30–34]. In addition, photoinduced deactivation of bacterial colonies is crucial for medical purposes. According to our knowledge, up to now, only Ma and Piccirillo investigated the photoinduced properties of Ag3PO4 composites under UV and visible radiation [24,25]. This paper is devoted to investigations on the stability of various Ag3PO4-based materials in the photodegradation of different organic pollutants under non-filtered and UV-filtered solarlike radiation. This work is crucial for the potential use of silver phosphate composites under open-air conditions and indoor applications where the portion of UVA irradiation is smaller than in the sunlight.

2.2. Photocatalytic and antimicrobial measurements The photocatalytic activity of Ag3PO4-based powders was evaluated in the decolorization of 10 mg/L methylene blue (MB) solution and in the mineralization of 0.05 M phenol solution under a metal-halogenide arc-lamp (HQI TS – OSRAM 400 W, λmax=525 nm) with spectral distribution characteristics and intensity comparable to natural sunlight (B-class simulator) [38]. The irradiation intensity in the range 335–380 nm was 2.0 mW/cm2. In the photocatalytic experiments, 100 mg of photocatalyst was used in 100 mL of organic substrate. Before irradiation, the suspensions were magnetically stirred for 30 min in the dark to reach the adsorption-desorption equilibrium on the surface of catalyst. The measurements were conducted by using either pyrex glass filter (λ > 300 nm) or UV cut-off filter (λ > 400 nm). The photocatalytic efficiency in MB decolorization was evaluated by following the decrease in MB concentration using UV–VIS spectrophotometry (Jasco V-530) in the range 200–800 nm. The mineralization of phenol was measured by total organic carbon (TOC) on Shimadzu TOC-5050 device. Antimicrobial properties of Ag3PO4-based powders were studied on strains of methicillin-resistant Staphylococus aureus K324 (MRSA) and Escherichia coli PCM 2427. The cultures were prepared using Tryptic Soy Agar (TSA). The bacterial suspension contained 107 colony-forming units (CFU mL−1) mixed with the photocatalyst at 1500 μg mL−1 and it was pre-incubated for 15 min in the dark. Then, the constantly stirred mixture of bacteria and photoactive powder was irradiated with a commercial 30-W Xenon lamp. After 30 and 60 min of irradiation, 1 mL of the treated bacterial suspension was taken and serially diluted (10−1–10−3). To quantify the results, 100 μL of each diluted solution was inoculated onto TSA plates and incubated for 24– 48 h at 37 °C, and then the colonies were counted to determine the survival numbers of bacteria. The bactericidal efficiency of irradiated photocatalysts was compared to analogue system kept in the dark. In addition, control experiments containing the single component including Aeroxide® P25, HA, and HAN, or without photoactive powder were performed (the photoactivity of pure AgI could not be evaluated since it is rapidly decomposed under irradiation). Each experiment was performed in triplicate. The t-test for dependent variables (Statistica 12, StatSoft, Poland) was used to compare results of the antimicrobial effects of the powders under light and dark conditions. Significance was accepted at a P-value < 0.05.

2. Experimental 2.1. Synthesis and characterization All chemical reagents used in this work, including trisodium phosphate dodecahydrate (Na3PO4·12H2O), disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O), silver nitrate (AgNO3), ammonia solution, potassium iodide (KI), Aeroxide® P25 (TiO2), hydroxyapatite (HA) and amorphous tricalcium phosphate (HAN) were of analytical grade without further purification. HA and HAN were provided by Cambioceramics. Pure silver phosphate was prepared by precipitation (AGP-p) and mechanical homogenization of powder precursors (AGP-m). In the precipitation synthesis, 0.75 M aqueous solution of Na3PO4·12H2O was added dropwise under vigorous magnetic stirring into 0.25 M aqueous solution of AgNO3. The stirring was continued for 12 h in the dark, and then the yellow precipitate was washed several times with distilled water. In the second case, AgNO3 and Na3PO4·12H2O powders were mixed in a mortar until a yellow paste arised which was subsequently

3. Results and discussion 3.1. Characterization of the photocatalytic powders The XRD of silver phosphate prepared using either precipitation or 2

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similar particle size (Fig. 2d). 3.2. Photocatalytic properties of Ag3PO4-based materials Fig. 3 shows the photocatalytic efficiency of the Ag3PO4-based powders in decolorization of methylene blue (MB) and mineralization of phenol. The calculated kinetic constants using pseudo-first order reaction for the 2 photodegradation processes (Fig. 3a and b) are summarized in Table 1 and the corresponding degradation curves are shown in Fig. 3c and d. To gain information about the photodegradation for all samples, a repeated run of photocatalytic measurement was performed. A striking characteristic between the 2 photocatalytic reactions is that the irradiation times needed to degrade phenol and MB were substantially different. Indeed the degradation of the organic dye chromophore occurred at higher kinetical rate than the full mineralization of phenol (Table 1). This is due to the fact that the aromatic ring needs to be destroyed at the phenol mineralization whereas for MB, the chromophore (saturated hydrocarbon) is first destroyed leaving the aromatic system intact [24]. At our conditions, MB is decolorized within 20 min whereas phenol is almost mineralized in approximately 4 h. The degradation of MB was followed by UV–VIS spectrophotometry at 664 nm and 291 nm. The decrease in absorbance of MB solution at 664 nm is usually taken as a reference for evaluating the decolorization rate of this organic dye i.e. the destruction of its chromophore. However, the absorbance of MB solution at 291 nm gives information about the degradation stage of the entire dye [24].

Fig. 1. XRD of a) silver phosphate prepared by precipitation (AGP-p), mechanical homogenization (AGP-m) and its composites with titania (AGP/P25), silver iodide (AGP/ AgI), hydroxyapatite (AGP/HA), and b) pure silver phosphate freshly prepared and after photocatalytic experiment.

3.2.1. Photocatalytic decolorization of methylene blue The decolorization of MB measured by absorbance at 664 nm (not shown) using pure Ag3PO4 (AGP-m and AGP-p) is completed within 20 min under both non-filtered and UV-filtered artificial solar-like radiation even after repeated run. This fact showed the extreme efficiency of pure silver phosphate in the destruction of the MB's chromophore. In addition, AGP-p exhibited higher kinetic rate in the decolorization of MB than AGP-m (Table 1). It can be expected that its morphology is more beneficial for MB degradation (Fig. 2a and b). However the efficiency of pure Ag3PO4 in full photocatalytic degradation of MB measured by spectrophotometry at 291 nm decreased in comparison with the decolorization rate measured at 664 nm (Fig. 3e). It indicates that the chromophore part of the dye is destroyed but organic intermediates are formed within 20 min. Nevertheless, the degradation rate of MB increased by a factor of 10% under UV-filtered light compared to that under unfiltered solar-like radiation (Fig. 3e). Therefore, the UV cut-off filter played a stabilizing role in the photocatalytic efficiency of Ag3PO4. The filtering-off of the UVA part of the irradiation led to a decrease in photocorrosion into metallic silver Ag°. For all the silver phosphate composites, the efficiency in MB decolorization is lower than that of pure silver phosphate (Fig. 3e). For AGP/P25 and AGP/AgI, even if the kinetical parameters are comparable to those of pure Ag3PO4 under unfiltered light (Table 1), the decolorization of MB is strongly decelerated and then stopped due to photocorrosion of Ag3PO4. Still under unfiltered solar-like radiation, negligible decrease in the decolorization is observed after repeated runs for AGP/P25 and AGP/AgI (Fig. 3e). This can be explained by the formation of AgxO and Ag° by self- and photo-corrosion that acts as cocatalyst although Ag3PO4 is deteriorated [19,24]. However, using UVfiltered light, the photoactivity dropped especially for AGP/AgI sample. This decrease caused by UV cut-off filter can be explained by the cutting of UVA part of the irradiation necessary for activation of titania part of the composite. Moreover, because of the photocorrosion at the surface of Ag3PO4 and AgI, the use of the filter logically decreased the energy of incident irradiation that penetrates the composites, thus decreasing their photoactivity. The results regarding AGP/AgI are in accordance with the work of H. Katsumata et al. where Ag3PO4/AgI composite is used for the degradation of Acide Orange 7 (AO7) [19].

mechanical homogenization (AGP-p and AGP-m) and Ag3PO4-based composites (AGP/X with X=P25, AgI, HA, and HAN) are presented in Fig. 1a. Each powder exhibited diffractions from Ag3PO4 (ICDD #00006-0505) since it represents its main component. In the composites, the intensities of silver phosphate are weaker due to a decrease in Ag3PO4 content. The strongest diffractions of Ag3PO4 in the composite are in AGP/P25 and the weakest ones in AGP/HAN, AGP/HA, and AGP/AgI (Fig. 1a) because the ratio of silver phosphate is about 33 wt% and 9 wt%, respectively. The quantity of Ag3PO4 in each composite was optimized according to [19,21,24]. For AGP/P25 composite, weak diffractions that belong to anatase (ICDD #01-086-1157) are present since Aeroxide® P25 (75% anatase and 25% rutile) was used to prepare this material. In AGP/AgI, silver iodide (ICDD #01-085-0801) and diffractions belonging to impurities are present in the diffractogram (Fig. 1a). The preparation method of the AGP/AgI composite allows the formation of compounds such as KAgO and K2AgI3 due to the ion exchange process and contaminate the material even after several cleaning processes. In both AGP/HA and AGP/HAN composites, diffractions from hydroxyapatite (ICDD #00-009-0432) are present (Fig. 1a). The diffraction intensities of hydroxyapatite decreased in AGP/HAN because of weakly developed crystalline structure of tricalcium phosphate (non-calcinated HA). The SEM images of the prepared photocatalysts are shown in Fig. 2. The AGP-p and AGP-m samples exhibited typical morphologies for pure Ag3PO4 where irregular cubo-octahedra (Fig. 2a-inset) and cubes (Fig. 2b-inset) can be observed, respectively. The particle size was about 1 µm in both AGP-p and AGP-m. In the composite samples (Fig 2c–f), the particle size is globally bigger than in pure Ag3PO4. However, the particle size of the silver phosphate component in the composite did not differ from that of pure Ag3PO4 i.e. around 1 µm. This is supported by back scattering electron (BSE) measurement shown in Fig. 2e for AGP/HA composite where good contrast is obtained between hydroxyapatite (dark particles) and silver phosphate (bright particle) and this can be observed in each composite as seen in Fig. 2. In fact, the Ag3PO4 particles are aggregated at the surface of agglomerated titania, calcinated and non-calcinated hydroxyapatite with a size ranging from 5 µm for P25 (Fig. 2c) to tenth of micrometers for HA and HAN (Fig. 2e–f). In AGP/AgI composite the two components showed 3

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Fig. 2. SEM images of a) AGP-p, b) AGP-m, c) AGP/P25, d) AGP/AgI, e) AGP/HA, and f) AGP/HAN.

m under unfiltered solar-like radiation, AGP-p showed here a decrease in mineralization rate compared to AGP-m (Fig. 3, Table 1). This can be explained by the morphology of the silver phosphate particles where AGP-p had irregular cubo-octahedron morphology while AGP-m was rather cubic (Fig. 2a and b). As a consequence, the morphology of the particles that is linked to the exposed facets showed selectivity in the type of organic substrate that is photodegraded at the surface of the photocatalyst [5]. After repeated run, AGP-p and AGP-m showed an important decrease in the mineralization of phenol from about 90% to around 55% under filtered light (Fig. 3f). This is due to unavoidable photocorrosion after 4 h necessary for the full mineralization of the organic substrate although this phenomenon is limited. In addition, after irradiation under either non-filtered or UV-filtered artificial solar-like radiation, the yellow colour of AGP-p and AGP-m powder turned black. The XRD (Fig. 1b) and SEM/EDX (not shown) of used Ag3PO4 samples confirmed the presence of metallic silver that was calculated to be at approximately 2 at%. It is worth to notice that the photocorrosion occurred at the surface of the photocatalyst, thus after 4 h of irradiation, the photocorrosion tends to reach a limit. It explains the similar mineralization rates after repeated run observed for either AGP-p or AGP-m (Fig. 3f). For the AGP/P25 and AGP/AgI composites, the mineralization rate of phenol is generally lower than that of pure silver phosphate. This is mainly due to 3 reasons that act together: (1) the quantity of silver phosphate is highly diluted in the composite, (2) titania is photoactive only under UV radiation and silver iodide is more susceptible to photocorrosion than silver phosphate, and (3) the particle size is bigger in the composite as shown in SEM images (Fig. 2). AGP/P25 and AGP/ AgI composites exhibited better reaction rate in photomineralization of

They explained the excellent reproducibility in reusing Ag3PO4/AgI photocatalyst under visible light by the formation of silver nanoparticles that acted synergically in charge carrier separation [19]. In the case of composites containing hydroxyapatite (AGP/HA and AGP/HAN), their photoactivity is very similar but the situation differs from the other composites because of a strong decrease in photoactivity is observed after repeated run (Fig. 3e) Among photocorrosion of Ag3PO4, the observed decrease is due mainly to the excellent adsorption properties of hydroxyapatite that traps MB molecules. Therefore, the photoactivity of AGP/HA and AGP/HAN composites was hindered by the saturation in MB molecules that cannot be either released or degraded. The obtained results are in contradiction to the work of Hong et al. where they reported that the formation of Ag3PO4/ hydroxyapatite composite led to enhanced photocatalytic activity and they attributed this fact to a decrease in particle size and to a limitation of the photocorrosion by hydroxyapatite that integrates the released silver in its lattice [21].

3.2.2. Photocatalytic mineralization of phenol To study further the effect of UV cut-off filter on the photocatalytic activity, mineralization of phenol was studied by TOC. Phenol is an excellent model pollutant since it contains an aromatic ring like most of important environmental pollutants. It is evident in Fig. 3d that the use of UV-filtered light promoted the efficiency of AGP-p and AGP-m photocatalyst in phenol mineralization by limiting the photocorrosion of silver(I). Therefore, it confirms the fact already observed and discussed in the Section 3.2.1. However, the methods employed for the preparation of pure silver phosphate influenced the photoactivity in phenol mineralization as it was the case in MB decolorization. Indeed, contrary to MB degradation where AGP-p was more efficient than AGP4

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Fig. 3. Pseudo first order kinetic rule of some representative samples illustrated for the degradation of a) methylene blue and b) phenol. The corresponding degradation curves are presented in c) for methylene blue and d) for phenol. The photocatalytic efficiency after the first and second run of all the Ag3PO4-based materials are represented in e) for methylene blue and in f) for phenol. The abbreviation “SUN” referred to unfiltered light and “VIS” to UV-filtered artificial solar-like radiation. The reused samples are labelled with “*”. Table 1 Kinetic constant calculated using pseudo-first order model for both degradation of MB and phenol. The kinetic constant are calculated for 100 mg photocatalyst under both simulated and UV-filtered sunlight after first and second runs. The abbreviation “SUN” referred to unfiltered light and “VIS” to UV-filtrated artificial solar-like radiation. k [min−1·mol−1] Sample

Methylene Blue

Phenol

1st run SUN-light AGP-m AGP-p AGP/P25 AGP/AgI AGP/HA AGP/HAN

4

4.4·10 8.1·104 5.1·104 6.4·104 1.8·104 1.4·104

2nd run VIS-light 4

24.5·10 36.2·104 3.5·104 1.4·104 1.3·104 1.3·104

SUN-light 4

4.1·10 6.4·104 4.8·104 5.2·104 0.9·104 0.8·104

1st run VIS-light 4

11.3·10 28.3·104 2.0·104 2.1·104 1.0·104 0.7·104

phenol at the first run under unfiltered light than under UV-filtered radiation (Fig. 3f, Table 1). This phenomenon is already discussed in Section 3.2.1. However, the beneficial role of the UV-filter is more accentuated in the case of phenol mineralization after repeated run for

2nd run

SUN-light

VIS-light

SUN-light

VIS-light

1.78 0.90 0.52 0.48 0.36 0.04

2.48 1.78 0.24 0.30 0.16 0.08

0.62 0.56 0.30 0.04 4.0·10−2 1.8·10−2

0.72 0.68 0.40 1.20 2.0·10−2 4.0·10−2

AGP/P25 and AGP/AgI (Fig. 3f, Table 1). The obtained results are in accordance with those obtained by Ma et al. where they found that Ag3PO4/TiO2 composite exhibited under UV-filtered light lower but stable photocatalytic efficiency than under UV irradiation [24]. 5

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of irradiation from 30 to 60 min, and only AGP/AgI composite inactivated 100% of staphylococci after 30 min of irradiation (Fig. 4a). The composites exhibited better deactivation in MRSA growth than the pure silver phosphate samples, especially for the AgP/AgI composite. So the tendency in biological inactivation is opposed to chemical degradation. It can be assumed that the content of released silver which is higher in the composites than in pure silver phosphate is strongly beneficial for the antimicrobial activity against MRSA. It is in accordance with the fact that the highest activity shows the AGP/AgI composite where high content of silver nanoparticles was found also in Ref. [19]. Gram-negative rods of E. coli were much more susceptible to photocatalytic inactivation by using Ag3PO4-based powders than Gram-positive staphylococci. We noticed rapid and significant (P < 0.0005) reduction of E. coli: after loading at 1500 µg mL−1 of either pure Ag3PO4 or any of its composite, 100% of the bacteria was inactivated even after 30 min of irradiation (Fig. 4b). These results demonstrated that silver phosphate and its composites have better bactericidal activity against Gram-negative than Gram-positive bacteria. Similar observations on systems containing Ag3PO4 have been reported by other investigators [25]. The basis of this phenomenon is presumably due to the difference in the structure of cell wall of E. coli and S. aureus. Gram-negative bacteria such as E. coli are characterized by a thinner layer of cell wall peptidoglycan that makes the bacteria more prone to aggressive factors of external environment. Photocatalytic inactivation of the tested strains can be explained by their susceptibility to oxidative stress, where the excitation of photosensitive molecules generates reactive radical species which damage cellular membranes that consequently leads to death of bacterial cells [40,41].

For the AGP/HA and AGP/HAN composites, the observed results in the mineralization rate of phenol are the lowest compared to the other composites and pure Ag3PO4 (Fig. 3f). To the adsorption properties of hydroxyapatite toward organic substrates, the weak TOC results are biased (Fig. 3b) by partial adsorption of CO2 on hydroxyapatite [39]. Indeed, CO2 that is released during the full mineralization of phenol can react with surface hydroxyl group of hydroxyapatite, and therefore CO2 is irreversibly chemisorbed on hydroxyapatite in the form of CO32− by the following reaction [39]: CO2 + 2OH− → CO32− + H2O As a consequence, the measured concentrations of CO2 did not reflect fairly the photocatalytic activity. Moreover, the weaker photocatalytic activity of AGP/HAN than that of AGP/HA can be attributed to higher content of carbonate ions in HAN [35,36].

3.3. Antimicrobial properties of Ag3PO4-based materials Although the main stress of this work is focused on the comparison in the photooxidative degradation of organic substrates under different irradiations, the antimicrobial properties of Ag3PO4-based materials were also investigated on E. coli and MRSA under non-filtered radiation (Fig. 4). The obtained results show that the sole irradiation (in the absence of photocatalysts) had no significant (P > 0.05) impact on the growth of E. coli and MRSA. Even the controls containing calcined and uncalcined hydroxyapatite (HA and HAN) did not significantly (P > 0.05) inhibit the bacterial growth under light exposure (Fig. 4). However, analyses revealed that photoinduced antibacterial activity of the control Aeroxide® P25, after 60 min of irradiation, was weak as seen in Fig. 4 but statistically significant (P < 0.0138 for E. coli; P < 0.0429 for MRSA). Pure Ag3PO4 and its composites (AGP-p, AGP-m, AGP/P25, AGP/ AgI, AGP/HA and AGP/HAN), despite the presence of silver in the materials, showed no significant (P > 0.05) bactericidal activity in the dark (Fig. 2). It can be assumed that the low activity of the investigated powders in the dark is caused by their great particle size and also by the low release of biocidal Ag(I) ions into the solution [43,44]. However in light, the presence of 1500 µg mL−1 of AGP/P25, AGP/ AgI, AGP/HA and AGP/HAN composites exhibited complete suppression of bacterial growth with 100% inactivation of MRSA after 60 min of irradiation from 6.2, 6.1, 6.3 and 6.0 log CFU mL−1, respectively, to 0 log CFU mL−1 (Fig. 4a). Under the same conditions, pure silver phosphate (AGP-p and AGP-m) showed significantly weaker bactericidal activity on MRSA growth (decrease from 6.2 to 4.6 and to 3.3 log CFU mL−1, respectively) (Fig. 4a). It can be observed that the bactericidal activity of Ag3PO4-based powders on MRSA increases with time

4. Conclusion Silver phosphate prepared by simple methods as precipitation or mechanical homogenization of powder precursors and composites of Ag3PO4 with titania, silver iodide, calcinated hydroxyapatite, and amorphous tricalcium phosphate have shown different photoactivity under non-filtered and UV-filtered artificial solar-like radiation. Pure Ag3PO4 exhibited the best degradation rate in decolorization of MB as well as in the mineralization of phenol. Due to limitation in photocorrosion under UV-filtered light, the photoactivity of pure silver phosphate was enhanced in the degradation of both organic substrates to reach more than 90% efficiency. However, the silver phosphatebased composites showed lower photocatalytic efficiency due, among others, to dilution of Ag3PO4 quantity in the materials and bigger crystallite size. Nevertheless after repeated run, for the composite containing AgI and TiO2, the use of UV cut off filter played a stabilizing role since mineralization rate of phenol and its kinetic are higher

Fig. 4. Antimicrobial properties of Ag3PO4-based materials on a) MRSA and b) E. coli.

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compared to those using unfiltered irradiation. For Ag3PO4 composites containing hydroxyapatite, the photocatalytic efficiency is rather low due to both photocorrosion of silver phosphate as well as high adsorption properties of hydroxyapatite toward organic substrates. Finally, all photocatalysts exhibited excellent antimicrobial properties where Gram-negative E. coli was more susceptible to photocatalytic deactivation than Gram-positive S. aureus (MRSA).

[19]

[20]

[21]

Acknowledgement

[22]

Authors appreciate the financial support granted by the Scientific Grant Agency of the Slovak Republic (Project VEGA 1/0276/15), by the Comenius University in Bratislava (Project for Young Scientists UK/ 45/2016 and UK/46/2016) and by the Wrocław Medical University (ST.A130.16.032). This work has been also supported by Operational Program Research and Development (project ITMS: 26210120010). Authors want also to thank Tomas Roch and Peter Billik for performing the XRD measurements, and Alicja Seniuk for her assistance in microbiological studies.

[23]

[24]

[25]

[26]

References [27] [1] M. Zhu, P. Chen, M. Liu, Visible-light-driven Ag/Ag3PO4-based plasmonic photocatalysts: enhanced photocatalytic performance by hybridization with grapheme oxide, Chin. Sci. Bull. 58 (1) (2013) 84–91. [2] J.A. Byrne, P.A. Ferdinandez-Ibanez, P.S.M. Dunlop, D.M.A. Alrousan, J.W.J. Hamilton, Photocatalytic enhancement for solar disinfection of water: a review, Int. J. Photoenergy 2011 (2011) 1–12. [3] G. Huang, Z.-L. Ma, W.-Q. Huang, Y. Tiang, C. Jiao, Z.-M. Yang, Z. Wan, A. Pan, Ag3PO4 semiconductor photocatalyst: possibilities and challenges, J. Nanomater. 2013 (2013) 1–8. [4] J. Xie, Y. Yang, H. He, D. Cheng, M. Mao, Q. Jiang, L. Song, J. Xiong, Facile synthesis of hierarchical Ag3PO4/TiO2 nanofiber heterostructures with highly enhanced visible light photocatalytic properties, Appl. Surf. Sci. 355 (2015) 921–929. [5] X. Chen, Y. Dai, X. Wang, Methods and mechanism for improvement of photocatalytic activity and stability of Ag3PO4: a review, J. Alloy. Compd. 649 (2015) 910–932. [6] A. Wu, C. Tian, W. Chang, Morphology-controlled synthesis of Ag3PO4 nano/ microcrystals and their antibacterial properties, Mater. Res. Bull. 48 (9) (2013) 3043–3048. [7] H.N. Ng, C. Calvo, R. Faggiani, A new investigation of the structure of silver orthophosphate, Acta Crystallogr. B 34 (1978) 898–899. [8] X. Ma, B. Lu, D. Li, R. Shi, C. Pan, Y. Zhu, Origin of photocatalytic activation of silver orthophosphate from first-principles, J. Phys. Chem. C 115 (2011) 4680–4687. [9] X. Cui, Y.F. Zheng, H. Zhou, H.Y. Yin, X.C. Song, The effect of synthesis temperature on the morphologies and visible light photocatalytic performance of Ag3PO4, J. Taiwan Inst. Chem. Eng. 60 (2016) 328–334. [10] Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. Stuart-Williams, H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, R.L. Withers, An orthophosphate semiconductor with photooxidation properties under visible-light irradiation, Nat. Mater. 9 (2010) 559–564. [11] T.L.R. Hewer, B.C. Machado, R.S. Freire, R. Guardani, Ag3PO4 sunlight-induced photocatalyst for degradation of phenol, RSC Adv. 4 (2014) 34674–34680. [12] R. Dhanabal, A. Chithambararaj, S. Velmathi, A. Chandra Bose, Visible light driven degradation of methylene blue dye using Ag3PO4, J. Environ. Chem. Eng. 3 (3) (2015) 1872–1881. [13] H. Wang, Y.S. Bai, J.T. Yang, X.F. Lang, J.H. Li, L. Guo, A facile way to rejuvenate Ag3PO4 as a recyclable highly efficient photocatalyst, Chem. Eur. J. 18 (18) (2012) (5524–2229). [14] H. Xu, H. Zhao, Y. Xu, Z. Chen, L. Huang, Y. Li, Y. Song, Q. Zhang, H. Li, Threedimensionally ordered macroporous WO3 modified Ag3PO4 with enhanced visible light photocatalytic performance, Ceram. Int. 42 (1) (2016) 1392–1398. [15] Z. Chen, W. Wang, Z. Zhang, X. Fang, High-efficiency visible-light-driven Ag3PO4/ AgI photocatalysts: Z-scheme photocatalytic mechanism for their enhanced photocatalytic activity, J. Phys. Chem. 117 (38) (2013) 19346–19352. [16] Y. Chai, J. Ding, L. Wang, Q. Liu, J. Ren, W.-L. Dai, Enormous enhancement in photocatalytic performance of Ag3PO4/HAp composite: a Z-scheme mechanism insight, Appl. Catal. B-Environ. 179 (2015) 29–36. [17] J. Guo, S. Ouyang, H. Zhou, T. Kako, J. Ye, Ag3PO4/In(OH)3 composite photocatalysts with adjustable surface-electric property for efficient photodegradation of organic dyes under simulated solar-light irradiation, J. Phys. Chem. C 117 (34) (2013) 11716–117724. [18] W. Liu, M. Wang, C. Xu, S. Chen, X. Fu, Ag3PO4/ZnO: an efficient visible-light-

[28] [29]

[30] [31]

[32]

[33]

[34]

[35]

[36] [37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

7

sensitized composite with its application in photocatalytic degradation of rhodamine B, Mater. Res. Bull. 48 (1) (2013) 106–113. H. Katsumata, T. Hayashi, M. Taniguchi, T. Suzuki, S. Kaneco, AgI/Ag3PO4 hybrids with highly efficient visible-light driven photocatalytic activity, Mater. Res. Bull. 63 (2015) 116–122. F.-M. Zhao, L. Pan, S. Wang, Q. Deng, J.-J. Zou, L. Wang, X. Zhang, Ag3PO4/TiO2 composite for efficient photodegradation of organic pollutants under visible light, Appl. Surf. Sci. 317 (2014) 833–838. X. Hong, X. Wu, Q. Zhang, M. Xiao, G. Yang, M. Qiu, G. Han, Hydroxyapatite supported Ag3PO4 nanoparticles with higher visible light photocatalytic activity, Appl. Surf. Sci. 258 (10) (2012) 4801–4805. C. Jia, X. Xie, M. Ge, Y. Zhao, H. Zhang, Z. Li, G. Cui, In situ ion exchange preparation of AgBr/Ag3PO4/HAP nanocomposite photocatalyst with enhanced visible light photocatalytic performance, Mat. Sci. Semicond. Process. 36 (2015) 71–77. W. Zhai, G. Li, P. Yu, l. Yang, l. Mao, Silver phosphate/carbon nanotube-stabilized pickering emulsion for highly efficient photocatalysis, J. Phys. Chem. C 117 (2013) 15183–15191. X. Ma, H. Li, Y. Wang, H. Li, B. Liu, S. Yin, T. Sato, Substantial change in phenomenon of « self-corrosion » on Ag3PO4/TiO2 compound photocatalyst, Appl. Catal. B-Environ. 158–159 (2014) 314–320. C. Piccirillo, R.A. Pinto, D.M. Tobaldi, R.C. Pullar, J.A. Labrincha, M.M.E. Pintado, P.M.L. Castro, Light induced antibacterial activity and photocatalytic properties of Ag/Ag3PO4-based material of marine origin, J. Photochem. Photobiol. A 296 (2015) 40–47. A.G. Mainous, V.A. Diaz, E.M. Matheson, S.H. Gregorie, W.J. Hueston, Trends in hospitalizations with antibiotic-resistant infections: U.S., 1997–2006, Public Health Rep. 126 (3) (2011) 354–360. O. Tolba, A. Loughrey, C.E. Goldsmith, B.C. Millar, P.J. Rooney, J.E. Moore, Survival of epidemic strains of healthcare (HA-MRSA) and community-associated (CA-MRSA) meticillin-resistant Staphylococcus aureus (MRSA) in river-, sea- and swimming pool water, Int. J. Hyg. Environ. Health 211 (3–4) (2008) 398–402. A. Caprioli, G. Scavia, S. Morabito, Public health microbiology of Shiga toxinproducing Escherichia coli, Microbiol. Spectr. 2 (6) (2014) (EHEC-0014-2013). S.M. Chekabab, J. Paquin-Veillette, C.M. Dozois, J. Harel, The ecological habitat and transmission of Escherichia coli O157:H7, FEMS Microbiol. Lett. 341 (1) (2013) 1–12. X. Xu, X. Yu, Y. Tang, C. Wu, Degradation of levofloxacin lactate using Ag/Ag3PO4 under visible light, J. Chin. Ceram. Soc. 40 (12) (2012) 1796–1801. M.E. Taheri, A. Petala, Z. Frontistis, D. Mantzavinos, D.I. Kondarides, Fast photocatalytic degradation of bisphenol A by Ag3PO4/TiO2 composites under solar radiation, Catal. Today (2016). http://dx.doi.org/10.1016/j.cattod.2016.05.047. F. Mendez-Arriaga, S. Esplugas, J. Gimenez, Photocatalytic degradation of nonsteroidal anti-inflammatory drugs with TiO2 and simulated solar irradiation, Water Res. 42 (3) (2008) 585–594. W.W.-P. Lai, H.H.-H. Lin, A.Y.-C. Lin, TiO2 photocatalytic degradation and transformation of oxazaphosphorine drugs in an aqueous environment, J. Hazard. Mater. 287 (2015) 133–141. M. El-Kemary, H. El-Shamy, I. El-Mehasseb, Photocatalytic degradation of ciprofloxacin drug in water using ZnO nanoparticles, J. Lumin. 130 (12) (2010) 2327–2331. N. Patel, I.R. Gibson, S. Ke, S.M. Best, W. Bonfield, Calcining influence on the powder properties of hydroxyapatite, J. Mater. Sci. Mater. Med. 12 (2) (2001) 181–188. Y.M.Z. Ahmed, S.M. El-Sheikh, Z.I. Zaki, Changes in hydroxyapatite powder properties via heat treatment, Bull. Mater. Sci. 38 (7) (2015) 1807–1819. R.-H. Lai, P.-J. Dong, Y.-L. Wang, J.-B. Luo, Redispersible and stable amorphous calcium phosphate nanoparticles functionalized by an organic bisphosphate, Chin. Chem. Lett. 25 (2) (2014) 295–298. X. Dong, Z. Sun, G.J. Nathan, P.J. Ashman, D. Gu, Time-resolved spectra of solar simulators employing metal halide and xenon arc lamps, Sol. Energy 115 (2015) 613–620. Z.H. Cheng, A. Yasukawa, K. Kandori, T. Ishikawa, FTIR study of adsorption of CO2 on nonstoichiometric calcium hydroxyapatite, Langmuir 14 (23) (1998) 6681–6686. J. Kiwi, V. Nadtochenko, Evidence for the mechanism of photocatalytic degradation of the bacterial wall membrane at the TiO2 interface by ATR-FTIR and laser kinetic spectroscopy, Langmuir 21 (10) (2005) 4631–4641. N.K. Eswar, P.C. Ramamurthy, G. Madras, Enhanced sunlight photocatalytic activity of Ag3PO4 decorated novel combustion synthesis derived TiO2 nanobelts for dye and bacterial degradation, Photochem. Photobiol. Sci. 14 (7) (2015) 1227–1237. M. Shekofteh-Gohari, A. Habibi-Yangjeh, Photosensitization of Fe3O4/ZnO by AgBr and Ag3PO4 to fabricate novel magnetically recoverable nanocomposites with significantly enhanced photocatalytic activity under visible-light irradiation, Ceram. Int. 42 (14) (2016) 15224–15234. A. Wu, Ch Tian, W. Chang, Morphology-controlled synthesis of Ag3PO4 nano/ microcrystals and their antibacterial properties, Mater. Res. Bull. 48 (2013) 3043–3048. P. Lalueza, M. Monzon, M. Arruebo, J. Santamarıa, Bactericidal effects of different silver-containing materials, Mater. Res. Bull. 46 (2011) 2070–2076.