Silver-modified ZnO highly UV-photoactive

Accepted Manuscript Title: Silver-modified ZnO highly UV-photoactive Authors: C. Jaramillo-P´aez, J.A. Nav´ıo, M.C. Hidalgo PII: DOI: Reference:

S1010-6030(17)31386-2 https://doi.org/10.1016/j.jphotochem.2017.12.044 JPC 11082

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

20-9-2017 20-12-2017 27-12-2017

Please cite this article as: C.Jaramillo-P´aez, J.A.Nav´ıo, M.C.Hidalgo, Silver-modified ZnO highly UV-photoactive, Journal of Photochemistry and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2017.12.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Silver-modified ZnO highly UV-photoactive C. Jaramillo-Páez1,2, J.A. Navío1,*, M.C. Hidalgo1 1Instituto

de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Sevilla, Spain de Química, Universidad del Tolima, Barrio Santa Elena, Ibagué, Colombia.

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2Departamento

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* Corresponding author: (J.A. Navío). E-mail address:[email protected]

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Graphical abstract

Highlights Ag-photodeposition on ZnO synthesized by a controlled precipitation method.

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Photocatalytic degradation of Rhodamine B, Methyl Orange and Phenol. Metalized sample, named as ZnO-Ag(x) showed high UV-photoactivity and good recycling stability.

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Abstract

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ZnO nanoparticles were successfully synthesized by a controlled precipitation procedure by mixing aqueous solutions of Zn(II) acetate and dissolved Na2CO3 at pH ca. 7.0 without template addition and ulterior calcination at 400ºC for 2 h. The Ag-ZnO catalysts (ranging from 0.5-10 Ag wt.-%) were obtained by photochemical deposition method at the surface of the prepared ZnO sample, using AgNO3 as precursor. The as-prepared catalysts (with and without silver) were characterized by XRD, BET, FE-SEM, TEM, and XPS and diffuse reflectance spectroscopy (DRS). The effect of Ag-phodeposition on the photocatalytic properties of ZnO nanoparticles was investigated. Three different probe molecules were used to evaluate the photocatalytic properties under UVillumination and visible illumination: Methyl Orange and Rhodamine B were chosen as hazardous dyes and Phenol as a transparent substrate. For each of the chosen substrates, it was observed that the UV-photocatalytic properties of ZnO improved with the amount of Ag deposited, up to an optimum percentage around 1-5 wt.-% Ag, being even better than the commercial Evonik-TiO2(P25) in the same conditions. Above this amount, the UV-photocatalytic properties of the Ag-ZnO samples remain unchanged, indicating a maximum for Ag-deposition. While ZnO and Ag-ZnO catalysts can photodegrade Rhodamine B, Methyl Orange and Phenol totally within 60 min under UV-illumination, the process is slightly faster for the case of Ag–ZnO nanoparticles. Under Visillumination, the silver-metalized samples did not present photocatalytic activity in the degradation of Methyl Orange. However, a very low photoactivity was present for phenol degradation (10% conversion) and a moderate conversion of ca. 70% for Rhodamine B degradation, after 120 min of Visible-illumination. High conversion values and a total organic carbon (TOC) removal of 86-97% were obtained over the Ag-ZnO photocatalysts after 120 min of UV-illumination, suggesting that these Ag-modified ZnO nanoparticles may have good applications in wastewater treatment, due to its reuse properties. Keywords: Zinc oxide; Ag-ZnO; Photocatalysis; Photocatalytic degradation; Phenol; Dyes

1. Introduction Heterogeneous photocatalysis is an effective and promising process for the destruction of contaminants such as microorganisms, phenolic compounds and dyes in water sources, by using solar or artificial light illumination. In particular, based semiconductor photocatalysis, has been widely applied in removing

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organic pollutants, both from air as well as from waters [1]. Semiconductor

materials like ZnO and TiO2 exhibit excellent photocatalytic activity under UV-

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light illumination, due to their relatively large band gap of about 3.37 and 3.20

eV, respectively. ZnO is a very attractive semiconductor photocatalyst, because its band gap energy is comparable to TiO2. In fact there are studies that indicate

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that the main advantage of ZnO is that it absorbs a larger fraction of the solar

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spectrum than TiO2 [2]. Due to the position of the valence band of ZnO, the

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photogenerated holes have strong enough oxidizing power to decompose great

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diversity of organic compounds [3]. Accordingly, ZnO has been suggested as an

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alternative to TiO2 photocatalysts due to its similar electronic properties and higher photoeffeciency [4]. In fact, today in the area of heterogeneous

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photocatalysis, ZnO has emerged as a good candidate as an efficient and promising material in heterogeneous photocatalysis overall because of its

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unique characteristics, such as direct and wide band gap in the near-UV spectral region, strong oxidation ability, good photocatalytic property, and a

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large free-exciton binding energy (60 meV) so that excitonic emission processes can persist at or even above room temperature [5]. On the other hand, modification of photocatalysts with noble metals such as Pt, Au and Pd, is a procedure used to obtain more efficient photocatalysts [5–11]. In a recent review, the concept of photo-deposition and the promise it might

hold for efficient preparation of co-catalytic nanoparticles on semiconductors has been well established [12]. Although the positive effects of the noble metal islands on photocatalytic activity are several [13–15], the amount of metal deposits cannot be increased

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indefinitely. In fact, negative effects of the presence of metal deposits on the photocatalyst surface have been described to lead to decreased photon

efficiency. In recent years, not only ZnO but metallic silver has caught the

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attention of a lot of researchers due to the increased photoactivity and bactericidal effect of ZnO-photocatalysts which contain it [16].

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In a previous work [17] we report that a ZnO photocatalyst, prepared by a facile

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and low cost method, is highly UV-effective for the degradation of Methyl

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Orange and Phenol. Also, it was reported that the UV-photocatalytic efficiency

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of the synthesized ZnO photocatalyst was remarkably enhanced in the

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presence of Ag+ cations and targeted Methyl Orange was completely degraded within 60 min, leading to a new photocatalyst (after recovering) which exhibits

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superior photocatalytic performance and excellent cycling stability for the UV-

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photocatalytic degradation of Methyl Orange. Taking into account the potential properties of the highly UV-photoactive ZnO previously reported [17], the main objective of the present study was to evaluate

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the photocatalytic activity of a highly UV-photoactive ZnO [17] now modified by Ag-photodeposition (with different nominal content of Ag) under both UV or visible-illumination. The activity of the materials prepared was evaluated by the use of three different substrates: Methyl Orange, Rhodamine B and Phenol. Commercial TiO2(P25, Evonik), was used as a reference photocatalyst.

2. Experimental details 2.1 Preparation of ZnO and Ag-ZnO Nanoparticles of ZnO were prepared by a facile surfactant-free chemical solution approach described in a paper previously published by us [17]. Briefly, 1.75 g of Zn(CH3COO)2.2H2O (Sigma-Aldrich, ≥ 99.0% purity) was totally

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dissolved in 50 mL of bidistilled water at pH=6.5 (solution A). Subsequently, 50

mL of a water solution containing 0.84 g of dissolved pure Na2CO3 (Panreac,

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99.0% purity) at pH=12 (solution B) was added in with stirring, being the final pH=6.8. During 24 h of ageing a white precipitated gel was progressively formed. The precipitate was centrifuged, washed with several portions of

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bidistilled water and dried at 100ºC 12 h. The sample thus prepared was

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annealed at 400°C for 2 h, since in our previous work [17], it was established

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the synthesized ZnO.

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that this treatment leads to an optimization of the photocatalytic properties of

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This as-prepared ZnO sample was modified by silver addition, using the photodeposition method. AgNO3, (Sigma-Aldrich 99.9%) was used as metal

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precursors for silver. Under an inert atmosphere (N2), a suspension of ZnO in bidistilled water containing isopropanol (Merck 99.8%) was prepared. Then, the

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appropriate amount of metal precursor was added in order to obtain nominal Ag loading from 0.5% to 10% weight total to ZnO. Photochemical deposition of Ag

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was then performed by illuminating the suspensions, using 2 h of illumination. The light intensity on the ZnO surface was 90 W/m2. After photodeposition, the powders were recovered by filtration and dried at 100°C overnight. The metalized samples were called ZnO-Ag(x), being x the nominal Ag loading in wt.-% to ZnO. Aliquots of the initial AgNO3 aqueous solution (without catalyst)

prior to the photodeposition process as well as the transparent liquid, remaining after settling of the photocatalyst, after the silver photodeposition process, were recovered and subjected to cations analysis by Atomic Emission Spectrometry with plasma ICP (Horiba Jobin Yvon, model ULTIMA 2). Commercial TiO2(P25) was used as reference material and was employed as

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received.

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2.2. Characterization of the photocatalysts

X-ray diffraction (XRD) patterns were obtained on a Siemens D-501 diffractometer with Ni filter and graphite monochromator using Cu Kα radiation.

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Crystallite sizes were calculated from the line broadening of the main X-ray

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were fitted by using a Voigt function.

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diffraction peaks, (100), (002) and (101) by using the Scherrer equation. Peaks

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BET surface areas (SBET) of all samples were evaluated by N 2 adsorption

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measurement with a Micromeritics ASAP 2010 instrument. Degasification of the samples was performed at 150ºC for 30 min in He flow.

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Chemical composition and total silver content in the Ag-metalized sample was determined by X-ray fluorescence spectrometry (XRF) in a Panalytical Axios

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sequential spectrophotometer equipped with a rhodium tube as the source of radiation. XRF measurements were performed onto pressed pellets (sample

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included in 10 wt.-% of wax).The morphology for all the samples was analyzed by field Scanning electron microscopy (SEM) using a Hitachi S 4800 microscope. Transmission electron microscopy (TEM) was performed in a Philips CM 200 microscope. The samples for the microscopic analyzes were dispersed in ethanol using an ultrasonicator and dropped on a carbon grid.

Light absorption properties of the samples were studied by UV–Vis spectroscopy. The Diffuse Reflectance UV–Vis Spectra (UV–Vis DRS) were recorded on a Varian spectrometer model Cary 100 equipped with an integrating sphere and using BaSO4 as reference. Band-gaps values were calculated from the corresponding Kubelka–Munk functions, F(R∞), which are

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proportional to the absorption of radiation, by plotting (F(R∞) ×hν)1/2 against hν.

Surface characterization by X-ray photoelectron spectroscopy (XPS) was

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conducted on a SPECS spectrometer, working with constant pass energy of 40

eV. The spectrometer main chamber was maintained at 5-6∙10-10 bar, and the machine was equipped with a PHOIBOS 150 9MCD hemispherical electron

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analyser, using Al Kα (hµ) 1486.6 eV at 250W and 12.5 Kv. Zn 2p3/2 signal

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(1022.4 eV) was used as the internal energy reference in all measurements. All

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2.3. Photocatalytic tests

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photoelectron spectra were analysed using Casa-XPS software.

The photocatalytic activity of the catalysts prepared was tested in the photo-

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assisted degradation of two chosen dyes, Methyl Orange or Rhodamine B and of a transparent substrate, the Phenol. Methyl Orange, Phenol and Rhodamine

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B [Reagent Plus >99%] were supplied by Sigma-Aldrich. From this section we will use the abbreviations of the reagents used, that will hereafter be named

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along the text, as MO (Methyl Orange), as RhB (Rhodamine B) and Phenol occasionally as Ph. Photocatalytic tests were carried out using a discontinuous batch system, this includes a 250 mL Pyrex reactor enveloped by an aluminum foil, filled with an aqueous suspension (100 mL) containing the single substrates (concentrations:

20 ppm of MO, 50 ppm of Phenol or 10 ppm of RhB) and the photocatalyst (1g/L). Illumination conditions, either for the photodeposition processes as well as for the photocatalytic tests, were achieved using an Osram Ultra-Vitalux lamp (300 W) with a sun-like spectrum and a main line in the UVA range at 365 nm. UV-

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conditions were obtained by illuminated through a UV-transparent Plexiglas®

top window (threshold absorption at 250 nm). The intensity of the incident UVA

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light on the solution was measured with a PMA 2200 UVA photometer (Solar

Light Co.) being ca 90 W/m2 (UVA PMA2110 sensor; spectral response 320– 400 nm). On the other hand, the intensity of light in the visible range measured

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in this case was 110 W/m2 (Photopic PMA2130 sensor; spectral response 400–

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700 nm).

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In order to favor the adsorption–desorption equilibrium, prior to illumination the

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suspension was magnetically stirred for 20 min in the dark. Magnetic stirring

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and a constant oxygen flow of 20 L/h, as an oxidant, were used to produce a homogeneous suspension of the photocatalyst in the solution. A tank bubbler

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was used as a source of oxygen. All photocatalytic tests started at pH ca. 5.5 and the total reaction time was 120 min.

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Concentrations of MO and RhB during the photodegradation reactions were analyzed by UV–Visible spectroscopy, considering the main peak of each dye in

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the visible range, located at 465 nm (for MO) and 554 nm (for RhB). For this analysis a UV–Vis spectrometry with a Cary 100 (Varian) spectrometer was used. Phenol concentrations and possible intermediates were followed using HPLC (Agilent Technologies, 1200 Series) equipped with UV-Vis detector using an Elipse XDB-C18 column (5 μm, 4.6 mm × 150 mm; Agilent) at 40ºC. Aliquots

(2 mL) were removed periodically during the experiments and filtered (Millipore Millex 25 0.45 mm membrane filter) previous to HPLC measurements. Mobile phase was water/methanol (65:35) at a flow rate of 0.8 mL/min. The initial reaction rates were calculated considering the first 15 minutes of reaction, and by using the equation r0 = K C0/t were K is the reaction constant, obtained from

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the slope of the degradation profile graph (substrate concentration Vs reaction

time), C0=starting concentration of the substrate [mg·L-1] and t is the time in

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seconds.

We have proven that the initial concentrations of the substrates do not suffer variations, either under direct photolysis or in the single presence of the

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catalyst. Reproducibility of the measurements was ensured by double testing of

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selected samples.

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Total organic carbon was followed by means of a TOC analyzer Shimadzu 500.

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Mineralization degrees (%) were evaluated by the TOC values upon 2 h of

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illumination, for all the photo-assisted processes studied, using the formula

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[1- (final TOC/initial TOC)]x100.

3. Results and discussion

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3.1. Characterization

Figure 1A displays the XRD pattern of the as-prepared ZnO sample and the

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metallized ones. All of the diffraction peaks were labeled and could be indexed to hexagonal wurtzite phase of ZnO (JCPDS Card file No. 079-2205). The diffraction peaks corresponding to (100), (002), (101), (102), (110), (103), (112), and (201) planes were found. The strong and sharp XRD peaks in the figure indicated that the bare ZnO and ZnO-Ag samples were highly crystalline. After

Ag-photodeposition on the surface of ZnO, four additional peaks of 38.1º, 44.3º, 64.4º and 77.4º can be observed (marked with asterisks), which can be attributed to the (111), (200), (220) and (311) crystal planes of cubic Ag (JCPDS 087-0719). These results indicated that no other impurities are found in the synthesized samples.

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In Figure 1B we represent the relative intensities of peaks corresponding to the (100) and (002) planes taking as reference the intensity of (101) plane, for the

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ZnO samples. By comparing the relative intensities for both peaks it can be observed that the difference between the relative intensities of I(100) and I(002) planes is slightly smaller for the ZnO sample in respect to the silver-doped ZnO,

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although not relevant differences exist on the ZnO-Ag(x) series. This fact would

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indicate that there are no significant variations in either the peak width or the

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peak intensity as the amount of Ag increases, which would occur in case the

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Ag+ ions were accommodated in the ZnO host lattice. This confirms that the

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photodeposition of Ag nanoparticles on the surface of ZnO neither affects the crystallinity nor induces strain in ZnO. Since the ionic radius of Ag+ (1.26 Å) is

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higher than that of Zn2+ (0.75 Å) the speculation that Ag+ ions are partially incorporated into the ZnO surface lattice by substituting Zn 2+ ions is not

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plausible, which in turn is reinforced by the non-distortion of the lattice parameters after the silver photodeposition.

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Scherrer’s formula [18] was used in verifying the average grain size of the Agdoped ZnO nanoparticles from the prominent (101) peak in the XRD and results are reported in Table 1 together with other physicochemical properties of the indicated samples, as well as the average crystallite size of the Ag-particles from the mean (111) peak.

Regarding the BET surface area, it is observed (Table 1) that the deposition of Ag on the surface of ZnO nanoparticles enables the slight decrease of the BET surface area in respect to the bare ZnO, probably because during the Agphotodeposition method, the Ag nanoparticles can aggregate forming small clusters, thus reducing the surface area of the resulting material. However, the

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metallized samples, above 0.5 wt.-% Ag, show few variations in their surface areas with the nominal Ag content; only a slight and insignificant reduction of

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the surface area is observed by increasing the nominal content of Ag. This fact could indicate that above a silver content of 1 wt.-%, Ag deposits grow on preformed metal cores, with little influence on the variations of surface areas.

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Figure 2 shows representative SEM micrographs images, of the different

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ZnO-Ag(x) materials. As we reported in a previous work [17], bare ZnO displays

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irregular shapes of particles with morphologies like rice-grains of average size

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ca. 25 nm. The photodeposition of Ag does not modify the initial morphology of

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bare ZnO although the formation of particle agglomerates is observed (Figure 2). It is interesting to note, however, that different distributions of silver are

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observed, depending on the amount of nominal silver photodeposited. Thus, in the ZnO-Ag(5) sample (Fig. 2D) an elemental distribution mapping shows that

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along with a quasi-homogeneous distribution of Ag, zones are present where the amount of Ag is greater (indicated in circles). However, this does not occur

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in the ZnO-Ag(10) samples where a homogeneous distribution of Ag is observed (Fig. 2F), although in the latter case with Ag particle sizes slightly higher than those observed for the ZnO-Ag(5) samples. This fact would indicate that the nominal amount of silver conditions the homogeneousheterogeneous distribution and/or the size of the deposited Ag particles.

Figures 3 give representative TEM images of ZnO-Ag(x) particles. For the case of ZnO-Ag(0.5) sample, few small Ag-nanoparticles were found which are inhomogeneously distributed (Figure 3A). Higher Ag nominal contents lead to a non-homogeneous distribution of the silver (Figures 3B and 3C), mostly growing in the form of clusters in different zones of the samples, with even Ag-free ZnO

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particles being observed (Fig. 3D).

Figure 4 shows the UV-vis DRS of the samples. All samples show absorption in

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the UV range (λ˂400 nm), corresponding to the intrinsic band gap transition of

the semiconductor related to the wurtzite crystal structure of ZnO [19]. The values of the band gap energy obtained are reported in Table 1. As can be

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seen, there are no significant differences in the band gap values that are around

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3.2 eV. Beyond the band-gap absorption threshold of ZnO at λ≤400 nm it is

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observed that the addition of silver ions and subsequent UV irradiation cause

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significant changes to the absorption spectrum of ZnO, resulting in high

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absorbance from 400 nm to the entire visible region. The absorption value is related to the amount of metal deposition, in other words the higher the amount

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of silver deposition the greater the absorption in the visible region. As expected, the absorption in the visible spectral range is directly proportional to the amount

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of Ag content on the different ZnO materials. The presence of Ag nanoparticles on ZnO materials leads to the increase of the absorption in the visible light

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range between 400 and 600 nm, due to the Ag surface plasmon band [20,21]. Regarding the position of the surface plasmon resonance, the absorption maximum for the ZnO-Ag(0.5) occur at 452.5 nm, shifting to smaller wavelengths as the nominal silver content increases (Figure 4 and Table 1). This shift may be attributed to the presence of Ag with different size and shape

as has been reported [22]. This underlines the fact that the metallic clusters on the photocatalyst surface are not homogeneous in size. It is known that lack of homogeneity is an inherent feature of wet deposition methods. The surface elemental composition and chemical status of the ZnO-Ag(x) samples were analyzed by XPS as shown in Fig. 5. Regarding the peak of

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O(1s) (Figure 5A, 5D and 5G) it is observed that this is formed by two contribution peaks whose relative percentages vary as the nominal silver

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content increases. The contribution of the peak at 530.4 eV should be assigned

to the lattice oxygen of the ZnO [23] while the peak at 531.6 eV could be assigned, in principle, to hydroxyl groups and/or chemisorbed oxygen.

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However, other works [24,25] revealed the presence of three components into

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the O(1s) peak, located at 530.2 eV, 531.8 eV and 533.4 eV respectively; the

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latter is usually attributed to the presence of loosely bound oxygen on the

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surface of ZnO [24,26]. In our case, the best Gaussian fitting revealed only two

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O(1s) contribution peaks. The fact that with an increase in the nominal silver content, the contribution of the peak at 530.4 eV slightly decreases while that of

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531.6 eV increases could be tentatively attributed to the formation of surface Ag/OH- or chemisorbed oxygen Ag/O. XPS observation of OH groups

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incorporated in an Ag(111) electrode has been reported [27]. Similarly, a mechanism of the Ag(111) sub-monolayer oxidation is suggested on the basis

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of combined evidence from cyclic voltammetry, in situ SERS, and ex situ XPS study [28]. The crystallite sizes of the Ag(111) are reported on Table 1. It is observed that this size increases as the nominal content of Ag increases. Thus, on the basis of our O(1s) XPS results and those reported on Ag(111) electrodes we can tentatively suggest the formation of a Ag(111) oxidized sub-monolayer

which increases as the crystallite size of the Ag(111) increases (as it is shown in Scheme 1). On the other hand, the peak of Zn 2p1/2 and Zn 2p3/2, located at 1045.5 and 1022.4 eV respectively, indicate the presence of Zn2+ on the surface of the samples [29,30]. The distance between the Zn 2p peaks was about 23 eV

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which was induced from the spin-orbit coupling.

In Figures 5C, 5F and 5I, the peaks centered at around 368.1 eV and 374.1 eV

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can be assigned to Ag 3d5/2 and Ag 3d3/2 respectively, with a spin-orbital splitting photoelectrons at 6.0 eV confirming that metallic-Ag particles have been successfully deposited on the surface of ZnO [31,32].

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Real Ag-content in the metallized samples was also determined by XRF and

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results are reported in Table 1. As it can be seen, the real metal content in the

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samples was lower than the nominal content indicating a partial reduction of the

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metal precursor during the photodeposition process carried out under the

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experimental conditions of this work. These results could be associated with the fact that the adsorption equilibriums of the Ag+ ions and the Zn2+ cations loss

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process may be related (Scheme 1), which in turn would condition the transfer and capture of the photogenerated electrons by the Ag + with the subsequent

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loss in effectiveness of the metallization process. Before and after the Ag photodeposition process, the liquid was subjected to

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plasma Atomic Emission Spectrometry analysis. With respect to the initial liquid, an increase of the zinc concentration and a decrease of the initial concentration of silver in the medium were detected, indicating that during silver metallization, Zn2+ leaching occurs, thus generating cationic defects on the surface of the ZnO, and simultaneously the incorporation of Ag at the ZnO surface. Therefore,

taking into account the plasma Atomic Emission Spectrometry analysis we have made an estimative calculation that implies that about 8-10% of the total Znatoms were lost by leaching. According to the characterization, results set forth above, several points can be established at this stage. First, during the Ag-photodeposition process, a part of

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the ZnO surface loses Zn2+ cations that pass into the medium, in which the Ag+ cations are present. Thus, the Zn2+ loss, in some way, would facilitate the

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adsorption of the Ag+ species to the surface.

This adsorption does not imply the incorporation of the Ag+ species in the cation vacancies left by Zn2+ losses, due to the large ionic radius difference between

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Zn2+ (0.75 Å) and Ag+ (1.26 Å). The not incorporation of the Ag+ ions in ZnO

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crystal lattice has been confirmed by XRD because non lattice distortions in

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ZnO crystal lattice are produced after Ag-incorporation on ZnO.

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Although the photodeposition is a very convincing method as it avoids the use

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of external reductants, the adhesion of metal deposits onto the semiconductor surface is however very poor. In the case of metal-photodeposition on ZnO, the

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photocorrosion of the ZnO surface could facilitate the adhesion of cationic metals at the surface and its ulterior photodeposition as metallic particles, as

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illustrated in some part of Scheme 1.

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3.2 Photocatalytic properties of ZnO-Ag(x) samples

Prior to photocatalytic experiments, the partial solubility of ZnO (leaching of Zn 2+ cations), as well as the photostability of bare ZnO was studied. Therefore, we have analyzed by Atomic Emission Spectrometry with ICP plasma (AES

technique): a) the amount of zinc present in the water before the incorporation of ZnO (0.01 ppm); b) the amount of zinc after continuous bubbling with oxygen in the dark (7.97 ppm) and finally c) the amount of zinc after 2 h of UV illumination, under continuous bubbling oxygen (7.90 ppm). From these results it is clear that there is a loss of zinc associated with the partial solubility of ZnO

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at pH=5.5. However, during the continuous bubbling process of oxygen in the

dark and/or under prolonged UV-illumination in continuous oxygen flow, no

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additional loss of zinc is produced, indicating a good photostability for the bare ZnO.

The photocatalytic efficiencies of the bare ZnO and Ag/ZnO photocatalysts with

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different Ag contents were evaluated by using three selected substrates (RhB,

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MO and Phenol) under two lighting conditions, UV and visible light. Figure 6,

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shows the conversion plots for photocatalytic transformation of RhB and MO

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and photocatalytic disappearance of Phenol under UV-illumination using the

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synthesized ZnO and ZnO-Ag(x) samples. As can be seen all the photocatalyst exhibited a 100% of conversions after a period of 60 min of UV-illumination.

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However, at shorter UV-illumination times (for instance 20 min), the conversion values were higher for RhB than for MO or for Phenol. Final mineralization

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degrees were calculated from the TOC measurements and values (data not shown) indicated a degree of mineralization ranging 86-97%, thus indicating a

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good UV-photocatalytic mineralization of the synthesized samples. In Figure 7, we show the conversion plots of the three selected substrates, using the ZnOAg(x) photocatalysts (x=1% and 5 wt.-% of Ag) under visible illumination. It is possible to observe that the initial concentration of MO, with the two selected photocatalysts, remains almost unaffected. Though, with both metallized

samples there is a slight improvement of RhB final conversion compared with that obtained for Phenol. In order to delve deeper into the photocatalytic properties of the prepared systems, under conditions of visible illumination, the degradation of the three

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chosen substrates has been studied, with the unmodified zinc oxide, under conditions of illumination in the visible. Only, Rhodamine B showed (dashed

lines) a degradation in the visible that is very similar (almost identical) to that

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observed under the same conditions with the metallized samples. However, no

MO-conversion was detected and only a very low conversion of phenol (<2%)

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was observed when using ZnO under visible illumination. These results indicate

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that the degradation of Rhodamine B with, either non-modified ZnO or Ag-ZnO

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in the visible, is associated with a photosensitizing effect, exerted by the RhB

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[33], of it rather than the effect of the metal loading. However, under conditions of illumination in the visible, the silver loading seems to have no effect on the

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degradation of the MO and a small effect on the phenol degradation.

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In a recent paper Sze-Mun Lam et al. [34] have reported the Visible-light responsive of Ag-doped flower-like ZnO (Ag/ZnO) photocatalysts with different

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loadings of Ag. These Authors report [34] that Ag/ZnO micro/nanoflowers enhanced visible light responsive photoactivity towards the degradation of Fast Green dye. They concluded that the sample 5 wt.-% Ag/ZnO samples showed

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excellent photoactivity in comparison with those of pure ZnO and commercial TiO2 (none specified). However, in this work [34], apart from not specifying the nature of the commercial TiO2, the comparison of the rate constants, standardized per unit area of the compared samples, is not made. Regardless of this, it is not clearly established if the photocatalytic activities observed in the

visible region are associated or not with a photosensitization process of Fast Green Dye, as in our case with Rhodamine B. Beyond these points, the absorption capacity in the visible region it could be associated with the morphology (micro/nanoflowers) of the samples.

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Figure 8A shows the values of initial reaction rates under UV-illumination. The initial reaction rates were calculated from the slopes of the conversion plots at

the first 15 min of reaction, and assuming zero-order kinetics at this stage of the

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reaction. Based on these results, it is clear that not only the ZnO synthesized but also the ZnO-Ag(x) present initial reaction rates, for the three chosen

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substrates, which are higher than those presented by the commercial

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TiO2(P25) in the same conditions of UV-illumination.

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However, surface area must be taken into account since surface area in

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catalysis is an important parameter. When initial reaction rates are estimated per surface area unit of catalyst (Figure 8B), it can be seen that the activity for

the substrate.

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ZnO and ZnO-Ag(x) samples are higher to that for TiO2(P25) independently of

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At the same time, it can be inferred that there is a clear dependence of the

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photocatalytic activity with the nominal (or real) content of Ag. For each substrate, the initial reaction rate standardized per unit area increases as the silver content increases, at least up to a value of 5% wt.- of Ag. Higher amounts

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of silver (e.g. 10% wt.-Ag) do not lead to any improvement. In this context, Y. Zhang et al., [35] reported that although uniform metallic silver plays an important role in improving the photocatalytic activity of ZnO, their performance reduces considerably once the content of Ag is higher than 3.9 wt.-%. Interestingly, in the present work, it’s found that although the nominal Ag-

photodeposited is higher than those values, the real ones for the optimum is 3.43 wt.-% according to XRF results (Table 1). Thus our results are in agreement to those found by Y. Zhang et al., [35]. Regarding the photocatalytic processes, both in the UV and the visible, the following proposal can be made: under UV-illumination, after the excitation of

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the semiconductor, charge carriers are generated, which are able to generate radical species susceptible to lead to the mineralization of the substrates. In this

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case, the incorporation of silver metal islands will lead to an increase in the photocatalytic process, as silver acts as an electron capturer, thus reducing the

rate of electron-hole recombination, favoring the generation of reactive radicals.

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However, under visible illumination, the activity found can be explained by the

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mechanism proposed by Bouzid et al., [36] in which the absorption in the visible

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of the plasmonic energy, lead to surface conduction electrons into the Ag

3.3 Stability tests

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nanoparticles and subsequent reactive radicals, as is illustrated in Scheme 1.

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Good recycling stability is a very important parameter for photocatalysts in the practical application. Photocatalysts stability (in particular those based on ZnO

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materials) is essential when dealing with aqueous effluents and a plausible (photo)-corrosion is expected. Moreover, the resistance to Zn 2+-leaching is

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crucial to maintain photocatalytic activity in this type of materials. Therefore, recycling

experiments

are

performed

testing

the

selected

ZnO-Ag(5)

photocatalyst to corroborate its resistance under reaction conditions. Stability tests are performed by using the photocatalysts several times by monitoring the percentage of degradation of MO (as selected substrate) after 60 min on UV-

illumination. Once the catalytic experiment is finished, the solid is separated and is reused by using MO as substrate reference under UV-illumination. Figure 9 shows the degradation percentages of MO aqueous solution after 60 min of UVillumination for 5 cycles, being practically of 100%, accompanied by a simultaneous high total organic removal (ca. 86%) in every cycle. Although

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Figure 9 shows the results obtained with the best of the metallized samples, ZnO-Ag(5), we have shown the results of the continuous reuses using the

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substrate with which the process is less efficient (Methyl Orange), only for

illustrative effect of the stability tests. Similar results are obtained when reusing

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the ZnO-Ag(1) photocatalyst.

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4. Conclusions

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In this work we have studied the effect of different nominal concentrations of Ag

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on the photocatalytic activity in the UV of a previously synthesized ZnO, which

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already had a high photocatalytic activity in the UV [17]. The method used has been the silver photodeposition on the surface of the pre-synthesized ZnO. The

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results obtained in this work allows to conclude that during the photodeposition process the amount of Zn2+ cations released into the medium facilitates the

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approach of the Ag+ cations to the surface of the ZnO where it is deposited (although precariously) given that the present amounts of silver are lower than

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the nominal contents. Our results confirm that although the Ag+ cations are not predisposed to be located in the cationic vacancies generated by the leaching of the surface Zn2+ cations, the incorporation of metallic silver nanoparticles on the surface of the ZnO is evidenced. This metallization process leads to

stabilization in the (photo)-corrosion capability inherent to ZnO leading to an improvement in photocatalytic activity and excellent reusing capacity. In any case, those ZnO-Ag(x) prepared samples showed photocatalytic activities much better than pure ZnO nanoparticles under UV-illumination, tested by using three different probe molecules (RhB, MO and Phenol). The

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photocatalytic activity of ZnO can be increased by metallization with Ag

nanoparticles, which produced a record of activities for the degradation of RhB,

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MO and Phenol, at least up to an optimum value of silver being superior to

those for TiO2(P25). Silver percentages higher than 1% wt. - do not lead to an improvement in photocatalytic activity.

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Under UV-illumination, with the use of the ZnO-Ag(x) photocatalysts (x=1-5 wt.-

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%) not only were high conversion values of the substrates used seen, but also

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a total organic carbon removal (TOC) of 86-97% was obtained as well as good

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recycling and photo-stability properties which suggests that these Ag-modified

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Acknowledgement

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ZnO nanoparticles may have good applications in wastewater treatment.

This work was supported by research fund from Project Ref. CTQ2015-64664-

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C2-2-P (MINECO/FEDER UE). Research services of CITIUS University of Seville are also acknowledged. We thank the University of Tolima for economic

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support in the studies commission of César Augusto Jaramillo Páez.

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Figure 1. (A) XRD patterns of the indicated samples; asterisk (*) denote the intensities of metallic silver peaks. (B) Evolution, as the nominal Ag-content increases, of the I100 and I002 relative intensities for the synthesized ZnO-Ag(x) samples.

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Figure 2. Selected SEM images corresponding to the ZnO-Ag(x) samples.

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Figure 3. Selected TEM images corresponding to the ZnO-Ag(x) samples.

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Figure 4. UV-Vis diffuses reflectance spectra of the bare ZnO and ZnO-Ag(x) samples.

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Figure 5. XPS results, in the O 1s, Zn 2p and Ag 3d region of the ZnO-Ag(x) samples: (A), (B) and (D) refer to ZnO-Ag(1); (D), (E) and (F) for ZnO-Ag(5); (G), (H) and (I) for ZnO-Ag(10).

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Figure 6. Conversion plots for photocatalytic transformations of Rhodamine B (RhB), Methyl Orange (MO) and Phenol (Ph) using the indicated ZnO and ZnOAg(x) catalysts under UV-illumination.

IP T SC R U N A M ED PT CC E A Figure 7. Conversion plots for photocatalytic transformation of Rhodamine B (RhB) and Phenol (Ph), under visible illumination, using two selected catalysts. The conversion of MO is not indicated because, under the experimental conditions it was close to cero. Dashed lines refer to RhB photodegradation over bare ZnO under visible illumination.

IP T SC R U N A M ED PT CC E A Figure 8. UV-photocatalytic activity of Methyl Orange (MO), Phenol (Ph) and Rhodamine B (RhB) using the indicated photocatalysts: (A) initial reaction rates and (B) initial reaction rates per surface area unit; results are compared with those obtained with the commercial oxide TiO2(P25) under the same conditions.

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Figure 9. Five repeated processes by using ZnO-Ag(5) as photocatalyst for degradation of Methyl Orange after 1 h of UV-illumination.

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Scheme 1. A schematic illustration of the UV-photodeposition of metallic Ag at the ZnO surface and the occurring Zn2+-leaching process. A tentative proposal for the photocatalysis of the ZnO-Ag(x) samples is also illustrated.

Table 1. Physicochemical parameters of the prepared ZnO-Ag(x) samples. SBET (m2 g-1)

30.2 27.5 24.9 23.8 24.2

Band Gap (eV)

3.25 3.23(452.56) 3.23(446.33) 3.21(433.74) 3.18(428.49)

XRF (%) O

Ag

Zn

--19.60 19.47 18.99 18.21

--0.35 0.98 3.43 7.34

--80.02 79.54 77.58 74.40

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ZnO ZnO-Ag(0.5) ZnO-Ag(1) ZnO-Ag(5) ZnO-Ag(10)

Crystallite size (nm) ZnO Ag (101) (111) 22.5 --26.8 40.0 30.4 57.6 30.6 52.2 31.6 38.0

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Sample