Scholarly article on topic 'Photocatalytic deposition of Ag nanoparticles on TiO2: Metal precursor effect on the structural and photoactivity properties'

Photocatalytic deposition of Ag nanoparticles on TiO2: Metal precursor effect on the structural and photoactivity properties Academic research paper on "Chemical sciences"

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{"Ag2O heterojunction" / "Silver nitrate" / "Silver acetylacetonate" / "Silver perchlorate" / "Ag nanoparticles"}

Abstract of research paper on Chemical sciences, author of scientific article — E. Albiter, M.A. Valenzuela, S. Alfaro, G. Valverde-Aguilar, F.M. Martínez-Pallares

Abstract A series of 1wt.% Ag–TiO2 photocatalysts were obtained by photodeposition using different organic (acetylacetonate, Ag-A) and inorganic (nitrate, Ag-N, and perchlorate, Ag-C) silver precursors in order to determinate the influence of the silver precursor on final properties of the photocatalysts. The resulting photocatalytic materials were characterized by different techniques (UV–Vis DRS, TEM/HRTEM and XPS) and their photocatalytic activity was evaluated in the degradation of rhodamine B (used as model pollutant) in aqueous solution under simulated solar light. The photocatalytic reduction of Ag species to Ag0 on TiO2 was higher with silver nitrate as precursor compared to acetylacetonate or perchlorate. All the Ag-modified TiO2 photocatalysts exhibited a surface plasmon resonance effect in the visible region (400–530nm) indicating different metal particle sizes depending on the Ag precursor used in their synthesis. A higher photocatalytic activity was obtained with all the Ag/TiO2 samples compared with non-modified TiO2. The descending order of photocatalytic activity was as follows: Ag-A/TiO2 ≈Ag-N/TiO2 >Ag-C/TiO2 >TiO2-P25. The enhanced photoactivity was attributed to the presence of different amounts Ag0 nanoparticles homogeneously distributed on Ag2O and TiO2, trapping the photogenerated electrons and avoiding charge recombination.

Academic research paper on topic "Photocatalytic deposition of Ag nanoparticles on TiO2: Metal precursor effect on the structural and photoactivity properties"

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Photocatalytic deposition of Ag nanoparticles on TiO2: metal precursor effect on the structural and photoactivity properties

Journal of Saudi Chemical Society

PII: DOI:

Reference:

E. Albiter, M.A. Valenzuela, S. Alfaro, G. Valverde-Aguilar, F.M. Martínez-Pallares

S1319-6103(15)00063-0 http://dx.doi.org/10.1016/joscs.2015.05.009 JSCS 739

To appear in:

Journal of Saudi Chemical Society

Received Date: 14 February 2015

Revised Date: 19 May 2015

Accepted Date: 22 May 2015

Please cite this article as: E. Albiter, M.A. Valenzuela, S. Alfaro, G. Valverde-Aguilar, F.M. Martínez-Pallares, Photocatalytic deposition of Ag nanoparticles on TiO2: metal precursor effect on the structural and photoactivity properties, Journal of Saudi Chemical Society (2015), doi: http://dx.doi.org/10.1016/jjscs.2015.05.009

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Photocatalytic deposition of Ag nanoparticles on TiO2: metal precursor effect on the structural and photoactivity properties

E. Albiter1'3*, M.A. Valenzuela1, S. Alfaro1, G.Valverde-Aguilar2, F. M. Martínez-P¡

llares1

:Lab. Catálisis y Materiales. ESIQIE-Instituto Politécnico Nacional. Zacatenco, 07738 México, D.F. M 2CICATA Unidad Legaria-Instituto Politécnico Nacional. Legaria 694, Col. Irrigación, 11500, México, DF, México. 3Centro de Ciencias Aplicadas y Desarrollo Tecnológico UNAM, Circuito Exterior S/N, Ciudad Universitaria, A. P.

70-186, México DF, México *Corresponding author: ealbitere@ipn.mx Telephone Number: +(52) 5557296000 ext. 55251

Keywords: Ag2Ü heterojunction, silver nitrate, silver acetylacetonate, silver perchlorate, Ag nanoparticles.

Abstract

A series of 1 wt. % Ag-TiO2 photocatalysts were obtained by photodeposition using different organic (acetylacetonate, Ag-A) and inorganic (nitrate, Ag-N, and perchlorate, Ag-C) silver precursors in order to determinate the influence of the silver precursor on the final properties of

the photocatalysts. The resulting photocatalytic materials were characterized by different techniques (UV-Vis DRS, TEM/HRTEM and XPS) and their photocatalytic activity was evaluated in the degradation of rhodamine B (used as model pollutant) in aqueous solution under simulated solar light. The photocatalytic reduction of Ag species to Ag° on TiO2 was higher with silver nitrate as precursor compared to acetylacetonate or perchlorate. All the Ag-modified TiO2 photocatalysts exhibited a surface plasmon resonance effect in the visible region (400-530 nm) indicating different metal particle size depending on the Ag precursor used in their synthesis. A higher photocatalytic activity was obtained with all the Ag/TiO2 samples compared with non-modified TiO2. The descending order of photocatalytic activity was as follows: Ag-A/TiO2 ~ Ag-N/TiO2 > Ag-C/TiO2 > TiO2-P25. The enhanced photoactivity was attributed to the presence of different amounts Ag0 nanoparticles homogeneously distributed on Ag2O and TiO2, trapping the photogenerated electrons and avoiding charge recombination.

Introduction

In recent years, noble metal nanoparticles (e.g. Ag, Au, Cu) have received much attention for new applications in biotechnology, catalysis, electronics, environmental and optics[1-9]. For instance, silver nanoparticles have been investigated in fields such as high-density information storage, photoluminescence and electroluminescence devices, surface-enhanced Raman scattering, heterogeneous catalysis, photocatalysis and disinfection[10-14]. In heterogeneous catalysis, supported silver catalysts have been successfully used at industrial scale for the oxidation of methanol to formaldehyde and ethylene to ethylene oxide[15]. Lately, Ag-doped semiconductor nanoparticles have had much interest in photocatalysis (i.e. degradation of organic pollutants, hydrogen production, CO2 photoreduction, disinfection), in order to improve the photoconversion yield and allowing the extension of the light absorption of wide band gap semiconductors to the visible light[4,11,16—19]. As it is well-known, Ag nanoparticles can trap the excited electrons from titanium dioxide and leave the holes for the degradation reaction of organic pollutants, improving the charge carrier separation[20]. On the other hand, silver nanoparticles can absorb visible light due to localized surface plasmon resonance[21], extending their wavelength response towards the visible region, leading to new applications such as antibacterial textiles, engineering materials, medical devices, food preparation surfaces, air conditioning filters and coated sanitary wares[22]. Concerning the photoactivity of Ag0/Ag2O deposited on TiO2, it has been proposed that the photoexcitation of Ag2O rather than Ag0 acts as active sites responsible for the enhanced photocatalytic activity, whereas Ag0 might contribute to the stability[23]. Also worth mentioning that the p-Ag2O/n-TiO2 nanoheterojunction have shown a significant improved photocatalytic activity under UV-Vis irradiation explained in terms of a better charge separation[24]. Recently,

it has been shown that a heterostructure type Ag-Ag2O/TiO2, synthesized by simple electrochemical method, resulted in a high active and stable photocatalyst under visible light, following a Z-scheme charge transfer mechanism[25].

The investigation of the relationship between the synthesis process parameters on the size and

morphology of the nanoparticles, which is connected to its optical and electronic properties, has led to a large number of preparation methods[10,11]. In a recent review concerning the synthesis and applications of silver nanoparticles, it has been reported that most synthesis processes produce spherical Ag nanoparticles with less than 20 nm of diameter; they are often synthesized via reduction of AgNO3 dissolved in water and using reducing agents such as NaBH4, among others compounds[26].

In particular, the photochemical and photocatalytic reduction have been studied extensively since the decade of the 80's, and they are considered as efficient ways to synthesize nanoparticles directly on semiconductor supports[27]. The photocatalytic deposition is carried out in presence of metal ions, semiconductor support and hole scavengers. After irradiation, the photogenerated electrons reduce the surface-adsorbed metal ions forming metal clusters, and then, Ag nanoparticles via a repeated reduction process[27]. We recently synthesized Ag/TiO2 composites by photocatalytic deposition that exhibit strong absorption centered at 420 nm with Ag nanoparticles (around 6-20 nm) uniformly deposited on the semiconductor[10]. As mentioned above, AgNO3 dissolved in water is the most common salt precursor used in the synthesis of Ag/TiO2 composites, even in the photochemical routes. Therefore, the present research was focused to the synthesis of Ag/TiO2 composites by photocatalytic deposition employing different organic (acetylacetonate) and inorganic (nitrate and perchlorate) silver precursors. The effect of silver precursor on structural characteristics and photoactivity under simulated solar light was

mainly studied.

with ethar carried out

Experimental section

Silver nitrate (Ag-N Fermont 99%), silver Perchlorate (Ag-C, Sigma-Aldrich 97%) and silver

acetylacetonate (Ag-A, Sigma Aldich, 98%) were used as silver precursors. Ethanol (absolute, Fermont) was used as solvent and commercial TiÜ2 (TiÜ2-P25, Evonik) was used as support. The supported Ag nanoparticles were obtained using the photocatalytic route as follows: an ethanolic solution of Ag precursor (0.5 mM) was mixed with TiO2-P25 in a 100 mL batch reactor and it was dispersed with ultrasonic irradiation. Then a nitrogen flow of 50 mL min-1 was bubbled throw the slurry to purge dissolved oxygen and to achieve an inert reaction atmosphere. The batch reactor was irradiated during 4 h using a LuzChem photoreactor (model LZC-4) equipped with 14 low pressure mercury lamps (8 W, Xmax = 360 nm). After the irradiation time, the obtained material was washed with ethanol, separated by centrifugation and it was dried at 50 °C in a convection oven.

The characterization was carried out by UV-Vis diffuse reflectance (GBC Cintra 20), X-ray photoelectron spectroscopy (XPS, ThermoScientific K-Alpha) and structural characterization was achieved from conventional TEM and HRTEM by means of a JEÜL FEG 2010 FasTem electron microscope with 1.9 A of resolution (point to point). For TEM and HRTEM studies, the samples were suspended in ethanol in order to disperse the powders then a drop of the sample was deposited on a lacey carbon copper grid as a TEM support.

The composites were tested in a model reaction such as Rhodamine B (RhB) degradation in aqueous solutions. Typically, powdered photocatalyst in the amount of 0.1 g L-1 was suspended in aqueous solution of 9.6 ppm of RhB. The suspension was magnetically stirred in the dark

during 30 min to achieve a complete adsorption/desorption equilibrium and then it was irradiated for 60 min with a solar light simulator (Newport model 67005) equipped with a 150 W Xe lamp and a power source which allows to change the light intensity of the lamp. The light intensity was measured using a digital light meter (A. W. Sperry SLM-110) and the measured light

intensity was 1.5 mW cm (10) at 20 cm from the source. The reaction temperature was kept constant at 25 °C during all the experiment and aliquots of the reaction medium were periodically sampled and filtered using a PTFE membrane filter (Millipore, 0.45 ^m) prior analysis. The RhB concentration was measured employing a spectrophotometric method by using a GBC Cintra 20 spectrophotometer and following the decrease in the absorbance of RhB at 550 nm.

Results and discussion

Figure 1 shows the UV-Vis diffuse reflectance spectra for Ag/TiO2 composites using silver acetylacetonate (Ag-A/TiO2), silver perchlorate (Ag-C/TiO2) and silver nitrate (Ag-N/TiO2) as Ag precursors and bare TiO2-P25. The absorption edge of all samples were practically located at k = 380 nm, which is consistent with the intrinsic band gap absorption of anatase TiO2 (3.2 eV). Note the broad absorption band from 400-800 nm for all Ag photocatalysts, highlighting the maxima at 470, 505 and 530 nm for Ag-C/TiO2, Ag-A/TiO2 and Ag-N/TiO2 respectively, attributed to the characteristic surface plasmon resonance (SPR) of metal cluster or Ag nanoparticles deposited on TiO2, depending of the Ag precursor. In fact, colloidal Ag nanoparticles prepared previously by our group, showed a narrow absorption peak with a maximum at 400-420 nm, resulting an average Ag particle size distribution of 2-5 nm[10]. Therefore, the results presented in Fig.1 could indicate that changes of the plasmon maxima

location from 400-420 nm to 530 nm, would be related to the size, distribution, and chemical interaction of the Ag NPs deposited on TiO2, induced by the type of Ag precursor used in the synthesis[28]. Indeed, the use of different silver precursors led to different coloration in the final photocatalyst, a pale brown color was observed with AgClO4 and certain shades of purple with

AgAc and AgNO3. In general, the red-shifted resonance wavelength is strongly related to the size of the nanoparticle[29], therefore, as we loaded the same nominal amount of silver (1 wt. %), then, the type of Ag precursor had an effect on the size of Ag nanoparticles. A possible explanation of this effect is the observed decrease of the photocatalytic efficiency of TiO2 in presence of inorganic salts, such as NO3 , Cl , SO4 [29]. At this point, comparing the area under the curve of Ag composite prepared with AgNO3 in Fig.1, it seems that Ag cations coming from AgNO3 were easier to reduce than those from AgClO4 and AcAg.

Figure 2 shows several HRTEM images for Ag-A/TiO2 photocatalyst. In Fig. 2 (a), the anatase phase was identified by the spacing d = 0.352 nm indexed as (101). In the second image, two NPs can be observed, one of them was TiO2 and another was AgO, this confirmed the deposit of Ag on the TiO2 surface. Also, two kinds of silver oxides, AgO and Ag2O were identified. The AgO phase was identified by two plane spacings, d = 0.241 nm and d = 0.263 nm, indexed as (111) and (100), respectively. The Ag2O was detected by the (002) and (100) reflections corresponding to the spacing d = 0.246 nm and d = 0.260 nm, respectively. The last high resolution image shows a well-defined hexagonal nanoparticle of Ag2O with a length of 15 nm and width of 12 nm approximately which had a spacing d = 0.263 nm indexed as (100). Fig. 2 (b) reveals similar crystallographic planes for Ag2O with the spacings d = 0.246 nm and d = 0.260 nm indexed as (002) and (100), respectively. For AgO, the reflection (111) was observed. However, in the two last images of Fig. 2(b), metallic silver (Ag0) was detected by the reflection

(200) with a spacing of d = 0.205 nm. Finally, in the Fig. 2(c) a higher region is presented where is possible to observe the same crystalline phases, belonging to TiO2, AgO and Ag0, as reported above, which revealed the formation of a composite material.

Figure 3 shows the TEM and HRTEM images for Ag-C/TiO2 photocatalyst. Figure 3 (a) displays

the TEM micrographs where it can be observed TiO2-P25 nanocrystals with sizes between 20 and 50 nm, also it is observed that the TiO2 nanoparticles are interconnected by channels with diameters of 20 nm. Here, the TiO2 nanoparticles presented a roughness on their surface that makes difficult to observe the Ag NPs that were identified by HRTEM in form of Ag2O. Probably, the TiO2 nanocrystals were attacked by the chlorine species provoking the oxidation of the Ag NPs. In the Fig. 3 (b) are presented HRTEM images belonging to the anatase phase, which is also identified by the spacing d = 0.352 nm and indexed as (101) and the silver oxide Ag2O was identified by the spacing d = 0.246 nm and indexed as (002).

Figure. 4 (a) shows a TEM and HRTEM micrographs of the Ag-N/TiO2 photocatalysts, in the first TEM image only the titania nanocrystals can be observed. The second HRTEM image shows a population of silver nanoparticles (dark particles) with average size of 1 or 2 nm both on the surface as inserted or embedded into the nanocrystals of TiO2. The Ag NPs deposited had a spheroid shape and a high population density on the TiO2 support. In the same way, as the other samples discussed above, in the Fig. 4 (b), it is shown the HRTEM micrographs which reveals the presence of the anatase phase identified by the spacing d = 0.352 nm and indexed as (101). Figure 4 (c) shows a HRTEM image of two coupled nanoparticles indicating the presence of a composite material formed by Ag2O, identified with the spacing d = 0.246 nm and indexed as (002), and another nanoparticle of the anatase phase identified by the spacing d = 0.352 nm and indexed as (101); the next HRTEM image only presents two silver oxides species, Ag2O and

AgO with the spacing d = 0.246 nm and d = 0.289 nm and indexed as (002) and (011), respectively.

The peak positions presented by the diffraction patterns were all in good agreement with those given in ASTM data cards PDF#211272 for anatase phase, PDF#011167 for Ag0, PDF#431( for AgO and PDF#191155 for Ag2O.

The high resolution XPS spectra of Ag 3d of Ag/TiO2 composites are shown in Fig. 5. As mentioned above, HRTEM results showed the presence of Ag0 and Ag2O, then, a deconvolution analysis of XPS raw spectra allowed us to estimate the presence of three Ag species. It is well known that the Ag (3d5/2) binding energies of Ag, Ag2O and AgO are located at 368.2, 367.8 and 367.4 eV, respectively[30]. Also, high resolution spectra of O 1s and Ti 2p of Ag/TiO2 composites are shown in Fig. S1 and S2 in the supporting information, respectively. All the composites presented similar spectra and, in the case of O 1s, it can be observed an asymmetrical peak centered at 529.9 eV which can be attributed to lattice oxygen and surface oxygen[31].In the Ti 2p spectra it can be observed two symmetrical peaks corresponding to Ti 2p3/2 (458.9 eV) and T 2p1/2 (464.6 eV) transitions which can be attributed to Ti4+ species[31]. Regarding to the Ag0 surface amount in all samples, it was slightly higher than the surface amount of Ag2O and the amount of AgO was negligible. According to the signals in XPS spectra, the ratio Ag0/Ag oxidized species was very similar for Ag-N and Ag-A (~1.3) and for Ag-C was lower (~1.1). These results agreed with other works where silver was also photodeposited, although the formation of Ag2O is not discussed[32]. Indeed, it is very difficult to conclude the exact amount of each of the Ag surface species due to the quite close values of their binding energy. However, after being exposed to air environment, our Ag/TiO2 samples began to change from their original color, slightly brown or purple, to dark purple, which could

be characteristic of the formation of silver oxides. Some authors have explained the formation AgiO deposited on TiÛ2, due to oxygen ions reverse-spill over as the dominant oxidation mechanism of deposited Ag0 (eqn. 1 and 2)[33]

4Ag0 + O"^ 2Ag2O + e" (1)

2Ag0 + OAg2O + e" (2)

Figure 6 shows the relative concentration (C/Co) versus irradiation time for the photocatalytic degradation of RhB with TiO2-P25 and Ag/TiO2 samples, under simulated solar light. As can be seen, all the samples showed a similar degradation profile reaching between 80 and 95 % degradation of RhB after 60 min. The Ag-A/TiO2 composite, prepared using silver acetylacetonate as precursor, showed the highest activity, closely followed by the Ag-N/TiO2 catalyst. The Ag-C/TiO2 sample showed a slightly better activity than TiO2-P25, which was the less active. The degradation profiles of RhB using all materials followed an apparent first order kinetic and the values of the calculated constants are shown in Fig. 7B. As expected, the obtained values followed the decreasing order: Ag-A/TiO2> Ag-N/TiO2> Ag-C/TiO2 > TiO2-P25. According to XPS analyses and TEM/HRTEM results, Ag and Ag2O were detected in all the Ag/TiO2 catalyst, which could explain the improved photo activity of these materials compared to that shown by TiO2-P25[34,35]. As it is well-known, metal nanoparticles deposited on TiO2 act as electron reservoirs, enhancing the electron-hole separation and, therefore, enhancing the photocatalytic performance[30,36]. Although all Ag/TiO2 materials contained a similar amount of Ag0 NPs, the observed differences between Ag-A/TiO2 or Ag-N/ TiO2 catalysts and Ag-C/TiO2 sample could be attributed to other factors, such as morphology and size of the Ag nanoparticles[37], the presence of surface plasmon resonance (SPR)[32] or metal-support

interactions[38-40].

The influence of SPR on the photocatalityc performance under visible light illumination, have been explained mainly by two mechanisms[32]: charge transfer and local electric field enhancement. In the charge transfer mechanism, the SPR excites the electrons in the Ag nanoparticles, which are transferred to the conduction band of TiO2, leaving a "plasmonic hole"[32] in the metal nanoparticle. According to this mechanism, it is not necessary to excite the TiO2 to produce charge carriers, which are produced in the metal nanoparticle under illumination with light corresponding to the SPR excitation wavelength and it has been found that these plasmonic charge carriers can participate in several redox reactions[32,41-43]. In the second mechanism, it is proposed that irradiation of metal nanoparticles with wavelengths near their SPR frequency can generate intense local electric fields near the metal-TiO2 interface. In this regions the generation rate of the charge carriers can be 1000 times higher than that generated in the bulk TiO2 and this enhanced generation rate is responsible of the improvement in the observed photoactivity[44-46].

Accordingly, our photoactivity results can be coarsely correlated with the ratio of surface species Ag0/Ag2O which were slightly higher with Ag-N/TiO2 and Ag-A/TiO2 catalysts. At this point, we can speculate during the reaction at being excited the photocatalyst, the photogenerated electrons can reduce Ag2O (E0 Ag+/Ag = +0.80 V) more efficiently than the oxygen reduction reaction (E0 O2/O2- = -0.33 V)[47], which, could explain an improved photocatalytic activity compared with bare TiO2 due to in situ formation of Ag nanoparticles. Additionally, silver oxide nanoparticles and TiO2 act as a Vis and UV light harvesters, respectively, which can transfer electrons from their conduction band mainly to surface silver nanoparticles, avoiding charge recombination.

In order to verify the influence of some reaction variables, a second set of experiments was carried out. Hereafter, the Ag-N/TiO2 photocatalyst was employed and the tested variables were RhB initial concentration, photocatalyst loading and light intensity. In Figure 7A, the relative RhB concentration profiles versus time, in function of the initial concentration of RhB, are

shown. As can be noted, the conversion after 60 min of irradiation time was inversely proportional to the RhB initial concentration. Also, it can be observed a slight change in the concentration profile above ~29 ppm. This change can be observed better in Fig. 7B, where the calculated pseudo-first order constants were plotted against the initial concentration of RhB. As it is well-known, the photocatalytic reactions can be described by the Langmuir-Hinshelwood (LH) model, which states that at lower concentrations of substrate a pseudo-first order kinetics is observed, and at higher concentrations, a pseudo-zero order kinetics is observed[48]. The change in the slope observed in Fig. 7B can be related to the change in the reaction order, suggesting that the photocatalytic degradation of RhB can be fitted to the LH model.

The influence of photocatalyst concentration in the degradation of RhB is shown in Fig. 8. It can be observed that the presence of the semiconductor was important because the photolysis of RhB was negligible under simulated solar light (see Fig. 8A). As the catalyst concentration was increased to 0.04 g L-1, the conversion after 60 min was increased to 70%, reaching a maximum of 85% at 0.3 g L-1 and above. It is to be noted that a linear relationship between the calculated pseudo-first order constant and the catalyst concentration was observed in the range of 0.04 to 0.1 g L-1 (see Fig. 8B). Above this value, the calculated constant was non-dependent of the catalyst concentration, due to the screening effect produced by an excess of suspended particles which resulted in an inefficient absorption of light by the photocatalyst, as observed in other photocatalytic systems[49].

Finally, the influence of the light intensity over the photocatalytic degradation of RhB is presented in the Fig. 9. The conversion at 35 min was proportional to the relative light intensity ranging from 50% to 95% with the higher light intensity (Fig. 9A). A linear relationship between the pseudo-first order constants and the relative light intensity was clearly observed (Fig. 9B).

This behaviour was been previously observed in other photochemical systems[48,50] where the reaction rates can be linear at low light intensities, square root dependent at intermediate light intensities and at high light intensities it remains constant.

Conclusions

It was studied the effect of the silver precursor on the structural and photocatalytic properties of Ag/TiO2 composites by using a photodeposition method. The photocatalytic reduction of Ag+ to Ag0 on TiO2 was higher with AgNO3 as precursor than AgClO4 or silver acetylacetonate. In all composites, a surface plasmon resonance was clearly observed in the range of X= 400-530 nm, with a colouration slightly brown to purple, denoting metal particle sizes from 2 to 20 nm. Bulk and surface analysis confirmed the presence of Ag and Ag2O and traces of AgO. Ag oxidized species were probably formed by the reaction of Ag° with oxygen species after being exposed to the environment. Ag0 dispersion on TiO2 was in the order: Ag-N/TiO2 > Ag-C/TiO2 > Ag-A/TiO2, however the ratio of Ag0/Ag2O was ~1.3 for Ag-N/TiO2 and Ag-A/TiO2 (~1.3) and for Ag-C/TiO2 was lower (~1.1). Upon modifying TiO2-P25 with Ag0/Ag2O, the rate constant of Rh-B degradation under simulated solar light was higher than pure TiO2. It seemed that the higher photocatalytic activity, obtained with silver acetylacetonate as precursor, was due to the higher amount of Ag surface species than to Ag particle size or metal dispersion.

Declaration of interest

The authors acknowledge the financial supports of Consejo Nacional de Ciencia y Tecnología (CONACyT) projects 166354 and 251151, Instituto Politécnico Nacional, projects SIP 201407 and 20150030.

140742 /

Acknowledgments

We thank to Luis Rendón (TEM, HRTEM) and Luis Lartundo (XPS) for his technical assistance.

E. Albiter wishes to thank CONACyT for its postgraduate scholarship support.

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Wavelength

Figure. 1: UV-Vis diffuse reflectance spectra of bare Ti02-P25 and Ag/ Ti02 composites prepared by photodepositing using different Ag precursors: A= acetylacetonate, C= Perchlorate, N= nitrate.

Figure 2. Representative HRTEM micrographs of Ag-A/TiO2 photocatalyst. HRTEM images shown impounds: a) anatase phase and two type of silver oxides Ag2O and AgO, B) metallic silver (Ago) silver oxide, c) anatase, silver oxide and metallic silver, in all cases forming nanocomposites, were identified. The reflections belong to all compounds are identified with white arrows and bars.

four com and silv

Figure 3. Representative micrographs: a) TEM images of Ag-C/TiO2 photocatalyst, b) HRTEM images showing anatase phase and the silver oxide (Ag2O) these compounds were detected by the (101) and (002) reflections, respectively. The reflections are identified with white arrows.

Figure 4. (a) TEM images of Ag-N/TiO2 photocatalysts showing first the nanocrystals of titania support, next HRTEM image showing a population of silver and silver oxide nanoparticles inserted on the TiO2

support. (b) HRTEM images shown nanoparticles of the anatase phase. (c) HRTEM images of anatase

phase and the silver oxides as Ag2O and AgO.

376 374 372 370 368 366 Binding energy (eV)

Figure 5: High resolution Ag 3d XPS spectra of Ag/ TiO2 composites prepared by photodeposition using different Ag precursors: a) Ag-A/TiO2, b ) Ag-N/TiO2, c) Ag-C/TiO2. A= acetylacetonate, C= perchlorate, N= nitrate.

40 60 80 100 Time (min)

ure 6: a) Relative RhB concentration profiles versus time for

Precursor Fig

ent Ag/TiO2 composites under

visible light irradiation. b) calculated pseudo-first order constants. A= acetylacetonate, C= perchlorate,

N= nitrate.

10 20 30 40 50 60 0 9.6 19 29 38 48

Time (min) Co (ppm)

Figure 7: a) Relative RhB concentration profiles versus time in function of RhB initial concentration. b)

calculated pseudo-first order constants. The photocatalyst used was Ag-N/TiO2 (N = nitrate).

st used was Ag-N/TiO2 (N = nitrate).

0 10 20 30 40

Time (min) l/lo

Figure 9: a) Relative RhB concentration profiles versus time in function of the relative light intensity. b)

Calculated pseudo-first order constants. The photocatalyst used was Ag-N/TiO2 (N = nitrate).

Figure Captions

Figure. 1: UV-Vis diffuse reflectance spectra of bare TiO2-P25 and Ag/ TiO2 composites prepared by photodepositing using different Ag precursors: A= acetylacetonate, C= perchlorate, N= nitrate.

Figure 2. Representative HRTEM micrographs of Ag-A/TiO2 photocatalyst. HRTEM images shown four compounds: a) anatase phase and two type of silver oxides Ag2O and AgO, B) metallic silver (Ago) and silver oxide, c) anatase, silver oxide and metallic silver, in all cases forming nanocomposites, were identified. The reflections belong to all compounds are identified with white arrows and bars.

Figure 3. Representative micrographs: a) TEM images of Ag-C/TiO2 photocatalyst, b) HRTEM images showing anatase phase and the silver oxide (Ag2O) these compounds were detected by the (101) and (002) reflections, respectively. The reflections are identified with white arrow

Fig. 4. (a) TEM images of Ag-N/TiO2 photocatalysts showing first the nanocrystals of titania support, next HRTEM image showing a population of silver and oxide silver nanoparticles inserted on the TiO2 support. (b) HRTEM images shown nanoparticles of the anatase phase. (c) HRTEM images of anatase phase and the silver oxides as Ag2O and AgO.

Figure 5: Figure 5: High resolution Ag 3d XPS spectra of Ag/ TiO2 composites prepared by photodepositing using different Ag precursors: a) Ag-A/TiO2, b ) Ag-N/TiO2, c) Ag-C/TiO2. A= acetylacetonate, C= perchlorate, N= nitrate

Figure 6: a) Relative RhB concentration profiles versus time for the different Ag/TiO2 composites under visible light irradiation. b) calculated pseudo-first order constants. A= acetylacetonate, C= perchlorate, N= nitrate.

Figure 7: a) Relative RhB concentration profiles versus time in function of RhB initial concentration. b) calculated pseudo-first order constants. The photocatalyst used was Ag-N/TiO2 (N = nitrate).

st order RhB con

Figure 8: a) Relative RhB concentration profiles versus time in function of photocatalyst loading. b) calculated pseudo-first constants. The photocatalyst used was Ag-N/TiO2 (N = nitrate).

Figure 9: a) Relative RhB concentration profiles versus time in function of the relative light intensity. b) Calculated pseudo-first order constants. The photocatalyst used was Ag-N/TiO2 (N = nitrate).