Journal of Saudi Chemical Society (2015) 19, 563–573

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Photocatalytic deposition of Ag nanoparticles on

TiO2: Metal precursor effect on the structural and

photoactivity properties

* Corresponding author at: Lab. Catalisis y Materiales, ESIQIE-Instituto Politecnico Nacional, Zacatenco, 07738 Mexico, DF,

Tel.: +52 5557296000x54251.

E-mail address: ealbitere@ipn.mx (E. Albiter).

Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.jscs.2015.05.0091319-6103 ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

E. Albitera,c,*, M.A. Valenzuela

a, S. Alfaro

a, G. Valverde-Aguilar

b,

F.M. Martınez-Pallares a

a Lab. Catalisis y Materiales, ESIQIE-Instituto Politecnico Nacional, Zacatenco, 07738 Mexico, DF, Mexicob CICATA Unidad Legaria-Instituto Politecnico Nacional, Legaria 694, Col. Irrigacion, 11500 Mexico, DF, Mexicoc Centro de Ciencias Aplicadas y Desarrollo Tecnologico UNAM, Circuito Exterior S/N, Ciudad Universitaria, A.P. 70-186,

Mexico, DF, Mexico

Received 14 February 2015; revised 19 May 2015; accepted 22 May 2015Available online 10 June 2015

KEYWORDS

Ag2O heterojunction;

Silver nitrate;

Silver acetylacetonate;

Silver perchlorate;

Ag nanoparticles

Abstract A series of 1 wt.% Ag–TiO2 photocatalysts were obtained by photodeposition using dif-

ferent 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 (400–530 nm) 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 photogen-

erated electrons and avoiding charge recombination.ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is

an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Mexico.

564 E. Albiter et al.

1. Introduction

In recent years, noble metal nanoparticles (e.g. Ag, Au, Cu)have received much attention for new applications in biotech-

nology, catalysis, electronics, environmental and optics [1–9].For instance, silver nanoparticles have been investigated infields such as high-density information storage, photolumines-

cence and electroluminescence devices, surface-enhancedRaman scattering, heterogeneous catalysis, photocatalysisand disinfection [10–14]. In heterogeneous catalysis, supportedsilver catalysts have been successfully used at industrial scale

for the oxidation of methanol to formaldehyde and ethyleneto ethylene oxide [15].

Lately, Ag-doped semiconductor nanoparticles have had

much interest in photocatalysis (i.e. degradation of organicpollutants, hydrogen production, CO2 photoreduction, disin-fection), in order to improve the photoconversion yield and

allow the extension of the light absorption of wide band gapsemiconductors 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 degradationreaction of organic pollutants, improving the charge carrierseparation [20]. On the other hand, silver nanoparticles canabsorb visible light due to localized surface plasmon resonance

[21], extending their wavelength response toward the visibleregion, leading to new applications such as antibacterial tex-tiles, 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 theenhanced photocatalytic activity, whereas Ag0 might con-tribute to the stability [23]. Also worth mentioning is that

the p-Ag2O/n-TiO2 nanoheterojunction has shown asignificant improved photocatalytic activity under UV–Visirradiation explained in terms of a better charge separation[24]. Recently, it has been shown that a heterostructure type

Ag–Ag2O/TiO2, synthesized by a simple electrochemicalmethod, resulted in a high active and stable photocatalystunder 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 electronicproperties, has led to a large number of preparation methods[10,11]. In a recent review concerning the synthesis and appli-cations of silver nanoparticles, it has been reported that most

synthesis processes produce spherical Ag nanoparticles withless than 20 nm of diameter; they are often synthesized viareduction of AgNO3 dissolved in water and using reducing

agents such as NaBH4, among other compounds [26].In particular, the photochemical and photocatalytic reduc-

tions have been studied extensively since the decade of the 80s,

and they are considered as efficient ways to synthesizenanoparticles directly on semiconductor supports [27]. Thephotocatalytic deposition is carried out in the presence of

metal ions, semiconductor support and hole scavengers.After irradiation, the photogenerated electrons reduce thesurface-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 nmwith Ag nanoparticles (around 6–20 nm) uniformly depositedon the semiconductor [10]. As mentioned above, AgNO3

dissolved in water is the most common salt precursor used inthe synthesis of Ag/TiO2 composites, even in the photochemi-cal routes. Therefore, the present research was focused to the

synthesis of Ag/TiO2 composites by photocatalytic depositionemploying different organic (acetylacetonate) and inorganic(nitrate and perchlorate) silver precursors. The effect of silver

precursor on structural characteristics and photoactivity undersimulated solar light was mainly studied.

2. Experimental section

Silver nitrate (Ag-N Fermont 99%), silver perchlorate (Ag-C,Sigma–Aldrich 97%) and silver acetylacetonate (Ag-A, Sigma

Aldrich, 98%) were used as silver precursors. Ethanol (abso-lute, Fermont) was used as solvent and commercial TiO2

(TiO2-P25, Evonik) was used as support. The supported Agnanoparticles were obtained using the photocatalytic route as

follows: an ethanolic solution of Ag precursor (0.5 mM) wasmixed with TiO2-P25 in a 100 mL batch reactor and it wasdispersed with ultrasonic irradiation. Then a nitrogen flow of

50 mL min�1 was bubbled through the slurry to purge dis-solved oxygen and to achieve an inert reaction atmosphere.The batch reactor was irradiated for 4 h using a LuzChem

photoreactor (model LZC-4) equipped with 14 low pressuremercury lamps (8 W, kmax = 360 nm). After the irradiationtime, the obtained material was washed with ethanol, sepa-rated 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 structuralcharacterization was achieved from conventional TEM andHRTEM by means of a JEOL FEG 2010 FasTem electron

microscope with 1.9 A of resolution (point to point). ForTEM and HRTEM studies, the samples were suspended inethanol in order to disperse the powders then a drop of the

sample was deposited on a lacey carbon copper grid as aTEM support.

The composites were tested in a model reaction such asRhodamine B (RhB) degradation in aqueous solutions.

Typically, powdered photocatalyst in the amount of0.1 g L�1 was suspended in aqueous solution of 9.6 ppm ofRhB. The suspension was magnetically stirred in the dark

during 30 min to achieve a complete adsorption/desorptionequilibrium and then it was irradiated for 60 min with a solarlight simulator (Newport model 67005) equipped with a 150 W

Xe lamp and a power source which allows to change the lightintensity of the lamp. The light intensity was measured using adigital light meter (A.W. Sperry SLM-110) and the measuredlight intensity was 1.5 mW cm�2 (I0) at 20 cm from the source.

The reaction temperature was kept constant at 25 �C during allthe experiments and aliquots of the reaction medium wereperiodically sampled and filtered using a PTFE membrane

filter (Millipore, 0.45 lm) prior to analysis. The RhBconcentration was measured employing a spectrophotometricmethod by using a GBC Cintra 20 spectrophotometer and

following the decrease in the absorbance of RhB at550 nm.

Photocatalytic deposition of Ag nanoparticles on TiO2 565

3. Results and discussion

Fig. 1 shows the UV–Vis diffuse reflectance spectra forAg/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 absorptionedge of all samples was practically located at k = 380 nm,

which is consistent with the intrinsic band gap absorption ofanatase TiO2 (3.2 eV). Note the broad absorption band from400 to 800 nm for all Ag photocatalysts, highlighting the max-ima at 470, 505 and 530 nm for Ag-C/TiO2, Ag-A/TiO2 and

Ag-N/TiO2 respectively, attributed to the characteristic surfaceplasmon resonance (SPR) of metal cluster or Ag nanoparticlesdeposited on TiO2, depending on 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 couldindicate that changes of the plasmon maxima location from400–420 nm to 530 nm, would be related to the size, distribu-

tion, and chemical interaction of the Ag NPs deposited onTiO2, induced by the type of Ag precursor used in the synthesis[28]. Indeed, the use of different silver precursors led to differ-ent coloration in the final photocatalyst, a pale brown color

was observed with AgClO4 and certain shades of purple withAgAc and AgNO3. In general, the red-shifted resonance wave-length 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 thesize of Ag nanoparticles. A possible explanation of this effect

is the observed decrease of the photocatalytic efficiency ofTiO2 in the presence of inorganic salts, such as NO3

�, Cl�,SO4

2� [29]. At this point, comparing the area under the curve

of Ag composite prepared with AgNO3 in Fig. 1, it seems thatAg cations coming from AgNO3 were easier to reduce thanthose from AgClO4 and AcAg.

Fig. 2 shows several HRTEM images for Ag-A/TiO2

photocatalyst. In Fig. 2a, the anatase phase was identified bythe spacing d = 0.352 nm indexed as (101). In the second

Figure 1 UV–Vis diffuse reflectance spectra of bare TiO2-P25

and Ag/TiO2 composites prepared by photodeposition using

different Ag precursors: A = acetylacetonate, C = perchlorate,

N = nitrate.

image, two NPs can be observed, one of them was TiO2 andanother was AgO, this confirmed the deposit of Ag on theTiO2 surface. Also, two kinds of silver oxides, AgO and

Ag2O were identified. The AgO phase was identified by twoplane 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 spacingd= 0.246 nm and d= 0.260 nm, respectively. The last highresolution image shows a well-defined hexagonal nanoparticle

of Ag2O with a length of 15 nm and width of 12 nm approxi-mately which had a spacing d= 0.263 nm indexed as (100).Fig. 2b reveals similar crystallographic planes for Ag2O withthe 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. 2b,metallic silver (Ag0) was detected by the reflection (200) with

a spacing of d= 0.205 nm. Finally, in Fig. 2c a higher regionis presented where it is possible to observe the same crystallinephases, belonging to TiO2, AgO and Ag0, as reported above,

which revealed the formation of a composite material.Fig. 3 shows the TEM and HRTEM images for Ag-C/TiO2

photocatalyst. Fig. 3a displays the TEM micrographs where it

can be observed TiO2-P25 nanocrystals with sizes between 20and 50 nm, also it is observed that the TiO2 nanoparticlesare interconnected by channels with diameters of 20 nm.Here, the TiO2 nanoparticles presented a roughness on their

surface that makes it difficult to observe the Ag NPs that wereidentified by HRTEM in the form of Ag2O. Probably, the TiO2

nanocrystals were attacked by the chlorine species provoking

the oxidation of the Ag NPs. In Fig. 3b are presentedHRTEM images belonging to the anatase phase, which is alsoidentified by the spacing d= 0.352 nm and indexed as (101)

and the silver oxide Ag2O was identified by the spacingd= 0.246 nm and indexed as (002).

Fig. 4a shows TEM and HRTEM micrographs of the

Ag-N/TiO2 photocatalysts, in the first TEM image only thetitania nanocrystals can be observed. The second HRTEMimage shows a population of silver nanoparticles (dark parti-cles) with average size of 1 or 2 nm both on the surface as

inserted or embedded into the nanocrystals of TiO2. The AgNPs deposited had a spheroid shape and a high populationdensity on the TiO2 support. In the same way, as the other

samples discussed above, in Fig. 4b the HRTEM micrographsreveal the presence of the anatase phase identified by the spac-ing d = 0.352 nm and indexed as (101). Fig. 4c shows a

HRTEM image of two coupled nanoparticles indicating thepresence of a composite material formed by Ag2O, identifiedwith the spacing d = 0.246 nm and indexed as (002), andanother nanoparticle of the anatase phase identified by the

spacing d = 0.352 nm and indexed as (101); the nextHRTEM image only presents two silver oxide species, Ag2Oand 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 the anatase phase, PDF#011167 forAg0, PDF#431038 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 esti-mate the presence of three Ag species. It is well-known that the

Figure 2 Representative HRTEM micrographs of Ag-A/TiO2 photocatalyst. HRTEM images show four compounds: (a) anatase phase

and two type of silver oxides Ag2O and AgO, (b) metallic silver (Ag0) and silver oxide, (c) anatase, silver oxide and metallic silver

nanocomposite. The reflections belonging to all compounds are identified with white arrows and bars.

566 E. Albiter et al.

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 res-olution spectra of O 1s and Ti 2p of Ag/TiO2 composites areshown in Figs. S1 and S2 in the Supporting information,

respectively. All the composites presented similar spectraand, in the case of O 1s, it can be observed an asymmetricalpeak centered at 529.9 eV which can be attributed to lattice

oxygen and surface oxygen [31]. In the Ti 2p spectra it canbe 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 the Ag0 surface amount in all samples, it was

slightly higher than the surface amount of Ag2O and theamount of AgO was negligible. According to the signals in

XPS spectra, the ratio Ag0/Ag oxidized species was verysimilar 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 isnot discussed [32]. Indeed, it is very difficult to conclude theexact amount of each of the Ag surface species due to the quite

close values of their binding energy. However, after beingexposed to air environment, our Ag/TiO2 samples began tochange from their original color, slightly brown or purple, to

dark purple, which could be characteristic of the formationof silver oxides. Some authors have explained the formationof Ag2O deposited on TiO2, due to oxygen ions reverse-spill

over as the dominant oxidation mechanism of deposited Ag0

(Eqs. (1) and (2)) [33]

4Ag0 þO�2 ! 2Ag2Oþ e� ð1Þ

2Ag0 þO� ! Ag2Oþ e� ð2Þ

Figure 3 Representative micrographs: (a) TEM images of Ag-C/TiO2 photocatalyst, (b) HRTEM images showing the anatase phase and

silver oxide (Ag2O). These compounds were detected by the (101) and (002) reflections, respectively. The reflections are identified with

white arrows.

Photocatalytic deposition of Ag nanoparticles on TiO2 567

Fig. 6 shows the relative concentration (C/C0) versus irradi-ation time for the photocatalytic degradation of RhB withTiO2-P25 and Ag/TiO2 samples, under simulated solar light.

As can be seen, all the samples showed a similar degradationprofile reaching between 80% and 95% degradation of RhBafter 60 min. The Ag-A/TiO2 composite, prepared using silveracetylacetonate 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, whichwas the less active. The degradation profiles of RhB using all

materials followed an apparent first order kinetic and the val-ues of the calculated constants are shown in Fig. 7b. Asexpected, 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, Ag0

and Ag2O were detected in all the Ag/TiO2 catalyst, whichcould 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 electronreservoirs, enhancing the electron–hole separation and, there-

fore, enhancing the photocatalytic performance [30,36].Although all Ag/TiO2 materials contained a similar amountof 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 ofthe Ag nanoparticles [37], the presence of surface plasmon res-onance (SPR) [32] or metal-support interactions [38–40].

The influence of SPR on the photocatalytic performance,under visible light illumination, has been explained mainlyby two mechanisms [32]: charge transfer and local electric fieldenhancement. In the charge transfer mechanism, the SPR

excites the electrons in the Ag nanoparticles, which are trans-ferred to the conduction band of TiO2, leaving a ‘‘plasmonichole’’ [32] in the metal nanoparticle. According to this mecha-

nism, it is not necessary to excite the TiO2 to produce chargecarriers, which are produced in the metal nanoparticle underillumination with light corresponding to the SPR excitation

wavelength and it has been found that these plasmonic chargecarriers can participate in several redox reactions [32,41–43]. Inthe second mechanism, it is proposed that irradiation of metalnanoparticles with wavelengths near their SPR frequency can

generate intense local electric fields near the metal–TiO2 inter-face. In these regions the generation rate of the charge carrierscan be 1000 times higher than that generated in the bulk TiO2

and this enhanced generation rate is responsible for theimprovement in the observed photoactivity [44–46].

Accordingly, our photoactivity results can be coarsely

correlated with the ratio of surface species Ag0/Ag2O which

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 show nanoparticles of the anatase

phase. (c) HRTEM images of the anatase phase and the silver oxides as Ag2O and AgO.

568 E. Albiter et al.

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 canreduce 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 Agnanoparticles. Additionally, silver oxide nanoparticles and

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.

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.

Photocatalytic deposition of Ag nanoparticles on TiO2 569

TiO2 act as Vis and UV light harvesters, respectively, which

can transfer electrons from their conduction band mainly tosurface 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 vari-

ables were RhB initial concentration, photocatalyst loadingand light intensity. In Fig. 7a, the relative RhB concentrationprofiles versus time, in function of the initial concentration of

RhB, are shown. As can be noted, the conversion after

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).

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).

570 E. Albiter et al.

60 min of irradiation time was inversely proportional to theRhB initial concentration. Also, it can be observed a slightchange in the concentration profile above �29 ppm. This

change can be observed better in Fig. 7b, where the calculatedpseudo-first order constants were plotted against the initialconcentration of RhB. As it is well-known, the photocatalytic

Photocatalytic deposition of Ag nanoparticles on TiO2 571

reactions can be described by the Langmuir–Hinshelwood(LH) model, which states that at lower concentrations ofsubstrate 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 relatedto the change in the reaction order, suggesting that the

photocatalytic degradation of RhB can be fitted to the LHmodel.

The influence of photocatalyst concentration in the degra-

dation of RhB is shown in Fig. 8. It can be observed thatthe presence of the semiconductor was important becausethe photolysis of RhB was negligible under simulated solarlight (see Fig. 8a). As the catalyst concentration was

increased to 0.04 g L�1, the conversion after 60 min wasincreased to 70%, reaching a maximum of 85% at0.3 g L�1 and above. It is to be noted that a linear relation-

ship between the calculated pseudo-first order constant andthe catalyst concentration was observed in the range of0.04–0.1 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 ofsuspended particles which resulted in an inefficient absorp-

tion of light by the photocatalyst, as observed in otherphotocatalytic systems [49].

Finally, the influence of the light intensity over thephotocatalytic degradation of RhB is presented in Fig. 9.

The conversion at 35 min was proportional to the relative lightintensity ranging from 50% to 95% with the higher light inten-sity (Fig. 9a). A linear relationship between the pseudo-first

order constants and the relative light intensity was clearlyobserved (Fig. 9b). This behavior was previously observed inother photochemical systems [48,50] where the reaction rates

can be linear at low light intensities, square root dependentat intermediate light intensities and at high light intensities itremains constant.

4. Conclusions

It was studied the effect of the silver precursor on the struc-

tural and photocatalytic properties of Ag/TiO2 compositesby using a photodeposition method. The photocatalyticreduction of Ag+ to Ag0 on TiO2 was higher with AgNO3

as a precursor than AgClO4 or silver acetylacetonate. In

all composites, a surface plasmon resonance was clearlyobserved in the range of k = 400–530 nm, with a colorationslightly brown to purple, denoting metal particle sizes from

2 to 20 nm. Bulk and surface analysis confirmed thepresence of Ag and Ag2O and traces of AgO. Ag oxidizedspecies were probably formed by the reaction of Ag0

with oxygen species after being exposed to the environ-ment. Ag0 dispersion on TiO2 was in the order:Ag-N/TiO2 > Ag-C/TiO2 > Ag-A/TiO2, however the ratioof Ag0/Ag2O was �1.3 for Ag-N/TiO2 and Ag-A/TiO2 and

for Ag-C/TiO2 was lower (�1.1). Upon modifying TiO2-P25with Ag0/Ag2O, the rate constant of Rh-B degradation undersimulated solar light was higher than pure TiO2. It seemed

that the higher photocatalytic activity, obtained with silveracetylacetonate as the precursor, was due to the higheramount of Ag surface species than to Ag particle size or metal

dispersion.

Declaration of interest

The authors acknowledge the financial supports of ‘‘ConsejoNacional de Ciencia y Tecnologıa’’ – Mexico (CONACyT)

Projects 166354, 153356 and 251151, ‘‘Instituto PolitecnicoNacional’’ – Mexico, Projects SIP 20150693 and 20150030.

Acknowledgments

We thank Luis Rendon (TEM, HRTEM) and Luis Lartundo

(XPS) for his technical assistance. E. Albiter wishes to thankCONACyT for its postdoctoral fellowship.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jscs.2015.05.009.

References

[1] V.K. Sharma, R.A. Yngard, Y. Lin, Silver nanoparticles: green

synthesis and their antimicrobial activities, Adv. Colloid

Interface Sci. 145 (2009) 83–96.

[2] M.M. Khan, S.A. Ansari, M.O. Ansari, B.K. Min, J. Lee, M.H.

Cho, Biogenic fabrication of Au@CeO2 nanocomposite with

enhanced visible light activity, J. Phys. Chem. C 118 (2014)

9477–9484.

[3] M.M. Khan, J. Lee, M.H. Cho, Au@TiO2 nanocomposites for

the catalytic degradation of methyl orange and methylene

blue: an electron relay effect, J. Ind. Eng. Chem. 20 (2014)

1584–1590.

[4] S. Oros-Ruiz, R. Zanella, R. Lopez, A. Hernandez-Gordillo, R.

Gomez, Photocatalytic hydrogen production by water/methanol

decomposition using Au/TiO2 prepared by deposition–

precipitation with urea, J. Hazard. Mater. 263 (Part 1) (2013)

2–10.

[5] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V.

Narayanan, A. Stephen, Enhanced photocatalytic activity of

ZnO/CuO nanocomposite for the degradation of textile dye on

visible light illumination, Mater. Sci. Eng., C 33 (2013) 91–98.

[6] R. Saravanan, V.K. Gupta, T. Prakash, V. Narayanan, A.

Stephen, Synthesis, characterization and photocatalytic activity

of novel Hg doped ZnO nanorods prepared by thermal

decomposition method, J. Mol. Liq. 178 (2013) 88–93.

[7] R. Saravanan, V.K. Gupta, V. Narayanan, A. Stephen, Visible

light degradation of textile effluent using novel catalyst ZnO/c-Mn2O3, J. Taiwan Inst. Chem. Eng. 45 (2014) 1910–1917.

[8] R. Saravanan, E. Thirumal, V.K. Gupta, V. Narayanan, A.

Stephen, The photocatalytic activity of ZnO prepared by simple

thermal decomposition method at various temperatures, J. Mol.

Liq. 177 (2013) 394–401.

[9] R. Saravanan, H. Shankar, T. Prakash, V. Narayanan, A.

Stephen, ZnO/CdO composite nanorods for photocatalytic

degradation of methylene blue under visible light, Mater.

Chem. Phys. 125 (2011) 277–280.

[10] E. Albiter, Z. Hai, S. Alfaro, H. Remita, M.A. Valenzuela, C.

Colbeau-Justin, A comparative study of photo-assisted

deposition of silver nanoparticles on TiO2, J. Nanosci.

Nanotechnol. 13 (2013) 4943–4948.

[11] E. Grabowska, A. Zaleska, S. Sorgues, M. Kunst, A.

Etcheberry, C. Colbeau-Justin, H. Remita, Modification of

572 E. Albiter et al.

titanium(IV) dioxide with small silver nanoparticles: application

in photocatalysis, J. Phys. Chem. C 117 (2013) 1955–1962.

[12] S. Linic, H. Piao, K. Adib, M.A. Barteau, Ethylene epoxidation

on Ag: identification of the crucial surface intermediate by

experimental and theoretical investigation of its electronic

structure, Angew. Chem. Int. Ed. 43 (2004) 2918–2921.

[13] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q.

Zhang, D. Qin, Y. Xia, Controlling the synthesis and assembly

of silver nanostructures for plasmonic applications, Chem. Rev.

111 (2011) 3669–3712.

[14] S. Scire, C. Crisafulli, S. Giuffrida, C. Mazza, P.M. Riccobene,

A. Pistone, G. Ventimiglia, C. Bongiorno, C. Spinella,

Supported silver catalysts prepared by deposition in aqueous

solution of Ag nanoparticles obtained through a photochemical

approach, Appl. Catal. Gen. 367 (2009) 138–145.

[15] A. Nagy, G. Mestl, High temperature partial oxidation reactions

over silver catalysts, Appl. Catal. Gen. 188 (1999) 337–353.

[16] Z. Shan, J. Wu, F. Xu, F.-Q. Huang, H. Ding, Highly effective

silver/semiconductor photocatalytic composites prepared

by a silver mirror reaction, J. Phys. Chem. C 112 (2008)

15423–15428.

[17] S.A. Ansari, M.M. Khan, M.O. Ansari, J. Lee, M.H. Cho,

Biogenic synthesis, photocatalytic, and photoelectrochemical

performance of Ag–ZnO nanocomposite, J. Phys. Chem. C 117

(2013) 27023–27030.

[18] M.M. Khan, S.A. Ansari, M.I. Amal, J. Lee, M.H. Cho, Highly

visible light active Ag@TiO2 nanocomposites synthesized using

an electrochemically active biofilm: a novel biogenic approach,

Nanoscale 5 (2013) 4427–4435.

[19] M.M. Khan, S.A. Ansari, J.-H. Lee, M.O. Ansari, J. Lee, M.H.

Cho, Electrochemically active biofilm assisted synthesis of

Ag@CeO2 nanocomposites for antimicrobial activity,

photocatalysis and photoelectrodes, J. Colloid Interface Sci.

431 (2014) 255–263.

[20] B. Ohtani, K. Iwai, S. Nishimoto, S. Sato, Role of platinum

deposits on titanium(IV) oxide particles: structural and kinetic

analyses of photocatalytic reaction in aqueous alcohol and

amino acid solutions, J. Phys. Chem. B. 101 (1997) 3349–3359.

[21] X. Zhou, G. Liu, J. Yu, W. Fan, Surface plasmon resonance-

mediated photocatalysis by noble metal-based composites under

visible light, J. Mater. Chem. 22 (2012) 21337–21354.

[22] A. Zielinska, E. Kowalska, J.W. Sobczak, I. acka, M. Gazda, B.

Ohtani, J. Hupka, A. Zaleska, Silver-doped TiO2 prepared by

microemulsion method: surface properties, bio- and

photoactivity, Sep. Purif. Technol. 72 (2010) 309–318.

[23] H. Zhang, G. Wang, D. Chen, X. Lv, J. Li, Tuning

photoelectrochemical performances of Ag–TiO2 nanocom-

posites via reduction/oxidation of Ag, Chem. Mater. 20 (2008)

6543–6549.

[24] D. Sarkar, C.K. Ghosh, S. Mukherjee, K.K. Chattopadhyay,

Three dimensional Ag2O/TiO2 Type-II (p–n) nanohetero-

junctions for superior photocatalytic activity, ACS Appl.

Mater. Interfaces 5 (2013) 331–337.

[25] C. Liu, C. Cao, X. Luo, S. Luo, Ag-bridged Ag2O nanowire

network/TiO2 nanotube array p–n heterojunction as a highly

efficient and stable visible light photocatalyst, J. Hazard. Mater.

285 (2015) 319–324.

[26] T.M. Tolaymat, A.M. El Badawy, A. Genaidy, K.G. Scheckel,

T.P. Luxton, M. Suidan, An evidence-based environmental

perspective of manufactured silver nanoparticle in syntheses and

applications: a systematic review and critical appraisal of

peer-reviewed scientific papers, Sci. Total Environ. 408 (2010)

999–1006.

[27] M. Sakamoto, M. Fujistuka, T. Majima, Light as a construction

tool of metal nanoparticles: synthesis and mechanism, J.

Photochem. Photobiol. C 10 (2009) 33–56.

[28] W. Xie, Y. Li, W. Sun, J. Huang, H. Xie, X. Zhao, Surface

modification of ZnO with Ag improves its photocatalytic

efficiency and photostability, J. Photochem. Photobiol. A 216

(2010) 149–155.

[29] C. Guillard, E. Puzenat, H. Lachheb, A. Houas, J.-M.

Herrmann, Why inorganic salts decrease the TiO2

photocatalytic efficiency, Int. J. Photoenergy 7 (2005) 1–9.

[30] P.V. Kamat, Meeting the clean energy demand: nanostructure

architectures for solar energy conversion, J. Phys. Chem. C 111

(2007) 2834–2860.

[31] H. Jensen, A. Soloviev, Z. Li, E.G. Søgaard, XPS and FTIR

investigation of the surface properties of different prepared

titania nano-powders, Appl. Surf. Sci. 246 (2005) 239–249.

[32] W. Hou, S.B. Cronin, A review of surface plasmon resonance-

enhanced photocatalysis, Adv. Funct. Mater. 23 (2013) 1612–

1619.

[33] R. Priya, K.V. Baiju, S. Shukla, S. Biju, M.L.P. Reddy, K. Patil,

K.G.K. Warrier, Comparing ultraviolet and chemical reduction

techniques for enhancing photocatalytic activity of silver oxide/

silver deposited nanocrystalline anatase titania, J. Phys. Chem.

C 113 (2009) 6243–6255.

[34] S. Sarina, E.R. Waclawik, H. Zhu, Photocatalysis on supported

gold and silver nanoparticles under ultraviolet and visible light

irradiation, Green Chem. 15 (2013) 1814–1833.

[35] D. Wang, G. Xue, Y. Zhen, F. Fu, D. Li, Monodispersed Ag

nanoparticles loaded on the surface of spherical Bi2WO6

nanoarchitectures with enhanced photocatalytic activities, J.

Mater. Chem. 22 (2012) 4751–4758.

[36] C. Yogi, K. Kojima, T. Takai, N. Wada, Photocatalytic

degradation of methylene blue by Au-deposited TiO2 film

under UV irradiation, J. Mater. Sci. 44 (2009) 821–827.

[37] R. Kaur, B. Pal, Co-catalysis effect of different morphological

facets of as prepared Ag nanostructures for the photocatalytic

oxidation reaction by Ag–TiO2 aqueous slurry, Mater. Chem.

Phys. 143 (2013) 393–399.

[38] L. Jiang, G. Zhou, J. Mi, Z. Wu, Fabrication of visible-light-

driven one-dimensional anatase TiO2/Ag heterojunction

plasmonic photocatalyst, Catal. Commun. 24 (2012) 48–51.

[39] H. Zhang, C. Liang, J. Liu, Z. Tian, G. Wang, W. Cai, Defect-

mediated formation of Ag cluster-doped TiO2 nanoparticles for

efficient photodegradation of pentachlorophenol, Langmuir 28

(2012) 3938–3944.

[40] A.A. Ismail, Facile synthesis of mesoporous Ag-loaded TiO2

thin film and its photocatalytic properties, Microporous

Mesoporous Mater. 149 (2012) 69–75.

[41] W.-T. Chen, Y.-J. Hsu, P.V. Kamat, Realizing visible

photoactivity of metal nanoparticles: excited-state behavior

and electron-transfer properties of silver (Ag8) clusters, J.

Phys. Chem. Lett. 3 (2012) 2493–2499.

[42] M.S. Devadas, K. Kwak, J.-W. Park, J.-H. Choi, C.-H. Jun,

E. Sinn, G. Ramakrishna, D. Lee, Directional electron

transfer in chromophore-labeled quantum-sized Au25 clusters:

Au25 as an electron donor, J. Phys. Chem. Lett. 1 (2010)

1497–1503.

[43] I. Dıez, R.H.A. Ras, Fluorescent silver nanoclusters, Nanoscale

3 (2011) 1963–1970.

[44] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal

nanostructures for efficient conversion of solar to chemical

energy, Nat. Mater. 10 (2011) 911–921.

[45] D.B. Ingram, P. Christopher, J.L. Bauer, S. Linic, Predictive

model for the design of plasmonic metal/semiconductor

composite photocatalysts, ACS Catal. 1 (2011) 1441–1447.

[46] Y. Lu, H. Yu, S. Chen, X. Quan, H. Zhao, Integrating

plasmonic nanoparticles with TiO2 photonic crystal for

enhancement of visible-light-driven photocatalysis, Environ.

Sci. Technol. 46 (2012) 1724–1730.

Photocatalytic deposition of Ag nanoparticles on TiO2 573

[47] Y. Nosaka, A.Y. Nosaka, Identification and roles of the active

species generated on various photocatalysts, in: P. Pichat (Ed.),

Photocatalytic Water Purifcation: From Fundamentals to

Recent Applications, Wiley-VCH Verlag GmbH & Co.,

Germany, 2013, pp. 1–24.

[48] J.M. Herrmann, Heterogeneous photocatalysis: state of the art

and present applications, Top. Catal. 34 (2005) 49–65.

[49] A. Mills, J. Wang, D.F. Ollis, Dependence of the kinetics of

liquid-phase photocatalyzed reactions on oxygen concentration

and light intensity, J. Catal. 243 (2006) 1–6.

[50] S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Kinetic analysis of

heterogeneous photocatalysis: role of hydroxyl radicals, Catal.

Rev. 55 (2013) 79–133.