Characterization of silver nanoparticles synthesized on titanium dioxide fine particles This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 Nanotechnology 19 065711 (http://iopscience.iop.org/0957-4484/19/6/065711) Download details: IP Address: 144.32.128.51 The article was downloaded on 21/05/2013 at 10:22 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Characterization of silver nanoparticles synthesized on titanium dioxide fine particles
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2008 Nanotechnology 19 065711
(http://iopscience.iop.org/0957-4484/19/6/065711)
Download details:
IP Address: 144.32.128.51
The article was downloaded on 21/05/2013 at 10:22
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Characterization of silver nanoparticlessynthesized on titanium dioxidefine particlesN Nino-Martınez1,2, G A Martınez-Castanon3,5, A Aragon-Pina2,F Martınez-Gutierrez4, J R Martınez-Mendoza1 andFacundo Ruiz1
1 Facultad de Ciencias, UASLP, Alvaro Obregon 64, CP 78000, San Luis Potosı, SLP, Mexico2 Instituto de Metalurgia, Facultad de Ingenierıa, UASLP, Alvaro Obregon 64, CP 78000,San Luis Potosı, SLP, Mexico3 Maestria en Ciencias Odontologicas, Facultad de Estomatologıa, UASLP, Avenida ManuelNava 2, Zona Universitaria, San Luis Potosı, SLP, Mexico4 Facultad de Ciencias Quımicas, UASLP, Alvaro Obregon 64, CP 78000, San Luis Potosı,SLP, Mexico
Received 5 September 2007, in final form 5 December 2007Published 23 January 2008Online at stacks.iop.org/Nano/19/065711
AbstractSilver nanoparticles with a narrow size distribution were synthesized over the surface of twodifferent commercial TiO2 particles using a simple aqueous reduction method. The reducingagent used was NaBH4; different molar ratios TiO2:Ag were also used. The nanocompositesthus prepared were characterized using transmission electron microscopy (TEM), scanningtransmission electron microscopy (STEM), scanning electron microscopy (SEM),energy-dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), x-raydiffraction (XRD), dynamic light scattering (DLS) and UV–visible (UV–vis) absorptionspectroscopy; the antibacterial activity was assessed using the standard microdilution method,determining the minimum inhibitory concentration (MIC) according to the National Committeefor Clinical Laboratory Standards. From the microscopy studies (TEM and STEM) weobserved that the silver nanoparticles are homogeneously distributed over the surface of TiO2
particles and that the TiO2:Ag molar ratio plays an important role. We used three differentTiO2Ag molar ratios and the size of the silver nanoparticles is 10, 20 and 80 nm, respectively. Itwas found that the antibacterial activity of the nanocomposites increases considerablycomparing with separated silver nanoparticles and TiO2 particles.
1. Introduction
Titanium dioxide (TiO2) is one of the most popularsemiconductor materials; it is commercially available andcan be used in many catalytic applications [1–8]. TiO2 hasa wide band gap (3 and 3.23 eV for anatase and rutile,respectively) which makes this material transparent to visiblelight, i.e., no photon absorption occurs at wavelengths beyond380 nm and catalytic reactions using pure TiO2 must becarried out using ultraviolet photons. Titanium dioxide is amaterial that also presents antibacterial activity [9–12]; thisantibacterial activity has been studied over E. coli and B.
5 Author to whom any correspondence should be addressed.
megaterium using environmental light [13]. Few studies haveinvestigated the application of TiO2 in life science [14]. Ithas been reported that the catalytic and bactericide propertiesof TiO2 can be improved by growing particles of a noblemetal (Ag, Au or Cu) over its surface [13], or inside itsmatrix as reported by Thiel et al [15]. In this work,silver nanoparticles were synthesized on the surface of TiO2
fine particles using a simple aqueous reduction method andthe composites thus obtained (TiO2@Ag) were characterizedusing TEM, STEM, SEM, EDS, XRD, UV–vis spectroscopy,DLS and XPS. An antibacterial activity test (NCCLS M7-A4, 1997) was conducted in order to confirm the improvedbactericide properties of the composites obtained.
TiO2 particles (DuPontTM Ti-Pure® R-902 and Degussa P25),AgNO3 (Sigma Aldrich, ACS Reagent), NaBH4 (SigmaAldrich, ACS Reagent) and NH4OH (30% w/w aqueoussolution, Sigma Aldrich, ACS Reagent) were used as receivedwithout further purification.
2.2. Synthesis method
For a typical procedure, 0.2000 g (2.5 mmol) of commercialTiO2 particles were dispersed in 100 ml of deionized water byusing an ultrasonic treatment for approximately five minutes;immediately afterwards 0.0169 g (0.1 mmol) or 0.0425 g(0.25 mmol) of AgNO3 was added. The solution wasmagnetically stirred for about 30 min at pH = 7. After this,0.1 mmol or 0.25 mmol of sodium borohydride, previouslydissolved in 10 ml of deionized water, was added as reducingagent. The pH of the reaction media was adjusted to 10by adding NH4OH, and finally the solution was magneticallystirred for 30 min. After several experiments these conditions(amount of sodium borohydride added, pH value and stirringtime) were chosen because they allow us to control the sizeand size distribution of the silver nanoparticles. The vigorouschemical reduction yields a brownish dispersion; there is achange of color (from white to brownish), and the reaction iscompleted after approximately 3 min; the additional 27 min ofmagnetic stirring allowed the narrowing of the size dispersion(Ostwald ripening) [16].
After this, the products obtained (TiO2@Ag) were filtered,washed and dried for further characterization. Hereafter,DuPontTM particles will be named as TiO2 1 and Degussaparticles will be named as TiO2 2. Three different sampleswere synthesized; the samples obtained using TiO2 1 andmolar ratios of 25:1 and 10:1 (TiO2:Ag) will be named asTiO2 1@Ag25 and TiO2 1@Ag10 respectively. The sampleprepared using TiO2 2 and a molar ratio of 10:1 (TiO2:Ag) willbe named as TiO2 2@Ag10 (see table 1).
2.3. Characterization
The composites produced were characterized by UV–visspectroscopy using an S2000-UV–vis spectrometer fromOceanOptics Inc. Dynamic light scattering analysis wasperformed in a Malvern Zetasizer Nano ZS. X-ray diffractionpatterns were obtained on a GBC-Difftech MMA model, withCu Kα irradiation at λ = 1.54 A. Transmission electronmicroscopy (TEM) analysis was performed on a JEOL JEM-1230 at an accelerating voltage of 100 kV, and the STEM
images were obtained on a JEOL 2010F. Scanning electronmicroscopy (SEM) analysis was performed on a Phillips XL-30 SEM equipped with an EDS spectrometer EDAX DX-4Model. XPS analysis of the powder samples was carriedout using a Kratos AXIS ULTRA XPS system fitted witha monochromated Al Kα x-ray source and a hemisphericalanalyzer with eight channeltrons. The source was operatedat 10 mA and 15 kV. UV–vis spectroscopy, SEM, EDS, XRDand XPS analyses were made using dried powders and TEM,STEM and DLS analyses were made using aqueous dispersionsof the TiO2@Ag composites.
2.4. Antibacterial test
The antimicrobial activity of the synthesized compositeswas tested using the standard microdilution method, whichdetermines the minimum inhibitory concentration (MIC)leading to inhibition of bacterial growth (NCCLS M7-A4,1997). Disposable microtitration plates were used for thetests. The composites in dispersion form were diluted 2–128 times with 100 μl of Mueller–Hinton broth inoculatedwith the tested bacteria at a concentration of 105 CFU ml−1.The minimum inhibitory concentration (MIC) was read after24 h of incubation at 37 ◦C as the MIC of the testedsubstance that inhibited the growth of the bacterial strain. Thedispersions were used in the form in which they had beenprepared. Therefore, control bactericidal tests of solutionswere performed containing all the reaction components.
3. Results and discussion
3.1. Synthesis
Silver ions (Ag+) can be deposited over the surface of TiO2
particles by cationic adsorption. TiO2 is an amphoteric oxidewith an isoelectronic point IEP = 6 [17]. When the pH valueof a TiO2 dispersion is lower than 6 the main surface speciesis –OH+
2 , and when the pH value of a TiO2 dispersion is biggerthan 6 the main surface species is –O−; in the latter case thesurface of TiO2 particles is negatively charged and silver ionscan be deposited over its surface [13]. In this work, in orderto ensure a complete adsorption of the silver ions, a mixtureof TiO2 particles and silver ions (added as silver nitrate) wasmagnetically stirred for about 30 min at pH = 7. After that, thereduction reaction proceeded on the surface of TiO2 particles.The synthesis of TiO2@Ag composites can be summarized asfollows:
(i) TiO2 + Ag+ → TiO2@(Ag+) at pH = 7
(ii) TiO2(Ag+) + BH−4 → TiO2@Ag at pH = 10.
3.2. SEM and EDS analysis
Figure 1 shows SEM images for TiO2 particles before andafter the silver synthesis. We can see that the TiO2 1 particlesare pseudospherical and their sizes range from 200 to 450 nm(figures 1(a)); TiO2 2 particles present strong agglomeration
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Nanotechnology 19 (2008) 065711 N Nino-Martınez et al
a) b)
c) d)
Figure 1. SEM images of the starting materials and the composites synthesized in this work: (a) TiO2 1, (b) TiO2 2, (c) TiO2 1@Ag25 and(d) TiO2 2@Ag10.
when seen by SEM (figures 1(b)); the morphology and particlesize of these particles cannot be resolved using this technique.The insets in figures 1(a) and (b) show EDS analysis results.The chemical elements present in TiO2 1 particles are Ti, O, Aland Si; TiO2 2 particles are composed of Ti and O. From SEMimages of the composites obtained it is evident that startingparticles do not change in morphology (figures 1(c) and (d));the deposited silver nanoparticles cannot be seen but theirpresence is detected using EDS analysis (insets in figures 1(c)and (d)).
3.3. TEM and STEM
Size distribution analysis was done on the Ag nanoparticlesprepared over the surface of TiO2 particles; each analysisaccounts for 200 particles, and the results are presented ashistograms in figures 2(c)–(e). The Ag nanoparticles in theTiO2 1@Ag25 sample have a mean size of 20.6 nm and astandard deviation of 5.1 nm; the Ag nanoparticles in theTiO2 1@Ag10 sample have a mean size of 77.3 nm and astandard deviation of 18.4 nm; finally, the Ag nanoparticlesin the TiO2 2@Ag10 sample have a mean size of 8.2 nm
and a standard deviation of 2.0 nm. Using TEM we canconfirm the size of the TiO2 1 particles, and the most importantinformation extracted from figure 2(a) is the irregular thinlayer observed on the surface of TiO2 particles; this couldbe a layer made of SiO2 and Al2O3 (according to the resultsobtained in the EDS analysis). It is shown in figure 2(b) thatTiO2 2 particles have a spherical morphology and a particlesize ranging from 15 to 70 nm.
Figures 2(c)–(e) show the images for TiO2 1@Ag25,TiO2 1@Ag10 and TiO2 2@Ag10 samples, respectively.Using TiO2 1 particles and increasing the amount of silvernitrate in reaction we cannot produce more Ag nanoparticlesas we expected; instead, silver nanoparticles already formed onthe surface of TiO2 1 grow (figures 2(c) and (d); figures 3(a)and (b)). The reason for this unexpected behavior could bethe presence of the irregular SiO2–Al2O3 thin layer on thesurface of TiO2 1 particles (the presence of these oxides inDuPontTM Ti-Pure® R-902 is also reported in the datasheetof the product). SiO2 and Al2O3 have no reactivity if theyare not activated with a more complicated process than justvarying the pH value [18, 19]; the presence of a transitionelement is very important, so Ag nanoparticles are formed only
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a) b)
c) d)
e)
10 15 20 25 30 350
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40 60 80 100 1200
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Figure 2. TEM images of the starting materials and the composites synthesized in this work: (a) TiO2 1, (b) TiO2 2, (c) TiO2 1@Ag25,(d) TiO2 1@Ag10 and (e) TiO2 2@Ag10. The arrow in (a) shows the irregular thin layer on the surface of these particles.
on the spots where there is no SiO2–Al2O3 thin layer; in ashort time, these spots are replete and the remaining silver ionsare deposited over the first silver nanoparticles formed, andfinally they grow. If we use TiO2 2 instead of TiO2 1 and if wemaintain the concentration of Ag+ as a constant, the amount ofsilver nanoparticles over the surface of TiO2 particles increasesconsiderably (figures 2(d) and (e)); the reason for this could be,
again, the presence of the SiO2–Al2O3 thin layer on the surfaceof TiO2 1 and the fact that Degussa P25 is reported as the mostreactive phase of TiO2 [20].
3.4. XRD analysis
Figure 4(a) shows the diffraction pattern obtained for theTiO2 1@Ag25 composite: only peaks from rutile phase
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Nanotechnology 19 (2008) 065711 N Nino-Martınez et al
a) b)
Figure 3. STEM images of the samples (a) TiO2 1@Ag25 and (b) TiO2 1@Ag10. These images show that the silver nanoparticles are biggerin sample TiO2 1@Ag10 than in sample TiO2 1@Ag25.
Figure 4. XRD analysis of samples (a) TiO2 1@Ag25, (b) TiO2 1@Ag10 and (c) TiO2 2@Ag10.
appear; no peaks from elemental silver appear, probably due
to the detection limit of the instrument. Figure 4(b) shows
the diffraction pattern obtained for the TiO2 1@Ag10 sample;
it presents peaks from rutile and silver. The amount of
AgNO3 used to prepare this sample is more than for sample
TiO2 1@Ag25; thus the silver amount present in the composite
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Nanotechnology 19 (2008) 065711 N Nino-Martınez et al
Figure 5. XPS analysis of the sample TiO2 1@Ag25.
is larger and can be detected by XRD. The x-ray diffractogramfor the TiO2 2@Ag10 sample shows reflections due to rutileand anatase phases of TiO2 and also shows reflections fromelemental silver (figure 4(c)). These results confirm that thenature of the synthesized particles is elemental silver.
3.5. XPS analysis
Because of the detection limit of the XRD instrument wewere unable to detect silver on sample TiO2 1@Ag25, so XPSanalyses were done in order to confirm the presence of Ag.Figure 5 shows the spectrum obtained: the peaks found at368.1 and 373.9 eV confirm the presence of silver in the formof Ag0 [21, 22]; these results agree with those obtained usingEDS analysis.
3.6. UV–vis analysis
The absorption spectra of the composites are presented infigure 6; they feature the band edge of TiO2 (415 nm forTiO2 1 particles and 380 nm for TiO2 2 particles). A weakband near to 450 nm corresponding to the signal of silvernanoparticles [23] is present in spectra of TiO2 1@Ag25 andTiO2 1@Ag10 composites; a well defined band at 450 nmarises in the spectrum of the TiO2 2@Ag10 sample. Thus,these composites could be used in photocatalysis without theuse of UV light.
3.7. DLS analysis
Figure 7 shows the DLS analyses made on TiO2 1 particlesbefore and after the synthesis of silver nanoparticles. Theseanalyses were used to probe whether silver nanoparticleswere really attached to the surface of TiO2 particles. Fromfigure 7(a), the TiO2 1 particles, before silver synthesis, havea mean diameter of 334 nm (93.1 nm width); after silversynthesis their mean diameter slightly grows (388 nm; 105 nmwidth) due to the presence of silver nanoparticles (figure 7(b)).This result confirms that silver nanoparticles really are attachedto TiO2 particles; if this were not the case we would see twopeaks in figure 7(b), one corresponding to silver nanoparticlesand the other one corresponding to TiO2 particles.
Figure 6. UV–vis spectra of the starting materials and thecomposites synthesized in this work.
Table 2. Minimum inhibition concentrations of TiO2 particles, Agnanoparticles and TiO2@Ag composites.
Material Minimum inhibition concentrationc (μg ml−1)
a No antibacterial activity was found with the concentrationstested in this work.b 20 nm Ag nanoparticles were synthesized under the sameconditions as the composites but without the presence of TiO2
particles.c Values in parentheses represent the calculated content of silverin the composites. This silver content was calculated using theresults from atomic absorption spectroscopy (AAS) performed onthe supernatant after filtering the composites. The Ag content inTiO2@Ag = Ag+ added –Ag in supernatant.
3.8. Antibacterial results
Minimum inhibitory concentration values were obtained forthe synthesized composites tested against E. coli (Gramnegative bacteria, ATCC 25922) and S. aureus (Gram positivebacteria, ATCC 25923). The results are presented as averagevalues ion table 2 (the Kruskal–Wallis test was applied).Control sample containing all the initial reaction componentsshowed no antibacterial activity.
The TiO2 1@Ag25 sample has higher antibacterialactivity than the other composites and presents higherantibacterial activity than TiO2 particles. If we compare theMIC of TiO2 1@Ag25 with that of silver nanoparticles we cansee that the MIC of the latter is lower, but the silver content inTiO2 1@Ag25 sample is much lower (almost 20 times) thanthe MIC of silver nanoparticles; thus, we can say that there is areal synergetic antibacterial activity in these composites. So farit is clear that bigger nanoparticles decrease the antibacterialactivity in our nanocomposites: TiO2 1@Ag10 composites
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Nanotechnology 19 (2008) 065711 N Nino-Martınez et al
a) b)
Figure 7. DLS analyses of sample TiO2 1: (a) before and (b) after the synthesis of silver nanoparticles.
present more silver content than TiO2 1@Ag25 composites buttheir size is bigger and the antibacterial activity decreases. Thefact that TiO2 1 and TiO2 2 particles showed no antibacterialactivity is due to the test conditions: the test was performedon dark. It is reported [8, 14] that the bactericide activity ofTiO2 is directly related to ultraviolet light absorption and theformation of free radicals, so in dark conditions TiO2 particlespresent no bactericide activity, which is consistent with ourresults. On the other hand, all the TiO2@Ag compositesshow antibacterial activity even though no light is present.The antibacterial mechanism of these composites is underinvestigation by our group.
Comparing our results with those reported by Thielet al (they use a different method to prepare nanocompositesbut perform similar evaluations of their bactericidal effects)we found that the best results obtained in their work inliquid medium were achieved using a concentration of thecomposite of 76 900 μg ml−1 (10 g in 130 ml) with 3 wt%of silver content (0.72 at.%), while the best bactericide effectfound in our work was achieved with a concentration of130.2 μg ml−1 with the sample TiO2 1@Ag25 with 0.5 wt%of silver content, i.e., although our composites are biggerin size the bactericide concentration is lower. The maindifference between these composites is that our compositespresent the silver nanoparticles on their surface, promotingcontact with bacteria, while Thiel’s composites have silverin their matrix which makes the direct contact of silver withbacteria cells difficult; this could be the main reason why bothnanocomposites present different antibacterial activities.
4. Conclusions
Silver nanoparticles were synthesized over the surface of twodifferent commercial TiO2 particles. The composites thus ob-tained were characterized: using XRD, XPS and UV–vis anal-ysis it was demonstrated that the nature of the nanoparticlesprepared is elemental silver; these silver nanoparticles are welldistributed over the surface of TiO2 particles and their averagesizes are 10, 20 and 80 nm, depending on the TiO2:Ag ratio.
The antibacterial activity of TiO2 nanoparticles was improvedand is dependent on the sort of TiO2 particles. In this work, thebest results were achieved using TiO2DupontTM particles anda TiO2:Ag molar ratio of 1:25.
Acknowledgments
This work was partially supported by Fondo de Apoyo a laInvestigacion (FAI) of Universidad Autonoma de San LuisPotosı (UASLP) and CONACYT-61257. N Nino-Mart ınezwould like to thank CONACYT for scholarship No. 185006.
References
[1] Kominami H, Murakami S, Kato J, Kera Y and Ohtani B 2002Correlation between some physical properties of titaniumdioxide particles and their photocatalytic activity for someprobe reactions in aqueous systems J. Phys. Chem. B106 10501–7
[2] Egerton T A and Tooley I R 2004 Effect of changes in TiO2
dispersion on its measured photocatalytic activity J. Phys.Chem. B 108 5066–72
[3] Noguchi T, Fujishima A, Sawunyama P and Hashimoto K 1998Photocatalytic degradation of gaseous formaldehyde usingTiO2 film Environ. Sci. Technol. 32 3831–3
[4] Karvinen S M 2003 The effects of trace element doping on theoptical properties and photocatalytic activity ofnanostructured titanium dioxide Ind. Eng. Chem. Res.42 1035–43
[5] Aarthi T and Madras G 2007 Photocatalytic degradation ofrhodamine dyes with nano-TiO2 Ind. Eng. Chem. Res.46 7–14
[6] Vinodgopal K, Wynkoop D E and Kamat P V 1996Environmental photochemistry on semiconductor surfaces:photosensitized degradation of a textile azo dye, acid orange7, on TiO2 particles using visible light Environ. Sci. Technol.30 1660–6
[7] Xu N, Shi Z, Fan Y, Dong J, Shi J and Hu M 1999 Effects ofparticle size of TiO2 on photocatalytic degradation ofmethylene blue in aqueous suspensions Ind. Eng. Chem. Res.38 373–9
[8] Wolfrum E J, Huang J, Blake D M, Maness P, Huang Z,Fiest J and Jacoby W A 2002 Photocatalytic oxidation of
Nanotechnology 19 (2008) 065711 N Nino-Martınez et al
bacteria, bacterial and fungal spores, and model biofilmcomponents to carbon dioxide on titanium dioxide-coatedsurfaces Environ. Sci. Technol. 36 3412–9
[9] Sunada K, Kikuchi Y, Hashimoto K and Fujishima A 1998Bactericidal and detoxification effects of TiO2 thin filmphotocatalysts Environ. Sci. Technol. 32 726–8
[10] Sunada K, Watanabe T and Hashimoto K 2003 Bactericidalactivity of copper-deposited TiO2 thin film under weak UVlight illumination Environ. Sci. Technol. 37 4785–9
[11] Jacoby W A, Maness P C, Wolfrum E J, Blake D M andFennell J A 2000 Mineralization of bacterial cell mass on aphotocatalytic surface in air Environ. Sci. Technol.32 2650–3
[12] Linkous C A, Carter G J, Locuson D B, Ouellette A J,Slattery D K and Smitha L A 2000 Photocatalytic inhibitionof algae growth using TiO2, WO3, and cocatalystmodifications Environ. Sci. Technol. 34 4754–8
[13] Fu G, Vary P S and Lin C 2005 Anatase TiO2 nanocompositesfor antimicrobial coatings J. Phys. Chem. B 109 8889–98
[14] Zhang A and Sun Y 2004 Photocatalytic killing effect of TiO2
nanoparticles on Ls-174-t human colon carcinoma cellsWorld J. Gastroenterol. 10 3191–3
[15] Thiel J, Pakstis L, Buzby S, Raffi M, Ni C, Pochan D J andIsmat Shah S 2007 Antibacterial properties of silver-dopedtitania Small 3 799–803
[16] Sugimoto T 2001 Monodispersed Particles (Amsterdam:Elsevier) pp 140–8
[17] Zanella R, Giorgio S, Henry C R and Louis C 2002 Alternativemethods for the preparation of gold nanoparticles supportedon TiO2 J. Phys. Chem. B 106 7634–42
[18] Pastoriza-Santos I, Gomez D, Perez-Juste J, Liz-Marzan L Mand Mulvaney P 2004 Optical properties of metalnanoparticle coated silica spheres: a simple effectivemedium approach Phys. Chem. Chem. Phys. 6 5056–60
[19] Tuval T and Gedanken A 2007 A microwave-assisted polyolmethod for the deposition of silver nanoparticles on silicaspheres Nanotechnology 18 255601
[20] Singh H K, Muneer M and Bahnemann D 2003 Photocatalyseddegradation of a herbicide derivative, bromacil, in aqueoussuspensions of titanium dioxide Photochem. Photobiol. Sci.2 151–6
[21] Bowering N, Croston D, Harrison P G and Walker G S 2007Silver modified Degussa P25 for the photocatalytic removalof nitric oxide Int. J. Photoenergy 2007 1–8
[22] Wagner C D, Riggs W, Davis L and Moulder J 1979 Handbookof X-Ray Photoelectron Spectroscopy ed G E Muilenberg(Minnesota: Perkin Elmer Corporation)
[23] Martınez-Castanon G A, Martınez-Mendoza J R,Ortega-Zarzosa G, Sanchez-Loredo M G and Ruiz F 2005Optical absorption of Ag particles dispersed in a SiO2
amorphous matrix J. Sol.–Gel Sci. Technol. 36 136–45
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