Journal of Colloid and Interface Science311(2007)
514–522
https://www.doczj.com/doc/f116464526.html,/locate/jcis
Synthesis,characterization,and visible light activity of new nanoparticle photocatalysts based on silver,carbon,and sulfur-doped TiO2
Dambar B.Hamal,Kenneth J.Klabunde?
Department of Chemistry,Kansas State University,KS66506,USA
Received27October2006;accepted2March2007
Available online12March2007
Abstract
New nanoparticle photocatalysts based on silver,carbon,and sulfur-doped TiO2having only the homogeneous anatase crystalline phase and high surface area were successfully synthesized by a modi?ed sol–gel route.The catalysts were characterized by EDX,XRD,BET,UV–vis, IR,and Raman spectroscopy.The effects of the experimental parameters on the visible light reactivity of the catalysts were evaluated for the photodegradation of gaseous acetaldehyde as a model indoor pollutant.The activity results show that the silver(I)ion,Ag+,doping signi?cantly promotes the visible light reactivities of carbon and sulfur-doped TiO2catalysts without any phase transformation from anatase to rutile.Moreover, Ag/(C,S)–TiO2photocatalysts degrade acetaldehyde10times faster in visible light and3times faster in UV light illuminations than the accred-ited photocatalyst P25–TiO2.The commendable visible photoactivities of Ag/(C,S)–TiO2new nanoparticle photocatalysts are predominantly attributable to an improvement in anatase crystallinity,high surface area,low band gap and nature of precursor materials used.
?2007Elsevier Inc.All rights reserved.
Keywords:Ag+-doped TiO2;Visible light activity;Titania;Doping;C-doped TiO2;Nanoparticles;Acetaldehyde;Photocatalysts;S-doped TiO2
1.Introduction
The phenomenon of photocatalysis is de?ned as the com-bination of photochemistry and catalysis.More precisely,the term“photocatalysis”herein implies that the light and a cat-alyst are essential to enhance the rates of thermodynamically favored but kinetically slow photophysical and photochemical transformations.Indeed,photocatalysis emerged as a new sci-enti?c area when Fujishima and Honda carried out photolysis of water into environmentally clean fuels(hydrogen and oxy-gen)using a titanium dioxide electrode in an electrochemical cell[1].Ever since,heterogeneous photocatalysis by means of TiO2has been widely accepted and exploited as an ef?cient technology for killing bacteria and degrading organic and in-organic pollutants[2–13].Moreover,titanium dioxide(TiO2) has been regarded as an excellent semiconductor photocata-lyst because of its performance,low cost,nontoxicity,stability, and availability.Unfortunately,because of its wide band gap *Corresponding author.Fax:+17855326666.
E-mail address:kenjk@https://www.doczj.com/doc/f116464526.html,(K.J.Klabunde).(anatase:3.2eV;rutile:3.0eV),the extensive exploitation of TiO2created an expectation to use merely3–4%UV light of the whole radiant solar energy.
Fortunately,there are,however,a number of empirical ways to design and develop a second generation of visible-light-sensitive photocatalysts of titanium dioxide by means of phys-ical and chemical processes[14,15].Of the various processes cited in the literature,ion implantation methods require more expensive and more sophisticated equipment,whereas chemi-cal methods are more economical and can be conducted at or near ambient temperatures.In context,a great deal of effort has shown that doping with transition metals,such as Cr,Co, V,and Fe,extends the spectral response of TiO2well into the visible region and enhances the photoreactivity[16–19].How-ever,transition metal ion-doped TiO2suffers from some seri-ous drawbacks,such as thermal instability and low quantum ef?ciency of the photoinduced charge carriers(electron–hole pairs)[20].Besides metal ion-doped TiO2systems,there are numerous recent reports on nonmetal-doped TiO2,for exam-ple,carbon,nitrogen,phosphorus,sulfur and?uorine doped photocatalysts[21–33].Basically,doping with carbon,nitrogen
0021-9797/$–see front matter?2007Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2007.03.001
D.B.Hamal,K.J.Klabunde/Journal of Colloid and Interface Science311(2007)514–522515
and sulfur effectively narrows the band gap of TiO2(<3.0eV) [34–36].Moreover,band gap narrowing emanates from the electronic perturbations caused by change of the lattice para-meters and/or the presence of the trap states within conduc-tion and valence bands of TiO2.Consequently,the photons of lower energy(λ>420nm)can induce electron–hole pairs in TiO2.These photoinduced electrons and holes,which are in fact very powerful reducing and oxidizing agents,migrate to the surface of TiO2and eventually become available for di-rect or indirect consecutive reduction and oxidation reactions. Furthermore,because of the presence some trap states within the band gap of titanium dioxide,the lifetime of the so-called photoinduced charge-carriers increases in such a way that it pre-dominates over the fast charge-recombination process,thereby resulting in an enhanced visible light reactivity.
Apart from doping TiO2with a single metal or nonmetal, it is highly anticipated that doping TiO2with an appropriate combination of metals and/or nonmetals would,of course,re-sult in more visible light sensitive photocatalysts for a desired application.In this context,Li et al.synthesized N–F-codoped TiO2photocatalysts by spray pyrolysis(SP)using TiCl3and NH4F precursors and observed an enhanced photoreactivity of the materials in visible light[37].Luo et al.prepared a Br and Cl-codoped TiO2system and demonstrated the ef?ciency of the material for photocatalytic splitting of water into H2and O2 in the presence of Pt co-catalyst and UV light irradiation[38]. These recent efforts and strategies have revealed that codoping TiO2with a metal and a nonmetal can result in the development of additional visible active photocatalysts[39–41].
It is well known that noble metals such as Ag,Au,and Pt possess unique electronic and catalytic properties.For example, Li reported that Au3+-doped TiO2exhibited visible light reac-tivity for the photodegradation of methylene blue[42].Like-wise,Kim et al.prepared Pt ion-doped TiO2,and examined its visible light activity for the photodegradation of chlorinated or-ganic compounds[43].From an economic viewpoint,gold and platinum are very expensive and unaffordable metals for ex-tensive use in https://www.doczj.com/doc/f116464526.html,pared to gold and platinum, silver is a more affordable metal and deserves further investi-gation.Munevver et al.reported a Ag–TiO2system for killing E.coli under UV light illumination[44].There have been some reports on the preparation of Ag+-doped TiO2?lms and nanoparticles that degrade some textile dyes(methyl orange, crystal violet,and methyl red)in aqueous medium[45–47]. Furthermore,silver halides,such as AgBr/SiO2and AgCl cata-lysts,have also been used in photocatalysis[48,49].It appears that doping with Ag+ions makes a dramatic improvement in the performance of TiO2photocatalysts;however,the afore-mentioned silver based photocatalysts function only under UV light.Earlier,researchers in our group have reported the synthe-sis and characterization of some nanocrystalline metal oxides and mixed metal oxides.It was discovered that materials in the nano-size domain exhibit a unique surface chemical reactivity for the destructive absorption of acid gas and chemical war-fare agents[50,51].Moreover,this unique surface reactivity of nanomaterials was attributed to the high surface areas and the presence of defects,edges,and corners.
Therefore,it is rationally more desirable to synthesize new nanoparticle visible-light-driven photocatalysts based on silver, carbon and/or sulfur-doped TiO2.To accomplish the desired work,a quest for a suitable precursor material seems indispens-able for carbon and sulfur dopants.On this line of research,Park et al.prepared carbon-doped TiO2nanotube arrays at an ele-vated temperature range500–800?C by using carbon monoxide precursor,and tangibly demonstrated the catalytic ef?ciency of the C-doped TiO2nanotubes for water splitting under visible light irradiation[52].Further,Choi et al.fabricated C-doped TiO2photocatalysts by oxidative annealing of TiC and ended up with the conclusion that C-doped TiO2powder exhibited su-perior photoactivity for the photodecomposition of methylene blue and water under UV light[53].Thus,carbon monoxide can be used,but it is rather dangerous and is not a suitable precur-sor for carbon dopant on a large-scale synthesis.Also,oxidative annealing of TiC requires hundreds of hours or progressively very high temperatures(600–750?C)for optimal activity of the materials and such a high temperature synthesis results in materials inclusive of anatase–rutile phases and lower surface areas,thereby making them less worthwhile for photocatalysis. We thus realize,in essence,the need for a more suitable pre-cursor for carbon and sulfur dopants other than TiC,CO,and NH2CSNH2.Herein,we express a strong preference for car-bon and sulfur as nonmetal dopants,because these elements can stabilize Ag+ions in doped TiO2systems.Moreover,the well-dispersed Ag+ions trap photoinduced electrons,leading to a substantial increase in electron–hole separation and a con-comitant decrease in charge-carrier recombination.
To the best of our knowledge,there has been no publication on the synthesis of a silver,carbon and sulfur-doped TiO2sys-tem and its photocatalytic performance in visible light.In this article,we report Ag/(C,S)-doped TiO2photocatalysts and the structural and photocatalytic properties of these new nanoparti-cle photocatalysts.The photoreactivity of the synthesized mate-rials was evaluated for the degradation of gaseous acetaldehyde
(a major indoor pollutant)under UV and visible light.
2.Experimental
2.1.Materials required
Ethanol(Absolute,200Proof,Aaper Alcohol and Chemical Co.),titanium(IV)isopropoxide(97%,Sigma–Aldrich),am-monium thiocyanate(97.5%,Alfa Aesar),thiourea(99%,Alfa Aesar),silver nitrate(99.9+%,Alfa Aesar),and ammonium hy-droxide(29.9%,Fisher)were used as received without further puri?cation.
2.2.Catalyst synthesis
Herein,we chose two different nonmetal precursors,ammo-nium thiocyanate(hereafter the samples are designated as–01) and thiourea(hereafter the samples are designated as–02)to elucidate the effect of precursor materials on the photoreactiv-ity of the doped TiO2.
516 D.B.Hamal,K.J.Klabunde/Journal of Colloid and Interface Science311(2007)514–522
Table1
EDX measurement of the various samples annealed at500?C/2h in air Samples a,b Ag(atm%)C(atm%)S(atm%) (C,S)–TiO2–01–500?C0.0 5.5 1.7
S–TiO2–02–500?C0.00.0 1.6
Ag/(C,S)–TiO2–01–500?C 1.07.70.4
Ag/(C,S)–TiO2–02–500?C 1.0 5.8 1.5
a01is designation for ammonium thiocyanate precursor.
b02is designation for thiourea precursor.
In a typical procedure,0.031mol of titanium(IV)isopropox-ide and0.124mol of ammonium thiocyanate or thiourea were dissolved200ml ethanol under vigorous stirring followed by drop-wise addition of0.125mol of de-ionized water containing 0.62ml of AgNO3(0.5M)and1ml of NH4OH for complete hydrolysis.After stirring for?ve minutes at room tempera-ture,the solvent was completely evaporated in a rotavapor.The as-synthesized samples(11.5–12g)were kept over night in a drying cabinet,and calcined at500?C for two hours in air at a heating rate of5?C/min using a Chamber Furnace(Carbolite, CWF-1100).The resulting yellow product was ground well into ?ne particles.For comparison,the same experimental proce-dure was used for the synthesis of only nonmetal-doped TiO2.
2.3.Catalyst characterization
A Scanning Electron Microscope-S-3500N and Absorbed Electron Detector-S-6542(Hitachi Science Systems,Ltd.), EDXA(Inca Energy,Oxford Instruments Microanalysis,Ltd.) were used to determine the surface composition of the sam-ples under the speci?ed conditions of20keV,15mm working distance,and4000×magni?cation.X-ray diffraction(XRD) patterns of samples were recorded on a Scintag XDS2000D8 diffractometer with Cu Kαradiation of wavelength of1.5406?and were analyzed from15?to75?(2θ)with a step size of 0.05?and step time of3s to assess the crystallinity of the cat-alysts under study.Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature(77K)on a Quan-tachrome Instrument(NOV A1000Series)and the speci?c sur-face areas were determined by the Brunauer–Emmett–Teller (BET)method.The Barrett–Joyner–Halenda(BJH)method was used to determine the pore size distributions derived from the BJH desorption isotherms.IR spectra were recorded on a Nicolet NEXUS670FTIR instrument to detect the presence of carbonate and sulfate by pelletizing with KBr as a refer-ence.Raman spectra were measured to assess the anatase crys-tallinity.The UV–vis absorption spectra were recorded from 200to800nm on a Cary500Scan UV–vis NIR Spectrometer with an integrating sphere attachment using PTFE powder as
a reference.
2.4.Kinetics of photocatalysis
Kinetic study of the photocatalysts for the photodegradation of gaseous acetaldehyde was performed in a305ml cylindrical air-?lled static glass reactor with a quartz window at room tem-perature.103mg of the catalyst was placed in a circular glass dish and mounted into the reactor.100μl of liquid acetaldehyde was added into the reactor,which vaporized,and the gaseous mixture of acetaldehyde and air was constantly stirred.Prior to light illumination,the concentration of the probe molecule was allowed to equilibrate for40min,and35μl of gaseous aliquot from the reactor was periodically extracted and analyzed on a GC-MS port(Shimadzu GCMS-QP5000).The temperatures of the column,injector and detector were maintained at40,200, and280?C,respectively.For the visible light experiment,the sample was illuminated with a1000W high-pressure mercury lamp(Oriel Corp.)at a distance of20cm from the top using combined?lters,one VIS-NIR long pass?lter(400nm)and another colored glass?lter(>420nm).Exactly,the same pro-cedure was followed for the UV light experiment by using two cut-off?lters(320<λ<420nm).
3.Results and discussion
3.1.Characterization
Table1shows the EDXA measurements of the doped TiO2 samples annealed at500?C/2h in air.It is worthwhile to note that both carbon(5.5atm%)and sulfur(1.7atm%)were si-multaneously incorporated into TiO2from NH4SCN precursor, whereas only sulfur(1.6atm%)could be incorporated into TiO2 from NH2CSNH2.Besides carbon and sulfur,no nitrogen and hydrogen were detected in energy dispersive X-ray analysis spectrum(EDXA).
The simultaneous incorporation of carbon and sulfur from the NH4SCN precursor is explained as follows.Ammonium thiocyanate,on the one hand,is an ionic compound and it is highly soluble in ethanol and,on the other hand,thiourea is an organic compound and is comparatively less soluble in the same volume of ethanol.Because of its ionic nature and the high solubility in the chosen solvent,NH4SCN forms elec-trically charged NH+
4
and SCN?ions,whereas NH2CSNH2 does not form electrically charged ions.For the time being,in case of NH4SCN,we can surmise that some of the NH+4and SCN?ions might have participated in forming a stable complex with Ti4+during solvent evaporation.This assumption might be correlated with the appearance of the yellow color of the as-synthesized sample after the complete evaporation of the sol-vent and its disappearance when dissolved into solvent again. On this basis,a portion of the yellow colored complex thus formed is formulated as[NH4][Ti(SCN)4·OH·H2O][54].Now, it is likely that during the calcination process,the combustion of the thiocyanate complex of Ti(IV)might,in part,lead to the substitution of carbon and sulfur for oxygen sites into TiO2.In contrast,because of a weaker ligand property of NH2CSNH2, the thiourea complex of Ti(IV)being very unstable undergoes a rapid combustion and decomposition,thereby resulting in only substitution of sulfur for oxygen sites into TiO2.Meanwhile, doping of TiO2with Ag+ions boosted the amount of the doped carbon along with a little amount of the doped sulfur from both the precursor materials.This is attributable to the greater af?n-ity of Ag+ions to exist in the forms of carbonate than sulfate. Moreover,we do believe that the nature and amounts of the
D.B.Hamal,K.J.Klabunde/Journal of Colloid and Interface Science311(2007)514–522
517
Fig.1.XRD pro?les of(a)C and/or S-doped TiO2and(b)Ag/(C,S)–TiO2–01.
Table2
Surface area and crystallite size of the catalysts
Catalysts samples BET surface area
(m2/g)
XRD crystallize size
(nm)
Degussa P25–TiO25123
(C,S)–TiO2–01–500?C758.5
S–TiO2–02–500?C677.0
Ag/(C,S)–TiO2–01–500?C865.3
Ag/(C,S)–TiO2–02–500?C716.8
Ag/(C,S)–TiO2–01–600?C3617
doped nonmetals entirely depend upon the source of the non-
metal precursors as well as the metal ions.
Fig.1a illustrates the X-ray diffraction(XRD)pro?les of
carbon and/or sulfur-doped TiO2(500?C/2h).Compared to
Degussa P25,which contains anatase(2θ=25.25?)and rutile
(2θ=27.5?)phases,the XRD patterns of the nonmetal-doped
TiO2samples revealed anatase as the predominant homoge-
neous crystalline phase(2θ=25.25?).Fig.1b illustrates the
XRD patterns of Ag/(C,S)–TiO2–01sample annealed progres-
sively at400,500,and600?C.Again,the analysis of the ob-
served peaks corroborated the homogeneous anatase crystalline
phase.Now,it is reasonable to infer that the doping with silver,
carbon and sulfur prevents the phase transformation of TiO2up
to600?C,whereas the polycrystalline TiO2prepared by a sol–
gel technique undergoes phase transformation above500?C
[55,56].Moreover,NH4SCN and Ag+ions precursor materi-
als promote the formation of the anatase in preference to rutile.
Further,no noticeable peaks of silver carbonate and silver metal
were observed in the X-ray diffractograms.
Table2shows the BET surface area and average crystallite
size of the various catalysts compared to Degussa P25.Ob-
viously,the speci?c surface areas of the doped TiO2samples
are higher than that of the undoped P-25.The average crystal-
lite sizes of the doped samples were determined by analyzing
the most intense(101)XRD peaks and using the well-known
Scherrer’s equation,t(size)=0.9λ/βcosθ[1],whereλis the
wavelength of the X-ray(nm),andβis the full width(radi-
ans)at half maximum of the signal.Up to500?C,the doped
samples are nanoparticles of average crystallite size of5–9nm,
whereas at600?C,the average crystallite size of the samples
was17nm.The BJH desorption isotherm shows that the doped
TiO2samples have microporous structures with an average pore
diameter of2–2.5nm.Of the various preparation parameters,
the calcination temperature has a profound effect on the BET
surface areas.It is understandable that the higher the temper-
ature and the longer the calcination time,the lower the BET
surface area because of sintering of the smaller particles into
the bigger particles.That is why Ag/(C,S)–TiO2–01catalysts
at600?C/2h have the least surface area(36m2/g).On the
other hand,(C,S)–TiO2–01samples(with and without Ag+)
calcined at500?C/2h have a little bit higher surface areas
than S–TiO2–02samples.This suggests that the ammonium
thiocyanate precursor prevents the sintering of particles more
effectively than does thiourea during the calcination step.For
the moment,we assume that this might be due to the differences
in reducing properties,doping modes,and heats of reactions of
ammonium thiocyanate and thiourea.
The IR measurement of the Ag/(C,S)–TiO2–01sample an-
nealed at500?C/2h shows the appearance of bands at1236,
1143,1038,761,and493cm?1.Slager et al.carried out the FT-
IR study of a pure silver(I)carbonate and observed four bands
at1410,1020,880,and690cm?1for Ag2CO3and only one
band at535cm?1for Ag2O[57].They also investigated that
the reaction of silver(I)carbonate with water vapor resulted in
the formation of basic silver carbonate(AgOHAg2CO3)hav-
ing two characteristic bands at1460and1480cm?https://www.doczj.com/doc/f116464526.html,-
pared to Slager’s results,the infrared spectrum of the Ag/
(C,S)–TiO2–01catalyst revealed that the two weak peaks ap-
pearing at1236,1038cm?1and one shoulder peak at761cm?1
could be from the presence of doped carbonate species.The
other infrared bands at1038and493cm?1are assigned to the
presence of characteristic bands of sulfate(ν3)and Ti–O–Ti–O
species.This implies that the carbon and sulfur doped into TiO2
are present,to some extent,in the form of carbonate and sul-
fate respectively.However,for future work,a meticulous FTIR
518 D.B.Hamal,K.J.Klabunde /Journal of Colloid and Interface Science 311(2007)514–522
study of the carbon and sulfur-doped TiO 2samples seems in-dispensable for a comprehensive understanding and a precise explanation of doping mechanisms.
Additionally,of the several spectroscopic techniques used to characterize TiO 2,Raman spectroscopy has been employed,because this technique provides a rapid way of understanding the doping mechanisms and obtaining the surface crystal mor-phologies of the TiO 2nanoparticles [58–60].Fig.2presents the Raman spectra of (C,S)–TiO 2–01and Ag/(C,S)–TiO 2–01catalysts after heat treatment at 500?C /2h.For comparison,the Raman spectrum of Degussa P25is also shown in the same ?gure.Obviously,the observed Raman peaks of the doped and undoped TiO 2samples match pretty well with each https://www.doczj.com/doc/f116464526.html,-pared to P25–TiO 2,a slight shift of some peak values is caused by the smaller crystallite sizes of the doped samples.Never-theless,these results con?rmed codoping of TiO 2with carbon,silver and sulfur.
UV–vis diffuse re?ectance spectroscopy directly provides some insight into the interactions of the photocatalyst materi-als with photon energies.Therefore,it is absolutely
imperative
Fig.2.Raman pro?les of (a)(C,S)–TiO 2–01and (b)Ag/(C,S)–TiO 2–01cata-lysts annealed at 500?C /2h in air.(The precursors for Ti,Ag,and C and S are Ti(ipr)4,AgNO 3,and NH 4SCN,respectively.)
to include this technique to examine for the visible light reac-tivity of synthesized photocatalysts.Fig.3a demonstrates the UV–vis absorption pro?les of carbon and/or sulfur-doped ti-tanium dioxide (calcined at 500?C /2h in air)compared to P25–TiO 2.Here,we perceived a noticeable shift of the optical absorption edges of the doped TiO 2systems toward the visi-ble regions of the solar spectrum.Notably,this shift towards the longer wavelengths originates from the band gap narrow-ing of titanium dioxide by carbon and/or sulfur doping [34,36]and the band gap energy of the doped samples was determined from the equation,Eg =1239.8/λ[2],where λis the wave-length (nm)of the exciting light [61].The band gap energies of the (C,S)–TiO 2–01,S–TiO 2–02,Ag/(C,S)–TiO 2–01,and Ag/(C,S)–TiO 2–02samples were found to be 2.77,2.79,2.75,and 2.76eV ,respectively.Because the doped samples have lower band gap energies than the undoped TiO 2(3.00–3.2eV),these photocatalysts are,therefore,likely to operate under visible light illumination.Again,comparing the band gap energy,we notice that only carbon and sulfur effectively contribute to the band gap narrowing of TiO 2.Despite somewhat lower band gap values of the silver,carbon,and sulfur-doped system,it clearly shows that doping of Ag +ions has,at most,a small contribution to the band gap reduction.Furthermore,doping TiO 2with car-bon and/or sulfur introduces some trap states (impurity levels)within the valence and conduction bands of titanium dioxide.Consequently,we observed a shoulder peaks in the UV–vis ab-sorption pro?les of the nonmetal-doped TiO 2samples.Fig.3b represents the profound effect of the calcination temperature in UV–vis absorption spectra of the Ag/(C,S)–TiO 2–01cata-lysts.
Compared to standard P25,Ag/(C,S)–TiO 2–01samples re-tain a substantial amount of the visible light absorption (λ>400nm)in the temperature range 400–600?C.However,the ability of these samples to absorb visible light effectively de-creases as the annealing temperature progressively increases.Therefore,Ag/(C,S)–TiO 2sample calcined at 600?C /2h shows the least visible light absorption compared to the other samples annealed below 600?C.In contrast,the sample
heat-
Fig.3.UV–visible absorption pro?les of (a)C and/or S-doped TiO 2,and (b)Ag/(C,S)–TiO 2–01catalysts at various temperatures compared to P25–TiO 2.
D.B.Hamal,K.J.Klabunde /Journal of Colloid and Interface Science 311(2007)514–522
519
Fig.4.Degradation of gaseous acetaldehyde on Ag/(C,S)–TiO 2–01catalyst (annealed at 500?C /2h in air)under (a)visible and (b)UV light.
treated at 400?C /2h showed a greatest shift of absorption edge toward the visible region and should be the most active catalyst in visible light.In fact,this could be due to the presence of some residual organic impurities left in doped TiO 2.On the other hand,the magnitude of the UV–vis absorption intensity depends on various factors such as crystal structures,band gap energy,thickness,and the presence and absence of defects and foreign elements in concerned semiconducting materials.Therefore we noticed that a difference between the UV–vis absorption inten-sities of the doped samples and P-25(Figs.3a and 3b )in the wavelength region less than 400nm.Herein,we assumed that the smaller UV–vis absorption intensity of the doped samples could be ascribed to the smaller band gap excitation energies (less than 3.0eV)whereas the higher the absorption intensity of the undoped P-25was attributed to a higher band gap excita-tion energy (3.0–3.2eV).3.2.Kinetics of photocatalysis
In order to assess the photocatalytic activity of the new syn-thesized materials,the photodegradation of gaseous acetalde-hyde by arti?cial light was performed at room temperature (298K).Figs.4a and 4b depict the photomineralization of the ubiquitous air pollutant acetaldehyde on Ag/(C,S)–TiO 2–01under visible light (λ>420nm)and UV light (320<λ<420nm)illuminations,respectively.From the concentration pro?les,it is now clearly seen that there was a noticeable con-sumption of acetaldehyde and a subsequent production of car-bon dioxide under UV and visible light irradiations.Both from the qualitative and quantitative viewpoints,the probe molecules degrade faster under UV light than under visible light illumina-tion.This is,indeed,because of the fact that the UV photons are more energetic and more penetrating (can pass through deeply),and produce more photoexcited electrons and holes for the sur-face catalysis from the doped TiO 2than the visible photons.On the other hand,no dark reaction was observed for the systems under study,indicating that the reaction was basically photo-catalytic in nature.Besides the photon energies,several other experimental parameters,especially the annealing temperature
and dopant concentration,could affect the photocatalytic per-formance of the materials.
Fig.5a shows the profound effect of annealing temperature on the production of carbon dioxide from the photodegradation of gaseous acetaldehyde under visible light irradiation.Based on the amount of CO 2produced,it is inferred that the visible light reactivity of Ag/(C,S)–TiO 2–01catalyst is enhanced as the annealing temperature increases from 400to 500?C,and thereafter the photocatalytic activity decreases further while ris-ing the temperature above 500?C.The optimal visible light activity of the sample at 500?C attributes to an improvement in crystallinity of the homogeneous anatase phase,whereas the lowest photoactivity at 600?C was because of the simultane-ous effects of the low surface area and the removal of some dopants from the doped catalysts.Fig.5b illustrates a simi-lar effect of Ag +ion concentration on the production of CO 2from CH 3CHO on Ag/(C,S)–TiO 2–01under visible light illu-mination.The concentration of silver ion was varied from 0.5to 10atm%.The optimal visible light reactivity of the catalyst was achieved for 1atm%of Ag +ion.This means that 1mol%Ag +ion concentration effectively suppresses the recombina-tion of the photogenerated charge-carriers on the surface of the catalyst in order that a large number of reactant molecules are absorbed and undergo subsequent oxidation and reduction re-actions.Unfortunately,an increase in Ag +ion concentration from 2to 10atm%has a deleterious effect on the photoactiv-ity of the catalysts.Conceivably,this happens because of the creation of recombination centers of charge-carriers at a higher loading of dopant concentration.Fig.5c demonstrates a simi-lar effect of SCN ?ion concentration on the production of CO 2from acetaldehyde on Ag/(C,S)–TiO 2–01catalyst.Here,we observed a marginal effect of SCN ?ion concentration.The optimum visible light reactivity of the sample was observed for 1:4mol ratio of Ti:SCN and samples containing the other mole ratios of Ti:SCN showed a little lower reactivity.Based on EDX analysis,carbon and sulfur have been introduced into TiO 2lattices from the NH 4SCN precursor.Of course,as the concentration of ammonium thiocyanate increases,the amount of carbon and sulfur contents in doped TiO 2also increases ac-
520 D.B.Hamal,K.J.Klabunde /Journal of Colloid and Interface Science 311(2007)
514–522
Fig.5.Effect of (a)calcination temperature,(b)Ag +ion and (c)SCN ?ion on CO 2production from CH 3CHO on Ag/(C,S)–TiO 2–01catalyst under visible
light.
Fig.6.Production of CO 2from CH 3CHO on various catalysts under (a)visible and (b)UV light.
cordingly,thereby accelerating the rate of the recombination of photoinduced charge-carriers in the framework of titania.
Figs.6a and 6b compare the amount of carbon dioxide pro-duced from the photomineralization of gaseous acetaldehyde on various catalysts under visible and UV light,respectively.Upon visible light illumination,it turns out that both Ag/(C,S)–TiO 2–01and Ag/(C,S)–TiO 2–02catalysts exhibit similar but higher photoactivity than P25,(C,S)–TiO 2–01and S–TiO 2–02in re-gard to the evolution of CO 2from CH 3CHO.In contrast,De-gussa P25showed the highest photocatalytic performance per-taining to the production of CO 2under the UV light,compared to the doped TiO 2.Speci?cally,P25–TiO 2is an accredited pho-
D.B.Hamal,K.J.Klabunde/Journal of Colloid and Interface Science311(2007)514–522521
Table3
Initial rates(mM/min)of production of CO2and degradation of CH3CHO on various catalysts under visible and UV light
Catalysts(CO2)vis(CH3CHO)vis(CO2)UV(CH3CHO)UV P25(TiO2)0.00020.0090.010.05
Ag/(C,S)–TiO2–010.0020.1100.0080.16
Ag/(C,S)–TiO2–020.0020.0600.0080.11
(C,S)–TiO2–010.00070.0530.0090.20
S–TiO2–020.00050.0460.0070.12 tocatalyst and it possesses a very good photoactivity.Therefore, it seems that the production of carbon dioxide from the pho-todegradation of acetaldehyde was more favored.Empirically, we have found that the photodegradation of acetaldehyde not only involved the formation of carbon dioxide but also some other minor products such as acetic and formic acids,which were below the detection limit to be quanti?ed.Therefore,for simplicity,the amounts of CH3CHO and CO2expressed in mM have been shown in Figs.4a and4b whereas only the amount of carbon dioxide produced in mM was shown in other?gures (Figs.5a–6b).
However,in photocatalysis,it would be more worthwhile to determine and compare the initial rates of the degradation of acetaldehyde and the subsequent production of carbon diox-ide(Table3).On comparing the initial rates,it is clearly seen that Ag/(C,S)–TiO2–01catalyst prepared by using NH4SCN as a nonmetal precursor degrades gaseous acetaldehyde10and 3times faster in visible light and UV light respectively than P25.Still,other doped TiO2catalysts have higher initial rates of degradation of CH3CHO and production of CO2than P25 both under UV and visible light.
The origin of the superior photoactivity of the doped TiO2 catalysts might be explained as follows.Herein,regardless of the magnetic and thermal properties,we consider that all pho-tocatalyst materials are assumed to being comprised of mainly two interlinked parts:the photopart and the catalysis part[62]. The photopart is intrinsically concerned with the interaction of the semiconducting materials with light,for example,ab-sorption of light,band gap,formation of electrons–holes,their dynamics and surface trapping,whereas the catalysis part in-cludes surface area,surface reactivity,radical formation and the heterogeneous interaction of the chemical species with catalyst surface.More importantly,the better the crystalline quality and the lower the band gap,the more improvement on the photopart leading to the higher visible light reactivity of the catalysts.On the other hand,the higher the surface area,the higher the reac-tivity.Fundamentally,photocatalysis is an interfacial reaction and a higher surface area of the material,of course,produces a greater number of accessible active sites,thereby yielding an enhanced reactivity.Moreover,the photocatalytic activity of any semiconductor photocatalyst is,in fact,the result of a com-promise and combination of these two structural parameters.In the case of pure TiO2,the crystalline anatase is the most active catalyst compared to rutile and brookite phases.Keeping this principle in mind,the doped TiO2catalysts under present study have high surface areas,low band gaps and only the anatase crystalline phases as con?rmed by the BET,UV–vis,Raman,and XRD measurement,respectively.As a result,these photo-catalysts exhibit better reactivity than P25–TiO2for the degra-dation of gaseous acetaldehyde under UV and visible light. Meanwhile,on comparing activity of the doped TiO2systems, it unequivocally shows that the Ag/(C,S)–TiO2–01catalyst ex-hibits the highest photoactivity because of its highest surface area(86m2/g)and lowest band gap(2.75eV).Nonetheless, the superior photoactivity of Ag/(C,S)–TiO2catalysts is also attributed to good dispersion of Ag+ions,synergistic effects of dopants,nature and source of precursor materials.However,it is interesting to note that doping TiO2only with Ag+ions by using AgNO3or Ag2O results in materials of almost no visible activity.
3.3.Mechanism for enhancing effect of silver dopant in photocatalysis
For a better understanding,we can propose a mechanism for the enhancing effect of silver ion dopant in photocatalysis.The chemical state of the Ag+ion in doped TiO2was was deter-mined by measuring the XPS of the best Ag/(C,S)–TiO2–01 catalyst before and after UV and visible light reactions.The XPS pro?le of the Ag/(C,S)–TiO2–01sample(not shown here) showed the presence of both Ag0and Ag+species before and after reaction.Before the light reaction,the presence of metallic silver particles was likely due to the thermal reduction of silver ions during calcination step at500?C/2h[63,64].In general, Ag+ions were easily reduced to Ag0particles by the photoex-cited conduction band electrons(e?CB)of Ag/(C,S)–TiO2–01 catalyst thereby resulting in the formation of more Ag0parti-cles.But after UV and visible light reactions,the XPS spectra of the Ag/(C,S)–TiO2–01sample remain almost the same.This implied that during photocatalytic reaction the reduced Ag0 particles were oxidized back to Ag+ions by the valence band holes(h+VB).Thus we can say that the presence of both Ag0and Ag+species facilitate the charge separation(electron–hole)and suppress the recombination of the photoexcited charge carriers thereby enhancing the catalytic property of the material.Based on these facts,the following mechanism was proposed to show a real photocatalytic activity of the Ag/(C,S)–TiO2–01catalyst for acetaldehyde photodegradation in gas phase.
(1) TiO2+hv→e?(CB)+h+(VB),
(2) Ag0+h+(VB)→Ag+,
(3) Ag++e?(CB)→Ag0.
4.Conclusions
For the?rst time,we report the synthesis and characteriza-tion of highly active(in visible light)new nanoparticle photo-catalysts based on silver,carbon and sulfur-doped TiO2.XRD and the BET measurements corroborate that these doped mate-rials are made up of the homogeneous anatase crystalline phase and have high surface areas.Furthermore,the UV–vis absorp-tion spectra substantiate the band gap tapering of TiO2by the doped carbon and/or sulfur along with a very little contribution
522 D.B.Hamal,K.J.Klabunde/Journal of Colloid and Interface Science311(2007)514–522
from the doped Ag+ions.Notably,the well-dispersed Ag+ions in(C,S)-doped TiO2signi?cantly promote the electron–hole separation and subsequently enhance the photoactivity.More-over,Ag/(C,S)–TiO2new nanoparticle photocatalysts degrade the gaseous acetaldehyde10and3times faster than P25–TiO2 under visible and UV light,https://www.doczj.com/doc/f116464526.html,pared to P25–TiO2,the commendable visible light activity of Ag/(C,S)–TiO2 nanoparticle photocatalysts is predominantly attributed to an improvement in anatase crystallinity,high surface area,low band gap and effect of precursor materials.Herein,we also re-port on codoping TiO2with carbon and sulfur from a single nonmetal precursor(NH4SCN)and this result is important for future photocatalyst preparations.
Acknowledgments
We acknowledge the Army Research Of?ce and National Science Foundation for partial support for the current work. For the XPS measurement,we are also thankful to Dr.Hohn, Keith L.and Chundi,Department of Chemical Engineering, Kansas State University.
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