Enhanced photocatalytic activity of C, F-codoped TiO2 loaded with AgCl

Enhanced photocatalytic activity of C,F-codoped TiO 2loaded with AgCl

Hongjian Yan a ,Saji Thomas Kochuveedu a ,Li Na Quan a ,Sang Soo Lee b ,Dong Ha Kim a ,?

a Department of Chemistry and Nano Science,College of Natural Sciences,Ewha Womans University,52,Ewhayeodae-gil,Seodaemun-gu,Seoul 120-750,Republic of Korea b

Polymer Hybrid Research Center,Korea Institute of Science and Technology,P.O.Box 131,Cheongryang,Seoul 130-650,Republic of Korea

a r t i c l e i n f o Article history:

Received 5November 2012

Received in revised form 20January 2013Accepted 21January 2013

Available online 13February 2013Keywords:

Co-doped TiO 2C,F-doping Photocatalysis

Visible light activity AgCl

a b s t r a c t

A protocol is reported for the development of a unique type of a co-doped TiO 2with two kinds of non-metal atoms,carbon (C)and ?uorine (F),as a visible-light active photocatalyst system.C,F-codoped TiO 2nanoobjects with anatase phase were synthesized by calcination of a mixture of TiOF 2and sulfuric-acid-treated melamine.The C,F co-doping resulted in a red-shift of the absorption edge to 510nm from the 390-nm value obtained for a control TiO 2photocatalyst.The C,F-codoped TiO 2shows photocatalytic activity under visible light irradiation.Furthermore,the loading of AgCl into the parent co-doped TiO 2enhanced its activity for degradation of methylene blue dye by about 3.5times in the ?rst hour irradiation.A brief mechanism responsible for the enhancement is proposed in terms of the transfer of holes from C,F-codoped TiO 2to the AgCl phase.

1.Introduction

The photocatalytic puri?cation and treatment of water and air using semiconductor nanostructures and sunlight is an effective and promising technology for low-cost environmental remedia-tion.Titanium dioxide (TiO 2)is a most promising semiconductor that has been investigated substantially for its excellent optical and electronic properties,long-term stability,low cost,and non-toxicity [1–3].However,most as-prepared TiO 2is only responsive to ultraviolet (UV)light due to its relatively large band gap (3.2eV for anatase)hindering the utilization of visible light,which occu-pies ca.43%of the entire solar spectrum.

There are several methods to improve the photocatalytic perfor-mance of TiO 2:(1)Chemical doping of TiO 2with metallic (Cr,Fe,V)[4,5]or non-metallic (N,C,B,S or F)elements [6–11]to modify the electronic structures of semiconductors as well as their surface properties,thus extending their visible light absorbance.For exam-ple,C,W-codoped TiO 2exhibited higher visible-light photocata-lytic activity than undoped TiO 2,C-doped TiO 2or W-doped TiO 2.The co-doping of C and W not only leads to the narrowing of the band gap of TiO 2,but also increases the separation ef?ciency of the photo-generated electrons-hole pairs [11];(2)Synthesizing TiO 2with certain exposed facets,which is more reactive due to the higher average surface energy (demonstrated with both theo-retical and experimental evidence)[12–14].TiO 2with certain ex-posed facets has a substantial effect on the surface separation

and transfer of photogenerated electron–hole pairs,resulting in a signi?cant enhancement of photocatalytic ef?ciency;(3)Coupling with other semiconductors to increase the separation ef?ciency of photogenerated electron–hole pairs during photocatalysis [15–20].Recently,the composites comprising TiO 2and other semiconduc-tors,such as WO 3[21],Co 3O 4[22],SiC [15],and Fe 2O 3[16],have been reported.Among these methods,doping TiO 2with a non-me-tal element is a promising approach to achieving visible light re-sponse in TiO 2photocatalysts.Furthermore,doping with two kinds of non-metal atoms has shown more bene?cial effects.How-ever,representative combinations of two non-metal atoms have mostly focused on S/N,C/N,B/N,and N/F pairs [23–27].To the best of our knowledge,there have been few reports on the use of C and F as co-dopants.

Silver chloride (AgCl),which has a direct band gap of 5.6eV and an indirect band gap of 3.25eV,is widely recognized as a photo-sensitive material,and is employed as a source material in photo-graphic ?lms [28,29].Recently,a series of visible-light active composite photocatalysts containing silver halides have been developed for the degradation of organic pollutants [30–32].In these photocatalysts,both metallic Ag nanoparticles and silver ha-lides were generated,and it was revealed that the metallic compo-nent was responsible for the absorption of visible light and photocatalytic activity due to its surface plasmon resonance effect.Thus,it is a necessary and meaningful task to elucidate the role of Ag in the form of silver halides on the photocatalytic activity.

We report on a facile synthetic protocol of TiO 2co-doped with C and F elements via calcination of a mixture of TiOF 2and sulfuric-acid-treated melamine in Ar atmosphere.Further,we demonstrate that the C,F-codoped TiO 2shows enhanced absorption in the

Corresponding author.Tel.:+82232774517;fax:+82232773419.

E-mail address:dhkim@ewha.ac.kr (D.H.Kim).

visible light region,leading to markedly enhanced photocatalytic activity,by loading a small amount of AgCl on its surface.

2.Experimental

2.1.Synthesis of C,F-codoped TiO2

TiOF2was synthesized via a facile one-step hydrothermal reaction according to previous reports[33,34].Typically,35.0ml of titanium butoxide(Ti(OBu)4),78.0ml of acetic acid(CH3COOH),and11mL of47%hydro?uoric acid solution were mixed in a Te?on-lined autoclave with a volume of150ml under stirring.The autoclave was kept at473K for12h in an oven.After cooling down unassisted to room tem-perature,the as-obtained products were collected and washed with water and eth-anol several times,and dried at373K for10h.

The as-prepared TiOF2was mixed with sulfuric-acid-treated melamine.Then, the mixture was heated at400°C for2h in Ar atmosphere,followed by heat treat-ment at400°C for1h in air.

2.2.Loading of AgCl

The loading of AgCl on C,F-codoped TiO2was performed by in situ precipi-tation method.Typically,0.2g of C,F-codoped TiO2powder was impregnated in an aqueous solution containing a certain amount of silver acetate(Ag(CH3COO)). After stirring for12h,NH4Cl solution was added.The solution was then evapo-rated over a water bath at80°C.Finally,the obtained powder was calcined in air at400°C for1h.

2.3.Photocatalytic experiment

The catalyst powder(10mg)was dispersed in30mL of methylene blue(MB) solution(10ppm).The samples were kept in the dark for1h before exposure to light,in order to ensure equilibrium between the adsorption and desorption of dye molecules on the surface of the catalyst.Then,the samples were irradiated un-der stirring by a300-W Xe lamp(Newport Co.,model66984)equipped with a420-nm cutoff?lter as a visible light source.A?xed amount of sample was withdrawn from the stock solution at regular intervals and centrifuged to remove the catalyst. To study the change in absorbance maxima of the dye,the speci?c characteristic absorbance was measured by UV–vis absorbance spectroscopy(Varian Cary5000 UV–vis–NIR spectrophotometer).

2.4.Characterization

The crystal phase of the samples was determined by X-ray diffractometry(D/ max RA,Rigaku)using nickel-?ltered copper radiation(Cu K a)at40kV and 30mA,over a2h range of10°–80°.The morphology of the catalyst was investigated using a JEOL JSM2100-F TEM microscope operated at100kV.X-ray photoelectron spectroscopy(XPS)spectra were collected on a PHI5000Versa Probe(Ulvac-PHI) system using an Al K a(1486.6eV)anode(25W,15kV).The binding energies were calibrated using the carbon C1s peak at284.6eV.UV–vis absorbance spectra were

as-prepared TiOF2precursors and C,F-codoped TiO2,respectively. As shown in Fig.1a,the diffraction peaks can be indexed as cubic phase with a space group of Pm-3m,which is in good agreement with the reported compound,cubic TiOF2(JCPDS No.77-0132). The characteristic peaks of TiOF2disappear,and new anatase peaks appear(Fig.1b)after heat treating the mixture of TiOF2and sulfu-ric-acid-treated melamine.This clearly indicates that the TiOF2 precursors changed to anatase TiO2phase.

The chemical nature of the as-prepared samples was then investigated by X-ray photoelectron spectroscopy(XPS).The sig-nals for C1s,F1s,Ti3d,and O1s,but not N1s,were clearly ob-served in the survey scan spectra in the region of0–1000eV (Fig.2a).The binding energy centered at around684.5eV,which is ascribed to F species,was observed for C,F-codoped TiO2sam-ples,indicating the incorporation of F in the TiO2lattice(Fig.2b). It is also observed in Fig.2c that the C1s peak exhibits two compo-nents with binding energy at284.6eV and288eV,respectively. The binding energy at284.6eV should be ascribed to ambient or-ganic impurities adsorbed on the surface of the sample,and the one at288eV may be associated with the carbon species as an interstitial dopant[35,36].No noticeable peaks due to nitrogen species were observed around399eV.These results lead us to con-clude that the TiO2was doped by C and F after heat treating the mixture of TiOF2and sulfuric-acid-treated melamine.

The loading of AgCl on C,F-codoped TiO2was performed by an in situ precipitation method.Fig.3shows the XRD patterns of sam-ples loaded with different amounts of AgCl,where typical XRD pat-terns of cubic AgCl(JCPDS No.31-1238)can be observed.It is also shown in Fig.3that the loading of AgCl does not change the crystal phase of the doped TiO2.Furthermore,the intensity of AgCl in-creases with the increasing amount of AgCl loaded.Fig.4shows the XPS analysis result of a representative C,F-codoped TiO2sam-ple loaded with AgCl.The C1s,F1s,Ti3d,O1s,Ag3d,and Cl2p signals can be clearly observed in the survey scan spectrum (Fig.4a).The peaks observed at267.2eV and373.2eV can be as-cribed to Ag(I)species.No signal due to Ag(0)could be found from the high-resolution Ag3d XPS spectrum.Therefore,the XRD and XPS results con?rm that cubic AgCl phase was deposited on the surface of C,F-codoped TiO2rather than metallic Ag.

The morphology of TiOF2,C,F-codoped TiO2,and AgCl-loaded C, F-codoped TiO2was characterized by TEM.In Fig.5a,it is observed that well-de?ned sub-micron cube structures with a uniform edge length of around300–400nm are developed for the TiOF2.After calcination with sulfuric-acid-treated melamine,part of the cubic TiOF2morphology was changed to particles,as shown in Fig.5b. Furthermore,some small particles of size around60–100nm could be observed.The interplanar spacing of C,F-codoped TiO2is3.52?(HRTEM,insert in Fig.5b),which corresponds to the d-spacing of the(101)plane from XRD measurements.As shown in Fig.5c, the loading of AgCl has little effect on the morphology of C, F-codoped TiO2.Small particles with4–10-nm diameter could be observed on the surface of C,F-codoped TiO2.Fig.5d shows the HRTEM of a selected particle loading on the surface of C,F-codoped TiO2.The interplanar spacing of the small particle is about2.0?, which corresponds to the(220)d-spacing of AgCl.Therefore,the TEM image clearly shows that the AgCl nanoparticles are loaded on the surface of C,F-codoped TiO2.

The absorption properties of TiOF2,C,F-codoped TiO2,and AgCl-loaded C,F-codoped TiO2were then investigated by UV–vis diffuse re?ectance spectroscopy.The absorption edge of the TiOF2sample occurs at ca.390nm,corresponding to a band gap of 3.2eV (Fig.6a).After calcination of the mixture of TiOF2and sulfuric-acid-treated melamine,the absorption edge of the resulting C,F-codoped TiO2sample was red-shifted to a lower energy region at about500nm,corresponding to a band gap of about 2.48eV (Fig.6b).The absorption edge has little change after loading with

H.Yan et al./Journal of Alloys and Compounds560(2013)20–2621

AgCl,although the intensity of the absorption was increased,as shown in Fig.6c.

Photocatalytic activity was demonstrated in terms of the degra-dation of methylene blue(MB)under visible light irradiation (Fig.7).The ratios of the intensities of the characteristic absor-bance peak of MB at a wavelength of about664nm were plotted after irradiation with visible light for a speci?c period of time(C) and prior to irradiation(C0),as shown in Fig.7e.All the samples show visible-light-responsive photocatalytic activity for the degra-dation of MB.The photocatalytic activity of C,F-codoped TiO2was further enhanced by loading with AgCl.With increasing the amount of AgCl,the photocatalytic activity is increased,and the performance achieves a maximum when the amount of AgCl is about7wt.%.Further increasing the amount of AgCl

crease in photocatalytic activity.The decrease in

activity at larger amounts of AgCl loading may

shielding effect of the visible light absorption by

band gap.The enhanced photocatalytic activity upon

AgCl was also examined in terms of the degradation

nol(see Fig.S1in the Supporting information).

The enhancement of the photocatalytic performance codoped TiO2by loading AgCl is attributed mainly

separation of photogenerated electron–hole pairs.

was supported by comparing the photoluminescence

of neat C,F-codoped TiO2with those of C,F-codoped

with7wt.%AgCl(Fig.S2).The PL of C,F-codoped

7wt.%AgCl was slightly quenched compared with

sample,indicating that the separation of photogenerated

tron–hole pairs is more effective in the presence

cise scheme for the visible-light-driven electron–hole

the AgCl-loaded C,F-codoped TiO2materials is proposed in Fig.8. In general,the conduction band(CB)and valence band(VB)of non-metal-doped TiO2is composed by Ti3d,and the hybrid of O 2p and the p orbital of the non-metal element,respectively.There-fore,the CB and VB of C,F-codoped TiO2are estimated to be à0.29eV and+2.19eV,respectively(with respect to the standard hydrogen electrode potential,SHE).For reference,it was reported that the CB and VB of AgCl are aboutà1.15eV and+2.1eV(versus SHE),respectively[21].The electron–hole pairs are?rst generated upon excitation by visible light absorbed in C,F-codoped TiO2,and the holes are subsequently transferred to the AgCl surface,thereby resulting in the oxidation of Clàions to Cl0atoms.The Cl0atoms in turn act as reactive species for the oxidation of dye molecules.At the same time,the photoinduced electrons are suspended in C,F-codoped TiO2and reduce the O2to Oà

radicals.However,since

2.The XPS spectra of C,F-codoped TiO2.(a)Survey scan;(b)high-resolution F1s;(c)high-resolution C1s and(d)high-resolution The XRD patterns of C,F-codoped TiO2loaded with different amounts of

the conduction band(CB)of AgCl(à1.15eV versus SHE)lies above the CB of C,F-codoped TiO2(à0.3eV versus SHE),the electrons can also reduce the Ag(I)to metallic Ag.

It is well known that Ag@AgCl has been shown to be an effective visible-light active photocatalyst for the surface plasmon reso-nance effect of Ag nanoparticles.However,there is an intrinsic

XPS spectra of C,F-codoped TiO2loaded with7wt.%AgCl.(a)Survey scan;(b)high-resolution Ag3d and(c)high-resolution

As shown in Fig.9,the photocatalytic activity decreases in the?rst

XPS spectrum(Fig.11c).The XPS results also con?rm existence of metallic Ag in the sample after irradiation,indicating part of the AgCl was reduced to metallic Ag during extended irradiation.The atomic concentrations of Ag and Cl in the catalysts and after irradiation were also analyzed by XPS.They determined to be0.58(before)and0.55(after)for Ag,and0.55 and0.21(after)for Cl,respectively.This indicates that leaching of Cl during the photocatalytic degradation of the The XRD patterns of C,F-codoped TiO2before and after irradiation

light for different amount of time.

activity than that of AgCl-loaded C,F-codoped TiO2,as shown in Fig.13,indicating that the incorporation of Ag element as a form of AgCl plays a signi?cant role in enhancing the C,F-codoped TiO2.

4.Conclusion

In conclusion,C,F-codoped TiO2photocatalysts with anatase phase were synthesized by calcination of the mixture of TiOF2 and sulfuric-acid-treated melamine.The C,F-codoping resulted in a red-shift of the absorption edge from390nm for TiO2to 510nm for C,F-codoped TiO2.The C,F-codoped TiO2shows effec-tive photocatalytic activity for the degradation of MB dye under visible light irradiation.The loading of AgCl further enhanced the visible light photocatalytic activity of C,F-codoped TiO2by the effective separation of photogenerated electron–hole pairs.This work demonstrated an unprecedented method for enhancing the photocatalytic activity of doped TiO2nanostructures by loading AgCl.Acknowledgment

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government(2011-0029409; 2012-0009649).H.Yan was supported by the RP-Grant2012of Ewha Womans University.

Appendix A.Supplementary material

Supplementary data associated with this article can be found, in the online version,at https://www.360docs.net/doc/427999933.html,/10.1016/j.jallcom.2013.

01.155.

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